Genetically Encoded Voltage Indicators Are Illuminating Subcellular Physiology of the Axon

Abstract

Everything we see and do is regulated by electrical signals in our nerves and muscle. Ion channels are crucial for sensing and generating electrical signals. Two voltage-dependent conductances, Na + and K + , form the bedrock of the electrical impulse in the brain known as the action potential. Several classes of mammalian neurons express combinations of nearly 100 different varieties of these two voltage-dependent channels and their subunits. Not surprisingly, this variability orchestrates a diversity of action potential shapes and firing patterns that have been studied in detail at neural somata. A remarkably understudied phenomena exists in subcellular compartments of the axon, where action potentials initiate synaptic transmission. Ion channel research was catalyzed by the invention of glass electrodes to measure electrical signals in cell membranes, however, progress in the field of neurobiology has been stymied by the fact that most axons in the mammalian CNS are far too small and delicate for measuring ion channel function with electrodes. These quantitative measurements of membrane voltage can be achieved within the axon using light. A revolution of optical voltage sensors has enabled exploring important questions of how ion channels regulate axon physiology and synaptic transmission. In this review we will consider advantages and disadvantages of different fluorescent voltage indicators and discuss particularly relevant questions that these indicators can elucidate for understanding the crucial relationship between action potentials and synaptic transmission.

Photocycle Dynamics of the Archaerhodopsin 3 Based Fluorescent Voltage Sensor QuasAr1

“…Tracking membrane potential of cells, especially neurons, using fluorescence methods is of high interest and is an active field of research (change of membrane voltage causes change of fluorescence efficiency) [1][2][3][4][5][6][7][8][9][10]. To determine membrane voltage, a variety of voltage sensitive dyes [11][12][13], genetically encoded calcium indicators (GECI) [4,14,15], and genetically encoded voltage indicators (GEVI) based on voltage sensing domains (VSD, composed of four trans-membrane helices and fused fluorescent proteins) [16][17][18][19][20] and on microbial rhodopsins (composed of seven trans-membrane α-helices with covalently bound retinal, using the intrinsic fluorescence of retinal [9,10,[21][22][23] or the modified fluorescence from attached fluorescent proteins [19,23,24] or dyes [12]) are in use.…”

Antidromic Analog Signaling

“…For technical reasons, the pace of research in the field of axon signaling has been relatively slow up to now. However, we predict that the development of new, brighter and faster, genetically encoded voltage indicators (for a recent review see Panzera and Hoppa, 2019) will soon open new avenues in the study of the axon, notably by allowing the simultaneous imaging of different axonal regions, far away from the somatic compartment of neurons.…”

High fidelity estimates of spikes and subthreshold waveforms from 1-photon voltage imagingin vivo

“…Precise spike waveforms are also useful to distinguish different spike types, e.g. complex vs. simple spikes, and subcellular spike modulations (Panzera and Hoppa, 2019). Accurate information on membrane voltages is essential for constraining detailed biophysical models of circuit function.…”