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doi: 10.1242/10.1242/jcs.00167
Commentary |
Department of Molecular Biology and Biochemistry, Osaka University Graduate School of Medicine/Faculty of Medicine, Suita 565-0871, Japan
* Author for correspondence (e-mail: ytakai{at}molbio.med.osaka-u-ac.jp)
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Summary |
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 Top Summary IntroductionMolecular structures of nectin...Intercellular adhesion activity...Roles of nectin and...Roles of nectin and...Roles of nectin and...Roles of nectin and...Role of nectin as...Conclusions and perspectivesReferences |
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Key words: Nectin, Afadin, Adherens junctions, Tight junctions, Puncta adherentia junctions, Sertoli-cell—spermatid junctions
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Introduction |
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TopSummaryIntroductionMolecular structures of nectin...Intercellular adhesion activity...Roles of nectin and...Roles of nectin and...Roles of nectin and...Roles of nectin and...Role of nectin as...Conclusions and perspectivesReferences |
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; Gumbiner,
1996; Vlemincks and Kemler,
1999; Angst et al.,
2000; Tepass et al.,
2000; Yagi and Takeichi,
2000). Intercellular junctions are generally associated with the
actin cytoskeleton, and this strengthens intercellular adhesion. In polarized
epithelial cells, intercellular adhesion is mediated through a junctional
complex comprising tight junctions (TJs), adherens junctions (AJs) and
desmosomes (DSs) (Farquhar and Palade,
1963) (Fig. 1A).
These junctional structures are typically aligned from the apical to basal
sides, although DSs are independently distributed in other areas. TJs are
likely to serve as a barrier that prevents solutes and water from passing
through the paracellular pathway and as a fence between the apical and
basolateral plasma membranes in epithelial cells (for reviews, see
Tsukita et al., 1999;
Tsukita et al., 2001). AJs
play key roles in the formation and maintenance of TJs and DSs and furthermore
in concentration of many biologically active molecules, such as membrane
receptors, signaling molecules and oncoproteins (for reviews, see
Tsukita et al., 1992;
Aberle et al., 1996;
Anastasiadis and Reynolds,
2000; Gumbiner,
2000; Nagafuchi,
2001). TJs also concentrate tumor suppressor proteins and cell
polarity proteins (Tsukita et al.,
1999; Tsukita et al.,
2001). However, how these molecules localize there remains
unclear. The formation and disruption of the junctional complex are
dynamically regulated by many extracellular and intracellular signals, such as
scatter factor/hepatocyte growth factor and Ras, Rac and Cdc42 small GTPases
(Gumbiner, 1996;
Gumbiner, 2000). Again, the
mechanisms that dynamically organize the junctional complex are largely
unknown.
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).
Synaptic junctions include presynaptic active zones, where synaptic vesicles
and Ca2+ channels localize, and post synaptic densities (PSDs),
where neurotransmitter receptors localize. Puncta adherentia junctions by
contrast are ultrastructurally similar to the AJs of epithelial cells. The
homotypic intercellular junctions are formed by homophilic interactions
between cell adhesion molecules (CAMs). This is in contrast to cell-matrix
AJs, which form through heterophilic interactions between CAMs and matrix
proteins.
In addition to the homotypic intercellular junctions described above,
heterotypic intercellular junctions also exist. These include those formed
between differentiating germ cells and their supporter Sertoli cells in the
seminiferous epithelium in the testis and between specialized sensory cells
and supporting cells in sensory epithelia. The seminiferous epithelium of the
testis contains two types of intercellular junction:
Sertoli-cell—Sertoli-cell junctions and Sertoli-cell—spermatid
junctions (for a review, see Russell and
Griswold, 1993) (Fig.
1C). Sertoli cells constitute the single-layered epithelium and
embrace and cultivate spermatogenic cells throughout their development
(spermatogenesis). During the latter half of spermiogenesis, spermatids form
prominent heterotypic intercellular junctions with Sertoli cells
(Sertoli-cell—spermatid junctions), which are downregulated when
spermatids are released as spermatozoa. Sertoli-cell—Sertoli cell
junctions, by contrast, are homotypic and similar to those in epithelial
cells.
We describe here an emerging intercellular adhesion system consisting of nectin, a Ca2+-independent immunoglobulin (Ig)-like CAM, and afadin, a nectin- and actin filament (F-actin)-binding protein that connects nectin to the actin cytoskeleton. This novel adhesion system plays roles in the organization of all of these homotypic, interneuronal and heterotypic junctions.
