1 The Burnham Institute, Neurobiology Program, La Jolla, CA 92037, USA
2 University of California, San Diego, Molecular Pathology Program, La Jolla, CA
92093, USA
Author for correspondence (e-mail:
elenap{at}burnham.org)
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Summary |
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Key words: Receptor tyrosine kinase, Growth cone, Axon guidance, Synapse, Angiogenesis, Rho, Ras
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Introduction |
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Regulation of Eph expression |
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Structural features |
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Recent studies show that engagement with an ephrin induces a conformational
change in the cytoplasmic portion of the Eph receptor
(Fig. 1B)
(Wybenga-Groot et al., 2001).
This is triggered by phosphorylation of two juxtamembrane tyrosine residues,
which relieves the inhibition of the juxtamembrane segment on the kinase
domain (Zisch et al., 2000
).
This phosphorylation also establishes binding sites for the SH2 domains of
several proteins. Occupancy of the juxtamembrane binding sites and additional
phosphorylation may further stabilize the active conformation of the Eph
receptor. The transmembrane ephrin-B ligands also become phosphorylated on
conserved cytoplasmic tyrosines upon binding to an EphB receptor
(Bruckner et al., 1997
;
Holland et al., 1996
;
Kalo et al., 2001
). This
tyrosine phosphorylation may induce a conformational change in the hairpin
structure in the C-terminal half of the cytoplasmic domain and allow binding
of SH2 domains (Song et al.,
2002
).
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Functional versatility of Eph proteins |
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Recent advances also suggest that Eph receptors and ephrins regulate the
development and function of neuromuscular junctions
(Lai et al., 2001) and central
synapses (Gerlai, 2001
).
During synaptogenesis, the Eph receptors help establish
(Dalva et al., 2000
) and
modify the postsynaptic specialization
(Ethell et al., 2001
) by
transmitting signals to the actin cytoskeleton through the Rho-family of small
GTPases (Irie and Yamaguchi,
2002
; Penzes et al.,
2003
). EphB receptors also modulate NMDA-receptor-mediated calcium
influx via a Src-family kinase pathway
(Takasu et al., 2002
) and
could have a direct impact on synaptic transmission
(Henderson et al., 2001
;
Contractor et al., 2002
).
Intriguingly, EphA/ephrin-A signals mediate a form of crosstalk between glial
cells and neurons, which regulates the morphology of excitatory synapses in
the mature hippocampus (Murai et al.,
2003
). Thus, both EphA and EphB receptors can regulate the
structural and functional properties of synapses and probably participate in
cognitive processes including learning and memory formation
(Murai and Pasquale,
2002
).
Eph proteins also play a critical role in the cellular organization and
function of non-neural tissues. In the cardiovascular system, ephrin-B2 and
EphB4 are preferentially expressed on arterial and venous endothelium,
respectively (Wang et al.,
1998). The similar phenotypes of EphB4- and ephrin-B2-knockout
mice suggest that reciprocal signaling is important for vascular development
and remodeling in the embryo (Adams et al.,
1999
; Gerety et al.,
1999
). EphB2, EphB3 and ephrin-B1 have also been implicated in
embryonic vascularization and their localization suggests that signaling
occurs not only between the arterial and venous compartments but also between
endothelial cells and the surrounding mesenchyme
(Adams et al., 1999
).
Intriguingly, EphB proteins may contribute to the organization of the vascular
network by mediating neuro-arterial interactions
(Mukouyama et al., 2002
).
These findings have important implications for understanding and treating
cancer because Eph proteins probably regulate angiogenic processes associated
with tumor growth (Brantley et al.,
2002
; Dodelet and Pasquale,
2000
; Ogawa et al.,
2000
; Pandey et al.,
1995
). Interestingly, EphB4 and ephrin-B2 also play a role in
erythropoiesis by influencing hematopoietic cell lineages that share a common
ancestry with endothelial cells (Suenobu
et al., 2002
; Wang et al.,
2002b
). Furthermore, Eph proteins play a role in platelet
clustering and hence may be important for blood clotting at sites of vascular
injury (Prevost et al.,
2002
).
