Pluripotent plasticity of stem cells and liver repopulation

Different types of stem cells have a role in liver regeneration or fibrous repair during and after several liver diseases. Otherwise, the origin of hepatic and/or extra‐hepatic stem cells in reactive liver repopulation is under controversy. The ability of the human body to self‐repair and replace the cells and tissues of some organs is often evident. It has been estimated that complete renewal of liver tissue takes place in about a year. Replacement of lost liver tissues is accomplished by proliferation of mature hepatocytes, hepatic oval stem cells differentiation, and sinusoidal cells as support. Hepatic oval cells display a distinct phenotype and have been shown to be a bipotential progenitor of two types of epithelial cells found in the liver, hepatocytes, and bile ductular cells. In gastroenterology and hepatology, the first attempts to translate stem cell basic research into novel therapeutic strategies have been made for the treatment of several disorders, such as inflammatory bowel diseases, diabetes mellitus, celiachy, and acute or chronic hepatopaties. In the future, pluripotent plasticity of stem cells will open a variety of clinical application strategies for the treatment of tissue injuries, degenerated organs. The promise of liver stem cells lie in their potential to provide a continuous and readily available source of liver cells that can be used for gene therapy, cell transplant, bio‐artificial liver‐assisted devices, drug toxicology testing, and use as an in vitro model to understand the developmental biology of the liver. Copyright © 2010 John Wiley & Sons, Ltd.     

Stem cell transplantation.

Modern physicians desire not only to treat but to cure congenital diseases. In a wide variety of diseases, bone marrow transplantation can be the tool of final cure. The limitations and risks of this procedure have motivated researchers to search for an earlier and safer method of treatment. Special features of fetal immune systems make it possible to perform the transplantation during fetal life using fetal hematopoietic stem cells, thus avoiding many of the side effects of bone marrow transplantation in neonatal life. We review the experimental work done with animal models in this field and the human trials that have been published recently.     

干细胞与心肌细胞替代治疗

胚胎干细胞及来源于骨髓、骨骼肌、血管、肝脏、皮肤、脂肪等组织器官的成体干细胞均有多向分化潜能。胚胎干细胞可分化为3个胚层的所有组织细胞。成体干细胞具有可塑性和转分化的潜能。在一定条件下,这些干细胞可被诱导分化为心肌细胞。成年心脏可能存在心肌干细胞,具有增殖和分化为包括跳动性心肌细胞的多种细胞的潜能。因此,干细胞可用于心肌细胞替代治疗,以替代死亡的心肌细胞,改善心脏功能,防治心肌梗塞后心衰、减少心肌重构等症状。本文对干细胞治疗心肌梗塞有关进展及问题作一综述。     

Stem cells and liver engineering

Human hepatocytes, suitable for treatment of patients with liver failure, for the creation of bioartificial (BAL) devices, or for studies for toxicity and metabolization studies in the pharmaceutical industry, are in short supply due to the lack of donor organs. Therefore, methods that allow ex vivo expansion of hepatocytes with mature function are being pursued. One cell source, believed to be a possible inexhaustible source of hepatocytes, is pluripotent stem cells (PSCs). However, directed differentiation of PSCs to cells with features of adult hepatocytes is not yet possible. Differentiated progeny remains mixed and PSC progeny does not have a number of the functional features of mature hepatocytes. In this review article, we will address tools being developed that allow for the identification of mature hepatocytes, in a non-invasive manner; to perform lineage tracing of PSC progeny; and novel culture systems being created for the in vitro differentiation of PSCs to hepatocyte like cells, and for the maintenance of primary liver derived hepatocytes or PSC-derived hepatic progeny in culture. As conventional two-dimensional (2D) static culture conditions poorly recapitulate the in vivo cellular environment, we will discuss bioreactor systems for liver tissue engineering, both macro-scale and micro-scale culture systems.     

Stem cell microencapsulation for phenotypic control,bioprocessing, and transplantation

Cell microencapsulation has been utilized for decades as a means to shield cells from the external environment while simultaneously permitting transport of oxygen, nutrients, and secretory molecules. In designing cell therapies, donor primary cells are often difficult to obtain and expand to appropriate numbers, rendering stem cells an attractive alternative due to their capacities for self‐renewal, differentiation, and trophic factor secretion. Microencapsulation of stem cells offers several benefits, namely the creation of a defined microenvironment which can be designed to modulate stem cell phenotype, protection from hydrodynamic forces and prevention of agglomeration during expansion in suspension bioreactors, and a means to transplant cells behind a semi‐permeable barrier, allowing for molecular secretion while avoiding immune reaction. This review will provide an overview of relevant microencapsulation processes and characterization in the context of maintaining stem cell potency, directing differentiation, investigating scalable production methods, and transplanting stem cells for clinically relevant disorders. Biotechnol. Bioeng. 2013; 110: 667–682. © 2012 Wiley Periodicals, Inc.     

