Tight Junctions/Adherens Junctions: Basic Structure and Function

Abstract

Adherens and tight junctions are intercellular junctions crucial for epithelial adhesion and barrier function in a wide variety of tissues and organisms. In stratifying epithelia, such as the epidermis, the role of adherens and tight junctions was considered less important owing to the abundance of desmosomes, mediating firm mechanical stability between the cells, and to the barrier function of the stratum corneum, respectively. This view has changed in recent years because of different studies that showed the importance of these structures for proper skin physiology and barrier function. The current review provides an overview of the crucial molecular constituents of these structures and highlights some recent results on their regulation. In particular, I will discuss their importance in skin biology.

Full text

Tight Junctions/Adherens Junctions: Basic Structure and
Function
Carien M. Niessen
1,2
Adherens and tight junctions are intercellular junctions crucial for epithelial adhesion and barrier function in a
wide variety of tissues and organisms. In stratifying epithelia, such as the epidermis, the role of adherens and
tight junctions was considered less important owing to the abundance of desmosomes, mediating firm
mechanical stability between the cells, and to the barrier function of the stratum corneum, respectively. This
view has changed in recent years because of different studies that showed the importance of these structures
for proper skin physiology and barrier function. The current review provides an overview of the crucial
molecular constituents of these structures and highlights some recent results on their regulation. In particular,
I will discuss their importance in skin biology.
Journal of Investigative Dermatology (2007) 127, 2525–2532; doi:10.1038/sj.jid.5700865
Introduction
An important property of epithelial and
endothelial cells is their assembly into
a physical and ion- and size-selective
barrier separating tissues. Intercellular
junctions, such as adherens and tight
junctions, play a crucial role in the
formation and maintenance of epithe-
lial and endothelial barriers. Adherens
and tight junctions were first identified
on the ultrastructural level as part of the
terminal bar, a tripartite junctional
complex bordering the apico-baso-
lateral membrane in a variety of polar-
ized simple epithelia and implicated in
barrier function (Farquhar and Palade,
1963). Desmosomes form the third
structure of this complex but will be
discussed in detail in another part of
this series. Tight junctions are the most
apical structure of the apical complex
demarcating the border between apical
and basolateral membrane domains.
The intercellular membrane space of
tight junctions is almost completely
obliterated, hence their alternative
name zonulae occludens. Adherens
junctions are positioned immediately
below tight junctions and character-
ized by two apposing membranes,
which are separated by B20 n
M, that
run parallel over a distance of
0.2–0.5 mm. Adherens junctions are
also found outside of the tripartite
complex in both epithelial and non-
epithelial cells, often showing a more
discontinuous or punctate pattern. Both
adherens and tight junctions are closely
associated with a circumferential belt
of actin.
Remarkably, without any know-
ledge at the time on the molecular
composition of the junctional complex,
scientists were able to make accurate
functional predictions based on this
ultrastructure. Tight junctions do pro-
vide epithelia with a semipermeable
size- and ion-specific barrier, which
varies depending on their exact mole-
cular composition (reviewed in Ander-
son et al., 2004). They also restrict the
diffusion of apical and basolateral
membrane components, the so-called
‘‘fence function.’’ Moreover, as pre-
dicted, adherens junctions are crucial
for the initiation and maintenance
of intercellular adhesion in a wide
variety of tissues and cell populations
(reviewed in Irie et al., 2004; Gumbi-
ner, 2005). Although their ultrastruc-
ture suggests that adherens and tight
junctions form stable structures, it is
now obvious that they are highly
dynamic complexes even in fully po-
larized epithelia.
A more recently identified function
of intercellular junctions is that they
provide the cell not only with structural
integrity but also function as land-
marks, spatially confining signaling
molecules and polarity cues as well as
serving as docking sites for vesicles
(reviewed in Nelson, 2003). This is
reflected in many components known
to be associated with adherens and
tight junctions. Whereas some of these
molecules are structural and form an
integral part of such junctions, others
are either transiently associated with
these junctions or found guilty through
association with one of the structural
junctional components. In addition, for
several structural components, novel
functions outside of the junctions have
been identified. It is often not clear if
such functions are directly coupled to
their junctional localization.
