HtrA1 inhibits mineral deposition by osteoblasts: requirement for the protease and PDZ domains

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.

Inhibition of the serine protease HtrA1 by SerpinE2 suggests an extracellular proteolytic pathway in the control of neural crest migration

We previously showed that SerpinE2 and the serine protease HtrA1 modulate fibroblast growth factor (FGF) signaling in germ layer specification and head-to-tail development of Xenopus embryos. Here, we present an extracellular proteolytic mechanism involving this serpin-protease system in the developing neural crest (NC). Knockdown of SerpinE2 by injected antisense morpholino oligonucleotides did not affect the specification of NC progenitors but instead inhibited the migration of NC cells, causing defects in dorsal fin, melanocyte, and craniofacial cartilage formation. Similarly, overexpression of the HtrA1 protease impaired NC cell migration and the formation of NC-derived structures. The phenotype of SerpinE2 knockdown was overcome by concomitant downregulation of HtrA1, indicating that SerpinE2 stimulates NC migration by inhibiting endogenous HtrA1 activity. SerpinE2 binds to HtrA1, and the HtrA1 protease triggers degradation of the cell surface proteoglycan Syndecan-4 (Sdc4). Microinjection of Sdc4 mRNA partially rescued NC migration defects induced by both HtrA1 upregulation and SerpinE2 downregulation. These epistatic experiments suggest a proteolytic pathway by a double inhibition mechanism.

SerpinE2 ┤HtrA1 protease ┤Syndecan-4 → NC cell migration.

Possible involvement of HtrA1 serine protease in the onset of osteoporotic bone extracellular matrix changes

High-temperature requirement A1 (HtrA1), a multidomain serine protease acting on Extracellular matrix (ECM) rearrangement, is also secreted by osteoblasts and osteoclasts. Recent and conflicting literature highlights HtrA1's role as a controller of bone remodeling, proposing it as a possible target for pathologies with unbalanced bone resorption, like Osteoporosis (OP). To add knowledge on this molecule function in bone physiopathology, here we compared HtrA1 distribution in the ECM of healthy (H) and OP bone tissue, also examining its localization in the sites of new bone formation. HtrA1 was homogeneously expressed in the mature bone ECM of H tissue showing a 55.6 ± 16.4% of the stained area, with a significant (p=0.0001) decrease in OP percentage stained area (21.1 ± 13.1). Moreover, HtrA1 was present in the endosteum and cells involved in osteogenesis, mainly in those "entrapped" in woven bone, whereas osteocytes in mature lamellar bone were negative. Based on our previous observation in OP tissue of a significantly increased expression of Decorin and Osteocalcin, both involved in bone mineralization and remodeling and equally substrates for HtrA1, we speculate that HtrA1 by controlling the proper amount of Decorin and Osteocalcin favors normal bone maturation and mineralization. Besides, we suggest that late-osteoblasts and pre-osteocytes secrete HtrA1 in the adjacent matrix whilst proceeding with their maturation and that HtrA1 expression is further modified during the remodeling from woven to the lamellar bone. Overall, our data suggest HtrA1 as a positive regulator of bone matrix formation and maturation: its reduced expression in mature OP bone, affecting protein content and distribution, could hamper correct bone remodeling and mineralization.

Development and validation of risk prediction and neural network models for dilated cardiomyopathy based on WGCNA

Background

Dilated cardiomyopathy (DCM) is a progressive heart condition characterized by ventricular dilatation and impaired myocardial contractility with a high mortality rate. The molecular characterization of DCM has not been determined yet. Therefore, it is crucial to discover potential biomarkers and therapeutic options for DCM.

Methods

The hub genes for the DCM were screened using Weighted Gene Co-expression Network Analysis (WGCNA) and three different algorithms in Cytoscape. These genes were then validated in a mouse model of doxorubicin (DOX)-induced DCM. Based on the validated hub genes, a prediction model and a neural network model were constructed and validated in a separate dataset. Finally, we assessed the diagnostic efficiency of hub genes and their relationship with immune cells.

Results

A total of eight hub genes were identified. Using RT-qPCR, we validated that the expression levels of five key genes (ASPN, MFAP4, PODN, HTRA1, and FAP) were considerably higher in DCM mice compared to normal mice, and this was consistent with the microarray results. Additionally, the risk prediction and neural network models constructed from these genes showed good accuracy and sensitivity in both the combined and validation datasets. These genes also demonstrated better diagnostic power, with AUC greater than 0.7 in both the combined and validation datasets. Immune cell infiltration analysis revealed differences in the abundance of most immune cells between DCM and normal samples.

Conclusion

The current findings indicate an underlying association between DCM and these key genes, which could serve as potential biomarkers for diagnosing and treating DCM.

Nupr1 deficiency downregulates HtrA1, enhances SMAD1 signaling, and suppresses age-related bone loss in male mice

Nuclear protein 1 (NUPR1) is a stress-induced protein activated by various stresses, such as inflammation and oxidative stress. We previously reported that Nupr1 deficiency increased bone volume by enhancing bone formation in 11-week-old mice. Analysis of differentially expressed genes between wild-type (WT) and Nupr1-knockout (Nupr1-KO) osteocytes revealed that high temperature requirement A 1 (HTRA1), a serine protease implicated in osteogenesis and transforming growth factor-β signaling was markedly downregulated in Nupr1-KO osteocytes. Nupr1 deficiency also markedly reduced HtrA1 expression, but enhanced SMAD1 signaling in in vitro-cultured primary osteoblasts. In contrast, Nupr1 overexpression enhanced HtrA1 expression in osteoblasts, suggesting that Nupr1 regulates HtrA1 expression, thereby suppressing osteoblastogenesis. Since HtrA1 is also involved in cellular senescence and age-related diseases, we analyzed aging-related bone loss in Nupr1-KO mice. Significant spine trabecular bone loss was noted in WT male and female mice during 6-19 months of age, whereas aging-related trabecular bone loss was attenuated, especially in Nupr1-KO male mice. Moreover, cellular senescence-related markers were upregulated in the osteocytes of 6-19-month-old WT male mice but markedly downregulated in the osteocytes of 19-month-old Nupr1-KO male mice. Oxidative stress-induced cellular senescence stimulated Nupr1 and HtrA1 expression in in vitro-cultured primary osteoblasts, and Nupr1 overexpression enhanced p16^ink4a expression in osteoblasts. Finally, NUPR1 expression in osteocytes isolated from the bones of patients with osteoarthritis was correlated with age. Collectively, these results indicate that Nupr1 regulates HtrA1-mediated osteoblast differentiation and senescence. Our findings unveil a novel Nupr1/HtrA1 axis, which may play pivotal roles in bone formation and age-related bone loss.

Current Views on Chr10q26 Contribution to Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is the leading cause of blindness in the global aging population. Familial aggregation and genome-wide association (GWA) studies have identified gene variants associated with AMD, implying a strong genetic contribution to AMD development. Two loci, on human Chr 1q31 and 10q26, respectively, represent the most influential of all genetic factors. While the role of CFH at Chr 1q31 is well established, uncertainty remains about the genes ARMS2 and HTRA1, at the Chr 10q26 locus. Since both genes are in strong linkage disequilibrium, assigning individual gene effects is difficult. In this chapter, we review current literature about ARMS2 and HTRA1 and their relevance to AMD risk. Future studies will be necessary to unravel the mechanisms by which they contribute to AMD.

