Cultured Astrocytes and Neurons Synthesize and Secrete Carboxypeptidase E, a Neuropeptide‐Processing Enzyme

Abstract

Carboxypeptidase E (EC 3.4.17.10) is a carboxypeptidase B-like enzyme associated with the biosynthesis of many peptide hormones and neurotransmitters. Media collected from cultured astrocytes contain a carboxypeptidase E-like activity. Cultured astrocytes secrete approximately 73% of their cellular level of carboxypeptidase E per hour, and secretion is not substantially influenced by 35 mM KCl. In contrast, neurons secrete only 29% of their cellular carboxypeptidase E per hour, but secretion increases to 86% on stimulation with 35 mM KCl. Secretion of carboxypeptidase E activity from both neuronal and astrocyte cultures is relatively selective; neither acid phosphatase or acetylglucosaminidase is secreted in appreciable amounts. Cultured neurons and astrocytes express a carboxypeptidase E mRNA of a similar size. The levels of this mRNA differ in astrocytes cultured from different brain regions, with high levels in striatal, cortical, hippocampal, and hypothalamic astrocytes and low levels in cerebellar astrocytes. The level of carboxypeptidase E mRNA in hypothalamic astrocyte cultures is four- to fivefold higher than the level in hypothalamic neuronal cultures. These results indicate that cultured astrocytes express carboxypeptidase E mRNA and enzymatic activity and thus contain one of the enzymes required in the biosynthesis of many peptide hormones and neurotransmitters.

Dissecting carboxypeptidase E: properties, functions and pathophysiological roles in disease

“…These results suggest that CPE mRNA localization in the central nervous system corresponds to the distribution of neurotransmitter-synthesis neurons, further indicating the role of CPE in neuropeptide processing (58). In addition, CPE mRNA is localized in reactive glia as well, which is consistent with a study showing that cultured astrocytes are able to synthesize and secrete CPE (60). Since cultured astrocytes can express a number of neuropeptides, such as proenkephlin (60), somatostatin (61) and angiotensinogen (62), this result suggests that CPE may process neuropeptides in glia.…”

“…In addition, CPE mRNA is localized in reactive glia as well, which is consistent with a study showing that cultured astrocytes are able to synthesize and secrete CPE (60). Since cultured astrocytes can express a number of neuropeptides, such as proenkephlin (60), somatostatin (61) and angiotensinogen (62), this result suggests that CPE may process neuropeptides in glia. However, high level of CPE mRNA is also shown in some neurons not presently known to express specific neuropeptides (59), implying that in these cells CPE may have functions other than peptide processing, such as transporting neurotransmitter vesicles, or there may be undiscovered peptides processed in these regions.…”

Amyloid-β Impairs Vesicular Secretion in Neuronal and Astrocyte Peptidergic Transmission

“…Stimuli that evoked robust CPE and SgIII release in neurons barely provoked a response in astrocytes. In good agreement with a seminal work analyzing secreted CPE enzymatic activity from Fricker’s lab (Vilijn et al, 1989), we observed no response of released CPE and SgIII to elevated [KCl] o from astrocytes. In addition, increasing [Ca 2+ ] i by ionophores caused variable and weak release responses in glial cells.…”

Neprilysin 4, a novel endopeptidase fromDrosophila melanogaster, displays distinct substrate specificities and exceptional solubility states

“…A similar activity can also be assumed for Drosophila NEP4 as its potential to degrade neuropeptides (Fig.6), together with a highly specific expression in glia cells (Fig.5N,O) strongly indicate such a physiological relevance. This hypothesis is further corroborated by the general functions of the glia cell subtypes NEP4 is expressed in: while cell body associated glia are structurally similar to mammalian astrocytes (Freeman and Doherty, 2006), lateral glia cells are similar to oligodendrocytes (Stork et al, 2008), with both mammalian cell types reported to be responsible for ion and neurotransmitter homeostasis (Mentlein and Dahms, 1994;Vilijn et al, 1989;Stacey et al, 2007). As no system responsible for the reuptake of neuropeptides at the nerve terminal is known in Drosophila, the biological activity of these transmitters is presumably controlled by extracellular degradation.…”