Hydrophilicity of a single residue within MscL correlates with increased channel mechanosensitivity

Mechanosensitive channel MscL gating transitions coupling with constriction point shift

The mechanosensitive channel of large conductance (MscL) acts as an "emergency release valve" that protects bacterial cells from acute hypoosmotic stress, and it serves as a paradigm for studying the mechanism underlying the transduction of mechanical forces. MscL gating is proposed to initiate with an expansion without opening, followed by subsequent pore opening via a number of intermediate substates, and ends in a full opening. However, the details of gating process are still largely unknown. Using in vivo viability assay, single channel patch clamp recording, cysteine cross-linking, and tryptophan fluorescence quenching approach, we identified and characterized MscL mutants with different occupancies of constriction region in the pore domain. The results demonstrated the shifts of constriction point along the gating pathway towards cytoplasic side from residue G26, though G22, to L19 upon gating, indicating the closed-expanded transitions coupling of the expansion of tightly packed hydrophobic constriction region to conduct the initial ion permeation in response to the membrane tension. Furthermore, these transitions were regulated by the hydrophobic and lipidic interaction with the constricting "hot spots". Our data reveal a new resolution of the transitions from the closed to the opening substate of MscL, providing insights into the gating mechanisms of MscL.

Mechanosensitive channel of large conductance enhances the mechanical stretching-induced upregulation of glycolysis and oxidative metabolism in Schwann cells

BACKGROUND

Physical exercise directly stretching the peripheral nerve promotes nerve regeneration; however, its action mechanism remains elusive. Our present study aimed to investigate the effects of mechanosensitive channel of large conductance (MscL) activated by mechanical stretching on the cultured Schwann cells (SCs) and explore the possible mechanism.

METHODS

Primary SCs from neonatal mice at 3-5 days of age were derived and transfected with the lentivirus vector expressing a mutant version of MscL, MscL-G22S. We first detected the cell viability and calcium ion (Ca2+) influx in the MscL-G22S-expressing SCs with low-intensity mechanical stretching and the controls. Proteomic and energy metabolomics analyses were performed to investigate the comprehensive effects of MscL-G22S activation on SCs. Measurement of glycolysis- and oxidative phosphorylation-related molecules and ATP production were respectively performed to further validate the effects of MscL-G22S activation on SCs. Finally, the roles of phosphatidylinositol-3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) signaling pathway in the mechanism of energy metabolism modulation of SCs by MscL-G22S activation was investigated.

RESULTS

Mechanical stretching-induced MscL-G22S activation significantly increased the cell viability and Ca2+ influx into the SCs. Both the proteomic and targeted energy metabolomics analysis indicated the upregulation of energy metabolism as the main action mechanism of MscL-G22S-activation on SCs. MscL-G22S-activated SCs showed significant upregulation of glycolysis and oxidative phosphorylation when SCs with stretching alone had only mild upregulation of energy metabolism than those without stimuli. MscL-G22S activation caused significant phosphorylation of the PI3K/AKT/mTOR signaling pathway and upregulation of HIF-1α/c-Myc. Inhibition of PI3K abolished the MscL-G22S activation-induced upregulation of HIF-1α/c-Myc signaling in SCs and reduced the levels of glycolysis- and oxidative phosphorylation-related substrates and mitochondrial activity.

CONCLUSION

Mechanical stretching activates MscL-G22S to significantly promote the energy metabolism of SCs and the production of energic substrates, which may be applied to enhance nerve regeneration via the glia-axonal metabolic coupling.

Mechanoresponsive Drug Delivery Systems for Vascular Diseases

Mechanoresponsive drug delivery systems (DDS) have emerged as promising candidates to improve the current effectiveness and lower the side effects typically associated with direct drug administration in the context of vascular diseases. Despite tremendous research efforts to date, designing drug delivery systems able to respond to mechanical stimuli to potentially treat these diseases is still in its infancy. By understanding relevant biological forces emerging in healthy and pathological vascular endothelium, it is believed that better-informed design strategies can be deduced for the fabrication of simple-to-complex macromolecular assemblies capable of sensing mechanical forces. These responsive systems are discussed through insights into essential parameter design (composition, size, shape, and aggregation state) , as well as their functionalization with (macro)molecules that are intrinsically mechanoresponsive (e.g., mechanosensitive ion channels and mechanophores). Mechanical forces, including the pathological shear stress and exogenous stimuli (e.g., ultrasound, magnetic fields), used for the activation of mechanoresponsive DDS are also introduced, followed by in vitro and in vivo experimental models used to investigate and validate such novel therapies. Overall, this review aims to propose a fresh perspective through identified challenges and proposed solutions that could be of benefit for the further development of this exciting field.

Mechanosensitive aquaporins

Cellular systems must deal with mechanical forces to satisfy their physiological functions. In this context, proteins with mechanosensitive properties play a crucial role in sensing and responding to environmental changes. The discovery of aquaporins (AQPs) marked a significant breakthrough in the study of water transport. Their transport capacity and regulation features make them key players in cellular processes. To date, few AQPs have been reported to be mechanosensitive. Like mechanosensitive ion channels, AQPs respond to tension changes in the same range. However, unlike ion channels, the aquaporin's transport rate decreases as tension increases, and the molecular features of the mechanism are unknown. Nevertheless, some clues from mechanosensitive ion channels shed light on the AQP-membrane interaction. The GxxxG motif may play a critical role in the water permeation process associated with structural features in AQPs. Consequently, a possible gating mechanism triggered by membrane tension changes would involve a conformational change in the cytoplasmic extreme of the single file region of the water pathway, where glycine and histidine residues from loop B play a key role. In view of their transport capacity and their involvement in relevant processes related to mechanical forces, mechanosensitive AQPs are a fundamental piece of the puzzle for understanding cellular responses.

A novel force transduction pathway from a tension sensor to the gate in the mechano-gating of MscL channel

The bacterial mechanosensitive channel of large conductance MscL is activated exclusively by increased tension in the membrane bilayer. Despite many proposed models for MscL opening, its precise mechano-gating mechanism, particularly how the received force at the tension sensor transmits to the gate remains incomplete. Previous studies have shown that along with amphipathic N-terminus located near the cytoplasmic surface of the membrane, Phe78 residue near the outer surface also acts as a "tension sensor," while Gly22 is a central constituent of the "hydrophobic gate." Present study focused on elucidating the force transmission mechanism from the sensor Phe78 in the outer transmembrane helix (TM2) to the gate in the inner transmembrane helix (TM1) of MscL by applying the patch clamp and molecular dynamics (MD) simulations to the wild type MscL channel and its single mutants at the sensor (F78N), the gate (G22N) and their combination (G22N/F78N) double mutant. F78N MscL resulted in a severe loss-of-function, while G22N MscL caused a gain-of-function channel exhibiting spontaneous openings at the resting membrane tension. We initially speculated that the spontaneous opening in G22N mutant might occur without tension acting on Phe78 residue. To test this hypothesis, we examined the (G22N/F78N) double mutant, which unexpectedly exhibited neither spontaneous activity nor activity by a relatively high membrane tension. To understand the underlying mechanism, we conducted MD simulations and analyzed the force transduction pathway. Results showed that the mutation at the tension sensor (F78N) in TM2 caused decreased interaction of this residue not only with lipids, but also with a group of amino acids (Ile32-Leu36-Ile40) in the neighboring TM1 helix, which resulted in an inefficient force transmission to the gate-constituting amino acids on TM1. This change also induced a slight tilting of TM1 towards the membrane plane and decreased the size of the channel pore at the gate, which seems to be the major mechanism for the inhibition of spontaneous opening of the double mutant channel. More importantly, the newly identified interaction between the TM2 (Phe78) and adjacent TM1 (Ile32-Leu36-Ile40) helices seems to be an essential force transmitting mechanism for the stretch-dependent activation of MscL given that substitution of any one of these four amino acids with Asn resulted in severe loss-of-function MscL as reported in our previous work.

