pig-1 MELK and ced-3 Caspase cooperate to control cell polarity in the C. elegans NSM neuroblast

Snail-like genes encode zinc-finger transcription factors that play essential roles in development, and one of their well-known functions is the epithelial-mesenchymal transition (EMT) induction. Many studies performed in organisms ranging from Drosophila melanogaster to mammals have reported that Snail transcription factors regulate various aspects of stem cell development, such as cell polarity and cell cycle progression. However, the mechanisms through which Snail-like genes regulate these developmental processes are not completely understood. To uncover these mechanisms, I studied the neurosecretory motor neuron neuroblast (NSMnb) lineage during C. elegans embryogenesis. In the NSMnb lineage, we have previously found that CES-1 Snail controls cell cycle progression by regulating expression of the gene cdc-25.2 CDC25. However, the mechanism by which ces-1 controls the asymmetric division of the NSMnb is unknown. By analyzing CES-1 ChIP-seq data acquired from the modENCODE Project, we identified more than 3,000 potential targets of CES-1 Snail. From the potential candidates that are involved in regulating asymmetric cell division, pig-1 was found to play an essential role in asymmetric NSMnb division. pig-1 encodes the sole C. elegans ortholog of Maternal Embryonic Leucine-zipper kinase (MELK) kinase. Through genetic studies, I confirmed that pig-1 acts downstream of ces-1 to control the asymmetric positioning of the NSMnb cleavage plane. Furthermore, by using a single-copy transcriptional reporter of pig-1, I observed that loss of ces-1 increases the transcriptional level of pig-1, while gain of ces-1 activity decreases the level of pig-1. Therefore, I conclude that CES-1 Snail regulates asymmetric positioning of the NSMnb cleavage plane by repressing expression of the gene pig-1. In the NSMnb, CES-1 Snail coordinates the cell cycle through cdc-25.2 and asymmetric positioning of the cleavage plane through pig-1 to ensure asymmetric cell division and the generation of two daughter cells of different sizes and fates: the larger NSM, which survives, and the smaller NSM sister cell (NSMsc), which dies. Apart from influencing the positioning of the cleavage plane, ces-1 and pig-1 also play roles in controlling the orientation of the NSMnb cleavage plane and in specifying the fate of the daughter cell, NSMsc. On the other hand, I show that ced-3, which encodes a Caspase and which usually executes cell death in C. elegans, also plays a role in regulating the asymmetric positioning of the NSMnb cleavage plane. Loss of ced-3 alone did not affect the asymmetric positioning of the NSMnb cleavage plane at lateral-dorsal side, but loss of both ced-3 and pig-1 reversed the cleavage plane to the medial-ventral side and generated a small NSM and a large NSMsc. This indicates that in the NSMnb lineage, ced-3 may have other functions in addition to executing cell death in the smaller daughter (NSMsc). Furthermore, I confirmed that this function is dependent on the Caspase activity of CED-3 protein. Taken together, ces-1 Snail and pig-1 MELK are two key factors that coordinate cell polarity and cell fate in the NSMnb lineage during C. elegans embryogenesis. In addition, ced-3 Caspase acts in parallel to pig-1 and ces-1 to promote the correct positioning of the cleavage plane in the NSMnb