Excessive bone loss in arthritic diseases is mostly due to abnormal activation of the immune system leading to stimulation of osteoclasts. While phospholipase Cγ (PLCγ) isoforms are known modulators of T and B lymphocyte-mediated immune responses, we found that blockade of PLCγ enzymatic activity also blocks early osteoclast development and function. Importantly, targeted deletion of Plcg2 in mice led to an osteopetrotic phenotype. PLCγ2, independent of PLCγ1, was required for receptor activator of NF-κB ligand-induced (RANKL-induced) osteoclastogenesis by differentially regulating nuclear factor of activated T cells c1 (NFATc1), activator protein-1 (AP1), and NF-κB. Specifically, we show that NFATc1 upregulation is dependent on RANKL-mediated phosphorylation of PLCγ2 downstream of Dap12/Fc receptor γ (Dap12/FcRγ) receptors and is blocked by the PLCγ inhibitor U73122. In contrast, activation of JNK and NF-κB was not affected by U73122 or Dap12/FcRγ deletion. Interestingly, we found that in osteoclasts, PLCγ2 formed a complex with the regulatory adapter molecule GAB2, was required for GAB2 phosphorylation, and modulated GAB2 recruitment to RANK. Thus, PLCγ2 mediates RANKL-induced osteoclastogenesis and is a potential candidate for antiresorptive therapy.

The marrow stromal cell is the principal source of the key osteoclastogenic cytokine receptor activator of NF-κB (RANK) ligand (RANKL). To individualize the role of marrow stromal cells in varying states of TNF-α-driven osteoclast formation in vivo, we generated chimeric mice in which wild-type (WT) marrow, immunodepleted of T cells and stromal cells, is transplanted into lethally irradiated mice deleted of both the p55 and p75 TNFR. As control, similarly treated WT marrow was transplanted into WT mice. Each group was administered increasing doses of TNF-α. Exposure to high-dose cytokine ex vivo induces exuberant osteoclastogenesis irrespective of in vivo TNF-α treatment or whether the recipient animals possess TNF-α-responsive stromal cells. In contrast, the osteoclastogenic capacity of marrow treated with lower-dose TNF-α requires priming by TNFR-bearing stromal cells in vivo. Importantly, the osteoclastogenic contribution of cytokine responsive stromal cells in vivo diminishes as the dose of TNF-α increases. In keeping with this conclusion, mice with severe inflammatory arthritis develop profound osteoclastogenesis and bone erosion independent of stromal cell expression of TNFR. The direct induction of osteoclast recruitment by TNF-α is characterized by enhanced RANK expression and sensitization of precursor cells to RANKL. Thus, osteolysis attending relatively modest elevations in ambient TNF-α depends upon responsive stromal cells. Alternatively, in states of severe periarticular inflammation, TNF-α may fully exert its bone erosive effects by directly promoting the differentiation of osteoclast precursors independent of cytokine-responsive stromal cells and T lymphocytes.

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Stress fractures of varying severity were created using a rat model of skeletal fatigue loading. Periosteal woven bone formed in proportion to the level of bone damage, resulting in the rapid recovery of whole bone strength independent of stress fracture severity.Introduction: A hard periosteal callus is a hallmark of stress fracture healing. The factors that regulate the formation of this woven bone callus are poorly understood. Our objective was to produce stress fractures of varying severity and to assess the woven bone response and recovery of bone strength. Materials and Methods: We used the forelimb compression model to create stress fractures of varying severity in 192 adult rats. Forelimbs were loaded in fatigue until the displacement reached 30%, 45%, 65%, or 85% of fracture. The osteogenic responses of loaded and contralateral control ulnas were assessed 7 and 14 days after loading using pQCT, CT, mechanical testing, histomorphometry, and Raman spectroscopy. Results: Loading stimulated the formation of periosteal woven bone that was maximal near the ulnar midshaft and transitioned to lamellar bone away from the midshaft. Woven bone area increased in a dose-response manner with increasing fatigue displacement. Whole bone strength was partially recovered at 7 days and fully recovered at 14 days, regardless of initial stress fracture severity. The density of the woven bone increased by 80% from 7 to 14 days, caused in part by a 30% increase in the mineral:collagen ratio of the woven bone tissue. Conclusions: Functional healing of a stress fracture, as evidenced by recovery of whole bone strength, occurred within 2 wk, regardless of stress fracture severity. Partial recovery of strength in the first week was attributed to the rapid formation of a collar of woven bone that was localized to the site of bone damage and whose size depended on the level of initial damage. Complete recovery of strength in the second week was caused by woven bone densification. For the first time, we showed that woven bone formation occurs as a dose-dependent response after damaging mechanical loading of bone.