& 2007 The Society for Investigative Dermatology www.jidonline.org 2525
PERSPECTIVE
Received 12 January 2007; revised 2 March 2007; accepted 14 March 2007
1
Center for Molecular Medicine Cologne, University of Cologne, Cologne, Germany and
2
Department of Dermatology, University of Cologne, Cologne,
Germany
Correspondence: Dr Carien M. Niessen, Center for Molecular Medicine, University of Cologne, LFI, 05, room 59, Joseph Stelzmannstrasse 9, Cologne D-50931,
Germany. E-mail: carien.niessen@uni-koeln.de
Abbreviations: EC, extracellular; p120cat, p120 catenin; JAMs, junctional adhesion molecules; GEFs, GTPase-specific nucleotide exchange factors

Broadly speaking, we can define the
junctional components in three general
categories: (1) structural proteins ne-
cessary for initiation of the junctions,
(2) plaque proteins associated with the
cytoskeleton, and (3) signaling/polarity
proteins. Although the division be-
tween the groups is actually a rather
gray area, it serves the purpose to
distinguish between protein complexes
that are required for the basic structure
of the junctions and, those that are
more likely involved in regulation of
junctional integrity or in intercellular
communication. This review discusses
the basic structural components of
adherens and tight junctions and their
importance in skin and will provide
some key examples of regulation.
Several excellent recent reviews exist
for a more comprehensive overview of
regulation of and signaling by junctions
(Matter and Balda, 2003; Irie et al.,
2004; Gumbiner, 2005; Halbleib and
Nelson, 2006; Perez-Moreno et al.,
2006).
Molecular composition of adherens
junctions
The adherens junction consists of two
basic adhesive units: the nectin–afadin
complex and the classical cadherin–
catenin complex (Figure 1). Both nectins
and cadherins are multimember fa-
milies and the cell-specific expression
of cadherins and nectins ultimately
determine the strength and adhesive
specificity of the adherens junctions.
The nectin–afadin complex. The nectin
family of IgG-like adhesion receptors
consists of four members, Nectin-1 to-
4. For each nectin, multiple splice
variants have been described. Nectins
form lateral homodimers that can en-
gage in both homophilic and hetero-
philic adhesion with other nectins or
nectin-like receptors, although with
variable specificity and affinity. Nectins
consist of an extracellular (EC) domain
comprising three IgG-like loops, a
single transmembrane region and a
cytoplasmic domain with a C-terminal
PDZ binding motif present in most
splice variants (Irie et al., 2004).
Nectins forms a structural adhesive
unit with the actin-binding protein
afadin, also known as ‘‘AF-6,’’ provid-
ing these adhesion molecules with a
direct link to the cytoskeleton. Afadin
was initially identified as a fusion
partner of ALL-1, a translocation found
in a subset of acute myeloid leukemias
(Prasad et al., 1993). This protein also
consists of a PDZ domain (to which
nectins bind), two Ras/Rap binding
domains, a dilute domain, a forkhead-
associated domain, and three proline
rich regions (in humans two), sugges
ting the potential to function as a signal
integrator at adherens junctions.
A smaller variant, S-Afadin, lacks the
actin-binding region. Unlike the longer
variant, knockdown of this variant did
not affect intercellular adhesion (Lorger
and Moelling, 2006).
Nectins may provide the first scaf-
fold for adherens and tight junction
formation. Utilizing many different in
vitro assays Takai and co-workers
(reviewed in Irie et al., 2004) showed
that cadherin mediated cell-cell adhe-
sion and tight junction formation was
dependent on nectins. Nevertheless,
single knockout mice of different nec-
tins did not reveal an essential role for
nectins in embryogenesis. This is likely
owing to compensation and/or redun-
dancy by other nectins because inacti-
vation of afadin did result in early
embryonic lethality due to perturbation
of intercellular junctions and polarity
(Ikeda et al., 1999; Zhadanov et al.,
1999). Moreover, phenotypes related to
junctional alterations have been ob-
served in specific organs in the different
existing nectin knockouts (Irie et al.,
2004).