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Molecular structures of nectin and afadin |
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TopSummaryIntroductionMolecular structures of nectin...Intercellular adhesion activity...Roles of nectin and...Roles of nectin and...Roles of nectin and...Roles of nectin and...Role of nectin as...Conclusions and perspectivesReferences |
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, nectin-1ß, nectin-1
, nectin-2,
nectin-2
, nectin-3, nectin-3ß and nectin-3
(Morrison and Racaniello,
1992; Aoki et al.,
1994; Lopez et al.,
1995; Eberlé et al.,
1995; Cocchi et al.,
1998; Takahashi et al.,
1999; Satoh-Horikawa et al.,
2000; Lopez et al.,
2001; Reymond et al.,
2001). All the members, except nectin-1, have an
extracellular region containing three Ig-like domains, a single transmembrane
region and a cytoplasmic region (Fig.
2). Nectin-1 is a secreted protein lacking the
transmembrane region. Nectin is conserved from humans to rodents. It is highly
homologous to the human poliovirus receptor (PVR), which has also four splice
variants: PVR, PVRß, PVR and PVR (Mendelson et al.,
1989; Koike et al., 1990).
PVR and PVR have a single transmembrane region, whereas
PVRß and PVR lack this. All the nectin family members, except
nectin-1ß, nectin-1, nectin-3 and nectin-4, have a
conserved four residue motif (Glu/Ala-X-Tyr-Val) that binds the PDZ domain of
afadin at their cytoplasmic C-termini
(Mandai et al., 1997;
Takahashi et al., 1999;
Satoh-Horikawa et al., 2000;
Reymond et al., 2001).
Although nectin-4 lacks this conserved motif, it nevertheless binds the PDZ
domain of afadin (Reymond et al.,
2001).
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Afadin has two splice variants: 1-afadin and s-afadin
(Mandai et al., 1997).
1-Afadin, the larger splice variant, is a nectin- and F-actin-binding protein
that has two Ras-association (RA) domains, a forkhead-associated (FHA) domain,
a DIL domain, a PDZ domain, three proline-rich (PR) domains and an
F-actin-binding domain (Mandai et al.,
1997; Takahashi et al.,
1999). 1-Afadin binds along the side of F-actin but not to the
ends of F-actin, although it does not have cross-linking activity. s-Afadin,
the smaller splice variant, lacks the F-actin-binding domain and the third
proline-rich domain. Human s-afadin is identical to the gene product of
AF-6, a gene that has been identified as an ALL-1 fusion
partner involved in acute myeloid leukemias
(Prasad et al., 1993).
AF-6/s-afadin has been reported to bind directly to RYK, a receptor tyrosine
kinase, and a subset of Eph receptor tyrosine kinases
(Hock et al., 1998;
Buchert et al., 1999;
Halford et al., 2000), but
Trivier et al. have recently shown that AF-6/s-afadin does not in fact bind to
RYK (Trivier and Ganesan,
2002). AF-6/s-afadin has also been reported to interact with a
deubiquitinating enzyme, Fam (Taya et al.,
1998), but Chen et al. have found no genetic interaction between
the Drosophila homologs of mammalian 1-afadin and Fam during its eye
development (Chen et al.,
2000). Unless otherwise specified, afadin refers to 1-afadin in
this article.
Nectin-1, nectin-2 and nectin-3 are ubiquitously expressed in a variety of
cells, including fibroblasts, epithelial cells and neurons
(Morrison and Racaniello,
1992; Aoki et al.,
1994; Lopez et al.,
1995; Lopez et al.,
1998; Eberlé et al.,
1995; Aoki et al.,
1997; Cocchi et al.,
1998; Takahashi et al.,
1999; Satoh-Horikawa et al.,
2000; Haarr et al.,
2001; Mizoguchi et al.,
2002). Nectin-2 and nectin-3 are also expressed in cells that lack
cadherins, such as B cells and monocytes, and spermatids
(Aoki et al., 1997;
Lopez et al., 1998;
Bouchard et al., 2000;
Ozaki-Kuroda et al., 2002).
Human nectin-4 is expressed mainly in the placenta
(Reymond et al., 2001).
1-Afadin is ubiquitously expressed, whereas s-afadin is mainly expressed in
neural tissue, although it is expressed at low levels in various other tissues
(Mandai et al., 1997;
Boettner et al., 2000).
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Intercellular adhesion activity of nectins |
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TopSummaryIntroductionMolecular structures of nectin...Intercellular adhesion activity...Roles of nectin and...Roles of nectin and...Roles of nectin and...Roles of nectin and...Role of nectin as...Conclusions and perspectivesReferences |
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; Gumbiner,
1996; Vlemincks and Kemler,
1999; Tepass et al.,
2000; Angst et al.,
2000; Yagi and Takeichi,
2000). It is a member of the cadherin superfamily, which consists
of >80 members. E-Cadherin has an extracellular region containing five
tandemly repeated domains (EC1-EC5), a single transmembrane region and a
cytoplasmic region (Fig. 3A).
