Autonomic Dysreflexia in Spinal Cord Injury: Mechanisms and Prospective Therapeutic Targets

High-level spinal cord injury (SCI) often results in cardiovascular dysfunction, especially the development of autonomic dysreflexia. This disorder, characterized as an episode of hypertension accompanied by bradycardia in response to visceral or somatic stimuli, causes substantial discomfort and potentially life-threatening symptoms. The neural mechanisms underlying this dysautonomia include a loss of supraspinal control to spinal sympathetic neurons, maladaptive plasticity of sensory inputs and propriospinal interneurons, and excessive discharge of sympathetic preganglionic neurons. While neural control of cardiovascular function is largely disrupted after SCI, the renin-angiotensin system (RAS), which mediates blood pressure through hormonal mechanisms, is up-regulated after injury. Whether the RAS engages in autonomic dysreflexia, however, is still controversial. Regarding therapeutics, transplantation of embryonic presympathetic neurons, collected from the brainstem or more specific raphe regions, into the injured spinal cord may reestablish supraspinal regulation of sympathetic activity for cardiovascular improvement. This treatment reduces the occurrence of spontaneous autonomic dysreflexia and the severity of artificially triggered dysreflexic responses in rodent SCI models. Though transplanting early-stage neurons improves neural regulation of blood pressure, hormonal regulation remains high and baroreflex dysfunction persists. Therefore, cell transplantation combined with selected RAS inhibition may enhance neuroendocrine homeostasis for cardiovascular recovery after SCI.

Targeting the LSD1/KDM1 Family of Lysine Demethylases in Cancer and Other Human Diseases

Lysine-specific demethylase 1 (LSD1) was the first histone demethylase discovered and the founding member of the flavin-dependent lysine demethylase family (KDM1). The human KDM1 family includes KDM1A and KDM1B, which primarily catalyze demethylation of histone H3K4me1/2. The KDM1 family is involved in epigenetic gene regulation and plays important roles in various biological and disease pathogenesis processes, including cell differentiation, embryonic development, hormone signaling, and carcinogenesis. Malfunction of many epigenetic regulators results in complex human diseases, including cancers. Regulators such as KDM1 have become potential therapeutic targets because of the reversibility of epigenetic control of genome function. Indeed, several classes of KDM1-selective small molecule inhibitors have been developed, some of which are currently in clinical trials to treat various cancers. In this chapter, we review the discovery, biochemical, and molecular mechanisms, atomic structure, genetics, biology, and pathology of the KDM1 family of lysine demethylases. Focusing on cancer, we also provide a comprehensive summary of recently developed KDM1 inhibitors and related preclinical and clinical studies to provide a better understanding of the mechanisms of action and applications of these KDM1-specific inhibitors in therapeutic treatment.

In Vitro Conditioning Determines the Capacity of Dental Pulp Stem Cells to Function as Pericyte-Like Cells

Dental pulp has been revealed as an accessible and a rich source of mesenchymal stem cells (MSCs) and its biological potential is currently under intense investigation. MSCs from dental pulp stem cells (DPSCs) have been indicated as a heterogeneous population oriented not only in repairing dentine but also in maintaining vascular and nervous homeostasis of the teeth. We sought to verify the phenotype of cells isolated from dental pulp of young donors and to investigate in vitro their role as pericyte-like cells. Specifically, we evaluated how culture conditions can modulate expression of pericyte markers in DPSCs and their capacity to stabilize endothelial tubes in vitro. DPSCs cultured in standard conditions expressed MSC markers and demonstrated to contain a population expressing the pericyte marker NG2. These DPSCs were associated with low sprouting capacity in extra-cellular (EC) Matrix and limited ability in retaining tubes formed by endothelial cells in a coculture angiogenesis model. When cultured in endothelial growth medium (EGM)-2, DPSCs significantly upregulated NG2, and partially alpha-smooth muscle actin. The resulting population conserved the stem marker CD73, but was negative for calponin and endothelial markers. EGM-2-conditioned DPSCs showed a higher sprouting ability in EC Matrix and efficient association with human umbilical vein endothelial cells allowing the partial retention of endothelial tubes for several days. Among growth factors contained in EGM-2 we identified basic fibroblast growth factor (bFGF) as mainly responsible for NG2 upregulation and long-term stabilization of endothelial tubes. According to the in vitro analysis, DPSCs represent an effective source of pericytes and the appropriate culture conditions could result in a population with a promising ability to stabilize vessels and promote vascular maturation.

