The cloning of a mechano-gated membrane ion channel

REVIEW ■ : Pain Mechanisms

Over the last few years, a new synthesis has emerged concerning the neural mechanisms of acute and chronic pain. This new model deals far more successfully than do classical models with the peculiarities of chronic pain syndromes seen in the clinic. As in earlier models, Aδ- and C-nociceptive afferents detect the initial noxious event. In addition, however, this input is now known to rapidly trigger a central hyperexcitability state ("central sensitization") that amplifies sensory signals subsequently entering the CNS along other afferent fiber types. As a result, in the presence of central sensitization, pain sensation is evoked by Aβ touch input, as well as by Aδ- and C-nociceptor input. Tenderness in subacute (e.g., inflammatory) pain, for example, is due to both peripherally sensitized nociceptors and centrally amplified, low-threshold input. The new synthesis also stresses the common ground between the subacute pain of injured tissue and the chronic pain that sometimes develops after nerve injury (i.e., neuropathic pain). In the event of neuropathic pain, the affected afferent axons and their sensory cell somata in the associated dorsal root ganglia (DRGs) become hyperexcitable to applied stimuli. Some even fire spontaneously. Hyperexcitability of the afferent neuron apparently results from specific changes in the regulation of membrane channel and receptor proteins. The resulting ectopic discharge (ectopia) contributes a direct neuropathic pain signal. In addition, neuropathic ectopia sets up and maintains a central sensitization state that amplifies ongoing pain and is responsible for pain on weak stimulation of adjacent areas of skin and deep tissues with residual innervation. The discovery that normal and ectopic Aβ touch input, as well as Aδ- and C-nociceptor input, contributes to subacute and chronic pathophysiological pain states opens previously unanticipated avenues for clinical pain control. NEUROSCIENTIST 2:233-244, 1996

Increased extracellular pressure stimulates tumor proliferation by a mechanosensitive calcium channel and PKC-β

Large tumors exhibit high interstitial pressure heightened by growth against the constraining stroma. Such pressures could stimulate tumor proliferation via a mechanosensitive ion channel. We studied the effects of 0-80 mmHg increased extracellular pressure for 24 h on proliferation of SW620, Caco-2, and CT-26 colon; MCF-7 breast; and MLL and PC3 prostate cancer cells, and delineated its mechanism in SW620 cells with specific inhibitors and siRNA. Finally, we compared NF-kB, phospho-IkB and cyclin D1 immunoreactivity in the high pressure centers and low pressure peripheries of human tumors. Pressure-stimulated proliferation in all cells. Pressure-driven SW620 proliferation required calcium influx via the T-type Ca(2+) channel Cav3.3, which stimulated PKC-β to invoke the IKK-IkB-NF-kB pathway to increase proliferation and S-phase fraction. The mitotic index and immunoreactivity of NF-kB, phospho-IkB, and cyclin D1 in the center of 28 large human colon, lung, and head and neck tumors exceeded that in tumor peripheries. Extracellular pressure increases [Ca(2+)]i via Cav3.3, driving a PKC-β- IKK- IkB-NF-kB pathway that stimulates cancer cell proliferation. Rapid proliferation in large stiff tumors may increase intratumoral pressure, activating this pathway to stimulate further proliferation in a feedback cycle that potentiates tumor growth. Targeting this pathway may inhibit proliferation in large unresectable tumors.

Sensory cilia in arthropods

In arthropods, the modified primary cilium is a structure common to all peripheral sensory neurons other than photoreceptors. Since its first description in 1958, it has been investigated in great detail in numerous sense organs (sensilla) of many insect species by means of electron microscopy and electrophysiology. The perfection of molecular biological methods has led to an enormous advance in our knowledge about development and function of sensory cilia in the fruitfly since the end of the last century. The cilia show a wealth of adaptations according to their different physiological roles: chemoreception, mechanoreception, hygroreception, and thermoreception. Divergent types of receptors and channels have evolved fulfilling these tasks. The number of olfactory receptor genes can be close to 300 in ants, whereas in crickets slightest mechanical stimuli are detected by the interaction of extremely sophisticated biomechanical devices with mechanosensory cilia. Despite their enormous morphological and physiological divergence, sensilla and sensory cilia develop according to a stereotyped pattern. Intraflagellar transport genes have been found to be decisive for proper development and function.

