Biosynthesis, secretion, and regulation of non-classical neuropeptides

Abstract

Peptides perform many roles in cell-cell signaling; examples include neuropeptides, hormones, and growth factors. Although the vast majority of known neuropeptides are produced in the secretory pathway, a number of bioactive peptides are derived from cytosolic proteins. Evidence now suggests that these peptides are commonly found in cells and may be involved in biological function. The hemopressins are a family of peptides derived from alpha and beta hemoglobin which bind to the CB1 cannabinoid receptor, functioning as agonists or antagonists depending on the size of the peptide. We hypothesize that bioactive peptides, such as the hemopressins, are "non-classical neuropeptides.".;In order for the hemopressins and other peptides derived from cytosolic proteins to be considered neuropeptide-like signaling molecules, they must be synthesized in brain, secreted in levels sufficient to produce effects, and either their synthesis or secretion should be regulated. Utilizing a quantitative peptidomics technique to identify peptides within tissues, we have been able to address these criteria with particular focus on the hemopressins.;It has recently been shown that hemoglobin mRNA and protein are present in neurons and glia. To evaluate whether the hemopressins are synthesized in brain, we compared the hemoglobin-derived peptidome in brain, blood, and heart. Many hemoglobin-derived peptides are found only in brain and not in blood, suggesting that the peptides detected in brain are produced from brain hemoglobin and not erythrocytes. We also find that some cytosolic peptides, including RVD-hemopressin, are selectively secreted from acute mouse brain slices.;To assess the criterion of regulation, we find that the levels of hemopressin and other hemoglobin-derived peptides are elevated in the brain of CPEfat/fat mice and ischemic mice as compared to wild-type controls.;Taken together, our data suggest that the hemopressins are non-classical neuropeptides because they are synthesized in brain, secreted and regulated. Although it is not yet clear what enzymes are responsible for the formation of the cytosolic peptides, this data furthers the possibility that cytosolic peptides found in cells are not just breakdown products, but are rather important and functional attributes to the cell.