Dual functions of dynamin in beta cell endocytosis and autophagy trafficking as a membrane fission molecule.

Abstract

Pancreatic β-cell exocytosis has been a central research theme for decades, but its endocytosis remains less known. Our work has investigated β-cell endocytosis by focusing on a subfamily of dynamin proteins and demonstrated their crucial role in regulating glucose-stimulated insulin secretion (GSIS) (J Clin Invest. 2015, PNAS 2021). Among three mammalian dynamin genes, DNM2 and DNM3 express abundantly in β cells of mice and humans, where they regulate endocytosis in a gene dose-dependent manner. Unlike neurons, most β-cells after tamoxifen-induced triple-dynamin knockout (TKO) are viable, which contrasts with the notion of the housekeeping role of dynamin and suggests an unappreciated role of dynamin-independent endocytosis. DNM2KO impairs the 2nd GSIS phase selectively, and TKO reduces both phases. Moreover, our recent experiments uncover dynamin's role in autophagy. LC3 (microtubule-associated protein 1A/1B light chain 3) staining reveals disrupted autophagic structures in KO β-cells, and these enlarged LC3+-structures are autolysosomes (ALS) rather than other autophagic intermediates as they are lamp1-positive. Starvation exacerbates the defect. Under TIRFM imaging, dynamin-2-EGFP can be recruited onto ALS to induce its fission, as evidenced by transient dynamin-signal increases at ALS fission sites right before ALS fission. This is done by expressing dynamin-2-EGFP in TKO β-cells, which provides a valuable platform for effectively monitoring dynamin dynamics/function because this method removes unlabeled endogenous dynamin molecules to compete with dynamin-2-EGFP. These new results indicate that dynamin regulates β-cell autophagy, in part by directly binding ALS, where it may act as a membrane remodeling and fission protein to transform large ALS into lysosomes. Together, these studies suggest that dynamin is a master regulator of β-cell membrane trafficking in both endocytosis and autophagy, and both processes involve the same intrinsic feature of dynamin as a membrane fission GTPase.