Human neural precursor cells express low levels of telomerase in vitro and show diminishing cell proliferation with extensive axonal outgrowth following transplantation

The mood stabilizer valproic acid stimulates GABA neurogenesis from rat forebrain stem cells

Valproate, an anticonvulsant drug used to treat bipolar disorder, was studied for its ability to promote neurogenesis from embryonic rat cortical or striatal primordial stem cells. Six days of valproate exposure increased by up to fivefold the number and percentage of tubulin beta III-immunopositive neurons, increased neurite outgrowth, and decreased by fivefold the number of astrocytes without changing the number of cells. Valproate also promoted neuronal differentiation in human fetal forebrain stem cell cultures. The neurogenic effects of valproate on rat stem cells exceeded those obtained with the neurotrophins brain-derived growth factor (BDNF) or NT-3, and slightly exceeded the effects obtained with another mood stabilizer, lithium. No effect was observed with carbamazepine. Most of the newly formed neurons were GABAergic, as shown by 10-fold increases in neurons that immunostained for GABA and the GABA-synthesizing enzyme GAD65/67. Double immunostaining for bromodeoxyuridine and tubulin beta III showed that valproate increased by four- to fivefold the proliferation of neuronal progenitors derived from rat stem cells and increased cyclin D2 expression. Valproate also regulated the expression of survival genes, Bad and Bcl-2, at different times of treatment. The expression of prostaglandin E synthase, analyzed by quantitative RT-PCR, was increased by ninefold as early as 6 h into treatment by valproate. The enhancement of GABAergic neuron numbers, neurite outgrowth, and phenotypic expression via increases in the neuronal differentiation of neural stem cell may contribute to the therapeutic effects of valproate in the treatment of bipolar disorder.

Isolation and characterization of tumorigenic, stem-like neural precursors from human glioblastoma

Transformed stem cells have been isolated from some human cancers. We report that, unlike other brain cancers, the lethal glioblastoma multiforme contains neural precursors endowed with all of the critical features expected from neural stem cells. Similar, yet not identical, to their normal neural stem cell counterpart, these precursors emerge as unipotent (astroglial) in vivo and multipotent (neuronal-astroglial-oligodendroglial) in culture. More importantly, these cells can act as tumor-founding cells down to the clonal level and can establish tumors that closely resemble the main histologic, cytologic, and architectural features of the human disease, even when challenged through serial transplantation. Thus, cells possessing all of the characteristics expected from tumor neural stem cells seem to be involved in the growth and recurrence of adult human glioblastomas multiforme.

Stem cell therapy for Parkinson?s disease: where do we stand?

A major neuropathological feature of Parkinson's disease (PD) is the loss of nigrostriatal dopaminergic neuron. Patients exhibit motor symptoms, including bradykinesia, rigidity, and tremor. Neural grafting has been reported to restore striatial dopaminergic neurotransmission and induce symptomatic relief. The major limitation of cell replacement therapy for PD is the shortage of suitable donor tissue. The present review describes the possible sources of cells, including embryonic stem cells and somatic adult stem cells, both of which potentially could be used in cell therapy for PD, and discusses the advantages and disadvantages of each cell type.

Telomere shortening and chromosomal instability abrogates proliferation of adult but not embryonic neural stem cells

Chromosome integrity is essential for cell viability and, therefore, highly proliferative cell types require active telomere elongation mechanisms to grow indefinitely. Consistently, deletion of telomerase activity in a genetically modified mouse strain results in growth impairments in all highly proliferative cell populations analyzed so far. We show that telomere attrition dramatically impairs the in vitro proliferation of adult neural stem cells (NSCs) isolated from the subventricular zone (SVZ) of telomerase-deficient adult mice. Reduced proliferation of postnatal neurogenic progenitors was also observed in vivo, in the absence of exogenous mitogenic stimulation. Strikingly, severe telomere erosion resulting in chromosomal abnormalities and nuclear accumulation of p53 did not affect the in vitro proliferative potential of embryonic NSCs. These results suggest that intrinsic differences exist between embryonic and adult neural progenitor cells in their response to telomere shortening, and that some populations of tissue-specific stem cells can bypass DNA damage check points.

Functional integration of embryonic stem cell-derived neurons in vivo

Pluripotency and the potential for continuous self-renewal make embryonic stem (ES) cells an attractive donor source for neuronal cell replacement. Despite recent encouraging results in this field, little is known about the functional integration of transplanted ES cell-derived neurons on the single-cell level. To address this issue, ES cell-derived neural precursors exhibiting neuron-specific enhanced green fluorescent protein (EGFP) expression were introduced into the developing brain. Donor cells implanted into the cerebral ventricles of embryonic rats migrated as single cells into a variety of brain regions, where they acquired complex morphologies and adopted excitatory and inhibitory neurotransmitter phenotypes. Synaptic integration was suggested by the expression of PSD-95 (postsynaptic density-95) on donor cell dendrites, which in turn were approached by multiple synaptophysin-positive host axon terminals. Ultrastructural and electrophysiological data confirmed the formation of synapses between host and donor cells. Ten to 21 d after birth, all EGFP-positive donor cells examined displayed active membrane properties and received glutamatergic and GABAergic synaptic input from host neurons. These data demonstrate that, at the single-cell level, grafted ES cell-derived neurons undergo morphological and functional integration into the host brain circuitry. Antibodies to the region-specific transcription factors Bf1, Dlx, En1, and Pax6 were used to explore whether functional donor cell integration depends on the acquisition of a regional phenotype. Our data show that incorporated neurons frequently exhibit a lacking or ectopic expression of these transcription factors. Thus, the lack of an appropriate regional "code" does not preclude morphological and synaptic integration of ES cell-derived neurons.

Combining Growth Factors, Stem Cells, and Gene Therapy for the Aging Brain

Stem cells have been suggested as a possible "fountain of youth" for replacing tissues lost during aging. In the brain, replacing lost neurons is a challenge, as they have to then be reconnected with their appropriate targets. Perhaps a more realistic and practical strategy for affecting the aging process would be to prevent the loss of neurons from occurring, thus retaining intact circuitry. Glial cell line-derived neurotrophic factor (GDNF) can reverse some aspects of aging in the monkey. Additionally, we have recently shown that GDNF directly infused into the human brain has significant effects on the symptoms of Parkinson disease. Human neural stem cells can be cultured, genetically modified, and transplanted. As such, these cells are ideal for ex vivo gene therapy, and may be used in the future as "minipumps" to release GDNF in vivo to protect aging neurons. Using such an approach could delay the effects of aging in the brain, giving a better quality of life. Stem cells might not be the fountain of youth, but provide a fountain of youth through the release of growth factors such as GDNF.

Long-term molecular and cellular stability of human neural stem cell lines

Human Neural Stem Cells (hNSCs) are excellent candidates for in vitro and in vivo molecular, cellular, and developmental research, and also for ex-vivo gene transfer and cell therapy in the nervous system. However, hNSCs are mortal somatic cells, and thus invariably enter an irreversible growth arrest after a finite number of cell divisions in culture. It has been proposed that this is due to telomere shortening. Here, we show that long-term cultured (up to 4 years) v-myc perpetuated hNSC lines do preserve short but stable and homogeneous telomeres (TRF and Q-FISH determinations). hNSC lines (but not strains) express high levels of telomerase activity, which is activated by v-myc, as demonstrated here. Telomerase activity is not constitutive, becoming non-detectable after differentiation (in parallel to v-myc down-regulation). hNSC lines also maintain a stable cell cycle length, mitotic potential, differentiation and neuron generation capacity, and do not express senescence-associated beta-galactosidase over years, as studied here. These data, collectively, help to explain the immortal nature of v-myc-perpetuated hNSC lines, and to establish them as excellent research tools for basic and applied neurobiological and translational studies.

