Interaction of collagen alpha1(X) containing engineered NC1 mutations with normal alpha1(X) in vitro. Implications for the molecular basis of schmid metaphyseal chondrodysplasia

Gene cloning to clinical trials-the trials and tribulations of a life with collagen

This review, based on the BSMB Fell-Muir Lecture I presented in July 2018 at the Matrix Biology Europe Conference in Manchester, gives a personal perspective of my own laboratory's contributions to research into type X collagen, metaphyseal chondrodysplasia type Schmid and potential treatments for this disorder that are currently entering clinical trial. I have tried to set the advances made in the context of the scientific technologies available at the time and how these have changed over the more than three decades of this research.

Genetic Disorders of the Extracellular Matrix

Mutations in the genes for extracellular matrix (ECM) components cause a wide range of genetic connective tissues disorders throughout the body. The elucidation of mutations and their correlation with pathology has been instrumental in understanding the roles of many ECM components. The pathological consequences of ECM protein mutations depend on its tissue distribution, tissue function, and on the nature of the mutation. The prevalent paradigm for the molecular pathology has been that there are two global mechanisms. First, mutations that reduce the production of ECM proteins impair matrix integrity largely due to quantitative ECM defects. Second, mutations altering protein structure may reduce protein secretion but also introduce dominant negative effects in ECM formation, structure and/or stability. Recent studies show that endoplasmic reticulum (ER) stress, caused by mutant misfolded ECM proteins, makes a significant contribution to the pathophysiology. This suggests that targeting ER‐stress may offer a new therapeutic strategy in a range of ECM disorders caused by protein misfolding mutations. Anat Rec, 2019. © 2019 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Competency for nonsense-mediated reduction in collagen X mRNA is specified by the 3' UTR and corresponds to the position of mutations in Schmid metaphyseal chondrodysplasia

Nonsense-mediated decay (NMD) is a eukaryotic cellular RNA surveillance and quality-control mechanism that degrades mRNA containing premature stop codons (nonsense mutations) that otherwise may exert a deleterious effect by the production of dysfunctional truncated proteins. Collagen X (COL10A1) nonsense mutations in Schmid-type metaphyseal chondrodysplasia are localized in a region toward the 3' end of the last exon (exon 3) and result in mRNA decay, in contrast to most other genes in which terminal-exon nonsense mutations are resistant to NMD. We introduce nonsense mutations into the mouse Col10a1 gene and express these in a hypertrophic-chondrocyte cell line to explore the mechanism of last-exon mRNA decay of Col10a1 and demonstrate that mRNA decay is spatially restricted to mutations occurring in a 3' region of the exon 3 coding sequence; this region corresponds to where human mutations have been described. This localization of mRNA-decay competency suggested that a downstream region, such as the 3' UTR, may play a role in specifying decay of mutant Col10a1 mRNA containing nonsense mutations. We found that deleting any of the three conserved sequence regions within the 3' UTR (region I, 23 bp; region II, 170 bp; and region III, 76 bp) prevented mutant mRNA decay, but a smaller 13 bp deletion within region III was permissive for decay. These data suggest that the 3' UTR participates in collagen X last-exon mRNA decay and that overall 3' UTR configuration, rather than specific linear-sequence motifs, may be important in specifying decay of Col10a1 mRNA containing nonsense mutations.

Surviving endoplasmic reticulum stress is coupled to altered chondrocyte differentiation and function

In protein folding and secretion disorders, activation of endoplasmic reticulum (ER) stress signaling (ERSS) protects cells, alleviating stress that would otherwise trigger apoptosis. Whether the stress-surviving cells resume normal function is not known. We studied the in vivo impact of ER stress in terminally differentiating hypertrophic chondrocytes (HCs) during endochondral bone formation. In transgenic mice expressing mutant collagen X as a consequence of a 13-base pair deletion in Col10a1 (13del), misfolded α1(X) chains accumulate in HCs and elicit ERSS. Histological and gene expression analyses showed that these chondrocytes survived ER stress, but terminal differentiation is interrupted, and endochondral bone formation is delayed, producing a chondrodysplasia phenotype. This altered differentiation involves cell-cycle re-entry, the re-expression of genes characteristic of a prehypertrophic-like state, and is cell-autonomous. Concomitantly, expression of Col10a1 and 13del mRNAs are reduced, and ER stress is alleviated. ERSS, abnormal chondrocyte differentiation, and altered growth plate architecture also occur in mice expressing mutant collagen II and aggrecan. Alteration of the differentiation program in chondrocytes expressing unfolded or misfolded proteins may be part of an adaptive response that facilitates survival and recovery from the ensuing ER stress. However, the altered differentiation disrupts the highly coordinated events of endochondral ossification culminating in chondrodysplasia.

