A Research Of The Relationship Between Bmi-1Gene And Brain Ageing

Background:Previous studies suggest that Bmi-1(B-cell specific moloney leukemia virus insertion site1), a transcriptional repressor of polycomb group, serves as the common targets of cell cycle, oxidative stress, DNA damage repair and mitochondrial function, thus playing a key role in the aging process. However, it warrants addressing the relationship between Bmi-1gene and brain ageing.Objective:In order to provide evidence for delaying aging via targeting Bmi-1gene, the aim of this study was to investigate the relationship between Bmi-1gene and brain senescence by analyzing physiological, biochemical and morphological phenotypes of the three different types of mouse models including Bmi-1gene knockout (Bmi-1-/-) mice, Bmi-1gene knockdown (Bmi-1+/-) mice and their littermates wild-type (Bmi-1+/+) mice.Methods:1. In the first section, we detected Bmi-1and p53protein in Bmi-1+/+mice at1,4,5,9,20months old by immunohistochemical staining and western blotting.2. In the second section, we measured levels of hydroxy radical and malondialdehyde and immunoactivities for MBP and GFAP between2and4week-old Bmi-1-/-mice and their littermates.3. In the third section, by use of8-month old Bmi-1+/-and Bmi-1+/+mice, we compared their spatial learning and memory capabilities by use of Morris water maze test, oxidative stress levels by flow cytometry and biochemical analysis, histopathological changes of neurons and glia by immunohistochemistry and electron microscope, and expression of proteins related to Bmi-1by western blotting.Results:1. Immunohistochemistry and western blotting showed that Bmi-1expression in the cortex, dentate gyrus, cerebellum and olfactory bulb gradually increased and reached the highest level at4months after birth, then decreased gradually with the ageing process. Relative protein level of p53was gradually increased in the aging brain.2. Levels of hydroxy radical and MDA were high in4-week old Bmi-1-/-mice that exhibited neurodegeneration compared with WT mice, but only high hydroxy radical level occurred in2-week old Bmi-1-/-mice.3. Compared with Bmi-1+/+mice,8-month old Bmi-1+/-mice showed mild, but non-significant deficits in learning and memory capability in the Morris water maze test. There were mild increases in oxidative indexes of reactive oxygen species and malonaldehyde, and mild decreases in antioxidative indexes such as superoxide dismutase and total antioxidant capacity in Bmi-1-/-mice. But it is interesting that levels of hydroxy radical and reduced glutathione significantly increased in Bmi-1+/-mice. The hippocampus from Bmi-1+/-mice showed increases in degenerated or apoptotic pyramidal cells and lipofuscin deposition in the neuronal soma, but decreased densities of synaptophysin immunoreactive products, synaptic vesicles and mitochondria. Furthermore, a mild activation of hippocampal astrocytes and microglia was also observed. In addition, Bmi-1downstream genes p19, p27, and p53were up-regulated and anti-apoptotic protein Bcl-2was down-regulated in Bmi-1+/-hippocampus.Conclusion:1. Bmi-1is down-regulated and p53up-regulated during the normal aging process.2. Absence of Bmi-1causes brain oxidative stress, which may be a primary mechanism for premature neurodegeneration. 3. Bmi-1gene knockdown causes early brain ageing phenotype in adult mice. However, there also exsit some compensatory mechanisms such as astrocytes undergo activated and increased reduced glutathione synthesis, which may partially decrease oxidative damage caused by down-regulated Bmi-1expression.