Mammalian Mechanoelectrical Transduction: Structure and Function of Force-Gated Ion Channels
2019; Cell Press; Volume: 179; Issue: 2 Linguagem: Inglês
10.1016/j.cell.2019.08.049
ISSN1097-4172
AutoresDominique Douguet, Éric Honoré,
Tópico(s)Connexins and lens biology
ResumoThe conversion of force into an electrical cellular signal is mediated by the opening of different types of mechanosensitive ion channels (MSCs), including TREK/TRAAK K2P channels, Piezo1/2, TMEM63/OSCA, and TMC1/2. Mechanoelectrical transduction plays a key role in hearing, balance, touch, and proprioception and is also implicated in the autonomic regulation of blood pressure and breathing. Thus, dysfunction of MSCs is associated with a variety of inherited and acquired disease states. Significant progress has recently been made in identifying these channels, solving their structure, and understanding the gating of both hyperpolarizing and depolarizing MSCs. Besides prototypical activation by membrane tension, additional gating mechanisms involving channel curvature and/or tethered elements are at play. The conversion of force into an electrical cellular signal is mediated by the opening of different types of mechanosensitive ion channels (MSCs), including TREK/TRAAK K2P channels, Piezo1/2, TMEM63/OSCA, and TMC1/2. Mechanoelectrical transduction plays a key role in hearing, balance, touch, and proprioception and is also implicated in the autonomic regulation of blood pressure and breathing. Thus, dysfunction of MSCs is associated with a variety of inherited and acquired disease states. Significant progress has recently been made in identifying these channels, solving their structure, and understanding the gating of both hyperpolarizing and depolarizing MSCs. Besides prototypical activation by membrane tension, additional gating mechanisms involving channel curvature and/or tethered elements are at play. Opening of mechanosensitive ion channels (MSCs) at the plasma membrane of mammalian cells, in the microsecond range, is the earliest event occurring upon mechanical stimulation (Chalfie, 2009Chalfie M. Neurosensory mechanotransduction.Nat. Rev. Mol. Cell Biol. 2009; 10: 44-52Google Scholar, Christensen and Corey, 2007Christensen A.P. Corey D.P. TRP channels in mechanosensation: direct or indirect activation?.Nat. Rev. Neurosci. 2007; 8: 510-521Google Scholar, Delmas and Coste, 2013Delmas P. Coste B. Mechano-gated ion channels in sensory systems.Cell. 2013; 155: 278-284Google Scholar, Kung, 2005Kung C. A possible unifying principle for mechanosensation.Nature. 2005; 436: 647-654Google Scholar, Murthy et al., 2017Murthy S.E. Dubin A.E. Patapoutian A. Piezos thrive under pressure: mechanically activated ion channels in health and disease.Nat. Rev. Mol. Cell Biol. 2017; 18: 771-783Google Scholar, Sachs and Morris, 1998Sachs F. Morris C.E. Mechanosensitive ion channels in nonspecialized cells.Rev. Physiol. Biochem. Pharmacol. 1998; 132: 1-77Google Scholar). Activation of MSCs by force is direct, without the involvement of a second messenger, causing a change in membrane potential and triggering biochemical responses (Kung, 2005Kung C. A possible unifying principle for mechanosensation.Nature. 2005; 436: 647-654Google Scholar, Sachs and Morris, 1998Sachs F. Morris C.E. Mechanosensitive ion channels in nonspecialized cells.Rev. Physiol. Biochem. Pharmacol. 