Endothelial Klf9 fine-tunes Akt signaling to act as a transcriptional brake restraining retinal angiogenesis.

Retinal angiogenesis requires precise transcriptional regulation. Krüppel-like factor 9 (Klf9) has been implicated in various biological processes; however, its specific role in retinal vascular development and ocular neovascular disease remains unclear. In this study, we identified Klf9 as a critical transcriptional regulator of retinal vascular homeostasis. Spatiotemporal transcriptomic and single-cell RNA sequencing analyses revealed that Klf9 was highly enriched in retinal endothelial cells and upregulated during vascular maturation. Using genetic mouse models, we demonstrated that endothelial-specific Klf9 deletion accelerated neonatal retinal vascular expansion and tip cell formation, whereas its overexpression delayed angiogenesis and disrupted barrier function. In oxygen-induced retinopathy, Klf9 loss exacerbated pathological neovascularization and leakage, while its overexpression conferred protection. Integrated RNA-seq and ATAC-seq profiling of human retinal microvascular endothelial cells revealed that Klf9 represses a network of genes involved in the PI3K-Akt pathway and focal adhesions. Key effectors, including AKT1, PTK2, and RAC1, were suppressed by reduced chromatin accessibility at their promoters. Both in vitro and in vivo rescue experiments confirmed that Akt activation reverses vascular hypoplasia caused by Klf9 overexpression, whereas Akt inhibition normalizes the hyper-angiogenic phenotype of the Klf9-deficient endothelium. Collectively, these findings establish Klf9 as a transcriptional brake on retinal angiogenesis, acting through chromatin-mediated suppression of the PI3K-Akt pathway, and provide new mechanistic insights and potential therapeutic targets for pathological retinal angiogenesis.