Injured adult motor and sensory axons regenerate into appropriate organotypic domains of neural progenitor grafts

Neural progenitor cell (NPC) transplantation has high therapeutic potential in neurological disorders. Functional restoration may depend on the formation of reciprocal connections between host and graft. While it has been reported that axons extending out of neural grafts in the brain form contacts onto phenotypically appropriate host target regions, it is not known whether adult, injured host axons regenerating into NPC grafts also form appropriate connections. We report that spinal cord NPCs grafted into the injured adult rat spinal cord self-assemble organotypic, dorsal horn-like domains. These clusters are extensively innervated by regenerating adult host sensory axons and are avoided by corticospinal axons. Moreover, host axon regeneration into grafts increases significantly after enrichment with appropriate neuronal targets. Together, these findings demonstrate that injured adult axons retain the ability to recognize appropriate targets and avoid inappropriate targets within neural progenitor grafts, suggesting that restoration of complex circuitry after SCI may be achievable.

Ecdysone and mediator change energy metabolism to terminate proliferation in Drosophila neural stem cells

Stem cells are highly abundant during early development but become a rare population in most adult organs. The molecular mechanisms causing stem cells to exit proliferation at a specific time are not well understood. Here, we show that changes in energy metabolism induced by the steroid hormone ecdysone and the Mediator initiate an irreversible cascade of events leading to cell-cycle exit in Drosophila neural stem cells. We show that the timely induction of oxidative phosphorylation and the mitochondrial respiratory chain are required in neuroblasts to uncouple the cell cycle from cell growth. This results in a progressive reduction in neuroblast cell size and ultimately in terminal differentiation. Brain tumor mutant neuroblasts fail to undergo this shrinkage process and continue to proliferate until adulthood. Our findings show that cell size control can be modified by systemic hormonal signaling and reveal a unique connection between metabolism and proliferation in stem cells.

Human dental mesenchymal stem cells and neural regeneration

Nerve tissue presents inherent difficulties for its effective regeneration. Stem cell transplantation is considered an auspicious treatment for neuronal injuries. Recently, human dental mesenchymal stem cells (DMSCs) have received extensive attention in the field of regenerative medicine due to their accessibility and multipotency. Since their origin is within the neural crest, they can be differentiated into neural crest-derived cells including neuron and glia cells both in vitro and in vivo. DMSCs are also able to secrete a wide variety of neurotrophins and chemokines, which promote neuronal cells to survival and differentiation. Experimental evidence has shown that human DMSCs engraftment recovered neuronal tissue damage in animal models of central nervous system injuries. Human DMSCs can be a new hope for treatment of nervous system diseases and deficits such as spinal cord injury, stroke and Parkinson's disease.

Identification and culture of olfactory neural progenitors from GFP mice

The olfactory epithelium (OE) is one of the best sources for obtaining adult stem cells from the nervous system, because it contains neural progenitors that regenerate continuously throughout life. The OE is accessible through the nasal cavity, which facilitates stem cell harvest for examination and transplantation. The mitotic activity of OE progenitors can be stimulated by intranasal irrigation with zinc sulfate (ZnSO4). In the study reported here, we focused on OE from a transgenic mouse line transfected with green fluorescent protein (GFP). Histological examination demonstrated the site of highest yield of OE in the transgenic and wild type littermates. Cultures were established from that site four days in vitro following ZnSO4 exposure. The GFP-derived primary cultures contained a heterogeneous population of fluorescent cells. After 10-12 days, a population of round, mitotically active cells emerged that formed fluorescent neurospheres. The neurosphere forming cells (NSFCs) were collected and subcultured up to four times. The NSFCs were primarily neuronal with only a few cells of glial lineage. Furthermore, the NSFCs were nestin positive and keratin negative, suggesting that they were neural progenitors. The endogenous GFP fluorescence of these cells provides a readily identifiable label that will facilitate their identification following transplantation into nontransfected hosts. They should provide a useful model for evaluating the potential therapeutic utility of OE progenitors in neurodegenerative diseases and neurotrauma repair.

