Time-course of mitochondrial gene expressions in mice brains: implications for mitochondrial dysfunction, oxidative damage, and cytochrome c in aging

The study of aging is critical for a better understanding of many age-related diseases. The free radical theory of aging, one of the prominent aging hypotheses, holds that during aging, increasing reactive oxygen species in mitochondria causes mutations in the mitochondrial DNA and damages mitochondrial components, resulting in senescence. Understanding a mitochondrial gene expression profile and its relationship to mitochondrial function becomes an important step in understanding aging. The objective of the present study was to determine mRNA expression of mitochondrial-encoded genes in brain slices from C57BL6 mice at four ages (2, 12, 18, and 24 months) and to determine how these altered mitochondrial genes influence age-related changes, including oxidative damage and cytochrome c in apoptosis. Using northern blot analysis, in situ hybridization, and immunofluorescence analyses, we analyzed changes in the expression of mitochondrial RNA encoding the mitochondrial genes, oxidative damage marker, 8-hydroxyguanosine (8-OHG), and cytochrome c in brain slices from the cortex of C57BL6 mice at each of the four ages. Our northern blot analysis revealed an increased expression of mitochondrial-encoded genes in complexes I, III, IV, and V of the respiratory chain in 12- and 18-month-old C57BL6 mice compared to 2-month-old mice, suggesting a compensatory mechanism that allows the production of proteins involved in the electron transport chain. In contrast to the up-regulation of mitochondrial genes in 12- and 18-month-old C57BL6 mice, mRNA expression in 24-month-old C57BL6 mice was decreased, suggesting that compensation maintained by the up-regulated genes cannot be sustained and that the down-regulation of expression results in the later stage of aging. Our in situ hybridization analyses of mitochondrial genes from the hippocampus and the cortex revealed that mitochondrial genes were over-expressed, suggesting that these brain areas are critical for mitochondrial functions. Our immunofluorescence analysis of 8-OHG and cytochrome c revealed increased 8-OHG and cytochrome c in 12-month-old C57BL6 mice, suggesting that age-related mitochondrial oxidative damage and apoptosis are associated with mitochondrial dysfunction. Our double-labeling analysis of in situ hybridization of ATPase 6 and our immunofluorescence analysis of 8-OHG suggest that specific neuronal populations undergo oxidative damage. Further, double-labeling analysis of in situ hybridization of ATPase 6 and immunofluorescence analysis of cytochrome c suggest cytochrome c release is related to mitochondrial dysfunction in the aging C57BL6 mouse brain. This study also suggests that these mitochondrial gene expression changes may relate to the role of mitochondrial dysfunction, oxidative damage, and cytochrome c in aging and in age-related diseases such as Alzheimer's disease and Parkinson's disease.     

Faithful tissue-specific expression of the human chromosome 21-linked <Emphasis Type="Italic">COL6A1</Emphasis> gene in BAC-transgenic mice

We created transgenic mice with a bacterial artificial chromosome (BAC) containing the human COL6A1 gene. In high-copy and low-copy transgenic lines, we found correct temporal and spatial expression of COL6A1 mRNA, paralleling the expression of the murine Col6a1 gene in a panel of nine adult and four fetal organs. The only exception was the fetal lung, in which the transgene was expressed poorly compared with the endogenous gene. Expression of COL6A1 mRNA from the transgene was copy number-dependent, and the increased gene dosage correlated with increased production of collagen VI alpha 1 in skin and heart, as indicated by Western blotting and immunohistochemistry. COL6A1 maps to Chromosome 21 and this gene has been a candidate for contributing to cardiac defects and skin abnormalities in Down syndrome. The low-copy and high-copy COL6A1 transgenics were born and survived in normal Mendelian proportions, without cardiac malformations or altered skin histology. These data indicate that the major promoter and enhancer sequences regulating COL6A1 expression are present in this 167-kb BAC clone. The lack of a strong cardiac or skin phenotype in the COL6A1 BAC-transgenic mice suggests that the increased expression of this gene does not, by itself, account for these phenotypes in Down syndrome.     

