Control of Mechanotransduction by Molecular Clutch Dynamics

Multicellular scale front-to-rear polarity in collective migration

Collective cell migration does not only reflect the migration of cells at a similar speed and in the same direction, it also implies the emergence of new properties observed at the level of the cell group. This collective behavior relies on interactions between the cells and the establishment of a hierarchy amongst cells with leaders driving the group of followers. Here, we make the parallel between the front-to-rear polarity axis in single cell and the front-to-rear multicellular-scale polarity of a migrating collective which established through exchange of biochemical and mechanical information from the front to the rear and vice versa. Such multicellular-scale polarity gives the migrating group the possibility to better sense and adapt to energy, biochemical and mechanical constraints and facilitates migration over long distances in complex and changing environments.

Cell mechanosensing is regulated by substrate strain energy rather than stiffness

The ability of cells to perceive the mechanical identity of extracellular matrix, generally known as mechanosensing, is generally depicted as a consequence of an intricate balance between pulling forces actuated by the actin fibers on the adhesion plaques and the mechanical reaction of the supporting material. However, whether the cell is sensitive to the stiffness or to the energy required to deform the material remains unclear. To address this important issue, here the cytoskeleton mechanics of BALB/3T3 and MC3T3 cells seeded on linearly elastic substrates under different levels of deformation were studied. In particular, the effect of prestrain on cell mechanics was evaluated by seeding cells both on substrates with no prestrain and on substrates with different levels of prestrain. Results indicated that cells recognize the existence of prestrain, exhibiting a stiffer cytoskeleton on stretched material compared to cells seeded on unstretched substrate. Cytoskeleton mechanics of cells seeded on stretched material were, in addition, comparable to those measured after the stretching of the substrate and cells together to the same level of deformation. This observation clearly suggests that cell mechanosensing is not mediated only by the stiffness of the substrate, as widely assumed in the literature, but also by the deformation energy associated with the substrate. Indeed, the clutch model, based on the exclusive dependence of cell mechanics upon substrate stiffness, fails to describe our experimental results. By modifying the clutch model equations to incorporate the dependence on the strain energy, we were able to correctly interpret the experimental evidence.

Cell Adhesion by Integrins

Integrins are heterodimeric cell surface receptors ensuring the mechanical connection between cells and the extracellular matrix. In addition to the anchorage of cells to the extracellular matrix, these receptors have critical functions in intracellular signaling, but are also taking center stage in many physiological and pathological conditions. In this review, we provide some historical, structural, and physiological notes so that the diverse functions of these receptors can be appreciated and put into the context of the emerging field of mechanobiology. We propose that the exciting journey of the exploration of these receptors will continue for at least another new generation of researchers.

Molecular Tension Sensors: Moving Beyond Force

Nearly all cellular processes are sensitive to mechanical inputs, and this plays a major role in diverse physiological processes. Mechanical stimuli are thought to be primarily detected through force-induced changes in protein structure. Approximately a decade ago, molecular tension sensors were created to measure forces across proteins within cells. Since then, an impressive assortment of sensors has been created and provided key insights into mechanotransduction, but comparisons of measurements between various sensors are challenging. In this review, we discuss the different types of molecular tension sensors, provide a system of classification based on their molecular-scale mechanical properties, and highlight how new applications of these sensors are enabling measurements beyond the magnitude of tensile load. We suggest that an expanded understanding of the functionality of these sensors, as well as integration with other techniques, will lead to consensus amongst measurements as well as critical insights into the underlying mechanisms of mechanotransduction.

Designing stem cell niches for differentiation and self-renewal

Mesenchymal stem cells, characterized by their ability to differentiate into skeletal tissues and self-renew, hold great promise for both regenerative medicine and novel therapeutic discovery. However, their regenerative capacity is retained only when in contact with their specialized microenvironment, termed the stem cell niche Niches provide structural and functional cues that are both biochemical and biophysical, stem cells integrate this complex array of signals with intrinsic regulatory networks to meet physiological demands. Although, some of these regulatory mechanisms remain poorly understood or difficult to harness with traditional culture systems. Biomaterial strategies are being developed that aim to recapitulate stem cell niches, by engineering microenvironments with physiological-like niche properties that aim to elucidate stem cell-regulatory mechanisms, and to harness their regenerative capacity in vitro In the future, engineered niches will prove important tools for both regenerative medicine and therapeutic discoveries.