Bacterial mechanosensitive channels--MscS: evolution's solution to creating sensitivity in function

The discovery of mechanosensing channels has changed our understanding of bacterial physiology. The mechanosensitive channel of small conductance (MscS) is perhaps the most intensively studied of these channels. MscS has at least two states: closed, which does not allow solutes to exit the cytoplasm, and open, which allows rapid efflux of solvent and solutes. The ability to appropriately open or close the channel (gating) is critical to bacterial survival. We briefly review the science that led to the isolation and identification of MscS. We concentrate on the structure-function relationship of the channel, in particular the structural and biochemical approaches to understanding channel gating. We highlight the troubling discrepancies between the various models developed to understand MscS gating.

Liposome reconstitution and modulation of recombinant N-methyl-D-aspartate receptor channels by membrane stretch

In this study, the heteromeric N-methyl-D-aspartate (NMDA) receptor channels composed of NR1a and NR2A subunits were expressed, purified, reconstituted into liposomes, and characterized by using the patch clamp technique. The protein exhibited the expected electrophysiological profile of activation by glutamate and glycine and internal Mg2+ blockade. We demonstrated that the mechanical energy transmitted to membrane-bound NMDA receptor channels can be exerted directly by tension developed in the lipid bilayer. Membrane stretch and application of arachidonic acid potentiated currents through NMDA receptor channels in the presence of intracellular Mg2+. The correlation of membrane tension induced by either mechanical or chemical stimuli with the physiological Mg2+ block of the channel suggests that the synaptic transmission can be altered if NMDA receptor complexes experience local changes in bilayer thickness caused by dynamic targeting to lipid microdomains, electrocompression, or chemical modification of the cell membranes. The ability to study gating properties of NMDA receptor channels in artificial bilayers should prove useful in further study of structure-function relationships and facilitate discoveries of new therapeutic agents for treatment of glutamate-mediated excitotoxicity or analgesic therapies.

Ion channels for mechanotransduction in the crayfish stretch receptor

Mechanosensitivity is found in almost every cell in all organisms from bacteria to vertebrates and covers a wide spectrum of function from osmosensing to mechanical sensing in the specialized receptors, such as the hair cells of the cochlea. The molecular substrate for such mechanosensitivity is thought to be mechanosensitive ion channels (MSCs). Because most development regarding the molecular aspects of the MSC has been made in nonsensory or sensory systems, which have not been accessible to recordings from ion channels, it is important to focus on the mechanosensitivity of sensory organs where their functional importance is undisputed. The stretch receptor organ (SRO) of the crustaceans is a suitable preparation for such studies. Each organ contains two receptors: one slowly and one rapidly adapting receptor neurons. The primary mechanosensitivity is generated by two types of MSC of hitherto unknown molecular type located in the neuronal dendrites, which are inserted into a receptor muscle fiber. In addition to the MSCs, the neurons contain voltage-gated Na(+) channels, which seem to be differently located in the slowly and rapidly adapting neurons. At least three types of voltage-gated K(+) channels are present in the sensory neurons, the location of which is not known. The spatial distribution of ion channels and the kinetics of the channels, together with the viscoelastic properties of the receptor muscles, determine the overall transducer properties and impulse firing of the two receptor neurons, including their typical adaptive characteristics.

Twenty odd years of stretch-sensitive channels

After formation of the giga-seal, the membrane patch can be stimulated by hydrostatic or osmotic pressure gradients applied across the patch. This feature led to the discovery of stretch-sensitive or mechanosensitive (MS) channels, which are now known to be ubiquitously expressed in cells representative of all the living kingdoms. In addition to mechanosensation, MS channels have been implicated in many basic cell functions, including regulation of cell volume, shape, and motility. The successful cloning, overexpression, and crystallization of bacterial MS channel proteins combined with patch clamp and modeling studies have provided atomic insight into the working of these nanomachines. In particular, studies of MS channels have revealed new understanding of how the lipid bilayer modulates membrane protein function. Three major membrane protein families, transient receptor potential, 2 pore domain K(+), and the epithelial Na(+) channels, have been shown to form MS channels in animal cells, and their polymodal activation embrace fields far beyond mechanosensitivity. The discovery of new drugs highly selective for MS channels ("mechanopharmaceutics") and the demonstration of MS channel involvement in several major human diseases ("mechanochannelopathies") provide added motivation for devising new techniques and approaches for studying MS channels.

