A novel carboxypeptidase B that processes native beta-amyloid precursor protein is present in human hippocampus

Brain Expression of CPB2 and Effects of Cpb2 Deficiency in Mouse Models of Behavior

BACKGROUND

 Procarboxypeptidase B2 (proCPB2 or TAFI) is a zymogen that after activation cleaves C-terminal basic residues from peptides or proteins with many identified targets. A splice variant of CPB2 has been found in the brain lacking essential residues for its carboxypeptidase function. The aim was to determine CPB2 expression in the brain and effects of CPB2 deficiency (Cpb2 -/-) on behavior.

MATERIALS AND METHODS

 Behavioral effects were tested by comparing Cpb2 -/- mice in short-term (open field and elevated zero maze tests) and long-term (Phenotyper) observations with wild-type (WT) controls.

RESULTS

 Long-term observation compared day 1 (acclimatizing to novel environment) to day 4 (fully acclimatized) with the inactive (day) and active (night) periods analyzed separately. Brain expression of CPB2 mRNA and protein was interrogated in publicly available databases. Long-term observation demonstrated differences between WT and Cpb2 -/- mice in several parameters. For example, Cpb2 -/- mice moved more frequently on both days 1 and 4, especially in the normally inactive periods. Cpb2 -/- mice spent more time on the shelter and less time in it. Differences were more pronounced on day 4 after the mice had fully acclimatized. In short-term observations, no differences were observed between Cpb2 -/- mice and WT mice. Brain expression of CBP2 was not detectable in the human protein atlas. Databases of single-cell RNAseq did not show expression of CPB2 mRNA in either human or mouse brain.

CONCLUSION

 Continuous observation of home-cage behavior suggests that Cpb2 -/- mice are more active than WT mice, show different day-night activity levels, and might have a different way of processing information.

Enhanced carboxypeptidase efficacies and differentiation of peptide epimers

Carboxypeptidases enzymatically cleaves the peptide bond of C-terminal amino acids within peptides. In humans, it is involved in enzymatic synthesis and maturation of proteins and peptides. Carboxypeptidases A and Y have difficulty hydrolyzing the peptide bond of dipeptides and some other amino acid sequences. Early investigations into different N-blocking groups concluded that larger moieties increased substrate susceptibility to peptide bond hydrolysis with carboxypeptidase. This study conclusively demonstrates that 6-aminoquinoline-N-hydroxysuccimidyl carbamate (AQC) as an N-blocking group greatly enhances substrate hydrolysis with carboxypeptidase. AQC addition to the N-terminus of amino acids and peptides also improves chromatographic peak shape and sensitivities via mass spectrometry detection. These enzymes have been used for amino acid sequence determination prior to the advent of modern proteomics. However, most modern proteomic methods assume that all peptides are comprised of l-amino acids and therefore cannot distinguish L-from d-amino acids within the peptide sequence. The majority of existing methods that allow for chiral differentiation either require synthetic standards or incur racemization in the process. This study highlights the resistance of d-amino acids within peptides to enzymatic hydrolysis by Carboxypeptidase Y. This stereoselectivity may be advantageous when screening low abundance peptide epimers.

Food protein amyloid fibrils: Origin, structure, formation, characterization, applications and health implications

Amyloid fibrils have traditionally been considered only as pathological aggregates in human neurodegenerative diseases, but it is increasingly becoming clear that the propensity to form amyloid fibrils is a generic property for all proteins, including food proteins. Differently from the pathological amyloid fibrils, those derived from food proteins can be used as advanced materials in biomedicine, tissue engineering, environmental science, nanotechnology, material science as well as in food science, owing to a combination of highly desirable feature such as extreme aspect ratios, outstanding stiffness and a broad availability of functional groups on their surfaces. In food science, protein fibrillization is progressively recognized as an appealing strategy to broaden and improve food protein functionality. This review article discusses the various classes of reported food protein amyloid fibrils and their formation conditions. It furthermore considers amyloid fibrils in a broad context, from their structural characterization to their forming mechanisms and ensued physical properties, emphasizing their applications in food-related fields. Finally, the biological fate and the potential toxicity mechanisms of food amyloid fibrils are discussed, and an experimental protocol for their health safety validation is proposed in the concluding part of the review.