The role of hepatocytes and oval cells in liver regeneration and repopulation

The liver has the unique capacity to regulate its growth and mass. In rodents and humans, it grows rapidly after resection of more than 50% of its mass. This growth process, as well as that following acute chemical injury is known as liver regeneration, although growth takes place by compensatory hyperplasia rather than true regeneration. In addition to hepatocytes and non-parenchymal cells, the liver contains intra-hepatic "stem" cells which can generate a transit compartment of precursors named oval cells. Liver regeneration after partial hepatectomy does not involve intra or extra-hepatic (hemopoietic) stem cells but depends on the proliferation of hepatocytes. Transplantation and repopulation experiments have demonstrated that hepatocytes, which are highly differentiated and long-lived cells, have a remarkable capacity for multiple rounds of replication. In this article, we review some aspects of the regulation of hepatocyte proliferation as well as the interrelationships between hepatocytes and oval cells in different liver growth processes. We conclude that in the liver, normally quiescent differentiated cells replicate rapidly after tissue resection, while intra-hepatic precursor cells (oval cells) proliferate and generate lineage only in situations in which hepatocyte proliferation is blocked or delayed. Although bone marrow stem cells can generate oval cells and hepatocytes, transdifferentiation is very rare and inefficient.     

Stem cell transplantation and MBL replacement therapy

Mannose-binding lectin (or mannan-binding lectin, MBL) may have an influence on susceptibility to infection in patients given chemotherapy to induce remission or as conditioning before stem cell transplantation. The most surprising finding reported from an inconsistent literature was the observation that mbl-2 gene mutations in donors could influence the risk of serious infections in recipients of allogeneic stem cell transplants. This could be explained if leukocytes in the stem cell preparations (or their derivatives) were able to synthesize and secrete MBL, but the available evidence seems to exclude that possibility. An alternative mechanism could involve MBL binding to autologous cells and inducing immunological maturation of those cells. MBL can certainly bind to various cell types via surface glycoconjugates and the possible significance of this for MBL replacement therapy will be discussed.     

Liver repopulation with hepatocyte transplantation: new avenues for gene and cell therapy

Liver-directed gene therapy is appropriate for many conditions. Recent work established that liver repopulation with transplanted cells can be effective in treating genetic disorders. Although hepatocytes express therapeutic genes with considerable efficiency, correction of genetic disorders is constrained by limitations in permanent gene transfer into hepatocytes and repopulation of the liver with transplanted cells. Adenoviral vectors are highly efficient for hepatic gene transfer but the onset of deleterious host immune responses against adenoviral vectors, along with clearance of transduced hepatocytes have caused problems. Nonetheless, recent work concerning engraftment and proliferation of transplanted hepatocytes in the liver has provided significant new information, which should refocus interest in hepatocyte-based therapies. Moreover, hepatocyte transplantation systems offer creative tools for defining critical mechanisms in gene regulation and survival of transduced cells.     

Stem cells in liver regeneration and therapy

The liver has adapted to the inflow of ingested toxins by the evolutionary development of unique regenerative properties and responds to injury or tissue loss by the rapid division of mature cells. Proliferation of the parenchymal cells, i.e. hepatocytes and epithelial cells of the bile duct, is regulated by numerous cytokine/growth-factor-mediated pathways and is synchronised with extracellular matrix degradation and restoration of the vasculature. Resident hepatic stem/progenitor cells have also been identified in small numbers in normal liver and implicated in liver tissue repair. Their putative role in the physiology, pathophysiology and therapy of the liver, however, is not yet precisely known. Hepatic stem/progenitor cells also known as “oval cells” in rodents have been implicated in liver tissue repair, at a time when the capacity for hepatocyte and bile duct replication is exhausted or experimentally inhibited (facultative stem/progenitor cell pool). Although much more has to be learned about the role of stem/progenitor cells in the physiology and pathophysiology of the liver, experimental analysis of the therapeutic value of these cells has been initiated. Transplantation of hepatic stem/progenitor cells or in vivo pharmacological activation of the pool of hepatic stem cells may provide novel modalities for the therapy of liver diseases. In addition, extrahepatic stem cells (e.g. bone marrow cells) are being investigated for their contribution to liver regeneration. Hepatic progenitor cells derived from embryonic stem cells are included in this review, which also discusses future perspectives of stem cell-based therapies for liver diseases.     

Stem cells do not play a significant role in repopulation of adult human cardiomyocytes

