Lysosomes Are the Major Vesicular Compartment Undergoing Ca2+-Regulated Exocytosis from Cortical Astrocytes

摘要

Although Ca²⁺-dependent exocytosis is considered to be a pathway for gliotransmitter release from astrocytes, the structural and functional bases of this process remain controversial. We studied the relationship between near-membrane Ca²⁺ elevations and the dynamics of single astroglial vesicles with styryl (FM) dyes. We show that cultured astrocytes, unlike neurons, spontaneously internalize FM dyes, resulting in the labeling of the entire acidic vesicle population within minutes. Interestingly, metabotropic glutamate receptor activation did not affect the FM labeling. Most FM-stained vesicles expressed sialin, CD63/LAMP3, and VAMP7, three markers for lysosomes and late endosomes. A subset of lysosomes underwent asynchronous exocytosis that required both Ca²⁺ mobilization from intracellular stores and Ca²⁺ influx across the plasma membrane. Lysosomal fusion occurred within seconds and was complete with no evidence for kiss and run. Our experiments suggest that astroglial Ca²⁺-regulated exocytosis is carried by lysosomes and operates on a timescale orders of magnitude slower than synaptic transmission.