Characterization of human type X procollagen and its NC-1 domain expressed as recombinant proteins in HEK293 cells

Preparation and characterization of a novel humanized collagen III with repeated fragments of Gly300-Asp329

Recombinant human collagens have attracted intensive interest in the past two decades, demonstrating considerable potential in medicine, tissue engineering, and cosmetics. Several humanized recombinant collagens have been produced, exhibiting similar characteristics as the native species. To get insight into the structural and bioactive properties of different parts of collagen, in this study, the segment of Gly300-Asp329 of type III collagen was first adopted and repeated 18 times to prepare a novel recombinant collagen (named rhCLA). RhCLA was successfully expressed in E. coli, and a convenient separation procedure was established through reasonably combining alkaline precipitation and acid precipitation, yielding crude rhCLA with a purity exceeding 90%. Additionally, a polishing purification step utilizing cation exchange chromatography was developed, achieving rhCLA purity surpassing 98% and an overall recovery of approximately 120 mg/L culture. Simultaneously, the contents of endotoxin, nucleic acids, and host proteins were reduced to extremely low levels. This fragmented type III collagen displayed a triple-helical structure and gel-forming capability at low temperatures. Distinct fibrous morphology was also observed through TEM analysis. In cell experiments, rhCLA exhibited excellent biocompatibility and cell adhesion properties. These results provide valuable insights for functional studies of type III collagen and a reference approach for the large-scale production of recombinant collagens.

Expression of type VI collagen α3 chain in canine mammary carcinomas

This study aimed to investigate the expression of type VI collagen α3 chain (COL6a3) in neoplastic cells of canine mammary gland carcinomas (CMGCs) using immunohistochemistry (IHC) and to evaluate the association between COL6a3 expression and tumor histological features, histological grades, and the differentiation status of neoplastic epithelial cells. COL6a3 expression in carcinoma cells was significantly associated with histologically low malignancy and low mitotic indices. In addition, COL6a3+ carcinoma cells were more frequently detected in simple carcinomas (tubular and tubulopapillary types) than in solid carcinomas. These findings indicate that reduced expression of COL6a3 in carcinoma cells contributes to the malignant phenotype in CMGCs. We also showed that COL6a3 expression in the carcinoma cells was more frequently detected in CK19+/CD49f + and/or CK19+/CK5+ tumors. In addition, COL6a3+/CK19+/CD49f + and COL6a3+/CK19+/CK5+ tumors consisted of CK19+/CD49f + and CK19+/CD49f- cells, and CK19+/CK5+ and CK19+/CK5- cells, respectively. Most of these tumors more frequently expressed GATA3, but not Notch1. These results indicate that COL6a3 is expressed in CMGCs containing both luminal progenitor-like and mature luminal-like cells and showing differentiation ability into mature luminal cells. It is possible that COL6 may be involved in the differentiation of luminal progenitor-like carcinoma cells into mature luminal-like carcinoma cells in CMGCs, which may suppresses the development of malignant phenotypes in CMGCs.

Synthetic biology-guided design and biosynthesis of protein polymers for delivery

Vehicles derived from genetically engineered protein polymers have gained momentum in the field of biomedical engineering due to their unique designability, remarkable biocompatibility and excellent biodegradability. However, the design and production of these protein polymers with on-demand sequences and supramolecular architectures remain underexplored, particularly from a synthetic biology perspective. In this review, we summarize the state-of-the art strategies for constructing the highly repetitive genes encoding the protein polymers, and highlight the advanced approaches for metabolically engineering expression hosts towards high-level biosynthesis of the target protein polymers. Finally, we showcase the typical protein polymers utilized to fabricate delivery vehicles.

Expression of receptor-type tumour endothelial marker 8 in carcinoma cells showing luminal progenitor-like phenotypes in canine mammary gland carcinomas

This study aimed to investigate the expression of receptor-type tumour endothelial marker 8 (TEM8RT) in canine mammary gland carcinomas (CMGCs) using immunohistochemistry and to evaluate the association between carcinoma cell TEM8RT expression and tumour histological features, histological grades and the differentiation status of neoplastic epithelial cells. TEM8RT expression was more frequently detected in simple carcinomas (tubular and tubulopapillary) than in solid carcinomas, and it was significantly correlated with histological grade Ⅰ tumours and a low mitotic index. Additionally, TEM8RT⁺ carcinoma cells were more frequently found in CMGCs showing luminal progenitor-like phenotypes, such as Notch1⁺, CK19⁺/CK5⁺/CD49f⁺ and CK19⁺/CK5^-/CD49f⁺. Double-labelling immunofluorescence detection techniques confirmed that most TEM8RT⁺ carcinoma cells expressed CD49f, Notch1 and CK19. However, TEM8RT immunoreactivity was not found in carcinoma cells expressing GATA3, which upregulates mature luminal cell differentiation. Furthermore, TEM8RT⁺ carcinoma cells were detected in a few CMGCs showing basal/stem cell-like phenotypes such as CK19^-/CK5⁺/CD49f⁺ and CK19^-/CK5⁺/CD49f^-. These findings indicate that TEM8RT is expressed in luminal progenitor-like carcinoma cells in CMGCs. Since TEM8 enhances self-renewal in human mammary stem/progenitor cells, it also may be involved in maintenance of luminal progenitor-like carcinoma cells, resulting in prevention of their transition to basal/stem cell-like carcinoma cells and development of less malignant CMGCs. Therefore, TEM8RT may be useful for indicating prognostic outcomes and identifying the possible ontogeny of carcinoma cells in mammary gland tumours.

Collagen- and hyaluronic acid-based hydrogels and their biomedical applications

Hydrogels have been widely investigated in biomedical fields due to their similar physical and biochemical properties to the extracellular matrix (ECM). Collagen and hyaluronic acid (HA) are the main components of the ECM in many tissues. As a result, hydrogels prepared from collagen and HA hold inherent advantages in mimicking the structure and function of the native ECM. Numerous studies have focused on the development of collagen and HA hydrogels and their biomedical applications. In this extensive review, we provide a summary and analysis of the sources, features, and modifications of collagen and HA. Specifically, we highlight the fabrication, properties, and potential biomedical applications as well as promising commercialization of hydrogels based on these two natural polymers.

