Title: Non-apoptotic Caspase-8 Signaling Mediates Retinal Angiogenesis
Description: The retina is one of the most metabolically active tissues in the body and the high energetic demand is met by a well-organized vascular network. Aberrant vasculature is a prominent feature of many vision-threatening diseases, and although angiogenic pathways have been extensively studied the limited efficacy of therapies currently available for the treatment of these diseases suggests that there is more to be elucidated. The caspase family of proteases is best known for their roles in programmed cell death and inflammation, however members of this family have been found to have essential functions independent of cell death. Caspase-8, in particular, has been previously shown to be essential for embryonic vascular development, however, a requirement for caspase-8 in postnatal vascular development has not been established and it is unclear how caspase-8 exerts its function. In this study, we investigate the cell specific roles of caspase-8 in the development of the retinal vasculature using the postnatal mouse retina as our model and identified endothelial caspase-8 as a mediator of canonical Wnt signaling.

Inducible endothelial cell-specific caspase-8 knockout (Casp8 iECKO) resulted in a delay in early angiogenesis and barrier establishment, and an increase in inflammation and premature vascular remodeling compared to littermate controls. Assessment of Lef1, a downstream effector of the Wnt pathway, confirmed that this phenotype was a result of inhibited Wnt signaling. We additionally show that caspase-8 mediates this pathway through degradation of its substrate HDAC7. HDAC7 has been shown previously to bind to β-catenin blocking its nuclear translocation. Caspase-8 mediated HDAC7 degradation restores β-catenin translocation and downstream Wnt signaling. We also explore the function of caspase-8 in myeloid cells – microglia and macrophages – during angiogenesis. We used an inducible myeloid-specific caspase-8 knockout (Casp8 imGKO) mouse and found that loss of caspase-8 in these cells did not affect angiogenesis. However, Casp8 imGKO resulted in a reduction in microglia number and a change in their morphology specifying a role for caspase-8 in mediating cell survival and activation in microglia.

Altogether we show that caspase-8 exerts cell specific functions during retinal angiogenesis that are independent of cell death. We elucidate a novel role of caspase-8 in mediating Wnt/β-catenin signaling, and implicate caspase-8 as a potential therapeutic target in pathological angiogenesis.