Complete amino acid sequence of human cartilage link protein (CRTL1) deduced from cDNA clones and chromosomal assignment of the gene

Hyaluronan in tissue injury and repair

A hallmark of tissue injury and repair is the turnover of extracellular matrix components. This review focuses on the role of the glycosaminoglycan hyaluronan in tissue injury and repair. Both the synthesis and degradation of extracellular matrix are critical contributors to tissue repair and remodeling. Fragmented hyaluronan accumulates during tissue injury and functions in ways distinct from the native polymer. There is accumulating evidence that hyaluronan degradation products can stimulate the expression of inflammatory genes by a variety of immune cells at the injury site. CD44 is the major cell-surface hyaluronan receptor and is required to clear hyaluronan degradation products produced during lung injury; impaired clearance of hyaluronan results in persistent inflammation. However, hyaluronan fragment stimulation of inflammatory gene expression is not dependent on CD44 in inflammatory macrophages. Instead, hyaluronan fragments utilize both Toll-like receptor (TLR) 4 and TLR2 to stimulate inflammatory genes in macrophages. Hyaluronan also is present on the cell surface of lung alveolar epithelial cells and provides protection against tissue damage by interacting with TLR2 and TLR4 on these parenchymal cells. The simple repeating structure of hyaluronan appears to be involved in a number of important aspects of noninfectious tissue injury and repair that are dependent on the size and location of the polymer as well as the interacting cells. Thus, the interactions between the endogenous matrix component hyaluronan and its signaling receptors initiate inflammatory responses, maintain structural cell integrity, and promote recovery from tissue injury.

B Cell Clonal Expansion and Somatic Hypermutation of Ig Variable Heavy Chain Genes in the Synovial Membrane of Patients with Osteoarthritis

Inflammatory mediators have been explored as possible factors in the initiation and/or progression of osteoarthritis (OA). This study shows that synovial infiltration by B lymphocytes is present in almost half of the knee OA cases. The degree of B lymphocyte infiltration is associated with more pronounced synovial inflammation and with the presence of plasma cells and lymphoid follicles in more severe cases. To examine whether these B cells are merely bystanders or could be involved in the pathogenesis of OA, we analyzed the Ig H chain variable region (V(H)) genes of B cells recovered from the synovial membrane of five OA patients with marked B cell infiltration. Sequence analysis of CDR3 regions of rearranged VDJ genes revealed clonal or oligoclonal B cell expansions in all cases. Expanded B cell clones in four of five OA patients showed clustered somatic mutations, occurring mainly in the CDRs and with a high replacement-to-silent ratio (>2.9), indicating that these cells are postgerminal center B cells that had been positively selected through their Ag receptor. These data demonstrate the presence in inflamed knee OA synovium of clonally expanded, Ag-driven B cells that may contribute to the development or progression of the disease.

Link protein can retard the degradation of hyaluronan in proteoglycan aggregates

OBJECTIVE

Loss of articular cartilage and intervertebral disc function in arthritis or disc degeneration is associated with degradation of the proteoglycan (PG) aggregates by either proteolysis of aggrecan or hyaluronan (HA) degradation. The aim of this work was to determine whether degradation of HA in PG aggregate degradation is influenced by link protein (LP) stabilization of the PG aggregates.

METHODS

Aggrecan and LP were prepared from fetal bovine epiphyseal cartilage, and PG aggregates were formed in the presence or absence of LP. The PG aggregates were exposed to hyaluronidase or free radicals to promote HA degradation. Degradation of HA, aggrecan and LP were assessed by gel filtration chromatography and polyacrylamide gel electrophoresis.

RESULTS

High concentrations of hyaluronidase cleaved both PG aggregates between each aggrecan molecule, whereas low concentrations gave much less cleavage of the LP-stabilized aggregate. High free radical concentrations gave extensive cleavage of all components of both PG aggregates, whereas low concentrations are more selective for HA damage and to a much lesser extent in the LP-stabilized aggregates. Thus the presence of LP caused a diminution in the capacity of both catabolic agents to degrade HA as long as levels of the degradative agents were not excessive.

