SNAP-25 regulation during adrenal gland development: Comparison with differentiation markers and other SNAREs

Synaptosomal-associated protein of 25 kDa (SNAP-25) is one of a limited num ber of soluble N-ethylmaleimide-sensitive fusion attachment protein recepto rs (SNAREs) that play a major role in membrane docking of synaptic vesicles and secretory granules during regulated exocytosis. We have previously sho wn that SNAP-25 levels differ between noradrenergic and adrenergic chromaff in cell populations of the adult adrenal gland. We examine SNAP-25 expressi on by immunofluoresence in cells of the sympathoadrenal lineage in the rat during late embryonic and postnatal development. In parallel, tyrosine hydr oxylase was used to identify sympathoadrenal cells, phenylethanolamine N-me thyltransferase to distinguish adrenergic from noradrenergic chromaffin cel ls, and chromogranin A to define the presence of secretory granules. In add ition, SNAP-25 protein and mRNA levels were followed in adrenal gland extra cts by immunoblotting and reverse transcription-polymerase chain reaction ( RT-PCR). Protein levels were compared with those of other molecules also im plicated in organelle trafficking, including syntaxin 1 and vesicle-associa ted membrane protein (VAMP-S) and the nonneuronal analogues SNAP-23 and cel lubrevin. This study provides evidence that SNAP-25 is expressed early duri ng development in sympathoadrenal neurons and migrating cells. It is detect ed in intra-adrenal chromoblasts as soon as they enter the adrenal primordi um. Its differential expression between catecholamine chromaffin cell pheno types is already evident from the 17th embryonic day, future noradrenergic cells appearing to express higher levels than adrenergic cells. The granule maturation marker chromogranin A is expressed in chromaffin cells later th an SNAP-25. Both SNAP-25 protein and mRNA increased rapidly in the adrenal gland in the perinatal period to peak during the first postnatal week, afte r which levels dropped dramatically to adult values. In contrast, levels of both syntaxin and SNAP-23 appeared to remain fairly constant throughout ad renal gland development. VAMP-2 expression increased gradually around birth to reach maximal levels during the first two postnatal weeks, and then dec reased slightly. Cellubrevin levels also appeared to increase gradually unt il adult values were attained by the end of the second postnatal week. The threefold increase of SNAP-25 mRNA shortly after birth compared to the low adult levels suggests that during this period SNAP-25 is implicated in addi tional functions than regulated secretion, possibly associated with cellula r growth or maturation. J. Comp. Neurol. 421:533-542, 2000. (C) 2000 Wiley- Liss, Inc.