Recording of mechanosensitive currents using piezoelectrically driven mechanostimulator

The activation thresholds and inactivation kinetics of poking-evoked PIEZO1 and PIEZO2 currents are sensitive to subtle variations in mechanical stimulation parameters

PIEZO1 and PIEZO2 are mechanically activated ion channels that confer mechanosensitivity to various cell types. PIEZO channels are commonly examined using the so-called poking technique, where currents are recorded in the whole-cell configuration of the patch-clamp technique, while the cell surface is mechanically stimulated with a small fire-polished patch pipette. Currently, there is no gold standard for mechanical stimulation, and therefore, stimulation protocols differ significantly between laboratories with regard to stimulation velocity, angle, and size of the stimulation probe. Here, we systematically examined the impact of variations in these three stimulation parameters on the outcomes of patch-clamp recordings of PIEZO1 and PIEZO2. We show that the inactivation kinetics of PIEZO1 and, to a lesser extent, of PIEZO2 change with the angle at which the probe that is used for mechanical stimulation is positioned and, even more prominently, with the size of its tip. Moreover, we found that the mechanical activation threshold of PIEZO2, but not PIEZO1, decreased with increasing stimulation speeds. Thus, our data show that two key outcome parameters of PIEZO-related patch-clamp studies are significantly affected by common variations in the mechanical stimulation protocols, which calls for caution when comparing data from different laboratories and highlights the need to establish a gold standard for mechanical stimulation to improve comparability and reproducibility of data obtained with the poking technique.

Cellular Mechanosensitivity: Validation of an Adaptable 3D-Printed Device for Microindentation

Mechanotransduction refers to the cellular ability to sense mechanical stimuli from the surrounding environment and convert them into biochemical signals that regulate cellular physiology and homeostasis. Mechanosensitive ion channels (MSCs), especially ones of Piezo family (Piezo1 and Piezo2), play a crucial role in mechanotransduction. These transmembrane proteins directly react to mechanical cues by triggering the onset of an ionic current. The relevance of this mechanism in driving physiology and pathology is emerging, and there is a growing need for the identification of an affordable and reliable assay to measure it. Setting up a mechanosensitivity assay requires exerting a mechanical stimulus on single cells while observing the downstream effects of channels opening. We propose an open-hardware approach to stimulate single adherent cells through controlled microindentation, using a 3D-printed actuation platform. We validated the device by measuring the mechanosensitivity of a neural mice cell line where the expression level and activity of Piezo1 were genetically and pharmacologically manipulated. Moreover, this extremely versatile device could be integrated with different read-out technologies, offering a new tool to improve the understanding of mechanotransduction in living cells.

Force From Filaments: The Role of the Cytoskeleton and Extracellular Matrix in the Gating of Mechanosensitive Channels

The senses of proprioception, touch, hearing, and blood pressure on mechanosensitive ion channels that transduce mechanical stimuli with high sensitivity and speed. This conversion process is usually called mechanotransduction. From nematode MEC-4/10 to mammalian PIEZO1/2, mechanosensitive ion channels have evolved into several protein families that use variant gating models to convert different forms of mechanical force into electrical signals. In addition to the model of channel gating by stretching from lipid bilayers, another potent model is the opening of channels by force tethering: a membrane-bound channel is elastically tethered directly or indirectly between the cytoskeleton and the extracellular molecules, and the tethering molecules convey force to change the channel structure into an activation form. In general, the mechanical stimulation forces the extracellular structure to move relative to the cytoskeleton, deforming the most compliant component in the system that serves as a gating spring. Here we review recent studies focusing on the ion channel mechanically activated by a tethering force, the mechanotransduction-involved cytoskeletal protein, and the extracellular matrix. The mechanosensitive channel PIEZO2, DEG/ENaC family proteins such as acid-sensing ion channels, and transient receptor potential family members such as NompC are discussed. State-of-the-art techniques, such as polydimethylsiloxane indentation, the pillar array, and micropipette-guided ultrasound stimulation, which are beneficial tools for exploring the tether model, are also discussed.

