The molecules of mechanosensation

Mechanosensing role of caveolae and caveolar constituents in human endothelial cells

A variety of evidence suggests that endothelial cell functions are impaired in altered gravity conditions. Nevertheless, the effects of hypergravity on endothelial cell physiology remain unclear. In this study we cultured primary human endothelial cells under mild hypergravity conditions for 24-48 h, then we evaluated the changes in cell cycle progression, caveolin1 gene expression and in the caveolae status by confocal microscopy. Moreover, we analyzed the activity of enzymes known to be resident in caveolae such as endothelial nitric oxide synthase (eNOS), cycloxygenase 2 (COX-2), and prostacyclin synthase (PGIS). Finally, we performed a three-dimensional in vitro collagen gel test to evaluate the modification of the angiogenic responses. Results indicate that hypergravity shifts endothelial cells to G(0)/G(1) phase of cell cycle, reducing S phase, increasing caveolin1 gene expression and causing an increased distribution of caveolae in the cell interior. Hypergravity also increases COX-2 expression, nitric oxide (NO) and prostacyclin (PGI2) production, and inhibits angiogenesis as evaluated by 3-D collagen gel test, through a pathway not involving apoptosis. Thus, endothelial cell caveolae may be responsible for adaptation of endothelium to hypergravity and the mechanism of adaptation involves an increased caveolin1 gene expression coupled to upregulation of vasodilators as NO and PGI2.

Mammalian TRPV4 (VR-OAC) directs behavioral responses to osmotic and mechanical stimuli in Caenorhabditis elegans

All animals detect osmotic and mechanical stimuli, but the molecular basis for these responses is incompletely understood. The vertebrate transient receptor potential channel vanilloid subfamily 4 (TRPV4) (VR-OAC) cation channel has been suggested to be an osmo/mechanosensory channel. To assess its function in vivo, we expressed TRPV4 in Caenorhabditis elegans sensory neurons and examined its ability to generate behavioral responses to sensory stimuli. C. elegans ASH neurons function as polymodal sensory neurons that generate a characteristic escape behavior in response to mechanical, osmotic, or olfactory stimuli. These behaviors require the TRPV channel OSM-9 because osm-9 mutants do not avoid nose touch, high osmolarity, or noxious odors. Expression of mammalian TRPV4 in ASH neurons of osm-9 worms restored avoidance responses to hypertonicity and nose touch, but not the response to odorant repellents. Mutations known to reduce TRPV4 channel activity also reduced its ability to direct nematode avoidance behavior. TRPV4 function in ASH required the endogenous C. elegans osmotic and nose touch avoidance genes ocr-2, odr-3, osm-10, and glr-1, indicating that TRPV4 is integrated into the normal ASH sensory apparatus. The osmotic and mechanical avoidance responses of TRPV4-expressing animals were different in their sensitivity and temperature dependence from the responses of wild-type animals, suggesting that the TRPV4 channel confers its characteristic properties on the transgenic animals' behavior. These results provide evidence that TRPV4 can function as a component of an osmotic/mechanical sensor in vivo.

Cardiac mechanotransduction and implications for heart disease

Mechanotransduction, the conversion of a mechanical stimulus into a cellular response, plays a fundamental role in cell volume regulation, fertilization, gravitaxis, proprioception, and the senses of hearing, touch, and balance. Mechanotransduction also fills important functions in the myocardium, where each cycle of contraction and relaxation leads to dynamic deformations. Since the initial observation of stretch induced muscle growth, our understanding of this complex field has been steadily growing, but remains incomplete. For example, the mechanism by which myocytes sense mechanical forces is still unknown. It is also unknown which mechanism converts such a stimulus into an electrochemical signal, and how this information is transferred to the nucleus. Is there a subpopulation of mechanosensing myocytes or mechanosensing cells in the myocardium? The following article offers an overview of the fundamental processes of mechanical stretch sensing in myocytes and recent advances in our understanding of this increasingly important field. Special emphasis is placed on the unique cardiac cytoskeletal structure and related Z-disc proteins.

Microtubule-associated [corrected] protein 7 increases the membrane expression of transient receptor potential vanilloid 4 (TRPV4)

The molecular mechanism of the transmission of changes in the shape of the cell surface to ion channels remains obscure. Ca2+ influx induced by cell deformity is inhibited by actin-freezing reagents, suggesting that the actin microfilament couples with an ion channel. Transient receptor potential vanilloid 4 (TRPV4) is a candidate in the calcium-permeable, swelling-activated mechanosensitive channel in heterogeneously expressed cells. To investigate the mechanosensitive molecular complex, we found that microtubule-associated protein 7 (MAP7) is the mouse TRPV4 C-terminal binding protein. MAP7 was coimmunoprecipitated with TRPV4. The results of a pull-down assay demonstrated that the alignment of amino acids 798-809 of TRPV4 was important in this interaction. TRPV4 and MAP7 colocalized in the lung and kidney. The coexpression of these two molecules resulted in the redistribution of TRPV4 toward the membrane and increased its functional expression. The alignment of amino acids 798-809 of TRPV4 was also important in the functional expression. The activated current was abolished by actin-freezing but not by microtubule-freezing reagents. We therefore believe that MAP7 may enhance the membrane expression of TRPV4 and possibly link cytoskeletal microfilaments.

Characterization of the transmembrane channel-like (TMC) gene family: functional clues from hearing loss and epidermodysplasia verruciformis

Mutations of TMC1 cause deafness in humans and mice. TMC1 and a related gene, TMC2, are the founding members of a novel gene family. Here we describe six additional TMC paralogs (TMC3 to TMC8) in humans and mice, as well as homologs in other species. cDNAs spanning the full length of the predicted open reading frames of the mammalian genes were cloned and sequenced. All are strongly predicted to encode proteins with 6 to 10 transmembrane domains and a novel conserved 120-amino-acid sequence that we termed the TMC domain. TMC1, TMC2, and TMC3 comprise a distinct subfamily expressed at low levels, whereas TMC4 to TMC8 are expressed at higher levels in multiple tissues. TMC6 and TMC8 are identical to the EVER1 and EVER2 genes implicated in epidermodysplasia verruciformis, a recessive disorder comprising susceptibility to cutaneous human papilloma virus infections and associated nonmelanoma skin cancers, providing additional genetic and tissue systems in which to study the TMC gene family.

Pharmacology of vagal afferent nerve activity in guinea pig airways

The excitability and activity of vagal afferent nerves innervating the airways can be pharmacologically increased and decreased. Autacoids released as a result of airway inflammation can lead to substantial increases in afferent nerve activity, consequently altering pulmonary reflex physiology. In a manner analogous to hyperalgesia associated with inflammation in the somato-sensory system, increases in vagal afferent nerve activity in inflamed airways may lead to a heightened cough reflex, and increases in autonomic activity in the airways. These effects may contribute to many of the symptoms of inflammatory airway disease. Here we provide a brief overview of some of the mechanisms by which the afferent activity in airway nerves can be pharmacologically modified.

