Differential regulation of midbrain dopaminergic neuron development by Wnt-1, Wnt-3a, and Wnt-5a

ING4 induces cell growth inhibition in human lung adenocarcinoma A549 cells by means of Wnt-1/beta-catenin signaling pathway

ING4, as a novel candidate tumor suppressor gene, has been implicated in several human malignances by tumor growth inhibition and apoptosis enhancement. The mechanism of ING4 remains largely unknown. The purpose of this study was to investigate the inhibitory tumor growth effects of ING4 on lung adenocarcinoma, and its mechanism, by ING4 cDNA transduction into A549 cells. Furthermore, the expression level of ING4 in lung adenocarcinoma tissues was examined. The expression of ING4 was markedly reduced in human lung adenocarcinoma tissues. Overexpression of ING4 can induce growth inhibition in A549 cells both in vitro and in vivo, and also induce up-regulation of p27, down-regulation of cyclinD1, SKP2, and Cox2, and inactivation of the Wnt-1/beta-catenin pathway. Moreover, overexpression of ING4 can enhance the sensitivity of A549 cells to radiotherapy and chemotherapy. Thus, ING4 may play an inhibitory role on A549 cell proliferation and tumor growth in lung adenocarcinoma by up-regulation or down-regulation of cell proliferation-regulating proteins such as p27, cyclinD1, SKP2, and Cox2 by means of inactivation of Wnt-1/beta-catenin signaling.

A mechanism misregulating p27 in tumors discovered in a functional genomic screen

The cyclin-dependent kinase inhibitor p27^KIP1 is a tumor suppressor gene in mice, and loss of p27 protein is a negative prognostic indicator in human cancers. Unlike other tumor suppressors, the p27 gene is rarely mutated in tumors. Therefore misregulation of p27, rather than loss of the gene, is responsible for tumor-associated decreases in p27 protein levels. We performed a functional genomic screen in p27^+/− mice to identify genes that regulate p27 during lymphomagenesis. This study demonstrated that decreased p27 expression in tumors resulted from altered transcription of the p27 gene, and the retroviral tagging strategy enabled us to pinpoint relevant transcription factors. inhibitor of DNA binding 3 (Id3) was isolated and validated as a transcriptional repressor of p27. We further demonstrated that p27 was a downstream target of Id3 in src-family kinase Lck-driven thymic lymphomagenesis and that p27 was an essential regulator of Lck-dependent thymic maturation during normal T-cell development. Thus, we have identified and characterized transcriptional repression of p27 by Id3 as a new mechanism decreasing p27 protein in tumors.

Many human cancers express abnormally low amounts of the p27 protein, and this is associated with aggressive tumor behavior and a poor clinical outcome. Surprisingly, the p27 gene is rarely mutated in these tumors and retains the potential to produce normal amounts of p27 protein. Therefore, understanding the pathways that cause the decrease of p27 protein in cancer cells may lead to the development of new therapies that restore p27 gene expression to normal levels. We undertook a survey of the mouse genome to identify genes that modulate p27 protein levels in lymphomas. Our analysis discovered inhibitor of DNA binding 3 (Id3) as a negative regulator of p27 gene expression. Additionally, we demonstrated that the p27 gene is controlled by Id3 during normal embryological development of the thymus. Our results underscore the fact that cancer cells frequently exploit normal developmental pathways as they evolve into increasingly aggressive transformed states.

Effects on differentiation of embryonic ventral midbrain progenitors by Lmx1a, Msx1, Ngn2, and Pitx3

Neurons derived from neural stem cells could potentially be used for cell therapy in neurodegenerative disorders, such as Parkinson's disease. To achieve controlled differentiation of neural stem cells, we expressed transcription factors involved in the development of midbrain dopaminergic neurons in rat and human neural progenitors. Using retroviral-mediated transgene delivery, we overexpressed Lmx1a (LIM homeobox transcription factor 1, alpha), Msx1 (msh homeobox homolog 1), Ngn2 (neurogenin 2), or Pitx3 (paired-like homeodomain transcription factor 3) in neurospheres derived from embryonic day 14.5 rat ventral mesencephalic progenitors. We also expressed either Lmx1a or Msx1 in the human embryonic midbrain-derived progenitor cell line NGC-407. Rat cells transduced with Ngn2 exited the cell cycle and expressed the neuronal marker microtubule-associated protein 2 and catecholamine-neuron protein vesicular monoamine transporter 2. Interestingly, Pitx3 downregulated the expression of SOX2 (SRY-box containing gene 2) and Nestin, altered cell morphology, but never induced neuronal or glial differentiation. Ngn2 exhibited a strong neuron-inducing effect. In contrast, few Lmx1a-transduced cells matured into neurons, and Msx1 overexpression promoted oligodendrogenesis rather than neuronal differentiation. Importantly, none of these four genes, alone or in combination, enhanced differentiation of rat neural stem cells into dopaminergic neurons. Notably, the overexpression of Lmx1a, but not Msx1, in human neural progenitors increased the yield of tyrosine hydroxylase-immunoreactive cells by threefold. Together, we demonstrate that induced overexpression of transcription factor genes has profound and specific effects on the differentiation of rat and human midbrain progenitors, although few dopamine neurons are generated.

Emerging restorative treatments for Parkinson's disease

Several exciting approaches for restorative therapy in Parkinson's disease have emerged over the past two decades. This review initially describes experimental and clinical data regarding growth factor administration. We focus on glial cell line-derived neurotrophic factor (GDNF), particularly its role in neuroprotection and in regeneration in Parkinson's disease. Thereafter, we discuss the challenges currently facing cell transplantation in Parkinson's disease and briefly consider the possibility to continue testing intrastriatal transplantation of fetal dopaminergic progenitors clinically. We also give a more detailed overview of the developmental biology of dopaminergic neurons and the potential of certain stem cells, i.e. neural and embryonic stem cells, to differentiate into dopaminergic neurons. Finally, we discuss adult neurogenesis as a potential tool for restoring lost dopamine neurons in patients suffering from Parkinson's disease.

Simvastatin-mediated upregulation of VEGF and BDNF, activation of the PI3K/Akt pathway, and increase of neurogenesis are associated with therapeutic improvement after traumatic brain injury

This study was undertaken to evaluate the effect of simvastatin, a cholesterol-lowering agent, on the Akt-mediated signaling pathway and neurogenesis in the dentate gyrus (DG) of the hippocampus in rats after traumatic brain injury (TBI). Adult male Wistar rats were divided into three groups: (1) sham group (n = 8); (2) saline control group (n = 40); and (3) simvastatin-treated group (n = 40). Controlled cortical impact (CCI) injury was performed over the left parietal lobe. Simvastatin was administered orally at a dose of 1 mg/kg starting at day 1 after TBI and then daily for 14 days. Bromodeoxyuridine (BrdU) was injected intraperitoneally into rats. A modified Morris Water Maze (WM) task was performed between 31 and 35 days after treatment to test spatial memory (n = 8/group). Animals were sacrificed at 1, 3, 7, 14, and 35 days after treatment (n = 8/group/time point). Western blot was utilized to investigate the changes in the Akt-mediated signaling pathway. Enzyme-linked immunosorbent assay (ELISA) analyses were employed to measure vascular endothelial growth factor (VEGF) and brain-derived neurotrophin factor (BDNF) expression. Immunohistochemical and fluorescent staining were performed to detect the BrdU- and neuronal nuclei (NeuN)/BrdU-positive cells. Our data show that simvastatin treatment increases phosphorylation of v-akt murine thymoma viral oncogene homolog (Akt), glycogen synthase kinase-3beta (GSK-3beta), and cAMP response element-binding proteins (CREB); elevates the expression of BDNF and VEGF in the DG; increases cell proliferation and differentiation in the DG; and enhances the recovery of spatial learning. These data suggest that the neurorestorative effect of simvastatin may be mediated through activation of the Akt-mediated signaling pathway, subsequently upregulating expression of growth factors and inducing neurogenesis in the DG of the hippocampus, thereby leading to restoration of cognitive function after TBI in rats.