Rational Design of Amyloid-Like Fibrillary Structures for Tailoring Food Protein Techno-Functionality and Their Potential Health Implications

To control and enhance protein functionality is a major challenge for food scientists. In this context, research on food protein fibril formation, especially amyloid fibril formation, holds much promise. We here first provide a concise overview of conditions, which affect amyloid formation in food proteins. Particular attention is directed towards amyloid core regions because these sequences promote ordered aggregation. Better understanding of this process will be key to tailor the fibril formation process. Especially seeding, that is, adding preformed protein fibrils to protein solutions to accelerate fibril formation holds promise to tailor aggregation and fibril techno-functionality. Some studies have already indicated that food protein fibrillation indeed improves their techno-functionality. However, much more research is necessary to establish whether protein fibrils are useful in complex food systems and whether and to what extent they resist food processing unit operations. In this review the effect of amyloid formation on gelation, interfacial properties, foaming, and emulsification is discussed. Despite their prevalent role as functional structures, amyloids also receive a lot of attention due to their association with protein deposition diseases, prompting us to thoroughly investigate the potential health impact of amyloid-like aggregates in food. A literature review on the effect of the different stages of the human digestive process on amyloid toxicity leads us to conclude that food-derived amyloid fibrils (even those with potential pathogenic properties) very likely have minimal impact on human health. Nevertheless, prior to wide-spread application of the technology, it is highly advisable to further verify the lack of toxicity of food-derived amyloid fibrils.

The mRNA encoding TAFI is alternatively spliced in different cell types and produces intracellular forms of the protein lacking TAFIa activity

TAFI (thrombin-activatable fibrinolysis inhibitor) is a pro-carboxypeptidase, encoded by the CPB2 gene in humans that links the coagulation cascade to fibrinolysis and inflammation. The liver is the main source for plasma TAFI, and TAFI expression has been documented in platelets and monocyte-derived macrophages. A recent study reported an alternatively spliced CPB2 mRNA variant lacking exon 7 (Δ7) in HepG2 cells and liver. Another study identified a CPB2 mRNA variant lacking exon 7 and a 52 bp deletion in exon 11 (Δ7+11) in human hippocampus. We have examined alternative splicing of CPB2 mRNA in various cell types by RT-PCR and have assessed the functional properties of TAFI variants encoded by these transcripts by recombinant expression in mammalian cells. We identified the Δ7 exon skipping event in liver, Dami megakaryoblasts, THP-1-derived macrophages, peripheral blood mononuclear cells, platelets, testis, cerebellum, and SH-SY5Y neuroblastoma cells. The Δ11 alternative splicing event was notably absent in liver cells. We also detected a novel exon Δ7+8 skipping event in liver and megakaryocytes. Of note, we detected non-alternatively spliced CPB2 transcripts in brain tissues, suggesting the expression of full-length TAFI in brain. Experiments using cultured mammalian cells transfected with wild-type CPB2-, Δ7-, Δ7+11 -, and_Δ11 -cDNA revealed that alternatively spliced TAFI is stored inside the cells, cannot be activated by thrombin-thrombomodulin, and does not have TAFIa activity. The alternative splicing events clearly do not give rise to a secreted protein with basic carboxy-peptidase activity, but the intracellular forms may possess novel functions related to intracellular proteolysis.

Structure-function relationships in thrombin-activatable fibrinolysis inhibitor

Thrombin-activatable fibrinolysis inhibitor (TAFI) is an important regulator in the balance of coagulation and fibrinolysis. TAFI is a metallocarboxypeptidase that circulates in plasma as zymogen. Activated TAFI (TAFIa) cleaves C-terminal lysine or arginine residues from peptide substrates. The removal of C-terminal lysine residues from partially degraded fibrin leads to reduced plasmin formation and thus attenuation of fibrinolysis. TAFI also plays a role in inflammatory processes via the removal of C-terminal arginine or lysine residues from bradykinin, thrombin-cleaved osteopontin, C3a, C5a and chemerin. TAFI has been studied extensively over the past three decades and recent publications provide a wealth of information, including crystal structures, mutants and structural data obtained with antibodies and peptides. In this review, we combined and compared available data on structure/function relationships of TAFI.

