Role of loop structures of neuropsin in the activity of serine protease and regulated secretion

Neuropsin promotes hippocampal synaptogenesis by regulating the expression and cleavage of L1CAM

During early postnatal brain development, the formation of proper synaptic connections between neurons is crucial for the development of functional neural networks. Recent studies have established the involvement of protease-mediated modulations of extracellular components in both synapse formation and elimination. The secretory serine protease neuropsin (also known as kallikrein-8) cleaves a few transmembrane or extracellular matrix proteins in a neural activity-dependent manner and regulates neural plasticity. However, neuropsin-dependent proteolysis of extracellular components and the involvement of these components in mouse brain development are poorly understood. We have observed that during hippocampus development, expression of neuropsin and levels of full-length or cleaved fragments of the neuropsin substrate protein L1 cell adhesion molecule (L1CAM) positively correlate with synaptogenesis. Our subcellular fractionation studies show that the expression of neuropsin and its proteolytic activity on L1CAM are enriched at developing hippocampal synapses. Activation of neuropsin expression upregulates the transcription and cleavage of L1CAM. Furthermore, blocking of neuropsin activity, as well as knockdown of L1CAM expression, significantly downregulates in vitro hippocampal synaptogenesis. Taken together, these findings provide evidence for the involvement of neuropsin activity-dependent regulation of L1CAM expression and cleavage in hippocampal synaptogenesis.

Kallikrein 8: A key sheddase to strengthen and stabilize neural plasticity

Neural networks are modified and reorganized throughout life, even in the matured brain. Synapses in the networks form, change, or disappear dynamically in the plasticity state. The pre- and postsynaptic signaling, transmission, and structural dynamics have been studied considerably well. However, not many studies have shed light on the events in the synaptic cleft and intercellular space. Neural activity-dependent protein shedding is a phenomenon in which (1) presynaptic excitation evokes secretion or activation of sheddases, (2) sheddases are involved not only in cleavage of membrane- or matrix-bound proteins but also in mechanical modulation of cell-to-cell connectivity, and (3) freed activity domains of protein factors play a role in receptor-mediated or non-mediated biological actions. Kallikrein 8/neuropsin (KLK8) is a kallikrein family serine protease rich in the mammalian limbic brain. Accumulated evidence has suggested that KLK8 is an important modulator of neural plasticity and consequently, cognition. Insufficiency, as well as excess of KLK8 may have detrimental effects on limbic functions.

Neuropsin in mental health

Neuropsin is a brain-expressed extracellular matrix serine protease that governs synaptic plasticity through activity-induced proteolytic cleavage of synaptic proteins. Its substrates comprise several molecules central to structural synaptic plasticity, and studies in rodents have documented its role in cognition and the behavioral and neurobiological response to stress. Intriguingly, differential usage of KLK8 (neuropsin gene) splice forms in the fetal and adult brain has only been reported in humans, suggesting that neuropsin may serve a specialized role in human neurodevelopment. Through systematic interrogation of large-scale genetic data, we review KLK8 regulation in the context of mental health and provide a summary of clinical and preclinical evidence supporting a role for neuropsin in the pathogenesis of mental illness.

Processing of neuregulin-1 by neuropsin regulates GABAergic neuron to control neural plasticity of the mouse hippocampus

Protease-mediated signaling is an important modulator of the nervous system. However, identifying the specific signaling substrates of such proteases is limited by the rapidity with which intermediate substrate forms are cleaved and released. Here, a screening method to detect noncleaved enzyme-bound forms was developed and used to identify a novel neuropsin/neuregulin-1 (NRG-1) proteolytic signaling system, which is specifically localized in the microdomain of synaptic cleft, in the mouse hippocampus. The extracellular protease, neuropsin, cleaved mature NRG-1 (comprising the extracellular domain of the NRG-1) at three newly identified sites to remove the heparin-binding domain of NRG-1. This released the ligand moiety from the matrix-glycosaminoglycan pool and enabled it to trigger the phosphorylation of NRG-1 receptor, p185 (ErbB4). Proteolysis of mature NRG-1 by neuropsin led to colocalization of the processed NRG-1 with ErbB4 in parvalbumin-positive hippocampal interneurons and consequent phosphorylation of tyrosine residues of proteins in the cells. Moreover, neuropsin knock-out mice exhibited impairments in Schaffer collateral early phase long-term potentiation, and application of the recombinant NRG-1 lacking heparin-binding activity reversed the effects through the activation of ErbB4 and GABA(A) receptors. Thus, ErbB4 signaling induced by neuropsin-dependent processing of NRG-1 contributes to the modulation of synaptic plasticity via regulation of GABAergic transmission. This signaling system may be involved in human cognition and mental disorders, such as schizophrenia and bipolar disorder, by its dysfunction.

