Nanoscale mechanobiology of cell adhesions

Superposition of Substrate Deformation Fields Induced by Molecular Clutches Explains Cell Spatial Sensing of Ligands

Cells can sense the physical properties of the extracellular matrices (ECMs), such as stiffness and ligand density, through cell adhesions to actively regulate their behaviors. Recent studies have shown that varying ligand spacing of ECMs can influence adhesion size, cell spreading, and even stem cell differentiation, indicating that cells have the spatial sensing ability of ECM ligands. However, the mechanism of the cells' spatial sensing remains unclear. In this study, we have developed a lattice-spring motor-clutch model by integrating cell membrane deformation, the talin unfolding mechanism, and the lattice spring for substrate ligand distribution to explore how the spatial distribution of integrin ligands and substrate stiffness influence cell spreading and adhesion dynamics. By applying the Gillespie algorithm, we found that large ligand spacing reduces the superposition effect of the substrate's displacement fields generated by pulling force from motor-clutch units, increasing the effective stiffness probed by the force-sensitive receptors; this finding explains a series of previous experiments. Furthermore, using the mean-field theory, we obtain the effective stiffness sensed by bound clutches analytically; our analysis shows that the bound clutch number and ligand spacing are the two key factors that affect the superposition effects of deformation fields and, hence, the effective stiffness. Overall, our study reveals the mechanism of cells' spatial sensing, i.e., ligand spacing changes the effective stiffness sensed by cells due to the superposition effect of deformation fields, which provides a physical clue for designing and developing biological materials that effectively control cell behavior and function.

Alternative molecular mechanisms for force transmission at adherens junctions via β-catenin-vinculin interaction

Force transmission through adherens junctions (AJs) is crucial for multicellular organization, wound healing and tissue regeneration. Recent studies shed light on the molecular mechanisms of mechanotransduction at the AJs. However, the canonical model fails to explain force transmission when essential proteins of the mechanotransduction module are mutated or missing. Here, we demonstrate that, in absence of α-catenin, β-catenin can directly and functionally interact with vinculin in its open conformation, bearing physiological forces. Furthermore, we found that β-catenin can prevent vinculin autoinhibition in the presence of α-catenin by occupying vinculin´s head-tail interaction site, thus preserving force transmission capability. Taken together, our findings suggest a multi-step force transmission process at AJs, where α-catenin and β-catenin can alternatively and cooperatively interact with vinculin. This can explain the graded responses needed to maintain tissue mechanical homeostasis and, importantly, unveils a force-bearing mechanism involving β-catenin and extended vinculin that can potentially explain the underlying process enabling collective invasion of metastatic cells lacking α-catenin.

Single-cell fluid-based force spectroscopy reveals near lipid size nano-topography effects on neural cell adhesion

Nano-roughness has shown great potential in enhancing high-fidelity electrogenic cell interfaces, owing to its characteristic topography comparable to proteins and lipids, which influences a wide range of cellular mechanical responses. Gaining a comprehensive understanding of how cells respond to nano-roughness at the single-cell level is not only imperative for implanted devices but also essential for tissue regeneration and interaction with complex biomaterial surfaces. In this study, we quantify cell adhesion and biomechanics of single cells to nano-roughened surfaces by measuring neural cell adhesion and biomechanics via fluidic-based single-cell force spectroscopy (SCFS). For this, we introduce nanoscale topographical features on polyimide (PI) surfaces achieving roughness up to 25 nm without chemical modifications. Initial adhesion experiments show cell-specific response to nano-roughness for neuroblastoma cells (SH-SY5Y) compared to human astrocytes (NHA) around 15 and 20 nm surface roughness. In addition, our SCFS measurements revealed a remarkable 2.5-fold increase in adhesion forces (150-164 nN) for SH-SY5Y cells cultured on roughened PI (rPI) surfaces compared to smooth surfaces (60-107 nN). Our data also shows that cells can distinguish changes in nano-roughness as small 2 nm (close to the diameter of a single lipid) and show roughness dependence adhesion while favoring 15 nm. Notably, this enhanced adhesion is accompanied by increased cell elongation upon cell detachment without any significant differences in cell area spreading. The study provides valuable insights into the interplay between nano-topography and cellular responses and offers practical implications for designing biomaterial surfaces with enhanced cellular interactions.