GAP43 stimulates inositol trisphosphate-mediated calcium release in response to hypotonicity

The identification of osmo/mechanosensory proteins in mammalian sensory neurons is still elusive. We have used an expression cloning approach to screen a human dorsal root ganglion cDNA library to look for proteins that respond to hypotonicity by raising the intracellular Ca(2+) concentration ([Ca(2+)](i)). We report the unexpected identification of GAP43 (also known as neuromodulin or B50), a membrane-anchored neuronal protein implicated in axonal growth and synaptic plasticity, as an osmosensory protein that augments [Ca(2+)](i) in response to hypotonicity. Palmitoylation of GAP43 plays an important role in the protein osmosensitivity. Depletion of intracellular stores or inhibition of phospholipase C (PLC) activity abrogates hypotonicity-evoked, GAP43-mediated [Ca(2+)](i) elevations. Notably, hypotonicity promoted the selective association of GAP43 with the PLC-delta(1) isoform, and a concomitant increase in inositol-1,4,5-trisphosphate (IP(3)) formation. Collectively, these findings indicate that hypo-osmotic activation of GAP43 induces Ca(2+) release from IP(3)-sensitive intracellular stores. The osmosensitivity of GAP43 furnishes a mechanistic framework that links axon elongation with phospho inositide metabolism, spontaneous triggering of cytosolic Ca(2+) transients and the regulation of actin dynamics and motility at the growth cone in response to temporal and local mechanical forces.

Vertebrate and invertebrate TRPV-like mechanoreceptors

Our senses of touch, hearing, and balance are mediated by mechanosensitive ion channels. In vertebrates, little is known about the molecular composition of these mechanoreceptors, an example of which is the transduction channel of the inner ear's receptor cells, hair cells. Members of the TRP family of ion channels are considered candidates for the vertebrate hair cell's mechanosensitive transduction channel and here we review the evidence for this candidacy. We start by examining the results of genetic screens in invertebrates that identified members of the TRP gene family as core components of mechanoreceptors. In particular, we discuss the Caenorhabditis elegans OSM-9 channel, an invertebrate TRPV channel, and the Drosophila melanogaster TRP channel NOMPC. We then evaluate basic features of TRPV4, a vertebrate member of the TRPV subfamily, which is gated by a variety of physical and chemical stimuli including temperature, osmotic pressure, and ligands. Finally, we compare the characteristics of all discussed mechanoreceptive TRP channels with the biophysical characteristics of hair cell mechanotransduction, speculating about the possible make-up of the elusive inner ear mechanoreceptor.

5-HT2 and 3 receptor antagonists suppress the response of rat type I slowly adapting mechanoreceptor: an in vitro study

Previous experiments have shown an increase in rat type I mechanoreceptor responsiveness during arterial serotonin (5-hydroxytryptamine) infusion and the presence of serotonin immunostaining in Merkel cells. The current findings demonstrate that the 5-HT(2) antagonists ritanserin and ketanserin, as well as the 5-HT(3) antagonist MDL 72222, reduce type I response to a standardized mechanical stimulus in an in vitro skin preparation. In addition, ritanserin blocked the enhancement of type I response produced by 5-HT. These experiments suggest that serotonin is released during mechanical distortion of the Merkel cell membrane and alters action potential generation by the type I ending. In addition, it is possible that serotonin, released from outside the type I complex, influences mechanoreceptor responsiveness. For example, serotonin generated during inflammatory events could enhance type I response to mechanical stimulation and thereby increase symptoms of mechanical allodynia.

Drosophila DEG/ENaC pickpocket genes are expressed in the tracheal system, where they may be involved in liquid clearance

The Drosophila tracheal system and mammalian airways are branching networks of tubular epithelia that deliver oxygen to the organism. In mammals, the epithelial Na(+) channel (ENaC) helps clear liquid from airways at the time of birth and removes liquid from the airspaces in adults. We tested the hypothesis that related Drosophila degenerin (DEG)/ENaC family members might play a similar role in the fly. Among 16 Drosophila DEG/ENaC genes, called pickpocket (PPK) genes, we found 9 expressed in the tracheal system. By in situ hybridization, expression appeared in late-stage embryos after tracheal tube formation, with individual PPK genes showing distinct temporal and spatial expression patterns as development progressed. Promoters for several PPK genes drove reporter gene expression in the larval and adult tracheal systems. Adding the DEG/ENaC channel blocker amiloride to the medium inhibited liquid clearance from the trachea of first instar larvae. Moreover, when RNA interference was used to silence PPK4 and PPK11, larvae failed to clear tracheal liquid. These data suggest substantial molecular diversity of DEG/ENaC channel expression in the Drosophila tracheal system where the PPK proteins likely play a role in Na(+) absorption. Extensive similarities between Drosophila and mammalian airways offer opportunities for genetic studies that may decipher further the structure and function of DEG/ENaC proteins and development of the airways.

Allometric scaling and proportion regulation in the freshwater planarian Schmidtea mediterranea

The regulation of scale and proportion in living organisms is an intriguing and enduring problem of biology. Regulatory mechanisms for controlling body size and proportion are clearly illustrated by the regeneration of missing body parts after amputation, in which the newly regenerated tissues ultimately attain a size that is anatomically congruent with the size of the rest of the organism. Understanding the molecular processes underpinning scaling would have deep consequences for our comprehension of tissue regeneration, developmental ontogeny, growth, and evolution. Although many theories have been put forward to explain this process, it is interesting that no satisfactory mechanistic explanation is currently available to explain scalar relationships. We chose to investigate the freshwater planarian, a commonly used model system for the study of metazoan regeneration, to delineate a strategy for the molecular dissection of scale and proportion mechanisms in metazoans. Here, we report on the cloning and discrete expression pattern of a novel planarian gene, which shares homology with the DEG/ENaC super-family of sodium channels. We have named H.112.3c cintillo ("head ribbon" in Spanish) and present a strategy for using the expression of this gene to monitor scale and proportion regulation during regeneration, growth and degrowth in the freshwater planarian Schmidtea mediterranea.

Regulation of swelling-activated chloride channels in embryonic chick heart cells

Swelling-activated Cl- currents, I(Cl,swell) were measured during hyposmotic shock in white Leghorn embryonic chick heart cells using the whole-cell recording of patch-clamp technique. Genistein, an inhibitor of protein tyrosine kinase (PTK), suppressed I(Cl,swell). Under isosmotic condition phorbol 12-myristate 13-acetate (PMA), an activator of PKC, elicited the Cl- current similar to that in hyposmotic solution, whereas hyposmotic shock did not elicit I(Cl,swell) in chelerythrine chloride(an inhibitor of PKC)-treated cells. Confocal microscopy experiments using FITC-phalloidin as a fluorescent label of F-actin showed that the actin network was moved from cortical region of the cell to the center after hyposmotic shock as compared with the image under isosmotic condition. When the cells were treated with cytochalasin B (CB) or cytochalasin D (CD) under isosmotic condition the disruption of the F-actin integrity was observed, and I(Cl,swell) was not elicited. With combination treatment of CB with PMA, hyposmotic solution could not elicited I(Cl,swell). The results suggested that the role of PTK, probably receptor tyrosine kinase, for regulation of I(Cl,swell) appeared to be at upstream site related to the role of F-actin. Then PKC signal pathway was activated somehow and finally change in the polymerization state of cytoskeleton led to activate the swelling-activated Cl- channels. These results demonstrate clearly that PTK, PKC and F-actin are important factors for regulation of I(Cl,swell), in embryonic chick heart cells as compared with often controversial results reported in different cell types.

