The secretory vesicle organelle is the primary site for neuropeptide biosynthesis. It is, therefore, important to understand the functional protein systems that allow neuropeptide to be produced. Knowledge of the secretory vesicle proteins and in vivo
intravesicular protein conditions via proteomic approaches can advance our understanding of neuropeptide biosynthetic mechanisms. Recent examination of proteins in model chromaffin secretory vesicles revealed several functional protein categories that together support secretory vesicle production of neuropeptides and bioactive catecholamines for cell-cell communication () (121
Proteomics reveals functional secretory vesicle protein systems for neuropeptide biosynthesis, storage, and secretion
Chromaffin granules represent model secretory vesicles for the produce, store, and secrete active enkephalin and related neuropeptides that function as peptide hormones and neurotransmitters for cell-cell communication. Protein systems involved in vesicular neuropeptide biosynthesis were examined in proteomic studies of soluble and membrane fractions of dense core secretory vesicles purified from neuroendocrine chromaffin cells. Proteins were separated by SDS-PAGE, and proteins from systematically sectioned gel lanes were identified by microcapillary LC-MS/MS (μLC-MS/MS) of tryptic peptides (121
). Proteomic results revealed functional categories of prohormones, proteases, catecholamine neurotransmitter metabolism, protein folding, redox regulation, ATPases, calcium regulation, signaling components, exocytotic mechanisms, and related functions. Several novel secretory vesicle components involved in proteolysis were identified consisting of cathepsin B, cathepsin D, cystatin C, ubiquitin, and TIMP, as well carboxypeptidase E/H and proprotein convertases that are known to participate in prohormone processing. Significantly, the membrane fraction exclusively contained an extensive number of GTP nucleotide-binding proteins related to Rab, Rho, and Ras signaling molecules, together with SNARE-related proteins and annexins that are involved in trafficking and exocytosis of secretory vesicle components. Membranes also preferentially contained ATPases that regulate proton translocation. These results implicate membrane-specific functions for signaling and exocytosis that allow secretory vesicles to produce, store, and secrete active peptide hormones and neurotransmitters released from adrenal medulla for the control of physiological functions.
The protein systems utilized in these chromaffin vesicles, representing dense core secretory vesicles (121
), resemble those of brain synaptic vesicles (122
). Proteomic studies provides inference for secretory vesicle protein systems utilized for functions of these vesicles including their biogenesis (121
) that are required for production of enkephalin and related neuropeptides in brain and endocrine tissues.
Secretory vesicles at synaptic nerve terminals in brain are essential for chemical neurotransmission among neurons. Proteomic studies of synaptic proteins have revealed their regulation by brain injury (126
), brain-derived neurotrophic factor (BDNF) (127
), as well as drug regulation by morphine (128
). The protein systems that support secretory vesicle exocytosis of peptide neurotransmitters and receptor activation at synaptic junctions of neurons function in concert to achieve neuropeptide-mediated communication in neural circuits.