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

Functional roles of Nurr1, Pitx3, and Lmx1a in neurogenesis and phenotype specification of dopamine neurons during in vitro differentiation of embryonic stem cells

To elucidate detailed functional mechanisms of key fate-determining transcription factors (eg, Nurr1, Pitx3, and Lmx1a) and their functional interplay for midbrain dopamine (mDA) neurons, we developed highly efficient gain-of-function system by transducing the neural progenitors (NPs) derived from embryonic stem cells (ESCs) with retroviral vectors, allowing the analysis of downstream molecular and cellular effects. Overexpression of each factors, Nurr1, Pitx3, and Lmx1a robustly promoted the dopaminergic differentiation of ESC-NP cells exposed to sonic hedgehog (SHH) and fibroblast growth factor 8 (FGF8). In addition, each of these factors directly interacts with potential binding sites within the tyrosine hydroxylase (TH) gene and activated its promoter activity. Interestingly, however, overexpression of Nurr1, but not of Pitx3 or Lmx1a, generated a significant number of nonneuronal TH-positive cells. In line with this, Pitx3 and Lmx1a, but not Nurr1, induced expression of the Ngn2 gene, which is critical for neurogenesis. We also observed that Pitx3 directly bound to its potential binding sites within the Ngn2 gene and the pan-neuronal marker β-tubulin III gene, suggesting that Pitx3 contributes to mDA neurogenesis by directly regulating these genes. Taken together, our data demonstrate that key mDA regulators (Nurr1, Pitx3, and Lmx1a) play overlapping as well as distinct roles during neurogenesis and neurotransmitter phenotype determination of mDA neurons.

ERKed by LRRK2: a cell biological perspective on hereditary and sporadic Parkinson's disease

The leucine rich repeat kinase 2 (LRRK2/dardarin) is implicated in autosomal dominant familial and sporadic Parkinson's disease (PD); mutations in LRRK2 account for up to 40% of PD cases in some populations. LRRK2 is a large protein with a kinase domain, a GTPase domain, and multiple potential protein interaction domains. As such, delineating the functional pathways for LRRK2 and mechanisms by which PD-linked variants contribute to age-related neurodegeneration could result in pharmaceutically tractable therapies. A growing number of recent studies implicate dysregulation of mitogen activated protein kinases 3 and 1 (also known as ERK1/2) as possible downstream mediators of mutant LRRK2 effects. As these master regulators of growth, differentiation, neuronal plasticity and cell survival have also been implicated in other PD models, a set of common cell biological pathways may contribute to neuronal susceptibility in PD. Here, we review the literature on several major cellular pathways impacted by LRRK2 mutations--autophagy, microtubule/cytoskeletal dynamics, and protein synthesis--in context of potential signaling crosstalk involving the ERK1/2 and Wnt signaling pathways. Emerging implications for calcium homeostasis, mitochondrial biology and synaptic dysregulation are discussed in relation to LRRK2 interactions with other PD gene products. It has been shown that substantia nigra neurons in human PD and Lewy body dementia patients exhibit cytoplasmic accumulations of ERK1/2 in mitochondria, autophagosomes and bundles of intracellular fibrils. Both experimental and human tissue data implicate pathogenic changes in ERK1/2 signaling in sporadic, toxin-based and mutant LRRK2 settings, suggesting engagement of common cell biological pathways by divergent PD etiologies.

Molecular signatures of tissue-specific microvascular endothelial cell heterogeneity in organ maintenance and regeneration

Microvascular endothelial cells (ECs) within different tissues are endowed with distinct but as yet unrecognized structural, phenotypic, and functional attributes. We devised EC purification, cultivation, profiling, and transplantation models that establish tissue-specific molecular libraries of ECs devoid of lymphatic ECs or parenchymal cells. These libraries identify attributes that confer ECs with their organotypic features. We show that clusters of transcription factors, angiocrine growth factors, adhesion molecules, and chemokines are expressed in unique combinations by ECs of each organ. Furthermore, ECs respond distinctly in tissue regeneration models, hepatectomy, and myeloablation. To test the data set, we developed a transplantation model that employs generic ECs differentiated from embryonic stem cells. Transplanted generic ECs engraft into regenerating tissues and acquire features of organotypic ECs. Collectively, we demonstrate the utility of informational databases of ECs toward uncovering the extravascular and intrinsic signals that define EC heterogeneity. These factors could be exploited therapeutically to engineer tissue-specific ECs for regeneration.

Correlation of Nr4a2 expression with the neuron progenitors in adult zebrafish brain

Our previous study showed that although Nr4a2b transcripts have little co-localization with tyrosine hydroxylase (TH) in the posterior tuberculum area, knockdown of Nr4a2 caused a decrease in the number of TH-positive (TH(+)) neurons in the posterior tuberculum area. It suggests that Nr4a2 expression in the progenitors may play an important role in regulating differentiation rather than survival of TH(+) progenitors in the posterior tuberculum area during early zebrafish embryogenesis. In this study, we determined the correlation between TH and Nr4a2 in adult zebrafish brain and found that Nr4a2b was co-localized with the spindle-shaped TH(+) cells in the posterior tuberculum area and some small round TH(+) cells in the pretectum area, but not with large pear-shaped TH(+) cells in adult zebrafish diencephalon. In the pretectum area, Nr4a2(+) cells were localized next to the dorsal side of TH(+) cells. Furthermore, we demonstrated that Nr4a2 was co-expressed with nestin in the progenitors of pretectum area and caudal periventricular hypothalamic zones with a lateral symmetry pattern beside the diencephalic ventricle. Co-expression of Nr4a2 and nestin in these areas was remarkably declined with aging. These findings indicate that Nr4a2 is expressed in the neuronal progenitors and plays a crucial role in the differentiation process of dopamine neuron from the stem cell. The change in Nr4a2 expression with aging suggests its possible association with neurodegenerative diseases.

Cellular and molecular basis of cerebellar development

Historically, the molecular and cellular mechanisms of cerebellar development were investigated through structural descriptions and studying spontaneous mutations in animal models and humans. Advances in experimental embryology, genetic engineering, and neuroimaging techniques render today the possibility to approach the analysis of molecular mechanisms underlying histogenesis and morphogenesis of the cerebellum by experimental designs. Several genes and molecules were identified to be involved in the cerebellar plate regionalization, specification, and differentiation of cerebellar neurons, as well as the establishment of cellular migratory routes and the subsequent neuronal connectivity. Indeed, pattern formation of the cerebellum requires the adequate orchestration of both key morphogenetic signals, arising from distinct brain regions, and local expression of specific transcription factors. Thus, the present review wants to revisit and discuss these morphogenetic and molecular mechanisms taking place during cerebellar development in order to understand causal processes regulating cerebellar cytoarchitecture, its highly topographically ordered circuitry and its role in brain function.

LRRK2: an éminence grise of Wnt-mediated neurogenesis?

