Abstract Body: Background
Ischemic white matter injury leads to long-term neurological deficits and lacks effective medication. Accumulating evidence suggests that microglia contribute to pro-remyelinating after brain injury. We previously obtained a picture of microglial dynamic changes after the transient middle cerebral artery occlusion (tMCAO) through RNA-seq and found that AXL, a member of the TAM family of receptor tyrosine kinases with phagocytosis and remyelinating functions, continued to increase until one-month post-tMCAO. This study examines the role of microglial Axl in mediating white matter damage after ischemic stroke.
Methods
Immunofluorescence staining and flow cytometry were used to verify the spatial and temporal characteristics of AXL expression. Inducible microglial-specific AXL knockout (Axl^flox/flox Cx3cr1^CreERT2+/−, cKO) and littermate control WT (CreERT2-/-) mice were administrated tamoxifen two weeks before tMCAO surgery. Neurobehavioral functions were evaluated by rotarod, adhesive removal, and Morris water maze tests. White matter injury and demyelination axons were examined by MRI, electron microscopy, and SMI32/MBP staining. Myelin debris accumulation and lipid droplet deposition were assessed by dMBP (degraded myelin basic protein), Oil Red O, FluoroMyelin, and BODIPY staining. Transcriptome analysis was conducted to explore mechanisms.
Results
AXL was mainly expressed in microglia in the peri-infarct area and was significantly up-regulated on the 7^th and 14^th day post-tMCAO. AXL⁺ microglia possessed stronger phagocytosis capacity and wrapped myelin debris most obviously in striatum after stroke. Compared to WT controls, AXL cKO mice exhibited worse sensorimotor and cognitive deficits in the chronic phase of stroke and more severe white matter injury four weeks after tMCAO. More myelin debris was accumulated in the peri-infarcted area and more lipid droplets were deposited in the microglia of AXL cKO mice. KEGG analysis showed “immune system” and “lipid metabolism” processes were changed significantly in AXL cKO mice. In vitro, the expression of cholesterol esterification and hydroxylase genes (Acat1, Ch25h, Dhcr24) were decreased, while the inflammation-related genes (Il6, Il1b, Tgfb, Tnf) were increased in AXL cKO microglia after the stimulation of myelin debris.
Conclusions
AXL improved white matter injury after ischemic stroke by promoting microglial phagocytosis of myelin debris and mediating intracellular cholesterol metabolism.