Advancing mechanobiology by performing whole-cell patch clamp recording on mechanosensitive cells subjected simultaneously to dynamic stretch events

A comprehensive understanding of mechano-electrical coupling requires continuous intracellular electrical recordings being performed on cells undergoing simultaneously in vivo like strain events. Here, we introduce a linear strain single-cell electrophysiology (LSSE) system that meets these requirements by delivering highly reproducible unidirectional strain events with magnitudes up to 12% and strain rates exceeding 200%s⁻¹ to adherent cells kept simultaneously in whole-cell patch-clamp recording configuration. Proof-of-concept measurements with NIH3T3 cells demonstrate that stable recording conditions are maintained over tens of strain cycles at maximal amplitudes and strain rates thereby permitting a full electrophysiological characterization of mechanically activated ion currents. Because mechano-electrical responses to predefined strain patterns can be investigated using any adherent wild-type or genetically modified cell type of interest, the LSSE system offers the perspective of providing advanced insights into mechanosensitive ion channel function that can finally be compared quantitatively among different types of channels and cells.

•

The methodology presented enables investigations of adherent mechanosensitive cells

•

Whole-cell patch-clamp recording is performed while cells are dynamically stretched

•

Continuous recording of sequences of physiological mechanical stimuli is practicable

•

Experiments with NIH3T3 cells reveal a robust atypical mechanosensitive current

A spinal organ of proprioception for integrated motor action feedback

Proprioception is essential for behavior and provides a sense of our body movements in physical space. Proprioceptor organs are thought to be only in the periphery. Whether the central nervous system can intrinsically sense its own movement remains unclear. Here we identify a segmental organ of proprioception in the adult zebrafish spinal cord, which is embedded by intraspinal mechanosensory neurons expressing Piezo2 channels. These cells are late-born, inhibitory, commissural neurons with unique molecular and physiological profiles reflecting a dual sensory and motor function. The central proprioceptive organ locally detects lateral body movements during locomotion and provides direct inhibitory feedback onto rhythm-generating interneurons responsible for the central motor program. This dynamically aligns central pattern generation with movement outcome for efficient locomotion. Our results demonstrate that a central proprioceptive organ monitors self-movement using hybrid neurons that merge sensory and motor entities into a unified network.

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.

Action potential alterations induced by single F11 neuronal cell loading

Several research programmes have demonstrated how Transcranial Ultrasound Stimulation (TUS) can non-invasively and reversibly mechanically perturb neuronal functions. However, the mechanisms through which such reversible and a priori non-damaging behaviour can be observed remain largely unknown. While several TUS protocols have demonstrated motor and behavioural alterations in in vivo models, in vitro studies remain scarce. In particular, an experimental framework able to load mechanically an individual neuron in a controlled manner and simultaneously measure the generation and evolution of action potentials before, during and after such load, while allowing for direct microscopy, has not been successfully proposed. To this end, we herein present a multiphysics setup combining nanoindentation and patch clamp systems, assembled in an inverted microscope for simultaneous bright-field or fluorescence imaging. We evaluate the potential of the platform with a set of experiments in which single dorsal root ganglion-derived neuronal cell bodies are compressed while their spontaneous activity is recorded. We show that these transient quasi-static mechanical loads reversibly affect the amplitude and rate of change of the neuronal action potentials, which are smaller and slower upon indentation, while irreversibly altering other features. The ability to simultaneously image, mechanically and electrically manipulate and record single cells in a perturbed mechanical environment makes this system particularly suitable for studying the multiphysics of the brain at the cell level.

Maturational Changes in Mouse Cutaneous Touch and Piezo2-Mediated Mechanotransduction

The age of studied animals has a profound impact on experimental outcomes in animal-based research. In mice, age influences molecular, morphological, physiological, and behavioral parameters, particularly during rapid postnatal growth and maturation until adulthood (at 12 weeks of age). Despite this knowledge, most biomedical studies use a wide-spanning age range from 4 to 12 weeks, raising concerns about reproducibility and potential masking of relevant age differences. Here, using mouse behavior and electrophysiology in cultured dorsal root ganglia (DRG), we reveal a decline in behavioral cutaneous touch sensitivity and Piezo2-mediated mechanotransduction in vitro during mouse maturation but not thereafter. In addition, we identify distinct transcript changes in individual Piezo2-expressing mechanosensitive DRG neurons by combining electrophysiology with single-cell RNA sequencing (patch-seq). Taken together, our study emphasizes the need for accurate age matching and uncovers hitherto unknown maturational plasticity in cutaneous touch at the level of behavior, mechanotransduction, and transcripts.