The importance of leucine-rich repeat kinase 2 (LRRK2) to mature neurons is well-established, since mutations in PARK8, the gene encoding LRRK2, are the most common known cause of Parkinson’s disease. Nonetheless, despite the LRRK2 knockout mouse having no overt neurodevelopmental defect, numerous lines of in vitro data point toward a central role for this protein in neurogenesis. Roles for LRRK2 have been described in many key processes, including neurite outgrowth and the regulation of microtubule dynamics. Moreover, LRRK2 has been implicated in cell cycle control, suggesting additional roles in neurogenesis that precede terminal differentiation. However, we contend that the suggested function of LRRK2 as a scaffolding protein at the heart of numerous Wnt signaling cascades provides the most tantalizing link to neurogenesis in the developing brain. Numerous lines of evidence show a critical requirement for multiple Wnt pathways in the development of certain brain regions, not least the dopaminergic neurons of the ventral mid-brain. In conclusion, these observations indicate a function of LRRK2 as a subtle yet critical mediator of the action of Wnt ligands on developing neurons. We suggest that LRRK2 loss- or gain-of-function are likely modifiers of developmental phenotypes seen in animal models of Wnt signaling deregulation, a hypothesis that can be tested by cross-breeding relevant genetically modified experimental strains.

The widely used Wnt1-Cre transgene causes developmental phenotypes by ectopic activation of Wnt signaling

The Wnt1-Cre transgenic mouse line is extensively used in the study of the development of the neural crest and its derivatives and the midbrain. The Wnt1 gene has important developmental roles in formation of the midbrain-hindbrain boundary, regulation of midbrain size, and neurogenesis of ventral midbrain dopaminergic (mDA) neurons. Here, we report that Wnt1-Cre transgenic mice exhibit phenotypes in multiple aspects of midbrain development. Significant expansion of the midbrain and increased proliferation in the developing inferior colliculus is associated with ectopic expression of Wnt1. Marked elevation of Wnt1 expression in the ventral midbrain is correlated with disruption of the differentiation program of ventral mDA neurons. We find that these phenotypes can be attributed to ectopic expression of Wnt1 from the Wnt1-Cre transgene leading to the ectopic activation of canonical Wnt/β-catenin signaling. Since these caveats could complicate the utility of Wnt1-Cre in some developmental circumstances, we report a new Wnt1-Cre2 transgenic mouse line that can serve the same purposes as the original without the associated phenotypic complications. These studies reveal an important caveat to a widely-used reagent, provide an improved version of this reagent, and indicate that the original Wnt1-Cre transgenic mouse line may be useful as a gain of function model for interrogating Wnt signaling mechanisms in multiple aspects of midbrain development.

Uncovering novel actors in astrocyte-neuron crosstalk in Parkinson's disease: the Wnt/β-catenin signaling cascade as the common final pathway for neuroprotection and self-repair

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by progressive loss of dopaminergic (DAergic) neuronal cell bodies in the substantia nigra pars compacta and gliosis. The cause and mechanisms underlying the demise of nigrostriatal DAergic neurons are ill-defined, but interactions between genes and environmental factors are recognized to play a critical role in modulating the vulnerability to PD. Current evidence points to reactive glia as a pivotal factor in PD pathophysiology, playing both protective and destructive roles. Here, the contribution of reactive astrocytes and their ability to modulate DAergic neurodegeneration, neuroprotection and neurorepair in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) rodent model of PD will be discussed in the light of novel emerging evidence implicating wingless-type mouse mammary tumor virus integration site (Wnt)/β-catenin signaling as a strong candidate in MPTP-induced nigrostriatal DAergic plasticity. In this work, we highlight an intrinsic Wnt1/frizzled-1/β-catenin tone that critically contributes to the survival and protection of adult midbrain DAergic neurons, with potential implications for drug design or drug action in PD. The dynamic interplay between astrocyte-derived factors and neurogenic signals in MPTP-induced nigrostriatal DAergic neurotoxicity and repair will be summarized, together with recent findings showing a critical role of glia-neural stem/progenitor cell (NPC) interactions aimed at overcoming neurodegeneration and inducing neurorestoration. Understanding the intrinsic plasticity of nigrostriatal DAergic neurons and deciphering the signals facilitating the crosstalk between astrocytes, microglia, DAergic neurons and NPCs may have major implications for the role of stem cell technology in PD, and for identifying potential therapeutic targets to induce endogenous neurorepair.

Midbrain dopaminergic neurons: a review of the molecular circuitry that regulates their development

Dopaminergic (DA) neurons of the ventral midbrain (VM) play vital roles in the regulation of voluntary movement, emotion and reward. They are divided into the A8, A9 and A10 subgroups. The development of the A9 group of DA neurons is an area of intense investigation to aid the generation of these neurons from stem cell sources for cell transplantation approaches to Parkinson's disease (PD). This review discusses the molecular processes that are involved in the identity, specification, maturation, target innervation and survival of VM DA neurons during development. The complex molecular interactions of a number of genetic pathways are outlined, as well as recent advances in the mechanisms that regulate subset identity within the VM DA neuronal pool. A thorough understanding of the cellular and molecular mechanisms involved in the development of VM DA neurons will greatly facilitate the use of cell replacement therapy for the treatment of PD.

Ryk, a receptor regulating Wnt5a-mediated neurogenesis and axon morphogenesis of ventral midbrain dopaminergic neurons

Ryk is an atypical transmembrane receptor tyrosine kinase that has been shown to play multiple roles in development through the modulation of Wnt signaling. Within the developing ventral midbrain (VM), Wnts have been shown to contribute to the proliferation, differentiation, and connectivity of dopamine (DA) neurons; however, the Wnt-related receptors regulating these events remain less well described. In light of the established roles of Wnt5a in dopaminergic development (regulating DA differentiation as well as axonal growth and repulsion), and its interaction with Ryk elsewhere within the central nervous system, we investigated the potential role of Ryk in VM development. Here we show temporal and spatial expression of Ryk within the VM, suggestive of a role in DA neurogenesis and axonal plasticity. In VM primary cultures, we show that the effects of Wnt5a on VM progenitor proliferation, DA differentiation, and DA axonal connectivity can be inhibited using an Ryk-blocking antibody. In support, Ryk knockout mice showed reduced VM progenitors and DA precursor populations, resulting in a significant decrease in DA cells. However, Ryk(-/-) mice displayed no defects in DA axonal growth, guidance, or fasciculation of the MFB, suggesting other receptors may be involved and/or compensate for the loss of this receptor. These findings identify for the first time Ryk as an important receptor for midbrain DA development.

The VPS35 gene and Parkinson's disease

Parkinson's disease (PD), the second most common age-related neurodegenerative disease, is characterized by loss of dopaminergic and nondopaminergic neurons, leading to a variety of motor and nonmotor symptoms. In addition to environmental factors, genetic predisposition and specific gene mutations have been shown to play an important role in the pathogenesis of this disorder. Recently, the identification of the vacuolar protein sorting 35 homolog gene (VPS35), linked to autosomal dominant late-onset PD, has provided new clues to the pathogenesis of PD. Here we discuss the VPS35 gene, its protein function, and various pathways involved in Wnt/β-catenin signaling and in the role of DMT1 mediating the uptake of iron and iron translocation from endosomes to the cytoplasm. Further understanding of these mechanisms will undoubtedly provide new insights into the pathogenic mechanisms of PD and may lead to prevention and better treatment of the disorder.

