Targeted and efficient activation of channelrhodopsins expressed in living cells via specifically-bound upconversion nanoparticles.

Optogenetics is an innovative technology now widely adopted by researchers in different fields of biological sciences. However, most light-sensitive proteins adopted in optogenetics are excited by ultraviolet or visible light which has a weak tissue penetration capability. Upconversion nanoparticles (UCNPs), which absorb near-infrared (NIR) light to emit shorter wavelength light, can help address this issue. In this report, we demonstrated the target selectivity by specifically conjugating the UCNPs with channelrhodopsin-2 (ChR2). We tagged the V5 epitope to the extracellular N-terminal of ChR2 (V5-ChR2m) and functionalized the surface of UCNPs with NeutrAvidin (NAv-UCNPs). After the binding of the biotinylated antibody against V5 onto the V5-ChR2m expressed in the plasma membrane of live HEK293T cells, our results showed that the NAv-UCNPs were specifically bound to the membrane of cells expressing V5-ChR2m. Without the V5 epitope or NAv modification, no binding of UCNPs onto the cell membrane was observed. For the cells expressing V5-ChR2m and bound with NAv-UCNPs, both 488 nm illumination and the upconverted blue emission from UCNPs by 980 nm excitation induced an inward current and elevated the intracellular Ca concentration. Our design reduces the distance between UCNPs and light-sensitive proteins to the molecular level, which not only minimizes the NIR energy required but also provides a way to guide the specific binding for optogenetics applications.