Stem cells and liver repair

Potential advantages of genetically modified mesenchymal stem cells in the treatment of acute and chronic liver diseases

Liver damage caused by toxicity can lead to various severe conditions, such as acute liver failure (ALF), fibrogenesis, and cirrhosis. Among these, liver cirrhosis (LC) is recognized as the leading cause of liver-related deaths globally. Unfortunately, patients with progressive cirrhosis are often on a waiting list, with limited donor organs, postoperative complications, immune system side effects, and high financial costs being some of the factors restricting transplantation. Although the liver has some capacity for self-renewal due to the presence of stem cells, it is usually insufficient to prevent the progression of LC and ALF. One potential therapeutic approach to improving liver function is the transplantation of gene-engineered stem cells. Several types of mesenchymal stem cells from various sources have been suggested for stem cell therapy for liver disease. Genetic engineering is an effective strategy that enhances the regenerative potential of stem cells by releasing growth factors and cytokines. In this review, we primarily focus on the genetic engineering of stem cells to improve their ability to treat damaged liver function. We also recommend further research into accurate treatment methods that involve safe gene modification and long-term follow-up of patients to increase the effectiveness and reliability of these therapeutic strategies.

Krüppel-like factor 15 in liver diseases: Insights into metabolic reprogramming

Liver diseases, characterized by metabolic disorder, have become a global public health problem with high morbidity and mortality. Krüppel-like factor 15 (KLF15) is a zinc-finger transcription factor mainly enriched in liver. Increasing evidence suggests that hepatic KLF15 is activated rapidly during fasting, and contributes to the regulation of gluconeogenesis, lipid, amino acid catabolism, bile acids, endobiotic and xenobiotic metabolism. This review summarizes the latest advances of KLF15 in metabolic reprogramming, and explore the function of KLF15 in acute liver injury, hepatitis B virus, and autoimmune hepatitis. which aims to evaluate the potential of KLF15 as a therapeutic target and prognostic biomarker for liver diseases.

A study about immunomodulatory effect and efficacy and prognosis of human umbilical cord mesenchymal stem cells in patients with chronic hepatitis B-induced decompensated liver cirrhosis

BACKGROUND AND AIM

The aim of our study was to investigate the immunomodulatory effect and short-term efficacy and long-term prognosis of decompensated liver cirrhosis patients caused by hepatitis B after a double transplantation with human umbilical cord mesenchymal stem cells (hUCMSCs).

METHODS

Fifty inpatients were recruited and given the same medical treatments, receiving hUCMSCs injection intravenously. Fifty-three patients (Group B) matched for age, sex, and baseline alanine aminotransferase, aspartate aminotransferase, albumin, total bilirubin, prothrombin time, and model for end-stage liver disease score and Child-Pugh classification, acted as the control group.

RESULTS

Interleukin-6 and tumor necrosis factor alpha levels markedly decreased, and interleukin-10 level apparently increased in Group A at 2 and 4 weeks after treatment. Transforming growth factor beta in Group A increased more remarkably at 2 weeks after treatment. T4 cells and Treg cells in Group A were apparently higher than those in Group B at 2 and 4 weeks after treatment, and T8 cells and B cells were significantly lower than those in Group B. Aspartate aminotransferase levels in Group A were dramatically declining at 8 and 12 weeks after treatment. Levels of albumin, total bilirubin, and prothrombin time in Group A were apparently improved from 4 to 12 weeks after treatment. The improvements in model for end-stage liver disease and Child-Pugh scores in Group A were notably superior to those in Group B from 4 to 36 weeks after treatment. There were no remarkable differences in the incidence of developing liver failure throughout the follow-up period, but the mortality rate of Group A was lower than that of Group B.

CONCLUSION

This therapeutic method may be an appropriate choice for patients with decompensated liver cirrhosis.

Current Perspectives Regarding Stem Cell-Based Therapy for Liver Cirrhosis

Liver cirrhosis is a major cause of mortality and a common end of various progressive liver diseases. Since the effective treatment is currently limited to liver transplantation, stem cell-based therapy as an alternative has attracted interest due to promising results from preclinical and clinical studies. However, there is still much to be understood regarding the precise mechanisms of action. A number of stem cells from different origins have been employed for hepatic regeneration with different degrees of success. The present review presents a synopsis of stem cell research for the treatment of patients with liver cirrhosis according to the stem cell type. Clinical trials to date are summarized briefly. Finally, issues to be resolved and future perspectives are discussed with regard to clinical applications.

Expansion and Hepatic Differentiation of Adult Blood-Derived CD34+ Progenitor Cells and Promotion of Liver Regeneration After Acute Injury

A new group of blood-derived CD34⁺ progenitor cells (BDPCs) with the ability to expand and differentiate into functional hepatocyte-like cells and promote liver regeneration is reported. With their ease of access, application through the peripheral blood, and the capability of rapid expansion and hepatic differentiation, BDPCs have great potential as a cell-based therapy for liver disease.

