CALCIUM-DEPENDENT REGULATED SECRETION IS CONTROLLED BY THE GTPASE RAB3 IN NEUROENDOCRINE CELLS

Changes in intracellular free calcium concentration ([Ca2+]i) play a f undamental role in the regulation of hormone secretion by endocrine an d neuronal cells. The increase in [Ca2+]i is one of the principle even ts in stimulus-secretion coupling. Secretory phenomena are subjected t o modulation via a large number of intracellular factors which can act on vesicular transport, exocytosis or endocytosis. Among these factor s are proteins able to bind and hydrolyse the nucleotide GTP belonging to the ras superfamily of small GTPases (or superfamily of small G pr oteins). GTPases of the Sec4/Ypt1/Rab family have been implicated in t he regulation of intracellular vesicle traffic. A role for Rab3 in reg ulating secretion was initially proposed because of its specific expre ssion in the nervous and endocrine systems, and its association with s ecretory vesicles. The function of Rab3 has been studied in two types of neuroendocrine cells: the anterior pituitary lactotroph cells and t he adrenal medulla chromaffin cells. Two techniques were combined: int racellular injection of antisense oligonucleotides and recombinant pro teins, followed by membrane capacitance measurements. The inhibition o f Rab3b synthesis in lactotroph cells by antisense oligonucleotides pr ovokes an inhibition of exocytosis. On the other hand, injection of an tisense oligonucleotides directed against rab3a into chromaffin cells led to a stimulation of the secretory response to successive depolariz ations of the cell membrane. These results indicate that Rab3 proteins control the calcium-dependent secretory process in neuroendocrine cel ls. In lactotroph cells Rab3b seems to play a stimulatory role, wherea s Rab3a acts as a negative regulator of catecholamine release in chrom affin cells.