Removal of apoptotic cells is a dynamic process coordinated by ligands on apoptotic cells, and receptors and other signaling proteins on the phagocyte. One of the fundamental challenges is to understand how different phagocyte proteins form specific and functional complexes to orchestrate the recognition/removal of apoptotic cells. One evolutionarily conserved pathway involves the proteins cell death abnormal (CED)-2/chicken tumor virus no. 10 (CT10) regulator of kinase (Crk)II, CED-5/180 kDa protein downstream of chicken tumor virus no. 10 (Crk) (Dock180), CED-12/engulfment and migration (ELMO) and MIG-2/RhoG, leading to activation of the small GTPase CED-10/Rac and cytoskeletal remodeling to promote corpse uptake. Although the role of ELMO : Dock180 in regulating Rac activation has been well defined, the function of CED-2/CrkII in this complex is less well understood. Here, using functional studies in cell lines, we observe that a direct interaction between CrkII and Dock180 is not required for efficient removal of apoptotic cells. Similarly, mutants of CED-5 lacking the CED-2 interaction motifs could rescue engulfment and migration defects in CED-5 deficient worms. Mutants of CrkII and Dock180 that could not biochemically interact could colocalize in membrane ruffles. Finally, we identify MIG-2/RhoG (which functions upstream of Dock180 : ELMO) as a possible point of crosstalk between these two signaling modules. Taken together, these data suggest that Dock180/ELMO and CrkII act as two evolutionarily conserved signaling submodules that coordinately regulate engulfment. Apoptotic cells are generated by diverse physiological processes, ranging from the elimination of damaged (or precancerous) cells to deletion of cells during developmental morphogenesis; 1,2 the culmination of the apoptotic program is the phagocytosis (or engulfment) of the apoptotic cell. In mammals, prompt removal of apoptotic cells is required to prevent the release of potential self-antigens and onset of autoimmune-like syndromes. [3][4][5] Removal of the apoptotic cell can be divided into several mechanistic steps: first, dying cells expose 'eat-me' signals, which are recognized by receptors on phagocytic cells. 6 These signals, both positive (such as phosphatidylserine and MFG-E8) 2 and negative (such as CD31 and CD47), are rapidly modified following the onset of apoptosis, facilitating removal of the apoptotic cell. A number of receptors have been identified that either directly or indirectly facilitate apoptotic cell recognition; however, signaling following corpse recognition is less well characterized.Recognition of the apoptotic cell by the phagocyte leads to the formation of an actin-rich phagocytic cup, followed by the internalization of the target. 7 Genetic studies in the nematode Caenorhabditis elegans (C. elegans) have led to the identification of eight genes that play a partially redundant role in removal of apoptotic cells. 2 Of these, cell death abnormal (CED)-2/chicken tumor virus no. 10 (CT10) regulator of kinase (Crk)II, C...

Cancer develops after the acquisition of a collection of mutations that together create the ‘cancer phenotype’. How collections of mutations work together within a cell, and whether there is selection for certain combinations of mutations, are not well understood. Using a Ras signaling network mathematical model we tested potential synergistic combinations within the Ras network. Intriguingly, our modeling, including a “computational random mutagenesis” approach, and subsequent experiments revealed that mutations of the tumor suppressor gene NF1 can amplify the effects of mutations in multiple other components of the Ras pathway, including weakly activating, noncanonical, Ras mutants. Since conventional wisdom holds that mutations within the same pathway do not co-occur, it was surprising that modeling and experiments both suggested a functional benefit for co-occurring Ras pathway mutations. Furthermore, we analyzed >3900 sequenced cancer specimens from the Cancer Cell Line Encyclopedia (CCLE) and The Cancer Genome Atlas (TCGA) and we uncovered an increased rate of co-occurrence between mutations the model predicted could display synergy. Overall, these data suggest that selective combinations of Ras pathway mutations could serve the role of cancer “driver”. More generally, this work presents a mechanism by which the context created by one mutation influences the evolutionary trajectories of cancer development, and this work suggests that mutations that result in “network instability” may promote cancer in a manner analogous to genomic instability.