Selective loss of dopaminergic neurons in the substantia nigra of Pitx3-deficient aphakia mice

Behavioral models of Parkinson's disease in rodents: a new look at an old problem

The circuitry important for voluntary movement is influenced by dopamine from the substantia nigra and regulated by the nigrostriatal system. The basal ganglia influence the pyramidal tract and other motor systems, such as the mesopontine nuclei and the rubrospinal tract. Although the neuroanatomical substrates underlying motor control are similar for humans and rodents, the behavioral repertoire mediated by those circuits is not. The principal aim of this review is to evaluate how injury to dopamine-mediated pathways in rodents gives rise to motor dysfunction that mimics human Parkinsonism. We will examine the behavioral tests in common use with rodent models of Parkinson's disease and critically evaluate the appropriateness of each test for detecting motor impairment. We will show how tests of motor performance must be guided by a thorough understanding of the clinical symptoms accompanying the disease, the circuitry mediating dopamine deficits in rodents, and familiarity with the rodent behavioral repertoire. We will explain how investigations in rodents of skilled forepaw actions, including placing, grooming, or foot faults, have clear correlates in Parkinson's disease, and are, therefore, the most sensitive ways of detecting motor impairment following dopamine loss from the basal ganglia of rodents.

Behavioral phenotypes and pharmacology in genetic mouse models of Parkinsonism

Prior to the discovery of genes associated with familial forms of Parkinson's disease, animal models of Parkinson's disease mainly consisted of toxin models based exclusively on the degeneration of nigrostriatal dopamine neurons. These traditional models have provided valuable insight into symptomatic treatments for Parkinson's disease; however, they lack the broad extra-nigral pathology and the progression that is observed in the disease. The novel genetic mouse models recently generated are advantageous because they have mutations that are known to cause familial Parkinson's disease and thus they have good construct validity. To maximize the utility of these models, a thoughtful phenotypical characterization is important. Our laboratory has assembled a battery of behavioral tests to assess sensorimotor function in genetic mouse models of Parkinsonism. This review discusses the sensitivity of these tests in different genetic mice in addition to their behavioral response to dopamine agonists.

Midbrain dopamine neuron differentiation: factors and fates

Dopaminergic neurons in the midbrain (mDNs) play a central role in the regulation of voluntary movement as well as other complex behaviors, and their loss is associated with Parkinson's disease (PD). The development of functional mDNs from multipotent progenitors is orchestrated by cell-intrinsic factors and cell-extrinsic environmental cues in a series of stages: early midbrain patterning, specification of mitotic precursors, postmitotic mDN development, and functional maturation. Of particular interest is how extracellular information is integrated with cell-intrinsic developmental programs. Cell fate mapping studies suggest that the stem-like progenitors for mDNs reside at the ventral midline floor plate, a region that also serves as a source of inductive signals for mDN specification such as Sonic Hedgehog (SHH). Cell replacement therapies, and in particular the use of embryonic or adult stem cell-derived dopaminergic neurons, offer potential novel treatment venues for PD, but such strategies require a detailed understanding of mDN development.

Animal models concerning the role of dopamine in attention-deficit hyperactivity disorder

Several models of attention-deficit hyperactivity disorder (ADHD) have been proposed, ranging from administration of neurotoxins to genetically manipulated models. These models are used to gain insight into ADHD as a disorder and assist in the discovery of new therapeutic strategies. However, the information gained from these models differs, depending to a large extent on the validity (or otherwise) of the model. Thus the insights gained from these models with respect to the pathophysiology and aetiology of ADHD remains inconclusive. No animal model resembles the clinical situation of ADHD perfectly but good animal models of ADHD should mimic its characteristics, confirm to an underlying theory of ADHD and ultimately make predictions of future therapies. While the involvement of dopamine (DA) in ADHD has been established, the evaluation of rodent models of ADHD particularly with respect to dopaminergic systems is attempted here. It is concluded that the neonatal 6-hydroxy-dopamine lesioned rat and DA transporter knockout/knockdown mice have the highest degree of validity for ADHD.

Induction of tyrosine hydroxylase expression by the transcription factor Pitx3

Transcription factors are fate determining regulatory factors in dopaminergic neuronal development and differentiation. Among them, Nurr1 is the most extensively studied, but the importance of Pitx3 has recently been appreciated. Over-expression of both factors has been utilized to enhance the dopaminergic differentiation of stem cells for transplantation into models of Parkinson's disease. Previous studies however have seen conflicting results regarding the induction of tyrosine hydroxylase expression and dopaminergic differentiation induced by over-expression of Pitx3. Here we show that over-expression of Pitx3 and Nurr1 induced endogenous tyrosine hydroxylase expression as well as a tyrosine hydroxylase promoter-reporter construct in a human non-neuronal and mouse embryonic stem cell lines. Combined simultaneous expression of Nurr1 and Pitx3 however did not lead to enhancement of tyrosine hydroxylase expression over that of either factor alone in either of the cell lines or with either method. These results suggest that other regulatory elements may also be involved in regulation of tyrosine hydroxylase expression. There was also a lack of a correlation between the expression levels of tyrosine hydroxylase with that of the transcription factor constructs. To yield a robust dopaminergic differentiation a combinatorial or successive treatment with different transcription factors may be more effective.

