Hypoxia in the regulation of neural stem cells

In aerobic organisms, oxygen is a critical factor in tissue and organ morphogenesis from embryonic development throughout post-natal life, as it regulates various intracellular pathways involved in cellular metabolism, proliferation, survival and fate. In the mammalian central nervous system, oxygen plays a critical role in regulating the growth and differentiation state of neural stem cells (NSCs), multipotent neuronal precursor cells that reside in a particular microenvironment called the neural stem cell niche and that, under certain physiological and pathological conditions, differentiate into fully functional mature neurons, even in adults. In both experimental and clinical settings, oxygen is one of the main factors influencing NSCs. In particular, the physiological condition of mild hypoxia (2.5–5.0% O₂) typical of neural tissues promotes NSC self-renewal; it also favors the success of engraftment when in vitro-expanded NSCs are transplanted into brain of experimental animals. In this review, we analyze how O₂ and specifically hypoxia impact on NSC self-renewal, differentiation, maturation, and homing in various in vitro and in vivo settings, including cerebral ischemia, so as to define the O₂ conditions for successful cell replacement therapy in the treatment of brain injury and neurodegenerative diseases.     

Cardiovascular development: towards biomedical applicability

Investigating the signalling pathways that regulate heart development is essential if stem cells are to become an effective source of cardiomyocytes that can be used for studying cardiac physiology and pharmacology and eventually developing cell-based therapies for heart repair. Here, we briefly describe current understanding of heart development in vertebrates and review the signalling pathways thought to be involved in cardiomyogenesis in multiple species. We discuss how this might be applied to stem cells currently thought to have cardiomyogenic potential by considering the factors relevant for each differentiation step from the undifferentiated cell to nascent mesoderm, cardiac progenitors and finally a fully determined cardiomyocyte. We focus particularly on how this is being applied to human embryonic stem cells and provide recent examples from both our own work and that of others.     

Crossing paths: interactions between the cell death machinery and growth factor survival signals

Cytokines and growth factors play a crucial role in the maintenance of haematopoietic homeostasis. They transduce signals that regulate the competing commitments of haematopoietic stem cells, quiescence or proliferation, retention of stem cell pluripotency or differentiation, and survival or demise. When the balance between these commitments and the requirements of the organisms is disturbed, particularly when it favours survival and proliferation, cancer may result. Cell death provoked by loss of growth factor signalling is regulated by the Bcl-2 family of apoptosis regulators, and thus survival messages transduced by growth factors must regulate the activity of these proteins. Many aspects of direct interactions between cytokine signalling and regulation of apoptosis remain elusive. In this review, we explore the mechanisms by which cytokines, in particular Interleukin-3 and granulocyte–macrophage colony-stimulating factor, promote cell survival and suppress apoptosis as models of how cytokine signalling and apoptotic pathways intersect.     

Molecular and Cellular Basis of Regeneration and Tissue Repair

Planarians possess amazing abilities to regulate tissue homeostasis and regenerate missing body parts. These features reside on the presence of a population of pluripotent/totipotent stem cells, the neoblasts, which are considered as the only planarian cells able to proliferate in the asexual strains. Neoblast distribution has been identified by mapping the cells incorporating bromodeoxyuridine, analyzing mitotic figures and using cell proliferation markers. Recently identified molecular markers specifically label subgroups of neoblasts, revealing thus the heterogeneity of the planarian stem cell population. Therefore, the apparent totipotency of neoblasts probably reflects the composite activities of multiple stem cell types. First steps have been undertaken to understand how neoblasts and differentiated cells communicate with each other to adapt the self-renewal and differentiation rates of neoblasts to the demands of the body. Moreover, the introduction of molecular resource database on planarians now paves the way to renewed strategies to understand planarian regeneration and stem cell-related issues.     