Emerging technologies in mechanotransduction research

Mechanotransduction research focuses on understanding how cells sense and respond to mechanical stimuli by converting mechanical signals into biochemical and biological responses. Cells have been shown to respond to mechanical stimuli through specialized biological machinery such as adhesion complexes. Research in the last two decades helped in identifying key components of cellular mechanotransduction. In recent years, integrated approaches, which are highlighted here, are emerging to provide new insights into the mechanistic and theoretical underpinnings of mechanotransduction. In particular, mathematical modeling has helped elucidate the mechanism underlining ligand spacing and distribution sensing, as well as sensing viscoelastic properties of the extracellular matrix. In addition, molecular tension sensors have helped dissect the forces involved in mechanotransduction at high spatial and temporal resolutions.

Microtubules at focal adhesions – a double-edged sword

Cell adhesion to the extracellular matrix is essential for cellular processes, such as migration and invasion. In response to cues from the microenvironment, integrin-mediated adhesions alter cellular behaviour through cytoskeletal rearrangements. The tight association of the actin cytoskeleton with adhesive structures has been extensively studied, whereas the microtubule network in this context has gathered far less attention. In recent years, however, microtubules have emerged as key regulators of cell adhesion and migration through their participation in adhesion turnover and cellular signalling. In this Review, we focus on the interactions between microtubules and integrin-mediated adhesions, in particular, focal adhesions and podosomes. Starting with the association of microtubules with these adhesive structures, we describe the classical role of microtubules in vesicular trafficking, which is involved in the turnover of cell adhesions, before discussing how microtubules can also influence the actin–focal adhesion interplay through RhoGTPase signalling, thereby orchestrating a very crucial crosstalk between the cytoskeletal networks and adhesions.

Building a microfluidic cell culture platform with stiffness control using Loctite 3525 glue

The study of mechanotransduction signals and cell response to mechanical properties requires designing culture substrates that possess some, or ideally all, of the following characteristics: (1) biological compatibility and adhesive properties, (2) stiffness control or tunability in a dynamic mode, (3) patternability on the microscale and (4) integrability in microfluidic chips. The most common materials used to address cell mechanotransduction are hydrogels, due to their softness. However, they may be impractical when complex scaffolds are sought and they lack viscous dissipative properties that are very important in mechanobiology. In this work, we show that an off-the-shelf, biocompatible photosensitive glue, Loctite 3525, may be used readily in mechanobiology assays without any special treatment prior to fabrication of cell culture platforms. Despite a high (MPa) stiffness easily tunable by UV exposure time at a fixed dose, 3T3 fibroblasts showed a response to the mechanics of the material similar to that obtained on much softer (kPa) hydrogels. Loctite's viscous dissipation properties indeed seemed to be responsible for such cell mechanical response, as suggested by recent works where more complex two-phase hydrogels were employed. More interestingly, it was possible to stiffen soft Loctite substrates by post-exposing them during cell culture, to observe changes in cell spreading caused by a dynamic stiffness modification. Thanks to Loctite 3525's patternability, micropillars were also fabricated to demonstrate the compatibility with traction force microscopy studies. Finally, the glue was used as an excellent adhesion layer for hydrogels on glass or PDMS, without the need for additional treatment, enabling the easy fabrication of microfluidic chips integrating hydrogels.