CONCLUSION

In addition to stabilizing the PG aggregates towards dissociation, LP may also help protect the PG aggregates from degradation under conditions where tissue catabolism is promoted.

Genetic rescue of chondrodysplasia and the perinatal lethal effect of cartilage link protein deficiency

The targeted disruption of cartilage link protein gene (Crtl1) in homozygous mice resulted in a severe chondrodysplasia and perinatal lethality. This raised the question of whether the abnormalities seen in Crtl1 null mice are all caused by the absence of link protein in cartilage or whether the deficiency of the protein in other tissues and organs contributed to the phenotype. To address this question we have generated transgenic mice overexpressing cartilage link protein under the control of a cartilage-specific promoter, and then these transgenic mice were used for a genetic rescue of abnormalities in Crtl1 null mice. While the overexpression of cartilage link protein resulted in no abnormal phenotype, the cartilage-specific transgene expression of link protein could completely prevent the perinatal mortality of link protein-deficient mice and, depending on the level of the link protein expression, rescue skeletal abnormalities. Although link protein was originally isolated from cartilage, we found and determined Crtl1 transcripts and corresponding proteins in every organ tested from mouse embryos to aging animals. We also identified three additional members of the link protein family, all co-localized with hyaluronic acid-binding proteoglycans in the mouse genome. The ubiquitous presence of link protein suggests a general and systemic function of link protein in the organization of extracellular matrix in a number of tissues, possibly interacting with other proteoglycans, such as versican, brevican, and neurocan.

Animal models of inflammatory spinal and sacroiliac joint diseases

Many cartilage matrix proteins or domains such as collagen types II, IX, and XI, GP39, AG1, VG1, and LP are potential antigens that might induce polyarthritis in susceptible animals (Table 1). Ordinarily, spondylitis is not a feature of polyarthritis induced with collagen types II, IX, and XI, GP39, cartilage matrix protein (matrilin-1) and cartilage LP. It seems that only the proteoglycans aggrecan and versican are capable of inducing sacroiliitis and spondylitis. Both molecules are structural proteins in intervertebral discs. Moreover, the arthritogenic or spondylitogenic epitopes of both molecules have been localized to the homologous N-terminal G1 globular domains. This region of versican and aggrecan is highly conserved, with 52% identity of amino acids. The homology is seen exclusively in the G1 domain and is concentrated between residues 115 and 332 (AG1 numbering) near the natural cleavage DIPEN site of aggrecan [84, 85]. Extra-articular pathology is often seen in rheumatic diseases, especially in AS. Other tissues, such as the sclera of the eye [86] and the media of the arteries [86, 87], also contain type II collagen, AG1, VG1, and LP, and versican is present in the central and peripheral nervous systems. Thus, there is the potential for an immune response against cartilage G1 and LP to be directed against related structures in extra-articular tissues. The presence of versican in the tendon and trochlea of the human superior oblique muscle might account for the occurrence of transient attacks of acquired Brown syndrome in patients with juvenile and adult forms of chronic RA [88]. Thus, it will be interesting to determine whether or not extra-articular expression of these cartilage proteins is closely related to extra-articular pathogenic expression in rheumatic diseases. Uveitis develops in VG1-immunized BALB/c mice, which is not seen in AG1-, and LP-treated animals. There is evidence that aggrecan and LP are also localized at these sites in the eye, but only immunity to versican can induce uveitis. In sacroiliitis and enthesitis of AS patients, the inflammation is associated with chondrometaplasia. In versican-induced sacroiliitis, replacement of cartilage by bone is seen with relatively little inflammation, somewhat resembling the situation in AS (Fig. 2). Versican can also stimulate chondrocyte proliferation [43]. Three conserved domains of human cartilage matrix molecules, namely VG1, AG1, and LP, show considerable homology [77, 79, 80, 89], and each is capable of inducing a unique inflammatory arthritis in BALB/c mice, with VG1 inducing only spondylitis [65], LP inducing peripheral arthritis with no spondylitis [90], and AG1 inducing axial and peripheral arthritis [66, 91]. It remains a mystery why such similar molecules cause different pathology in different target tissues. The exact immunopathogenic mechanisms deserve further study.

