The secreted glycoprotein lubricin protects cartilage surfaces and inhibits synovial cell overgrowth

Expression and mapping of lubricin in canine flexor tendon

Matrix composition and the biomechanical environment are intimately interdependent in most connective tissues. Lubricin has many distinct biological functions, including lubrication, antiadhesion, and cytoprotection in cartilage, tendons, and other tissues. This study investigated the distribution of lubricin in the canine flexor digitorum profundus (FDP) tendon by immunohistochemistry. Lubricin was found both on the tendon surface and at the interface of collagen fiber bundles within the tendon, where the cells are subjected to shear force in addition to tension and compression. The expression of lubricin in regions of the canine flexor tendon that differ in mechanical or nutritional environment was also investigated using RT-PCR. Six N-terminal splicing variants were identified from six distinct anatomical regions of flexor tendon. The variants with larger sizes were noted in regions subjected to significant shear and compressive forces. Lubricin is ubiquitous in the FDP tendon, with variations in distribution and splicing that appear to correspond to discrete anatomic locations that differ by mechanical or nutritional environment.

Expression of superficial zone protein in mandibular condyle cartilage

OBJECTIVE

Superficial zone protein (SZP) has been shown to function in the boundary lubrication of articular cartilages of the extremities. However, the expression of SZP has not been clarified in mandibular cartilage which is a tissue that includes a thick fibrous layer on the surface. This study was conducted to clarify the distribution of SZP on the mandibular condyle and the regulatory effects of humoral factors on the expression in both explants and fibroblasts derived from mandibular condyle.

METHODS

The distribution of SZP was determined in bovine mandibular condyle cartilage, and the effects of interleukin-1beta (IL-1beta) and transforming growth factor-beta (TGF-beta) on SZP expression were examined in condyle explants and fibroblasts derived from the fibrous zone of condyle cartilage.

RESULTS

SZP was highly distributed in the superficial zone of intact condyle cartilage. The SZP expression was up-regulated by TGF-beta in both explants and cultured fibroblasts, whereas the expression was slightly down-regulated by IL-1beta. A significant increase in accumulation of SZP protein was also observed in the culture medium of the fibroblasts treated with TGF-beta.

CONCLUSIONS

These results suggest that SZP plays an important role in boundary lubrication of mandible condylar cartilage, is synthesized locally within the condyle itself, and exhibits differential regulation by cell mediators relevant to mandibular condyle repairing and pathologies.

Mapping lubricin in canine musculoskeletal tissues

Lubricin, also known as superficial zone protein or PRG4, has many distinct biological functions, including lubrication, antiadhesion, and as a regulator of cell growth. This study investigated lubricin in canine musculoskeletal tissues using RT-PCR and immunohistochemistry. One or more variants were noted in canine flexor digitorum profundus (FDP) tendon, Achilles tendon, patellar tendon, A2 pulley, anterior cruciate ligament (ACL), knee lateral collateral ligament (LCL), articular cartilage, meniscus, muscle, and skin. We found 6 N-terminal lubricin splicing variants. The variants with larger sizes were identified in FDP tendon, ACL, LCL, A2 pulley, and cartilage. Lubricin was distributed both on the tissue surfaces and at the interface of fiber bundles within tissues, but this distribution varied by tissue type. We conclude that lubricin is present in many tissues; variations in splicing and physical distribution suggest that the variants of lubricin may play different roles in different locations.

Dynamic shear stimulation of bovine cartilage biosynthesis of proteoglycan 4

OBJECTIVE

The boundary lubrication function of articular cartilage is mediated in part by proteoglycan 4 (PRG4) molecules at the articular surface and in synovial fluid. The objective of this study was to determine the effects of dynamic shear stimulation on PRG4 biosynthesis by bovine cartilage explants.

METHODS

Cartilage disks with intact articular surfaces were harvested from immature bovines. Some disks were subjected to 24 hours (day 1) of loading, consisting of a step load of 20% static compression either alone or with superimposed dynamic shear (3% amplitude and 0.1 Hz), while other disks were cultured free-swelling as controls. After the 24-hour loading period, disks were terminated or were further incubated for up to 72 hours (days 2-4) in free-swelling culture to assess chondrocyte responses to, and following, unloading. PRG4 products secreted into culture medium were quantified by enzyme-linked immunosorbent assay and characterized by Western blotting. Chondrocytes expressing PRG4 were localized by immunohistochemistry, and depth-associated variations in chondrocyte PRG4 expression were quantified by image analysis.

RESULTS

Dynamic shear stimulation increased PRG4 secretion to 3-4 times that of unloaded controls and statically compressed samples. Sheared cartilage secreted more PRG4 of 345 kd relative to smaller molecular weight species, as compared with unloaded controls. Immunohistochemistry revealed that shear stimulation also increased the total number of cells expressing PRG4 by inducing expression by cells at a depth of 200-400 microm.

CONCLUSION

The paradigm that certain mechanical stimuli up-regulate biosynthesis in cartilage appears operative not only for load-bearing matrix constituents, but also for PRG4 molecules that mediate lubrication.

Consequences of Disease-causing Mutations on Lubricin Protein Synthesis, Secretion, and Post-translational Processing

Lubricin, a protein product of the gene PRG4, is a secreted mucin-like proteoglycan that is a major lubricant in articulating joints. Mutations in PRG4 cause the autosomal recessive, human disorder camptodactyly-arthropathy-coxa vara-pericarditis syndrome. We developed rabbit polyclonal antibodies against human lubricin to determine the consequence of disease-causing mutations at the protein level and to study the protein's normal post-translational processing. Antiserum generated against an epitope in the amino-terminal portion of lubricin detected protein in wild-type synovial fluid and in conditioned media from wild-type cultured synoviocytes. However, the antiserum did not detect lubricin in synovial fluid or cultured synoviocytes from several patients with frameshift or nonsense mutations in PRG4. Antiserum generated against an epitope in the protein's carboxyl-terminal, hemopexin-like domain identified a post-translational cleavage event in wild-type lubricin, mediated by a subtilisin-like proprotein convertase (SPC). Interestingly, in contrast to wild-type lubricin, one disease-causing mutation that removes the last 8 amino acids of the protein, including a conserved cysteine residue, was not cleaved within the hemopexin-like domain when expressed in COS-7 cells. This suggests that formation of an intrachain disulfide bond is required for SPC-mediated cleavage and that SPC-mediated cleavage is essential to protein function.

Transcriptional control of chondrocyte fate and differentiation

Chondrogenesis is an essential process in vertebrates. It leads to the formation of cartilage growth plates, which drive body growth and have primary roles in endochondral ossification. It also leads to the formation of permanent cartilaginous tissues that provide major structural support in the articular joints and respiratory and auditory tracts throughout life. Defects in chondrogenesis cause chondrodysostoses and chondrodysplasias. These skeletal malformation diseases account for a significant proportion of birth defects in humans and can dramatically affect a person's expectancy and quality of life. Chondrogenesis occurs when pluripotent mesenchymal cells commit to the chondrocyte lineage, and through a series of differentiation steps build and eventually remodel cartilage. This review summarizes and discusses our current knowledge and lack of knowledge about the chondrocyte differentiation pathway, from mesenchymal cells to growth plate and articular chondrocytes, with a main focus on how it is controlled by tissue patterning and cell differentiation transcription factors, such as, but not limited to, Pax1 and Pax9, Nkx3.1 and Nkx3.2, Sox9, Sox5 and Sox6, Runx2 and Runx3, and c-Maf.