The cadherin–catenin complex. The
type I classical cadherins belong to a
large super family of proteins, the
characteristic of which is the cadherin
EC repeat. The desmosomal cadherins
are also part of the cadherin super
family. Cadherins are considered
homophilic adhesion molecules
although recent data indicate that
binding can be more promiscuous
(Niessen and Gumbiner, 2002; Duguay
et al., 2003). If cadherins can mediate
heterophilic adhesion and what the
exact molecular requirements of the
EC domain for adhesion are, are
both subjects of an ongoing debate
(reviewed in Gumbiner, 2005). Inacti-
vation of different cadherins in a variety
of organisms have shown their impor-
tance in tissue morphogenesis (Gumbiner,
2005; Halbleib and Nelson, 2006).
Figure 1. Schematic representation of the basic structural components of the adherens junctions.
Shown are the cadherin–catenin complex and the nectin–afadin complex and their potential interactions
with actin (see text).
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Classical cadherins form a basic
complex with the catenins, a-, b-, and
p120 catenin (p120ctn). Both p120ctn
and b-catenin bind directly to the
cadherin via their armadillo repeats,
whereas a-catenin connects via
b-catenin. Plakoglobin, also known as
g-catenin, is a close relative of b-
catenin. Although primarily associated
with desmosomal cadherins, it can
substitute for b-catenin in the classical
cadherin/catenin complex. Binding of
b-catenin to cadherin is crucial for full
adhesive function and this is largely
dependent on its ability to bridge the
cadherin with the actin-binding pro-
tein, a-catenin (reviewed in Aberle
et al., 1996). This makes b-catenin an
excellent candidate to mediate signal-
induced changes in cadherin adhesive
contacts. b-catenin can directly bind to
several signaling proteins, for example,
the EGF receptor or tyrosine phospha-
tases. Moreover, growth factor-induced
tyrosine phosphorylation of the cadher-
in–catenin complex is associated with
changes in intercellular adhesion con-
comitant with changes in complex
composition (reviewed in Nelson and
Nusse, 2004). In vitro studies using
cadherin–a-catenin fusion mutants
indeed suggested a role for b-catenin
in regulation of intercellular motility
(Nagafuchi et al., 1994). However,
both in vitro and in vivo studies using
similar constructs showed no b-cate-
nin-dependent impairment of adhesion
regulation (Takeda et al., 1995; Pac-
quelet and Rorth, 2005). The signifi-
cance of b-catenin in regulation of
adhesion is therefore still under debate.
As b-catenin is the central player in
Wnt signaling, a pathway that regulates
cell fate determination, it is tempting to
speculate that its function in adherens
junctions relates to the coordination of
morphogenetic movements with cell
fate determination. This is corroborated
by a multitude of studies showing a
close reciprocal relationship between
intercellular adhesion and Wnt signal-
ing (Nelson and Nusse, 2004; Brem-
beck et al., 2006).
P120ctn is the poster child of a
subfamily of Armadillo repeat-contain-
ing proteins, which also includes
d-catenin, Armadillo-Repeat gene de-
leted in Velo-Cardio Facial syndrome
(ARVCF), and plakophilins. The first
three members appear to be function-
ally redundant with respect to classical
cadherin binding. Plakophilins are pre-
dominantly found at desmosomes. The
cadherin interaction with p120ctn is
crucial for cell-surface stability by
regulating endocytosis (Xiao et al.,
2007). In the absence of p120ctn
cadherin, cell-surface expression is
strongly diminished. Furthermore,
p120ctn has emerged as a major
regulator and integrator of signaling
by the Rho family of small GTPases
(Anastasiadis, 2007), and this is at least
partially dependent on its interaction
with the cadherin (Wildenberg et al.,
2006).
Actin binding at the adherens junction.
Adherens junctions are closely con-
nected to the actin cytoskeleton as their
disturbance perturbs the actin cytoske-
leton. Because a-catenin can directly
bind either b-catenin or actin, it was
considered textbook knowledge that a-
catenin provided the connection of
cadherins to actin. Indeed, many other
in vitro and in vivo studies corrobo-
rated such a direct link. However,
recent studies from the Nelson and
Weis groups found no in vitro evidence
for the existence of a ternary cadherin-
b–a-catenin–actin complex (Yamada
et al., 2005). Instead, binding of
a-catenin to actin or b-catenin is
mutually exclusive (Drees et al., 2005).
