Mapping the ligand-binding sites and disease-associated mutations on the most abundant protein in the human, type I collagen

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.

Non-enzymatic glycation of type I collagen diminishes collagen-proteoglycan binding and weakens cell adhesion

Non-enzymatic glycation of type I collagen occurs in aging and diabetes, and may affect collagen solubility, charge, polymerization, and intermolecular interactions. Proteoglycans(1) (PGs) bind type I collagen and are proposed to regulate fibril assembly, function, and cell-collagen interactions. Moreover, on the collagen fibril a keratan sulfate (KS) PG binding region overlaps with preferred collagen glycation sites. Thus, we examined the effect of collagen modified by simple glycation on PG-collagen interactions. By affinity coelectrophoresis (ACE), we found reduced affinities of heparin and KSPGs for glycated but not normal collagen, whereas the dermatan sulfate (DS)PGs decorin and biglycan bound similarly to both, and that the affinity of heparin for normal collagen decreased with increasing pH. Circular dichroism (CD) spectroscopy revealed normal and glycated collagens to assume triple helical conformations, but heparin addition caused precipitation and decreased triple helical content-effects that were more marked with glycated collagen. A spectrophotometric assay revealed slower polymerization of glycated collagen. However, ultrastructural analyses indicated that fibrils assembled from normal and glycated collagen exhibited normal periodicity, and had similar structures and comparable diameter distributions. B-cells expressing the cell surface heparan sulfate PG syndecan-1 adhered well to normal but not glycated collagen, and endothelial cell migration was delayed on glycated collagen. We speculate that glycation diminishes the electrostatic interactions between type I collagen and PGs, and may interfere with core protein-collagen associations for KSPGs but not DSPGs. Therefore in vivo, collagen glycation may weaken PG-collagen interactions, thereby disrupting matrix integrity and cell-collagen interactions, adhesion, and migration.

Recombinant collagen studies link the severe conformational changes induced by osteogenesis imperfecta mutations to the disruption of a set of interchain salt bridges

The clinical severity of Osteogenesis Imperfecta (OI), also known as the brittle bone disease, relates to the extent of conformational changes in the collagen triple helix induced by Gly substitution mutations. The lingering question is why Gly substitutions at different locations of collagen cause different disruptions of the triple helix. Here, we describe markedly different conformational changes of the triple helix induced by two Gly substitution mutations placed only 12 residues apart. The effects of the Gly substitutions were characterized using a recombinant collagen fragment modeling the 63-residue segment of the alpha1 chain of type I collagen containing no Hyp (residues 877-939) obtained from Escherichia coli. Two Gly --> Ser substitutions at Gly-901 and Gly-913 associated with, respectively, mild and severe OI variants were introduced by site-directed mutagenesis. Biophysical characterization and limited protease digestion experiments revealed that while the substitution at Gly-901 causes relatively minor destabilization of the triple helix, the substitution at Gly-913 induces large scale unfolding of an unstable region C-terminal to the mutation site. This extensive unfolding is caused by the intrinsic low stability of the C-terminal region of the helix and the mutation induced disruption of a set of salt bridges, which functions to lock this unstable region into the triple helical conformation. The extensive conformational changes associated with the loss of the salt bridges highlight the long range impact of the local interactions of triple helix and suggest a new mechanism by which OI mutations cause severe conformational damages in collagen.

The metabolism of proline as microenvironmental stress substrate

Proline, a unique proteogenic secondary amino acid, has its own metabolic system with special features. Recent findings defining the regulation of this system led us to propose that proline is a stress substrate in the microenvironment of inflammation and tumorigenesis. The criteria for proline as a stress substrate are: 1) the enzymes utilizing proline respond to stress signaling; 2) there is a large, mobilizable pool of proline; and 3) the metabolism of proline serves special stress functions. Studies show that the proline-utilizing enzyme, proline oxidase (POX)/proline dehydrogenase (PRODH), responds to genotoxic, inflammatory, and nutrient stress. Proline as substrate is stored as collagen in extracellular matrix, connective tissue, and bone and it is rapidly released from this reservoir by the sequential action of matrix metalloproteinases, peptidases, and prolidase. Special functions include the use of proline by POX/PRODH to generate superoxide radicals that initiate apoptosis by intrinsic and extrinsic pathways. Under conditions of nutrient stress, proline is an energy source. It provides carbons for the tricarboxylic acid cycle and also participates in the proline cycle. The latter, catalyzed by mitochondrial POX and cytosolic pyrroline-5-carboxylate reductase, shuttles reducing potential from the pentose phosphate pathway into mitochondria to generate ATP and oxidizing potential to activate the cytosolic pentose phosphate pathway.

Left ventricular assist device-induced molecular changes in the failing myocardium

PURPOSE OF REVIEW

There is considerable increase in the use of left ventricular assist devices for the treatment of severe heart failure. Traditionally viewed as a bridge to transplantation and more recently as a destination therapy, left ventricular assist device support is now recognized to offer potential for myocardial recovery through reverse remodeling, a potential that is further enhanced by combination with pharmacologic therapy. In this study, we examine the molecular changes associated with left ventricular assist device support and how these may contribute to the recovery process.

RECENT FINDINGS

Studies in both patients and experimental models have demonstrated that improved function is associated with alterations in several key pathways including cell survival, cytokine signaling, calcium handling, adrenergic receptor signaling, cytoskeletal and contractile proteins, energy metabolism, extracellular matrix, and endothelial and microvascular functions. Moreover, the unique research opportunities offered by left ventricular assist device analysis are beginning to distinguish changes associated with recovery from those of mechanical unloading alone and identify potential predictors and novel therapeutic targets capable of enhancing myocardial repair.

SUMMARY

Significant progress has been made toward revealing molecular changes associated with myocardial recovery from heart failure. These studies also offer new insight into the pathogenesis of heart failure and point to novel therapeutic strategies.

Collagen fibrillogenesis: fibronectin, integrins, and minor collagens as organizers and nucleators

Collagens are triple helical proteins that occur in the extracellular matrix (ECM) and at the cell–ECM interface. There are more than 30 collagens and collagen-related proteins but the most abundant are collagens I and II that exist as D-periodic (where D = 67 nm) fibrils. The fibrils are of broad biomedical importance and have central roles in embryogenesis, arthritis, tissue repair, fibrosis, tumor invasion, and cardiovascular disease. Collagens I and II spontaneously form fibrils in vitro, which shows that collagen fibrillogenesis is a selfassembly process. However, the situation in vivo is not that simple; collagen I-containing fibrils do not form in the absence of fibronectin, fibronectin-binding and collagen-binding integrins, and collagen V. Likewise, the thin collagen II-containing fibrils in cartilage do not form in the absence of collagen XI. Thus, in vivo, cellular mechanisms are in place to control what is otherwise a protein self-assembly process. This review puts forward a working hypothesis for how fibronectin and integrins (the organizers) determine the site of fibril assembly, and collagens V and XI (the nucleators) initiate collagen fibrillogenesis.

