Subcellular localization suggests novel functions for prolyl endopeptidase in protein secretion

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.

The effect of prolyl oligopeptidase inhibition on extracellular acetylcholine and dopamine levels in the rat striatum

Prolyl oligopeptidase (PREP, EC 3.4.21.26) inhibitors have potential as cognition enhancers, but the mechanism of action behind the cognitive effects remains unclear. Since acetylcholine (ACh) and dopamine (DA) are known to be associated with the regulation of cognitive processes, we investigated the effects of two PREP inhibitors on the extracellular levels of ACh and DA in the rat striatum using in vivo microdialysis. KYP-2047 and JTP-4819 were administered either as a single systemic dose (50 μmol/kg∼17 mg/kg i.p.) or directly into the striatum by retrodialysis via the microdialysis probe (12.5, 37.5 or 125 μM at 1.5 μl/min for 60 min). PREP inhibitors had no significant effect on striatal DA levels after systemic administration. JTP-4819 significantly decreased ACh levels both after systemic (by ∼25%) and intrastriatal (by ∼30-50%) administration. KYP-2047 decreased ACh levels only after intrastriatal administration by retrodialysis (by ∼40-50%) when higher drug levels were reached, indicating that higher brain drug levels are needed to modulate ACh levels than to inhibit PREP. This result does not support the earlier hypothesis that the positive cognitive effects of PREP inhibitors in rodents would be mediated through the cholinergic system. In vitro specificity studies did not reveal any obvious off-targets that could explain the observed effect of KYP-2047 and JTP-4819 on ACh levels, instead confirming the concept that these compounds have a high selectivity towards PREP.

Heteroarylketones inhibit astroglial interleukin-6 expression via a STAT3/NF-κB signaling pathway

Background

Elevated brain levels of the pleiotropic cytokine interleukin-6, which is mainly secreted from activated local astrocytes, contribute to pathological events including neuroinflammation and neurodegeneration. Thus, inhibition of pathological IL-6 expression provides a rationale strategy for targeting the onset or further progression of neurological disorders including Alzheimer's disease, multiple sclerosis, Parkinson's disease and traumatic brain injury. The purpose of this study was to identify and to characterize new potent inhibitors of astrocytic IL-6 expression for further therapeutic development of novel anti-inflammatory and neuroprotective drugs.

Methods

Oncostatin M (OSM)-treated human glioma U343 cells were used as model for induction of astrocytic IL-6 expression. This model was characterized by immunoblotting, siRNA technique, ELISA and qRT-PCR and used to screen low molecular weight compound libraries for IL-6-lowering effects. To validate bioactive compounds identified from library screens, bacterial lipopolysaccharide was used to induce IL-6 expression in cultivated primary astrocytes and in mice in vivo. To dissect underlying molecular mechanisms, protein extracts from OSM-treated U343 cells were analyzed by phospho-specific immunoblotting and immunocytochemistry as well as by co-immunoprecipitation.

Results

OSM-treatment (100 ng/ml; 24 h) led to 30-fold increase of IL-6 secretion from U343 cells. The temporal profile of IL-6 mRNA induction displayed a biphasic induction pattern with peak synthesis at 1 h (6.5-fold) and 16 h (5.5-fold) post stimulation. IL-6 protein release did not show that biphasic pattern and was detected as early as 3 h post stimulation reaching a maximum at 24 h. The screen of compound libraries identified a set of heteroarylketones (HAKs) as potent inhibitors of IL-6 secretion. HAK compounds affected the second peak in IL-6 mRNA synthesis, whereas the first peak was insensitive to HAK treatment. HAK compounds also suppressed lipopolysaccharide-induced IL-6 expression in primary murine astrocytes as well as in brain and plasma samples from lipopolysaccharide-treated mice. Finally, HAK compounds were demonstrated to specifically suppress the OSM-induced phosphorylation of STAT3 at serine 727 and the physical interaction of pSTAT3^S727 with p65.

Conclusion

Heteroarylketone compounds are potent inhibitors of IL-6 expression in vitro and in vivo and may represent a new class of potent anti-inflammatory and neuroprotective drugs.

