Mitochondrial translation failure represses cholesterol gene expression via Pyk2-Gsk3ß-Srebp2 axis.

Neurodegenerative diseases and other age-related disorders are closely associated with mitochondrial dysfunction. We previously showed that mice with neuron-specific deficiency of mitochondrial translation exhibit leukoencephalopathy because of demyelination. Reduced cholesterol metabolism has been associated with demyelinating diseases of the brain such as Alzheimer's disease. However, the molecular mechanisms involved and relevance to the pathogenesis remained unknown. In this study, we show that inhibition of mitochondrial translation significantly reduced expression of the cholesterol synthase genes and degraded their sterol-regulated transcription factor, sterol regulatory element-binding protein 2 (Srebp2). Furthermore, the phosphorylation of Pyk2 and Gsk3ß was increased in the white matter of p32cKO mice. We observed that Pyk2 inhibitors reduced the phosphorylation of Gsk3ß and that GSK3ß inhibitors suppressed degradation of the transcription factor Srebp2. The Pyk2-Gsk3ß axis is involved in the ubiquitination of Srebp2 and reduced expression of cholesterol gene. These results suggest that inhibition of mitochondrial translation may be a causative mechanism of neurodegenerative diseases of aging. Improving the mitochondrial translation or effectiveness of Gsk3ß inhibitors is a potential therapeutic strategy for leukoencephalopathy.

Improving lysosomal ferroptosis with NMN administration protects against heart failure.

Myocardial mitochondria are primary sites of myocardial energy metabolism. Mitochondrial disorders are associated with various cardiac diseases. We previously showed that mice with cardiomyocyte-specific knockout of the mitochondrial translation factor p32 developed heart failure from dilated cardiomyopathy. Mitochondrial translation defects cause not only mitochondrial dysfunction but also decreased nicotinamide adenine dinucleotide (NAD+) levels, leading to impaired lysosomal acidification and autophagy. In this study, we investigated whether nicotinamide mononucleotide (NMN) administration, which compensates for decreased NAD+ levels, improves heart failure because of mitochondrial dysfunction. NMN administration reduced damaged lysosomes and improved autophagy, thereby reducing heart failure and extending the lifespan in p32cKO mice. We found that lysosomal damage due to mitochondrial dysfunction induced ferroptosis, involving the accumulation of iron in lysosomes and lipid peroxide. The ameliorative effects of NMN supplementation were found to strongly affect lysosomal function rather than mitochondrial function, particularly lysosome-mediated ferroptosis. NMN supplementation can improve lysosomal, rather than mitochondrial, function and prevent chronic heart failure.

Mitochondrial dysfunction-induced high hCG associated with development of fetal growth restriction and pre-eclampsia with fetal growth restriction.

Fetal growth restriction (FGR) and pre-eclampsia with fetal growth restriction (PE/FGR) are high-risk perinatal diseases that may involve high levels of human chorionic gonadotropin (hCG) and mitochondrial dysfunction. However, little is known about how these factors affect placental function. We investigated how mitochondrial dysfunction and high hCG expression affected placental function in unexplained FGR and PE/FGR. We observed elevated expression of hCGß and growth differentiation factor 15 mRNA and protein levels in the placenta with both diseases. Likewise, antiangiogenic factors, such as Ang2, IP10, sFlt1, IL8, IL1B, and TNFα, were also upregulated at the mRNA level. In addition, the expression of COXI and COXII which encoded by mitochondrial DNA were significantly decreased in both diseases, suggesting that mitochondrial translation was impaired. Treatment with hCG increased Ang2, IP10, IL8, and TNFα mRNA levels in a dose-dependent manner via the p38 and JNK pathways. Mitochondrial translation inhibitors increased hCGß expression through stabilization of HIF1α, and increased IL8 and TNFα mRNA expression. These results revealed that high expression of hCG due to mitochondrial translational dysfunction plays an important role in the pathogenesis of FGR and PE/FGR.

Mitochondrial translation deficiency impairs NAD+ -mediated lysosomal acidification.

