Mechanosensing Mechanisms Research Articles

Bioinspired robot skin with mechanically gated electron channels for sliding tactile perception.

Human-like tactile perception is critical for promoting robotic intelligence. However, reproducing tangential "sliding" perception of human skin is still struggling. Inspired by the lateral gating mechanosensing mechanism of mechanosensory cells, which perceives mechanical stimuli by lateral tension-induced opening-closing of ion channels, we report a robot skin (R-skin) with mechanically gated electron channels, achieving ultrasensitive and fast-response sliding tactile perception via pyramidal artificial fingerprint-triggered opening-closing of electron gates (E-gates, namely, customized V-shaped cracks within embedded mesh electron channels). By imitating cytomembrane to modulate membrane mechanics, local strain is enhanced at E-gates to effectively regulate electron pathways for high sensitivity while weakened at other positions to suppress random cracks for robust stability. The R-skin can directly recognize ultrafine surface microstructure (5 μm) at a response frequency (485 Hz) outshining humans and achieve human-like sliding perception functions, including dexterously distinguishing texture of complex-shaped objects and providing real-time feedback for grasping.

Thick filament activation is different in fast- and slow-twitch skeletal muscle.

The contractile properties of fast-twitch and slow-twitch skeletal muscles are primarily determined by the myosin isoform content and modulated by a variety of sarcomere proteins. X-ray diffraction studies of regulatory mechanisms in muscle contraction have focused predominately on fast- or mixed-fibre muscle with slow muscle being much less studied. Here, we used time-resolved X-ray diffraction to investigate the dynamic behaviour of the myofilament proteins in relatively pure slow-twitch-fibre rat soleus (SOL) and pure fast-twitch-fibre rat extensor digitorum longus (EDL) muscle during twitch and tetanic contractions at optimal length. During twitch contractions the diffraction signatures indicating a transition in the myosin heads from ordered OFF states, where heads are held close to the thick filament backbone, to disordered ON states, where heads are free to bind to thin filaments, were found in EDL and not in SOL muscle. During tetanic contraction, changes in the disposition of myosin heads as active tension develops is a quasi-stepwise process in EDL muscle whereas in SOL muscle this relationship appears to be linear. The observed reduced extensibility of the thick filaments in SOL muscle as compared to EDL muscles indicates a molecular basis for this behaviour. These data indicate that for the EDL, thick filament activation is a cooperative strain-induced mechano-sensing mechanism, whereas for the SOL, thick filament activation has a more graded response. These different approaches to thick filament regulation in fast- and slow-twitch muscles may be adaptations for short-duration, strong contractions versus sustained, finely controlled contractions, respectively. KEY POINTS: Fast-twitch muscle and slow-twitch muscle are optimized for strong, short-duration contractions and for tonic postural activity, respectively. Structural events (OFF to ON transitions) in the myosin-containing thick filaments in fast muscle help determine the timing and strength of contractions, but these have not been studied in slow-twitch muscle. The X-ray diffraction signatures of structural OFF to ON transitions are different in fast extensor digitorum longus (EDL) and slow soleus (SOL) muscle, being completely absent during twitches in soleus muscle and blunted during tetanic contractions SOL as compared to EDL Quasi-stepwise thick filament structural OFF to ON transitions in fast twitch muscle may be an adaptation for rapid, ballistic movements, whereas more graded OFF to ON structural transitions in slow-twitch muscle may be an adaptation for slower, finer motions.

Abstract 14818: Precision Nanomedicine Targeting Novel Endothelial Mechano-Sensing Mechanisms and Treating Vascular Complications in vivo

