Thermostability Gradient in the Collagen Triple Helix Reveals its Multi-domain Structure

Waste-to-resource: Extraction and transformation of aquatic biomaterials for regenerative medicine

The fisheries and aquaculture industry are known for generating substantial waste or by-products, often underutilized, or relegated to low-value purposes. However, this overlooked segment harbors a rich repository of valuable bioactive materials of which have a broad-spectrum of high-value applications. As the blue economy gains momentum and fisheries expand, sustainable exploitation of these aquatic resources is increasingly prioritized. In this review, we present a comprehensive overview of technology-enabled methods for extracting and transforming aquatic waste into valuable biomaterials and their recent advances in regenerative medicine applications, focusing on marine collagen, chitin/chitosan, calcium phosphate and bioactive-peptides. We discuss the inherent bioactive qualities of these "waste-to-resource" aquatic biomaterials and identify opportunities for their use in regenerative medicine to advance healthcare while achieving the Sustainable Development Goals.

Human cartilage model of the precocious osteoarthritis-inducing COL2A1 p.Arg719Cys reveals pathology-driving matrix defects and a failure of the ER proteostasis network to recognize the defective procollagen-II

Objectives

Mutations in the procollagen-II gene (COL2A1) often cause chondrodysplasias, including the precocious osteoarthritis-inducing p.Arg719Cys. Understanding the molecular basis of such diseases has long been challenging, owing to a lack of models accurately reflecting disease genotypes and phenotypes. To address this challenge, we develop and characterize in vitro human cartilage derived from wild-type and disease-causing Arg719Cys COL2A1 isogenic induced pluripotent stem cell (iPSC) lines.

Methods

Using directed differentiation of iPSCs to chondrocytes, we generated cartilage from wild-type and Arg719Cys COL2A1 lines. We compared the resulting protein, cell, and tissue properties using immunohistochemistry, electron microscopy, SDS-PAGE, RNA-sequencing, and quantitative interactomics.

Results

While both wild-type and disease lines deposited a cartilage matrix, the Arg719Cys matrix was deficient. Arg719Cys collagen-II was excessively post-translationally modified and modestly intracellularly retained, leading to endoplasmic reticulum (ER) distention suggestive of an ER storage defect. Interactomic studies indicated that Arg719Cys procollagen-II was not differentially engaged by the ER proteostasis network. RNA-sequencing showed that the ER storage defect engendered by Arg719Cys procollagen-II also did not activate cellular stress responses, including the unfolded protein response. These data suggest that cells fail to properly recognize Arg719Cys-associated procollagen-II defects.

Conclusions

A failure to identify and rectify defective procollagen-II folding in cells expressing Arg719Cys procollagen-II leads to the deposition of a sparse and defective collagen-II matrix, culminating in pathology. Combined with the highly expandable human cartilage disease model reported here, this work provides motivation and a platform to discover therapeutic strategies targeting procollagen folding, quality control, and secretion in this collagenopathy and others.

Integrated analysis of COL2A1 variant data and classification of type II collagenopathies

The COL2A1 gene encodes the alpha-1 chain of type II procollagen. Type II collagen, comprised of three identical alpha-1 chains, is the major component of cartilage. COL2A1 gene variants are the etiologies of genetic diseases, termed type II collagenopathies, with a wide spectrum of clinical presentations. To date, at least 460 distinct COL2A1 mutations, identified in 663 independent probands, and 21 definite disorders have been reported. Nevertheless, a well-defined genotype-phenotype correlation has not been established, and few hot spots of mutation have been reported. In this study, we analyzed data of COL2A1 variants and clinical information of patients obtained from the Leiden Open Variation Database 3.0, as well as the currently available relevant literature. We determined the characteristics of the COL2A1 variants and distributions of the clinical manifestations in patients, and identified four likely genotype-phenotype correlations. Moreover, we classified 21 COL2A1-related disorders into five categories, which may assist clinicians in understanding the essence of these complex phenotypes and prompt genetic screening in clinical practice.

