The genetics of heteromeric amino acid transporters

20:112-124, 2005. doi:10.1152/physiol.00051.2004 PhysiologyGasol, Marta Pineda, Lidia Feliubadaló, Josep Chillarón and Antonio Zorzano Manuel Palacín, Virginia Nunes, Mariona Font-Llitjós, Maite Jiménez-Vidal, Joana Fort, EmmaThe Genetics of Heteromeric Amino Acid Transporters

You might find this additional information useful...

103 articles, 39 of which you can access free at: This article cites http://physiologyonline.physiology.org/cgi/content/full/20/2/112#BIBL

on the following topics: http://highwire.stanford.edu/lists/artbytopic.dtlcan be found at Medline items on this article's topics

Medicine .. Urine Formation Medicine .. Renal Aminoaciduria Physiology .. Mice Biochemistry .. Transporters Immunology .. Transgenic/Knockout Mice Developmental Biology .. Cystinuria

including high-resolution figures, can be found at: Updated information and services http://physiologyonline.physiology.org/cgi/content/full/20/2/112

can be found at: Physiologyabout Additional material and information http://www.the-aps.org/publications/physiol

This information is current as of November 15, 2005 .

http://www.the-aps.org/.Copyright © 2005 by the American Physiological Society. ISSN: 1548-9213, ESSN: 1548-9221. Visit our website at 20814-3991.bimonthly in February, April, June, August, October, and December by the American Physiological Society, 9650 Rockville Pike, Bethesda MD

) publishes brief review articles on major physiological developments. It is publishedNews in Physiological Science (formerly published as Physiology

on Novem

ber 15, 2005 physiologyonline.physiology.org

nloaded from

acid transporters; CATs) and related proteins,

which on average show <25% amino acid identity

to the light subunits of HATs.

The general features of HATs are as follows

(reviewed in Refs. 15, 21, 61, and 100–102; only very

recent references are cited):

• The heavy subunits (molecular mass of ~90

and ~80 kDa for rBAT and 4F2hc, respec-

tively) are type II membrane N-glycopro-

teins with a single transmembrane domain,

an intracellular NH2

terminus, and an extra-

cellular COOH terminus significantly

homologous to insect and bacterial glucosi-

dases (FIGURE 1). Recently, X-ray diffrac-

tion of the extracellular domain of human

4F2hc revealed a three-dimensional struc-

ture similar to that of bacterial glucosidases

{a triose phosphate isomerase (TIM) barrel

Heteromeric Amino AcidTransporters

Heteromeric amino acid transporters (HATs) are

composed of a heavy subunit and a light subunit

(Table 1) (15, 21, 100–102). Two homologous heavy

subunits (HSHATs) from the SLC3 family have been

cloned and are called rBAT (i.e., related to b0,+

amino acid transport) and 4F2hc [i.e., heavy chain

of the surface antigen 4F2hc, also named CD98 or

fusion regulatory protein 1 (FRP1)]. Nine light sub-

units (LSHATs; SLC7 family members from SLC7A5

to SLC7A13) have been identified. Six of them are

partners of 4F2hc (LAT1, LAT2, y+LAT1, y+LAT2,

asc1, and xCT); one forms a heterodimer with rBAT

(b0,+AT); and two (asc2 and AGT-1) seem to interact

with as yet unknown heavy subunits (100).

Members SLC7A1–SLC7A4 of family SLC7 corre-

spond to system y+ isoforms (i.e., cationic amino

REVIEWS

The Genetics of Heteromeric Amino AcidTransporters

Manuel Palacín,1 Virginia Nunes,2

Mariona Font-Llitjós,2

Maite Jiménez-Vidal,1,2 Joana Fort,1

Emma Gasol,1 Marta Pineda,1

Lidia Feliubadaló,2 Josep Chillarón,1

and Antonio Zorzano1

1Department of Biochemistry and Molecular Biology, Facultyof Biology and Institut de Recerca Biomedica de Barcelona,

Barcelona Science Park, University of Barcelona, and 2Centre de Genètica Mèdica i Molecular, Institut de

Recerca Oncològica, Barcelona, Spain.mpalacin@pcb.ub.edu

Heteromeric amino acid transporters (HATs) are composed of a heavy (SLC3 family)

and a light (SLC7 family) subunit. Mutations in system b0,+ (rBAT-b0,+AT) and in sys-

tem y+L (4F2hc-y+LAT1) cause the primary inherited aminoacidurias (PIAs) cystinuria

and lysinuric protein intolerance, respectively. Recent developments [including the

identification of the first Hartnup disorder gene (B0AT1; SLC6A19)] and knockout

mouse models have begun to reveal the basis of renal and intestinal reabsorption of

amino acids in mammals.

PHYSIOLOGY 20: 112–124, 2005; 10.1152/physiol.00051.2004

Table 1. Heteromeric amino acid transporters

Heavy Chain Light Chain Gene Amino Acid Human Inherited (HSHAT) (LSHAT) Transport Chromosome Aminoaciduria

4F2hc SLC3A2 11q13

y+LAT1 SLC7A7 y+L 14q11.2 LPI

y+LAT2 SLC7A6 y+L 16q22.1

LAT1 SLC7A5 L 16q24.3

LAT2 SLC7A8 L 14q11.2

asc1 SLC7A10 asc 19q12-13

xCT SLC7A11 xc– 4q28-q32

rBAT SLC3A1 2p16.3 cystinuria

b0,+AT SLC7A9 b0,+ 19q12-13 cystinuria

AGT1 SLC7A13 new 8q21.3

asc2 SCL7A12 asc not present

Heterometric amino acid transporters (HATs) are composed of a heavy and a light chain. Heavy subunits belongto solute carrier family SLC3, and light subunits belong to SLC7. AGT and asc2 heterodimerize with unknownheavy subunits. A functional asc2 gene is not present in the human genome. LPI, lysinuric protein intolerance.

on Novem

nloaded from

[(��)8] and eight antiparallel �-strands;

FIGURE 1} (unpublished observations).

• The light subunits (~50 kDa) are highly

hydrophobic and not glycosylated. This

results in anomalously high mobility in SDS-

PAGE (35–40 kDa). Recent cysteine-scanning

mutagenesis studies support a 12-transmem-

brane-domain topology, with the NH2

COOH terminals located inside the cell and

with a reentrant-like structure in the intracel-

lular loop IL2-3 for xCT, as a model for the

light subunits of HATs (FIGURE 1) (31, 38).

• The light and the corresponding heavy sub-

unit are linked by a disulfide bridge (FIGURE

1). For this reason, HATs are also named gly-

coprotein-associated amino acid trans-

porters (21, 102). The intervening cysteine

residues are located in the putative extracel-

lular loop EL3-4 of the light subunit and a

few residues away from the transmembrane

domain of the heavy subunit (FIGURE 1).

• The light subunit cannot reach the plasma

membrane unless it interacts with the heavy

subunit.

• The light subunit confers specific amino

acid transport activity to the heteromeric

complex (LAT1 and LAT2 for system L iso-

forms, y+LAT1 and y+LAT2 for system y+L

isoforms, asc1 and asc2 for system asc iso-

forms, xCT for system xc– isoforms, b0,+AT for

system b0,+ isoforms, and AGT-1 for a system

serving aspartate and glutamate transport

(Table 1). Moreover, reconstitution in lipo-

somes shows that the light subunit b0,+AT is

fully functional in the absence of the heavy

subunit rBAT (71).

• HAT transport activities are, with the excep-

tion of system asc isoforms, tightly coupled

amino acid antiporters (67).

Primary Inherited Aminoacidurias

Primary inherited aminoacidurias (PIAs) are caused

by defective amino acid transport activities, which

113PHYSIOLOGY • Volume 20 • April 2005 • www.physiologyonline.org

REVIEWS

Extracellular

Cytosolic

Disulfide

bridge

COOHNH2

FIGURE 1. A heteromeric amino acid transporter The heavy subunit (pink) and the light subunit (blue) are linked by a disulfide bridge (yellow) with conserved cysteineresidues (cysteine 158 for the human xCT and cysteine 109 for human 4F2hc). The heavy subunits (4F2hc or rBAT) aretype II membrane glycoproteins with an intracellular NH2 terminus, a single transmembrane domain, and a bulkyCOOH terminal. This part of the protein shows homology with bacterial glycosidases. The membrane topology of thelight subunit xCT, as model of the other light subunit, shows 12 transmembrane domains, with the NH2 and COOHterminals located intracellularly and with a reentrant loop-like structure in the intracellular loop IL2-3 (His110 corre-sponds to the apex of the loop) (31). Residues with external (black) or internal (red) accessibility are shown.

on Novem

nloaded from

remains to be identified.

Cystinuria

Cystinuria is an autosomal inherited disorder,

characterized by impaired transport of cystine and

dibasic amino acids in the proximal renal tubule

and the gastrointestinal tract [first described by Sir

Archivald Garrod in 1908 (30) and reviewed in Refs.

60 and 80]. The overall prevalence of the disease is

1/7,000 neonates, ranging from 1/2,500 neonates

in Libyan Jews to 1/100,000 among Swedes (60).

Patients present normal to low-normal levels in

blood, hyperexcretion in urine, and intestinal mal-

absorption of these amino acids. High cystine con-

centration in the urinary tract most often causes

the formation of recurring cystine stones (i.e.,

urolithiasis) due to the low solubility of this amino

acid. This is the only symptom associated with the

disease. Therefore, treatment attempts to increase

cystine solubility in urine (high hydratation, urine

alkalinization, and formation of soluble cystine

adducts with thiol drugs) (60). Cystinuria is not

accompanied by malnutrition, suggesting that

intestinal malabsorption is not severe. Absorption

of di- and tripeptides via PepT1 may prevent mal-

nutrition in cystinuria (17).