The multifaced role of HtrA1 in the development of joint and skeletal disorders

HtrA1 (High temperature requirement A1) family proteins include four members, widely conserved from prokaryotes to eukaryotes, named HtrA1, HtrA2, HtrA3 and HtrA4. HtrA1 is a serine protease involved in a variety of biological functions regulating many signaling pathways degrading specific components and playing key roles in many human diseases such as neurodegenerative disorders, pregnancy complications and cancer. Due to its role in the breakdown of many ExtraCellular Matrix (ECM) components of articular cartilage such as fibronectin, decorin and aggrecan, HtrA1 encouraged many researches on studying its role in several skeletal diseases (SDs). These studies were further inspired by the fact that HtrA1 is able to regulate the signaling of one of the most important cytokines involved in SDs, the TGFβ-1. This review aims to summarize the data currently available on the role of HtrA1 in skeletal diseases such as Osteoporosis, Rheumatoid Arthritis, Osteoarthritis and Intervertebral Disc Degeneration (IDD). The use of HtrA1 as a marker of frailty in geriatric medicine would represent a powerful tool for identifying older individuals at risk of developing skeletal disorders, evaluating an appropriate intervention to improve quality care in these people avoiding or improving age-related SDs in the elderly population.

Ongoing controversies and recent insights of the ARMS2-HTRA1 locus in age-related macular degeneration

Age-related macular degeneration (AMD) is the most common cause of central vision loss among elderly populations in industrialized countries. Genome-wide association studies have consistently associated two genomic loci with progression to late-stage AMD: the complement factor H (CFH) locus on chromosome 1q31 and the age-related maculopathy susceptibility 2-HtrA serine peptidase 1 (ARMS2-HTRA1) locus on chromosome 10q26. While the CFH risk variant has been shown to alter complement activity, the ARMS2-HTRA1 risk haplotype remains enigmatic due to high linkage disequilibrium and inconsistent functional findings spanning two genes that are plausibly causative for AMD risk. In this review, we detail the genetic and functional evidence used to support either ARMS2 or HTRA1 as the causal gene for AMD risk, emphasizing both the historical development and the current understanding of the ARMS2-HTRA1 locus in AMD pathogenesis. We conclude by summarizing the evidence in favor of HTRA1 and present our hypothesis whereby HTRA1-derived ECM fragments mediate AMD pathogenesis.

The interplay of oxidative stress and ARMS2-HTRA1 genetic risk in neovascular AMD

Age-related macular degeneration (AMD) is the leading cause of vision loss in adults over 60 years old globally. There are two forms of advanced AMD: “dry” and “wet”. Dry AMD is characterized by geographic atrophy of the retinal pigment epithelium and overlying photoreceptors in the macular region; whereas wet AMD is characterized by vascular penetrance from the choroid into the retina, known as choroidal neovascularization (CNV). Both phenotypes eventually lead to loss of central vision. The pathogenesis of AMD involves the interplay of genetic polymorphisms and environmental risk factors, many of which elevate retinal oxidative stress. Excess reactive oxygen species react with cellular macromolecules, forming oxidation-modified byproducts that elicit chronic inflammation and promote CNV. Additionally, genome-wide association studies have identified several genetic variants in the age-related maculopathy susceptibility 2/high-temperature requirement A serine peptidase 1 (ARMS2-HTRA1) locus associated with the progression of late-stage AMD, especially the wet subtype. In this review, we will focus on the interplay of oxidative stress and HTRA1 in drusen deposition, chronic inflammation, and chronic angiogenesis. We aim to present a multifactorial model of wet AMD progression, supporting HTRA1 as a novel therapeutic target upstream of vascular endothelial growth factor (VEGF), the conventional target in AMD therapeutics. By inhibiting HTRA1’s proteolytic activity, we can reduce pro-angiogenic signaling and prevent proteolytic breakdown of the blood-retina barrier. The anti-HTRA1 approach offers a promising alternative treatment option to wet AMD, complementary to anti-VEGF therapy.

New Technologies to Study Functional Genomics of Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is the most common cause of irreversible vision loss in people over 50 years old in developed countries. Currently, we still lack a comprehensive understanding of the genetic factors contributing to AMD, which is critical to identify effective therapeutic targets to improve treatment outcomes for AMD patients. Here we discuss the latest technologies that can facilitate the identification and functional study of putative genes in AMD pathology. We review improved genomic methods to identify novel AMD genes, advances in single cell transcriptomics to profile gene expression in specific retinal cell types, and summarize recent development of in vitro models for studying AMD using induced pluripotent stem cells, organoids and biomaterials, as well as new molecular technologies using CRISPR/Cas that could facilitate functional studies of AMD-associated genes.

RUNX2 co-operates with EGR1 to regulate osteogenic differentiation through Htra1 enhancers

Runt-related transcription factor 2 (Runx2) has been shown to regulate osteoblast differentiation by directly or indirectly regulating numerous osteoblast-related genes. However, our understanding of the transcriptional mechanisms of RUNX2 is mainly restricted to its transactivation, while the mechanism underlying its inhibitory effect during osteoblast differentiation remains largely unknown. Here, we incorporated the anti-RUNX2 chromatin immunoprecipitation (ChIP) sequencing in MC3T3-E1 cells and RNA-sequencing of parietal bone from Runx2 heterozygous mutant mice, to identify the putative genes negatively regulated by RUNX2. We identified HtrA serine peptidase 1 (Htra1) as a target gene and found ten candidate Htra1 enhancers potentially regulated by RUNX2, among which seven were verified by dual-luciferase assays. Furthermore, we investigated the motifs in the vicinity of RUNX2-binding sites and identified early growth response 1 (EGR1) as a potential partner transcription factor (TF) potentially regulating Htra1 expression, which was subsequently confirmed by Re-ChIP assays. RUNX2 and EGR1 co-repressed Htra1 and increased the expression levels of other osteoblast marker genes, such as osterix, osteocalcin, and osteoprotegerin at the messenger RNA and protein level. Moreover, Alizarin red staining combined with alkaline phosphatase (ALP) staining showed decreased calcified nodules and ALP activity in the siRUNX2+siEGR1 group compared with siRUNX2 group. Our findings revealed the detailed mechanism of the inhibitory function of RUNX2 towards its downstream genes, along with its partner TFs, to promote osteoblast differentiation.

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.

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.

Low Density Receptor-Related Protein 1 Interactions With the Extracellular Matrix: More Than Meets the Eye

The extracellular matrix (ECM) is a biological substrate composed of collagens, proteoglycans and glycoproteins that ensures proper cell migration and adhesion and keeps the cell architecture intact. The regulation of the ECM composition is a vital process strictly controlled by, among others, proteases, growth factors and adhesion receptors. As it appears, ECM remodeling is also essential for proper neuronal and glial development and the establishment of adequate synaptic signaling. Hence, disturbances in ECM functioning are often present in neurodegenerative diseases like Alzheimer’s disease. Moreover, mutations in ECM molecules are found in some forms of epilepsy and malfunctioning of ECM-related genes and pathways can be seen in, for example, cancer or ischemic injury. Low density lipoprotein receptor-related protein 1 (Lrp1) is a member of the low density lipoprotein receptor family. Lrp1 is involved not only in ligand uptake, receptor mediated endocytosis and lipoprotein transport—functions shared by low density lipoprotein receptor family members—but also regulates cell surface protease activity, controls cellular entry and binding of toxins and viruses, protects against atherosclerosis and acts on many cell signaling pathways. Given the plethora of functions, it is not surprising that Lrp1 also impacts the ECM and is involved in its remodeling. This review focuses on the role of Lrp1 and some of its major ligands on ECM function. Specifically, interactions with two Lrp1 ligands, integrins and tissue plasminogen activator are described in more detail.

