Vascular Endothelium - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 520

9.6.4.1 Actin-Myosin Cytoskeleton and Partners

Ca 2 + ] i induced by
various mediators that regulate arrangement of the cytoskeletal mesh and quality of
cell junctions. The primary determinant of endothelial permeability is the state of
the actin cytoskeleton. Myosin sliding along actin filaments generates contraction.
Cytoskeleton contraction induces small gaps between endothelial cells. It then
raises extravasation of macromolecules [ 954 ]. Two myosin light chains (MLC), one
structural and one regulatory, are bound to the myosin heavy chain (MHC).
Actin polymerization is controlled by various processes, such as nucleation,
elongation, severing, and crosslinking of actin filaments. These mechanisms involve
a huge set of regulators (

The endothelium permeability is enhanced by transient rise in [

160). Concomitant formation of actin stress fibers,
actomyosin cross-bridging, and destabilization of microtubules increase endothelial
permeability. Microtubule destablization induces activation of P38MAPK and
microtubule-associated RhoGEF1 or RasGRF1 that targets RhoA GTPase [ 854 ].
Endothelium resistance to configuration change depends on adhesions of endothelial
cells to both adjoining cells and matrix.
Actomyosin-mediated contraction that primes formation of intercellular gaps
requires mono- or diphosphorylation of regulatory myosin light chain (MLC) by
myosin light chain kinase. Ca 2 + -calmodulin-dependent MLCKs phosphorylate
regulatory MLCs that interact with actin filaments. On the other hand, myosin-
1-associated protein phosphatase balances by dephosphorylation of MLCs the
phosphorylation effect of MLCKs. Phosphatase must then be inhibited. Agent
cAMP reduces MLC phosphorylation to preserve endothelial barrier integrity.
In addition, PKA-anchoring proteins (AKAPs) and vasoactive stimulatory phos-
phoprotein (VASP) are required for cAMP-induced Rac1 activation that stabilizes
endothelial barrier. 85
Myosin light chain kinase can be phosphorylated by protein kinase-A and
P21-activated kinase that reduce its catalytic activity, as well as extracellular signal-
regulated kinases ERK1 and ERK2 and Src kinase (Table 9.21 ). Conversely, protein
phosphatases dephosphorylate MLC P . Cytosolic Ser/Thr phosphatase PP3, but nei-
ther PP1 nor PP2, is a Ca 2 + -dependent phosphatase (a.k.a. calcineurin). Inhibition
of phosphatase activity increases endothelial permeability. Phosphorylation of PP1
by RoCK kinase specifically targeted by GTPase RhoA hinders PP1 activity.
Actomyosin-mediated contraction results from an increase in cytosolic calcium
concentration from endoplasmic reticulum stores followed by a sustained activity
of transient receptor potential channels in endothelial cell plasma membrane
(Sect. 9.6.4.2 ).
Calcium sensitization corresponds to increased myosin light chain phosphoryla-
tion, and hence cell contraction, in response to an external stimulus independently
of changes in intracellular Ca 2 + concentration. This process happens mainly by

∼

85 VASP may act as a component of a scaffold complex that controls GEFs or GAPs dedicated to
Rac1 GTPase.