Wnt5a-treated midbrain neural stem cells improve dopamine cell replacement therapy in parkinsonian mice

Dopamine (DA) cell replacement therapy in Parkinson disease (PD) can be achieved using human fetal mesencephalic tissue; however, limited tissue availability has hindered further developments. Embryonic stem cells provide a promising alternative, but poor survival and risk of teratoma formation have prevented their clinical application. We present here a method for generating large numbers of DA neurons based on expanding and differentiating ventral midbrain (VM) neural stem cells/progenitors in the presence of key signals necessary for VM DA neuron development. Mouse VM neurospheres (VMNs) expanded with FGF2, differentiated with sonic hedgehog and FGF8, and transfected with Wnt5a (VMN-Wnt5a) generated 10-fold more DA neurons than did conventional FGF2-treated VMNs. VMN-Wnt5a cells exhibited the transcriptional and biochemical profiles and intrinsic electrophysiological properties of midbrain DA cells. Transplantation of these cells into parkinsonian mice resulted in significant cellular and functional recovery. Importantly, no tumors were detected and only a few transplanted grafts contained sporadic nestin-expressing progenitors. Our findings show that Wnt5a improves the differentiation and functional integration of stem cell-derived DA neurons in vivo and define Wnt5a-treated neural stem cells as an efficient and safe source of DA neurons for cell replacement therapy in PD.

Meningeal cells induce dopaminergic neurons from embryonic stem cells

Neural induction of midbrain dopaminergic (DA) neurons from embryonic stem (ES) cells can be achieved by culturing them on a bone marrow-derived stromal cell line, PA6, which possesses stromal cell-derived inducing activity (SDIA). The mechanism of SDIA is unknown, but clinical application of ES cell transplantation requires the use of defined factors for DA neuron induction. Here, we demonstrate that meningeal cells harvested from the developing dura can induce DA neuron differentiation from mouse and human ES cells, as assessed by midbrain DA marker expression and secretion of DA in response to potassium stimuli. Intriguingly, the inductive strength of meningeal cells depends on their developmental stage, with those harvested from embryonic day 18 embryos showing the highest activity. Among six soluble factors known to be involved in DA neuron differentiation, only Wnt-5a and transforming growth factor-beta3 were expressed by both meningeal and PA6 cells, and the expression of Wnt-5a correlated with the DA neuron induction activity of these cells. Furthermore, the induction of DA neuron differentiation by PA6 cell-conditioned medium was reversed by addition of a Wnt-5a neutralizing antibody, whereas recombinant Wnt-5a promoted DA neuron induction when cells were cultured on Matrigel. These results indicate that meningeal cells can be used as feeders to induce DA neurons from ES cells, and that Wnt-5a plays an important role in DA neuron induction by SDIA.

Establishment of immortalized multipotent hematopoietic progenitor cell lines by retroviral-mediated gene transfer of beta-catenin

OBJECTIVE

Hematopoietic stem cells (HSCs) are characterized by their ability to give rise to all mature blood lineages and to renew themselves. In the present study, we show that beta-catenin plays an essential role in the immortalization of hematopoietic progenitor cells (HPCs).

MATERIALS AND METHODS

Cellular, molecular, and cytogenetic properties of the immortalized HPCs were determined using flow cytometry (immunophenotyping), microscopy (telomere length, spectral karyotyping), telomere repeat amplification protocol assay (telomerase activity), real-time polymerase chain reaction (expression studies), and in vitro and in vivo differentiation assays.

RESULTS

Retroviral-mediated overexpression of human beta-catenin in lin(-)- and lin(-)Sca-1+c-kit+ hematopoietic cells led to generation of novel, murine interleukin-3-, and stem cell factor-dependent hematopoietic multipotent progenitor cell lines (DK1mix and DK2mix) with stable surface antigen expression of the stem cell markers Sca-1 and c-kit (>92%) in long-term culture. Further, immunophenotypic characterization revealed a CD244+CD150(-)CD48(-) phenotype of DKmix cells, which is consistent with the expression profile of multipotential hematopoietic progenitors. Upon exposure to selective hematopoietic cytokines in vitro, and upon transplantation into irradiated congenic recipients, DKmix cells generated progenies of lymphoid and myeloid lineages. Continuous in vitro proliferation and expansion of DKmix cells were associated with stabilized telomere length and consistent telomerase activity. Interestingly, constitutive expression of beta-catenin was not required for sustained long-term viability and proliferation of immortalized DKmix cells.

CONCLUSION

In summary, our findings define beta-catenin as an important regulator in HPC self-renewal and function. Further, our results distinguish the importance of beta-catenin in the immortalization process of HPCs, from its dispensable role in their maintenance.

Targeting of neurotrophic factors, their receptors, and signaling pathways in the developmental neurotoxicity of organophosphates in vivo and in vitro

Neurotrophic factors control neural cell differentiation and assembly of neural circuits. We previously showed that organophosphate pesticides differentially regulate members of the fibroblast growth factor (fgf) gene family. We administered chlorpyrifos and diazinon to neonatal rats on postnatal days 1-4 at doses devoid of systemic toxicity or growth impairment, and spanning the threshold for barely-detectable cholinesterase inhibition. We evaluated the impact on gene families for different classes of neurotrophic factors. Using microarrays, we examined the regional expression of mRNAs encoding the neurotrophins (ntfs), brain-derived neurotrophic factor (bdnf), nerve growth factor (ngf), the wnt and fzd gene families and the corresponding receptors. Chlorpyrifos and diazinon both had widespread effects on the fgf, ntf, wnt and fzd families but much less on the bdnf and ngf groups. However, the two organophosphates showed disparate effects on a number of key neurotrophic factors. To determine if the actions were mediated directly on differentiating neurons, we tested chlorpyrifos in PC12 cells, an in vitro model of neural cell development. Effects in PC12 cells mirrored many of those for members of the fgf, ntf and wnt families, as well as the receptors for the ntfs, especially during early differentiation, the stage known to be most susceptible to disruption by organophosphates. Our results suggest that actions on neurotrophic factors provide a mechanism for the developmental neurotoxicity of low doses of organophosphates, and, since effects on expression of the affected genes differed with test agent, may help explain regional disparities in effects and critical periods of vulnerability.

