Transient anabolic effects accompany epidermal growth factor receptor signal activation in articular cartilage in vivo

Abstract

IntroductionSignals from the epidermal growth factor receptor (EGFR) have typically been considered to provide catabolic activities in articular cartilage, and accordingly have been suggested to have a causal role in osteoarthritis progression. The aim of this study was to determine in vivo roles for endogenous EGFR signal activation in articular cartilage.MethodsTransgenic mice with conditional, limb-targeted deletion of the endogenous intracellular EGFR inhibitor Mig-6 were generated using CreLoxP (Mig-6-flox; Prx1Cre) recombination. Histology, histochemical staining and immunohistochemistry were used to confirm activation of EGFR signaling in the articular cartilage and joints, and to analyze phenotypic consequences of Mig-6 loss on articular cartilage morphology, proliferation, expression of progenitor cell markers, presence of chondrocyte hypertrophy and degradation of articular cartilage matrix.ResultsThe articular cartilage of Mig-6-conditional knockout (Mig-6-cko) mice was dramatically and significantly thicker than normal articular cartilage at 6 and 12 weeks of age. Mig-6-cko articular cartilage contained a population of chondrocytes in which EGFR signaling was activated, and which were three to four times more proliferative than normal Mig-6-flox articular chondrocytes. These cells expressed high levels of the master chondrogenic regulatory factor Sox9, as well as high levels of putative progenitor cell markers including superficial zone protein (SZP), growth and differentiation factor-5 (GDF-5) and Notch1. Expression levels were also high for activated β-catenin and the transforming growth factor beta (TGF-β) mediators phospho-Smad2/3 (pSmad2/3). Anabolic effects of EGFR activation in articular cartilage were followed by catabolic events, including matrix degradation, as determined by accumulation of aggrecan cleavage fragments, and onset of hypertrophy as determined by type × collagen expression. By 16 weeks of age, the articular cartilage of Mig-6-cko knees was no longer thickened and was degenerating.ConclusionsThese results demonstrate unexpected anabolic effects of EGFR signal activation in articular cartilage, and suggest the hypothesis that these effects may promote the expansion and/or activity of an endogenous EGFR-responsive cell population within the articular cartilage.

Reduction in Disease Progression by Inhibition of Transforming Growth Factor α–CCL2 Signaling in Experimental Posttraumatic Osteoarthritis

“…In one context, highly increased EGFR activity leads to increased OA severity (e.g., in the Mig-6 knockout model [18]), in agreement with the deleterious effects in cartilage demonstrated in the present study after exogenous stimulation by TGFa. In the developmental context, EGFR activity stimulates increased cartilage thickness and chondrocyte proliferation (38). Indeed, we have reported increased chondrocyte proliferation in articular cartilage treated with exogenous TGFa (9), and cartilage-specific knockout of Mig-6 also leads to excessive articular chondrocyte proliferation (39,40).…”

EGFR signaling is critical for maintaining the superficial layer of articular cartilage and preventing osteoarthritis initiation

“…Previous studies of EGFR signaling in articular cartilage development and degeneration focused on an indirect approach analyzing mice lacking Mig-6, an EGFR inhibitor (9)(10)(11)(12). Because Mig-6 is not specific to EGFR, those results are too complicated to clearly interpret EGFR function.…”

“…Mig-6 null mice develop a severe OA-like joint degeneration phenotype at an early adult stage (9). However, mice with limb-(Prx1-Cre) or cartilage-(Col2-Cre) specific deletion of Mig-6 showed a much milder OA phenotype (10)(11)(12). Indeed, a…”

Cartilage-specific deletion of Mig-6 results in osteoarthritis-like disorder with excessive articular chondrocyte proliferation

“…The formation of osteophytes is likely the result of inappropriate cell proliferation in the absence of Mig-6; such proliferative activity was also seen around the articular cartilage. Recently, it was reported that specific deletion of Mig-6 in the limbs (Mig-6-flox;Prx1 Cre mice) results in a similar articular cartilage thickening and increased proliferation of chondrocytes (27). Mig-6 deficiency may also lead to defects in ECM remodeling by affecting the production of anabolic and catabolic factors as well as proteases from the chondrocytes, thereby affecting the health of the articular cartilage.…”