A novel 3-hydroxyproline (3Hyp)-rich motif marks the triple-helical C terminus of tendon type I collagen

Loss of the long form of Plod2 phenocopies contractures of Bruck syndrome-osteogenesis imperfecta

Bruck syndrome is an autosomal recessive form of osteogenesis imperfecta caused by biallelic variants in PLOD2 or FKBP10 and is characterized by joint contractures, bone fragility, short stature, and scoliosis. PLOD2 encodes LH2, which hydroxylates type I collagen telopeptide lysines, a critical step for collagen crosslinking. The Plod2 global knockout mouse model is limited by early embryonic lethality, and thus, the role of PLOD2 in skeletogenesis is not well understood. We generated a novel Plod2 mouse line modeling a variant identified in two unrelated individuals with Bruck syndrome: PLOD2 c.1559dupC, predicting a frameshift and loss of the long isoform LH2b. In the mouse, the duplication led to loss of LH2b mRNA as well as significantly reduced total LH2 protein. This model, Plod2fs/fs, survived up to E18.5 although in non-Mendelian genotype frequencies. The homozygous frameshift model recapitulated the joint contractures seen in Bruck syndrome and had indications of absent type I collagen telopeptide lysine hydroxylation in bone. Genetically labeling tendons with Scleraxis-GFP in Plod2fs/fs mice revealed the loss of extensor tendons in the forelimb by E18.5, and developmental studies showed extensor tendons developed through E14.5 but were absent starting at E16.5. Second harmonic generation showed abnormal tendon type I collagen fiber organization, suggesting structurally abnormal tendons. Characterization of the skeleton by μCT and Raman spectroscopy showed normal bone mineralization levels. This work highlights the importance of properly crosslinked type I collagen in tendon and bone, providing a promising new mouse model to further our understanding of Bruck syndrome.

Impact of Bioinformatics Search Parameters for Peptides' Identification and Their Post-Translational Modifications: A Case Study of Proteolysed Gelatines from Beef, Pork, and Fish

Bioinformatics software, allowing the identification of peptides by the comparison of peptide fragmentation spectra obtained by mass spectrometry versus targeted databases or directly by de novo sequencing, is now mandatory in peptidomics/proteomics approaches. Programming the identification software requires specifying, among other things, the mass measurement accuracy of the instrument and the digestion enzyme used with the number of missed cleavages allowed. Moreover, these software algorithms are able to identify a large number of post-translational modifications (PTMs). However, peptide and PTM identifications are challenging in the agrofood field due to non-specific cleavage sites of physiological- or food-grade enzymes and the number and location of PTMs. In this study, we show the importance of customized software programming to obtain a better peptide and PTM identification rate in the agrofood field. A gelatine product and one industrial gelatine hydrolysate from three different sources (beef, pork, and fish), each digested by simulated gastrointestinal digestion, MS-grade trypsin, or both, were used to perform the comparisons. Two main points are illustrated: (i) the impact of the set-up of specific enzyme versus no specific enzyme use and (ii) the impact of a maximum of six PTMs allowed per peptide versus the standard of three. Prior knowledge of the composition of the raw proteins is an important asset for better identification of peptide sequences.

Tissue-specific collagen hydroxylation at GEP/GDP triplets mediated by P4HA2

Collagen, the most abundant organic compound of vertebrate organisms, is a supramolecular, protein-made polymer. Details of its post-translational maturation largely determine the mechanical properties of connective tissues. Its assembly requires massive, heterogeneous prolyl-4-hydroxylation (P4H), catalyzed by Prolyl-4-hydroxylases (P4HA1-3), providing thermostability to its elemental, triple helical building block. So far, there was no evidence of tissue-specific regulation of P4H, nor of a differential substrate repertoire of P4HAs. Here, the post-translational modifications of collagen extracted from bone, skin, and tendon were compared, revealing lower hydroxylation of most GEP/GDP triplets, together with fewer other residue positions along collagen α chains, in the tendon. This regulation is mostly conserved in two distant homeotherm species, mouse and chicken. The comparison of detailed P4H patterns in both species suggests a two-step mechanism of specificity. P4ha2 expression is low in tendon and its genetic invalidation in the ATDC5 cellular model of collagen assembly specifically mimics the tendon-related P4H profile. Therefore, P4HA2 has a better ability than other P4HAs to hydroxylate the corresponding residue positions. Its local expression participates in determining the P4H profile, a novel aspect of the tissue specificities of collagen assembly. Data availability: Proteomics data are available via ProteomeXchange with the identifier PXD039221. Reviewer account details.

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.

Age-related type I collagen modifications reveal tissue-defining differences between ligament and tendon

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Tendon and ligament collagens differ in their post-translational lysine and cross-linking chemistry.

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In ligament collagen, hydroxylysyl aldehyde, permanent cross-linking dominates.

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Tendon collagen has a mix of cross-links based on lysyl and hydroxylysyl aldehydes.

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The profile in tendon appears more adapted to facilitating growth, structural remodeling and repair of the fibrillar matrix.

Tendons and ligaments tend to be pooled into a single category as dense elastic bands of collagenous connective tissue. They do have many similar properties, for example both tissues are flexible cords of fibrous tissue that join bone to either muscle or bone. Tendons and ligaments are both prone to degenerate and rupture with only limited capacity to heal, although tendons tend to heal faster than ligaments. Type I collagen constitutes about 80% of the dry weight of tendons and ligaments and is principally responsible for the core strength of each tissue. Collagen synthesis is a complex process with multiple steps and numerous post-translational modifications including proline and lysine hydroxylation, hydroxylysine glycosylation and covalent cross-linking. The chemistry, placement and quantity of intramolecular and intermolecular cross-links are believed to be key contributors to the tissue-specific variations in material strength and biological properties of collagens. As tendons and ligaments grow and develop, the collagen cross-links are known to chemically mature, strengthen and change in profile. Accordingly, changes in cross-linking and other post-translational modifications are likely associated with tissue development and degeneration. Using mass spectrometry, we have compared tendon and ligaments from fetal and adult bovine knee joints to investigate changes in collagen post-translational properties. Although hydroxylation levels at the type I collagen helical cross-linking lysine residues were similar in all adult tissues, ligaments had significantly higher levels of glycosylation at these sites compared to tendon. Differences in lysine hydroxylation were also found between the tissues at the telopeptide cross-linking sites. Total collagen cross-linking analysis, including mature trivalent cross-links and immature divalent cross-links, revealed unique cross-linking profiles between tendon and ligament tissues. Tendons were found to have a significantly higher frequency of smaller diameter collagen fibrils compared with ligament, which we suspect is functionally associated with the unique cross-linking profile of each tissue. Understanding the specific molecular characteristics that define and distinguish these specialized tissues will be important to improving the design of orthopedic treatment approaches.

