Endogenous Piezo1 Can Confound Mechanically Activated Channel Identification and Characterization

Microscale geometrical modulation of PIEZO1 mediated mechanosensing through cytoskeletal redistribution

The microgeometry of the cellular microenvironment profoundly impacts cellular behaviors, yet the link between it and the ubiquitously expressed mechanosensitive ion channel PIEZO1 remains unclear. Herein, we describe a fluorescent micropipette aspiration assay that allows for simultaneous visualization of intracellular calcium dynamics and cytoskeletal architecture in real-time, under varied micropipette geometries. By integrating elastic shell finite element analysis with fluorescent lifetime imaging microscopy and employing PIEZO1-specific transgenic red blood cells and HEK cell lines, we demonstrate a direct correlation between the microscale geometry of aspiration and PIEZO1-mediated calcium signaling. We reveal that increased micropipette tip angles and physical constrictions lead to a significant reorganization of F-actin, accumulation at the aspirated cell neck, and subsequently amplify the tension stress at the dome of the cell to induce more PIEZO1's activity. Disruption of the F-actin network or inhibition of its mobility leads to a notable decline in PIEZO1 mediated calcium influx, underscoring its critical role in cellular mechanosensing amidst geometrical constraints.

Tentonin 3 is a pore-forming subunit of a slow inactivation mechanosensitive channel

Mechanically activating (MA) channels transduce numerous physiological functions. Tentonin 3/TMEM150C (TTN3) confers MA currents with slow inactivation kinetics in somato- and barosensory neurons. However, questions were raised about its role as a Piezo1 regulator and its potential as a channel pore. Here, we demonstrate that purified TTN3 proteins incorporated into the lipid bilayer displayed spontaneous and pressure-sensitive channel currents. These MA currents were conserved across vertebrates and differ from Piezo1 in activation threshold and pharmacological response. Deep neural network structure prediction programs coupled with mutagenetic analysis predicted a rectangular-shaped, tetrameric structure with six transmembrane helices and a pore at the inter-subunit center. The putative pore aligned with two helices of each subunit and had constriction sites whose mutations changed the MA currents. These findings suggest that TTN3 is a pore-forming subunit of a distinct slow inactivation MA channel, potentially possessing a tetrameric structure.

Piezo1 - Serine/threonine-protein phosphatase 2A - Cofilin1 biochemical mechanotransduction axis controls F-actin dynamics and cell migration

This study sheds light on a ground-breaking biochemical mechanotransduction pathway and reveals how Piezo1 channels orchestrate cell migration. We observed an increased cell migration rate in HEK293T (HEK) cells treated with Yoda1, a Piezo1 agonist, or in HEK cells overexpressing Piezo1 (HEK + P). Conversely, a significant reduction in cell motility was observed in HEK cells treated with GsMTx4 (a channel inhibitor) or upon silencing Piezo1 (HEK-P). Our findings establish a direct correlation between alterations in cell motility, Piezo1 expression, abnormal F-actin microfilament dynamics, and the regulation of Cofilin1, a protein involved in severing F-actin microfilaments. Here, the conversion of inactive pCofilin1 to active Cofilin1, mediated by the serine/threonine-protein phosphatase 2A catalytic subunit C (PP2AC), resulted in increased severing of F-actin microfilaments and enhanced cell migration in HEK + P cells compared to HEK controls. However, this effect was negligible in HEK-P and HEK cells transfected with hsa-miR-133b, which post-transcriptionally inhibited PP2AC mRNA expression. In summary, our study suggests that Piezo1 regulates cell migration through a biochemical mechanotransduction pathway involving PP2AC-mediated Cofilin1 dephosphorylation, leading to changes in F-actin microfilament dynamics.

TMC7 functions as a suppressor of Piezo2 in primary sensory neurons blunting peripheral mechanotransduction

The transmembrane channel-like (TMC) protein family comprises eight members, with TMC1 and TMC2 being extensively studied. This study demonstrates substantial co-expression of TMC7 with the mechanosensitive channel Piezo2 in somatosensory neurons. Genetic deletion of TMC7 in primary sensory ganglia neurons in vivo enhances sensitivity in both physiological and pathological mechanosensory transduction. This deletion leads to an increase in proportion of rapidly adapting (RA) currents conducted by Piezo2 in dorsal root ganglion (DRG) neurons and accelerates RA deactivation kinetics. In HEK293 cells expressing both proteins, TMC7 significantly suppresses the current amplitudes of co-expressed Piezo2. Our findings reveal that TMC7 and Piezo2 exhibit physical interactions, and both proteins also physically interact with cytoskeletal β-actin. We hypothesize that TMC7 functions as an inhibitory modulator of Piezo2 in DRG neurons, either through direct inhibition or by disrupting the transmission of mechanical forces from the cytoskeleton to the channel.

Towards an ion-channel-centric approach to ultrasound neuromodulation

Ultrasound neuromodulation is a promising technology that could revolutionize study and treatment of brain conditions ranging from mood disorders to Alzheimer's disease and stroke. An understanding of how ultrasound directly modulates specific ion channels could provide a roadmap for targeting specific neurological circuits and achieving desired neurophysiological outcomes. Although experimental challenges make it difficult to unambiguously identify which ion channels are sensitive to ultrasound in vivo, recent progress indicates that there are likely several different ion channels involved, including members of the K2P, Piezo, and TRP channel families. A recent result linking TRPM2 channels in the hypothalamus to induction of torpor by ultrasound in rodents demonstrates the feasibility of targeting a specific ion channel in a specific population of neurons.

Endophilin A2 controls touch and mechanical allodynia via kinesin-mediated Piezo2 trafficking

BACKGROUND

Tactile and mechanical pain are crucial to our interaction with the environment, yet the underpinning molecular mechanism is still elusive. Endophilin A2 (EndoA2) is an evolutionarily conserved protein that is documented in the endocytosis pathway. However, the role of EndoA2 in the regulation of mechanical sensitivity and its underlying mechanisms are currently unclear.

METHODS

Male and female C57BL/6 mice (8-12 weeks) and male cynomolgus monkeys (7-10 years old) were used in our experiments. Nerve injury-, inflammatory-, and chemotherapy-induced pathological pain models were established for this study. Behavioral tests of touch, mechanical pain, heat pain, and cold pain were performed in mice and nonhuman primates. Western blotting, immunostaining, co-immunoprecipitation, proximity ligation and patch-clamp recordings were performed to gain insight into the mechanisms.

RESULTS

The results showed that EndoA2 was primarily distributed in neurofilament-200-positive (NF200+) medium-to-large diameter dorsal root ganglion (DRG) neurons of mice and humans. Loss of EndoA2 in mouse NF200+ DRG neurons selectively impaired the tactile and mechanical allodynia. Furthermore, EndoA2 interacted with the mechanically sensitive ion channel Piezo2 and promoted the membrane trafficking of Piezo2 in DRG neurons. Moreover, as an adaptor protein, EndoA2 also bound to kinesin family member 5B (KIF5B), which was involved in the EndoA2-mediated membrane trafficking process of Piezo2. Loss of EndoA2 in mouse DRG neurons damaged Piezo2-mediated rapidly adapting mechanically activated currents, and re-expression of EndoA2 rescued the MA currents. In addition, interference with EndoA2 also suppressed touch sensitivity and mechanical hypersensitivity in nonhuman primates.