Impact of Arginine to Cysteine Mutations in Collagen II on Protein Secretion and Cell Survival

Inherited point mutations in collagen II in humans affecting mainly cartilage are broadly classified as chondrodysplasias. Most mutations occur in the glycine (Gly) of the Gly-X-Y repeats leading to destabilization of the triple helix. Arginine to cysteine substitutions that occur at either the X or Y position within the Gly-X-Y cause different phenotypes like Stickler syndrome and congenital spondyloepiphyseal dysplasia (SEDC). We investigated the consequences of arginine to cysteine substitutions (X or Y position within the Gly-X-Y) towards the N and C terminus of the triple helix. Protein expression and its secretion trafficking were analyzed. Substitutions R75C, R134C and R704C did not alter the thermal stability with respect to wild type; R740C and R789C proteins displayed significantly reduced melting temperatures (T_m) affecting thermal stability. Additionally, R740C and R789C were susceptible to proteases; in cell culture, R789C protein was further cleaved by matrix metalloproteinases (MMPs) resulting in expression of only a truncated fragment affecting its secretion and intracellular retention. Retention of misfolded R740C and R789C proteins triggered an ER stress response leading to apoptosis of the expressing cells. Arginine to cysteine mutations towards the C-terminus of the triple helix had a deleterious effect, whereas mutations towards the N-terminus of the triple helix (R75C and R134C) and R704C had less impact.

A novel de novo mutation in COL2A1 leading to spondyloepiphyseal dysplasia congenita in a Chinese family

Spondyloepiphyseal dysplasia congenita (SEDC) is an extremely rare autosomal dominant chondrodysplasia that is usually caused by substitution of glycine with another amino acid in the triple helical region of COL2A1. Herein, we describe a case of SEDC in a Chinese family with a novel de novo mutation in the COL2A1 gene, c.1150G>A (p.Gly384Ser), which may impair protein stability and lead to dysfunction of type II collagen.

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.

Identification of an Electrostatic Ruler Motif for Sequence-Specific Binding of Collagenase to Collagen

Sequence-specific cleavage of collagen by mammalian collagenase plays a pivotal role in cell function. Collagenases are matrix metalloproteinases that cleave the peptide bond at a specific position on fibrillar collagen. The collagenase Hemopexin-like (HPX) domain has been proposed to be responsible for substrate recognition, but the mechanism by which collagenases identify the cleavage site on fibrillar collagen is not clearly understood. In this study, Brownian dynamics simulations coupled with atomic-detail and coarse-grained molecular dynamics simulations were performed to dock matrix metalloproteinase-1 (MMP-1) on a collagen IIIα1 triple helical peptide. We find that the HPX domain recognizes the collagen triple helix at a conserved R-X11-R motif C-terminal to the cleavage site to which the HPX domain of collagen is guided electrostatically. The binding of the HPX domain between the two arginine residues is energetically stabilized by hydrophobic contacts with collagen. From the simulations and analysis of the sequences and structural flexibility of collagen and collagenase, a mechanistic scheme by which MMP-1 can recognize and bind collagen for proteolysis is proposed.

Development and characterization of a eukaryotic expression system for human type II procollagen

Background

Triple helical collagens are the most abundant structural protein in vertebrates and are widely used as biomaterials for a variety of applications including drug delivery and cellular and tissue engineering. In these applications, the mechanics of this hierarchically structured protein play a key role, as does its chemical composition. To facilitate investigation into how gene mutations of collagen lead to disease as well as the rational development of tunable mechanical and chemical properties of this full-length protein, production of recombinant expressed protein is required.