Enhanced Collagen-like Protein for Facile Biomaterial Fabrication

We present a collagen-mimetic protein of bacterial origin based upon a modified subdomain of the collagen-like Sc12 protein from Streptococcus pyogenes, as an alternative collagen-like biomaterial platform that is highly soluble, forms stable, homogeneous, fluid-like solutions at elevated concentrations, and that can be efficiently fabricated into hydrogel materials over a broad range of pH conditions. This extended bacterial collagen-like (eBCL) protein is expressed in a bacterial host and purified as a trimeric assembly exhibiting a triple helical secondary structure in its collagen-like subdomain that is stable near physiological solution conditions (neutral pH and 37 °C), as well as over a broad range of pH conditions. We also show how this sequence can be modified to include biofunctional attributes, in particular, the Arg-Gly-Asp (RGD) sequence to elicit integrin-specific cell binding, without loss of structural function. Furthermore, through the use of EDC-NHS chemistry, we demonstrate that members of this eBCL protein system can be covalently cross-linked to fabricate transparent hydrogels with high protein concentrations (at least to 20% w/w). These hydrogels are shown to possess material properties and resistance to enzymatic degradation that are comparable or superior to a type I collagen control. Moreover, such hydrogels containing the constructs with the RGD integrin-binding sequence are shown to promote the adhesion, spreading, and proliferation of C2C12 and 3T3 cells in vitro. Due to its enhanced solubility, structural stability, fluidity at elevated concentrations, ease of modification, and facility of cross-linking, this eBCL collagen-mimetic system has potential for numerous biomedical material applications, where the ease of processing and fabrication and the facility to tailor the sequence for specific biological functionality are desired.

Exploring the potential of the recombinant human collagens for biomedical and clinical applications: a short review

Natural animal collagen and its recombinant collagen are favourable replacements in human tissue engineering due to their remarkable biomedical property. However, this exploitation is largely restricted due to the potential of immunogenicity and virus contamination. Exploring new ways to produce human collagen is fundamental to its biomedical and clinical application. All human fibrillar collagen molecules have three polypeptide chains constructed from a repeating Gly-Xaa-Yaa triplet, where Xaa and Yaa represent one random amino acid. Using cDNA techniques to modify several repeat sequences of the cDNA fragment, a novel human collagen, named recombinant human-like collagen (rHLC), with low immunogenicity and little risk from hidden virus can be engineered and notably tailored to specific applications. Human-like collagen (HLC) was initially used as a coating to modify the tissue engineering scaffold, and then used as the scaffold after cross-link agents were added to increase its mechanical strength. Due to its good biocompatibility, low immunogenicity, stabilised property, and the ability of mass production, HLC has been widely used in skin injury treatments, vascular scaffolds engineering, cartilage, bone defect repair, skincare, haemostatic sponge, and drug delivery, including coating with medical nanoparticles. In this review, we symmetrically reviewed the development, recent advances in design and application of HLC, and other recombinant human collagen-based biomedicine potentials. At the end, future improvements are also discussed.

Advances in Plant-Derived Scaffold Proteins

Scaffold proteins form critical biomatrices that support cell adhesion and proliferation for regenerative medicine and drug screening. The increasing demand for such applications urges solutions for cost effective and sustainable supplies of hypoallergenic and biocompatible scaffold proteins. Here, we summarize recent efforts in obtaining plant-derived biosynthetic spider silk analogue and the extracellular matrix protein, collagen. Both proteins are composed of a large number of tandem block repeats, which makes production in bacterial hosts challenging. Furthermore, post-translational modification of collagen is essential for its function which requires co-transformation of multiple copies of human prolyl 4-hydroxylase. We discuss our perspectives on how the GAANTRY system could potentially assist the production of native-sized spider dragline silk proteins and prolyl hydroxylated collagen. The potential of recombinant scaffold proteins in drug delivery and drug discovery is also addressed.

Blood and urinary collagen markers in osteoarthritis: markers of tissue turnover and disease activity

Introduction: The need for diagnostic markers in osteoarthritis (OA) is acute and immediate, as sensitive and precise tools that monitor disease activity and treatment response are lacking. Collagens - types I, II, and III - are the skeleton of the extracellular matrix of joint tissues. Joint collagens are generally turned over at a low rate, but the balance between formation and degradation is disturbed, leading to the loss of, for example, cartilage.Areas covered: We discuss the markers reflecting collagen turnover and provide examples of how they have been applied in OA research, as well as how we believe these should be used in the future. We have searched PubMed for full-text articles written in English using different combinations of the following terms: OA, biomarker, and collagen. The result is a narrative review that gives examples from the literature.Expert opinion: Collagen markers show promise, as they are direct measures of tissue balance. Until now, collagen markers have mainly been tested in observational cohorts, which may provide insights into the association between the candidate marker and clinical variables; however, these do not advance the development of qualified markers that can be used for drug development or in clinical practice.

Expression of Tumour Endothelial Marker 8 in Canine Mammary Gland Tumour Cells

The aim of this study was to investigate the expression of tumour endothelial marker 8 (TEM8) in canine mammary gland tumours (MGTs) by immunohistochemistry and to evaluate the association between tumour cell TEM8 expression and tumour histological features, histological grades and expression of luminal and basal/myoepithelial cell markers. TEM8 expression was detected in >60 % of neoplastic epithelial cells in all simple adenomas (n = 25), simple carcinomas (n = 43) and invasive micropapillary carcinomas (n = 5) studied. Six of the 18 solid carcinomas studied showed TEM8 expression in >60% of carcinoma cells present in solid structures and in 12 of the 18 solid carcinomas, <30% of the luminal structure-forming carcinoma cells showed TEM8 expression. TEM8 expression in the neoplastic cells was not associated with histological malignancy in canine MGTs. TEM8⁺ tumour cells frequently showed the luminal-like phenotype cytokeratin (CK)19⁺/p63^-/α-smooth muscle actin (SMA)^-, while most TEM8^- tumour cells exhibited the basal-like phenotype CK19^-/p63⁺/αSMA^-. These findings indicate that TEM8 may be involved in maintaining the characteristics of luminal cells in canine MGTs and that TEM8 would be useful in identifying the type of neoplastic epithelial cell in MGTs.

The Collagen Suprafamily: From Biosynthesis to Advanced Biomaterial Development

Collagen is the oldest and most abundant extracellular matrix protein that has found many applications in food, cosmetic, pharmaceutical, and biomedical industries. First, an overview of the family of collagens and their respective structures, conformation, and biosynthesis is provided. The advances and shortfalls of various collagen preparations (e.g., mammalian/marine extracted collagen, cell-produced collagens, recombinant collagens, and collagen-like peptides) and crosslinking technologies (e.g., chemical, physical, and biological) are then critically discussed. Subsequently, an array of structural, thermal, mechanical, biochemical, and biological assays is examined, which are developed to analyze and characterize collagenous structures. Lastly, a comprehensive review is provided on how advances in engineering, chemistry, and biology have enabled the development of bioactive, 3D structures (e.g., tissue grafts, biomaterials, cell-assembled tissue equivalents) that closely imitate native supramolecular assemblies and have the capacity to deliver in a localized and sustained manner viable cell populations and/or bioactive/therapeutic molecules. Clearly, collagens have a long history in both evolution and biotechnology and continue to offer both challenges and exciting opportunities in regenerative medicine as nature's biomaterial of choice.