In addition, actin dynamics at intercellu-
lar adhesive contact sites were very
different to those of the cadherin
complex, suggesting the absence of a
stable interaction. Regardless, both
genetic and cell biological data
strongly indicate that regulation of
actin polymerization does take place
at or in close vicinity of the adherens
junctions. This is at least partially
dependent on a-catenin (reviewed in
Gates and Peifer, 2005; Scott and Yap,
2006). The actin-binding protein afadin
is another candidate to connect adhe-
rens junctions directly to actin.
The actin nucleating proteins formin
and Arp2/3 are associated with the
cadherin complex (Kobielak et al.,
2004; Verma et al., 2004). Thus it is
possible that both Arp2/3-dependent
actin branching activity, important for
lammelipodia formation, and formin-
dependent linear actin filament poly-
merization actin cables, important for
fillapodia formation, occur at or near
adherens junctions. How and which
factors regulate local activity is less
clear. A wide variety of actin-binding
and regulatory proteins, such as ZO-1,
vinculin, spectrin, cortactin, moesin,
a-actinin, Ena/Vasp, Wave, and Wasp,
can associate with and affect adherens
junctions and may thus be responsible
(reviewed in Gates and Peifer, 2005;
Scott and Yap, 2006). Overall, the
results suggest a dynamic interaction
of adherens junctions with the cyto-
skeleton (Figure 1).
Cooperation of nectins and cadherins
in adherens junction formation. Many
potential physical links between the
nectin–cadherin adhesion systems have
been identified, the most direct one
being those between afadin and a-
catenin or p120ctn (Hoshino et al.,
2005). However, thus far, it has not
been possible to show the existence of
a cadherin/nectin-containing complex
in vivo. Nevertheless, both systems
appear to be required for formation
and function of adherens junctions. For
example, both adhesion complexes
cooperate in the formation and plasti-
city of synaptic junctions, a specialized
form of adherens junctions (Togashi
et al., 2006).
Either cadherin or nectin enga-
gement can control the activity of the
Rho family of small GTPases, which is
a crucial regulator of actin dynamics.
Activated forms of these GTPases affect
cadherin activity and junction stability
(Braga and Yap, 2005), suggesting a
close reciprocal relationship. One
model suggests that initial engagement
of cells occurs via nectins that activate
Rac and Cdc42, which then stimulate
the formation of lammelapodial protru-
sions and cadherin binding (Irie et al.,
2004). In addition, Rho GTPases may
regulate actomyosin contractions,
which is important for junctional re-
arrangements during morphogenetic
movements (Bertet et al., 2004). Local
regulation of small GTPase activity is
most likely mediated by the recruit-
ment of Rho family GTPase-specific
nucleotide exchange factors (GEFs) and
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CM Niessen
Tight and Adherens Junctions

GTPase-activating proteins to adherens
junctions, several of which interact
with core components. Adherens junc-
tions may regulate Rho small GTPases
via their upstream regulators, the Rap
small GTPases. RapGEFs directly inter-
act with E-cadherin and engagement of
E-cadherin activates Rap. Afadin is an
effector of Rap activity, suggesting the
possibility of a positive loop reinforcing
intercellular adhesion (reviewed in
Kooistra et al., 2007).
Molecular composition of tight junctions
Transmembrane components. Three
types of structural transmembrane com-
ponents that are enriched at tight
junctions have the potential to mediate
cell–cell adhesion (Figure 2): the IgG-
like family of junctional adhesion
molecules (JAMs), the claudin, and
occludin families of four transmem-
brane spanning molecules (Schneeber-
ger and Lynch, 2004; Furuse and
Tsukita, 2006).
Occludin was the first identified
transmembrane component of tight
junctions. Although tight junctions
without occludin are rare, its physio-
logical function in tight junctions is still
unclear. Tight junctional strands and
barrier function were still present in
cells and epithelial tissues deficient for
occludin. Nevertheless, the mice ex-
hibit several different phenotypes, such
as growth retardation, mineral deposits
in the brain, male sterility, and gastritis,
suggesting barrier impairment (Saitou
et al., 2000). Alternatively, occludin
serves an as yet unidentified function
independent of the classical tight junc-
tional barrier and fence function. In-
deed, studies done using cells in which
occludin expression was knocked
down by RNAi indicate a crucial role
for this protein in the communication
of apoptosis to surrounding cells (Yu
et al., 2005). Only recently a protein,
tricellulin, was identified with structur-
al similarity to occludin. Unlike other
tight junctional proteins, tricellulin is
enriched only at tricellular tight junc-
tions, where it enforces the barrier
function of epithelial cell sheets (Ike-
nouchi et al., 2005). Its mutation
contributes to deafness in humans
(Riazuddin et al., 2006). This finding
further illustrates the growing molecu-
lar complexity of tight junctions.
