From tissue mechanics to transcription factors
Introduction
Many cell types are highly sensitive to physical as well as chemical stimuli. Transduction of physical stimuli into biochemical signaling pathways has been well characterized in systems such as the response of retinal cells to the absorption of photons, or the activation of ion channels by deflection of actin bundles in the hair cell due to acoustic waves. However, much less is known about how cells respond when the forces they generate internally are opposed by passive, viscoelastic properties of the extracellular matrix (ECM) or by neighboring cells within tissues. This brief review summarizes recent progress in identifying intracellular signaling pathways, and especially transcriptional programs, that are differentially activated when cells adhere to materials with different mechanical properties or when they are subject to tension arising from externally applied forces.
Mechanical signals can be generally classified as those that arise from the passive mechanical resistance to forces generated by the cell, and those that arise when a cell is actively subjected to forces applied by other cells, fluid flow, or gravity. In all situations, these forces are resisted by the stiffness of the ECM and the glycocalyx that surrounds cells. Changes in tissue stiffness are frequently associated with diseases such as cancer (Levental et al., 2010), fibrosis (Wells, 2008), and atherosclerosis (Duprez and Cohn, 2007, Kothapalli et al., 2012, Mitchell et al., 2010). Some studies suggest that, in addition to resulting from pathology, mechanical changes may play a role akin to soluble factors in causing the progression of such diseases. For example, measurements of the viscoelasticity of liver in experimentally-induced liver fibrosis in rats showed that the stiffness of liver, as quantified by shear elastic modulus, increased before histologically-detectable increases in ECM deposition or myofibroblast differentiation (Georges et al., 2007) (Perepelyuk et al., 2013). These results suggest that changes in tissue mechanics that can activate liver myofibroblast precursors- hepatic stellate cells (Olsen et al., 2011) and portal fibroblasts (Li et al., 2007) - precede and therefore may cause or at least contribute to development of pathosis. Increased tissue stiffness also appears to contribute to the development and spread of cancer in some models (Levental et al., 2010); the response of cells to abnormal ECM stiffness may then render them resistant to chemotherapeutic agents, possibly because of changes in the cytoskeleton-membrane interface at cell adhesions (Schrader et al., 2011).
Many cell types alter their structure and function in response to the mechanical properties of the materials to which they adhere (Pelham and Wang, 1997) and the type of adhesion receptor by which they bind (Byfield et al., 2009, Chopra et al., 2012, Ganz et al., 2006). Mechanical stimuli can act in concert with or in some cases override or prevent chemical stimulation (Wells and Discher, 2008). In vivo, cells engage their ECM both by mechanosensitive adhesion complexes and by other surface receptors, including those for growth factors and inflammatory mediators, that cannot act as adhesive anchors but that potentially modify the mechanical signals transduced at the cell/ECM interface. The cellular response to substrate stiffness in vitro or to changes in the mechanical properties of tissues during development, injury, or disease can be context-dependent, with different cell types being maximally sensitive to widely different ranges of substrate stiffness (Georges and Janmey, 2005). Substrate stiffness can be sensed by cells within 2 min of their adhesion to substrates with similar surface topologies and adhesion protein densities but different elastic moduli (Yeung et al., 2005). Pioneering studies of substrate stiffness sensing showed that this response does not require protein synthesis (Pelham and Wang, 1997), indicating that the initial response of cells to substrate mechanics requires only signals that are acutely produced in response to tension. There is no obvious universal response to substrate stiffness, but increasing stiffness commonly correlates with increased actomyosin contractility, activation of the small GTPase RhoA, increased tyrosine phosphorylation of numerous proteins, activation of focal adhesion kinase (FAK), and increased Ca2+ influx through mechanosensitive channels. How these initial signals integrate with each other and are translated into changes in cytoskeletal structure such as increased synthesis of α-smooth muscle actin (α-SMA) (Hinz et al., 2001), which is a common downstream effect of increased stiffness, and other morphological and functional responses is currently an active area of research.
