A ced-3 caspase – ect-2 RhoGEF axis coordinates functional interactions between the apoptotic pathway and cell size in Caenorhabditis elegans

Programmed cell death via apoptosis is a common cell fate during animal development and its mis-regulation can have serious implications in diseases and disorders. Therefore, it is of high importance to study to apoptosis. The highly conserved central apoptotic pathway was initially discovered in C. elegans and consists of four genes acting in a sequence – egl-1 BH3-only, ced-9 Bcl-2, ced-4 Apaf-1 and ced-3 caspase. The most downstream gene in the pathway, ced-3, encodes for a cysteine protease called caspase, and is essential in the execution of apoptosis. A previous study in our lab had uncovered a novel non-apoptotic role of ced-3 caspase in promoting asymmetric division of C. elegans neuroblasts (Mishra et al., 2018). However, the mechanism by which ced-3 caspase promotes asymmetric division still remained to be elucidated. Thus, in my study, I aimed to decipher the mechanism(s) by which the apoptotic gene, ced-3 caspase, promotes asymmetric cell division. To that end, I first demonstrated that CED-3 caspase protein physically and directly interacts with a regulator of actomyosin contractility, called ECT-2 RhoGEF (Rho guanine-nucleotide exchange factor). Furthermore, using the NSM (neurosecretory motor neuron) lineage in C. elegans, I found that ECT-2 RhoGEF is asymmetrically enriched in the NSM neuroblast, which is the mother of the apoptotic cell. I also found that the asymmetric enrichment of ECT-2 RhoGEF depends on ced-3 caspase activity. Next, by analysing the cell size ratios of the daughters of the NSM neuroblast, my colleagues and I found that genetically, ced-3 caspase acts upstream of ect-2 RhoGEF to promote the asymmetric division by size of the NSM neuroblast. We refer to this as the ced-3-ect-2 axis. Based on these findings, we propose that the ced-3-ect-2 axis promotes polar actomyosin contractility in the NSM neuroblast, which results in its asymmetric division by size and thereby the formation of its smaller apoptotic daughter cell called the NSMsc (NSM sister cell). Molecularly, we propose that CED-3 de-recruits ECT-2 from the dorsal cortex of the NSM neuroblast before metaphase, and that this de-recruitment of ECT-2 is important for the NSM neuroblast to divide asymmetrically. 6 Next, my colleagues and I investigated the effect of the size of the smaller daughter cell, the NSMsc, on its apoptotic fate. We found that increasing the size of the NSMsc by reducing ect-2 activity decreases its probability to undergo apoptosis. Conversely, for the first time, we showed that decreasing the size of the NSMsc by hyperactivation of ect-2 can increase its probability to undergo apoptosis. Thus, we propose that cell size and apoptosis are inversely corelated – larger cells are more prone to survive and smaller cells are more prone to die. Taken together, the findings from this study have found reciprocal interactions between the apoptotic pathway and cell size. In the NSM neuroblast, the apoptotic pathway acts upstream of cell size i.e. ced-3 promotes asymmetric division of the NSM neuroblast and the formation of a smaller NSMsc. Conversely, in the NSMsc, cell size acts upstream of the apoptotic pathway i.e. the small size of the NSMsc promotes the activation/activity of CED-3 and thereby its apoptosis