Introduction: Endothelial mechano-transduction mechanisms are instrumental to vascular health and disease. Novel strategies targeting disease-causing mechano-sensitive pathways in dysfunctional endothelial cells could revolutionize future cardiovascular therapeutics. Vascular complications such as atherosclerosis and stenosis preferentially develop at arterial curvatures and bifurcations where endothelial cells are activated by local disturbed blood flow, leading to peripheral artery disease, carotid artery disease and ischemic stroke. Hypothesis: Current vascular therapies mainly target systematic risk factors (e.g. hypercholesterolemia and hypertension) but not the diseased vasculature, distinct molecular/cellular signatures of which can be targeted by innovated precision nanomedicine approaches. Methods: We first elucidated novel mechano-sensitive molecular mechanisms in endothelium activated by disturbed flow (DF) and then engineered rationally-designed nano-materials with purposed-constructed functionalities to deliver therapeutic nucleotides to DF-activated endothelial cells. Results: Our results elucidated previously unrecognized endothelial mechano-sensitive pathways in endothelial activation, with emphasis upon cellular metabolism (DF-induced glycolysis), human genetic variants (DF-induced suppression of PLPP3, a CAD GWAS gene), miRNA, protein stability (DF-induced NOS3 protein degradation via TXNDC5) and mRNA chemical modification/epi-transcriptome (DF-induced suppression of m7G). VCAM1-targeting nanoparticles were engineered to deliver therapeutic nucleotides such as mRNA, miRNA inhibitor, or CRISPR/Cas9 constructs specifically to inflamed endothelial cells to intervene aforementioned mechano-sensitive pathways, effectively reducing atherosclerosis and stenosis in mice. Similar approaches were very effective to promote endothelial health and lessen acute respiratory distress syndrome (ARDS) in mice induced by influenza or SARS-CoV-2 viruses. Conclusions: These results elucidate novel endothelial mechano-sensing mechanisms and provide a proof of concept of innovative targeted nanomedicine approaches, addressing an unmet medical need in vascular therapies.

The glycocalyx affects the mechanotransductive perception of the topographical microenvironment

The cell/microenvironment interface is the starting point of integrin-mediated mechanotransduction, but many details of mechanotransductive signal integration remain elusive due to the complexity of the involved (extra)cellular structures, such as the glycocalyx. We used nano-bio-interfaces reproducing the complex nanotopographical features of the extracellular matrix to analyse the glycocalyx impact on PC12 cell mechanosensing at the nanoscale (e.g., by force spectroscopy with functionalised probes). Our data demonstrates that the glycocalyx configuration affects spatio-temporal nanotopography-sensitive mechanotransductive events at the cell/microenvironment interface. Opposing effects of major glycocalyx removal were observed, when comparing flat and specific nanotopographical conditions. The excessive retrograde actin flow speed and force loading are strongly reduced on certain nanotopographies upon strong reduction of the native glycocalyx, while on the flat substrate we observe the opposite trend. Our results highlight the importance of the glycocalyx configuration in a molecular clutch force loading-dependent cellular mechanism for mechanosensing of microenvironmental nanotopographical features.Graphical

Impairment of urinary bladder mechanical properties in rat model of type 2 diabetes.

The urinary bladder is a mechanosensitive organ that accumulates, stores, and expels considerable amounts of fluid. While the neuronal bladder control via the CNS is well defined, the data on the mechanisms of local mechanical sensitivity of the bladder wall are either insufficient or contradictory. Here we compared the mechanical properties of bladder wall of normal rats and rats with modeled type 2 diabetes (T2D). T2D was modeled in 3-month-old Wistar male rats by combined administration of nicotinamide (230 mg/kg) and streptozotocin (65 mg/kg). Cystometry of isolated, denervated whole bladders and stress-strain tensiometry on detrusor smooth muscle (DSM) strips were used to assess the mechanical properties of bladder wall tissues from control and diabetic animals on 10th week after induction. The pressure-volume cystometrograms of both control and T2D bladders featured a quasi plateau between ascending sections. T2D cystometrograms revealed markedly elevated intravesicular pressure (~100% at 1 ml) and a shortened plateau, consistent with decreased bladder wall elasticity and reduced structural bladder capacity versus control. Experiments on urothelium-intact and urothelium-devoid DSM strips have shown that the decrease of bladder walls elasticity in T2D can be explained by the switch of stretched urothelium from inducing DSM relaxation to inducing DSM contraction due to a change in the prevalent release of contractile versus relaxing urothelial factor(s). The decreased elasticity of the bladder walls in T2D results from alterations in urothelium-dependent mechanosensory mechanisms. Elevated intravesical pressure in T2D may contribute to urge incontinence and/or symptoms of upper urinary tract damage.

