1. Light-activated ion channels for remote control of neuronal firing
Banghart, M. et al
Choi, Yu Yong

2. Background
Techniques for controlling neural activity have considerable limitations
Traditional electrical and neural chemical methods requires invasive electrodes and chemical delivery systems that cannot control patterns of activity in densely packed neural tissue
Optical techniques utilizing caged neurotransmitters are less invasive and more precise, but reversal of the effects of the uncaged transmitter is limited by its diffusion kinetics
Exogenous expression of genes encoding ion channels has been used to influence electrical activity in specific neurons, but the onset and reversal of gene expression is slow
Photic regulation has been conferred on neurons by introducing a rhodopsin-based signal transduction-based cascade, This technique requires coordinated exogenous expression of three different genes and produces light responses that can be slow in onset and offset and variable in different neurons, possibly because the nature of the native ion channels that are regulated by the cascade can vary between neurons
This light-activated ion channels allow rapid, remote, and noninvasive control. Because the light-activated gate is covalently linked to the ion channel, and because the ion-channel is integral to the neuronal cell membrane, control over individual neurons is spatially accurate and does not rely on a diffusible ligand. In addition, the gate can be reversly photo-switched, allowing recurrent control of neural activity

3. Rod cell
Figure Details of phototransduction in rod photoreceptors. (A) The molecular structure of rhodopsin, the pigment in rods. (B) The second messenger cascade of phototransduction. Light stimulation of rhodopsin in the receptor disks leads to the activation of a G-protein (transducin), which in turn activates a phosphodiesterase (PDE). The phosphodiesterase hydrolyzes cGMP, reducing its concentration in the outer segment and leading to the closure of sodium channels in the outer segment membrane

4. Action potential

5. ion channels
Three physical models for the opening and closing of ion channels
A localized conformational change occurs in one region of the channel
A generalized structural change occurs along the length of the channel
A blocking particle swings into and out of the channel mouth
Types of
Ligand-gated
Voltage-gated
Stretch or pressure-gated

6. Shaker K+ channel
Typical voltage-gated K+ channels is an assembly of four identical (or similar) transmembrane subunits surrounding a central pore. Each subunit has six transmembrane (S1-S6), with both N- and C- termini on the intracellular side of the membrane (top left panel). The narrowest part of the pore, the selectivity filter, is formed by a loop between S5 and S6; the voltage sensor includes the S4 region with its multiple positive charges (Yellen, G., Nature )

7. Shaker K+ channel
Liu, Y. et al Neuron

8. Blaustein, R. et al. 2000. Nat. Struct. Biol.
Shaker K+ channel
Blaustein, R. et al Nat. Struct. Biol.

9. Patch Clamp http://www.iac-usnc.org/Methods/wholecell/equipment.html
Planar patch clamp

10. Xenopus laevis

11. Patch Clamp
Note that only the on-cell patch and the whole-cell patch are associated with the complete neuron. The others are only concerned with a portion of membrane. Only the whole-cell approach actually accesses one of the plasmas within the neuron directly. In the case of the on-cell approach, the impedance of the membrane must be considered in the overall test set design.
Planar patch clamp

13. Photocontrol of MAL-AZO-QA-modified Shaker channels in X
Photocontrol of MAL-AZO-QA-modified Shaker channels in X. laevis oocytes

16. Discussion
By combining synthetic chemistry and protein mutagenesis, the Shaker channel has been reengineered
This technique lies in its spatial and temporal accuracy, its noninvasiveness and its reversibility.
and may provide an effective means for controlling the activity of specific neurons downstream from sites of neural damage or degeneration
and may be used to restore light-regulated activity in healthy retinal neurons after degeneration of rods and cones, the native phtoreceptors
and may have application in fields of nanotechnology, bioelectronics and material sciences