Tissue-specific Forms of Type IX Collagen-Proteoglycan Arise from the Use of Two Widely Separated Promoters

The Versatility of Collagen in Pharmacology: Targeting Collagen, Targeting with Collagen

Collagen, a versatile family of proteins with 28 members and 44 genes, is pivotal in maintaining tissue integrity and function. It plays a crucial role in physiological processes like wound healing, hemostasis, and pathological conditions such as fibrosis and cancer. Collagen is a target in these processes. Direct methods for collagen modulation include enzymatic breakdown and molecular binding approaches. For instance, Clostridium histolyticum collagenase is effective in treating localized fibrosis. Polypeptides like collagen-binding domains offer promising avenues for tumor-specific immunotherapy and drug delivery. Indirect targeting of collagen involves regulating cellular processes essential for its synthesis and maturation, such as translation regulation and microRNA activity. Enzymes involved in collagen modification, such as prolyl-hydroxylases or lysyl-oxidases, are also indirect therapeutic targets. From another perspective, collagen is also a natural source of drugs. Enzymatic degradation of collagen generates bioactive fragments known as matrikines and matricryptins, which exhibit diverse pharmacological activities. Overall, collagen-derived peptides present significant therapeutic potential beyond tissue repair, offering various strategies for treating fibrosis, cancer, and genetic disorders. Continued research into specific collagen targeting and the application of collagen and its derivatives may lead to the development of novel treatments for a range of pathological conditions.

Exome-wide copy number variation analysis identifies a COL9A1 in frame deletion that is associated with hearing loss

Pathogenic variants in COL9A1 are primarily associated with autosomal recessive Stickler syndrome. Patients with COL9A1-associated Stickler syndrome (STL) present hearing loss (HL), ophthalmic manifestations and skeletal abnormalities. However, the clinical spectrum of patients with COL9A1 variants can also include multiple epiphyseal dysplasia, as well as non-syndromic HL that was observed in one previously reported proband. Exome sequencing was performed on the genomic DNA of an Iranian patient and his affected brother who both report non-syndromic HL. A 44.6 kb homozygous in-frame deletion spanning exons 6 to 33 of COL9A1 was detected via exome-based copy number variation analysis. The deleted exons were confirmed by PCR in the patient and his affected brother, who both have non-syndromic HL. Segregation analysis via qPCR confirmed the parents as heterozygous deletion carriers. Breakpoint analysis mapped the homozygous deletion spanning introns 5 to 33 (g.70,948,188_70,997,277del, NM_001851.4(COL9A1):c.697-3754_2112+769del, p.(Phe233_Ser704del), with an additional 67 bp of inserted intronic sequence that may have originated due to a fork stalling and template switching/microhomology-mediated break-induced replication (FoSTeS/MMBIR) mechanism. This mechanism has not been previously implicated in HL or STL. This is also the first reported copy number variation in COL9A1 that was identified through an exome data set in an Iranian family with apparent non-syndromic HL. The present study emphasizes the importance of exome-wide copy number variation analysis in molecular diagnosis and provides supporting evidence to associate COL9A1 with autosomal recessive non-syndromic HL.

Isolation and Characterisation of Major and Minor Collagens from Hyaline Cartilage of Hoki (Macruronus novaezelandiae)

The composition and properties of collagen in teleost (bony fish) cartilage have never been studied. In this study, we aimed to identify and characterise all collagen species in the nasal cartilage of hoki (Macruronus novaezelandiae). Four native collagen species were extracted using two techniques, and isolated with differential salt precipitation. We were able to assign the identity of three of these collagen species on the basis of solubility, SDS-PAGE and amino acid analyses. We found that hoki cartilage contains the major collagen, type II, and the minor collagens, type IX and type XI, which are homologous to those found in mammal and chicken cartilage. Using these extraction protocols, we also isolated a full-length type IX collagen from cartilage for the first time. In addition, we detected a 90 kDa, highly glycosylated collagen that has not been identified in any other species. For each isolate, structural and biochemical characterisations were performed using circular dichroism and Fourier transform infrared spectroscopy analyses, and the thermal denaturation properties were determined. Our results showed that the properties of hoki cartilage-derived collagens are similar to those of collagens in mammalian cartilage, indicating that teleost cartilage could provide biological ingredients for the development of biomaterials to treat cartilage-related illnesses.

Ageing of the vitreous: From acute onset floaters and flashes to retinal detachment

Floaters and flashes are most commonly symptoms of age-related degenerative changes in the vitreous body and posterior vitreous detachment. The etiology and pathogenesis of floaters' formation is still not well understood. Patients with acute-onset floaters, flashes and defects in their visual field, represent a medical emergency with the need for same day referral to an ophthalmologist. Indirect ophthalmoscopy with scleral indentation is needed in order to find possible retinal break(s), on-time treatment and prevention of retinal detachment. The molecular and genetic pathogenesis, as well as the epidemiology of the ageing changes of the vitreous is summarized here, with view on the several treatment modalities in relation to their success rate and side-effects.

Type IX collagen interacts with fibronectin providing an important molecular bridge in articular cartilage

Type IX collagen is covalently bound to the surface of type II collagen fibrils within the cartilage extracellular matrix. The N-terminal, globular noncollagenous domain (NC4) of the α1(IX) chain protrudes away from the surface of the fibrils into the surrounding matrix and is available for molecular interactions. To define these interactions, we used the NC4 domain in a yeast two-hybrid screen of a human chondrocyte cDNA library. 73% of the interacting clones encoded fibronectin. The interaction was confirmed using in vitro immunoprecipitation and was further characterized by surface plasmon resonance. Using whole and pepsin-derived preparations of type IX collagen, the interaction was shown to be specific for the NC4 domain with no interaction with the triple helical collagenous domains. The interaction was shown to be of high affinity with nanomolar K(d) values. Analysis of the fibronectin-interacting clones indicates that the constant domain is the likely site of interaction. Type IX collagen and fibronectin were shown to co-localize in cartilage. This novel interaction between the NC4 domain of type IX collagen and fibronectin may represent an in vivo interaction in cartilage that could contribute to the matrix integrity of the tissue.

A haplotype at the COL9A2 gene locus contributes to the genetic risk for lumbar spinal stenosis in the Korean population

STUDY DESIGN

We conducted a cross-sectional, genotyping study in patients with lumbar spinal stenosis (LSS) and controls.

OBJECTIVE

To determine the contribution of COL9A2 polymorphisms to LSS development in the Korean population.

SUMMARY OF BACKGROUND DATA

Because congenital spinal stenosis is typically associated with chondrodysplasias, which are genetic disorders, genetic factors may also play a role in degenerative LSS. A recent Finnish study reported a splice site mutation in COL9A2, leading to premature translation termination. However, a few studies on the genetic association of single nucleotide polymorphisms (SNPs) or haplotypes with LSS have appeared.

METHODS

We studied 205 symptomatic patients with radiographically proven LSS and 101 volunteers with no history of back problems from our institution. Magnetic resonance images were obtained for all the patients and controls. Quantitative image evaluation for LSS was performed to evaluate the severity of LSS. All patients and controls were genotyped for COL9A2 allele variations, using a polymerase chain reaction-based technique.

RESULTS

We found no causal SNPs in COL9A2 that were significantly associated with LSS, even after phenotypic subgrouping. Haplotype analysis showed that the "GCAGCG" haplotype (HAP2) was overrepresented in LSS patients (P = 0.023, odds ratio [OR] = 1.86), especially in those with severe stenosis (P = 0.018, OR = 1.98). In addition, the "TCAGCG" haplotype (HAP4) was overrepresented in controls (P = 0.042, OR = 0.52).

CONCLUSION

Although no SNPs in COL9A2 were associated with LSS, a COL9A2 haplotype (HAP2) was significantly associated with LSS in the Korean population, whereas another haplotype (HAP4) may play a protective role against LSS development. However, the genetic functions of COL9A2 haplotypes in LSS remain to be determined.

Trabecular bone deterioration in col9a1+/- mice associated with enlarged osteoclasts adhered to collagen IX-deficient bone

INTRODUCTION

Short collagen IX, the exclusive isoform expressed by osteoblasts, is synthesized through alternative transcription of the col9a1 gene. The function of short collagen IX in bone was characterized in col9a1-null mutant mice.