Results

Here, we present a human type II procollagen expression system that produces full-length procollagen utilizing a previously characterized human fibrosarcoma cell line for production. The system exploits a non-covalently linked fluorescence readout for gene expression to facilitate screening of cell lines. Biochemical and biophysical characterization of the secreted, purified protein are used to demonstrate the proper formation and function of the protein. Assays to demonstrate fidelity include proteolytic digestion, mass spectrometric sequence and posttranslational composition analysis, circular dichroism spectroscopy, single-molecule stretching with optical tweezers, atomic-force microscopy imaging of fibril assembly, and transmission electron microscopy imaging of self-assembled fibrils.

Conclusions

Using a mammalian expression system, we produced full-length recombinant human type II procollagen. The integrity of the collagen preparation was verified by various structural and degradation assays. This system provides a platform from which to explore new directions in collagen manipulation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-015-0228-7) contains supplementary material, which is available to authorized users.

Structural and mechanical differences between collagen homo- and heterotrimers: relevance for the molecular origin of brittle bone disease

Collagen constitutes one-third of the human proteome, providing mechanical stability, elasticity, and strength to organisms. Normal type I collagen is a heterotrimer triple-helical molecule consisting of two α-1 chains and one α-2 chain. The homotrimeric isoform of type I collagen, which consists of three α-1 chains, is only found in fetal tissues, fibrosis, and cancer in humans. A mouse model of the genetic brittle bone disease, osteogenesis imperfect, oim, is characterized by a replacement of the α-2 chain by an α-1 chain, resulting also in a homotrimer collagen molecule. Experimental studies of oim mice tendon and bone have shown reduced mechanical strength compared to normal mice. The relationship between the molecular content and the decrease in strength is, however, still unknown. Here, fully atomistic simulations of a section of mouse type I heterotrimer and homotrimer collagen molecules are developed to explore the effect of the substitution of the α-2 chain. We calculate the persistence length and carry out a detailed analysis of the structure to determine differences in structural and mechanical behavior between hetero- and homotrimers. The results show that homotrimer persistence length is half of that of the heterotrimer (96 Å vs. 215 Å), indicating it is more flexible and confirmed by direct mechanical testing. Our structural analyses reveal that in contrast to the heterotrimer, the homotrimer easily forms kinks and freely rotates with angles much larger than heterotrimer. These local kinks may explain the larger lateral distance between collagen molecules seen in the fibrils of oim mice tendon and could have implications for reducing the intermolecular cross-linking, which is known to reduce the mechanical strength.

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.

Association of a p.Pro786Leu variant in COL2A1 with mild spondyloepiphyseal dysplasia congenita in a three-generation family

Heterozygous sequence variants of the COL2A1 gene cause a phenotypic spectrum collectively called type II collagenopathies. Here, we describe a COL2A1 sequence variant, c.2957C>T, p.Pro986Leu in the triple helical domain, which is a Y-position substitution in exon 41 of the repeating triplet sequence Gly-X-Y of the proα1(II) chain. This sequence variant was associated with a mild spondyloepiphyseal dysplasia phenotype in three individuals in a three-generation family. On clinical examination at the age of 19 months, the proband had a flat face, bifid uvula, and a protruding abdomen. Radiographically, he had rhizomelia, mesomelia, and ovoid-shaped vertebrae. He also had absent mineralization of the epiphyses, the os pubis, tali, and calcanei. His mother had myopia, mild lumbar lordosis, and mild coxa vara. She had a detached retina repaired at age 24 years. The maternal grandmother had cataracts but has had no kyphoscoliosis or lordosis. All three had disproportionate short stature. None had arthritis or hearing loss. The sequence variant in this family is the only reported Y-position proline substitution in the triple helical domain (Gly-X-Y) of the proα1(II) coded by the COL2A1 gene.

