BDNF-TrkB Signaling Pathway Is Implicated In Dendritic Spine Development And Abnormal Behaviors In Fragile X Syndrome Mouse Model

Objective:ragile X syndrome(FXS)is a common mental disorder neurodevelopmental disease caused by the loss of fragile X mental retardation protein(FMRP),which is characterized by developmental disorders of dendritic spines.Brain-derived neurotrophic factor(BDNF)is considered as a key growth-promoting signal involved in neuronal survial,morphogenesis,differentiation and synapse formation.Its functions are mainly performed by activating its high-affinity receptor tyrosine kinase B(TrkB)and its downstream pathways.However,the alternation of BDNF-TrkB signaling pathway in FXS are still unkown.Whether the alternation of BDNF-TrkB signaling pathway were involved in the developmental disorder of denditic spines in FXS? Whether it could be a target for correcting the developmental disorder of denditic spines and abnormal behavior in FXS? Our study focuses on the alternation of BDNF-TrkB signaling pathway in Fmr1 knockout(Fmr1 KO)mice during the developmental stage.And also we investigates the association between the BDNF/TrkB signaling pathway and abnormal dendritic spine and behaviors in Fmr1 KO mice during the developmental stage.In addition,7,8-dihydroxyflavone(7,8-DHF)is used to activate TrkB and we detect the role of 7,8-DHF in dendritic spine development and behaviors in FXS.Methods:The postal development stage(P7,P10,P14,P21 and P30)of Fmr1 KO mice(FVB 129 P2-Pde6b+Tyrc-ch-Fmr1tm1Cgr/J)and wild-type(WT)mice(FVB.129P2-Pde6bTyrc-ch/AntJ)at the postnatal development stage(P7,P10,P14,P21 and P30)were used in this study.1)The pathological changes of dendritic spines were detected by Golgi staining and transmission electron microscope.2)The mRNA and protein levels of BDNF in hippocampus were analyzed by qRT-PCR and western blot.Moreover,the key proteins in BDNF/TrkB signaling pathway were detected by western blot.3)Synaptic plasticity was tested by electrophysiological analysis.4)Morris water maze test were used to examine the spatial memory functions.5)Intraperitoneal injection of 7,8-DHF(5mg/kg)were performed in mice from P14 to P30,then the experiments ablove were performed.Results:The study found that 1)The protein levels of BDNF,p-TrkB(Tyr816),p-PLC?1(Tyr783)and p-CaMKII(Thr286)were decreased in the early developmental stage of Fmr1 KO mice(P7,P10,P14,P21,P30),while the mRNA levels of BDNF were significantly increased;2)The number and length of immature dendritic filopodium were incresed and the number of mature dendritc spines were reduced;3)Spatial learning and memory were impared in Fmr1 KO mice;4)TrkB receptor agonist enhanced the expression of of p-TrkB(Tyr816),p-PLC?1(Tyr783)and p-CaMK?(Thr286)in Fmr1 KO mice,reduced the number of dendritic filopodium,incresed the number of mushroom dendritic spines,incresed the number of synapse in the hippocampal CA1 region,recovered the LTD impairment,and improved the spatial learning and memory in Fmr1 KO mice.Conclusion:BDNF/TrkB signaling pathway is abnormal at the early postnatal development stage of Fmr1 KO mice(P7,P10,P14,P21,P30),and the alternation of BDNF/TrkB signaling pathway is assocciated with the abnormity of hippocampus dendritic spines and bahavior.7,8-DHF could activate BDNF-TrkB signaling pathway and correct the abnormalities of dendritic spines and behavoir in Fmr1 KO mice.