Thy-1 (CD90)-regulated cell adhesion and migration of mesenchymal cells: insights into adhesomes, mechanical forces, and signaling pathways

Cell adhesion and migration depend on the assembly and disassembly of adhesive structures known as focal adhesions. Cells adhere to the extracellular matrix (ECM) and form these structures via receptors, such as integrins and syndecans, which initiate signal transduction pathways that bridge the ECM to the cytoskeleton, thus governing adhesion and migration processes. Integrins bind to the ECM and soluble or cell surface ligands to form integrin adhesion complexes (IAC), whose composition depends on the cellular context and cell type. Proteomic analyses of these IACs led to the curation of the term adhesome, which is a complex molecular network containing hundreds of proteins involved in signaling, adhesion, and cell movement. One of the hallmarks of these IACs is to sense mechanical cues that arise due to ECM rigidity, as well as the tension exerted by cell-cell interactions, and transduce this force by modifying the actin cytoskeleton to regulate cell migration. Among the integrin/syndecan cell surface ligands, we have described Thy-1 (CD90), a GPI-anchored protein that possesses binding domains for each of these receptors and, upon engaging them, stimulates cell adhesion and migration. In this review, we examine what is currently known about adhesomes, revise how mechanical forces have changed our view on the regulation of cell migration, and, in this context, discuss how we have contributed to the understanding of signaling mechanisms that control cell adhesion and migration.

Organization, dynamics and mechanoregulation of integrin-mediated cell-ECM adhesions

The ability of animal cells to sense, adhere to and remodel their local extracellular matrix (ECM) is central to control of cell shape, mechanical responsiveness, motility and signalling, and hence to development, tissue formation, wound healing and the immune response. Cell-ECM interactions occur at various specialized, multi-protein adhesion complexes that serve to physically link the ECM to the cytoskeleton and the intracellular signalling apparatus. This occurs predominantly via clustered transmembrane receptors of the integrin family. Here we review how the interplay of mechanical forces, biochemical signalling and molecular self-organization determines the composition, organization, mechanosensitivity and dynamics of these adhesions. Progress in the identification of core multi-protein modules within the adhesions and characterization of rearrangements of their components in response to force, together with advanced imaging approaches, has improved understanding of adhesion maturation and turnover and the relationships between adhesion structures and functions. Perturbations of adhesion contribute to a broad range of diseases and to age-related dysfunction, thus an improved understanding of their molecular nature may facilitate therapeutic intervention in these conditions.

Zyxin regulates embryonic stem cell fate by modulating mechanical and biochemical signaling interface

Biochemical signaling and mechano-transduction are both critical in regulating stem cell fate. How crosstalk between mechanical and biochemical cues influences embryonic development, however, is not extensively investigated. Using a comparative study of focal adhesion constituents between mouse embryonic stem cell (mESC) and their differentiated counterparts, we find while zyxin is lowly expressed in mESCs, its levels increase dramatically during early differentiation. Interestingly, overexpression of zyxin in mESCs suppresses Oct4 and Nanog. Using an integrative biochemical and biophysical approach, we demonstrate involvement of zyxin in regulating pluripotency through actin stress fibres and focal adhesions which are known to modulate cellular traction stress and facilitate substrate rigidity-sensing. YAP signaling is identified as an important biochemical effector of zyxin-induced mechanotransduction. These results provide insights into the role of zyxin in the integration of mechanical and biochemical cues for the regulation of embryonic stem cell fate.

Analysing Mechanically Evoked Currents at Cell-Substrate Junctions

The precise study of mechanically activated ion channels requires a combination of electrophysiology to directly measure channel-mediated ionic flux and a means to apply meaningful mechanical stimuli to activate the channel. In metazoans, individual cells in vivo experience mechanical inputs at the cell-substrate interface where cells form connections to the local microenvironment. To study such processes in vitro, a technique is required where mechanical stimuli can be applied to cells via connections with an underlying substrate. Here, we outline the methodology for combining whole-cell patch-clamp electrophysiology (to monitor transmembrane currents) with elastomer pillar arrays that can be deflected (to apply stimuli to cells). This quantitative technique can be used to assess changes in sensitivity and kinetics of mechanically evoked currents when cell intrinsic or cell extrinsic factors are manipulated.

