Using genetic mutations to study the neural basis of behavior.

These examples clearly portend a bright future for the use of genetic mutations to address the function of neural systems. Commenting on his tendency to dream about future scientific technologies, the late Carl Sagan suggested, “Dreams are maps.” Here, in the spirit of Sagan's comment, we will ignore limitations or apparent feasibility and will dream about types of mutations that could help revolutionize systems-level neuroscience. We do not imagine that these particular mice comprise a list that is either comprehensive or optimal, nor do we imagine that they are possible in the near future. They are simply our maps.One mouse is inspired by the immunotoxin-mediated cell targeting technique of Watanabe et al. 1998xWatanabe, D., Inokawa, H., Hashimoto, K., Suzuki, N., Kano, M., Shigemoto, R., Hirano, T., Toyama, K., Kaneko, S., Yokoi, M. et al. Cell. 1998; 95: 17–27Abstract | Full Text | Full Text PDF | PubMed | Scopus (135)See all ReferencesWatanabe et al. 1998. The goal is to achieve functional inactivation of a particular cell type with extremely rapid onset and offset. An approach of this type has been employed with great success in certain invertebrate systems where particular cells can be functionally removed from the circuit via direct hyperpolarization through a microelectrode. The cell is still alive, but it cannot communicate with other neurons because the hyperpolarization prevents action potentials. Although this technique affords superb temporal specificity, its application is limited to the number of cells that can be simultaneously impaled with microelectrodes.Perhaps entire classes of cells could be functionally removed from a circuit with cell type–specific expression of a recombinant protein comprised of a known potassium or chloride channel and a receptor for an artificial ligand. If the appropriate artificial receptor–ligand combination could be developed, cells expressing the transgene could be functionally removed from the circuit via hyperpolarization that is activated by injection of the ligand. Combining such a transgene with a cell type–specific promoter could produce a rapid onset and reversible variant of the immunotoxin-mediated technique. Cells could be rapidly removed and, with an artificial competitive antagonist, rapidly reinstated in the circuit. By potentially eliminating compensatory changes such as those characterized in the cerebellum by Watanabe et al. 1998xWatanabe, D., Inokawa, H., Hashimoto, K., Suzuki, N., Kano, M., Shigemoto, R., Hirano, T., Toyama, K., Kaneko, S., Yokoi, M. et al. Cell. 1998; 95: 17–27Abstract | Full Text | Full Text PDF | PubMed | Scopus (135)See all ReferencesWatanabe et al. 1998, such a “hyperpolarization” mouse would provide a tremendous degree of component and temporal specificity.Another type of futuristic mouse might move away from using targeted mutations as lesions (see Siegel and Isacoff 1998xSiegel, M.S. and Isacoff, E.Y. Neuron. 1998; 19: 735–741Abstract | Full Text | Full Text PDF | Scopus (239)See all ReferencesSiegel and Isacoff 1998). Perhaps a recombinant protein could be expressed to record activity of cells. Such a mouse could be used, for example, to determine the cells that are active during a particular form of learning. The recombinant protein might be constructed with three properties: (1) the ability to bind some activity-dependent molecule such as elevated free calcium, (2) the ability to bind an initiator molecule, and (3) when these two binding sites are occupied to undergo a detectable conformational change or to generate a measurable product.The idea would be that the initiator molecule could be injected or otherwise delivered in a temporally discrete manner, and that this would initiate the “recording” process. From that time on, binding at the second, activity-dependent site would generate a measurable product in the cell. The keys would be the temporal specificity of the initiator molecule, as well as the specificity and stringency of the binding site detecting neural activity. If the expression of this molecule could be limited to synapses, the resolution of activity-dependent accumulation of the marker molecule could be at the level of synapses rather than cells. In this way mutant mice could be used not only to identify the neural components responsible for a particular behavior, but also to study how those components interact to produce the input/output properties of the system.Finding synapses that undergo plasticity during learning is a fundamental but technically demanding goal for neurobiologists studying the neural basis of learning. As more is learned about the molecules required for particular forms of synaptic plasticity, it might even be possible to limit the accumulation of marker to synapses that have undergone that form of plasticity since the injection of the initiator molecule. Such a mouse could be used to establish links between forms of plasticity and forms of learning at a level of certainty not currently approachable. A mouse could be injected with the initiator molecule and then trained in a specific task. Subsequent histological analysis could reveal synapses with marker accumulation, and thus synapses that had undergone plasticity during that form of learning could be identified.As systems-level neuroscientists, it is hard to guess whether mice like these are many decades away or just around the corner. Either way, there is no doubt that continuing advances in gene targeting technology, when combined with robust behavioral analysis, will help revolutionize our ability to study and to understand brain systems.