Mechanical communication-associated cell directional migration and branching connections mediated by calcium channels, integrin β1, and N-cadherin.

Cell–cell mechanical communications at a large spatial scale (above hundreds of micrometers) have been increasingly recognized in recent decade, which shows importance in tissue-level assembly and morphodynamics. The involved mechanosensing mechanism and resulted physiological functions are still to be fully understood. Recent work showed that traction force sensation in the matrix induces cell communications for self-assembly. Here, based on the experimental model of cell directional migration on Matrigel hydrogel, containing 0.5 mg/ml type I collagen, we studied the mechano-responsive pathways for cell distant communications. Airway smooth muscle (ASM) cells assembled network structure on the hydrogel, whereas stayed isolated individually when cultured on glass without force transmission. Cell directional migration, or network assembly was significantly attenuated by inhibited actomyosin activity, or inhibition of inositol 1,4,5-trisphosphate receptor (IP3R) calcium channel or SERCA pump on endoplasmic reticulum (ER) membrane, or L-type calcium channel on the plasma membrane. Inhibition of integrin β1 with siRNA knockdown reduced cell directional migration and branching assembly, whereas inhibition of cell junctional N-cadherin with siRNA had little effect on distant attractions but blocked branching assembly. Our work demonstrated that the endoplasmic reticulum calcium channels and integrin are mechanosensing signals for cell mechanical communications regulated by actomyosin activity, while N-cadherin is responsible for traction force-induced cell stable connections in the assembly.

A mechano-sensing mechanism for waving in plant roots

Arabidopsis roots grown on inclined agar surfaces exhibit unusual sinusoidal patterns known as root-waving. The origin of these patterns has been ascribed to both genetic and environmental factors. Here we propose a mechano-sensing model for root-waving, based on a combination of friction induced by gravitropism, the elasticity of the root and the anchoring of the root to the agar by thin hairs, and demonstrate its relevance to previously obtained experimental results. We further test the applicability of this model by performing experiments in which we measure the effect of gradually changing the inclination angles of the agar surfaces on the wavelength and other properties of the growing roots. We find that the observed dynamics is different than the dynamics reported in previous works, but that it can still be explained using the same mechano-sensing considerations. This is supported by the fact that a scaling relation derived from the model describes the observed dependence of the wavelength on the tilt angle for a large range of angles. We also compare the prevalence of waving in different plant species and show that it depends on root thickness as predicted by the model. The results indicate that waving can be explained using mechanics and gravitropism alone and that mechanics may play a greater role in root growth and form than was previously considered.

Abstract 139: Precision Nanomedicine Targeting Novel Endothelial Mechano-sensing Mechanisms To Treat Vascular Diseases

Endothelial mechano-transduction mechanisms are instrumental to vascular health and disease but targeting disease-causing mechano-sensing pathways remains extremely challenging. Atherosclerosis preferentially develops at arterial curvatures and bifurcations where disturbed blood flow activates endothelium, leading to peripheral artery disease, carotid artery disease and ischemic stroke. Nevertheless; current atherosclerosis therapies mainly target systematic risk factors but not the vasculature per se . This underscores the significance and unique opportunity to identify and target novel mechanosensitive mechanisms in activated endothelium subjected to disturbed flow. Our objectives are to (1) delineate novel endothelial mechano-sensing mechanisms and (2) devise innovative precision nanomedicine approaches targeting these disease-causing mechano-sensitive pathways. Using multiple new transgenic mouse lines, our studies demonstrated that disturbed flow (DF) significantly increases the endothelial expression of TXNDC5 (thioredoxin domain containing 5) and microRNA-92a (miR-92a), two atherogenic molecules driving atherogenesis. Mechanistically, DF-induced TXNDC5 increases proteasome-mediated degradation of heat shock factor 1 and consequently promotes eNOS/NOS3 protein degradation. Meanwhile, DF induces endothelial miR-92a to suppress key anti-inflammatory molecules such as KLF2, KLF4, and PLPP3. We successfully formulated nanoparticles to deliver TXNDC5-targeting CRISPR/Cas9 plasmids driven by an endothelium-specific promoter (CDH5), which effectively deleted endothelial TXNDC5 expression and reduced atherosclerosis in Apoe -/- mice. Moreover, VCAM1-tageting polyelectrolyte complex micelles were engineered to selectively deliver miR-92a inhibitors to inflamed endothelium, which markedly decreased arterial stenosis and atherosclerosis in Apoe -/- mice. These results elucidate novel endothelial mechano-sensing mechanisms and provide a proof of concept of innovative targeted nanomedicine approaches, addressing an unmet medical need in vascular therapies.