The low availability of functional hepatocytes has been an unmet demand for basic scientific research, new drug development, and cell-based clinical applications for decades. Because of the inability to expand hepatocytes in vitro, alternative sources of hepatocytes are a focus of liver regenerative medicine. We report a new group of blood-derived CD34⁺ progenitor cells (BDPCs) that have the ability to expand and differentiate into functional hepatocyte-like cells and promote liver regeneration. BDPCs were obtained from the peripheral blood of an adult mouse with expression of surface markers CD34, CD45, Sca-1, c-kit, and Thy1.1. BDPCs can proliferate in vitro and differentiate into hepatocyte-like cells expressing hepatocyte markers, including CK8, CK18, CK19, α-fetoprotein, integrin-β1, and A6. The differentiated BDPCs (dBDPCs) also display liver-specific functional activities, such as glycogen storage, urea production, and albumin secretion. dBDPCs have cytochrome P450 activity and express specific hepatic transcription factors, such as hepatic nuclear factor 1α. To demonstrate liver regenerative activity, dBDPCs were injected into mice with severe acute liver damage caused by a high-dose injection of carbon tetrachloride (CCl₄). dBDPC treatment rescued the mice from severe acute liver injury, increased survival, and induced liver regeneration. Because of their ease of access and application through peripheral blood and their capability of rapid expansion and hepatic differentiation, BDPCs have great potential as a cell-based therapy for liver disease.

Significance

Hematopoietic stem/progenitor cell expansion and tissue-specific differentiation in vitro are challenges in regenerative medicine, although stem cell therapy has raised hope for the treatment of liver diseases by overcoming the scarcity of hepatocytes. This study identified and characterized a group of blood-derived progenitor cells (BDPCs) from the peripheral blood of an adult mouse. The CD34⁺ progenitor-dominant BDPCs were rapidly expanded and hepatically differentiated into functional hepatocyte-like cells with our established coculture system. BDPC treatment increased animal survival and produced full regeneration in a severe liver injury mouse model caused by CCl₄. BDPCs could have potential for liver cell therapies.

Cross effects of resveratrol and mesenchymal stem cells on liver regeneration and homing in partially hepatectomized rats

In this study, we examined the effect of preoperatively administered resveratrol (RV) and mesenchymal stem cells (MSCs) on regeneration of partially hepatectomized rat liver. We also evaluated the effect of RV on homing of MSCs. MSCs were isolated from bone marrow and cultured in vitro. Wistar albino rats were randomly divided into four groups. In groups, rats received (1) no treatment, (2) single dose RV, (3) MSCs and (4) RV plus MSCs before partial hepatectomy (PH). Injected MSCs were traced by labeling them with green fluorescent protein, and liver regeneration was determined by comparison of liver weight gain, histological examination and immunohistochemical staining with proliferating cell nuclear antigen (PCNA) for mitotic cells. The expression levels of tumor necrosis factor -alpha (TNF-α), interleukin-6 (IL-6) and hepatocyte growth factor (HGF) were also determined in the parafin sections of liver specimens with immunohistochemical staining. Administration of RV and MSCs separately or together enhanced liver regeneration despite decreasing the TNF-α and IL-6 expression. This positive contribution was probably due to direct raising effect on HGF for RV and HGF expression for MSCs that we demonstrated with immunohistochemical staining. Additionally, RV increased the homing of MSCs in liver probably related to life prolonging effect on MSCs. These results indicate that preoperative RV as well as MSCs application enhances liver regeneration after partial hepatectomy in rats. Paying attention to RV about the effect on liver regeneration and homing of MSCs might be the goal of further investigations.

Identification and Characterization of Lineage(-)CD45(-)Sca-1(+) VSEL Phenotypic Cells Residing in Adult Mouse Bone Tissue