Stem and progenitor cell therapies: recent progress for spinal cord injury repair

Mechanical trauma to the spinal cord is often accompanied by irreversible tissue damage, limited endogenous repair and permanent loss of motor, sensory and autonomic function. The implantation of exogenous cells or the stimulation of endogenous cells, to repopulate and replace or to provide a conducive environment for repair, offers a promising therapeutic direction for overcoming the multitude of obstacles facing successful recovery from spinal cord injury. Although relatively new to the scene of cell based therapies for reparative medicine, stem cells and their progenitors have been labeled as the 'cell of the future' for revolutionizing the treatment of CNS injury and neurodegenerative disorders. The following review examines the different types of stem cells and their progenitors, their utility in experimental models of spinal cord injury and explores the outstanding issues that still need to be addressed before they move towards clinical implementation.

Glial precursor cell transplantation therapy for neurotrauma and multiple sclerosis

Traumatic injury to the brain or spinal cord and multiple sclerosis (MS) share a common pathophysiology with regard to axonal demyelination. Despite advances in central nervous system (CNS) repair in experimental animal models, adequate functional recovery has yet to be achieved in patients in response to any of the current strategies. Functional recovery is dependent, in large part, upon remyelination of spared or regenerating axons. The mammalian CNS maintains an endogenous reservoir of glial precursor cells (GPCs), capable of generating new oligodendrocytes and astrocytes. These GPCs are upregulated following traumatic or demyelinating lesions, followed by their differentiation into oligodendrocytes. However, this innate response does not adequately promote remyelination. As a result, researchers have been focusing their efforts on harvesting, culturing, characterizing, and transplanting GPCs into injured regions of the adult mammalian CNS in a variety of animal models of CNS trauma or demyelinating disease. The technical and logistic considerations for transplanting GPCs are extensive and crucial for optimizing and maintaining cell survival before and after transplantation, promoting myelination, and tracking the fate of transplanted cells. This is especially true in trials of GPC transplantation in combination with other strategies such as neutralization of inhibitors to axonal regeneration or remyelination. Overall, such studies improve our understanding and approach to developing clinically relevant therapies for axonal remyelination following traumatic brain injury (TBI) or spinal cord injury (SCI) and demyelinating diseases such as MS.

Injuring neurons induces neuronal differentiation in a population of hippocampal precursor cells in culture

A novel population of hippocampal precursor cells (HPCs) that can be induced to differentiate into astrocytes and oligodendrocytes can be derived from hippocampal cultures grown in serum-free media. The HPCs are PDGF-responsive, do not proliferate with bFGF, and grow as sheets of cells rather than gathering into neurospheres. The HPCs share many markers (A2B5, GD3, poly-sialylated neuronal common adhesion molecule (PSA-NCAM), and NG2) with oligodendrocyte precursor cells (OPCs). The HPCs do not express markers for mature neurons, astrocytes, or oligodendrocytes. Like OPCs, the HPCs differentiate into glial fibrillary acidic protein (GFAP)+ astrocytes and GalC+ oligodendrocytes with the addition of bone morphogenetic protein-4 (BMP-4) and triiodothyronine (T3), respectively. They do not differentiate into neurons with the addition or withdrawal of basic fibroblast growth factor (bFGF), brain-derived neurotrophic factor (BDNF), or retinoic acid (RA). These HPCs can be stimulated to differentiate into neuron-like cells by the induction of neuronal injury or cell death in nearby cultured neurons or by conditioned medium from injured neuronal cultures. Under these conditions, HPCs grow larger, develop more extensive dendritic processes, become microtubule-associated protein-2-immunoreactive, express large voltage-dependent sodium currents, and form synaptic connections. The conversion of endogenous pluripotent precursor cells into neurons in response to local brain injury may be an important component of central nervous system homeostasis.

