Biosensing Motor Neuron Membrane Potential in Live Zebrafish Embryos
Summary
Protocols described here allow for the study of the electrical properties of excitable cells in the most non-invasive physiological conditions by employing zebrafish embryos in an in vivo system together with a fluorescence resonance energy transfer (FRET)-based genetically encoded voltage indicator (GEVI) selectively expressed in the cell type of interest.
Abstract
The protocols described here are designed to allow researchers to study cell communication without altering the integrity of the environment in which the cells are located. Specifically, they have been developed to analyze the electrical activity of excitable cells, such as spinal neurons. In such a scenario, it is crucial to preserve the integrity of the spinal cell, but it is also important to preserve the anatomy and physiological shape of the systems involved. Indeed, the comprehension of the manner in which the nervous system-and other complex systems-works must be based on a systemic approach. For this reason, the live zebrafish embryo was chosen as a model system, and the spinal neuron membrane voltage changes were evaluated without interfering with the physiological conditions of the embryos.
Here, an approach combining the employment of zebrafish embryos with a FRET-based biosensor is described. Zebrafish embryos are characterized by a very simplified nervous system and are particularly suited for imaging applications thanks to their transparency, allowing for the employment of fluorescence-based voltage indicators at the plasma membrane during zebrafish development. The synergy between these two components makes it possible to analyze the electrical activity of the cells in intact living organisms, without perturbing the physiological state. Finally, this non-invasive approach can co-exist with other analyses (e.g., spontaneous movement recordings, as shown here).
Introduction
In vivo systemic component analysis allows scientists to investigate cellular behavior in the most reliable way. This is particularly true when the activity under scrutiny is heavily influenced by cell-cell interactions (both contact- and non-contact-dependent), as in the nervous system, where membrane voltage changes drive the communication among excitable cells. The comprehension of the information encoded by these electrical signals is the key to understanding the way the nervous system works in both physiological and disease states.
In order to study cell electrical properties in the most non-invasive physiological conditions, several genetically encoded voltage indicators have been recently developed1. As opposed to the previous generations of optical voltage sensors (mainly voltage-sensitive dyes)2, GEVIs allow for in vivo analyses of the intact neural system, and their expression can be limited to specific cell types or populations.
The zebrafish embryo is the in vivo "substrate" of choice to take advantage of the great potential attributed to GEVIs. In fact, thanks to its optical clarity and its simplified yet evolutionarily conserved nervous system, the zebrafish model allows for the straightforward identification and manipulation of every cellular component in a network. Indeed, the employment of the FRET-based GEVI Mermaid3 led to the identification of pre-symptomatic alterations in spinal motor neuron behavior in a zebrafish model of amyotrophic lateral sclerosis (ALS)4.
The following in vivo protocol describes how to monitor the electrical properties of spinal motor neurons in intact zebrafish embryos expressing Mermaid in a neuronal-specific manner. Moreover, it demonstrates how pharmacologically induced changes in such electrical properties can be associated with alterations in the frequency of embryonic spontaneous coilings, the stereotypic motor activity that characterizes the movement behavior of the zebrafish at very early stages of development.
Protocol
Representative Results
Discussion
The protocol presented here allowed us to explore the association between the electrical properties of zebrafish embryo spinal motor neurons and the spontaneous coiling behavior, the earliest stereotypic motor activity, which appears around 17 hpf of embryonic development and lasts until 24 hpf10.
Our approach provides researchers with a tool to study the neural system of intact embryos, fully preserving the complexity of the interactions between cells in a developing f…
Disclosures
The authors have nothing to disclose.
Acknowledgements
The authors would like to thank Simona Rodighiero for her priceless support with the FRET imaging analysis.
Materials
| Low Melting Point Agarose | Sigma-Aldrich | A9414 |
| DMSO | Sigma-Aldrich | W387520 |
| Riluzole | Sigma-Aldrich | R116 |
| Pfu Ultra HQ DNA polymerase | Agilent Technologies – Stratagene Products Division | 600389 |
| T3 Universal primer | Sigma-Aldrich | |
| Wizard SV Gel and PCR Clean-Up system | Promega | A9280 |
| Universal SmaI primer | Eurofins | |
| StrataClone Mammalian Expression Vector System / pCMV-SC blunt vector | Agilent Technologies – Stratagene Products Division | 240228 |
| SmaI | New England Biolabs | R0141S |
| T4 DNA ligase | Promega | M1801 |
| SalI | New England Biolabs | R0138S |
| EcoRV | New England Biolabs | R0195S |
| 35 mm, glass-bottomed imaging dish | Ibidi | 81151 |
| forceps | Sigma-Aldrich | F6521 |
| Stereomicroscope | Leica Microsystems | M10 F |
| Digital camera | Leica Microsystems | DFC 310 FX |
| Leica Application Suite 4.7.1 software | Leica Microsystems | |
| QuickTime Player, v10.4 | Apple | |
| Confocal microscope (inverted) | Leica Microsystems | TCS SP5 |
| Microinjector | Eppendorf | Femtojet |
| ImageJ macro Biosensor_FRET | ||
| GraphPad Prism 6.0c | GraphPad Software, Inc |
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