Transforming Growth Factor β Is Required for Differentiation of Mouse Mesencephalic Progenitors into Dopaminergic Neurons In Vitro and In Vivo: Ectopic Induction in Dorsal Mesencephalon

Tissue engineering is a prerequisite for cell replacement as therapeutic strategy for degenerative diseases, such as Parkinson's disease. In the present study, we investigated regional identity of mesencephalic neural progenitors and characterized their development toward ventral mesencephalic dopaminergic neurons. We show that neural progenitors from ventral and dorsal mouse embryonic day 12 mesencephalon exhibit regional identity in vitro. Treatment of ventral midbrain dissociated neurospheres with transforming growth factor beta (TGF-beta) increased the number of Nurr1- and tyrosine hydroxylase (TH)-immunoreactive cells, which can be further increased when the spheres are treated with TGF-beta in combination with sonic hedgehog (Shh) and fibroblast growth factor 8 (FGF8). TGF-beta differentiation signaling is TGF-beta receptor-mediated, involving the Smad pathway, as well as the p38 mitogen-activated protein kinase pathway. In vivo, TGF-beta2/TGF-beta3 double-knockout mouse embryos revealed significantly reduced numbers of TH labeled cells in ventral mesencephalon but not in locus coeruleus. TH reduction in Tgfbeta2(-/-)/Tgfbeta3(+/-) was higher than in Tgf-beta2(+/-)/Tgf-beta3(-/-). Most importantly, TGF-beta may ectopically induce TH-immunopositive cells in dorsal mesencephalon in vitro, in a Shh- and FGF8-independent manner. Together, the results clearly demonstrate that TGF-beta2 and TGF-beta3 are essential signals for differentiation of midbrain progenitors toward neuronal fate and dopaminergic phenotype.

Transcriptional control of midbrain dopaminergic neuron development

Although loss of midbrain dopaminergic neurons is associated with one of the most common human neurological disorders, Parkinson's disease, little is known about the specification of this neuronal subtype. Hence, the recent identification of major transcriptional determinants regulating the development of these neurons has brought much excitement and encouragement to this field. These new findings will help to elucidate the genetic program that promotes the generation of midbrain dopaminergic neurons. Importantly, these discoveries will also significantly advance efforts to differentiate stem cells into midbrain dopaminergic neurons that can be used for therapeutic use in treating Parkinson's disease.

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.

Pitx3 deficiency in mice affects cholinergic modulation of GABAergic synapses in the nucleus accumbens

We investigated to what extent Pitx3 deficiency, causing hyperdopaminergic transmission in the nucleus accumbens microcircuitry, may lead to developmental changes. First, spontaneous firing activity of cholinergic interneurons in the nucleus accumbens was recorded in vitro. Firing patterns in the Pitx3-deficient mice were more variable and intrinsically different from those observed in wild-type mice. Next, to test whether the irregular firing patterns observed in mutant mice affected the endogenous nicotinic modulation of the GABAergic input of medium spiny neurons, we recorded spontaneous GABAergic inputs to these cells before and after the application of the nicotinic receptor blocker mecamylamine. Effects of mecamylamine were found in slices of either genotype, but in a rather inconsistent manner. Possibly this was attributable to heterogeneity in firing of nearby cholinergic interneurons. Thus paired recordings of cholinergic interneurons and medium spiny neurons were performed to more precisely control the experimental conditions of the cholinergic modulation of GABAergic synaptic transmission. We found that controlling action potential firing in cholinergic neurons leads to a conditional increase in GABAergic input frequency in wild-type mice but not in Pitx3-deficient mice. We conclude that Pitx3-deficient mice have neural adaptations at the level of the nucleus accumbens microcircuitry that in turn may have behavioral consequences. It is discussed to what extent dopamine release in the nucleus accumbens may be a long-term gating mechanism leading to alterations in cholinergic transmission in the nucleus accumbens, in line with previously reported neural adaptations found as consequences of repeated drug treatment in rodents.

Strategies to unravel molecular codes essential for the development of meso-diencephalic dopaminergic neurons

Understanding the development of neuronal systems has become an important asset in the attempt to solve complex questions about neuropathology as found in Parkinson's disease, schizophrenia and other complex neuronal diseases. The development of anatomical and functional divergent structures in the brain is achieved by a combination of early anatomical patterning and highly coordinated neuronal migration and differentiation events. Fundamental to the existence of divergent structures in the brain is the early region-specific molecular programming. Neuronal progenitors located along the neural tube can still adapt many different identities. Their exact position in the developing brain, however, determines early molecular specification by region-specific signalling molecules. These signals determine time and region-specific expression of early regulatory genes, leading to neuronal differentiation. Here, we focus on a well-described neuronal group, the meso-diencephalic dopaminergic neurons, of which heterogeneity based on anatomical position could account for the difference in vulnerability of specific subgroups as observed in Parkinson's disease. The knowledge of their molecular coding helps us to understand how the meso-diencephalic dopaminergic system is built and could provide clues that unravel mechanisms associated with the neuropathology in complex diseases such as Parkinson's disease.

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.