Neural precursor cells derived from human embryonic brain retain regional specificity

Recent studies have revealed that neural precursor cells can be expanded not only from the subventricular zone and hippocampus but also from other regions of the human embryonic brain. To determine the regional differences of these precursor cells, we divided the brain of a 9-week-old human embryo into four parts, i.e., telencephalon, diencephalon, mesencephalon, and rhombencephalon. All cultures of the tissues yielded neurospheres, and these spheres gave rise to neurons, astrocytes, and oligodendrocytes. An analysis of clonal populations revealed that these precursor cells were multipotent, and two region-specific differences in neural precursor cells were revealed: 1) The precursor cells from the rostral part of the brain tended to proliferate faster than those from the caudal part, and 2) the precursor cells from the diencephalon and mesencephalon gave rise to more tyrosine hydoxylase (TH)-positive neurons than those from the telencephalon and rhombencephalon. When 50-day-cultured spheres were caused to differentiate, the percentage of TH-positive cells per total cell population was 1.2% for diencephalic and mesencephalic precursors, whereas it was 0.4% for telencephalic and rhombencephalic ones. Furthermore, the TH-positive cells from diencephalic and mesencephalic precursors were large, multipolar, and gamma-aminobutyric acid (GABA)-negative, which suggested that these cells were midbrain dopaminergic neurons. In contrast, TH-positive cells from telencephalic and rhombencephalic precursors were small, bipolar, and GABA-positive. These results suggest that human neural precursor cells might have the potential to differentiate into a variety of cells but retain regional specificity.

Progress in cerebral transplantation of expanded neuronal stem cells

OBJECT

Given the success and limitations of human fetal primary neural tissue transplantation, neuronal stem cells (NSCs) that can be adequately expanded in culture have been the focus of numerous attempts to develop a superior source of replacement cells for restorative neurosurgery. To clarify recent progress toward this goal, the transplantation into the adult brain of NSCs, expanded in vitro before grafting, was reviewed.

METHODS

Neuronal stem cells can be expanded from a variety of sources, including embryos, fetuses, adult bone marrow, and adult brain tissue. Recent investigations of each of these expanded stem cell types have generated a large body of information along with a great number of unanswered questions regarding the ability of these cells to replace damaged neurons. Expanded NSCs offer many advantages over their primary tissue predecessors, but also may exhibit different functional abilities as grafted cells. Because expanded NSCs will most likely ultimately replace primary tissue grafting in clinical trials, this review was undertaken to focus solely on this distinct body of work and to summarize clearly the existing preclinical data regarding the in vivo successes, limits, and unknowns of using each expanded NSC type when transplanted into the adult brain.

CONCLUSIONS

Embryonic stem cell-derived cells have demonstrated appropriate neuronal phenotypes after transplantation into nonneurogenic areas of the adult brain. Understanding the mechanisms responsible for this may lead to similar success with less studied adult neuronal progenitor cells, which offer the potential for autologous NSC transplantation with less risk of tumorigenesis.

Neuronal and glial differentiation following culture of the human embryonic cortical stem cells

OBJECTIVE

To set up long-term in vitro culture system of the human neural stem cells (hNSC) and to study their biological properties.

METHODS

Human fetuses aged about 20 weeks following spontaneous abortion were adopted. A serum-free medium containing basic fibroblast growth factor and epidermal growth factor was used to make the hNSCs divide continuously in the culture. The growth curve of continually passaged cells was examined. The effects of long-term culture on the cell cycle, cell differentiation were analyzed. The cell cycles of these cells were analyzed using flow cytometry.

RESULTS

The cells from the human embryonic cortical tissue could be maintained and propagated in the presence of growth factors. Neurospheres were generated continually. Only one month after the primary culture, the precursors could be effectively discarded. The cells could be cultured for ten months. The cells had an exponential, consistent growth. The cell cycle analysis indicated that the hNSCs maintained remarkable proliferation. Upon differentiation, the hNSCs gave rise to mature cells. The multi-lineage potential of differentiation after different passages remained unchanged.

CONCLUSION

The hNSCs isolated from the human embryonic tissues retained their biological features after long-term culture in vitro.

Improving the survival of human CNS precursor-derived neurons after transplantation

We have examined the effects of predifferentiation and energy substrate deprivation on long-term expanded human neural precursor cells (HNPCs). The pre-differentiation of HNPC cultures produced large numbers of neurons (>60%) and mature glial cells capable of generating glycogen stores that protected the neuronal population from experimental metabolic stress. When predifferentiated HNPCs were transplanted into intact adult rat hippocampus, fewer cells survived compared to undifferentiated HNPC transplants. This cell death was completely attenuated, however, when predifferentiated HNPC cultures were pretreated to boost glial energy stores and resulted in greatly increased neuronal survival in vivo. The transplanted cells primarily engrafted within the granular layer of the dentate gyrus, where a large proportion of the predifferentiated HNPCs co-expressed neuronal markers whereas most HNPCs outside of the neuronal layer did not, indicating that the predifferentiated cells remained capable of responding to local cues in the adult brain. Undifferentiated HNPCs migrated more widely in the brain after grafting than did the predifferentiated cells, which generally remained within the hippocampus.

Comparison of pure and mixed populations of human fetal-derived neural progenitors transplanted into intact adult rat brain

We examined the influence of initial graft composition on the number, type, and distribution of human progenitor cells after transplantation into the anterior subventricular zone (SVZa) of normal adult rats. The grafted populations were derived from 19-week-old human cortical tissue grown under adherent conditions in the presence of fibroblast growth factor (FGF) and from a subpopulation of nestin-expressing cells, isolated using negative immunoselection methods, which exhibited properties of neural progenitors. Identical numbers of each were transplanted and the number and location of engrafted cells were compared 4 weeks later. We found a significantly greater number of presumptive neurons and astrocytes in animals that received mixed grafts compared to those enriched for progenitors. In addition, the number of human cells undergoing division was significantly greater in animals that received mixed grafts. The spatial distribution of grafted cells was not significantly different, suggesting that the patterns of cell migration were unaffected by transplant composition, whereas, a greater proportion of neurons was observed in the neurogenic areas of animals that received progenitor-enriched grafts. From a clinical perspective, our results suggest that the cellular composition of human fetal-derived transplants may be an important parameter that influences the number and pattern of differentiation of engrafted cells following transplantation in the mature CNS.

