The effect of prolyl oligopeptidase inhibitors on alpha-synuclein aggregation and autophagy cannot be predicted by their inhibitory efficacy

Missense variants in CMS22 patients reveal that PREPL has both enzymatic and nonenzymatic functions

Congenital myasthenic syndrome-22 (CMS22, OMIM 616224) is a rare genetic disorder caused by deleterious genetic variation in the prolyl endopeptidase-like (PREPL) gene. Previous reports have described patients with deletions and nonsense variants in PREPL, but nothing is known about the effect of missense variants in the pathology of CMS22. In this study, we have functionally characterized missense variants in PREPL from 3 patients with CMS22, all with hallmark phenotypes. Biochemical evaluation revealed that these missense variants do not impair hydrolase activity, thereby challenging the conventional diagnostic criteria and disease mechanism. Structural analysis showed that the variants affect regions most likely involved in intraprotein or protein-protein interactions. Indeed, binding to a selected group of known interactors was differentially reduced for the 3 variants. The importance of nonhydrolytic functions of PREPL was investigated in catalytically inactive PREPL p.Ser559Ala cell lines, which showed that hydrolytic activity of PREPL is needed for normal mitochondrial function but not for regulating AP1-mediated transport in the transgolgi network. In conclusion, these studies showed that CMS22 can be caused not only by deletion and truncation of PREPL but also by missense variants that do not necessarily result in a loss of hydrolytic activity of PREPL.

Study of the Conformational Dynamics of Prolyl Oligopeptidase by Mass Spectrometry: Lessons Learned

Ion mobility mass spectrometry (IM-MS) can be used to analyze native proteins according to their size and shape. By sampling individual molecules, it allows us to study mixtures of conformations, as long as they have different collision cross sections and maintain their native conformation after dehydration and vaporization in the mass spectrometer. Even though conformational heterogeneity of prolyl oligopeptidase has been demonstrated in solution, it is not detectable in IM-MS. Factors that affect the conformation in solution, binding of an active site ligand, the stabilizing Ser554Ala mutation, and acidification do not qualitatively affect the collision-induced unfolding pattern. However, measuring the protection of accessible cysteines upon ligand binding provides a principle for the development of MS-based ligand screening methods.

Prolyl Endopeptidase Is Involved in Filaggrinolysis and Cornification

FLG is a well-known biomarker of atopic dermatitis and skin dryness. Its full proteolysis (or filaggrinolysis) produces the major constituents of the natural moisturizing factor. Some proteases/peptidases remain to be identified in this multistep process. Mining 16 omics analyses, we identified prolyl endopeptidase (PREP) as a candidate peptidase. Indirect immunofluorescence and confocal analysis demonstrated its localization in the granular and deep cornified layers, where it colocalized with FLG. Tandem mass spectroscopy and fluorescent quenching activity assays showed that PREP cleaved several synthetic peptides derived from the FLG sequence, at the carboxyl side of an internal proline. Deimination of these peptides increased PREP enzymatic efficiency. Specific inhibition of PREP in reconstructed human epidermis using benzyloxycarbonyl-pro-prolinal induced the accumulation of FLG monomers. Downregulation of PREP expression in reconstructed human epidermis using RNA interference confirmed the impact of PREP on FLG metabolism and highlighted a more general role of PREP in keratinocyte differentiation. Indeed, quantitative global proteomic, western blotting, and RT-qPCR analyses showed a strong reduction in the expression of bleomycin hydrolase, known to be involved in filaggrinolysis, and of several other actors of cornification such as loricrin. Consequently, at the functional level, the transepidermal electric resistance was drastically reduced.

