The Interaction Between BDNF/TrkB And MyD88/NF-?B Signal Pathways In Pneumococcal Meningitis

Objective:Pneumococcal meningitis acts as a serious pediatric inflammatory disease in central nervous system,with high occurrences of mortality and neurological sequelae.The underlying physiopathologic mechanism is not clear,which is associated with excessive activated inflammatory response,inducing irreversible brain damage.Brainderived neurotrophic factor(BDNF)is the member of neurotrophic family and is widely expressed in nervous system,which plays important roles in neuronal survival,maturity,and development.Our previous studies have found antibiotic treatment attenuated the increased expression of BDNF following pneumococcal meningitis,and application of exogenous BDNF showed obvious neuroprotective effects.Based on these results,our present study aims to investigate if activation of the classical inflammatory signaling pathway,namely the MyD88/NF-?B pathway,regulates BDNF expression in pneumococcal meningitis,and whether BDNF/TrkB interaction could have effects on MyD88/NF-?B-signaling pathways,thus clarify the interaction between BDNF/TrkB and MyD88/ NF-?B signaling pathways in pneumococcal meningitis.Methods: Part 1: MyD88 knockout(MyD88-/-)mice and wild-type littermates were infected intracisternally with S.pneumoniae suspension.24 hours after inoculation,histopathology of brains was evaluated.Cytokine and chemokine in brains and spleens was analyzed using ELISA.NF-?B activation was evaluated using EMSA.Hippocampal and cortical BDNF was assessed using RT-PCR,western blot and ELISA,respectively.BDNF promoter activity was evaluated using ChIP-PCR.Part 2: SD rats were pretreated with exogenous BDNF or TrkB inhibitor,prior to intracisternal infection with live S.pneumoniae.At 24 h post-infection,pathological severity of brain sections was evaluated by using histological staining.Cytokine and chemokine levels in the hippocampus and cortex were evaluated by ELISA.The DNA-binding activityof NF-?B was measured by EMSA.Key molecules associated with the related signaling pathway,namely TrkB,MyD88,and PI3K/AKT,were analyzed by western blot.Part 3: Wister infant rats were used to construct the model of pneumococcal meningitis,adjuvant therapy with 7,8-DHF or placebo DMSO were injected intraperitoneally.Two major brain injuries were evaluated by Nissl staining,Auditory brainstem responses was performed to assess the function of cochlea neurons.Results: Part 1: 24 h post infection,myd88-/-mice showed obviously worse clinical parameters,neuron injury,and hippocampal apoptosis when compared with WT infected mice.Inflammatory response was extremely activated in cortexs and spleens of WT infected mice,however,MyD88 deficiency significantly attenuated the production of inflammatory mediators.Meanwhile,the increased expression of BDNF during acute phase of meningitis was obviously decreased in myd88-/-infected mice.NF-?B binding activation was significantly enhanced after 24 h of infection in WT infected mice,however,this bacteria-induced NF-?B binding activation was markedly attenuated in myd88-/-mice.We also found there is a consensus NF-?B binding site present in the BDNF promoter region.Part 2: Rats pretreated with BDNF exhibited reduced clinical impairment,pathological severity,and hippocampal apoptosis after pneumococcal meningitis.Furthermore,BDNF pretreatment suppressed the expression of inflammatory factors,and increased the expression of the anti-inflammatory factor.Moreover,BDNF pretreatment increased TrkB expression,inhibited MyD88/NF-?B signaling pathway,and activated downstream PI3K/AKT signaling.Conversely,administration of TrkB inhibitor attenuated the anti-inflammatory and anti-apoptotic effects of BDNF.Part 3: Adjuvant therapy with 7,8-DHF could significantly decreased necrosis of the cortex,hippocampal apoptosis,and improved hearing loss following pneumococcal meningitis.Conclusion:(1)The activity of MyD88/NF-?B signaling induced innate immune response during pneumococcal meningitis,and this pathway is also required for BDNF expression.(2)BDNF/TrkB interaction exerted anti-inflammatory and anti-apoptotic effects in pneumococcal meningitis via modulation of MyD88/NF-?B-and PI3K/AKT-signaling pathways.(4)7,8-DHF acts as a small molecule agonist of TrkB,is expected to solve the problem of clinical application of exogenous BDNF and become a potential therapy for bacterial meningitis.