Metabolic and non-metabolic liver zonation is established non-synchronously and requires sinusoidal Wnts

Generation of in vivo-like multicellular liver organoids by mimicking developmental processes: A review

Liver is involved in metabolic reactions, ammonia detoxification, and immunity. Multicellular liver tissue cultures are more desirable for drug screening, disease modeling, and researching transplantation therapy, than hepatocytes monocultures. Hepatocytes monocultures are not stable for long. Further, hepatocyte-like cells induced from pluripotent stem cells and in vivo hepatocytes are functionally dissimilar. Organoid technology circumvents these issues by generating functional ex vivo liver tissue from intrinsic liver progenitor cells and extrinsic stem cells, including pluripotent stem cells. To function as in vivo liver tissue, the liver organoid cells must be arranged precisely in the 3-dimensional space, closely mimicking in vivo liver tissue. Moreover, for long term functioning, liver organoids must be appropriately vascularized and in contact with neighboring epithelial tissues (e.g., bile canaliculi and intrahepatic bile duct, or intrahepatic and extrahepatic bile ducts). Recent discoveries in liver developmental biology allows one to successfully induce liver component cells and generate organoids. Thus, here, in this review, we summarize the current state of knowledge on liver development with a focus on its application in generating different liver organoids. We also cover the future prospects in creating (functionally and structurally) in vivo-like liver organoids using the current knowledge on liver development.

The thrombopoietin mimetic JNJ-26366821 reduces the late injury and accelerates the onset of liver recovery after acetaminophen-induced liver injury in mice

Acetaminophen (APAP)-induced hepatotoxicity is comprised of an injury and recovery phase. While pharmacological interventions, such as N-acetylcysteine (NAC) and 4-methylpyrazole (4-MP), prevent injury there are no therapeutics that promote recovery. JNJ-26366821 (TPOm) is a novel thrombopoietin mimetic peptide with no sequence homology to endogenous thrombopoietin (TPO). Endogenous thrombopoietin is produced by hepatocytes and the TPO receptor is present on liver sinusoidal endothelial cells in addition to megakaryocytes and platelets, and we hypothesize that TPOm activity at the TPO receptor in the liver provides a beneficial effect following liver injury. Therefore, we evaluated the extent to which TPOm, NAC or 4-MP can provide a protective and regenerative effect in the liver when administered 2 h after an APAP overdose of 300 mg/kg in fasted male C57BL/6J mice. TPOm did not affect protein adducts, oxidant stress, DNA fragmentation and hepatic necrosis up to 12 h after APAP. In contrast, TPOm treatment was beneficial at 24 h, i.e., all injury parameters were reduced by 42-48%. Importantly, TPOm enhanced proliferation by 100% as indicated by PCNA-positive hepatocytes around the area of necrosis. When TPOm treatment was delayed by 6 h, there was no effect on the injury, but a proliferative effect was still evident. In contrast, 4MP and NAC treated at 2 h after APAP significantly attenuated all injury parameters at 24 h but failed to enhance hepatocyte proliferation. Thus, TPOm arrests the progression of liver injury by 24 h after APAP and accelerates the onset of the proliferative response which is essential for liver recovery.

Development of a human liver microphysiological coculture system for higher throughput chemical safety assessment

Chemicals in the systemic circulation can undergo hepatic xenobiotic metabolism, generate metabolites, and exhibit altered toxicity compared with their parent compounds. This article describes a 2-chamber liver-organ coculture model in a higher-throughput 96-well format for the determination of toxicity on target tissues in the presence of physiologically relevant human liver metabolism. This 2-chamber system is a hydrogel formed within each well consisting of a central well (target tissue) and an outer ring-shaped trough (human liver tissue). The target tissue chamber can be configured to accommodate a three-dimensional (3D) spheroid-shaped microtissue, or a 2-dimensional (2D) cell monolayer. Culture medium and compounds freely diffuse between the 2 chambers. Human-differentiated HepaRG liver cells are used to form the 3D human liver microtissues, which displayed robust protein expression of liver biomarkers (albumin, asialoglycoprotein receptor, Phase I cytochrome P450 [CYP3A4] enzyme, multidrug resistance-associated protein 2 transporter, and glycogen), and exhibited Phase I/II enzyme activities over the course of 17 days. Histological and ultrastructural analyses confirmed that the HepaRG microtissues presented a differentiated hepatocyte phenotype, including abundant mitochondria, endoplasmic reticulum, and bile canaliculi. Liver microtissue zonation characteristics could be easily modulated by maturation in different media supplements. Furthermore, our proof-of-concept study demonstrated the efficacy of this coculture model in evaluating testosterone-mediated androgen receptor responses in the presence of human liver metabolism. This liver-organ coculture system provides a practical, higher-throughput testing platform for metabolism-dependent bioactivity assessment of drugs/chemicals to better recapitulate the biological effects and potential toxicity of human exposures.

Perturbed liver gene zonation in a mouse model of non-alcoholic steatohepatitis

Liver zonation characterizes the separation of metabolic pathways along the lobules and is required for optimal hepatic function. Wnt signaling is a master regulator of spatial liver zonation. A perivenous-periportal Wnt activity gradient orchestrates metabolic zonation by activating gene expression in perivenous hepatocytes, while suppressing gene expression in their periportal counterparts. However, the understanding as to the liver gene zonation and zonation regulators in diseases is limited. Non-alcoholic steatohepatitis (NASH) is a chronic liver disease characterized by fat accumulation, inflammation, and fibrosis. Here, we investigated the perturbation of liver gene zonation in a mouse NASH model by combining spatial transcriptomics, bulk RNAseq and in situ hybridization. Wnt-target genes represented a major subset of genes showing altered spatial expression in the NASH liver. The altered Wnt-target gene expression levels and zonation spatial patterns were in line with the up regulation of Wnt regulators and the augmentation of Wnt signaling. Particularly, we found that the Wnt activator Rspo3 expression was restricted to the perivenous zone in control liver but expanded to the periportal zone in NASH liver. AAV8-mediated RSPO3 overexpression in controls resulted in zonation changes, and further amplified the disturbed zonation of Wnt-target genes in NASH, similarly Rspo3 knockdown in Rspo3+/- mice resulted in zonation changes of Wnt-target genes in both chow and HFD mouse. Interestingly, there were no impacts on steatosis, inflammation, or fibrosis NASH pathology from RSPO3 overexpression nor Rspo3 knockdown. In summary, our study demonstrated the alteration of Wnt signaling in a mouse NASH model, leading to perturbed liver zonation.

GSK3β/NF-κB -dependent transcriptional regulation of homeostatic hepatocyte Tnf production

Maintenance of hepatocyte homeostasis plays an important role in mediating the pathogenesis of many diseases. A growing body of literature has established a critical role played by tumor necrosis factor-α (TNFα) in maintaining hepatocyte homeostasis; however, the transcriptional mechanisms underlying constitutive Tnf expression are unknown. Whole liver fractions and primary hepatocytes from adult control C57BL/6 mice and the murine hepatocyte cell line AML12 were assessed for constitutive Tnf expression. Impacts of glycogen synthase kinase-3 β (GSK3β) and nuclear factor κB (NF-κB) inhibition on constitutive Tnf expression were assessed in AML12 cells. Finally, AML12 cell proliferation following GSK3β and NF-κB inhibition was evaluated. Constitutive Tnf gene expression is present in whole liver, primary hepatocytes, and cultured AML12 hepatocytes. Cytokine-induced Tnf gene expression is regulated by NF-κB activation. Pharmacological inhibition of GSK3β resulted in a time- and dose-dependent inhibition of Tnf gene expression. GSK3β inhibition decreased nuclear levels of the NF-κB subunits p65 and p50. We determined that NF-κB transcription factor subunit p65 binds to consensus sequence elements present in the murine TNFα promoter and inhibition of GSK3β decreases binding and subsequent Tnf expression. Finally, AML12 cell growth was significantly reduced following GSK3β and NF-κB inhibition. These results demonstrate that GSK3β and NF-κB are essential for mediating Tnf expression and constitutive hepatocyte cell growth. These findings add to a growing body of literature on TNFα mediated hepatocyte homeostasis and identify novel molecular mechanisms involved in mediating response to various disease states in the liver.NEW & NOTEWORTHY Maintenance of hepatocyte homeostasis plays an important role in controlling the pathogenesis of many diseases. Our findings add to a growing body of literature on tumor necrosis factor-α (TNFα)-mediated hepatocyte homeostasis and identify novel molecular mechanisms involved in regulating this response.