Mechanosensitive channels: multiplicity of families and gating paradigms

Mechanosensitive ion channels are the primary transducers that convert mechanical force into an electrical or chemical signal in hearing, touch, and other mechanical senses. Unlike vision, olfaction, and some types of taste, which all use similar kinds of primary heterotrimeric GTP-binding protein-coupled receptors, mechanosensation relies on diverse types of transducer molecules. Unrelated types of channels can be used for the perception of various mechanical stimuli, not only in distant groups of organisms, but also in separate locations of the same organism. The extreme sensitivity of the transduction mechanism in the auditory system, which relies on an elaborate structure of rigid cilia, filamentous links, and molecular motors to focus force on transduction channels, contrasts with that of the bacterial channel MscL, which is opened by high lateral tension in the membrane and fulfills a safety-valve rather than a sensory function. The spatial scales of conformational movement and force in these two systems are described, and are shown to be consistent with a general physical description of mechanical channel gating. We outline the characteristics of several types of mechanosensitive channels and the functional contexts in which they participate in signaling and cellular regulation in sensory and nonsensory cells.

Purification of the small mechanosensitive channel of Escherichia coli (MscS): the subunit structure, conduction, and gating characteristics in liposomes

The small mechanosensitive channel, MscS, is a part of the turgor-driven solute efflux system that protects bacteria from lysis in the event of osmotic downshift. It has been identified in Escherichia coli as a product of the orphan yggB gene, now called mscS (Levina et al., 1999, EMBO J. 18:1730). Here I show that that the isolated 31-kDa MscS protein is sufficient to form a functional mechanosensitive channel gated directly by tension in the lipid bilayer. MscS-6His complexes purified in the presence of octylglucoside and lipids migrate in a high-resolution gel-filtration column as particles of approximately 200 kDa. Consistent with that, the protein cross-linking patterns predict a hexamer. The channel reconstituted in soybean asolectin liposomes was activated by pressures of 20-60 mm Hg and displayed the same asymmetric I-V curve and slight anionic preference as in situ. At the same time, the single-channel conductance is proportional to the buffer conductivity in a wide range of salt concentrations. The rate of channel activation in response to increasing pressure gradient across the patch was slower than the rate of closure in response to decreasing steps of pressure gradient. Therefore, the open probability curves were recorded with descending series of pressures. Determination of the curvature of patches by video imaging permitted measurements of the channel activity as a function of membrane tension (gamma). Po(gamma) curves had the midpoint at 5.5 +/- 0.1 dyne/cm and gave estimates for the energy of opening DeltaG = 11.4 +/- 0.5 kT, and the transition-related area change DeltaA = 8.4 +/- 0.4 nm(2) when fitted with a two-state Boltzmann model. The correspondence between channel properties in the native and reconstituted systems is discussed.

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.

Induced membrane hypo/hyper-mechanosensitivity: a limitation of patch-clamp recording

Practical limitations of the patch-clamp technique when recording mechanogated membrane ion channels are considered. Mechanical overstimulation of the patch or the cell from excessive suction/pressure protocols induces morphological and functional changes. In particular, the plasma membrane becomes decoupled from the underlying cytoskeleton to form either membrane blebs (cell-attached) or ghosts (whole cell). As a consequence, a membrane ion channel may show either a decrease or an increase in its native mechanosensitivity or even acquire mechanosensitivity. The effect varies with ion channel and cell type and presumably arises because of a disruption of membrane-cytoskeleton interactions. We consider that such disruptions are a pathological consequence of excessive mechanical stress, either during or after seal formation, rather than an immutable consequence of patch-clamp recording. By careful attention to the suction/pressure protocols during sealing and throughout recording, such artifacts can be avoided.

Mechanosensitive potassium channels in rat colon sensory neurons

Single-channel recording techniques were used to characterize mechanosensitive channels in identified (1.1'-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine methanesulfonate labeled) colon sensory neurons dissociated from adult S1 dorsal root ganglia. Channels were found in 30% (7/23) of patches in a cell-attached configuration and in 43% (48/111) of excised inside-out patches. Channels were highly selective for K(+), had a slope conductance of 54 pS in symmetrical solutions, and were blocked by tetraethylammonium, amiloride, and benzamil. Channels were also seen under Ca(2+)-free conditions. Gadolinium (Gd(3+)), a known blocker of mechanosensitive ion channels, did not block channel activity. Tetrodotoxin and 4-aminopyridine were also ineffective. The cytoskeletal disrupters colchicine and cytochalasin D reduced the percentage of patches containing mechanosensitive channels. These results indicate that rat colon sensory neurons contain K(+)-selective mechanosensitive channels that may modulate the membrane excitability induced by colonic distension.

