Structural and functional analysis of HtrA1 and its subdomains

Cystine-knot peptide inhibitors of HTRA1 bind to a cryptic pocket within the active site region

Cystine-knot peptides (CKPs) are naturally occurring peptides that exhibit exceptional chemical and proteolytic stability. We leveraged the CKP carboxypeptidase A1 inhibitor as a scaffold to construct phage-displayed CKP libraries and subsequently screened these collections against HTRA1, a trimeric serine protease implicated in age-related macular degeneration and osteoarthritis. The initial hits were optimized by using affinity maturation strategies to yield highly selective and potent picomolar inhibitors of HTRA1. Crystal structures, coupled with biochemical studies, reveal that the CKPs do not interact in a substrate-like manner but bind to a cryptic pocket at the S1' site region of HTRA1 and abolish catalysis by stabilizing a non-competent active site conformation. The opening and closing of this cryptic pocket is controlled by the gatekeeper residue V221, and its movement is facilitated by the absence of a constraining disulfide bond that is typically present in trypsin fold serine proteases, thereby explaining the remarkable selectivity of the CKPs. Our findings reveal an intriguing mechanism for modulating the activity of HTRA1, and highlight the utility of CKP-based phage display platforms in uncovering potent and selective inhibitors against challenging therapeutic targets.

Disulfide-constrained peptide scaffolds enable a robust peptide-therapeutic discovery platform

Peptides present an alternative modality to immunoglobulin domains or small molecules for developing therapeutics to either agonize or antagonize cellular pathways associated with diseases. However, peptides often suffer from poor chemical and physical stability, limiting their therapeutic potential. Disulfide-constrained peptides (DCP) are naturally occurring and possess numerous desirable properties, such as high stability, that qualify them as drug-like scaffolds for peptide therapeutics. DCPs contain loop regions protruding from the core of the molecule that are amenable to peptide engineering via direct evolution by use of phage display technology. In this study, we have established a robust platform for the discovery of peptide therapeutics using various DCPs as scaffolds. We created diverse libraries comprising seven different DCP scaffolds, resulting in an overall diversity of 2 x 1011. The effectiveness of this platform for functional hit discovery has been extensively evaluated, demonstrating a hit rate comparable to that of synthetic antibody libraries. By utilizing chemically synthesized and in vitro folded peptides derived from selections of phage displayed DCP libraries, we have successfully generated functional inhibitors targeting the HtrA1 protease. Through affinity maturation strategies, we have transformed initially weak binders against Notch2 with micromolar Kd values to high-affinity ligands in the nanomolar range. This process highlights a viable hit-to-lead progression. Overall, our platform holds significant potential to greatly enhance the discovery of peptide therapeutics.

HTRA1 disaggregates α-synuclein amyloid fibrils and converts them into non-toxic and seeding incompetent species

Parkinson's disease (PD) is closely linked to α-synuclein (α-syn) misfolding and accumulation in Lewy bodies. The PDZ serine protease HTRA1 degrades fibrillar tau, which is associated with Alzheimer's disease, and inactivating mutations to mitochondrial HTRA2 are implicated in PD. Here, we report that HTRA1 inhibits aggregation of α-syn as well as FUS and TDP-43, which are implicated in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia. The protease domain of HTRA1 is necessary and sufficient for inhibiting aggregation, yet this activity is proteolytically-independent. Further, HTRA1 disaggregates preformed α-syn fibrils, rendering them incapable of seeding aggregation of endogenous α-syn, while reducing HTRA1 expression promotes α-syn seeding. HTRA1 remodels α-syn fibrils by targeting the NAC domain, the key domain catalyzing α-syn amyloidogenesis. Finally, HTRA1 detoxifies α-syn fibrils and prevents formation of hyperphosphorylated α-syn accumulations in primary neurons. Our findings suggest that HTRA1 may be a therapeutic target for a range of neurodegenerative disorders.

Structural insight into CD93 recognition by IGFBP7

The CD93/IGFBP7 axis proteins are key factors expressed in endothelial cells (EC) that mediate EC angiogenesis and migration. Their upregulation contributes to tumor vascular abnormality and a blockade of this interaction promotes a favorable tumor microenvironment for therapeutic interventions. However, the interactions of these proteins with each other remain unclear. In this study, we determined a partial structure of the human CD93-IGFBP7 complex comprising the EGF1 domain of CD93 and the IB domain of IGFBP7. Mutagenesis studies confirmed interactions and specificities. Cellular and mouse tumor studies demonstrated the physiological relevance of the CD93-IGFBP7 interaction in EC angiogenesis. Our study provides leads for the development of therapeutic agents to precisely disrupt unwanted CD93-IGFBP7 signaling in the tumor microenvironment. Additionally, analysis of the CD93 full-length architecture provides insights into how CD93 protrudes on the cell surface and forms a flexible platform for binding to IGFBP7 and other ligands.

Endoplasmic reticulum stress impairs trophoblast syncytialization through upregulation of HtrA4 and causes early-onset preeclampsia

OBJECTIVE

Syncytiotrophoblasts form via mononuclear cytotrophoblast fusion during placentation and play a critical role in maternal-fetal communication. Impaired syncytialization inevitably leads to pregnancy-associated complications, including preeclampsia. Endoplasmic reticulum stress (ERS) is reportedly linked with preeclampsia, but little is known about its association with syncytialization. High temperature requirement factor A4 (HtrA4), a placental-specific protease, is responsible for protein quality control and placental syncytialization. This study aimed to investigate the relationship among HtrA4, ERS, and trophoblast syncytialization in the development of early-onset preeclampsia (EO-PE).

METHODS

HtrA4 expression and ERS in preeclamptic placentas and control placentas were analyzed by Western blotting and qRT-PCR. HtrA4 and ERS localization in placentas was determined by immunohistochemistry and immunofluorescence. BeWo cells were used to stimulate the effects of HtrA4 and ERS on syncytialization.

RESULTS

HtrA4 expression was upregulated in EO-PE and positively correlated with ERS. HtrA4 activity was increased in preeclampsia. Under normoxia, HtrA4 overexpression in BeWo cells did not alter the ERS level. In addition, treatment with hypoxia/reoxygenation (H/R) or an ERS inducer increased HtrA4 expression. HtrA4 upregulation suppressed the levels of syncytin-2 and β-HCG in the presence of forskolin (FSK), and this change was exaggerated after ERS activation. In addition, treatment with an ERS inhibitor markedly suppressed FSK-treated cell fusion in a manner related to downregulation of HtrA4 expression.

CONCLUSION

Our results suggest that ERS enables syncytialization of placental development by upregulating HtrA4, but that excessive HtrA4 expression and preexisting ERS impair syncytialization and cause EO-PE.

10q26 - The enigma in age-related macular degeneration

Despite comprehensive research efforts over the last decades, the pathomechanisms of age-related macular degeneration (AMD) remain far from being understood. Large-scale genome wide association studies (GWAS) were able to provide a defined set of genetic aberrations which contribute to disease risk, with the strongest contributors mapping to distinct regions on chromosome 1 and 10. While the chromosome 1 locus comprises factors of the complement system with well-known functions, the role of the 10q26-locus in AMD-pathophysiology remains enigmatic. 10q26 harbors a cluster of three functional genes, namely PLEKHA1, ARMS2 and HTRA1, with most of the AMD-associated genetic variants mapping to the latter two genes. High linkage disequilibrium between ARMS2 and HTRA1 has kept association studies from reliably defining the risk-causing gene for long and only very recently the genetic risk region has been narrowed to ARMS2, suggesting that this is the true AMD gene at this locus. However, genetic associations alone do not suffice to prove causality and one or more of the 14 SNPs on this haplotype may be involved in long-range control of gene expression, leaving HTRA1 and PLEKHA1 still suspects in the pathogenic pathway. Both, ARMS2 and HTRA1 have been linked to extracellular matrix homeostasis, yet their exact molecular function as well as their role in AMD pathogenesis remains to be uncovered. The transcriptional regulation of the 10q26 locus adds an additional level of complexity, given, that gene-regulatory as well as epigenetic alterations may influence expression levels from 10q26 in diseased individuals. Here, we provide a comprehensive overview on the 10q26 locus and its three gene products on various levels of biological complexity and discuss current and future research strategies to shed light on one of the remaining enigmatic spots in the AMD landscape.

