FTY720 Improves Social Interaction And Cognitive Deficits By Enhancing Remyelination In Mice With Chronic Unpredictable Mild Stress

The medial prefrontal cortex is an important brain area involved in cognitive and emotional activities,and is also the main target area of stress injury.Sphinogosine-1-phosphate(S1P)is an important signal molecule with many biological activities that has attracted much attention in recent years.The sphingosine 1-phosphate receptor(S1PRs)modulator FTY720 can regulate the periphery lymphocyte migration and inhibit immune response,and is a clinical first-line drug for the treatment of multiple sclerosis(MS).Studies have shown that FTY720 can act on the immune system to inhibit lymphocyte migration and improve white matter myelin injury.In addition to the well-known immunosuppressive effects,recent studies have shown that FTY720 can also directly act on the central nervous system,so this study aimed to investigate whether FTY720 could improve social interaction and cognitive behaviors of chronic stress mice and the possible mechanisms.We used a chronic unpredictable mild stress(CUMS)model.After adult ICR male(18-22g)was acclimated,2-3 kinds of stimuli were randomly given everyday for 4 weeks,and no repeat stimuli was given for two consecutive days,intraperitoneally injection of FTY720(1mg/kg)or an equal volume of vehicle per day during this period,at a dose of 10ml/kg.Three-chamber social interaction test and novel objection recognition test were used to evaluate the sociability and cognitive functions of the mice.Immunofluorescence was used to observe the myelinated axonal density in the mPFC tissue,the number of oligodendrocyte precursor cells,the number of mature oligodendrocyte cells,the number of astrocytes,and the expression of S1PR1 on oligodendrocyte precursor cells.The morphology of myelin in mPFC tissue was observed by electron microscopy.The protein expressions of S1PR1,S1PR3,MBP,NG2,CNPase,GFAP,BMP4,P-Smad1/5/8,Id2,p-Akt,p-mTOR were detected by Western Blotting.The mRNA expression level of S1 PRs was detected by qRT-PCR.The main findings are as follows:1.FTY720 ameliorates social and cognitive dysfunctions in CUMS mice: CUMS mice did not show the preference for mouse-containing cage in the first social interaction period,whereas FTY720 treatment significantly increased this preference;CUMS mice did not show the preference for unfamiliar mouse-containing cage in the second social interaction period,whereas FTY720 treatment significantly increased this preference.In novel objection recognition test,compared to control mice,CUMS mice did not show the preference for novel objection,but FTY720 treatment significantly increased the preference for novel objection.2.FTY720 ameliorates demyelination injury in the mPFC of CUMS mice: The immunofluorescence in the mPFC of CUMS mice showed that the density of myelinated axons was significantly decreased,the protein expression of myelin basic protein(MBP)was significantly down-regulated,and electron microscopy images showed obvious demyelination injury and thinner myelin sheath in the mPFC in CUMS mice,FTY720 treatment significantly improved demyelinating damage,increased myelin thickness and MBP protein expression,increased axonal myelination.3.FTY720 improves social and cognitive dysfunction caused by LPC-induced demyelination in the mPFC: stereotactic injection of LPC in the mPFC region,after 14 days,LPC mice had apparently social interaction and cognitive dysfunctins,indicating demyelination in the mPFC could lead to behavioral dysfunctions,and FTY720 could improve behavioral changes by promoting remyelination.4.FTY720 enhances OPCs differentiation in the mPFC of CUMS mice: OPCs were significantly increased in the mPFC of CUMS mice,but mature oligodendrocytes(CNPase+/Oilg2+)were significantly reduced,and the number of(GFAP+)astrocytes had significantly increased.FTY720 treatment reduced the number of OPCs,and increased the amount of mature oligodendrocytes,and decreased the number of astrocytes,so improved the differentiation of OPCs,promoting remyelination.5.FTY720 promotes OPCs differentiation in the mPFC of CUMS mice through S1PR1: q-RT-PCR results showed that the mRNA levels of S1PR1,S1PR2,S1PR3,S1PR5 in the mPFC tissue of all groups mice had not significant differences,and CUMS and FTY720 treatment did not affect the mRNA expression level of S1 PRs.Western blotting results showed that there was no significant change in the expression of S1PR3 protein between control mice and CUMS mice,S1PR1 protein level was significantly up-regulated in CUMS mice,and FTY720 treatment down-regulated S1PR1 protein expression.And S1PR1 was highly expressed in OPCs of mPFC in CUMS mice,and FTY720 significantly down-regulated the expression of S1PR1 on OPCs.Specific modulator of S1PR1(SEW2871)ameliorated LPC-induced demyelination injury and then improvd social interaction and cognitive dysfunctions through enhancing OPCs differentiation.6.FTY720 inhibits BMP/Smad pathway and activates ATK/mTOR signaling pathway: Western blotting analysis showed that BMP4 was up-regulated in the mPFC of CUMS mice,and the expression of downstream P-Smad1/5/8 and Id2 were both significantly increased.FTY720 did not affect the expression of BMP4 but inhibited the up-regulation of P-Smad1/5/8 and Id2.Similarly,P-AKT and P-mTOR were down-regulated in the mPFC of CUMS mice,AKT was not significantly changed,mTOR was significantly up-regulated;FTY720 significantly up-regulated P-AKT,P-mTOR,and did not affect AKT,and decreased mTOR.In the LPC demyelinating model,BMP4 was not significantly changed,but P-Smad1/5/8 and Id2 were significantly up-regulated in the mPFC of LPC mice,and FTY720 or SEW2871 treatment both significantly decreased P-Smad1/5/8 and Id2.Conclusion: FTY720 can enhance remyelination and improve social and cognitive dysfunction in CUMS mice by regulating BMP/Smad and ATK/mTOR signaling pathway through S1PR1.Our study suggests that S1 P receptor modulator may be a potential protection against chronic stress-induced m PFC damage.