Calpain-mediated proteolysis of talin regulates adhesion dynamics

Unveiling the role of astrogliosis in Alzheimer's disease Pathology: Insights into mechanisms and therapeutic approaches

Alzheimer's disease (AD) is one of the most debilitating age-related disorders that affect people globally. It impacts social and cognitive behavior of the individual and is characterized by phosphorylated tau and Aβ accumulation. Astrocytesmaintain a quiescent, anti-inflammatory state on anatomical level, expressing few cytokines and exhibit phagocytic activity to remove misfolded proteins. But in AD, in response to specific stimuli, astrocytes overstimulate their phagocytic character with overexpressing cytokine gene modules. Upon interaction with generated Aβ and neurofibrillary tangle, astrocytes that are continuously activated release a large number of inflammatory cytokines. This cytokine storm leads to neuroinflammation which is also one of the recognizable features of AD. Astrogliosis eventually promotes cholinergic dysfunction, calcium imbalance, oxidative stress and excitotoxicity. Furthermore, C5aR1, Lcn2/, BDNF/TrkB and PPARα/TFEB signaling dysregulation has a major impact on the disease progression. This review clarifies numerous ways that lead to astrogliosis, which is stimulated by a variety of processes that exacerbate AD pathology and make it a suitable target for AD treatment. Drugs under clinical and preclinical investigations that target several pathways managing astrogliosis and are efficacious in ameliorating the pathology of the disease are also included in this study. D-ALA2GIP, TRAM-34, Genistein, L-serine, MW150 and XPro1595 are examples of few drugs targeting astrogliosis. Therefore, this study may aid in the development of a potent therapeutic agent for ameliorating astrogliosis mediated AD progression.

Depletion of calpain2 accelerates epithelial barrier establishment and reduces growth factor-induced cell scattering

Calpain2 is a conventional member of the non-lysosomal calpain protease family that has been shown to affect the dynamics of focal and cell-cell adhesions by proteolyzing the components of adhesion complexes. Here, we inactivated calpain2 using CRISPR/Cas9 in epithelial MDCK cells. We show that depletion of calpain2 has multiple effects on cell morphology and function. Calpain2-depleted cells develop epithelial shape, however, they cover a smaller area, and cell clusters are more compact. Inactivation of calpain2 enhanced restoration of transepithelial electrical resistance after calcium switch, decreased cell migration, and delayed cell scattering induced by HGF/SF. In addition, calpain2 depletion prevented morphological changes induced by ERK2 overexpression. Interestingly, proteolysis of several calpain2 targets, including E-cadherin, β-catenin, talin, FAK, and paxillin, was not discernibly affected by calpain2 depletion. Taken together, these data suggest that calpain2 regulates the stability of cell-cell and cell-substratum adhesions indirectly without affecting the proteolysis of these adhesion complexes.

Enzyme Engineering by Force: DNA Springs for the Modulation of Biocatalytic Trajectories

The engineering of enzymatic activity generally involves alteration of the protein primary sequences, which introduce structural changes that give rise to functional improvements. Mechanical forces have been used to interrogate protein biophysics, leading to deep mechanistic insights in single-molecule studies. Here, we use simple DNA springs to apply small pulling forces to perturb the active site of a thermostable alcohol dehydrogenase. Methods were developed to enable the study of different spring lengths and spring orientations under bulk catalysis conditions. Tension applied across the active site expanded the binding pocket volume and shifted the preference of the enzyme for longer chain-length substrates, which could be tuned by altering the spring length and the resultant applied force. The substrate specificity changes did not occur when the DNA spring was either severed or rotated by ∼90°. These findings demonstrate an alternative approach in protein engineering, where active site architectures can be dynamically and reversibly remodeled using applied mechanical forces.

Foxp3-mediated blockage of ryanodine receptor 2 underlies contact-based suppression by regulatory T cells

The suppression mechanism of Tregs remains an intensely investigated topic. As our focus has shifted toward a model centered on indirect inhibition of DCs, a universally applicable effector mechanism controlled by the transcription factor forkhead box P3 (Foxp3) expression has not been found. Here, we report that Foxp3 blocked the transcription of ER Ca2+-release channel ryanodine receptor 2 (RyR2). Reduced RyR2 shut down basal Ca2+ oscillation in Tregs, which reduced m-calpain activities that are needed for T cells to disengage from DCs, suggesting a persistent blockage of DC antigen presentation. RyR2 deficiency rendered the CD4+ T cell pool immune suppressive and caused it to behave in the same manner as Foxp3+ Tregs in viral infection, asthma, hypersensitivity, colitis, and tumor development. In the absence of Foxp3, Ryr2-deficient CD4+ T cells rescued the systemic autoimmunity associated with scurfy mice. Therefore, Foxp3-mediated Ca2+ signaling inhibition may be a central effector mechanism of Treg immune suppression.

Calpain Promotes LPS-induced Lung Endothelial Barrier Dysfunction via Cleavage of Talin

Acute lung injury (ALI) is characterized by lung vascular endothelial cell (EC) barrier compromise resulting in increased endothelial permeability and pulmonary edema. The infection of gram-negative bacteria that produce toxins like LPS is one of the major causes of ALI. LPS activates Toll-like receptor 4, leading to cytoskeleton reorganization, resulting in lung endothelial barrier disruption and pulmonary edema in ALI. However, the signaling pathways that lead to the cytoskeleton reorganization and lung microvascular EC barrier disruption remain largely unexplored. Here we show that LPS induces calpain activation and talin cleavage into head and rod domains and that inhibition of calpain attenuates talin cleavage, RhoA activation, and pulmonary EC barrier disruption in LPS-treated human lung microvascular ECs in vitro and lung EC barrier disruption and pulmonary edema induced by LPS in ALI in vivo. Moreover, overexpression of calpain causes talin cleavage and RhoA activation, myosin light chain (MLC) phosphorylation, and increases in actin stress fiber formation. Furthermore, knockdown of talin attenuates LPS-induced RhoA activation and MLC phosphorylation and increased stress fiber formation and mitigates LPS-induced lung microvascular endothelial barrier disruption. Additionally, overexpression of talin head and rod domains increases RhoA activation, MLC phosphorylation, and stress fiber formation and enhances lung endothelial barrier disruption. Finally, overexpression of cleavage-resistant talin mutant reduces LPS-induced increases in MLC phosphorylation in human lung microvascular ECs and attenuates LPS-induced lung microvascular endothelial barrier disruption. These results provide the first evidence that calpain mediates LPS-induced lung microvascular endothelial barrier disruption in ALI via cleavage of talin.

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.

Joining forces: crosstalk between mechanosensitive PIEZO1 ion channels and integrin-mediated focal adhesions

Both integrin-mediated focal adhesions (FAs) and mechanosensitive ion channels such as PIEZO1 are critical in mechanotransduction processes that influence cell differentiation, development, and cancer. Ample evidence now exists for regulatory crosstalk between FAs and PIEZO1 channels with the molecular mechanisms underlying this process remaining unclear. However, an emerging picture is developing based on spatial crosstalk between FAs and PIEZO1 revealing a synergistic model involving the cytoskeleton, extracellular matrix (ECM) and calcium-dependent signaling. Already cell type, cell contractility, integrin subtypes and ECM composition have been shown to regulate this crosstalk, implying a highly fine-tuned relationship between these two major mechanosensing systems. In this review, we summarize the latest advances in this area, highlight the physiological implications of this crosstalk and identify gaps in our knowledge that will improve our understanding of cellular mechanosensing.