The presence of functional tight
junctions in the absence of occludin
leads the group of Tsukita to the
identification of two other tight junc-
tional transmembrane proteins, named
claudin-1 and -2. Around the same
time other groups independently found
that mutation or inactivation of certain
proteins, now recognized as claudins,
resulted in diseases such as hypomag-
nesaemia (claudin-16), deafness (clau-
din-14), and absence of central nervous
system myelin and sertoli cell tight
junction strands (claudin-11) (reviewed
in Furuse and Tsukita, 2006).
The claudin family consists of at
least 24 members, with each showing a
specific organ and tissue distribution.
Exogenous expression of claudins in
fibroblasts not only induced Ca
2 þ
-
independent cell–cell adhesion but also
resulted in the formation of tight junc-
tion fibers, indicating its crucial role in
tight junction formation. More impor-
tantly, experimentally manipulating the
type of claudin expression directly
affected paracellular ion and/or size
selectivity (Van Itallie et al., 2001; Nitta
et al., 2003). As the EC loops of
occludin contain very few charged
amino acids, these loops are charged
in claudins and their isoelectric point
varies widely between the different
claudins. Changing the charge in the
first EC loop of claudin-15 altered
barrier ion specificity (Colegio et al.,
2002). It is now widely recognized that
the large variety in strength, size, and
ion specificity of tight junctional bar-
riers in different epithelia and endo-
thelia is largely due to the type of
claudin(s) found at specific tight junc-
tions (Anderson et al., 2004; Furuse and
Tsukita, 2006).
The IgG-like family of JAMs is the
third group of transmembrane receptors
found at tight junctions. The family
consist of the closely related molecules
JAM-A, -B and, -C and the more
distantly related Coxsackie and adeno-
virus receptor, endothelial cell-selec-
tive adhesion molecule, and JAM-4
(Ebnet et al., 2004). JAMs can be
engaged in homophilic and heterophi-
lic adhesion but do not induce the
formation of tight junctional strands
when expressed in fibroblasts. JAMs are
not exclusively found on cells that form
tight junctions but also on cells such as
leukocytes, thereby contributing to
their transendothelial migration (Ebnet
et al., 2004). In addition, Jam-C reg-
ulates polarization and differentiation
of spermatids by recruiting polarity
protein complexes (Gliki et al., 2004).
Plasma
membrane
Plasma
membrane
JAM-1
JAM-1
Claudins
JAM-1
JAM-1
Occludin/
tricellulin
Actin
MAGI
ZO1/2
ZO1/2
ZO1/2
Cingulin
ZO-1/2/3
Figure 2. Schematic representation of the basic structural transmembrane components of tight
junctions. ZO-1 or ZO-2 is important for clustering of claudins and occludin, resulting in the formation
of tight junctional strands. The role of the other scaffolding proteins (ZO-3/MAGI/MUP1) is less clear.
The ZOs and cingulin can provide a direct link to the actin cytoskeleton.
2528 Journal of Investigative Dermatology (2007), Volume 127
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Scaffolding proteins. The incorporation
and association of occludin, claudins,
and JAMs in tight junctional strands
require local clustering of these pro-
teins. As no direct interactions have
been found between occludins, clau-
dins, and JAMs, cytoplasmic binding
partners must fulfill this scaffolding
function (Figure 2). An important group
of tight junctional scaffolding mole-
cules are the zonula occludens proteins
ZO-1, ZO-2, and ZO-3. These proteins
belong to the membrane-associated
guanylate kinase-like homologs family
and are characterized by three
N-terminal PDZ domains, an SH3 domain
followed by the GUK domain. These
proteins can interact directly with
occludin and claudins via their PDZ
domains, whereas their C-terminus can
associate with actin, thus providing a
direct link with the cytoskeleton
(Schneeberger and Lynch, 2004).
