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

Whey-based sports supplements: Heat damage and protein breakdown after in vitro gastrointestinal digestion

This study was aimed to evaluate the effect of heat damage on the release of total amino acids (AA), essential AA (EAA), branched-chain AA (BCAA) and bioactive peptides following in vitro static simulated gastrointestinal digestion (SGID) of four commercial whey-protein based sports supplements. The extent of protein glycation and denaturation was evaluated through the determination of the content of furosine and soluble whey proteins. The strongest protein breakdown (41.3 %) and the highest release of AA, EAA and BCAA (36.20, 27.78, and 11.30 g/100 g protein, respectively) was observed in the sports supplement characterised by the lowest (52.5 %) level of soluble whey proteins; whereas the protein glycation had a negligible impact on the studied parameters. The SGID also led to the release of several peptides with various reported bioactivities that may be beneficial to sports activity.

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.

Exploring bi-carbazole-linked triazoles as inhibitors of prolyl endo peptidase via integrated in vitro and in silico study

A serine protease called prolyl endopeptidase (PEP) hydrolyses the peptide bonds on the carboxy side of the proline ring. The excessive PEP expression in brain results in neurodegenerative illnesses like dementia, Alzheimer's disease, and Parkinson's disease. Results of the prior studies on antioxidant activity, and the non-cytotoxic effect of bi-carbazole-linked triazoles, encouraged us to extend our studies towards its anti-diabetic potential. Hence, for this purpose all compounds 1-9 were evaluated to reveal their anti-prolyl endo peptidase activity. Fortunately, seven compounds resulted into significant inhibitory capability ranging from 26 to 63 µM. Among them six compounds 4-9 exhibited more potent inhibitory activity with IC50 values 46.10 ± 1.16, 42.30 ± 1.18, 37.14 ± 1.21, 26.29 ± 0.76, 28.31 ± 0.64 and 31.11 ± 0.84 µM respectively, while compound 3 was the least active compound in the series with IC50 value 63.10 ± 1.58 µM comparing with standard PEP inhibitor bacitracin (IC50 = 125 ± 1.50 µM). Moreover, mechanistic study was performed for the most active compounds 7 and 8 with Ki values 24.10 ± 0.0076 and 23.67 ± 0.0084 µM respectively. Further, the in silico studies suggested that the compounds exhibited potential interactions and significant molecular conformations, thereby elucidating the structural basis for their inhibitory effects.

Pharmacological Potential of Bioactive Peptides for the Treatment of Diseases Associated with Alzheimer's and Brain Disorders

Bioactive peptides are a promising class of therapeutics for the treatment of diseases associated with Alzheimer's and brain disorders. These peptides are derived from naturally occurring proteins and have been shown to possess a variety of beneficial properties. They may modulate neurotransmitter systems, reduce inflammation, and improve cognitive performance. In addition, bioactive peptides have the potential to target specific molecular pathways involved in the pathogenesis of Alzheimer's and brain disorders. For example, peptides have been shown to interact with amyloid-beta, a major component of amyloid plaques found in Alzheimer's disease, and have been shown to reduce its accumulation in the brain. Furthermore, peptides have been found to modulate the activity of glutamate receptors, which are important for memory and learning, as well as to inhibit the activity of enzymes involved in the formation of toxic amyloid-beta aggregates. Finally, bioactive peptides have the potential to reduce oxidative stress and inflammation, two major components of many neurological disorders. These peptides could be used alone or in combination with traditional pharmacological treatments to improve the management of diseases associated with Alzheimer's and brain disorders.

Thymus transplantation regulates blood pressure and alleviates hypertension-associated heart and kidney damage via transcription factors FoxN1 pathway

Previous studies have found that thymus is involved in the process of hypertension. However, whether thymus transplantation alleviates target organ damage in hypertensive mice remains unknown. The aim of this study was to evaluate the effects of thymus transplantation on blood pressure and target organ changes in mice with hypertension. Mice were randomly divided into normal control group (Con), hypertensive group (HTN) and thymus transplantation group (HTN + Trans). Thymus of neonatal mice was transplanted into the renal capsule of the transplantation group. After transplantation, the mouse tail noninvasive pressure was measured and heart function was evaluated weekly. Then mice were euthanized and organs or tissues were harvested at 4 weeks post-transplantation. The blood pressure of HTN + Trans group was lower than that in the HTN group. The expression of FoxN1, Aire, ATRAP, thymosin β4 and the content of sjTREC in thymus of HTN group was decreased and the number of naïve T cells in HTN group was lower compared with other two groups. The ratio of cTEC/mTEC in HTN group was higher than that in Con group and lower than that in HTN + Trans group. Cardiac pathology showed cardiac hypertrophy and fibrosis in HTN group whereas thymus transplantation improved heart function and structure. Altogether, our findings demonstrated thymus transplantation could improve thymus function of hypertensive mice, which increased the expression of thymus transcription factor FoxN1, affected the proportion of T cell subsets, and increased thymosin β4 thereby reducing blood pressure and reversing the progression of target organ damage.

Proteomic Analysis Reveals Differential Protein Expression Induced by Inhibition of Prolyl Oligopeptidase in Filarial Parasites

Prolyl oligopeptidase (POP) plays a crucial role in the processing and degradation of neuropeptides and regulates inositol trisphosphate (IP3) signaling in mammals. We have reported that POP inhibition leads to IP3-mediated calcium efflux leading to mitochondrial-mediated apoptosis in the filarial parasite Setaria cervi. This study further elucidates the effect of altered calcium homeostasis on the proteome of filarial parasites. Adult parasites were treated with POP's specific inhibitor, Z-Pro-prolinal (ZPP), for 7 h. Cytosolic and mitochondrial proteome was analyzed using 2D gel electrophoresis coupled with MALDI-MS/MS. Phosphoproteins were also analyzed in the cytosolic fraction of the parasites. The phosphoprotein analysis revealed 7, and 9 spots in the cytosolic fraction of control and ZPP-treated parasites, respectively. The two identified protein spots in the treated set were found to be involved in G protein signaling. In cytosolic fraction, 109 and 112 protein spots were observed in control and treated parasites, respectively. Of these, 56 upregulated and 32 downregulated protein spots were observed in the treated set. On the other hand, 50 and 47 protein spots were detected in the mitochondrial fraction of control and treated parasites, respectively. Of these spots, 18 upregulated and 12 down-regulated protein spots were found in treated parasites. In silico analysis showed that the identified proteins were involved in energy metabolism, calcium signaling, stress response, and cytoskeleton organization. These findings correlate with our previous results suggesting the important regulatory role of POP in signaling and different metabolic pathways of filarial parasites.

