Molecular basis of mechanosensory transduction

Lipid-protein interactions: Lessons learned from stress

Biological membranes are essential for normal function and regulation of cells, forming a physical barrier between extracellular and intracellular space and cellular compartments. These physical barriers are subject to mechanical stresses. As a consequence, nature has developed proteins that are able to transpose mechanical stimuli into meaningful intracellular signals. These proteins, termed Mechanosensitive (MS) proteins provide a variety of roles in response to these stimuli. In prokaryotes these proteins form transmembrane spanning channels that function as osmotically activated nanovalves to prevent cell lysis by hypoosmotic shock. In eukaryotes, the function of MS proteins is more diverse and includes physiological processes such as touch, pain and hearing. The transmembrane portion of these channels is influenced by the physical properties such as charge, shape, thickness and stiffness of the lipid bilayer surrounding it, as well as the bilayer pressure profile. In this review we provide an overview of the progress to date on advances in our understanding of the intimate biophysical and chemical interactions between the lipid bilayer and mechanosensitive membrane channels, focusing on current progress in both eukaryotic and prokaryotic systems. These advances are of importance due to the increasing evidence of the role the MS channels play in disease, such as xerocytosis, muscular dystrophy and cardiac hypertrophy. Moreover, insights gained from lipid-protein interactions of MS channels are likely relevant not only to this class of membrane proteins, but other bilayer embedded proteins as well. This article is part of a Special Issue entitled: Lipid-protein interactions.

A short splice form of Xin-actin binding repeat containing 2 (XIRP2) lacking the Xin repeats is required for maintenance of stereocilia morphology and hearing function

Approximately one-third of known deafness genes encode proteins located in the hair bundle, the sensory hair cell's mechanoreceptive organelle. In previous studies, we used mass spectrometry to characterize the hair bundle's proteome, resulting in the discovery of novel bundle proteins. One such protein is Xin-actin binding repeat containing 2 (XIRP2), an actin-cross-linking protein previously reported to be specifically expressed in striated muscle. Because mutations in other actin-cross-linkers result in hearing loss, we investigated the role of XIRP2 in hearing function. In the inner ear, XIRP2 is specifically expressed in hair cells, colocalizing with actin-rich structures in bundles, the underlying cuticular plate, and the circumferential actin belt. Analysis using peptide mass spectrometry revealed that the bundle harbors a previously uncharacterized XIRP2 splice variant, suggesting XIRP2's role in the hair cell differs significantly from that reported in myocytes. To determine the role of XIRP2 in hearing, we applied clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9-mediated genome-editing technology to induce targeted mutations into the mouse Xirp2 gene, resulting in the elimination of XIRP2 protein expression in the inner ear. Functional analysis of hearing in the resulting Xirp2-null mice revealed high-frequency hearing loss, and ultrastructural scanning electron microscopy analyses of hair cells demonstrated stereocilia degeneration in these mice. We thus conclude that XIRP2 is required for long-term maintenance of hair cell stereocilia, and that its dysfunction causes hearing loss in the mouse.

Substrate-bound protein gradients to study haptotaxis

Cells navigate in response to inhomogeneous distributions of extracellular guidance cues. The cellular and molecular mechanisms underlying migration in response to gradients of chemical cues have been investigated for over a century. Following the introduction of micropipettes and more recently microfluidics for gradient generation, much attention and effort was devoted to study cellular chemotaxis, which is defined as guidance by gradients of chemical cues in solution. Haptotaxis, directional migration in response to gradients of substrate-bound cues, has received comparatively less attention; however, it is increasingly clear that in vivo many physiologically relevant guidance proteins - including many secreted cues - are bound to cellular surfaces or incorporated into extracellular matrix and likely function via a haptotactic mechanism. Here, we review the history of haptotaxis. We examine the importance of the reference surface, the surface in contact with the cell that is not covered by the cue, which forms a gradient opposing the gradient of the protein cue and must be considered in experimental designs and interpretation of results. We review and compare microfluidics, contact printing, light patterning, and 3D fabrication to pattern substrate-bound protein gradients in vitro. The range of methods to create substrate-bound gradients discussed herein makes possible systematic analyses of haptotactic mechanisms. Furthermore, understanding the fundamental mechanisms underlying cell motility will inform bioengineering approaches to program cell navigation and recover lost function.

Optical mechanotransduction with carbazole-based luminescent liquid single-crystal elastomers

Carbazole-based liquid single-crystal elastomers (LSCEs) are valuable fluorescent flexible materials to perform optical mechanotransduction under ambient conditions. Indeed, the covalent incorporation of carbazole derivatives into nematic LSCEs allows to tune their luminescence on demand under mechanical control in a quick and reversible fashion. Specifically, the fluorescence intensity for these materials can be switched back and forth in less than a second. Moreover, such a process can be performed several times without detecting any sign of fatigue in the system. In addition, these materials show excellent resistance to aging; 2 years after their preparation they exhibit the very same mechanofluorescent behavior as when freshly prepared. In fact, the here reported fluorescent systems are highly sensitive; the application of a force of 70 mN decreases the fluorescence in the elastomeric material by 7%. Thus, mechanical forces are attractive external stimuli to modulate the fluorescence of nematic elastomers rapidly and reversibly enabling thereby mechanotransduction.

Mechanochemistry in Polymers with Supramolecular Mechanophores

Mechanochemistry is a burgeoning field of materials science. Inspired by nature, many scientists have looked at different ways to introduce weak bonds into polymeric materials to impart them with function and in particular mechano-responsiveness. In the following sections, the incorporation of some of the weakest bonds, i.e. non-covalent bonds, into polymeric solids is being surveyed. This review covers sequentially π-π interactions, H-bonding and metal-ligand coordination bonds and tries to highlight some of the advantages and limitations of such systems, while providing some key perspective of what may come next in this tantalizing field.