Stress fractures of varying severity were created using a rat model of skeletal fatigue loading. Periosteal woven bone formed in proportion to the level of bone damage, resulting in the rapid recovery of whole-bone strength independent of stress fracture severity.Introduction-A hard periosteal callus is a hallmark of stress fracture healing. The factors that regulate the formation of this woven bone callus are poorly understood. Our objective was to produce stress fractures of varying severity and to assess the woven bone response and recovery of bone strength.Materials and Methods-We used the forelimb compression model to create stress fractures of varying severity in 192 adult rats. Forelimbs were loaded in fatigue until the displacement reached 30, 45, 65 or 85% of fracture. The osteogenic responses of loaded and contralateral control ulnae were assessed 7 and 14 days after loading using pQCT, microCT, mechanical testing, histomorphometry, and Raman spectroscopy.Results-Loading stimulated the formation of periosteal woven bone that was maximal near the ulnar mid-shaft and transitioned to lamellar bone away from the mid-shaft. Woven bone area increased in a dose-response manner with increasing fatigue displacement. Whole-bone strength was partially recovered at 7 days and fully recovered at 14 days, regardless of initial stress fracture severity. The density of the woven bone increased by 80% from 7 to 14 days, due in part to a 30% increase in the mineral:collagen ratio of the woven bone tissue.Conclusion-Functional healing of a stress fracture, as evidenced by recovery of whole-bone strength, occurred within 2 weeks, regardless of stress fracture severity. Partial recovery of strength in the first week was attributed to the rapid formation of a collar of woven bone that was localized to the site of bone damage and whose size depended on the level of initial damage. Complete recovery of strength in the second week was due to woven bone densification. For the first time we have shown that woven bone formation occurs as a dose-dependent response following damaging mechanical loading of bone.

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FHL2, a molecule that interacts with many integrins and transcription factors, was found to play an important role in osteoblast differentiation. Overexpression of FHL2 increases the accumulation of osteoblast differentiation markers and matrix mineralization, whereas FHL2 deficiency results in inhibition of osteoblast differentiation and decreased bone formation.Introduction: Integrin-matrix interaction plays a critical role in osteoblast function. It has been shown that the cytoplasmic domains of integrin ␤ subunits mediate signal transduction induced by integrin-matrix interaction. We reasoned that the identification of proteins interacting with ␤-cytoplasmic tails followed by analysis of the function of these proteins would enhance our understanding on integrin signaling and the roles of these proteins in osteoblast activities. Materials and Methods: Yeast two hybrid assay was used to identify proteins interacting with the cytoplasmic domain of integrin ␤5 subunit. The association of these proteins with integrin ␣v␤5 was confirmed by confocal analysis and co-immunoprecipitation. A stable MC3T3-E1 cells line overexpressing Four and Half Lim Protein 2 (FHL2) and mouse osteoblasts deficient in FHL2 were used to study the roles of FHL2 in osteoblast differentiation and bone formation. Matrix protein expression was determined by mRNA analysis and Western blotting. Matrix mineralization was detected by Alizarin red staining. Alkaline phosphatase activity was also measured. CT was used to determine bone histomorphometry. Results and Conclusions: FHL2 and actin-binding proteins, palladin and filamin A, were identified as proteins interacting with ␤5 cytoplasmic domain. FHL2 co-localized with ␣v␤5 at the focal adhesion sites in association with palladin and filamin A. FHL2 was also present in nuclei. Osteoblasts overexpressing FHL2 exhibited increased adhesion to and migration on matrix proteins. Conversely, FHL2 stimulation of CREB activity was dependent on integrin function because it was inhibited by Gly-Arg-Gly-Asp-Ser (GRGDS) peptide. The expression of osteoblast differentiation markers and Msx2 was upregulated, and bone matrix mineralization was increased in FHL2 overexpressing cells. In contrast, FHL2-deficient bone marrow cells and osteoblasts displayed decreased osteoblast colony formation and differentiation, respectively, compared with wildtype cells. Moreover, FHL2-deficient female mice exhibited greater bone loss than the wildtype littermates after ovariectomy. Thus, FHL2 plays an important role in osteoblast differentiation and bone formation.

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