MATERIALS AND METHODS

Trabecular bone morphometry of lumbar bones and tibias was evaluated by muCT and nondecalcified histology. Osteoblastic and osteoclastic activities were evaluated by PCR- and microarray-based gene expression assays and TRACP-5b and C-terminal telopeptide (CTX) assays, as well as in vitro using bone marrow stromal cells and splenocytes. The effect of col9a1(+/-) mutation on osteoclast morphology was evaluated using RAW264.7-derived osteoclastic cells cultured on the mutant or wildtype calvarial bone substrates.

RESULTS

Col9a1 knockout mutation caused little effects on the skeletal development; however, young adult female col9a1(-/-) and col9a1(+/-) mice exhibited significant loss of trabecular bone. The trabecular bone architecture was progressively deteriorated in both male and female heterozygous col9a1(+/-) mice while aging. The aged mutant mice also exhibited signs of thoracic kyphosis and weight loss, resembling the clinical signs of osteoporosis. The col9a1(+/-) osteoblasts synthesized short col9a1 transcripts at decreased rates. Whereas bone formation activities in vitro and in vivo were not affected, the mutant osteoblast expressed the elevated ratio of RANKL/osteoprotegerin. Increased serum TRACP-5b and CTX levels were found in col9a1(+/-) mice, whose bone surface was associated with osteoclastic cells that were abnormally flattened and enlarged. The mutant and wildtype splenocytes underwent similar osteoclastogenesis in vitro; however, RAW264.7-derived osteoclastic cells, when cultured on the col9a1(+/-) calvaria, widely spread over the bone surface and formed large resorption pits. The surface of col9a1(+/-) calvaria was found to lack the typical nanotopography.

CONCLUSIONS

The mineralized bone matrix deficient of short collagen IX may become susceptible to osteoclastic bone resorption, possibly through a novel non-cell-autonomous mechanism. The data suggest the involvement of bone collagen IX in the pathogenesis of osteoporosis.

Structural and functional characterization of recombinant matrilin-3 A-domain and implications for human genetic bone diseases

Mutations in matrilin-3 result in multiple epiphyseal dysplasia, which is characterized by delayed and irregular bone growth and early onset osteoarthritis. The majority of disease-causing mutations are located within the beta-sheet of the single A-domain of matrilin-3, suggesting that they disrupt the structure and/or function of this important domain. Indeed, the expression of mutant matrilin-3 results in its intracellular retention within the rough endoplasmic reticulum of cells, where it elicits an unfolded protein response. To understand the folding characteristics of the matrilin-3 A-domain we determined its structure using CD, analytical ultracentrifugation, and dual polarization interferometry. This study defined novel structural features of the matrilin-3 A-domain and identified a conformational change induced by the presence or the absence of Zn(2+). In the presence of Zn(2+) the A-domain adopts a more stable "tighter" conformation. However, after the removal of Zn(2+) a potential structural rearrangement of the metal ion-dependent adhesion site motif occurs, which leads to a more "relaxed" conformation. Finally, to characterize the interactions of the matrilin-3 A-domain we performed binding studies on a BIAcore using type II and IX collagen and cartilage oligomeric matrix protein. We were able to demonstrate that it binds to type II and IX collagen and cartilage oligomeric matrix protein in a Zn(2+)-dependent manner. Furthermore, we have also determined that the matrilin-3 A-domain appears to bind exclusively to the COL3 domain of type IX collagen and that this binding is abolished in the presence of a disease causing mutation in type IX collagen.

Association study of COL9A2 with lumbar disc disease in the Japanese population

Lumbar disc disease (LDD) is a common musculo-skeletal disease with strong genetic determinants. In a Finnish population, a single nucleotide polymorphism (SNP) causing an amino-acid substitution (Trp2 allele) in COL9A2, which encodes the alpha2 (IX) chain of type IX collagen, has been reported to associate with LDD. However, replication studies in different populations have produced controversial results. To further investigate the association of COL9A2 with LDD in Japanese, we examined SNPs in COL9A2, including Trp2, in 470 LDD patients (mean age 35) along with 658 controls (mean age 48). We identified a total of 43 sequence variations in COL9A2. Nine SNPs, including Trp2, were selected and genotyped. After Bonferroni's correction, none of these SNPs showed association. Unlike observations in the Finnish population, Trp2 was common in Japanese, and no association with LDD was apparent. However, we did see association of a COL9A2 specific haplotype with LDD (P=0.025; permutation test); this association is more significant in patients with severe lumbar disc degeneration (P=0.011). Thus, the association of Trp2 with LDD was not replicated, but COL9A2 susceptibility allele(s) other than Trp2 may be present in Japanese LDD.

Characterization of an abundant COL9A1 transcript in the cochlea with a novel 3' UTR: Expression studies and detection of miRNA target sequence

EST N66408 represents one of several large unique clusters expressed in the Morton human fetal cochlear cDNA library. N66408 is 575 bp in size and initial BLAST analysis of this sequence showed no homology to any known genes or expressed sequence tags (ESTs) from other organs or tissues. Sequence of the original cochlear clone from which N66408 was derived revealed that the corresponding cDNA was about 700 bp in size, including 125 bp at its 5' end with homology to the 3' end of COL9A1 in addition to 575 bp of novel sequence. RT-PCR analysis using primers specific to COL9A1 isoforms 1 and 2 detected expression of both isoforms in human fetal cochlea. Tissue in situ hybridization using the novel 3' UTR sequence as probe showed abundant expression in spiral limbus and spiral ligament, and a moderate level of expression in the organ of Corti. dbEST analysis of ESTs specific to the 3' UTR of COL9A1 showed 19 ESTs derived from various tissues; three polyadenylation sites were identified and the majority of these ESTs were derived from overlapping polyadenylation signals at the second site (position 749-758). Comparison of the 3' UTR of human COL9A1 with its orthologs as well as with dbEST uncovered a highly conserved region around the overlapping polyadenylation signals at position 749-758 in mammals. A search of the microRNA database revealed a highly conserved target sequence for miR-9 immediately preceding the overlapping polyadenylation signals in the novel 3' UTR of COL9A1, suggesting its role in posttranscriptional regulation of COL9A1.

Collagen and proteoglycan turnover in focally damaged human ankle cartilage: evidence for a generalized response and active matrix remodeling across the entire joint surface

OBJECTIVE

Although cartilage lesions occur in the ankles, osteoarthritis rarely develops in the ankles, suggesting that ankle cartilage can up-regulate mechanisms to repair the damaged matrix. To define these processes, we compared cartilage samples obtained from normal tali and from lesional sites of damaged tali.

METHODS

Cartilage samples were obtained from the tali of normal ankles and from 3 sites on tali with lesions (the lesion, adjacent to the lesion, and far removed from the lesion). Cartilage was analyzed for type II collagen (CII) messenger RNA, C-terminal type II procollagen propeptide (CPII), the collagenase cleavage neoepitope (Col2-3/4C(short)), and the denaturation epitope (Col2-3/4m). For the assessment of type IX collagen, the COL2 and NC4 domains were evaluated. The cartilage samples were also assayed for glycosaminoglycans, epitope 846 of aggrecan, and DNA.

RESULTS

The DNA content, epitope 846, COL2(IX), and the denaturation epitope were significantly increased in lesional cartilage. Although there was a tendency toward an increase in CII content and CPII, the increase did not reach significance. Neither the NC4(IX) domain nor Col2-3/4C was elevated. Surprisingly, changes in cartilage both adjacent to and remote from the lesion were similar to those in the lesion.

CONCLUSION

The changes observed in cartilage obtained from the lesion and from sites adjacent to the lesion were not surprising; however, the changes in cartilage obtained from sites remote from the lesion were unexpected. This up-regulation of matrix turnover in ankles with degenerative lesions may indicate a physiologic response of the entire articular surface to repair the damaged matrix, which is not restricted to the lesion site. This suggests that there may be some mechanism of communication across the cartilage. The response by ankle cartilage obtained from a site remote from the lesion has not been observed in the knee.

