Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
Journ
alof
Cell
Scie
nce
The in vivo function of mammalian cell and tissuepolarity regulators – how to shape and maintain theepidermal barrier
Michaela T. Niessen1,2, Sandra Iden3 and Carien M. Niessen1,3,*1Department of Dermatology, Center for Molecular Medicine, Robert Kochstrasse 21, 50931 Cologne, Germany2International Graduate School for Genetics and Functional Genomics (IGS-GFG), Graduate School for Biological Sciences, Institute for Genetics,Zulpicher Strasse 47a, 50674 Cologne, Germany3Cologne Excellence Cluster on Cellular Stress Responses in Aging-associated Diseases (CECAD), Institute for Genetics, Zulpicher Strasse 47a,50674 Cologne, Germany
*Author for correspondence ([email protected])
Journal of Cell Science 125, 3501–3510� 2012. Published by The Company of Biologists Ltddoi: 10.1242/jcs.092890
SummaryThe establishment and maintenance of cell and tissue polarity is crucial for a range of biological processes, such as oriented division,migration, adhesion and barrier function. The molecular pathways that regulate cell and tissue polarity have been extensively studied inlower organisms as well as in mammalian cell culture. By contrast, relatively little is still known about how polarization regulates the in
vivo formation and homeostasis of mammalian tissues. Several recent papers have identified crucial roles for mammalian polarityproteins in a range of in vivo processes, including stem cell behavior, cell fate determination, junction formation and maintenance andorgan development. Using the epidermis of the skin as a model system, this Commentary aims to discuss the in vivo significance of cell
and tissue polarity in the regulation of mammalian tissue morphogenesis, homeostasis and disease. Specifically, we discuss themechanisms by which the molecular players previously identified to determine polarity in vitro and/or in lower organisms regulateepidermal stratification; orient cell division to drive cell fate determination within the epidermal lineage; and orient hair follicles. Wealso describe how altered polarity signaling contributes to skin cancer.
Key words: Polarity, Cytoskeleton, Signaling, Adhesion, Epidermis, Hair follicle, Barrier function, Cancer
IntroductionPolarity is defined as the unequal distribution of molecules
(RNAs, lipids, proteins) within a cell to produce asymmetry in its
structure and function at the cellular, tissue and organismal level.
Such asymmetry is a basic feature of almost all cells, and is
important for a wide range of processes, including cell fate
determination, adhesion, migration, differentiation and stem cell
maintenance (reviewed by Nelson, 2003; St Johnston and
Ahringer, 2010). Polarity comes in two main forms: cell
polarity and tissue polarity. In cell polarity, asymmetry is
achieved in individual cells, such as for instance the leading and
trailing edge of a migrating cell or apicobasal polarity (Fig. 1A).
In tissue polarity, also known as planar cell polarity (PCP),
subcellular structures and/or cells are aligned in the plane of the
tissue (reviewed by Bayly and Axelrod, 2011; Klein and
Mlodzik, 2005), as seen, for example, in the positioning of
actin-based hair in Drosophila wing cells (Fig. 2A). Although it
was initially mainly studied in lower organisms or in epithelial
cell culture systems, it is now clear that polarization is a
fundamental requirement for the proper functioning of cells and
tissues and that inappropriate function of polarity pathways
disturbs tissue homeostasis, leading to a variety of human
diseases, such as kidney disease, hearing impairment and cancer
(Bulgakova and Knust, 2009; Huang and Muthuswamy, 2010;
Lee and Vasioukhin, 2008; McNeill, 2010).
In this Commentary, we will briefly introduce the major
molecular mediators that are important to establish cell and tissue
polarity and then discuss how different types of polarity are
established in a stratifying epithelium, the epidermis of the skin,
and which molecular players have been implicated in the
regulation of these different processes.
Core molecular regulators of polarityPolarity is established and maintained by the activity of a core set
of polarity proteins, most of which were first identified in
Drosophila melanogaster or Caenorhabditis elegans, and are
highly conserved throughout metazoa. The basic molecular
players that determine either cell or tissue polarity are not
identical. Cell polarity is regulated by a core set of proteins and
protein complexes (Table 1). The Par (partitioning-defective)
proteins were the first cell polarity proteins to be identified based
on their role in asymmetric cell division of the C. elegans zygote
(Kemphues et al., 1988). The PAR genes encode either adaptor
proteins, such as the PDZ-domain-containing proteins Par3 and
Par6 or the 14-3-3 protein Par5, or serine and threonine kinases
such as Par1 and Par4. Par3 and Par6 form a dynamic complex
with atypical protein kinase C (aPKC) (Suzuki and Ohno, 2006;
McCaffrey and Macara, 2009). Two additional cell polarity
complexes regulate a range of asymmetric cell processes: the
transmembrane protein Crumbs forms a complex with the PDZ
Commentary 3501
Journ
alof
Cell
Scie
nce
Fig. 1. Functional similarities and differences of polarity in simple epithelia and stratifying epithelia. At present, it is unclear whether the same molecular
mechanisms controlling apico-basolateral polarity in simple epithelia are involved in the establishment of epidermal polarity. (A) apico-basolateral polarity in
simple epithelia. The apical junctional complex, consisting of tight junctions (TJs), adherens junctions (AJs) and desmosomes (DSs) forms a border to establish
apico-basolateral polarity. Mutual interactions between polarity proteins and complexes regulate apico-basolateral polarity and barrier formation in simple
epithelia. (B) Polarity in the mouse epidermis. In contrast to simple epithelia, the epidermis has no distinct apical and basolateral membrane domains, but displays
apico-basolateral polarity over the multilayered tissue with the stratum granulosum forming the viable apical border. This is reflected in differential expression or
localization of polarity proteins, for example PAR3, PAR6, LGL1, Crumbs and aPKC (black lines indicate localization patterns in the different layers of the skin),
and adhesive junctions. Moreover, lamellar bodies (blue circles) and keratohyalin granules (purple asterisks) are targeted (indicated by arrows) towards the upper
layers to form the cornified envelope. Par, partitioning defective; Dlg, discs large; Lgl, lethal giant larvae; aPKC, atypical protein kinase C; Patj, protein-
associated with tight junction; Pals1, proteins associated with Lin Seven-1.
Stratum corneumStratum granulosumStratum spinosumStratum basale
Sebaceousgland
Hairshaft
Wild type
Vangl2 mutant, Celsr1 mutant
Anterior Posterior
45°
D
Fig. 2. Mechanisms and mediators of planar cell polarity in the
Drosophila wing and mouse epidermis. (A) Single actin-based hairs
in wing cells of Drosophila polarize to localize at the proximal side in
all cells of the tissue (shown on the left). On the right, a schematical
top view on Drosophila wing cells at pupal stages is shown
illustrating the asymmetric distribution of core PCP proteins Pk–
Vang–Fmi group (proximal) and the Dsh–Dgo–Fz–Fmi group
(distal). (B) Polarized distribution of PCP proteins mediate
orientation of developing mouse hair follicles. In the embryonic
epidermal basal layer, E-cadherin is expressed at all cell–cell contact
sites, whereas CELSR1, VANGL2 and FZ are exclusively localized
at anterior and/or posterior lateral membranes already at placode
stage, whereas the budding hair follicle does not reveal any obvious
asymmetry. At hair germ stage, PCP signaling determines anterior
constriction of cells resulting in asymmetry, characterized within the
hair germ by anterior localized ZO-1 and P-cadherin and posterior
enrichment of NCAM and E-cadherin. (C) Orientation of hairs on
dorsal skin and paws of control Fz6+/2 and Fz62/2 (Frizzled-6
knockout) mice. Loss of FZ6 interferes with normal hair patterning
and leads to random waves, swirls and whorls in back skin (top panel)
and paws of mutant mice (bottom panel). Image kindly provided by
Jeremy Nathans. (D) Orientation of hair follicles in the skin. In dorsal
skin of newborn mice, hair grows uniformly in a posterior direction
and hair follicle angles encompass a range of roughly 6 45˚ relative
to the anterior–posterior (A-P) axis (left). In mice with mutations in
Vangl2 or Celsr1, this planar orientation of hair follicles is lost
(right). Pk, Prickle; Vang, Van Gogh; Fmi, Flamingo; Dsh,
Dishevelled; Dgo, Diego; Fz, Frizzled; Celsr1, cadherin EGF-like,
LAG-like, seven pass receptor (murine homolog of Drosophila
flamingo); VANGL2, murine homolog of Van Gogh; ZO-1, zona
occludens-1; NCAM, Neural cell adhesion molecule.
