Hematopoietic origin of hepatic stellate cells in the adult liver

Plasticity, heterogeneity, and multifunctionality of hepatic stellate cells in liver pathophysiology

HSCs, the resident pericytes of the liver, have consistently been at the forefront of liver research due to their crucial roles in various hepatic pathological processes. Prior literature often depicted HSCs in a binary framework, categorizing them as either quiescent or activated. However, recent advances in HSC research, particularly the advent of single-cell RNA-sequencing, have revolutionized our understanding of these cells. This sophisticated technique offers an unparalleled, high-resolution insight into HSC populations, uncovering a spectrum of diversity and functional heterogeneity across various physiological states of the liver, ranging from liver development to the liver aging process. The single-cell RNA-sequencing revelations have also highlighted the intrinsic plasticity of HSCs and underscored their complex roles in a myriad of pathophysiological processes, including liver injury, repair, and carcinogenesis. This review aims to integrate and clarify these recent discoveries, focusing on how the inherent plasticity of HSCs is central to their dynamic roles both in maintaining liver homeostasis and orchestrating responses to liver injury. Future research will clarify whether findings from rodent models can be translated to human livers and guide how these insights are harnessed to develop targeted therapeutic interventions.

Perivascular Mesenchymal Stem/Stromal Cells, an Immune Privileged Niche for Viruses?

Mesenchymal stem cells (MSCs) play a critical role in response to stress such as infection. They initiate the removal of cell debris, exert major immunoregulatory activities, control pathogens, and lead to a remodeling/scarring phase. Thus, host-derived ‘danger’ factors released from damaged/infected cells (called alarmins, e.g., HMGB1, ATP, DNA) as well as pathogen-associated molecular patterns (LPS, single strand RNA) can activate MSCs located in the parenchyma and around vessels to upregulate the expression of growth factors and chemoattractant molecules that influence immune cell recruitment and stem cell mobilization. MSC, in an ultimate contribution to tissue repair, may also directly trans- or de-differentiate into specific cellular phenotypes such as osteoblasts, chondrocytes, lipofibroblasts, myofibroblasts, Schwann cells, and they may somehow recapitulate their neural crest embryonic origin. Failure to terminate such repair processes induces pathological scarring, termed fibrosis, or vascular calcification. Interestingly, many viruses and particularly those associated to chronic infection and inflammation may hijack and polarize MSC’s immune regulatory activities. Several reports argue that MSC may constitute immune privileged sanctuaries for viruses and contributing to long-lasting effects posing infectious challenges, such as viruses rebounding in immunocompromised patients or following regenerative medicine therapies using MSC. We will herein review the capacity of several viruses not only to infect but also to polarize directly or indirectly the functions of MSC (immunoregulation, differentiation potential, and tissue repair) in clinical settings.

In vitro and in vivo study: Ethanolic extract leaves of Azadirachta indica Juss. variant of Indonesia and Philippines suppresses tumor growth of hepatocellular carcinoma by inhibiting IL-6/STAT3 signaling

Background: Azadirachta indica Juss. has been shown to suppress cancer progression through a variety of mechanisms. In order to treat cancer progression, cancer immunotherapy is used to stimulate the immune system where immunosuppression is present in tumor microenvironments. Many cancer cells produce a lot of interleukin-6 (IL-6) and signal transducer activator of transcription 3 (STAT3). STAT3 plays a key role in suppressing the expression of critical immune activation regulators. IL-6-mediated STAT3 activation is common in the tumor microenvironment. Inhibiting the IL-6/STAT3 signaling pathway has become a therapeutic option for cancer progression. As vimentin is also expressed in hepatic stellate cells boosting cancer survival. We focused on the precise effect of extract from leaves of Azadirachta indica Juss, on inhibiting the IL-6/STAT3 signaling cascade on hepatocellular carcinoma by in vitro and in vivo study. Methods: In the in vitro study, the effect of Azadirachta indica Juss. variant Indonesia and Philippines against the expression of IL-6 and STAT3 was examined in liver cancer cell line. In the in vivo study, 24 male rats ( Rattus norvegicus) strain Wistar were induced by diethylnitrosamine and carbon tetrachloride (CCl 4). Based on the therapy given, the groups were divided into negative control, positive control, Indonesia extract, and Philippine extract. Expression of IL-6, STAT3, and vimentin were tested using immunohistochemistry staining. The data were analyzed using analysis of variance, which was then followed by the Tukey test. Results: Statistically significant difference in IL-6 and STAT3 was observed between the treatment groups and positive control group by in vitro study and in vivo study. Generally, there is no significant difference between treatment using Indonesian and Philippine leaves. Conclusion: Both therapy doses of Azadirachta indica variant in Indonesia and Philippines were able to reduce IL-6, STAT3 and vimentin expression of hepatocellular carcinoma cell by in vitro and in vivo experiment.

A Novel Secondary Neoplasm Following Allogeneic Hematopoietic Stem Cell Transplant: Mixed Donor-Recipient Primitive Mesenchymal Proliferation of the Liver

Post-hematopoietic stem cell transplant secondary solid neoplasms are uncommon and usually host-derived. We describe a 6-year-old female who developed a mixed donor-recipient origin mesenchymal stromal tumor-like lesion in the liver following an unrelated hematopoietic stem cell transplant complicated by severe graft-versus-host disease. This lesion arose early post-transplant in association with hepatic graft-versus-host disease. At 12 years post-transplant, the neoplasm has progressively shrunken in size and the patient remains well with no neoplasm-associated sequelae. This report characterizes a novel lesion of mixed origin post-transplant and offers unique insights into the contribution of bone marrow-derived cells to extra-medullary tissues.

A non-selective endothelin receptor antagonist bosentan modulates kinetics of bone marrow-derived cells in ameliorating pulmonary hypertension in mice

The aim of this study was to investigate whether a dual endothelin receptor antagonist bosentan modulates the kinetics of bone marrow-derived stem cells in inhibiting the development of pulmonary hypertension. Bone marrow chimeric mice, transplanted with enhanced green fluorescent protein (eGFP)-positive bone marrow mononuclear cells, were exposed to hypobaric hypoxia or kept in the ambient air, and were daily treated with bosentan sodium salt or saline for 21 days. After the treatment period, right ventricular pressure was measured and pulmonary vascular morphometry was conducted. Incorporation of bone marrow-derived cells was analyzed by immunohistochemistry. Gene expression and protein level in the lung tissue were evaluated by quantitative real-time PCR and western blotting, respectively. The results showed that, in hypoxic mice, right ventricular pressure and the percentage of muscularized vessel were increased and pulmonary vascular density was decreased, each of which was reversed by bosentan. Bone marrow-derived endothelial cells and macrophages in lungs were increased by hypoxia. Bosentan promoted bone marrow-derived endothelial cell incorporation but inhibited macrophage infiltration into lungs. Quantitative real-time PCR analysis revealed that interleukin 6, stromal cell-derived factor-1, and monocyte chemoattractant protein-1 were upregulated by hypoxia, in which interleukin 6 and monocyte chemoattractant protein-1 were downregulated and stromal cell-derived factor-1 was upregulated by bosentan. Protein level of endothelial nitric oxide synthase (eNOS) in the whole lung was significantly upregulated by hypoxia, which was further upregulated by bosentan. Bosentan modulated kinetics of bone marrow-derived ECs and macrophages and related gene expression in lungs in ameliorating pulmonary hypertension in mice. Altered kinetics of bone marrow-derived stem cells may be a novel mechanism of the endothelin receptor blockade in vivo and confer a new understanding of the therapeutic basis for pulmonary hypertension.