Localization of ZO-1 to tight junctions
requires its actin-binding domain (Fan-
ning et al., 2002). In addition, ZO-1
can also directly interact with JAMs and
form homodimers or heterodimers with
either ZO-2 or ZO-3. Only very
recently, it was shown that either ZO-1
or ZO-2, but not ZO-3, is crucial for
clustering of claudins, strand forma-
tion, and barrier function (Umeda
et al., 2006). Surprisingly, apico-baso-
lateral polarity was not obviously dis-
turbed in the absence of tight
junctional strands, suggesting that tight
junctions are not involved in the
separation of the apical and basolateral
membrane domains (the fence func-
tion). One should keep in mind that
these studies were done under 2-D
culture conditions that provide external
polarity cues. More stringent tests, such
as 3-D culture conditions or animal
models, will thus be necessary to rule
out a function for tight junctions in
apico-basolateral polarity and fence
function (Shin and Margolis, 2006).
Several other PDZ-containing scaf-
folding proteins, such MUPP1 and
MAGI proteins are associated with the
tight junctional cytosolic plaque and
can directly interact with one or more
of the tight junctional transmembrane
components (Schneeberger and Lynch,
2004). It is at present unclear if these
molecules are directly involved in the
formation of the tight junctions or serve
a more regulatory function.
Cingulin, a non-PDZ tight junctional
plaque protein, interacts with ZOs,
JAMs, and actin via its head domain,
whereas its central rod domain is
required for homodimerization and
interacts with myosin. As such, this
protein may be an important regulator
of tight junctional dynamics during
actomyosin contraction (Clayburgh
et al., 2005). However, cells with a
deletion of the cingulin head domain
showed no obvious disturbance in tight
junctional strands (Guillemot et al.,
2004). Instead, cingulin may couple
junctional integrity to cytoskeletal reg-
ulation and proliferation by binding to
the Rho-specific exchange factor GEF-
H1. Not only does GEF-H1 affects
paracellular permeability but it also
regulates Rho activity and G1/S transi-
tion (Aijaz et al., 2005).
Regulation of intercellular junctions
A large group of molecules can either
directly interact with components of
adherens or tight junctions or are
localized at these junctions. Generally,
they are either involved in the dynamic
regulation of intercellular adhesion and
junction assembly/disassembly or in
communicating signals from the junc-
tions. The mechanisms that regulate the
formation and dynamic maintenance of
adherens junctions can occur on multi-
ple levels varying from transcriptional
regulation to more local regulation,
such as cytoskeletal dynamics, proteol-
ytic cleavage by proteases, phosphory-
lation of key components by growth
factors or endocytosis.
Formation of adherens junctions
facilitates the assembly of tight junc-
tions. This is reflected by several
interactions between core adherens
and tight junctional components, such
as those between ZO-1 and a-catenin
or afadin. ZO-1 is recruited to early
cadherin-containing intercellular con-
tacts (Itoh et al., 1997), thus providing a
first scaffold for the tight junctions.
Interference with nectin or cadherin-
based adhesion disturbed tight junc-
tions (Irie et al., 2004; Tunggal et al.,
2005) but absence of both ZO-1/ZO-2
had no effect on adherens junctions
(Umeda et al., 2006).
Studies in Drosophila made the
important observation that junction
formation is functionally coupled to
the establishment of polarity (Nelson,
2003). Several different multiprotein
complexes regulate the set-up of polar-
ity by specifying membrane domains.
In mammalian polarized simple epithe-
lial cells, apical domain polarity com-
plexes, such as Par3/Par6/aPKC and
Crumbs/Pals/Patj, are localized at tight
junctions. More importantly, functional
interference with any of these proteins
affects paracellular permeability, indi-
cating their importance in the assembly
of functional tight junctions (Anderson
et al., 2004; Shin et al., 2006). Vice
versa, loss of ZO1/ZO-2 did result in a
more lateral distribution of Par3 but
without obvious loss of apico-basolat-
eral polarity (Umeda et al., 2006). The
polarity protein scribble, important for
basolateral membrane domain identity
in Drosophila, is recruited to E-cadher-
in junctions in mammalian cells and its
downregulation affects cell–cell adhe-
sion (Qin et al., 2005). Many direct
interactions exists between polarity
proteins and core junctional compo-
nents (reviewed in Shin et al., 2006),
but their exact contribution to the
molecular mechanisms that underlie
the interplay between intercellular
junctions and polarity is far from clear.
