Dynamic force sensing of filamin revealed in single-molecule experiments

Filamin protects myofibrils from contractile damage through changes in its mechanosensory region

Filamins are mechanosensitive actin crosslinking proteins that organize the actin cytoskeleton in a variety of shapes and tissues. In muscles, filamin crosslinks actin filaments from opposing sarcomeres, the smallest contractile units of muscles. This happens at the Z-disc, the actin-organizing center of sarcomeres. In flies and vertebrates, filamin mutations lead to fragile muscles that appear ruptured, suggesting filamin helps counteract muscle rupturing during muscle contractions by providing elastic support and/or through signaling. An elastic region at the C-terminus of filamin is called the mechanosensitive region and has been proposed to sense and counteract contractile damage. Here we use molecularly defined mutants and microscopy analysis of the Drosophila indirect flight muscles to investigate the molecular details by which filamin provides cohesion to the Z-disc. We made novel filamin mutations affecting the C-terminal region to interrogate the mechanosensitive region and detected three Z-disc phenotypes: dissociation of actin filaments, Z-disc rupture, and Z-disc enlargement. We tested a constitutively closed filamin mutant, which prevents the elastic changes in the mechanosensitive region and results in ruptured Z-discs, and a constitutively open mutant which has the opposite elastic effect on the mechanosensitive region and gives rise to enlarged Z-discs. Finally, we show that muscle contraction is required for Z-disc rupture. We propose that filamin senses myofibril damage by elastic changes in its mechanosensory region, stabilizes the Z-disc, and counteracts contractile damage at the Z-disc.

Pathological mechanisms of cold and mechanical stress in modulating cancer progression

Environmental temperature and cellular mechanical force are the inherent factors that participate in various biological processes and regulate cancer progress, which have been hot topics worldwide. They occupy a dominant part in the cancer tissues through different approaches. However, extensive investigation regarding pathological mechanisms in the carcinogenic field. After research, we found cold stress via two means to manipulate tumors: neuroscience and mechanically sensitive ion channels (MICHs) such as TRP families to regulate the physiological and pathological activities. Excessive cold stimulation mediated neuroscience acting on every cancer stage through the hypothalamus-pituitary-adrenocorticoid (HPA) to reach the target organs. Comparatively speaking, mechanical force via Piezo of MICHs controls cancer development. The progression of cancer depends on the internal activation of proto-oncogenes and the external tumorigenic factors; the above two means eventually lead to genetic disorders at the molecular level. This review summarizes the interaction of bidirectional communication between them and the tumor. It covers the main processes from cytoplasm to nucleus related to metastasis cascade and tumor immune escape.

Mechanotransduction through protein stretching

Cells sense and respond to subtle changes in their physicality, and via a myriad of different mechanosensitive processes, convert these physical cues into chemical and biochemical signals. This process, called mechanotransduction, is possible due to a highly sophisticated machinery within cells. One mechanism by which this can occur is via the stretching of mechanosensitive proteins. Stretching proteins that contain force-dependent regions results in altered geometry and dimensions of the connections, as well as differential spatial organization of signals bound to the stretched protein. The purpose of this mini-review is to discuss some of the intense recent activity in this area of mechanobiology that strives to understand how protein stretching can influence signaling outputs and cellular responses.

Use of DNA forceps to measure receptor-ligand dissociation equilibrium constants in a single-molecule competition assay

The ability of biophysicists to decipher the behavior of individual biomolecules has steadily improved over the past thirty years. However, it still remains unclear how an ensemble of data acquired at the single-molecule level compares with the data acquired on an ensemble of the same molecules. We here propose an assay to tackle this question in the context of dissociation equilibrium constant measurements. A sensor is built by engrafting a receptor and a ligand onto a flexible dsDNA scaffold and mounting this assembly on magnetic tweezers. This way, looking at the position of the magnetic bead enables one to determine in real-time if the two molecular partners are associated or not. Next, to quantify the affinity of the scrutinized single-receptor for a given competitor, various amounts of the latter molecule are introduced in solution and the equilibrium response of the sensor is monitored throughout the titration protocol. Proofs of concept are established for the binding of three rapamycin analogs to the FKBP12 cis-trans prolyl isomerase. For each of these drugs the mean affinity constant obtained on a ten of individual receptors agrees with the one previously determined in a bulk assay. Furthermore, experimental contingencies are sufficient to explain the dispersion observed over the single-molecule values.

The Role of Mechanotransduction in Contact Inhibition of Locomotion and Proliferation

Contact inhibition (CI) represents a crucial tumor-suppressive mechanism responsible for controlling the unbridled growth of cells, thus preventing the formation of cancerous tissues. CI can be further categorized into two distinct yet interrelated components: CI of locomotion (CIL) and CI of proliferation (CIP). These two components of CI have historically been viewed as separate processes, but emerging research suggests that they may be regulated by both distinct and shared pathways. Specifically, recent studies have indicated that both CIP and CIL utilize mechanotransduction pathways, a process that involves cells sensing and responding to mechanical forces. This review article describes the role of mechanotransduction in CI, shedding light on how mechanical forces regulate CIL and CIP. Emphasis is placed on filamin A (FLNA)-mediated mechanotransduction, elucidating how FLNA senses mechanical forces and translates them into crucial biochemical signals that regulate cell locomotion and proliferation. In addition to FLNA, trans-acting factors (TAFs), which are proteins or regulatory RNAs capable of directly or indirectly binding to specific DNA sequences in distant genes to regulate gene expression, emerge as sensitive players in both the mechanotransduction and signaling pathways of CI. This article presents methods for identifying these TAF proteins and profiling the associated changes in chromatin structure, offering valuable insights into CI and other biological functions mediated by mechanotransduction. Finally, it addresses unanswered research questions in these fields and delineates their possible future directions.

Force-Dependent Structural Changes of Filamin C Rod Domains Regulated by Filamin C Dimer

Filamin C (FLNC), a large dimeric actin-binding protein in muscle cells, plays a critical role in transmitting force in the cytoskeleton and that between membrane receptors and the cytoskeleton. It performs crucial mechanosensing and downstream mechanotransduction functions via force-dependent interactions with signaling proteins. Mutations in FLNC have been linked to muscle and heart diseases. The mechanical responses of the force-bearing elements in FLNC have not been determined. This study investigated the mechanical responses of FLNC domains and their dimerization interface using magnetic tweezers. Results showed high stability of the N-terminal domains in the rod-1 segment but significant changes in the rod-2 domains in response to forces of a few piconewtons (pN). The dimerization interface, formed by the R24 domain, has a lifetime of seconds to tens of seconds at pN forces, and it dissociates within 1 s at forces greater than 14 pN. The findings suggest the FLNC dimerization interface provides sufficient mechanical stability that enables force-dependent structural changes in rod-2 domains for signaling protein binding and maintains structural integrity of the rod-1 domains.

