Study On Effect Of Pubertal Bisphenol-A Exposure On Age-Related Cognitive Changes And Its Mechanism In Mice

BackgroundDue to the increasing number of elderly individuals in the world,age-related cognitive decline as well as the neurodegenerative disorders,such as Alzheimer's disease,has become a prominent public health problem.However,there is no effective treatment for these diseases.Thus,it is extremely important to explore the factors and the mechanisms responsible for age-related disseases.Mounting evidence suggests that exposure to environmental endocrine disruptors(EEDs)during the stages of brain development has been associated with neurodegenerative disorders.Among EEDs,bisphenol-a(BPA)is one of the chemicals produced in the highest volumes worldwide.Many findings from animal studies have documented that perinatal exposure to BPA may disrupt neurodevelopment,affect neuronal plasticity in the hippocampus,and thereby cause impaired cognitive function of the offspring.In recent years,there also has been a growing interest in the negative effects of BPA exposure during puberty,which is regarded as the most important period of postnatal neurodevelopment.Studies have shown that pubertal BPA exposure is associated with lower cognitive performance tested in adolescence and adulthood.However,no studies to date have investigated the association into old age.Therefore,an experimental study on the effect of pubertal BPA exposure on age-related cognitive and biochemical changes would help us to understand the mechanisms of the brain aging and provide new insight for treatment and prevention of neurodegenerative diseases.ObjectiveThe aim of this study is to explore the effect of pubertal BPA exposure on the age-related cognitive and biochemical changes in the CD-1 mice,and preliminary exploration of epigenetic modification mechanisms whether involved in these changes.Methods(1)After acclimatization for one week,three-week-old male and female CD-1mice were randomly divided into an old control group and two BPA groups,with 16mice(8 males,8 females)in each group.The mice in the low-dose BPA and high-dose BPA groups received water with BPA(1 and 100 mg/L,respectively)as the sole drinking fluid for 4 weeks during puberty(from postnatal days 28 to 56).When the mice reached old age,they underwent behavioral testing at 18 months of age.Any animals with movement disorders,hair removal,or any visible tumor were excluded before the tests.The final number of mice was 11 in the old control group(6 males,5females),14 in the low-dose BPA group(7 males,7 females),and 13 in the high-dose BPA group(7 males,6 females).Moreover,in order to analyze the age effect,12 mice(6 males,6 females)at the age of 6-month undertook the same tests as young controls.The behavioral tests included: sensorimotor tasks(beam walking),spontaneous exploration and anxiety tasks(open field,elevated plus maze),spatial learning and memory(novel location recognition,radial six-arm water maze).(2)The mice were anesthetized with halothane and killed 15 days after the behavioral tests.The blood samples were obtained by quickly removing the eyeball from the socket.After decapitation,the brain tissues were rapidly removed from the skull and embedded with paraffin into blocks for immunohistochemistry.The serum concentrations of sex hormones [testosterone(only in males)and estradiol(only infemales)] and thyroid hormones [free triiodothyronine(FT3)and free thyroxin(FT4)in all mice] were tested by enzyme-linked immunosorbent assay.The levels of the astrocyte marker glial fibrillary acidic protein(GFAP),the presynaptic proteins synaptotagmin(Syt)-1 and syntaxin-1 in the different layers of hippocampus were detected by immunohistochemical staining.(3)The levels of total histone H3,total histone H4,histones H3 lysine 9 and H4 lysine 8 acetylation(H3K9ac,H4K8ac),histone H3 lysine 9 tri-methylation(H3K9me3),and histone H3 serine 10 phosphorylation(H3S10phos)were detected by immunohistochemistry in the cell layer of dentate gyrus(DG),Cornu Ammonis(CA)1and CA3 of hippocampus.Results(1)The CD-1 mice displayed age-related decline in sensorimotor and spatial cognitive abilitiy,and increase in anxiety.Pubertal BPA exposure could accelerate the age-related spatial cognitive impairment and increased anxiety,and the 100 mg/l BPA group showed more significant impact.(2)The CD-1 mice displayed age-related decrease in the levels of serum sex and thyroid hormones and hippocampal syntaxin-1,and increases in the hippocampal levels of GFAP and Syt-1.Pubertal BPA exposure accelerated these age-related changes,and the 100 mg/l BPA group showed more significant impact.The levels of serum FT4 as well as hippocampal Syt-1 and syntaxin-1 significantly correlated with the spatial memory performances.(3)The CD-1 mice displayed age-related decrease in the hippocampal levels of H3K9 ac,H4K8ac and H3S10 phos,and increase in hippocampal H3K9me3 level.Pubertal BPA exposure accelerated age-related decrease in the levels of H3K9 ac andH4K8ac of the hippocampus,and increase in hippocampal H3K9me3 level,and the 100mg/l BPA group showed more significant impact.The hippocampal levels of H3K9 ac,H4K8ac and H3K9me3 were significantly correlated with the cognitive ability.SummaryPubertal BPA exposure could accelerate the behavioral and neurobiochemical changes in the CD-1 mice.Epigenetic modification mechanisms maybe involved in these changes.