Heterogeneity in proline hydroxylation of fibrillar collagens observed by mass spectrometry

Collagen is the major protein in the extracellular matrix and plays vital roles in tissue development and function. Collagen is also one of the most processed proteins in its biosynthesis. The most prominent post-translational modification (PTM) of collagen is the hydroxylation of Pro residues in the Y-position of the characteristic (Gly-Xaa-Yaa) repeating amino acid sequence of a collagen triple helix. Recent studies using mass spectrometry (MS) and tandem MS sequencing (MS/MS) have revealed unexpected hydroxylation of Pro residues in the X-positions (X-Hyp). The newly identified X-Hyp residues appear to be highly heterogeneous in location and percent occupancy. In order to understand the dynamic nature of the new X-Hyps and their potential impact on applications of MS and MS/MS for collagen research, we sampled four different collagen samples using standard MS and MS/MS techniques. We found considerable variations in the degree of PTMs of the same collagen from different organisms and/or tissues. The rat tail tendon type I collagen is particularly variable in terms of both over-hydroxylation of Pro in the X-position and under-hydroxylation of Pro in the Y-position. In contrast, only a few unexpected PTMs in collagens type I and type III from human placenta were observed. Some observations are not reproducible between different sequencing efforts of the same sample, presumably due to a low population and/or the unpredictable nature of the ionization process. Additionally, despite the heterogeneous preparation and sourcing, collagen samples from commercial sources do not show elevated variations in PTMs compared to samples prepared from a single tissue and/or organism. These findings will contribute to the growing body of information regarding the PTMs of collagen by MS technology, and culminate to a more comprehensive understanding of the extent and the functional roles of the PTMs of collagen.

Reticulocalbin 3 is involved in postnatal tendon development by regulating collagen fibrillogenesis and cellular maturation

Tendon plays a critical role in the joint movement by transmitting force from muscle to bone. This transmission of force is facilitated by its specialized structure, which consists of highly aligned extracellular matrix consisting predominantly of type I collagen. Tenocytes, fibroblast-like tendon cells residing between the parallel collagen fibers, regulate this specialized tendon matrix. Despite the importance of collagen structure and tenocyte function, the biological mechanisms regulating fibrillogenesis and tenocyte maturation are not well understood. Here we examine the function of Reticulocalbin 3 (Rcn3) in collagen fibrillogenesis and tenocyte maturation during postnatal tendon development using a genetic mouse model. Loss of Rcn3 in tendon caused decreased tendon thickness, abnormal tendon cell maturation, and decreased mechanical properties. Interestingly, Rcn3 deficient mice exhibited a smaller collagen fibril distribution and over-hydroxylation in C-telopeptide cross-linking lysine from α1(1) chain. Additionally, the proline 3-hydroxylation sites in type I collagen were also over-hydroxylated in Rcn3 deficient mice. Our data collectively suggest that Rcn3 is a pivotal regulator of collagen fibrillogenesis and tenocyte maturation during postnatal tendon development.

Use of osteoblast-derived matrix to assess the influence of collagen modifications on cancer cells

Collagenous stromal accumulations predict a worse clinical outcome in a variety of malignancies. Better tools are needed to elucidate the way in which collagen influences cancer cells. Here, we report a method to generate collagenous matrices that are deficient in key post-translational modifications and evaluate cancer cell behaviors on those matrices. We utilized genetic and biochemical approaches to inhibit lysine hydroxylation and glucosylation on collagen produced by MC-3T3-E1 murine osteoblasts (MC cells). Seeded onto MC cell-derived matrix surface, multicellular aggregates containing lung adenocarcinoma cells alone or in combination with cancer-associated fibroblasts dissociated with temporal and spatial patterns that were influenced by collagen modifications. These findings demonstrate the feasibility of generating defined collagen matrices that are suitable for cell culture studies.

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Feasibility of culturing multicellular aggregates on matrices with defined collagen modifications.

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Collagen modifications influence cancer cell behavior.

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This methodology is a useful tool for cancer researchers.

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.

Down-Regulation of Collagen Hydroxylation in Colorectal Liver Metastasis

Collagen is significantly upregulated in colorectal liver metastasis (CRLM) compared to liver tissue. Expression levels of specific collagen types in CRLM resemble those in colorectal cancer (CRC) and colon tissue. We investigated whether the collagen hydroxylation pattern from the primary tumor also migrates with the metastatic tumor. The degree of collagen alpha-1(I) hydroxylation in colon, CRC, liver, and CRLM tissue of the same individuals (n = 14) was studied with mass spectrometry. The degree of hydroxylation was investigated in 36 collagen alpha-1(I) peptides, covering 54% of the triple helical region. The degree of hydroxylation in liver tissue was similar to that in colon tissue. The overall degree of hydroxylation was significantly lower (9 ± 14%) in CRC tissue and also significantly lower (12 ± 22%) in CRLM tissue compared to colon. Furthermore, eleven peptides with a specific number of hydroxylations are significantly different between CRLM and liver tissue; these peptides could be candidates for the detection of CRLM. For one of these eleven peptides, a matching naturally occurring peptide in urine has been identified as being significantly different between patients suffering from CRLM and healthy controls. The hydroxylation pattern in CRLM resembles partly the pattern in liver, primary colorectal cancer and colon.