Brain pharmacokinetics of two prolyl oligopeptidase inhibitors, JTP-4819 and KYP-2047, in the rat

Prolyl oligopeptidase (PREP) inhibitors are potential drug candidates for the treatment of neurological disorders, but little is known about their ability to cross the blood-brain barrier and to reach the target site. This study characterizes brain pharmacokinetics of two potent PREP inhibitors, JTP-4819 and KYP-2047. Firstly, the in vitro permeability (P(app) ) of JTP-4819 and KYP-2047 through a bovine brain microvessel endothelial cell monolayer was assessed. Then, the in vivo brain/blood ratio was determined for the total brain and plasma concentrations and also for the unbound extracellular drug concentrations after a single dose (50 μmol/kg i.p.). KYP-2047 had a significantly higher P(app) than JTP-4819. In vivo, KYP-2047 had higher total and unbound brain/blood ratios. KYP-2047 was equally distributed between the cortex, hippocampus and striatum. In the case of JTP-4819, the unbound brain extracellular concentrations could not be readily predicted from the unbound blood levels, probably because of its poor membrane penetration properties. KYP-2047 displayed a better ability to reach the intracellularly located brain PREP, and it inhibited this enzyme more effectively than JTP-4819 after an equimolar single dose. In conclusion, KYP-2047 showed better brain penetration characteristics than JTP-4819 both in vitro and in vivo. KYP-2047 is a brain-penetrating, potent and long-acting PREP inhibitor; thus, it represents a convenient pharmacological tool for assessing the potential of PREP as a drug target.

Use of Umbrella Sampling to Calculate the Entrance/Exit Pathway for Z-Pro-Prolinal Inhibitor in Prolyl Oligopeptidase

Prolyl oligopeptidase (POP), a member of the prolyl endopeptidase family, is known to play a role in several neurological disorders. Its primary function is to cleave a wide range of small oligopeptides, including neuroactive peptides. We have used force biased molecular dynamics simulation to study the binding mechanism of POP. We examined three possible binding pathways using Steered Molecular Dynamics (SMD) and Umbrella Sampling (US) on a crystal structure of porcine POP with bound Z-pro-prolinal (ZPP). Using SMD, an exit pathway between the first and seventh blade of the β-propeller domain of POP was found to be a nonviable route. US on binding pathways through the β-propeller tunnel and the TYR190-GLN208 flexible loop at the interface between both POP domains allowed us to isolate the flexible loop pathway as the most probable. Further analysis of that pathway suggests a long-range covariation of the interdomain H-bond network, which indicates the possibility of large-scale domain reorientation observed in bacterial homologues and hypothesized to also occur in human POP.

Prolyl endopeptidase is involved in cellular signalling in human neuroblastoma SH-SY5Y cells

Prolyl endopeptidase (PREP), probably acting through the inositol cycle, has been implicated in memory and learning. However, the physiological role of PREP is unknown. It has been shown that PREP expression, regulated in cerebellar granule cells, has probably a role in cell proliferation and differentiation. Here, we report the levels and subcellular distribution of PREP in human neuroblastoma SH-SY5Y cells in proliferating conditions and under differentiation induced by retinoic acid (RA). We analysed the levels of cell signalling intermediates, growth behavior and gene expression, and differentiation morphology changes, upon PREP inhibition. After induction of differentiation, PREP activity was found decreased in the nucleus but increased to high levels in the cytoplasm, due in part to increased PREP transcription. The levels of inositol (1,4,5)-trisphosphate revealed no correlation with PREP activity, but phosphorylated extracellular signal-regulated kinases 1 and 2 were decreased by PREP inhibition during early stages of differentiation. Morphological evaluation indicated that PREP inhibition retarded the onset of differentiation. PREP activity regulated gene expression of protein synthesis machinery, intracellular transport and kinase complexes. We conclude that PREP is a regulatory target and a regulatory element in cell signalling. This is the first report of a direct influence of a cell signalling molecule, RA, on PREP expression.