In summary, the Eph receptors and ephrins display extensive functional versatility. Their activities result from bi-directional signals propagated downstream of the ligand-receptor complex. In the next sections we summarize some of the most recent findings on Eph receptor forward signaling, ephrin-mediated reverse signaling, and crosstalk with other receptors.
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Forward signaling |
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Eph signaling through Rho family GTPases
Increasing evidence indicates that the Eph receptors regulate actin
dynamics through small GTPases of the Rho family (Rho, Rac and Cdc42). Rho
GTPases cycle between an active, GTP-bound conformation and an inactive
GDP-bound conformation. They control cell shape and movement by promoting the
formation of stress fibers (Rho), lamellipodia (Rac) and filopodia (Cdc42)
(Nobes and Hall, 1995). In
neurons, Rho activation inhibits neurite outgrowth and promotes growth cone
collapse and axon retraction (Dickson,
2001
; Luo, 2000
;
Yuan et al., 2003
). Rac and
Cdc42 play an antagonistic role to Rho in neuronal morphology by enhancing the
formation of the F-actin meshwork in growth cone lamellipodia (Rac) and
promoting the extension of filopodia (Cdc42)
(Kozma et al., 1997
;
Yuan et al., 2003
).
EphA receptors can directly activate Rho GTPases through the recently
identified exchange factor Ephexin
(Shamah et al., 2001).
Exchange factors activate GTPases by catalyzing the replacement of GDP with
GTP. Ephexin is preferentially expressed in the nervous system and
constitutively binds the kinase domain of EphA receptors. Interestingly,
ephrin-A1 treatment of cultured neurons potentiates Ephexin-mediated exchange
on Rho. The activation of Rho and its downstream effectors, Rho-associated
kinases, propagate ephrin-A-induced signals to initiate growth cone collapse
(Wahl et al., 2000
). In 293T
and melanoma cells ephrin-A-induced Rho activation causes the retraction of
cell processes, cell rounding/detachment, and membrane blebbing, and this
appears to depend on the adaptor protein Crk
(Lawrenson et al., 2002
). Rho
activity may mediate these events by stabilizing actin filaments and promoting
actomyosin contractility (Dickson,
2001
; Luo, 2000
).
Recent data show that actin filament stabilization alone can cause actin
redistribution from the outer edges to the central domain of the growth cone
and axon retraction through basal myosin-driven contractility
(Gallo et al., 2002
;
Jurney et al., 2002
;
Meima et al., 1997a
;
Meima et al., 1997b
).
Concomitantly with the activation of Rho, ephrin-A ligands can inhibit Rac
activation in retinal and cortical neurons
(Shamah et al., 2001;
Wahl et al., 2000
).
Furthermore, the kinase Pak, a major downstream effector of Rac and Cdc42, is
also inhibited by ephrin-A treatment. However, it is unclear whether Rac
inactivation is a direct consequence of Rho activation or is mediated through
an independent signaling pathway. Jurney et al.
(Jurney et al., 2002
) have
shown that, in cultures of retinal neurons, Rac is only briefly inactivated by
ephrin-A ligands, and recovers within minutes at the onset of growth cone
collapse. Indeed, Rac activity is required for reorganizing actin filaments to
the center of the collapsing growth cone and for driving endocytosis of the
plasma membrane. Further experiments are needed to elucidate the details of
the connection between Eph receptors and the Rac signaling pathway.
Remarkably, EphB receptors seem to interact with a different group of
exchange factors for Rho family GTPases. EphB2 has recently been shown to
associate with the exchange factors intersectin
(Irie and Yamaguchi, 2002) and
kalirin (Penzes et al.,
2003
). Intersectin activates Cdc42 and its activity is
synergistically enhanced by EphB2 and WASP, an adaptor that promotes branching
and elongation of actin filaments through the Arp2/3 complex
(Cory et al., 2002
;
Hussain et al., 1999
).
