The Role And Underlying Mechanisms Of AMPK And Sclerostin In Osteoarthritis Development

Osteoarthritis(OA)is a progressive degenerative disease of the joints that is associated with ageing.There are still no effective disease-modifying OA therapies to date,so a better understanding of the mechanism of OA pathogenesis may allow for the development of new therapies.Part 1 AMPK deficiency in chondrocytes accelerated the progression of instability-induced and ageing-associated osteoarthritis in adult miceAMP-activated protein kinase(AMPK)is an evolutionarily conserved serine/threonine kinase and exists as heterotrimeric complexes containing one catalytic subunit(encoded by ?1 or ?2)and two regulatory subunits.AMPK activity is constitutively present in normal articular chondrocytes.There is evidence that reduced AMPK activity in chondrocytes is associated with OA and senescence.Reduced AMPK? phosphorylation was noted in mouse surgical instability-induced and human OA knee cartilage.AMPK? phosphorylation in aged mouse knee cartilage was also reduced.Interleukin-1?(IL-1?)or tumor necrosis factor ?(TNF?)downregulated the activity of AMPK in chondrocytes,and upregulating AMPK activity attenuated IL-1?or TNF?-induced catabolic gene expression in chondrocytes in vitro.Consistently,AMPK pharmacological activators exerted a chondroprotective effect in vivo.However,a recent study reported that chondrocyte-specific ablation of AMPKal failed to affect OA pathogenesis in a surgically induced OA mouse model.In the present study,adult cartilage-specific AMPK?1 cKO,AMPK?2 cKO and AMPKa cDKO mice were generated by Cre-Loxp technology.We investigated the role of AMPK in maintaining a healthy state of articular cartilage and in OA development.Compared with WT littermates,mutant mice displayed accelerated severity of surgically induced OA,especially AMPKa cDKO mice.Furthermore,male but not female AMPKa cDKO mice exhibited severely spontaneous ageing-associated OA lesions at 12 months of age.The chondrocytes isolated from AMPKa cDKO mice resulted in an enhanced interleukin-1?(IL-1?)-stimulated catabolic response.In addition,upregulated expression of matrix metalloproteinase-3(MMP-3),MMP-13 and phospho-nuclear factor-?B(phospho-NF-?B)p65 and increased levels of apoptotic markers were detected in the cartilage of AMPKa cDKO mice compared with their WT littermates in vivo.Taken together,our findings suggest that AMPK activity in chondrocytes is important in maintaining joint homeostasis and OA development.Part 2 Accelerated development of instability-induced osteoarthritis in transgenic mice overexpressing SOSTThe Wnt/?-catenin signaling pathway is a key regulator in bone and cartilage homeostasis and has been linked to the cartilage degradation in OA.Sclerostin(SOST)is a potent soluble antagonist of Wnt signaling and a regulator of bone homeostasis.While SOST has been widely viewed as a secreted peptide produced by osteocytes,recent studies have shown that hypertrophic chondrocytes are capable of producing SOST protein.The number of SOST-positive chondrocytes increases in surgery-induced OA in mice and sheep,as well as late stage human OA.What is more,interleukin-1?(IL-1?)upregulates the mRNA level of SOST.Recently,studies has documented that the increased SOST could protect against cartilage damage.SOST dose-dependently inhibited IL-la-stimulated aggrecanolysis in vitro,while mice with deficiency of Sost have accelerated surgery-induced cartilage damage.This study was performed to assess whether overexpressing SOST could decelerate the progression of OA using SOST Tg mice.Compared with WT littermates,SOST Tg mice display accelerated severity of surgically induced OA.In addition,SOST Tg mice had reduced subchondral bone plate thickness relative to littermates in the sham-operated knees,while SOST Tg mice resulted in significantly greater subchondral bone thickness in the DMM-operated groups compared with sham-operated groups at 8 weeks post-surgery.The accelerated OA in SOST Tg mice may be associated with reduced ?-catenin signaling and increased chondrocyte apoptosis.Overexpressing SOST led to accelerated development of instability-induced OA and our data further highlight that cartilage homeostasis requires finely tuned Wnt signaling.