Murine osteoclasts secrete serine protease HtrA1 capable of degrading osteoprotegerin in the bone microenvironment

Osteoclasts are multinucleated cells responsible for bone resorption. The differentiation of osteoclasts from bone marrow macrophages (BMMs) is induced by receptor activator of NF-κB ligand (RANKL). Osteoprotegerin (OPG), a decoy receptor of RANKL, inhibits osteoclastogenesis by blocking RANKL signaling. Here we investigated the degradation of OPG in vitro. Osteoclasts, but not BMMs, secreted OPG-degrading enzymes. Using mass spectrometry and RNA-sequencing analysis, we identified high-temperature requirement A serine peptidase 1 (HtrA1) as an OPG-degrading enzyme. HtrA1 did not degrade OPG pre-reduced by dithiothreitol, suggesting that HtrA1 recognizes the three-dimensional structure of OPG. HtrA1 initially cleaved the amide bond between leucine 90 and glutamine 91 of OPG, then degraded OPG into small fragments. Inhibitory activity of OPG on RANKL-induced osteoclastogenesis was suppressed by adding HtrA1 in RAW 264.7 cell cultures. These results suggest that osteoclasts potentially prepare a microenvironment suitable for osteoclastogenesis. HtrA1 may be a novel drug target for osteoporosis.

High-temperature Requirement Protein A1 Regulates Odontoblastic Differentiation of Dental Pulp Cells via the Transforming Growth Factor Beta 1/Smad Signaling Pathway

INTRODUCTION

Dentinogenesis includes odontoblast differentiation and extracellular matrix maturation as well as dentin mineralization. It is regulated by numerous molecules. High-temperature requirement protein A1 (HtrA1) plays crucial roles in bone mineralization and development and is closely associated with the transforming growth factor beta (TGF-β) signal in osteogenesis differentiation. Simultaneously, the TGF-β1/small mother against decapentaplegic (Smad) signaling pathway is an important signaling pathway in various physiological processes and as a downstream regulation factor of HtrA1. However, the role of HtrA1 and its relationship with the TGF-β1/Smad signaling pathway in dentin mineralization is unknown.

METHODS

We detected the role of HtrA1 and its relationship with the TGF-β1/Smad signaling pathway in odontoblastic differentiation of human dental pulp cells (hDPCs) in this study. First, hDPCs were cultured in mineralized medium, and odontoblastic differentiation was confirmed by investigating mineralized nodule formation, alkaline phosphatase (ALP) activity, and the expression of mineral-associated genes, including ALP, collagen I, and dentin sialophosphoprotein. Then, the expression of HtrA1 and TGF-β1/Smad in hDPCs was investigated in hDPCs during mineralized induction. After HtrA1 knockdown by lentivirus, the mineralized nodule formation, ALP activity, and expression of mineral-associated genes and TGF-β1/Smad genes were investigated to confirm the effect of HtrA1 on odontoblastic differentiation and its relationship with the TGF-β1/Smad signaling pathway.

RESULTS

The expression of HtrA1 and TGF-β1 was increased during odontoblastic differentiation of hDPCs along with the messenger RNA expression of downstream factors of the TGF-β1/Smad signaling pathway. In addition, lentivirus-mediated HtrA1 knockdown inhibited the process of mineralization and the expression of HtrA1 and TGF-β1/Smad genes.

CONCLUSIONS

These findings suggest that HtrA1 might positively regulate odontoblastic differentiation of hDPCs through activation of the TGF-β1/Smad signaling pathway.

BMP-Responsive Protease HtrA1 Is Differentially Expressed in Astrocytes and Regulates Astrocytic Development and Injury Response

Astrocytes perform a wide array of physiological functions, including structural support, ion exchange, and neurotransmitter uptake. Despite this diversity, molecular markers that label subpopulations of astrocytes are limited, and mechanisms that generate distinct astrocyte subtypes remain unclear. Here we identified serine protease high temperature requirement A 1 (HtrA1), a bone morphogenetic protein 4 signaling regulated protein, as a novel marker of forebrain astrocytes, but not of neural stem cells, in adult mice of both sexes. Genetic deletion of HtrA1 during gliogenesis accelerates astrocyte differentiation. In addition, ablation of HtrA1 in cultured astrocytes leads to altered chondroitin sulfate proteoglycan expression and inhibition of neurite extension, along with elevated levels of transforming growth factor-β family proteins. Brain injury induces HtrA1 expression in reactive astrocytes, and loss of HtrA1 leads to an impairment in wound closure accompanied by increased proliferation of endothelial and immune cells. Our findings demonstrate that HtrA1 is differentially expressed in adult mouse forebrain astrocytes, and that HtrA1 plays important roles in astrocytic development and injury response.SIGNIFICANCE STATEMENT Astrocytes, an abundant cell type in the brain, perform a wide array of physiological functions. Although characterized as morphologically and functionally diverse, molecular markers that label astrocyte subtypes or signaling pathways that lead to their diversity remain limited. Here, after examining the expression profile of astrocytes generated in response to bone morphogenetic protein signaling, we identify high temperature requirement A 1 (HtrA1) as an astrocyte-specific marker that is differentially expressed in distinct adult mouse brain regions. HtrA1 is a serine protease that has been linked to cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy, a small blood vessel disease in humans. Understanding the role of HtrA1 during development and after injury will provide insights into how distinct astrocyte populations are generated and their unique roles in injury and disease.

Role of HTRA1 in bone formation and regeneration: In vitro and in vivo evaluation

The role of mammalian high temperature requirement protease A1 (HTRA1) in somatic stem cell differentiation and mineralized matrix formation remains controversial, having been demonstrated to impart either anti- or pro-osteogenic effects, depending on the in vitro cell model used. The aim of this study was therefore to further evaluate the role of HTRA1 in regulating the differentiation potential and lineage commitment of murine mesenchymal stem cells in vitro, and to assess its influence on bone structure and regeneration in vivo. Our results demonstrated that short hairpin RNA-mediated ablation of Htra1 in the murine mesenchymal cell line C3H10T1/2 increased the expression of several osteogenic gene markers, and significantly enhanced matrix mineralization in response to BMP-2 stimulation. These effects were concomitant with decreases in the expression of chondrogenic gene markers, and increases in adipogenic gene expression and lipid accrual. Despite the profound effects of loss-of-function of HTRA1 on this in vitro osteochondral model, these were not reproduced in vivo, where bone microarchitecture and regeneration in 16-week-old Htra1-knockout mice remained unaltered as compared to wild-type controls. By comparison, analysis of femurs from 52-week-old mice revealed that bone structure was better preserved in Htra1-knockout mice than age-matched wild-type controls. These findings therefore provide additional insights into the role played by HTRA1 in regulating mesenchymal stem cell differentiation, and offer opportunities for improving our understanding of how this multifunctional protease may act to influence bone quality.