Combining DNA scaffolds and acoustic force spectroscopy to characterize individual protein bonds

Single-molecule data are of great significance in biology, chemistry, and medicine. However, new experimental tools to characterize, in a multiplexed manner, protein bond rupture under force are still needed. Acoustic force spectroscopy is an emerging manipulation technique which generates acoustic waves to apply force in parallel on multiple microbeads tethered to a surface. We here exploit this configuration in combination with the recently developed modular junctured-DNA scaffold that has been designed to study protein-protein interactions at the single-molecule level. By applying repetitive constant force steps on the FKBP12-rapamycin-FRB complex, we measure its unbinding kinetics under force at the single-bond level. Special efforts are made in analyzing the data to identify potential pitfalls. We propose a calibration method allowing in situ force determination during the course of the unbinding measurement. We compare our results with well-established techniques, such as magnetic tweezers, to ensure their accuracy. We also apply our strategy to study the force-dependent rupture of a single-domain antibody with its antigen. Overall, we get a good agreement with the published parameters that have been obtained at zero force and population level. Thus, our technique offers single-molecule precision for multiplexed measurements of interactions of biotechnological and medical interest.

Force: A messenger of axon outgrowth

The axon is a sophisticated macromolecular machine composed of interrelated parts that transmit signals like spur gears transfer motion between parallel shafts. The growth cone is a fine sensor that integrates mechanical and chemical cues and transduces these signals through the generation of a traction force that pushes the tip and pulls the axon shaft forward. The axon shaft, in turn, senses this pulling force and transduces this signal in an orchestrated response, coordinating cytoskeleton remodeling and intercalated mass addition to sustain and support the advancing of the tip. Extensive research suggests that the direct application of active force is per se a powerful inducer of axon growth, potentially bypassing the contribution of the growth cone. This review provides a critical perspective on current knowledge of how the force is a messenger of axon growth and its mode of action for controlling navigation, including aspects that remain unclear. It also focuses on novel approaches and tools designed to mechanically manipulate axons, and discusses their implications in terms of potential novel therapies for re-wiring the nervous system.

Filamin-A-interacting protein 1 (FILIP1) is a dual compartment protein linking myofibrils and microtubules during myogenic differentiation and upon mechanical stress

Variations in the gene encoding filamin-A-interacting protein 1 (FILIP1) were identified to be associated with a combination of neurological and muscular symptoms. While FILIP1 was shown to regulate motility of brain ventricular zone cells, a process important for corticogenesis, the function of the protein in muscle cells has been less well characterized. The expression of FILIP1 in regenerating muscle fibres predicted a role in early muscle differentiation. Here we analysed expression and localization of FILIP1 and its binding partners filamin-C (FLNc) and microtubule plus-end-binding protein EB3 in differentiating cultured myotubes and adult skeletal muscle. Prior to the development of cross-striated myofibrils, FILIP1 is associated with microtubules and colocalizes with EB3. During further myofibril maturation its localization changes, and FILIP1 localizes to myofibrillar Z-discs together with the actin-binding protein FLNc. Forced contractions of myotubes by electrical pulse stimulation (EPS) induce focal disruptions in myofibrils and translocation of both proteins from Z-discs to these lesions, suggesting a role in induction and/or repair of these structures. The immediate proximity of tyrosylated, dynamic microtubules and EB3 to lesions implies that also these play a role in these processes. This implication is supported by the fact that in nocodazole-treated myotubes that lack functional microtubules, the number of lesions induced by EPS is significantly reduced. In summary, we here show that FILIP1 is a cytolinker protein that is associated with both microtubules and actin filaments, and might play a role in the assembly of myofibrils and their stabilization upon mechanical stress to protect them from damage.

Identification of Filamin A Mechanobinding Partner III: SAV1 Specifically Interacts with Filamin A Mechanosensitive Domain 21

Filamin A (FLNA) cross-links actin filaments and mediates mechanotransduction by force-induced conformational changes of its domains. FLNA's mechanosensitive immunoglobulin-like repeats (R) interact with each other to create cryptic binding sites, which can be exposed by physiologically relevant mechanical forces. Using the FLNA mechanosensing domains as an affinity ligand followed by stable isotope labeling by amino acids in cell culture (SILAC)-based proteomics, we recently identified smoothelin and fimbacin as FLNA mechanobinding proteins. Here, using the mechanosensing domain as an affinity ligand and two labeled amino acids, we identify salvador homologue 1 (SAV1), a component of the Hippo pathway kinase cascade, as a new FLNA mechanobinding partner. We demonstrate that SAV1 specifically interacts with the cryptic C-D cleft of FLNA R21 and map the FLNA-binding site on SAV1. We show that point mutations on the R21 C strand block the SAV1 interaction and find that SAV1 contains a FLNA-binding motif in the central region (¹¹⁶Phe-¹²⁴Val). Point mutations F116A and T118A (FT/AA) disrupt the interaction. A proximity ligation assay reveals that their interaction occurs in the cytosol in an actin polymerization-dependent manner. Although SAV1 is typically found in the cytosol, disrupting the interaction between SAV1 and FLNA causes SAV1 to diffuse to the nucleus and YAP1 to diffuse to the cytosol in an inverse relationship. These results suggest that FLNA mediates regulation of the Hippo pathway through actin polymerization-dependent interaction with SAV1.

The force-dependent filamin A-G3BP1 interaction regulates phase-separated stress granule formation

Filamin A (FLNA) is an actin crosslinking protein that mediates mechanotransduction. External and internal mechanical forces, through the actin cytoskeleton, can induce conformational changes of the FLNA molecule to expose cryptic binding sites for its binding partners. Here, we identified Ras GTPase-activating protein SH3 domain-binding protein 1 (G3BP1) as a new FLNA mechanobinding partner. Unlike other FLNA binding partners to the mechanosensing domain repeat 21 (R21), G3BP1 requires an additional neighboring repeat R22 to interact. We demonstrated that their interaction occurs in the cytosol of living cells in an actin polymerization-dependent manner. We also mapped the FLNA-binding site on G3BP1 and found that a F360A point mutation in the RNA recognition motif disrupts the interaction. RNA interfered with the FLNA-G3BP1 interaction, and FLNA did not localize in RNA-rich stress granules (SGs). Disruption of the interaction was sufficient to promote phase-separated SG formation, and arsenite treatment further stimulated the formation of SGs. Taken together, these data identify G3BP1 as a new mechanobinding protein that interacts with the FLNA mechanosensing domain R21 and suggest that SG formation is partially regulated by mechanical force.

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.

Piconewton forces mediate GAIN domain dissociation of the latrophilin-3 adhesion GPCR

Latrophilins are adhesion G-protein coupled receptors (aGPCRs) that control excitatory synapse formation. aGPCRs, including latrophilins, are autoproteolytically cleaved at their GPCR-Autoproteolysis Inducing (GAIN) domain, but the two resulting fragments remain associated on the cell surface. It is thought that force-mediated dissociation of the fragments exposes a peptide that activates G-protein signaling of aGPCRs, but whether GAIN domain dissociation can occur on biologically relevant timescales and at physiological forces is unknown. Here, we show using magnetic tweezers that physiological forces dramatically accelerate the dissociation of the latrophilin-3 GAIN domain. Forces in the 1-10 pN range were sufficient to dissociate the GAIN domain on a seconds-to-minutes timescale, and the GAIN domain fragments reversibly reassociated after dissociation. Thus, mechanical force may be a key driver of latrophilin signaling during synapse formation, suggesting a physiological mechanism by which aGPCRs may mediate mechanically-induced signal transduction.

