Monitoring of cell-cell communication and contact history in mammals

Monitoring of cell-cell communication in multicellular organisms is fundamental to understanding diverse biological processes such as embryogenesis and tumorigenesis. To track cell-cell contacts in vivo, we developed an intercellular genetic technology to monitor cell-cell contact and to trace cell contact histories by permanently marking contacts between cells. In mice, we engineered an artificial Notch ligand into one cell (the sender cell) and an artificial receptor into another cell (the receiver cell). Contact between the sender and receiver cells triggered a synthetic Notch signaling that activated downstream transcriptional programs in the receiver cell, thereby transiently or permanently labeling it. In vivo cell-cell contact was observed during development, tissue homeostasis, and tumor growth. This technology may be useful for studying dynamic in vivo cell-cell contacts and cell fate plasticity. Description Tracing a history of cell-cell contact The ability to monitor the history of cell-cell contact could benefit our understanding of cellular interactions in many biological contexts, from development to various disease states. Zhang et al. modified synthetic receptor systems that could be targeted to cells of interest in mice. Binding of the designed ligand on a sender cell to a compatible receptor on a receiver cell permanently marked the receiver cell through a change in expression of a fluorescent reporter. For example, they could detect endothelial cells that moved during development from the heart to the liver. —LBR An intercellular genetic technology monitors cell-cell contact and traces cell contact histories by permanently marking contact between cells. INTRODUCTION Cell-cell communication through direct contact is pervasive in multicellular organisms and is essential in many fundamental biological processes. The ability to experimentally track such cell-cell communication signaling could substantially advance our understanding of diverse biological processes from embryogenesis to tumorigenesis. The existing technologies are not suitable to monitor and trace cell-cell contact for long-term in vivo studies, because many biological processes such as embryogenesis, tumorigenesis, and tissue regeneration develop over time after the initial cell-cell contact. RATIONALE The Notch pathway transmits signaling through direct cell-cell contact in many cellular processes during development and homeostasis. In the canonical Notch pathway, upon cell contact, the Notch ligand on one cell binds to the Notch receptor on another cell to trigger a signaling pathway that leads to transcription activation of particular genes. To understand the dynamic in vivo cell-cell communications over time, we developed an intercellular genetic approach using the synthetic Notch pathway (synNotch) that converts a cellular contact event into a controllable transcriptional program. We engineered in mice an artificial Notch ligand, a membrane-tethered green fluorescent protein (GFP), into one cell type (the sender cell) and an artificial receptor in which the extracellular and intracellular domains of Notch were replaced with an anti-GFP nanobody and the tetracycline transactivator, respectively, into another cell type (the receiver cell). Contact between the sender and receiver cells triggered synNotch signaling that activated the downstream transcriptional programs in the receiver cell in vivo. To reveal the ongoing cell-cell contact, as reflected by synNotch activation in a receiver cell after direct contact with a sender cell, we used a tet-off system to express detectable reporters. To trace cell contact history, we used the Cre-loxP system to genetically fate map receiver cells, along with their progenies, permanently after cell contact. RESULTS In the intercellular genetic system, we demonstrated that endothelial cells (receiver cells) in the developing heart were genetically labeled after contact with neighboring cardiomyocytes (sender cells). The endothelial cells that had contact with cardiomyocytes in early embryogenesis were permanently tagged with the genetic reporter. Their progenies migrated into liver and subsequently formed a substantial portion of the vasculature there, suggesting that part of the liver vasculature originates from the developing heart during embryogenesis. Application of these synNotch mice in tumorigenesis revealed the contact history between tumor cells (sender cells) and endothelial cells (receiver cells) during tumor growth and revealed that tumor vessels not only expanded within the tumor but also outgrew into the periphery of the tumor and had strong angiogenic properties. Upon contacting tumor cells, these endothelial cells gained properties in angiogenic, migratory, and inflammatory responses. Additionally, we generated mice for Cre-induced synNotch ligand or receptor expression, enabling broadly applicable approaches for genetic labeling of cell-cell contact and study of cell contact signaling in vivo. Engineering both the synNotch ligand and receptor, as well as different genetic readouts, in one mouse, we demonstrated simultaneous yet distinct recording of not only ongoing cell-cell contact but also historical cell-cell contact. CONCLUSION Our work provides a genetic system for recording cell-cell contact and cell contact history in vivo. The implications of our findings are that endothelial cells in the developing heart migrate and contribute to the liver vasculature, whereas endothelial cells in tumors not only expand within the tumor but also grow outward into the boundary-adjacent normal tissue with robust angiogenesis. The suite of new synNotch mouse lines provides a toolbox for genetic labeling and tracing of contacts between any cell type, offering a useful approach for studying dynamic in vivo cell-cell communications and the resulting cell fate plasticity in diverse life science fields. Monitoring of cell-cell contact in the heart. (A) Whole-mount fluorescence image of a mouse embryo showing that cardiomyocytes and endothelial cells express the synNotch ligand (green) and the synNotch receptor (purple). (B to D) Whole-mount images of synNotch neonatal hearts shown in green (B), blue (C), and red (D) fluorescence channels. Present and past cell contact signaling are displayed by blue and red fluorescence, respectively.