Oleuropein as a Potent Compound against Neurological Complications Linked with COVID-19: A Computational Biology Approach

The association of COVID-19 with neurological complications is a well-known fact, and researchers are endeavoring to investigate the mechanistic perspectives behind it. SARS-CoV-2 can bind to Toll-like receptor 4 (TLR-4) that would eventually lead to α-synuclein aggregation in neurons and stimulation of neurodegeneration pathways. Olive leaves have been reported as a promising phytotherapy or co-therapy against COVID-19, and oleuropein is one of the major active components of olive leaves. In the current study, oleuropein was investigated against SARS-CoV-2 target (main protease 3CL^pro), TLR-4 and Prolyl Oligopeptidases (POP), to explore oleuropein potency against the neurological complications associated with COVID-19. Docking experiments, docking validation, interaction analysis, and molecular dynamic simulation analysis were performed to provide insight into the binding pattern of oleuropein with the three target proteins. Interaction analysis revealed strong bonding between oleuropein and the active site amino acid residues of the target proteins. Results were further compared with positive control lopinavir (3CL^pro), resatorvid (TLR-4), and berberine (POP). Moreover, molecular dynamic simulation was performed using YASARA structure tool, and AMBER14 force field was applied to examine an 100 ns trajectory run. For each target protein-oleuropein complex, RMSD, RoG, and total potential energy were estimated, and 400 snapshots were obtained after each 250 ps. Docking analyses showed binding energy as −7.8, −8.3, and −8.5 kcal/mol for oleuropein-3CL^pro, oleuropein-TLR4, and oleuropein-POP interactions, respectively. Importantly, target protein-oleuropein complexes were stable during the 100 ns simulation run. However, an experimental in vitro study of the binding of oleuropein to the purified targets would be necessary to confirm the present study outcomes.

Potential neurotoxic activity of diverse molecules released by microglia

Microglia are the professional immune cells of the brain, which support numerous physiological processes. One of the defensive functions provided by microglia involves secretion of cytotoxins aimed at destroying invading pathogens. It is also recognized that the adverse activation of microglia in diseased brains may lead to secretion of cytotoxic molecules, which could be damaging to the surrounding cells, including neurons. Several of these toxins, such as reactive oxygen and nitrogen species, L-glutamate, and quinolinic acid, are widely recognized and well-studied. This review is focused on a structurally diverse group of less-established microglia neurotoxins, which were selected by applying the two criteria that these molecules 1) can be released by microglia, and 2) have the potential to be directly harmful to neurons. The following 11 molecules are discussed in detail: amyloid beta peptides (Aβ); cathepsin (Cat)B and CatD; C-X-C motif chemokine ligand (CXCL)10 and CXCL12 (5-67); high mobility group box (HMGB)1; lymphotoxin (LT)-α; matrix metalloproteinase (MMP)-2 and MMP-9; platelet-activating factor (PAF); and prolyl endopeptidase (PEP). Molecular mechanisms of their release by microglia and neurotoxicity, as well as available evidence implicating their involvement in human neuropathologies are summarized. Further studies on several of the above molecules are warranted to confirm either their microglial origin in the brain or direct neurotoxic effects. In addition, investigations into the differential secretion patterns of neurotoxins by microglia in response to diverse stimuli are required. This research could identify novel therapeutic targets for neurological disorders involving adverse microglial activation.

2020 update on the renin-angiotensin-aldosterone system in pediatric kidney disease and its interactions with coronavirus

The last decade was crucial for our understanding of the renin-angiotensin-aldosterone system (RAAS) as a two-axis, counter-regulatory system, divided into the classical axis, formed by angiotensin-converting enzyme (ACE), angiotensin II (Ang II), and the angiotensin type 1 receptor (AT1R), and the alternative axis comprising angiotensin-converting enzyme 2 (ACE2), angiotensin-(1-7) (Ang-(1-7)), and the Mas receptor. Breakthrough discoveries also took place, with other RAAS endopeptides being described, including alamandine and angiotensin A. In this review, we characterize the two RAAS axes and the role of their components in pediatric kidney diseases, including childhood hypertension (HTN), pediatric glomerular diseases, congenital abnormalities of the kidney and urinary tract (CAKUT), and chronic kidney disease (CKD). We also present recent findings on potential interactions between the novel coronavirus, SARS-CoV-2, and components of the RAAS, as well as potential implications of coronavirus disease 2019 (COVID-19) for pediatric kidney diseases.

Prolyl endopeptidase inhibitor Y-29794 blocks the IRS1-AKT-mTORC1 pathway and inhibits survival and in vivo tumor growth of triple-negative breast cancer

Prolyl endopeptidase (PREP), also known as prolyl oligopeptidase (POP), is an enzyme that cleaves short peptides (<30 amino acids in length) on the C-terminal side of proline. PREP is highly expressed in multiple carcinomas and is a potential target for cancer therapy. A potent inhibitor of PREP, Y-29794, causes long-lasting inhibition of PREP in mouse tissues. However, there are no reports on Y-29794 effects on cancer cell and tumor proliferation. Using cell line models of aggressive triple-negative breast cancer (TNBC), we show here that Y-29794 inhibited proliferation and induced death in multiple TNBC cell lines. Cell death induced by Y-29794 coincided with inhibition of the IRS1-AKT-mTORC1 survival signaling pathway, although stable depletion of PREP alone was not sufficient to reduce IRS1-AKT-mTORC1 signaling or induce death. These results suggest that Y-29794 elicits its cancer cell killing effect by targeting other mechanisms in addition to PREP. Importantly, Y-29794 inhibited tumor growth when tested in xenograft models of TNBC in mice. Induction of cell death in culture and inhibition of xenograft tumor growth support the potential utility of Y-29794 or its derivatives as a treatment option for TNBC tumors.

