Membrane proximal lysosomes are the major vesicles responsible for calcium-dependent exocytosis in nonsecretory cells

TLDR

Calcium triggers exocytosis in nonsecretory cells, a process essential for repairing membrane ruptures. TIRF microscopy revealed that numerous organelles—including ER, post‑Golgi vesicles, late and early endosomes, and lysosomes—reside within 100 nm of the plasma membrane, yet only lysosomes undergo calcium‑dependent exocytosis. Elevated cytosolic calcium induced exocytosis exclusively of predocked lysosomes at the plasma membrane, showing that calcium triggers fusion rather than recruitment of lysosomes.

Abstract

Similar to its role in secretory cells, calcium triggers exocytosis in nonsecretory cells. This calcium-dependent exocytosis is essential for repair of membrane ruptures. Using total internal reflection fluorescence microscopy, we observed that many organelles implicated in this process, including ER, post-Golgi vesicles, late endosomes, early endosomes, and lysosomes, were within 100 nm of the plasma membrane (in the evanescent field). However, an increase in cytosolic calcium led to exocytosis of only the lysosomes. The lysosomes that fused were predominantly predocked at the plasma membrane, indicating that calcium is primarily responsible for fusion and not recruitment of lysosomes to the cell surface.