Alternative uses for co-products: Harnessing the potential of valuable compounds from meat processing chains

Opportunities for exploiting the inherent value of protein-rich meat processing co-products, in the context of increased global demand for protein and for sustainable processing systems, are discussed. While direct consumption maybe the most profitable route for some, this approach is influenced greatly by local and cultural traditions. A more profitable and sustainable approach may be found in recognizing this readily available and under-utilised resource can provide high value components, such as proteins, with targeted high value functionality of relevance to a variety of sectors. Applications in food & beverages, petfood biomedical and nutrition arenas are discussed. Utilization of the raw material in its entirety is a necessary underlying principle in this approach to help maintain minimum waste generation. Understanding consumer attitudes to these products, in particular when used in food or beverage systems, is critical in optimizing commercialization strategies.

The minor collagens in articular cartilage

Articular cartilage is a connective tissue consisting of a specialized extracellular matrix (ECM) that dominates the bulk of its wet and dry weight. Type II collagen and aggrecan are the main ECM proteins in cartilage. However, little attention has been paid to less abundant molecular components, especially minor collagens, including type IV, VI, IX, X, XI, XII, XIII, and XIV, etc. Although accounting for only a small fraction of the mature matrix, these minor collagens not only play essential structural roles in the mechanical properties, organization, and shape of articular cartilage, but also fulfil specific biological functions. Genetic studies of these minor collagens have revealed that they are associated with multiple connective tissue diseases, especially degenerative joint disease. The progressive destruction of cartilage involves the degradation of matrix constituents including these minor collagens. The generation and release of fragmented molecules could generate novel biochemical markers with the capacity to monitor disease progression, facilitate drug development and add to the existing toolbox for in vitro studies, preclinical research and clinical trials.

Bioengineered Collagens

There is a great deal of interest in obtaining recombinant collagen as an alternative source of material for biomedical applications and as an approach for obtaining basic structural and biological information. However, application of recombinant technology to collagen presents challenges, most notably the need for post-translational hydroxylation of prolines for triple-helix stability. Full length recombinant human collagens have been successfully expressed in cell lines, yeast, and several plant systems, while collagen fragments have been expressed in E. coli. In addition, bacterial collagen-like proteins can be expressed in high yields in E. coli and easily manipulated to incorporate biologically active sequences from human collagens. These expression systems allow manipulation of biologically active sequences within collagen, which has furthered our understanding of the relationships between collagen sequences, structure and function. Here, recombinant studies on collagen interactions with cell receptors, extracellular matrix proteins, and matrix metalloproteinases are reviewed, and discussed in terms of their potential biomaterial and biomedical applications.

In vitro calcite crystal morphology is modulated by otoconial proteins otolin-1 and otoconin-90

Otoconia are formed embryonically and are instrumental in detecting linear acceleration and gravity. Degeneration and fragmentation of otoconia in elderly patients leads to imbalance resulting in higher frequency of falls that are positively correlated with the incidence of bone fractures and death. In this work we investigate the roles otoconial proteins Otolin-1 and Otoconin 90 (OC90) perform in the formation of otoconia. We demonstrate by rotary shadowing and atomic force microscopy (AFM) experiments that Otolin-1 forms homomeric protein complexes and self-assembled networks supporting the hypothesis that Otolin-1 serves as a scaffold protein of otoconia. Our calcium carbonate crystal growth data demonstrate that Otolin-1 and OC90 modulate in vitro calcite crystal morphology but neither protein is sufficient to produce the shape of otoconia. Coadministration of these proteins produces synergistic effects on crystal morphology that contribute to morphology resembling otoconia.

The EMILIN/Multimerin family

Elastin microfibrillar interface proteins (EMILINs) and Multimerins (EMILIN1, EMILIN2, Multimerin1, and Multimerin2) constitute a four member family that in addition to the shared C-terminus gC1q domain typical of the gC1q/TNF superfamily members contain a N-terminus unique cysteine-rich EMI domain. These glycoproteins are homotrimeric and assemble into high molecular weight multimers. They are predominantly expressed in the extracellular matrix and contribute to several cellular functions in part associated with the gC1q domain and in part not yet assigned nor linked to other specific regions of the sequence. Among the latter is the control of arterial blood pressure, the inhibition of Bacillus anthracis cell cytotoxicity, the promotion of cell death, the proangiogenic function, and a role in platelet hemostasis. The focus of this review is to highlight the multiplicity of functions and domains of the EMILIN/Multimerin family with a particular emphasis on the regulatory role played by the ligand-receptor interactions of the gC1q domain. EMILIN1 is the most extensively studied member both from the structural and functional point of view. The structure of the gC1q of EMILIN1 solved by NMR highlights unique characteristics compared to other gC1q domains: it shows a marked decrease of the contact surface of the trimeric assembly and while conserving the jelly-roll topology with two β-sheets of antiparallel strands it presents a nine-stranded β-sandwich fold instead of the usual 10-stranded fold. This is likely due to the insertion of nine residues that disrupt the ordered strand organization and forma a highly dynamic protruding loop. In this loop the residue E933 is the site of interaction between gC1q and the α4β1 and α9β1 integrins, and contrary to integrin occupancy that usually upregulates cell growth, when gC1q is ligated by the integrin the cells reduce their proliferative activity.

Natural and Genetically Engineered Proteins for Tissue Engineering

To overcome the limitations of traditionally used autografts, allografts and, to a lesser extent, synthetic materials, there is the need to develop a new generation of scaffolds with adequate mechanical and structural support, control of cell attachment, migration, proliferation and differentiation and with bio-resorbable features. This suite of properties would allow the body to heal itself at the same rate as implant degradation. Genetic engineering offers a route to this level of control of biomaterial systems. The possibility of expressing biological components in nature and to modify or bioengineer them further, offers a path towards multifunctional biomaterial systems. This includes opportunities to generate new protein sequences, new self-assembling peptides or fusions of different bioactive domains or protein motifs. New protein sequences with tunable properties can be generated that can be used as new biomaterials. In this review we address some of the most frequently used proteins for tissue engineering and biomedical applications and describe the techniques most commonly used to functionalize protein-based biomaterials by combining them with bioactive molecules to enhance biological performance. We also highlight the use of genetic engineering, for protein heterologous expression and the synthesis of new protein-based biopolymers, focusing the advantages of these functionalized biopolymers when compared with their counterparts extracted directly from nature and modified by techniques such as physical adsorption or chemical modification.

