Oophorectomy‐induced bone loss is attenuated in MAGP1‐deficient mice

Microfibrils are extracellular matrix (ECM) structures that impart strength to tissue, and regulate growth factor signaling by sequestering ligands within the matrix. The constitutive components of these fibers in vertebrates are the fibrillins (FBN-1,-2,-3) and MAGPs (MAGP-1,-2), [Sakai et al., 1986, Cleary and Gibson, 1983]. Importantly, microfibrils are known to interact with numerous other proteins, including ligands of the TGFβ superfamily [Dallas et al., 2000, Gregory et al., 2005]. Microfibrils are abundantly expressed in bone, and can be found in the periosteal matrix, surrounding osteocytes, chondrocytes and osteons, on the endochondral surface, and within the trabecular matrix [Arteage-Solis and Ramirez, 2008]. Disruption of the fibrillins and MAGP1 induces an array of phenotypes that can affect both the appendicular and axial skeleton [Craft et al., 2010, Ramirez et al., 2008, Weinbaum et al., 2008]. A phenotype common to fibrillin and MAGP1 mutation/deletions is diminished bone mass. However, the mechanisms whereby microfibrils regulate bone remodeling have been largely unappreciated. Bone remodeling, the balance of bone resorption and new bone formation, is necessary for maintaining bone quality. This process is coordinated by the coupling of osteoclasts (resorption) and osteoblasts (formation). Bone loss associated with postmenopausal osteoporosis, rheumatoid arthritis and lytic bone metastases are typically the consequence of excess osteoclast activation, and thus increased bone resorption [Katagiri and Takahashi, 2002, Takayanagi et al., 2000, Rodan and Martin, 2000]. The receptor activator of NF-κB (RANK) signaling pathway is necessary for osteoclastogenesis [Boyce and Xing, 2007]. Osteoblasts stimulate osteoclastogenesis through RANK ligand (RANKL) expression, and block it by osteoprotegerin production (OPG, the decoy RANKL receptor). Because of their regulatory roles, the RANKL:OPG ratio is considered an important predictor of skeletal dynamics [Hofbauer and Schoppet, 2004]. We recently reported that MAGP1 deficient (MAGP1Δ) mice develop progressive bone loss and have reduced whole bone strength [Craft et al., 2010]. Utilizing in vitro and in vivo assays, we found no significant difference in MAGP1Δ osteoblast differentiation or function (mineralization capacity). However, significantly more osteoclasts are found in MAGP1Δ bone. This difference is due to MAGP1Δ bone marrow macrophages (BMM) being sensitized to RANKL as demonstrated by more MAGP1Δ BMMs differentiating to osteoclasts than do WT cells when exposed to a similar amount of RANKL. Nistala et al. [Nistala et al., 2010, Nistala et al., 2010] subsequently demonstrated enhanced osteoclastogenesis in fibrillin-deficient mice. Together, these studies show that osteoblasts derived from mice deficient in MAGP1, FBN1 or FBN2 all express higher levels of RANKL [Craft et al., 2010, Nistala et al., 2010, Nistala et al., 2010] and establish that microfibrils are important regulators of RANKL-RANK signaling and thus bone homeostasis. RANKL expression is elevated in women with postmenopausal osteoporosis, as well as, in oophorectomized rodents [Eghbali-Fatourechi et al., 2003, Ominsky et al., 2008]. Further, RANKL inhibition blunts oophorectomy (OVX)-induced bone loss [Ominsky et al., 2008, Samadfam et al., 2007]. If increased expression or sensitivity to RANKL is a cause for the low bone mass of MAGP1Δ mice [Craft et al., 2010], then it is possible that MAGP1Δ mice will have an attenuated response to oophorectomy because of an inability to raise RANKL expression above their already high levels. Here we show that the increase in RANKL expression in MAGP1Δ mice following oophorectomy is less than that seen in oophorectomized WT mice and that the mutant mice lose less bone following oophorectomy, relative to WT mice. We also establish a link between elevated RANKL expression and TGF-β signaling in MAGP1 deficiency.