Interestingly, tissue growth factor-
b-induced disassembly of tight junc-
tions during epithelial to mesenchymal
transition appears to require interac-
tions of its receptor with the polarity
protein Par6 and occludin (Barrios-
Rodiles et al., 2005).
The importance of adherens and tight
junctions in the skin
The importance of adherens and tight
junctions in skin physiology and
pathology is best illustrated by studies
from junctional component knockout
mice, which revealed crucial roles for
these structures in the epidermis.
Although other cell compartments do
form adherens and tight junctions, their
presence has only been poorly studied
in the context of skin function and
disease and will thus not be discussed.
The epidermis forms an important
barrier that protects the organism
from the outside while also preventing
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CM Niessen
Tight and Adherens Junctions

unnecessary loss of water. Although
cells are not polarized in an apico-
basolateral sense, the tissue shows a
polarized distribution of intercellular
junction components, differentiation
markers and polarity proteins. In addi-
tion, since this is a self-renewing tissue
with a continuous upward movement of
cells, intercellular junctions must dyna-
mically rearrange without losing their
adhesive strength or barrier properties.
The best indication that adherens
junctions play a crucial role in the
mechanical stability of keratinocytes
comes from mice carrying an epider-
mal specific deletion of a-catenin
(Vasioukhin et al., 2001a). This caused
detachment of the epidermis associated
with impaired intercellular adhesion
and loss of adherens junctions. In
addition, epidermal loss of E-cadherin,
expressed on all viable layers of the
epidermis, resulted in hair loss owing
to impaired intercellular adhesion
(Young et al., 2003; Tinkle et al., 2004).
Several results suggest that adherens
junctions and desmosomes cooperate
to assure proper epidermal cohesion.
P-cadherin loss alone has no obvious
skin phenotype but enhances the
blistering defects caused by the absence
of the desmosomal cadherin desmogle-
in-3 (Lenox et al., 2000). Lack of a
desmosomal plaque protein, desmopla-
kin, in keratinocytes not only impaired
desmosomes but also adherens junc-
tions (Vasioukhin et al., 2001b).
Adherens junctions may have other
functions next to their structural inter-
cellular adhesive role. Loss of a-catenin
in the epidermis results in overgrowth
and the formation of epithelial cysts,
associated with alterations in growth
factor signaling (Vasioukhin et al.,
2001a). Thus, a-catenin may be an
important regulator for signaling recep-
tors, and this may depend on its
association with adherens junction.
Alternatively, a-catenin-dependent over-
growth may result from aberrations in
stem cell division. For example, a-
catenin appears to be required for the
proper positioning of aPKC in cells
undergoing asymmetric cell division in
the basal layer of the epidermis (Lech-
ler and Fuchs, 2005), and this likely
contributes to the balance between
stem cells and differentiated cells.
Deletion of p120ctn in the epider-
mis activates an inflammatory response
in mice, which is linked to increased
NF-kB signaling in keratinocytes (Per-
ez-Moreno et al., 2006). Interestingly,
expression analysis of a-catenin-defi-
cient keratinocytes also revealed upre-
gulation of this pathway (Kobielak and
Fuchs, 2006). Together, these reports
suggest that adherens junction compo-
nents may couple regulation of inflam-
mation to structural integrity of the
epidermis, even though the molecular
mechanisms are far from clear.
The stratum corneum physically
separates the organism from its envi-
ronment and protects it from unneces-
sary water loss as well as detrimental
influences from the outside. Owing to
the existence of this barrier, it was
assumed that tight junctions would not
contribute to the epidermal barrier,
despite expression of its components
in the epidermis (Brandner et al.,
2006). This perception changed when
extensive epidermal water loss was
observed in claudin-1-deficient mice
(Furuse et al., 2002). These mice had
an apparently normal functioning stra-
tum corneum but a dysfunctional oc-
cludin-positive barrier in the granular
layer. The ultrastructural observation of
continuous strands in the epidermal
granular layer indeed showed the pre-
sence of an integral tight junctional
barrier (Schluter et al., 2004). The
importance of tight junctions for phy-
siological barrier function is further
underscored by the observation that
claudin-1 mutations are found in neo-
natal ichtyosis-sclerosing cholangitis
syndrome (Hadj-Rabia et al., 2004).