Label-free Single-Molecule Quantification of Rapamycin-induced FKBP-FRB Dimerization for Direct Control of Cellular Mechanotransduction

Chemically induced dimerization (CID) has been applied to study numerous biological processes and has important pharmacological applications. However, the complex multistep interactions under various physical constraints involved in CID impose a great challenge for the quantification of the interactions. Furthermore, the mechanical stability of the ternary complexes has not been characterized; hence, their potential application in mechanotransduction studies remains unclear. Here, we report a single-molecule detector that can accurately quantify almost all key interactions involved in CID and the mechanical stability of the ternary complex, in a label-free manner. Its application is demonstrated using rapamycin-induced heterodimerization of FRB and FKBP as an example. We revealed the sufficient mechanical stability of the FKBP/rapamycin/FRB ternary complex and demonstrated its utility in the precise switching of talin-mediated force transmission in integrin-based cell adhesions.

A simulation of the random and directed motion of dendritic cells in chemokine fields

Dendritic cells (DCs) are the most effective professional antigen-presenting cell. They ferry antigen from the extremities to T cells and are essential for the initiation of an adaptive immune response. Despite interest in how DCs respond to chemical stimuli, there have been few attempts to model DC migration. In this paper, we simulate the motility of DCs by modeling the generation of forces by filopodia and a force balance on the cell. The direction of fliopodial extension is coupled to differential occupancy of cognate chemokine receptors across the cell. Our model simulates chemokinesis and chemotaxis in a variety of chemical and mechanical environments. Simulated DCs undergoing chemokinesis were measured to have a speed of 5.1 ± 0.07 μm·min-1 and a persistence time of 3.2 ± 0.46 min, consistent with experiment. Cells undergoing chemotaxis exhibited a stronger chemotactic response when exposed to lower average chemokine concentrations, also consistent with experiment. We predicted that when placed in two opposing gradients, cells will cluster in a line, which we call the "line of equistimulation;" this clustering has also been observed. We calculated the effect of varying gradient steepness on the line of equistimulation, with steeper gradients resulting in tighter clustering. Moreover, gradients are found to be most potent when cells are in a gradient of chemokine whose mean concentration is close to the binding of the Kd to the receptor, and least potent when the mean concentration is 0.1Kd. Comparing our simulations to experiment, we can give a quantitative measure of the strength of certain chemokines relative to others. Assigning the signal of CCL19 binding CCR7 a baseline strength of 1, we found CCL21 binding CCR7 had a strength of 0.28, and CXCL12 binding CXCR4 had a strength of 0.30. These differences emerge despite both chemokines having virtually the same Kd, suggesting a mechanism of signal amplification in DCs requiring further study.

Mechanosensing through immunoreceptors

The immune response is orchestrated by a variety of immune cells. The function of each cell is determined by the collective signals from various immunoreceptors, whose expression and activity depend on the developmental stages of the cell and its environmental context. Recent studies have highlighted the presence of mechanical force on several immunoreceptor-ligand pairs and the important role of force in regulating their interaction and function. In this Perspective, we use the T cell antigen receptor as an example with which to review the current understanding of the mechanosensing properties of immunoreceptors. We discuss the types of forces that immunoreceptors may encounter and the effects of force on ligand bonding, conformational change and the triggering of immunoreceptors, as well as the effects of force on the downstream signal transduction, cell-fate decisions and effector function of immune cells.

Unfolding of polymers tethered to viscoelastic substrates

The problem of globular polymer unfolding under applied force is a widely-studied fundamental topic in biological and chemical physics, with important applications in cell biology. Much of the existing theoretical and experimental literature focuses on the case where force is applied while fixing the opposite end of the polymer chain in space. However, in a realistic biological microenvironment, forces will be applied against viscoelastic references, and the deformation of the folded polymer chain will be combined with the deformation of viscoelastic substrate. In this paper, we consider several simple viscoelastic models for the substrate, and show that its relaxation properties determine the unfolding kinetics. In particular, for low pulling forces, substrates with longer relaxation times cause lower unfolding rates for the pulled polymer chain, whereas for high forces, those substrates with longer relaxation times instead produce higher unfolding rates.