Looking at an oft-overlooked part of the eye: a new perspective on ciliary body development in chick

The ciliary body is an essential tissue for the development and homeostasis of the vertebrate eye. Embryonically, the epithelial portion of the ciliary body derives from the neuroepithelium of the optic cup, however, it differentiates into a secretory tissue and produces an aqueous humor that sustains the lens and cornea, and maintains the requisite pressure within the orb. The unique differentiation of this portion of the optic cup is little understood. This article reviews what is known about the development of the ciliary body and presents some preliminary findings that may lead to a new model for the formation of the ciliary body.

Cellular invasion of the chicken corneal stroma during development: regulation by multiple matrix metalloproteases and the lens

Avian corneal development requires cellular invasion into the acellular matrix of the primary stroma. Previous results show that this invasion is preceded by the removal of the fibril-associated type IX collagen, which possibly stabilizes matrices through interfibrillar cross-bridges secured by covalent crosslinks. In the present study, we provide evidence for the expression of three matrix metalloproteinases (MMPs) in early corneas, two of which act cooperatively to selectively remove type IX collagen in situ. In organ cultures, MMP inhibitors (either TIMP-2 or a synthetic inhibitor) resulted in arrested development, in which collagen IX persisted, and the stroma remained compact and acellular. We also show that blocking covalent crosslinking of collagen allows for cellular invasion to occur, even when the removal of type IX collagen is prevented. Thus, one factor regulating corneal invasion is the physical structure of the matrix, which can be modified by either selective proteolysis or reducing interfibrillar cross-bridges. We also detected another level of regulation of cellular invasion involving inhibition by the underlying lens. This block, which seems to influence invasive behavior independently of matrix modification, is a transient event that is released in ovo just before invasion proceeds.

Complete primary structure and genomic organization of the mouse Col14a1 gene

The entire mouse cDNA sequence for type XIV collagen was determined using overlapping PCR products. The 6456 nucleotide (nt) cDNA sequence contains a 5391-nt open reading frame encoding 1797 amino acid residues. The amino terminus has a 28-residue signal peptide that is followed by the mature polypeptide of 1769 amino acid residues with a calculated molecular mass of 193.2 kDa. The mouse alpha1(XIV) collagen chain is predicted to contain all the structural domains described for the polypeptide in chicken and human. These include fibronectin type III repeats, von Willebrand factor A domains, thrombospondin-N-terminal-like domains and two triple-helical domains similar to those of other collagen family members. The amino acid residue sequence of human alpha1(XIV) collagen showed an overall identity of 74% to the chicken sequence and 88% to the human sequence. The entire mouse genomic structure has been determined and is made up of 48 exons. Alternatively spliced forms of mouse type XIV, collagen were not identified corresponding to the findings for the human form.

Collagens and collagen-related matrix components in the human and mouse eye

The three-dimensional structure of the eye plays an important role in providing a correct optical environment for vision. Much of this function is dependent on the unique structural features of ocular connective tissue, especially of the collagen types and their supramolecular structures. For example, the organization of collagen fibrils is largely responsible for transparency and refraction of cornea, lens and vitreous body, and collagens present in the sclera are largely responsible for the structural strength of the eye. Phylogenetically, most of the collagens are highly conserved between different species, which suggests that collagens also share similar functions in mice and men. Despite considerable differences between the mouse and the human eye, particularly in the proportion of the different tissue components, the difficulty of performing systematic histologic and molecular studies on the human eye has made mouse an appealing alternative to studies addressing the role of individual genes and their mutations in ocular diseases. From a genetic standpoint, the mouse has major advantages over other experimental animals as its genome is better known than that of other species and it can be manipulated by the modern techniques of genetic engineering. Furthermore, it is easy, quick and relatively cheap to produce large quantities of mice for systematic studies. Thus, transgenic techniques have made it possible to study consequences of specific mutations in genes coding for structural components of ocular connective tissues in mice. As these changes in mice have been shown to resemble those in human diseases, mouse models are likely to provide efficient tools for pathogenetic studies on human disorders affecting the extracellular matrix. This review is aimed to clarify the role of collagenous components in the mouse and human eye with a closer look at the new findings of the collagens in the cartilage and the eye, the so-called "cartilage collagens".

Regulation of protein diversity by alternative pre-mRNA splicing with specific focus on chondrogenesis

Analysis of the human genome has dramatically demonstrated that the majority of protein diversity is generated by alternative splicing of pre-mRNA. This powerful and versatile mechanism controls the synthesis of functionally different protein isoforms that may be required during specific stages of development from a single gene. Consequently, ubiquitous and/or tissue-specific RNA splicing factors that regulate this splicing mechanism provide the basis for defining phenotypic characteristics of cells during differentiation. In this review, we will introduce the basic mechanisms of pre-mRNA alternative splicing, describe how this process is regulated by specific RNA splicing factors, and relate this to various systems of cell differentiation. Chondrogenesis, a well-defined differentiation pathway necessary for skeletogenesis, will be discussed in detail, with focus on some of the alternatively-spliced proteins known to be expressed during cartilage development. We propose a heuristic view that, ultimately, it is the regulation of these RNA splicing factors that determines the differentiation status of a cell. Studying regulation at the level of pre-mRNA alternative splicing will provide invaluable insights into how many developmental mechanisms are controlled, thus enabling us to manipulate a system to select for a specific differentiation pathway.

Intervertebral disc collagen. Usage of the short form of the alpha1(IX) chain in bovine nucleus pulposus

Nucleus pulposus, the central zone of the intervertebral disc, is gel-like and has a similar collagen phenotype to that of hyaline cartilage. Amino-terminal protein sequence analysis of the alpha1(IX)COL3 domain purified from bovine nucleus pulposus gave a different sequence to that of the long alpha1(IX) transcript expressed in hyaline cartilage and matched the predicted sequence of short alpha1(IX). The findings indicate that the matrix of bovine nucleus pulposus contains only the short form of alpha1(IX) that lacks the NC4 domain. The sequence encoded by exon 7, predicted from human COL9A1, is absent from both short and long forms of alpha1(IX) from bovine nucleus pulposus and articular cartilage. A structural analysis of the cross-linking sites occupied in type IX collagen from nucleus pulposus showed that usage of the short alpha1(IX) transcript in disc tissue had no apparent effect on cross-linking behavior. As in cartilage, type IX collagen of nucleus pulposus was heavily cross-linked to type II collagen and to other molecules of type IX collagen with a similar site occupancy.

Complete primary structure and genomic organization of the mouse Col14a1 gene

The entire mouse cDNA sequence for type XIV collagen was determined using overlapping PCR products. The 6456 nucleotide (nt) cDNA sequence contains a 5391-nt open reading frame encoding 1797 amino acid residues. The amino terminus has a 28-residue signal peptide that is followed by the mature polypeptide of 1769 amino acid residues with a calculated molecular mass of 193.2 kDa. The mouse alpha1(XIV) collagen chain is predicted to contain all the structural domains described for the polypeptide in chicken and human. These include fibronectin type III repeats, von Willebrand factor A domains, thrombospondin-N-terminal-like domains and two triple-helical domains similar to those of other collagen family members. The amino acid residue sequence of human alpha1(XIV) collagen showed an overall identity of 74% to the chicken sequence and 88% to the human sequence. The entire mouse genomic structure has been determined and is made up of 48 exons. Alternatively spliced forms of mouse type XIV, collagen were not identified corresponding to the findings for the human form.

Type II and type IX collagen transcript isoforms are expressed during mouse testis development

Mutations in the transcription factor SOX9 give rise to campomelic dysplasia, a syndrome characterized by skeletal abnormalities and XY sex reversal. Sox9 is expressed at sites of chondrogenesis and in the developing testis, and, thus, it plays a role in two overtly different pathways of differentiation. Previous studies have identified the gene for type II collagen, Col2a1, as a target of Sox9 in mouse chondrocytes and implicated Col9a3 as a Sox9 target in testis. Using differential expression analysis combined with reverse transcription-polymerase chain reaction and whole-mount in situ hybridization, we have identified nonchondrocytic collagen transcript isoforms that are expressed in the early male mouse gonad. Male-specific, gonadal expression of nonchondrocytic Col2a1 was first seen at 11.5 days postcoitum (dpc) and was undetectable by 13.5 dpc. This was accompanied by increasing expression of nonchondrocytic Col9a1, Col9a2, and Col9a3, first detected at 11.5 dpc. Expression was analyzed in testes that had been depleted of germ cells by the cytotoxic drug busulfan. These studies showed Col9a3 and Col2a1 to be expressed in Sertoli cells within the developing testis cords. Nonchondrocytic type II collagen contains a cysteine-rich domain that has been shown to bind members of the transforming growth factor beta superfamily of signaling molecules. Thus, this interaction may play a role in the morphogenesis and differentiation of the testis.

