Myocyte shearing, myocardial sheets, and microtubules.

The cytoskeleton of cardiac myocytes is a complex network of many interacting protein filaments and associated proteins with multiple functions: it forms structures responsible for maintaining cell shape and contractile filament registration, structural integrity, internal transport and cell division, and scaffolding for several putative signaling cascades.1 The tight coupling between the extracellular matrix and components of the cytoskeleton at the cell membrane suggests that, in addition to transmitting contractile forces generated by the myofilaments to the matrix and the cardiac chambers, the cytoskeleton may also be important for propagating external physical signals into the cell.2 Mutations in a growing list of cytoskeletal genes are associated with cardiomyopathies. Disruption of desmin, plakoglobin, N-cadherin, plectin, and vinculin all produce a dilated cardiac phenotype with impaired function, either in fetal development or after birth.3,4,5 These are elements of the cytoskeleton that connect intracellular structures with the extracellular matrix, and thus are likely force-transmitting components in the myocyte. Originally, Chien6 proposed that defects in the cytoskeletal component of the myocyte result in a dilated cardiomyopathic phenotype, whereas mutations in the sarcomeric proteins typically generate a pattern of hypertrophic cardiomyopathy with preserved ventricular systolic function in addition to the myocyte hypertrophy and disarray.7 But as more data have accumulated, the picture has become more complex, and cytoskeletal defects have been implicated in hypertrophic as well as dilated cardiomyopathies.8 Cytoskeletal proteins have been implicated in several load-sensing pathways, and there is evidence that cytoskeletal proteins play a critical role in biomechanical signaling and may be involved with ventricular dilation and heart failure.9,10 These studies have led to the hypothesis that there may exist a common pathway to cardiac dilation and failure, and the critical components of this “failure transition”11 are likely to be structural themselves or closely related to force transmitting elements. The complex structure and multiple functions of the cytoskeleton make it inherently difficult to study on a quantitative scale.12 Its physical and signaling properties rely not only on its molecular structure but its dynamic threedimensional organization. Experimental interventions that disrupt components of the cytoskeleton can alter the distribution of stresses within the other components. Separating the “inside-out” force transmission functions of the cytoskeleton from the “outside-in” mechanotransduction and signaling roles is challenging, especially in vivo. Yet it is likely that both play a significant role in cardiac remodeling and heart failure. Defects in the actin-associated cytoskeleton causing dilated cardiomyopathy, such as the deletion of muscle LIM protein (MLP)13—a Z-disc–associated structural protein interacting with actin, titin, and alpha-actinin14—have been shown to result in alterations both to biomechanical signal transduction and to diastolic mechanical properties.15 Understanding the precise molecular mechanisms of these alterations will require a comprehensive theoretical framework for the biophysics and mechanical properties of the cytoskeletal network within the context of the intact myocyte. Both from a molecular viewpoint and a biophysical and mechanotransduction perspective, the microtubule network is among the better-studied components in the cytoskeleton.16 A substantial body of evidence implicates microtubule polymerization and depolymerization in myocyte dysfunction in some models of ventricular hypertrophy,17 though not all. Studies have shown that microtubules contribute significantly to the viscoelastic properties of myocytes,18 especially their passive viscosity.19 With few exceptions, studies of the mechanical properties of myocytes and the contributions of the microtubules have used a single uniaxial mechanical test, most commonly measuring the longitudinal elastic stiffness or viscosity of the cell. However, the resting and active myocardium is well known to be anisotropic.20 Even the mechanotransduction responses of micropatterned cultured myocytes to stretch in vitro are anisotropic, with differential induction of hypertrophic gene expression and protein synthesis by longitudinal versus transverse stretch.21 One isolated cell study measured the axial and transverse mechanical properties of single myocytes,22 but the intact myocardium also experiences substantial shearing stresses and strains in vivo.23 Tagawa and colleagues24 used magnetic twisting rheometry to probe the viscoelastic properties of the myocyte cytoskeleton. However, information on the anisotropic material properties of myocytes under multi-axial tension, compression and shear, and the contributions of cytoskeletal structures, remains sparse. In this issue of Circulation Research, Nishimura and coworkers25 describe a novel combination of single cell mechanical tests that they have used to probe the elastic properties of isolated adult rat ventricular myocytes under axial tension, transverse compression, as well as axial and longitudinal shearing. Using a combination of flexible and rigid carbon microfibers, latex microspheres and thin glass plates together with piezoelectric translators, these authors The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Department of Bioengineering, University of California San Diego, La Jolla, Calif. Correspondence to Andrew McCulloch, Department of Bioengineering, 0412, La Jolla, CA 92093. E-mail [email protected] (Circ Res. 2006;98:1-3.) © 2006 American Heart Association, Inc.