Autoantibody production and immune complex (IC) formation are frequently observed in autoimmune diseases associated with bone loss. However, it has been poorly understood whether ICs regulate bone metabolism directly. Here we show that the level of osteoclastogenesis is determined by the strength of FcRg signalling, which is dependent on the relative expression of positive and negative FcgRs (FcgRI/III/IV and IIB, respectively) as well as the availability of their ligands, ICs. Under physiological conditions, unexpectedly, FcgRIII inhibits osteoclastogenesis by depriving other osteoclastogenic Ig-like receptors of FcRg. Fcgr2b À / À mice lose bone upon the onset of a hypergammaglobulinemia or the administration of IgG1 ICs, which act mainly through FcgRIII. The IgG2 IC activates osteoclastogenesis by binding to FcgRI and FcgRIV, which is induced under inflammatory conditions. These results demonstrate a link between the adaptive immunity and bone, suggesting a regulatory role for ICs in bone resorption in general, and not only in inflammatory diseases.

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TAK1 (encoded by Map3k7) is a mitogen-activated protein kinase kinase kinase (MAP3K), which activates the transcription factors AP-1 and NF-κB in response to receptor activator of NF-κB ligand (RANKL) stimulation, thus constituting a key regulator of osteoclast differentiation. Here we report the functional relevance of the kinase activity of TAK1 in the late stage of osteoclast differentiation in vivo using Ctsk-Cre mice and TAK1 mutant mice in which the TAK1 kinase domain was flanked by loxP. The Map3k7(flox/kd)Ctsk(Cre/+) mice displayed a severe osteopetrotic phenotype due to a marked decrease in osteoclast number. RANKL-induced activation of MAPK and NF-κB was impaired in the late stage of osteoclast differentiation. The absence of suppressive effect of an administered NF-κB inhibitor on the late stage of osteoclastogenesis led us to investigate unknown TAK1 targets in osteoclast differentiation. We performed a phosphoproteomic analysis of RANKL-stimulated osteoclast precursor cells from Map3k7(flox/kd)Ctsk(Cre/+) mice, revealing multiple targets regulated by TAK1 during osteoclastogenesis. Thus, TAK1 functions as a critical regulator of the phosophorylation status of various cellular proteins that govern osteoclastogenesis.

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Gametogenesis in budding yeast involves large-scale rearrangement of membrane traffic to allow de novo formation of a membrane, called the prospore membrane (PSM). However, the mechanism underlying this event is not fully elucidated. Here, we show that the number of endoplasmic reticulum exit sites (ERES) per cell fluctuates and switches from decreasing to increasing upon the onset of PSM formation. Reduction in ERES number is accompanied by a transient stall in membrane traffic, resulting in loss of the preexisting Golgi apparatus from the cell, as well as local ERES regeneration, leading to Golgi reassembly in nascent spores. We have revealed that protein phosphatase-1 (PP-1) and its development-specific subunit, Gip1, promote ERES regeneration through Sec16 foci formation. Furthermore, a mutant with impaired ERES formation showed defects in PSM growth and spore formation. Thus, ERES regeneration in nascent spores facilitates the segregation of membrane traffic organelles, leading to PSM growth.