Hepatic nutrient and hormone signaling to mTORC1 instructs the postnatal metabolic zonation of the liver

The metabolic functions of the liver are spatially organized in a phenomenon called zonation, linked to the differential exposure of portal and central hepatocytes to nutrient-rich blood. The mTORC1 signaling pathway controls cellular metabolism in response to nutrients and insulin fluctuations. Here we show that simultaneous genetic activation of nutrient and hormone signaling to mTORC1 in hepatocytes results in impaired establishment of postnatal metabolic and zonal identity of hepatocytes. Mutant hepatocytes fail to upregulate postnatally the expression of Frizzled receptors 1 and 8, and show reduced Wnt/β-catenin activation. This defect, alongside diminished paracrine Wnt2 ligand expression by endothelial cells, underlies impaired postnatal maturation. Impaired zonation is recapitulated in a model of constant supply of nutrients by parenteral nutrition to piglets. Our work shows the role of hepatocyte sensing of fluctuations in nutrients and hormones for triggering a latent metabolic zonation program.

Endothelial Slc35a1 Deficiency Causes Loss of LSEC Identity and Exacerbates Neonatal Lipid Deposition in the Liver in Mice

BACKGROUND & AIMS

The functional maturation of the liver largely occurs after birth. In the early stages of life, the liver of a newborn encounters enormous high-fat metabolic stress caused by the consumption of breast milk. It is unclear how the maturing liver adapts to high lipid metabolism. Liver sinusoidal endothelial cells (LSECs) play a fundamental role in establishing liver vasculature and are decorated with many glycoproteins on their surface. The Slc35a1 gene encodes a cytidine-5'-monophosphate (CMP)-sialic acid transporter responsible for transporting CMP-sialic acids between the cytoplasm and the Golgi apparatus for protein sialylation. This study aimed to determine whether endothelial sialylation plays a role in hepatic vasculogenesis and functional maturation.

METHODS

Endothelial-specific Slc35a1 knockout mice were generated. Liver tissues were collected for histologic analysis, lipidomic profiling, RNA sequencing, confocal immunofluorescence, and immunoblot analyses.

RESULTS

Endothelial Slc35a1-deficient mice exhibited excessive neonatal hepatic lipid deposition, severe liver damage, and high mortality. Endothelial deletion of Slc35a1 led to sinusoidal capillarization and disrupted hepatic zonation. Mechanistically, vascular endothelial growth factor receptor 2 (VEGFR2) in LSECs was desialylated and VEGFR2 signaling was enhanced in Slc35a1-deficient mice. Inhibition of VEGFR2 signaling by SU5416 alleviated lipid deposition and restored hepatic vasculature in Slc35a1-deficient mice.

CONCLUSIONS

Our findings suggest that sialylation of LSECs is critical for maintaining hepatic vascular development and lipid homeostasis. Targeting VEGFR2 signaling may be a new strategy to prevent liver disorders associated with abnormal vasculature and lipid deposition.

Region-specific cellular and molecular basis of liver regeneration after acute pericentral injury

Liver injuries often occur in a zonated manner. However, detailed regenerative responses to such zonal injuries at cellular and molecular levels remain largely elusive. By using a fate-mapping strain, Cyp2e1-DreER, to elucidate liver regeneration after acute pericentral injury, we found that pericentral regeneration is primarily compensated by the expansion of remaining pericentral hepatocytes, and secondarily by expansion of periportal hepatocytes. Employing single-cell RNA sequencing, spatial transcriptomics, immunostaining, and in vivo functional assays, we demonstrated that the upregulated expression of the mTOR/4E-BP1 axis and lactate dehydrogenase A in hepatocytes contributes to pericentral regeneration, while activation of transforming growth factor β (TGF-β1) signaling in the damaged area mediates fibrotic responses and inhibits hepatocyte proliferation. Inhibiting the pericentral accumulation of monocytes and monocyte-derived macrophages through an Arg-Gly-Asp (RGD) peptide-based strategy attenuates these cell-derived TGF-β1 signalings, thus improving pericentral regeneration. Our study provides integrated and high-resolution spatiotemporal insights into the cellular and molecular basis of pericentral regeneration.

Compartmentalization, cooperation, and communication: The 3Cs of Hepatocyte zonation

The unique architecture of the liver allows for spatial compartmentalization of its functions, also known as liver zonation. In contrast to organelles and cells, this compartment is devoid of a surrounding membrane, rendering traditional biochemical tools ineffective for studying liver zonation. Recent advancements in tissue imaging and single-cell technologies have provided new insights into the complexity of tissue organization, rich cellular composition, and the gradients that shape zonation. Hepatocyte gene expression profiles and metabolic programs differ based on their location. Non-parenchymal cells further support hepatocytes from different zones through local secretion of factors that instruct hepatocyte activities. Collectively, these elements form a cohesive and dynamic network of cell-cell interactions that vary across space, time, and disease states. This review will examine the cell biology of hepatocytes in vivo, presenting the latest discoveries and emerging principles that govern tissue-level and sub-cellular compartmentalization.

Wnt-β-catenin in hepatobiliary homeostasis, injury, and repair

Wnt-β-catenin signaling has emerged as an important regulatory pathway in the liver, playing key roles in zonation and mediating contextual hepatobiliary repair after injuries. In this review, we will address the major advances in understanding the role of Wnt signaling in hepatic zonation, regeneration, and cholestasis-induced injury. We will also touch on some important unanswered questions and discuss the relevance of modulating the pathway to provide therapies for complex liver pathologies that remain a continued unmet clinical need.

Nuclear Acly protects the liver from ischemia-reperfusion injury

BACKGROUND AND AIMS

Hepatic ischemia-reperfusion (IR) injury is the most common complication that occurs in liver surgery and hemorrhagic shock. ATP citrate lyase (Acly) plays a pivotal role in chromatin modification via generating acetyl-CoA for histone acetylation to influence biological processes. We aim to examine the roles of Acly, which is highly expressed in hepatocytes, in liver IR injury.

APPROACH AND RESULTS

The functions of Acly in hepatic IR injury were examined in the mouse model with a hepatocyte-specific knockout of Acly . The Acly target genes were analyzed by CUT&RUN assay and RNA sequencing. The relationship between the susceptibility of the steatotic liver to IR and Acly was determined by the gain of function studies in mice. Hepatic deficiency of Acly exacerbated liver IR injury. IR induced Acly nuclear translocation in hepatocytes, which spatially fueled nuclear acetyl-CoA. This alteration was associated with enhanced acetylation of H3K9 and subsequent activation of the Foxa2 signaling pathway. Nuclear localization of Acly enabled Foxa2-mediated protective effects after hypoxia-reperfusion in cultured hepatocytes, while cytosolic Acly demonstrated no effect. The presence of steatosis disrupted Acly nuclear translocation. In the steatotic liver, restoration of Acly nuclear localization through overexpression of Rspondin-1 or Rspondin-3 ameliorated the IR-induced injury.

CONCLUSIONS

Our results indicate that Acly regulates histone modification by means of nuclear AcCoA production in hepatic IR. Disruption of Acly nuclear translocation increases the vulnerability of the steatotic liver to IR. Nuclear Acly thus may serve as a potential therapeutic target for future interventions in hepatic IR injury, particularly in the context of steatosis.

