1.N.1 The Osteoclast Fusion Complex (OFC) Family 

Osteoclast cell fusion to form syncytia is promoted by several proteins.  Most importantly are DC-STAMP and OC-STAMP, but other proteins such as the V-type proton ATPase subunit d2, ATP6v0d2 (TC# 3.A.2.2.6), CD47, a leucocyte surface antigen of 323 aas (Q08722) and syncytin-1 (TC# 1.G.9.1.1) may play roles (Møller et al. 2016). The process is regulated by NFATc1, a master regulator of nuclear factor kappaB ligand-induced osteoclast differentiation. Kim et al. 2008 demonstrated a role for NFATc1 as a positive regulator of nuclear factor kappaB ligand-mediated osteoclast fusion as well as other fusion-inducing factors such as TNF-alpha. Exogenous overexpression of a constitutively active form of NFATc1 in bone marrow-derived monocyte/macrophage cells (BMMs) induces formation of multinucleated osteoclasts as well as the expression of fusion-mediating molecules such as the d2 isoform of vacuolar ATPase V(o) domain (Atp6v0d2) and the dendritic cell-specific transmembrane protein (DC-STAMP). Inactivation of NFATc1 by cyclosporin A treatment attenuates expression of Atp6v0d2 and DC-STAMP and the subsequent fusion of osteoclasts. Kim et al. 2008 showed that NFATc1 binds to the promoter regions of Atp6v0d2 and DC-STAMP in osteoclasts and directly induces their expression. Furthermore, overexpression of Atp6v0d2 and DC-STAMP rescues cell-cell fusion of preosteoclasts despite reduced NFATc1 activity. Thus, the NFATc1/Atp6v0d2 and DC-STAMP signaling axis plays a key role in the osteoclast multinucleation process, which is essential for efficient bone resorption.

The osteoclast is a bone-resorbing cell, which degrades bone tissues by secreting protons and lysosomal enzymes into the resorption pit. The plasma membrane facing bone tissue (ruffled border), generated partly by fusion of lysosomes, may mimic H⁺ flux mechanisms regulating acidic vesicles. Kuno 2018 identified three electrogenic H⁺-fluxes in osteoclast plasma membranes, a vacuolar H⁺- V-type ATPase (TC# 3.A.2), a voltage-gated proton Hv channel (TC# 1.A.51) and an acid-inducible H⁺-leak. The V-ATPase mediates active H⁺ efflux, acidifying the resorption pit, while the acid-inducible H⁺ leak, activated at an extracellular pH < 5.5, diminishes pit acidification, possibly to protect bone from excess degradation. The two-way H⁺ flux mechanisms may have advantages in fine regulation of pit pH. The Hv channel mediates passive H⁺ efflux. Although its working ranges are limited, the amount of H⁺ extrusion is 100 times larger than those of the V-ATPase and may support reactive oxygen species production during osteoclastogenesis. Extracellular Ca²⁺, H⁺ and inorganic phosphate, which accumulate in the resorption pit, either stimulate or inhibit these H⁺ fluxes (Kuno 2018).

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