Objective Congenital deficiency of the principal boundary lubricant in cartilage (lubricin, encoded by the gene PRG4) increases joint friction and causes progressive joint failure. This study was undertaken to determine whether restoring lubricin expression would prevent, delay, or reverse the disease process caused by congenital deficiency. Methods Using genetically engineered lubricin deficient mice, we restored gene function before conception or at 3 weeks, 2 months, or 6 months after birth. We evaluated the effect of restoring gene function (i.e., expressing lubricin) on the tibial-femoral-patellar joints of mice, histologically and by ex vivo biomechanical testing. Results Restoring gene function prior to conception prevented disease. Restoring gene function at 3 weeks of age improved, but did not normalize, joint histology and whole joint friction; cyclic loading produced fewer activated caspase-3 containing chondrocytes when lubricin expression was restored at three weeks of age compared to non-restored littermates. Restoring lubricin expression in 2-month-old or 6-month-old mice had no beneficial effect on histology, whole joint friction, or activation of caspase-3 compared to non-restored littermates. Conclusions When boundary lubrication is congenitally deficient and cartilage becomes damaged, the window of opportunity for restoring lubrication and slowing disease progression is limited.

Fibrosis in the synovium and infrapatellar fat pad (IFP) is frequently observed in knee osteoarthritis (OA) and is often correlated with joint pain and stiffness.However, the mechanisms underpinning the development of knee fibrosis in OA are relatively poorly understood. In this study, we used a combination of histological, immunohistochemical, and multiplex gene expression analyses to characterize the fibrosis that develops in a mouse model of load-induced OA. Histological evaluation showed the time-dependent development of fibrosis in the synovium, capsule, and IFP of loaded limbs of male 11-week-old mice. The development of load-induced fibrosis was accompanied primarily by proliferation, expansion, and activation of the stromal compartment, and by increased macrophage presence evidenced by increased F4/80 and MAC2 positive immunostaining. The presence of B and T-cells was minimal in both control and loaded limbs, but CD3-positive immunostaining was significantly higher in C57BL/6J at 2 weeks after loading, indicating an increased presence of T-cells. Using NanoString gene expression analyses of human and mouse tissues, we found that mice subjected to cyclic loading recapitulated the gene expression profile observed in human fibrotic tissues, including increased expression of collagen genes. Together, our results indicate that this well-controlled nonsurgical mouse model can be used to study the mechanisms underpinning the development of knee fibrosis, and potentially to test targeted strategies to prevent the development of fibrosis and stiffness of the knee.

Growth of the axial and appendicular skeleton depends on endochondral ossification, which is controlled by tightly regulated cell–cell interactions in the developing growth plates. Previous studies have uncovered an important role of a disintegrin and metalloprotease 17 (ADAM17) in the normal development of the mineralized zone of hypertrophic chondrocytes during endochondral ossification. ADAM17 regulates EGF-receptor signaling by cleaving EGFR-ligands such as TGFα from their membrane-anchored precursor. The activity of ADAM17 is controlled by two regulatory binding partners, the inactive Rhomboids 1 and 2 (iRhom1, 2), raising questions about their role in endochondral ossification. To address this question, we generated mice lacking iRhom2 (iR2−/−) with floxed alleles of iRhom1 that were specifically deleted in chondrocytes by Col2a1-Cre (iR1∆Ch). The resulting iR2−/−iR1∆Ch mice had retarded bone growth compared to iR2−/− mice, caused by a significantly expanded zone of hypertrophic mineralizing chondrocytes in the growth plate. Primary iR2−/−iR1∆Ch chondrocytes had strongly reduced shedding of TGFα and other ADAM17-dependent EGFR-ligands. The enlarged zone of mineralized hypertrophic chondrocytes in iR2−/−iR1∆Ch mice closely resembled the abnormal growth plate in A17∆Ch mice and was similar to growth plates in Tgfα−/− mice or mice with EGFR mutations. These data support a model in which iRhom1 and 2 regulate bone growth by controlling the ADAM17/TGFα/EGFR signaling axis during endochondral ossification.