Current Stem Cell Research & Therapy - Volume 4, Issue 2, 2009

Embryonic stem (ES) cells do not exist in nature but, usually produced from the inner cell mass (ICM) of the blastocyst, are considered equivalent to ICM cells captured during a short period of transient self-renewal and pluripotency capability. Although, artificial, ES cells represent a formidable model to investigate fundamental aspects of cell stemness and early embryo development. ES cells are indeed the only stem cell type able to indefinite self-renewal and to differentiate into cellular derivates of ectodermal, mesodermal and endodermal lineages. Recent extensive studies have revealed that ES cells maintain self-renewal and pluripotency because of a self-organizing network of transcription factors and intracellular pathways activated by extracellular signalling that together prevent their differentiation and promote their proliferation, and because of epigenetic processes that maintain the chromatin in a plastic differentiation status. Primordial germ cells (PGCs), the embryonic precursors of gametes, because of their unique ability to retain true developmental totipotency, are considered the mother of all stem cells. Despite several similarities with ES cells, they display only transient self-renewal capability and distinct lineage-specific characteristics. In fact, in normal condition PGCs are believed to differentiate into germ cells only, oogonia/oocytes in the female, and prospermatogonia in the male which ultimately produce eggs and sperm, respectively. It is not until the fertilization of the egg or parthenogenesis that the intrinsic germ cell totipotency program is revealed. Many aspects of the extrinsic factors and signalling required for ES cell self-renewal and pluripotency have been identified and dissected. On the other hand, several extrinsic factors controlling PGC development have been identified, but the underlying molecular signalling remains little defined. In the present review, by comparing the available information about signalling elicited by four growth factors such as leukaemia inhibitory factor (LIF), bone morphogenic protein 4 (BMP4), fibroblast growth factor 2 (FGF2) and kit ligand (KL) in mouse ES cells and PGCs, on which most of such studies have been performed, we aimed to give clues for the molecular understanding of the similarities and differences between these two unique cell types and to explain how apparent contradictory properties such as lineage-specific characteristics and pluripotency may coexist within PGCs. The first two growth factors have been demonstrated to control key aspects of the self-renewal and pluripotency of ES cells. BMP4 and KL are known for their crucial role in regulating various process of PGC development in the embryo from the formation of PGC precursors and PGC specification (BMP4) to their survival, proliferation and migration (KL). Moreover, the combined action of LIF, FGF2 and KL is necessary and sufficient for PGC transformation into ES-like cells termed embryonic germ (EG) cells.

One of the key characteristics of human embryonic stem cells (hESC) is their ability to proliferate for an indefinite period of time. Previous studies have shown that a unique network of transcription factors are involved in hESC self renewal. Since hESC lines have the potential to differentiate into cells of all three germ layers, cells derived from hESC may be useful for the treatment of a variety of inherited or acquired diseases. The molecular signal required to differentiate hESC into a particular cell type has not been defined. It is expected that global gene expression profiling of hESC may provide an insight into the critical genes involved in maintaining pluripotency of hESC and genes that are modulated when hESCs differentiate. Several groups have utilized a variety of high throughput techniques and performed gene expression profiling of undifferentiated hESCs and mouse ES cells (mESC) to identify a set of genes uniquely expressed in ES cells but not in mature cells and defined them as “stemness” genes. These molecular techniques include DNA microarray, EST-enumeration, MPSS profiling, and SAGE. Irrespective of the molecular technique used, highly expressed genes showed similar expression pattern in several ES cell lines supporting their importance. A set of approximately 100 genes were identified, which are highly expressed in ES cells and considered to be involved in maintaining pluripotency and self renewal of ES cells. Various studies have also reported on the gene expression profiling of differentiated embryoid bodies (EB) derived from hESCs and mESCs. When hESCs are differentiated, “stemness” genes are down-regulated and a set of genes are up-regulated. Together with down-modulation of “stemness” genes and upregulation of new genes may provide a new insight into the molecular pathways of hESC differentiation and study of these genes may be useful in the characterization of differentiated cells.