Depletion of IQ motif-containing GTPase activating protein 2 (IQGAP2) reduces hepatic glycogen and impairs insulin signaling

The liver is critical in maintaining metabolic homeostasis, regulating both anabolic and catabolic processes. Scaffold protein IQ motif-containing GTPase activating protein 2 (IQGAP2) is highly expressed in the liver and implicated in fatty acid uptake. However, its role in coordinating either fed or fasted responses is not well understood. Here we report that IQGAP2 is widely expressed in the liver that is pronounced in the pericentral region. Although control and IQGAP2 knockout mouse model showed comparable hepatic gene expression in the fasted state, we found significant defects in fed state responses. Glycogen levels were reduced in the periportal region when IQGAP2 was deleted. Consistently, we observed a decrease in phosphorylated glycogen synthase kinase 3α and total glycogen synthase protein in the fed IQGAP2 knockout mice which suggest inadequate glycogen synthesis. Moreover, immunoprecipitation of IQGAP2 revealed its interaction with GSK3 and GYS. Furthermore, our study demonstrated that knocking down IQGAP2 in vitro significantly decreased the phosphorylation of AKT and forkhead box O3 proteins downstream of insulin signaling. These findings suggest that IQGAP2 contributes to liver fed state metabolism by interacting with glycogen synthesis regulators and affecting the phosphorylation of insulin pathway components. Our results suggest that IQGAP2 plays a role in regulating fed state metabolism.

Extracellular matrix modulates the spatial hepatic features in hepatocyte-like cells derived from human embryonic stem cells

BACKGROUND

Human pluripotent stem cell (hPSC)-derived hepatocyte-like cells (HLCs) can provide a valuable in vitro model for disease modelling and drug development. However, generating HLCs with characteristics comparable to hepatocytes in vivo is challenging. Extracellular matrix (ECM) plays an important role in supporting liver development and hepatocyte functions, but their impact on hepatocyte differentiation and maturation during hPSC differentiation remains unclear. Here, we investigate the effects of two ECM components-Matrigel and type I collagen on hepatic differentiation of human embryonic stem cells (hESCs).

METHODS

hESC-derived HLCs were generated through multistage differentiation in two-dimensional (2D) and three-dimensional (3D) cultures, incorporating either type I collagen or Matrigel during hepatic specification and maturation. The resulting HLCs was characterized for their gene expression and functionality using various molecular and cellular techniques.

RESULTS

Our results showed that HLCs cultured with collagen exhibited a significant increase in albumin and alpha-1 anti-trypsin expression with reduced AFP compared to HLCs cultured with Matrigel. They also secreted more urea than Matrigel cultures. However, these HLCs exhibited lower CYP3A4 activity and glycogen storage than those cultured with Matrigel. These functional differences in HLCs between collagen and Matrigel cultures closely resembled the hepatocytes of periportal and pericentral zones, respectively.

CONCLUSION

Our study demonstrates that Matrigel and collagen have differential effects on the differentiation and functionality of HLCs, which resemble, to an extent, hepatic zonation in the liver lobules. Our finding has an important impact on the generation of hPSC-HLCs for biomedical and medical applications.

Humanized mouse liver reveals endothelial control of essential hepatic metabolic functions

Hepatocytes, the major metabolic hub of the body, execute functions that are human-specific, altered in human disease, and currently thought to be regulated through endocrine and cell-autonomous mechanisms. Here, we show that key metabolic functions of human hepatocytes are controlled by non-parenchymal cells (NPCs) in their microenvironment. We developed mice bearing human hepatic tissue composed of human hepatocytes and NPCs, including human immune, endothelial, and stellate cells. Humanized livers reproduce human liver architecture, perform vital human-specific metabolic/homeostatic processes, and model human pathologies, including fibrosis and non-alcoholic fatty liver disease (NAFLD). Leveraging species mismatch and lipidomics, we demonstrate that human NPCs control metabolic functions of human hepatocytes in a paracrine manner. Mechanistically, we uncover a species-specific interaction whereby WNT2 secreted by sinusoidal endothelial cells controls cholesterol uptake and bile acid conjugation in hepatocytes through receptor FZD5. These results reveal the essential microenvironmental regulation of hepatic metabolism and its human-specific aspects.

Single-cell sequencing reveals the existence of fetal vascular endothelial stem cell-like cells in mouse liver

BACKGROUND

A resident vascular endothelial stem cell (VESC) population expressing CD157 and CD200 has been identified recently in the adult mouse. However, the origin of this population and how it develops has not been characterized, nor has it been determined whether VESC-like cells are present during the perinatal period. Here, we investigated the presence of perinatal VESC-like cells and their relationship with the adult VESC-like cell population.

METHODS

We applied single-cell RNA sequencing of endothelial cells (ECs) from embryonic day (E) 14, E18, postnatal day (P) 7, P14, and week (W) 8 liver and investigated transcriptomic changes during liver EC development. We performed flow cytometry, immunofluorescence, colony formation assays, and transplantation assays to validate the presence of and to assess the function of CD157+ and CD200+ ECs in the perinatal period.

RESULTS

We identified CD200- expressing VESC-like cells in the perinatal period. These cells formed colonies in vitro and had high proliferative ability. The RNA velocity tool and transplantation assay results indicated that the projected fate of this population was toward adult VESC-like cells expressing CD157 and CD200 1 week after birth.

CONCLUSION

Our study provides a comprehensive atlas of liver EC development and documents VESC-like cell lineage commitment at single-cell resolution.

Comprehensive Strategy for Identifying Extracellular Vesicle Surface Proteins as Biomarkers for Non-Alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is a liver disorder that has become a global health concern due to its increasing prevalence. There is a need for reliable biomarkers to aid in the diagnosis and prognosis of NAFLD. Extracellular vesicles (EVs) are promising candidates in biomarker discovery, as they carry proteins that reflect the pathophysiological state of the liver. In this review, we developed a list of EV proteins that could be used as diagnostic biomarkers for NAFLD. We employed a multi-step strategy that involved reviewing and comparing various sources of information. Firstly, we reviewed papers that have studied EVs proteins as biomarkers in NAFLD and papers that have studied circulating proteins as biomarkers in NAFLD. To further identify potential candidates, we utilized the EV database Vesiclepedia.org to qualify each protein. Finally, we consulted the Human Protein Atlas to search for candidates' localization, focusing on membrane proteins. By integrating these sources of information, we developed a comprehensive list of potential EVs membrane protein biomarkers that could aid in diagnosing and monitoring NAFLD. In conclusion, our multi-step strategy for identifying EV-based protein biomarkers for NAFLD provides a comprehensive approach that can also be applied to other diseases. The protein candidates identified through this approach could have significant implications for the development of non-invasive diagnostic tests for NAFLD and improve the management and treatment of this prevalent liver disorder.

Distinct hepatocyte identities in liver homeostasis and regeneration

The process of metabolic liver zonation is spontaneously established by assigning distributed tasks to hepatocytes along the porto-central blood flow. Hepatocytes fulfil critical metabolic functions, while also maintaining hepatocyte mass by replication when needed. Recent technological advances have enabled us to fine-tune our understanding of hepatocyte identity during homeostasis and regeneration. Subsets of hepatocytes have been identified to be more regenerative and some have even been proposed to function like stem cells, challenging the long-standing view that all hepatocytes are similarly capable of regeneration. The latest data show that hepatocyte renewal during homeostasis and regeneration after liver injury is not limited to rare hepatocytes; however, hepatocytes are not exactly the same. Herein, we review the known differences that give individual hepatocytes distinct identities, recent findings demonstrating how these distinct identities correspond to differences in hepatocyte regenerative capacity, and how the plasticity of hepatocyte identity allows for division of labour among hepatocytes. We further discuss how these distinct hepatocyte identities may play a role during liver disease.