Cigarette smoke-induced lung emphysema in mice is associated with prolyl endopeptidase, an enzyme involved in collagen breakdown

There is increasing evidence that the neutrophil chemoattractant proline-glycine-proline (PGP), derived from the breakdown of the extracellular matrix, plays an important role in neutrophil recruitment to the lung. PGP formation is a multistep process involving neutrophils, metalloproteinases (MMPs), and prolyl endopeptidase (PE). This cascade of events is now investigated in the development of lung emphysema. A/J mice were whole body exposed to cigarette smoke for 20 wk. After 20 wk or 8 wk after smoking cessation, animals were killed, and bronchoalveolar lavage fluid and lung tissue were collected to analyze the neutrophilic airway inflammation, the MMP-8 and MMP-9 levels, the PE activity, and the PGP levels. Lung tissue degradation was assessed by measuring the mean linear intercept. Additionally, we investigated the effect of the peptide L-arginine-threonine-arginine (RTR), which binds to PGP sequences, on the smoke-induced neutrophil influx in the lung after 5 days of smoke exposure. Neutrophilic airway inflammation was induced by cigarette smoke exposure. MMP-8 and MMP-9 levels, PE activity, and PGP levels were elevated in the lungs of cigarette smoke-exposed mice. PE was highly expressed in epithelial and inflammatory cells (macrophages and neutrophils) in lung tissue of cigarette smoke-exposed mice. After smoking cessation, the neutrophil influx, the MMP-8 and MMP-9 levels, the PE activity, and the PGP levels were decreased or reduced to normal levels. Moreover, RTR inhibited the smoke-induced neutrophil influx in the lung after 5 days' smoke exposure. In the present murine model of cigarette smoke-induced lung emphysema, it is demonstrated for the first time that all relevant components (neutrophils, MMP-8, MMP-9, PE) involved in PGP formation from collagen are upregulated in the airways. Together with MMPs, PE may play an important role in the formation of PGP and thus in the pathophysiology of lung emphysema.

Sequential expression, activity and nuclear localization of prolyl oligopeptidase protein in the developing rat brain

Prolyl oligopeptidase (POP) is a serine protease that hydrolyzes peptides shorter than 30-mer. Some evidence has recently been obtained that POP can generate protein-protein interactions and therefore participate in various physiological and pathological events. Several studies have reported that POP may be involved in neurogenesis since its activity increases during development and can be found in the nucleus of proliferating tissues. In cell cultures, POP has been shown to be localized in the nucleus, but only early in the development, since during maturation it is moved to the cytosol. We have now studied for the first time the expression of POP protein, its enzymatic activity and nuclear localization in vivo in the developing rat brain. We observed that enzymatic activity of POP is highest on embryonic day 18 while the protein amounts reach their peak at birth. Furthermore, POP is located in the nucleus only early in the development but is transferred to the cytosol already before parturition. Our in vivo results confirm the previous cell culture results supporting the role of POP in neurogenesis. A discordance of antenatal protein amounts and enzymatic activities is suggesting a tight regulation of POP activity and possibly even a nonhydrolytic role at that stage.

GAP43 shows partial co-localisation but no strong physical interaction with prolyl oligopeptidase

It has recently been proposed that prolyl oligopeptidase (POP), the cytosolic serine peptidase with neurological implications, binds GAP43 (Growth-Associated Protein 43) and is implicated in neuronal growth cone formation, axon guidance and synaptic plasticity. We investigated the interaction between GAP43 and POP with various biophysical and biochemical methods in vitro and studied the co-localisation of the two proteins in differentiated HeLa cells. GAP43 and POP showed partial co-localisation in the cell body as well as in the potential growth cone structures. We could not detect significant binding between the recombinantly expressed POP and GAP43 using gel filtration, CD, ITC and BIACORE studies, pull-down experiments, glutaraldehyde cross-linking and limited proteolysis. However, glutaraldehyde cross-linking suggested a weak and transient interaction between the proteins. Both POP and GAP43 interacted with artificial lipids in our in vitro model system, but the presence of lipids did not evoke binding between them. In native polyacrylamide gel electrophoresis, GAP43 interacted with one of the three forms of a polyhistidine-tagged prolyl oligopeptidase. The interaction of the two proteins was also evident in ELISA and we have observed co-precipitation of the two proteins during co-incubation at higher concentrations. Our results indicate that there is no strong and direct interaction between POP and GAP43 at physiological conditions.