Force-dependent focal adhesion assembly and disassembly: A computational study

Cells interact with the extracellular matrix (ECM) via cell-ECM adhesions. These physical interactions are transduced into biochemical signals inside the cell which influence cell behaviour. Although cell-ECM interactions have been studied extensively, it is not completely understood how immature (nascent) adhesions develop into mature (focal) adhesions and how mechanical forces influence this process. Given the small size, dynamic nature and short lifetimes of nascent adhesions, studying them using conventional microscopic and experimental techniques is challenging. Computational modelling provides a valuable resource for simulating and exploring various "what if?" scenarios in silico and identifying key molecular components and mechanisms for further investigation. Here, we present a simplified mechano-chemical model based on ordinary differential equations with three major proteins involved in adhesions: integrins, talin and vinculin. Additionally, we incorporate a hypothetical signal molecule that influences adhesion (dis)assembly rates. We find that assembly and disassembly rates need to vary dynamically to limit maturation of nascent adhesions. The model predicts biphasic variation of actin retrograde velocity and maturation fraction with substrate stiffness, with maturation fractions between 18-35%, optimal stiffness of ∼1 pN/nm, and a mechanosensitive range of 1-100 pN/nm, all corresponding to key experimental findings. Sensitivity analyses show robustness of outcomes to small changes in parameter values, allowing model tuning to reflect specific cell types and signaling cascades. The model proposes that signal-dependent disassembly rate variations play an underappreciated role in maturation fraction regulation, which should be investigated further. We also provide predictions on the changes in traction force generation under increased/decreased vinculin concentrations, complementing previous vinculin overexpression/knockout experiments in different cell types. In summary, this work proposes a model framework to robustly simulate the mechanochemical processes underlying adhesion maturation and maintenance, thereby enhancing our fundamental knowledge of cell-ECM interactions.

A genome-wide in vivo CRISPR screen identifies essential regulators of T cell migration to the CNS in a multiple sclerosis model

Multiple sclerosis (MS) involves the infiltration of autoreactive T cells into the CNS, yet we lack a comprehensive understanding of the signaling pathways that regulate this process. Here, we conducted a genome-wide in vivo CRISPR screen in a rat MS model and identified 5 essential brakes and 18 essential facilitators of T cell migration to the CNS. While the transcription factor ETS1 limits entry to the CNS by controlling T cell responsiveness, three functional modules, centered around the adhesion molecule α4-integrin, the chemokine receptor CXCR3 and the GRK2 kinase, are required for CNS migration of autoreactive CD4+ T cells. Single-cell analysis of T cells from individuals with MS confirmed that the expression of these essential regulators correlates with the propensity of CD4+ T cells to reach the CNS. Our data thus reveal key regulators of the fundamental step in the induction of MS lesions.

The US21 viroporin of human cytomegalovirus stimulates cell migration and adhesion

The human cytomegalovirus (HCMV) US12 gene family contributes to virus-host interactions by regulating the virus' cell tropism and its evasion of host innate immune responses. US21, one of the 10 US12 genes (US12-US21), is a descendant of a captured cellular transmembrane BAX inhibitor motif-containing gene. It encodes a 7TMD endoplasmic reticulum (ER)-resident viroporin (pUS21) capable of reducing the Ca2+ content of ER stores, which, in turn, protects cells against apoptosis. Since regulation of Ca2+ homeostasis affects a broad range of cellular responses, including cell motility, we investigated whether pUS21 might also interfere with this cytobiological consequence of Ca2+ signaling. Indeed, deletion of the US21 gene impaired the ability of HCMV-infected cells to migrate, whereas expression of US21 protein stimulated cell migration and adhesion, as well as focal adhesion (FA) dynamics, in a way that depended on its ability to manipulate ER Ca2+ content. Mechanistic studies revealed pUS21-mediated cell migration to involve calpain 2 activation since its inhibition prevented the viroporin's effects on cell motility. Pertinently, pUS21 expression stimulated a store-operated Ca2+ entry (SOCE) mechanism that may determine the activation of calpain 2 by promoting Ca2+ entry. Furthermore, pUS21 was observed to interact with talin-1, a calpain 2 substrate, and crucial protein component of FA complexes. A functional consequence of this interaction was confirmed by talin-1 knockdown, which abrogated the pUS21-mediated increase in cell migration. Together, these results indicate the US21-encoded viroporin to be a viral regulator of cell adhesion and migration in the context of HCMV infection. IMPORTANCE Human cytomegalovirus (HCMV) is an opportunistic pathogen that owes part of its success to the capture, duplication, and tuning of cellular genes to generate modern viral proteins which promote infection and persistence in the host by interfering with many cell biochemical and physiological pathways. The US21 viral protein provides an example of this evolutionary strategy: it is a cellular-derived calcium channel that manipulates intracellular calcium homeostasis to confer edges to HCMV replication. Here, we report on the characterization of a novel function of the US21 protein as a viral regulator of cell migration and adhesion through mechanisms involving its calcium channel activity. Characterization of HCMV multifunctional regulatory proteins, like US21, supports the better understanding of viral pathogenesis and may open avenues for the design of new antiviral strategies that exploit their functions.

Membrane adhesion junctions regulate airway smooth muscle phenotype and function

The local environment surrounding airway smooth muscle (ASM) cells has profound effects on the physiological and phenotypic properties of ASM tissues. ASM is continually subjected to the mechanical forces generated during breathing and to the constituents of its surrounding extracellular milieu. The smooth muscle cells within the airways continually modulate their properties to adapt to these changing environmental influences. Smooth muscle cells connect to the extracellular cell matrix (ECM) at membrane adhesion junctions that provide mechanical coupling between smooth muscle cells within the tissue. Membrane adhesion junctions also sense local environmental signals and transduce them to cytoplasmic and nuclear signaling pathways in the ASM cell. Adhesion junctions are composed of clusters of transmembrane integrin proteins that bind to ECM proteins outside the cell and to large multiprotein complexes in the submembranous cytoplasm. Physiological conditions and stimuli from the surrounding ECM are sensed by integrin proteins and transduced by submembranous adhesion complexes to signaling pathways to the cytoskeleton and nucleus. The transmission of information between the local environment of the cells and intracellular processes enables ASM cells to rapidly adapt their physiological properties to modulating influences in their extracellular environment: mechanical and physical forces that impinge on the cell, ECM constituents, local mediators, and metabolites. The structure and molecular organization of adhesion junction complexes and the actin cytoskeleton are dynamic and constantly changing in response to environmental influences. The ability of ASM to rapidly accommodate to the ever-changing conditions and fluctuating physical forces within its local environment is essential for its normal physiological function.