Tight hydrophobic core and flexible helices yield MscL with a high tension gating threshold and a membrane area mechanical strain buffer

The mechanosensitive (MS) channel of large conductance, MscL, is the high-tension threshold osmolyte release valve that limits turgor pressure in bacterial cells in the event of drastic hypoosmotic shock. Despite MscL from Mycobacterium tuberculosis (TbMscL) being the first structurally characterized MS channel, its protective mechanism of activation at nearly-lytic tensions has not been fully understood. Here, we describe atomistic simulations of expansion and opening of wild-type (WT) TbMscL in comparison with five of its gain-of-function (GOF) mutants. We show that under far-field membrane tension applied to the edge of the periodic simulation cell, WT TbMscL expands into a funnel-like structure with trans-membrane helices bent by nearly 70°, but does not break its 'hydrophobic seal' within extended 20 μs simulations. GOF mutants carrying hydrophilic substitutions in the hydrophobic gate of increasing severity (A20N, V21A, V21N, V21T and V21D) also quickly transition into funnel-shaped conformations but subsequently fully open within 1-8 μs. This shows that solvation of the de-wetted (vapor-locked) constriction is the rate-limiting step in the gating of TbMscL preceded by area-buffering silent expansion. Pre-solvated gates in these GOF mutants reduce this transition barrier according to hydrophilicity and the most severe V21D eliminates it. We predict that the asymmetric shape-change of the periplasmic side of the channel during the silent expansion provides strain-buffering to the outer leaflet thus re-distributing the tension to the inner leaflet, where the gate resides.

Approaches for the modulation of mechanosensitive MscL channel pores

MscL was the first mechanosensitive ion channel identified in bacteria. The channel opens its large pore when the turgor pressure of the cytoplasm increases close to the lytic limit of the cellular membrane. Despite their ubiquity across organisms, their importance in biological processes, and the likelihood that they are one of the oldest mechanisms of sensory activation in cells, the exact molecular mechanism by which these channels sense changes in lateral tension is not fully understood. Modulation of the channel has been key to understanding important aspects of the structure and function of MscL, but a lack of molecular triggers of these channels hindered early developments in the field. Initial attempts to activate mechanosensitive channels and stabilize functionally relevant expanded or open states relied on mutations and associated post-translational modifications that were often cysteine reactive. These sulfhydryl reagents positioned at key residues have allowed the engineering of MscL channels for biotechnological purposes. Other studies have modulated MscL by altering membrane properties, such as lipid composition and physical properties. More recently, a variety of structurally distinct agonists have been shown bind to MscL directly, close to a transmembrane pocket that has been shown to have an important role in channel mechanical gating. These agonists have the potential to be developed further into antimicrobial therapies that target MscL, by considering the structural landscape and properties of these pockets.

Potassium Binding Interactions with Aliphatic Amino Acids: Thermodynamic and Entropic Effects Analyzed via a Guided Ion Beam and Computational Study

Noncovalent interactions between alkali metals and amino acids are critical for many biological processes, especially for proper function of protein ion channels; however, many precise binding affinities between alkali metals and amino acids still need to be measured. This study addresses this need by using threshold collision-induced dissociation with a guided ion beam tandem mass spectrometer to measure binding affinities between potassium cations and the aliphatic amino acids: Gly, Ala, hAla, Val, Leu, and Ile. These measurements are supplemented by theoretical calculations and include commentary on effects of enthalpy, entropy, and structural preference. Notably, all levels of theory indicate that the lowest-lying isomers at 298 K have K⁺ binding to the carbonyl oxygen in either a monodentate ([CO]) or bidentate ([CO,OH]) fashion, isomers that are linked in a double-well potential. This complicates the analysis of the data, although does not greatly influence the final results. Analysis of the resulting cross sections includes accounting for multiple ion-molecule collisions, internal energy of reactant ions, and unimolecular decay rates. The resulting experimental bond dissociation energies generally increase as the polarizability of the amino acid increases, results that agree well with quantum chemical calculations done at the B3LYP, B3P86, and MP2(full) levels of theory, with B3LYP-GD3BJ predicting systematically larger values.

Building programmable multicompartment artificial cells incorporating remotely activated protein channels using microfluidics and acoustic levitation

Intracellular compartments are functional units that support the metabolism within living cells, through spatiotemporal regulation of chemical reactions and biological processes. Consequently, as a step forward in the bottom-up creation of artificial cells, building analogous intracellular architectures is essential for the expansion of cell-mimicking functionality. Herein, we report the development of a droplet laboratory platform to engineer complex emulsion-based, multicompartment artificial cells, using microfluidics and acoustic levitation. Such levitated models provide free-standing, dynamic, definable droplet networks for the compartmentalisation of chemical species. Equally, they can be remotely operated with pneumatic, heating, and magnetic elements for post-processing, including the incorporation of membrane proteins; alpha-hemolysin; and mechanosensitive channel of large-conductance. The assembly of droplet networks is three-dimensionally patterned with fluidic input configurations determining droplet contents and connectivity, whilst acoustic manipulation can be harnessed to reconfigure the droplet network in situ. The mechanosensitive channel can be repeatedly activated and deactivated in the levitated artificial cell by the application of acoustic and magnetic fields to modulate membrane tension on demand. This offers possibilities beyond one-time chemically mediated activation to provide repeated, non-contact, control of membrane protein function. Collectively, this expands our growing capability to program and operate increasingly sophisticated artificial cells as life-like materials.

Asymmetric effects of amphipathic molecules on mechanosensitive channels

Mechanosensitive (MS) ion channels are primary transducers of mechanical force into electrical and/or chemical intracellular signals. Many diverse MS channel families have been shown to respond to membrane forces. As a result of this intimate relationship with the membrane and proximal lipids, amphipathic compounds exert significant effects on the gating of MS channels. Here, we performed all-atom molecular dynamics (MD) simulations and employed patch-clamp recording to investigate the effect of two amphipaths, Fluorouracil (5-FU) a chemotherapy agent, and the anaesthetic trifluoroethanol (TFE) on structurally distinct mechanosensitive channels. We show that these amphipaths have a profound effect on the bilayer order parameter as well as transbilayer pressure profile. We used bacterial mechanosensitive channels (MscL/MscS) and a eukaryotic mechanosensitive channel (TREK-1) as force-from-lipids reporters and showed that these amphipaths have differential effects on these channels depending on the amphipaths' size and shape as well as which leaflet of the bilayer they incorporate into. 5-FU is more asymmetric in shape and size than TFE and does not penetrate as deep within the bilayer as TFE. Thereby, 5-FU has a more profound effect on the bilayer and channel activity than TFE at much lower concentrations. We postulate that asymmetric effects of amphipathic molecules on mechanosensitive membrane proteins through the bilayer represents a general regulatory mechanism for these proteins.