Embryonic stem cells are by definition the master cells capable of differentiating into every type of cells either in vitro or in vivo. Several lines of evidence suggest, however, that adult stem cells and even terminally differentiated somatic cells under appropriate microenvironmental cues are able to be reprogrammed and contribute to a much wider spectrum of differentiated progeny than previously anticipated. This has been demonstrated by using tissue- specific stem cells, which like embryonic stem cells do not express CD45 as an exclusive hematopoietic marker (skin, adipose, cord blood and bone marrow- derived stem cells). On the other side, there is a great number of reports which demonstrate that hematopoietic cells (CD45+) from different sources (peripheral blood, cord blood, bone marrow) are also able to cross the tissue boundaries and give rise to the cells of the other germ layers. Herein we discuss the differentiation and reprogramming potential of both hematopoietic and non- hematopoietic stem cells along endodermal, mesodermal and neuroectodermal lineage and their importance for regenerative medicine.

Many retinal dystrophies are associated with photoreceptor loss, which causes irreversible blindness. The recent identification of various sources of stem cells in the mammalian retina has raised the possibility that cell-based therapies might be efficient strategies to treat a wide range of incurable eye diseases. A first step towards the successful therapeutic exploitation of these cells is to unravel intrinsic and extrinsic regulators that control their proliferation and cell lineage determination. In this review, we provide an overview of the different types and molecular fingerprints of retinal stem cells identified so far. We also detail the current knowledge on molecular cues that influence their self-renewal and proliferation capacity. In particular, we focus on recent data implicating developmental signaling pathways, such as Wnt, Notch and Hedeghog, both in the normal and regenerating retina in different animal models. Last, we discuss the potential of ES cells and various adult stem cells for retinal repair.

The origin of vascular smooth muscle cells that accumulate in the neointima in vascular diseases such as transplant arteriosclerosis, atherosclerosis and restenosis remains subject to much debate. Smooth muscle cells are a highly heterogeneous cell population with different characteristics and markers, and distinct phenotypes in physiological and pathological conditions. Several studies have reported a role for bone marrow-derived progenitor cells in vascular maintenance and repair. Moreover, bone marrow-derived smooth muscle progenitor cells have been detected in human atherosclerotic tissue as well as in in vivo mouse models of vascular disease. However, it is not clear whether smooth muscle progenitor cells can be regarded as a ‘friend’ or ‘foe’ in neointima formation. In this review we will discuss the heterogeneity of smooth muscle cells, the role of smooth muscle progenitor cells in vascular disease, potential mechanisms that could regulate smooth muscle progenitor cell contribution and the implications this may have on designing novel therapeutic tools to prevent development and progression of vascular disease.

Liver failure is a catastrophic illness associated with the death of many patients who are waiting for transplantation. Currently there are no effective treatments for this disease, therefore scientists have paid their attention to the field of stem cells, which has helped to understand the pathogenesis of liver disease, expanded the drug discovery processes, and could potentially be used as an alternative therapy. Recent reports demonstrating the production of liver like cells derived from bone marrow and embryonic stem cells, have established a better understanding of the soluble factors and biochemical compounds that are essential in liver development. Although considerable progress has been made in differentiating stem cells into liver cells, current protocols have not yet produced cells with the phenotype of a complete mature hepatocyte. Therefore, the proper criteria for defining what constitutes a functional human stem cell-derived hepatocyte are required. This review describes the current challenges and future opportunities in Embryonic Stem cell differentiation to liver cells, and the appropriate characteristics needed for their future clinical use in the treatment of liver disease.

