The long-term goal of this research is to devise molecular sensors for binding, visualizing, and quantifying mobile zinc in neurobiological systems. Endogenous stores of zinc in presynaptic vesicles of hippocampal neurons in the brain are released upon physiological stimulation to perform an incompletely defined role in learning and memory. Similar mobile zinc stores present in the olfactory bulb (OB) process odorant information trans- mitted from the nose in a more direct signaling pathway. Uncontrolled Zn2+ release in the brain is associated with damage following seizure, ischemia, or blunt head trauma. In order to study these neurochemical phenomena, Zn2+ responsive sensors are required that can track the spatial and temporal distribution of the mobilized ion in response to physiological and pathological stimuli. The design, synthesis, evaluation, and optimization of the sensors constitute the major components of this research project. Each sensor will have up to three modules. Minimally, there will be zinc-binding and zinc-reporting units. The binding modules typically comprise multidentate ligands with variable Zn2+ affinity, selectivity for Zn2+ over competing ions in neuronal tissue, and fast, reversible coordination to monitor biological changes on the ms time scale. The zinc-reporting module will be either fluorescent or phosphorescent, for use in optical imaging (OI) experiments, or capable of altering water relaxation rates, for use in magnetic resonance imaging (MRI) studies. Fluorescent reporters include xanthenone and single-walled carbon nanotube derivatives. Phosphorescent sensors are based on cyclometalated iridium(III) complexes. MRI constructs utilize manganese(III) porphyrins. Strategies are adopted for attaching an optional third module to localize photoluminescence-based zinc sensors to programmed cellular targets to investigate Zn2+ dynamics at specific sites in a signal transduction pathway following physiological or pathological stimulation. An associated objective is to prepare zinc-selective, rapid chelating agents to be used in conjunction with investigations of the biological functions of mobile Zn2+. Thermodynamic, kinetic, photophysical, and theoretical studies of the zinc sensors and chelators will guide synthetic directions for making improvements to optimize their utility in applications. Specific applications include the evaluation by OI of hypotheses concerning the roles of mobile zinc in neurotransmission at mossy fiber synapses in the hippocampus and at glomeruli in the OB and the visualization by MRI of mobile zinc activity in the hippocampus under physiological and pathological conditions. This project is relevant to public health, for it will provide the means to test theories about the functions of mobile Zn2+ in the brain as well as the means by which to assess the postulated association of uncontrolled zinc levels with neurodegenerative diseases, such as Alzheimer's, and with more acute toxic encephalopathies. The chemistry devised will also facilitate the development of tools to measure mobile zinc stores that occur in other tissues such as the prostate and pancreas, where quantitation of mobile Zn2+ has the potential for early detection of diseases involving these organs.
This research involves the synthesis, characterization, and application of optical and magnetic resonance imaging tools to investigate the underlying mechanisms of mobile zinc in the brain. Signal transduction by mobile zinc is implicated in learning and memory and in the processing of odorant information from the nose. Uncontrolled zinc levels contribute to neurodegenerative diseases, which may be responsive to zinc-selective chelating agents also devised in the project.
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|Loas, Andrei; Radford, Robert J; Lippard, Stephen J (2014) Addition of a second binding site increases the dynamic range but alters the cellular localization of a red fluorescent probe for mobile zinc. Inorg Chem 53:6491-3|
|Radford, Robert J; Chyan, Wen; Lippard, Stephen J (2014) Peptide Targeting of Fluorescein-Based Sensors to Discrete Intracellular Locales. Chem Sci 5:4512-4516|
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