Candidate cell and matrix interaction domains on the collagen fibril, the predominant protein of vertebrates

Type I collagen, the predominant protein of vertebrates, polymerizes with type III and V collagens and non-collagenous molecules into large cable-like fibrils, yet how the fibril interacts with cells and other binding partners remains poorly understood. To help reveal insights into the collagen structure-function relationship, a data base was assembled including hundreds of type I collagen ligand binding sites and mutations on a two-dimensional model of the fibril. Visual examination of the distribution of functional sites, and statistical analysis of mutation distributions on the fibril suggest it is organized into two domains. The "cell interaction domain" is proposed to regulate dynamic aspects of collagen biology, including integrin-mediated cell interactions and fibril remodeling. The "matrix interaction domain" may assume a structural role, mediating collagen cross-linking, proteoglycan interactions, and tissue mineralization. Molecular modeling was used to superimpose the positions of functional sites and mutations from the two-dimensional fibril map onto a three-dimensional x-ray diffraction structure of the collagen microfibril in situ, indicating the existence of domains in the native fibril. Sequence searches revealed that major fibril domain elements are conserved in type I collagens through evolution and in the type II/XI collagen fibril predominant in cartilage. Moreover, the fibril domain model provides potential insights into the genotype-phenotype relationship for several classes of human connective tissue diseases, mechanisms of integrin clustering by fibrils, the polarity of fibril assembly, heterotypic fibril function, and connective tissue pathology in diabetes and aging.

Spatio-temporal modification of collagen scaffolds mediated by triple helical propensity

Functionalized collagen that incorporates exogenous compounds may offer new and improved biomaterials applications, especially in drug-delivery, multifunctional implants, and tissue engineering. To that end, we developed a specific and reversible collagen modification technique utilizing associative chain interactions between synthetic collagen mimetic peptide (CMP) [(ProHypGly) chi; Hyp = hydroxyproline] and type I collagen. Here we show temperature-dependent collagen binding and subsequent release of a series of CMPs with varying chain lengths indicating a triple helical propensity driven binding mechanism. The binding took place when melted, single-strand CMPs were allowed to fold while in contact with reconstituted type I collagens. The binding affinity is highly specific to collagen as labeled CMP bound to nanometer scale periodic positions on type I collagen fibers and could be used to selectively image collagens in ex vivo human liver tissue. When heated to physiological temperature, bound CMPs discharged from the collagen at a sustained rate that correlated with CMP's triple helical propensity, suggesting that sustainability is mediated by dynamic collagen-CMP interactions. We also report on the spatially defined modification of collagen film with linear and multi-arm poly(ethylene glycol)-CMP conjugates; at 37 degrees C, these PEG-CMP conjugates exhibited temporary cell repelling activity lasting up to 9 days. These results demonstrate new opportunities for targeting pathologic collagens for diagnostic or therapeutic applications and for fabricating multifunctional collagen coatings and scaffolds that can temporally and spatially control the behavior of cells associated with the collagen matrices.

Matrix metalloproteinases and bone

Matrix metalloproteinases (MMPs) are members of a family of zinc-dependent proteolytic enzymes. Several of the MMPs are expressed at high levels in bone and cartilage in mammals including humans and mice and are capable of cleaving native, undenatured collagens with long uninterrupted triple helices; these MMPs therefore potentially function as collagenases in vivo. Several MMPs expressed in the skeleton appear to function in endochondral ossification during embryonic development and in modeling and remodeling of bone postnatally and later in life. Different functions of MMPs have been elucidated through observations of spontaneous mutations in MMP genes in humans and of targeted mutations in Mmp genes and collagen (substrate) genes in mice. Potential mechanisms to account for effects of these mutations are considered in this review.

ER stress-mediated apoptosis in a new mouse model of osteogenesis imperfecta

Osteogenesis imperfecta is an inherited disorder characterized by increased bone fragility, fractures, and osteoporosis, and most cases are caused by mutations affecting the type I collagen genes. Here, we describe a new mouse model for Osteogenesis imperfecta termed Aga2 (abnormal gait 2) that was isolated from the Munich N-ethyl-N-nitrosourea mutagenesis program and exhibited phenotypic variability, including reduced bone mass, multiple fractures, and early lethality. The causal gene was mapped to Chromosome 11 by linkage analysis, and a C-terminal frameshift mutation was identified in the Col1a1 (procollagen type I, alpha 1) gene as the cause of the disorder. Aga2 heterozygous animals had markedly increased bone turnover and a disrupted native collagen network. Further studies showed that abnormal proα1(I) chains accumulated intracellularly in Aga2/+ dermal fibroblasts and were poorly secreted extracellularly. This was associated with the induction of an endoplasmic reticulum stress-specific unfolded protein response involving upregulation of BiP, Hsp47, and Gadd153 with caspases-12 and −3 activation and apoptosis of osteoblasts both in vitro and in vivo. These studies resulted in the identification of a new model for Osteogenesis imperfecta, and identified a role for intracellular modulation of the endoplasmic reticulum stress-associated unfolded protein response machinery toward osteoblast apoptosis during the pathogenesis of disease.

Osteogenesis imperfecta (OI) is a heterogeneous collection of connective tissue disorders typically caused by mutations in the COL1A1/2 genes that encode the chains of type I collagen, the principle structural protein of bone. Phenotypic expression in OI depends on the nature of the mutation, causing a clinical heterogeneity ranging from a mild risk of fractures to perinatal lethality. Here, we describe a new OI mouse model with a dominant mutation in the terminal C-propeptide domain of Col1a1 generated using the N-ethyl-N-nitrosourea (ENU) mutagenesis strategy. Heterozygous animals developed severe-to-lethal phenotypes that were associated with endoplasmic reticulum stress, and caspases-12 and −3 activation within calvarial osteoblasts. We provide evidence for endoplasmic reticulum stress–associated apoptosis as a key component in the pathogenesis of disease.