Stick-slip dynamics of migrating cells on viscoelastic substrates

Stick-slip motion, a common phenomenon observed during crawling of cells, is found to be strongly sensitive to the substrate stiffness. Stick-slip behaviors have previously been investigated typically using purely elastic substrates. For a more realistic understanding of this phenomenon, we propose a theoretical model to study the dynamics on a viscoelastic substrate. Our model, based on a reaction-diffusion framework, incorporates known important interactions such as retrograde flow of actin, myosin contractility, force-dependent assembly, and disassembly of focal adhesions coupled with cell-substrate interaction. We show that consideration of a viscoelastic substrate not only captures the usually observed stick-slip jumps but also predicts the existence of an optimal substrate viscosity corresponding to maximum traction force and minimum retrograde flow which was hitherto unexplored. Moreover, our theory predicts the time evolution of individual bond force that characterizes the stick-slip patterns on soft versus stiff substrates. Our analysis also elucidates how the duration of the stick-slip cycles are affected by various cellular parameters.

Matrix Nanopatterning Regulates Mesenchymal Differentiation through Focal Adhesion Size and Distribution According to Cell Fate

Extracellular matrix remodeling plays a pivotal role during mesenchyme patterning into different lineages. Tension exerted from cell membrane receptors bound to extracellular matrix ligands is transmitted by the cytoskeleton to the cell nucleus inducing gene expression. Here, we used dendrimer-based arginine–glycine–aspartic acid (RGD) uneven nanopatterns, which allow the control of local surface adhesiveness at the nanoscale, to unveil the adhesive requirements of mesenchymal tenogenic and osteogenic commitments. Cell response was found to depend on the tension resulting from cell–substrate interactions, which affects nuclear morphology and is regulated by focal adhesion size and distribution.

Costameres, dense plaques and podosomes: the cell matrix adhesions in cardiovascular mechanosensing

The stiffness of the cardiovascular environment changes during ageing and in disease and contributes to disease incidence and progression. For instance, increased arterial stiffness can lead to atherosclerosis, while stiffening of the heart due to fibrosis can increase the chances of heart failure. Cells can sense the stiffness of the extracellular matrix through integrin adhesions and other mechanosensitive structures and in response to this initiate mechanosignalling pathways that ultimately change the cellular behaviour. Over the past decades, interest in mechanobiology has steadily increased and with this also our understanding of the molecular basis of mechanosensing and transduction. However, much of our knowledge about the mechanisms is derived from studies investigating focal adhesions in non-muscle cells, which are distinct in several regards from the cell-matrix adhesions in cardiomyocytes (costameres) or vascular smooth muscle cells (dense plaques or podosomes). Therefore, we will look here first at the evidence for mechanical sensing in the cardiovascular system, before comparing the different cytoskeletal arrangements and adhesion sites in cardiomyocytes and vascular smooth muscle cells and what is known about mechanical sensing through the various structures.

Mechanical feedback enables catch bonds to selectively stabilize scanning microvilli at T-cell surfaces

T-cells use microvilli to search the surfaces of antigen-presenting cells for antigenic ligands. The active motion of scanning microvilli provides a force-generating mechanism that is intriguing in light of single-molecule experiments showing that applied forces increase the lifetimes of stimulatory receptor–ligand bonds (catch-bond behavior). In this work, we introduce a theoretical framework to explore the motion of a microvillar tip above an antigen-presenting surface when receptors on the tip stochastically bind to ligands on the surface and dissociate from them in a force-dependent manner. Forces on receptor-ligand bonds impact the motion of the microvillus, leading to feedback between binding and microvillar motion. We use computer simulations to show that the average microvillar velocity varies in a ligand-dependent manner; that catch bonds generate responses in which some microvilli almost completely stop, while others move with a broad distribution of velocities; and that the frequency of stopping depends on the concentration of stimulatory ligands. Typically, a small number of catch bonds initially immobilize the microvillus, after which additional bonds accumulate and increase the cumulative receptor-engagement time. Our results demonstrate that catch bonds can selectively slow and stabilize scanning microvilli, suggesting a physical mechanism that may contribute to antigen discrimination by T-cells.