The role of single protein elasticity in mechanobiology

In addition to biochemical signals and genetic considerations, mechanical forces are rapidly emerging as a master regulator of human physiology. Yet the molecular mechanisms that regulate force-induced functionalities across a wide range of scales, encompassing the cell, tissue or organ levels, are comparatively not so well understood. With the advent, development and refining of single molecule nanomechanical techniques, enabling to exquisitely probe the conformational dynamics of individual proteins under the effect of a calibrated force, we have begun to acquire a comprehensive knowledge on the rich plethora of physicochemical principles that regulate the elasticity of single proteins. Here we review the major advances underpinning our current understanding of how the elasticity of single proteins regulates mechanosensing and mechanotransduction. We discuss the present limitations and future challenges of such a prolific and burgeoning field.

Interaction of LARP4 to filamin A mechanosensing domain regulates cell migrations

Filamin A (FLNA) is an actin cross-linking protein that mediates mechanotransduction. Force-dependent conformational changes of FLNA molecule expose cryptic binding site of FLNA, allowing interaction with partners such as integrin, smoothelin, and fimbacin. Here, we identified La-related protein 4 (LARP4) as a new FLNA mechanobinding partner. LARP4 specifically interacts with the cleft formed by C and D strands of immunoglobulin-like repeat 21 (R21) which is blocked by A strand of R20 without force. We validated the interaction between LARP4 and FLNA R21 both in vivo and in vitro. We also determined the critical amino acid that is responsible for the interaction and generated the non-FLNA-binding mutant LARP4 (F277A in human: F273A in mouse Larp4) that disrupts the interaction. Fluorescence recovery after photobleaching (FRAP) of GFP-labeled LARP4 in living cells demonstrated that mutant LARP4 diffuses faster than WT LARP4. Proximity ligation assay (PLA) also confirmed their interaction and disruption of actin polymerization diminishes the interaction. Data mining of RNAseq analysis of LARP4 knockdown (KD) HEK293T cells suggested that LARP4 is involved in morphogenesis and cell motility. Consistent with this prediction, we found that KD of LARP4 increases cell migration speed and expression of the F277A mutant LARP4 in LARP4-KD cells also leads to a higher cell migration speed compared to WT LARP4. These results demonstrated that the LARP4 interaction with FLNA regulates cell migration.

Determination of protein-protein interactions at the single-molecule level using optical tweezers

Biomolecular interactions are at the base of all physical processes within living organisms; the study of these interactions has led to the development of a plethora of different methods. Among these, single-molecule (in singulo) experiments have become relevant in recent years because these studies can give insight into mechanisms and interactions that are hidden for ensemble-based (in multiplo) methods. The focus of this review is on optical tweezer (OT) experiments, which can be used to apply and measure mechanical forces in molecular systems. OTs are based on optical trapping, where a laser is used to exert a force on a dielectric bead; and optically trap the bead at a controllable position in all three dimensions. Different experimental approaches have been developed to study protein–protein interactions using OTs, such as: (1) refolding and unfolding in trans interaction where one protein is tethered between the beads and the other protein is in the solution; (2) constant force in cis interaction where each protein is bound to a bead, and the tension is suddenly increased. The interaction may break after some time, giving information about the lifetime of the binding at that tension. And (3) force ramp in cis interaction where each protein is attached to a bead and a ramp force is applied until the interaction breaks. With these experiments, parameters such as kinetic constants (k_off, k_on), affinity values (K_D), energy to the transition state ΔG^≠, distance to the transition state Δx^≠ can be obtained. These parameters characterize the energy landscape of the interaction. Some parameters such as distance to the transition state can only be obtained from force spectroscopy experiments such as those described here.

A-to-I RNA editing of Filamin A regulates cellular adhesion, migration and mechanical properties

A-to-I RNA editing by ADARs is an abundant epitranscriptomic RNA-modification in metazoa. In mammals, Flna pre-mRNA harbours a single conserved A-to-I RNA editing site that introduces a Q-to-R amino acid change in Ig repeat 22 of the encoded protein. Previously, we showed that FLNA editing regulates smooth muscle contraction in the cardiovascular system and affects cardiac health. The present study investigates how ADAR2-mediated A-to-I RNA editing of Flna affects actin crosslinking, cell mechanics, cellular adhesion and cell migration. Cellular assays and AFM measurements demonstrate that the edited version of FLNA increases cellular stiffness and adhesion but impairs cell migration in both, mouse fibroblasts and human tumour cells. In vitro, edited FLNA leads to increased actin crosslinking, forming actin gels of higher stress resistance. Our study shows that Flna RNA editing is a novel regulator of cytoskeletal organisation, affecting the mechanical property and mechanotransduction of cells.

Folding Free Energy Determination of an RNA Three-Way Junction Using Fluctuation Theorems

Nonequilibrium work relations and fluctuation theorems permit us to extract equilibrium information from nonequilibrium measurements. They find application in single-molecule pulling experiments where molecular free energies can be determined from irreversible work measurements by using unidirectional (e.g., Jarzynski’s equality) and bidirectional (e.g., Crooks fluctuation theorem and Bennet’s acceptance ratio (BAR)) methods. However, irreversibility and the finite number of pulls limit their applicability: the higher the dissipation, the larger the number of pulls necessary to estimate ΔG within a few kBT. Here, we revisit pulling experiments on an RNA three-way junction (3WJ) that exhibits significant dissipation and work-distribution long tails upon mechanical unfolding. While bidirectional methods are more predictive, unidirectional methods are strongly biased. We also consider a cyclic protocol that combines the forward and reverse work values to increase the statistics of the measurements. For a fixed total experimental time, faster pulling rates permit us to efficiently sample rare events and reduce the bias, compensating for the increased dissipation. This analysis provides a more stringent test of the fluctuation theorem in the large irreversibility regime.

β2-Integrins – Regulatory and Executive Bridges in the Signaling Network Controlling Leukocyte Trafficking and Migration

Leukocyte trafficking is an essential process of immunity, occurring as leukocytes travel within the bloodstream and as leukocyte migration within tissues. While it is now established that leukocytes can utilize the mesenchymal migration mode or amoeboid migration mode, differences in the migratory behavior of leukocyte subclasses and how these are realized on a molecular level in each subclass is not fully understood. To outline these differences, first migration modes and their dependence on parameters of the extracellular environments will be explained, as well as the intracellular molecular machinery that powers migration in general. Extracellular parameters are detected by adhesion receptors such as integrins. β2-integrins are surface receptors exclusively expressed on leukocytes and are essential for leukocytes exiting the bloodstream, as well as in mesenchymal migration modes, however, integrins are dispensable for the amoeboid migration mode. Additionally, the balance of different RhoGTPases – which are downstream of surface receptor signaling, including integrins – mediate formation of membrane structures as well as actin dynamics. Individual leukocyte subpopulations have been shown to express distinct RhoGTPase profiles along with their differences in migration behavior, which will be outlined. Emerging aspects of leukocyte migration include signal transduction from integrins via actin to the nucleus that regulates DNA status, gene expression profiles and ultimately leukocyte migratory phenotypes, as well as altered leukocyte migration in tumors, which will be touched upon.