1998; 132: 1-77Google Scholar). In mammalian cells, two types of MSCs co-exist at the plasma membrane: depolarizing cationic non-selective channels (permeable to Na+, K+, and Ca2+) and hyperpolarizing K+-selective stretch-activated channels (Delmas and Coste, 2013Delmas P. Coste B. Mechano-gated ion channels in sensory systems.Cell. 2013; 155: 278-284Google Scholar, Murthy et al., 2017Murthy S.E. Dubin A.E. Patapoutian A. Piezos thrive under pressure: mechanically activated ion channels in health and disease.Nat. Rev. Mol. Cell Biol. 2017; 18: 771-783Google Scholar) (Figure 1; Table S1). MSCs open in response to a variety of mechanical stimuli, including local membrane stretching, cell squeezing, shear stress, cell swelling, deflection of hair cell bundles, and substrate deformation (Delmas and Coste, 2013Delmas P. Coste B. Mechano-gated ion channels in sensory systems.Cell. 2013; 155: 278-284Google Scholar, Wu et al., 2017aWu J. Lewis A.H. Grandl J. Touch, tension, and transduction—the function and regulation of Piezo ion channels.Trends Biochem. Sci. 2017; 42: 57-71Google Scholar). In this review, taking advantage of recent molecular, structural, and functional studies, we will discuss the common properties, as well as the specific gating mechanisms that are involved in the activation of the different types of mammalian MSCs by force, including TREK/TRAAK K2P channels, Piezo1/2, TMEM63/OSCA, and TMC1/2 (Figure 1; Table S1), in light of their physiological role and association with various disease states. The prototypical bacterial MscL channel, acting as an osmotic valve, is a very large conductance (3 nS) ionic pore permeable to both ions (without selectivity) and osmolytes that is gated by membrane tension (Kung, 2005Kung C. A possible unifying principle for mechanosensation.Nature. 2005; 436: 647-654Google Scholar, Sukharev et al., 1994Sukharev S.I. Blount P. Martinac B. Blattner F.R. Kung C. A large-conductance mechanosensitive channel in E. coli encoded by mscL alone.Nature. 1994; 368: 265-268Google Scholar) (Figure 2B). Activation involves a major iris-like expansion of its transmembrane segments (TMs) (Perozo et al., 2002Perozo E. Cortes D.M. Sompornpisut P. Kloda A. Martinac B. Open channel structure of MscL and the gating mechanism of mechanosensitive channels.Nature. 2002; 418: 942-948Google Scholar). Force is directly transmitted to the channel through the membrane (“force-from-lipid principle”) (Kung, 2005Kung C. A possible unifying principle for mechanosensation.Nature. 2005; 436: 647-654Google Scholar, Martinac et al., 1990Martinac B. Adler J. Kung C. Mechanosensitive ion channels of E. coli activated by amphipaths.Nature. 1990; 348: 261-263Google Scholar). A large increase in channel cross-sectional area (about 20 nm2 for MscL), caused by an increase in membrane tension, confers a high mechanosensitivity, proportional to the steepness of the sigmoidal gating curve, primarily reflecting the dimensional changes of the mechanosensitive channel upon force activation (Sukharev et al., 1994Sukharev S.I. Blount P. Martinac B. Blattner F.R. Kung C. A large-conductance mechanosensitive channel in E. coli encoded by mscL alone.Nature. 1994; 368: 265-268Google Scholar) (Figure 2A). In comparison, for small conductance (about 100 pS, 30-fold less than MscL) mammalian TREK-1/TREK-2/TRAAK K2P channels, only a modest increase in cross-sectional area (1.8, 2.7, and 4.7 nm2 for TREK-1, TRAAK, and TREK-2, respectively) occurs (Brohawn et al., 2014aBrohawn S.G. Campbell E.B. MacKinnon R. Physical mechanism for gating and mechanosensitivity of the human TRAAK K+ channel.Nature. 2014; 516: 126-130Google Scholar, Dong et al., 2015Dong Y.Y. Pike A.C. Mackenzie A. McClenaghan C. Aryal P. Dong L. Quigley A. Grieben M. Goubin S. Mukhopadhyay S. et al.K2P channel gating mechanisms revealed by structures of TREK-2 and a complex with Prozac.Science. 