Progress in metallocarboxypeptidases and their small molecular weight inhibitors

In what corresponds to a life span, metallocarboxypeptidases (MCPs) have jumped from being mere contaminants in animal pancreas powders (in depression year 1929) to be key players in cellular and molecular processes (in yet-another-depression years 2009-2010). MCPs are unique zinc-dependent enzymes that catalyze the breakdown of the amide bond at the C-terminus of peptide and protein substrates and participate in the recovery of dietary amino acids, tissue organogenesis, neurohormone and cytokine maturation and other important physiological processes. More than 26 genes code for MCPs in the human genome, many of them still waiting to be fully understood in terms of physiological function. A variety of MCPs have been linked to diseases in man: acute pancreatitis and pancreas cancer, type 2 diabetes, Alzheimer's Disease, various types of cancer, and fibrinolysis and inflammation. Many of these discoveries have been made possible thanks to recent advances, as exemplified by plasma carboxypeptidases N and B, known for fifty and twenty years, respectively, which have had their structures released only very recently. Plasma carboxypeptidase B is a biological target for therapy because of its involvement in the coagulation/fibrinolysis processes. Besides, the widespread use of carboxypeptidase A as a benchmark metalloprotease since the early days of Biochemistry has allowed the identification and design of an increasingly vast repertory of small molecular weight inhibitors. With these two examples we wish to emphasize that MCPs have become part of the drug discovery portfolio of pharmaceutical companies and academic research laboratories. This paper will review key developments in the discovery and design of MCP small molecular weight inhibitors, with an emphasis on the discovery of chemically diverse entities. Although encouraging advances have been achieved in the last few years, the specificity and oral bioavailability of the new chemotherapeutic agents seem to pose a challenge to medicinal chemists.

Delivery of peptide and protein drugs over the blood-brain barrier

Peptide and protein (P/P) drugs have been identified as showing great promises for the treatment of various neurodegenerative diseases. A major challenge in this regard, however, is the delivery of P/P drugs over the blood-brain barrier (BBB). Intense research over the last 25 years has enabled a better understanding of the cellular and molecular transport mechanisms at the BBB, and several strategies for enhanced P/P drug delivery over the BBB have been developed and tested in preclinical and clinical-experimental research. Among them, technology-based approaches (comprising functionalized nanocarriers and liposomes) and pharmacological strategies (such as the use of carrier systems and chimeric peptide technology) appear to be the most promising ones. This review combines a comprehensive overview on the current understanding of the transport mechanisms at the BBB with promising selected strategies published so far that can be applied to facilitate enhanced P/P drug delivery over the BBB.

Structure of activated thrombin-activatable fibrinolysis inhibitor, a molecular link between coagulation and fibrinolysis

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a metallocarboxypeptidase (MCP) that links blood coagulation and fibrinolysis. TAFI hampers fibrin-clot lysis and is a pharmacological target for the treatment of thrombotic conditions. TAFI is transformed through removal of its prodomain by thrombin-thrombomodulin into TAFIa, which is intrinsically unstable and has a short half-life in vivo. Here we show that purified bovine TAFI activated in the presence of a proteinaceous inhibitor renders a stable enzyme-inhibitor complex. Its crystal structure reveals that TAFIa conforms to the alpha/beta-hydrolase fold of MCPs and displays two unique flexible loops on the molecular surface, accounting for structural instability and susceptibility to proteolysis. In addition, point mutations reported to enhance protein stability in vivo are mainly located in the first loop and in another surface region, which is a potential heparin-binding site. The protein inhibitor contacts both the TAFIa active site and an exosite, thus contributing to high inhibitory efficiency.

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.

The Purkinje cell degeneration (pcd) mouse: an unexpected molecular link between neuronal degeneration and regeneration

The spontaneous autosomal recessive mouse mutation, Purkinje cell degeneration (pcd), was first identified through its ataxic behavior. Since its discovery in the 1970s, the strain has undergone extensive investigation, although another quarter century elapsed until the mutant gene (agtpbp1 a.k.a. Nna1) underlying the pcd phenotype was identified. As Nna1 was initially discovered as a gene induced in motor neurons following axotomy the finding that its loss leads to selective neuronal degeneration points to a novel and unexpected common molecular mechanism contributing to the apparently opposing processes of degeneration and regeneration. The elucidation of this mechanism may of course have significant implications for an array of neurological disorders. Here we will first review the principle features of the pcd phenotype and then discuss the functional implications of more recent findings emanating from the characterization of Nna1, the protein that is lost in pcd. We also provide new data on the genetic dissection of the cell death pathways operative in pcd(3J) mice, proving that granule cell death and Purkinje cell death in these mice have distinct molecular bases. We also provide new information on the structure of mouse Nna1 as well as Nna1 protein levels in pcd(3J) mice.

