The National Science Foundation recently featured research from the Spudich laboratory on light-sensors in microorganisms. The NSF article (1) highlights the seminal role of basic research on phototaxis by microorganisms in the development of optogenetics, a new biotechnology that has revolutionized research on neural circuitry and holds promise for gene therapy for neurological diseases.

The highlighted article (2) reported the identification of the long-sought phototaxis receptors in Chlamydomonas algae as two membrane proteins, each containing a 7-helix photoactive domain similar in structure to visual pigments. The Spudich lab used RNAi suppression analysis and electrophysiology to demonstrate that the receptors mediate light-induced ion currents depolarizing the algal plasma membrane, which in turn control the cell’s swimming trajectory.  The two receptors, now called channelrhodopsin-1 and -2 (ChR1 and ChR2), were found to differ in spectral sensitivity and light intensity operating ranges.  Extending their function beyond the algae, the rhodopsin-like domains were subsequently shown to mediate membrane depolarization in Xenopus oocytes by Ernst Bamberg and coworkers in Germany, and Stanford neuroscientists Ed Boyden and Karl Deisseroth expressed the most effective of the two (ChR2) in neurons demonstrating that it enabled precise photocontrol of neuron firing, coining in 2005 the term “optogenetics”.  Searching for “channelrhodopsin OR optogenetics” pulls up 843 articles on PubMed and 511 grants on NIH RePORTER, and optogenetics is one of the key areas of President Obama’s 10-year initiative called “Brain Research through Advancing Innovative Neurotechnologies (BRAIN)”.

The algal receptors mediate phototaxis by altering the cell’s swimming direction by controlling the electrical potential of its plasma membrane.  Light absorption by the receptor proteins opens cation-conductive channels allowing passive influx of ions (e.g. H+, Na+, Ca+), which depolarizes the membrane.  By modulating the membrane potential, the channelrhodopsins provide a continuous cellular measurement of light intensity.  The cell uses this information together with an intracellular organelle (the stigma or “eye-spot”) that periodically shades the receptors as the cell rotates, to orient the cell’s trajectory along the direction of a light beam.

When channelrhodopsins are expressed in excitable cells such as neurons, their light-gated cation channel activity enables the experimenter to use light to activate the cell, i.e. induce action potentials. With this method, neuroscientists gain unparalleled control of neuron firing to selectively activate a genetically targeted cell type with the unmatched temporal and spatial precision of light. Complementing this photocontrol, other microbial rhodopsins that carry out light-driven ion transport enable light-induced hyperpolarization of the membrane, i.e. neuron silencing.

Reproducing a quote from the NSF article: “The development of optogenetics is yet one more beautiful example of a revolutionary biotechnology growing out of purely basic research.”

References:

1.  http://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=129298

2. Sineshchekov OA, Jung K-H, and Spudich JL. (2002) Two rhodopsins mediate phototaxis to low and high intensity light in Chlamydomonas reinhardtii. Proc. Natl. Acad. Sci. USA, 99, 8689-8694.

For more recent articles from the Spudich lab on optogenetic rhodopsins, see the following and references therein:

Govorunova EG, Sineshchekov OA, Li H, Janz R, and Spudich JL (2013) Characterization of a highly efficient blue-shifted channelrhodopsin from the marine alga Platymonas subcordiformis.  J Biol Chem 288:29911-29922.

 

Sineshchekov OA, Govorunova EG, Wang J, Li H, Spudich JL (2013) Intramolecular proton transfer in channelrhodopsins.  Biophys J. 2013 104:807-817.

 

Spudich JL, Sineshchekov OA, and Govorunova EG. (2013) Mechanism divergence in microbial rhodopsins.  BBA Bioenerg. doi:pii:S0005-2728(13)00103-5. 10.1016/j.bbabio.2013.06.006.

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