Discovery of a Tunable Heterocyclic Electrophile 4-Chloro-pyrazolopyridine That Defines a Unique Subset of Ligandable Cysteines

Electrophilic small molecules with novel reactivity are powerful tools that enable activity-based protein profiling and covalent inhibitor discovery. Here, we report a reactive heterocyclic scaffold, 4-chloro-pyrazolopyridine (CPzP) for selective modification of proteins via a nucleophilic aromatic substitution (SNAr) mechanism. Chemoproteomic profiling reveals that CPzPs engage cysteines within functionally diverse protein sites including ribosomal protein S5 (RPS5), inosine monophosphate dehydrogenase 2 (IMPDH2), and heat shock protein 60 (HSP60). Through the optimization of appended recognition elements, we demonstrate the utility of CPzP for covalent inhibition of prolyl endopeptidase (PREP) by targeting a noncatalytic active-site cysteine. This study suggests that the proteome reactivity of CPzPs can be modulated by both electronic and steric features of the ring system, providing a new tunable electrophile for applications in chemoproteomics and covalent inhibitor design.

5-Aminothiazoles Reveal a New Ligand-Binding Site on Prolyl Oligopeptidase Which is Important for Modulation of Its Protein-Protein Interaction-Derived Functions

A series of novel 5-aminothiazole-based ligands for prolyl oligopeptidase (PREP) comprise selective, potent modulators of the protein-protein interaction (PPI)-mediated functions of PREP, although they are only weak inhibitors of the proteolytic activity of PREP. The disconnected structure-activity relationships are significantly more pronounced for the 5-aminothiazole-based ligands than for the earlier published 5-aminooxazole-based ligands. Furthermore, the stability of the 5-aminothiazole scaffold allowed exploration of wider substitution patterns than that was possible with the 5-aminooxazole scaffold. The intriguing structure-activity relationships for the modulation of the proteolytic activity and PPI-derived functions of PREP were elaborated by presenting a new binding site for PPI modulating PREP ligands, which was initially discovered using molecular modeling and later confirmed through point mutation studies. Our results suggest that this new binding site on PREP is clearly more important than the active site of PREP for the modulation of its PPI-mediated functions.

VHL-Modified PROteolysis TArgeting Chimeras (PROTACs) as a Strategy to Evade Metabolic Degradation in In Vitro Applications

PROteolysis TArgeting Chimeras (PROTACs) are tripartite molecules consisting of a linker connecting a ligand for a protein of interest to an E3 ligase recruiter, whose rationale relies on proteasome-based protein degradation. PROTACs have expanded as a therapeutic strategy to open new avenues for unmet medical needs. Leveraging our expertise, we undertook a series of in vitro experiments aimed at elucidating PROTAC metabolism. In particular, we focused on PROTACs recruiting the von Hippel-Lindau (VHL) E3 ligase. After high-resolution mass spectrometry measurements, a characteristic metabolite with mass reduction of 200 units was detected and successively confirmed as a product deriving from the cleavage of the VHL ligand moiety. Subsequently, we identified hepatic and extrahepatic prolyl endopeptidases as the main putative metabolic enzymes involved. Finally, we designed and synthesized analogs of the VHL ligands that we further exploited for the synthesis of novel VHL-directed PROTACs with an improved metabolic stability in in vitro applications.

The Prolyl Oligopeptidase Inhibitor KYP-2047 Is Cytoprotective and Anti-Inflammatory in Human Retinal Pigment Epithelial Cells with Defective Proteasomal Clearance

Increased oxidative stress, dysfunctional cellular clearance, and chronic inflammation are associated with age-related macular degeneration (AMD). Prolyl oligopeptidase (PREP) is a serine protease that has numerous cellular functions, including the regulation of oxidative stress, protein aggregation, and inflammation. PREP inhibition by KYP-2047 (4-phenylbutanoyl-L-prolyl1(S)-cyanopyrrolidine) has been associated with clearance of cellular protein aggregates and reduced oxidative stress and inflammation. Here, we studied the effects of KYP-2047 on inflammation, oxidative stress, cell viability, and autophagy in human retinal pigment epithelium (RPE) cells with reduced proteasomal clearance. MG-132-mediated proteasomal inhibition in ARPE-19 cells was used to model declined proteasomal clearance in the RPEs of AMD patients. Cell viability was assessed using LDH and MTT assays. The amounts of reactive oxygen species (ROS) were measured using 2',7'-dichlorofluorescin diacetate (H2DCFDA). ELISA was used to determine the levels of cytokines and activated mitogen-activated protein kinases. The autophagy markers p62/SQSTM1 and LC3 were measured with the western blot method. MG-132 induced LDH leakage and increased ROS production in the ARPE-19 cells, and KYP-2047 reduced MG-132-induced LDH leakage. Production of the proinflammatory cytokine IL-6 was concurrently alleviated by KYP-2047 when compared with cells treated only with MG-132. KYP-2047 had no effect on autophagy in the RPE cells, but the phosphorylation levels of p38 and ERK1/2 were elevated upon KYP-2047 exposure, and the inhibition of p38 prevented the anti-inflammatory actions of KYP-2047. KYP-2047 showed cytoprotective and anti-inflammatory effects on RPE cells suffering from MG-132-induced proteasomal inhibition.