Traditionally, three types of cystinuria have been

recognized in humans: type I, type II, and type III

114 PHYSIOLOGY • Volume 20 • April 2005 • www.physiologyonline.org

affect renal reabsorption of amino acids and may

also affect intestinal absorption of amino acids and

transport function in other organs. Several PIA have

been described (Table 2). Reabsorption of dibasic

amino acids is defective in lysinuric protein intoler-

ance [LPI, also named hyperdibasic aminoaciduria

type 2 or familial protein intolerance (MIM 222700)

and first described by Perheentupa et al. in 1965

(63); reviewed in Ref. 86], autosomal dominant

hyperdibasic aminoaciduria type I (MIM 222690)

(106), and isolated lysinuria (56). Reabsorption of

cystine and dibasic amino acids is defective in

cystinuria (reviewed in Ref. 60). Hyperexcretion of

cystine occurs only in isolated cystinuria (MIM

238200) (8). Reabsorption of zwitterionic amino

acids (i.e., neutral amino acids at physiological pH)

is defective in Hartnup disorder (MIM 234500) (3)

and iminoglycinuria (MIM 242600) (72). Finally,

reabsorption of glutamate and aspartate is defective

in dicarboxylic aminoaciduria (MIM 222730) (53,

HATs are involved in PIA, because mutations in

rBAT (SLC3A1) and b0,+AT (SLC7A9) cause cystin-

uria and mutations in y+LAT1 cause LPI. The rela-

tionship between LPI, autosomal dominant hyper-

dibasic aminoaciduria type I, and isolated lysinuria

at the molecular level, if any, is unknown. Similarly,

the gene responsible for isolated cystinuria

REVIEWS

Table 2. Primary inherited aminoacidurias

Prevalence Type of Gene Chromosome Mutations Transport Inheritance System

Cystinuria* 1/7,000Type I AR SLC3A1 2p16.3 103

SLC7A9 19q13.1 12 b0,+

Non-type I ADIP SLC7A9 66SLC3A1 1

Isolated very rare AR? ? ? ? ?cystinuria

LPI >100 cases AR SLC7A7 14q11 31 y+L

Hyperdibasic very rare AD ? ? ? ?aminoaciduriatype 1

Isolated very rare AR? ? ? ? ?lysinuria

Hartnup 1/30,000 AR SLC6A19 5p15 10 B0

disorder

Renal familial 1/15,000 AR ? ? ? Imino(?)†iminoglycinuria

Dicarboxylic very rare AR? SLC1A1 9p24 KO null‡ XAG–

aminoaciduria ? ?

AR, autosomal recessive; ADIP, autosomal dominant with incomplete penetrance; AD, autosomal dominant;AR?, familial studies in the very few cases described for these diseases suggest an autosomal recessive modeof inheritance. *Among the 103 SLC3A1 and 66 SLC7A9 mutations described in cystinuria (which preferentiallyassociated with phenotypes I and non-I, respectively), 12 SLC7A9 mutations associated to phenotype I in someheterozygotes, and 1 SLC3A1 mutation (dupE5–E9) associated in some heterozygotes with phenotype I (28).†The amino acids hyperexcreted in renal familial iminoglycinuria (glycine and proline) suggest that the defec-tive transport system corresponds to system Imino, but no experimental evidence or candidate gene is avail-able. ‡Slc1a1-null knockout mice present dicarboxylic aminoaciduria (62), suggesting this gene as a candidatefor the human disease.

on Novem

nloaded from

(80). This classification correlates poorly with

molecular findings, and it has recently been revised

to type I (MIM 220100) and non-type I (MIM

600918) cystinuria (with the latter corresponding to

old types II and III). These two are distinguished on

the basis of the cystine and dibasic aminoaciduria

of the obligate heterozygotes (60): type I heterozy-

gotes are silent, whereas non-type I heterozygotes

present a variable degree of urinary hyperexcretion

of cystine and dibasic amino acids that is higher in

type II than in type III. This indicates that type I

cystinuria is transmitted as an autosomal recessive

trait, whereas non-type I is transmitted dominant-

ly, with incomplete penetrance (27). Not surpris-

ingly, urolithiasis has been described in a minority

of non-type I heterozygotes (Table 3). Patients with

a mixed type, inheriting type I and non-type I alle-

les from either parent, have also been described

(33). Data on the relative proportion of the two

types in specific populations are scarce. In 97 well-

characterized families of the International

Cystinuria Consortium (ICC) cohort of patients,

mainly from Italy, Spain, and Israel, 38, 47, and 14%

transmit type I, non-type I, and mixed cystinuria,

respectively (Table 3) (28). This cohort is not a reg-

istry, and therefore it might not represent the whole

population within those countries (20).

LPI is a PIA with an autosomal recessive mode of

inheritance (reviewed in Ref. 86). LPI is predomi-

nantly reported in Finland, with a prevalence of

1/60,000. LPI also occurs in Southern Italy and

Japan (prevalence of 1/50,000 in the northern part

of Iwate) (see Ref. 46 and reviewed in Ref. 58).

In LPI there is massive excretion of dibasic amino

acids, especially lysine, and the intestinal absorp-

tion of these amino acids is poor (reviewed in Ref.

57). As a result, plasma levels of dibasic amino

acids are low. Arginine and ornithine are interme-

diates of the urea cycle that provide the carbon

skeleton. Their reduced availability is thought to

produce a functional deficiency of the urea cycle.

Protein malnutrition and lysine deficiency con-

tribute to the patient’s failure to thrive.

Several organs are affected in LPI (reviewed in

Ref. 86). Patients with LPI are usually asymptomatic

while being breast fed, and symptoms (e.g., vomit-

ing, diarrhea, and hyperammonemic coma when

force-fed high-protein food) appear only after

weaning. After infancy, patients with LPI reject a

high-protein diet and show delay in bone growth,

osteoporosis, hepatosplenomegaly, muscle hypoto-

nia, and sparse hair. Most patients show normal

mental development, but some may have moderate

retardation. About two-thirds of the patients have

interstitial changes in chest radiographs, some-

times with acute or chronic respiratory insufficien-

cy that can lead to fatal pulmonary alveolar pro-

teinosis and to multiple-organ dysfunction syn-

drome. Further symptoms, such as glomeru-

lonephritis and erythroblastophagia, suggest that

the immune system is affected in some patients.

Low-protein diet and citrulline (i.e., a urea cycle

intermediate that is not a substrate of system y+L) are

used to correct the functional deficiency of the urea

cycle (reviewed in Ref. 57). The final height in treated

patients is slightly subnormal. This treatment neither

corrects hepatosplenomegaly nor delays bone age or

osteoporosis, probably due to lysine deficiency.

The transport characteristics of two of the LSHAT-

associated transport systems are relevant to the

inherited aminoacidurias cystinuria and LPI (Table

1): system b0,+ [due to the rBAT (SLC3A1) and b0,+AT

(SLC7A9) heterodimer] is a tertiary active mecha-

nism of renal reabsorption and intestinal absorption

REVIEWS

Table 3. Genotype and phenotype classification of cystinurial probands

Phenotype

Genotype I Non-I Non-I Carriers Mixed Total Probands %ab

AA 29 2 31 39.7

AA(B) 1 1 1.3

BB 1 34 7 42 53.8

B+ 3 3 3.8

BB(A) 1 1 1.3

Total Probands 30 34 3 11 78 100 % 38.5 43.6 3.8 14.1 100

Among the 164 probands studied by the International Cystinuria Consortium, 78 are fully genotyped andphenotyped (28). Thirty-eight probands are not fully genotyped [1 mutated SLC3A1 allele (A?), 1 mutat-ed SLC7A9 allele (B?), and no mutated allele (??) have been identified in 11, 22, and 5 probands, respec-tively). Sixty-seven probands (25 AA, 23 BB, 5 A?, 11 B?, and 3 ??) have not been phenotyped due tolack or incoherent information on the amino acid excretion in urine of the obligate heterozygotes. AA, 2mutated SLC3A1 alleles; BB, 2 mutated SLC7A9 alleles; B+, mutated SLC7A9 carrier; AA(B), 2 mutatedSLC3A1 alleles and 1 mutated SLC7A9 allele; BB(A), 2 mutated SLC7A9 alleles and 1 mutated SLC3A1allele.

on Novem

nloaded from

ed to this gene as a candidate for cystinuria. In 1994

it was demonstrated that mutations in SLC3A1 cause

type I cystinuria (11). Since then 103 distinct rBAT

mutations have been described, including non-

sense, missense, splice-site, and frame-shift muta-

tions, as well as large deletions and chromosome

rearrangements (see Ref. 28 and references therein).

Cystinuria resembling type I due to mutations in

canine SLC3A1 has been reported in Newfoundland

dogs (37). Similarly, Pebbels mice (homozygous for

the rBAT mutation D140G) develop type I cystinuria

with urolithiasis (64).

Functional analysis of the most common SLC3A1

mutation, M467T, revealed a trafficking defect, with

the protein reaching the plasma membrane ineffi-

ciently (14). A trafficking defect, also suggested for

other SLC3A1 mutations (14, 74), is consistent with

the proposed role of rBAT as an ancillary subunit of

b0,+AT. SLC3A1 mutations may also affect transport

properties of the holotransporter b0,+: the cystin-

uria-specific mutation R365W, in addition to tem-

perature-sensitive protein stability and trafficking

defect, shows a defect in the efflux of arginine but

not in its influx (68). This suggests two transport

pathways for the light subunit b0,+AT, one for influx

of dibasic amino acid and cystine in the apical plas-

ma membrane. It mediates the electrogenic

exchange of dibasic amino acids (influx) for neutral

amino acids (efflux) (reviewed in Ref. 59) (FIGURE

2). This exchange is favored by the membrane

potential (negative inside the cell) and the high

intracellular concentration of neutral amino acids,

which is the result of the activity of sodium-depend-

ent transport systems for neutral amino acids in the

apical and basolateral domains of the epithelial

cells. The influx of cystine through system b0,+ is

favored by the intracellular reduction of cystine to

cysteine. System y+L (due to a 4F2hc-y+LAT1 het-

erodimer) mediates the electroneutral exchange of

dibasic amino acids (efflux) for neutral amino acids

plus sodium (influx) (10, 13, 39). It is assumed that

this transport system allows the efflux of dibasic

amino acids against the membrane potential at the

basolateral domain of epithelial cells (FIGURE 2).

The 4F2hc-y+LAT2 heterodimer mediates system y+L

in many other cell types (39, 65, 96).

