Conditional deletion of NMNAT1 in the murine cone photoreceptor leads to severe cone degeneration with distinct neural circuitry defects

Abstract

Purpose : Mutations in the nuclear-specific NAD⁺ synthase nicotinamide mononucleotide adenylyltransferase 1 (NMNAT1) result in severe vision loss in humans. Our previous study demonstrates that loss of NMNAT1 in the murine retina displays defects in the terminal differentiation of photoreceptor and bipolar cells due to distinct changes in metabolic and transcriptional profiling. We sought to investigate the role of NMNAT1 in cone development, functions, and retinal structural integrity in a cone-lacking NMNAT1 mouse model.

Methods : A cone-specific conditional animal model was generated by crossing a floxed NMNAT1 (NMNAT1^fl/fl) mouse line with transgenic mice expressing cone-specific cre recombinase under cone-specific promoter. The progressive cone degeneration was assessed at various ages by immunocytochemistry (ICC) followed by confocal imaging. The scotopic and photopic responses were assessed by electroretinography (ERG). Protein localization and expression were analyzed by ICC and immunoblotting (IB). Ultrastructural analysis was performed using transmission electron microscopy (TEM).

Results : Conditional deletion of NMNAT1 in murine cone photoreceptors displays progressive and complete cone degeneration, and dysmorphic outer plexiform layer (OPL) between postnatal ages 5 to 30 (P5- P30) was observed. No detectable photopic ERG response at P20 with reduced scotopic b-wave was detected. We observed a significant loss and mislocalization of cone-specific proteins with normal expression of rod-specific proteins. The ultrastructure of NMNAT1 deficient cones (P10-P30) showed severe cone degeneration and dysmorphic OPL.

Conclusions : Our results show that NMNAT1 is indispensable for cone photoreceptor function and survival. The loss of NMNAT1 in cones leads to severe defects in OPL which affect scotopic b-wave despite having a normal a-wave amplitude. The combination of ICC, IB, and ultrastructure analysis allows for a thorough characterization of nuclear-specific NMNAT1 role in cone function and neural circuitry maintenance. Collectively, our study demonstrates that NMNAT1 is critical not only for cone photoreceptor function and survival but also for rod function.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.