Transgenic mouse lines expressing hybrid voltage sensors

Genetically encoded optical voltage sensors expand the optogenetic toolkit to enable the imaging of electrical activity from genetically defined populations of neurons. In a recent paper that appeared in the Journal of Neurophysiology, Wang et al reported the imaging of electrical activity in hippocampal slices from transgenic mice expressing hybrid voltage sensors (hVoS). hVoS probes are membrane targeted fluorescent proteins that have been optimized for a FRET interaction with dipicrylamine, a lipophilic molecule that partitions into lipid bilayers. A change in voltage alters the FRET interaction between the fluorescent protein and dipicrylamine to produce an optical signal that can be imaged.

Schematic illustration of the hVoS method. A fluorescent protein (XFP) resides at the inner surface of a cell membrane. Negative potentials (left) drive negatively charged DPA (red circles) toward the outer membrane surface, away from the XFP. Depolarization (right) pulls DPA toward the XFP, thus increasing FRET efficiency. From Wang et al., 2010.

Among the various genetically encoded voltage sensors currently under development in various labs, hVOS probes have a signal amplitude comparable to other probes (20-30% for 100 mV), but a very rapid response time (~0.5 msec). Thus, these probes are rapid enough to detect action potentials. Wang et al generated transgenic mice with two different high-performance hVoS probes under control of a neuron-specific thy-1 promoter. Hippocampal slices from these animals present distinct spatial patterns of expression, and electrical stimulation evoked fluorescence changes as high as 3%.

Signal spread upon electrical stimulation of the CA1 region. The electrode was positioned in the stratum radiatum. Stimulations evoked responses throughout the stratum radiatum and latency increased with distance. Left: the CA1 region is shown in a slice from the hVOS 2.0 line with fluorescence traces superimposed. The stratum pyramidale is highlighted by the red curve and the stimulation electrode is faintly visible (*). Right: selected traces from locations indicated in D (single trial responses without averaging). From Wang et al., 2012.

In some instances, clear responses were recorded in a single trial without averaging. One of the hVoS probes was found to target axons preferentially, due to the presence of an axonal membrane targeting motif (from GAP-43). This probe introduces the possibility of subcellular targeting to the field of optogenetics. The high density of probe-expressing neurons meant that optical signals reported voltage changes from populations of cells rather than single neurons. It will be important to test sparser targeting methods to determine whether optical signals can be detected from single cells. This study represents an important step toward the study of functional activity in networks of genetically defined populations of neurons in intact neural circuits.

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