Regulation of prolyl endopeptidase activity by the intracellular redox state

Prolyl Oligopeptidase Regulates Dopamine Transporter Oligomerization and Phosphorylation in a PKC- and ERK-Independent Manner

Prolyl oligopeptidase (PREP) is a serine protease that binds to alpha-synuclein (aSyn) and induces its aggregation. PREP inhibitors have been shown to have beneficial effects in Parkinson's disease models by enhancing the clearance of aSyn aggregates and modulating striatal dopamine. Additionally, we have shown that PREP regulates phosphorylation and internalization of dopamine transporter (DAT) in mice. In this study, we clarified the mechanism behind this by using HEK-293 and PREP knock-out HEK-293 cells with DAT transfection. We tested the effects of PREP, PREP inhibition, and alpha-synuclein on PREP-related DAT regulation by using Western blot analysis and a dopamine uptake assay, and characterized the impact of PREP on protein kinase C (PKC) and extracellular signal-regulated kinase (ERK) by using PKC assay and Western blot, respectively, as these kinases regulate DAT phosphorylation. Our results confirmed our previous findings that a lack of PREP can increase phosphorylation and internalization of DAT and decrease uptake of dopamine. PREP inhibition had a variable impact on phosphorylation of ERK dependent on the metabolic state of cells, but did not have an effect on phosphorylation or function of DAT. PREP modifications did not affect PKC activity either. Additionally, a lack of PREP elevated a DAT oligomerization that is associated with intracellular trafficking of DAT. Our results suggest that PREP-mediated phosphorylation, oligomerization, and internalization of DAT is not dependent on PKC or ERK.

Issues about the physiological functions of prolyl oligopeptidase based on its discordant spatial association with substrates and inconsistencies among mRNA, protein levels, and enzymatic activity

Prolyl oligopeptidase (POP) is a serine endopeptidase that hydrolyses proline-containing peptides shorter than 30 amino acids. POP may be associated with cognitive functions, possibly via the cleavage of neuropeptides. Recent studies have also suggested novel non-hydrolytic and non-catalytic functions for POP. Moreover, POP has also been proposed as a regulator of inositol 1,4,5-triphosphate signaling and several other functions such as cell proliferation and differentiation, as well as signal transduction in the central nervous system, and it is suspected to be involved in pathological conditions such as Parkinson's and Alzheimer's diseases and cancer. POP inhibitors have been developed to restore the depleted neuropeptide levels encountered in aging or in neurodegenerative disorders. These compounds have shown some antiamnesic effects in animal models. However, the mechanisms of these hypothesized actions are still far from clear. Moreover, the physiological role of POP has remained unknown, and a lack of basic studies, including its distribution, is obvious. The aim of this review is to gather information about POP and to propose some novel roles for this enzyme based on its distribution and its discordant spatial association with its best known substrates.

On the role of prolyl oligopeptidase in health and disease

Prolyl oligopeptidase (POP) is a serine peptidase which digests small peptide-like hormones, neuroactive peptides, and various cellular factors. Therefore, this peptidase has been implicated in many physiological processes as well as in some psychiatric disorders, most probably through interference in inositol cycle. Intense research has been performed to elucidate, on the one hand, the basic structure, ligand binding, and kinetic properties of POP, and on the other, the pharmacology of its inhibitors. There is fairly strong evidence of in vivo importance of POP on substance P, arginine vasopressin, thyroliberin and gonadoliberin metabolism. However, information about the biological relevance of POP is not yet conclusive. Evidence regarding the physiological role of POP is lacking, which is surprising considering that peptidase inhibitors have been exploited for drug development, some of which are currently in clinical trials as memory enhancers for the aged and in a variety of neurological disorders. Here we review the recent progress on POP research and evaluate the relevance of the peptidase in the metabolism of various neuropeptides. The recognition of novel forms and relatives of POP may improve our understanding of how this family of proteins functions in normal and in neuropathological conditions.

Pharmacogenetics in model systems: defining a common mechanism of action for mood stabilisers

Defining the underlying causes of psychiatric disorders has provided an ongoing and intractable problem. The analysis of the genetic basis of manic depression, in particular, has been impeded by the absence of a suitable model system and by the lack of candidate causative genes. One recent approach to overcome these problems has involved identifying those genes which control the sensitivity to anti-manic drugs in a model organism. Characterisation of the role of these genes and their encoded proteins in this model has allowed the analysis of their mammalian homologues to elucidate the therapeutic role of these drugs and the possible aetiology of manic depression. This approach has been used successfully with the cellular slime mould, Dictyostelium discoideum. This article introduces the use of model systems for pharmacogenetics research. It describes the identification of prolyl oligopeptidase in D. discoideum as a modulator of inositol phosphate signalling, and the subsequent identification of a common mechanism of action of three anti-manic drugs in mammalian neurons. The use of pharmacogenetics in model systems will provide a powerful tool for the ongoing analysis of both the treatment and cause of psychiatric disorders.