The physical imperative in circadian rhythm: a cytoskeleton-related physically resettable clock mechanism hypothesis

Organisms maintaining circadian rhythmicity are responding to physical constraint of a 24-hour cycle. Time-cue sensing is fundamental to the clock existence, and entrainment of circadian rhythm is indeed accessible to a wide variety of geophysical stimuli. Light-dark and temperature changes are the main time-cues. Additional physical forces such as barometric pressure, electrostatic and electromagnetic fields and gravity force, display a daily cyclic behavior and can function as secondary time-cues. A conceptual framework that contains explanations to all circadian properties including cell autonomous, environmental responsiveness and self-sustained character, is still lacking. It is argued that clock responsiveness to external cues is central to the cellular clock mechanism, and therefore, the nature of the time-cues and the pathways that enable the cell to respond to physical stimuli are of central importance. A role for cytoskeleton in clock entrainment mechanism is suggested in light of cytoskeleton's major involvement in cellular mechanotransduction.

Energetic and spatial parameters for gating of the bacterial large conductance mechanosensitive channel, MscL

MscL is multimeric protein that forms a large conductance mechanosensitive channel in the inner membrane of Escherichia coli. Since MscL is gated by tension transmitted through the lipid bilayer, we have been able to measure its gating parameters as a function of absolute tension. Using purified MscL reconstituted in liposomes, we recorded single channel currents and varied the pressure gradient (P) to vary the tension (T). The tension was calculated from P and the radius of curvature was obtained using video microscopy of the patch. The probability of being open (Po) has a steep sigmoidal dependence on T, with a midpoint (T1/2) of 11.8 dyn/cm. The maximal slope sensitivity of Po/Pc was 0.63 dyn/cm per e-fold. Assuming a Boltzmann distribution, the energy difference between the closed and fully open states in the unstressed membrane was DeltaE = 18.6 kBT. If the mechanosensitivity arises from tension acting on a change of in-plane area (DeltaA), the free energy, TDeltaA, would correspond to DeltaA = 6.5 nm2. MscL is not a binary channel, but has four conducting states and a closed state. Most transition rates are independent of tension, but the rate-limiting step to opening is the transition between the closed state and the lowest conductance substate. This transition thus involves the greatest DeltaA. When summed over all transitions, the in-plane area change from closed to fully open was 6 nm2, agreeing with the value obtained in the two-state analysis. Assuming a cylindrical channel, the dimensions of the (fully open) pore were comparable to DeltaA. Thus, the tension dependence of channel gating is primarily one of increasing the external channel area to accommodate the pore of the smallest conducting state. The higher conducting states appear to involve conformational changes internal to the channel that don't involve changes in area.

A hexameric transmembrane pore revealed by two-dimensional crystallization of the large mechanosensitive ion channel (MscL) of Escherichia coli

We have established a reconstitution method of the detergent-solubilized recombinant large mechanosensitive ion channel of Escherichia coli (MscL) that yielded two-dimensional crystals. For that purpose, we have developed a new protocol using Triton X-100 to solubilize and purify the MscL protein. This protocol not only allowed an increase in the protein yield but also made it possible to obtain a homogeneous delipidated and reproducible preparation of the purified protein. When examined by the patch-clamp method MscL channels were found to be fully functional, exhibiting characteristic conductance and activation by pressure. For electron crystallography the homogeneous Triton X-100-purified recombinant MscL was further reconstituted at low lipid-to-protein ratios using Bio-Beads SM2 to remove the detergent. Two-dimensional crystals, exhibiting a p6 plane group symmetry, have been produced and examined by negative stain electron microscopy. Image processing of selected micrographs yielded a projection map at 15-A resolution that provided the first explicit structural information about the molecular boundary and homohexameric organization of the MscL channels in the membrane bilayer.