Wnt Signaling Stimulates Cooperation between GREB1 and HNF4α to Promote Proliferation in Hepatocellular Carcinoma

Wnt signaling is known to maintain two cell states, hepatocyte differentiation and proliferation, in hepatocellular carcinoma (HCC). On the other hand, activation of Wnt signaling in colon cancer promotes uncontrollable stereotypic proliferation, whereas cells remain undifferentiated. To elucidate the unique mode of Wnt signaling in HCC, we comprehensively investigated HCC-specific Wnt pathway target genes and identified GREB1. Wnt signaling induced expression of GREB1 coupled with HNF4α and FOXA2, master transcription factors that maintain hepatic differentiation. Moreover, GREB1 was enriched at the regulatory region of atypical HNF4α target genes, including progrowth genes, thereby stimulating HCC proliferation. Therefore, GREB1 acts as a unique mediator of versatile Wnt signaling in HCC progression, bridging the roles of the Wnt pathway in differentiation and proliferation.

SIGNIFICANCE

GREB1 is a liver cancer-specific Wnt signaling target gene that induces an oncogenic shift of HNF4α, a putative tumor suppressor, and may represent a therapeutic target in Wnt-activated hepatocellular carcinoma.

Solidified saturated fats coating subunit vaccines greatly extended vaccine booster release and contributed to a Th1/Th2 mixed immune response in mice

Delayed release of vaccine coupled with a soluble vaccine acts as a primer and a booster with only a single administration, which would be very beneficial to livestock producers. We developed a subdermal pellet consisting of solid-phase pure stearic acid (SA) or palmitic acid (PA) that was used to encapsulate a small volume liquid vaccine consisting of fluorescently labeled *Ovalbumin (Cy5-*OVA) formulated with Emulsigen-D +/- Poly I:C (EMP) adjuvants. Mice were also immunized via the subcutaneous route with Cy5-*OVA-EMP (soluble liquid). The vaccine leached out of the pellet with very little dissolution of the fat itself resulting in the sustained subdermal delivery of antigens and adjuvants. Cy5-*OVA was still visible 60 days post administration in mice immunized with stearic acid-coated or palmitic acid-coated pellets. In these mice, persistently high IgG1 and IgG2a antibody titres were detected as well as significant IFNγ production at least 60 days post-injection. These responses were significantly higher than those observed after a single subcutaneous injection of the vaccine. A repeat trial with the pellets alone +/- the soluble vaccine showed comparable immune responses after surgical implantation of the pellet, suggesting that pellet alone may be sufficient. The PA-coated vaccines led to dermal inflammation in the mice that would limit usefulness of this vehicle, but this was largely absent when SA was used to coat the pellets. These data suggest that the SA-coated adjuvanted vaccine prolonged the release of the vaccine and triggered a comparable immune response to the mice that received the two liquid injections, and a single pellet vaccine should be tested as a novel immunization method for livestock.

Characterisation of hepatic lipid signature distributed across the liver zonation using mass spectrometry imaging

Background & Aims

Lipid metabolism plays an important role in liver pathophysiology. The liver lobule asymmetrically distributes oxygen and nutrition, resulting in heterogeneous metabolic functions. Periportal and pericentral hepatocytes have different metabolic functions, which lead to generating liver zonation. We developed spatial metabolic imaging using desorption electrospray ionisation mass spectrometry to investigate lipid distribution across liver zonation with high reproducibility and accuracy.

Methods

Fresh frozen livers from healthy mice with control diet were analysed using desorption electrospray ionisation mass spectrometry imaging. Imaging was performed at 50 μm × 50 μm pixel size. Regions of interest (ROIs) were manually created by co-registering with histological data to determine the spatial hepatic lipids across liver zonation. The ROIs were confirmed by double immunofluorescence. The mass list of specific ROIs was automatically created, and univariate and multivariate statistical analysis were performed to identify statistically significant lipids across liver zonation.

Results

A wide range of lipid species was identified, including fatty acids, phospholipids, triacylglycerols, diacylglycerols, ceramides, and sphingolipids. We characterised hepatic lipid signatures in three different liver zones (periportal zone, midzone, and pericentral zone) and validated the reproducibility of our method for measuring a wide range of lipids. Fatty acids were predominantly detected in the periportal region, whereas phospholipids were distributed in both the periportal and pericentral zones. Interestingly, phosphatidylinositols, PI(36:2), PI(36:3), PI(36:4), PI(38:5), and PI(40:6) were located predominantly in the midzone (zone 2). Triacylglycerols and diacylglycerols were detected mainly in the pericentral region. De novo triacylglycerol biosynthesis appeared to be the most influenced pathway across the three zones.

Conclusions

The ability to accurately assess zone-specific hepatic lipid distribution in the liver could lead to a better understanding of lipid metabolism during the progression of liver disease.

Impact and Implications

Zone-specific hepatic lipid metabolism could play an important role in lipid homoeostasis during disease progression. Herein, we defined the zone-specific references of hepatic lipid species in the three liver zones using molecular imaging. The de novo triacylglycerol biosynthesis was highlighted as the most influenced pathway across the three zones.

Temporal Dynamics of Metabolic Acquisition in Grafted Engineered Human Liver Tissue

Liver disease affects millions globally, and end-stage liver failure is only cured by organ transplant. Unfortunately, there is a growing shortage of donor organs as well as inequitable access to transplants across populations. Engineered liver tissue grafts that supplement or replace native organ function can address this challenge. While engineered liver tissues have been successfully engrafted previously, the extent to which these tissues express human liver metabolic genes and proteins remains unknown. Here, it is built engineered human liver tissues and characterized their engraftment, expansion, and metabolic phenotype at sequential stages post-implantation by RNA sequencing, histology, and host serology. Expression of metabolic genes is observed at weeks 1-2, followed by the cellular organization into hepatic cords by weeks 4-9.5. Furthermore, grafted engineered tissues exhibited progressive spatially restricted expression of critical functional proteins known to be zonated in the native human liver. This is the first report of engineered human liver tissue zonation after implantation in vivo, which can have important translational implications for this field.

Hepatocyte β-catenin loss is compensated by Insulin-mTORC1 activation to promote liver regeneration

BACKGROUND AND AIMS

Liver regeneration (LR) following partial hepatectomy (PH) occurs via activation of various signaling pathways. Disruption of a single pathway can be compensated by activation of another pathway to continue LR. The Wnt-β-catenin pathway is activated early during LR and conditional hepatocyte loss of β-catenin delays LR. Here, we study mechanism of LR in the absence of hepatocyte-β-catenin.

APPROACH AND RESULTS

Eight-week-old hepatocyte-specific Ctnnb1 knockout mice (β-catenin ΔHC ) were subjected to PH. These animals exhibited decreased hepatocyte proliferation at 40-120 h and decreased cumulative 14-day BrdU labeling of <40%, but all mice survived, suggesting compensation. Insulin-mediated mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) activation was uniquely identified in the β-catenin ΔHC mice at 72-96 h after PH. Deletion of hepatocyte regulatory-associated protein of mTOR (Raptor), a critical mTORC1 partner, in the β-catenin ΔHC mice led to progressive hepatic injury and mortality by 30 dys. PH on early stage nonmorbid Raptor ΔHC -β-catenin ΔHC mice led to lethality by 12 h. Raptor ΔHC mice showed progressive hepatic injury and spontaneous LR with β-catenin activation but died by 40 days. PH on early stage nonmorbid Raptor ΔHC mice was lethal by 48 h. Temporal inhibition of insulin receptor and mTORC1 in β-catenin ΔHC or controls after PH was achieved by administration of linsitinib at 48 h or rapamycin at 60 h post-PH and completely prevented LR leading to lethality by 12-14 days.

CONCLUSIONS

Insulin-mTORC1 activation compensates for β-catenin loss to enable LR after PH. mTORC1 signaling in hepatocytes itself is critical to both homeostasis and LR and is only partially compensated by β-catenin activation. Dual inhibition of β-catenin and mTOR may have notable untoward hepatotoxic side effects.

