Mechanotransduction in Caenorhabditis elegans: the role of DEG/ENaC ion channels

Axonal mitochondria regulate gentle touch response through control of axonal actin dynamics

Actin in neuronal processes is both stable and dynamic. The origin & functional roles of the different pools of actin is not well understood. We find that mutants that lack mitochondria, ric-7 and mtx-2; miro-1, in neuronal processes also lack dynamic actin. Mitochondria can regulate actin dynamics upto a distance ~80 μm along the neuronal process. Absence of axonal mitochondria and dynamic actin does not markedly alter the Spectrin Membrane Periodic Skeleton (MPS) in touch receptor neurons (TRNs). Restoring mitochondria inTRNs cell autonomously restores dynamic actin in a sod-2 dependent manner. We find that dynamic actin is necessary and sufficient for the localization of gap junction proteins in the TRNs and for the C. elegans gentle touch response. We identify an in vivo mechanism by which axonal mitochondria locally facilitate actin dynamics through reactive oxygen species that we show is necessary for electrical synapses & behaviour.

Arterial myogenic response and aging

The arterial myogenic response is an inherent property of resistance arteries. Myogenic tone is crucial for maintaining a relatively constant blood flow in response to changes in intraluminal pressure and protects delicate organs from excessive blood flow. Although this fundamental physiological phenomenon has been extensively studied, the underlying molecular mechanisms are largely unknown. Recent studies identified a crucial role of mechano-activated angiotensin II type 1 receptors (AT1R) in this process. The development of myogenic response is affected by aging. In this review, we summarize recent progress made to understand the role of AT1R and other mechanosensors in the control of arterial myogenic response. We discuss age-related alterations in myogenic response and possible underlying mechanisms and implications for healthy aging.

A sensory-motor neuron type mediates proprioceptive coordination of steering in C. elegans via two TRPC channels

Animal locomotion is mediated by a sensory system referred to as proprioception. Defects in the proprioceptive coordination of locomotion result in uncontrolled and inefficient movements. However, the molecular mechanisms underlying proprioception are not fully understood. Here, we identify two transient receptor potential cation (TRPC) channels, trp-1 and trp-2, as necessary and sufficient for proprioceptive responses in C. elegans head steering locomotion. Both channels are expressed in the SMDD neurons, which are required and sufficient for head bending, and mediate coordinated head steering by sensing mechanical stretches due to the contraction of head muscle and orchestrating dorsal head muscle contractions. Moreover, the SMDD neurons play dual roles to sense muscle stretch as well as to control muscle contractions. These results demonstrate that distinct locomotion patterns require dynamic and homeostatic modulation of feedback signals between neurons and muscles.

Proprioception provides the nervous system with feedback about body posture in animals and is essential for the generation of coherent locomotive behaviors, such as walking, running, or crawling. However, little is known about the identity of proprioceptive receptors that sense body movement to regulate locomotion and the extent to which proprioception modulates sensorimotor coordination. Here, we analyze the molecular mechanisms that control head steering locomotion of Caenorhabditis elegans. We show that this movement is regulated by the transient receptor potential cation (TRPC) channels TRP-1 and TRP-2 and the SMDD proprioceptive neurons. We observe that mutant animals for both channels are defective in head steering locomotion and that ectopic expression of TRP-1 or TPR-2 in a C. elegans chemosensory neuron confers head bending–dependent responses, suggesting roles for these channels in proprioception. We also find that SMDD neurons are both necessary and sufficient to generate head steering locomotion via the two channels. Moreover, we demonstrate that the proprioceptive system mediates locomotion coordination by desynchronizing activities in motor systems. We conclude that two TRPC channels in collaboration with the proprioceptive receptor SMDD neurons control head steering in worms during forward locomotion.