The cellular repair of the brain in Parkinson's disease—past, present and future

Damage to the central nervous system was once considered irreparable. However, there is now growing optimism that neural transplant therapies may one day enable complete circuit reconstruction and thus functional benefit for patients with neurodegenerative conditions such as Parkinson's disease (PD), and perhaps even those with more widespread damage such as stroke patients. Indeed, since the late 1980s hundreds of patients with Parkinson's disease have received allografts of dopamine-rich embryonic human neural tissue. The grafted tissue has been shown to survive and ameliorate many of the symptoms of the disease, both in the clinical setting and in animal models of the disease. However, practical problems associated with tissue procurement and storage, and ethical concerns over using aborted human fetal tissue have fuelled a search for alternative sources of suitable material for grafting. In particular, stem cells and xenogeneic embryonic dopamine-rich neural tissue are being explored, both of which bring their own practical and ethical dilemmas. Here we review the progress made in neural transplantation, both in the laboratory and in the clinic with particular attention to the development of stem cell and xenogeneic tissue based therapy.

Differentiation and migration of long term expanded human neural progenitors in a partial lesion model of Parkinson’s disease

Human neural progenitor cells (HNPCs) can be expanded in large numbers for significant periods of time to provide a reliable source of neural cells for transplantation in neurodegenerative disorders such as Parkinson's disease (PD). In the present study, HNPCs isolated from embryonic cortex were expanded as neurospheres in cell culture for 10 months. Just prior to transplantation, a proportion of the HNPCs were treated in a "predifferentiation" protocol in combination with the neurotropic factor NT4, in order to yield significant numbers of neurons. For transplantation, either undifferentiated HNPCs, or predifferentiated HNPCs were transplanted into the substantia nigra of a rat model of Parkinson's disease. At 12 weeks, there was good survival with proliferation of transplanted HNPCs occurring after transplantation but ceasing before the animals were sacrificed. Transplants of predifferentiated cells contained a higher proportion of neurons. The presence of a lesion in the striatum had a significant influence on the migration of transplanted cells from the substantia nigra into the striatum. There was no significant behavioural recovery or effect of transplanted HNPCs on the loss of dopaminergic cells from the host brain. In conclusion, HNPCs may provide a source of cells for use in the treatment of Parkinson's disease.

Telomerase immortalization of neuronally restricted progenitor cells derived from the human fetal spinal cord

Lineage-restricted progenitors of the central nervous system (CNS) are not readily expandable because their mitotic competence is limited. Here we used retroviral overexpression of human telomerase reverse transcriptase (hTERT) to immortalize progenitors from human fetal spinal cord. The hTERT-immortalized cells divided in basic fibroblast growth factor (bFGF) expressed high telomerase activity, and gave rise to phenotypically restricted subpopulations of either glia or neurons. The latter included a prototypic line, hSC11V-TERT, that gave rise only to neurons. These included both chx10(+) interneurons and Islet1(+)/Hb9(+)/ChAT(+) motor neurons; the latter were recognized by green fluorescent protein (GFP) driven by the Hb9 enhancer. The neurons were postmitotic and achieved electrophysiologic competence. Upon xenograft to both fetal rat brain and injured adult spinal cord, they matured as neurons and survived for 6 months, with no evident tumorigenesis. The cells have survived >168 doublings in vitro, with karyotypic normalcy and without replicative senescence. hTERT overexpression thus permits the generation of progenitor lines able to give rise to phenotypically restricted neurons.

Telomerase immortalization of human neural progenitor cells

It is necessary to expand human neural progenitor cells in vitro to obtain large numbers for research purposes and cell transplantation. A potential obstacle to in vitro expansion, however, is that neural progenitor cells have a limited replication life-span and gradually lose their differentiation potential. We report here that ectopic expression of the catalytic subunit of human telomerase (hTERT) gene in neural progenitor cells could induce telomerase activity, stabilize telomeres and extend their replicative life-spans. The telomerase-immortalized cells (hNPC-TERT) maintained the normal diploid karyotype, expressed the markers of human neural progenitor cells and meanwhile held the differentiation potential in vitro for up to 120 population doublings. This study provides a new approach for obtaining unlimited quantities of normal phenotypic and homogeneous human neural progenitor cells in vitro.

Studies on the differentiation of dopaminergic traits in human neural progenitor cells in vitro and in vivo

The development of cell replacement therapies for the treatment of neurodegenerative disorders such as Parkinson's disease (PD) may depend upon the successful differentiation of human neural stem/progenitor cells into dopamine (DA) neurons. We show here that primary human neural progenitors (HNPs) can be expanded and maintained in culture both as neurospheres (NSPs) and attached monolayers where they develop into neurons and glia. When transplanted into the 6-hydroxydopamine-lesioned rat striatum, undifferentiated NSPs survive longer (60% graft survival at 8-16 weeks vs. 30% graft survival at 8-13 weeks) and migrate farther than their attached counterparts. While both NSP and attached cells continue to express neuronal traits after transplantation, the spontaneous expression of differentiated transmitter-related traits is not observed in either cell type. However, following predifferentiation in culture using a previously described cocktail of reagents, approximately 25% of HNPs can permanently express the DA enzyme tyrosine hydroxylase (TH), even following replating and removal of the DA differentiation cocktail. When these predifferentiated HNPs are transplanted into the brain, however, TH staining is not observed, either because expression is lost or TH-expressing cells preferentially die. Consistent with the latter view is a decrease in total cell survival and migration, and an enhanced glial response in these grafts. In contrast, we found that the overall survival of HNPs is improved when cells engraft near blood vessels or CSF compartments or when they are placed into an intact unlesioned brain, suggesting that there are factors, as yet unidentified, that can better support the development of engrafted HNPs.

Embryonic Stem Cells for Neural Replacement Therapy: Prospects and Challenges

Injury or degeneration of the vertebrate central nervous system often disrupts neuronal circuitry that is built by projection neurons during early embryonic life. Repair of neural network through regeneration of these early-born projection neurons in adult life often fails since stem cells residing in the adult brain are generally programmed to give rise to late-born interneurons. Thus, exogenous cells are needed to rebuild the neural circuitry. Nevertheless, cell replacement in the brain remains a challenging goal because of the lack of safe and effective donor cells, as well as difficulty in remodeling the nonneurogenic adult CNS environment. Here I will concentrate on the donor side and discuss how recent advancement in stem cell technology offers hope for transplant therapy, with a focus on the potentials and hurdles of human embryonic stem cells as a sustainable source.

Toward cell replacement therapy: promises and caveats

Studies in animal models have suggested a role for stem cells in repair and regeneration of the nervous system. Human equivalents of stem and precursor cells have been isolated and their efficacy is being evaluated in rodent and primate models. Difficulties exist in translating results of these preclinical models to therapy in humans. Evolutionary differences among rodents, primates, and humans; fundamental differences in the anatomy and physiology; differences in immune responses in xenotransplant models; the paucity of good transplant models of chronic disease; and allelic variability in the cells themselves make any study evaluating the efficacy of cells in transplant models difficult to interpret. As no better alternatives to testing in animals exist, we suggest that at this early stage a considered step-by-step approach to testing and comparison of different transplant strategies in isolation will prepare us better for clinical trials than simple evaluation of functional outcomes in various models of disease. We emphasize that we do not recommend delaying or abandoning clinical trials; rather, we suggest that one anticipate failures and design experiments and data collection such that we learn from these failures to ensure future success in as rapid a time frame as possible.