Hepatocyte-Macrophage Acetoacetate Shuttle Protects against Tissue Fibrosis

Metabolic plasticity has been linked to polarized macrophage function, but mechanisms connecting specific fuels to tissue macrophage function remain unresolved. Here we apply a stable isotope tracing, mass spectrometry-based untargeted metabolomics approach to reveal the metabolome penetrated by hepatocyte-derived glucose and ketone bodies. In both classically and alternatively polarized macrophages, [¹³C]acetoacetate (AcAc) labeled ∼200 chemical features, but its reduced form D-[¹³C]β-hydroxybutyrate (D-βOHB) labeled almost none. [¹³C]glucose labeled ∼500 features, and while unlabeled AcAc competed with only ∼15% of them, the vast majority required the mitochondrial enzyme succinyl-coenzyme A-oxoacid transferase (SCOT). AcAc carbon labeled metabolites within the cytoplasmic glycosaminoglycan pathway, which regulates tissue fibrogenesis. Accordingly, livers of mice lacking SCOT in macrophages were predisposed to accelerated fibrogenesis. Exogenous AcAc, but not D-βOHB, ameliorated diet-induced hepatic fibrosis. These data support a hepatocyte-macrophage ketone shuttle that segregates AcAc from D-βOHB, coordinating the fibrogenic response to hepatic injury via mitochondrial metabolism in tissue macrophages.

[Mechanisms of hepatic stellate cell activation as a therapeutic target for the treatment of non-alcoholic steatohepatitis]

Nonalcoholic fatty liver disease (NAFLD) is a rising cause of chronic liver disease worldwide. Although majority of patients with NAFLD are benign and non-progressive, having only steatosis, some fraction of patients develop non-alcoholic steatohepatitis (NASH), which can lead to cirrhosis, hepatocellular carcinoma, and eventually increased liver-related mortality. Among histological features of NAFLD, it has been reported that liver fibrosis is the most important predictor of long-term outcomes. Liver fibrosis is a dynamic process characterized by the over-accumulation of extracellular matrix due to chronic liver injury resulting from any etiology including not only NASH, but also viral infection and alcoholic liver disease. Activation of hepatic stellate cells (HSCs) has been well established as a central driver of fibrosis in experimental animal models and human liver injury. It is a transdifferentiation of quiescent, vitamin-A‑storing cells into proliferative and fibrogenic myofibroblasts. However, the discovery of novel pathways and mediators reveals the complexity of HSC activation. These emerging pathways include hedgehog, autophagy, free cholesterol, YAP1, hepcidin, and nuclear/G-protein coupled receptor-mediated signals. In addition, pathways of HSC clearance have been uncovered such as apoptosis, senescence, and reversion to an inactivated state. Thus, clarifying the underlying mechanisms of HSC activation could lead to the identification of novel therapeutic targets for NASH, and several drug candidates are currently being developed in clinical trials.

Stellate Cells in Tissue Repair, Inflammation, and Cancer

Stellate cells are resident lipid-storing cells of the pancreas and liver that transdifferentiate to a myofibroblastic state in the context of tissue injury. Beyond having roles in tissue homeostasis, stellate cells are increasingly implicated in pathological fibrogenic and inflammatory programs that contribute to tissue fibrosis and that constitute a growth-permissive tumor microenvironment. Although the capacity of stellate cells for extracellular matrix production and remodeling has long been appreciated, recent research efforts have demonstrated diverse roles for stellate cells in regulation of epithelial cell fate, immune modulation, and tissue health. Our present understanding of stellate cell biology in health and disease is discussed here, as are emerging means to target these multifaceted cells for therapeutic benefit.

Proceedings of the signature series event of the international society for cellular therapy: "Advancements in cellular therapies and regenerative medicine in digestive diseases," London, United Kingdom, May 3, 2017

A summary of the First Signature Series Event, "Advancements in Cellular Therapies and Regenerative Medicine for Digestive Diseases," held on May 3, 2017, in London, United Kingdom, is presented. Twelve speakers from three continents covered major topics in the areas of cellular therapy and regenerative medicine applied to liver and gastrointestinal medicine as well as to diabetes mellitus. Highlights from their presentations, together with an overview of the global impact of digestive diseases and a proposal for a shared online collection and data-monitoring platform tool, are included in this proceedings. Although growing evidence demonstrate the feasibility and safety of exploiting cell-based technologies for the treatment of digestive diseases, regulatory and methodological obstacles will need to be overcome before the successful implementation in the clinic of these novel attractive therapeutic strategies.

Hepatic stellate cells derived from the nestin-positive cells in septum transversum during rat liver development

Hepatic stellate cells (HSCs) play a principal role in Vitamin A metabolism and are considered the major matrix-producing cell type in the diseased liver. Rat HSCs are identified by immunohistochemistry with myogenic or mesenchymal (desmin, vimentin, and alpha-smooth muscle actin) or neural (e.g., GFAP or neuronal cell adhesion molecule) markers. Embryonic origin of rat HSCs was determined using these markers. Nestin, an intermediate filament protein originally identified in neuronal stem or progenitor cells, is widely used as a stem cell marker, including hepatic stem cells in adult rat livers. Additionally, nestin is reportedly expressed in activated HSCs during liver injury and hepatic regeneration. However, little is known about nestin expression in rat fetal liver HSCs. The present study aimed to clarify nestin-positive HSC expression during rat liver development. At embryonic day (ED) 10.5, nestin expression in mesenchymal cells adjacent to the liver bud was detected by immunohistochemistry. At ED 11.5, nestin-positive cells were also detected in desmin-positive cells appearing and increasing in intensity by ED 16.5. However, nestin-positive cells in the parenchyma decreased by ED 20.5 or later. These findings reveal that the nestin-positive HSCs during rat liver development originate from nestin-positive mesenchymal cells in the septum transversum.

Fibrosis in small syngeneic rat liver grafts because of damaged bone marrow stem cells from chronic alcohol consumption

A patient with liver failure due to chronic and acute alcohol abuse under consideration for an urgent liver transplant shortly after stopping alcohol may have residual abnormalities that threaten transplant success, particularly for a small graft. To address this, we studied a model in which reduced-size (50%) Lewis rat livers are transplanted into green fluorescence protein transgenic Lewis recipients after they are fed alcohol or a control diet for 5 weeks. Here we show that normal small Lewis grafts transplanted to alcohol-fed Lewis hosts developed fibrosis, whereas no fibrosis was observed in control-fed recipients. Host-derived CD133 + 8-hydroxy-2'-deoxyguanosine (8-OHdG) cells were significantly increased in livers recovered from both alcohol-fed and control recipients, but only alcohol-fed recipients demonstrated co-staining (a marker of oxidative DNA damage). α smooth muscle actin (α-SMA) staining, a marker for myofibroblasts, also co-localized with CD133 + cells only in the livers of alcohol-fed recipients. Immunostaining and polymerase chain reaction analysis confirmed that chronic alcohol consumption decreased the proportion of bone marrow stem cells (BMSCs) expressing CD133, c-Kit, and chemokine (C-X-C motif) receptor 4 markers and caused oxidative mitochondria DNA (mtDNA) damage. Culture of CD133 + cells from normal rats with medium containing 3% ethanol for 48 hours resulted in elevated mitochondrial 8-OHdG and mtDNA deletion, and ethanol exposure diminished CD133 expression but dramatically increased α-SMA expression. In conclusion, oxidative mtDNA damage and deletions occur in BMSCs of chronic alcohol-fed recipients, and these damaged cells mobilize to the small liver grafts and become myofibroblasts where they play a key role in the subsequent development of fibrosis. Liver Transplantation 23 1564-1576 2017 AASLD.

Healing of Preterm Ruptured Fetal Membranes

Preterm premature rupture of membrane (pPROM) is associated with 30–40% of preterm births. Infection is considered a leading cause of pPROM due to increased levels of proinflammatory cytokines in amniotic fluid. Only 30%, however, are positive for microbial organisms by amniotic fluid culture. Interestingly, in some pregnancies complicated by preterm premature rupture of membranes (pPROM), membranes heal spontaneously and pregnancy continues until term. Here, we investigated mechanisms of amnion healing. Using a preclinical mouse model, we found that small ruptures of the fetal membrane closed within 72 h whereas healing of large ruptures was only 40%. Small rupture induced transient upregulation of cytokines whereas large ruptures elicited sustained upregulation of proinflammatory cytokines in the fetal membranes. Fetal macrophages from amniotic fluid were recruited to the wounded amnion where macrophage adhesion molecules were highly expressed. Recruited macrophages released limited and well-localized amounts of IL-1β and TNF which facilitated epithelial-mesenchymal transition (EMT) and epithelial cell migration. Arg1 + macrophages dominated within 24 h. Migration and healing of the amnion mesenchymal compartment, however, remained compromised. These findings provide novel insights regarding unique healing mechanisms of amnion.