Tension can directly suppress Aurora B kinase-triggered release of kinetochore-microtubule attachments

Chromosome segregation requires sister kinetochores to attach microtubules emanating from opposite spindle poles. Proper attachments come under tension and are stabilized, but defective attachments lacking tension are released, giving another chance for correct attachments to form. This error correction process depends on Aurora B kinase, which phosphorylates kinetochores to destabilize their microtubule attachments. However, the mechanism by which Aurora B distinguishes tense versus relaxed kinetochores remains unclear because it is difficult to detect kinase-triggered detachment and to manipulate kinetochore tension in vivo. To address these challenges, we apply an optical trapping-based assay using soluble Aurora B and reconstituted kinetochore-microtubule attachments. Strikingly, the tension on these attachments suppresses their Aurora B-triggered release, suggesting that tension-dependent changes in the conformation of kinetochores can regulate Aurora B activity or its outcome. Our work uncovers the basis for a key mechano-regulatory event that ensures accurate segregation and may inform studies of other mechanically regulated enzymes.

A tethered ligand assay to probe SARS-CoV-2:ACE2 interactions

In the dynamic environment of the airways, where SARS-CoV-2 infections are initiated by binding to human host receptor ACE2, mechanical stability of the viral attachment is a crucial fitness advantage. Using single-molecule force spectroscopy techniques, we mimic the effect of coughing and sneezing, thereby testing the force stability of SARS-CoV-2 RBD:ACE2 interaction under physiological conditions. Our results reveal a higher force stability of SARS-CoV-2 binding to ACE2 compared to SARS-CoV-1, causing a possible fitness advantage. Our assay is sensitive to blocking agents preventing RBD:ACE2 bond formation. It will thus provide a powerful approach to investigate the modes of action of neutralizing antibodies and other agents designed to block RBD binding to ACE2 that are currently developed as potential COVID-19 therapeutics.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections are initiated by attachment of the receptor-binding domain (RBD) on the viral Spike protein to angiotensin-converting enzyme-2 (ACE2) on human host cells. This critical first step occurs in dynamic environments, where external forces act on the binding partners and avidity effects play an important role, creating an urgent need for assays that can quantitate SARS-CoV-2 interactions with ACE2 under mechanical load. Here, we introduce a tethered ligand assay that comprises the RBD and the ACE2 ectodomain joined by a flexible peptide linker. Using magnetic tweezers and atomic force spectroscopy as highly complementary single-molecule force spectroscopy techniques, we investigate the RBD:ACE2 interaction over the whole physiologically relevant force range. We combine the experimental results with steered molecular dynamics simulations and observe and assign fully consistent unbinding and unfolding events across the three techniques, enabling us to establish ACE2 unfolding as a molecular fingerprint. Measuring at forces of 2 to 5 pN, we quantify the force dependence and kinetics of the RBD:ACE2 bond in equilibrium. We show that the SARS-CoV-2 RBD:ACE2 interaction has higher mechanical stability, larger binding free energy, and a lower dissociation rate compared to SARS-CoV-1, which helps to rationalize the different infection patterns of the two viruses. By studying how free ACE2 outcompetes tethered ACE2, we show that our assay is sensitive to prevention of bond formation by external binders. We expect our results to provide a way to investigate the roles of viral mutations and blocking agents for targeted pharmaceutical intervention.

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.

New insights into regulation of αIIbβ3 integrin signaling by filamin A

Background

Filamin (FLN) regulates many cell functions through its scaffolding activity cross‐linking cytoskeleton and integrins. FLN was shown to inhibit integrin activity, but the exact mechanism remains unclear.

Objectives

The aim of this study was to evaluate the role of filamin A (FLNa) subdomains on the regulation of integrin αIIbβ3 signaling.

Methods

Three FLNa deletion mutants were overexpressed in the erythro‐megakaryocytic leukemic cell line HEL: Del1, which lacks the N‐terminal CH1‐CH2 domains mediating the FLNa‐actin interaction; Del2, lacking the Ig‐like repeat 21, which mediates the FLNa‐β3 interaction; and Del3, lacking the C‐terminal Ig repeat 24, responsible for FLNa dimerization and interaction with the small Rho guanosine triphosphatase involved in actin cytoskeleton reorganisation. Fibrinogen binding to HEL cells in suspension and talin‐β3 proximity in cells adherent to immobilized fibrinogen were assessed before and after αIIbβ3 activation by the protein kinase C agonist phorbol 12‐myristate 13‐acetate.

Results

Our results show that FLNa‐actin and FLNa‐β3 interactions negatively regulate αIIbβ3 activation. Moreover, FLNa‐actin interaction represses Rac activation, contributing to the negative regulation of αIIbβ3 activation. In contrast, the FLNa dimerization domain, which maintains Rho inactive, was found to negatively regulate αIIbβ3 outside‐in signaling.

Conclusion

We conclude that FLNa negatively controls αIIbβ3 activation by regulating actin polymerization and restraining activation of Rac, as well as outside‐in signaling by repressing Rho.

Molten globule-like transition state of protein barnase measured with calorimetric force spectroscopy

Understanding the molecular forces driving the unfolded polypeptide chain to self-assemble into a functional native structure remains an open question. However, identifying the states visited during protein folding (e.g., the transition state between the unfolded and native states) is tricky due to their transient nature. Here, we introduce calorimetric force spectroscopy in a temperature jump optical trap to determine the enthalpy, entropy, and heat capacity of the transition state of protein barnase. We find that the transition state has the properties of a dry molten globule, that is, high free energy and low configurational entropy, being structurally similar to the native state. This experimental single-molecule study characterizes the thermodynamic properties of the transition state in funneled energy landscapes.

Understanding how proteins fold into their native structure is a fundamental problem in biophysics, crucial for protein design. It has been hypothesized that the formation of a molten globule intermediate precedes folding to the native conformation of globular proteins; however, its thermodynamic properties are poorly known. We perform single-molecule pulling experiments of protein barnase in the range of 7 ^∘C to 37 ^∘C using a temperature-jump optical trap. We derive the folding free energy, entropy and enthalpy, and heat capacity change (ΔCp = 1,050 ± 50 cal/mol·K) at low ionic strength conditions. From the measured unfolding and folding kinetic rates, we also determine the thermodynamic properties of the transition state, finding a significant change in ΔCp (∼90%) between the unfolded and the transition states. In contrast, the major change in enthalpy (∼80%) occurs between the transition and native states. These results highlight a transition state of high energy and low configurational entropy structurally similar to the native state, in agreement with the molten globule hypothesis.