A review on mechanobiology of cell adhesion networks in different stages of sporadic colorectal cancer to explain its tumorigenesis

Sporadic colorectal cancer (CRC) is strongly linked to extraneous factors, like poor diet and lifestyle, but not to inherent factors like familial genetics. The changes at the epigenomics and signalling pathways are known across the sporadic CRC stages. The catch is that temporal information of the onset, the feedback loop, and the crosstalk of signalling and noise are still unclear. This makes it challenging to diagnose and treat colon cancer effectively with no relapse. Various microbial cells and native cells of the colon, contribute to sporadic CRC development. These cells secrete autocrine and paracrine for their bioenergetics and communications with other cell types. Imbalances of the biochemicals affect the epithelial lining of colon. One side of this epithelial lining is interfacing the dense colon tissue, while the other side is exposed to microbiota and excrement from the lumen. Hence, the epithelial lining is prone to tumorigenesis due to the influence of both biochemical and mechanical cues from its complex surrounding. The role of physical transformations in tumorigenesis have been limitedly discussed. In this context, cellular and tissue structures, and force transductions are heavily regulated by cell adhesion networks. These networks include cell anchoring mechanism to the surrounding, cell structural integrity mechanism, and cell effector molecules. This review will focus on the progression of the sporadic CRC stages that are governed by the underlaying cell adhesion networks within the epithelial cells. Additionally, current and potential technologies and therapeutics that target cell adhesion networks for treatments of sporadic CRC will be incorporated.

Scanning Tunneling Microscopy of Biological Structures: An Elusive Goal for Many Years

Scanning tunneling microscopy (STM) is a technique that can be used to directly observe individual biomolecules at near-molecular scale. Within this framework, STM is of crucial significance because of its role in the structural analysis, the understanding the imaging formation, and the development of relative techniques. Four decades after its invention, it is pertinent to ask how much of the early dream has come true. In this study, we aim to overview different analyses for DNA, lipids, proteins, and carbohydrates. The relevance of STM imaging is exhibited as an opportunity to assist measurements and biomolecular identification in nanobiotechnology, nanomedicine, biosensing, and other cutting-edge applications. We believe STM research is still an entire science research ecosystem for joining several areas of expertise towards a goal settlement that has been elusive for many years.

Adaptation of cell spreading to varying fibronectin densities and topographies is facilitated by β1 integrins

Cells mechanical behaviour in physiological environments is mediated by interactions with the extracellular matrix (ECM). In particular, cells can adapt their shape according to the availability of ECM proteins, e.g., fibronectin (FN). Several in vitro experiments usually simulate the ECM by functionalizing the surfaces on which cells grow with FN. However, the mechanisms underlying cell spreading on non-uniformly FN-coated two-dimensional substrates are not clarified yet. In this work, we studied cell spreading on variously functionalized substrates: FN was either uniformly distributed or selectively patterned on flat surfaces, to show that A549, BRL, B16 and NIH 3T3 cell lines are able to sense the overall FN binding sites independently of their spatial arrangement. Instead, only the total amount of available FN influences cells spreading area, which positively correlates to the FN density. Immunocytochemical analysis showed that β1 integrin subunits are mainly responsible for this behaviour, as further confirmed by spreading experiments with β1-deficient cells. In the latter case, indeed, cells areas do not show a dependency on the amount of available FN on the substrates. Therefore, we envision for β1 a predominant role in cells for sensing the number of ECM ligands with respect to other focal adhesion proteins.

Emerging interplay of cytoskeletal architecture, cytomechanics and pluripotency

Pluripotent stem cells (PSCs) are capable of differentiating into all three germ layers and trophoblasts, whereas tissue-specific adult stem cells have a more limited lineage potency. Although the importance of the cytoskeletal architecture and cytomechanical properties in adult stem cell differentiation have been widely appreciated, how they contribute to mechanotransduction in PSCs is less well understood. Here, we discuss recent insights into the interplay of cellular architecture, cell mechanics and the pluripotent states of PSCs. Notably, the distinctive cytomechanical and morphodynamic profiles of PSCs are accompanied by a number of unique molecular mechanisms. The extent to which such mechanobiological signatures are intertwined with pluripotency regulation remains an open question that may have important implications in developmental morphogenesis and regenerative medicine.

