Rat tripeptidyl peptidase I: molecular cloning, functional expression, tissue localization and enzymatic characterization

The effects of para-chloromercuribenzoic acid and different oxidative and sulfhydryl agents on a novel, non-AT1, non-AT2 angiotensin binding site identified as neurolysin

A novel, non-AT1, non-AT2 brain binding site for angiotensin peptides that is unmasked by p-chloromercuribenzoate (PCMB) has been identified as a membrane associated variant of neurolysin. The ability of different organic and inorganic oxidative and sulfhydryl reactive agents to unmask or inhibit 125I-Sar1Ile8 angiotensin II (SI-Ang II) binding to this site was presently examined. In tissue membranes from homogenates of rat brain and testis incubated in assay buffer containing losartan (10 μM) and PD123319 (10 μM) plus 100 μM PCMB, 5 of the 39 compounds tested inhibited 125I-SI Ang II binding in brain and testis. Mersalyl acid, mercuric chloride (HgCl2) and silver nitrate (AgNO3) most potently inhibited 125I-SI Ang II binding with IC50s ~1-20 μM. This HgCl2 inhibition was independent of any interaction of HgCl2 with angiotensin II (Ang II) based on the lack of effect of HgCl2 on the dipsogenic effects of intracerebroventricularly administered Ang II and 125I-SI Ang II binding to AT1 receptors in the liver. Among sulfhydryl reagents, cysteamine and reduced glutathione (GSH), but not oxidized glutathione (GSSG) up to 1mM, inhibited PCMB-unmasked 125I-SI Ang II binding in brain and testis. Thimerosal and 4-hydroxymercuribenzoate moderately inhibited PCMB-unmasked 125I-SI Ang II binding in brain and testis at 100 μM; however, they also unmasked non-AT1, non-AT2 binding independent of PCMB. 4-Hydroxybenzoic acid did not promote 125 I-SI Ang II binding to this binding site indicating that only specific organomercurial compounds can unmask the binding site. The common denominator for all of these interacting substances is the ability to bind to protein cysteine sulfur. Comparison of cysteines between neurolysin and the closely related enzyme thimet oligopeptidase revealed an unconserved cysteine (cys650, based on the full length variant) in the proposed ligand binding channel (Brown et al., 2001) [45] near the active site of neurolysin. It is proposed that the mercuric ion in PCMB and closely related organomercurial compounds binds to cys650, while the acidic anion forms an ionic bond with a nearby arginine or lysine along the channel to effect a conformational change in neurolysin that promotes Ang II binding.

Dipeptidyl-peptidase I does not functionally compensate for the loss of tripeptidyl-peptidase I in the neurodegenerative disease late-infantile neuronal ceroid lipofuscinosis

LINCL (late-infantile neuronal ceroid lipofuscinosis) is a fatal neurodegenerative disease resulting from mutations in the gene encoding the lysosomal protease TPPI (tripeptidyl-peptidase I). TPPI is expressed ubiquitously throughout the body but disease appears restricted to the brain. One explanation for the absence of peripheral pathology is that in tissues other than brain, other proteases may compensate for the loss of TPPI. One such candidate is another lysosomal aminopeptidase, DPPI (dipeptidyl-peptidase I), which appears to have overlapping substrate specificity with TPPI and is expressed at relatively low levels in brain. Compensation for the loss of TPPI by DPPI may have therapeutic implications for LINCL and, in the present study, we have investigated this possibility using mouse genetic models. Our rationale was that if DPPI could compensate for the loss of TPPI in peripheral tissues, then its absence should exacerbate disease in an LINCL mouse model but, conversely, increased CNS (central nervous system) expression of DPPI should ameliorate disease. By comparing TPPI and DPPI single mutants with a double mutant lacking both proteases, we found that the loss of DPPI had no effect on accumulation of storage material, disease severity or lifespan of the LINCL mouse. Transgenic expression of DPPI resulted in a approximately 2-fold increase in DPPI activity in the brain, but this had no significant effect on survival of the LINCL mouse. These results together indicate that DPPI cannot functionally compensate for the loss of TPPI. Therapeutic approaches to increase neuronal expression of DPPI are therefore unlikely to be effective for treatment of LINCL.

