Genetically encoded fluorescent calcium indicators (GECIs) allow for imaging of neuronal calcium activity with high spatiotemporal resolution. However, to date only very few GECIs were developed in the near-infrared (NIR) range of 700 to 900 nm, where optical scattering and attenuation are minimal in tissue. NIR GECIs generally suffer from low brightness and weak fluorescence responses and are thus not deemed as suitable for in vivo imaging.

NIR-GECO2G is a recently developed GECI with excitation and emission maxima at 678 nm and 704 nm, respectively. Using widefield fluorescence imaging in the live the mouse brain, we demonstrate several-fold improved response magnitude compared to the original NIR-GECO1 variant. We further show several-fold increased NIR-GECO2G brightness levels in Blvra-/- mice, where high concentrations of biliverdin (BV) result from deleting the gene for the enzyme that aids in the breakdown of BV.

The inherent advantages of red-shifted fluorescent proteins and fluorescent protein-based biosensors for the study of signaling processes in neurons and other tissues have motivated the development of a plethora of new tools. Relative to green fluorescent proteins (GFPs) and other blue-shifted alternatives, red fluorescent proteins (RFPs) provide the inherent advantages of lower phototoxicity, lower autofluorescence, and deeper tissue penetration associated with longer wavelength excitation light. All other factors being the same, the multiple benefits of using RFPs make these tools seemingly ideal candidates for use in neurons and, ultimately, the brain. However, for many applications, the practical utility of RFPs still falls short of the preferred GFPs. We present an overview of RFPs and RFP-based biosensors, with an emphasis on their reported applications in neuroscience.