Traction Force and Mechanosensing can be Functionally Distinguished Through the Use of Specific Domains of the Calpain Small Subunit

Cell migration is a fundamental process pertaining to many critical physiological events. The ability to form and release adhesion structures is necessary for cell migration. The Calpain family of cysteine proteases are known to target adhesion proteins as their substrates and modulate adhesion dynamics. The two best studied Calpains, Calpain 1 and Calpain 2 form catalytically active holoenzymes through heterodimerization with a common non-catalytic regulatory small subunit known as Calpain 4. In previous studies, we determined that calpains are important in the production of traction forces and in the sensing of localized mechanical stimulation from the external environment. We found that perturbation of either Calpain 1 or 2 had no effect on the generation of traction forces. However, traction forces were weak when Calpain 4 was silenced. On the other hand, silencing of Calpain 1, 2, or 4 resulted in deficient sensing of external mechanical stimuli. These results together suggest that Calpain 4 functions independent of the catalytic large subunits in the generation of traction forces but functions together with either catalytic subunit in sensing external mechanical stimuli. The small subunit Calpain 4 contains 268 a.a. and is composed of 2 domains, the N-terminal domain V and C-terminal domain VI. Domain VI is a calmodulinlike domain containing five consecutive EF-hand motifs, of which the fifth one heterodimerizes with a large subunit. Moreover, domain V contains the common sequence GTAMRILGGVI that suggests cell membrane interactions. Given these attributes of domain V and VI of Calpain 4, we speculated that an individual domain might provide the functional properties for either traction or sensing. Therefore, each domain was cloned and expressed individually in Capn4-/- cells and assayed for traction and sensing. Results revealed that over-expression of domain V was sufficient to rescue the traction forces defect in Capn4-/- cells while overexpression of domain VI did not rescue the traction force. Consistent with our hypothesis, overexpression of domain VI rescued the sensing defect in Capn4-/- cells while overexpression of domain V had no effect. These results suggest that individual domains of Calpain 4 do indeed function independently to regulate either traction force or the sensing of external stimuli. We speculate that membrane association of Calpain 4 is required for the regulation of traction force and its association with a catalytic subunit is necessary for mechanosensing.

The βI domain promotes active β1 integrin clustering into mature adhesion sites

Modulation of integrin function is required in many physiological and pathological settings, such as angiogenesis and cancer. Integrin allosteric changes, clustering, and trafficking cooperate to regulate cell adhesion and motility on extracellular matrix proteins via mechanisms that are partly defined. By exploiting four monoclonal antibodies recognizing distinct conformational epitopes, we show that in endothelial cells (ECs), the extracellular βI domain, but not the hybrid or I-EGF2 domain of active β1 integrins, promotes their FAK-regulated clustering into tensin 1-containing fibrillar adhesions and impairs their endocytosis. In this regard, the βI domain-dependent clustering of active β1 integrins is necessary to favor fibronectin-elicited directional EC motility, which cannot be effectively promoted by β1 integrin conformational activation alone.

Cell shape and tension alter focal adhesion structure

Cells are not only anchored to the extracellular matrix via the focal adhesion complex, the focal adhesion complex also serves as a sensor for force transduction. How tension influences the structure of focal adhesions is not well understood. Here, we analyse the effect of tension on the location of key focal adhesion proteins, namely vinculin, paxillin and actin. We use micropatterning on gold surfaces to manipulate the cell shape, to create focal adhesions at specific cell areas, and to perform metal-induced energy transfer (MIET) measurements on the patterned cells. MIET resolves the different protein locations with respect to the gold surface with nanometer accuracy. Further, we use drugs influencing the cellular motor protein myosin or mechanosensitive ion channels to get deeper insight into focal adhesions at different tension states. We show here that in particular actin is affected by the rationally tuned force balance. Blocking mechanosensitive ion channels has a particularly high influence on the actin and focal adhesion architecture, resulting in larger focal adhesions with elevated paxillin and vinculin and strongly lowered actin stress fibres. Our results can be explained by a balance of adhesion tension with cellular tension together with ion channel-controlled focal adhesion homeostasis, where high cellular tension leads to an elevation of vinculin and actin, while high adhesion tension lowers these proteins.

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.

Force- and cell state-dependent recruitment of Piezo1 drives focal adhesion dynamics and calcium entry

Mechanosensing is an integral part of many physiological processes including stem cell differentiation, fibrosis, and cancer progression. Two major mechanosensing systems-focal adhesions and mechanosensitive ion channels-can convert mechanical features of the microenvironment into biochemical signals. We report here unexpectedly that the mechanosensitive calcium-permeable channel Piezo1, previously perceived to be diffusive on plasma membranes, binds to matrix adhesions in a force-dependent manner, promoting cell spreading, adhesion dynamics, and calcium entry in normal but not in most cancer cells tested except some glioblastoma lines. A linker domain in Piezo1 is needed for binding to adhesions, and overexpression of the domain blocks Piezo1 binding to adhesions, decreasing adhesion size and cell spread area. Thus, we suggest that Piezo1 is a previously unidentified component of focal adhesions in nontransformed cells that catalyzes adhesion maturation and growth through force-dependent calcium signaling, but this function is absent in most cancer cells.

The interplay between physical cues and mechanosensitive ion channels in cancer metastasis

Physical cues have emerged as critical influencers of cell function during physiological processes, like development and organogenesis, and throughout pathological abnormalities, including cancer progression and fibrosis. While ion channels have been implicated in maintaining cellular homeostasis, their cell surface localization often places them among the first few molecules to sense external cues. Mechanosensitive ion channels (MICs) are especially important transducers of physical stimuli into biochemical signals. In this review, we describe how physical cues in the tumor microenvironment are sensed by MICs and contribute to cancer metastasis. First, we highlight mechanical perturbations, by both solid and fluid surroundings typically found in the tumor microenvironment and during critical stages of cancer cell dissemination from the primary tumor. Next, we describe how Piezo1/2 and transient receptor potential (TRP) channels respond to these physical cues to regulate cancer cell behavior during different stages of metastasis. We conclude by proposing alternative mechanisms of MIC activation that work in tandem with cytoskeletal components and other ion channels to bestow cells with the capacity to sense, respond and navigate through the surrounding microenvironment. Collectively, this review provides a perspective for devising treatment strategies against cancer by targeting MICs that sense aberrant physical characteristics during metastasis, the most lethal aspect of cancer.

Local synthesis of the phosphatidylinositol-3,4-bisphosphate lipid drives focal adhesion turnover

Focal adhesions are multifunctional organelles that couple cell-matrix adhesion to cytoskeletal force transmission and signaling and to steer cell migration and collective cell behavior. Whereas proteomic changes at focal adhesions are well understood, little is known about signaling lipids in focal adhesion dynamics. Through the characterization of cells from mice with a kinase-inactivating point mutation in the class II PI3K-C2β, we find that generation of the phosphatidylinositol-3,4-bisphosphate (PtdIns(3,4)P₂) membrane lipid promotes focal adhesion disassembly in response to changing environmental conditions. We show that reduced growth factor signaling sensed by protein kinase N, an mTORC2 target and effector of RhoA, synergizes with the adhesion disassembly factor DEPDC1B to induce local synthesis of PtdIns(3,4)P₂ by PI3K-C2β. PtdIns(3,4)P₂ then promotes turnover of RhoA-dependent stress fibers by recruiting the PtdIns(3,4)P₂-dependent RhoA-GTPase-activating protein ARAP3. Our findings uncover a pathway by which cessation of growth factor signaling facilitates cell-matrix adhesion disassembly via a phosphoinositide lipid switch.

The key role of Calpain in COVID-19 as a therapeutic strategy

COVID-19 is one of the viral diseases that has caused many deaths and financial losses to humans. Using the available information, this virus appears to activate the host cell-death mechanism through Calpain activation. Calpain inhibition can stop its downstream cascade reactions that cause cell death. Given the main roles of Calpain in the entry and pathogenicity of the SARS-CoV-2, its inhibition can be effective in controlling the COVID-19. This review describes how the virus activates Calpain by altering calcium flow. When Calpain was activated, the virus can enter the target cell. Subsequently, many complications of the disease, such as inflammation, cytokine storm and pulmonary fibrosis, are caused by virus-activated Calpain function. Calpain inhibitors appear to be a potential drug to control the disease and prevent death from COVID-19.