Hematopoietic-to-mesenchymal transition of adipose tissue macrophages is regulated by integrin β1 and fabricated fibrin matrices

Some bona fide adult adipocytes arise de novo from a bone marrow-derived myeloid lineage. These studies further demonstrate that adipose tissue stroma contains a resident population of myeloid cells capable of adipocyte and multilineage mesenchymal differentiation. These resident myeloid cells lack hematopoietic markers and express mesenchymal and progenitor cell markers. Because bone marrow mesenchymal progenitor cells have not been shown to enter the circulation, we hypothesized that myeloid cells acquire mesenchymal differentiation capacity in adipose tissue. We fabricated a 3-dimensional fibrin matrix culture system to define the adipose differentiation potential of adipose tissue-resident myeloid subpopulations, including macrophages, granulocytes and dendritic cells. Our data show that multilineage mesenchymal potential was limited to adipose tissue macrophages, characterized by the acquisition of adipocyte, osteoblast, chondrocyte and skeletal muscle myocyte phenotypes. Fibrin hydrogel matrices stimulated macrophage loss of hematopoietic cell lineage determinants and the expression of mesenchymal and progenitor cell markers, including integrin β1. Ablation of integrin β1 in macrophages inhibited adipocyte specification. Therefore, some bona fide adipocytes are specifically derived from adipose tissue-resident macrophages via an integrin β1-dependent hematopoietic-to-mesenchymal transition, whereby they become capable of multipotent mesenchymal differentiation. The requirement for integrin β1 highlights this molecule as a potential target for controlling the production of marrow-derived adipocytes and their contribution to adipose tissue development and function.

Mechanisms of hepatic stellate cell activation

Hepatic fibrosis is a dynamic process characterized by the net accumulation of extracellular matrix resulting from chronic liver injury of any aetiology, including viral infection, alcoholic liver disease and NASH. Activation of hepatic stellate cells (HSCs) - transdifferentiation of quiescent, vitamin-A-storing cells into proliferative, fibrogenic myofibroblasts - is now well established as a central driver of fibrosis in experimental and human liver injury. Yet, the continued discovery of novel pathways and mediators, including autophagy, endoplasmic reticulum stress, oxidative stress, retinol and cholesterol metabolism, epigenetics and receptor-mediated signals, reveals the complexity of HSC activation. Extracellular signals from resident and inflammatory cells including macrophages, hepatocytes, liver sinusoidal endothelial cells, natural killer cells, natural killer T cells, platelets and B cells further modulate HSC activation. Finally, pathways of HSC clearance have been greatly clarified, and include apoptosis, senescence and reversion to an inactivated state. Collectively, these findings reinforce the remarkable complexity and plasticity of HSC activation, and underscore the value of clarifying its regulation in hopes of advancing the development of novel diagnostics and therapies for liver disease.

Eya2, a Target Activated by Plzf, Is Critical for PLZF-RARA-Induced Leukemogenesis

PLZF is a transcription factor that confers aberrant self-renewal in leukemogenesis, and the PLZF-RARA fusion gene causes acute promyelocytic leukemia (APL) through differentiation block. However, the molecular mechanisms of aberrant self-renewal underlying PLZF-mediated leukemogenesis are poorly understood. To investigate these mechanisms, comprehensive expression profiling of mouse hematopoietic stem/progenitor cells transduced with Plzf was performed, which revealed the involvement of a key transcriptional coactivator, Eya2, a target molecule shared by Plzf and PLZF-RARA, in the aberrant self-renewal. Indeed, PLZF-RARA as well as Plzf rendered those cells immortalized through upregulation of Eya2. Eya2 also led to immortalization without differentiation block, while depletion of Eya2 suppressed clonogenicity in cells immortalized by PLZF-RARA without influence on differentiation and apoptosis. Interestingly, cancer outlier profile analysis of human samples of acute myeloid leukemia (AML) in The Cancer Genome Atlas (TCGA) revealed a subtype of AML that strongly expressed EYA2 In addition, gene set enrichment analysis of human AML samples, including TCGA data, showed that this subtype of AML was more closely associated with the properties of leukemic stem cells in its gene expression signature than other AMLs. Therefore, EYA2 may be a target for molecular therapy in this subtype of AML, including PLZF-RARA APL.

The stellate cell system (vitamin A-storing cell system)

Past, present, and future research into hepatic stellate cells (HSCs, also called vitamin A-storing cells, lipocytes, interstitial cells, fat-storing cells, or Ito cells) are summarized and discussed in this review. Kupffer discovered black-stained cells in the liver using the gold chloride method and named them stellate cells (Sternzellen in German) in 1876. Wake rediscovered the cells in 1971 using the same gold chloride method and various modern histological techniques including electron microscopy. Between their discovery and rediscovery, HSCs disappeared from the research history. Their identification, the establishment of cell isolation and culture methods, and the development of cellular and molecular biological techniques promoted HSC research after their rediscovery. In mammals, HSCs exist in the space between liver parenchymal cells (PCs) or hepatocytes and liver sinusoidal endothelial cells (LSECs) of the hepatic lobule, and store 50-80% of all vitamin A in the body as retinyl ester in lipid droplets in the cytoplasm. SCs also exist in extrahepatic organs such as pancreas, lung, and kidney. Hepatic (HSCs) and extrahepatic stellate cells (EHSCs) form the stellate cell (SC) system or SC family; the main storage site of vitamin A in the body is HSCs in the liver. In pathological conditions such as liver fibrosis, HSCs lose vitamin A, and synthesize a large amount of extracellular matrix (ECM) components including collagen, proteoglycan, glycosaminoglycan, and adhesive glycoproteins. The morphology of these cells also changes from the star-shaped HSCs to that of fibroblasts or myofibroblasts.

Hepatic Stellate Cells and microRNAs in Pathogenesis of Liver Fibrosis

microRNAs (miRNAs) are small non-coding RNAs that regulate gene expression by either blocking translation or inducing degradation of target mRNA. miRNAs play essential roles in diverse biological and pathological processes, including development of hepatic fibrosis. Hepatic stellate cells (HSCs) play a central role in development of hepatic fibrosis and there are intricate regulatory effects of miRNAs on their activation, proliferation, collagen production, migration, and apoptosis. There are multiple differentially expressed miRNAs in activated HSCs, and in this review we aim to summarize current data on miRNAs that participate in the development of hepatic fibrosis. Based on this review, miRNAs may serve as biomarkers for diagnosis of liver disease, as well as markers of disease progression. Most importantly, dysregulated miRNAs may potentially be targeted by novel therapies to treat and reverse progression of hepatic fibrosis.

Isolation and Colony Formation of Murine Bone and Bone Marrow Cells

Adult homeostasis is dependent on normal Wt1 expression. Loss of Wt1 expression in adult mice causes rapid loss of the mesenchymal tissues, fat and bone, amongst other phenotypes. Bone and bone marrow mesenchymal stromal cells can be studied by cell isolation and expansion. The stemness of these cells can then be characterized by carrying out a colony-forming unit-fibroblast assay and observing clonogenic capabilities.