Delta-like 1 participates in the specification of ventral midbrain progenitor derived dopaminergic neurons

Delta-like 1 (Dlk1), a member of the Delta/Notch protein family, is expressed in the mouse ventral midbrain (VM) as early as embryonic day 11.5 (E11.5) followed by exclusive expression in tyrosine 3-monooxygenase (TH) positive neurons from E12.5 onwards. To further elucidate the yet unknown function of Dlk1 in VM neuron development, we investigated the effect of soluble Dlk1 protein as well as the intrinsic Dlk1 function in the course of VM progenitor expansion and dopaminergic (DA) neuron differentiation in vitro. Dlk1 treatment during expansion increased DA progenitor proliferation and the proportion of NR4A2+ neurons expressing TH after differentiation, whereas Dlk1 treatment during the course of DA precursor differentiation did not alter TH+ neuron counts. In contrast, silencing of endogenously expressed Dlk1 prior to DA precursor differentiation partially prevented the expression of DA neuron markers, which was not accompanied with alteration of overall or local proliferation. Due to the latter finding in combination with the absence of Dlk1 negative DA neurons in differentiated cultures, we suggest that Dlk1 expression might have a permissive effect on DA neuron differentiation in vitro. The study presented here is the first publication identifying Dlk1 effects on ventral midbrain-derived DA precursor differentiation.

Role of Wnt5a in the proliferation of human glioblastoma cells

Wnt5a operates as either a tumor suppressor or a tumor stimulator, according to tumor type. The functions of Wnt5a in human glioblastoma (GBM) have yet to be determined. We initially evaluated the expression of Wnt5a in human glioma. The results of immunohistochemical analyses have revealed that Wnt5a expression was higher in human GBM than in normal brain tissue and low-grade astrocytoma. In order to assess the role of Wnt5a on proliferation in human glioblastoma cells, we employed U87MG and GBM-05, a newly established GBM cell line. GBM-05 was established from a patient diagnosed with GBM. GBM-05 cells were shown to express Nestin, but did not express GFAP and Map2ab. GBM-05 cells formed infiltrating brain tumors after being intracerebrally transplanted into nude mice, and xenotransplanted GBM-05 cells were observed to differentiate into neuronal and astrocyte lineages. Wnt5a expression in the xenotransplanted tumors was higher than that detected in the surrounding brain tissues. The overexpression of Wnt5a increased the proliferation of GBM-05 and U87MG in vitro. By way of contrast, the downregulation of Wnt5a expression as the result of RNA interference reduced proliferation from GBM-05 and U87MG cells in vitro, and reduced tumorigenicity in vivo. Our data indicate that Wnt5a signaling is an important regulator in the proliferation of human glioma cells.

Human embryonic stem cell-derived dopaminergic neurons reverse functional deficit in parkinsonian rats

We show that human embryonic stem cell-derived dopaminergic neurons survived transplantation to the neurotoxin 6-hydroxydopamine-lesioned rat striatum and, in combination with the cells newly differentiated from their progenitors, contributed to locomotive function recovery at 5 months. The animal behavioral improvement was correlated with the dopamine neurons present in the graft. Although the donor cells contained forebrain and midbrain dopamine neurons, the dopamine neurons present in the graft mainly exhibited a midbrain, or nigra, phenotype, suggesting the importance of midbrain dopamine neurons in functional repair. Furthermore, progenies of grafted cells were neurons and glia with greatly diminished mitotic activity by 5 months. Thus, the in vitro-produced human dopamine neurons can functionally engraft in the brain.

Stem-cell-based strategies for the treatment of Parkinson's disease

BACKGROUND

Cell transplantation to replace lost neurons in neurodegenerative diseases such as Parkinson's disease (PD) offers a hopeful prospect for many patients. Previously, fetal grafts have been shown to survive, integrate and induce functional recovery in PD patients. However, limited tissue availability has haltered the widespread use of this therapy and begs the demand for alternative tissue sources. In this regard, stem cells may constitute one such source.

OBJECTIVE/METHODS

In this review we outline various types of stem cells currently available and provide an overview of their possible application for PD. We address not only the obvious possibility of using stem cells in cell replacement therapy but also the benefits of stem cell lines in drug discovery.

RESULTS/CONCLUSION

Stem cells carrying reporters or mutations in genes linked to familial PD are likely to contribute to the identification of new drug targets and subsequent development of new drugs for PD. Thus, stem cells are, and will be more so in the future, invaluable tools in the quest for new therapies against neurodegenerative diseases such as PD.

A Wnt signal regulates stem cell fate and differentiation in vivo

Our knowledge about the normal generation of midbrain dopaminergic neurons in vivois still rudimentary, despite many attempts to recapitulate the underlying events in vitro. Because the loss of these neurons is implicated in Parkinson's disease, this lack of information is one of the major drawbacks in the development of better therapies for this severe human neurological disorder. Recently, substantial advances have been made by demonstrating that the secreted molecule Wnt1 regulates a genetic network, including the transcription factors Otx2 and Nkx2-2, for the initial establishment of the dopaminergic progenitor domain in the mammalian ventral midbrain. In addition, Wnt1 appears to regulate the differentiation of the postmitotic progeny of these precursors by initiating the expression of midbrain dopaminergic-specific transcription factors. A genetic cascade controlled by the secreted molecule Sonic hedgehog, including the transcription factors Lmx1a, Msx1 and Nkx6-1, acts in parallel with the Wnt1-regulated network to establish the midbrain dopaminergic progenitor domain. The Sonic-hedgehog-controlled cascade may diverge from the Wnt1-regulated network at later stages of neural development through induction of proneural transcription factors required for the acquisition of generic neuronal properties by the midbrain dopaminergic progeny. Here we provide a brief overview of these regulatory gene networks.

Retinal stem cells: promising candidates for retina transplantation

Stem cell transplantation is widely considered as a promising therapeutic approach for photoreceptor degeneration, one of the major causes of blindness. In this review, we focus on the biology of retinal stem cells (RSCs) and progenitor cells (RPCs) isolated from fetal, postnatal, and adult animals, with emphasis on those from rodents and humans. We discuss the origin of RSCs/RPCs, the markers expressed by these cells and the conditions for the isolation, culture, and differentiation of these cells in vitro or in vivo by induction with exogenous stimulation.

Wnt signaling and neural stem cells: caught in the Wnt web

Wnt proteins have now been identified as major physiological regulators of multiple aspects of stem cell biology, from self-renewal and pluripotency to precursor cell competence and terminal differentiation. Neural stem cells are the cellular building blocks of the developing nervous system and provide the basis for continued neurogenesis in the adult mammalian central nervous system. Here, we outline the most recent advances in the field about the critical factors and regulatory networks involved in Wnt signaling and discuss recent findings on how this increasingly intricate pathway contributes to the shaping of the developing and adult nervous system on the level of the neural stem cell.

Reconstruction of dopaminergic neural network and locomotion function in planarian regenerates

Planarian, an invertebrate flatworm, has a high capacity for regeneration when compared with other worms and animals. We show here for the first time that the reconstructed dopamine (DA) neural network regulates locomotion and behavior in planarian regenerates. The gene encoding tyrosine hydroxylase in the planarian Dugesia japonica (DjTH) was identified. DjTH protein was coexpressed with aromatic amino acid decarboxylase-like A (DjAADCA) in the planarian central nervous system (CNS). In addition, DjTH-knockdown planarians lost the ability to synthesize DA, but showed no change in 5-hydroxytryptamine synthesis. When the planarian body was amputated, DjTH-positive neurons were regenerated in the brain newly rebuilt from the tail piece at Day 3, and the DjTH-positive axonal and dendritic neural network in the CNS (dopaminergic tiara) was reconstructed at Days 5-7. At that time, autonomic locomotion and methamphetamine-induced hyperkinesia were also suppressed in DjTH-knockdown planarians. Planarian locomotion and behavior seem to be regulated in both cilia- and muscle-dependent manners. In DjTH-knockdown planarians, muscle-mediated locomotion and behavior were significantly attenuated. These results suggest that DA neurons play a key role in the muscle-mediated movement in planarians.

