Assembly in vitro of thin and thick fibrils of collagen II from recombinant procollagen II. The monomers in the tips of thick fibrils have the opposite orientation from monomers in the growing tips of collagen I fibrils

Collagen type II: From biosynthesis to advanced biomaterials for cartilage engineering

Collagen type II is the major constituent of cartilage tissue. Yet, cartilage engineering approaches are primarily based on collagen type I devices that are associated with suboptimal functional therapeutic outcomes. Herein, we briefly describe cartilage's development and cellular and extracellular composition and organisation. We also provide an overview of collagen type II biosynthesis and purification protocols from tissues of terrestrial and marine species and recombinant systems. We then advocate the use of collagen type II as a building block in cartilage engineering approaches, based on safety, efficiency and efficacy data that have been derived over the years from numerous in vitro and in vivo studies.

Type-I collagen fibrils: From growth morphology to local order

The length of type-I collagen fibrils in solution increases through the development and progress of pointed tips appearing successively at the two ends of an axis-symmetric shaft with constant diameter. Those tips, respectively fine ([Formula: see text]) or coarse ([Formula: see text]) have opposite molecular orientations. The [Formula: see text]-pointed tips, the first to appear, are particularly remarkable as they all show, on most of their length, a common parabolic profile which stays constant during the growth. Assuming that the latter occurs by lateral accretion of individual molecules in staggered configuration, we propose to give account of this prominent morphological feature along a purely geometrical argument, the profile of a tip being linked to the shape of the trajectories followed all along the accretion process. Among several possible trajectories, Fermat spirals lead to a parabolic profile in perfect agreement with the one observed for [Formula: see text]-pointed tips. This is to be put in relation with the presence of such spirals in phyllotactic patterns which ensure the best packing efficiency in cases of axis-symmetry, which is indeed that of dense collagen fibrils. Moreover, those patterns are structured by concentric circles of dislocations, constitutive of the structure itself, whose behaviour might contribute to the mechanical properties of the fibrils.

To achieve self-assembled collagen mimetic fibrils using designed peptides

It has proven challenging to obtain collagen-mimetic fibrils by protein design. We recently reported the self-assembly of a mini-fibril showing a 35 nm, D-period like, axially repeating structure using the designed triple helix Col108. Peptide Col108 was made by bacterial expression using a synthetic gene; its triple helix domain consists of three pseudo-identical units of amino acid sequence arranged in tandem. It was postulated that the 35 nm d-period of Col108 mini-fibrils originates from the periodicity of the Col108 primary structure. A mutual staggering of one sequence unit of the associating Col108 triple helices can maximize the inter-helical interactions and produce the observed 35 nm d-period. Based on this unit-staggered model, a triple helix consisting of only two sequence units is expected to have the potential to form the same d-periodic mini-fibrils. Indeed, when such a peptide, peptide 2U108, was made it was found to self-assemble into mini-fibrils having the same d-period of 35 nm. In contrast, no d-periodic mini-fibrils were observed for peptide 1U108, which does not have long-range repeating sequences in its primary structure. The findings of the periodic mini-fibrils of Col108 and 2U108 suggest a way forward to create collagen-mimetic fibrils for biomedical and industrial applications.

Molecular assembly of recombinant chicken type II collagen in the yeast Pichia pastoris