Journal of Cell Science 125 (15)3502
Journ
alof
Cell
Scie
nce
adaptor proteins Patj (protein-associated with tight junction) and
Lin7, as well as the membrane-associated guanylate kinase
(MAGUK) scaffold Pals1 (proteins associated with Lin7)
(Assemat et al., 2008; Bulgakova and Knust, 2009). The other
complex consists of the leucine-rich repeat and PDZ (LAP)
protein Scribble, the WD40 repeat-containing protein Lgl (lethal
giant larvae) and the MAGUK protein Dlg (discs large) (Bilder,
2004; Humbert et al., 2008).
To establish and maintain cell polarity, these cell polarity
proteins function in an integrated network that controls their
subcellular localization through multiple positive- and negative-
feedback loops that are tightly regulated in a spatiotemporal
manner (Fig. 1A). This network interacts with small GTPases,
adhesive junctions, the cytoskeleton and vesicle transport to
regulate the establishment and maintenance of cell polarity
(Iden and Collard, 2008; Nelson, 2003). Although the basic
mechanisms are similar, each type of cell polarity requires the
engagement of different cell polarity proteins.
Tissue polarity is regulated by planar polarity proteins, which
engage in signal transduction to coordinate changes in cell shape
and polarity across cells in the plane of the tissue (McNeill, 2010;
Simons and Mlodzik, 2008). Currently, two main core PCP
pathways have been described, the Frizzled (Fz)–Dishevelled
(Dsh)–Van Gogh (Vang) pathway and the Fat–Dachsous (Ds)–
Four-joint (Fj) pathway, and their functions have been best
characterized in Drosophila (McNeill, 2010; Simons and
Mlodzik, 2008). Fz–Dsh–Vang proteins form asymmetric
domains within cells in the plane of the tissue (Fig. 2A),
whereas the Fat–Ds–Fj pathway establishes gradients over a
range of cells (Table 2). Because tissue polarity requires the
polarization of individual cells, the different polarity signal
pathways are integrated to coordinate cell with tissue polarity
(Simons and Mlodzik, 2008). For example, the cell polarity
protein Scribble genetically interacts with the PCP protein vang-
like 2 (VANGL2) in mice (Montcouquiol et al., 2003), and loss
of Scribble results in classical PCP phenotypes, such as neural
tube closure defects and impaired embryonic wound healing
(Caddy et al., 2010; Dow et al., 2007; Murdoch et al., 2003). For
an in-depth discussion of the function and interactions of core
polarity proteins, we refer the reader to several excellent reviews
that cover different aspects of polarity (Assemat et al., 2008;
Bilder, 2004; Bulgakova and Knust, 2009; Goldstein and Macara,
2007; Humbert et al., 2008; Iden and Collard, 2008; Knoblich,
2008; Li and Bowerman, 2010; Martin-Belmonte and Mostov,
2008; McNeill, 2010; Nelson, 2003; Shin et al., 2006; Simons
and Mlodzik, 2008; St Johnston and Ahringer, 2010).
Whereas in lower organisms most of the core polarity proteins
are unique, in mammals, several orthologs exist for nearly all
core polarity proteins (Tables 1, 2) (for references, see reviews
above). Redundancy or compensation might thus obscure the in
vivo relevance of a particular polarity protein when mouse
knockout models of these proteins are examined. Nevertheless,
the complete inactivation of several polarity proteins has already
revealed their conserved role in mammalian polarity-dependent
processes, such as FAT4, whose inactivation disturbs oriented
cell division and tubule elongation, resulting in cystic kidney
disease (Saburi et al., 2008).
The skin as an in vivo model system for polarity-driven processesMammalian skin protects the organism from dehydration and
provides a barrier against harmful influences, such as UV,
temperature and microbes. The epidermis forms the outermost
layer and consists of the interfollicular epidermis (IFE) and
Table 1. Cell polarity proteinsa
Original symbol Mammalian symbol Function
Par1b MARK2 Ser/Thr kinaseMARK1, MARK3, MARK4, MARK5?
Par3b PAR3A (ASIP) PDZ-containing scaffoldPAR3B
Par4b LKB1 Ser/Thr kinasePar5b 14-3-3 family (7 members, b, c, e, f, t, s, g) PhosphoSer/Thr binding scaffoldPar6b PAR6A
PAR6B PDZ-containing scaffoldPAR6C
aPKC (atypical protein kinase C)d aPKCl (m), aPKCi (h) Ser/Thr kinaseaPKCf
Lgl (Lethal giant larvae)c LGL1 (Hugl1) WD40-repeat-containing scaffoldLGL2
Scribblec Scribble (Vartul, Crib1, LAP4) LAP protein or scaffoldDlg (Discs Large)c DLG1 (Sap97), DLG2 (PSD-93), DLG3 (SAP-102),
DLG4 (PSD95, SAP90), DLG5MAGUK protein or scaffold
Crb (Crumbs)c CRB1 (RP12, LCA8), CRB2, CRB3 TM protein or scaffoldStd (Stardust)c PALS1 (MMP5), PALS2 (MMP6, VAM-1, p55T)
MMP1–MMP3, MMP7MAGUK protein or scaffold
Patj (Drosophila Pals1-associatedtight junction protein)c
PATJ (INADL), MUP1 PDZ-containing scaffold
Lin7b LIN7A (Velis1, MALS-1), LIN7B (Velis2, MALS-2),LIN7C (Velis3, MALS-3)
PDZ-containing scaffold
aOriginal symbols of polarity proteins, the organism in which they were first identified, symbols of the mammalian orthologs and known structural function.bIdentified in C. elegans.cIdentified in Drosophila.dIdentified in mammals.N.D., not determined; TM, transmembrane protein; h, human; m, mouse.
Polarity signaling in the epidermis 3503
Journ
alof
Cell
Scie
nce
epidermal appendages, such as the hair follicles, and sebaceous
and sweat glands (Koster, 2009). Epidermal keratinocytes
balance life-long self-renewal in the proliferating basal layer
with a strictly spatiotemporally regulated terminal differentiation
program that is necessary to form the stratum corneum, a dead,
cornified and water-impermeable cell layer (Koster, 2009).
Different populations of stem and progenitor cells that are
located in the basal layer of the IFE and in specific areas of hair
follicles guarantee constant self-renewal under steady state
conditions and ensure sufficient plasticity for the fast
replacement of lost tissue in case of injury (Blanpain and
Fuchs, 2009; Watt and Jensen, 2009).
The epidermis is therefore an excellent cell biology model
system to study a range of processes that require polarization,
such as (1) formation and maintenance of a life-long self-
renewing stratifying epithelium and its appendages; (2) the
behavior of different populations of stem or progenitor cells that
drive epidermal self-renewal and cyclic regeneration of the hair
follicle; (3) coupling of cell fate decisions to oriented cell
division; (4) the highly organized spatial distribution of hair
follicles and sebaceous glands; and (5) wound healing as a model
for migration and tissue regeneration. Another advantage of this
model system is that primary keratinoyctes can be used to study
differentiation, stratification and junction formation. However,
under 2D cell culture conditions, these cells will not form a
stratum corneum. Therefore, using epidermis and keratinocytes,
several key questions concerning polarity can be investigated,
such as how a life-long self-renewing tissue regulates the balance
between asymmetric and symmetric divisions and how this is
coupled to cell fate; how multicellular structures, such as hair
follicles, align in the plane of the tissue; and how apico-
basolateral polarity is established over multiple layers. In the
following sections, we highlight and discuss recent research on
the epidermis that provides novel insight into these questions.