Epigenetic patterns at the mouse prolyl oligopeptidase gene locus suggest the CpG island in the gene body to be a novel regulator for gene expression

Prolyl oligopeptidase (POP) is a widely distributed serine peptidase which hydrolyzes small peptides on the carboxyl side of an internal proline residue. While its physiological role has been intensely studied, the regulatory mechanism of the gene expression is poorly understood. This time we assessed the POP mRNA expression in mouse embryos and tissues related to reproduction and development and found that POP mRNA was highly expressed in the ovarian granulosa cell, placental spongiotrophoblast, and blastocyst embryo. To elucidate the mechanism by which POP expression is regulated, we investigated DNA methylation and histone modification patterns of the two CpG islands (CGIs) found at the mouse POP locus. Whereas the CGI including the POP promoter (CGI-1) was completely hypomethylated in all the tissues examined, DNA methylation level of the CGI in the gene body (CGI-2) was lower in the granulosa cell, placenta, and blastocyst than in the liver. Some specific CpGs in CGI-2 were significantly demethylated in the three tissues. An in vitro reporter analysis indicated that CGI-2 enhanced POP promoter activity and its effect was significantly reduced by DNA methylation. Moreover, histone H3 acetylation and H3K4 methylation levels of CGI-2 were higher in the granulosa cell than liver. The results suggest that the CGI-2 region is a cis-element for the POP gene expression.

Prolyl oligopeptidase is inhibited in relapsing-remitting multiple sclerosis

Background

Multiple sclerosis (MS) is a complex, inflammatory and neurodegenerative disease of the central nervous system leading to long-term disability. Recent studies indicate a close association between inflammation and neurodegeneration in all lesions and disease stages of MS. Prolyl oligopeptidase (POP) is a proline-specific serine protease that cleaves several neuroactive peptides. This peptidase has been implicated in neurodegeneration, as well as in the modulation of the inflammatory response.

Methods

We examined plasma POP and the levels of an endogenous POP inhibitor from relapsing remitting MS patients and compared these with healthy controls, by monitoring the fluorescent changes due to standard fluorescently labelled substrate cleavage. We analysed the data in relationship to patient age and disease disability status.

Results

We observed a significant decrease in POP activity in plasma of relapsing remitting MS patients relative to healthy controls, coupled with an increase of POP endogenous inhibitor. The POP activity was also correlated with patient age and disability status. The lowered POP activity from plasma of MS patients could be rescued by reductants

Conclusions

The decrease in circulating POP activity measured in MS is reverted by reductants. This suggests that POP inactivation in MS might be a result of the oxidative conditions prevailing in the plasma of the diseased patients. Plasma levels of POP activity as well as those of their endogenous inhibitor are suggested as biomarkers of inflammation and oxidative stress in MS.

Increased prolyl endopeptidase activity in human neoplasia

Prolyl endopeptidase (EC 3.4.21.26) (PEP) is a serine peptidase that converts several biologically active peptides. This enzyme has been linked to several neurological, digestive, cardiovascular and infectous disorders. However, little is known about its involvement in neoplastic processes. This study analyzes fluorimetrically cytosolic and membrane-bound PEP activity in a large series (n=122) of normal and neoplastic tissues from the kidney, colon, oral cavity, larynx, thyroid gland and testis. Cytosolic PEP activity significantly increased in clear cell renal cell carcinoma, urothelial carcinoma of the renal pelvis and head and neck squamous cell carcinoma. Both cytosolic and membrane-bound PEP activity were also increased in colorectal adenomatous polyps. These data suggest the involvement of PEP in some mechanisms that underlie neoplastic processes.

Prolyl endopeptidase mRNA expression in the central nervous system during rat development

Prolyl endopeptidase (PEP) is a serine protease that cleaves small peptides at the carboxyl side of L-proline. PEP has been reported to have important functions in the brain being implicated in learning and memory processes, psychological disorders and neurodegenerative diseases. Several PEP substrates have been shown to play a role during brain development and this observation led us to investigate the expression of PEP mRNA in the rat brain and spinal cord, from embryo to adult stages. In situ hybridization revealed that PEP mRNA is expressed early, from embryonic day 15, notably in germinative areas including the neocortical, hippocampal, pallidal, thalamic, anterior hypothalamic, tectal, cerebellar, pontine and medullary neuroepithelia. PEP mRNA was also found in the differentiating fields of the olfactory bulb, the orbital and cingulate cortex, the hippocampal formation, the cortical plate and the subventricular zone of the cortex. Quantitative RT-PCR analysis in various brain areas and the spinal cord showed that PEP mRNA levels are more abundant during the perinatal stages, coinciding with a period of neuronal migration and differentiation. From then on, PEP mRNA expression decreased, reaching its lowest levels at adulthood. Overall, the present data support the possibility that PEP exerts specific functions related to neurodevelopment besides those proposed to date.