A prolyl oligopeptidase inhibitor reduces tau pathology in cellular models and in mice with tauopathy

Tauopathies are neurodegenerative diseases that are characterized by accumulation of hyperphosphorylated tau protein, higher-order aggregates, and tau filaments. Protein phosphatase 2A (PP2A) is a major tau dephosphorylating phosphatase, and a decrease in its activity has been demonstrated in tauopathies, including Alzheimer's disease. Prolyl oligopeptidase is a serine protease that is associated with neurodegeneration, and its inhibition normalizes PP2A activity without toxicity under pathological conditions. Here, we assessed whether prolyl oligopeptidase inhibition could protect against tau-mediated toxicity in cellular models in vitro and in the PS19 transgenic mouse model of tauopathy carrying the human tau-P301S mutation. We show that inhibition of prolyl oligopeptidase with the inhibitor KYP-2047 reduced tau aggregation in tau-transfected HEK-293 cells and N2A cells as well as in human iPSC-derived neurons carrying either the P301L or tau-A152T mutation. Treatment with KYP-2047 resulted in increased PP2A activity and activation of autophagic flux in HEK-293 cells and N2A cells and in patient-derived iNeurons, as indicated by changes in autophagosome and autophagy receptor markers; this contributed to clearance of insoluble tau. Furthermore, treatment of PS19 transgenic mice for 1 month with KYP-2047 reduced tau burden in the brain and cerebrospinal fluid and slowed cognitive decline according to several behavioral tests. In addition, a reduction in an oxidative stress marker was seen in mouse brains after KYP-2047 treatment. This study suggests that inhibition of prolyl oligopeptidase could help to ameliorate tau-dependent neurodegeneration.

The biological role of prolyl oligopeptidase and the procognitive potential of its peptidic inhibitors from food proteins

Prolyl oligopeptidase (POP) is a conserved serine protease belonging to proline-specific peptidases. It has both enzymatic and non-enzymatic activity and is involved in numerous biological processes in the human body, playing a role in e.g., cellular growth and differentiation, inflammation, as well as the development of some neurodegenerative and neuropsychiatric disorders. This article describes the physiological and pathological aspects of POP activity and the state-of-art of its peptidic inhibitors originating from food proteins, with a particular focus on their potential as cognition-enhancing agents. Although some milk, meat, fish, and plant protein-derived peptides have the potential to be applied as natural, procognitive nutraceuticals, their effectiveness requires further evaluation, especially in clinical trials. We demonstrated that the important features of the most promising POP-inhibiting peptides are very short sequence, high content of hydrophobic amino acids, and usually the presence of proline residue.

Cross-talk between α-synuclein and the microtubule cytoskeleton in neurodegeneration