9-Methyl-beta-carboline up-regulates the appearance of differentiated dopaminergic neurones in primary mesencephalic culture

beta-Carbolines (BCs) derive from tryptophan and its derivatives. They are formed endogenously in humans and mammals and occur inter alia in cooked meat and tobacco smoke. They have been detected in human brain, cerebrospinal fluid, and plasma. Up to now they were predominantly identified as compounds exhibiting neurotoxic actions. Since significantly higher amounts are present in parkinsonian patients, they are regarded as potential pathogenetic factors in Parkinson's disease. We identified for the first time a BC (9-methyl-BC; 9-me-BC) exerting neuroprotective and neuron-differentiating effects. Treatment of primary mesencephalic dopaminergic cultures with 9-me-BC inhibited the basal release of lactate dehydrogenase and reduced the number of cells stained with propidium iodide. Caspase-3 activity was decreased, the total protein content was unchanged and ATP content was increased. Furthermore, the expression of inflammation-related genes was reduced. The number of differentiated dopaminergic neurones was significantly increased and a wide array of neurotrophic/transcription factors (Shh, Wnt1, Wnt5a, En1, En2, Nurr1, Pitx3) and marker genes (Th, Dat, Aldh1a1) decisive for dopaminergic differentiation was stimulated. Consistently, the dopamine content was slightly, although non-significantly, increased and the dopamine uptake capacity was elevated. An anti-proliferative effect was observed in human neuroblastoma SH-SY5Y cells which is consistent with a reduced incorporation of bromodesoxyuridine into the DNA of primary mesencephalic cells. Whether the additional dopaminergic neurones in primary culture derive from dopaminergic precursor cells, previously tyrosine hydroxylase negative dopaminergic neurones or are the result of a transdifferentiation process remains to be established.

A regulatory circuit mediating convergence between Nurr1 transcriptional regulation and Wnt signaling

The orphan nuclear receptor Nurr1 is essential for the development and maintenance of midbrain dopaminergic neurons, the cells that degenerate during Parkinson's disease, by promoting the transcription of genes involved in dopaminergic neurotransmission. Since Nurr1 lacks a classical ligand-binding pocket, it is not clear which factors regulate its activity and how these factors are affected during disease pathogenesis. Since Wnt signaling via beta-catenin promotes the differentiation of Nurr1(+) dopaminergic precursors in vitro, we tested for functional interactions between these systems. We found that beta-catenin and Nurr1 functionally interact at multiple levels. In the absence of beta-catenin, Nurr1 is associated with Lef-1 in corepressor complexes. Beta-catenin binds Nurr1 and disrupts these corepressor complexes, leading to coactivator recruitment and induction of Wnt- and Nurr1-responsive genes. We then identified KCNIP4/calsenilin-like protein as being responsive to concurrent activation by Nurr1 and beta-catenin. Since KCNIP4 interacts with presenilins, the Alzheimer's disease-associated proteins that promote beta-catenin degradation, we tested the possibility that KCNIP4 induction regulates beta-catenin signaling. KCNIP4 induction limited beta-catenin activity in a presenilin-dependent manner, thereby serving as a negative feedback loop; furthermore, Nurr1 inhibition of beta-catenin activity was absent in PS1(-/-) cells or in the presence of small interfering RNAs specific to KCNIP4. These data describe regulatory convergence between Nurr1 and beta-catenin, providing a mechanism by which Nurr1 could be regulated by Wnt signaling.

Integrative genomic and functional analyses reveal neuronal subtype differentiation bias in human embryonic stem cell lines

The self-renewal and differentiation potential of human embryonic stem cells (hESCs) suggests that hESCs could be used for regenerative medicine, especially for restoring neuronal functions in brain diseases. However, the functional properties of neurons derived from hESC are largely unknown. Moreover, because hESCs were derived under diverse conditions, the possibility arises that neurons derived from different hESC lines exhibit distinct properties, but this possibility remains unexplored. To address these issues, we developed a protocol that allows stepwise generation from hESCs of cultures composed of approximately 70-80% human neurons that exhibit spontaneous synaptic network activity. Comparison of neurons derived from the well characterized HSF1 and HSF6 hESC lines revealed that HSF1- but not HSF6-derived neurons exhibit forebrain properties. Accordingly, HSF1-derived neurons initially form primarily GABAergic synaptic networks, whereas HSF6-derived neurons initially form glutamatergic networks. microRNA profiling revealed significant expression differences between the two hESC lines, suggesting that microRNAs may influence their distinct differentiation properties. These observations indicate that although both HSF1 and HSF6 hESCs differentiate into functional neurons, the two hESC lines exhibit distinct differentiation potentials, suggesting that they are preprogrammed. Information on hESC line-specific differentiation biases is crucial for neural stem cell therapy and establishment of novel disease models using hESCs.

Distinct but redundant expression of the Frizzled Wnt receptor genes at signaling centers of the developing mouse brain

The establishment of the regional subdivisions of the vertebrate CNS is accomplished through the activity of different neuroepithelial organizing centers. The wingless/int (Wnt) family of secreted glycoproteins, among other factors, plays a crucial role in signaling from these centers. Wnt1 secreted from the boundary between the mid- and hindbrain, for instance, controls the development of this brain region and of associated neuronal populations. Different Wnts secreted from the caudomedial pallium, the cortical hem, pattern the adjacent hippocampal field. The first step in Wnt signal transduction is binding of the Wnt ligand to its receptors, the seven-pass transmembrane Frizzled proteins. Inactivation of different Frizzled genes in mice have revealed an extensive functional redundancy between these receptors. In order to discriminate between a possible participation of different Frizzled receptors in the transduction of Wnt signals at the mid-/hindbrain boundary and the cortical hem, we have performed a detailed expression study of the 10 known murine Frizzled genes at crucial stages of mouse embryonic development. Our analysis reveals a highly dynamic yet distinct expression pattern of individual Frizzled genes in the anterior neural tube of the developing mouse embryo. The overlapping spatio-temporal expression of at least two and up to six Frizzled genes in any region of the developing mouse brain, however, also suggests a vast functional redundancy of the murine Frizzled receptors. This redundancy has to be taken into consideration for future analyses of Frizzled receptor function at these signaling centers in the mouse.

Glycogen synthase kinase 3beta (GSK3beta) regulates differentiation and proliferation in neural stem cells from the rat subventricular zone

On the basis of its inhibition by SB216763, we identified the multifunctional enzyme Glycogen Synthase Kinase 3beta (GSK3beta) as a central regulator for differentiation and cell survival of adult neural stem cells. Detected by proteomic approaches, members of the Wnt/beta-catenin signaling pathway appear to participate in enhanced neuronal differentiation and activated transcription of beta-catenin target genes during GSK3beta inhibition, associated with decreased apoptosis.

FGF signalling pathways in development of the midbrain and anterior hindbrain

Development of the central nervous system is coordinated by intercellular signalling centres established within the neural tube. The isthmic organizer (IsO), located between the midbrain and anterior hindbrain, is one such centre. Important signal molecules secreted by the IsO include members of the fibroblast growth factor and Wnt families. These signals are integrated with dorsally and ventrally derived signals to regulate development of the midbrain and rhombomere 1 of the hindbrain. The IsO is operational for a remarkably long period of time. Depending on the developmental stage, it controls a variety of processes such as cell survival, cell identity, neural precursor proliferation, neuronal differentiation and axon guidance. This review focuses on the fibroblast growth factor signalling, its novel molecular regulatory mechanisms and how this pathway regulates multiple aspects of cell behaviour in the developing midbrain and anterior hindbrain.