Electromechanical coupling model of gating the large mechanosensitive ion channel (MscL) of Escherichia coli by mechanical force

We have developed a theoretical electromechanical coupling (EMC) model of gating of the large-conductance mechanosensitive ion channel (MscL). The model presents the first attempt to explain the pressure-dependent transitions between the closed and open channel conformations on a molecular level by assuming 1) a homohexameric structural model of the channel, 2) electrostatic interactions between various domains of the homohexamer, 3) structural flexibility of the N-terminal portion of the monomer, and 4) mechanically and electrostatically induced displacement of the N-terminal domain relative to other structural domains of the protein. In the EMC model, 12 membrane-spanning alpha-helices (six each of the M1 and M2 transmembrane domains of the MscL monomer), are envisaged to line the channel pore with a diameter of 40 A, whereas the N- and C-termini are oriented toward each other inside the pore when the channel is closed. The model proposes that stretching the membrane bilayer by mechanical force causes the monomers to be pulled away from and slightly tilted toward each other. This relative movement of alpha-helices could serve as a trigger to initiate a "swing-like" motion of the N-terminus around the glycine residue G14 that may act as a pivot. The analysis of the attractive and repulsive coulomb forces between all domains of the channel homohexamer suggested that an inclination angle of approximately 3.0 degrees - 4.1 degrees between the oppositely oriented channel monomers should suffice for the N-terminus to turn away from other domains causing the channel to open. According to the EMC model the minimal free energy change, deltaG, that could initiate the opening of the channel was 2 kT. Also, the model predicted that the negative pressure required for channel open probability, Po = 0.5, should be between 50 and 80 mmHg. These values were in a good agreement with the experimentally estimated pressures of 60-70 mmHg obtained with the MscL reconstituted in liposomes. Furthermore, consistent with a notion that the N-terminus may present a mechanosensitive structural element providing a mechanism to open the MscL by mechanical force, the model provides a simple explanation for the variations in pressure sensitivity observed with several MscL mutants having either deletions or substitutions in N- or C-terminus, or site-directed mutations in the S2-S3 loop.

Mutations in a bacterial mechanosensitive channel change the cellular response to osmotic stress

MscL is a channel found in bacterial plasma membranes that opens a large pore in response to mechanical stress. Here we demonstrate that some mutations within this channel protein (K31D and K31E) evoke a cellular phenotype in which the growth rate is severely depressed. Increasing the osmolarity of the growth medium partially rescues this "slowed growth" phenotype and decreases an abnormal cytosolic potassium loss observed in cells expressing the mutants. In addition, upon sudden decrease in osmolarity (osmotic downshock) more cytoplasmic potassium is released from cells expressing the mutants than cells expressing wild-type MscL. After osmotic downshock, all cells remained viable; hence, the differences in potassium efflux observed are not due to cell lysis but instead appear to be an exaggeration of the normal response to this sudden change in environmental osmolarity. Patch clamp studies in native bacterial membranes substantiate the hypothesis that these mutant channels are more sensitive to mechanical stresses, especially at voltages approaching those estimated for bacterial membrane potentials. These data are consistent with a crucial role for MscL in the adaptation to large osmotic downshock and suggest that if the normally tight regulation of MscL gating is disrupted, cell growth can be severely inhibited.

Estimation of the pore size of the large-conductance mechanosensitive ion channel of Escherichia coli

The open channel diameter of Escherichia coli recombinant large-conductance mechanosensitive ion channels (MscL) was estimated using the model of Hille (Hille, B. 1968. Pharmacological modifications of the sodium channels of frog nerve. J. Gen. Physiol. 51:199-219) that relates the pore size to conductance. Based on the MscL conductance of 3.8 nS, and assumed pore lengths, a channel diameter of 34 to 46 A was calculated. To estimate the pore size experimentally, the effect of large organic ions on the conductance of MscL was examined. Poly-L-lysines (PLLs) with a diameter of 37 A or larger significantly reduced channel conductance, whereas spermine (approximately 15 A), PLL19 (approximately 25 A) and 1,1'-bis-(3-(1'-methyl-(4,4'-bipyridinium)-1-yl)-propyl)-4,4'-b ipyridinium (approximately 30 A) had no effect. The smaller organic ions putrescine, cadaverine, spermine, and succinate all permeated the channel. We conclude that the open pore diameter of the MscL is approximately 40 A, indicating that the MscL has one of the largest channel pores yet described. This channel diameter is consistent with the proposed homohexameric model of the MscL.