Age-related liver endothelial zonation triggers steatohepatitis by inactivating pericentral endothelium-derived C-kit

Aging leads to systemic metabolic disorders, including steatosis. Here we show that liver sinusoidal endothelial cell (LSEC) senescence accelerates liver sinusoid capillarization and promotes steatosis by reprogramming liver endothelial zonation and inactivating pericentral endothelium-derived C-kit, which is a type III receptor tyrosine kinase. Specifically, inhibition of endothelial C-kit triggers cellular senescence, perturbing LSEC homeostasis in male mice. During diet-induced nonalcoholic steatohepatitis (NASH) development, Kit deletion worsens hepatic steatosis and exacerbates NASH-associated fibrosis and inflammation. Mechanistically, C-kit transcriptionally inhibits chemokine (C-X-C motif) receptor (CXCR)4 via CCAAT enhancer-binding protein α (CEBPA). Blocking CXCR4 signaling abolishes LSEC-macrophage-neutrophil cross-talk and leads to the recovery of C-kit-deficient mice with NASH. Of therapeutic relevance, infusing C-kit-expressing LSECs into aged mice or mice with diet-induced NASH counteracts age-associated senescence and steatosis and improves the symptoms of diet-induced NASH by restoring metabolic homeostasis of the pericentral liver endothelium. Our work provides an alternative approach that could be useful for treating aging- and diet-induced NASH.

Loss of Hepatic Leucine-Rich Repeat-Containing G-Protein Coupled Receptors 4 and 5 Promotes Nonalcoholic Fatty Liver Disease

The roof plate-specific spondin-leucine-rich repeat-containing G-protein coupled receptor 4/5 (LGR4/5)-zinc and ring finger 3 (ZNRF3)/ring finger protein 43 (RNF43) module is a master regulator of hepatic Wnt/β-catenin signaling and metabolic zonation. However, its impact on nonalcoholic fatty liver disease (NAFLD) remains unclear. The current study investigated whether hepatic epithelial cell-specific loss of the Wnt/β-catenin modulator Lgr4/5 promoted NAFLD. The 3- and 6-month-old mice with hepatic epithelial cell-specific deletion of both receptors Lgr4/5 (Lgr4/5dLKO) were compared with control mice fed with normal diet (ND) or high-fat diet (HFD). Six-month-old HFD-fed Lgr4/5dLKO mice developed hepatic steatosis and fibrosis but the control mice did not. Serum cholesterol-high-density lipoprotein and total cholesterol levels in 3- and 6-month-old HFD-fed Lgr4/5dLKO mice were decreased compared with those in control mice. An ex vivo primary hepatocyte culture assay and a comprehensive bile acid (BA) characterization in liver, plasma, bile, and feces demonstrated that ND-fed Lgr4/5dLKO mice had impaired BA secretion, predisposing them to develop cholestatic characteristics. Lipidome and RNA-sequencing analyses demonstrated severe alterations in several lipid species and pathways controlling lipid metabolism in the livers of Lgr4/5dLKO mice. In conclusion, loss of hepatic Wnt/β-catenin activity by Lgr4/5 deletion led to loss of BA secretion, cholestatic features, altered lipid homeostasis, and deregulation of lipoprotein pathways. Both BA and intrinsic lipid alterations contributed to the onset of NAFLD.

Spatiotemporal Metabolic Liver Zonation and Consequences on Pathophysiology

Hepatocytes are the main workers in the hepatic factory, managing metabolism of nutrients and xenobiotics, production and recycling of proteins, and glucose and lipid homeostasis. Division of labor between hepatocytes is critical to coordinate complex complementary or opposing multistep processes, similar to distributed tasks at an assembly line. This so-called metabolic zonation has both spatial and temporal components. Spatial distribution of metabolic function in hepatocytes of different lobular zones is necessary to perform complex sequential multistep metabolic processes and to assign metabolic tasks to the right environment. Moreover, temporal control of metabolic processes is critical to align required metabolic processes to the feeding and fasting cycles. Disruption of this complex spatiotemporal hepatic organization impairs key metabolic processes with both local and systemic consequences. Many metabolic diseases, such as nonalcoholic steatohepatitis and diabetes, are associated with impaired metabolic liver zonation. Recent technological advances shed new light on the spatiotemporal gene expression networks controlling liver function and how their deregulation may be involved in a large variety of diseases. We summarize the current knowledge about spatiotemporal metabolic liver zonation and consequences on liver pathobiology.

Wnt signaling in liver regeneration, disease, and cancer

The liver exhibits the highest recovery rate from acute injuries. However, in chronic liver disease, the long-term loss of hepatocytes often leads to adverse consequences such as fibrosis, cirrhosis, and liver cancer. The Wnt signaling plays a pivotal role in both liver regeneration and tumorigenesis. Therefore, manipulating the Wnt signaling has become an attractive approach to treating liver disease, including cancer. Nonetheless, given the crucial roles of Wnt signaling in physiological processes, blocking Wnt signaling can also cause several adverse effects. Recent studies have identified cancer-specific regulators of Wnt signaling, which would overcome the limitation of Wnt signaling target approaches. In this review, we discussed the role of Wnt signaling in liver regeneration, precancerous lesion, and liver cancer. Furthermore, we summarized the basic and clinical approaches of Wnt signaling blockade and proposed the therapeutic prospects of cancer-specific Wnt signaling blockade for liver cancer treatment.

Mapping transcriptional heterogeneity and metabolic networks in fatty livers at single-cell resolution

Non-alcoholic fatty liver disease is a heterogeneous disease with unclear underlying molecular mechanisms. Here, we perform single-cell RNA sequencing of hepatocytes and hepatic non-parenchymal cells to map the lipid signatures in mice with non-alcoholic fatty liver disease (NAFLD). We uncover previously unidentified clusters of hepatocytes characterized by either high or low srebp1 expression. Surprisingly, the canonical lipid synthesis driver Srebp1 is not predictive of hepatic lipid accumulation, suggestive of other drivers of lipid metabolism. By combining transcriptional data at single-cell resolution with computational network analyses, we find that NAFLD is associated with high constitutive androstane receptor (CAR) expression. Mechanistically, CAR interacts with four functional modules: cholesterol homeostasis, bile acid metabolism, fatty acid metabolism, and estrogen response. Nuclear expression of CAR positively correlates with steatohepatitis in human livers. These findings demonstrate significant cellular differences in lipid signatures and identify functional networks linked to hepatic steatosis in mice and humans.

The Vasculature in Pulmonary Fibrosis

Purpose of Review

The current paradigm of idiopathic pulmonary fibrosis (IPF) pathogenesis involves recurrent injury to a sensitive alveolar epithelium followed by impaired repair responses marked by fibroblast activation and deposition of extracellular matrix. Multiple cell types are involved in this response with potential roles suggested by advances in single-cell RNA sequencing and lung developmental biology. Notably, recent work has better characterized the cell types present in the pulmonary endothelium and identified vascular changes in patients with IPF.

Recent Findings

Lung tissue from patients with IPF has been examined at single-cell resolution, revealing reductions in lung capillary cells and expansion of a population of vascular cells expressing markers associated with bronchial endothelium. In addition, pre-clinical models have demonstrated a fundamental role for aging and vascular permeability in the development of pulmonary fibrosis.

Summary

Mounting evidence suggests that the endothelium undergoes changes in the context of fibrosis, and these changes may contribute to the development and/or progression of pulmonary fibrosis. Additional studies will be needed to further define the functional role of these vascular changes.