Apico-basolateral polarity in a stratifyingepitheliumSimple epithelial cells polarize to establish two different
membrane domains – the apical and basolateral membranes –
which have distinct structural and functional characteristics and
are separated by the apical junctional complex (Niessen and
Gottardi, 2008). This complex consists of tight junctions, which
form an ion and size barrier and provide a fence that prevents
mixing of apical and basolateral membrane components,
adherens junctions and desmosomes (Fig. 1A). Apico-
basolateral polarity is essential for the formation of a protective
barrier, and for vectorial functions, such as the directed secretion
of components or transport of fluids. The establishment of apico-
basolateral polarity in simple mammalian epithelial cells is
achieved by complex agonistic and antagonistic interactions of
cell polarity protein complexes that allow for the spatial targeting
of polarity complexes to different domains within the cell
(Fig. 1A) (Nelson, 2009; Roh and Margolis, 2003; St Johnston
and Ahringer, 2010).
The stratifying epidermis is not a classically polarized
epithelium in which tight junctions separate basolateral and
apical membrane proteins and lipids. Instead, the epidermis
establishes polarity along the basal to apical axis of the tissue,
with the stratum granulosum forming the viable apical boundary
(Fig. 1B). The formation of the uppermost layer, the stratum
corneum, depends on the fusion of lamellar bodies (LBs) and
keratohyalin granules with the more apical plasma membranes at
the transition between stratum granulosum and corneum layers.
Interestingly, functional tight junctions are found in the stratum
granulosum and, as in simple epithelia, these might serve as a
fence that is necessary for restricting ‘apical’ targeting of protein
and lipid vesicles at the transition between the granular and
cornified layer.
The mechanisms that regulate the formation of stratifying
apico-basolateral tissue polarity are largely unknown. If
mechanisms similar to those in simple epithelia are in place,
the mutual antagonistic actions of polarity complexes have to be
established over several cell layers. A relatively simple system
could consist of counter-gradients of mutually inhibiting
complexes over the basal–apical axis of the epidermis
(Fig. 1B). From C. elegans to humans, in simple epithelia, the
formation and maintenance of intercellular junctions and apical
membrane domain identity is tightly linked to the activity of cell
polarity proteins, such as Par4 (liver kinase B1 in mammals,
LKB1), the Par–aPKC complex and the Crumbs homolog 1
(Crb1) complex (Goldstein and Macara, 2007; Nelson, 2003;
Bulgakova and Knust, 2009). Loss of either Crb1 or LKB1 does
not obviously impair epidermal barrier function, suggesting that
these proteins are dispensable, although their barrier properties
Table 2. Planar cell polarity proteins
Original symbol Mammalian symbol Function
Fz (Frizzled)a FZ3 7-TM receptorFZ6
Dsh (Dishevelled)a DVL1, DVL2, DVL3PDZ, DIX and DEP domain scaffold
Fmi (Flamingo), Starry knighta CELSR1 7-TM proteinVang (Van Gogh), Strabismusa VANGL1 4-TM protein
VANGL2 (looptail mutant)Dg (Diego)a Diversin (ANKRD6) Ankyrin-repeat containing scaffold
Inversin (INVS)Ft (Fat)a FAT4 Cadherin or TM proteinDs (Dachsous)a ND Cadherin or TM proteinFj (Four-jointed) ND TM serine/threonine kinasePk (Prickle)a PRICKLE1, PRICKLE2 LI- and PET-domain-containing protein
aIdentified in Drosophila.ND, not determined; TM, transmembrane protein.
Journal of Cell Science 125 (15)3504
Journ
alof
Cell
Scie
nce
were not specifically assessed in these studies (Gurumurthy et al.,
2008; van de Pavert et al., 2004). Interestingly, the small GTPase
Rac and aPKC activity are necessary for tight junction barrier
function in stratifying keratinocytes in vitro (Helfrich et al., 2007;
Mertens et al., 2005). In addition, loss of cell adhesion molecules,
such as E-cadherin (Tunggal et al., 2005) or CD44 (Kirschner
et al., 2011), impairs skin barrier function that is associated with
an altered activity or localization of the Par–aPKC complex and,
in the case of CD44, with the loss of apical LBs. Taken together,
these results suggest that adhesive contacts and polarity signal
pathways cooperate in the skin in a similar manner to those in
simple epithelia to drive epithelial barrier function.
Regulation of cell fate and stem cell behavior inthe epidermisCell division not only generates daughter cells, but through the
control of orientation can also regulate their position within the
tissue, and/or their cell fate. Cells can divide either symmetrically
(symmetric cell division, SCD) resulting in two daughter cells
with similar fate, or asymmetrically (asymmetric cell division,
ACD) leading to two daughter cells with differential fate
(Fig. 3A,B). Organisms use this process not only to generate
different cell types during development but also for tissue
homeostasis and regeneration to replace cells that are turned over
or lost (Knoblich, 2010). For example, different populations of
stem and/or progenitor cells use ACD to allow for self-renewal
and differentiation at the same time (Farkas and Huttner, 2008).
One of the mechanistically best understood types of ACD
occurs in the Drosophila neuroblast, in which all divisions are
asymmetric, generating another neuroblast progenitor and a
ganglion mother cell. Neuroblast ACD requires the differential
partitioning of fate-determining proteins (Fig. 3A) (Knoblich,
2008), which is achieved by differential polarization of two
opposite membrane domains within the cell coupled with correct
positioning and orientation of the mitotic spindle. This will
ensure the appropriate partitioning of fate-determining factors
Fig. 3. Mechanisms of asymmetric cell division. Schematic overview of asymmetric localization of polarity proteins and spindle orientation regulators during
asymmetric cell division in Drosophila and in the interfollicular epidermis, illustrating that similar molecular mediators are involved in the establishment of
asymmetric cell divisions of neuroblasts and keratinocytes. (A) Asymmetric cell division (ACD) in Drosophila neuroblasts. The apical aPKC–Baz–Par6 complex
is connected to the Pins–Ga1–MUD complex through Inscuteable (Insc). This complex directs the asymmetric basal localization of the cell fate
determinants Numb, Brat and Prospero. GMC, ganglion mother cell. (B) ACD in the developing IFE. ACD contribute to stratification by producing one basal,
proliferating cell (light green) and one suprabasal cell (dark green), whereas symmetric cell divisions (SCD) result in two daughter cells residing in the basal layer.
aPKC–Par3, INSC and Ga1–LGN–NUMA–DCTN1 localize to one side of the dividing cell and are important for the establishment of epidermal ACD, as reported
for their Drosophila homologs in neuroblast ACD. Suprabasal activity of the Notch signaling pathway (indicated by nuclei positive for HES1, a well-known Notch
target) are crucial for the regulation of this process. (C) Effects of deletion or overexpression of molecular mediators on the ratio between ACD and
SCD and protein localization in the developing IFE. Epidermal deletion of aPKCl (left panel) results in an increase in perpendicular spindle orientation in the
epidermis. Forced induction of mouse INSC (middle panel) induces increased asymmetric spindle orientation in the epidermis shortly after induction (early),
whereas 3 days after induction, this effect is reversed (late) (Poulson and Lechler, 2010). Upon in vivo knockdown of LGN or NUMA (right panel), epidermal
basal cells are biased towards symmetric spindle orientation (Williams et al., 2011). Baz, Bazooka, Drosophila homolog of Par3; Par, partitioning defective;
aPKC, atypical protein kinase C; Pins, partner of Inscuteable (Drosophila homolog of LGN); Mud, mushroom body defect (Drosophila homolog of NUMA); Brat,
brain tumor; INSC, Inscuteable; NUMA, nuclear mitotic apparatus protein1; DCTN1, dynactin1.
Polarity signaling in the epidermis 3505
Journ
alof
Cell
Scie
nce
during cell division (Pearson and Bloom, 2004). Recently, many
authors have also demonstrated the importance of spindleorientation in regulating morphogenetic processes in mammals.