The brain link protein-1 (BRAL1): cDNA cloning, genomic structure, and characterization as a novel link protein expressed in adult brain

We report here molecular cloning and expression analysis of the gene for a novel human brain link protein-1 (BRAL1) which is predominantly expressed in brain. The predicted open reading frame of human brain link protein-1 encoded a polypeptide of 340 amino acids containing three protein modules, the immunoglobulin-like fold and proteoglycan tandem repeat 1 and 2 domains, with an estimated mass of 38 kDa. The brain link protein-1 mRNA was exclusively present in brain. When analyzed during mouse development, it was detected solely in the adult brain. Concomitant expression pattern of mRNAs for brain link protein-1 and various lectican proteoglycans in brain suggests a possibility that brain link protein-1 functions to stabilize the binding between hyaluronan and brevican. The human BRAL1 gene contained 7 exons and spanned approximately 6 kb. The entire immunoglobulin-like fold was encoded by a single exon and the proteoglycan tandem repeat 1 and 2 domains were encoded by a single and two exons, respectively. The deduced amino acid sequence of human brain link protein-1 exhibited 45% identity with human cartilage link protein-1 (CRTL1), previously reported as link protein to stabilize aggregates of aggrecan and hyaluronan in cartilage. These results suggest that brain link protein-1 may have distinct function from cartilage link protein-1 and play specific roles, especially in the adult brain.

The matrilins: a novel family of oligomeric extracellular matrix proteins

The matrilin family at present has four members that all share a structure made up of von Willebrand factor A domains, epidermal growth factor-like domains and a coiled coil alpha-helical module. The first member of the family, matrilin-1 (previously called cartilage matrix protein or CMP), is expressed mainly in cartilage. Matrilin-3 has a similar tissue distribution, while matrilin-2 and -4 occur in a wide variety of extracellular matrices. Matrilin-1 is associated with cartilage proteoglycans as well as being a component of both collagen-dependent and collagen-independent fibrils and on the basis of the related structures other matrilins may play similar roles. The matrilin genes are strictly and differently regulated and their expression may serve as markers for cellular differentiation.

The identification of proteoglycan-associated mRNAs in human dental pulp cells

Characterization of the dental pulp proteoglycans has been largely confined to the glycosaminoglycan component of the proteoglycan molecule, while the protein core has received little attention. This study was conducted to identify mRNAs of previously well-characterized proteoglycans-biglycan, decorin and versican-and link protein in dental pulp cells. Dermal fibroblasts were used as a positive control. Oligonucleotide probes were constructed based on published sequences for the four proteins from human tissues. Total RNA was isolated from cultured human pulp and dermal cells, separated according to size by formaldehyde gel electrophoresis and subsequently transferred to a nylon filter. Northern hybridizations using the oligonucleotide probes revealed the expression of biglycan, decorin, versican and link protein mRNAs. Biglycan and decorin are small proteoglycans that have a regulatory effect on collagen fibrillogenesis. Assuming expression of link protein and versican in vivo, the larger proteoglycans in the dental pulp are capable of forming large proteoglycan aggregates.

Bovine chondrocyte link protein cDNA sequence: interspecies conservation of primary structure and mRNA untranslated regions

The sequence for bovine link protein cDNA, including 108 bases of the 5' untranslated region (UTR) and 768 nucleotides of the 3' UTR, was determined from polymerase chain reaction products and bovine articular chondrocyte cDNA clones. The deduced primary structure for bovine link protein predicts a protein 354 amino acid residues in length. Comparative analysis with link protein sequence from several other species revealed overall high conservation of protein coding sequence. High nucleotide sequence conservation was observed within the extensive 5' and 3' UTRs of bovine, human, pig, chick and rat link protein mRNA. As evidence that the UTRs might play a role in regulation of link protein mRNA turnover, multiple occurrences of the adenosine-uridine binding factor motif A(Ua)A were found to be conserved between species within 3' UTRs. A polyadenylation signal was conserved between the bovine and chicken sequence, use of which would result in the smallest of multiple bovine link protein mRNA species observed by Northern blot analysis.

