Effect And Mechanism Of CD200 In Chronic Stress-induced Depressive-like Behaviors Of Mice

Part I The role of CD200 in chronic stress-induced depressive-like behaviors in miceObjective: A large number of studies have shown that neuroinflammation is a key mechanism of depression.Cluster of differentiation 200(CD200)and its receptor CD200 R are one of the modulatory immunity system.CD200 is expressed on membrane glycoprotein on neurons,and plays a role of immunosuppression via CD200R1 that expressed in microglia.CD200 interacts with CD200 R to maintain microglia in the quiescent state.Microglia is activated when the interaction is disrupted.Numerous studies have shown that CD200 is closely related to the pathogenesis of inflammatory diseases involved with Alzheimer's disease(AD),aging,and multiple sclerosis.Recent researches also suggest that the activation of microglia may be associated with the onset of depression.Therefore,we used chronic social defeat stress(CSDS)to induce depressive-like behaviors,and explored the role of CD200 in the pathogenesis of depression.Methods: After exposure to 10 days of CSDS,we detected susceptible and resilient mice using sucrose preference(SPT),social interaction(SI)and tail suspension test(TST)behaviors.Quantitative real-time PCR(q PCR)was used to detect the relative expression of CD200 and CD200R1 m RNA in different brain regions to identify the brain region with the most significant change.Western blotting was applied to examine the expression of CD200 and CD200R1 protein in dentate gyrus(DG)region.With lentivirus(LV)expressing si CD200 to silence CD200 and microinjection of CD200 Fc into the DG,we evaluated the role of CD200 in depression-like behavior induced by CSDS.Results:(1)After exposure to 10 days of CSDS,the susceptible mice showed obvious depressive-like behaviors: the decreased social interaction ratio(Control: 1.43 ± 0.048,Susceptible: 0.61 ± 0.050,P < 0.001 vs Control)and sucrose preference(Control: 90.00 ± 0.96%,Susceptible: 68.22 ± 2.76%,P < 0.001 vs Control)and the immobility time of TST(Control: 102.60 ± 7.87 s,Susceptible: 132.50 ± 8.92 s,P < 0.05 vs Control).(2)The real-time quantitative PCR(q PCR)detected the m RNA expression in different regions: compared to control(1.00 ± 0.075),CSDS significantly reduced the relative expression of CD200 in peripheral blood(0.58 ± 0.081,P < 0.01),but not CD200R1;We also found that CD200 m RNA expression was down-regulated in hippocampus and DG regions(Hip,Control: 1.00 ± 0.11,CSDS: 0.76 ± 0.030,P < 0.05;DG,Control: 1.00 ± 0.028,CSDS: 0.77 ± 0.039,P < 0.01).However,CD200 and CD200R1 m RNA expression had no significant discrepancy in the medial prefrontal cortex(m PFC).(3)The expression of CD200 protein in DG region significantly reduced in susceptible mice(Control: 1.00 ± 0.038,Susceptible: 0.81 ± 0.074,Resilient: 0.99 ± 0.062,P < 0.05),and the CD200R1 level have no statistical differences.(4)Injection with CD200 Fc into DG region significantly improved depressive-like behaviors induced by CSDS,including increased social interaction(Ctrl+ACSF: 1.07 ± 0.12 s,Ctrl+CD200Fc: 0.96 ± 0.090 s,CSDS+ACSF: 0.62 ± 0.11 s,CSDS+CD200Fc: 1.37 ± 0.29 s,P = 0.0021 vs CSDS+ACSF),decreased immobility time in the tail suspension and forced swimming test(TST: Ctrl+ACSF: 131.60 ± 8.09 s,Ctrl+CD200Fc: 138.30 ± 9.45 s,CSDS+ACSF: 162.30 ± 13.43 s,CSDS+CD200Fc: 118.20 ± 12.87 s,P = 0.0083 vs CSDS+ACSF;FST: Ctrl+ACSF: 55.25 ± 14.8 s,Ctrl+CD200Fc: 72.44 ± 18.54 s,CSDS+ACSF: 122.60 ± 13.36 s,CSDS+CD200Fc: 69.22 ± 12.93 s,P = 0.017 vs CSDS+ACSF).