Role of Top2b in retinal development and homeostasis: Insights from a conditional knockout mouse model

Rod photoreceptors, constituting approximately 70% of retinal cells, have traditionally been viewed as a homogeneous population that utilizes rhodopsin (RHO) for dim-light vision. This dissertation challenges that view by reporting that conditional deletion of Top2b uncovers unexpected molecular heterogeneity within this lineage.DNA topoisomerase II beta (TOP2B) is a critical enzyme involved in DNA topology and gene regulation, with significant implications for retinal development and maintenance. Using a rod-specific Top2b conditional knockout (cKO) mouse model, we demonstrate that TOP2B is essential for maintaining rod integrity and function. Top2b cKO mice exhibit late-onset retinal degeneration, characterized by disrupted outer segment morphology, impaired synaptic structure and a progressive loss of both rod and cone function measured by electroretinography.Crucially, single-cell RNA sequencing analyses revealed distinct rod subpopulations. Transcriptional state transitions are detected at as early as two months of age, preceding the onset of rod structural degeneration and functional decline. These subpopulations exhibit divergent transcriptional responses to Top2b deletion, including the emergence of discrete stress-adaptive, degenerative, and pathological states.Collectively, these findings redefine rod heterogeneity, emphasizing TOP2B’s role in regulating transcriptional homeostasis, and offer new insights into the mechanisms of retinal degeneration.