In the current model for its intercellular adhesion activity, which is based
on crystallographic analysis of the EC1 and EC2 domains of E-cadherin and
N-cadherin and of the whole domains (EC1-EC5) of C-cadherin
(Shapiro et al., 1995;
Nagar et al., 1996;
Tamura et al., 1998;
Pertz et al., 1999;
Boggon et al., 2002), cadherin
first forms a cis-dimer, in which the monomers are aligned in a parallel
orientation, and then a trans-dimer, in which cis-dimers from opposing cell
surfaces interact in an anti-parallel orientation. The formation of the
cis-dimer appears to be essential for the formation of the trans-dimer
(Tamura et al., 1998;
Pertz et al., 1999).
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In contrast to cadherins, the intercellular adhesion activity of nectins is
Ca2+ independent (Aoki et al.,
1997; Takahashi et al.,
1999; Miyahara et al.,
2000; Satoh-Horikawa et al.,
2000). However, in common with E-cadherin, it is likely that
nectin first forms a cis-dimer and then a trans-dimer
(Fig. 3B)
(Lopez et al., 1998;
Miyahara et al., 2000;
Satoh-Horikawa et al., 2000;
Momose et al., 2002). Analysis
of point and truncated mutants of nectin reveals that the formation of a
cis-dimer is essential for the formation of a trans-dimer, whereas the latter
is not essential for formation of the former. Each nectin family member forms
a homo-cis-dimer but not a hetero-cis-dimer
(Satoh-Horikawa et al., 2000),
although the splice variants nectin-2 and nectin-2 form a
hetero-cis-dimer (Lopez et al.,
1998). Each member also forms a homo-trans-dimer
(Aoki et al., 1997;
Lopez et al., 1998;
Takahashi et al., 1999;
Miyahara et al., 2000;
Satoh-Horikawa et al., 2000;
Reymond et al., 2001;
Momose et al., 2002).
Nectin-3, however, can form a hetero-trans-dimer with either nectin-1 or
nectin-2, and these hetero-trans-dimers are much stronger than
homo-trans-dimers (Satoh-Horikawa et al.,
2000). Nectin-3 also forms a hetero-trans-dimer with PVR
(Reymond et al., 2001), and
nectin-4 forms a hetero-trans-dimer with nectin-1
(Reymond et al., 2001).
However, nectin-1 and nectin-2 do not form a hetero-trans-dimer
(Satoh-Horikawa et al., 2000;
Reymond et al., 2001). In this
respect, nectins differ from cadherins, which form mainly homo-trans-dimers
(Takeichi, 1991;
Gumbiner, 1996;
Vlemincks and Kemler, 1999;
Angst et al., 2000;
Tepass et al., 2000;
Yagi and Takeichi, 2000).
A mutant of nectin lacking the second Ig-like domain does not form a
cis-dimer, indicating that the second Ig-like domain is necessary for the
formation of the cis-dimer (Momose et al.,
2002). Nevertheless, a fragment of the first Ig-like domain can
form a cis—dimer (Krummenacher et
al., 2002). This domain is thus also likely to be involved in the
formation of the cisdimer (Fabre et al.,
2002; Krummenacher et al.,
2002). In addition, the first Ig-like domain is necessary for the
formation of the trans-dimer (Aoki et al.,
1997; Miyahara et al.,
2000; Sakisaka et al.,
2001; Reymond et al.,
2001; Momose et al.,
2002; Krummenacher et al.,
2002). The first Ig-like domains of nectin-3 and nectin-4 bind to
the same region of the first Ig-like domain of nectin-1 to form the respective
hetero-transdimers, although nectin-3 shows higher affinity than nectin-4
(Fabre et al., 2002). The
function of the third Ig-like domain is currently unknown. Note that the
interaction between nectin and afadin is not essential for the formation of
the cis-dimer or the trans-dimer (Miyahara
et al., 2000).
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Roles of nectin and afadin in organization of AJs in fibroblasts |
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TopSummaryIntroductionMolecular structures of nectin...Intercellular adhesion activity...Roles of nectin and...Roles of nectin and...Roles of nectin and...Roles of nectin and...Role of nectin as...Conclusions and perspectivesReferences |
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; Takahashi et al.,
1999). Neither nectin-2 nor afadin is found at cell-matrix AJs
(Mandai et al., 1997;
Takahashi et al., 1999).