Gene expression in human neural stem cells: effects of leukemia inhibitory factor

Human neural precursor cells grown in culture provide a source of tissue for drug screening, developmental studies and cell therapy. However, mechanisms underlying their growth and differentiation are poorly understood. We show that epidermal growth factor (EGF) responsive precursors derived from the developing human cortex undergo senescence after 30-40 population doublings. Leukemia inhibitory factor (LIF) increased overall expansion rates, prevented senescence and allowed the growth of a long-term self renewing neural stem cell (ltNSCctx) for up to 110 population doublings. We established basal gene expression in ltNSCctx using Affymetrix oligonucleotide microarrays that delineated specific members of important growth factor and signaling families consistently expressed across three separate lines. Following LIF withdrawal, 200 genes showed significant decreases. Protein analysis confirmed LIF-regulated expression of glial fibrillary acidic protein, CD44, and major histocompatibility complex I. This study provides the first molecular profile of human ltNSCctx cultures capable of long-term self renewal, and reveals specific sets of genes that are directly or indirectly regulated by LIF.

Human neural stem cell transplantation in the MPTP-lesioned mouse

Human neural stem cells have exhibited a remarkable versatility to respond to environmental signals. Their characterization in models of neurotoxic injury may provide insight into human disease treatment paradigms. This study investigates the survival and migration of transplanted human stem cells and tyrosine hydroxylase immunoreactivity in the parkinsonian 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned mouse model, using antisera recognizing human nuclear protein (hNuc) and tyrosine hydroxylase (TH). Our results indicate long-term (up to 90 days) survival of human stem cell xenograft in the MPTP-lesioned mouse and the presence of hNuc-immunoreactive cells at sites distal to the transplant core. Few TH-positive cells are identified in the striatum by immunoperoxidase staining and using immunofluorescent double labeling, infrequent TH-immunoreactive, transplanted cells are identified.

Identification and isolation of multipotential neural progenitor cells from the subcortical white matter of the adult human brain

The subcortical white matter of the adult human brain harbors a pool of glial progenitor cells. These cells can be isolated by fluorescence-activated cell sorting (FACS) after either transfection with green fluorescent protein (GFP) under the control of the CNP2 promoter, or A2B5-targeted immunotagging. Although these cells give rise largely to oligodendrocytes, in low-density culture we observed that some also generated neurons. We thus asked whether these nominally glial progenitors might include multipotential progenitor cells capable of neurogenesis. We found that adult human white-matter progenitor cells (WMPCs) could be passaged as neurospheres in vitro and that these cells generated functionally competent neurons and glia both in vitro and after xenograft to the fetal rat brain. WMPCs were able to produce neurons after their initial isolation and did not require in vitro expansion or reprogramming to do so. These experiments indicate that an abundant pool of mitotically competent neurogenic progenitor cells resides in the adult human white matter.

Recent advances in stem cell neurobiology

1. Neural stem cells can be cultured from the CNS of different mammalian species at many stages of development. They have an extensive capacity for self-renewal and will proliferate ex vivo in response to mitogenic growth factors or following genetic modification with immortalising oncogenes. Neural stem cells are multipotent since their differentiating progeny will give rise to the principal cellular phenotypes comprising the mature CNS: neurons, astrocytes and oligodendrocytes. 2. Neural stem cells can also be derived from more primitive embryonic stem (ES) cells cultured from the blastocyst. ES cells are considered to be pluripotent since they can give rise to the full cellular spectrum and will, therefore, contribute to all three of the embryonic germ layers: endoderm, mesoderm and ectoderm. However, pluripotent cells have also been derived from germ cells and teratocarcinomas (embryonal carcinomas) and their progeny may also give rise to the multiple cellular phenotypes contributing to the CNS. In a recent development, ES cells have also been isolated and grown from human blastocysts, thus raising the possibility of growing autologous stem cells when combined with nuclear transfer technology. 3. There is now an emerging recognition that the adult mammalian brain, including that of primates and humans, harbours stem cell populations suggesting the existence of a previously unrecognised neural plasticity to the mature CNS, and thereby raising the possibility of promoting endogenous neural reconstruction. 4. Such reports have fuelled expectations for the clinical exploitation of neural stem cells in cell replacement or recruitment strategies for the treatment of a variety of human neurological conditions including Parkinson's disease (PD), Huntington's disease, multiple sclerosis and ischaemic brain injury. Owing to their migratory capacity within the CNS, neural stem cells may also find potential clinical application as cellular vectors for widespread gene delivery and the expression of therapeutic proteins. In this regard, they may be eminently suitable for the correction of genetically-determined CNS disorders and in the management of certain tumors responsive to cytokines. Since large numbers of stem cells can be generated efficiently in culture, they may obviate some of the technical and ethical limitations associated with the use of fresh (primary) embryonic neural tissue in current transplantation strategies. 5. While considerable recent progress has been made in terms of developing new techniques allowing for the long-term culture of human stem cells, the successful clinical application of these cells is presently limited by our understanding of both (i) the intrinsic and extrinsic regulators of stem cell proliferation and (ii) those factors controlling cell lineage determination and differentiation. Although such cells may also provide accessible model systems for studying neural development, progress in the field has been further limited by the lack of suitable markers needed for the identification and selection of cells within proliferating heterogeneous populations of precursor cells. There is a further need to distinguish between the committed fate (defined during normal development) and the potential specification (implying flexibility of fate through manipulation of its environment) of stem cells undergoing differentiation. 6. With these challenges lying ahead, it is the opinion of the authors that stem-cell therapy is likely to remain within the experimental arena for the foreseeable future. In this regard, few (if any) of the in vivo studies employing neural stem cell grafts have shown convincingly that behavioural recovery can be achieved in the various model paradigms. Moreover, issues relating to the quality control of cultured cells and their safety following transplantation have only begun to be addressed. 7. While on the one hand cell biotechnologists have been quick to realise the potential commercial value, human stem cell research and its clinical applications has been the subject of intense ethical and legislative considerations. The present chapter aims to review some recent aspects of stem cell research applicable to developmental neurobiology and the potential applications in clinical neuroscience.

Mammalian neural stem-cell renewal: nature versus nurture

Recent data show that the final events of mammalian brain organogenesis may depend in part on the direct control of neural stem cell (NSC) proliferation and survival. Environmental and intrinsic factors play a role throughout development and during adulthood to regulate NSC proliferation. The NSCs acquire new competences throughout development, including adulthood, and this change in competence is region-specific. The factors controlling NSC survival, undifferentiated state, proliferation, and cell-cycle number are beginning to be identified, but the links between them remain unclear. However, current knowledge should help to formulate an understanding of how a stem cell can generate a new stem cell.

Telomerase mediates the cell survival-promoting actions of brain-derived neurotrophic factor and secreted amyloid precursor protein in developing hippocampal neurons

Telomerase, a reverse transcriptase that maintains chromosome ends (telomeres) during successive cell divisions in mitotic cells is present in neuroblasts and early postmitotic embryonic neurons but is absent from adult neurons. The signals that control telomerase levels during development are unknown, as are the functions of telomerase in developing neurons. We now report that telomerase activity and levels of its catalytic subunit telomerase reverse transcriptase (TERT) are increased in embryonic hippocampal neurons by brain-derived neurotrophic factor (BDNF) and a secreted form of beta-amyloid precursor protein (sAPP). BDNF and sAPP promote the survival of the embryonic neurons, and these trophic effects are blocked when TERT production is suppressed using antisense technology. Telomerase is required for the long-term survival of early postmitotic neurons during a time window of approximately 1 week in culture; telomerase is then downregulated and is not required for BDNF and sAPP survival signaling in mature neurons. The increase in telomerase activity and trophic effects of BDNF and sAPP are mediated by phosphatidylinositol-3 kinase and p42/p44 MAP kinases. Our findings demonstrate a requirement for telomerase in the cell survival-promoting actions of BDNF and sAPP in early postmitotic hippocampal neurons, suggesting a previously unknown role for telomerase in mediating the biological actions of neurotrophic factors during brain development.