Differential responses of abomasal transcriptome to Haemonchus contortus infection between Haemonchus-selected and Trichostrongylus-selected merino sheep

Haemonchus contortus is the most prevalent and pathogenic gastrointestinal nematode infecting sheep and goats. The two CSIRO sheep resource flocks, the Haemonchus-selected flock (HSF) and Trichostrongylus-selected flock (TSF) were developed for research on host resistance or susceptibility to gastrointestinal nematode infection. A recent study focused on the gene expression differences between resistant and susceptible sheep within each flock, with lymphatic and gastrointestinal tissues. To identify features in the host transcriptome and understand the molecular differences underlying host resistance to H. contortus between flocks with different selective breeding and genetic backgrounds, we compared the abomasal transcriptomic responses of the resistant or susceptible animals between HSF and TSF flocks. A total of 11 and 903 differentially expressed genes were identified in the innate infection treatment in HSF and TSF flocks between resistant and susceptible sheep respectively, while 52 and 485 genes were identified to be differentially expressed in the acquired infection treatment, respectively. Among them, 294 genes had significantly different gene expression levels between HSF and TSF flock animals within the susceptible sheep by both the innate and acquired infections. Moreover, similar expression patterns of the 294 genes were observed, with 273 genes more highly expressed in HSF and 21 more highly expressed in the TSF within the abomasal transcriptome of the susceptible animals. Gene ontology enrichment of the differentially expressed genes identified in this study predicted the likely differing function between the two flock's susceptible lines in response to H. contortus infection. Nineteen pathways were significantly enriched in both the innate and adaptive immune responses in susceptible animals, which indicated that these pathways likely contribute to the host resistance development to H. contortus infection in susceptible sheep. Biological networks built for the set of genes differentially abundant in susceptible animals identified hub genes of PRKG1, PRKACB, PRKACA, and ITGB1 for the innate immune response, and CALM2, MYL1, COL1A1, ITGB1 and ITGB3 for the adaptive immune response, respectively. Our results offered a quantitative snapshot of host transcriptomic changes induced by H. contortus infection between flocks with different selective breeding and genetic backgrounds and provided novel insights into molecular mechanisms of host resistance.

Adhesion and proliferation properties of type I collagen-derived peptide for possible use in skin tissue engineering application

The surface properties of three-dimensional scaffolds are improved by coating or covalently linking certain adhesion-promoting proteins or peptides. In the present study, the effect of type I collagen-derived peptide (GKNGDDGEA) on adhesion and proliferation of HaCaT keratinocytes and NIH3T3 murine fibroblast cell lines was studied to assess its suitability for possible skin tissue engineering applications. Cell adhesion and proliferation of HaCaT and NIH3T3 were found to be enhanced by peptide coating. The optimum peptide coating densities to obtain the best cell adhesion and proliferation were found to be 0.827 µmoles/cm² and 0.62 µmoles/cm² for HaCaT and NIH3T3, respectively. Cell adhesion, in the presence of anti-integrin α1 antibody, inhibited attachment of NIH3T3 cells indicating the involvement of integrin α1 receptor. However, the attachment of HaCaT cells was not affected by anti-integrin treatment. The higher expression of paxillin confirmed the effect of the peptide in mediating focal adhesion kinases (FAKs) in cell adhesion and proliferation. Gene expression analysis was performed on cell migration proteins like Rho, Rac, Cdc42, integrin receptor α1, and β1, and the extracellular matrix modulating proteins like MMP2, TIMP, and COL1A1 to validate their role on the peptide-mediated cell proliferation. Immunofluorescence analysis showed the distribution and localisation of phospho-FAK on cells cultured on the peptide-coated surfaces. Results support the role of peptides in enhancing cell adhesion and proliferation properties.

Isoform-specific functions of synaptopodin-2 variants in cytoskeleton stabilization and autophagy regulation in muscle under mechanical stress

Protein homeostasis (proteostasis) in multicellular organisms depends on the maintenance of force-bearing and force-generating cellular structures. Within myofibrillar Z-discs of striated muscle, isoforms of synaptopodin-2 (SYNPO2/myopodin) act as adapter proteins that are engaged in proteostasis of the actin-crosslinking protein filamin C (FLNc) under mechanical stress. SYNPO2 directly binds F-actin, FLNc and α-actinin and thus contributes to the architectural features of the actin cytoskeleton. By its association with autophagy mediating proteins, i.e. BAG3 and VPS18, SYNPO2 is also engaged in protein quality control and helps to target mechanical unfolded and damaged FLNc for degradation. Here we show that deficiency of all SYNPO2-isoforms in myotubes leads to decreased myofibrillar stability and deregulated autophagy under mechanical stress. In addition, isoform-specific proteostasis functions were revealed. The PDZ-domain containing variant SYNPO2b and the shorter, PDZ-less isoform SYNPO2e both localize to Z-discs. Yet, SYNPO2e is less stably associated with the Z-disc than SYNPO2b, and is dynamically transferred into FLNc-containing myofibrillar lesions under mechanical stress. SYNPO2e also recruits BAG3 into these lesions via interaction with the WW domain of BAG3. Our data provide evidence for a role of myofibrillar lesions as a transient quality control compartment essential to prevent and repair contraction-induced myofibril damage in muscle and indicate an important coordinating activity for SYNPO2 therein.

Intramolecular quality control: HIV-1 envelope gp160 signal-peptide cleavage as a functional folding checkpoint

Removal of the membrane-tethering signal peptides that target secretory proteins to the endoplasmic reticulum is a prerequisite for proper folding. While generally thought to be removed co-translationally, we report two additional post-targeting functions for the HIV-1 gp120 signal peptide, which remains attached until gp120 folding triggers its removal. First, the signal peptide improves folding fidelity by enhancing conformational plasticity of gp120 by driving disulfide isomerization through a redox-active cysteine. Simultaneously, the signal peptide delays folding by tethering the N terminus to the membrane, until assembly with the C terminus. Second, its carefully timed cleavage represents intramolecular quality control and ensures release of (only) natively folded gp120. Postponed cleavage and the redox-active cysteine are both highly conserved and important for viral fitness. Considering the ∼15% proteins with signal peptides and the frequency of N-to-C contacts in protein structures, these regulatory roles of signal peptides are bound to be more common in secretory-protein biogenesis.

Strain induced mechanoresponse depends on cell contractility and BAG3-mediated autophagy

Basically all mammalian tissues are constantly exposed to a variety of environmental mechanical signals. Depending on the signal strength, mechanics intervenes in a multitude of cellular processes and is thus capable to induce simple cellular adaptations but also complex differentiation processes and even apoptosis. The underlying recognition typically depends on mechanosensitive proteins, which most often sense the mechanical signal for the induction of a cellular signaling cascade by changing their protein conformation. However, the fate of mechanosensors after mechanical stress application is still poorly understood and it remains unclear whether protein degradation pathways affect the mechanosensitivity of cells. Here, we show that cyclic stretch induces autophagosome formation in a time-dependent manner. Formation depends on the cochaperone BAG3 and thus likely involves BAG3-mediated chaperone-assisted selective autophagy. Furthermore, we demonstrate that strain-induced cell reorientation is clearly delayed upon inhibition of autophagy, suggesting a bidirectional crosstalk between mechanotransduction and autophagic degradation. The strength of the observed delay depends on stable adhesion structures and stress fiber formation in a RhoA-dependent manner.