A morphometric analysis of the osteocyte canaliculus using applied automatic semantic segmentation by machine learning.

Osteocytes play a role as mechanosensory cells by sensing flow-induced mechanical stimuli applied on their cell processes. High-resolution imaging of osteocyte processes and the canalicular wall are necessary for the analysis of this mechanosensing mechanism. Focused ion beam-scanning electron microscopy (FIB-SEM) enabled the visualization of the structure at the nanometer scale with thousands of serial-section SEM images. We applied machine learning for the automatic semantic segmentation of osteocyte processes and canalicular wall and performed a morphometric analysis using three-dimensionally reconstructed images. Six-week-old-mice femur were used. Osteocyte processes and canaliculi were observed at a resolution of 2nm/voxel in a 4 × 4μm region with 2000 serial-section SEM images. Machine learning was used for automatic semantic segmentation of the osteocyte processes and canaliculi from serial-section SEM images. The results of semantic segmentation were evaluated using the dice similarity coefficient (DSC). The segmented data were reconstructed to create three-dimensional images and a morphological analysis was performed. The DSC was > 83%. Using the segmented data, a three-dimensional image of approximately 3.5μm in length was reconstructed. The morphometric analysis revealed that the median osteocyte process diameter was 73.8 ± 18.0nm, and the median pericellular fluid space around the osteocyte process was 40.0 ± 17.5nm. We used machine learning for the semantic segmentation of osteocyte processes and canalicular wall for the first time, and performed a morphological analysis using three-dimensionally reconstructed images.

Studying Murine Small Bowel Mechanosensing of Luminal Particulates.

Gastrointestinal (GI) motility is critical for normal digestion and absorption. In the small bowel, which absorbs nutrients, motility optimizes digestion and absorption. For this reason, some of the motility patterns in the small bowel include segmentation for mixing of luminal contents and peristalsis for their propulsion. Physical properties of luminal contents modulate the patterns of small bowel motility. The mechanical stimulation of GI mechanosensory circuits by transiting luminal contents and underlying gut motility initiate and modulate complex GI motor patterns. Yet, the mechanosensory mechanisms that drive this process remain poorly understood. This is primarily due to a lack of tools to dissect how the small bowel handles materials of different physical properties. To study how the small bowel handles particulates of varying sizes, we have modified an established in vivo method to determine small bowel transit. We gavage live mice with fluorescent liquid or tiny fluorescent beads. After 30 minutes, we dissect out the bowels to image the distribution of fluorescent contents across the entirety of the GI tract. In addition to high-resolution measurements of the geometric center, we use variable size binning and spectral analysis to determine how different materials affect small bowel transit. We have explored how a recently discovered "gut touch" mechanism affects small bowel motility using this approach.

Mechanical force can enhance c-Src kinase activity by impairing autoinhibition

Cellular mechanosensing is pivotal for virtually all biological processes, and many molecular mechano-sensors and their way of function are being uncovered. In this work, we suggest that c-Src kinase acts as a direct mechano-sensor. c-Src is responsible for, among others, cell proliferation, and shows increased activity in stretched cells. In its native state, c-Src has little basal activity, because its kinase domain binds to an SH2 and SH3 domain. However, it is known that c-Src can bind to p130Cas, through which force can be transmitted to the membrane. Using molecular dynamics simulations, we show that force acting between the membrane-bound N-terminus of the SH3 domain and p130Cas induces partial SH3 unfolding, thereby impeding rebinding of the kinase domain onto SH2/SH3 and effectively enhancing kinase activity. Forces involved in this process are slightly lower or similar to the forces required to pull out c-Src from the membrane through the myristoyl linker, and key interactions involved in this anchoring are salt bridges between negative lipids and nearby basic residues in c-Src. Thus, c-Src appears to be a candidate for an intriguing mechanosensing mechanism of impaired kinase inhibition, which can be potentially tuned by membrane composition and other environmental factors.