Development of an in vivo cleavable donor plasmid for targeted transgene integration by CRISPR-Cas9 and CRISPR-Cas12a

The CRISPR-Cas system is widely used for genome editing of cultured cells and organisms. The discovery of a new single RNA-guided endonuclease, CRISPR-Cas12a, in addition to the conventional CRISPR-Cas9 has broadened the number of editable target sites on the genome. Here, we developed an in vivo cleavable donor plasmid for precise targeted knock-in of external DNA by both Cas9 and Cas12a. This plasmid, named pCriMGET_9-12a (plasmid of synthetic CRISPR-coded RNA target sequence-equipped donor plasmid-mediated gene targeting via Cas9 and Cas12a), comprises the protospacer-adjacent motif sequences of Cas9 and Cas12a at the side of an off-target free synthetic CRISPR-coded RNA target sequence and a multiple cloning site for donor cassette insertion. pCriMGET_9-12a generates a linearized donor cassette in vivo by both CRISPR-Cas9 and CRISPR-Cas12a, which resulted in increased knock-in efficiency in culture cells. This method also achieved > 25% targeted knock-in of long external DNA (> 4 kb) in mice by both CRISPR-Cas9 and CRISPR-Cas12a. The pCriMGET_9-12a system expands the genomic target space for transgene knock-in and provides a versatile, low-cost, and high-performance CRISPR genome editing tool.

Single-cell spatial transcriptomics reveals a dynamic control of metabolic zonation and liver regeneration by endothelial cell Wnt2 and Wnt9b

The conclusive identity of Wnts regulating liver zonation (LZ) and regeneration (LR) remains unclear despite an undisputed role of β-catenin. Using single-cell analysis, we identified a conserved Wnt2 and Wnt9b expression in endothelial cells (ECs) in zone 3. EC-elimination of Wnt2 and Wnt9b led to both loss of β-catenin targets in zone 3, and re-appearance of zone 1 genes in zone 3, unraveling dynamicity in the LZ process. Impaired LR observed in the knockouts phenocopied models of defective hepatic Wnt signaling. Administration of a tetravalent antibody to activate Wnt signaling rescued LZ and LR in the knockouts and induced zone 3 gene expression and LR in controls. Administration of the agonist also promoted LR in acetaminophen overdose acute liver failure (ALF) fulfilling an unmet clinical need. Overall, we report an unequivocal role of EC-Wnt2 and Wnt9b in LZ and LR and show the role of Wnt activators as regenerative therapy for ALF.

Modelling urea cycle disorders using iPSCs

The urea cycle is a liver-based pathway enabling disposal of nitrogen waste. Urea cycle disorders (UCDs) are inherited metabolic diseases caused by deficiency of enzymes or transporters involved in the urea cycle and have a prevalence of 1:35,000 live births. Patients present recurrent acute hyperammonaemia, which causes high rate of death and neurological sequelae. Long-term therapy relies on a protein-restricted diet and ammonia scavenger drugs. Currently, liver transplantation is the only cure. Hence, high unmet needs require the identification of effective methods to model these diseases to generate innovative therapeutics. Advances in both induced pluripotent stem cells (iPSCs) and genome editing technologies have provided an invaluable opportunity to model patient-specific phenotypes in vitro by creating patients’ avatar models, to investigate the pathophysiology, uncover novel therapeutic targets and provide a platform for drug discovery. This review summarises the progress made thus far in generating 2- and 3-dimensional iPSCs models for UCDs, the challenges encountered and how iPSCs offer future avenues for innovation in developing the next-generation of therapies for UCDs.

Simulation of the crosstalk between glucose and acetaminophen metabolism in a liver zonation model

The liver metabolizes a variety of substances that sometimes interact and regulate each other. The modeling of a single cell or a single metabolic pathway does not represent the complexity of the organ, including metabolic zonation (heterogeneity of functions) along with liver sinusoids. Here, we integrated multiple metabolic pathways into a single numerical liver zonation model, including drug and glucose metabolism. The model simulated the time-course of metabolite concentrations by the combination of dynamic simulation and metabolic flux analysis and successfully reproduced metabolic zonation and localized hepatotoxicity induced by acetaminophen (APAP). Drug metabolism was affected by nutritional status as the glucuronidation reaction rate changed. Moreover, sensitivity analysis suggested that the reported metabolic characteristics of obese adults and healthy infants in glucose metabolism could be associated with the metabolic features of those in drug metabolism. High activities of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphate phosphatase in obese adults led to increased APAP oxidation by cytochrome P450 2E1. In contrast, the high activity of glycogen synthase and low activities of PEPCK and glycogen phosphorylase in healthy infants led to low glucuronidation and high sulfation rates of APAP. In summary, this model showed the effects of glucose metabolism on drug metabolism by integrating multiple pathways into a single liver metabolic zonation model.

The RSPO-LGR4/5-ZNRF3/RNF43 module in liver homeostasis, regeneration, and disease

WNT/β-catenin signaling plays pivotal roles during liver development, homeostasis, and regeneration. Likewise, its deregulation disturbs metabolic liver zonation and is responsible for the development of a large number of hepatic tumors. Liver fibrosis, which has become a major health burden for society and a hallmark of NASH, can also be promoted by WNT/β-catenin signaling. Upstream regulatory mechanisms controlling hepatic WNT/β-catenin activity may constitute targets for the development of novel therapies addressing these life-threatening conditions. The R-spondin (RSPO)-leucine-rich repeat-containing G protein-coupled receptor (LGR) 4/5-zinc and ring finger (ZNRF) 3/ring finger 43 (RNF43) module is fine-tuning WNT/β-catenin signaling in several tissues and is essential for hepatic WNT/β-catenin activity. In this review article, we recapitulate the role of the RSPO-LGR4/5-ZNRF3/RNF43 module during liver development, homeostasis, metabolic zonation, regeneration, and disease. We further discuss the controversy around LGR5 as a liver stem cell marker.

A strategy of vascular-targeted therapy for liver fibrosis

BACKGROUND AND AIMS

No effective treatments are available for liver fibrosis. Angiogenesis is deeply involved in liver fibrogenesis. However, current controversial results suggest it is difficult to treat liver fibrosis through vascular targeting. There are three different microvessels in liver: portal vessels, liver sinusoids, and central vessels. The changes and roles for each of the three different vessels during liver fibrogenesis are unclear. We propose that they play different roles during liver fibrogenesis, and a single vascular endothelial cell (EC) regulator is not enough to fully regulate these three vessels to treat liver fibrosis. Therefore, a combined regulation of multiple different EC regulatory signaling pathway may provide new strategies for the liver fibrosis therapy. Herein, we present a proof-of-concept strategy by combining the regulation of leukocyte cell-derived chemotaxin 2 (LECT2)/tyrosine kinase with immunoglobulin-like and epidermal growth factor-like domains 1 signaling with that of vascular endothelial growth factor (VEGF)/recombinant VEGF (rVEGF) signaling.

APPROACH AND RESULTS

The CCl₄ -induced mouse liver fibrosis model and NASH model were both used. During fibrogenesis, vascular changes occurred at very early stage, and different liver vessels showed different changes and played different roles: decreased portal vessels, increased sinusoid capillarization and the increased central vessels the increase of portal vessels alleviates liver fibrosis, the increase of central vessels aggravates liver fibrosis, and the increase of sinusoid capillarization aggravates liver fibrosis. The combinational treatment of adeno-associated viral vector serotype 9 (AAV9)-LECT2-short hairpin RNA (shRNA) and rVEGF showed improved therapeutic effects, but it led to serious side effects. The combination of AAV9-LECT2-shRNA and bevacizumab showed both improved therapeutic effects and decreased side effects.

CONCLUSIONS

Liver vascular changes occurred at very early stage of fibrogenesis. Different vessels play different roles in liver fibrosis. The combinational treatment of AAV9-LECT2-shRNA and bevacizumab could significantly improve the therapeutic effects on liver fibrosis.