RNA cytosine methyltransferase Nsun3 regulates embryonic stem cell differentiation by promoting mitochondrial activity

Chemical modifications of RNA have been attracting increasing interest because of their impact on RNA fate and function. Therefore, the characterization of enzymes catalyzing such modifications is of great importance. The RNA cytosine methyltransferase NSUN3 was recently shown to generate 5-methylcytosine in the anticodon loop of mitochondrial tRNA^Met. Further oxidation of this position is required for normal mitochondrial translation and function in human somatic cells. Because embryonic stem cells (ESCs) are less dependent on oxidative phosphorylation than somatic cells, we examined the effects of catalytic inactivation of Nsun3 on self-renewal and differentiation potential of murine ESCs. We demonstrate that Nsun3-mutant cells show strongly reduced mt-tRNA^Met methylation and formylation as well as reduced mitochondrial translation and respiration. Despite the lower dependence of ESCs on mitochondrial activity, proliferation of mutant cells was reduced, while pluripotency marker gene expression was not affected. By contrast, ESC differentiation was skewed towards the meso- and endoderm lineages at the expense of neuroectoderm. Wnt3 was overexpressed in early differentiating mutant embryoid bodies and in ESCs, suggesting that impaired mitochondrial function disturbs normal differentiation programs by interfering with cellular signalling pathways. Interestingly, basal levels of reactive oxygen species (ROS) were not altered in ESCs, but Nsun3 inactivation attenuated induction of mitochondrial ROS upon stress, which may affect gene expression programs upon differentiation. Our findings not only characterize Nsun3 as an important regulator of stem cell fate but also provide a model system to study the still incompletely understood interplay of mitochondrial function with stem cell pluripotency and differentiation.     

Nestin expression – a property of multi-lineage progenitor cells?

Tissue-specific progenitor cells are characterized by proliferation and differentiation, but, in contrast to embryonic stem (ES) cells, have limited capacities for self-renewal and no tumourigenic potential. These latter traits make progenitor cells an ideal source for regenerative cell therapies. In this review, we describe what is currently known about nestin, an intermediate filament first identified in neuroepithelial stem cells. During embryogenesis, nestin is expressed in migrating and proliferating cells, whereas in adult tissues, nestin is mainly restricted to areas of regeneration. We show that nestin is abundant in ES-derived progenitor cells that have the potential to develop into neuroectodermal, endodermal and mesodermal lineages. Although it remains unclear what factors regulate in vitro and in vivo expression of nestin, we conclude that nestin represents a characteristic marker of multi-lineage progenitor cells and suggest that its presence in cells may indicate multi-potentiality and regenerative potential.     

Peripheral B cell survival

Recent findings suggest that lymphocyte survival is a continuous active process and support the role of B cell receptor engagement in B cell survival. In this context the conflict of survival interests between the diverse B cells gives rise to a pattern of interactions which mimics the behavior of complex ecological systems. In response to competition lymphocytes modify their survival requirements and diverge to occupy different immunological niches through differentiation. Thus naive and memory-activated B cell populations show independent homeostatic regulation. We discuss how niche differentiation allows the coexistence of different cell types and guarantees both repertoire diversity and efficient immune responses.     

Genetically determined epithelial dysfunction and its consequences for microflora–host interactions

The intestinal epithelium forms a highly active functional interface between the relatively sterile internal body surfaces and the enormously complex and diverse microbiota that are contained within the lumen. Genetic models that allow for manipulation of genes specifically in the intestinal epithelium have provided an avenue to understand the diverse set of pathways whereby intestinal epithelial cells (IECs) direct the immune state of the mucosa associated with homeostasis versus either productive or non-productive inflammation as occurs during enteropathogen invasion or inflammatory bowel disease (IBD), respectively. These pathways include the unfolded protein response (UPR) induced by stress in the endoplasmic reticulum (ER), autophagy, a self-cannibalistic pathway important for intracellular bacterial killing and proper Paneth cell function as well as the interrelated functions of NOD2/NF-κB signaling which also regulate autophagy induction. Multiple genes controlling these IEC pathways have been shown to be genetic risk factors for human IBD. This highlights the importance of these pathways not only for proper IEC function but also suggesting that IECs may be one of the cellular originators of organ-specific and systemic inflammation as in IBD.     

Tumor suppressive pathways in the control of neurogenesis

The generation of specialized neural cells in the developing and postnatal central nervous system is a highly regulated process, whereby neural stem cells divide to generate committed neuronal progenitors, which then withdraw from the cell cycle and start to differentiate. Cell cycle checkpoints play a major role in regulating the balance between neural stem cell expansion and differentiation. Loss of tumor suppressors involved in checkpoint control can lead to dramatic alterations of neurogenesis, thus contributing to neoplastic transformation. Here we summarize and critically discuss the existing literature on the role of tumor suppressive pathways and their regulatory networks in the control of neurogenesis and transformation.     