Murine bone marrow (BM)-derived very small embryonic-like stem cells (BM VSELs), defined by a lineage-negative (Lin(-)), CD45-negative (CD45(-)), Sca-1-positive (Sca-1(+)) immunophenotype, were previously reported as postnatal pluripotent stem cells (SCs). We developed a highly efficient method for isolating Lin(-)CD45(-)Sca-1(+) small cells using enzymatic treatment of murine bone. We designated these cells as bone-derived VSELs (BD VSELs). The incidences of BM VSELs in the BM-derived nucleated cells and that of BD VSELs in bone-derived nucleated cells were 0.002% and 0.15%, respectively. These BD VSELs expressed a variety of hematopoietic stem cell (HSC), mesenchymal stem cell (MSC), and endothelial cell markers. The gene expression profile of the BD VSELs was clearly distinct from those of HSCs, MSCs, and ES cells. In the steady state, the BD VSELs proliferated slowly, however, the number of BD VSELs significantly increased in the bone after acute liver injury. Moreover, green fluorescent protein-mouse derived BD VSELs transplanted via tail vein injection after acute liver injury were detected in the liver parenchyma of recipient mice. Immunohistological analyses suggested that these BD VSELs might transdifferentiate into hepatocytes. This study demonstrated that the majority of the Lin(-)CD45(-)Sca-1(+) VSEL phenotypic cells reside in the bone rather than the BM. However, the immunophenotype and the gene expression profile of BD VSELs were clearly different from those of other types of SCs, including BM VSELs, MSCs, HSCs, and ES cells. Further studies will therefore be required to elucidate their cellular and/or SC characteristics and the potential relationship between BD VSELs and BM VSELs.

Stem cell-based regenerative opportunities for the liver: State of the art and beyond

The existing mismatch between the great demand for liver transplants and the number of available donor organs highlights the urgent need for alternative therapeutic strategies in patients with acute or chronic liver failure. The rapidly growing knowledge on stem cell biology and the intrinsic repair processes of the liver has opened new avenues for using stem cells as a cell therapy platform in regenerative medicine for hepatic diseases. An impressive number of cell types have been investigated as sources of liver regeneration: adult and fetal liver hepatocytes, intrahepatic stem cell populations, annex stem cells, adult bone marrow-derived hematopoietic stem cells, endothelial progenitor cells, mesenchymal stromal cells, embryonic stem cells, and induced pluripotent stem cells. All these highly different cell types, used either as cell suspensions or, in combination with biomaterials as implantable liver tissue constructs, have generated great promise for liver regeneration. However, fundamental questions still need to be addressed and critical hurdles to be overcome before liver cell therapy emerges. In this review, we summarize the state-of-the-art in the field of stem cell-based therapies for the liver along with existing challenges and future perspectives towards a successful liver cell therapy that will ultimately deliver its demanding goals.

Mesenchymal stem cell therapy for liver fibrosis

Currently, the most effective treatment for end-stage liver fibrosis is liver transplantation; however, transplantation is limited by a shortage of donor organs, surgical complications, immunological rejection, and high medical costs. Recently, mesenchymal stem cell (MSC) therapy has been suggested as an effective alternate approach for the treatment of hepatic diseases. MSCs have the potential to differentiate into hepatocytes, and therapeutic value exists in their immune-modulatory properties and secretion of trophic factors, such as growth factors and cytokines. In addition, MSCs can suppress inflammatory responses, reduce hepatocyte apoptosis, increase hepatocyte regeneration, regress liver fibrosis and enhance liver functionality. Despite these advantages, issues remain; MSCs also have fibrogenic potential and the capacity to promote tumor cell growth and oncogenicity. This paper summarizes the properties of MSCs for regenerative medicine and their therapeutic mechanisms and clinical application in the treatment of liver fibrosis. We also present several outstanding risks, including their fibrogenic potential and their capacity to promote pre-existing tumor cell growth and oncogenicity.

Hepatic progenitor cells in liver regeneration: current advances and clinical perspectives

When there is a massive loss of hepatocytes and/or an inhibition in the proliferative capacity of the mature hepatocytes, activation of a dormant cell population of resident hepatic progenitor cells (HPCs) occurs. Depending on the type of liver damage HPCs generate new hepatocytes and biliary cells to repopulate the liver placing them as potential candidates for cell therapy in human liver failure. Liver injury specific mechanisms through which HPCs differentiate towards mature epithelial cell types are recently become understood. Such new insights will enable us not only to direct HPCs behaviour for therapeutic purposes, but also to develop clinically feasible methods for in vivo differentiation of other stem cell types towards functional hepatocytes. This review aimed to provide the current improved knowledge of the role of HPCs niche and its signals in directing the behaviour and fate of HPCs and to translate this basic knowledge of HPCs activation/differentiation into its clinical applications.