Collagen analysis with mass spectrometry

Mass spectrometry-based techniques can be applied to investigate collagen with respect to identification, quantification, supramolecular organization, and various post-translational modifications. The continuous interest in collagen research has led to a shift from techniques to analyze the physical characteristics of collagen to methods to study collagen abundance and modifications. In this review, we illustrate the potential of mass spectrometry for in-depth analyses of collagen.

Conformational landscape of substituted prolines

The cyclic side chain of the amino acid proline confers unique conformational restraints on its backbone and side chain dihedral angles. This affects two equilibria-one at the backbone (cis/trans) and the other at the side chain (endo/exo). Substitutions on the proline ring impose additional steric and stereoelectronic effects that can further modulate both these equilibria, which in turn can also affect the backbone dihedral angle (ϕ, ψ) preferences. In this review, we have explored the conformational landscape of several termini capped mono-(2-, 3-, 4-, and 5-) substituted proline derivatives in the Cambridge Structural Database, correlating observed conformations with the nature of substituents and deciphering the underlying interactions for the observed structural biases. The impact of incorporating these derivatives within model peptides and proteins are also discussed for selected cases. Several of these substituents have been used to introduce bioorthogonal functionality and modulate structure-specific ligand recognition or used as spectroscopic probes. The incorporation of these diversely applicable functional groups, coupled with their ability to define an amino acid conformation via stereoelectronic effects, have a broad appeal among chemical biologists, molecular biophysicists, and medicinal chemists.

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.

Quantitative proteomic profiling of extracellular matrix and site-specific collagen post-translational modifications in an in vitro model of lung fibrosis

Lung fibrosis is characterized by excessive deposition of extracellular matrix (ECM), in particular collagens, by fibroblasts in the interstitium. Transforming growth factor-β1 (TGF-β1) alters the expression of many extracellular matrix (ECM) components produced by fibroblasts, but such changes in ECM composition as well as modulation of collagen post-translational modification (PTM) levels have not been comprehensively investigated. Here, we performed mass spectrometry (MS)-based proteomics analyses to assess changes in the ECM deposited by cultured lung fibroblasts from idiopathic pulmonary fibrosis (IPF) patients upon stimulation with transforming growth factor β1 (TGF-β1). In addition to the ECM changes commonly associated with lung fibrosis, MS-based label-free quantification revealed profound effects on enzymes involved in ECM crosslinking and turnover as well as multiple positive and negative feedback mechanisms of TGF-β1 signaling. Notably, the ECM changes observed in this in vitro model correlated significantly with ECM changes observed in patient samples. Because collagens are subject to multiple PTMs with major implications in disease, we implemented a new bioinformatic platform to analyze MS data that allows for the comprehensive mapping and site-specific quantitation of collagen PTMs in crude ECM preparations. These analyses yielded a comprehensive map of prolyl and lysyl hydroxylations as well as lysyl glycosylations for 15 collagen chains. In addition, site-specific PTM analysis revealed novel sites of prolyl-3-hydroxylation and lysyl glycosylation in type I collagen. Interestingly, the results show, for the first time, that TGF-β1 can modulate prolyl-3-hydroxylation and glycosylation in a site-specific manner. Taken together, this proof of concept study not only reveals unanticipated TGF-β1 mediated regulation of collagen PTMs and other ECM components but also lays the foundation for dissecting their key roles in health and disease.

The proteomic data has been deposited to the ProteomeXchange Consortium via the MassIVE partner repository with the data set identifier MSV000082958.

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Quantitative proteomics of TGF-β-induced changes in ECM composition and collagen PTM in pulmonary fibroblasts

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TGF-β promotes crosslinking and turnover as well as complex feedback mechanisms that alter fibroblast ECM homeostasis.

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A novel bioinformatic workflow for MS data analysis enabled global mapping and quantitation of known and novel collagen PTMs

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Quantitative assessment of prolyl-3-hydroxylation site occupancy and lysine-O-glycosylation microheterogeneity

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TGF-β1 modulates collagen PTMs in a site-specific manner that may favor collagen accumulation in lung fibrosis

Identification of 4-Hydroxyproline at the Xaa Position in Collagen by Mass Spectrometry

Collagen has a triple helix form, structured by a [-Gly-Xaa-Yaa-] repetition, where Xaa and Yaa are amino acids. This repeating unit can be post-translationally modified by enzymes, where proline is often hydroxylated into hydroxyproline (Hyp). Two Hyp isomers occur in collagen: 4-hydroxyproline (4Hyp, Gly-Xaa-Pro, substrate for 4-prolyl hydroxylase) and 3-hydroxyproline (3Hyp, Gly-Pro-4Hyp, substrate for 3-prolyl hydroxylase). If 4Hyp is lacking at the Yaa position, then Pro at the Xaa position should remain unmodified. Nevertheless, in literature 41 positions have been described where Hyp occurs at the Xaa position (?xHyp) lacking an adjacent 4Hyp. We report four additional positions in liver and colorectal liver metastasis tissue (CRLM). We studied the sequence commonalities between the 45 known positions of ?xHyp. Alanine and glutamine were frequently present adjacent to ?xHyp. We showed that proline, position 584 in COL1A2, had a lower rate of modification in CRLM than in healthy liver. The isomeric identity of ?xHyp, that is, 3- and/or 4Hyp, remains unknown. We present a proof of principle identification of ?xHyp. This identification is based on liquid chromatography retention time differences and mass spectrometry using ETD-HCD fragmentation, complemented by ab initio calculations. Both techniques identify ?xHyp at position 584 in COL1A2 as 4-hydroxyproline (4xHyp).

