All cells secrete proteins constitutively, but specialized polypeptide- secreting neuroendocrine cells have developed an additional pathway of storage of peptide hormones in granules whose exocytosis is acutely regulated by specific physiologic stimuli. Since storage granules are enriched in proteins different from those secreted constitutively and those concentrated in lysosomes, professional secretory cells must internally distinguish these different classes of proteins by processes that are collectively known as molecular sorting. The vertebrate phylum in general, and man in particular, have adapted peptide-hormone secretion as a major line of defense against disturbances in metabolic balance; thus it appears that the molecular sorting of polypeptides in endocrine cells, coupled with the signalling that triggers secretagogue action, represent the precision tuning upon which endocrine homeostasis is based. In an array of human and animal diseases including certain known genetic mutations, alterations in molecular sorting of the above classes of molecules are associated with abnormalities ranging from subtle phenotypic defects to lack of viability of the organism. It is evident that in order to elucidate the underlying basis for these sorting defects, we must have a working knowledge of the molecular mechanisms involved in sorting processes. However, up to now, there has been great controversy over conflicting hypotheses created to explain how peptide hormones are normally targeted and concentrated in secretory granules. In a nutshell, these views can be divided into a """"""""sorting for entry"""""""" hypothesis in which proteins are pre-selected in the Golgi complex for entry into storage granules, versus a """"""""sorting by retention"""""""" hypothesis in which unsorted content proteins enter forming granules as a soluble mixture; then non- stored proteins are selectively (and non-selectively) removed while regulated secretory proteins are retained by condensation into an insoluble core. Although the prevailing scientific view has been that regulated secretory proteins are sorted for entry into the storage granule pathway, this grant application takes the point of view of, and intends to further examine, the sorting-by-retention hypothesis. A review of background information as well as several important preliminary findings lay the groundwork and establish the feasibility of novel experiments whose outcome could cause us to revise substantially the way we think about how peptide hormones are sorted and stored.
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