CONCLUSIONS

Our data reveal that the KIF5B/EndoA2/Piezo2 complex is essential for Piezo2 trafficking and for sustaining transmission of touch and mechanical hypersensitivity signals. EndoA2 regulates touch and mechanical allodynia via kinesin-mediated Piezo2 trafficking in sensory neurons. Our findings identify a potential new target for the treatment of mechanical pain.

Quantitative mechanical stimulation of GPR68 using a novel 96 well flow plugin

Mechanosensitive proteins play a crucial role in a range of physiological processes, including hearing, tactile sensation and regulating blood flow. While previous work has demonstrated the mechanosensitivity of several proteins, the ability to apply precisely defined mechanical forces to cells in a consistent, replicable manner remains a significant challenge. In this work we present a novel 96-well plate-compatible plugin device for generating highly-controlled flow-based mechanical simulation of cells, which enables quantitative assessment of mechanosensitive protein function. The device is used to mechanically stimulate HEK 293T cells expressing the mechanosensitive protein GPR68, a G protein-coupled receptor. By assaying intracellular calcium levels during flow-based cell stimulation, we determine that GPR68 signalling is a function of the applied shear-force. As this approach is compatible with conventional cell culture plates and allows for simultaneous readout in a conventional fluorescence plate reader, this represents a valuable new tool to investigate mechanotransduction.

PIEZO ion channels: force sensors of the interoceptive nervous system

Many organs are designed to move: the heart pumps each second, the gastrointestinal tract squeezes and churns to digest food, and we contract and relax skeletal muscles to move our bodies. Sensory neurons of the peripheral nervous system detect signals from bodily tissues, including the forces generated by these movements, to control physiology. The processing of these internal signals is called interoception, but this is a broad term that includes a wide variety of both chemical and mechanical sensory processes. Mechanical senses are understudied, but rapid progress has been made in the last decade, thanks in part to the discovery of the mechanosensory PIEZO ion channels (Coste et al., 2010). The role of these mechanosensors within the interoceptive nervous system is the focus of this review. In defining the transduction molecules that govern mechanical interoception, we will have a better grasp of how these signals drive physiology.

Touch sensation requires the mechanically gated ion channel ELKIN1

Touch perception is enabled by mechanically activated ion channels, the opening of which excites cutaneous sensory endings to initiate sensation. In this study, we identify ELKIN1 as an ion channel likely gated by mechanical force, necessary for normal touch sensitivity in mice. Touch insensitivity in Elkin1-/- mice was caused by a loss of mechanically activated currents (MA currents) in around half of all sensory neurons activated by light touch (low-threshold mechanoreceptors). Reintroduction of Elkin1 into sensory neurons from Elkin1-/- mice restored MA currents. Additionally, small interfering RNA-mediated knockdown of ELKIN1 from induced human sensory neurons substantially reduced indentation-induced MA currents, supporting a conserved role for ELKIN1 in human touch. Our data identify ELKIN1 as a core component of touch transduction in mice and potentially in humans.

Photolipid excitation triggers depolarizing optocapacitive currents and action potentials

Optically-induced changes in membrane capacitance may regulate neuronal activity without requiring genetic modifications. Previously, they mainly relied on sudden temperature jumps due to light absorption by membrane-associated nanomaterials or water. Yet, nanomaterial targeting or the required high infrared light intensities obstruct broad applicability. Now, we propose a very versatile approach: photolipids (azobenzene-containing diacylglycerols) mediate light-triggered cellular de- or hyperpolarization. As planar bilayer experiments show, the respective currents emerge from millisecond-timescale changes in bilayer capacitance. UV light changes photolipid conformation, which awards embedding plasma membranes with increased capacitance and evokes depolarizing currents. They open voltage-gated sodium channels in cells, generating action potentials. Blue light reduces the area per photolipid, decreasing membrane capacitance and eliciting hyperpolarization. If present, mechanosensitive channels respond to the increased mechanical membrane tension, generating large depolarizing currents that elicit action potentials. Membrane self-insertion of administered photolipids and focused illumination allows cell excitation with high spatiotemporal control.

Stretch response of the mechano-gated channel TMEM63A in membrane patches and single cells

The recently discovered ion channel TMEM63A has biophysical features distinctive for mechano-gated cation channels, activating at high pressures with slow kinetics while not inactivating. However, some biophysical properties are less clear, including no information on its function in whole cells. The aim of this study is to expand the TMEM63A biophysical characterization and examine the function in whole cells. Piezo1-knockout HEK293T cells were cotransfected with human TMEM63A and green fluorescent protein (GFP), and macroscopic currents in cell-attached patches were recorded by high-speed pressure clamp at holding voltages from -120 to -20 mV with 0-100 mmHg patch suction for 1 s. HEK293 cells cotransfected with TMEM63A and GCaMP5 were seeded onto polydimethylsiloxane (PDMS) membrane, and the response to 3-12 s of 1%-15% whole cell isotropic (equi-biaxial) stretch induced by an IsoStretcher was measured by the change in intracellular calcium ([Ca2+]i) and presented as (ΔF/F0 > 1). Increasing patch pressures activated TMEM63A currents with accelerating activation kinetics and current amplitudes that were pressure dependent but voltage independent. TMEM63A currents were plateaued within 2 s, recovered quickly, and were sensitive to Gd3+. In whole cells stretched on flexible membranes, radial stretch increased the [Ca2+]i responses in a larger proportion of cells cotransfected with TMEM63A and GCaMP5 than GCaMP5-only controls. TMEM63A currents are force activated and voltage insensitive, have a high threshold for pressure activation with slow activation and deactivation, and lack inactivation over 5 s. TMEM63A has the net polarity and kinetics that would depolarize plasma membranes and increase inward currents, contributing to a sustained [Ca2+]i increase in response to high stretch.NEW & NOTEWORTHY TMEM63A has biophysical features distinctive for mechano-gated cation channels, but some properties are less clear, including no functional information in whole cells. We report that pressure-dependent yet voltage-independent TMEM63A currents in cell membrane patches correlated with cell size. In addition, radial stretch of whole cells on flexible membranes increased the [Ca2+]i responses more in TMEM63A-transfected cells. Inward TMEM63A currents in response to high stretch can depolarize plasma membranes and contribute to a sustained [Ca2+]i increase.

