Correlates between hematopoiesis and neuropoiesis: neural stem cells

The aryl hydrocarbon receptor has a normal function in the regulation of hematopoietic and other stem/progenitor cell populations

The aryl hydrocarbon receptor (AhR) is known mainly as the mediator for the toxicity of certain xenobiotics. However, there is also much information to indicate that this transcription factor has important biological functions. Here we review the evidence that the AhR has a significant role in the regulation of hematopoietic stem cells (HSCs). Data to support this come from studies with xenobiotic AhR ligands, phenotypic analyses of mice lacking AhR, examining the presence and regulation of the AhR within HSCs, knowledge of genes and signaling pathways regulated by the AhR, and investigations of hematopoietic disorders. Based on this information, we hypothesize that AhR expression is necessary for the proper maintenance of quiescence in HSCs, and that AhR down-regulation is essential for "escape" from quiescence and subsequent proliferation of these cells. This implicates the AhR as a negative regulator of hematopoiesis with a function of curbing excessive or unnecessary proliferation. This provides an important advantage by preventing the premature exhaustion of HSCs and sensitivity to genetic alterations, thus preserving HSC function and long-term multi-lineage generation over the lifespan of the organism. This also implicates a role of the AhR in aging processes. AhR dysregulation may result in the altered ability of HSCs to sense appropriate signals in the bone marrow microenvironment leading to hematopoietic disease. It is also reasonable to hypothesize that this protein has an important function in the regulation of other tissue stem cell populations. Suggestive evidence is consistent with a role in skin and neural stem cells.

SOX2, a persistent marker for multipotential neural stem cells derived from embryonic stem cells, the embryo or the adult

Multipotent neural stem cells are present throughout the development of the central nervous system (CNS), persist into adulthood in defined locations and can be derived from more primitive embryonic stem cells. We show that SOX2, an HMG box transcription factor, is expressed in multipotent neural stem cells at all stages of mouse ontogeny. We have generated transgenic mice expressing enhanced green fluorescent protein (EGFP) under the control of the endogenous locus-regulatory regions of the Sox2 gene to prospectively identify neural stem/progenitor cells in vivo and in vitro. Fluorescent cells coexpress SOX2 protein, and EGFP fluorescence is detected in proliferating neural progenitor cells of the entire anterior-posterior axis of the CNS from neural plate stages to adulthood. SOX2-EGFP cells can form neurospheres that can be passaged repeatedly and can differentiate into neurons, astrocytes and oligodendrocytes. Moreover, prospective clonal analysis of SOX2-EGFP-positive cells shows that all neurospheres, whether isolated from the embryonic CNS or the adult CNS, express SOX2-EGFP. In contrast, the pattern of SOX2-EGFP expression using randomly integrated Sox2 promoter/reporter construct differs, and neurospheres are heterogeneous for EGFP expression. These studies demonstrate that SOX2 may meet the requirements of a universal neural stem cell marker and provides a means to identify cells which fulfill the basic criteria of a stem cell: self-renewal and multipotent differentiation.

Signal transduction pathways involved in the lineage-differentiation of NSCs: can the knowledge gained from blood be used in the brain?

Neural stem cells (NSC) are capable of differentiating toward neuronal, astrocytic, oligodendrocytic and glial lineages, depending on their spatial location within the central nervous system (CNS). Although, a lot of knowledge has been gained in the understanding of differentiation-specific signaling in hematopoietic (HSC) and mesenchymal (MSC) counterparts, the molecular mechanisms underlying lineage commitment in NSCs are just beginning to be understood. Furthermore, it is not well comprehended as to how the specification of one cell lineage can result in the suppression of parallel pathways in the NSCs. Thus, a thorough understanding of various signal transduction cascades activated via cytokines and growth factors, and the confounding effects of different CNS microenvironments are critically required to determine the full potential of NSCs. Our knowledge on the clonogenic ability, differentiation potential, and the inherent plasticity in both HSCs and MSCs may facilitate the understanding of lineage commitment in the NSCs as well. The information available from the marrow-derived stem cells may be extrapolated toward the similar signaling pathways in the neural precursors. From a number of previous studies, it is apparent that four distinctly different subsets of ligand-receptor superfamilies are involved in determining the fate of NSCs. These include 1) the transforming growth factor type-beta-1 (TGF-beta1) and bone morphogenetic protein (BMP) superfamily; 2) the platelet-derived and epidermal (PDGF/EGF) growth factors; 3) the interleukin-6, leukemia inhibitory factor, and ciliary neurotrophic factor (IL-6/LIF/CNTF) superfamily; and 4) the EGF-like Notch/Delta group of extracellular ligands. Ligand binding to the cell surface receptor activates the receptor's cytosolic catalytic domain and/or the receptor-associated protein-kinases, which in turn activate intracellular second messengers and different sets of transcription factors. Transcription factor oligomerization, nuclear localization, followed by their recognition of DNA elements, leads to the expression of lineage-specific genes. Association between different groups of transcription factors can also regulate their ability to transcriptionally activate different genes. The limited availability of coactivators and cosuppressors, which can sequester the transcription factor complexes toward or away from a specific gene locus, further adds to the complexity in the cross talk between different signaling cascades. Both concerted actions of temporally regulated signals and convergent effects of different signaling cascades can thus ultimately precipitate the phenotypic changes. It is beginning to be realized that in addition to the cytokines and growth factors, cell-to-cell and cell-to-extracellular matrix (ECM) interactions, are also important within the molecular scenario linked to both proliferation and differentiation of the stem cells. The cell surface molecules, which include cell adhesion molecules (CAMs), integrins, selectins, and the immunoglobulins, are well known to regulate HSC and MSC commitment within different tissue microenvironments and may have direct implications in understanding the NSC cell fate determination within different regions of the brain.