The N-terminal extension is essential for the formation of the active dimeric structure of liver peroxisomal alanine:glyoxylate aminotransferase

Alanine:glyoxylate aminotransferase (AGT) is a pyridoxal-phosphate (PLP)-dependent enzyme. Its deficiency causes the hereditary kidney stone disease primary hyperoxaluria type 1. AGT is a highly stable compact dimer and the first 21 residues of each subunit form an extension which wraps over the surface of the neighboring subunit. Naturally occurring and artificial amino acid replacements in this extension create changes in the functional properties of AGT in mammalian cells, including relocation of the enzyme from peroxisomes to mitochondria. In order to elucidate the structural and functional role of this N-terminal extension, we have analyzed the consequences of its removal using a variety of biochemical and cell biological methods. When expressed in Escherichia coli, the N-terminal deleted form of AGT showed the presence of the protein but in an insoluble form resulting in only a 10% soluble yield as compared to the full-length version. The purified soluble fraction showed reduced affinity for PLP and greatly reduced catalytic activity. Although maintaining a dimer form, it was highly prone to self-aggregation. When expressed in a mammalian cell line, the truncated construct was normally targeted to peroxisomes, where it formed large stable but catalytically inactive aggregates. These results suggest that the N-terminal extension plays an essential role in allowing AGT to attain its correct conformation and functional activity. The precise mechanism of this effect is still under investigation.

Matrix recruitment and calcium sequestration for spatial specific otoconia development

Otoconia are bio-crystals anchored to the macular sensory epithelium of the utricle and saccule in the inner ear for motion sensing and bodily balance. Otoconia dislocation, degeneration and ectopic calcification can have detrimental effects on balance and vertigo/dizziness, yet the mechanism underlying otoconia formation is not fully understood. In this study, we show that selected matrix components are recruited to form the crystal matrix and sequester Ca(2+) for spatial specific formation of otoconia. Specifically, otoconin-90 (Oc90) binds otolin through both domains (TH and C1q) of otolin, but full-length otolin shows the strongest interaction. These proteins have much higher expression levels in the utricle and saccule than other inner ear epithelial tissues in mice. In vivo, the presence of Oc90 in wildtype (wt) mice leads to an enrichment of Ca(2+) in the luminal matrices of the utricle and saccule, whereas absence of Oc90 in the null mice leads to drastically reduced matrix-Ca(2+). In vitro, either Oc90 or otolin can increase the propensity of extracellular matrix to calcify in cell culture, and co-expression has a synergistic effect on calcification. Molecular modeling and sequence analysis predict structural features that may underlie the interaction and Ca(2+)-sequestering ability of these proteins. Together, the data provide a mechanism for the otoconial matrix assembly and the role of this matrix in accumulating micro-environmental Ca(2+) for efficient CaCO(3) crystallization, thus uncover a critical process governing spatial specific otoconia formation.

Matrix biochemistry and cell biology of dedifferentiated chondrosarcomas

Dedifferentiated chondrosarcoma is an uncommon mesenchymal neoplasm comprised of two different components, low-grade conventional chondrosarcoma and high-grade non-cartilaginous sarcoma. In order to gain better insight into the biology of this tumor, we investigated a large series of dedifferentiated chondrosarcomas by looking at the composition of the extracellular tumor matrix within each of the distinct histological components. Our results showed that the well-differentiated portion of the tumors showed matrix components largely similar to conventional chondrosarcomas or enchondromas. In contrast, the high-grade portions showed a variety of staining patterns related to the matrix being formed. Cartilage-specific proteoglycans and collagens were consistently absent, except in areas showing a chondroblastic osteosarcoma histomorphology. Instead, the most dominant immunostaining was received for type I collagen. Type III and VI collagens were concentrated in the areas showing a fibroblastic phenotype. Our results lend further support to the notion that dedifferentiated chondrosarcoma represents transdifferentiation of a cell towards various blastic mesenchymal cell lineages, most commonly osteoblastic and fibroblastic, but occasionally chondroblastic as well. There was no difference in the clinical outcome of patients with differing high-grade tumor types, emphasizing that grade is a more important predictor of biological behavior than the direction of tumor differentiation.

The collagen V homotrimer [alpha1(V)](3) production is unexpectedly favored over the heterotrimer [alpha1(V)](2)alpha2(V) in recombinant expression systems

Collagen V, a fibrillar collagen with important functions in tissues, assembles into distinct chain associations. The most abundant and ubiquitous molecular form is the heterotrimer [α1(V)]₂α2(V). In the attempt to produce high levels of recombinant collagen V heterotrimer for biomedical device uses, and to identify key factors that drive heterotrimeric chain association, several cell expression systems (yeast, insect, and mammalian cells) have been assayed by cotransfecting the human proα1(V) and proα2(V) chain cDNAs. Suprisingly, in all recombinant expression systems, the formation of [α1(V)]₃ homotrimers was considerably favored over the heterotrimer. In addition, pepsin-sensitive proα2(V) chains were found in HEK-293 cell media indicating that these cells lack quality control proteins preventing collagen monomer secretion. Additional transfection with Hsp47 cDNA, encoding the collagen-specific chaperone Hsp47, did not increase heterotrimer production. Double immunofluorescence with antibodies against collagen V α-chains showed that, contrary to fibroblasts, collagen V α-chains did not colocalized intracellularly in transfected cells. Monensin treatment had no effect on the heterotrimer production. The heterotrimer production seems to require specific machinery proteins, which are not endogenously expressed in the expression systems. The different constructs and transfected cells we have generated represent useful tools to further investigate the mechanisms of collagen trimer assembly.

Demonstration of the interaction of transforming growth factor beta 2 and type X collagen using a modified tandem affinity purification tag

Like other members of the transforming growth factor beta (TGF-beta) family of growth factors, the biological activity of TGF-beta2 is believed to be regulated by the formation and dissociation of multiprotein complexes. To isolate the molecular complex formed by TGF-beta2 secreted by hypertrophic chondrocytes we have used expression of TGF-beta2 fused with the humanized, tandem affinity purification (hTAP) tag and mass spectrometry for the identification of interacting proteins. The hTAP synthetic gene was assembled by systematically replacing the rare codons of the original TAP tag with codons most preferred in highly expressed human genes to circumvent the poor translation efficiency of the original TAP tag in animal cells. TGF-beta2 was shown to interact with Type X collagen and this interaction confirmed using V5 tagged TGF-beta2. Functional interaction was suggested by the inhibition of TGF-beta2 activity by type X collagen in culture and the influence of a mutation in type X collagen on the distribution of TGF-beta2 in growth cartilage.