HtrA1 prevents and reverses α-synuclein aggregation, rendering it non-toxic and seeding incompetent

Parkinson disease (PD) is closely linked to the misfolding and accumulation of α-synuclein (α-syn) into Lewy bodies. HtrA1 is a PDZ serine protease that degrades fibrillar tau, which is associated with Alzheimer disease (AD). Further, inactivating mutations to mitochondrial HtrA2 have been implicated in PD. Here, we establish that HtrA1 inhibits the aggregation of α-syn as well as FUS and TDP-43, which are implicated in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). We demonstrate that the protease domain of HtrA1 is necessary and sufficient for inhibition of aggregation, yet this activity is independent of HtrA1 proteolytic activity. Further, we find that HtrA1 also disaggregates preformed α-syn fibrils, which may promote their clearance. Treatment of α-syn fibrils with HtrA1 renders α-syn incapable of seeding the aggregation of endogenous α-syn in mammalian biosensor cells. We find that HtrA1 remodels α-syn by specifically targeting the NAC domain, which is the key domain that catalyzes α-syn oligomerization and fibrillization. Finally, in a primary neuron model of α-syn aggregation, we show that HtrA1 and its proteolytically inactive form both detoxify α-syn and prevent the formation of hyperphosphorylated α-syn accumulations. Our findings suggest that HtrA1 prevents aggregation and promotes disaggregation of multiple disease-associated proteins, and may be a therapeutic target for treating a range of neurodegenerative disorders.

Inhibiting HIF-1 signaling alleviates HTRA1-induced RPE senescence in retinal degeneration

BACKGROUND

Age-related macular degeneration (AMD), characterized by the degeneration of retinal pigment epithelium (RPE) and photoreceptors, is the leading cause of irreversible vision impairment among the elderly. RPE senescence is an important contributor to AMD and has become a potential target for AMD therapy. HTRA1 is one of the most significant susceptibility genes in AMD, however, the correlation between HTRA1 and RPE senescence hasn't been investigated in the pathogenesis of AMD.

METHODS

Western blotting and immunohistochemistry were used to detect HTRA1 expression in WT and transgenic mice overexpressing human HTRA1 (hHTRA1-Tg mice). RT-qPCR was used to detect the SASP in hHTRA1-Tg mice and ARPE-19 cells infected with HTRA1. TEM, SA-β-gal was used to detect the mitochondria and senescence in RPE. Retinal degeneration of mice was investigated by fundus photography, FFA, SD-OCT and ERG. The RNA-Seq dataset of ARPE-19 cells treated with adv-HTRA1 versus adv-NC were analyzed. Mitochondrial respiration and glycolytic capacity in ARPE-19 cells were measured using OCR and ECAR. Hypoxia of ARPE-19 cells was detected using EF5 Hypoxia Detection Kit. KC7F2 was used to reduce the HIF1α expression both in vitro and in vivo.

RESULTS

In our study, we found that RPE senescence was facilitated in hHTRA1-Tg mice. And hHTRA1-Tg mice became more susceptible to NaIO₃ in the development of oxidative stress-induced retinal degeneration. Similarly, overexpression of HTRA1 in ARPE-19 cells accelerated cellular senescence. Our RNA-seq revealed an overlap between HTRA1-induced differentially expressed genes associated with aging and those involved in mitochondrial function and hypoxia response in ARPE-19 cells. HTRA1 overexpression in ARPE-19 cells impaired mitochondrial function and augmented glycolytic capacity. Importantly, upregulation of HTRA1 remarkably activated HIF-1 signaling, shown as promoting HIF1α expression which mainly located in the nucleus. HIF1α translation inhibitor KC7F2 significantly prevented HTRA1-induced cellular senescence in ARPE-19 cells, as well as improved the visual function in hHTRA1-Tg mice treated with NaIO₃.

CONCLUSIONS

Our study showed elevated HTRA1 contributes to the pathogenesis of AMD by promoting cellular senescence in RPE through damaging mitochondrial function and activating HIF-1 signaling. It also pointed out that inhibition of HIF-1 signaling might serve as a potential therapeutic strategy for AMD. Video Abstract.

Structural insight into CD93 recognition by IGFBP7

The CD93/IGFBP7 axis are key factors expressed in endothelial cells (EC) that mediate EC angiogenesis and migration. Upregulation of them contributes to tumor vascular abnormality and blockade of this interaction promotes a favorable tumor microenvironment for therapeutic interventions. However, how these two proteins associated to each other remains unclear. In this study, we solved the human CD93-IGFBP7 complex structure to elucidate the interaction between the EGF 1 domain of CD93 and the IB domain of IGFBP7. Mutagenesis studies confirmed the binding interactions and specificities. Cellular and mouse tumor studies demonstrated the physiological relevance of the CD93-IGFBP7 interaction in EC angiogenesis. Our study provides hints for development of therapeutic agents to precisely disrupt unwanted CD93-IGFBP7 signaling in the tumor microenvironment. Additionally, analysis of the CD93 full-length architecture provides insights into how CD93 protrudes on the cell surface and forms a flexible platform for binding to IGFBP7 and other ligands.

Heterozygous Pathogenic and Likely Pathogenic Symptomatic HTRA1 Variant Carriers in Cerebral Small Vessel Disease

High temperature requirement serine peptidase A1 (HTRA1) related cerebral small vessel disease (CSVD) includes both symptomatic heterozygous HTRA1 variant carrier and cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL) patients. Presently, most reported symptomatic heterozygous HTRA1 variant carrier cases are sporadic family reports with a lack of specific characteristics. Additionally, the molecular mechanism of heterozygous HTRA1 gene variants is unclear. We conducted this review to collect symptomatic carriers of heterozygous HTRA1 gene variants reported as of 2022, analyzed all pathogenicity according to American College of Medical Genetics and Genomics (ACMG) variant classification, and summarized the cases with pathogenic and likely pathogenic HTRA1 variants gender characteristics, age of onset, geographical distribution, initial symptoms, clinical manifestations, imaging signs, HTRA1 gene variant information and to speculate its underlying pathogenic mechanisms. In this review, we summarized the following characteristics of pathogenic and likely pathogenic symptomatic HTRA1 variant carriers: to date, the majority of reported symptomatic HTRA1 carriers are in European and Asian countries, particularly in China which was found to have the highest number of reported cases. The age of first onset is mostly concentrated in the fourth and fifth decades. The heterozygous HTRA1 gene variants were mostly missense variants. The two variant sites, 166-182 aa and 274-302 aa, were the most concentrated. Clinicians need to pay attention to de novo data and functional data, which may affect the pathogenicity analysis. The decrease in HtrA1 protease activity is currently the most important explanation for the genetic pathogenesis.

Allosteric inhibition of HTRA1 activity by a conformational lock mechanism to treat age-related macular degeneration

The trimeric serine protease HTRA1 is a genetic risk factor associated with geographic atrophy (GA), a currently untreatable form of age-related macular degeneration. Here, we describe the allosteric inhibition mechanism of HTRA1 by a clinical Fab fragment, currently being evaluated for GA treatment. Using cryo-EM, X-ray crystallography and biochemical assays we identify the exposed LoopA of HTRA1 as the sole Fab epitope, which is approximately 30 Å away from the active site. The cryo-EM structure of the HTRA1:Fab complex in combination with molecular dynamics simulations revealed that Fab binding to LoopA locks HTRA1 in a non-competent conformational state, incapable of supporting catalysis. Moreover, grafting the HTRA1-LoopA epitope onto HTRA2 and HTRA3 transferred the allosteric inhibition mechanism. This suggests a conserved conformational lock mechanism across the HTRA family and a critical role of LoopA for catalysis, which was supported by the reduced activity of HTRA1-3 upon LoopA deletion or perturbation. This study reveals the long-range inhibition mechanism of the clinical Fab and identifies an essential function of the exposed LoopA for activity of HTRA family proteases.

Novel mutations in HTRA1-related cerebral small vessel disease and comparison with CADASIL

Objective

There is evidence showing both heterozygous HTRA1 and homozygous HTRA1 mutations as causal for familial cerebral small vessel disease (CSVD). The clinical and neuroimaging signs of heterozygous HTRA1‐related CSVD can mimic cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). We aimed to characterize the genotypic and phenotypic features of HTRA1‐related CSVD, and we compared the features of heterozygous HTRA1‐related CSVD and CADASIL.

Methods

We carried out genetic sequencing in a series of unrelated patients with suspected familial CSVD from China. Clinical and imaging characteristics of heterozygous HTRA1‐related CSVD and CADASIL were compared.

Results

We identified nine heterozygous HTRA1 mutations and one homozygous HTRA1 mutation, seven of which are novel. Compared with CADASIL, patients with heterozygous HTRA1‐related CSVD had a higher proportion of spine disorders and a lower proportion of white matter hyperintensities involving the anterior temporal lobe (p < 0.001).