A glance into somatic stem cell biology: basic principles, new concepts, and clinical relevance

Somatic stem cells are undifferentiated cells with a high capacity for self-renewal that can give rise to one or more specialized cell types with specific functions in the body. Profound characterization of these cells has been difficult due to the fact that their frequency in different tissues of the body is extremely low; furthermore, their identification is not based on their morphology but on immunophenotypic and functional assays. Nevertheless, significant advances in the study of these cells at both cellular and molecular levels have been achieved during the last decade. The majority of what we know concerning somatic stem cell biology has come from work on hematopoietic stem cells. More recently, however, there has been a great amount of information on neural and epithelial stem cells. The importance of stem cell research has gone beyond basic biology and is currently contributing to the development of new medical approaches for treatment of hematologic, neurologic, autoimmune, and metabolic disorders (cellular therapy).

Potential use of stem cells in neuroreplacement therapies for neurodegenerative diseases

The use of stem cells for neuroreplacement therapy is no longer science fiction--it is science fact. We have succeeded in the development of neural and mesenchymal stem cell transplantation to produce neural cells in the brain. We have also seen improvement in cognitive function following stem cell transplantation in a memory-impaired aged animal model. These results promise a bright future for stem cell therapies in neurodegenerative diseases. Before we begin to think about clinical applications beyond the present preclinical studies, we have to consider the pathophysiological environment of individual diseases and weigh the factors that affect stem cell biology. Here, I not only review potential therapeutic applications of stem cell strategies in neurodegenerative diseases, but also discuss stem cell biology regarding factors that are altered under disease conditions.

Region-specific generation of cholinergic neurons from fetal human neural stem cells grafted in adult rat

Pluripotent or multipotent stem cells isolated from human embryos or adult central nervous system (CNS) may provide new neurons to ameliorate neural disorders. A major obstacle, however, is that the majority of such cells do not differentiate into neurons when grafted into non-neurogenic areas of the adult CNS. Here we report a new in vitro priming procedure that generates a nearly pure population of neurons from fetal human neural stem cells (hNSCs) transplanted into adult rat CNS. Furthermore, the grafted cells differentiated by acquiring a cholinergic phenotype in a region-specific manner. This technology may advance stem cell-based therapy to replace lost neurons in neural injury or neurodegenerative disorders.

Induction of telomerase activity during an early burst of proliferation in pancreatic regeneration

Telomerase activity (TA) and telomerase reverse transcriptase (rTERT) were investigated in Sprague-Dawley rats, killed at 6 and 12 h, and 1, 2, 3, 7, and 14 days after 90% pancreatectomy (px), by TRAPeze enzyme-linked immunosorbent assay telomerase detection and reverse transcriptase-polymerase chain reaction. TA increased at 2 days and reached a maximum at 3 days after px, when the small ductules showed the highest proliferation activity. After 3 days, TA decreased to basal levels as the ductules differentiated into new endocrine and exocrine pancreas. The expression of rTERT showed a correlation with TA. These results suggest telomerase is actively regulated during pancreatic regeneration.

Properties of a fetal multipotent neural stem cell (NEP cell)

Multipotent neural stem cells (NSCs) present in the developing neural tube (E10.5, neuroepithelial cells; NEP) were examined for the expression of candidate stem cell markers, and the expression of these markers was compared with later appearing precursor cells (E14.5) that can be distinguished by the expression of embryonic neural cell adhesion molecule (E-NCAM) and A2B5. NEP cells possess gap junctions, express connexins, and appear to lack long cilia. Most candidate markers, including Nestin, Presenilin, Notch, and Numb, were expressed by both NEP cells as well as other cell populations. Fibroblast growth factor receptor 4 (FGFR4), Frizzled 9 (Fz9), and SRY box-containing gene 2 (Sox2) as assessed by immunocytochemistry and in situ hybridization are markers that appear to distinguish NSCs from other precursor cells. Neither Hoechst 33342 nor rhodamine-123 staining, telomerase (Tert) expression, telomerase activity, or breakpoint cluster region protein 1 (Bcrp1) transporter expression could be used to distinguish NEP stem cells from other dividing cells. NEP cells, however, lacked expression of several lineage markers that are expressed by later appearing cells. These included absence of expression of CD44, E-NCAM, A2B5, epidermal growth factor receptor (EGFR), and platelet-derived growth factor receptor-alpha (PDGFR alpha), suggesting that negative selection using cell surface epitopes could be used to isolate stem cell populations from mixed cultures of cells. Using mixed cultures of cells isolated from E14.5 stage embryos, we show that NEP cells can be enriched by depleting differentiating cells that express E-NCAM or A2B5 immunoreactivity. Overall, our results show that a spectrum of markers used in combination can reliably distinguish multipotent NSCs from other precursor cells as well as differentiated cells present in the CNS.

Evaluation of in vitro proliferative activity of human fetal neural stem/progenitor cells using indirect measurements of viable cells based on cellular metabolic activity

To scale up human neural stem/progenitor cell (NSPC) cultures for clinical use, we need to know how long these cells can live ex vivo without losing their ability to proliferate and differentiate; thus, a convenient method is needed to estimate the proliferative activity of human NSPCs grown in neurosphere cultures, as direct cell counting is laborious and potentially inaccurate. Here, we isolated NSPCs from human fetal forebrain and prepared neurosphere cultures. We determined the number of viable cells and estimated their proliferative activity in long-term culture using two methods that measure viable cell numbers indirectly, based on their metabolic activity: the WST-8 assay, in which a formazan dye is produced upon reduction of the water-soluble tetrazolium salt WST-8 by dehydrogenase activity, and the ATP assay, which measures the ATP content of the total cell plasma. We compared the results of these assays with the proliferative activity estimated by DNA synthesis using the 5-bromo-2'-deoxyuridine incorporation assay. We found the numbers of viable human NSPCs to be directly proportional to the metabolic reaction products obtained in the WST-8 and ATP assays. Both methods yielded identical cell growth curves, showing an exponentially proliferative phase and a change in the population doubling time in long-term culture. They also showed that human NSPCs could be expanded for up to 200 days ex vivo without losing their ability to proliferate and differentiate. Our findings indicated that indirect measurements of viable cells based on metabolic activity, especially the ATP assay, are very effective and reproducible ways to determine the numbers of viable human NSPCs in intact neurospheres.