Looking at the puzzle that depicts the molecular determinants in neurodegeneration, many pieces are lacking and multiple interconnections among key proteins and intracellular pathways still remain unclear. Here we focus on the concerted action of α-synuclein and the microtubule cytoskeleton, whose interplay, indeed, is emerging but remains largely unexplored in both its physiology and pathology. α-Synuclein is a key protein involved in neurodegeneration, underlying those diseases termed synucleinopathies. Its propensity to interact with other proteins and structures renders the identification of neuronal death trigger extremely difficult. Conversely, the unbalance of microtubule cytoskeleton in terms of structure, dynamics and function is emerging as a point of convergence in neurodegeneration. Interestingly, α-synuclein and microtubules have been shown to interact and mediate cross-talks with other intracellular structures. This is supported by an increasing amount of evidence ranging from their direct interaction to the engagement of in-common partners and culminating with their respective impact on microtubule-dependent neuronal functions. Last, but not least, it is becoming even more clear that α-synuclein and tubulin work synergically towards pathological aggregation, ultimately resulting in neurodegeneration. In this respect, we supply a novel perspective towards the understanding of α-synuclein biology and, most importantly, of the link between α-synuclein with microtubule cytoskeleton and its impact for neurodegeneration and future development of novel therapeutic strategies.

MiR-17-5p Targets and Downregulates CADM2, Activating the Malignant Phenotypes of Colon Cancer Cells

Accumulating studies have demonstrated that CADM2 modulated malignant phenotype of various cancer cells, while its regulatory function and mechanism have not yet been reported. In this study, qRT-PCR was utilized to measure CADM2 mRNA level in normal cells and colon cancer cells, also, IHC and WB were applied to detect CADM2 protein expression in colon tissues, exhibiting low mRNA and protein levels of CADM2 in colon cancer. Applying cell function experiments, the impacts of CADM2 on colon cell phenotypes were examined, and the results illustrated that upregulating CADM2 remarkably repressed proliferation, invasion, migration, cell cycle of colon cancer cells, and facilitated cell apoptosis. Thus, it could be considered that CADM2 served as a tumor repressor gene in colon cancer. Moreover, the outcomes of dual-luciferase assay displayed that miR-17-5p could target CADM2, and overexpressing miR-17-5p could notably inhibit the mRNA and protein expression levels of CADM2. We, therefore, assumed that CADM2 was a downstream target of miR-139-5p. qRT-PCR was conducted to assess miR-17-5p level in colon cancer cells and normal cells, verifying a high miR-17-5p expression in the cancer cells. The effects of miR-17-5p on colon cell phenotypes were examined as well, where we determined that miR-17-5p served as a tumor-promoting factor. Finally, the rescue experiments exhibited that miR-17-5p could activate tumor-promoting phenotypes, while such activating effects could be reversed by upregulating CADM2. In short, the study proved that miR-17-5p facilitated malignant progression of colon cancer through targeting CADM2 at a post-transcriptional level. Our findings offer new insight into molecular therapy of colon cancer patients.

Inhibition-mediated changes in prolyl oligopeptidase dynamics possibly related to α-synuclein aggregation

The formation of protein aggregates is one of the leading causes of neuronal malfunction and subsequent brain damage in many neurodegenerative diseases. In Parkinson's disease, α-synucleins are involved in the accumulation of aggregates. The origin of aggregation is unknown, but there is convincing evidence that it can be reduced by prolyl oligopeptidase (PREP) inhibition. This effect cannot simply be related to the inhibition of the enzyme's catalytic function since not all PREP inhibitors stop α-synuclein aggregation. Finding differences in the dynamics of the enzyme inhibited by different compounds would allow us to identify the protein regions involved in the interaction between PREP and α-synuclein. Here, we investigate the effects of three PREP inhibitors, each of which affects α-synuclein aggregation to a different extent. We use molecular dynamics modelling to identify the molecular mechanisms underlying PREP inhibition and find structural differences between inhibitor-PREP systems. We suggest that even subtle variations in enzyme dynamics affect its interactions with α-synucleins. Our identification of these regions may therefore be biologically relevant in preventing α-synuclein aggregate formation.