2015; 347: 1256-1259Google Scholar, Honoré et al., 2006Honoré E. Patel A.J. Chemin J. Suchyna T. Sachs F. Desensitization of mechano-gated K2P channels.Proc. Natl. Acad. Sci. U S A. 2006; 103: 6859-6864Google Scholar) (Figure 2B). Thus, large-conductance MSCs are sensitive to smaller changes in membrane tension than small-conductance MSCs. If the MSC in the bilayer is approximated to a cylindrical plug, the free energy difference between the open and closed states (ΔG = ΔG0 − TΔA) is given by ΔG0 (the difference in the absence of force) minus membrane tension (T) times the change in cross-sectional channel area occurring between the closed and the open state (ΔA) (Liang and Howard, 2018Liang X. Howard J. Structural biology: Piezo senses tension through curvature.Curr. Biol. 2018; 28: R357-R359Google Scholar, Sachs and Morris, 1998Sachs F. Morris C.E. Mechanosensitive ion channels in nonspecialized cells.Rev. Physiol. Biochem. Pharmacol. 1998; 132: 1-77Google Scholar). Tension causes an expansion (ΔA) of the mechanosensitive channel (inducing pore opening) by an energy equal to −TΔA. Protein area expansion (ΔA) is proportional to the energy that controls activation of the channel. Thus, a smaller area change (ΔA) requires a broader range of tension (between 0.5 and 12 mN/m for TRAAK) for channel activation (shallow gating curve) (Brohawn et al., 2014aBrohawn S.G. Campbell E.B. MacKinnon R. Physical mechanism for gating and mechanosensitivity of the human TRAAK K+ channel.Nature. 2014; 516: 126-130Google Scholar, Dong et al., 2015Dong Y.Y. Pike A.C. Mackenzie A. McClenaghan C. Aryal P. Dong L. Quigley A. Grieben M. Goubin S. Mukhopadhyay S. et al.K2P channel gating mechanisms revealed by structures of TREK-2 and a complex with Prozac.Science. 2015; 347: 1256-1259Google Scholar, Honoré et al., 2006Honoré E. Patel A.J. Chemin J. Suchyna T. Sachs F. Desensitization of mechano-gated K2P channels.Proc. Natl. Acad. Sci. U S A. 2006; 103: 6859-6864Google Scholar) (Figure 2A). Notably, both positive and negative pressure (causing a similar increase in membrane tension) activates reconstituted TRAAK channels (Brohawn et al., 2014bBrohawn S.G. Su Z. MacKinnon R. Mechanosensitivity is mediated directly by the lipid membrane in TRAAK and TREK1 K+ channels.Proc. Natl. Acad. Sci. U S A. 2014; 111: 3614-3619Google Scholar). In summary, two MSC classes that appear to have different degrees of cross-sectional area changes, bacterial MscL and mammalian mechanosensitive TREK/TRAAK K2P channels, offer good examples of how variations in this key parameter that controls “force-from-lipid” are manifest (Figure 2A). Besides the steepness of the gating curve (related to the difference in cross-sectional area changes), another striking difference between MscL and mechanosensitive K2P channels is the threshold for mechanical activation, which is very high for MscL (10 mN/m) but remarkably low for TRAAK (0.5 mN/m) (Aryal et al., 2017Aryal P. Jarerattanachat V. Clausen M.V. Schewe M. McClenaghan C. Argent L. Conrad L.J. Dong Y.Y. Pike A.C.W. Carpenter E.P. et al.Bilayer-mediated structural transitions control mechanosensitivity of the TREK-2 K2P channel.Structure. 2017; 25: 708-718.e2Google Scholar, Brohawn et al., 2014aBrohawn S.G. Campbell E.B. MacKinnon R. Physical mechanism for gating and mechanosensitivity of the human TRAAK K+ channel.Nature. 2014; 516: 126-130Google Scholar, Brohawn et al., 2014bBrohawn S.G. Su Z. MacKinnon R. Mechanosensitivity is mediated directly by the lipid membrane in TRAAK and TREK1 K+ channels.Proc. Natl. Acad. Sci. U S A. 2014; 111: 3614-3619Google Scholar, Sukharev et al., 1994Sukharev S.I. Blount P. Martinac B. Blattner F.R. Kung C. A large-conductance mechanosensitive channel in E. coli encoded by mscL alone.Nature. 1994; 368: 265-268Google Scholar) (Figure 2A). The mid-point of the gating curve depends on channel pre-stress (i.e., the resting tension acting on the channel), influenced by the stiffness of the membrane and the cytoskeleton linkages (Sachs and Morris, 1998Sachs F. Morris C.E. Mechanosensitive ion channels in nonspecialized cells.Rev. Physiol. Biochem. Pharmacol. 