Interpretation

This study shows that most HTRA1‐related CSVD patients in China carry heterozygous HTRA1 mutations. The specific extra‐neurological features and neuroimaging features reveal informative differences between heterozygous HTRA1‐related CSVD and CADASIL. We expand the mutational spectrum of HTRA1.

An allosteric HTRA1-calpain 2 complex with restricted activation profile

SignificanceClassic serine proteases are synthesized as inactive precursors that are proteolytically processed, resulting in irreversible activation. We report an alternative and reversible mechanism of activation that is executed by an inactive protease. This mechanism involves a protein complex between the serine protease HTRA1 and the cysteine protease calpain 2. Surprisingly, activation is restricted as it improves the proteolysis of soluble tau protein but not the dissociation and degradation of its amyloid fibrils, a task that free HTRA1 is efficiently performing. These data exemplify a challenge for protein quality control proteases in the clearing of pathogenic fibrils and suggest a potential for unexpected side effects of chemical modulators targeting PDZ or other domains located at a distance to the active site.

Interplay between HTRA1 and classical signalling pathways in organogenesis and diseases

The high temperature requirement factor A1 (HTRA1) is a serine protease which modulates an array of signalling pathways driving basal biological processes. HTRA1 plays a significant role in cell proliferation, migration and fate determination, in addition to controlling protein aggregates through refolding, translocation or degradation. The mutation of HTRA1 has been implicated in a plethora of disorders and this has also led to its growing interest as drug therapy target. This review details the involvement of HTRA1 in certain signalling pathways, namely the transforming growth factor beta (TGF-β), canonical Wingless/Integrated (WNT) and NOTCH signalling pathways during organogenesis and various disease pathogenesis such as preeclampsia, age-related macular degeneration (AMD), small vessel disease and cancer. We have also explored possible avenues of exploiting the serine proteases for therapeutic management of these disorders.

Are apolipoprotein E fragments a promising new therapeutic target for Alzheimer’s disease?

Human apolipoprotein E (ApoE) is a 299-amino acid secreted glycoprotein that binds cholesterol and phospholipids. ApoE exists as three common isoforms (ApoE2, ApoE3, and ApoE4) and heterozygous carriers of the ε4 allele of the gene encoding ApoE (APOE) have a fourfold greater risk of developing Alzheimer’s disease (AD). The enzymes thrombin, cathepsin D, α-chymotrypsin-like serine protease, and high-temperature requirement serine protease A1 are responsible for ApoE proteolytic processing resulting in bioactive C-terminal-truncated fragments that vary depending on ApoE isoforms, brain region, aging, and neural injury. The objectives of the present narrative review were to describe ApoE processing, discussing current hypotheses about the potential role of various ApoE fragments in AD pathophysiology, and reviewing the current development status of different anti-ApoE drugs. The exact mechanism by which APOE gene variants increase/decrease AD risk and the role of ApoE fragments in the deposition are not fully understood, but APOE is known to directly affect tau-mediated neurodegeneration. ApoE fragments co-localize with neurofibrillary tangles and amyloid β (Aβ) plaques, and may cause neurodegeneration. Among anti-ApoE approaches, a fascinating strategy may be to therapeutically overexpress ApoE2 in APOE ε4/ε4 carriers through vector administration or liposomal delivery systems. Another approach involves reducing ApoE4 expression by intracerebroventricular antisense oligonucleotides that significantly decreased Aβ pathology in transgenic mice. Differences in the proteolytic processing of distinct ApoE isoforms and the use of ApoE fragments as mimetic peptides in AD treatment are also under investigation. Treatment with peptides that mimic the structural and biological properties of native ApoE may reduce Aβ deposition, tau hyperphosphorylation, and glial activation in mouse models of Aβ pathology. Alternative strategies involve the use of ApoE4 structure correctors, passive immunization to target a certain form of ApoE, conversion of the ApoE4 aminoacid sequence into that of ApoE3 or ApoE2, and inhibition of the ApoE-Aβ interaction.

Course monitoring of membranous nephropathy: Both autoantibodies and podocytes require multidimensional attention

A variety of podocyte antigens have been identified in human membranous nephropathy (MN), which is divided into various antigen-dominated subtypes, confirming the concept that MN is the common pattern of glomerular injury in multiple autoimmune responses. The detection of autoantibodies has been widely used, which promoted the clinical practice of MN toward personalized precision medicine. However, given the potential risks of immunosuppressive therapy, more autoantibodies and biomarkers need to be identified to predict the prognosis and therapeutic response of MN more accurately. In this review, we attempted to summarize the autoantigens/autoantibodies and autoimmune mechanisms that can predict disease states based on the current understanding of MN pathogenesis, especially the podocyte injury manifestations. In conclusion, both the autoimmune response and podocyte injury require multidimensional attention in the disease course of MN.

Whole-exome sequencing reveals a role of HTRA1 and EGFL8 in brain white matter hyperintensities

White matter hyperintensities (WMH) are among the most common radiological abnormalities in the ageing population and an established risk factor for stroke and dementia. While common variant association studies have revealed multiple genetic loci with an influence on their volume, the contribution of rare variants to the WMH burden in the general population remains largely unexplored. We conducted a comprehensive analysis of this burden in the UK Biobank using publicly available whole-exome sequencing data (n up to 17 830) and found a splice-site variant in GBE1, encoding 1,4-alpha-glucan branching enzyme 1, to be associated with lower white matter burden on an exome-wide level [c.691+2T>C, β = -0.74, standard error (SE) = 0.13, P = 9.7 × 10-9]. Applying whole-exome gene-based burden tests, we found damaging missense and loss-of-function variants in HTRA1 (frequency of 1 in 275 in the UK Biobank population) to associate with an increased WMH volume (P = 5.5 × 10-6, false discovery rate = 0.04). HTRA1 encodes a secreted serine protease implicated in familial forms of small vessel disease. Domain-specific burden tests revealed that the association with WMH volume was restricted to rare variants in the protease domain (amino acids 204-364; β = 0.79, SE = 0.14, P = 9.4 × 10-8). The frequency of such variants in the UK Biobank population was 1 in 450. The WMH volume was brought forward by ∼11 years in carriers of a rare protease domain variant. A comparison with the effect size of established risk factors for WMH burden revealed that the presence of a rare variant in the HTRA1 protease domain corresponded to a larger effect than meeting the criteria for hypertension (β = 0.26, SE = 0.02, P = 2.9 × 10-59) or being in the upper 99.8% percentile of the distribution of a polygenic risk score based on common genetic variants (β = 0.44, SE = 0.14, P = 0.002). In biochemical experiments, most (6/9) of the identified protease domain variants resulted in markedly reduced protease activity. We further found EGFL8, which showed suggestive evidence for association with WMH volume (P = 1.5 × 10-4, false discovery rate = 0.22) in gene burden tests, to be a direct substrate of HTRA1 and to be preferentially expressed in cerebral arterioles and arteries. In a phenome-wide association study mapping ICD-10 diagnoses to 741 standardized Phecodes, rare variants in the HTRA1 protease domain were associated with multiple neurological and non-neurological conditions including migraine with aura (odds ratio = 12.24, 95%CI: 2.54-35.25; P = 8.3 × 10-5]. Collectively, these findings highlight an important role of rare genetic variation and the HTRA1 protease in determining WMH burden in the general population.

Overview of Human HtrA Family Proteases and Their Distinctive Physiological Roles and Unique Involvement in Diseases, Especially Cancer and Pregnancy Complications

The mammalian high temperature requirement A (HtrA) proteins are a family of evolutionarily conserved serine proteases, consisting of four homologs (HtrA1-4) that are involved in many cellular processes such as growth, unfolded protein stress response and programmed cell death. In humans, while HtrA1, 2 and 3 are widely expressed in multiple tissues with variable levels, HtrA4 expression is largely restricted to the placenta with the protein released into maternal circulation during pregnancy. This limited expression sets HtrA4 apart from the rest of the family. All four HtrAs are active proteases, and their specific cellular and physiological roles depend on tissue type. The dysregulation of HtrAs has been implicated in many human diseases such as cancer, arthritis, neurogenerative ailments and reproductive disorders. This review first discusses HtrAs broadly and then focuses on the current knowledge of key molecular characteristics of individual human HtrAs, their similarities and differences and their reported physiological functions. HtrAs in other species are also briefly mentioned in the context of understanding the human HtrAs. It then reviews the distinctive involvement of each HtrA in various human diseases, especially cancer and pregnancy complications. It is noteworthy that HtrA4 expression has not yet been reported in any primary tumour samples, suggesting an unlikely involvement of this HtrA in cancer. Collectively, we accentuate that a better understanding of tissue-specific regulation and distinctive physiological and pathological roles of each HtrA will improve our knowledge of many processes that are critical for human health.