Structural insights into the activation mechanisms of human HtrA serine proteases

Human HtrA1-4 proteins belong to the HtrA family of evolutionarily conserved serine proteases and function as important modulators of many physiological processes, including maintenance of mitochondrial homeostasis, cell signaling and apoptosis. Disturbances in their action are linked to severe diseases, including oncogenesis and neurodegeneration. The HtrA1-4 proteins share structural and functional features of other members of the HtrA protein family, however there are several significant differences in structural architecture and mechanisms of action which makes each of them unique. Our goal is to present recent studies regarding human HtrAs. We focus on their physiological functions, structure and regulation, and describe current models of activation mechanisms. Knowledge of molecular basis of the human HtrAs' action is a subject of great interest; it is crucial for understanding their relevance in cellular physiology and pathogenesis as well as for using them as targets in future therapies of diseases such as neurodegenerative disorders and cancer.

Loss-of-Function of HtrA1 Abrogates All-Trans Retinoic Acid-Induced Osteogenic Differentiation of Mouse Adipose-Derived Stromal Cells Through Deficiencies in p70S6K Activation

All-trans retinoic acid (ATRA) is a potent inducer of osteogenic differentiation in mouse adipose-derived stromal cells (mASCs), although the underlying mechanisms responsible for its mode of action have yet to be completely elucidated. High temperature requirement protease A1 (HtrA1) is a newly recognized modulator of human multipotent stromal cell (MSC) osteogenesis and as such, may play a role in regulating ATRA-dependent osteogenic differentiation of mASCs. In this study, we assessed the influence of small interfering RNA (siRNA)-induced repression of HtrA1 production on mASC osteogenesis and examined its effects on ATRA-mediated mammalian target of rapamycin (mTOR) signaling. Inhibition of HtrA1 production in osteogenic mASCs resulted in a significant reduction of alkaline phosphatase activity and mineralized matrix formation. Western blot analyses revealed the rapid activation of Akt (Ser473) and p70S6K (Thr389) in ATRA-treated mASCs, and that levels of phosphorylated p70S6K were noticeably reduced in HtrA1-deficient mASCs. Further studies using mTOR inhibitor rapamycin and siRNA specific for the p70S6K gene Rps6kb1 confirmed ATRA-mediated mASC osteogenesis as being dependent on p70S6K activation. Finally, transfection of cells with a constitutively active rapamycin-resistant p70S6K mutant could restore the mineralizing capacity of HtrA1-deficient mASCs. These findings therefore lend further support for HtrA1 as a positive mediator of MSC osteogenesis and provide new insights into the molecular mode of action of ATRA in regulating mASC lineage commitment.

Nascent osteoblast matrix inhibits osteogenesis of human mesenchymal stem cells in vitro

INTRODUCTION

Adult mesenchymal stem cells (MSCs) are considered promising candidates for cell-based therapies. Their potential utility derives primarily from their immunomodulatory activity, multi-lineage differentiation potential, and likely progenitor cell function in wound healing and repair of connective tissues. However, in vitro, MSCs often senesce and spontaneously differentiate into osteoblasts after prolonged expansion, likely because of lack of regulatory microenvironmental signals. In vivo, osteoblasts that line the endosteal bone marrow surface are in close proximity to MSCs in the marrow stroma and thus may help to regulate MSC fate.

METHODS

We examined here how osteogenic differentiation of MSCs in vitro is affected by exposure to osteoblastic cells (OBCs). Human bone marrow MSCs were exposed to OBCs, derived by induced osteogenic differentiation of MSCs, either directly in contact co-cultures, or indirectly to OBC-conditioned medium or decellularized OBC extracellular matrix (ECM).

RESULTS

Our results showed that OBCs can act as negative regulators of MSC osteogenesis. mRNA expression profiling revealed that OBCs did not affect MSC osteogenesis in direct contact cultures or via secreted factors. However, seeding MSCs on decellularized OBC ECM significantly decreased expression of several osteogenic genes and maintained their fibroblastic morphologies. Proteomic analysis identified some of the candidate protein regulators of MSC osteogenesis.

CONCLUSIONS

These findings provide the basis for future studies to elucidate the signaling mechanisms responsible for osteoblast matrix-mediated regulation of MSC osteogenesis and to better manipulate MSC fate in vitro to minimize their spontaneous differentiation.

Synonymous variants in HTRA1 implicated in AMD susceptibility impair its capacity to regulate TGF-β signaling

High-temperature requirement A1 (HTRA1) is a secreted serine protease reported to play a role in the development of several cancers and neurodegenerative diseases. Still, the mechanism underlying the disease processes largely remains undetermined. In age-related macular degeneration (AMD), a common cause of vision impairment and blindness in industrialized societies, two synonymous polymorphisms (rs1049331:C>T, and rs2293870:G>T) in exon 1 of the HTRA1 gene were associated with a high risk to develop disease. Here, we show that the two polymorphisms result in a protein with altered thermophoretic properties upon heat-induced unfolding, trypsin accessibility and secretion behavior, suggesting unique structural features of the AMD-risk-associated HTRA1 protein. Applying MicroScale Thermophoresis and protease digestion analysis, we demonstrate direct binding and proteolysis of transforming growth factor β1 (TGF-β1) by normal HTRA1 but not the AMD-risk-associated isoform. As a consequence, both HTRA1 isoforms strongly differed in their ability to control TGF-β mediated signaling, as revealed by reporter assays targeting the TGF-β1-induced serpin peptidase inhibitor (SERPINE1, alias PAI-1) promoter. In addition, structurally altered HTRA1 led to an impaired autocrine TGF-β signaling in microglia, as measured by a strong down-regulation of downstream effectors of the TGF-β cascade such as phosphorylated SMAD2 and PAI-1 expression. Taken together, our findings demonstrate the effects of two synonymous HTRA1 variants on protein structure and protein interaction with TGF-β1. As a consequence, this leads to an impairment of TGF-β signaling and microglial regulation. Functional implications of the altered properties on AMD pathogenesis remain to be clarified.

HtrA1 may regulate the osteogenic differentiation of human periodontal ligament cells by TGF-β1

Periodontal ligament cells (PDLCs) in periodontal ligament (PDL) can differentiate into osteoblasts, while physiologically PDL remains non-mineralized space although located two hard tissues. But the exact mechanism of which is still unclear. High-temperature requirement protein A1 (HtrA1) is a key mineralization regulator and could inhibit the osteogenesis by transforming growth factor-β (TGF-β) signaling. However, the role of HtrA1 in PDLCs osteogenic differentiation has yet to be clarified. We assume HtrA1 may play an important role in maintaining the balance of PDL mineralization, and may regulate human periodontal ligament cells (hPDLCs) osteogenic differentiation by TGF-β1. Firstly we confirmed the mRNA expression of HtrA1 and TGF-β1 in hPDLCs by RT-PCR, then QDs-based immunofluorescence demonstrated the co-localization of them in the cytoplasm, and co-immunoprecipitation further confirmed the interaction between them. Lentivirus-mediated HtrA1 overexpression enhanced the osteogenic differentiation of hPDLCs, as well as up-regulation of TGF-β1. In contrast, knockdown of HtrA1 suppressed the osteogenic differentiation with down-regulation of TGF-β1. These findings suggested that HtrA1 plays a positive role in hPDLCs osteogenic differentiation and may regulate this process by TGF-β1.

The autolysis of human HtrA1 is governed by the redox state of its N-terminal domain

Human HtrA1 (high-temperature requirement protein A1) belongs to a conserved family of serine proteases involved in protein quality control and cell fate. The homotrimeric ubiquitously expressed protease has chymotrypsin-like specificity and primarily targets hydrophobic stretches in selected or misfolded substrate proteins. In addition, the enzyme is capable of exerting autolytic activity by removing the N-terminal insulin-like growth factor binding protein (IGFBP)/Kazal-like tandem motif without affecting the protease activity. In this study, we have addressed the mechanism governing the autolytic activity and find that it depends on the integrity of the disulfide bonds in the N-terminal IGFBP/Kazal-like domain. The specificity of the autolytic cleavage reveals a strong preference for cysteine in the P1 position of HtrA1, explaining the lack of autolysis prior to disulfide reduction. Significantly, the disulfides were reduced by thioredoxin, suggesting that autolysis of HtrA1 in vivo is linked to the endogenous redox balance and that the N-terminal domain acts as a redox-sensing switch.

