An Experimental Study Of Age-related Myelination Decline On Memory Capacity In Adult Brains

Aging represents one challenge for the public healthcare system throughout the world.Statistics from the United Nations Department of Economic and Social Affairs,the number of people aged 60 years or over is predicted to grow by 56% from 901 million to 1.4 billion by 2030.Similarly,the elder population aged 65 and above in China reached 176.03 million in 2019,accounting for 12.6%.With life expectancy increasing,cognitive decline and dementia are more common than ever.Cognitive frailty is a great threat to human's health in twenty-first century.Magnetic resonance imaging(MRI)could find hyperintensities of white-matter in elderly brain.There are associations between the presence and severity of white-matter hyperintensities and cognitive function found by population-based cross-sectional studies.White-matter consists of myelinated nerve fiber in CNS.The myelin sheath around neuronal axons is formed by cell membrane emitted by oligodendrocytes.Most myelinated Oligdentrocytes differentiate from Oligdentrocyte precursor cells(OPCs)in perinatal period(Mice in the first 4 weeks after birth).The myelin sheath not only ensures rapid and accurate transmission of action potentials,but also provides nutritional support to neurons and regulates synaptic development.After adulthood,OPCs continue to proliferate and differentiate into new myelin.So what happens to myelination during aging?Is there a link with age-related cognitive decline?For this reason,we study the changes of myelination and its function in the aging process from the following four aspects.(1)Changes in myelin sheath and cognitive function during aging;(2)Changes of newly formed myelin in aging mice;(3)Using the conditonal knock-out Olig2 mouse model to simulate the formation of myelination in aging mice and to study the effect of insufficient myelination on cognitive function;(4)Using conditonal knock-out Chrm1 mouse and Clemastine to promote myelination and to test cognitive function.We used immunofluorescence and transmission electron microscope(TEM)to study myelination during aging and used Morris water maze to test memory function.The main results are as follows:1.Myelin degenerates and cognitive declines in aging mice.MBP expression and CC1 positive OLs declined with age,especially after 13-month-old.Myelin sheaths in the corpus callosum were observed by transmission electron microscopy(TEM)and found the myelinated axons' density in the corpus callosum was gradually decreasing from 3 to 22 months and the average g-ratio values were increasing with ages.4-and 13-month-old mice were challenged with the water maze task and a deficit in learning and memory was detected in the aging mice.2.Myelinogenesis is vanishing in aging brains.Studying the Cspg4-CreERT;Tau-mGFP transgenic mice,we found numerous mGFP-positive myelin sheaths were seen in the 6-or 8-month-old corpus callosum.By contrast,mGFPpositive myelin sheaths were sparsely distributed in the 18-and 22-month-old corpus callosum,even 5 months after recombination.we used PLP-CreERt;mTomato/mGFP(mT/mG)line to observe new myelin.Consistently,far fewer myelin sheaths were generated in the 13-month-old corpus callosum compared with the 4-and 8-month-old brains 10 d after recombination.As hippocampus is crucial for memory storage,we examined myelination in the hippocampal CA1 region.Our results indicate that newly formed myelin sheaths are greatly decreased in the CA1 region and neighboring corpus callosum in the middle-aged mice.To assess the stability of pre-existing myelin sheaths in mature adult mice,we used PLP-CreERt;mTomato/mGFP(mT/mG)line and found that the mGFP-positive areas were significantly decreased by ?10% in layers I–III of the 3+10-month cortex,suggesting that the majority of myelin sheaths that are generated before 3 months are stable over the subsequent 10 months.3.Active myelination is required for spatial memory capacity.To specifically manipulate myelination in mature adult mice,we used the Cspg4-CreERt;Mapt-mGFP;Olig2fl/fl transgenic mice to delete Olig2 in adult OPCs and also visualize newly generated myelin in 4-month-old mice.Conditional deletion of Olig2 in adult OPCs resulted in a significant decrease in mGFP-positive oligodendrocytes and myelin sheaths 4weeks after recombination in the corpus callosum,cortex and hippocampus of the Olig2 conditional knockout(cKO)brains.when compared with the heterozygote littermates.The Olig2 deletion did not significantly alter the number of mature oligodendrocytes(CC1-positive),OPCs(Cspg4-positive)or microglia(Iba1-positive)in the brains.After 3 days of training in the water maze,the Olig2 cKO mice exhibited a significantly lower frequency of crossing in the target sector.Consistently,the distance traveled in the platform quadrant and the time in the target sector were also significantly decreased in the Olig2 c KO mice compared with the littermate controls.These results demonstrate that active myelination is required for spatial memory.4.Enhancing myelination reverses the spatial memory deficit in aged mice.We next asked whether enhancing oligodendroglial differentiation and myelination represented a means to improve memory function in older mice.Our previous work implicates the muscarinic acetylcholine receptor 1(Chrm1)as a potent negative regulator of oligodendroglial differentiation and myelination.The Chrm1 deletion in OPCs resulted in a ?5-fold increase in mGFP-myelin expression in the corpus callosum,cortex and hippocampus at 18 months compared with age-matched wild types without affecting vascular density(CD31-positive)and microglia(Iba1-positive),indicating that the deletion of Chrm1 in OPCs can enhance differentiation and myelination even in the aging CNS.In keeping with the suggestion that age-related synaptic loss is an important indicator of declining hippocampal function,we found that the density of synaptic puncta(both synapsin1 and vGlut1)was remarkably increased in the CA1 region of the Chrm1 cKO hippocampus compared with the wild types.The Chrm1 cKO mice displayed significant improvement in water maze task performance,showing increased frequency of target sector crossing,and greater time and distance traveled in the platform quadrant.We treated 12-month-old mice with clemastine for 4 months.Clemastine treatment not only significantly enhanced myelination,but also preserved spatial memory capacity in the water maze test.Collectively,our data indicate that rejuvenating myelination can rescue synaptic loss in the hippocampus and improve memory function in aging mice.Our results suggest that both myelin degeneration and diminished renewal may contribute to the age-related decrease of white matter volume.Additionally,our findings indicate that spatial memory function also requires dynamic myelination in mature adult brains.Thus,the decline in memory can be attributed,at least in part,to diminished myelination in aging mice.Here,we find that enhancing endogenous myelination by Chrm1 deletion improves spatial memory function in old mice.As a proof of concept,we have shown that clemastine treatment reverses age-related memory decline.For all these reasons,our study found myelination is requisite for cognitive function and enhancing myelination represents a promising strategy for age-related cognitive decline.