Bone Marrow Stem Cell Contribution to Pulmonary Homeostasis and Disease

The understanding of bone marrow stem cell plasticity and contribution of bone marrow stem cells to pathophysiology is evolving with the advent of innovative technologies. Recent data has led to new mechanistic insights in the field of mesenchymal stem cell (MSC) research, and an increased appreciation for the plasticity of the hematopoietic stem cell (HSC). In this review, we discuss current research examining the origin of pulmonary cell types from endogenous lung stem and progenitor cells as well as bone marrow-derived stem cells (MSCs and HSCs) and their contributions to lung homeostasis and pathology. We specifically highlight recent findings from our laboratory that demonstrate an HSC origin for pulmonary fibroblasts based on transplantation of a clonal population of cells derived from a single HSC. These findings demonstrate the importance of developing an understanding of the sources of effector cells in disease state. Finally, a perspective is given on the potential clinical implications of these studies and others addressing stem cell contributions to lung tissue homeostasis and pathology.

The development of hepatic stellate cells in normal and abnormal human fetuses - an immunohistochemical study

The precise embryological origin and development of hepatic stellate cells is not established. Animal studies and observations on human fetuses suggest that they derive from posterior mesodermal cells that migrate via the septum transversum and developing diaphragm to form submesothelial cells beneath the liver capsule, which give rise to mesenchymal cells including hepatic stellate cells. However, it is unclear if these are similar to hepatic stellate cells in adults or if this is the only source of stellate cells. We have studied hepatic stellate cells by immunohistochemistry, in developing human liver from autopsies of fetuses with and without malformations and growth restriction, using cellular Retinol Binding Protein-1 (cRBP-1), Glial Fibrillary Acidic Protein (GFAP), and α-Smooth Muscle Actin (αSMA) antibodies, to identify factors that influence their development. We found that hepatic stellate cells expressing cRBP-1 are present from the end of the first trimester of gestation and reduce in density throughout gestation. They appear abnormally formed and variably reduced in number in fetuses with abnormal mesothelial Wilms Tumor 1 (WT1) function, diaphragmatic hernia and in ectopic liver nodules without mesothelium. Stellate cells showed similarities to intravascular cells and their presence in a fetus with diaphragm agenesis suggests they may be derived from circulating stem cells. Our observations suggest circulating stem cells as well as mesothelium can give rise to hepatic stellate cells, and that they require normal mesothelial function for their development.

Experimental models of liver fibrosis

Hepatic fibrosis is a wound healing response to insults and as such affects the entire world population. In industrialized countries, the main causes of liver fibrosis include alcohol abuse, chronic hepatitis virus infection and non-alcoholic steatohepatitis. A central event in liver fibrosis is the activation of hepatic stellate cells, which is triggered by a plethora of signaling pathways. Liver fibrosis can progress into more severe stages, known as cirrhosis, when liver acini are substituted by nodules, and further to hepatocellular carcinoma. Considerable efforts are currently devoted to liver fibrosis research, not only with the goal of further elucidating the molecular mechanisms that drive this disease, but equally in view of establishing effective diagnostic and therapeutic strategies. The present paper provides a state-of-the-art overview of in vivo and in vitro models used in the field of experimental liver fibrosis research.

Pathophysiology of Portal Hypertension

The bases of our current knowledge on the physiology of the hepatic portal system are largely owed to the work of three pioneering vascular researchers from the sixteenth and the seventeenth centuries: A. Vesalius, W. Harvey, and F. Glisson. Vesalius is referred to as the founder of modern human anatomy, and in his influential book, De humani corporis fabrica libri septem, he elaborated the first anatomical atlas of the hepatic portal venous system (Vesalius 2013). Sir William Harvey laid the foundations of modern cardiovascular research with his Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (Harvey 1931) in which he established the nature of blood circulation. Finally, F. Glisson characterized the gastrointestinal-hepatic vascular system (Child 1955). These physiological descriptions were later complemented with clinical observations. In the eighteenth and nineteenth centuries, Morgagni, Puckelt, Cruveilhier, and Osler were the first to make the connection between common hepatic complications – ascites, splenomegaly, and gastrointestinal bleeding – and obstruction of the portal system (Sandblom 1993). These were the foundations that allowed Gilbert, Villaret, and Thompson to establish an early definition of portal hypertension at the beginning of the twentieth century. In this period, Thompson performed the first direct measurement of portal pressure by laparotomy in some patients (Gilbert and Villaret 1906; Thompson et al. 1937). Considering all these milestones, and paraphrasing Sir Isaac Newton, if hepatologists have seen further, it is by standing on the shoulders of giants.

Nowadays, our understanding of the pathogenesis of portal hypertension has largely improved thanks to the progress in preclinical and clinical research. However, this field is ever-changing and hepatologists are continually identifying novel pathological mechanisms and developing new therapeutic strategies for this clinical condition. Hence, the aim of this chapter is to summarize the current knowledge about this clinical condition.

Early to Phase II drugs currently under investigation for the treatment of liver fibrosis

INTRODUCTION

Chronic liver diseases represent a high unmet medical need and are characterized by persistent inflammation, parenchymal damage and fibrotic remodeling, leading eventually to cirrhosis and hepatic failure. Besides the persisting high prevalence of chronic viral hepatitis B and C, the dramatic increase in nonalcoholic steatohepatitis is now considered to be a major pathophysiologic driver for fibrosis development and subsequently cirrhosis. Increasing evidence suggests that also liver cirrhosis can regress when treated adequately.

AREAS COVERED

Herein, the authors review the underlying pathophysiologic mechanisms leading to fibrotic remodeling in the liver. They also highlight the options for novel treatment strategies by using molecular targeted agents.

EXPERT OPINION

New in vitro and preclinical animal models, and the careful selection of patients with high disease dynamics for clinical studies, provide a sound basis for the clinical development of antifibrotic agents in humans. Surrogate parameters of liver function, inflammation, tissue remodeling and damage, as well as noninvasive imaging techniques, can be applied in clinical trials to provide fast readouts and novel and reliable endpoints for trial design, and provide an attractive regulatory environment for this emerging disease area.

Cell-type-resolved quantitative proteomics of murine liver

Mass spectrometry (MS)-based proteomics provides a powerful approach to globally investigate the biological function of individual cell types in mammalian organs. Here, we applied this technology to the in-depth analysis of purified hepatic cell types from mouse. We quantified 11,520 proteins, making this the most comprehensive proteomic resource of any organ to date. Global protein copy number determination demonstrated that a large proportion of the hepatocyte proteome is dedicated to fatty acid and xenobiotic metabolism. We identified as-yet-unknown components of the TGF-β signaling pathway and extracellular matrix in hepatic stellate cells, uncovering their regulative role in liver physiology. Moreover, our high-resolution proteomic data set enabled us to compare the distinct functional roles of hepatic cell types in cholesterol flux, cellular trafficking, and growth factor receptor signaling. This study provides a comprehensive resource for liver biology and biomedicine.

Hepatic stellate cells and liver fibrosis

Hepatic stellate cells are resident perisinusoidal cells distributed throughout the liver, with a remarkable range of functions in normal and injured liver. Derived embryologically from septum transversum mesenchyme, their precursors include submesothelial cells that invade the liver parenchyma from the hepatic capsule. In normal adult liver, their most characteristic feature is the presence of cytoplasmic perinuclear droplets that are laden with retinyl (vitamin A) esters. Normal stellate cells display several patterns of intermediate filaments expression (e.g., desmin, vimentin, and/or glial fibrillary acidic protein) suggesting that there are subpopulations within this parental cell type. In the normal liver, stellate cells participate in retinoid storage, vasoregulation through endothelial cell interactions, extracellular matrix homeostasis, drug detoxification, immunotolerance, and possibly the preservation of hepatocyte mass through secretion of mitogens including hepatocyte growth factor. During liver injury, stellate cells activate into alpha smooth muscle actin-expressing contractile myofibroblasts, which contribute to vascular distortion and increased vascular resistance, thereby promoting portal hypertension. Other features of stellate cell activation include mitogen-mediated proliferation, increased fibrogenesis driven by connective tissue growth factor, and transforming growth factor beta 1, amplified inflammation and immunoregulation, and altered matrix degradation. Evolving areas of interest in stellate cell biology seek to understand mechanisms of their clearance during fibrosis resolution by either apoptosis, senescence, or reversion, and their contribution to hepatic stem cell amplification, regeneration, and hepatocellular cancer.

