Myelin Renewal In Optic Nerve During Aging And Its Functional Concequence

In light of the increasing size of the aged population,aging turns to be an important risk factor for the decline of visual function.To retard the visual functional deficits during aging,many efforts have been made to investigate the various factors that influence age-related structural and functional changes in the optic nerve.However,the mechanism of age-related changes in the optic nerve remains unclear.RGCs are the projection neurons of the retina.Its bodies are located in the retina GCL layer,and its axons converge to form the optic nerve.Oligodendrocytes concentrically and repeatedly wrapping axons and then myelin sheaths generated.Studies in recent years have shown that action potential can be transmitted rapidly and efficiently through myelin sheaths,moreover,myelin sheaths can provide nutrition and support for axons.Previous studies of our group found that myelin degeneration and reduced myelin regeneration can lead to age-related memory deficits,suggesting that myelin plays an important role in the aging of the nervous system.However,the specific role of myelin sheath in the decline of visual function remains unclear.Therefore,in view of the role of myelin in the decline of visual function during aging,the following three questions are proposed in this paper:The change of optic nerve myelin during aging?The effects of declined newly-formed myelin on retinal and optic nerve structure and function?The effect of myelination drugs treatment on optic nerve structure and function.Three aspects were carried out.(1)With Cspg4-CreERT;mT/mG mouse and wildtype mouse,we can investigate age-associated changes of oligodendrocyte,oligodendrocyte precursor cells,RGCs axons,newly-formed myelin and visual conduction ability of mice.(2)Through Olig2-Knockout mouse,we can investigate the relationship between impaired myelination and optic function.(3)Myelination drugs clemastine were treated with aged mice and explore its effect on optic function.In this thesis,immunofluorescent staining was mainly used for investigating the changes of optic nerve myelin,fVEP was mainly used for investigating the changes of visual function,OCT and immunostaining in Whole-mount retina were mainly used for investigating the changes of axon.Main results were listed as follows:1.The regeneration ability of optic nerve myelin decreased and the visual conduction function was impaired in aging mice.Firstly,through immunostaining in Whole-mount retina and optic nerve,compared with4 months group,no significant difference was found in NG2~+cell(OPCs)counting number in13 months group,but CC1~+cells(OLs)counting number,axon area ofβ-tublin~+(RGCs),the newly-formed myelin was significantly decreased in 13 months group.Next,to verify the changes of visual conduction function,fVEP test was carried out,and the latency of P2 wave in 13 months group was prolonged.Moreover,to verify the number of newly-formed myelin,we used NG2 staining on the Cspg4-CreERT;mT/mG mouse,and newly-formed myelin in 4months and 13 months mice were compared,then we found newly formed myelin was significantly decreased in 13 month mice.2.Inhibition of myelination of optic nerve in mice resulted in structural and functional damage of optic nerve.In order to clarify the role of myelin regeneration in aging process,Olig2,a key factor of regulating differentiation,was conditionally knocked out in oligodendrocyte progenitor cells,and tau-MGFP was used to observe the changes of myelin.Knocking out for 1 month,we found almost 80%newly-formed myelin decreased,confirming that Olig2 knockout can reduce optic nerve remyelination.Next,knocking out for 9 month,whole-mount retina immunostaining,fVEP and OCT were tested,and we found the P2 latency in Olig2-knockout mice was significantly prolonged,the retinal RNFL layer was thinner and axon area ofβ-tublin~+decreased too.In conclusion,Olig2 knockout significantly reduces the number of newly-formed myelin sheath of optic nerve,and damages the structure and functions of optic nerve.3.Clemastine enhances myelination and partially rescues age-related visual function declines.We next asked whether enhancing oligodendroglial myelination represents an approach to improve visual function in aged mice.13-month-old Cspg4-CreERT;Tau-mGFP mice were continuously treated with clemastine for 1 months.Consistent with previous evidence that clemastine could promote oligodendroglial differentiation and myelination,we found clemastine treatment significantly increase the newly-formed myelin sheath in optic nerve.Then 13-month-old wildtype mice were continuously treated with clemastine for 5 months,and the latency of P2 wave in mice treated with clemastine was significantly shortened.Meanwhile,theβ-tublin staining onsequence indicate that the loss of RGCs axons was reduced after clemastine treatment.In conclusion,clemastine can significantly increase the amount of newly-formed myelin in the optic nerve and partly reduce the functional impairment of retinal and optic nerve structures during aging.In conclusion,at the beginning we demonstrated age-related changes in the structure and function of the optic nerve;Next we discover age-related decline of newly-formed myelin in optic nerve;Then to directly access the specific functional significance of myelination in optic nerve,we use Olig2 conditional knockout mice,and the result shows hypomyelination alone is sufficient to impair visual function;Next,We further identified that latency of P2 wave shortened in mice treated with clemastine,suggesting enhancing myelination is a feasible strategy for visual function aging deficit.Taking together,our study found myelination is an important regulator for visual function aging deficit and enhancing myelination represents a promising strategy for structural and functional recovery in visual function aging deficit.