Feeling the force: Multiscale force sensing and transduction at the cell-cell interface

A universal principle of all living cells is the ability to sense and respond to mechanical stimuli which is essential for many biological processes. Recent efforts have identified critical mechanosensitive molecules and response pathways involved in mechanotransduction during development and tissue homeostasis. Tissue-wide force transmission and local force sensing need to be spatiotemporally coordinated to precisely regulate essential processes during development such as tissue morphogenesis, patterning, cell migration and organogenesis. Understanding how cells identify and interpret extrinsic forces and integrate a specific response on cell and tissue level remains a major challenge. In this review we consider important cellular and physical factors in control of cell-cell mechanotransduction and discuss their significance for cell and developmental processes. We further highlight mechanosensitive macromolecules that are known to respond to external forces and present examples of how force responses can be integrated into cell and developmental programs.

Junctional ER Organization Affects Mechanotransduction at Cadherin-Mediated Adhesions

Cadherin-mediated adhesions (also known as adherens junctions) are adhesive complexes that connect neighboring cells in a tissue. While the role of the actin cytoskeleton in withstanding tension at these sites of contact is well documented, little is known about the involvement of microtubules and the associated endoplasmic reticulum (ER) network in cadherin mechanotransduction. Therefore, we investigated how the organization of ER extensions in close proximity of cadherin-mediated adhesions can affect such complexes, and vice versa. Here, we show that the extension of the ER to cadherin-mediated adhesions is tension dependent and appears to be cadherin-type specific. Furthermore, the different structural organization of the ER/microtubule network seems to affect the localization of ER-bound PTP1B at cadherin-mediated adhesions. This phosphatase is involved in the modulation of vinculin, a molecular clutch which enables differential engagement of the cadherin-catenin layer with the actomyosin cytoskeleton in response to tension. This suggests a link between structural organization of the ER/microtubule network around cadherin-specific adhesions, to control the mechanotransduction of adherens junctions by modulation of vinculin conformational state.

Focal adhesion dynamics in cellular function and disease

Acting as a bridge between the cytoskeleton of the cell and the extra cellular matrix (ECM), the cell-ECM adhesions with integrins at their core, play a major role in cell signalling to direct mechanotransduction, cell migration, cell cycle progression, proliferation, differentiation, growth and repair. Biochemically, these adhesions are composed of diverse, yet an organised group of structural proteins, receptors, adaptors, various enzymes including protein kinases, phosphatases, GTPases, proteases, etc. as well as scaffolding molecules. The major integrin adhesion complexes (IACs) characterised are focal adhesions (FAs), invadosomes (podosomes and invadopodia), hemidesmosomes (HDs) and reticular adhesions (RAs). The varied composition and regulation of the IACs and their signalling, apart from being an integral part of normal cell survival, has been shown to be of paramount importance in various developmental and pathological processes. This review per-illustrates the recent advancements in the research of IACs, their crucial roles in normal as well as diseased states. We have also touched on few of the various methods that have been developed over the years to visualise IACs, measure the forces they exert and study their signalling and molecular composition. Having such pertinent roles in the context of various pathologies, these IACs need to be understood and studied to develop therapeutical targets. We have given an update to the studies done in recent years and described various techniques which have been applied to study these structures, thereby, providing context in furthering research with respect to IAC targeted therapeutics.

Differential cellular responses to adhesive interactions with galectin-8- and fibronectin-coated substrates

The mechanisms underlying the cellular response to extracellular matrices (ECMs) that consist of multiple adhesive ligands are still poorly understood. Here, we address this topic by monitoring specific cellular responses to two different extracellular adhesion molecules - the main integrin ligand fibronectin and galectin-8, a lectin that binds β-galactoside residues  - as well as to mixtures of the two proteins. Compared with cell spreading on fibronectin, cell spreading on galectin-8-coated substrates resulted in increased projected cell area, more-pronounced extension of filopodia and, yet, the inability to form focal adhesions and stress fibers. These differences can be partially reversed by experimental manipulations of small G-proteins of the Rho family and their downstream targets, such as formins, the Arp2/3 complex and Rho kinase. We also show that the physical adhesion of cells to galectin-8 was stronger than adhesion to fibronectin. Notably, galectin-8 and fibronectin differently regulate cell spreading and focal adhesion formation, yet act synergistically to upregulate the number and length of filopodia. The physiological significance of the coherent cellular response to a molecularly complex matrix is discussed. This article has an associated First Person interview with the first author of the paper.

