Mouse epidermal growth factor-responsive neural precursor cells increase the survival and functional capacity of embryonic rat dopamine neurons in vitro

mGluR1-Dependent Long Term Depression in Rodent Midbrain Dopamine Neurons Is Regulated by Neuregulin 1/ErbB Signaling

Increasing evidence demonstrates that the neurotrophic factor Neuregulin 1 (NRG1) and its receptors, ErbB tyrosine kinases, modulate midbrain dopamine (DA) transmission. We have previously reported that NRG1/ErbB signaling is essential for proper metabotropic glutamate receptors 1 (mGluR1) functioning in midbrain DA neurons, thus the functional interaction between ErbB receptors and mGluR1 regulates neuronal excitation and in vivo striatal DA release. While it is widely recognized that mGluR1 play a pivotal role in long-term modifications of synaptic transmission in several brain areas, specific mGluR1-dependent forms of synaptic plasticity in substantia nigra pars compacta (SNpc) DA neurons have not been described yet. Here, first we aimed to detect and characterize mGluR1-dependent glutamatergic long-term depression (LTD) in SNpc DA neurons. Second, we tested the hypothesis that endogenous ErbB signaling, by affecting mGluR1, fine-tunes glutamatergic synaptic plasticity in DA cells. We found that either pharmacological or synaptic activation of mGluR1 causes an LTD of AMPAR-mediated transmission in SNpc DA neurons from mice and rat slices, which is reliant on endogenous NRG1/ErbB signaling. Indeed, LTD is counteracted by a broad spectrum ErbB inhibitor. Moreover, the intracellular injection of pan-ErbB- or ErbB2 inhibitors inside DA neurons reduces mGluR1-dependent LTD, suggesting an involvement of ErbB2/ErbB4-containing receptors. Interestingly, exogenous NRG1 fosters LTD expression during minimal mGluRI activation. These results enlarge our cognizance on mGluR1 relevance in the induction of a novel form of long-term synaptic plasticity in SNpc DA neurons and describe a new NRG1/ErbB-dependent mechanism shaping glutamatergic transmission in DA cells. This might have important implications either in DA-dependent behaviors and learning/memory processes or in DA-linked diseases.

A Simple Breeding Protocol for the Procurement of Accurately Staged Rat Donor Embryos for Neural Transplantation

Obtaining accurately staged rat embryos can be difficult because of the variety of breeding protocols employed and because precise staging cannot be confirmed until excision of the embryos from the dam. The detection of estrus, pairing of animals, and confirmation of pregnancies is generally left to commercial suppliers, as in-house breeding can be laborious and unpredictable. Here we describe a simple, reliable in-house breeding protocol for the generation of accurately staged embryos as assessed by measurements of average crown to rump length (CRL).

In situ hybridization reveals developmental regulation of ErbB1-4 mRNA expression in mouse midbrain: implication of ErbB receptors for dopaminergic neurons

Although epidermal growth factor (EGF) and neuregulin-1 are neurotrophic factors for mesencephalic dopaminergic neurons and implicated in schizophrenia, the cellular localization and developmental regulation of their receptors (ErbB1-4) remain to be characterized. Here we investigated the distributions of mRNA for ErbB1-4 in the midbrain of the developing mouse with in situ hybridization and immunohistochemistry. The expression of ErbB1 and ErbB2 mRNAs was relatively high at the perinatal stage and frequently colocalized with mRNA for S100beta and Olig2, markers for immature astrocytes or oligodendrocyte precursors. Modest signal for ErbB1 mRNA was also detected in a subset of dopaminergic neurons. ErbB3 mRNA was detectable at postnatal day 10, peaked at postnatal day 18, and colocalized with 2',3'-cyclic nucleotide 3'-phosphodiesterase, a marker for oligodendrocytes. In contrast, ErbB4 mRNA was exclusively localized in neurons throughout development. Almost all of ErbB4 mRNA-expressing cells (94%-96%) were positive for tyrosine hydroxylase in the substantia nigra pars compacta but 66%-78% in the ventral tegmental area and substantia nigra pars lateralis. Conversely, 92%-99% of tyrosine hydroxylase-positive cells expressed ErbB4 mRNA. The robust and restricted expression of ErbB4 mRNA in the midbrain dopaminergic neurons suggests that ErbB4 ligands, neuregulin-1 and other EGF-related molecules, contribute to development or maintenance of this neuronal population.