A high-throughput electrophysiology assay to study the response of PIEZO1 to mechanical stimulation

PIEZO1 channels are mechanically activated cation channels that play a pivotal role in sensing mechanical forces in various cell types. Their dysfunction has been associated with numerous pathophysiological states, including generalized lymphatic dysplasia, varicose vein disease, and hereditary xerocytosis. Given their physiological relevance, investigating PIEZO1 is crucial for the pharmaceutical industry, which requires scalable techniques to allow for drug discovery. In this regard, several studies have used high-throughput automated patch clamp (APC) combined with Yoda1, a specific gating modifier of PIEZO1 channels, to explore the function and properties of PIEZO1 in heterologous expression systems, as well as in primary cells. However, a combination of solely mechanical stimulation (M-Stim) and high-throughput APC has not yet been available for the study of PIEZO1 channels. Here, we show that optimization of pipetting parameters of the SyncroPatch 384 coupled with multihole NPC-384 chips enables M-Stim of PIEZO1 channels in high-throughput electrophysiology. We used this approach to explore differences between the response of mouse and human PIEZO1 channels to mechanical and/or chemical stimuli. Our results suggest that applying solutions on top of the cells at elevated pipetting flows is crucial for activating PIEZO1 channels by M-Stim on the SyncroPatch 384. The possibility of comparing and combining mechanical and chemical stimulation in a high-throughput patch clamp assay facilitates investigations on PIEZO1 channels and thereby provides an important experimental tool for drug development.

TMEM120A/TACAN: A putative regulator of ion channels, mechanosensation, and lipid metabolism

TMEM120A (TACAN) is an enigmatic protein with several seemingly unconnected functions. It was proposed to be an ion channel involved in sensing mechanical stimuli, and knockdown/knockout experiments have implicated that TMEM120A may be necessary for sensing mechanical pain. TMEM120A's ion channel function has subsequently been challenged, as attempts to replicate electrophysiological experiments have largely been unsuccessful. Several cryo-EM structures revealed TMEM120A is structurally homologous to a lipid modifying enzyme called Elongation of Very Long Chain Fatty Acids 7 (ELOVL7). Although TMEM120A's channel function is debated, it still seems to affect mechanosensation by inhibiting PIEZO2 channels and by modifying tactile pain responses in animal models. TMEM120A was also shown to inhibit polycystin-2 (PKD2) channels through direct physical interaction. Additionally, TMEM120A has been implicated in adipocyte regulation and in innate immune response against Zika virus. The way TMEM120A is proposed to alter each of these processes ranges from regulating gene expression, acting as a lipid modifying enzyme, and controlling subcellular localization of other proteins through direct binding. Here, we examine TMEM120A's structure and proposed functions in diverse physiological contexts.

Computational modeling of stretch induced calcium signaling at the apical membrane domain in umbrella cells

The urinary bladder epithelium comprises a specialised population of superficially placed cells called the umbrella cells. The apical membrane domain of umbrella cells has several intriguing morphological properties and is the site for various signaling activities. A key function of umbrella cells is to sense mechanical stimuli as the bladder stretches in response to filling. More specifically, the mechanotransduction of stretch into subcellular signals is brought about by the activation of Piezo1 channels that mediate calcium into the cell interior. The incoming calcium is critical to several aspects of umbrella cell signaling, including regulation of exocytosis, ATP release and downstream purinergic signaling. We report here a computational framework that models stretch-induced mechanotransduction via Piezo1 channels and the resulting calcium signaling in umbrella cells factoring in morphological details of subcellular compartment volumes. Our results show the following: (i) activation of Piezo1 conductance in response to stretch; (ii) development of varying Piezo1 mediated [Ca2+] profiles in subcellular compartments, namely, the apical sub-plasma membrane space, cytosol and mitochondria. The varying calcium amplitudes and temporal profiles in the subcellular compartments indicate highly specialised roles for stretch-mediated calcium in umbrella cells, including its potential effect on the energetics of mitochondria and the regulation of exocytosis.

Force-induced motions of the PIEZO1 blade probed with fluorimetry

Mechanical forces are thought to activate mechanosensitive PIEZO channels by changing the conformation of a large transmembrane blade domain. Yet, whether different stimuli induce identical conformational changes in this domain remains unclear. Here, we repurpose a cyclic permuted green fluorescent protein as a conformation-sensitive probe to track local rearrangements along the PIEZO1 blade. Two independent probes, one inserted in an extracellular site distal to the pore and the other in a distant intracellular proximal position, elicit sizable fluorescence signals when the tagged channels activate in response to fluid shear stress of low intensity. Neither cellular indentations nor osmotic swelling of the cell elicit detectable fluorescence signals from either probe, despite the ability of these stimuli to activate the tagged channels. High-intensity flow stimuli are ineffective at eliciting fluorescence signals from either probe. Together, these findings suggest that low-intensity fluid shear stress causes a distinct form of mechanical stress to the cell.

Photolipid excitation triggers depolarizing optocapacitive currents and action potentials

Optically-induced changes in membrane capacitance may regulate neuronal activity without requiring genetic modifications. Previously, they mainly relied on sudden temperature jumps due to light absorption by membrane-associated nanomaterials or water. Yet, nanomaterial targeting or the required high infrared light intensities obstruct broad applicability. Now, we propose a very versatile approach: photolipids (azobenzene-containing diacylglycerols) mediate light-triggered cellular de- or hyperpolarization. As planar bilayer experiments show, the respective currents emerge from millisecond-timescale changes in bilayer capacitance. UV light changes photolipid conformation, which awards embedding plasma membranes with increased capacitance and evokes depolarizing currents. They open voltage-gated sodium channels in cells, generating action potentials. Blue light reduces the area per photolipid, decreasing membrane capacitance and eliciting hyperpolarization. If present, mechanosensitive channels respond to the increased mechanical membrane tension, generating large depolarizing currents that elicit action potentials. Membrane self-insertion of administered photolipids and focused illumination allows cell excitation with high spatiotemporal control.

Improved PIEZO1 agonism through 4-benzoic acid modification of Yoda1

BACKGROUND AND PURPOSE

The protein PIEZO1 forms mechanically activated, calcium-permeable, non-selective cation channels in numerous cell types from several species. Options for pharmacological modulation are limited and so we modified a small-molecule agonist at PIEZO1 channels (Yoda1) to increase the ability to modulate these channels.

EXPERIMENTAL APPROACH

Medicinal chemistry generated Yoda1 analogues that were tested in intracellular calcium and patch-clamp assays on cultured cells exogenously expressing human or mouse PIEZO1 or mouse PIEZO2. Physicochemical assays and wire myography assays on veins from mice with genetic disruption of PIEZO1.

KEY RESULTS

A Yoda1 analogue (KC159) containing 4-benzoic acid instead of the pyrazine of Yoda1 and its potassium salt (KC289) have equivalent or improved reliability, efficacy and potency, compared with Yoda1 in functional assays. Tested against overexpressed mouse PIEZO1 in calcium assays, the order of potency (as EC50 values, nM) was KC289, 150 > KC159 280 > Yoda1, 600). These compounds were selective for PIEZO1 over other membrane proteins, and the physicochemical properties were more suited to physiological conditions than those of Yoda1. The vasorelaxant effects were consistent with PIEZO1 agonism. In contrast, substitution with 2-benzoic acid failed to generate a modulator.

CONCLUSION AND IMPLICATIONS

4-Benzoic acid modification of Yoda1 improves PIEZO1 agonist activity at PIEZO1 channels. We suggest naming this new modulator Yoda2. It should be a useful tool compound in physiological assays and facilitate efforts to identify a binding site. Such compounds may have therapeutic potential, for example, in diseases linked genetically to PIEZO1 such as lymphatic dysplasia.