Understanding the cellular responses based on low-density electrospun fiber networks

Fibers produced from electrospinning are well-known to be extremely fine with diameters ranging from tens of nanometers to a few microns. Such ultrafine fibers not only allow for engineering scaffolds resembling the ultrastructure of the native extracellular matrix, but also offer possibility to explore the remodeling behavior of cells in vitro, due to their mechanically 'adequate' softness endowed by their ultrafine fineness. However, the remodeling effect of cells on the biomimicking fibrous substrates remains to be understood, because the crisscrossing and entangling among nanofibers in those tightly packed fibrous mats ultimately lead to merely a topological phenomenon, similar to that of the nanofiber-like topography embossed on the surface of a solid matter. In this study, the effect of nanofiber density on cellular response behavior was investigated by reducing the density of electrospun fiber networks. Using polycaprolactone (PCL) as a model polymer, randomly oriented fiber networks with various densities, namely, 37.7 ± 16.3 μg/cm² (D1), 103.8 ± 16.3 μg/cm² (D2), 198.2 ± 40.0 μg/cm² (D3), and 471.8 ± 32.7 μg/cm² (D4), were prepared by electrospinning for varied collection durations (10 s, 50 s, 100 s, and 10 min, respectively). By examining the responsive behavior of the human induced pluripotent stem cell-derived mesenchymal stem cells (hiPS-MSCs) cultured on these nanofibrous networks, we showed that the fiber network with a moderate density (D2) is beneficial to the cell attachment, spreading, actin polymerization, contractility and migration. There also showed an increased tendency in nuclear localization of the Yes-associated protein (YAP) and subsequent activation of YAP responsive gene transcription, and cell proliferation and collagen synthesis were also enhanced on the D2. However, further increasing the fiber density (D3, D4) gave rise to weakened induction effect of fibers on the cellular responses. These results enrich our understanding on the effect of fiber density on cell behavior, and disclose the dependence of cellular responses on fiber density. This study paves the way to precisely design biomimetic fibrous scaffolds for achieving enhanced cell-scaffold interactions and tissue regeneration.

Nanointeraction: The profound influence of nanostructured and nano-drug delivery biomedical implant surfaces on cell behavior

Nanostructured surfaces feature promising biological properties on biomaterials attracting large interest at basic research, implant industry development, and bioengineering applications. Thou, nanoscale interactions at a molecular and cellular level are not yet completely understood and its biological and clinical implications need to be further elucidated. As follows, the aim of this comprehensive review was to evaluate nanostructured surfaces at biomedical implants focusing on surface development, nanostructuration, and nanoengineered drug delivery systems that can induce specific cell interactions in all relevant aspects of biological, reparative, anti-bacterial, anti-inflammatory and clinical processes. The methods and the physio-chemical properties involved in nanotopography performance, the main cellular characteristics involved at surface/cell interaction, and a summary of results and outlooks reported in studies applying nanostructured surfaces and nano-drug delivery systems is presented. The future prospects and commercial translation of this developing field, particularly concerning multifunctional nanostructured surfaces and its clinical implications are further discussed. At a cellular level, nanostructured biomedical implant surfaces can enhance osteogenesis by targeting osteoblasts, osteocytes, and mesenchymal cells, stimulate fibroblast/epithelial cells proliferation and adherence, inhibit bacterial cell proliferation and biofilm accumulation, and act as immune-modulating surfaces targeting macrophages and reducing pro-inflammatory cytokine expression. Moreover, several methodological options to create drug-delivery systems on metallic implant surfaces are available, however, the clinical translation is yet incomplete. The efficiency of which nanostructured/nano-delivery surfaces may target specific cell interactions and favor clinical outcomes needs to be further elucidated in pre-clinical and clinical studies, along with engineering solutions for commercial translation and approval of controlling agencies.