Several lines of evidence indicate that the nectin-afadin and
E-cadherin—catenin systems are physically and functionally associated
and that the systems play cooperative roles in the organization of AJs
(Fig. 4A). Studies using L
cells that express exogenous full-length or mutant nectin-1 or nectin-2 and
E-cadherin have revealed that nectin recruits ß-catenin through
-catenin and E-cadherin through the -catenin and ß-catenin
complex to nectin-based junctions
(Tachibana et al., 2000).
Moreover, a recombinant extracellular fragment of nectin-3 fused to the Fc
domain of IgG (Nef-3) forms a trans-dimer with nectin-1 and thereby inhibits
the formation of the nectin-1-based intercellular junctions in EL cells
(Honda et al., 2003). This
inhibits the formation of the E-cadherin-based intercellular junctions.
Conversely, Nef-3 coated on microbeads first recruits the
nectin-1—afadin complex and then the E-cadherin—catenin complex to
bead-cell contact sites in EL cells that stably express exogenous nectin-1
(nectin-1-EL cells) (Honda et al.,
2003). An extracellular fragment of E-cadherin fused to the Fc
domain of IgG (Cef) coated on microbeads recruits not only the
E-cadherin—catenin complex but also the nectin-1—afadin complex to
the bead-cell contact sites in nectin-1-EL cells
(Honda et al., 2003).
Kinetically, the nectin-based junctions form first and their formation is
followed by formation of the E-cadherin-based junctions.
How the nectin-afadin system is physically associated with the
E-cadherin—catenin system is not known, but both afadin and
-catenin have been shown to be essential for the association of nectin
and E-cadherin. The cytoplasmic tail of E-cadherin and ß-catenin might be
involved in, but is not essential for, this association. Afadin directly binds
-catenin in vitro, but this binding is not strong
(Tachibana et al., 2000;
Pokutta et al., 2002). The
direct binding of these proteins may occur in vivo, but it is more likely that
a post-translational modification(s) of either or both proteins and/or an
unidentified molecule(s) are required for the binding of -catenin to
afadin (Tachibana et al.,
2000; Pokutta et al.,
2002). -Catenin also binds to ZO-1 — an AJ component
in fibroblasts (Itoh et al.,
1991; Itoh et al.,
1997; Imamura et al.,
1999; Gumbiner,
2000; Nagafuchi,
2001). Furthermore, ZO-1 binds to afadin
(Yamamoto et al., 1997). ZO-1
might thus be involved in the association of nectin and E-cadherin, but this
possibility is unlikely given that the direct binding of afadin to ZO-1 has
not been reproduced (Sakisaka et al.,
1999; Yokoyama et al.,
2001) and nectin recruits ZO-1 to the nectin-based junctions
through afadin in an -catenin-independent manner
(Yokoyama et al., 2001).
Ponsin, an afadin- and vinculin-binding protein
(Mandai et al., 1999), and
vinculin colocalize with nectin and afadin at intercellular AJs and
furthermore localize to cell-matrix AJs
(Mandai et al., 1999).
Vinculin directly binds to -catenin
(Tsukita et al., 1992;
Aberle et al., 1996;
Gumbiner, 2000;
Nagafuchi, 2001). Vinculin and
ponsin might thus also be involved in the association of nectin and
E-cadherin. However, ponsin forms a binary complex with either afadin or
vinculin and does not form a ternary complex — at least in a cell-free
assay system (Mandai et al.,
1999). Ponsin and vinculin are not essential for the association
of nectin and E-cadherin (Tachibana et
al., 2000).
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Roles of nectin and afadin in organization of the junctional complex in epithelial cells |
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TopSummaryIntroductionMolecular structures of nectin...Intercellular adhesion activity...Roles of nectin and...Roles of nectin and...Roles of nectin and...Roles of nectin and...Role of nectin as...Conclusions and perspectivesReferences |
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; Takahashi et al.,
1999; Satoh-Horikawa et al.,
2000) (Fig. 4A).
Immunoelectron microscopy reveals that nectin-2 is confined to AJs, under
which F-actin bundles lie. This distribution pattern is different from that of
E-cadherin, which, although concentrated at AJs, is widely distributed from
the apical to the basal sides of the lateral membrane
(Tsukita et al., 1992;
Gumbiner, 1996). Neither
nectin-2 nor afadin is found at cell-matrix AJs
(Mandai et al., 1997;
Takahashi et al., 1999;
Sakisaka et al., 1999;
Asakura et al., 1999).
