Metavinculin: New insights into functional properties of a muscle adhesion protein

Metavinculin modulates force transduction in cell adhesion sites

Vinculin is a ubiquitously expressed protein, crucial for the regulation of force transduction in cells. Muscle cells express a vinculin splice-isoform called metavinculin, which has been associated with cardiomyopathies. However, the molecular function of metavinculin has remained unclear and its role for heart muscle disorders undefined. Here, we have employed a set of piconewton-sensitive tension sensors to probe metavinculin mechanics in cells. Our experiments reveal that metavinculin bears higher molecular forces but is less frequently engaged as compared to vinculin, leading to altered force propagation in cell adhesions. In addition, we have generated knockout mice to investigate the consequences of metavinculin loss in vivo. Unexpectedly, these animals display an unaltered tissue response in a cardiac hypertrophy model. Together, the data reveal that the transduction of cell adhesion forces is modulated by expression of metavinculin, yet its role for heart muscle function seems more subtle than previously thought.

Metavinculin Tunes the Flexibility and the Architecture of Vinculin-Induced Bundles of Actin Filaments

Vinculin is an abundant protein found at cell-cell and cell-extracellular matrix junctions. In muscles, a longer splice isoform of vinculin, metavinculin, is also expressed. The metavinculin-specific insert is part of the C-terminal tail domain, the actin-binding site of both isoforms. Mutations in the metavinculin-specific insert are linked to heart disease such as dilated cardiomyopathies. Vinculin tail domain (VT) both binds and bundles actin filaments. Metavinculin tail domain (MVT) binds actin filaments in a similar orientation but does not bundle filaments. Recently, MVT was reported to sever actin filaments. In this work, we asked how MVT influences F-actin alone or in combination with VT. Cosedimentation and limited proteolysis experiments indicated a similar actin binding affinity and mode for both VT and MVT. In real-time total internal reflection fluorescence microscopy experiments, MVT's severing activity was negligible. Instead, we found that MVT binding caused a 2-fold reduction in F-actin's bending persistence length and increased susceptibility to breakage. Using mutagenesis and site-directed labeling with fluorescence probes, we determined that MVT alters actin interprotomer contacts and dynamics, which presumably reflect the observed changes in bending persistence length. Finally, we found that MVT decreases the density and thickness of actin filament bundles generated by VT. Altogether, our data suggest that MVT alters actin filament flexibility and tunes filament organization in the presence of VT. Both of these activities are potentially important for muscle cell function. Perhaps MVT allows the load of muscle contraction to act as a signal to reorganize actin filaments.

Mechanical dynamics in live cells and fluorescence-based force/tension sensors

Three signaling systems play the fundamental roles in modulating cell activities: chemical, electrical, and mechanical. While the former two are well studied, the mechanical signaling system is still elusive because of the lack of methods to measure structural forces in real time at cellular and subcellular levels. Indeed, almost all biological processes are responsive to modulation by mechanical forces that trigger dispersive downstream electrical and biochemical pathways. Communication among the three systems is essential to make cells and tissues receptive to environmental changes. Cells have evolved many sophisticated mechanisms for the generation, perception and transduction of mechanical forces, including motor proteins and mechanosensors. In this review, we introduce some background information about mechanical dynamics in live cells, including the ubiquitous mechanical activity, various types of mechanical stimuli exerted on cells and the different mechanosensors. We also summarize recent results obtained using genetically encoded FRET (fluorescence resonance energy transfer)-based force/tension sensors; a new technique used to measure mechanical forces in structural proteins. The sensors have been incorporated into many specific structural proteins and have measured the force gradients in real time within live cells, tissues, and animals.

Integrating actin dynamics, mechanotransduction and integrin activation: the multiple functions of actin binding proteins in focal adhesions

Focal adhesions are clusters of integrin transmembrane receptors that mechanically couple the extracellular matrix to the actin cytoskeleton during cell migration. Focal adhesions sense and respond to variations in force transmission along a chain of protein-protein interactions linking successively actin filaments, actin binding proteins, integrins and the extracellular matrix to adapt cell-matrix adhesion to the composition and mechanical properties of the extracellular matrix. This review focuses on the molecular mechanisms by which actin binding proteins integrate actin dynamics, mechanotransduction and integrin activation to control force transmission in focal adhesions.

Costamere remodeling with muscle loading and unloading in healthy young men

Costameres are mechano-sensory sites of focal adhesion in the sarcolemma that provide a structural anchor for myofibrils. Their turnover is regulated by integrin-associated focal adhesion kinase (FAK). We hypothesized that changes in content of costamere components (beta 1 integrin, FAK, meta-vinculin, gamma-vinculin) with increased and reduced loading of human anti-gravity muscle would: (i) relate to changes in muscle size and molecular parameters of muscle size regulation [p70S6K, myosin heavy chain (MHC)1 and MHCIIA]; (ii) correspond to adjustments in activity and expression of FAK, and its negative regulator, FRNK; and (iii) reflect the temporal response to reduced and increased loading. Unloading induced a progressive decline in thickness of human vastus lateralis muscle after 8 and 34 days of bedrest (−4% and −14%, respectively; n = 9), contrasting the increase in muscle thickness after 10 and 27 days of resistance training (+5% and +13%; n = 6). Changes in muscle thickness were correlated with changes in cross-sectional area of type I muscle fibers (r = 0.66) and beta 1 integrin content (r = 0.76) at the mid-point of altered loading. Changes in meta-vinculin and FAK-pY397 content were correlated (r = 0.85) and differed, together with the changes of beta 1 integrin, MHCI, MHCII and p70S6K, between the mid- and end-point of resistance training. By contrast, costamere protein level changes did not differ between time points of bedrest. The findings emphasize the role of FAK-regulated costamere turnover in the load-dependent addition and removal of myofibrils, and argue for two phases of muscle remodeling with resistance training, which do not manifest at the macroscopic level.