Large-Scale Differentiation and Site Specific Discrimination of Hydroxyproline Isomers by Electron Transfer/Higher-Energy Collision Dissociation (EThcD) Mass Spectrometry

3- and 4-Hydroxyprolines (HyP) are regioisomers that play different roles in various species and organs. Despite their distinct functions inside cells, they are generally considered indistinguishable using mass spectrometry due to their identical masses. Here, we demonstrate, for the first time, that characteristic w ions can be produced by electron-transfer/higher energy collision dissociation (EThcD) dual fragmentation technique to confidently discriminate 3-HyP/4-HyP isomers. An integrated and high throughput strategy was developed which combined online LC separation with EThcD for large-scale differentiation of 3-HyP/4-HyP in complex samples. An automated algorithm was developed for charge state dependent characterization of 3-HyP/4-HyP isomers. Using this combined discrimination approach, we identified 108 3-HyP sites and 530 4-HyP sites from decellularized pancreas, allowing more than 5-fold increase of both 3-HyP and 4-HyP identifications compared to previous reports. This approach outperformed ETD and HCD in the analysis of HyP-containing peptides with unique capacity to generate w ions for HyP discrimination, improved fragmentation of precursor ions, as well as unambiguous localization of modifications. A high content of 3-HyP was observed in the C-terminal (GPP)n domain of human CO1A1, which was previously only identified in vertebrate fibrillar collagens from tendon. Unexpectedly, some unusual HyP sites at Xaa position in Gly-HyP-Ala, Gly-HyP-Val, Gly-HyP-Gln, Gly-HyP-Ser, and Gly-HyP-Arg were also confirmed to be 3-hydroxylated, whose functions and enzymes are yet to be discovered. Overall, this novel discrimination strategy can be readily implemented into de novo sequencing or other proteomic search engines.

The triple helix of collagens - an ancient protein structure that enabled animal multicellularity and tissue evolution

The cellular microenvironment, characterized by an extracellular matrix (ECM), played an essential role in the transition from unicellularity to multicellularity in animals (metazoans), and in the subsequent evolution of diverse animal tissues and organs. A major ECM component are members of the collagen superfamily -comprising 28 types in vertebrates - that exist in diverse supramolecular assemblies ranging from networks to fibrils. Each assembly is characterized by a hallmark feature, a protein structure called a triple helix. A current gap in knowledge is understanding the mechanisms of how the triple helix encodes and utilizes information in building scaffolds on the outside of cells. Type IV collagen, recently revealed as the evolutionarily most ancient member of the collagen superfamily, serves as an archetype for a fresh view of fundamental structural features of a triple helix that underlie the diversity of biological activities of collagens. In this Opinion, we argue that the triple helix is a protein structure of fundamental importance in building the extracellular matrix, which enabled animal multicellularity and tissue evolution.

Distinct post-translational features of type I collagen are conserved in mouse and human periodontal ligament

BACKGROUND AND OBJECTIVE

Specifics of the biochemical pathways that modulate collagen cross-links in the periodontal ligament (PDL) are not fully defined. Better knowledge of the collagen post-translational modifications that give PDL its distinct tissue properties is needed to understand the pathogenic mechanisms of human PDL destruction in periodontal disease. In this study, the post-translational phenotypes of human and mouse PDL type I collagen were surveyed using mass spectrometry. PDL is a highly specialized connective tissue that joins tooth cementum to alveolar bone. The main function of the PDL is to support the tooth within the alveolar bone while under occlusal load after tooth eruption. Almost half of the adult population in the USA has periodontal disease resulting from inflammatory destruction of the PDL, leading to tooth loss. Interestingly, PDL is unique from other ligamentous connective tissues as it has a high rate of turnover. Rapid turnover is believed to be an important characteristic for this specialized ligament to function within the oral-microbial environment. Like other ligaments, PDL is composed predominantly of type I collagen. Collagen synthesis is a complex process with multiple steps and numerous post-translational modifications including hydroxylation, glycosylation and cross-linking. The chemistry, placement and quantity of intermolecular cross-links are believed to be important regulators of tissue-specific structural and mechanical properties of collagens.

MATERIAL AND METHODS

Type I collagen was isolated from several mouse and human tissues, including PDL, and analyzed by mass spectrometry for post-translational variances.

RESULTS

The collagen telopeptide cross-linking lysines of PDL were found to be partially hydroxylated in human and mouse, as well as in other types of ligament. However, the degree of hydroxylation and glycosylation at the helical Lys87 cross-linking residue varied across species and between ligaments. These data suggest that different types of ligament collagen, notably PDL, appear to have evolved distinctive lysine/hydroxylysine cross-linking variations. Another distinguishing feature of PDL collagen is that, unlike other ligaments, it lacks any of the known prolyl 3-hydroxylase 2-catalyzed 3-hydroxyproline site modifications that characterize tendon and ligament collagens. This gives PDL a novel modification profile, with hybrid features of both ligament and skin collagens.

CONCLUSION

This distinctive post-translational phenotype may be relevant for understanding why some individuals are at risk of rapid PDL destruction in periodontal disease and warrants further investigation. In addition, developing a murine model for studying PDL collagen may be useful for exploring potential clinical strategies for promoting PDL regeneration.

Genome-wide analysis identifies differential promoter methylation of Leprel2, Foxf1, Mmp25, Igfbp6, and Peg12 in murine tendinopathy

We have used a murine Achilles tendinopathy model to investigate whether tissue changes (such as collagen disorganization, chondroid metaplasia, and loss of tensile properties) which are broadly characteristic of human tendinopathies, are accompanied by changes in the expression of chromatin-modifying enzymes and the methylation status of promoter regions of tendon cell DNA. Tendinopathy was induced by two intra-tendinous TGF-β1 injections followed by cage activity or treadmill running for up to 28 days. Activation of DNA methyltransferases occurred at 3 days after the TGF-β1 injections and also at 14 days, but only with treadmill activity. Genome-wide Methyl Mini-Seq™ analysis identified 19 genes with differentially methylated promoters, five of which perform functions with an apparent direct relevance to tendinopathy (Leprel2, Foxf1, Mmp25, Igfbp6, and Peg12). The functions of the genes identified included collagen fiber assembly and pericellular interactions, therefore their perturbation could play a role in the characteristic disorganization of fibers in affected tendons. We postulate that a study of the functional genomics of these genes in animal and human tendon could further delineate the pathogenesis of this multi-factorial complex disease. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:947-955, 2017.

