Mechanoregulation of Exocytosis Rates by Vesicle-Membrane Merging Kinetics

Abstract

Exocytosis is central to neurotransmission, hormone secretion and other fundamental processes. Neurons and secretory cells subject to repeated stimulus typically exhibit facilitated or depressed release rates. Such regulation can be due to depletion of the readily releasable pool (RRP) of release-competent vesicles, and the actin cortex is thought to play a role by presenting a functional barrier between vesicles and the plasma membrane (PM) (Vitale et al. 1995) and by regulating PM tension that promotes contents release (Bretou et al., 2014). Here we propose that an important mechanical factor regulating exocytosis rates is post-fusion vesicle-PM merging, whose kinetics limit release at high stimulus frequencies. Using a continuum model describing membrane tension dynamics and lipid flow, we show that repeated delivery of secretory vesicles to a release site retards vesicle-PM merging and depresses release rates by lowering membrane tension locally which weakens the net mechanical force driving vesicle merging. We find vesicle-PM merging is driven by mechanical forces: osmotic squeezing, membrane tension, and PM-cortex adhesion. Osmotic squeezing abolishes vesicle membrane tension, creating tension gradients that reel vesicles into the PM, assisted by membrane-actin cortex adhesion. In chromaffin cells, we predict merging on ∼1 sec time scales, agreeing with experiment (Shin et al, 2020). However, above a critical exocytosis rate, membrane tension near a release site cannot recover, as each repeated vesicle merging lowers the tension and slows subsequent merger. This dramatically reduces the merging driving force and the merging rate, a negative feedback that depresses exocytosis rates. We predict a critical rate that is comparable to the exocytosis rate at hotspots recently measured in INS-1 cells (Yuan et al., 2015). Thus, we propose that exocytic depression previously attributed to RRP depletion may originate in part from these ubiquitous biophysical mechanical effects.