Persistence of intracellular and extracellular changes after incompletely suppressing expression of the R789C (p.R989C) and R992C (p.R1192C) collagen II mutants

Mutations in COL2A1 produce a spectrum of disorders whose hallmark feature is alterations in skeletal development. Attempts to counteract the effects of collagen mutations at the molecular level have been relatively ineffective due to the inability to selectively suppress a mutant allele, and failure to deliver a sufficient number of cells expressing wild-type collagen. Moreover, these approaches are hampered because the minimal therapeutic conditions that would allow extracellular matrix remodeling and recovery of cells from stress are not known. Here, we employed a tetracycline-inducible system for expressing the R789C or R992C collagen II mutants, allowing us to decrease the production of mutant proteins by 25, 50, 75, or 100% with respect to their initial production. Through analysis of intracellular and extracellular parameters we have shown that affected cell/matrix systems are able to recover from mutation-induced aberrations only when 100% expression of mutant collagens is shut off, but not if the expression of small amounts of mutant molecules persists in the system. Our data suggest that efficient remodeling of tissues affected by the presence of thermolabile collagen mutants may depend on their complete elimination rather than on partial reduction.

Dissecting a bacterial collagen domain from Streptococcus pyogenes: sequence and length-dependent variations in triple helix stability and folding

To better investigate the relationship between sequence, stability, and folding, the Streptococcus pyogenes collagenous domain CL (Gly-Xaa-Yaa)(79) was divided to create three recombinant triple helix subdomains A, B, and C of almost equal size with distinctive amino acid features: an A domain high in polar residues, a B domain containing the highest concentration of Pro residues, and a very highly charged C domain. Each segment was expressed as a monomer, a linear dimer, and a linear trimer fused with the trimerization domain (V domain) in Escherichia coli. All recombinant proteins studied formed stable triple helical structures, but the stability varied depending on the amino acid sequence in the A, B, and C segments and increased as the triple helix got longer. V-AAA was found to melt at a much lower temperature (31.0 °C) than V-ABC (V-CL), whereas V-BBB melted at almost the same temperature (∼36-37 °C). When heat-denatured, the V domain enhanced refolding for all of the constructs; however, the folding rate was affected by their amino acid sequences and became reduced for longer constructs. The folding rates of all the other constructs were lower than that of the natural V-ABC protein. Amino acid substitution mutations at all Pro residues in the C fragment dramatically decreased stability but increased the folding rate. These results indicate that the thermostability of the bacterial collagen is dominated by the most stable domain in the same manner as found with eukaryotic collagens.

Reducing the effects of intracellular accumulation of thermolabile collagen II mutants by increasing their thermostability in cell culture conditions

Mutations in collagen II are associated with spondyloepiphyseal dysplasia, a group of heritable diseases whose common features include aberrations of skeletal growth. The mechanisms through which mutations in collagen II affect the cartilaginous tissues are complex and include both intracellular and extracellular processes. One of those mechanisms involves cellular stress caused by excessive accumulation of misfolded collagen II mutants. We investigated whether stabilizing the structure of thermolabile R789C and R992C collagen II mutants would improve their secretion from cells, thereby reducing cellular stress and apoptosis. Employing glycerol and trimethylamine N-oxide (TMAO), chemicals that increase the thermostability of collagen triple helices, we demonstrated that those compounds function as chaperones and stabilize the R789C and R992C mutants, accelerate their secretion, and improve cell survival. Our study provides a scientific basis for considering misfolded triple helices of collagen mutants a target for reducing the deleterious effects caused by their excessive intracellular accumulation.

R992C (p.R1192C) Substitution in collagen II alters the structure of mutant molecules and induces the unfolded protein response

We investigated the molecular bases of spondyloepiphyseal dysplasia (SED) associated with the R992C (p.R1192C) substitution in collagen II. At the protein level, we analyzed the structure and integrity of mutant molecules, and at the cellular level, we specifically studied the effects of the presence of the R992C collagen II on the biological processes taking place in host cells. Our studies demonstrated that mutant collagen II molecules were characterized by altered electrophoretic mobility, relatively low thermostability, the presence of atypical disulfide bonds, and slow rates of secretion into the extracellular space. Analyses of cellular responses to the presence of the mutant molecules showed that excessive accumulation of thermolabile collagen II was associated with the activation of an "unfolded protein response" and an increase in apoptosis of host cells. Collectively, these data suggest that molecular mechanisms of SED may be driven not only by structural changes in the architecture of extracellular collagenous matrices, but also by intracellular processes activated by the presence of mutant collagen II molecules.