Mechanical stimulation of neurites generates an inward current in putative aortic baroreceptor neurons in vitro

We investigated the responses of putative aortic baroreceptor neurons to mechanical stimulation of their processes. Putative aortic baroreceptor neurons were identified by applying the carbocyanine dye DiI to the adventitia of the aortic arch of anesthetized rats. After at least 1 week, the nodose ganglia were removed and the neurons were cultured. Within 2-3 days, neurite outgrowth was evident on many neurons. The soma was voltage-clamped using whole cell patch clamp techniques while the neurites were deformed with pneumatic ejection of bath solution at 5-15 psi using a glass pipette (7-15 microm) positioned at least 50 microm from the neurite. Mechanical stimulation induced an inward current in 15 out of 17 putative aortic baroreceptor neurons. The magnitude of the current was related to the intensity of stimulation. The current was blocked by 20 microM gadolinium (n = 11), a reported blocker of mechanically sensitive ion channels, or by incubating the cells overnight in 10 microM phalloidin, which binds to actin filaments (n = 5). We conclude that mechanical deformation of neurites of putative baroreceptor neurons activates a mechanosensitive inward current in the soma and that the cytoskeletal actin filaments are involved in the generation of this current.

Molecular dissection of the large mechanosensitive ion channel (MscL) of E. coli: mutants with altered channel gating and pressure sensitivity

In the search for the essential functional domains of the large mechanosensitive ion channel (MscL) of E. coli, we have cloned several mutants of the mscL gene into a glutathione S-transferase fusion protein expression system. The resulting mutated MscL proteins had either amino acid additions, substitutions or deletions in the amphipathic N-terminal region, and/or deletions in the amphipathic central or hydrophilic C-terminal regions. Proteolytic digestion of the isolated fusion proteins by thrombin yielded virtually pure recombinant MscL proteins that were reconstituted into artificial liposomes and examined for function by the patch-clamp technique. The addition of amino acid residues to the N-terminus of the MscL did not affect channel activity, whereas N-terminal deletions or changes to the N-terminal amino acid sequence were poorly tolerated and resulted in channels exhibiting altered pressure sensitivity and gating. Deletion of 27 amino acids from the C-terminus resulted in MscL protein that formed channels similar to the wild-type, while deletion of 33 C-terminal amino acids extinguished channel activity. Similarly, deletion of the internal amphipathic region of the MscL abolished activity. In accordance with a recently proposed spatial model of the MscL, our results suggest that (i) the N-terminal portion participates in the channel activation by pressure, and (ii) the essential channel functions are associated with both, the putative central amphipathic alpha-helical portion of the protein and the six C-terminal residues RKKEEP forming a charge cluster following the putative M2 membrane spanning alpha-helix.

The cellular and molecular response of cardiac myocytes to mechanical stress

External load plays a critical role in determining muscle mass and its phenotype in cardiac myocytes. Cardiac myocytes have the ability to sense mechanical stretch and convert it into intracellular growth signals, which lead to hypertrophy. Mechanical stretch of cardiac myocytes in vitro causes activation of multiple second messenger systems that are very similar to growth factor-induced cell signaling systems. Stretch of neonatal rat cardiac myocytes stimulates a rapid secretion of angiotensin II which, together with other growth factors, mediates stretch-induced hypertrophic responses in vitro. In this review, various cell signaling mechanisms initiated by mechanical stress on cardiac myocytes are summarized with emphasis on potential mechanosensing mechanisms and the relationship between mechanical loading and the cardiac renin-angiotensin system.

Mechanosensitive channels of Escherichia coli: the MscL gene, protein, and activities

