Lipid interconversions and endocytic machinery mediate secretory granule fusion and fission during their biogenesis
2Laboratory of Membrane Trafficking Mechanisms, Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Japan
3Departments of Pathology and of Microbiology and Immunology, Sean N. Parker Center for Allergy and Asthma Research, Stanford University School of Medicine, USA
Homotypic fusion of secretory granules (SGs) is an intermediate step in SG biogenesis in professional secretory cells, including endocrine and exocrine cells, and cells of the immune system, such as the mast cells (MCs). However, the precise molecular mechanisms that underlie these processes have only recently begun to clarify. Previously, we have shown that the small GTPase Rab5 regulates both homotypic fusion of the SGs and their heterotypic fusion with endosomes. Here we demonstrate that Rab5-stimulated activation of phosphatidylinositol 3 kinases (PI3Ks) and the consequent formation of PI(3,4,5)P3 are required for the activation of the non-receptor protein tyrosine phosphatase PTPN9. We show that PTPN9 is a critical player in this chain of events; first, by mediating the fusion of endosomes with autophagosomes, that then fuse with the SGs, as prerequisite for their subsequent homotypic fusion. Second, by mediating the homotypic fusion of the hybrid, LC3-positive SGs, a process that strictly depends on CD63-mediated apposition of PI4K. Finally, we show that MC SGs also undergo fission that is mediated by dynamin recruitment to SG PI(4,5)P2.Taken together, our results identify PI3 and PI4 lipid kinases, CD63 and PTPN9 as Rab5 downstream mediators of SG fusion during their biogenesis, and dynamin as a regulator of their fission. Our results also reveal that fusion and fission events, that control the size and number of the SGs during their biogenesis, are regulated by lipid interconversions.