Collective cell migration induced by mechanical stress and substrate adhesiveness

Mechanical stress normal to the boundary of a tissue sheet can arise in both constrained as well as unconstrained epithelial layers through pushing and pulling of surrounding tissue and substrate adhesiveness, respectively. A continuum model is used to investigate how such stress influences the epithelial dynamics. Four types of spreading and motility can be identified: a uniformly stretched stationary state, uniform sheet migration, active stress compensation by polarization close to the boundary, and a wormlike progression by deformation waves. Analytical and numerical solutions are presented along with bifurcation diagrams using normal stress and active force as control parameters.

A multi-scale computational assessment of channel gating assumptions within the Meissner corpuscle

From the macroscopic mechanical deformation of skin to the feeling of touch is a chain of complex events whereby information is converted from one form to another between different scales. An important link in this chain is receptor activation, which requires incorporation of microanatomical, cellular and ion channel transduction models. Of particular interest is the deformations at the axon membrane bi-layer, which are believed to be involved in mechanoelectrical signal transduction by activation of ion channels. We present a fully coupled multi-scale finite element analysis of the finger pad during tactile exploration, whereby the Meissner corpuscle, which is modeled as a single representative volume element (RVE) at the microscopic level, interacts with the macroscopic finger model. Maximum values of local stretching and compression occurring at the bi-layer are monitored for finger models with and without fingerprints, the presence of which generates a remarkable amplification of the signal. The contours of the surface being explored are well represented by the maximal peaks observed within the membrane.

Mechanotransduction in intervertebral discs

Mechanotransduction plays a critical role in intracellular functioning—it allows cells to translate external physical forces into internal biochemical activities, thereby affecting processes ranging from proliferation and apoptosis to gene expression and protein synthesis in a complex web of interactions and reactions. Accordingly, aberrant mechanotransduction can either lead to, or be a result of, a variety of diseases or degenerative states. In this review, we provide an overview of mechanotransduction in the context of intervertebral discs, with a focus on the latest methods of investigating mechanotransduction and the most recent findings regarding the means and effects of mechanotransduction in healthy and degenerative discs. We also provide some discussion of potential directions for future research and treatments.

Magnetic nanoparticles for ultrafast mechanical control of inner ear hair cells

We introduce cubic magnetic nanoparticles as an effective tool for precise and ultrafast control of mechanosensitive cells. The temporal resolution of our system is ∼1000 times faster than previously used magnetic switches and is comparable to the current state-of-the-art optogenetic tools. The use of a magnetism-gated switch reported here can address the key challenges of studying mechanotransduction in biological systems. The cube-shaped magnetic nanoparticles are designed to bind to components of cellular membranes and can be controlled with an electromagnet to exert pico-Newtons of mechanical force on the cells. The cubic nanoparticles can thus be used for noncontact mechanical control of the position of the stereocilia of an inner ear hair cell, yielding displacements of tens of nanometers, with sub-millisecond temporal resolution. We also prove that such mechanical stimulus leads to the influx of ions into the hair cell. Our study demonstrates that a magnetic switch can yield ultrafast temporal resolution, and has capabilities for remote manipulation and biological specificity, and that such magnetic system can be used for the study of mechanotransduction processes of a wide range of sensory systems.

Mechanochemistry with metallosupramolecular polymers

The transduction of mechanical force into useful chemical reactions is an emerging design approach to impart soft materials with new functions. Here, we report that mechanochemical transductions can be achieved in metallosupramolecular polymers. We show that both reversible and irreversible reactions are possible and useful to create mechanically responsive materials that display new functions. The metallopolymer studied was a cross-linked network assembled from a europium salt and a telechelic poly(ethylene-co-butylene) with 2,6-bis(1'-methylbenzimidazolyl)pyridine (Mebip) ligands at the termini. The Eu(3+) complexes serve both as mechanically responsive binding motifs and as built-in optical probes that can monitor the extent of (dis)assembly due to their characteristic photoluminescent properties. Indeed, dose-dependent and reversible metal-ligand dissociation occurs upon exposure to ultrasound in solution. The absence of ultrasound-induced dissociation of a low-molecular weight model complex and in-depth studies of temperature effects confirm that the dissociation is indeed the result of mechanical activation. The influence of the strength of the metal-ligand interactions on the mechanically induced dissociation was also explored. Metallopolymers in which the Mebip ligands were substituted with more strongly coordinating dipicolinate (dpa) ligands do not dissociate upon exposure to ultrasound. Finally, we show that mechanochemical transduction in metallosupramolecular polymers is also possible in the solid state. We demonstrate mending of damaged objects through ultrasound as well as mechanochromic behavior based on metal-exchange reactions in metallopolymers imbibed with an auxiliary metal salt.

Mechanical signaling via nonlinear wavefront propagation in a mechanically excitable medium

Models that invoke nonlinear wavefront propagation in a chemically excitable medium are rife in the biological literature. Indeed, the idea that wavefront propagation can serve as a signaling mechanism has often been invoked to explain synchronization of developmental processes. In this paper we suggest a kind of signaling based not on diffusion of a chemical species but on the propagation of mechanical stress. We construct a theoretical approach to describe mechanical signaling as a nonlinear wavefront propagation problem and study its dependence on key variables such as the effective elasticity and damping of the medium.