Activation and Migration of Human Skeletal Muscle Stem Cells In Vitro Differently Rely on Calcium Signals

Muscle regeneration is essential for proper muscle homeostasis and relies primarily on muscle stem cells (MuSC). MuSC are maintained quiescent in their niche and can be activated following muscle injury. Using an in vitro model of primary human quiescent MuSC (called reserve cells, RC), we analyzed their Ca²⁺ response following their activation by fetal calf serum and assessed the role of Ca²⁺ in the processes of RC activation and migration. The results showed that RC displayed a high response heterogeneity in a cell-dependent manner following serum stimulation. Most of these responses relied on inositol 1,4,5-trisphosphate (IP₃)-dependent Ca²⁺ release associated with Ca²⁺ influx, partly due to store-operated calcium entry. Our study further found that blocking the IP₃ production, Ca²⁺ influx, or both did not prevent the activation of RC. Intra- or extracellular Ca²⁺ chelation did not impede RC activation. However, their migration potential depended on Ca²⁺ responses displayed upon stimulation, and Ca²⁺ blockers inhibited their movement. We conclude that the two major steps of muscle regeneration, namely the activation and migration of MuSC, differently rely on Ca²⁺ signals.

Focal adhesion-mediated cell anchoring and migration: from in vitro to in vivo

Cell-extracellular matrix interactions have been studied extensively using cells cultured in vitro. These studies indicate that focal adhesion (FA)-based cell-extracellular matrix interactions are essential for cell anchoring and cell migration. Whether FAs play a similarly important role in vivo is less clear. Here, we summarize the formation and function of FAs in cultured cells and review how FAs transmit and sense force in vitro. Using examples from animal studies, we also describe the role of FAs in cell anchoring during morphogenetic movements and cell migration in vivo. Finally, we conclude by discussing similarities and differences in how FAs function in vitro and in vivo.

Cytoskeletal protein degradation in brain death donor kidneys associates with adverse posttransplant outcomes

In brain death, cerebral injury contributes to systemic biological dysregulation, causing significant cellular stress in donor kidneys adversely impacting the quality of grafts. Here, we hypothesized that donation after brain death (DBD) kidneys undergo proteolytic processes that may deem grafts susceptible to posttransplant dysfunction. Using mass spectrometry and immunoblotting, we mapped degradation profiles of cytoskeletal proteins in deceased and living donor kidney biopsies. We found that key cytoskeletal proteins in DBD kidneys were proteolytically cleaved, generating peptide fragments, predominantly in grafts with suboptimal posttransplant function. Interestingly, α-actinin-4 and talin-1 proteolytic fragments were detected in brain death but not in circulatory death or living donor kidneys with similar donor characteristics. As talin-1 is a specific proteolytic target of calpain-1, we investigated a potential trigger of calpain activation and talin-1 degradation using human ex vivo precision-cut kidney slices and in vitro podocytes. Notably, we showed that activation of calpain-1 by transforming growth factor-β generated proteolytic fragments of talin-1 that matched the degradation fragments detected in DBD preimplantation kidneys, also causing dysregulation of the actin cytoskeleton in human podocytes; events that were reversed by calpain-1 inhibition. Our data provide initial evidence that brain death donor kidneys are more susceptible to cytoskeletal protein degradation. Correlation to posttransplant outcomes may be established by future studies.

Application of piconewton forces to individual filopodia reveals mechanosensory role of L-type Ca2+ channels

Filopodia are ubiquitous membrane projections that play crucial role in guiding cell migration on rigid substrates and through extracellular matrix by utilizing yet unknown mechanosensing molecular pathways. As recent studies show that Ca²⁺ channels localized to filopodia play an important role in regulation of their formation and since some Ca²⁺ channels are known to be mechanosensitive, force-dependent activity of filopodial Ca²⁺ channels might be linked to filopodia's mechanosensing function. We tested this hypothesis by monitoring changes in the intra-filopodial Ca²⁺ level in response to application of stretching force to individual filopodia of several cell types using optical tweezers. Results show that stretching forces of tens of pN strongly promote Ca²⁺ influx into filopodia, causing persistent Ca²⁺ oscillations that last for minutes even after the force is released. Several known mechanosensitive Ca²⁺ channels, such as Piezo 1, Piezo 2 and TRPV4, were found to be dispensable for the observed force-dependent Ca²⁺ influx, while L-type Ca²⁺ channels appear to be a key player in the discovered phenomenon. As previous studies have shown that intra-filopodial transient Ca²⁺ signals play an important role in guidance of cell migration, our results suggest that the force-dependent activation of L-type Ca²⁺ channels may contribute to this process. Overall, our study reveals an intricate interplay between mechanical forces and Ca²⁺ signaling in filopodia, providing novel mechanistic insights for the force-dependent filopodia functions in guidance of cell migration.

Calpain as a therapeutic target in cancer

INTRODUCTION

Calpain-1 and calpain-2 are prototypical classical isoforms of the calpain family of calcium-activated cysteine proteases. Their substrate proteins participate in a wide range of cellular processes, including transcription, survival, proliferation, apoptosis, migration, and invasion. Dysregulated calpain activity has been implicated in tumorigenesis, suggesting that calpains may be promising therapeutic targets.

AREAS COVERED

This review covers clinical and basic research studies implicating calpain-1 and calpain-2 expression and activity in tumorigenesis and metastasis. We highlight isoform specific functions and provide an overview of substrates and cancer-related signalling pathways affected by calpain-mediated proteolytic cleavage. We also discuss efforts to develop clinically relevant calpain specific inhibitors and spotlight the challenges facing inhibitor development.

EXPERT OPINION

Rationale for targeting calpain-1 and calpain-2 in cancer is supported by pre-clinical and clinical studies demonstrating that calpain inhibition has the potential to attenuate carcinogenesis and block metastasis of aggressive tumors. The wide range of substrates and cleavage products, paired with inconsistencies in model systems, underscores the need for more complete understanding of physiological substrates and how calpain cleavage alters their function in cellular processes. The development of isoform specific calpain inhibitors remains an important goal with therapeutic potential in cancer and other diseases.

Metastasising Fibroblasts Show an HDAC6-Dependent Increase in Migration Speed and Loss of Directionality Linked to Major Changes in the Vimentin Interactome

Metastasising cells express the intermediate filament protein vimentin, which is used to diagnose invasive tumours in the clinic. We aimed to clarify how vimentin regulates the motility of metastasising fibroblasts. STED super-resolution microscopy, live-cell imaging and quantitative proteomics revealed that oncogene-expressing and metastasising fibroblasts show a less-elongated cell shape, reduced cell spreading, increased cell migration speed, reduced directionality, and stronger coupling between these migration parameters compared to normal control cells. In total, we identified and compared 555 proteins in the vimentin interactome. In metastasising cells, the levels of keratin 18 and Rab5C were increased, while those of actin and collagen were decreased. Inhibition of HDAC6 reversed the shape, spreading and migration phenotypes of metastasising cells back to normal. Inhibition of HDAC6 also decreased the levels of talin 1, tropomyosin, Rab GDI β, collagen and emilin 1 in the vimentin interactome, and partially reversed the nanoscale vimentin organisation in oncogene-expressing cells. These findings describe the changes in the vimentin interactome and nanoscale distribution that accompany the defective cell shape, spreading and migration of metastasising cells. These results support the hypothesis that oncogenes can act through HDAC6 to regulate the vimentin binding of the cytoskeletal and cell–extracellular matrix adhesion components that contribute to the defective motility of metastasising cells.