Elucidating the molecular basis of spontaneous activation in an engineered mechanosensitive channel

Mechanosensitive channel of large conductance (MscL) detects and responds to changes in the pressure profile of cellular membranes and transduces the mechanical energy into electrical and/or chemical signals. MscL can be activated using ultrasonic or chemical activation methods to improve the absorption of medicines and bioactive compounds into cells. However, re-engineering chemical signals such as pH change can trigger channel activation in MscL. This study elucidates the activation mechanism of an engineered MscL at an atomic level through a combination of equilibrium and non-equilibrium (NE) molecular dynamics (MD) simulations. Comparing the wild-type (WT) and engineered MscL activation processes suggests that the two systems are likely associated with different active states and different transition pathways. These findings indicate that (1) periplasmic loops play a key role in the activation process of MscL, (2) the loss of various backbone-backbone hydrogen bonds and salt bridge interactions in the engineered MscL channel causes the spontaneous opening of the channel, and (3) the most significant interactions lost during the activation process are between the transmembrane helices 1 and 2 in engineered MscL channel. The orientation-based biasing approach for producing and optimizing an open MscL model used in this work is a promising way to characterize unknown protein functional states and investigate the activation processes in ion channels and transmembrane proteins in general. This work paves the way for a computational framework for engineering more efficient pH-sensing mechanosensitive channels.

2021 Nobel Prize for mechanosensory transduction

Written by someone who has worked in the mechanobiology field for close to 40 years, this commentary describes some historical background to the recent award of one-half of the Nobel Prize for Physiology or Medicine to Ardem Patapoutian for his discovery of the family of mechanosensitive Piezo ion channels, which function as mechanoreceptors feeling the environment in senses such as touch, pain, and proprioception.

Cyclodextrins for structural and functional studies of mechanosensitive channels

Mechanosensitive (MS) channels that are activated by the 'force-from-lipids' (FFL) principle rest in the membrane in a closed state but open a transmembrane pore in response to changes in the transmembrane pressure profile. The molecular implementations of the FFL principle vary widely between different MS channel families. The function of MS channels is often studied by patch-clamp electrophysiology, in which mechanical force or amphipathic molecules are used to activate the channels. Structural studies of MS channels in states other than the closed resting state typically relied on the use of mutant channels. Cyclodextrins (CDs) were recently introduced as a relatively easy and convenient approach to generate membrane tension. The principle is that CDs chelate hydrophobic molecules and can remove lipids from membranes, thus forcing the remaining lipids to cover more surface area and creating tension for membrane proteins residing in the membranes. CDs can be used to study the structure of MS channels in a membrane under tension by using single-particle cryo-electron microscopy to image the channels in nanodiscs after incubation with CDs as well as to characterize the function of MS channels by using patch-clamp electrophysiology to record the effect of CDs on channels inserted into membrane patches excised from proteoliposomes. Importantly, because incubation of membrane patches with CDs results in the activation of MscL, an MS channel that opens only shortly before membrane rupture, CD-mediated lipid removal appears to generate sufficient force to open most if not all types of MS channels that follow the FFL principle.

Cyclodextrins increase membrane tension and are universal activators of mechanosensitive channels

The bacterial mechanosensitive channel of small conductance (MscS) has been extensively studied to understand how mechanical forces are converted into the conformational changes that underlie mechanosensitive (MS) channel gating. We showed that lipid removal by β-cyclodextrin can mimic membrane tension. Here, we show that all cyclodextrins (CDs) can activate reconstituted Escherichia coli MscS, that MscS activation by CDs depends on CD-mediated lipid removal, and that the CD amount required to gate MscS scales with the channel's sensitivity to membrane tension. Importantly, cholesterol-loaded CDs do not activate MscS. CD-mediated lipid removal ultimately causes MscS desensitization, which we show is affected by the lipid environment. While many MS channels respond to membrane forces, generalized by the "force-from-lipids" principle, their different molecular architectures suggest that they use unique ways to convert mechanical forces into conformational changes. To test whether CDs can also be used to activate other MS channels, we chose to investigate the mechanosensitive channel of large conductance (MscL) and demonstrate that CDs can also activate this structurally unrelated channel. Since CDs can open the least tension-sensitive MS channel, MscL, they should be able to open any MS channel that responds to membrane tension. Thus, CDs emerge as a universal tool for the structural and functional characterization of unrelated MS channels.

Targeted Neurostimulation in Mouse Brains with Non-invasive Ultrasound

Recently developed brain stimulation techniques have significantly advanced our ability to manipulate the brain's function. However, stimulating specific neurons in a desired region without significant surgical invasion remains a challenge. Here, we demonstrate a neuron-specific and region-targeted neural excitation strategy using non-invasive ultrasound through activation of heterologously expressed mechanosensitive ion channels (MscL-G22S). Low-intensity ultrasound is significantly better at inducing Ca²⁺ influx and neuron activation in vitro and at evoking electromyogram (EMG) responses in vivo in targeted cells expressing MscL-G22S. Neurons in the cerebral cortex or dorsomedial striatum of mice are made to express MscL-G22S and stimulated ultrasonically. We find significant upregulation of c-Fos in neuron nuclei only in the regions expressing MscL-G22S compared with the non-MscL controls, as well as in various other regions in the same brain. Thus, we detail an effective approach for activating specific regions and cell types in intact mouse brains by sensitizing them to ultrasound using a mechanosensitive ion channel.

In-Lipid Structure of Pressure-Sensitive Domains Hints Mechanosensitive Channel Functional Diversity

The mechanosensitive channel of large conductance (MscL) from Mycobacterium tuberculosis has been used as a structural model for rationalizing functional observations in multiple MscL orthologs. Although these orthologs adopt similar structural architectures, they reportedly present significant functional differences. Subtle structural discrepancies on mechanosensitive channel nanopockets are known to affect mechanical gating and may be linked to large variability in tension sensitivity among these membrane channels. Here, we modify the nanopocket regions of MscL from Escherichia coli and M. tuberculosis and employ PELDOR/DEER distance and 3pESEEM deuterium accessibility measurements to interrogate channel structure within lipids, in which both channels adopt a closed conformation. Significant in-lipid structural differences between the two constructs suggest a more compact E. coli MscL at the membrane inner-leaflet, as a consequence of a rotated TM2 helix. Observed differences within lipids could explain E. coli MscL’s higher tension sensitivity and should be taken into account in extrapolated models used for MscL gating rationalization.

