Development of a hepatic cryoinjury model to study liver regeneration.

The liver is a remarkable organ that can regenerate in response to injury. Depending on the extent of injury, the liver can undergo compensatory hyperplasia or fibrosis. Despite decades of research, the molecular mechanisms underlying these processes are poorly understood. Here, we developed a new model to study liver regeneration based on cryoinjury. To visualise liver regeneration at cellular resolution, we adapted the CUBIC tissue-clearing approach. Hepatic cryoinjury induced a localised necrotic and apoptotic lesion characterised by inflammation and infiltration of innate immune cells. Following this initial phase, we observed fibrosis, which resolved as regeneration re-established homeostasis in 30 days. Importantly, this approach enables the comparison of healthy and injured parenchyma with an individual animal, providing unique advantages to previous models. In summary, the hepatic cryoinjury model provides a fast and reproducible method for studying the cellular and molecular pathways underpinning fibrosis and liver regeneration.

Development of a hepatic cryoinjury model to study liver regeneration.

The liver is a remarkable organ that can regenerate in response to injury. Depending on the extent of injury, the liver can undergo compensatory hyperplasia or fibrosis. Despite decades of research, the molecular mechanisms underlying these processes are poorly understood. Here, we developed a new model to study liver regeneration based on cryoinjury. To visualise liver regeneration at cellular resolution, we adapted the CUBIC tissue-clearing approach. Hepatic cryoinjury induced a localised necrotic and apoptotic lesion characterised by inflammation and infiltration of innate immune cells. Following this initial phase, we observed fibrosis, which resolved as regeneration re-established homeostasis in 30 days. Importantly, this approach enables the comparison of healthy and injured parenchyma with an individual animal, providing unique advantages to previous models. In summary, the hepatic cryoinjury model provides a fast and reproducible method for studying the cellular and molecular pathways underpinning fibrosis and liver regeneration.

SLAM-ITseq identifies that Nrf2 induces liver regeneration through the pentose phosphate pathway.

The liver exhibits a remarkable capacity to regenerate following injury. Despite this unique attribute, toxic injury is a leading cause of liver failure. The temporal processes by which the liver senses injury and initiates regeneration remain unclear. Here, we developed a transgenic zebrafish model wherein hepatocyte-specific expression of uracil phosphoribosyltransferase (UPRT) enabled the implementation of SLAM-ITseq to investigate the nascent transcriptome during initiation of liver injury and regeneration. Using this approach, we identified a rapid metabolic transition from the fed to the fasted state that was followed by induction of the nuclear erythroid 2-related factor (Nrf2) antioxidant program. We find that activation of Nrf2 in hepatocytes is required to induce the pentose phosphate pathway (PPP) and improve survival following liver injury. Mechanistically, we demonstrate that inhibition of the PPP disrupts nucleotide biosynthesis to prevent liver regeneration. Together, these studies provide fundamental insights into the mechanism by which early metabolic adaptation to injury facilitates tissue regeneration.

Vasculature is getting Hip(po): Hippo signaling in vascular development and disease.

The Hippo signaling pathway regulates developmental organ growth, regeneration, and cell fate decisions. Although the role of the Hippo pathway, and its transcriptional effectors YAP and TAZ, has been well documented in many cell types and species, only recently have the roles for this pathway come to light in vascular development and disease. Experiments in mice, zebrafish, and in vitro have uncovered roles for the Hippo pathway, YAP, and TAZ in vasculogenesis, angiogenesis, and lymphangiogenesis. In addition, the Hippo pathway has been implicated in vascular cancers and cardiovascular diseases, thus identifying it as a potential therapeutic target for the treatment of these conditions. However, despite recent advances, Hippo's role in the vasculature is still underappreciated compared with its role in epithelial tissues. In this review, we appraise our current understanding of the Hippo pathway in blood and lymphatic vessel development and highlight the current knowledge gaps and opportunities for further research.

The KEAP1-NRF2 pathway regulates TFEB/TFE3-dependent lysosomal biogenesis.