Calpain-2 regulates hypoxia/HIF-induced plasticity toward amoeboid cancer cell migration and metastasis

Hypoxia, through hypoxia inducible factor (HIF), drives cancer cell invasion and metastatic progression in various cancer types. In epithelial cancer, hypoxia induces the transition to amoeboid cancer cell dissemination, yet the molecular mechanisms, relevance for metastasis, and effective intervention to combat hypoxia-induced amoeboid reprogramming remain unclear. Here, we identify calpain-2 as a key regulator and anti-metastasis target of hypoxia-induced transition from collective to amoeboid dissemination of breast and head and neck (HN) carcinoma cells. Hypoxia-induced amoeboid dissemination occurred through low extracellular matrix (ECM)-adhesive, predominantly bleb-based amoeboid movement, which was maintained by a low-oxidative and -glycolytic energy metabolism ("eco-mode"). Hypoxia induced calpain-2-mediated amoeboid conversion by deactivating β1 integrins through enzymatic cleavage of the focal adhesion adaptor protein talin-1. Consequently, targeted downregulation or pharmacological inhibition of calpain-2 restored talin-1 integrity and β1 integrin engagement and reverted amoeboid to elongated phenotypes under hypoxia. Calpain-2 activity was required for hypoxia-induced amoeboid conversion in the orthotopic mouse dermis and upregulated in invasive HN tumor xenografts in vivo, and attenuation of calpain activity prevented hypoxia-induced metastasis to the lungs. This identifies the calpain-2/talin-1/β1 integrin axis as a druggable mechanosignaling program that conserves energy yet enables metastatic dissemination that can be reverted by interfering with calpain activity.

Cancer biology: Hypoxia-induced talin tail-docking sparks cancer metastasis

Hypoxia drives cancer metastasis and induces cancer cells to switch from collective to amoeboid migration. A new study identifies a molecular pathway in which hypoxia stimulates calpain-2-mediated cleavage of talin-1, resulting in a reduction of integrin β1 activity and the promotion of blebbing amoeboid cancer cell migration and metastasis.

Cleavage of talin by calpain promotes platelet-mediated fibrin clot contraction

Calpain-catalyzed talin cleavage in platelets promotes fibrin clot contraction.

Calpain cleaves talin in proximity to vinculin binding sites, likely initiating vinculin binding to talin to promote clot contraction.

Blood clot contraction is driven by traction forces generated by the platelet cytoskeleton that are transmitted to fibrin fibers via the integrin αIIbβ3. Here we show that clot contraction is impaired by inhibitors of the platelet cytosolic protease calpain. We used subtiligase-mediated labeling of amino termini and mass spectrometry to identify proteolytically cleaved platelet proteins involved in clot contraction. Of 32 calpain-cleaved proteins after TRAP stimulation, 14 were cytoskeletal, most prominently talin and vinculin. A complex of talin and vinculin constitutes a mechanosensitive clutch connecting integrins bound to the extracellular matrix with the actin cytoskeleton. Accordingly, we focused on talin and vinculin. Talin is composed of an N-terminal head domain and a C-terminal rod domain organized into a series of 4- and 5-helix bundles. The bundles contain 11 vinculin binding sites (VBSs), each of which is an α-helix packed into a bundle interior and requiring structural rearrangement to initiate vinculin binding. We detected 8 calpain-mediated cleavages in talin, 2 previously identified in unstructured regions and 6 in α-helical regions in proximity to a VBS. There is evidence in vitro that applying mechanical force across talin enables vinculin binding to the talin rod. However, we found that inhibiting platelet cytoskeletal contraction had no effect on talin cleavage, indicating that talin cleavage by calpain in platelets does not require cytoskeleton-generated tensile force. Therefore, it is likely that calpain acts in the later stages of clot retraction through focal adhesion disassembly.

Talin in mechanotransduction and mechanomemory at a glance

Talins are cytoskeletal linker proteins that consist of an N-terminal head domain, a flexible neck region and a C-terminal rod domain made of 13 helical bundles. The head domain binds integrin β-subunit cytoplasmic tails, which triggers integrin conformational activation to increase affinity for extracellular matrix proteins. The rod domain links to actin filaments inside the cell to transmit mechanical loads and serves as a mechanosensitive signalling hub for the recruitment of many other proteins. The α-helical bundles function as force-dependent switches - proteins that interact with folded bundles are displaced when force induces unfolding, exposing previously cryptic binding sites for other ligands. This leads to the notion of a talin code. In this Cell Science at a Glance article and the accompanying poster, we propose that the multiple switches within the talin rod function to process and store time- and force-dependent mechanical and chemical information.

Nonlinear effects of environmental salinity on the gill transcriptome versus proteome of Oreochromis niloticus modulate epithelial cell turnover

A data-independent acquisition (DIA) assay library for targeted quantitation of thousands of Oreochromis niloticus gill proteins using a label- and gel-free workflow was generated and used to compare protein and mRNA abundances. This approach generated complimentary rather than redundant data for 1899 unique genes in gills of tilapia acclimated to freshwater and brackish water. Functional enrichment analyses identified mitochondrial energy metabolism, serine protease and immunity-related functions, and cytoskeleton/ extracellular matrix organization as major processes controlled by salinity in O. niloticus gills. Non-linearity in salinity-dependent transcriptome versus proteome regulation was revealed for specific functional groups of genes. The relationship was more linear for other molecular functions/ cellular processes, suggesting that the salinity-dependent regulation of O. niloticus gill function relies on post-transcriptional mechanisms for some functions/ processes more than others. This integrative systems biology approach can be adopted for other tissues and organisms to study cellular dynamics for many changing ecological contexts.

Pleural mesothelial cell migration into lung parenchyma by calpain contributes to idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is defined as a specific form of chronic, progressive fibrosing interstitial pneumonia. It is unknown why fibrosis in IPF distributes in the peripheral or named sub-pleural area. Migration of pleural mesothelial cells (PMC) should contribute to sub-pleural fibrosis. Calpain is known to be involved in cell migration, but the role of calpain in PMC migration has not been investigated. In this study, we found that PMCs migrated into lung parenchyma in patients with IPF. Then using Wt1^{tm1(EGFP/Cre)Wtp} /J knock-in mice, we observed PMC migration into lung parenchyma in bleomycin-induced pleural fibrosis models, and calpain inhibitor attenuated pulmonary fibrosis with prevention of PMC migration. In vitro studies revealed that bleomycin and transforming growth factor-β1 increased calpain activity in PMCs, and activated calpain-mediated focal adhesion (FA) turnover as well as cell migration, cell proliferation, and collagen-I synthesis. Furthermore, we determined that calpain cleaved FA kinase in both C-terminal and N-terminal regions, which mediated FA turnover. Lastly, the data revealed that activated calpain was also involved in phosphorylation of cofilin-1, and p-cofilin-1 induced PMC migration. Taken together, this study provides evidence that calpain mediates PMC migration into lung parenchyma to promote sub-pleural fibrosis in IPF.

Altered endocytosis in cellular senescence

Cellular senescence occurs in response to diverse stresses (e.g., telomere shortening, DNA damage, oxidative stress, oncogene activation). A growing body of evidence indicates that alterations in multiple components of endocytic pathways contribute to cellular senescence. Clathrin-mediated endocytosis (CME) and caveolae-mediated endocytosis (CavME) represent major types of endocytosis that are implicated in senescence. More recent research has also identified a chromatin modifier and tumor suppressor that contributes to the induction of senescence via altered endocytosis. Here, molecular regulators of aberrant endocytosis-induced senescence are reviewed and discussed in the context of their capacity to serve as senescence-inducing stressors or modifiers.