Fibrogenesis and Carcinogenesis in Nonalcoholic Steatohepatitis (NASH): Involvement of Matrix Metalloproteinases (MMPs) and Tissue Inhibitors of Metalloproteinase (TIMPs)

Nonalcoholic steatohepatitis (NASH) is emerging worldwide because life-styles have changed to include much over-eating and less physical activity. The clinical and pathophysiological features of NASH are very different from those of HBV- and HCV-chronic liver diseases. The prognosis of NASH is worse among those with nonalcoholic fatty liver diseases (NAFLD), and some NASH patients show HCC with or without cirrhosis. In the present review we discuss fibrogenesis and the relationship between fibrosis and HCC occurrence in NASH to clarify the role of MMPs and TIMPs in both mechanisms. Previously we proposed MMP and TIMP expression in the multi-step occurrence of HCC from the literature based on viral-derived HCC. We introduce again these expressions during hepatocarcinogenesis and compare them to those in NASH-derived HCC, although the relationship with hepatic stem/progenitor cells (HPCs) invasion remains unknown. Signal transduction of MMPs and TIMPs is also discussed because it is valuable for the prevention and treatment of NASH and NASH-derived HCC.

Monocytes infiltrate the pancreas via the MCP-1/CCR2 pathway and differentiate into stellate cells

Recent studies have shown that monocytes possess pluripotent plasticity. We previously reported that monocytes could differentiate into hepatic stellate cells. Although stellate cells are also present in the pancreas, their origin remains unclear. An accumulation of enhanced green fluorescent protein (EGFP)⁺CD45^– cells was observed in the pancreases and livers of chimeric mice, which were transplanted with a single hematopoietic stem cell isolated from EGFP-transgenic mice and treated with carbon tetrachloride (CCl₄). Because the vast majority of EGFP⁺CD45^– cells in the pancreas expressed stellate cell-associated antigens such as vimentin, desmin, glial fibrillary acidic protein, procollagen-I, and α-smooth muscle actin, they were characterized as pancreatic stellate cells (PaSCs). EGFP⁺ PaSCs were also observed in CCl₄-treated mice adoptively transferred with monocytes but not with other cell lineages isolated from EGFP-transgenic mice. The expression of monocyte chemoattractant protein-1 (MCP-1) and angiotensin II (Ang II) increased in the pancreas of CCl₄-treated mice and their respective receptors, C-C chemokine receptor 2 (CCR2) and Ang II type 1 receptor (AT1R), were expressed on Ly6C^high monocytes isolated from EGFP-transgenic mice. We examined the effect of an AT1R antagonist, irbesartan, which is also a CCR2 antagonist, on the migration of monocytes into the pancreas. Monocytes migrated toward MCP-1 but not Ang II in vitro. Irbesartan inhibited not only their in vitro chemotaxis but also in vivo migration of adoptively transferred monocytes from peripheral blood into the pancreas. Irbesartan treatment significantly reduced the numbers of EGFP⁺F4/80⁺CCR2⁺ monocytic cells and EGFP⁺ PaSCs in the pancreas of CCl₄-treated chimeric mice receiving EGFP⁺ bone marrow cells. A specific CCR2 antagonist RS504393 inhibited the occurrence of EGFP⁺ PaSCs in injured mice. We propose that CCR2⁺ monocytes migrate into the pancreas possibly via the MCP-1/CCR2 pathway and give rise to PaSCs.

Roles of EXTL2, a member of the EXT family of tumour suppressors, in liver injury and regeneration processes

The gene products of two members of the EXT (exostosin) gene family, EXT1 and EXT2, function together as a polymerase in the biosynthesis of heparan sulfate. EXTL2 (EXT-like 2), one of the three EXTL genes in the human genome that are homologous to EXT1 and EXT2, encodes an N-acetylhexosaminyltransferase. We have demonstrated that EXTL2 terminates chain elongation of GAGs (glycosaminoglycans), and thereby regulates GAG biosynthesis. The abnormal GAG biosynthesis caused by loss of EXTL2 had no effect on normal development or normal adult homoeostasis. Therefore we examined the role of EXTL2 in CCl4 (carbon tetrachloride)-induced liver failure, a model of liver disease. On the fifth day after CCl4 administration, the liver/body weight ratio was significantly smaller for EXTL2-knockout mice than for wild-type mice. Consistent with this observation, hepatocyte proliferation following CCl4 treatment was lower in EXTL2-knockout mice than in wild-type mice. EXTL2-knockout mice experienced less HGF (hepatocyte growth factor)-mediated signalling than wild-type mice specifically because GAG synthesis was altered in these mutant mice. In addition, GAG synthesis in hepatic stellate cells was up-regulated during liver repair in EXTL2-knockout mice. Taken together, the results of the present study indicated that EXTL2-mediated regulation of GAG synthesis was important to the tissue regeneration processes that follow liver injury.

Plzf drives MLL-fusion-mediated leukemogenesis specifically in long-term hematopoietic stem cells

Oncogenic transformation requires unlimited self-renewal. Currently, it remains unclear whether a normal capacity for self-renewal is required for acquiring an aberrant self-renewal capacity. Our results in a new conditional transgenic mouse showed that a mixed lineage leukemia (MLL) fusion oncogene, MLL-ENL, at an endogenous-like expression level led to leukemic transformation selectively in a restricted subpopulation of hematopoietic stem cells (HSCs) through upregulation of promyelocytic leukemia zinc finger (Plzf). Interestingly, forced expression of Plzf itself immortalized HSCs and myeloid progenitors in vitro without upregulation of Hoxa9/Meis1, which are well-known targets of MLL fusion proteins, whereas its mutant lacking the BTB/POZ domain did not. In contrast, depletion of Plzf suppressed the MLL-fusion-induced leukemic transformation of HSCs in vitro and in vivo. Gene expression analyses of human clinical samples showed that a subtype of PLZF-high MLL-rearranged myeloid leukemia cells was closely associated with the gene expression signature of HSCs. These findings suggested that MLL fusion protein enhances the self-renewal potential of normal HSCs to develop leukemia, in part through a Plzf-driven self-renewal program.

Hepatic stellate cells in liver development, regeneration, and cancer

Hepatic stellate cells are liver-specific mesenchymal cells that play vital roles in liver physiology and fibrogenesis. They are located in the space of Disse and maintain close interactions with sinusoidal endothelial cells and hepatic epithelial cells. It is becoming increasingly clear that hepatic stellate cells have a profound impact on the differentiation, proliferation, and morphogenesis of other hepatic cell types during liver development and regeneration. In this Review, we summarize and evaluate the recent advances in our understanding of the formation and characteristics of hepatic stellate cells, as well as their function in liver development, regeneration, and cancer. We also discuss how improved knowledge of these processes offers new perspectives for the treatment of patients with liver diseases.

In vivo tracking and comparison of the therapeutic effects of MSCs and HSCs for liver injury

Background

Mesenchymal stem cells (MSCs) and hematopoietic stem cells (HSCs) have been studied for damaged liver repair; however, the conclusions drawn regarding their homing capacity to the injured liver are conflicting. Besides, the relative utility and synergistic effects of these two cell types on the injured liver remain unclear.

Methodology/Principal Findings

MSCs, HSCs and the combination of both cells were obtained from the bone marrow of male mice expressing enhanced green fluorescent protein(EGFP)and injected into the female mice with or without liver fibrosis. The distribution of the stem cells, survival rates, liver function, hepatocyte regeneration, growth factors and cytokines of the recipient mice were analyzed. We found that the liver content of the EGFP-donor cells was significantly higher in the MSCs group than in the HSCs or MSCs+HSCs group. The survival rate for the MSCs group was significantly higher than that of the HSCs or MSCs+HSCs group; all surpassed the control group. After MSC-transplantation, the injured livers were maximally restored, with less collagen than the controls. The fibrotic areas had decreased to a lesser extent in the mice transplanted with HSCs or MSCs+HSCs. Compared with mice in the HSCs group, the mice that received MSCs had better improved liver function. MSCs exhibited more remarkable paracrine effects and immunomodulatory properties on hepatic stellate cells and native hepatocytes in the treatment of the liver pathology. Synergistic actions of MSCs and HSCs were most likely not observed because the stem cells in liver were detected mostly as single cells, and single MSCs are insufficient to provide a beneficial niche for HSCs.

