Biology of Sialyl GlycansPart 3: New thinking in function and glycan biosynthesis

Joseph Lau

ST6Gal-1

X3Q O H X1a X1bL X2 IIII II IV V VI

P3 P4 P2b P1 P2a P2c

• A single copy gene (Siat1)

• Transcription starts from six physically distinct promoters

• Generation of the identical ST6Gal-1 protein

• Differential regulation in different tissues and cells

• Feature conserved across mammalian species

α2,6

β1,4test

es

kid

ne

y

mu

scle

liv

er

lun

g

sple

en

bra

in

he

art

P1

ST6Gal-1 deficient mice have moresevere acute inflammatory response

Ce

lls

x 1

0-3

* p <0.003

*

*

0

5 00

1 00 0

1 50 0

2 00 0

t ota l eos neu CD8 CD4 B DC

w t

dP1

Eos Ly6Gpos CCR3pos

Neu Ly6Gpos CCR3neg

;

;

Bone niches govern

hematopoietic stem cell activities-- interactions between “seeds” and “soil”--

- from Yin & Li (2006) J Clin Invest. 116: 1195-1201

;

Hematopoietic stem and progenitor cells

LSKLin neg

cKit ++

Sca-1 +

LKLin neg

cKit +

Sca-1 neg

Lin pos

Lin neg

cKit lo

Sca-1 lo

;

HSPCs from ST6Gal-1 deficient (∆P1) animals

proliferate and differentiate faster ex vivo

Total

Lin - - + +

Sca1 + - - -

cKit + + + -

WT

∆P1

*

*

**

A B

cell

s x 1

05

differentiation

HSC MPP

CLP

GMP

MKEPCMP

OPPLSK

LK

lymph

neu

mono

RBC

platelet

differentiation

;

Recombinant ST6Gal-1 can suppress ex vivo

HSPC proliferation and differentiation !!

cells x 106

Total LSK LKCD11b+

cKit +

CD11b+

cKit -

no addition

rST6Gal

differentiation

HSC MPP

CLP

GMP

MKEPCMP

OPPLSK

LK

lymph

neu

mono

RBC

platelet

differentiation

;

ST6Gal-1 Expression

ST6Gal-1

circulation

liver

α2,6 sialylated

hepatic glycoproteins

bone marrow

Proliferation, Differentiation

;

Challenging the Traditional ParadigmChallenging the Traditional Paradigm

T T

T

T

TTT

� Each cell always makes its own glycans

� Glycans can be made by non-self enzymes!

T T

T

T

TTT

secreted ST6

;

HSPCs have cell surface α2,6-sialic acids,

but do not express the ST6Gal-1 to make them

rela

tive

ab

un

da

nce

ST6Gal-1 mRNA

ST6Gal-1 enzyme activity

Cell surface α2,6 Sia

;

Cell surface α2,6-SA is constructed by

extrinsic ST6Gal-1

α2,6 sialic acid on cells?

KO > KO

WT > KO WT > WT

KO > WT

Mice strains:WT= wild-type mice with intact ST6Gal-1KO= mutant mice with defective ST6Gal-1

KO

donor

recipient

KO

WT

donor

recipient

KO

WT

donor

recipient

WT

KO

donor

recipient

WT

;

SummaryGlycans as mediators between cell (“seed”) and niche (“soil”)

SummaryGlycans as mediators between cell (“seed”) and niche (“soil”)

� Glycans occupy critical intersection between cells and their outside

environment.

� Existence of a novel, “extrinsic” glycosylation pathway

� Extrinsic ST6Gal-1 can alter hematopoietic activity

Current Questions:

� What is mechanism of ST6Gal-1 action?

• How can this be exploited for therapeutic benefit?

� Do other glycosyltransferases act by the “extrinsic” pathway?

• Do they possess clinically useful properties?