The molecular identification of ion channels in internal membranes has made scant progress compared with the study of plasma membrane ion channels. We investigated a prominent voltagedependent, cation-selective, and calcium-activated vacuolar ion conductance of 320 pS (yeast vacuolar conductance, YVC1) in Saccharomyces cerevisiae. Here we report on a gene, the deduced product of which possesses significant homology to the ion channel of the transient receptor potential (

Life's origin entails enclosing a compartment to hoard material, energy, and information. The envelope necessarily comprises amphipaths, such as prebiotic fatty acids, to partition the two aqueous domains. The self-assembled lipid bilayer comes with a set of properties including its strong anisotropic internal forces that are chemically or physically malleable. Added bilayer stretch can alter force vectors on embedded proteins to effect conformational change. The force-from-lipid principle was demonstrated 25 y ago when stretches opened purified Escherichia coli MscL channels reconstituted into artificial bilayers. This reductionistic exercise has rigorously been recapitulated recently with two vertebrate mechanosensitive K + channels (TREK1 and TRAAK). Membrane stretches have also been known to activate various voltage-, ligand-, or Ca 2+ -gated channels. Careful analyses showed that Kv, the canonical voltage-gated channel, is in fact exquisitely sensitive even to very small tension. In an unexpected context, the canonical transient-receptor-potential channels in the Drosophila eye, long presumed to open by ligand binding, is apparently opened by membrane force due to PIP 2 hydrolysis-induced changes in bilayer strain. Being the intimate medium, lipids govern membrane proteins by physics as well as chemistry. This principle should not be a surprise because it parallels water's paramount role in the structure and function of soluble proteins. Today, overt or covert mechanical forces govern cell biological processes and produce sensations. At the genesis, a bilayer's response to osmotic force is likely among the first senses to deal with the capricious primordial sea. mechanosensitivity | force sensing | channel gating | bilayer mechanics | touch

Focus on touch and hearing distracts attention from numerous subconscious force sensors such as the vital control of blood pressure, systemic osmolarity, etc. and sensors in non-animals. Multifarious manifestations should not obscure invariant and fundamental physico-chemical principles. We advocate that force-from-lipid (FFL) is one such principle. It is based on the fact that the self-assembled bilayer necessitates inherent forces that are large, and anisotropic, even at life’s origin. Functional response of membrane proteins is governed by bilayer-force changes. Added stress can redirect these forces, leading to geometric changes of embedded proteins such as ion channels. The FFL principle was first demonstrated when purified bacterial MscL remained mechanosensitive (MS) after reconstituting into bilayers. This key experiment has recently been unequivocally replicated with two vertebrate MS K2p channels. Even the canonical Kv and the Drosophila TRPCs have now been shown to be MS in biophysical and in physiological contexts, supporting the universality of the FFL paradigm. We also review the deterministic role of mechanical force during stem-cell differentiation as well as the cell-cell and cell-matrix tethers that provide force communications. In both the ear hair cell and the worm’s touch neuron, deleting the cadherin or microtubule tethers reduces but does not eliminate MS-channel activities. We found no evidence to distinguish whether these tethers directly pulls on the channel protein or a surrounding lipid platform. Regardless of the implementation, pulling tether tenses up the bilayer. Membrane tenting is directly visible at the apexes of the stereocilia.

Whether animal ion channels functioning as mechanosensors are directly activated by stretch force or indirectly by ligands produced by the stretch is a crucial question. TRPV4, a key molecular model, can be activated by hypotonicity, but the mechanism of activation is unclear. One model has this channel being activated by a downstream product of phospholipase A 2 , relegating mechanosensitivity to the enzymes or their regulators. We expressed rat TRPV4 in Xenopus oocytes and repeatedly examined >200 excised patches bathed in a simple buffer. We found that TRPV4 can be activated by tens of mm Hg pipette suctions with open probability rising with suction even in the presence of relevant enzyme inhibitors. Mechanosensitivity of TRPV4 provides the simplest explanation of its various forcerelated physiological roles, one of which is in the sensing of weight load during bone development. Gain-of-function mutants cause heritable skeletal dysplasias in human. We therefore examined the brachyolmia-causing R616Q gain-of-function channel and found increased whole-cell current densities compared with wild-type channels. Single-channel analysis revealed that R616Q channels maintain mechanosensitivity but have greater constitutive activity and no change in unitary conductance or rectification.In stark contrast with vision, smell, and most tastes, which are based on G-protein-coupled receptors, the mechanical senses (hearing, touch, balance, monitoring blood pressure or systemic osmolarity etc.) are poorly understood in molecular terms. Although prokaryotic mechanosensitive channels are clearly activated directly by membrane stretch force (6), how eukaryotic mechanosensitive channels are activated is still under debate. Models range from direct activation by stretches from membrane and/or cytoskeleton to indirect activation through stretch-produced ligands (7,8). The mammalian twopore K ϩ channels TREK1 and TAASK are activated by both membrane stretch and polyunsaturated fatty acids (PUFAs) 2 (9). There, cytoskeletal disruption actually increases mechanosensitivity, suggesting that the cytoskeleton does not transmit stretch force to these channels (10).Among the Ca 2ϩ -permeable transient receptor potential channels, yeast TRPY1 (11) and animal TRPV4 (transient receptor potential channel subtype V4) (12) have been studied extensively for their response to osmotic or mechanical stimuli. TRPY1 can be activated directly by suctions applied to excised membrane patches (11, 13). However, results from such a test from limited preliminary studies found in the TRPV4 literature are inconsistent and even contradictory (1, 2, 14), likely reflecting mechanical complexities of patches excised from animal cells and/or molecular heterogeneity.Rodent TRPV4 channels were first cloned repeatedly by following hypotonicity-induced Ca 2ϩ signals (1, 2). Rat trpv4 complements the mechano-and osmosensitivity defects of the osm-9 mutant worm (15). How osmotic force activates TRPV4 is unclear, however. Besides hypotonicity, this polymodal channel is a...

Ca 2؉ is released from the vacuole into the yeast cytoplasm on an osmotic upshock, but how this upshock is perceived was unknown. We found the vacuolar channel, Yvc1p, to be mechanosensitive, showing that the Ca 2؉ conduit is also the sensing molecule. Although fragile, the yeast vacuole allows limited direct mechanical examination. Pressures at tens of millimeters of Hg (1 mmHg ‫؍‬ 133 Pa) activate the 400-pS Yvc1p conductance in whole-vacuole recording mode as well as in the excised cytoplasmic-side-out mode. Raising the bath osmolarity activates this channel and causes vacuolar shrinkage and deformation. It appears that, on upshock, a transient osmotic force activates Yvc1p to release Ca 2؉ from the vacuole. Mechanical activation of Yvc1p occurs regardless of Ca 2؉ concentration and is apparently independent of its known Ca 2؉ activation, which we now propose to be an amplification mechanism (Ca 2؉ -induced Ca 2؉ release). Yvc1p is a member of the transient receptor potential-family channels, several of which have been associated with mechanosensation in animals. The possible use of Yvc1p as a molecular model to study mechanosensation in general is discussed.