Prolyl oligopeptidase inhibition by KYP-2407 increases alpha-synuclein fibril degradation in neuron-like cells

Growing evidence emphasizes insufficient clearance of pathological alpha-synuclein (αSYN) aggregates in the progression of Parkinson's disease (PD). Consequently, cellular degradation pathways represent a potential therapeutic target. Prolyl oligopeptidase (PREP) is highly expressed in the brain and has been suggested to increase αSYN aggregation and negatively regulate the autophagy pathway. Inhibition of PREP with a small molecule inhibitor, KYP-2407, stimulates autophagy and reduces the oligomeric species of αSYN aggregates in PD mouse models. However, whether PREP inhibition has any effects on intracellular αSYN fibrils has not been studied before. In this study, the effect of KYP2407 on αSYN preformed fibrils (PFFs) was tested in SH-SY5Y cells and human astrocytes. Immunostaining analysis revealed that both cell types accumulated αSYN PFFs intracellularly but KYP-2047 decreased intracellular αSYN deposits only in SH-SY5Y cells, as astrocytes did not show any PREP activity. Western blot analysis confirmed the reduction of high molecular weight αSYN species in SH-SY5Y cell lysates, and secretion of αSYN from SH-SY5Y cells also decreased in the presence of KYP-2407. Accumulation of αSYN inside the SH-SY5Y cells resulted in an increase of the auto-lysosomal proteins p62 and LC3BII, as well as calpain 1 and 2, which have been shown to be associated with PD pathology. Notably, treatment with KYP-2407 significantly reduced p62 and LC3BII levels, indicating an increased autophagic flux, and calpain 1 and 2 levels returned to normal in the presence of KYP-2407. Our findings indicate that PREP inhibition can potentially be used as therapy to reduce the insoluble intracellular αSYN aggregates.

A molecular mechanism of mouse placental spongiotrophoblast differentiation regulated by prolyl oligopeptidase

SummaryIn eutherian mammals, the placenta plays a critical role in embryo development by supplying nutrients and hormones and mediating interaction with the mother. To establish the fine connection between mother and embryo, the placenta needs to be formed normally, but the mechanism of placental differentiation is not fully understood. We previously revealed that mouse prolyl oligopeptidase (POP) plays a role in trophoblast stem cell (TSC) differentiation into two placental cell types, spongiotrophoblasts (SpT) and trophoblast giant cells. Here, we focused on SpT differentiation and attempted to elucidate a molecular mechanism. For Ascl2, Arnt, and Egfr genes that are indispensable for SpT formation, we found that a POP-specific inhibitor, SUAM-14746, significantly decreased Ascl2 expression, which was consistent with a significant decrease in expression of Flt1, a gene downstream of Ascl2. Although this downregulation was unlikely to be mediated by the PI3K-Akt pathway, our results indicated that POP controls TSC differentiation into SpT by regulating the Ascl2 gene.

Transcriptomics and iTRAQ-Proteomics Analyses of Bovine Mammary Tissue with Streptococcus agalactiae-Induced Mastitis

Mastitis is a highly prevalent disease in dairy cows that causes large economic losses. Streptococcus agalactiae is a common contagious pathogen and a major cause of bovine mastitis. The immune response to intramammary infection with S. agalactiae in dairy cows is a very complex biological process. To understand the host immune response to S. agalactiae-induced mastitis, mammary gland of lactating Chinese Holstein cows was challenged with S. agalactiae via nipple tube perfusion. Visual inspection, analysis of milk somatic cell counts, histopathology, and transmission electron microscopy of mammary tissue were performed to confirm S. agalactiae-induced mastitis. Microarray and isobaric tags for relative and absolute quantitation (iTRAQ) were used to compare the transcriptomes and proteomes of healthy and mastitic mammary tissue. Compared with healthy tissue, a total of 129 differentially expressed genes (DEGs, fold change >2, p < 0.05) and 144 differentially expressed proteins (DEPs, fold change >1.2, p < 0.05) were identified in mammary tissue from S. agalactiae-challenged cows. Among the concordant 18 DEGs/DEPs, immunoglobulin M precursor, cathelicidin-7 precursor, integrin alpha-5, and complement C4-A-like isoform X1 were associated with mastitis. Intramammary infection with S. agalactiae triggered a complex host innate immune response that involved complement and coagulation cascades, ECM-receptor interaction, focal adhesion, and phagosome and bacterial invasion of epithelial cells pathways. These results provide candidate genes or proteins for further studies in the context of prevention and targeted treatment of bovine mastitis.

Alzheimer-associated cerebrospinal fluid fragments of neurogranin are generated by Calpain-1 and prolyl endopeptidase

Background

Neurogranin (Ng) is a small 7.6 kDa postsynaptic protein that has been detected at elevated concentrations in cerebrospinal fluid (CSF) of patients with Alzheimer’s disease (AD), both as a full-length molecule and as fragments from its C-terminal half. Ng is involved in postsynaptic calcium (Ca) signal transduction and memory formation via binding to calmodulin in a Ca-dependent manner. The mechanism of Ng secretion from neurons to CSF is currently unknown, but enzymatic cleavage of Ng may be of relevance. Therefore, the aim of the study was to identify the enzymes responsible for the cleavage of Ng, yielding the Ng fragment pattern of C-terminal fragments detectable and increased in CSF of AD patients.

Methods

Fluorigenic quenched FRET probes containing sequences of Ng were utilized to identify Ng cleaving activities among enzymes known to have increased activity in AD and in chromatographically fractionated mouse brain extracts.

Results

Human Calpain-1 and prolyl endopeptidase were identified as the candidate enzymes involved in the formation of endogenous Ng peptides present in CSF, cleaving mainly in the central region of Ng, and between amino acids 75_76 in the Ng sequence, respectively. The cleavage by Calpain-1 affects the IQ domain of Ng, which may deactivate or change the function of Ng in Ca²⁺/calmodulin -dependent signaling for synaptic plasticity. While shorter Ng fragments were readily cleaved in vitro by prolyl endopeptidase, the efficiency of cleavage on larger Ng fragments was much lower.

Conclusions

Calpain-1 and prolyl endopeptidase cleave Ng in the IQ domain and near the C-terminus, respectively, yielding specific fragments of Ng in CSF. These fragments may give clues to the roles of increased activities of these enzymes in the pathophysiology of AD, and provide possible targets for pharmacologic intervention.

