Synthesis of fusogenic lipids through activation of phospholipase D₁ by GTPases and the kinase RSK2 is required for calcium-regulated exocytosis in neuroendocrine cells

Exocytosis of hormones occurs through the fusion of large dense-core secretory vesicles with the plasma membrane. This highly regulated process involves key proteins such as SNAREs (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptors) and also specific lipids at the site of membrane fusion. Among the different lipids required for exocytosis, our recent observations have highlighted the crucial role of PA (phosphatidic acid) in the late stages of membrane fusion in various exocytotic events. An RNAi (RNA interference) strategy coupled with the detection of PA in living cells has pointed to plasma membrane-associated PLD1 (phospholipase D₁) as the main producer of PA in response to secretagogue stimulation. We have identified several GTPases which regulate the activation level of PLD₁ in neuroendocrine cells. Finally, RSK2 (ribosomal S6 kinase 2) appears to phosphorylate and regulate the activity of PLD₁ in a calcium-dependent manner. Altogether our results have unravelled a complex set of regulatory pathways controlling the synthesis of fusogenic lipids at the secretory granule fusion site by PLD₁.