The Molecular Basis of Cystinuria

The above-mentioned characteristics of rBAT point-

REVIEWS

aa+ , CssC aa–

(di, tri)-aa

Sing leaas

H+Na+Na+

Na+ Na+

aa–aa+

aa+ , CssC

Apical

Baso lateral

Blo o d

EAAT3 PepT

4F2hcATPase ?

Taa–

rBATbo,+AT

y+LAT1

T?TAT1

B0AT1 ASCT2

FIGURE 2. Transporters involved in the renal and intestinal reabsorption of amino acidsTransporters with a probed role in renal reabsorption or intestinal absorption of amino acids are colored. Transporterspresent in the apical or basolateral plasma membrane of the epithelial cells of the proximal convoluted tubule or ofthe small intestine, but with no direct experimental evidence supporting their role in reabsorption, are shown uncol-ored. Fluxes of amino acids in the reabsorption direction are in red. PepT (H+-dependent peptide transporter PepT1in small intestine and Pept2 in kidney) cotransports di- and tripeptides [(di,tri)-aa] with protons. Then, intracellularhydrolysis renders single amino acids. The high intracellular concentration of neutral amino acids (aa0) due to thetransport activity of apical (B0AT1) and basolateral (y+LAT1-4F2hc, T) transporters drives the active reabsorption ofdibasic amino acids (aa+) and cystine (CssC), together with the membrane potential and reduction of CssC to cysteine(CSH). Basolateral efflux transporters for dicarboxylic amino acids are unknown (Taa

–). T represents basolateral Na+-dependent transporters with undefined roles in reabsoption (systems A and ASC).

on Novem

nloaded from

and the other for efflux. This scenario is consistent

with two additional sets of results: 1) the chicken

intestinal system b0,+ has a sequential mechanism

of exchange, compatible with the formation of a

ternary complex (i.e., the transporter bound to its

intracellular and extracellular amino acid sub-

strates) (95); and 2) the analog aminoisobutyrate

(AIB) induces an unequal exchange with other sub-

strates through the rBAT-induced system b0,+ in

oocytes (i.e., using the endogenous b0,+AT subunit)

(16). The oligomeric structure of system b0,+ (i.e.,

rBAT-b0,+AT heteromer) is unknown. Functional

coordination of two rBAT-b0,+AT heterodimers in a

heterotetrameric structure would explain these

results. If this is not the case, the transport defect

associated with mutation R365W would suggest

that a single b0,+AT subunit contains two transloca-

tion pathways.

The gene causing non-type I cystinuria was

assigned to 19q12-13.1 by linkage analysis (5, 89,

103). In 1999 the non-type I cystinuria gene was

identified as SLC7A9 (24). SLC7A9 was a positional

candidate gene for non-type I cystinuria because it

has the appropriate chromosomal location, rBAT-

associated amino acid transport activity (system

b0,+), and tissue expression (mainly in kidney and

small intestine). The protein product encoded by

SLC7A9 was termed b0,+AT for b0,+ amino acid trans-

porter. Sixty-six SLC7A9 mutations causing cystin-

uria have been described (see Ref. 28 and refer-

ences therein). Mutation G105R is the most fre-

quent SLC7A9 mutation in the ICC cohort of

patients (27.4% of the identified SLC7A9 alleles).

Similarly to the human disease, the Slc7a9-knock-

out mouse presents non-type I cystinuria with

urolithiasis (23).

Several cystinuria-specific SLC7A9 missense

mutations have been reported to lead to a transport

function defect (27). Among these mutations,

reconstitution in proteoliposomes showed that

A182T-mutated b0,+AT is active but with a trafficking

defect to the plasma membrane, whereas mutation

A354T renders the transporter inactive (71).

Explained alleles in cystinuria

Very recently, the ICC performed an exhaustive

mutational analysis on 164 probands (28): 86.8% of

the independent alleles were identified. The cover-

age of identified alleles was similar in all cystinuria

types (type I, 90.5%; non-type I, 87.6%; mixed,

89.3%; untyped patients, 83.6%).

The unidentified alleles (13.2%) may be due to

mutations in intronic or promoter regions (e.g.,

two cystinuria-specific sequence variants in the

promoter region of SLC3A1 have been reported in

Ref. 7), to SLC3A1 or SLC7A9 polymorphisms in

combination with cystinuria-specific mutations in

the other allele [as suggested by Schmidt et al. (76)],

or to unidentified genes. These three possibilities

have not been confirmed or ruled out. Functional

studies of promoter sequence variants have not

been reported, and cystinuria association studies

and functional analysis of candidate polymor-

phisms are required to demonstrate their role in

the disease. Of particular interest is the possibility

of a third cystinuria gene. In this sense, Goodyer’s

group proposed SLC7A10 as a candidate (50).

SLC7A10 is in the vicinity of the cystinuria gene

SLC7A9 on chromosome 19q13.1 and codes for the

renal light subunit asc1, which, with 4F2hc, forms

the holotransporter asc with substrate specificity

for cysteine and other small neutral amino acids.

Moreover, these authors found the missense muta-

tion E112D associated with cystinuria. In contrast,

recent studies have ruled out this hypothesis: 1)

cystinuria-specific mutations are not found in

patients with alleles not explained by mutations in

the two cystinuria genes (67, 78), 2) the conserva-

tive mutation E112D does not affect transport of

4F2hc-asc1 (67), and 3) asc1 mRNA is expressed in

the distal tubule where renal reabsorption of amino

acids is not relevant but where asc1 may have a role

in the regulation of osmosis (67). Another candi-

date is a missing light subunit of rBAT.

Coimmunoprecipitation studies and the SLC7A9

knockout model demonstrated that the protein

products of SLC3A1 (rBAT) and SLC7A9 (b0,+AT)

heterodimerize in mouse kidney brush border

membranes, but a significant part of rBAT het-

erodimerizes with an unknown light subunit (X)

(23, 25). Indeed, a heterodimer of rBAT is present in

kidney brush-border membranes of the Slc7a9-

knockout mice (23). The protein X may also be

present in human kidney brush borders, but a final

demonstration is needed (25). Therefore, the gene

coding for X could be a candidate for cystinuria.

However, the similar hyperexcretion of amino acids

detected in urine of patients with mutations in

SLC3A1 or in SLC7A9 does not support this hypoth-

esis (28). If X were coded by a cystinuria gene, one

would expect higher aminoaciduria in patients

with mutations in SLC3A1 than in patients with

mutations in SLC7A9: b0,+AT mutations would

affect system b0,+ only, whereas rBAT mutations

would affect system b0,+ and the transport activity

of the rBAT/X heteromeric complex. In summary,

the possibility of a third cystinuria gene cannot be

discarded, but it would be relegated to a very small

proportion of patients and seems improbable

(Only 3% of the probands of the ICC show no muta-

tion in either of the two cystinuria genes; Ref. 28).

Genotype-phenotype correlations in cystinuria

Initial data suggested a one-to-one correlation

between the phenotype and the mutated gene

REVIEWS

on Novem

nloaded from

in SLC3A1 (i.e., individuals AA); 2) all non-type I

patients (including type non-type I heterozygotes

with urolithiasis) have mutations in SLC7A9 (i.e.,

individuals BB and B+); 3) patients with mixed

cystinuria carry mutations in SLC3A1 (2 probands

AA) or in SLC7A9 (7 probands BB); and 4) 2 out of 126

fully genotyped probands carry mutations in both

genes [probands AA(B) and BB(A)], suggesting a role

for digenic inheritance in cystinuria (see below).

Urolithiais shows a clear gender and individual

variability among cystinuria patients (20). In the

ICC cohort, the age of onset (first stone) ranges

from 2–40 yr with a median of 12 and 15 yr for

males and females, respectively. The incidence of

onset before the age of 3 is lower in females than

males. Similarly, the number of total stone events

(i.e., spontaneously emitted stones plus those sur-

gically removed) is higher in males than females

(0.42 and 0.21 events per year in males and females,

respectively). Of the 224 patients studied, ten with

aminoaciduria and full genetic confirmation of the

disease did not develop renal stones, and two of

them are over 40 years of age. In contrast, clinical

symptoms (i.e., urolithiais and its consequences)

are almost identically represented in the two

cystinuria types when either the clinical or the

genetic classification is considered. The differences

in severity between the genders and marked differ-

ences between siblings sharing the same mutations

(19, 20) suggest that other lithogenic factors, genet-

ic or environmental, contribute to the urolithiasis

phenotype. Moreover, only about half of the

Slc7a9-knockout mice, which have a mixed genetic

background, develop urolithiasis (23). Moreover,

lithiasic and nonlithiasic Slc7a9-knockout mice

hyperexcrete similar levels of cystine. Studies in

Slc7a9-knockout mice within different genetic

backgrounds may unravel the genetic factors con-

tributing to urolithiasis besides mutations in Slc7a9

and the cystine levels in urine.

Digenic inheritance causing partial phenotype

To our knowledge, only four patients with muta-

tions in both cystinuria genes have been reported

(28, 35). There is no report of the urine phenotype

of the Swedish patient AA(B). In the ICC database,

only two sisters AA(B) and one male BB(A) from 2

families out of 126 fully genotyped families have

been identified. They are classified as mixed cystin-

uria patients (i.e., each of the two mutated alleles in

the same gene is associated with phenotype I or

non-phenotype I in the obligate heterozygotes).

The aminoaciduria levels of these patients and

their double-heterozygote (i.e., AB) relatives indi-

cate that digenic inheritance in cystinuria has only

a partial effect on the phenotype, restricted to a

variable impact on the aminoaciduria. Indeed,

(mutations in SLC3A1 resulted in type I, and muta-

tions in SLC7A9 resulted in non-type I) (12, 32). In

contrast to this simple view, recent data show a

more complex scenario. On the one hand, all

SLC3A1 mutations in well-characterized families

cause type I cystinuria, with the exception of muta-

tion dupE5–E9, which shows the non-type I pheno-

type in four out of six heterozygotes studied (28).