Cellular and transcriptional heterogeneity in the intrahepatic biliary epithelium

Epithelial tissues comprise heterogeneous cellular subpopulations, which often compartmentalize specialized functions like absorption and secretion to distinct cell types. In the liver, hepatocytes and biliary epithelial cells (BECs; also called cholangiocytes) are the two major epithelial lineages and play distinct roles in (1) metabolism, protein synthesis, detoxification, and (2) bile transport and modification, respectively. Recent technological advances, including single cell transcriptomic assays, have shed new light on well-established heterogeneity among hepatocytes, endothelial cells, and immune cells in the liver. However, a "ground truth" understanding of molecular heterogeneity in BECs has remained elusive, and the field currently lacks a set of consensus biomarkers for identifying BEC subpopulations. Here, we review long-standing definitions of BEC heterogeneity as well as emerging studies that aim to characterize BEC subpopulations using next generation single cell assays. Understanding cellular heterogeneity in the intrahepatic bile ducts holds promise for expanding our foundational mechanistic knowledge of BECs during homeostasis and disease.

BASS: multi-scale and multi-sample analysis enables accurate cell type clustering and spatial domain detection in spatial transcriptomic studies

Spatial transcriptomic studies are reaching single-cell spatial resolution, with data often collected from multiple tissue sections. Here, we present a computational method, BASS, that enables multi-scale and multi-sample analysis for single-cell resolution spatial transcriptomics. BASS performs cell type clustering at the single-cell scale and spatial domain detection at the tissue regional scale, with the two tasks carried out simultaneously within a Bayesian hierarchical modeling framework. We illustrate the benefits of BASS through comprehensive simulations and applications to three datasets. The substantial power gain brought by BASS allows us to reveal accurate transcriptomic and cellular landscape in both cortex and hypothalamus.

Specific Temporal Requirement of Prox1 Activity During Pancreatic Acinar Cell Development

BACKGROUND AND AIMS

An interactive regulatory network assembled through the induction and downregulation of distinct transcription factors governs acinar cell maturation. Understanding how this network is built is relevant for protocols of directed pancreatic acinar differentiation. The murine transcription factor Prox1 is highly expressed in multipotent pancreatic progenitors and in various mature pancreatic cell types except for acinar cells. In this study, we investigated when is Prox1 expression terminated in developing acinar cells and the potential involvement of its activity in acinar cell specification/differentiation. We also investigated the effects of sustained Prox1 expression in acinar maturation and maintenance.

METHODS

Prox1 acinar expression was analyzed by immunofluorescence and confocal microscopy. Prox1-null embryos (Prox1GFPCre/Δ), Prox1AcOE transgenic mice, histologic and immunostaining methods, transmission electron microscopy, functional assays, and quantitative RNA and RNA-sequencing methods were used to investigate the effects of Prox1 functional deficiency and sustained Prox1 expression in acinar maturation and homeostasis.

RESULTS

Immunostaining results reveal transient Prox1 expression in newly committed embryonic acinar cells. RNA-sequencing demonstrate precocious expression of multiple "late" acinar genes in the pancreas of Prox1GFPCre/Δ embryos. Prox1AcOE transgenic mice carrying sustained Prox1 acinar expression have relatively normal pancreas development. In contrast, Prox1AcOE adult mice have severe pancreatic alterations involving reduced acinar gene expression, abnormal acinar secretory granules, acinar atrophy, increased endoplasmic reticulum stress, and mild chronic inflammation.

CONCLUSION

Prox1 transient expression in early acinar cells is necessary for correct sequential gene expression. Prox1 expression is terminated in developing acinar cells to complete maturation and to preserve homeostasis.

Dual Cre and Dre recombinases mediate synchronized lineage tracing and cell subset ablation in vivo

Genetic technology using site-specific recombinases (SSR), such as the Cre-loxP system, has been widely employed for labelling specific cell populations and for studying their functions in vivo. To enhance the precision of cell lineage tracing and functional study, a similar SSR system termed Dre-rox has been recently used in combination with Cre-loxP. To enable more specific cell lineage tracing and ablation through dual recombinase activity, we generated two mouse lines that render Dre- or Dre+Cre-mediated recombination to excise a stop codon sequence that prevents the expression of diphtheria toxin receptor (DTR) knocked into the ubiquitously expressed and safe Rosa26 locus. Using different Dre- and Cre-expressing mouse lines, we showed that the surrogate gene reporter tdTomato and DTR were simultaneously expressed in target cells and in their descendants, and observed efficient ablation of tdTomato⁺ cells after diphtheria toxin administration. These mouse lines were used to simultaneously trace and deplete target cells of interest through the inducible expression of a reporter and DTR using dual Cre and Dre recombinases, allowing more precise and efficient study of the role of specific cell subsets within a heterogeneous population in pathophysiological conditions in vivo.

Liver Endothelial Heg Regulates Vascular/Biliary Network Patterning and Metabolic Zonation Via Wnt Signaling

BACKGROUND & AIMS

The liver has complex interconnecting blood vessel and biliary networks; however, how the vascular and biliary network form and regulate each other and liver function are not well-understood. We aimed to examine the role of Heg in mammalian liver development and functional maintenance.

METHODS

Global (Heg^-/-) or liver endothelial cell (EC)-specific deletion of Heg (Lyve1-Cre;Heg^fl/fl ) mice were used to study the in vivo function of Heg in the liver. Carbon-ink anterograde and retrograde injection were used to visualize the 3-dimensional patterning of liver portal and biliary networks, respectively. RNA sequencing, histology, and molecular and biochemical assays were used to assess liver gene expression, protein distribution, liver injury response, and function.

RESULTS

Heg deficiency in liver ECs led to a sparse liver vascular and biliary network. This network paucity does not compromise liver function under baseline conditions but did alter liver zonation. Molecular analysis revealed that endothelial Heg deficiency decreased expression of Wnt ligands/agonists including Wnt2, Wnt9b, and Rspo3 in ECs, which limits Axin2 mediated canonical Wnt signaling and the expression of cytochrome P450 enzymes in hepatocytes. Under chemical-induced stressed conditions, Heg-deficiency in liver ECs protected mice from drug-induced liver injuries.

CONCLUSION

Our study found that endothelial Heg is essential for the 3-D patterning of the liver vascular and indirectly regulates biliary networks and proper liver zonation via its regulation of Wnt ligand production in liver endothelial cells. The endothelial Heg-initiated changes of the liver metabolic zonation and metabolic enzyme expression in hepatocytes was functionally relevant to xenobiotic metabolism and drug induced liver toxicity.

Hepatocyte generation in liver homeostasis, repair, and regeneration

The liver has remarkable capability to regenerate, employing mechanism to ensure the stable liver-to-bodyweight ratio for body homeostasis. The source of this regenerative capacity has received great attention over the past decade yet still remained controversial currently. Deciphering the sources for hepatocytes provides the basis for understanding tissue regeneration and repair, and also illustrates new potential therapeutic targets for treating liver diseases. In this review, we describe recent advances in genetic lineage tracing studies over liver stem cells, hepatocyte proliferation, and cell lineage conversions or cellular reprogramming. This review will also evaluate the technical strengths and limitations of methods used for studies on hepatocyte generation and cell fate plasticity in liver homeostasis, repair and regeneration.

Development of new HPLC method for identification of metabolic degradation of N-pyrrolylhydrazide hydrazones with determined MAO- B activity in cellular cultures

In this research, a new rapid PR- HPLC method was developed for the determination of metabolites in isolated rat hapatocytes. The chromatographic parameters, including the stationary and mobile phases, outlet pressure, temperature and flow rate, were optimized. The method identified two initial from the synthesis molecules in higher concentration and one new unidentified structure as products of the hepatocytic processing of the evaluated analyte. The results identified as first step of metabolism the hydrolysis of the hydrazone group. Further investigations should be aimed into determining the next metabolic transformations, predicted by the in silico application of the web server SMARTCyp.