Neutrophils contain prolyl endopeptidase and generate the chemotactic peptide, PGP, from collagen

Prolyl endopeptidase (PE), a protease that cleaves after proline residues in oligopeptides, is highly active in brain and degrades neuropeptides in vitro. We have recently demonstrated that PE, in concert with MMP's, can generate PGP (proline-glycine-proline), a novel, neutrophil chemoattractant, from collagen. In this study, we demonstrate that human peripheral blood neutrophils contain PE, which is constitutively active, and can generate PGP de novo from collagen after activation with LPS. This novel, pro-inflammatory role for PE raises the possibility of a self-sustaining pathway of neutrophilic inflammation and may provide biomarkers and therapeutic targets for diseases caused by chronic, neutrophilic inflammation.

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.

Prolyl oligopeptidase binds to GAP-43 and functions without its peptidase activity

Inhibitors of the enzyme prolyl oligopeptidase (PO) improve performance in rodent learning and memory tasks. PO inhibitors are also implicated in the action of drugs used to treat bipolar disorder: they reverse the effects of three mood stabilizers on the dynamic behaviour of neuronal growth cones. PO cleaves prolyl bonds in short peptides, suggesting that neuropeptides might be its brain substrates. PO is located in the cytosol, however, where it would not contact neuropeptides. Here, we show that mice with a targeted PO null-mutation have altered growth cone dynamics. The wild-type phenotype is restored by PO cDNAs encoding either native or a catalytically-dead enzyme. In addition, we show that PO binds to the growth-associated protein GAP-43, which is a key regulator of synaptic plasticity. Taken together, our results show that peptidase activity is not required for PO function in neurons and suggest that PO instead acts by binding to cytosolic proteins that control growth cone and synaptic function.

Prolyl oligopeptidase stimulates the aggregation of alpha-synuclein

Despite its thorough enzymological and biochemical characterization the exact function of prolyl oligopeptidase (PO, E.C. 3.4.21.26) remains unclear. The positive effect of PO inhibitors on learning and memory in animal models for amnesia, enzyme activity measurements in patient samples and (neuro)peptide degradation studies link the enzyme with neurodegenerative disorders. The brain protein alpha-synuclein currently attracts much attention because of its proposed role in the pathology of Parkinson's disease. A fundamental question concerns how the essentially disordered protein is transformed into the highly organized fibrils that are found in Lewy bodies, the hallmarks of Parkinson's disease. Using gel electrophoresis and MALDI TOF/TOF mass spectrometry we investigated the possibility of alpha-synuclein as a PO substrate. We found that in vitro incubation of the protein with PO did not result in truncation of full-length alpha-synuclein. Surprisingly, however, we found an acceleration of the aggregation process of alpha-synuclein using turbidity measurements that was reversed by specific inhibitors of PO enzymatic activity. If PO displays this activity also in vivo, PO inhibitors might have an effect on neurodegenerative disorders through a decrease in the aggregation of alpha-synuclein.

Expression and traffic of cellular prolyl oligopeptidase are regulated during cerebellar granule cell differentiation, maturation, and aging

Prolyl oligopeptidase (POP) is an endopeptidase which cleaves short proline-containing neuropeptides, and it is involved in memory and learning. POP also has an intercellular function mediated through the inositol pathway, and has been involved in cell death. POP has been early considered as a housekeeping enzyme, but the recent research indicates that POP expression is regulated across tissues and intracellularly. In the brain, POP is exclusively expressed in neurons and most abundantly in pyramidal neurons of cerebral cortex, in the CA1 field neurons of hippocampus and in cerebellar Purkinje's cells. Intracellularly, POP is mainly present in the cytoplasm and some in intracellular membranes, like rough endoplasmic reticulum and Golgi apparatus. In this paper, we systematically studied the levels of expression of POP along the life of cerebellar granule cells (CGC) in culture and the distribution of POP within different intracellular compartments. We used the tight-binding inhibitor JTP-4819 covalently coupled with fluorescein (FJTP) as a tool to study the changes on expression and localization of POP protein. Our results indicate that POP activity levels are regulated during the life of the neurons. POP was found mainly in cytoplasm and neuronal projections, but at an early developmental phase significant amounts were found also in nuclei. Along the life of the neurons, POP activity fluctuated in 7-day cycles. In young neurons, the cytosolic POP activity was low but increased by maturation so that the activity peak coincided with full differentiation. Over aging, cytoplasmic POP was concentrated around nucleus, but the activity decreased with time. POP was also present in vesicles across the neuron. No major changes were seen in the nuclear or membrane bound POP over aging until activity disappeared upon neuronal death. This is the first time when POP was found in the nuclei of human neuronal cells.

