Distribution and quantification of neuropeptide processing enzyme mRNA in the hippocampus of postmortem depressed subjects
Waselus M, Medina AM, Turner CA, Bunney WE, Myers RM, Schatzberg AF, Barchas JD, Lee FS, Akil H, Watson SJ Jr
Society for Neuroscience. 2018.
Neuropeptides are one of a number of substrates that have been implicated in the pathophysiology of major depressive disorder (MDD). Unlike the biosynthesis of classical neurotransmitters, neuropeptides undergo a series of processing steps to release active neuropeptide(s) from their larger, inactive precursors. The first enzymatic step in this process is carried out by proprotein/prohormone convertases (PCs), which typically cleave inactive peptide precursors C-terminal to paired basic amino acid residues (e.g., Lys/Arg). Two members of this family, PC1/3 and PC2, are stored in secretory granules and act on peptide precursors processed within the regulatory secretory pathway. We have previously shown that PC1/3 and PC2 expression levels are decreased at the level of the anterior hippocampus of MDD subjects when compared to non-psychiatric control subjects. At the level of the posterior hippocampus, MDDs had decreased 7B2 (a modulator of PC2) expression compared to controls, and together with findings in the anterior hippocampus suggest a dysregulation in the initial peptide processing steps in MDD. Here, we extend these studies to examine expression of two additional peptide processing molecules that function subsequent to the PCs in the generation of active neuropeptides: carboxypeptidase E (CPE) and peptidyl-glycine alpha-amidating monooxygenase (PAM). CPE is the primary carboxypeptidase involved in the biosynthesis of regulated secretory pathway neuropeptides, is broadly distributed in the brain and functions in mature secretory granules to remove C-terminal basic residues on intermediate cleavage products previously generated by PCs. PAM is a bifunctional molecule and sole known enzyme responsible for peptide amidation which is required for full functionality of around half of known neuropeptides. In this study, frozen 10µm sections through the hippocampus of MDD and non-psychiatric control subjects were processed for in situ hybridization using radiolabeled cRNA probes for CPE or PAM. Quantitative measurements of CPE or PAM mRNA expression using ImageJ are currently ongoing in the anterior and posterior hippocampal formation (HF). Initial evaluation of the distribution of CPE and PAM mRNA indicates that both are expressed in all regions of the HF, with CPE more abundantly expressed than PAM, consistent with previous studies carried out in the rat. Together with our previous findings, we anticipate that differences in the expression of CPE and/or PAM will contribute to a more thorough understanding of the molecular and anatomical specificity underlying the dysregulation of neuropeptides in the postmortem HF of depressed individuals.