Lineage-restricted neural precursors survive, migrate, and differentiate following transplantation into the injured adult spinal cord

Fetal spinal cord from embryonic day 14 (E14/FSC) has been used for numerous transplantation studies of injured spinal cord. E14/FSC consists primarily of neuronal (NRP)- and glial (GRP)-restricted precursors. Therefore, we reasoned that comparing the fate of E14/FSC with defined populations of lineage-restricted precursors will test the in vivo properties of these precursors in CNS and allow us to define the sequence of events following their grafting into the injured spinal cord. Using tissue derived from transgenic rats expressing the alkaline phosphatase (AP) marker, we found that E14/FSC exhibited early cell loss at 4 days following acute transplantation into a partial hemisection injury, but the surviving cells expanded to fill the entire injury cavity by 3 weeks. E14/FSC grafts integrated into host tissue, differentiated into neurons, astrocytes, and oligodendrocytes, and demonstrated variability in process extension and migration out of the transplant site. Under similar grafting conditions, defined NRP/GRP cells showed excellent survival, consistent migration out of the injury site and robust differentiation into mature CNS phenotypes, including many neurons. Few immature cells remained at 3 weeks in either grafts. These results suggest that by combining neuronal and glial restricted precursors, it is possible to generate a microenvironmental niche where emerging glial cells, derived from GRPs, support survival and neuronal differentiation of NRPs within the non-neurogenic and non-permissive injured adult spinal cord, even when grafted into acute injury. Furthermore, the NRP/GRP grafts have practical advantages over fetal transplants, making them attractive candidates for neural cell replacement.

Functional SDF1 alpha/CXCR4 signaling in the developing spinal cord

Stromal cell-derived factor (SDF1) and its cognate receptor CXCR4 have been shown to play a central role in the development of the cerebellum, hippocampus, and neocortex. However, little is known about the functions of SDF1/CXCR4 in early spinal cord progenitor cell differentiation. Here, we show that a functional SDF1alpha/CXCR4 signaling pathway is present in developing spinal cord cells (a spliced variant of SDF1). RT-PCR analysis of SDF1alpha and CXCR4 showed that they were present in E10.5 neural tube and their expression increased as neuroepithelial cells differentiated into more committed spinal cord progenitors. Stimulation of the more differentiated progenitors (E14.5) with SDF1alpha resulted in rapid activation of the extracellular signal-regulated kinase (ERK)1/2. This SDF1alpha-induced ERK activity was dose dependent and could be inhibited by pre-treatment of the cells with either pertussis toxin, an inactivator of G-protein-coupled receptors, or PD98059, a MEK1 inhibitor. Concomitant with ERK activation, SDF1alpha also activated the downstream transcription factor Ets, a substrate for ERK phosphorylation. Further, downstream activation of genes associated with cell survival, differentiation and migration was assessed using a G-protein-coupled receptor pathway-focused microarray. We found that 23 genes, including PDK1, Egr-1, Grm5, and E-selectin, were up-regulated by SDF1alpha. Furthermore, SDF1alpha induced chemotaxis in both neural and glial progenitors in in vitro migration assays. Pre-treatment of the cells with either pertussis toxin or PD98059 completely inhibited SDF1alpha-induced chemotaxis. Thus, our data suggest that SDF1alpha may function through a CXCR4/ERK/Ets-linked signalling pathway in spinal cord neural development to modulate migration of progenitor cells.

Aging and neuronal replacement

Neural stem cells contribute to neurogenesis in both the embryonic and adult brain. However, while adult neural stem cells produce new neurons that populate the olfactory bulb and the granule cell layer of the hippocampus, they do not normally participate in reparative neurogenesis following injury or disease affecting regions distant from the subventricular zone or the dentate gyrus. Here we review differences between neural stem cells found in the embryo and the adult, and describe factors that enhance neuronal output from these cells in vivo. Additionally, we review evidence that neural stem cells can be transplanted into injured regions of the adult brain to enhance compensatory neurogenesis from endogenous precursors. Pre-differentiation of neural stem cells into immature neurons prior to transplantation can also aid in functional recovery following injury or disease.