Effects of Thiazolidinediones on Differentiation, Proliferation, and Apoptosis

Thiazolidinediones induce adipocyte differentiation and thereby limit proliferative potential; hence, early investigations focused on their ability to modulate cellular proliferation and apoptosis. Several lines of evidence indicate significant thiazolidinedione-mediated antitumor activity. An emerging view is that some antitumor effects are totally or partially peroxisome proliferator-activated receptor-γ (PPARγ) dependent, whereas others are PPARγ independent. The aim of this review is to examine the current evidence about the molecular mechanisms by which thiazolidinediones augment cellular differentiation, inhibit cellular proliferation, and induce apoptosis. We first address the role of thiazolidinediones and/or PPARγ on Wnt/β-catenin signaling pathway as it affects cellular differentiation and then discuss other pathways that are also involved in differentiation as well as proliferation and apoptosis. (Mol Cancer Res 2007;5(6):523–30)

Activation of the canonical Wnt pathway by the antipsychotics haloperidol and clozapine involves dishevelled-3

Protein kinase B (Akt), glycogen synthase kinase-3 (GSK-3) and members of the Wnt signal transduction pathway were recently found to be altered in schizophrenia and targeted by antipsychotic drugs. In the current study, selected Wnt signalling proteins were investigated to determine if they are altered by the antipsychotics clozapine or haloperidol in the rat prefrontal cortex. Pheochromocytoma (PC12) and neuroblastoma (SH-SY5Y) cells were also used to elucidate how antipsychotics generated the pattern of changes observed in vivo. Western blotting (WB) revealed that treatment with haloperidol or clozapine caused an up-regulation of Wnt-5a, dishevelled-3, Axin, total and phosphorylated GSK-3 and beta-catenin protein levels. Treatment of PC12 and SH-SY5Y cells with a variety of pharmacological agents as well as the over-expression of several Wnt related proteins failed to mimic the pattern observed in vivo following antipsychotic treatment. However, the over-expression of dishevelled-3 nearly perfectly duplicated the changes observed in vivo. Immunoprecipitations (IP) conducted using protein isolated from the rat prefrontal cortex indicated that dishevelled-3 is associated with the D2 dopamine receptor thereby suggesting that antipsychotics may act on dishevelled-3 via D2 dopamine receptors to initiate a cascade of downstream changes involving Axin, GSK-3 and beta-catenin that may help to alleviate psychosis in schizophrenic patients.

Transcriptional regulation of neurogenesis: potential mechanisms in cerebral ischemia

Recent data provides evidence that new neurons are born in cerebral ischemia. Although ultimate evidence for their functional importance is lacking, correlational data suggest that they contribute to recovery. Therefore, the underlying mechanisms of neurogenesis are interesting as a basis for pharmacological enhancement of the phenomenon. Neurogenesis is a multistep process that includes proliferation of precursor cells, migration of the newborn cells to the site of lesion, differentiation, integration into neuronal circuits, and survival. All these steps rely on gene transcription. However, only preliminary data about the specific transcriptional control of neurogenesis in cerebral ischemia have been obtained so far. To promote this investigation, we review currently available information on six pathways (Notch, Wnt/beta-catenin, NF-kappaB, signal transducers and activators of transcription (STA) 3, HIF-1, and cyclic AMP response element-binding protein [CREB]) that have been shown to regulate transcription in neurogenesis and that have been implicated in cerebral ischemia. With the exception of CREB, direct involvement in postischemic neurogenesis is quite conjectural and much more must be learned to draw practical conclusions.

Wnt expression in the adult rat subventricular zone after stroke

Neurogenesis occurs in adult brain neural progenitor cells (NPCs) located in the subventricular zone (SVZ) of the lateral ventricle and the subgrandular zone of the hippocampal dentate gyrus. After ischemic stroke, NPCs in the SVZ proliferate and migrate towards the ischemic boundary region to replenish damaged neurons. During development, the Wnt pathways contribute to stem cell maintenance and promote neurogenesis. We hypothesized that stroke up regulates Wnt family genes in SVZ cells. Non-ischemic and ischemic cultured SVZ cells and a single population of non-ischemic and ischemic SVZ cells isolated by laser capture microdissection (LCM) were analyzed for Wnt pathway expression using real-time RT-PCR and immunostaining. The number of neurospheres increased significantly (p<0.05) in SVZ cells derived from ischemic (32+/-4.7/rat) compared with the number in non-ischemic SVZ cells (18+/-3/rat). Wnt family gene mRNA levels were detected in SVZ cells isolated from both cultured and LCM SVZ cells, however there was no up regulation between non-ischemic and ischemic SVZ cells. Immunostaining on brain sections also demonstrated no up regulation of Wnt pathway protein between ischemic and non-ischemic SVZ cells. Expression of the Wnt family genes in SVZ cells support the hypothesis that the Wnt pathway may be involved in neurogenesis in the adult brain. However, ischemia does not up regulate Wnt family gene expression.

How to make a mesodiencephalic dopaminergic neuron

Dopaminergic neurons located in the ventral mesodiencephalon are essential for the control of voluntary movement and the regulation of emotion, and are severely affected in neurodegenerative diseases such as Parkinson's disease. Recent advances in molecular biology and mouse genetics have helped to unravel the mechanisms involved in the development of mesodiencephalic dopaminergic (mdDA) neurons, including their specification, migration and differentiation, as well as the processes that govern axonal pathfinding and their specific patterns of connectivity and maintenance. Here, we follow the developmental path of these neurons with the goal of generating a molecular code that could be exploited in cell-replacement strategies to treat diseases such as Parkinson's disease.

Wnt-5a induces Dishevelled phosphorylation and dopaminergic differentiation via a CK1-dependent mechanism

Previously, we have shown that Wnt-5a strongly regulates dopaminergic neuron differentiation by inducing phosphorylation of Dishevelled (Dvl). Here, we identify additional components of the Wnt-5a-Dvl pathway in dopaminergic cells. Using in vitro gain-of-function and loss-of-function approaches, we reveal that casein kinase 1 (CK1) delta and CK1epsilon are crucial for Dvl phosphorylation by non-canonical Wnts. We show that in response to Wnt-5a, CK1epsilon binds Dvl and is subsequently phosphorylated. Moreover, in response to Wnt-5a or CK1epsilon, the distribution of Dvl changed from punctate to an even appearance within the cytoplasm. The opposite effect was induced by a CK1epsilon kinase-dead mutant or by CK1 inhibitors. As expected, Wnt-5a blocked the Wnt-3a-induced activation of beta-catenin. However, both Wnt-3a and Wnt-5a activated Dvl2 by a CK1-dependent mechanism in a cooperative manner. Finally, we show that CK1 kinase activity is necessary for Wnt-5a-induced differentiation of primary dopaminergic precursors. Thus, our data identify CK1 as a component of Wnt-5a-induced signalling machinery that regulates dopaminergic differentiation, and suggest that CK1delta/epsilon-mediated phosphorylation of Dvl is a common step in both canonical and non-canonical Wnt signalling.