Cooperative transcription activation by Nurr1 and Pitx3 induces embryonic stem cell maturation to the midbrain dopamine neuron phenotype

Midbrain dopamine (DA) neurons play a central role in the regulation of voluntary movement, and their degeneration is associated with Parkinson's disease. Cell replacement therapies, and in particular embryonic stem (ES) cell-derived DA neurons, offer a potential therapeutic venue for Parkinson's disease. We sought to identify genes that can potentiate maturation of ES cell cultures to the midbrain DA neuron phenotype. A number of transcription factors have been implicated in the development of midbrain DA neurons by expression analyses and loss-of-function knockout mouse studies, including Nurr1, Pitx3, Lmx1b, Engrailed-1, and Engrailed-2. However, none of these factors appear sufficient alone to induce the mature midbrain DA neuron phenotype in ES cell cultures in vitro, suggesting a more complex regulatory network. Here we show that Nurr1 and Pitx3 cooperatively promote terminal maturation to the midbrain DA neuron phenotype in murine and human ES cell cultures.

Neurogenin 2 is required for the development of ventral midbrain dopaminergic neurons

Proneural genes are crucial regulators of neurogenesis and subtype specification in many areas of the nervous system; however, their function in dopaminergic neuron development is unknown. We report that proneural genes have an intricate pattern of expression in the ventricular zone of the ventral midbrain, where mesencephalic dopaminergic neurons are generated. Neurogenin 2(Ngn2) and Mash1 are expressed in the ventral midline, while Ngn1, Ngn2 and Mash1 are co-localized more laterally in the ventricular zone. Ngn2 is also expressed in an intermediate zone immediately adjacent to the ventricular zone at the ventral midline. To examine the function of these genes, we analyzed mutant mice in which one or two of these genes were deleted (Ngn1, Ngn2 and Mash1) or substituted (Mash1 in the Ngn2 locus). Our results demonstrate that Ngn2 is required for the differentiation of Sox2+ ventricular zone progenitors into Nurr1+postmitotic dopaminergic neuron precursors in the intermediate zone, and that it is also likely to be required for their subsequent differentiation into tyrosine hydroxylase-positive dopaminergic neurons in the marginal zone. Although Mash1 normally has no detectable function in dopaminergic neuron development, it could partially rescue the generation of dopaminergic neuron precursors in the absence of Ngn2. These results demonstrate that Ngn2 is uniquely required for the development of midbrain dopaminergic neurons.

Genetic mouse models of parkinsonism: strengths and limitations

Parkinson's disease (PD) is a progressive neurodegenerative disorder. Patients with PD display a combination of motor symptoms including resting tremor, rigidity, bradykinesia, and postural instability that worsen over time. These motor symptoms are related to the progressive loss of dopamine neurons in the substantia nigra pars compacta. PD patients also suffer from nonmotor symptoms that may precede the cardinal motor symptoms and that are likely related to pathology in other brain regions. Traditional toxin models of PD have focused on the nigrostriatal pathway and the loss of dopamine neurons in this region, and these models have been important in our understanding of PD and in the development of symptomatic treatments for the disease. However, they are limited in that they do not reproduce the full pathology and progression seen in PD, thus creating a need for better models. The recent discovery of specific genes causing familial forms of PD has contributed to the development of novel genetic mouse models of PD. This review discusses the validity, benefits, and limitations of these new models.

The role of Pitx3 in survival of midbrain dopaminergic neurons

Dopamine belongs to the most intensively studied neurotransmitters of the brain, because of its implications in psychiatric and neurological disorders. Although, clinical relevance of midbrain dopaminergic (mDA) neurons is well recognized and dopaminergic dysfunction may have a genetic component, the genetic cascades underlying developmental processes are still largely unknown. With the advances in molecular biology, mDA neurons and their involvement in psychiatric and neurological disorders are now subject of studies that aim to delineate the fundamental neurobiology of these neurons. These studies are concerned with developmental processes, cell-specific gene expression and regulation, molecular pharmacology, and genetic association of dopamine-related genes and mDA-associated disorders. Several transcription factors implicated in the post-mitotic mDA development, including Nurr1, Lmx1b, Pitx3, and En1/En2 have contributed to the understanding of how mDA neurons are generated in vivo. Furthermore, these studies provide insights into new strategies for future therapies of Parkinson's Disease (PD) using stem cells for engineering DA neurons in vitro. Here, we will discuss the role of Pitx3 in molecular mechanisms involved in the regional specification, neuronal specification and differentiation of mDA neurons.

Developmental origin and fate of meso-diencephalic dopamine neurons

Specific vulnerability of substantia nigra compacta neurons as compared to ventral tegmental area neurons, as emphasized in Parkinson's disease, has been studied for many years and is still not well understood. The molecular codes and mechanisms that drive development of these structures have recently been studied through the use of elegant genetic ablation experiments. The data suggested that specific genes at specific anatomical positions in the ventricular zone are crucial to drive development of young neurons into the direction of the dopaminergic phenotype. In addition, it has become clear the these dopaminergic neurons are present in the diencephalon and in the mesencephalon and that they may contain a specific molecular signature that defines specific subsets in terms of position and function. The data indicate that these specific subsets may explain the specific response of these neurons to toxins and genetic ablation.