One Disease with two Faces: Semidominant Inheritance of a Novel HTRA1 Mutation in a Consanguineous Family

OBJECTIVES

To identify the underlying genetic defect for a consanguineous family with an unusually high number of members affected by cerebral small vessel disease.

MATERIALS AND METHODS

A total of 6 individuals, of whom 3 are severely affected, from the family were clinically and radiologically evaluated. SNP genotyping was performed in multiple members to demonstrate genome-wide runs-of-homozygosity. Coding variants in the most likely candidate gene, HTRA1 were explored by Sanger sequencing. Published HTRA1-related phenotypes were extensively reviewed to explore the effect of number of affected alleles on phenotypic expression.

RESULTS

Genome-wide homozygosity mapping identified a 3.2 Mbp stretch on chromosome 10q26.3 where HTRA1 gene is located. HTRA1 sequencing revealed an evolutionarily conserved novel homozygous c.824C>T (p.Pro275Leu) mutation, affecting the serine protease domain of HtrA1. Early-onset of cognitive and motor deterioration in homozygotes are in consensus with CARASIL. However, there was a clear phenotypic variability between homozygotes which includes alopecia, a suggested hallmark of CARASIL. All heterozygotes, presenting as CADASIL type 2, had spinal disk degeneration and several neuroimaging findings, including leukoencephalopathy and microhemorrhage despite a lack of severe clinical presentation.

CONCLUSION

Here, we clearly demonstrate that CARASIL and CADASIL type 2 are two clinical consequences of the same disorder with different severities thorough the evaluation of the largest collection of homozygotes and heterozygotes segregating in a family. Considering the semi-dominant inheritance of HTRA1-related phenotypes, genetic testing and clinical follow-up must be offered for all members of a family with HTRA1 mutations regardless of symptoms.

Cryo-EM structures of inhibitory antibodies complexed with arginase 1 provide insight into mechanism of action

Human Arginase 1 (hArg1) is a metalloenzyme that catalyzes the hydrolysis of L-arginine to L-ornithine and urea, and modulates T-cell-mediated immune response. Arginase-targeted therapies have been pursued across several disease areas including immunology, oncology, nervous system dysfunction, and cardiovascular dysfunction and diseases. Currently, all published hArg1 inhibitors are small molecules usually less than 350 Da in size. Here we report the cryo-electron microscopy structures of potent and inhibitory anti-hArg antibodies bound to hArg1 which form distinct macromolecular complexes that are greater than 650 kDa. With local resolutions of 3.5 Å or better we unambiguously mapped epitopes and paratopes for all five antibodies and determined that the antibodies act through orthosteric and allosteric mechanisms. These hArg1:antibody complexes present an alternative mechanism to inhibit hArg1 activity and highlight the ability to utilize antibodies as probes in the discovery and development of peptide and small molecule inhibitors for enzymes in general.

Serine Protease HTRA1 as a Novel Target Antigen in Primary Membranous Nephropathy

BACKGROUND

Identification of target antigens PLA2R, THSD7A, NELL1, or Semaphorin-3B can explain the majority of cases of primary membranous nephropathy (MN). However, target antigens remain unidentified in 15%-20% of patients.

METHODS

A multipronged approach, using traditional and modern technologies, converged on a novel target antigen, and capitalized on the temporal variation in autoantibody titer for biomarker discovery. Immunoblotting of human glomerular proteins followed by differential immunoprecipitation and mass spectrometric analysis was complemented by laser-capture microdissection followed by mass spectrometry, elution of immune complexes from renal biopsy specimen tissue, and autoimmune profiling on a protein fragment microarray.

RESULTS

These approaches identified serine protease HTRA1 as a novel podocyte antigen in a subset of patients with primary MN. Sera from two patients reacted by immunoblotting with a 51-kD protein within glomerular extract and with recombinant human HTRA1, under reducing and nonreducing conditions. Longitudinal serum samples from these patients seemed to correlate with clinical disease activity. As in PLA2R- and THSD7A- associated MN, anti-HTRA1 antibodies were predominantly IgG4, suggesting a primary etiology. Analysis of sera collected during active disease versus remission on protein fragment microarrays detected significantly higher titers of anti-HTRA1 antibody in active disease. HTRA1 was specifically detected within immune deposits of HTRA1-associated MN in 14 patients identified among three cohorts. Screening of 118 "quadruple-negative" (PLA2R-, THSD7A-, NELL1-, EXT2-negative) patients in a large repository of MN biopsy specimens revealed a prevalence of 4.2%.

CONCLUSIONS

Conventional and more modern techniques converged to identify serine protease HTRA1 as a target antigen in MN.

Insulin-Like Growth Factor-1 (IGF-1) and Its Monitoring in Medical Diagnostic and in Sports

Insulin-like growth factor-1 (IGF-1) is the principal mediator of growth hormone (GH), plays a crucial role in promoting cell growth and differentiation in childhood and continues to have an anabolic effect in adults. IGF-1 is part of a wide network of growth factors, receptors and binding proteins involved in mediating cellular proliferation, differentiation and apoptosis. Bioavailability of IGF-1 is affected by insulin-like growth factor binding proteins (IGFBPs) which bind IGF-1 in circulation with an affinity equal to or greater than that of the IGF-1 receptor (IGF-1R). The six IGFBPs serve as carrier proteins and bind approximately 98% of all circulating IGF-1. Other proteins known to bind IGF-1 include ten IGFBP-related proteins (IGFBP-rPs), albeit with lower affinities than the IGFBPs. IGF-1 expression levels vary in a number of clinical conditions suggesting it has the potential to provide crucial information as to the state of an individual’s health. IGF-1 is also a popular doping agent in sport and has featured in many high-profile doping cases in recent years. However, the existence of IGFBPs significantly reduces the levels of immunoreactive IGF-1 in samples, requiring multiple pre-treatment steps that reduce reproducibility and complicates interpretation of IGF-1 assay results. Here we provide an overview of the IGF network of growth factors, their receptors and the entirety of the extended family of IGFBPs, IGFBP-rPs, E peptides as well as recombinant IGF-1 and their derivatives. We also discuss issues related to the detection and quantification of bioavailable IGF-1.

Identification and Characterization of Plasmin-Independent Thrombolytic Enzymes

Supplemental Digital Content is available in the text.

Rationale:

Current thrombolytic agents activate plasminogen to plasmin which triggers fibrinolysis to dissolve thrombi. Since plasmin is a nonspecific proteolytic enzyme, all of the current plasmin-dependent thrombolytics lead to serious hemorrhagic complications, demanding a new class of fibrinolytic enzymes independent from plasmin activation and undesirable side effects. We speculated that the mammalian version of bacterial heat-shock proteins could selectively degrade intravascular thrombi, a typical example of a highly aggregated protein mixture.

Objective:

The objective of this study is to identify enzymes that can dissolve intravascular thrombi specifically without affecting fibrinogen and fibronectin so that the wound healing processes remain uninterrupted and tissues are not damaged. In this study, HtrA (high-temperature requirement A) proteins were tested for its specific proteolytic activity on intravascular thrombi independently from plasmin activation.

Methods and Results:

HtrA1 and HtrA2/Omi proteins, collectively called as HtrAs, lysed ex vivo blood thrombi by degrading fibrin polymers. The thrombolysis by HtrAs was plasmin-independent and specific to vascular thrombi without causing the systemic activation of plasminogen and preventing nonspecific proteolysis of other proteins including fibrinogen and fibronectin. As expected, HtrAs did not disturb clotting and wound healing of excised wounds from mouse skin. It was further confirmed in a tail bleeding and a rebleeding assay that HtrAs allowed normal clotting and maintenance of clot stability in wounds, unlike other thrombolytics. Most importantly, HtrAs completely dissolved blood thrombi in tail thrombosis mice, and the intravenous injection of HtrAs to mice with pulmonary embolism completely dissolved intravascular thrombi and thus rescued thromboembolism.

Conclusions:

Here, we identified HtrA1 and HtrA2/Omi as plasmin-independent and highly specific thrombolytics that can dissolve intravascular thrombi specifically without bleeding risk. This work is the first report of a plasmin-independent thrombolytic pathway, providing HtrA1 and HtrA2/Omi as ideal therapeutic candidates for various thrombotic diseases without hemorrhagic complications.

