Brain function relies on synaptic vesicle (SV) exocytosis and endocytosis. These cell physiological processes are crucial for neurotransmission. Ca2+ triggers the exocytic fusion of synaptic vesicles, which leads to the release of neurotransmitter. To sustain neurotransmission, SV exocytosis must be spatiotemporally coupled to a qualitatively and quantitatively corresponding retrieval of membrane and SV proteins via compensatory endocytosis. Yet, it is unknown how neurons balance SV exo- and endocytosis to maintain presynaptic membrane homeostasis und thereby sustain brain function. This study identifies Synaptotagmin 1 (Syt1), the main Ca2+ sensor and key factor for synchronous neurotransmission, as a homeostatic, post-fusion trigger for compensatory endocytosis. Severe conserved defects in neurotransmission are caused by genetic loss, mutation, or acute inactivation of Syt1. As an SV protein, Syt1 interacts via its two C2 domains with proteins of both, the exo- and endocytic machinery, and furthermore mediates neurotransmission upon binding to charged phospholipids of the membrane. We demonstrate that Syt1 couples SV exocytosis and compensatory endocytosis by triggering the local, activity-dependent synthesis of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] at presynaptic sites. Exocytosed Syt1 on the neuronal plasma membrane recruits phosphatidylinositol 4-phosphate [PI(4)P] 5-kinase Iγ (PIPKIγ), the main PI(4,5)P2 synthesizing enzyme at the synapse. The Syt1-dependent recruitment of PIPKIγ results in elevated levels of presynaptic PI(4,5)P2. As an important signaling lipid and driving force for SV endocytosis, we detect Syt1-dependent facilitation of SV endocytosis. Genetic interference with Syt1/ PIPKIγ complex formation selectively impairs PI(4,5)P2-triggered SV endocytosis but not exocytic SV fusion. We show the Syt1-triggered synthesis of signaling lipids to couple SV exo- and endocytosis across a wide range of physiological stimulation paradigms. Considering Syt1 and PI(4,5)P2 being associated with various physiological as well as pathophysiological processes, we predict similar mechanisms to couple fusion and retrieval in other cell types undergoing regulated secretion.