Striatal neuroadaptation and rescue of locomotor deficit by l-dopa in aphakia mice, a model of Parkinson's disease

Preferential neurodegeneration of dopaminergic neurons in the ventral substantia nigra of the midbrain is a hallmark of Parkinson's disease. The homeobox transcription factor Pitx3 is similarly and selectively expressed in the same neurons. Pitx3 deficiency in a natural mouse mutant, the aphakia mouse, was correlated with the loss of these neurons and with a deficit in locomotor activity. We now report that the locomotor deficit of aphakia mice is established by 40 days of age and that it can be rescued by injection of l-dopa. We further show that downstream striatal correlates of the midbrain neuronal losses in aphakia mice, as assessed by dopamine transporter binding and expression of dopamine receptors, enkephalin, dynorphin and neurotensin, are highly similar to neuroadaptive responses observed following rapid neurodegeneration induced by neurotoxin administration in adult animals or following the progressive neurodegenerative processes as seen in Parkinson patients. Taken collectively, these data support the idea that the aphakia mice represent a selective model of dopaminergic deficiency that closely resembles the midbrain and striatal neuropathology associated with Parkinson's disease, and this suggests that these mice are a good model to assess therapies for Parkinson's disease as well as to understand the susceptibility of these neurons to neurodegeneration.

Purified mouse dopamine neurons thrive and function after transplantation into brain but require novel glial factors for survival in culture

Cell replacement therapy in Parkinson's disease depends on a reliable source of purified dopamine (DA) neurons (PDN) and the identification of factors relevant to their survival. Our goal was to genetically tag and purify by flow cytometry embryonic midbrain DA neurons from a transgenic mouse line carrying 11 kb of human tyrosine hydroxylase promoter driving expression of the enhanced green fluorescent protein (GFP) for studies in vivo and in vitro. A 99% purification of GFP(+) cells was achieved. When transplanted into 6-hydroxydopamine-treated rat striatum, PDN survived, became well-integrated and produced recovery from amphetamine-induced motor behaviors. However, when grown in culture, PDN died within days of plating. No known growth factors prevented PDN death as did incubation with novel factors in glia/glial-conditioned media. We conclude that GFP-tagged DA neurons can be purified to homogeneity and can survive and function when grown with glial factors in vitro or after transplantation in vivo.

3,4-dihydroxyphenylalanine reverses the motor deficits in Pitx3-deficient aphakia mice: behavioral characterization of a novel genetic model of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease characterized by a loss of dopaminergic neurons in the substantia nigra. There is a need for genetic animal models of PD for screening and in vivo testing of novel restorative therapeutic agents. Although current genetic models of PD produce behavioral impairment and nigrostriatal dysfunction, they do not reproduce the loss of midbrain dopaminergic neurons and 3,4-dihydroxyphenylalanine (L-DOPA) reversible behavioral deficits. Here, we demonstrate that Pitx3-deficient aphakia (ak) mice, which have been shown previously to exhibit a major loss of substantia nigra dopaminergic neurons, display motor deficits that are reversed by L-DOPA and evidence of "dopaminergic supersensitivity" in the striatum. Thus, ak mice represent a novel genetic model exhibiting useful characteristics to test the efficacy of symptomatic therapies for PD and to study the functional changes in the striatum after dopamine depletion and L-DOPA treatment.

The role of Nurr1 in the development of dopaminergic neurons and Parkinson's disease

Nurr1, a transcription factor belonging to the orphan nuclear receptor superfamily, is critical in the development and maintenance of the dopaminergic system and as such it may have role in the pathogenesis of Parkinson' disease (PD). Human Nurr1 gene has been mapped to chromosome 2q22-23 and Nurr1 protein is predominantly expressed in central dopaminergic neurons. Nurr1 interacts with other factors critical for the survival of mensencephalic dopaminergic neurons and it appears to regulate the expression of tyrosine hydroxylase (TH), dopamine transporter (DAT), vesicular monoamine transporter 2 (VMAT2), and l-aromatic amino acid decarboxylase (AADC), all of which are important in the synthesis and storage of dopamine. Experimental studies in Nurr1 knock-out mice indicate that Nurr1 deficiency results in impaired dopaminergic function and increased vulnerability of those midbrain dopaminergic neurons that degenerate in PD. Decreased Nurr1 expression is found in the autopsied PD midbrains, particularly in neurons containing Lewy bodies, as well as in peripheral lymphocytes of patients with parkinsonian disorders. Several variants in Nurr1 gene have been reported in association with PD. All these studies suggest that Nurr1 is not only essential in the development of mensencephalic dopaminergic neurons and maintenance of their functions, but it may also play a role in the pathogenesis of PD.

Signalling through phospholipase C beta 4 is not essential for midbrain dopaminergic neuron survival

The most prominent progressive neurodegenerative movement disorder, Parkinson's disease, is attributed to selective loss of dopamine neurons in the substantia nigra pars compacta, resulting in severe deficiency of dopamine. The homeo-domain gene, Pit x 3, is essential for proper development of midbrain dopaminergic neurons in the substantia nigra pars compacta and might be involved in midbrain dopaminergic survival pathways. The mGluR1-signaling downstream-effector phospholipase C beta 4 was identified in a suppression subtractive hybridization screen comparing wild-type and Pit x 3-deficient Aphakia midbrain dopaminergic neurons. Expression pattern analysis revealed that phospholipase C beta 4 was expressed in midbrain dopaminergic neurons of the substantia nigra pars compacta and part of the ventral tegmental area, whereas expression of mGluR1alpha was predominantly observed in the more vulnerable midbrain dopaminergic neurons in the lateral substantia nigra pars compacta. However, clear expression of phospholipase C beta 4 in spared midbrain dopaminergic neurons of Aphakia mice located in the ventral tegmental area, indicated that induction and maintenance of phospholipase C beta 4 expression is Pit x 3-independent in these neurons. Furthermore, we report here a normal distribution of midbrain dopaminergic cell bodies and axonal projection to the striatum in phospholipase C beta 4-/- mice, indicating that signaling of phospholipase C beta 4 is not essential for the survival of midbrain dopaminergic neurons.

