ions including cl-, and glutamate. mscl: large conductance mechanosensitive channel. the closed structure of mscl in bacteria mscl was the first ms channels cloned and sequenced, and is by far one of the most studied channels. the gene encoding mscl protein is trka and it is located in the inner membrane of the e. coli. the protein is 17 kda, and consists of 136 amino acids; mostly hydrophobic residues resulting in two hydrophobic segments, however molecular weight of the functional channel is presumed to be 60-70 kda from gel filtration experiments, suggesting oligomerization. as a common feature no cysteines residues are present in this channel. in 1998 the homolog mscl from mycobacterium tuberculosis tb-mscl was elucidated at closed state by x ray crystallography at 3.5 å resolution. the protein is a homopentamer composed mostly of helical regions trans orientation of the helices with respect to the bilayer, with two domains: the cytoplasmic and the transmembrane. the channel is 85 å in length, 35 å and 50 å for the cytoplasmic transmembrane domain respectively and 50 å in diameter. the helices cross the membrane twice with both the c-terminal and the n-terminal, thus having two transmembrane domains tm1 and tm2 being tm1 the most conserved region among mscl proteins especially at the n-terminal region. it is located in the cytoplasm and forms a α-hydrophobic helix called s1; the region between the transmembrane domains form a loop that is divided into two regions: s2 a glycine-proline rich region and s3 a short helical section. the secondary structure of the protein is resistant to thermal denaturation still in the presence of sds. during the activation of the prokaryotic mscl by tension in the lipid bilayer an intermediate state was determined. the s1 segments form a bundle when the structure is in the closed state, and the crosslinking of s1 segments prevents the opening of the channel. when tension is applied to the membrane the transmembrane barrel-like structure expand and stretch apart the region s1-tm1 allowing the channel to open. the size of the pore at open state is approximately 25å. the transition from closed to intermediate state is accompanied by small movements of the tm1; further transitions to the open stated are characterized by big rearrangements in both the tm1 and tm2. role of lipid bilayer in ms the lipid bilayer is an important structure in all living cells; it has many functions such as separation of compartments, and signaling among others. in the case of the prokaryotic protein channels mscs and mscl both are gated by tension in the lipid bilayer, thus suggesting an important role in such a complex structures. the tension in the membrane bilayer has been extensively studied, simple intrinsic properties of the lipids can account for the contributions in the free energy of the open, intermediate, and close state of the ms channels. the bilayer possess different features that allows it to transduce tension and to prevent exhaustive deformations, the first one is “in plane fluidity of the lipid bilayer” meaning that any in plane tension in the lipid bilayer is felt homogenously in the absence of cytoskeleton interactions. the lipid molecules have specific spaces in between them which prevent changes in the lipid bilayer. the contribution of membrane deformation in the gating of ms channels can be divided in two types: the deformation of the plane of the bilayer, and the deformation of the thickness of the bilayer. also during any process involving changes in the structure, the free energy of the process itself is also an important factor. during gating the major processes that account for this event are: hydrophobic mismatch, and membrane curvature. it has been calculated that the free energy of the tension in the lipid bilayer is similar to the energy needed for gating the channels. a different study showed that the length of the hydrophobic tail affects its functioning as well as supporting the different states, phosphatidylcholine (pc) 18 stabilizes better the open state of the mscl channel, pc 14 stabilizes the intermediate state, and a mixture of pc 18 and lysophosphatidylcholine (lpc) stabilizes the closed state, suggesting that the bilayer thickness (for carbon tail lengths of 16, 18 and 20) affects channel function. in conclusion the energy from the environment of the membrane plays an important role in the total energy of channel gating. eukaryotes in eukaryotes, two of the best known mechanosensitive ion channels are the potassium channels trek-1 and traak, both of which are found in mammalian neurons. recently, a new mechanosensitive ion channel family was cloned, with two mammalian members, piezo1 and piezo2. both these channels are expressed in the lungs and bladder, organs with important mechanosensory functions. piezo1 is also expressed in the skin, and in red blood cells, and its gain of function mutations cause hereditary xerocytosis. piezo2 is expressed in sensory neurons of the dorsal root and trigeminal ganglia indicating that it may play a role in touch sensation. mutations in piezo2 are associated with a human disease named distal arthrogryposis. physiological role of ms ms channels are ubiquitously expressed in the membrane of prokaryotes suggesting their significance. in bacteria and archaea the function of these channels is conserved and it has been demonstrated that they play a role in turgor regulation. in eukarya ms channels are involved in all five senses. the main family is trp, and one good example is hair cells involved in the hearing process. when a wave of sound deflects the stereocilia, the channel opens. this is an instance of the spring-like tether gating mechanism. recent studies have revealed a new role of mechanosensitive pathways in which naive mesenchymal stem cells are committed to a particular lineage based on the elasticity of its surrounding matrix. some ms channels that have been cloned and characterized. data adapted from martinac, 2001 channel source gating mechanism physiological role msclbacterialipid bilayerturgor regulation and cell growth mscsbacterialipid bilayerturgor regulation and cell growth mscmjarchaealipid bilayerturgor regulation mec4c. eleganstethertouch trpyfungibilayerturgor regulation treck-1mammalianbilayerresting membrane potential ms have also been suggested as a potential target for antibiotics, the reasoning behind this idea is that both mcss and mscl are highly conserved among prokaryotes, but their homologs have not been found in animals making them an exceptional potential for further studies. in mammalian neurons, opening of the ion channels depolarizes the afferent neuron producing an action potential with sufficient depolarization. channels open in response to two different mechanisms: the prokaryotic model