Mechanical regulation of myofibroblast phenoconversion and collagen contraction

Activated fibroblasts promote physiological wound repair following tissue injury. However, dysregulation of fibroblast activation contributes to the development of fibrosis by enhanced production and contraction of collagen-rich extracellular matrix. At the peak of their activities, fibroblasts undergo phenotypic conversion into highly contractile myofibroblasts by developing muscle-like features, including formation of contractile actin-myosin bundles. The phenotype and function of fibroblasts and myofibroblasts are mechanically regulated by matrix stiffness using a feedback control system that is integrated with the progress of tissue remodelling. The actomyosin contraction machinery and cell-matrix adhesion receptors are critical elements that are needed for mechanosensing by fibroblasts and the translation of mechanical signals into biological responses. Here, we focus on mechanical and chemical regulation of collagen contraction by fibroblasts and the involvement of these factors in their phenotypic conversion to myofibroblasts.

Signal transduction via integrin adhesion complexes

Integrin adhesion complexes (IACs) have evolved over millions of years to integrate metazoan cells physically with their microenvironment. It is presumed that the simultaneous interaction of thousands of integrin receptors to binding sites in anisotropic extracellular matrix (ECM) networks enables cells to assemble a topological description of the chemical and mechanical properties of their surroundings. This information is then converted into intracellular signals that influence cell positioning, differentiation and growth, but may also influence other fundamental processes, such as protein synthesis and energy regulation. In this way, changes in the microenvironment can influence all aspects of cell phenotype. Current concepts envisage cell fate decisions being controlled by the integrated signalling output of myriad receptor clusters, but the mechanisms are not understood. Analyses of the adhesome, the complement of proteins attracted to the vicinity of IACs, are now providing insights into some of the primordial links connecting these processes. This article reviews recent advances in our understanding of the composition of IACs, the mechanisms used to transduce signals through these junctions, and the links between IACs and cell phenotype.

Influence of the extracellular matrix on cell-intrinsic circadian clocks

Cell-autonomous circadian clocks coordinate tissue homeostasis with a 24-hourly rhythm. The molecular circadian clock machinery controls tissue- and cell type-specific sets of rhythmic genes. Disruptions of clock mechanisms are linked to an increased risk of acquiring diseases, especially those associated with aging, metabolic dysfunction and cancer. Despite rapid advances in understanding the cyclic outputs of different tissue clocks, less is known about how the clocks adapt to their local niche within tissues. We have discovered that tissue stiffness regulates circadian clocks, and that this occurs in a cell-type-dependent manner. In this Review, we summarise new work linking the extracellular matrix with differential control of circadian clocks. We discuss how the changes in tissue structure and cellular microenvironment that occur throughout life may impact on the molecular control of circadian cycles. We also consider how altered clocks may have downstream impacts on the acquisition of diseases.

Soft Hyaluronic Gels Promote Cell Spreading, Stress Fibers, Focal Adhesion, and Membrane Tension by Phosphoinositide Signaling, Not Traction Force

Cells respond to both physical and chemical aspects of their substrate. Whether intracellular signals initiated by physical stimuli are fundamentally different from those elicited by chemical stimuli is an open question. Here, we show that the requirement for a stiff substrate (and, therefore, high cellular tension) for cells to produce large focal adhesions and stress fibers is obviated when a soft substrate contains both hyaluronic acid (HA) and an integrin ligand (collagen I). HA is a major extracellular matrix component that is often up-regulated during wound healing and tumor growth. HA, together with collagen I, promotes hepatocellular carcinoma cell (Huh7) spreading on very soft substrates (300 Pa), resulting in morphology and motility similar to what these cells develop only on stiff substrates (>30 kPa) formed by polyacrylamide that contains collagen but not HA. The effect of HA requires turnover of polyphosphoinositides and leads to the activation of Akt. The inhibition of polyphosphoinositide turnover causes Huh7 cells and fibroblasts to decrease spreading and detach, whereas cells on stiffer substrates show almost no response. Traction force microscopy shows that the cell maintains a low strain energy and net contractile moment on HA substrates compared to stiff polyacrylamide substrates. Membrane tension measured by tether pulling is similar on soft HA and stiff polyacrylamide substrates. These results suggest that simultaneous signaling stimulated by HA and an integrin ligand can generate phosphoinositide-mediated signals to the cytoskeleton that reproduce those generated by high cellular tension.