Epithelial sodium channel/degenerin family of ion channels: a variety of functions for a shared structure

The recently discovered epithelial sodium channel (ENaC)/degenerin (DEG) gene family encodes sodium channels involved in various cell functions in metazoans. Subfamilies found in invertebrates or mammals are functionally distinct. The degenerins in Caenorhabditis elegans participate in mechanotransduction in neuronal cells, FaNaC in snails is a ligand-gated channel activated by neuropeptides, and the Drosophila subfamily is expressed in gonads and neurons. In mammals, ENaC mediates Na+ transport in epithelia and is essential for sodium homeostasis. The ASIC genes encode proton-gated cation channels in both the central and peripheral nervous system that could be involved in pain transduction. This review summarizes the physiological roles of the different channels belonging to this family, their biophysical and pharmacological characteristics, and the emerging knowledge of their molecular structure. Although functionally different, the ENaC/DEG family members share functional domains that are involved in the control of channel activity and in the formation of the pore. The functional heterogeneity among the members of the ENaC/DEG channel family provides a unique opportunity to address the molecular basis of basic channel functions such as activation by ligands, mechanotransduction, ionic selectivity, or block by pharmacological ligands.

Essential hydrophilic carboxyl-terminal regions including cysteine residues of the yeast stretch-activated calcium-permeable channel Mid1

The yeast Saccharomyces cerevisiae MID1 gene encodes a stretch-activated Ca(2+)-permeable nonselective cation channel composed of 548 amino acid residues. A physiological role of the Mid1 channel is known to maintain the viability of yeast cells exposed to mating pheromone, but its structural basis remains to be clarified. To solve this problem, we identified the mutation sites of mid1 mutant alleles generated by in vivo ethyl methanesulfonate mutagenesis and found that two mid1 alleles have nonsense mutations at the codon for Trp(441), generating a truncated Mid1 protein lacking two-thirds of the intracellular carboxyl-terminal region from Asn(389) to Thr(548). In vitro random mutagenesis with hydroxylamine also showed that the carboxyl-terminal region is essential. To identify the functional portion of the carboxyl-terminal region in detail, we performed a progressive carboxyl-terminal truncation followed by functional analyses and found that the truncated protein produced from the mid1 allele bearing the amber mutation at the codon for Phe(522) (F522Am) complemented the mating pheromone-induced death phenotype of the mid1 mutant and increased its Ca(2+) uptake activity to a wild-type level, whereas N521Am did not. This result indicates that the carboxyl-terminal domain spanning from Asn(389) to Asn(521) is required for Mid1 function. Interestingly, this domain is cysteine-rich, and alanine-scanning mutagenesis revealed that seven out of 10 cysteine residues are unexchangeable. These results clearly indicate that the carboxyl-terminal domain including the cysteine residues is important for Mid1 function.

[Nociceptors and mediators in acute inflammatory pain]

OBJECTIVE

To bring together the most recent data concerning the physiology of nociceptors at a time when there has been significant progress in the understanding of these.

DATA SOURCES

References were obtained from computerised bibliographic data banks (Medline and others) and the authors' personal documents.

DATA SYNTHESIS

Nociceptive impulses are generated at the periphery in unmyelinated fibres called nociceptors, the responses of which depend on the tissue environment. Numerous mediators can activate, sensitise or "wake up" nociceptor: kinins (bradykinin, kallidin and their metabolites), pro-inflammatory cytokines (TNF alpha, IL-6, IL-1 beta, IL-8), anti-inflammatory cytokines (IL-4, IL-6, IL-10, IL-12, IL-13), prostanoids (PGE2, PGI2), lipo-oxygenases (leucotrienes such LTB4 or 15-HETE), the "central mediators of the immune response" (NF-kappa B), growth factors such as neurotrophins (NGF, BDNF, NT-3 and NT-4/5), peptides (substance P, CGRP, Neurokinin A), nitric oxide, histamine, serotonin, proteases, excitatory amino acids, adrenergic amines and opioids. The release of neuromediators by primary afferent fibres in the spinal cord may be summarised by successively considering calcium channels, presynaptic receptors, excitatory amino acids and peptides.

CONCLUSION

Sensitisation phenomena are not exclusively peripheral but also central in origin and these are interlinked.

Specificity of cold thermotransduction is determined by differential ionic channel expression

Sensations of cold are mediated by specific thermoreceptor nerve endings excited by low temperature and menthol. Here we identify a population of cold-sensitive cultured mouse trigeminal ganglion neurons with a unique set of biophysical properties. Their impulse activity during cooling and menthol application was similar to that of cold thermoreceptor fibers in vivo. We show that cooling closes a background K+ channel, causing depolarization and firing that is limited by the slower reduction of a cationic inward current (Ih). In cold-insensitive neurons, firing is prevented by a slow, transient, 4-AP-sensitive K+ current (IKD) that acts as an excitability brake. In addition, pharmacological blockade of IKD induced thermosensitivity in cold-insensitive neurons, a finding that may explain cold allodynia in neuropathic pain. These results suggest that cold sensitivity is not associated to a specific transduction molecule but instead results from a favorable blend of ionic channels expressed in a small subset of sensory neurons.

Mechanosensitive ion channels in cultured sensory neurons of neonatal rats

Mechanosensitive (MS) ion channels are present in a variety of cells. However, very little is known about the ion channels that account for mechanical sensitivity in sensory neurons. We identified the two most frequently encountered but distinct types of MS channels in 1390 of 2962 membrane patches tested in cultured dorsal root ganglion neurons. The two MS channels exhibited different thresholds, thus named as low-threshold (LT) and high-threshold (HT) MS channels, and sensitivity to pressure. The two channels retained different single-channel conductances and current-voltage relationships: LT and HT channels elicited large- and small-channel conductance with outwardly rectifying and linear I-V relationships, respectively. Both LT and HT MS channels were permeable to monovalent cations and Ca2+ and were blocked by gadolinium, a blocker of MS channels. Colchicine and cytochalasin D markedly reduced the activities of the two MS channels, indicating that cytoskeletal elements support the mechanosensitivity. Both types of MS channels were found primarily in small sensory neurons with diameters of <30 microm. Furthermore, HT MS channels were sensitized by a well known inducer of mechanical hyperalgesia, prostaglandin E2, via the protein kinase A pathway. We identified two distinct types of MS channels in sensory neurons that probably give rise to the observed MS whole-cell currents and transduce mechanical stimuli to neural signals involved in somatosensation, including pain.

Functional implications of the localization and activity of acid-sensitive channels in rat peripheral nervous system

Acid-sensitive ion channels (ASIC) are proton-gated ion channels expressed in neurons of the mammalian central and peripheral nervous systems. The functional role of these channels is still uncertain, but they have been proposed to constitute mechanoreceptors and/or nociceptors. We have raised specific antibodies for ASIC1, ASIC2, ASIC3, and ASIC4 to examine the distribution of these proteins in neurons from dorsal root ganglia (DRG) and to determine their subcellular localization. Western blot analysis demonstrates that all four ASIC proteins are expressed in DRG and sciatic nerve. Immunohistochemical experiments and functional measurements of unitary currents from the ASICs with the patch-clamp technique indicate that ASIC1 localizes to the plasma membrane of small-, medium-, and large-diameter cells, whereas ASIC2 and ASIC3 are preferentially in medium to large cells. Neurons coexpressing ASIC2 and ASIC3 form predominantly heteromeric ASIC2-3 channels. Two spliced forms, ASIC2a and ASIC2b, colocalize in the same population of DRG neurons. Within cells, the ASICs are present mainly on the plasma membrane of the soma and cellular processes. Functional studies indicate that the pH sensitivity for inactivation of ASIC1 is much higher than the one for activation; hence, increases in proton concentration will inactivate the channel. These functional properties and localization in DRG have profound implications for the putative functional roles of ASICs in the nervous system.