Synthesis and use of 4-peptidylhydrazido-N-hexyl-1,8-naphthalimides as fluorogenic histochemical substrates for dipeptidyl peptidase IV and tripeptidyl peptidase I

Gly-Pro-, Gly-Pro-Met- and Ala-Ala-Phe-N'-(2-hexyl-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)-hydrazides are synthesized by guanidinium/uronium type condensing reagent and used as fluorogenic substrates to localize dipeptidyl peptidase IV and tripeptidyl peptidase I activities in mammalian tissue sections. Enzyme hydrolysis releases 2-hexyl-6-hydrazino-1H-benzo[de]isoquinoline-1,3(2H)-dione, which couples with piperonal to form insoluble fluorescent hydrazone, precipitating on the enzyme locations and marking them. The fluorescent technique reveals precisely the enzymes locations at the lack of background noise in a single incubation step. It avoids most of the drawbacks of the previously proposed fluorescent histochemical techniques and can be valuable for the in situ studies of these enzymes in norm and pathology.

The significance of brain aminopeptidases in the regulation of the actions of angiotensin peptides in the brain

From the outset, the concept of a brain renin-angiotensin system (RAS) has been controversial and this controversy continues to this day. In addition to the unresolved questions as to the means by which, and location(s) where brain Ang II is synthesized, and the uncertainties regarding the functionality of the different subtypes of Ang II receptors in the brain, a new controversy has arisen with respect to the identity of the angiotensin peptide(s) that activate brain AT(1) receptors. While it has been known for some time that Ang III can activate Ang II receptors with equivalent or near-equivalent efficacy to Ang II, it has been proposed that in the brain, only Ang III is active. This proposal, which we have named "The Angiotensin III Hypothesis" states that Ang II must be converted to Ang III in order to activate brain AT(1) receptors. This review examines several aspects of the controversies regarding the brain RAS with a special focus on brain aminopeptidases, studies that either support or refute The Angiotensin III Hypothesis, and the implications of The Angiotensin III Hypothesis for the activity of the brain RAS. It also addresses the need for further research that can test The Angiotensin III Hypothesis and definitively identify the angiotensin peptide(s) that activate brain AT(1) receptor-mediated effects.

Neuropeptidases

Neuropeptides are neurotransmitters and modulators distributed in the central nervous system (CNS) and peripheral nervous system. Their abnormalities cause neurological and mental diseases. Neuropeptidases are enzymes crucial for the biosynthesis and biodegradation of neuropeptides. We here focus on the peptidases involved in the metabolism of the well-studied opioid peptides. Bioactive enkephalins are formed from propeptides by processing enzymes—prohormone thiol protease, prohormone convertase 1 and 2 (PC 1 and 2), carboxypeptidase H/E, and Arg/Lys aminopeptidase. After they exert their biological effects, enkephalins are likely to be inactivated by degrading enzymes—angiotensin-converting enzyme (ACE), aminopeptidase N (APN), puromycin-sensitive aminopeptidase (PSA), and endopeptidase 24.11. Recently, a neuron-specific aminopeptidase (NAP), which was a putative enkephalin-inactivating enzyme at the synapses, was found. Neuropeptidases are useful drug targets and their inhibitors can be therapeutic. Synthetic anti-enkephalinases and anti-aminopeptidases are being developed. They are potent analgesics but have fewer side effects than the opiates.

Characterisation of lipofuscin-like lysosomal inclusion bodies from human placenta

A structural hallmark of lysosomes is heterogeneity of their contents. We describe a method for isolation of particulate materials from human placental lysosomes. After a methionine methyl ester-induced disruption of lysosomes and two density gradient centrifugations we obtained a homogeneous membrane fraction and another one enriched in particulate inclusions. The latter exhibited a yellow-brown coloration and contained bodies lacking a delimiting membrane, which were characterised by a granular pattern and high electron density. The lipofuscin-like inclusion materials were rich in tripeptidyl peptidase I, beta-glucuronidase, acid ceramidase and apolipoprotein D and contained proteins originating from diverse subcellular localisations. Here we show that human term placenta contains lipofuscin-like lysosomal inclusions, a phenomenon usually associated with senescence in postmitotic cells. These findings imply that a simple pelleting of a lysosomal lysate is not appropriate for the isolation of lysosomal membranes, as the inclusions tend to be sedimented with the membranes.