Type I Collagen Purification from Rat Tail Tendons

Type I collagen, or collagen I, is the most abundant protein in the human body and provides strength and resiliency to tissues such as bone, tendons, ligaments, and skin. Collagen I forms macromolecular networks in which resident mesenchymal cells are embedded. Cell-extracellular matrix interactions are critical not only for maintenance of tissue properties but also for guiding and orienting the phenotype of resident cells. Cues from the extracellular matrix have been shown to be critical in pathophysiologies such as fibrosis, aging, and cancer. Hence, the details of these interactions are being scrutinized to better understand the mechanisms of such diseases and conditions. Many in vitro assays, such as cell-embedded collagen lattices, preparation of hydrogels, adhesion assays, etc., have been developed to study various aspects of cell-extracellular matrix interactions. All these in vitro models rely on utilizing high-quality purified collagen I. Here, we provide state-of-the-art collagen I extraction protocol and useful tips to produce high-quality purified collagen I solutions. We also provide a detailed protocol for pepsin digestion of collagen I, for a highly reliable collagen concentration assay, and guidelines for conducting quality controls to validate purified collagen solutions. Collagen I prepared with these procedures is highly suitable for many in vitro applications.

Molecular insights into prolyl and lysyl hydroxylation of fibrillar collagens in health and disease

Collagen is a macromolecule that has versatile roles in physiology, ranging from structural support to mediating cell signaling. Formation of mature collagen fibrils out of procollagen α-chains requires a variety of enzymes and chaperones in a complex process spanning both intracellular and extracellular post-translational modifications. These processes include modifications of amino acids, folding of procollagen α-chains into a triple-helical configuration and subsequent stabilization, facilitation of transportation out of the cell, cleavage of propeptides, aggregation, cross-link formation, and finally the formation of mature fibrils. Disruption of any of the proteins involved in these biosynthesis steps potentially result in a variety of connective tissue diseases because of a destabilized extracellular matrix. In this review, we give a revised overview of the enzymes and chaperones currently known to be relevant to the conversion of lysine and proline into hydroxyproline and hydroxylysine, respectively, and the O-glycosylation of hydroxylysine and give insights into the consequences when these steps are disrupted.

The cleavage site preference of the porcine pepsin on the N-terminal α1 chain of bovine type I collagen: a focal analysis with mass spectrometry

Bovine type I collagen consists of two α1 and one α2 chains, containing the internal triple helical regions and the N- and C-terminal telopeptides. In industries, it is frequently digested with porcine pepsin to produce a triple helical collagen without the telopeptides. However, the digestion mechanism is not precisely understood. Here, we performed a mass spectrometric analysis of the pepsin digest of the N-terminal telopeptide pQLSYGYDEKSTGISVP (1-16) in the α1 chain. When purified collagen was digested, pQLSYGY (1-6) and pQLSYGYDEKSTG (1-12) were identified, while DEKSTG (7-12) was not. When the N-terminal telopeptide mimetic synthetic peptide pQLSK(MOCAc)GYDEKSTGISK(Dnp)P-NH₂ was digested, pQLSK(MOCAc)GYDEKSTG (1-12) and ISK(Dnp)P-NH₂ (13-16) were readily identified, pQLSK(MOCAc)GY (1-6) and DEKSTGISK(Dnp)P-NH₂ (7-16) were weakly detected, and DEKSTG (7-12) was hardly identified. These results suggest that pepsin preferentially cleaves Tyr6-Asp7 and less preferentially Gly12-Ile13. They also suggest that the former cleavage requires native collagen structure, while the latter cleavage does not.

Collagen cross-linking: insights on the evolution of metazoan extracellular matrix

Collagens constitute a large family of extracellular matrix (ECM) proteins that play a fundamental role in supporting the structure of various tissues in multicellular animals. The mechanical strength of fibrillar collagens is highly dependent on the formation of covalent cross-links between individual fibrils, a process initiated by the enzymatic action of members of the lysyl oxidase (LOX) family. Fibrillar collagens are present in a wide variety of animals, therefore often being associated with metazoan evolution, where the emergence of an ancestral collagen chain has been proposed to lead to the formation of different clades. While LOX-generated collagen cross-linking metabolites have been detected in different metazoan families, there is limited information about when and how collagen acquired this particular modification. By analyzing telopeptide and helical sequences, we identified highly conserved, potential cross-linking sites throughout the metazoan tree of life. Based on this analysis, we propose that they have importantly contributed to the formation and further expansion of fibrillar collagens.

The supramolecular structure of bone: X-ray scattering analysis and lateral structure modeling

The evolution of vertebrates required a key development in supramolecular evolution: internally mineralized collagen fibrils. In bone, collagen molecules and mineral crystals form a nanocomposite material comparable to cast iron in tensile strength, but several times lighter and more flexible. Current understanding of the internal nanoscale structure of collagen fibrils, derived from studies of rat tail tendon (RTT), does not explain how nucleation and growth of mineral crystals can occur inside a collagen fibril. Experimental obstacles encountered in studying bone have prevented a solution to this problem for several decades. This report presents a lateral packing model for collagen molecules in bone fibrils, based on the unprecedented observation of multiple resolved equatorial reflections for bone tissue using synchrotron small-angle X-ray scattering (SAXS; ∼1 nm resolution). The deduced structure for pre-mineralized bone fibrils includes features that are not present in RTT: spatially discrete microfibrils. The data are consistent with bone microfibrils similar to pentagonal Smith microfibrils, but are not consistent with the (nondiscrete) quasi-hexagonal microfibrils reported for RTT. These results indicate that collagen fibrils in bone and tendon differ in their internal structure in a manner that allows bone fibrils, but not tendon fibrils, to internally mineralize. In addition, the unique pattern of collagen cross-link types and quantities in mineralized tissues can be can be accounted for, in structural/functional terms, based on a discrete microfibril model.

