Genetically Encoded Biosensors Reveal Spatial Differences in the Metabolism of Retinal Cells

Vision is fundamental for human life, and any impairment significantly reduces an individual’s quality of life. However, vision impairment places also a burden on our society, with an estimated 1.8 billion people expected to be affected by 2050. As the global population continues to age, age-related vision loss caused by glaucoma or age-related macular degeneration (AMD) is expected to become a major public health concern. Retinal degeneration caused by AMD, glaucoma as well as inherited retinal diseases are linked to impairment of retinal energy metabolism. Therefore, understanding the underlying pathologies and the role of energy metabolism in disease development are crucial for the development of new treatments. Here, we investigated differences in energy metabolism between rod photoreceptors and inner retinal neurons. Many studies proposed that rod photoreceptors are the main driver of aerobic glycolysis in the retina and data on inner retinal cells is scarce. Therefore, we utilized genetically encoded biosensors and two-photon fluorescence lifetime microscopy to monitor metabolite dynamics on a cellular level in acute eye sections. Additionally, we also investigated alterations of energy metabolism during degeneration in the RhoP23H/+ mouse, a model for retinitis pigmentosa. The utilization of genetically encoded sensors and pharmacological protocols allowed us to dissect energy metabolism on a cellular level. We could show that rod photoreceptors rely mainly on glucose for energy production and utilize it via aerobic glycolysis and OXPHOS. In contrast, inner retinal neurons could maintain ATP levels in the absence of glucose but presence of lactate, suggesting that these cells rely mostly on OXPHOS for energy production. Additionally, the determination of the NADH/NAD+ ratio confirmed that rod photoreceptors are more glycolytic and inner retinal neurons more oxidative. Further, we were able to demonstrate that both rod photoreceptors and inner retinal neurons produce lactate, however rod photoreceptors generated more lactate and at a higher rate. Surprisingly, our data revealed only minor metabolic alterations in the degenerating rods, suggesting a switch to a higher contribution of aerobic glycolysis to ATP production. In summary, our data aligns with previous published data that rod photoreceptors are the main driver of glycolysis in the retina. Furthermore, the results and tools presented here expand our knowledge on photoreceptor and inner retinal neurons in physiology and pathophysiology with a spatial and temporal resolution outperforming other approaches.