From sense of touch to tactile intelligence: Thoughts from the 2021 Nobel Prize in Physiology or Medicine

<p indent=0mm>Human beings have highly developed sense of touch. Through touching we can perceive heat, pain, mechanical stimuli, recognize features of an object such as texture and hardness, and even understand emotion of a person. Hence, human touch is one of the most important ways to sense and adapt to the world around us. However, we had little understanding of how cutaneous perception was formed in our nervous system, until the groundbreaking discoveries were made by this year’s Nobel Prize laureates in Physiology or Medicine, David Julius from University of California, San Francisco, and Ardem Patapoutian from Scripps Research. They found that heat and mechanical stimuli can initiate specific nerve impulses for temperature, pain, and touch sensations. The quest to understand temperature sensation and pain spans millennia. In the early 20th century, the Greek physician Galen proposed that sensations were transmitted along pathways that connect the organs to the brain. It was not until 1997 that the true underlying molecular mechanism was revealed by Julius in a breakthrough work. Julius and his team found that a receptor on the cell’s membrane arranges itself from one side to the other, so that it creates a passage called a transient receptor potential (TRP) channel. This ion channel opens or closes in response to heat. Inspired by Julius’s work, Patapoutian in the early 21st century explored a mechanosensitive ion channel, called the Piezo channel, and identified that it is essential for the sense of touch. Patapoutian and his team also found that the Piezo channel is responsible for the sensing of position and motion of our body, which explains why we can balance on a steep stair. Not only did these discoveries elucidate the physiological understanding of human somatosensory, they also enlightened the researchers who work on reproducing such human functions in robotics and prosthetics. The molecular mechanisms of thermo-sensation and mechano-sensation provided a new paradigm for designing artificial cutaneous perception and constructing electronic skin for robots. Once fully armed by tactile perception and haptic feedback, smart robots and artificial intelligence may start to have consciousness and feel emotions like we humans do. With the awarding of the 2021 Nobel Prize in Physiology or Medicine to research in how humans perceive touch and pain, the research in tactile intelligence now has a clearer roadmap and gains a greater momentum. We foresee many exciting breakthroughs in this field in the near future.

Glaucoma and biomechanics.

Biomechanics is an important aspect of the complex family of diseases known as the glaucomas. Here, we review recent studies of biomechanics in glaucoma. Several tissues have direct and/or indirect biomechanical roles in various forms of glaucoma, including the trabecular meshwork, cornea, peripapillary sclera, optic nerve head/sheath, and iris. Multiple mechanosensory mechanisms and signaling pathways continue to be identified in both the trabecular meshwork and optic nerve head. Further, the recent literature describes a variety of approaches for investigating the role of tissue biomechanics as a risk factor for glaucoma, including pathological stiffening of the trabecular meshwork, peripapillary scleral structural changes, and remodeling of the optic nerve head. Finally, there have been advances in incorporating biomechanical information in glaucoma prognoses, including corneal biomechanical parameters and iridial mechanical properties in angle-closure glaucoma. Biomechanics remains an active aspect of glaucoma research, with activity in both basic science and clinical translation. However, the role of biomechanics in glaucoma remains incompletely understood. Therefore, further studies are indicated to identify novel therapeutic approaches that leverage biomechanics. Importantly, clinical translation of appropriate assays of tissue biomechanical properties in glaucoma is also needed.