Effective treatment of rheumatoid arthritis can be mediated by native chicken type II collagen (nCCII), recombinant peptide containing nCCII tolerogenic epitopes (CTEs), or a therapeutic DNA vaccine encoding the full-length CCOL2A1 cDNA. As recombinant CCII (rCCII) might avoid potential pathogenic virus contamination during nCCII preparation or chromosomal integration and oncogene activation associated with DNA vaccines, here we evaluated the importance of propeptide and telopeptide domains on rCCII triple helix molecular assembly. We constructed pC- and pN-procollagen (without N- or C-propeptides, respectively) as well as CTEs located in the triple helical domain lacking both propeptides and telopeptides, and expressed these in yeast Pichia pastoris host strain GS115 (his4, Mut⁺) simultaneously with recombinant chicken prolyl-4-hydroxylase α and β subunits. Both pC- and pN-procollagen monomers accumulated inside P. pastoris cells, whereas CTE was assembled into homotrimers with stable conformation and secreted into the supernatants, suggesting that the large molecular weight pC-or pN-procollagens were retained within the endoplasmic reticulum whereas the smaller CTEs proceeded through the secretory pathway. Furthermore, resulting recombinant chicken type II collagen pCα1(II) can induced collagen-induced arthritis (CIA) rat model, which seems to be as effective as the current standard nCCII. Notably, protease digestion assays showed that rCCII could assemble in the absence of C- and N-propeptides or telopeptides. These findings provide new insights into the minimal structural requirements for rCCII expression and folding.

Target-Specific Delivery of an Antibody That Blocks the Formation of Collagen Deposits in Skin and Lung

Regardless of the cause of organ fibrosis, its main unwanted consequence is the formation of collagen fibril-rich deposits that hamper the structure and function of affected tissues. Although many strategies have been proposed for the treatment of fibrotic diseases, no therapy has been developed, which can effectively block the formation of collagen fibril deposits. With this in mind, we recently developed an antibody-based therapy to block key interactions that drive collagen molecules into fibrils. In this study, we analyzed target specificity, which is a main parameter that defines the safe use of all antibody-based therapies in humans. We hypothesized that, regardless of the route of administration, our antibody would preferentially bind to free collagen molecules synthesized at the sites of fibrosis and have minimal off-target interactions when applied in various tissues. To test this hypothesis, we used two experimental models of organ fibrosis: (1) a keloid model, in which antibody constructs were directly implanted under the skin of nude mice and (2) an experimental model of pulmonary fibrosis, in which our antibody was administered systemically by intravenous injection. Following administration, we studied the distribution of our antibody within target and off-target sites as well as analyzed its effects on fibrotic tissue formation. We found that local and systemic application of our antibody had high specificity for targeting collagen fibrillogenesis and also appeared safe and therapeutically effective. In summary, this study provides the basis for further testing our antifibrotic antibody in a broad range of disease conditions and suggests that this treatment approach will be effective if delivered by local or systemic administration.

Fell Muir Lecture: Collagen fibril formation in vitro and in vivo

It is a great honour to be awarded the Fell Muir Prize for 2016 by the British Society of Matrix Biology. As recipient of the prize, I am taking the opportunity to write a minireview on collagen fibrillogenesis, which has been the focus of my research for 33 years. This is the process by which triple helical collagen molecules assemble into centimetre-long fibrils in the extracellular matrix of animals. The fibrils appeared a billion years ago at the dawn of multicellular animal life as the primary scaffold for tissue morphogenesis. The fibrils occur in exquisite three-dimensional architectures that match the physical demands of tissues, for example orthogonal lattices in cornea, basket weaves in skin and blood vessels, and parallel bundles in tendon, ligament and nerves. The question of how collagen fibrils are formed was posed at the end of the nineteenth century. Since then, we have learned about the structure of DNA and the peptide bond, understood how plants capture the sun's energy, cloned animals, discovered antibiotics and found ways of editing our genome in the pursuit of new cures for diseases. However, how cells generate tissues from collagen fibrils remains one of the big unsolved mysteries in biology. In this review, I will give a personal account of the topic and highlight some of the approaches that my research group are taking to find new insights.

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.

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.

Testing the anti-fibrotic potential of the single-chain Fv antibody against the α2 C-terminal telopeptide of collagen I

Abstract This study focuses on the single-chain fragment variable (scFv) variant of the original IgA-type antibody, recognizing the α2 C-terminal telopeptide (α2Ct) of human collagen I, designed to inhibit post-traumatic localized fibrosis via blocking the formation of collagen-rich deposits. We have demonstrated that the scFv construct expressed in yeast cells was able to fold into an immunoglobulin-like conformation, but it was prone to forming soluble aggregates. Functional assays, however, indicate that the scFv construct specifically binds to the α2Ct epitope and inhibits collagen fibril formation both in vitro and in a cell culture model representing tissues that undergo post-traumatic fibrosis. Thus, the presented study demonstrates the potential of the scFv variant to serve as an inhibitor of the excessive formation of collagen-rich fibrotic deposits, and it reveals certain limitations associated with the current stage of development of this antibody construct.

