Liver Regeneration: Different Sub-Populations of Parenchymal Cells at Play Choreographed by an Injury-Specific Microenvironment

Acquisition of epithelial plasticity in human chronic liver disease

For many adult human organs, tissue regeneration during chronic disease remains a controversial subject. Regenerative processes are easily observed in animal models, and their underlying mechanisms are becoming well characterized1-4, but technical challenges and ethical aspects are limiting the validation of these results in humans. We decided to address this difficulty with respect to the liver. This organ displays the remarkable ability to regenerate after acute injury, although liver regeneration in the context of recurring injury remains to be fully demonstrated. Here we performed single-nucleus RNA sequencing (snRNA-seq) on 47 liver biopsies from patients with different stages of metabolic dysfunction-associated steatotic liver disease to establish a cellular map of the liver during disease progression. We then combined these single-cell-level data with advanced 3D imaging to reveal profound changes in the liver architecture. Hepatocytes lose their zonation and considerable reorganization of the biliary tree takes place. More importantly, our study uncovers transdifferentiation events that occur between hepatocytes and cholangiocytes without the presence of adult stem cells or developmental progenitor activation. Detailed analyses and functional validations using cholangiocyte organoids confirm the importance of the PI3K-AKT-mTOR pathway in this process, thereby connecting this acquisition of plasticity to insulin signalling. Together, our data indicate that chronic injury creates an environment that induces cellular plasticity in human organs, and understanding the underlying mechanisms of this process could open new therapeutic avenues in the management of chronic diseases.

Toward Transplantation of Liver Organoids: From Biology and Ethics to Cost-effective Therapy

Liver disease is a common cause of morbidity and mortality, and many patients would benefit from liver transplantation. However, because of a shortage of suitable donor livers, even of those patients who are placed on the donor liver waiting list, many do not survive the waiting time for transplantation. Therefore, alternative treatments for end-stage liver disease need to be explored. Recent advances in organoid technology might serve as a solution to overcome the donor liver shortage in the future. In this overview, we highlight the potential of organoid technology for cell therapy and tissue engineering approaches. Both organoid-based approaches could be used as treatment for end-stage liver disease patients. Additionally, organoid-based cell therapy can also be used to repair liver grafts ex vivo to increase the supply of transplantable liver tissue. The potential of both approaches to become clinically available is carefully assessed, including their clinical, ethical, and economic implications. We provide insight into what aspects should be considered further to allow alternatives to donor liver transplantation to be successfully clinically implemented.

Cardiomyocyte Ploidy, Metabolic Reprogramming and Heart Repair

The adult heart is made up of cardiomyocytes (CMs) that maintain pump function but are unable to divide and form new myocytes in response to myocardial injury. In contrast, the developmental cardiac tissue is made up of proliferative CMs that regenerate injured myocardium. In mammals, CMs during development are diploid and mononucleated. In response to cardiac maturation, CMs undergo polyploidization and binucleation associated with CM functional changes. The transition from mononucleation to binucleation coincides with unique metabolic changes and shift in energy generation. Recent studies provide evidence that metabolic reprogramming promotes CM cell cycle reentry and changes in ploidy and nucleation state in the heart that together enhances cardiac structure and function after injury. This review summarizes current literature regarding changes in CM ploidy and nucleation during development, maturation and in response to cardiac injury. Importantly, how metabolism affects CM fate transition between mononucleation and binucleation and its impact on cell cycle progression, proliferation and ability to regenerate the heart will be discussed.

A hepatoprotective role of peritumoral non-parenchymal cells in early liver tumorigenesis

Various 3D models of hepatocytes (HCs) have been established to assess liver functions in vitro. The contribution of the hepatic non-parenchymal cells (NPCs), however, is largely neglected in these models. Here, we report a comparative study of hepatic spheroids generated from freshly isolated mouse whole liver cells (WLCs) and HCs (referred to as SphWLC and SphHC, respectively). We found that HC differentiation was preserved better in SphWLC than in SphHC, and, when co-cultured with liver tumor spheroids (SphT), SphWLC showed more potent suppression of SphT growth compared to SphHC. Histological characterization revealed marked activation and accumulation of hepatic stellate cells (HSCs) at the SphWLC:SphT interface. We found that mixing HSCs in both 3D and 2D HC:tumor co-cultures provided potent protection to HCs against tumor-induced cell death. Activation of HSCs at the tumor border was similarly found in liver tumors from both mice and patients. Overall, our study suggests a hepatoprotective role of peritumoral HSCs in liver tumorigenesis and the potential application of SphWLC as a useful 3D model for dissecting the liver's response to tumorigenesis in vitro.

