Stathmin-2 mediates paracrine hormone regulation of glucagon through lysosomal trafficking in αTC1-6 cells.

The secretion of glucagon from the pancreatic alpha (α) cell within the islets of Langerhans is physiologically regulated by nutrients (glucose, amino acids, fatty acids), neurotransmitters and paracrine hormones. Insulin and somatostatin form an intra-islet paracrine network to control glucagon secretion through direct inhibitory effects on α cell secretory granule exocytosis. In a potential new cellular pathway for the regulation of glucagon secretion, we have previously identified the neuronal trafficking protein Stathmin-2 (Stmn2) as a negative regulator of glucagon trafficking and secretion by directing glucagon to degradative lysosomes. In this study, we examined whether insulin and somatostatin direct glucagon to lysosomes in a Stmn2-dependent manner as part of their paracrine mechanisms. Using the αTC1-6 glucagon-secreting cell line and confocal microscopy of both fixed and live cells, we show that insulin and somatostatin direct glucagon, glucagon+LAMP1+ vesicles, and LAMP1-RFP to the intracellular region, away from sites of exocytosis. As visualized in live cells, insulin treatment resulted in the rapid retrograde transport of lysosomes from the cell periphery, and this effect was lost under siRNA-mediated silencing of Stmn2. Somatostatin appeared to enhance the intracellular retention of lysosomes, also in a Stmn2-dependent manner. We determined a possible mechanism for Stmn2 in the regulation of lysosome transport in αTC1-6 cells through the Arf-like small GTPase Arl8, indicating that Stmn2 may function in lysosomal positioning along microtubules. We propose that Stmn2-mediated lysosomal transport may be a potential new pathway through which insulin and somatostatin regulate glucagon secretion in the pancreatic α cell. KEY POINTS: Glucagon is produced by the pancreatic islet alpha cell to maintain blood glucose levels, and its secretion is tightly regulated by the islet hormones, insulin and somatostatin. Insulin and somatostatin are known to inhibit glucagon secretory granule exocytosis. In this study, we report a novel pathway through which insulin and somatostatin may inhibit glucagon secretion by directing glucagon to lysosomes through the actions of stathmin-2, a negative regulator of glucagon secretion. We show that insulin directs glucagon and lysosomes away from sites of exocytosis in a stathmin-2-dependent manner, while somatostatin retains lysosomes intracellularly in a stathmin-2-dependent manner. These results demonstrate a new pathway by which insulin and somatostatin regulate glucagon secretion, furthering our understanding of alpha cell physiology.