Neurospheres modified to produce glial cell line-derived neurotrophic factor increase the survival of transplanted dopamine neurons

Glial cell line-derived neurotrophic factor (GDNF) has been shown to increase the survival of dopamine neurons in a variety of in vitro and in vivo model systems. Therefore, it constitutes an important therapeutic protein with the potential to ameliorate dopamine neuronal degeneration in Parkinson's disease or to support dopamine neuronal replacement strategies. However, biophysical and practical considerations present obstacles for the direct delivery of the GDNF protein to CNS neurons. Here we show that rodent neural precursor cells isolated and expanded in culture as neurospheres (NS) can be genetically modified to express green fluorescent protein (GFP) or to release GDNF using lentiviral constructs. GDNF-NS increased the fibre outgrowth of primary embryonic dopamine neurons in cocultures, showing that the protein was released in biologically significant quantities. Furthermore, after transplantation into the 6-hydroxydopamine-lesioned rat striatum, GDNF-NS significantly increased the survival of cografted primary dopamine neurons. However, this was not reflected in behavioural recovery in these animals. We found that, by 6 weeks, few cells expressed GDNF or GFP, suggesting either that transgene expression was down-regulated over time or that the cells died. This may explain the initial effects on dopamine neuronal survival within the graft but the lack of long-term effect on subsequent fibre outgrowth and behaviour. Providing sustained levels of neural precursor-mediated transgene expression can be achieved following transplantation in the future; this approach may prove beneficial as an alternative therapeutic strategy in the cell-based management of Parkinson's disease.

AF5, a CNS cell line immortalized with an N-terminal fragment of SV40 large T: growth, differentiation, genetic stability, and gene expression

Central nervous system progenitor cells that are self-renewing in culture and also differentiate under controlled conditions are potentially useful for developmental studies and for cell-based therapies. We characterized growth and plasticity properties and gene expression in a rat mesencephalic cell line, AF5, that was immortalized with an N-terminal fragment of SV40 large T (T155g). For over 150 population doublings in culture, the growth rate of AF5 cells remained steady, the cells remained responsive to bFGF, and telomerase activity and telomere lengths were unchanged. While karyotype analyses revealed some chromosomal abnormalities, these were also unchanged over time; additionally, no mutations in p53 gene sequences were found, and wild-type p53 activation was normal. AF5 cells produced PDGF, TGFbeta1, TGFbeta2, and bFGF. Similar to primary progenitor cells, AF5 cells retained their plasticity in culture; they could be propagated in an undifferentiated state as "neurospheres" in serum-free media or as adherent cultures in serum-containing media, and they differentiated when allowed to become confluent. Adherent subconfluent actively growing cultures expressed a marker for immature neurons, nestin, while few cells expressed the mature neuronal cell marker betaIII-tubulin. Confluent cultures ceased growing, developed differentiated morphologies, contained few or no nestin-expressing cells, and acquired betaIII-tubulin expression. Global gene expression was examined using a 15,000 gene microarray, comparing exponential growth with and without bFGF stimulation, and the differentiated state. The AF5 cell line exhibited stable genetic and growth properties over extended periods of time, while retaining the ability to differentiate in vitro. These data suggest that the AF5 cell line may be useful as an in vitro model system for studies of neural differentiation.

Differentiation of the RN33B cell line into forebrain projection neurons after transplantation into the neonatal rat brain

The rat neural cell line RN33B has a remarkable ability to undergo region-specific neuronal differentiation after transplantation into the CNS. To further study its neurogenic properties in vivo, we used a recombinant lentiviral vector to genetically label the cells with the Green Fluorescent Protein (GFP) gene before implantation into the striatum/cortex, hippocampus, or mesencephalon of newborn rats. Three weeks after implantation, about 1-2% of the GFP-expressing cells had developed morphologies typical of neurons, astrocytes, or oligodendrocytes, the rest remained as either immature or undifferentiated nestin-positive cells. At 15-17 weeks postgrafting, the immature cells had disappeared in most graft recipients and only cells with neuronal or glial morphologies remained in similar numbers as at 3 weeks. The GFP distributed throughout the expressing cells, revealing fine morphological details, including dendrites with spines and extensive axonal projections. In all forebrain regions, the grafted cells differentiated into neurons with morphologies characteristic for each site, including large numbers of pyramidal-like cells in the cortex and the hippocampus, giving rise to dense projections to normal cortical target regions and to the contralateral hippocampus, respectively. In lower numbers, it was also possible to identify GFP-positive granulelike cells in the hippocampus, as well as densely spiny neurons in the striatum. In the mesencephalon by contrast, cells with astrocytic features predominated. The ability of the grafted RN33B cells to undergo region-specific differentiation into highly specialized types of forebrain projection neurons and establish connections with appropriate targets suggests that cues present in the microenvironment of the neonatal rat brain can effectively guide the development of immature progenitors, also in the absence of ongoing neurogenesis.

Stem cell repair of central nervous system injury

Neural stem cells (NSCs) have great potential as a therapeutic tool for the repair of a number of CNS disorders. NSCs can either be isolated from embryonic and adult brain tissue or be induced from both mouse and human ES cells. These cells proliferate in vitro through many passages without losing their multipotentiality. Following engraftment into the adult CNS, NSCs differentiate mainly into glia, except in neurogenic areas. After engraftment into the injured and diseased CNS, their differentiation is further retarded. In vitro manipulation of NSC fate prior to transplantation and/or modification of the host environment may be necessary to control the terminal lineage of the transplanted cells to obtain functionally significant numbers of neurons. NSCs and a few types of glial precursors have shown the capability to differentiate into oligodendrocytes and to remyeliate the demyelinated axons in the CNS, but the functional extent of remyelination achieved by these transplants is limited. Manipulation of endogenous neural precursors may be an alternative therapy or a complimentary therapy to stem cell transplantation for neurodegenerative disease and CNS injury. However, this at present is challenging and so far has been unsuccessful. Understanding mechanisms of NSC differentiation in the context of the injured CNS will be critical to achieving these therapeutic strategies.

The controlled conversion of human neural progenitor cells derived from foetal ventral mesencephalon into dopaminergic neurons in vitro

The expansion and differentiation of neural progenitor cells in vitro provides an approach to study the development and differentiation of neurons. The ventral mesencephalic area of the brain is an important source of neural progenitor cells and the differentiated neural progenitor cell has paramount potential for use in transplant therapies such as those used in the treatment of neurodegenerative diseases. Here, the controlled conversion of human foetal progenitor cells derived from ventral mesencephalon into dopaminergic neurons is reported. The immunoreactivity to tyrosine hydroxylase (TH) and levels of dopamine (DA) and its metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), secreted into culture medium, were used to assess dopaminergic neuronal phenotype. Expansion of the neural progenitor cells for 3 weeks in the presence of basic fibroblast growth factor (2 ng/ml) followed by its withdrawal resulted in approximately 60% of cells staining positive for TH, when challenged in concert with brain-derived neurotrophic factor (50 ng/ml), DA (10 microM) and forskolin (10 microM) for a further 3 weeks. A corresponding 41-fold increase in DA and DOPAC was measured in the incubation medium by HPLC. Therefore, the successful conversion of human foetal progenitor cells in vitro resulting in the desired dopaminergic neuronal phenotype, could provide a solution to the problem of limited availability of human foetuses for clinical surgical transplantation therapies, which are currently in progress for the treatment of neurodegenerative diseases such as Parkinson's disease.

