Prolyl 3-hydroxylase 1 null mice display abnormalities in fibrillar collagen-rich tissues such as tendons, skin, and bones

Pan-Cancer Analysis of P3H1 and Experimental Validation in Renal Clear Cell Carcinoma

Prolyl 3-hydroxylase 1 (P3H1) has been implicated in cancer development, but no pan-cancer analysis has been conducted on P3H1. In this study, for the first time, aspects associated with P3H1, such as the mRNA expression, any mutation, promoter methylation, and prognostic significance, the relationship between P3H1 and clinicopathological parameters, drug sensitivity, and immune cell infiltration were investigated by searching several databases including The Cancer Genome Atlas (TCGA), Genotype-Tissue Expression (GTEx), cBioPortal, and The Tumor Immune Evaluation Resource (TIMER2.0) using bioinformatics tools. The findings indicate significant differential expression of P3H1 in most tumors when compared to normal tissues, with a strong association with clinical prognosis. A pan-cancer Cox regression analysis revealed that high P3H1 expression is significantly associated with low overall survival in patients with brain lower grade glioma, kidney clear cell carcinoma, adrenocortical cancer, liver hepatocellular carcinoma, mesothelioma, sarcoma, uveal melanoma, bladder urothelial carcinoma, kidney papillary cell carcinoma, kidney chromophobe, thymoma, and thyroid carcinoma. A negative correlation was observed between P3H1 DNA methylation and its expression. P3H1 is significantly associated with infiltrating cells, immune-related genes, tumor mutation burden, microsatellite instability, and mismatch repair. Finally, A significant correlation was found between P3H1 expression and sensitivity to nine drugs. Thus, enhanced P3H1 expression is associated with poor prognosis in a variety of tumors, which may be due to its role in tumor immune regulation and tumor microenvironment. This pan-cancer analysis provides insight into the function of P3H1 in tumorigenesis of different cancers and provides a theoretical basis for further in-depth studies to follow.

Mmp14 is required for matrisome homeostasis and circadian rhythm in fibroblasts

The circadian clock in tendon regulates the daily rhythmic synthesis of collagen-I and the appearance and disappearance of small-diameter collagen fibrils in the extracellular matrix. How the fibrils are assembled and removed is not fully understood. Here, we first showed that the collagenase, membrane type I-matrix metalloproteinase (MT1-MMP, encoded by Mmp14), is regulated by the circadian clock in postnatal mouse tendon. Next, we generated tamoxifen-induced Col1a2-Cre-ERT2::Mmp14 KO mice (Mmp14 conditional knockout (CKO)). The CKO mice developed hind limb dorsiflexion and thickened tendons, which accumulated narrow-diameter collagen fibrils causing ultrastructural disorganization. Mass spectrometry of control tendons identified 1195 proteins of which 212 showed time-dependent abundance. In Mmp14 CKO mice 19 proteins had reversed temporal abundance and 176 proteins lost time dependency. Among these, the collagen crosslinking enzymes lysyl oxidase-like 1 (LOXL1) and lysyl hydroxylase 1 (LH1; encoded by Plod2) were elevated and had lost time-dependent regulation. High-pressure chromatography confirmed elevated levels of hydroxylysine aldehyde (pyridinoline) crosslinking of collagen in CKO tendons. As a result, collagen-I was refractory to extraction. We also showed that CRISPR-Cas9 deletion of Mmp14 from cultured fibroblasts resulted in loss of circadian clock rhythmicity of period 2 (PER2), and recombinant MT1-MMP was highly effective at cleaving soluble collagen-I but less effective at cleaving collagen pre-assembled into fibrils. In conclusion, our study shows that circadian clock-regulated Mmp14 controls the rhythmic synthesis of small diameter collagen fibrils, regulates collagen crosslinking, and its absence disrupts the circadian clock and matrisome in tendon fibroblasts.

Cell differentiation and matrix organization are differentially affected during bone formation in osteogenesis imperfecta zebrafish models with different genetic defects impacting collagen type I structure

Osteogenesis imperfecta (OI) is a family of rare heritable skeletal disorders associated with dominant mutations in the collagen type I encoding genes and recessive defects in proteins involved in collagen type I synthesis and processing and in osteoblast differentiation and activity. Historically, it was believed that the OI bone phenotype was only caused by abnormal collagen type I fibrils in the extracellular matrix, but more recently it became clear that the altered bone cell homeostasis, due to mutant collagen retention, plays a relevant role in modulating disease severity in most of the OI forms and it is correlated to impaired bone cell differentiation. Despite in vitro evidence, in vivo data are missing. To better understand the physiopathology of OI, we used two zebrafish models: Chihuahua (Chi/+), carrying a dominant p.G736D substitution in the α1 chain of collagen type I, and the recessive p3h1-/-, lacking prolyl 3-hydroxylase (P3h1) enzyme. Both models share the delay of collagen type I folding, resulting in its overmodification and partial intracellular retention. The regeneration of the bony caudal fin of Chi/+ and p3h1-/- was employed to investigate the impact of abnormal collagen synthesis on bone cell differentiation. Reduced regenerative ability was evident in both models, but it was associated to impaired osteoblast differentiation and osteoblastogenesis/adipogenesis switch only in Chi/+. On the contrary, reduced osteoclast number and activity were found in both models during regeneration. The dominant OI model showed a more detrimental effect in the extracellular matrix organization. Interestingly, the chemical chaperone 4-phenylbutyrate (4-PBA), known to reduce cellular stress and increase collagen secretion, improved bone formation only in p3h1-/- by favoring caudal fin growth without affecting bone cell markers expression. Taken together, our in vivo data proved the negative impact of structurally abnormal collagen type I on bone formation but revealed a gene mutation-specific effect on bone cell differentiation and matrix organization in OI. These, together with the distinct ability to respond to the chaperone treatment, underline the need for precision medicine approaches to properly treat the disease.

Perturbed post-translational modification (PTM) network atlas of collagen I during stent-induced neointima formation

Myocardial infarction (MI) leading to heart failure contributes to almost 85% of deaths associated with CVDs. MI results from plaque formation in the coronary artery which leads to a lack of oxygen and nutrients in the myocardium. To date, stenting is a widely used gold-standard technique to maintain the proper blood flow through coronary circulation in the myocardium. Bare metal stents (BMS) and drug-eluting stents (DES) are majorly used in implantation. However, BMS and DES both can induce neointima formation by depositing excessive collagens in the coronary arteries leading to restenosis. Identification and quantitative analysis of site-specific post-translational modifications (PTMs) of deposited COL1A1 from neointima ECM are not known. Applying our in-house workflow, we re-analyzed a previously published mass-spectrometry data set to comprehensively map site-specific prolyl-hydroxylation, lysyl hydroxylation, and O-glycosylation sites in COL1A1 from neointima ECM. Furthermore, we quantitated the occupancy level of 9 3-hydroxyproline (3-HyP) sites, 2 hydroxylysine sites, and glycosylation microheterogeneity on 6 lysine sites of COL1A1. Although the total level of COL1A1 was decreased in DES-induced neointima, the occupancy levels of 2 3-HyP sites (P⁸⁷², and P⁸⁸¹) and 2 HyK (K⁴³⁵ and K⁷⁶⁸) sites of COL1A1 were significantly (p < 0.05) elevated in DES-induced neointima compared to BMS-induced neointima. We also found O-glycosylation to be significantly elevated on 3 lysine sites (K⁵⁷³, K³³⁹, and K and K⁸⁴⁹) of COL1A1 in DES-induced neointima compared to BMS-induced neointima. Taken together, our first comprehensive PTM analysis of COL1A1 reflected significant site-specific alterations that may play a very important role in the ECM remodeling during stent-induced neointima formation in MI patients. SIGNIFICANCE: The knowledge about site-specific post-translational modifications (PTMs) of collagen 1 deposited in the neointima ECM during the post-stenting restenosis process is absent. Here for the first time, we report the altered levels of COL1A1 PTMs during metal stent and drug-eluting stent-induced neointima formation. Our study showcases a novel ECM remodeling through site-specific collagen PTMs during stent-induced restenosis.