Wnt5a cooperates with canonical Wnts to generate midbrain dopaminergic neurons in vivo and in stem cells

Wnts are a family of secreted proteins that regulate multiple steps of neural development and stem cell differentiation. Two of them, Wnt1 and Wnt5a, activate distinct branches of Wnt signaling and individually regulate different aspects of midbrain dopaminergic (DA) neuron development. However, several of their functions and interactions remain to be elucidated. Here, we report that loss of Wnt1 results in loss of Lmx1a and Ngn2 expression, as well as agenesis of DA neurons in the midbrain floor plate. Remarkably, a few ectopic DA neurons still emerge in the basal plate of Wnt1(-/-) mice, where Lmx1a is ectopically expressed. These results indicate that Wnt1 orchestrates DA specification and neurogenesis in vivo. Analysis of Wnt1(-/-);Wnt5a(-/-) mice revealed a greater loss of Nurr1(+) cells and DA neurons than in single mutants, indicating that Wnt1 and Wnt5a interact genetically and cooperate to promote midbrain DA neuron development in vivo. Our results unravel a functional interaction between Wnt1 and Wnt5a resulting in enhanced DA neurogenesis. Taking advantage of these findings, we have developed an application of Wnts to improve the generation of midbrain DA neurons from neural and embryonic stem cells. We thus show that coordinated Wnt actions promote DA neuron development in vivo and in stem cells and suggest that coordinated Wnt administration can be used to improve DA differentiation of stem cells and the development of stem cell-based therapies for Parkinson's disease.

A novel strategy for intrastriatal dopaminergic cell transplantation: sequential "nest" grafting influences survival and behavioral recovery in a rat model of Parkinson's disease

Neural transplantation in experimental parkinsonism (PD) is limited by poor survival of grafted embryonic dopaminergic (DA) cells. In this proof-of-principle study we hypothesized that a first regular initial graft may create a "dopaminergic" environment similar to the perinatal substantia nigra and consequently stimulate a subsequent graft. Therefore, we grafted ventral mesencephalic neurons sequentially at different time intervals into the same target localization. Rats with a unilateral lesion of the dopamine neurons produced by injections of 6-hydroxydopamine (6-OHDA) received E14 ventral mesencephalon derived grafts into the DA-depleted striatum. In the control group we grafted all 6 deposits on the first day (d0). The other 4 groups received four graft deposits distributed over 2 implantation tracts followed by a second engraftment injected into the same site 3, 6, 14 and 21 days later. Quantitative assessment of the survival of tyrosine hydroxylase-immunoreactive neurons and graft volume revealed best results for those DA grafts implanted 6 days after the first one. In the present study, a model of short-interval sequential transplantation into the same target-site, so called "nest" grafts were established in the 6-OHDA rat model of PD which might become a useful tool to further elucidate the close neurotrophic and neurotopic interactions between the immediate graft vicinity and the cell suspension graft. In addition, we could show that the optimal milieu was established around the sixth day after the initial transplantation. This may also help to further optimize current transplantation strategies to restore the DA system in patients with PD.

Cell fate analysis of embryonic ventral mesencephalic grafts in the 6-OHDA model of Parkinson's disease

Evidence from carefully conducted open label clinical trials suggested that therapeutic benefit can be achieved by grafting fetal dopaminergic (DAergic) neurons derived from ventral mesencephalon (VM) into the denervated striatum of Parkinson's disease (PD) patients. However, two double-blind trials generated negative results reporting deleterious side effects such as prominent dyskinesias. Heterogeneous composition of VM grafts is likely to account for suboptimal clinical efficacy.We consider that gene expression patterns of the VM tissue needs to be better understood by comparing the genetic signature of the surviving and functioning grafts with the cell suspensions used for transplantation. In addition, it is crucial to assess whether the grafted cells exhibit the DAergic phenotype of adult substantia nigra pars compacta (SNpc). To investigate this further, we used a GFP reporter mouse as source of VM tissue that enabled the detection and dissection of the grafts 6 weeks post implantation. A comparative gene expression analysis of the VM cell suspension and grafts revealed that VM grafts continue to differentiate post-implantation. In addition, implanted grafts showed a mature SNpc-like molecular DAergic phenotype with similar expression levels of TH, Vmat2 and Dat. However, by comparing gene expression of the adult SNpc with dissected grafts we detected a higher expression of progenitor markers in the grafts. Finally, when compared to the VM cell suspension, post-grafting there was a higher expression of markers inherent to glia and other neuronal populations.In summary, our data highlight the dynamic development of distinctive DAergic and non-DAergic gene expression markers associated with the maturation of VM grafts in vivo. The molecular signature of VM grafts and its functional relevance should be further explored in future studies aimed at the optimization of DAergic cell therapy approaches in PD.

Immortalization of neuronal progenitors using SV40 large T antigen and differentiation towards dopaminergic neurons

Transplantation is common in clinical practice where there is availability of the tissue and organ. In the case of neurodegenerative disease such as Parkinson's disease (PD), transplantation is not possible as a result of the non-availability of tissue or organ and therefore, cell therapy is an innovation in clinical practice. However, the availability of neuronal cells for transplantation is very limited. Alternatively, immortalized neuronal progenitors could be used in treating PD. The neuronal progenitor cells can be differentiated into dopaminergic phenotype. Here in this article, the current understanding of the molecular mechanisms involved in the differentiation of dopaminergic phenotype from the neuronal progenitors immortalized with SV40 LT antigen is discussed. In addition, the methods of generating dopaminergic neurons from progenitor cells and the factors that govern their differentiation are elaborated. Recent advances in cell-therapy based transplantation in PD patients and future prospects are discussed.

Tiam1 regulates the Wnt/Dvl/Rac1 signaling pathway and the differentiation of midbrain dopaminergic neurons

Understanding the mechanisms that drive the differentiation of dopaminergic (DA) neurons is crucial for successful development of novel therapies for Parkinson's disease, in which DA neurons progressively degenerate. However, the mechanisms underlying the differentiation-promoting effects of Wnt5a on DA precursors are poorly understood. Here, we present the molecular and functional characterization of a signaling pathway downstream of Wnt5a, the Wnt/Dvl/Rac1 pathway. First, we characterize the interaction between Rac1 and Dvl and identify the N-terminal part of Dvl3 as necessary for Rac1 binding. Next, we show that Tiam1, a Rac1 guanosine exchange factor (GEF), is expressed in the ventral midbrain, interacts with Dvl, facilitates Dvl-Rac1 interaction, and is required for Dvl- or Wnt5a-induced activation of Rac1. Moreover, we show that Wnt5a promotes whereas casein kinase 1 (CK1), a negative regulator of the Wnt/Dvl/Rac1 pathway, abolishes the interactions between Dvl and Tiam1. Finally, using ventral midbrain neurosphere cultures, we demonstrate that the generation of DA neurons in culture is impaired after Tiam1 knockdown, indicating that Tiam1 is required for midbrain DA differentiation. In summary, our data identify Tiam1 as a novel regulator of DA neuron development and as a Dvl-associated and Rac1-specific GEF acting in the Wnt/Dvl/Rac1 pathway.

Activation of Wnt/β-catenin pathway by exogenous Wnt1 protects SH-SY5Y cells against 6-hydroxydopamine toxicity

Wnt1, initially described as a modulator of embryonic development, has recently been discovered to exert cytoprotective effects in cellular models of several diseases, including Parkinson's disease (PD). We, therefore, examined the neuroprotective effects of exogenous Wnt1 on dopaminergic SH-SY5Y cells treated with 6-hydroxydopamine (6-OHDA). Here, we show that 10-500 μM 6-OHDA treatment decreased cell viability and increased lactate dehydrogenase (LDH) leakage. SH-SY5Y cells treated with 100 μM 6-OHDA for 24 h showed reduced Wnt/β-catenin activity, decreased mitochondrial transmembrane potential, elevated levels of reactive oxidative species (ROS) and phosphatidylserine (PS) extraversion, increased levels of Chop and Bip/GRP78 and reduced level of p-Akt (Ser473). In contrast, exogenous Wnt1 attenuated 6-OHDA-induced changes. These results suggest that activation of the Wnt/β-catenin pathway by exogenous Wnt1 protects against 6-OHDA-induced changes by restoring mitochondria and endoplasmic reticulum (ER) function.

