Mitofusin 2 (MFN2) is a protein of the mitochondrial outer membrane that belongs to a family of highly conserved dynamin-related GTPases. It is implicated in several intracellular pathways; however, its main role is the regulation of mitochondrial dynamics, in particular mitochondrial fusion. Mutations in MFN2 are associated with Charcot–Marie–Tooth disease type 2A (CMT2A), a neurological disorder characterized by a wide spectrum of clinical features, primarily a motor sensory neuropathy. The cellular and molecular mechanisms by which MFN2 mutations lead to neuronal degeneration are largely unknown, and there is currently no cure for patients. Here, we present the most recent in vitro and in vivo models of CMT2A and the more promising therapeutic approaches under development. These models and therapies may represent relevant tools for the study and recovery of defective mitochondrial dynamics that seem to play a significant role in the pathogenesis of other more common neurodegenerative diseases.
Kordelia BarbullushiElena AbatiFederica RizzoNereo BresolinGiacomo P. ComiStefania Corti
Mitofusin 2 (MFN2) plays critical roles in both mitochondrial fusion and the establishment of mitochondria-endoplasmic reticulum (ER) interactions. Hypothalamic ER stress has emerged as a causative factor for the development of leptin resistance, but the underlying mechanisms are largely unknown. Here, we show that mitochondria-ER contacts in anorexigenic pro-opiomelanocortin (POMC) neurons in the hypothalamus are decreased in diet-induced obesity. POMC-specific ablation of Mfn2 resulted in loss of mitochondria-ER contacts, ER stress-induced leptin resistance, hyperphagia, reduced energy expenditure, and obesity. Pharmacological relieve of hypothalamic ER stress reversed these metabolic alterations. Our data establish MFN2 in POMC neurons as an essential regulator of systemic energy balance by fine-tuning the mitochondrial-ER axis homeostasis and function. This previously unrecognized role for MFN2 argues for a crucial involvement in mediating ER stress-induced leptin resistance.[Schneeberger M, Dietrich MO, Sebastián D, Imbernón M, Castaño C, Garcia A, Esteban Y, Gonzalez-Franquesa A, Rodríguez IC, Bortolozzi A, Garcia-Roves PM, Gomis R, Nogueiras R, Horvath TL, Zorzano A, Claret M. Mitofusin 2 in POMC Neurons Connects ER Stress with Leptin Resistance and Energy Imbalance. Cell. 2013 Sep 26;155(1):172-87. doi: 10.1016/j.cell.2013.09.003. PubMed PMID: 24074867]
Mitochondria are key organelles in the maintenance of cellular energy metabolism and integrity. Here, we show that mitochondria number decrease but their size increase in orexigenic agouti-related protein (Agrp) neurons during the transition from fasted to fed to overfed state. These fusion-like dynamic changes were cell-type specific, as they occurred in the opposite direction in anorexigenic pro-opiomelanocortin (POMC) neurons. Interfering with mitochondrial fusion mechanisms in Agrp neurons by cell-selectively knocking down mitofusin 1(Mfn1) or mitofusin 2 (Mfn2) resulted in altered mitochondria size and density in these cells. Deficiency in mitofusins impaired the electric activity of Agrp neurons during high-fat diet (HFD), an event reversed by cell-selective administration of ATP. Agrp-specific Mfn1 or Mfn2 knockout mice gained less weight when fed a HFD due to decreased fat mass. Overall, our data unmask an important role for mitochondrial dynamics governed by Mfn1 and Mfn2 in Agrp neurons in central regulation of whole-body energy metabolism.
[Dietrich MO, Liu ZW, Horvath TL. Mitochondrial dynamics controlled by mitofusins regulate agrp neuronal activity and diet-induced obesity. Cell. 2013 Sep 26;155(1):188-99. doi: 10.1016/j.cell.2013.09.004. PubMed PMID: 24074868]
Type 2 diabetes mellitus (T2DM) is a major health issue that has reached epidemic status worldwide. Resistance to the pleiotropic effects of insulin represents a primary process in the development of the disease, but the molecular mechanisms leading to insulin resistance have not been elucidated completely. Literature data showed that MicroRNA-106b (miR-106b) is correlated closely with skeletal muscle insulin resistance and type 2 diabetes. In this study, the authors identiï¬ÂÂÂÂÂed Mfn2 as a direct target of miR-106b. Overexpression of miR-106b resulted in mitochondrial dysfunction and insulin resistance in C2C12 myotubes. MiR-106b was increased in insulin-resistant cultured C2C12 myotubes induced by TNF-a, and accompanied by increasing Mfn2 level, miR-106b loss of function improved mitochondrial function and insulin sensitivity impaired by TNF-a in C2C12 myotubes. In addition, MiR-106b targeted Mfn2 and regulated skeletal muscle mitochondrial function and insulin sensitivity. and demonstrated that miR-106b negatively regulated Mfn2 and skeletal muscle insulin sensitivity induced insulin resistance. These results suggest that miR-106b may represent a potential therapeutic target for the treatment of insulin resistance.
