In vivo characterization of genetic expression of virus-transduced calcium indicators in retinal ganglion cells using a low-cost funduscope.

Virus-transduced calcium indicators are effective reporters of neural activity, offering the advantage of cell-specific labeling. To track the level of in vivo expression of genetically encoded calcium indicators (GECIs) in rodent retina, we developed a noninvasive imaging approach based on a custom-modified low-cost and simple fundus system that enabled us to monitor and characterize in vivo bright-field and fluorescence retinal image. The system clearly resolves individual retinal ganglion cells (RGCs) and axons. RGC fluorescence intensity and number of observable fluorescent cells show a consistent rising trend from week 1 to week 3 after viral injection, indicating a uniform increase of GCaMP6f expression. At defined time points, we prepared wholemount retina mounted on a transparent multielectrode array (MEA) and used calcium imaging to identify the optimal time for studying the responsiveness of RGCs to external electrical stimulation. The results show that the fluorescence-endoscopy fundus system is a powerful and widely accessible tool for evaluating in vivo fluorescence reporter expression.