Carboxypeptidase E/NFα1: A New Neurotrophic Factor against Oxidative Stress-Induced Apoptotic Cell Death Mediated by ERK and PI3-K/AKT Pathways

Niamh X. Cawley,Yong Cheng,Y. Peng Loh,Michal Hetman

doi:10.1371/journal.pone.0071578

Abstract

Mice lacking Carboxypeptidase E (CPE) exhibit degeneration of hippocampal neurons caused by stress at weaning while over-expression of CPE in hippocampal neurons protect them against hydrogen peroxide-induced cell death. Here we demonstrate that CPE acts as an extracellular trophic factor to protect neurons. Rat hippocampal neurons pretreated with purified CPE protected the cells against hydrogen peroxide-, staurosporine- and glutamate-induced cell death. This protection was observed even when hippocampal neurons were treated with an enzymatically inactive mutant CPE or with CPE in the presence of its inhibitor, GEMSA. Purified CPE added to the culture medium rescued CPE knock-out hippocampal neurons from cell death. Both ERK and AKT were phosphorylated within 15 min after CPE treatment of hippocampal neurons and, using specific inhibitors, both signaling pathways were shown to be required for the neuroprotective effect. The expression of the anti-apoptotic protein, B-cell lymphoma 2 (BCL-2), was up-regulated after hippocampal neurons were treated with CPE. Furthermore, hydrogen peroxide induced down-regulation of BCL-2 protein and subsequent activation of caspase-3 were inhibited by CPE treatment. Thus, this study has identified CPE as a new neurotrophic factor that can protect neurons against degeneration through the activation of ERK and AKT signaling pathways to up-regulate expression of BCL-2.

Highlights

Neurological diseases such as Alzheimer’s disease and Parkinson’s disease, as well as various types of stress including excess glucocorticoids, glutamate neurotoxicity and ischemia lead to neuronal cell death [1,2,3,4]
Our previous study showed that transduction of carboxypeptidase E (CPE) into primary cultured hippocampal neurons protected them against H2O2 induced neurotoxicity [17]
Our findings identified a new role for CPE, as a neurotrophic factor that functions extracellularly to protect hippocampal neurons against oxidative stressinduced, staurosporine-induced and glutamate-induced apoptotic cell death and is consistent with the observation that CPE is secreted from neurons (Fig. 1A and [19])

Summary

Introduction

Neurological diseases such as Alzheimer’s disease and Parkinson’s disease, as well as various types of stress including excess glucocorticoids, glutamate neurotoxicity and ischemia lead to neuronal cell death [1,2,3,4]. Recent studies have suggested that carboxypeptidase E (CPE) is involved in neuroprotection [5]. CPE was first discovered as an enkephalin convertase in 1982 [6,7] and was subsequently found to be the enzyme that cleaves the Cterminally extended basic residues from peptide intermediates in endocrine cells and neuropeptides in peptidergic neurons (for review see [8]). Various non-enzymatic roles of CPE have been found. CPE acts as a sorting receptor to target proneuropeptides and pro-brain-derived neurotrophic factor (proBDNF) to the regulated secretory pathway [9,10]. The cytoplasmic tail of CPE mediates BDNF vesicle transport [11] and synaptic vesicle localization to the nerve terminal preactive zone [12]

Methods

Results

Conclusion