The Regulation of Intestinal Mucosal Barrier by Myosin Light Chain Kinase/Rho Kinases

Abstract

The intestinal epithelial apical junctional complex, which includes tight and adherens junctions, contributes to the intestinal barrier function via their role in regulating paracellular permeability. Myosin light chain II (MLC-2), has been shown to be a critical regulatory protein in altering paracellular permeability during gastrointestinal disorders. Previous studies have demonstrated that phosphorylation of MLC-2 is a biochemical marker for perijunctional actomyosin ring contraction, which increases paracellular permeability by regulating the apical junctional complex. The phosphorylation of MLC-2 is dominantly regulated by myosin light chain kinase- (MLCK-) and Rho-associated coiled-coil containing protein kinase- (ROCK-) mediated pathways. In this review, we aim to summarize the current state of knowledge regarding the role of MLCK- and ROCK-mediated pathways in the regulation of the intestinal barrier during normal homeostasis and digestive diseases. Additionally, we will also suggest potential therapeutic targeting of MLCK- and ROCK-associated pathways in gastrointestinal disorders that compromise the intestinal barrier.

Keywords: adherens junction; apical junction complex; myosin light chain kinase; rho-associated coiled-coil containing protein kinase; tight junction.

Figure 1

The role of phosphorylation of myosin light chain-2 (MLC-2) in the apical junctional complex in the intestinal epithelial paracellular pathway. The apical junctional complex consists of tight junctions (TJs) and adherens junctions (AJs). TJs consist of transmembrane proteins (e.g., claudins and occludin), and scaffold PDZ domain-expressing zonula occludin (ZO) proteins, which serve to form complex linking transmembrane proteins to the cytoskeletal actomyosin ring. AJs are located below the TJ and are comprised of two families of adhesive units: The E-cadherin/catenin family and nectin/afadin complexes. These proteins are dynamically regulated to maintain epithelial integrity. In intestinal disorders, increased phosphorylation of MLC-2 induces contraction of the perijunctional actomyosin ring and disrupt the apical junctional complex. This results in elevated paracellular permeability during the pathogenesis of various digestive diseases.

Figure 2

Regulation of phosphorylation of myosin light chain-2 (MLC-2) by myosin light chain kinase (MLCK)/Rho-associated coiled coil containing protein kinase (ROCK) to regulate AJC barrier in intestinal epithelial cells. Paracellular permeability is principally determined by the phosphorylation level of the regulatory light chain of myosin, MLC-2, which is regulated by the enzymes MLCK and ROCK. Increased intracellular Ca²⁺ levels stimulate MLCK activity, which directly phosphorylates MLC-2. Enhanced ROCK activity also directly phosphorylates MLC-2 and inhibits MCL phosphatase (MLCP) activity by phosphorylating the myosin phosphatase target subunit 1 (MYPT1). There are pharmaceutical inhibitors for MLCK and ROCK listed.