Collagen's triglycine repeat number and phylogeny suggest an interdomain transfer event from a Devonian or Silurian organism into Trichodesmium erythraeum

Two competing effects at two vastly different scales may explain collagen’s current translation length. The necessity to have long molecules for maintaining mechanical integrity at the organism and supraorganism scales may be limited by the need to have small molecules capable of robust self-assembly at the nanoscale. The triglycine repeat regions of all 556 currently cataloged organisms with collagen-like genes were ranked by length. This revealed a sharp boundary in the GXY transcript number at 1032 amino acids (344 GXY repeats). An anomalous exception, however, is the intron-free Trichodesmium erythraeum collagen gene. Immunogold atomic force microscopy reveals, for the first time, the presence of a collagen-like protein in T. erythraeum. A phylogenetic protein sequence analysis which includes vertebrates, nonvertebrates, shrimp white spot syndrome virus, Streptococcus equi, and Bacillus cereus predicts that the collagen-like sequence may have emerged shortly after the divergence of fibrillar and nonfibrillar collagens. The presence of this anomalously long collagen gene within a prokaryote may represent an interdomain transfer from eukaryotes into prokaryotes that gives T. erythraeum the ability to form blooms that cover hundreds of square kilometers of ocean. We propose that the collagen gene entered the prokaryote intron-free only after it had been molded by years of mechanical selective pressure in larger organisms and only after large, dense food sources such as marine vertebrates became available. This anomalously long collagen-like sequence may explain T. erythraeum’s ability to aggregate and thus concentrate its toxin for food-source procurement.

Mapping of SPARC/BM-40/osteonectin-binding sites on fibrillar collagens

The 33-kDa matrix protein SPARC (BM-40, osteonectin) binds several collagen types with moderate affinity. The collagen-binding site resides in helix alphaA of the extracellular calcium-binding domain of SPARC and is partially masked by helix alphaC. Previously, we found that the removal of helix alphaC caused a 10-fold increase in the affinity of SPARC for collagen, and we identified amino acids crucial for binding by site-directed mutagenesis. In this study, we used rotary shadowing, CNBr peptides, and synthetic peptides to map binding sites of SPARC onto collagens I, II, and III. Rotary shadowing and electron microscopy of SPARC-collagen complexes identified a major binding site approximately 180 nm from the C terminus of collagen. SPARC binding was also detected with lower frequency near the matrix metalloproteinase cleavage site. These data fit well with our analysis of SPARC binding to CNBr peptides, denaturation of which abolished binding, indicating triple-helical conformation of collagen to be essential. SPARC binding was substantially decreased in two of seven alpha2(I) mutant procollagen I samples and after N-acetylation of Lys/Hyl side chains in wild-type collagen. Synthetic peptides of collagen III were used to locate the binding sites, and we found SPARC binding activity in a synthetic triple-helical peptide containing the sequence GPOGPSGPRGQOGVMGFOGPKGNDGAO (where O indicates 4-hydroxyproline), with affinity for SPARC comparable with that of procollagen III. This sequence is conserved among alpha chains of collagens I, II, III, and V. In vitro collagen fibrillogenesis was delayed in the presence of SPARC, suggesting that SPARC might modulate collagen fibril assembly in vivo.

The importance of proline residues in the structure, stability and susceptibility to proteolytic degradation of collagens

Collagens are among proteins that undergo several post-translational modifications, such as prolyl hydroxylation, that occur during elongation of the nascent chains in the endoplasmic reticulum. The major structural collagens, types I, II and III, have large, uninterrupted triple helices, comprising three polyproline II-like chains supercoiled around a common axis. The structure has a requirement for glycine, as every third residue, and is stabilized by the high content of proline and 4-hydroxyproline residues. Action of prolyl hydroxylases is critical. Spontaneous or targeted genetic defects in prolyl hydroxylases can be lethal or result in severe osteogenesis imperfecta. Prolines, as determinants of substrate specificity and susceptibility, also play a role in degradation of collagen by collagenolytic matrix metalloproteinases (MMPs). Targeted mutations in mice in the collagenase cleavage domain have profound effects on collagen turnover and the function of connective tissues. Prolines are thus critical determinants of collagen structure and function.

Predicting the clinical lethality of osteogenesis imperfecta from collagen glycine mutations

Osteogenesis imperfecta (OI), or brittle bone disease, often results from missense mutation of one of the conserved glycine residues present in the repeating Gly-X-Y sequence characterizing the triple-helical region of type I collagen. A composite model was developed for predicting the clinical lethality resulting from glycine mutations in the alpha1 chain of type I collagen. The lethality of mutations in which bulky amino acids are substituted for glycine is predicted by their position relative to the N-terminal end of the triple helix. The effect of a Gly --> Ser mutation is modeled by the relative thermostability of the Gly-X-Y triplet on the carboxy side of the triplet containing the substitution. This model also predicts the lethality of Gly --> Ser and Gly --> Cys mutations in the alpha2 chain of type I collagen. The model was validated with an independent test set of six novel Gly --> Ser mutations. The hypothesis derived from the model of an asymmetric interaction between a Gly --> Ser mutation and its neighboring residues was tested experimentally using collagen-like peptides. Consistent with the prediction, a significant decrease in stability, calorimetric enthalpy, and folding time was observed for a peptide with a low-stability triplet C-terminal to the mutation compared to a similar peptide with the low-stability triplet on the N-terminal side. The computational and experimental results together relate the position-specific effects of Gly --> Ser mutations to the local structural stability of collagen and lend insight into the etiology of OI.

Cell–collagen interactions: the use of peptide Toolkits to investigate collagen–receptor interactions

Fibrillar collagens provide the most fundamental platform in the vertebrate organism for the attachment of cells and matrix molecules. We have identified specific sites in collagens to which cells can attach, either directly or through protein intermediaries. Using Toolkits of triple-helical peptides, each peptide comprising 27 residues of collagen primary sequence and overlapping with its neighbours by nine amino acids, we have mapped the binding of receptors and other proteins on to collagens II or III. Integrin α2β1 binds to several GXX′GER motifs within the collagens, the affinities of which differ sufficiently to control cell adhesion and migration independently of the cellular regulation of the integrin. The platelet receptor, Gp (glycoprotein) VI binds well to GPO (where O is hydroxyproline)-containing model peptides, but to very few Toolkit peptides, suggesting that sequence in addition to GPO triplets is important in defining GpVI binding. The Toolkits have been applied to the plasma protein vWF (von Willebrand factor), which binds to only a single sequence, identified by truncation and amino acid substitution within Toolkit peptides, as GXRGQOGVMGFO in collagens II and III. Intriguingly, the receptor tyrosine kinase, DDR2 (discoidin domain receptor 2) recognizes three sites in collagen II, including its vWF-binding site, although the amino acids that support the interaction differ slightly within this motif. Furthermore, the secreted protein BM-40 (basement membrane protein 40) also binds well to this same region. Thus the availability of extracellular collagen-binding proteins may be important in regulating and facilitating direct collagen–receptor interaction.