Computational framework for analyzing flow-induced strain on osteocyte as modulated by microenvironment

Osteocytes buried in bone matrix are major mechanosensory cells that regulate bone remodeling in response to interstitial fluid flow in a lacuno-canalicular porosity. To gain an understanding of the mechanism of osteocyte mechanosensing, it is important to be able to evaluate the local strain on the osteocyte process membrane induced by the interstitial fluid flow. The microenvironment of the osteocytes, including the pericellular matrix (PCM) and canalicular ultrastructure, is a key modulator of the flow-induced strain on the osteocyte process membrane because it produces heterogeneous flow patterns in the pericellular space. To investigate the effect of changes in the microenvironment of osteocytes on the flow-induced strain, we developed a novel computational framework for analyzing the fluid–structure interaction. Computer simulations based on the proposed framework enabled evaluation of the spatial distribution of flow-induced strain on the osteocyte process membrane according to changes in the PCM density and canalicular curvature. The simulation results reveal that a decrease in PCM density and an increase in canalicular curvature, each of which is associated with aging and bone disease, have the notable effect of enhancing local flow-induced strain on the osteocyte process membrane. We believe that the proposed computational framework is a promising framework for investigating cell-specific mechanical stimuli and that it has the potential to accelerate the mechanobiological study of osteocytes by providing a deeper understanding of their mechanical environment in living bone tissue.

Biomechanical model of cells probing the myosin-II-independent mechanosensing mechanism.

Mechanosensing of cells to extracellular matrix (ECM) is highly active and plays a crucial role in various physiological processes. Growing numbers of studies provide evidence that cell sensitivity to ECM stiffness is a complex stress-strain feedback process. However, the mechanisms that rule this process are still not fully known. Here, an alternative mechanosensing scheme of cells, which is different from the previous myosin-II-based mechanisms, is proposed by employing the tension in cortical cytoskeletons (CSKs) as a force module to probe the substrate. The molecular mechanotransduction from cortical CSKs, through actin filaments and focal adhesions, and finally to the substrate, is mechanically modeled to scale the dynamic traction forces of cells. The developed model captures the characteristic spread of cells with respect to ECM stiffness whereby the spread is fully developed on a stiff substrate but not on a soft one. Furthermore, durotactic migration of cells on an elastic-gradient substrate is successfully modeled by the current method. The cells are concluded to migrate, actuated by the polarized traction forces from the stiffness gradient of the substrate and the stiffness matching between cells and substrate. Finally, the cells are proposed to actively target the preferred substrate by following a rule of mechanical matching between cells and substrate. This study provides a theoretical tool to advance our knowledge regarding the passive mechanical properties and the active sensing of cells, and further promotes the discovery of mechanosensing mechanisms as well as the material design for embryonic development and tissue homeostasis.

Molecular Spring Constant Analysis by Biomembrane Force Probe Spectroscopy.

A biomembrane force probe (BFP) has recently emerged as a native-cell-surface or in situ dynamic force spectroscopy (DFS) nanotool that can measure single-molecular binding kinetics, assess mechanical properties of ligand-receptor interactions, visualize protein dynamic conformational changes and more excitingly elucidate receptor mediated cell mechanosensing mechanisms. More recently, BFP has been used to measure the spring constant of molecular bonds. This protocol describes the step-by-step procedure to perform molecular spring constant DFS analysis. Specifically, two BFP operation modes are discussed, namely the Bead-Cell and Bead-Bead modes. This protocol focuses on deriving spring constants of the molecular bond and cell from DFS raw data.

The 'Yin and Yang' of Cancer Cell Growth and Mechanosensing.

Simple SummaryCells can sense their physical environment, or extracellular matrix (ECM), in a process called ‘mechanosensing’. They do this via adhesion sites which allow them to attach to the ECM and to simultaneously obtain information about its mechanical properties. This mechanical information is converted to biochemical signals that affect cell behavior in various ways. Stiff surroundings tend to promote proliferation of normal cells, whereas soft substrates can lead to their death. Cancer cells, however, can survive on soft substrates, while maintaining increased proliferation on stiff surfaces. This observation is interesting because it suggests that cancer cells potentially ignore negative physical cues, such as a soft ECM, and exploit positive stimuli in the form of a stiff ECM. This brings to mind a ‘yin and yang’ equilibrium which is ever shifting in favor of cancer cell survival and growth. We propose that different rigidities, which cancer cells may encounter in different regions of the body, can lead to modifications in the interactions and adhesions of the cells with the ECM, thus activating signaling pathways that boost cancer cell proliferation. Here, we discuss these interactions through the lens of mechanosensing, and its abnormal function in cancer.In cancer, two unique and seemingly contradictory behaviors are evident: on the one hand, tumors are typically stiffer than the tissues in which they grow, and this high stiffness promotes their malignant progression; on the other hand, cancer cells are anchorage-independent—namely, they can survive and grow in soft environments that do not support cell attachment. How can these two features be consolidated? Recent findings on the mechanisms by which cells test the mechanical properties of their environment provide insight into the role of aberrant mechanosensing in cancer progression. In this review article, we focus on the role of high stiffness on cancer progression, with particular emphasis on tumor growth; we discuss the mechanisms of mechanosensing and mechanotransduction, and their dysregulation in cancerous cells; and we propose that a ‘yin and yang’ type phenomenon exists in the mechanobiology of cancer, whereby a switch in the type of interaction with the extracellular matrix dictates the outcome of the cancer cells.