Electronic supplementary material

The online version of this article (10.1186/s13024-018-0279-z) contains supplementary material, which is available to authorized users.

Mechanisms in hypertension and target organ damage: Is the role of the thymus key? (Review)

A variety of cells and cytokines have been shown to be involved in the whole process of hypertension. Data from experimental and clinical studies on hypertension have confirmed the key roles of immune cells and inflammation in the process. Dysfunction of the thymus, which modulates the development and maturation of lymphocytes, has been shown to be associated with the severity of hypertension. Furthermore, gradual atrophy, functional decline or loss of the thymus has been revealed to be associated with aging. The restoration or enhancement of thymus function via upregulation in the expression of thymus transcription factors forkhead box N1 or thymus transplantation may provide an option to halt or reverse the pathological process of hypertension. Therefore, the thymus may be key in hypertension and associated target organ damage, and may provide a novel treatment strategy for the clinical management of patients with hypertension in addition to different commercial drugs. The purpose of this review is to summarize and discuss the advances in our understanding of the impact of thymus function on hypertension from data from animal and human studies, and the potential mechanisms.

Implication of the Kallikrein-Kinin system in neurological disorders: Quest for potential biomarkers and mechanisms

Neurological disorders represent major health concerns in terms of comorbidity and mortality worldwide. Despite a tremendous increase in our understanding of the pathophysiological processes involved in disease progression and prevention, the accumulated knowledge so far resulted in relatively moderate translational benefits in terms of therapeutic interventions and enhanced clinical outcomes. Aiming at specific neural molecular pathways, different strategies have been geared to target the development and progression of such disorders. The kallikrein-kinin system (KKS) is among the most delineated candidate systems due to its ubiquitous roles mediating several of the pathophysiological features of these neurological disorders as well as being implicated in regulating various brain functions. Several experimental KKS models revealed that the inhibition or stimulation of the two receptors of the KKS system (B1R and B2R) can exhibit neuroprotective and/or adverse pathological outcomes. This updated review provides background details of the KKS components and their functions in different neurological disorders including temporal lobe epilepsy, traumatic brain injury, stroke, spinal cord injury, Alzheimer's disease, multiple sclerosis and glioma. Finally, this work will highlight the putative roles of the KKS components as potential neurotherapeutic targets and provide future perspectives on the possibility of translating these findings into potential clinical biomarkers in neurological disease.

Identification and characterization of a novel prolyl oligopeptidase in filarial parasite Setaria cervi

A 75 kDa serine protease having prolyl oligopeptidase activity has been purified from Setaria cervi, a bovine filarial parasite. The MALDI-MS/MS analysis of the purified protein revealed 6 peptides showing nearest match S9A (prolyl oligopeptidase) family protein from Plesiocystis pacifica. The ScPOP was found to be unique compared to mammalian POP with respect to its kinetic properties. To elucidate its role, filarial parasites were exposed to specific inhibitor of POP, Z-Pro-prolinal (ZPP) for 8 h. The inhibition of POP induced calcium signaling via phospholipase c stimulation which further triggered mitochondrial mediated apoptosis in filarial parasites.

Peptidome profiles and bioactivity elucidation of buffalo-milk dairy products after gastrointestinal digestion

Buffalo milk is highly appreciated for its nutritive properties and highly employed in dairy products, despite this the release of bioactive peptides has not been investigated thoroughly. The aim of this work was to characterize in detail the bioaccesible peptides from buffalo-milk dairy products. Six products were subjected to in vitro simulated gastrointestinal digestion and then analyzed by LC-HRMS. The identified peptides were 165 in Yoghurt, 152 in Scamorza, 146 in Mozzarella, 136 in Grana and Ricotta, 120 in Ice Cream samples, belonging to both buffalo caseins (α_s1-, β-, k-CN) and whey proteins (α-LA, β-LG). The identified peptide sequences were subjected to a database driven bioactivity search. Results highlighted a wide range of potential bioactive peptides, including antihypertensive, immunomodulatory, antimicrobial, antidiabetic, anticancer and antioxidant activity. These data evidence the content of healthy peptides released from buffalo-milk dairy products and suggest that the specific technological process influence their bioaccessibility.

Mechanism of Action of Prolyl Oligopeptidase (PREP) in Degenerative Brain Diseases: Has Peptidase Activity Only a Modulatory Role on the Interactions of PREP with Proteins?

In the aging brain, the correct balance of neural transmission and its regulation is of particular significance, and neuropeptides have a significant role. Prolyl oligopeptidase (PREP) is a protein highly expressed in brain, and evidence indicates that it is related to aging and in neurodegenration. Although PREP is regarded as a peptidase, the physiological substrates in the brain have not been defined, and after intense research, the molecular mechanisms where this protein is involved have not been defined. We propose that PREP functions as a regulator of other proteins though peptide gated direct interaction. We speculate that, at least in some processes where PREP has shown to be relevant, the peptidase activity is only a consequence of the interactions, and not the main physiological activity.

Prolyl oligopeptidase attenuates hepatic stellate cell activation through induction of Smad7 and PPAR-γ

Prolyl oligopeptidase (POP) is a serine endopeptidase widely distributed in vivo with high activity in the liver. However, its biological functions in the liver have remained largely elusive. A previous study by our group has shown that POP produced N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP) and thereby exerted an anti-fibrogenic effect on hepatic stellate cells (HSCs) in vitro. It was therefore hypothesized that POP may affect the activation state of HSCs and has an important role in liver fibrosis. The HSC-T6 immortalized rat liver stellate cell line was treated with the POP inhibitor S17092 or transfected with recombinant lentivirus to overexpress POP. Cell proliferation and apoptosis were determined using a Cell Counting Kit-8 and flow cytometry, respectively. The activation status of HSCs was determined by examination of the expression of α-smooth muscle actin (α-SMA), collagen I, monocyte chemoattractant protein-1 (MCP-1), transforming growth factor (TGF)-β-Smad signaling and peroxisome proliferator activated receptor-γ (PPAR-γ). Inhibition by S17092 decreased, whereas lentiviral expression increased the activity of POP and cell proliferation, while neither of the treatments affected cell apoptosis. Of note, S17092 significantly increased, whereas POP overexpression decreased the expression of α-SMA and MCP-1 without affecting the expression of collagen I and TGF-β1. Furthermore, S17092 caused a reduction, whereas POP overexpression caused an upregulation of Smad7 protein and PPAR-γ, but not phosphorylated-Smad2/3 expression. In conclusion, POP attenuated the activation of HSCs through inhibition of TGF-β signaling and induction of PPAR-γ, which may have therapeutic potential in liver fibrosis.

