Fluorescent biosensors for subcellular pharmacokinetics

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Drug development for the central nervous system (CNS), especially for psychiatry, has slowed, partially because we do not know the mechanisms by which some drugs exert their therapeutic or harmful effects. The project provides data to test the hypothesis that several CNS drugs act, in addition to their acute effects, in a slower, “inside-out” fashion. The drugs would start by binding to their classical molecular targets, but in organelles. By measuring neural drugs, and their target interactions, within organelles of living cells, this project helps to test inside-out pharmacology. The experiments invent, then exploit, genetically encoded fluorescent biosensors to measure drugs in organelles. The biosensors are bacterial and archaeal periplasmic binding proteins (PBPs), fused to circularly permuted green fluorescent protein (cp-GFP). Sub-Approach A is a solution-based screen of drugs x existing biosensors. The library of 92 compounds includes many orally available drugs approved for various indications, but emphasizing psychiatry. The collection of 60 purified biosensor proteins comprises five existing families, which now sense glutamate, dopamine, GABA, and serotonergic drugs. Sub-Approach B utilizes “directed evolution” to improve the “hits”, toward the goal of detecting the drugs at pharmacologically appropriate sub-micromolar concentrations. The major tools—site- saturation mutagenesis, atomic-scale structure, computational docking, and high-through fluorescence screening--are expected to converge on appropriate biosensors. Sub-Approach C expresses the refined biosensors in ER and performs live-cell, time-resolved imaging while the drugs are applied extracellularly. We begin with the simple questions, “does the drug enter the ER, and how quickly?” We then analyze signals within organelles that also express the classical targets for the drugs. We expect a rich set of data on “kinetic buffering” of diffusion by binding to the targets within ER, thus revealing drug-receptor interaction within organelles of live cells. The sub-approach then graduates to mouse preparations, using viral vectors, brain slices, and two-photon imaging in intact animals will be employed. Sub-Approaches D and E complement each other. D extends subcellular pharmacokinetics to acidic organelles, including secretory granules and neurotransmitter vesicles already suspected of accumulating drugs via “acid trapping”. We'll retain the PBP portions of the biosensors, but employ additional cp-fluorescent proteins, known to function at low pH, and also modify linkers. The result will become a collection of fluorescent biosensor platforms, each specialized to perform best within, and targeted to, a class of organelles. Sub-approach E extends the drug biosensor strategy to new classes of PBPs, and to new classes of drugs. We will retain the cp-fluorescent protein part of the biosensors, but optimize the new PBPs and linkers. The transformative overall results will produce at least ten, and as many as 100, biosensors to detect drugs within organelles, and a clear roadmap for subcellular pharmacokinetics as a robust research tool. Data could suggest transformative therapeutic strategies for psychiatry, addiction, and neurodegeneration.