Milder presentation of osteogenesis imperfecta type VIII due to compound heterozygosity for a predicted loss-of-function variant and novel missense variant in P3H1-further expansion of the phenotypic spectrum

Osteogenesis imperfecta (OI) is a heritable disorder of bone metabolism characterized by multiple fractures with minimal trauma. Autosomal recessive OI type VIII is associated with biallelic pathogenic variants in P3H1 and classically characterized by skeletal anomalies in addition to significant bone fragility, sometimes presenting with in utero fractures and/or neonatal lethality. P3H1 encodes a collagen prolyl hydroxylase that critically 3-hydroxylates proline residue 986 on the α chain of collagen types I and II to achieve proper folding and assembly of mature collagen and is present in a complex with CRTAP and CypB. Most individuals with OI type VIII have had biallelic predicted loss-of-function variants leading to reduced or absent levels of P3H1 mRNA. The reported missense variants have all fallen in the catalytic domain of the protein and are thought to be associated with a milder phenotype. Here, we describe an infant presenting with five long bone fractures in the first year of life found to have a novel missense variant in trans with a nonsense variant in P3H1 without any other bony anomalies on imaging. We hypothesize that missense variants in the catalytic domain of P3H1 lead to decreased but not absent hydroxylation of Pro986, with preserved KDEL retention signal and complex stability, causing an attenuated phenotype.

A Founder Intronic Variant in P3H1 Likely Results in Aberrant Splicing and Protein Truncation in Patients of Karen Descent with Osteogenesis Imperfecta Type VIII

One of the most important steps in post-translational modifications of collagen type I chains is the hydroxylation of carbon-3 of proline residues by prolyl-3-hydroxylase-1 (P3H1). Genetic variants in P3H1 have been reported to cause autosomal recessive osteogenesis imperfecta (OI) type VIII. Clinical and radiographic examinations, whole-exome sequencing (WES), and bioinformatic analysis were performed in 11 Thai children of Karen descent affected by multiple bone fractures. Clinical and radiographic findings in these patients fit OI type VIII. Phenotypic variability is evident. WES identified an intronic homozygous variant (chr1:43212857A > G; NM_022356.4:c.2055 + 86A > G) in P3H1 in all patients, with parents in each patient being heterozygous for the variant. This variant is predicted to generate a new "CAG" splice acceptor sequence, resulting in the incorporation of an extra exon that leads to a frameshift in the final exon and subsequent non-functional P3H1 isoform a. Alternative splicing of P3H1 resulting in the absence of functional P3H1 caused OI type VIII in 11 Thai children of Karen descent. This variant appears to be specific to the Karen population. Our study emphasizes the significance of considering intronic variants.

Lysyl hydroxylase 3-mediated post-translational modifications are required for proper biosynthesis of collagen α1α1α2(IV)

Collagens are the most abundant proteins in the body and among the most biosynthetically complex. A molecular ensemble of over 20 endoplasmic reticulum resident proteins participates in collagen biosynthesis and contributes to heterogeneous post-translational modifications. Pathogenic variants in genes encoding collagens cause connective tissue disorders, including osteogenesis imperfecta, Ehlers-Danlos syndrome, and Gould syndrome (caused by mutations in COL4A1 and COL4A2), and pathogenic variants in genes encoding proteins required for collagen biosynthesis can cause similar but overlapping clinical phenotypes. Notably, pathogenic variants in lysyl hydroxylase 3 (LH3) cause a multisystem connective tissue disorder that exhibits pathophysiological features of collagen-related disorders. LH3 is a multifunctional collagen-modifying enzyme; however, its precise role(s) and substrate specificity during collagen biosynthesis has not been defined. To address this critical gap in knowledge, we generated LH3 KO cells and performed detailed quantitative and molecular analyses of collagen substrates. We found that LH3 deficiency severely impaired secretion of collagen α1α1α2(IV) but not collagens α1α1α2(I) or α1α1α1(III). Amino acid analysis revealed that LH3 is a selective LH for collagen α1α1α2(IV) but a general glucosyltransferase for collagens α1α1α2(IV), α1α1α2(I), and α1α1α1(III). Importantly, we identified rare variants that are predicted to be pathogenic in the gene encoding LH3 in two of 113 fetuses with intracranial hemorrhage-a cardinal feature of Gould syndrome. Collectively, our findings highlight a critical role of LH3 in α1α1α2(IV) biosynthesis and suggest that LH3 pathogenic variants might contribute to Gould syndrome.

An Update on Animal Models of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a heterogeneous disorder characterized by bone fragility, multiple fractures, bone deformity, and short stature. In recent years, the application of next generation sequencing has triggered the discovery of many new genetic causes for OI. Until now, more than 25 genetic causes of OI and closely related disorders have been identified. However, the mechanisms of many genes on skeletal fragility in OI are not entirely clear. Animal models of OI could help to understand the cellular, signaling, and metabolic mechanisms contributing to the disease, and how targeting these pathways can provide therapeutic targets. To date, a lot of animal models, mainly mice and zebrafish, have been described with defects in 19 OI-associated genes. In this review, we summarize the known genetic causes and animal models that recapitulate OI with a main focus on engineered mouse and zebrafish models. Additionally, we briefly discuss domestic animals with naturally occurring OI phenotypes. Knowledge of the specific molecular basis of OI will advance clinical diagnosis and potentially stimulate targeted therapeutic approaches.

Cell surface accumulation of intracellular leucine proline-enriched proteoglycan 1 enhances odontogenic potential of human dental pulp stem cells

Primary dental pulp cells can be differentiated into odontoblast-like cells, which are responsible for dentin formation and mineralization. Successful differentiation of primary dental pulp cells can be verified using a few markers. However, odontoblast-specific cell surface markers have not been fully studied yet. LEucine PRoline-Enriched Proteoglycan 1 (LEPRE1) is a basement membrane-associated proteoglycan. LEPRE1 protein levels are increased during odontoblastic differentiation of human dental pulp cells. Intracellular and cell surface accumulation of this protein completely disappeared during dentin maturation and mineralization. Cell surface binding of an anti-LEPRE1 monoclonal antibody that could recognize an extracellular region was gradually increased in the odontoblastic stage. Overexpression and knock-down experiments showed that accumulation of intracellular LEPRE1 could lead to inefficient odontoblastic differentiation and that the movement of LEPRE1 from intracellular region to the cell surface was required for odontoblastic differentiation. Indeed, when LEPRE1 already located on the cell surface was blocked by the anti-LEPRE1 monoclonal antibody, odontoblastic differentiation of human dental pulp cells was inhibited. In this study, we looked at other aspects of LEPRE1 function as a cell surface molecule rather than its known intracellular hydroxylase activity. Our results indicate that this protein has potential as a specific cell surface marker in odontoblastic differentiation.

Curative Cell and Gene Therapy for Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) describes a series of genetic bone fragility disorders that can have a substantive impact on patient quality of life. The multidisciplinary approach to management of children and adults with OI primarily involves the administration of antiresorptive medication, allied health (physiotherapy and occupational therapy), and orthopedic surgery. However, advances in gene editing technology and gene therapy vectors bring with them the promise of gene-targeted interventions to provide an enduring or perhaps permanent cure for OI. This review describes emergent technologies for cell- and gene-targeted therapies, major hurdles to their implementation, and the prospects of their future success with a focus on bone disorders. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Characterization of the microRNA transcriptomes and proteomics of cochlear tissue-derived small extracellular vesicles from mice of different ages after birth

The cochlea is an important sensory organ for both balance and sound perception, and the formation of the cochlea is a complex developmental process. The development of the mouse cochlea begins on embryonic day (E)9 and continues until postnatal day (P)21 when the hearing system is considered mature. Small extracellular vesicles (sEVs), with a diameter ranging from 30 to 200 nm, have been considered a significant medium for information communication in both physiological and pathological processes. However, there are no studies exploring the role of sEVs in the development of the cochlea. Here, we isolated tissue-derived sEVs from the cochleae of FVB mice at P3, P7, P14, and P21 by ultracentrifugation. These sEVs were first characterized by transmission electron microscopy, nanoparticle tracking analysis, and western blotting. Next, we used small RNA-seq and mass spectrometry to characterize the microRNA transcriptomes and proteomes of cochlear sEVs from mice at different ages. Many microRNAs and proteins were discovered to be related to inner ear development, anatomical structure development, and auditory nervous system development. These results all suggest that sEVs exist in the cochlea and are likely to be essential for the normal development of the auditory system. Our findings provide many sEV microRNA and protein targets for future studies of the roles of cochlear sEVs.

