Mapping the Nervous System: Tracing Neural Circuits with Color Changing Proteins

by Christine Zhang

Stepping out the front door of my dorm, I am frequently greeted by a sharp gust of wind that convinces me to turn back and grab a coat. The reaction is almost instantaneous. But in that split-second, the action of turning around requires 100 billion action potentials and the signal transmits over 20 quadrillion synapses1. With the complexity of the nervous system, it can be difficult to pinpoint the origin of neural circuits. Yet these details are critical to understanding and treating neurodegenerative diseases. There has been substantial research in developing mechanisms to identify active neural circuits, and presently, research in synapses looks the most promising.

There are billions of neurons in the human body lined side-by-side to each other, separated by a microscopic gap known as a synapse. As the action potential arrives at the end of a neuron, calcium channels in that neuron open and the ion rushes in, triggering the release of neurotransmitters to carry the signal over the synapse2. The difference in calcium levels inside and outside of the neuron creates a concentration gradient that activates the neurotransmitter; thus, calcium levels and the intensity of neuron activity are strongly correlated.

At present, there are two widely accepted methods of detecting neural activity: genetically encoded calcium indicators (GECIs) and immediate early genes (IEGs). Both GECIs and IEGs monitor neuron functioning at the molecular level to give estimates of neural activity. However, the two processes are incomplete in design, hindered by complex set-ups and limited efficacy. GECIs can track calcium concentrations directly but require sophisticated machinery, physical restraint of the subject, and only provide limited scopes of view3. These complex requirements make GECIs feasible in few situations. In contrast, IEGs can be monitored in a larger time window and can be observed in free moving bodies, but cannot monitor neural activity directly. Rather than tracking calcium levels, IEGs record the expression of intermediate genes, which is at best weakly correlated with neural electrical activity3.

In light of these difficulties, Benjamin Fosque, Yi Sun, and Hod Dana, researchers in the Department of Biochemistry and Molecular Biology at the University of Chicago, developed a new mechanism to monitor neural behavior. Their idea features a fluorescent protein that changes color from green to red under violet light. Fosque, Sun, and Dana constructed a mutagenic fluorescent protein, called CaMPARI, to undergo this color change only in the presence of calcium. CaMPARI, or calcium-modulated photoactivatable ratiometric integrator, changes color 21 times faster in the presence of calcium than in its absence3. The rate of fluorescent conversion coupled with the intensity of fluorescence of CaMPARI conveys unprecedented levels of information on cell-type identification and examination and has groundbreaking potential.

When tested in Drosophila melanogsaster and larval zebrafish in vivo to track whole-brain activity and neural pathways, CaMPARI continued to prove to be highly successful3. It combined the advantages of traditional neural activity tests without the drawbacks. The method employed direct targeting of calcium as in GECIs and the flexible time window and freedom of movement of IEGs. As an additional benefit, CaMPARI also enables the possibility for follow-up experiments including electrical recordings, antigen detections, and genetic profiling of cells.

With the enhanced tracking of neural activity, neurologists can more rigorously study neurons on the molecular level and observe individual cell behaviors. Given its in vivo applications, CaMPARI can also contribute to developing personalized cures for patients with neurodegenerative diseases and to understanding the exact mechanisms in which neural diseases affect the body. With a reliable and easily employable neural monitoring mechanism, the potential scientific and medical gains are endless.

Christine Zhang ‘18 is a freshman in Thayer Hall.

Works Cited

  1. Bryant, A. What is the synaptic firing rate of the human brain? Stanford Neuroblog, Aug. 27, 2013.
  2. Sudhof, T.; Malenka, R. Neuron 2008, 60(3), 469-476.
  3. Fosque, B. et al. Science 2015, 347(6223), 755-760.
Mapping the Nervous System: Tracing Neural Circuits with Color Changing Proteins
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