For the past three decades, the O’Halloran group has investigated how fluctuations in metal ions inside cells influence key cellular decisions. Using genetic, chemical, structural, mechanistic and biological imaging methods, the group has uncovered new types of metal receptors and tied their function to a number of disease-related physiological processes, including diabetes, fungal infections, malaria and cancer.
Figure 1. The E. coli Metallome. The total metal content of E. coli grown in minimal media (grey) as compared with the total metal content of media alone (white) determined by inductively coupled mass spectrometry (ICP-MS) (Outten et al., 2001).
The O’Halloran group identified many of examples of metalloregulatory proteins–factors that govern metal responsive gene expression. Through structure, function, genetic and thermodynamic studies the group has established the molecular basis of allosteric control and metal ion selectivity. This work revealed that the pool of free zinc or copper ions in the cellular cytoplasm under resting conditions is vanishingly small, challenging long-held views that Fe, Cu and Zn are trace elements in cells. In 2001 Caryn Outten was the first to analytically define the cellular metallome (Figure 1) and showed that the total content of essential transition metals is tightly controlled in the millimolar range.
The O’Halloran group also discovered a new family of intracellular metal trafficking proteins called metallochaperone proteins. The group defined the structures, functions and mechanisms of a number of these proteins that control intracellular distribution of copper. These freely diffusing proteins traffic copper to specific cytosolic targets, and have since been found in many kingdoms of life.
- Gilston BA, O’Halloran T. “Mechanisms Controlling the Cellular Metal Economy”. In: Cullota V, Scott R, editors. The Encyclopedia of Inorganic and Bioinorganic Chemistry: Metals in Cells. Chichester, UK: John Wiley & Sons, Ltd.; 2013. p. 3-14.
- Outten CE, O’Halloran TV. “Femtomolar sensitivity of metalloregulatory proteins controlling zinc homeostasis”. Science. 2001;292(5526):2488-92.
- Lin, S., Pufahl, R., Dancis, A., O’Halloran, T.V., Culotta, V.C. “A Role for the Saccharomyces cerevisiae ATXI Gene in Copper Trafficking and Iron Transport.” J. Biological Chemistry, 1997, 272, 9215-9220.
- Pufahl RA, Singer CP, Peariso KL, Lin SJ, Schmidt PJ, Fahrni CJ, Culotta VC, Penner-Hahn JE, O’Halloran TV. “Metal ion chaperone function of the soluble Cu(I) receptor Atx1”. Science. 1997;278(5339):853-6.
- O’Halloran, T. V. “Transition Metals in Control of Gene Expression.” Science.1993; 261, 715-725.
The Metallome in Disease
Figure 2: Zinc is localized within the Plasmodium falciparum plasma membrane. The parasite accrues labile zinc within internal compartments separate from the food vacuole and nuclear regions. B & W images: transmitted light microscopy. Green= zinc imaged using the fluorescent zinc dye Zinbo-7. Red = DNA imaged with the DNA specific dye Syto-21.
The O’Halloran group identified atypical cellular metallomes in several pathologies with the goal of developing new therapeutic strategies to combat these diseases. Disruption of the metallome can lead to functional impairment of insulin producing β-cells that may be associated with type-2 diabetes mellitus. Furthermore, there may be a potential role for zinc fluxes in clearing insulin from the bloodstream. The group published the first studies demonstrating a direct role for a copper transporter in fungal disease and developed a method to identify small molecules that target the zinc homeostasis machinery, aiming to develop antifungal agents. The group also ascertained that zinc fluxes are essential for the normal life cycle of the malaria causing parasite, Plasmodium falciparum (Figure 2).
Current studies focus on the ionic composition of nuclei in cancer and how the metallome modulates chromatin conformation and nuclear shape, and ultimately, gene expression and the emergence of cancer phenotypes. The group is testing the hypothesis that ion imbalances are significant contributors to the transformed phenotype of metastatic breast cancer, glioblastoma and multiple myeloma cancer cells. The overarching goal of this project is to translate these physical science insights into the role of the intracellular ion concentrations in modulating tumor cell behavior into the development of new targets for therapeutic intervention in metastatic disease.
- El Muayed M, Raja MR, Zhang X, Macrenaris KW, Bhatt S, Chen X, Urbanek M, O’Halloran TV, Lowe WL Jr. “Accumulation of cadmium in insulin-producing B cells.” Islets, 2012 Nov 1;4(6):405-16.
- Raja MR, Waterman SR, Qiu J, Bleher R, Williamson PR, O’Halloran TV. “A copper hyperaccumulation phenotype correlates with pathogenesis in Cryptococcus neoformans”. Metallomics. 2013;5(4):363-71.
- O’Halloran TV, Kebede M, Philips SJ, Attie AD. “Zinc, insulin, and the liver: a menage a trois”. Journal of Clinical Investigation. 2013;123(10):4136-9.
- Marvin RG, Wolford JL, Kidd MJ, Murphy S, Ward J, Que EL, Mayer ML, Penner-Hahn JE, Haldar K, O’Halloran TV. “Fluxes in “free” and total zinc are essential for progression of intraerythrocytic stages of Plasmodium falciparum”. Chemistry and Biology. 2012;19(6):731-41.
- Waterman SR, Park YD, Raja M, Qiu J, Hammoud DA, O’Halloran TV, Williamson PR. Role of CTR4 in the Virulence of Cryptococcus neoformans. mBio. 2012;3(5).
- Simm C, Luan CH, Weiss E, O’Halloran T. High-throughput screen for identifying small molecules that target fungal zinc homeostasis. PloS one. 2011;6(9):e25136. Epub 2011/10/08.
- Furukawa, Y., Torres, A.S., O’Halloran, T.V. “Oxygen-induced Maturation of SOD1: A Key Role for Disulfide Formation by the Copper Chaperone CCS.” European Molecular Biology Organization (EMBO) Journal. 2004, 23, 2872-2881.
- Rae, T.D., Schmidt, P.J., Pufahl, R.A., Culotta, V.C., and O’Halloran, T.V. “Undetectable Free Intracellular Copper: the Requirement of a Copper Chaperone for Superoxide Dismutase.” Science. 1999; 284, 805-808.