HTRA1-related autosomal dominant cerebral small vessel disease

BACKGROUND

Homozygous or compound heterozygous mutations in high temperature requirement serine peptidase A1 (HTRA1) gene are responsible for cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL). Recently, increasing evidence has shown that heterozygous HTRA1 mutations are also associated with cerebral small vessel disease (CSVD) with an autosomal dominant pattern of inheritance. This study was aimed to analyze the genetic and clinical characteristics of HTRA1-related autosomal dominant CSVD.

METHODS

We presented three new Chinese cases of familial CSVD with heterozygous HTRA1 mutations and reviewed all clinical case reports and articles on HTRA1-related autosomal dominant CSVD included in PUBMED by the end of March 1, 2020. CARASIL probands with genetic diagnosis reported to date were also reviewed. The genetic and clinical characteristics of HTRA1-related autosomal dominant CSVD were summarized and analyzed by comparing with CARASIL.

RESULTS

Forty-four HTRA1-related autosomal dominant CSVD probands and 22 CARASIL probands were included. Compared with typical CARASIL, HTRA1-related autosomal dominant probands has a higher proportion of vascular risk factors (P < 0.001), a later onset age (P < 0.001), and a relatively slower clinical progression. Alopecia and spondylosis can be observed, but less than those in the typical CARASIL. Thirty-five heterozygous mutations in HTRA1 were reported, most of which were missense mutations. Amino acids located close to amino acids 250-300 were most frequently affected, followed by these located near 150∼200. While amino acids 250∼300 were also the most frequently affected region in CARASIL patients, fewer mutations precede the 200th amino acids were detected, especially in the Kazal-type serine protease domain.

CONCLUSIONS

HTRA1-related autosomal dominant CSVD is present as a mild phenotype of CARASIL. The trend of regional concentration of mutation sites may be related to the concentration of key sites in these regions which are responsible for pathogenesis of HTRA1-related autosomal dominant CSVD.

Activity-Based Probes for the High Temperature Requirement A Serine Proteases

The high temperature requirement A (HTRA) family of serine proteases mediates protein quality control. These proteins process misfolded proteins in several diseases including Alzheimer's disease (AD) and Parkinson's disease (PD). While their structures and activation mechanisms have been studied, the precise details of the regulation of their activity under physiological conditions have not been completely elucidated, partly due to the lack of suitable chemical probes. In the present study, we developed novel activity-based probes (ABPs) targeting the HTRAs and demonstrated their utility in the monitoring and quantification of changes in enzyme activity in live cells. Using our probes, we found the activity of HTRA1 to be highly elevated in an AD-like cell-based model. We also observed the active HTRA2 in live cells by using a mitochondrion-targeted probe. We believe that our probes can serve as a useful tool to study the role of human HTRAs in neurodegenerative diseases.

The 10q26 Risk Haplotype of Age-Related Macular Degeneration Aggravates Subretinal Inflammation by Impairing Monocyte Elimination

A minor haplotype of the 10q26 locus conveys the strongest genetic risk for age-related macular degeneration (AMD). Here, we examined the mechanisms underlying this susceptibility. We found that monocytes from homozygous carriers of the 10q26 AMD-risk haplotype expressed high amounts of the serine peptidase HTRA1, and HTRA1 located to mononuclear phagocytes (MPs) in eyes of non-carriers with AMD. HTRA1 induced the persistence of monocytes in the subretinal space and exacerbated pathogenic inflammation by hydrolyzing thrombospondin 1 (TSP1), which separated the two CD47-binding sites within TSP1 that are necessary for efficient CD47 activation. This HTRA1-induced inhibition of CD47 signaling induced the expression of pro-inflammatory osteopontin (OPN). OPN expression increased in early monocyte-derived macrophages in 10q26 risk carriers. In models of subretinal inflammation and AMD, OPN deletion or pharmacological inhibition reversed HTRA1-induced pathogenic MP persistence. Our findings argue for the therapeutic potential of CD47 agonists and OPN inhibitors for the treatment of AMD.

A distinct concerted mechanism of structural dynamism defines activity of human serine protease HtrA3

Human HtrA3 (high-temperature requirement protease A3) is a trimeric multitasking propapoptotic serine protease associated with critical cellular functions and pathogenicity. Implicated in diseases including cancer and pre-eclampsia, its role as a tumor suppressor and potential therapeutic target cannot be ignored. Therefore, elucidating its mode of activation and regulatory switch becomes indispensable towards modulating its functions with desired effects for disease intervention. Using computational, biochemical and biophysical tools, we delineated the role of all domains, their combinations and the critical phenylalanine residues in regulating HtrA3 activity, oligomerization and specificity. Our findings underline the crucial roles of the N-terminus as well as the PDZ domain in oligomerization and formation of a catalytically competent enzyme, thus providing new insights into its structure–function coordination. Our study also reports an intricate ligand-induced allosteric switch, which redefines the existing hypothesis of HtrA3 activation besides opening up avenues for modulating protease activity favorably through suitable effector molecules.

Development of a therapeutic anti-HtrA1 antibody and the identification of DKK3 as a pharmacodynamic biomarker in geographic atrophy

Genetic polymorphisms in the region of the trimeric serine hydrolase high-temperature requirement 1 (HTRA1) are associated with increased risk of age-related macular degeneration (AMD) and disease progression, but the precise biological function of HtrA1 in the eye and its contribution to disease etiologies remain undefined. In this study, we have developed an HtrA1-blocking Fab fragment to test the therapeutic hypothesis that HtrA1 protease activity is involved in the progression of AMD. Next, we generated an activity-based small-molecule probe (ABP) to track target engagement in vivo. In addition, we used N-terminomic proteomic profiling in preclinical models to elucidate the in vivo repertoire of HtrA1-specific substrates, and identified substrates that can serve as robust pharmacodynamic biomarkers of HtrA1 activity. One of these HtrA1 substrates, Dickkopf-related protein 3 (DKK3), was successfully used as a biomarker to demonstrate the inhibition of HtrA1 activity in patients with AMD who were treated with the HtrA1-blocking Fab fragment. This pharmacodynamic biomarker provides important information on HtrA1 activity and pharmacological inhibition within the ocular compartment.

NBCZone: Universal three-dimensional construction of eleven amino acids near the catalytic nucleophile and base in the superfamily of (chymo)trypsin-like serine fold proteases

(Chymo)trypsin-like serine fold proteases belong to the serine/cysteine proteases found in eukaryotes, prokaryotes, and viruses. Their catalytic activity is carried out using a triad of amino acids, a nucleophile, a base, and an acid. For this superfamily of proteases, we propose the existence of a universal 3D structure comprising 11 amino acids near the catalytic nucleophile and base - Nucleophile-Base Catalytic Zone (NBCZone). The comparison of NBCZones among 169 eukaryotic, prokaryotic, and viral (chymo)trypsin-like proteases suggested the existence of 15 distinct groups determined by the combination of amino acids located at two "key" structure-functional positions 54_T and 55_T near the catalytic base His57_T. Most eukaryotic and prokaryotic proteases fell into two major groups, [ST]A and TN. Usually, proteases of [ST]A group contain a disulfide bond between cysteines Cys42_T and Cys58_T of the NBCZone. In contrast, viral proteases were distributed among seven groups, and lack this disulfide bond. Furthermore, only the [ST]A group of eukaryotic proteases contains glycine at position 43_T, which is instrumental for activation of these enzymes. In contrast, due to the side chains of residues at position 43_T prokaryotic and viral proteases do not have the ability to carry out the structural transition of the eukaryotic zymogen-zyme type.

The thrombospondin module 1 domain of the matricellular protein CCN3 shows an atypical disulfide pattern and incomplete CWR layers

The members of the CCN (Cyr61/CTGF/Nov) family are a group of matricellular regulatory proteins that are essential to a wide range of functional pathways in cell signalling. Through interacting with extracellular matrix components and growth factors via one of their four domains, the CCN proteins are involved in critical biological processes such as angiogenesis, cell proliferation, bone development, fibrogenesis and tumorigenesis. Here, the crystal structure of the thrombospondin module 1 (TSP1) domain of CCN3 (previously known as Nov) is presented, which shares a similar three-stranded fold with the thrombospondin type 1 repeats of thrombospondin-1 and spondin-1, but with variations in the disulfide connectivity. Moreover, the CCN3 TSP1 domain lacks the typical π-stacked ladder of charged and aromatic residues on one side of the domain that is seen in other TSP1 domains. Using conservation analysis among orthologous domains, it is shown that a charged cluster in the centre of the domain is the most conserved site and this cluster is predicted to be a potential functional epitope for heparan sulfate binding. This variant TSP1 domain has also been used to revise the sequence determinants of TSP1 domains and to derive improved Pfam sequence profiles for the identification of novel TSP1 domains in more than 10 000 proteins across diverse phyla.