Several lines of evidence indicate that, as in fibroblasts, the
nectin-afadin and E-cadherin—catenin systems are physically and
functionally associated in epithelial cells. Firstly, during the formation of
the junctional complex involving AJs and TJs in Madin Darby canine kidney
(MDCK) cells that stably express exogenous nectin-1 (nectin-1-MDCK cells),
E-cadherin is recruited to the nectin-1-based junctions where afadin
colocalizes (Honda et al.,
2003). Moreover, Nef-3 inhibits the formation of nectin-1-based
intercellular junctions and thereby the recruitment of E-cadherin, and this
inhibits formation of E-cadherin-based AJs in wild-type and nectin-1-MDCK
cells (Honda et al., 2003). In
the latter, Nef-3 coated on microbeads first recruits the
nectin-1—afadin complex and then the E-cadherin—catenin complex to
the bead-cell contact sites (Honda et al.,
2003).
Further evidence is provided by the gastric cancer cell line, HSC-39. These
cells express E-cadherin but do not form intercellular junctions
(Yanagihara et al., 1991).
Overexpression of nectin-2 or nectin-3 leads to intercellular adhesion
(Peng et al., 2002). In
keratinocytes an N-cadherin mutant that lacks the extracellular region acts as
a dominant negative mutant and reduces the E-cadherin-based adhesion activity
(Fujimori and Takeichi et al., 1993). Overexpression of nectin-3 reverses this
inhibitory action of the N-cadherin mutant (Y. Tanaka, H. Nakanishi, S.
Kakunaga et al., unpublished). Overexpressed nectin-3 recruits the N-cadherin
mutant to the nectin-3-based junctions. The N-cadherin mutant probably
associates with endogenous nectin and therefore prevents it from associating
with endogenous E-cadherin.
Afadin-deficient mice show developmental defects at stages around
gastrulation, including disorganization of the ectoderm, impaired migration of
the mesoderm, and loss of somites and other structures that are derived from
both the ectoderm and the mesoderm (Ikeda
et al., 1999; Zhadanov et al.,
1999). Furthermore, cystic embryoid bodies derived from
afadin-deficient embryonic stem cells show disorganization of the ectoderm
(Ikeda et al., 1999). In the
ectoderm of afadin-deficient mice and embryoid bodies, the organization of AJs
is highly impaired. Again, these results strongly implicate the nectin-afadin
system in the organization of intercellular junctions in these tissues.
Single-molecule image analysis of nectin-2 and E-cadherin in mouse mammary
tumor (MTD-1A) cells indicates that nectin-2 moves more rapidly than
E-cadherin on the free surface of the plasma membrane (Katsuno et al.,
manuscript in preparation). These results suggest that the nectin-afadin and
E-cadherin—catenin systems cooperatively organize AJs. The molecular
mechanism by which the nectin-afadin and E-cadherin—catenin systems
associate in epithelial cells thus appears to be similar to that in
fibroblasts but has not fully been elucidated.
At TJs, three CAMs have thus far been identified
(Fig. 4A). Claudin functions as
a Ca2+-independent CAM (Tsukita
et al., 1999; Tsukita et al.,
2001) and constitutes a superfamily consisting of over 20 members.
Occludin is another transmembrane protein at TJs, but its function has not yet
been established. Junctional adhesion molecule (JAM) represents a
Ca2+-independent Ig-like CAM family of at least three members at
TJs (Martin-Padura et al.,
1998; Ozaki et al.,
1999; Palmeri et al.,
2000; Liang et al.,
2002). Claudin and occludin have four transmembrane regions,
whereas JAM has an extracellular region that has two Ig-like domains, a single
transmembrane region and a cytoplasmic region. Claudin, occludin and JAM are
linked to the actin cytoskeleton through peripheral membrane proteins,
including ZO-1, ZO-2 and ZO-3 (Tsukita et
al., 1999; Tsukita et al.,
2001; Bazzoni et al.,
2000; Ebnet et al.,
2000). ZO-1 and ZO-2 are F-actin-binding proteins and form a dimer
with ZO-3 (Tsukita et al.,
1999; Tsukita et al.,
2001). The association of these CAMs with the actin cytoskeleton
has been proposed to strengthen the intercellular adhesion of TJs
(Tsukita et al., 1999;
Tsukita et al., 2001).
Although the formation of TJs is dependent on the formation of
E-cadherin-based AJs (Takeichi,
1991; Gumbiner,
1996; Vlemincks and Kemler,
1999), accumulating evidence suggests that the nectin-afadin
system plays a role in the organization of TJs. During the formation of the
junctional complex of AJs and TJs in nectin-1-MDCK cells, claudin, occludin
and JAM are recruited to the apical side of the nectin-1-based junctions,
where both afadin and ZO-1 colocalize
(Fukuhara et al., 2002a;
Fukuhara et al., 2002b). After
they are recruited, ZO-1 translocates from the nectin-1-based junctions to
their apical side. Recruitment of claudin, occludin and JAM is inhibited by
Nef-3 (Fukuhara et al., 2002a;
Fukuhara et al., 2002b). Nef-3
coated on microbeads recruits not only the nectin-1-afadin complex but also
ZO-1 and JAM to the bead-cell contact sites in nectin-1-MDCK cells
(Fukuhara et al., 2002a).