Differential expression of the homeobox gene Pitx3 in midbrain dopaminergic neurons

The transcription factor Pitx3 is expressed selectively in the midbrain and regulates the differentiation and survival of dopaminergic neurons. Lack of this factor results in a degeneration similar to that seen in Parkinson's disease. We have studied the pattern and the level of expression of Pitx3 in dopaminergic neurons of 3- to 4-week-old Wistar rats. We report Pitx3 expression in almost all dopaminergic substantia nigra (SN) and ventral tegmental area (VTA) neurons. It is coexpressed with the neuroprotective marker calbindin (CB) in a larger population of VTA (43%) than SN (16%) dopaminergic neurons. The level of Pitx3 mRNA, determined by semiquantitative RT-PCR, is approximately 6x higher in VTA than in SN single neurons. In the VTA but not in SN the level of Pitx3 is associated with the presence of CB: in CB-positive neurons the expression of Pitx3 mRNA is 3.6x higher than in CB-negative cells. CB is expressed in a larger population of VTA than SN neurons and the relative level of CB expression is 4x higher in VTA than in SN. A higher Pitx3 expression level and higher coexpression of Pitx3 and CB in VTA than in SN neurons may contribute to the different vulnerability of these dopaminergic nuclei to neurodegeneration.

Molecular and cellular alterations in the Pitx3-deficient midbrain dopaminergic system

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by loss of midbrain dopaminergic (mDA) neurons in the substantia nigra compacta (SNc). In order to provide insights into adaptive mechanisms of the mDA system in pathology, specific molecular and cellular parameters of the mDA system were studied in Pitx3-deficient Aphakia (ak) mice, which suffer from severe developmental failure of SNc mDA neurons. Here, we demonstrate differential changes in striatal gene expression, reflecting the specific neuronal loss in these mice. In addition, the neuronal activity of remaining mDA neurons in the ventral tegmental area (VTA) was significantly increased in ak mice. In conclusion, ak mice display specific molecular and cellular alterations in the mDA system that provide new insights in compensatory mechanisms present in mDA-associated disorders such as PD.

Midbrain dopaminergic development in vivo and in vitro from embryonic stem cells

The midbrain dopaminergic (mDA) neurons play a key role in the function of a variety of brain systems, including motor control and reward pathways. This has led to much interest in these neurons as targets for intervention in human disorders such as Parkinson's disease and schizophrenia. A major area of interest is to direct embryonic stem (ES) cells to differentiate into mDA neurons in vitro, which can then be used for cell therapy or drug screening. At present, our understanding of mDA development in vivo is limited. However, recent studies have identified a number of regulatory factors that influence the development of mDA neurons in vivo. Such studies will not only increase our understanding of mDA development in vivo, they may also promote new paradigms for regulating mDA production from ES cells in vitro. Here we review the current knowledge on mDA development in vivo and mDA differentiation.

Stromal cell-derived inducing activity, Nurr1, and signaling molecules synergistically induce dopaminergic neurons from mouse embryonic stem cells

To induce differentiation of embryonic stem cells (ESCs) into specialized cell types for therapeutic purposes, it may be desirable to combine genetic manipulation and appropriate differentiation signals. We studied the induction of dopaminergic (DA) neurons from mouse ESCs by overexpressing the transcription factor Nurr1 and coculturing with PA6 stromal cells. Nurr1-expressing ESCs (N2 and N5) differentiated into a higher number of neurons (approximately twofold) than the naïve ESCs (D3). In addition, N2/N5-derived cells contained a significantly higher proportion (>50%) of tyrosine hydroxylase (TH)+ neurons than D3 (<30%) and an even greater proportion of TH+ neurons (approximately 90%) when treated with the signaling molecules sonic hedgehog, fibroblast growth factor 8, and ascorbic acid. N2/N5-derived cells express much higher levels of DA markers (e.g., TH, dopamine transporter, aromatic amino acid decarboxylase, and G protein-regulated inwardly rectifying K+ channel 2) and produce and release a higher level of dopamine, compared with D3-derived cells. Furthermore, the majority of generated neurons exhibited electrophysiological properties characteristic of midbrain DA neurons. Finally, transplantation experiments showed efficient in vivo integration/generation of TH+ neurons after implantation into mouse striatum. Taken together, our results show that the combination of genetic manipulation(s) and in vitro cell differentiation conditions offers a reliable and effective induction of DA neurons from ESCs and may pave the way for future cell transplantation therapy in Parkinson's disease.