Role of mechanosensitive ion channels in the sensation of pain

Our ability to sense mechanical cues from our environment depend on the capacity of molecular sensor capable of converting mechanical energy into biochemical or electrical signals. This process, termed mechanotransduction, relies on the activity of mechanosensitive ion channels (MSCs) that are expressed in most tissues, including cells of the inner and outer ear, sensory and sympathetic neurons, and vascular cells. However, the precise role these channels play in the physiology of the cells and organs, where they are expressed is not completely understood. In this review, we will explore some of the recent findings on the role of MSCs to our sense of mechanical pain.

TMEM150C/Tentonin3 Is a Regulator of Mechano-gated Ion Channels

Neuronal mechano-sensitivity relies on mechano-gated ion channels, but pathways regulating their activity remain poorly understood. TMEM150C was proposed to mediate mechano-activated current in proprioceptive neurons. Here, we studied functional interaction of TMEM150C with mechano-gated ion channels from different classes (Piezo2, Piezo1, and the potassium channel TREK-1) using two independent methods of mechanical stimulation. We found that TMEM150C significantly prolongs the duration of the mechano-current produced by all three channels, decreases apparent activation threshold in Piezo2, and induces persistent current in Piezo1. We also show that TMEM150C is co-expressed with Piezo2 in trigeminal neurons, expanding its role beyond proprioceptors. Finally, we cloned TMEM150C from the trigeminal neurons of the tactile-foraging domestic duck and showed that it functions similarly to the mouse ortholog, demonstrating evolutionary conservation among vertebrates. Our studies reveal TMEM150C as a general regulator of mechano-gated ion channels from different classes.

Piezo2 integrates mechanical and thermal cues in vertebrate mechanoreceptors

Tactile information is detected by thermoreceptors and mechanoreceptors in the skin and integrated by the central nervous system to produce the perception of somatosensation. Here we investigate the mechanism by which thermal and mechanical stimuli begin to interact and report that it is achieved by the mechanotransduction apparatus in cutaneous mechanoreceptors. We show that moderate cold potentiates the conversion of mechanical force into excitatory current in all types of mechanoreceptors from mice and tactile-specialist birds. This effect is observed at the level of mechanosensitive Piezo2 channels and can be replicated in heterologous systems using Piezo2 orthologs from different species. The cold sensitivity of Piezo2 is dependent on its blade domains, which render the channel resistant to cold-induced perturbations of the physical properties of the plasma membrane and give rise to a different mechanism of mechanical activation than that of Piezo1. Our data reveal that Piezo2 is an evolutionarily conserved mediator of thermal-tactile integration in cutaneous mechanoreceptors.

Roles of ASICs in Nociception and Proprioception

Acid-sensing ion channels (ASICs) are a group of proton-gated ion channels belonging to the degenerin/epithelial sodium channel (DED/ENaC) family. There are at least six ASIC subtypes - ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3, and ASIC4 - all expressed in somatosensory neurons. ASIC3 is the most abundant in dorsal root ganglia (DRG) and the most sensitive to extracellular acidification. ASICs were found as the major player involved in acid-induced pain in humans. Accumulating evidence has further shown ASIC3 as the molecular determinant involved in pain-associated tissue acidosis in rodent models. Besides having a role in nociception, members of the DEG/ENaC family have been demonstrated as essential mechanotransducers in the nematode Caenorhabditis elegans and fly Drosophila melanogaster. ASICs are mammalian homologues of DEG/ENaC and therefore may play a role in mechanotransduction. However, the role of ASICs in neurosensory mechanotransduction is disputed. Here we review recent studies to probe the roles of ASICs in acid nociception and neurosensory mechanotransduction. In reviewing genetic models and delicate electrophysiology approaches, we show ASIC3 as a dual-function protein for both acid-sensing and mechano-sensing in somatosensory nerves and therefore involved in regulating both nociception and proprioception.