To Be or Not to Be an Ion Channel: Cryo-EM Structures Have a Say

Ion channels are the second largest class of drug targets after G protein-coupled receptors. In addition to well-recognized ones like voltage-gated Na/K/Ca channels in the heart and neurons, novel ion channels are continuously discovered in both excitable and non-excitable cells and demonstrated to play important roles in many physiological processes and diseases such as developmental disorders, neurodegenerative diseases, and cancer. However, in the field of ion channel discovery, there are an unignorable number of published studies that are unsolid and misleading. Despite being the gold standard of a functional assay for ion channels, electrophysiological recordings are often accompanied by electrical noise, leak conductance, and background currents of the membrane system. These unwanted signals, if not treated properly, lead to the mischaracterization of proteins with seemingly unusual ion-conducting properties. In the recent ten years, the technical revolution of cryo-electron microscopy (cryo-EM) has greatly advanced our understanding of the structures and gating mechanisms of various ion channels and also raised concerns about the pore-forming ability of some previously identified channel proteins. In this review, we summarize cryo-EM findings on ion channels with molecular identities recognized or disputed in recent ten years and discuss current knowledge of proposed channel proteins awaiting cryo-EM analyses. We also present a classification of ion channels according to their architectures and evolutionary relationships and discuss the possibility and strategy of identifying more ion channels by analyzing structures of transmembrane proteins of unknown function. We propose that cross-validation by electrophysiological and structural analyses should be essentially required for determining molecular identities of novel ion channels.

Deciphering mechanically activated ion channels at the single-channel level in dorsal root ganglion neurons

Mechanically activated (MA) ion channels confer somatosensory neurons with the ability to sense a wide range of mechanical stimuli. MA ion channel activity in somatosensory neurons is best described by the electrophysiological recordings of MA currents in cultured dorsal root ganglion (DRG) neurons. Biophysical and pharmacological characterization of DRG MA currents has guided the field in screening/confirming channel candidates that induce the currents and facilitate the mechanosensory response. But studies on DRG MA currents have relied mostly on whole-cell macroscopic current properties obtained by membrane indentation, and little is known about the underlying MA ion channels at the single-channel level. Here, by acquiring indentation-induced macroscopic currents as well as stretch-activated single-channel currents from the same cell, we associate macroscopic current properties with single-channel conductance. This analysis reveals the nature of the MA channel responsible for the ensemble response. We observe four different conductances in DRG neurons with no association with a specific type of macroscopic current. Applying this methodology to a Piezo2 expressing DRG neuronal subpopulation allows us to identify PIEZO2-dependent stretch-activated currents and conductance. Moreover, we demonstrate that upon Piezo2 deletion, the remaining macroscopic responses are predominantly mediated by three different single-channel conductances. Collectively, our data predict that at least two other MA ion channels exist in DRG neurons that remain to be discovered.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Mechanosensitive membrane proteins: Usual and unusual suspects in mediating mechanotransduction

This Viewpoint, which accompanies a Special Issue focusing on membrane mechanosensors, discusses unifying and unique features of both established and emerging mechanosensitive (MS) membrane proteins, their distribution across protein families and phyla, and current and future challenges in the study of these important proteins and their partners. MS membrane proteins are essential for tissue development, cellular motion, osmotic homeostasis, and sensing external and self-generated mechanical cues like those responsible for touch and proprioception. Though researchers' attention and this Viewpoint focus on a few famous ion channels that are considered the usual suspects as MS mechanosensors, we also discuss some of the more unusual suspects, such as G-protein coupled receptors. As the field continues to grow, so too will the list of proteins suspected to function as mechanosensors and the diversity of known MS membrane proteins.

The energetics of rapid cellular mechanotransduction

Cells throughout the human body detect mechanical forces. While it is known that the rapid (millisecond) detection of mechanical forces is mediated by force-gated ion channels, a detailed quantitative understanding of cells as sensors of mechanical energy is still lacking. Here, we combine atomic force microscopy with patch-clamp electrophysiology to determine the physical limits of cells expressing the force-gated ion channels (FGICs) Piezo1, Piezo2, TREK1, and TRAAK. We find that, depending on the ion channel expressed, cells can function either as proportional or nonlinear transducers of mechanical energy and detect mechanical energies as little as ~100 fJ, with a resolution of up to ~1 fJ. These specific energetic values depend on cell size, channel density, and cytoskeletal architecture. We also make the surprising discovery that cells can transduce forces either nearly instantaneously (<1 ms) or with a substantial time delay (~10 ms). Using a chimeric experimental approach and simulations, we show how such delays can emerge from channel-intrinsic properties and the slow diffusion of tension in the membrane. Overall, our experiments reveal the capabilities and limits of cellular mechanosensing and provide insights into molecular mechanisms that different cell types may employ to specialize for their distinct physiological roles.

Lack of evidence for participation of TMEM150C in sensory mechanotransduction

The membrane protein TMEM150C has been proposed to form a mechanosensitive ion channel that is required for normal proprioceptor function. Here, we examined whether expression of TMEM150C in neuroblastoma cells lacking Piezo1 is associated with the appearance of mechanosensitive currents. Using three different modes of mechanical stimuli, indentation, membrane stretch, and substrate deflection, we could not evoke mechanosensitive currents in cells expressing TMEM150C. We next asked if TMEM150C is necessary for the normal mechanosensitivity of cutaneous sensory neurons. We used an available mouse model in which the Tmem150c locus was disrupted through the insertion of a LacZ cassette with a splice acceptor that should lead to transcript truncation. Analysis of these mice indicated that ablation of the Tmem150c gene was not complete in sensory neurons of the dorsal root ganglia (DRG). Using a CRISPR/Cas9 strategy, we made a second mouse model in which a large part of the Tmem150c gene was deleted and established that these Tmem150c^−/− mice completely lack TMEM150C protein in the DRGs. We used an ex vivo skin nerve preparation to characterize the mechanosenstivity of mechanoreceptors and nociceptors in the glabrous skin of the Tmem150c^−/− mice. We found no quantitative alterations in the physiological properties of any type of cutaneous sensory fiber in Tmem150c^−/− mice. Since it has been claimed that TMEM150C is required for normal proprioceptor function, we made a quantitative analysis of locomotion in Tmem150c^−/− mice. Here again, we found no indication that there was altered gait in Tmem150c^−/− mice compared to wild-type controls. In summary, we conclude that existing mouse models that have been used to investigate TMEM150C function in vivo are problematic. Furthermore, we could find no evidence that TMEM150C forms a mechanosensitive channel or that it is necessary for the normal mechanosensitivity of cutaneous sensory neurons.

Shear stress activates nociceptors to drive Drosophila mechanical nociception

Mechanical nociception is essential for animal survival. However, the forces involved in nociceptor activation and the underlying mechanotransduction mechanisms remain elusive. Here, we address these problems by investigating nocifensive behavior in Drosophila larvae. We show that strong poking stimulates nociceptors with a mixture of forces including shear stress and stretch. Unexpectedly, nociceptors are selectively activated by shear stress, but not stretch. Both the shear stress responses of nociceptors and nocifensive behavior require transient receptor potential A1 (TrpA1), which is specifically expressed in nociceptors. We further demonstrate that expression of mammalian or Drosophila TrpA1 in heterologous cells confers responses to shear stress but not stretch. Finally, shear stress activates TrpA1 in a membrane-delimited manner, through modulation of membrane fluidity. Together, our study reveals TrpA1 as an evolutionarily conserved mechanosensitive channel specifically activated by shear stress and suggests a critical role of shear stress in activating nociceptors to drive mechanical nociception.