The PDZ domain protein PICK1 and the sodium channel BNaC1 interact and localize at mechanosensory terminals of dorsal root ganglion neurons and dendrites of central neurons

Members of the BNaC/ASIC family of ion channels have been implicated in mechanotransduction and nociception mediated by dorsal root ganglion (DRG) neurons. These ion channels are also expressed in the CNS. We identified the PDZ domain protein PICK1 as an interactor of BNaC1(ASIC2) in a yeast two-hybrid screen. We show by two-hybrid assays, glutathione S-transferase pull-down assays, and coimmunoprecipitations that the BNaC1-PICK1 interaction is specific, and that coexpression of both proteins leads to their clustering in intracellular compartments. The interaction between BNaC1 and PICK1 requires the PDZ domain of PICK1 and the last four amino acids of BNaC1. BNaC1 is similar to two other BNaC/ASIC family members, BNaC2 (ASIC1) and ASIC4, at its extreme C terminus, and we show that PICK1 also interacts with BNaC2. We found that PICK1, like BNaC1 and BNaC2, is expressed by DRG neurons and, like the BNaC1alpha isoform, is present at their peripheral mechanosensory endings. Both PICK1 and BNaC1alpha are also coexpressed by some pyramidal neurons of the cortex, by pyramidal neurons of the CA3 region of hippocampus, and by cerebellar Purkinje neurons, localizing to their dendrites and cell bodies. Therefore, PICK1 interacts with BNaC/ASIC channels and may regulate their subcellular distribution or function in both peripheral and central neurons.

Mechanotransduction: all signals point to cytoskeleton, matrix, and integrins

Mechanical stresses modulate cell function by either activating or tuning signal transduction pathways. Mechanotransduction, the process by which cells convert mechanical stimuli into a chemical response, occurs both in cells specialized for sensing mechanical cues and in parenchymal cells whose primary function is not mechanosensory. However, common among the various responses to mechanical stress is the importance of direct or indirect connections between the internal cytoskeleton, the extracellular matrix (ECM), and traditional signal transducing molecules. In many instances, these elements converge at focal adhesions, sites of structural attachment between the cytoskeleton and ECM that are anchored by cell surface integrin receptors. Alenghat and Ingber discuss the accumulating evidence for the central role of cytoskeleton, ECM, and integrin-anchored focal adhesions in several mechanotransduction pathways.

Interaction of the synaptic protein PICK1 (protein interacting with C kinase 1) with the non-voltage gated sodium channels BNC1 (brain Na+ channel 1) and ASIC (acid-sensing ion channel)

Neuronal members of the degenerin/epithelial Na(+) channel (DEG/ENaC) family of cation channels include the mammalian brain Na(+) channel 1 (BNC1), acid-sensing ion channel (ASIC) and dorsal-root acid-sensing ion channel (DRASIC). Their response to acidic pH, their sequence similarity to nematode proteins involved in mechanotransduction and their modulation by neuropeptides suggest that they may function as receptors for a number of different stimuli. Using the yeast two-hybrid assay, we found that the PDZ domain-containing protein PICK1 (protein interacting with C kinase) interacts specifically with the C-termini of BNC1 and ASIC, but not DRASIC or the related alphaENaC or betaENaC. In both the yeast two-hybrid system and mammalian cells, deletion of the BNC1 and ASIC C-termini abolished the interaction with PICK1. Likewise, mutating the PDZ domain in PICK1 abolished its interaction with BNC1 and ASIC. In addition, in a heterologous expression system PICK1 altered the distribution of BNC1 channels; this effect was dependent on the PDZ domain of PICK1 and the C-terminus of BNC1. We found crude synaptosomal fractions of brain to be enriched in ASIC, suggesting a possible synaptic localization. Moreover, in transfected hippocampal neurons ASIC co-localized with PICK1 in a punctate pattern at synapses. These data suggest that PICK1 binds ASIC and BNC1 via its PDZ domain. This interaction may be important for the localization and/or function of these channels in both the central and peripheral nervous systems.

Functional effects of a monoclonal antibody on mechanoelectrical transduction in outer hair cells

The functional effect of a monoclonal antibody, RA6.3, on mechanoelectrical transduction (MET) of outer hair cells (OHCs) isolated from the adult guinea-pig cochlea was investigated. This antibody was raised by an antiidiotypic approach against amiloride binding sites. RA6.3 irreversibly reduced the receptor current, independent of membrane potential. The time course of the functional block was independent of dilution (1:100, 50 and 10), beginning 1.2+/-0.5 min after the start of application and decreasing exponentially with a time constant of 0.29+/-0.18 min to 53+/-8% of the control current. The residual current was reversibly blocked by amiloride (300 microM), mainly at negative membrane potentials. Block of control current by amiloride was competitively inhibited by simultaneous application of RA6.3. These results suggest that RA6.3 binds to or in close proximity to amiloride receptor sites associated with the MET channels. Irreversibility, incompleteness, independence of membrane potential and independence of antibody dilution of the functional block can all be explained by irreversible binding of one antibody molecule to a receptor site, yielding a non-blocked state, followed by a relatively slow, reversible transition to a blocked state. It is proposed that the reversible transition might represent intramolecular binding of the second antibody combining site to the second receptor site.

pH alterations "reset" Ca2+ sensitivity of brain Na+ channel 2, a degenerin/epithelial Na+ ion channel, in planar lipid bilayers

Members of the degenerin/epithelial Na(+) channel superfamily of ion channels subserve many functions, ranging from whole body sodium handling to mechanoelectrical transduction. We studied brain Na(+) channel 2 (BNaC-2) in planar lipid bilayers to examine its single channel properties and regulation by Ca(2+). Upon incorporation of vesicles made from membranes of oocytes expressing either wild-type (WT) BNaC-2 or BNaC-2 with a gain-of-function (GF) point mutation (G433F), functional channels with different properties were obtained. WT BNaC-2 resided in a closed state with short openings, whereas GF BNaC-2 was constitutively activated; a decrease in the pH in the trans compartment of the bilayer activated WT BNaC-2 and decreased its permeability for Na(+) over K(+). Moreover, these maneuvers made the WT channel more resistant to amiloride. In contrast, GF BNaC-2 did not respond to a decrease in pH, and its amiloride sensitivity and selectivity for Na(+) over K(+) were unaffected by this pH change. Buffering the bathing solutions with EGTA to reduce the free [Ca(2+)] to <10 nm increased WT single channel open probability 10-fold, but not that of GF BNaC-2. Ca(2+) blocked both WT and GF BNaC-2 in a dose- and voltage-dependent fashion; single channel conductances were unchanged. A drop in pH reduced the ability of Ca(2+) to inhibit these channels. These results show that BNaC-2 is an amiloride-sensitive sodium channel and suggest that pH activation of these channels could be, in part, a consequence of H(+) "interference" with channel regulation by Ca(2+).

Inhibition of mechanical activation of guinea-pig airway afferent neurons by amiloride analogues

1. The aim of this study was to investigate a role for Epithelial Sodium Channels (ENaCs) in the mechanical activation of low-threshold vagal afferent nerve terminals in the guinea-pig trachea/bronchus. 2. Using extracellular single-unit recording techniques, we found that the ENaC blocker amiloride, and its analogues dimethylamiloride and benzamil caused a reduction in the mechanical activation of guinea-pig airway afferent fibres. 3. Amiloride and it analogues also reduced the sensitivity of afferent fibres to electrical stimulation such that greater stimulation voltages were required to induce action potentials from their peripheral terminals within the trachea/bronchus. 4. The relative potencies of these compounds for inhibiting electrical excitability of afferent nerves were similar to that observed for inhibition of mechanical stimulation (dimethylamiloride approximately benzamil > amiloride). This rank order of potency is incompatible with the known rank order of potency for blockade of ENaCs (benzamil > amiloride >> dimethylamiloride). 5. As voltage-gated sodium channels play an important role in determining the electrical excitability of neurons, we used whole-cell patch recordings of nodose neuron cell bodies to investigate the possibility that amiloride analogues caused blockade of these channels. At the concentration required to inhibit mechanical activation of vagal nodose afferent fibres (100 microM), benzamil caused significant inhibition of voltage-gated sodium currents in neuronal cell bodies acutely isolated from guinea-pig nodose ganglia. 6. Combined, our findings suggest that amiloride and its analogues did not selectively block mechanotransduction in airway afferent neurons, but rather they reduced neuronal excitability, possibly by inhibiting voltage-gated sodium currents.

