Transient phospholipid asymmetry as a regulatory switch coupling exocytosis to compensatory endocytosis.

Plasma membrane phospholipid asymmetry is usually described as a homeostatic arrangement that preserves surface identity. In secretory cells, however, this arrangement is briefly and locally relaxed when intracellular calcium rises. Evidence from adrenal chromaffin cells and cerebellar synapses now places this transient remodeling at a specific point in the vesicle cycle: it is largely dispensable for membrane fusion, yet it is required for efficient compensatory endocytosis. In both systems, loss of phospholipid scramblase 1 (PLSCR1) prevents stimulus-dependent phosphatidylserine exposure without substantially reducing the initial exocytic response. The consequences emerge during membrane retrieval, with defective recapture of secretory-granule membrane proteins in chromaffin cells, delayed synaptic-vesicle endocytosis in neurons, and a failure to sustain neurotransmission during high-frequency activity. These findings shift the emphasis from phosphatidylserine exposure as a passive correlate of secretion to lipid asymmetry as a dynamic regulatory variable. Several non-exclusive mechanisms may account for this effect. Scrambling may relax leaflet stress, lower membrane stiffness, accelerate lateral diffusion, preserve the identity of recently fused vesicular membrane, and reorganize electrostatic interactions that govern actin and endocytic protein recruitment. At the same time, the molecular status of PLSCR1 remains unsettled. Its direct lipid-translocating activity has not been established in intact cells, and it may instead act through oligomerization, an unidentified transporter, or a signaling scaffold. This review integrates the neuroendocrine and synaptic evidence, identifies the strongest mechanistic inferences, and outlines experiments needed to distinguish lipid transport from lipid-dependent signaling in the coupling of exocytosis to endocytosis.