A two-phase response of endothelial cells to hydrostatic pressure

The vascular endothelium is exposed to three types of mechanical forces: blood flow-mediated shear stress, vessel-diameter dependent wall tension and hydrostatic pressure. Despite considerable variations of blood pressure in normal and pathological physiology, little is known about the acute molecular and cellular effects of hydrostatic pressure on endothelial cells. Here, we used a combination of quantitative fluorescence microscopy, atomic force microscopy and molecular perturbations to characterize the specific response of endothelial cells to pressure application. We identified a two-phase response of endothelial cells to acute (1 h) vs. chronic (24 h) pressure application (100 mmHg). While both regimes induce cortical stiffening, the acute response is linked to calcium-mediated myosin activation, whereas the chronic cell response is dominated by increased cortical actin density and a loss in endothelial barrier function. GsMTx-4 and amiloride inhibit the acute pressure response, which suggest the sodium channel ENaC as key player in endothelial pressure sensing. The described two-phase pressure response may participate in the differential effects of transient changes in blood pressure and hypertension.

Cadherin-11 regulates both mesenchymal stem cell differentiation into smooth muscle cells and the development of contractile function in vivo

Although soluble factors, such as transforming growth factor β1 (TGF-β1), induce mesenchymal stem cell (MSC) differentiation towards the smooth muscle cell (SMC) lineage, the role of adherens junctions in this process is not well understood. In this study, we found that cadherin-11 but not cadherin-2 was necessary for MSC differentiation into SMCs. Cadherin-11 regulated the expression of TGF-β1 and affected SMC differentiation through a pathway that was dependent on TGF-β receptor II (TGFβRII) but independent of SMAD2 or SMAD3. In addition, cadherin-11 activated the expression of serum response factor (SRF) and SMC proteins through the Rho-associated protein kinase (ROCK) pathway. Engagement of cadherin-11 increased its own expression through SRF, indicative of the presence of an autoregulatory feedback loop that committed MSCs to the SMC fate. Notably, SMC-containing tissues (such as aorta and bladder) from cadherin-11-null (Cdh11(-/-)) mice showed significantly reduced levels of SMC proteins and exhibited diminished contractility compared with controls. This is the first report implicating cadherin-11 in SMC differentiation and contractile function in vitro as well as in vivo.

The molecular basis of mechanosensory transduction

Multiple senses, including hearing, touch and osmotic regulation, require the ability to convert force into an electrical signal: A process called mechanotransduction. Mechanotransduction occurs through specialized proteins that open an ion channel pore in response to a mechanical stimulus. Many of these proteins remain unidentified in vertebrates, but known mechanotransduction channels in lower organisms provide clues into their identity and mechanism. Bacteria, fruit flies and nematodes have all been used to elucidate the molecules necessary for force transduction. This chapter discusses many different mechanical senses and takes an evolutionary approach to review the proteins responsible for mechanotransduction in various biological kingdoms.

Sensory functions for degenerin/epithelial sodium channels (DEG/ENaC)

All animals use a sophisticated array of receptor proteins to sense their external and internal environments. Major advances have been made in recent years in understanding the molecular and genetic bases for sensory transduction in diverse modalities, indicating that both metabotropic and ionotropic pathways are important in sensory functions. Here, I review the historical background and recent advances in understanding the roles of a relatively newly discovered family of receptors, the degenerin/epithelial sodium channels (DEG/ENaC). These animal-specific cation channels show a remarkable sequence and functional diversity in different species and seem to exert their functions in diverse sensory modalities. Functions for DEG/ENaC channels have been implicated in mechanosensation as well as chemosensory transduction pathways. In spite of overall sequence diversity, all family members share a unique protein topology that includes just two transmembrane domains and an unusually large and highly structured extracellular domain, that seem to be essential for both their mechanical and chemical sensory functions. This review will discuss many of the recent discoveries and controversies associated with sensory function of DEG/ENaC channels in both vertebrate and invertebrate model systems, covering the role of family members in taste, mechanosensation, and pain.