Genetic dissection of midbrain dopamine neuron development in vivo

Midbrain dopamine (MbDA) neurons are partitioned into medial and lateral cohorts that control complex functions. However, the genetic underpinnings of MbDA neuron heterogeneity are unclear. While it is known that Wnt1-expressing progenitors contribute to MbDA neurons, the role of Wnt1 in MbDA neuron development in vivo is unresolved. We show that mice with a spontaneous point mutation in Wnt1 have a unique phenotype characterized by the loss of medial MbDA neurons concomitant with a severe depletion of Wnt1-expressing progenitors and diminished LMX1a-expressing progenitors. Wnt1 mutant embryos also have alterations in a hierarchical gene regulatory loop suggesting multiple gene involvement in the Wnt1 mutant MbDA neuron phenotype. To investigate this possibility, we conditionally deleted Gbx2, Fgf8, and En1/2 after their early role in patterning and asked whether these genetic manipulations phenocopied the depletion of MbDA neurons in Wnt1 mutants. The conditional deletion of Gbx2 did not result in re-positioning or distribution of MbDA neurons. The temporal deletion of Fgf8 did not result in the loss of either LMX1a-expressing progenitors nor the initial population of differentiated MbDA neurons, but did result in a complete loss of MbDA neurons at later stages. The temporal deletion and species specific manipulation of En1/2 demonstrated a continued and species specific role of Engrailed genes in MbDA neuron development. Notably, our conditional deletion experiments revealed phenotypes dissimilar to Wnt1 mutants indicating the unique role of Wnt1 in MbDA neuron development. By placing Wnt1, Fgf8, and En1/2 in the context of their temporal requirement for MbDA neuron development, we further deciphered the developmental program underpinning MbDA neuron progenitors.

Expression of early developmental markers predicts the efficiency of embryonic stem cell differentiation into midbrain dopaminergic neurons

Dopaminergic neurons derived from pluripotent stem cells are among the best investigated products of in vitro stem cell differentiation owing to their potential use for neurorestorative therapy of Parkinson's disease. However, the classical differentiation protocols for both mouse and human pluripotent stem cells generate a limited percentage of dopaminergic neurons and yield a considerable cellular heterogeneity comprising numerous scarcely characterized cell populations. To improve pluripotent stem cell differentiation protocols for midbrain dopaminergic neurons, we established extensive and strictly quantitative gene expression profiles, including markers for pluripotent cells, neural progenitors, non-neural cells, pan-neuronal and glial cells, neurotransmitter phenotypes, midbrain and nonmidbrain populations, floor plate and basal plate populations, as well as for Hedgehog, Fgf, and Wnt signaling pathways. The profiles were applied to discrete stages of in vitro differentiation of mouse embryonic stem cells toward the dopaminergic lineage and after transplantation into the striatum of 6-hydroxy-dopamine-lesioned rats. The comparison of gene expression in vitro with stages in the developing ventral midbrain between embryonic day 11.5 and 13.5 ex vivo revealed dynamic changes in the expression of transcription factors and signaling molecules. Based on these profiles, we propose quantitative gene expression milestones that predict the efficiency of dopaminergic differentiation achieved at the end point of the protocol, already at earlier stages of differentiation.

The importance of Wnt signalling for neurodegeneration in Parkinson's disease

PD (Parkinson's disease) is a devastating progressive motor disorder with no available cure. Over the last two decades, an increasing number of genetic defects have been found that cause familial and idiopathic forms of PD. In parallel, the importance of Wnt signalling pathways for the healthy functioning of the adult brain and the dysregulation of these pathways in neurodegenerative disease has become apparent. Cell biological functions disrupted in PD are partially controlled by Wnt signalling pathways and proteins encoded by PARK genes have been shown to modify Wnt signalling. This suggests the prospect of targeting Wnt signalling pathways to modify PD progression.

Cellular programming and reprogramming: sculpting cell fate for the production of dopamine neurons for cell therapy

Pluripotent stem cells are regarded as a promising cell source to obtain human dopamine neurons in sufficient amounts and purity for cell replacement therapy. Importantly, the success of clinical applications depends on our ability to steer pluripotent stem cells towards the right neuronal identity. In Parkinson disease, the loss of dopamine neurons is more pronounced in the ventrolateral population that projects to the sensorimotor striatum. Because synapses are highly specific, only neurons with this precise identity will contribute, upon transplantation, to the synaptic reconstruction of the dorsal striatum. Thus, understanding the developmental cell program of the mesostriatal dopamine neurons is critical for the identification of the extrinsic signals and cell-intrinsic factors that instruct and, ultimately, determine cell identity. Here, we review how extrinsic signals and transcription factors act together during development to shape midbrain cell fates. Further, we discuss how these same factors can be applied in vitro to induce, select, and reprogram cells to the mesostriatal dopamine fate.

Elucidating the sources of β-catenin dynamics in human neural progenitor cells

Human neural progenitor cells (hNPCs) form a new prospect for replacement therapies in the context of neurodegenerative diseases. The Wnt/β-catenin signaling pathway is known to be involved in the differentiation process of hNPCs. RVM cells form a common cell model of hNPCs for in vitro investigation. Previous observations in RVM cells raise the question of whether observed kinetics of the Wnt/β-catenin pathway in later differentiation phases are subject to self-induced signaling. However, a concern when investigating RVM cells is that experimental results are possibly biased by the asynchrony of cells w.r.t. the cell cycle. In this paper, we present, based on experimental data, a computational modeling study on the Wnt/β-catenin signaling pathway in RVM cell populations asynchronously distributed w.r.t. to their cell cycle phases. Therefore, we derive a stochastic model of the pathway in single cells from the reference model in literature and extend it by means of cell populations and cell cycle asynchrony. Based on this, we show that the impact of the cell cycle asynchrony on wet-lab results that average over cell populations is negligible. We then further extend our model and the thus-obtained simulation results provide additional evidence that self-induced Wnt signaling occurs in RVM cells. We further report on significant stochastic effects that directly result from model parameters provided in literature and contradict experimental observations.

LRRK2 functions as a Wnt signaling scaffold, bridging cytosolic proteins and membrane-localized LRP6

Mutations in PARK8, encoding leucine-rich repeat kinase 2 (LRRK2), are a frequent cause of Parkinson's disease (PD). Nonetheless, the physiological role of LRRK2 remains unclear. Here, we demonstrate that LRRK2 participates in canonical Wnt signaling as a scaffold. LRRK2 interacts with key Wnt signaling proteins of the β-catenin destruction complex and dishevelled proteins in vivo and is recruited to membranes following Wnt stimulation, where it binds to the Wnt co-receptor low-density lipoprotein receptor-related protein 6 (LRP6) in cellular models. LRRK2, therefore, bridges membrane and cytosolic components of Wnt signaling. Changes in LRRK2 expression affects pathway activity, while pathogenic LRRK2 mutants reduce both signal strength and the LRRK2–LRP6 interaction. Thus, decreased LRRK2-mediated Wnt signaling caused by reduced binding to LRP6 may underlie the neurodegeneration observed in PD. Finally, a newly developed LRRK2 kinase inhibitor disrupted Wnt signaling to a similar extent as pathogenic LRRK2 mutations. The use of LRRK2 kinase inhibition to treat PD may therefore need reconsideration.

