Regulation of Osteoclast Function

Abstract

Osteoclastic bone resorption consists of several complicated processes: osteoclast development, attachment of osteoclasts to calcified tissues, development of a ruffled border and clear zone, followed by the secretion of acids and lysosomal enzymes into the space beneath the ruffled border. During the past decade, several new approaches have been taken to investigate osteoclast function. Highly purified functionally active osteoclasts can be isolated, which allows osteoclast function to be studied biochemically. The mechanisms of reduced bone resorption have been elucidated in several types of congenital osteopetrotic mutant mice. For example, the important role of macrophage-colony stimulating factor (M-CSF, colony-stimulating factor 1) in osteoclast development has been established in the osteopetrotic (op/op) mutant mice. Another osteopetrotic mutant mouse model has been generated by targeted disruption of the proto-oncogene c-src. Studies using this model showed that tyrosine kinase–mediated signals are essentially involved in osteoclast function. Using a purified osteoclast preparation, we showed that osteoclast function was activated by osteoblasts through a mechanism involving cell-to-cell contact. On the basis of these new approaches and experimental findings, we examined several fundamental issues regarding osteoclastic bone resorption. We considered how osteoclasts form ruffled borders and clear zones, which signals are necessary for osteoclast polarization, which mechanisms regulate the life span of osteoclasts, how calcitonin and bisphosphonates inactivate osteoclast function, and how osteoblasts regulate osteoclast function. In this article, we review the regulatory mechanisms of osteoclast function mainly on the basis of the experimental results obtained in our murine culture system. Multinucleated osteoclasts responsible for bone resorption are derived from hemopoietic cells of the monocyte-macrophage lineage.1-4 The progenitor cells are recruited from the hemopoietic tissues such as bone marrow and splenic tissues to bone via the circulating blood stream. They proliferate and differentiate into osteoclasts through a mechanism involving cell-to-cell interaction with osteoblastic stromal cells.4 We have developed a mouse marrow culture system and a coculture system of mouse osteoblastic cells and hemopoietic cells in which osteoclast-like multinucleated cells are formed in response to bone-resorbing factors such as 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3), parathyroid hormone (PTH), prostaglandin E2 (PGE2), and interleukin-6 (IL-6) plus soluble IL-6 receptors.4 The multinucleated cells formed in the mouse culture systems satisfied major criteria of osteoclasts such as tartrate-resistant acid phosphatase (TRAP, a marker enzyme of osteoclasts) activity, calcitonin receptors, p60c-src, vitronectin receptors (αvβ3), and bone-resorbing activity. Recent evidence has suggested that those bone-resorbing hormones and cytokines commonly and directly act on osteoblastic cells for osteoclast development in cocultures of mouse osteoblastic and spleen cells.4, 5 Osteoclasts have several unique characteristics for resorbing bone. The most characteristic morphological features of osteoclasts are the presence of ruffled borders and clear zones.1-4 The clear zone serves in the attachment of osteoclasts to the bone surface. The resorbing area under the ruffled border of osteoclasts is acidic, which favors dissolution of bone mineral. Protons are provided by carbonic anhydrase II in bone-resorbing osteoclasts.6 The vacuolar type proton ATPase exists in the ruffled border membranes of osteoclasts.7, 8 The transport of protons into resorption lacunae is mediated by this vacuolar type proton ATPase. Lysosomal enzymes of osteoclasts are also secreted into a resorbing area to degrade the organic matrix of bone. Cysteine proteinases such as cathepsin K (OC-2)9, 10 and metalloproteinases (MMPs) such as MMP-911 have been reported to be present in osteoclasts, which degrade type I collagen of demineralized bone in an acidic condition. To organize highly polarized cytoplasmic structures such as ruffled borders and clear zones, osteoclasts must adhere to the bone surface. Therefore, bone matrix components were considered essential for inducing osteoclast polarization. However, Jones et al.12, 13 reported that osteoclasts could resorb not only bone and dentine, but also enamel, avian egg shells, and oyster shells. Chambers et al.14 also reported that rabbit osteoclasts could form resorption pits on inorganic bone in the presence of serum. Moreover, Silver et al.15 demonstrated that the space between firmly attached osteoclasts and glass cover slips is maintained at a lower pH than that of culture medium. These findings suggest that bone matrix components are not necessarily required for osteoclasts to obtain polarity and exhibit bone-resorptive activity. Bearing these findings in mind, we examined the factor(s) that would be