Tissue engineering the mechanosensory circuit of the stretch reflex arc: sensory neuron innervation of intrafusal muscle fibers

The sensory circuit of the stretch reflex arc, composed of specialized intrafusal muscle fibers and type Ia proprioceptive sensory neurons, converts mechanical information regarding muscle length and stretch to electrical action potentials and relays them to the central nervous system. Utilizing a non-biological substrate, surface patterning photolithography and a serum-free medium formulation a co-culture system was developed that facilitated functional interactions between intrafusal muscle fibers and sensory neurons. The presence of annulospiral wrappings (ASWs) and flower-spray endings (FSEs), both physiologically relevant morphologies in sensory neuron-intrafusal fiber interactions, were demonstrated and quantified using immunocytochemistry. Furthermore, two proposed components of the mammalian mechanosensory transduction system, BNaC1 and PICK1, were both identified at the ASWs and FSEs. To verify functionality of the mechanoreceptor elements the system was integrated with a MEMS cantilever device, and Ca(2+) currents were imaged along the length of an axon innervating an intrafusal fiber when stretched by cantilever deflection. This system provides a platform for examining the role of this mechanosensory complex in the pathology of myotonic and muscular dystrophies, peripheral neuropathy, and spasticity inducing diseases like Parkinson's. These studies will also assist in engineering fine motor control for prosthetic devices by improving our understanding of mechanosensitive feedback.

Mechanotransduction - a field pulling together?

Mechanical stresses are ever present in the cellular environment, whether through external forces that are applied to tissues or endogenous forces that are generated within the active cytoskeleton. Despite the wide array of studies demonstrating that such forces affect cellular signaling and function, it remains unclear whether mechanotransduction in different contexts shares common mechanisms. Here, I discuss possible mechanisms by which applied forces, cell-generated forces and changes in substrate mechanics could exert changes in cell function through common mechanotransduction machinery. I draw from examples that are primarily focused on the role of adhesions in transducing mechanical forces. Based on this discussion, emerging themes arise that connect these different areas of inquiry and suggest multiple avenues for future studies.

Primary processes in sensory cells: current advances

In the course of evolution, the strong and unremitting selective pressure on sensory performance has driven the acuity of sensory organs to its physical limits. As a consequence, the study of primary sensory processes illustrates impressively how far a physiological function can be improved if the survival of a species depends on it. Sensory cells that detect single-photons, single molecules, mechanical motions on a nanometer scale, or incredibly small fluctuations of electromagnetic fields have fascinated physiologists for a long time. It is a great challenge to understand the primary sensory processes on a molecular level. This review points out some important recent developments in the search for primary processes in sensory cells that mediate touch perception, hearing, vision, taste, olfaction, as well as the analysis of light polarization and the orientation in the Earth's magnetic field. The data are screened for common transduction strategies and common transduction molecules, an aspect that may be helpful for researchers in the field.

CFTR and Wnt/beta-catenin signaling in lung development

BACKGROUND

Cystic fibrosis transmembrane conductance regulator (CFTR) was shown previously to modify stretch induced differentiation in the lung. The mechanism for CFTR modulation of lung development was examined by in utero gene transfer of either a sense or antisense construct to alter CFTR expression levels. The BAT-gal transgenic reporter mouse line, expressing beta-galactosidase under a canonical Wnt/beta-catenin-responsive promoter, was used to assess the relative roles of CFTR, Wnt, and parathyroid hormone-related peptide (PTHrP) in lung organogenesis. Adenoviruses containing full-length CFTR, a short anti-sense CFTR gene fragment, or a reporter gene as control were used in an intra-amniotic gene therapy procedure to transiently modify CFTR expression in the fetal lung.

RESULTS

A direct correlation between CFTR expression levels and PTHrP levels was found. An inverse correlation between CFTR and Wnt signaling activities was demonstrated.

CONCLUSION

These data are consistent with CFTR participating in the mechanicosensory process essential to regulate Wnt/beta-Catenin signaling required for lung organogenesis.

Cell mechanics: integrating cell responses to mechanical stimuli

Forces are increasingly recognized as major regulators of cell structure and function, and the mechanical properties of cells are essential to the mechanisms by which cells sense forces, transmit them to the cell interior or to other cells, and transduce them into chemical signals that impact a spectrum of cellular responses. Comparison of the mechanical properties of intact cells with those of the purified cytoskeletal biopolymers that are thought to dominate their elasticity reveal the extent to which the studies of purified systems can account for the mechanical properties of the much more heterogeneous and complex cell. This review summarizes selected aspects of current work on cell mechanics with an emphasis on the structures that are activated in cell-cell contacts, that regulate ion flow across the plasma membrane, and that may sense fluid flow that produces low levels of shear stress.