For example, ACD promotes the exit of one daughter cell from
the epicardium into the myocardium during heart morphogenesis(Wu et al., 2010), and regulate the differentiation of mammalian
neuronal progenitors (Zhong and Chia, 2008).
Asymmetric and symmetric divisions in the epidermis
In stratifying epithelia, spindle orientations that are either in
parallel or perpendicular to the basement membrane were firstobserved in the IFE (Smart, 1970), and later in the cornea
(Lamprecht, 1990), hair follicles (Blanpain and Fuchs, 2009;
Zhang et al., 2009) and in sebaceous gland development(Frances and Niemann, 2012) (Fig. 3B, Fig. 4). In the cornea,
perpendicular cell divisions are associated with a differential size
and morphology of the daughter cells (Lamprecht, 1990),suggesting that these divisions are asymmetric ones that
promote differential cell fate, whereas parallel divisions are
symmetric and produce two basal daughters that would increasethe surface area of the cornea. Lechler and Fuchs subsequently
showed that the switch from predominant symmetric (parallel)
divisions to asymmetric (perpendicular) divisions coincides withthe onset of stratification, providing indirect evidence that ACD
promotes epidermal differentiation (Lechler and Fuchs, 2005).
Indeed, whereas the basal daughter remains positive for the basalcell marker keratin 14, the suprabasal daughter originating from
ACD expresses keratin 10, a marker for suprabasal cell identity
(Poulson and Lechler, 2010).
An elegant in vivo lentiviral knockdown system used to
interfere with the molecular machinery that determines spindlepositioning in Drosophila neuroblasts, such as microtubule-
associated dynein binding protein (MUD) and partner of
inscuteable (PINS), provided first mechanistic insight into how
ACD is established in the murine epidermis. For example,knockdown of the mammalian PINS homolog LGN (ten leucine-
glycine-asparagine tripeptides in its N-terminal region) or the
MUD homolog NUMA (nuclear mitotic apparatus protein),results in a strong decrease of ACD, concomitant with an increase
in SCD (Fig. 3C) (Williams et al., 2011). These changes are
accompanied by a thinner epidermis that has a decreased numberof suprabasal layers with impaired expression of differentiation
markers, as well as a slight increase in basal cell number. This
observation links key regulators of spindle positioning todifferentiation in mammalian epithelial cells.
Unlike in Drosophila neuroblasts, not all epidermal divisionsare asymmetric. Moreover, the decision to divide either
symmetrically or asymmetrically is not predetermined in single
epidermal progenitors (Poulson and Lechler, 2010), indicatingthat the microenvironment plays an important role in the overall
outcome of cell divisions. In agreement with this model, loss of
cell adhesive cues, such as b1 integrins or the adherens junctionmolecule a-catenin in mouse epidermis, results in a random
spindle orientation that leads to altered differentiation patterns
(Lechler and Fuchs, 2005).
Polarity proteins in asymmetric and symmetric celldivision in skin
What is the importance of polarity signaling in regulating
asymmetric divisions in mammals? In Drosophila neuroblasts,
the initial polarization cue comes from the apical enrichment ofthe polarity proteins Par3, Par6 and aPKC. This apical
distribution is essential for asymmetric localization of cell fate
determinants, which is coupled to spindle orientation by bindingto the adaptor protein Inscuteable (Insc) (Fig. 3A). Insc then
recruits a protein complex consisting of the heterotrimeric G
Fig. 4. Symmetric and asymmetric divisions in different
compartments of the epidermis. In addition to ACD
occurring in the IFE (see Fig. 2B), ACD-like divisions have
been observed in the region where hair follicle stem cells
move outwards of the bulge region (hair germ) suggesting
that they fuel hair follicle anagen growth (black arrows),
whilst replenishing bulge cells. By contrast, inside the bulge,
symmetric cell divisions serve to replenish the stem cell pool
during anagen phases of the hair cycle. Asymmetric
divisions are also found in the bulb of the anagen hair
follicle positioned close to the dermal papilla and might
drive differentiation of the different hair follicle layers. BM,
basement membrane; SG, sebaceous gland; DP, dermal
papilla.
Journal of Cell Science 125 (15)3506
Journ
alof
Cell
Scie
nce
protein a1-subunit (Ga1), PINS and MUD, which providesattachment sites for astral microtubules (Knoblich, 2010).
Polarized distribution of the aPKC–Par complex is inheritedfrom the epithelial cell, from which the neuroblast arose afterdelamination (Prehoda, 2009). Similarly, in both mouse neuronsand in the epidermis, PAR3 and aPKC show an apical
distribution that is independent of cell division (Lechler andFuchs, 2005). In the epidermis, this apical polarity might havebeen inherited from the polarized single layer epithelium before
the onset of stratification. Whereas in Drosophila neuroblasts theapical membrane domain determines the future neuroblast stemcell, in the epidermis, the aPKC- and Par3-enriched domain
marks the future differentiated suprabasal daughter cell (Poulsonand Lechler, 2010). In addition, Par3 and aPKC also show anapical localization in cells that undergo symmetric divisions,indicating that differential Par3 and aPKC localization alone is
not sufficient to drive ACD. In line with this observation is thefinding that the mammalian PAR–PAR6–aPKC complexregulates spindle positioning in SCD, as recently demonstrated
in 3D cell culture using knockdown approaches (Durgan et al.,2011; Hao et al., 2010).
In the developing mammalian brain, either loss or
overexpression of PAR3 increases SCD, but whereas PAR3overexpression increases the number of cells with progenitorradial glial cell fate, knockdown of PAR3 results in a greater
number of cells with differentiated fate (Bultje et al., 2009).Interestingly, this indicates that levels of PAR3 might directlydetermine cell fate. Similarly, loss of LGL1 in mice causes braindysplasia as a result of an increase in SCD (Klezovitch et al.,
2004). Thus, as in lower organisms, LGL1 and PAR3 in the brainare essential for ACD and work in an antagonistic manner toregulate differential cell fate. It is not yet clear whether PAR3
and LGL1 play similar roles in the developing epidermis.
The role of the two mammalian isoforms of aPKC – aPKCland aPKCf – in the regulation of ACD and SCD is more
controversial. Knockdown of aPKCl in ex vivo cultured mouseembryos alters cell fate (Dard et al., 2009), whereas in vivoinactivation of either aPKCl in developing neurons (Imai et al.,2006) or both mammalian aPKC isoforms in the hematopoetic
system (Sengupta et al., 2011) does not alter cell fate, suggestingthat, in mammals, aPKCs do not control ACD and thereby cellfate in vivo. By contrast, epidermal inactivation of aPKCl results
in an increase in asymmetric divisions in the interfollicularepidermis and the hair follicle that is associated with altereddifferentiation in both epidermal compartment (M.T.N., J. Scott
and C.M.N., unpublished results). This implies that aPKCl mightin fact be a negative regulator of asymmetric spindle orientation(Fig. 3C).
Knockdown of NUMA or LGN in the epidermis does not
affect aPKC or PAR3 localization (Williams et al., 2011)(Fig. 3C). A similar observation was made in mice with anepidermal deletion of serum response factor (SRF), a
transcription factor that activates gene expression of a range ofactin regulators (Luxenburg et al., 2011). In these mice, spindlepositioning is random and is accompanied by random cortical
localization of LGN, even though PAR3 remains apical(Luxenburg et al., 2011). Together, these data suggest that,similar to the process in neuroblasts, aPKC and PAR3 either act
upstream of LGN and NUMA, or that the apical localization ofthese two complexes serves different functions; for example, thePar–aPKC-mediated localization of thus far unknown cell fate
determinants versus LGN–NUMA-mediated spindle orientation.
SRF loss also alters the basal cell architecture that is associatedwith reduced cortical localization of the actinomyosincytoskeleton and the phosphorylated forms of the actin-
associated proteins ezrin, moesin and radixin (collectivelytermed ERM). Interestingly, loss of the PAR3 binding proteinmerlin (also known as NF2), an ERM-related protein, also resultsin altered spindle positioning that is accompanied by impaired
differentiation (Gladden et al., 2010). Thus, perhaps PAR3regulates the recruitment of spindle position regulators to theapical membrane through regulation of ERM–merlin and
actinomyosin.