Cartilage proteoglycans: structure and potential functions

Hyaline cartilage contains five well-characterized proteoglycans in its extracellular matrix, and it is likely that others exist. The largest in size and most abundant by weight is aggrecan, a proteoglycan that possesses over 100 chondroitin sulfate and keratan sulfate chains. Aggrecan is also characterized by its ability to interact with hyaluronic acid to form large proteoglycan aggregates. Both the high anionic charge on the individual aggrecan molecules endowed by the sulfated glycosaminoglycan chains and the localization within the matrix endowed by aggregate formation are essential for aggrecan function. The molecule provides cartilage with its osmotic properties, which give articular cartilage its ability to resist compressive loads. The other proteoglycans are characterized by their ability to interact with collagen. They are much smaller than aggrecan in size but may be present in similar molar amounts. Decorin, biglycan, and fibromodulin are closely related in protein structure but differ in glycosaminoglycan composition and function. Decorin and biglycan possess one and two dermatan sulfate chains, respectively, whereas fibromodulin bears several keratan sulfate chains. Decorin and fibromodulin both interact with the type II collagen fibrils in the matrix and may play a role in fibrillogenesis and interfibril interactions. Biglycan is preferentially localized in the pericellular matrix, where it may interact with type VI collagen. Finally, type IX collagen can also be considered as a proteoglycan, as its alpha 2(IX) chain may bear a glycosaminoglycan chain. It may serve as a bridge between the collagen fibrils or with the interspersed aggrecan network.

The expression of functional link protein in a baculovirus system: analysis of mutants lacking the A, B and B' domains

Functional recombinant human link protein has been produced using a baculovirus expression system. In addition to the intact link protein, three mutant forms have also been expressed. Each mutant bears a deletion equivalent to the protein encoded by one exon in the gene. These deletions represent the A domain, which is thought to be responsible for interaction with aggrecan, and the B or B' domains, which are associated with the interaction with hyaluronate. Such deletions split codons spanning exon boundaries, but maintain the reading frame of the protein and result in the correct amino acid being present at the splice junction. All the recombinant proteins appear as two components upon SDS/PAGe, though the abundance of the two forms does vary between preparations, as a result of variable substitution by N-linked oligosaccharides. The recombinant intact link protein was able to interact with both hyaluronate and aggrecan, showing that the baculovirus system is able to produce functional molecules. All of the recombinant mutant link proteins were also able to interact with hyaluronate, indicating that both the B and B' domains can function independently. The recombinant mutant link proteins were also able to interact with aggrecan, with the exception of the mutant lacking the A domain, confirming that this ability resides entirely within this domain.

The link proteins

Aggregates of chondroitin-keratan sulfate proteoglycan (aggrecan) and hyaluronic acid (hyaluronan) are the major space-filling components of cartilage. A glycoprotein, link protein (LP; 40-48 kDa) stabilizes the aggregate by binding to both hyaluronic acid and aggrecan. In the absence of LP, aggregates are smaller (as estimated by rotary shadowing of electron micrographs) and less stable (they dissociate at pH 5) than they are in the presence of LP. The proteoglycan aggregate, including LP, is dissociated in the presence of chaotropes such as 4 M guanidine hydrochloride. On removal of the chaotrope, the complex will reassociate. This forms the basis of the isolation of LP from cartilage and has been described in detail elsewhere. Tryptic digestion of the proteoglycan aggregates results in a high molecular weight product that consists of hyaluronic acid to which is bound LP and the N-terminal globular domain of aggrecan (hyaluronic acid binding region; HABR) in a 1:1 stoichiometry. The amino acid sequences of LP and HABR are surprisingly similar. The amino acid sequence can be divided into three domains; an N-terminal domain that falls into the immunoglobulin super-family and two C-terminal domains that are similar to each other. The DNA structure echoes this similarity, in that the major domains are reflected in three separate exons in both LP and HABR. The two C-terminal domains are largely responsible for the association with HA and are related to two recently described hyaluronate-binding proteins, CD44 and TSG-6. A variety of approaches, including analysis of the forms of LP in vivo, rotary shadowing and analysis of the sequence in the immunoglobulin-like domain, have shed considerable light on the structure-function relationships of LP. This review describes the structure and function of LP in detail, focusing on what can be inferred from the similarity of LP, HABR and related molecules such as immunoglobulins and lymphocyte HA-receptors.