Wnt7a overexpression delays beta-tubulin III expression in transgenic mouse embryos

Wnt7a and HA-tagged Wnt7a have previously been shown to promote or delay neuronal differentiation respectively. In this study, we show that embryonic days 9.5 and 10.5 transgenic mouse embryos overexpressing Wnt7a specifically in nestin-positive neural stem/progenitor cells displayed a delay in neuronal differentiation, assayed by beta-tubulin III expression. Our results corroborate previous studies using HA-Wnt7a, and suggest a critical role for Wnt7a in control of neuronal progenitor maturation.

BMPs, FGF8 and Wnts regulate the differentiation of locus coeruleus noradrenergic neuronal precursors

In the present study, we investigated the involvement of rhombomere 1 patterning proteins in the regulation of the major noradrenergic centre of the brain, the locus coeruleus. Primary cultures of rat embryonic day 13.5 locus coeruleus were treated with fibroblast growth factor-8, noggin and members of the bone morphogenetic and Wnt protein families. We show that bone morphogenetic proteins 2, 5 and 7 increase and noggin decreases the number of tyrosine hydroxylase-positive locus coeruleus neurons. Interestingly, from all Wnts expressed in the first rhombomere by embryonic day 12.5 in the mice, we only found expression of wnt5a mRNA in the vicinity of the locus coeruleus. In agreement with this finding, from all Wnts studied in vitro, only Wnt5a increased the number of tyrosine hydroxylase-positive neurons in locus coeruleus cultures. Finally, we also found that fibroblast growth factor-8 increased the number of tyrosine hydroxylase-positive cells in locus coeruleus cultures. Neither of the identified factors affected the survival of tyrosine hydroxylase-positive locus coeruleus noradrenergic neurons or the proliferation of their progenitors or neurogenesis. Instead, our results suggest that these patterning signals of rhombomere 1 may work to promote the differentiation of noradrenergic progenitors at later stages of development.

Canonical Wnt signaling transiently stimulates proliferation and enhances neurogenesis in neonatal neural progenitor cultures

Canonical Wnt signaling triggers the formation of heterodimeric transcription factor complexes consisting of beta-catenin and T cell factors, and thereby controls the execution of specific genetic programs. During the expansion and neurogenic phases of embryonic neural development canonical Wnt signaling initially controls proliferation of neural progenitor cells, and later neuronal differentiation. Whether Wnt growth factors affect neural progenitor cells postnatally is not known. Therefore, we have analyzed the impact of Wnt signaling on neural progenitors isolated from cerebral cortices of newborn mice. Expression profiling of pathway components revealed that these cells are fully equipped to respond to Wnt signals. However, Wnt pathway activation affected only a subset of neonatal progenitors and elicited a limited increase in proliferation and neuronal differentiation in distinct subsets of cells. Moreover, Wnt pathway activation only transiently stimulated S-phase entry but did not support long-term proliferation of progenitor cultures. The dampened nature of the Wnt response correlates with the predominant expression of inhibitory pathway components and the rapid actuation of negative feedback mechanisms. Interestingly, in differentiating cell cultures activation of canonical Wnt signaling reduced Hes1 and Hes5 expression suggesting that during postnatal neural development, Wnt/beta-catenin signaling enhances neurogenesis from progenitor cells by interfering with Notch pathway activity.

Canonical Wnt signaling through Lef1 is required for hypothalamic neurogenesis

Although the functional importance of the hypothalamus has been demonstrated throughout vertebrates, the mechanisms controlling neurogenesis in this forebrain structure are poorly understood. We report that canonical Wnt signaling acts through Lef1 to regulate neurogenesis in the zebrafish hypothalamus. We show that Lef1 is required for proneural and neuronal gene expression, and for neuronal differentiation in the posterior hypothalamus. Furthermore, we find that this process is dependent on Wnt8b, a ligand of the canonical pathway expressed in the posterior hypothalamus, and that both Wnt8b and Lef1 act to mediate beta-catenin-dependent transcription in this region. Finally, we show that Lef1 associates in vivo with the promoter of sox3, which depends on Lef1 for its expression and can rescue neurogenesis in the absence of Lef1. The conserved presence of this pathway in other vertebrates suggests a common mechanism for regulating hypothalamic neurogenesis.

Microarray analyses support a role for Nurr1 in resistance to oxidative stress and neuronal differentiation in neural stem cells

Nurr1 is an orphan nuclear receptor required for the development of midbrain dopaminergic neurons. To better understand the molecular consequences of Nurr1 expression, we compared the transcriptomes of two independent control and Nurr1-expressing NSC lines using Affymetrix cDNA microarrays. These data reveal the regulation of genes involved in promoting cell survival (trophic/growth factors and stress response genes) and in preventing cell death (decreased caspase-3 and caspase-11 expression). We found that conditioned medium from Nurr1-expressing NSC lines enhanced the survival of midbrain dopaminergic neurons in primary cultures and that Nurr1-expressing NSC lines themselves were more resistant to oxidative stress. These findings are accompanied by a dynamic pattern of gene regulation that is consistent with a role for Nurr1 in promoting both the acquisition of brain-region-specific identity (Engrailed-1) and neuronal differentiation (tubulin beta III). Interestingly, our gene expression profiles suggested that tenascin-C was regulated by Nurr1 in developing dopaminergic neurons. This was further confirmed in vitro and in Nurr1 knockout mice where low levels of tenascin-C mRNA were observed. Analysis of tenascin-C-null mice revealed an increase in the number of Nurr1(+) cells that become tyrosine hydroxylase-positive (TH(+)) dopaminergic neurons at embryonic day 11.5, suggesting that tenascin-C normally delays the acquisition of TH by Nurr1(+) precursors. Thus, our results confirm the presence of both secreted and cell-intrinsic survival signals modulated by Nurr1 and suggest that Nurr1 is a key regulator of both survival and dopaminergic differentiation.

Impaired neural development caused by inducible expression of Axin in transgenic mice

Ablations of the Axin family genes demonstrated that they modulate Wnt signaling in key processes of mammalian development. The ubiquitously expressed Axin1 plays an important role in formation of the embryonic neural axis, while Axin2 is essential for craniofacial skeletogenesis. Although Axin2 is also highly expressed during early neural development, including the neural tube and neural crest, it is not essential for these processes, apparently due to functional redundancy with Axin1. To further investigate the role of Wnt signaling during early neural development, and its potential regulation by Axins, we developed a mouse model for conditional gene activation in the Axin2-expressing domains. We show that gene expression can be successfully targeted to the Axin2-expressing cells in a spatially and temporally specific fashion. High levels of Axin in this domain induce a region-specific effect on the patterning of neural tube. In the mutant embryos, only the development of midbrain is severely impaired even though the transgene is expressed throughout the neural tube. Axin apparently regulates beta-catenin in coordinating cell cycle progression, cell adhesion and survival of neuroepithelial precursors during development of ventricles. Our data support the conclusion that the development of embryonic neural axis is highly sensitive to the level of Wnt signaling.

