TABLE 2.

Osteoclasts and the bone resorption phase.

Origin of osteoclasts	Differentiation stages: Hematopoietic stem cell precursors differentiate into monocyte and macrophage, and then they fuse into end-differentiated multinucleated (bone resorbing) cells (Tanaka et al., 1993; Quinn et al., 1998; Roodman, 1999; Udagawa et al., 1999; Holmbeck and Szabov, 2006; Bar-Shavit, 2007; Bruzzaniti and Baron, 2006). Osteocyte apoptosis is thought to contribute to the recruitment of osteoclast precursors by diminishing the secretion of osteocyte-derived factors (e.g., TGF-β) that have inhibitory effect on osteoclast formation (Heino et al., 2002; Aguirre et al., 2006).
Main factors involved in osteoclastogenesis	Osteoblasts, osteocytes, RANKL, M-CSF, OPG, TNF, ILs, mineralized bone particles containing osteocalcin, DC-STAMP, OC-STAMP (Tanaka et al., 1993; Roach, 1994; Wiebe et al., 1996; Kotake et al., 1999; Udagawa et al., 1999; Marie, 2003; Miyamoto, 2006; Kim et al., 2011; Hienz et al., 2015; Plotkin and Bruzzaniti, 2019).
Key signaling events involved in osteoclastogenesis	After the induction of PU.1, the stem cell precursor is determined to the osteoclastic lineage (Tondravi et al., 1997). Then, cell proliferation is induced following expression and activation of c-fms by the precursor. RANK is subsequently expressed and activated by RANKL, after which RANK interacts with the TRAF family members (e.g., TRAF2, TRAF6) and lead to downstream activation of MAP kinases and NF-kβ. This process is aided by co-signaling from other receptors (such as TREM2, OSCAR, DAP 12, and FcRγ) (Koga et al., 2004; Mocsai et al., 2004). The interaction between immunoreceptors (e.g., TREM2, OSCAR) and FcRγ/FcRc adapters activates Syk kinases, leading to PLCγ activation. Ca(II), which is mobilized from the intracellular stores, activates calcineurin, resulting in dephosphorylation of NFATc1. Moreover, the activation of calcineurin involves the activation of phospholipase-Cγ and Tec kinases (Mocsai et al., 2004; Faccio et al., 2005; Wada et al., 2005). In general, most signaling pathways (MAPKs, NF-κB, AP-1, Ca(II), Src/PI3K/AKt) which are activated in the osteoclast converge to induce the activity of NFATc1 (Gori et al., 2000; Ishida et al., 2002; Takayanagi et al., 2002; Matsuo et al., 2004; Paiva and Granjeiro, 2017; Plotkin and Bruzzaniti, 2019; Zheng et al., 2019). Upon translocation to the nucleus, NFATc1 acts together with c-fos to promote the expression of key osteoclast genes. Some of the osteoclast differentiation genes to which NFATc1 binds directly are OSCAR (Kim Y. et al., 2005), cathepsin K (Matsumoto et al., 2004), calcitonin receptor (Matsuo et al., 2004), integrin β3 (Crotti et al., 2006, 2008), MMP-9 (Sundaram et al., 2007), and TRAP (Matsuo et al., 2004; Paiva and Granjeiro, 2017). Of note, another factor which controls NFATc1 is OPG, which functions as a decoy receptor for RANKL, thus inhibiting the differentiation of osteoclasts (Lacey et al., 1998). Osteoclastogenesis is regulated by the RANKL/OPG balance. Opposing effects on RANK during osteoclast differentiation is exerted by LGR4 which signals through G-protein or Wnt signaling pathways (Luo et al., 2016). Cytokines which inhibit RANK signaling on osteoclasts are IL-10, IFNs (α, β), and GM-CSF.
Mechanisms that underlie the action of osteoclasts	During initiation of the resorption phase, the mature osteoclasts (1-2% of bone cells) attach to the bone surface via αvβ3, αvβ5, α2β1, and αvβ1 integrins (Vaananen and Horton, 1995; Datta et al., 2008; Rauner et al., 2012; Plotkin and Bruzzaniti, 2019). At the bone/osteoclast surface, a ruffled border which is entirely surrounded by a sealing zone is formed, thereby creating an isolated resorption (Howship’s) lacuna (i.e., scalloped erosion) (Miyauchi et al., 1991; Mimura et al., 1994; Teitelbaum, 2000; Teitelbaum and Ross, 2003). Osteoclasts dissolve mineral (hydroxyapatite) and organic components (e.g., type I collagen) of the bone matrix in the resorption lacuna (Teitelbaum et al., 1995; Rauner et al., 2012). This resorption process is mediated by the secretion of hydrogen ions, to acidify the resorption compartment beneath osteoclasts and dissolve hydroxyapatite crystals (Blair et al., 1989; Teti et al., 1989). Hydrogen ions, supplied by the reaction of water and carbon dioxide and catalyzed by carbonic anhydrase II, are transported into the resorption lacuna by ATPases located in the ruffled border of osteoclasts (Baron, 1989; Mattsson et al., 1994; Li et al., 1999; Bruzzaniti and Baron, 2006; Hienz et al., 2015). Hydrochloric acid formed with chloride ions pumped into the resorption lacuna dissolves the mineralized bone matrix (Silver et al., 1988; Plotkin and Bruzzaniti, 2019). In addition, lysosomal enzymes (e.g., cathepsin K), bone-derived collagenases, and other proteinases (e.g., tartrate-resistant acid phosphatase) act in concert to mediate the resorption process (Bord et al., 1996; Gelb et al., 1996; Saftig et al., 1998; Boyle et al., 2003; Teitelbaum, 2007; Hienz et al., 2015). Osteoclast-mediated bone resorption, which takes a few (2-4) weeks during each remodeling cycle, results in Howship’s lacuna on the surface of trabecular bone and cylindrical Haversian canals in cortical bone (Bruzzaniti and Baron, 2006; Teitelbaum, 2007; Hienz et al., 2015). After one resorption lacuna is completed, the osteoclast cells die by apoptosis (Plotkin and Bruzzaniti, 2019) or move along the bone surface to resume resorption. This phase lasts approximately 8-10 days (Teitelbaum, 2007).
Systemic and local factors that stimulate bone resorption	Osteocytes as the major source of RANKL; thyroid hormones; PTH/PTHrP; calcitriol; glucocorticoids; growth factors (FGF, PDGF, EGF); TNF-α; colony-stimulating factors (M-CSF, GM-CSF); IL-1, -6, -7, -8, -11, -15, -17; PGE1, 2, 12; PGH2 (MacDonald, 1986; Dempster et al., 1993; Raisz, 1993; Kawaguchi et al., 1994, 1995; Nash et al., 1994; Holt et al., 1996; Lanske et al., 1999; Roodman, 1999; Lam et al., 2000; Compston, 2001; Ragab et al., 2002; Sher et al., 2004; Eijken et al., 2005; Dai et al., 2006; Zhang et al., 2008; Kini and Nandeesh, 2012; Rauner et al., 2012; Parra-Torres et al., 2013; Paiva and Granjeiro, 2017; Hachemi et al., 2018; Bellido and Gallant, 2019).

