Cell membranes consist of heterogeneous lipid domains that influence key cellular processes, including signal transduction, endocytosis, and electrical excitability. The goal of this study was to assess the size of cholesterol-enriched ordered membrane domains (OMD) in various cell types. Multiple cell types were tested using fluorescence lifetime imaging microscopy (FLIM) and Förster resonance energy transfer (FRET), whereby small nociceptor DRG neurons and cardiac pacemaker cells displayed the highest FRET intensities. This implies that electrically active cells tend to have large OMDs. Treatment of cells with the cholesterol-extracting reagent β-cyclodextrin (β-CD) led to a decrease in FRET, indicating a reduction in the OMD size, whereas detergents known to promote domain coalescence in artificial membranes increased OMD size. In an fatty liver model, palmitate supplementation increased FRET whereas oleate supplementation decreased FRET in isolated primary murine hepatocytes, highlighting the importance of unsaturated lipid tails in lipid domain organization. Disruption of OMD using β-CD potentiated action potential firing in nociceptor DRG neurons and decreased the free energy needed for opening native hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. After disrupting the OMD, HCN channels exhibited an increased relative open probability at the resting membrane potential (RMP). A significant reduction in FRET was observed in both a chemotherapy-induced neuropathic pain model and a spared nerve injury model of neuropathic pain, consistent with disrupted or shrunken OMD in these models. Collectively, these findings show that disturbances in lipid domains may contribute to the progression of neuropathic pain, and they suggest new therapeutic strategies to achieve pain relief.

Phosphoinositide membrane lipids are ubiquitous low-abundance signaling molecules. They direct many physiological processes that involve ion channels, membrane identification, fusion of membrane vesicles, and vesicular endocytosis. Pools of these lipids are continually broken down and refilled in living cells, and the rates of some of these reactions are strongly accelerated by physiological stimuli. Recent biophysical experiments described here measure and model the kinetics and regulation of these lipid signals in intact cells. Rapid on-line monitoring of phosphoinositide metabolism is made possible by optical tools and electrophysiology. The experiments reviewed here reveal that as for other cellular second messengers, the dynamic turnover and lifetimes of membrane phosphoinositides are measured in seconds, controlling and timing rapid physiological responses, and the signaling is under strong metabolic regulation. The underlying mechanisms of this metabolic regulation remain questions for the future.

β-Secretase 1 (β-site amyloid precursor protein [APP]-cleaving enzyme 1, BACE1) plays a crucial role in the amyloidogenesis of Alzheimer’s disease (AD). BACE1 was also discovered to act like an auxiliary subunit to modulate neuronal KCNQ2/3 channels independently of its proteolytic function. BACE1 is palmitoylated at its carboxyl-terminal region, which brings BACE1 to ordered, cholesterol-rich membrane microdomains (lipid rafts). However, the physiological consequences of this specific localization of BACE1 remain elusive. Using spectral Förster resonance energy transfer (FRET), BACE1 and KCNQ2/3 channels were confirmed to form a signaling complex, a phenomenon that was relatively independent of the palmitoylation of BACE1. Nevertheless, palmitoylation of BACE1 was required for recruitment of KCNQ2/3 channels to lipid-raft domains. Two fluorescent probes, designated L10 and S15, were used to label lipid-raft and non-raft domains of the plasma membrane, respectively. Coexpressing BACE1 substantially elevated FRET between L10 and KCNQ2/3, whereas the BACE1-4C/A quadruple mutation failed to produce this effect. In contrast, BACE1 had no significant effect on FRET between S15 probes and KCNQ2/3 channels. A reduction of BACE1-dependent FRET between raft-targeting L10 probes and KCNQ2/3 channels by applying the cholesterol-extracting reagent methyl-β-cyclodextrin (MβCD), raft-disrupting general anesthetics, or pharmacological inhibitors of palmitoylation, all supported the hypothesis of the palmitoylation-dependent and raft-specific localization of KCNQ2/3 channels. Furthermore, mutating the four carboxyl-terminal cysteines (4C/A) of BACE1 abolished the BACE1-dependent increase of FRET between KCNQ2/3 and the lipid raft-specific protein caveolin 1. Taking these data collectively, we propose that the AD-related protein BACE1 underlies the localization of a neuronal potassium channel.

Pacemaker hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels exhibit a reversed voltage-dependent gating, activating by membrane hyperpolarization instead of depolarization. Sea urchin HCN (spHCN) channels also undergo inactivation with hyperpolarization which occurs only in the absence of cyclic nucleotide. Here we applied transition metal ion FRET, patch-clamp fluorometry and Rosetta modeling to measure differences in the structural rearrangements between activation and inactivation of spHCN channels. We found that removing cAMP produced a largely rigid-body rotation of the C-linker relative to the transmembrane domain, bringing the A’ helix of the C-linker in close proximity to the voltage-sensing S4 helix. In addition, rotation of the C-linker was elicited by hyperpolarization in the absence but not the presence of cAMP. These results suggest that – in contrast to electromechanical coupling for channel activation – the A’ helix serves to couple the S4-helix movement for channel inactivation, which is likely a conserved mechanism for CNBD-family channels.