Hypotonic stress influence the membrane potential and alter the proliferation of keratinocytes in vitro

Keratinocyte proliferation and differentiation is strongly influenced by mechanical forces. We investigated the effect of osmotic changes in the development of HaCaT cells in culture using intracellular calcium measurements, electrophysiological recordings and molecular biology techniques. The application of hypotonic stress (174 mOsmol/l) caused a sustained hyperpolarization of HaCaT cells from a resting potential of -27 +/- 4 to -51 +/- 9 mV. This change was partially reversible. The surface membrane channels involved in the hyperpolarization were identified as chloride channels due to the lack of response in the absence of the anion. Cells responded with an elevation of intracellular calcium concentration to hypotonic stress, which critically depended on external calcium. The presence of phorbol-12-myristate-13-acetate in the culture medium for 12 h augmented the subsequent response to hypotonic stress. A sudden switch from iso- to hypotonic solution increased cell proliferation and suppressed the production of involucrin, filaggrin and transglutaminase, markers of keratinocyte differentiation. It is concluded that sudden mechanical forces increase the proliferation of keratinocytes through alterations in their membrane potential and intracellular calcium concentration. These changes together with additional modifications in channel expression and intracellular signalling mechanisms could underlie the increased proliferation of keratinocytes in hyperproliferative skin diseases.

siRNA knock down of casein kinase 2 increases force and cross-bridge cycling rates in vascular smooth muscle

Contraction of smooth muscle involves myosin light chain (MLC) kinase catalyzed phosphorylation of the regulatory MLC, activation of myosin, and the development of force. However, this cannot account for all aspects of a smooth muscle contraction, suggesting that other regulatory mechanisms exist. One potentially important technique to study alternative sites of contractile regulation is the use of small interfering RNA (siRNA). The goal of this study was to determine whether siRNA technology can decrease the levels of a specific protein and allow for the determination of how that protein affects contractile regulation. To achieve this goal, we tested the hypothesis that casein kinase 2 (CK2) is part of the complex regulatory scheme present in vascular smooth muscle. Using intact strips of swine carotid artery, we determined that siRNA against CK2 produced a tissue that resulted in a ∼60% knockdown after 4 days in organ culture. Intact strips of vascular tissue depleted of CK2 produced greater levels of force and exhibited an increased sensitivity to all stimuli tested. This was accompanied by an increase in cross-bridge cycling rates but not by a change in MLC phosphorylation levels. α-Toxin-permeabilized vascular tissue depleted of CK2 also showed an increased sensitivity to calcium compared with control tissues. Our results demonstrate that siRNA is a viable technique with which to study regulatory pathways in intact smooth muscle tissue. Our results also demonstrate that CK2 plays an important role in the mechanism(s) responsible for the development of force and cross-bridge cycling by a MLC phosphorylation-independent pathway.

Ion-channel regulation of chondrocyte matrix synthesis in 3D culture under static and dynamic compression

Inhibition of various ion channels alters chondrocyte mechanotransduction in monolayer, but the mechanisms involved in chondrocyte mechanotransduction in three- dimensional culture remain unclear. The objective of this study was to investigate the effects of inhibiting putative ion-channel influenced mechanotransduction mechanisms on the chondrocyte responses to static and dynamic compression in three-dimensional culture. Bovine articular cartilage explants were used to investigate the dose-dependent inhibition and recovery of protein and sulfated glycosaminoglycan (sGAG) syntheses by four ion-channel inhibitors: 4-Aminopyridine (4AP), a K(+) channel blocker; Nifedipine (Nf), a Ca(2+) channel blocker; Gadolinium (Gd), a stretch-activated channel blocker; and Thapsigargin (Tg), which releases intracellular Ca(2+) stores by inhibiting ATP-dependent Ca(2+) pumps. Chondrocyte-seeded agarose gels were used to examine the influence of 20 h of static and dynamic loading in the presence of each of the inhibitors. Overall, treatment with the ion-channel inhibitors had a greater effect on sGAG synthesis, with the exception of Nf, which more substantially affected protein synthesis. Treatment with Tg significantly impaired both overall protein and sGAG synthesis, with a drastic reduction in sGAG synthesis. The inhibitors differentially influenced the responses to mechanical stimuli. Dynamic compression significantly upregulated protein synthesis but did not significantly affect sGAG synthesis with Nf or Tg treatment. Dynamic compression significantly upregulated both protein and sGAG synthesis rates with Gd treatment. There was no significant stimulation of either protein or sGAG synthesis by dynamic compression with 4AP treatment. Interruption of many ion-channel signaling mechanisms affected sGAG synthesis, suggesting a complicated, multi-pathway signaling process. Also, Ca(2+) signaling may be critical for the transduction of mechanical stimulus in regulating sGAG synthesis. This modulation potentially occurs through direct interactions with the extracellular matrix.