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

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

Kuskokwim syndrome, a recessive congenital contracture disorder, extends the phenotype of FKBP10 mutations

Recessive mutations in FKBP10 at 17q21.2, encoding FKBP65, cause both osteogenesis imperfecta (OI) and Bruck syndrome (OI plus congenital contractures). Contractures are a variable manifestation of null/missense FKBP10 mutations. Kuskokwim syndrome (KS) is an autosomal recessive congenital contracture disorder found among Yup'ik Eskimos. Linkage mapping of KS to chromosome 17q21, together with contractures as a feature of FKBP10 mutations, made FKBP10 a candidate gene. We identified a homozygous three-nucleotide deletion in FKBP10 (c.877_879delTAC) in multiple Kuskokwim pedigrees; 3% of regional controls are carriers. The mutation deletes the highly conserved p.Tyr293 residue in FKBP65's third peptidyl-prolyl cis-trans isomerase domain. FKBP10 transcripts are normal, but mutant FKBP65 is destabilized to a residual 5%. Collagen synthesized by KS fibroblasts has substantially decreased hydroxylation of the telopeptide lysine crucial for collagen cross-linking, with 2%-10% hydroxylation in probands versus 60% in controls. Matrix deposited by KS fibroblasts has marked reduction in maturely cross-linked collagen. KS collagen is disorganized in matrix, and fibrils formed in vitro had subtle loosening of monomer packing. Our results imply that FKBP10 mutations affect collagen indirectly, by ablating FKBP65 support for collagen telopeptide hydroxylation by lysyl hydroxylase 2, thus decreasing collagen cross-links in tendon and bone matrix. FKBP10 mutations may also underlie other arthrogryposis syndromes.

Strategies for enhancing the accumulation and retention of extracellular matrix in tissue-engineered cartilage cultured in bioreactors

Production of tissue-engineered cartilage involves the synthesis and accumulation of key constituents such as glycosaminoglycan (GAG) and collagen type II to form insoluble extracellular matrix (ECM). During cartilage culture, macromolecular components are released from nascent tissues into the medium, representing a significant waste of biosynthetic resources. This work was aimed at developing strategies for improving ECM retention in cartilage constructs and thus the quality of engineered tissues produced in bioreactors. Human chondrocytes seeded into polyglycolic acid (PGA) scaffolds were cultured in perfusion bioreactors for up to 5 weeks. Analysis of the size and integrity of proteoglycans in the constructs and medium showed that full-sized aggrecan was being stripped from the tissues without proteolytic degradation. Application of low (0.075 mL min(-1)) and gradually increasing (0.075-0.2 mL min(-1)) medium flow rates in the bioreactor resulted in the generation of larger constructs, a 4.0-4.4-fold increase in the percentage of GAG retained in the ECM, and a 4.8-5.2-fold increase in GAG concentration in the tissues compared with operation at 0.2 mL min(-1). GAG retention was also improved by pre-culturing seeded scaffolds in flasks for 5 days prior to bioreactor culture. In contrast, GAG retention in PGA scaffolds infused with alginate hydrogel did not vary significantly with medium flow rate or pre-culture treatment. This work demonstrates that substantial improvements in cartilage quality can be achieved using scaffold and bioreactor culture strategies that specifically target and improve ECM retention.

