Myosin light chain kinase in microvascular endothelial barrier function

Abstract

Microvascular barrier dysfunction is implicated in the initiation and progression of inflammation, posttraumatic complications, sepsis, ischaemia-reperfusion injury, atherosclerosis, and diabetes. Under physiological conditions, a precise equilibrium between endothelial cell-cell adhesion and actin-myosin-based centripetal tension tightly controls the semi-permeability of microvascular barriers. Myosin light chain kinase (MLCK) plays an important role in maintaining the equilibrium by phosphorylating myosin light chain (MLC), thereby inducing actomyosin contractility and weakening endothelial cell-cell adhesion. MLCK is activated by numerous physiological factors and inflammatory or angiogenic mediators, causing vascular hyperpermeability. In this review, we discuss experimental evidence supporting the crucial role of MLCK in the hyperpermeability response to key cell signalling events during inflammation. At the cellular level, in vitro studies of cultured endothelial monolayers treated with MLCK inhibitors or transfected with specific inhibiting peptides have demonstrated that induction of endothelial MLCK activity is necessary for hyperpermeability. Ex vivo studies of live microvessels, enabled by development of the isolated, perfused venule method, support the importance of MLCK in endothelial permeability regulation in an environment that more closely resembles in vivo tissues. Finally, the role of MLCK in vascular hyperpermeability has been confirmed with in vivo studies of animal disease models and the use of transgenic MLCK210 knockout mice. These approaches provide a more complete view of the role of MLCK in vascular barrier dysfunction.

Figure 1

Control of actin–myosin contraction in endothelium. Increased MLC phosphorylation in response to MLCK activation by Ca²⁺-calmodulin binding and src kinase activity, or to inhibition of MLCP by ROCK activation downstream of RhoA, increases MLC ATPase-driven force generation relative to actin.

Figure 2

MLCK activation in endothelial hyperpermeability. Endothelial MLCK is activated in response to multiple cell signalling events including elevated intracellular Ca⁺⁺, protein kinase C (PKC) activation, and signalling through nitric oxide synthase (NOS)- dependent production of NO, and guanylate cyclase (GC)-dependent production of cGMP. Activation of cGMP-dependent protein kinase (PKG) activates MLCK through activation of MEK1 and ERK1/2. Possible direct activation by PKC, PKG, or ERK1/2 is represented by question marks.