Maintaining proteostasis under mechanical stress

Cell survival, tissue integrity and organismal health depend on the ability to maintain functional protein networks even under conditions that threaten protein integrity. Protection against such stress conditions involves the adaptation of folding and degradation machineries, which help to preserve the protein network by facilitating the refolding or disposal of damaged proteins. In multicellular organisms, cells are permanently exposed to stress resulting from mechanical forces. Yet, for long time mechanical stress was not recognized as a primary stressor that perturbs protein structure and threatens proteome integrity. The identification and characterization of protein folding and degradation systems, which handle force-unfolded proteins, marks a turning point in this regard. It has become apparent that mechanical stress protection operates during cell differentiation, adhesion and migration and is essential for maintaining tissues such as skeletal muscle, heart and kidney as well as the immune system. Here, we provide an overview of recent advances in our understanding of mechanical stress protection.

Protein nanomechanics in biological context

How proteins respond to pulling forces, or protein nanomechanics, is a key contributor to the form and function of biological systems. Indeed, the conventional view that proteins are able to diffuse in solution does not apply to the many polypeptides that are anchored to rigid supramolecular structures. These tethered proteins typically have important mechanical roles that enable cells to generate, sense, and transduce mechanical forces. To fully comprehend the interplay between mechanical forces and biology, we must understand how protein nanomechanics emerge in living matter. This endeavor is definitely challenging and only recently has it started to appear tractable. Here, I introduce the main in vitro single-molecule biophysics methods that have been instrumental to investigate protein nanomechanics over the last 2 decades. Then, I present the contemporary view on how mechanical force shapes the free energy of tethered proteins, as well as the effect of biological factors such as post-translational modifications and mutations. To illustrate the contribution of protein nanomechanics to biological function, I review current knowledge on the mechanobiology of selected muscle and cell adhesion proteins including titin, talin, and bacterial pilins. Finally, I discuss emerging methods to modulate protein nanomechanics in living matter, for instance by inducing specific mechanical loss-of-function (mLOF). By interrogating biological systems in a causative manner, these new tools can contribute to further place protein nanomechanics in a biological context.

The role of filamins in mechanically stressed podocytes

Glomerular hypertension induces mechanical load to podocytes, often resulting in podocyte detachment and the development of glomerulosclerosis. Although it is well known that podocytes are mechanosensitive, the mechanosensors and mechanotransducers are still unknown. Since filamin A, an actin-binding protein, is already described to be a mechanosensor and mechanotransducer, we hypothesized that filamins could be important for the outside-in signaling as well as the actin cytoskeleton of podocytes under mechanical stress. In this study, we demonstrate that filamin A is the main isoform of the filamin family that is expressed in cultured podocytes. Together with filamin B, filamin A was significantly up-regulated during mechanical stretch (3 days, 0.5 Hz, and 5% extension). To study the role of filamin A in cultured podocytes under mechanical stress, filamin A was knocked down (Flna KD) by specific siRNA. Additionally, we established a filamin A knockout podocyte cell line (Flna KO) by CRISPR/Cas9. Knockdown and knockout of filamin A influenced the expression of synaptopodin, a podocyte-specific protein, focal adhesions as well as the morphology of the actin cytoskeleton. Moreover, the cell motility of Flna KO podocytes was significantly increased. Since the knockout of filamin A has had no effect on cell adhesion of podocytes during mechanical stress, we simultaneously knocked down the expression of filamin A and B. Thereby, we observed a significant loss of podocytes during mechanical stress indicating a compensatory mechanism. Analyzing hypertensive mice kidneys as well as biopsies of patients suffering from diabetic nephropathy, we found an up-regulation of filamin A in podocytes in contrast to the control. In summary, filamin A and B mediate matrix-actin cytoskeleton interactions which are essential for the adaptation of cultured podocyte to mechanical stress.

SlyD Accelerates trans-to-cis Prolyl Isomerization in a Mechanosignaling Protein under Load

Prolyl isomerization is recognized as one of the key regulatory mechanisms, which plays a crucial role in cell signaling, ion channel gating, phage virus infection, and molecular timing. This isomerization is usually slow but often accelerated by an enzyme, called peptidyl-prolyl isomerase (PPIase). In the current project, we investigate using single-molecule force spectroscopy (SMFS) the impact of a bacterial PPIase, SlyD, on the cis-trans isomerization of the proline 2225 (P2225) in an isolated 20th domain of a cytoskeletal mechanosensing protein filamin-A (FlnA20). To explore the FlnA20-PPIase interaction, we have used multiple SMFS modes, like constant velocity, constant distance, and jumping trap experiments. In our previous study, we reported the unique nature of the P2225, which is conserved in all naturally occurring filamins and can slowly (minutes) interconvert between cis-trans isomers, in absence of any PPIase. Our current results show a staggering 25-fold acceleration of the trans-to-cis isomerization rate in the presence of saturating SlyD concentration (7.25 μM) compared to the unenzymatic condition. A SlyD concentration-dependent depletion of the trans isomeric lifetime was also observed. Additionally, we observed that SlyD stabilizes the cis-isomer in the native state of FlnA20 by ∼2 k_BT. This is the first single-molecule observation of the cis-trans isomerization catalysis by a PPIase in a mechanosensing protein.

ECM remodeling in stem cell culture: a potential target for regulating stem cell function

Stem cells (SCs) hold great potential for regenerative medicine, tissue engineering and cell therapy. The clinical applications of SCs require both high quality and quantity of transplantable cells. However, during conventional in vitro expansion, SCs tend to lose properties that make them amenable for cell therapies. Extracellular matrix (ECM) serves an essential regulatory part in the growth, differentiation and homeostasis of all cells in vivo. when signals transmitted to cells, they do not respond passively. Many cell types can remodel pericellular matrix to meet their specific needs. This reciprocal cell-ECM interaction is crucial for the conservation of cell and tissue functions and homeostasis. In vitro ECM remodeling also plays a key role in regulating the lineage fate of SCs. A deeper understanding of in vitro ECM remodeling may provide new perspectives for the maintenance of SC function. In this review, we critically examined three ways that cells can be used to influence the pericellular matrix: (i) exerting tensile force on the ECM, (ii) secreting a variety of ECM proteins, and (iii) degrading the surrounding matrix, and its impact on SC lineage fate. Finally, we describe the deficiencies of current studies and what needs to be done next to further understand the role of ECM remodeling in ex vivo SC cultures.

Myopathy associated LDB3 mutation causes Z-disc disassembly and protein aggregation through PKCα and TSC2-mTOR downregulation

Mechanical stress induced by contractions constantly threatens the integrity of muscle Z-disc, a crucial force-bearing structure in striated muscle. The PDZ-LIM proteins have been proposed to function as adaptors in transducing mechanical signals to preserve the Z-disc structure, however the underlying mechanisms remain poorly understood. Here, we show that LDB3, a well-characterized striated muscle PDZ-LIM protein, modulates mechanical stress signaling through interactions with the mechanosensing domain in filamin C, its chaperone HSPA8, and PKCα in the Z-disc of skeletal muscle. Studies of Ldb3Ala165Val/+ mice indicate that the myopathy-associated LDB3 p.Ala165Val mutation triggers early aggregation of filamin C and its chaperones at muscle Z-disc before aggregation of the mutant protein. The mutation causes protein aggregation and eventually Z-disc myofibrillar disruption by impairing PKCα and TSC2-mTOR, two important signaling pathways regulating protein stability and disposal of damaged cytoskeletal components at a major mechanosensor hub in the Z-disc of skeletal muscle.

