Redistribution of cholesterol from vesicle to plasmalemma controls fusion pore geometry

Abstract

Highlights 36  Intravesicular cholesterol redistributes to the outer leaflet of the plasmalemma. 37 3  Cholesterol depletion widens the fusion pore, whereas cholesterol enrichment constricts the fusion 38 pore. 39  A model of cholesterol-dependent force preventing fusion pore widening is developed. 40  Disease-related increase in vesicle cholesterol constricts the fusion pore. 41 ABSTRACT 42 Eukaryotic vesicles fuse with the plasmalemma to form the fusion pore, previously considered to be 43 unstable with widening of the pore diameter. Recent studies established that the pore diameter is 44 stable, reflecting balanced forces of widening and closure. Proteins are considered key regulators of 45 the fusion pore, whereas the role of membrane lipids remains unclear. Super-resolution microscopy 46 revealed that lactotroph secretory vesicles discharge cholesterol after stimulation of exocytosis; 47 subsequently, vesicle cholesterol redistributes to the outer leaflet of the plasmalemma. Cholesterol 48 depletion in lactotrophs and astrocytes evokes release of vesicle hormone, indicating that cholesterol 49 constricts the fusion pore. A new model of cholesterol-dependent fusion pore diameter regulation is 50 proposed. High-resolution measurements of fusion pore conductance confirmed that the fusion pore 51 widens with cholesterol depletion and constricts with cholesterol enrichment. In fibroblasts lacking 52 the Npc1 protein, in which cholesterol accumulates in vesicles, the fusion pore is narrower than in 53 controls, showing that cholesterol regulates fusion pore geometry. 54 65 to the membrane area, and is affected by vesicle fusion and fission (Neher and Marty, 1982).66

Calcium-dependent subquantal peptide release from single docked lawn-resident vesicles of pituitary lactotrophs

“…Ionic gradients as well as inter/intra redistribution and/or orientation of membrane proteins, lipids and sterols are well suited to provide for membrane curvature driving fusion pore activity. Recently, the idea that lipid redistribution is crucial for the regulation of fusion pore activity has been reinforced by the demonstration that membrane cholesterol regulates fusion pore activity in pituitary lactotrophs [79,80]. Given its extreme mobility in lipid systems, cholesterol has always been considered likely to partake in Ca 2+ -regulated exocytosis.…”

“…Accordingly, the mobility of cholesterol from lipidic to aqueous environments is determined by its concentration gradient and by the membrane curvature, because cholesterol retention increases with enhancement of surface curvature [82]. The transfer of vesicle cholesterol to the outer leaflet of the plasma membrane through the fusion pore orifice causes a radial force constricting the fusion pore in lactotrophs actively engaged in prolactin secretion [79,80]. Cholesterol transfer from secretory vesicles to the plasma membrane reduced fusion pore conductance [80].…”

Exocytotic fusion pore under stress

“…In the latter case, as already discussed in the previous section, membrane rafts enriched with cholesterol may generate a force on the neighbouring membrane regions with heterogeneous membrane structure [52], and these forces may play a role in membrane fusion [42]. However, direct demonstration of such a role of cholesterol in membrane fusion has only recently been addressed [72].…”