Importin-alpha transports Norrin to the nucleus to promote proliferation and Notch signaling in glioblastoma stem cells.

Norrin, a secreted protein encoded by NDP gene, is recognized for its established role as a paracrine canonical Frizzled-4/Wnt ligand that mediates angiogenesis and barrier function in the brain. However, emerging evidence suggests that Norrin possesses Frizzled-4-independent functions, notably impacting Notch activation and proliferation of cancer stem cells. We conducted a BioID protein-proximity screen to identify Norrin-interacting proteins. Surprisingly, a significant proportion of the proteins we identified were nuclear. Through comprehensive tagging and proximity ligation assays, we demonstrate that Norrin is transported to the nucleus through KPNA2 (member of the Importin-alpha family). Subsequently, we demonstrate that KPNA2 loss of function in patient-derived primary glioblastoma stem cells results in a nuclear to cytoplasmic shift of Norrin distribution, and a complete abrogation of its function in stimulating Notch signaling and cellular proliferation. These results indicate that Norrin is actively transported into the nucleus to regulate vital signaling pathways and cellular functions.