and the mammalian hair cell model. stretch-activated ion channels have been shown to detect vibration, pressure, stretch, touch, sounds, tastes, smell, heat, volume, and vision. stretch-activated ion channels have been categorized into three distinct "superfamilies": the enac/deg family, the trp family, and the k1 selective family. these channels are involved with bodily functions such as blood pressure regulation. they are shown to be associated with many cardiovascular diseases. stretch-activated channels were first observed in chick skeletal muscles by falguni guharay and frederick sachs in 1983 and the results were published in 1984. since then stretch-activated channels have been found in cells from bacteria to humans as well as plants. the opening of these channels is central to a neuron's response to pressure, often osmotic pressure and blood pressure, to regulate ionic flow in internal environments. techniques used to study ms this is a short list of the most frequently techniques used to study the properties, function, mechanism and other features of these channels: patch-clamp: single cell recording. epr molecular dynamics simulation: determination of the atomic fluctuation of the system. atomic force microscopy: mechanical forces of the membrane. micropipette aspiration: pressure to cells. 3d simulations finite element model of mscl, a bacterial channel. this figure is similar to that in tang et al. mutagenesis through experiments performed on the cytoskeleton and extra-cytoplasmic matrix of stretch-activated ion channels, these structures have been shown to play significant roles in mechanotransduction. in one such experiment on adult heart cells, whole cell recordings were taken on cells being squeezed with two pipettes at 1 hz/1 um. this squeezing produced no current until five minutes in when a large depolarization was observed. hereafter, the cell became extremely responsive to every compression and gradually decreased sensitivity over the next few minutes. researchers hypothesized that, initially, the cytoskeleton was buffering the mechanical deformation of the squeezing from the channel. the depolarization at five minutes was the cytoskeleton snapping which subsequently caused the channel to sense the mechanical deformations and thereby respond to the stimuli. researchers believe that over the few minutes where the channel repaired itself the cytoskeleton must be repairing itself and newly adapting to the squeezing stimuli. structure asic there are six known asic subunits, asic1a, asic1b, asic2a, asic2b, asic3, and asic4, which have two transmembrane domains, extracellular and intracellular loops, and c and n termini. these asic subunits likely form tetramers with varying kinetics, ph sensitivity, tissue distribution, and pharmacological properties. trp superfamily there are seven subfamilies within the trp superfamily: trpc (canonical), trpv (vanilloid), trpm (melastatin), trpp (polycystin), trpml (mucolipin), trpa (ankyrin), and trpn (nompc-like). trp proteins typically consist of six transmembrane domains, s1, s2, s3, s4, s5, and s6, with a pore between s5 and s6. these contain intracellular n and c termini, which form tetramers and vary in length and domain. within the channel there are ankyrins, which are structural proteins that mediate protein-protein interactions, and are thought to contribute to the tether model of stretch-activated channel opening. nompc, identified in d. melanogaster mechanotransduction and a member of the trpn subfamily, contains a relatively high number of ankyrins. k1-selective superfamily k2p channels consist of six subfamilies and contain four transmembrane domains, which form two pores each between domains 1–2 and 3–4. k2p channels also contain a short n terminal domain and a c terminal which varies in length. there is also a large extracellular linker region between domain 1 and the first pore formed between domains 1–2. examples trp channels are typically non-selective, although a few are selective for calcium or hydrated magnesium ions, and are composed of integral membrane proteins. although many trp channels are activated by voltage change, ligand binding, or temperature change, some trp channels have been hypothesized to be involved in mechanotransduction. some examples are trpv4, which mediates mechanical load in a variety of tissues, including the liver, heart, lung, trachea, testis, spleen, salivary glands, cochlea, and vascular endothelial cells, as well as trpc1 and trpc6, which are involved in muscle mechanosensation. trpc1 is expressed in the myocytes of the heart, arteries, and skeletal muscle. trpc1 is widely considered to be a non-selective “store-operated ion channel” (soc) involved in the calcium influx following calcium depletion of the endoplasmic reticulum of the cell. trpc6 is a calcium-permeable non-selective cation channel expressed in the cardiovascular system. trpc6 is potentially a sensor of mechanically and osmotically induced membrane stretch, and is possibly directly gated by membrane tension. other examples include trek-1 and traak which are found in mammalian neurons and are classified as potassium channels in the tandem pore domain class and "mid-1" (also known as "mclc" or clcc1.) the six k2p channel subfamilies are regulated by various physical, cellular, and pharmacological stimulants, including membrane stretch, heat, ph change, calcium flux, and protein kinases. clinical relevance stretch-activated ion channels perform important functions in many different areas of our body. pressure-dependent myogenic constriction resistance arteries require these channels for regulation in the smooth muscle of the arteries. they have been found to be used for volume sensing in animals and blood pressure regulation. bacteria have been shown to relieve hydrostatic pressure through mscl and mscs channels. pathologies associated with stretch-activated ion channels stretch-activated ion channels have been correlated with major pathologies. some of these pathologies include cardiac arrhythmia (such as atrial fibrillation), cardiac hypertrophy, duchenne muscular dystrophy, and other cardiovascular diseases. blocking stretch-activated ion channels gadolinium (gd3+) and other lanthanides have been shown to block stretch-activated ion channel function. the peptide toxin isolated from the chilean rose tarantula (grammostola rosea, synonym g. spatulata), mechanotoxin 4 (gsmtx4) has been shown to inhibit these channels from the extracellular side, but it does not inhibit all stretch-activated ion channels and particularly has no effect on 2p channels. list of diseases associated with mechanosensitive channels polycystic kidney disease. atrial fibrillation abnormalities in the function of ms channels can cause: neuronal disease muscular degeneration.