Osteogenesis imperfecta

Osteogenesis imperfecta is a phenotypically and molecularly heterogeneous group of inherited connective tissue disorders that share similar skeletal abnormalities causing bone fragility and deformity. Previously, the disorder was thought to be an autosomal dominant bone dysplasia caused by defects in type I collagen, but in the past 10 years discoveries of novel (mainly recessive) causative genes have lent support to a predominantly collagen-related pathophysiology and have contributed to an improved understanding of normal bone development. Defects in proteins with very different functions, ranging from structural to enzymatic and from intracellular transport to chaperones, have been described in patients with osteogenesis imperfecta. Knowledge of the specific molecular basis of each form of the disorder will advance clinical diagnosis and potentially stimulate targeted therapeutic approaches. In this Seminar, together with diagnosis, management, and treatment, we describe the defects causing osteogenesis imperfecta and their mechanism and interrelations, and classify them into five groups on the basis of the metabolic pathway compromised, specifically those related to collagen synthesis, structure, and processing; post-translational modification; folding and cross-linking; mineralisation; and osteoblast differentiation.

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.

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.

Increased C-telopeptide cross-linking of tendon type I collagen in fibromodulin-deficient mice

The controlled assembly of collagen monomers into fibrils, with accompanying intermolecular cross-linking by lysyl oxidase-mediated bonds, is vital to the structural and mechanical integrity of connective tissues. This process is influenced by collagen-associated proteins, including small leucine-rich proteins (SLRPs), but the regulatory mechanisms are not well understood. Deficiency in fibromodulin, an SLRP, causes abnormal collagen fibril ultrastructure and decreased mechanical strength in mouse tendons. In this study, fibromodulin deficiency rendered tendon collagen more resistant to nonproteolytic extraction. The collagen had an increased and altered cross-linking pattern at an early stage of fibril formation. Collagen extracts contained a higher proportion of stably cross-linked α1(I) chains as a result of their C-telopeptide lysines being more completely oxidized to aldehydes. The findings suggest that fibromodulin selectively affects the extent and pattern of lysyl oxidase-mediated collagen cross-linking by sterically hindering access of the enzyme to telopeptides, presumably through binding to the collagen. Such activity implies a broader role for SLRP family members in regulating collagen cross-linking placement and quantity.

Evolutionary origins of C-terminal (GPP)n 3-hydroxyproline formation in vertebrate tendon collagen

Approximately half the proline residues in fibrillar collagen are hydroxylated. The predominant form is 4-hydroxyproline, which helps fold and stabilize the triple helix. A minor form, 3-hydroxyproline, still has no clear function. Using peptide mass spectrometry, we recently revealed several previously unknown molecular sites of 3-hydroxyproline in fibrillar collagen chains. In fibril-forming A-clade collagen chains, four new partially occupied 3-hydroxyproline sites were found (A2, A3, A4 and (GPP)_n) in addition to the fully occupied A1 site at Pro986. The C-terminal (GPP)_n motif has five consecutive GPP triplets in α1(I), four in α2(I) and three in α1(II), all subject to 3-hydroxylation. The evolutionary origins of this substrate sequence were investigated by surveying the pattern of its 3-hydroxyproline occupancy from early chordates through amphibians, birds and mammals. Different tissue sources of type I collagen (tendon, bone and skin) and type II collagen (cartilage and notochord) were examined by mass spectrometry. The (GPP)_n domain was found to be a major substrate for 3-hydroxylation only in vertebrate fibrillar collagens. In higher vertebrates (mouse, bovine and human), up to five 3-hydroxyproline residues per (GPP)_n motif were found in α1(I) and four in α2(I), with an average of two residues per chain. In vertebrate type I collagen the modification exhibited clear tissue specificity, with 3-hydroxyproline prominent only in tendon. The occupancy also showed developmental changes in Achilles tendon, with increasing 3-hydroxyproline levels with age. The biological significance is unclear but the level of 3-hydroxylation at the (GPP)_n site appears to have increased as tendons evolved and shows both tendon type and developmental variations within a species.

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.

Solid-state NMR study reveals collagen I structural modifications of amino acid side chains upon fibrillogenesis

In vivo, collagen I, the major structural protein in human body, is found assembled into fibrils. In the present work, we study a high concentrated collagen sample in its soluble, fibrillar, and denatured states using one and two dimensional {(1)H}-(13)C solid-state NMR spectroscopy. We interpret (13)C chemical shift variations in terms of dihedral angle conformation changes. Our data show that fibrillogenesis increases the side chain and backbone structural complexity. Nevertheless, only three to five rotameric equilibria are found for each amino acid residue, indicating a relatively low structural heterogeneity of collagen upon fibrillogenesis. Using side chain statistical data, we calculate equilibrium constants for a great number of amino acid residues. Moreover, based on a (13)C quantitative spectrum, we estimate the percentage of residues implicated in each equilibrium. Our data indicate that fibril formation greatly affects hydroxyproline and proline prolyl pucker ring conformation. Finally, we discuss the implication of these structural data and propose a model in which the attractive force of fibrillogenesis comes from a structural reorganization of 10 to 15% of the amino acids. These results allow us to further understand the self-assembling process and fibrillar structure of collagen.

Collagen prolyl 3-hydroxylation: a major role for a minor post-translational modification?