Effect of bone marrow mesenchymal stem cell transplantation on acute hepatic failure in rats

The aim of the present study was to investigate the effectiveness of bone marrow mesenchymal stem cell (BMSC) transplantation in the treatment of acute hepatic failure (AHF) in rats. BMSCs were isolated from rat bone marrow, cultured and analyzed by flow cytometry. Following BMSC transplantation into rats with AHF, the levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), albumin (ALB), direct bilirubin (DBIL) and indirect bilirubin (IBIL) in the serum were measured using an automatic biochemical analyzer. Hematoxylin and eosin (H&E) staining and a terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay were performed to analyze the pathological changes and apoptosis rate. Levels of cluster of differentiation (CD)163 and interleukin (IL)-10 in the serum and liver tissue were detected by an enzyme-linked immunosorbent assay (ELISA) assay and western blot analysis. Compared with the levels in the control group, the serum levels of ALT, AST, DBIL, IBIL, CD163 and IL-10 in the BMSC transplantation groups were significantly lower at 120 and 168 h, while the serum levels of ALB were significantly higher at 168 h after BMSC transplantation. The pathological features of liver failure were alleviated by BMSC transplantation. The expression levels of CD163 and IL-10 in the liver tissue were also significantly decreased following transplantation. The results indicate that BMSCs have a therapeutic effect on AHF in rats, and CD163 and IL-10 may be used as sensitive serum prognosis indicators in the early assessment of patients following liver transplantation.

Stem cells for liver regeneration

The liver has a unique capacity to repair following injury, which is largely achieved by proliferation of hepatocytes. However, in situations of chronic or overwhelming liver injury, additional repair mechanisms, namely liver progenitor or oval cells, are activated. These cells, located in the canals of Hering, express markers for both hepatocytes and biliary cells and have the capacity to differentiate down both hepatocyte and biliary lineages. Previous work has suggested that the administration of autologous or allogeneic cell therapies such as haematopoietic or mesenchymal stem cells can augment liver repair by either stimulating endogenous repair mechanisms or by suppressing ongoing damage. A better understanding of how cell therapies can promote liver regeneration will lead to the refinement of these therapeutic approaches and also develop new pharmacological agents for liver repair.

Unbalanced distribution of materials: the art of giving rise to hepatocytes from liver stem/progenitor cells

Liver stem/progenitor cells (LSPCs) are able to duplicate themselves and differentiate into each type of cells in the liver, including mature hepatocytes and cholangiocytes. Understanding how to accurately control the hepatic differentiation of LSPCs is a challenge in many fields from preclinical to clinical treatments. This review summarizes the recent advances made to control the hepatic differentiation of LSPCs over the last few decades. The hepatic differentiation of LSPCs is a gradual process consisting of three main steps: initiation, progression and accomplishment. The unbalanced distribution of the affecting materials in each step results in the hepatic maturation of LSPCs. As the innovative and creative works for generating hepatocytes with full functions from LSPCs are gradually accumulated, LSPC therapies will soon be a new choice for treating liver diseases.

The road to regenerative liver therapies: the triumphs, trials and tribulations

The liver is one of the few organs that possess a high capacity to regenerate after liver failure or liver damage. The parenchymal cells of the liver, hepatocytes, contribute to the majority of the regeneration process. Thus, hepatocyte transplantation presents an alternative method to treating liver damage. However, shortage of hepatocytes and difficulties in maintaining primary hepatocytes still remain key obstacles that researchers must overcome before hepatocyte transplantation can be used in clinical practice. The unique properties of pluripotent stem cells (PSCs) and induced pluripotent stem cells (iPSCs) have provided an alternative approach to generating enough functional hepatocytes for cellular therapy. In this review, we will present a brief overview on the current state of hepatocyte differentiation from PSCs and iPSCs. Studies of liver regenerative processes using different cell sources (adult liver stem cells, hepatoblasts, hepatic progenitor cells, etc.) will be described in detail as well as how this knowledge can be applied towards optimizing culture conditions for the maintenance and differentiation of these cells towards hepatocytes. As the outlook of stem cell-derived therapy begins to look more plausible, researchers will need to address the challenges we must overcome in order to translate stem cell research to clinical applications.

Implication of hepatic stem cells in functional liver repopulation

The liver has an enormous potential to restore the parenchymal tissue loss due to injury. This is accomplished by the proliferation of either the hepatocytes or liver progenitor cells in cases where massive damage prohibits hepatocytes from entering the proliferative response. Under debate is still whether hepatic stem cells are involved in liver tissue maintenance and regeneration or even whether they exist at all. The definition of an adult tissue-resident stem cell comprises basic functional stem cell criteria like the potential of self-renewal, multipotent, i.e. at least bipotent differentiation capacity and serial transplantability featuring the ability of functional tissue repopulation. The relationship between a progenitor and its progeny should exemplify the lineage commitment from the putative stem cell to the differentiated cell. This is mainly assessed by lineage tracing and immunohistochemical identification of markers specific to progenitors and their descendants. Flow cytometry approaches revealed that the liver stem cell population in animals is likely to be heterogeneous giving rise to progeny with different molecular signatures, depending on the stimulus to activate the putative stem cell compartment. The stem cell criteria are met by a variety of cells identified in the fetal and adult liver both under normal and injury conditions. It is the purpose of this review to verify hepatic stem cell candidates in the light of the stem cell definition criteria mentioned. Also from this point of view adult stem cells from non-hepatic tissues such as bone marrow, umbilical cord blood or adipose tissue, have the potential to differentiate into cells featuring functional hepatocyte characteristics. This has great impact because it opens the possibility of generating hepatocyte-like cells from adult stem cells in a sufficient amount and quality for their therapeutical application to treat end-stage liver diseases by stem cell-based hepatocytes in place of whole organ transplantation.

