We have generated RANK (receptor activator of NF-B) nullizygous mice to determine the molecular genetic interactions between osteoprotegerin, osteoprotegerin ligand, and RANK during bone resorption and remodeling processes. RANK ؊/؊ mice lack osteoclasts and have a profound defect in bone resorption and remodeling and in the development of the cartilaginous growth plates of endochondral bone. The osteopetrosis observed in these mice can be reversed by transplantation of bone marrow from rag1 ؊/؊ (recombinase activating gene 1) mice, indicating that RANK ؊/؊ mice have an intrinsic defect in osteoclast function. Calciotropic hormones and proresorptive cytokines that are known to induce bone resorption in mice and human were administered to RANK ؊/؊ mice without inducing hypercalcemia, although tumor necrosis factor ␣ treatment leads to the rare appearance of osteoclast-like cells near the site of injection. Osteoclastogenesis can be initiated in RANK ؊/؊ mice by transfer of the RANK cDNA back into hematopoietic precursors, suggesting a means to critically evaluate RANK structural features required for bone resorption. Together these data indicate that RANK is the intrinsic cell surface determinant that mediates osteoprotegerin ligand effects on bone resorption and remodeling as well as the physiological and pathological effects of calciotropic hormones and proresorptive cytokines.B one remodeling and homeostasis is an essential function that regulates skeletal integrity throughout adult life in higher vertebrates and mammals. The maintenance of skeletal mass is controlled by the activities of specialized cells within the bone that have seemingly antagonistic activities: bone synthesis and bone resorption. Osteoblastic cells of mesenchymal origin synthesize and deposit bone matrix and increase bone mass. Osteoclastic cells are large, multinucleated phagocytes of hematopoietic origin that resorb both mature and newly synthesized bone upon activation. Bone synthesis and resorption processes are highly coordinated and are regulated by osteotropic and calciotropic hormones during physiological and pathological conditions (1, 2). Increased bone resorption and turnover mediated by activated osteoclasts is known to occur in various crippling diseases, such as osteoporosis and arthritis, and can lead to pathological decreases in bone mass and skeletal integrity.Recently, two critical extracellular regulators of osteoclast differentiation and activation have been identified: osteoprotegerin (OPG) (3) and OPG ligand (OPGL) (4). OPGL is a tumor necrosis factor (TNF)-related cytokine that stimulates osteoclast differentiation from hematopoietic precursor cells and activation of mature osteoclasts in vitro and in vivo. Mice lacking OPGL also lack osteoclasts and have defects in bone remodeling processes that leads to severe osteopetrosis (5). OPG is a secreted TNF receptor (TNFR)-related protein that binds to and neutralizes OPGL bioactivity. Transgenic mice that overexpress OPG also have defects in osteoclastogenesis similar t...

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Osteoprotegerin ligand (OPGL) targets osteoclast precursors and osteoclasts to enhance differentiation and activation, however, little is known about OPGL effects on osteoclast survival. In vitro, the combination of OPGL ؉ colony-stimulating factor-1 (CSF-1) is required for optimal osteoclast survival. Ultrastructurally, apoptotic changes were observed in detached cells and culture lysates exhibited elevated caspase 3 activity, particularly in cultures lacking CSF-1. DEVD-FMK (caspase 3 inhibitor) partially protected cells when combined with OPGL, but not when used alone or in combination with CSF-1. CSF-1 maintained NF-B activation and increased the expression of bcl-2 and bcl-X L mRNA, but had no effect on JNK activation. In contrast, OPGL enhanced both NF-B and JNK kinase activation and increased the expression of c-src, but not bcl-2 and bcl-X L mRNA. These data suggest that aspects of both OPGL's and CSF-1's signaling/survival pathways are required for optimal osteoclast survival. In mice, a single dose of OPG, the OPGL decoy receptor, led to a >90% loss of osteoclasts because of apoptosis within 48 hours of exposure without impacting osteoclast precursor cells. Therefore, OPGL is essential, but not sufficient, for osteoclast survival and endogenous CSF-1 levels are insufficient to maintain osteoclast viability in the absence of OPGL. Osteoclasts mediate the resorption component of bone modeling and remodeling, which together are pivotal to the formation and maintenance of the mammalian skeleton. These specialized members of the monocyte-macrophage family arise from hematopoietic precursors with the location and magnitude of their activity guided by cells that surface the bone matrix. The bone lining cells, which include osteoblasts, endosteal, and periosteal lining cells, seem to mediate the local, systemic, physiological, and/or pathological stimuli impinging on them and consequently provide molecular signals that eventuate in osteoclast-mediated bone resorption.

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Oncostatin M (OSM) is a member of a family of cytokines that includes ciliary neurotrophic factor, interleukin-6, interleukin-11, cardiotrophin-1, and leukemia inhibitory factor (LIF). The receptors for these cytokines consist of a common signaling subunit, gp130, to which other subunits are added to modify ligand specificity. We report here the isolation and characterization of a cDNA encoding a subunit of the mouse OSM receptor. In NIH 3T3 cells (which endogenously express gp130, LIF receptor ␤ [LIFR␤], and the protein product, c12, of the cDNA described here), mouse LIF, human LIF, and human OSM signaled through receptors containing the LIFR␤ and gp130 but not through the mouse OSM receptor. Mouse OSM, however, signaled only through a c12-gp130 complex; it did not use the LIF receptor. Binding studies demonstrated that mouse OSM associated directly with either the c12 protein or gp130. These data highlight the species-specific differences in receptor utilization and signal transduction between mouse and human OSM. In mouse cells, only mouse OSM is capable of activating the mouse OSM receptor; human OSM instead activates the LIF receptor. Therefore, these data suggest that all previous studies with human OSM in mouse systems did not elucidate the biology of OSM but, rather, reflected the biological actions of LIF.