Mechanosensing refers to the ability of cells to translate mechanical forces into biochemical signals that regulate cellular responses. In living tissues, these mechanical stimuli come mostly from the extracellular matrix (ECM), which acts as a potent regulator of cell behavior as well as providing essential structural support.1 Normal tissue homeostasis is a dynamic, bidirectional process in which cells both respond to mechanical cues and adjust the characteristics of the ECM to maintain a healthy state.1,2
The mechanical properties of individual cells are largely determined by their cytoskeletal organization, and dynamic rearrangements of the actin cytoskeleton are therefore an important aspect of environmental adaptation.3 Actin forms semiflexible filaments that can adopt multiple architectures with different mechanical properties, meaning that the actomyosin cytoskeleton can serve as an “active spring” or “shock absorber” in response to the cellular environment.4
At the cell surface, mechanosensing relies on certain plasma membrane receptors that recognize specific ECM components to initiate downstream formation of large protein complexes known as adhesions (Figure 1).5 The major class of mechanoreceptors is the integrins, which are heterodimeric structures containing various α- and β- subunits in different combinations that dictate their ligand binding specificity.6 When activated, integrins become physically connected to the actin cytoskeleton through a range of linking factors including the vital adapter protein talin,2 which engages the cytoplasmic tail of β-integrins and unfolds under force to reveal multiple interaction sites for the actin-binding protein vinculin.7
Integrin-mediated binding to the ECM initially occurs through small receptor clusters to give reversible, short-lived “nascent adhesions.” With sufficient mechanical loading, integrins transition to extended conformations that promote the binding of cytoplasmic adapter proteins and formation of longer-lived “focal complexes” that experience higher contractile forces. These can mature further into more stable “focal adhesions” (FAs) that are anchored to large actin fibers and can be used, for example, to generate traction forces and drive cellular movement. Importantly, two-way signaling operates throughout the aforementioned network, and mechanotransduction is therefore referred to as either “outside-in” or “inside-out” depending on the directionality.6
Figure 1. Schematic of mechanosensing recognition by integrins from the ECM. Upon activation, the integrins signal to actin via actin-binding proteins like vinculin and talin
The laminins are a family of important ECM components involved in outside-in mechanosensing. They are all trimeric protein complexes containing α-, β-, and γ-chains, and 16 such combinations are found in humans.8 Laminin-binding mechanoreceptors include α6β1 integrin, which is essential for basal mammary epithelial cell function via binding to laminin-111 (i.e., laminin-α1β1γ1).9 Also, α6β4 integrin is involved in regulating endothelial cell responses to shear stress under blood flow, and evidently signals to multiple factors involved in actin cytoskeleton regulation.10 The α6-integrin subunit has been identified as a mediator of myofibroblast invasion in lung fibrosis, through its role in mechanosensing of matrix stiffness in the laminin-rich pulmonary basement membrane.11
Within the cell, additional adapter proteins—such as the linker of nucleoskeleton and cytoskeleton (LINC) complex—enable extracellular mechanical forces to be relayed all the way to the nucleus to effect transcriptional changes.12 Well-studied mechanoresponsive transcriptional regulators include YAP1 and WWTR1 (TAZ). Nuclear translocation of YAP1 occurs above a force threshold (across integrins) of around 50 pN,13 and downstream YAP1/WWTR1 signaling influences many cellular processes including proliferation and differentiation. This pathway is highly mechanosensitive and specifically requires the integrity and tension of the actin cytoskeleton for signal transduction.14,15 In one functional study in mice, surgical tendon detensioning resulted in YAP1/WWTR1-mediated derepression of matrix metalloprotease (MMP) genes to induce ECM degradation.16
Fibronectin is a large, multidomain protein that exists in various isoforms due to alternative splicing17 and represents another major constituent of the ECM. It mainly binds to α5β1 and αvβ3 integrins, and via these interactions, it serves as an important scaffold for migration of many different cell types.18 In an example of inside-out mechanotransduction, fibronectin can assemble into fibrils in the ECM. Intracellular tension generated by actin cytoskeletal remodeling is transmitted through integrins to bound fibronectin molecules, resulting in force-induced unfolding to expose cryptic self-association sites and drive fibril assembly.19 Regulation of fibronectin fibrillogenesis is important in many processes, including both normal and pathological angiogenesis.20 In this context, fibronectin fibrils provide vital extracellular mechanical and chemical cues to ensure correct alignment of endothelial cells in the vasculature.21 Conversely, in early atherosclerosis, excessive α5β1 integrin-mediated fibronectin deposition promotes vascular remodeling and proinflammatory NF-κB signaling.22
Mechanosensing is a key component of multiple biological processes including embryonic morphogenesis, wound healing, vascular remodeling, and bone homestasis.23 Unsurprisingly, it is particularly important in tissues that frequently experience mechanical stress, such as muscle, cartilage, and blood vessels.24 On the other hand, dysregulated or maladaptive mechanical responses are associated with various pathologies including atherosclerosis, fibrosis, and cancer.
For example, mutations in β2-integrin cause type 1 leukocyte adhesion deficiency (LAD-I), a rare phagocytic disorder associated with recurrent bacterial infections and high infant mortality.25 Similarly, mutations in α3-integrin can lead to severe renal disease, since α3β1 is the most highly expressed integrin in the kidneys.26 It acts as a laminin receptor to facilitate podocyte adhesion to the laminin-rich glomerular basement membrane, and is therefore essential for normal ultrafiltration of the blood.
In a notable recent report, the relatively understudied transcription factor VGLL3 was newly identified as a mechanosensitive promoter of cardiac fibrosis, through upregulation of collagen production in response to ECM stiffness following myocardial infarction (MI). The associated signaling pathway involved integrin-β1, Rho GTPase activation, and actin polymerization. Significantly, knockout mice showed improved cardiac function after MI, suggesting VGLL3 as a target for much-needed new antifibrotic drugs.27
It has been known for some time that cellular microtubule mass is sensitive to mechanical forces and ECM composition,28 yet relatively little is known about the role of microtubules in mechanosensing. However, recent findings suggest fundamental crosstalk between the actin and tubulin networks, since talin- and actin-dependent mechanosensing upregulates microtubule acetylation, a stabilizing posttranslational modification (PTM), and promotes RhoA activation and actomyosin contractility.29
Other evidence suggests that tendon fibroblasts extend microtubule-rich projections during muscle contraction to both sense force and control ECM organization,30 and the microtubule cytoskeleton also seems to play a prominent role in cardiomyocytes. In these cells, detyrosination of α-tubulin (another well-known microtubule PTM) was found to modulate vital functional properties including stiffness, viscoelasticity, and stretch-dependent mechanosignaling.31 Finally, microtubules and the actin cytoskeleton both seem to be important for mechanotransduction to the nucleus, with synergistic roles in regulating chromatin accessibility and mediating transcriptional changes in response to mechanical force.32
More generally, while mechanosensing has largely been associated with the plasma membrane (and to some extent, the nucleus), emerging evidence suggests it also operates at the level of individual organelles.33 Current investigations involve functional and morphological responses to mechanical cues for the endoplasmic reticulum, Golgi apparatus, mitochondria, and endolysosomal system. This exciting new research area will further extend our understanding of mechanobiology and its crucial implications in both health and disease.