The assembly and folding of secreted proteins in the endoplasmic reticulum (ER) is exquisitely regulated by a complex mechanism that maintains an equilibrium between folded and unfolded proteins. Perturbation of this homeostasis induces ER stress, which, if not alleviated through ER stress signaling (ERSS), ultimately triggers cell death. Normal bone growth occurs through a highly coordinated differentiation program that yields specialized cartilage cells (chondrocytes); when this program is disrupted, chondrodysplasia, or malformed skeletons, can result. Chondrodysplasias caused by mutations that affect protein assembly and secretion are characterized by a disorganization of bony growth plates and distension of the ER. We tested whether these chondrodysplasia characteristics were linked to ERSS. By investigating the impact of ER stress on the cell fate of hypertrophic chondrocytes (HCs) in transgenic mice expressing mutations in collagen that prevent proper folding, we revealed a novel adaptive mechanism that helps alleviate the unfolded protein load. Instead of undergoing apoptosis, the HCs undergoing ER stress adapt, re-enter the cell cycle, and revert to a less-mature state in which expression of the mutant collagen is reduced. Our findings have broad implications for adaptive mechanisms to ER stress in vivo and for the pathophysiology underlying chondrodysplasias caused by mutations that impact on protein assembly and secretion.

When subjected to ER stress (by expression of misfolded or unfolded proteins), hypertrophic chondrocytes undergo alterations to their developmental program that may be part of an adaptive response.

COL10A1 nonsense and frame-shift mutations have a gain-of-function effect on the growth plate in human and mouse metaphyseal chondrodysplasia type Schmid

Missense, nonsense and frame-shift mutations in the collagen X gene (COL10A1) result in metaphyseal chondrodysplasia type Schmid (MCDS). Complete degradation of mutant COL10A1 mRNA by nonsense-mediated decay in human MCDS cartilage implicates haploinsufficiency in the pathogenesis for nonsense mutations in vivo. However, the mechanism is unclear in situations where the mutant mRNA persist. We show that nonsense/frame-shift mutations can elicit a gain-of-function effect, affecting chondrocyte differentiation in the growth plate. In an MCDS proband, heterozygous for a p.Y663X nonsense mutation, the growth plate cartilage contained 64% wild-type and 36% mutant mRNA and the hypertrophic zone was disorganized and expanded. The in vitro translated mutant collagen X chains, which are truncated, were misfolded, unable to assemble into trimers and interfered with the assembly of normal alpha1(X) chains into trimers. Unlike Col10a1 null mutants, transgenic mice (FCdel) bearing the mouse equivalent of a human MCDS p.P620fsX621 mutation, displayed typical characteristics of MCDS with disproportionate shortening of limbs and early onset coxa vara. In FCdel mice, the degree of expansion of the hypertrophic zones was transgene-dosage dependent, being most severe in mice homozygous for the transgene. Chondrocytes in the lower region of the expanded hypertrophic zone expressed markers uncharacteristic of hypertrophic chondrocytes, indicating that differentiation was disrupted. Misfolded FCdel alpha1(X) chains were retained within the endoplasmic reticulum of hypertrophic chondrocytes, activating the unfolded protein response. Our findings provide strong in vivo evidence for a gain-of-function effect that is linked to the activation of endoplasmic reticulum-stress response and altered chondrocyte differentiation, as a possible molecular pathogenesis for MCDS.

