Preparation and characterization of an antiserum against purified proteoglycan link proteins from avian cartilage

Link protein shows species variation in its susceptibility to proteolysis

Human cartilage link protein exists as three native components, while equine, bovine, and porcine cartilage link protein exist as two and Swarm rat chondrosarcoma link protein exists as only one component. These nonhuman link protein components represent intact protein structures, and there is little evidence for proteolytically modified forms in nonhuman tissues. In human cartilage, the proteolytic production of modified link proteins increases with age, whereas high amounts of such products were not seen in the nonhuman tissues. However, the small amounts of link protein fragments that were observed in the nonhuman cartilages were of a similar size to their human counterparts. On digestion of human proteoglycan aggregate with stromelysin, rapid modification of the link protein components occurred, whereas the aggregates from nonhuman cartilages showed incomplete cleavage of their link protein components. The relative resistance of nonhuman link protein to stromelysin may in part be due to a unique amino acid substitution present near the enzymic cleave site.

The chicken embryonic mesonephros synthesizes link protein, an extracellular matrix molecule usually found in cartilage

Link protein is a macromolecule that is relatively abundant in the extracellular matrix of cartilage, where it acts as a stabilizing component in aggregates of the large chondroitin sulfate proteoglycan and hyaluronic acid. In the present study, link protein transcripts were demonstrated in the chicken embryonic mesonephros by RNA in situ hybridization using a cartilage link protein cDNA probe. The link protein transcripts of the mesonephros are of the same size as those seen in cartilage. In addition, mesonephroi contain a protein that is immunologically reactive with a link protein polyclonal antiserum and this protein is identical in size to link protein isolated from cartilage. No transcripts for cartilage proteoglycan core protein were detected in the mesonephros. Type II collagen and cartilage matrix protein transcripts were also not detectable in the mesonephros. From previous data on chondrogenesis in the developing limb bud, the transcription of link protein and the proteoglycan core protein genes appeared to be spatially and temporally regulated in a coordinated fashion. However, the presence of link protein transcripts in the mesonephros, independent of cartilage proteoglycan core protein gene expression, indicates that these genes can be regulated independently of each other.

Expression of cartilage-matrix genes and localization of their translation products in the embryonic chick eye

The presence of cartilage-matrix molecules in the embryonic chick eye was shown by means of measuring RNA expression and immunostaining studies. The molecules examined were proteoglycan core protein (PG-core), link protein (LP), cartilage-matrix protein (CMP) and type II collagen. The genes encoding these proteins are expressed in 8- to 11-day embryonic chick eyes at constant steady-state levels. Immunopositive stained tissues include sclera, choroid, cornea, lens capsule (PG-core, LP, CMP), lens epithelium, lens fibers (LP, CMP) and the membranes in the retina (LP). In addition, data from comparative studies employing the eyes from the proteoglycan core-protein-deficient mutant, nanomelia, indicate that levels of mRNA for core protein are almost absent and that CMP is reduced in the above-mentioned tissues.

The tandemly repeated sequences of cartilage link protein contain the sites for interaction with hyaluronic acid

The ternary complex involving link protein (LP), proteoglycan monomer, and hyaluronic acid (HA) is an important component of the extracellular matrix of cartilage. LP contains tandemly repeated sequences that were tested for their ability to interact with HA. A solid-phase assay was developed in which LP could specifically bind to immobilized HA. Detection of LP was by means of an antiserum directed against a peptide from the NH2-terminal half of LP. LP binding to HA could be inhibited with mAb 8A4 (Caterson, B., J. R. Baker, J. E. Christner, Y. Lee, and M. Lentz. 1985. J. Biol. Chem. 260:11348-11356). Using synthetic peptides that correspond to specific amino acid sequences of chicken LP (Deák, F., I. Kiss, K. J. Sparks, W. S. Argraves, G. Hampikian, and P. F. Goetinck. 1986. Proc. Natl. Acad. Sci. USA. 83:3766-3770) the epitopes for mAb 8A4 were determined to reside in peptides Gly217-Pro226 and Arg316-Arg325. These two peptides were also capable of inhibiting the interaction between LP and HA. The peptide Trp242-Val251 and Pro339-Val348 could also inhibit the interaction between LP and HA. All four peptides reside in the tandemly repeated domains of LP and they contain clusters of positively charged amino acids. Polylysine could not inhibit the interaction of LP with HA. The results indicate that the sites for interaction with HA are in the tandemly repeated sequences of LP and that there are four potential sites available for that interaction.

Structural features of cartilage matrix protein deduced from cDNA

cDNAs encoding the Mr 54,000 chicken cartilage matrix protein (CMP) were selected from a cartilage cDNA expression library by immunological means. Antibodies elicited against insert-encoded protein purified from one of the clones reacted specifically with chicken CMP in immunoblots of total cartilage extract, providing positive identification of the cDNA clones isolated. The cDNAs detect a 3.4-kilobase transcript that was present in sternal cartilage and in cartilaginous but not in precartilaginous embryonic limb tissues. The cDNAs code for 416 amino acids of the chicken CMP, including its COOH terminus. There are two striking features in the deduced CMP amino acid sequence: first, it contains a region with significant homologies to repeat sequences in the precursor for epidermal growth factor; and second, it is made up of two large homologous repeat sequences. These results provide the first detailed structural information on the CMP and establish it as a developmentally regulated marker of cartilage differentiation.

Identification of link proteins in canine synovial cell cultures and canine articular cartilage

Link proteins are glycoproteins in cartilage that are involved in the stabilization of aggregates of proteoglycans and hyaluronic acid. We have identified link proteins in synovial cell cultures form normal canine synovium using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunofluorescence, and immunolocation with specific antibodies by electrophoretic transfer. We have also found evidence for the synthesis of link proteins in these cultures by fluorography of radiolabeled synovial cell extracts. We have identified a 70,000 mol-wt protein in canine synovial cell culture extracts that has antigenic cross-reactivity with the 48,000-mol-wt link protein. Three link proteins were identified in normal canine articular cartilage. These results indicate that link proteins are more widely distributed in connective tissues than previously recognized and may have biological functions other than aggregate stabilization.