Predicting flexible loop regions that interact with ligands: the challenge of accurate scoring

Flexible loop regions play a critical role in the biological function of many proteins and have been shown to be involved in ligand binding. In the context of structure-based drug design, using or predicting an incorrect loop configuration can be detrimental to the study if the loop is capable of interacting with the ligand. Three protein systems, each with at least one flexible loop region in close proximity to the known binding site, were selected for loop prediction using the CorLps program; a six residue loop region from phosphoribosylglycinamide formyltransferase (GART), two nine residue loop regions from cytochrome P450 (CYP) 119, and an 11 residue loop region from enolase were selected for loop prediction. The results of this study indicate that the statistically based DFIRE scoring function implemented in the CorLps program did not accurately rank native-like predicted loop configurations in any protein system. In an attempt to improve the ranking of the native-like predicted loop configurations, the MM/GBSA and the optimized MM/GBSA-dsr scoring functions were used to re-rank the predicted loops with and without bound ligand. In general, single snapshot MM/GBSA scoring provided the best ranking of native-like loop configurations. Based on the scoring function analyses presented, the optimal ranking of native-like loop configurations is still a difficult challenge and the choice of the "best" scoring function appears to be system dependent.

New computational method for prediction of interacting protein loop regions

Flexible loop regions of proteins play a crucial role in many biological functions such as protein-ligand recognition, enzymatic catalysis, and protein-protein association. To date, most computational methods that predict the conformational states of loops only focus on individual loop regions. However, loop regions are often spatially in close proximity to one another and their mutual interactions stabilize their conformations. We have developed a new method, titled CorLps, capable of simultaneously predicting such interacting loop regions. First, an ensemble of individual loop conformations is generated for each loop region. The members of the individual ensembles are combined and are accepted or rejected based on a steric clash filter. After a subsequent side-chain optimization step, the resulting conformations of the interacting loops are ranked by the statistical scoring function DFIRE that originated from protein structure prediction. Our results show that predicting interacting loops with CorLps is superior to sequential prediction of the two interacting loop regions, and our method is comparable in accuracy to single loop predictions. Furthermore, improved predictive accuracy of the top-ranked solution is achieved for 12-residue length loop regions by diversifying the initial pool of individual loop conformations using a quality threshold clustering algorithm.

Functional characterization of the human-specific (type II) form of kallikrein 8, a gene involved in learning and memory

Kallikrein 8 (KLK8) is a serine protease functioning in the central nervous system, and essential in many aspects of neuronal activities. Sequence comparison and gene expression analysis among diverse primate species identified a human-specific splice form of KLK8 (type II) with preferential expression in the human brain, which may contribute to the origin of human cognition. To gain insights into the physiological and biochemical role of this novel form, we conducted functional analyses of human type II KLK8. Our results show that type II KLK8 is abundantly expressed in human embryonic stem cells and in embryo brain samples, suggesting a potential role in embryogenesis. There are dramatic expression variations in different individuals and brain regions, which is a reflection of its dynamic role in neural activities. Furthermore, the transcription start site (TSS) of KLK8 is tissue-specific, with a brain-specific TSS found in humans indicating functional specialization. Our in vitro biochemical assay shows that there is a type II-specific intermediate protein form, although the processed end-point enzymes are the same for both type I and type II KLK8, suggesting that the emergence of type II KLK8 in the human brain likely leads to functional modifications of KLK8.