Next to claudin-1, other claudins are
expressed in skin and they may be
important for the selective transport of
small solutes through the skin.
The adherens junction protein E-
cadherin is important for functional
epidermal tight junctions since early
epidermal deletion of this protein
largely phenocopied the claudin-1-de-
ficient mice. This was associated with
inappropriate localization of polarity
proteins like aPKC, which is localized
not only at tight junctions but at
cell–cell contacts in all viable epider-
mal layers (Tunggal et al., 2005).
Blocking aPKC function in keratino-
cytes interfered with in vitro barrier
formation, suggesting that polarity pro-
teins also contribute to epidermal
barrier formation, similar to simple
epithelia. Interestingly, tight junction
assembly was not visible disturbed,
suggesting that aPKC regulates a late
step in the formation of functional tight
junctions (Suzuki et al., 2002; Helfrich
et al., 2007).
Several papers indicate that the
stratum corneum and tight junctions
cooperate in the formation of a func-
tional skin barrier. Overexpression of
claudin-6 under the involucrin promo-
tor in mice results in epidermal barrier
defects associated with changes in both
the tight junctional and stratum cor-
neum barrier (Turksen and Troy, 2002).
Inactivation of the membrane-
anchored channel-activating serine pro-
tease 1/Prss8 disturbed both barriers,
although the molecular mechanism is
unclear (Leyvraz et al., 2005). Because
of the importance of the lipid composi-
tion in the stratum corneum, and the
presumed fence function of tight junc-
tions in simple epithelia, it is tempting to
speculate that tight junctions in the
stratum granulosum regulate ‘‘apical’’
protein and lipid vesicle targeting to-
ward the stratum corneum. However,
recent data suggest that that the fence
function is independent of tight junc-
tions (Umeda et al., 2006). Addressing
the relationship between the two bar-
riers may thus also contribute to solving
the question if tight junctions do or do
not contribute to epithelial fence func-
tion (Shin and Margolis, 2006).
A concept potentially important for
skin pathology is the observation that
junctions and their associated proteins
are hijacked by a variety of viruses and
bacteria to obtain entry into cells and/
or replicate. This happens on at least
two different levels. First, certain
viruses or bacteria use transmembrane
components of cellular junctions as
receptors. For example, nectin-1 is a
receptor for herpes simplex virus.
Second, bacteria and viruses modulate
junctional structures resulting in at least
a partial disruption of such structures.
This can happen either by inserting
effectors into the cell or activating
signals that locally regulate the actin
cytoskeleton or by direct binding to
2530 Journal of Investigative Dermatology (2007), Volume 127
CM Niessen
Tight and Adherens Junctions

junctional components (Sousa et al.,
2005).
Concluding remarks
The last 10 years have brought dra-
matic new insights into the function of
adherens and tight junctions in skin
physiology and pathology. It is now
obvious that these structures are crucial
components of skin barrier function,
and perhaps link structural integrity to
proliferation and inflammatory re-
sponses of the skin. It will be important
to dissect how tight junctions only
seem to form a structural barrier in
the granular layer of the epidermis,
even though many of its components
are already at sites of intercellular
contacts in the spinous and/or basal
layer. Another exciting direction is the
dissection of the role of intercellular
junctions in inflammatory diseases as-
sociated with impaired barrier func-
tion, such as psoriasis, and if mutations
in junctional components underlie un-
characterized barrier diseases.
CONFLICT OF INTEREST
The authors state no conflict of interest.
ACKNOWLEDGMENTS
I thank all the members of my lab for helpful
discussions. I especially appreciated the critical
feedback on the manuscript from Christian
Michels, Drs Jeanie Scott and Beate Eckes. Work
in the laboratory is funded by the German Research
foundation (DGF), SFB589 and Ko
¨
ln Fortune.
I apologize to those whose work is not described
or properly cited due to space limitations.
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