1998; 132: 1-77Google Scholar). For mechanosensitive TREK/TRAAK K2P channels, modest channel area expansion together with a reduced lipid deformation are proposed to synergistically promote channel activation in the lower tension range (Brohawn et al., 2014aBrohawn S.G. Campbell E.B. MacKinnon R. Physical mechanism for gating and mechanosensitivity of the human TRAAK K+ channel.Nature. 2014; 516: 126-130Google Scholar). Thus, both protein expansion (ΔA) and pre-stress are key parameters conditioning the gating of MSCs. Opening of TREK/TRAAK K2P channels results in an efflux of potassium, causing cell hyperpolarization and a consequent decrease in cell excitability (for a review, see Honoré, 2007Honoré E. The neuronal background K2P channels: focus on TREK1.Nat. Rev. Neurosci. 2007; 8: 251-261Google Scholar). TREK/TRAAK are polymodal K+ channels that are stimulated by protons, heat, stretch, and a variety of lipids, including free polyunsaturated fatty acids (PUFAs), lysophospholipids, PIP2, lysophosphatidic acid (LPA), and phosphatidic acid, as well as pharmacological agents such as volatile general anesthetics (for a review, see Honoré, 2007Honoré E. The neuronal background K2P channels: focus on TREK1.Nat. Rev. Neurosci. 2007; 8: 251-261Google Scholar). On the contrary, TREK-1 is negatively regulated through phosphorylation by protein kinases A and C (for a review, see Honoré, 2007Honoré E. The neuronal background K2P channels: focus on TREK1.Nat. Rev. Neurosci. 2007; 8: 251-261Google Scholar). Both TREK-1/2 and TRAAK are highly expressed in neurons, including sensory dorsal root ganglion (DRG) neurons (Table S1). TREK-1/TRAAK knockout mice show enhanced sensitivity to painful stimuli, including mechanical allodynia, as well as heat hyperalgesia (Alloui et al., 2006Alloui A. Zimmermann K. Mamet J. Duprat F. Noël J. Chemin J. Guy N. Blondeau N. Voilley N. Rubat-Coudert C. et al.TREK-1, a K+ channel involved in polymodal pain perception.EMBO J. 2006; 25: 2368-2376Google Scholar, Noël et al., 2009Noël J. Zimmermann K. Busserolles J. Deval E. Alloui A. Diochot S. Guy N. Borsotto M. Reeh P. Eschalier A. Lazdunski M. The mechano-activated K+ channels TRAAK and TREK-1 control both warm and cold perception.EMBO J. 2009; 28: 1308-1318Google Scholar). Moreover, TREK-1 knockout mice are sensitized to inflammatory pain (Alloui et al., 2006Alloui A. Zimmermann K. Mamet J. Duprat F. Noël J. Chemin J. Guy N. Blondeau N. Voilley N. Rubat-Coudert C. et al.TREK-1, a K+ channel involved in polymodal pain perception.EMBO J. 2006; 25: 2368-2376Google Scholar, Noël et al., 2009Noël J. Zimmermann K. Busserolles J. Deval E. Alloui A. Diochot S. Guy N. Borsotto M. Reeh P. Eschalier A. Lazdunski M. The mechano-activated K+ channels TRAAK and TREK-1 control both warm and cold perception.EMBO J. 2009; 28: 1308-1318Google Scholar). Along the same line, TREK-1 plays a role in migraine (Royal et al., 2019Royal P. Andres-Bilbe A. Avalos Prado P. Verkest C. Wdziekonski B. Schaub S. Baron A. Lesage F. Gasull X. Levitz J. et al.Migraine-associated TRESK mutations increase neuronal excitability through alternative translation initiation and inhibition of TREK.Neuron. 