Acid sensitive background potassium channels K2P3.1 and K2P9.1 undergo rapid dynamin-dependent endocytosis

Fluvastatin inhibits Rab5-mediated IKs internalization caused by chronic Ca2+-dependent PKC activation

Statins, in addition to their cholesterol lowering effects, can prevent isoprenylation of Rab GTPase proteins, a key protein family for the regulation of protein trafficking. Rab-GTPases have been shown to be involved in the control of membrane expression level of ion channels, including one of the major cardiac repolarizing channels, IKs. Decreased IKs function has been observed in a number of disease states and associated with increased propensity for arrhythmias, but the mechanism underlying IKs decrease remains elusive. Ca²⁺-dependent PKC isoforms (cPKC) are chronically activated in variety of human diseases and have been suggested to acutely regulate IKs function. We hypothesize that chronic cPKC stimulation leads to Rab-mediated decrease in IKs membrane expression, and that can be prevented by statins. In this study we show that chronic cPKC stimulation caused a dramatic Rab5 GTPase-dependent decrease in plasma membrane localization of the IKs pore forming subunit KCNQ1, reducing IKs function. Our data indicates fluvastatin inhibition of Rab5 restores channel localization and function after cPKC-mediated channel internalization. Our results indicate a novel statin anti-arrhythmic effect that would be expected to inhibit pathological electrical remodeling in a number of disease states associated with high cPKC activation. Because Rab-GTPases are important regulators of membrane trafficking they may underlie other statin pleiotropic effects.

Endocytosis as a mode to regulate functional expression of two-pore domain potassium (K₂p) channels

Two-pore domain potassium (K_2P) channels are implicated in an array of physiological and pathophysiological roles. As a result of their biophysical properties, these channels produce a background leak K⁺ current which has a direct effect on cellular membrane potential and activity. The regulation of potassium leak from cells through K_2P channels is of critical importance to cell function, development and survival. Controlling the cell surface expression of these channels is one mode to regulate their function and is achieved through a balance between regulated channel delivery to and retrieval from the cell surface. Here, we explore the modes of retrieval of K_2P channels from the plasma membrane and observe that K_2P channels are endocytosed in both a clathrin-mediated and clathrin-independent manner. K_2P channels use a variety of pathways and show altered internalisation and sorting in response to external cues. These pathways working in concert, equip the cell with a range of approaches to maintain steady state levels of channels and to respond rapidly should changes in channel density be required.

Cooperative endocytosis of the endosomal SNARE protein syntaxin-8 and the potassium channel TASK-1

SNARE proteins can have functions unrelated to membrane fusion. The unassembled form of the SNARE protein syntaxin-8 interacts with the K⁺ channel TASK-1; both proteins are internalized via clathrin-mediated endocytosis in a cooperative manner. This is a novel mechanism for the control of endocytosis by cargo proteins.

The endosomal SNARE protein syntaxin-8 interacts with the acid-sensitive potassium channel TASK-1. The functional relevance of this interaction was studied by heterologous expression of these proteins (and mutants thereof) in Xenopus oocytes and in mammalian cell lines. Coexpression of syntaxin-8 caused a fourfold reduction in TASK-1 current, a corresponding reduction in the expression of TASK-1 at the cell surface, and a marked increase in the rate of endocytosis of the channel. TASK-1 and syntaxin-8 colocalized in the early endosomal compartment, as indicated by the endosomal markers 2xFYVE and rab5. The stimulatory effect of the SNARE protein on the endocytosis of the channel was abolished when both an endocytosis signal in TASK-1 and an endocytosis signal in syntaxin-8 were mutated. A syntaxin-8 mutant that cannot assemble with other SNARE proteins had virtually the same effect as wild-type syntaxin-8. Total internal reflection fluorescence microscopy showed formation and endocytosis of vesicles containing fluorescence-tagged clathrin, TASK-1, and/or syntaxin-8. Our results suggest that the unassembled form of syntaxin-8 and the potassium channel TASK-1 are internalized via clathrin-mediated endocytosis in a cooperative manner. This implies that syntaxin-8 regulates the endocytosis of TASK-1. Our study supports the idea that endosomal SNARE proteins can have functions unrelated to membrane fusion.