In Drosophila neuroblasts, the Par–aPKC complex is notabsolutely required for correct spindle orientation, because flieshave an alternative mechanism that involves microtubules and
the MUD complex, which is activated in telophase when theapical determinant complex is either missing or does not functionproperly (reviewed by Knoblich, 2010). Recent data indicate that
the epidermis might also correct potential imbalances in divisionorientation, albeit through a very different mechanism. Epidermaloverexpression of mouse INSC initially promotes asymmetric
divisions, but this effect is reversed upon prolonged INSCexpression, and is accompanied by the dissociation of NUMAfrom the apically localized INSC and LGN proteins (Fig. 3C)
(Poulson and Lechler, 2010). This result suggests that epidermalprogenitors can sense and correct disturbances in the balance ofACD versus SCD (Ray and Lechler, 2011). Taken together, theserecent findings suggest that although the core machinery that
controls ACD in Drosophila is conserved in the mammalianepidermis, there are also important differences with regard to theregulation of ACD and SCD.
Asymmetric divisions and cell fate determination:outstanding questions
One important question that remains to be answered is whether
epidermal ACD is directly coupled to the asymmetric segregationof cell fate determinants, as is the case in Drosophila neuroblasts,and/or whether differential cell fate is achieved through unequal
positioning of cells, which then receive diverse signals from thelocal environment to regulate their fate. Some evidence for thelatter possibility is provided by the observation that the strongdecrease in ACD upon in vivo knockdown of LGN in the
epidermis is associated with a reduction in Notch signaling,which is necessary in suprabasal layers to promote epidermaldifferentiation (Williams et al., 2011).
Another important remaining question is whether asymmetricdivisions regulate cell fate decisions of stem or progenitor cellbehavior in adult skin. Lineage-tracing studies in mice combinedwith mathematical modeling imply that random SCD and ACD
occur in the adult IFE (Clayton et al., 2007). However, it has beendebated whether these cells truly represent long-lived stem cells.In mice, the hair follicle bulge harbours the most pluripotent stem
cells and lineage tracing experiments in this system suggest thatsymmetric divisions replenish these cells (Petersson et al., 2011;Zhang et al., 2010). This is consistent with findings that hair
follicle stem cells do not oblige with the ‘immortal strand’hypothesis (Sotiropoulou et al., 2008; Waghmare et al., 2008).This proposes that stem cells asymmetrically segregate their
chromosomes to retain the same strand of DNA over severaldivisions in order to avoid accumulating DNA mutations.Progenitors located just below the bulge in the hair germ
Polarity signaling in the epidermis 3507
Journ
alof
Cell
Scie
nce
(Greco et al., 2009) can divide asymmetrically (Zhang et al.,2010) and some recent evidence suggest that these cells replenish
bulge stem cells while at the same time providing enough cells todrive anagen hair follicle growth (Hsu et al., 2011) (Fig. 4).Perpendicular spindles have also been observed in the hair bulb(Blanpain and Fuchs, 2009), suggesting that such asymmetric
divisions might regulate the differentiation of the various hairfollicle layers. One important caveat in all these studies is thattracing cell divisions cannot yet be correlated with tracing of the
differential cell fate. Nevertheless, indirect evidence from globalRNA expression data suggests that dividing stem cells in thebulge retain their stem cell signature in agreement with SCD for
these cells (Zhang et al., 2009). It will be important to furtherexamine the in vivo role of polarity proteins in the regulation ofcell fate in the epidermis and its appendages.
Orientation of hair folliclesThe orientation and alignment of hair follicles along the bodyaxis is an example of tissue polarity in the adult skin. In mice,
hair follicles and hairs are inserted with an angle of around 45–55˚ relative to the anterior–posterior (A-P) body axis (Fig. 2D).Unlike single-cell appendages, such as the hair on a Drosophila
wing or the stereocilia in the inner ear sensory hair cell, hairfollicles consist of hundreds of cells. The individual cell positionnot only needs to be aligned with their direct neighbours, but thecells of an individual hair follicle also have to act as a unit to
communicate their position with adjacent hair follicles to achievethis global anterior-posterior hair follicle pattern. Hair folliclemorphogenesis starts at embryonic day 14.5 (E14.5) with the
budding of a placode from the basal epidermal layer in responseto inductive signals from the dermis (Schneider et al., 2009).Initially, placode formation is symmetric with first signs of A-P
asymmetry observed later at the hair germ stage, whendeveloping follicles adopt their polarized position owing tobasal constriction of HF cells at the anterior side, concomitant
with posterior cells adopting a columnar shape (Fig. 2B). This isaccompanied by differential localization of several proteins eitherat the anterior or posterior side of the hair follicle (Devenport andFuchs, 2008).
Several recent papers have provided substantial evidence thatthis coordinated polarization of hair follicles is driven by theactivity of core PCP proteins. Upon loss of one of the PCP
transmembrane component, either mammalian FZ6 (Guo et al.,2004) or the mammalian Flamingo ortholog CELSR1 cadherin(Ravni et al., 2009), hair patterning is disturbed, resulting in hair
swirls (Fig. 2C). Closer analysis of FZ6-deficient mice indicatesthat hair follicle and shaft orientation is regulated by at least twosystems, an early-acting global orientation that depends on FZ6signaling and a later-acting FZ6-independent local PCP system
(Wang and Nathans, 2007). How is asymmetry established in theearly hair follicle? Recent data indicate that FZ6 probably actstogether with CELSR1 and VANGL2 in a non-cell-autonomous
fashion early in hair follicle morphogenesis to globally orient hairfollicles. One day before placode formation (E13.5), FZ6,VANGL2 and CELSR1 exhibit a distinct localization pattern,
with VANGL2 enriched on anterior basal membranes, FZ6 onposterior ones and CELSR1 on both (Devenport and Fuchs,2008), which is a similar pattern to that observed for these
molecules in, for example, Drosophila wing cells (Fig. 2A,B).Mutations in genes encoding CELSR1 or VANGL2 not onlydisturb anterior and/or posterior localization of VANGL2, FZ6
and CELSR1 (Devenport and Fuchs, 2008), indicating that theirlocalization is mutually dependent, but also prevent constriction
and hair follicle asymmetry (Fig. 2D). PCP proteins areinternalized during mitosis in a manner that is dependent onCELSR1, and segregate symmetrically upon division, after whichthey regain their polarized distribution (Devenport et al., 2011).
This mechanism probably serves as a protective mechanismagainst improper planar signaling across different cells and hairfollicles, when cells round up during mitosis. Prevention of their
internalization during development interferes with the orientationof hair follicles (Devenport et al., 2011). Together, these resultsindicate that the core FZ6–VANGL2 pathway regulates the
individual and global polarization of hair follicles.
Altered polarity – a hallmark of cancerHallmarks of cancer are unrestricted cell division, an altered
cyto-architecture and a gain of migratory and invasive capacities,all properties in which polarity proteins have a key role. Ingeneral, loss or reduced expression of polarity proteins (e.g.
LKB1, Scribble, DLG1, LGL2, DSH, PAR3 and Crumbs) hasbeen associated with tumor initiation and/or progression(Hawkins and Russell, 2008; Huang and Muthuswamy, 2010;
Humbert et al., 2008; Lee and Vasioukhin, 2008), suggestingthat they might have tumor suppressive function. The E6oncoprotein of human papillomavirus (HPV) strains associatedwith high carcinogenesis risk target human Scribble and DLG1
for degradation (Gardiol et al., 1999; Nakagawa and Huibregtse,2000), in agreement with such a tumor suppressive function. Inmice, loss of Scribble indeed results in prostate neoplasia
(Pearson et al., 2011). However, only LKB1 mutations havebeen directly linked to human cancer thus far (Jansen et al.,2009). Epidermal inactivation of LKB1 predisposes mice to non-
melanoma skin cancer, thus providing direct evidence that LKB1serves as a cell-autonomous tumor suppressor in the skin(Gurumurthy et al., 2008). Epidermal loss of PAR3 also
inhibits formation and growth of papillomas. However, suchmice are predisposed to the formation of keratoacanthomas (S.I.and J. G. Collard, unpublished). These tumors are thought toderive from a different cellular subpopulation within the
epidermis than papilloma (Perez-Losada and Balmain, 2003),thus suggesting that the cellular context might determine whetherPAR3 functions as a tumor promoter or suppressor.