2019; 101: 232-245.e6Google Scholar). Opening of hyperpolarizing K2P channels limits (acting as an electrical brake) the depolarization mediated by the activation of other types of excitatory cationic non-selective channels, including MSCs and heat-sensitive TPRV channels. Thus, TREK/TRAAK MSCs are involved in polymodal pain perception (Alloui et al., 2006Alloui A. Zimmermann K. Mamet J. Duprat F. Noël J. Chemin J. Guy N. Blondeau N. Voilley N. Rubat-Coudert C. et al.TREK-1, a K+ channel involved in polymodal pain perception.EMBO J. 2006; 25: 2368-2376Google Scholar, Noël et al., 2009Noël J. Zimmermann K. Busserolles J. Deval E. Alloui A. Diochot S. Guy N. Borsotto M. Reeh P. Eschalier A. Lazdunski M. The mechano-activated K+ channels TRAAK and TREK-1 control both warm and cold perception.EMBO J. 2009; 28: 1308-1318Google Scholar). Of note, within the CNS, TREK-1 is also involved in neuroprotection and general anesthesia and plays an important role in depression (for a review, see Honoré, 2007Honoré E. The neuronal background K2P channels: focus on TREK1.Nat. Rev. Neurosci. 2007; 8: 251-261Google Scholar). Interestingly, missense mutations (Ala172Glu and Ala244Pro, in TM3 and TM4, respectively) in TRAAK cause a syndrome including facial dysmorphism hypertrichosis, epilepsy, intellectual disability/developmental delay, and gingival overgrowth (Bauer et al., 2018Bauer C.K. Calligari P. Radio F.C. Caputo V. Dentici M.L. Falah N. High F. Pantaleoni F. Barresi S. Ciolfi A. et al.Mutations in KCNK4 that affect gating cause a recognizable neurodevelopmental syndrome.Am. J. Hum. Genet. 2018; 103: 621-630Google Scholar) (Table S1). Those mutations produce a basal gain of function (GOF) that impairs sensitivity to mechanical stimulation, as well as activation by arachidonic acid. Molecular dynamics (MD) simulations predict that the TRAAK disease-causing mutations seal lateral fenestrations (Aryal et al., 2017Aryal P. Jarerattanachat V. Clausen M.V. Schewe M. McClenaghan C. Argent L. Conrad L.J. Dong Y.Y. Pike A.C.W. Carpenter E.P. et al.Bilayer-mediated structural transitions control mechanosensitivity of the TREK-2 K2P channel.Structure. 2017; 25: 708-718.e2Google Scholar, Bauer et al., 2018Bauer C.K. Calligari P. Radio F.C. Caputo V. Dentici M.L. Falah N. High F. Pantaleoni F. Barresi S. Ciolfi A. et al.Mutations in KCNK4 that affect gating cause a recognizable neurodevelopmental syndrome.Am. J. Hum. Genet. 2018; 103: 621-630Google Scholar, Brohawn et al., 2014aBrohawn S.G. Campbell E.B. MacKinnon R. Physical mechanism for gating and mechanosensitivity of the human TRAAK K+ channel.Nature. 2014; 516: 126-130Google Scholar) (Figure 1A; see below). Thus, hyperpolarizing mechanosensitive K2P channels play a major pathophysiological role and are implicated in a variety of inherited and acquired disease states. The K2P channels TREK-1/TREK-2/TRAAK are made of a dimer of subunits, each including four TMs and two pore domains (P1 and P2) in tandem (Brohawn et al., 2012Brohawn S.G. del Mármol J. MacKinnon R. Crystal structure of the human K2P TRAAK, a lipid- and mechano-sensitive K+ ion channel.Science. 2012; 335: 436-441Google Scholar, Brohawn et al., 2014aBrohawn S.G. Campbell E.B. MacKinnon R. Physical mechanism for gating and mechanosensitivity of the human TRAAK K+ channel.Nature. 2014; 516: 126-130Google Scholar) (Figure 1A). A large extracellular cap with a bifurcated ionic pathway lies on top of the transmembrane channel core, thus explaining their resistance to K+ channel pore blockers (Brohawn et al., 2012Brohawn S.G. del Mármol J. MacKinnon R. Crystal structure of the human K2P TRAAK, a lipid- and mechano-sensitive K+ ion channel.Science. 2012; 335: 436-441Google Scholar) (Figures 1A and 3). Notably, K2P channels lack a classical bundle-crossing inner gate, as found in other types of K+ channels. An upward movement of the inner end of TM4 due to a rotation (25°) around a central hinge glycine, together with a rotation (15°) of the TM2 and TM3 segments, obstructs two large intramembrane-side fenestrations (Brohawn et al., 2014aBrohawn S.G. Campbell E.B. MacKinnon R. Physical mechanism for gating and mechanosensitivity of the human TRAAK K+ channel.Nature. 