Modulation of collagen II fiber formation in 3-D porous scaffold environments

Collagen II, a major extracellular matrix component in cartilaginous tissues, undergoes fibrillogenesis under physiological conditions. The present study explored collagen II fiber formation in solution and in two- (coverslip) and three-dimensional (scaffold) environments under different incubation conditions. These conditions include variations in adsorption buffers, the presence of 1-ethyl-3-(3-dimenthylaminopropyl) carbodiimide/N-hydroxysuccinimide crosslinker and the nature of the material surfaces. We extend our observations of collagen II fiber formation in two dimensions to develop an approach for the formation of a fibrillar collagen II network throughout surface-modified polylactide-co-glycolide porous scaffolds. Morphologically, the collagen II network is similar to that present in native articular cartilage. Biological validation of the resultant optimized functional scaffold, using rat bone marrow-derived mesenchymal stem cells, shows appreciable cell infiltration throughout the scaffold with enhanced cell spreading at 24h post-seeding. This economic and versatile approach is thus believed to have significant potential in cartilage tissue engineering applications.

Collagen fibril formation. A new target to limit fibrosis

We present a concept for reducing formation of fibrotic deposits by inhibiting self-assembly of collagen molecules into fibrils, a main component of fibrotic lesions. Employing monoclonal antibodies that bind to the telopeptide region of a collagen molecule, we found that blocking telopeptide-mediated collagen/collagen interactions reduces the amount of collagen fibrils accumulated in vitro and in keloid-like organotypic constructs. We conclude that inhibiting extracellular steps of the fibrotic process provides a novel approach to limit fibrosis in a number of tissues and organs.

Influences of hyaluronan on type II collagen fibrillogenesis in vitro

The effect to the kinetics of type II collagen fibrillogenesis with the addition of hyaluronan (HA), (Mw of 1.8x10(6) Da), at various concentrations of HA (0.01, 0.05 and 0.1 wt.%) for a series of fibril formation systems was examined in this study. Evidences deduced from the turbidity-time curves revealed that the inclusion of HA had minor or no impact to the fibrillogenesis of type II collagen (collagen conc. at 0.2 mg/mL). The apparent rate constants, klag (lag phase) increased slightly but kg (growth phase) decreased not very significantly with addition of HA, as compared to the case of pure collagen. This leads us to believe tentatively that, with the addition of HA to collagen solutions, the nucleation process of the fibril formation might have been sped up slightly whereas the growth process slowed up slightly. However, data from TEM observations on the resulting fibrils indicated that the presence of HA did not significantly affect the diameters and the characteristic D-banding periods of the collagen fiber formed. And, from the statistical analyses, we found only insignificant difference (P>0.05) between the specimens from the various experimental groups. It seems to indicate that the ultimate packing of collagen monomers was probably not interfered or affected significantly by the presence of HA in vitro.

Collagen XI chain misassembly in cartilage of the chondrodysplasia (cho) mouse

Molecular mechanisms controlling the assembly of cartilage-specific types II, IX and XI collagens into a heteropolymeric network of uniformly thin, unbanded fibrils are not well understood, but collagen XI has been implicated. The present study on cartilage from the homozygous chondrodysplasia (cho/cho) mouse adds support to this concept. In the absence of alpha1(XI) collagen chains, thick, banded collagen fibrils are formed in the extracellular matrix of cho/cho cartilage. A functional knock-out of the type XI collagen molecule has been assumed. We have re-examined this at the protein level to see if, rather than a complete knock-out, alternative type XI chain assemblies were formed. Mass spectrometry of purified triple-helical collagen from the rib cartilage of cho/cho mice identified alpha1(V) and alpha2(XI) chains. These chains were recovered in roughly equal amounts based on Coomassie Blue staining of SDS-PAGE gels, in addition to alpha1(II)/alpha3(XI) collagen chains. Using telopeptide-specific antibodies and Western blot analysis, it was further shown that type V/XI trimers were present in the matrix cross-linked to each other and to type II collagen molecules to form heteropolymers. Cartilage from heterozygous (cho/+) mice contained a mix of alpha1(V) and alpha1(XI) chains and a mix of thin and thick fibrils on transmission electron microscopy. In summary, the results imply that native type XI collagen molecules containing an alpha1(XI) chain are required to form uniformly thin fibrils and support a role for type XI collagen as the template for the characteristic type II collagen fibril network of developing cartilage.

