Carboxypeptidase E (CPE), also referred to as
enkephalin convertase or
carboxypeptidase H (EC 3.4.17.10), is present in
neurotransmitter secretory granules and can remove C-terminal basic residues following
endopeptidase cleavage during
peptide processing. Using in situ hybridization with 35S-labeled
oligonucleotide probes, we have mapped the localization of CPE
mRNA in the rat brain. Specificity for CPE was confirmed by control experiments, which included production of identical patterns hybridization with 3 different
antisense oligonucleotide probes, loss of label with
RNase pretreatment of sections or co-incubation with excess unlabeled probe, and lack of labeling with sense orientation probes. In addition, the regional distribution of CPE
mRNA by Northern blot analysis corresponded with distribution of labeling by in situ hybridization. The highest levels of CPE
mRNA were found to be present in the pyramidal cells of the hippocampus, the pituitary anterior and intermediate lobes, the ependymal cells of the lateral ventricle, the endopiriform nucleus, the basolateral amygdala, the supraoptic nucleus, and the paraventricular nucleus. Intermediate levels were present in the thalamus, medial geniculate nucleus, lateral septal nucleus, piriform and entorhinal cortex, nucleus of the tractus solitarius, cerebellar cortex, pontine nuclei, and inferior olive. The lowest levels were found in the hippocampal granule cell layer, lateral hypothalamus, globus pallidus, and brain stem reticular formation.
Ibotenic acid lesions of the hippocampus eliminated the majority of the label, which had been present over pyramidal cells, though labeling was increased over areas of reactive
gliosis, suggesting that activated astrocytes can also synthesize CPE
mRNA. In general, the localization of CPE
mRNA in the rat brain corresponded to the distribution of
enkephalin and other
peptide neurotransmitter-synthesizing neurons, though CPE
mRNA was also present in neurons that do not secrete known
peptides and in reactive glia. The widespread yet specific localization of CPE
mRNA in the rat brain suggests it may be an excellent marker for
peptide synthesizing cells in the CNS.