Functional biology of the neuronal ceroid lipofuscinoses (NCL) proteins

Neuronal ceroid lipofucinoses (NCLs) are a group of severe neurodegenerative disorders characterized by accumulation of autofluorescent ceroid lipopigment in patients' cells. The different forms of NCL share many similar pathological features but result from mutations in different genes. The genes affected in NCLs encode both soluble and transmembrane proteins and are localized to ER or to the endosomes/lysosomes. Due to selective vulnerability of the central nervous system in the NCL disorders, the corresponding proteins are proposed to have important, tissue specific roles in the brain. The pathological similarities of the different NCLs have led not only to the grouping of these disorders but also to suggestion that the NCL proteins function in the same biological pathway. Despite extensive research, including the development of several model organisms for NCLs and establishment of high-throughput techniques, the precise biological function of many of the NCL proteins has remained elusive. The aim of this review is to summarize the current knowledge of the functions, or proposed functions, of the different NCL proteins.

Tripeptidyl-peptidase I in health and disease

The lysosomal lumen contains numerous acidic hydrolases involved in the degradation of carbohydrates, lipids, proteins, and nucleic acids, which are basic cell components that turn over continuously within the cell and/or are ingested from outside of the cell. Deficiency in almost any of these hydrolases causes accumulation of the undigested material in secondary lysosomes, which manifests itself as a form of lysosomal storage disorder (LSD). Mutations in tripeptidyl-peptidase I (TPP I) underlie the classic late-infantile form of neuronal ceroid lipofuscinoses (CLN2), the most common neurodegenerative disorders of childhood. TPP I is an aminopeptidase with minor endopeptidase activity and Ser475 serving as an active-site nucleophile. The enzyme is synthesized as a highly glycosylated precursor transported by mannose-6-phosphate receptors to lysosomes, where it undergoes proteolytic maturation. This review summarizes recent progress in understanding of TPP I biology and molecular pathology of the CLN2 disease process, including distribution of the enzyme, its biosynthesis, glycosylation, transport and activation, as well as catalytic mechanisms and their potential implications for pathogenesis and treatment of the underlying disease. Promising data from gene and stem cell therapy in laboratory animals raise hope that CLN2 will be the first neurodegenerative LSD for which causative treatment will become available for humans.

Glycosaminoglycans Modulate Activation, Activity, and Stability of Tripeptidyl-peptidase I in Vitro and in Vivo

Tripeptidyl-peptidase I (TPP I, CLN2 protein) is a lysosomal exopeptidase that sequentially removes tripeptides from the N termini of polypeptides and shows a minor endoprotease activity. Mutations in TPP I lead to classic late-infantile neuronal ceroid lipofuscinosis, a neurodegenerative lysosomal storage disease. TPP I proenzyme is converted in lysosomes into a mature enzyme with the assistance of another protease and is able to autoactivate in acidic pH in vitro via a unimolecular mechanism. Because autoactivation in vitro at the pH values reported for lysosomes generated inactive enzyme, we intended to determine whether physiologically relevant factors can modify this process to also make it plausible in vivo. Here, we report that high ionic strength and glycosaminoglycans (GAGs) increase yields (ionic strength) or yields and rates (GAGs) of activation, enhance degradation of liberated TPP I prosegment fragments, and switch effective autoactivation of TPP I proenzyme toward less acidic pH values (up to pH 6.0). Although ionic strength and GAGs also inhibited TPP I activity in vitro and in living cells, the degree of inhibition (from 20 to 60%) appears to be of rather limited functional significance. Importantly, binding to GAGs improved thermal stability of TPP I and protected the enzyme against alkaline pH-induced denaturation in vitro (t((1/2)) of mature enzyme at pH 7.4 increased by approximately 8-fold in the presence of heparin) and in vivo ( approximately 2-fold higher loss of TPP I in cells deficient in GAGs than in control cells after bafilomycin A1 treatment). These findings elucidate a potent physiologically relevant mechanism of TPP I regulation by GAGs and suggest that generation of the active enzyme via autoactivation can be accomplished not only in vitro but in vivo as well.