Characterization of membrane-bound prolyl endopeptidase from brain

Prolyl oligopeptidase (POP) is a serine protease that cleaves small peptides at the carboxyl side of an internal proline residue. Substance P, arginine-vasopressin, thyroliberin and gonadoliberin are proposed physiological substrates of this protease. POP has been implicated in a variety of brain processes, including learning, memory, and mood regulation, as well as in pathologies such as neurodegeneration, hypertension, and psychiatric disorders. Although POP has been considered to be a soluble cytoplasmic peptidase, significant levels of activity have been detected in membranes and in extracellular fluids such as serum, cerebrospinal fluid, seminal fluid, and urine, suggesting the existence of noncytoplasmic forms. Furthermore, a closely associated membrane prolyl endopeptidase (PE) activity has been previously detected in synaptosomes and shown to be different from the cytoplasmic POP activity. Here we isolated, purified and characterized this membrane-bound PE, herein referred to as mPOP. Although, when attached to membranes, mPOP presents certain features that distinguish it from the classical POP, our results indicate that this protein has the same amino acid sequence as POP except for the possible addition of a hydrophobic membrane anchor. The kinetic properties of detergent-soluble mPOP are fully comparable to those of POP; however, when attached to the membranes in its natural conformation, mPOP is significantly less active and, moreover, it migrates anomalously in SDS/PAGE. Our results are the first to show that membrane-bound and cytoplasmic POP are encoded by variants of the same gene.

The natural product berberine is a human prolyl oligopeptidase inhibitor

Prolyl oligopeptidase is a cytosolic serine peptidase that hydrolyzes proline-containing peptides at the carboxy terminus. This peptidase has been associated with schizophrenia, bipolar affective disorder, and related neuropsychiatric disorders, and therefore may have important clinical implications. Among the strategies used to find novel prolyl oligopeptidase inhibitors, traditional Chinese medicinal plants provide a rich source of unexplored compounds. We used (19)F NMR spectroscopy to search for new prolyl oligopeptidase inhibitors in a library of traditional Chinese medicine plant extracts. Several extracts were identified as powerful inhibitors of this peptidase. The alkaloid berberine was the prolyl oligopeptidase inhibitory molecule isolated from Rhizoma coptidis extract. Berberine inhibited prolyl oligopeptidase in a dose-dependent manner. As berberine is a natural compound that has been safely administered to humans, it opens up new perspectives for the treatment of neuropsychiatric diseases. The results described herein suggest that the initiation of clinical trials in patients with schizophrenia, bipolar affective disorder, or related diseases in which cognitive capabilities are affected should be undertaken with either the extract or pure BBR.

Cellular and subcellular distribution of rat brain prolyl oligopeptidase and its association with specific neuronal neurotransmitters

Prolyl oligopeptidase (POP) is a serine endopeptidase that hydrolyzes proline-containing peptides shorter than 30-mer. It has been suggested that POP is associated with cognitive functions and inositol 1,4,5-triphosphate (IP(3)) signaling. However, little is known about the distribution and physiological role of POP in the brain. We used immunohistochemistry to determine the cellular and subcellular distribution of POP in the rat brain. POP was specifically expressed in the glutamatergic pyramidal neurons of the cerebral cortex, particularly in the primary motor and somatosensory cortices, and also in the CA1 field of hippocampus. Purkinje cells of the cerebellum were also intensively immunostained for POP. Double immunofluorescence indicated that POP was present in the gamma-aminobutyric acid (GABA)ergic and cholinergic interneurons of the thalamus and cortex but not in the nigrostriatal dopaminergic neurons. POP did not colocalize with astrocytic markers in any part of the rat brain. We used postembedding immunoelectron microscopy to determine the distribution of POP at the subcellular level. POP was mainly present in neuronal cytosol and membranes, hardly at all in neuronal plasma membrane, but more extensively in intracellular membranes such as the rough endoplasmic reticulum and Golgi apparatus. Our findings point to a role for POP--evidently modifying neuropeptide levels--in excitatory and inhibitory neurotransmission in the central nervous system via glutamatergic, GABAergic, and cholinergic neurotransmission systems. Furthermore, according to our results, POP may be involved in thalamocortical neurotransmission, memory and learning functions of the hippocampal formation, and GABAergic regulation of voluntary movements. Subcellular distribution of POP points to a role in protein processing and secretion.