The maintenance of redox and metabolic homeostasis is integral to embryonic development. Nuclear factor erythroid 2-related factor 2 (NRF2) is a stress-induced transcription factor that plays a central role in the regulation of redox balance and cellular metabolism. Under homeostatic conditions, NRF2 is repressed by Kelch-like ECH-associated protein 1 (KEAP1). Here, we demonstrate that Keap1 deficiency induces Nrf2 activation and postdevelopmental lethality. Loss of viability is preceded by severe liver abnormalities characterized by an accumulation of lysosomes. Mechanistically, we demonstrate that loss of Keap1 promotes aberrant activation of transcription factor EB (TFEB)/transcription factor binding to IGHM Enhancer 3 (TFE3)-dependent lysosomal biogenesis. Importantly, we find that NRF2-dependent regulation of lysosomal biogenesis is cell autonomous and evolutionarily conserved. These studies identify a role for the KEAP1-NRF2 pathway in the regulation of lysosomal biogenesis and suggest that maintenance of lysosomal homeostasis is required during embryonic development.

YAP regulates an SGK1/mTORC1/SREBP-dependent lipogenic program to support proliferation and tissue growth.

The coordinated regulation of growth control and metabolic pathways is required to meet the energetic and biosynthetic demands associated with proliferation. Emerging evidence suggests that the Hippo pathway effector Yes-associated protein 1 (YAP) reprograms cellular metabolism to meet the anabolic demands of growth, although the mechanisms involved are poorly understood. Here, we demonstrate that YAP co-opts the sterol regulatory element-binding protein (SREBP)-dependent lipogenic program to facilitate proliferation and tissue growth. Mechanistically, YAP stimulates de novo lipogenesis via mechanistic target of rapamcyin (mTOR) complex 1 (mTORC1) signaling and subsequent activation of SREBP. Importantly, YAP-dependent regulation of serum- and glucocorticoid-regulated kinase 1 (SGK1) is required to activate mTORC1/SREBP and stimulate de novo lipogenesis. We also find that the SREBP target genes fatty acid synthase (FASN) and stearoyl-CoA desaturase (SCD) are conditionally required to support YAP-dependent proliferation and tissue growth. These studies reveal that de novo lipogenesis is a metabolic vulnerability that can be targeted to disrupt YAP-dependent proliferation and tissue growth.

Pharmacologic Reduction of Mitochondrial Iron Triggers a Noncanonical BAX/BAK-Dependent Cell Death.

Cancer cell metabolism is increasingly recognized as providing an exciting therapeutic opportunity. However, a drug that directly couples targeting of a metabolic dependency with the induction of cell death in cancer cells has largely remained elusive. Here we report that the drug-like small-molecule ironomycin reduces the mitochondrial iron load, resulting in the potent disruption of mitochondrial metabolism. Ironomycin promotes the recruitment and activation of BAX/BAK, but the resulting mitochondrial outer membrane permeabilization (MOMP) does not lead to potent activation of the apoptotic caspases, nor is the ensuing cell death prevented by inhibiting the previously established pathways of programmed cell death. Consistent with the fact that ironomycin and BH3 mimetics induce MOMP through independent nonredundant pathways, we find that ironomycin exhibits marked in vitro and in vivo synergy with venetoclax and overcomes venetoclax resistance in primary patient samples. SIGNIFICANCE: Ironomycin couples targeting of cellular metabolism with cell death by reducing mitochondrial iron, resulting in the alteration of mitochondrial metabolism and the activation of BAX/BAK. Ironomycin induces MOMP through a different mechanism to BH3 mimetics, and consequently combination therapy has marked synergy in cancers such as acute myeloid leukemia. This article is highlighted in the In This Issue feature, p. 587.

Identification of NQO2 As a Protein Target in Small Molecule Modulation of Hepatocellular Function.

The utility of in vitro human disease models is mainly dependent on the availability and functional maturity of tissue-specific cell types. We have previously screened for and identified small molecules that can enhance hepatocyte function in vitro. Here, we characterize the functional effects of one of the hits, FH1, on primary human hepatocytes in vitro, and also in vivo on primary hepatocytes in a zebrafish model. Furthermore, we conducted an analogue screen to establish the structure-activity relationship of FH1. We performed affinity-purification proteomics that identified NQO2 to be a potential binding target for this small molecule, revealing a possible link between inflammatory signaling and hepatocellular function in zebrafish and human hepatocyte model systems.