Wnt signal pathways and neural stem cell differentiation

Self-renewal, migration and differentiation of neural progenitor cells are controlled by a variety of pleiotropic signal molecules. Members of the morphogen family of Wnt molecules play a crucial role for developmental and repair mechanisms in the embryonic and adult nervous system. A strategy of disclosure of the role of different canonical (glycogen synthase kinase-3beta/beta-catenin-dependent) and noncanonical (Ca2+- and JNK-dependent) signal pathways for progenitor cell expansion and differentiations is illustrated at the example of the rat striatal progenitor cell line ST14A that is immortalized by stable retroviral transfection with a temperature-sensitive mutant of the SV40 large T antigen. A shift from permissive 33 degrees C to nonpermissive 39 degrees C leads to proliferation stop and start of differentiation into glial and neuronal cells. Investigation of expression of Wnts, Wnt receptors and Wnt-dependent signal pathway assay point to a stage-dependent involvement of canonical and noncanonical signaling in proliferation and differentiation of ST14A cells, whereby a mutual suppression of pathway activities is likely. Canonical Wnt molecules are not detected in proliferating and differentiating ST14A cells except Wnt2. The noncanonical Wnt molecules Wnt4, Wnt5a and Wnt11 are expressed in proliferating cells and increase during differentiation, whereas cellular beta-catenin decreases in the early phase and is restored in the late phase of differentiation. Accumulation of beta-catenin at the membrane in undifferentiated proliferating cells and its nuclear localization in nondividing undifferentiated cells under differentiation conditions argues for a distinct spatially regulated role of the molecule in the proliferation and early differentiation phase. Ca2+-dependent and JNK-dependent noncanonical Wnt signaling is not detected during differentiation of ST14A cells. Complete exploration of the role of Wnt pathways, for differentiation of the neural progenitor cells ST14A will require Wnt overexpression and exposure of ST14A cells to exogenous Wnts either with purified Wnts or by co-cultures with Wnt producers.

Neural subtype specification from embryonic stem cells

One of the keys to using embryonic stem cells (ESCs) in brain research and potential application in neurological diseases is directed differentiation of neuronal and glial subtypes. This may be achieved by application of developmental principles in guiding cell lineage specification from naïve stem cells. Establishment of defined ESC differentiation models that recapitulate in vivo development, especially from human ESCs, will most likely provide a dynamic tool for dissecting molecular mechanisms underlying early embryonic development that is otherwise not readily obtainable. This is also a rational and realistic way of producing enriched populations of functional neurons and glia for pathological analyses as well as possible therapeutic applications.

Wnt-3a utilizes a novel low dose and rapid pathway that does not require casein kinase 1-mediated phosphorylation of Dvl to activate beta-catenin

The current view of canonical Wnt signalling is that following Wnt binding to its receptors (Frizzled-Lrp5/6), dishevelled (Dvl) becomes hyperphosphorylated, and the signal is transduced to the APC-GSK3beta-axin-beta-catenin multiprotein complex, which subsequently dissociates. As a result beta-catenin is not phosphorylated, escapes proteosomal degradation and activates its target genes after translocation to the nucleus. Here, we analyzed the importance of the Wnt-3a-induced phosphorylation and shift in electrophoretic migration of Dvl (PS-Dvl) for the activation of beta-catenin. Analysis of Wnt-3a time- and dose-responses in a dopaminergic cell line showed that beta-catenin is activated rapidly (within minutes) and at a low dose of Wnt-3a (1 ng/ml). Surprisingly, PS-Dvl appeared only after 30 min and at greater doses (> or =20 ng/ml) of Wnt-3a. Moreover, we found that a casein kinase 1 inhibitor (D4476) or siRNA for casein kinase 1 delta/epsilon (CK1delta/epsilon) blocked the Wnt-3a-induced PS-Dvl. Interestingly, CK1 inhibition or siRNA for CK1delta/epsilon did not ablate the activation of beta-catenin by Wnt-3a, indicating that there is a PS-Dvl-independent path to activate beta-catenin. The increase in beta-catenin activation by Wnt-3a (PS-Dvl-dependent or -independent) were blocked by Dickkopf1 (Dkk1), suggesting that the effect of Wnt-3a is in both cases mediated by Lrp5/6 receptors. Thus, our results show that Wnt-3a rapidly induce a partial activation of beta-catenin in the absence of PS-Dvl at low doses, while at high doses induce a full activation of beta-catenin in a PS-Dvl-dependent manner.

Neural stem cell properties of Müller glia in the mammalian retina: regulation by Notch and Wnt signaling

The retina in adult mammals, unlike those in lower vertebrates such as fish and amphibians, is not known to support neurogenesis. However, when injured, the adult mammalian retina displays neurogenic changes, raising the possibility that neurogenic potential may be evolutionarily conserved and could be exploited for regenerative therapy. Here, we show that Müller cells, when retrospectively enriched from the normal retina, like their radial glial counterparts in the central nervous system (CNS), display cardinal features of neural stem cells (NSCs), i.e., they self-renew and generate all three basic cell types of the CNS. In addition, they possess the potential to generate retinal neurons, both in vitro and in vivo. We also provide direct evidence, by transplanting prospectively enriched injury-activated Müller cells into normal eye, that Müller cells have neurogenic potential and can generate retinal neurons, confirming a hypothesis, first proposed in lower vertebrates. This potential is likely due to the NSC nature of Müller cells that remains dormant under the constraint of non-neurogenic environment of the adult normal retina. Additionally, we demonstrate that the mechanism of activating the dormant stem cell properties in Müller cells involves Wnt and Notch pathways. Together, these results identify Müller cells as latent NSCs in the mammalian retina and hence, may serve as a potential target for cellular manipulation for treating retinal degeneration.

Stable expression of a neuronal dopaminergic progenitor phenotype in cell lines derived from human amniotic fluid cells

Cells from human amniotic fluid derived from the fetus are considered a source of multipotent cells. Their properties have not been fully exploited, partially because unlike other embryonic sources such as embryonic stem (ES) cells, cell lines from amniocentesis samples have not been generated. We have established and characterized the properties of eight individual cell lines. Flow cytometry using several cell surface markers showed that all cell lines generated consisted of homogeneous populations that lack HLAII antigenicity. Using a combination of immunocytochemistry, Western blotting, and RT-PCR, we found weak expression of Oct4 and nestin and strong expression of tubulin-betaIII, MAP2, and tau. Specific markers for cholinergic, (nor)adrenergic, and GABAergic neurons or glia were weakly expressed or absent, whereas expression of factors implicated in early induction of dopaminergic neurons, TGF-beta3 and beta-catenin were present. Further analysis showed strong expression of EN-1, c-RET, PTX3, and NURR1 essential for induction and survival of midbrain dopaminergic neurons, TH, AADC, and VMAT2 components of dopamine synthesis and secretion, and syntaxin1A and SNAP-25 necessary for neurotransmitter exocytosis. This phenotype was retained throughout passages and up to the current passage 36. Expression of neuronal and dopaminergic markers in individual AF cell lines was comparable to expression in neurons induced from ES cells and in IMR-32 and SH-SY5Y neuroblastomas. Our data show that cell lines can be derived from subcultures of amniocentesis, and are primarily composed of a population of progenitors with a phenotype similar to that of committed mesencephalic dopaminergic neurons.

