ABSTRACT
Eukaryotic vesicles fuse with the plasmalemma to form the fusion pore, previously considered to be unstable with widening of the pore diameter. Recent studies established that the pore diameter is stable, reflecting balanced forces of widening and closure. Proteins are considered key regulators of the fusion pore, whereas the role of membrane lipids remains unclear. Super-resolution microscopy revealed that lactotroph secretory vesicles discharge cholesterol after stimulation of exocytosis; subsequently, vesicle cholesterol redistributes to the outer leaflet of the plasmalemma. Cholesterol depletion in lactotrophs and astrocytes evokes release of vesicle hormone, indicating that cholesterol constricts the fusion pore. A new model of cholesterol-dependent fusion pore diameter regulation is proposed. High-resolution measurements of fusion pore conductance confirmed that the fusion pore widens with cholesterol depletion and constricts with cholesterol enrichment. In fibroblasts lacking the Npc1 protein, in which cholesterol accumulates in vesicles, the fusion pore is narrower than in controls, showing that cholesterol regulates fusion pore geometry.
Graphical AbstractTop: stages through which a vesicle interacts with the plasmalemma. Stage A denotes hemifusion, which proceeds to stage B, with a narrow fusion pore, which can then reversibly open (stage C), before widening fully (stage D). Bottom: redistribution of cholesterol from the vesicle to the outer leaflet of the plasmalemma controls fusion pore constriction.
In Brief A membrane pore is formed when the vesicle membrane fuses with the plasmalemma. Proteins were considered key regulators of the opening and closing of this fusion pore. Here, evidence is provided to show that cholesterol, a membrane constituent, determines a radial force constricting the fusion pore, revealing that the fusion pore functions as a proteolipidic structure.
Highlights
Intravesicular cholesterol redistributes to the outer leaflet of the plasmalemma.
Cholesterol depletion widens the fusion pore, whereas cholesterol enrichment constricts the fusion pore.
A model of cholesterol-dependent force preventing fusion pore widening is developed.
Disease-related increase in vesicle cholesterol constricts the fusion pore.
