Bioluminescence activation of light-sensing molecules

Introduction Optogenetic approaches provide control of neuronal activity in rapid, millisecond timescales achieved by physical light activation (Boyden et al., 2005; Li et al., 2005). However, many applications, specifically in vivo, often do not require the highest temporal resolution; instead, the usefulness of opsins for in vivo studies within the brain has been limited by the need for optical fibers for light delivery, specifically regarding the number and the location(s) of neurons that can be photostimulated. Because of the small dimensions of the optical fibers commonly used for optogenetic photostimulation, as well as the attenuation of light in brain tissue due to light scattering, the volume over which neurons can be excited is much smaller than the volume of most major brain structures; this means that only a small fraction of the relevant neurons will be activated. Simultaneous photostimulation of multiple locations can increase this number, but requires multiple light sources, which is often impractical. Finally, insertion of light guides or optic fibers in order to access deep brain structures is technically demanding and causes damage to brain tissue. To allow manipulation of the activity of dispersed neuronal populations using optogenetic probes without fiber-optic implants, we developed an approach whereby bioluminescence – biological light produced by enzymatic reaction between a protein, luciferase and its diffusible substrate, luciferin – activates the opsin, which is tethered to the luciferase, creating a luminescent opsin, or luminopsin (LMO) (Figure 11.1) (Berglund et al., 2013). After injection into the peripheral bloodstream, luciferin reaches a target in the brain because it crosses the blood–brain barrier (Birkner et al., 2014). Light is generated by the luciferase and then activates the opsin, resulting in activation (in case of channelrhodopsins) or inhibition (in case of proton or chloride pumps or chloride channels) of the target neurons. This strategy takes full advantage of optogenetic elements – direct translation of light activation into neural effects, effective in any neuron or cell type and an ever-expanding repertoire of functionality – by using them as the common denominator, but switching out the light source from an invasive, physical one to a non-invasive, biological one (i.e. a light-producing protein, or a luciferase). This approach shares non-invasive neuronal modulation with other approaches, referred to as “chemogenetics” (Sternson and Roth, 2014), in which a genetically targeted actuator interacts with a systemically administered small molecule.