In vitro differentiation of size-sieved stem cells into electrically active neural cells

Size-sieved stem (SS) cells isolated from human bone marrow and propagated in vitro are a population of cells with consistent marker typing, and can form bone, fat, and cartilage. In this experiment, we demonstrated that SS cells could be induced to differentiate into neural cells under experimental cell culture conditions. Five hours after exposure to antioxidant agents (beta-mercaptoethanol +/- retinoic acid) in serum-free conditions, SS cells expressed the protein for nestin, neuron-specific enolase (NSE), neuron-specific nuclear protein (NeuN), and neuron-specific tubulin-1 (TuJ-1), and the mRNA for NSE and Tau. Immunofluorescence showed that almost all the cells (>98%) expressed NeuN and TuJ-1. After 5 days of beta-mercaptoethanol treatment, the SS cells expressed neurofilament high protein but not mitogen-activated protein-2, glial filament acidic protein, and galactocerebroside. For such long-term-treated cells, voltage-sensitive ionic current could be detected by electrophysiological recording, and the intracellular calcium ion, Ca(2+), concentration can be elevated by high potassium (K(+)) buffer and glutamate. These findings suggest that SS cells may be an alternative source of undifferentiated cells for cell therapy and gene therapy in neural dysfunction.

Neurogenin3 participates in gliogenesis in the developing vertebrate spinal cord

To study the role of basic helix-loop-helix (bHLH) transcription factors in gliogenesis, we examined whether bHLH transcription factors were expressed in glial precursor cells and participated in regulating oligodendrocyte and astrocyte development. As assessed by reverse transcription-polymerase chain reaction (RT-PCR), Neurogenin3 (Ngn3) was transiently expressed in bipotential glial cells fated to become either oligodendrocytes or astrocytes. Mice lacking Ngn3 displayed a loss of Nkx2.2 expression, a transcription factor required for proper oligodendrogliogenesis. Furthermore, a reduction in the expression of myelin basic protein (MBP), proteolipid protein (PLP), and glial fibrillary acidic protein (GFAP), markers for mature oligodendrocytes and astrocytes, was observed in the Ngn3 null mice. Overexpression of Ngn3 was sufficient to drive expression from the PLP promoter in transient cotransfection assays. Overall, the data suggest that Ngn3 may regulate glial differentiation at a developmental stage prior to the segregation of the oligodendrocyte and astrocyte lineage.

Microarray analysis of selected genes in neural stem and progenitor cells

To access and compare gene expression in fetal neuroepithelial cells (NEPs) and progenitor cells, we have used microarrays containing approximately 500 known genes related to cell cycle regulation, apoptosis, growth and differentiation. We have identified 152 genes that are expressed in NEPs and 209 genes expressed by progenitor cells. The majority of genes (141) detected in NEPs are also present in progenitor populations. There are 68 genes specifically expressed in progenitors with little or no expression in NEPs, and a few genes that appear to be present exclusively in NEPs. Using cell sorting, RT-PCR, in situ hybridization or immunocytochemistry, we have examined the segregation of expression to neuronal and glial progenitors, and identified several that appeared to be enriched in neuronal (e.g. CDK5, neuropilin, EphrinB2, FGF11) or glial (e.g. CXCR4, RhoC, CD44, tenascin C) precursors. Our data provide a first report of gene expression profiles of neural stem and progenitor cells at early stages of development, and provide evidence for the potential roles of specific cell cycle regulators, chemokines, cytokines and extracellular matrix molecules in neural development and lineage segregation.

Isolation of a glial-restricted tripotential cell line from embryonic spinal cord cultures

Neuroepithelial stem cells (NEPs), glial-restricted precursors (GRPs), and neuron-restricted precursors (NRPs) are present during early differentiation of the spinal cord and can be identified by cell surface markers. In this article, we describe the properties of GRP cells that have been immortalized using a regulatable v-myc retrovirus construct. Immortalized GRP cells can be maintained in an undifferentiated dividing state for long periods and can be induced to differentiate into two types of astrocytes and into oligodendrocytes in culture. A clonal cell line prepared from immortalized GRP cells, termed GRIP-1, was also shown to retain the properties of a glial-restricted tripotential precursor. Transplantation of green fluorescent protein (GFP)-labeled subclones of the immortalized cells into the adult CNS demonstrates that this cell line can also participate in the in vivo development of astrocytes and oligodendrocytes. Late passages of the immortalized cells undergo limited transdifferentiation into neurons as assessed by expression of multiple neuronal markers. The availability of a conditionally immortalized cell line obviates the difficulties of obtaining a large and homogeneous population of GRPs that can be used for studying the mechanism and signals for glial cell differentiation as well as their application in transplantation protocols.