Mitochondrial translation dysfunction is associated with neurodegenerative and cardiovascular diseases. Cells eliminate defective mitochondria by the lysosomal machinery via autophagy. The relationship between mitochondrial translation and lysosomal function is unknown. In this study, mitochondrial translation-deficient hearts from p32-knockout mice were found to exhibit enlarged lysosomes containing lipofuscin, suggesting impaired lysosome and autolysosome function. These mice also displayed autophagic abnormalities, such as p62 accumulation and LC3 localization around broken mitochondria. The expression of genes encoding for nicotinamide adenine dinucleotide (NAD+ ) biosynthetic enzymes-Nmnat3 and Nampt-and NAD+ levels were decreased, suggesting that NAD+ is essential for maintaining lysosomal acidification. Conversely, nicotinamide mononucleotide (NMN) administration or Nmnat3 overexpression rescued lysosomal acidification. Nmnat3 gene expression is suppressed by HIF1α, a transcription factor that is stabilized by mitochondrial translation dysfunction, suggesting that HIF1α-Nmnat3-mediated NAD+ production is important for lysosomal function. The glycolytic enzymes GAPDH and PGK1 were found associated with lysosomal vesicles, and NAD+ was required for ATP production around lysosomal vesicles. Thus, we conclude that NAD+ content affected by mitochondrial dysfunction is essential for lysosomal maintenance.

Genomic characteristics revealed by targeted exon sequencing of testicular germ cell tumors in Japanese men.

OBJECTIVE: To investigate the somatic mutation profiles of testicular germ cell tumors in Japanese men. METHODS: We analyzed the somatic missense mutation profile of testicular germ cell tumors among 21 Japanese men with seminoma (n = 14), pure embryonic carcinoma (n = 3) and mixed testicular germ cell tumor (n = 4) by targeted next-generation sequencing of 409 cancer-related genes covering 1.23 Mb of the genome. RESULTS: We identified a total of 22 missense mutations in 21 primary testicular germ cell tumor samples (0.89 mutations/Mb), of which seven mutations were confirmed to be absent from the germline. KIT:p.Asn822Tyr, KIT:p.Leu576Pro, PIK3CA:p.Glu542Lys and FBXW7:p.Arg505His were statistically and functionally potential. A total of 18 missense mutations were previously unknown in testicular germ cell tumors. PDGFRA amplification from one patient with seminoma was detected. KIT, BCR,PIK3CG, PIK3CA and PDGFRA mutations involved in aberrant signaling of the KIT-PI3K-AKT pathway was detected in 27.3% of detected mutations. CONCLUSIONS: The present investigation identified a low mutation rate in testicular germ cell tumors among Asian patients, 18 novel mutations and PDGFRA amplification. Limitations of the present study are the small sample and missing normal DNA for some testicular germ cell tumors.

Mitochondrial translation inhibition triggers ATF4 activation, leading to integrated stress response but not to mitochondrial unfolded protein response.

Mitochondrial-nuclear communication, known as retrograde signaling, is important for regulating nuclear gene expression in response to mitochondrial dysfunction. Previously, we have found that p32/C1qbp-deficient mice, which have a mitochondrial translation defect, show endoplasmic reticulum (ER) stress response and integrated stress response (ISR) gene expression in the heart and brain. However, the mechanism by which mitochondrial translation inhibition elicits these responses is not clear. Among the transcription factors that respond to mitochondrial stress, activating transcription factor 4 (ATF4) is a key transcription factor in the ISR. Herein, chloramphenicol (CAP), which inhibits mitochondrial DNA (mtDNA)-encoded protein expression, induced eukaryotic initiation factor 2 α subunit (eIF2α) phosphorylation and ATF4 induction, leading to ISR gene expression. However, the expression of the mitochondrial unfolded protein response (mtUPR) genes, which has been shown in Caenorhabditis elegans, was not induced. Short hairpin RNA-based knockdown of ATF4 markedly inhibited the CAP-induced ISR gene expression. We also observed by ChIP analysis that induced ATF4 bound to the promoter region of several ISR genes, suggesting that mitochondrial translation inhibition induces ISR gene expression through ATF4 activation. In the present study, we showed that mitochondrial translation inhibition induced the ISR through ATF4 activation rather than the mtUPR.