Validating Fluorescent Chrnb4.EGFP Mouse Models for the Study of Cone Photoreceptor Degeneration

To validate the application of a known transgenic mouse line with green fluorescent cones (Chrnb4.EGFP) to study cone photoreceptor biology and function in health and disease.

Chrnb4.EGFP retinas containing GFP⁺ cones were compared with retinas without the GFP transgene via immunohistochemistry, quantitative real-time polymerase chain reaction, electroretinograms, and flow cytometry. The Chrnb4.EGFP line was backcrossed to the mouse models of cone degeneration, Pde6c and Gnat2 , generating the new lines Gnat2.GFP and Pde6c.GFP, which were also studied as described.

GFP expression spanned the length of the cone cell in the Chrnb4.EGFP line, as well as in the novel Gnat2.GFP and Pde6c.GFP lines. The effect of GFP expression showed no significant changes to outer nuclear layer cell death, cone-specific gene expression, and immune response activation. A temporal decrease in GFP expression over time was observed, but GFP fluorescence was still detected through flow cytometry as late as 6 months. Furthermore, a functional analysis of photopic and scotopic electroretinogram responses of the Chrnb4 mouse showed no significant difference between GFP^- and GFP⁺ mice, whereas electroretinogram recordings for the Pde6c.GFP and Gnat2.GFP lines matched previous reports from the original lines.

This study demonstrates that the Chrnb4.EGFP mouse can be a powerful tool to overcome the limitations of studying cone biology, including the use of this line to study different types of cone degeneration.

This work validates research tools that could potentially offer more reliable preclinical data in the development of treatments for cone-mediated vision loss conditions, shortening the gap to clinical translation.