Synthetic dysmobility screen unveils an integrated STK40-YAP-MAPK system driving cell migration

Integrating signals is essential for cell survival, leading to the concept of synthetic lethality. However, how signaling is integrated to control cell migration remains unclear. By conducting a "two-hit" screen, we revealed the synergistic reduction of cell migration when serine-threonine kinase 40 (STK40) and mitogen-activated protein kinase (MAPK) were simultaneously suppressed. Single-cell analyses showed that STK40 knockdown reduced cell motility and coordination by strengthening focal adhesion (FA) complexes. Furthermore, STK40 knockdown reduced the stability of yes-associated protein (YAP) and subsequently decreased YAP transported into the nucleus, while MAPK inhibition further weakened YAP activities in the nucleus to disturb FA remodeling. Together, we unveiled an integrated STK40-YAP-MAPK system regulating cell migration and introduced "synthetic dysmobility" as a novel strategy to collaboratively control cell migration.

The proprotein convertase furin inhibits IL-13-induced inflammation in airway smooth muscle by regulating integrin-associated signaling complexes

Furin is a proprotein convertase that regulates the activation and the inactivation of multiple proteins including matrix metalloproteinases, integrins, and cytokines. It is a serine endoprotease that localizes to the plasma membrane and can be secreted into the extracellular space. The role of furin in regulating inflammation in isolated canine airway smooth muscle tissues was investigated. The treatment of airway tissues with recombinant furin (rFurin) inhibited the activation of Akt and eotaxin secretion induced by IL-13, and it prevented the IL-13-induced suppression of smooth muscle myosin heavy chain expression. rFurin promoted a differentiated phenotype by activating β₁-integrin proteins and stimulating the activation of the adhesome proteins vinculin and paxillin by talin. Activated paxillin induced the binding of Akt to β-parvin IPP [integrin-linked kinase (ILK), PINCH, parvin] complexes, which inhibits Akt activation. Treatment of tissues with a furin inhibitor or the depletion of endogenous furin using shRNA resulted in Akt activation and inflammatory responses similar to those induced by IL-13. Furin inactivation or IL-13 caused talin cleavage and integrin inactivation, resulting in the inactivation of vinculin and paxillin. Paxillin inactivation resulted in the coupling of Akt to α-parvin IPP complexes, which catalyze Akt activation and an inflammatory response. The results demonstrate that furin inhibits inflammation in airway smooth muscle induced by IL-13 and that the anti-inflammatory effects of furin are mediated by activating integrin proteins and integrin-associated signaling complexes that regulate Akt-mediated pathways to the nucleus. Furin may have therapeutic potential for the treatment of inflammatory conditions of the lungs and airways.

Store Operated Calcium Entry in Cell Migration and Cancer Metastasis

Ca²⁺ signaling is ubiquitous in eukaryotic cells and modulates many cellular events including cell migration. Directional cell migration requires the polarization of both signaling and structural elements. This polarization is reflected in various Ca²⁺ signaling pathways that impinge on cell movement. In particular, store-operated Ca²⁺ entry (SOCE) plays important roles in regulating cell movement at both the front and rear of migrating cells. SOCE represents a predominant Ca²⁺ influx pathway in non-excitable cells, which are the primary migrating cells in multicellular organisms. In this review, we summarize the role of Ca²⁺ signaling in cell migration with a focus on SOCE and its diverse functions in migrating cells and cancer metastasis. SOCE has been implicated in regulating focal adhesion turnover in a polarized fashion and the mechanisms involved are beginning to be elucidated. However, SOCE is also involved is other aspects of cell migration with a less well-defined mechanistic understanding. Therefore, much remains to be learned regarding the role and regulation of SOCE in migrating cells.

Mechanically induced integrin ligation mediates intracellular calcium signaling with single pulsating cavitation bubbles

Therapeutic ultrasound or shockwave has shown its great potential to stimulate neural and muscle tissue, where cavitation microbubble induced Ca²⁺ signaling is believed to play an important role. However, the pertinent mechanisms are unknown, especially at the single-cell level. Particularly, it is still a major challenge to get a comprehensive understanding of the effect of potential mechanosensitive molecular players on the cellular responses, including mechanosensitive ion channels, purinergic signaling and integrin ligation by extracellular matrix. Methods: Here, laser-induced cavitation microbubble was used to stimulate individual HEK293T cells either genetically knocked out or expressing Piezo1 ion channels with different normalized bubble-cell distance. Ca²⁺ signaling and potential membrane poration were evaluated with a real-time fluorescence imaging system. Integrin-binding microbeads were attached to the apical surface of the cells at mild cavitation conditions, where the effect of Piezo1, P2X receptors and integrin ligation on single cell intracellular Ca²⁺ signaling was assessed. Results: Ca²⁺ responses were rare at normalized cell-bubble distances that avoided membrane poration, even with overexpression of Piezo1, but could be increased in frequency to 42% of cells by attaching integrin-binding beads. We identified key molecular players in the bead-enhanced Ca²⁺ response: increased integrin ligation by substrate ECM triggered ATP release and activation of P2X-but not Piezo1-ion channels. The resultant Ca²⁺ influx caused dynamic changes in cell spread area. Conclusion: This approach to safely eliciting a Ca²⁺ response with cavitation microbubbles and the uncovered mechanism by which increased integrin-ligation mediates ATP release and Ca²⁺ signaling will inform new strategies to stimulate tissues with ultrasound and shockwaves.

Store operated calcium channels in cancer progression

In recent decades cancer emerged as one of the leading causes of death in the developed countries, with some types of cancer contributing to the top 10 causes of death on the list of the World Health Organization. Carcinogenesis, a malignant transformation causing formation of tumors in normal tissues, is associated with changes in the cell cycle caused by suppression of signaling pathways leading to cell death and facilitation of those enhancing proliferation. Further progression of cancer, during which benign tumors acquire more aggressive phenotypes, is characterized by metastatic dissemination through the body driven by augmented motility and invasiveness of cancer cells. All these processes are associated with alterations in calcium homeostasis in cancer cells, which promote their proliferation, motility and invasion, and dissuade cell death or cell cycle arrest. Remodeling of store-operated calcium entry (SOCE), one of the major pathways regulating intracellular Ca²⁺ concentration ([Ca²⁺]_i), manifests a key event in many of these processes. This review systematizes current knowledge on the mechanisms recruiting SOCE-related proteins in carcinogenesis and cancer progression.

Inhibition of Shear-Induced Platelet Aggregation by Xueshuantong via Targeting Piezo1 Channel-Mediated Ca2+ Signaling Pathway

XueShuanTong (XST) comprising therapeutically active ginsenosides, a lyophilized extract of Panax notoginseng roots, is extensively used in traditional Chinese medicine to treat ischemic heart and cerebrovascular diseases. Our recent study shows that treatment with XST inhibits shear-induced thrombosis formation but the underlying mechanism remained unclear. This study aimed to investigate the hypothesis that XST inhibited shear-induced platelet aggregation via targeting the mechanosensitive Ca²⁺-permeable Piezo1 channel by performing platelet aggregation assay, Ca²⁺ imaging and Western blotting analysis. Exposure to shear at physiologically (1,000–2000 s⁻¹) and pathologically related rates (4,000–6,000 s⁻¹) induced platelet aggregation that was inhibited by treatment with GsMTx-4. Exposure to shear evoked robust Ca²⁺ responses in platelets that were inhibited by treatment with GsMTx-4 and conversely enhanced by treatment with Yoda1. Treatment with XST at a clinically relevant concentration (0.15 g L⁻¹) potently inhibited shear-induced Ca²⁺ responses and platelet aggregation, without altering vWF-mediated platelet adhesion and rolling. Exposure to shear, while resulting in no effect on the calpain-2 expression in platelets, induced calpain-2-mediated cleavage of talin1 protein, which is known to be critical for platelet activation. Shear-induced activation of calpain-2 and cleavage of talin1 were attenuated by treatment with XST. Taken together, our results suggest that XST inhibits shear-induced platelet aggregation via targeting the Piezo1 channel to prevent Piezo1-mediated Ca²⁺ signaling and downstream calpain-2 and talin1 signal pathway, thus providing novel insights into the mechanism of the therapeutic action of XST on platelet aggregation and thrombosis formation.