Neuropsin (KLK8)-dependent and -independent synaptic tagging in the Schaffer-collateral pathway of mouse hippocampus

Hippocampal early long-term potentiation (LTP) elicited by a weak (one or two) tetanic stimulus normally fades away within 90 min. Late LTP elicited by strong (four) stimuli lasts >180 min and requires new protein synthesis to persist. If a strong tetanus is injected once into a synapse, even a weak tetanus injected into another synapse can evoke persistent LTP. It was hypothesized that a synaptic tag enables capture of newly synthesized synaptic molecules. Here, we found two synaptic capture mechanisms for a weakly stimulated synapse to acquire persistency (i.e., neuropsin dependent and independent). The single tetanus evokes a neuropsin-dependent form that follows downstream signaling into integrin/actin signal and L-type voltage-dependent Ca2+ channel (LVDCC) pathway. Additionally, a neuropsin-independent form of synaptic capture is evoked by a stronger (two) tetanus than the former. Both forms converging on LVDCC might serve different associative memories depending on their input strength. Our study strongly supports the hypothesis of synaptic tagging and demonstrates that neuropsin-dependent late associativity is particularly important in nonstressful associative memory.

Aberrant human tissue kallikrein levels in the stratum corneum and serum of patients with psoriasis: dependence on phenotype, severity and therapy

BACKGROUND

Human tissue kallikreins (KLKs) are a family of 15 trypsin-like or chymotrypsin-like secreted serine proteases (KLK1-KLK15). Multiple KLKs have been quantitatively identified in normal stratum corneum (SC) and sweat as candidate desquamation-related proteases.

OBJECTIVES

To quantify KLK5, KLK6, KLK7, KLK8, KLK10, KLK11, KLK13 and KLK14 in the SC and serum of patients with psoriasis, and their variation between lesional and nonlesional areas and with phenotype, therapy and severity. The overall SC serine protease activities were also measured.

METHODS

Enzyme-linked immunosorbent assays and enzymatic assays were used.

RESULTS

The lesional SC of psoriasis generally contained significantly higher levels of all KLKs. KLK6, KLK10 and KLK13 levels were significantly elevated even in the nonlesional SC. The overall trypsin-like, plasmin-like and furin-like activities were significantly elevated in the lesional SC. Plasmin-like activity was significantly elevated also in the nonlesional SC. The SC chymotrypsin-like activity was only slightly elevated in psoriasis. KLK7 serum levels did not differ between normal volunteers and patients with psoriasis. Serum KLK6, KLK8, KLK10 and KLK13 levels in patients with untreated psoriasis significantly correlated with Psoriasis Area and Severity Index score. Serum KLK5 and KLK11 levels decreased in patients with psoriasis after therapy, especially with etretinate. Patients with erythrodermic psoriasis exhibited significantly higher serum KLK levels than normal subjects or patients with psoriasis vulgaris or arthropathic psoriasis.

CONCLUSIONS

We found aberrant KLK levels in the SC and serum of patients with psoriasis and suggest that KLKs might be involved in the pathogenesis of this disease.

Role of neuropsin in formation and maturation of Schaffer-collateral L1cam-immunoreactive synaptic boutons

We report that neuropsin is involved in the synaptogenesis/maturation of orphan and small synaptic boutons in the Schaffer-collateral pathway. Most non-synaptic orphan boutons and a number of immature small synaptic boutons expressed the cell adhesion molecule L1 in presynaptic Schaffer-collateral terminals, whereas mature large boutons on mushroom spines were devoid of L1. The number of L1-immunoreactive boutons was markedly higher in neuropsin-deficient mice than in wild-type mice, whereas there were far fewer mature large boutons. L1-immunoreactive boutons were hypertrophied in the mutant mice. When a recombinant active neuropsin was microinjected into the mutant hippocampus, the number of immunoreactive synaptic boutons reverted to wild-type levels after one day. These results strongly suggest that enzymatically active neuropsin allows a maturational change of L1-immunoreactive small boutons, both orphan and synaptic, and this step may be important in synaptic plasticity based on activity-dependent structural change.