Molecular structure and the role of high-temperature requirement protein 1 in skeletal disorders and cancers

Human high‐temperature requirement protein 1 (HTRA1) is a member of serine proteases and consists of four well‐defined domains—an IGFBP domain, a Kazal domain, a protease domain and a PDZ domain. HTRA1 is a secretory protein and also present intracellularly and associated with microtubules. HTRA1 regulates a broad range of physiological processes via its proteolytic activity. This review examines the role of HTRA1 in bone biology, osteoarthritis, intervertebral disc (IVD) degeneration and tumorigenesis. HTRA1 mediates diverse pathological processes via a variety of signalling pathways, such as TGF‐β and NF‐κB. The expression of HTRA1 is increased in arthritis and IVD degeneration, suggesting that HTRA1 protein is attributed to cartilage degeneration and disease progression. Emerging evidence also suggests that HTRA1 has a role in tumorigenesis. Further understanding the mechanisms by which HTRA1 displays as an extrinsic and intrinsic regulator in a cell type–specific manner will be important for the development of HTRA1 as a therapeutic target.

Surface loops of trypsin-like serine proteases as determinants of function

Trypsin and chymotrypsin-like serine proteases from family S1 (clan PA) constitute the largest protease group in humans and more generally in vertebrates. The prototypes chymotrypsin, trypsin and elastase represent simple digestive proteases in the gut, where they cleave nearly any protein. Multidomain trypsin-like proteases are key players in the tightly controlled blood coagulation and complement systems, as well as related proteases that are secreted from diverse immune cells. Some serine proteases are expressed in nearly all tissues and fluids of the human body, such as the human kallikreins and kallikrein-related peptidases with specialization for often unique substrates and accurate timing of activity. HtrA and membrane-anchored serine proteases fulfill important physiological tasks with emerging roles in cancer. The high diversity of all family members, which share the tandem β-barrel architecture of the chymotrypsin-fold in the catalytic domain, is conferred by the large differences of eight surface loops, surrounding the active site. The length of these loops alters with insertions and deletions, resulting in remarkably different three-dimensional arrangements. In addition, metal binding sites for Na+, Ca2+ and Zn2+ serve as regulatory elements, as do N-glycosylation sites. Depending on the individual tasks of the protease, the surface loops determine substrate specificity, control the turnover and allow regulation of activation, activity and degradation by other proteins, which are often serine proteases themselves. Most intriguingly, in some serine proteases, the surface loops interact as allosteric network, partially tuned by protein co-factors. Knowledge of these subtle and complicated molecular motions may allow nowadays for new and specific pharmaceutical or medical approaches.

ER stress-induced aggresome trafficking of HtrA1 protects against proteotoxicity

High temperature requirement A1 (HtrA1) belongs to an ancient protein family that is linked to various human disorders. The precise role of exon 1-encoded N-terminal domains and how these influence the biological functions of human HtrA1 remain elusive. In this study, we traced the evolutionary origins of these N-terminal domains to a single gene fusion event in the most recent common ancestor of vertebrates. We hypothesized that human HtrA1 is implicated in unfolded protein response. In highly secretory cells of the retinal pigmented epithelia, endoplasmic reticulum (ER) stress upregulated HtrA1. HtrA1 co-localized with vimentin intermediate filaments in highly arborized fashion. Upon ER stress, HtrA1 tracked along intermediate filaments, which collapsed and bundled in an aggresome at the microtubule organizing center. Gene silencing of HtrA1 altered the schedule and amplitude of adaptive signaling and concomitantly resulted in apoptosis. Restoration of wild-type HtrA1, but not its protease inactive mutant, was necessary and sufficient to protect from apoptosis. A variant of HtrA1 that harbored exon 1 substitutions displayed reduced efficacy in rescuing cells from proteotoxicity. Our results illuminate the integration of HtrA1 in the toolkit of mammalian cells against protein misfolding and the implications of defects in HtrA1 in proteostasis.

HTRA1 Mutations Identified in Symptomatic Carriers Have the Property of Interfering the Trimer-Dependent Activation Cascade

Background: Mutations in the high-temperature requirement A serine peptidase 1 (HTRA1) cause cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL). Most carriers for HTRA1 mutations are asymptomatic, but more than 10 mutations have been reported in symptomatic carriers. The molecular differences between the mutations identified in symptomatic carriers and mutations identified only in CARASIL patients are unclear. HTRA1 is a serine protease that forms homotrimers, with each HTRA1 subunit activating the adjacent HTRA1 via the sensor domain of loop 3 (L3) and the activation domain of loop D (LD). Previously, we analyzed four HTRA1 mutant proteins identified in symptomatic carriers and found that they were unable to form trimers or had mutations in the LD or L3 domain. The mutant HTRA1s with these properties are presumed to inhibit trimer-dependent activation cascade. Indeed, these mutant HTRA1s inhibited wild-type (WT) protease activity. In this study, we further analyzed 15 missense HTRA1s to clarify the molecular character of mutant HTRA1s identified in symptomatic carriers.

Methods: We analyzed 12 missense HTRA1s identified in symptomatic carriers (hetero-HTRA1) and three missense HTRA1s found only in CARASIL (CARASIL-HTRA1). The protease activity of the purified recombinant mutant HTRA1s was measured using fluorescein isothiocyanate-labeled casein as substrate. Oligomeric structure was evaluated by size-exclusion chromatography. The protease activities of mixtures of WT with each mutant HTRA1 were also measured.

Results: Five hetero-HTRA1s had normal protease activity and were excluded from further analysis. Four of the seven hetero-HTRA1s and one of the three CARASIL-HTRA1s were unable to form trimers. The other three hetero-HTRA1s had mutations in the LD domain. Together with our previous work, 10 of 11 hetero-HTRA1s and two of six CARASIL-HTRA1s were either defective in trimerization or had mutations in the LD or L3 domain (P = 0.006). By contrast, eight of 11 hetero-HTRA1s and two of six CARASIL-HTRA1 inhibited WT protease activity (P = 0.162).

Conclusions: HTRA1 mutations identified in symptomatic carriers have the property of interfering the trimer-dependent activation cascade of HTRA1.

Discerning the mechanism of action of HtrA4: a serine protease implicated in the cell death pathway

High-temperature requirement protease A4 (HtrA4) is a secretary serine protease whose expression is up-regulated in pre-eclampsia (PE) and hence is a possible biomarker of PE. It has also been altered in cancers such as glioblastoma, breast carcinoma, and prostate cancer making it an emerging therapeutic target. Among the human HtrAs, HtrA4 is the least characterized protease pertaining to both structure and its functions. Although the members of human HtrA family share a significant structural and functional conservation, subtle structural changes have been associated with certain distinct functional requirements. Therefore, intricate dissection of HtrA4 structural and functional properties becomes imperative to understand its role in various biological and pathophysiological processes. Here, using inter-disciplinary approaches including in silico, biochemical and biophysical studies, we have done a domain-wise dissection of HtrA4 to delineate the roles of the domains in regulating oligomerization, stability, protease activity, and specificity. Our findings distinctly demonstrate the importance of the N-terminal region in oligomerization, stability and hence the formation of a functional enzyme. In silico structural comparison of HtrA4 with other human HtrAs, enzymology studies and cleavage assays with X-linked inhibitor of apoptosis protein (XIAP) show overall structural conservation and allosteric mode of protease activation, which suggest functional redundancy within this protease family. However, significantly lower protease activity as compared with HtrA2 indicates an additional mode of regulation of the protease activity in the cellular milieu. Overall, these studies provide first-hand information on HtrA4 and its interaction with antiapoptotic XIAP thus implicating its involvement in the apoptotic pathway.