[Zhang Y, Yang L, Gao YF, Fan ZM, Cai XY, Liu MY, Guo XR, Gao CL, Xia ZK. MicroRNA-106b induces mitochondrial dysfunction and insulin resistance in C2C12 myotubes by targeting mitofusin-2. Mol Cell Endocrinol. 2013 Aug 14;381(1-2):230-240]
Alterations in mitochondrial dynamics (fission, fusion, and movement) are implicated in many neurodegenerative diseases, from rare genetic disorders such as Charcot-Marie-Tooth disease, to common conditions including Alzheimer's disease. However, the relationship between altered mitochondrial dynamics and neurodegeneration is incompletely understood. Here we show that disease associated MFN2 proteins suppressed both mitochondrial fusion and transport, and produced classic features of segmental axonal degeneration without cell body death, including neurofilament filled swellings, loss of calcium homeostasis, and accumulation of reactive oxygen species. By contrast, depletion of Opa1 suppressed mitochondrial fusion while sparing transport, and did not induce axonal degeneration. Axon degeneration induced by mutant MFN2 proteins correlated with the disruption of the proper mitochondrial positioning within axons, rather than loss of overall mitochondrial movement, or global mitochondrial dysfunction. We also found that augmenting expression of MFN1 rescued the axonal degeneration caused by MFN2 mutants, suggesting a possible therapeutic strategy for Charcot-Marie-Tooth disease. These experiments provide evidence that the ability of mitochondria to sense energy requirements and localize properly within axons is key to maintaining axonal integrity, and may be a common pathway by which disruptions in axonal transport contribute to neurodegeneration.
[Mitofusin2 Mutations Disrupt Axonal Mitochondrial Positioning and Promote Axon Degeneration. J Neurosci. 2012 Mar 21;32(12):4145-55. Albert L. Misko, Yo Sasaki, Elizabeth Tuck, Jeffrey Milbrand, and Robert H. Baloh
MFN2 and OPA1 genes encode two dynamin-like GTPase proteins involved in the fusion of the mitochondrial membrane. They have been associated with Charcot-Marie-Tooth disease type 2A and autosomal dominant optic atrophy, respectively. We report a large family with optic atrophy beginning in early childhood, associated with axonal neuropathy and mitochondrial myopathy in adult life. The clinical presentation looks like the autosomal dominant optic atrophy 'plus' phenotype linked to OPA1 mutations but is associated with a novel MFN2 missense mutation (c.629A>T, p.D210V). Multiple mitochondrial DNA deletions were found in skeletal muscle and this observation makes MFN2 a novel gene associated with 'mitochondrial DNA breakage' syndrome. Contrary to previous studies in patients with Charcot-Marie-Tooth disease type 2A, fibroblasts carrying the MFN2 mutation present with a respiratory chain deficiency, a fragmentation of the mitochondrial network and a significant reduction of MFN2 protein expression. Furthermore, we show for the first time that impaired mitochondrial fusion is responsible for a deficiency to repair stress-induced mitochondrial DNA damage. It is likely that defect in mitochondrial DNA repair is due to variability in repair protein content across the mitochondrial population and is at least partially responsible for mitochondrial DNA instability.
[The MFN2 gene is responsible for mitochondrial DNA instability and optic atrophy ‘plus’ phenotype. Brain. 2012 Jan;135(Pt 1):23-34. Cecile Rouzier, Sylvie Bannwarth, Annabelle Chaussenot, Arnaud Chevrollier, Annie Verschueren, Nathalie Bonello-Palot, Konstantina Fragaki, Aline Cano, Jean Pouget, Jean-Francois Pellissier, Vincent Procaccio, Brigitte Chabrol and Veronique Paquis-Flucklinger]