Collagen fibril architecture, domain organization, and triple-helical conformation govern its proteolysis

We describe the molecular structure of the collagen fibril and how it affects collagen proteolysis or "collagenolysis." The fibril-forming collagens are major components of all mammalian connective tissues, providing the structural and organizational framework for skin, blood vessels, bone, tendon, and other tissues. The triple helix of the collagen molecule is resistant to most proteinases, and the matrix metalloproteinases that do proteolyze collagen are affected by the architecture of collagen fibrils, which are notably more resistant to collagenolysis than lone collagen monomers. Until now, there has been no molecular explanation for this. Full or limited proteolysis of the collagen fibril is known to be a key process in normal growth, development, repair, and cell differentiation, and in cancerous tumor progression and heart disease. Peptide fragments generated by collagenolysis, and the conformation of exposed sites on the fibril as a result of limited proteolysis, regulate these processes and that of cellular attachment, but it is not known how or why. Using computational and molecular visualization methods, we found that the arrangement of collagen monomers in the fibril (its architecture) protects areas vulnerable to collagenolysis and strictly governs the process. This in turn affects the accessibility of a cell interaction site located near the cleavage region. Our observations suggest that the C-terminal telopeptide must be proteolyzed before collagenase can gain access to the cleavage site. Collagenase then binds to the substrate's "interaction domain," which facilitates the triple-helix unwinding/dissociation function of the enzyme before collagenolysis.

Natural variation in four human collagen genes across an ethnically diverse population

Collagens are members of one of the most important families of structural proteins in higher organisms. There are 28 types of collagens encoded by 43 genes in humans that fall into several different functional protein classes. Mutations in the major fibrillar collagen genes lead to osteogenesis imperfecta (COL1A1 and COL1A2 encoding the chains of Type I collagen), chondrodysplasias (COL2A1 encoding the chains of Type II collagen), and vascular Ehlers-Danlos syndrome (COL3A1 encoding the chains of Type III collagen). Over the past 2 decades, mutations in these collagen genes have been catalogued, in hopes of understanding the molecular etiology of diseases caused by these mutations, characterizing the genotype-phenotype relationships, and developing robust models predicting the molecular and clinical outcomes. To achieve these goals better, it is necessary to understand the natural patterns of variation in collagen genes in human populations. We screened exons, flanking intronic regions, and conserved noncoding regions for variations in COL1A1, COL1A2, COL2A1, and COL3A1 in 48 individuals from each of four ethnically diverse populations. We identified 459 single-nucleotide polymorphisms (SNPs), more than half of which were novel and not found in public databases. Of the 52 SNPs found in coding regions, 15 caused amino acid substitutions while 37 did not. Although the four collagens have similar gene and protein structures, they have different molecular evolutionary characteristics. For example, COL1A1 appears to have been under substantially stronger negative selection than the rest. Phylogenetic analysis also suggests that the four genes have very different evolutionary histories among the different ethnic groups. Our observations suggest that the study of collagen mutations and their relationships with disease phenotypes should be performed in the context of the genetic background of the subjects.

Heparin intercalation into reconstituted collagen I fibrils: Impact on growth kinetics and morphology

Collagen type I fibrils, reconstituted in vitro in the presence of heparin, exhibit an unusually thick and straight shape. A detailed structural analysis by scanning force and scanning electron microscopy revealed a non-linear dependence in size distribution, width-to-length ratio, and morphology over a wide range of glycosaminoglycan (GAG) concentrations. By varying molecular weight, degree of sulphation, charge, and concentration of different GAGs we are able to correlate the morphological data with kinetic turbidimetric measurements, and quantitation of fibril-bound GAG. The experiments imply a pronounced impact of the prenucleation phase on the cofibril morphology as a result of the strong electrostatic interaction of heparin with tropocollagen. Heparin is assumed to stabilize the collagen microfibrils and to enhance their parallel accretion during cofibrillogenesis with preservation of the typical asymmetric collagen banding pattern. The heparin quantitation data show heparin to be intercalated as a linker molecule with one specific binding site inside the cofibrils. The reconstituted cofibrils with their unusual morphology and GAG intercalation-a phenomenon not reported in vivo-can be expected to exhibit interesting mechanical and biochemical behaviours as a biomaterial for extracellular matrix scaffolds.

Structural heterogeneity of type I collagen triple helix and its role in osteogenesis imperfecta

We investigated regions of different helical stability within human type I collagen and discussed their role in intermolecular interactions and osteogenesis imperfecta (OI). By differential scanning calorimetry and circular dichroism, we measured and mapped changes in the collagen melting temperature (DeltaTm) for 41 different Gly substitutions from 47 OI patients. In contrast to peptides, we found no correlations of DeltaTm with the identity of the substituting residue. Instead, we observed regular variations in DeltaTm with the substitution location in different triple helix regions. To relate the DeltaTm map to peptide-based stability predictions, we extracted the activation energy of local helix unfolding (DeltaG) from the reported peptide data. We constructed the DeltaG map and tested it by measuring the H-D exchange rate for glycine NH residues involved in interchain hydrogen bonds. Based on the DeltaTm and DeltaG maps, we delineated regional variations in the collagen triple helix stability. Two large, flexible regions deduced from the DeltaTm map aligned with the regions important for collagen fibril assembly and ligand binding. One of these regions also aligned with a lethal region for Gly substitutions in the alpha1(I) chain.

Mutations in type I collagen genes in Japanese osteogenesis imperfecta patients

BACKGROUND

Osteogenesis imperfecta (OI) is an autosomal dominant disorder of connective tissue characterized by bone fragility and low bone mass. COL1A1 and COL1A2 genes are very large and have been rarely analyzed systematically in Japan. The aim of this project was to develop an effective and convenient method of finding mutations in the COL1A1 and COL1A2 gene by using denaturing high-performance liquid chromatography (DHPLC).

METHODS

Polymerase chain reaction (PCR) amplicons of genomic DNA from the COL1A1 or COL1A2 gene were followed by heteroduplex analysis by DHPLC. Products containing heteroduplexes were then sequenced.

RESULTS

Twenty-two OI families were analyzed, and 193 of the 1122 PCR products in the COL1A1 gene, all containing heteroduplexes, were sequenced. Sixty-two samples had single-base substitutions or single-base deletions or insertions within introns. Eight had single-base substitutions in exons. Six were pathogenic mutations, and two were silent mutations. In 16 families not identified with pathogenic mutation in COL1A1, COL1A2 was similarly analyzed. A total of 138 of the 848 PCR products were sequenced, and 46 samples had single-base substitutions, or single-base deletions or insertions within introns. Twenty-four samples had single-base substitutions in exons. Three were pathogenic mutations and the others silent.

