Recent advances toward understanding osteoclast physiology

Osteoclasts develop from precursor cells of the monocyte series. However, specialized differentiation for efficient bone degradation separates the osteoclast from the macrophage. The physical reasons for these differences are emerging from the study of osteoclastic physiology and biochemistry. Key osteoclast specializations are multinucleation, formation of a tightly sealed extracellular compartment on bone, and high-capacity secretion of HCl and acid proteases into this extracellular site. Multinucleation increases efficiency of extracellular attachment processes. The attachment process is mediated by cell membrane integrins, and is sensitive to changes in intracellular or extracellular calcium. Acid production exploits carbonic acid as the source of acid and conjugate base equivalents, reflected in abundant osteoclastic carbonic anhydrase type II expression. Secretion of acid involves extremely high expression of vacuolar-type H⁺-ATPase and a chloride channel in the cell's specialized acid secreting organelle, the ruffled membrane, which is polarized to the osteoclast's bone attachment. Acid secretion is balanced by chloride-bicarbonate exchange in the cell's non-bone attached membranes; this functionally resembles the band 3 chloride- bicarbonate exchanger of the red cell carbon dioxide transport system. Bone collagen is degraded by acid proteases secreted into the acid degradation site via the mannose-6-phosphate receptor system, which is targeted to lysosomes in other cells. Functional deficits, as in osteopetrosis, may affect any of the elements involved in osteoclast differentiation. Furthermore, new antiosteoclastic therapeutic agents may inhibit osteoclast biochemistry intentionally, such as for the control of hypercalcemia of malignancy.