Stimulation by caveolin-1 of the hypotonicity-induced release of taurine and ATP at basolateral, but not apical, membrane of Caco-2 cells

Regulatory volume decrease (RVD) is a protective mechanism that allows mammalian cells to restore their volume when exposed to a hypotonic environment. A key component of RVD is the release of K(+), Cl(-), and organic osmolytes, such as taurine, which then drives osmotic water efflux. Previous experiments have indicated that caveolin-1, a coat protein of caveolae microdomains in the plasma membrane, promotes the swelling-induced Cl(-) current (I(Cl,swell)) through volume-regulated anion channels. However, it is not known whether the stimulation by caveolin-1 is restricted to the release of Cl(-) or whether it also affects the swelling-induced release of other components, such as organic osmolytes. To address this problem, we have studied I(Cl,swell) and the hypotonicity-induced release of taurine and ATP in wild-type Caco-2 cells that are caveolin-1 deficient and in stably transfected Caco-2 cells that express caveolin-1. Electrophysiological characterization of wild-type and stably transfected Caco-2 showed that caveolin-1 promoted I(Cl,swell), but not cystic fibrosis transmembrane conductance regulator currents. Furthermore, caveolin-1 expression stimulated the hypotonicity-induced release of taurine and ATP in stably transfected Caco-2 cells grown as a monolayer. Interestingly, the effect of caveolin-1 was polarized because only the release at the basolateral membrane, but not at the apical membrane, was increased. It is therefore concluded that caveolin-1 facilitates the hypotonicity-induced release of Cl(-), taurine, and ATP, and that in polarized epithelial cells, the effect of caveolin-1 is compartmentalized to the basolateral membrane.

Extracellular zinc and ATP-gated P2X receptor calcium entry channels: New zinc receptors as physiological sensors and therapeutic targets

In this review, we focus on two attributes of P2X receptor channel function, one essential and one novel. First, we propose that P2X receptors are extracellular sensors as well as receptors and ion channels. In particular, the large extracellular domain (that comprises 70% of the molecular mass of the receptor channel protein) lends itself to be a cellular sensor. Moreover, its exquisite sensitivity to extracellular pH, ionic strength, and multiple ligands evokes the function of a sensor. Second, we propose that P2X receptors are extracellular zinc receptors as well as receptors for nucleotides. We provide novel data in multiple publications and illustrative data in this invited review to suggest that zinc triggers ATP-independent activation of P2X receptor channel function. In this light, P2X receptors are the cellular site of integration between autocrine and paracrine zinc signaling and autocrine and paracrine purinergic signaling. P2X receptors may sense changes in these ligands as well as in extracellular pH and ionic strength and transduce these sensations via calcium and/or sodium entry and changes in membrane potential.

A Drosophila DEG/ENaC channel subunit is required for male response to female pheromones

Odorants and pheromones as well as sweet- and bitter-tasting small molecules are perceived through activation of G protein-coupled chemosensory receptors. In contrast, gustatory detection of salty and sour tastes may involve direct gating of sodium channels of the DEG/ENaC family by sodium and hydrogen ions, respectively. We have found that ppk25, a Drosophila melanogaster gene encoding a DEG/ENaC channel subunit, is expressed at highest levels in the male appendages responsible for gustatory and olfactory detection of female pheromones: the legs, wings, and antennae. Mutations in the ppk25 gene reduce or even abolish male courtship response to females in the dark, conditions under which detection of female pheromones is an essential courtship-activating sensory input. In contrast, the same mutations have no effect on other behaviors tested. Importantly, ppk25 mutant males that show no response to females in the dark execute all of the normal steps of courtship behavior in the presence of visible light, suggesting that ppk25 is required for activation of courtship behavior by chemosensory perception of female pheromones. Finally, a ppk25 mutant allele predicted to encode a truncated protein has dominant-negative properties, suggesting that the normal Ppk25 protein acts as part of a multiprotein complex. Together, these results indicate that ppk25 is necessary for response to female pheromones by D. melanogaster males, and suggest that members of the DEG/ENaC family of genes play a wider role in chemical senses than previously suspected.

