DISTRIBUTION AND REGULATION OF THE CANDIDATE PROHORMONE PROCESSING ENZYMES SPC2 AND SPC3 IN ADULT-RAT BRAIN

A number of candidate mammalian prohormone processing enzymes related to the yeast Kex2 endoprotease have been cloned and demonstrated to cl eave several prohormone precursors at single, pairs and tetra basic am ino acid processing sites. We have mapped the distribution of the mRNA s encoding two of these endoproteases in adult rat brain. SPC3 message levels showed a more restricted distribution and generally lower leve ls than SPC2 transcripts. The highest levels of SPC2 mRNA were found i n the pyramidal cells of the hippocampus, several thalamic nuclei, the habenula and selected nuclei in the hypothalamus. SPC3 mRNA was most abundant in dentate gyrus granule cells, the habenula and selected hyp othalamic nuclei. In the hypothalamus overlapping and unique distribut ions of the two transcripts were seen in the paraventricular nucleus w ith SPC3 mRNA predominantly expressed in lateral magnocellular cells. Both SPC2 and SPC3 mRNA were upregulated in the paraventricular and su praoptic hypothalamic nuclei following chronic salt loading. Combined immunocytochemistry/in situ hybridization histochemistry demonstrated that SPC2 and SPC3 transcripts were both expressed in the vasopressine rgic subpopulation of magnocellular neurons in the supraoptic nucleus. SPC3 mRNA, but not SPC2 transcripts, also colocalized with immunoreac tive vasopressin-associated neurophysin in the suprachiasmatic nucleus . These results remain consistent with roles for SPC2 and SPC3 in the biosynthesis of neuropeptides and for a specific role for SPC3 in the processing of provasopressin. Increased levels of SPC2 and SPC3 transc ripts following a chronic osmotic stimulus suggests these proteases ar e coregulated with prohormone substrates and may be useful as an indic ator of peptidergic activity.