Impaired Oligodendrocyte Development Regulated By Schizophrenia Risk Factors Leads To Mental Disorder Like Behaviors

Schizophrenia is one of the most severe psychiatric disorders.Even though both environmental factors and genetic factors have been proved to contribute to schizophrenia onset,the etiology is still elusive and becomes a prevailing issue.As we know,except for neuronal abnormalities during the pathogenesis of schizophrenia,the contribution of oligodendrocyte(OL)and myelin on the complex etiology has been well-recognized.Oligodendrocytes are differentiated from oligodendrocyte precursor cells(OPCs),which can proliferate and bidirectionally differentiate into OLs or astrocytes.The maturation of OL is necessary for myelination in central nervous system(CNS).Except for the traditional function of OL,which is to form myelin sheath around axons,recent studies uncovered that OL can provide trophic support to neurons and regulate neuronal functions.Moreover,myelin plasticity has been considered to play important roles in some higher brain functions,such as motor skill learning.Abnormalities of OL and myelin have not only been found in demyelinating disease,such as multiple sclerosis and leukodystrophies,but also in major mental illnesses.Decreased OL density and myelin related genes expression have been reported in postmortem brains from schizophrenia,bipolor disorder and depression.Moreover,abnormal myelin structure and white matter disruptions have been identified in brain imaging studies.In fact,the first episode schizophrenia patients already displayed compromised white matter integrity in cortical and subcortical brain regions,suggesting that OL impairment is involved in early schizophrenia pathogenesis.However,the contribution of OL or myelin abnormalities on the etiology of schizophrenia is still lack of solid experimental evidence.Notably,accumulating studies identified numerous schizophrenia risk genes,supporting the idea that the genetic factors are involved in the pathogenesis of schizophrenia.Some schizophrenia risk genes are specifically expressed in OL,which plays important roles in OL and myelin development,such as Olig2.While whether malfunction of these OL genes could cause psychiatric symptoms is unknown.Besides,the function of schizophrenia risk genes has been primarily investigated in neurons.Whether malfunction of the same risk genes underlies OL impairment in schizophrenia remains elusive.For example,FEZ1(Fasciculation and Elongation Protein Zeta-1),a well-defined schizophrenia risk gene,has been found to play important roles in neuronal axon and dendrite development.However,whether FEZ1 is expressed in OL and whether FEZ1 deficiency in schizophrenia could interfere with OL development or function is vastly unknown.In this study,we specifically knocked out schizophrenia risk gene Olig2 in OL,causing OL and myelin impairments.Using this mouse model,we explored how OL or myelin deficit caused by Olig2 deficiency could affect brain function and psychiatric behaviors at different ages.Besides,we identified the expression and function of schizophrenia risk gene FEZ1 in OL and decoded the coordination of multiple schizophrenia-affected genes that converge on regulation of FEZ1 espression in OL,offering a model to explain how malfunction of distinct risk factors can lead to common abnormalities in the pathogenesis of psychiatric disorders.Main results are listed below:1.Deleting Olig2 impaired OL development and caused aberrantly increased cortical glutamate and anxiety-like behaviors in juvenile miceWe specifically knocked out Olig2 in OL by cross CNP-Cre mice with floxed Olig2 mice.Ablating Olig2 impaired OL maturation and led to a robust decrease of mature OL density in cortex from juvenile mice(postnatal day21,P21).Taking advantage of 1H-MRS,we found that glutamate level was aberrantly increased in cortex from juvenile Olig2 c KO mice.Moreover,glutamatergic synapses of Olig2 c KO cortical neurons contained higher density of synaptic vehicles,implying increased glutamate release upon synaptic activation.Importantly,Juvenile Olig2 c KO mice showed anxious phenotype in both open field test and elevated plus maze test(EPM),and impulsivity-like behaviors in cliff avoidance reaction test(CAR).These results indicated that impaired OL maturation could interfere with glutamatergic