Kalirin, an exchange factor for Rac, co-clusters with activated EphB2 and
seems to localize activated Rac to sites of EphB-ephrin interaction without
changing the overall level of Rac activation
(Irie and Yamaguchi, 2002
;
Penzes et al., 2003
). The
intersectin-Cdc42-WASP-actin and kalirin-Rac-Pak-actin pathways have been
recently proposed to regulate the EphB-receptor-mediated morphogenesis and
maturation of dendritic spines in cultured hippocampal and cortical neurons
(Irie and Yamaguchi, 2002
;
Penzes et al., 2003
). It will
be interesting to determine whether activation of Pak downstream of Cdc42 and
Rac opposes actomyosin contractility, since Pak phosphorylates and inhibits
myosin light chain kinase (Sanders et
al., 1999
). Perhaps activation of a Cdc42/Rac-Pak signaling
pathway accounts for the lack of neurite retraction downstream of EphB
receptors (Meima et al.,
1997b
). Furthermore, it may explain the appearance of numerous
hair-like structures protruding from the cell body of neurons treated with
ephrin-B1 (Meima et al.,
1997b
) and the ephrin-B1-dependent increase in the density of
dendritic spine protrusions (Penzes et
al., 2003
).
Differential signaling may account for the ability of ephrin-BEphB
interactions to cause modifications to cell morphology different from those
induced by ephrin-AEphA interactions. For example, in primary cortical
neurons ephrin-B1 causes growth cone collapse without causing extensive
neurite retraction (Meima et al.,
1997b). Furthermore, the neurites become thinner and develop large
swellings along their lengths, which are classic morphological signs of
microtubule depolymerization (Baas and
Ahmad, 2001
). Indeed, ephrin-B1 causes microtubule
depolymerization (Meima et al.,
1997b
). By contrast, the actin filaments remain intact but are
partially redistributed to the neuronal cell body. It will be interesting to
determine whether these signaling differences account for the disparity in the
in vivo guidance activity of EphA and EphB receptors. For example, during the
establishment of visual system topography, EphA receptors mediate axon
repulsion and retraction whereas EphB receptors appear to provide a stop
signal limited to the growth cone (Hindges
et al., 2002
; Mann et al.,
2002
).
The Ras family
Eph receptors also regulate the activities of small GTPases of the Ras
family. The best-characterized member of this family, H-Ras, activates a MAP
kinase cascade culminating in the phosphorylation and activation of the
Erk1/Erk2 MAP kinases (Chang and Karin,
2001; Johnson and Lapadat,
2002
). Although transcriptional regulation and increased cell
proliferation are major outputs of the RasMAP-kinase pathway, this
pathway is also important for cell migration, neurite outgrowth and axon
guidance (Borasio et al., 1989
;
Forcet et al., 2002
). Indeed,
MAP kinases can phosphorylate cytoskeletal targets such as microtubules and
microfilaments, in addition to myosin light chain kinase
(Gundersen and Cook, 1999
;
Klemke et al., 1997
). Eph
receptors negatively regulate the RasMAP-kinase pathway in most cell
types, with a few exceptions (Elowe et
al., 2001
; Miao et al.,
2001
; Pratt and Kinch,
2002
; Zisch et al.,
2000
). For example, EphB2 transiently downregulates H-Ras activity
and MAP kinase phosphorylation and induces neurite retraction in the NG108
neuronal cell line (Elowe et al.,
2001
; Tong et al.,
2003
). Furthermore, ephrin-B2 stimulation inhibits VEGF (vascular
endothelial cell growth factor) and angiopoietin-1-mediated cell migration
while downregulating the RasMAP-kinase pathway
(Kim et al., 2002
). Likewise,
EphA2 downregulates the RasMAP-kinase pathway in fibroblasts,
endothelial cells, epithelial cells and tumor cells
(Miao et al., 2001
).
Remarkably, the Eph receptors can attenuate RasMAP-kinase signaling
downstream of other receptors, such as integrins and various families of
receptor tyrosine kinase (Elowe et al.,
2001
; Grunwald et al.,
2001
; Kim et al.,
2002
; Miao et al.,
2001
). However, it remains to be determined whether this may be a
result of crosstalk between activated H-Ras and Rho family proteins
(Bar-Sagi and Hall, 2000
) or
other mechanisms.