Prolyl 3-hydroxylation is a rare but conserved post-translational modification in many collagen types and, when defective, may be linked to a number of human diseases with musculoskeletal and potentially ocular and renal pathologies. Prolyl 3-hydroxylase-1 (P3H1), the enzyme responsible for converting proline to 3-hydroxyproline (3Hyp) in type I collagen, requires the coenzyme CRTAP for activity. Mass spectrometric analysis showed that the Crtap-/- mouse was missing 3-hydroxyproline in type I collagen α-chains. This finding led to the discovery of mutations in genes encoding the P3H1 complex as a cause of recessively inherited osteogenesis imperfecta (brittle bone disease). Since then, many additional 3Hyp sites have been identified in various collagen types and classified based on observed substrate and tissue specificity. P3H1 is part of a family of gene products that also includes isoenzymes P3H2 and P3H3 as well as CRTAP and Sc65. It is believed these isoenzymes and coenzyme proteins have evolved different collagen substrate site and tissue specificities in their activities. The post-translational fingerprinting of collagens will be essential in understanding the basic role and extent of regulated variations of prolyl 3-hydroxylation in collagen. We believe that prolyl 3-hydroxylation is a functionally significant collagen post-translational modification and can be a cause of disease when absent.

Proteomic profiling of breast tissue collagens and site-specific characterization of hydroxyproline residues of collagen alpha-1-(I)

In a quantitative proteomics-based breast cancer study of complementary normal and tumor biopsies, 22 collagen isoforms were detected by LC-MALDI TOF/TOF MS. By applying proline oxidation, representing hydroxyproline, in database search parameters a substantial increase in assigned MS/MS was achieved, boosting the average (three experiments) number of peptides from 306 to 8126 for collagen alpha-1(I). The plethora of peptide identities for alpha-1(I) was disproportionate with full length protein sequence coverage which only increased from 28.3 to 64.4%. The peptides, in fact, constituted an extensive two-dimensional array of isomers exhibiting heterogeneity in degree and location of hydroxyproline residues. A total of 3433 peptides, scores>36 (p<0.01), constituting 94% of the triple helix region of collagen alpha-1(I) provided a census of proline hydroxylation levels defined as the rate of site occupancy for each peptide isomer (r) and the total site occupancy for each proline residue (t). MS/MS and MS/MS/MS analysis, by MALDI-QIT-TOF MS, was used to corroborate site-specific proline hydroxylation of the original data. In addition, iTRAQ data for each collagen isoform in each of 10 patients (grouped by disease) was determined and indicated an increase in fibrillar collagens in invasive carcinoma but little change in fibroadenoma or DCIS.

Absence of FKBP10 in recessive type XI osteogenesis imperfecta leads to diminished collagen cross-linking and reduced collagen deposition in extracellular matrix

Recessive osteogenesis imperfecta (OI) is caused by defects in genes whose products interact with type I collagen for modification and/or folding. We identified a Palestinian pedigree with moderate and lethal forms of recessive OI caused by mutations in FKBP10 or PPIB, which encode endoplasmic reticulum resident chaperone/isomerases FKBP65 and CyPB, respectively. In one pedigree branch, both parents carry a deletion in PPIB (c.563_566delACAG), causing lethal type IX OI in their two children. In another branch, a child with moderate type XI OI has a homozygous FKBP10 mutation (c.1271_1272delCCinsA). Proband FKBP10 transcripts are 4% of control and FKBP65 protein is absent from proband cells. Proband collagen electrophoresis reveals slight band broadening, compatible with ≈10% over-modification. Normal chain incorporation, helix folding, and collagen T(m) support a minimal general collagen chaperone role for FKBP65. However, there is a dramatic decrease in collagen deposited in culture despite normal collagen secretion. Mass spectrometry reveals absence of hydroxylation of the collagen telopeptide lysine involved in cross-linking, suggesting that FKBP65 is required for lysyl hydroxylase activity or access to type I collagen telopeptide lysines, perhaps through its function as a peptidylprolyl isomerase. Proband collagen to organics ratio in matrix is approximately 30% of normal in Raman spectra. Immunofluorescence shows sparse, disorganized collagen fibrils in proband matrix.

Comprehensive mass spectrometric mapping of the hydroxylated amino acid residues of the α1(V) collagen chain

BACKGROUND

α1(V) is an extensively modified collagen chain important in disease.

RESULTS

Comprehensive mapping of α1(V) post-translational modifications reveals unexpectedly large numbers of X-position hydroxyprolines in Gly-X-Y amino acid triplets.

CONCLUSION

The unexpected abundance of X-position hydroxyprolines suggests a mechanism for differential modification of collagen properties.

SIGNIFICANCE

Positions, numbers, and occupancy of modified sites can provide insights into α1(V) biological properties. Aberrant expression of the type V collagen α1(V) chain can underlie the connective tissue disorder classic Ehlers-Danlos syndrome, and autoimmune responses against the α1(V) chain are linked to lung transplant rejection and atherosclerosis. The α1(V) collagenous COL1 domain is thought to contain greater numbers of post-translational modifications (PTMs) than do similar domains of other fibrillar collagen chains, PTMs consisting of hydroxylated prolines and lysines, the latter of which can be glycosylated. These types of PTMs can contribute to epitopes that underlie immune responses against collagens, and the high level of PTMs may contribute to the unique biological properties of the α1(V) chain. Here we use high resolution mass spectrometry to map such PTMs in bovine placental α1(V) and human recombinant pro-α1(V) procollagen chains. Findings include the locations of those PTMs that vary and those PTMs that are invariant between these α1(V) chains from widely divergent sources. Notably, an unexpectedly large number of hydroxyproline residues were mapped to the X-positions of Gly-X-Y triplets, contrary to expectations based on previous amino acid analyses of hydrolyzed α1(V) chains from various tissues. We attribute this difference to the ability of tandem mass spectrometry coupled to nanoflow chromatographic separations to detect lower-level PTM combinations with superior sensitivity and specificity. The data are consistent with the presence of a relatively large number of 3-hydroxyproline sites with less than 100% occupancy, suggesting a previously unknown mechanism for the differential modification of α1(V) chain and type V collagen properties.