Mutations of COL10A1 in Schmid metaphyseal chondrodysplasia

Schmid metaphyseal chondrodysplasia (SMCD) is a dominantly inherited cartilage disorder caused by mutations in the gene for the hypertrophic cartilage extracellular matrix structural protein, collagen X (COL10A1). Thirty heterozygous mutations have been described, about equally divided into two mutation types, missense mutations, and mutations that introduce premature termination signals. The COL10A1 mutations are clustered (33/36) in the 3' region of exon 3, which codes for the C-terminal NC1 trimerization domain. The effect of COL10A1 missense mutations have been examined by in vitro expression and assembly assays and cell transfection studies, which suggest that a common consequence is the disruption of collagen X trimerization and secretion, with consequent intracellular degradation. The effect of COL10A1 nonsense mutations in cartilage tissue has been examined in two patients, demonstrating that the mutant mRNA is completely removed by nonsense mediated mRNA decay. Thus for both classes of mutations, functional haploinsufficiency is the most probable cause of the clinical phenotype in SMCD.

Deletions in the COL10A1 gene are not associated with skeletal changes in dogs

Type 10 collagen alpha 1 (COL10A1) is a short-chain collagen of cartilage synthesized by chondrocytes during the growth of long bones. COL10A1 mutations, which frequently result in COL10A1 haploinsufficiency, have been identified in patients with Schmid metaphyseal chondrodysplasia (SMCD), a cartilage disorder characterized by short-limbed short stature and bowed legs. Similarities between SMCD and short stature in various dog breeds suggested COL10A1 as a candidate for canine skeletal dysplasia. We report the sequencing of the exons and promoter region of the COL10A1 gene in dog breeds fixed for a specific type of skeletal dysplasia known as chondrodysplasia, breeds that segregate the skeletal dysplasia phenotype, and control dogs of normal stature. Thirteen single nucleotide polymorphisms (SNPs), one insertion, and two deletions, one of which introduces a premature stop codon and likely results in nonsense-mediated decay and the degradation of the mutant allele product, were identified in the coding region. There appear to be no causal relationships between the polymorphisms identified in this study and short stature in dogs. Although COL10A1 haploinsufficiency is an important cause of SMCD in humans, it does not seem to be responsible for the skeletal dysplasia phenotype in these dog breeds. In addition, homozygosity for the nonsense allele does not result in the observed canine skeletal dysplasia phenotype.

Disease mechanisms in late-onset retinal macular degeneration associated with mutation in C1QTNF5

Late-onset retinal macular degeneration (L-ORMD) is an autosomal dominant condition resembling age-related macular degeneration (AMD) in which a key pathological feature is a thick extracellular sub-retinal pigment epithelial (RPE) deposit. L-ORMD is caused by mutation in the C1QTNF5 (CTRP5) short-chain collagen gene, but the disease mechanism is unknown. Here, we first show that wild-type C1QTNF5 is secreted, whereas mutant C1QTNF5 is misfolded and retained within the endoplasmic reticulum (ER). Secondly, the ER retained mutant protein has a shorter half-life than wild-type C1QTNF5 and is preferentially degraded by proteasomes. Thirdly, C1QTNF5 is shown to interact with the membrane-type frizzled related protein (MFRP), on the basis of yeast two-hybrid, protein pull-down and co-immunoprecipitation assays and RPE co-localization. These data suggest that L-ORMD is due to insufficient levels of secreted C1QTNF5, compromised RPE cell function resulting from ER retention of the mutant protein or both mechanisms.

Schmid type of metaphyseal chondrodysplasia and COL10A1 mutations--findings in 10 patients

The Schmid type of metaphyseal chondrodyplasia (MCDS) is characterized by short stature, widened growth plates, and bowing of the long bones. It results from autosomal dominant mutations of COL10A1, the gene which encodes alpha1(X) chains of type X collagen. We report the clinical and radiographic findings in 10 patients with MCDS and COL10A1 mutations. Six patients had lower limb deformities, which necessitated orthopedic surgeries in all of them. One patient demonstrated no deformities and normal stature at age 11 years (height -1.2 SDS) while the others manifested severe short stature (<-3.5 SDS). Radiographs showed metaphyseal changes which were most pronounced at the hips and knees. Five of the identified 10 mutations in COL10A1 were novel. Six mutations resulted in truncation of the NC1 domain while four mutations were single amino-acid substitutions. Our findings suggest that COL10A1 mutations result in a uniform pattern of growth plate abnormalities. However, the clinical variability in severity among affected individuals is greater than previously thought.

