Barrier Maintenance in Neovessels - Mechanical and Chemical Signaling in Angiogenesis - page 212

Concerning basal endothelial barrier regulation, much attention has been given
to the relationship between VE-cadherin, the catenins and Rho GTPases. After
cadherin engagement p120ctn binding activates Vav2, a GEF for Cdc42 and Rac1.
This GEF positively regulates Cdc42 and Rac1 activity. Both GTPases bind
IQGAP (IQ motif containing GTPase-activating protein 1) [ 24 , 25 ]. Unlike the
name suggests IQGAP1 does not have GAP activity, meaning that it does not
enhance the GTP hydrolytic activity of the GTPases. Instead its binding to Cdc42
and Rac1 inhibits their intrinsic GTPase activity, thus keeping them in their active,
GTP-bound state. By recruiting IQGAP1, Rac1 and Cdc42 inhibit the interaction
of IQGAP1 with b-catenin thereby initiating the binding of a- and b-catenin. This
cross-links the complex with the actin cytoskeleton and stabilises the AJ [ 25 ]. In
contrast, inactivation of Rac1 and Cdc42 causes dissociation from IQGAP1 that
then binds to b-catenin at the AJ complex. As a result a- and b-catenin dissociate,
leading to weaker adhesion and inter-endothelial gap formation [ 26 ].
Another AJ protein is the endothelial-specific HEG (heart of glass) trans-
membrane receptor, which interacts with a currently unknown extracellular ligand
to activate a coordinated signalling response through inhibiting the activity of
RhoA. This response requires the cerebral cavernous malformations proteins
CCM1 (or KRIT1), CCM2 and CCM3 and leads to a cellular response that may
include induction of endothelial vacuolisation and homotypic endothelial cell-to-
cell junction formation. These cellular events are required for normal vascular
development and maintenance of vascular integrity. Defects in these steps result in
enhanced activation of RhoA and may lead to the pathological changes associated
with hereditary cerebral cavernous malformations in humans [ 27 , 28 ].
In conclusion, the maintenance of the endothelial barrier depends on RhoA,
Cdc42 and Rac1 activation. RhoA activity decreases upon gap closure due to
VE-cadherin and p120ctn engagement. The binding of the AJs to the actin cyto-
skeleton involves Cdc42, which is kept in its active state by binding to IQGAP1.
Activation of Epac and RapGEF2 stimulate Rap1, which via Vav2 activates Rac1.
Rac1 activation results in the inhibition of p115RhoGEF and thus a suppression of
RhoA. Vav2 activity is further enhanced by p120ctn binding to the junctional
complexes. The latter also directly activates p190RhoGAP thereby further inhib-
iting RhoA activity [ 26 ].
3.2 Regulation by Caveolae
A close interrelationship between paracellular and transcellular pathways has been
surmised for a long time, but was not well understood. A coupling between
caveolae-mediated endocytosis to increased junctional permeability was indicated
by the finding that loss of caveolin-1 results in not only the disappearance of caveolar
structures but also destabilization of inter-endothelial junctions and formation of
inter-endothelial gaps [ 29 ]. Recently more direct evidence has been obtained
[ 30 , 31 ]. Paracellular permeability of the endothelium is finely regulated by