Imaging Mass Spectrometry Reveals Tumor Metabolic Heterogeneity.

Malignant tumors exhibit high degrees of genomic heterogeneity at the cellular level, leading to the view that subpopulations of tumor cells drive growth and treatment resistance. To examine the degree to which tumors also exhibit metabolic heterogeneity at the level of individual cells, we employed multi-isotope imaging mass spectrometry (MIMS) to quantify utilization of stable isotopes of glucose and glutamine along with a label for cell division. Mouse models of melanoma and malignant peripheral nerve sheath tumors (MPNSTs) exhibited striking heterogeneity of substrate utilization, evident in both proliferating and non-proliferating cells. We identified a correlation between metabolic heterogeneity, proliferation, and therapeutic resistance. Heterogeneity in metabolic substrate usage as revealed by incorporation of glucose and glutamine tracers is thus a marker for tumor proliferation. Collectively, our data demonstrate that MIMS provides a powerful tool with which to dissect metabolic functions of individual cells within the native tumor environment.

YAP Regulates Hematopoietic Stem Cell Formation in Response to the Biomechanical Forces of Blood Flow.

Hematopoietic stem and progenitor cells (HSPCs), first specified from hemogenic endothelium (HE) in the ventral dorsal aorta (VDA), support lifelong hematopoiesis. Their de novo production promises significant therapeutic value; however, current in vitro approaches cannot efficiently generate multipotent long-lived HSPCs. Presuming this reflects a lack of extrinsic cues normally impacting the VDA, we devised a human dorsal aorta-on-a-chip platform that identified Yes-activated protein (YAP) as a cyclic stretch-induced regulator of HSPC formation. In the zebrafish VDA, inducible Yap overexpression significantly increased runx1 expression in vivo and the number of CD41+ HSPCs downstream of HE specification. Endogenous Yap activation by lats1/2 knockdown or Rho-GTPase stimulation mimicked Yap overexpression and induced HSPCs in embryos lacking blood flow. Notably, in static human induced pluripotent stem cell (iPSC)-derived HE culture, compound-mediated YAP activation enhanced RUNX1 levels and hematopoietic colony-forming potential. Together, our findings reveal a potent impact of hemodynamic Rho-YAP mechanotransduction on HE fate, relevant to de novo human HSPC production.

Mutations in RABL3 alter KRAS prenylation and are associated with hereditary pancreatic cancer.

Pancreatic ductal adenocarcinoma is an aggressive cancer with limited treatment options1. Approximately 10% of cases exhibit familial predisposition, but causative genes are not known in most families2. We perform whole-genome sequence analysis in a family with multiple cases of pancreatic ductal adenocarcinoma and identify a germline truncating mutation in the member of the RAS oncogene family-like 3 (RABL3) gene. Heterozygous rabl3 mutant zebrafish show increased susceptibility to cancer formation. Transcriptomic and mass spectrometry approaches implicate RABL3 in RAS pathway regulation and identify an interaction with RAP1GDS1 (SmgGDS), a chaperone regulating prenylation of RAS GTPases3. Indeed, the truncated mutant RABL3 protein accelerates KRAS prenylation and requires RAS proteins to promote cell proliferation. Finally, evidence in patient cohorts with developmental disorders implicates germline RABL3 mutations in RASopathy syndromes. Our studies identify RABL3 mutations as a target for genetic testing in cancer families and uncover a mechanism for dysregulated RAS activity in development and cancer.

Yap1 promotes sprouting and proliferation of lymphatic progenitors downstream of Vegfc in the zebrafish trunk.

Lymphatic vascular development involves specification of lymphatic endothelial progenitors that subsequently undergo sprouting, proliferation and tissue growth to form a complex second vasculature. The Hippo pathway and effectors Yap and Taz control organ growth and regulate morphogenesis and cellular proliferation. Yap and Taz control angiogenesis but a role in lymphangiogenesis remains to be fully elucidated. Here we show that YAP displays dynamic changes in lymphatic progenitors and Yap1 is essential for lymphatic vascular development in zebrafish. Maternal and Zygotic (MZ) yap1 mutants show normal specification of lymphatic progenitors, abnormal cellular sprouting and reduced numbers of lymphatic progenitors emerging from the cardinal vein during lymphangiogenesis. Furthermore, Yap1 is indispensable for Vegfc-induced proliferation in a transgenic model of Vegfc overexpression. Paracrine Vegfc-signalling ultimately increases nuclear YAP in lymphatic progenitors to control lymphatic development. We thus identify a role for Yap in lymphangiogenesis, acting downstream of Vegfc to promote expansion of this vascular lineage.

