Rapid conversion of force into a biological signal enables living cells to respond to mechanical forces in their environment. The force is believed to initially affect the plasma membrane and then alter the behavior of membrane proteins. Phospholipase D2 (PLD2) is a mechanosensitive enzyme that is regulated by a structured membrane-lipid site comprised of cholesterol and saturated ganglioside (GM1). Here we show stretch activation of TWIK-related K+ channel (TREK-1) is mechanically evoked by PLD2 and spatial patterning involving ordered GM1 and 4,5-bisphosphate (PIP2) clusters in mammalian cells. First, mechanical force deforms the ordered lipids, which disrupts the interaction of PLD2 with the GM1 lipids and allows a complex of TREK-1 and PLD2 to associate with PIP2 clusters. The association with PIP2 activates the enzyme, which produces the second messenger phosphatidic acid (PA) that gates the channel. Co-expression of catalytically inactive PLD2 inhibits TREK-1 stretch currents in a biological membrane. Cellular uptake of cholesterol inhibits TREK-1 currents in culture and depletion of cholesterol from astrocytes releases TREK-1 from GM1 lipids in mouse brain. Depletion of the PLD2 ortholog in flies results in hypersensitivity to mechanical force. We conclude PLD2 mechanosensitivity combines with TREK-1 ion permeability to elicit a mechanically evoked response.

We thank van Swinderen and Hines (1) for their response to our recent article, where we find that anesthetics disrupt lipid rafts and activate phospholipase D (PLD) through a membrane-mediated mechanism (2). Here we take the opportunity to address some minor misunderstandings and clarify our experimental procedures. Our chloroform induction in Drosophila is part of a larger study (3) which we reference in our study (reference 40 in ref. 2), referring the reader to a detailed description of the animal experiments, including the genotype of the wild-type flies (w1118), and full dose–response curves for three anesthetics (chloroform, isoflurane, … [↵][1]1To whom correspondence may be addressed. Email: shansen{at}scripps.edu or rlerner{at}scripps.edu. [1]: #xref-corresp-1-1

TWIK related K+ channel (TREK-1) is a mechano- and anesthetic sensitive channel that when activated attenuates pain and causes anesthesia. Recently the enzyme phospholipase D2 (PLD2) was shown to bind to the channel and generate a local high concentration of phosphatidic acid (PA), an anionic signaling lipid that gates TREK-1. In a biological membrane, the cell harnesses lipid heterogeneity (lipid compartments) to control gating of TREK-1 using palmitate-mediated localization of PLD2. Here we discuss the ability of mechanical force and anesthetics to disrupt palmitate-mediated localization of PLD2 giving rise to TREK-1's mechano- and anesthetic-sensitive properties. The likely consequences of this indirect lipid-based mechanism of activation are discussed in terms of a putative model for excitatory and inhibitory mechano-effectors and anesthetic sensitive ion channels in a biological context. Lastly, we discuss the ability of locally generated PA to reach mM concentrations near TREK-1 and the biophysics of localized signaling. Palmitate-mediated localization of PLD2 emerges as a central control mechanism of TREK-1 responding to mechanical force and anesthetic action. This article is part of a Special Issue entitled: Molecular biophysics of membranes and membrane proteins.

Inhaled anesthetics are a chemically diverse collection of hydrophobic molecules that robustly activate TWIK-related K+ channels (TREK-1) and reversibly induce loss of consciousness. For 100 y, anesthetics were speculated to target cellular membranes, yet no plausible mechanism emerged to explain a membrane effect on ion channels. Here we show that inhaled anesthetics (chloroform and isoflurane) activate TREK-1 through disruption of phospholipase D2 (PLD2) localization to lipid rafts and subsequent production of signaling lipid phosphatidic acid (PA). Catalytically dead PLD2 robustly blocks anesthetic TREK-1 currents in whole-cell patch-clamp recordings. Localization of PLD2 renders the TRAAK channel sensitive, a channel that is otherwise anesthetic insensitive. General anesthetics, such as chloroform, isoflurane, diethyl ether, xenon, and propofol, disrupt lipid rafts and activate PLD2. In the whole brain of flies, anesthesia disrupts rafts and PLDnull flies resist anesthesia. Our results establish a membrane-mediated target of inhaled anesthesia and suggest PA helps set thresholds of anesthetic sensitivity in vivo.