Nitric oxide synthase inhibitors enhance mechanosensitive Ca(2+) influx in cultured dorsal root ganglion neurons

Nitric oxide (NO) can have opposite effects on peripheral sensory neuron sensitivity depending on the concentration and source of NO, and the experimental setting. The aim of this study was to determine the role of endogenous NO production in the regulation of mechanosensitive Ca(2+) influx of dorsal root ganglion (DRG) neurons. Adult mouse DRG neurons were grown in primary culture for 2-5 days, loaded with Fura-2, and tested for mechanically mediated changes in [Ca(2+)](i) by fluorescent ratio imaging. In the presence of the NOS inhibitors L-NAME, TRIM, or 7-NI, but not the inactive analogue D-NAME, peak [Ca(2+)](i) transients to mechanical stimulation were increased more than 2-fold. Neither La(3+) (25 microM), an inhibitor of voltage activated Ca(2+) channels, or tetrodotoxin (TTX, 1 microM), a selective inhibitor of voltage-gated Na(+) channels, had an effect on mechanically activated [Ca(2+)](i) transients under control conditions. However, in the presence of L-NAME, both La(3+) and TTX partially blocked the [Ca(2+)](i) response. Addition of Gd(3+), a blocker of mechanosensitive cation channels and L-type Ca(2+) channels, at a concentration (100 microM) that markedly inhibited the mechanical response under control conditions, only partially inhibited the response in the presence of L-NAME. The combination of either La(3+) or TTX with Gd(3+) caused near complete inhibition of mechanically stimulated [Ca(2+)](i) transients in the presence of L-NAME. We conclude that focal mechanical stimulation of DRG neurons causes Ca(2+) influx occurs primarily through mechanosensitive cation channels under control conditions. In the presence of NOS inhibitors, additional Ca(2+) influx occurs through voltage-sensitive Ca(2+) channels. These results suggest that endogenously produced NO in cultured DRG neurons decreases mechanosensitivity by inhibiting voltage-gated Na(+) and Ca(2+) channels.

Transport and localization of the DEG/ENaC ion channel BNaC1alpha to peripheral mechanosensory terminals of dorsal root ganglia neurons

Mammalian brain sodium channel (BNaC, also known as BNC/ASIC) proteins form acid-sensitive and amiloride-blockable sodium channels that are related to putative mechanosensory channels. Certain BNaC isoforms are expressed exclusively in dorsal root ganglia (DRG) and have been proposed to form the ion channels mediating tissue acidosis-induced pain. With antibody labeling, we find that the BNaC1alpha isoform is expressed by most large DRG neurons (low-threshold mechanosensors not involved in acid-induced nociception) and few small nociceptor neurons (which include high-threshold mechanoreceptors). BNaC1alpha is transported from DRG cell bodies to sensory terminals in the periphery, but not to the spinal cord, and is located specifically at specialized cutaneous mechanosensory terminals, including Meissner, Merkel, penicillate, reticular, lanceolate, and hair follicle palisades as well as some intraepidermal and free myelinated nerve endings. Accordingly, BNaC1alpha channels might participate in the transduction of touch and painful mechanical stimuli.

ENaC subunits are molecular components of the arterial baroreceptor complex

Mechanosensation is essential to the perception of our environment. It is required for hearing, touch, balance, proprioception, and blood pressure homeostasis. Yet little is known about the identity of ion-channel complexes that transduce mechanical stimuli into neuronal responses. Genetic studies in Caenorhabditis elegans suggest that members of the DEG/ENaC family may be mechanosensors. Therefore we tested the hypothesis that mammalian epithelial Na(+)-channel (ENaC) subunits contribute to the mechanosensor in baroreceptor neurons. The data presented here show that ENaC transcripts and proteins are expressed in mechanosensory neurons and at the putative sites of mechanotransduction in baroreceptor sensory-nerve terminals. Additionally, known ENaC inhibitors, amiloride and benzamil, disrupt mechanotransduction in arterial baroreceptor neurons. These data are consistent with the hypothesis that DEG/ENaC proteins are components of mechanosensitive ion-channel complexes.

Structural and functional differences between two homologous mechanosensitive channels of Methanococcus jannaschii

We report the molecular cloning and characterization of MscMJLR, a second type of mechanosensitive (MS) channel found in the archaeon Methanococcus jannaschii. MscMJLR is structurally very similar to MscMJ, the MS channel of M.jannaschii that was identified and cloned first by using the TM1 domain of Escherichia coli MscL as a genetic probe. Although it shares 44% amino acid sequence identity and similar cation selectivity with MscMJ, MscMJLR exhibits other major functional differences. The conductance of MscMJLR of approximately 2 nS is approximately 7-fold larger than the conductance of MscMJ and rectifies with voltage. The channel requires approximately 18 kT for activation, which is three times the amount of energy required to activate MscMJ, but is comparable to the activation energy of Eco-MSCL: Our study indicates that a multiplicity of conductance-wise and energetically well-tuned MS channels in microbial cell membranes may provide for cell survival by the sequential opening of the channels upon challenge with different osmotic cues.

Molecular basis of mechanotransduction in living cells

The simplest cell-like structure, the lipid bilayer vesicle, can respond to mechanical deformation by elastic membrane dilation/thinning and curvature changes. When a protein is inserted in the lipid bilayer, an energetic cost may arise because of hydrophobic mismatch between the protein and bilayer. Localized changes in bilayer thickness and curvature may compensate for this mismatch. The peptides alamethicin and gramicidin and the bacterial membrane protein MscL form mechanically gated (MG) channels when inserted in lipid bilayers. Their mechanosensitivity may arise because channel opening is associated with a change in the protein's membrane-occupied area, its hydrophobic mismatch with the bilayer, excluded water volume, or a combination of these effects. As a consequence, bilayer dilation/thinning or changes in local membrane curvature may shift the equilibrium between channel conformations. Recent evidence indicates that MG channels in specific animal cell types (e.g., Xenopus oocytes) are also gated directly by bilayer tension. However, animal cells lack the rigid cell wall that protects bacteria and plants cells from excessive expansion of their bilayer. Instead, a cortical cytoskeleton (CSK) provides a structural framework that allows the animal cell to maintain a stable excess membrane area (i.e., for its volume occupied by a sphere) in the form of membrane folds, ruffles, and microvilli. This excess membrane provides an immediate membrane reserve that may protect the bilayer from sudden changes in bilayer tension. Contractile elements within the CSK may locally slacken or tighten bilayer tension to regulate mechanosensitivity, whereas membrane blebbing and tight seal patch formation, by using up membrane reserves, may increase membrane mechanosensitivity. In specific cases, extracellular and/or CSK proteins (i.e., tethers) may transmit mechanical forces to the process (e.g., hair cell MG channels, MS intracellular Ca(2+) release, and transmitter release) without increasing tension in the lipid bilayer.