Therapeutic potential of mesenchymal stem cells overexpressing human forkhead box A2 gene in the regeneration of damaged liver tissues

BACKGROUND AND AIM

Although a liver transplantation is considered to be the only effective long-term treatment in many cases of liver diseases, it is limited by a lack of donor organs and immune rejection. As an autologous stem cell approach, this study was conducted to assess whether forkhead box A2 (Foxa2) gene overexpression in bone marrow-derived mesenchymal stem cells (MSC) could protect the liver from hepatic diseases by stimulating tissue regeneration after cell transplantation.

METHODS

Rat MSC (rMSC) were isolated, characterized, and induced to hepatocytes that expressed liver-specific markers. Four different treatments (control [phosphate-buffered saline], rMSC alone, rMSC/pIRES-enhanced green fluorescent protein (EGFP) vector, and rMSC/pIRES-EGFP/human Foxa2) were injected into the spleen of carbon tetrachloride-injured rats. Biochemical and histological analyses on days 30, 60, and 90 post-transplantation were performed to evaluate the therapeutic capacities of MSC overexpressing hFoxa2.

RESULTS

rMSC transfected with hFoxa2 were induced into hepatogenic linage and expressed several liver-specific genes, such as, Foxa2, α-fetoprotein, cytokeratin-18, hepatocyte nuclear factor-1α, and hepatocyte growth factor. A group of animals treated with MSC/hFoxa2 showed significant recovery of liver-specific enzyme expressions to normal levels at the end of the study (90 days). Furthermore, when compared to the fibrotic areas of the samples treated with MSC alone or MSC/vector, the fibrotic area of the samples treated with rMSC/hFoxa2 for 90 days significantly decreased, until they were completely gone.

CONCLUSIONS

Human Foxa2 efficiently promoted the incorporation of MSC into liver grafts, suggesting that hFoxa2 genes could be used for the structural or functional recovery of damaged liver cells.

Copper-induced hepatitis: the COMMD1 deficient dog as a translational animal model for human chronic hepatitis

Chronic inflammatory liver disease regardless of aetiology leads to failing regeneration and fibrosis, ending in cirrhosis. Both in man and in animals this worldwide health problem has no definitive cure. Chronic liver injury causes hepatic stellate cells to proliferate and differentiate into matrix-producing cells. New therapeutic options will be developed upon detailed understanding of the molecular mechanisms driving liver fibrosis. This may lead to new anti-fibrotic therapies which need to be tested in suitable models before application in the veterinary and human clinic. On the other side, to restore the failing regenerative capacity of the diseased liver cells, adult progenitor cells are of interest, as an alternative to whole organ transplantation. In order to find the most suitable large animal model it is important to recognise that the typical histopathological reaction pattern of the liver can differ between mammalian species. It is therefore imperative that specialists in veterinary internal medicine and pathology, being familiar with the diseases and pathologies of the liver in different animal species, are teaming-up in finding the best models for veterinary and human liver diseases. Several large animal models have been mentioned, like pigs, sheep, and dogs. Based on the observations that man and dog share the same hepatopathies and have identical clinical, pathological and pathogenetic reaction patterns during the development of liver disease, the dog seems to be a properly suited species to test new therapeutic strategies for pets and their best friends.

Analysis of biodistribution and engraftment into the liver of genetically modified mesenchymal stromal cells derived from adipose tissue

Presently, orthotopic liver transplant is the major therapeutic option for patients affected by primary liver diseases. This procedure is characterized by major invasive surgery, scarcity of donor organs, high costs, and lifelong immunosuppressive treatment. Transplant of hepatic precursor cells represents an attractive alternative. These cells could be used either for allogeneic transplantation or for autologous transplant after ex vivo genetic modification. We used stromal cells isolated from adipose tissue (AT-SCs) as platforms for autologous cell-mediated gene therapy. AT-SCs were transduced with lentiviral vectors expressing firefly luciferase, allowing for transplanted cell tracking by bioluminescent imaging (BLI). As a complementary approach, we followed circulating human α1-antitrypsin (hAAT) levels after infusion of AT-SCs overexpressing hAAT. Cells were transplanted into syngeneic mice after CCl(4)-induced hepatic injury. Luciferase bioluminescence signals and serum hAAT levels were measured at different time points after transplantation and demonstrate persistence of transplanted cells for up to 2 months after administration. These data, along with immunohistochemical analysis, suggest engraftment and repopulation of injured livers by transplanted AT-SCs. Moreover, by transcriptional targeting using cellular tissue-specific regulatory sequences, we confirmed that AT-SCs differentiate towards a hepatogenic-like phenotype in vitro and in vivo. Additionally, in transplanted cells reisolated from recipient animals' livers, we detected activation of the α-fetoprotein (AFP) promoter. This promoter is normally transcriptionally silenced in adult tissues but can be reactivated during liver regeneration, suggesting commitment towards hepatogenic-like differentiation of engrafted cells in vivo. Our data support AT-SC-mediated gene therapy as an innovative therapeutic option for disorders of liver metabolism.