While attachment of connective tissue cells to the ECM is generally reliant on the formation and remodeling of integrin-mediated, actomyosin-linked adhesions, connective tissue cells can also adhere to each other by intercellular adhesive molecules (e.g. cadherins) that may act as force (Ko et al., 2001) and stiffness (Chopra et al., 2011, Ganz et al., 2006) sensors and regulate gene expression. N-cadherin-mediated adherens junctions are influenced by integrins; fibroblasts may therefore integrate mechanical signals from intercellular and matrix adhesion systems to coordinate gene responses that are involved in differentiation, organogenesis, and wound healing (Linask et al., 2005). Mechanotransduction may not be a single isolated process involving integrins or cadherins (Ko et al., 2001, Potard et al., 1997) but instead may result from a concatenation of processes that require the recruitment of attachment, cytoskeletal, and signaling proteins. Conceivably these proteins then form docking/signaling complexes that are arranged in time and space to regulate transcription.
Section snippets
Models of transcriptional response to tissue mechanics
Most studies and models of mechanically triggered transcriptional changes suggest that the transition from physical to biochemical events occurs at the plasma membrane or within the cytoplasm and leads to biochemical changes in transcription factors or regulators that cause their entry into the nucleus to regulate gene expression. While the molecular identity of specific mechanotransducers in mammals has not been clearly defined, analysis of Caenorhabditis elegans (Syntichaki and Tavernarakis,
Substrate stiffness effects on cell proliferation and differentiation in vitro
One of the major downstream effects of mechanical signaling is altered proliferation. Indeed, the proliferation of many cell types increases with ECM stiffness. This effect is clearly seen by measuring entry into the S phase of the cell cycle or by determining the transcriptional activation of mechanosensitive cell cycle genes as described below. Proximally, these transcriptional changes result from mechanosensitive alterations in the assembly and/or phosphorylation of focal adhesion proteins
Mechanotranscription and differentiation in tissues in response to external stress
Forces affect the metabolic responses and gene expression repertoires of many tissues. Altered gene expression in response to physical signals can manifest as stimulation of bone formation (Martin, 2007, Skerry and Suva, 2003, Turner and Robling, 2004), enhanced turnover of connective tissue matrices (Ozaki et al., 2005), remodeling of the periodontium during orthodontic treatment and dental occlusal trauma (Krishnan and Davidovitch, 2009, Rygh, 1973), induction of cardiac hypertrophy by volume
Conclusion
The physical environment of a cell is as rich a source of potential signals and information as its chemical environment, and the responses of cells to physical stimuli are also likely to be as complex as their biochemical and genetic signaling pathways. Since many cells are embedded within materials composed of ECM molecules populated by neighboring force-generating cells, the homeostatic state of any single cell depends on inputs from multiple mechanosensitive signaling systems. Perturbations
References (120)
- et al.
The influence of hydrogel modulus on the proliferation and differentiation of encapsulated neural stem cells
Biomaterials
(2009) - et al.
Elements of the smooth muscle alpha-actin promoter required in cis for transcriptional activation in smooth muscle. Evidence for cell type-specific regulation
Journal of Biological Chemistry
(1992) - et al.
Absence of filamin A prevents cells from responding to stiffness gradients on gels coated with collagen but not fibronectin
Biophysical Journal
(2009) - et al.
Force-induced myofibroblast differentiation through collagen receptors is dependent on mammalian diaphanous (mDia)
Journal of Biological Chemistry
(2010) - et al.
Hippo pathway-independent restriction of TAZ and YAP by angiomotin
Journal of Biological Chemistry
(2011) - et al.
From mechanotransduction to extracellular matrix gene expression in fibroblasts
Biochimica et Biophysica Acta
(2009) - et al.
Reprogramming cardiomyocyte mechanosensing by crosstalk between integrins and hyaluronic acid receptors
Journal of Biomechanics
(2012) New TRP channels in hearing and mechanosensation
Neuron
(2003)- et al.
Myocardin-related transcription factors A and B are key regulators of TGF-beta1-induced fibroblast to myofibroblast differentiation
Journal of Investigative Dermatology
(2011) - et al.
Interaction of p38 and Sp1 in a mechanical force-induced, beta 1 integrin-mediated transcriptional circuit that regulates the actin-binding protein filamin-A
Journal of Biological Chemistry
(2002)