Polarity control of spindle positioning in the C. elegans embryo

Cell and tissue polarity guide a large variety of developmental processes, in-cluding the choice between symmetric and asymmetric cell division. Asymmetric divisions create cell diversity and are needed for maintenance of tissue specific stem cells. Symmetric divisions, on the other hand, promote exponential cell pro-liferation. Polarized cells can divide symmetrically, when cleavage takes place along the polarity axis. Alternatively, polarized cells can divide asymmetrically, . To control this decision, developmental cues position the mitotic spindle, which instructs the plane of cell cleavage. In animal cells, positioning of the spindle de-pends on evolutionarily conserved interactions between a heterotrimeric G protein alpha subunit, TPR-GoLoco domain protein, and NuMA-related coiled-coil pro-tein. This trimeric complex recruits the dynein microtubule motor and captures as-tral microtubules at the cortex. The interplay between dynein and depolymerizing microtubules generates cortical pulling forces that promote aster movement and spindle positioning. Through mechanisms that are poorly understood, cell polarity and other developmental signals control the microtubule-pulling forces to instruct the orientation and plane of cell division. In this chapter, we review the current understanding of the connection between cell polarity and spindle positioning, with a focus on studies of the early C. elegans embryo. The nematode C. elegans develops through highly reproducible symmetric and asymmetric cell divisions and has contributed significantly to the knowledge of cell polarity and spindle po-sitioning in asymmetric cell division.