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Neuropeptides are vital for cell-cell communication and function in the regulation of the nervous and endocrine systems. They are generated by post-translational modification (PTM) steps resulting in small active peptides generated from prohormone precursors. Phosphorylation is a significant PTM for the bioactivity of neuropeptides. From the known diversity of distinct neuropeptide functions, it is hypothesized that the extent of phosphorylation varies among different neuropeptides. To assess this hypothesis, neuropeptide-containing dense core secretory vesicles from bovine adrenal medullary chromaffin cells were subjected to global phosphopeptidomics analyses by liquid chromatography (LC)-mass spectrometry (MS/MS). Phosphopeptides were identified directly by LC-MS/MS and indirectly by phosphatase treatment followed by LC-MS/MS. The data identified numerous phosphorylated peptides derived from neuropeptide precursors such as chromogranins, secretogranins, proenkephalin and pro-NPY. Phosphosite occupancies were observed at high and low levels among identified peptides and many of the high occupancy phosphopeptides represent prohormone-derived peptides with currently unknown bioactivities. Peptide sequence analyses demonstrated SxE as the most prevalent phosphorylation site motif, corresponding to phosphorylation sites of the Fam20C protein kinase known to be present in the secretory pathway. The range of high to low phosphosite occupancies for neuropeptides demonstrates cellular regulation of neuropeptide phosphorylation. Graphical Abstract ᅟ.

Diverse neuropeptides participate in cell–cell communication to coordinate neuronal and endocrine regulation of physiological processes in health and disease. Neuropeptides are short peptides ranging in length from ~3 to 40 amino acid residues that are involved in biological functions of pain, stress, obesity, hypertension, mental disorders, cancer, and numerous health conditions. The unique neuropeptide sequences define their specific biological actions. Significantly, this review article discusses how the neuropeptide field is at the crest of expanding knowledge gained from mass-spectrometry-based neuropeptidomic studies, combined with proteomic analyses for understanding the biosynthesis of neuropeptidomes. The ongoing expansion in neuropeptide diversity lies in the unbiased and global mass-spectrometry-based approaches for identification and quantitation of peptides. Current mass spectrometry technology allows definition of neuropeptide amino acid sequence structures, profiling of multiple neuropeptides in normal and disease conditions, and quantitative peptide measures in biomarker applications to monitor therapeutic drug efficacies. Complementary proteomic studies of neuropeptide secretory vesicles provide valuable insight into the protein processes utilized for neuropeptide production, storage, and secretion. Furthermore, ongoing research in developing new computational tools will facilitate advancements in mass-spectrometry-based identification of small peptides. Knowledge of the entire repertoire of neuropeptides that regulate physiological systems will provide novel insight into regulatory mechanisms in health, disease, and therapeutics.