An important question for the future is how altered polaritysignaling inhibits and/or promotes (skin) carcinogenesis. This islikely to occur on different levels. For instance, tumor initiation
might occur through alterations in the extent of ACD comparedwith SCD. In Drosophila neuroblasts, switching from ACD toSCD promotes unrestricted growth and allows for tumorformation (Januschke and Gonzalez, 2008). An increase in
ACD is therefore predicted to inhibit mammalian tumorigenesis,whereas an increase in SCD could promote carcinogenesis (Leeand Vasioukhin, 2008). Although this has not been rigorously
tested in mammals, it is interesting to note that overexpression ofPAR6 or aPKCi is associated with a range of human carcinomas(Fields et al., 2007; Nolan et al., 2008). In addition, expression
of a constitutive membrane-bound aPKC promotes symmetricdivisions and overgrowth in Drosophila neuroblasts (Lee et al.,2006). By contrast, loss of aPKCl almost completely prevents
tumorigenesis in either mouse models of colon cancer (Murrayet al., 2009) or lung carcinoma (Regala et al., 2009). PAR6–aPKC might also drive tumorigenesis through interactions with
Journal of Cell Science 125 (15)3508
Journ
alof
Cell
Scie
nce
kinase receptors, because for example, transformation driven by
the tyrosine kinase ERBB2 depends on PAR6–aPKC (Aranda
et al., 2006). Similarly, many in vitro studies have shown that
polarity proteins have a crucial role in the assembly and
disassembly of cell–cell junctions (Feigin and Muthuswamy,
2009; Suzuki and Ohno, 2006), as well as in migration (Caddy
et al., 2010; Dow et al., 2007; Pegtel et al., 2007; Tscharntke
et al., 2007). This is not only essential for wound healing (Caddy
et al., 2010), but also is a key determinant for invasion of tumor
cells.
Conclusion and PerspectivesThe last decade has brought tremendous progress in our insights
into how cell and tissue polarity proteins regulate a range of
biological essential processes. Only recently, in vivo tissue-
specific mammalian models have become available that now
allow us to tease out the cell-autonomous and non-cell-
autonomous functions of these proteins in different tissues. As
highlighted here for the epidermis, these studies started to reveal
important insights into their function during morphogenesis.
Because many core polarity proteins have multiple mammalian
orthologs, a future challenge will be to identify their unique and
overlapping roles, as well as to determine tissue-specific
requirements and link them to human disease. In this context, it
is important to note that polarity protein functions not only
determine cell and tissue architecture, but have also been
implicated in the regulation of a variety of cellular pathways
that are crucial for growth, differentiation, metabolic activity and
innate immunity. Thus, polarity signal pathways could turn out to
be central integrators of cyto-architecture and tissue homeostasis.
AcknowledgementsWe would like to apologize to all colleagues whose original work wecould not cite because of space limitations. We would like to thankJeremy Nathans (Johns Hopkins University, Baltimore) for providingus with the images for Fig. 2C. We thank Susanne Vorhagen andRehan Villani for helpful discussions.
FundingC.M.N. is funded by the Deutsche Krebshilfe and the DeutscheForschunsgemeinschaft (DFG) [grant numbers SFB829 A1, Z2 andSFB832 A3 and Z3]. S.I. is supported by the DFG [grant numbersSFB829 and SFB832], and by the Stiftung Kolner Krebsforschung.
ReferencesAranda, V., Haire, T., Nolan, M. E., Calarco, J. P., Rosenberg, A. Z., Fawcett, J. P.,
Pawson, T. and Muthuswamy, S. K. (2006). Par6-aPKC uncouples ErbB2 induced
disruption of polarized epithelial organization from proliferation control. Nat. Cell
Biol. 8, 1235-1245.
Assemat, E., Bazellieres, E., Pallesi-Pocachard, E., Le Bivic, A. and Massey-
Harroche, D. (2008). Polarity complex proteins. Biochim. Biophys. Acta 1778, 614-
630.
Bayly, R. and Axelrod, J. D. (2011). Pointing in the right direction: new developments
in the field of planar cell polarity. Nat. Rev. Genet. 12, 385-391.
Bilder, D. (2004). Epithelial polarity and proliferation control: links from the Drosophila
neoplastic tumor suppressors. Genes Dev. 18, 1909-1925.
Blanpain, C. and Fuchs, E. (2009). Epidermal homeostasis: a balancing act of stem
cells in the skin. Nat. Rev. Mol. Cell Biol. 10, 207-217.
Bulgakova, N. A. and Knust, E. (2009). The Crumbs complex: from epithelial-cell
polarity to retinal degeneration. J. Cell Sci. 122, 2587-2596.
Bultje, R. S., Castaneda-Castellanos, D. R., Jan, L. Y., Jan, Y. N., Kriegstein, A. R.
and Shi, S. H. (2009). Mammalian Par3 regulates progenitor cell asymmetric division
via notch signaling in the developing neocortex. Neuron 63, 189-202.
Caddy, J., Wilanowski, T., Darido, C., Dworkin, S., Ting, S. B., Zhao, Q., Rank, G.,
Auden, A., Srivastava, S., Papenfuss, T. A. et al. (2010). Epidermal wound repair is
regulated by the planar cell polarity signaling pathway. Dev. Cell 19, 138-147.
Clayton, E., Doupe, D. P., Klein, A. M., Winton, D. J., Simons, B. D. and Jones,
P. H. (2007). A single type of progenitor cell maintains normal epidermis. Nature
446, 185-189.
Dard, N., Le, T., Maro, B. and Louvet-Vallee, S. (2009). Inactivation of aPKClreveals a context dependent allocation of cell lineages in preimplantation mouseembryos. PLoS ONE 4, e7117.
Devenport, D. and Fuchs, E. (2008). Planar polarization in embryonic epidermisorchestrates global asymmetric morphogenesis of hair follicles. Nat. Cell Biol. 10,1257-1268.
Devenport, D., Oristian, D., Heller, E. and Fuchs, E. (2011). Mitotic internalization ofplanar cell polarity proteins preserves tissue polarity. Nat. Cell Biol. 13, 893-902.
Dow, L. E., Kauffman, J. S., Caddy, J., Zarbalis, K., Peterson, A. S., Jane, S. M.,Russell, S. M. and Humbert, P. O. (2007). The tumour-suppressor Scribble dictatescell polarity during directed epithelial migration: regulation of Rho GTPaserecruitment to the leading edge. Oncogene 26, 2272-2282.
Durgan, J., Kaji, N., Jin, D. and Hall, A. (2011). Par6B and atypical PKC regulatemitotic spindle orientation during epithelial morphogenesis. J. Biol. Chem. 286,12461-12474.
Farkas, L. M. and Huttner, W. B. (2008). The cell biology of neural stem andprogenitor cells and its significance for their proliferation versus differentiationduring mammalian brain development. Curr. Opin. Cell Biol. 20, 707-715.
Feigin, M. E. and Muthuswamy, S. K. (2009). Polarity proteins regulate mammaliancell-cell junctions and cancer pathogenesis. Curr. Opin. Cell Biol. 21, 694-700.
Fields, A. P., Frederick, L. A. and Regala, R. P. (2007). Targeting the oncogenicprotein kinase Ci signalling pathway for the treatment of cancer. Biochem. Soc. Trans.
35, 996-1000.
Frances, D. and Niemann, C. (2012). Stem cell dynamics in sebaceous glandmorphogenesis in mouse skin. Dev. Biol. 363, 138-146.
