HMGB1/RAGE axis mediates stress-induced RVLM neuroinflammation in mice via impairing mitophagy flux in microglia

Abstract

Background: Microglial mediated neuroinflammation in the rostral ventrolateral medulla (RVLM) plays roles in the etiology of stress-induced hypertension (SIH). It was reported that autophagy influenced inflammation via immunophenotypic switching of microglia. High-mobility group box 1 (HMGB1) acts as a regulator of autophagy and initiates the production of proinflammatory cytokines (PICs), but the underlying mechanisms remain unclear. Methods: The stressed mice were subjected to intermittent electric foot shocks plus noises administered for 2 h twice daily for 15 consecutive days. In mice, blood pressure (BP) and renal sympathetic nerve activity (RSNA) were monitored by noninvasive tail-cuff method and platinum-iridium electrodes placed respectively. Microinjection of siRNA-HMGB1 (siHMGB1) into the RVLM of mice to study the effect of HMGB1 on microglia M1 activation was done. mRFP-GFP-tandem fluorescent LC3 (tf-LC3) vectors were transfected into the RVLM to evaluate the process of autolysosome formation/autophagy flux. The expression of RAB7, lysosomal-associated membrane protein 1 (LAMP1), and lysosomal pH change were used to evaluate lysosomal function in microglia. Mitophagy was identified by transmission electron microscopic observation or by checking LC3 and MitoTracker colocalization under a confocal microscope. Results: We showed chronic stress increased cytoplasmic translocations of HMGB1 and upregulation of its receptor RAGE expression in microglia. The mitochondria injury, oxidative stress, and M1 polarization were attenuated in the RVLM of stressed Cre-CX3CR1/RAGE fl/fl mice. The HMGB1/RAGE axis increased at the early stage of stress-induced mitophagy flux while impairing the late stages of mitophagy flux in microglia, as revealed by decreased GFP fluorescence quenching of GFP-RFP-LC3-II puncta and decreased colocalization of lysosomes with mitochondria. The expressions of RAB7 and LAMP1 were decreased in the stressed microglia, while knockout of RAGE reversed these effects and caused an increase in acidity of lysosomes. siHMGB1 in the RVLM resulted in BP lowering and RSNA decreasing in SIH mice. When the autophagy inducer, rapamycin, is used to facilitate the mitophagy flux, this treatment results in attenuated NF-κB activation and reduced PIC release in exogenous disulfide HMGB1 (ds-HMGB1)-stimulated microglia. Conclusions: Collectively, we demonstrated that inhibition of the HMGB1/RAGE axis activation led to increased stress-induced mitophagy flux, hence reducing the activity of microglia-mediated neuroinflammation and consequently reduced the sympathetic vasoconstriction drive in the RVLM.

Promiscuous Roles of Autophagy and Proteasome in Neurodegenerative Proteinopathies

“…Conversely, autophagy induction promotes microglia polarization toward the M2 phenotype, thus blunting inflammation and neurotoxicity [223]. In this context, AGE-modified misfolded proteins may play a key role, as RAGE activation within microglia impairs lysosome acidification and autophagy flux, while promoting microglia-mediated neuro-inflammation [224]. However, conflicting results still exist on the role of RAGE signaling upon autophagy activity.…”

High Mobility Group Box 1 in Human Cancer

“…Although many of the preclinical studies on HMGB1-mediated neuro-inflammation have focused on the pathogenesis of neurological disorders [ 166 , 167 ], they are also likely to be implicated in establishing an immune microenvironment in the brain that favors immunosuppression and tumor growth, possibly driven by microglia and M2-like macrophages [ 168 , 169 ], exacerbated by HMGB1-activated NET formation by infiltrating neutrophils [ 170 ]. Clearly, however, further research is needed to unravel the precise involvement of the HMGB1/TLR4/RAGE axis in this scenario.…”

Merging the Multi-Target Effects of Phytochemicals in Neurodegeneration: From Oxidative Stress to Protein Aggregation and Inflammation

“…These can either aggregate and persist in the donor cell or move out to spread the disease within various RAGE-expressing recipient cells, including brain endothelial cells, microglia, astrocytes, and neurons [ 10 , 38 ]. The binding of AGEs with RAGEs triggers a variety of transduction mechanisms, which may bridge alterations in cell-clearing mechanisms with oxidative, and apoptotic events [ 44 , 47 , 48 ]. In detail, RAGEs trigger activation of PKC, NF-kB, JAK2/STAT1, and AKT/mTOR pathways, which promote a vicious cycle of inflammatory and oxidative reactions while altering autophagy and the proteasome [ 10 , 44 ].…”

“…This is associated with enhanced antigen processing of endogenous proteins such as α-syn, and subsequent activation of T-cell responses against glial and neuronal cells, which under excessive pro-inflammatory conditions do upregulate MHC molecules just like antigen-presenting cells [ 228 , 229 ]. Remarkably, molecular pathways that recruit the immunoproteasome, such as PKC, JAK-STAT, NF-κB, RAGE/TLR, and mTORC1 are known to impinge on the autophagy machinery [ 10 , 18 , 28 , 44 , 46 , 47 , 48 ]. The very same immunoproteasome may contribute to impair autophagy through ATG5 and PTEN degradation [ 231 , 232 ].…”

“…The irreversible glycation and oxidation of proteins and lipids produce AGEs which, similar to fragments of mtDNA and HMGB1, are released extracellularly either as free compounds or via exosomes that derive from the fusion of autophagy-lysosome vacuoles with the plasma membrane [ 10 , 38 , 45 ]. In neighboring cells, including neurons and glia, these DAMPs bind to toll-like receptors (TLRs) and AGE receptors (RAGEs), thus activating downstream oxidative and inflammatory signaling pathways which converge in altering cell-clearing systems, such as nuclear factor (NF)-κB, JAK/STAT, protein kinase C (PKC), and phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) [ 28 , 44 , 46 , 47 , 48 ]. In this way, indigested DAMPs may perpetuate oxidative and inflammatory damage in the surrounding neuronal milieu.…”