Purification and characterization of a novel anti-coagulant from the leech Hirudinaria manillensis

Protease inhibitors have been reported rarely from the leech Hirudinaria manillensis. In this study, we purified a novel protease inhibitor (bdellin-HM-2) with anticoagulant properties from H. manillensis. With a molecular weight of 1.4x10⁴, bdellin-HM-2 was also characterized with three intra-molecular disulfide bridges at the N-terminus and multiple HHXDD and HXDD motifs at the C-terminus. cDNA cloning revealed that the putative nucleotide-encoding protein of bdellin-HM-2 contained 132 amino acids and was encoded by a 399 bp open reading frame (ORF). Sequence alignment showed that bdellin-HM-2 shared similarity with the “non-classical” Kazal-type serine protease inhibitors, but had no inhibitory effect on trypsin, elastase, chymotrypsin, kallikrein, factor XIIa (FXIIa), factor XIa (FXIa), factor Xa (FXa), thrombin, or plasmin. Bdellin-HM-2 showed anticoagulant effects by prolonging the activated partial thromboplastin time (aPTT), indicating a role in enabling H. manillensis to obtain a blood meal from its host. Our results suggest that bdellin-HM-2 may play a crucial role in blood-sucking in this leech species and may be a potential candidate for the development of clinical anti-thrombotic drugs.

Anti-rheumatoid arthritis effects in adjuvant-induced arthritis in rats and molecular docking studies of Polygonum orientale L. extracts

BACKGROUND AND PURPOSE

Polygonum orientale L. (family: Polygonaceae), named Hongcao in China, has effects of dispelling wind and dampness, promoting blood circulation, and relieving pain. Our group has already studied and confirmed that POEa and POEe (ethyl acetate and ethyl ether extract of P. orientale, respectively) had anti-inflammatory and analgesic effects in early research, which was mainly relevant to the existence of flavonoids. According to the clinical application of P. orientale in traditional Chinese medicine, it has long been used for rheumatic arthralgia and rheumatoid arthritis. Therefore, our group further explored whether flavonoids of P. orientale have anti-rheumatoid arthritis effect and how does they play this role.

METHODS

Dried small pieces of the stems and leaves of P. orientale were decocted with water and partitioned successively to obtain POEa and POEe, respectively. The anti-rheumatoid arthritis effect of P. orientale was studied by using a Freund's complete adjuvant (FCA)-induced arthritis (AIA) in a rat model. The levels of PGE2, TNF-α, and IL-1β in serum of AIA rats were detected by enzyme linked immunosorbent assay (ELISA) to explore its mechanisms. In addition, we computationally studied the relationships between the 15 chemical components of POEa and POEe, and the currently focused 9 target proteins of rheumatoid arthritis by molecular docking.

RESULTS

Pharmacological experiments showed that POEa and POEe significantly ameliorate symptoms of rheumatoid arthritis via reducing paw swelling volume, arthritis score, and thymus and spleen indices, as well as increasing body weight in AIA rats. Simultaneously, the concentrations of PGE2, TNF-α, and IL-1β were significantly decreased by POEa and POEe. Histopathology revealed noticeable reduction in bone and cartilage, synovial hyperplasia, inflammatory cell infiltration, cartilage surface erosion, and joint degeneration by POEa and POEe treatment. In addition, the molecular docking studies showed that docking scores of 14 chemical compositions (including 12 flavonoids and 2 phenolic acids) of POEa and POEe with anti-rheumatoid arthritis protein targets were better than the complexed ligands of the anti-rheumatoid arthritis protein targets. Among them, six flavonoids in POEa and POEe had more docking protein targets (n ≥ 3). Five anti-rheumatoid arthritis targets including high-temperature requirement A1 protease (HtrA1), janus kinase 1 (JAK1), cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (i-NOS), and prostaglandin E2 (PGE2) had better docking score compared with the complexed ligands. Moreover, most of the chemical components in POEa and POEe showed strong interaction with HtrA1.

CONCLUSIONS

The flavonoids of P. orientale have anti-rheumatoid arthritis effect. In addition, the molecular docking results indicate that quercetin, catechol, orientin, and other six flavonoids may be closely related to HtrA1, JAK1, COX-2, i-NOS, and PGE2 protein target receptors. It suggests that these chemical compositions form strong protein-ligand complexes with these protein targets, especially HtrA1 to exert anti-rheumatoid arthritis. Further experimental studies show that mechanisms of anti-rheumatoid arthritis effects may also be relevant to inhibit the levels of PGE2, TNF-α, and IL-1β in serum. Therefore, our group can further explore the possible active ingredients and mechanisms of the anti-rheumatoid arthritis effects of flavonoids, and focus on the inhibition of the expression of inflammatory factors and the TGF-β1/Smad signaling pathway associated with HtrA1 protein target receptors, which can provide a direction and powerful reference for the action mechanism and drug research of anti-rheumatoid arthritis of flavonoids in P. orientale.

Structural basis of inactivation of human counterpart of mouse motor neuron degeneration 2 mutant in serine protease HtrA2

Serine protease high temperature requirement protease A2 (HtrA2) is involved in apoptosis and protein quality control. However, one of its murine inactive mutants (S276C aka mnd2) is associated with motor neuron degeneration 2. Similarly, this conserved mutation in human HtrA2 (hHtrA2) also renders the protease inactive, implicating pathogenicity. However, the structural determinants for its inactivation have not yet been elucidated. Here, using multidisciplinary approach, we studied the structural basis of inactivity associated with this mutation in hHtrA2. Characterization of secondary and tertiary structural properties, protein stability, oligomeric properties, and enzyme activity for both wild-type and mutant has been performed using biophysical and functional enzymology studies. The structural comparison at atomic resolution has been carried out using X-ray crystallography. While enzyme kinetics showed inactivity, spectroscopic probes did not identify any significant secondary structural changes in the mutant. X-ray crystallographic analysis of the mutant protein at 2 Å resolution highlighted the significance of a water molecule that plays important role in mediating intermolecular interactions for maintaining the functional ensemble of the protease. Overall, the crystallographic data along with biophysical and enzymology studies helped decipher the structural basis of inactivity of hHtrA2S276C, which might pave way toward further investigating its correlation with aberration of normal cellular functions, hence pathogenicity.

A Chinese CARASIL Patient Caused by Novel Compound Heterozygous Mutations in HTRA1

Our objective is to reported a Chinese CARASIL patient caused by novel compound heterozygous mutations in HTRA1. Detailed clinical and neuroimaging examination were conducted in proband and her available family members. Sanger sequencing of NOTCH3 and HTRA1 was used to investigate causative mutations. The patient was born in an outbred family. She experienced recurrent transient ischemic attacks, hair loss, and low back pain. Brain magnetic resonance imaging showed multiple lacunar infarctions, diffuse leukoencephalopathy, and multiple microbleeds of white matter. A compound heterozygous mutation, c.958G > A (p.D320N) and c.1021G > A (p.G341J), were identified in the proband. This report highlights that screening of HTRA1 should be considered in young SVD patient despite from outbred families.

Structures of insect Imp-L2 suggest an alternative strategy for regulating the bioavailability of insulin-like hormones

The insulin/insulin-like growth factor signalling axis is an evolutionary ancient and highly conserved hormonal system involved in the regulation of metabolism, growth and lifespan in animals. Human insulin is stored in the pancreas, while insulin-like growth factor-1 (IGF-1) is maintained in blood in complexes with IGF-binding proteins (IGFBP1-6). Insect insulin-like polypeptide binding proteins (IBPs) have been considered as IGFBP-like structural and functional homologues. Here, we report structures of the Drosophila IBP Imp-L2 in its free form and bound to Drosophila insulin-like peptide 5 and human IGF-1. Imp-L2 contains two immunoglobulin-like fold domains and its architecture is unrelated to human IGFBPs, suggesting a distinct strategy for bioavailability regulation of insulin-like hormones. Similar hormone binding modes may exist in other insect vectors, as the IBP sequences are highly conserved. Therefore, these findings may open research routes towards a rational interference of transmission of diseases such as malaria, dengue and yellow fevers.

N-Terminomics identifies HtrA1 cleavage of thrombospondin-1 with generation of a proangiogenic fragment in the polarized retinal pigment epithelial cell model of age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of irreversible blindness in the elderly population. Variants in the HTRA1-ARMS2 locus have been linked to increased AMD risk. In the present study we investigated the impact of elevated HtrA1 levels on the retina pigment epithelial (RPE) secretome using a polarized culture system. Upregulation of HtrA1 alters the abundance of key proteins involved in angiogenesis and extracellular matrix remodeling. Thrombospondin-1, an angiogenesis modulator, was identified as a substrate for HtrA1 using terminal amine isotope labeling of substrates in conjunction with HtrA1 specificity profiling. HtrA1 cleavage of thrombospondin-1 was further corroborated by in vitro cleavage assays and targeted proteomics together with small molecule inhibition of HtrA1. While thrombospondin-1 is anti-angiogenic, the proteolytically released N-terminal fragment promotes the formation of tube-like structure by endothelial cells. Taken together, our findings suggest a mechanism by which increased levels of HtrA1 may contribute to AMD pathogenesis. The proteomic data has been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the data set identifier. For quantitative secretome analysis, project accession: PXD007691, username: reviewer45093@ebi.ac.uk, password: 1FUpS6Yq. For TAILS analysis, project accession: PXD007139, username: reviewer76731@ebi.ac.uk, password: sNbMp7xK.