Expression, crystallization, and three-dimensional structure of the catalytic domain of human plasma kallikrein

Plasma kallikrein is a serine protease that has many important functions, including modulation of blood pressure, complement activation, and mediation and maintenance of inflammatory responses. Although plasma kallikrein has been purified for 40 years, its structure has not been elucidated. In this report, we described two systems (Pichia pastoris and baculovirus/Sf9 cells) for expression of the protease domain of plasma kallikrein, along with the purification and high resolution crystal structures of the two recombinant forms. In the Pichia pastoris system, the protease domain was expressed as a heterogeneously glycosylated zymogen that was activated by limited trypsin digestion and treated with endoglycosidase H deglycosidase to reduce heterogeneity from the glycosylation. The resulting protein was chromatographically resolved into four components, one of which was crystallized. In the baculovirus/Sf9 system, homogeneous, crystallizable, and nonglycosylated protein was expressed after mutagenizing three asparagines (the glycosylation sites) to glutamates. When assayed against the peptide substrates, pefachrome-PK and oxidized insulin B chain, both forms of the protease domain were found to have catalytic activity similar to that of the full-length protein. Crystallization and x-ray crystal structure determination of both forms have yielded the first three-dimensional views of the catalytic domain of plasma kallikrein. The structures, determined at 1.85 A for the endoglycosidase H-deglycosylated protease domain produced from P. pastoris and at 1.40 A for the mutagenically deglycosylated form produced from Sf9 cells, show that the protease domain adopts a typical chymotrypsin-like serine protease conformation. The structural information provides insights into the biochemical and enzymatic properties of plasma kallikrein and paves the way for structure-based design of protease inhibitors that are selective either for or against plasma kallikrein.

Serine proteases regulating synaptic plasticity

A number of molecules have been postulated to be involved in long-term potentiation, an experimental model for learning and short-term memory. Although the molecular mechanisms of the long-term potentiation have been considerably well understood, it is not yet known why and how real memory can last very long with outstanding stability. A mechanical change of synaptic morphology at acquisition, consolidation and retention of memory is hypothesized to explain long-lasting memory. Changes in the synaptic morphology may be due, at least in part, to local extracellular proteolysis of cell adhesion and extracellular matrix molecules. Some extracellular serine proteases of the Clan PA family may modulate synaptic adhesion and associate with long-term potentiation and learning behavior. In the present review, candidate proteases that are involved in the hippocampal memory are overviewed.

The emerging roles of human tissue kallikreins in cancer

Human tissue kallikreins (hKs), which are encoded by the largest contiguous cluster of protease genes in the human genome, are secreted serine proteases with diverse expression patterns and physiological roles. Although primarily known for their clinical applicability as cancer biomarkers, recent evidence implicates hKs in many cancer-related processes, including cell-growth regulation, angiogenesis, invasion and metastasis. They have been shown to promote or inhibit neoplastic progression, acting individually and/or in cascades with other hKs and proteases, and might represent attractive targets for therapeutic intervention.

Human Tissue Kallikreins: Physiologic Roles and Applications in Cancer

Tissue kallikreins are members of the S1 family (clan SA) of trypsin-like serine proteases and are present in at least six mammalian orders. In humans, tissue kallikreins (hK) are encoded by 15 structurally similar, steroid hormone–regulated genes (KLK) that colocalize to chromosome 19q13.4, representing the largest cluster of contiguous protease genes in the entire genome. hKs are widely expressed in diverse tissues and implicated in a range of normal physiologic functions from the regulation of blood pressure and electrolyte balance to tissue remodeling, prohormone processing, neural plasticity, and skin desquamation. Several lines of evidence suggest that hKs may be involved in cascade reactions and that cross-talk may exist with proteases of other catalytic classes. The proteolytic activity of hKs is regulated in several ways including zymogen activation, endogenous inhibitors, such as serpins, and via internal (auto)cleavage leading to inactivation. Dysregulated hK expression is associated with multiple diseases, primarily cancer. As a consequence, many kallikreins, in addition to hK3/PSA, have been identified as promising diagnostic and/or prognostic biomarkers for several cancer types, including ovarian, breast, and prostate. Recent data also suggest that hKs may be causally involved in carcinogenesis, particularly in tumor metastasis and invasion, and, thus, may represent attractive drug targets to consider for therapeutic intervention.

NMDA-dependent proteolysis of presynaptic adhesion molecule L1 in the hippocampus by neuropsin

Synaptic plasticity requires an activity-dependent, rapid, and long-lasting modification of synaptic character, including morphology and coupling strength. Here we show that a serine protease, neuropsin, directly and specifically modifies the synaptic adhesion molecule L1, which was localized to the presynaptic site of the asymmetric synapse in the mouse hippocampus. Increased neural activity triggered the rapid, transient activation of the precursor form of neuropsin in an NMDA receptor-dependent manner. The activated neuropsin immediately cleaved L1 and released a neuropsin-specific extracellular 180 kDa fragment. This neuropsin-specific L1-cleaving system is involved in NMDA receptor-dependent synaptic plasticity, such as the Schaffer collateral long-term potentiation.