Deficient activity of mammalian prolyl oligopeptidase on the immunoactive peptide digestion in coeliac disease

OBJECTIVE

Gliadin digestion-resistant peptides are harmful in coeliac disease (CD), and initiate an autoimmune reaction that cause a cascade of symptoms. The role of the endogenous prolyl oligopeptidase (POP) is still not clear, and its activity over gliadin immunoactive peptides has not been fully established. Our objective was therefore to determine the endogenous POP protein level, tissue distribution and total activity in normal and CD epithelia, to evaluate tissue peptidase activity over gliadin peptides, and compare this with activities of mammalian POP and rat intestinal extracts.

MATERIAL AND METHODS

POP was assayed in biopsy preparations enzymatically and by Western blot analysis. Distribution was studied by immunohistochemistry using a specific POP antibody. Peptide cleavage was followed by mass spectroscopy-high-performance liquid chromatography (MS-HPLC).

RESULTS

There was no difference in POP activity between normal and CD samples, but those from active CD subjects had an even higher ability to degrade the 33-mer peptide than those from treated CD and healthy humans. POP locates intracellularly in epithelia, similarly to dipeptidyl peptidase IV (DPPIV), but the latter is clearly found in normal microvilli but less so in diseased microvilli. Mammalian POP is unable to digest 33-mer peptide, which, conversely, is a POP inhibitor. Rat intestine is more effective than human intestine in cleaving the 33-mer peptide. However, the products are still harmful epitopes. A surplus of POP eliminates 12-mer and 19-mer peptide products.

CONCLUSIONS

The results rule out a causative role of POP in the pathogenesis of CD and strongly suggest that other peptidases are needed to eliminate gliadin-derived, immunoactive and toxic peptides larger than 33-mer, which is a POP inhibitor.

Distribution of immunoreactive prolyl oligopeptidase in human and rat brain

Prolyl oligopeptidase (POP) is a serine endoprotease that hydrolyses peptides shorter than 30-mer. POP may have a role in inositol 1,4,5-triphosphate (IP(3)) signaling and in the actions of antidepressants, and POP inhibitors have exhibited antiamnesic and neuroprotective properties. However, little is known about the distribution of POP protein in the brain. We used immunohistochemistry to localize POP enzyme in the human whole hemisphere and in the rat whole brain. In humans, the highest POP densities were observed in caudate nucleus and putamen, hippocampus and cortex. In the rat, the highest POP densities were found in substantia nigra, hippocampus, cerebellum and caudate putamen. In general, the distribution of POP in human and rat brains was very similar and resembled that of IP(3) receptors. Our findings are support for a role of POP in movement regulation, cognition and possibly in IP(3) signaling. The expression of POP in processing nuclei further supports its function beyond neuropeptide metabolism.

2(S)-(Cycloalk-1-enecarbonyl)-1-(4-phenyl-butanoyl)pyrrolidines and 2(S)-(aroyl)-1-(4-phenylbutanoyl)pyrrolidines as prolyl oligopeptidase inhibitors

In order to replace the P2-P1 amide group, different 1-cycloalkenyls and 2-aryls were studied in the place of the P1 pyrrolidine group of a 4-phenylbutanoyl-L-Pro-pyrrolidine structure, which is a well-known prolyl oligopeptidase inhibitor SUAM-1221. The 1-cyclopentenyl and the 2-thienyl groups gave novel compounds, which were equipotent with the corresponding pyrrolidine-analog SUAM-1221. It was shown that the P2-P1 amide group of POP inhibitors can be replaced by an alpha,beta-unsaturated carbonyl group or the aryl conjugated carbonyl group.