Gardiol, D., Kuhne, C., Glaunsinger, B., Lee, S. S., Javier, R. and Banks, L. (1999).Oncogenic human papillomavirus E6 proteins target the discs large tumour suppressorfor proteasome-mediated degradation. Oncogene 18, 5487-5496.
Gladden, A. B., Hebert, A. M., Schneeberger, E. E. and McClatchey, A. I. (2010).The NF2 tumor suppressor, Merlin, regulates epidermal development through theestablishment of a junctional polarity complex. Dev. Cell 19, 727-739.
Goldstein, B. and Macara, I. G. (2007). The PAR proteins: fundamental players inanimal cell polarization. Dev. Cell 13, 609-622.
Greco, V., Chen, T., Rendl, M., Schober, M., Pasolli, H. A., Stokes, N., Dela Cruz-Racelis, J. and Fuchs, E. (2009). A two-step mechanism for stem cell activationduring hair regeneration. Cell Stem Cell 4, 155-169.
Guo, N., Hawkins, C. and Nathans, J. (2004). Frizzled6 controls hair patterning inmice. Proc. Natl. Acad. Sci. USA 101, 9277-9281.
Gurumurthy, S., Hezel, A. F., Sahin, E., Berger, J. H., Bosenberg, M. W. and
Bardeesy, N. (2008). LKB1 deficiency sensitizes mice to carcinogen-inducedtumorigenesis. Cancer Res. 68, 55-63.
Hao, Y., Du, Q., Chen, X., Zheng, Z., Balsbaugh, J. L., Maitra, S., Shabanowitz, J.,
Hunt, D. F. and Macara, I. G. (2010). Par3 controls epithelial spindle orientation byaPKC-mediated phosphorylation of apical Pins. Curr. Biol. 20, 1809-1818.
Hawkins, E. D. and Russell, S. M. (2008). Upsides and downsides to polarity andasymmetric cell division in leukemia. Oncogene 27, 7003-7017.
Helfrich, I., Schmitz, A., Zigrino, P., Michels, C., Haase, I., le Bivic, A., Leitges, M.
and Niessen, C. M. (2007). Role of aPKC isoforms and their binding partners Par3and Par6 in epidermal barrier formation. J. Invest. Dermatol. 127, 782-791.
Hsu, Y. C., Pasolli, H. A. and Fuchs, E. (2011). Dynamics between stem cells, niche,and progeny in the hair follicle. Cell 144, 92-105.
Huang, L. and Muthuswamy, S. K. (2010). Polarity protein alterations in carcinoma: afocus on emerging roles for polarity regulators. Curr. Opin. Genet. Dev. 20, 41-50.
Humbert, P. O., Grzeschik, N. A., Brumby, A. M., Galea, R., Elsum, I. and
Richardson, H. E. (2008). Control of tumourigenesis by the Scribble/Dlg/Lglpolarity module. Oncogene 27, 6888-6907.
Iden, S. and Collard, J. G. (2008). Crosstalk between small GTPases and polarityproteins in cell polarization. Nat. Rev. Mol. Cell Biol. 9, 846-859.
Imai, F., Hirai, S., Akimoto, K., Koyama, H., Miyata, T., Ogawa, M., Noguchi, S.,
Sasaoka, T., Noda, T. and Ohno, S. (2006). Inactivation of aPKCl results in the lossof adherens junctions in neuroepithelial cells without affecting neurogenesis in mouseneocortex. Development 133, 1735-1744.
Jansen, M., Ten Klooster, J. P., Offerhaus, G. J. and Clevers, H. (2009). LKB1 andAMPK family signaling: the intimate link between cell polarity and energymetabolism. Physiol. Rev. 89, 777-798.
Januschke, J. and Gonzalez, C. (2008). Drosophila asymmetric division, polarity andcancer. Oncogene 27, 6994-7002.
Kemphues, K. J., Priess, J. R., Morton, D. G. and Cheng, N. S. (1988). Identificationof genes required for cytoplasmic localization in early C. elegans embryos. Cell 52,311-320.
Kirschner, N., Haftek, M., Niessen, C. M., Behne, M. J., Furuse, M., Moll, I. andBrandner, J. M. (2011). CD44 regulates tight-junction assembly and barrierfunction. J. Invest. Dermatol. 131, 932-943.
Klein, T. J. and Mlodzik, M. (2005). Planar cell polarization: an emerging model pointsin the right direction. Annu. Rev. Cell Dev. Biol. 21, 155-176.
Klezovitch, O., Fernandez, T. E., Tapscott, S. J. and Vasioukhin, V. (2004). Loss ofcell polarity causes severe brain dysplasia in Lgl1 knockout mice. Genes Dev. 18,559-571.
Knoblich, J. A. (2008). Mechanisms of asymmetric stem cell division. Cell 132, 583-597.
Polarity signaling in the epidermis 3509
Journ
alof
Cell
Scie
nce
Knoblich, J. A. (2010). Asymmetric cell division: recent developments and theirimplications for tumour biology. Nat. Rev. Mol. Cell Biol. 11, 849-860.
Koster, M. I. (2009). Making an epidermis. Ann. N. Y. Acad. Sci. 1170, 7-10.Lamprecht, J. (1990). Symmetric and asymmetric cell division in rat corneal
epithelium. Cell Tissue Kinet. 23, 203-216.Lechler, T. and Fuchs, E. (2005). Asymmetric cell divisions promote stratification and
differentiation of mammalian skin. Nature 437, 275-280.Lee, C. Y., Robinson, K. J. and Doe, C. Q. (2006). Lgl, Pins and aPKC regulate
neuroblast self-renewal versus differentiation. Nature 439, 594-598.Lee, M. and Vasioukhin, V. (2008). Cell polarity and cancer–cell and tissue polarity as
a non-canonical tumor suppressor. J. Cell Sci. 121, 1141-1150.Li, R. and Bowerman, B. (2010). Symmetry breaking in biology. Cold Spring Harb.
Perspect. Biol. 2, a003475.Luxenburg, C., Pasolli, H. A., Williams, S. E. and Fuchs, E. (2011). Developmental
roles for Srf, cortical cytoskeleton and cell shape in epidermal spindle orientation.Nat. Cell Biol. 13, 203-214.
Martin-Belmonte, F. and Mostov, K. (2008). Regulation of cell polarity duringepithelial morphogenesis. Curr. Opin. Cell Biol. 20, 227-234.
McCaffrey, L. M. and Macara, I. G. (2009). Widely conserved signaling pathways inthe establishment of cell polarity. Cold Spring Harb. Perspect. Biol. 1, a001370.
McNeill, H. (2010). Planar cell polarity: keeping hairs straight is not so simple. Cold
Spring Harb. Perspect. Biol. 2, a003376.Mertens, A. E., Rygiel, T. P., Olivo, C., van der Kammen, R. and Collard, J. G.
(2005). The Rac activator Tiam1 controls tight junction biogenesis in keratinocytesthrough binding to and activation of the Par polarity complex. J. Cell Biol. 170, 1029-1037.
Montcouquiol, M., Rachel, R. A., Lanford, P. J., Copeland, N. G., Jenkins, N. A.and Kelley, M. W. (2003). Identification of Vangl2 and Scrb1 as planar polaritygenes in mammals. Nature 423, 173-177.
Murdoch, J. N., Henderson, D. J., Doudney, K., Gaston-Massuet, C., Phillips, H. M.,Paternotte, C., Arkell, R., Stanier, P. and Copp, A. J. (2003). Disruption ofscribble (Scrb1) causes severe neural tube defects in the circletail mouse. Hum. Mol.
Genet. 12, 87-98.Murray, N. R., Weems, J., Braun, U., Leitges, M. and Fields, A. P. (2009). Protein
kinase C bII and PKCi/l: collaborating partners in colon cancer promotion andprogression. Cancer Res. 69, 656-662.
Nakagawa, S. and Huibregtse, J. M. (2000). Human scribble (Vartul) is targeted forubiquitin-mediated degradation by the high-risk papillomavirus E6 proteins and theE6AP ubiquitin-protein ligase. Mol. Cell. Biol. 20, 8244-8253.
