SNAP-25 IS EXPRESSED IN ISLETS OF LANGERHANS AND IS INVOLVED IN INSULIN RELEASE

SNAP-25 is known as a neuron specific molecule involved in the fusion of small synaptic vesicles with the presynaptic plasma membrane. By im munolocalization and Western blot analysis, it is now shown that SNAP- 25 is also expressed in pancreatic endocrine cells. Botulinum neurotox ins (BoNT) A and E were used to study the role of SNAP-25 in insulin s ecretion. These neurotoxins inhibit transmitter release by cleaving SN AP-25 in neurons. Cells from a pancreatic B cell line (HIT) and primar y rat islet cells were permeabilized with streptolysin-O to allow toxi n entry. SNAP-25 was cleaved by BoNT/A and BoNT/E, resulting in a mole cular mass shift of similar to 1 and 3 kD, respectively. Cleavage was accompanied by an inhibition of Ca++-stimulated insulin release in bot h cell types. In HIT cells, a concentration of 30-40 nM BoNT/E gave ma ximal inhibition of stimulated insulin secretion of similar to 60%, co inciding with essentially complete cleavage of SNAP-25. Half maximal e ffects in terms of cleavage and inhibition of insulin release were obt ained at a concentration of 5-10 nM. The A type toxin showed maximal a nd half-maximal effects at concentrations of 4 and 2 nM, respectively. In conclusion, the results suggest a role for SNAP-25 in fusion of de nse core secretory granules with the plasma membrane in an endocrine c ell type-the pancreatic B cell.