Differential roles of ApoE isoforms in regulation of inflammatory cytokine expression

Abstract

AbstractBackgroundApoE Ɛ4 has long been known as a top genetic risk factor in the development of late‐onset Alzheimer’s disease (LOAD). More recently the genetic variants of the innate immune receptor TREM2 such as R47H, which confers a similar risk as ApoE4 in LOAD, have been identified as genetic risk factors in the development of many neurodegenerative diseases with dysregulated neuroinflammation. TREM2 is expressed highly on microglia and macrophages and plays key roles in inflammation regulation. Our previous study (Dorey et al. 2017) has shown that ApoE isoforms play differential roles in Aβ‐induced inflammatory response. Atagi et al (2015) identified that ApoE is a ligand for TREM2. Given the physiological relationship between ApoE and TREM2 while also considering the three isoforms of ApoE, it is crucial to investigate and understand the mechanisms underlying the possibility differential ApoE isoform signaling through TREM2 surface receptor.MethodTHP‐1 (human monocytic leukemia cell line) cells were differentiated into macrophages (M0) using PMA and subsequently polarized to M1 and M2 using IFN‐γ/LPS and IL‐4/IL‐13, respectively. Following differentiation, 3uM of ApoE2, E3, or E4 was added to polarization media. RNA was extracted following 2 day polarization and used for RT‐qPCR in relative gene expression quantification.ResultApoE2 strongly stimulated the expression of anti‐inflammatory cytokines IL‐10 and TARC/CCL17 in M1 and M2 macrophages as compared to control, ApoE3 and ApoE4. While ApoE4 showed a massive reduction in these same transcripts in M0 and M2 macrophages. In contrast, ApoE4 strongly promoted the expression of pro‐inflammatory cytokines IL‐1β and TNF‐α in M0, M1, and M2 macrophages as compared to control, ApoE3 and ApoE2.ConclusionThe ability for ApoE2 to promote the expression of anti‐inflammatory cytokines in macrophages, suggests that ApoE2 may play a neuroprotective role; while ApoE4’s ability to promote inflammatory cytokine expression indicates its pro‐inflammatory role. Both ApoE2 and ApoE4 may play their roles via interacting with TREM2.