Dynamic molecular processes mediate cellular mechanotransduction

Abstract

Cellular responses to mechanical forces are crucial in embryonic development and adult physiology, and are involved in numerous diseases, including atherosclerosis, hypertension, osteoporosis, muscular dystrophy, myopathies and cancer. These responses are mediated by load-bearing subcellular structures, such as the plasma membrane, cell-adhesion complexes and the cytoskeleton. Recent work has demonstrated that these structures are dynamic, undergoing assembly, disassembly and movement, even when ostensibly stable. An emerging insight is that transduction of forces into biochemical signals occurs within the context of these processes. This framework helps to explain how forces of varying strengths or dynamic characteristics regulate distinct signalling pathways.

Figure1.. Switch-like models of mechanotransduction.

a) Cells are mechanically integrated structures, where the extracellular matrix (ECM) and actin cytoskeleton are connected by integrins and focal adhesion (FA) proteins. Microtubules and many ion channels are also integrated with this network. Forces can be directly applied through the ECM or transmitted through the cytoskeleton to mechanosensitive components, such as FAs, to mediate cellular response to forces. b) Immunostaining of a vascular smooth muscle cell. F-actin filaments (red) link to variably-sized, punctuate FAs visualized by vinculin (green) and phosphorylated focal adhesion kinase (pFAK) staining. The variable amount of pFAK staining in the FAs is indicative of different local signaling environments that are likely linked to distinct mechanical signals. c) A common mechanism of mechanotransduction is force-induced conformational changes. For example, membrane tension can cause ion channel opening. Also, talin connects the integrin cytoplasmic tail to F-actin; tension on talin reveals cryptic vinculin binding sites, subsequent binding of vinculin reinforces the linkage.

Figure 2.. The focal adhesion clutch.

Due to forces from actin polymerization and myosin-dependent contractility, actin filaments flow rearward over FAs toward the nucleus. Through FA proteins that link actin to integrins, force is applied to the ECM. Force-dependent changes in FA exchange rates (represented as green arrows, see Box 1 for details) alter dynamics and size of FAs. On soft surfaces or where external force is applied slowly, on- and off-rates are moderately fast and FAs have moderate lifetimes. However, on stiff surfaces or when external forces are applied quickly, distinct behaviors can be observed. In the absence of FA strengthening, molecular linker dissociation rates increase (slip bond behavior). However, these proteins are sometimes replaced through re-binding, resulting in large exchange rates and short-lived FAs. FA strengthening is associated with catch bonds that slow FA protein dissociation under force, with exposure of cryptic binding sites to recruit new proteins that reinforce the adhesion, and with conformational changes in FA proteins that activate signaling pathways to recruit additional molecular linkers. Any way, large, long-lived FAs are the result.

Figure 3.. Dynamic aspects of mechanotransduction.

The endothelial cells that line blood vessel walls are subject to both static and cyclic stretch. Even when the signal strengths are matched, dynamically distinct mechanical stimuli can activate common or unique signaling pathways based on the strength of the proteins, specifically resistance to conformation changes, and the dynamic nature of the structures bearing load. In dynamic structures, like nascent adhesions, and stable structures, like mature adhesions and stress fibers, cyclic stretch does not apply forces for sufficient time to induce conformation changes in strong proteins, but weak proteins will signal. In response to static stretch dynamic structures can readily adapt, and there is no long term signaling. In stable structures the long force application causes conformational changes in both weak and strong proteins. Thus signaling pathways that are preferentially activated by cyclic stretch are likely induced by weak proteins that localize exclusively to dynamic structures. Pathways selectively activated by static stretch are expected to contain mechanosensors that are strong proteins in stable structures. And pathways activated by both types of signals are likely due to weak proteins that localize to both dynamic and stable structures. Stars indicate active proteins.