Distribution of proSAAS-derived peptides in rat neuroendocrine tissues

Using a technique to identify substrates of the peptide processing enzyme carboxypeptidase E (CPE), several novel peptides were detected in the brain and pituitary of Cpe^fat/Cpe^fat mice and found to be derived from a single precursor, termed proSAAS. In order to gain further information regarding the potential physiological roles of these peptides, we have examined the distribution of two proSAAS-derived peptides, ARPVKEPRSLSAASAPLAETSTPLRL (SAAS) and LENSSPQAPARRLLPP (LEN), in rat neuroendocrine tissues using immunohistochemistry. Both peptides are detected throughout the brain, with the highest concentrations of SAAS peptide in the hypothalamus. In the hippocampus, both peptides are co-localized with prohormone convertase 1 in the dentate gyrus and CA1-3 region. In cerebellum, SAAS peptide is co-localized with prohormone convertase 1 in Purkinje and granular cells, whereas LEN is much more abundant in the Purkinje cells relative to the granular cells. Similarly, SAAS and prohormone convertase 1 are co-localized in the dorsal horn of the spinal cord, while LEN is mainly restricted to fibers of the white matter. In the pituitary, SAAS, LEN, and prohormone convertase 1 are detected in all three lobes. In the pancreas, SAAS, LEN, and prohormone convertase 1 are only detected in the islets, although the peptides are enriched in the peripheral cells (alpha and/or delta) while prohormone convertase 1 is only expressed in the inner cells (beta). Both SAAS and LEN are present in the adrenal medulla along with prohormone convertase 1. Taken together, these data are consistent with the proposed role for proSAAS as an endogenous inhibitor of prohormone convertase 1 in many, but not all cell types. However, the broader localization of the peptides allows for the possibility that they perform additional functions.