Post-translational modifications of human thrombin-activatable fibrinolysis inhibitor (TAFI): evidence for a large shift in the isoelectric point and reduced solubility upon activation

Thrombin-activable fibrinolysis inhibitor (TAFI) is distinct from pancreatic procarboxypeptidase B in several ways. The enzymatic activity of TAFIa is unstable and decays with a half-life of a few minutes. During this study, we observed that (i) the isoelectric point (pI) of TAFI shifts dramatically from pH 5 toward pH 8 upon activation and (ii) TAFIa is significantly less soluble than TAFI. The structural bases for these observations were investigated by characterizing all post-translational modifications, including attached glycans and disulfide connectivity. The analyses revealed that all five potential N-glycosylation sites were utilized including Asn22, Asn51, Asn63, Asn86 (located in the activation peptide), and Asn219 (located in the catalytic domain). Asn219 was also found in an unglycosylated variant. Four of the glycans, Asn51, Asn63, Asn86, and Asn219 displayed microheterogeneity, while the glycan attached to Asn22 appeared to be homogeneous. In addition, bisecting GlcNAc attached to the trimannose core was detected, suggesting an origin other than the liver. Monosaccharide composition and LC-MS/MS analyses did not produce evidence for O glycosylation. TAFI contains eight cysteine residues, of which two, Cys69 and Cys383, are not involved in disulfides and contain free sulfhydryl groups. The remaining six cystines form disulfides, including Cys156-Cys169, Cys228-Cys252, and Cys243-Cys257. This pattern is homologous to pancreatic procarboxypeptidase B, and it is therefore unlikely that permutations in the cysteine connectivity are responsible for the enzymatic instability. LC-MS/MS analyses covering more than 90% of the TAFI amino acid sequence revealed no additional modifications. When these results are taken together, they suggest that the inherent instability of TAFIa is not caused by post-translational modifications. However, after activation, TAFIa loses 80% of the attached glycans, generating a large shift in pI and a propensity to precipitate. These changes are likely to significantly affect the properties of TAFIa as compared to TAFI.

β-Amyloid (Aβ) causes detachment of N1E-115 neuroblastoma cells by acting as a scaffold for cell-associated plasminogen activation

A major component of neuritic plaques in brain tissue of Alzheimer's disease patients is the beta-amyloid peptide (Abeta). Accumulation of Abeta has been associated with increased neuronal cell death and cognitive decline. We have previously shown that amyloid peptides like Abeta bind tissue-type plasminogen activator (tPA) and stimulate plasmin production. Here we investigated how Abeta regulates plasmin formation by N1E-115 neuroblastoma cells and the effects of Abeta-mediated plasmin formation on cell attachment and cell survival. We find that Abeta induces excessive cell-associated plasmin generation that causes cell detachment. Cell detachment is inhibited by carboxypeptidase B (CPB), an enzyme that blocks plasmin formation by cleaving off C-terminal lysine residues. Plasmin and CPB control Abeta-induced cell detachment independently of direct effects on cell viability. Abeta40 as well as oligomeric and fibrillar forms of Abeta42 stimulated tPA-mediated plasminogen activation and cell detachment. Our results suggest that plasmin-mediated cell detachment could contribute to the pathological effects of Abeta in diseased brain.

Amyloid-β-stimulated plasminogen activation by tissue-type plasminogen activator results in processing of neuroendocrine factors

Alzheimer's disease brain is characterized by the abundant presence of amyloid deposits. Accumulation of the major constituent of these deposits, amyloid-beta (Abeta), has been associated with decreased neurotransmission, increased neuronal cell death, and with cognitive decline. The mechanisms underlying these phenomena have not yet been fully elucidated. We have previously shown that amyloid peptides like Abeta bind tissue-type plasminogen activator (tPA) and cause enhanced plasmin production. Here we describe the identification of five major neuronal cell-produced Abeta-associated proteins and how Abeta-stimulated plasmin formation affects their processing. These five proteins are all neuroendocrine factors (NEFs): chromogranins A, B and C; truncated chromogranin B; and VGF. Plasminogen caused processing of Abeta-bound (but not soluble) tPA, chromogranin B and VGF and the degradation products were released from Abeta. Processing of the neuroendocrine factors was dependent on tPA as it was largely abrogated in tPA-/- cells or in the presence of a specific tPA-inhibitor. If plasmin indeed produces NEF-derived peptides in vivo, some of these peptides may have biological activity, for instance in regulating neurotransmitter release that may affect the pathology of Alzheimer's disease.