The assembly and remodeling of the extracellular matrix in the growth plate in relationship to mineral deposition and cellular hypertrophy: an in situ study of collagens II and IX and proteoglycan

The recent development of new specific immunoassays has provided an opportunity to study the assembly and resorption of type II and IX collagens of the extracellular matrix in relationship to endochondral calcification in situ. Here, we describe how in the bovine fetal physis prehypertrophic chondrocytes deposit an extensive extracellular matrix that, initially, is rich in both type II and type IX collagens and proteoglycan (PG; principally, aggrecan). The majority of the alpha1(IX)-chains lack the NC4 domain consistent with our previous studies with cultured chondrocytes. During assembly, the molar ratio of type II/COL2 domain of the alpha1(IX)-chain varied from 8:1 to 25:1. An increase in the content of Ca2+ and inorganic phosphate (Pi) was initiated in the prehypertrophic zone when the NC4 domain was removed selectively from the alpha1(IX)-chain. This was followed by the progressive loss of the alpha1(IX) COL2 domain and type II collagen. In the hypertrophic zone, the Ca2+/Pi molar ratio ranged from 1.56 to a maximum of 1.74, closely corresponding to that of mature hydroxyapatite (1.67). The prehypertrophic zone had an average ratio Ca2+/Pi ranging from 0.25 to 1, suggesting a phase transformation. At hypertrophy, when mineral content was maximal, type II collagen was reduced maximally in content coincident with a peak of cleavage of this molecule by collagenase when matrix metalloproteinase 13 (MMP-13) expression was maximal. In contrast, PG (principally aggrecan) was retained when hydroxyapatite was formed consistent with the view that this PG does not inhibit and might promote calcification in vivo. Taken together with earlier studies, these findings show that matrix remodeling after assembly is linked closely to initial changes in Ca2+ and Pi to subsequent cellular hypertrophy and mineralization. These changes involve a progressive and selective removal of types II and IX collagens with the retention of the PG aggrecan.

Chondrocyte phenotype and cell survival are regulated by culture conditions and by specific cytokines through the expression of Sox-9 transcription factor

OBJECTIVE

To investigate the effects of culture conditions, serum and specific cytokines such as insulin-like growth factor (IGF) 1 and interleukin (IL) 1alpha on phenotype and cell survival in cultures of Syrian hamster embryonic chondrocyte-like cells (DES4(+).2).

METHODS

Proteins and RNA extracted from subconfluent and confluent early- and late-passage DES4(+).2 cells cultured in the presence or absence of serum and IL-1alpha or IGF-1 or both cytokines together were analysed for the expression of chondrocyte-specific genes and for the chondrogenic transcription factor Sox-9 by Western and Northern blotting. Apoptosis was assessed by agarose gel electrophoresis of labelled low-molecular weight DNA extracted from DES4(+).2 cells and another Syrian hamster embryonic chondrocyte-like cell line, 10W(+).1, cultured under the different conditions and treatments.

RESULTS

Early passage DES4(+).2 cells expressed chondrocyte-specific molecules such as collagen types alpha1(II) and alpha1(IX), aggrecan, biglycan and link protein and collagen types alpha1(I) and alpha1(X) mRNAs, suggesting a prehypertrophic chondrocyte-like phenotype. The expression of all genes investigated was cell density- and serum-dependent and was low to undetectable in cell populations from later passages. Early-passage DES4(+).2 and 10W(+).1 cells survived when cultured at low cell density, but died by apoptosis when cultured at high cell density in the absence of serum or IGF-1. IGF-1 and IL-1alpha had opposite and antagonistic effects on the chondrocyte phenotype and survival. Whereas IL-1alpha acting alone suppressed cartilage-specific gene expression without significantly affecting cell survival, IGF-1 increased the steady-state mRNA levels and relieved the IL-1alpha-induced suppression of all the chondrocyte-specific genes investigated; it also enhanced chondrocyte survival. Suppression of the chondrocyte phenotype by the inflammatory cytokine IL-1alpha correlated with marked down-regulation of the transcription factor Sox-9, which was relieved by IGF-1. The expression of the Sox9 gene was closely correlated with the expression of the chondrocyte-specific genes under all conditions and treatments.

CONCLUSIONS

The results suggest that the effects of cartilage anabolic and catabolic cytokines IGF-1 and IL-1alpha on the expression of the chondrocyte phenotype are mediated by Sox-9. As Sox-9 appears to be essential for matrix production, the potent effect of IL-1alpha in suppressing Sox-9 expression may limit the ability of cartilage to repair during inflammatory joint diseases.

Analysis of the promoter region of human cartilage oligomeric matrix protein (COMP)

Cartilage oligomeric matrix protein (COMP) is an extracellular matrix protein expressed in cartilage, ligament, and tendon. The importance of COMP in the matrix of these cells is underscored by the discovery that mutations in COMP cause the skeletal dysplasias, pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (EDM1). Here, we present the first report on the analysis of the human COMP promoter region in cartilage, ligament, and tendon cells. A 1.7-kb region of the COMP promoter has been cloned and sequenced and no TATA or CAAT boxes were found. Primer extension identified multiple transcription start sites. All four transcription start sites were utilized in chondrocytes with only three of them utilized in tendon and ligament cells. Differential regulation was observed for different parts of this 1.7-kb region with the 370-bp proximal region conveying the strongest promoter activity. The highest activity was observed in tendon and ligament. Finally, we provide evidence that the DNA binding protein SP1 plays a role in the regulation of COMP expression. These results indicate that COMP expression within these cells is regulated in a unique manner that differs from the expression of other extracellular matrix genes.

Selective assembly and remodelling of collagens II and IX associated with expression of the chondrocyte hypertrophic phenotype

The assembly and resorption of the extracellular matrix in the physis of the growth plate are poorly understood. By examining isolated fetal growth plate chondrocytes in culture and using immunochemical methods we show that type II collagen, proteoglycan aggrecan, and type IX collagen are assembled into a matrix that is initially enriched in type II collagen over proteoglycan and type IX collagen. When compared to the content of the COL2 domain in the alpha(1)(IX) chain it is evident that the majority ( 90%) of type IX molecules lack the NC4 domain unlike in articular cartilage. During matrix assembly the molar ratio of type II/COL2 of alpha(1)(IX) varied from 25:1 to 2.5:1. Following expression of the hypertrophic phenotype (initiation of type X collagen synthesis) there are parallel changes in both collagen and proteoglycan contents (inversely related to collagenase cleavage of type II collagen). The NC4 domain is then selectively, rapidly and irreversibly removed as mineralization is initiated, leaving the alpha(1)(IX) chain COL2 domain. Subsequently as mineralization progresses type II and type IX collagen (COL2 domain), but not the proteoglycan aggrecan, are resorbed coincident with a markedly increased cleavage of type II collagen by collagenase as mineral is deposited in the matrix. This study, therefore reveals a carefully orchestrated series of events in matrix assembly and resorption that prepares the extracellular matrix for mineralization.

