Restoring mitofusin balance prevents axonal degeneration in a Charcot-Marie-Tooth type 2A model
Yueqin Zhou, … , Cathleen M. Lutz, Robert H. Baloh
Yueqin Zhou, … , Cathleen M. Lutz, Robert H. Baloh
Published March 18, 2019
Citation Information: J Clin Invest. 2019;129(4):1756-1771. https://doi.org/10.1172/JCI124194.
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Research Article Neuroscience

Restoring mitofusin balance prevents axonal degeneration in a Charcot-Marie-Tooth type 2A model

Abstract

Mitofusin-2 (MFN2) is a mitochondrial outer-membrane protein that plays a pivotal role in mitochondrial dynamics in most tissues, yet mutations in MFN2, which cause Charcot-Marie-Tooth disease type 2A (CMT2A), primarily affect the nervous system. We generated a transgenic mouse model of CMT2A that developed severe early onset vision loss and neurological deficits, axonal degeneration without cell body loss, and cytoplasmic and axonal accumulations of fragmented mitochondria. While mitochondrial aggregates were labeled for mitophagy, mutant MFN2 did not inhibit Parkin-mediated degradation, but instead had a dominant negative effect on mitochondrial fusion only when MFN1 was at low levels, as occurs in neurons. Finally, using a transgenic approach, we found that augmenting the level of MFN1 in the nervous system in vivo rescued all phenotypes in mutant MFN2R94Q-expressing mice. These data demonstrate that the MFN1/MFN2 ratio is a key determinant of tissue specificity in CMT2A and indicate that augmentation of MFN1 in the nervous system is a viable therapeutic strategy for the disease.

Authors

Yueqin Zhou, Sharon Carmona, A.K.M.G. Muhammad, Shaughn Bell, Jesse Landeros, Michael Vazquez, Ritchie Ho, Antonietta Franco, Bin Lu, Gerald W. Dorn II, Shaomei Wang, Cathleen M. Lutz, Robert H. Baloh

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Figure 6

Increased expression of MFN1 rescued the mitochondrial accumulation, axon degeneration, gliosis, and transcriptomic changes seen in MFN2R94Q mice.

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Increased expression of MFN1 rescued the mitochondrial accumulation, axo...
(A) Mitochondrial accumulation labeled with CFP-COX8A in vivo (7-month-old mice). Scale bar: 100 μM. n = 2 mice/genotype. The photograph for MFN2R94Q:CFP-COX8A is the same as in Figure 3A. (B) Colocalization of clustered mitochondria with p62. Flag-MFN2R94Q (red), p62 (green), DAPI (blue). Scale bar: 50 μM. n = 3–4 mice/genotype. (C) Degenerating axons stained with Fluoro-Jade in the spinal cord (5-month-old mice). Scale bar: 50 μM. n = 2–3 mice/genotype. (D) Neurofilament in cross sections of optic nerve stained with SMI32 antibody. Scale bar: 50 μM (3-month-old mice). n = 3–4 mice/genotype. (E) Astrogliosis in brains (brain stem shown here) of 5-month-old mice. Gfap (green), Flag (red), and DAPI (blue). Scale bar: 50 μM. n = 3 mice/genotype. (F) Microglial staining in brains (cortex shown here) of 5-month-old mice. Iba1 (green), Flag (red), and DAPI (blue). Scale bar: 100 μM. n = 3 mice/genotype. (G) PCA of RNA-Seq data of lumbar spinal cord (10 months old). n = 3–5 mice/genotype. (H) Schematic diagram of a model for MFN1:MFN2 heterotypic interactions in cis in a torus-like conformation. The balance of MFN1 and MFN2 on mitochondria in different cell types determines the sensitivity to a dominant negative MFN2 mutant within a multimolecular complex (WT MFN shown with circular head, mutant MFN2 as a diamond). Altering the MFN1:MFN2 balance through increasing MFN1 protein in neurons restores mitochondrial fusion and other MFN functions despite the continued presence of mutant MFN2R94Q.