CONCLUSIONS

Mutations were identified in nine COL1A1/COL1A2 associated with OI type I-IV genes by scanning with DHPLC. Software was used to detect point mutation and large deletions/insertions in COL1A1 and COL1A2 genes.

Ultrastructural Study on the Specific Binding of Genetically Engineered Epidermal Growth Factor to Type I Collagen Fibrils

In an attempt to develop collagen-growth factor composites for use in tissue engineering, chimeric proteins consisting of epidermal growth factor and collagen binding domains derived from von Willebrand factor or fibronectin were synthesized by means of recombinant technology. These chimeric proteins were bound to type I collagen fibrils, and the ultrastructures of composites were analyzed by transmission electron microscopy combined with the gold nanoparticle labeling technique. The results of the ultrastructural study revealed that chimeric proteins were densely assembled on collagen fibrils through the specific recognition of binding sites, producing the ordered array of chimeric proteins.

Designed triple-helical peptides as tools for collagen biochemistry and matrix engineering

Collagens, characterized by a unique triple-helical structure, are the predominant component of extracellular matrices (ECMs) existing in all multicellular animals. Collagens not only maintain structural integrity of tissues and organs, but also regulate a number of biological events, including cell attachment, migration and differentiation, tissue regeneration and animal development. The specific functions of collagens are generally triggered by specific interactions of collagen-binding molecules (membrane receptors, soluble factors and other ECM components) with certain structures displayed on the collagen triple helices. Thus, synthetic triple-helical peptides that mimic the structure of native collagens have been used to investigate the individual collagen-protein interactions, as well as collagen structure and stability. The first part of this article illustrates the design of various collagen-mimetic peptides and their recent applications in matrix biology. Collagen is also acknowledged as one of the most promising biomaterials in regenerative medicine and tissue engineering. However, the use of animal-derived collagens in human could put the recipients at risks of pathogen transmission or allergic reactions. Hence, the production of safe artificial collagen surrogates is currently of considerable interest. The latter part of this article reviews recent attempts to develop artificial collagens as novel biomaterials.

Photocontrol of the collagen triple helix: synthesis and conformational characterization of bis-cysteinyl collagenous peptides with an azobenzene clamp

For the photomodulation of the collagen triple helix with an azobenzene clamp, we investigated various collagenous peptides consisting of ideal (Gly-Pro-Hyp) repeats and containing cysteine residues in various positions for a side chain-to-side chain crosslink with a suitable chromophore derivative. Comparative conformational analysis of these cysteine peptides indicated an undecarepeat peptide with two cysteine residues located in the central portion in i and i+7 positions and flanked by (Gly-Pro-Hyp) repeat sequences as the most promising for the cross-bridging experiments. In aqueous alcoholic solution the azobenzene-undecarepeat peptide formed a stable triple helix in equilibrium with the monomeric species as a trans-azobenzene isomer, whereas photoisomerization to the cis isomer leads to unfolding of at least part of the triple helix. Furthermore, the residual supercoiled structure acts like an intermolecular knot, thus making refolding upon cis-to-trans isomerization a concentration-independent fast event. Consequently, these photoswitchable collagenous systems should be well suited for time-resolved studies of folding/unfolding of the collagen triple helix under variable thermodynamic equilibria.

Consortium for osteogenesis imperfecta mutations in the helical domain of type I collagen: regions rich in lethal mutations align with collagen binding sites for integrins and proteoglycans

Osteogenesis imperfecta (OI) is a generalized disorder of connective tissue characterized by fragile bones and easy susceptibility to fracture. Most cases of OI are caused by mutations in type I collagen. We have identified and assembled structural mutations in type I collagen genes (COL1A1 and COL1A2, encoding the proalpha1(I) and proalpha2(I) chains, respectively) that result in OI. Quantitative defects causing type I OI were not included. Of these 832 independent mutations, 682 result in substitution for glycine residues in the triple helical domain of the encoded protein and 150 alter splice sites. Distinct genotype-phenotype relationships emerge for each chain. One-third of the mutations that result in glycine substitutions in alpha1(I) are lethal, especially when the substituting residues are charged or have a branched side chain. Substitutions in the first 200 residues are nonlethal and have variable outcome thereafter, unrelated to folding or helix stability domains. Two exclusively lethal regions (helix positions 691-823 and 910-964) align with major ligand binding regions (MLBRs), suggesting crucial interactions of collagen monomers or fibrils with integrins, matrix metalloproteinases (MMPs), fibronectin, and cartilage oligomeric matrix protein (COMP). Mutations in COL1A2 are predominantly nonlethal (80%). Lethal substitutions are located in eight regularly spaced clusters along the chain, supporting a regional model. The lethal regions align with proteoglycan binding sites along the fibril, suggesting a role in fibril-matrix interactions. Recurrences at the same site in alpha2(I) are generally concordant for outcome, unlike alpha1(I). Splice site mutations comprise 20% of helical mutations identified in OI patients, and may lead to exon skipping, intron inclusion, or the activation of cryptic splice sites. Splice site mutations in COL1A1 are rarely lethal; they often lead to frameshifts and the mild type I phenotype. In alpha2(I), lethal exon skipping events are located in the carboxyl half of the chain. Our data on genotype-phenotype relationships indicate that the two collagen chains play very different roles in matrix integrity and that phenotype depends on intracellular and extracellular events.

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.

Effects of fibronectin and laminin on structural, mechanical and transport properties of 3D collageneous network

Recent studies, on cells cultured in 3D collagen gels, have shown that, beside from their well known biochemical role, fibronectin (FN) and laminin (LM) affect cell functions via a modification of mechanical and structural properties of matrix due to interaction with collagen molecules. Though biochemical properties of FN and LM have been widely studied, little is known about their role in collagen matrix assembly. The aim of this work was to characterize FN- and LM-based collagen semi-interpenetrating polymer networks (semi-IPNs), in order to understand how these biomacromolecular species can affect collagen network assembly and properties. Morphology, viscoelasticity and diffusivity of collagen gels and FN- and LM-based collagen semi-IPNs were analysed by Confocal Laser Scanning microscopy (CLSM), Environmental Scanning Electron microscopy (ESEM), Transmission Electron microscopy (TEM), Rheometry and Fluorescence Recovery After Photobleaching (FRAP) techniques. It was found that FN and LM were organized in aggregates, interspersed in collagen gel, and in thin fibrils, distributed along collagen fibres. In addition, high FN and LM concentrations affected collagen fibre assembly and structure and induced drastic effects on rheological and transport properties.