PIEZO channels and newcomers in the mammalian mechanosensitive ion channel family

Many ion channels have been described as mechanosensitive according to various criteria. Most broadly defined, an ion channel is called mechanosensitive if its activity is controlled by application of a physical force. The last decade has witnessed a revolution in mechanosensory physiology at the molecular, cellular, and system levels, both in health and in diseases. Since the discovery of the PIEZO proteins as prototypical mechanosensitive channel, many proteins have been proposed to transduce mechanosensory information in mammals. However, few of these newly identified candidates have all the attributes of bona fide, pore-forming mechanosensitive ion channels. In this perspective, we will cover and discuss new data that have advanced our understanding of mechanosensation at the molecular level.

TMEM120A/TACAN inhibits mechanically activated PIEZO2 channels

PIEZO2 channels mediate rapidly adapting mechanically activated currents in peripheral sensory neurons of the dorsal root ganglia (DRG), and they are indispensable for light touch and proprioception. Relatively little is known about what other proteins regulate PIEZO2 activity in a cellular context. TMEM120A (TACAN) was proposed to act as a high threshold mechanically activated ion channel in nociceptive DRG neurons. Here, we find that Tmem120a coexpression decreased the amplitudes of mechanically activated PIEZO2 currents and increased their threshold of activation. TMEM120A did not inhibit mechanically activated PIEZO1 and TREK1 channels and TMEM120A alone did not result in the appearance of mechanically activated currents above background. Tmem120a and Piezo2 expression in mouse DRG neurons overlapped, and siRNA-mediated knockdown of Tmem120a increased the amplitudes of rapidly adapting mechanically activated currents and decreased their thresholds to mechanical activation. Our data identify TMEM120A as a negative modulator of PIEZO2 channel activity, and do not support TMEM120A being a mechanically activated ion channel.

The Piezo1 ion channel in glaucoma: a new perspective on mechanical stress

Glaucomatous optic nerve damage caused by pathological intraocular pressure elevation is irreversible, and its course is often difficult to control. This group of eye diseases is closely related to biomechanics, and the correlation between glaucoma pathogenesis and mechanical stimulation has been studied in recent decades. The nonselective cation channel Piezo1, the most important known mechanical stress sensor, is a transmembrane protein widely expressed in various cell types. Piezo1 has been detected throughout the eye, and the close relationship between Piezo1 and glaucoma is being confirmed. Pathological changes in glaucoma occur in both the anterior and posterior segments of the eye, and it is of great interest for researchers to determine whether Piezo1 plays a role in these changes and how it functions. The elucidation of the mechanisms of Piezo1 action in nonocular tissues and the reported roles of similar mechanically activated ion channels in glaucoma will provide an appropriate basis for further investigation. From a new perspective, this review provides a detailed description of the current progress in elucidating the role of Piezo1 in glaucoma, including relevant questions and assumptions, the remaining challenging research directions and mechanism-related therapeutic potential.

Molecular determinants of mechanosensation in the muscle spindle

The muscle spindle (MS) provides essential sensory information for motor control and proprioception. The Group Ia and II MS afferents are low threshold slowly-adapting mechanoreceptors and report both static muscle length and dynamic muscle movement information. The exact molecular mechanism by which MS afferents transduce muscle movement into action potentials is incompletely understood. This short review will discuss recent evidence suggesting that PIEZO2 is an essential mechanically sensitive ion channel in MS afferents and that vesicle-released glutamate contributes to maintaining afferent excitability during the static phase of stretch. Other mechanically gated ion channels, voltage-gated sodium channels, other ion channels, regulatory proteins, and interactions with the intrafusal fibers are also important for MS afferent mechanosensation. Future studies are needed to fully understand mechanosensation in the MS and whether different complements of molecular mediators contribute to the different response properties of Group Ia and II afferents.

Endogenous ion channels expressed in human embryonic kidney (HEK-293) cells

Mammalian expression systems, particularly the human embryonic kidney (HEK-293) cells, combined with electrophysiological studies, have greatly benefited our understanding of the function, characteristic, and regulation of various ion channels. It was previously assumed that the existence of endogenous ion channels in native HEK-293 cells could be negligible. Still, more and more ion channels are gradually reported in native HEK-293 cells, which should draw our attention. In this regard, we summarize the different ion channels that are endogenously expressed in HEK-293 cells, including voltage-gated Na⁺ channels, Ca²⁺ channels, K⁺ channels, Cl^- channels, nonselective cation channels, TRP channels, acid-sensitive ion channels, and Piezo channels, which may complicate the recording of the heterogeneously expressed ion channels to a certain degree. We noted that the expression patterns and channel profiles varied with different studies, which may be due to the distinct originality of the cells, cell culture conditions, passage numbers, and different recording protocols. Therefore, a better knowledge of endogenous ion channels may help minimize potential problems in characterizing heterologously expressed ion channels. Based on this, it is recommended that HEK-293 cells from unknown sources should be examined before transfection for the characterization of their functional profile, especially when the expression level of exogenous ion channels does not overwhelm the endogenous ion channels largely, or the current amplitude is not significantly higher than the native currents.

Förster Resonance Energy Transfer-Based Single-Cell Imaging Reveals Piezo1-Induced Ca2+ Flux Mediates Membrane Ruffling and Cell Survival

A mechanosensitive ion channel, Piezo1 induces non-selective cation flux in response to various mechanical stresses. However, the biological interpretation and underlying mechanisms of cells resulting from Piezo1 activation remain elusive. This study elucidates Piezo1-mediated Ca²⁺ influx driven by channel activation and cellular behavior using novel Förster Resonance Energy Transfer (FRET)-based biosensors and single-cell imaging analysis. Results reveal that extracellular Ca²⁺ influx via Piezo1 requires intact caveolin, cholesterol, and cytoskeletal support. Increased cytoplasmic Ca²⁺ levels enhance PKA, ERK, Rac1, and ROCK activity, which have the potential to promote cancer cell survival and migration. Furthermore, we demonstrate that Piezo1-mediated Ca²⁺ influx upregulates membrane ruffling, a characteristic feature of cancer cell metastasis, using spatiotemporal image correlation spectroscopy. Thus, our findings provide new insights into the function of Piezo1, suggesting that Piezo1 plays a significant role in the behavior of cancer cells.