The Plasma Membrane Ca2+ Pump PMCA4b Regulates Melanoma Cell Migration through Remodeling of the Actin Cytoskeleton

Simple Summary

Earlier we demonstrated that the plasma membrane Ca²⁺ pump PMCA4b inhibits migration and metastatic activity of BRAF mutant melanoma cells, however, the exact mechanism has not been fully understood. Here we demonstrate that PMCA4b acted through actin cytoskeleton remodeling in generating a low migratory melanoma cell phenotype resulting in increased cell–cell connections, lamellipodia and stress fiber formation. Both proper trafficking and calcium transporting activity of the pump were essential to complete these tasks indicating that controlling Ca²⁺ concentration levels at specific plasma membrane locations such as the cell front played a role. Our findings suggest that PMCA4b downregulation is likely one of the mechanisms that leads to the perturbed cancer cell cytoskeleton organization resulting in enhanced melanoma cell migration and metastasis.

Abstract

We demonstrated that the plasma membrane Ca²⁺ ATPase PMCA4b inhibits migration and metastatic activity of BRAF mutant melanoma cells. Actin dynamics are essential for cells to move, invade and metastasize, therefore, we hypothesized that PMCA4b affected cell migration through remodeling of the actin cytoskeleton. We found that expression of PMCA4b in A375 BRAF mutant melanoma cells induced a profound change in cell shape, cell culture morphology, and displayed a polarized migratory character. Along with these changes the cells became more rounded with increased cell–cell connections, lamellipodia and stress fiber formation. Silencing PMCA4b in MCF-7 breast cancer cells had a similar effect, resulting in a dramatic loss of stress fibers. In addition, the PMCA4b expressing A375 cells maintained front-to-rear Ca²⁺ concentration gradient with the actin severing protein cofilin localizing to the lamellipodia, and preserved the integrity of the actin cytoskeleton from a destructive Ca²⁺ overload. We showed that both PMCA4b activity and trafficking were essential for the observed morphology and motility changes. In conclusion, our data suggest that PMCA4b plays a critical role in adopting front-to-rear polarity in a normally spindle-shaped cell type through F-actin rearrangement resulting in a less aggressive melanoma cell phenotype.

Manipulation of Focal Adhesion Signaling by Pathogenic Microbes

Focal adhesions (FAs) serve as dynamic signaling hubs within the cell. They connect intracellular actin to the extracellular matrix (ECM) and respond to environmental cues. In doing so, these structures facilitate important processes such as cell-ECM adhesion and migration. Pathogenic microbes often modify the host cell actin cytoskeleton in their pursuit of an ideal replicative niche or during invasion to facilitate uptake. As actin-interfacing structures, FA dynamics are also intimately tied to actin cytoskeletal organization. Indeed, exploitation of FAs is another avenue by which pathogenic microbes ensure their uptake, survival and dissemination. This is often achieved through the secretion of effector proteins which target specific protein components within the FA. Molecular mimicry of the leucine-aspartic acid (LD) motif or vinculin-binding domains (VBDs) commonly found within FA proteins is a common microbial strategy. Other effectors may induce post-translational modifications to FA proteins through the regulation of phosphorylation sites or proteolytic cleavage. In this review, we present an overview of the regulatory mechanisms governing host cell FAs, and provide examples of how pathogenic microbes have evolved to co-opt them to their own advantage. Recent technological advances pose exciting opportunities for delving deeper into the mechanistic details by which pathogenic microbes modify FAs.

On the adhesion-velocity relation and length adaptation of motile cells on stepped fibronectin lanes

The biphasic adhesion-velocity relation is a universal observation in mesenchymal cell motility. It has been explained by adhesion-promoted forces pushing the front and resisting motion at the rear. Yet, there is little quantitative understanding of how these forces control cell velocity. We study motion of MDA-MB-231 cells on microlanes with fields of alternating Fibronectin densities to address this topic and derive a mathematical model from the leading-edge force balance and the force-dependent polymerization rate. It reproduces quantitatively our measured adhesion-velocity relation and results with keratocytes, PtK1 cells, and CHO cells. Our results confirm that the force pushing the leading-edge membrane drives lamellipodial retrograde flow. Forces resisting motion originate along the whole cell length. All motion-related forces are controlled by adhesion and velocity, which allows motion, even with higher Fibronectin density at the rear than at the front. We find the pathway from Fibronectin density to adhesion structures to involve strong positive feedbacks. Suppressing myosin activity reduces the positive feedback. At transitions between different Fibronectin densities, steady motion is perturbed and leads to changes of cell length and front and rear velocity. Cells exhibit an intrinsic length set by adhesion strength, which, together with the length dynamics, suggests a spring-like front-rear interaction force. We provide a quantitative mechanistic picture of the adhesion-velocity relation and cell response to adhesion changes integrating force-dependent polymerization, retrograde flow, positive feedback from integrin to adhesion structures, and spring-like front-rear interaction.

Critical role of lipid membranes in polarization and migration of cells: a biophysical view

Cell migration plays vital roles in many biologically relevant processes such as tissue morphogenesis and cancer metastasis, and it has fascinated biophysicists over the past several decades. However, despite an increasing number of studies highlighting the orchestration of proteins involved in different signaling pathways, the functional roles of lipid membranes have been essentially overlooked. Lipid membranes are generally considered to be a functionless two-dimensional matrix of proteins, although many proteins regulating cell migration gain functions only after they are recruited to the membrane surface and self-organize their functional domains. In this review, we summarize how the logistical recruitment and release of proteins to and from lipid membranes coordinates complex spatiotemporal molecular processes. As predicted from the classical framework of the Smoluchowski equation of diffusion, lipid/protein membranes serve as a 2D reaction hub that contributes to the effective and robust regulation of polarization and migration of cells involving several competing pathways.

Cross-talk between the calcium channel TRPV4 and reactive oxygen species interlocks adhesive and degradative functions of invadosomes

Invadosomes support cell invasion by coupling both acto-adhesive and extracellular matrix degradative functions, which are apparently antagonistic. β1-integrin dynamics regulate this coupling, but the actual sensing mechanism and effectors involved have not yet been elucidated. Using genetic and reverse genetic approaches combined with biochemical and imaging techniques, we now show that the calcium channel TRPV4 colocalizes with β1-integrins at the invadosome periphery and regulates its activation and the coupling of acto-adhesive and degradative functions. TRPV4-mediated regulation of podosome function depends on its ability to sense reactive oxygen species (ROS) in invadosomes' microenvironment and involves activation of the ROS/calcium-sensitive kinase Ask1 and binding of the motor MYO1C. Furthermore, disease-associated TRPV4 gain-of-function mutations that modulate ECM degradation are also implicated in the ROS response, which provides new perspectives in our understanding of the pathophysiology of TRPV4 channelopathies.

