Specific RANK Cytoplasmic Motifs Drive Osteoclastogenesis

Upon receptor activator of NF-κB ligand (RANKL) binding, RANK promotes osteoclast formation through the recruitment of tumor necrosis factor (TNF) receptor-associated factors (TRAFs). In vitro assays identified two RANK intracellular motifs that bind TRAFs: PVQEET^560-565 (Motif 2) and PVQEQG^604-609 (Motif 3), which potently mediate osteoclast formation in vitro. To validate the in vitro findings, we have generated knock-in (KI) mice harboring inactivating mutations in RANK Motifs 2 and 3. Homozygous KI (RANK^KI/KI ) mice are born at the predicted Mendelian frequency and normal in tooth eruption. However, RANK^KI/KI mice exhibit significantly more trabecular bone mass than age- and sex-matched heterozygous KI (RANK^+/KI ) and wild-type (RANK^+/+ ) counterparts. Bone marrow macrophages (BMMs) from RANK^KI/KI mice do not form osteoclasts when they are stimulated with macrophage colony-stimulating factor (M-CSF) and RANKL in vitro. RANKL is able to activate the NF-κB, ERK, p38, and JNK pathways in RANK^KI/KI BMMs, but it cannot stimulate c-Fos or NFATc1 in the RANK^KI/KI cells. Previously, we showed that RANK signaling plays an important role in Porphyromonas gingivalis (Pg)-mediated osteoclast formation by committing BMMs into the osteoclast lineage. Here, we show that RANKL-primed RANK^KI/KI BMMs are unable to differentiate into osteoclasts in response to Pg stimulation, indicating that the two RANK motifs are required for Pg-induced osteoclastogenesis. Mechanistically, RANK Motifs 2 and 3 facilitate Pg-induced osteoclastogenesis by stimulating c-Fos and NFATc1 expression during the RANKL pretreatment phase as well as rendering c-Fos and NFATc1 genes responsive to subsequent Pg stimulation. Cell-penetrating peptides (CPPs) conjugated with RANK segments containing Motif 2 or 3 block RANKL- and Pg-mediated osteoclastogenesis. The CPP conjugates abrogate RANKL-stimulated c-Fos and NFATc1 expression but do not affect RANKL-induced activation of NF-κB, ERK, p38, JNK, or Akt signaling pathway. Taken together, our current findings demonstrate that RANK Motifs 2 and 3 play pivotal roles in osteoclast formation in vivo and mediate Pg-induced osteoclastogenesis in vitro.