Genetic variants at the solute carrier family 39 member 8 (SLC39A8) gene locus are associated with the regulation of whole-blood manganese (Mn) and multiple physiological traits. SLC39A8 encodes ZIP8, a divalent metal ion transporter best known for zinc transport. Here, we hypothesized that ZIP8 regulates Mn homeostasis and Mn-dependent enzymes to influence metabolism. We generated Slc39a8-inducible global-knockout (ZIP8-iKO) and liver-specific–knockout (ZIP8-LSKO) mice and observed markedly decreased Mn levels in multiple organs and whole blood of both mouse models. By contrast, liver-specific overexpression of human ZIP8 (adeno-associated virus–ZIP8 [AAV-ZIP8]) resulted in increased tissue and whole blood Mn levels. ZIP8 expression was localized to the hepatocyte canalicular membrane, and bile Mn levels were increased in ZIP8-LSKO and decreased in AAV-ZIP8 mice. ZIP8-LSKO mice also displayed decreased liver and kidney activity of the Mn-dependent enzyme arginase. Both ZIP8-iKO and ZIP8-LSKO mice had defective protein N-glycosylation, and humans homozygous for the minor allele at the lead SLC39A8 variant showed hypogalactosylation, consistent with decreased activity of another Mn-dependent enzyme, β-1,4-galactosyltransferase. In summary, hepatic ZIP8 reclaims Mn from bile and regulates whole-body Mn homeostasis, thereby modulating the activity of Mn-dependent enzymes. This work provides a mechanistic basis for the association of SLC39A8 with whole-blood Mn, potentially linking SLC39A8 variants with other physiological traits.
Wen Lin, David R. Vann, Paschalis-Thomas Doulias, Tao Wang, Gavin Landesberg, Xueli Li, Emanuela Ricciotti, Rosario Scalia, Miao He, Nicholas J. Hand, Daniel J. Rader
(A) qPCR analysis of Slc39a8 expression in male Slc39a8fl/fl and ZIP8-iKO mice injected with tamoxifen at 8 weeks of age and sacrificed 5 weeks after the injection (n = 3–6). (B) ICP-OES analysis of Mn levels in male Slc39a8fl/fl and ZIP8-iKO mice injected with tamoxifen at 8 weeks of age and sacrificed 5 weeks after injection (n = 5–6). (C) qPCR analysis of Slc39a8 expression in 12- to 14-week-old male WT and ZIP8 Het mice (n = 4–6). (D) ICP-OES analysis of Mn levels in 12- to 14-week-old male WT and ZIP8 Het mice (n = 4–5). qPCR results were normalized to Gapdh. ICP-OES results were normalized to wet tissue weight. Mn levels in the tissues were normalized to the average of the control group. Data on the absolute Mn content can be found in the Supplemental Table. All data represent the mean ± SD. ***P ≤ 0.001, **P ≤ 0.01, and *P ≤ 0.05, by Student’s t test.