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Title:Mechanisms underlying polyene macrolide mediated rescue of growth in ion channel deficient yeast
Author(s):Hou, Jennifer
Director of Research:Burke, Martin D.
Doctoral Committee Chair(s):Burke, Martin D.
Doctoral Committee Member(s):Gennis, Robert B.; Orlean, Peter; Procko, Erik
Department / Program:Biochemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):yeast physiology
protein deficiency
small molecule
ion channel
ion homeostasis
cell growth
Abstract:Human diseases caused by missing or dysfunctional protein ion channels, known as channelopathies, affect several organs and systems of the body including that of the heart, lungs, kidneys, and nervous system. Channelopathies can result in devastating and potentially life-threatening diseases. To date, more than 30 human channelopathies are incurable. Although significant advances have been made in both gene therapy and protein replacement therapies, there is still a major need for new treatment strategies. We questioned whether an imperfect small molecule mimic of a missing protein ion channel could be sufficient to restore physiology in protein-deficient systems. Small molecules possess many qualities as potential therapeutics including that they are orally bio-available, cell permeable, and minimally immunogenic. Specifically for small molecule inhibitors, their primary mechanism of action is to bind to and block over-active proteins. However, in the case of unexpressed or degraded proteins where no target is available, this approach is futile. We hypothesized that through functionally interfacing with inherent networks of protein ion pumps and transporters, an imperfect small molecule ion channel could be enough to rescue physiology. Amphotericin B (AmB) is a prime example of an ion channel forming small molecule. Clinically for more than 60 years, medical providers have utilized AmB to treat patients suffering from invasive, systemic fungal infections. Previously, AmB’s mode of cell killing was thought to be through ion channel formation. However, The Burke Group at The University of Illinois at Urbana-Champaign showed that AmB kills yeast through sterol extraction. Moreover, we found that AmB only caused cell death when the ratio of AmB exceeded that of sterols; therefore, pre-complexation of AmB with sterols serves as a strategy to ameliorate toxicity. Alternatively, to harness AmB’s ion channel forming activity, we employed this small molecule at low concentrations. Further characterization showed that AmB small molecule ion channels are unselective, wherein they conduct potassium (K+), sodium (Na+), chloride (Cl-), and bicarbonate (HCO3-) ions. We employed Saccharomyces cerevisiae (S.cerevisiae) as an outstanding model organism because this unicellular eukaryote is genetically tractable, well-annotated, and amenable to genetic deletion. Furthermore, yeast have been widely utilized to elucidate several, fundamental biological processes including cell cycle regulation, telomere maintenance, and vesicle trafficking, and to characterize the pathophysiology of human disease. Yeast express two primary K+ transporters, Trk1 and Trk2 that are localized to the plasma membrane. Proton (H+)-ATPases Pma1 and V-ATPase utilize the energy from ATP hydrolysis to drive H+ out of cells or into vacuoles respectively, thereby producing favorable electrochemical gradients. These gradients promote K+ flow through Trk1 and Trk2 passive transporters into cells, to achieve cytosolic concentrations around 200-300 mM and cation storage into vacuoles. For cellular physiology, K+ is crucial for retaining intracellular pH, sustaining plasma membrane potential, and promoting enzyme activity. Deficiencies of these integral K+ transporters result in a loss of cell growth under standard laboratory conditions (10-15 mM K+). We found that an imperfect small molecule mimic of a missing protein ion channel, a.k.a. molecular prosthetic is sufficient to restore physiology in a protein-deficient model organism. We demonstrated that AmB both vigorously and sustainably restored physiology in K+ transporter deficient yeast (trk1Δtrk2Δ). Furthermore, we showed that AmB mediated rescue is ion channel dependent. Contributing to the lab’s molecular prostheses program, this thesis work has made four major contributions. First, we predicted that small molecule mediated rescue is generalizable. Initially, we reconfirmed literature reports and demonstrated that Nystatin A1, Candicidin, and Mepartricin permeabilize yeast. Then to test our prediction, we employed two assays: the disc diffusion and micro-broth dilution assay. We observed no rescue of growth with a Natamycin control (does not form ion channels). In contrast, we observed restored cell growth for Nystatin A1 (optimal at 1 μM), Candicidin (8 nM), and Mepartricin (8 nM). These findings suggest the versatility of trk1Δtrk2Δ homeostatic mechanisms to functionally interface with different imperfect small molecule mimics, and thereby rescue physiology. Second, we elucidated a mechanism by which