At least in some cases, the ability of Eph receptors to regulate the
RasMAP-kinase pathway seems to depend on the Ras GTPase-activating
protein, Ras-GAP (Tong et al.,
2003). Although RasGAP binds to activated EphB receptors
(Becker et al., 2000
;
Hock et al., 1998
;
Holland et al., 1997
;
Kim et al., 2002
), it is not
clear whether direct physical interaction is essential
(Miao et al., 2001
;
Tong et al., 2003
). This is
further complicated by the ephrin-induced association of RasGAP with
p62Dok (a negative regulator of the MAP kinase pathway)
(Jones and Dumont, 1999
) and
the connection of RasGAP with RhoGAP
(Holland et al., 1997
;
Leblanc et al., 1998
).
R-Ras is another Ras protein whose function is suppressed downstream of Eph
receptors. This relies on a novel regulatory mechanism involving tyrosine
phosphorylation of the effector domain of R-Ras by EphB2
(Zou et al., 1999). R-Ras
positively regulates integrin-mediated adhesion
(Zhang et al., 1996
), and
R-Ras tyrosine phosphorylation decreases adhesion and rounding of 293 cells
transfected with EphB2. This is presumably caused by interfering with the
ability of R-Ras to bind effector proteins. EphB receptors can also regulate
Rap1 in human aortic endothelial cells
(Nagashima et al., 2002
).
Rap1, like R-Ras, positively modulates integrin-mediated adhesion
(Caron et al., 2000
;
Reedquist et al., 2000
).
Ephrin-B1 treatment of human aortic endothelial cells, which express EphB1,
causes Crk-dependent Rap1 activation and cell spreading. A possible link
between Eph receptors and R-Ras/Rap1 is SHEP1. SHEP1 contains an SH2 domain
that binds activated Eph receptors and a guanine nucleotide exchange
factor-like domain that binds R-Ras, Rap1
(Dodelet et al., 1999
) and the
docking protein Cas (Sakakibara and
Hattori, 2000
). SHEP1 promotes Rap1 activation through a complex
with Cas, the adaptor Crk, and its associated exchange factor C3G
(Sakakibara et al.,
2002
).
Other signaling pathways downstream of Eph receptors
Eph receptors also influence other signaling molecules that regulate cell
behavior. However, the information available is not yet sufficient to develop
a coherent model of these Eph receptor pathways. Several Eph receptors have
been reported to suppress (Miao et al.,
2000; Zou et al.,
1999
) or promote (Huynh-Do et
al., 1999
; Nagashima et al.,
2002
; Stein et al.,
1998b
) integrin activity. Focal adhesion kinase (FAK), a critical
element in integrin signaling, may connect Eph receptors with integrins
(Miao et al., 2000
). In PC-3
prostate carcinoma cells, ligand activation of EphA2 causes dissociation of
FAK and the transient recruitment of the phosphotyrosine phosphatase Shp2,
which dephosphorylates FAK and its substrate paxillin. This correlates with
inhibition of integrin-mediated adhesion, cell spreading and cell migration.
However, this mechanism may be cell-type specific because EphA2 activity can
increase FAK phosphorylation in NIH3T3 cells and enhance cell spreading in a
FAK-dependent fashion (Carter et al.,
2002
).
EphA2-mediated cell spreading on adhesive substrates depends not only on
FAK, but also on Rho and the FAK-binding protein Cas
(Carter et al., 2002). Cas is
an adaptor that mediates assembly of structural and signaling proteins at
FAK-containing integrin adhesion sites. Some of the Cas interactions depend on
Cas tyrosine phosphorylation. Indeed, EphA2 activity increases Cas tyrosine
phosphorylation in NIH 3T3 cells (Carter
et al., 2002
). Similarly, EphB1 activation in human aortic
endothelial cells increases Cas tyrosine phosphorylation while also promoting
an association with the Crk adaptor protein
(Nagashima et al., 2002
). This
Cas/Crk pathway may be important for Rap1 activation in membrane ruffles after
ephrin-B1 treatment. Interestingly, SHEP1 binds to both Cas and Rap1
(Dodelet et al., 1999
) and may
link Eph receptors and these proteins during adhesion and migration.