Brain regeneration in physiology and pathology: the immune signature driving therapeutic plasticity of neural stem cells

Regenerative processes occurring under physiological (maintenance) and pathological (reparative) conditions are a fundamental part of life and vary greatly among different species, individuals, and tissues. Physiological regeneration occurs naturally as a consequence of normal cell erosion, or as an inevitable outcome of any biological process aiming at the restoration of homeostasis. Reparative regeneration occurs as a consequence of tissue damage. Although the central nervous system (CNS) has been considered for years as a "perennial" tissue, it has recently become clear that both physiological and reparative regeneration occur also within the CNS to sustain tissue homeostasis and repair. Proliferation and differentiation of neural stem/progenitor cells (NPCs) residing within the healthy CNS, or surviving injury, are considered crucial in sustaining these processes. Thus a large number of experimental stem cell-based transplantation systems for CNS repair have recently been established. The results suggest that transplanted NPCs promote tissue repair not only via cell replacement but also through their local contribution to changes in the diseased tissue milieu. This review focuses on the remarkable plasticity of endogenous and exogenous (transplanted) NPCs in promoting repair. Special attention will be given to the cross-talk existing between NPCs and CNS-resident microglia as well as CNS-infiltrating immune cells from the circulation, as a crucial event sustaining NPC-mediated neuroprotection. Finally, we will propose the concept of the context-dependent potency of transplanted NPCs (therapeutic plasticity) to exert multiple therapeutic actions, such as cell replacement, neurotrophic support, and immunomodulation, in CNS repair.

Three-dimensional reconstructions of intrahepatic bile duct tubulogenesis in human liver

Background

During liver development, intrahepatic bile ducts are thought to arise by a unique asymmetric mode of cholangiocyte tubulogenesis characterized by a series of remodeling stages. Moreover, in liver diseases, cells lining the Canals of Hering can proliferate and generate new hepatic tissue. The aim of this study was to develop protocols for three-dimensional visualization of protein expression, hepatic portal structures and human hepatic cholangiocyte tubulogenesis.

Results

Protocols were developed to digitally visualize portal vessel branching and protein expression of hepatic cell lineage and extracellular matrix deposition markers in three dimensions. Samples from human prenatal livers ranging from 7 weeks + 2 days to 15½ weeks post conception as well as adult normal and acetaminophen intoxicated liver were used. The markers included cytokeratins (CK) 7 and 19, the epithelial cell adhesion molecule (EpCAM), hepatocyte paraffin 1 (HepPar1), sex determining region Y (SRY)-box 9 (SOX9), laminin, nestin, and aquaporin 1 (AQP1).

Digital three-dimensional reconstructions using CK19 as a single marker protein disclosed a fine network of CK19 positive cells in the biliary tree in normal liver and in the extensive ductular reactions originating from intrahepatic bile ducts and branching into the parenchyma of the acetaminophen intoxicated liver. In the developing human liver, three-dimensional reconstructions using multiple marker proteins confirmed that the human intrahepatic biliary tree forms through several developmental stages involving an initial transition of primitive hepatocytes into cholangiocytes shaping the ductal plate followed by a process of maturation and remodeling where the intrahepatic biliary tree develops through an asymmetrical form of cholangiocyte tubulogenesis.

Conclusions

The developed protocols provide a novel and sophisticated three-dimensional visualization of vessels and protein expression in human liver during development and disease.

Fetal liver cell transplantation as a potential alternative to whole liver transplantation?

