As above-mentioned, the binding of RANKL to its receptor RANK, which is encoded by
Tnfrsf11a, leads to generate downstream RANKL-RANK signaling cascades, which trigger pre-OCs to differentiate into mature OCs [
4]. RANKL is expressed in two distinct forms including (1) a membrane-bound form expressed in cell surface of osteoblasts (OBs) and a soluble form. The membrane-bound RANKL is cleaved into the soluble form by metalloproteinases such as matrix metalloproteinase (MMP)-14 [
43,
44]. Although both forms function as agonistic ligands for RANK receptor, the membrane-bound RANKL is believed to be more efficient. Mice with the genetic deletions of
Tnfsf11 or
Tnfsf11a exhibit severe osteopetrosis accompanied by defective tooth eruption owing to the mortality of their OCs [
43,
45]. On the other hand, RANKL-RANK signaling is also suppressed by
Tnfsf11b- encoded osteoprotegerin, which is a soluble decoy receptor for RANKL preventing RANKL from binding to RANK receptor [
4]. RANK is a member of the tumor necrosis factor receptor (TNFR) superfamily that lacks intrinsic enzymatic activity required for directly activating its downstream signaling molecules. Consequently, following the RANKL-RANK interaction, RANK transduces intracellular signals by recruiting adaptor molecules such as TNFR-associated factors (TRAFs), which then trimerize cytosolic tail of RANK receptor, and activate MAPKs, NF-кB, activator protein-1 (AP-1) and finally nuclear factor of activated T-cells cytosolic 1 (NFATc1), resulting in the commitment of monocyte/macrophage precursor cells to the OC lineage and the activation of mature OCs [
7]. In the late stage of OC differentiation, the osteoclastogenic genes required for bone resorption are transcribed by NFATc1 [
7]. More particularly, activation of TRAF6 leads to upregulate the NF-кB transcriptional activity by enhancing IкB kinase (IKK) complex that is mediated by either atypical protein kinase C (PKC) or TGFβ-activated kinase 1 (TAK1)-dependent phosphorylation [
46]. The interaction of scaffolding protein p62 with TRAF6 and aPKCs brings out the establishment of a multimeric protein complex that regulates IKKβ [
47]. In addition, TRAF6 also forms complexes with TAK1 and the adaptor proteins TAK1-binding protein 1 (TAB1) and TAB2, which enables TAK1 to phosphorylate IKK complex and subsequently activate NF-κB downstream signaling cascades [
48]. It is best characterized that NF-κB signaling is indispensable for osteoclastogenesis. That is, NF-κB p50/p52 double-knockout mice exhibit considerable osteopetrosis owing to the deterioration in OC formation [
49]. AP-1 transcription factor comprising Fos (c-Fos, FosB, Fra-1, and Fra-2), Jun (c-Jun, JunB, and JunD), and ATF (ATFa, ATF, ATF2, ATF4, and B-ATF) family members is also activated via the induction of c-Fos by the adaptor proteins [
50]. c-Fos knockout mice and transgenic mice that overexpress dominant negative c-Jun exhibit severe osteopetrosis, indicating that the transcription factors, NF-κB and AP-1, are crucial for activating the downstream targets of RANKL signaling pathway at the early stage of OC differentiation [
7]. Furthermore, the recruitment of adaptor proteins activates MAPKs such as c-Jun N-terminal kinase (JNK), p38, and extracellular signal-regulated kinase, and Akt/PKB [
51].