Mesenchymal stem cells and their microenvironment

Mesenchymal stem cells (MSCs), coming from a wide range of sources, have multi-directional differentiation ability. MSCs play vital roles in immunomodulation, hematopoiesis and tissue repair. The microenvironment of cells often refers to the intercellular matrix, other cells, cytokines and humoral components. It is also the place for cells’ interaction. The stability of the microenvironment is pivotal for maintaining cell proliferation, differentiation, metabolism and functional activities. Abnormal changes in microenvironment components can interfere cell functions. In some diseases, MSCs can interact with the microenvironment and accelerate disease progression. This review will discuss the characteristics of MSCs and their microenvironment, as well as the interaction between MSCs and microenvironment in disease.

Protective effects of exosomes derived from lyophilized porcine liver against acetaminophen damage on HepG2 cells

Background

Recently, extracellular vesicles have come to the fore following their emerging role in cell communication, thanks to their ability to reach cells into the human body without dissipating their cargo, transferring biological active molecules, such as proteins, nucleic acids, lipids, etc. They appear as a promising tool in medicine, because of their capability to modulate cellular response in recipient cells. Moreover, a considerable number of publications suggests that exosome uptake is selective but not specific, and it can cross species and cell-type boundaries. This study aims to explore the potential role of porcine liver derived extracellular vesicles, exosomes in particular, to protect human cells from acute damage induced by acetaminophen.

Methods

Extracellular vesicles were isolated from porcine lyophilized liver using polymer-based precipitation and a further enrichment was performed using affinity beads. The effects of obtained fractions, total extracellular vesicles and enriched extracellular vesicles, were assessed on human liver derived HepG2 cells. Cell growth and survival were tested, with MTT and area coverage analysis designed by us, as well as protein expression, with immunofluorescence and Western blot. Oxidative stress in live cells was also measured with fluorogenic probes.

Results

After proving that porcine extracellular vesicles did not have a toxic effect on HepG2, quite the contrary total extracellular vesicle fraction improved cell growth, we investigated their protective capability with a preconditioning strategy in APAP-induced damage. EVs displayed not only the ability to strongly modulate cell survival responses, but they also were able to boost cell cycle progression.

Conclusions

Extracellular vesicles derived from farm animal food derivatives are able to modulate human hepatic cell metabolism, also improving cell survival in a damaged context.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12906-021-03476-y.

Role of the Microenvironment in Mesenchymal Stem Cell-Based Strategies for Treating Human Liver Diseases

Liver disease is a severe health problem that endangers human health worldwide. Mesenchymal stem cell (MSC) therapy is a novel treatment for patients with different liver diseases due to its vast expansion potential and distinctive immunomodulatory properties. Despite several preclinical trials having confirmed the considerable efficacy of MSC therapy in liver diseases, the questionable safety and efficacy still limit its application. As a precursor cell, MSCs can adjust their characteristics in response to the surrounding microenvironment. The microenvironment provides physical and chemical factors essential for stem cell survival, proliferation, and differentiation. However, the mechanisms are still not completely understood. We, therefore, summarized the mechanisms underlying the MSC immune response, especially the interaction between MSCs and the liver microenvironment, discussing how to achieve better therapeutic effects.

