Peroxisome Proliferator-Activated Receptor-δ Deficiency in Microglia Results in Exacerbated Axonal Injury and Tissue Loss in Experimental Autoimmune Encephalomyelitis
Ellinore R. Doroshenko, Paulina C. Drohomyrecky, Annette Gower, Heather Whetstone, Lindsay S. Cahill, Milan Ganguly, Shoshana Spring, Tae Joon Yi, John G. Sled, Shannon E. Dunn
Abstract
Peroxisome proliferator-activated receptor (PPAR)-δ is a nuclear receptor that functions to maintain metabolic homeostasis, regulate cell growth, and limit the development of excessive inflammation during immune responses. Previously, we reported that PPAR-δ-deficient mice develop a more severe clinical course of experimental autoimmune encephalomyelitis (EAE); however, it was difficult to delineate the role that microglia played in this disease phenotype since PPAR-δ-deficient mice exhibited a number of immune defects that enhanced CNS inflammation upstream of microglia activation. Here, we specifically investigated the role of PPAR-δ in microglia during EAE by using mice where excision of a floxed Ppard allele was driven by expression of a tamoxifen (TAM)-inducible CX3C chemokine receptor 1 promoter-Cre recombinase transgene ( Cx3cr1 CreERT2 : Ppard fl/fl ). We observed that by 30 days of TAM treatment, Cx3cr1 CreERT2 : Ppard fl/fl mice exhibited Cre-mediated deletion primarily in microglia and this was accompanied by efficient knockdown of Ppard expression in these cells. Upon induction of EAE, TAM-treated Cx3cr1 CreERT2 : Ppard fl/fl mice presented with an exacerbated course of disease compared to TAM-treated Ppard fl/fl controls. Histopathological and magnetic resonance (MR) studies on the spinal cord and brains of EAE mice revealed increased Iba-1 immunoreactivity, axonal injury and CNS tissue loss in the TAM-treated Cx3cr1 CreERT2 : Ppard fl/fl group compared to controls. In early EAE, a time when clinical scores and the infiltration of CD45 + leukocytes was equivalent between Cx3cr1 CreERT2 : Ppard fl/fl and Ppard fl/fl mice, Ppard -deficient microglia exhibited a more reactive phenotype as evidenced by a shorter maximum process length and lower expression of genes associated with a homeostatic microglia gene signature. In addition, Ppard -deficient microglia exhibited increased expression of genes associated with reactive oxygen species generation, phagocytosis and lipid clearance, M2-activation, and promotion of inflammation. Our results therefore suggest that PPAR-δ has an important role in microglia in limiting bystander tissue damage during neuroinflammation.