CH391L/S12/LightSensors

Introduction

Molecules that respond to light are increasingly being used as input domains to facilitate the non-invasive control of variously complex gene circuits. For the purposes of this wiki, we will focus on few types of light-sensitive proteins: photocaging, photoreceptors, two-component systems, and opsins.

Photocaging

Photocaging involves the chemical addition of a small molecule effector (e.g., IPTG or doxycyline) onto another molecule (the cage) that upon addition of light such as UV, releases the effector molecule to perform a task. (Mechanism) This permits the control of gene expression upon light stimulation. The caging of IPTG, for instance, permits the spatiotemporal control of genes under the control of the Lac operator. Alternatively, one might cage a small molecule within a protein blocking its activity, that upon light-stimulus permits the protein to function. Amino acids such as Tyrosine have been caged with the active site of a Polymerase, that upon light expression, permits gene expression.

Photogacing

Optogenetics

The 2019 Oxford English dictionary definition of Optogenetics was fancifully written as:

"the branch of biotechnology which combines genetic engineering with optics to observe and control the function of genetically targeted groups of cells with light, often in the intact animal"[1]

This means scientists can switch on brains cells simply with light. Optogenetics is having the biggest impact in the field of neuroscience, where it is used to activate biochemical processes in neurons at millisecond timescales in live cells or animals. Genes such as halorrhodopsin, react to light and trigger an action potential in neurons, activating the cells. These light reactive opsins are essentially ion channels. Delivery of light specific neurons is done through a fiber-optic cable into a mouse's brain, if done on live animals.

[Optogenetics video Nature Method of the year 2010]

The devices used for this technique are generally encoded directly into the DNA using genetic engineering techniques. The types of protein devices used can be categorized into actuators, which drive light commands into processes, and the sensors, which emit signals as an output for study (ie, GFP).

Optogenetics

Actuators

Depolarizing

• chARGe, P2X2, TRPV1, TRPM8, channelrhodopsin-2, LiGluR

Hyperpolarizing

• SPARK, halorhodopsin

Sensors

Membrane Potential

• FlaSh, SPARC, VSFP, Mermaid

Calcium

• cameleon, camagaroo, pericam, G-CaMP

Synaptic transmission

• synapto-pHluorin, sypHy

applications

Last year, researchers used viral delivery (rAAV) of channelrhodopsin-2 into the eyes of blind mice to restore many vision functions. [2]

References

<biblio>

1. Drepper2011 pmid=21336931

//Lights on and action! Controlling microbial gene expression by light.

1. Miesenbock2009 pmid=19833960

//The optogenetic catechism.

1. Dorouddchi2011 pmid=21505421

//Virally delivered channelrhodopsin-2 safely and effectively restores visual function in multiple mouse models of blindness.