Graft outcomes influenced by co-expression of Pax7 in graft and host tissue

Cell replacement therapies offer promise in the treatment of neurotrauma and neurodegenerative disorders and have concentrated on the use of primary fetal brain tissue. However, there is a growing promise of using neural stem cells, in which case other factors may be important in their successful engraftment. We therefore investigated whether the co-expression of the major developmental transcription factor (Pax7 in this study) of donor tissue to graft site influences transplant survival and differentiation in the rat midbrain. Neural progenitor cells were prepared from either the Pax7-expressing dorsal (DM) or non-Pax7-expressing ventral mesencephalon (VM) of embryonic EGFP(+/+) rats. Cells were dissociated and grafted into the adult rat superior colliculus (SC) lesioned with quinolinic acid 3 days previously, a time shown to be associated with the up-regulation of Pax7. Grafts were then examined 4 weeks later. Our results suggest the origin of the graft tissue did not alter graft survival in the SC; however, dorsal grafts appear to have a higher incidence of neuronal survival, whereas ventral grafts have a higher incidence of astrocytic survivors.

Isolation of progenitor cells from GFP-transgenic pigs and transplantation to the retina of allorecipients

Work in rodents has demonstrated that progenitor transplantation can achieve limited photoreceptor replacement in the mammalian retina; however, replication of these findings on a clinically relevant scale requires a large animal model. To evaluate the ability of porcine retinal progenitor cells to survival as allografts and integrate into the host retinal architecture, we isolated donor cells from fetal green fluorescent protein (GFP)-transgenic pigs. Cultures were propagated from the brain, retina, and corneo-scleral limbus. GFP expression rapidly increased with time in culture, although lower in conjunction with photoreceptor markers and glial fibrillary acid protein (GFAP), thus suggesting downregulation of GFP during differentiation. Following transplantation, GFP expression allowed histological visualization of integrated cells and extension of fine processes to adjacent plexiform layers. GFP expression in subretinal grafts was high in cells expressing vimentin and lower in cells expressing photoreceptor markers, again consistent with possible downregulation during differentiation. Cells survived transplantation to the injured retina of allorecipients at all time points examined (up to 10 weeks) in the absence of exogenous immune suppression without indications of rejection. These findings demonstrate the feasibility of allogeneic progenitor transplantation in a large mammal and the utility of the pig in ocular regeneration studies.

Remyelination protects axons from demyelination-associated axon degeneration

In multiple sclerosis, demyelination of the CNS axons is associated with axonal injury and degeneration, which is now accepted as the major cause of neurological disability in the disease. Although the kinetics and the extent of axonal damage have been described in detail, the mechanisms by which it occurs are as yet unclear; one suggestion is failure of remyelination. The goal of this study was to test the hypothesis that failure of prompt remyelination contributes to axonal degeneration following demyelination. Remyelination was inhibited by exposing the brain to 40 Gy of X-irradiation prior to cuprizone intoxication and this resulted in a significant increase in the extent of axonal degeneration and loss compared to non-irradiated cuprizone-fed mice. To exclude the possibility that this increase was a consequence of the X-irradiation and to highlight the significance of remyelination, we restored remyelinating capacity to the X-irradiated mouse brain by transplanting of GFP-expressing embryo-derived neural progenitors. Restoring the remyelinating capacity in these mice resulted in a significant increase in axon survival compared to non-transplanted, X-irradiated cuprizone-intoxicated mice. Our results support the concept that prompt remyelination protects axons from demyelination-associated axonal loss and that remyelination failure contributes to the axon loss that occurs in multiple sclerosis.