Currently, there are two points of view on the ability of adult human heart to regenerate. One of them holds that the myocardium has a poor ability to regenerate. According to the other, the myocardium can rapidly regenerate due to the presence of resident stem cells in it. The purpose of this study was to test these hypotheses by investigating the distribution of cardiomyocytes by size and ploidy in human beings of different age. Using cytofluorometry and interferometry, we determined the dry weight, volume, and ploidy of myocytes isolated from the left ventricle of a normal heart of 12 men at the age of 20–30 (n = 7) and 40–50 (n = 5) years. The mean dry weight of cardiomyocytes was 6906 ± 182 pg (10^–12 g) in the 20- to 30-yearold men and 9126 ± 263 pg in 40- to 50-year-old men; the myocyte volume was 55250 ± 1457 and 73005 ± 2106 µm³, respectively. Cells with volumes intermediate between the cells at the stage of “dividing myocytes” and mature myocytes were absent. The number of cardiomyocytes in the left ventricle was (3.18 ± 0.05) × 10⁹ in the 20–30-year-old age group and (2.06 ± 0.6) × 10⁹ in the 40–50-year-old group. The largest subset (41.3%) of the myocyte population was represented by mononuclear cells with tetraploid nuclei. The proportion of myocytes of different ploidy classes and their mean ploidy did not change in the range of 20–50 years. On the basis on these data, we concluded that stem cells do not play a significant role in restoring the number of lost myocytes. Hypertrophy of myocytes caused by the increase in their cytoplasm is the main mechanism of compensation of the function of the left ventricle of the heart in aging human beings.     

Stem cell transplantation for rheumatic autoimmune diseases

Immunoablative therapy and hematopoietic stem cell transplantation (HSCT) is an intensive treatment modality aimed at 'resetting' the dysregulated immune system of a patient with immunoablative therapy and allow outgrowth of a nonautogressive immune system from reinfused hematopoietic stem cells, either from the patient (autologous HSCT) or a healthy donor (allogeneic HSCT). HSCT has been shown to induce profound alterations of the immune system affecting B and T cells, monocytes, and natural killer and dendritic cells, resulting in elimination of autoantibody-producing plasma cells and in induction of regulatory T cells. Most of the available data have been collected through retrospective cohort analyses of autologous HSCT, case series, and translational studies in patients with refractory autoimmune diseases. Long-term and marked improvements of disease activity have been observed, notably in systemic sclerosis, systemic lupus erythematosus, and juvenile idiopathic arthritis, and treatment-related morbidity and mortality have improved due to better patient selection and modifications of transplant regimens. Treatment-related mortality has decreased to approximately 7%. Prospective, randomised, controlled clinical trials are ongoing or planned in systemic sclerosis, systemic lupus erythematosus, and several nonrheumatological conditions.     

Potential of bioengineering processes for therapeutic repopulation of the liver with cells

Multiple unique aspects of liver biology make this organ an excellent paradigm for novel cell and gene therapy applications. In recent years, insights were obtained into how transplanted cells engraft and proliferate in the liver, including in the context of pre-existing disease. Also, a variety of animal models were studied to establish the basis of cell and gene therapy applications in specific disorders. Through ongoing research activity, additional mechanisms in liver repopulation have been uncovered, where manipulation of specific cell compartments and cellular processes,e.g., those aimed at extracellular matrix component receptors or soluble signals in transplanted and native cells can be exploited for enhancing cell engraftment and proliferation. Such studies demonstrate the possibility of applying biotechnology and/or bioengineering principles to organ replacement aimed at cell and gene therapy. Joining of these disciplines with research in stem cell biology, particularly in efforts concerning targeting of transplanted stem cells to given organs with achievement of lineage-specific cell differentiation and function, will be particularly important for future cell and gene therapy applications. This review offers an overview of relevant mechanisms in liver repopulation.     

Stem cell niches: famished paneth cells, gluttonous stem cells

Adult tissue stem cells adjust to environmental changes. A new study in the?mouse intestine reveals that caloric restriction causes Paneth cells to repress mTORC1 signaling; this in turn stimulates proliferation of neighboring stem cells.     

Stem and progenitor cells in liver regeneration and repair

Mammalian liver has a unique capacity to regenerate following resection or injury, and recovery of liver mass is mainly through proliferation of remaining adult hepatocytes. However, in pathologic conditions, especially during acute liver failure (ALF) and advanced stages of chronic liver disease (CLD), regeneration eventually fails and orthothopic liver transplantation (OLT) represents the only curative approach. The clinical scenario of a world-wide increasing incidence of end-stage CLD and an associated lack of organ availability has led several laboratories to explore the feasibility and efficiency of experimental alternatives to OLT involving cellular therapy. This review presents experimental and clinical studies performed in the last 10-15 years where adult and embryonic hepatocytes, hepatic stem/progenitor cells and extrahepatic stem cells have been used as transplantable cell sources.     

Stem cell transplantation for a myelodysplastic syndrome in children

The myelodysplastic syndrome (MDS) is a rare, clonal disorder of pluripotent stem cells in children and is characterized by ineffective haematopoiesis, morphologic abnormalities in one or more cell lines in a usually cellular bone marrow, and by predilection for the acute leukaemia. A large proportion of children with MDS present associated clinical abnormalities. Allogeneic stem cell transplantation (SCT) is the only definitive cure for this heterogeneous group of lethal disorders. Results with SCT have been difficult to interpret due to the variability of conditioning regimens, types of donors, and pretransplant therapy. In many series, the outcome with donors other than matched siblings has been extremely poor. The optimal pre-transplant therapy and conditioning regimen for SCT in MDS have not yet been defined. The establishment of several international working groups will eventually help to elucidate the pathogenesis of childhood MDS and will evaluate new treatment strategies to improve their clinical outcome.