Phf15—a novel transcriptional repressor regulating inflammation in mouse microglia

Aim: Excessive microglial inflammation has emerged as a key player in mediating the effects of aging and neurodegeneration on brain dysfunction. Thus, there is great interest in discovering transcriptional repressors that can control this process. We aimed to examine whether Phf15, one of the top differentially expressed genes in microglia during aging in humans, could regulate transcription of pro-inflammatory mediators in microglia. Methods: RT-qPCR was used to assess Phf15 mRNA expression in mouse brain during aging. Loss-of-function (shRNA-mediated knockdown (KD) and CRISPR/Cas9-mediated knockout (KO) of Phf15) and gain-of-function (retroviral overexpression (OE) of murine Phf15 cDNA) studies in a murine microglial cell line (SIM-A9) followed by immune activation with lipopolysaccharide (LPS) were used to determine the effect of Phf15 on pro-inflammatory factor (Tnfaplha;, Il-1beta, Nos2) mRNA expression. RNA-sequencing was used to determine global transcriptional changes after Phf15 knockout under basal conditions and after LPS stimulation. Results: Phf15 expression increases in mouse brain during aging, similar to humans. KD, KO and OE studies determined that Phf15 represses mRNA expression levels of pro-inflammatory mediators such as Tnfalpha, Il-1beta and Nos2. Global transcriptional changes after Phf15 KO showed that Phf15 specifically represses genes related to the antiviral (type I interferon) response and cytokine production in microglia. Conclusion: We provide the first evidence that Phf15 is an important transcriptional repressor of microglial inflammation, regulating the antiviral response and pro-inflammatory cytokine production. Importantly, Phf15 regulates both basal and signal-dependent activation and controls the magnitude and duration of the microglial inflammatory response. Keywords: Phf15, microglia, transcriptional repression, neuroinflammation