Membrane protein mediated bilayer communication in networks of droplet interface bilayers

Droplet interface bilayers (DIBs) are model membranes formed between lipid monolayer-encased water droplets in oil. Compared to conventional methods, one of the most unique properties of DIBs is that they can be connected together to generate multi-layered ‘tissue-like’ networks, however introducing communication pathways between these compartments typically relies on water-soluble pores that are unable to gate. Here, we show that network connectivity can instead be achieved using a water-insoluble membrane protein by successfully reconstituting a chemically activatable mutant of the mechanosensitive channel MscL into a network of DIBs. Moreover, we also show how the small molecule activator can diffuse through an open channel and across the neighbouring droplet to activate MscL present in an adjacent bilayer. This demonstration of membrane protein mediated bilayer communication could prove key toward developing the next generation of responsive bilayer networks capable of defining information flow inside a minimal tissue.

Life with Bacterial Mechanosensitive Channels, from Discovery to Physiology to Pharmacological Target

General principles in biology have often been elucidated from the study of bacteria. This is true for the bacterial mechanosensitive channel of large conductance, MscL, the channel highlighted in this review. This channel functions as a last-ditch emergency release valve discharging cytoplasmic solutes upon decreases in osmotic environment. Opening the largest gated pore, MscL passes molecules up to 30 Å in diameter; exaggerated conformational changes yield advantages for study, including in vivo assays. MscL contains structural/functional themes that recur in higher organisms and help elucidate how other, structurally more complex, channels function. These features of MscL include (i) the ability to directly sense, and respond to, biophysical changes in the membrane, (ii) an α helix ("slide helix") or series of charges ("knot in a rope") at the cytoplasmic membrane boundary to guide transmembrane movements, and (iii) important subunit interfaces that, when disrupted, appear to cause the channel to gate inappropriately. MscL may also have medical applications: the modality of the MscL channel can be changed, suggesting its use as a triggered nanovalve in nanodevices, including those for drug targeting. In addition, recent studies have shown that the antibiotic streptomycin opens MscL and uses it as one of the primary paths to the cytoplasm. Moreover, the recent identification and study of novel specific agonist compounds demonstrate that the channel is a valid drug target. Such compounds may serve as novel-acting antibiotics and adjuvants, a way of permeabilizing the bacterial cell membrane and, thus, increasing the potency of commonly used antibiotics.

Characterizing the mechanosensitive response of Paraburkholderia graminis membranes

Bacterial mechanosensitive channels gate in response to membrane tension, driven by shifts in environmental osmolarity. The mechanosensitive channels of small conductance (MscS) and large conductance (MscL) from Escherichia coli (Ec) gate in response to mechanical force applied to the membrane. Ec-MscS is the foundational member of the MscS superfamily of ion channels, a diverse family with at least fifteen subfamilies identified by homology to the pore lining helix of Ec-MscS, as well as significant diversity on the N- and C-termini. The MscL family of channels are homologous to Ec-MscL. In a rhizosphere associated bacterium, Paraburkholderia graminis C4D1M, mechanosensitive channels are essential for cell survival during changing osmotic environments such as a rainstorm. Utilizing bioinformatics, we predicted six MscS superfamily members and a single MscL homologue. The MscS superfamily members fall into at least three subfamilies: bacterial cyclic nucleotide gated, multi-TM, and extended N-terminus. Osmotic downshock experiments show that wildtype P. graminis cells contain a survival mechanism that prevents cell lysis in response to hypoosmotic shock. To determine if this rescue is due to mechanosensitive channels, we developed a method to create giant spheroplasts of P. graminis to explore the single channel response to applied mechanical tension. Patch clamp electrophysiology on these spheroplasts shows two unique conductances: MscL-like and MscS-like. These conductances are due to likely three unique proteins. This indicates that channels that gate in response to mechanical tension are present in the membrane. Here, we report the first single channel evidence of mechanosensitive ion channels from P. graminis membranes.

Allosteric activation of an ion channel triggered by modification of mechanosensitive nano-pockets

Lipid availability within transmembrane nano-pockets of ion channels is linked with mechanosensation. However, the effect of hindering lipid-chain penetration into nano-pockets on channel structure has not been demonstrated. Here we identify nano-pockets on the large conductance mechanosensitive channel MscL, the high-pressure threshold channel. We restrict lipid-chain access to the nano-pockets by mutagenesis and sulfhydryl modification, and monitor channel conformation by PELDOR/DEER spectroscopy. For a single site located at the entrance of the nano-pockets and distal to the channel pore we generate an allosteric response in the absence of tension. Single-channel recordings reveal a significant decrease in the pressure activation threshold of the modified channel and a sub-conducting state in the absence of applied tension. Threshold is restored to wild-type levels upon reduction of the sulfhydryl modification. The modification associated with the conformational change restricts lipid access to the nano-pocket, interrupting the contact between the membrane and the channel that mediates mechanosensitivity.

Temperature-sensitive mutants of MscL mechanosensitive channel

MscL is a mechanosensitive channel that undergoes a global conformational change upon application of membrane stretching. To elucidate how the structural stability and flexibility occur, we isolated temperature-sensitive (Ts) mutants of Escherichia coli MscL that allowed cell growth at 32°C but not at 42°C. Two Ts mutants, L86P and D127V, were identified. The L86P mutation occurred in the second transmembrane helix, TM2. Substitution of residues neighbouring L86 with proline also led to a Ts mutation, but the substitution of L86 with other amino acids did not result in a Ts phenotype, indicating that the Ts phenotype was due to a structural change of TM2 helix by the introduction of a proline residue. The D127V mutation was localized in the electrostatic belt of the bundle of cytoplasmic helices, indicating that stability of the pentameric bundle of the cytoplasmic helix affects MscL structure. Together, this study described a novel class of MscL mutations that were correlated with the thermodynamic stability of the MscL structure.

Mutation of a single residue promotes gating of vertebrate and invertebrate two-pore domain potassium channels

Mutations that modulate the activity of ion channels are essential tools to understand the biophysical determinants that control their gating. Here, we reveal the conserved role played by a single amino acid position (TM2.6) located in the second transmembrane domain of two-pore domain potassium (K2P) channels. Mutations of TM2.6 to aspartate or asparagine increase channel activity for all vertebrate K2P channels. Using two-electrode voltage-clamp and single-channel recording techniques, we find that mutation of TM2.6 promotes channel gating via the selectivity filter gate and increases single channel open probability. Furthermore, channel gating can be progressively tuned by using different amino acid substitutions. Finally, we show that the role of TM2.6 was conserved during evolution by rationally designing gain-of-function mutations in four Caenorhabditis elegans K2P channels using CRISPR/Cas9 gene editing. This study thus describes a simple and powerful strategy to systematically manipulate the activity of an entire family of potassium channels.

Mutations that modulate the activity of ion channels are essential tools to understand the biophysical determinants that control their gating. Here authors reveal the role played by a single residue in the second transmembrane domain of vertebrate and invertebrate two-pore domain potassium channels.

The voltage-dependence of MscL has dipolar and dielectric contributions and is governed by local intramembrane electric field

Channels without canonical voltage sensors can be modulated by voltage acting on other domains. Here we show that besides protein dipoles, pore hydration can be affected by electric fields. In patches, both WT MscL and its V23T mutant show a decrease in the tension midpoint with hyperpolarization. The mutant exhibits a stronger parabolic dependence of transition energy on voltage, highly consistent with the favourable dielectric contribution from water filling the expanding pore. Purified V23T MscL in DPhPC droplet interface bilayers shows a similar voltage dependence. When reconstituted in an asymmetric DOPhPC/DPhPC bilayer carrying a permanent bias of ~130 mV due to a dipole potential difference between the interfaces, the channel behaved as if the local intramembrane electric field sets the tension threshold for gating rather than just the externally applied voltage. The data emphasize the roles of polarized water in the pore and interfacial lipid dipoles in channel gating thermodynamics.

