P3504Near-Infrared photoactivation via upconversion nanoparticles promotes new advances in cardiac optogenetics toolbox
Authors: Wang X I , Cheng Y , Rao P , Wang L 
Published: October 2019

Abstract

Background
The optogenetics manipulation of the heart based on the visible light is limited in the therapeutic potential because of the low tissue penetration. Near-infrared (NIR) light has deeper tissue penetration capabilities but radiates at unsuitable wavelengths, while upconversion nanoparticles (UCNPs) absorb NIR light to convert visible light.

Purpose
We aimed to investigate the efficient NIR control of the rat heart in vivo via UCNPs mediated cardiac optogenetics.

Methods
Systemic delivery via jugular vein injection of (AAV9-CAG-hChR2 (H134R)-mCherry) were performed in SD rats to achieve sufficient Channelrhodopsin-2 (ChR2) transfer throughout the whole heart. UCNPs of NaYF4:Yb/Tm with optimal excitation wavelength at 975nm were chosen to emit upconverted blue light. Different concentrations of UCNPs cyclohexane solution were embed in composite polydimethylsiloxane films to make flexible substrates for cardiac optogenetics study in open-chest rats (n=3). The UCNPs film was attached to the right ventricle and the 980nm NIR illumination was applied.

Results
The upconversion luminescence spectra of four concentrations (2.5, 5, 10 and 20mg/ml) of NaYF4:Yb/Tm scanned under 980nm excitation at 0.5w showed similar peaks around 475, 645 and 695nm. Emission intensity increased with the UCNPs concentration (Figure 1). The NIR-upconverted blue light from the freestanding films embedded with 2.5 and 5mg/ml UCNPs failed to capture the heart till the peak output power of the NIR laser, and the hearts were successfully captured and paced by the upconverted blue light from 10 and 20mg/mL UCNPs films (20 pulses in 8Hz with 20ms duration were repeated 3 times with the interval of 1s). However the NIR power was lower on 10mg/mL UCNPs film than the 20mg/mL one (0.93±0.11w vs 1.71±0.75w). Therefore, UCNPs film with concentration of 10mg/mL NaYF4:Yb/Tm were used for efficient cardiac optogenetic pacing by NIR light from a 400um optical fiber. Optogenetics capture of the ventricle was achieved at different NIR power, pulse duration and flash frequency. The strength-duration curve summarized the minimal NIR irradiance power of 8Hz flash required for 100% capture at different pulse duration (2, 5, 10, 20 and 50ms). Notably the longer the pulse duration was, the lower the light intensity required. Furthermore, the increasing flash frequency (6, 7, 8 and 10Hz) of the NIR light setting at 1.66w (2-fold threshold power) and 20ms duration induced sufficient cardiac pacing (Figure 2).
The efficient NIR control of the heart

Conclusion
We demonstrated the successful NIR photo-activation of ChR2 expressed in the heart by the upconverted blue light via UCNPs, which resulted in a flexible UCNPs-assisted cardiac optogenetic approach for optical control of heart activity. We believe that these advances in cardiac optogenetic toolbox not only represent a novel practical application of UCNPs, but also open up new possibilities for remote or tissue penetrating heart control.

Acknowledgement/Funding
The national natural science foundation of China (81772044)