Distinct sites of origin of oligodendrocytes and somatic motoneurons in the chick spinal cord: oligodendrocytes arise from Nkx2.2-expressing progenitors by a Shh-dependent mechanism

In the vertebrate spinal cord, oligodendrocytes arise from the ventral part of the neuroepithelium, a region also known to generate somatic motoneurons. The emergence of oligodendrocytes, like that of motoneurons, depends on an inductive signal mediated by Sonic hedgehog. We have defined the precise timing of oligodendrocyte progenitor specification in the cervico-brachial spinal cord of the chick embryo. We show that ventral neuroepithelial explants, isolated at various development stages, are unable to generate oligodendrocytes in culture until E5 but become able to do so in an autonomous way from E5.5. This indicates that the induction of oligodendrocyte precursors is a late event that occurs between E5 and E5.5, precisely at the time when the ventral neuroepithelium stops producing somatic motoneurons. Analysis of the spatial restriction of oligodendrocyte progenitors, evidenced by their expression of O4 or PDGFR(α), indicate that they always lie within the most ventral Nkx2.2-expressing domain of the neuroepithelium, and not in the adjacent domain characterized by Pax6 expression from which somatic motoneurons emerge. We then confirm that Shh is necessary between E5 and E5.5 to specify oligodendrocyte precursors but is no longer required beyond this stage to maintain ongoing oligodendrocyte production. Furthermore, Shh is sufficient to induce oligodendrocyte formation from ventral neuroepithelial explants dissected at E5. Newly induced oligodendrocytes expressed Nkx2.2 but not Pax6, correlating with the in vivo observation. Altogether, our results show that, in the chick spinal cord, oligodendrocytes originate from Nkx2.2-expressing progenitors.

Comparison of neural precursor cell fate in second trimester human brain and spinal cord

Neural transplantation holds promise for the treatment of traumatic brain and spinal cord injury by replacing lost cellular elements as well as repairing neural damage. Fetal human stem cells derived from central nervous system (CNS) tissue are potential transplantable sources for all cell types found in the mature human nervous system including neurons, astrocytes and oligodendroglia. Although nearly all areas of the fetal human neuraxis contain undifferentiated neural precursor cells, the phenotypic fate of the daughter cells might vary from one region to another during a specific developmental period. The purpose of this study was to compare the various cell types derived from neural precursors cultured from second trimester fetal human brain and spinal cord. To this end, brains (n = 8) and spinal cords (n = 8) of 15-24 week fetuses were dissociated and grown in culture medium supplemented with epidermal growth factor (EGF), basic fibroblast growth factor (FGF) and leukemia inhibitory factor (LIF). The proliferating precursor cells from both brain and spinal cord grew as spherical masses that were plated on laminin-coated dishes after seven days in culture. During the next 5-7 days, the cells that emerged from these spheres were fixed and processed for immunocytochemistry. Brain derived spheres gave rise to cells expressing antigens specific for neurons (MAP-2ab and neuron specific-intermediate filaments), astrocytes (GFAP) and oligodendrocytes (A007). In contrast, cells that emerged from spinal cord derived spheres were only immunoreactive for GFAP. These data suggest that neuroepithelial precursor cells from different CNS regions, although similar in their responsiveness to proliferative growth factors, might differ in their ability to generate different cell types in the adult CNS.