function and cause anxiety related phenotypes.2.Ablating Olig2 caused myelin deficits and led to cognitive impairments and schizophrenia like behaviorsSpecifically deleting Olig2 in OL caused persistent myelin developmental deficiency in Olig2 c KO mice.Robust loss of myelin sheath was found in anterior cingulate cortex(ACC)from adult Olig2 c KO mice(P56).Moreover,myelin deficits led to dysregulation of nodes of Ranvier related genes in ACC.Adult Olig2 c KO mice showed intact locomotor activity but mild anxiety.Most importantly,adult Olig2 c KO mice clearly showed sensorimotor gating deficit and non-spatial memory impairment,strongly suggesting cognitive deficits and schizophrenia like behaviors.Besides,as compared with WT littermates,Olig2 KO mice showed increased risk of social withdrawal after social isolation during adolescence,indicating that developmental myelin deficit could increase the risk of pivotal phenotypes reminiscent of schizophrenia.3.Schizophrenia risk gene FEZ1 is necessary for OL processes developmentImmunostaining of brain slices and cultured cells showed that FEZ1 protein was expressed in mouse,rat and human OLs at various developmental stages.FEZ1 protein was located in cytoplasm,deposited to the growing processes and enriched in the enlarged tips.Based on Z-stack analyses,we found FEZ1 to be colocalized with F-actin and microtubule tract,indicating that FEZ1 could interact with cytoskeleton in OL.Besides,FEZ1 was upregulated during OL and myelin development.Functionally,knockdown of FEZ1 attenuated OL processes complexity,even though proliferation of OPC was not affected.These results strongly proved that FEZ1 was expressed in OL and necessary for processes development.4.FEZ1 expression in OL is regulated by a functional interplay between histone deacetylation and transcription factors affected in psychiatric diseasesHistone deacetylase(HDAC)inhibitor(Trichostatin A,TSA)treatment caused an increase of histone aceylation at Fez1 promoter and a decrease of FEZ1 expression in OLs,indicating that FEZ1 transcription was mainly driven by transcription repressors.However,expression of FEZ1 in neuron was not affected by TSA treatment.Furthermore,we searched for transcription factors(TFs)that may bind rat Fez1 promoter,especially focusing on repressor TFs of which expressions declined during OPC differentiation.Surprisingly,most of TFs were involved in major mental illnesses(schizophrenia,bipolor disorder and depression).Besides,TSA could significantly decrease expression of disease related activators,whereas increase expression of repressors.Moreover,we cloned and overexpressed schizophrenia related repressor Id4 and activator Sox10,and ensured their opposite regulatory roles on FEZ1 transcription.Besides,Id4 could downregulate expression of several activators.These results showed how transcription of the Fez1 gene in OL cells is governed by a sophisticated functional interplay between histone acetylation-mediated chromatin modification and transcription factors which are dysregulated in schizophrenia.5.Schizophrenia risk factor Quaking is essential for FEZ1 expression in OLQuaking(QKI),a RNA binding protein,is a glia specific schizophrenia risk factor.Previous study found that quaking played key roles in controlling m RNA abundance and promoting OL development.In this study,we found that human,mouse and rat FEZ1 m RNA 3' untranslated regions(3'UTRs)harbor two consensus Quaking Response Elements(QREs).In fact,FEZ1 m RNA was co-immunoprecipitated with QKI in OL cytoplasma.Moreover,FEZ1 m RNA was markedly reduced in optic nerves of the homozygous quakingviable(qkv)hypomyelination mutant mice(q/q)that harbor OL-specific QKI deficiency.And FEZ1 protein was diminished in OL from q/q mice corpus callosum.The reduction of FEZ1 in q/q optic nerves was completely rescued by Flag-QKI 6(q/qtg).Unlike FEZ1,DISC1 m RNA was not regulated by QKI.Moreover,in contrast to optic nerves,FEZ1 expression was not affected by QKI deficiency in the hippocampus of the q/q mutant mice.These results suggest that QKI binds and stabilizes FEZ1 m RNA in the OL.Our results present direct evidenc to confirm the hypothesis that OL and/or myelin deficits were involved in the etiology of schizophrenia,which may provide new insights into the therapeutic stategies for schizophrenia and other psychiatric disorders.