Intracellular signalling during inflammation-induced osteoclastogenesis. RANKL (receptor activator of NF-kB ligand) binds to its receptor RANK and induces the key regulator of osteoclast differentiation NFATc1 (Nuclear factor of activated T cells, cytoplasmic 1) through two different signalling pathways. On the one hand, RANK recruits TRAF6 (TNF receptor-associated factor 6) and activates NF-κB, JNK, p38, c-fos and AP-1. On the other hand, NFATc1 is activated by calcineurin that is in turn activated by elevated calcium in the cytoplasm. Phospholipase C (PLC)γ mediates the release of calcium in the cytoplasm; PLCγ is activated by RANK through Btk/Tec and by OSCAR (osteoclast-associated receptor) and TREM-2 (triggering receptor expressed by myeloid cells 2) through the Fc receptor gamma chain (FcRγ), DAP12 (DNAX-activating protein of 12 kDa) and Syk signalling. TNF binds to its receptor TNFR1, which recruits TRADD (TNFR-associated DD protein) and RIP-1 (Receptor interacting protein-1). TNF receptor-associated factor (TRAF)2, TRAF5 and TRAF6 mediate further signalling through NF-κB, JNK and p38. The IL-1 receptor IL-1R1 binds MYD88 (myeloid differentiation primary response gene 88) and RAK4, which activates TRAF6 through phosphorylation of IL-1 receptor-activating protein kinase (IRAK)2 and IRAK1. After binding IL-6, the IL-6 receptor recruits two gp130 molecules and activates the signal transducer and activator of transcription (STAT) pathway and the mitogen-activated protein kinase (MAPK) pathway (JNK, p38, ERK) through gp130. MMP, matrix metalloproteinase.