Molecular cloning, sequencing, and tissue and developmental expression of mouse cartilage oligomeric matrix protein (COMP)

Mouse cartilage oligomeric matrix protein cDNA was cloned and sequenced by a reverse transcription-polymerase chain reaction. The open reading frame encoded a product of 755 amino acids that shares a high degree of identity to and possesses all the characteristic molecular features of both rat and human cartilage oligomeric matrix protein. This suggests that cartilage oligomeric matrix protein is highly conserved during evolution. The clone was 83, 84, and 95% identical to human, bovine, and rat cartilage oligomeric matrix protein cDNA, respectively. In tissues from the adult mouse, cartilage oligomeric matrix protein was expressed not only in cartilage and tendon but in trachea, bone, skeletal muscle, eye, heart, and placenta as well, and no expression was found in other tissues. Immunohistology revealed that cartilage oligomeric matrix was deposited as early as 10 days post coitus in predifferentiated mouse embryo mesenchyme. It was detected in all cartilaginous tissues and in the skeletal muscles of the embryo at day 13. As development progressed, accumulation of cartilage oligomeric matrix protein was marked in the growth plate. At 19 days post coitus, it was prominently deposited in the hypertrophic zone of the growth plate, perichondrium, and periosteum and in the superficial layer of the articular cartilage surface but was absent in the more central areas of the epiphyseal cartilage. The restricted tissue distribution and expression of cartilage oligomeric matrix protein in developing as well as adult mouse tissues suggest the regulation of this protein at the transcriptional level. The findings reported herein are the first detailed characterization of the distribution of cartilage oligomeric matrix protein during early skeletal development of the mouse.

Structural macromolecules and supramolecular organisation of the vitreous gel

The vitreous gel is a transparent extracellular matrix that fills the cavity behind the lens of the eye and is surrounded by and attached to the retina. This gel liquefies during ageing and in 25-30% of the oppulation the residual gel structure eventually collapses away from the posterior retina in a process called posterior retina in a process called posterior vitreous detachment. This process plays a pivotal role in a number of common blinding conditions including rhegmatogenous retinal detachment, proliferative diabetic retinopathy and macular hole formation. In order to understand the molecular events underlying vitreous liquefaction and posterior vitreous detachment and to develop new therapies it is important to understand the molecular basis of normal vitreous gel structure and how this is altered during ageing. It has previously been established that a dilute dispersion of thin (heterotypic) collagen fibrils is essential to the gel structure and that age-related vitreous liquefaction is intimately related to a process whereby these collagen fibrils aggregate. Collagen fibrils have a natural tendency to aggregate so a key question that has to be addressed is: what normally maintains the spacing of the collagen fibrils? In mammalian vitreous a network of hyaluronan normally fills the spaces between these collagen fibrils. This hyaluronan network can be removed without destroying the gel structure, so the hyaluronan is not essential for maintaining the spacing of the collagen fibrils although it probably does increase the mechanical resilience of the gel. The thin heterotypic collagen fibrils have a coating of non-covalently bound macromolecules which, along with the surface features of the collagen fibrils themselves, probably play a fundamental role in maintaining gel stability. They are likely to both maintain the short-range spacing of vitreous collagen fibrils and to link the fibrils together to form a contiguous network. A collagen fibril-associated macromolecule that may contribute to the maintenance of short-range spacing is opticin, a newly discovered extracellular matrix leucine-rich repeat protein. In addition, surface features of the collagen fibrils such as the chondroitin sulphate glycosaminoglycan chains of type IX collagen proteoglycan may also play an important role in maintaining fibril spacing. Furthering our knowledge of these and other components related to the surface of the heterotypic collagen fibrils will allow us to make important strides in understanding the macromolecular organisation of this unique and fascinating tissue. In addition, it will open up new therapeutic opportunities as it will allow the development of therapeutic reagents that can be used to modulate vitreous gel structure and thus treat a number of common, potentially blinding, ocular conditions.

Mammalian skeletogenesis and extracellular matrix: what can we learn from knockout mice?

Formation of the vertebrate skeleton and the proper functions of bony and cartilaginous elements are determined by extracellular, cell surface and intracellular molecules. Genetic and biochemical analyses of human heritable skeletal disorders as well as the generation of knockout mice provide useful tools to identify the key players of mammalian skeletogenesis. This review summarises our recent work with transgenic animals carrying ablated genes for cartilage extracellular matrix proteins. Some of these mice exhibit a lethal phenotype associated with severe skeletal defects (type II collagen-null, perlecan-null), whereas others show mild (type IX collagen-null) or no skeletal abnormalities (matrilin-1-null, fibromodulin-null, tenascin-C-null). The appropriate human genetic disorders are discussed and contrasted with the knockout mice phenotypes.

A mutation in the alpha 3 chain of type IX collagen causes autosomal dominant multiple epiphyseal dysplasia with mild myopathy

Multiple epiphyseal dysplasia (MED) is a degenerative cartilage condition shown in some cases to be caused by mutations in genes encoding cartilage oligomeric matrix protein or type IX collagen. We studied a family with autosomal dominant MED affecting predominantly the knee joints and a mild proximal myopathy. Genetic linkage to the COL9A3 locus on chromosome 20q13.3 was established with a peak log(10) odds ratio for linkage score of 3.87 for markers D20S93 and D20S164. Reverse transcription-PCR performed on the muscle biopsy revealed aberrant mRNA lacking exon 3, which predicted a protein lacking 12 amino acids from the COL3 domain of alpha3(IX) collagen. Direct sequencing of genomic DNA confirmed the presence of a splice acceptor mutation in intron 2 of the COL9A3 gene (intervening sequence 2, G-A, -1) only in affected family members. By electron microscopy, chondrocytes from epiphyseal cartilage exhibited dilated rough endoplasmic reticulum containing linear lamellae of alternating electron-dense and electron-lucent material, reflecting abnormal processing of mutant protein. Type IX collagen chains appeared normal in size and quantity but showed defective cross-linking by Western blotting. The novel phenotype of MED and mild myopathy is likely caused by a dominant-negative effect of the exon 3-skipping mutation in the COL9A3 gene. Patients with MED and a waddling gait but minimal radiographic hip involvement should be evaluated for a primary myopathy and a mutation in type IX collagen.

A short isoform of Col9a1 supports alveolar bone repair

Bone wound created in intramembranous alveolar bone heals without the formation of cartilage precursor tissue. However, the expression of cartilage collagen mRNAs has been suggested. In this report, we examined the expression and the potential role of type IX collagen in bone restoration and remodeling. The sequence specific polymerase chain reaction demonstrated the exclusive expression of short transcriptional isoform of alpha1(IX) collagen (Col9a1) in alveolar bone wound healing, while the long isoform of Col9a1 transcript was absent. Type IX collagen was immunolocalized in the preliminary matrix organized in granulation tissue before trabecular bone formation in tooth extraction socket. In Col9a1-null mutant mice, there were considerable variations in alveolar bone wound healing with the absence of or abnormally organized trabecular bone. Occasionally, unusual apposition of cortical-bone-like layers in bone marrow space was observed. The Col9a1-null mice indicated no growth retardation, and their facial and long bones maintained the normal size and shape. However, the primary spongiosa region of adult Col9a1 mutant mice showed an abnormal trabecular bone structure associated with abnormal immunostaining with the hypertrophic cartilage specific type X collagen antibody. These data suggest that type IX collagen short transcriptional variant is involved in the restoration and remodeling processes of trabecular bone.

Interaction of Ihh and BMP/Noggin signaling during cartilage differentiation

Bone morphogenetic proteins (BMPs) have been implicated in regulating multiple stages of bone development. Recently it has been shown that constitutive activation of the BMP receptor-IA blocks chondrocyte differentiation in a similar manner as misexpression of Indian hedgehog. In this paper we analyze the role of BMPs as possible mediators of Indian hedgehog signaling and use Noggin misexpression to gain insight into additional roles of BMPs during cartilage differentiation. We show by comparative analysis of BMP and Ihh expression domains that the borders of Indian hedgehog expression in the chondrocytes are reflected in changes of the expression level of several BMP genes in the adjacent perichondrium. We further demonstrate that misexpression of Indian hedgehog appears to directly upregulate BMP2 and BMP4 expression, independent of the differentiation state of the flanking chondrocytes. In contrast, changes in BMP5 and BMP7 expression in the perichondrium correspond to altered differentiation states of the flanking chondrocytes. In addition, Noggin and Chordin, which are both expressed in the developing cartilage elements, also change their expression pattern after Ihh misexpression. Finally, we use retroviral misexpression of Noggin, a potent antagonist of BMP signaling, to gain insight into additional roles of BMP signaling during cartilage differentiation. We find that BMP signaling is necessary for the growth and differentiation of the cartilage elements. In addition, this analysis revealed that the members of the BMP/Noggin signaling pathway are linked in a complex autoregulatory network.

