Extracellular Microfibrils Control Osteoblast-supported Osteoclastogenesis by Restricting TGFβ Stimulation of RANKL Production*

Mutations in fibrillin-1 or fibrillin-2, the major structural components of extracellular microfibrils, cause pleiotropic manifestations in Marfan syndrome and congenital contractural arachnodactyly, respectively. We recently found that fibrillin-1 and fibrillin-2 control bone formation by regulating osteoblast differentiation through the differential modulation of endogenous TGFβ and bone morphogenetic protein signals. Here, we describe in vivo and ex vivo experiments that implicate the fibrillins as negative regulators of bone resorption. Adult Fbn2−/− mice display a greater than normal osteolytic response to locally implanted lipopolysaccharide-coated titanium particles. Although isolated cultures of Fbn2−/− preosteoclasts exhibited normal differentiation and activity, these features were substantially augmented when mutant or wild-type preosteoclasts were co-cultured with Fbn2−/− but not wild-type osteoblasts. Greater osteoclastogenic potential of Fbn2−/− osteoblasts was largely accounted for by up-regulation of the Rankl gene secondary to heightened TGFβ activity. This conclusion was based on the findings that blockade of TGFβ signaling blunts Rankl up-regulation in Fbn2−/− osteoblasts and bones and that systemic TGFβ antagonism improves locally induced osteolysis in Fbn2−/− mice. Abnormally high Rankl expression secondary to elevated TGFβ activity was also noted in cultured osteoblasts from Fbn1−/− mice. Collectively our data demonstrated that extracellular microfibrils balance local catabolic and anabolic signals during bone remodeling in addition to implying distinct mechanisms of bone loss in Marfan syndrome and congenital contractural arachnodactyly.