Biological evidence to define a vitamin A deficiency cutoff using total liver vitamin A reserves

Vitamin A is a fat-soluble vitamin involved in essential functions including growth, immunity, reproduction, and vision. The vitamin A Dietary Reference Intakes (DRIs) for North Americans suggested that a minimally acceptable total liver vitamin A reserve (TLR) is 0.07 µmol/g, which is not explicitly expressed as a vitamin A deficiency cutoff. The Biomarkers of Nutrition for Development panel set the TLR cutoff for vitamin A deficiency at 0.1 µmol/g based on changes in biological response of several physiological parameters at or above this cutoff. The criteria used to formulate the DRIs include clinical ophthalmic signs of vitamin A deficiency, circulating plasma retinol concentrations, excretion of vitamin A metabolites in the bile, and long-term storage of vitamin A as protection against vitamin A deficiency during times of low dietary intake. This review examines the biological responses that occur as TLRs are depleted. In consideration of all of the DRI criteria, the review concludes that induced biliary excretion and long-term vitamin A storage do not occur until TLRs are >0.10 µmol/g. If long-term storage is to continue to be part of the DRI criteria, vitamin A deficiency should be set at a minimum cutoff of 0.10 µmol/g and should be set higher during times of enhanced requirements where TLRs can be rapidly depleted, such as during lactation or in areas with high infection burden. In population-based surveys, cutoffs are important when using biomarkers of micronutrient status to define the prevalence of deficiency and sufficiency to inform public health interventions. Considering the increasing use of quantitative biomarkers of vitamin A status that indirectly assess TLRs, i.e. the modified-relative-dose response and retinol-isotope dilution tests, setting a TLR as a vitamin A deficiency cutoff is important for users of these techniques to estimate vitamin A deficiency prevalence. Future researchers and policymakers may suggest that DRIs should be set with regard to optimal health and not merely to prevent a micronutrient deficiency.

nCP:Fe Nanocontrast Agent for Magnetic Resonance Imaging-Based Early Detection of Liver Cirrhosis and Hepatocellular Carcinoma

Early detection of liver tumors and cirrhotic lesions by magnetic resonance imaging (MRI) remains a great challenge. Here, we report a biomineral nanocontrast agent based on iron-doped nanocalcium phosphate (nCP:Fe-CA) for magnetic resonance imaging of early-stage liver cirrhotic and hepatocellular carcinoma nodules using rat models. We have optimized an intravenously injectable, aqueous suspension of nCP:Fe-CA having an average size of 137.6 nm, a spherical shape, magnetic relaxivity of 63 mM^-1S^-1, and colloidal stability for 48 h, post-resuspension in an aqueous phase. Compared to superparamagnetic iron oxide nanoparticles (SPIONs), the optimized nCP:Fe-CA could detect liver tumor lesions as small as ∼0.25 cm, whereas the current clinical detection limit is ∼1 cm. In addition, multiple cirrhotic nodules of size <0.2 cm could be detected by nCP:Fe-CA-assisted MRI. The number of nodules observed after injecting nCP:Fe-CA was ∼3 times higher than that without CA (5-10 nodules). A biocompatibility study on healthy rats injected with nCP:Fe-CA showed unaltered liver transaminases, blood urea nitrogen, serum creatinine, and insignificant hemolysis. Furthermore, hepatobiliary clearance of nCP:Fe-CA was observed in 72 h compared to prolonged retention of SPIONs for 30 days when tested under identical conditions. Overall, the nCP:Fe-CA nanoparticles showed promising results as a biocompatible, MR contrast (T2) agent for the early-stage imaging of liver cirrhosis and hepatocellular carcinoma.

Molecular pathways of liver regeneration: A comprehensive review

The liver is a unique parenchymal organ with a regenerative capacity allowing it to restore up to 70% of its volume. Although knowledge of this phenomenon dates back to Greek mythology (the story of Prometheus), many aspects of liver regeneration are still not understood. A variety of different factors, including inflammatory cytokines, growth factors, and bile acids, promote liver regeneration and control the final size of the organ during typical regeneration, which is performed by mature hepatocytes, and during alternative regeneration, which is performed by recently identified resident stem cells called “hepatic progenitor cells”. Hepatic progenitor cells drive liver regeneration when hepatocytes are unable to restore the liver mass, such as in cases of chronic injury or excessive acute injury. In liver maintenance, the body mass ratio is essential for homeostasis because the liver has numerous functions; therefore, a greater understanding of this process will lead to better control of liver injuries, improved transplantation of small grafts and the discovery of new methods for the treatment of liver diseases. The current review sheds light on the key molecular pathways and cells involved in typical and progenitor-dependent liver mass regeneration after various acute or chronic injuries. Subsequent studies and a better understanding of liver regeneration will lead to the development of new therapeutic methods for liver diseases.