Role and Regulation of Wnt/β-Catenin in Hepatic Perivenous Zonation and Physiological Homeostasis

Metabolic heterogeneity or functional zonation is a key characteristic of the liver that allows different metabolic pathways to be spatially regulated within the hepatic system and together contribute to whole body homeostasis. These metabolic pathways are segregated along the portocentral axis of the liver lobule into three hepatic zones: periportal, intermediate or midzonal, and perivenous. The liver performs complementary or opposing metabolic functions within different hepatic zones while synergistic functions are regulated by overlapping zones, thereby maintaining the overall physiological stability. The Wnt/β-catenin signaling pathway is well known for its role in liver growth, development, and regeneration. In addition, the Wnt/β-catenin pathway plays a fundamental and dominant role in hepatic zonation and signals to orchestrate various functions of liver metabolism and pathophysiology. The β-catenin protein is the central player in the Wnt/β-catenin signaling cascade, and its activation is crucial for metabolic patterning of the liver. However, dysregulation of Wnt/β-catenin signaling is also implicated in different liver pathologies, including those associated with metabolic syndrome. β-Catenin is preferentially localized in the central region of the hepatic lobule surrounding the central vein and regulates multiple functions of this region. This review outlines the role of Wnt/β-catenin signaling pathway in controlling the different metabolic processes surrounding the central vein and its relation to liver homeostasis and dysfunction.

Blood Supply and Oxygenation Status of the Liver: From Physiology to Malignancy

To maintain a multitude of vital functions, blood flow to the normal liver and the hepatic oxygenation status has to be kept on a high level (1.0-1.2 mL/g/min and 30-40 mmHg, respectively). There is a longitudinal oxygen partial pressure (pO2) gradient within the liver sinusoids between periportal inflow and outflow into the central vein leading to a zonation of the O2 status, which is associated with a zoning of liver functions. Oxygenation of metastatic lesions of colorectal cancers in the liver is poor due to a dysfunctional vascularity and inadequate blood supply. Hepatocellular carcinomas (HCCs) are highly vascularised (arterialised), metabolically very active and present with a predominantly arterial blood supply. HCCs are generally believed to be very hypoxic. However, confirmation of severe hypoxia based on reliable, direct pO2 measurements in HCCs is still missing.

The Scavenger Function of Liver Sinusoidal Endothelial Cells in Health and Disease

The aim of this review is to give an outline of the blood clearance function of the liver sinusoidal endothelial cells (LSECs) in health and disease. Lining the hundreds of millions of hepatic sinusoids in the human liver the LSECs are perfectly located to survey the constituents of the blood. These cells are equipped with high-affinity receptors and an intracellular vesicle transport apparatus, enabling a remarkably efficient machinery for removal of large molecules and nanoparticles from the blood, thus contributing importantly to maintain blood and tissue homeostasis. We describe here central aspects of LSEC signature receptors that enable the cells to recognize and internalize blood-borne waste macromolecules at great speed and high capacity. Notably, this blood clearance system is a silent process, in the sense that it usually neither requires or elicits cell activation or immune responses. Most of our knowledge about LSECs arises from studies in animals, of which mouse and rat make up the great majority, and some species differences relevant for extrapolating from animal models to human are discussed. In the last part of the review, we discuss comparative aspects of the LSEC scavenger functions and specialized scavenger endothelial cells (SECs) in other vascular beds and in different vertebrate classes. In conclusion, the activity of LSECs and other SECs prevent exposure of a great number of waste products to the immune system, and molecules with noxious biological activities are effectively “silenced” by the rapid clearance in LSECs. An undesired consequence of this avid scavenging system is unwanted uptake of nanomedicines and biologics in the cells. As the development of this new generation of therapeutics evolves, there will be a sharp increase in the need to understand the clearance function of LSECs in health and disease. There is still a significant knowledge gap in how the LSEC clearance function is affected in liver disease.

Metabolic Contributions of Wnt Signaling: More Than Controlling Flight

The canonical Wnt signaling pathway is ubiquitous throughout the body and influences a diverse array of physiological processes. Following the initial discovery of the Wnt signaling pathway during wing development in Drosophila melanogaster, it is now widely appreciated that active Wnt signaling in mammals is necessary for the development and growth of various tissues involved in whole-body metabolism, such as brain, liver, pancreas, muscle, and adipose. Moreover, elegant gain- and loss-of-function studies have dissected the tissue-specific roles of various downstream effector molecules in the regulation of energy homeostasis. This review attempts to highlight and summarize the contributions of the Wnt signaling pathway and its downstream effectors on whole-body metabolism and their influence on the development of metabolic diseases, such as diabetes and obesity. A better understanding of the Wnt signaling pathway in these tissues may aid in guiding the development of future therapeutics to treat metabolic diseases.

Spatial Reconstruction of the Early Hepatic Transcriptomic Landscape After an Acetaminophen Overdose Using Single-Cell RNA-Sequencing

An acetaminophen (APAP) overdose is the most common cause of acute liver failure in the United States. A hallmark characteristic of APAP hepatotoxicity is centrilobular necrosis. General, innate mechanisms such as lower amounts of GSH and higher cytochrome P450 2e1 expression in pericentral (PC) hepatocytes are known to contribute to the differences in susceptibility to cell injury between periportal (PP) hepatocytes and PC hepatocytes. Although a sequence of molecular events involving formation of the reactive metabolite N-acetyl-p-benzoquinone imine, GSH depletion, oxidative stress, and c-Jun N-terminal kinase activation define the early cell stress trajectory following APAP exposure, their activation in PC versus PP hepatocytes is not well characterized. By using single-cell RNA-sequencing, we provide the first reconstruction of the early transcriptomic APAP liver lobule after validation of our methodology using human liver single-cell RNA-sequencing data. Two hours after APAP treatment, we find that PP hepatocytes progress along the APAP stress axis to oxidative stress, before resolving injury due to innate and adaptive mechanisms. However, PC hepatocytes continue along this stress axis as indicated by activation of mitogen-activated protein kinase genes, which is absent in PP hepatocytes. We also identify a population of glutamine synthetase enriched PC hepatocytes in close proximity to the central vein, where a stepwise induction of a stress program culminated in cell death. Collectively, these findings elucidate a molecular sequence of events distinguishing the differential response to APAP exposure between PP and PC hepatocytes and identify a subset of uniquely susceptible PC hepatocytes.

Generation of Hepatobiliary Cell Lineages from Human Induced Pluripotent Stem Cells: Applications in Disease Modeling and Drug Screening

The possibility to reproduce key tissue functions in vitro from induced pluripotent stem cells (iPSCs) is offering an incredible opportunity to gain better insight into biological mechanisms underlying development and disease, and a tool for the rapid screening of drug candidates. This review attempts to summarize recent strategies for specification of iPSCs towards hepatobiliary lineages -hepatocytes and cholangiocytes-and their use as platforms for disease modeling and drug testing. The application of different tissue-engineering methods to promote accurate and reliable readouts is discussed. Space is given to open questions, including to what extent these novel systems can be informative. Potential pathways for improvement are finally suggested.

Roles of Estrogens in the Healthy and Diseased Oviparous Vertebrate Liver

The liver is a vital organ that sustains multiple functions beneficial for the whole organism. It is sexually dimorphic, presenting sex-biased gene expression with implications for the phenotypic differences between males and females. Estrogens are involved in this sex dimorphism and their actions in the liver of several reptiles, fishes, amphibians, and birds are discussed. The liver participates in reproduction by producing vitellogenins (yolk proteins) and eggshell proteins under the control of estrogens that act via two types of receptors active either mainly in the cell nucleus (ESR) or the cell membrane (GPER1). Estrogens also control hepatic lipid and lipoprotein metabolisms, with a triglyceride carrier role for VLDL from the liver to the ovaries during oogenesis. Moreover, the activation of the vitellogenin genes is used as a robust biomarker for exposure to xenoestrogens. In the context of liver diseases, high plasma estrogen levels are observed in fatty liver hemorrhagic syndrome (FLHS) in chicken implicating estrogens in the disease progression. Fishes are also used to investigate liver diseases, including models generated by mutation and transgenesis. In conclusion, studies on the roles of estrogens in the non-mammalian oviparous vertebrate liver have contributed enormously to unveil hormone-dependent physiological and physiopathological processes.