Glycogen synthase kinase-3 inhibition promotes proliferation and neuronal differentiation of human-induced pluripotent stem cell-derived neural progenitors

Human-induced pluripotent stem cell-derived neural progenitors (hiPSC-NPs) have the ability to self-renew and differentiate into glial and neuronal lineages, which makes them an invaluable source in cell replacement therapy for neurological diseases. Therefore, their enhanced proliferation and neuronal differentiation are pivotal features that can be used in repairing neurological injuries. One of the main regulators of neural development is Wnt signaling, which results in the inhibition of glycogen synthase kinase 3 (GSK-3). Here, we assess the impact of GSK-3 inhibition by the small molecule CHIR99021 on the expansion and differentiation of hiPSC-NPs in an adherent condition and a defined medium. Cell proliferation analyses have revealed that inhibition of GSK-3 in the presence of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) increased the proliferation of hiPSC-NPs across 10 passages. The inhibition of β-catenin signaling by XAV and NOTCH signaling by DAPT reversed CHIR impact on hiPSC-NPs proliferation. The target genes of β-catenin, C-MYC and CYCLIN D1 as well as NOTCH target genes, HES1 and HES5 were upregulated. The treatment of NPs by CHIR in the absence of bFGF and EGF resulted in an increase of neuronal differentiation rather than proliferation by stabilization of β-catenin regardless of the NOTCH pathway. Thus, GSK-3 inhibition has been shown to promote proliferation of the NPs by activating β-catenin and NOTCH-related cell cycle genes in the presence of bFGF and EGF. Additionally, during GSK-3 inhibition, an absence of these growth factors allows for the switch to neuronal differentiation with a bias toward a dopaminergic fate. This may provide desired cells that can be used in therapeutic applications and offer insights into the etiology of some neurological disorders.

SFRP1 and SFRP2 dose-dependently regulate midbrain dopamine neuron development in vivo and in embryonic stem cells

Secreted Frizzled related proteins (sFRPs) are a family of proteins that modulate Wnt signaling, which in turn regulates multiple aspects of ventral midbrain (VM) and dopamine (DA) neuron development. However, it is not known which Wnt signaling branch and what aspects of midbrain DA neuron development are regulated by sFRPs. Here, we show that sFRP1 and sFRP2 activate the Wnt/planar-cell-polarity/Rac1 pathway in DA cells. In the developing VM, sFRP1 and sFRP2 are expressed at low levels, and sFRP1-/- or sFRP2-/- mice had no detectable phenotype. However, compound sFRP1-/-;sFRP2-/- mutants revealed a Wnt/PCP phenotype similar to that previously described for Wnt5a-/- mice. This included an anteroposterior shortening of the VM, a lateral expansion of the Shh domain and DA lineage markers (Lmx1a and Th), as well as an accumulation of Nurr1+ precursors in the VM. In vitro experiments showed that, while very high concentrations of SFRP1 had a negative effect on cell survival, low/medium concentrations of sFRP1 or sFRP2 promoted the DA differentiation of progenitors derived from primary VM cultures or mouse embryonic stem cells (ESCs), mimicking the effects of Wnt5a. We thus conclude that the main function of sFRP1 and sFRP2 is to enhance Wnt/PCP signaling in DA cells and to regulate Wnt/PCP-dependent functions in midbrain development. Moreover, we suggest that low-medium concentrations of sFRPs may be used to enhance the DA differentiation of ESCs and improve their therapeutic application.

Transcription factor TCF4 maintains the properties of human corneal epithelial stem cells

TCF4, a key transcription factor of Wnt signaling system, has been recently found to be essential for maintaining stem cells. However, its signaling pathway is not well elucidated. This study was to explore the functional roles and signaling pathway of TCF4 in maintaining adult stem cell properties using human corneal epithelial stem cells as a model. With immunofluorescent staining and real-time polymerase chain reaction, we observed that TCF4 was exclusively expressed in the basal layer of human limbal epithelium where corneal epithelial stem cells reside. TCF4 was found to be well colocalized with ABCG2 and p63, two recognized epithelial stem/progenitor cell markers. Using in vitro culture models of primary human corneal epithelial cells, we revealed that TCF4 mRNA and protein were upregulated by cells in exponential growth stage, and RNA interference by small interfering RNA-TCF4 (10-50 nM) transfection blocked TCF4 signaling and suppressed cell proliferation as measured by WST-1 assay. TCF4 silence was found to be accompanied by downregulated proliferation-associated factors p63 and survivin, as well as upregulated cyclin-dependent kinase inhibitor 1C (p57). By creating a wound healing model in vitro, we identified upregulation and activation of β-catenin/TCF4 with their protein translocation from cytoplasm to nuclei, as evaluated by reverse transcription-quantitative real-time polymerase chain reaction, immunostaining, and Western blotting. Upregulated p63/survivin and downregulated p57 were further identified to be TCF4 downstream molecules that promote cell migration and proliferation in wound healing process. These findings demonstrate that transcription factor TCF4 plays an important role in determining or maintaining the phenotype and functional properties of human corneal epithelial stem cells.

Review of the neuroanatomic landscape implicated in glucose sensing and regulation of nutrient signaling: immunophenotypic localization of diabetes gene Tcf7l2 in the developing murine brain

Genetic variants in the transcription factor 7-like 2(Tcf7l2) gene have been found to confer a significant risk of type 2 diabetes and attenuated insulin secretion. Based on its genomic wide association Tcf7l2 is considered the single most important predictor of diabetes to date. Previous studies of Tcf7l2 mRNA localization in the adult brain suggest a putative role of Tcf7l2 in the CNS regulation of energy homeostasis. The present study further characterizes the immunophenotypic distribution of peptide expression in the brains of Tcf7l2 progeny during developmental time periods between E12.5 and P1. Tcf7l2(-/-) is lethal beyond P1. Results show that while negligible TCF7L2 expression is found in the developing brains of Tcf7l2(-/-)mice, TCF7L2 protein is relatively widespread and robustly expressed in the brain by E18.5 and exhibits specific expression within neuronal populations and regions of the brain in Tcf7l2(+/-) and Tcf7l2(+/+) progeny. Strong immunophenotypic labeling was found in the diencephalic structure of the thalamus that suggests a role of Tcf7l2 in the development and maintenance of thalamic activity. Strongly expressed TCF7L2 was localized in select hypothalamic and preoptic nuclei indicative of Tcf7l2 function within neurons controlling energy balance. Definitive neuronal staining for TCF7L2 within nuclei of the brain stem and circumventricular organs extends TCF7L2 localization within autonomic neurons and its potential integration with autonomic function. In addition robust TCF7L2 expression was found in the tectal and tegmental structures of the superior and inferior colliculi as well as transient expression in neuroepithelium of the cerebral and hippocampal cortices of E16 and E18.5. Patterns of TCF7L2 peptide localization when compared to the adult protein synthetic chemical/anatomical landscape of glucose sensing exhibit a good correlational fit between its expression and regions, nuclei, and pathways regulating energy homeostasis via integration and response to peripheral endocrine, metabolic and neuronal signaling. TCF was also found co-localized with peptides that regulate energy homeostasis including AgRP, POMC and NPY. TCF7l2, some variants of which have been shown to impair GLP-1-induced insulin secretion, was also found co-localize with GLP-1 in adult TCF wild type progeny. Impaired Tcf7l2-mediated neural regulation may contribute to the risk and/or underlying pathophysiology of type 2 diabetes that has found high expression in genomic studies of Tcf7l2 variants.