Structure-guided examination of the mechanogating mechanism of PIEZO2

Piezo channels are mechanically activated ion channels that confer mechanosensitivity to a variety of different cell types. Piezos oligomerize as propeller-shaped homotrimers that are thought to locally curve the membrane into spherical domes that project into the cell. While several studies have identified domains and amino acids that control important properties such as ion permeability and selectivity as well as inactivation kinetics and voltage sensitivity, only little is known about intraprotein interactions that govern mechanosensitivity-the most unique feature of PIEZOs. Here we used site-directed mutagenesis and patch-clamp recordings to investigate the mechanogating mechanism of PIEZO2. We demonstrate that charged amino acids at the interface between the beam domain-i.e., a long α-helix that protrudes from the intracellular side of the "propeller" blade toward the inner vestibule of the channel-and the C-terminal domain (CTD) as well as hydrophobic interactions between the highly conserved Y2807 of the CTD and pore-lining helices are required to ensure normal mechanosensitivity of PIEZO2. Moreover, single-channel recordings indicate that a previously unrecognized intrinsically disordered domain located adjacent to the beam acts as a cytosolic plug that limits ion permeation possibly by clogging the inner vestibule of both PIEZO1 and PIEZO2. Thus, we have identified several intraprotein domain interfaces that control the mechanical activation of PIEZO1 and PIEZO2 and which might thus serve as promising targets for drugs that modulate the mechanosensitivity of Piezo channels.

Deep Sequencing of Somatosensory Neurons Reveals Molecular Determinants of Intrinsic Physiological Properties

Dorsal root ganglion (DRG) sensory neuron subtypes defined by their in vivo properties display distinct intrinsic electrical properties. We used bulk RNA sequencing of genetically labeled neurons and electrophysiological analyses to define ion channel contributions to the intrinsic electrical properties of DRG neuron subtypes. The transcriptome profiles of eight DRG neuron subtypes revealed differentially expressed and functionally relevant genes, including voltage-gated ion channels. Guided by these data, electrophysiological analyses using pharmacological and genetic manipulations as well as computational modeling of DRG neuron subtypes were undertaken to assess the functions of select voltage-gated potassium channels (Kv1, Kv2, Kv3, and Kv4) in shaping action potential (AP) waveforms and firing patterns. Our findings show that the transcriptome profiles have predictive value for defining ion channel contributions to sensory neuron subtype-specific intrinsic physiological properties. The distinct ensembles of voltage-gated ion channels predicted to underlie the unique intrinsic physiological properties of eight DRG neuron subtypes are presented.

Cell Mechanotransduction With Piconewton Forces Applied by Optical Tweezers

Mechanical stresses are always present in the cellular environment and mechanotransduction occurs in all cells. Although many experimental approaches have been developed to investigate mechanotransduction, the physical properties of the mechanical stimulus have yet to be accurately characterized. Here, we propose a mechanical stimulation method employing an oscillatory optical trap to apply piconewton forces perpendicularly to the cell membrane, for short instants. We show that this stimulation produces membrane indentation and induces cellular calcium transients in mouse neuroblastoma NG108-15 cells dependent of the stimulus strength and the number of force pulses.

Nanoscale Mechanical Stimulation Method for Quantifying C. elegans Mechanosensory Behavior and Memory

Withdrawal escape response of C. elegans to nonlocalized vibration is a useful behavioral paradigm to examine mechanisms underlying mechanosensory behavior and its memory-dependent change. However, there are very few methods for investigating the degree of vibration frequency, amplitude and duration needed to induce behavior and memory. Here, we establish a new system to quantify C. elegans mechanosensory behavior and memory using a piezoelectric sheet speaker. In the system, we can flexibly change the vibration properties at a nanoscale displacement level and quantify behavioral responses under each vibration property. This system is an economic setup and easily replicated in other laboratories. By using the system, we clearly detected withdrawal escape responses and confirmed habituation memory. This system will facilitate the understanding of physiological aspects of C. elegans mechanosensory behavior in the future.

Studying Mechanosensitivity of Two-Pore Domain K+ Channels in Cellular and Reconstituted Proteoliposome Membranes

Mechanical force sensation is fundamental to a wide breadth of biology from the classic senses of touch, pain, hearing, and balance to less conspicuous sensations of proprioception, blood pressure, and osmolarity and basic aspects of cell growth, differentiation, and development. These diverse and essential systems use force-gated (or mechanosensitive) ion channels that convert mechanical stimuli into cellular electrical signals. TRAAK, TREK1, and TREK2 are K+-selective ion channels of the two-pore domain K+ (K2P) family that are mechanosensitive: they are gated open by increasing membrane tension. TRAAK and TREK channels are thought to play roles in somatosensory and other mechanosensory processes in neuronal and non-neuronal tissues. Here, we present protocols for three assays to study mechanical activation of these channels in cell membranes: (1) cell swelling, (2) cell poking, and (3) patched membrane stretching. Patched membrane stretching is also applicable to the study of mechanosensitive K2P channel activity in a cell-free system and a procedure for proteoliposome reconstitution and patching is also presented. These approaches are also readily applicable to the study of other mechanosensitive ion channels.

