Seeing Ghosts with c-fos-derived Tools

"Progress in science depends on new techniques, new discoveries and new ideas, probably in that order." — Sydney Brenner

Bread, beer, and the flu are only a few of the many ways microbes welcome us to their worlds. By looking into the microbial realms, modern biology has found the genome-engineering tools to put the right transgenes in the right cells. These tools have changed the way researchers study neural circuits. Nowadays, to study the neurons responsible for specific behaviors, researchers typically screen through a library of pre-engineered animals, each expressing transgenes in a distinct group of neurons, with the hope that one of these neuronal populations controls the behavior of interest. Together with other tools, this library approach accelerated the discoveries of specific neurons controlling feeding, drinking, fighting, and parental behaviors. However, despite great effort, this strategy is still limited by the transgenic lines available and helpless when the relevant neuronal populations do not share a common genetic locus. For example, what if one wishes to study how higher-order, olfactory-processing neurons respond to the smell of apple pie? How can we control these pie-smelling neurons if we don’t know who they are?

Manipulating specific neurons without genetic access is like dancing with ghosts that cannot be seen. However, if we know enough about those ghosts, we may be able to monitor them indirectly by, for example, examining their surrounding air flow with smoke. In the case of neurons, the air flow is calcium and the smoke is c-fos. c-fos belongs to a group of genes called immediate early genes1, which are turned on quickly and transiently by elevated calcium in the cell, a proxy for neural activity. In the pie case, the pie-smelling neurons will become active when presented with a pie, and the resulting build-up of calcium in these neurons will turn on c-fos only in these cells. In this way, researchers can take advantage of the calcium-dependent c-fos promoter to drive expression of cell-manipulating tools, such as the light-activated channel Channelrhodopsin-22,3, in neurons that respond to the pie smells. In this way, researchers can gain control of the pie-smelling neurons, and study their circuitry by re-activating them after the initial labeling, only this time with light instead of the pie.

What else can be done with these c-fos-derived tools? Virtually anything that can be done with standard genetic tools. Researchers have already used optogenetics to both activate and inhibit the labeled neurons2–4. In the future, one might be able to use c-fos-driven reporters of cell activity to have a slow-motion view of the dynamics of only the labeled neurons during a behavior. One could use genetically encoded gene-manipulation tools to knock down or knock out genes in labeled neurons. One could use c-fos tools to study anatomy in an activity-dependent manner. One could even use c-fos derived tools to discover new drug targets by performing molecular profiling in neurons active in the process of interest.

Sometime in the future, calcium-dependent genetic tools may also be used to treat patients with hyperactive neurons (e.g. seizure). One could use c-fos to deliver hyperpolarizing channels only in hyperactive excitatory neurons. In this way, by allowing cell-specific tailoring of hyperpolarization drive, one could selectively suppress the activity of seizing neurons while leaving the rest to function normally.

References:

  1. Sheng, M. & Greenberg, M. E. The regulation and function of c-fos and other immediate early genes in the nervous system. Neuron 4, 477–485 (1990).
  2. Gore, F. et al. Neural Representations of Unconditioned Stimuli in Basolateral Amygdala Mediate Innate and Learned Responses. Cell 162, 134–145 (2015).
  3. Liu, X. et al. Optogenetic stimulation of a hippocampal engram activates fear memory recall. Nature 484, 381–5 (2012).
  4. Tanaka, K. Z. et al. Cortical representations are reinstated by the hippocampus during memory retrieval. Neuron 84, 347–54 (2014).