Cooperative electrogenic proton transport pathways in the plasma membrane of the proton-secreting osteoclast.

A proton is a ubiquitous signaling ion. Many transmembrane H+ transport pathways either maintain pH homeostasis or generate acidic compartments. 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. We identified three electrogenic H+-fluxes in osteoclast plasma membranes-a vacuolar H+-ATPase (V-ATPase), a voltage-gated proton channel (Hv channel) and an acid-inducible H+-leak-whose electrophysiological profiles and regulation mechanisms differed. V-ATPase and Hv channel, both may have intracellular reservoirs, but the recruitment/internalization is regulated independently. V-ATPase mediates active H+ efflux, acidifying the resorption pit, while 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 in opposite directions may have advantages in fine regulation of pit pH. 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, will either stimulate or inhibit these H+ fluxes. Skeletal integration is disrupted by too much or too less of bone resorption. Diversities in plasma membrane H+ flux pathways, which may co-operate or compete, are essential to adjust osteoclast functions in variable conditions.