Dissecting metabolism using zebrafish models of disease.

Zebrafish (Danio rerio) are becoming an increasingly powerful model organism to study the role of metabolism in disease. Since its inception, the zebrafish model has relied on unique attributes such as the transparency of embryos, high fecundity and conservation with higher vertebrates, to perform phenotype-driven chemical and genetic screens. In this review, we describe how zebrafish have been used to reveal novel mechanisms by which metabolism regulates embryonic development, obesity, fatty liver disease and cancer. In addition, we will highlight how new approaches in advanced microscopy, transcriptomics and metabolomics using zebrafish as a model system have yielded fundamental insights into the mechanistic underpinnings of disease.

Estrogen Activation of G-Protein-Coupled Estrogen Receptor 1 Regulates Phosphoinositide 3-Kinase and mTOR Signaling to Promote Liver Growth in Zebrafish and Proliferation of Human Hepatocytes.

BACKGROUND & AIMS: Patients with cirrhosis are at high risk for hepatocellular carcinoma (HCC) and often have increased serum levels of estrogen. It is not clear how estrogen promotes hepatic growth. We investigated the effects of estrogen on hepatocyte proliferation during zebrafish development, liver regeneration, and carcinogenesis. We also studied human hepatocytes and liver tissues. METHODS: Zebrafish were exposed to selective modifiers of estrogen signaling at larval and adult stages. Liver growth was assessed by gene expression, fluorescent imaging, and histologic analyses. We monitored liver regeneration after hepatocyte ablation and HCC development after administration of chemical carcinogens (dimethylbenzanthrazene). Proliferation of human hepatocytes was measured in a coculture system. We measured levels of G-protein-coupled estrogen receptor (GPER1) in HCC and nontumor liver tissues from 68 patients by immunohistochemistry. RESULTS: Exposure to 17ß-estradiol (E2) increased proliferation of hepatocytes and liver volume and mass in larval and adult zebrafish. Chemical genetic and epistasis experiments showed that GPER1 mediates the effects of E2 via the phosphoinositide 3-kinase-protein kinase B-mechanistic target of rapamycin pathway: gper1-knockout and mtor-knockout zebrafish did not increase liver growth in response to E2. HCC samples from patients had increased levels of GPER1 compared with nontumor tissue samples; estrogen promoted proliferation of human primary hepatocytes. Estrogen accelerated hepatocarcinogenesis specifically in male zebrafish. Chemical inhibition or genetic loss of GPER1 significantly reduced tumor development in the zebrafish. CONCLUSIONS: In an analysis of zebrafish and human liver cells and tissues, we found GPER1 to be a hepatic estrogen sensor that regulates liver growth during development, regeneration, and tumorigenesis. Inhibitors of GPER1 might be developed for liver cancer prevention or treatment. TRANSCRIPT PROFILING: The accession number in the Gene Expression Omnibus is GSE92544.

Yap regulates glucose utilization and sustains nucleotide synthesis to enable organ growth.

The Hippo pathway and its nuclear effector Yap regulate organ size and cancer formation. While many modulators of Hippo activity have been identified, little is known about the Yap target genes that mediate these growth effects. Here, we show that yap-/- mutant zebrafish exhibit defects in hepatic progenitor potential and liver growth due to impaired glucose transport and nucleotide biosynthesis. Transcriptomic and metabolomic analyses reveal that Yap regulates expression of glucose transporter glut1, causing decreased glucose uptake and use for nucleotide biosynthesis in yap-/- mutants, and impaired glucose tolerance in adults. Nucleotide supplementation improves Yap deficiency phenotypes, indicating functional importance of glucose-fueled nucleotide biosynthesis. Yap-regulated glut1 expression and glucose uptake are conserved in mammals, suggesting that stimulation of anabolic glucose metabolism is an evolutionarily conserved mechanism by which the Hippo pathway controls organ growth. Together, our results reveal a central role for Hippo signaling in glucose metabolic homeostasis.