2014; 516: 126-130Google Scholar) (Figures 1A and 3A). When TM4 is in the down state (dashed magenta line), lateral windows are wide open (yellow circle), and it was hypothesized that they might be filled with lipids (in yellow) extending to the central cavity underneath the selectivity filter, presumably blocking the flow of K+ ions (Brohawn et al., 2014aBrohawn S.G. Campbell E.B. MacKinnon R. Physical mechanism for gating and mechanosensitivity of the human TRAAK K+ channel.Nature. 2014; 516: 126-130Google Scholar) (Figures 1A, middle, and 3A). In response to an increase in membrane tension, it is predicted that lateral fenestrations close because of an upward movement of TM4 (solid magenta line), thereby preventing the entry of blocking lipids and allowing the flow of K+ (Brohawn et al., 2014aBrohawn S.G. Campbell E.B. MacKinnon R. Physical mechanism for gating and mechanosensitivity of the human TRAAK K+ channel.Nature. 2014; 516: 126-130Google Scholar) (Figures 1A, right panel, and 3A). However, MD simulations of TREK-2 predict that phospholipid tails appear to be too short to reach into the cavity (Aryal et al., 2017Aryal P. Jarerattanachat V. Clausen M.V. Schewe M. McClenaghan C. Argent L. Conrad L.J. Dong Y.Y. Pike A.C.W. Carpenter E.P. et al.Bilayer-mediated structural transitions control mechanosensitivity of the TREK-2 K2P channel.Structure. 2017; 25: 708-718.e2Google Scholar). Moreover, intracellular Rb+ activates TREK-1 by directly influencing a voltage-dependent gate within the selectivity filter, even in the absence of stretch (i.e., when TM4 is in the down state and when lateral fenestrations are opened) (Aryal et al., 2017Aryal P. Jarerattanachat V. Clausen M.V. Schewe M. McClenaghan C. Argent L. Conrad L.J. Dong Y.Y. Pike A.C.W. Carpenter E.P. et al.Bilayer-mediated structural transitions control mechanosensitivity of the TREK-2 K2P channel.Structure. 2017; 25: 708-718.e2Google Scholar, Schewe et al., 2016Schewe M. Nematian-Ardestani E. Sun H. Musinszki M. Cordeiro S. Bucci G. de Groot B.L. Tucker S.J. Rapedius M. Baukrowitz T. A non-canonical voltage-sensing mechanism controls gating in K2P K(+) channels.Cell. 2016; 164: 937-949Google Scholar). Thus, these electrophysiological data are not consistent with a lipid occlusion model because intracellular Rb+ would be unable to reach the selectivity filter if lipids block the permeation pathway (Aryal et al., 2017Aryal P. Jarerattanachat V. Clausen M.V. Schewe M. McClenaghan C. Argent L. Conrad L.J. Dong Y.Y. Pike A.C.W. Carpenter E.P. et al.Bilayer-mediated structural transitions control mechanosensitivity of the TREK-2 K2P channel.Structure. 2017; 25: 708-718.e2Google Scholar). Although a large conformational change in TM4 (controlling the opening of lateral fenestrations) occurs upon channel activation, more recent findings strongly support the notion that the activation gate is located at the level of the selectivity filter (Lolicato et al., 2017Lolicato M. Arrigoni C. Mori T. Sekioka Y. Bryant C. Clark K.A. Minor Jr., D.L. K2P2.1 (TREK-1)-activator complexes reveal a cryptic selectivity filter binding site.Nature. 2017; 547: 364-368Google Scholar) (Figure 3B). Norfluoxetine (Prozac) binds and inhibits TREK-2 only when the lateral fenestrations are opened (i.e., when TM4 is down) (Dong et al., 2015Dong Y.Y. Pike A.C. Mackenzie A. McClenaghan C. Aryal P. Dong L. Quigley A. Grieben M. Goubin S. Mukhopadhyay S. et al.K2P channel gating mechanisms revealed by structures of TREK-2 and a complex with Prozac.Science. 2015; 347: 1256-1259Google Scholar, McClenaghan et al., 2016McClenaghan C. Schewe M. Aryal P. Carpenter E.P. Baukrowitz T. Tucker S.J. Polymodal activation of the TREK-2 K2P channel produces structurally distinct open states.J. Gen. Physiol. 