Graphene oxide exposure suppresses immune responses and increases the sensitivities of zebrafishes to lipopolysaccharides via the down-regulation of Toll-like receptors

Besides the roles in regulating immune responses, recent studies have also suggested a pivotal role of Toll-like receptors (TLRs) in lipid metabolism. Previously we reported that graphene oxide (GO) suppresses TLR signaling pathways. In this study, we further investigated the in vivo toxicity of GO in zebrafish larvae via TLRs. Zebrafish embryos (5 h post fertilization; hpf) were exposed to 0.01, 0.1, 1 or 10 mg/L GO for up to 5 days, and the results revealed that GO induced developmental toxicity, in the form of delayed hatching time and increased deformity, mortality and heart rate; blood flow and linear velocity, however, remained unchanged. Meanwhile, the locomotor activities of larvae were also increased with GO exposure. As the possible mechanism for these effects, GO exposure significantly decreased TLR1, TLR4 and TLR5 expression, leading to suppressed expression of immune genes, including interleukin-1β (IL-1β), IL-6, IL-8, and tumor necrosis factor α (TNFα). Furthermore, the expression of perilipin 2 (PLIN2) was also decreased, which consequently led to decreased lipid droplet (LD) staining and triglyceride (TG) content. Interestingly, GO-induced deformity was significantly increased by TLR2/TLR4 agonist lipopolysaccharides (LPS), but less effectively promoted by TLR3 agonist poly inosinic: cytidylic acid (IC). The results from this study suggested that GO exposure induces developmental toxicity to zebrafish larvae via the suppression of TLR signaling pathways, likely due to immune inhibition and dysfunction of LD biogenesis.