EphA8 has also been reported to regulate integrin function in both NIH3T3
and 293 cells through a constitutive interaction with the p110 subunit
of PI 3-kinase (Choi and Park,
1999
; Gu and Park,
2001
). Interestingly, it appears that EphA8 inhibits cell adhesion
in 293 cells but stimulates adhesion of NIH 3T3 cells. Activation of PI
3-kinase is also important for endothelial cell migration and proliferation
downstream of EphB4 (Steinle et al.,
2002
). The SH2 domain of the p85 regulatory subunit of PI 3-kinase
also interacts with activated EphA2, but the significance of this interaction
is not clear (Pandey et al.,
1994
).
In addition, Eph receptors can modify cell behavior by signaling through
other SH2-domain-containing adaptor proteins. It was recently reported that
the SH2 domain of Grb7 binds to the tyrosine phosphorylated SAM domain of
EphB1 and this association can modify cell migration
(Han et al., 2002;
Han et al., 2001
).
Interestingly, this interaction appears to be selective, as Grb7 does not bind
the related receptor EphB3. The same phosphorylated tyrosine motif of EphB1
and EphB2 mediates binding of the low molecular weight phosphotyrosine
phosphatase (LMW-PTP), an interaction that regulates cell adhesion
(Stein et al., 1998b
). The
adaptors Shc, Grb2, Grb10 and Nck can also interact with Eph receptors
(Pratt and Kinch, 2002
;
Stein et al., 1996
;
Stein et al., 1998a
). The
Nck/NIK (Nck-interacting Ste20 kinase) pathway is important to activate JNK
and upregulate integrin-mediated adhesion
(Becker et al., 2000
). Nck
could also link Eph receptors to the regulation of the actin cytoskeleton by
cooperating with Rho proteins. It interacts with downstream effectors of Rac
and Cdc42, including Pak3 and WASP, which influence actin polymerization
(Becker et al., 2000
;
Holland et al., 1997
).
Additionally, some Eph receptors seem to selectively bind the ubiquitin ligase
and adaptor protein Cbl. EphB6 clustering in Jurkat cells leads to Cbl
dephosphorylation (Freywald et al.,
2002
; Luo et al.,
2001
). Cbl may couple EphB6 to Grb2 and Crk, and promote EphB6
degradation via the proteasome pathway. The interaction of Cbl with EphA2
instead occurs only after ephrin binding and promotes activation-dependent
EphA2 degradation (Walker-Daniels et al.,
2002
; Wang et al.,
2002a
).
A number of additional signaling proteins have been linked to Eph receptor
downstream signaling pathways. For example, the Src and Abl family cytoplasmic
tyrosine kinases associate with activated Eph receptors and this may
contribute to the regulation of cytoskeletal organization, cell migration and
axon guidance (Kalo and Pasquale,
1999). Dominant negative approaches have suggested that signaling
by the Src family kinase Fyn plays an important role in linking EphA8
signaling to cell attachment responses
(Choi and Park, 1999
).
Interestingly, while Eph receptors are thought to activate Src proteins, the
EphB2 receptor inhibits the in vitro activity of Abl
(Yu et al., 2001
;
Zisch et al., 1998
).
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Reverse signaling |
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The first studies to suggest that the ephrins are more than merely ligands
for the Eph receptors were in the mid 1990s, when Brambilla et al. found that
the cytoplasmic portion of ephrin-B1 can negatively regulate transformation of
NIH-3T3 cells transfected with EphB-Trk chimeric receptors
(Brambilla et al., 1995). This
was followed by two seminal reports showing that the extracellular portions of
EphB receptors can induce ephrin-B tyrosine phosphorylation
(Bruckner et al., 1997
;
Holland et al., 1996
). Src
family kinases are responsible for ephrin-B phosphorylation upon Eph receptor
engagement (Holland et al.,
1996
; Palmer et al.,
2002
), and three conserved tyrosines in the cytoplasmic domain of
ephrin-B1 have been identified as in vivo phosphorylation sites
(Kalo et al., 2001
). This
suggests a receptor-like property for ephrin-B molecules, and the possibility
of transmitting signals through proteins containing SH2 domains.