Bacterial mechanosensitive channels: models for studying mechanosensory transduction

SIGNIFICANCE

Sensations of touch and hearing are manifestations of mechanical contact and air pressure acting on touch receptors and hair cells of the inner ear, respectively. In bacteria, osmotic pressure exerts a significant mechanical force on their cellular membrane. Bacteria have evolved mechanosensitive (MS) channels to cope with excessive turgor pressure resulting from a hypo-osmotic shock. MS channel opening allows the expulsion of osmolytes and water, thereby restoring normal cellular turgor and preventing cell lysis.

RECENT ADVANCES

As biological force-sensing systems, MS channels have been identified as the best examples of membrane proteins coupling molecular dynamics to cellular mechanics. The bacterial MS channel of large conductance (MscL) and MS channel of small conductance (MscS) have been subjected to extensive biophysical, biochemical, genetic, and structural analyses. These studies have established MscL and MscS as model systems for mechanosensory transduction.

CRITICAL ISSUES

In recent years, MS ion channels in mammalian cells have moved into focus of mechanotransduction research, accompanied by an increased awareness of the role they may play in the pathophysiology of diseases, including cardiac hypertrophy, muscular dystrophy, or Xerocytosis.

FUTURE DIRECTIONS

A recent exciting development includes the molecular identification of Piezo proteins, which function as nonselective cation channels in mechanosensory transduction associated with senses of touch and pain. Since research on Piezo channels is very young, applying lessons learned from studies of bacterial MS channels to establishing the mechanism by which the Piezo channels are mechanically activated remains one of the future challenges toward a better understanding of the role that MS channels play in mechanobiology.

Physiological and pathological functions of mechanosensitive ion channels

Rapid sensation of mechanical stimuli is often mediated by mechanosensitve ion channels. Their opening results from conformational changes induced by mechanical forces. It leads to membrane permeation of selected ions and thereby to electrical signaling. Newly identified mechanosensitive ion channels are emerging at an astonishing rate, including some that are traditionally assigned for completely different functions. In this review, we first provide a brief overview of ion channels that are known to play a role in mechanosensation. Next, we focus on three representative ones, including the transient receptor potential channel V4 (TRPV4), Kv1.1 voltage-gated potassium (Kv) channel, and Piezo channels. Their structures, biophysical properties, expression and targeting patterns, and physiological functions are highlighted. The potential role of their mechanosensation in related diseases is further discussed. In sum, mechanosensation appears to be achieved in a variety of ways by different proteins and plays a fundamental role in the function of various organs under normal and abnormal conditions.

The extracellular matrix protein artichoke is required for integrity of ciliated mechanosensory and chemosensory organs in Drosophila embryos

Sensory cilia are often encapsulated by an extracellular matrix (ECM). In Caenorhabditis elegans, Drosophila melanogaster, and vertebrates, this ECM is thought to be directly involved in ciliary mechanosensing by coupling external forces to the ciliary membrane. Drosophila mechano- and chemosensory cilia are both associated with an ECM, indicating that the ECM may have additional roles that go beyond mechanosensory cilium function. Here, we identify Artichoke (ATK), an evolutionarily conserved leucine-rich repeat ECM protein that is required for normal morphogenesis and function of ciliated sensilla in Drosophila. atk is transiently expressed in accessory cells in all ciliated sensory organs during their late embryonic development. Antibody stainings show ATK protein in the ECM that surrounds sensory cilia. Loss of ATK protein in atk null mutants leads to cilium deformation and disorientation in chordotonal organs, apparently without uncoupling the cilia from the ECM, and consequently to locomotion defects. Moreover, impaired chemotaxis in atk mutant larvae suggests that, based on ATK protein localization, the ECM is also crucial for the correct assembly of chemosensory receptors. In addition to defining a novel ECM component, our findings show the importance of ECM integrity for the proper morphogenesis of ciliated organs in different sensory modalities.

Mechanosensitive channels are activated by stress in the actin stress fibres, and could be involved in gravity sensing in plants

Mechanosensitive (MS) channels are expressed in a variety of cells. The molecular and biophysical mechanism involved in the regulation of MS channel activities is a central interest in basic biology. MS channels are thought to play crucial roles in gravity sensing in plant cells. To date, two mechanisms have been proposed for MS channel activation. One is that tension development in the lipid bilayer directly activates MS channels. The second mechanism proposes that the cytoskeleton is involved in the channel activation, because MS channel activities are modulated by pharmacological treatments that affect the cytoskeleton. We tested whether tension in the cytoskeleton activates MS channels. Mammalian endothelial cells were microinjected with phalloidin-conjugated beads, which bound to stress fibres, and a traction force to the actin cytoskeleton was applied by dragging the beads with optical tweezers. MS channels were activated when the force was applied, demonstrating that a sub-pN force to the actin filaments activates a single MS channel. Plants may use a similar molecular mechanism in gravity sensing, since the cytoplasmic Ca(2+) concentration increase induced by changes in the gravity vector was attenuated by potential MS channel inhibitors, and by actin-disrupting drugs. These results support the idea that the tension increase in actin filaments by gravity-dependent sedimentation of amyloplasts activates MS Ca(2+) -permeable channels, which can be the molecular mechanism of a Ca(2+) concentration increase through gravistimulation. We review recent progress in the study of tension sensing by actin filaments and MS channels using advanced biophysical methods, and discuss their possible roles in gravisensing.