Fighting the Fiber: Targeting Collagen in Lung Fibrosis

Organ fibrosis is characterized by epithelial injury and aberrant tissue repair, where activated effector cells, mostly fibroblasts and myofibroblasts, excessively deposit collagen into the extracellular matrix. Fibrosis frequently results in organ failure and has been estimated to contribute to at least one third of all global deaths. Also lung fibrosis, in particular idiopathic pulmonary fibrosis (IPF), is a fatal disease with rising incidence worldwide. As current treatment options targeting fibrogenesis are insufficient, there is an urgent need for novel therapeutic strategies. During the last decade, several studies have proposed to target intra- and extracellular components of the collagen biosynthesis, maturation, and degradation machinery. This includes intra- and extracellular targets directly acting on collagen gene products, but also such that anabolize essential building blocks of collagen, in particular glycine and proline biosynthetic enzymes. Collagen, however, is a ubiquitous molecule in the body and fulfils essential functions as a macromolecular scaffold, growth factor reservoir, and receptor binding site in virtually every tissue. This review summarizes recent advances and future directions in this field. Evidence for the proposed therapeutic targets and where they currently stand in terms of clinical drug development for treatment of fibrotic disease is provided. The drug targets are furthermore discussed in light of (1) specificity for collagen biosynthesis, maturation and degradation, and (2) specificity for disease-associated collagen. As therapeutic success and safety of these drugs may largely depend on targeted delivery, different strategies for specific delivery to the main effector cells and to the extracellular matrix are discussed.

Tendon and motor phenotypes in the Crtap-/- mouse model of recessive osteogenesis imperfecta

Osteogenesis imperfecta (OI) is characterized by short stature, skeletal deformities, low bone mass, and motor deficits. A subset of OI patients also present with joint hypermobility; however, the role of tendon dysfunction in OI pathogenesis is largely unknown. Using the Crtap-/- mouse model of severe, recessive OI, we found that mutant Achilles and patellar tendons were thinner and weaker with increased collagen cross-links and reduced collagen fibril size at 1- and 4-months compared to wildtype. Patellar tendons from Crtap-/- mice also had altered numbers of CD146+CD200+ and CD146-CD200+ progenitor-like cells at skeletal maturity. RNA-seq analysis of Achilles and patellar tendons from 1-month Crtap-/- mice revealed dysregulation in matrix and tendon marker gene expression concomitant with predicted alterations in TGF-β, inflammatory, and metabolic signaling. At 4-months, Crtap-/- mice showed increased αSMA, MMP2, and phospho-NFκB staining in the patellar tendon consistent with excess matrix remodeling and tissue inflammation. Finally, a series of behavioral tests showed severe motor impairments and reduced grip strength in 4-month Crtap-/- mice - a phenotype that correlates with the tendon pathology.

Collagen Biosynthesis, Processing, and Maturation in Lung Ageing

In addition to providing a macromolecular scaffold, the extracellular matrix (ECM) is a critical regulator of cell function by virtue of specific physical, biochemical, and mechanical properties. Collagen is the main ECM component and hence plays an essential role in the pathogenesis and progression of chronic lung disease. It is well-established that many chronic lung diseases, e.g., chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF) primarily manifest in the elderly, suggesting increased susceptibility of the aged lung or accumulated alterations in lung structure over time that favour disease. Here, we review the main steps of collagen biosynthesis, processing, and turnover and summarise what is currently known about alterations upon lung ageing, including changes in collagen composition, modification, and crosslinking. Recent proteomic data on mouse lung ageing indicates that, while the ER-resident machinery of collagen biosynthesis, modification and triple helix formation appears largely unchanged, there are specific changes in levels of type IV and type VI as well as the two fibril-associated collagens with interrupted triple helices (FACIT), namely type XIV and type XVI collagens. In addition, levels of the extracellular collagen crosslinking enzyme lysyl oxidase are decreased, indicating less enzymatically mediated collagen crosslinking upon ageing. The latter contrasts with the ageing-associated increase in collagen crosslinking by advanced glycation endproducts (AGEs), a result of spontaneous reactions of protein amino groups with reactive carbonyls, e.g., from monosaccharides or reactive dicarbonyls like methylglyoxal. Given the slow turnover of extracellular collagen such modifications accumulate even more in ageing tissues. In summary, the collective evidence points mainly toward age-induced alterations in collagen composition and drastic changes in the molecular nature of collagen crosslinks. Future work addressing the consequences of these changes may provide important clues for prevention of lung disease and for lung bioengineering and ultimately pave the way to novel targeted approaches in lung regenerative medicine.

Upregulated LEPRE1 correlates with poor outcome and its knockdown attenuates cells proliferation, migration and invasion in osteosarcoma

Leucine proline-enriched proteoglycan 1 has been extensively explored because of its protective function in cell homeostasis and correlation with osteogenesis imperfect. Human osteosarcoma is the most common primary malignant tumor of bone with multiple and complex genomic aberrations. However, the functional role of leucine proline-enriched proteoglycan 1 is still unknown in osteosarcoma. Thus we performed this study to explain the leucine proline-enriched proteoglycan 1 effect in osteosarcoma. Gene arrays of human osteosarcoma were downloaded from the Gene Expression Omnibus database. Quantitative real-time PCR was conducted to assess the expression of leucine proline-enriched proteoglycan 1 in osteosarcoma cell lines. Then we attenuated leucine proline-enriched proteoglycan 1 expression in MG63 cells by siRNA strategy and assessed the effect of leucine proline-enriched proteoglycan 1 on cell proliferation, migration and invasion through in-vitro experiments. Additionally, we detected the role of leucine proline-enriched proteoglycan 1 knockdown on PI3K/AKT pathway-related proteins using western blotting. Leucine proline-enriched proteoglycan 1 was increased in osteosarcoma tissues and cells. The overall survival curve demonstrated that high-regulated leucine proline-enriched proteoglycan 1 was linked with poor prognosis of patients with osteosarcoma. The capabilities of proliferation, migration and invasion were all inhibited in MG63 cell because of the downregulation of leucine proline-enriched proteoglycan 1. Furthermore, the expression of phosphorylated PI3K and AKT was impaired after knockdown the leucine proline-enriched proteoglycan 1 as well as P70S6K. In conclusion, leucine proline-enriched proteoglycan 1 might function as an important therapeutic factor in human osteosarcoma through regulating the PI3K/AKT signaling pathway.

Aberrant binding of mutant HSP47 affects posttranslational modification of type I collagen and leads to osteogenesis imperfecta

Heat shock protein 47 (HSP47), encoded by the SERPINH1 gene, is a molecular chaperone essential for correct folding of collagens. We report a homozygous p.(R222S) substitution in HSP47 in a child with severe osteogenesis imperfecta leading to early demise. p.R222 is a highly conserved residue located within the collagen interacting surface of HSP47. Binding assays show a significantly reduced affinity of HSP47-R222S for type I collagen. This altered interaction leads to posttranslational overmodification of type I procollagen produced by dermal fibroblasts, with increased glycosylation and/or hydroxylation of lysine and proline residues as shown by mass spectrometry. Since we also observed a normal intracellular folding and secretion rate of type I procollagen, this overmodification cannot be explained by prolonged exposure of the procollagen molecules to the modifying hydroxyl- and glycosyltransferases, as is commonly observed in other types of OI. We found significant upregulation of several molecular chaperones and enzymes involved in procollagen modification and folding on Western blot and RT-qPCR. In addition, we showed that an imbalance in binding of HSP47-R222S to unfolded type I collagen chains in a gelatin sepharose pulldown assay results in increased binding of other chaperones and modifying enzymes. The elevated expression and binding of this molecular ensemble to type I procollagen suggests a compensatory mechanism for the aberrant binding of HSP47-R222S, eventually leading to overmodification of type I procollagen chains. Together, these results illustrate the importance of HSP47 for proper posttranslational modification and provide insights into the molecular pathomechanisms of the p.(R222S) alteration in HSP47, which leads to a severe OI phenotype.

Heat shock protein 47 (HSP47) is essential for correct collagen folding. We report a homozygous p.(R222S) substitution in HSP47 in a child with severe osteogenesis imperfecta. The highly conserved p.R222 residue is located within the collagen interacting surface and HSP47-R222S shows a significantly reduced affinity for type I collagen. This altered interaction leads to posttranslational overmodification of type I procollagen. In contrast to other types of OI, this overmodification is not caused by prolonged exposure of procollagen to modifying enzymes, since the intracellular folding rate of type I procollagen appears to be normal. We show significant upregulation of several molecular chaperones and collagen-modifying enzymes and increased binding of several of these molecules to unfolded type I collagen chains upon abnormal HSP47-R222S binding. This suggests a compensatory mechanism for aberrant HSP47-R222S binding, eventually leading to overmodification of type I procollagen chains, and underscores the importance of HSP47 for proper posttranslational modification.