The Kinetics and the Permeation Properties of Piezo Channels

Piezo channels are eukaryotic, cation-selective mechanosensitive channels (MSCs), which show rapid activation and voltage-dependent inactivation. The kinetics of these channels are largely consistent across multiple cell types and different stimulation paradigms with some minor variability. No accessory subunits that associate with Piezo channels have been reported. They are homotrimers and each ∼300kD monomer has an N-terminal propeller blade-like mechanosensing module, which can confer mechanosensing capabilities on ASIC-1 (the trimeric non-MSC, acid-sensing ion channel-1) and a C-terminal pore module, which influences conductance, selectivity, and channel inactivation. Repeated stimulation can cause domain fracture and diffusion of these channels leading to synchronous loss of inactivation. The reconstituted channels spontaneously open only in asymmetric bilayers but lack inactivation. Mutations that cause hereditary xerocytosis alter PIEZO1 kinetics. The kinetics of the wild-type PIEZO1 and alterations thereof in mutants (M2225R, R2456K, and DhPIEZO1) are summarized in the form of a quantitative model and hosted online. The pore is permeable to alkali ions although Li⁺ permeates poorly. Divalent cations, notably Ca²⁺, traverse the channel and inhibit the flux of monovalents. The large monovalent organic cations such as tetramethyl ammonium and tetraethyl ammonium can traverse the channel, but slowly, suggesting a pore diameter of ∼8Å, and the estimated in-plane area change upon opening is around 6-20nm². Ruthenium red can enter the channel only from the extracellular side and seems to bind in a pocket close to residue 2496.

A critical role for Piezo2 channels in the mechanotransduction of mouse proprioceptive neurons

Proprioceptors are responsible for the conscious sensation of limb position and movement, muscle tension or force, and balance. Recent evidence suggests that Piezo2 is a low threshold mechanosensory receptor in the peripheral nervous system, acting as a transducer for touch sensation and proprioception. Thus, we characterized proprioceptive neurons in the mesencephalic trigeminal nucleus that are involved in processing proprioceptive information from the face and oral cavity. This is a specific population of neurons that produce rapidly adapting mechanically-activated currents that are fully dependent on Piezo2. As such, we analyzed the deficits in balance and coordination caused by the selective deletion of the channel in proprioceptors (conditional knockout). The data clearly shows that Piezo2 fulfills a critical role in a defined homogeneous population of proprioceptor neurons that innervate the head muscles, demonstrating that this ion channel is essential for mammalian proprioceptive mechanotransduction.

Evidence for the involvement of ASIC3 in sensory mechanotransduction in proprioceptors

Acid-sensing ion channel 3 (ASIC3) is involved in acid nociception, but its possible role in neurosensory mechanotransduction is disputed. We report here the generation of Asic3-knockout/eGFPf-knockin mice and subsequent characterization of heterogeneous expression of ASIC3 in the dorsal root ganglion (DRG). ASIC3 is expressed in parvalbumin (Pv+) proprioceptor axons innervating muscle spindles. We further generate a floxed allele of Asic3 (Asic3^f/f) and probe the role of ASIC3 in mechanotransduction in neurite-bearing Pv+ DRG neurons through localized elastic matrix movements and electrophysiology. Targeted knockout of Asic3 disrupts spindle afferent sensitivity to dynamic stimuli and impairs mechanotransduction in Pv+ DRG neurons because of substrate deformation-induced neurite stretching, but not to direct neurite indentation. In behavioural tasks, global knockout (Asic3^−/−) and Pv-Cre::Asic3^f/f mice produce similar deficits in grid and balance beam walking tasks. We conclude that, at least in mouse, ASIC3 is a molecular determinant contributing to dynamic mechanosensitivity in proprioceptors.

Acid-sensing ion channel 3 (ASIC3) is known to play a role in nociception, but its role in low threshold neurosensory mechanotransduction is unclear. Here, the authors target ASIC3 expression in dorsal root ganglion parvalbumin positive neurons and find ASIC3 contributes to dynamic proprioception responses.

