I recently went to a talk given by Karl Deisseroth of Stanford University whose lab has been in the forefront of developing tools that allow neurons to be activated or de-activated using pulses of light in combination with expressed opsin transgenes. The opsins are basically photoactivated proteins ( like the rhodopsin in our retina) and they are activated by a single photon of light . Deisseroths lab had a while ago developed the use of a Channel rhodopsin- ChR2 which showed a light activated cationic (positive ion) current . Thus, when ChR2 was expressed in neurons a flash of blue light triggered a current of sodium and potassium ions which resembled the neuron firing or the action potential. The group set out looking for opsins that had the opposite effect , i.e they accelerated the suppression of the neuronal action potential ( or brought the neuron back to rest) in response to a light photon. The protein had be functional in neurons and also bring about the light activated depolarization at a time scale of the action potential .
After trolling through a few opsins the Deisseroth group in close collaboration with the Georg Nagel lab at the University of Wuerzburg in Germany zeroed in on the opsin from Naturomonas pharaonis (NpHR or Halo) an archaebacteria that lives in transient salt lakes in the Egyptian dessert in salt concentrations as high as 3.5 M NaCl and a pH of 11. The bug possibly uses the opsins to capture the energy from sunlight and uses the energy to drive the uptake of chloride which allows it to survive in the very salty lake it lives in.
This new opsin now was able to drive a chloride current on photoactivation which effectively turned the neuron off. Fortunately for the group the NpHr opsin was also activated at a wavelength entirely different from the “on switch” ChR2 opsin.
Concurrently with the publication in Nature the group released a video of a worm expressing both opsins which could be paralysed and tickled into movement by pulses of light that stimulated NpHr and CpHr respectively. In the talk Deisseroth also spoke of developments to move the experiments into a mouse model where a transgenic mouse would have a fibre optic probe inserted into its head that would then allow light to stimulate a precice area of the brain ( like a pulse of light would cause the neuron that controlled whisker movement to turn on and the mouse would twitch its whisker).
All of this clearly opens the road to other such opsins that can possibly respond to other wavelengths and drive excitatory and stimulatory currents with different properties. Interestingly way back in 2004 the metagenomics initiative by Craig Venter (the Sorcerer II expedition) published details of several novel opsins from the sargaso sea mass sequencing samples. Who knows , maybe the next such opsin may come some deep sea archaeon and have totally different spectral and kinetic properties which would allow an added level of control to the optical querying of neuronal circuits.
References and additional material
- Video of light stimulated supression of worm twitching upon NpHr (Halo) activation
- NpHr genome project page
- Free full text PlosBiology paper on NpHR by Xue Han and Edward Boyden
- Image link from an article in the MIT technology review
- MIT technology review article on ChR2 and NpHr ( worm video link )
- Request the ChR2 and NpHR (Halo) plasmids deposited by the Boyden lab at addgene
- The TED talks have an interesting talk by Craig Venter : around minute 3:46 he talks about the opsins
- Full text ( free access) PLOS Biology article on Sorcerer II data.. one of the figures contains an analysis of the spectral characteristics of proteorhodopsins from the metgagenomics dataset