Stromal factors SDF1α, sFRP1, and VEGFD induce dopaminergic neuron differentiation of human pluripotent stem cells

Human embryonic stem cell (hESC)-derived dopaminergic (DA) neurons hold potential for treating Parkinson's disease (PD) through cell replacement therapy. Generation of DA neurons from hESCs has been achieved by coculture with the stromal cell line PA6, a source of stromal cell-derived inducing activity (SDIA). However, the factors produced by stromal cells that result in SDIA are largely undefined. We previously reported that medium conditioned by PA6 cells can generate functional DA neurons from NTera2 human embryonal carcinoma stem cells. Here we show that PA6-conditioned medium can induce DA neuronal differentiation in both NTera2 cells and the hESC I6 cell line. To identify the factor(s) responsible for SDIA, we used large-scale microarray analysis of gene expression combined with mass spectrometric analysis of PA6-conditioned medium (CM). The candidate factors, hepatocyte growth factor (HGF), stromal cell-derived factor-1 α (SDF1α), secreted frizzled-related protein 1 (sFRP1), and vascular endothelial growth factor D (VEGFD) were identified, and their concentrations in PA6 CM were established by immunoaffinity capillary electrophoresis. Upon addition of SDF1α, sFRP1, and VEGFD to the culture medium, we observed an increase in the number of cells expressing tyrosine hydroxylase (a marker for DA neurons) and βIII-tubulin (a marker for immature neurons) in both the NTera2 and I6 cell lines. These results indicate that SDF1α, sFRP1, and VEGFD are major components of SDIA and suggest the potential use of these defined factors to elicit DA differentiation of pluripotent human stem cells for therapeutic intervention in PD.

Cell replacement therapy for Parkinson's disease: how close are we to the clinic?

Cell replacement therapy (CRT) offers great promise as the future of regenerative medicine in Parkinson´s disease (PD). Three decades of experiments have accumulated a wealth of knowledge regarding the replacement of dying neurons by new and healthy dopaminergic neurons transplanted into the brains of animal models and affected patients. The first clinical trials provided the proof of principle for CRT in PD. In these experiments, intrastriatal transplantation of human embryonic mesencephalic tissue reinnervated the striatum, restored dopamine levels and showed motor improvements. Sequential controlled studies highlighted several problems that should be addressed prior to the wide application of CRT for PD patients. Moreover, owing to ethical and practical problems, embryonic stem cells require replacement by better-suited stem cells. Several obstacles remain to be surpassed, including identifying the best source of stem cells for A9 dopaminergic neuron generation, eliminating the risk of tumor formation and the development of graft-induced dyskinesias, and standardizing dopaminergic cell production in order to enable clinical application. In this article, we present an update on CRT for PD, reviewing the research milestones, various stem cells used and tailored differentiation methods, and analyze the information gained from the clinical trials.

Ectopic Wnt/beta-catenin signaling induces neurogenesis in the spinal cord and hindbrain floor plate

The most ventral structure of the developing neural tube, the floor plate (FP), differs in neurogenic capacity along the neuraxis. The FP is largely non-neurogenic at the hindbrain and spinal cord levels, but generates large numbers of dopamine (mDA) neurons at the midbrain levels. Wnt1, and other Wnts are expressed in the ventral midbrain, and Wnt/beta catenin signaling can at least in part account for the difference in neurogenic capacity of the FP between midbrain and hindbrain levels. To further develop the hypothesis that canonical Wnt signaling promotes mDA specification and FP neurogenesis, we have generated a model wherein beta–catenin is conditionally stabilized throughout the FP. Here, we unambiguously show by fate mapping FP cells in this mutant, that the hindbrain and spinal cord FP are rendered highly neurogenic, producing large numbers of neurons. We reveal that a neurogenic hindbrain FP results in the altered settling pattern of neighboring precerebellar neuronal clusters. Moreover, in this mutant, mDA progenitor markers are induced throughout the rostrocaudal axis of the hindbrain FP, although TH+ mDA neurons are produced only in the rostral aspect of rhombomere (r)1. This is, at least in part, due to depressed Lmx1b levels by Wnt/beta catenin signaling; indeed, when Lmx1b levels are restored in this mutant, mDA are observed not only in rostral r1, but also at more caudal axial levels in the hindbrain, but not in the spinal cord. Taken together, these data elucidate both patterning and neurogenic functions of Wnt/beta catenin signaling in the FP, and thereby add to our understanding of the molecular logic of mDA specification and neurogenesis.

Molecular organization and timing of Wnt1 expression define cohorts of midbrain dopamine neuron progenitors in vivo

Midbrain dopamine (MbDA) neurons are functionally heterogeneous and modulate complex functions through precisely organized anatomical groups. MbDA neurons are generated from Wnt1-expressing progenitors located in the ventral mesencephalon (vMes) during embryogenesis. However, it is unclear whether the progenitor pool is partitioned into distinct cohorts based on molecular identity and whether the timing of gene expression uniquely identifies subtypes of MbDA neurons. In this study we show that Wnt1-expressing MbDA progenitors from embryonic day (E)8.5-12.5 have dynamic molecular identities that correlate with specific spatial locations in the vMes. We also tested the hypothesis that the timing of Wnt1 expression in progenitors is related to the distribution of anatomically distinct cohorts of adult MbDA neurons using genetic inducible fate mapping (GIFM). We demonstrate that the Wnt1 lineage contributes to specific cohorts of MbDA neurons during a 7-day epoch and that the contribution to MbDA neurons predominates over other ventral Mb domains. In addition, we show that calbindin-, GIRK2-, and calretinin-expressing MbDA neuron subtypes are derived from Wnt1-expressing progenitors marked over a broad temporal window. Through GIFM and quantitative analysis we demonstrate that the Wnt1 lineage does not undergo progressive lineage restriction, which eliminates a restricted competence model of generating MbDA diversity. Interestingly, we uncover that two significant peaks of Wnt1 lineage contribution to MbDA neurons occur at E9.5 and E11.5. Collectively, our findings delineate the temporal window of MbDA neuron generation and show that lineage and timing predicts the terminal distribution pattern of MbDA neurons.

Neural stem cell niches in health and diseases

Presence of neural stem cells in adult mammalian brains, including human, has been clearly demonstrated by several studies. The functional significance of adult neurogenesis is slowly emerging as new data indicate the sensitivity of this event to several "every day" external stimuli such as physical activity, learning, enriched environment, aging, stress and drugs. In addition, neurogenesis appears to be instrumental for task performance involving complex cognitive functions. Despite the growing body of evidence on the functional significance of NSC and despite the bulk of data concerning the molecular and cellular properties of NSCs and their niches, several critical questions are still open. In this work we review the literature describing i) old and new sites where NSC niche have been found in the CNS; ii) the intrinsic factors regulating the NSC potential; iii) the extrinsic factors that form the niche microenvironment. Moreover, we analyse NSC niche activation in iv) physiological and v) pathological conditions. Given the not static nature of NSCs that continuously change phenotype in response to environmental clues, a unique "identity card" for NSC identification is still lacking. Moreover, the multiple location of NSC niches that increase in diseases, leaves open the question of whether and how these structures communicate throughout long distance. We propose a model where all the NSC niches in the CNS may be connected in a functional network using the threads of the meningeal net as tracks.