Manipulation of mitochondrial DNA gene expression in the mouse

Mitochondrial dysfunction due to impaired respiratory chain function is increasingly recognized as an important cause of human disease. Mitochondrial disorders are relatively common and have an estimated incidence of 1:10,000 live births. There are more than 100 different point mutations and numerous large rearrangements of mitochondrial DNA (mtDNA; mainly single deletions) that cause human disease. We aimed at obtaining an animal model to study physiological aspects of mtDNA mutation disorders. There are as yet unsolved technical problems associated with transfection of mammalian mitochondria. We therefore choose to manipulate mtDNA expression by targeting of the nuclear gene encoding Tfam. We utilised the cre-loxP recombination system to disrupt Tfam since this system allows manipulation of respiratory chain function in selected mouse tissues. We have found increased cell death or apoptosis induction in both germ line and tissue-specific Tfam knockouts. Our results further suggest that increased production of reactive oxygen species (ROS) is not a prominent feature in cells with impaired mtDNA expression.     

Abnormal mitochondria in Down syndrome iPSC-derived GABAergic interneurons and organoids

Down syndrome (DS) is caused by trisomy 21, and it is characterized by developmental brain disorders and neurological dysfunction. Clinical studies and basic research have revealed that defects in mitochondrial function contribute to the pathogenesis of DS. However, the underlying mechanisms of mitochondrial dysfunction in DS remain unclear. In this study, we first generated GABAergic interneurons and medial ganglionic eminence (MGE) organoids from DS patients and control induced pluripotent stem cells. The mitochondria were abnormally clustered in the perinuclear region of GABA neurons and cell in MGE organoids from DS patients, which exhibited impaired mitochondrial function as assessed by seahorse oxidative phosphorylation assay. Inhibition of the DSCAM-PAK1 pathway by gene editing or treatment with a small molecule corrected mitochondrial perinuclear aggregation in cells from DS patients. Therefore, our study provides insight into the potential mechanism of mitochondrial dysfunction in DS.     

Mitochondrial diseases and ATPase defects of nuclear origin

Dysfunctions of the F(1)F(o)-ATPase complex cause severe mitochondrial diseases affecting primarily the paediatric population. While in the maternally inherited ATPase defects due to mtDNA mutations in the ATP6 gene the enzyme is structurally and functionally modified, in ATPase defects of nuclear origin mitochondria contain a decreased amount of otherwise normal enzyme. In this case biosynthesis of ATPase is down-regulated due to a block at the early stage of enzyme assembly-formation of the F(1) catalytic part. The pathogenetic mechanism implicates dysfunction of Atp12 or other F(1)-specific assembly factors. For cellular energetics, however, the negative consequences may be quite similar irrespective of whether the ATPase dysfunction is of mitochondrial or nuclear origin.     

Growth failure in second-trimester fetuses with trisomy 21

Growth failure in the Down syndrome is common postnatally, but is thought to be less consistent in fetuses and newborns. We describe the growth of individual organs in 53 second-trimester abortuses with trisomy 21 and compare the organ weights to organ weights from 432 spontaneously aborted, but otherwise normal control specimens. Using multiple regression analysis, we found body weight to be the most significant predictor of all organ weights in normal fetuses; therefore, this variable was used to generate the regression lines to which the organ weights of trisomic specimens were compared. All trisomic fetal organs were found to be small, with an abnormal karyotype being a significant predictor of low organ weight. However, the effect on individual organs was variable, with some organs differing only minimally from the controls. Placental weights were not affected by fetal trisomy. This study demonstrates the presence of well-established, although variably severe, growth retardation in second-trimester fetuses with Down syndrome.     

Social smiling in normal and down syndrome infants: A comparative study

The aim of this work has been to compare social smiling in Down syndrome (mongolism) and normal infants, attending especially to the brow movements that appear before it. Facial responses of eight Down syndrome and eight normal infants from three to five months were analized by means of an anatomically based measurement technique during face-to-face interactions with their mothers. Despite their mental retardation, Down syndrome infants showed identical muscle movements as normal infants before and during smiling. However, some differences were found in smile frequency and leght, as well as in the brow movements frequency before smiling. Results are discussed in terms of the psychophisiological dysfunction of Down syndrome infants that are originated by a chromosome imbalance.