Multiscale Mechanobiology in Brain Physiology and Diseases

Increasing evidence suggests that mechanics play a critical role in regulating brain function at different scales. Downstream integration of mechanical inputs into biochemical signals and genomic pathways causes observable and measurable effects on brain cell fate and can also lead to important pathological consequences. Despite recent advances, the mechanical forces that influence neuronal processes remain largely unexplored, and how endogenous mechanical forces are detected and transduced by brain cells into biochemical and genetic programs have received less attention. In this review, we described the composition of brain tissues and their pronounced microstructural heterogeneity. We discuss the individual role of neuronal and glial cell mechanics in brain homeostasis and diseases. We highlight how changes in the composition and mechanical properties of the extracellular matrix can modulate brain cell functions and describe key mechanisms of the mechanosensing process. We then consider the contribution of mechanobiology in the emergence of brain diseases by providing a critical review on traumatic brain injury, neurodegenerative diseases, and neuroblastoma. We show that a better understanding of the mechanobiology of brain tissues will require to manipulate the physico-chemical parameters of the cell microenvironment, and to develop three-dimensional models that can recapitulate the complexity and spatial diversity of brain tissues in a reproducible and predictable manner. Collectively, these emerging insights shed new light on the importance of mechanobiology and its implication in brain and nerve diseases.

Nobel somatosensations and pain

The Nobel prices 2021 for Physiology and Medicine have been awarded to David Julius and Ardem Patapoutian "for their discoveries of receptors for temperature and touch", TRPV1 and PIEZO1/2. The present review tells the past history of the capsaicin receptor, covers further selected TRP channels, TRPA1 in particular, and deals with mechanosensitivity in general and mechanical hyperalgesia in particular. Other achievements of the laureates and translational aspects of their work are shortly treated.

Mechanobiology of Microvascular Function and Structure in Health and Disease: Focus on the Coronary Circulation

The coronary microvasculature plays a key role in regulating the tight coupling between myocardial perfusion and myocardial oxygen demand across a wide range of cardiac activity. Short-term regulation of coronary blood flow in response to metabolic stimuli is achieved via adjustment of vascular diameter in different segments of the microvasculature in conjunction with mechanical forces eliciting myogenic and flow-mediated vasodilation. In contrast, chronic adjustments in flow regulation also involve microvascular structural modifications, termed remodeling. Vascular remodeling encompasses changes in microvascular diameter and/or density being largely modulated by mechanical forces acting on the endothelium and vascular smooth muscle cells. Whereas in recent years, substantial knowledge has been gathered regarding the molecular mechanisms controlling microvascular tone and how these are altered in various diseases, the structural adaptations in response to pathologic situations are less well understood. In this article, we review the factors involved in coronary microvascular functional and structural alterations in obstructive and non-obstructive coronary artery disease and the molecular mechanisms involved therein with a focus on mechanobiology. Cardiovascular risk factors including metabolic dysregulation, hypercholesterolemia, hypertension and aging have been shown to induce microvascular (endothelial) dysfunction and vascular remodeling. Additionally, alterations in biomechanical forces produced by a coronary artery stenosis are associated with microvascular functional and structural alterations. Future studies should be directed at further unraveling the mechanisms underlying the coronary microvascular functional and structural alterations in disease; a deeper understanding of these mechanisms is critical for the identification of potential new targets for the treatment of ischemic heart disease.

Role of Mechanoinsensitive Nociceptors in Painful Diabetic Peripheral Neuropathy

The cutaneous mechanisms that trigger spontaneous neuropathic pain in diabetic peripheral neuropathy (PDPN) are far from clear. Two types of nociceptors are found within the epidermal and dermal skin layers. Small-diameter lightly myelinated Aδ and unmyelinated C cutaneous mechano and heat-sensitive (AMH and CMH) and C mechanoinsensitive (CMi) nociceptors transmit pain from the periphery to central nervous system. AMH and CMH fibers are mainly located in the epidermis, and CMi fibers are distributed in the dermis. In DPN, dying back intra-epidermal AMH and CMH fibers leads to reduced pain sensitivity, and the patients exhibit significantly increased pain thresholds to acute pain when tested using traditional methods. The role of CMi fibers in painful neuropathies has not been fully explored. Microneurography has been the only tool to access CMi fibers and differentiate AMH, CMH, and CMi fiber types. Due to the complexity, its use is impractical in clinical settings. In contrast, a newly developed diode laser fiber selective stimulation (DLss) technique allows to safely and selectively stimulate Aδ and C fibers in the superficial and deep skin layers. DLss data demonstrate that patients with painful DPN have increased Aδ fiber pain thresholds, while C-fiber thresholds are intact because, in these patients, CMi fibers are abnormally spontaneously active. It is also possible to determine the involvement of CMi fibers by measuring the area of DLss-induced neurogenic axon reflex flare. The differences in AMH, CMH, and CMi fibers identify patients with painful and painless neuropathy. In this review, we will discuss the role of CMi fibers in PDPN.

Piezo1 controls cell volume and migration by modulating swelling-activated chloride current through Ca2+ influx

Regulatory volume decrease (RVD), a homeostatic process responsible for the re-establishment of the original cell volume upon swelling, is critical in controlling several functions, including migration. RVD is mainly sustained by the swelling-activated Cl^- current (I_Cl,swell ), which can be modulated by cytoplasmic Ca²⁺ . Cell swelling also activates mechanosensitive channels, including the ubiquitously expressed Ca²⁺ -permeable channel Piezo1. We hypothesized that, by controlling cytoplasmic Ca²⁺ and in turn I_Cl,swell , Piezo1 is involved in the fine regulation of RVD and cell migration. We compared RVD and I_Cl,swell in wild-type (WT) HEK293T cells, which express endogenous levels of Piezo1, and in cells overexpressing (OVER) or knockout (KO) for Piezo1. Compared to WT, RVD was markedly increased in OVER, while virtually absent in KO cells. Consistently, I_Cl,swell amplitude was highest in OVER and lowest in KO cells, with WT cells displaying an intermediate level, suggesting a Ca²⁺ -dependent modulation of the current by Piezo1 channels. Indeed, in the absence of external Ca²⁺ , I_Cl,swell in both WT and OVER cells, as well as the RVD probed in OVER cells, were significantly lower than in the presence of Ca²⁺ and no longer different compared to KO cells. However, the Piezo-mediated Ca²⁺ influx was ineffective in enhancing I_Cl,swell in the absence of releasable Ca²⁺ from intracellular stores. The different expression levels of Piezo1 affected also cell migration which was strongly enhanced in OVER, while reduced in KO cells, as compared to WT. Taken together, our data indicate that Piezo1 controls RVD and migration in HEK293T cells by modulating I_Cl,swell through Ca²⁺ influx.

Piezo1 and Piezo2 foster mechanical gating of K2P channels

Mechanoelectrical transduction is mediated by the opening of different types of force-sensitive ion channels, including Piezo1/2 and the TREK/TRAAK K2P channels. Piezo1 curves the membrane locally into an inverted dome that reversibly flattens in response to force application. Moreover, Piezo1 forms numerous preferential interactions with various membrane lipids, including cholesterol. Whether this structural architecture influences the functionality of neighboring membrane proteins is unknown. Here, we show that Piezo1/2 increase TREK/TRAAK current amplitude, slow down activation/deactivation, and remove inactivation upon mechanical stimulation. These findings are consistent with a mechanism whereby Piezo1/2 cause a local depletion of membrane cholesterol associated with a prestress of TREK/TRAAK channels. This regulation occurs in mouse fibroblasts between endogenous Piezo1 and TREK-1/2, both channel types acting in concert to delay wound healing. In conclusion, we demonstrate a community effect between different structural and functional classes of mechanosensitive ion channels.

