Peroxisome Proliferator-Activated Receptor-δ Deficiency in Microglia Results in Exacerbated Axonal Injury and Tissue Loss in Experimental Autoimmune Encephalomyelitis

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 <i>Ppard</i> allele was driven by expression of a tamoxifen (TAM)-inducible CX3C chemokine receptor 1 promoter-Cre recombinase transgene (<i>Cx3cr1</i>^CreERT2: <i>Ppard</i>^fl/fl). We observed that by 30 days of TAM treatment, <i>Cx3cr1</i>^CreERT2: <i>Ppard</i>^fl/fl mice exhibited Cre-mediated deletion primarily in microglia and this was accompanied by efficient knockdown of <i>Ppard</i> expression in these cells. Upon induction of EAE, TAM-treated <i>Cx3cr1</i>^CreERT2: <i>Ppard</i>^fl/fl mice presented with an exacerbated course of disease compared to TAM-treated <i>Ppard</i>^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 <i>Cx3cr1</i>^CreERT2: <i>Ppard</i>^fl/fl group compared to controls. In early EAE, a time when clinical scores and the infiltration of CD45⁺ leukocytes was equivalent between <i>Cx3cr1</i>^CreERT2: <i>Ppard</i>^fl/fl and <i>Ppard</i>^fl/fl mice, <i>Ppard</i>-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, <i>Ppard</i>-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.