Prolyl Oligopeptidase Regulates Dopamine Transporter Phosphorylation in the Nigrostriatal Pathway of Mouse

Alpha-synuclein is the main component of Lewy bodies, a histopathological finding of Parkinson's disease. Prolyl oligopeptidase (PREP) is a serine protease that binds to α-synuclein and accelerates its aggregation in vitro. PREP enzyme inhibitors have been shown to block the α-synuclein aggregation process in vitro and in cellular models, and also to enhance the clearance of α-synuclein aggregates in transgenic mouse models. Moreover, PREP inhibitors have induced alterations in dopamine and metabolite levels, and dopamine transporter immunoreactivity in the nigrostriatal tissue. In this study, we characterized the role of PREP in the nigrostriatal dopaminergic and GABAergic systems of wild-type C57Bl/6 and PREP knockout mice, and the effects of PREP overexpression on these systems. Extracellular concentrations of dopamine and protein levels of phosphorylated dopamine transporter were increased and dopamine reuptake was decreased in the striatum of PREP knockout mice, suggesting increased internalization of dopamine transporter from the presynaptic membrane. Furthermore, PREP overexpression increased the level of dopamine transporters in the nigrostriatal tissue but decreased phosphorylated dopamine transporters in the striatum in wild-type mice. Our results suggest that PREP regulates the function of dopamine transporter, possibly by controlling the phosphorylation and transport of dopamine transporter into the striatum or synaptic membrane.

Prolyl Oligopeptidase Inhibition Attenuates Steatosis in the L02 Human Liver Cell Line

Background

Prolyl oligopeptidase (POP) is a serine endopeptidase that is widely distributed in vivo, particularly in the liver. Significant changes in functional mitochondrial proteins involved with mitochondrial oxidoreductases/transporters and nucleic acid binding proteins were observed after POP inhibition in the liver, which suggested a role of POP in regulating liver energy metabolism. Steatosis in nonalcoholic fatty liver disease (NAFLD) is associated with disturbances in lipid and energy metabolism in hepatocytes. Here, we aimed to study the effect of POP on hepatocyte steatosis.

Methods

The human liver cell line L02 was used to investigate the biological effects of POP. An in vitro cell model of steatosis was successfully induced with oleic acid and palmitic acid. L02 cells were also subjected to S17092 (a POP inhibitor) at different concentrations for 24 or 48 h. Ac-SDKP levels and POP activity were measured to assess the rate of inhibition of POP by S17092. The POP gene and protein expression levels were detected using real-time PCR and Western blots, respectively. Oil red O staining was performed and the triglyceride levels in the L02 cells were also measured. Cell proliferation and apoptosis were detected using CCK-8 and flow cytometry, respectively. The expression of genes involved in lipid metabolism was detected using real-time PCR. The effects of POP inhibition on LC3B II were detected by Western blot.

Results

Compared with the control, the POP mRNA levels increased by approximately 30%, and the POP protein levels increased by almost 60% in the steatotic L02 cells. After S17092 (0.026~130 μM) incubation for 24 or 48 h, cell proliferation was significantly decreased in the free fatty acid (FFA)-treated cells at 26–130 μM; however, S17092 did not affect the proliferation of L02 cells after 24 h of incubation with S17092 at 0.026–65 μM without FFA treatment. S17092 treatment (13 and 26 μM) also elicited no significant effect on apoptosis in normal L02 cells, but FFA treatment increased cell apoptosis, which was attenuated by S17092 incubation. S17092 treatment inhibited intracellular POP activity and decreased the AcSDKP level at the concentration of 0.026–26 μM. After treatment with FFA for 24 h, oil red O staining revealed significant lipid accumulation in the cells in the model group compared with the controls; however, lipid accumulation was suppressed after the administration of S17092 (13 and 26 μM). Accordingly, the triglyceride levels in the FFA-treated cells were approximately 5-fold greater than those of the controls and were decreased by approximately 25% and 45% after the administration of S17092 at 13 and 26 μM, respectively. The mRNA levels of FASN, PPAR-γ, and SREBP-1c were higher in the FFA-treated cells than in the normal controls, and all of these levels were significantly inhibited in the presence of S17092 at both 13 and 26 μM. S17092 treatment did not affect LC3B II in the FFA-treated cells compared with FFA treatment alone.

Conclusion

The expression of POP increases with hepatocyte steatosis, and POP inhibitors can significantly reduce intracellular lipid accumulation, which might be related to the inhibition of genes involved in lipid synthesis.

The expression levels of prolyl oligopeptidase responds not only to neuroinflammation but also to systemic inflammation upon liver failure in rat models and cirrhotic patients

Background

Liver failure in experimental animals or in human cirrhosis elicits neuroinflammation. Prolyl oligopeptidase (PREP) has been implicated in neuroinflammatory events in neurodegenerative diseases: PREP protein levels are increased in brain glial cells upon neuroinflammatory insults, but the circulating PREP activity levels are decreased in multiple sclerosis patients in a process probably mediated by bioactive peptides. In this work, we studied the variation of PREP levels upon liver failure and correlated it with several inflammatory markers to conclude on the relation of PREP with systemic and/or neuroinflammation.

Methods

PREP enzymatic activity and protein levels measured with immunological techniques were determined in the brain and plasma of rats with portacaval shunt (PCS) and after treatment with ibuprofen. Those results were compared with the levels of PREP measured in plasma from cirrhotic patients with or without minimal hepatic encephalopathy (MHE). Levels of several pro-inflammatory cytokines and those of NO/cGMP homeostasis metabolites were measured in PCS rats and cirrhotic patients to conclude on the role of PREP in inflammation.

