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Master’s & Doctoral Defenses

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The Public presentation portion of a defense is open to everyone and is an especially valuable opportunity for graduate students to experience the process firsthand.

Note: All information is provided by the academic units.

Amanda Drewicz

Title: Stable Isotope Geochemistry of Bioapatite
Program: Doctor of Philosophy in Geoscience
Advisor: Dr. Matthew Kohn, Geosciences
Committee: Dr. Christopher Hill, Anthropology, Marion Lytle, Geosciences, and Dr. Jennifer Pierce, Geosciences
Date: August 22, 2018
Time: 3:00 p.m.
Location: Engineering Building, Room 110

Read Amanda Drewicz's Abstract Here

Although past climates cannot be used as direct analogs for future climate change, understanding how previous environments responded to changing climates can help inform policy surrounding future climate change. Presented here are reconstructed climates within the interior western United States, from two different geologic time periods. Each had a different climate that differed greatly from modern day environments from the same locations. A new approach for understanding climate is also presented using hydrogen isotopes in tooth enamel. Expanding our isotopic toolbox for climate reconstructions allows for more certain interpretations, and the use of tooth enamel stable hydrogen (δD), oxygen (δ18O), and carbon (δ13C) compositions allow for more sound climate reconstructions.
The mid-Miocene Climatic Optimum (MMCO), between c. 17 and c. 14 Ma, represents the warmest period on Earth in the last 35 Ma, and is thought to reflect a high partial pressure of atmospheric CO2 (pCO2). Using tooth enamel δ13C values from the interior Pacific Northwest, mean annual precipitation (MAP) was estimated before, during, and following the MMCO, to test whether MAP tracks pCO2 levels. This work speculates high pCO2 contributed to higher MAP at c. 28 and 15.1 Ma, and lower pCO2 contributed to lower MAP for other time periods. Terrestrial climates during the MMCO were likely more dynamic than originally considered, with wet-warm and cool-dry cycles reflecting 20-, 40-, and 100-ka Milankovitch cycles. Modern climate models predict that the Pacific Northwest will become wetter and warmer with increased CO2 levels, and this climate projection is consistent with MMCO climates associated with high pCO2 levels.
Tooth enamel δ18O and δ13C values and tufa δ18O values from well-dated late Pleistocene deposits in the Las Vegas Wash (LVW), Nevada, were used to reconstruct past precipitation seasonality, where enhanced net precipitation aided in the expansion of desert wetlands. Low late Pleistocene water δ18O values, inferred from tufa and tooth enamel, indicate that paleowetland expansion likely resulted from increased winter precipitation derived from high latitudes of the Pacific Ocean. Low tooth enamel δ13C and inferred %C4 grass values are consistent with an increase in proportion of winter precipitation. Increased winter precipitation diverges from late Pleistocene climate reconstructions at lower latitudes in the American Southwest and modern-day climes that receive nearly equal proportions of winter and summer moisture.
Stable hydrogen and oxygen isotope compositions correlate between organic tissues and meteoric water. This correlation was tested for the first time in inorganic modern herbivore tooth enamel by measuring oxygen and hydrogen isotope compositions from localities were water compositions are well known. Against expectations, δD and δ18O values of modern tooth enamel do not align with the Global Meteoric Water Line (GMWL) and hydrogen isotope compositions display little isotopic variation (c. 35‰) between vastly different geographic locations, but a strong correlation (R2 = 0.84) indicates a coupling between these two isotopes. Tooth enamel δD values were compared to local precipitation and surface water compositions, which generally correlate (R2 = 0.71), suggesting tooth enamel hydrogen somewhat reflects biogenic water compositions. However, when hydrogen peak area and hydrogen amount (H mg/sample mg) are compared to sample weight (mg), it is evident that additional, labile hydrogen, is adsorbed onto bioapatite crystallites, and c. 80% of measured hydrogen from “tooth enamel” is actually labile hydrogen. The timing of exchange between adsorbed water onto bioapatite crystallites and water vapor was extracted by equilibrating powdered samples with very high and low water vapor compositions for 48 hours, and then exposing samples to laboratory air for up to 8 hours. In both experiments, adsorbed water and laboratory water vapor equilibrate within 1 to 2 hours of exposure. Rather quick equilibration rates complicate stable hydrogen isotope interpretations, because the almost instantaneous equilibration between adsorbed water (onto bioapatite) and distinct local water vapor would result in different measured δD values for a single specimen among laboratories. Powdered enamel was then heated to 70 °C for 48 hours in attempt to remove labile hydrogen. Measured δD values from tooth enamel, that were immediately removed from the oven, were c. 8 to 30‰ lower than samples equilibrated with laboratory air for 8 hours. The lower δD values of heated tooth enamel do not result from the elimination of adsorbed water molecules, but may rather represent a different, temperature-dependent partition coefficient between adsorbed water (on apatite) and water vapor.