The discoidin domain receptor DDR2 is a receptor for type X collagen

During endochondral ossification, collagen X is deposited in the hypertrophic zone of the growth plate. Our previous results have shown that collagen X is capable of interacting directly with chondrocytes, primarily via integrin alpha2beta1. In this study, we determined whether collagen X could also interact with the non-integrin collagen receptors, discoidin domain receptors (DDRs), DDR1 or DDR2. The widely expressed DDRs are receptor tyrosine kinases that are activated by a number of different collagen types. Collagen X was found to be a much better ligand for DDR2 than for DDR1. Collagen X bound to the DDR2 extracellular domain with high affinity and stimulated DDR2 autophosphorylation, the first step in transmembrane signalling. Expression of DDR2 in the epiphyseal plate was confirmed by RT-PCR and immunohistochemistry. The spatial expression of DDR2 in the hypertrophic zone of the growth plate is consistent with a physiological interaction of DDR2 with collagen X. Surprisingly, the discoidin domain of DDR2, which fully contains the binding sites for the fibrillar collagens I and II, was not sufficient for collagen X binding. The nature of the DDR2 binding site(s) within collagen X was further analysed. In addition to a collagenous domain, collagen X contains a C-terminal NC1 domain. DDR2 was found to recognise the triple-helical region of collagen X as well as the NC1 domain. Binding to the collagenous region was dependent on the triple-helical conformation. DDR2 autophosphorylation was induced by the collagen X triple-helical region but not the NC1 domain, indicating that the triple-helical region of collagen X contains a specific DDR2 binding site that is capable of receptor activation. Our study is the first to describe a non-fibrillar collagen ligand for DDR2 and will form the basis for further studies into the biological function of collagen X during endochondral ossification.

Misfolding of collagen X chains harboring Schmid metaphyseal chondrodysplasia mutations results in aberrant disulfide bond formation, intracellular retention, and activation of the unfolded protein response

Collagen X is a short chain collagen expressed specifically by the hypertrophic chondrocytes of the cartilage growth plate during endochondral bone formation. Accordingly, COL10A1 mutations disrupt growth plate function and cause Schmid metaphyseal chondrodysplasia (SMCD). SMCD mutations are almost exclusively located in the NC1 domain, which is crucial for both trimer formation and extracellular assembly. Several mutations are expected to reduce the level of functional collagen X due to NC1 domain misfolding or exclusion from stable trimer formation. However, other mutations may be tolerated within the structure of the assembled NC1 trimer, allowing mutant chains to exert a dominant-negative impact within the extracellular matrix. To address this, we engineered SMCD mutations that are predicted either to prohibit subunit folding and assembly (NC1del10 and Y598D, respectively) or to allow trimerization (N617K and G618V) and transfected these constructs into 293-EBNA and SaOS-2 cells. Although expected to form stable trimers, G618V and N617K chains (like Y598D and NC1del10 chains) were secreted very poorly compared with wild-type collagen X. Interestingly, all mutations resulted in formation of an unusual SDS-stable dimer, which dissociated upon reduction. As the NC1 domain sulfhydryl group is not solvent-exposed in the correctly folded NC1 monomer, disulfide bond formation would result only from a dramatic conformational change. In cells expressing mutant collagen X, we detected significantly increased amounts of the spliced form of X-box DNA-binding protein mRNA and up-regulation of BiP, two key markers for the unfolded protein response. Our data provide the first clear evidence for misfolding of SMCD collagen X mutants, and we propose that solvent exposure of the NC1 thiol may trigger the recognition and degradation of mutant collagen X chains.

Distribution of Type VI Collagen in the Cartilaginous Tissue of the Proximal Tibia in the Domestic Cat

To investigate the distribution of the early stage chondrocytes during the formation and closure of epiphyseal growth plate (EGP) of the domestic cat, we examined the EGP of proximal tibiae by immunohistochemistry for type VI collagen. In the epiphyseal cartilage without the secondary ossification center (SOC) and EGP in newborn cats aged 1 and 10 days, type VI collagen-positive chondrocytes were located around the cartilage canals and articular surface. In the epiphyseal cartilage with the SOC and EGP in young cats aged 1 to 3 months, type VI collagen-positive chondrocytes were located in the upper resting zone of the EGP, and then increased throughout the resting zone along with maturation. In the adult cats with the partially closed EGP, type VI collagen-positive chondrocytes were distributed throughout the remaining EGP. These findings indicate that the early stage chondrocytes characterized with type VI collagen are continuously located in the EGP during maturation. In addition, the increase of the early stage chondrocytes and the decrease of the reserve chondrocytes in the EGP along with maturation may cause the cessation of the longitudinal growth of the EGP, and finally bring about the EGP closure.

Expression and Supramolecular Assembly of Recombinant α1(VIII) and α2(VIII) Collagen Homotrimers

Collagen VIII is an extracellular matrix macromolecule comprising two polypeptide chains, alpha1(VIII) and alpha2(VIII), that can form homotrimers in vitro and in vivo. Here, recombinant collagen VIII was expressed to study its supramolecular assembly following secretion. Cells transfected with alpha1(VIII) or alpha2(VIII) assembled and secreted homotrimers that were stable in denaturing conditions and had a molecular mass of approximately 180 kDa on SDS-PAGE gels. Co-transfection with prolyl 4-hydroxylase generated homotrimers with stable pepsin-resistant triple-helical domains. Size fractionation of native recombinant collagen VIII molecules expressed with or without prolyl 4-hydroxylase identified urea-sensitive high molecular mass assemblies eluting in the void volume of a Superose 6HR 10/30 column and urea-resistant assemblies of approximately 700 kDa, all of which were composed of homotrimers. Immunofluorescence analysis highlighted the extracellular deposition of recombinant alpha1(VIII)(3), alpha2(VIII)(3), and co-expressed alpha1(VIII)(3)/alpha2(VIII)(3). Microscopy analysis of recombinant collagen VIII identified rod-like molecules of 134 nm in length that assembled into angular arrays with branching angles of approximately 114 degrees and extensive networks. Based on these data, we propose a model of collagen VIII assembly in which four homotrimers form a tetrahedron stabilized by central interacting C-terminal NC1 trimers. Tetrahedrons may then act as building blocks of three-dimensional hexagonal lattices generated by secondary interactions involving terminal and helical sequences.