Therapeutic Effect of Novel Cyanopyrrolidine-Based Prolyl Oligopeptidase Inhibitors in Rat Models of Amnesia

Prolyl oligopeptidase (POP) is a large cytosolic serine peptidase that is altered in patients with Alzheimer’s disease, Parkinsonian syndrome, muscular dystrophies, and other denervating diseases. Thus, POP may represent a relevant therapeutic target for treatment of neuropsychiatric disorders and neurodegenerative diseases. Here, we report the characterization of five novel cyanopyrrolidine-based compounds (BocTrpPrdN, BocGlyPrdN, CbzMetPrdN, CbzGlnPrdN, and CbzAlaPrdN) and show that they are potent inhibitors of POP and are predicted to penetrate the blood-brain barrier (BBB). Indeed, we show that CbzMetPrdN penetrates the rat BBB and effectively inhibits POP in the brain when administered intraperitoneally. Furthermore, molecular modeling confirmed these compounds likely inhibit POP via interaction with the POP catalytic site. We evaluated protective effects of the cyanopyrrolidine-based POP inhibitors using scopolamine- and maximal electroshock-induced models of amnesia in rats and showed that BocTrpPrdN, BocGlyPrdN, CbzMetPrdN, and CbzGlnPrdN significantly prolonged conditioned passive avoidance reflex (CPAR) retention time when administered intraperitoneally (1 and 2 mg/kg) before evaluation in both models of amnesia, although CbzAlaPrdN was not effective in scopolamine-induced amnesia. Our data support previous reports on the antiamnesic effects of prolinal-based POP inhibitors and indicate an important role of POP in the regulation of learning and memory processes in the CNS.

Increased Expression of Prolyl Endopeptidase Induced by Oxidative Stress in Nucleus Pulposus Cells Aggravates Intervertebral Disc Degeneration

A healthy microenvironment of the intervertebral disc tissue is characterized by hypoxia owing to its sparse vascular distribution. Oxidative stress plays a pivotal role in the pathological development of intervertebral disc degeneration (IVDD). We found that the expression of prolyl endopeptidase (PREP) increased in degenerative nucleus pulposus (NP) tissues. The purpose of this study was to determine whether PREP is involved in oxidative-stress-induced IVDD. Tertbutyl hydroperoxide can inhibit the expression of PREP by activating the PI3K/AKT signaling pathway at low concentrations in NP cells. Knockdown of PREP protected NP cells from apoptosis induced by oxidative stress, whereas overexpression of PREP exacerbated the apoptosis of NP cells. We also investigated the connection between the PI3K/AKT signaling pathway and PREP and found that the activation of the PI3K/AKT signaling pathway downregulated the expression of PREP by inhibiting p53. As a crucial transcription factor, p53 binds to the PREP promoter region and promotes its transcription. Overexpression of PREP also impairs protein secretion in the extracellular matrix of NP cells. Furthermore, the in vivo knockout of PREP could attenuate puncture-induced IVDD. These findings suggested that the downregulation of PREP might maintain the viability of NP cells and attenuate IVDD under oxidative stress.

Autophagy-Lysosomal Pathway as Potential Therapeutic Target in Parkinson's Disease

Cellular quality control systems have gained much attention in recent decades. Among these, autophagy is a natural self-preservation mechanism that continuously eliminates toxic cellular components and acts as an anti-ageing process. It is vital for cell survival and to preserve homeostasis. Several cell-type-dependent canonical or non-canonical autophagy pathways have been reported showing varying degrees of selectivity with regard to the substrates targeted. Here, we provide an updated review of the autophagy machinery and discuss the role of various forms of autophagy in neurodegenerative diseases, with a particular focus on Parkinson's disease. We describe recent findings that have led to the proposal of therapeutic strategies targeting autophagy to alter the course of Parkinson's disease progression.

Inhibition of prolyl oligopeptidase: A promising pathway to prevent the progression of age-related macular degeneration