This mutation consists of a gene rearrangement

c.(891+1524_1618-1600)dup (initially published in

Ref. 77), which results in the duplication of exons

5–9 and the corresponding in-frame duplication of

amino acid residues E298–D539 of rBAT, as shown

by RNA studies (28, 77). Functional studies are nec-

essary to explain the dominant negative effect of

dupE5–E9 mutation on the rBAT/b0,+AT heteromer-

ic complex. On the other hand, most of the het-

erozygotes carrying a SLC7A9 mutation have a non-

type I urine phenotype (i.e., hyperexcretion of

dibasic amino acids and cystine), but heterozy-

gotes carrying SLC7A9 mutations may also have a

type I phenotype (i.e., silent heterozygotes).

Approximately 14% of the SLC7A9 heterozygotes

have phenotype I in the ICC cohort of patients and

their relatives (20). SLC7A9 mutations that have

been found associated with phenotype I in some

families are I44T, G63R, G105R, T123M, A126T,

V170M (the Libyan Jewish mutation), A182T,

G195R, Y232C, P261L, W69X, and c.614dupA (28,

49). There is no clear explanation of why these

mutations associate with phenotype I, since some

proteins show residual transport activity when

expressed in heterologous expression systems but

others do not (27). A182T is the most frequent

SLC7A9 mutation associated with phenotype I (i.e.,

6 out of 11 A182T heterozygotes in the ICC cohort),

and this mutation leads to a protein with 50%

residual transport activity at the plasma membrane

(27, 71). Moreover, mixed cystinuria patients with

two mutations in SLC7A9 presented a level of

aminoaciduria in the lower range of non-type I

patients with two mutations in this gene (28). This

suggests that, in addition to individual and popula-

tion variability, mild SLC7A9 mutations may be

more prone to associate with silent phenotype in

heterozygotes.

The lack of a direct relationship between the

mutated cystinuria gene and the type of cystinuria

led the ICC to propose two parallel classifications to

describe cystinuria (20) that are 1) based on the

urine phenotype of the obligate heterozygotes (type

I, non-type I, and mixed, as described above) and 2)

based on the genotype of the patients (type A due to

mutations in SLC3A1, type B due to mutations in

SLC7A9, and type AB to define a possible digenic

cystinuria). Table 3 summarizes the double classifi-

cation for 78 cystinuria probands by the ICC (28) as

follows: 1) most type I patients have two mutations

REVIEWS

on Novem

nloaded from

none of the individuals AB presented urolithiasis.

Given that the frequencies of type A and B alleles

are similar in this cohort, if digenic inheritance was

the rule in cystinuria, we would expect a quarter of

patients to be AA, a quarter of patients to be BB,

and half of patients to be AB. This indicates that

digenic inheritance affecting phenotype is an

exception in cystinuria. However, we cannot rule

out the possibility that some combinations of

mutations A and B might produce enough cystine

hyperexcretion to cause urolithiasis.

A working hypothesis on the biogenesis of the

rBAT-b0,+AT heterodimer and the urine phenotypes

in cystinuria may explain the apparent lack of full

digenic inheritance in cystinuria: the rBAT protein

is produced in excess in kidney, and therefore nei-

ther an rBAT mutation in heterozygosis [e.g., mouse

cystinuria model with the SLC3A1 mutation D140G

(64)] nor any of the human cystinuria rBAT muta-

tions except dupE5–E9 (28) lead to hyperexcretion

of amino acids (phenotype I). Also, b0,+AT controls

the expression of the functional rBAT-b0,+AT het-

erodimeric complex: interaction with b0,+AT stabi-

lizes rBAT, and the excess of rBAT is degraded, as

shown in transfected cells (4, 71). As a result, a half

dose of b0,+AT [heterozygotes of severe human

SLC7A9 mutations or of the SLC7A9 knockout mice

(23, 28)] results in a significant decrease in the

expression of rBAT-b0,+AT heterodimer complex

(system b0,+), which causes hyperexcretion of cys-

tine and dibasic amino acids. In this scenario, the

lack of a full phenotype because of digenic inheri-

tance indicates that in double heterozygotes (AB),

the mutated rBAT, with trafficking defects (14, 68,

74), does not interact and/or does not compromise

the heterodimerization and trafficking to the plas-

ma membrane of the half dose of wild-type b0,+AT

with the half dose of wild-type rBAT. Thus individu-

als AB behave as heterozygotes B with a variable

degree of aminoaciduria, which could be greater

than that of single heterozygotes within the family,

depending on the particular combination of muta-

tions. Demonstration of this hypothesis requires a

deep study of both the impact of cystinuria-specific

rBAT and b0,+AT mutations (including the dominant

negative rBAT mutation dupE5–E9) in the biogene-

sis and the functional and structural stoichiometry

of subunits of the heteromeric complex rBAT/b0,+AT

both in cell culture studies and in vivo with the help

of Slc3a1 and Slc7a9 double-mutant mice.

The Molecular Basis of LPI

The gene responsible for LPI was localized to

14q11.2 in Finnish and non-Finnish populations

(47, 48). The cloning of y+LAT1, encoded by SLC7A7

(96), revealed characteristics that made this gene

an excellent candidate for LPI:

• y+LAT1 heterodimerizes with 4F2hc to

express system y+L amino acid transport

activity in the basolateral plasma mem-

brane of the epithelial cells of the renal

proximal tubule and the small intestine (see

above).

• SLC7A7 is expressed in tissues affected in

LPI (kidney, small intestine, lung, and white

blood cells).

• SLC7A7 maps to the correct location for LPI.

Torrents and co-workers (97) performed muta-

tional analysis of SLC7A7 in 1 Spanish and 31

Finnish LPI patients. A single Finnish mutant allele

(1181-2A>T) was found, in which the splice site

acceptor of intron 6 is inactivated and a cryptic

acceptor 10 bp downstream is activated, with the

result that 10 bp of the open reading frame are

deleted and the reading frame is shifted. This

mutation has been found in all Finnish LPI patients

(i.e., “the Finnish mutation”) (54, 97). The Spanish

patient with LPI was a genetic compound of two

SLC7A7 mutations (a missense mutation, L334R,

and a 4-bp deletion, 1291delCTTT). Simul-

taneously and independently, Borsani and co-

workers (6) identified the Finnish mutation in four

Finnish patients and found two additional SLC7A7

mutations (1625insATAC and 242del543) in five

Italian patients with LPI. These two studies estab-

lished that mutations in SLC7A7 cause LPI.

Additional studies showed the nonsense mutation

W242X and the insertion 1625insATAC as the most

prevalent mutations in the south of Italy (88) and

the nonsense mutation R410X as the most preva-

lent in Japan (55).

A total of 31 SLC7A7 mutations have been

described in 113 patients with LPI (222 explained

alleles from a total of 226 studied alleles, including

consanguineous ones; reviewed in Ref. 58).

Identified SLC7A7 mutations include missense,

nonsense, and splicing mutations, insertions, dele-

tions, and large genomic rearrangements. No LPI-

associated mutation has been found in SLC3A2,

which encodes the heavy subunit of y+LAT1

(4F2hc). This strongly suggests that SLC7A7 is the

only gene involved in the primary cause of LPI. It is

believed that mutations in 4F2hc would be delete-

rious, and indeed, the targeted disruption of Slc3a2

in mouse is lethal during embryonic life (98). 4F2hc

serves as the heavy subunit of six other heteromer-

ic amino acid transporters (see above). Therefore, a

defect in 4F2hc would result in six defective amino

acid transport activities expressed in many cell

types and tissues. Moreover, 4F2hc is a multifunc-

tional protein with a putative role in �1-integrin

function and cellular fusion (reviewed in Ref. 15).

Thus defective 4F2hc would be life threatening.

Functional studies in oocytes and transfected

REVIEWS

on Novem

nloaded from

port systems in addition to system b0,+ contribute to

the renal reabsorption of dibasic amino acids. The

study of the dibasic amino acid transport activities

in the Slc7a9-knockout mice should help to identify

those transporters. Similar data are not available for

LPI patients, but the experiments of the Finnish

group (70) suggest that system y+L is the main baso-

lateral absorption system for dibasic amino acids in

the small intestine: an oral load with the dipeptide

lysyl-glycine increased glycine plasma concentra-

tions, but plasma lysine remained almost

unchanged in patients with LPI, whereas both

amino acids increased in plasma of control subjects

and in patients with cystinuria. This demonstrated

the basolateral defect in LPI and points to a role of

PepT1 in amino acid assimilation. Indeed, as a con-

sequence of the renal and intestinal defects in LPI,

plasma levels of dibasic amino acids are 1/2 to 1/3

of normal levels (reviewed in Ref. 86), whereas in

human and mouse cystinuria cystine and dibasic

amino acid levels in plasma are only moderately

(20–30%) reduced (reviewed in Ref. 57). In all, these

results suggest that system y+L has a higher impact

than system b0,+ on renal and intestinal reabsorp-

tion of dibasic amino acids.

An understanding of the molecular basis of zwit-

terionic renal and intestinal reabsorption was

helped by the identification of B0AT1 (SLC6A19) as

the gene causing Hartnup disorder (45, 81). B0AT1

corresponds to system B0 [also named system neu-

tral brush border (system NBB)] and catalyzes the

Na+-dependent transport of most neutral amino

acids (9). Patients with Hartnup disorder present

hyperexcretion and malabsorption of neutral

amino acids (reviewed in Ref. 51). Interestingly,

they also show moderate hyperexcretion of cystine

and dibasic amino acids, suggesting a metabolic

link with system b0,+ (reviewed in Ref. 81) (FIGURE

2). The basolateral Na+-K+-ATPase generates the

electrochemical gradient of Na+, which is used by

apical (system B0; i.e., B0AT1) and basolateral (sys-

tems A and ASC) Na+ cotransporters to generate a

high intraepithelial concentration of zwitterionic

amino acids. A reduced influx of zwitterionic

amino acids when B0AT1 is defective (i.e., Hartnup

disorder) would reduce the substrates for exchange

with cystine and dibasic amino acids via system

The above-mentioned transporters with a role in

reabsorption of amino acids are highly expressed in

the corresponding apical or basolateral plasma

membrane of the epithelial cells of the proximal

convoluted tubules (S1 and S2 segments) in kidney

and of the enterocytes of the small intestine (18, 25,

29). The heterodimer 4F2hc-LAT2 has a similar pat-

tern of expression in kidney and small intestine,

where it is located in the basolateral plasma mem-

brane (18, 66). 4F2hc-LAT2 is an exchanger with

cells showed that frameshift mutations (e.g.,

1291delCTTT, 1548delC, and the Finnish mutation)

produce a severe defect in trafficking to the plasma

membrane (54, 94). In contrast, the missense

mutations G54V and L334R inactivate the trans-

porter (e.g., the mutated proteins reach the plasma

membrane when coexpressed with 4F2hc, but no

transport activity is elicited) (54, 94). Recently,

functional studies in oocytes showed that the LPI-

specific SLC7A7 mutations M1L, M50K, T188I,

W242X, S386R, Y457X, and c.1471delTTCT lead to

loss of transport function. In this last study, no

attempt was made to elucidate whether these

mutations affect trafficking or inactivate the trans-

porter (Sperandeo MP and Sebastio G, personal

communication).

