Article by Martin Liberman & Roberta Lock
Optical Nanotransformer Enabled Light Control of Cell Behavior
Subcellular Optogenetics Enacted by Targeted Nanotransformers of Near-Infrared Light, ACS Photonics, 2017
Artem Pliss et al., Paras N. Prasad Lab
Optogenetic systems use light activated proteins to control cell behavior. To be light-responsive, target cells must be genetically engineered to make the specific proteins that can respond to a specific wavelength of light. Compared to many other methods of controlling cell behavior, a light-based approach is attractive because it is very precise, targeting only specific cells with the necessary protein (spatially precise), and only when the light is actively on (temporally precise). This has many interesting potential future applications, like a pacemaker to pace the heart that uses light instead of electronics. However, one of the largest difficulties with the translation of optogenetic tools from lab-based cell culture studies into larger, more complex animal models is the penetration of visible light through tissues. Blue light (~470 nm), which is needed to excite most optogenetic proteins, has very poor tissue penetration due to high absorption and scattering by biomolecules. This recent study tackles this obstacle.
What did these researchers do?
A solution that this paper presents is upconversion nanoparticles (UCNPs) that convert near-infrared (980nm, NIR) light, which has high tissue penetration characteristics, into blue light. This lab group used UCNP that contained rare earth metals like Ytterbium (Yb3+) and Thulium (Tm3+) to upconvert the NIR to blue light. This paper showed that the UNCPs upconverted blue light elicited the same effect as direct blue light stimulation in controlling cell behavior. In this case, the light-responsive protein in the cell is called channelrhodopsin2 (ChR2), which when stimulated with blue light, opens an ion channel in the cell membrane, allowing sodium and calcium ions to enter and activate the cell.
Transmission Electron Microscopy Image of Synthesized Core/Shell Nanoparticles
Why is this important?
Overcoming the obstacle of tissue penetration will help expand the applications of optogenetics into more in vivo models, where light has to travel further distances to its intended target.
How did the researchers do this?
The upconversion nanoparticles were synthesized using a method called thermal decomposition (where heat breaks down a starting compound), to create nanoparticles where the rare metal ions are enclosed in a protective casing (called a core-shell format). To make sure the UCNPs went to the targeted cells, they were then tagged with folic acid to ensure uptake only by cells with a high presence of the corresponding receptors. The nanoparticle experiments stimulated the cells using a near infrared laser (980 nm), whereas the control experiments (no nanoparticles) were illuminated with blue light (476 nm wavelength) to stimulate ChR2 directly. The resulting cell response of calcium ion movement was detected using a fluorescent dye, which visualizes calcium activity using a 543 nm light.
What comes next?
The UCNPs localized preferentially into certain areas within individual cells, causing ChR2 stimulation and Ca2+ influx to be increased in areas near the nanoparticles, while no activation was detected in regions of the same cell that contained an insignificant number of nanoparticles. This high specificity and subcellular precision allow for increased manipulation and study of cell signaling, though this must be further studied in future experiments. Additionally, UCNPs take longer to activate and deactivate their fluorescent signal than directly using a laser, meaning a slight decrease in their efficiency and precision. The next steps would be to incubate the UCNPs into conducting cells like neurons and attempt to trigger an action potential, and then using them in an animal model to see if these UCNPs really help the optogenetic system to overcome the challenge of light penetration that blue light faces.