The potential for circuit reconstruction by expanded neural precursor cells explored through porcine xenografts in a rat model of Parkinson's disease

Neural precursors with the properties of neural stem cells can be isolated from the developing brain, can be expanded in culture, and have been suggested as a potential source of cells for neuronal replacement therapies in degenerative disorders such as Parkinson's disease (PD). Under such conditions an improved spectrum of functional benefit may be obtained through homotypic reconstruction of degenerated neural circuitry, and to this end we have investigated the potential of expanded neural precursor cells (ENPs) to form long axonal projections following transplantation in the 6-hydroxydopamine-lesioned rat model of PD. ENPs have been isolated from the embryonic pig, since implantation in a xenograft environment is thought to favor axonal growth. These porcine ENPs possessed similar properties in vitro to those described in other species: they proliferated in response to epidermal and fibroblast growth factor-2, expressed the neuroepithelial marker nestin, and differentiated into neurons, astrocytes, and occasional oligodendrocytes on mitogen withdrawal. The use of pig-specific markers following xenotransplantion into cyclosporin A-immunosuppressed rats revealed that many cells differentiated into neurons and displayed extensive axogenesis, such that when placed in the region of the substantia nigra fibers projected throughout the striatal neuropil. These neurons were not restricted in the targets to which they could project since following intrastriatal grafting fibers were seen in the normal striatal targets of the pallidum and substantia nigra. Staining for a pig-specific synaptic marker suggested that synapses were formed in these distant sites. A small number of these cells differentiated spontaneously to express a catecholaminergic phenotype, but were insufficient to mediate behavioral recovery. Our results suggest that when the efficiency of neurochemical phenotype induction is increased, ENP-derived neurons have the potential to be a uniquely flexible source of cells for therapeutic cell replacement where anatomical reconstruction is advantageous.

Neural cells from primary human striatal xenografts migrate extensively in the adult rat CNS

Primary neural cells do not appear to migrate significantly following transplantation into the adult rodent CNS, which is in contrast to expanded neural precursor cells where migration is well-documented. However, most transplant studies of primary neural tissue have been performed in an allograft situation in which it is difficult to identify graft-derived cells. We have, therefore, used a xenograft paradigm to investigate the potential for cells derived from grafts of primary human fetal striatal tissue (gestational age of 66-72 days) to migrate following intrastriatal transplantation in an athymic adult rat model of Huntington's disease. The use of an antibody specific to human nuclear antigen enabled clear identification of graft-derived cells within the host brain, and specific neural phenotypes were determined using human-specific tau for neurons, glial fibrillary acidic protein for mature astrocytes and Ki67 for proliferative cells. At 6 weeks, the graft mass was very dense with a high proliferative index, few cells had migrated away from the graft, and the cells that had differentiated both within and away from the graft were mainly neurons. In contrast, at 6 months, the graft core was dispersed significantly more and a large number of graft-derived cells had migrated throughout the brain as far rostral as the olfactory bulb and as caudal as the substantia nigra. Cells had differentiated into both neurons and astrocytes and the level of proliferation was significantly lower within the graft. These results demonstrate that primary neural xenografts contain proliferative cells that possess the ability to migrate and differentiate into both neurons and astrocytes, and suggest that these cells could contribute to normal graft function. This property may be a consequence of the xenograft situation and could potentially be exploited to provide the opportunity to target regions of distant pathology in neurodegenerative diseases using xenotransplantation of embryonic neural tissue.

Genetic perpetuation of in vitro expanded human neural stem cells: cellular properties and therapeutic potential

Long-term propagated human neural stem cells (self-renewing and multipotent) allow for the unlimited and predictable generation of different types of human neural cells in vitro. In addition, these cell lines may be of help for the elucidation of basic neuro-developmental issues, and also for the development of innovative therapeutic strategies for nervous system disorders (through cell replacement and/or gene transfer-based therapies). In this article we summarize our current knowledge about these long-term cultured cells, particularly that of immortalized cells, with the aim of critically addressing their usefulness and potential for therapeutic use. Perpetuation methods and in vitro properties of immortalized cells are analyzed. Although reports on in vivo studies are scarce, present data on survival, integration, migration, and differentiation of the cells indicate that they may be useful for the development of cellular and genetic therapies, in various models of neurodegeneration. A great deal of basic and applied research remains to be done in order to fully explore, understand, and exploit the therapeutic potential of human neural precursor cells.

Stem cells in the treatment of Parkinson's disease

Stem cells have been suggested as candidate therapeutic tools for neurodegenerative disorders, given their ability to give rise to the appropriate cell types after grafting in vivo. In this review I summarize some of the evidence currently available concerning two approaches for the treatment of Parkinson's disease: (1) The generation of dopaminergic neurons from embryonic stem cells, multipotent stem cells, and neuronal progenitor cells for cell replacement therapy. (2) The engineering of multipotent stem cells to release glial cell-line derived neurotrophic factor, a potent neurotrophic factor for dopaminergic neurons, in a neuroprotective and neuroregenerative approach to the treatment of Parkinson's disease.

Regional specification of rodent and human neurospheres

Neural precursor cells were isolated from various regions of the developing rat and human brain and grown in culture as aggregates termed neurospheres. We asked whether cells within human and rodent neurospheres are identical, or whether they have species specific characteristics or differences based on their region of origin. Under our culture conditions, rodent neurospheres isolated from the cortex (ctxNS) and striatum (strNS) grew faster than those from the mesencephalon (mesNS), but stopped growing after only eight to ten population doublings. In contrast, human neurospheres under identical culture conditions, continued to grow for over 40 population doublings. Following migration and differentiation of both rodent and human cultures, ctxNS and strNS generated high numbers of small neurons whereas mesNS generated small numbers of large neurons with many long fibres. Only very rare neurons from mesNS expressed dopaminergic markers, and thus may require further signals to fully mature. While the rat neurospheres generated high numbers of oligodendrocytes, very few were found to develop from human neurospheres from any region after a few weeks of passaging. FACS analysis revealed a unique population of smaller cells within human strNS and ctxNS, which appeared to be neuronal progenitors. However, large cells within neurospheres were capable of generating these small neuronal progenitors following further proliferation. Together, our data show that rat and human neurospheres have unique characteristics with regard to growth and differentiation, and that the majority of precursor cells within neurospheres are regionally specified to generate set numbers of neurons. These findings have important implications for understanding the nature of proliferating neural precursors isolated from the developing CNS, and their potential for brain repair.

Migration patterns and phenotypic differentiation of long-term expanded human neural progenitor cells after transplantation into the adult rat brain

We have examined long-term growth-factor expanded human neural progenitors following transplantation into the adult rat brain. Cells, obtained from the forebrain of a 9-week old fetus, propagated in the presence of epidermal growth factor, basic fibroblast growth factor, and leukemia inhibitory factor were transplanted into the striatum, subventricular zone (SVZ), and hippocampus. At 14 weeks, implanted cells were identified using antisera recognizing human nuclei and the reporter gene green fluorescent protein. Different migration patterns of the grafted cells were observed: (i) target-directed migration of doublecortin (DCX, a marker for migrating neuroblasts)-positive cells along the rostral migratory stream to the olfactory bulb and into the granular cell layer following transplantation into the SVZ and hippocampus, respectively; (ii) non-directed migration of DCX-positive cells in the grey matter in striatum and hippocampus, and (iii) extensive migration of above all nestin-positive/DCX-negative cells within white matter tracts. At the striatal implantation site, neuronal differentiation was most pronounced at the graft core with axonal projections extending along the internal capsule bundles. In the hippocampus, cells differentiated primarily into interneurons both in the dentate gyrus and in the CA1-3 regions as well as into granule-like neurons. In the striatum and hippocampus, a significant proportion of the grafted cells differentiated into glial cells, some with long processes extending along white matter tracts. Although the survival time was over 3 months in the present study a large fraction of the grafted cells remained undifferentiated in a stem or progenitor cell stage as revealed by the expression of nestin and/or GFAP.

