Chapter 3: Osteoclast Biology and Bone Resorption F. Patrick Ross

Figure 1 Figure 1 The osteoclast as a resorptive cell. Transmission electron microscopy of a multinucleated primary rat osteoclast on bone. Note the extensive ruffled border, close apposition of the cell to bone and the partially degraded matrix between the sealing zones. Courtesy of H. Zhao. © 2008 American Society for Bone and Mineral Research From the Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, 7 th Edition.

Figure 2 Figure 2 Role of cytokines, hormones, steroids, and prostaglandins in osteoclast formation. Under the influence of other cytokines (data not shown), hematopoietic stem cells (HSCs) commit to the myeloid lineage, express c-Fms and RANK, the receptors for M-CSF and RANKL, respectively, and differentiate into osteoclasts. Mesenchymal cells in the marrow respond to a range of stimuli, secreting a mixture of pro- and anti-osteoclastogenic proteins, the latter primarily OPG. Glucocorticoids suppress bone resorption indirectly but possibly also target osteoclasts and/or their precursors. Estrogen, by a complex mechanism, inhibits activation of T cells, decreasing their secretion of RANKL and TNF-α; the sex steroid also inhibits osteoblast and osteoclast differentiation and lifespan. A key factor regulating bone resorption is the RANKL/OPG ratio. © 2008 American Society for Bone and Mineral Research From the Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, 7 th Edition.

Figure 3 Figure 3 Mechanism of osteoclastic bone resorption. (A) The osteoclast adheres to bone through the integrin αvβ3, creating a sealing zone, into which is secreted hydrochloric acid and acidic proteases such as cathepsin K, MMP9, and MMP13. The acid is generated by the combined actions of a vacuolar H+ ATPase; it coupled chloride channel and a basolateral chloride-bicarbonate exchanger. Carbonic anhydrase converts CO2 into H+ and HCO3− (data not shown). (B) Integrin engagement results in signals that target acidifying vesicles ( = proton pump complex) containing specific cargo (black dots) to the bone- apposed face of the cell. Fusion of these vesicles with the plasma membrane generates a polarized cell capable of secreting the acid and proteases required for bone resorption. © 2008 American Society for Bone and Mineral Research From the Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, 7 th Edition.

Figure 4 Figure 4 Regulation and role of small GTPases in osteoclasts. Signals from αvβ3 and/or receptor tyrosine kinases (RTKs) activate small GTPases of the Rho family in a c-src–dependent manner. Bisphosphonates, the potent antiresorptive drugs, block addition of hydrophobic moieties onto the GTPases, preventing their membrane targeting and activation. The active GTPases also regulate cell viability and thus bisphosphonates induce osteoclast death. © 2008 American Society for Bone and Mineral Research From the Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, 7 th Edition.

Figure 5 Figure 5 Cell–cell interactions in bone marrow. Hematopoietic stem cells (H), the precursors of both T cells (T) and osteoclasts (OC), reside in a stem cell niche provided by osteoblasts (OB), which, together with stromal cells (S), derive from mesenchymal stem cells (M). Bone degradation results in release of matrix-associated growth factors (thick vertical line), which stimulate mesenchymal cells and thus bone formation. This “coupling” is an essential consequence of osteoclast activity.(28) After activation, T cells secrete molecules that stimulate osteoclastogenesis and function. Cancer cells (C) release cytokines that activate bone resorption; in turn, matrix-derived factors stimulate cancer cell proliferation, the so- called “vicious cycle.” © 2008 American Society for Bone and Mineral Research From the Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, 7 th Edition.

Figure 6 Figure 6 Osteoclast signaling pathways. Summary of the major receptors, downstream kinases, and effector transcription factors that regulate osteoclast formation and function. Proliferation (P) of precursors is driven chiefly through ERKs and their downstream cyclin targets and E2F; maximal activation of this pathway requires combined signals from c-Fms and the integrin αvβ3. As expected, the cytoskeleton (C) is independent of nuclear control but depends on a series of kinases and their cytoskeletal-regulating targets, whereas differentiation (D) is regulated largely by controlling gene expression. The calcium/calmodulin (CaM)/calcineurin (CN) axis enhances nuclear translocation of NFAT1c, the most distal transcription factor characterized to date. © 2008 American Society for Bone and Mineral Research From the Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, 7 th Edition.