Biomimetic soluble collagen purified from bones

Type I collagen has been extensively exploited as a biomaterial for biomedical applications and drug delivery; however, small molecular alterations occurring during the isolation procedure and its interaction with residual bone extracellular matrix molecules or proteins might affect the overall material biocompatibility and performance. The aim of the current work is to study the potential alterations in collagen properties and organization associated with the absence of proteoglycans, which mimic pathological conditions associated with age-related diseases. A new approach for evaluating the effect of proteoglycans on the properties of isolated type I collagen from the bone matrix is described. Additional treatment with guanidine hydrochloride was introduced to remove residual proteoglycans from the collagen matrix. The properties of the isolated collagen with/without guanidine hydrochloride treatment were investigated and compared with a commercial rabbit collagen as control. We demonstrate that the absence of proteoglycans in the isolated type I collagen affects its thermal properties, the extraction into its native structure, and its ability to hydrate and self-assemble into fibers. The fine control and tuning of all these features, linked to the absence of non-collagenous proteins as proteoglycans, offer the possibility of designing new strategies and biomaterials with advanced biomimetic properties aimed at regenerating bone tissue in the case of fragility and/or defects.

Murine tendon function is adversely affected by aggrecan accumulation due to the knockout of ADAMTS5

The present study examined the effect of ADAMTS5 (TS5) knockout on the properties of murine flexor digitorum longus (FDL) and Achilles tendons. FDL and Achilles tendons were analyzed using biomechanical testing, histology, and immunohistochemistry; further characterization of FDL tendons was conducted using transmission electron microscopy (collagen fibril ultrastructure), SDS-PAGE (collagen content and type), fluorescence-assisted carbohydrate electrophoresis for chondroitin sulfate and hyaluronan, and Western blotting for aggrecan, versican, and decorin abundance and distribution. FDL tendons of TS5(-/-) mice showed a 33% larger cross-sectional area, increased collagen fibril area fraction, and decreased material properties relative to those of wild type mice. In TS5(-/-) mice, aggrecan accumulated in the pericellular matrix of tendon fibroblasts. In Achilles tendons, cross-sectional area, stress relaxation, and structural properties were similar in TS5(-/-) and wild type mice; however, the TS5(-/-) tendons exhibited a higher tensile modulus and a weakened enthesis. These results demonstrate that TS5 deficiency disturbs normal tendon collagen organization and alters biomechanical properties. Hence, the role of ADAMTS5 in tendon is to remove pericellular and interfibrillar aggrecan to maintain the molecular architecture responsible for normal tissue function.

Peptidyl 3-hydroxyproline binding properties of type I collagen suggest a function in fibril supramolecular assembly

Proline residues in collagens are extensively hydroxylated post-translationally. A rare form of this modification, (3S,2S)-l-hydroxyproline (3Hyp), remains without a clear function. Disruption of the enzyme complex responsible for prolyl 3-hydroxylation results in severe forms of recessive osteogenesis imperfecta (OI). These OI types exhibit a loss of or reduction in the level of 3-hydroxylation at two proline residues, α1(I) Pro986 and α2(I) Pro707. Whether the resulting brittle bone phenotype is caused by the lack of the 3-hydroxyl addition or by another function of the enzyme complex is unknown. We have speculated that the most efficient mechanism for explaining the chemistry of collagen intermolecular cross-linking is for pairs of collagen molecules in register to be the subunit that assembles into fibrils. In this concept, the exposed hydroxyls from 3Hyp are positioned within mutually interactive binding motifs on adjacent collagen molecules that contribute through hydrogen bonding to the process of fibril supramolecular assembly. Here we report observations on the physical binding properties of 3Hyp in collagen chains from experiments designed to explore the potential for interaction using synthetic collagen-like peptides containing 3Hyp. Evidence of self-association was observed between a synthetic peptide containing 3Hyp and the CB6 domain of the α1(I) chain, which contains the single fully 3-hydroxylated proline. Using collagen from a case of severe recessive OI with a CRTAP defect, in which Pro986 was minimally 3-hydroxylated, such binding was not observed. Further study of the role of 3Hyp in supramolecular assembly is warranted for understanding the evolution of tissue-specific variations in collagen fibril organization.

A role for prolyl 3-hydroxylase 2 in post-translational modification of fibril-forming collagens

The fibrillar collagen types I, II, and V/XI have recently been shown to have partially 3-hydroxylated proline (3Hyp) residues at sites other than the established primary Pro-986 site in the collagen triple helical domain. These sites showed tissue specificity in degree of hydroxylation and a pattern of D-periodic spacing. This suggested a contributory role in fibril supramolecular assembly. The sites in clade A fibrillar α1(II), α2(V), and α1(I) collagen chains share common features with known prolyl 3-hydroxylase 2 (P3H2) substrate sites in α1(IV) chains implying a role for this enzyme. We pursued this possibility using the Swarm rat chondrosarcoma cell line (RCS-LTC) found to express high levels of P3H2 mRNA. Mass spectrometry determined that all the additional candidate 3Hyp substrate sites in the pN type II collagen made by these cells were highly hydroxylated. In RNA interference experiments, P3H2 protein synthesis was suppressed coordinately with prolyl 3-hydroxylation at Pro-944, Pro-707, and the C-terminal GPP repeat of the pNα1(II) chain, but Pro-986 remained fully hydroxylated. Furthermore, when P3H2 expression was turned off, as seen naturally in cultured SAOS-2 osteosarcoma cells, full 3Hyp occupancy at Pro-986 in α1(I) chains was unaffected, whereas 3-hydroxylation of residue Pro-944 in the α2(V) chain was largely lost, and 3-hydroxylation of Pro-707 in α2(V) and α2(I) chains were sharply reduced. The results imply that P3H2 has preferred substrate sequences among the classes of 3Hyp sites in clade A collagen chains.