Recent advances in bioprinting technologies for engineering hepatic tissue

In the sphere of liver tissue engineering (LTE), 3D bioprinting has emerged as an effective technology to mimic the complex in vivo hepatic microenvironment, enabling the development of functional 3D constructs with potential application in the healthcare and diagnostic sector. This review gears off with a note on the liver's microscopic 3D architecture and pathologies linked to liver injury. The write-up is then directed towards unmasking recent advancements and prospects of bioprinting for recapitulating 3D hepatic structure and function. The article further introduces available stem cell opportunities and different strategies for their directed differentiation towards various hepatic stem cell types, including hepatocytes, hepatic sinusoidal endothelial cells, stellate cells, and Kupffer cells. Another thrust of the article is on understanding the dynamic interplay of different hepatic cells with various microenvironmental cues, which is crucial for controlling differentiation, maturation, and maintenance of functional hepatic cell phenotype. On a concluding note, various critical issues and future research direction towards clinical translation of bioprinted hepatic constructs are discussed.

Multi-omics analysis of hiPSCs-derived HLCs matured on-chip revealed patterns typical of liver regeneration

Maturation of human-induced pluripotent stem cells (hiPSCs)-derived hepatocytes-like cells (HLCs) toward a complete hepatocyte phenotype remains a challenge as primitiveness patterns are still commonly observed. In this study, we propose a modified differentiation protocol for those cells which includes a prematuration in Petri dishes and a maturation in microfluidic biochip. For the first time, a large range of biomolecular families has been extracted from the same sample to combine transcriptomic, proteomic, and metabolomic analysis. After integration, these datasets revealed specific molecular patterns and highlighted the hepatic regeneration profile in biochips. Overall, biochips exhibited processes of cell proliferation and inflammation (via TGFB1) coupled with anti-fibrotic signaling (via angiotensin 1-7, ATR-2, and MASR). Moreover, cultures in this condition displayed physiological lipid-carbohydrate homeostasis (notably via PPAR, cholesterol metabolism, and bile synthesis) coupled with cell respiration through advanced oxidative phosphorylation (through the overexpression of proteins from the third and fourth complex). The results presented provide an original overview of the complex mechanisms involved in liver regeneration using an advanced in vitro organ-on-chip technology.

Longitudinal imaging and femtosecond laser manipulation of the liver: How to generate and trace single-cell-resolved micro-damage in vivo

The liver is known to possess extensive regenerative capabilities, the processes and pathways of which are not fully understood. A necessary step towards a better understanding involves the analysis of regeneration on the microscopic level in the in vivo environment. We developed an evaluation method combining longitudinal imaging analysis in vivo with simultaneous manipulation on single cell level. An abdominal imaging window was implanted in vivo in Balb/C mice for recurrent imaging after implantation. Intravenous injection of Fluorescein Isothiocyanate (FITC)-Dextran was used for labelling of vessels and Rhodamine 6G for hepatocytes. Minimal cell injury was induced via ablation with a femtosecond laser system during simultaneous visualisation of targeted cells using multiphoton microscopy. High-resolution imaging in vivo on single cell level including re-localisation of ablated regions in follow-up measurements after 2-7 days was feasible. Targeted single cell manipulation using femtosecond laser pulses at peak intensities of 3-6.6 μJ led to enhancement of FITC-Dextran in the surrounding tissue. These reactions reached their maxima 5-15 minutes after ablation and were no longer detectable after 24 hours. The procedures were well tolerated by all animals. Multiphoton microscopy in vivo, combined with a femtosecond laser system for single cell manipulation provides a refined procedure for longitudinal evaluation of liver micro-regeneration in the same region of interest. Immediate reactions after cell ablation and tissue regeneration can be analysed.