Clinical proteomics in lung diseases

Proteomics is a relatively new approach for understanding the pathology and pathogenesis of various diseases. It has also been used for characterizing the modifications in protein expression during the development of interstitial lung diseases, in lung tumors, or following exposure to exogenous stress signals. We compared the protein composition of primary human lung fibroblasts derived from patients with lung fibrosis and control fibroblasts of unaffected lung tissues. We found a predominant modulation in proteins related to the cytoskeleton, including decreased expression of vimentin and lamin A/C, and increased expression of moesin. Furthermore, we observed lower levels of components of the antioxidative system, such as omega class glutathione S-transferase and an up-regulation of an intracellular chloride channel. In fibroblasts obtained from fibrotic lungs, the expression of a major histocompatibility complex class I C was decreased, and so was the expression of tripeptidyl-peptidase-I-precursor, a collagen-degrading exopeptidase. Our results and the studies reviewed in this paper represent just the beginning of detailed studies that should unravel the relevance and the functional consequences of differential protein expressions in the diseased lung.

Proteome comparison of alveolar macrophages with monocytes reveals distinct protein characteristics

Alveolar macrophages (AMs) are a subset of tissue macrophages situated in the alveolar milieu. Compared with their precursor blood monocytes, AMs exhibit distinct physiologic functions unique to their anatomic location. However, the molecular details that control monocyte differentiation into AMs remain unknown. This study employed a proteomic approach to define protein characteristics that distinguish AMs from monocytes. AMs and monocytes were obtained from six nonsmoking, healthy donors. Whole cell lysates from each donor's AMs and monocytes were analyzed by two-dimensional (2D) gel electrophoreses. The protein density for each protein spot in a 2D gel was compared between these two cell types. Proteins that demonstrated consistent level changes of greater than 2.5-fold in all six donors were subjected to tandem mass spectrometry for protein identity. Using this process, we revealed proteome changes in AMs that relate to their physiologic roles in proteolysis, actin reorganization, and cellular adaptation in the unique alveolar milieu. By comparison, blood monocytes displayed higher levels of the proteins involved in transcription, metabolism, inflammation, and in the control of proteolysis. These results provide new insights into the biology of mononuclear phagocytes and set a basis for future causality studies.

The intracellular location and function of proteins of neuronal ceroid lipofuscinoses

Neuronal ceroid lipofuscinoses are a group of diseases characterized by accumulation of hydrophobic proteins in lysosomes of neurons and other types of cells. NCLs are caused by at least 8 mutant genes (CLN1-CLN8), though CLN4 and CLN7 have not yet been identified. Except for Cln1p, the protein encoded by CLN1, the defective proteins are associated with lysosomal accumulation of mitochondrial ATP synthase subunit c. Cln1p and Cln2p are soluble lysosomal enzymes, targeted to lysosomes in a mannose 6-phosphate dependent manner. Mutations in the lysosomal protease cathepsin D cause another NCL. Cln3p, Cln5p, Cln6p and Cln8p are thought to be transmembrane proteins. Cln3p and Cln5p are localized in the endosome-lysosomal compartment. Deficiency of endosomal membrane protein CLC-3, a member of the chloride channel family, causes NCL-like phenotype and lysosomal storage of subunit c. Herein, we review the features of NCL and NCL-related proteins and discuss the involvement of the proteins in lysosomal degradation of subunit c.