Three arginine to cysteine substitutions in the pro-alpha (I)-collagen chain cause Ehlers-Danlos syndrome with a propensity to arterial rupture in early adulthood

Mutations in the COL1A1 and COL1A2 genes, encoding the proalpha1 and 2 chains of type I collagen, cause osteogenesis imperfecta (OI) or Ehlers-Danlos syndrome (EDS) arthrochalasis type. Although the majority of missense mutations in the collagen type I triple helix affect glycine residues in the Gly-Xaa-Yaa repeat, few nonglycine substitutions have been reported. Two arginine-to-cysteine substitutions in the alpha1(I)-collagen chain are associated with classic EDS [R134C (p.R312C)] or autosomal dominant Caffey disease with mild EDS features [R836C (p.R1014C)]. Here we show alpha1(I) R-to-C substitutions in three unrelated patients who developed iliac or femoral dissection in early adulthood. In addition, manifestations of classic EDS in Patient 1 [c.1053C>T; R134C (p.R312C); X-position] or osteopenia in Patients 2 [c.1839C>T; R396C (p.R574C); Y-position] and 3 [c.3396C>T; R915C (p.R1093C); Y-position] are seen. Dermal fibroblasts from the patients produced disulfide-bonded alpha1(I)-dimers in approximately 20% of type I collagen, which were efficiently secreted into the medium in case of the R396C and R915C substitution. Theoretical stability calculations of the collagen type I heterotrimer and thermal denaturation curves of monomeric mutant alpha1(I)-collagen chains showed minor destabilization of the collagen helix. However, dimers were shown to be highly unstable. The R134C and R396C caused delayed procollagen processing by N-proteinase. Ultrastructural findings showed collagen fibrils with variable diameter and irregular interfibrillar spaces, suggesting disturbed collagen fibrillogenesis. Our findings demonstrate that R-to-C substitutions in the alpha1(I) chain may result in a phenotype with propensity to arterial rupture in early adulthood. This broadens the phenotypic range of nonglycine substitutions in collagen type I and has important implications for genetic counseling and follow-up of patients carrying this type of mutation.

Functional impact of missense variants in BRCA1 predicted by supervised learning

Many individuals tested for inherited cancer susceptibility at the BRCA1 gene locus are discovered to have variants of unknown clinical significance (UCVs). Most UCVs cause a single amino acid residue (missense) change in the BRCA1 protein. They can be biochemically assayed, but such evaluations are time-consuming and labor-intensive. Computational methods that classify and suggest explanations for UCV impact on protein function can complement functional tests. Here we describe a supervised learning approach to classification of BRCA1 UCVs. Using a novel combination of 16 predictive features, the algorithms were applied to retrospectively classify the impact of 36 BRCA1 C-terminal (BRCT) domain UCVs biochemically assayed to measure transactivation function and to blindly classify 54 documented UCVs. Majority vote of three supervised learning algorithms is in agreement with the assay for more than 94% of the UCVs. Two UCVs found deleterious by both the assay and the classifiers reveal a previously uncharacterized putative binding site. Clinicians may soon be able to use computational classifiers such as those described here to better inform patients. These classifiers can be adapted to other cancer susceptibility genes and systematically applied to prioritize the growing number of potential causative loci and variants found by large-scale disease association studies.

Heparin-mediated conformational changes in fibronectin expose vascular endothelial growth factor binding sites

Regulation of angiogenesis involves interactions between vascular endothelial growth factor (VEGF) and components of the extracellular matrix, including fibronectin and heparan sulfate. In the present study, we identified two classes of VEGF binding sites on fibronectin. One was constitutively available whereas the availability of the other was modulated by the conformational state of fibronectin. Atomic force microscopy studies revealed that heparin and hydrophilic substrates promoted the extended conformation of fibronectin, leading to increased VEGF binding. The ability of heparin to enhance VEGF binding to fibronectin was dependent on the chemical composition and chain length of heparin, since long (>22 saccharides) heparin chains with sulfation on the 6-O and N positions of glucosamine units were required for full activity. Treatment of the complex endothelial extracellular matrix with heparin also increased VEGF binding, suggesting that heparin/heparan sulfate might regulate VEGF interactions within the extracellular matrix by controlling the structure and organization of fibronectin matrices.

CRTAP Is Required for Prolyl 3- Hydroxylation and Mutations Cause Recessive Osteogenesis Imperfecta

Prolyl hydroxylation is a critical posttranslational modification that affects structure, function, and turnover of target proteins. Prolyl 3-hydroxylation occurs at only one position in the triple-helical domain of fibrillar collagen chains, and its biological significance is unknown. CRTAP shares homology with a family of putative prolyl 3-hydroxylases (P3Hs), but it does not contain their common dioxygenase domain. Loss of Crtap in mice causes an osteochondrodysplasia characterized by severe osteoporosis and decreased osteoid production. CRTAP can form a complex with P3H1 and cyclophilin B (CYPB), and Crtap-/- bone and cartilage collagens show decreased prolyl 3-hydroxylation. Moreover, mutant collagen shows evidence of overmodification, and collagen fibrils in mutant skin have increased diameter consistent with altered fibrillogenesis. In humans, CRTAP mutations are associated with the clinical spectrum of recessive osteogenesis imperfecta, including the type II and VII forms. Hence, dysregulation of prolyl 3-hydroxylation is a mechanism for connective tissue disease.

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.

Simultaneous proteomic profiling of four different growth states of human fibroblasts, using amine-reactive isobaric tagging reagents and tandem mass spectrometry

In general, permanent growth arrest due to exhaustive cell replication can be induced prematurely by either stress or overexpression of selected oncogenes. In an attempt to examine key proteins involved in achieving premature senescence, and how they differ from those in serially passaged, replicatively exhausted cells, we used a novel proteomic profiling approach, isobaric tagging for relative and absolute quantitation (iTRAQ), to perform simultaneous four-way comparison of replicatively senescent fibroblasts, oxidatively stressed prematurely senescent fibroblasts, and their young replicating and quiescent counterparts. Two hundred and forty proteins were identified and quantified simultaneously; data analysis reveals: (1) groups of proteins whose expressions are uniformly either up- or down-regulated in all three growth arrest states; (2) signature proteins which may serve as candidate proteomic markers to differentiate the quiescent state from permanent growth arrest by either exhaustive replication or stress induction and (3) that while oxidative stress-induced, prematurely senescent fibroblasts morphologically resemble their replicatively exhausted counterparts, they exhibit different protein expression patterns. Results from simultaneous proteomic profiling were validated by Western blotting for selected proteins: collagen type I, HSP90 and vimentin. In conclusion, this report shows that iTRAQ proteomic profiling is a powerful technique for globally mapping protein signatures for different culture growth states.