Ultrasonic Control of Neural Activity through Activation of the Mechanosensitive Channel MscL

Externally controlling the excitation of a neuronal subset through ion channels activation can modulate the firing pattern of an entire neural circuit in vivo. As nanovalves in the cell membrane, ion channels can be opened by light (optogenetics) or ultrasonic (sonogenetics) means. A thoroughly analyzed force sensor is the Escherichia coli mechano sensitive channel of large conductance (MscL). Here we expressed MscL in rat hippocampal neurons in a primary culture and showed that it could be activated by low-pressure ultrasound pulses. The gain-of-function mutation, I92L, sensitized MscL's sonic response, triggering action potentials at a peak negative pressure as low as 0.25 MPa. Further, the I92L MscL reliably elicited individual spikes by timed brief pulses, making excitation programmable. Because MscL opens to tension in the lipid bilayer, requiring no other proteins or ligands, it could be developed into a general noninvasive sonogenetic tool to manipulate the activities of neurons or other cells and potential nanodevices.

Xenon-inhibition of the MscL mechano-sensitive channel and the CopB copper ATPase under different conditions suggests direct effects on these proteins

Xenon is frequently used as a general anesthetic in humans, but the mechanism remains an issue of debate. While for some membrane proteins, a direct interaction of xenon with the protein has been shown to be the inhibitory mechanism, other membrane protein functions could be affected by changes of membrane properties due to partitioning of the gas into the lipid bilayer. Here, the effect of xenon on a mechanosensitive ion channel and a copper ion-translocating ATPase was compared under different conditions. Xenon inhibited spontaneous gating of the Escherichia coli mechano-sensitive mutant channel MscL-G22E, as shown by patch-clamp recording techniques. Under high hydrostatic pressure, MscL-inhibition was reversed. Similarly, the activity of the Enterococcus hirae CopB copper ATPase, reconstituted into proteoliposomes, was inhibited by xenon. However, the CopB ATPase activity was also inhibited by xenon when CopB was in a solubilized state. These findings suggest that xenon acts by directly interacting with these proteins, rather than via indirect effects by altering membrane properties. Also, inhibition of copper transport may be a novel effect of xenon that contributes to anesthesia.

Engineering a pH-Sensitive Liposomal MRI Agent by Modification of a Bacterial Channel

MscL is a bacterial mechanosensitive channel that serves as a cellular emergency release valve, protecting the cell from lysis upon a drop in external osmolarity. The channel has an extremely large pore (30 Å) and can be purified and reconstituted into artificial membranes. Moreover, MscL is modified to open in response to alternative external stimuli including changes in pH. These properties suggest this channel's potential as a triggered "nanopore" for localized release of vesicular contents such as magnetic resonance imaging (MRI) contrast agents and drugs. Toward this end, several variants of pH-triggered MscL nanovalves are engineered. Stealth vesicles previously been shown to evade normal in vivo clearance and passively accumulate in inflamed and malignant tissues are reconstituted. These vesicles are loaded with 1,4,7,10-tetraazacyclododecane tetraacetic acid gadolinium complex (Gd-DOTA), an MRI contrast reagent, and the resulting nanodevices tested for their ability to release Gd-DOTA as evidenced by enhancement of the longitudinal relaxation rate (R₁ ) of the bulk water proton spins. Nanovalves that are responsive to physiological pH changes are identified, but differ in sensitivity and efficacy, thus giving an array of nanovalves that could potentially be useful in different settings. These triggered nanodevices may be useful in delivering both diagnostic and therapeutic agents.

Mechano-sensitization of mammalian neuronal networks through expression of the bacterial large-conductance mechanosensitive ion channel

Development of remote stimulation techniques for neuronal tissues represents a challenging goal. Among the potential methods, mechanical stimuli are the most promising vectors to convey information non-invasively into intact brain tissue. In this context, selective mechano-sensitization of neuronal circuits would pave the way to develop a new cell-type-specific stimulation approach. We report here, for the first time, the development and characterization of mechano-sensitized neuronal networks through the heterologous expression of an engineered bacterial large-conductance mechanosensitive ion channel (MscL). The neuronal functional expression of the MscL was validated through patch-clamp recordings upon application of calibrated suction pressures. Moreover, we verified the effective development of in-vitro neuronal networks expressing the engineered MscL in terms of cell survival, number of synaptic puncta and spontaneous network activity. The pure mechanosensitivity of the engineered MscL, with its wide genetic modification library, may represent a versatile tool to further develop a mechano-genetic approach.This article has an associated First Person interview with the first author of the paper.

Bacterial Mechanosensitive Channels

Mechanosensitive (MS) channels protect bacteria against hypo-osmotic shock and fulfil additional functions. Hypo-osmotic shock leads to high turgor pressure that can cause cell rupture and death. MS channels open under these conditions and release unspecifically solutes and consequently the turgor pressure. They can recognise the raised pressure via the increased tension in the cell membrane. Currently, a better understanding how MS channels can sense tension on molecular level is developing because the interaction of the lipid bilayer with the channel is being investigated in detail. The MS channel of large conductance (MscL) and of small conductance (MscS) have been distinguished and studied in molecular detail. In addition, larger channels were found that contain a homologous region corresponding to MscS so that MscS represents a family of channels. Often several members of this family are present in a species. The importance of this family is underlined by the fact that members can be found not only in bacteria but also in higher organisms. While MscL and MscS have been studied for years in particular by electrophysiology, mutagenesis, molecular dynamics, X-ray crystallography and other biophysical techniques, only recently more details are emerging about other members of the MscS-family.

Bacterial Mechanosensors

Bacteria represent one of the most evolutionarily successful groups of organisms to inhabit Earth. Their world is awash with mechanical cues, probably the most ancient form of which are osmotic forces. As a result, they have developed highly robust mechanosensors in the form of bacterial mechanosensitive (MS) channels. These channels are essential in osmoregulation, and in this setting, provide one of the simplest paradigms for the study of mechanosensory transduction. We explore the past, present, and future of bacterial MS channels, including the alternate mechanosensory roles that they may play in complex microbial communities. Central to all of these functions is their ability to change conformation in response to mechanical stimuli. We discuss their gating according to the force-from-lipids principle and its applicability to eukaryotic MS channels. This includes the new paradigms emerging for bilayer-mediated channel mechanosensitivity and how this molecular detail may provide advances in both industry and medicine.