Genomic Characterization of Human DSPG3

DSPG3, the human homolog to chick PG-Lb, is a member of the small leucine-rich repeat proteoglycan (SLRP) family, including decorin, biglycan, fibromodulin, and lumican. In contrast to the tissue distribution of the other SLRPs, DSPG3 is predominantly expressed in cartilage. In this study, we have determined that the human DSPG3 gene is composed of seven exons: Exon 2 ofDSPG3 includes the start codon, exons 4–7 code for the leucine-rich repeats, exons 3 and 7 contain the potential glycosaminoglycan attachment sites, and exon 7 contains the potential N-glycosylation sites and the stop codon. We have identified two polymorphic variations, an insertion/deletion composed of 19 nucleotides in intron 1 and a tetranucleotide (TATT)n repeat in intron 5. Analysis of 1.6 kb of upstream promoter sequence ofDSPG3 reveals three TATA boxes, one of which is 20 nucleotides before the transcription start site. The transcription start site precedes the translation start site by 98 nucleotides. There are 14 potential binding sites for SOX9, a transcription factor present in cartilage, in the promoter, and in the first intron of DSPG3. We have examined the evolution of the SLRP gene family and found that gene products clustered together in the evolutionary tree are encoded by genes with similarities in genomic structure. Hence, it appears that the majority of the introns in the SLRP genes were inserted after the differentiation of the SLRP genes from an ancestral gene that was most likely composed of 2–3 exons.

[The sequence data described in this paper have been submitted to GenBank under accession nos.AF031658 and U63814.]

Revised genomic organization of FBN1 and significance for regulated gene expression

FBN1 encodes fibrillin-1, an extracellular matrix protein that is defective in Marfan syndrome. This gene is divided into 65 exons and was previously reported to be approximately 110 kb in length. The existence of 3 exons upstream of the exon containing the putative initiating methionine left open the possibility of alternative fibrillin-1 isoforms that vary at their N-termini. Detailed examination of YACs containing human FBN1 reveal that the gene is 200 kb, almost twice as large as previously thought. Characterization of the porcine FBN1 cDNA and 5' flanking sequence demonstrates extreme conservation between the pig and the human predicted proteins and argues against the possibility of alternative amino-terminal coding sequence. These data further our understanding of the regulatory requirements for gene expression and establish a framework for recombinant expression of fibrillin-1.

Current perspectives in residual ridge remodeling and its clinical implications: a review

PURPOSE

This article reviews the current understanding of the biology of tooth extraction wound healing and residual ridge remodeling.

METHODS

The review of the biology of tooth extraction wound healing involves a discussion of the different cells populating the tooth extraction wound, the matrix formation, and the control of the repair process in the short-term. Defects in socket matrix formation or cellular activity will lead to stalled healing. The review of residual ridge remodeling describes the long-term result of tooth extraction and formation of residual ridges, in which the quantity of bone tissue continuously decreases. This may suggest that any potential regulatory factors of residual ridge resorption should have an adverse effect either on the increased catabolic activity by osteoclasts or on the decreased anabolic activity by osteoblasts. Both short-term tooth extraction healing and long-term residual ridge remodeling processes are interdependent. Furthermore, any potential genetic and environmental regulatory factors can affect the quality and quantity of bone by altering the gene expression events taking place in bone cells.

RESULTS

The intent of this article was to review the current progresses of biologic research on residual ridge remodeling and to relate the changes at molecular, cellular, and tissue levels. The understanding of residual ridge remodeling may provide a sound scientific basis for improved restorative and therapeutic treatments of the edentulous population.

Human COL9A1 and COL9A2 genes. Two genes of 90 and 15 kb code for similar polypeptides of the same collagen molecule

Here we report the complete structure for the human COL9A1 and the complete sequence for the human COL9A2 genes. The COL9A1 gene is about 90 kb and consists of 38 exons. The COL9A2 gene is only about 15 kb, and it contains 32 exons. Sequence analysis of the promoter regions for the human COL9A2, the mouse Col9a2 and the human COL2A1 genes identified a conserved 14 bp sequence. The data also indicated that the alternative exon 1* found in intron 6 of the COL9A1 gene is separated from exon 7 only by a short intron in the chick, human, mouse and rat genes probably explaining why transcripts from exon 1* are spliced directly to exon 8.

Collagen type IX and developmentally regulated swelling of the avian primary corneal stroma

A critical event in avian corneal development occurs when the acellular primary stroma swells and becomes populated by mesenchymal cells that migrate from the periphery. These cells then deposit the mature stromal matrix that exhibits the unique features necessary for corneal function. Our previous work correlated the disappearance of collagen type IX immunoreactivity at stage 27 (5 1/2-6 days) with matrix swelling and invasion. To investigate further the mechanism of this disappearance, we employed immunohistochemistry after tissue fixation with Histochoice, a non-crosslinking fixative, immunoblot analysis of protein extracts, and gel substrate chromatography (zymography) to detect endogenous proteolytic activity. We found that corneas fixed in Histochoice retain immunoreactivity for type IX collagen for 1-2 days after corneal swelling. This immunoreactivity, however, becomes extractable from tissue sections of unfixed corneas at the time of initiation of stromal swelling and mesenchymal cell invasion. Immunoblot analysis confirmed that, following swelling, immunoreactivity for collagen IX decreased substantially in corneas, but not in the vitreous body, which served as a comparison. Analysis of ammonium sulfate (AS) fractions of such extracts indicated that, at the time of swelling, much of the immunoreactivity for type IX collagen in cornea shifted from the AS precipitate (containing high molecular weight molecules) to the AS supernatant (containing smaller fragments). In contrast, collagen IX immunoreactivity from the vitreous was precipitated by ammonium sulfate throughout the period of study. Collagen type II, a major fibrillar collagen in both the corneal stroma and vitreous, remained in the high molecular weight fraction at all times examined. Zymography detected the presence of the latent (proenzyme) form of gelatinase A (MMP-2) before corneal swelling and invasion (4 days), and both the latent and active forms of the enzyme after corneal swelling. This suggests tissue-specific, developmentally regulated proteolysis of collagen IX as a trigger for corneal matrix swelling.

Pyruvate dehydrogenase E1 alpha isoform in rat testis: cDNA cloning, characterization, and biochemical comparison of the recombinant testis and liver enzymes

Previous data indicated a tissue-specific regulation of mitochondrial pyruvate dehydrogenase (PDH) complex, especially in the brain and testis. The lack of biochemical data on the rat testis PDH limits comparative analysis between testis and liver enzymes. Therefore, we have isolated a cDNA clone encoding rat testis PDH E1 alpha isoform, determined its nucleotide sequence, studied the tissue-specific expression, and characterized the recombinant protein produced in bacteria, compared to the liver counterpart. Our cDNA clone (2.2 kb) contained the identical open reading frame (from nt 974 to 2149) with that previously reported (Cullingford et al., 1993 Biochim Biophys Acta 1216:149-153) but contained a long 5' untranslated region, which has little identity to the other clone. Northern blot confirmed testis-specific expression of this isoform. Genomic DNA analyses by PCR amplification suggested this clone is a gene product distinct from its X-linked somatic counterpart. Our biochemical and kinetic analyses revealed that the purified recombinant rat testis PDH E1 (containing both E1 alpha and E1 beta subunits) was enzymatically active and phosphorylated in vitro by purified PDH-kinase p48 or p45, similar to the recombinant human liver enzyme. Our current data thus indicate that the differential regulation of testis PDH observed in the animal model may result from differential modulation of PDH-kinase or -phosphatase in this tissue rather than the presence of functionally different PDH E1 subunit.