Podocytes respond to mechanical stress in vitro

Glomerular capillary pressure is thought to affect the structure and function of glomerular cells. However, it is unknown whether podocytes are intrinsically sensitive to mechanical forces. In the present study, differentiated mouse podocytes were cultured on flexible silicone membranes. Biaxial cyclic stress (0.5 Hz and 5% linear strain) was applied to the membranes for up to 3 d. Mechanical stress reduced the size of podocyte cell bodies, and processes became thin and elongated. Podocytes did not align in the inhomogeneous force field. Whereas the network of microtubules and that of the intermediate filament vimentin exhibited no major changes, mechanical stress induced a reversible reorganization of the actin cytoskeleton: transversal stress fibers (SF) disappeared and radial SF that were connected to an actin-rich center (ARC) formed. Epithelial and fibroblast cell lines did not exhibit a comparable stress-induced reorganization of the F-actin. Confocal and electron microscopy revealed an ellipsoidal and dense filamentous structure of the ARC. Myosin II, alpha-actinin, and the podocyte-specific protein synaptopodin were present in radial SF, but, opposite to F-actin, they were not enriched in the ARC. The formation of the ARC and of radial SF in response to mechanical stress was inhibited by nonspecific blockade of Ca(2+) influx with Ni(2+) (1 mM), by Rho kinase inhibition with Y-27632 (10 microM), but not by inhibition of stretch-activated cation channels with Gd(3+) (50 microM). In summary, mechanical stress induces a unique reorganization of the actin cytoskeleton in podocytes, featuring radial SF and an ARC, which differ in protein composition. The F-actin reorganization in response to mechanical stress depends on Ca(2+) influx and Rho kinase. The present study provides the first direct evidence that podocytes are mechanosensitive.

Ion channels in airway afferent neurons

Action potentials initiated at the peripheral terminal of an afferent nerve are conducted to the central nervous system therein causing release of neurotransmitters that excite secondary neurons in the brain stem or spinal cord. Various chemicals, extremes in osmolarity and pH as well as mechanical stimuli are sensed by primary afferent nerves that innervate the airways. The processes leading to excitation of afferent nerve endings, conduction of action potentials along axons, transmitter secretion, and neuronal excitability are regulated by ions flowing through channels in the nerve membrane. Voltage-gated ion channels selective for K+ and Na+ ions allow the generation and conduction of action potentials and along with families of ion channels selective for other ions such as Ca2+ or Cl- are thought to play distinctive roles in regulating neuronal excitability and transmitter secretion. Here we discuss, in general terms, the roles played by various classes of ion channels in the activation, neurotransmitter secretion and excitability of primary afferent neurons.

Nociceptors for the 21st century

This review summarizes recent developments in the context of the neurochemical classification of nociceptors and explores the relationships between functionally and neurochemically defined subgroups. Although the complete picture is not yet available, several lines of intriguing evidence suggest that despite the complexity and diversity of nociceptor properties, a relatively "simple" neurochemical classification fits well with several recently identified molecular characteristics.

Regional and subunit-specific downregulation of acid-sensing ion channels in the pilocarpine model of epilepsy

Acid-sensing ion channels (ASICs) constitute a recently discovered family of excitatory cation channels, structurally related to the superfamily of degenerin/epithelial sodium channels. ASIC1b and ASIC3 are highly expressed in primary sensory neurons and are thought to play a role in pain transmission related to acidosis. ASIC1a, ASIC2a, and ASIC2b are also distributed in the central nervous system where their function remains unclear. We investigated here the regulation of their expression during status epilepticus (SE), a condition in which neuronal overexcitation leads to acidosis. In animals treated with pilocarpine (380 mg/kg) to induce SE, we observed a marked decrease of ASIC2b mRNA levels in all hippocampal areas and of ASIC1a mRNA levels in the CA1-2 fields. These changes were also observed after protective treatment from neuronal cell death with diazepam (10 mg/kg) and pentobarbital (30 mg/kg). These findings suggest a key role of channels containing ASIC1a and ASIC2b subunits in both normal and pathological activity of hippocampus.

Swelling-activated calcium signalling in cultured mouse primary sensory neurons

The effects of hypo-osmotic membrane stretch on intracellular calcium concentration ([Ca(2+)](i)), cell volume and cellular excitability were investigated in cultured mouse primary sensory trigeminal neurons. Hypotonic solutions (15--45%) led to rapid cell swelling in all neurons. Swelling was accompanied by dose-dependent elevations in [Ca(2+)](i) in a large fraction of neurons. Responses could be classified into three categories. (i) In 57% of the neurons [Ca(2+)](i) responses had a slow rise time and were generally of small amplitude. (ii) In 21% of the neurons, responses had a faster rise and were larger in amplitude. (iii) The remaining cells (22%) did not show [Ca(2+)](i) responses to hypo-osmotic stretch. Slow and fast [Ca(2+)](i) changes were observed in trigeminal neurons of different sizes with variable responses to capsaicin (0.5 microM). The swelling-induced [Ca(2+)](i) responses were not abolished after depletion of intracellular Ca2+ stores with cyclopiazonic acid or preincubation in thapsigargin, but were suppressed in the absence of external Ca(2+). They were strongly attenuated by extracellular nickel and gadolinium. Hypotonic stimulation led to a decrease in input resistance and to membrane potential depolarization. Under voltage-clamp, the [Ca(2+)](i) elevation produced by hypotonic stimulation was accompanied by the development of an inward current and a conductance increase. The time course and amplitude of the [Ca(2+)](i) response to hypo-osmotic stimulation showed a close correlation with electrophysiological properties of the neurons. Fast [Ca(2+)](i) responses were characteristic of trigeminal neurons with short duration action potentials and marked inward rectification. These findings suggest that hypo-osmotic stimulation activates several Ca(2+)-influx pathways, including Gd(3+)-sensitive stretch-activated ion channels, in a large fraction of trigeminal ganglion neurons. Opening of voltage-gated Ca(2+) channels also contributes to the response. The pattern and rate of Ca(2+) influx may be correlated with functional subtypes of sensory neurons.

Molecular identification of a mechanosensitive channel in archaea

The TM1 domain of the large conductance mechanosensitive (MS) channel of Escherichia coli was used as a genetic probe to search the genomic database of the archaeon Methanoccoccus jannashii for MscL homologs. We report that the hypothetical protein MJ0170 of M. jannashii exhibited 38.5% sequence identity with the TM1 domain of Eco-MscL. Moreover, MJ0170 was found to be a conserved homolog of MscS, the second type of E. coli MS channel encoded by the yggB gene. Furthermore, we identified a cluster of charged residues KIKEE in the C-terminus of MJ0170 that strikingly resembled the charged C-terminal amino acid cluster present in Eco-MscL (RKKEE). We cloned and expressed MJ0170 in E. coli, which when reconstituted into liposomes or expressed in the cell membrane of giant E. coli spheroplasts, exhibited similar activity to the bacterial MS channels. Our study suggests that the M. jannashii MS channel and its homologs evolved as a result of gene duplication of the ancestral MscL-like molecule with the TM1 domain remaining the most conserved structural motif among prokaryotic MS channels.