The Renal and IntestinalReabsorption of Amino Acids

The renal reabsorption of amino acids occurs in the

proximal convoluted tubule (85), and the absorp-

tion of amino acids occurs in the small intestine

(52). Most of the transporters responsible for these

functions are the same in kidney and intestine

(FIGURE 2). The most striking exception to this rule

is the proton-dependent peptide transporters.

PepT1 and PepT2 are expressed in small intestine

and in kidney, respectively. The physiological role

of PepT2 in kidney is unknown, and the quantita-

tive contribution of PepT1 to assimilation of amino

acids has not yet been evaluated in mammals or

humans (17). A deeper study of the phenotype of

the PepT2-knockout mice (73) and generation and

study of the PepT1 may answer these questions.

The molecular basis of cystinuria and LPI teaches

us about the molecular bases of cystine and dibasic

amino acid reabsorption. Mutations of the apical

exchanger b0,+ (heterodimer rBAT-b0,+AT) lead to

hyperexcretion of cystine and dibasic amino acids,

as shown in human (11, 24), mouse (23, 64), and

canine (37) cystinuria. Mutations of the basolateral

exchanger y+L (heterodimer 4F2hc-y+LAT1) pro-

duce hyperexcretion of dibasic amino acids (6, 97).

Similarly, there is intestinal malabsorption of cys-

tine and dibasic amino acids in cystinuria and of

dibasic amino acids in LPI (reviewed in Refs. 80 and

86). Thus the sequential transport activities of sys-

tems b0,+ (apical) and y+L (basolateral) play a major

role in renal and intestinal reabsorption of amino

acids. Besides these two players, not much is known

about other transporters with a role in dibasic

amino acid reabsorption. Cystinuria patients may

show almost null cystine reabsorption in kidney,

whereas dibasic reabsorption in kidney remains sig-

nificant (reviewed in Ref. 25). This suggests that sys-

tem b0,+ is the main (if not the only) apical reab-

sorption system for cystine, but other apical trans-

REVIEWS

on Novem

nloaded from

broad specificity for small and large zwitterionic

amino acids with characteristics of system L (66).

This suggests that 4F2hc-LAT2 may have a role in

reabsorption of zwitterionic amino acids. Indeed,

antisense experiments in the polarized opossum

kidney cell line OK, derived from proximal convo-

luted epithelial cells, demonstrated a role of LAT2

(SLC7A8) in the transepithelial flux of cystine, and

the basolateral efflux of cysteine and influx of ala-

nine, serine, and threonine (intracellular concen-

tration of cysteine increases, and that of alanine,

serine, and threonine decreases in LAT2-antisense

OK cells in polarized culture) (26). To our knowl-

edge, no inherited human disease has yet been

related to LAT2 mutations. A final demonstration of

the role of LAT2 in reabsorption would only be pos-

sible after the generation of LAT2-knockout mouse

models.

There is evidence for phenotypic variants of

Hartnup disorder, suggesting that 5p15.33

(SLC6A19) is not the only locus to affect or modify

the phenotype (79, 92). Two of the families with

Hartnup disorder apparently present no mutations

for SLC6A19, and linkage to 5p15 has been exclud-

ed in one of them (45). This suggests that a defect in

other amino acid transporters may result in

Hartnup disorder. Several candidates are already

available:

• Within the SLC6 family there are two

B0AT1sequence-related orphan trans-

porters, XT2 (SLC6A18) and XT3 (SLC6A20).

These three genes belong to a branch of

amino acid transporters within the Na+-Cl–-

dependent neurotransmitter family (9).

SLC6A18 is located in the vicinity of

SLC6A19 in 5p15, and therefore it might be

excluded from linkage to Hartnup disorder

(45). Functional and tissue-expression stud-

ies are needed to ascertain the role of XT2

and XT3 in reabsorption of amino acids.

• The amino acid transporter ASCT2 (also

named ATB0) (SLC1A5) is expressed in the

apical plasma membrane of the renal proxi-

mal convoluted and small intestine epithe-

lial cells (1). This transporter exchanges

most of the zwitterionic amino acids in a

Na+-dependent manner (40, 42, 99). The

role of ASCT2 in amino acid reabsorption

has not been evaluated.

• The metabolic link between systems b0,+,

B0, and y+L requires a basolateral transport

system to mediate the efflux of neutral

amino acids (FIGURE 2).

A defective amino acid transport system for

basolateral efflux of zwitterionic amino acids

would increase the intracellular concentration of

these amino acids, resulting in their hyperexcretion

in urine and intestinal malabsorption. Candidate

transporters for this function may be found within

families SLC16 and SLC43. Amino acid transporters

in these families mediate facilitated diffusion and

may therefore mediate the efflux of zwitterionic

amino acids from the high intracellular concentra-

tion to the interstitial space. T-type amino acid

transporter 1 (TAT1; SLC16A10) transports aromat-

ic amino acids in a Na+- and H+-independent man-

ner (43, 44). TAT1 is expressed in human kidney

and small intestine with a basolateral location. The

SLC16 family (also named MCT for monocarboxy-

late transporters) contains members transporting

monocarboxylates and thyroid hormones as well.

Eight transporters within this family are orphan

transporters (MCT5–7, MCT9, and MCT11–14) (34).

Knockout mouse models for TAT1, and their related

orphan transporters expressed in kidney cortex

and small intestine, may help to identify basolater-

al transporters involved in reabsorption of zwitteri-

onic amino acids. LAT3 (2) and LAT4 (5a) within

family SLC43 mediates facilitated diffusion of zwit-

terionic amino acids with characteristics of system

L. Neither of these two transporters is expressed in

epithelial cells of the renal proximal convoluted

tubule or the small intestine, but the SLC43 family

has a third member with no identified transport

function (EEG1; Ref. 90). Functional and tissue-

expression studies are needed to ascertain the role

of EEG1 in reabsorption of amino acids.

The molecular basis of the renal and intestinal

reabsorption of acidic amino acids is less known.

The bulk (>90%) of filtered acidic amino acids is

reabsorbed within segment S1 (i.e., the first part of

the proximal convoluted tubule) (83, 84). Two api-

cal acidic transport systems have been described:

one of high capacity and low affinity and the other

of low capacity and high affinity (reviewed in 36).

The Na+/K+-dependent acidic amino acid trans-

porter EAAT3, which localized to chromosome

9p24 (87) (also named EAAC1; SLC1A1) (system

–) is expressed in the brush-border membranes

of segments S2 and S3 of the nephron (82). The

transport characteristics of SLC1A1 correspond to

the high-affinity system (41). The Slc1a1-knockout

mice develop dicarboxylic aminoaciduria (62),

demonstrating the role of this transporter in renal

reabsorption of acidic amino acids. In contrast, a

direct demonstration of the role of SLC1A1 in

human dicarboxylic aminoaciduria is lacking. The

apical low-affinity transport system for acidic

amino acids in kidney has been determined in

brush-border membrane preparations (104), but

its molecular entity remained elusive (36). At renal

basolateral plasma membranes, a high-affinity

Na+/K+-dependent transport system for acidic

amino acids has been reported (75), but its molec-

REVIEWS

on Novem

nloaded from

10. Broer A, Wagner CA, Lang F, and Broer S. The heterodimericamino acid transporter 4F2hc/y+LAT2 mediates arginineefflux in exchange with glutamine. Biochem J 349: 787–795,2000.

11. Calonge MJ, Gasparini P, Chillaron J, Chillon M, Gallucci M,Rousaud F, Zelante L, Testar X, Dallapiccola B, Di Silverio F,Barcelo P, Estivill X, Zorzano A, Nunes V, and Palacin M.Cystinuria caused by mutations in rBAT, a gene involved inthe transport of cystine. Nat Genet 6: 420–425, 1994.

12. Calonge MJ, Volpini V, Bisceglia L, Rousaud F, de Sanctis L,Beccia E, Zelante L, Testar X, Zorzano A, Estivill X, GaspariniP, Nunez V, and Palacin M. Genetic heterogeneity in cystin-uria: the SLC3A1 gene is linked to type I but not to type IIIcystinuria. Proc Natl Acad Sci USA 92: 9667–9671, 1995.

13. Chillaron J, Estevez R, Mora C, Wagner CA, Suessbrich H,Lang F, Gelpi JL, Testar X, Busch AE, Zorzano A, and PalacinM. Obligatory amino acid exchange via systems b0,+-like andy+L-like. A tertiary active transport mechanism for renal reab-sorption of cystine and dibasic amino acids. J Biol Chem 271:17761–17770, 1996.

14. Chillaron J, Estevez R, Samarzija I, Waldegger S, Testar X,Lang F, Zorzano A, Busch A, and Palacin M. An intracellulartrafficking defect in type I cystinuria rBAT mutants M467T andM467K. J Biol Chem 272: 9543–9549, 1997.

15. Chillaron J, Roca R, Valencia A, Zorzano A, and Palacin M.Heteromeric amino acid transporters: biochemistry, genetics,and physiology. Am J Physiol Renal Physiol 281: F995–F1018,2001.

16. Coady MJ, Chen XZ, and Lapointe JY. rBAT is an amino acidexchanger with variable stoichiometry. J Membr Biol 149:1–8, 1996.

