THE ANATOMICAL RECORD 297:1758–1769 (2014) The Role of Mechanotransduction on Vascular Smooth Muscle Myocytes Cytoskeleton and Contractile Function GEORGE J.C. YE, ALEXANDER P. NESMITH, AND KEVIN KIT PARKER* Disease Biophysics Group, Wyss Institute for Biologically Inspired Engineering and the School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts ABSTRACT Smooth muscle (SM) exhibits a highly organized structural hierarchy that extends over multiple spatial scales to perform a wide range of func- tions at the cellular, tissue, and organ levels. Early efforts primarily focused on understanding vascular SM (VSM) function through biochemi- cal signaling. However, accumulating evidence suggests that mechano- transduction, the process through which cells convert mechanical stimuli into biochemical cues, is requisite for regulating contractility. Cytoskeletal proteins that comprise the extracellular, intercellular, and intracellular domains are mechanosensitive and can remodel their structure and func- tion in response to external mechanical cues. Pathological stimuli such as malignant hypertension can act through the same mechanotransductive pathways to induce maladaptive remodeling, leading to changes in cellu- lar shape and loss of contractile function. In both health and disease, the cytoskeletal architecture integrates the mechanical stimuli and mediates structural and functional remodeling in the VSM. Anat Rec, 297:1758– 1769, 2014. VC 2014 Wiley Periodicals, Inc. Key words: mechanotransduction; tissue mechanics; smooth muscle; cytoskeleton Smooth muscle (SM) structure and function interact VSM tone. A well-studied example of this process is the over many orders of spatial magnitude, ranging from the myogenic response, where small arteries contract to coun- centimeter-length scale of vessels to the nanometer- teract increased intraluminal pressure, protecting the length scale of cytoskeletal proteins. These relationships blood vessel from potential hypertensive injury (Davis, are maintained in vivo in several ways, one of which is 2012). Hence, the ability of VSM to sense and respond to the dynamic responsiveness to mechanical forces at the mechanical forces experienced in normal physiology and tissue, cellular, and subcellular spatial scales through in injury is critical for proper regulation of vascular tone. mechanotransduction, the translation of mechanical stim- It is now widely accepted that the cellular cytoskele- uli into biochemical reactions within a cell. In one exam- ton plays a critical role in mediating mechanotransduc- ple, cyclic cardiac pumping exposes vascular SM (VSM) tion (Wang et al., 1993b; Alenghat and Ingber, 2002; (Fig. 1) to a number of mechanical stimuli, such as trans- mural pressure, vascular shear strain induced by pulsa- tile pressure, and circumferential wall tension (Osol, *Correspondence to: Kevin Kit Parker, Harvard School of 1995). The resultant changes within the VSM are cytos- Engineering and Applied Sciences, 29 Oxford St, Pierce Hall keletal remodeling (Hayakawa et al., 2001; Cunningham 321, Cambridge, MA 02138. Fax: 617-496-1793. E-mail: et al., 2002; Gunst and Zhang, 2008), altered membrane [email protected] conductance (Sparks, 1964; Kirber et al., 1992; Langton, Received 24 May 2014; Accepted 6 June 2014. 1993), and biochemical signal activation (Mills et al., DOI 10.1002/ar.22983 1990; Kulik et al., 1991; Pirola et al., 1994; Cattaruzza Published online in Wiley Online Library (wileyonlinelibrary. et al., 2004), that ultimately lead to functional changes in com). VC 2014 WILEY PERIODICALS, INC. ROLE OF MECHANOTRANSDUCTION ON VSM MYOCYTES CYTOSKELETON & CONTRACTILE FUNCTION 1759 Fig. 1. Hierarchical organization of vascular tissue spans multiple spatial scales from nanometers to meters. Vascular smooth muscle cells assemble into muscle tissue that forms the media layer of the elas- tic and muscular arteries. The spindle-shaped cells contain nanometer-scale protein complexes that allow it to respond to mechanical cues in the cellular microenvironment. ADAPTED in part from Servier Medical Art (reproduction permitted: http://creativecommons.org/licenses/by/3.0/). Ingber, 2006). In cardiomyocytes, mechanosensitive pro- zation gives rise to cellular architecture; and, redistribu- teins embedded in the cytoskeletal network adapt their tion of the cytoskeletal network as a result of polymerization states and distributions in response to mechanotransduction leads to changes in cellular and mechanical cues, which eventually translates into func- tissue structure. Hence, understanding the interaction of tional changes (McCain and Parker, 2011; Sheehy et al., the mechanotransductive machinery may provide new 2012). In both striated and SM cells, cytoskeletal organi- insights into health and disease. 