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.

Homozygous expression of the myofibrillar myopathy-associated p.W2710X filamin C variant reveals major pathomechanisms of sarcomeric lesion formation

Filamin C (FLNc) is mainly expressed in striated muscle cells where it localizes to Z-discs, myotendinous junctions and intercalated discs. Recent studies have revealed numerous mutations in the FLNC gene causing familial and sporadic myopathies and cardiomyopathies with marked clinical variability. The most frequent myopathic mutation, p.W2710X, which is associated with myofibrillar myopathy, deletes the carboxy-terminal 16 amino acids from FLNc and abolishes the dimerization property of Ig-like domain 24. We previously characterized "knock-in" mice heterozygous for this mutation (p.W2711X), and have now investigated homozygous mice using protein and mRNA expression analyses, mass spectrometry, and extensive immunolocalization and ultrastructural studies. Although the latter mice display a relatively mild myopathy under normal conditions, our analyses identified major mechanisms causing the pathophysiology of this disease: in comparison to wildtype animals (i) the expression level of FLNc protein is drastically reduced; (ii) mutant FLNc is relocalized from Z-discs to particularly mechanically strained parts of muscle cells, i.e. myotendinous junctions and myofibrillar lesions; (iii) the number of lesions is greatly increased and these lesions lack Bcl2-associated athanogene 3 (BAG3) protein; (iv) the expression of heat shock protein beta-7 (HSPB7) is almost completely abolished. These findings indicate grave disturbances of BAG3-dependent and -independent autophagy pathways that are required for efficient lesion repair. In addition, our studies reveal general mechanisms of lesion formation and demonstrate that defective FLNc dimerization via its carboxy-terminal domain does not disturb assembly and basic function of myofibrils. An alternative, more amino-terminally located dimerization site might compensate for that loss. Since filamins function as stress sensors, our data further substantiate that FLNc is important for mechanosensing in the context of Z-disc stabilization and maintenance.

Mechanotransduction, nanotechnology, and nanomedicine

Mechanotransduction, a conversion of mechanical forces into biochemical signals, is essential for human development and physiology. It is observable at all levels ranging from the whole body, organs, tissues, organelles down to molecules. Dysregulation results in various diseases such as muscular dystrophies, hypertension-induced vascular and cardiac hypertrophy, altered bone repair and cell deaths. Since mechanotransduction occurs at nanoscale, nanosciences and applied nanotechnology are powerful for studying molecular mechanisms and pathways of mechanotransduction. Atomic force microscopy, magnetic and optical tweezers are commonly used for force measurement and manipulation at the single molecular level. Force is also used to control cells, topographically and mechanically by specific types of nano materials for tissue engineering. Mechanotransduction research will become increasingly important as a sub-discipline under nanomedicine. Here we review nanotechnology approaches using force measurements and manipulations at the molecular and cellular levels during mechanotransduction, which has been increasingly play important role in the advancement of nanomedicine.

Structure and Function of Filamin C in the Muscle Z-Disc

Filamin C (FLNC) is one of three filamin proteins (Filamin A (FLNA), Filamin B (FLNB), and FLNC) that cross-link actin filaments and interact with numerous binding partners. FLNC consists of a N-terminal actin-binding domain followed by 24 immunoglobulin-like repeats with two intervening calpain-sensitive hinges separating R15 and R16 (hinge 1) and R23 and R24 (hinge-2). The FLNC subunit is dimerized through R24 and calpain cleaves off the dimerization domain to regulate mobility of the FLNC subunit. FLNC is localized in the Z-disc due to the unique insertion of 82 amino acid residues in repeat 20 and necessary for normal Z-disc formation that connect sarcomeres. Since phosphorylation of FLNC by PKC diminishes the calpain sensitivity, assembly, and disassembly of the Z-disc may be regulated by phosphorylation of FLNC. Mutations of FLNC result in cardiomyopathy and muscle weakness. Although this review will focus on the current understanding of FLNC structure and functions in muscle, we will also discuss other filamins because they share high sequence similarity and are better characterized. We will also discuss a possible role of FLNC as a mechanosensor during muscle contraction.

Probing nanomechanical responses of cell membranes

Despite the importance in various cellular processes, the nanomechanical responses of the living cell membrane have been elusive due to complexities in the membrane associated with the hidden architecture of multiple molecular components, including the lipid bilayer. Here, combined experimental and theoretical frameworks that can probe and interpret nanomechanical responses of the cell membrane are demonstrated. A magnetic tweezer assay was introduced to apply pico-Newton scale forces to lipids and E-cadherin molecules at the living cell surface. Two unique classes of force-extension curves were identified: one with a deflection transition (Type I) and another with a discontinuous transition (Type II). The repeated observations of these responses, regardless of cell type and targeted cell surface molecule, suggest the Type I and II curves are the primary nanomechanical responses of cell membranes. To reproduce these responses in vitro, a model system using synthetic lipid vesicles was also developed. Together with a finite element model of lipid bilayers, the reproduced responses suggest that the confined fluidity and curvature constraints imposed on the lipid bilayer components of the cell membrane are the main parameters responsible for the generation of these responses. This work provides an insight into how forces on membrane molecules propagate to the lipid bilayer components to generate specific nanomechanical responses. In addition, the consistent results obtained using different methodologies demonstrate that the presented force-probing assays and the theoretical model can serve a combined testbed to investigate nanoscale mechanics of the living cell membrane.

Filamin A: key actor in platelet biology

Filamins (FLNs) are large dimeric actin-binding proteins that regulate actin cytoskeleton remodeling. In addition, FLNs serve as scaffolds for signaling proteins, such as tyrosine kinases, GTPases, or phosphatases, as well as for adhesive receptors, such as integrins. Thus, they connect adhesive receptors to signaling pathways and to cytoskeleton. There are 3 isoforms of FLN (filamin a [FLNa], FLNb, FLNc) that originate from 3 homologous genes. FLNa has been the recent focus of attention because its mutations are responsible for a wide spectrum of defects called filaminopathies A, affecting brain (peri-ventricular nodular heterotopia), heart (valve defect), skeleton, gastrointestinal tract, and, more recently, the megakaryocytic lineage. This review will focus on the physiological and pathological roles of FLNa in platelets. Indeed, FLNa mutations alter platelet production from their bone marrow precursors, the megakaryocytes, yielding giant platelets in reduced numbers (macrothrombocytopenia). In platelets per se, FLNa mutations may lead to impaired αIIbβ3 integrin activation or in contrast, increased αIIbβ3 activation, potentially enhancing the risk of thrombosis. Experimental work delineating the interaction of FLNa with its platelet partners, including αIIbβ3, the von Willebrand factor receptor GPIb-IX-V, the tyrosine kinase Syk, and the signaling pathway of the collagen receptor GPVI, will also be reviewed.