Gene expression profiling of embryonic human neural stem cells and dopaminergic neurons from adult human substantia nigra

Neural stem cells (NSC) with self-renewal and multipotent properties serve as an ideal cell source for transplantation to treat neurodegenerative insults such as Parkinson's disease. We used Agilent's and Illumina Whole Human Genome Oligonucleotide Microarray to compare the genomic profiles of human embryonic NSC at a single time point in culture, and a multicellular tissue from postmortem adult substantia nigra (SN) which are rich in dopaminergic (DA) neurons. We identified 13525 up-regulated genes in both cell types of which 3737 (27.6%) genes were up-regulated in the hENSC, 4116 (30.4%) genes were up-regulated in the human substantia nigra dopaminergic cells, and 5672 (41.93%) were significantly up-regulated in both cell population. Careful analysis of the data that emerged using DAVID has permitted us to distinguish several genes and pathways that are involved in dopaminergic (DA) differentiation, and to identify the crucial signaling pathways that direct the process of differentiation. The set of genes expressed more highly at hENSC is enriched in molecules known or predicted to be involved in the M phase of the mitotic cell cycle. On the other hand, the genes enriched in SN cells include a different set of functional categories, namely synaptic transmission, central nervous system development, structural constituents of the myelin sheath, the internode region of axons, myelination, cell projection, cell somata, ion transport, and the voltage-gated ion channel complex. Our results were also compared with data from various databases, and between different types of arrays, Agilent versus Illumina. This approach has allowed us to confirm the consistency of our obtained results for a large number of genes that delineate the phenotypical differences of embryonic NSCs, and SN cells.

Otx genes in neurogenesis of mesencephalic dopaminergic neurons

Mesencephalic-diencephalic dopaminergic (mdDA) neurons play a relevant role in the control of movement, behavior, and cognition. Indeed loss and/or abnormal functioning of mdDA neurons are responsible for Parkinson's disease as well as for addictive and psychiatric disorders. In the last years a wealth of information has been provided on gene functions controlling identity, fate, and proliferation of mdDA progenitors. This review will focus on the role exerted by Otx genes in early decisions regulating sequential steps required for the neurogenesis of mesencephalic dopaminergic (mesDA) neurons. In this context, the regulatory network involving Otx functional interactions with signaling molecules and transcription factors required to promote or prevent the development of mesDA neurons will be analyzed in detail.

Wnt signaling promotes neuronal differentiation from mesenchymal stem cells through activation of Tlx3

Wnt/β-catenin signaling promotes neural differentiation by activation of the neuron-specific transcription factors, Neurogenin1 (Ngn1), NeuroD, and Brn3a, in the nervous system. As neurons in cranial sensory ganglia and dorsal root ganglia transiently express Ngn1, NeuroD, and Brn3a during embryonic development, we hypothesized that Wnt proteins could instructively promote a sensory neuronal fate from mesenchymal stem cells (MSCs) directed to differentiate into neurons. Consistent with our hypothesis, Wnt1 induced expression of sensory neuron markers including Ngn1, NeuroD, and Brn3a, as well as glutamatergic markers in neurally induced MSCs in vitro and promoted engraftment of transplanted MSCs in the inner ear bearing selective loss of sensory neurons in vivo. Given the consensus function of T-cell leukemia 3 (Tlx3), as a glutamatergic selector gene, we postulated that the effects of canonical Wnt signaling on sensory neuron and glutamatergic marker gene expression in MSCs may be mediated by Tlx3. We first confirmed that Wnt1 indeed upregulates Tlx3 expression, which can be suppressed by canonical Wnt inhibitors. Next, our chromatin immunoprecipitation assays revealed that T-cell factor 3/4, Wnt-activated DNA binding proteins, interact with a regulatory region of Tlx3 in MSCs after neural induction. Furthermore, we demonstrated that forced expression of Tlx3 in MSCs induced sensory and glutamatergic neuron markers after neural induction. Together, these results identify Tlx3 as a novel target for canonical Wnt signaling that confers somatic stem cells with a sensory neuron phenotype upon neural induction.

Quantitative and kinetic profile of Wnt/β-catenin signaling components during human neural progenitor cell differentiation

ReNcell VM is an immortalized human neural progenitor cell line with the ability to differentiate in vitro into astrocytes and neurons, in which the Wnt/β-catenin pathway is known to be involved. However, little is known about kinetic changes of this pathway in human neural progenitor cell differentiation. In the present study, we provide a quantitative profile of Wnt/β-catenin pathway dynamics showing its spatio-temporal regulation during ReNcell VM cell differentiation. We show first that T-cell factor dependent transcription can be activated by stabilized β-catenin. Furthermore, endogenous Wnt ligands, pathway receptors and signaling molecules are temporally controlled, demonstrating changes related to differentiation stages. During the first three hours of differentiation the signaling molecules LRP6, Dvl2 and β-catenin are spatio-temporally regulated between distinct cellular compartments. From 24 h onward, components of the Wnt/β-catenin pathway are strongly activated and regulated as shown by mRNA up-regulation of Wnt ligands (Wnt5a and Wnt7a), receptors including Frizzled-2, -3, -6, -7, and -9, and co-receptors, and target genes including Axin2. This detailed temporal profile of the Wnt/β-catenin pathway is a first step to understand, control and to orientate, in vitro, human neural progenitor cell differentiation.

A Wnt1 regulated Frizzled-1/β-Catenin signaling pathway as a candidate regulatory circuit controlling mesencephalic dopaminergic neuron-astrocyte crosstalk: Therapeutical relevance for neuron survival and neuroprotection

BACKGROUND

Dopamine-synthesizing (dopaminergic, DA) neurons in the ventral midbrain (VM) constitute a pivotal neuronal population controlling motor behaviors, cognitive and affective brain functions, which generation critically relies on the activation of Wingless-type MMTV integration site (Wnt)/β-catenin pathway in their progenitors. In Parkinson's disease, DA cell bodies within the substantia nigra pars compacta (SNpc) progressively degenerate, with causes and mechanisms poorly understood. Emerging evidence suggests that Wnt signaling via Frizzled (Fzd) receptors may play a role in different degenerative states, but little is known about Wnt signaling in the adult midbrain. Using in vitro and in vivo model systems of DA degeneration, along with functional studies in both intact and SN lesioned mice, we herein highlight an intrinsic Wnt1/Fzd-1/β-catenin tone critically contributing to the survival and protection of adult midbrain DA neurons.

RESULTS

In vitro experiments identifie Fzd-1 receptor expression at a mRNA and protein levels in dopamine transporter (DAT) expressing neurons, and demonstrate the ability of exogenous Wnt1 to exert robust neuroprotective effects against Caspase-3 activation, the loss of tyrosine hydroxylase-positive (TH+) neurons and [3H] dopamine uptake induced by different DA-specific insults, including serum and growth factor deprivation, 6-hydroxydopamine and MPTP/MPP+. Co-culture of DA neurons with midbrain astrocytes phenocopies Wnt1 neuroprotective effects, whereas RNA interference-mediated knockdown of Wnt1 in midbrain astrocytes markedly reduces astrocyte-induced TH+ neuroprotection. Likewise, silencing β-catenin mRNA or knocking down Fzd-1 receptor expression in mesencephalic neurons counteract astrocyte-induced TH+ neuroprotection. In vivo experiments document Fzd-1 co-localization with TH+ neurons within the intact SNpc and blockade of Fzd/β-catenin signaling by unilateral infusion of a Fzd/β-catenin antagonist within the SN induces reactive astrocytosis and acutely inhibits TH+ neuron survival in ipsilateral SNpc, an effect efficiently prevented by pharmacological activation of β-catenin signaling within the SNpc.