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

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

Impaired multimerization of human adiponectin mutants associated with diabetes. Molecular structure and multimer formation of adiponectin

Adiponectin is an adipocyte-derived hormone, which has been shown to play important roles in the regulation of glucose and lipid metabolism. Eight mutations in human adiponectin have been reported, some of which were significantly related to diabetes and hypoadiponectinemia, but the molecular mechanisms of decreased plasma levels and impaired action of adiponectin mutants were not clarified. Adiponectin structurally belongs to the complement 1q family and is known to form a characteristic homomultimer. Herein, we demonstrated that simple SDS-PAGE under non-reducing and non-heat-denaturing conditions clearly separates multimer species of adiponectin. Adiponectin in human or mouse serum and adiponectin expressed in NIH-3T3 or Escherichia coli formed a wide range of multimers from trimers to high molecular weight (HMW) multimers. A disulfide bond through an amino-terminal cysteine was required for the formation of multimers larger than a trimer. An amino-terminal Cys-Ser mutation, which could not form multimers larger than a trimer, abrogated the effect of adiponectin on the AMP-activated protein kinase pathway in hepatocytes. Among human adiponectin mutations, G84R and G90S mutants, which are associated with diabetes and hypoadiponectinemia, did not form HMW multimers. R112C and I164T mutants, which are associated with hypoadiponectinemia, did not assemble into trimers, resulting in impaired secretion from the cell. These data suggested impaired multimerization and/or the consequent impaired secretion to be among the causes of a diabetic phenotype or hypoadiponectinemia in subjects having these mutations. In conclusion, not only total concentrations, but also multimer distribution should always be considered in the interpretation of plasma adiponectin levels in health as well as various disease states.

Preparative procedures and purity assessment of collagen proteins

Collagens represent a large family (25 members identified so far) of closely related proteins. While the preparative procedures for the members that are ubiquitous and present in tissues in large quantities (typically fibre and network forming collagens types I, II, III, IV and V) are well established, the procedures for more recently discovered minor collagen types, namely those possessing large non-collagenous domain(s) in their molecule, are mostly micropreparative and for some collagenous proteins even do not exist. The reason is that the proof of their existence is based on immunochemical staining of tissue slices and nucleic database searching. Methods of preparation and identification of constituting alpha-polypeptide chains as well as collagenous and non-collagenous domains are also reviewed. Methods for revealing non-enzymatic posttranslational modifications (particularly of the fibre forming collagen types) are briefly described as well.

Collagen X chains harboring Schmid metaphyseal chondrodysplasia NC1 domain mutations are selectively retained and degraded in stably transfected cells

Collagen X is a short chain, homotrimeric collagen expressed specifically by hypertrophic chondrocytes during endochondral bone formation and growth. Although the exact role of collagen X remains unresolved, mutations in the COL10A1 gene disrupt growth plate function and result in Schmid metaphyseal chondrodysplasia (SMCD). With the exception of two mutations that impair signal peptide cleavage during alpha1(X) chain biosynthesis, SMCD mutations are clustered within the carboxyl-terminal NC1 domain. The formation of stable NC1 domain trimers is a critical stage in collagen X assembly, suggesting that mutations within this domain may result in subunit mis-folding or reduce trimer stability. When expressed in transiently transfected cells, alpha1(X) chains containing SMCD mutations were unstable and presumed to be degraded intracellularly. More recently, in vitro studies have shown that certain missense mutations may exert a dominant negative effect on alpha1(X) chain assembly by formation of mutant homotrimers and normal-mutant heterotrimers. In contrast, analysis of cartilage tissue from two SMCD patients revealed that the truncated mutant message was fully degraded, resulting in 50% reduction of functional collagen X within the growth plate. Therefore, in the absence of data that conclusively demonstrates the full cellular response to mutant collagen X chains, the molecular mechanisms underlying SMCD remain controversial. To address this, we closely examined the effect of two NC1 domain mutations, one frameshift mutation (1963del10) and one missense mutation (Y598D), using both semi-permeabilized cell and stable cell transfection expression systems. Although able to assemble to a limited extent in both systems, we show that, in intact cells, collagen X chains harboring both SMCD mutations did not evade quality control mechanisms within the secretory pathway and were degraded intracellularly. Furthermore, co-expression of wild-type and mutant chains in stable transfected cells demonstrated that, although wild-type chains were secreted, mutant chains were largely excluded from hetero-trimer formation. Our data indicate, therefore, that the predominant effect of the NC1 mutations Y598D and 1963del10 is a reduction in the amount of functional collagen X within the growth cartilage extracellular matrix.