The CAR T-Cell Mechanoimmunology at a Glance

Chimeric antigen receptor (CAR) T-cell transfer is a novel paradigm of adoptive T-cell immunotherapy. When coming into contact with a target cancer cell, CAR T-cell forms a nonclassical immunological synapse with the cancer cell and dynamically orchestrates multiple critical forces to commit cytotoxic immune function. Such an immunologic process involves a force transmission in the CAR and a spatiotemporal remodeling of cell cytoskeleton to facilitate CAR activation and CAR T-cell cytotoxic function. Yet, the detailed understanding of such mechanotransduction at the interface between the CAR T-cell and the target cell, as well as its molecular structure and signaling, remains less defined and is just beginning to emerge. This article summarizes the basic mechanisms and principles of CAR T-cell mechanoimmunology, and various lessons that can be comparatively learned from interrogation of mechanotransduction at the immunological synapse in normal cytotoxic T-cell. The recent development and future application of novel bioengineering tools for studying CAR T-cell mechanoimmunology is also discussed. It is believed that this progress report will shed light on the CAR T-cell mechanoimmunology and encourage future researches in revealing the less explored yet important mechanosensing and mechanotransductive mechanisms involved in CAR T-cell immuno-oncology.

Cellular dynamics of myogenic cell migration: molecular mechanisms and implications for skeletal muscle cell therapies

Directional cell migration is a critical process underlying morphogenesis and post-natal tissue regeneration. During embryonic myogenesis, migration of skeletal myogenic progenitors is essential to generate the anlagen of limbs, diaphragm and tongue, whereas in post-natal skeletal muscles, migration of muscle satellite (stem) cells towards regions of injury is necessary for repair and regeneration of muscle fibres. Additionally, safe and efficient migration of transplanted cells is critical in cell therapies, both allogeneic and autologous. Although various myogenic cell types have been administered intramuscularly or intravascularly, functional restoration has not been achieved yet in patients with degenerative diseases affecting multiple large muscles. One of the key reasons for this negative outcome is the limited migration of donor cells, which hinders the overall cell engraftment potential. Here, we review mechanisms of myogenic stem/progenitor cell migration during skeletal muscle development and post-natal regeneration. Furthermore, strategies utilised to improve migratory capacity of myogenic cells are examined in order to identify potential treatments that may be applied to future transplantation protocols.

K+ Channel Tetramerization Domain 5 (KCTD5) Protein Regulates Cell Migration, Focal Adhesion Dynamics and Spreading through Modulation of Ca2+ Signaling and Rac1 Activity

Cell migration is critical for several physiological and pathophysiological processes. It depends on the coordinated action of kinases, phosphatases, Rho-GTPases proteins, and Ca²⁺ signaling. Interestingly, ubiquitination events have emerged as regulatory elements of migration. Thus, the role of proteins involved in ubiquitination processes could be relevant to a complete understanding of pro-migratory mechanisms. KCTD5 is a member of Potassium Channel Tetramerization Domain (KCTD) proteins that have been proposed as a putative adaptor for Cullin3-E3 ubiquitin ligase and a novel regulatory protein of TRPM4 channels. Here, we study whether KCTD5 participates in cell migration-associated mechanisms, such as focal adhesion dynamics and cellular spreading. Our results show that KCTD5 CRISPR/Cas9- and shRNA-based depletion in B16-F10 cells promoted an increase in cell migration and cell spreading, and a decrease in the focal adhesion area, consistent with an increased focal adhesion disassembly rate. The expression of a dominant-negative mutant of Rho-GTPases Rac1 precluded the KCTD5 depletion-induced increase in cell spreading. Additionally, KCTD5 silencing decreased the serum-induced Ca²⁺ response, and the reversion of this with ionomycin abolished the KCTD5 knockdown-induced decrease in focal adhesion size. Together, these data suggest that KCTD5 acts as a regulator of cell migration by modulating cell spreading and focal adhesion dynamics through Rac1 activity and Ca²⁺ signaling, respectively.

Developing fluorescence sensor probe to capture activated muscle-specific calpain-3 (CAPN3) in living muscle cells

Calpain-3 (CAPN3) is a muscle-specific type of calpain whose protease activity is triggered by Ca2+ Here, we developed CAPN3 sensor probes (SPs) to detect activated-CAPN3 using a fluorescence/Förster resonance energy transfer (FRET) technique. In our SPs, partial amino acid sequence of calpastatin, endogenous CAPN inhibitor but CAPN3 substrate, is inserted between two different fluorescence proteins that cause FRET. Biochemical and spectral studies revealed that CAPN3 cleaved SPs and changed emission wavelengths of SPs. Importantly, SPs were scarcely cleaved by CAPN1 and CAPN2. Furthermore, our SP successfully captured the activation of endogenous CAPN3 in living myotubes treated with ouabain. Our SPs would become a promising tool to detect the dynamics of CAPN3 protease activity in living cells.

Cell stretching is amplified by active actin remodelling to deform and recruit proteins in mechanosensitive structures

Detection and conversion of mechanical forces into biochemical signals controls cell functions during physiological and pathological processes. Mechanosensing is based on protein deformations and reorganizations, yet the molecular mechanisms are still unclear. Using a cell-stretching device compatible with super-resolution microscopy and single-protein tracking, we explored the nanoscale deformations and reorganizations of individual proteins inside mechanosensitive structures. We achieved super-resolution microscopy after live stretching on intermediate filaments, microtubules and integrin adhesions. Simultaneous single-protein tracking and stretching showed that while integrins followed the elastic deformation of the substrate, actin filaments and talin also displayed lagged and transient inelastic responses associated with active acto-myosin remodelling and talin deformations. Capturing acute reorganizations of single molecules during stretching showed that force-dependent vinculin recruitment is delayed and depends on the maturation of integrin adhesions. Thus, cells respond to external forces by amplifying transiently and locally cytoskeleton displacements, enabling protein deformation and recruitment in mechanosensitive structures.

Insights and perspectives on calcium channel functions in the cockpit of cancerous space invaders

Development of metastasis causes the most serious clinical consequences of cancer and is responsible for over 90 % of cancer-related deaths. Hence, a better understanding of the mechanisms that drive metastasis formation appears critical for drug development designed to prevent the spread of cancer and related mortality. Metastasis dissemination is a multistep process supported by the increased motility and invasiveness capacities of tumor cells. To succeed in overcoming the mechanical constraints imposed by the basement membrane and surrounding tissues, cancer cells reorganize their focal adhesions or extend acto-adhesive cellular protrusions, called invadosomes, that can both contact the extracellular matrix and tune its degradation through metalloprotease activity. Over the last decade, accumulating evidence has demonstrated that altered Ca²⁺ channel activities and/or expression promote tumor cell-specific phenotypic changes, such as exacerbated migration and invasion capacities, leading to metastasis formation. While several studies have addressed the molecular basis of Ca²⁺ channel-dependent cancer cell migration, we are still far from having a comprehensive vision of the Ca²⁺ channel-regulated mechanisms of migration/invasion. This is especially true regarding the specific context of invadosome-driven invasion. This review aims to provide an overview of the current evidence supporting a central role for Ca²⁺ channel-dependent signaling in the regulation of these dynamic degradative structures. It will present available data on the few Ca²⁺ channels that have been studied in that specific context and discuss some potential interesting actors that have not been fully explored yet.