Nectin-2 recruits both -catenin and ZO-1 at the same time to the
nectin-2-based intercellular adhesion sites in L cells that stably express
exogenous nectin-2 (nectin-2-L cells)
(Yokoyama et al., 2001). This
recruitment requires afadin but not -catenin, ponsin or vinculin. In
the ectoderm of afadin-deficient mice and embryoid bodies, organization of not
only AJs but also TJs is highly impaired
(Ikeda et al., 1999).
Furthermore, mutations in the nectin-1 gene are responsible for cleft
lip/palate ectodermal dysplasia — Margarita island ectodermal dysplasia
and Zlotogora-Ogür syndrome — which is characterized by cleft
lip/palate, syndactyly and ectodermal dysplasia
(Suzuki et al., 2000;
Sozen et al., 2001). These
results suggest that the nectin-afadin system plays a role in the organization
of TJs. It is unknown how the nectin-afadin system regulates the organization
of TJs, but ZO-1, ZO-2 and ZO-3 associated with the nectin-afadin system may
play a role in recruiting JAM, claudin and occludin. It also remains unknown
whether JAM is involved in the localization of claudin and occludin for
organization of TJs.
At the initial stage of formation of AJs and TJs, primordial spot-like
junctions first form at the tips of the cellular protrusions that radiate from
adjacent cells (Yonemura et al.,
1995; Adams et al.,
1998; Vasioukhin et al.,
2000). Components of the E-cadherin—catenin and
nectin-afadin systems, ZO-1 and JAM colocalize to these spot-like junctions,
at which neither claudin or occludin is concentrated
(Ando-Akatsuka et al., 1996;
Asakura et al., 1999;
Sakisaka et al., 1999;
Ebnet et al., 2001). The
spot-like junctions begin to fuse to form short line-like junctions, at which
claudin and occludin do accumulate. Although the precise timing of arrival of
each component at the junctional complex during its formation is unclear, we
can propose the following speculative model for formation of the junctional
complex.
In our model, all CAMs, including nectin, E-cadherin, JAM, claudin and occludin, are diffusely distributed on the free surface of the plasma membranes of migrating cells (Fig. 5A). When the two migrating cells contact through protrusions such as filopodia and lamellipodia, nectin and E-cadherin separately form trans-dimers that form micro-clusters at intercellular contact sites. Because, kinetically, nectin forms micro-clusters more rapidly than E-cadherin, the nectin-based micro-clusters are mainly formed at the initial stage. The nectin-based microclusters then recruit E-cadherin, which results in the formation of a mixture of nectin- and E-cadherin-based microclusters. The nectin and E-cadherin molecules in these microclusters are associated through afadin and catenins that are linked to the actin cytoskeleton. E-Cadherin-based micro-clusters that form slowly and independently of the nectin-based microclusters rapidly recruit the nectin-afadin complex to form other primordial spot-like junctions. These primordial junctions fuse with each other to form short line-like junctions, which develop into more matured AJs. During the formation of AJs, JAM is first assembled at the apical side of AJs, and this is followed by the recruitment of claudin and occludin presumably through ZO-1, ZO-2 and ZO—3, which eventually leads to the establishment of claudin-based TJs (Fig. 5B).
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Roles of nectin and afadin in organization of synapses |
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TopSummaryIntroductionMolecular structures of nectin...Intercellular adhesion activity...Roles of nectin and...Roles of nectin and...Roles of nectin and...Roles of nectin and...Role of nectin as...Conclusions and perspectivesReferences |
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). Nectin-1 and nectin-3 localize asymmetrically at the
presynaptic and postsynaptic sides, respectively, of the plasma membranes of
the puncta adherentia junctions and form hetero-trans-dimers
(Mizoguchi et al., 2002)
(Fig. 4B). Afadin, N-cadherin
and N-catenin localize symmetrically at both sides
(Nishioka et al., 2000;
Mizoguchi et al., 2002).
s-Afadin, but not l-afadin, localizes at PSDs
(Buchert et al., 1999;
Nishioka et al., 2000). In at
least some brain regions other than the CA3 area of the hippocampus, such as
the optic tectum, N-cadherin localizes at both synaptic junctions and puncta
adherentia junctions (Yamagata et al.,
1995).