Two novel COL1A1 mutations in patients with osteogenesis imperfecta (OI) affect the stability of the collagen type I triple-helix

Osteogenesis imperfecta (OI) is a bone dysplasia caused by mutations in the COL1A1 and COL1A2 genes. Although the condition has been intensely studied for over 25 years and recently over 800 novel mutations have been published, the relation between the location of mutations and clinical manifestation is poorly understood. Here we report missense mutations in COL1A1 of several OI patients. Two novel mutations were found in the D1 period. One caused a substitution of glycine 200 by valine at the N-terminus of D1 in OI type I/IV, lowering collagen stability by 50% at 34 degrees C. The other one was a substitution of valine 349 by phenylalanine at the C-terminus of D1 in OI type I, lowering collagen stability at 37.5 degrees C. Two other mutations, reported before, changed amino residues in D4. One was a lethal substitution changing glycine 866 to serine in genetically identical twins with OI type II. That mutated amino acid was near the border of D3 and D4. The second mutation changed glycine 1040 to serine located at the border of D4 and D0.4, in a proband manifesting OI type III, and lowered collagen stability at 39 degrees C (2 degrees C lower than normal). Our results confirm the hypothesis on a critical role of the D1 and D4 regions in stabilization of the collagen triple-helix. The defect in D1 seemed to produce a milder clinical type of OI, whereas the defect in the C-terminal end of collagen type caused the more severe or lethal types of OI.

Extracellular matrix: from atomic resolution to ultrastructure

The extracellular matrix (ECM) is a highly organized multimolecular structure, essential for life in higher organisms. Although substantial high-resolution structural information is available for relatively small fragments of ECM components, the inherent difficulty in preparing and analyzing samples of large, fibrous polymers impedes structural efforts. Here, we review recent advances in understanding the structure of three important ECM components: collagen, fibrillin and fibronectin. Emphasis is placed on the key role of intermolecular interactions in assembling larger, microm scale, structures.

Czech dysplasia metatarsal type: another type II collagen disorder

Czech dysplasia metatarsal type is an autosomal-dominant disorder characterized by an early-onset, progressive spondyloarthropathy with normal stature. Shortness of third and/or fourth toes is a frequently observed clinical feature. Similarities between individuals with this dysplasia and patients with an R275C mutation in the COL2A1 gene, prompted us to analyze the COL2A1 gene in the original families reported with Czech dysplasia. Targeted sequencing of exon 13 of the COL2A1 gene was performed, followed by sequencing of the remaining exons in case the R275C mutation was not identified. We identified the R275C substitution in two of the original patients reported with Czech dysplasia and three additional patients. All affected individuals had a similar phenotype characterized by normal height, spondyloarthropathy, short postaxial toes and absence of ocular and orofacial anomalies. The R275C mutation was excluded in a third patient reported with Czech dysplasia. However, the identification of the Y1391C mutation in this patient with disproportionate short stature made the diagnosis of spondyloperipheral dysplasia (SPD) more probable. The Y1391C mutation is located in the C-propeptide of the procollagen chain and has been reported before in a patient with the Torrance type of lethal platyspondylic skeletal dysplasia (PLSD-T). Our observation of the same Y1391C mutation in an additional unrelated patient with SPD further supports the evidence that PLSD-T and SPD represent a phenotypic continuum. The R275C mutation in the COL2A1 gene causes a specific type II collagen disorder that was recently delineated as Czech dysplasia.