17. Daniel H. Molecular and integrative physiology of intestinalpeptide transport. Annu Rev Physiol 66: 361–384, 2004.

18. Dave MH, Schulz N, Zecevic M, Wagner CA, and Verrey F.Expression of heteromeric amino acid transporters along themurine intestine. J Physiol 558: 597–610, 2004.

19. Dello Strologo L, Carbonari D, Gallucci M, Gasparini P,Bisceglia L, Zelante L, Rousaud F, Nunes V, Palacín M, andRizzoni G. Inter and intrafamilial clinical variability in patientswith cystinuria type I and identified mutation (Abstract). J AmSoc Nephrol 8: 388A, 1997.

20. Dello Strologo L, Pras E, Pontesilli C, Beccia E, Ricci-Barbini V,de Sanctis L, Ponzone A, Gallucci M, Bisceglia L, Zelante L,Jimenez-Vidal M, Font M, Zorzano A, Rousaud F, Nunes V,Gasparini P, Palacin M, and Rizzoni G. Comparison betweenSLC3A1 and SLC7A9 cystinuria patients and carriers: a needfor a new classification. J Am Soc Nephrol 13: 2547–2553,2002.

21. Deves R and Boyd CA. Surface antigen CD98(4F2): not a sin-gle membrane protein, but a family of proteins with multiplefunctions. J Membr Biol 173: 165–177, 2000.

22. Fan MZ, Matthews JC, Etienne NM, Stoll B, Lackeyram D, andBurrin DG. Expression of apical membrane L-glutamate trans-porters in neonatal porcine epithelial cells along the smallintestinal crypt-villus axis. Am J Physiol Gastrointest LiverPhysiol 287: G385–G398, 2004.

23. Feliubadalo L, Arbones ML, Manas S, Chillaron J, Visa J,Rodes M, Rousaud F, Zorzano A, Palacin M, and Nunes V.Slc7a9-deficient mice develop cystinuria non-I and cystineurolithiasis. Hum Mol Genet 12: 2097–2108, 2003.

24. Feliubadalo L, Font M, Purroy J, Rousaud F, Estivill X, NunesV, Golomb E, Centola M, Aksentijevich I, Kreiss Y, Goldman B,Pras M, Kastner DL, Pras E, Gasparini P, Bisceglia L, Beccia E,Gallucci M, de Sanctis L, Ponzone A, Rizzoni GF, Zelante L,Bassi MT, George AL Jr, and Palacin M. Non-type I cystinuriacaused by mutations in SLC7A9, encoding a subunit (b0,+AT)of rBAT. International Cystinuria Consortium. Nat Genet 23:52–57, 1999.

25. Fernandez E, Carrascal M, Rousaud F, Abian J, Zorzano A,Palacin M, and Chillaron J. rBAT-b0,+AT heterodimer is themain apical reabsorption system for cystine in the kidney. AmJ Physiol Renal Physiol 283: F540–F548, 2002.

26. Fernandez E, Torrents D, Chillaron J, Martin Del Rio R,Zorzano A, and Palacin M. Basolateral LAT-2 has a major rolein the transepithelial flux of L-cystine in the renal proximaltubule cell line OK. J Am Soc Nephrol 14: 837–847, 2003.

ular structure has not been identified. Another

member of the SLC1 family, GLT1 [i.e., the glial

high-affinity glutamate transporter (69), also

named EAAT2; SLC1A2] is expressed in rat kidney

cortex and porcine small intestine (22, 105). To our

knowledge, the expression of GLT1 protein has not

been studied in kidney or intestine. Slc1a2-knock-

out mice show lethal spontaneous epileptic

seizures (91). Reabsorption of acidic amino acids

could be studied in these mice to ascertain the role

of GLT1, although this is difficult because only 50%

survive longer than 6 wk. �

We thank Robin Rycroft for editorial help.

Studies in our laboratory referred to above were sup-ported in part by the Spanish Ministry of Science andTechnology (SAF2003-08940-01/02), the European Union(EUGINDAT; LSHM-CT-2003-502852), the SpanishInstituto de Salud Carlos III (networks G03/054, C03/07and C03/08), the Generalitat de Catalunya (2001SGR00399; 2001 SGR00118), and the Comissionat per aUniversitats i Recerca (Generalitat de Catalunya). M. Font-Llitjós was a recipient of a CIRIT fellowship. M. Jiménez-Vidal was supported by BIOMED BMH4 C798-3154. L.Feliubadaló was supported by EUGINDAT.

References

1. Avissar NE, Ryan CK, Ganapathy V, and Sax HC. Na+-depend-ent neutral amino acid transporter ATB0 is a rabbit epithelialcell brush-border protein. Am J Physiol Cell Physiol 281:C963–C971, 2001.

2. Babu E, Kanai Y, Chairoungdua A, Kim DK, Iribe Y,Tangtrongsup S, Jutabha P, Li Y, Ahmed N, Sakamoto S, AnzaiN, Nagamori S, and Endou H. Identification of a novel systemL amino acid transporter structurally distinct from het-erodimeric amino acid transporters. J Biol Chem 278:43838–43845, 2003.

3. Baron DN, Dent CE, Harris H, Hart EW, and Jepson JB.Hereditary pellagra-like skin rash with temporary cerebellarataxia, constant renal amino-aciduria, and other bizarre bio-chemical features. Lancet 271: 421–428, 1956.

4. Bauch C and Verrey F. Apical heterodimeric cystine andcationic amino acid transporter expressed in MDCK cells. AmJ Physiol Renal Physiol 283: F181–F189, 2002.

5. Bisceglia L, Calonge MJ, Totaro A, Feliubadalo L, MelchiondaS, Garcia J, Testar X, Gallucci M, Ponzone A, Zelante L,Zorzano A, Estivill X, Gasparini P, Nunes V, and Palacin M.Localization, by linkage analysis, of the cystinuria type III geneto chromosome 19q13.1. Am J Hum Genet 60: 611–616,1997.

5a. Bodoy S, Martin L, Zorzano A, Palacin M, Esteves R, andBertran J. Identification of LAT4, a novel amino acid trans-porter with system L activity. J Biol Chem (January 19, 2005);doi:10.1074/ jbc.M408638200.

6. Borsani G, Bassi MT, Sperandeo MP, De Grandi A,Buoninconti A, Riboni M, Manzoni M, Incerti B, Pepe A,Andria G, Ballabio A, and Sebastio G. SLC7A7, encoding aputative permease-related protein, is mutated in patientswith lysinuric protein intolerance. Nat Genet 21: 297–301,1999.

7. Boutros M, Ong P, Saadi I, Hiou-Tim F, Vicanek C, Rozen R,and Goodyer P. The human rBAT promoter mutations incystinuria (Abstract). Am J Hum Genet 65, Suppl: A94, 1999.

8. Brodehl J, Gellissen K, and Kowalewski S. An isolated defectof the tubular cystine reabsorption in a family with idiopathichypoparathyroidism. Klin Wochenschr 45: 38–40, 1967.

9. Broer A, Klingel K, Kowalczuk S, Rasko JE, Cavanaugh J, andBroer S. Molecular cloning of mouse amino acid transportsystem B0, a neutral amino acid transporter related toHartnup disorder. J Biol Chem 279: 24467–24476, 2004.

REVIEWS

on Novem

nloaded from

REVIEWS

42. Kekuda R, Torres-Zamorano V, Fei YJ, Prasad PD,Li HW, Mader LD, Leibach FH, and Ganapathy V.Molecular and functional characterization ofintestinal Na+-dependent neutral amino acidtransporter B0. Am J Physiol 272: G1463–G1472,1997.

43. Kim DK, Kanai Y, Chairoungdua A, Matsuo H, ChaSH, and Endou H. Expression cloning of a Na+-independent aromatic amino acid transporterwith structural similarity to H+/monocarboxylatetransporters. J Biol Chem 276: 17221–17228,2001.

44. Kim DK, Kanai Y, Matsuo H, Kim JY, ChairoungduaA, Kobayashi Y, Enomoto A, Cha SH, Goya T,Endou H. The human T-type amino acid trans-porter-1: characterization, gene organization, andchromosomal location. Genomics 79: 95–103,2002.

45. Kleta R, Romeo E, Ristic Z, Ohura T, Stuart C,Arcos-Burgos M, Dave MH, Wagner CA,Camargo SR, Inoue S, Matsuura N, Helip-WooleyA, Bockenhauer D, Warth R, Bernardini I, Visser G,Eggermann T, Lee P, Chairoungdua A, Jutabha P,Babu E, Nilwarangkoon S, Anzai N, Kanai Y,Verrey F, Gahl WA, and Koizumi A. Mutations inSLC6A19, encoding B0AT1, cause Hartnup disor-der. Nat Genet 36: 999–1002, 2004.

46. Koizumi A, Matsuura N, Inoue S, Utsunomiya M,Nozaki J, Inoue K, and Takasago Y. Evaluation ofa mass screening program for lysinuric proteinintolerance in the northern part of Japan. MassScreening Group. Genet Test 7: 29–35, 2003.

47. Lauteala T, Mykkanen J, Sperandeo MP, GaspariniP, Savontaus ML, Simell O, Andria G, Sebastio G,and Aula P. Genetic homogeneity of lysinuric pro-tein intolerance. Eur J Hum Genet 6: 612–615,1998.

48. Lauteala T, Sistonen P, Savontaus ML, MykkanenJ, Simell J, Lukkarinen M, Simell O, and Aula P.Lysinuric protein intolerance (LPI) gene maps tothe long arm of chromosome 14. Am J HumGenet 60: 1479–1486, 1997.

49. Leclerc D, Boutros M, Suh D, Wu Q, Palacin M,Ellis JR, Goodyer P, and Rozen R. SLC7A9 muta-tions in all three cystinuria subtypes. Kidney Int62: 1550–1559, 2002.

50. Leclerc D, Wu Q, Ellis JR, Goodyer P, and RozenR. Is the SLC7A10 gene on chromosome 19 a can-didate locus for cystinuria? Mol Genet Metab 73:333–339, 2001.