Interaction between amino propeptides of type XI procollagen alpha1 chains

Type XI collagen is a quantitatively minor yet essential constituent of the cartilage extracellular matrix. The amino propeptide of the alpha1 chain remains attached to the rest of the molecule for a longer period of time after synthesis than the other amino propeptides of type XI collagen and has been localized to the surface of thin collagen fibrils. Yeast two-hybrid system was used to demonstrate that a homodimer of alpha1(XI) amino propeptide (alpha1(XI)Npp) could form in vivo. Interaction was also confirmed using multi-angle laser light scattering, detecting an absolute weight average molar mass ranging from the size of a monomer to the size of a dimer (25,000-50,000 g/mol), respectively. Binding was shown to be saturable by ELISA. An interaction between recombinant alpha1(XI)Npp and the endogenous alpha1(XI)Npp was observed, and specificity for alpha1(XI)Npp but not alpha2(XI)Npp was demonstrated by co-precipitation. The interaction between the recombinant form of alpha1(XI)Npp and the endogenous alpha1(XI)Npp resulted in a stable association during the regeneration of cartilage extracellular matrix by fetal bovine chondrocytes maintained in pellet culture, generating a protein that migrated with an apparent molecular mass of 50-60 kDa on an SDS-polyacrylamide gel.

Type XIX collagen purified from human umbilical cord is characterized by multiple sharp kinks delineating collagenous subdomains and by intermolecular aggregates via globular, disulfide-linked, and heparin-binding amino termini

Type XIX collagen was discovered from the sequence of rhabdomyosarcoma cDNA clones. The chain is composed of a 268-residue amino terminus, an 832-residue discontinuous collagenous region, and a 19-residue carboxyl peptide. Light microscopy immunohistochemistry of adult human tissues demonstrated that type XIX is localized in vascular, neuronal, mesenchymal, and some epithelial basement membrane zones. It also appears to be involved in events linked to skeletal myogenesis. In this report, we have presented the first direct evidence for the molecular structure of type XIX collagen. Using human umbilical cord, native type XIX was purified by neutral salt extraction and by ion exchange and antibody affinity chromatography. Type XIX was found to represent only approximately 10(-6)% of the dry weight of tissue, making it by far the least abundant collagen ever isolated. Transmission electron microscopy after rotary shadowing revealed the appearance of rodlike structures with multiple sharp bends, a small nodule at one end of the molecule, and a total length of 240 nm. Domain-specific antibodies were used to identify the nodule as the noncollagenous amino terminus, whereas the location of most kinks corresponds to major interruptions separating the five collagenous subdomains. More than half of the type XIX molecules observed were present in oligomers of different size and complexity, resulting from association of the amino-terminal domains. Biochemical analysis demonstrated that these supramolecular aggregates are dependent upon and/or stabilized by intermolecular disulfide cross-links and that the globular amino terminus contains a high affinity, heparin-binding site. The polymorphic conformational states of this rare collagen, and its ability to self-assemble into a higher order structure provide focal points for future determination of biologically significant functions in cell-cell and/or cell-matrix interactions.

Characterization of recombinant soluble macrophage scavenger receptor MARCO

MARCO is a type II transmembrane protein of the class A scavenger receptor family. It has a short N-terminal cytoplasmic domain, a transmembrane domain, and a large extracellular part composed of a 75-residue long spacer domain, a 270-residue collagenous domain, and a 99-residue long scavenger receptor cysteine-rich (SRCR) domain. Previous studies have indicated a role for this receptor in anti-microbial host defense functions. In this work we have produced the extracellular part of MARCO as a recombinant protein, and analyzed its binding properties. The production of this protein, soluble MARCO (sMARCO), has made it possible for the first time to study MARCO and its binding properties in a cell-free system. Using circular dichroism analyses, a protease-sensitive assay, and rotary shadowing electron microscopy, sMARCO was shown to have a triple-helical collagenous structure. Rotary shadowing also demonstrated that the molecules often associate with each other via the globes. sMARCO was found to bind avidly both heat-killed and living bacteria. Lipopolysaccharide, an important component of the outer membrane of Gram-negative bacteria, was shown to be a ligand of MARCO. Studies with different bacterial strains indicated that the O-side chain of lipopolysaccharide is not needed for the bacterial recognition. Finally, the C-terminal SRCR domain was also produced as a recombinant protein, and its bacteria-binding capability was studied. Although the transfection experiments with transmembrane MARCO variants have indicated a crucial role for this domain in bacterial binding, the monomeric domain exhibited low, barely detectable bacteria-binding activity. Thus, it is possible that cooperation between the SRCR domain and the collagenous domain is needed for high-affinity bacterial binding, or that the SRCR domain has to be in a trimeric form to effectively bind to bacteria.

Insight into Schmid metaphyseal chondrodysplasia from the crystal structure of the collagen X NC1 domain trimer

Collagen X is expressed specifically in the growth plate of long bones. Its C1q-like C-terminal NC1 domain forms a stable homotrimer and is crucial for collagen X assembly. Mutations in the NC1 domain cause Schmid metaphyseal chondrodysplasia (SMCD). The crystal structure at 2.0 A resolution of the human collagen X NC1 domain reveals an intimate trimeric assembly strengthened by a buried cluster of calcium ions. Three strips of exposed aromatic residues on the surface of NC1 trimer are likely to be involved in the supramolecular assembly of collagen X. Most internal SMCD mutations probably prevent protein folding, whereas mutations of surface residues may affect the collagen X suprastructure in a dominant-negative manner.

The amino-terminal heptad repeats of the coiled-coil neck domain of pulmonary surfactant protein d are necessary for the assembly of trimeric subunits and dodecamers

Pulmonary surfactant protein D (SP-D), a lung host defense protein, is assembled as multimers of trimeric subunits. Trimerization of SP-D monomers is required for high affinity saccharide binding, and the oligomerization of trimers is required for many of its functions. A peptide containing the alpha-helical neck region can spontaneously trimerize in vitro. However, it is not known whether this sequence is necessary for the complete cellular assembly of disulfide-cross-linked, trimeric subunits and dodecamers. For the present studies, we synthesized mutant cDNAs with deletions or site-directed substitutions in the neck domain of rat SP-D, and examined the assembly of the newly synthesized proteins after transfection of CHO-K1 cells. The neck domain contains three "classical" heptad repeat motifs with leucine residues at the "d position," and a distinctive C-terminal repeat previously suggested to drive trimeric chain association. Deletion of the highly conserved core of the latter repeat (FSRYLKK) did not interfere with the secretion of dodecamers with lectin activity. By contrast, deletion of the entire neck domain or deletion of one or two amino-terminal repeats resulted in defective molecular assembly. The secreted proteins eluted in the position of monomers by gel filtration under nondenaturing conditions. In addition, the neck + carbohydrate recognition domain of SP-D was necessary and sufficient for the trimerization of a heterologous collagen sequence located amino-terminal to the trimeric coiled-coil. These studies provide strong evidence that the amino-terminal heptad repeats of the neck domain are necessary for the intracellular, trimeric association of SP-D monomers and for the assembly and secretion of functional dodecamers.