Region-specific and epileptogenic-dependent expression of six subtypes of alpha2,3-sialyltransferase in the adult mouse brain

Sialylated glycoconjugates play important roles in various biological functions. The structures are also observed in brains and it has been proposed that sialylation may affect neural plasticity. To clarify the effects of sialylation in the brain, particular neurons that exhibit sialylation should first be determined. Using in situ hybridization, we performed systematic surveys of the localization of mRNAs encoding the six alpha2,3-sialyltransferases (ST3Gal I-VI) in the adult mouse brain with or without physiological stimulation. First, striking region-specific patterns of expression were observed: While ST3Gal II, III, and V mRNAs were in neuronal cells throughout the brain, ST3Gal I, IV, and VI mRNAs were in restricted brain regions. Next, to assess whether the expression of the six mRNAs can be regulated, we examined the effect of kindling epileptogenesis on the six mRNA levels. Of the six subtypes, upregulation in the ST3Gal IV level in the thalamus was most pronounced; the number of ST3Gal IV-expressing neurons in the anterior thalamic nuclei increased from 2% to 21% in a time-dependent manner during epileptogenesis. Western blot analysis evaluated the increase of the end-products in the thalamus. These findings provide a molecular basis to clarify when and where sialylated glycoconjugates function accompanied by neural plasticity.

Post-translational modification of bone morphogenetic protein-1 is required for secretion and stability of the protein

Bone morphogenetic protein (BMP)-1 is a glycosylated metalloproteinase that is fundamental to the synthesis of a normal extracellular matrix because it cleaves type I procollagen, as well as other precursor proteins. Sequence analysis suggests that BMP-1 has six potential N-linked glycosylation sites (i.e. NXS/T) namely: Asn(91) (prodomain), Asn(142) (metalloproteinase domain), Asn(332) and Asn(363) (CUB1 domain), Asn(599) (CUB3 domain), and Asn(726) in the C-terminal-specific domain. In this study we showed that all these sites are N-glycosylated with complex-type oligosaccharides containing sialic acid, except Asn(726) presumably because proline occurs immediately C-terminal of threonine in the consensus sequence. Recombinant BMP-1 molecules lacking all glycosylation sites or the three CUB-specific sites were not secreted. BMP-1 lacking CUB glycosylation was translocated to the proteasome for degradation. BMP-1 molecules lacking individual glycosylation sites were efficiently secreted and exhibited full procollagen C-proteinase activity, but N332Q and N599Q exhibited a slower rate of cleavage. BMP-1 molecules lacking any one of the CUB-specific glycosylation sites were sensitive to thermal denaturation. The study showed that the glycosylation sites in the CUB domains of BMP-1 are important for secretion and stability of the molecule.

Crystal structure and biochemical characterization of human kallikrein 6 reveals that a trypsin-like kallikrein is expressed in the central nervous system

The human kallikreins are a large multigene family of closely related serine-type proteases. In this regard, they are similar to the multigene kallikrein families characterized in mice and rats. There is a much more extensive body of knowledge regarding the function of mouse and rat kallikreins in comparison with the human kallikreins. Human kallikrein 6 has been proposed as the homologue to rat myelencephalon-specific protease, an arginine-specific degradative-type protease abundantly expressed in the central nervous system and implicated in demyelinating disease. We present the x-ray crystal structure of mature, active recombinant human kallikrein 6 at 1.75-A resolution. This high resolution model provides the first three-dimensional view of one of the human kallikreins and one of only a few structures of serine proteases predominantly expressed in the central nervous system. Enzymatic data are presented that support the identification of human kallikrein 6 as the functional homologue of rat myelencephalon-specific protease and are corroborated by a molecular phylogenetic analysis. Furthermore, the x-ray data provide support for the characterization of human kallikrein 6 as a degradative protease with structural features more similar to trypsin than the regulatory kallikreins.