This is different for other proteins involved in this process.

The apical Par proteins are also involved in epithelial polarity and cell migration. Mutating centrosomal proteins like Asp (Aspm in mice) ( Fish et al., 2006 and Fish et al., 2008) or Cnn (CDK5RAP2 in mice) ( Barrera et al., 2010) might affect signaling pathways by disrupting primary cilia and will influence centrosome asymmetry, which was proposed to be important in cortical neurogenesis ( Wang et al., 2009). Mutating dynein-binding proteins like Lis1 causes defects in spindle morphology and cell migration ( Yingling ISRIB datasheet et al., 2008). Therefore, our mInsc knockout and mInsc-overexpression mice are particularly specific tools to analyze spindle orientation. The spindle orientation defects we observe in mInsc-deficient mice are different from the one previously reported for LGN, the mouse homolog of the Insc-binding partner Pins. In LGN knockouts, the orientation of the mitotic spindle

is randomized while lack of mInsc causes almost all mitotic spindles to assume a planar orientation. This is in agreement with the functions reported for the two genes in flies and explains why the two genes have different effects on cortical neurogenesis ( Konno et al., 2008 and Shitamukai et al., 2011) (and this study). Our results suggest that intermediate progenitors are more likely to arise from oblique or horizontal divisions (in which the spindle BI 6727 manufacturer is oriented oblique or vertical, respectively). First, increasing or decreasing mInsc unless expression elevates or reduces the number of neurons, respectively. At the same time, both the total number of apical progenitors and the number of mitotic cells in the VZ remain constant. Second, mInsc levels affect

the number of Tbr2-positive intermediate progenitors and the number of cells dividing outside the VZ. And finally, apical progenitors labeled by electroporation of RFP-expressing plasmids are more likely to give rise to Tbr2-positive intermediate progenitors when mInsc levels are increased. We propose a model in which mInsc influences spindle orientation and thereby regulates the balance between direct and indirect neurogenesis ( Figure 8). Whether or not mInsc is required for generating all or most BPs is not clear. It is remarkable that the terminal forebrain phenotype of mInsc mice is similar to the one observed for Tbr2, in which intermediate progenitors are essentially absent ( Arnold et al., 2008 and Sessa et al., 2008): in both cases, thickness of the CP is reduced by about 40%. While the outer layers are more affected in Tbr2−/− mice, however, NesCre/+;mInscfl/fl mice show similar defects across all layers. This could be explained if intermediate progenitors initially form through a spindle orientation-dependent mechanism, but later neurogenesis can also proceed through a partially redundant, mInsc-independent mechanism.