Pulling MscL open via N-terminal and TM1 helices: A computational study towards engineering an MscL nanovalve

There are great opportunities in the manipulation of bacterial mechanosensitive (MS) ion channels for specific and targeted drug delivery purposes. Recent research has shown that these ion channels have the potential to be converted into nanovalves through clever use of magnetic nanoparticles and magnetic fields. Using a combination of molecular dynamics (MD) simulations and the finite element (FE) modelling, this study investigates the theoretical feasibility of opening the MscL channel (MS channel of large conductance of E. coli) by applying mechanical force directly to its N-terminus. This region has already been reported to function as a major mechanosensor in this channel. The stress-strain behaviour of each MscL helix was obtained using all atom MD simulations. Using the same method, we simulated two models, the wild-type (WT) MscL and the G22N mutant MscL, both embedded in a POPE lipid bilayer. In addition to indicating the main interacting residues at the hydrophobic pore, their pairwise interaction energies were monitored during the channel gating. We implemented these inputs into our FE model of MscL using curve-fitting codes and continuum mechanics equations. In the FE model, the channel could be fully opened via pulling directly on the N-terminus and bottom of TM1 by mutating dominant van der Waals interactions in the channel pore; otherwise the stress generated on the channel protein can irreversibly unravel the N-secondary structure. This is a significant finding suggesting that applying force in this manner is sufficient to open an MscL nanovalve delivering various drugs used, for example, in cancer chemotherapy. More importantly, the FE model indicates that to fully operate an MscL nanovalve by pulling directly on the N-terminus and bottom of TM1, gain-of-function (GOF) mutants (e.g., G22N MscL) would have to be employed rather than the WT MscL channel.

Synapses in the spotlight with synthetic optogenetics

Membrane receptors and ion channels respond to various stimuli and relay that information across the plasma membrane by triggering specific and timed processes. These include activation of second messengers, allowing ion permeation, and changing cellular excitability, to name a few. Gaining control over equivalent processes is essential to understand neuronal physiology and pathophysiology. Recently, new optical techniques have emerged proffering new remote means to control various functions of defined neuronal populations by light, dubbed optogenetics. Still, optogenetic tools do not typically address the activity of receptors and channels native to neurons (or of neuronal origin), nor gain access to their signaling mechanisms. A related method-synthetic optogenetics-bridges this gap by endowing light sensitivity to endogenous neuronal receptors and channels by the appending of synthetic, light-receptive molecules, or photoswitches. This provides the means to photoregulate neuronal receptors and channels and tap into their native signaling mechanisms in select regions of the neurons, such as the synapse. This review discusses the development of synthetic optogenetics as a means to study neuronal receptors and channels remotely, in their natural environment, with unprecedented spatial and temporal precision, and provides an overview of tool design, mode of action, potential clinical applications and insights and achievements gained.

Activation of the mechanosensitive ion channel MscL by mechanical stimulation of supported Droplet-Hydrogel bilayers

The droplet on hydrogel bilayer (DHB) is a novel platform for investigating the function of ion channels. Advantages of this setup include tight control of all bilayer components, which is compelling for the investigation of mechanosensitive (MS) ion channels, since they are highly sensitive to their lipid environment. However, the activation of MS ion channels in planar supported lipid bilayers, such as the DHB, has not yet been established. Here we present the activation of the large conductance MS channel of E. coli, (MscL), in DHBs. By selectively stretching the droplet monolayer with nanolitre injections of buffer, we induced quantifiable DHB tension, which could be related to channel activity. The MscL activity response revealed that the droplet monolayer tension equilibrated over time, likely by insertion of lipid from solution. Our study thus establishes a method to controllably activate MS channels in DHBs and thereby advances studies of MS channels in this novel platform.

Voltage-Dependent Inactivation of MscS Occurs Independently of the Positively Charged Residues in the Transmembrane Domain

MscS (mechanosensitive channel of small conductance) is ubiquitously found among bacteria and plays a major role in avoiding cell lysis upon rapid osmotic downshock. The gating of MscS is modulated by voltage, but little is known about how MscS senses membrane potential. Three arginine residues (Arg-46, Arg-54, and Arg-74) in the transmembrane (TM) domain are possible to respond to voltage judging from the MscS structure. To examine whether these residues are involved in the voltage dependence of MscS, we neutralized the charge of each residue by substituting with asparagine (R46N, R54N, and R74N). Mechanical threshold for the opening of the expressed wild-type MscS and asparagine mutants did not change with voltage in the range from -40 to +100 mV. By contrast, inactivation process of wild-type MscS was strongly affected by voltage. The wild-type MscS inactivated at +60 to +80 mV but not at -60 to +40 mV. The voltage dependence of the inactivation rate of all mutants tested, that is, R46N, R54N, R74N, and R46N/R74N MscS, was almost indistinguishable from that of the wild-type MscS. These findings indicate that the voltage dependence of the inactivation occurs independently of the positive charges of R46, R54, and R74.

Nanomechanical properties of MscL α helices: A steered molecular dynamics study

Gating of mechanosensitive (MS) channels is driven by a hierarchical cascade of movements and deformations of transmembrane helices in response to bilayer tension. Determining the intrinsic mechanical properties of the individual transmembrane helices is therefore central to understanding the intricacies of the gating mechanism of MS channels. We used a constant-force steered molecular dynamics (SMD) approach to perform unidirectional pulling tests on all the helices of MscL in M. tuberculosis and E. coli homologs. Using this method, we could overcome the issues encountered with the commonly used constant-velocity SMD simulations, such as low mechanical stability of the helix during stretching and high dependency of the elastic properties on the pulling rate. We estimated Young's moduli of the α-helices of MscL to vary between 0.2 and 12.5 GPa with TM2 helix being the stiffest. We also studied the effect of water on the properties of the pore-lining TM1 helix. In the absence of water, this helix exhibited a much stiffer response. By monitoring the number of hydrogen bonds, it appears that water acts like a 'lubricant' (softener) during TM1 helix elongation. These data shed light on another physical aspect underlying hydrophobic gating of MS channels, in particular MscL.

In situ, Reversible Gating of a Mechanosensitive Ion Channel through Protein-Lipid Interactions

Understanding the functioning of ion channels, as well as utilizing their properties for biochemical applications requires control over channel activity. Herein we report a reversible control over the functioning of a mechanosensitive ion channel by interfering with its interaction with the lipid bilayer. The mechanosensitive channel of large conductance from Escherichia coli is reconstituted into liposomes and activated to its different sub-open states by titrating lysophosphatidylcholine (LPC) into the lipid bilayer. Activated channels are closed back by the removal of LPC out of the membrane by bovine serum albumin (BSA). Electron paramagnetic resonance spectra showed the LPC-dose-dependent gradual opening of the channel pore in the form of incrementally increasing spin label mobility and decreasing spin-spin interaction. A method to reversibly open and close mechanosensitive channels to distinct sub-open conformations during their journey from the closed to the fully open state enables detailed structural studies to follow the conformational changes during channel functioning. The ability of BSA to revert the action of LPC opens new perspectives for the functional studies of other membrane proteins that are known to be activated by LPC.

From membrane tension to channel gating: A principal energy transfer mechanism for mechanosensitive channels

Mechanosensitive (MS) channels are evolutionarily conserved membrane proteins that play essential roles in multiple cellular processes, including sensing mechanical forces and regulating osmotic pressure. Bacterial MscL and MscS are two prototypes of MS channels. Numerous structural studies, in combination with biochemical and cellular data, provide valuable insights into the mechanism of energy transfer from membrane tension to gating of the channel. We discuss these data in a unified two-state model of thermodynamics. In addition, we propose a lipid diffusion-mediated mechanism to explain the adaptation phenomenon of MscS.

