Enhanced liver X receptor signalling reduces brain injury and promotes tissue regeneration following experimental intracerebral haemorrhage: roles of microglia/macrophages

Abstract

Background: Inflammation-exacerbated secondary brain injury and limited tissue regeneration are barriers to favourable prognosis after intracerebral haemorrhage (ICH). As a regulator of inflammation and lipid metabolism, Liver X receptor (LXR) has the potential to alter microglia/macrophage (M/M) phenotype, and assist tissue repair by promoting cholesterol efflux and recycling from phagocytes. To support potential clinical translation, the benefits of enhanced LXR signalling are examined in experimental ICH.

Methods: Collagenase-induced ICH mice were treated with the LXR agonist GW3965 or vehicle. Behavioural tests were conducted at multiple time points. Lesion and haematoma volume, and other brain parameters were assessed using multimodal MRI with T2-weighted, diffusion tensor imaging and dynamic contrast-enhanced MRI sequences. The fixed brain cryosections were stained and confocal microscopy was applied to detect LXR downstream genes, M/M phenotype, lipid/cholesterol-laden phagocytes, oligodendrocyte lineage cells and neural stem cells. Western blot and real-time qPCR were also used. CX3CR1^CreER: Rosa26^iDTR mice were employed for M/M-depletion experiments.

Results: GW3965 treatment reduced lesion volume and white matter injury, and promoted haematoma clearance. Treated mice upregulated LXR downstream genes including ABCA1 and Apolipoprotein E, and had reduced density of M/M that apparently shifted from proinflammatory interleukin-1β⁺ to Arginase1⁺CD206⁺ regulatory phenotype. Fewer cholesterol crystal or myelin debris-laden phagocytes were observed in GW3965 mice. LXR activation increased the number of Olig2⁺PDGFRα⁺ precursors and Olig2⁺CC1⁺ mature oligodendrocytes in perihaematomal regions, and elevated SOX2⁺ or nestin⁺ neural stem cells in lesion and subventricular zone. MRI results supported better lesion recovery by GW3965, and this was corroborated by return to pre-ICH values of functional rotarod activity. The therapeutic effects of GW3965 were abrogated by M/M depletion in CX3CR1^CreER: Rosa26^iDTR mice.

Conclusions: LXR agonism using GW3965 reduced brain injury, promoted beneficial properties of M/M and facilitated tissue repair correspondent with enhanced cholesterol recycling.

Keywords: hemorrhage; inflammatory response; stroke.