The stem-cell menagerie

The numbers, types and locations of stem cells in the nervous system have been the subject of much discussion. This review summarizes data on the types of stem cell present at different stages of development and in the adult brain, and the markers suggested to distinguish between the various possibilities that have been reported. We present evidence that more than one class of stem cell is present in the developing and adult nervous systems, and that it might be possible to distinguish between stem-cell populations and to localize the cell of origin of a particular neurosphere, based on markers that persist in culture and by using universal stem-cell markers prospectively to identify stem cells in vivo.

Intraarterial administration of marrow stromal cells in a rat model of traumatic brain injury

To test the efficacy of various delivery routes of stem cells to treat cerebral injury, we investigated the parenchymal distribution of marrow stromal cells (MSCs) injected into the internal carotid artery (ICA) of the adult rat after traumatic brain injury (TBI). Bromodeoxyuridine (BrdU)-labeled MSCs were injected via the ipsilateral ICA at 24 h after TBI. Using histology and immunohistochemistry, the distribution of implanted MSCs was analyzed at 7 days after transplantation. Four groups (n = 4/group) were studied: group 1, animals transplanted with MSCs cultured with NGF and BDNF at 24 h after TBI; group 2, animals transplanted with MSCs cultured without NGF and BDNF; group 3, animals injected with a placebo, phosphate buffered saline into the ICA at 24 h after TBI; and group 4, rats subjected to TBI only. In groups 1 and 2, BrdU-positive cells were localized to the boundary zone of the lesion, corpus callosum and cortex of the ipsilateral hemisphere. The number of BrdU-positive cells was significantly higher in the ipsilateral hemisphere than in the contralateral hemisphere. More MSCs infused intraarterially engrafted in group 1 (18.9%) than in group 2 (14.4%, p < 0.05). Using double staining, BrdU-positive cells expressed MAP-2, NeuN, and GFAP in both groups 1 and 2, with this expression being greater in group 1 and the difference between two groups reaching statistical significance in case of MAP-2. Our data suggest that intraarterial transplantation of MSCs is a viable route for the intracerebral administration of MSCs for the treatment of TBI, since MSCs infused intraarterially after TBI survive and migrate into the brain. Some implanted MSCs express proteins specific to neurons and astrocytes. The addition of NGF and BDNF promote migration of MSCs into the brain and subsequent expression of neuronal protein MAP-2.

From hematopoiesis to neuropoiesis: evidence of overlapping genetic programs

It is reasonable to propose that gene expression profiles of purified stem cells could give clues for the molecular mechanisms of stem cell behavior. We took advantage of cDNA subtraction to identify a set of genes selectively expressed in mouse adult hematopoietic stem cells (HSC) as opposed to bone marrow (BM). Analysis of HSC-enriched genes revealed several key regulatory gene candidates, including two novel seven transmembrane (7TM) receptors. Furthermore, by using cDNA microarray techniques we found a large set of HSC-enriched genes that are expressed in mouse neurospheres (a population greatly enriched for neural progenitor cells), but not present in terminally differentiated neural cells. In situ hybridization demonstrated that many of them, including one HSC-enriched 7TM receptor, were selectively expressed in the germinal zones of fetal and adult brain, the regions harboring mouse neural stem cells. We propose that at least some of the transcripts that are selectively and commonly expressed in two or more types of stem cells define a functionally conserved group of genes evolved to participate in basic stem cell functions, including stem cell self-renewal.

Flt-3 and its ligand are expressed in neural crest-derived tumors and promote survival and proliferation of their cell lines

Flt-3 ligand (FL) is a cytokine that promotes the survival, proliferation, and differentiation of hematopoietic progenitors in synergy with other growth factors, such as stem cell factor. Previously we have demonstrated that stem cell factor and its receptor c-kit are expressed in neural crest-derived tumor cells and that a c-kit block induces their apoptosis. Here we have evaluated the expression of flt-3 and its ligand in 12 neuroectodermal tumor cell lines from neuroblastoma (NB), neuroepithelioma (NE), Ewing sarcoma (ES), and peripheral neuroectodermal tumor (PNET) and in 38 biopsies: 19 from NB and 19 from ES and PNET. RT-PCR demonstrated the expression of flt-3 and FL in all lines. Coexpression was observed in 42% of NB and in 74% of ES and PNET biopsies. Flow cytometry confirmed the presence of membrane and cytoplasmic flt-3 and membrane FL in all lines, whereas soluble FL protein was not measurable in their supernatants. Microphysiometric demonstration of acidification of the medium provided evidence of the specific response of cell lines to FL stimulation. Specific flt-3 phosphorylation after FL treatment was also demonstrated by Western blotting analysis. In cells growing in RPMI plus 1% fetal calf serum, FL revealed a significant proliferating activity, more evident in NB and NE lines (mean increase of viable cells, 73 +/- 26% after 1 day). Treatment with flt-3 antisense oligonucleotides significantly inhibited cell growth. FL also displayed an antiapoptotic activity: after a 12-hour culture in the presence of 0.1% fetal calf serum, FL caused a 50% reduction of apoptotic cells. These results provide further evidence that neuroectodermal and hematopoietic cells share common regulatory pathways, and could be of interest in the clinical management of neuroectodermal tumors.