Molecular mechanisms of triggering, amplifying and targeting RANK signaling in osteoclasts

Figure 1 Three phases of receptor activator of nuclear factor-κB signaling during osteoclast differentiation. Osteoclast differentiation downstream of receptor activator of nuclear factor-κB (RANK) signaling is divided into triggering, amplifying and targeting phases, based on the nuclear factor of activated T-cells cytoplasmic 1 activation state. ITAM: Immunoreceptor tyrosine-based activation motif; TRAF6: Tumor necrosis factor receptor-associated factor 6; Gab2: Grb-2-associated binder-2; PLCγ2: Phospholipase C γ2; RANKL: RANK ligand; Ig: Immunoglobulin.

Figure 2 Triggering phase. Trimerization of receptor activator of nuclear factor-κB (RANK) by binding of RANK ligand (RANKL) immediately activates mitogen-activated protein kinases (MAPKs), nuclear factor (NF)-κB, and activator protein-1 (AP-1). An adaptor molecule complex including tumor necrosis factor receptor-associated factor 6 (TRAF6), Grb-2-associated binder-2 (Gab2) and phospholipase C (PLC)γ2 on TRAF6 binding sites of RANK is essential to induce the triggering phase. HCR: Highly conserved domain in RANK; JNK: c-Jun N-terminal kinase; Erk: Extracellular signal-regulated kinase.

Figure 3 Amplifying phase. Both Ca²⁺ oscillation-dependent and -independent nuclear factor of activated T-cells cytoplasmic 1 (NFATc1) amplification are induced. Highly conserved domain in receptor activator of nuclear factor-κB (RANK)-mediated RANK signaling and immunoreceptor tyrosine-based activation motif (ITAM) signaling lead to continuous phospholipase C (PLC)γ2 activation. Regulator of G-protein signaling 10 (RGS10) determines the Ca²⁺ oscillation pattern through control of PLCγ2 by competitive binding of Ca²⁺/calmodulin and phosphatidylinositol 3, 4, 5-trisphosphate (PIP₃). Sustained Ca²⁺ oscillation contributes to NFATc1 amplification mediated by transcriptional auto-induction. In the Ca²⁺ oscillation-independent pathway, Cot kinase enhances NFATc1 stabilization through direct phosphorylation and contributes to its accumulation. ER: Endoplasmic reticulum; BLNK: B cell linker protein; SLP76: Src homology 2 domain-containing leukocyte protein of 76 kD; TRAF6: Tumor necrosis factor receptor-associated factor 6; Gab2: Grb-2-associated binder-2; IP₃R: IP₃ receptor; CaM: Calmodulin; Btk: Bruton's tyrosine kinase.

Figure 4 Targeting phase. Negative regulation of nuclear factor of activated T-cells cytoplasmic 1 (NFATc1) is indicated by blue lines and activation of its targets, shown in boxes, is indicated by red arrows. RANK: Receptor activator of nuclear factor-κB; ITAM: Immunoreceptor tyrosine-based activation motif; OSCAR: Osteoclast-associated receptor; Blimp1: B-lymphocyte-induced maturation protein-1; DC-STAMP: Dendritic cell-specific transmembrane protein; TRAP: Tartrate-resistant acid phosphatase; IRF-8: Interferon regulatory factor-8; MafB: V-maf musculoaponeurotic fibrosarcoma oncogene family, protein B; Bcl6: B cell lymphoma 6; CLC: Chloride channel.