PI(4,5)P2 regulates the gating of NaV1.4 channels

Voltage-gated sodium (NaV) channels are densely expressed in most excitable cells and activate in response to depolarization, causing a rapid influx of Na+ ions that initiates the action potential. The voltage-dependent activation of NaV channels is followed almost instantaneously by fast inactivation, setting the refractory period of excitable tissues. The gating cycle of NaV channels is subject to tight regulation, with perturbations leading to a range of pathophysiological states. The gating properties of most ion channels are regulated by the membrane phospholipid, phosphatidylinositol (4,5) bisphosphate (PI(4,5)P2). However, it is not known whether PI(4,5)P2 modulates the activity of NaV channels. Here, we utilize optogenetics to activate specific, membrane-associated phosphoinositide (PI)-phosphatases that dephosphorylate PI(4,5)P2 while simultaneously recording NaV1.4 channel currents. We show that dephosphorylating PI(4,5)P2 left-shifts the voltage-dependent gating of NaV1.4 to more hyperpolarized membrane potentials, augments the late current that persists after fast inactivation, and speeds the rate at which channels recover from fast inactivation. These effects are opposed by exogenous diC8PI(4,5)P2. We provide evidence that PI(4,5)P2 is a negative regulator that tunes the gating behavior of NaV1.4 channels.

Imaging Membrane Proteins Using Total Internal Reflection Fluorescence Microscopy (TIRFM) in Mammalian Cells

The cell surfaceome is of vital importance across physiology, developmental biology, and disease states alike. The precise identification of proteins and their regulatory mechanisms at the cell membrane has been challenging and is typically determined using confocal microscopy, two-photon microscopy, or total internal reflection fluorescence microscopy (TIRFM). Of these, TIRFM is the most precise, as it harnesses the generation of a spatially delimited evanescent wave at the interface of two surfaces with distinct refractive indices. The limited penetration of the evanescent wave illuminates a narrow specimen field, which facilitates the localization of fluorescently tagged proteins at the cell membrane but not inside of the cell. In addition to constraining the depth of the image, TIRFM also significantly enhances the signal-to-noise ratio, which is particularly valuable in the study of live cells. Here, we detail a protocol for micromirror TIRFM analysis of optogenetically activated protein kinase C-ε in HEK293-T cells, as well as data analysis to demonstrate the translocation of this construct to the cell-surface following optogenetic activation. Graphic abstract.

The Pharmacology of Two-Pore Domain Potassium Channels

Two-pore domain potassium channels are formed by subunits that each contain two pore-loops moieties. Whether the channels are expressed in yeast or the human central nervous system, two subunits come together to form a single potassium selective pore. TOK1, the first two-domain channel was cloned from Saccharomyces cerevisiae in 1995 and soon thereafter, 15 distinct K2P subunits were identified in the human genome. The human K2P channels are stratified into six K2P subfamilies based on sequence as well as physiological or pharmacological similarities. Functional K2P channels pass background (or "leak") K+ currents that shape the membrane potential and excitability of cells in a broad range of tissues. In the years since they were first described, classical functional assays, latterly coupled with state-of-the-art structural and computational studies have revealed the mechanistic basis of K2P channel gating in response to specific physicochemical or pharmacological stimuli. The growing appreciation that K2P channels can play a pivotal role in the pathophysiology of a growing spectrum of diseases makes a compelling case for K2P channels as targets for drug discovery. Here, we summarize recent advances in unraveling the structure, function, and pharmacology of the K2P channels.