Expression and distribution of carboxypeptidase B in the hippocampal subregions of normal and Alzheimer's disease brain

Earlier neurochemical studies suggested that human brain carboxypeptidase B may play a significant role in the degradation of amyloid-beta1-42 in the brain. Using an immimohistochemical technique we report here on the neuronal expression and distribution of this enzyme in the segments (CA1a, CA1b and CA1c) of the CA1 subfield and in area CA4 of the hippocampus in normal and Alzheimer's disease brain samples. Its distribution was compared with the appearance of neurofibrillary tangles in the same brain sample. For immunohistochemical localization of carboxypeptidase B, a specific C14-module antibody was applied, together with the Gallyas silver impregnation technique for the demonstration of neurofibrillary tangles. The results revealed that, in the control samples, most of the immunoreactivity appeared in segment CA1a in the pyramidal cells, less in segment CA1b and least in segment CA1c. In the Alzheimer's disease samples, there was no particular immunostaining in the neurons, but, a large number of silver-impregnated degenerated neurons appeared. The results support the suggestion that carboxypeptidase B may play a significant role in elimination of the intracellular accumulation and toxicity of amyloid-beta in the human brain and thereby protect the neurons from degeneration.

Roles of amyloid precursor protein and its fragments in regulating neural activity, plasticity and memory

Amyloid-beta precursor protein (APP) is a membrane-spanning protein with a large extracellular domain and a much smaller intracellular domain. It is the source of the amyloid-beta (Abeta) peptide found in neuritic plaques of Alzheimer's disease (AD) patients. Because Abeta shows neurotoxic properties, and because familial forms of AD promote Abeta accumulation, a massive international research effort has been aimed at understanding the mechanisms of Abeta generation, catabolism and toxicity. APP, however, is an extremely complex molecule that may be a functionally important molecule in its full-length configuration, as well as being the source of numerous fragments with varying effects on neural function. For example, one fragment derived from the non-amyloidogenic processing pathway, secreted APPalpha (sAPPalpha), is neuroprotective, neurotrophic and regulates cell excitability and synaptic plasticity, while Abeta appears to exert opposing effects. Less is known about the neural functions of other fragments, but there is a growing interest in understanding the basic biology of APP as it has become recognized that alterations in the functional activity of the APP fragments during disease states will have complex effects on cell function. Indeed, it has been proposed that reductions in the level or activity of certain APP fragments, in addition to accumulation of Abeta, may play a critical role in the cognitive dysfunction associated with AD, particularly early in the course of the disease. To test and modify this hypothesis, it is important to understand the roles that full-length APP and its fragments normally play in neuronal structure and function. Here we review evidence addressing these fundamental questions, paying particular attention to the contributions that APP fragments play in synaptic transmission and neural plasticity, as these may be key to understanding their effects on learning and memory. It is clear from this literature that APP fragments, including Abeta, can exert a powerful regulation of key neural functions including cell excitability, synaptic transmission and long-term potentiation, both acutely and over the long-term. Furthermore, there is a small but growing literature confirming that these fragments correspondingly regulate behavioral learning and memory. These data indicate that a full account of cognitive dysfunction in AD will need to incorporate the actions of the full complement of APP fragments. To this end, there is an urgent need for a dedicated research effort aimed at understanding the behavioral consequences of altered levels and activity of the different APP fragments as a result of experience and disease.

Human brain carboxypeptidase B, which cleaves β-amyloid peptidesin vitro, is expressed in the endoplasmic reticulum of neurons

