Muller glial responses compensate for degenerating photoreceptors in retinitis pigmentosa

Retinitis pigmentosa: Focus on genes suggests possible treatments Insights into the genetic and metabolic changes in retinitis pigmentosa, a rare genetic disorder that leads to increasingly worse vision, suggest a new approach for treating degenerative diseases of the retina. Researchers in the US led by Zhongiie Fu at Boston Children's Hospital/Harvard Medical School, Boston, analyzed the activity of genes in the retinal cells of mice used to model retinitis pigmentosa in humans. They detected a significant decrease in the activity of several genes involved in light reception, subsequent reduced nerve signalling activities and metabolism in the rod and cone cells of the retina. These cells facilitate vision at low and high light levels, respectively. Increased gene activity in retinal cells called the Muller glial cluster partially compensated for these defects. The results suggest possible gene and protein targets for new treatments. Photoreceptor degeneration caused by genetic defects leads to retinitis pigmentosa, a rare disease typically diagnosed in adolescents and young adults. In most cases, rod loss occurs first, followed by cone loss as well as altered function in cells connected to photoreceptors directly or indirectly. There remains a gap in our understanding of retinal cellular responses to photoreceptor abnormalities. Here, we utilized single-cell transcriptomics to investigate cellular responses in each major retinal cell type in retinitis pigmentosa model (P23H) mice vs. wild-type littermate mice. We found a significant decrease in the expression of genes associated with phototransduction, the inner/outer segment, photoreceptor cell cilium, and photoreceptor development in both rod and cone clusters, in line with the structural changes seen with immunohistochemistry. Accompanying this loss was a significant decrease in the expression of genes involved in metabolic pathways and energy production in both rods and cones. We found that in the Muller glia/astrocyte cluster, there was a significant increase in gene expression in pathways involving photoreceptor maintenance, while concomitant decreases were observed in rods and cones. Additionally, the expression of genes involved in mitochondrial localization and transport was increased in the Muller glia/astrocyte cluster. The Muller glial compensatory increase in the expression of genes downregulated in photoreceptors suggests that Muller glia adapt their transcriptome to support photoreceptors and could be thought of as general therapeutic targets to protect against retinal degeneration.