SummaryTranscription factor Nrf2 and its repressor Keap1 regulate a network of cytoprotective genes involving more than 1% of the genome, their best known targets being drug-metabolizing and antioxidant genes. Here we demonstrate a novel role for this pathway in directly regulating mitochondrial bioenergetics in murine neurons and embryonic fibroblasts. Loss of Nrf2 leads to mitochondrial depolarisation, decreased ATP levels and impaired respiration, whereas genetic activation of Nrf2 increases the mitochondrial membrane potential and ATP levels, the rate of respiration and the efficiency of oxidative phosphorylation. We further show that Nrf2-deficient cells have increased production of ATP in glycolysis, which is then used by the F1Fo-ATPase for maintenance of the mitochondrial membrane potential. While the levels and in vitro activities of the respiratory complexes are unaffected by Nrf2 deletion, their activities in isolated mitochondria and intact live cells are substantially impaired. In addition, the rate of regeneration of NADH after inhibition of respiration is much slower in Nrf2-knockout cells than in their wild-type counterparts. Taken together, these results show that Nrf2 directly regulates cellular energy metabolism through modulating the availability of substrates for mitochondrial respiration. Our findings highlight the importance of efficient energy metabolism in Nrf2-mediated cytoprotection.

See Dasgupta et al. (doi:) for a scientific commentary on this article. Shahni et al. identify a novel mutation in STAT2, which encodes a component of the JAK-STAT cytokine signalling pathway, in three patients with severe neurological deterioration following viral infection. STAT2 is shown to regulate mitochondrial fission, suggesting a new avenue for treatment of mitochondrial diseases and possibly common neurodegenerative disorders.

Objective: Charcot-Marie-Tooth (CMT) disease is the most common inherited neuromuscular disorder, affecting 1 in 2,500 individuals. Mitochondrial DNA (mtDNA) mutations are not generally considered within the differential diagnosis of patients with uncomplicated inherited neuropathy, despite the essential requirement of ATP for axonal function. We identified the mtDNA mutation m.9185TϾC in MT-ATP6, encoding the ATP6 subunit of the mitochondrial ATP synthase (OXPHOS complex V), at homoplasmic levels in a family with mitochondrial disease in whom a severe motor axonal neuropathy was a striking feature. This led us to hypothesize that mutations in the 2 mtDNA complex V subunit encoding genes, MT-ATP6 and MT-ATP8, might be an unrecognized cause of isolated axonal CMT and distal hereditary motor neuropathy (dHMN). Methods:A total of 442 probands with CMT type 2 (CMT2) (270) and dHMN (172) were screened for MT-ATP6/8 mutations after exclusion of mutations in known CMT2/dHMN genes. Mutation load was quantified using restriction endonuclease analysis. Blue-native gel electrophoresis (BN-PAGE) was performed to analyze the effects of m.9185TϾC on complex V structure and function.Results: Three further probands with CMT2 harbored the m.9185TϾC mutation. Some relatives had been classified as having dHMN. Patients could be separated into 4 groups according to their mutant m.9185TϾC levels. BN-PAGE demonstrated both impaired assembly and reduced activity of the complex V holoenzyme. Conclusions:We have shown that m.9185TϾC in MT-ATP6 causes CMT2 in 1.1% of genetically undefined cases. This has important implications for diagnosis and genetic counseling. Recognition that mutations in MT-ATP6 cause CMT2 enhances current understanding of the pathogenic basis of axonal neuropathy. Neurology ® 2012;79:1145-1154 GLOSSARY BN-PAGE ϭ blue-native polyacrylamide gel electrophoresis; CMT ϭ Charcot-Marie-Tooth; CMT2 ϭ Charcot-Marie-Tooth type 2; dHMN ϭ distal hereditary motor neuropathy; LS ϭ Leigh syndrome; mtDNA ϭ mitochondrial DNA; NARP ϭ neurogenic muscle weakness, ataxia, and retinitis pigmentosa; NCS ϭ nerve conduction studies; OXPHOS ϭ oxidative phosphorylation; UMN ϭ upper motor neuron.Mitochondrial ATP generation by oxidative phosphorylation (OXPHOS) underpins key molecular processes that are essential for normal central and peripheral nervous system axonal function. Axonal peripheral neuropathies are a well-recognized complication of primary mitochondrial DNA (mtDNA) mutations; however, the neuropathy is rarely the presenting or predominant clinical manifestation of the disease. [1][2][3][4][5][6][7][8] In contrast, mutations in the nuclearencoded mitochondrial genes MFN2 9 and GDAP1, 10 which encode outer mitochondrial membrane proteins, usually present with isolated peripheral neuropathy and are now recognized to be important causes of both the axonal and demyelinating forms of Charcot-Marie-Tooth (CMT) disease.One of the major unresolved challenges in neuromuscular diseases, such as CMT type 2 (CMT2) and distal heredit...

