Channelrhodopsin

Channelrhodopsins ( German also: channelrhodopsins ) are transport proteins whose conductivity for certain ions controlled by light (English light - gated ) is. Channelrhodopsins thereby allow the influence of pH in the cytosol, the calcium import and the electrical excitability of cells by incident light.

So far (as of 2008 ) are known three channelrhodopsins: Channelrhodopsin -1 ( ChR1 ) and channelrhodopsin -2 ( ChR2 ) are light-gated proton channels and serve green algae of the genus Chlamydomonas as sensory photoreceptors; they control negative and positive phototaxische reactions at high light. Channelrhodopsin -2 may also for cations (positively charged ions) are permeable. VChR1 was found in the multicellular alga Volvox; its absorption maximum is at a higher wavelength than ChR1 and ChR2. It shows 80 percent homology to the ChR1 group.

Structure and function

Channelrhodopsins is attributed to a comparable rhodopsin structure composed of seven helical transmembrane domains. As the chromophore, the isomerized light by all-trans Retinal, a vitamin A derivative is covalently bound within the protein molecule. While most G protein- coupled receptors open ( including rhodopsin ) ion channels indirectly via second messengers, forms at the channelrhodopsins the protein itself a pore. This structure enables a very fast and reliable depolarization of the cell. The absorption maximum of ChR2 is approximately 460 nm in the blue. In combination with the chloride pump halorhodopsin, activated by yellow light, the use of ChR2 in the experiment ( over-expression of these proteins in neurons in cell culture ) allows the optical activation and deactivation of neuronal activity by illumination with light of different colors.

Channelrhodopsins (CHR) are, like other rhodopsins also helical proteins having seven transmembrane domains and a retinal chromophore attached as a protonated Schiff base covalently linked to the protein. The absorption maximum of ChR2 is approximately 460-470 nm in the blue. When the all-trans -retinal protein absorbs light in the retinal complex, it isomerizes into a 13 -cis-retinal, thereby causing a conformational change of the protein. This leads to opening of the pore in the protein, its diameter is at least 0.6 nm, the 13-cis -retinal relaxes after some time back to all-trans -retinal, thus increasing the pore closes and the ion flow is interrupted. The structure of seven transmembrane domains, as found in Channelrhodopsin -2, is rare in ion channels, which are usually composed of identical subunits.

Applications in research

While the N -terminus includes the seven transmembrane domain, the C-terminal end of the ChR2 protein extends into the intracellular space and can be replaced or modified without impairing the function of the protein as an ion channel. Channelrhodopsins you with a number of transfection techniques ( viral transfection, electroporation, gene gun ) was expressed in excitable cells such as neurons ( produced). Vitamin A, the precursor of the light-absorbing chromophore retinal in vertebrate cells usually already present, so that excitable cells, which express channelrhodopsin, can be easily depolarize by lighting.

Due to these characteristics, bioengineering and neuroscience interested in the use of channelrhodopsins, for example, for applications such as photostimulation of neurons. The blue-sensitive ChR2 in combination with the yellow light by aktiviertierbaren chloride pump halorhodopsin allow the switching on and off of neuronal activity within milliseconds. The emerging discipline that deals with the control of genetically modified cells by light is called optogenetics.

If ChR2 labeled with a fluorescent label excited axons and synapses can be identified in the intact brain tissue by light. This technique can be used to educate the mokularen events during the induction of synaptic plasticity. With the help of ChR2 extensive neuronal pathways have been mapped in the brain.

Is that the behavior of transgenic animals expressing ChR2 in a proportion of their neurons can be controlled without contact by intense illumination with blue light, has already been shown for nematodes, fruit flies, zebrafish and mice.

The visual function of blind mice could be restored by expression of ChR2 in bipolar cells of the retina in the eye part. It is also conceivable a future medical use of ChR2 in certain forms of retinal degeneration or to stimulation of deep brain sections.