Results

In PCA rats, we found that PREP levels are significantly increased in the hippocampus, striatum and cerebellum, that in the cerebellum the PREP increase was significantly found in the extracellular space and that the levels were restored to those measured in control rats after administration of an anti-inflammatory agent, ibuprofen. In cirrhotic patients, circulatory PREP activity was found to correlate to systemic and neuroinflammatory markers and had a negative correlation with the severity of the disease, although no clear relation to MHE.

Conclusions

These results support the idea that PREP levels could be used as indicators of cirrhosis severity in humans, and using other markers, it might contribute to assessing the level of neuroinflammation in those patients. This work reports, for the first time, that PREP is secreted to the extracellular space in the cerebellum most probably due to glial activation and supports the role of the peptidase in the inflammatory response.

Suppression of tumor growth in mice by rationally designed pseudopeptide inhibitors of fibroblast activation protein and prolyl oligopeptidase

Tumor microenvironments (TMEs) are composed of cancer cells, fibroblasts, extracellular matrix, microvessels, and endothelial cells. Two prolyl endopeptidases, fibroblast activation protein (FAP) and prolyl oligopeptidase (POP), are commonly overexpressed by epithelial-derived malignancies, with the specificity of FAP expression by cancer stromal fibroblasts suggesting FAP as a possible therapeutic target. Despite overexpression in most cancers and having a role in angiogenesis, inhibition of POP activity has received little attention as an approach to quench tumor growth. We developed two specific and highly effective pseudopeptide inhibitors, M83, which inhibits FAP and POP proteinase activities, and J94, which inhibits only POP. Both suppressed human colon cancer xenograft growth > 90% in mice. By immunohistochemical stains, M83- and J94-treated tumors had fewer microvessels, and apoptotic areas were apparent in both. In response to M83, but not J94, disordered collagen accumulations were observed. Neither M83- nor J94-treated mice manifested changes in behavior, weight, or gastrointestinal function. Tumor growth suppression was more extensive than noted with recently reported efforts by others to inhibit FAP proteinase function or reduce FAP expression. Diminished angiogenesis and the accompanying profound reduction in tumor growth suggest that inhibition of either FAP or POP may offer new therapeutic approaches that directly target TMEs.

Prolyl oligopeptidase enhances α-synuclein dimerization via direct protein-protein interaction

Prolyl oligopeptidase (PREP) accelerates the aggregation of α-synuclein (aSyn), a key protein involved in development of Parkinson disease and other synucleinopathies. PREP inhibitors reduce aSyn aggregation, but the mechanism has remained unknown. We have now used protein-fragment complementation assays (PCA) and microscale thermophoresis in parallel to show that PREP interacts directly with aSyn in both intact cells and in a cell-free system. Using split luciferase-based PCA, we first showed that PREP enhances the formation of soluble aSyn dimers in live Neuro-2A neuroblastoma cells. A PREP inhibitor, KYP-2047, reduced aSyn dimerization in PREP-expressing cells but not in cells lacking PREP expression. aSyn dimerization was also enhanced by PREP(S554A), an enzymatically inactive PREP mutant, but this was not affected by KYP-2047. PCA and microscale thermophoresis studies showed that aSyn interacts with both PREP and PREP(S554A) with low micromolar affinity. Neither the proline-rich, C-terminal domain of aSyn nor the hydrolytic activity of PREP was required for the interaction with PREP. Our results show that PREP binds directly to aSyn to enhance its dimerization and may thus serve as a nucleation point for aSyn aggregation. Native gel analysis showed that KYP-2047 shifts PREP to a compact monomeric form with reduced ability to promote aSyn nucleation. As PREP inhibition also enhances autophagic clearance of aSyn, PREP inhibitors may reduce accumulation of aSyn inclusions via a dual mechanism and are thus a novel therapeutic candidate for synucleinopathies. Our results also suggest that PREP has other cellular functions in addition to its peptidase activity.

Hypothalamic prolyl endopeptidase (PREP) regulates pancreatic insulin and glucagon secretion in mice

Prolyl endopeptidase (PREP) has been implicated in neuronal functions. Here we report that hypothalamic PREP is predominantly expressed in the ventromedial nucleus (VMH), where it regulates glucose-induced neuronal activation. PREP knockdown mice (Prep(gt/gt)) exhibited glucose intolerance, decreased fasting insulin, increased fasting glucagon levels, and reduced glucose-induced insulin secretion compared with wild-type controls. Consistent with this, central infusion of a specific PREP inhibitor, S17092, impaired glucose tolerance and decreased insulin levels in wild-type mice. Arguing further for a central mode of action of PREP, isolated pancreatic islets showed no difference in glucose-induced insulin release between Prep(gt/gt) and wild-type mice. Furthermore, hyperinsulinemic euglycemic clamp studies showed no difference between Prep(gt/gt) and wild-type control mice. Central PREP regulation of insulin and glucagon secretion appears to be mediated by the autonomic nervous system because Prep(gt/gt) mice have elevated sympathetic outflow and norepinephrine levels in the pancreas, and propranolol treatment reversed glucose intolerance in these mice. Finally, re-expression of PREP by bilateral VMH injection of adeno-associated virus-PREP reversed the glucose-intolerant phenotype of the Prep(gt/gt) mice. Taken together, our results unmask a previously unknown player in central regulation of glucose metabolism and pancreatic function.

A long non-coding RNA transcribed from conserved non-coding sequences contributes to the mouse prolyl oligopeptidase gene activation

Prolyl oligopeptidase (POP) is a multifunctional protease which is involved in many physiological events, but its gene regulatory mechanism is poorly understood. To identify novel regulatory elements of the POP gene, we compared the genomic sequences at the mouse and human POP loci and found six conserved non-coding sequences (CNSs) at adjacent intergenic regions. From these CNSs, four long non-coding RNAs (lncRNAs) were transcribed and the expression pattern of one (lncPrep+96kb) was correlated with that of POP. lncPrep+96kb was transcribed as two forms due to the different transcriptional start sites and was localized at the nucleus and cytoplasm, although more was present at the nucleus. When we knocked down lncPrep+96kb in the primary ovarian granulosa cell and a hepatic cell line, the POP expression was decreased in both cells. In contrast, overexpression of lncPrep+96kb increased the POP expression only in the granulosa cell. Because lncPrep+96kb was upregulated with the same timing as POP in the hormone-treated ovary, this lncRNA could play a role in the POP gene activation in the granulosa cell. Moreover, a downstream region of the human POP gene was also transcribed. We propose a novel mechanism for the POP gene activation.