Fetal striatum- and ventral mesencephalon-derived expanded neurospheres rescue dopaminergic neurons in vitro and the nigro-striatal system in vivo

The pathogenesis of Parkinson's disease (PD) involves ongoing apoptotic loss of dopaminergic neurons in the substantia nigra pars compacta. Local delivery of the trophic factors can rescue dopaminergic neurons and halt the progression of PD. In this study we show that fetal E11 striatum-derived neurospheres and E14.5 ventral mesencephalon (VM) -derived neurospheres (NS E11 and NSvm, respectively) are a source of factors that rescue dopaminergic neurons. First, long-term expanded NS E11 and NSvm rescued primary dopaminergic neurons from serum-deprivation induced apoptosis and promoted survival of dopaminergic neurons for 14 days in vitro and this effect was due to soluble contact-independent factor/s. Second, green fluorescent protein-expressing NS E11 and NSvm grafted into the midbrain of mice with unilateral 6-hydroxydopamine-induced Parkinsonism resulted in partial rescue of the nigro-striatal system and improvement of the hypo-dopaminergic behavioral deficit. Reverse transcription-polymerase chain reaction (RT-PCR) analysis demonstrated that intact NS E11 and NSvm expressed fibroblast growth factor-2, brain-derived neurotrophic factor (BDNF), pleiotrophin, neurotrophin-3, but not glial cell line-derived neurotrophic factor (GDNF). GDNF expression was also undetectable in vivo in grafted NS E11 and NSvm suggesting that NS-derived factor/s other than GDNF mediated the rescue of nigral dopaminergic neurons. Identification of NS-derived soluble factor(s) may lead to development of novel neuroprotective therapies for PD. An unexpected observation of the present study was the detection of the ectopic host-derived tyrosine hydroxylase (TH) -expressing cells in sham-grafted mice and NS E11- and NSvm -grafted mice. We speculate that injury-derived signals (such as inflammatory cytokines that are commonly released during transplantation) induce TH expression in susceptible cells.

Directing the differentiation of embryonic stem cells to neural stem cells

Embryonic stem cells (ESCs) are a potential source of neural derivatives that can be used in stem cell-based therapies designed to treat neurological disorders. The derivation of specific neuronal or glial cell types from ESCs invariably includes the production of neural stem cells (NSCs). We describe the basic mechanisms of neural induction during vertebrate embryogenesis and how this information helped formulate several protocols used to generate NSCs from ESCs. We highlight the advantages and disadvantages of each approach and review what has been learned about the intermediate stages in the transition from ESC to NSC. Recent data describing how specific growth factors and signaling molecules regulate production of NSCs are described and a model synthesizing this information is presented.

Control of neurogenesis and tyrosine hydroxylase expression in neural progenitor cells through bHLH proteins and Nurr1

The production of dopamine (DA) neurons from neural progenitor cells (NPC) is of particular interest as these neurons degenerate in Parkinson's disease. Here, we report that the characteristics of NPC from the ventral midbrain (NPC(VM)) and the striatum (NPC(STR)) are intrinsically determined. A detailed analysis of the VM during development revealed Ngn2 and Mash1 expression in a DA progenitor domain. Interestingly, over-expression of either Ngn2 or Mash1 induced neurogenesis from expanded NPC(VM). Whereas Ngn2 inhibited cell division and the production of neurons even in the presence of mitogens, Mash1 allowed the progenitors to divide while retaining neurogenic potential. However, none of the new neurons derived by over-expressing Ngn2 or Mash1 were positive for DA neuronal markers such as tyrosine hydroxylase. Nurr1 over-expression increased TH levels in a dose-dependant manner within both neurons and glia, suggesting a non-neuronal-specific activation of this enzyme by Nurr1. Double infection with Nurr1 and either Ngn2 or Mash1 resulted in the production of small numbers of TH+ neurons, which were larger in size when derived from NPC(VM) compared to NPC(STR). These data provide proof of concept that over-expression of multiple transcription factors can drive the fate of NPC first towards neurons, and then towards the DA phenotype. However, further factors may be required to generate fully functional DA neurons.