Because organ shortage is the fundamental limitation of whole liver transplantation, novel therapeutic options, especially the possibility of restoring liver function through cell transplantation, are urgently needed to treat end-stage liver diseases. Groundbreaking in vivo studies have shown that transplanted hepatocytes are capable of repopulating the rodent liver. The two best studied models are the urokinase plasminogen activator (uPA) transgenic mouse and the fumarylacetoacetate hydrolase (FAH)-deficient mouse, in which genetic modifications of the recipient liver provide a tissue environment in which there is extensive liver injury and selection pressure favoring the proliferation and survival of transplanted hepatocytes. Because transplanted hepatocytes do not significantly repopulate the (near-)normal liver, attention has been focused on finding alternative cell types, such as stem or progenitor cells, that have a higher proliferative potential than hepatocytes. Several sources of stem cells or stem-like cells have been identified and their potential to repopulate the recipient liver has been evaluated in certain liver injury models. However, rat fetal liver stem/progenitor cells (FLSPCs) are the only cells identified to date that can effectively repopulate the (near-)normal liver, are morphologically and functionally fully integrated into the recipient liver, and remain viable long-term. Even though primary human fetal liver cells are not likely to be routinely used for clinical liver cell repopulation in the future, using or engineering candidate cells exhibiting the characteristics of FLSPCs suggests a new direction in developing cell transplantation strategies for therapeutic liver replacement. This review will give a brief overview concerning the existing animal models and cell sources that have been used to restore normal liver structure and function, and will focus specifically on the potential of FLSPCs to repopulate the liver.

Clinical trials for stem cell therapies

In recent years, clinical trials with stem cells have taken the emerging field in many new directions. While numerous teams continue to refine and expand the role of bone marrow and cord blood stem cells for their vanguard uses in blood and immune disorders, many others are looking to expand the uses of the various types of stem cells found in bone marrow and cord blood, in particular mesenchymal stem cells, to uses beyond those that could be corrected by replacing cells in their own lineage. Early results from these trials have produced mixed results often showing minor or transitory improvements that may be attributed to extracellular factors. More research teams are accelerating the use of other types of adult stem cells, in particular neural stem cells for diseases where beneficial outcome could result from either in-lineage cell replacement or extracellular factors. At the same time, the first three trials using cells derived from pluripotent cells have begun.

Epithelial cell adhesion molecule (EpCAM) marks hepatocytes newly derived from stem/progenitor cells in humans

Epithelial cell adhesion molecule (EpCAM) is a surface marker on human hepatic stem/progenitor cells that is reported as absent on mature hepatocytes. However, it has also been noted that in cirrhotic livers of diverse causes, many hepatocytes have EpCAM surface expression; this may represent aberrant EpCAM expression in injured hepatocytes or, as we now hypothesize, persistence of EpCAM in hepatocytes that have recently derived from hepatobiliary progenitors. To evaluate this concept, we investigated patterns of EpCAM expression in hepatobiliary cell compartments of liver biopsy specimens from patients with all stages of chronic hepatitis B and C, studying proliferation, senescence and telomere lengths. We found that EpCAM(+) hepatocytes were rare in early stages of disease, became increasingly prominent in later stages in parallel with the emergence of ductular reactions, and were consistently arrayed around the periphery of cords of keratin 19(+) hepatobiliary cells of the ductular reaction, with which they shared EpCAM expression. Proliferating cell nuclear antigen (proliferation marker) and p21 (senescence marker) were both higher in hepatocytes in cirrhosis than in normal livers, but ductular reaction hepatobiliary cells had the highest proliferation rate, in keeping with being stem/progenitor cell-derived transit amplifying cells. Telomere lengths in EpCAM(+) hepatocytes in cirrhosis were higher than EpCAM(-) hepatocytes (P < 0.046), and relatively shorter than those in the corresponding ductular reaction hepatobiliary cells (P = 0.057).

CONCLUSION

These morphologic, topographic, immunophenotypic, and molecular data support the concept that EpCAM(+) hepatocytes in chronic viral hepatitis are recent progeny of the hepatobiliary stem/progenitor cell compartment through intermediates of the transit amplifying, ductular reaction hepatobiliary cells.

The effect of recombinant E-cadherin substratum on the differentiation of endoderm-derived hepatocyte-like cells from embryonic stem cells

Generation of specific lineages of cells from embryonic stem (ES) cells is pre-requisite to use these cells in pre-clinical applications. Here, we developed a recombinant E-cadherin substratum for generation of hepatic progenitor populations at single cell level. This artificial acellular feeder layer supports the stepwise differentiation of ES cells to cells with characteristics of definitive endoderm, hepatic progenitor cells, and finally cells with phenotypic and functional characteristics of hepatocytes. The efficient differentiation of hepatic endoderm cells (approximately 55%) together with the absence of neuroectoderm and mesoderm markers suggests the selective induction of endoderm differentiation. The co-expression of E-cahderin and alpha-fetoprotein (approximately 98%) suggests the important role of E-cadherin as a surface marker for the enrichment of hepatic progenitor cells. With extensive expansion, approximately 92% albumin expressing cells can be achieved without any enzymatic stress and cell sorting. Furthermore, these mouse ES cell-derived hepatocyte-like cells showed higher morphological similarities to primary hepatocytes. In conclusion, we demonstrated that E-cadherin substratum can guide differentiation of ES cells into endoderm-derived hepatocyte-like cells. This recombinant extracellular matrix could be effectively used as an in vitro model for studying the mechanisms of early stages of liver development even at single cell level.