TRPs in mechanosensing and volume regulation

Mechanosensitive channels allow cells to respond to changes in membrane stretch that occur due to external stimuli like pressure or flow or that occur because of osmotically induced cell swelling or shrinkage. Ion fluxes through the channels change the membrane potential and ion concentrations and link the stretch to cellular signalling. Changes in cellular activity evoked by mechanical stimuli can be used to elicit local tissue responses or can be transmitted further to generate more widespread responses. Channels can respond directly to membrane stress, can be conferred mechanosensitive by interaction with structural proteins, or can be activated by mechanosensitive signalling pathways. Because mechanosensitive channels are often nonselective cation channels, and invertebrate TRP isoforms are involved in mechanosensation, many of the mammalian TRP isoforms have been investigated with regard to their mechanosensitivity. There is evidence that members of the TRPC, TRPV, TRPM, TRPA and TRPP subfamilies could be in some way mechanosensitive, and each of the activation mechanisms described above is used by a TRP channel. TRP channels may be involved in mechanosensitive processes ranging from flow and pressure sensing in the vasculature and other organs to mechanosensation in sensory neurones and sensory organs. There is also evidence for a role of mechano- or osmosensitive TRP isoforms in osmosensing and the regulation of cell volume. Often, a number of different TRP isoforms have been implicated in a single type of mechanosensitive response. In many cases, the involvement of the isoforms needs to be confirmed, and their exact role in the signalling process determined.

Role of endogenous ENaC and TRP channels in the myogenic response of rat posterior cerebral arteries

AIMS

Mechanogated ion channels are predicted to mediate pressure-induced myogenic vasoconstriction in small resistance arteries. Recent findings have indicated that transient receptor potential (TRP) channels and epithelial sodium channels (ENaC) are involved in mechanotransduction. The purpose of this study was to investigate the role of TRP channels and ENaC in the myogenic response. Our previous study suggested that ENaC could be a component of the mechanosensitive ion channels in rat posterior cerebral arteries (PCA). However, the specific ion channel proteins mediating myogenic constriction are unknown. Here we found, for the first time, that ENaC interacted with TRPM4 but not with TRPC6 using immunoprecipitation and confocal microscopy.

METHODS AND RESULTS

Treatment with a specific βENaC inhibitor, amiloride, a specific TRPM4 inhibitor, 9-phenanthrol, and a TRPC6 inhibitor, SKF96365, resulted in inhibition of the pressure-induced myogenic response. Moreover, the myogenic response was inhibited in rat PCA transfected with small interfering RNA of βENaC, TRPM4, and TRPC6. Co-treatment with amiloride and 9-phenanthrol showed a similar inhibitory effect on myogenic contraction compared to single treatment with amiloride or 9-phenanthrol. The myogenic response was not affected by 9-phenanthrol or amiloride treatment in PCA transfected with βENaC or TRPM4 siRNA, respectively. However, pressure-induced myogenic response was fully inhibited by co-treatment with amiloride, 9-phenanthrol, and SKF96365, and by treatment with SKF96365 in PCA transfected with βENaC siRNA.

CONCLUSION

Our results suggest that ENaC, TRPM4, and TRPC6 play important roles in the pressure-induced myogenic response, and that ENaC and TRPM4 interact in rat PCA.

SelR reverses Mical-mediated oxidation of actin to regulate F-actin dynamics

Actin's polymerization properties are markedly altered by oxidation of its conserved Met 44 residue. Mediating this effect is a specific oxidation-reduction (redox) enzyme, Mical, that works with Semaphorin repulsive guidance cues and selectively oxidizes Met 44. We now find that this actin-regulatory process is reversible. Employing a genetic approach, we identified a specific methionine sulfoxide reductase (MsrB) enzyme SelR that opposes Mical redox activity and Semaphorin-Plexin repulsion to direct multiple actin-dependent cellular behaviours in vivo. SelR specifically catalyses the reduction of the R isomer of methionine sulfoxide (methionine-R-sulfoxide) to methionine, and we found that SelR directly reduced Mical-oxidized actin, restoring its normal polymerization properties. These results indicate that Mical oxidizes actin stereospecifically to generate actin Met-44-R-sulfoxide (actin(Met(R)O-44)), and also implicate the interconversion of specific Met/Met(R)O residues as a precise means to modulate protein function. Our results therefore uncover a specific reversible redox actin regulatory system that controls cell and developmental biology.

Mechanosensation: tethered ion channels

Two new studies show that the Drosophila transient receptor potential (TRP) family member NOMPC forms both a mechanically gated ion channel and a fine filament that, by tethering the protein to microtubules, might act as a gating spring.

Mechanical-stretch of C2C12 myoblasts inhibits expression of Toll-like receptor 3 (TLR3) and of autoantigens associated with inflammatory myopathies

Recent studies in patients suffering from inflammatory autoimmune myopathies suggested that moderate exercise training improves or at least stabilizes muscle strength and function without inducing disease flares. However, the precise mechanisms involved in this beneficial effect have not been extensively studied. Here we used a model of in vitro stretched C2C12 myoblasts to investigate whether mechanical stretch could influence myoblast proliferation or the expression of proinflammatory genes. Our results demonstrated that cyclic mechanical stretch stimulated C2C12 cell cycling and early up-regulation of the molecules related to mechanical-stretch pathway in muscle (calmodulin, nNOS, MMP-2, HGF and c-Met). Unexpectedly, mechanical stretch also reduced the expression of TLR3 and of proteins known to represent autoantigens in inflammatory autoimmune myopathies (Mi-2, HRS, DNA-PKcs, U1-70). Interestingly, stimulation or inhibition of calmodulin, NOS, HGF or c-Met molecules in vitro affected the expression of autoantigens and TLR3 proteins confirming their role in the inhibition of autoantigens and TLR3 during mechanical stretch. Overall, this study demonstrates for the first time that mechanical stretch could be beneficial by reducing expression of muscle autoantigens and of pro-inflammatory TLR3 and may provide new insight to understand how resistance training can reduce the symptoms associated with myositis.