Conclusions/Significance

MSCs exhibited a greater homing capability for the injured liver and modulated fibrosis and inflammation more effectively than did HSCs. Synergistic effects of MSCs and HSCs were not observed in liver injury.

Hematopoietic stem cells are pluripotent and not just "hematopoietic"

Over a decade ago, several preclinical transplantation studies suggested the striking concept of the tissue-reconstituting ability (often referred to as HSC plasticity) of hematopoietic stem cells (HSCs). While this heralded an exciting time of radically new therapies for disorders of many organs and tissues, the concept was soon mired in controversy and remained dormant for almost a decade. This commentary provides a concise review of evidence for HSC plasticity, including more recent findings based on single HSC transplantation in mouse and clinical transplantation studies. There is strong evidence for the concept that HSCs are pluripotent and are the source for the majority, if not all, of the cell types in our body. Also discussed are some biological and experimental issues that need to be considered in the future investigation of HSC plasticity.

Bone marrow progenitor cells do not contribute to liver fibrogenic cells

AIM

To investigate the contribution of bone marrow (BM) cells to hepatic fibrosis.

METHODS

To establish a model of chimerism, C57Bl/6 female mice were subjected to full-body irradiation (7 Gy) resulting in BM myeloablation. BM mononuclear cells obtained from male transgenic mice expressing enhanced green fluorescent protein (GFP) were used for reconstitution. Engraftment was confirmed by flow cytometry. To induce liver injury, chimeric animals received carbon tetrachloride (CCl(4)) 0.5 mL/kg intraperitoneally twice a week for 30 d (CCl(4) 30 d) and age-matched controls received saline (Saline 30 d). At the end of this period, animals were sacrificed for post mortem analysis. Liver samples were stained with hematoxylin and eosin to observe liver architectural changes and with Sirius red for collagen quantification by morphometric analysis. α-smooth muscle actin (α-SMA) was analyzed by confocal microscopy to identify GFP+ cells with myofibroblast (MF) characteristics. Liver tissue, BM and peripheral blood were collected and prepared for flow cytometric analysis using specific markers for detection of hepatic stellate cells (HSCs) and precursors from the BM.

RESULTS

Injury to the liver induced changes in the hepatic parenchymal architecture, as reflected by the presence of inflammatory infiltrate and an increase in collagen deposition (Saline 30 d = 11.10% ± 1.12% vs CCl(4) 30 d = 12.60% ± 0.73%, P = 0.0329). Confocal microscopy revealed increased reactivity against α-SMA in CCl(4) 30 d compared to Saline 30 d, but there was no co-localization with GFP+ cells, suggesting that cells from BM do not differentiate to MFs. Liver flow cytometric analysis showed a significant increase of CD45+/GFP+ cells in liver tissue (Saline 30 d = 3.2% ± 2.2% vs CCl(4) 30 d = 5.8% ± 1.3%, P = 0.0458), suggesting that this increase was due to inflammatory cell infiltration (neutrophils and monocytes). There was also a significant increase of common myeloid progenitor cells (CD117+/CD45+) in the livers of CCl(4)-treated animals (Saline 30 d = 2.16% ± 1.80% vs CCl(4) 30 d = 5.60% ± 1.30%, P = 0.0142). In addition the GFP-/CD38+/CD45- subpopulation was significantly increased in the CCl(4) 30 d group compared to the Saline 30 d group (17.5% ± 3.9% vs 9.3% ± 2.4%, P = 0.004), indicating that the increase in the activated HSC subpopulation was not of BM origin.

CONCLUSION

BM progenitor cells do not contribute to fibrosis, but there is a high recruitment of inflammatory cells that stimulates HSCs and MFs of liver origin.

Sustained telomere length in hepatocytes and cholangiocytes with increasing age in normal liver

Telomeres, a validated biomarker of aging, comprise multiple nucleotide repeats capping chromosomes that shorten with each cell cycle until a critical length is achieved, precipitating cell senescence. Only two previous studies focused on the effect of aging in "normal" liver tissue, but these studies were compromised by small sample size, limited age range, tissue derived from individuals with an increased risk of senescence, and the use of liver homogenates. We developed a robust large-volume, four-color quantitative fluorescent in situ hybridization technique to measure telomere length in large numbers of hepatocytes, Kupffer cells, hepatic stellate cells, CD4-positive and CD8-positive lymphocytes, and cholangiocytes. Following validation against the gold standard (Southern blotting), the technique was applied to normal archived paraffin-embedded liver tissue obtained following reperfusion of implanted donor liver. We studied 73 highly selected donors aged 5-79 years with a short medical illness preceding death and no history of liver disease, reperfusion injury, or steatosis and normal graft function 1-year posttransplantation. Cholangiocytes had significantly longer telomeres compared with all other intrahepatic lineages over a wide age range (P < 0.05). Age-related telomere attrition was restricted to sinusoidal cells (i.e., Kupffer cells [P = 0.0054] and stellate cells [P = 0.0001]). Cholangiocytes and hepatocytes showed no age-related telomere shortening.

CONCLUSION

In normal liver and over a broad age range, cholangiocytes have longer telomeres than all other intrahepatic lineages. Age-related telomere length decline is restricted to Kupffer cells and stellate cells.

The differentiation of MSCs into functional hepatocyte-like cells in a liver biomatrix scaffold and their transplantation into liver-fibrotic mice

Hepatocytes derived from mesenchymal stem cells (MSCs) hold great potential for cell-based therapies for liver diseases. The cell-based therapies are critically dependent on the hepatic differentiation of the MSCs with a high efficiency and on a considerable scale. Recent results have shown that decellularized organs provide a three-dimensional extracellular matrix for the lineage restriction of stem cell maturation. In this study, we compared the cell proliferation and hepatic differentiation of murine MSCs in a biomatrix scaffold from rat liver and in the presence and absence growth factors (GF) with a two-dimensional substrate. In the absence or presence of GF, the dynamic cultured scaffold (DCS) stimulated the MSCs to express endodermal and hepatocyte-specific genes and proteins associated with improved functions, and the cells exhibited the ultrastructural characteristics of mature hepatocytes. When transplanted into CCl(4)-injured mice, the cells pretreated with a combination of the DCS and GF exhibited increased survival, liver function, engraftment into the host liver and further hepatic differentiation. The paracrine effect of the transplanted cells on hepatic stellate cells and native hepatocytes played a key role in the treatment of the liver pathology. These studies define an effective method that facilitates the hepatic differentiation of MSCs exhibiting extensive functions and support further research into the use of a decellularized liver matrix as a bioscaffold for liver tissue engineering.

Effect of Yiguanjian decoction on cell differentiation and proliferation in CCl4-treated mice

AIM: To investigate the cellular mechanisms of action of Yiguanjian (YGJ) decoction in treatment of chronic hepatic injury.

METHODS: One group of mice was irradiated, and received enhanced green fluorescent protein (EGFP)-positive bone marrow transplants followed by 13 wk of CCl₄ injection and 6 wk of oral YGJ administration. A second group of Institute for Cancer Research mice was treated with 13 wk of CCl₄ injection and 6 wk of oral YGJ administration. Liver function, histological changes in the liver, and Hyp content were analyzed. The expression of α-smooth muscle actin (α-SMA), F4/80, albumin (Alb), EGFP, mitogen-activated protein kinase-2 (PKM2), Ki-67, α fetoprotein (AFP), monocyte chemotaxis protein-1 and CC chemokine receptor 2 were assayed.

