In Vivo Tracking and 3D Mapping of Cell Death in Regeneration and Cancer Using Trypan Blue.

Tracking cell death in vivo can enable a better understanding of the biological mechanisms underlying tissue homeostasis and disease. Unfortunately, existing cell death labeling methods lack compatibility with in vivo applications or suffer from low sensitivity, poor tissue penetration, and limited temporal resolution. Here, we fluorescently labeled dead cells in vivo with Trypan Blue (TBlue) to detect single scattered dead cells or to generate whole-mount three-dimensional maps of large areas of necrotic tissue during organ regeneration. TBlue effectively marked different types of cell death, including necrosis induced by CCl4 intoxication in the liver, necrosis caused by ischemia-reperfusion in the skin, and apoptosis triggered by BAX overexpression in hepatocytes. Moreover, due to its short circulating lifespan in blood, TBlue labeling allowed in vivo "pulse and chase" tracking of two temporally spaced populations of dying hepatocytes in regenerating mouse livers. Additionally, upon treatment with cisplatin, TBlue labeled dead cancer cells in livers with cholangiocarcinoma and dead thymocytes due to chemotherapy-induced toxicity, showcasing its utility in assessing anticancer therapies in preclinical models. Thus, TBlue is a sensitive and selective cell death marker for in vivo applications, facilitating the understanding of the fundamental role of cell death in normal biological processes and its implications in disease.

Single-cell spatial multi-omics and deep learning dissect enhancer-driven gene regulatory networks in liver zonation.

In the mammalian liver, hepatocytes exhibit diverse metabolic and functional profiles based on their location within the liver lobule. However, it is unclear whether this spatial variation, called zonation, is governed by a well-defined gene regulatory code. Here, using a combination of single-cell multiomics, spatial omics, massively parallel reporter assays and deep learning, we mapped enhancer-gene regulatory networks across mouse liver cell types. We found that zonation affects gene expression and chromatin accessibility in hepatocytes, among other cell types. These states are driven by the repressors TCF7L1 and TBX3, alongside other core hepatocyte transcription factors, such as HNF4A, CEBPA, FOXA1 and ONECUT1. To examine the architecture of the enhancers driving these cell states, we trained a hierarchical deep learning model called DeepLiver. Our study provides a multimodal understanding of the regulatory code underlying hepatocyte identity and their zonation state that can be used to engineer enhancers with specific activity levels and zonation patterns.

A Novel Irreversible TEAD Inhibitor, SWTX-143, Blocks Hippo Pathway Transcriptional Output and Causes Tumor Regression in Preclinical Mesothelioma Models.

The Hippo pathway and its downstream effectors, the YAP and TAZ transcriptional coactivators, are deregulated in multiple different types of human cancer and are required for cancer cell phenotypes in vitro and in vivo, while largely dispensable for tissue homeostasis in adult mice. YAP/TAZ and their main partner transcription factors, the TEAD1-4 factors, are therefore promising anticancer targets. Because of frequent YAP/TAZ hyperactivation caused by mutations in the Hippo pathway components NF2 and LATS2, mesothelioma is one of the prime cancer types predicted to be responsive to YAP/TAZ-TEAD inhibitor treatment. Mesothelioma is a devastating disease for which currently no effective treatment options exist. Here, we describe a novel covalent YAP/TAZ-TEAD inhibitor, SWTX-143, that binds to the palmitoylation pocket of all four TEAD isoforms. SWTX-143 caused irreversible and specific inhibition of the transcriptional activity of YAP/TAZ-TEAD in Hippo-mutant tumor cell lines. More importantly, YAP/TAZ-TEAD inhibitor treatment caused strong mesothelioma regression in subcutaneous xenograft models with human cells and in an orthotopic mesothelioma mouse model. Finally, SWTX-143 also selectively impaired the growth of NF2-mutant kidney cancer cell lines, suggesting that the sensitivity of mesothelioma models to these YAP/TAZ-TEAD inhibitors can be extended to other tumor types with aberrations in Hippo signaling. In brief, we describe a novel and specific YAP/TAZ-TEAD inhibitor that has potential to treat multiple Hippo-mutant solid tumor types.

Regeneration Defects in Yap and Taz Mutant Mouse Livers Are Caused by Bile Duct Disruption and Cholestasis.

BACKGROUND AND AIMS: The Hippo pathway and its downstream effectors YAP and TAZ (YAP/TAZ) are heralded as important regulators of organ growth and regeneration. However, different studies provided contradictory conclusions about their role during regeneration of different organs, ranging from promoting proliferation to inhibiting it. Here we resolve the function of YAP/TAZ during regeneration of the liver, where Hippo's role in growth control has been studied most intensely. METHODS: We evaluated liver regeneration after carbon tetrachloride toxic liver injury in mice with conditional deletion of Yap/Taz in hepatocytes and/or biliary epithelial cells, and measured the behavior of different cell types during regeneration by histology, RNA sequencing, and flow cytometry. RESULTS: We found that YAP/TAZ were activated in hepatocytes in response to carbon tetrachloride toxic injury. However, their targeted deletion in adult hepatocytes did not noticeably impair liver regeneration. In contrast, Yap/Taz deletion in adult bile ducts caused severe defects and delay in liver regeneration. Mechanistically, we showed that Yap/Taz mutant bile ducts degenerated, causing cholestasis, which stalled the recruitment of phagocytic macrophages and the removal of cellular corpses from injury sites. Elevated bile acids activated pregnane X receptor, which was sufficient to recapitulate the phenotype observed in mutant mice. CONCLUSIONS: Our data show that YAP/TAZ are practically dispensable in hepatocytes for liver development and regeneration. Rather, YAP/TAZ play an indirect role in liver regeneration by preserving bile duct integrity and securing immune cell recruitment and function.

Peritumoral activation of the Hippo pathway effectors YAP and TAZ suppresses liver cancer in mice.

The Hippo signaling pathway and its two downstream effectors, the YAP and TAZ transcriptional coactivators, are drivers of tumor growth in experimental models. Studying mouse models, we show that YAP and TAZ can also exert a tumor-suppressive function. We found that normal hepatocytes surrounding liver tumors displayed activation of YAP and TAZ and that deletion of Yap and Taz in these peritumoral hepatocytes accelerated tumor growth. Conversely, experimental hyperactivation of YAP in peritumoral hepatocytes triggered regression of primary liver tumors and melanoma-derived liver metastases. Furthermore, whereas tumor cells growing in wild-type livers required YAP and TAZ for their survival, those surrounded by Yap- and Taz-deficient hepatocytes were not dependent on YAP and TAZ. Tumor cell survival thus depends on the relative activity of YAP and TAZ in tumor cells and their surrounding tissue, suggesting that YAP and TAZ act through a mechanism of cell competition to eliminate tumor cells.