Mechanotransduction down to individual actin filaments

The actin cytoskeleton plays an essential role in a cell's ability to generate and sense forces, both internally and in interaction with the outside world. The transduction of mechanical cues into biochemical reactions in cells, in particular, is a multi-scale process which requires a variety of approaches to be understood. This review focuses on understanding how mechanical stress applied to an actin filament can affect its assembly dynamics. Today, experiments addressing this issue at the scale of individual actin filaments are emerging and bring novel insight into mechanotransduction. For instance, recent data show that actin filaments can act as mechanosensors, as an applied tension or curvature alters their conformation and their affinity for regulatory proteins. Filaments can also transmit mechanical tension to other proteins, which consequently change the way they interact with the filaments to regulate their assembly. These results provide evidence for mechanotransduction at the scale of individual filaments, showing that forces participate in the regulation of filament assembly and organization. They bring insight into the elementary events coupling mechanics and biochemistry in cells. The experiments presented here are linked to recent technical developments, and certainly announce the advent of more exciting results in the future.

Differentiating haemostasis from thrombosis for therapeutic benefit

The central role of platelets in the formation of the primary haemostatic plug as well as in the development of arterial thrombosis is well defined. In general, the molecular events underpinning these processes are broadly similar. Whilst it has long been known that disturbances in blood flow, changes in platelet reactivity and enhanced coagulation reactions facilitate pathological thrombus formation, the precise details underlying these events remain incompletely understood. Intravital microscopy studies have highlighted the dynamic and heterogeneous nature of thrombus development and demonstrated that there are considerable spatiotemporal differences in the activation states of platelets within a forming thrombus. In this review we will consider the factors regulating the activation state of platelets in a developing thrombus and discuss how specific prothrombotic factors may influence this process, leading to excessive thrombus propagation. We will also discuss some potentially novel therapeutic approaches that may reduce excess thrombus development whilst minimising bleeding risk.

Tonotopic morphometry of the lamina reticularis of the guinea pig cochlea with associated microstructures and related mechanical implications

Morphometry of the lamina reticularis of the guinea pig cochlea was performed using scanning electron microscopy. Seventy-four geometrical parameters of the lamina reticularis, the bundles of stereocilia, and individual stereocilia, in all rows of hair cells and within the individual hair cells, were measured at ten equally spaced locations along the longitudinal direction of the cochlea. Variations of the parameters versus the longitudinal coordinate were statistically analyzed and fitted with polynomials (constant, linear, or quadratic). Our data show that a unique set of geometrical parameters of inner and outer hair cells is typical for every frequency-dependent position at the lamina reticularis. Morphology of the outer hair cell structures varies more than respective parameters of the inner hair cells. Mechanical modeling using the obtained geometrical parameters provides a novel glance at the mechanical characteristics with respect to the cochlear tonotopy.

Piezo1-dependent stretch-activated channels are inhibited by Polycystin-2 in renal tubular epithelial cells

Mechanical forces associated with fluid flow and/or circumferential stretch are sensed by renal epithelial cells and contribute to both adaptive or disease states. Non-selective stretch-activated ion channels (SACs), characterized by a lack of inactivation and a remarkably slow deactivation, are active at the basolateral side of renal proximal convoluted tubules. Knockdown of Piezo1 strongly reduces SAC activity in proximal convoluted tubule epithelial cells. Similarly, overexpression of Polycystin-2 (PC2) or, to a greater extent its pathogenic mutant PC2-740X, impairs native SACs. Moreover, PC2 inhibits exogenous Piezo1 SAC activity. PC2 coimmunoprecipitates with Piezo1 and deletion of its N-terminal domain prevents both this interaction and inhibition of SAC activity. These findings indicate that renal SACs depend on Piezo1, but are critically conditioned by PC2.

Mechano-regulation of alternative splicing

Alternative splicing contributes to the complexity of proteome by producing multiple mRNAs from a single gene. Affymetrix exon arrays and experiments in vivo or in vitro demonstrated that alternative splicing was regulated by mechanical stress. Expression of mechano-growth factor (MGF) which is the splicing isoform of insulin-like growth factor 1(IGF-1) and vascular endothelial growth factor (VEGF) splicing variants such as VEGF₁₂₁, VEGF_165, VEGF₂₀₆, VEGF₁₈₉, VEGF₁₆₅ and VEGF₁₄₅ are regulated by mechanical stress. However, the mechanism of this process is not yet clear. Increasing evidences showed that the possible mechanism is related to Ca²⁺ signal pathway and phosphorylation signal pathway. This review proposes possible mechanisms of mechanical splicing regulation. This will contribute to the biomechanical study of alternative splicing.

Sound response mediated by the TRP channels NOMPC, NANCHUNG, and INACTIVE in chordotonal organs of Drosophila larvae

Mechanical stimuli, including tactile and sound signals, convey a variety of information important for animals to navigate the environment and avoid predators. Recent studies have revealed that Drosophila larvae can sense harsh or gentle touch with dendritic arborization (da) neurons in the body wall and can detect vibration with chordotonal organs (Cho). Whether they can also detect and respond to vibration or sound from their predators remains an open question. Here we report that larvae respond to sound of wasps and yellow jackets, as well as to pure tones of frequencies that are represented in such natural sounds, with startle and burrowing behaviors. The larval response to sound/vibration requires Cho neurons and, to a lesser extent, class IV da neurons. Our calcium imaging and electrophysiological experiments reveal that Cho neurons, but not class IV da neurons, are excited by natural sounds or pure tones, with tuning curves and intensity dependence appropriate for the behavioral responses. Furthermore, our study implicates the transient receptor potential (TRP) channels NOMPC, NANCHUNG, and INACTIVE, but not the dmPIEZO channel, in the mechanotransduction and/or signal amplification for the detection of sound by the larval Cho neurons. These findings indicate that larval Cho, like their counterparts in the adult fly, use some of the same mechanotransduction channels to detect sound waves and mediate the sensation akin to hearing in Drosophila larvae, allowing them to respond to the appearance of predators or other environmental cues at a distance with behaviors crucial for survival.