RESULTS: As hepatic damage progressed, EGFP-positive marrow cells migrated into the liver and were mainly distributed along the fibrous septa. They showed a conspicuous coexpression of EGFP with α-SMA and F4/80 but no coexpression with Alb. Moreover, the expression of PKM2, AFP and Ki-67 was enhanced dynamically and steadily over the course of liver injury. YGJ abrogated the increases in the number of bone marrow-derived fibrogenic cells in the liver, inhibited expression of both progenitor and mature hepatocyte markers, and reduced fibrogenesis.

CONCLUSION: YGJ decoction improves liver fibrosis by inhibiting the migration of bone marrow cells into the liver as well as inhibiting their differentiation and suppressing the proliferation of both progenitors and hepatocytes in the injured liver.

Interplay between bone marrow and liver in the pathogenesis of hepatic fibrosis

Recent advances in the technologies of both molecular biology and regenerative medicine have made it possible to identify bone marrow (BM)-derived cells migrating into various fibrotic organs including the liver. A number of studies have reported that BM-derived cells migrating into fibrotic liver tissue exhibit a myofibroblast-like phenotype and may participate in the progression of liver fibrosis. On the other hand, it has also been shown that BM-derived cells express matrix metalloproteinases and contribute to the regression of experimental liver fibrosis. These contradictory results may arise, at least in part, from the uncertainty of various different methods that have been used in those studies. In this review article, we describe the interplay between BM and liver in the progression and regression of liver fibrosis, with an emphasis on the necessity of qualified methods with high specificity and sensitivity to evaluate the role of BM-derived cells in collagen production.

Hepatic stellate cell progenitor cells

Hepatic stellate cells (HSCs) are recognized as a major player in liver fibrogenesis. Upon liver injury, HSCs differentiate into myofibroblasts and participate in progression of fibrosis and cirrhosis. Additional cell types such as resident liver fibroblasts/myofibroblasts or bone marrow cells are also known to generate myofibroblasts. One of the major obstacles to understanding the mechanism of liver fibrogenesis is the lack of knowledge regarding the developmental origin of HSCs and other liver mesenchymal cells. Recent cell lineage analyses demonstrate that HSCs are derived from mesoderm during liver development. MesP1-expressing mesoderm gives rise to the septum transversum mesenchyme before liver formation and then to the liver mesothelium and mesenchymal cells, including HSCs and perivascular mesenchymal cells around the veins during liver development. During the growth of embryonic liver, the mesothelium, consisting of mesothelial cells and submesothelial cells, migrates inward from the liver surface and gives rise to HSCs and perivascular mesenchymal cells, including portal fibroblasts, smooth muscle cells around the portal vein, and fibroblasts around the central vein. Cell lineage analyses indicate that mesothelial cells are HSC progenitor cells capable of differentiating into HSCs and other liver mesenchymal cells during liver development.

Acute multiple organ failure in adult mice deleted for the developmental regulator Wt1

There is much interest in the mechanisms that regulate adult tissue homeostasis and their relationship to processes governing foetal development. Mice deleted for the Wilms' tumour gene, Wt1, lack kidneys, gonads, and spleen and die at mid-gestation due to defective coronary vasculature. Wt1 is vital for maintaining the mesenchymal–epithelial balance in these tissues and is required for the epithelial-to-mesenchyme transition (EMT) that generates coronary vascular progenitors. Although Wt1 is only expressed in rare cell populations in adults including glomerular podocytes, 1% of bone marrow cells, and mesothelium, we hypothesised that this might be important for homeostasis of adult tissues; hence, we deleted the gene ubiquitously in young and adult mice. Within just a few days, the mice suffered glomerulosclerosis, atrophy of the exocrine pancreas and spleen, severe reduction in bone and fat, and failure of erythropoiesis. FACS and culture experiments showed that Wt1 has an intrinsic role in both haematopoietic and mesenchymal stem cell lineages and suggest that defects within these contribute to the phenotypes we observe. We propose that glomerulosclerosis arises in part through down regulation of nephrin, a known Wt1 target gene. Protein profiling in mutant serum showed that there was no systemic inflammatory or nutritional response in the mutant mice. However, there was a dramatic reduction in circulating IGF-1 levels, which is likely to contribute to the bone and fat phenotypes. The reduction of IGF-1 did not result from a decrease in circulating GH, and there is no apparent pathology of the pituitary and adrenal glands. These findings 1) suggest that Wt1 is a major regulator of the homeostasis of some adult tissues, through both local and systemic actions; 2) highlight the differences between foetal and adult tissue regulation; 3) point to the importance of adult mesenchyme in tissue turnover.

It is important to understand the cellular and molecular pathways that regulate the maintenance and turnover of adult tissues. These processes often go awry in diseases and are likely to deteriorate with ageing. Here we show that removal of a single gene, the Wilms' Tumour gene, Wt1, in the adult mouse leads to the extremely rapid deterioration of multiple tissues. Within 7–9 days after gene removal kidneys fail, the pancreas and spleen suffer severe atrophy, there is widespread loss of bone and body fat, and red blood cells are no longer produced. Our findings reveal the vulnerability of adult tissues, while opening up avenues for dissecting the pathways controlling tissue turnover. Further experiments showed that the tissue failure we observed is due both to local defects of stem/progenitor cell activities and to significant changes in the serum levels of some key master regulators. In particular there is a dramatic reduction in the levels of IGF-1, a key regulator of homeostasis and aging. Our studies also show that the control of adult tissue turnover may be different from that during foetal development. These findings have important implications for understanding and treating common human diseases.

Durable donor engraftment after radioimmunotherapy using α-emitter astatine-211-labeled anti-CD45 antibody for conditioning in allogeneic hematopoietic cell transplantation

To reduce toxicity associated with external γ-beam radiation, we investigated radioimmunotherapy with an anti-CD45 mAb labeled with the α-emitter, astatine-211 ((211)At), as a conditioning regimen in dog leukocyte antigen-identical hematopoietic cell transplantation (HCT). Dose-finding studies in 6 dogs treated with 100 to 618 μCi/kg (211)At-labeled anti-CD45 mAb (0.5 mg/kg) without HCT rescue demonstrated dose-dependent myelosuppression with subsequent autologous recovery, and transient liver toxicity in dogs treated with (211)At doses less than or equal to 405 μCi/kg. Higher doses of (211)At induced clinical liver failure. Subsequently, 8 dogs were conditioned with 155 to 625 μCi/kg (211)At-labeled anti-CD45 mAb (0.5 mg/kg) before HCT with dog leukocyte antigen-identical bone marrow followed by a short course of cyclosporine and mycophenolate mofetil immunosuppression. Neutropenia (1-146 cells/μL), lymphopenia (0-270 cells/μL), and thrombocytopenia (1500-6560 platelets/μL) with prompt recovery was observed. Seven dogs had long-term donor mononuclear cell chimerism (19%-58%), whereas 1 dog treated with the lowest (211)At dose (155 μCi/kg) had low donor mononuclear cell chimerism (5%). At the end of follow-up (18-53 weeks), only transient liver toxicity and no renal toxicity had been observed. In conclusion, conditioning with (211)At-labeled anti-CD45 mAb is safe and efficacious and provides a platform for future clinical trials of nonmyeloablative transplantation with radioimmunotherapy-based conditioning.

Recruitment of bone marrow-derived valve interstitial cells is a normal homeostatic process

Advances in understanding of the maintenance of the cardiac valves during normal cardiac function and response to injury have led to several novel findings, including that there is contribution of extra-cardiac cells to the major cellular population of the valve: the valve interstitial cell (VIC). While suggested to occur in human heart studies, we have been able to experimentally demonstrate, using a mouse model, that cells of bone marrow hematopoietic stem cell origin engraft into the valves and synthesize collagen type I. Based on these initial findings, we sought to further characterize this cell population in terms of its similarity to VICs and begin to elucidate its contribution to valve homeostasis. To accomplish this, chimeric mice whose bone marrow was repopulated with enhanced green fluorescent protein (EGFP) expressing total nucleated bone marrow cells were used to establish a profile of EGFP(+) valve cells in terms of their expression of hematopoietic antigens, progenitor markers, fibroblast- and myofibroblast-related molecules, as well as their distribution within the valves. Using this profile, we show that normal (non-irradiated, non-transplanted) mice have BM-derived cell populations that exhibit identical morphology and phenotype to those observed in transplanted mice. Collectively, our findings establish that the engraftment of bone marrow-derived cells occurs as part of normal valve homeostasis. Further, our efforts demonstrate that the use of myeloablative irradiation, which is commonly employed in studies involving bone marrow transplantation, does not elicit changes in the bone marrow-derived VIC phenotype in recipient mice.

