Highlights d A PCR-based electroporation screen yielded an improved voltage indicator, ASAP3 d ASAP3 shows larger voltage responses than other fluorescent protein-based sensors d Ultrafast local volume excitation (ULoVE) boosts randomaccess two-photon signals d ASAP3 and ULoVE report subthreshold and spiking potentials in deep brain regions

Monitoring voltage dynamics in defined neurons deep in the brain is critical for unraveling the function of neuronal circuits but is challenging due to the limited performance of existing tools. In particular, while genetically encoded voltage indicators have shown promise for optical detection of voltage transients, many indicators exhibit low sensitivity when imaged under two-photon illumination. Previous studies thus fell short of visualizing voltage dynamics in individual neurons in single trials. Here, we report ASAP2s, a novel voltage indicator with improved sensitivity. By imaging ASAP2s using random-access multi-photon microscopy, we demonstrate robust single-trial detection of action potentials in organotypic slice cultures. We also show that ASAP2s enables two-photon imaging of graded potentials in organotypic slice cultures and in Drosophila. These results demonstrate that the combination of ASAP2s and fast two-photon imaging methods enables detection of neural electrical activity with subcellular spatial resolution and millisecond-timescale precision.DOI: http://dx.doi.org/10.7554/eLife.25690.001

Understanding information processing in the brain requires us to monitor neural activity at high spatiotemporal resolution. Using an ultrafast two-photon fluorescence microscope (2PFM) empowered by all-optical laser scanning, we imaged neural activity in vivo at up to 3,000 frames per second and submicron spatial resolution. This ultrafast imaging method enabled monitoring of both supra- and sub-threshold electrical activity down to 345 μm below the brain surface in head-fixed awake mice.

Imaging of transmembrane voltage deep in brain tissue with cellular resolution has the potential to reveal information processing by neuronal circuits in living animals with minimal perturbation. Multi-photon voltage imaging in vivo, however, is currently limited by speed and sensitivity of both indicators and imaging methods. Here, we report the engineering of an improved genetically encoded voltage indicator, ASAP3, which exhibits up to 51% fluorescence responses in the physiological voltage range, sub-millisecond activation kinetics, and full responsivity under two-photon illumination. We also introduce an ultrafast local volume excitation (ULOVE) two-photon scanning method to sample ASAP3 signals in awake mice at kilohertz rates with increased stability and sensitivity. ASAP3 and ULOVE allowed continuous single-trial tracking of spikes and subthreshold events for minutes in deep locations, with subcellular resolution, and with repeated sampling over multiple days. By imaging voltage in visual cortex neurons, we found evidence for cell type-dependent subthreshold modulation by locomotion. Thus, ASAP3 and ULOVE enable continuous high-speed highresolution imaging of electrical activity in deeply located genetically defined neurons during awake behavior.shaping 7,8 . However, parallel excitation is problematic for imaging, as scattered fluorescence signals from simultaneously excited cells would become intermixed, hindering detection of small responses. Furthermore, as GEVIs reside in the membrane rather than the cytosol, the number of indicator molecules that can be excited in an optical section through a mammalian cell body is typically smaller for GEVIs than for GECIs 4,6 .Given these limitations, the development of GEVIs with larger two-photon responses to electrical events of interest is highly desirable. Currently, the GEVIs with the largest responses to both subthreshold changes and APs are based on two types of voltage-sensing domains: seven-transmembrane helix opsins and four-transemembrane helix voltage-sensing domains (VSDs). Opsin-based GEVIs have been used in vivo with one-photon excitation to report electrical activity of superficially located neurons 9,10 , but their responsivity is severely attenuated under twophoton excitation 4,11 . In contrast, ASAP-family GEVIs, composed of a circularly permuted green fluorescent protein variant inserted within the VSD of G. gallus voltage-sensing phosphatase (Fig. 1a), are fully responsive under twophoton excitation 11 . In particular, ASAP2s demonstrates the largest response per AP of fluorescent protein-based GEVIs, but its kinetics are actually slower than earlier ASAP variants 11 . If ASAP-family kinetics and/or overall responsivity could be improved, then electrical events could be more easily detected by two-photon imaging.Here we report an improved indicator, ASAP3, resulting from novel methods for generation and screening of GEVI libraries in mammalian cells. ASAP3 features the largest responses of fluorescent GEVIs to either steady-state voltages or ...

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