Tentonin 3/TMEM150C regulates glucose-stimulated insulin secretion in pancreatic β-cells

Glucose homeostasis is initially regulated by the pancreatic hormone insulin. Glucose-stimulated insulin secretion in β-cells is composed of two cellular mechanisms: a high glucose concentration not only depolarizes the membrane potential of the β-cells by ATP-sensitive K⁺ channels but also induces cell inflation, which is sufficient to release insulin granules. However, the molecular identity of the stretch-activated cation channel responsible for the latter pathway remains unknown. Here, we demonstrate that Tentonin 3/TMEM150C (TTN3), a mechanosensitive channel, contributes to glucose-stimulated insulin secretion by mediating cation influx. TTN3 is expressed specifically in β-cells and mediates cation currents to glucose and hypotonic stimulations. The glucose-induced depolarization, firing activity, and Ca²⁺ influx of β-cells were significantly lower in Ttn3^-/- mice. More importantly, Ttn3^-/- mice show impaired glucose tolerance with decreased insulin secretion in vivo. We propose that TTN3, as a stretch-activated cation channel, contributes to glucose-stimulated insulin secretion.

Patch-seq of mouse DRG neurons reveals candidate genes for specific mechanosensory functions

A variety of mechanosensory neurons are involved in touch, proprioception, and pain. Many molecular components of the mechanotransduction machinery subserving these sensory modalities remain to be discovered. Here, we combine recordings of mechanosensitive (MS) currents in mechanosensory neurons with single-cell RNA sequencing. Transcriptional profiles are mapped onto previously identified sensory neuron types to identify cell-type correlates between datasets. Correlation of current signatures with single-cell transcriptomes provides a one-to-one correspondence between mechanoelectric properties and transcriptomically defined neuronal populations. Moreover, a gene-expression differential comparison provides a set of candidate genes for mechanotransduction complexes. Piezo2 is expectedly found to be enriched in rapidly adapting MS current-expressing neurons, whereas Tmem120a and Tmem150c, thought to mediate slow-type MS currents, are uniformly expressed in all mechanosensory neuron subtypes. Further knockdown experiments disqualify them as mediating MS currents in sensory neurons. This dataset constitutes an open resource to explore further the cell-type-specific determinants of mechanosensory properties.

Cardiovascular mechanosensitive ion channels-Translating physical forces into physiological responses

Cells and tissues are constantly exposed to mechanical stress. In order to respond to alterations in mechanical stimuli, specific cellular machinery must be in place to rapidly convert physical force into chemical signaling to achieve the desired physiological responses. Mechanosensitive ion channels respond to such physical stimuli in the order of microseconds and are therefore essential components to mechanotransduction. Our understanding of how these ion channels contribute to cellular and physiological responses to mechanical force has vastly expanded in the last few decades due to engineering ingenuities accompanying patch clamp electrophysiology, as well as sophisticated molecular and genetic approaches. Such investigations have unveiled major implications for mechanosensitive ion channels in cardiovascular health and disease. Therefore, in this chapter I focus on our present understanding of how biophysical activation of various mechanosensitive ion channels promotes distinct cell signaling events with tissue-specific physiological responses in the cardiovascular system. Specifically, I discuss the roles of mechanosensitive ion channels in mediating (i) endothelial and smooth muscle cell control of vascular tone, (ii) mechano-electric feedback and cell signaling pathways in cardiomyocytes and cardiac fibroblasts, and (iii) the baroreflex.

Mechanical and chemical activation of GPR68 probed with a genetically encoded fluorescent reporter

G-protein-coupled receptor (GPCR) 68 (GPR68, or OGR1) couples extracellular acidifications and mechanical stimuli to G-protein signaling and plays important roles in vascular physiology, neuroplasticity and cancer progression. Inspired by previous GPCR-based reporters, here, we inserted a cyclic permuted fluorescent protein into the third intracellular loop of GPR68 to create a genetically encoded fluorescent reporter of GPR68 activation we call 'iGlow'. iGlow responds to known physiological GPR68 activators such as fluid shear stress and extracellular acidifications. In addition, iGlow responds to Ogerin, a synthetic GPR68-selective agonist, but not to a non-active Ogerin analog, showing the specificity of iGlow-mediated fluorescence signals. Flow-induced iGlow activation is not eliminated by pharmacological modulation of downstream G-protein signaling, disruption of actin filaments or application of GsMTx4, an inhibitor of certain mechanosensitive ion channels activated by membrane stretch. Deletion of the conserved helix 8, proposed to mediate mechanosensitivity in certain GPCRs, does not eliminate flow-induced iGlow activation. iGlow could be useful to investigate the contribution of GPR68-dependent signaling in health and disease.

The Atr-Chek1 pathway inhibits axon regeneration in response to Piezo-dependent mechanosensation

Atr is a serine/threonine kinase, known to sense single-stranded DNA breaks and activate the DNA damage checkpoint by phosphorylating Chek1, which inhibits Cdc25, causing cell cycle arrest. This pathway has not been implicated in neuroregeneration. We show that in Drosophila sensory neurons removing Atr or Chek1, or overexpressing Cdc25 promotes regeneration, whereas Atr or Chek1 overexpression, or Cdc25 knockdown impedes regeneration. Inhibiting the Atr-associated checkpoint complex in neurons promotes regeneration and improves synapse/behavioral recovery after CNS injury. Independent of DNA damage, Atr responds to the mechanical stimulus elicited during regeneration, via the mechanosensitive ion channel Piezo and its downstream NO signaling. Sensory neuron-specific knockout of Atr in adult mice, or pharmacological inhibition of Atr-Chek1 in mammalian neurons in vitro and in flies in vivo enhances regeneration. Our findings reveal the Piezo-Atr-Chek1-Cdc25 axis as an evolutionarily conserved inhibitory mechanism for regeneration, and identify potential therapeutic targets for treating nervous system trauma.

From stretch to deflection: the importance of context in the activation of mammalian, mechanically activated ion channels

The ability of cells to convert mechanical perturbations into biochemical information is an essential aspect of mammalian physiology. The molecules that mediate such mechanotransduction include mechanically activated ion channels, which directly convert mechanical inputs into electrochemical signals. The unifying feature of these channels is that their open probability increases with the application of a mechanical input. However, the structure, activation profile and sensitivity of distinct mechanically activated ion channels vary from channel to channel. In this review, we discuss how ionic currents can be mechanically evoked and monitored in vitro, and describe the distinct activation profiles displayed by a range of mammalian channels. In addition, we discuss the various mechanisms by which the best-characterized mammalian, mechanically activated ion channel, PIEZO1, can be modulated. The diversity of activation and modulation of these mammalian ion channels suggest that these molecules may facilitate a finely controlled and diverse ability to sense mechanical inputs in mammalian cells.

