Author ORCID Identifier
Date of Graduation
5-2024
Document Type
Dissertation (PhD)
Program Affiliation
Neuroscience
Degree Name
Doctor of Philosophy (PhD)
Advisor/Committee Chair
Jaroslaw Aronowski
Committee Member
Jian Hu
Committee Member
Jun Li
Committee Member
John William Lindsey
Committee Member
Jack Waymire
Committee Member
Jin Seon Im
Abstract
After stroke, microglia (MG) and blood-derived macrophages, together (MF), clear dead cells and cellular debris in the infarcted brain through phagocytosis. However, the phagocytic capability of MF declines with age. Age-related changes in MF phenotype also include overactive inflammatory responses to stroke-induced brain injury, altogether resulting in poor recovery after stroke. Retinoid-X-receptor (RXR) is a pleiotropic transcription factor. Our studies suggest that RXRa enhances MF phagocytic functions, reduces inflammatory responses, and improves post-stroke recovery. To establish phenotypes of aging MF, MG were MACS-sorted from the brains of young adults (2-4 months old) and aged (18-20 months old) mice for transcriptomic analysis. RXR agonist bexarotene (BEX; 5 mg/kg) or vehicle were intraperitoneally (i.p.) injected daily for three days prior to harvest (n=3/group). We found that compared to young adults, MG from the aged brains expressed higher levels of pro-inflammatory genes and lower levels of phagocytosis-facilitating genes (Cd206 and Cd36). BEX treatment was able to restore these age-associated phenotypes. Also, in agreement with gene expression profiles, using erythrocytes as targets for phagocytosis, we showed that BEX indeed enhanced the phagocytic ability of MF in the aged brain. Finally, to establish the role of RXR activation in MF plays in long-term recovery after stroke, aged 18-20 months old, myeloid-specific RXRa knockout mice (Mac-RXRa-/-) and littermate control (RXRafl/fl) of both sexes were subject to transient middle cerebral artery (MCA) occlusion to induce ischemic stroke. To activate RXR, BEX (5 mg/kg) or vehicle control was i.p. injected 24 hours after stroke onset and then daily for seven days. To assess the neurological recovery outcome, sensorimotor function (foot fault and corner turn tests on days 3, 7, 14, and 28) and cognitive function (Y maze, Barnes maze, and novel object recognition test on day 30) were evaluated. We found that BEX significantly improved sensorimotor and cognitive functions in RXRafl/fl mice. The beneficial effect of BEX on neurological recovery was mitigated in Mac-RXRa-/- mice, suggesting that RXR signaling in MF plays an essential role in regulating recovery after stroke. In conclusion, our data show that activating RXRa partially restores age-related MF dysfunctions and that RXRa deficiency in MF significantly restricts the therapeutic effect of RXR activation in improving post-stroke recovery in the aged brain.
Keywords
Ischemic Stroke, Microglia, Aging, Neuroimmunology, Bexarotene, Phagocytosis, Post-stroke Recovery