Peptide POP inhibitors for the treatment of the cognitive symptoms of schizophrenia

Schizophrenia is a serious life-long disease that affects a significant part of the adult population. Although there is considerably effective medication for the positive symptoms of the disease, none are available for the associated cognitive deficits. These deficits are a core feature of schizophrenia, and they severely impair the functionality and social integration of patients. POP is a promising target for the treatment of the cognitive deficits of schizophrenia. Inhibitors of this peptidase show cognition-enhancing properties, act through a complex mechanism and have suitable pharmacological properties. Nevertheless, several studies must be carried out in order to improve the design and clinical evaluation of these substances. Permeability to the brain, appropriate animal models and suitable indications are the main issues that must be addressed. However, current information supports the potential of POP as an interesting drug target for the treatment of the cognitive deficits related to schizophrenia.

Alteration of prolyl oligopeptidase and activated α-2-macroglobulin in multiple sclerosis subtypes and in the clinically isolated syndrome

Prolyl oligopeptidase (PREP) has been considered as a drug target for the treatment of neurodegenerative diseases. In plasma, PREP has been found altered in several disorders of the central nervous system including multiple sclerosis (MS). Oxidative stress and the levels of an endogenous plasma PREP inhibitor have been proposed to decrease PREP activity in MS. In this work, we measured the circulating levels of PREP in patients suffering of relapsing remitting (RR), secondary progressive (SP), primary progressive (PP) MS, and in subjects with clinically isolated syndrome (CIS). We found a significantly lower PREP activity in plasma of RRMS as well as in PPMS patients and a trend to reduced activity in subjects diagnosed with CIS, compared to controls. No signs of oxidative inactivation of PREP, and no correlation with the endogenous PREP inhibitor, identified as activated α-2-macroglobulin (α2M*), were observed in any of the patients studied. However, a significant decrease of α2M* was recorded in MS. In cell cultures, we found that PREP specifically stimulates immune active cells possibly by modifying the levels of fibrinogen β, thymosin β4, and collagen. Our results open new lines of research on the role of PREP and α2M* in MS, aiming to relate them to the diagnosis and prognosis of this devastating disease.

Dysregulated bile acid synthesis, metabolism and excretion in a high fat-cholesterol diet-induced fibrotic steatohepatitis in rats

BACKGROUND AND AIMS

Cholesterol over-intake is involved in the onset of nonalcoholic steatohepatitis (NASH), and hepatocellular bile acid (BA) accumulation correlates with liver injuries. However, how dietary cholesterol influences cholesterol and BA kinetics in NASH liver remains ambiguous and needs to be clarified.

METHODS

Molecular markers involved in cholesterol and BA kinetics were investigated at protein and mRNA levels in an already-established stroke-prone spontaneously hypertensive 5/Dmcr rat model with fibrotic steatohepatitis, by feeding a high fat-cholesterol (HFC) diet.

RESULTS

Unlike the control diet, the HFC diet deposited cholesterol greatly in rat livers, where 3-hydroxy-3-methylglutaryl CoA reductase, low-density lipoprotein (LDL) receptor and LDL receptor-related protein-1 were expectedly downregulated, especially at 8 and 14 weeks, suggesting that cholesterol synthesis and uptake in response to cholesterol accumulation may not be disorganized. The HFC diet did not upregulate liver X receptor-α, conversely, it enhanced classic BA synthesis by upregulating cholesterol 7α-hydroxylase but downregulating sterol 12α-hydroxylase, and influenced alternative synthesis by downregulating sterol 27-hydroxylase but upregulating oxysterol 7α-hydroxylase, mainly at 8 and 14 weeks, indicating that there were different productions of primary BA species. Unexpectedly, no feedback inhibition of BA synthesis by farnesoid X receptor occurred. Additionally, the HFC diet impaired BA detoxification by UDP-glucuronosyltransferase and sulfotransferase 2A1, and decreased excretion by bile salt export pump at 8 and 14 weeks, although it induced compensatory export by multidrug resistance-associated protein-3. The disturbed BA detoxification may correlate with suppressed pregnane X receptor and constitutive androstane receptor.

CONCLUSIONS

The HFC diet may accumulate BA in rat livers, which influences fibrotic steatohepatitis progression.

P2-substituted N-acylprolylpyrrolidine inhibitors of prolyl oligopeptidase: biochemical evaluation, binding mode determination, and assessment in a cellular model of synucleinopathy

We have investigated the effect of regiospecifically introducing substituents in the P2 part of the typical dipeptide derived basic structure of PREP inhibitors. This hitherto unexplored modification type can be used to improve target affinity, selectivity, and physicochemical parameters in drug discovery programs focusing on PREP inhibitors. Biochemical evaluation of the produced inhibitors identified several substituent types that significantly increase target affinity, thereby reducing the need for an electrophilic "warhead" functionality. Pronounced PREP specificity within the group of Clan SC proteases was generally observed. Omission of the P1 electrophilic function did not affect the overall binding mode of three representative compounds, as studied by X-ray crystallography, while the P2 substituents were demonstrated to be accommodated in a cavity of PREP that, to date, has not been probed by inhibitors. Finally, we report on results of selected inhibitors in a SH-SY5Y cellular model of synucleinopathy and demonstrate a significant antiaggregation effect on α-synuclein.

MicroRNA-324-3p promotes renal fibrosis and is a target of ACE inhibition

The contribution of microRNA (miRNA) to the pathogenesis of renal fibrosis is not well understood. Here, we investigated whether miRNA modulates the fibrotic process in Munich Wistar Fromter (MWF) rats, which develop spontaneous progressive nephropathy. We analyzed the expression profile of miRNA in microdissected glomeruli and found that miR-324-3p was the most upregulated. In situ hybridization localized miR-324-3p to glomerular podocytes, parietal cells of Bowman's capsule, and most abundantly, cortical tubules. A predicted target of miR-324-3p is prolyl endopeptidase (Prep), a serine peptidase involved in the metabolism of angiotensins and the synthesis of the antifibrotic peptide N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP). In cultured tubular cells, transient transfection with a miR-324-3p mimic reduced Prep protein and activity, validating Prep as a target of this miRNA. In MWF rats, upregulation of miR-324-3p associated with markedly reduced expression of Prep in both glomeruli and tubules, low urine Ac-SDKP, and increased deposition of collagen. ACE inhibition downregulated glomerular and tubular miR-324-3p, promoted renal Prep expression, increased plasma and urine Ac-SDKP, and attenuated renal fibrosis. In summary, these results suggest that dysregulation of the miR-324-3p/Prep pathway contributes to the development of fibrosis in progressive nephropathy. The renoprotective effects of ACE inhibitors may result, in part, from modulation of this pathway, suggesting that it may hold other potential therapeutic targets.

