Rho-Kinase Inhibition of Active Force and Passive Tension in Airway Smooth Muscle: A Strategy for Treating Airway Hyperresponsiveness in Asthma

Abstract

Rho-kinase inhibitors have been identified as a class of potential drugs for treating asthma because of their ability to reduce airway inflammation and active force in airway smooth muscle (ASM). Past research has revealed that, besides the effect on the ASM's force generation, rho-kinase (ROCK) also regulates actin filament formation and filament network architecture and integrity, thus affecting ASM's cytoskeletal stiffness. The present review is not a comprehensive examination of the roles played by ROCK in regulating ASM function but is specifically focused on passive tension, which is partially determined by the cytoskeletal stiffness of ASM. Understanding the molecular basis for maintaining active force and passive tension in ASM by ROCK will allow us to determine the suitability of ROCK inhibitors and its downstream enzymes as a class of drugs in treating airway hyperresponsiveness seen in asthma. Because clinical trials using ROCK inhibitors in the treatment of asthma have yet to be conducted, the present review focuses on the in vitro effects of ROCK inhibitors on ASM's mechanical properties which include active force generation, relaxation, and passive stiffness. The review provides justification for future clinical trials in the treatment of asthma using ROCK inhibitors alone and in combination with other pharmacological and mechanical interventions.

Keywords: asthma; cytoskeletal stiffness; mechano-pharmacology; rho-kinase; smooth muscle tone.

Figure 1

Simplified signaling pathways regulating smooth muscle activation and contraction. The pathway on the right (blue) mainly regulates phosphorylation of MLC20. This is achieved through activation of MLCK by calcium and calmodulin. Once MLC20 is phosphorylated, the myosin crossbridges are able to interact with actin filaments to generate force. The pathway on the left (green) regulates actin polymerization and actin filament stabilization which are important in strengthening the cytoskeleton that facilitates force generation and transmission. The small GTPase RhoA activates ROCK, which in turn activates LIMK leading to inhibition of cofilin and its actin filament severing function. This pathway therefore promotes actin filament formation and stabilization. The other branch of ROCK signaling is through activation of p21-activated kinase (PAK), which leads to phosphorylation of paxillin, activation of the small GTPase cDc42, and N-WASP activation, and eventually actin filament formation and stabilization. More details of the description of the signaling pathways are presented in the main text. +, stimulatory; -, inhibitory; ROCK: Rho-kinase; MLC20: regulatory myosin light chain; MLCK: myosin light chain kinase; MLCP: myosin light chain phosphatase; LIMK: LIM kinase; N-WASP: neuronal Wiskott–Aldrich syndrome protein; PAK: p21-activated kinase.