SUN1/2 Are Essential for RhoA/ROCK-Regulated Actomyosin Activity in Isolated Vascular Smooth Muscle Cells

Vascular smooth muscle cells (VSMCs) are the predominant cell type in the blood vessel wall. Changes in VSMC actomyosin activity and morphology are prevalent in cardiovascular disease. The actin cytoskeleton actively defines cellular shape and the LInker of Nucleoskeleton and Cytoskeleton (LINC) complex, comprised of nesprin and the Sad1p, UNC-84 (SUN)-domain family members SUN1/2, has emerged as a key regulator of actin cytoskeletal organisation. Although SUN1 and SUN2 function is partially redundant, they possess specific functions and LINC complex composition is tailored for cell-type-specific functions. We investigated the importance of SUN1 and SUN2 in regulating actomyosin activity and cell morphology in VSMCs. We demonstrate that siRNA-mediated depletion of either SUN1 or SUN2 altered VSMC spreading and impaired actomyosin activity and RhoA activity. Importantly, these findings were recapitulated using aortic VSMCs isolated from wild-type and SUN2 knockout (SUN2 KO) mice. Inhibition of actomyosin activity, using the rho-associated, coiled-coil-containing protein kinase1/2 (ROCK1/2) inhibitor Y27632 or blebbistatin, reduced SUN2 mobility in the nuclear envelope and decreased the association between SUN2 and lamin A, confirming that SUN2 dynamics and interactions are influenced by actomyosin activity. We propose that the LINC complex exists in a mechanical feedback circuit with RhoA to regulate VSMC actomyosin activity and morphology.

The Hippo network kinase STK38 contributes to protein homeostasis by inhibiting BAG3-mediated autophagy

Chaperone-assisted selective autophagy (CASA) initiated by the cochaperone Bcl2-associated athanogene 3 (BAG3) represents an important mechanism for the disposal of misfolded and damaged proteins in mammalian cells. Under mechanical stress, the cochaperone cooperates with the small heat shock protein HSPB8 and the cytoskeleton-associated protein SYNPO2 to degrade force-unfolded forms of the actin-crosslinking protein filamin. This is essential for muscle maintenance in flies, fish, mice and men. Here, we identify the serine/threonine protein kinase 38 (STK38), which is part of the Hippo signaling network, as a novel interactor of BAG3. STK38 was previously shown to facilitate cytoskeleton assembly and to promote mitophagy as well as starvation and detachment induced autophagy. Significantly, our study reveals that STK38 exerts an inhibitory activity on BAG3-mediated autophagy. Inhibition relies on a disruption of the functional interplay of BAG3 with HSPB8 and SYNPO2 upon binding of STK38 to the cochaperone. Of note, STK38 attenuates CASA independently of its kinase activity, whereas previously established regulatory functions of STK38 involve target phosphorylation. The ability to exert different modes of regulation on central protein homeostasis (proteostasis) machineries apparently allows STK38 to coordinate the execution of diverse macroautophagy pathways and to balance cytoskeleton assembly and degradation.

A modular DNA scaffold to study protein-protein interactions at single-molecule resolution

The residence time of a drug on its target has been suggested as a more pertinent metric of therapeutic efficacy than the traditionally used affinity constant. Here, we introduce junctured-DNA tweezers as a generic platform that enables real-time observation, at the single-molecule level, of biomolecular interactions. This tool corresponds to a double-strand DNA scaffold that can be nanomanipulated and on which proteins of interest can be engrafted thanks to widely used genetic tagging strategies. Thus, junctured-DNA tweezers allow a straightforward and robust access to single-molecule force spectroscopy in drug discovery, and more generally in biophysics. Proof-of-principle experiments are provided for the rapamycin-mediated association between FKBP12 and FRB, a system relevant in both medicine and chemical biology. Individual interactions were monitored under a range of applied forces and temperatures, yielding after analysis the characteristic features of the energy profile along the dissociation landscape.

Identification of Filamin A Mechanobinding Partner II: Fimbacin Is a Novel Actin Cross-Linking and Filamin A Binding Protein

Filamin A (FLNA), an actin cross-linking protein, acts as a mechanosensor and mechanotransducer by exposing the cryptic binding site on repeat 21 (R21) to interact with integrin. Here, we investigated if any other biological molecule interacts with the cryptic binding site. Using proteomics and an in silico screening for a FLNA-binding motif, we identified and characterized a protein termed fimbacin (filamin mechanobinding actin cross-linking protein), encoded in the LUZP1 gene, as a novel FLNA-binding partner. Fimbacin does not interact with canonical full-length FLNA, but the exposure of a cryptic integrin-binding site of FLNA R21 enables fimbacin to interact. We have identified two FLNA binding sites on fimbacin and determined critical amino acid residues for the interaction. We also found that fimbacin itself is a new actin cross-linking protein and mapped the actin-binding site on amino acid residues 400-500. Fimbacin oligomerizes (estimated as an octamer on size exclusion chromatography) through the amino-terminal domain that is predicted to be a coiled-coil to cross-link actin filaments. When expressed, fimbacin localized to actin stress fibers in tissue culture cells. Although the interaction with FLNA is not necessary for fimbacin to colocalize with F-actin, fluorescent recovery after photobleaching (FRAP) revealed that their interaction stabilizes fimbacin on the actin cytoskeleton and that inhibition of Rho-kinase, an upstream activator of myosin II, also decreases the interaction presumably due to a loss of internal mechanical stress. Taken together, these data identify fimbacin as a new actin cross-linking protein that interacts with the FLNA mechanosensing domain R21.

Identification of Filamin A Mechanobinding Partner I: Smoothelin Specifically Interacts with the Filamin A Mechanosensitive Domain 21

Filamin A (FLNA) is a ubiquitously expressed actin cross-linking protein and a scaffold of numerous binding partners to regulate cell proliferation, migration, and survival. FLNA is a homodimer, and each subunit has an N-terminal actin-binding domain followed by 24 immunoglobulin-like repeats (R). FLNA mediates mechanotransduction by force-induced conformational changes of its cryptic integrin-binding site on R21. Here, we identified two novel FLNA-binding partners, smoothelins (SMTN A and B) and leucine zipper protein 1 (LUZP1), using stable isotope labeling by amino acids in cell culture (SILAC)-based proteomics followed by an in silico screening for proteins having a consensus FLNA-binding domain. We found that, although SMTN does not interact with full-length FLNA, it binds to FLNA variant 1 (FLNAvar-1) that exposes the cryptic CD cleft of R21. Point mutations on the C strand that disrupt the integrin binding also block the SMTN interaction. We identified FLNA-binding domains on SMTN using mutagenesis and used the mutant SMTN to investigate the role of the FLNA-SMTN interaction on the dynamics and localization of SMTN in living cells. Fluorescence recovery after photobleaching (FRAP) of GFP-labeled SMTN in living cells demonstrated that the non-FLNA-binding mutant SMTN diffuses faster than wild-type SMTN. Moreover, inhibition of Rho-kinase using Y27632 also increases the diffusion. These data demonstrated that SMTN specifically interacts with FLNAvar-1 and mechanically activated FLNA in cells. The companion report (Wang and Nakamura, 2019) describes the interactions of FLNA with the transcript of the LUZP1 gene.