Zinc is an essential human nutrient that must be tightly regulated, as both zinc excess and deficiency are deleterious. Sophisticated mechanisms of zinc homeostasis allow animals to sense and respond to imbalances in zinc at the cellular level. However, the understanding of these mechanisms remains incomplete. Our recent discoveries in the roundworm C. elegans have suggested new models for mechanisms of zinc homeostasis that have important implications for human biology. C. elegans has proven to be an ideal model organism for the study of zinc biology thanks to new methods in manipulating dietary zinc in media, newly described metal- related phenotypes, and genetic manipulations common in the worm community. In this system, we discovered that the High Zinc Activation (HZA) element mediates the transcriptional response of multiple genes in response to high dietary zinc. Further, we discovered the high zinc activated nuclear receptor (HIZR-1), which is the master regulator of zinc homeostasis, as hizr-1(lf) mutants fail to induce zinc response genes and these mutants are hypersensitive to zinc toxicity. The HIZR-1 DNA binding domain (DBD) directly binds the HZA and the HIZR-1 ligand-binding domain (LBD) directly binds zinc. HIZR-1 responds to high dietary zinc by accumulating in the nucleus. Most interestingly, a chimeric Gal4(DBD)::HIZR-1(LBD) fusion protein confers zinc responsiveness in human cell culture. These observations suggested two exciting hypotheses: (1) HIZR-1 responds to high levels of zinc because specific cysteine, histidine and/or acidic amino acids in the ligand- binding domain directly coordinate zinc. (2) The high zinc homeostasis response in human cells is regulated by a functional human ortholog of hizr-1. I will test these hypotheses by, (1) defining the mechanism of zinc binding by structure-function analysis of the HIZR-1 ligand binding domain, and, (2) determining if a human orphan nuclear receptor senses high zinc by directly binding zinc in vitro and in vivo. These studies will be impactful by defining the mechanism of action of a new high zinc sensor and potentially identifying a human high zinc sensor with implications for treating human diseases of abnormal zinc homeostasis. The ability of animals to sense high and low dietary zinc and respond by adjusting uptake, storage and excretion is critical for homeostasis. The proposed experiments build on our exciting preliminary results that generated innovative new hypotheses about mechanisms of zinc homeostasis. I will directly test these hypotheses in worms and human cells. Aberrant zinc accumulation is implicated in several human diseases, and the results may suggest new therapeutic strategies for addressing disorders of zinc metabolism in humans.
Zinc homeostasis is an essential process for all life on earth, with limitations of homeostasis resulting in a milieu of diseases both from lack of zinc and excess zinc in a biological system. We have discovered a significant part of the mechanism of cellular zinc homeostasis in the roundworm C. elegans, which uses a nuclear receptor to sense excess zinc and mediate a transcriptional response to return zinc to the proper balance. By characterizing the mechanism of zinc binding in nuclear receptors and establishing the orthologous zinc homeostasis system in humans, we will enable the development of new therapeutic strategies to treat the disease milieu of zinc dysregulation and disorder.