Cartilage tissue engineering: molecular control of chondrocyte differentiation for proper cartilage matrix reconstruction

BACKGROUND

Articular cartilage defects are a veritable therapeutic problem because therapeutic options are very scarce. Due to the poor self-regeneration capacity of cartilage, minor cartilage defects often lead to osteoarthritis. Several surgical strategies have been developed to repair damaged cartilage. Autologous chondrocyte implantation (ACI) gives encouraging results, but this cell-based therapy involves a step of chondrocyte expansion in a monolayer, which results in the loss in the differentiated phenotype. Thus, despite improvement in the quality of life for patients, reconstructed cartilage is in fact fibrocartilage. Successful ACI, according to the particular physiology of chondrocytes in vitro, requires active and phenotypically stabilized chondrocytes.

SCOPE OF REVIEW

This review describes the unique physiology of cartilage, with the factors involved in its formation, stabilization and degradation. Then, we focus on some of the most recent advances in cell therapy and tissue engineering that open up interesting perspectives for maintaining or obtaining the chondrogenic character of cells in order to treat cartilage lesions.

MAJOR CONCLUSIONS

Current research involves the use of chondrocytes or progenitor stem cells, associated with "smart" biomaterials and growth factors. Other influential factors, such as cell sources, oxygen pressure and mechanical strain are considered, as are recent developments in gene therapy to control the chondrocyte differentiation/dedifferentiation process.

GENERAL SIGNIFICANCE

This review provides new information on the mechanisms regulating the state of differentiation of chondrocytes and the chondrogenesis of mesenchymal stem cells that will lead to the development of new restorative cell therapy approaches in humans. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.

Differential proteomic analysis of synovial fluid from rheumatoid arthritis and osteoarthritis patients

Background

Rheumatoid arthritis and osteoarthritis are two common musculoskeletal disorders that affect the joints. Despite high prevalence rates, etiological factors involved in these disorders remain largely unknown. Dissecting the molecular aspects of these disorders will significantly contribute to improving their diagnosis and clinical management. In order to identify proteins that are differentially expressed between these two conditions, a quantitative proteomic profiling of synovial fluid obtained from rheumatoid arthritis and osteoarthritis patients was carried out by using iTRAQ labeling followed by high resolution mass spectrometry analysis.

Results

We have identified 575 proteins out of which 135 proteins were found to be differentially expressed by ≥3-fold in the synovial fluid of rheumatoid arthritis and osteoarthritis patients. Proteins not previously reported to be associated with rheumatoid arthritis including, coronin-1A (CORO1A), fibrinogen like-2 (FGL2), and macrophage capping protein (CAPG) were found to be upregulated in rheumatoid arthritis. Proteins such as CD5 molecule-like protein (CD5L), soluble scavenger receptor cysteine-rich domain-containing protein (SSC5D), and TTK protein kinase (TTK) were found to be upregulated in the synovial fluid of osteoarthritis patients. We confirmed the upregulation of CAPG in rheumatoid arthritis synovial fluid by multiple reaction monitoring assay as well as by Western blot. Pathway analysis of differentially expressed proteins revealed a significant enrichment of genes involved in glycolytic pathway in rheumatoid arthritis.

Conclusions

We report here the largest identification of proteins from the synovial fluid of rheumatoid arthritis and osteoarthritis patients using a quantitative proteomics approach. The novel proteins identified from our study needs to be explored further for their role in the disease pathogenesis of rheumatoid arthritis and osteoarthritis.

Sartaj Ahmad and Raja Sekhar Nirujogi contributed equally to this article.

A novel mutation of the high-temperature requirement A serine peptidase 1 (HTRA1) gene in a Chinese family with cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL)

OBJECTIVE

Mutations in the high-temperature requirement A serine peptidase 1 (HTRA1) gene were studied in a Chinese family with cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL).

METHODS

Exons 1-9 of the HTRA1 gene were amplified and bidirectionally sequenced in a Chinese family with CARASIL. Mutation effects were analysed by three-dimensional modelling of the serine protease HTRA1 protein.

RESULTS

The proband was found to be homozygous for a novel missense mutation (c.854 C > T) identified in exon 4 of the HTRA1 gene; the parents of the proband were heterozygous for the same missense mutation. This c.854 C > T mutation resulted in a change from proline to leucine (p.P285L) in serine protease HTRA1, and was absent in 260 control chromosomes. Three-dimensional models showed that the change from proline to leucine (p.P285L) could attenuate the hydrogen bond between S284 and S287 residues, which might affect function of serine protease HTRA1.

CONCLUSION

Discovery of a novel missense mutation (c.854C>T) associated with CARASIL expands the known CARASIL-related mutations in HTRA1.

Borrelia burgdorferi BbHtrA degrades host ECM proteins and stimulates release of inflammatory cytokines in vitro

The Lyme disease spirochaete, Borrelia burgdorferi, causes damage to diverse host tissues and induces inflammation but the mechanisms of injury are poorly understood. We recently reported that a surface-exposed B. burgdorferi protease, which is expressed during human disease and is conserved within the major Lyme disease spirochaete species, degrades the extracellular matrix proteoglycan, aggrecan. Here we demonstrate that BbHtrA also degrades fibronectin and numerous proteoglycans found in skin, joints and neural tissues. BbHtrA degradation of fibronectin released known pro-inflammatory fibronectin fragments FnIII(13-14) and Fnf-29, which may amplify the inflammatory processes triggered by the presence of the bacteria. When this hypothesis was tested directly by exposing chondrocytes to BbHtrA in vitro, inflammatory cytokines (sICAM-1 and IL-6) and chemokines (CXCL1, CCL1, CCL2 and CCL5) that are hallmarks of Lyme disease were induced. These results provide the first evidence that, by utilizing BbHtrA, B. burgdorferi may actively participate in its dissemination and in the tissue damage and inflammation observed in Lyme disease.

Serine protease HTRA1 antagonizes transforming growth factor-β signaling by cleaving its receptors and loss of HTRA1 in vivo enhances bone formation

HTRA1 is a member of the High Temperature Requirement (HTRA1) family of serine proteases, which play a role in several biological and pathological processes. In part, HTRA1 regulation occurs by inhibiting the TGF-β signaling pathway, however the mechanism of inhibition has not been fully defined. Previous studies have shown that HTRA1 is expressed in a variety of tissues, including sites of skeletal development. HTRA1 has also been implicated in the process of bone formation, although the precise manner of regulation is still unknown. This study investigated how HTRA1 regulates TGF-β signaling and examined the in vivo effects of the loss of HTRA1. We demonstrated that recombinant HTRA1 was capable of cleaving both type II and type III TGF-β receptors (TβRII and TβRIII) in vitro in a dose-dependent manner, but it did not affect the integrity of TβRI or TGF-β. Overexpression of HTRA1 led to decreased levels of both TβRII and III on the cell surface but had no effect on TβRI. Silencing HTRA1 expression significantly increased TGF-β binding to the cell surface and TGF-β responsiveness within the cell. To examine the role of HTRA1 in vivo, we generated mice with a targeted gene deletion of HTRA1. Embryonic fibroblasts isolated from these mice displayed an increase in TGF-β-induced expression of several genes known to promote bone formation. Importantly, the loss of HTRA1 in the knockout mice resulted in a marked increase in trabecular bone mass. This study has identified a novel regulatory mechanism by which HTRA1 antagonizes TGF-β signaling, and has shown that HTRA1 plays a key role in the regulation of bone formation.