Tissue mechanics govern the rapidly adapting and symmetrical response to touch

Interactions with the physical world are deeply rooted in our sense of touch and depend on ensembles of somatosensory neurons that invade and innervate the skin. Somatosensory neurons convert the mechanical energy delivered in each touch into excitatory membrane currents carried by mechanoelectrical transduction (MeT) channels. Pacinian corpuscles in mammals and touch receptor neurons (TRNs) in Caenorhabditis elegans nematodes are embedded in distinctive specialized accessory structures, have low thresholds for activation, and adapt rapidly to the application and removal of mechanical loads. Recently, many of the protein partners that form native MeT channels in these and other somatosensory neurons have been identified. However, the biophysical mechanism of symmetric responses to the onset and offset of mechanical stimulation has eluded understanding for decades. Moreover, it is not known whether applied force or the resulting indentation activate MeT channels. Here, we introduce a system for simultaneously recording membrane current, applied force, and the resulting indentation in living C. elegans (Feedback-controlled Application of mechanical Loads Combined with in vivo Neurophysiology, FALCON) and use it, together with modeling, to study these questions. We show that current amplitude increases with indentation, not force, and that fast stimuli evoke larger currents than slower stimuli producing the same or smaller indentation. A model linking body indentation to MeT channel activation through an embedded viscoelastic element reproduces the experimental findings, predicts that the TRNs function as a band-pass mechanical filter, and provides a general mechanism for symmetrical and rapidly adapting MeT channel activation relevant to somatosensory neurons across phyla and submodalities.

Contribution of mechanosensitive ion channels to somatosensation

Mechanotransduction, the conversion of a mechanical stimulus into an electrical signal, is a central mechanism to several physiological functions in mammals. It relies on the function of mechanosensitive ion channels (MSCs). Although the first single-channel recording from MSCs dates back to 30 years ago, the identity of the genes encoding MSCs has remained largely elusive. Because these channels have an important role in the development of mechanical hypersensitivity, a better understanding of their function may lead to the identification of selective inhibitors and generate novel therapeutic pathways in the treatment of chronic pain. Here, I will describe our current understanding of the role MSCs may play in somatosensation and the potential candidate genes proposed to encode them.

Transduction and encoding sensory information by skin mechanoreceptors

Physical contact with the external world occurs through specialized neural structures called mechanoreceptors. Cutaneous mechanoreceptors provide information to the central nervous system (CNS) about touch, pressure, vibration, and skin stretch. The physiological function of these mechanoreceptors is to convert physical forces into neuronal signals. Key questions concern the molecular identity of the mechanoelectric transducer channels and the mechanisms by which the physical parameters of the mechanical stimulus are encoded into patterns of action potentials (APs). Compelling data indicate that the biophysical traits of mechanosensitive channels combined with the collection of voltage-gated channels are essential to describe the nature of the stimulus. Recent research also points to a critical role of the auxiliary cell-nerve ending communication in encoding stimulus properties. This review describes the characteristics of ion channels responsible for translating mechanical stimuli into the neural codes that underlie touch perception and pain.

Using injectoporation to deliver genes to mechanosensory hair cells

Mechanosensation, the transduction of mechanical force into electrochemical signals, allows organisms to detect touch and sound, to register movement and gravity, and to sense changes in cell volume and shape. The hair cells of the mammalian inner ear are the mechanosensors for the detection of sound and head movement. The analysis of gene function in hair cells has been hampered by the lack of an efficient gene transfer method. Here we describe a method termed injectoporation that combines tissue microinjection with electroporation to express cDNAs and shRNAs in mouse cochlear hair cells. Injectoporation allows for gene transfer into dozens of hair cells, and it is compatible with the analysis of hair cell function using imaging approaches and electrophysiology. Tissue dissection and injectoporation can be carried out within a few hours, and the tissue can be cultured for days for subsequent functional analyses.

Touch sense: functional organization and molecular determinants of mechanosensitive receptors

Cutaneous mechanoreceptors are localized in the various layers of the skin where they detect a wide range of mechanical stimuli, including light brush, stretch, vibration and noxious pressure. This variety of stimuli is matched by a diverse array of specialized mechanoreceptors that respond to cutaneous deformation in a specific way and relay these stimuli to higher brain structures. Studies across mechanoreceptors and genetically tractable sensory nerve endings are beginning to uncover touch sensation mechanisms. Work in this field has provided researchers with a more thorough understanding of the circuit organization underlying the perception of touch. Novel ion channels have emerged as candidates for transduction molecules and properties of mechanically gated currents improved our understanding of the mechanisms of adaptation to tactile stimuli. This review highlights the progress made in characterizing functional properties of mechanoreceptors in hairy and glabrous skin and ion channels that detect mechanical inputs and shape mechanoreceptor adaptation.