Cancer stem cells (CSC), also called tumor initiating cells (TIC), are considered to be the origin of replicating malignant tumor cells in a variety of human cancers. Their presence in the tumor may herald malignancy potential, mediate resistance to conventional chemotherapy or radiotherapy, and confer poor survival outcomes. Thus, CSC may serve as critical cellular targets for treatment. The ability to therapeutically target CSC hinges upon identifying their unique cell surface markers and the underlying survival signaling pathways. While accumulating evidence suggests cell-surface antigens (such as CD44, CD133) as CSC markers for several tumor tissues, emerging clinical needs exist for the identification of new markers to completely separate CSC from normal stem cells. Recent studies have demonstrated the critical role of the tumor suppressor PTEN/PI3 kinase pathway in regulating TIC in leukemia, brain, and intestinal tissues. The successful eradication of tumors by therapies targeting CSC will require an in-depth understanding of the molecular mechanisms governing CSC self renewal, differentiation, and escape from conventional therapy. Here we review recent progress from brain tumor and intestinal stem cell research with a focus on the PTEN-Akt-Wnt pathway, and how the components of CSC pathways may serve as biomarkers for diagnosis, prognosis, and therapeutics.

Chronic myeloproliferative disorders (cMPDs) are clonal hemopoietic malignancies arising at the multipotent stem cell level. These conditions are characterized by increased blood count, marrow hyperplasia and extramedulary hemopoiesis. Vascular events might complicate their course, and transformation to either acute leukemia or myelofibrosis can finally occur. Among cMPDs, Polycythemia Vera (PV), Essential Thrombocythemia (ET) and Primary Myelofibrosis (PMF) belong to the group of Ph-negative cMPDs. Although they share common pathogenetic features, these entities have a quite different prognosis. The common pathogenetic basis of Ph-negative cMPDs was recognized long ago, and it was suggested that a stimulating factor might enhance bone marrow hemopoietic activity. Hemopoietic progenitors from cMPDs show hypersensitivity to low levels of a variety of hemopoietic cytokines. The independency of erythroid precursors from erythropoietin became the first surrogate marker of an abnormal hemopoietic clone. This clone is characterized by increased proliferation and survival, as well as by decreased apoptosis, leading to the accumulation of mature blood cells that additionally show a phenotype of activated cells. Recently four independent groups have described an activating point mutation in the JAK2 kinase as a key pathogenetic event in Ph-negative cMPDs. JAK2 is a tyrosine kinase that acts as a second intracellular messenger for many hemopoietic cytokine receptors. It is now believed that jacking up hemopoiesis can explain many features of myeloproliferation. Interestingly, some features are associated with intracellular levels of mutated JAK2 (the “dosage hypothesis”). The mutation in JAK2 kinase is not an example of a genetic defect leading to a single disease, since it occurs in many other myeloid disorders, and probably represents a secondary hit in a multistep ongogenetic process. Nevertheless, it has changed the way we approach cMPD patients and has clarified many aspects of their biology.

The term late-onset non-infectious pulmonary complications (LONIPCs) has been used to refer to events occurring later than 3 months after allogeneic hematopoietic stem transplant (HSCT), such as bronchiolitis obliterans, bronchiolitis obliterans with organizing pneumonia, and lymphocytic or idiopathic interstitial pneumonia. The incidence of LONIPCs varies widely, ranging between 10% and 26%. Median time for LONIPC development is about 8-12 months after HSCT. Clinical symptoms may be insidious and non specific at the beginning and can be present in different types of infections. The diagnosis is made on the basis of thoracic high-resolution computed tomography and pulmonary function tests (PFT). It usually requires that standard cultures for infective agents on bronchoalveolar lavage are negative and is confirmed by transbronchial or lung biopsy, whenever possible. Total body irradiation and high doses of drugs used in the conditioning regimens , HLA disparity between donor and recipient, and chronic graft-versus-host disease (GVHD) are the main risk factors for LONIPCs. Since patients with LONIPCs have an increased risk of mortality because of infections or respiratory failure, pre- and post-transplant PFTs are strongly recommended in order to timely identify affected patients. The administration of antithymocyte globulin before unrelated donor transplants and slow taper of cyclosporine after transplant have been shown to prevent chronic GVHD and, therefore, the occurrence of LONIPCs.