Though widely used, the mechanisms of sedation from compounds like ethanol and volatile anesthetics are not well understood. Drosophila melanogaster has been identified as an ideal model organism for studying phenotypic responses to ethanol and other anesthetics due to its low cost, similar phenotypic response, and many available genetic tools. Previously, measuring the effects of anesthetics on Drosophila has required labor‐intensive methods often using large cohorts of flies, making them cumbersome and inefficient. Here, we describe the setup and experimental protocol for Volatile Anesthetic Position Recording (VAPR), a high‐throughput technique to measure the effect of ethanol vapor and other volatile drugs on Drosophila behavior. VAPR utilizes flow meters to adjust the relative concentration of aerosolized drugs which are then passed through chambers containing individual flies. Automated video tracking records the position and movement of the flies over time, which we analyze to quantify hyperactivity, sedation, and other treatment related behavioral changes. Using VAPR, we found flies lacking the enzyme phospholipase D (PLD) are insensitive to ethanol sedation but appear unaffected in their sensitivity to the anesthetics chloroform and isoflurane. Support or Funding Information National Science Foundation (NSF) Summer Undergraduate Research Fellowship (SURF) grant 1359369. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

The transduction of force into a biological signal is critical to all living organisms. Recently, disruption of ordered lipids has emerged as an ‘atypical’ force sensor in biological membranes; however, disruption has yet to link with canonical channel mechanosensation. We show here that force‐induced disruption and lipid mixing activates TREK‐1. This activation is dependent on PLD2 which transduces the force into the chemical signal phosphatidic acid that is then sensed by TREK‐1 with a latency of <3 ms. TREK‐1 then produces a mechanically induced change in membrane potential. These experiments establish the ordered lipid as the force sensor, PLD2 as the primary chemical transducer, and the ‘mechanosensitive’ ion channel TREK‐1 as a downstream effector of mechanical transduction. Confirming a central role for PA singling in force transduction, genetic deletion of PLD2 decreases mechanosensitivity and pain thresholds in D. melanogaster.Support or Funding InformationNATIONAL INSTITUTE OF NEUROLOGICAL DISORDERS AND STROKENATIONAL INSTITUTE OF AGEINGHOWARD HUGHES MEDICAL INSTITUTEThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Despite the widespread consumption of ethanol, mechanisms underlying its anesthetic effects remain uncertain. n-Alcohols induce anesthesia up to a specific chain length and then lose potency—an observation known as the “chain-length cutoff effect.” This cutoff effect is thought to be mediated by alcohol binding sites on proteins such as ion channels, but where these sites are for long-chain alcohols and how they mediate a cutoff remain poorly defined. In animals, the enzyme phospholipase D (PLD) has been shown to generate alcohol metabolites (e.g., phosphatidylethanol) with a cutoff, but no phenotype has been shown connecting PLD to an anesthetic effect. Here we show loss of PLD blocks ethanol-mediated hyperactivity in Drosophila melanogaster (fruit fly), demonstrating that PLD mediates behavioral responses to alcohol in vivo. Furthermore, the metabolite phosphatidylethanol directly competes for the endogenous PLD product phosphatidic acid at lipid-binding sites within potassium channels [e.g., TWIK-related K+ channel type 1 (K2P2.1, TREK-1)]. This gives rise to a PLD-dependent cutoff in TREK-1. We propose an alcohol pathway where PLD produces lipid-alcohol metabolites that bind to and regulate downstream effector molecules including lipid-regulated potassium channels.

BACKGROUND: Local anesthetics cause reversible block of pain and robustly inhibit TWIK-related K+ channel (TREK-1) currents. Before local anesthesia onset, injection of local anesthetics can cause unwanted transient pain. TREK-1 is an anesthetic-sensitive potassium channel that when inhibited produces pain. A disordered C-terminal loop of TREK-1 is thought to contribute to anesthetic sensitivity, but the molecular basis for TREK-1 inhibition by local anesthetics is unknown. Phospholipase D2 (PLD2) is an enzyme that produces phosphatidic acid (PA) required for TREK-1 activation and also binds to the channel’s C terminus. METHODS: Here, we use biophysical and cellular techniques to characterize direct and indirect lipid-mediated mechanism for TREK-1 inhibition (respectively). We characterized direct binding of local anesthetic to TREK-1 by reconstituting the purified channel into artificial membranes and measuring ion flux. We characterized indirect PA-mediated inhibition of TREK-1 by monitoring lipid production in live whole cells using a fluorescent PLD2 product release assay and ion channel current using live whole-cell patch-clamp electrophysiology. We monitored anesthetic-induced nanoscale translocation of PLD2 to TREK-1 channels with super-resolution direct stochastic reconstruction microscopy (dSTORM). RESULTS: We find local anesthetics tetracaine, lidocaine, and bupivacaine directly bind to and inhibit PLD2 enzymatic activity. The lack of PLD2 activity indirectly inhibited TREK-1 currents. Select local anesthetics also partially blocked the open pore of TREK-1 through direct binding. The amount of pore block was variable with tetracaine greater than bupivacaine and lidocaine exhibiting a minor effect. Local anesthetics also disrupt lipid rafts, a mechanism that would normally activate PLD2 were it not for their direct inhibition of enzyme catalysis. CONCLUSIONS: We propose a mechanism of TREK-1 inhibition comprised of (1) primarily indirect PLD2-dependent inhibition of lipid catalysis and (2) limited direct inhibition for select local anesthetics through partial open pore block. The inhibition through PLD2 explains how the C terminus can regulate the channel despite being devoid of structure and putative binding sites for local anesthetics.