LRP1 protects the vasculature by regulating levels of connective tissue growth factor and HtrA1

OBJECTIVE

Low-density lipoprotein receptor-related protein 1 (LRP1) is a large endocytic and signaling receptor that is abundant in vascular smooth muscle cells. Mice in which the lrp1 gene is deleted in smooth muscle cells (smLRP1(-/-)) on a low-density lipoprotein receptor-deficient background display excessive platelet derived growth factor-signaling, smooth muscle cell proliferation, aneurysm formation, and increased susceptibility to atherosclerosis. The objectives of the current study were to examine the potential of LRP1 to modulate vascular physiology under nonatherogenic conditions.

APPROACH AND RESULTS

We found smLRP1(-/-) mice to have extensive in vivo aortic dilatation accompanied by disorganized and degraded elastic lamina along with medial thickening of the arterial vessels resulting from excess matrix deposition. Surprisingly, this was not attributable to excessive platelet derived growth factor-signaling. Rather, quantitative differential proteomic analysis revealed that smLRP1(-/-) vessels contain a 4-fold increase in protein levels of high-temperature requirement factor A1 (HtrA1), which is a secreted serine protease that is known to degrade matrix components and to impair elastogenesis, resulting in fragmentation of elastic fibers. Importantly, our study discovered that HtrA1 is a novel LRP1 ligand. Proteomics analysis also identified excessive accumulation of connective tissue growth factor, an LRP1 ligand and a key mediator of fibrosis.

CONCLUSIONS

Our findings suggest a critical role for LRP1 in maintaining the integrity of vessels by regulating protease activity as well as matrix deposition by modulating HtrA1 and connective tissue growth factor protein levels. This study highlights 2 new molecules, connective tissue growth factor and HtrA1, which contribute to detrimental changes in the vasculature and, therefore, represent new target molecules for potential therapeutic intervention to maintain vessel wall homeostasis.

HtrA1 is induced by oxidative stress and enhances cell senescence through p38 MAPK pathway

Genetic predisposition and senescence of retinal pigment epithelium induced by oxidative stress are major contributors to age-related macular degeneration (AMD). Single-nucleotide polymorphisms in HTRA1 are strongly linked to the onset of AMD. In this study, we examine the role of HtrA1 in premature senescence and cell death induced by oxidative stress. HtrA1 mRNA and protein were up-regulated during premature senescence induced by H2O2 in both mouse embryonic fibroblasts (MEFs) and ARPE-19 cells. Expression of the senescence markers p21(CIP1/WAF1) and p16(INK4a), and SA-β-galactosidase activity, were higher in HtrA1+/- MEFs than in HtrA1-/- MEFs. HtrA1+/+ and HtrA1+/- MEFs were more resistant than HtrA1-/- MEFs to H2O2-induced cell death. Activation of p38 MAPK by oxidative stress was quicker in HtrA1+/- MEFs than in HtrA1-/- MEFs. The effects of excess HtrA1 were examined by transient transfection of cells with HtrA1 expression vectors or by addition of recombinant proteins. Excess wild type HtrA1 accelerated premature senescence of MEFs and ARPE-19 cells, while the protease-inactive HtrA1 S328A did not. HtrA1-induced senescence was abrogated by inhibition of p38 MAPK. We conclude that HtrA1 is induced by oxidative stress and promotes premature cell senescence through p38 MAPK in a protease activity-dependent manner.

Inflammatory markers associated with osteoarthritis after destabilization surgery in young mice with and without Receptor for Advanced Glycation End-products (RAGE)

HtrA1, Ddr-2, and Mmp-13 are reliable biomarkers for osteoarthritis (OA), yet the exact mechanism for the upregulation of HtrA-1 is unknown. Some have shown that chondrocyte hypertrophy is associated with early indicators of inflammation including TGF-β and the Receptor for Advanced Glycation End-products (RAGE). To examine the correlation of inflammation with the expression of biomarkers in OA, we performed right knee destabilization surgery on 4-week-old-wild type and RAGE knock-out (KO) mice. We assayed for HtrA-1, TGF-β1, Mmp-13, and Ddr-2 in articular cartilage at 3, 7, 14, and 28 days post-surgery by immunohistochemistry on left and right knee joints. RAGE KO and wild type mice both showed staining for key OA biomarkers. However, RAGE KO mice were significantly protected against OA compared to controls. We observed a difference in the total number of chondrocytes and percentage of chondrocytes staining positive for OA biomarkers between RAGE KO and control mice. The percentage of cells staining for OA biomarkers correlated with severity of cartilage degradation. Our results indicate that the absence of RAGE did protect against the development of advanced OA. We conclude that HtrA-1 plays a role in lowering TGF-β1 expression in the process of making articular cartilage vulnerable to damage associated with OA progression.

The emerging roles of HTRA1 in musculoskeletal disease

High-temperature requirement serine protease A1 (HTRA1) is one of four known proteases belonging to the broadly conserved family of HTRA proteins. Although it was originally considered as representing an important modulator of tumorigenesis, an increasing number of reports have suggested that its influence on human disease may extend beyond cancer. HTRA1 has the capacity to degrade numerous extracellular matrix proteins, and as such, its potential involvement in diseases of the musculoskeletal system has been gaining increased attention. Musculoskeletal disease constitutes a wide variety of degenerative conditions that can manifest themselves in different ways such as joint and back pain, as well as deficiencies in skeletal bone quality, and ultimately result in significant suffering and reduced quality of life. Convincing data now exist to support a detrimental role for HTRA1 in the pathogenesis of joint and intervertebral disk degeneration. However, the function of HTRA1 in other closely related musculoskeletal diseases affecting bone and muscle remains unclear and largely unexplored. To help set the stage for future research, we discuss here some of the recent advances in our understanding of the role played by HTRA1 in musculoskeletal pathology.

Human serine protease HTRA1 positively regulates osteogenesis of human bone marrow-derived mesenchymal stem cells and mineralization of differentiating bone-forming cells through the modulation of extracellular matrix protein

Mammalian high-temperature requirement serine protease A1 (HTRA1) is a secreted member of the trypsin family of serine proteases which can degrade a variety of bone matrix proteins and as such has been implicated in musculoskeletal development. In this study, we have investigated the role of HTRA1 in mesenchymal stem cell (MSC) osteogenesis and suggest a potential mechanism through which it controls matrix mineralization by differentiating bone-forming cells. Osteogenic induction resulted in a significant elevation in the expression and secretion of HTRA1 in MSCs isolated from human bone marrow-derived MSCs (hBMSCs), mouse adipose-derived stromal cells (mASCs), and mouse embryonic stem cells. Recombinant HTRA1 enhanced the osteogenesis of hBMSCs as evidenced by significant changes in several osteogenic markers including integrin-binding sialoprotein (IBSP), bone morphogenetic protein 5 (BMP5), and sclerostin, and promoted matrix mineralization in differentiating bone-forming osteoblasts. These stimulatory effects were not observed with proteolytically inactive HTRA1 and were abolished by small interfering RNA against HTRA1. Moreover, loss of HTRA1 function resulted in enhanced adipogenesis of hBMSCs. HTRA1 Immunofluorescence studies showed colocalization of HTRA1 with IBSP protein in osteogenic mASC spheroid cultures and was confirmed as being a newly identified HTRA1 substrate in cell cultures and in proteolytic enzyme assays. A role for HTRA1 in bone regeneration in vivo was also alluded to in bone fracture repair studies where HTRA1 was found localized predominantly to areas of new bone formation in association with IBSP. These data therefore implicate HTRA1 as having a central role in osteogenesis through modification of proteins within the extracellular matrix.