Molecular Mechanism of α1(I)-Osteogenesis Imperfecta/Ehlers-Danlos Syndrome

We demonstrate that 85 N-terminal amino acids of the alpha1(I) chain participate in a highly stable folding domain, acting as the stabilizing anchor for the amino end of the type I collagen triple helix. This anchor region is bordered by a microunfolding region, 15 amino acids in each chain, which include no proline or hydroxyproline residues and contain a chymotrypsin cleavage site. Glycine substitutions and amino acid deletions within the N-anchor domain induce its reversible unfolding above 34 degrees C. The overall triple helix denaturation temperature is reduced by 5-6 degrees C, similar to complete N-anchor removal. N-propeptide partially restores the stability of mutant procollagen but not sufficiently to prevent N-anchor unfolding and a conformational change at the N-propeptide cleavage site. The ensuing failure of N-proteinase to cleave at the misfolded site leads to incorporation of pN-collagen into fibrils. Similar, but weaker, effects are caused by G88E substitution in the adjacent triplet, which appears to alter N-anchor structure as well. As in Ehlers-Danlos syndrome (EDS) VIIA/B, fibrils containing pN-collagen are thinner and weaker causing EDS-like laxity of large and small joints and paraspinal ligaments. However, distinct structural consequences of N-anchor destabilization result in a distinct alpha1(I)-osteogenesis imperfecta (OI)/EDS phenotype.

Association of bovine dentine phosphophoryn with collagen fragments

Bovine dentine phosphophoryn (BDP), a protein rich in aspartyl (Asp) and O-phosphoseryl (Ser(P)) residues, is synthesized by odontoblasts and believed to be involved in matrix-mediated biomineralization of dentine. Phosphophoryn was purified from bovine dentine using EDTA extraction, Ca(2+) precipitation, anion exchange and size exclusion chromatography. The purified protein migrated on SDS-PAGGE as a single band. The protein was dephosphorylated using a chelex alkaline dialysis procedure, repurified using anion exchange and size exclusion chromatography and then subjected to cleavage with trypsin. The digest was subjected to reversed-phase HPLC and analysed by Q-TOF mass spectrometry. The only non-trypsin peptides that could be identified were two collagen Type I alpha2 peptides whose sequence was determined by fragmentation analysis. The association of collagen fragments with highly purified phosphophoryn suggests that the EDTA extraction method yields BDP that is strongly bound to collagen fragments. This association now helps explain discrepancies in molecular weight and amino acid composition data for various phosphophoryn preparations compared with the same data calculated from the C-terminal extension of mouse, rat and human dentine sialophosphoprotein (DSPP) gene products. Analysis of the mutation pattern of the clinical disorder Osteogenesis Imperfecta within the region enclosed by the identified collagen fragments reveals that phosphophoryn associates with a segment of collagen that is crucial for structure and/or function.

Fluorescently labeled collagen binding proteins allow specific visualization of collagen in tissues and live cell culture

Visualization of the formation and orientation of collagen fibers in tissue engineering experiments is crucial for understanding the factors that determine the mechanical properties of tissues. In this study, collagen-specific fluorescent probes were developed using a new approach that takes advantage of the inherent specificity of collagen binding protein domains present in bacterial adhesion proteins (CNA35) and integrins (GST-alpha1I). Both collagen binding domains were obtained as fusion proteins from an Escherichia coli expression system and fluorescently labeled using either amine-reactive succinimide (CNA35) or cysteine-reactive maleimide (GST-alpha1I) dyes. Solid-phase binding assays showed that both protein-based probes are much more specific than dichlorotriazinyl aminofluorescein (DTAF), a fluorescent dye that is currently used to track collagen formation in tissue engineering experiments. The CNA35 probe showed a higher affinity for human collagen type I than did the GST-alpha1I probe (apparent K(d) values of 0.5 and 50 microM, respectively) and showed very little cross-reactivity with noncollagenous extracellular matrix proteins. The CNA35 probe was also superior to both GST-alpha1I and DTAF in visualizing the formation of collagen fibers around live human venous saphena cells. Immunohistological experiments on rat tissue showed colocalization of the CNA35 probe with collagen type I and type III antibodies. The fluorescent probes described here have important advantages over existing methods for visualization of collagen, in particular for monitoring the formation of collagen in live tissue cultures over prolonged time periods.

Resolution of the vertical and horizontal heterogeneity of adsorbed collagen layers by combination of QCM-D and AFM

Collagen (type I from calf skin) adsorption on polystyrene (PS) and plasma-oxidized polystyrene (PSox) was studied, using a quartz crystal microbalance with energy dissipation measurements (QCM-D) and atomic force microscopy (AFM) in tapping mode. Radio-labeled collagen was used to measure the adsorbed amount and the ability of adsorbed collagen to exchange with molecules in the solution. The results show that the collagen adlayer consists of two parts: a dense and thin sheet in which fibrils are formed (directly observed by AFM) and an overlying thick layer (up to 200 nm) containing protruding molecules or bundles which are in very low concentration but modify noticeably the local viscosity. The thickness and viscosity of the semi-liquid adlayer both increase with adsorption time and collagen concentration. Fibril formation near the surface also increases with time and collagen concentration and occurs more readily on PS compared to PSox. Radiochemical measurements show that this may be related to the larger mobility of molecules adsorbed on PS, presumably owing to a smaller number of binding points.

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.

Structure, stability and interactions of type I collagen with GLY349-CYS substitution in alpha 1(I) chain in a murine Osteogenesis Imperfecta model

Here we report the structural and functional studies of collagen from the Brtl mouse, a heterozygous knock-in model for Osteogenesis Imperfecta, which has a G349C substitution introduced in one col1a1 allele. We observed that 25+/-5% of alpha 1(I) chains in different tissues and in different extracts from matrix deposited by cultured cells were S-S-linked mutant dimers. Apparently mutant and normal molecules are equally well incorporated into the matrix and they form mature covalent crosslinks with the same efficiency. We found different extents of post-translational overmodification of mutant molecules in different tissues, but we found no consistent differences between lethal and non-lethal animals. We did not detect any changes in the thermal stability or rate of thermal denaturation of mutant collagen. We also did not detect any changes in collagen-collagen recognition and interactions except for disruption of quasi-crystalline lateral packing of molecules in tendons from some, mostly prepubertal, mutant animals. In contrast, alpha 1(I)(3) collagen from the oim mouse--the only other non-lethal murine OI model studied by similar techniques--has altered stability, fibrillogenesis, collagen-collagen interactions and produces a more consistent and more pronounced disruption of tendon crystallinity. Nevertheless, while the G349C substitution causes moderate or lethal OI, heterozygous oim mice are much less affected. Overall, our results suggest that OI symptoms and phenotype variation in G349C animals are related to abnormal interactions of mutant collagen helices with other matrix molecules or abnormal function of osteoblasts rather than to abnormal structure, physical properties or interactions between mutant collagen helices.