MR-Based Electrical Conductivity Imaging of Liver Fibrosis in an Experimental Rat Model

BACKGROUND

Liver fibrosis is characterized by the excessive accumulation of extracellular matrix proteins. Electrical conductivity imaging at low frequency can provide novel contrast because the contrast mechanisms originate from the changes in the concentration and mobility of ions in the extracellular space.

PURPOSE

To evaluate the feasibility of an MR-based electrical conductivity imaging that can detect the changes in a tissue condition associated with the progression of liver fibrosis.

STUDY TYPE

Prospective phantom and animal study.

ANIMAL MODEL

Fibrosis was induced by weekly intraperitoneal injection of dimethylnitrosamine (DMN) in 45 male Sprague-Dawley rats.

FIELD STRENGTH/SEQUENCE

3T MRI with a multispin-echo pulse sequence.

ASSESSMENT

The percentage change of conductivity (Δσ, %) in the same region-of-interest (ROI) was calculated from the DMN-treated rats based on the values of the normal control rats. The percentage change was also calculated between the ROIs in each DMN-treated group.

STATISTICAL TESTS

One-way analysis of variance (ANOVA) and a two-sample t-test were performed.

RESULTS

Liver tissues in normal control rats showed a uniform conductivity distribution of 56.6 ± 4.4 (mS/m). In rats more than 5 weeks after induction, the fibrous region showed an increased conductivity of ≥12% compared to that of the corresponding normal control rats. From regional comparisons in the same liver, the fibrous region showed an increased conductivity of ≥11% compared to the opposite, less induced region of rats more than 5 weeks after induction. Liver samples from the fibrous region represent tissue damages such as diffuse centrilobular congestion with marked dilatation of central veins from the histological findings. Immunohistochemistry revealed significant levels of attenuated fibrosis and increased inflammatory response.

DATA CONCLUSION

The increased conductivity in the fibrous region is related to the changes of the extracellular space. The correlation between the collagen deposition and conductivity changes is essential for future clinical studies. Level of Evidence 2 Technical Efficacy Stage 2 J. MAGN. RESON. IMAGING 2021;53:554-563.

Human Liver-Derived Extracellular Matrix for the Culture of Distinct Human Primary Liver Cells

The lack of robust methods to preserve, purify and in vitro maintain the phenotype of the human liver’s highly specialized parenchymal and non-parenchymal cell types importantly hampers their exploitation for the development of research and clinical applications. There is in this regard a growing interest in the use of tissue-specific extracellular matrix (ECM) to provide cells with an in vitro environment that more closely resembles that of the native tissue. In the present study, we have developed a method that allows for the isolation and downstream application of the human liver’s main cell types from cryopreserved material. We also isolated and solubilized human liver ECM (HL-ECM), analyzed its peptidomic and proteomic composition by mass spectrometry and evaluated its interest for the culture of distinct primary human liver cells. Our analysis of the HL-ECM revealed proteomic diversity, type 1 collagen abundance and partial loss of integrity following solubilization. Solubilized HL-ECM was evaluated either as a coating or as a medium supplement for the culture of human primary hepatocytes, hepatic stellate cells and liver sinusoidal endothelial cells. Whereas the solubilized HL-ECM was suitable for cell culture, its impact on the phenotype and/or functionality of the human liver cells was limited. Our study provides a first detailed characterization of solubilized HL-ECM and a first report of its influence on the culture of distinct human primary liver cells.

Regulation of Genomic Output and (Pluri)potency in Regeneration

Regeneration is a remarkable phenomenon that has been the subject of awe and bafflement for hundreds of years. Although regeneration competence is found in highly divergent organisms throughout the animal kingdom, recent advances in tools used for molecular and genomic characterization have uncovered common genes, molecular mechanisms, and genomic features in regenerating animals. In this review we focus on what is known about how genome regulation modulates cellular potency during regeneration. We discuss this regulation in the context of complex tissue regeneration in animals, from Hydra to humans, with reference to ex vivo-cultured cell models of pluripotency when appropriate. We emphasize the importance of a detailed molecular understanding of both the mechanisms that regulate genomic output and the functional assays that assess the biological relevance of such molecular characterizations.