Studies using antibodies to N-cadherin and antagonistic peptides indicate
that N-cadherin plays a role in the organization of synapses
(Yamagata et al., 1995;
Tang et al., 1998). Inhibition
of nectin-1- and nectin-3-based adhesion by an inhibitor of nectin-1 (gD) in
cultured rat hippocampal neurons results in a decrease in size and a
concomitant increase in the number of synapses
(Mizoguchi et al., 2002). The
exact mechanism by which this occurs remains to be clarified but reflects
partial inhibition of the formation of the hetero-transdimer between nectin-1
and nectin-3, which may affect N-cadherin-mediated adhesion and eventually
lead to formation of smaller synapses. Why the number of synapses increases is
not known either but may be due to a failure of neurons to determine the
proper positions of synapses or compensation for functionally less competent
smaller synapses. Thus, it is likely that the formation of the
hetero-trans-dimer between nectin-1 and nectin-3 plays an important role in
the determination of the position and the size of synapses — in
cooperation with the N-cadherin—catenin system. This role of the
nectin-afadin system is consistent with the finding that mutations in the
nectin-1 gene are responsible for cleft lip/palate ectodermal dysplasia
— Zlotogora-Ogür syndrome — which is characterized by mental
retardation in addition to ectodermal dysplasia
(Suzuki et al., 2000).
Synapses are formed by the meeting of axons and dendrites during their
maturation. At primitive synapses, synaptic junctions and puncta adherentia
junctions are not morphologically differentiated, but during the maturation of
synapses membrane domain specialization gradually occurs
(Amaral and Dent, 1981). This
neural membrane domain specialization appears to be similar to that found
during formation of the junctional complex in epithelial cells with respect to
the dynamic localization patterns of the junctional proteins. It is postulated
that primordial junctions form first, followed by transport of the components
of active zones on dense core vesicles and subsequent formation of active
zones at the presynaptic side (for reviews, see
Desbach et al., 2001;
Ziv and Garner, 2001). At the
postsynaptic side, the components of PSDs are assembled, and membrane
receptors on vesicles are transported to this region. We imagine that, in
synaptogenesis, the nectin-afadin unit first forms primordial junctions, and
this is followed by the recruitment of the N-cadherin—catenin unit. The
components of active zones would then be recruited to the primordial junctions
to form active zones at the presynaptic side. At the postsynaptic side, the
components of PSDs would be assembled, and membrane receptors would be
recruited. The membrane domains, comprising synaptic junctions and puncta
adherentia junctions, would then gradually become segregated, and this would
be followed by maturation of synapses.
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Roles of nectin and afadin in organization of Sertoli-cell—spermatid junctions |
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TopSummaryIntroductionMolecular structures of nectin...Intercellular adhesion activity...Roles of nectin and...Roles of nectin and...Roles of nectin and...Roles of nectin and...Role of nectin as...Conclusions and perspectivesReferences |
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;
Vogl et al., 2000). ES
contains F-actin bundles, which are arranged at regular intervals beneath the
plasma membrane, and a flattened cistern of the endoplasmic reticulum, which
is connected to microtubules (Fig.
4C). This is supposed to function as a scaffold that stabilizes an
adhesive domain in the plasma membrane of Sertoli cells. Vinculin is
concentrated at Sertoli-cell—Sertoli-cell junctions and
Sertoli-cell—spermatid junctions
(Grove and Vogl, 1989;
Pfeiffer and Vogl, 1991).
Since the ES is formed only in Sertoli cells, Sertoli-cell—spermatid
junctions are asymmetric. Moreover, unlike at typical AJs, the existence of
the cadherin-catenin system has been questioned at
Sertoli-cell—spermatid junctions
(Byers et al., 1991;
Cyr et al., 1992; Anderson et
al., 1994). Thus, little is known about the intercellular CAMs responsible for
Sertoli-cell—spermatid junctions. In contrast to
Sertoli-cell—spermatid junctions, Sertoli-cell—Sertoli-cell
junctions are equipped with TJs as well as AJs, which serve as the
`blood-testis' barrier.
Nectin-2, nectin-3 and afadin colocalize with the F-actin that underlies
Sertoli-cell—spermatid junctions
(Ozaki-Kuroda et al., 2002).