Biologic and clinical significance of cytogenetic and molecular cytogenetic abnormalities in benign and malignant cartilaginous lesions

Cartilaginous neoplasms are often histologically and therapeutically challenging. Predicting biologic behavior can be difficult. In this study, 120 nonneoplastic, benign, and malignant cartilaginous lesions from 103 patients were cytogenetically analyzed in a 6-year period after short-term culture. For selected cases, fluorescent in situ hybridization (FISH) techniques using chromosome-specific probes were performed on metaphase/interphase preparations and on paraffin-embedded tissue sections. Clonal abnormalities of chromosomes 2, 3, 5, 7, 8, and 12 were most frequently observed. Involvement of chromosomes 5, 8, and 12 may be etiologically significant because of the gene localizations for the human cartilage link protein, Langer-Giedion syndrome (a rare syndrome characterized by multiple exostoses), and type II collagen (a major component of normal cartilage) respectively, to these three chromosomes. That chromosome 7 abnormalities were observed only in malignant tumors is of diagnostic value. The identity of three marker chromosomes and the significance of trisomy 7 (a finding of controversial meaning), were determined with FISH. That the presence of chromosome aberrations and increasing histologic grade strongly correlated (p = 0.001) is of prognostic importance. Moreover, complex aberrations were observed nearly exclusively in high-grade tumors (p = 0.001). The data show that nonrandom chromosome loci are aberrantly affected in cartilaginous lesions and that these abnormalities may be of significant histopathogenetic consequence. In addition, these chromosome abnormalities appear to be diagnostically and prognostically valuable in classifying and grading chondromatous neoplasms.

Exclusion of candidate genes from a role in cleft lip with or without cleft palate: linkage and association studies

Candidate genes and marker loci for cleft lip/palate (CL/P) were tested using linkage analyses and association studies. Eight British families with apparent autosomal dominant inheritance of non-syndromic CL/P participated in the linkage analyses while the association analyses involved 61 unrelated British white people with CL/P and 60 controls. The report of an association between RARA (17q21) and unrelated Australian persons with CL/P (p = 0.016) was not confirmed in British CL/P persons (chi 2 = 0.954, p > 0.1). There was also no evidence of linkage between RARA and the eight CL/P families (Z = -3.211, theta = 0.001). Linkage was excluded between familial CL/P and F13A1 (map position 6p24-25) with an observed maximum lod score of Z = -2.052 at theta = 0.05. No association was found between alleles at VIM (10p13) and the British CL/P subjects (chi 2 = 0.110, p > 0.5). Multipoint analysis excluded linkage between familial CL/P and the markers D1S65 and D1S58 which flank the Van der Woude syndrome locus with a maximum lod score of Z = -4.0. This suggests that the genetic defect underlying VWS is not the same as in non-syndromic CL/P. There was no evidence of linkage between CRTL1 (5q15) and the eight CL/P families (Z = -3.466, theta = 0.05).