Hepatoblast-like progenitor cells derived from embryonic stem cells can repopulate livers of mice

BACKGROUND & AIMS

Hepatocyte-like cells can be derived from pluripotent stem cells such as embryonic stem (ES) cells, but ES cell-derived hepatic cells with extensive capacity to repopulate liver have not been identified. We aimed to identify and purify ES cell-derived hepatoblast-like progenitor cells and to explore their capacity for liver repopulation in mice after in vitro expansion.

METHODS

Unmanipulated mouse ES cells were cultured under defined conditions and allowed to undergo stepwise hepatic differentiation. The derived hepatic cells were examined by morphologic, fluorescence-activated cell sorting, gene expression, and clonal expansion analyses. The capacities of ES cell-derived hepatic progenitor cells to repopulate liver were investigated in mice that were deficient in fumarylacetoacetate hydrolase (Fah) (a model of liver injury).

RESULTS

Mouse ES cells were induced to differentiate into a population that contained hepatic progenitor cells; this population included cells that expressed epithelial cell adhesion molecule (EpCAM) but did not express c-Kit. Clonal hepatic progenitors that arose from single c-Kit(-)EpCAM(+) cells could undergo long-term expansion and maintain hepatoblast-like characteristics. Enriched c-Kit(-)EpCAM(+) cells and clonally expanded hepatic progenitor cells repopulated the livers of Fah-deficient mice without inducing tumorigenesis.

CONCLUSIONS

ES cell-derived c-Kit(-)EpCAM(+) cells contain a population of hepatoblast-like progenitor cells that can repopulate livers of mice.

Stem cell-based therapies for liver diseases: state of the art and new perspectives

Millions of patients worldwide suffer from end-stage liver pathologies, whose only curative therapy is liver transplantation (OLT). Given the donor organ shortage, alternatives to OLT have been evaluated, including cell therapies. Hepatocyte transplantation has been attempted to cure metabolic liver disorders and end-stage liver diseases. The evaluation of its efficacy is complicated by the shortage of human hepatocytes and their difficult expansion and cryopreservation. Recent advances in cell biology have led to the concept of "regenerative medicine", based on the therapeutic potential of stem cells (SCs). Different types of SCs are theoretically eligible for liver cell replacement. These include embryonic and fetal SCs, induced pluripotent cells, annex SCs, endogenous liver SCs, and extrahepatic adult SCs. Aim of this paper is to critically analyze the possible sources of SCs suitable for liver repopulation and the results of the clinical trials that have been published until now.

Emerging use of stem cells in regenerative medicine

Stem cells represent a unique opportunity for regenerative medicine to cure a broad number of diseases for which current treatment only alleviates symptoms or retards further disease progression. However, the number of stem cells available has speedily increased these past 10 years and their diversity presents new challenges to clinicians and basic scientists who intend to use them in clinics or to study their unique properties. In addition, the recent possibility to derive pluripotent stem cells from somatic cells using epigenetic reprogramming has further increased the clinical interest of stem cells since induced pluripotent stem cells could render personalized cell-based therapy possible. The present review will attempt to summarize the advantages and challenges of each type of stem cell for current and future clinical applications using specific examples.

The fatty acid constitution and ordering state of membranes in dominant temperature-sensitive lethal mutation and wild-type Drosophila melanogaster larvae

The ordering state and changes in fatty acid composition of microsomal (MS) and mitochondrial (MC) membranes of two dominant temperature-sensitive (DTS) lethal mutations and the wild-type Oregon-R strain larvae of Drosophila melanogaster have been studied at 18 and 29 degrees C and after temperature-shift experiments. The membranes of wild-type larvae have a stable ordering state, with "S" values between 0.6 (18 degrees C) and 0.5 (29 degrees C) in both membranes which remained unchanged in shift experiments, although the ratios of saturated/unsaturated fatty acids were changed as expected. The strongly DTS mutation 1(2) 10DTS forms very rigid membranes at the restrictive temperature (29 degrees C) which cannot be normalized after shift down, while shift up or development at the permissive temperature results in normal ordering state. This mutant is less able to adjust MS and MC fatty acid composition in response to the growth temperature than the wild type. The less temperature-sensitive 1(2)2DTS allele occupies an intermediate state between Oregon-R and 1(2)10DTS in both respects. We assume and the genetical data suggest that the DTS mutant gene product is in competition with the wild-type product, resulting in a membrane structure which is not able to accommodate to the restrictive temperature.