Disorders of coenzyme Q(10) (CoQ(10)) biosynthesis represent the most treatable subgroup of mitochondrial diseases. Neurological involvement is frequently observed in CoQ(10) deficiency, typically presenting as cerebellar ataxia and/or seizures. The aetiology of the neurological presentation of CoQ(10) deficiency has yet to be fully elucidated and therefore in order to investigate these phenomena we have established a neuronal cell model of CoQ(10) deficiency by treatment of neuronal SH-SY5Y cell line with para-aminobenzoic acid (PABA). PABA is a competitive inhibitor of the CoQ(10) biosynthetic pathway enzyme, COQ2. PABA treatment (1 mM) resulted in a 54 % decrease (46 % residual CoQ(10)) decrease in neuronal CoQ(10) status (p < 0.01). Reduction of neuronal CoQ(10) status was accompanied by a progressive decrease in mitochondrial respiratory chain enzyme activities, with a 67.5 % decrease in cellular ATP production at 46 % residual CoQ(10). Mitochondrial oxidative stress increased four-fold at 77 % and 46 % residual CoQ(10). A 40 % increase in mitochondrial membrane potential was detected at 46 % residual CoQ(10) with depolarisation following oligomycin treatment suggesting a reversal of complex V activity. This neuronal cell model provides insights into the effects of CoQ(10) deficiency on neuronal mitochondrial function and oxidative stress, and will be an important tool to evaluate candidate therapies for neurological conditions associated with CoQ(10) deficiency.

BackgroundThe autosomal-recessive cerebellar ataxias (ARCA) are a clinically and genetically heterogeneous group of neurodegenerative disorders. The large number of ARCA genes leads to delay and difficulties obtaining an exact diagnosis in many patients and families. Ubiquinone (CoQ10) deficiency is one of the potentially treatable causes of ARCAs as some patients respond to CoQ10 supplementation. The AarF domain containing kinase 3 gene (ADCK3) is one of several genes associated with CoQ10 deficiency. ADCK3 encodes a mitochondrial protein which functions as an electron-transfer membrane protein complex in the mitochondrial respiratory chain (MRC).MethodsWe report two siblings from a consanguineous Pakistani family who presented with cerebellar ataxia and severe myoclonus from adolescence. Whole exome sequencing and biochemical assessment of fibroblasts were performed in the index patient.ResultsA novel homozygous frameshift mutation in ADCK3 (p.Ser616Leufs*114), was identified in both siblings. This frameshift mutation results in the loss of the stop codon, extending the coding protein by 81 amino acids. Significant CoQ10 deficiency and reduced MRC enzyme activities in the index patient's fibroblasts suggested that the mutant protein may reduce the efficiency of mitochondrial electron transfer. CoQ10 supplementation was initiated following these genetic and biochemical analyses. She gained substantial improvement in myoclonic movements, ataxic gait and dysarthric speech after treatment.ConclusionThis study highlights the importance of diagnosing ADCK3 mutations and the potential benefit of treatment for patients. The identification of this new mutation broadens the phenotypic spectrum associated with ADCK3 mutations and provides further understanding of their pathogenic mechanism.