Project Summary Cancer remains a menace because of our inability to tackle metastatic disease. Metastasis is the culmination of a long history of interactions between cancer cells and the microenvironments of invaded tissues, resulting in a disease that is highly heterogeneous intra- and interpatient. For this reason, the currently available treatments for metastatic disease show limited efficacy since a large fraction of tumors develop escape strategies. Therefore, targeting a new type of cancer commonality could lead to new treatments for a significant portion of metastatic patients. It has been recognized for more than a century that the invaded tissue (“soil”) plays a role in metastasis as important as that of the metastatic cells (“seed”). However, technical limitations have kept most research focused on the primary disease or intrinsic properties of the “seed”. Yet colonized organs are not passive; they contain complex sets of cell types which must interact with invading cells to allow invasion and expansion. I propose a new conceptual framework in which molecular changes arising in metastasis-interacting resident cells (MiRCs) are a general feature of metastasis and could serve as new therapeutic targets. As such alterations may be common across cancer types and patients, targeting them could lead to Extensive Medicine approaches to complement Precision Medicine. To fulfill this vision, a new in vivo strategy to label and track interactions between metastatic cells and MiRCs is required. We have developed such a system, based on synthetic Notch (SynNotch) receptors. We have used this new SynNotch MiRC Labeling System, and our expertise in liver biology (the liver is the most common site of metastasis across cancer types), to label and isolate metastasis-interacting hepatocytes (MiHs). By using this approach, our first goal is to identify the molecular alterations in MiHs across multiple murine and human liver metastatic models; candidate genes will be tested for ability to reduce metastasis. In addition, to realize the full potential of this novel conceptual framework, we will generate a genetic toolbox to allow the analysis of any MiRC in any metastatic model. Finally, we will invert the SynNotch System to track metastatic cells which have interacted with a tissue resident cell of choice; this will help address questions such as whether metastatic progression occurs sequentially or by developing from the primary tumor, and whether invasion of lymph nodes is a prerequisite for metastasis. This proposal aims to provide the conceptual and experimental foundation for a new approach to tackle metastatic disease based on the molecular characterization of MiRCs using our new SynNotch in vivo System. Tools and techniques established here can also be applied in metastatic cells to aid in resolving fundamental questions about the metastatic process. Finally, these same tools and techniques can be used in other diseases or biological processes invol...