Cholesterol effects on nicotinic acetylcholine receptor.

This short overview addresses the effects of cholesterol (Chol) on the stability, supramolecular organization, and function of the murine muscle-type nicotinic acetylcholine receptor (AChR). The biophysical properties of Chol-AChR interaction are discussed first, as they constitute, in our view, the very source of the modulation exerted by Chol on receptor structure and function. In the absence of innervation, AChRs occur in the form of aggregates that remain stable at the cell-surface membrane of mammalian model cells over a period of hours. Acute Chol depletion drastically reduces (approximately 50%) the number of receptor aggregates by accelerating the rate of endocytosis in a receptor-expressing clonal cell line. Chol depletion also results in ion channel gain-of-function of the remaining cell-surface AChRs, whereas Chol enrichment has the opposite effect. Wide-field and confocal microscopy show AChR clusters as diffraction-limited puncta of approximately 200 nm diameter. Stimulated emission depletion fluorescence microscopy resolves these puncta into nanoclusters with an average diameter of approximately 55 nm. Exploiting the enhanced resolution, the effect of acute Chol depletion is shown to alter the short- and long-range organization of AChR nanoclusters. In the short range, the 50% AChRs remaining at the cell surface form larger nanoclusters. On larger scales (0.5-3.5 microm) Ripley's K-test on stimulated emission depletion images reveal changes in nanocluster distribution, attributable to the Chol-related abolition of cytoskeletal physical barriers normally preventing the lateral diffusion of AChR nanoclusters. We discuss these observations in the light of the so-called 'raft' hypothesis and surmise that Chol content at the plasmalemma homeostatically modulates cell-surface organization and stability of receptor nanodomains and fine tunes receptor channel function to temporarily compensate for acute AChR loss from the cell surface.