The ion channels to cytoskeleton connection as potential mechanism of mechanosensitivity

As biological force-sensing systems mechanosensitive (MS) ion channels present the best example of coupling molecular dynamics of membrane proteins to the mechanics of the surrounding cell membrane. In animal cells MS channels have over the past two decades been very much in focus of mechanotransduction research. In recent years this helped to raise awareness of basic and medical researchers about the role that abnormal MS channels may play in the pathophysiology of diseases, such as cardiac hypertrophy, atrial fibrillation, muscular dystrophy or polycystic kidney disease. To date a large number of MS channels from organisms of diverse phylogenetic origins have been identified at the molecular level; however, the structure of only few of them has been determined. Although their function has extensively been studied in a great variety of cells and tissues by different experimental approaches it is, with exception of bacterial MS channels, very little known about how these channels sense mechanical force and which cellular components may contribute to their function. By focusing on MS channels found in animal cells this article discusses the ways in which the connections between cytoskeleton and ion channels may contribute to mechanosensory transduction in these cells. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Identification and analysis of putative homologues of mechanosensitive channels in pathogenic protozoa

Mechanosensitive channels play important roles in the physiology of many organisms, and their dysfunction can affect cell survival. This suggests that they might be therapeutic targets in pathogenic organisms. Pathogenic protozoa lead to diseases such as malaria, dysentery, leishmaniasis and trypanosomiasis that are responsible for millions of deaths each year worldwide. We analyzed the genomes of pathogenic protozoa and show the existence within them of genes encoding putative homologues of mechanosensitive channels. Entamoeba histolytica, Leishmania spp., Trypanosoma cruzi and Trichomonas vaginalis have genes encoding homologues of Piezo channels, while most pathogenic protozoa have genes encoding homologues of mechanosensitive small-conductance (MscS) and K⁺-dependent (MscK) channels. In contrast, all parasites examined lack genes encoding mechanosensitive large-conductance (MscL), mini-conductance (MscM) and degenerin/epithelial Na⁺ (DEG/ENaC) channels. Multiple sequence alignments of evolutionarily distant protozoan, amoeban, plant, insect and vertebrate Piezo channel subunits define an absolutely conserved motif that may be involved in channel conductance or gating. MscS channels are not present in humans, and the sequences of protozoan and human homologues of Piezo channels differ substantially. This suggests the possibility for specific targeting of mechanosensitive channels of pathogens by therapeutic drugs.

A novel role of cytosolic protein synthesis inhibition in aminoglycoside ototoxicity

Ototoxicity is a main dose-limiting factor in the clinical application of aminoglycoside antibiotics. Despite longstanding research efforts, our understanding of the mechanisms underlying aminoglycoside ototoxicity remains limited. Here we report the discovery of a novel stress pathway that contributes to aminoglycoside-induced hair cell degeneration. Modifying the previously developed bioorthogonal noncanonical amino acid tagging method, we used click chemistry to study the role of protein synthesis activity in aminoglycoside-induced hair cell stress. We demonstrate that aminoglycosides inhibit protein synthesis in hair cells and activate a signaling pathway similar to ribotoxic stress response, contributing to hair cell degeneration. The ability of a particular aminoglycoside to inhibit protein synthesis and to activate the c-Jun N-terminal kinase (JNK) pathway correlated well with its ototoxic potential. Finally, we report that a Food and Drug Administration-approved drug known to inhibit ribotoxic stress response also prevents JNK activation and improves hair cell survival, opening up novel strategies to prevent and treat aminoglycoside ototoxicity.

Hybrid mechanoresponsive polymer wires under force activation

Force activation is triggered in a stretched polymer wire with color changes produced as a consequence of the molecules undergoing structural and conformational changes. A markedly increased efficiency of force activation is achieved by decreasing the diameter of the wires. The hybrid mechanosensitive polymer wire can function as micro- and nanoscale force sensor.

Mechano-sensing by actin filaments and focal adhesion proteins

Mechanosensitive ion channels have long been the only established molecular class of cell mechanosensors with known molecular entities. However, recent advances in the state-of-the-art techniques, including single-molecule manipulation and imaging, have enabled an investigation of non-channel type cell mechanosensors and the underlying biophysical mechanisms of their activation. To date, two focal adhesion proteins, talin and p130Cas, have been postulated to act as putative mechanosensors, acting through mechano-induced unfolding of their particular soft domain(s) susceptible to phosphorylation. More recently, the actin filament has been demonstrated to act as a mechanosensor in the presence of the soluble actin-severing protein, cofilin. The cofilin severing activity negatively depends on the tension in the actin filament through tension-dependent binding/unbinding of cofilin to/from the actin filament. As a result, relaxed actin filaments are severed, while tensed ones are either not severed or severed after a long delay. Here we review the latest progress in the mechanosensing by non-channel type proteins and discuss the possible physiological roles of the mechanosensing performed by actin filaments in the course of cell migration.