The mammalian sodium channel BNC1 is required for normal touch sensation

Of the vertebrate senses, touch is the least understood at the molecular level The ion channels that form the core of the mechanosensory complex and confer touch sensitivity remain unknown. However, the similarity of the brain sodium channel 1 (BNC1) to nematode proteins involved in mechanotransduction indicated that it might be a part of such a mechanosensor. Here we show that disrupting the mouse BNC1 gene markedly reduces the sensitivity of a specific component of mechanosensation: low-threshold rapidly adapting mechanoreceptors. In rodent hairy skin these mechanoreceptors are excited by hair movement. Consistent with this function, we found BNC1 in the lanceolate nerve endings that lie adjacent to and surround the hair follicle. Although BNC1 has been proposed to have a role in pH sensing, the acid-evoked current in cultured sensory neurons and the response of acid-stimulated nociceptors were normal in BNC1 null mice. These data identify the BNC1 channel as essential for the normal detection of light touch and indicate that BNC1 may be a central component of a mechanosensory complex.

Gating energies and forces of the mammalian hair cell transducer channel and related hair bundle mechanics

We quantified the molecular energies and forces involved in opening and closing of mechanoelectrical transducer channels in hair cells using a novel generally applicable method. It relies on a thermodynamic description of the free energy of an ion channel in terms of its open probability. The molecular gating force per channel as reflected in hair bundle mechanics is shown to equal kT/I(X) x dI(X)/dX, where I is the transducer current and X the deflection of the hair bundle. We applied the method to previously measured I(X) curves in mouse outer hair cells (OHCs) and vestibular hair cells (VHCs). Contrary to current models of transduction, gating of the transducer channel was found to involve only a finite range of free energy (< 10 kT), a consequence of our observation that the channel has a finite minimum open probability of ca. 1% for inhibitory bundle deflections. The maximum gating forces per channel of both cell types were found to be comparable (ca. 300-500 fN). Because of differences in passive restoring forces, gating forces result in very limited mechanical nonlinearity in OHC bundles compared to that in VHC bundles. A kinetic model of channel activation is proposed that accounts for the observed transducer currents and gating forces. It also predicts adaptation-like effects and spontaneous bundle movements ensuing from changes in state energy gaps possibly related to interactions of the channel with calcium ions.

Responses of nerve fibres of the rat saphenous nerve neuroma to mechanical and chemical stimulation: an in vitro study

The response of neuroma nerve endings to different stimuli was studied in a saphenous nerve neuroma preparation in vitro. Electrical activity was recorded from 141 single fibres dissected of saphenous nerve. One-third (27 %) displayed spontaneous activity. Based on their response to mechanical and chemical stimuli, neuroma nerve fibres were classified as mechanosensory fibres (47.5 %), mechanically insensitive chemosensory fibres (17.0 %), polymodal nociceptor fibres (28.4 %) and unresponsive fibres (7.1 %). Mechanosensory and polymodal neuroma endings responded to von Frey hair stimulation either with a few impulses (phasic units) or a sustained discharge (tonic units). Polymodal units were additionally activated by at least one of the following stimuli: acidic solutions; a combination of bradykinin, prostaglandin E2, serotonin, substance P and histamine (all at 1 microM) plus 7 mM KCl (inflammatory soup); 600 mM NaCl and capsaicin. Low pH solutions increased the firing discharge of polymodal endings proportionally to the proton concentration. The 'inflammatory soup' evoked a firing response characterized by the absence of tachyphylaxis, which appeared when its components were applied separately. Both stimuli sensitized polymodal fibres to mechanical stimulation. Hypertonic NaCl (600 mM) and capsaicin (3.3 mM) induced a prolonged discharge that outlasted the stimulus duration. Mechanically insensitive chemosensory neuroma fibres exhibited responses to chemical stimuli analogous to polymodal fibres. They became mechanically sensitive after chemical stimulation. These findings show that neuroma nerve endings in the rat saphenous nerve neuroma in vitro are functionally heterogeneous and exhibit properties reminiscent of those in intact mechanosensory, polymodal and 'silent' nociceptor sensory afferents, including their sensitization by algesic chemicals.

Insect mechanoreception: what a long, strange TRP it's been

Insect bristles are model mechanosensory organs. An ion channel of the TRP superfamily has recently been identified which is required for production of mechanoreceptor currents by insect bristles, and seems likely to represent a new kind of mechanically gated channel.

In vivo structure-function analyses of Caenorhabditis elegans MEC-4, a candidate mechanosensory ion channel subunit

Mechanosensory signaling mediated by mechanically gated ion channels constitutes the basis for the senses of touch and hearing and contributes fundamentally to the development and homeostasis of all organisms. Despite this profound importance in biology, little is known of the molecular identities or functional requirements of mechanically gated ion channels. We report a genetically based structure-function analysis of the candidate mechanotransducing channel subunit MEC-4, a core component of a touch-sensing complex in Caenorhabditis elegans and a member of the DEG/ENaC superfamily. We identify molecular lesions in 40 EMS-induced mec-4 alleles and further probe residue and domain function using site-directed approaches. Our analysis highlights residues and subdomains critical for MEC-4 activity and suggests possible roles of these in channel assembly and/or function. We describe a class of substitutions that disrupt normal channel activity in touch transduction but remain permissive for neurotoxic channel hyperactivation, and we show that expression of an N-terminal MEC-4 fragment interferes with in vivo channel function. These data advance working models for the MEC-4 mechanotransducing channel and identify residues, unique to MEC-4 or the MEC-4 degenerin subfamily, that might be specifically required for mechanotransducing function. Because many other substitutions identified by our study affect residues conserved within the DEG/ENaC channel superfamily, this work also provides a broad view of structure-function relations in the superfamily as a whole. Because the C. elegans genome encodes representatives of a large number of eukaryotic channel classes, we suggest that similar genetic-based structure-activity studies might be generally applied to generate insight into the in vivo function of diverse channel types.

Molecular events caused by mechanical stress in bone

The shape of bone changes as a result of bone remodeling corresponding to physical circumstances such as mechanical stress. The tissue which receives the loaded mechanical stress most efficiently is bone matrix. Recent studies revealed the function of osteocytes as mechanosensors in the early stage of bone remodeling. Loaded mechanical stress is converted to a series of biochemical reactions, and finally activates osteoclasts and osteoblasts to cause bone resorption and formation. Biochemical and molecular biological studies have recently resulted in the identification of the gene of which expression level is changed by mechanical stress. Nitric oxide (NO) and cAMP is secreted in response to mechanical stress in the immediate early stage. Genes encoding enzymes such as glutamate/aspartate transporter (GLAST), nitric oxide synthetase (NOS) and prostaglandin G/H synthetase (PGHS-2) are identified as mechanical stress-responsive. The expression level of IGF-I is enhanced under the control of PTH/PTHrP. The expression of c-fos is increased by loading of mechanical stress. AP1, a heterodimer of c-FOS/c-JUN, functions as a transcription factor of downstream gene(s). Elements including AP1 sites, cyclic AMP response elements (CRE) and shear stress response elements (SSRE) are found in the promoter region of mechanical stress-response genes. The enhanced expression of osteopontin (OPN) in the osteocytes of bone resorption sites was demonstrated by in situ hybridization and immunohistochemistry and transdifferentiation of chondrocytes with the abundant expression of BMP-2 and -4 in the process of distraction osteogenesis was observed.

A Drosophila mechanosensory transduction channel

Mechanosensory transduction underlies a wide range of senses, including proprioception, touch, balance, and hearing. The pivotal element of these senses is a mechanically gated ion channel that transduces sound, pressure, or movement into changes in excitability of specialized sensory cells. Despite the prevalence of mechanosensory systems, little is known about the molecular nature of the transduction channels. To identify such a channel, we analyzed Drosophila melanogaster mechanoreceptive mutants for defects in mechanosensory physiology. Loss-of-function mutations in the no mechanoreceptor potential C (nompC) gene virtually abolished mechanosensory signaling. nompC encodes a new ion channel that is essential for mechanosensory transduction. As expected for a transduction channel, D. melanogaster NOMPC and a Caenorhabditis elegans homolog were selectively expressed in mechanosensory organs.