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

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

A dominant interference collagen X mutation disrupts hypertrophic chondrocyte pericellular matrix and glycosaminoglycan and proteoglycan distribution in transgenic mice

Collagen X transgenic (Tg) mice displayed skeleto-hematopoietic defects in tissues derived by endochondral skeletogenesis.(1) Here we demonstrate that co-expression of the transgene product containing truncated chicken collagen X with full-length mouse collagen X in a cell-free translation system yielded chicken-mouse hybrid trimers and truncated chicken homotrimers; this indicated that the mutant could assemble with endogenous collagen X and thus had potential for dominant interference. Moreover, species-specific collagen X antibodies co-localized the transgene product with endogenous collagen X to hypertrophic cartilage in growth plates and ossification centers; proliferative chondrocytes also stained diffusely. Electron microscopy revealed a disrupted hexagonal lattice network in the hypertrophic chondrocyte pericellular matrix in Tg growth plates, as well as altered mineral deposition. Ruthenium hexamine trichloride-positive aggregates, likely glycosaminoglycans (GAGs)/proteoglycans (PGs), were also dispersed throughout the chondro-osseous junction. These defects likely resulted from transgene co-localization and dominant interference with endogenous collagen X. Moreover, altered GAG/PG distribution in growth plates of both collagen X Tg and null mice was confirmed by a paucity of staining for hyaluronan and heparan sulfate PG. A provocative hypothesis links the disruption of the collagen X pericellular network and GAG/PG decompartmentalization to the potential locus for hematopoietic failure in the collagen X mice.

Renal fibrosis: collagen composition and assembly regulates epithelial-mesenchymal transdifferentiation

Type IV collagen is a major component of basement membranes and it provides structural and functional support to various cell types. Type IV collagen exists in a highly complex suprastructure form and recent studies implicate that protomer (the trimeric building unit of type IV collagen) assembly is mediated by the NC1 domain present in the C-terminus of each collagen alpha-chain polypeptide. Here we show that type IV collagen contributes to the maintenance of the epithelial phenotype of proximal tubular epithelial cells, whereas type I collagen promotes epithelial-to-mesenchymal transdifferentiation (EMT). In addition, the recombinant human alpha1NC1 domain inhibits assembly of type IV collagen NC1 hexamers and potentially disrupts the deposition of type IV collagen, facilitating EMT in vitro. Inhibition of type IV collagen assembly by the alpha1NC1 domain up-regulates the production of transforming growth factor-beta1 in proximal tubular epithelial cells, an inducer of EMT. These results strongly suggest that basement membrane architecture is pivotal for the maintenance of epithelial phenotype and that changes in basement membrane architecture potentially lead to up-regulation of transforming growth factor-beta1, which contributes to EMT during renal fibrosis.

Aberrant signal peptide cleavage of collagen X in Schmid metaphyseal chondrodysplasia. Implications for the molecular basis of the disease

Schmid metaphyseal chondrodysplasia results from mutations in the collagen X (COL10A1) gene. With the exception of two cases, the known mutations are clustered in the C-terminal nonhelical (NC1) domain of the collagen X. In vitro and cell culture studies have shown that the NC1 mutations result in impaired collagen X trimer assembly and secretion. In the two other cases, missense mutations that alter Gly(18) at the -1 position of the putative signal peptide cleavage site were identified (Ikegawa, S., Nakamura, K., Nagano, A., Haga, N., and Nakamura, Y. (1997) Hum. Mutat. 9, 131-135). To study their impact on collagen X biosynthesis using in vitro cell-free translation in the presence of microsomes, and cell transfection assays, these two mutations were created in COL10A1 by site-directed mutagenesis. The data suggest that translocation of the mutant pre-alpha1(X) chains into the microsomes is not affected, but cleavage of the signal peptide is inhibited, and the mutant chains remain anchored to the membrane of microsomes. Cell-free translation and transfection studies in cells showed that the mutant chains associate into trimers but cannot form a triple helix. The combined effect of both the lack of signal peptide cleavage and helical configuration is impaired secretion. Thus, despite the different nature of the NC1 and signal peptide mutations in collagen X, both result in impaired collagen X secretion, probably followed by intracellular retention and degradation of mutant chains, and causing the Schmid metaphyseal chondrodysplasia phenotype.