N-glycosylation is crucial for folding, trafficking, and stability of human tripeptidyl-peptidase I

Tripeptidyl-peptidase I (TPP I) is a lysosomal serine-carboxyl peptidase that sequentially removes tripeptides from polypeptides. Naturally occurring mutations in TPP I are associated with the classic late infantile neuronal ceroid lipofuscinosis. Human TPP I has five potential N-glycosylation sites at Asn residues 210, 222, 286, 313, and 443. To analyze the role of N-glycosylation in the function of the enzyme, we obliterated each N- glycosylation consensus sequence by substituting Gln for Asn, either individually or in combinations, and expressed mutated cDNAs in Chinese hamster ovary and human embryonic kidney 293 cells. Here, we demonstrate that human TPP I in vivo utilizes all five N-glycosylation sites. Elimination of one of these sites, at Asn-286, dramatically affected the folding of the enzyme. However, in contrast to other misfolded proteins that are retained in the endoplasmic reticulum, only a fraction of misfolded TPP I mutant expressed in Chinese hamster ovary cells, but not in human embryonic kidney 293 cells, was arrested in the ER, whereas its major portion was secreted. Secreted proenzyme formed non-native, interchain disulfide bridges and displayed only residual TPP I activity upon acidification. A small portion of TPP I missing Asn-286-linked glycan reached the lysosome and was processed to an active species; however, it showed low thermal and pH stability. N-Glycans at Asn-210, Asn-222, Asn-313, and Asn-443 contributed slightly to the specific activity of the enzyme and its resistance to alkaline pH-induced inactivation. Phospholabeling experiments revealed that N-glycans at Asn-210 and Asn-286 of TPP I preferentially accept a phosphomannose marker. Thus, a dual role of oligosaccharide at Asn-286 in folding and lysosomal targeting could contribute to the unusual, but cell type-dependent, fate of misfolded TPP I conformer and represent the molecular basis of the disease process in subjects with naturally occurring missense mutation at Asn-286.

Inhibition of recombinant dipeptidyl peptidase III by synthetic hemorphin-like peptides

In order to find the most effective antagonist for dipeptidyl peptidase III degrading enkephalin, we synthesized hemorphin-like pentapeptides with aliphatic or aromatic amino acids at the N-termini, such as VVYPW, LVYPW, IVYPW, YVYPW, FVYPW and WVYPW. Among those pentapeptides, IVYPW and WVYPW showed the strongest inhibitory activity toward rDPP III. The K(i) values of IVYPW and WVYPW were 0.100+/-0.011 and 0.126+/-0.015 microM (mean+/-S.E.), respectively. The order of K(i) values was Ile> or =Trp>Phe> or =Tyr>Leu>Ala>Val>Ser>Gly. rDPP III activity is inhibited in a non-competitive manner by these peptides. The peptide VYPW did not inhibit rDPP III activity, but the sequence is essential for the expression of inhibitory activity.

Biosynthesis, glycosylation, and enzymatic processing in vivo of human tripeptidyl-peptidase I

Human tripeptidyl-peptidase I (TPP I, CLN2 protein) is a lysosomal serine protease that removes tripeptides from the free N termini of small polypeptides and also shows a minor endoprotease activity. Due to various naturally occurring mutations, an inherited deficiency of TPP I activity causes a fatal lysosomal storage disorder, classic late infantile neuronal ceroid lipofuscinosis (CLN2). In the present study, we analyzed biosynthesis, glycosylation, transport, and proteolytic processing of this enzyme in stably transfected Chinese hamster ovary cells as well as maturation of the endocytosed proenzyme in CLN2 lymphoblasts, fibroblasts, and N2a cells. Human TPP I was initially identified as a single precursor polypeptide of approximately 68 kDa, which, within a few hours, was converted to the mature enzyme of approximately 48 kDa. Compounds affecting the pH of intracellular acidic compartments, those interfering with the intracellular vesicular transport as well as inhibition of the fusion between late endosomes and lysosomes by temperature block or 3-methyladenine, hampered the conversion of TPP I proenzyme into the mature form, suggesting that this process takes place in lysosomal compartments. Digestion of immunoprecipitated TPP I proenzyme with both N-glycosidase F and endoglycosidase H as well as treatment of the cells with tunicamycin reduced the molecular mass of TPP I proenzyme by approximately 10 kDa, which indicates that all five potential N-glycosylation sites in TPP I are utilized. Mature TPP I was found to be partially resistant to endo H treatment; thus, some of its N-linked oligosaccharides are of the complex/hybrid type. Analysis of the effect of various classes of protease inhibitors and mutation of the active site Ser(475) on human TPP I maturation in cultured cells demonstrated that although TPP I zymogen is capable of autoactivation in vitro, a serine protease that is sensitive to AEBSF participates in processing of the proenzyme to the mature, active form in vivo.