Matrix stiffness mechanically conditions EMT and migratory behavior of oral squamous cell carcinoma

Tumors are composed of heterogeneous phenotypes, each having different sensitivities to the microenvironment. One microenvironment characteristic - matrix stiffness - helps to regulate malignant transformation and invasion in mammary tumors, but its influence on oral squamous cell carcinoma (OSCC) is unclear. We observed that, on stiff matrices, a highly invasive OSCC cell line (SCC25) comprising a low E-cad to N-cad ratio (Inv^H/E:N^L; SCC25) had increased migration velocity and decreased adhesion strength compared to a less invasive OSCC cell line (Cal27) with high E-cad to N-cad ratio (Inv^L/E:N^H; Cal27). However, Inv^L/E:N^H cells acquire a mesenchymal signature and begin to migrate faster when exposed to prolonged time on a stiff niche, suggesting that cells can be mechanically conditioned. Owing to increased focal adhesion assembly, Inv^L/E:N^H cells migrated faster, which could be reduced when increasing integrin affinity with high divalent cation concentrations. Mirroring these data in human patients, we observed that collagen organization, an indicator of matrix stiffness, was increased with advanced disease and correlated with early recurrence. Consistent with epithelial tumors, our data suggest that OSCC cells are mechanically sensitive and that their contribution to tumor progression is mediated in part by this sensitivity.This article has an associated First Person interview with the first author of the paper.

Integrin activation by talin, kindlin and mechanical forces

Integrins are the major family of adhesion molecules that mediate cell adhesion to the extracellular matrix. They are essential for embryonic development and influence numerous diseases, including inflammation, cancer cell invasion and metastasis. In this Perspective, we discuss the current understanding of how talin, kindlin and mechanical forces regulate integrin affinity and avidity, and how integrin inactivators function in this framework.

Where No Hand Has Gone Before: Probing Mechanobiology at the Cellular Level

Physical forces and other mechanical stimuli are fundamental regulators of cell behavior and function. Cells are also biomechanically competent: they generate forces to migrate, contract, remodel, and sense their environment. As the knowledge of the mechanisms of mechanobiology increases, the need to resolve and probe increasingly small scales calls for novel technologies to mechanically manipulate cells, examine forces exerted by cells, and characterize cellular biomechanics. Here, we review novel methods to quantify cellular force generation, measure cell mechanical properties, and exert localized piconewton and nanonewton forces on cells, receptors, and proteins. The combination of these technologies will provide further insight on the effect of mechanical stimuli on cells and the mechanisms that convert these stimuli into biochemical and biomechanical activity.

Non-canonical Wnt signals regulate cytoskeletal remodeling in osteoclasts

Osteoclasts are multinucleated cells responsible for bone resorption. Osteoclasts adhere to the bone surface through integrins and polarize to form actin rings, which are formed by the assembly of podosomes. The area contained within actin rings (also called sealing zones) has an acidic pH, which causes dissolution of bone minerals including hydroxyapatite and the degradation of matrix proteins including type I collagen by the protease cathepsin K. Osteoclasts resorb bone matrices while moving on bone surfaces. Osteoclasts change their cell shapes and exhibit three modes for bone resorption: motile resorbing mode for digging trenches, static resorbing mode for digging pits, and motile non-resorbing mode. Therefore, the actin cytoskeleton is actively remodeled in osteoclasts. Recent studies have revealed that many molecules, such as Rac, Cdc42, Rho, and small GTPase regulators and effectors, are involved in actin cytoskeletal remodeling during the formation of actin rings and resorption cavities on bone slices. In this review, we introduce how these molecules and non-canonical Wnt signaling regulate the bone-resorbing activity of osteoclasts.