2016; 147: 497-505Google Scholar) (Figure 3). Taking advantage of the state-dependent inhibition of TREK-2 by norfluoxetine, it was demonstrated that TREK-2 can be conductive both when TM4 is down or up (Lolicato et al., 2014Lolicato M. Riegelhaupt P.M. Arrigoni C. Clark K.A. Minor Jr., D.L. Transmembrane helix straightening and buckling underlies activation of mechanosensitive and thermosensitive K(2P) channels.Neuron. 2014; 84: 1198-1212Google Scholar, McClenaghan et al., 2016McClenaghan C. Schewe M. Aryal P. Carpenter E.P. Baukrowitz T. Tucker S.J. Polymodal activation of the TREK-2 K2P channel produces structurally distinct open states.J. Gen. Physiol. 2016; 147: 497-505Google Scholar) (Figure 3B). Additional findings indicate that the selectivity filter of mechanosensitive K2P channels acts as an activation gate (C-type gate) (Bagriantsev et al., 2011Bagriantsev S.N. Peyronnet R. Clark K.A. Honoré E. Minor Jr., D.L. Multiple modalities converge on a common gate to control K2P channel function.EMBO J. 2011; 30: 3594-3606Google Scholar, Bagriantsev et al., 2012Bagriantsev S.N. Clark K.A. Minor Jr., D.L. Metabolic and thermal stimuli control K(2P)2.1 (TREK-1) through modular sensory and gating domains.EMBO J. 2012; 31: 3297-3308Google Scholar, Lolicato et al., 2017Lolicato M. Arrigoni C. Mori T. Sekioka Y. Bryant C. Clark K.A. Minor Jr., D.L. K2P2.1 (TREK-1)-activator complexes reveal a cryptic selectivity filter binding site.Nature. 2017; 547: 364-368Google Scholar, McClenaghan et al., 2016McClenaghan C. Schewe M. Aryal P. Carpenter E.P. Baukrowitz T. Tucker S.J. Polymodal activation of the TREK-2 K2P channel produces structurally distinct open states.J. Gen. Physiol. 2016; 147: 497-505Google Scholar) (Figure 3B). Notably, binding of small-molecule TREK-1 activators (ML335 and ML402) to a cryptic binding pocket (between P1 pore helix and TM4) is predicted to restrict the movement behind the selectivity filter, enhancing K+ permeation (Lolicato et al., 2017Lolicato M. Arrigoni C. Mori T. Sekioka Y. Bryant C. Clark K.A. Minor Jr., D.L. K2P2.1 (TREK-1)-activator complexes reveal a cryptic selectivity filter binding site.Nature. 2017; 547: 364-368Google Scholar) (Figure 3B). Moreover, MD simulations predict that membrane stretch increases K+ occupancy within the selectivity filter of TREK-2 (Aryal et al., 2017Aryal P. Jarerattanachat V. Clausen M.V. Schewe M. McClenaghan C. Argent L. Conrad L.J. Dong Y.Y. Pike A.C.W. Carpenter E.P. et al.Bilayer-mediated structural transitions control mechanosensitivity of the TREK-2 K2P channel.Structure. 2017; 25: 708-718.e2Google Scholar). Notably, mechanosensitive K2P currents show a relaxation during maintained pressure stimulation, both in transfected cells and upon reconstitution into artificial bilayers (Brohawn et al., 2014bBrohawn S.G. Su Z. MacKinnon R. Mechanosensitivity is mediated directly by the lipid membrane in TRAAK and TREK1 K+ channels.Proc. Natl. Acad. Sci. U S A. 2014; 111: 3614-3619Google Scholar, Honoré et al., 2006Honoré E. Patel A.J. Chemin J. Suchyna T. Sachs F. Desensitization of mechano-gated K2P channels.Proc. Natl. Acad. Sci. U S A. 2006; 103: 6859-6864Google Scholar). This inactivation mechanism (τ = 45 ms) is intrinsic to the channel and is sensitive to pre-stress (resting tension) and resembles the C-type inactivation of Kv channels, a process by which K+ flux is blocked by conformational changes of the selectivity filter (Brohawn et al., 2014bBrohawn S.G. Su Z. MacKinnon R. Mechanosensitivity is mediated directly by the lipid membrane in TRAAK and TREK1 K+ channels.Proc. Natl. Acad. Sci. U S A. 2014; 111: 3614-3619Google Scholar, Honoré et al., 2006Honoré E. Patel A.J. Chemin J. Suchyna T. Sachs F. Desensitization of mechano-gated K2P channels.Proc. Natl. Acad. Sci. U S A. 2006; 103: 6859-6864Google Scholar). Altogether, those findings indicate the contribution of a filter C-type gate, as well as of a large conformational change of the TM4 helix, in the gating of mechanosensitive K2P channels (Brohawn et al., 2014aBrohawn S.G. Campbell E.B. MacKinnon R. Physical mechanism for gating and mechanosensitivity of the human TRAAK K+ channel.Nature. 