Collagen IX: evidence for a structural association between NC4 domains in cartilage and a novel cleavage site in the alpha 1(IX) chain

Collagen IX, a structural component of the extracellular matrix of connective tissues, is synthesized as long and short forms which contain or lack, respectively, a 27 kDa non-collagenous (NC) 4 domain at the N-terminus of the alpha 1(IX) chain of the molecule. The long form occurs in cartilage and developing cornea, but not in vitreous, suggesting a specialized function for the NC4 domain, perhaps by interacting with other macromolecules. To test this hypothesis, embryonic chick cartilage was treated with DTSSP, dissociated with bacterial collagenase, and the NC4-containing DTSSP-cross-linked protein complexes examined and purified. Analysis of cartilage extracts using an anti-NC4 antibody, and of purified NC4-containing complexes, identified a predominant NC4 dimer. A naturally-occurring N-terminal fragment of the alpha 1(IX) chain, whose size is equivalent to the NC4-COL3-NC3 domains of the chain, was identified. Association of collagen IX molecules via NC4 domains and the existence of a cleavage site close to the NC3 domain of the molecule are likely to be of primary importance in the growth and remodeling processes of cartilage, in health and disease.

Murine laminin alpha3A and alpha3B isoform chains are generated by usage of two promoters and alternative splicing

We already identified two distinct laminin alpha3A and alpha3B chain isoforms which differ in their amino-terminal ends and display different tissue-specific expression patterns. In this study we have investigated whether these two different isoforms are products of the same laminin alpha3 (lama3) gene and transcribed from one or two separate promoters. Genomic clones were isolated that encompass the sequences upstream to the 5' ends of both the alpha3A and the alpha3B cDNAs. Sequence analysis of the region upstream to the alpha3A open reading frame revealed the presence of a TATA box and potential binding sites for responsive elements. By primer extension analysis, the transcription start site of the alpha3B mRNA isoform was defined. The sequences upstream to the alpha3B mRNA transcription start site do not contain a TATA box near the transcription initiation sites, but AP-1, AP-2, and Sp1 consensus binding site sequences were identified. The genomic regions located immediately upstream of the alpha3A and alpha3B transcription start sites were shown to possess promoter activities in transfection experiments. In the promoter regions, response elements for the acute phase reactant signal and NF-interleukin 6 were found, and their possible relevance in the context of inflammation and wound healing is discussed. Our results demonstrate that the lama3 gene produces the two polypeptides by alternative splicing and contains two promoters, which regulate the production of the two isoforms alpha3A and alpha3B.

Effect of ovariectomy on the local residual ridge remodeling

STATEMENT OF PROBLEM

Osteoporosis and edentulism are two disease processes that affect a large group of elderly people in the United States (24 and 25 million, respectively). These two diseases are independent of each other; however, they have several pathologic symptoms in common, such as reduction in bone mass.

PURPOSE

The purpose of this study was to determine whether estrogen deficiency or its replacement therapy have any effect on the phenomenon of residual ridge remodeling.

MATERIAL AND METHODS

Three animal groups were formed that consisted of six female Sprague-Dawley rats each. The two groups had ovariectomy and received either a vehicle solution or a daily dose (1.5 micrograms/day) of 17 beta-estradiol delivered through osmotic pumps. The control group underwent sham surgery and received a vehicle solution. Animals were pair fed throughout the experiment. Unilateral molar extraction was performed in the maxilla, which produced a suitable site for examination of histologic characteristics and molecular biologic analyses. At the 4-week postextraction period the bone remodeling activity was noted at the surface of the residual ridge in the control group.

RESULTS

The ovariectomized group showed increased bone resorption activity, whereas the surface of the residual ridge alveolar bone of the ovariectomized and estrogen-treated group was covered by a layer of hyaline tissue. Poly(A)+ ribonucleic acid samples were isolated from the remodeling residual ridge tissues. Expression of alpha 2(I), alpha 1(II), alpha 1(IX), and alpha 2(X) collagens were examined by ribonucleic acid transfer dot blots. Compared with the control group, ovariectomized animals showed a reduction in bone formation with decreased expressions of type I and II collagens. In contrast, the estrogen-treatment group showed decreased formation of type I collagen with a much increased expression of type II collagen. Further examination of type II collagen formation on the ovariectomized and estrogen-treated group by means of in situ hybridization revealed the notable labeling by the type IIA collagen probe, which was associated with the surface tissue of the residual ridge alveolar bone.

CONCLUSION

These findings suggest that estrogen deficiency and its replacement therapy seem to affect the activity of residual ridge bone remodeling at the molecular level.

Collagen IX

Collagen IX was identified as a distinct component of cartilage about 10 years ago. Composed of three polypeptide chains, its heterotrimeric molecules are located on the surface of type II collagen fibrils in cartilage. The interaction between collagens II and IX is stabilized by covalent crosslinks. The location of collagen IX molecules on fibril surfaces suggests that they represent macromolecular bridges between fibrils and other matrix components in cartilage, and that collagen IX is important for the cohesive and compressive properties of cartilage. Transgenic mice with mutations in a type IX gene develop normally, but show degenerative changes in articular cartilage after birth. Additional evidence for the importance of collagen IX in human articular cartilage comes from the recent finding that a mutation in one of the collagen IX genes causes multiple epiphyseal dysplasia. The mild complaints in affected individuals with the mutation suggest that mutations in type IX collagen genes may represent genetic risk factors for late onset osteoarthritis.

Biosynthesis and processing of type XVI collagen in human fibroblasts and smooth muscle cells

The alpha 1(XVI) collagen chain, recently identified by cDNA cloning, exhibits structural similarity to a subgroup of collagens that associate with collagen fibrils. Recombinant alpha 1(XVI) collagen chains produced in embryonic kidney cells are able to form stable homotrimers, which are rapidly converted into smaller polypeptides after secretion into the culture medium. In this study, we investigated the biosynthesis of native type XVI collagen by immunoprecipitation of metabolically labeled human cells. Dermal fibroblasts and arterial smooth muscle cells were precipitated with three antibodies raised against distinct regions in the N- and C-terminal part of the human alpha 1(XVI) collagen chain. A disulfide-bonded polypeptide of 220 kDa was obtained from the culture medium, cells and extracellular matrix with all three antibodies. This polypeptide is sensitive to bacterial collagenase digestion and partially resistant to pepsin digestion, suggesting that it is the endogenous alpha 1(XVI) collagen chain. Pulse/chase experiments showed that the newly synthesized alpha 1(XVI) chains are secreted into the medium and deposited in the extracellular matrix in a time-dependent manner. Unlike the recombinant chain, the native type XVI collagen does not undergo extensive proteolytic processing upon secretion. Both cell types deposit a substantial amount of the newly synthesized alpha 1(XVI) chain into the extracellular matrix, in which the 220-kDa polypeptide is the only product immunoprecipitated. There is little evidence for the presence of another constituent chain. The data are consistent with a nomotrimeric chain composition for type XVI collagen. No apparent difference exists in the rate of synthesis and secretion between fibroblasts and smooth muscle cells. Indirect immunofluorescence microscopy showed an extracellular distribution of type XVI collagen, which is located close to cells but not associated with fibrillar structures.

Regulation of rate of cartilage differentiation by Indian hedgehog and PTH-related protein

Proper regulation of chondrocyte differentiation is necessary for the morphogenesis of skeletal elements, yet little is known about the molecular regulation of this process. A chicken homolog of Indian hedgehog (Ihh), a member of the conserved Hedgehog family of secreted proteins that is expressed during bone formation, has now been isolated. Ihh has biological properties similar to those of Sonic hedgehog (Shh), including the ability to regulate the conserved targets Patched (Ptc) and Gli. Ihh is expressed in the prehypertrophic chondrocytes of cartilage elements, where it regulates the rate of hypertrophic differentiation. Misexpression of Ihh prevents proliferating chondrocytes from initiating the hypertrophic differentiation process. The direct target of Ihh signaling is the perichondrium, where Gli and Ptc flank the expression domain of Ihh. Ihh induces the expression of a second signal, parathyroid hormone-related protein (PTHrP), in the periarticular perichondrium. Analysis of PTHrP (-/-) mutant mice indicated that the PTHrP protein signals to its receptor in the prehypertrophic chondrocytes, thereby blocking hypertrophic differentiation. In vitro application of Hedgehog or PTHrP protein to normal or PTHrP (-/-) limb explants demonstrated that PTHrP mediates the effects of Ihh through the formation of a negative feedback loop that modulates the rate of chondrocyte differentiation.

