Mechanism of neuroinflammatory injury induced by lead and lipopolysaccharide in mice

Background Both lead and lipopolysaccharide (LPS) exposures can result in neuroinflammation and microglia polarization, but it is not clear whether the combined exposure to lead and LPS will aggravate neuroinflammation and the polarization of microglia.

Objective The in vivo experiment investigates the effect and mechanism of combined exposure to lead and LPS on neuroinflammation in mice.

Methods A total of 64 SPF male C57 mice were randomly divided into eight groups: control group, LPS group, low lead group, medium lead group, high lead group, low lead+LPS group, medium lead+LPS group, and high lead+LPS group. Mice in the lead exposure groups were given 0.25, 0.50, and 1.00 g·L^-1 lead acetate through drinking water for nine weeks respectively. The LPS treatment was administered at a dose of 250 g·kg^-1 (by weight) for 7 d at the 9th week of lead exposure. The animals were sacrified 24 h after the last injection. The changes of inflammatory factors tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6) in the cortex were detected by ELLSA. The expressions of M1 markersinduced nitric oxide synthase (iNOS), NADPH oxidase 2 (NOX2), andchemokine receptor 7 (CCR7), M2 markersarginase 1 (Arg-1), transforming growth factor-β (TGF-β), and chemokine receptor 2 (CCR2), triggering receptor expressed on myeloid cells 2 (TREM2) mRNA and microRNA-34a (miR-34a) were detected by quantitative PCR. The protein expressions of TREM2, iNOS, and Arg-1 were detected by Western blotting.

Results Compared with the control group, the levels of inflammatory cytokines TNF-α, IL-1β, and IL-6 in the cortex of mice were increased in the LPS group, the medium lead group, and the high lead group. Compared with the LPS group or the corresponding lead group, the levels of TNF-α (7.82±0.60, 8.79±1.62), IL^-1β (8.80±0.95, 11.18±0.92), and IL-6 (7.53±0.90, 8.66±0.93) were elevated in the medium lead+LPS group and the high lead+LPS group. The expression levels of M1 markers iNOS, NOX2, and CCR7 mRNA and iNOS protein in the cortex of the LPS group, the medium lead group, and the high lead group were higher than those of the control group, while the expression levels of M2 markers Arg-1, TGF-β, and CCR2 mRNA and Arg-1 protein were lower, especially the expression of CCR2 mRNA (P < 0.05). Compared with the LPS group or the corresponding lead group, the mRNA and protein expression levels of iNOS and the CCR7 mRNA expression levels of the medium lead+LPS group and the high lead+LPS group were increased, while the expression levels of Arg-1 mRNA were decreased. In addition, the TREM2 mRNA (0.20±0.03, 0.04±0.01) and protein (0.32±0.02, 0.21±0.02) expression levels in the cortex of the medium lead+LPS and the high lead+LPS groups were lower than those of the LPS group or the corresponding lead group, while the expression level of miR-34a was increased (P < 0.05).

Conclusion Combined exposure to lead and LPS aggravates neuroinflammatory injury and increases M1 polarization of microglia, which may be related to the increased expression of TREM2 up-regulated by miR-34a.