The mechanosensitive nature of TRPV channels

Transient receptor potential vanilloid (TRPV) channels are widely expressed in both sensory and nonsensory cells. Whereas the channels display a broad diversity to activation by chemical and physical stimuli, activation by mechanical stimuli is common to many members of this group in both lower and higher organisms. Genetic screening in Caenorhabditis elegans has demonstrated an essential role for two TRPV channels in sensory neurons. OSM-9 and OCR-2, for example, are essential for both osmosensory and mechanosensory (nose-touch) behaviors. Likewise, two Drosophila TRPV channels, NAN and IAV, have been shown to be critical for hearing by the mechanosensitive chordotonal organs located in the fly's antennae. The mechanosensitive nature of the channels appears to be conserved in higher organisms for some TRPV channels. Two vertebrate channels, TRPV2 and TRPV4, are sensitive to hypotonic cell swelling, shear stress/fluid flow (TRPV4), and membrane stretch (TRPV2). In the osmosensing neurons of the hypothalamus (circumventricular organs), TRPV4 appears to function as an osmoreceptor, or part of an osmoreceptor complex, in control of vasopressin release, whereas in inner ear hair cells and vascular baroreceptors a mechanosensory role is suggestive, but not demonstrated. Finally, in many nonsensory cells expressing TRPV4, such as vascular endothelial cells and renal tubular epithelial cells, the channel exhibits well-developed local mechanosensory transduction processes where both cell swelling and shear stress/fluid flow lead to channel activation. Hence, many TRPV channels, or combinations of TRPV channels, display a mechanosensitive nature that underlies multiple mechanosensitive processes from worms to mammals.

Hypotonic stress induces E-cadherin expression in cultured human keratinocytes

Human epidermis marks the interface between internal and external environments with the major task being to maintain body hydration. Alternating exposure of skin to a dry or humid environment is likely to cause changes in the epidermal water gradient resulting in osmotic alterations of epidermal keratinocytes. The present in vitro approach studied the effect of hypotonicity on cell-cell contact. It was demonstrated that hypotonic stress applied to human epithelial cells (HaCaT, A-431) induced upregulation of E-cadherin at both, the protein and mRNA level. 5'-deletional mutants of the E-cadherin promoter identified an element ranging from -53 to +31 that conveyed strong transactivation under hypotonic stress. In order to define relevant upstream regulators members of the MAP kinase family, the epidermal growth factor receptor (EGFR) and protein kinase B/Akt (PKB/Akt) were investigated. Hypotonic conditions led to a fast activation of ERK1/2, SAPK/JNK, p38, EGFR and PKB/Akt with distinct activation patterns. Experiments using specific inhibitors showed that p38 contributes to the E-cadherin transactivation under hypotonic conditions. Further upstream, adhesion was found to be a prerequisite for E-cadherin transactivation in this model. In summary, the present study provides evidence that E-cadherin is an osmo-sensitive gene that responds to hypotonic stress. The function of this regulation may be found in morphological changes induced by cell swelling. It is likely that induction of E-cadherin contributes to the stabilization between adjacent cells in order to withstand the physical forces induced by hypotonicity.