Human bone marrow stem cells exhibit neural phenotypes and ameliorate neurological deficits after grafting into the ischemic brain of rats

There is now evidence to suggest that bone marrow mesenchymal stem cells (MSCs) not only differentiate into mesodermal cells, but can also adopt the fate of endodermal and ectodermal cell types. In this study, we addressed the hypotheses that human MSCs can differentiate into neural cells when implanted in the brain and restore sensorimotor function after experimental stroke. Purified human MSCs were grafted into the cortex surrounding the area of infarction 1 week after cortical brain ischemia in rats. Two and 6 weeks after transplantation animals were assessed for sensorimotor function and then sacrificed for histological examination. Ischemic rats that received human MSCs exhibited significantly improved functional performance in limb placement test. Histological analyses revealed that transplanted human MSCs expressed markers for astrocytes (GFAP(+)), oligodendroglia (GalC(+)), and neurons (beta III(+), NF160(+), NF200(+), hNSE(+), and hNF70(+)). The morphological features of the grafted cells, however, were spherical in nature with few processes. Therefore, it is unlikely that the functional recovery observed by the ischemic rats with human MSC grafts was mediated by the integration of new "neuronal" cells into the circuitry of the host brain. The observed functional improvement might have been mediated by proteins secreted by transplanted hMSCs, which could have upregulated host brain plasticity in response to experimental stroke.

Transduction of human neural progenitor cells using recombinant adeno-associated viral vectors

Human neural progenitor cells (hNPCs) represent an attractive source for cell therapy of neurological disorders. Genetic modification of hNPCs may allow a controlled release of therapeutic proteins, suppress immune rejection, or produce essential neurotransmitters. In search of an effective gene delivery vehicle, we evaluated the efficiency of a recombinant adeno-associated viral (rAAV) vector expressing enhanced green fluorescent protein (CAGegfp). Our study demonstrated that CAGegfp efficiently transduced both proliferating and differentiated hNPCs in vitro. EGFP expression was detected as early as 1 day after exposure to CAGegfp and was detectable for up to 4 months. Following transduction, the growth rate of hNPCs slowed down, but they were still able to differentiate into neurons and glia. Furthermore, CAGegfp-modified hNPCs survived, differentiated and expressed EGFP after transplanting into spinal cord of adult rats. Our results indicated that rAAV vectors might be a useful tool in hNPC-based cell and gene therapy for neurological disorders.

Neuronal target genes of the neuron-restrictive silencer factor in neurospheres derived from fetuses with Down's syndrome: a gene expression study

BACKGROUND

Identification of genes and characterisation of their function is an essential step towards understanding complex pathophysiological abnormalities in Down's syndrome. We did a study to investigate abnormalities in gene expression in human neuronal stem cells and progenitor cells from Down's syndrome and control post-mortem human fetal tissue.

METHODS

Indexing-based differential display PCR was done on neuronal precursor cells derived from the cortex of a fetus with Down's syndrome, and findings were compared with those of two control samples. Findings were validated against neurosphere preparations from three independent Down's syndrome fetuses and five independent controls by real-time quantitative PCR.

FINDINGS

Results of differential display PCR analysis showed that SCG10--a neuron--specific growth-associated protein regulated by the neuron-restrictive silencer factor REST-was almost undetectable in the Down's syndrome sample. This finding was validated by real-time PCR. We also found that other genes regulated by the REST transcription factor were selectively repressed, whereas non-REST-regulated genes with similar functions were unaffected. Changes in expression of several key developmental genes in the Down's syndrome stem-cell and progenitor-cell pool correlated with striking changes in neuron morphology after differentiation.

INTERPRETATION

Our findings suggest a link between dysregulation of the REST transcription factor and some of the neurological deficits seen in Down's syndrome. Experimental REST downregulation has been shown to trigger apoptosis, which could account for the striking and selective loss of neurons in the differentiated Down's syndrome cell preparations.

Telomerase as a therapeutic target for malignant gliomas

Telomerase, a ribonucleoprotein enzyme, is considered as a potential target of cancer therapy because of its preferential expression in tumors. In particular, malignant gliomas are one of the best candidates for telomerase-targeted therapy. It is because malignant gliomas are predominantly telomerase-positive, while normal brain tissues do not express telomerase. In theory, there are two telomerase-associated therapeutic approaches for telomerase-positive tumors. One approach is the anti-telomerase cancer therapy to directly inhibit telomerase activity, resulting in apoptotic cell death or growth arrest. Two major components of the telomerase holoenzyme complex, the RNA template (hTER) and catalytic subunit (reverse transcriptase, hTERT) are well considered as therapeutic targets. The other approach is the telomerase-specific cancer therapy by targeting telomerase-expressing tumor cells as a means to directly kill the cells. Strategies using the transfer of therapeutic gene under the hTERT promoter system as well as immunotherapy directed against telomerase-positive cells are generally included. These telomerase-associated therapies are very promising for the treatment of malignant gliomas.

Porcine neural xenografts in the immunocompetent rat: immune response following grafting of expanded neural precursor cells

Intracerebral neural xenografts elicit a host immune response that results in their rapid rejection. This forms a key barrier to the therapeutic use of xenogeneic tissue transplantation for conditions such as Parkinson's disease. The current study sought to provide insight into the cellular components of donor cell suspensions that are important in stimulating the host rejection response and thereby to suggest rational manipulations of xenogeneic donor tissue that might ultimately enhance its clinical utility. The neural stem cell mitogens, epidermal growth factor and fibroblast growth factor-2, have been used to isolate and expand populations of primordial neural precursor cells from the embryonic pig brain. The immune response elicited by these cells on transplantation into the non-immunosuppressed rat has been fully characterised. In the first experiments, expanded neural precursors were grafted into the hemi-parkinsonian, non-immunosuppressed Sprague-Dawley rat and graft status and host response examined 10, 21, 35 and 60 days post-transplantation. While equivalent primary tissue grafts were completely eliminated at 35 days, grafts of expanded neural precursors with healthy neurofilament-positive projections were present at all time-points, and two large grafts remained even at 60 days. Some grafts appeared to elicit minimal host immune responses at the time-points they were examined, although most did appear to be undergoing a rejection process since a co-ordinated response involving host cytotoxic T-lymphocytes, microglia/macrophages, immunoglobulin M and complement could be demonstrated to varying degrees. Subsequent experiments went on to demonstrate further that expanded precursor populations and primary tissue suspensions differed in their immunogenic profile. Firstly, when primary tissue was injected intraperitoneally into immunocompetent rats a vigorous primary humoral response was generated. No such response was detected following injection of expanded neural precursors. Secondly, flow cytometric analysis revealed small but significant levels of class II porcine major histocompatibility complex expression in primary cell suspensions but no such expression in expanded precursor populations.The results of this study therefore demonstrate that the immunogenicity of porcine neural cell suspensions used for intracerebral grafting is reduced when neural stem cell mitogens are used to expand precursor cells. The implications of these findings in the development of novel xenogeneic cellular therapies for neurodegenerative conditions such as Parkinson's disease are discussed.