(5)Injection with LV-GFP and LV-si CD200 into DG region and then exposure to subthreshold stress,the mice with LV-si CD200 showed a remarkable increase in susceptibility to stress: the time in interaction zone with target was significantly reduced(LV-GFP-Stress: 55.98 ± 1.42 s,LV-si CD200-Stress: 37.21 ± 1.27 s,P < 0.001),and the immobility time in the tail suspension test was increased(LV-GFP-Stress: 122.60 ± 14.33 s,LV-si CD200-Stress: 181.60 ± 12.68 s,P = 0.0044).Conclusion: Expressions of CD200 protein and m RNA in the DG region of susceptible mice were significantly reduced;Decreased CD200 in DG region increased the susceptibility of mice to the stress;Injection with CD200 Fc into DG region rescued depressive-like behaviors of mice.Part II The mechanism of CD200 in chronic stress-induced depressive behaviors of miceObjective: It has been reported that chronic stress induces a decline in hippocampal neurogenesis and activates microglia.CD200 Fc can improve hippocampal neurogenesis,and ameliorate the activated microglia by acting on CD200R1,which is expressed on microglia.However,it remains unknown whether CD200 exerts the antidepressant action through promoting neurogenesis and ininhibiting microglia activation.Therefore,we examined the role of CD200 in hippocampal neurogenesis and microglia activation induced by CSDS.Methods: By knockdown of CD200R1 in DG region and then microinjection of CD200 Fc into the DG region,we detected depressive-like behaviors of mice by using social interaction,tail suspension and forced swimming tests.With immunofluorescence method,we examined the hippocampal neurogenesis and microglia activation after injection of CD200 Fc into DG.Finally,we used classical antidepressants fluoxetine to test the expression of CD200 protein in DG region.Results:(1)Knockdown of CD200R1 in DG region followed by intra-DG injection of CD200 Fc could not reverse depressive-like behaviors induced by CSDS.Compared with CSDS-LV-GFP+CD200Fc,CD200R1 knockdown mice(CSDS-LV-si CD200R1)with CD200 Fc injection did not improve the social avoidance behavior,represented as decreased social interaction ratio(Ctrl-LV-GFP+CD200Fc: 1.01 ± 0.12,Ctrl-LV-si CD200R1+CD200Fc: 0.87 ± 0.058,CSDS-LV-GFP+CD200Fc: 1.23 ± 0.24,CSDS-LV-si CD200R1+CD200Fc: 0.67 ± 0.12,P < 0.05,CSDS-LV-si CD200R1+CD200Fc vs CSDS-LV-GFP+CD200Fc);In the tail suspension test and forced swimming test,the immobility time was also prolonged(TST,CSDS-LV-GFP+CD200Fc: 97.75 ± 11.54 s,CSDS-LV-si CD200R1+CD200Fc: 156.50 ± 10.14 s,P < 0.01;FST,CSDS-LV-GFP+CD200Fc: 61.38 ± 15.71 s,CSDS-LV-si CD200R1+CD200Fc: 106.90 ± 15.70 s,P < 0.05).(2)CSDS led to the impairment of neurogenesis in DG region: the number of proliferating cells and immature neurons were decreased(Brd U+: Control+ACSF: 1491.00 ± 103.50,CSDS+ACSF: 861.30 ± 63.07,P < 0.01;Brd U+DCX+: Control+ACSF: 619.30 ± 52.95,CSDS+ACSF: 358.10 ± 42.00,P < 0.05),and injection of CD200 Fc into lateral ventricle rescued the impaired neurogenesis induced by CSDS(Brd U+: CSDS+ACSF: 861.30 ± 63.07,CSDS+CD200Fc: 2743.00 ± 343.30,P < 0.001;Brd U+DCX+: CSDS+ACSF: 358.10 ± 42.00,CSDS+CD200Fc: 1230.00 ± 221.10,P < 0.001).(3)CSDS induced microglia activation in DG regions,including the increased number of microglia,the extended cell body,and the shortened branches,which can be reversed by CD200 Fc.The percentage of activated microglia was decreased after CD200 Fc infusion(CSDS+ACSF: 70.70 ± 5.88%,CSDS+CD200Fc: 35.30 ± 4.49%,P = 0.0014).(4)Treatment with fluoxetine for 14 consecutive days significantly improved depressive-like behaviors induced by CSDS,including the increased social interaction ratio(Ctrl+Sal: 1.32 ± 0.15,Ctrl+Flu: 1.15 ± 0.14,CSDS+Sal: 0.44 ± 0.06,CSDS+Flu: 1.37 ± 0.15,P < 0.001,CSDS+Flu vs CSDS+Sal),the decreased immobility time in tail suspension test(Ctrl+Sal: 122.40 ± 19.83 s,Ctrl+Flu: 139.80 ± 18.36 s,CSDS+Sal: 184.0 ± 11.78 s,CSDS+Flu: 58.76 ± 11.77 s,P < 0.001,CSDS+Flu vs CSDS+Sal).Furthermore,fluoxetine treatment prevented the decline of CD200 protein induced by CSDS in DG region(Ctrl+Sal: 1.01 ± 0.03,Ctrl+Flu: 0.96 ± 0.03,CSDS+Sal: 0.89 ± 0.02,CSDS+Flu: 1.10 ± 0.050,P < 0.01,CSDS+Flu vs CSDS+Sal).Conclusion: CD200 Fc exerted antidepression action through CD200R1,promoted neurogenesis induced by stress in DG region,and inhibited activation of microglia induced by CSDS.