Dry age-related macular degeneration (AMD) is a currently untreatable vision threatening disease. Impaired proteasomal clearance and autophagy in the retinal pigment epithelium (RPE) and subsequent photoreceptor damage are connected with dry AMD, but detailed pathophysiology is still unclear. In this paper, we discover inhibition of cytosolic protease, prolyl oligopeptidase (PREP), as a potential pathway to treat dry AMD. We showed that PREP inhibitor exposure induced autophagy in the RPE cells, shown by increased LC3-II levels and decreased p62 levels. PREP inhibitor treatment increased total levels of autophagic vacuoles in the RPE cells. Global proteomics was used to examine the phenotype of a commonly used cell model displaying AMD characteristics, oxidative stress and altered protein metabolism, in vitro. These RPE cells displayed induced protein aggregation and clear alterations in macromolecule metabolism, confirming the relevance of the cell model. Differences in intracellular target engagement of PREP inhibitors were observed with cellular thermal shift assay (CETSA). These differences were explained by intracellular drug exposure (the unbound cellular partition coefficient, Kp_uu). Importantly, our data is in line with previous observations regarding the discrepancy between PREP's cleaving activity and outcomes in autophagy. This highlights the need to further explore PREP's role in autophagy so that more effective compounds can be designed to battle diseases in which autophagy induction is needed. The present work is the first report investigating the PREP pathway in the RPE and we predict that the PREP inhibitors can be further optimized for treatment of dry AMD.

2-Imidazole as a Substitute for the Electrophilic Group Gives Highly Potent Prolyl Oligopeptidase Inhibitors

Different five-membered nitrogen-containing heteroaromatics in the position of the typical electrophilic group in prolyl oligopeptidase (PREP) inhibitors were investigated and compared to tetrazole. The 2-imidazoles were highly potent inhibitors of the proteolytic activity. The binding mode for the basic imidazole was studied by molecular docking as it was expected to differ from the acidic tetrazole. A new putative noncovalent binding mode with an interaction to His680 was found for the 2-imidazoles. Inhibition of the proteolytic activity did not correlate with the modulating effect on protein–protein-interaction-derived functions of PREP (i.e., dimerization of alpha-synuclein and autophagy). Among the highly potent PREP inhibiting 2-imidazoles, only one was also a potent modulator of PREP-catalyzed alpha-synuclein dimerization, indicating that the linker length on the opposite side of the molecule from the five-membered heteroaromatic is critical for the disconnected structure–activity relationships.

Prolyl oligopeptidase inhibition reduces alpha-synuclein aggregation in a cellular model of multiple system atrophy

Multiple system atrophy (MSA) is a fatal neurodegenerative disease where the histopathological hallmark is glial cytoplasmic inclusions in oligodendrocytes, rich of aggregated alpha‐synuclein (aSyn). Therefore, therapies targeting aSyn aggregation and toxicity have been studied as a possible disease‐modifying therapy for MSA. Our earlier studies show that inhibition of prolyl oligopeptidase (PREP) with KYP‐2047 reduces aSyn aggregates in several models. Here, we tested the effects of KYP‐2047 on a MSA cellular models, using rat OLN‐AS7 and human MO3.13 oligodendrocyte cells. As translocation of p25α to cell cytosol has been identified as an inducer of aSyn aggregation in MSA models, the cells were transiently transfected with p25α. Similar to earlier studies, p25α increased aSyn phosphorylation and aggregation, and caused tubulin retraction and impaired autophagy in OLN‐AS7 cells. In both cellular models, p25α transfection increased significantly aSyn mRNA levels and also increased the levels of inactive protein phosphatase 2A (PP2A). However, aSyn or p25α did not cause any cellular death in MO3.13 cells, questioning their use as a MSA model. Simultaneous administration of 10 µM KYP‐2047 improved cell viability, decreased insoluble phosphorylated aSyn and normalized autophagy in OLN‐AS7 cells but similar impact was not seen in MO3.13 cells.

Deletion or inhibition of prolyl oligopeptidase blocks lithium-induced phosphorylation of GSK3b and Akt by activation of protein phosphatase 2A

