Type I phosphotidylinosotol 4-phosphate 5-kinase γ regulates osteoclasts in a bifunctional manner.

Type 1 phosphotidylinosotol-4 phosphate 5 kinase γ (PIP5KIγ) is central to generation of phosphotidylinosotol (4,5)P(2) (PI(4,5)P(2)). PIP5KIγ also participates in cytoskeletal organization by delivering talin to integrins, thereby enhancing their ligand binding capacity. As the cytoskeleton is pivotal to osteoclast function, we hypothesized that absence of PIP5KIγ would compromise their resorptive capacity. Absence of the kinase diminishes PI(4,5) abundance and desensitizes precursors to RANK ligand-stimulated differentiation. Thus, PIP5KIγ(-/-) osteoclasts are reduced in number in vitro and confirm physiological relevance in vivo. Despite reduced numbers, PIP5KIγ(-/-) osteoclasts surprisingly have normal cytoskeletons and effectively resorb bone. PIP5KIγ overexpression, which increases PI(4,5)P(2), also delays osteoclast differentiation and reduces cell number but in contrast to cells lacking the kinase, its excess disrupts the cytoskeleton. The cytoskeleton-disruptive effects of excess PIP5KIγ reflect its kinase activity and are independent of talin recognition. The combined arrested differentiation and disorganized cytoskeleton of PIP5KIγ-transduced osteoclasts compromises bone resorption. Thus, optimal PIP5KIγ and PI(4,5)P(2) expression, by osteoclasts, are essential for skeletal homeostasis.

Oophorectomy-induced bone loss is attenuated in MAGP1-deficient mice.

Microfibril-associated glycoprotein-1 (MAGP1), together with the fibrillins, are constitutive components of vertebrate microfibrils. Mice deficient in MAGP1 (murine MAGP1 knockout animals (Mfap2(-/-)); MAGP1Δ) is appropriate develop progressive osteopenia and reduced whole bone strength, and have elevated numbers of osteoclasts lining the bone surface. Our previous studies suggested that the increased osteoclast population was associated with elevated levels of receptor activator of NF-κB ligand (RANKL), a positive regulator of osteoclast differentiation. To explore the relationship between RANKL expression and osteoclast differentiation in MAGP1 deficiency, oophorectomy (OVX) was used to stimulate RANKL expression in both WT and MAGP1Δ animals. Bone loss following OVX was monitored using whole body DEXA and in vivo µCT. While WT mice exhibited significant bone loss following OVX, percent bone loss was reduced in MAGP1Δ mice. Further, serum RANKL levels rose significantly in OVX WT mice, whereas, there was only a modest increase in RANKL following OVX in the mutant mice due to already high baseline levels. Elevated RANKL expression was normalized when cultured MAGP1Δ osteoblasts were treated with a neutralizing antibody targeting free TGFß. These studies provide support for increased RANKL expression associated with MAGP1 deficiency and provide a link to altered TGF-ß signaling as a possible causative signaling pathway regulating RANKL expression in MAGP1Δ osteoblasts.

IL-17 mediates estrogen-deficient osteoporosis in an Act1-dependent manner.

Estrogen-deficient osteoporosis may be an inflammatory disorder and we therefore asked if IL-17 participates in its pathogenesis. Deletion of the principal IL-17 receptor (IL-17RA) protects mice from ovariectomy (OVX)-induced bone loss. Further supporting a central role of IL-17 in its pathogenesis, OVX-induced osteoporosis is prevented by a blocking antibody targeting the cytokine. IL-17 promotes osteoclastogenesis by stimulating RANK ligand (RANKL) expression by osteoblastic cells, mediated by the IL-17RA SEFIR/TILL domain. Estrogen deprivation, however does not enhance IL-17RA mRNA expression by osteoblasts or in bone, but augments that of Act1, an IL-17RA-interacting protein and signaling mediator. Similar to IL-17RA(-/-) mice, those lacking Act1 are protected from OVX-induced bone loss. Also mirroring IL-17RA-deficiency, absence of Act1 in osteoblasts, but not osteoclasts, impairs osteoclastogenesis via dampened RANKL expression. Transduction of WT Act1 into Act1(-/-) osteoblasts substantially rescues their osteoclastogenic capacity. The same construct, however, lacking its E3 ligase U-box or its SEFIR domain, which interacts with its counterpart in IL-17RA, fails to do so. Estrogen deprivation, therefore, promotes RANKL expression and bone resorption in association with upregulation of the IL-17 effector, Act1, supporting the concept that post-menopausal osteoporosis is a disorder of innate immunity.

Fyn promotes proliferation, differentiation, survival and function of osteoclast lineage cells.

c-Src and Lyn are the only Src family kinases (SFKs) with established activity in osteoclasts (OCs). c-Src promotes function via cytoskeletal organization of the mature resorptive cell while Lyn is a negative regulator of osteoclastogenesis. We establish that Fyn, another SFK, also impacts the OC, but in a manner distinctly different than c-Src and Lyn. Fyn deficiency principally alters cells throughout the osteoclastogenic process, resulting in diminished numbers of resorptive polykaryons. Arrested OC formation in the face of insufficient Fyn reflects reduced proliferation of precursors, in response to M-CSF and retarded RANK ligand (RANKL)-induced differentiation, attended by suppressed activation of the osteoclastogenic signaling molecules, c-Jun, and NF-κB. The anti-apoptotic properties of RANKL are also compromised in cells deleted of Fyn, an event mediated by increased Bim expression and failed activation of Akt. The defective osteoclastogenesis of Fyn-/- OCs dampens bone resorption, in vitro. Finally, while Fyn deficiency does not regulate basal osteoclastogenesis, in vivo, it reduces that stimulated by RANKL by ~2/3. Thus, Fyn is a pro-resorptive SFK, which exerts its effects by prompting proliferation and differentiation while attenuating apoptosis of OC lineage cells.

c-Src links a RANK/αvß3 integrin complex to the osteoclast cytoskeleton.

RANK ligand (RANKL), by mechanisms unknown, directly activates osteoclasts to resorb bone. Because c-Src is key to organizing the cell's cytoskeleton, we asked if the tyrosine kinase also mediates RANKL-stimulated osteoclast activity. RANKL induces c-Src to associate with RANK(369-373) in an αvß3-dependent manner. Furthermore, RANK(369-373) is the only one of six putative TRAF binding motifs sufficient to generate actin rings and activate the same cytoskeleton-organizing proteins as the integrin. While c-Src organizes the cell's cytoskeleton in response to the cytokine, it does not participate in RANKL-stimulated osteoclast formation. Attesting to their collaboration, αvß3 and activated RANK coprecipitate, but only in the presence of c-Src. c-Src binds activated RANK via its Src homology 2 (SH2) domain and αvß3 via its SH3 domain, suggesting the kinase links the two receptors. Supporting this hypothesis, deletion or inactivating point mutation of either the c-Src SH2 or SH3 domain obviates the RANK/αvß3 association. Thus, activated RANK prompts two distinct signaling pathways; one promotes osteoclast formation, and the other, in collaboration with c-Src-mediated linkage to αvß3, organizes the cell's cytoskeleton.