Pitx3 regulates tyrosine hydroxylase expression in the substantia nigra and identifies a subgroup of mesencephalic dopaminergic progenitor neurons during mouse development

Recent studies of mouse mutant aphakia have implicated the homeobox gene Pitx3 in the survival of substantia nigra dopaminergic neurons, the degeneration of which causes Parkinson's disease. To directly investigate a role for Pitx3 in midbrain DA neuron development, we have analysed a line of Pitx3-null mice that also carry an eGFP reporter under the control of the endogenous Pitx3 promoter. We show that the lack of Pitx3 resulted in a loss of nascent substantia nigra dopaminergic neurons at the beginning of their final differentiation. Pitx3 deficiency also caused a loss of tyrosine hydroxylase (TH) expression specifically in the substantia nigra neurons. Therefore, our study provides the first direct evidence that the aphakia allele of Pitx3 is a hypomorph and that Pitx3 is required for the regulation of TH expression in midbrain dopaminergic neurons as well as the generation and/or maintenance of these cells. Furthermore, using the targeted GFP reporter as a midbrain dopaminergic lineage marker, we have identified previously unrecognised ontogenetically distinct subpopulations of dopaminergic cells within the ventral midbrain based on their temporal and topographical expression of Pitx3 and TH. Such an expression pattern may provide the molecular basis for the specific dependence of substantia nigra DA neurons on Pitx3.

The homeodomain transcription factor Pitx3 facilitates differentiation of mouse embryonic stem cells into AHD2-expressing dopaminergic neurons

The A9 dopaminergic (DA) neuronal group projecting to the dorsal striatum is the most vulnerable in Parkinson's disease (PD). We genetically engineered mouse embryonic stem (ES) cells to express the transcription factors Nurr1 or Pitx3. After in vitro differentiation of Pitx3-expressing ES cells, the proportion of DA neurons expressing aldehyde dehydrogenase 2 (AHD2) increased, while the total number of DA neurons remained the same. The highest levels of AHD2 expression were observed in mouse A9 DA neurons projecting to the dorsal striatum. Furthermore, real-time PCR analyses of in vitro differentiated Pitx3-expressing ES cells revealed that genes highly expressed in A9 DA neurons were up-regulated. When transplanted into the mouse striatum, Pitx3-expressing cells generated an increased proportion of AHD2-expressing DA neurons. Contrastingly, in Nurr1-expressing ES cells, increases of all midbrain DA markers were observed, resulting in a higher total number of DA neurons in vitro and in vivo, whereas the proportion of AHD2-expressing DA neurons was not changed. Our data, using gain-of-function analysis of ES cells, suggest that Pitx3 may be important for specification and/or maintenance of A9-like neuronal properties, while Nurr1 influences overall midbrain DA specification. These findings may be important for modifying ES cells to generate an optimal cell source for transplantation therapy of PD.

Early and progressive sensorimotor anomalies in mice overexpressing wild-type human alpha-synuclein

Accumulation of alpha-synuclein in brain is a hallmark of synucleinopathies, neurodegenerative diseases that include Parkinson's disease. Mice overexpressing alpha-synuclein under the Thy-1 promoter (ASO) show abnormal accumulation of alpha-synuclein in cortical and subcortical regions of the brain, including the substantia nigra. We examined the motor deficits in ASO mice with a battery of sensorimotor tests that are sensitive to alterations in the nigrostriatal dopaminergic system. Male wild-type and ASO mice were tested every 2 months for 8 months for motor performance and coordination on a challenging beam, inverted grid, and pole, sensorimotor deficits in an adhesive removal test, spontaneous activity in a cylinder, and gait. Fine motor skills were assessed by the ability to grasp cotton from a bin. ASO mice displayed significant impairments in motor performance and coordination and a reduction in spontaneous activity as early as 2 months of age. Motor performance and coordination impairments became progressively worse with age and sensorimotor deficits appeared at 6 months. Fine motor skills were altered at 4 months and worsened at 8 months. These data indicate that overexpression of alpha-synuclein induced an early and progressive behavioral phenotype that can be detected in multiple tests of sensorimotor function. These behavioral deficits provide a useful way to assess novel drug therapy in genetic models of synucleinopathies.

Genetic control of dopaminergic neuron differentiation

The mesencephalic dopaminergic (mesDA) system controls movement and emotional behaviour, and its degeneration causes Parkinson's disease and other psychiatric disorders. Recent findings are leading to better understanding of the genetic control of generation and functioning of the mesDA system. This advancement could disclose new perspectives for therapeutic approaches of mesDA-related disorders.

Genetics of subthalamic nucleus in development and disease

The subthalamic nucleus (STN) is a crucial node in the basal ganglia. Clinical success in targeting the STN for deep brain stimulation in Parkinson's disease patients has prompted increased interest in understanding STN biology. In this report, we discuss recent evidence for transcription factor mediated regulation of STN development. We also review STN developmental neurobiology and known patterns of gene expression in the developing and mature STN.

Striatal expression of GDNF and differential vulnerability of midbrain dopaminergic cells

Glial cell line-derived neurotrophic factor (GDNF) is a member of the transforming growth factor-beta superfamily that when exogenously administrated exerts a potent trophic action on dopaminergic (DA) cells. Although we know a lot about its signalling mechanisms and pharmacological effects, physiological actions of GDNF on the adult brain remain unclear. Here, we have used morphological and molecular techniques, and an experimental model of Parkinson's disease in rats, to investigate whether GDNF constitutively expressed in the adult mesostriatal system plays a neuroprotective role on midbrain DA cells. We found that although all midbrain DA cells express both receptor components of GDNF (GFRalpha1 and Ret), those in the ventral tegmental area (VTA) and rostromedial substantia nigra (SNrm) also contain GDNF but not GDNFmRNA. The levels of GDNFmRNA are significantly higher in the ventral striatum (vSt), the target region of VTA and SNrm cells, than in the dorsal striatum (dSt), the target region of DA cells in the caudoventral substantia nigra (SNcv). After fluoro-gold injection in striatum, VTA and SNrm DA cells show triple labelling for tyrosine hydroxylase, GDNF and fluoro-gold, and after colchicine injection in the lateral ventricle, they become GDNF-immunonegative, suggesting that GDNF in DA somata comes from their striatal target. As DA cells in VTA and SNrm are more resistant than those in SNcv to intracerebroventricular injection of 6-OHDA, as occurs in Parkinson's disease, we can suggest that the fact that they project to vSt, where GDNF expression is significantly higher than in the dSt, is a neuroprotective factor involved in the differential vulnerability of midbrain DA neurons.