Nelson, W. J. (2003). Adaptation of core mechanisms to generate cell polarity. Nature
422, 766-774.Nelson, W. J. (2009). Remodeling epithelial cell organization: transitions between front-
rear and apical-basal polarity. Cold Spring Harb. Perspect. Biol. 1, a000513.Niessen, C. M. and Gottardi, C. J. (2008). Molecular components of the adherens
junction. Biochim. Biophys. Acta 1778, 562-571.Nolan, M. E., Aranda, V., Lee, S., Lakshmi, B., Basu, S., Allred, D. C. and
Muthuswamy, S. K. (2008). The polarity protein Par6 induces cell proliferation andis overexpressed in breast cancer. Cancer Res. 68, 8201-8209.
Pearson, C. G. and Bloom, K. (2004). Dynamic microtubules lead the way for spindlepositioning. Nat. Rev. Mol. Cell Biol. 5, 481-492.
Pearson, H. B., Perez-Mancera, P. A., Dow, L. E., Ryan, A., Tennstedt, P., Bogani,
D., Elsum, I., Greenfield, A., Tuveson, D. A., Simon, R. et al. (2011). SCRIBexpression is deregulated in human prostate cancer, and its deficiency in micepromotes prostate neoplasia. J. Clin. Invest. 121, 4257-4267.
Pegtel, D. M., Ellenbroek, S. I., Mertens, A. E., van der Kammen, R. A., de Rooij, J.
and Collard, J. G. (2007). The Par-Tiam1 complex controls persistent migration bystabilizing microtubule-dependent front-rear polarity. Curr. Biol. 17, 1623-1634.
Perez-Losada, J. and Balmain, A. (2003). Stem-cell hierarchy in skin cancer. Nat. Rev.
Cancer 3, 434-443.Petersson, M., Brylka, H., Kraus, A., John, S., Rappl, G., Schettina, P. and
Niemann, C. (2011). TCF/Lef1 activity controls establishment of diverse stem andprogenitor cell compartments in mouse epidermis. EMBO J. 30, 3004-3018.
Poulson, N. D. and Lechler, T. (2010). Robust control of mitotic spindle orientation inthe developing epidermis. J. Cell Biol. 191, 915-922.
Prehoda, K. E. (2009). Polarization of Drosophila neuroblasts during asymmetricdivision. Cold Spring Harb. Perspect. Biol. 1, a001388.
Ravni, A., Qu, Y., Goffinet, A. M. and Tissir, F. (2009). Planar cell polarity cadherin
Celsr1 regulates skin hair patterning in the mouse. J. Invest. Dermatol. 129, 2507-
2509.
Ray, S. and Lechler, T. (2011). Regulation of asymmetric cell division in the epidermis.
Cell Div. 6, 12.
Regala, R. P., Davis, R. K., Kunz, A., Khoor, A., Leitges, M. and Fields, A. P. (2009).
Atypical protein kinase Ci is required for bronchioalveolar stem cell expansion and
lung tumorigenesis. Cancer Res. 69, 7603-7611.
Roh, M. H. and Margolis, B. (2003). Composition and function of PDZ protein
complexes during cell polarization. Am. J. Physiol. Renal Physiol. 285, F377-F387.
Saburi, S., Hester, I., Fischer, E., Pontoglio, M., Eremina, V., Gessler, M., Quaggin,
S. E., Harrison, R., Mount, R. and McNeill, H. (2008). Loss of Fat4 disrupts PCP
signaling and oriented cell division and leads to cystic kidney disease. Nat. Genet. 40,
1010-1015.
Schneider, M. R., Schmidt-Ullrich, R. and Paus, R. (2009). The hair follicle as a
dynamic miniorgan. Curr. Biol. 19, R132-R142.
Sengupta, A., Duran, A., Ishikawa, E., Florian, M. C., Dunn, S. K., Ficker, A. M.,
Leitges, M., Geiger, H., Diaz-Meco, M., Moscat, J. et al. (2011). Atypical protein
kinase C (aPKCf and aPKCl) is dispensable for mammalian hematopoietic stem cell
activity and blood formation. Proc. Natl. Acad. Sci. USA 108, 9957-9962.
Shin, K., Fogg, V. C. and Margolis, B. (2006). Tight junctions and cell polarity. Annu.
Rev. Cell Dev. Biol. 22, 207-235.
Simons, M. and Mlodzik, M. (2008). Planar cell polarity signaling: from fly
development to human disease. Annu. Rev. Genet. 42, 517-540.
Smart, I. H. (1970). Variation in the plane of cell cleavage during the process of
stratification in the mouse epidermis. Br. J. Dermatol. 82, 276-282.
Sotiropoulou, P. A., Candi, A. and Blanpain, C. (2008). The majority of multipotent
epidermal stem cells do not protect their genome by asymmetrical chromosome
segregation. Stem Cells 26, 2964-2973.
St Johnston, D. and Ahringer, J. (2010). Cell polarity in eggs and epithelia: parallels
and diversity. Cell 141, 757-774.
Suzuki, A. and Ohno, S. (2006). The PAR-aPKC system: lessons in polarity. J. Cell Sci.
119, 979-987.
Tscharntke, M., Pofahl, R., Chrostek-Grashoff, A., Smyth, N., Niessen, C.,
Niemann, C., Hartwig, B., Herzog, V., Klein, H. W., Krieg, T. et al. (2007).
Impaired epidermal wound healing in vivo upon inhibition or deletion of Rac1. J. Cell
Sci. 120, 1480-1490.
Tunggal, J. A., Helfrich, I., Schmitz, A., Schwarz, H., Gunzel, D., Fromm, M.,
Kemler, R., Krieg, T. and Niessen, C. M. (2005). E-cadherin is essential for in vivo
epidermal barrier function by regulating tight junctions. EMBO J. 24, 1146-1156.
van de Pavert, S. A., Kantardzhieva, A., Malysheva, A., Meuleman, J., Versteeg, I.,
Levelt, C., Klooster, J., Geiger, S., Seeliger, M. W., Rashbass, P. et al. (2004).
Crumbs homologue 1 is required for maintenance of photoreceptor cell polarization
and adhesion during light exposure. J. Cell Sci. 117, 4169-4177.
Waghmare, S. K., Bansal, R., Lee, J., Zhang, Y. V., McDermitt, D. J. and Tumbar,
T. (2008). Quantitative proliferation dynamics and random chromosome segregation
of hair follicle stem cells. EMBO J. 27, 1309-1320.
Wang, Y. and Nathans, J. (2007). Tissue/planar cell polarity in vertebrates: new
insights and new questions. Development 134, 647-658.
Watt, F. M. and Jensen, K. B. (2009). Epidermal stem cell diversity and quiescence.
EMBO Mol. Med. 1, 260-267.
Williams, S. E., Beronja, S., Pasolli, H. A. and Fuchs, E. (2011). Asymmetric cell
divisions promote Notch-dependent epidermal differentiation. Nature 470, 353-358.
Wu, M., Smith, C. L., Hall, J. A., Lee, I., Luby-Phelps, K. and Tallquist, M. D.
(2010). Epicardial spindle orientation controls cell entry into the myocardium. Dev.
Cell 19, 114-125.
Zhang, Y. V., Cheong, J., Ciapurin, N., McDermitt, D. J. and Tumbar, T. (2009).
Distinct self-renewal and differentiation phases in the niche of infrequently dividing
hair follicle stem cells. Cell Stem Cell 5, 267-278.
Zhang, Y. V., White, B. S., Shalloway, D. I. and Tumbar, T. (2010). Stem cell
dynamics in mouse hair follicles: a story from cell division counting and single cell
lineage tracing. Cell Cycle 9, 1504-1510.
Zhong, W. and Chia, W. (2008). Neurogenesis and asymmetric cell division. Curr.
Opin. Neurobiol. 18, 4-11.
Journal of Cell Science 125 (15)3510