PAR-1 promotes primary neurogenesis and asymmetric cell divisions via control of spindle orientation

In both invertebrate and vertebrate embryonic central nervous systems, deep cells differentiate while superficial (ventricular) epithelial cells remain in a proliferative, stem cell state. The conserved polarity protein PAR-1, which is basolaterally localised in epithelia, promotes and is required for differentiating deep layer cell types, including ciliated cells and neurons. It has recently been shown that atypical protein kinase C (aPKC), which is apically enriched, inhibits neurogenesis and acts as a nuclear determinant, raising the question of how PAR-1 antagonises aPKC activity to promote neurogenesis. Here we show that PAR-1 stimulates the generation of deep cell progeny from the superficial epithelium of the neural plate and that these deep cells have a corresponding (i.e. deep cell) neuronal phenotype. We further show that gain- and loss-of-function of PAR-1 increase and decrease, respectively, the proportion of epithelial mitotic spindles with a vertical orientation, thereby respectively increasing and decreasing the number of cleavages that generate deep daughter cells. PAR-1 is therefore a crucial regulator of the balance between symmetric (two superficial daughters) and asymmetric (one superficial and one deep daughter) cell divisions. Vertebrate PAR-1 thus antagonises the anti-neurogenic influence of apical aPKC by physically partitioning cells away from it in vivo.