Ly6C(+) monocytes are extrahepatic precursors of hepatic stellate cells in the injured liver of mice

OBJECTIVE

We previously reported that hepatic stellate cells (HpSCs) are of hematopoietic origin in liver injury. However, the immediate precursors of HpSCs remain unknown. This study was conducted to elucidate whether terminally differentiated blood cells can differentiate into HpSCs.

MATERIALS AND METHODS

We adoptively transferred a variety of cells isolated from enhanced green fluorescent protein (EGFP)-transgenic mice into carbon tetrachloride (CCl(4))-treated nontransgenic mice twice weekly for 2 weeks. We examined the presence of EGFP(+) HpSCs in the injured liver using immunofluorescence analysis.

RESULTS

Monocytes, neutrophils, eosinophils, B cells, or T cells from EGFP mice were transferred into CCl(4)-treated mice. Thirty percent of EGFP(+) cells in the livers of mice given Ly6C(high)c-kit(-) monocytes were negative for CD45, but were positive for glial fibrillary acidic protein, desmin, CD146, ADAMTS13, and α-smooth muscle actin, well-known markers of HpSCs. EGFP(+)CD45(-) cells were predominantly positive for glial fibrillary acidic protein. Although 48% of EGFP(+) cells were positive for procollagen type I, half of them were CD45(-). In the livers of mice given neutrophils, eosinophils, B cells, or T cells, all of the EGFP(+) cells were CD45(+). The majority of EGFP(+) cells in the nonparenchymal cell fraction purified from the livers of mice given Ly6C(high)c-kit(-) monocytes contained lipid droplets and were positive for glial fibrillary acidic protein, desmin, ADAMTS13, and procollagen type I. When Ly6C(+) monocyte-depleted peripheral blood total nucleated cells were adoptively transferred into CCl(4)-treated mice, we found no EGFP(+)CD45(-) cells in the liver.

CONCLUSIONS

These results suggest that Ly6C(+) monocytes can become HpSCs in the injured liver.

Hepatic stellate cell (vitamin A-storing cell) and its relative--past, present and future

HSCs (hepatic stellate cells) (also called vitamin A-storing cells, lipocytes, interstitial cells, fat-storing cells or Ito cells) exist in the space between parenchymal cells and liver sinusoidal endothelial cells of the hepatic lobule and store 50-80% of vitamin A in the whole body as retinyl palmitate in lipid droplets in the cytoplasm. In physiological conditions, these cells play pivotal roles in the regulation of vitamin A homoeostasis. In pathological conditions, such as hepatic fibrosis or liver cirrhosis, HSCs lose vitamin A and synthesize a large amount of extracellular matrix components including collagen, proteoglycan, glycosaminoglycan and adhesive glycoproteins. Morphology of these cells also changes from the star-shaped SCs (stellate cells) to that of fibroblasts or myofibroblasts. The hepatic SCs are now considered to be targets of therapy of hepatic fibrosis or liver cirrhosis. HSCs are activated by adhering to the parenchymal cells and lose stored vitamin A during hepatic regeneration. Vitamin A-storing cells exist in extrahepatic organs such as the pancreas, lungs, kidneys and intestines. Vitamin A-storing cells in the liver and extrahepatic organs form a cellular system. The research of the vitamin A-storing cells has developed and expanded vigorously. The past, present and future of the research of the vitamin A-storing cells (SCs) will be summarized and discussed in this review.

Interplay of cysteinyl leukotrienes and TGF-β in the activation of hepatic stellate cells from Schistosoma mansoni granulomas

Hepatic stellate cells (HSCs) have a critical role in liver physiology, and in the pathogenesis of liver inflammation and fibrosis. Here, we investigated the interplay between leukotrienes (LT) and TGF-β in the activation mechanisms of HSCs from schistosomal granulomas (GR-HSCs). First, we demonstrated that GR-HSCs express 5-lipoxygenase (5-LO), as detected by immunolocalization in whole cells and confirmed in cell lysates through western blotting and by mRNA expression through RT-PCR. Moreover, mRNA expression of 5-LO activating protein (FLAP) and LTC(4)-synthase was also documented, indicating that GR-HSCs have the molecular machinery required for LT synthesis. Morphological analysis of osmium and Oil-Red O-stained HSC revealed large numbers of small lipid droplets (also known as lipid bodies). We observed co-localization of lipid droplet protein marker (ADRP) and 5-LO by immunofluorescence microscopy. We demonstrated that GR-HSCs were able to spontaneously release cysteinyl-LTs (CysLTs), but not LTB(4,) into culture supernatants. CysLT production was highly enhanced after TGF-β-stimulation. Moreover, the 5-LO inhibitor zileuton and 5-LO gene deletion were able to inhibit the TGF-β-stimulated proliferation of GR-HSCs, suggesting a role for LTs in HSC activation. Here, we extend the immunoregulatory function of HSC by demonstrating that HSC from liver granulomas of schistosome-infected mouse are able to release Cys-LTs in a TGF-β-regulated manner, potentially impacting pathogenesis and liver fibrosis in schistosomiasis.

Evolving challenges in hepatic fibrosis

Continued elucidation of the mechanisms of hepatic fibrosis has yielded a comprehensive and nuanced portrait of fibrosis progression and regression. The paradigm of hepatic stellate cell (HSC) activation remains the foundation for defining events in hepatic fibrosis and has been complemented by progress in a number of new areas. Cellular sources of extracellular matrix beyond HSCs have been identified. In addition, the role of chemokine, adipokine, neuroendocrine, angiogenic and NAPDH oxidase signaling in the pathogenesis of hepatic fibrosis has been uncovered, as has the contribution of extracellular matrix stiffness to fibrogenesis. There is also increased awareness of the contribution of innate immunity and greater understanding of the complexity of gene regulation in HSCs and myofibroblasts. Finally, both apoptosis and senescence have been recognized as orchestrated programs that eliminate fibrogenic cells during resolution of liver fibrosis. Ironically, the progress that has been made has highlighted the growing disparity between advances in the experimental setting and their translation into new diagnostic tools and treatments. As a result, focus is shifting towards overcoming key translational challenges in order to accelerate the development of new therapies for patients with chronic liver disease.

Stem cell plasticity: recapping the decade, mapping the future

In slightly more than a decade of stem cell plasticity research, 24 peer-reviewed articles have demonstrated plasticity across organ and/or embryonic lineage boundaries at the single-cell level, with only 1 article showing negative results. These data, taken together with data about reversibility of gene restrictions that have also accumulated during the same period, indicate that postnatal cells, even "terminally differentiated" ones, have a degree of plasticity not appreciated previously. This review looks back at the four known pathways of cell plasticity and at previously described "plasticity principles" of Genomic Completeness, Cellular Uncertainty, Stochasticity of Cell Origin and Fate, relating these to issues of experimental design and discourse that are key to understanding and evaluating plasticity data. Although the physiologic roles played by such plasticity may still be debated, the manipulations of these phenomena for therapeutic or industrial purposes should finally be considered ripe for exploration. For the future, plasticity, indeed all stem cell biology, must be considered as part of a larger web of cell-to-cell and cell-to-matrix interactions that function fully only at the tissue level; thus, the success of stem cell biology necessarily must involve assembling data from cell and molecular biology research into systems of interactions that might be reasonably called "tissue biology." Interdisciplinary collaborations with complexity and chaos theorists, using mathematical/computer modeling of cell behaviors, will be vital to fully exploring stem cell behaviors in the coming decades.