Characterization of the neuronal marker NeuN as a multiply phosphorylated antigen with discrete subcellular localization

NeuN (neuronal nuclei) is an antigen used widely in research and diagnostics to identify postmitotic neurons. The present study aims at an initial understanding of the molecular nature and functional significance of this as yet ill-defined antigen. Using isoelectric focusing, both the 46- and 48-kDa isoforms of NeuN can be separated in multiple spots spanning a pH range of 8-10.5, suggesting that they might be phosphorylated. Enzymatic dephosphorylation abolishes NeuN immunoreactivity, confirming that NeuN is indeed a phosphoprotein, and establishing that binding of the defining antibody depends on its state of phosphorylation. Combined biochemical and immunohistochemical analysis show that both the 46- and the 48-kDa NeuN isoforms can be localized to the cell nucleus as well as in the neuronal cytoplasm. Their relative concentration in these compartments is distinct, however, with the 48-kDa isoform being the predominant isoform in the cytoplasm. Within the nucleus, NeuN is found preferentially in areas of low chromatin density and virtually excluded from areas containing densely packed DNA. The present identification of multiple differentially phosphorylated isoforms of NeuN, together with recent reports on the dependence of NeuN immunoreactivity levels on a variety of physiologic or pathologic signals, suggests a previously unappreciated level of complexity in the regulation of this enigmatic, neuron-specific antigen.

Localization of immunoreactivity for Deleted in Colorectal Cancer (DCC), the receptor for the guidance factor netrin-1, in ventral tier dopamine projection pathways in adult rodents

DCC (deleted in colorectal cancer)-the receptor of the netrin-1 neuronal guidance factor-is expressed and is active in the central nervous system (CNS) during development, but is down-regulated during maturation. The substantia nigra contains the highest level of netrin-1 mRNA in the adult rodent brain, and corresponding mRNA for DCC has also been detected in this region but has not been localized to any particular neuron type. In this study, an antibody raised against DCC was used to determine if the protein was expressed by adult dopamine neurons, and identify their distribution and projections. Significant DCC-immunoreactivity was detected in midbrain, where it was localized to ventrally displaced A9 dopamine neurons in the substantia nigra, and ventromedial A10 dopamine neurons predominantly situated in and around the interfascicular nucleus. Strong immunoreactivity was not detected in dopamine neurons found elsewhere, or in non-dopamine-containing neurons in the midbrain. Terminal fields selectively labeled with DCC antibody corresponded to known nigrostriatal projections to the dorsolateral striatal patches and dorsomedial shell of the accumbens, and were also detected in prefrontal cortex, septum, lateral habenular and ventral pallidum. The unique distribution of DCC-immunoreactivity in adult ventral midbrain dopamine neurons suggests that netrin-1/DCC signaling could function in plasticity and remodeling previously identified in dopamine projection pathways. In particular, a recent report that DCC is regulated through the ubiquitin-proteosome system via Siah/Sina proteins, is consistent with a potential involvement in genetic and sporadic forms of Parkinson's disease.

Differentiation of Human Embryonic Stem Cells to Dopaminergic Neurons in Serum-Free Suspension Culture

The use of human embryonic stem cells (hESCs) as a source of dopaminergic neurons for Parkinson's disease cell therapy will require the development of simple and reliable cell differentiation protocols. The use of cell cocultures, added extracellular signaling factors, or transgenic approaches to drive hESC differentiation could lead to additional regulatory as well as cell production delays for these therapies. Because the neuronal cell lineage seems to require limited or no signaling for its formation, we tested the ability of hESCs to differentiate to form dopamine-producing neurons in a simple serum-free suspension culture system. BG01 and BG03 hESCs were differentiated as suspension aggregates, and neural progenitors and neurons were detectable after 2-4 weeks. Plated neurons responded appropriately to electrophysiological cues. This differentiation was inhibited by early exposure to bone morphogenic protein (BMP)-4, but a pulse of BMP-4 from days 5 to 9 caused induction of peripheral neuronal differentiation. Real-time polymerase chain reaction and whole-mount immunocytochemistry demonstrated the expression of multiple markers of the midbrain dopaminergic phenotype in serum-free differentiations. Neurons expressing tyrosine hydroxylase (TH) were killed by 6-hydroxydopamine (6-OHDA), a neurotoxic catecholamine. Upon plating, these cells released dopamine and other catecholamines in response to K+ depolarization. Surviving TH+ neurons, derived from the cells differentiated in serum-free suspension cultures, were detected 8 weeks after transplantation into 6-OHDA-lesioned rat brains. This work suggests that hESCs can differentiate in simple serum-free suspension cultures to produce the large number of cells required for transplantation studies.

