The Role And Mechanism Of HMGB1 Derived From Astrocytes In Experimental Autoimmune Encephalomyelitis

Background: Multiple sclerosis(MS)is an autoimmune encephalomyelitis characterized by inflammation,demyelination and neurodegeneration in the central nervous system(CNS).And the specific pathogenesis of MS needs to be further investigated.High mobility group box 1(HMGB1)is located in the nuclei of cells under physiological conditions and participates in the transcription,replication and repair of DNA.While under pathological conditions,HMGB1 can be released to extracellular space and play its role as a damage associated molecular pattern(DAMP)molecule.Our previous studies revealed that specific neutralization of local HMGB1 in the CNS significantly alleviated experimental autoimmune encephalomyelitis(EAE)in mice,while the cellular source and exact mechanism of HMGB1 in EAE were unclear.Astrocytes are the most abundant cell populations in the CNS with wide distribution and diverse functions.HMGB1 is highly expressed in astrocytes and its expression pattern is closely related to the progression of EAE.While the exact role of astrocytic HMGB1 in EAE is unknown.Objective: To explore the role and mechanism of HMGB1 derived from astrocytes in EAE.Methods: Astrocytic HMGB1 was knocked out through injecting GFAP-CreERT2-HMGB1f/f mice with Tamoxifen.Experiments including transcriptome sequence,flow cytometry,histological staining,Western Blot,Real time PCR and so on were conducted in vivo and in vitro to explore in depth.Results: 1.Results in vivo showed astrocytic HMGB1 conditional knockout had following effects on EAE mice:(1)Decreased the morbidity,delayed the onset time,reduced disease score and demyelination.(2)Suppressed the activation of astrocytes and microglia,decreased the number of neurotoxic astrocytes while increased the number of neuroprotective astrocytes.(3)Reduced the pathogenic immune cells infiltrated in the CNS through increasing tight junction protein Claudin5 and decreasing intercellular adhesion molecule 1(ICAM1)and vascular cell adhesion molecule 1(VCAM1)to improve the immune cells infiltration associated functions of the blood brain barrier(BBB).(4)Improved the cholesterol level of the CNS by recovering cholesterol synthesis in astrocytes,and then promoted the proliferation and differentiation of oligodendrocyte progenitor cells(OPCs)and improved remyelination.2.In vitro experiments exploring mechanism found that:(1)Astrocytes released HMGB1 actively during EAE.(2)HMGB1 decreased Claudin5 through toll-like receptor 4(TLR4)and increased ICAM1 and VCAM1 through receptor of advanced glycation endproducts(RAGE)to affect the immune cells infiltration associated functions of brain microvascular endothelial cells(BMECs).(3)HMGB1 influenced cholesterol synthesis of astrocytes via decreasing transcription factor sterol regulatory element binding protein 2(SREBP2)through TLR4.And HMGB1-knockout astrocytes promoted the proliferation and differentiation of OPCs through synthesizing and releasing more cholesterol under EAE stimulation.Conclusion: Focusing on the functions of BBB and cholesterol metabolism,our present study revealed the dual functions of astrocytic HMGB1 in inflammatory damage and remyelination recovery during EAE in depth: on the one hand,it promoted inflammatory damage through affecting the immune cells infiltration associated functions of BMECs;on the other hand,it inhibited remyelination via influencing the cholesterol synthesis of astrocytes.