Costimulatory signals are required for activation of immune cells, but it is not known whether they contribute to other biological systems. The development and homeostasis of the skeletal system depend on the balance between bone formation and resorption. Receptor activator of NF-kappaB ligand (RANKL) regulates the differentiation of bone-resorbing cells, osteoclasts, in the presence of macrophage-colony stimulating factor (M-CSF). But it remains unclear how RANKL activates the calcium signals that lead to induction of nuclear factor of activated T cells c1, a key transcription factor for osteoclastogenesis. Here we show that mice lacking immunoreceptor tyrosine-based activation motif (ITAM)-harbouring adaptors, Fc receptor common gamma subunit (FcRgamma) and DNAX-activating protein (DAP)12, exhibit severe osteopetrosis owing to impaired osteoclast differentiation. In osteoclast precursor cells, FcRgamma and DAP12 associate with multiple immunoreceptors and activate calcium signalling through phospholipase Cgamma. Thus, ITAM-dependent costimulatory signals activated by multiple immunoreceptors are essential for the maintenance of bone homeostasis. These results reveal that RANKL and M-CSF are not sufficient to activate the signals required for osteoclastogenesis.

No abstract

Cloning and sequence analysis of cDNA for the Electrophorus electricus electroplax sodium channel indicate that this protein, consisting of 1,820 amino acid residues, exhibits four repeated homology units, which are presumably oriented in a pseudosymmetric fashion across the membrane. Each homology unit contains a unique segment with clustered positively charged residues, which may be involved in the gating structure, possibly in conjunction with negatively charged residues clustered elsewhere.

Despite its widespread distribution on both lymphoid and myeloid cells, the biological role of the low-affinity immunoglobulin-G receptor, Fc gamma RII, is not fully understood. Defects in this receptor or its signalling pathway in B cells result in perturbations in immune-complex-mediated feedback inhibition of antibody production. We now report that Fc gamma RII-deficient animals display elevated immunoglobulin levels in response to both thymus-dependent and thymus-independent antigens. Additionally, the effector arm of the allergic response is perturbed in these mice. Mast cells from Fc gamma RII-/- are highly sensitive to IgG-triggered degranulation, in contrast to their wild-type counterparts. Fc gamma RII-deficient mice demonstrate an enhanced passive cutaneous analphylaxis reaction, the result of a decreased threshold for mast-cell activation by Fc gamma RIII cross-linking. These results demonstrate that Fc gamma RII acts as a general negative regulator of immune-complex-triggered activation in vivo for both the afferent and efferent limbs of the immune response. Exploiting this property offers new therapeutic opportunities for the treatment of allergic and autoimmune disorders.

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Distinct classes of acetylcholine receptor channels are formed when Xenopus oocytes are injected with combinations of the bovine alpha-, beta-, gamma- and delta- or the alpha-, beta-, gamma- and epsilon-subunit-specific messenger RNAs. The conductance and gating properties of the two classes of channels, in conjunction with the developmental changes in the muscular contents of the mRNAs, suggest that replacement of the gamma-subunit by the epsilon-subunit is responsible for the functional alteration of the receptor during muscle development.

Immunoreceptor tyrosine-based activation motifs (ITAMs) are crucial in antigen receptor signaling in acquired immunity. Although receptors associated with the ITAM-bearing adaptors FcRgamma and DAP12 on myeloid cells have been suggested to activate innate immune responses, the mechanism coupling those receptors to 'downstream' signaling events is unclear. The CARMA1-Bcl-10-MALT1 complex is critical for the activation of transcription factor NF-kappaB in lymphocytes but has an unclear function in myeloid cells. Here we report that deletion of the gene encoding the Bcl-10 adaptor-binding partner CARD9 resulted in impaired myeloid cell activation of NF-kappaB signaling by several ITAM-associated receptors. Moreover, CARD9 was required for Toll-like receptor-induced activation of dendritic cells through the activation of mitogen-activated protein kinases. Although Bcl10-/- and Card9-/- mice had similar signaling impairment in myeloid cells, Card11-/- (CARMA1-deficient) myeloid cell responses were normal, and although Card11-/- lymphocytes were defective in antigen receptor-mediated activation, Card9-/- lymphocytes were not. Thus, the activation of lymphoid and myeloid cells through ITAM-associated receptors or Toll-like receptors is regulated by CARMA1-Bcl-10 and CARD9-Bcl-10, respectively.

Certain autoimmune diseases result in abnormal bone homeostasis, but association of immunodeficiency with bone is poorly understood. Osteoclasts, which derive from bone marrow cells, are under the control of the immune system. Differentiation of osteoclasts is mainly regulated by signaling pathways activated by RANK and immune receptors linked to ITAM-harboring adaptors. However, it is unclear how the two signals merge to cooperate in osteoclast differentiation. Here we report that mice lacking the tyrosine kinases Btk and Tec show severe osteopetrosis caused by a defect in bone resorption. RANK and ITAM signaling results in formation of a Btk(Tec)/BLNK(SLP-76)-containing complex and PLCgamma-mediated activation of an essential calcium signal. Furthermore, Tec kinase inhibition reduces osteoclastic bone resorption in models of osteoporosis and inflammation-induced bone destruction. Thus, this study reveals the importance of the osteoclastogenic signaling complex composed of tyrosine kinases, which may provide the molecular basis for a new therapeutic strategy.