Can Nanofluidic Chemical Release Enable Fast, High Resolution Neurotransmitter-Based Neurostimulation?

Artificial chemical stimulation could provide improvements over electrical neurostimulation. Physiological neurotransmission between neurons relies on the nanoscale release and propagation of specific chemical signals to spatially-localized receptors. Current knowledge of nanoscale fluid dynamics and nanofluidic technology allows us to envision artificial mechanisms to achieve fast, high resolution neurotransmitter release. Substantial technological development is required to reach this goal. Nanofluidic technology—rather than microfluidic—will be necessary; this should come as no surprise given the nanofluidic nature of neurotransmission. This perspective reviews the state of the art of high resolution electrical neuroprostheses and their anticipated limitations. Chemical release rates from nanopores are compared to rates achieved at synapses and with iontophoresis. A review of microfluidic technology justifies the analysis that microfluidic control of chemical release would be insufficient. Novel nanofluidic mechanisms are discussed, and we propose that hydrophobic gating may allow control of chemical release suitable for mimicking neurotransmission. The limited understanding of hydrophobic gating in artificial nanopores and the challenges of fabrication and large-scale integration of nanofluidic components are emphasized. Development of suitable nanofluidic technology will require dedicated, long-term efforts over many years.

Mechanisms of mechanosensing - mechanosensitive channels, function and re-engineering

Sensing and responding to mechanical stimuli is an ancient behavior and ubiquitous to all forms of life. One of its players 'mechanosensitive ion channels' are involved in processes from osmosensing in bacteria to pain in humans. However, the mechanism of mechanosensing is yet to be elucidated. This review describes recent developments in the understanding of a bacterial mechanosensitive channel. Force from the lipid principle of mechanosensation, new methods to understand protein-lipid interactions, the role of water in the gating, the use of engineered mechanosensitive channels in the understanding of the gating mechanism and application of the accumulated knowledge in the field of drug delivery, drug design and sensor technologies are discussed.

Osmotic Stress

Escherichia coli and Salmonella encounter osmotic pressure variations in natural environments that include host tissues, food, soil, and water. Osmotic stress causes water to flow into or out of cells, changing their structure, physics, and chemistry in ways that perturb cell functions. E. coli and Salmonella limit osmotically induced water fluxes by accumulating and releasing electrolytes and small organic solutes, some denoted compatible solutes because they accumulate to high levels without disturbing cell functions. Osmotic upshifts inhibit membrane-based energy transduction and macromolecule synthesis while activating existing osmoregulatory systems and specifically inducing osmoregulatory genes. The osmoregulatory response depends on the availability of osmoprotectants (exogenous organic compounds that can be taken up to become compatible solutes). Without osmoprotectants, K+ accumulates with counterion glutamate, and compatible solute trehalose is synthesized. Available osmoprotectants are taken up via transporters ProP, ProU, BetT, and BetU. The resulting compatible solute accumulation attenuates the K+ glutamate response and more effectively restores cell hydration and growth. Osmotic downshifts abruptly increase turgor pressure and strain the cytoplasmic membrane. Mechanosensitive channels like MscS and MscL open to allow nonspecific solute efflux and forestall cell lysis. Research frontiers include (i) the osmoadaptive remodeling of cell structure, (ii) the mechanisms by which osmotic stress alters gene expression, (iii) the mechanisms by which transporters and channels detect and respond to osmotic pressure changes, (iv) the coordination of osmoregulatory programs and selection of available osmoprotectants, and (v) the roles played by osmoregulatory mechanisms as E. coli and Salmonella survive or thrive in their natural environments.

Scanning MscL Channels with Targeted Post-Translational Modifications for Functional Alterations

Mechanosensitive channels are present in all living organisms and are thought to underlie the senses of touch and hearing as well as various important physiological functions like osmoregulation and vasoregulation. The mechanosensitive channel of large conductance (MscL) from Escherichia coli was the first protein shown to encode mechanosensitive channel activity and serves as a paradigm for how a channel senses and responds to mechanical stimuli. MscL plays a role in osmoprotection in E. coli, acting as an emergency release valve that is activated by membrane tension due to cell swelling after an osmotic down-shock. Using an osmotically fragile strain in an osmotic down-shock assay, channel functionality can be directly determined in vivo. In addition, using thiol reagents and expressed MscL proteins with a single cysteine substitution, we have shown that targeted post-translational modifications can be performed, and that any alterations that lead to dysfunctional proteins can be identified by this in vivo assay. Here, we present the results of such a scan performed on 113 MscL cysteine mutants using five different sulfhydryl-reacting probes to confer different charges or hydrophobicity to each site. We assessed which of these targeted modifications affected channel function and the top candidates were further studied using patch clamp to directly determine how channel activity was affected. This comprehensive screen has identified many residues that are critical for channel function as well as highlighted MscL domains and residues that undergo the most drastic environmental changes upon gating.

Mutations in a Conserved Domain of E. coli MscS to the Most Conserved Superfamily Residue Leads to Kinetic Changes

In Escherichia coli (E. coli) the mechanosensitive channel of small conductance, MscS, gates in response to membrane tension created from acute external hypoosmotic shock, thus rescuing the bacterium from cell lysis. E. coli MscS is the most well studied member of the MscS superfamily of channels, whose members are found throughout the bacterial and plant kingdoms. Homology to the pore lining helix and upper vestibule domain of E. coli MscS is required for inclusion into the superfamily. Although highly conserved, in the second half of the pore lining helix (TM3B), E. coli MscS has five residues significantly different from other members of the superfamily. In superfamilies such as this, it remains unclear why variations within such a homologous region occur: is it tolerance of alternate residues, or does it define functional variance within the superfamily? Point mutations (S114I/T, L118F, A120S, L123F, F127E/K/T) and patch clamp electrophysiology were used to study the effect of changing these residues in E. coli MscS on sensitivity and gating. The data indicate that variation at these locations do not consistently lead to wildtype channel phenotypes, nor do they define large changes in mechanosensation, but often appear to effect changes in the E. coli MscS channel gating kinetics.

Functional similarities between heterogeneously and homogenously expressed MscL constructs

The mechanosensitive channel of large conductance MscL is a well-characterized mechanically gated non-selective ion channel, which often serves as a prototype mechanosensitive channel for mechanotransduction studies. However, there are some discrepancies between MscL constructs used in these studies, most notably unintended heterogeneous expression from some MscL expression constructs. In this study we investigate the possible cause of this expression pattern, and compare the original non-homogenously expressing constructs with our new homogeneously expressing one to confirm that there is little functional difference between them. In addition, a new MscL construct has been developed with an improved molar extinction coefficient at 280 nm, enabling more accurate protein quantification.