Receiving mixed signals: uncoupling oligodendrocyte differentiation and myelination

The development and maturation of an oligodendroglial cell is comprised of three intimately related processes that include proliferation, differentiation, and myelination. Here we review how proliferation and differentiation are controlled by distinct molecular mechanisms and discuss whether differentiation is merely a default of inhibited proliferation. We then address whether differentiation and myelination can be uncoupled in a similar manner. This task is particularly challenging because an oligodendrocyte cannot myelinate without first differentiating, and these processes are therefore not mutually exclusive. Is it solely the presence of the axon that distinguishes a differentiated oligodendrocyte from a myelinating one? Uncoupling these two processes requires identifying specific signals that regulate myelination without affecting the differentiation process. We will review current understanding of the relationship between differentiation and myelination and discuss whether these two processes can truly be uncoupled.     

mTOR signaling in neural stem cells: from basic biology to disease

The mammalian target of rapamycin (mTOR) pathway is a central controller of growth and homeostasis, and, as such, is implicated in disease states where growth is deregulated, namely cancer, metabolic diseases, and hamartoma syndromes like tuberous sclerosis complex (TSC). Accordingly, mTOR is also a pivotal regulator of the homeostasis of several distinct stem cell pools in which it finely tunes the balance between stem cell self-renewal and differentiation. mTOR hyperactivation in neural stem cells (NSCs) has been etiologically linked to the development of TSC-associated neurological lesions, such as brain hamartomas and benign tumors. Animal models generated by deletion of mTOR upstream regulators in different types of NSCs reproduce faithfully some of the TSC neurological alterations. Thus, mTOR dysregulation in NSCs seems to be responsible for the derangement of their homeostasis, thus leading to TSC development. Here we review recent advances in the molecular dissection of the mTOR cascade, its involvement in the maintenance of stem cell compartments, and in particular the implications of mTOR hyperactivation in NSCs in vivo and in vitro.     

Stem cells and their niche: a matter of fate

Embryonic stem cells provide an in vitro model for developmental biologists to study cell fate decisions during ontogenesis, while somatic stem cells allow physiologists to understand tissue homeostasis in the adult. The behavior of stem cells is dependent on an intimate relationship with a supportive niche. This brief review highlights some of the most important recent trends in stem cell biology, focusing in particular on the supportive microenvironments for both embryonic and adult stem cells. Known intrinsic and extrinsic molecular players from the best-characterized stem cell types are summarized, illuminating a number of shared environmental cues among tissues originating from all three embryonic germ layers. Received 6 October 2005; received after revision 27 December 2005; accepted 17 January 2006     

Cadherin 22 participates in the self-renewal of mouse female germ line stem cells via interaction with JAK2 and β-catenin

The self-renewal capacity of the stem cell pool determines tissue function and health. Cadherin-22 (Cdh22), a member of the cadherin superfamily, has two splicing patterns in rats, and the short type that lacks a catenin binding domain is closely related to spermatogonial stem cell self-renewal. Previously, we reported that CDH22 was highly expressed in mouse ovary germ cells, especially in female germ line stem cells (FGSCs). However, its underlying function in FGSCs is still not clear. Here, we found that Cdh22 encodes only one type of protein product in mice and demonstrated that CDH22 was required for FGSC self-renewal. In addition, JAK2 and β-catenin were found to interact with CDH22 and be involved in CDH22 signaling in mouse FGSCs. Moreover, extrinsic CDH22 was identified as a potential molecule that participates in FGSC adhesion and is pivotal for FGSC maintenance and self-renewal. These results reveal that CDH22 functions as an essential molecule in FGSC maintenance and self-renewal via different mechanisms, including interaction with the JAK-STAT signaling pathway and β-catenin.     

Nestin-expressing progenitor cells: function,identity and therapeutic implications

The neuroepithelial stem cell protein, or Nestin, is a cytoskeletal intermediate filament initially characterized in neural stem cells. However, current extensive evidence obtained in in vivo models and humans shows presence of Nestin⁺ cells with progenitor and/or regulatory functions in a number of additional tissues, remarkably bone marrow. This review presents the current knowledge on the role of Nestin in essential stem cell functions, including self-renewal/proliferation, differentiation and migration, in the context of the cytoskeleton. We further discuss the available in vivo models for the study of Nestin⁺ cells and their progeny, their function and elusive nature in nervous system and bone marrow, and their potential mechanistic role and promising therapeutic value in preclinical models of disease. Future improved in vivo models and detection methods will allow to determine the true essence of Nestin⁺ cells and confirm their potential application as therapeutic target in a range of diseases.