Characterization of the mechanisms by which gelatinase A, neutrophil collagenase, and membrane-type metalloproteinase MMP-14 recognize collagen I and enzymatically process the two alpha-chains

The turnover of native collagen has been ascribed to different members of the matrix metalloproteinase (MMP) family. Here, the mechanisms by which neutrophil collagenase (MMP-8), gelatinase A (MMP-2), and the ectodomain of MT1-MMP (ectMMP-14) degrade fibrillar collagen were examined. In particular, the hydrolysis of type I collagen at 37 degrees C was investigated to identify functional differences in the processing of the two alpha-chain types of fibrillar collagen. Thermodynamic and kinetic parameters were used for a quantitative comparison of the binding, unwinding, and hydrolysis of triple helical collagen. We demonstrate that the MMP family has developed at least two distinct mechanisms for collagen unwinding and cleavage. MMP-8 and ectMMP-14 display a similar mechanism (although with different catalytic parameters), which is characterized by binding (likely through the hemopexin-like domain) and cleavage of alpha-1 and/or alpha-2 chains without distinguishing between them and keeping the gross conformation of the triple helix (at least during the first cleavage step). On the other hand, MMP-2 binds preferentially the alpha-1 chains (likely through the fibronectin-like domain, which is not present in MMP-8 and ectMMP-14), grossly altering the whole triple helical arrangement of the collagen molecule and cleaving preferentially the alpha-2 chain. These distinctive mechanisms underly a drastically different mode of interaction with triple helical fibrillar collagen I, according to which the MMP domain is involved in binding. These findings can be related to the different role exerted by these MMPs on collagen homeostasis in the extracellular matrix.

Molecular basis of organization of collagen fibrils

The collagen fibrils are formed by self-assembly of individual collagen molecules, but the mechanism that drives their orderly packing during fibril formation is not clearly defined. To identify structural determinants critical for the D-periodic alignment of collagen molecules we employed three sets of genetically engineered collagen II variants: (i) a set in which domains corresponding to the specific D periods have been purposely deleted, (ii) a set of collagen variants consisting of tandem repeats of a specific D period, and (iii) a set lacking definite fragments of the D4 period. All collagen variants were analyzed for their ability to assemble into D-periodic fibrils. Even though all genetically engineered collagen variants differ significantly from the wild-type collagen II, most of them were able to form filamentous structures. The D-periodic banding pattern, an indication of the staggered arrangement of collagen monomers, however, occurred only when the D1, D4, and D0.4 domains of interacting collagen monomers could potentially cluster together to form a triad through telopeptide-mediated binding. Our results identify a critical step in the formation of collagenous matrices and provide experimental evidence for the active involvement of the N-terminal and C-terminal regions of fibrillar collagens in this process.

Testing the utility of rationally engineered recombinant collagen-like proteins for applications in tissue engineering

Collagens are attractive proteins as materials for tissue engineering. Over the last decade, significant progress has been made in developing technologies for large-scale production of native-like human recombinant collagens. Yet, the rational design of customized collagen-like proteins for smart biomaterials to enhance the quality of engineered tissues has not been explored. We mapped the D4 domain of human collagen II as most critical for supporting migration of chondrocytes and used this information to genetically engineer a collagen-like protein consisting of tandem repeats of the D4 domain (mD4 collagen). This novel collagen has been utilized to fabricate a scaffold for support of chondrocytes. We determined superior qualities of cartilaginous constructs created by chondrocytes cultured in scaffolds containing the mD4 collagen in comparison to those formed by chondrocytes cultured in bare scaffolds or those coated with wild-type collagen II. Our results are a first attempt to rationally engineer collagen-like proteins with characteristics tailored for specific needs of cartilage engineering and provide a basis for rational engineering of similar proteins for a variety of biomedical applications.

