Insulin secretory dysfunction of the pancreatic ␤-cell in type-2 diabetes is thought to be due to defective nutrient sensing and/or deficiencies in the mechanism of insulin exocytosis. Previous studies have indicated that the GTP-binding protein, Rab3A, plays a mechanistic role in insulin exocytosis. Here, we report that Rab3A ؊/؊ mice develop fasting hyperglycemia and upon a glucose challenge show significant glucose intolerance coupled to ablated first-phase insulin release and consequential insufficient insulin secretion in vivo, without insulin resistance. The in vivo insulin secretory response to arginine was similar in Rab3A ؊/؊ mice as Rab3A ؉/؉ control animals, indicating a phenotype reminiscent of insulin secretory dysfunction found in type-2 diabetes. However, when a second arginine dose was given 10 min after, there was a negligible insulin secretory response in Rab3A ؊/؊ mice, compared with that in Rab3A ؉/؉ animals, that was markedly increased above that to the first arginine stimulus. There was no difference in ␤-cell mass or insulin production between Rab3A ؊/؊ and Rab3A ؉/؉ mice. However, in isolated islets, secretagogue-induced insulin release (by glucose, GLP-1, glyburide, or fatty acid) was ϳ60 -70% lower in Rab3A

The specific biochemical steps required for glucoseregulated insulin exocytosis from ␤-cells are not well defined. ] i increased. Inhibitors of PP2B specifically reduced the second, microtubule-dependent, phase of insulin secretion, suggesting that dephosphorylation of KHC was required for transport of ␤-granules from the storage pool to replenish the readily releasable pool of ␤-granules. This is distinct from synaptic vesicle exocytosis, because neurotransmitter release from synaptosomes did not require a Ca 2؉ -dependent KHC dephosphorylation. These results suggest a novel mechanism for regulating KHC function and ␤-granule transport in ␤-cells that is mediated by casein kinase 2 and PP2B. They also implicate a novel regulatory role for PP2B/calcineurin in the control of insulin secretion downstream of a rise in [Ca 2؉ ] i .