51. Levy HL. Hartnup disorder. In: The Metabolic andMolecular Bases of Inherited Disease, edited byScriver CR, Beaudet AL, Sly SW, and Valle D. NewYork: McGraw Hill, 2001, p. 4957–4969

52. Mariotti F, Huneau JF, Mahe S, and Tome D.Protein metabolism and the gut. Curr Opin ClinNutr Metab Care 3: 45–50, 2000.

53. Melançon SB, Dallaire L, Lemieux B, Robitaille P,and Potier M. Dicarboxylic aminoaciduria: aninborn error of amino acid conservation. J Pediatr91: 422–427, 1977.

54. Mykkanen J, Torrents D, Pineda M, Camps M,Yoldi ME, Horelli-Kuitunen N, Huoponen K,Heinonen M, Oksanen J, Simell O, Savontaus ML,Zorzano A, Palacin M, and Aula P. Functionalanalysis of novel mutations in y(+)LAT-1 aminoacid transporter gene causing lysinuric proteinintolerance (LPI). Hum Mol Genet 9: 431–438,2000.

55. Noguchi A, Shoji Y, Koizumi A, Takahashi T,Matsumori M, Kayo T, Ohata T, Wada Y,Yoshimura I, Maisawa S, Konishi M, Takasago Y,and Takada G. SLC7A7 genomic structure andnovel variants in three Japanese lysinuric proteinintolerance families. Hum Mutat 15: 367–372,2000.

56. Omura K, Yamanaka N, Higami S, Matsuoka O,Fujimoto A. Lysine malabsorption syndrome: anew type of transport defect. Pediatrics 57:102–105, 1976.

27. Font MA, Feliubadalo L, Estivill X, Nunes V,Golomb E, Kreiss Y, Pras E, Bisceglia L, d’AdamoAP, Zelante L, Gasparini P, Bassi MT, George AL Jr,Manzoni M, Riboni M, Ballabio A, Borsani G, ReigN, Fernandez E, Zorzano A, Bertran J, and PalacinM. Functional analysis of mutations in SLC7A9,and genotype-phenotype correlation in non-TypeI cystinuria. International Cystinuria Consortium.Hum Mol Genet 10: 305–316, 2001.

28. Font-Llitjós M, Jiménez-Vidal M, Bisceglia L, DiPerna M, de Sanctis L, Rousaud F, Zelante L,PalacínM, and Nunes V. New insights into cystin-uria: 40 new mutations, genotype-phenotype cor-relation, and digenic inheritance causing partialphenotype. J Med Genet 42: 58–68, 2005.

29. Furriols M, Chillaron J, Mora C, Castello A,Bertran J, Camps M, Testar X, Vilaro S, Zorzano A,and Palacin M. rBAT, related to L-cysteine trans-port, is localized to the microvilli of proximalstraight tubules, and its expression is regulated inkidney by development. J Biol Chem 268:27060–27068, 1993.

30. Garrod AE. Inborn errors of metabolism (lecturesI–IV). Lancet 2: 1–214, 1908.

31. Gasol E, Jimenez-Vidal M, Chillaron J, Zorzano A,and Palacin M. Membrane topology of system xc-light subunit reveals a re-entrant loop with sub-strate-restricted accessibility. J Biol Chem 279:31228–31236, 2004.

32. Gasparini P, Calonge MJ, Bisceglia L, Purroy J,Dianzani I, Notarangelo A, Rousaud F, Gallucci M,Testar X, and Ponzone A. Molecular genetics ofcystinuria: identification of four new mutationsand seven polymorphisms, and evidence forgenetic heterogeneity. Am J Hum Genet 57:781–788, 1995.

33. Goodyer PR, Clow C, Reade T, and Girardin C.Prospective analysis and classification of patientswith cystinuria identified in a newborn screeningprogram. J Pediatr 122: 568–572, 1993.

34. Halestrap AP and Meredith D. The SLC16 genefamily–from monocarboxylate transporters(MCTs) to aromatic amino acid transporters andbeyond. Pflügers Arch 447: 619–628, 2004.

35. Harnevik L, Fjellstedt E, Molbaek A, Denneberg T,and Soderkvist P. Mutation analysis of SLC7A9 incystinuria patients in Sweden. Genet Test 7:13–20, 2003.

36. Hediger MA. Glutamate transporters in kidneyand brain. Am J Physiol Renal Physiol 277:F487–F492, 1999.

37. Henthorn PS, Liu J, Gidalevich T, Fang J, CasalML, Patterson DF, and Giger U. Canine cystinuria:polymorphism in the canine SLC3A1 gene andidentification of a nonsense mutation in cystinuricNewfoundland dogs. Hum Genet 107: 295–303,2000.

38. Jimenez-Vidal M, Gasol E, Zorzano A, Nunes V,Palacin M, and Chillaron J. Thiol modification ofcysteine 327 in the eighth transmembranedomain of the light subunit xCT of the het-eromeric cystine/glutamate antiporter suggestsclose proximity to the substrate binding site/per-meation pathway. J Biol Chem 279:11214–11221, 2004.

39. Kanai Y, Fukasawa Y, Cha SH, Segawa H,Chairoungdua A, Kim DK, Matsuo H, Kim JY,Miyamoto K, Takeda E, and Endou H. Transportproperties of a system y+L neutral and basicamino acid transporter. Insights into the mecha-nisms of substrate recognition. J Biol Chem 275:20787–20793, 2000.

40. Kanai Y and Hediger MA. The glutamate and neu-tral amino acid transporter family: physiologicaland pharmacological implications. Eur JPharmacol 479: 237–247, 2003.

41. Kanai Y and Hediger MA. Primary structure andfunctional characterization of a high-affinity gluta-mate transporter. Nature 360: 467–471, 1992.

57. Palacin M, Bertran J, Chillaron J, Estevez R, andZorzano A. Lysinuric protein intolerance: mecha-nisms of pathophysiology. Mol Genet Metab 81,Suppl 1: S27–S37, 2004.

58. Palacin M, Borsani G, and Sebastio G. The molec-ular bases of cystinuria and lysinuric protein intol-erance. Curr Opin Genet Dev 11: 328–335, 2001.

59. Palacin M, Estevez R, Bertran J, and Zorzano A.Molecular biology of mammalian plasma mem-brane amino acid transporters. Physiol Rev 78:969–1054, 1998.

60. Palacin M, Goodyer P, Nunes V, and Gasparini P.Cystinuria. In: The Metabolic and Molecular Basesof Inherited Disease, edited by Scriver CR,Beaudet AL, Sly SW, and Valle D. New York:McGraw Hill, 2001, chapt. 191, p. 4957–4969.[Updated chapter available at http://genetics.accessmedicine.com]

61. Palacin M and Kanai Y. The ancillary proteins ofHATs: SLC3 family of amino acid transporters.Pflügers Arch 447: 490–494, 2004.

62. Peghini P, Janzen J, and Stoffel W. Glutamatetransporter EAAC-1-deficient mice develop dicar-boxylic aminoaciduria and behavioral abnormali-ties but no neurodegeneration. EMBO J 16:3822–3832, 1997.

63. Perheentupa J and Visakorpi JK. Protein intoler-ance with deficient transport of basic aminoacids. Lancet 2: 813–816, 1965.

64. Peters T, Thaete C, Wolf S, Popp A, Sedlmeier R,Grosse J, Nehls MC, Russ A, and Schlueter V. Amouse model for cystinuria type I. Hum MolGenet 12: 2109–2120, 2003.

65. Pfeiffer R, Rossier G, Spindler B, Meier C, Kuhn L,and Verrey F. Amino acid transport of y+L-type byheterodimers of 4F2hc/CD98 and members ofthe glycoprotein-associated amino acid trans-porter family. EMBO J 18: 49–57, 1999.

66. Pineda M, Fernandez E, Torrents D, Estevez R,Lopez C, Camps M, Lloberas J, Zorzano A, andPalacin M. Identification of a membrane protein,LAT-2, that Co-expresses with 4F2 heavy chain, anL-type amino acid transport activity with broadspecificity for small and large zwitterionic aminoacids. J Biol Chem 274: 19738–19744, 1999.

67. Pineda M, Font M, Bassi MT, Manzoni M, BorsaniG, Marigo V, Fernandez E, Rio RM, Purroy J,Zorzano A, Nunes V, and Palacin M. The aminoacid transporter asc-1 is not involved in cystinuria.Kidney Int 66: 1453–1464, 2004.

68. Pineda M, Wagner CA, Broer A, Stehberger PA,Kaltenbach S, and Gelpi JL, Martin Del Rio R,Zorzano A, Palacin M, Lang F, and Broer S.Cystinuria-specific rBAT(R365W) mutation revealstwo translocation pathways in the amino acidtransporter rBAT-b0,+AT. Biochem J 377: 665–674,2004.

69. Pines G, Danbolt NC, Bjoras M, Zhang Y,Bendahan A, Eide L, Koepsell H, Storm-MathisenJ, Seeberg E, and Kanner BI. Cloning and expres-sion of a rat brain L-glutamate transporter. Nature360: 464–467, 1992.

70. Rajantie J, Simell O, and Perheentupa J.Basolateral-membrane transport defect for lysinein lysinuric protein intolerance. Lancet 1:1219–1221, 1980.

71. Reig N, Chillaron J, Bartoccioni P, Fernandez E,Bendahan A, Zorzano A, Kanner B, Palacin M, andBertran J. The light subunit of system b0,+ is fullyfunctional in the absence of the heavy subunit.EMBO J 21: 4906–4914, 2002.

72. Rosenberg LE, Durant JL, and Elsas LJ. Familialiminoglycinuria. An inborn error of renal tubulartransport. N Engl J Med 278: 1407–1413, 1968.

