Aging impairs the osteocytic regulation of collagen integrity and bone quality

Poor bone quality is a major factor in skeletal fragility in elderly individuals.The molecular mechanisms that establish and maintain bone quality,independent of bone mass,are unknown but are thought to be primarily determined by osteocytes.We hypothesize that the age-related decline in bone quality results from the suppression of osteocyte perilacunar/canalicular remodeling(PLR),which maintains bone material properties.We examined bones from young and aged mice with osteocyte-intrinsic repression of TGFβsignaling(TβRII^(ocy−/−))that suppresses PLR.The control aged bone displayed decreased TGFβsignaling and PLR,but aging did not worsen the existing PLR suppression in male TβRII^(ocy−/−)bone.This relationship impacted the behavior of collagen material at the nanoscale and tissue scale in macromechanical tests.The effects of age on bone mass,density,and mineral material behavior were independent of osteocytic TGFβ.We determined that the decline in bone quality with age arises from the loss of osteocyte function and the loss of TGFβ-dependent maintenance of collagen integrity.

Osteocyte dysfunction promotes osteoarthritis through MMP13-dependent suppression of subchondral bone homeostasis

Osteoarthritis(OA),long considered a primary disorder of articular cartilage,is commonly associated with subchondral bone sclerosis.However,the cellular mechanisms responsible for changes to subchondral bone in OA,and the extent to which these changes are drivers of or a secondary reaction to cartilage degeneration,remain unclear.In knee joints from human patients with end-stage OA,we found evidence of profound defects in osteocyte function.Suppression of osteocyte perilacunar/canalicular remodeling(PLR)was most severe in the medial compartment of OA subchondral bone,with lower protease expression,diminished canalicular networks,and disorganized and hypermineralized extracellular matrix.As a step toward evaluating the causality of PLR suppression in OA,we ablated the PLR enzyme MMP13 in osteocytes while leaving chondrocytic MMP13 intact,using Cre recombinase driven by the 9.6-kb DMP1 promoter.Not only did osteocytic MMP13 deficiency suppress PLR in cortical and subchondral bone,but it also compromised cartilage.Even in the absence of injury,osteocytic MMP13 deficiency was sufficient to reduce cartilage proteoglycan content,change chondrocyte production of collagen II,aggrecan,and MMP13,and increase the incidence of cartilage lesions,consistent with early OA.Thus,in humans and mice,defects in PLR coincide with cartilage defects.Osteocyte-derived MMP13 emerges as a critical regulator of cartilage homeostasis,likely via its effects on PLR.Together,these findings implicate osteocytes in bone-cartilage crosstalk in the joint and suggest a causal role for suppressed perilacunar/canalicular remodeling in osteoarthritis.