Genetic Models of Mechanotransduction: The NematodeCaenorhabditis elegans

Mechanotransduction, the conversion of a mechanical stimulus into a biological response, constitutes the basis for a plethora of fundamental biological processes such as the senses of touch, balance, and hearing and contributes critically to development and homeostasis in all organisms. Despite this profound importance in biology, we know remarkably little about how mechanical input forces delivered to a cell are interpreted to an extensive repertoire of output physiological responses. Recent, elegant genetic and electrophysiological studies have shown that specialized macromolecular complexes, encompassing mechanically gated ion channels, play a central role in the transformation of mechanical forces into a cellular signal, which takes place in mechanosensory organs of diverse organisms. These complexes are highly efficient sensors, closely entangled with their surrounding environment. Such association appears essential for proper channel gating and provides proximity of the mechanosensory apparatus to the source of triggering mechanical energy. Genetic and molecular evidence collected in model organisms such as the nematode worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse highlight two distinct classes of mechanically gated ion channels: the degenerin (DEG)/epithelial Na+channel (ENaC) family and the transient receptor potential (TRP) family of ion channels. In addition to the core channel proteins, several other potentially interacting molecules have in some cases been identified, which are likely parts of the mechanotransducing apparatus. Based on cumulative data, a model of the sensory mechanotransducer has emerged that encompasses our current understanding of the process and fulfills the structural requirements dictated by its dedicated function. It remains to be seen how general this model is and whether it will withstand the impiteous test of time.

Diabetes decreases rabbit bladder smooth muscle contraction while increasing levels of myosin light chain phosphorylation

The effect of diabetes mellitus on the regulation of urinary bladder smooth muscle contraction was studied. Diabetes was induced in the rabbit by alloxan injection followed by 16 wk of housing. The bladder was harvested and strips of wall devoid of both mucosa and serosa were examined. Intact strips of bladder smooth muscle from diabetic animals produced less stress in response to membrane depolarization than muscle from control animals; sensitivity to KCl was not changed. Carbachol responses were similar in muscle strips from the two animal groups. Basal myosin light chain (MLC) phosphorylation levels were significantly elevated in response to most stimuli in muscle strips from diabetic animals, although levels of stress were either unchanged or lower. alpha-Toxin-permeabilized strips that allow for control of the intracellular environment while maintaining excitation-contraction coupling showed increased levels of MLC phosphorylation but decreased sensitivity to activator Ca2+ in smooth muscle from diabetic animals. MLC phosphatase contents were similar in smooth muscle from the two animal groups; however, MLC phosphatase activity was greater in muscle from control compared with diabetic animals. These results suggest that diabetes mellitus uncouples basal MLC phosphorylation from force in the bladder smooth muscle cell.

Patch clamp study of the UNC-105 degenerin and its interaction with the LET-2 collagen in Caenorhabditis elegans muscle

Degenerins have emerged from genetic studies in Caenorhabditis elegans as candidate mechanically gated amiloride-sensitive ion channels for transducing mechanical stimuli into cellular responses. In C. elegans muscle, the existence of a genetic interaction between the unc-105 degenerin gene and let-2, a gene encoding an alpha2(IV) collagen, raised the possibility that UNC-105 may function as a mechanically gated channel in a stretch receptor complex. However, to date, ion channel activity of UNC-105 has only been recorded in a gain-of-function mutant form in heterologous expression systems. In this study we investigated the in situ properties of UNC-105 using the whole cell configuration of the patch clamp technique on body wall muscle cells from acutely dissected C. elegans. Amiloride was found to be without effect on membrane potential of wild-type muscle cells, suggesting that the UNC-105 degenerin is electrically silent in resting muscle. Hypo-osmotic shocks induced a reversible depolarization of muscle cells but which was not affected by amiloride. Deformation of the cells by applying tension to the filamentous complex on which muscle cells remained attached or by ejecting external solution under pressure failed to induce any change of membrane potential. In gain-of-function unc-105(n506) mutant cells, an amiloride-sensitive inward Na(+) current was found to be constitutively active, leading to maintained muscle depolarization. An associated mutation in the alpha2(IV) collagen LET-2 led to the closure of the mutant UNC-105(n506) channel while a collagenase treatment of these double mutant cells caused it to re-open, giving evidence for a functional interaction between LET-2 collagen and mutant UNC-105 channel.