The link proteins

Aggregates of chondroitin-keratan sulfate proteoglycan (aggrecan) and hyaluronic acid (hyaluronan) are the major space-filling components of cartilage. A glycoprotein, link protein (LP; 40-48 kDa) stabilizes the aggregate by binding to both hyaluronic acid and aggrecan. In the absence of LP, aggregates are smaller (as estimated by rotary shadowing of electron micrographs) and less stable (they dissociate at pH 5) than they are in the presence of LP. The proteoglycan aggregate, including LP, is dissociated in the presence of chaotropes such as 4 M guanidine hydrochloride. On removal of the chaotrope, the complex will reassociate. This forms the basis of the isolation of LP from cartilage and has been described in detail elsewhere. Tryptic digestion of the proteoglycan aggregates results in a high molecular weight product that consists of hyaluronic acid to which is bound LP and the N-terminal globular domain of aggrecan (hyaluronic acid binding region; HABR) in a 1:1 stoichiometry. The amino acid sequences of LP and HABR are surprisingly similar. The amino acid sequence can be divided into three domains; an N-terminal domain that falls into the immunoglobulin super-family and two C-terminal domains that are similar to each other. The DNA structure echoes this similarity, in that the major domains are reflected in three separate exons in both LP and HABR. The two C-terminal domains are largely responsible for the association with HA and are related to two recently described hyaluronate-binding proteins, CD44 and TSG-6. A variety of approaches, including analysis of the forms of LP found in vivo, rotary shadowing and analysis of the sequence in the immunoglobulin-like domain, have shed considerable light on the structure-function relationships of LP. This review describes the structure and function of LP in detail, focusing on what can be inferred from the similarity of LP, HABR and related molecules such as immunoglobulins and lymphocyte HA-receptors.

A recurrent chondromyxoid fibroma with chromosome aberrations ins(5;2)(q13;p21p25) and 2p deletion: a case report

We report a patient with a recurrent chondromyxoid fibroma, a rare benign tumor of the bone with clonal aberrations in chromosomes 2 and 5. Karyotyping, chromosome painting, interphase cytogenetics by in situ hybridization, and DNA flow cytometry were used. The karyotype was interpreted as 46,XX,der(2)ins(5;2)(q13;p21p25),der(2)ins(5;2)(q13;p21p25), der(5)ins(5;2) (q13;p21p25).

Exclusion of human proteoglycan link protein (CRTL1) and type II collagen (COL2A1) genes in pseudoachondroplasia

Patients with pseudoachondroplasia have a skeletal dysplasia with marked short stature. The most common cause of this condition is an autosomal dominant mutation, although autosomal recessive inheritance has been reported. Linkage to 2 cartilage-specific candidate genes, type II collagen (COL2A1) and proteoglycan link protein genes (CRTL1), was tested in 9 autosomal dominant families with pseudoachondroplasia. Tight linkage to these candidate genes was excluded with LOD scores for COL2A1 of -2.45 at theta = 0.05 and for CRTL1 of -7.28 at theta = 0.001. Discordant inheritance of the disease phenotype with each of these genes was also observed. Thus, these 2 candidate genes can be excluded as the cause of disease in these families.

Distribution of cartilage proteoglycan (aggrecan) core protein and link protein gene expression during human skeletal development

The distribution of cartilage proteoglycan core protein (aggrecan) and cartilage proteoglycan link protein was investigated by in situ hybridization during different stages of human skeletal development. Aggrecan and link protein expression were confined to chondrocytes of the developing skeleton and other cartilaginous structures. Distribution and intensity of the signal was identical with aggrecan as compared to link protein probes. Parallel to the calcification of cartilaginous matrix, chondrocytes of this area lost the expression of aggrecan and link protein specific mRNA and stayed negative throughout the following stages of skeletal development. Highest expression was found in the lower proliferative and upper hypertrophic zone whereas the resting zone showed less expression. Aggrecan gene expression was additionally investigated in iliac crest biopsies of 3 patients with pseudoachondroplasia and compared to age-matched controls. Distribution and intensity of staining revealed no abnormalities. Thus, the phenotypic changes during chondrocyte maturation are accompanied by distinct changes in aggrecan and link protein gene expression. This pattern was maintained in the growth plate of patients with pseudoachondroplasia.