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.

Influence of fibril taper on the function of collagen to reinforce extracellular matrix

Collagen fibrils provide tensile reinforcement for extracellular matrix. In at least some tissues, the fibrils have a paraboloidal taper at their ends. The purpose of this paper is to determine the implications of this taper for the function of collagen fibrils. When a tissue is subjected to low mechanical forces, stress will be transferred to the fibrils elastically. This process was modelled using finite element analysis because there is no analytical theory for elastic stress transfer to a non-cylindrical fibril. When the tissue is subjected to higher mechanical forces, stress will be transferred plastically. This process was modelled analytically. For both elastic and plastic stress transfer, a paraboloidal taper leads to a more uniform distribution of axial tensile stress along the fibril than would be generated if it were cylindrical. The tapered fibril requires half the volume of collagen than a cylindrical fibril of the same length and the stress is shared more evenly along its length. It is also less likely to fracture than a cylindrical fibril of the same length in a tissue subjected to the same mechanical force.

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.

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.

Procollagen VII self-assembly depends on site-specific interactions and is promoted by cleavage of the NC2 domain with procollagen C-proteinase

Procollagen VII is a homotrimer of 350-kDa proalpha1(VII) chains. Each chain has a central collagenous domain flanked by a noncollagenous amino-terminal NC1 domain and a carboxy-terminal NC2 domain. After secretion from cells, procollagen VII molecules form antiparallel dimers with a 60 nm overlap. These dimers are stabilized by disulfide bonds formed between cysteines present in the NC2 domain and cysteines present in the triple-helical domain. Electron microscopy has provided direct evidence for the existence of collagen VII dimers, but the dynamic process of dimer formation is not well understood. In the present study, we tested the hypothesis that, during dimer formation, the NC2 domain of one procollagen VII molecule specifically recognizes and binds to the triple-helical region adjacent to Cys-2625 of another procollagen VII molecule. We also investigated the role of processing of the NC2 domain by the procollagen C-proteinase/BMP-1 in dimer assembly. We engineered mini mouse procollagen VII variants consisting of intact NC1 and NC2 domains and a shortened triple helix in which the C-terminal region encompassing Cys-2625 was either preserved or substituted with the region encompassing Cys-1448 derived from the N-terminal part of the triple-helical domain. The results indicate that procollagen VII self-assembly depends on site-specific interactions between the NC2 domain and the triple-helical region adjacent to Cys-2625 and that this process is promoted by the cleavage of the NC2 by procollagen C-proteinase/BMP1.

Procollagen II amino propeptide processing by ADAMTS-3. Insights on dermatosparaxis

The amino and carboxyl propeptides of procollagens I and II are removed by specific enzymes as a prerequisite for fibril assembly. Null mutations in procollagen I N-propeptidase (ADAMTS-2) cause dermatosparaxis in cattle and the Ehlers-Danlos syndrome (dermatosparactic type) in humans by preventing proteolytic excision of the N-propeptide of procollagen I. We have found that procollagen II is processed normally in dermatosparactic nasal cartilage, suggesting the existence of another N-propeptidase(s). We investigated such a role for ADAMTS-3 in Swarm rat chondrosarcoma RCS-LTC cells, which fail to process the procollagen II N-propeptide. Stable transfection of RCS-LTC cells with bovine ADAMTS-2 or human ADAMTS-3 partially rescued the processing defect, suggesting that ADAMTS-3 has procollagen II N-propeptidase activity. Human skin and skin fibroblasts showed 30-fold higher mRNA levels of ADAMTS-2 than ADAMTS-3, whereas ADAMTS-3 mRNA was 5-fold higher than ADAMTS-2 mRNA in human cartilage. We propose that both ADAMTS-2 and ADAMTS-3 process procollagen II, but ADAMTS-3 is physiologically more relevant, given its preferred expression in cartilage. The findings provide an explanation for the sparing of cartilage in dermatosparaxis and, perhaps, for the relative sparing of some procollagen I-containing tissues.

