Full-fusion and kiss-and-run in chromaffin cells controlled by irreversible vesicle size-dependent fusion pore transitions

Abstract

Exocytosis operates through two distinct modes. Full-fusion leads to rapid expulsion of the entire content of a vesicle; kiss-and-run leads to slow and partial expulsion. These two modes have important biological consequences for endocrine regulation and synaptic transmission. Amperometry recordings of catecholamine release from chromaffin cells reveal single-vesicle fusion events corresponding to both of these modes, but classification is often difficult. This study introduces a new method of analyzing amperometry data to improve this classification. The ratio of the average amplitude to the peak amplitude differs between full-fusion and kiss-and-run, and the probability distribution of this ratio is well fitted by a double-Gaussian. Kiss-and-run events identified by this method have fusion pores with kinetic properties different from pores associated with full-fusion. They have slower transition rates and lifetime distributions indicative of irreversible transitions. The total-charge of an amperometric spike is expected to scale with vesicle volume during a full-fusion event. The cube root of this quantity should therefore scale with diameter, but the distribution of this quantity differs from the distribution of vesicle diameter seen in the electron microscope. Fusion pore lifetimes associated with full-fusion depend on vesicle size, and this makes the choice of mode size dependent. The fusion pore thus bifurcates after opening, and vesicle size influences this choice. The secretory vesicle protein synaptophysin influences the size dependence of fusion pore lifetime and the choice of release mode. Incorporating vesicle size into an analysis of release mode reconciled the kinetics of fusion pores, as well as the distributions of vesicle diameter and catecholamine content. Thus, the initial fusion pore emerges as a critical focus in endocrine regulation. By modulating the size-dependence of the mode of exocytosis, changes in the molecular makeup of the exocytotic apparatus can impact the shape and size of an amperometric event, and the speed and composition of secretion.