Intracellular mechanics: connecting rheology and mechanotransduction

Cell mechanics is crucial for a wide range of cell functions, including proliferation, polarity, migration and differentiation. Cells sense external physical cues and translate them into a cellular response. While force sensing occurs in the vicinity of the plasma membrane, forces can reach deep in the cell interior and to the nucleus. We review here the recent developments in the field of intracellular mechanics. We focus first on intracellular rheology, the study of the mechanical properties of the cell interior, and recapitulate the contribution of active mechanisms, the cytoskeleton and intracellular organelles to cell rheology. We then discuss how forces are transmitted inside the cell during mechanotransduction events, through direct force transmission and biochemical signaling, and how intracellular rheology and mechanotransduction are connected.

The Actin Cytoskeleton: A Mechanical Intermediate for Signal Integration at the Immunological Synapse

The immunological synapse (IS) is a specialized structure that serves as a platform for cell-cell communication between a T cell and an antigen-presenting cell (APC). Engagement of the T cell receptor (TCR) with cognate peptide-MHC complexes on the APC activates the T cell and instructs its differentiation. Proper T cell activation also requires engagement of additional receptor-ligand pairs, which promote sustained adhesion and deliver costimulatory signals. These events are orchestrated by T cell actin dynamics, which organize IS components and facilitate their signaling. The actin network flows from the edge of the cell inward, driving the centralization of TCR microclusters and providing the force to activate the integrin LFA-1. We recently showed that engagement of LFA-1 slows actin flow, and that this affects TCR signaling. This study highlights the physical nature of the IS, and contributes to a growing appreciation in the field that mechanosensing and mechanotransduction are essential for IS function. Additionally, it is becoming clear that there are multiple types of actin structures at the IS that promote signaling in distinct ways. How the different actin structures contribute to force production and mechanotransduction is just beginning to be explored. In this Perspective, we will feature recent work from our lab and others, that collectively points toward a model in which actin dynamics drive mechanical signaling and receptor crosstalk during T cell activation.

Actin-Based Cell Protrusion in a 3D Matrix

Cell migration controls developmental processes (gastrulation and tissue patterning), tissue homeostasis (wound repair and inflammatory responses), and the pathobiology of diseases (cancer metastasis and inflammation). Understanding how cells move in physiologically relevant environments is of major importance, and the molecular machinery behind cell movement has been well studied on 2D substrates, beginning over half a century ago. Studies over the past decade have begun to reveal the mechanisms that control cell motility within 3D microenvironments – some similar to, and some highly divergent from those found in 2D. In this review we focus on migration and invasion of cells powered by actin, including formation of actin-rich protrusions at the leading edge, and the mechanisms that control nuclear movement in cells moving in a 3D matrix.

Cell migration has been well studied in 2D, but how this relates to movement in physiological 3D tissues and matrix is not clear, particularly in vertebrate interstitial matrix.

In 3D matrix cells actin polymerisation directly contributes to the formation of lamellipodia to facilitate migration and invasion (mesenchymal movement), analogous to 2D migration; actomyosin contractility promotes bleb formation to indirectly promote protrusion (amoeboid movement).

Mesenchymal migration can be characterised by polymerisation of actin to form filopodial protrusions, in the absence of lamellipodia.

Translocation of the nucleus is emerging as a critical step due to the constrictive environment of 3D matrices, and the mechanisms that transmit force to the nucleus and allow movement are beginning to be uncovered.

Role of mechanical cues in shaping neuronal morphology and connectivity

Neuronal circuits, the functional building blocks of the nervous system, assemble during development through a series of dynamic processes including the migration of neurons to their final position, the growth and navigation of axons and their synaptic connection with target cells. While the role of chemical cues in guiding neuronal migration and axonal development has been extensively analysed, the contribution of mechanical inputs, such as forces and stiffness, has received far less attention. In this article, we review the in vitro and more recent in vivo studies supporting the notion that mechanical signals are critical for multiple aspects of neuronal circuit assembly, from the emergence of axons to the formation of functional synapses. By combining live imaging approaches with tools designed to measure and manipulate the mechanical environment of neurons, the emerging field of neuromechanics will add a new paradigm in our understanding of neuronal development and potentially inspire novel regenerative therapies.