Sonic hedgehog promotes G(1) cyclin expression and sustained cell cycle progression in mammalian neuronal precursors

Sonic hedgehog (Shh) signal transduction via the G-protein-coupled receptor, Smoothened, is required for proliferation of cerebellar granule neuron precursors (CGNPs) during development. Activating mutations in the Hedgehog pathway are also implicated in basal cell carcinoma and medulloblastoma, a tumor of the cerebellum in humans. However, Shh signaling interactions with cell cycle regulatory components in neural precursors are poorly understood, in part because appropriate immortalized cell lines are not available. We have utilized primary cultures from neonatal mouse cerebella in order to determine (i) whether Shh initiates or maintains cell cycle progression in CGNPs, (ii) if G(1) regulation by Shh resembles that of classical mitogens, and (iii) whether individual D-type cyclins are essential components of Shh proliferative signaling in CGNPs. Our results indicate that Shh can drive continued cycling in immature, proliferating CGNPs. Shh treatment resulted in sustained activity of the G(1) cyclin-Rb axis by regulating levels of cyclinD1, cyclinD2, and cyclinE mRNA transcripts and proteins. Analysis of CGNPs from cyclinD1(-/-) or cyclinD2(-/-) mice demonstrates that the Shh proliferative pathway does not require unique functions of cyclinD1 or cyclinD2 and that D-type cyclins overlap functionally in this regard. In contrast to many known mitogenic pathways, we show that Shh proliferative signaling is mitogen-activated protein kinase independent. Furthermore, protein synthesis is required for early effects on cyclin gene expression. Together, our results suggest that Shh proliferative signaling promotes synthesis of regulatory factor intermediates that upregulate or maintain cyclin gene expression and activity of the G(1) cyclin-Rb axis in proliferating granule neuron precursors.

Region-specific differentiation of neural tube-derived neuronal restricted progenitor cells after heterotopic transplantation

Spinal cord neuronal restricted progenitor (NRP) cells, when transplanted into the neonatal anterior forebrain subventricular zone, migrate to distinct regions throughout the forebrain including the olfactory bulb, frontal cortex, and occipital cortex but not to the hippocampus. Their migration pattern and differentiation potential is distinct from anterior forebrain subventricular zone NRPs. Irrespective of their final destination, NRP cells do not differentiate into glia. Rather they synthesize neurotransmitters, acquire region-specific phenotypes, and receive synapses from host neurons after transplantation. Spinal cord NRPs express choline acetyl transferase even in regions where host neurons do not express this marker. The restricted distribution of transplanted spinal cord NRP cells and their acquisition of varied region-specific phenotypes suggest that their ultimate fate and phenotype is dictated by a combination of intrinsic properties and extrinsic cues from the host.

Molecular genetic classification of central nervous system malformations

Traditional schemes of classifying nervous system malformations are based on descriptive morphogenesis of anatomic processes of ontogenesis, such as neurulation, neuroblast migration, and axonal pathfinding. This proposal is a first attempt to incorporate the recent molecular genetic data that explain programming of development etiologically. A scheme based purely on genetic mutations would not be practical, in part because only in a few dysgeneses are the specific defects known, but also because several genes might be involved sequentially and many genes inhibit or augment the expression of others. The same genes serve different functions at different stages and are involved in multiple organ systems. Some complex malformations, such as holoprosencephaly, result from several unrelated defective genes. Finally, a pure genetic classification would be too inflexible to incorporate some anatomic criteria. The basis for the proposed scheme is, therefore, disturbances in patterns of genetic expression; polarity gradients of the axes of the neural tube (eg, upregulation or downregulation of genetic influences); segmentation (eg, deletions of specific neuromeres, ectopic expression); mutations that cause change in cell lineage (eg, dysplastic gangliocytoma of cerebellum, myofiber differentiation within brain); and specific genes or molecules that mediate neuroblast migration in its early (eg, filamin-1), middle (eg, LIS1, double-cortin), or late course (eg, reelin, L1-CAM). The proposed scheme undoubtedly will undergo many future revisions, but it provides a starting point using currently available data.

Neurons from stem cells: Implications for understanding nervous system development and repair

Neurodegenerative diseases cost the economies of the developed world billions of dollars per annum. Given ageing population profiles and the increasing extent of this problem, there has been a surge of interest in neural stem cells and in neural differentiation protocols that yield neural cells for therapeutic transplantation. Due to the oncogenic potential of stem cells a better characterisation of neural differentiation, including the identification of new neurotrophic factors, is required. Stem cell cultures undergoing synchronous in vitro neural differentiation provide a valuable resource for gene discovery. Novel tools such as microarrays promise to yield information regarding gene expression in stem cells. With the completion of the yeast, C. elegans, Drosophila, human, and mouse genome projects, the functional characterisation of genes using genetic and bioinformatic tools will aid in the identification of important regulators of neural differentiation.Key words: neural differentiation, neural precursor cell, brain repair, central nervous system repair, CNS.