Hypoxia-driven secretion of extracellular matrix proteins in the exosomes reflects the asymptomatic pathology of rotator cuff tendinopathies

The major hallmark of rotator cuff tendinopathies (RCT) is the disorganization of the tendon extracellular matrix (ECM), which is due to a decrease in the ratio of collagen I to collagen III. In addition, the pathology of the tendon matrisome remains asymptomatic, and hypoxia has been identified to be the priming signal to initiate the molecular pathology of RCT. Also, the secretome content of hypoxia-challenged tendon cells (tenocytes) reflects the pathological status of RCT. With this background, the present study was designed to establish the expression status and molecular crosstalk of the ECM component proteins contained in the exosomes of the hypoxia-challenged swine tenocytes. The mass spectrometry analysis revealed the upregulation of COL1A2, P4HA1, PRDX2, P3H1, COL6A1, PPIB, LCN1, and COL3A1 and the downregulation of COLA12, PDIA4, COLG, FN1, CTSK, and TNC in the exosomes of hypoxic tenocytes. These proteins interact with diverse proteins and operate multiple pathways associated with ECM homeostasis and repair as determined by NetworkAnalyst. The functional analysis of these proteins reflects the pathology of tendon ECM, which is correlated with the asymptomatic phase of RCT. Understanding the signaling mediated by these proteins would reveal the underlying molecular pathology and offers translational significance in the diagnosis and management of RCT.

Prolyl and lysyl hydroxylases in collagen synthesis

Collagens are the most abundant proteins in the extracellular matrix. They provide a framework to build organs and tissues and give structural support to make them resistant to mechanical load and forces. Several intra- and extracellular modifications are needed to make functional collagen molecules, intracellular post-translational modifications of proline and lysine residues having key roles in this. In this article, we provide a review on the enzymes responsible for the proline and lysine modifications, that is collagen prolyl 4-hydroxylases, 3-hydroxylases and lysyl hydroxylases, and discuss their biological functions and involvement in diseases.

Effects of phosphate ion concentration on in-vitro fibrillogenesis of sturgeon type I collagen

Nonmammalian collagens have attracted significant attention owing to their potential for use as a source of cell scaffolds for tissue engineering. Since the morphology of collagen fibrils controls cell proliferation and differentiation, its regulation is essential for fabricating scaffolds with desirable characteristics. In this study, we evaluated the effects of the phosphate ion (Pi) concentration on the characteristics of fibrils formed from swim bladder type I collagen (SBC) and skin type I collagen (SC) from the Bester sturgeon. An increase in the Pi concentration decreased the fibril formation rate, promoted the formation of thick fibrils, and increased the thermal stability of the fibrils for both SBC and SC. However, the SBC and SC fibrils exhibited different fibril formation rates, degrees of fibrillogenesis, morphologies, and denaturation temperatures for the same reaction conditions. Finally, by regulating the Pi concentration, various types of SBC and SC fibrils could be coated on cell culture wells, and fibroblasts could be cultured on them. The results showed that thin fibrils enhance fibroblast extension and proliferation, whereas thick fibrils restrain fibroblast extension but orient them in the same direction. The results of this study suggest that SBC fibrils, which exhibit diverse morphologies, are suitable for use as a novel scaffold material, whose characteristics can be tailored readily by varying the Pi concentration.

Substitution of murine type I collagen A1 3-hydroxylation site alters matrix structure but does not recapitulate osteogenesis imperfecta bone dysplasia

Null mutations in CRTAP or P3H1, encoding cartilage-associated protein and prolyl 3-hydroxylase 1, cause the severe bone dysplasias, types VII and VIII osteogenesis imperfecta. Lack of either protein prevents formation of the ER prolyl 3-hydroxylation complex, which catalyzes 3Hyp modification of types I and II collagen and also acts as a collagen chaperone. To clarify the role of the A1 3Hyp substrate site in recessive bone dysplasia, we generated knock-in mice with an α1(I)P986A substitution that cannot be 3-hydroxylated. Mutant mice have normal survival, growth, femoral breaking strength and mean bone mineralization. However, the bone collagen HP/LP crosslink ratio is nearly doubled in mutant mice, while collagen fibril diameter and bone yield energy are decreased. Thus, 3-hydroxylation of the A1 site α1(I)P986 affects collagen crosslinking and structural organization, but its absence does not directly cause recessive bone dysplasia. Our study suggests that the functions of the modification complex as a collagen chaperone are thus distinct from its role as prolyl 3-hydroxylase.

Characterization of PPIB interaction in the P3H1 ternary complex and implications for its pathological mutations

The P3H1/CRTAP/PPIB complex is essential for prolyl 3-hydroxylation and folding of procollagens in the endoplasmic reticulum (ER). Deficiency in any component of this ternary complex is associated with the misfolding of collagen and the onset of osteogenesis imperfecta. However, little structure information is available about how this ternary complex is assembled and retained in the ER. Here, we assessed the role of the KDEL sequence of P3H1 and probed the spatial interactions of PPIB in the complex. We show that the KDEL sequence is essential for retaining the P3H1 complex in the ER. Its removal resulted in co-secretion of P3H1 and CRTAP out of the cell, which was mediated by the binding of P3H1 N-terminal domain with CRTAP. The secreted P3H1/CRTAP can readily bind PPIB with their C-termini close to PPIB in the ternary complex. Cysteine modification, crosslinking, and mass spectrometry experiments identified PPIB surface residues involved in the complex formation, and showed that the surface of PPIB is extensively covered by the binding of P3H1 and CRTAP. Most importantly, we demonstrated that one disease-associated pathological PPIB mutation on the binding interface did not affect the PPIB prolyl-isomerase activity, but disrupted the formation of P3H1/CRTAP/PPIB ternary complex. This suggests that defects in the integrity of the P3H1 ternary complex are associated with pathological collagen misfolding. Taken together, these results provide novel structural information on how PPIB interacts with other components of the P3H1 complex and indicate that the integrity of P3H1 complex is required for proper collagen formation.

Suitability and limitations of mesenchymal stem cells to elucidate human bone illness

Functional impairment of mesenchymal stem cells (MSCs), osteoblast progenitor cells, has been proposed to be a pathological mechanism contributing to bone disorders, such as osteoporosis (the most common bone disease) and other rare inherited skeletal dysplasias. Pathological bone loss can be caused not only by an enhanced bone resorption activity but also by hampered osteogenic differentiation of MSCs. The majority of the current treatment options counteract bone loss, and therefore bone fragility by blocking bone resorption. These so-called antiresorptive treatments, in spite of being effective at reducing fracture risk, cannot be administered for extended periods due to security concerns. Therefore, there is a real need to develop osteoanabolic therapies to promote bone formation. Human MSCs emerge as a suitable tool to study the etiology of bone disorders at the cellular level as well as to be used for cell therapy purposes for bone diseases. This review will focus on the most relevant findings using human MSCs as an in vitro cell model to unravel pathological bone mechanisms and the application and outcomes of human MSCs in cell therapy clinical trials for bone disease.

Role of prolyl hydroxylation in the molecular interactions of collagens

Co- and post-translational hydroxylation of proline residues is critical for the stability of the triple helical collagen structure. In this review, we summarise the biology of collagen prolyl 4-hydroxylases and collagen prolyl 3-hydroxylases, the enzymes responsible for proline hydroxylation. Furthermore, we describe the potential roles of hydroxyproline residues in the complex interplay between collagens and other proteins, especially integrin and discoidin domain receptor type cell adhesion receptors. Qualitative and quantitative regulation of collagen hydroxylation may have remarkable effects on the properties of the extracellular matrix and consequently on the cell behaviour.

The Role of Matrix Composition in the Mechanical Behavior of Bone

PURPOSE OF REVIEW

While thinning of the cortices or trabeculae weakens bone, age-related changes in matrix composition also lower fracture resistance. This review summarizes how the organic matrix, mineral phase, and water compartments influence the mechanical behavior of bone, thereby identifying characteristics important to fracture risk.

RECENT FINDINGS

In the synthesis of the organic matrix, tropocollagen experiences various post-translational modifications that facilitate a highly organized fibril of collagen I with a preferred orientation giving bone extensibility and several toughening mechanisms. Being a ceramic, mineral is brittle but increases the strength of bone as its content within the organic matrix increases. With time, hydroxyapatite-like crystals experience carbonate substitutions, the consequence of which remains to be understood. Water participates in hydrogen bonding with organic matrix and in electrostatic attractions with mineral phase, thereby providing stability to collagen-mineral interface and ductility to bone. Clinical tools sensitive to age- and disease-related changes in matrix composition that the affect mechanical behavior of bone could potentially improve fracture risk assessment.