Alterations in prolyl oligopeptidase (PREP) activity have been connected, for example, with bipolar and major depressive disorder, and several studies have reported that lack or inhibition of PREP blocks the effects of lithium on inositol 1,4,5-triphosphate (IP₃ ) levels. However, the impact of PREP modulation on other intracellular targets of lithium, such as glycogen synthase kinase 3 beta (GSK3b) or protein kinase B (Akt), has not been studied. We recently found that PREP regulates protein phosphatase 2A (PP2A), and because GSK3b and Akt are PP2A substrates, we studied if PREP-related lithium insensitivity is dependent on PP2A. To assess this, HEK-293 and SH-SY5Y cells with PREP deletion or PREP inhibition (KYP-2047) were exposed to lithium, and thereafter, the phosphorylation levels of GSK3b and Akt were measured by Western blot. As expected, PREP deletion and inhibition blocked the lithium-induced phosphorylation on GSK3b and Akt in both cell lines. When lithium exposure was combined with okadaic acid, a PP2A inhibitor, KYP-2047 did not have effect on lithium-induced GSK3b and Akt phosphorylation. Therefore, we conclude that PREP deletion or inhibition blocks the intracellular effects of lithium on GSK3b and Akt via PP2A activation.

Prolyl oligopeptidase inhibition reduces oxidative stress via reducing NADPH oxidase activity by activating protein phosphatase 2A

Oxidative stress (OS) is a common toxic feature in various neurodegenerative diseases. Therefore, reducing OS could provide a potential approach to achieve neuroprotection. Prolyl oligopeptidase (PREP) is a serine protease that is linked to neurodegeneration, as endogenous PREP inhibits autophagy and induces the accumulation of detrimental protein aggregates. As such, inhibition of PREP by a small-molecular inhibitor has provided neuroprotection in preclinical models of neurodegenerative diseases. In addition, PREP inhibition has been shown to reduce production of reactive oxygen species (ROS) and the absence of PREP blocks stress-induced ROS production. However, the mechanism behind PREP-related ROS regulation is not known. As we recently discovered PREP's physiological role as a protein phosphatase 2A (PP2A) regulator, we wanted to characterize PREP inhibition as an approach to reduce OS. We studied the impact of a PREP inhibitor, KYP-2047, on hydrogen peroxide and ferrous chloride induced ROS production and on cellular antioxidant response in HEK-293 and SH-SY5Y cells. In addition, we used HEK-293 and SH-SY5Y PREP knock-out cells to validate the role of PREP on stress-induced ROS production. We were able to show that absence of PREP almost entirely blocks the stress-induced ROS production in both cell lines. Reduced ROS production and smaller antioxidant response was also seen in both cell lines after PREP inhibition by 10 μM KYP-2047. Our results also revealed that the OS reducing mechanism of PREP inhibition is related to reduced activation of ROS producing NADPH oxidase through enhanced PP2A activation. In conclusion, our results suggest that PREP inhibition could also provide neuroprotection by reducing OS, thus broadening the scope of its beneficial effects on neurodegeneration.

Prolyl Endopeptidase-Like Facilitates the α-Synuclein Aggregation Seeding, and This Effect Is Reverted by Serine Peptidase Inhibitor PMSF

The aggregation of α-synuclein (α-Syn) is a characteristic of Parkinson’s disease (PD). α-Syn oligomerization/aggregation is accelerated by the serine peptidase, prolyl oligopeptidase (POP). Factors that affect POP conformation, including most of its inhibitors and an impairing mutation in its active site, influence the acceleration of α-Syn aggregation resulting from the interaction of these proteins. It is noteworthy, however, that α-Syn is not cleaved by POP. Prolyl endopeptidase-like (PREPL) protein is structurally related to the serine peptidases belonging to the POP family. Based on the α-Syn–POP studies and knowing that PREPL may contribute to the regulation of synaptic vesicle exocytosis, when this protein can encounter α-Syn, we investigated the α-Syn–PREPL interaction. The binding of these two human proteins was observed with an apparent affinity constant of about 5.7 μM and, as in the α-Syn assays with POP, the presence of PREPL accelerated the oligomerization/aggregation events, with no α-Syn cleavage. Furthermore, despite this lack of hydrolytic cleavage, the serine peptidase active site inhibitor phenylmethylsulfonyl fluoride (PMSF) abolished the enhancement of the α-Syn aggregation by PREPL. Therefore, given the attention to POP inhibitors as potential drugs to treat synucleinopathies, the present data point to PREPL as another potential target to be explored for this purpose.