2014; 516: 126-130Google Scholar, Lolicato et al., 2014Lolicato M. Riegelhaupt P.M. Arrigoni C. Clark K.A. Minor Jr., D.L. Transmembrane helix straightening and buckling underlies activation of mechanosensitive and thermosensitive K(2P) channels.Neuron. 2014; 84: 1198-1212Google Scholar, Lolicato et al., 2017Lolicato M. Arrigoni C. Mori T. Sekioka Y. Bryant C. Clark K.A. Minor Jr., D.L. K2P2.1 (TREK-1)-activator complexes reveal a cryptic selectivity filter binding site.Nature. 2017; 547: 364-368Google Scholar, McClenaghan et al., 2016McClenaghan C. Schewe M. Aryal P. Carpenter E.P. Baukrowitz T. Tucker S.J. Polymodal activation of the TREK-2 K2P channel produces structurally distinct open states.J. Gen. Physiol. 2016; 147: 497-505Google Scholar) (Figures 1A, 3A, and 3B). In addition, the TREK-1 intracellular C-terminal cytosolic domain is a major regulatory element of the channel and relays mechanical, thermal, and acidic modulations to the filter C-type gate through transmembrane helix TM4 and pore helix 1 (Bagriantsev et al., 2011Bagriantsev S.N. Peyronnet R. Clark K.A. Honoré E. Minor Jr., D.L. Multiple modalities converge on a common gate to control K2P channel function.EMBO J. 2011; 30: 3594-3606Google Scholar, Bagriantsev et al., 2012Bagriantsev S.N. Clark K.A. Minor Jr., D.L. Metabolic and thermal stimuli control K(2P)2.1 (TREK-1) through modular sensory and gating domains.EMBO J. 2012; 31: 3297-3308Google Scholar) (Figure 3C, dashed arrows). Conversely, phosphorylation of the Ct domain at Ser 300 and Ser 333 by protein kinases A or C inhibits channel activity (for review, see Honoré, 2007Honoré E. The neuronal background K2P channels: focus on TREK1.Nat. Rev. Neurosci. 2007; 8: 251-261Google Scholar) (Figure 3C). Across the lipid bilayer there is repulsion between the head groups and between the lipid tails, generating positive pressure (push), while there are two regions of high negative pressure (pull) around the glycerol backbone that is required to prevent water entry through the membrane (Aryal et al., 2017Aryal P. Jarerattanachat V. Clausen M.V. Schewe M. McClenaghan C. Argent L. Conrad L.J. Dong Y.Y. Pike A.C.W. Carpenter E.P. et al.Bilayer-mediated structural transitions control mechanosensitivity of the TREK-2 K2P channel.Structure. 2017; 25: 708-718.e2Google Scholar, Clausen et al., 2017Clausen M.V. Jarerattanachat V. Carpenter E.P. Sansom M.S.P. Tucker S.J. Asymmetric mechanosensitivity in a eukaryotic ion channel.Proc. Natl. Acad. Sci. U S A. 2017; 114: E8343-E8351Google Scholar, Martinac et al., 2018Martinac B. Bavi N. Ridone P. Nikolaev Y.A. Martinac A.D. Nakayama Y. Rohde P.R. Bavi O. Tuning ion channel mechanosensitivity by asymmetry of the transbilayer pressure profile.Biophys. Rev. 2018; 10: 1377-1384Google Scholar) (Figure 2D). Upon stretch of an ideal membrane, thickness is reduced, the positive pressure in the middle of the bilayer is lowered, and the peaks of negative pressure are increased (Aryal et al., 2017Aryal P. Jarerattanachat V. Clausen M.V. Schewe M. McClenaghan C. Argent L. Conrad L.J. Dong Y.Y. Pike A.C.W. Carpenter E.P. et al.Bilayer-mediated structural transitions control mechanosensitivity of the TREK-2 K2P channel.Structure. 2017; 25: 708-718.e2Google Scholar) (Figure 2D). In MD simulations of TREK-2, a change in the lateral pressure profile (by lowering the number of phospholipids in the inner leaflet, but not i
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