Heritable Skeletal Disorders Arising from Defects in Processing and Transport of Type I Procollagen from the ER: Perspectives on Possible Therapeutic Approaches

Rare bone disorders are a heterogeneous group of diseases, initially associated with mutations in type I procollagen (PC) genes. Recent developments from dissection at the molecular and cellular level have expanded the list of disease-causing proteins, revealing that disruption of the machinery that handles protein secretion can lead to failure in PC secretion and in several cases result in skeletal dysplasia. In parallel, cell-based in vitro studies of PC trafficking pathways offer clues to the identification of new disease candidate genes. Together, this raises the prospect of heritable bone disorders as a paradigm for biosynthetic protein traffic-related diseases, and an avenue through which therapeutic strategies can be explored.Here, we focus on human syndromes linked to defects in type I PC secretion with respect to the landscape of biosynthetic and protein transport steps within the early secretory pathway. We provide a perspective on possible therapeutic interventions for associated heritable craniofacial and skeletal disorders, considering different orders of complexity, from the cellular level by manipulation of proteostasis pathways to higher levels involving cell-based therapies for bone repair and regeneration.

Genetic causes and mechanisms of Osteogenesis Imperfecta

Osteogenesis Imperfecta (OI) is a genetic disorder characterized by various clinical features including bone deformities, low bone mass, brittle bones, and connective tissue manifestations. The predominant cause of OI is due to mutations in the two genes that encode type I collagen. However, recent advances in sequencing technology has led to the discovery of novel genes that are implicated in recessive and dominant OI. These include genes that regulate the post-translational modification, secretion and processing of type I collagen as well as those required for osteoblast differentiation and bone mineralization. As such, OI has become a spectrum of genetic disorders informing about the determinants of both bone quantity and quality. Here we summarize the known genetic causes of OI, animal models that recapitulate the human disease and mechanisms that underlie disease pathogenesis. Additionally, we discuss the effects of disrupted collagen networks on extracellular matrix signaling and its impact on disease progression.

Ziploc-ing the structure 2.0: Endoplasmic reticulum-resident peptidyl prolyl isomerases show different activities toward hydroxyproline

Extracellular matrix proteins are biosynthesized in the rough endoplasmic reticulum (rER), and the triple-helical protein collagen is the most abundant extracellular matrix component in the human body. Many enzymes, molecular chaperones, and post-translational modifiers facilitate collagen biosynthesis. Collagen contains a large number of proline residues, so the cis/trans isomerization of proline peptide bonds is the rate-limiting step during triple-helix formation. Accordingly, the rER-resident peptidyl prolyl cis/trans isomerases (PPIases) play an important role in the zipper-like triple-helix formation in collagen. We previously described this process as "Ziploc-ing the structure" and now provide additional information on the activity of individual rER PPIases. We investigated the substrate preferences of these PPIases in vitro using type III collagen, the unhydroxylated quarter fragment of type III collagen, and synthetic peptides as substrates. We observed changes in activity of six rER-resident PPIases, cyclophilin B (encoded by the PPIB gene), FKBP13 (FKBP2), FKBP19 (FKBP11), FKBP22 (FKBP14), FKBP23 (FKBP7), and FKBP65 (FKBP10), due to posttranslational modifications of proline residues in the substrate. Cyclophilin B and FKBP13 exhibited much lower activity toward post-translationally modified substrates. In contrast, FKBP19, FKBP22, and FKBP65 showed increased activity toward hydroxyproline-containing peptide substrates. Moreover, FKBP22 showed a hydroxyproline-dependent effect by increasing the amount of refolded type III collagen in vitro and FKBP19 seems to interact with triple helical type I collagen. Therefore, we propose that hydroxyproline modulates the rate of Ziploc-ing of the triple helix of collagen in the rER.

Osteogenesis imperfecta: new genes reveal novel mechanisms in bone dysplasia

Osteogenesis imperfecta (OI) is a skeletal dysplasia characterized by fragile bones and short stature and known for its clinical and genetic heterogeneity which is now understood as a collagen-related disorder. During the last decade, research has made remarkable progress in identifying new OI-causing genes and beginning to understand the intertwined molecular and biochemical mechanisms of their gene products. Most cases of OI have dominant inheritance. Each new gene for recessive OI, and a recently identified gene for X-linked OI, has shed new light on its (often previously unsuspected) function in bone biology. Here, we summarize the literature that has contributed to our current understanding of the pathogenesis of OI.

Fibrillar Collagens

Fibrillar collagens (types I, II, III, V, XI, XXIV and XXVII) constitute a sub-group within the collagen family (of which there are 28 types in humans) whose functions are to provide three-dimensional frameworks for tissues and organs. These networks confer mechanical strength as well as signalling and organizing functions through binding to cellular receptors and other components of the extracellular matrix (ECM). Here we describe the structure and assembly of fibrillar collagens, and their procollagen precursors, from the molecular to the tissue level. We show how the structure of the collagen triple-helix is influenced by the amino acid sequence, hydrogen bonding and post-translational modifications, such as prolyl 4-hydroxylation. The numerous steps in the biosynthesis of the fibrillar collagens are reviewed with particular attention to the role of prolyl 3-hydroxylation, collagen chaperones, trimerization of procollagen chains and proteolytic maturation. The multiple steps controlling fibril assembly are then discussed with a focus on the cellular control of this process in vivo. Our current understanding of the molecular packing in collagen fibrils, from different tissues, is then summarized on the basis of data from X-ray diffraction and electron microscopy. These results provide structural insights into how collagen fibrils interact with cell receptors, other fibrillar and non-fibrillar collagens and other ECM components, as well as enzymes involved in cross-linking and degradation.

Animal models of osteogenesis imperfecta: applications in clinical research

Osteogenesis imperfecta (OI), commonly known as brittle bone disease, is a genetic disease characterized by extreme bone fragility and consequent skeletal deformities. This connective tissue disorder is caused by mutations in the quality and quantity of the collagen that in turn affect the overall mechanical integrity of the bone, increasing its vulnerability to fracture. Animal models of the disease have played a critical role in the understanding of the pathology and causes of OI and in the investigation of a broad range of clinical therapies for the disease. Currently, at least 20 animal models have been officially recognized to represent the phenotype and biochemistry of the 17 different types of OI in humans. These include mice, dogs, and fish. Here, we describe each of the animal models and the type of OI they represent, and present their application in clinical research for treatments of OI, such as drug therapies (ie, bisphosphonates and sclerostin) and mechanical (ie, vibrational) loading. In the future, different dosages and lengths of treatment need to be further investigated on different animal models of OI using potentially promising treatments, such as cellular and chaperone therapies. A combination of therapies may also offer a viable treatment regime to improve bone quality and reduce fragility in animals before being introduced into clinical trials for OI patients.

Intracellular mechanisms of molecular recognition and sorting for transport of large extracellular matrix molecules

Extracellular matrix (ECM) proteins are biosynthesized in the rough endoplasmic reticulum (rER) and transported via the Golgi apparatus to the extracellular space. The coat protein complex II (COPII) transport vesicles are approximately 60-90 nm in diameter. However, several ECM molecules are much larger, up to several hundreds of nanometers. Therefore, special COPII vesicles are required to coat and transport these molecules. Transmembrane Protein Transport and Golgi Organization 1 (TANGO1) facilitates loading of collagens into special vesicles. The Src homology 3 (SH3) domain of TANGO1 was proposed to recognize collagen molecules, but how the SH3 domain recognizes various types of collagen is not understood. Moreover, how are large noncollagenous ECM molecules transported from the rER to the Golgi? Here we identify heat shock protein (Hsp) 47 as a guide molecule directing collagens to special vesicles by interacting with the SH3 domain of TANGO1. We also consider whether the collagen secretory model applies to other large ECM molecules.

Non-Lethal Type VIII Osteogenesis Imperfecta Has Elevated Bone Matrix Mineralization

CONTEXT

Type VIII osteogenesis imperfecta (OI; OMIM 601915) is a recessive form of lethal or severe OI caused by null mutations in P3H1, which encodes prolyl 3-hydroxylase 1.

OBJECTIVES

Clinical and bone material description of non-lethal type VIII OI.

DESIGN

Natural history study of type VIII OI.

SETTING

Pediatric academic research centers.