Although mechanosensory responses are ubiquitous and diverse, the molecular bases of mechanosensation in most cases remain mysterious MscL, a mechanosensitive channel of large conductance of Escherichia coli and its bacterial homologues are the first and currently only channel molecules shown to directly sense mechanical stretch of the membrane. In response to the tension conveyed via the lipid bilayer, MscL increases its open probability by several orders of magnitude. In the present review we describe the identification, cloning, and first sets of biophysical and structural data on this simplest mechanosensory molecule. We discovered a 2.5-ns mechanosensitive conductance in giant E. coli spheroplasts. Using chromatographies to enrich the target and patch clamp to assay the channel activity in liposome-reconstituted fractions, we identified the MscL protein and cloned the mscL gene. MscL comprises 136 amino acid residues (15 kDa), with two highly hydrophobic regions, and resides in the inner membrane of the bacterium. PhoA-fusion experiments indicate that the protein spans the membrane twice with both termini in the cytoplasm. Spectroscopic techniques show that it is highly helical. Expression of MscL tandems and covalent cross-linking suggest that the active channel complex is a homo-hexamer. We have identified several residues, which when deleted or substituted, affect channel kinetics or mechanosensitivity. Although unique when discovered, highly conserved MscL homologues in both gram-negative and gram-positive bacteria have been found, suggesting their ubiquitous importance among bacteria.

Mechanotransducing ion channels in C6 glioma cells

Mechanotransducing (MS) ion channels and images of the patch membrane were studied in cell-attached patches in C6 glioma cells. MS channel density was approximately 0.08 to 0.5 channels/microns2, channel conductance was approximately 40 pS (at -40 mV), and the reversal potential was +15 mV. Replacement of NaCl with KCl, CsCl, or Na gluconate in the pipette solution was without substantial effect on the current-voltage relationship. Replacement of NaCl with NMDG (N-Methyl-D-Glucamine) Cl or reducing NaCl decreased the amplitude of inward currents at negative membrane potentials and caused the reversal potential to shift in the negative direction. Rapid application of suction to the back of the pipette usually elicited a fast (< 0.1 s) appearance of channel activity. The peak (phasic) in channel activity was followed by a decrease to a constant (tonic) level of activity. The reduction in channel activity--called adaptation--was reduced at depolarizing membrane potentials and disappeared if too much pressure was applied. Positive pressure caused the patch membrane to curve toward the pipette tip, move in the direction of the tip, and evoke MS channel activity. Removal of the positive pressure caused the patch to move back to the original position. Conversely, negative pressure caused the patch membrane to curve away from the pipette tip, move away from the tip, and elicit MS channel activity. Gigohm seal resistances were always maintained during translational movement of the patch membrane. Tonic MS channel activity was not associated with translational movements of the patch membrane. Phasic and tonic channel activity were independent of the sign of curvature of the patch membrane. C6 glioma cells have rapidly adapting voltage-dependent MS ion channels, which are non-selective for monovalent cations, and belong to the stretch-activating class of mechanosensory ion channels. Adaptation in MS channels may allow the cell to limit the influx of cations in response to mechanical input. The selective loss of adaptation suggests that the MS channel's gate receives input from two sources. A minimal viscoelastic mechanical model of adaptation and two alternative models for translational movement of the patch are presented.

Molecular genetic approaches to nociceptor development and function

The activation of peripheral nociceptors is the subject of intense scrutiny, because of its significance in pain regulation. Genetic approaches, including homology cloning, difference cloning and transgenic manipulation of mice are providing useful insights into nociceptor function. Recent work suggests that transcriptional regulators (for example, islet-I), which are expressed relatively selectively in sensory neurones, play a crucial role in defining cellular phenotype. Difference cloning has identified genes which encode both ligand-gated and voltage-gated ion channels expressed by small-diameter sensory neurones. The role of inflammatory mediators such as NGF in regulating nociceptor function has been clarified in mis-expression and deletion studies. An understanding of the mechanisms that regulate gene expression in nociceptors should provide new ways to manipulate nociceptor sensitivity, with potential significance for pain therapy.

Purification and functional reconstitution of the recombinant large mechanosensitive ion channel (MscL) of Escherichia coli

The large mechanosensitive ion channel (MscL) of Escherichia coli was expressed on a plasmid encoding MscL as a fusion protein with glutathione S-transferase in an Escherichia coli strain containing a disruption in the chromosomal mscL gene. After purification of the fusion protein using glutathione-coated beads, thrombin cleavage allowed recovery of the MscL protein. The purified protein was reconstituted into artificial liposomes and found to be fully functional when examined with the patch-clamp technique. The reconstituted recombinant MscL protein formed ion channels that exhibited characteristic conductance and pressure sensitivity and were blocked by the mechanosensitive ion channel inhibitor gadolinium. The recombinant MscL protein was also used to raise specific anti-MscL polyclonal antibodies which abolished channel activity when preincubated with the MscL protein.