At least one SH2-domain-containing protein binds to tyrosine-phosphorylated
ephrin-B1 (Cowan and Henkemeyer,
2001). The adaptor protein Grb4, which has three SH3 domains and
an SH2 domain, could link ephrin-Bs to a vast signaling network that modifies
cell morphology through reorganization of the actin cytoskeleton. Indeed,
activation of ephrin-B1 increases Fak activity, redistributes the Fak-binding
protein paxillin, and leads to disassembly of focal adhesions. A mechanism
that may serve to turn off phosphorylation-dependent ephrin-B reverse signals
involves the delayed recruitment of the phosphotyrosine phosphatase PTP-BL,
which can dephosphorylate the ephrin-B cytoplasmic domain and inactivate Src
family kinases (Palmer et al.,
2002
). The interaction of PTB-BL with ephrin-B1 is mediated by the
PDZ domain of the phosphatase, which binds the C-terminal PDZ-binding motif of
ephrin-B molecules. Interestingly, phosphorylation of this region may modulate
ephrin-B binding to PDZ domain-containing proteins.
Ephrin-Bs can also initiate reverse signaling through PDZ-domain-mediated
associations (Lu et al.,
2001). The GTPase-activating protein PDZ-RGS3, which catalyzes the
hydrolysis of GTP to GDP in the
subunits of heterotrimeric G proteins,
binds to the PDZ-binding motif of ephrin-B molecules. Signaling through
PDZ-RGS3 mediates the de-adhesion of Xenopus embryo cells expressing
ephrin-B1. It also inhibits SDF1 (stromal cell derived factor 1)-mediated
cerebellar granule cell chemotaxis through the CXCR4 G-protein-coupled
chemokine receptor. During cerebellar development, ephrin-B activation by EphB
receptors may attenuate granule cell attraction to SDF-1, which is expressed
at the pial surface, and allow cells to migrate from the external to the
internal granule cell layer (Lu et al.,
2001
). This signaling mechanism may have broad implications for
cell migratory behavior in other systems as well.
In vivo evidence also supports the importance of reverse signaling through
ephrins. Intriguingly, replacement of the wild-type receptor with a
signaling-deficient mutant can rescue the axon guidance defects observed in
several knockout lines (Birgbauer et al.,
2000; Henderson et al.,
2001
; Henkemeyer et al.,
1996
; Kullander et al.,
2001
). Furthermore, removal of the ephrin-B2 cytoplasmic domain in
mice causes vascular defects similar to those found in ephrin-B2-knockout mice
(Adams et al., 2001
). This
suggests that reverse signaling is essential for sculpting the developing
vasculature. Ephrin-B signaling also promotes neovascularization in a corneal
micropocket assay, and enhances endothelial cell attachment and migration
(Huynh-Do et al., 2002
). These
affects are believed to be mediated through integrin signaling and are
accompanied by JNK activation. The PDZ-domain-binding site of ephrin-B1, in
particular, appears to be important for this function.
Ephrin-B signaling is also necessary for boundary formation. In Zebrafish
animal cap studies, the bi-directional signaling through ephrin-B2 and EphB2
restricts cell intermingling, while unidirectional signaling through either
ephrin-B2 or EphB2 prevents cell communication through gap junctions. The
formation of rhombomeres also relies on the activation of ephrin signaling
(Mellitzer et al., 1999;
Xu et al., 1999
). This could
be accomplished through Eph-ephrin-induced modifications of adhesive or
migratory properties of cells during development. Thus, concomitant activation
of an Eph receptor and its ephrin ligand is likely to be necessary for
cellular organization and maintenance in many tissues.
Ephrin-A ligands can also convey reverse signals that modify cell behavior.
The ephrin-A molecules, like many GPI-anchored proteins, are targeted to lipid
rafts, where they presumably assemble into protein complexes that transduce
intracellular signals. Indeed, clustering of ephrin-A molecules with EphA-Fc
fusion proteins recruits the Src family kinase Fyn to lipid rafts
(Davy et al., 1999). This is
accompanied by the redistribution of vinculin, activation of MAP kinase,
tyrosine phosphorylation of a 120 Kd lipid raft protein, and increased cell
substrate adhesion (Davy et al.,
1999
; Huai and Drescher,
2001
). Interestingly, in C. elegans the primary function
of the only Eph receptor, Vab-1, may be to activate reverse signals through
GPI-linked ephrins (Wang et al.,
1999
).
