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

Area of Science:

• Cell Biology
• Mitochondrial Dynamics
• Skeletal Biology

Background:

• Mitofusins (MFNs) regulate mitochondrial networks, impacting cell functions like energy production and calcium signaling.
• Osteoclasts, critical for bone resorption, possess numerous mitochondria, yet their MFN-dependent functions remain largely uncharacterized.
• MFN expression increases during osteoclastogenesis, suggesting a role in this process.

Purpose of the Study:

• To investigate the role of mitofusins (MFNs), specifically MFN1 and MFN2, in osteoclast differentiation and function.
• To determine whether MFN-mediated mitochondrial tethering or mitophagy is essential for osteoclastogenesis.
• To elucidate the impact of MFN deficiency on bone mass and susceptibility to osteolysis.

Main Methods:

• Conditional deletion of MFN1 and MFN2 (double conditional KO, dcKO) in murine osteoclast precursors.
• Overexpression of MFN2 and MFN1 in dcKO precursors to assess rescue effects.
• Generation of mice lacking only MFN2 in osteoclasts.
• Analysis of osteoclast differentiation, bone mass, and response to RANKL-induced osteolysis.
• Assessment of mitochondrial tethering and mitophagy functions using MFN2 variants.
• Measurement of calcium signaling, including store-operated calcium entry and endoplasmic reticulum calcium release.

Main Results:

• Conditional deletion of MFN1 and MFN2 in osteoclast precursors abolished differentiation in vitro and increased bone mass in young female mice.
• Overexpression of MFN2, but not MFN1, rescued the defective osteoclastogenesis.
• MFN2-deficient mice exhibited increased bone mass and resistance to RANKL-induced osteolysis.
• MFN-mediated mitochondrial-endoplasmic reticulum tethering, not mitophagy, was essential for osteoclast differentiation.
• MFN deficiency impaired store-operated calcium entry, which was restored by MFN2 re-expression, normalizing NFATc1 levels.

Conclusions:

• MFN2 is critical for osteoclast differentiation and function, primarily through its role in maintaining mitochondrion-endoplasmic reticulum tethering.
• MFN2 deficiency in osteoclasts leads to increased bone mass and protection against bone loss.
• Targeting MFN2 in osteoclasts represents a potential therapeutic strategy for bone diseases characterized by excessive bone resorption.