Class IX includes the microbial opsins NOP-1 and ORP-1.
By turning discharges in specific groups of cells on or off, using light-responsive microbial opsins such as channelrhodopsins and halorhodopsins, it is now possible, in principle, to determine how the activity of each cell type contributes to the global computations of a neural circuit (Deisseroth, 2011; Peron and Svoboda, 2011).
This work demonstrates an efficient strategy for engineering non-natural, tailored properties into microbial opsins, properties relevant for imaging and interrogating biological systems.
Optogenetics uses genetic encoding of light sensitivity (by the expression of microbial opsins) to provide such capabilities for manipulation, recording, and control by light with cell specificity and high spatiotemporal resolution.
Recent advances in understanding microbial opsins have been driven by molecular engineering for optogenetics and by comparative genomics.
Until 2005, when Deisseroth's group discovered that these microbial opsins could precisely control neurons in response to light and, in 2006, showed that even adult vertebral tissues, including the brain, express natural all-trans-retinal.
The single-component microbial opsins, although more limited than multicomponent cascades in processing, display unparalleled compactness and speed.
"We were brainstorming in the Spring of 2000, and we stumbled across some papers on so-called microbial opsins," Boyden explains.
Deisseroth's group subsequently demonstrated the use of microbial opsins for neuronal control in freely moving mammals.
Recent experiments in rodents have restored light-sensitivity by expressing melanopsin or microbial opsins either broadly throughout the retina or selectively in the inner segments of surviving cones or in bipolar cells.