HtrA1 is a novel antagonist controlling fibroblast growth factor (FGF) signaling via cleavage of FGF8

Accumulating evidence suggests that HtrA1 (high-temperature requirement A1) is involved in modulating crucial cellular processes and implicated in life-threatening diseases, such as cancer and neuropathological disorders; however, the exact functions of this protease in vivo remain unknown. Here, we show that loss of HtrA1 function increases fibroblast growth factor 8 (FGF8) mRNA levels and triggers activation of FGF signaling, resulting in dorsalization in zebrafish embryos. Notably, HtrA1 directly cleaves FGF8 in the extracellular region, and this cleavage results in decreased activation of FGF signaling, which is essential for many physiological processes. Therefore, HtrA1 is indispensable for dorsoventral patterning in early zebrafish embryogenesis and serves as a key upstream regulator of FGF signaling through the control of FGF levels. Furthermore, this study offers insight into new strategies to control human diseases associated with HtrA1 and FGF signaling.

Temporal profiling and pulsed SILAC labeling identify novel secreted proteins during ex vivo osteoblast differentiation of human stromal stem cells

It is well established that bone forming cells (osteoblasts) secrete proteins with autocrine, paracrine, and endocrine function. However, the identity and functional role for the majority of these secreted and differentially expressed proteins during the osteoblast (OB) differentiation process, is not fully established. To address these questions, we quantified the temporal dynamics of the human stromal (mesenchymal, skeletal) stem cell (hMSC) secretome during ex vivo OB differentiation using stable isotope labeling by amino acids in cell culture (SILAC). In addition, we employed pulsed SILAC labeling to distinguish genuine secreted proteins from intracellular contaminants. We identified 466 potentially secreted proteins that were quantified at 5 time-points during 14-days ex vivo OB differentiation including 41 proteins known to be involved in OB functions. Among these, 315 proteins exhibited more than 2-fold up or down-regulation. The pulsed SILAC method revealed a strong correlation between the fraction of isotope labeling and the subset of proteins known to be secreted and involved in OB differentiation. We verified SILAC data using qRT-PCR analysis of 9 identified potential novel regulators of OB differentiation. Furthermore, we studied the biological effects of one of these proteins, the hormone stanniocalcin 2 (STC2) and demonstrated its autocrine effects in enhancing osteoblastic differentiation of hMSC. In conclusion, combining complete and pulsed SILAC labeling facilitated the identification of novel factors produced by hMSC with potential role in OB differentiation. Our study demonstrates that the secretome of osteoblastic cells is more complex than previously reported and supports the emerging evidence that osteoblastic cells secrete proteins with endocrine functions and regulate cellular processes beyond bone formation.

Detrimental role for human high temperature requirement serine protease A1 (HTRA1) in the pathogenesis of intervertebral disc (IVD) degeneration

Human HTRA1 is a highly conserved secreted serine protease that degrades numerous extracellular matrix proteins. We have previously identified HTRA1 as being up-regulated in osteoarthritic patients and as having the potential to regulate matrix metalloproteinase (MMP) expression in synovial fibroblasts through the generation of fibronectin fragments. In the present report, we have extended these studies and investigated the role of HTRA1 in the pathogenesis of intervertebral disc (IVD) degeneration. HTRA1 mRNA expression was significantly elevated in degenerated disc tissue and was associated with increased protein levels. However, these increases did not correlate with the appearance of rs11200638 single nucleotide polymorphism in the promoter region of the HTRA1 gene, as has previously been suggested. Recombinant HTRA1 induced MMP production in IVD cell cultures through a mechanism critically dependent on MEK but independent of IL-1β signaling. The use of a catalytically inactive mutant confirmed these effects to be primarily due to HTRA1 serine protease activity. HTRA1-induced fibronectin proteolysis resulted in the generation of various sized fragments, which when added to IVD cells in culture, caused a significant increase in MMP expression. Furthermore, one of these fragments was identified as being the amino-terminal fibrin- and heparin-binding domain and was also found to be increased within HTRA1-treated IVD cell cultures as well as in disc tissue from patients with IVD degeneration. Our results therefore support a scenario in which HTRA1 promotes IVD degeneration through the proteolytic cleavage of fibronectin and subsequent activation of resident disc cells.

Human high temperature requirement serine protease A1 (HTRA1) degrades tau protein aggregates

Protective proteases are key elements of protein quality control pathways that are up-regulated, for example, under various protein folding stresses. These proteases are employed to prevent the accumulation and aggregation of misfolded proteins that can impose severe damage to cells. The high temperature requirement A (HtrA) family of serine proteases has evolved to perform important aspects of ATP-independent protein quality control. So far, however, no HtrA protease is known that degrades protein aggregates. We show here that human HTRA1 degrades aggregated and fibrillar tau, a protein that is critically involved in various neurological disorders. Neuronal cells and patient brains accumulate less tau, neurofibrillary tangles, and neuritic plaques, respectively, when HTRA1 is expressed at elevated levels. Furthermore, HTRA1 mRNA and HTRA1 activity are up-regulated in response to elevated tau concentrations. These data suggest that HTRA1 is performing regulated proteolysis during protein quality control, the implications of which are discussed.

Structural and functional analysis of HtrA1 and its subdomains

The homotrimeric human serine protease HtrA1 is homologous to bacterial HtrA proteases regarding the trypsin-like catalytic and PDZ domains but differs by the presence of an N-terminal domain with IGFBP and Kazal homology. The crystal structures and SAXS analysis presented herein reveal the rare tandem of IGFBP- and Kazal-like modules, a protease active site that adopts a competent conformation in the absence of substrate or inhibitor and a model for the intact protein in solution. Highly sensitive enzymatic assays and binding studies demonstrate that the N-terminal tandem has no apparent effect on protease activity, and in accordance with the structure-based predictions, neither the IGFBP- nor Kazal-like module retains the function of their prototype proteins. Our structures of the unliganded HtrA1 active site suggest two-state equilibrium and a "conformational selection" model, in which substrate binds to the active conformer.

Quantitative determination of high-temperature requirement protein A1 and its possible associated molecules during induced reparative dentin formation

INTRODUCTION

The equilibrium of inhibitors and inducers plays an important role in the mineralization of dentin during dentinogenesis. High-temperature requirement protein A1 (HtrA1) is a novel bone mineralization inhibitor involved in physiological and pathological bone formation. However, the expression of HtrA1 in tooth mineralization is unknown. The purpose of the present study was to investigate the localization and quantity of HtrA1 and its possible related molecules during induced reparative dentin formation.

METHODS

Rats were randomly sacrificed after direct pulp capping on days 0, 7, 14, and 21. Maxillary segments were obtained and routinely prepared for histological analysis, immunohistochemistry, quantum dots-based double immunofluorescence, and CRi's Nuance imaging system (CRI, Woburn, MA)-based quantitative determination.