Sheenah Bryant

Title: Novel Regulatory Pathways of Protein Channels
Program: Doctor of Philosophy in Biomolecular Sciences
Advisor: Dr. Daniel Fologea, Physics
Committee: Dr. Julia Thom Oxford, Biological Sciences, Dr. Kenneth A. Cornell, Chemistry and Biochemistry, and Dr. Denise G. Wingett, Biological Sciences
Date: September 10, 2018
Time: 9:30 p.m.
Location: Mutlipurpose Room 108

Read Sheenah Bryant's Abstract Here

Since the proposal of the fluid mosaic model of a cell membrane, substantial scientific evidence has shown that the cell membrane is not simply an inert structure with the sole role of separating two chemically different environments. The cell membrane dynamically satisfies basic needs; such as water, ion and nutrient transport; without which the cell could not survive. It is a structure which actively participates in a great variety of physiological functions. The activity of the cell membrane is responsible for the contraction of our muscles and information processing of our brains. In order to participate in such a wide range of biological processes, the cell membrane incorporates an extensive variety of protein transporters in its structure. These transporters contribute to the selective barrier function of the membrane, and are highly regulated.
It is this regulation that enables certain complex physiological functions. The mechanisms of regulation of membrane transporters are obvious in the case of ion channels, which are transmembrane protein transporters facilitating controlled transport of specific ions across the membrane. Their regulation is mediated by specific physical or chemical stimuli, of which voltage, ligands, temperature, light and pressure are most common. However, recent reports indicate that regulation of such transporters may also be achieved by other environmental factors which are not easy to identify in the complex biochemical environment of the cell. Understanding these novel environmental factors and how they modulate the transport across membranes may be a crucial step to better understand the functionality of transmembrane transporters in health and disease.
In this respect, the work presented here employs a highly regulated transmembrane transporter; Lysenin, a pore forming toxin extracted from red earthworms, shares many of the fundamental features of ion channels, such as voltage and ligand regulation. In addition to these features, lysenin accumulates in lipid rafts (which are ubiquitous in animal cells). This model transporter offers opportunities to investigate novel regulatory pathways that are otherwise very difficult to identify in a living cell. In the work presented in this dissertation, I investigate how specific physical and chemical determinants of the membrane and surrounding solution, as well as the gating mechanism itself, may contribute to the emergence of unexpected cellular functionalities.
In this endeavor, I showed that increasing the local density of lysenin channels in a target membrane substantially changed the voltage-induced regulation, and that this density can be simply manipulated by altering the membrane’s lipid composition. Next, I demonstrated that the macroergic molecule ATP plays an important role in adjusting the conductance of pore-forming transporters and modulates their biological activity. These observations expand the well-established role of ATP as a signaling molecule, which has been proposed and well-studied for the last several decades. Finally, based on experimental observations that lysenin is endowed with molecular memory, I hypothesized a gating mechanism capable of explaining such a novel and unexpected feature. For these investigations, I focused my work on understanding the influence of multivalent cations on lysenin, which are capable of modulating the voltage-induced gating by electrostatic screening of the voltage domain sensor. The proposed gating mechanism, in which the voltage domain sensor moves into the hydrophobic core of the membrane upon gating, is supported by experimental evidence showing that anion binding to the channel lumen presents qualitative and quantitative differences in voltage regulation, as opposed to binding to the voltage domains sensor.
Therefore, the work presented here advances our knowledge with respect to how transmembrane transporters are influenced by frequently overlooked environmental factors, and how this may significantly contribute to the achievement of novel physiological functions. This level of understanding may prove crucial for determining potential connections between metabolic pathways and channelopathies which are commonly attributed to genetic defects of ion channels.