RANK, receptor activator of nuclear factor kappa B; RANKL, receptor activator of nuclear factor kappa B ligand; GM-CSF, granulocyte-macrophage colony-stimulating factor; OPG, osteoprotegerin; TNF, tumor necrosis factor; IL, interleukin; DC-STAMP, dendritic-cell specific transmembrane protein; OC-STAMP, osteoclast stimulatory transmembrane protein; NFkB, nuclear-factor kappa B; TRAF6, TNF receptor-associated factor 6; TREM2, triggering receptor expressed on myeloid cells-2; OSCAR, osteoclast-associated receptor; DAP, DNAX-activating protein; FcRγ, Fc common receptor γ chain; FcRc, soluble Fc receptor from a group C streptococcus; Syk, spleen tyrosine kinase; PLC, phospholipase C; NFATc1, nuclear factor of activated T cell cytoplasmic 1; Tec, tyrosine protein kynase; AP, activator protein; Src, steroid receptor coactivator; PI3K, phosphatidylinositol 3-phosphate kinase; TRAP, tartrate-resistant acid phosphatase; LGR, leucine-rich repeat-containing G protein-coupled receptor; IFN, interferon; PTH, parathyroid hormone; PTHrP, PTH-related protein; FGF, fibroblast growth factor; PDGF, platelet-derived growth factor; EGF epidermal growth factor; M-CSF, macrophage colony-stimulating factor; PGE, prostaglandin E; PGH, prostaglandin H.