Alignment and assembly of adsorbed collagen molecules induced by anisotropic chemical nanopatterns

Collagen, a protein widely used to control cell-material interactions, is known to self-assemble in solution. Supramolecular structures also form on material surfaces following collagen adsorption. Herein, we report the use of anisotropic, flat, surface chemical nanopatterns, which consist of alkyl-terminated tracks drawn in an oligo(ethylene glycol)-terminated matrix, to direct collagen adsorption. As revealed by atomic force microscopy, the spontaneous collagen adsorption performed on such patterned substrates results in the accumulation of collagen on the hydrophobic tracks. Moreover, the width of the tracks (30-90 nm), which is much smaller than the length of the collagen molecule (approximately 300 nm), is the origin of preferential alignment of the molecules and of their assembly into continuous bundles of adsorbed collagen. This chemical guidance effect due to self-confinement of proteins upon adsorption may bring novel and valuable applications, specifically in biomaterials science and cell growth control.

Identifying the SPARC Binding Sites on Collagen I and Procollagen I by Atomic Force Microscopy

SPARC (secreted protein acidic and rich in cysteine) is a matricellular protein associated with the extracellular matrix (ECM). It has been found that the production of collagen I is a requisite for the association of SPARC with ECM, and studies with SPARC-null mice indicate that SPARC plays a role in modifying the structure of collagen fibers. It is not known, however, whether SPARC interacts with the collagen I precursor, procollagen I. In this study, the binding of SPARC to collagen I and procollagen I was verified by surface plasmon resonance. The SPARC-binding sites on collagen I and procollagen I were identified by directly visualizing their complexes using tapping-mode atomic force microscopy (TM-AFM). The characteristic chain end feature in collagen I is not readily detected by AFM, so unambiguous location of the binding sites relative to the C- or N-termini is difficult. In contrast, procollagen I, with its large globular C-propeptide, permits easy identification of the C-terminus. Histograms were constructed and compared based on the distances of the bound SPARC to the C-terminus of procollagen I and to the closest end of collagen I. There is a broad distribution of SPARC binding sites on procollagen I with the most preferred binding region located approximately 1/3 from the C-terminus. Characterization of the SPARC-binding sites on collagen I and procollagen I provides useful information for further understanding of the functional implications of their interactions.

Involvement of the collagen I-binding motif in the anti-angiogenic activity of pigment epithelium-derived factor

Pigment epithelium-derived factor (PEDF) is the most potent endogenous inhibitor of angiogenesis in age-related macular degeneration and tumors. However, the molecular mechanism of the anti-angiogenic activity of PEDF is poorly understood. PEDF interacts with the extracellular matrix (ECM) in vitro. Here, we investigated the possible involvement of the motif for ECM interaction in the anti-angiogenic activity of PEDF. The growth rates of HeLa cells in culture were not affected by transfection of PEDF, indicating that PEDF did not suppress tumor cell growth directly. In tumor xenografts, the overexpression of wild-type PEDF significantly suppressed tumor growth, whereas a mutant of the collagen I-binding site of PEDF (Col-mut PEDF) did not inhibit tumor growth. A mutant of the heparin-binding site of PEDF (Hep-mut PEDF) suppressed tumor growth. Histological analysis showed that the density and area of microvasculatures in either PEDF or Hep-mut PEDF were suppressed when compared with those in either vector or Col-mut PEDF. Our data indicate that PEDF inhibits tumor growth via its anti-angiogenic activity, and the collagen I-binding motif of PEDF is involved in the biological activity.

A novel COL1A1 mutation in infantile cortical hyperostosis (Caffey disease) expands the spectrum of collagen-related disorders

Infantile cortical hyperostosis (Caffey disease) is characterized by spontaneous episodes of subperiosteal new bone formation along 1 or more bones commencing within the first 5 months of life. A genome-wide screen for genetic linkage in a large family with an autosomal dominant form of Caffey disease (ADC) revealed a locus on chromosome 17q21 (LOD score, 6.78). Affected individuals and obligate carriers were heterozygous for a missense mutation (3040Ctwo head right arrowT) in exon 41 of the gene encoding the alpha1(I) chain of type I collagen (COL1A1), altering residue 836 (R836C) in the triple-helical domain of this chain. The same mutation was identified in affected members of 2 unrelated, smaller families with ADC, but not in 2 prenatal cases and not in more than 300 chromosomes from healthy individuals. Fibroblast cultures from an affected individual produced abnormal disulfide-bonded dimeric alpha1(I) chains. Dermal collagen fibrils of the same individual were larger, more variable in shape and size, and less densely packed than those in control samples. Individuals bearing the mutation, whether they had experienced an episode of cortical hyperostosis or not, had joint hyperlaxity, hyperextensible skin, and inguinal hernias resembling symptoms of a mild form of Ehlers-Danlos syndrome type III. These findings extend the spectrum of COL1A1-related diseases to include a hyperostotic disorder.

Influence of the aggregation state in solution on the supramolecular organization of adsorbed type I collagen layers

In the last years, adsorbed collagen was shown to form layers with a supramolecular organization depending on the substrate surface properties and on the preparation procedure. If the concentration of collagen and the duration of adsorption are sufficient, fibrillar collagen structures are formed, corresponding to assemblies of a few molecules. This occurs more readily on hydrophobic compared to hydrophilic surfaces. This study aims at understanding the origin of such fibrillar structures and in particular at determining whether they result from the deposition of fibrils formed in solution or from the building of assemblies at the interface. Therefore, type I collagen solutions with an increasing degree of aggregation were prepared, using the "neutral-start" approach, by ageing pH 5.8 solutions at 37 degrees C for 15 min, 2 or 7 days. The obtained solutions were used to investigate the influence of collagen aggregation in solution on the supramolecular organization of adsorbed collagen layers, which was characterized by X-ray photoelectron spectroscopy and atomic force microscopy. Polystyrene and plasma-oxidized polystyrene were chosen as substrates for the adsorption. The size and the density of collagen fibrils at the interface decreased upon increasing the degree of aggregation of collagen in solution. This is explained by a competitive adsorption process between monomers and aggregates of the solution, turning at the advantage of the monomers. More aggregated solutions, which are thus depleted in free monomers, behave like less concentrated solutions, i.e. lead to a lower adsorbed amount and less fibril formation at the interface. This study shows that the supramolecular fibrils observed in adsorbed collagen layers, especially on hydrophobic substrates, are not formed in the solution, prior to adsorption, but are built at the interface, through the assembly of free segments of adsorbed molecules.