Molecular dynamics study on protein-water interplay in the mechanogating of the bacterial mechanosensitive channel MscL

One of the goals of mechanosensitive channel (MSC) studies is to understand the underlying molecular and biophysical mechanisms of the mechano-gating process from force sensing to gate opening. We focus on the latter process and investigate the role of water in the bacterial MSC MscL, which is activated by membrane tension. We analyze the interplay between water and the gate-constituting amino acids, Leu19-Gly26, through molecular dynamics simulations. To highlight the role of water, specifically hydration of the gate, in MscL gating, we restrain lateral movements of the water molecules along the water-vapor interfaces at the top and bottom of the vapor bubble, plugging the closed gate. The gating behaviors in this model and the normal MscL model, in which water movements are unrestrained, are compared. In the normal model, increased membrane tension breaks the hydrogen bond between Leu19 and Val 23 of the inner helix, exposing the backbone carbonyl oxygen of Leu19 to the water-accessible lumen side of the gate. Associated with this activity, water comes to access the vapor region and stably interacts with the carbonyl oxygen to induce a dewetting to wetting transition that facilitates gate expansion toward channel opening. By contrast, in the water-restrained model, carbonyl oxygen is also exposed, but no further conformational changes occur at the gate. This suggests that gate opening relies on a conformational change initiated by wetting. The penetrated water weakens the hydrophobic interaction between neighboring transmembrane inner helices called the "hydrophobic lock" by wedging into the space between their interacting portions.

Negative and positive temperature dependence of potassium leak in MscS mutants: Implications for understanding thermosensitive channels

Bacterial mechanosensitive channel of small conductance (MscS) is a protein, whose activity is modulated by membrane tension, voltage and cytoplasmic crowding. MscS is a homoheptamer and each monomer consists of three transmembrane helices (TM1-3). Hydrophobic pore of the channel is made of TM3s surrounded by peripheral TM1/2s. MscS gating is a complex process, which involves opening and inactivation in response to the increase of membrane tension. A number of MscS mutants were isolated. Among them mutants affecting gating have been found including gain-of-function (GOF) and loss-of-function (LOF) that open at lower or at higher thresholds, respectively. Previously, using an in vivo screen we isolated multiple MscS mutants that leak potassium and some of them were GOF or LOF. Here we show that for a subset of these mutants K+ leak is negatively (NTD) or positively (PTD) temperature dependent. We show that temperature reliance of these mutants does not depend on how MS gating is affected by a particular mutation. Instead, we argue that NTD or PTD leak is due to the opposite allosteric coupling of the structures that determine the temperature dependence to the channel gate. In PTD mutants an increased hydration of the pore vestibule is directly coupled to the increase in the channel conductance. In NTD mutants, at higher temperatures an increased hydration of peripheral structures leads to complete separation of TM3 and a pore collapse.

Lipid-protein interactions: Lessons learned from stress

Biological membranes are essential for normal function and regulation of cells, forming a physical barrier between extracellular and intracellular space and cellular compartments. These physical barriers are subject to mechanical stresses. As a consequence, nature has developed proteins that are able to transpose mechanical stimuli into meaningful intracellular signals. These proteins, termed Mechanosensitive (MS) proteins provide a variety of roles in response to these stimuli. In prokaryotes these proteins form transmembrane spanning channels that function as osmotically activated nanovalves to prevent cell lysis by hypoosmotic shock. In eukaryotes, the function of MS proteins is more diverse and includes physiological processes such as touch, pain and hearing. The transmembrane portion of these channels is influenced by the physical properties such as charge, shape, thickness and stiffness of the lipid bilayer surrounding it, as well as the bilayer pressure profile. In this review we provide an overview of the progress to date on advances in our understanding of the intimate biophysical and chemical interactions between the lipid bilayer and mechanosensitive membrane channels, focusing on current progress in both eukaryotic and prokaryotic systems. These advances are of importance due to the increasing evidence of the role the MS channels play in disease, such as xerocytosis, muscular dystrophy and cardiac hypertrophy. Moreover, insights gained from lipid-protein interactions of MS channels are likely relevant not only to this class of membrane proteins, but other bilayer embedded proteins as well. This article is part of a Special Issue entitled: Lipid-protein interactions.

Breaking the hydrophobicity of the MscL pore: insights into a charge-induced gating mechanism

The mechanosensitive channel of large conductance (MscL) is a protein that responds to membrane tension by opening a transient pore during osmotic downshock. Due to its large pore size and functional reconstitution into lipid membranes, MscL has been proposed as a promising artificial nanovalve suitable for biotechnological applications. For example, site-specific mutations and tailored chemical modifications have shown how MscL channel gating can be triggered in the absence of tension by introducing charged residues at the hydrophobic pore level. Recently, engineered MscL proteins responsive to stimuli like pH or light have been reported. Inspired by experiments, we present a thorough computational study aiming at describing, with atomistic detail, the artificial gating mechanism and the molecular transport properties of a light-actuated bacterial MscL channel, in which a charge-induced gating mechanism has been enabled through the selective cleavage of photo-sensitive alkylating agents. Properties such as structural transitions, pore dimension, ion flux and selectivity have been carefully analyzed. Besides, the effects of charge on alternative sites of the channel with respect to those already reported have been addressed. Overall, our results provide useful molecular insights into the structural events accompanying the engineered MscL channel gating and the interplay of electrostatic effects, channel opening and permeation properties. In addition, we describe how the experimentally observed ionic current in a single-subunit charged MscL mutant is obtained through a hydrophobicity breaking mechanism involving an asymmetric inter-subunit motion.

Ion-mediated conformational switches

Molecular switches are ubiquitous in Nature and provide the basis of many forms of transport and signalling. Single synthetic molecules that change conformation, and thus function, reversibly in a stimulus-dependent manner are of great interest not only to chemists but society in general; myriad applications exist in storage, display, sensing and medicine. Here we describe recent developments in the area of ion-mediated switching.

Hydrophobic gating in ion channels

Biological ion channels are nanoscale transmembrane pores. When water and ions are enclosed within the narrow confines of a sub-nanometer hydrophobic pore, they exhibit behavior not evident from macroscopic descriptions. At this nanoscopic level, the unfavorable interaction between the lining of a hydrophobic pore and water may lead to stochastic liquid-vapor transitions. These transient vapor states are "dewetted", i.e. effectively devoid of water molecules within all or part of the pore, thus leading to an energetic barrier to ion conduction. This process, termed "hydrophobic gating", was first observed in molecular dynamics simulations of model nanopores, where the principles underlying hydrophobic gating (i.e., changes in diameter, polarity, or transmembrane voltage) have now been extensively validated. Computational, structural, and functional studies now indicate that biological ion channels may also exploit hydrophobic gating to regulate ion flow within their pores. Here we review the evidence for this process and propose that this unusual behavior of water represents an increasingly important element in understanding the relationship between ion channel structure and function.

Improving the Design of a MscL-Based Triggered Nanovalve

The mechanosensitive channel of large conductance, MscL, has been proposed as a triggered nanovalve to be used in drug release and other nanodevices. It is a small homopentameric bacterial protein that has the largest gated pore known: greater than 30 Å. Large molecules, even small proteins can be released through MscL. Although MscL normally gates in response to membrane tension, early studies found that hydrophilic or charged residue substitutions near the constriction of the channel leads to pore opening. Researchers have successfully changed the modality of MscL to open to stimuli such as light by chemically modifying a single residue, G22, within the MscL pore. Here, by utilizing in vivo, liposome efflux, and patch clamp assays we compared modification of G22 with that of another neighboring residue, G26, and demonstrate that modifying G26 may be a better choice for triggered nanovalves used for triggered vesicular release of compounds.