Cyclic nucleotide-gated channel α-3 (CNGA3) interacts with stereocilia tip-link cadherin 23 + exon 68 or alternatively with myosin VIIa, two proteins required for hair cell mechanotransduction

Previously, we obtained evidence for a photoreceptor/olfactory type of CNGA3 transcript in a purified teleost vestibular hair cell preparation with immunolocalization of CNGA3 protein to stereocilia of teleost vestibular and mammalian cochlear hair cells. The carboxyl terminus of highly Ca(2+)-permeable CNGA3 expressed in the mammalian organ of Corti and saccular hair cells was found to interact with an intracellular domain of microfibril interface-located protein 1 (EMILIN 1), a member of the elastin superfamily, also immunolocalizd to hair cell stereocilia (Selvakumar, D., Drescher, M. J., Dowdall, J. R., Khan, K. M., Hatfield, J. S., Ramakrishnan, N. A., and Drescher, D. G. (2012) Biochem. J. 443, 463-476). Here, we provide evidence for organ of Corti proteins, of Ca(2+)-dependent binding of the amino terminus of CNGA3 specifically to the carboxyl terminus of stereocilia tip-link protein CDH23 +68 (cadherin 23 with expressed exon 68) by yeast two-hybrid mating and co-transformation protocols, pulldown assays, and surface plasmon resonance analysis. Myosin VIIa, required for adaptation of hair cell mechanotransduction (MET) channel(s), competed with CDH23 +68, with direct Ca(2+)-dependent binding to the amino terminus of CNGA3. Based upon the premise that hair cell stereocilia tip-link proteins are closely coupled with MET, these results are consistent with the possibility that CNGA3 participates in hair-cell MET. Together with the demonstration of protein-protein interaction between HCN1 and tip-link protein protocadherin 15 CD3 (Ramakrishnan, N. A., Drescher, M. J., Barretto, R. L., Beisel, K. W., Hatfield, J. S., and Drescher, D. G. (2009) J. Biol. Chem. 284, 3227-3238; Ramakrishnan, N. A., Drescher, M. J., Khan, K. M., Hatfield, J. S., and Drescher, D. G. (2012) J. Biol. Chem. 287, 37628-37646), a protein-protein interaction for CNGA3 and a second tip-link protein, CDH23 +68, further suggests possible association of two different channels with a single stereocilia tip link.

The iso-response method: measuring neuronal stimulus integration with closed-loop experiments

Throughout the nervous system, neurons integrate high-dimensional input streams and transform them into an output of their own. This integration of incoming signals involves filtering processes and complex non-linear operations. The shapes of these filters and non-linearities determine the computational features of single neurons and their functional roles within larger networks. A detailed characterization of signal integration is thus a central ingredient to understanding information processing in neural circuits. Conventional methods for measuring single-neuron response properties, such as reverse correlation, however, are often limited by the implicit assumption that stimulus integration occurs in a linear fashion. Here, we review a conceptual and experimental alternative that is based on exploring the space of those sensory stimuli that result in the same neural output. As demonstrated by recent results in the auditory and visual system, such iso-response stimuli can be used to identify the non-linearities relevant for stimulus integration, disentangle consecutive neural processing steps, and determine their characteristics with unprecedented precision. Automated closed-loop experiments are crucial for this advance, allowing rapid search strategies for identifying iso-response stimuli during experiments. Prime targets for the method are feed-forward neural signaling chains in sensory systems, but the method has also been successfully applied to feedback systems. Depending on the specific question, “iso-response” may refer to a predefined firing rate, single-spike probability, first-spike latency, or other output measures. Examples from different studies show that substantial progress in understanding neural dynamics and coding can be achieved once rapid online data analysis and stimulus generation, adaptive sampling, and computational modeling are tightly integrated into experiments.

Developmental expression patterns of kainate receptors in the mouse spinal cord

Kainate receptors, a subtype of ionotropic glutamate receptors, perform important functions in the spinal cord. This study aimed to examine the expression pattern of various kainate receptor subunits in the spinal cord over different stages of development. The regional distribution and levels of Grik1-5 mRNAs, which encode kainate receptor subunits, were examined in the spinal cord of embryonic, perinatal, and adult mice using in-situ hybridization and real-time PCR. At different developmental stages, the expression of Grik1-5 genes showed different regional distributions in the spinal cord. At E16.5, Grik2 and Grik3 were mainly expressed in the dorsal horns whereas Grik5 was expressed in the entire spinal cord. At P0 and P7, Grik2 expression accumulated at laminae II-IV, whereas Grik1 accumulated at the superficial laminae of the dorsal horns. At P30 and P60, the expression of Grik1-5 was concentrated in the superficial laminae of the dorsal horns. Development-related changes were observed in the expression pattern of Grik1-5. Grik5 was expressed in the entire spinal cord up to the perinatal period, whereas from P7 to adult stages, Grik5 expression was almost exclusively restricted to the dorsal horns. Similar observations were present with Grik1, Grik2, and Grik3. Consistently, quantitative determination of the expression levels of Grik1-5 was in accordance with the in-situ hybridization results. This age-related dynamic expression of kainate receptors may act as one driving force for the development of the anatomofunctional pattern and the maturation of the somatosensory circuitry in the spinal cord.

A mechanical model of stereocilia that demonstrates a shift in the high-sensitivity region due to the interplay of a negative stiffness and an adaptation mechanism