Expression of bone-specific genes by hypertrophic chondrocytes: implication of the complex functions of the hypertrophic chondrocyte during endochondral bone development

Endochondral bone formation is one of the most extensively examined developmental sequences within vertebrates. This process involves the coordinated temporal/spatial differentiation of three separate tissues (cartilage, bone, and the vasculature) into a variety of complex structures. The differentiation of chondrocytes during this process is characterized by a progressive morphological change associated with the eventual hypertrophy of these cells. These cellular morphological changes are coordinated with proliferation, a columnar orientation of the cells, and the expression of unique phenotypic properties including type X collagen, high levels of bone, liver, and kidney alkaline phosphatase, and mineralization of the cartilage matrix. Several studies indicate that hypertrophic chondrocytes also express osteocalcin, osteopontin, and bone sialoprotein, three proteins which until very recently were widely believed to be restricted in their expression to osteoblasts. Recent studies suggest that the hypertrophic chondrocytes are regulated by the calcitropic hormones, morphogenic steroids, and local tissue factors. These considerations are based on the regulation by 1,25 (OH)2D3 and retinoids of the cartilage specific genes as well as osteopontin and osteocalcin expression in hypertrophic chondrocytes. They are also based on the effects on growth plate development caused by 1) transgenic ablation of autocrine/paracrine regulators such as PTHrP and of the transcriptional regulator c-fos and 2) naturally occurring genetic mutations of the FGF receptor. These studies further suggest that specific transcriptional factors mediate exogenous regulatory signals in a coordinated manner with the development of bone. While it has been widely demonstrated that the majority of hypertrophic chondrocytes undergo apoptosis during terminal stages of the developmental sequence, their response to specific exogenous regulatory signals and their expression of bone-specific proteins give rise to questions about whether all growth chondrocytes have the same developmental fates and have identical functions. Furthermore, specific questions arise as to whether there are similar mechanisms of regulation for commonly expressed genes found in both cartilage and bone or whether these genes have unique regulatory mechanisms in these different tissues. These recent findings suggest that hypertrophic chondrocytes are functionally coupled during endochondral bone formation to the recruitment of osteoblasts, vascular cells, and osteoclasts.

Identification of three N-terminal ends of type XVIII collagen chains and tissue-specific differences in the expression of the corresponding transcripts. The longest form contains a novel motif homologous to rat and Drosophila frizzled proteins

Transcripts for the alpha 1 chain of mouse type XVIII collagen were found to be heterogeneous at their 5'-ends and to encode three variant N-terminal sequences of the ensuing 1315-, 1527-, or 1774-residue collagen chains. The variant mRNAs appeared to originate from the use of two alternate promoters of the alpha 1(XVIII) chain gene, resulting in the synthesis of either short or long N-terminal non-collagenous NC1 domains, the latter being further subject to modification due to alternative splicing of the transcripts. As a result, the 1527- and 1774-residue polypeptides share the same signal peptide, and the lengths of their NC1 domains are 517 or 764 amino acid residues, respectively, while the 1315-residue polypeptide has a different signal peptide and a 301-residue NC1 domain. The longest NC1 domain was strikingly characterized by a 110-residue sequence with 10 cysteines, which was found to be homologous with the previously identified frizzled proteins belonging to the family of G-protein-coupled membrane receptors. Thus, it is proposed that the cysteine-rich motif, termed fz, represents a new sequence motif that can be found in otherwise unrelated proteins. Tissues containing mainly one or two NC1 domain mRNA variants or all three NC1 domains were identified, indicating that there is tissue-specific utilization of two alternate promoters and alternative splicing of alpha 1(XVIII) transcripts.

Recombinant analysis of human alpha 1 (XVI) collagen. Evidence for processing of the N-terminal globular domain

The N-terminal non-collagenous domain NC11 of the human collagen alpha 1 (XVI) chain was obtained as a recombinant 35-kDa protein from stably transfected kidney cell clones. This form had undergone proteolytic trimming at a basic cleavage motif indicating a similar release in vivo. Domain NC11 showed a globular shape after rotary shadowing and was resistant to neutral proteases. Specific antibodies could be raised against recombinant NC11 and were used for the analysis of other cell clones transfected with the full-length alpha 1 (XVI) chain. Immunoprecipitation of detergent extracts of metabolically labelled cells demonstrated the presence of disulfide-bonded 200-kDa polypeptides possessing NC11 epitopes. This material was partially resistant to pepsin, indicating the formation of alpha 1 (XVI) chain homotrimers with a triple-helical conformation. Yet a substantial proportion of these homotrimers was degraded to fragments of variable size (35-150 kDa) when secreted into the culture medium. Several of these fragments could be obtained on a semi-preparative scale from cells grown in hollow fiber cassettes and showed substantial hydroxylation of proline, consistent with triple-helix formation. Edman degradation demonstrated the origin of some from the N-terminal and of one from a more C-terminal position of collagen XVI. This extensive degradation may be explained by the release of NC11 and by further cleavages within some of the nine interruptions of the triple-helical domain of the alpha 1(XVI) chain. Whether this process also occurs in situ remains to be shown.

Chondrocyte differentiation

Data obtained while investigating growth plate chondrocyte differentiation during endochondral bone formation both in vivo and in vitro indicate that initial chondrogenesis depends on positional signaling mediated by selected homeobox-containing genes and soluble mediators. Continuation of the process strongly relies on interactions of the differentiating cells with the microenvironment, that is, other cells and extracellular matrix. Production of and response to different hormones and growth factors are observed at all times and autocrine and paracrine cell stimulations are key elements of the process. Particularly relevant is the role of the TGF-beta superfamily, and more specifically of the BMP subfamily. Other factors include retinoids, FGFs, GH, and IGFs, and perhaps transferrin. The influence of local microenvironment might also offer an acceptable settlement to the debate about whether hypertrophic chondrocytes convert to bone cells and live, or remain chondrocytes and die. We suggest that the ultimate fate of hypertrophic chondrocytes may be different at different microanatomical sites.

Analysis of transcriptional isoforms of collagen types IX, II, and I in the developing avian cornea by competitive polymerase chain reaction

The genes for the alpha 1(IX), alpha 1(II), and alpha 2(I) collagen chains can give rise to different isoforms of mRNA, generated by alternative promotor usage [for alpha 1(IX) and alpha 2(I)] or alternative splicing [for alpha 1(II)]. In this study, we employed competitive reverse transcriptase PCR to quantitate the amounts of transcriptional isoforms for these genes in the embryonic avian cornea from its inception (about 3 1/2 days of development) to 11 days. In order to compare values at different time points, the results were normalized to those obtained for the "housekeeping" enzyme, glycerol-3-phosphate dehydrogenase (G3PDH). These values were compared to those obtained from other tissues (anterior optic cup and cartilage) that synthesize different combinations of the collagen isoforms. We found that, in the cornea, transcripts from the upstream promotor of alpha 1(IX) collagen (termed "long IX") were predominant at stage 18-20 (about 3 1/2 days), but then fell rapidly, and remained at a low level. By 5 days (just before stromal swelling) the major mRNA isoform of alpha 1(IX) was from the downstream promoter (termed "short IX"). The relative amount of transcript for the short form of type IX collagen rose to a peak at about 6 days of development, and then declined. Throughout this period, the predominant transcriptional isoform of the collagen type II gene was IIA (i.e., containing the alternatively spliced exon 2). This indicates that the molecules of type II collagen that are assembled into heterotypic fibrils with type I collagen possess, at least transiently, an amino-terminal globular domain similar to that found in collagen types I, III, and V. For type I, the "bone/tendon" mRNA isoform of the alpha 2(I) collagen gene was predominant; transcripts from the downstream promotor were at basal levels. In other tissues expressing collagen types IX and II, long IX was expressed predominantly with the IIA form in the anterior optic cup at stage 22/23; in 14 1/2 day cartilage, long IX was expressed predominantly along with the IIB form of alpha 1(II). The downstream transcript of the alpha 2(I) gene (Icart) was found at high levels only in cartilage.