Impact of Zn and the Zn transporter ZIP8 on M. tuberculosis infection of human macrophages
Eusondia Arnett1, Miranda Lumbreras1, Charlie Pyle2, Abul Azad1, Deandra Smith3, Varun Dwivedi1, Joanne Turner1, Daren Knoell3, Larry S Schlesinger1
1 Texas Biomedical Research Institute, San Antonio, TX
2 Duke University School of Medicine, Durham, NC
3 University of Nebraska Medical Center, Omaha, NE
Zinc (Zn) is an essential micronutrient whose concentration and location within cells is tightly regulated by Zn transporters (ZIPs and ZnTs that increase and decrease cytosolic Zn levels, respectively). ZIP8 is the only Zn transporter highly induced following M. tuberculosis (M.tb) infection of human macrophages. M.tb is a highly adept intracellular pathogen, residing within distinctive phagosomes. The capacity of M.tb to persevere in the phagosome depends in part on access to micronutrients. There is a fundamental gap in knowledge regarding the importance of Zn at the M.tb-host interface. Resolving this gap is critical since dietary Zn deficiency and polymorphic variants of ZIP8 are associated with increased susceptibility to TB and other infectious diseases. Our data indicate that ZIP8 is enriched at the M.tb phagosome in human macrophages and that myeloid-specific ZIP8 knockout mice exhibit reduced M.tb burden compared to wild type mice, whereas Zn deficient mice exhibit increased M.tb burden. Our central hypothesis is that M.tb exploits ZIP8 to enhance infection, and that ZIP8 mediates transport of Zn from the phagosome to the cytosol, thereby protecting bacteria from excessive Zn levels and Zn poisoning while leaving low Zn levels in the phagosome for M.tb growth, similar to Zn deficiency. Further, we hypothesize that ZIP8 increases cytosolic Zn levels to moderately dampen NFkB activation and cytokine production and enhance Metal Regulatory Transcription Factor 1 (MTF-1) signaling to enhance matrix metalloproteinase (MMP) production, thereby enabling M.tb growth and persistence. This is exacerbated during Zn deficiency, which severely lowers cytosolic Zn levels, leading to dysregulated NFkB and MTF-1 activation, mimicking the excessive inflammation seen in Zn deficient individuals with active TB. Thus, Zn intake and cellular concentration and location are expected to critically impact the immune response. We have confirmed that ZIP8 deficient macrophages have reduced M.tb burden relative to wild type and that pro-inflammatory cytokines are altered under this condition. These studies will provide new understanding of the Zn and ZIP8-dependent host response to M.tb infection and foster new micronutrient-based intervention strategies to improve prevention and treatment of TB.