High-resolution image-based simulation reveals membrane strain concentration on osteocyte processes caused by tethering elements

Osteocytes are vital for regulating bone remodeling by sensing the flow-induced mechanical stimuli applied to their cell processes. In this mechanosensing mechanism, tethering elements (TEs) connecting the osteocyte process with the canalicular wall potentially amplify the strain on the osteocyte processes. The ultrastructure of the osteocyte processes and canaliculi can be visualized at a nanometer scale using high-resolution imaging via ultra-high voltage electron microscopy (UHVEM). Moreover, the irregular shapes of the osteocyte processes and the canaliculi, including the TEs in the canalicular space, should considerably influence the mechanical stimuli applied to the osteocytes. This study aims to characterize the roles of the ultrastructure of osteocyte processes and canaliculi in the mechanism of osteocyte mechanosensing. Thus, we constructed a high-resolution image-based model of an osteocyte process and a canaliculus using UHVEM tomography and investigated the distribution and magnitude of flow-induced local strain on the osteocyte process by performing fluid–structure interaction simulation. The analysis results reveal that local strain concentration in the osteocyte process was induced by a small number of TEs with high tension, which were inclined depending on the irregular shapes of osteocyte processes and canaliculi. Therefore, this study could provide meaningful insights into the effect of ultrastructure of osteocyte processes and canaliculi on the osteocyte mechanosensing mechanism.

The Synergic Role of Actomyosin Architecture and Biased Detachment in Muscle Energetics: Insights in Cross Bridge Mechanism beyond the Lever-Arm Swing

Muscle energetics reflects the ability of myosin motors to convert chemical energy into mechanical energy. How this process takes place remains one of the most elusive questions in the field. Here, we combined experimental measurements of in vitro sliding velocity based on DNA-origami built filaments carrying myosins with different lever arm length and Monte Carlo simulations based on a model which accounts for three basic components: (i) the geometrical hindrance, (ii) the mechano-sensing mechanism, and (iii) the biased kinetics for stretched or compressed motors. The model simulations showed that the geometrical hindrance due to acto-myosin spatial mismatching and the preferential detachment of compressed motors are synergic in generating the rapid increase in the ATP-ase rate from isometric to moderate velocities of contraction, thus acting as an energy-conservation strategy in muscle contraction. The velocity measurements on a DNA-origami filament that preserves the motors’ distribution showed that geometrical hindrance and biased detachment generate a non-zero sliding velocity even without rotation of the myosin lever-arm, which is widely recognized as the basic event in muscle contraction. Because biased detachment is a mechanism for the rectification of thermal fluctuations, in the Brownian-ratchet framework, we predict that it requires a non-negligible amount of energy to preserve the second law of thermodynamics. Taken together, our theoretical and experimental results elucidate less considered components in the chemo-mechanical energy transduction in muscle.

LIM domain proteins in cell mechanobiology.

The actin cytoskeleton is important for maintaining mechanical homeostasis in adherent cells, largely through its regulation of adhesion and cortical tension. The LIM (Lin-11, Isl1, MEC-3) domain-containing proteins are involved in a myriad of cellular mechanosensitive pathways. Recent work has discovered that LIM domains bind to mechanically stressed actin filaments, suggesting a novel and widely conserved mechanism of mechanosensing. This review summarizes the current state of knowledge of LIM protein mechanosensitivity.