PATIENTS

Five patients with non-lethal type VIII OI, and one patient with lethal type VIII OI.

INTERVENTIONS

None.

MAIN OUTCOME MEASURES

Clinical examinations included bone mineral density, radiographs, and serum and urinary metabolites. Bone biopsy samples were analyzed for histomorphometry and bone mineral density distribution by quantitative backscattered electron imaging microscopy. Collagen biochemistry was examined by mass spectrometry, and collagen fibrils were examined by transmission electron microscopy.

RESULTS

Type VIII OI patients have extreme growth deficiency, an L1-L4 areal bone mineral density Z-score of -5 to -6, and normal bone formation markers. Collagen from bone and skin tissue and cultured osteoblasts and fibroblasts have nearly absent 3-hydroxylation (1-4%). Collagen fibrils showed abnormal diameters and irregular borders. Bone histomorphometry revealed decreased cortical width and very thin trabeculae with patches of increased osteoid, although the overall osteoid surface was normal. Quantitative backscattered electron imaging showed increased matrix mineralization of cortical and trabecular bone, typical of other OI types. However, the proportion of bone with low mineralization was increased in type VIII OI bone, compared to type VII OI.

CONCLUSIONS

P3H1 is the unique enzyme responsible for collagen 3-hydroxylation in skin and bone. Bone from non-lethal type VIII OI children is similar to type VII, especially bone matrix hypermineralization, but it has distinctive features including extremely thin trabeculae, focal osteoid accumulation, and an increased proportion of low mineralized bone.

Developmental Stage-dependent Regulation of Prolyl 3-Hydroxylation in Tendon Type I Collagen

3-Hydroxyproline (3-Hyp), which is unique to collagen, is a fairly rare post-translational modification. Recent studies have suggested a function of prolyl 3-hydroxylation in fibril assembly and its relationships with certain disorders, including recessive osteogenesis imperfecta and high myopia. However, no direct evidence for the physiological and pathological roles of 3-Hyp has been presented. In this study, we first estimated the overall alterations in prolyl hydroxylation in collagens purified from skin, bone, and tail tendon of 0.5-18-month-old rats by LC-MS analysis with stable isotope-labeled collagen, which was recently developed as an internal standard for highly accurate collagen analyses. 3-Hyp was found to significantly increase in tendon collagen until 3 months after birth and then remain constant, whereas increased prolyl 3-hydroxylation was not observed in skin and bone collagen. Site-specific analysis further revealed that 3-Hyp was increased in tendon type I collagen in a specific sequence region, including a previously known modification site at Pro(707) and newly identified sites at Pro(716) and Pro(719), at the early ages. The site-specific alterations in prolyl 3-hydroxylation with aging were also observed in bovine Achilles tendon. We postulate that significant increases in 3-Hyp at the consecutive modification sites are correlated with tissue development in tendon. The present findings suggest that prolyl 3-hydroxylation incrementally regulates collagen fibril diameter in tendon.

2-Oxoglutarate-dependent dioxygenases are sensors of energy metabolism, oxygen availability, and iron homeostasis: potential role in the regulation of aging process

Recent studies have revealed that the members of an ancient family of nonheme Fe(2+)/2-oxoglutarate-dependent dioxygenases (2-OGDO) are involved in the functions associated with the aging process. 2-Oxoglutarate and O2 are the obligatory substrates and Fe(2+) a cofactor in the activation of 2-OGDO enzymes, which can induce the hydroxylation of distinct proteins and the demethylation of DNA and histones. For instance, ten-eleven translocation 1-3 (TET1-3) are the demethylases of DNA, whereas Jumonji C domain-containing histone lysine demethylases (KDM2-7) are the major epigenetic regulators of chromatin landscape, known to be altered with aging. The functions of hypoxia-inducible factor (HIF) prolyl hydroxylases (PHD1-3) as well as those of collagen hydroxylases are associated with age-related degeneration. Moreover, the ribosomal hydroxylase OGFOD1 controls mRNA translation, which is known to decline with aging. 2-OGDO enzymes are the sensors of energy metabolism, since the Krebs cycle intermediate 2-oxoglutarate is an activator whereas succinate and fumarate are the potent inhibitors of 2-OGDO enzymes. In addition, O2 availability and iron redox homeostasis control the activities of 2-OGDO enzymes in tissues. We will briefly elucidate the catalytic mechanisms of 2-OGDO enzymes and then review the potential functions of the above-mentioned 2-OGDO enzymes in the control of the aging process.

Protein Hydroxylation Catalyzed by 2-Oxoglutarate-dependent Oxygenases

The post-translational hydroxylation of prolyl and lysyl residues, as catalyzed by 2-oxoglutarate (2OG)-dependent oxygenases, was first identified in collagen biosynthesis. 2OG oxygenases also catalyze prolyl and asparaginyl hydroxylation of the hypoxia-inducible factors that play important roles in the adaptive response to hypoxia. Subsequently, they have been shown to catalyze N-demethylation (via hydroxylation) of N(ϵ)-methylated histone lysyl residues, as well as hydroxylation of multiple other residues. Recent work has identified roles for 2OG oxygenases in the modification of translation-associated proteins, which in some cases appears to be conserved from microorganisms through to humans. Here we give an overview of protein hydroxylation catalyzed by 2OG oxygenases, focusing on recent discoveries.

Mechanistic and therapeutic insights gained from studying rare skeletal diseases

Rare bone diseases account for 5% of all birth defects and can cause significant morbidity throughout patients' lives. Significant progress is being made to elucidate the pathophysiological mechanisms underlying these diseases. This paper summarizes presentation highlights of a workshop on Rare Skeletal Diseases convened to explore how the study of rare diseases has influenced the field's understanding of bone anabolism and catabolism and directed the search for new therapies benefiting patients with rare conditions as well as patients with common skeletal disorders.

Post-translationally abnormal collagens of prolyl 3-hydroxylase-2 null mice offer a pathobiological mechanism for the high myopia linked to human LEPREL1 mutations

Myopia, the leading cause of visual impairment worldwide, results from an increase in the axial length of the eyeball. Mutations in LEPREL1, the gene encoding prolyl 3-hydroxylase-2 (P3H2), have recently been identified in individuals with recessively inherited nonsyndromic severe myopia. P3H2 is a member of a family of genes that includes three isoenzymes of prolyl 3-hydroxylase (P3H), P3H1, P3H2, and P3H3. Fundamentally, it is understood that P3H1 is responsible for converting proline to 3-hydroxyproline. This limited additional knowledge also suggests that each isoenzyme has evolved different collagen sequence-preferred substrate specificities. In this study, differences in prolyl 3-hydroxylation were screened in eye tissues from P3h2-null (P3h2(n/n)) and wild-type mice to seek tissue-specific effects due the lack of P3H2 activity on post-translational collagen chemistry that could explain myopia. The mice were viable and had no gross musculoskeletal phenotypes. Tissues from sclera and cornea (type I collagen) and lens capsule (type IV collagen) were dissected from mouse eyes, and multiple sites of prolyl 3-hydroxylation were identified by mass spectrometry. The level of prolyl 3-hydroxylation at multiple substrate sites from type I collagen chains was high in sclera, similar to tendon. Almost every known site of prolyl 3-hydroxylation in types I and IV collagen from P3h2(n/n) mouse eye tissues was significantly under-hydroxylated compared with their wild-type littermates. We conclude that altered collagen prolyl 3-hydroxylation is caused by loss of P3H2. We hypothesize that this leads to structural abnormalities in multiple eye tissues, but particularly sclera, causing progressive myopia.

Ziploc-ing the structure: Triple helix formation is coordinated by rough endoplasmic reticulum resident PPIases

BACKGROUND

Protein folding is crucial for proteins' specific functions and is facilitated by various types of enzymes and molecular chaperones. The peptidyl prolyl cis/trans isomerases (PPIase) are one of these families of enzymes. They ubiquitously exist inside the cell and there are eight PPIases in the rough endoplasmic reticulum (rER), a compartment where the folding of most secreted proteins occurs.

SCOPE OF REVIEW

We review the functional and structural aspects of individual rER resident PPIases. Furthermore, we specifically discuss the role of these PPIases during collagen biosynthesis, since collagen is the most abundant protein in humans, is synthesized in the rER, and contains a proportionally high number of proline residues.

MAJOR CONCLUSIONS

The rER resident PPIases recognize different sets of substrates and facilitate their folding. Although they are clearly catalysts for protein folding, they also have more broad and multifaceted functions. We propose that PPIases coordinate collagen biosynthesis in the rER.