Genetic networks controlling the development of midbrain dopaminergic neurons

Recent data have substantially advanced our understanding of midbrain dopaminergic neuron development. Firstly, a Wnt1-regulated genetic network, including Otx2 and Nkx2-2, and a Shh-controlled genetic cascade, including Lmx1a, Msx1 and Nkx6-1, have been unravelled, acting in parallel or sequentially to establish a territory competent for midbrain dopaminergic precursor production at relatively early stages of neural development. Secondly, the same factors (Wnt1 and Lmx1a/Msx1) appear to regulate midbrain dopaminergic and/or neuronal fate specification in the postmitotic progeny of these precursors by controlling the expression of midbrain dopaminergic-specific and/or general proneural factors at later stages of neural development. For the first time, early inductive events have thus been linked to later differentiation processes in midbrain dopaminergic neuron development. Given the pivotal importance of this neuronal population for normal function of the human brain and its involvement in severe neurological and psychiatric disorders such as Parkinson's Disease, these advances open new prospects for potential stem cell-based therapies. We will summarize these new findings in the overall context of midbrain dopaminergic neuron development in this review.

Wnt expression and canonical Wnt signaling in human bone marrow B lymphopoiesis

BACKGROUND

The early B lymphopoiesis in mammals is regulated through close interactions with stromal cells and components of the intracellular matrix in the bone marrow (BM) microenvironment. Although B lymphopoiesis has been studied for decades, the factors that are implicated in this process, both autocrine and paracrine, are inadequately explored. Wnt signaling is known to be involved in embryonic development and growth regulation of tissues and cancer. Wnt molecules are produced in the BM, and we here ask whether canonical Wnt signaling has a role in regulating human BM B lymphopoiesis.

RESULTS

Examination of the mRNA expression pattern of Wnt ligands, Fzd receptors and Wnt antagonists revealed that BM B progenitor cells and stromal cells express a set of ligands and receptors available for induction of Wnt signaling as well as antagonists for fine tuning of this signaling. Furthermore, different B progenitor maturation stages showed differential expression of Wnt receptors and co-receptors, beta-catenin, plakoglobin, LEF-1 and TCF-4 mRNAs, suggesting canonical Wnt signaling as a regulator of early B lymphopoiesis. Exogenous Wnt3A induced stabilization and nuclear accumulation of beta-catenin in primary lineage restricted B progenitor cells. Also, Wnt3A inhibited B lymphopoiesis of CD133+CD10- hematopoietic progenitor cells and CD10+ B progenitor cells in coculture assays using a supportive layer of stromal cells. This effect was blocked by the Wnt antagonists sFRP1 or Dkk1. Examination of early events in the coculture showed that Wnt3A inhibits cell division of B progenitor cells.

CONCLUSION

These results indicate that canonical Wnt signaling is involved in human BM B lymphopoiesis where it acts as a negative regulator of cell proliferation in a direct or stroma dependent manner.

A Wnt1-regulated genetic network controls the identity and fate of midbrain-dopaminergic progenitors in vivo

Midbrain neurons synthesizing the neurotransmitter dopamine play a central role in the modulation of different brain functions and are associated with major neurological and psychiatric disorders. Despite the importance of these cells, the molecular mechanisms controlling their development are still poorly understood. The secreted glycoprotein Wnt1 is expressed in close vicinity to developing midbrain dopaminergic neurons. Here, we show that Wnt1 regulates the genetic network, including Otx2 and Nkx2-2, that is required for the establishment of the midbrain dopaminergic progenitor domain during embryonic development. In addition, Wnt1 is required for the terminal differentiation of midbrain dopaminergic neurons at later stages of embryogenesis. These results identify Wnt1 as a key molecule in the development of midbrain dopaminergic neurons in vivo. They also suggest the Wnt1-controlled signaling pathway as a promising target for new therapeutic strategies in the treatment of Parkinson's disease.

The dopamine D2 receptor regulates the development of dopaminergic neurons via extracellular signal-regulated kinase and Nurr1 activation

Because the dopaminergic pathways in the midbrain have been closely associated with serious neuropsychiatric disorders, the elucidation of the mechanisms underlying dopaminergic neuronal development should provide some important clues for related disorders. In mice lacking the dopamine D2 receptor (D2R-/-), stereological cell counting analysis showed that the number of mesencephalic tyrosine hydroxylase (TH) cells was significantly low during ontogeny, compared with that observed in wild-type (WT) mice, thereby indicating an alteration in dopaminergic neuronal development in the absence of D2R. The results of immunohistochemical and reverse transcription-PCR analyses revealed that the expression of Nurr1, an orphan nuclear receptor, as well as Ptx3 expression, was selectively reduced in D2R-/- mice during the embryonic stage. A reporter gene assay using the Nur response element linked to the luciferase reporter gene indicated that the stimulation of D2R results in the activation of the Nurr1-mediated reporter gene. This D2R-mediated Nur response element-dependent transcriptional activity was regulated via the activation of extracellular signal-regulated kinase (ERK). Furthermore, quinpirole treatment was shown to elicit an increase in the number of TH-positive neurons, as well as the neuritic extension of TH neurons, coupled with ERK activation and Nurr1 activation in the TH-positive neurons in primary mesencephalic cultures from WT mice. However, this regulation was not detected in the D2R-/- mice. These results suggest that signaling through D2R in association with Nurr1 using ERK, plays a critical role in mesencephalic dopaminergic neuronal development.

Influence of retinoic acid and lithium on proliferation and dopaminergic potential of human NT2 cells

Our laboratory is working with the human NTera2/D1 (NT2) cell line, which has properties similar to those of progenitor cells in the central nervous system (CNS). These neural-like precursor cells can differentiate into all three major lineages, neurons, astrocytes, and oligodendrocytes. The pure neuronal population, hNT neurons, possess characteristics of dopamine (DA) cells. First, we analyzed whether the retinoic acid (RA)-treated hNT neurons and the NT2 precursor cells expressed two transcription factors required for development of the midbrain DA neurons. We report that NT2 cells endogenously expressed Engrailed-1 and Ptx3, whereas RA-treated hNT neurons did not express Engrailed-1 or Ptx3. Next we examined the influence of lithium treatment on Engrailed-1 and Ptx3 as well as another critical transcription factor, Nurr1. Previous research has shown that lithium can mimic the Wnt pathway, which is important for the induction of these transcription factors. Finally, we investigated the effect of lithium treatment on the viability and proliferation of NT2 cells, because lithium has been shown to stimulate neurogenesis in adult neural precursors. Lithium treatment increased the viability and proliferation of NT2 cells. The expression of transcription factors essential for the induction and maintenance of the DA phenotype was not increased in NT2 after lithium treatment. We conclude that the NT2 cell line is an excellent in vitro model system for studying the influence of pharmalogical agents on proliferation, differentiation, and apoptosis of a human neural progenitor cell line.

Function of Wnts in Dopaminergic Neuron Development

Dopaminergic (DA) neurons in the ventral midbrain (VM) are one of the major cell types lost in Parkinson's disease (PD). Proof of principle exists for cell replacement therapies for PD, but wider application is halted by the unavailability of abundant sources of DA neurons. Stem cells might constitute one of these sources. However, efficient protocols promoting their specific differentiation into a DA neuronal phenotype are required. In this review, we summarize the latest findings concerning the contribution of the Wnt family of glycolipoproteins in the development of VM DA neurons. Regulators of Wnt signaling are involved in several neurodevelopmental processes. Recent results indicate that Wnts are key regulators of proliferation and differentiation of DA precursors during VM neurogenesis and different Wnts have specific and unique activity profiles. Interestingly, chemical inhibitors of glycogen synthase kinase-3beta stabilize beta-catenin and increase DA differentiation in VM precursor cultures. We hereby propose that Wnts are likely to contribute in the future to improve stem/precursor cell replacement therapy approaches to PD.