Desensitization of mechano-gated K _2P channels

Abstract

The neuronal mechano-gated K2P channels TREK-1 and TRAAK show pronounced desensitization within 100 ms of membrane stretch. Desensitization persists in the presence of cytoskeleton disrupting agents, upon patch excision, and when channels are expressed in membrane blebs. Mechanosensitive currents evoked with a variety of complex stimulus protocols were globally fit to a four-state cyclic kinetic model in detailed balance, without the need to introduce adaptation of the stimulus. However, we show that patch stress can be a complex function of time and stimulation history. The kinetic model couples desensitization to activation, so that gentle conditioning stimuli do not cause desensitization. Prestressing the channels with pressure, amphipaths, intracellular acidosis, or the E306A mutation reduces the peak-to-steady-state ratio by changing the preexponential terms of the rate constants, increasing the steady-state current amplitude. The mechanical responsivity can be accounted for by a change of in-plane area of Ϸ2 nm 2 between the closed and open conformations. Desensitization and its regulation by chemical messengers is predicted to condition the physiological role of K2P channels.M ost receptors, including ligand-gated ion channels, desensitize with continuous stimulation. Desensitization is a reversible, use-dependent, form of signal plasticity critical for maintaining the dynamic sensitivity over a wide dynamic range. Desensitization also protects cells from inappropriate persistent activation. Many mechano-gated ion channels desensitize (1-5). There are two basic mechanisms that account for desensitization: adaptation and inactivation. Adaptation refers to the uncoupling of the stimulus from the channel, whereas inactivation refers to a block of the permeation path. In hair cells of the inner ear, deflection opens a mechano-gated channel at the tip of the stereocilium leading to depolarization (6). A fast adaptation mechanism (0.3-5 ms) is attributed to the binding of calcium near the channel after permeation through the open ionic pore (6), i.e., Ca 2ϩ block. A slower adaptation (10-100 ms) occurs when the calcium-dependent molecular motor myosin-1c, which pulls on the channels, slips down the actin cytoskeleton reducing the force applied to the channel (6).Rapid desensitization also occurs in mechanosensitive channels expressed in nonspecialized cells. For instance, the stretchactivated cation channel of Xenopus oocyte opens transiently in response to a step change in patch pressure (5). The decrease in current has been attributed to relaxation of the mechanical stimulus, although as shown in this work, that interpretation may not be a good discriminator. Desensitization of the oocyte channel is fragile and may disappear when patches are repetitively stimulated (5). This fragility was attributed to decoupling of the bilayer from the cytoskeleton (5).Stretch-activated K ϩ channels are present in neurons of both the central and the peripheral nervous systems (7-9), and they also have been described in nonne...

Feeling the hidden mechanical forces in lipid bilayer is an original sense

“…The bilayer crenators (trinitrophenol, lysolecithin) activate, whereas cup formers (chlorpromazine, tetracaine) inhibit TREK-1 (36, 37), consistent with the bilayer-coupling hypothesis (14,26). TREK-1 is estimated to expand by ∼2 to 4 nm 2 in the membrane plane when opens (45). It remains reasonable that the inner helices (S2 and/or S4) converge to close and diverge to open like other K + channels as was supposed by earlier homology models (50), even if the inner gate cooperates with the filter gate.…”

“…These authors reasoned that open probability has been maximized by inherent tension in the patches (41). Such a tension is likely generated during the gigaOhm-seal formation, bonding the bilayer lipids to the pipet glass surface (42)(43)(44)(45).…”

A common mechanism underlies stretch activation and receptor activation of TRPC6 channels

“…2C; 7 sec.). The continuous activation of current may reflect residual tension in the patch resulting from membrane adhesion to the inside surface of the pipette that occurred during pressure application (22). The difference in the rate of channel response to shorter and longer pressure pulses may reflect potentiation by locally elevated Ca 2ϩ entering through TRPC6 channels which are known to be poten- tiated by Ca 2ϩ (21).…”

Mechanosensitivity is mediated directly by the lipid membrane in TRAAK and TREK1 K + channels

“…Difficulties in high level expression, purification, and reconstitution have precluded such an analysis of mechanosensitivity in eukaryotic ion channels with the same rigor as applied to MscL and MscS (15,16). Whereas mechanosensitivity of eukaryotic channels has been demonstrated by poking cell membranes under whole-cell voltage clamp and by pressure activation of channels in patches excised from cells or membrane blebs from cells (17), these experiments have not distinguished a direct membrane-mediated mechanism from other mechanisms that would rely on additional macromolecular components inescapably present in the cell-based assays. TRAAK (K2P4.1) and TREK channels are members of the two-pore domain K + (K2P) ion channels.…”