Isolation and characterization of EMILIN-2, a new component of the growing EMILINs family and a member of the EMI domain-containing superfamily

EMILIN (elastin microfibril interfase located Protein) is an elastic fiber-associated glycoprotein consisting of a self-interacting globular C1q domain at the C terminus, a short collagenous stalk, an extended region of potential coiled-coil structure, and an N-terminal cysteine-rich domain (EMI domain). Using the globular C1q domain as a bait in the yeast two-hybrid system, we have isolated a cDNA encoding a novel protein. Determination of the entire primary structure demonstrated that this EMILIN-binding polypeptide is highly homologous to EMILIN. The domain organization is superimposable, one important difference being a proline-rich (41%) segment of 56 residues between the potential coiled-coil region and the collagenous domain absent in EMILIN. The entire gene (localized on chromosome 18p11.3) was isolated from a BAC clone, and it is structurally almost identical to that of EMILIN (8 exons, 7 introns with identical phases at the exon/intron boundaries) but much larger (about 40 versus 8 kilobases) than that of EMILIN. Given these findings we propose to name the novel protein EMILIN-2 and the prototype member of this family EMILIN-1 (formerly EMILIN). The mRNA expression of EMILIN-2 is more restricted compared with that of EMILIN-1; highest levels are present in fetal heart and adult lung, whereas, differently from EMILIN-1, adult aorta, small intestine, and appendix show very low expression, and adult uterus and fetal kidney are negative. Finally, the EMILIN-2 protein is secreted extracellularly by in vitro-grown cells, and in accordance with the partial coexpression in fetal and adult tissues, the two proteins shown extensive but not absolute immunocolocalization in vitro.

Conformational requirements of collagenous peptides for recognition by the chaperone protein HSP47

The collagen binding chaperone HSP47 interacts with procollagen in the endoplasmic reticulum and plays a crucial role in the biosynthesis of collagen. We recently demonstrated that typical collagen model peptides, (Pro-Pro-Gly)(n), possess sufficient structural information for interaction with HSP47 (Koide, T., Asada, S., and Nagata, K. (1999) J. Biol. Chem. 274, 34523-34526). Here we show that binding of (Gly-Pro-Pro)(n) peptides to HSP47 can be detected using the two-hybrid system in yeast if a trimerizing domain is fused to the C termini of the peptides. Some peptides interacted with HSP47 at a lowered assay temperature at 24 degrees C but not at 30 degrees C, indicating the importance of conformational change of the substrate peptides. To analyze the spectrum of HSP47 substrate sequences, we performed two-hybrid screening of collagen-like peptides in designed random peptide libraries using HSP47 as a bait. In selected peptides, the enrichment ratio calculated for each amino acid residue correlated strongly with the contribution of the residue to triple-helix stability independently determined using synthetic collagen model peptides. Taken together, our results suggest that HSP47 preferentially recognizes collagenous Gly-X-Y repeats in triple-helical conformation. We also demonstrated that screening of combinatorial peptide libraries is a powerful strategy to determine conformational requirements as well as the elucidation of binding motifs in primary structure.

Self-assembly and supramolecular organization of EMILIN

The primary structure of human Elastin microfibril interface-located protein (EMILIN), an elastic fiber-associated glycoprotein, consists of a globular C1q domain (gC1q) at the C terminus, a short collagenous stalk, a long region with a high potential for forming coiled-coil alpha helices, and a cysteine-rich N-terminal sequence. It is not known whether the EMILIN gC1q domain is involved in the assembly process and in the supramolecular organization as shown for the similar domain of collagen X. By employing the yeast two-hybrid system the EMILIN gC1q domains interacted with themselves, proving for the first time that this interaction occurs in vivo. The gC1q domain formed oligomers running as trimers in native gels that were less stable than the comparable trimers of the collagen X gC1q domain since they did not withstand heating. The collagenous domain was trypsin-resistant and migrated at a size corresponding to a triple helix under native conditions. In reducing agarose gels, EMILIN also migrated as a trimer, whereas under non-reducing conditions it formed polymers of many millions of daltons. A truncated fragment lacking gC1q and collagenous domains assembled to a much lesser extent, thus deducing that the C-terminal domain(s) are essential for the formation of trimers that finally assemble into large EMILIN multimers.

The EMILIN protein family

The EMILINs are a new family of glycoproteins of the extracellular matrix. The prototype of this family is the chicken EMILIN that was originally identified in extracts of aortas; it was then found to be widely distributed in several tissues associated with elastin and localized at the interface between amorphous elastin and microfibrils. Based on peptide sequences, chicken and human cDNAs coding for EMILIN were isolated by RT/PCR by screening kidney and heart cDNA libraries. By using a C-terminal fragment of human EMILIN-1 as a bait in the yeast two-hybrid system, a second family member, EMILIN-2, has also been isolated. EMILINs are characterized by a C-terminal gC1q globular domain, a short collagenous sequence, a long coiled-coil region and a new cysteine-rich N-terminal domain that can be considered a hallmark of the family being present also in multimerin. The gene for EMILIN-1 was mapped on chromosome 2p23 overlapping with the promoter region of the ketohexokinase gene. The gC1q domain of EMILIN-1 can form relatively stable and compact homotrimers and this association is then followed by a multimeric assembly of disulfide-bonded protomers. Recombinant EMILIN-1 purified from the supernatant of 293 cells represents a very efficient ligand for cell adhesion of several cell types.

Schmid's metaphyseal chondrodysplasia mutations interfere with folding of the C-terminal domain of human collagen X expressed in Escherichia coli

Human collagen X contains a highly conserved 161-amino acid C-terminal non-triple helical domain that is homologous to the C-terminal domain of collagen VIII and to the C1q module of the human C1 enzyme. We have expressed this domain (residues 545-680) in Escherichia coli as a glutathione S-transferase fusion protein. The purified fusion protein trimerizes spontaneously in vitro, and after thrombin cleavage, the purified C-terminal domain trimer (46.2 kDa) is extremely stable and trypsin-resistant. Mutations within the C-terminal domain have been observed in patients with Schmid's metaphyseal chondrodysplasia (SMCD). Some of these mutations (Y598D, G618V, W651X, or H669X; X is the stop codon) were constructed by site-directed mutagenesis. Each mutation had identical consequences regarding the fusion protein: 1) absence of trimeric formation, 2) copurification of the approximately 60-kDa GroEL chaperone protein, and 3) sensitivity of the monomeric fusion protein to trypsin digestion. These results show that the C-terminal domain of collagen X is sufficient to produce a very stable and compact trimer in the absence of collagen Gly-X-Y repeats. Moreover, mutations causing SMCD interfere in this system with the correct folding of the C-terminal domain. The existence of a similar mechanism in chondrocytes might explain the relative homogeneity of phenotypes in SMCD despite the diversity of mutations.

