A major obstacle to achieving long-term cancer remission is the ability of some cancer cells to resist therapy. Strikingly, the fundamental cell biology underlying therapy resistance in human tumors is remarkably similar to that which confers incredible regenerative capacity in many planarian species. In these flatworm species, even a small piece of excised tissue can recreate an entirely new animal. This remarkable ability relies on the maintenance of a heterogeneous pool of planarian stem cells that bear many similarities to cancer stem cells. For example, a subset of these cells can tolerate high doses of γ-radiation and restore the entire stem cell population. Yet little is known about the mechanisms that confer this tolerance and almost nothing is known about their resistance to chemotherapies. Preliminary research revealed that planarians show resistance to the drug cisplatin. Treatment of planarians with 80-100μM cisplatin induced a phenotype that broadly mimicked the development of chemoresistance in human cancers: animals exhibited an initial loss of tissue followed by regeneration. Based on these data and known effects of cisplatin, the central hypothesis is that cisplatin differentially affects mitochondrial function across the stem and progenitor populations, which differentially alters a conserved chromatin signature that marks tumor suppressor genes i.e., broad H3 lysine 4 trimethylation (H3K4me3), increases cellular heterogeneity, and promotes chemoresistance. The goal of this study is to leverage the remarkable functional similarities between human tumors and the planarian stem cell population to uncover fundamental mechanisms that link the development of chemoresistance to metabolic and epigenetic regulation in vivo through the following specific aims: 1) To identify cisplatin-responsive differences in mitochondrial function in stem and progenitor cell populations using mitochondrial probes, high resolution imaging, and biochemical assays. This study uses state-of the-art approaches to determine the metabolic heterogeneity of stem cells in vivo. 2) To establish mechanistic links between metabolism, chromatin state, and chemoresistance by perturbing metabolite availability and measuring the impact on H3K4me3, transcriptional heterogeneity, and chemoresistance. This study will reveal critical links between metabolic and epigenetic regulation in chemoresistance. 3) To assess the conservation of metabolic H3K4me3 regulatory mechanisms in lung cancer using established airway organoid models. This study will test the hypothesis that resistant lung cancer cells respond to cisplatin-induced ROS by reducing H3K4me3. The proposed research will uncover novel mechanisms and target genes underlying chemoresistance.