Intracellular localization of novel human brain carboxypeptidase B (HBCPB) was investigated in human hippocampus, using immunohistochemistry by confocal laser microscopy and biochemical purification of the homogenate by density gradient ultracentrifugation. The former revealed that the majority of HBCPB was expressed in the endoplasmic reticulum, in which the HBCPB-specific C14-module immunoreactivity was colocalized with GRP78 immunoreactivity, a stress 70 heat shock protein specifically expressed in the endoplasmic reticulum. The latter showed that anti-C14-module immunoreactivity and prepro-HBCPB immunoreactivity were both enriched in the microsome fraction, especially in that of the endoplasmic reticulum-density fraction of normal human hippocampal homogenates from various sources. However, HBCPB prepared from human hippocampus showed exopeptidase activity for synthetic beta-amyloid 1-42 peptide, in which Abeta X-42 C-terminus immunoreactivity was decreased in a fashion dose-dependent of the amount of the protease added. These findings indicate that HBCPB, which is expressed in the endoplasmic reticulum of a group of neuronal perikarya, may play an important physiological role in degradation of beta-amyloid 1-42, which is specifically generated in the endoplasmic reticulum of human and rodent neurons and is also regarded as the most pathogenic and aggregatable species among all beta-amyloid peptides.

Expression of human brain carboxypeptidase B, a possible cleaving enzyme for beta-amyloid precursor protein, in peripheral fluids

Human brain carboxypeptidase B (HBCPB) is a novel brain protease that processes native brain beta-amyloid precursor protein (APP) in vitro. Immunoblot analysis of human serum and cerebrospinal fluid (CSF) using anti C14-module antibody, which recognizes the C-terminal peptide unique to HBCPB, detected the 30 and 40 kDa immunoreactive bands. Analysis of HBCPB prepared from both serum and CSF demonstrated proteolytic activities for brain APP. Protease inhibitor spectrum analysis also supports that these bands correspond to the mature form and and prepro form of HBCPB, respectively. As is the case in brain parenchyma, the prepro-form is dominant in CSF. In serum, however, the majority of HBCPB exists in the mature form, possibly due to an abundant trypsin-like proteolytic activity in serum. HBCPB expressed in serum and CSF, therefore, may have a significance as a peripheral marker of the brain protease, which participates in APP processing in human brain.

Regenerating motor neurons express Nna1, a novel ATP/GTP-binding protein related to zinc carboxypeptidases

To identify genes involved in axon regeneration, differential screening was applied to RNA isolated from spinal cord of mice subjected to sciatic nerve transection or crush injury. A 4-kb transcript, termed nna1, was identified that was rapidly induced in affected motor neurons in both paradigms. The levels of nna1 transcript levels declined in motor neurons within 1-2 weeks after nerve crush, coincident with target reinnervation. If reinnervation was blocked by nerve cut and ligation, nna1 was continuously expressed in motor neurons. In addition, in situ analysis of developing embryonic nervous tissue showed nna1 was highly expressed in differentiating neurons, but not proliferating populations. Nna1 is predicted to be a zinc carboxypeptidase that contains nuclear localization signals and an ATP/GTP binding motif. Cultured neurons transfected with green fluorescent protein (GFP)-nna1 expressed GFP-Nna1 in cytoplasmic and nuclear compartments. Thus, Nna1 may contribute to nuclear signaling events in differentiating and regenerating neurons.

A novel isoform of a kallikrein-like protease, TLSP/hippostasin, (PRSS20), is expressed in the human brain and prostate

cDNAs encoding two splicing variants of a serine protease, termed hippostasin, were isolated by a PCR-based cloning strategy. The difference of 5' nucleotide sequence resulted in the variation in the amino terminal ends of the two, brain and prostate, types of human hippostasin. The longest ORF of the brain-type was 250 amino acids with a putative signal peptide, while that of the prostate-type was 282 amino acids. Homology search using the amino acid sequence revealed that prostate-type hippostasin was identical to TLSP (PRSS20), which is expressed in human primary keratinocytes (1). Transient expression analysis showed that both brain- and prostate-type TLSP/hippostasin were secreted into the conditioned medium as about 40 kDa proteins. Human TLSP/hippostasin showed 47% and 45% identity to trypsinogen II and kallikrein, respectively. In fact, the recombinant human TLSP/hippostasin efficiently cleaved Bz-Phe-Arg-4-methylcoumaryl-7-amide, a kallikrein substrate, and weakly cleaved other substrates for kallikrein and trypsin. Northern blot analysis detected a 1.3 kb band in the whole brain and a 1.4 kb band in the prostate and the lung. In situ hybridization revealed that it was expressed preferentially by the pyramidal neurons in the human hippocampus and secretory epithelial cells in the prostate. These results indicated that TLSP/hippostasin is involved in the functions of the human central nervous system and prostate and that it is a multifunctional protease present in various organs.