Transitin, a nestin-like intermediate filament protein, mediates cortical localization and the lateral transport of Numb in mitotic avian neuroepithelial cells.

Neuroepithelium is an apicobasally polarized tissue that contains neural stem cells and gives rise to neurons and glial cells of the central nervous system. The cleavage orientation of neural stem cells is thought to be important for asymmetric segregation of fate-determinants, such as Numb. Here, we show that an intermediate filament protein, transitin, colocalizes with Numb in the cell cortex of mitotic neuroepithelial cells, and that transitin anchors Numb via a physical interaction. Detailed immunohistological and time-lapse analyses reveal that basal Numb-transitin complexes shift laterally during mitosis, allowing asymmetric segregation of Numb-transitin to one of the daughter cells, even when the cell cleavage plane is perpendicular to the ventricular surface. In addition, RNA interference (RNAi) knockdown of the transitin gene reveals its involvement in neurogenesis. These results indicate that transitin has important roles in determining the intracellular localization of Numb, which regulates neurogenesis in the developing nervous system of avian embryos.