Summary During development, all cells make the decision to live or die. While the molecular mechanisms that execute the apoptotic program are well defined, less is known about how cells decide whether to live or die. In C. elegans, this decision is linked to how cells divide asymmetrically [1, 2]. Several classes of molecules are known to regulate asymmetric cell divisions in metazoans, yet these molecules do not appear to control C. elegans divisions that produce apoptotic cells [3]. We identified CNT-2, an Arf GAP protein of the AGAP family, as a novel regulator of this type of neuroblast division. Loss of CNT-2 altered daughter cell size and caused the apoptotic cell to adopt the fate of its sister cell, resulting in extra neurons. CNT-2’s Arf GAP activity was essential for its function in these divisions. The N-terminus of CNT-2, which contains a GTPase-like domain that defines the AGAP class of Arf GAPs, negatively regulates CNT-2’s function. We provide evidence that CNT-2 regulates receptor-mediated endocytosis and consider the implications of its role in asymmetric cell divisions.

Neuroblast divisions in the nematode Caenorhabditis elegans often give rise to a larger neuron and a smaller cell that dies. We have previously identified genes that, when mutated, result in neuroblast divisions that generate daughter cells that are more equivalent in size. This effect correlates with the survival of daughter cells that would normally die. We now describe a role for the DEP domain-containing protein TOE-2 in promoting the apoptotic fate in the Q lineage. TOE-2 localized at the plasma membrane and accumulated in the cleavage furrow of the Q.a and Q.p neuroblasts, suggesting that TOE-2 might position the cleavage furrow asymmetrically to generate daughter cells of different sizes. This appears to be the case for Q.a divisions where loss of TOE-2 led to a more symmetric division and to survival of the smaller Q.a daughter. Localization of TOE-2 to the membrane is required for this asymmetry, but, surprisingly, the DEP domain is dispensable. By contrast, loss of TOE-2 led to loss of the apoptotic fate in the smaller Q.p daughter but did not affect the size asymmetry of the Q.p daughters. This function of TOE-2 required the DEP domain but not localization to the membrane. We propose that TOE-2 ensures an apoptotic fate for the small Q.a daughter by promoting asymmetry in the daughter cell sizes of the Q.a neuroblast division but by a mechanism that is independent of cell size in the Q.p division.

Cytohesins are Arf guanine nucleotide exchange factors (GEFs) that regulate membrane trafficking and actin cytoskeletal dynamics. We report here that GRP-1, the sole Caenorhabditis elegans cytohesin, controls the asymmetric divisions of certain neuroblasts that divide to produce a larger neuronal precursor or neuron and a smaller cell fated to die. In the Q neuroblast lineage, loss of GRP-1 led to the production of daughter cells that are more similar in size and to the transformation of the normally apoptotic daughter into its sister, resulting in the production of extra neurons. Genetic interactions suggest that GRP-1 functions with the previously described Arf GAP CNT-2 and two other Arf GEFs, EFA-6 and BRIS-1, to regulate the activity of Arf GTPases. In agreement with this model, we show that GRP-1's GEF activity, mediated by its SEC7 domain, is necessary for the posterior Q cell (Q.p) neuroblast division and that both GRP-1 and CNT-2 function in the Q.posterior Q daughter cell (Q.p) to promote its asymmetry. Although functional GFP-tagged GRP-1 proteins localized to the nucleus, the extra cell defects were rescued by targeting the Arf GEF activity of GRP-1 to the plasma membrane, suggesting that GRP-1 acts at the plasma membrane. The detection of endogenous GRP-1 protein at cytokinesis remnants, or midbodies, is consistent with GRP-1 functioning at the plasma membrane and perhaps at the cytokinetic furrow to promote the asymmetry of the divisions that require its function.M ETAZOAN development relies on the coordinated behavior of individual cells that either adopt morphogenetic behaviors such as proliferation, migration, adhesion, and differentiation, or die at precise times and places. Despite extensive characterization of the molecular mechanisms associated with the execution of programmed cell death (PCD) during development, less is known about how a given cell chooses to live or die. PCD execution often relies on the activation of versatile proteases, the caspases, by an evolutionarily conserved molecular cascade. In the nematode Caenorhabditis elegans, somatic PCD is largely determined by asymmetric cell divisions that produce a surviving daughter cell and a daughter cell fated to die (Frank et al. 2005;Cordes et al. 2006;Hatzold and Conradt 2008;Ou et al. 2010;Singhvi et al. 2011). The invariant lineage that produces these dying cells makes C. elegans a powerful system to explore the mechanisms involved in PCD specification.Although several studies point to the cell-specific transcriptional control of EGL-1, a BH3-only protein that can activate the caspase cascade, as a mechanism of PCD specification (Potts and Cameron 2011), other data suggest that daughter cell-size asymmetry regulates PCD (Frank et al. 2005;Cordes et al. 2006;Hatzold and Conradt 2008;Ou et al. 2010;Singhvi et al. 2011). Indeed, C. elegans divisions that generate dying cells are generally asymmetric, producing a larger surviving daughter and a smaller daughter fated to die. Several mutants affecting this size difference al...

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