Selenoprotein H is an essential regulator of redox homeostasis that cooperates with p53 in development and tumorigenesis.

Selenium, an essential micronutrient known for its cancer prevention properties, is incorporated into a class of selenocysteine-containing proteins (selenoproteins). Selenoprotein H (SepH) is a recently identified nucleolar oxidoreductase whose function is not well understood. Here we report that seph is an essential gene regulating organ development in zebrafish. Metabolite profiling by targeted LC-MS/MS demonstrated that SepH deficiency impairs redox balance by reducing the levels of ascorbate and methionine, while increasing methionine sulfoxide. Transcriptome analysis revealed that SepH deficiency induces an inflammatory response and activates the p53 pathway. Consequently, loss of seph renders larvae susceptible to oxidative stress and DNA damage. Finally, we demonstrate that seph interacts with p53 deficiency in adulthood to accelerate gastrointestinal tumor development. Overall, our findings establish that seph regulates redox homeostasis and suppresses DNA damage. We hypothesize that SepH deficiency may contribute to the increased cancer risk observed in cohorts with low selenium levels.

Yap reprograms glutamine metabolism to increase nucleotide biosynthesis and enable liver growth.

The Hippo pathway is an important regulator of organ size and tumorigenesis. It is unclear, however, how Hippo signalling provides the cellular building blocks required for rapid growth. Here, we demonstrate that transgenic zebrafish expressing an activated form of the Hippo pathway effector Yap1 (also known as YAP) develop enlarged livers and are prone to liver tumour formation. Transcriptomic and metabolomic profiling identify that Yap1 reprograms glutamine metabolism. Yap1 directly enhances glutamine synthetase (glul) expression and activity, elevating steady-state levels of glutamine and enhancing the relative isotopic enrichment of nitrogen during de novo purine and pyrimidine biosynthesis. Genetic or pharmacological inhibition of GLUL diminishes the isotopic enrichment of nitrogen into nucleotides, suppressing hepatomegaly and the growth of liver cancer cells. Consequently, Yap-driven liver growth is susceptible to nucleotide inhibition. Together, our findings demonstrate that Yap1 integrates the anabolic demands of tissue growth during development and tumorigenesis by reprogramming nitrogen metabolism to stimulate nucleotide biosynthesis.

Mitochondrial dysfunction remodels one-carbon metabolism in human cells.

Mitochondrial dysfunction is associated with a spectrum of human disorders, ranging from rare, inborn errors of metabolism to common, age-associated diseases such as neurodegeneration. How these lesions give rise to diverse pathology is not well understood, partly because their proximal consequences have not been well-studied in mammalian cells. Here we provide two lines of evidence that mitochondrial respiratory chain dysfunction leads to alterations in one-carbon metabolism pathways. First, using hypothesis-generating metabolic, proteomic, and transcriptional profiling, followed by confirmatory experiments, we report that mitochondrial DNA depletion leads to an ATF4-mediated increase in serine biosynthesis and transsulfuration. Second, we show that lesioning the respiratory chain impairs mitochondrial production of formate from serine, and that in some cells, respiratory chain inhibition leads to growth defects upon serine withdrawal that are rescuable with purine or formate supplementation. Our work underscores the connection between the respiratory chain and one-carbon metabolism with implications for understanding mitochondrial pathogenesis.

The lure of zebrafish in liver research: regulation of hepatic growth in development and regeneration.

The liver is an essential organ that plays a pivotal role in metabolism, digestion and nutrient storage. Major efforts have been made to develop zebrafish (Danio rerio) as a model system to study the pathways regulating hepatic growth during liver development and regeneration. Zebrafish offer unique advantages over other vertebrates including in vivo imaging at cellular resolution and the capacity for large-scale chemical and genetic screens. Here, we review the cellular and molecular mechanisms that regulate hepatic growth during liver development in zebrafish. We also highlight emerging evidence that developmental pathways are reactivated following liver injury to facilitate regeneration. Finally, we discuss how zebrafish have transformed drug discovery efforts and enabled the identification of drugs that stimulate hepatic growth and provide hepatoprotection in pre-clinical models of liver injury, with the ultimate goal of identifying novel therapeutic approaches to treat liver disease.