CONCLUSION

These results defining a novel Wnt1/Fzd-1/β-catenin astrocyte-DA autoprotective loop provide a new mechanistic inside into the regulation of pro-survival processes, with potentially relevant consequences for drug design or drug action in Parkinson's disease.

Wnt-3a and Wnt-3 differently stimulate proliferation and neurogenesis of spinal neural precursors and promote neurite outgrowth by canonical signaling

Wnt factors regulate neural stem cell development and neuronal connectivity. Here we investigated whether Wnt-3a and Wnt-3, expressed in the developing spinal cord, regulate proliferation and the neuronal differentiation of spinal cord neural precursors (SCNP). Wnt-3a promoted a sustained increase of SCNP proliferation and decreased the expression of cyclin-dependent kinase inhibitors. In contrast, Wnt-3 transiently enhanced SCNP proliferation and increased neurogenesis through β-catenin signaling. Furthermore, both Wnt-3a and Wnt-3 stimulated neurite outgrowth in SCNP-derived neurons through β-catenin- and TCF4-dependent transcription. Glycogen synthase kinase-3β inhibitors mimicked Wnt signaling and promoted neurite outgrowth in established cultures. We conclude that Wnt-3a and Wnt-3 factors signal through the canonical Wnt/β-catenin pathway to regulate different aspects of SCNP development. These findings may be of therapeutic interest for the treatment of neurodegenerative diseases and nerve injury.

ES cell-derived renewable and functional midbrain dopaminergic progenitors

During early development, midbrain dopaminergic (mDA) neuronal progenitors (NPs) arise from the ventral mesencephalic area by the combined actions of secreted factors and their downstream transcription factors. These mDA NPs proliferate, migrate to their final destinations, and develop into mature mDA neurons in the substantia nigra and the ventral tegmental area. Here, we show that such authentic mDA NPs can be efficiently isolated from differentiated ES cells (ESCs) using a FACS method combining two markers, Otx2 and Corin. Purified Otx2(+)Corin(+) cells coexpressed other mDA NP markers, including FoxA2, Lmx1b, and Glast. Using optimized culture conditions, these mDA NPs continuously proliferated up to 4 wk with almost 1,000-fold expansion without significant changes in their phenotype. Furthermore, upon differentiation, Otx2(+)Corin(+) cells efficiently generated mDA neurons, as evidenced by coexpression of mDA neuronal markers (e.g., TH, Pitx3, Nurr1, and Lmx1b) and physiological functions (e.g., efficient DA secretion and uptake). Notably, these mDA NPs differentiated into a relatively homogenous DA population with few serotonergic neurons. When transplanted into PD model animals, aphakia mice, and 6-OHDA-lesioned rats, mDA NPs differentiated into mDA neurons in vivo and generated well-integrated DA grafts, resulting in significant improvement in motor dysfunctions without tumor formation. Furthermore, grafted Otx2(+)Corin(+) cells exhibited significant migratory function in the host striatum, reaching >3.3 mm length in the entire striatum. We propose that functional and expandable mDA NPs can be efficiently isolated by this unique strategy and will serve as useful tools in regenerative medicine, bioassay, and drug screening.

En1 and Wnt signaling in midbrain dopaminergic neuronal development

Dopaminergic neurons of the ventral mesodiencephalon are affected in significant health disorders such as Parkinson's disease, schizophrenia, and addiction. The ultimate goal of current research endeavors is to improve the clinical treatment of such disorders, such as providing a protocol for cell replacement therapy in Parkinson's disease that will successfully promote the specific differentiation of a stem cell into a dopaminergic neuronal phenotype. Decades of research on the developmental mechanisms of the mesodiencephalic dopaminergic (mdDA) system have led to the identification of many signaling pathways and transcription factors critical in its development. The unraveling of these pathways will help fill in the pieces of the puzzle that today dominates neurodevelopment research: how to make and maintain a mdDA neuron. In the present review, we provide an overview of the mdDA system, the processes and signaling molecules involved in its genesis, with a focus on the transcription factor En1 and the canonical Wnt pathway, highlighting recent findings on their relevance--and interplay--in the development and maintenance of the mdDA system.

Wnt5a regulates midbrain dopaminergic axon growth and guidance

During development, precise temporal and spatial gradients are responsible for guiding axons to their appropriate targets. Within the developing ventral midbrain (VM) the cues that guide dopaminergic (DA) axons to their forebrain targets remain to be fully elucidated. Wnts are morphogens that have been identified as axon guidance molecules. Several Wnts are expressed in the VM where they regulate the birth of DA neurons. Here, we describe that a precise temporo-spatial expression of Wnt5a accompanies the development of nigrostriatal projections by VM DA neurons. In mice at E11.5, Wnt5a is expressed in the VM where it was found to promote DA neurite and axonal growth in VM primary cultures. By E14.5, when DA axons are approaching their striatal target, Wnt5a causes DA neurite retraction in primary cultures. Co-culture of VM explants with Wnt5a-overexpressing cell aggregates revealed that Wnt5a is capable of repelling DA neurites. Antagonism experiments revealed that the effects of Wnt5a are mediated by the Frizzled receptors and by the small GTPase, Rac1 (a component of the non-canonical Wnt planar cell polarity pathway). Moreover, the effects were specific as they could be blocked by Wnt5a antibody, sFRPs and RYK-Fc. The importance of Wnt5a in DA axon morphogenesis was further verified in Wnt5a^−/− mice, where fasciculation of the medial forebrain bundle (MFB) as well as the density of DA neurites in the MFB and striatal terminals were disrupted. Thus, our results identify a novel role of Wnt5a in DA axon growth and guidance.

International Union of Basic and Clinical Pharmacology. LXXX. The class Frizzled receptors

The receptor class Frizzled, which has recently been categorized as a separate group of G protein-coupled receptors by the International Union of Basic and Clinical Pharmacology, consists of 10 Frizzleds (FZD(1-10)) and Smoothened (SMO). The FZDs are activated by secreted lipoglycoproteins of the Wingless/Int-1 (WNT) family, whereas SMO is indirectly activated by the Hedgehog (HH) family of proteins acting on the transmembrane protein Patched (PTCH). Recent years have seen major advances in our knowledge about these seven-transmembrane-spanning proteins, including: receptor function, molecular mechanisms of signal transduction, and the receptor's role in embryonic patterning, physiology, cancer, and other diseases. Despite intense efforts, many question marks and challenges remain in mapping receptor-ligand interaction, signaling routes, mechanisms of specificity and how these molecular details underlie disease and also the receptor's important role in physiology. This review therefore focuses on the molecular aspects of WNT/FZD and HH/SMO signaling discussing receptor structure, mechanisms of signal transduction, accessory proteins, receptor dynamics, and the possibility of targeting these signaling pathways pharmacologically.