Ephrin-A reverse signals may be modulated by cell surface shedding of the
ligand through its association with the metalloprotease Adam10/Kuzbanian
(Hattori et al., 2000). Upon
binding of EphA receptors, Adam10 cleaves ephrin-A2 from the cell surface.
This could serve a dual function. Ephrin-A cleavage from the cell surface
allows Eph-receptor-bearing structures such as growth cones to change their
response to ephrin-A molecules from adhesion to repulsion. In addition, the
cleaved ligand is no longer able to transmit signals or activate EphA
receptors, and hence both reverse and forward signaling is terminated.
![]() |
Crosstalk |
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Another potentially interesting form of crosstalk is that of EphB receptors
with the multi-transmembrane protein ARMS (ankyrin repeat-rich membrane
spanning). ARMS associates with the p75 and Trk nerve growth factor receptors,
and is a substrate of both Eph and Trk receptors
(Kong et al., 2001). It will
be interesting to determine whether ARMS links the Eph and nerve growth factor
receptor signaling pathways during axon outgrowth, synaptogenesis and
plasticity.
More recently, EphB receptors have been shown to associate directly with
NMDA receptors at synapses. Ephrin-B-induced activation of EphB receptors
causes NMDA receptor clustering, potentially helping to initiate the
development of the postsynaptic specialization
(Dalva et al., 2000). This
interaction is functionally important because EphB2 activation enhances
Src-mediated NMDA receptor phosphorylation, which results in increased
glutamate-induced calcium influx through the NMDA receptor
(Takasu et al., 2002
). This
is consistent with the reduced NMDA-mediated currents in EphB2 knock out mice
(Henderson et al., 2001
).
Thus, crosstalk between Eph and NMDA receptors could be important for early
events during synaptogenesis and in modifying the physiological properties of
synapses.
Clues are also beginning to emerge that some Eph receptors form complexes
with other Eph receptors. An example is EphB6, which has no detectable
catalytic activity but forms a hetero-receptor complex with EphB1 and is
transphosphorylated as a result (Freywald
et al., 2002; Gurniak and
Berg, 1996
; Matsuoka et al.,
1997
). EphB6 also seems to convey signals through association with
other types of cell surface receptor. Antibody-mediated clustering of EphB6 in
Jurkat cells results in the co-clustering of the T-cell receptor and promotes
the proliferation of normal T cells (Luo
et al., 2002
). Furthermore, EphB6 enhances the production of
certain lymphokines and promotes Fas-mediated apoptosis when co-clustered with
the T-cell receptor (Luo et al.,
2001
). These studies suggest that EphB6 serves to reduce the
threshold for T-cell receptor activation in lymphocytes. Thus, even in the
absence of kinase activity, Eph receptors can communicate with each other and
with other cell surface proteins to modify signaling pathways.
Ephrins may also exhibit crosstalk with receptor tyrosine kinases. For
example, the PDGF receptor phosphorylates the ephrin-B cytoplasmic domain
(Bruckner et al., 1997), and
the activated FGF receptor associates with Xenopus ephrin-B1 and
phosphorylates it (Chong et al.,
2000
). Interestingly, phosphorylation by the FGF receptor reverses
the effects of ephrin-B1 on cell dissociation in Xenopus embryos
(Chong et al., 2000
).
Ephrin-B1 is also a substrate of the Tie-2 receptor, at least in vitro
(Adams et al., 1999
).
![]() |
Perspectives |
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Acknowledgments |
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Footnotes |
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![]() |
References |
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Adams, R. H., Wilkinson, G. A., Weiss, C., Diella, F., Gale, N.
W., Deutsch, U., Risau, W. and Klein, R. (1999). Roles of
ephrinB ligands and EphB receptors in cardiovascular development: demarcation
of arterial/venous domains, vascular morphogenesis, and sprouting
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