Recent Advances in Angiogenesis and ... - Bentham Science

Plexins and Neuropilins Regulate Integrin Recent Advances in Angiogenesis and Antiangiogenesis, 2009 3
belonging to four classes (A–D), contain a divergent
extracellular Sema domain and a conserved
cytoplasmic domain unique to Plx with sequence
similarity to a group of Ras-family-specific GTPaseactivating
protein (GAP) activity. In addition to Plx,
Sema holoreceptor complexes contain numerous
coreceptors. Some of them are modulatory subunits
(for example, Ig superfamily cell adhesion molecules),
or provide further diversity to semaphorin function,
such as such as CD72, T cell immunoglobulin and
mucin-domain-containing 2 (Tim-2), vascularendothelial
growth factor (VEGF) receptor R-2, c-Met
and Erb-2 receptors, tyrosine kinase- like
transmembrane protein offtrack or integrins [10-15].
However, most secreted Sema, including Sema3A and
Sema3F do not bind directly to Plx receptors, but they
use Nrp-1 or Nrp-2 as coreceptor ligand-binding
subunits [16]. Interestingly, Nrp-1 and -2 are
coreceptors of VEGFR-1,-2 and -3 engaged by
different VEGF forms [17-19]. The extracellular
region of Nrp1 contains two repeated complementbinding
domains (CUB domains; a1–a2 domains), two
coagulation factor-like domains (b1–b2 domains), and
a juxtamembrane 5 meprin/A5/μ-phosphatase (MAM)
homology domain. The Nrp1 intracellular region is
only 50 amino acids long, and its function is poorly
characterized [8, 9]. Domains b1-b2 have been
implicated in the binding with VEGF-A165 [20], a1a2
and b1-b2 domains with Sema3A. The MAM/c
domain instead mediates the Sema3A elicited Nrp1
oligomerization [8, 9] that is required for Sema3A
SEMA binding
VEGF binding/
Sema binding
Clusterization
1 2
45%
48%
35%
55%
MAM domain
CUB (a1,a2)
FVI / FVII (b1,b2)
Fig. (2). Neuropilins structure. For details see text. Numbers
indicate the homology percentage between Nrp-1 and Nrp-2.
biological activity. The cytoplasmic tail includes about
42–44 amino acids and does not display catalytic
activity of its own, but presents a binding site for the
PDZ domain of Nrp-1-interacting protein [21] (Fig. 2).
3. ROLE OF SEMAPHORINS IN VAS-
CULAR SYSTEM
The first evidences for a role of Sema/Nrp/Plx system
in vascular biology were provided by the groups
Klagsbrun and Fujisawa, which respectively
demonstrated that in ECs Nrp-1 acts a VEGFR-2 coreceptor
[19] and found that Nrp-1 is required for
mouse cardiovascular development [22]. Afterwards,
several reports confirmed and extended these
observations. In zebrafish, knockdown of sema3aa
affects the migration of Nrp-1 + angioblasts, finally
impairing dorsal aorta formation and normal
circulation [23]. Additionally, single morpholino
knockdown of either sema3aa or sema3ab in
TG(fli1:EGFP) y1 embryos results in a less dramatic
phenotype with patterning defects of intersomitic
vessels [24]. Knockdown of Sema3a gene in outbred
CD-1 mouse strain and over-expression of dominant
negative Sema3 receptor mutants [25] or delivery of
anti-Sema3A antibodies [26] in chick embryos were
found to cause angiogenic remodeling defects. By
studying the positive effect exerted by a specific
Sema3A inhibitor in axonal regeneration into the
injured spinal cord it has been observed an
enhancement of angiogenesis in the injured tissue,
reinforcing the concept of the negative modulatory
role of Sema3A in angiogenesis [27]. In ECs
PlxD1[28] and, albeit to lesser extent, PlxA2 [29] are
the most abundant Plxs. Both Sema3A and Sema3C
bind with a significantly higher affinity to a receptor
complex formed by the association of Nrp-1 and/or -2
with PlxD1 than to a complex in which Nrps
associates with PlxA1 [28].
Therefore, the Nrp/PlxD1 complex could represent the
most efficient transducer of the chemorepulsive effect
of Sema3A [30, 31]. Different from other Sema3,
Sema3E can directly bind to PlxD1 [32]. Mainly based
on defects in the intersomitic vessel patterning of
Sema3E and PlxD1, Sema3E/PlxD1 has been
proposed to be the major signaling pathway regulating
vascular development [32, 33]. However, while
Sema3e null mice are viable and do not show any
gross abnormality [32], all PlxD1 -/- pups become
cyanotic shortly after birth and succumb within 24 hrs
because of severe cardiovascular defects [28].
Therefore, it is likely that in ECs PlxD1 transduces
signals not only from Sema3E, but also from other
Sema3 likely employing Nrp as co-receptors, as
originally proposed by Epstein and colleagues [28].
Based on the fact that ECs express high levels of both
Nrp1 and PlxD1 and on observations that Sema3E
promotes tumor angiogenesis [34, 35], at present these