Comparative analysis of the substrate preferences of two post-proline cleaving endopeptidases, prolyl oligopeptidase and fibroblast activation protein α

Post-proline cleaving peptidases are promising therapeutic targets for neurodegenerative diseases, psychiatric conditions, metabolic disorders, and many cancers. Prolyl oligopeptidase (POP; E.C. 3.4.21.26) and fibroblast activation protein α (FAP; E.C. 3.4.24.B28) are two post-proline cleaving endopeptidases with very similar substrate specificities. Both enzymes are implicated in numerous human diseases, but their study is impeded by the lack of specific substrate probes. We interrogated a combinatorial library of proteolytic substrates and identified novel and selective substrates of POP and FAP. These new sequences will be useful as probes for fundamental biochemical study, scaffolds for inhibitor design, and triggers for controlled drug delivery.

Distribution of prolyl oligopeptidase in human peripheral tissues and in ovarian and colorectal tumors

Prolyl oligopeptidase (PREP) is a serine protease that hydrolyzes peptides shorter than 30-mer, and it has been connected with multiple physiological and pathological conditions. PREP has been mostly studied in the brain, but significant PREP activities have been measured in peripheral tissues. Moreover, increased PREP activities have been found in tumors. In this study, the authors studied the immunohistochemical distribution of PREP protein in human peripheral tissues and in ovarian and colorectal tumors. PREP was found to be widely distributed in human peripheral tissues and specifically in certain cells. The most intense PREP expression was seen in the testis, ovaries, liver, and some parts of the skin. At the cellular level, high PREP levels were seen as a rule in secreting epithelial cells and cells involved in reproduction. Increased PREP expression was seen in most of the tumors studied. PREP expression was higher in malignant than benign tumors, and in ovarian epithelial cancers, there was a trend for increased PREP staining with increased malignancy grade. Results suggest that PREP may be associated with secretory processes as well as in reproduction. A more abundant expression of PREP in malignant than benign tumors suggests that PREP may be associated with expansion and metastasis of tumors.

Prolyl oligopeptidase induces angiogenesis both in vitro and in vivo in a novel regulatory manner

BACKGROUND AND PURPOSE A serine protease, prolyl oligopeptidase (POP) has been reported to be involved in the release of the pro-angiogenic tetrapeptide acetyl-N-Ser-Asp-Lys-Pro (Ac-SDKP) from its precursor, 43-mer thymosin β4 (Tβ4). Recently, it was shown that both POP activity and the levels of Ac-SDKP are increased in malignant tumours. The aim of this study was to clarify the release of Ac-SDKP, and test if POP and a POP inhibitor, 4-phenyl-butanoyl-L-prolyl-2(S)-cyanopyrrolidine (KYP-2047), can affect angiogenesis. EXPERIMENTAL APPROACH We used HPLC for bioanalytical and an enzyme immunoassay for pharmacological analysis. Angiogenesis of human umbilical vein endothelial cells was assessed in vitro using a 'tube formation' assay and in vivo using a Matrigel plug assay (BD Biosciences, San Jose, CA, USA) in adult male rats. Moreover, co-localization of POP and blood vessels was studied. KEY RESULTS We showed the sequential hydrolysis of Tβ4: the first-step hydrolysis by proteases to <30-mer peptides is followed by an action of POP. Unexpectedly, POP inhibited the first hydrolysis step, revealing a novel regulation system. POP with Tβ4 significantly induced, while KYP-2047 effectively prevented, angiogenesis in both models compared with Tβ4 addition itself. POP and endothelial cells were abundantly co-localized in vivo. CONCLUSIONS AND IMPLICATIONS We have now revealed that POP is a second-step enzyme in the release of Ac-SDKP from Tβ4, and it has novel autoregulatory effect in the first step. Our results also advocate a role for Ac-SDKP in angiogenesis, and suggest that POP has a pro-angiogenic role via the release of Ac-SDKP from its precursor Tβ4 and POP inhibitors can block this action.

Low molecular weight inhibitors of Prolyl Oligopeptidase: a review of compounds patented from 2003 to 2010

INTRODUCTION

Prolyl Oligopeptidase (POP) is a serine peptidase that cleaves post-proline bonds in short peptides. Besides the direct hydrolytic regulation function over peptides, neuropeptides and peptide hormones, POP is probably involved in the regulation of the inositol pathway and participates in protein-protein interactions. Experimental data show that POP inhibitors have neuroprotective, anti-amnesic and cognition-enhancing properties. These compounds are considered therapeutic agents of interest for the treatment of cognitive deficits related to neuropsychiatric and neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease. Recent findings pointed to the involvement of POP in angiogenesis, although the exact mechanism is still under study.

AREAS COVERED

This review comprises patents and patent applications involving POP inhibitors patented between 2003 and 2010, classified as peptidomimetics, heteroaryl ketones and alkaloids. The binding processes and the mechanisms of inhibition of these inhibitors are also discussed, together with their in vivo effects.

EXPERT OPINION

The major part of the repertory of POP inhibitors derived from systematical modification of the canonical compound benzyloxycarbonyl-prolyl-prolinal (ZPP). Nevertheless, only two of them have progressed into the clinical trials. One possible reason for this failure is the lack of studies concerning pharmacodynamics, pharmacokinetics and toxicity, together with the absence of suitable animal models. Moreover, POP is still not a well-defined therapeutic target. Further studies are required for the elucidation of the biological role of POP and to validate the therapeutic action of inhibitors in cognitive processes. In contrast, the involvement of POP in protein-protein interactions together with the recent evidences in angiogenesis opens alternative approaches to the traditional active site-directed inhibitors, as well as new therapeutic applications.