Stereocilia are the basic sensory units of nature's inertial sensors and are highly sensitive over broad dynamic ranges, which is a major challenge in the design of conventional engineering sensors. The high sensitivity that is maintained by stereocilia was hypothesized to exist due to a combination of adaptation and negative stiffness mechanisms, which shift the region of highest sensitivity toward the active operation range of the stereocilia bundle. To examine the adaptation hypothesis in terms of its potential applicability to future applications regarding the design of inertial sensors, we developed a mechanical mimicry of the interplay between negative stiffness and the adaptation of the stereocilia that produces spontaneous oscillation of the hair bundle. The mechanical model consists of an inverted pendulum and a fixed T-bar that mimic the interaction of two adjacent stereocilia. To focus on the interaction of one gating spring and the corresponding adaptation motor without the effect of coupling from the other gating springs attached to the neighboring stereocilia, we fixed one bar that contains the adaptation motor. To emulate the negative resistance of the tip-link due to the transient stiffness softening by the gating ion channel, a magnet pair was attached to the top of the inverted pendulum and the fixed T-bar. Readjustment of the tip-link tension by the 'slipping down and climbing up' motion of the adaptation molecular motors was demonstrated by the side-to-side movement of the magnet by a step motor. The negative stiffness region was observed near the equilibrium position and shifted with the activation of the adaptation motor. The temporal demonstration of the stiffness shift was measured as a spontaneous oscillation. The results showed that the interplay between the negative stiffness and the adaptation mechanism was mechanically produced by the combination of a repulsive force and its continuous readjustment and is better understood through a parameter study of a biomimetic mechanical system.

Sensory cilia in arthropods

In arthropods, the modified primary cilium is a structure common to all peripheral sensory neurons other than photoreceptors. Since its first description in 1958, it has been investigated in great detail in numerous sense organs (sensilla) of many insect species by means of electron microscopy and electrophysiology. The perfection of molecular biological methods has led to an enormous advance in our knowledge about development and function of sensory cilia in the fruitfly since the end of the last century. The cilia show a wealth of adaptations according to their different physiological roles: chemoreception, mechanoreception, hygroreception, and thermoreception. Divergent types of receptors and channels have evolved fulfilling these tasks. The number of olfactory receptor genes can be close to 300 in ants, whereas in crickets slightest mechanical stimuli are detected by the interaction of extremely sophisticated biomechanical devices with mechanosensory cilia. Despite their enormous morphological and physiological divergence, sensilla and sensory cilia develop according to a stereotyped pattern. Intraflagellar transport genes have been found to be decisive for proper development and function.

Evidence of interlinks between bioelectromagnetics and biomechanics: from biophysics to medical physics

A vast literature on electromagnetic and mechanical bioeffects at the bone and soft tissue level, as well as at the cellular level (osteoblasts, osteoclasts, keratinocytes, fibroblasts, chondrocytes, nerve cells, endothelial and muscle cells) has been reviewed and analysed in order to show the evident connections between both types of physical energies. Moreover, an intimate link between the two is suggested by transduction phenomena (electromagnetic-acoustic transduction and its reverse) occurring in living matter, as a sound biophysical literature has demonstrated. However, electromagnetic and mechanical signals are not always interchangeable, depending on their respective intensity. Calculations are reported in order to show in which cases (read: for which values of electric field in V/m and of mechanical pressure in Pa) a given electromagnetic or mechanical bioeffect is only due to the directly impinging energy or even to the indirect transductional energy. The relevance of the treated item for the applications of medical physics to regenerative medicine is stressed.

Immunohistochemical localization of acid-sensing ion channel 2 (ASIC2) in cutaneous Meissner and Pacinian corpuscles of Macaca fascicularis

Acid-sensing ion channel 2 (ASIC2) is a member of the degenerin/epithelial sodium channel superfamily, presumably involved mechanosensation. Expression of ASIC2 has been detected in mechanosensory neurons as well as in both axons and Schwann-like cells of cutaneous mechanoreceptors. In these studies we analysed expression of ASIC2 in the cutaneous sensory corpuscles of Macaca fascicularis using immunohistochemistry and laser confocal-scanner microscopy. ASIC2 immunoreactivity was detected in both Meissner and Pacinian corpuscles. It was found to co-localize with neuron-specific enolase and RT-97, but not with S100 protein, demonstrating that ASIC2 expression is restricted to axons supplying mechanoreceptors. These results demonstrate for the first time the presence of the protein ASIC2 in cutaneous rapidly adapting low-threshold mechanoreceptors of monkey, suggesting a role of this ion channel in touch sense.

Cellular mechanism of mechanotranscription in colonic smooth muscle cells

Mechanical stretch in obstruction induces expression of cyclooxygenase-2 (COX-2) in gut smooth muscle cells (SMCs). The stretch-induced COX-2 plays a critical role in motility dysfunction in obstructive bowel disorders (OBDs). The aims of the present study were to investigate the intracellular mechanism of mechanotranscription of COX-2 in colonic SMCs and to determine whether inhibition of mechanotranscription has therapeutic benefits in OBDs. Static stretch was mimicked in vitro in primary culture of rat colonic circular SMCs (RCCSMCs) and in colonic circular muscle strips. Partial obstruction was surgically induced with a silicon band in the distal colon of rats and COX-2-deficient mice. Static stretch of RCCSMCs significantly induced expression of COX-2 mRNA and protein and activated MAP kinases ERKs, p38, and JNKs. ERKs inhibitor PD98059, p38 inhibitor SB203580, and JNKs inhibitor SP600125 significantly blocked stretch-induced COX-2 expression. Pharmacological and molecular inhibition of stretch-activated ion channels (SACs) and integrins significantly suppressed stretch-induced expression of COX-2. SAC blockers inhibited stretch-activated ERKs, p38, and JNKs, but inhibition of integrins attenuated p38 activation only. In colonic circular muscle strips, stretch led to activation of MAPKs, induction of COX-2, and suppression of contractility. Inhibition of p38 with SB203580 blocked COX-2 expression and restored muscle contractility. Administration of SB203580 in vivo inhibited obstruction-induced COX-2 and improved motility function. Stretch-induced expression of COX-2 in RCCSMCs depends on mechanosensors, SACs, and integrins and an intracellular signaling mechanism involving MAPKs ERKs, p38, and JNKs. Inhibitors of the mechanotranscription pathway have therapeutic potentials for OBDs.