Tripeptidyl peptidase-I is essential for the degradation of sulphated cholecystokinin-8 (CCK-8S) by mouse brain lysosomes

Tripeptidyl peptidase-I (TPP-I) is a lysosomal exopeptidase which removes tripeptides from the N-terminus of small proteins. Mutations in the TPP-I gene result in a lethal neurodegenerative disease, late infantile neuronal ceroid lipofuscinosis. The pathological consequences of loss of activity are only manifested in neuronal cells suggesting that TPP-I may be involved in the lysosomal degradation of neuropeptides. We have investigated the degradation of the C-terminal octapeptide of sulphated cholecystokinin (CCK-8S) by a lysosomal fraction purified from mouse brain. Degradation products were characterised by reversed phase HPLC and mass spectrometry. Incubation of CCK-8S with brain lysosomes results in the sequential removal of the tripeptides DY(SO(3)H)M and Glycl-Tryptophanyl-Methionine from the N-terminus of CCK-8S. Degradation of CCK-8S in the isolated lysosomal fraction is completely prevented by Ala-Ala-Phe-chloromethyl ketone, an inhibitor of TPP-I. Butabindide, a specific inhibitor of TPP-II, a cell surface peptidase which also cleaves CCK-8S, inhibits TPP-I but kinetic studies indicate that the Ki for inhibition of TPP-I is 1000-fold higher than the Ki for the inhibition of TPP-II. Consequently, higher concentrations of butabindide are required for the inhibition of CCK-8S degradation by TPP-I than by TPP-II. These results indicate that whereas cell surface TPP-II is responsible for regulating extracellular CCK-8S levels, lysosomal TPP-I is largely responsible for the degradation of CCK-8S which enters the cell by receptor-mediated endocytosis.

Lysosomal degradation of cholecystokinin-(29-33)-amide in mouse brain is dependent on tripeptidyl peptidase-I: implications for the degradation and storage of peptides in classical late-infantile neuronal ceroid lipofuscinosis

Tripeptidyl peptidase-I (TPP-I) is a lysosomal exopeptidase which removes tripeptides from the N-terminus of small peptides. Mutations in the TPP-I gene result in a lethal neurodegenerative disease, classical late-infantile neuronal ceroid lipofuscinosis (CLN2). This disease is characterized by the accumulation of proteinaceous and autofluorescent material within the lysosomes of neurons, which undergo massive cell death during the course of the disease. The absence of TPP-I may result in the lysosomal accumulation of small peptides and proteins, which eventually compromises lysosomal functions critical to the survival of neurons. To investigate the metabolism of small peptides, we have studied the degradation of cholecystokinin-(29-33)-amide (GWMDF-NH2; cholecystokinin C-terminal pentapeptide) by lysosomal fractions isolated from mouse brain and several other tissues. GWMDF-NH2 is cleaved at only one peptide bond by brain lysosomes, to produce GWM and DF-NH2. Inhibitor studies demonstrate that this reaction is catalysed by TPP-I. In contrast, lysosomal fractions from other mouse tissues additionally cleave a second peptide bond to produce GW and MDF-NH2. Inhibitor studies indicate that this reaction is catalysed by dipeptidyl peptidase-I (DPP-I; cathepsin C). Inhibitors of TPP-I are sufficient to completely block the degradation of GWMDF-NH2 by brain, but inhibitors of both TPP-I and DPP-I are required to completely inhibit the degradation of GWMDF-NH2 by other mouse tissues. Enzyme assays confirm the low activity of DPP-I in brain. An unrelated neuropeptide, neuromedin B, is degraded by a pathway that is partially dependent on TPP-I. These results indicate that TPP-I is required for the partial or complete digestion of certain neuropeptides by brain lysosomes. In the absence of TPP-I, neuropeptides or their degradation products will accumulate in brain lysosomes and may contribute to the pathogenesis of CLN2. Other tissues are spared because they express another peptidase, DPP-I, which has extensive activity on peptides and can compensate for the loss of TPP-I.