A Silicon-Rhodamine Chemical-Genetic Hybrid for Far Red Voltage Imaging from Defined Neurons in Brain Slice

We describe the design, synthesis, and application of voltage-sensitive silicon rhodamines. Based on the Berkeley Red Sensor of Transmembrane potential, or BeRST, scaffold, the new dyes possess an isomeric molecular wire for improved alignment in the plasma membrane and contain 2' carboxylic acids for ready functionalization. Conjugation with secondary amines affords tertiary amides that localize to cellular membranes and respond to voltage changes with a 24% ΔF/F per 100 mV. When combined with a flexible polyethyleneglycol (PEG) linker and a chloroalkane HaloTag ligand, the new indicators, or isoBeRST dyes, enable voltage imaging from genetically defined cells and neurons. Covalent ligation of isoBeRST to cell surface-expressed HaloTag enzymes provides up to 3-fold improved labeling over previous, rhodamine-based hybrid strategies. We show that isoBeRST-Halo hybrid indicators achieve single-trial voltage imaging of membrane potential dynamics from dissociated rat hippocampal neurons or mouse cortical neurons in brain slices. With far-red/near infrared excitation and emission, turn-on response to action potentials, effective cell labeling in thick tissue, and excellent photostability, the new isoBeRST-Halo derivatives provide an important complement to voltage imaging in neurobiology.