73. Rubio-Aliaga I, Frey I, Boll M, Groneberg DA,Eichinger HM, Balling R, and Daniel H. Targeteddisruption of the peptide transporter Pept2 genein mice defines its physiological role in the kidney.Mol Cell Biol 23: 3247–3252, 2003.

on Novem

nloaded from

85. Silbernagl S. The renal handling of amino acidsand oligopeptides. Physiol Rev 68: 911–1007,1988.

86. Simell O. Lysinuric protein intolerance and othercationic aminoacidurias. In: The Metabolic andMolecular Bases of Inherited Disease, edited byScriver CR, Beaudet AL, Sly SW, Valle D. NewYork: McGraw Hill, 2001, chapt. 192, p.4933–4956. [Updated chapter available athttp://genetics. accessmedicine.com]

87. Smith CP, Weremowicz S, Kanai Y, Stelzner M,Morton CC, and Hediger MA. Assignment of thegene coding for the human high-affinity gluta-mate transporter EAAC1 to 9p24: potential rolein dicarboxylic aminoaciduria and neurodegener-ative disorders. Genomics 20: 335–336, 1994.

88. Sperandeo MP, Bassi MT, Riboni M, Parenti G,Buoninconti A, Manzoni M, Incerti B, Larocca MR,Di Rocco M, Strisciuglio P, Dianzani I, Parini R,Candito M, Endo F, Ballabio A, Andria G, SebastioG, and Borsani G. Structure of the SLC7A7 geneand mutational analysis of patients affected bylysinuric protein intolerance. Am J Hum Genet 66:92–99, 2000.

89. Stoller ML, Bruce JE, Bruce CA, Foroud T,Kirkwood SC, and Stambrook PJ. Linkage of typeII and type III cystinuria to 19q13.1: codominantinheritance of two cystinuric alleles at 19q13.1produces an extreme stone-forming phenotype.Am J Med Genet 86: 134–139, 1999.

90. Stuart RO, Pavlova A, Beier D, Li Z, Krijanovski Y,and Nigam SK. EEG1, a putative transporterexpressed during epithelial organogenesis: com-parison with embryonic transporter expressionduring nephrogenesis. Am J Physiol Renal Physiol281: F1148–F1156, 2001.

91. Tanaka K, Watase K, Manabe T, Yamada K,Watanabe M, Takahashi K, Iwama H, Nishikawa T,Ichihara N, Kikuchi T, Okuyama S, Kawashima N,Hori S, Takimoto M, and Wada K. Epilepsy andexacerbation of brain injury in mice lacking theglutamate transporter GLT-1. Science 276:1699–1702, 1997.

92. Tarlow MJ, Seakins JW, Lloyd JK, Matthews DM,Cheng B, and Thomas AJ. Absorption of aminoacids and peptides in a child with a variant ofHartnup disease and coexistent coeliac disease.Arch Dis Child 47: 798–803, 1972.

93. Teijema HL, van Gelderen HH, Giesberts MA, andLaurent de Angulo MS. Dicarboxylicaminoaciduria: an inborn error of glutamate andaspartate transport with metabolic implications,in combination with a hyperprolinemia.Metabolism 23: 115–123, 1974.

94. Toivonen M, Mykkanen J, Aula P, Simell O,Savontaus ML, and Huoponen K. Expression ofnormal and mutant GFP-tagged y+L amino acidtransporter-1 in mammalian cells. BiochemBiophys Res Commun 291: 1173–1179, 2002.

74. Saadi I, Chen XZ, Hediger M, Ong P, Pereira P,Goodyer P, and Rozen R. Molecular genetics ofcystinuria: mutation analysis of SLC3A1 and evi-dence for another gene in type I (silent) pheno-type. Kidney Int 54: 48–55, 1998.

75. Sacktor B, Rosenbloom IL, Liang CT, and Cheng L.Sodium gradient- and sodium plus potassium gra-dient-dependent L-glutamate uptake in renalbasolateral membrane vesicles. J Membr Biol 60:63–71, 1981.

76. Schmidt C, Tomiuk J, Botzenhart E, Vester U,Halber M, Hesse A, Wagner C, Lahme S, Lang F,Zerres K, Eggermann T, Bachmann H, BokenkampA, Fischbach M, Frund S, Pistor KG, and ZappelHF. Genetic variations of the SLC7A9 gene: alleledistribution of 13 polymorphic sites in Germancystinuria patients and controls. Arbeitsgemein-schaft fur Padiatrische Nephrologie. Clin Nephrol59: 353–359, 2003.

77. Schmidt C, Vester U, Wagner CA, Lahme S,Hesse A, Hoyer P, Lang F, Zerres K, andEggermann T. Significant contribution of genom-ic rearrangements in SLC3A1 and SLC7A9 to theetiology of cystinuria. Arbeitsgemeinschaft furPadiatrische Nephrologie. Kidney Int 64:1564–1572, 2003.

78. Schmidt C, Vester U, Zerres K, and Eggermann T.No evidence for a role of SLC7A10 in 19q13 inthe etiology of cystinuria. Clin Nephrol 62: 71–73,2004.

79. Scriver CR, Mahon B, Levy HL, Clow CL, ReadeTM, Kronick J, Lemieux B, and Laberge C. TheHartnup phenotype: Mendelian transport disor-der, multifactorial disease. Am J Hum Genet 40:401–412, 1987.

80. Segal S and Thier SO. CHAPTER TITLE??? In: TheMetabolic and Molecular Bases of InheritedDiseases (7th ed.), edited by Scriver CH, BeaudetAL, Sly WS, and Valle D. New York: McGraw-Hill,1995, vol. III, chapt. 117, p. 3581–3601.

81. Seow HF, Broer S, Broer A, Bailey CG, Potter SJ,Cavanaugh JA, and Rasko JE. Hartnup disorder iscaused by mutations in the gene encoding theneutral amino acid transporter SLC6A19. NatGenet 36: 1003–1007, 2004.

82. Shayakul C, Kanai Y, Lee WS, Brown D, RothsteinJD, and Hediger MA. Localization of the high-affinity glutamate transporter EAAC1 in rat kid-ney. Am J Physiol Renal Physiol 273: F1023–F1029, 1997.

83. Silbernagl S and Volkl H. Molecular specificity ofthe tubular resorption of “acidic” amino acids. Acontinuous microperfusion study in rat kidney invivo. Pflügers Arch 396: 225–230, 1983.

84. Silbernagl S. Kinetics and localization of tubularresorption of “acidic” amino acids. A microperfu-sion and free flow micropuncture study in rat kid-ney. Pflügers Arch 396: 218–224, 1983.

95. Torras-Llort M, Torrents D, Soriano-Garcia JF,Gelpi JL, Estevez R, Ferrer R, Palacin M, andMoreto M. Sequential amino acid exchangeacross b0,+-like system in chicken brush borderjejunum. J Membr Biol 180: 213–220, 2001.

96. Torrents D, Estevez R, Pineda M, Fernandez E,Lloberas J, Shi YB, Zorzano A, and Palacin M.Identification and characterization of a membraneprotein (y+L amino acid transporter-1) that associ-ates with 4F2hc to encode the amino acid trans-port activity y+L. A candidate gene for lysinuricprotein intolerance. J Biol Chem 273:32437–32445, 1998.

97. Torrents D, Mykkanen J, Pineda M, Feliubadalo L,Estevez R, de Cid R, Sanjurjo P, Zorzano A, NunesV, Huoponen K, Reinikainen A, Simell O,Savontaus ML, Aula P, and Palacin M.Identification of SLC7A7, encoding y+LAT-1, asthe lysinuric protein intolerance gene. Nat Genet21: 293–296, 1999.

98. Tsumura H, Suzuki N, Saito H, Kawano M, OtakeS, Kozuka Y, Komada H, Tsurudome M, and Ito Y.The targeted disruption of the CD98 gene resultsin embryonic lethality. Biochem Biophys ResCommun 308: 847–851, 2003.

99. Utsunomiya-Tate N, Endou H, and Kanai Y.Cloning and functional characterization of a sys-tem ASC-like Na+-dependent neutral amino acidtransporter. J Biol Chem 271: 14883–14890,1996.

100. Verrey F, Closs EI, Wagner CA, Palacin M, EndouH, and Kanai Y. CATs and HATs: the SLC7 family ofamino acid transporters. Pflügers Arch 447:532–542, 2004.

101. Verrey F, Jack DL, Paulsen IT, Saier MH Jr, andPfeiffer R. New glycoprotein-associated amino acidtransporters. J Membr Biol 172: 181–192, 1999.

102. Verrey F, Meier C, Rossier G, and Kuhn LC.Glycoprotein-associated amino acid exchangers:broadening the range of transport specificity.Pflügers Arch 440: 503–512, 2000.

103. Wartenfeld R, Golomb E, Katz G, Bale SJ,Goldman B, Pras M, Kastner DL, and Pras E.Molecular analysis of cystinuria in Libyan Jews:exclusion of the SLC3A1 gene and mapping of anew locus on 19q. Am J Hum Genet 60: 617–624,1997.

104. Weiss SD, McNamara PD, Pepe LM, and Segal S.Glutamine and glutamic acid uptake by rat renalbrushborder membrane vesicles. J Membr Biol43: 91–105, 1978.

105. Welbourne TC and Matthews JC. Glutamatetransport and renal function. Am J Physiol RenalPhysiol 277: F501–F505, 1999.

106. Whelan DT and Scriver CR. Hyperdibasic-aminoaciduria: an inherited disorder of aminoacid transport. Pediatr Res 2: 525–534, 1968.

REVIEWS

on Novem

nloaded from

The genetics of heteromeric amino acid transporters

Documents

The Genetics of Human Longevity

Mexican River Otter Genetics

Genetics and conservation biology

Building Blocks of Population Genetics

Plant ATP-Binding Cassette Transporters

Naftalin De Felice 2012 Transporters and co transporters in theory and practice annotated

Pharmacogenetics of Metformin Transporters Suggests No

Auxin transporters--why so many?

Genetics of early miscarriage

Genetics Slides

Ch.13. WATERSCAPE GENETICS – APPLICATIONS OF LANDSCAPE GENETICS TO RIVERS, LAKES, AND SEAS (in \"Landscape Genetics: Concepts, Methods, Applications\")

Host Genetics and COVID-19

Genetics and African Cattle Domestication

Mendel's Genetics

Cell Biology \u0026 Genetics Cellular Biology and Genetics

Forensic genetics

Genetics of Estrogen-Related Traits;

Mendelian Genetics

Citrus Genetics, Breeding and Biotechnology

Carbohydrate-based synthetic ion transporters