RESULTS

The square measure values of reparative dentin significantly increased on day 7 and continued to increase until day 21. HtrA1, matrix Gla protein (MGP), nestin, and bone sialoprotein were positively stained and colocalized in the odontoblasts and/or odontoblast-like cells zone and reparative dentin during induced reparative dentin formation. The expressions of HtrA1 and MGP were significantly enhanced after direct pulp capping on day 7 and did not significantly change between days 7, 14, and day 21. Both expressions of HtrA1 and MGP were positively correlated with the square measure values of reparative dentin; however, no correlation was found between the expressions of HtrA1 and MGP.

CONCLUSIONS

HtrA1 could be observed and might possibly be involved in the process of reparative dentin formation associated with MGP.

HTRA1 in Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is a leading cause of severe visual impairment and irreversible blindness in most developed countries, affecting more than 50 million of elderly people worldwide. Current treatments, such as intravitreal injection of antiangiogenic agents, mitigate the effect of advanced AMD but cannot completely cure the disease. Comprehensive understanding of the AMD pathological mechanisms is important for the development of new therapies. Previously, we identified a single-nucleotide polymorphism (rs11200638) in the promoter region of the high temperature requirement factor A1 (HTRA1) gene on chromosome 10q26 to be associated with exudative AMD. In further biological studies, we have provided evidence that HTRA1 could be a potential disease-causing gene within the 10q26 locus. In this review, we summarize the pathology of AMD and the molecular function of the HtrA1 protein. Also evaluated are the genetic effects of HTRA1 polymorphism on AMD in different populations and interactions with other AMD-associated genes, especially with the complement factor H (CFH) gene, which was identified for nonexudative AMD. The biological roles of HtrA1 are exhaustively examined on its contribution to the multifactorial pathogenic mechanism of AMD. Although HtrA1 should play a part in AMD pathogenesis, a host of other genetic and environmental factors, known and unknown, is involved and warrants intensive future research.

Unveiling novel genes upregulated by both rhBMP2 and rhBMP7 during early osteoblastic transdifferentiation of C2C12 cells

FINDINGS

We set out to analyse the gene expression profile of pre-osteoblastic C2C12 cells during osteodifferentiation induced by both rhBMP2 and rhBMP7 using DNA microarrays. Induced and repressed genes were intercepted, resulting in 1,318 induced genes and 704 repressed genes by both rhBMP2 and rhBMP7. We selected and validated, by RT-qPCR, 24 genes which were upregulated by rhBMP2 and rhBMP7; of these, 13 are related to transcription (Runx2, Dlx1, Dlx2, Dlx5, Id1, Id2, Id3, Fkhr1, Osx, Hoxc8, Glis1, Glis3 and Cfdp1), four are associated with cell signalling pathways (Lrp6, Dvl1, Ecsit and PKCδ) and seven are associated with the extracellular matrix (Ltbp2, Grn, Postn, Plod1, BMP1, Htra1 and IGFBP-rP10). The novel identified genes include: Hoxc8, Glis1, Glis3, Ecsit, PKCδ, LrP6, Dvl1, Grn, BMP1, Ltbp2, Plod1, Htra1 and IGFBP-rP10.

BACKGROUND

BMPs (bone morphogenetic proteins) are members of the TGFβ (transforming growth factor-β) super-family of proteins, which regulate growth and differentiation of different cell types in various tissues, and play a critical role in the differentiation of mesenchymal cells into osteoblasts. In particular, rhBMP2 and rhBMP7 promote osteoinduction in vitro and in vivo, and both proteins are therapeutically applied in orthopaedics and dentistry.

CONCLUSION

Using DNA microarrays and RT-qPCR, we identified both previously known and novel genes which are upregulated by rhBMP2 and rhBMP7 during the onset of osteoblastic transdifferentiation of pre-myoblastic C2C12 cells. Subsequent studies of these genes in C2C12 and mesenchymal or pre-osteoblastic cells should reveal more details about their role during this type of cellular differentiation induced by BMP2 or BMP7. These studies are relevant to better understanding the molecular mechanisms underlying osteoblastic differentiation and bone repair.

Cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL): from discovery to gene identification

Cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL) is a single-gene disorder directly affecting the cerebral small blood vessels, that is caused by mutations in the HTRA1 gene encoding HtrA serine peptidase/protease 1 (HTRA1). CARASIL is the second known genetic form of ischemic, nonhypertensive, cerebral small-vessel disease with an identified gene, along with cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). The exact prevalence of CARASIL is currently unknown, and to date approximately 50 patients have been reported, most of them from Japan and two from China. Genetically, no founder haplotype has been identified, and thus the disease is expected to be found more widely. The main clinical manifestations of CARASIL are ischemic stroke or stepwise deterioration in brain functions, progressive dementia, premature baldness, and attacks of severe low back pain or spondylosis deformans/disk herniation. The most characteristic findings on brain magnetic resonance imaging are diffuse white matter changes and multiple lacunar infarctions in the basal ganglia and thalamus. Histopathologically, CARASIL is characterized by intense arteriosclerosis, mainly in the small penetrating arteries, without granular osmiophilic materials or amyloid deposition. CARASIL is a prototype single-gene disorder of cerebral small vessels secondary to and distinct from CADASIL. CARASIL-associated mutant HTRA1 exhibited decreased protease activity and failed to repress transforming growth factor-β family signaling, indicating that the increased signaling causes arteriopathy in CARASIL. Therefore, HTRA1 represents another new gene to be considered in future studies of cerebral small-vessel diseases, as well as alopecia and degenerative vertebral/disk diseases.

Matrix remodeling as stem cell recruitment event: a novel in vitro model for homing of human bone marrow stromal cells to the site of injury shows crucial role of extracellular collagen matrix

The goal of the present study was to devise an in vitro model suitable for investigations of the homing of mesenchymal stem cells to sites of injury. Such a model was designed on the basis of a "transwell" assay, with an insert seeded with human bone marrow stromal cells and a well with a desired cell type. To mimic physiological environment and to simulate "injury", cells in a well were maintained not only on tissue culture plastic but also on collagens I and IV, major matrix components in musculoskeletal and adipose tissues respectively, and subjected to a severe thermal stress. The results obtained showed a massive translocation of bone marrow stromal cells through the inserts' membrane toward the "injury" site. Unexpectedly, it emerged that collagen matrix is essential in producing such a migration. The results obtained suggest that upon injury cells secrete a substance which interacts with collagen matrix to produce a homing agent. The substance in question appears to be a protease and its interaction with the collagen matrix appears to be a digestion of the latter into fragments shown to be chemotactic. Both AEBSF, an inhibitor of serine proteases, and leupeptin, an inhibitor of cysteine proteases as well as of trypsin-like serine proteases, but not the broad spectrum MMP inhibitor marimastat, significantly inhibit the observed homing effect and this inhibition is not due to cytotoxicity. Moreover, immunoprecipitation of HTRA1, a trypsin-like serine protease known to be secreted by cells differentiating into all three major mesenchymal lineages and by stressed cells in general and shown to degrade a number of matrix proteins including collagen, significantly diminished the homing effect. The data suggest that this protease is a major contributor to the observed chemotaxis of bone marrow stromal cells. The present study indicates that collagen fragments can mediate the migration of bone marrow stromal cells. The results also suggest that, at least in musculoskeletal and in adipose tissues, matrix remodeling occurrences, usually closely associated with tissue remodeling, should also be regarded as potential stem cells recruitment events.