Since their discovery more than 150 years ago, anesthetics were speculated to target the cellular membranes. Hitherto, there is no plausible mechanism that explains how anesthetics perturbation on membrane could to lead to an effect. Here we show that the volatile anesthetics chloroform and isoflurane activate TREK‐1 channels through disruption of ordered lipid domains (rafts). Employing super resolution microscopy, we found that raft disruption by anesthetics cause the raft‐localized enzyme phospholipase D2 (PLD2) to translocate and activate TREK‐1. A catalytically dead PLD2 significantly decrease the anesthetic‐specific TREK‐1 currents. Additionally, transfer of the PLD2 binding‐site to an anesthetic‐insensitive channel, TRAAK, results in anesthetic‐sensitive currents. The general anesthetics chloroform, isoflurane, diethyl ether, xenon, and propofol were all found to activate PLD2 in cellular membranes indicating a broadly applicable mechanism. We propose a two‐step model of anesthetic TREK‐1 activation. First, inhaled anesthetics disrupt lipid rafts. Second, translocation and PLD2‐dependent production of anionic lipid activate associated channels. This model suggests anesthetics could indirectly activate ion channel in contrast to direct binding models. Support or Funding Information This work was supported by a Director's New Innovator Award (1DP2NS087943‐01 to S.B.H.) from the NIH, a graduate fellowship from the Joseph B. Scheller & Rita P. Scheller Charitable Foundation to E.N.P. We are grateful to the Iris and Junming Le Foundation for funds to purchasea super‐resolution microscope, making this study possible. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

Volatile anesthetics are compounds which are commonly used to induce a reversable loss of consciousness (LOC) in animals. The molecular mechanism of how anesthetics induce LOC is largely unknown. However, observations have been made which show that there are genetically-encoded traits which influence the effective concentration of anesthetics in the inducement of LOC. Despite this long-term observation, little progress has been made in identifying genes involved in anesthetic sensitivity. One reason for this is that many techniques to test anesthetic sensitivity are technically challenging and are inhibitory for high-throughput studies. Here we introduce a technique for testing volatiles and aerosols with positional recording (VAAPR), a method which allows for high-throughput testing of the effect of anesthetics and other aerosolized drugs using Drosophila. Using VAAPR we show that the enzyme phospholipase D (PLD) significantly shifts the concentration of diethyl ether, chloroform, and isoflurane needed to induce LOC in Drosophila. We also show that PLD is required for a paradoxical hyperactivity phenotype. We expect that this technique will allow for additional genes to be found which control anesthetic sensitivity as well as other behavioral phenotypes.

The signaling lipid phosphatidylcholine 4,5 bisphosphate (PIP2) clusters with proteins in the plasma membrane away from saturated domains formed by GM1 lipids and cholesterol (GM1 domains or ‘lipid rafts’). The lipid metabolizing enzyme phospholipase D2 (PLD2) localizes to both lipid raft and PIP2 clusters creating a competition for localization. We show that disruption of GM1 domains by mechanical force or inhaled anesthetics causes PLD2 to translocate to PIP2 clusters where it activates twik related potassium (TREK‐1) channels giving rise to the channel's mechano and anesthetic sensitivity. We conclude cells have evolved to (i.) sense and respond to disruption of lipid order and (ii.) utilize lipid order to dynamically regulate enzymes and ion channels through substrate presentation.Support or Funding InformationThis work was supported by a Director's New Innovator Award (1DP2NS087943‐01 to S.B.H.) from the NIH, a graduate fellowship from the Joseph B. Scheller & Rita P. Scheller Charitable Foundation to E.N.P. We are grateful to the Iris and Junming Le Foundation for funds to purchase a super‐resolution microscope, making this study possible.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

The sensing of physical force, mechanosensation, underlies two of five human senses—touch and hearing. How transduction of force in a membrane occurs remains unclear. We asked if a biological membrane could employ kinetic energy to transduce a signal absent tension. Here we show that lipid rafts are dynamic compartments that inactivate the signalling enzyme phospholipase D2 (PLD2) by sequestering the enzyme from its substrate. Mechanical disruption of the lipid rafts activates PLD2 by mixing the enzyme with its substrate to produce the signalling lipid phosphatidic acid (PA). We calculate a latency time of <650 μs for PLD activation by mixing. Our results establish a fast, non-tension mechanism for mechanotransduction where disruption of ordered lipids initiates a mechanosensitive signal for cell growth through mechanical mixing.

Although the ribosome is a very general catalyst, it cannot synthesize all protein sequences equally well. For example, ribosomes stall on the secretion monitor (SecM) leader peptide to regulate expression of a downstream gene. Using a genetic selection in Escherichia coli, we identified additional nascent peptide motifs that stall ribosomes. Kinetic studies show that some nascent peptides dramatically inhibit rates of peptide release by release factors. We find that residues upstream of the minimal stalling motif can either enhance or suppress this effect. In other stalling motifs, peptidyl transfer to certain aminoacyl-tRNAs is inhibited. In particular, three consecutive Pro codons pose a challenge for elongating ribosomes. The translation factor elongation factor P, which alleviates pausing at polyproline sequences, has little or no effect on other stalling peptides. The motifs that we identified are underrepresented in bacterial proteomes and show evidence of stalling on endogenous E. coli proteins.