Bone development

Early development of the vertebrate skeleton depends on genes that pattern the distribution and proliferation of cells from cranial neural crest, sclerotomes, and lateral plate mesoderm into mesenchymal condensations at sites of future skeletal elements. Within these condensations, cells differentiate to chondrocytes or osteoblasts and form cartilages and bones under the control of various transcription factors. In most of the skeleton, organogenesis results in cartilage models of future bones; in these models cartilage is replaced by bone by the process of endochondral ossification. Lastly, through a controlled process of bone growth and remodeling the final skeleton is shaped and molded. Significant and exciting insights into all aspects of vertebrate skeletal development have been obtained through molecular and genetic studies of animal models and humans with inherited disorders of skeletal morphogenesis, organogenesis, and growth.

Collagens and collagen-related diseases

The collagen superfamily of proteins plays a dominant role in maintaining the integrity of various tissues and also has a number of other important functions. The superfamily now includes more than 20 collagen types with altogether at least 38 distinct polypeptide chains, and more than 15 additional proteins that have collagen-like domains. Most collagens form polymeric assemblies, such as fibrils, networks and filaments, and the superfamily can be divided into several families based on these assemblies and other features. All collagens also contain noncollagenous domains, and many of these have important functions that are distinct from those of the collagen domains. Major interest has been focused on endostatin, a fragment released from type XVIII collagen, which potently inhibits angiogenesis and tumour growth. Collagen synthesis requires eight specific post-translational enzymes, some of which are attractive targets for the development of drugs to inhibit collagen accumulation in fibrotic diseases. The critical roles of collagens have been clearly illustrated by the wide spectrum of diseases caused by the more than 1,000 mutations that have thus far been identified in 22 genes for 12 out of the more than 20 collagen types. These diseases include osteogenesis imperfecta, many chondrodysplasias, several subtypes of the Ehlers-Danlos syndrome, Alport syndrome, Bethlem myopathy, certain subtypes of epidermolysis bullosa, Knobloch syndrome and also some cases of osteoporosis, arterial aneurysms, osteoarthrosis, and intervertebral disc disease. The characterization of mutations in additional collagen genes will probably add further diseases to this list. Mice with genetically engineered collagen mutations have proved valuable for defining the functions of various collagens and for studying many aspects of the related diseases.

Self-assembly and supramolecular organization of EMILIN

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

Growth plate compressions and altered hematopoiesis in collagen X null mice

A variable skeleto-hematopoietic phenotype was observed in collagen X null mice which mirrored the defects in transgenic (Tg) mice with dominant interference collagen X mutations (Jacenko, O., P. LuValle, and B.R. Olsen. 1993. Nature. 365:56-61). Specifically, perinatal lethality was seen in approximately 10.8% of null mutants at week three after birth, and in another subset by 12 wk. In perinatal lethal mutants, growth plates were compressed, trabecular bone reduced, and hematopoietic aplasia and erythrocyte-filled vascular sinusoids were apparent in marrows. Lymphatic organs, reduced to approximately 80% that of controls, displayed altered architecture and lymphocyte content. In thymuses, a paucity of cortical CD3(+)/CD4(+)/CD8(+) lymphocytes was consistent with the marrow's inability to replenish maturing T cells. In spleens, an unaltered T cell distribution was coupled with diffuse staining for IgD(+)/B220(+) B cells, whose reduction was prominent in poorly organized lymphatic nodules. Disorderly arrays of splenic macrophages surrounding periarteriolar lymphatic sheaths and a red pulp depletion further complemented the Tg perinatal lethal phenotype. Moreover, subtle growth plate compressions and hematopoietic changes were seen in all null mice. Data from Tg and null mice implicate the disruption of collagen X function in the observed skeleto-hematopoietic defects, and suggest that hypertrophic cartilage and endochondral skeletogenesis may contribute to the marrow microenvironment prerequisite for blood cell differentiation.