PIEZO ion channel is required for root mechanotransduction in Arabidopsis thaliana

Plant roots adapt to the mechanical constraints of the soil to grow and absorb water and nutrients. As in animal species, mechanosensitive ion channels in plants are proposed to transduce external mechanical forces into biological signals. However, the identity of these plant root ion channels remains unknown. Here, we show that Arabidopsis thaliana PIEZO1 (PZO1) has preserved the function of its animal relatives and acts as an ion channel. We present evidence that plant PIEZO1 is expressed in the columella and lateral root cap cells of the root tip, which are known to experience robust mechanical strain during root growth. Deleting PZO1 from the whole plant significantly reduced the ability of its roots to penetrate denser barriers compared to wild-type plants. pzo1 mutant root tips exhibited diminished calcium transients in response to mechanical stimulation, supporting a role of PZO1 in root mechanotransduction. Finally, a chimeric PZO1 channel that includes the C-terminal half of PZO1 containing the putative pore region was functional and mechanosensitive when expressed in naive mammalian cells. Collectively, our data suggest that Arabidopsis PIEZO1 plays an important role in root mechanotransduction and establish PIEZOs as physiologically relevant mechanosensitive ion channels across animal and plant kingdoms.

Inactivation Kinetics and Mechanical Gating of Piezo1 Ion Channels Depend on Subdomains within the Cap

Piezo1 ion channels are activated by mechanical stimuli and mediate the sensing of blood flow. Although cryo-electron microscopy (cryo-EM) structures have revealed the overall architecture of Piezo1, the precise domains involved in activation and subsequent inactivation have remained elusive. Here, we perform a targeted chimeric screen between Piezo1 and the closely related isoform Piezo2 and use electrophysiology to characterize their inactivation kinetics during mechanical stimulation. We identify three small subdomains within the extracellular cap that individually can confer the distinct kinetics of inactivation of Piezo2 onto Piezo1. We further show by cysteine crosslinking that conformational flexibility of these subdomains is required for mechanical activation to occur and that electrostatic interactions functionally couple the cap to the extensive blades, which have been proposed to function as sensors of membrane curvature and tension. This study provides a demonstration of internal gating motions involved in mechanotransduction by Piezo1.

Lewis and Grandl combine a chimeric screen and cysteine crosslinking to identify small subdomains of the cap of mechanically activated Piezo1 ion channels that must have conformational flexibility for mechanical gating. They further show that electrostatic interactions couple one of these domains to the channel blade.

Stretch-activated ion channels identified in the touch-sensitive structures of carnivorous Droseraceae plants

In response to touch, some carnivorous plants such as the Venus flytrap have evolved spectacular movements to capture animals for nutrient acquisition. However, the molecules that confer this sensitivity remain unknown. We used comparative transcriptomics to show that expression of three genes encoding homologs of the MscS-Like (MSL) and OSCA/TMEM63 family of mechanosensitive ion channels are localized to touch-sensitive trigger hairs of Venus flytrap. We focus here on the candidate with the most enriched expression in trigger hairs, the MSL homolog FLYCATCHER1 (FLYC1). We show that FLYC1 transcripts are localized to mechanosensory cells within the trigger hair, transfecting FLYC1 induces chloride-permeable stretch-activated currents in naïve cells, and transcripts coding for FLYC1 homologs are expressed in touch-sensing cells of Cape sundew, a related carnivorous plant of the Droseraceae family. Our data suggest that the mechanism of prey recognition in carnivorous Droseraceae evolved by co-opting ancestral mechanosensitive ion channels to sense touch.

Tentonin 3/TMEM150C senses blood pressure changes in the aortic arch

The baroreceptor reflex is a powerful neural feedback that regulates arterial pressure (AP). Mechanosensitive channels transduce pulsatile AP to electrical signals in baroreceptors. Here we show that tentonin 3 (TTN3/TMEM150C), a cation channel activated by mechanical strokes, is essential for detecting AP changes in the aortic arch. TTN3 was expressed in nerve terminals in the aortic arch and nodose ganglion (NG) neurons. Genetic ablation of Ttn3 induced ambient hypertension, tachycardia, AP fluctuations, and impaired baroreflex sensitivity. Chemogenetic silencing or activation of Ttn3+ neurons in the NG resulted in an increase in AP and heart rate, or vice versa. More important, overexpression of Ttn3 in the NG of Ttn3-/- mice reversed the cardiovascular changes observed in Ttn3-/- mice. We conclude that TTN3 is a molecular component contributing to the sensing of dynamic AP changes in baroreceptors.

A New Hope in Spinal Degenerative Diseases: Piezo1

As a newly discovered mechanosensitive ion channel protein, the piezo1 protein participates in the transmission of mechanical signals on the cell membrane and plays a vital role in mammalian biomechanics. Piezo1 has attracted widespread attention since it was discovered in 2010. In recent years, studies on piezo1 have gradually increased and deepened. In addition to the discovery that piezo1 is expressed in the respiratory, cardiovascular, gastrointestinal, and urinary systems, it is also stably expressed in cells such as mesenchymal stem cells (MSCs), osteoblasts, osteoclasts, chondrocytes, and nucleus pulposus cells that constitute vertebral bodies and intervertebral discs. They can all receive external mechanical stimulation through the piezo1 protein channel to affect cell proliferation, differentiation, migration, and apoptosis to promote the occurrence and development of lumbar degenerative diseases. Through reviewing the relevant literature of piezo1 in the abovementioned cells, this paper discusses the effect of piezo1 protein expression under mechanical stress stimuli on spinal degenerative disease, providing the molecular basis for the pathological mechanism of spinal degenerative disease and also a new basis, ideas, and methods for the prevention and treatment of this degenerative disease.

Stretch and poke stimulation for characterizing mechanically activated ion channels

Quantitative functional characterization of mechanically activated ion channels is most commonly achieved by a combination of patch-clamp electrophysiology and stimulation by stretch (or pressure-clamp) and poke (or cell-indentation). A variety of stretch and poke protocols can be used to make measurements of many ion channel properties, including channel number, unitary conductance, ion selectivity, stimulus threshold and sensitivity, stimulus adaptation, and gating kinetics (activation, deactivation, inactivation, recovery from inactivation). Here, we review the general methods of stretch and poke stimulation and discuss the advantages and disadvantages of each. We use the vertebrate mechanically activated ion channel Piezo1 to explain equipment components and calibration, demonstrate experimental protocols and data analyses, and discuss underlying concepts and mechanistic interpretations.

Optimal pulse length of insonification for Piezo1 activation and intracellular calcium response

Ultrasound (US) neuromodulation, especially sonogenetics, has been demonstrated with potential applications in noninvasive and targeted treatment of various neurological disorders. Despite the growing interest, the mechanism for US neuromodulation remains elusive, and the optimal condition for eliciting a neural response with minimal adverse effect has not been identified. Here, we investigate the Piezo1 activation and intracellular calcium response elicited by acoustical streaming induced shear stress under various US exposure conditions. We find that Piezo1 activation and resultant intracellular calcium response depend critically on shear stress amplitude and pulse length of the stimulation. Under the same insonification acoustic energy, we further identify an optical pulse length that leads to maximum cell deformation, Piezo1 activation, and calcium response with minimal injury, confirmed by numerical modeling of Piezo1 channel gating dynamics. Our results provide insight into the mechanism of ultrasonic activation of Piezo1 and highlight the importance of optimizing US exposure conditions in sonogenetics applications.