1760 YE ET AL. Fig. 2. Mechanotransductive cytoskeletal proteins in vascular smooth muscle cells. Integrin links extrac- ellular matrix proteins such as collagen to actin fibers, allowing extracellular mechanical signals to be directly transmitted into the cell. Actin responds to mechanical input to the cells by rapidly changing the F- to G-actin ratios and also acts as an intracellular sensor. Cadherin junctions provide mechanical links between adjacent cells, allowing forces to be transmitted between cells. In this review, we examine the contribution of mecha- tons typically clustered at the focal adhesion complex notransduction on VSM cytoskeletal organization and via the short cytoplasmic tail of the b-subunit (Fig. 3). contractile function. We first discuss the collective net- Functionally, integrins transduce both “outside-in” and work of mechanosensitive cytoskeletal proteins in the “inside-out” mechanical signals in many different cell VSM extracellular, intercellular, and intracellular types including VSMCs (Baker and Zaman, 2010). To domains that enable translation and integration of date, 24 integrins have been described and among them, mechanical stimuli into structural or biochemical 13 out of 24 are found in VSMCs (a1b1, a2b1, a3b1, a4b1, changes. We then draw on evidence found from in vitro a5b1, a6b1, a7b1, a8b1, a9b1, avb1, avb3, avb5, and a6bv) studies to show the responses of the VSM cytoskeleton (Glukhova et al., 1991; Moiseeva, 2001). Herein, we dis- to external mechanical cues and how this lead changes cuss how integrin subtypes are sensitive to ECM compo- in VSM contractile function. sition for transducing mechanical stimuli and perform contractile functions requisite for maintenance of proper MECHANOSENSITIVE CONTRACTILE vascular tone in vivo. CYTOSKELETON IN VASCULAR SMOOTH In vitro studies using isolated arterioles and VSMCs strongly suggest that integrins are crucial mechano- MUSCLE CELLS transductive elements for VSMCs. Wilson and coworkers VSM experiences a wide range of mechanical stimuli (1995) demonstrated that the mitogenic response of the throughout the cardiovascular system such as trans- VSM to strain was dependent on the composition of the mural pressure, pulsatile pressure, and shear stress extracellular matrix to which it was adhered. Specifi- (Osol, 1995). These mechanical signals can propagate cally, culturing VSMCs on fibronectin elicited the most within and between cells (Fig. 2). For example, integrin significant mitogenic response to strain which corre- proteins directly connect the extracellular matrix to sponded with increased integrin binding. Further, solu- actin filaments within the cell, allowing forces to be ble fibronectin, integrin binding peptide GRGDTP, and transmitted from outside to inside the cells (Wiesner antibodies to b3oravb5 integrins all independently et al., 2005). Cadherin junctions directly couple adjacent blocked the mitogenic response of newborn rat VSMCs VSM cells (VSMCs) together and propagate mechanical normally induced by mechanical strain, while soluble signals from one cell to the next (Philippova et al., laminin, the inactive peptide GRGESP, and the antibody 1998). Actin, intermediate filaments, and microtubules to the b1 integrin did not alter the mitogenic response to propagate mechanical signals through common hubs strain. Hence, specific integrin subunits sense and trans- named dense plaques that are distributed throughout duce the mechanical strain requisite for induction of the the VSMC cytoplasm (Gimbrone Jr. and Cotran, 1975). myogenic response in VSM (Wilson et al., 1995). Other In the following sections, we will briefly review the studies subsequently showed that an integrin- VSMC cytoskeletal components in extracellular, intercel- recognizing synthetic RGD peptide can cause sustained lular, and intracellular domains that contribute to vasodilation (Mogford et al., 1996) and decreased intra- mechanotransduction and modulate contractile cellular Ca21 level in rat VSMCs (D’Angelo et al., 1997). functions. These early studies demonstrated that integrins play an important role in transducing mechanical cues to intra- Mechanical Signaling through the Integrin- cellular signals that produce functional adaptive responses. More recently, studies on isolated rat arte- Extracellular Matrix Interface riole tissue and VSMCs showed that antibody blocking Integrin proteins are transmembrane, heterodimeric of a5b1 and avb3 integrins significantly inhibits myo- receptors comprising a- and b-subunits. They connect genic constriction (Martinez-Lemus et al., 2005; the extracellular matrix (ECM) to the internal cytoskele- Sun et al., 2008). However, pulling on fibronectin and ROLE OF