A role for mechanosensitive channels in survival of stationary phase: regulation of channel expression by RpoS

The mechanosensitive (MS) channels MscS and MscL are essential for the survival of hypoosmotic shock by Escherichia coli cells. We demonstrate that MscS and MscL are induced by osmotic stress and by entry into stationary phase. Reduced levels of MS proteins and reduced expression of mscL- and mscS-LacZ fusions in an rpoS mutant strain suggested that the RNA polymerase holoenzyme containing sigmaS is responsible, at least in part, for regulating production of MS channel proteins. Consistent with the model that the effect of sigmaS is direct, the MscS and MscL promoters both use RNA polymerase containing sigmaS in vitro. Conversely, clpP or rssB mutations, which cause enhanced levels of sigmaS, show increased MS channel protein synthesis. RpoS null mutants are sensitive to hypoosmotic shock upon entry into stationary phase. These data suggest that MscS and MscL are components of the RpoS regulon and play an important role in ensuring structural integrity in stationary phase bacteria.

Hypotonicity induces membrane protrusions and actin remodeling via activation of small GTPases Rac and Cdc42 in Rat-1 fibroblasts

An important consequence of cell swelling is the reorganization of the F-actin cytoskeleton in different cell types. We demonstrate in this study by means of rhodamine-phalloidin labeling and fluorescence microscopy that a drastic reorganization of F-actin occurs in swollen Rat-1 fibroblasts: stress fibers disappear and F-actin patches are formed in peripheral extensions at the cell border. Moreover, we demonstrate that activation of both Rac and Cdc42, members of the family of small Rho GTPases, forms the link between the hypotonic stimulation and F-actin reorganization. Indeed, inhibition of the small GTPases RhoA, Rac, and Cdc42 (by Clostridium difficile toxin B) prevents the hypotonicity-induced reorganization of the actin cytoskeleton, whereas inhibition of RhoA alone (by C. limosum C3 exoenzyme) does not preclude this rearrangement. Second, a direct activation and translocation toward the actin patches underneath the plasma membrane is observed for endogenous Rac and Cdc42 (but not for RhoA) during cell swelling. Finally, transfection of Rat-1 fibroblasts with constitutively active RhoA, dominant negative Rac, or dominant negative Cdc42 abolishes the swelling-induced actin reorganization. Interestingly, application of cRGD, a competitor peptide for fibronectin-integrin association, induces identical membrane protrusions and changes in the F-actin cytoskeleton that are also inhibited by C. difficile toxin B and dominant negative Rac or Cdc42. Moreover, cRGD also induces a redistribution of endogenous Rac and Cdc42 to the newly formed submembranous F-actin patches. We therefore conclude that hypotonicity and cRGD remodel the F-actin cytoskeleton in Rat-1 fibroblasts in a Rac/Cdc42-dependent way.

Quinine inhibits vascular contraction independent of effects on calcium or myosin phosphorylation

This report contains results of studies designed to determine whether quinine has direct effects on myofilament Ca2+ sensitization in addition to effects on Ca2+. Quinine decreased the EC50 value and maximal contraction of intact arterial strips to histamine. Incubation of arterial strips with indomethacin or 1H-[1,2,4]oxadiazole[4,3-alpha]quinoxalin-1-one did not alter quinine inhibition, suggesting that the effect is not mediated via cyclooxygenase or cGMP. Pretreatment of strips with quinine had no effect on the histamine-dependent increases in myosin light chain phosphorylation levels. Quinine inhibited Ca2+-induced contraction in alpha-toxin permeabilized strips, but not the Ca2+-induced contraction in Triton X-100 permeabilized strips. Pretreatment of the alpha-toxin permeabilized strips with quinine before stimulation with guanosine-5'-O-(3-thio)triphosphate (GTPgammaS) did not have any effect on the response. In conclusion, quinine inhibited Ca2+-dependent contractions of the alpha-toxin permeabilized strips, which retain modulatory pathways both upstream and downstream from the contractile proteins but did not inhibit GTPgammaS-dependent contraction of the alpha-toxin permeabilized preparation important in upstream modulation of the contraction. Moreover, quinine did not inhibit the Ca2+-dependent contractions of the Triton X-100 permeabilized strips, which are devoid of all modulatory pathways. This suggests that quinine does not act upstream from or directly on the contractile proteins. A more likely site of action may be downstream of the contractile proteins and specifically at the coupling of the contractile proteins with the physiological endpoint of force development.