Design for Fast Optogenetic Screen In Mammalian Cells For Next Gen Ca2+ Sensors

Genetically encoded fluorescent biosensors are powerful tools for studying complex signaling in the nervous system, and now both Ca2+ and voltage sensors are available to study the signaling behavior of entire neural circuits. There is a pressing need for improved sensors to properly interrogate these systems. Improving them is challenging because testing them involves low throughput, labor-intensive processes. Our goal was to create a live cell system in HEK293 cells that use a simple, reproducible, optogenetic process for testing prototypes of genetically encoded biosensors. In this live cell system, blue light activates an adenylyl cyclase enzyme (bPAC) that increases intracellular cAMP [1]. In turn, the cAMP opens a cAMP gated ion channel (olfactory cyclic nucleotide-gated channel, CNG, or the hyperpolarization-activated cyclic nucleotide-gated channel, HCN2). This produces slow, whole-cell Ca2+ transients and voltage changes. To increase the speed of these transients, we added the inwardly rectifying potassium channel Kir2.1, the bacterial voltage-gated sodium channel NAVROSD, and Connexin-43. This is a modular system in which the types of channels, and their relative amounts, can be tuned to produce the cellular behavior that is crucial for screening biosensors. The result is a highly reproducible, high-throughput live cell system that can be used to screen voltage and Ca2+ sensors in multiple fluorescent wavelengths simultaneously.