Peptides play many important physiological roles in most organisms. Neuropeptides and peptide hormones function in cell-cell signaling and are involved with a wide variety of biological functions including feeding and body weight regulation, fear, anxiety, pain, circadian rhythms, memory, reward mechanisms, and many others. Recently, we have developed a method to isolate and quantify peptides from mouse tissues. This method has led to the identification of a large number of novel peptides. Many of the novel peptides are produced from cytosolic proteins, and not from secretory pathway proteins that are the precursors of classical neuropeptides. Some of the peptides derived from cytosolic proteins are secreted and bind to extracellular receptors; these are putative “nonclassical” neuropeptides, a novel class of cell-cell signaling molecule. Further studies are aimed at understanding the mechanisms by which these peptides are produced, secreted, and regulated.
We are also interested in the enzymes that produce neuropeptides. Peptide processing enzymes include endopeptidases which initially cleave the peptide precursors at specific sites (usually pairs of the basic amino acids lysine and arginine), and then carboxypeptidases which selectively remove the basic amino acids remaining on the C-terminus. One carboxypeptidase responsible for the formation of many peptide hormones and neurotransmitters is carboxypeptidase E. We identified a strain of mouse (named fat/fat) that does not produce active carboxypeptidase E due to a point mutation; these mice are obese, sterile, hyperglycemic, and have neurological impairments.
In addition to neuropeptide processing enzymes, several other cellular peptidases are being studied in the laboratory. Current projects use peptidomics and other techniques to identify the physiological function of the peptidase. One of the enzymes being studied is cytosolic carboxypeptidase 1; mice lacking this enzyme show abnormal movement due to neurodegeneration of cerebellar Purkinje cells. Another enzyme currently being studied is carboxypeptidase A6; humans with mutations in this enzyme develop epilepsy. We are studying the role of carboxypeptidase A6 in animal models, with a focus on understanding how mutations in the protein lead to epilepsy.