On the mechanical response of the actomyosin cortex during cell indentations

A mechanical model is presented to analyze the mechanics and dynamics of the cell cortex during indentation. We investigate the impact of active contraction on the cross-linked actin network for different probe sizes and indentation rates. The essential molecular mechanisms of filament stretching, cross-linking and motor activity, are represented by an active and viscous mechanical continuum. The filaments behave as worm-like chains linked either by passive rigid linkers or by myosin motors. In the first example, the effects of probe size and loading rate are evaluated using the model for an idealized rounded cell shape in which properties are based on the results of parallel-plate rheometry available in the literature. Extreme cases of probe size and indentation rate are taken into account. Afterward, AFM experiments were done by engaging smooth muscle cells with both sharp and spherical probes. By inverse analysis with finite element software, our simulations mimicking the experimental conditions show the model is capable of fitting the AFM data. The results provide spatiotemporal dependence on the size and rate of the mechanical stimuli. The model captures the general features of the cell response. It characterizes the actomyosin cortex as an active solid at short timescales and as a fluid at longer timescales by showing (1) higher levels of contraction in the zones of high curvature; (2) larger indentation forces as the probe size increases; and (3) increase in the apparent modulus with the indentation depth but no dependence on the rate of the mechanical stimuli. The methodology presented in this work can be used to address and predict microstructural dependence on the force generation of living cells, which can contribute to understanding the broad spectrum of results in cell experiments.

New perspectives on integrin-dependent adhesions

Integrins are heterodimeric transmembrane receptors that connect the extracellular matrix environment to the actin cytoskeleton via adaptor molecules through assembly of a range of adhesion structures. Recent advances in biochemical, imaging and biophysical methods have enabled a deeper understanding of integrin signalling and their associated regulatory processes. The identification of the consensus integrin-based 'adhesomes' within the last 5 years has defined common core components of adhesion complexes and associated partners. These approaches have also uncovered unexpected adhesion protein behaviour and molecules recruited to adhesion sites that have expanded our understanding of the molecular and physical control of integrin signalling.

Influence of Platelet Lysate on 2D and 3D Amniotic Mesenchymal Stem Cell Cultures

The mechanobiological behavior of mesenchymal stem cells (MSCs) in two- (2D) or three-dimensional (3D) cultures relies on the formation of actin filaments which occur as stress fibers and depends on mitochondrial dynamics involving vimentin intermediate filaments. Here we investigate whether human platelet lysate (HPL), that can potentially replace fetal bovine serum for clinical-scale expansion of functional cells, can modulate the stress fiber formation, alter mitochondrial morphology, change membrane elasticity and modulate immune regulatory molecules IDO and GARP in amnion derived MSCs. We can provide evidence that culture supplementation with HPL led to a reduction of stress fiber formation in 2D cultured MSCs compared to a conventional growth medium (MSCGM). 3D MSC cultures, in contrast, showed decreased actin concentrations independent of HPL supplementation. When stress fibers were further segregated by their binding to focal adhesions, a reduction in ventral stress fibers was observed in response to HPL in 2D cultured MSCs, while the length of the individual ventral stress fibers increased. Dorsal stress fibers or transverse arcs were not affected. Interestingly, ventral stress fiber formation did not correlate with membrane elasticity. 2D cultured MSCs did not show differences in the Young's modulus when propagated in the presence of HPL and further cultivation to passage 3 also had no effect on membrane elasticity. In addition, HPL reduced the mitochondrial mass of 2D cultured MSCs while the mitochondrial mass in 3D cultured MSCs was low initially. When mitochondria were segregated into punctuate, rods and networks, a cultivation-induced increase in punctuate and network mitochondria was observed in 2D cultured MSCs of passage 3. Finally, mRNA and protein expression of the immunomodulatory molecule IDO relied on stimulation of 2D culture MSCs with pro-inflammatory cytokines IFN-γ and TNF-α with no effect upon HPL supplementation. GARP mRNA and surface expression was constitutively expressed and did not respond to HPL supplementation or stimulation with IFN-γ and TNF-α. In conclusion, we can say that MSCs cultivated in 2D and 3D are sensitive to medium supplementation with HPL with changes in actin filament formation, mitochondrial dynamics and membrane elasticity that can have an impact on the immunomodulatory function of MSCs.

