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revised on 21.01.2020 and posted on 22.01.2020by Tomas Fiala, Jihang Wang, Matthew Dunn, Peter Šebej, Se Joon Choi, Ekeoma Nwadibia, Eva Fialova, Diana M. Martinez, Claire E. Cheetham, Keri J. Fogle, Michael J. Palladino, Zachary Freyberg, David Sulzer, Dalibor Sames
Voltage sensitive fluorescent dyes (VSDs) are important tools for probing signal transduction in neurons and other excitable cells. These sensors, rendered highly lipophilic to anchor the conjugated pi-wire molecular framework in the membrane, offer several favorable functional parameters including fast response kinetics and high sensitivity to membrane potential changes. The impact of VSDs has, however, been limited due to the lack of cell-specific targeting methods in brain tissue or living animals. We address this key challenge by introducing a non-genetic molecular platform for cell- and molecule-specific targeting of synthetic voltage sensitive dyes in the brain. We employ a dextran polymer particle to overcome the inherent lipophilicity of voltage sensitive dyes by dynamic encapsulation, and high-affinity ligands to target the construct to specific neuronal cells utilizing only native components of the neurotransmission machinery at physiological expression levels. Dichloropane, a monoamine transporter ligand, enables targeting of dense dopaminergic axons in the mouse striatum and sparse noradrenergic axons in the mouse cortex in acute brain slices. PFQX in conjunction with ligand-directed acyl imidazole chemistry enables covalent labeling of AMPA-type glutamate receptors in the same brain regions. Probe variants bearing either a classical electrochromic ANEP dye or state-of-the-art VoltageFluor-type dye respond to membrane potential changes in a similar manner to the parent dyes, as shown by whole-cell patch recording. We demonstrate the feasibility of optical voltage recording with our probes in brain tissue with one-photon and two-photon fluorescence microscopy and define the signal limits of optical voltage imaging with synthetic sensors under a low photon budget determined by the native expression levels of the target proteins. We envision that modularity of our platform will enable its application to a variety of molecular targets and sensors, as well as lipophilic drugs and signaling modulators. This work demonstrates the feasibility of a chemical targeting approach and expands the possibilities of cell-specific imaging and pharmacology.
T.F. is the recipient of the Alfred Bader Fellowship in Organic Chemistry. This work was supported by the G. Harold & Leila Y. Mathers Charitable Foundation (to D. Sames) and the Collaborative and Multidisciplinary Pilot Research (CaMPR) award (to D.M.M.), NIDA R0107418 (to D. Sulzer) and the JPB Foundation (to D. Sulzer). The work in Drosophila brain was supported by John F. and Nancy A. Emmerling Fund of The Pittsburgh Foundation (to Z.F.), start-up funds from the University of Pittsburgh Department of Psychiatry (to Z.F.), start-up funds from the University of Pittsburgh Department of Neurobiology (to C.E.C. and Z.F.), and funding from the National Institutes of Health: R21AG059386 (to M.J.P.), R01GM108073 (to M.J.P.), and R21NS095614 (to M.J.P.).