Single amino acid substitutions in the C-terminus of collagen II alter its affinity for collagen IX

The structural integrity of cartilage depends on the presence of extracellular matrices (ECM) formed by heterotypic fibrils composed of collagen II, collagen IX, and collagen XI. The formation of these fibrils depends on the site-specific binding between relatively small regions of interacting collagen molecules. Single amino acid substitutions in collagen II change the physicochemical and structural characteristics of those sites, thereby leading to an alteration of intermolecular collagen II/collagen IX interaction. Employing a biosensor to study interactions between R75C, R789C or G853E collagen II mutants and collagen IX, we demonstrated significant changes in the binding affinities. Moreover, analyses of computer models representing mutation sites defined exact changes in physicochemical characteristics of collagen II mutants. Our study shows that changes in collagen II/collagen IX affinity could represent one of the steps in a cascade of changes occurring in the ECM of cartilage as a result of single amino acid substitutions in collagen II.

Guilty by association: some collagen II mutants alter the formation of ECM as a result of atypical interaction with fibronectin

Among the structural components of extracellular matrices (ECM) fibrillar collagens play a critical role, and single amino acid substitutions in these proteins lead to pathological changes in tissues in which they are expressed. Employing a biologically relevant experimental model consisting of cells expressing R75C, R519C, R789C, and G853E procollagen II mutants, we found that the R789C mutation causing a decrease in the thermostability of collagen not only alters individual collagen molecules and collagen fibrils, but also has a negative impact on fibronectin. We propose that thermolabile collagen molecules are able to bind to fibronectin, thereby altering intracellular and extracellular processes in which fibronectin takes part, and we postulate that such an atypical interaction could change the architecture of the ECM of affected tissues in patients harboring mutations in genes encoding fibrillar collagens.

Position of single amino acid substitutions in the collagen triple helix determines their effect on structure of collagen fibrils

Collagen II fibrils are a critical structural component of the extracellular matrix of cartilage providing the tissue with its unique biomechanical properties. The self-assembly of collagen molecules into fibrils is a spontaneous process that depends on site-specific binding between specific domains belonging to interacting molecules. These interactions can be altered by mutations in the COL2A1 gene found in patients with a variety of heritable cartilage disorders known as chondrodysplasias. Employing recombinant procollagen II, we studied the effects of R75C or R789C mutations on fibril formation. We determined that both R75C and R789C mutants were incorporated into collagen assemblies. The effects of the R75C and R789C substitutions on fibril formation differed significantly. The R75C substitution located in the thermolabile region of collagen II had no major effect on the fibril formation process or the morphology of fibrils. In contrast, the R789C substitution located in the thermostable region of collagen II caused profound changes in the morphology of collagen assemblies. These results provide a basis for identifying pathways leading from single amino acid substitutions in collagen II to changes in the structure of individual fibrils and in the organization of collagenous matrices.

The D2 period of collagen II contains a specific binding site for the human discoidin domain receptor, DDR2

The human discoidin domain receptors (DDRs), DDR1 and DDR2, are expressed widely and, uniquely among receptor tyrosine kinases, activated by the extracellular matrix protein collagen. This activation is due to a direct interaction of collagen with the DDR discoidin domain. Here, we localised a specific DDR2 binding site on the triple-helical region of collagen II. Collagen II was found to be a much better ligand for DDR2 than for DDR1. As expected, DDR2 binding to collagen II was dependent on triple-helical collagen and was mediated by the DDR2 discoidin domain. Collagen II served as a potent stimulator of DDR2 autophosphorylation, the first step in transmembrane signalling. To map the DDR2 binding site(s) on collagen II, we used recombinant collagen II variants with specific deletions of one of the four repeating D periods. We found that the D2 period of collagen II was essential for DDR2 binding and receptor autophosphorylation, whereas the D3 and D4 periods were dispensable. The DDR2 binding site on collagen II was further defined by recombinant collagen II-like proteins consisting predominantly of tandem repeats of the D2 or D4 period. The D2 construct, but not the D4 construct, mediated DDR2 binding and receptor autophosphorylation, demonstrating that the D2 period of collagen II harbours a specific DDR2 recognition site. The discovery of a site-specific interaction of DDR2 with collagen II gives novel insight into the nature of the interaction of collagen II with matrix receptors.

