Zijing Sheng

Programmed cell death triggered by nucleotide pool damage and its prevention by MutT homolog-1 (MTH1) with oxidized purine nucleoside triphosphatase.

Accumulation of oxidized bases such as 8-oxoguanine in either nuclear or mitochondrial DNA triggers various cellular dysfunctions including mutagenesis, and programmed cell death or senescence. Recent studies have revealed that oxidized nucleoside triphosphates such as 8-oxo-dGTP in the nucleotide pool are the main source of oxidized bases accumulating in the DNA of cells under oxidative stress. To counteract such deleterious effects of nucleotide pool damage, mammalian cells possess MutT homolog-1 (MTH1) with oxidized purine nucleoside triphosphatase and related enzymes, thus minimizing the accumulation of oxidized bases in cellular DNA. Depletion or increased expression of the MTH1 protein have revealed its significant roles in avoiding programmed cell death or senescence as well as mutagenesis, and accumulating evidences indicate that MTH1 is involved in suppression of degenerative disorders such as neurodegeneration.

8-Oxoguanine accumulation in mitochondrial DNA causes mitochondrial dysfunction and impairs neuritogenesis in cultured adult mouse cortical neurons under oxidative conditions

Oxidative stress and mitochondrial dysfunction are implicated in aging-related neurodegenerative disorders. 8-Oxoguanine (8-oxoG), a common oxidised base lesion, is often highly accumulated in brains from patients with neurodegenerative disorders. MTH1 hydrolyses 8-oxo-2′-deoxyguanosine triphosphate (8-oxo-dGTP) to 8-oxo-dGMP and pyrophosphate in nucleotide pools, while OGG1 excises 8-oxoG paired with cytosine in DNA, thereby minimising the accumulation of 8-oxoG in DNA. Mth1/Ogg1-double knockout (TO-DKO) mice are highly susceptible to neurodegeneration under oxidative conditions and show increased accumulation of 8-oxoG in mitochondrial DNA (mtDNA) in neurons, suggesting that 8-oxoG accumulation in mtDNA causes mitochondrial dysfunction. Here, we evaluated the contribution of MTH1 and OGG1 to the prevention of mitochondrial dysfunction during neuritogenesis in vitro. We isolated cortical neurons from adult wild-type and TO-DKO mice and maintained them with or without antioxidants for 2 to 5 days and then examined neuritogenesis. In the presence of antioxidants, both TO-DKO and wild-type neurons exhibited efficient neurite extension and arborisation. However, in the absence of antioxidants, the accumulation of 8-oxoG in mtDNA of TO-DKO neurons was increased resulting in mitochondrial dysfunction. Cells also exhibited poor neurite outgrowth with decreased complexity of neuritic arborisation, indicating that MTH1 and OGG1 are essential for neuritogenesis under oxidative conditions.

Molecular pathophysiology of impaired glucose metabolism, mitochondrial dysfunction, and oxidative DNA damage in Alzheimer's disease brain

In normal brain, neurons in the cortex and hippocampus produce insulin, which modulates glucose metabolism and cognitive functions. It has been shown that insulin resistance impairs glucose metabolism and mitochondrial function, thus increasing production of reactive oxygen species. Recent progress in Alzheimers disease (AD) research revealed that insulin production and signaling are severely impaired in AD brain, thereby resulting in mitochondrial dysfunction and increased oxidative stress. Among possible oxidative DNA lesions, 8-oxoguanine (8-oxoG) is highly accumulated in the brain of AD patients. Previously we have shown that incorporating 8-oxoG in nuclear and mitochondrial DNA promotes MUTYH (adenine DNA glycosylase) dependent neurodegeneration. Moreover, cortical neurons prepared from MTH1 (8-oxo-dGTPase)/OGG1 (8-oxoG DNA glycosylase)-double deficient adult mouse brains is shown to exhibit significantly poor neuritogenesis in vitro with increased 8-oxoG accumulation in mitochondrial DNA in the absence of antioxidants. Therefore, 8-oxoG can be considered involved in the neurodegenerative process in AD brain. In mild cognitive impairment, mitochondrial dysfunction and oxidative damage may induce synaptic dysfunction due to energy failures in neurons thus resulting in impaired cognitive function. If such abnormality lasts long, it can lead to vicious cycles of oxidative damage, which may then trigger the neurodegenerative process seen in Alzheimer type dementia.

Mitochondrial MUTYH-dependent striatal degeneration

DJ-1 was originally identified as a novel oncogene in our laboratory and later found to be a causative gene of a familial form of Parkinson’s disease (PD), PARK7. DJ-1 plays multiple roles in cell survival under the oxidative condition through transcriptional regulation, anti-oxidant activity, chaperone activity, translational regulation and maintenance of mitochondrial integrity. Consistent with these functions, DJ-1 localizes both in the nucleus and cytoplasm and also oxidative stress has shown to lead DJ-1 translocation to mitochondria. The detailed molecular function of DJ-1, however, remains unclear.In animal models, inhibition of mitochondrial complex I by rotenone, complex I inhibitor, causes Parkinsonism, suggesting that production of reactive oxygen species by electron leakage following complex I dysfunction causes a selective neuronal cell death.We have previously reported that reduced expression of DJ-1 by RNAi decreased mitochondrial complex I activity and that DJ-1 directly binds to NDUFA4, a components of complex I, suggesting that DJ-1 has a role in regulating mitochondria complex I activity through the direct interaction to the complex.To further investigate functions of mitochondrial DJ-1, we analyzed the mitochondrial membrane potential in DJ-1-deficient cells using TMRM in the presense or absence of oligomycin and found that significant decrease of membrane potential was observed in DJ-1-deficient fibroblast. We are now addressing if DJ-1 deficiency affects functions of other PD genes.

Mitochondrial toxin, 3-nitropropionic acid induces MUTYH-dependent striatal degeneration

ALS is a progressive disease with selective motor neuron <MN> death. The cell death of MN in ALS is supposed to be not fully consistent with apoptosis, necrosis, or autophagy. Recently, it was reported that the shift of balance between YAP Cs as prosurvival factors and activated p73 promotes apoptosis in transcriptional repression-induced atypical death <TRIAD>. In this study, G93ASOD1 Tg mice were examined to investigate for possible relationships between the mechanism of MN death in ALS and TRIAD. The levels of YAP Cs in the spinal cords of Tg mice decreased with disease progression, whereas FL-YAP, a p73 cofactor that promotes apoptosis, was preserved. Also, the ratio of phosphorylation of p73 increased in Tg mice. Our results suggest that the progressive decrease in the levels of YAP Cs and the relative increase in phosphorylated p73 are correlated with disease progression in ALS model mice.