Human neural progenitors deliver glial cell line-derived neurotrophic factor to parkinsonian rodents and aged primates

Glial cell line-derived neurotrophic factor (GDNF) has been shown to increase the survival and functioning of dopamine neurons in a variety of animal models and some recent human trials. However, delivery of any protein to the brain remains a challenge due to the blood/brain barrier. Here we show that human neural progenitor cells (hNPC) can be genetically modified to release glycosylated GDNF in vitro under an inducible promoter system. hNPC-GDNF were transplanted into the striatum of rats 10 days following a partial lesion of the dopamine system. At 2 weeks following transplantation, the cells had migrated within the striatum and were releasing physiologically relevant levels of GDNF. This was sufficient to increase host dopamine neuron survival and fiber outgrowth. At 5 weeks following grafting there was a strong trend towards functional improvement in transplanted animals and at 8 weeks the cells had migrated to fill most of the striatum and continued to release GDNF with transport to the substantia nigra. These cells could also survive and release GDNF 3 months following transplantation into the aged monkey brain. No tumors were found in any animal. hNPC can be genetically modified, and thereby represent a safe and powerful option for delivering growth factors to specific targets within the central nervous system for diseases such as Parkinson's.

Neuroprotective properties of cultured neural progenitor cells are associated with the production of sonic hedgehog

Numerous studies have shown that abnormal motor behavior improves when neural progenitor cells (NPCs) are transplanted into animal models of neurodegeneration. The mechanisms responsible for this improvement are not fully understood. Indirect anatomical evidence suggests that attention of abnormal motor behavior is attributed, at least in part, to the secretion of trophic factors from the transplanted NPCs. However, there is little direct evidence supporting this hypothesis. Here we show that NPCs isolated from the subventricular zone (SVZ) of neonatal mice are highly teratogenic when transplanted into the neural tube of developing chick embryos and are neuroprotective for fetal dopaminergic neurons in culture because they release sonic hedgehog (Shh). In addition, the neuroprotective properties of NPCs can be exploited to promote better long-term survival of transplanted fetal neurons in an animal model of Parkinson's disease. Thus, cultured NPCs isolated from the SVZ can secrete at least one potent mitogen (Shh) that dramatically affects the fate of neighboring cells. This trait may account for some of the improvement in motor behavior often reported in animal models of neurodegeneration after transplantation of cultured NPCs that were isolated from the SVZ.

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.

Elimination of serotonergic cells induces a marked increase in generation of dopaminergic neurons from mesencephalic precursors

Production of dopaminergic (DA) neurons from stem/precursor cells for transplantation in Parkinson's disease has become a major focus of research. However, the inductive signals mediating the production of DA neurons remain poorly understood, and the influence of other cell populations simultaneously generated within the cell aggregates has not been studied. We investigated whether DA phenotype (i.e. tyrosine hydroxylase-immunoreactive, TH-ir), serotonergic, floor plate (FP4-ir), and fibroblast growth factor 8 (FGF-8)-ir cells differentiate from proliferating cell aggregates obtained from rat mesencephalic precursors, and we also investigated the effects of serotonergic cells on differentiation of DA cells. We observed FP4-ir, FGF-8-ir, TH-ir and serotonergic cells within the aggregates. The TH-ir cells appeared within or in close proximity to a central FP4-ir core, and then concentrated peripherally forming a cap that surrounded the central FP4-ir area. The serotonergic cells and fibers formed a cap surrounding that of TH-ir neurons. Cell aggregates treated with an antibody against FGF-4 or with the serotonergic toxin 5,7-dyhydroxytryptamine or the serotonin synthesis inhibitor dl-p-chlorophenylalanine showed a marked decrease in the number of 5-HT-ir cells (10-20% of controls) and a marked increase in that of TH-ir neurons (700-900% of controls). The present results show that manipulation of other cell populations in the cell aggregates, particularly the serotonergic population, may be an effective method of increasing the production of DA neurons from stem/precursor cells.

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.

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.