Mechanisms of cell corpse clearance in tissue turnover and regeneration using planarians and zebrafish

Project Abstract Every day, adult humans create and destroy tens of billions of cells. We know that imbalances in tissue homeostasis can lead to too many new cells, such as in tumorigenesis, and disruptions in dying cell clearance can cause inflammation, tissue damage, and autoimmune conditions. In species with the ability to regenerate

from acute injury, waves of cell death are observed and are thought trigger appropriate levels of proliferation and inflammation. While decades of work have elucidated the pathways of cell death and corpse clearance during animal development, together called efferocytosis, relatively little is known about the dynamics of cell

corpses in adult tissues or how efferocytosis machinery may function in the regeneration process. Major outstanding questions include: are cell corpses digested in place, or are they physically removed from tissues?; what cells do the engulfing in a given context?; and what happens to tissue turnover and

regeneration when engulfment genes are disrupted? To uncover mechanisms of cell corpse clearance in adult regenerative animals, we utilize the complementary advantages of two model systems: planarians and zebrafish. The freshwater planarian, a flatworm from the phylum Platyhelminthes, has significant advantages to studying gene function in adult stem cell biology and

regeneration in vivo. On the other hand, zebrafish are one of the best genetic models of regeneration and provide direct vertebrate relevance and the ability to watch cellular processes using transgenic lines. When combined, planarians and zebrafish can be used for gene discovery and genetic mechanisms of regeneration.

In the current proposal, we developed a way to selectively kill off multiple cell types in planarians and can observe those cells transit from the periphery of the animal and be excreted through the gut. In our supporting data, we show that the gene engulfment and cell motility (ELMO) is required to clear cell corpses, but we do

not understand how or through what cell types. Further, we developed a model of spinal cord regeneration using zebrafish, and show that multiple elmo genes are expressed in phagocytic immune cells as well as cell types in the spinal cord. Building on these data, in AIM 1, we will test how different cell types in planarians use

the ELMO pathway to clear cell corpses in several contexts. In AIM 2, we will identify feedback signals from dying cells to stem cells and test how ELMO functions during regeneration. In AIM 3, we will use our zebrafish model of spinal cord regeneration to test how the ELMO pathway functions in cell corpse clearance and

regenerative outcomes. Given that the ELMO pathway is highly conserved, our work will illuminate fundamental aspects of how defects in cell clearance influence regeneration outcomes and stem cell proliferation control, which will have high relevance to multiple human diseases.