Effect of Aging on Rat Tissue Peptidase Activities

The process of aging is known to involve alterations in the activity of peptidases and proteases. However, the precise changes in the activity of many peptidases in aged tissues have not yet been fully characterized, and both decreases and increases in both peptidase activity and peptide levels have been reported to occur during the aging process. In the present study, we measured the activity of several peptidases in selected tissues (brain cortex, brain stem, liver, kidney, heart, and lung) of the young adult (3 months old) and aged (18 months old and 22 months old) rat. The activities of prolyl endopeptidase, pyroglutamyl peptidase I, puromycin sensitive aminopeptidase, and aminopeptidase N were assayed using beta-naphthylamine aminoacidic derivatives as substrates. The activity of the soluble fractions of prolyl endopeptidase was found to be reduced in the lungs of aged animals, while reduced activity of soluble pyroglutamyl peptidase I and also aminopeptidase N was measured in the aged kidney and heart, respectively. In contrast, increased activity of particulate prolyl endopeptidase was measured in the brain stem of older animals. Since most of these changes can be correlated with known alterations in the levels of peptides controlled by each enzyme, the results of the present study indicate that the studied peptidases may play an important role in regulating tissue peptide levels during aging.

Affected enzyme activities in Alzheimer's disease are sensitive to antemortem hypoxia

Many enzyme activities in Alzheimer's disease (AD) are changed. Some of these enzyme activities are related to certain neurotransmitter systems. Enzymes in the brain can also be sensitive to antemortem hypoxia. In the present study it was determined if enzyme activities that are altered in AD are also subject to alteration by antemortem hypoxia. As an indicator of antemortem hypoxia brain lactate concentration was used. Enzyme activities measured were those of prolyl endopeptidase (PE), aminopeptidase (AP), phosphatidylinositol (PI) kinase, phosphatidylinositol phosphate kinase, alpha-ketoglutarate dehydrogenase (alpha-KGDH), choline acetyltransferase and beta-glucuronidase. All of these enzyme activities have been measured in AD patients before and several of them have been found to be decreased. In accordance with previous findings, PE, alpha-KGDH and ChAT activities were reduced in AD patients. PI kinase and beta-glucuronidase activities, however, were not reduced, contrary to previous findings. All enzyme activities, except that of beta-glucuronidase, correlated with brain lactate concentration, suggesting that antemortem hypoxia has a major influence on the activity of enzymes in the brain. PE, AP, alpha-KGDH and ChAT activities were still different between AD and control samples when these were matched for lactate concentration. The enzyme activities that were changed in AD were also significantly correlated with lactate concentration, an indicator of antemortem hypoxia, in brain specimens. This suggests that antemortem hypoxia and AD have some factor in common that may be responsible for changes in enzyme activities. Since both PE and alpha-KGDH are known to be sensitive to oxidative stress this factor could be oxidative stress.

Regulation of prolyl oligopeptidase activity in regenerating rat liver

We have previously shown that the naturally occurring polyamines, spermidine and spermine, reverse effectively the in vitro inhibition of prolyl oligopeptidase (POPase) by its endogenous inhibitor by forming a kinetically significant complex (Soeda et al., J. Neurochem. (1986) 46, 1304-1307). In this study, we examined changes in the activities of POPase and its endogenous inhibitor and in the concentrations of polyamines during the regeneration of rat liver. POPase activity in the liver cytosol peaked 2 days after partial hepatectomy and then decreased near to control activity by 9 days, without its altered synthetic levels. Total polyamine concentrations also peaked at 2 days and remained elevated by 9 days, while cytosolic POPase inhibitor activity was minimal (56% of control) at 2 days. Treatment of the animals with a synthetic POPase inhibitor, Z-Gly-Pro-CHN2 (4 mg/kg), resulted in an obvious suppression of the liver regeneration. These results imply that the activity of POPase involved in nonlysosomal proteolytic pathway is exquisitely regulated by changes not only in its endogenous inhibitor levels but also in intracellular cationic potentials such as polyamines, and that POPase plays a crucial role for the growth and differentiation of liver cell.

Trinucleotide repeats in neurologic diseases: an hypothesis concerning the pathogenesis of Huntington's disease, Kennedy's disease, and spinocerebellar ataxia type I

Three neurodegenerative diseases, Huntington's disease (HD), Kennedy's disease (hereditary spinobulbar muscular atrophy, SBMA), and type 1 spinocerebellar ataxia (SCA-1) have been found to share a common genetic defect: an unstable region of repeated CAG trinucleotides which are thought to be translated into a polyglutamine moiety. The unstable repeat regions occur near the N-termini of the predicted proteins for HD and SBMA, but the location of the CAG repeat region is not known for SCA-1. Each disease is notable for a relatively circumscribed region of central nervous system pathology, and the lack of predicted similarity of the abnormal proteins makes a common mechanism related to the function of each protein unlikely. In order to reconcile the similar genetic abnormalities with the disparities in phenotypes, we suggest a common thread with regard to the pathogenesis of neuronal death. We hypothesize that the mechanism of neurotoxicity in these diseases occurs not through the production of abnormal proteins, but by the generation of abnormal posttranslational cleavage products. These products, in part consisting of abnormally large polyglutamine moieties, act to disturb the cellular and mitochondrial milieu such that energy metabolism is impaired, rendering specific regions of the nervous system vulnerable, and resulting in the clinical phenotypes of HD, SBMA, and SCA-1. We offer this interpretation of recent genetic findings from a neurobiologic perspective, in addition to suggesting testable hypotheses concerning potential disease mechanisms.