Innovative Genetic Tool Developed for Observing Neurons

MIT and Boston University researchers create a new way to see neurons in action.

Posted Oct 20, 2019

TheDigitalArtist/Pixabay
Source: TheDigitalArtist/Pixabay

One of the biggest challenges in neuroscience is having the ability to observe the brain’s neuronal activity across different brain regions in live, awake mammals. Exciting progress has been made using genetic techniques to aid in observing brain activity. Researchers at the Massachusetts Institute of Technology (MIT) and Boston University (BU) have created a new tool that enables the imaging of groups of neurons in the brains of mice using genomics and a light microscope.

In a study published in Nature on October 9, 2019, senior authors Edward Boyden of MIT, Xue Han of BU, and their colleagues reveal how the brain activity of awake mice can be observed using a genetically encoded fluorescent voltage indicator they developed, called SomArchon.

Conventional methods of using thin electrodes inserted into the brain to measure electrical activity of mammalian neurons is a painstaking process limited to measuring very few neurons, typically just one, during any one period. What sets this new tool apart from prior methods is that multiple neurons can be observed simultaneously, versus just a single neuron at a time.

“Under conventional one-photon microscopy, SomArchon enables the routine population analysis of around 13 neurons at once, in multiple brain regions (cortex, hippocampus, and striatum) of head-fixed, awake, behaving mice,” the researchers wrote.

Multiple electrical sensors in a tightly spaced array — microelectrode arrays (MEAs) — can record the electrical activity of multiple neurons. However, microelectrode arrays detect the spikes, also called action potentials, rather than the sub-threshold voltage changes. Unlike microelectrode arrays, this new tool is able to report the changes in both spikes and the sub-threshold voltage.

“We also examined how spikes relate to the subthreshold theta oscillations of individual hippocampal neurons, with SomArchon showing that the spikes of individual neurons are more phase locked to their own subthreshold theta oscillations than to local field potential theta oscillations,” the researchers reported. “Thus, SomArchon reports both spikes and subthreshold voltage dynamics in awake, behaving mice.”

Calcium imaging is an existing method of brain imaging that enables researchers to monitor brain activity, albeit somewhat inefficiently and circuitously. In mammalian neurons, calcium ions (Ca2+) are intracellular messengers. When neurons are in an excited state, the intracellular concentration of calcium can increase 10 to 100 times the resting state calcium concentration of approximately 50–100 nM.

The researchers report that the new tool has advantages over calcium imaging. “Using SomArchon, we detected both positive and negative responses of striatal neurons during movement, as previously reported by electrophysiology but not easily detected using modern calcium imaging techniques, highlighting the power of voltage imaging to reveal bidirectional modulation,” the team wrote.

SomArchon is the modified version of Archon1, a molecule created by robotic directed evolution that can be inserted into neurons genetically and fluoresce when the neuron’s electrical activity increases.

Archon1 is an opsin-based fluorescent voltage reporter that can image spiking and millivolt-scale subthreshold and synaptic activity. Archon1 was tested on mice brains in vitro, zebrafish in vivo, as well as worms. Edward Boyden and his colleagues published their work on Archon1 in Nature Chemical Biology in February 2018.

The researchers created SomArchon by modifying Archon1 so that it adheres to the central cell bodies of the neurons, versus axons and dendrites, so that the resulting images would be clearer. It’s analogous to leaving the city for the wilderness in order to better see the illumination of the stars in the night sky. The city lights are like the axons and dendrites that make it more difficult to view the stars, or main neuron bodies. Aptly named, soma means “body” in Greek, and the modern definition refers to the cell body sans axons and dendrites.

SomArchon is compatible with optogenetics, an emerging field in biotechnology in which light is used to control cells in living tissues. In optogenetics, genetic code is added to target tissue, typically a neuron, which enables it to make light-responsive proteins called opsins. In August 2005, Boyden, along with scientists Karl Deisseroth, Feng Zhang, Georg Nagel, and Ernst Bamberg, made history with an optogenetics study published in Nature Neuroscience. The team demonstrated the ability to switch neurons on and off by shining a blue light on neurons that were inserted with opsin from green algae Chlamydomonas reinhardtii, called channelrhodopsin-2 (ChR2).

The researchers also enabled SomArchon to work with expansion microscopy. In January 2019, Boyden, along with Eric Betzig, Ruixuan Gao, Shoh Asano, Srigokul Upadhyayula, and their research colleagues, published in Science a groundbreaking method of mapping the brain at high resolution by expanding the brain tissue and lattice light-sheet microscopy—a 3D microscopy technique.

These are electrifying times for neuroscience. There are many unanswered questions when it comes to the human brain. The recent innovative tools and techniques for brain imaging are accelerating research and science for the benefit of humanity at large.

Copyright © 2019 Cami Rosso All rights reserved.

References

Piatkevich, Kiryl D., Bensussen, Seth, Tseng, Hua-an, Shroff, Sanaya N., Lopez-Huerta, Violeta Gisselle, Park, Demian, Jung, Erica E., Shemesh, Or A., Straub, Christoph, Gritton, Howard J., Romano, Michael F., Costa, Emma, Sabatini, Bernardo L., Fu, Zhanyan, Boyden, Edward S., Han, Xue. “Population imaging of neural activity in awake behaving mice.” Nature. 09 October 2019.

Boyden, Edward, et al. “A robotic multidimensional directed evolution approach applied to fluorescent voltage reporters.” Nature Chemical Biology. 26 February 2018.

Betzig, Eric, Boyden, Edward, et al. “Cortical column and whole-brain imaging with molecular contrast and nanoscale resolution.” Science. January 18, 2019.

Grienberger, Christine, Konnerth, Arthur. “Imaging Calcium in Neurons.” Neuron. March 08, 2012.