Molecular Organization of Integrin-Based Adhesion Complexes in Mouse Embryonic Stem Cells

The mechanical microenvironment serves as an important factor influencing stem cell differentiation. Mechanobiological responses depend strongly on actomyosin contractility and integrin-based cell-extracellular matrix (ECM) interactions mediated by adhesive structures such as focal adhesions (FAs). While the roles of FAs in mechanobiology have been intensively studied in many mesenchymal and migratory cell types, recently it has been recognized that certain pluripotent stem cells (PSCs) exhibited significantly attenuated FA-mediated mechanobiological responses. FAs in such PSCs are sparsely distributed and much less prominent in comparison to "classical" FAs of typical adherent cells. Despite these differences, insights into how FAs in PSCs are structurally organized to perform their functions are still elusive. Using mouse embryonic stem cells (mESCs) to study PSC-ECM interactions, here we surveyed the molecular composition and nanostructural organization of FAs. We found that, despite being small in size, mESC FAs appeared to be compositionally mature, containing markers such as vinculin, zyxin, and α-actinin, and dependent on myosin II contractility. Using super-resolution microscopy, we revealed that mESC FAs were organized into a conserved multilayer nanoscale architecture. However, the nanodomain organization was compressed in mESCs, with the force transduction layer spanning ∼10 nm, significantly more compact than in FAs of other cell types. Furthermore, we found that the position and orientation of vinculin, a key mechanotransduction protein, were modulated in an ECM-dependent manner. Our analysis also revealed that while most core FA genes were expressed, the expression of LIM domain proteins was comparatively lower in PSCs. Altogether our results suggest that while core structural and mechanosensitive elements are operational in mESC FAs, their structural organization and regulatory aspects may diverge significantly from "classical" FAs, which may account for the attenuated mechanobiological responses of these cell types.

Visualizing the ‘backbone’ of focal adhesions

To understand how complex machines perform their functions, it is essential to map out the ‘blueprints’ of how their internal components are organized. Focal adhesions (FAs) are complex mechanobiological structures involved in a plethora of cell biological processes. The application of super-resolution microscopy in concert with protein engineering offers one approach to unravel the complexity of how individual proteins are organized within FAs. In our recent application, the FA protein talin was found to form a direct structural and physical link between integrin and actin. Interestingly, engineered talin constructs with alternate lengths rescaled the FA nanostructure accordingly. This helped establish that talin could be analogous to the backbone of FAs, serving as the mechanosensitive master coordinator of FA structural organization.

Computational modeling of single-cell mechanics and cytoskeletal mechanobiology

Cellular cytoskeletal mechanics plays a major role in many aspects of human health from organ development to wound healing, tissue homeostasis and cancer metastasis. We summarize the state-of-the-art techniques for mathematically modeling cellular stiffness and mechanics and the cytoskeletal components and factors that regulate them. We highlight key experiments that have assisted model parameterization and compare the advantages of different models that have been used to recapitulate these experiments. An overview of feed-forward mechanisms from signaling to cytoskeleton remodeling is provided, followed by a discussion of the rapidly growing niche of encapsulating feedback mechanisms from cytoskeletal and cell mechanics to signaling. We discuss broad areas of advancement that could accelerate research and understanding of cellular mechanobiology. A precise understanding of the molecular mechanisms that affect cell and tissue mechanics and function will underpin innovations in medical device technologies of the future. WIREs Syst Biol Med 2018, 10:e1407. doi: 10.1002/wsbm.1407 This article is categorized under: Models of Systems Properties and Processes > Mechanistic Models Physiology > Mammalian Physiology in Health and Disease Models of Systems Properties and Processes > Cellular Models.

Bridging the gap: Super-resolution microscopy of epithelial cell junctions

Cell junctions are critical for cell adhesion and communication in epithelial tissues. It is evident that the cellular distribution, size, and architecture of cell junctions play a vital role in regulating function. These details of junction architecture have been challenging to elucidate in part due to the complexity and size of cell junctions. A major challenge in understanding these features is attaining high resolution spatial information with molecular specificity. Fluorescence microscopy allows localization of specific proteins to junctions, but with a resolution on the same scale as junction size, rendering internal protein organization unobtainable. Super-resolution microscopy provides a bridge between fluorescence microscopy and nanoscale approaches, utilizing fluorescent tags to reveal protein organization below the resolution limit. Here we provide a brief introduction to super-resolution microscopy and discuss novel findings into the organization, structure and function of epithelial cell junctions.