Genetics of chemotaxis and thermotaxis in the nematode Caenorhabditis elegans

Molecular genetic analysis of chemotaxis and theramotaxis in Caenorhabditis elegans has revealed the molecular bases of olfaction, taste, and thermosensation, which, in turn, has demonstrated that sensory signaling in C. elegans is very similar to that in vertebrates. A cyclic nucleotide-gated channel (TAX-2/TAX-4) that is highly homologous to the olfactory and photoreceptor channels in vertebrates is required for taste and thermosensation, in addition to olfaction. A cation channel (OSM-9) that is closely related to a capsaicin receptor channel is required for olfactory adaptation in one olfactory neuron and olfactory sensation in the other olfactory neuron. A novel G alpha protein (ODR-3) is essential for olfactory responses in all olfactory neurons and aversive responses in a polymodal sensory neuron. A G protein-coupled seven-transmembrane receptor (ODR-10) is the first olfactory receptor whose ligand was elucidated. Using chemotaxis and thermotaxis as behavioral paradigms, neural plasticity including learning and memory can be studied genetically in C. elegans.

Contributions of the different extramembranous domains of the mechanosensitive ion channel MscL to its response to membrane tension

MscL is a mechanosensitive channel that is gated by tension in the membrane bilayer alone. It is a homo-oligomer of a protein comprising two transmembrane segments connected by an external loop, with the NH(2) and COOH termini located in the cytoplasm. The contributions of the extramembranous domains of the channel to its activity were investigated by specific proteolysis during patch-clamp experiments. Limited proteolysis of the COOH terminus or the NH(2) terminus increased the mechanosensitivity of the channel without changing its conductance. Strikingly, after cleavage of the external loop of each monomer, the channel was still functional, and its mechanosensitivity was increased dramatically, indicating that the loop acts as a spring that resists the opening of the channel and promotes its closure when it is open. These results indicate that the integrity of most of the extramembranous domains is not essential for mechanosensitivity. They suggest that these domains counteract the movement of the transmembrane helices to which they are connected, thus setting the level of sensitivity of the channel to tension.

Mechanoreceptors in calanoid copepods: designed for high sensitivity

The mechanoreceptors of the first antennae of Pleuromamma xiphias, a mesopelagic calanoid copepod, are critical for the detection of potential threats. These receptors exceed the physiological performance of other crustacean mechanoreceptors in sensitivity to water velocities as well as in frequency response. A study of these receptors was initiated to elucidate structure-function relationships. Morphologically, the receptors resemble the arthropod scolopidial organs by the presence of a scolopale tube. However, the rigidity of the copepod receptors greatly exceeds those described for crustaceans and other arthropods. The scolopale tube completely encloses the distal dendrites and it is firmly anchored to the cuticle. Microtubules are organized in register and arise from microtubule subfibers associated with crescent-shaped rods which extend from the basal body region to the setal socket. The distal dendrites are filled with a large number of cross-linked microtubules. Termination of the distal dendrites inside the lumen of the setae is gradual with a firm anchoring to the cuticle. A likely mechanism for mechanotransduction would involve a linkage between individual microtubules and mechano-gated channels in the dendritic membrane. The rigidity probably contributes to the high frequency sensitivity, and termination of the dendrite inside the seta increases the overall sensitivity of these receptors.

Mechanical transduction by rat dorsal root ganglion neurons in vitro

Although it is generally presumed that mechanical sensitivity of somatosensory nerve fibers results from the activation of mechanosensitive ion channels, a mechanically-gated whole-cell current has never been demonstrated in dorsal root ganglion (DRG) neurons. We performed patch clamp experiments on rat DRG neurons in culture, and report the first mechanically-activated current in somatosensory neurons (I(mech)). This whole-cell current is observed in most dorsal root ganglion neurons but not in non-sensory sympathetic ganglion neurons. The current-voltage relation of I(mech) indicates that it is a non-selective cation current. Sensitivity of I(mech) to block by gadolinium suggests that it may be mediated by a member of a family of mechanosensitive non-selective cation channels observed in many cell types. Sensitivity to benzamil supports this idea, and further suggests that the current might be mediated by a member of the degenerin/ epithelial sodium channel (DEG/ENaC) family.

Epithelial sodium channel in human epidermal keratinocytes: expression of its subunits and relation to sodium transport and differentiation

The amiloride-sensitive epithelial sodium channel (ENaC) is a main determinant of sodium absorption in renal and colonic epithelial cells. Surprisingly, it is also expressed in non-transporting epithelia such as the epidermis. To gain insight into the putative role of ENaC in keratinocytes, we have evaluated its expression in human skin and in cultured human keratinocytes. Our results indicate that (1) ENaC is expressed in the epidermis and in cultured keratinocytes, at the mRNA and at the protein levels, (2) the ratio of expression of the different ENaC subunits is drastically modified at the protein level during cell growth and differentiation, with a selective upregulation of the β subunit, (3) no transepithelial sodium transport function is apparent in cultured keratinocytes, but patch-clamp recordings indicate the existence of functional sodium channels with properties similar to those of the cloned ENaC and (4) ENaC inhibition does not alter keratinocyte proliferation, but it significantly decreases the frequency of dome formation in confluent keratinocyte cultures. These results document for the first time the characteristics of ENaC subunit expression in human keratinocytes, and suggest that ENaC may be important during differentiation.

Feeling the vibes: chordotonal mechanisms in insect hearing

To hear, insects use diverse external structures, which transform acoustic signals to mechanical ones, coupled to astonishingly uniform mechanosensory transducers, the chordotonal organs. New evidence showing that chordotonal organs and vertebrate auditory hair cells are developmentally related and that chordotonal organs and insect bristle organs are mechanistically related suggests that all these ciliated mechanoreceptors may be derived from the same ancestral molecular mechanotransduction complex. Identification of these elusive molecules will settle this issue.

Stomatin, a MEC-2 like protein, is expressed by mammalian sensory neurons

The molecular mechanism whereby vertebrate primary sensory neurons convert mechanical energy at their receptive fields into action potentials is unknown. In recent years, genetic screens for touch insensitive mutants of the nematode worm Caenorhabditis elegans have led to the identification of several genes required for mechanical sensitivity. A model has been proposed in which a mechanically gated ion channel is connected both to the extracellular matrix and to the cytoskeleton. Displacement of the membrane is proposed to produce a shearing force that pulls the channel open. MEC-2 is thought to play an important role in this complex by linking the ion channel to the cytoskeleton. MEC-2 is highly homologous to a vertebrate protein called stomatin. Stomatin was first isolated from erythrocytes where it is a major integral membrane protein. To date, however, no data on neuronal expression of stomatin in the peripheral nervous system (PNS) or central nervous system (CNS) is available. Here, we have used RT-PCR, in situ hybridization, Northern blotting, and immunocytochemistry to demonstrate that stomatin is expressed by all sensory neurons in mouse dorsal root ganglia. Indirect immunofluorescence together with transfection of cultured adult sensory neurons with epitope-tagged stomatin show that stomatin is localized in spots on somatic and axonal membranes. During development, stomatin begins to be expressed by sensory neurons only as target innervation occurs. The onset of expression of stomatin thus coincides with the onset of functional mechanical sensitivity. Together, our data suggest that stomatin, like the C. elegans MEC-2 gene, is expressed in an appropriate temporal and spatial manner to participate in a putative vertebrate mechanotransduction complex.