The tethering function of mitofusin2 controls osteoclast differentiation by modulating the Ca2+–NFATc1 axis

Abstract

Dynamic regulation of the mitochondrial network by mitofusins (MFNs) modulates energy production, cell survival, and many intracellular signaling events, including calcium handling. However, the relative importance of specific mitochondrial functions and their dependence on MFNs vary greatly among cell types. Osteoclasts have many mitochondria, and increased mitochondrial biogenesis and oxidative phosphorylation enhance bone resorption, but little is known about the mitochondrial network or MFNs in osteoclasts. Because expression of each MFN isoform increases with osteoclastogenesis, we conditionally deleted MFN1 and MFN2 (double conditional KO (dcKO)) in murine osteoclast precursors, finding that this increased bone mass in young female mice and abolished osteoclast precursor differentiation into mature osteoclasts in vitro. Defective osteoclastogenesis was reversed by overexpression of MFN2 but not MFN1; therefore, we generated mice lacking only MFN2 in osteoclasts. MFN2-deficient female mice had increased bone mass at 1 year and resistance to Receptor Activator of NF-κB Ligand (RANKL)-induced osteolysis at 8 weeks. To explore whether MFN-mediated tethering or mitophagy is important for osteoclastogenesis, we overexpressed MFN2 variants defective in either function in dcKO precursors and found that, although mitophagy was dispensable for differentiation, tethering was required. Because the master osteoclastogenic transcriptional regulator nuclear factor of activated T cells 1 (NFATc1) is calcium-regulated, we assessed calcium release from the endoplasmic reticulum and store-operated calcium entry and found that the latter was blunted in dcKO cells. Restored osteoclast differentiation by expression of intact MFN2 or the mitophagy-defective variant was associated with normalization of store-operated calcium entry and NFATc1 levels, indicating that MFN2 controls mitochondrion–endoplasmic reticulum tethering in osteoclasts.

Mitochondrial Sirt3 contributes to the bone loss caused by aging or estrogen deficiency

“…We next examined whether Sirt3 deletion altered the abundance of proteins involved in mitochondria dynamics and mitophagy. Mitofusin2 mediates the tethering of adjacent mitochondria, as well as the tethering of mitochondria to the endoplasmic reticulum (ER), and promotes osteoclast differentiation ( 35 ). Sirt3 deletion had no impact on the protein levels of Mitofusin2 in BMM stimulated with RANKL ( Figure 6E ).…”

Dynamin‐related protein 1 positively regulates osteoclast differentiation and bone loss

“…The role of Mfns in osteoclasts has been previously reported [24,25]. Dual deletion of Mfn1 and Mfn2 in the osteoclast lineage leads to increased bone mass in female mice in vivo [25].…”

“…Dual deletion of Mfn1 and Mfn2 in the osteoclast lineage leads to increased bone mass in female mice in vivo [25]. Loss of Mfn2 also leads to increased bone mass in vivo [25]. Mechanically, Mfn2 mediates connections between endoplasmic reticulum and mitochondria, which generates large fluctuations in cytoplasmic Ca 2+ that drives NFATc1 activation [24,25].…”

“…Loss of Mfn2 also leads to increased bone mass in vivo [25]. Mechanically, Mfn2 mediates connections between endoplasmic reticulum and mitochondria, which generates large fluctuations in cytoplasmic Ca 2+ that drives NFATc1 activation [24,25]. Since Mfn2 acts as the molecular tether that connects mitochondria to the endo/sarcoplasmic reticulum, in addition to promoting mitochondrial fusion [26], it is unclear whether the mitochondrial fusion function of Mfn2 plays some role during osteoclast formation.…”

Bone Disease in Nephropathic Cystinosis: Beyond Renal Osteodystrophy

“…In line with these observations, osteoclasts are multinucleated giant cells that contain a very high density of mitochondria, and increased mitochondria biogenesis and oxidative phosphorylation increase bone resorption. Interestingly enough, very recent studies revealed that mitofusin 2 facilitates osteoclastogenesis by modulating calcium -calcineurin -NFATc1 (Nuclear factor of activated T-cell cytoplasm1); thus, it is tempting to propose that mitochondria function might be damaged in CTNS deficient osteoclasts [37,38]. The study of proximal tubular kidney cells obtained from healthy donors and patients with cystinosis reveals that cystinosin deficiency is associated with altered mammalian target of rapamycin complex 1 (mTORC1) signaling, characterized by abnormal lysosomal retention of mTORC1 during starvation followed by delayed reactivation of mTORC1 [39].…”