A model for type II collagen fibrils: distinctive D-band patterns in native and reconstituted fibrils compared with sequence data for helix and telopeptide domains

The periodical D-band pattern is generally considered a unique ultrastructural feature shared by all fibril-forming collagens, which correlates with the intrafibril, paracrystalline array of tropocollagen monomers. Distinct band patterns have been reported, however, for collagen stained long-spacing (SLS) crystallites of genetic types I, II, and III. Moreover, D-band patterns of negatively stained, native type II collagen fibrils were found to be not identical to those of type I in our previous research. Because of (a) these distinctive features, (b) tropocollagen heterotrimeric conditions (type I) vs homotrimeric conditions (type II), and (c) different lengths and poor homology between extrahelical telopeptides, the molecular array or telopeptide conformation within the extensively studied type I collagen fibrils could be not the same as those in the very much less intensively studied type II collagen fibrils. In this investigation, a distinctive positive-staining D-band pattern was found for type II collagen fibrils obtained from human cartilages. A fibril model was developed by analyzing actual D-band patterns, and matching them against simulated patterns based on the primary structure of extrahelical and helical domains in human type II tropocollagen. In particular, a more prominent b(1) band was apparent in native type II collagen fibrils than in type I. This distinctive feature was also observed for native-type collagen fibrils reconstituted from purified type II collagen, i.e., free from associated minor type XI collagen. On modeling possible monomer arrays, the best fit between microdensitograms and simulation traces was found for 234 amino acid staggering, as is also the case for type I collagen fibrils. On comparing this model with an analogous one for type I collagen fibrils, there was a higher intraband distribution of charged residues for band b(1), consistent with the higher electrondensity observed for this band in type II collagen fibrils. N- and C-telopeptide displacement in the model corresponded to D-locations of a c(2) subband, which we named c(2.0), and band a(3), respectively. In simulation profiles, c(2.0) -like and a(3) -like peaks mimicked the corresponding peaks in microdensitograms when molecular reversals were adopted at positions 10N-12N, 12C-14C, and 17C-19C for N- and C-telopeptides. Hydrophobic interactions and algorithmic predictions of protein secondary structure, according to Chou and Fasman and Rost and Sander criteria, were consistent with these conformational models, and suggest that an additional molecular reversal may occur at positions 3N-5N. These telopeptide "S-fold" conformations, interpreted as axial projections of tridimensional conformation, may represent starting points for further investigation into the still unresolved tridimensional conformation of telopeptides in monomers arrayed within type II collagen fibrils.

Collagen XI nucleates self-assembly and limits lateral growth of cartilage fibrils

Fibrils of embryonic cartilage are heterotypic alloys formed by collagens II, IX, and XI and have a uniform diameter of approximately 20 nm. The molecular basis of this lateral growth control is poorly understood. Collagen II subjected to fibril formation in vitro produced short and tapered tactoids with strong D-periodic banding. The maximal width of these tactoids varied over a broad range. By contrast, authentic mixtures of collagens II, IX, and XI yielded long and weakly banded fibrils, which, strikingly, had a uniform width of about 20 nm. The same was true for mixtures of collagens II and XI lacking collagen IX as long as the molar excess of collagen II was less than 8-fold. At higher ratios, the proteins assembled into tactoids coexisting with cartilage-like fibrils. Therefore, diameter control is an inherent property of appropriate mixtures of collagens II and XI. Collagen IX is not essential for this feature but strongly increases the efficiency of fibril formation. Therefore, this protein may be an important stabilizing factor of cartilage fibrils.