Glycosaminoglycan from Apostichopus japonicus inhibits hepatic glucose production via activating Akt/FoxO1 and inhibiting PKA/CREB signaling pathways in insulin resistant hepatocytes

The aim of this study was to elucidate the effect and the underlying mechanism of glycosaminoglycan from Apostichopus japonicus (AHG) on hepatic glucose production (HGP) in insulin resistant hepatocytes. Insulin resistance was induced with high glucose (HG) for 24 h in primary hepatocytes. The results showed that AHG exhibited hypoglycemic activity at a relatively low concentration (1 μg mL^-1) and revealed non-toxic activity to insulin resistant hepatocytes even at 500 μg mL^-1 concentration. The HGP test showed that the treatment of AHG (10 μg mL^-1) for 3 h decreased HGP by 25% in insulin resistant hepatocytes. Quantitative PCR and western blot analysis revealed that AHG also ameliorated phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase). The data revealed the mechanism of AHG in alleviating HGP via activating the Akt/FoxO1 signaling pathway and suppressing the PKA/CREB signaling pathway in insulin resistant hepatocytes. This finding suggests that AHG could be a potential marine natural product for the treatment of dysregulating glucose homeostasis.

Invasive Ductular Reaction Operates Hepatobiliary Junctions upon Hepatocellular Injury in Rodents and Humans

Ductular reaction (DR) is observed in virtually all liver diseases in both humans and rodents. Depending on the injury, DR is confined within the periportal area or invades the parenchyma. On severe hepatocellular injury, invasive DR has been proposed to arise for supplying the liver with new hepatocytes. However, experimental data evidenced that DR contribution to hepatocyte repopulation is at the most modest, unless replicative capacity of hepatocytes is abrogated. Herein, we proposed that invasive DR could contribute to operating hepatobiliary junctions on hepatocellular injury. The choline-deficient ethionine-supplemented mouse model of hepatocellular injury and human liver samples were used to evaluate the hepatobiliary junctional role of the invasive form of DR. Choline-deficient ethionine-supplemented-induced DR expanded as biliary epithelium into the lobule and established new junctions with the canaliculi. By contrast, no new ductular-canalicular junctions were observed in mouse models of biliary obstructive injury exhibiting noninvasive DR. Similarly, in humans, an increased number of hepatobiliary junctions were observed in hepatocellular diseases (viral, drug induced, or metabolic) in which DR invaded the lobule but not in biliary diseases (obstruction or cholangitis) in which DR was contained within the portal mesenchyme. In conclusion, our data in rodents and humans support that invasive DR plays a hepatobiliary junctional role to maintain structural continuity between hepatocytes and ducts in disorders affecting hepatocytes.

Human Liver Regeneration: An Etiology Dependent Process

Regeneration of the liver has been an interesting and well-investigated topic for many decades. This etiology and time-dependent mechanism has proven to be extremely challenging to investigate, certainly in human diseases. A reason for this challenge is found in the numerous interactions of different cell components, of which some are even only temporarily present (e.g., inflammatory cells). To orchestrate regeneration of the epithelial cells, their interaction with the non-epithelial components is of utmost importance. Hepatocytes, cholangiocytes, liver progenitor cells, and peribiliary glands have proven to be compartments of regeneration. The ductular reaction is a common denominator in virtually all liver diseases; however, it is predominantly found in late-stage hepatic and biliary diseases. Ductular reaction is an intriguing example of interplay between epithelial and non-epithelial cells and encompasses bipotential liver progenitor cells which are able to compensate for the loss of the exhausted hepatocytes and cholangiocytes in biliary and hepatocytic liver diseases. In this manuscript, we focus on the etiology-specific damage that is observed in different human diseases and how the liver regulates the regenerative response in an acute and chronic setting. Furthermore, we describe the importance of morphological keynotes in different etiologies and how spatial information is of relevance for every basic and translational research of liver regeneration.