Molecular Structure of the Collagen Triple Helix

The molecular conformation of the collagen triple helix confers strict amino acid sequence constraints, requiring a (Gly-X-Y)(n) repeating pattern and a high content of imino acids. The increasing family of collagens and proteins with collagenous domains shows the collagen triple helix to be a basic motif adaptable to a range of proteins and functions. Its rodlike domain has the potential for various modes of self-association and the capacity to bind receptors, other proteins, GAGs, and nucleic acids. High-resolution crystal structures obtained for collagen model peptides confirm the supercoiled triple helix conformation, and provide new information on hydrogen bonding patterns, hydration, sidechain interactions, and ligand binding. For several peptides, the helix twist was found to be sequence dependent, and such variation in helix twist may serve as recognition features or to orient the triple helix for binding. Mutations in the collagen triple-helix domain lead to a variety of human disorders. The most common mutations are single-base substitutions that lead to the replacement of one Gly residue, breaking the Gly-X-Y repeating pattern. A single Gly substitution destabilizes the triple helix through a local disruption in hydrogen bonding and produces a discontinuity in the register of the helix. Molecular information about the collagen triple helix and the effect of mutations will lead to a better understanding of function and pathology.

Novel amino acid substitution in the Y-position of collagen type II causes spondyloepimetaphyseal dysplasia congenita

We report on monozygotic twins with short stature and severe spondyloepimetaphyseal dysplasia congenita (SEMDC) from the Polish population. Phenotype of the twin girls resembles spondyloepiphyseal dysplasia congenita Spranger-Wiedemann (SEDC-SW), but shortening of the stature is more severe and the cranioface is normal. The distinctive radiographic features, in spite of similarity to SEDC-SW, indicate different spinal and, notably, severe metaphyseal involvement. Molecular analysis of the COL2A1 gene revealed an A to G transition at nucleotide +79 of exon 41 that converted the codon for arginine at amino acid 792 to a codon for glycine (Arg792Gly). The twins were heterozygous for the mutation and neither parent had this change. The Arg792Gly substitution is located at the Y-position of Gly-X-Y triplet, and it is likely that this substitution decreased the thermal stability of the triple helix and may affect fibril growth by replacement of an arginine residue, which is important for a conformation of the triple helix.

Single Amino Acid Substitutions in Procollagen VII Affect Early Stages of Assembly of Anchoring Fibrils

Procollagen VII is a homotrimer of 350-kDa pro-alpha1(VII) chains, each consisting of a central collagenous domain flanked by the noncollagenous N-terminal NC1 domain and the C-terminal NC2 domain. After secretion from cells, procollagen VII molecules form anti-parallel dimers with a C-terminal 60-nm overlap. Characteristic alignment of procollagen VII monomers forming a dimer depends on site-specific binding between the NC2 domain and the triple-helical region adjacent to Cys-2634 of the interacting procollagen VII molecules. Formation of the intermolecular disulfide bonds between Cys-2634 and either Cys-2802 or Cys-2804 is promoted by the cleavage of the NC2 domain by procollagen C-proteinase. By employing recombinant procollagen VII variants harboring G2575R, R2622Q, or G2623C substitutions previously disclosed in patients with dystrophic epidermolysis bullosa, we studied how these amino acid substitutions affect intermolecular interactions. Binding assays utilizing an optical biosensor demonstrated that the G2575R substitution increased affinity between mutant molecules. In contrast, homotypic binding between the R2622Q or G2623C molecules was not detected. In addition, kinetics of heterotypic binding of all analyzed mutants to wild type collagen VII were different from those for binding between wild type molecules. Moreover, solid-state binding assays demonstrated that R2622Q and G2623C substitutions prevent formation of stable assemblies of procollagen C-proteinase-processed mutants. These results indicate that single amino acid substitutions in procollagen VII alter its self-assembly and provide a basis for understanding the pathomechanisms leading from mutations in the COL7A1 gene to fragility of the dermal-epidermal junction seen in patients with dystrophic forms of epidermolysis bullosa.