Collagen II containing a Cys substitution for Arg-alpha1-519. Analysis by atomic force microscopy demonstrates that mutated monomers alter the topography of the surface of collagen II fibrils

A recombinant human procollagen II was prepared that contained a substitution of Cys for Arg at alpha1-519 and that was found in five families with early onset generalized osteoarthritis with or without features of a mild chondrodysplasia. Previously, the presence of mutated monomers in mixtures with wildtype collagen II was shown to increase the lag period for fibril assembly. Also, the fibrils were more loosely packed and some thick fibrils lacked a D-periodic banding pattern. Here we re-examined the fibrils using a combination of transmission electron microscopy and atomic force microscopy. The presence of the mutated monomers increased the diameter of the thin filaments that were consistently formed in association with the thick fibrils of collagen II. In addition, the presence of the mutated monomers increased the depth of the gap regions in all fibrils with a distinct D-periodic banding pattern. The results, therefore, may indicate that the mutated monomers formed two or three additional outer layers of monomers in 0D-period staggers on the surface of the fibrils. Apparently, the mutated monomers were bound on the surface through intermolecular disulfide bonds.

Growth of sea cucumber collagen fibrils occurs at the tips and centers in a coordinated manner

Collagen fibrils are the principle source of mechanical strength in the mutable dermis of the sea cucumber Cucumaria frondosa. To obtain information about the mechanism by which collagen molecules self-assemble into fibrils, we have isolated single intact fibrils with lengths in the range 14-444 microm. These fibrils have been studied by scanning transmission electron microscopy, yielding data that show how cross-sectional mass, and hence the number of molecules in the cross-section, depend on axial location. In an individual fibril, the two ends always display similar mass distributions. The two tips of each fibril must therefore maintain identity in shape and size throughout growth. The linear relationship between cross-sectional mass and distance from the adjacent end shows that a growing tip is (like the tip of a vertebrate collagen fibril) paraboloidal in shape. Comparison of data from many different fibrils, over a wide range of lengths, however, revealed that the paraboloidal tip becomes blunter as the fibril grows in length. In contrast to vertebrate fibrils, those from C. frondosa do not have a central shaft region of constant cross-sectional mass. Rather, the cross-sectional mass increases to a maximum in the center of each fibril. The maximum cross-sectional mass of the fibrils increases exponentially with increasing fibril length. The centrosymmetry, the paraboloidal shape of the tips, and the hyperbolic increase in maximum cross-sectional mass with fibril length, is evidence for a co-ordinated regulation of length and diameter, which differs from the kind of regulation that gives rise to collagen fibrils in vertebrates (chickens and mice).

Inhibition of the self-assembly of collagen I into fibrils with synthetic peptides. Demonstration that assembly is driven by specific binding sites on the monomers

A series of experiments were carried out to test the hypothesis that the self-assembly of collagen I monomers into fibrils depends on the interactions of specific binding sites in different regions of the monomer. Six synthetic peptides were prepared with sequences found either in the collagen triple helix or in the N- or C-telopeptides of collagen I. The four peptides with sequences found in the telopeptides were found to inhibit self-assembly of collagen I in a purified in vitro system. At concentrations of 2.5 mM, peptides with sequences in the C-telopeptides of the alpha1(I) and alpha2(I) chain inhibited assembly at about 95%. The addition of the peptide with the alpha2-telopeptide sequence was effective in inhibiting assembly if added during the lag phase and early propagation phase but not later in the assembly process. Experiments with biotinylated peptides indicated that both the N- and C-telopeptides bound to a region between amino acid 776 and 822 of the alpha(I) chain. A fragment of nine amino acids with sequences in the alpha2-telopeptide was effective in inhibiting fibril assembly. Mutating two aspartates in the 9-mer peptide to serine had no effect on inhibition of fibril assembly, but mutating two tyrosine residues and one phenylalanine residue abolished the inhibitory action. Molecular modeling of the binding sites demonstrated favorable hydrophobic and electrostatic interactions between the alpha2telopeptide and residues 781-794 of the alpha(I) chain.