GENERAL SIGNIFICANCE

This review expands our understanding of collagen biosynthesis by explaining the influence of novel indirect mechanisms of regulating folding and this is also explored for PPIases. We also suggest future directions of research to obtain a better understanding of collagen biosynthesis and functions of PPIases in the rER. This article is part of a Special Issue entitled Proline-directed Foldases: Cell Signaling Catalysts and Drug Targets.

Targeting the LRP5 pathway improves bone properties in a mouse model of osteogenesis imperfecta

The cell surface receptor low-density lipoprotein receptor-related protein 5 (LRP5) is a key regulator of bone mass and bone strength. Heterozygous missense mutations in LRP5 cause autosomal dominant high bone mass (HBM) in humans by reducing binding to LRP5 by endogenous inhibitors, such as sclerostin (SOST). Mice heterozygous for a knockin allele (Lrp5(p.A214V) ) that is orthologous to a human HBM-causing mutation have increased bone mass and strength. Osteogenesis imperfecta (OI) is a skeletal fragility disorder predominantly caused by mutations that affect type I collagen. We tested whether the LRP5 pathway can be used to improve bone properties in animal models of OI. First, we mated Lrp5(+/p.A214V) mice to Col1a2(+/p.G610C) mice, which model human type IV OI. We found that Col1a2(+/p.G610C) ;Lrp5(+/p.A214V) offspring had significantly increased bone mass and strength compared to Col1a2(+/p.G610C) ;Lrp5(+/+) littermates. The improved bone properties were not a result of altered mRNA expression of type I collagen or its chaperones, nor were they due to changes in mutant type I collagen secretion. Second, we treated Col1a2(+/p.G610C) mice with a monoclonal antibody that inhibits sclerostin activity (Scl-Ab). We found that antibody-treated mice had significantly increased bone mass and strength compared to vehicle-treated littermates. These findings indicate increasing bone formation, even without altering bone collagen composition, may benefit patients with OI.

Abnormal type I collagen post-translational modification and crosslinking in a cyclophilin B KO mouse model of recessive osteogenesis imperfecta

Cyclophilin B (CyPB), encoded by PPIB, is an ER-resident peptidyl-prolyl cis-trans isomerase (PPIase) that functions independently and as a component of the collagen prolyl 3-hydroxylation complex. CyPB is proposed to be the major PPIase catalyzing the rate-limiting step in collagen folding. Mutations in PPIB cause recessively inherited osteogenesis imperfecta type IX, a moderately severe to lethal bone dysplasia. To investigate the role of CyPB in collagen folding and post-translational modifications, we generated Ppib^−/− mice that recapitulate the OI phenotype. Knock-out (KO) mice are small, with reduced femoral areal bone mineral density (aBMD), bone volume per total volume (BV/TV) and mechanical properties, as well as increased femoral brittleness. Ppib transcripts are absent in skin, fibroblasts, femora and calvarial osteoblasts, and CyPB is absent from KO osteoblasts and fibroblasts on western blots. Only residual (2–11%) collagen prolyl 3-hydroxylation is detectable in KO cells and tissues. Collagen folds more slowly in the absence of CyPB, supporting its rate-limiting role in folding. However, treatment of KO cells with cyclosporine A causes further delay in folding, indicating the potential existence of another collagen PPIase. We confirmed and extended the reported role of CyPB in supporting collagen lysyl hydroxylase (LH1) activity. Ppib^−/− fibroblast and osteoblast collagen has normal total lysyl hydroxylation, while increased collagen diglycosylation is observed. Liquid chromatography/mass spectrometry (LC/MS) analysis of bone and osteoblast type I collagen revealed site-specific alterations of helical lysine hydroxylation, in particular, significantly reduced hydroxylation of helical crosslinking residue K87. Consequently, underhydroxylated forms of di- and trivalent crosslinks are strikingly increased in KO bone, leading to increased total crosslinks and decreased helical hydroxylysine- to lysine-derived crosslink ratios. The altered crosslink pattern was associated with decreased collagen deposition into matrix in culture, altered fibril structure in tissue, and reduced bone strength. These studies demonstrate novel consequences of the indirect regulatory effect of CyPB on collagen hydroxylation, impacting collagen glycosylation, crosslinking and fibrillogenesis, which contribute to maintaining bone mechanical properties.

Osteogenesis imperfecta (OI), or brittle bone disease, is characterized by susceptibility to fractures from minimal trauma and growth deficiency. Deficiency of components of the collagen prolyl 3-hydroxylation complex, CRTAP, P3H1 and CyPB, cause recessive types VII, VIII and IX OI, respectively. We have previously shown that mutual protection within the endoplasmic reticulum accounts for the overlapping severe phenotype of patients with CRTAP and P3H1 mutations. However, the bone dysplasia in patients with CyPB deficiency is distinct in terms of phenotype and type I collagen biochemistry. Using a knock-out mouse model of type IX OI, we have demonstrated that CyPB is the major, although not unique, peptidyl prolyl cis-trans isomerase that catalyzes the rate-limiting step in collagen folding. CyPB is also required for activity of the collagen prolyl 3-hydroxylation complex; collagen α1(I) P986 modification is lost in the absence of CyPB. Unexpectedly, CyPB was found to also influence collagen helical lysyl hydroxylation in a tissue-, cell- and residue-specific manner. Thus CyPB facilitates collagen folding directly, but also indirectly regulates collagen hydroxylation, glycosylation, crosslinking and fibrillogenesis through its interactions with other collagen modifying enzymes in the endoplasmic reticulum.

Connective tissue alterations in Fkbp10-/- mice

Osteogenesis imperfecta (OI) is an inherited brittle bone disorder characterized by bone fragility and low bone mass. Loss of function mutations in FK506-binding protein 10 (FKBP10), encoding the FKBP65 protein, result in recessive OI and Bruck syndrome, of which the latter is additionally characterized by joint contractures. FKBP65 is thought to act as a collagen chaperone, but it is unknown how loss of FKBP65 affects collagen synthesis and extracellular matrix formation. We evaluated the developmental and postnatal expression of Fkbp10 and analyzed the consequences of its generalized loss of function. Fkbp10 is expressed at low levels in E13.5 mouse embryos, particularly in skeletal tissues, and steadily increases through E17.5 with expression in not only skeletal tissues, but also in visceral tissues. Postnatally, expression is limited to developing bone and ligaments. In contrast to humans, with complete loss of function mutations, Fkbp10(-/-) mice do not survive birth, and embryos present with growth delay and tissue fragility. Type I calvarial collagen isolated from these mice showed reduced stable crosslink formation at telopeptide lysines. Furthermore, Fkbp10(-/-) mouse embryonic fibroblasts show retention of procollagen in the cell layer and associated dilated endoplasmic reticulum. These data suggest a requirement for FKBP65 function during embryonic connective tissue development in mice, but the restricted expression postnatally in bone, ligaments and tendons correlates with the bone fragility and contracture phenotype in humans.

Differential effects of collagen prolyl 3-hydroxylation on skeletal tissues

Mutations in the genes encoding cartilage associated protein (CRTAP) and prolyl 3-hydroxylase 1 (P3H1 encoded by LEPRE1) were the first identified causes of recessive Osteogenesis Imperfecta (OI). These proteins, together with cyclophilin B (encoded by PPIB), form a complex that 3-hydroxylates a single proline residue on the α1(I) chain (Pro986) and has cis/trans isomerase (PPIase) activity essential for proper collagen folding. Recent data suggest that prolyl 3-hydroxylation of Pro986 is not required for the structural stability of collagen; however, the absence of this post-translational modification may disrupt protein-protein interactions integral for proper collagen folding and lead to collagen over-modification. P3H1 and CRTAP stabilize each other and absence of one results in degradation of the other. Hence, hypomorphic or loss of function mutations of either gene cause loss of the whole complex and its associated functions. The relative contribution of losing this complex's 3-hydroxylation versus PPIase and collagen chaperone activities to the phenotype of recessive OI is unknown. To distinguish between these functions, we generated knock-in mice carrying a single amino acid substitution in the catalytic site of P3h1 (Lepre1^H662A). This substitution abolished P3h1 activity but retained ability to form a complex with Crtap and thus the collagen chaperone function. Knock-in mice showed absence of prolyl 3-hydroxylation at Pro986 of the α1(I) and α1(II) collagen chains but no significant over-modification at other collagen residues. They were normal in appearance, had no growth defects and normal cartilage growth plate histology but showed decreased trabecular bone mass. This new mouse model recapitulates elements of the bone phenotype of OI but not the cartilage and growth phenotypes caused by loss of the prolyl 3-hydroxylation complex. Our observations suggest differential tissue consequences due to selective inactivation of P3H1 hydroxylase activity versus complete ablation of the prolyl 3-hydroxylation complex.