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.

Ontogeny of prolyl endopeptidase and pyroglutamyl peptidase I in rat tissues

Prolyl endopeptidase and pyroglutamyl peptidase I are enzymes which participate in the degradation of thyrotropin-releasing hormone (TRH), a hormone which is thought to play an important role in the development of organs and tissues. Here, we have characterized the ontogeny of TRH degrading enzyme activity in the brain cortex, lung, heart, kidney and liver. Overall, prolyl endopeptidase activity was found to be 2 to 5 fold higher in newborn vs. adult rat tissues, with the exception of the soluble form in the liver and the particulate form in the lung. In contrast, the developmental profile of pyroglutamyl peptidase I activity was found to be more variable and tissue dependent. These results corroborate the idea that both enzymes play important, tissue-specific roles during the development and maturation of rat organs.

Ontogeny of prolyl endopeptidase and pyroglutamyl peptidase I in rat tissues

Prolyl endopeptidase and pyroglutamyl peptidase I are enzymes which participate in the degradation of thyrotropin-releasing hormone (TRH), a hormone which is thought to play an important role in the development of organs and tissues. Here, we have characterized the ontogeny of TRH degrading enzyme activity in the brain cortex, lung, heart, kidney and liver. Overall, prolyl endopeptidase activity was found to be 2 to 5 fold higher in newborn vs. adult rat tissues, with the exception of the soluble form in the liver and the particulate form in the lung. In contrast, the developmental profile of pyroglutamyl peptidase I activity was found to be more variable and tissue dependent. These results corroborate the idea that both enzymes play important, tissue-specific roles during the development and maturation of rat organs.

An introduction of a pyridine group into the structure of prolyl oligopeptidase inhibitors

A series of ionizable prolyl oligopeptidase inhibitors were developed through the introduction of a pyridyl group to the P3 position of the prolyl oligopeptidase inhibitor structure. The study was performed on previously developed prolyl oligopeptidase inhibitors with proline mimetics at the P2 position. The 3-pyridyl group resulted in equipotent compounds as compared to the parent compounds. It was shown that the pyridyl group improves water solubility and, in combination with a 5(R)-tert-butyl-l-prolyl group at the P2 position, good lipophilicity can be achieved.

Suggested functions for prolyl oligopeptidase: a puzzling paradox

Prolyl oligopeptidase (PO, E.C. 3.4.21.26) is a post-proline cleaving enzyme with endopeptidase activity towards peptides not longer than 30 amino acids. It has been purified and characterized from various mammalian and bacterial sources, but despite its thorough enzymological and structural characterization, the exact function of PO remains obscure. Many investigations have addressed the physiological role of this enzyme, mainly by the use of specific PO inhibitors, activity measurements in clinical samples and (neuro)peptide degradation studies. From the combined results emerges a puzzling paradox: how can an intracellular, cytoplasmatic oligopeptidase affect not only the amount of extracellular neuropeptides but also signal transduction and secretion? This report provides a review of the literature on the suggested functions for PO, highlighting possible pitfalls and contradictions.

Brain Prolyl Endopeptidase Expression in Aging, APP Transgenic Mice and Alzheimer’s Disease

Prolyl endopeptidase (PEP) is believed to inactivate neuropeptides that are present in the extracellular space. However, the intracellular localization of PEP suggests additional, yet unidentified physiological functions for this enzyme. Here we studied the expression, enzymatic activity and subcellular localization of PEP in adult and aged mouse brain as well as in brains of age-matched APP transgenic Tg2576 mice and in brains of Alzheimer's disease patients. In mouse brain PEP was exclusively expressed by neurons and displayed region- and age-specific differences in expression levels, with the highest PEP activity being present in cerebellum and a significant increase in hippocampal but not cortical or cerebellar PEP activity in aged mouse brain. In brains of young APP transgenic Tg2576 mice, hippocampal PEP activity was increased compared to wild-type littermates in the pre-plaque phase but not in aged mice with beta-amyloid plaque pathology. This "accelerated aging" with regard to hippocampal PEP expression in young APP transgenic mice might be one factor contributing to the observed cognitive deficits in these mice in the pre-plaque phase and could also explain in part the cognition-enhancing effects of PEP inhibitors in several experimental paradigms.