Structurally abnormal type II collagen in a severe form of Kniest dysplasia caused by an exon 24 skipping mutation

Type II collagen mutations have been identified in a phenotypic continuum of chondrodysplasias that range widely in clinical severity. They include achondrogenesis type II, hypochondrogenesis, spondyloepiphyseal dysplasia congenita, spondyloepimetaphyseal dysplasia, Kniest dysplasia, and Stickler syndrome. We report here results that define the underlying genetic defect and consequent altered structure of assembled type II collagen in a neonatal lethal form of Kniest dysplasia. Electrophoresis of a cyanogen bromide (CNBr) (CB) digest of sternal cartilage revealed an alpha1(II)CB11 peptide doublet and a slightly retarded mobility for all major CB peptides, which implied post-translational overmodification. Further peptide mapping and sequence analysis of CB11 revealed equal amounts of a normal alpha1(II) sequence and a chain lacking the 18 residues (361-378 of the triple helical domain) corresponding to exon 24. Sequence analysis of an amplified genomic DNA fragment identified a G to A transition in the +5 position of the splice donor consensus sequence of intron 24 in one allele. Cartilage matrix analysis showed that the short alpha1(II) chain was present in collagen molecules that had become cross-linked into fibrils. Trypsin digestion of the pepsin-extracted native type II collagen selectively cleaved the normal length alpha1(II) chains within the exon 24 domain. These findings support a hypothesis that normal and short alpha-chains had combined to form heterotrimeric molecules in which the chains were in register in both directions from the deletion site, accommodated effectively by a loop out of the normal chain exon 24 domain. Such an accommodation, with potential overall shortening of the helical domain and hence misalignment of intermolecular relationships within fibrils, offers a common molecular mechanism by which a group of different mutations might act to produce the Kniest phenotype.

Collagen II containing a Cys substitution for arg-alpha1-519. Homotrimeric monomers containing the mutation do not assemble into fibrils but alter the self-assembly of the normal protein

A recombinant system was used to prepare human type II procollagen containing the substitution of Cys for Arg at alpha1-519 found in three unrelated families with early onset generalized osteoarthritis together with features of a mild chondrodysplasia probably best classified as spondyloepiphyseal dysplasia. In contrast to mutated procollagens containing Cys substitutions for obligatory Gly residues, the Cys substitution at alpha1-519 did not generate any intramolecular disulfide bonds. The results were consistent with computer modeling experiments that demonstrated that the alpha carbon distances were shorter with Cys substitutions for obligatory Gly residues than with Cys substitutions in the Y position residues in repeating -Gly-X-Y- sequences of the collagen triple helix. The mutated collagen did not assemble into fibrils under conditions in which the normal monomers polymerized. However, the presence of the mutated monomer in mixtures with normal collagen II increased the lag time for fibril assembly and altered the morphology of the fibrils formed.

Collagen fibrillogenesis in situ: fibril segments become long fibrils as the developing tendon matures

Tissue architecture, stability, and mechanical attributes are all determined by the structure and organization of collagen fibrils. Therefore, the characterization of fibril growth steps and determination of how this growth is regulated is essential to the elucidation of how tissues are assembled. We have proposed that fibril segments are intermediates in the formation of mature fibrils. The purpose of this study was to determine the length and structure of fibrils within a relatively mature tendon. The in situ determination of length performed here was only the second direct determination of fibril length in a vertebrate connective tissue and the first for a relatively mature tissue. The data demonstrate that the fibrils were discontinuous at 18 days of tendon development. However, both ends were not present in any of the analyzed fibrils within the 18-day tendon. Because the data set was 50-60 microm, this indicates a mean fibril length greater than 60 microm. These data are in contrast to data from the 14-day tendon, in which 80% of the fibrils had both ends in a 26-microm data set and the mean segment length was shown to be 10-30 microm. There were equal numbers of alpha and beta ends in the 18-day tendon. The structure of the ends was comparable to that in the less mature tendon. The data also indicate that fibril asymmetry and structure were maintained. The increase in fibril length is interpreted as being the result of a post-depositional, regulated assembly of segments via a lateral association/fusion to form mature fibrils. This hypothesis predicts an increase in diameter at this stage of development. The diameter increases have been documented, but this is the first demonstration of increases in length and maintenance of segment structure during this important stage of tendon development.