Runx1 controls terminal morphology and mechanosensitivity of VGLUT3-expressing C-mechanoreceptors

VGLUT3-expressing unmyelinated low-threshold mechanoreceptors (C-LTMRs) are proposed to mediate pleasant touch and/or pain, but the molecular programs controlling C-LTMR development are unknown. Here, we performed genetic fate mapping, showing that VGLUT3 lineage sensory neurons are divided into two groups, based on transient or persistent VGLUT3 expression. VGLUT3-transient neurons are large- or medium-diameter myelinated mechanoreceptors that form the Merkel cell-neurite complex. VGLUT3-persistent neurons are small-diameter unmyelinated neurons that are further divided into two subtypes: (1) tyrosine hydroxylase (TH)-positive C-LTMRs that form the longitudinal lanceolate endings around hairs, and (2) TH-negative neurons that form epidermal-free nerve endings. We then found that VGLUT3-persistent neurons express the runt domain transcription factor Runx1. Analyses of mice with a conditional knock-out of Runx1 in VGLUT3 lineage neurons demonstrate that Runx1 is pivotal to the development of VGLUT3-persistent neurons, such as the expression of VGLUT3 and TH and the formation of the longitudinal lanceolate endings. Furthermore, Runx1 is required to establish mechanosensitivity in C-LTMRs, by controlling the expression of the mechanically gated ion channel Piezo2. Surprisingly, both acute and chronic mechanical pain was largely unaffected in these Runx1 mutants. These findings appear to argue against the recently proposed role of VGLUT3 in C-LTMRs in mediating mechanical hypersensitivity induced by nerve injury or inflammation. Thus, our studies provide new insight into the genetic program controlling C-LTMR development and call for a revisit for the physiological functions of C-LTMRs.

Neurosensory mechanotransduction through acid-sensing ion channels

Acid-sensing ion channels (ASICs) are voltage-insensitive cation channels responding to extracellular acidification. ASIC proteins have two transmembrane domains and a large extracellular domain. The molecular topology of ASICs is similar to that of the mechanosensory abnormality 4- or 10-proteins expressed in touch receptor neurons and involved in neurosensory mechanotransduction in nematodes. The ASIC proteins are involved in neurosensory mechanotransduction in mammals. The ASIC isoforms are expressed in Merkel cell-neurite complexes, periodontal Ruffini endings and specialized nerve terminals of skin and muscle spindles, so they might participate in mechanosensation. In knockout mouse models, lacking an ASIC isoform produces defects in neurosensory mechanotransduction of tissue such as skin, stomach, colon, aortic arch, venoatrial junction and cochlea. The ASICs are thus implicated in touch, pain, digestive function, baroreception, blood volume control and hearing. However, the role of ASICs in mechanotransduction is still controversial, because we lack evidence that the channels are mechanically sensitive when expressed in heterologous cells. Thus, ASIC channels alone are not sufficient to reconstruct the path of transducing molecules of mechanically activated channels. The mechanotransducers associated with ASICs need further elucidation. In this review, we discuss the expression of ASICs in sensory afferents of mechanoreceptors, findings of knockout studies, technical issues concerning studies of neurosensory mechanotransduction and possible missing links. Also we propose a molecular model and a new approach to disclose the molecular mechanism underlying the neurosensory mechanotransduction.

Piezo-electrically driven mechanical stimulation of sensory neurons

Mechanotransduction, the conversion of a mechanical stimulus into a biological response, constitutes the basis of a variety of physiological functions such as the senses of touch, balance, proprioception, blood pressure, and hearing. In vertebrates, mechanosensation is mediated by mechanosensory neurons, whose cell bodies are located in trigeminal and dorsal root ganglia. Here, we describe an in vitro model of mechanotransduction that provides an opportunity to explore the properties of mechanosensitive channels in mammalian sensory neurons. The mechano-clamp method allows applying local force on plasma membrane of whole-cell patch-clamped sensory neurons. This technique uses a mechanical probe driven by a computer-assisted piezoelectric microstage to repeatedly stimulate sensory neurons with accurate control of stimulus strength, duration, and speed.