Immunocytochemical and physiological characterization of a population of cultured human neural precursors

Human neural precursor cells (HNPC) have recently become commercially available. In an effort to determine the usefulness of these cells for in vitro studies, we have grown cultured HNPCs (cHNPCs) according to the supplier specifications. Here we report our characterization of cHNPCs under nondifferentiating and differentiating growth conditions and make a comparison to primary HNPCs (pHNPCs) obtained at the same developmental time point from a different commercial supplier. We found that under nondifferentiating conditions, cHNPCs expressed nestin, divided rapidly, expressed few markers of differentiated cells, and displayed both 4-aminopyridine (4-AP)-sensitive and delayed-rectifier type K(+) currents. No inward currents were observed. On changing to differentiating culture conditions, a majority of the cells expressed neuronal markers, did not divide, expressed inward and outward time- and voltage-dependent currents, and responded to the application of the neurotransmitters acetylcholine and glutamate. The outward current densities were indistinguishable from those in undifferentiated cells. The inward currents included TTX-sensitive and -resistant Na(+) currents, sustained Ca(2+) currents, and an inwardly rectifying K(+) current. Comparison of the properties of differentiated cells from cHNPCs with neurons obtained from primary fetal cultures (pHNPCs) revealed two major differences: the differentiated cHNPCs did not express embryonic neural cell adhesion molecule (E-NCAM) immunoreactivity but did co-express GFAP immunoreactivity. The co-expression of neuronal and glial markers was likely due to the growth of cells in serum containing medium as the pHNPCs that were never exposed to serum did express E-NCAM and did not co-express glial fibrillary acidic protein (GFAP). The relevance of these results is discussed and compared with results from other neuronal progenitor populations and cultured human neuronal cells.

Gliogenesis in the central nervous system

Multipotential neuroepithelial stem cells are thought to give rise to all the differentiated cells of the central nervous system (CNS). The developmental potential of these multipotent stem cells becomes more restricted as they differentiate into progressively more committed cells and ultimately into mature neurons and glia. In studying gliogenesis, the optic nerve and spinal cord have become invaluable models and the progressive stages of differentiation are being clarified. Multiple classes of glial precursors termed glial restricted precursors (GRP), oligospheres, oligodendrocyte-type2 astrocyte (O-2A) and astrocyte precursor cells (APC) have been identified. Similar classes of precursor cells can be isolated from human neural stem cell cultures and from embryonic stem (ES) cell cultures providing a non-fetal source of such cells. In this review, we discuss gliogenesis, glial stem cells, putative relationships of these cells to each other, factors implicated in gliogenesis, and therapeutic applications of glial precursors.

Multipotent and restricted precursors in the central nervous system

Acquisition of cell type-specific properties in the nervous system is likely a process of sequential restriction in developmental potential. At least two classes of pluripotent stem cells, neuroepithelial (NEP) stem cells and EGF-dependent neurosphere stem cells, have been identified in distinct spatial and temporal domains. Pluripotent stem cells likely generate central nervous system (CNS) and peripheral nervous system (PNS) derivatives via the generation of intermediate lineage-restricted precursors that differ from each other and from multipotent stem cells. Neuronal precursors termed neuronal-restricted precursors (NRPs), multiple classes of glial precursors termed glial-restricted precursors (GRPs), oligodendrocyte-type 2 astrocytes (O2As), astrocyte precursor cells (APCs), and PNS precursors termed neural crest stem cells (NCSCs) have been identified. Multipotent stem cells and restricted precursor cells can be isolated from embryonic stem (ES) cell cultures providing a non-fetal source of such cells. Analysis in multiple species illustrates similarities between rat, mouse, and human cell differentiation raising the possibility that similar factors and markers may be used to isolate precursor cells from human tissue or ES cells. Anat Rec (New Anat): 257:137-143, 1999.