(Fig. 4C). Nectin-2 and
nectin-3 reside specifically in Sertoli cells and spermatids, respectively,
which suggests the formation of a hetero-trans-dimer from nectin-3 in
spermatids and nectin-2 in Sertoli cells. The nectin-based adhesive membrane
microdomains show one-to-one linkage with each F-actin bundle at Sertoli cell
—spermatid junctions. Nectin-2 and afadin localize at
Sertoli-cell—Sertoli-cell junctions. Nectin-2-deficient mice exhibit
male-specific infertility and have defects in the later steps of sperm
morphogenesis, including distorted nuclei and an abnormal distribution of
mitochondria (Bouchard et al.,
2000). In these mice, the structure of
Sertoli-cell—spermatid junctions is severely impaired, and the
localization of nectin-3 and afadin is disorganized, whereas
Sertoli-cell—Sertoli-cell junctions are apparently normal
(Ozaki-Kuroda et al., 2002).
The loosened adhesion and the lack of an F-actin scaffold due to
mislocalization of afadin at Sertoli-cell—spermatid junctions may act
together to render the contact site weak and convoluted and also produce the
drastic condensation of spermatid nuclei.
On the basis of these observations, we propose a model for the molecular organization of Sertoli-cell—spermatid junctions in which nectin-2 on the Sertoli cell membrane forms a hetero-trans-dimer with nectin—3 on the spermatid membrane to form discrete adhesive membrane domains (Fig. 4C). In this model, the cytoplasmic region of nectin-2 in Sertoli cells binds to afadin to connect F-actin bundles to the membrane. The localization of afadin in spermatids remains unknown. It is likely that Sertoli-cell—spermatid junctions rely largely on the nectin-afadin system, whereas Sertoli-cell—Sertoli-cell junctions are formed through cooperation of multiple intercellular adhesion systems.
|
Role of nectin as a receptor for -herpes viruses for entry and
intercellular spreading
|
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TopSummaryIntroductionMolecular structures of nectin...Intercellular adhesion activity...Roles of nectin and...Roles of nectin and...Roles of nectin and...Roles of nectin and...Role of nectin as...Conclusions and perspectivesReferences |
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). Nectin-2 was originally isolated as the murine homolog of
human PVR (Morrison and Racaniello,
1992) but turned out to be another poliovirus-receptor-related
protein and was named PRR-2 (Eberlé
et al., 1995). Neither PRR-1 nor PRR-2 has thus far been shown to
serve as a PVR. They were later shown to serve as receptors for -herpes
viruses, facilitating their entry and intercellular spreading, and renamed
HveC and HveB, respectively (Cocchi et
al., 1998; Cocchi et al.,
2000; Geraghty et al.,
1998; Warner et al.,
1998; Sakisaka et al.,
2001). It remains unknown whether nectin-3 and nectin-4 serve as
receptors for viruses. Human nectin-1 allows entry of all -herpes
viruses tested so far, including herpes simplex virus (HSV) type 1, HSV type 2
and pseudorabies virus (Spear et al.,
2000; Campadelli-Fiume et al.,
2000). Human nectin-2 can mediate entry of a restricted number of
-herpes viruses (Spear et al.,
2000; Campadelli-Fiume et al.,
2000). The usual manifestations of HSV disease are mucocutaneous
lesions. HSV disease establishes latent infection of neurons in sensory
ganglia and causes recurrent lesions at the sites of primary infection. In HSV
disease, the intercellular spreading significantly contributes to the
pathogenesis. The underlying mechanisms for entry and intercellular spreading
of HSVs through nectin-1 are not fully understood, but at least four viral
glycoproteins, gD, gB, and the gH-gL heterodimer, participate in the entry of
HSV-1 (Spear et al., 2000;
Campadelli-Fiume et al.,
2000). Recombinant gD binds to nectin-1 in host cells and inhibits
HSV-1 infection, which indicates that gD serves as a viral component that
specifically interacts with its cellular receptor, nectin-1. The first Ig-like
domain of nectin-1 is sufficient for the binding of gD and entry of HSV-1, and
the second and third Ig-like domains increase the efficiency of entry
(Cocchi et al., 1998). The
interaction of nectin-1 with afadin increases the efficiency of intercellular
spreading, but not the entry, of HSV-1
(Sakisaka et al., 2001). The
E-cadherin—catenin system increases efficiency of both entry and
intercellular spreading of HSV-1, which provides an additional line of
evidence that nectin and E-cadherin are physically and functionally associated
(Sakisaka et al., 2001). |
Conclusions and perspectives |
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TopSummaryIntroductionMolecular structures of nectin...Intercellular adhesion activity...Roles of nectin and...Roles of nectin and...Roles of nectin and...Roles of nectin and...Role of nectin as...Conclusions and perspectivesReferences |
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References |
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TopSummaryIntroductionMolecular structures of nectin...Intercellular adhesion activity...Roles of nectin and...Roles of nectin and...Roles of nectin and...Roles of nectin and...Role of nectin as...Conclusions and perspectivesReferences |
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