The crystal structure of an essential high-temperature requirement protein HtrA1 (Rv1223) from Mycobacterium tuberculosis reveals its unique features

High-temperature requirement A (HtrA) proteins, which are members of the heat-shock-induced serine protease family, are involved in extracytoplasmic protein quality control and bacterial survival strategies under stress conditions, and are associated with the virulence of several pathogens; they are therefore major drug targets. Mycobacterium tuberculosis possesses three putative HtrAs: HtrA1 (Rv1223), HtrA2 (Rv0983) and HtrA3 (Rv0125). Each has a cytoplasmic region, a transmembrane helix and a periplasmic region. Here, the crystal structure of the periplasmic region consisting of a protease domain (PD) and a PDZ domain from an M. tuberculosis HtrA1 mutant (mHtrA1^S387A) is reported at 2.7 Å resolution. Although the mHtrA1^S387A PD shows structural features similar to those of other HtrAs, its loops, particularly L3 and LA, display different conformations. Loop L3 communicates between the PDs of the trimer and the PDZ domains and undergoes a transition from an active to an inactive conformation, as reported for an equivalent HtrA (DegS). Loop LA, which is responsible for higher oligomer formation owing to its length (50 amino acids) in DegP, is very short in mHtrA1^S387A (five amino acids), as in mHtrA2 (also five amino acids), and therefore lacks essential interactions for the formation of higher oligomers. Notably, a well ordered loop known as the insertion clamp in the PDZ domain interacts with the protease domain of the adjacent molecule, which possibly aids in the stabilization of a trimeric functional unit of this enzyme. The three-dimensional structure of mHtrA1^S387A presented here will be useful in the design of enzyme-specific antituberculosis inhibitors.

High-Temperature Requirement A1 (Htra1) - A Novel Regulator of Canonical Wnt Signaling

Different cancer types as well as many other diseases are caused by aberrant activation of the canonical Wnt signal transduction pathway, and it is especially implicated in the development and progression of colorectal cancer (CRC). The main effector protein of the canonical Wnt signaling cascade is β-catenin, which binds to the T- cell factor/lymphoid enhancer factor (TCF/LEF) and triggers the activation of Wnt target genes. Here, we identify the serine protease High-Temperature Requirement A1 (HTRA1) as a novel component of the canonical Wnt pathway. We show that the HTRA1 protein inhibits the Wnt/β-catenin signaling, in both paracrine and autocrine manners, and affects the expression of several Wnt target genes. Moreover, HTRA1 forms a complex with β-catenin and reduces the proliferation rates of cells. Taken together, our findings indicate that HTRA1 functions as a novel suppressor of the canonical Wnt signaling pathway.

HtrA1 Mediated Intracellular Effects on Tubulin Using a Polarized RPE Disease Model

Age-related macular degeneration (AMD) is the leading cause of irreversible vision loss. The protein HtrA1 is enriched in retinal pigment epithelial (RPE) cells isolated from AMD patients and in drusen deposits. However, it is poorly understood how increased levels of HtrA1 affect the physiological function of the RPE at the intracellular level. Here, we developed hfRPE (human fetal retinal pigment epithelial) cell culture model where cells fully differentiated into a polarized functional monolayer. In this model, we fine-tuned the cellular levels of HtrA1 by targeted overexpression. Our data show that HtrA1 enzymatic activity leads to intracellular degradation of tubulin with a corresponding reduction in the number of microtubules, and consequently to an altered mechanical cell phenotype. HtrA1 overexpression further leads to impaired apical processes and decreased phagocytosis, an essential function for photoreceptor survival. These cellular alterations correlate with the AMD phenotype and thus highlight HtrA1 as an intracellular target for therapeutic interventions towards AMD treatment.

Macrophage migration inhibitory factor (MIF) modulates trophic signaling through interaction with serine protease HTRA1

Macrophage migration inhibitory factor (MIF), a small conserved protein, is abundant in the immune- and central nervous system (CNS). MIF has several receptors and binding partners that can modulate its action on a cellular level. It is upregulated in neurodegenerative diseases and cancer although its function is far from clear. Here, we report the finding of a new binding partner to MIF, the serine protease HTRA1. This enzyme cleaves several growth factors, extracellular matrix molecules and is implicated in some of the same diseases as MIF. We show that the function of the binding between MIF and HTRA1 is to inhibit the proteolytic activity of HTRA1, modulating the availability of molecules that can change cell growth and differentiation. MIF is therefore the first endogenous inhibitor ever found for HTRA1. It was found that both molecules were present in astrocytes and that the functional binding has the ability to modulate astrocytic activities important in development and disease of the CNS.

HtrA1 activation is driven by an allosteric mechanism of inter-monomer communication

The human protease family HtrA is responsible for preventing protein misfolding and mislocalization, and a key player in several cellular processes. Among these, HtrA1 is implicated in several cancers, cerebrovascular disease and age-related macular degeneration. Currently, HtrA1 activation is not fully characterized and relevant for drug-targeting this protease. Our work provides a mechanistic step-by-step description of HtrA1 activation and regulation. We report that the HtrA1 trimer is regulated by an allosteric mechanism by which monomers relay the activation signal to each other, in a PDZ-domain independent fashion. Notably, we show that inhibitor binding is precluded if HtrA1 monomers cannot communicate with each other. Our study establishes how HtrA1 trimerization plays a fundamental role in proteolytic activity. Moreover, it offers a structural explanation for HtrA1-defective pathologies as well as mechanistic insights into the degradation of complex extracellular fibrils such as tubulin, amyloid beta and tau that belong to the repertoire of HtrA1.

Molecular motion regulates the activity of the Mitochondrial Serine Protease HtrA2

HtrA2 (high-temperature requirement 2) is a human mitochondrial protease that has a role in apoptosis and Parkinson's disease. The structure of HtrA2 with an intact catalytic triad was determined, revealing a conformational change in the active site loops, involving mainly the regulatory LD loop, which resulted in burial of the catalytic serine relative to the previously reported structure of the proteolytically inactive mutant. Mutations in the loops surrounding the active site that significantly restricted their mobility, reduced proteolytic activity both in vitro and in cells, suggesting that regulation of HtrA2 activity cannot be explained by a simple transition to an activated conformational state with enhanced active site accessibility. Manipulation of solvent viscosity highlighted an unusual bi-phasic behavior of the enzymatic activity, which together with MD calculations supports the importance of motion in the regulation of the activity of HtrA2. HtrA2 is an unusually thermostable enzyme (T_M=97.3 °C), a trait often associated with structural rigidity, not dynamic motion. We suggest that this thermostability functions to provide a stable scaffold for the observed loop motions, allowing them a relatively free conformational search within a rather restricted volume.

Heterozygous mutations of HTRA1 gene in patients with familial cerebral small vessel disease

AIMS

Cerebral small vessel disease (SVD) is the leading cause of vascular dementia. Although the most of cases are sporadic, familial monogenic causes have been identified in a growing minority of patients. CADASIL, due to mutations of NOTCH3 gene, is the most common genetic SVD, and CARASIL, linked to HTRA1 gene mutations, is a rare but well known autosomal recessive SVD. Recently, also heterozygous HTRA1 mutations have been described in patients with familial SVD. To detect a genetic cause of familial SVD, we performed mutational analysis of HTRA1 gene in a large cohort of Italian NOTCH3-negative patients.

METHODS

We recruited 142 NOTCH3-negative patients and 160 healthy age-matched controls. Additional control data were obtained from five pathogenicity prediction software.

RESULTS

Five different HTRA1 heterozygous mutations were detected in nine patients from five unrelated families. Clinical phenotype was typical of SVD, and the onset was presenile. Brain magnetic resonance imaging (MRI) showed a subcortical leukoencephalopathy, with involvement of the external and internal capsule, corpus callosum, and multiple lacunar infarcts. Cerebral microbleeds were also seen, while anterior temporal lobes involvement was not present.

CONCLUSION

Our observation further supports the pathogenic role of the heterozygous HTRA1 mutations in familial SVD.