Optogenetic mice 'light up' neuroscience research
In 2010, the interdisciplinary journal "Nature Methods" chose optogenetics as the "Method of the Year" across all fields of science and engineering (Wikipedia). What's optogenetics? It's a technology that combines genetics and optics to control and track – with split-second timing – specific events and functions of living cells, tissues and organisms. Incorporated into mouse models, optogenetic technology is being used to map and analyze complex neural circuitries and to develop innovative therapies for many neurological disorders, including depression, narcolepsy, Parkinson's disease, blindness, addiction and memory loss. Our glowing collection of diverse optogenetic mouse models is growing. Some of its features are described below.
Our optogenetic rhodopsin models are engineered to express light-sensitive ion channels called opsins, which are derived from either algae or archaebacteria, in specific brain cell populations. Opsins are transmembrane, retinal-binding proteins that combine a light-sensitive domain with an ion channel or pump. Using the right combinations of these mice and light wavelengths, you can either turn on or turn off specific subsets of neurons within the animal.
Stimulatory rhodopsin models. We distribute 15 stimulatory rhodopsin models. Some contain opsins like channelrhodopsin 2 (ChR2, or COP4) from the algae Chlamydomonas reinhardtii. They are sodium ion channels and respond to blue light. When COP4-expressing neurons are stimulated by pulses of blue light of approximately 470nm, sodium ions flood the cells, depolarize them and cause action potentials to fire. Some of our activation rhodopsin models contain COP4 variants engineered to contain mammalian codon replacements, gain-of-function mutations, endoplasmic reticulum (ER)-signaling motif or golgi-signaling motifs which either improve COP4 expression, make it more sensitive to certain absorption wavelengths or change its channel opening dynamics. For example, the mhChR2 variant, also called hChR2 or ChR2(H134R), contains a gain-of-function H134R substitution that results in larger stationary photocurrents and a wider light-activation spectral range (~450-490nm) than ChR2. The channelrhodopsin-1 (VChR1 or COP3) from the alga Volvox carteri, on the other hand, has a red-shifted action spectrum that responds to green light (~535 nm) instead of blue.
Inhibitory rhodopsin models. We distribute four inhibitory rhodopsin models. They express opsins that hyperpolarize cells and prevent action potentials from firing. Three express halorhodopsin (NpHR, or HOP), derived from the archaeobacterium Natronomas pharaonis. HOP is an inward chloride ion pump sensitive to yellow light of approximately 580nm. The fourth expresses archaerhodopsin-3 (Arch, aR-3, or AOP3), derived from the halobacterium Halorubrum sodomense. In contrast to halorhodopsin, archaerhodopsin encodes an outward proton pump and is sensitive to yellow light of approximately 575nm. It also has slower decay kinetics than halorhodopsin, meaning that the channels, once activated, stay open longer (~ 2X).
Calcium and chloride sensors
One of our optogenetic mice, B6;129S-Gt(ROSA)26Sortm38(CAG-GCaMP3)Hze/J (014538), expresses the fusion protein GCaMP3, a calcium sensor. In the presence of calcium binding, the protein fluoresces brightly; in the absence of calcium binding, it fluoresces dimly. Three of our optogenetic models express the chloride sensor clomeleon, a fusion protein that contains CFP and YFP. When the chloride ion concentration is low, clomeleon's YFP component fluoresces; when the chloride ion concentration increases, YFP fluorescence decreases and CFP fluorescence increases. By using an appropriate calibration curve, researchers can convert the ratio of YFP to CFP fluorescence to chloride ion concentration.
Several of our optogenetic mouse strains are Cre recombinase-dependent tool strains: when mated to mice that express Cre recombinase, they produce offspring that express an optogenetic effector protein. These mice can be used in combination with the wide variety of available Cre strains to generate mice that express an optogenetic protein of interest only in a specific neuronal population. To find an appropriate JAX mouse that expresses Cre in a particular neuron of choice, consult our "Cre Strains for Neurobiology" website. You can check for additional Cre mice with an appropriate expression distribution in the published literature via the Mouse Genome Informatics website’s Cre Portal.