EMILIN, a component of the elastic fiber and a new member of the C1q/tumor necrosis factor superfamily of proteins

EMILIN (elastin microfibril interface located protein) is an extracellular matrix glycoprotein abundantly expressed in elastin-rich tissues such as blood vessels, skin, heart, and lung. It occurs associated with elastic fibers at the interface between amorphous elastin and microfibrils. Avian EMILIN was extracted from 19-day-old embryonic chick aortas and associated blood vessels and purified by ion-exchange chromatography and gel filtration. Tryptic peptides were generated from EMILIN and sequenced, and degenerate inosine-containing oligonucleotide primers were designed from some peptides. A set of primers allowed the amplification of a 360-base pair reverse transcription polymerase chain reaction product from chick aorta mRNA. A probe based on a human homologue selected by comparison of the chick sequence with EST data base was used to select overlapping clones from both human aorta and kidney cDNA libraries. Here we present the cDNA sequence of the entire coding region of human EMILIN encompassing an open reading frame of 1016 amino acid residues. There was a high degree of homology (76% identity and 88% similarity) between the chick C terminus and the human sequence as well as between the N terminus of the mature chick protein where 10 of 12 residues, as determined by N-terminal sequencing, were identical or similar to the deduced N terminus of human EMILIN. The domain organization of human EMILIN includes a C1q-like globular domain at the C terminus, a collagenous stalk, and a longer segment in which at least four heptad repeats and a leucine zipper can be identified with a high potential for forming coiled-coil alpha helices. At the N terminus there is a cysteine-rich sequence stretch similar to a region of multimerin, a platelet and endothelial cell component, containing a partial epidermal growth factor-like motif. The native state of the recombinantly expressed EMILIN C1q-like domain to be used in cell adhesion was determined by CD spectra analysis, which indicated a high value of beta-sheet conformation. The EMILIN C1q-like domain promoted a high cell adhesion of the leiomyosarcoma cell line SK-UT-1, whereas the fibrosarcoma cell line HT1080 was negative.

Goodpasture antigen: expression of the full-length alpha3(IV) chain of collagen IV and localization of epitopes exclusively to the noncollagenous domain

BACKGROUND

Tissue injury in Goodpasture (GP) syndrome (rapidly progressive glomerular nephritis and pulmonary hemorrhage) is mediated by antibasement membrane antibodies that are targeted to the alpha3(IV) chain of type IV collagen, one of five alpha(IV) chains that occur in the glomerular basement membrane. GP antibodies are known to bind epitopes within the carboxyl terminal noncollagenous domain (NC1) of the alpha3(IV) chain, termed the GP autoantigen. Whether epitopes also exist in the 1400-residue collagenous domain is unknown because studies to date have focused solely on the NC1 domain. A knowledge of GP epitopes is important for the understanding of the etiology and pathogenesis of the disease and for the development of therapeutic strategies.

METHODS

A cDNA construct was prepared for the full-length human alpha3(IV) chain. The construct was stably transfected into human embryonic kidney 293 cells. The purified full-length r-alpha3(IV) chain was characterized by electrophoresis and electron microscopy. The capacity of this chain for binding of GP antibodies from five patients was compared with that of the human r-alpha3(IV)NC1 domain by competitive enzyme-linked immunosorbent assay.

RESULTS

The r-alpha3(IV) chain was secreted from 293 cells as a single polypeptide chain that did not spontaneously undergo assembly into a triple-helical molecule. An analysis of GP-antibody binding to the full-length r-alpha3(IV) chain showed binding exclusively to the globular NC1 domain.

CONCLUSION

The full-length human alpha3(IV) chain possesses the capacity to bind GP autoantibodies. The epitope(s) is found exclusively on the nontriple-helical NC1 domain of the alpha3(IV) chain, indicating the presence of specific immunogenic properties. The alpha3(IV) chain alone does not spontaneously undergo assembly into a triple-helical homotrimeric molecule, suggesting that coassembly with either the alpha4(IV) and/or the alpha5(IV) chain may be required for triple-helix formation.

Metaphyseal chondrodysplasia type Schmid mutations are predicted to occur in two distinct three-dimensional clusters within type X collagen NC1 domains that retain the ability to trimerize

Metaphyseal chondrodysplasia type Schmid (MCDS) is caused by mutations in COL10A1 that are clustered in the carboxyl-terminal non-collagenous (NC1) encoding domain. This domain is responsible for initiating trimerization of type X collagen during biosynthesis. We have built a molecular model of the NC1 domain trimer based on the crystal structure coordinates of the highly homologous trimeric domain of ACRP30 (adipocyte complement-related protein of 30 kDa or AdipoQ). Mapping of the MCDS mutations onto the structure reveals two specific clusters of residues as follows: one on the surface of the monomer which forms a tunnel through the center of the assembled trimer and the other on a patch exposed to solvent on the exterior surface of each monomeric unit within the assembled trimer. Biochemical studies on recombinant trimeric NC1 domain show that the trimer has an unusually high stability not exhibited by the closely related ACRP30. The high thermal stability of the trimeric NC1 domain, in comparison with ACRP30, appears to be the result of a number of factors including the 17% greater total buried solvent-accessible surface and the increased numbers of hydrophobic contacts formed upon trimerization. The 27 amino acid sequence present at the amino terminus of the NC1 domain, which has no counterpart in ACRP30, also contributes to the stability of the trimer. We have also shown that NC1 domains containing the MCDS mutations Y598D and S600P retain the ability to homotrimerize and heterotrimerize with wild type NC1 domain, although the trimeric complexes formed are less stable than those of the wild type molecule. These studies suggest strongly that the predominant mechanism causing MCDS involves a dominant interference of mutant chains on wild type chain assembly.

The alpha1(VIII) and alpha2(VIII) chains of type VIII collagen can form stable homotrimeric molecules

Type VIII collagen is a short chain collagen. Two chains have been described, alpha1(VIII) and alpha2(VIII), but the chain composition of type VIII collagen is far from resolved. To address this question, we have expressed full-length alpha1(VIII) and alpha2(VIII) chains in an in vitro translation system supplemented with semipermeabilized cells. Both chains gave a translation product of approximately 80 kDa that could be shown to produce a chymotrypsin/trypsin-resistant product of approximately 60 kDa, indicating that both chains could form homotrimers. Hydroxylation of proline residues was a prerequisite for stable trimer formation. The melting temperature for the alpha1(VIII) homotrimer was 45 degreesC, whereas that for alpha2(VIII) was 42 degreesC. The ability of both chains of type VIII collagen to form stable triple helices suggests that there may be different forms of this collagen and that cells may modulate the chain composition in response to different biological conditions.