imperfect small molecule ion channels rescue physiology in yeast. We hypothesized that small molecule mimics harness favorable electrochemical gradients to drive K+ into cells and back to physiological levels, thereby restoring cell growth in trk1Δtrk2Δ yeast. We have obtained evidence in support of each of these steps. Consistent with prior reports, trk1Δtrk2Δ yeast are hyperpolarized compared to that of WT. These gradients drive K+ into cells as demonstrated by radioactive rubidium (86Rb+, surrogate tracer for K+) influx assays. Next, we showed that through working together with the cell’s endogenous network of protein ion pumps, small molecule ion channels significantly restored total cellular K+ levels in trk1Δtrk2Δ back to that found in WT, in both a dose- and time-dependent manner. In contrast, Natamycin did not restore total cellular K+ content nor cell growth. Furthermore, chemical inhibition of H+-ATPases abrogated restoration of total cellular K+ content and growth. Thus, these results provide mechanistic evidence for how small molecule ion channels rescue physiology. Third, our findings suggest that imperfect small molecule mediated rescue of physiology is possible through functional collaboration with the cell’s inherent network of protein ion pumps and transporters. AmB and other polyene macrolide ion channels are unregulated, unrectified, unselective, and passive. However, we hypothesized that through working together with H+-ATPases (drivers) and transporters (correctors) that imperfect small molecule ion channels are sufficient to restore physiology. V-ATPase and Pma1 generate favorable electrochemical gradients. These gradients promote K+ flux through Trk1 and Trk2 and into vacuoles. We predicted that inhibiting V-ATPase or Pma1 would abrogate small molecule mediated rescue of trk1Δtrk2Δ. To test our hypothesis, we employed three chemical inhibitors: Nocodazole (impedes microtubule dynamics), Bafilomycin (blocks V-ATPase), and Ebselen (inhibits Pma1) against small molecule ion channel treated WT and trk1Δtrk2Δ. We observed exceptional sensitivity in small molecule rescued trk1Δtrk2Δ cells to Bafilomycin and to Ebselen blockage compared to that observed in WT. In contrast, minor differences were observed between these two treatment groups, to Nocodazole inhibition. Therefore, these results suggest that cells require active V-ATPase and Pma1, possibly to drive K+ through small molecule ion channels, for rescue of trk1Δtrk2Δ cell growth. Next, we hypothesized that endogenous protein transporters are crucial for correcting the lack of ion selectivity of small molecule ion channels. Yeast express plasma membrane localized voltage gated K+ efflux pump Tok1, Na+(K+)/H+ antiporter Nha1, and Na+-ATPase Ena1-5 that extrude excess K+ and Na+. We acquired a series of isogenic yeast mutants and conducted the liquid broth rescue assay. We observed that small molecule mimics did not recover growth in trk1Δtrk2Δnha1Δ yeast for any concentrations screened. Importantly, the nha1Δ mutant showed similar growth to that of WT. In contrast, deletion of either Tok1 (trk1Δtrk2Δtok1Δ) or Ena1-5 (trk1Δtrk2Δena1-5Δ) did not hamper small molecule mediated rescue. Thus, these findings suggest that Nha1 is important for expelling excess Na+ and/or K+ from small molecule treated trk1Δtrk2Δ. Last, in collaboration with The Mitchell Group, we have identified several potential and uncharacterized small molecule polyene macrolides through biosynthetic gene cluster analysis. We hypothesized through interrogating the gene clusters of several, highly conserved domains including the aminotransferase, ketosynthase, etc. that we can identify novel producing organisms of polyene macrolides. Through utilizing a moderate throughput yeast screening platform, we have found several organisms that produce extracts that may contain novel polyene macrolides. In summary, this thesis work culminates into four major findings: first, small molecule mediated rescue is generalizable to other polyene macrolide family members; second, a primary mechanism by which small molecule ion channels rescue cell growth in trk1Δtrk2Δ yeast is through restored cellular K+ content; third, small molecule mimics work in conjunction with the cell’s inherent network of protein ion pumps and transporters to restore physiology; and last, we have identified several potential and uncharacterized polyene macrolides through biosynthetic gene cluster analysis. In conclusion, these results demonstrate the potential for imperfect small molecule mimics to provide function in the case of protein ion channel deficiencies. Furthermore, these findings showcase an in-depth mechanistic pathway for how imperfect small molecule mimics restore cellular physiology in protein-deficient yeast. Intriguingly, the same protein characters responsible for restoring physiology in yeast may also be at play in other eukaryotic systems.
Issue Date:2018-06-27
Rights Information:Copyright 2018 Jennifer Hou
Date Available in IDEALS:2018-09-27
Date Deposited:2018-08

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