Regulation of bone turnover by calcium-regulated calcium channels

Abstract

Calcium plays multiple roles in osteoclast formation, survival, and activity. Intracellular calcium is determined both by the release of intracellular stores and the influx of extracellular calcium through a variety of calcium channels. Osteoclasts express several classes of calcium channels, including ryanodine receptors (RyRs), inositol-1,4,5-trisphosphate receptors (IP(3)Rs), and calcium release-activated calcium channels (CRACs), which respond to depletion of intracellular stores. IP(3)R2 is expressed in osteoclast precursors and activated by cytokines that stimulate osteoclast differentiation. In mature osteoclasts, the IP(3)R1 isoform is highly expressed and is implicated in nitric oxide-cGMP-stimulated processes. RyR calcium channels may contribute to the release of intracellular calcium stores, while RyR2 in the plasma membrane may act to limit osteoclast activity based on extracellular calcium concentration. Orai, through regulation by endoplasmic reticular store-sensing proteins, including Stim-1, may also mediate calcium influx and act as a signal amplifier for calcium release by other calcium channels. Together, these receptors allow intracellular Ca(2+) signals to modulate bone turnover and, through calcium-sensing functions, allow coupling of osteoclast activity to extracellular conditions and integrating additional cytokine and nitric oxide signals via transient intracellular calcium signals.

Figure 1.

Regulation of osteoclast motility by calcium and NO. NO is an autocrine and paracrine stimulus, being produced by mesenchymal cells including chondrocytes and osteoblasts, as well as a regulated product of monocyte family cells including osteoclasts. NO causes cGMP synthesis, activating PKG I and leading to reorganization of osteoclast attachment proteins. This can allow the cell to move, or it may precede apoptosis. PKG I mediates actin disassembly by phosphorylation of intermediate proteins. VASP is a major target; the integrin-related assembly includes αvβ3, migfilin, and other proteins. PKG I and VASP are essential for a local Ca²⁺ signal. This Ca²⁺ signal depends on the cytoskeletal assembly and cannot be produced without VASP and src. The inositol-1,4,5 trisphosphate receptor-1 is the key Ca²⁺ pulse generator, and Ca²⁺ pulses activate μ-calpain, which is also required for motility. Calmodulin activated proteins also include calcineurin, a phosphodiesterase, and a Ca2⁺-ATPase (not shown), which dephosphorylate proteins and reduce cGMP and Ca²⁺ terminating the motility cycle. By homology with other cells, PKG1β may downregulate the calcium signal as well by phosphorylating IP3R1, although intermediate protein-localized PKG1β activity in osteoclasts is uncharacterized.

Figure 2.

At least two large and highly regulated calcium channels mediate osteoclast retraction and detachment. The RyR2 is, in the osteoclast, a plasma membrane receptor that under some circumstances colocalizes with cell attachment. In contrast, the IP3R1 is localized to endoplasmic reticulum, where one of its major functions is transducing calcium signals secondary to nitric oxide. It is probable, although yet poorly studied, that calcium-activated calcium release, including that via Stim1 and Orai, may contribute to downstream calcium activation in some cases.