The prolyl 3-hydroxylase complex serves to hydroxylate a single residue in type I collagen and also serves as a collagen chaperone. The complex is comprised of prolyl 3-hydroxylase 1, cartilage associated protein, and cyclophilin B. Mutations have been identified in the genes encoding the complex members in patients with recessive Osteogenesis Imperfecta. Recent data suggest that prolyl 3-hydroxylation of collagen does not alter the stability of collagen but may rather mediate protein-protein interactions. Additionally, the collagen chaperoning function of the complex is an important rate limiting step in the modification of type I collagen. Irrespective of whether patients with mutations in the genes encoding the members of the prolyl 3-hydroxylase complex have hypomorphic or complete loss of function alleles, either circumstance leads to the loss of both functions of the prolyl 3-hydroxylase complex. Thus, it is unknown how collagen chaperoning and/or hydroxylation affect bone and cartilage homeostasis. In this study, we generated a mouse model lacking the prolyl 3-hydroxylation activity of the complex while maintaining the chaperoning function. We found that the hydroxylase mutant mice have normal cartilage and normal cortical bone but decreased trabecular bone, suggesting that there is a differential requirement for hydroxylation in different tissues.

Biological role of prolyl 3-hydroxylation in type IV collagen

Collagens constitute nearly 30% of all proteins in our body. Type IV collagen is a major and crucial component of basement membranes. Collagen chains undergo several posttranslational modifications that are indispensable for proper collagen function. One of these modifications, prolyl 3-hydroxylation, is accomplished by a family of prolyl 3-hydroxylases (P3H1, P3H2, and P3H3). The present study shows that P3H2-null mice are embryonic-lethal by embryonic day 8.5. The mechanism of the unexpectedly early lethality involves the interaction of non-3-hydroxylated embryonic type IV collagen with the maternal platelet-specific glycoprotein VI (GPVI). This interaction results in maternal platelet aggregation, thrombosis of the maternal blood, and death of the embryo. The phenotype is completely rescued by producing double KOs of P3H2 and GPVI. Double nulls are viable and fertile. Under normal conditions, subendothelial collagens bear the GPVI-binding sites that initiate platelet aggregation upon blood exposure during injuries. In type IV collagen, these sites are normally 3-hydroxylated. Thus, prolyl 3-hydroxylation of type IV collagen has an important function preventing maternal platelet aggregation in response to the early developing embryo. A unique link between blood coagulation and the ECM is established. The newly described mechanism may elucidate some unexplained fetal losses in humans, where thrombosis is often observed at the maternal/fetal interface. Moreover, epigenetic silencing of P3H2 in breast cancers implies that the interaction between GPVI and non-3-hydroxylated type IV collagen might also play a role in the progression of malignant tumors and metastasis.

Molecular properties and fibril ultrastructure of types II and XI collagens in cartilage of mice expressing exclusively the α1(IIA) collagen isoform

Until now, no biological tools have been available to determine if a cross-linked collagen fibrillar network derived entirely from type IIA procollagen isoforms, can form in the extracellular matrix (ECM) of cartilage. Recently, homozygous knock-in transgenic mice (Col2a1(+ex2), ki/ki) were generated that exclusively express the IIA procollagen isoform during post-natal development while type IIB procollagen, normally present in the ECM of wild type mice, is absent. The difference between these Col2a1 isoforms is the inclusion (IIA) or exclusion (IIB) of exon 2 that is alternatively spliced in a developmentally regulated manner. Specifically, chondroprogenitor cells synthesize predominantly IIA mRNA isoforms while differentiated chondrocytes produce mainly IIB mRNA isoforms. Recent characterization of the Col2a1(+ex2) mice has surprisingly shown that disruption of alternative splicing does not affect overt cartilage formation. In the present study, biochemical analyses showed that type IIA collagen extracted from ki/ki mouse rib cartilage can form homopolymers that are stabilized predominantly by hydroxylysyl pyridinoline (HP) cross-links at levels that differed from wild type rib cartilage. The findings indicate that mature type II collagen derived exclusively from type IIA procollagen molecules can form hetero-fibrils with type XI collagen and contribute to cartilage structure and function. Heteropolymers with type XI collagen also formed. Electron microscopy revealed mainly thin type IIA collagen fibrils in ki/ki mouse rib cartilage. Immunoprecipitation and mass spectrometry of purified type XI collagen revealed a heterotrimeric molecular composition of α1(XI)α2(XI)α1(IIA) chains where the α1(IIA) chain is the IIA form of the α3(XI) chain. Since the N-propeptide of type XI collagen regulates type II collagen fibril diameter in cartilage, the retention of the exon 2-encoded IIA globular domain would structurally alter the N-propeptide of type XI collagen. This structural change may subsequently affect the regulatory function of type XI collagen resulting in the collagen fibril and cross-linking differences observed in this study.

An additional function of the rough endoplasmic reticulum protein complex prolyl 3-hydroxylase 1·cartilage-associated protein·cyclophilin B: the CXXXC motif reveals disulfide isomerase activity in vitro

Collagen biosynthesis occurs in the rough endoplasmic reticulum, and many molecular chaperones and folding enzymes are involved in this process. The folding mechanism of type I procollagen has been well characterized, and protein disulfide isomerase (PDI) has been suggested as a key player in the formation of the correct disulfide bonds in the noncollagenous carboxyl-terminal and amino-terminal propeptides. Prolyl 3-hydroxylase 1 (P3H1) forms a hetero-trimeric complex with cartilage-associated protein and cyclophilin B (CypB). This complex is a multifunctional complex acting as a prolyl 3-hydroxylase, a peptidyl prolyl cis-trans isomerase, and a molecular chaperone. Two major domains are predicted from the primary sequence of P3H1: an amino-terminal domain and a carboxyl-terminal domain corresponding to the 2-oxoglutarate- and iron-dependent dioxygenase domains similar to the α-subunit of prolyl 4-hydroxylase and lysyl hydroxylases. The amino-terminal domain contains four CXXXC sequence repeats. The primary sequence of cartilage-associated protein is homologous to the amino-terminal domain of P3H1 and also contains four CXXXC sequence repeats. However, the function of the CXXXC sequence repeats is not known. Several publications have reported that short peptides containing a CXC or a CXXC sequence show oxido-reductase activity similar to PDI in vitro. We hypothesize that CXXXC motifs have oxido-reductase activity similar to the CXXC motif in PDI. We have tested the enzyme activities on model substrates in vitro using a GCRALCG peptide and the P3H1 complex. Our results suggest that this complex could function as a disulfide isomerase in the rough endoplasmic reticulum.

Posttranslational modifications in type I collagen from different tissues extracted from wild type and prolyl 3-hydroxylase 1 null mice

Type I collagen extracted from tendon, skin, and bone of wild type and prolyl 3-hydroxylase 1 (P3H1) null mice shows distinct patterns of 3-hydroxylation and glycosylation of hydroxylysine residues. The A1 site (Pro-986) in the α1-chain of type I collagen is almost completely 3-hydroxylated in every tissue of the wild type mice. In contrast, no 3-hydroxylation of this proline residue was found in P3H1 null mice. Partial 3-hydroxylation of the A3 site (Pro-707) was present in tendon and bone, but absent in skin in both α-chains of the wild type animals. Type I collagen extracted from bone of P3H1 null mice shows a large reduction in 3-hydroxylation of the A3 site in both α-chains, whereas type I collagen extracted from tendon of P3H1 null mice shows little difference as compared with wild type. These results demonstrate that the A1 site in type I collagen is exclusively 3-hydroxylated by P3H1, and presumably, this enzyme is required for the 3-hydroxylation of the A3 site of both α-chains in bone but not in tendon. The increase in glycosylation of hydroxylysine in P3H1 null mice in bone was found to be due to an increased occupancy of normally glycosylated sites. Despite the severe disorganization of collagen fibrils in adult tissues, the D-period of the fibrils is unchanged. Tendon fibrils of newborn P3H1 null mice are well organized with only a slight increase in diameter. The absence of 3-hydroxyproline and/or the increased glycosylation of hydroxylysine in type I collagen disturbs the lateral growth of the fibrils.