Homeobox gene Pitx3 and its role in the development of dopamine neurons of the substantia nigra

The homeobox gene Pitx3 plays an important part in the development and function of vertebrate midbrain dopaminergic neurons. Re-localization of the genetic defect in the mouse mutant aphakia to the Pitx3 locus, together with the subsequent identification of two deletions causing the gene to be silent, has been the hallmark of several studies into the role of Pitx3. In this review, we summarize the data and reflect on the role of Pitx3 in the development of dopamine neurons in the midbrain. The data indicate that Pitx3 is essential for the survival of dopamine neurons located in the substantia nigra compacta during development. Molecular analysis of the underlying mechanisms might provide new insights for understanding the selective degeneration observed in Parkinson patients.

Transcription factors specifying dopamine phenotype are decreased in cocaine users

During development, survival of midbrain dopamine neurons and specification of their phenotype are dependent upon the intracellular expression of a number of transcription factors, including Engrailed 1, Pitx3, and Nurr1. The role of these transcription factors in the maintenance of the dopaminergic phenotype is less clear. In the present study, we show that each of these transcription factors is robustly expressed in adult dopamine neurons in human midbrain, and that cocaine abuse is associated with a significant decrease in the abundance of Nurr1 and Pitx3 in these cells. These data suggest that cocaine abuse leads to a partial loss of dopaminergic phenotype.

Generation of embryonic stem cells and transgenic mice expressing green fluorescence protein in midbrain dopaminergic neurons

We have generated embryonic stem (ES) cells and transgenic mice with green fluorescent protein (GFP) inserted into the Pitx3 locus via homologous recombination. In the central nervous system, Pitx3-directed GFP was visualized in dopaminergic (DA) neurons in the substantia nigra and ventral tegmental area. Live primary DA neurons can be isolated by fluorescence-activated cell sorting from these transgenic mouse embryos. In culture, Pitx3-GFP is coexpressed in a proportion of ES-derived DA neurons. Furthermore, ES cell-derived Pitx3-GFP expressing DA neurons responded to neurotrophic factors and were sensitive to DA-specific neurotoxin N-4-methyl-1, 2, 3, 6-tetrahydropyridine. We anticipate that the Pitx3-GFP ES cells could be used as a powerful model system for functional identification of molecules governing mDA neuron differentiation and for preclinical research including pharmaceutical drug screening and transplantation. The Pitx3 knock-in mice, on the other hand, could be used for purifying primary neurons for molecular studies associated with the midbrain-specific DA phenotype at a level not previously feasible. These mice would also provide a useful tool to study DA fate determination from embryo- or adult-derived neural stem cells.

Early developmental failure of substantia nigra dopamine neurons in mice lacking the homeodomain gene Pitx3

The mesencephalic dopamine (mesDA) system is involved in the control of movement and behavior. The expression of Pitx3 in the brain is restricted to the mesDA system and the gene is induced relatively late, at E11.5, a time when tyrosine hydroxylase (Th) gene expression is initiated. We show here that, in the Pitx3-deficient aphakia (ak) mouse mutant, the mesDA system is malformed. Owing to the developmental failure of mesDA neurons in the lateral field of the midbrain, mesDA neurons are not found in the SNc and the projections to the caudate putamen are selectively lost. However, Pitx3 is expressed in all mesDA neurons in control animals. Therefore, mesDA neurons react specifically to the loss of Pitx3. Defects of motor control where not seen in the ak mice, suggesting that other neuronal systems compensate for the absence of the nigrostriatal pathway. However, an overall lower activity was observed. The results suggest that Pitx3 is specifically required for the formation of the SNc subfield at the onset of dopaminergic neuron differentiation.

PITX2 is required for normal development of neurons in the mouse subthalamic nucleus and midbrain

Pitx2, a homeodomain transcription factor, is essential for normal development of the pituitary gland, craniofacial region, eyes, heart, abdominal viscera, and limbs. Complete loss of Pitx2 in mice (Pitx2(-/-)) results in embryonic lethality by approximately e15 due to cardiac defects, whereas embryos with partial loss of function (Pitx2(neo/-) or Pitx2(neo/neo)) survive until later in development (e17-e19). Pitx2 is expressed in discrete populations of postmitotic neurons in the mouse brain, but its role in mammalian central nervous system (CNS) development is not known. We undertook an analysis of Pitx2-deficient embryos to determine whether loss of Pitx2 affects CNS development. The CNS is normal in hypomorphic e16.5 Pitx2(neo/-) and e18.5 Pitx2(neo/neo) embryos, with no evidence of midline or other defects. Midgestation (e10.5) Pitx2(-/-) embryos have normally formed neural tube structures and cerebral vesicles, whereas older (e14.5) Pitx2(-/-) embryos exhibit loss of gene expression and axonal projections in the subthalamic nucleus (a group of cells in the ventrolateral thalamus) and in the developing superior colliculus of dorsal midbrain. Our results suggest a role for Pitx2 in regulating regionally specific terminal neuronal differentiation in the developing ventrolateral thalamus and midbrain.

Making connections: the development of mesencephalic dopaminergic neurons

The disorders of two adjacent sets of mesencephalic dopaminergic (MDNs) are associated with two significant health problems: Parkinson's disease and drug addiction. Because of this, a great deal of research has focused on understanding the growth, development and maintenance of MDNs. Many transcription factors and signaling pathways are known to be required for normal MDNs formation, but a unified model of MDN development is still unclear. The long-term goal is to design therapeutic strategies to: (i) nurture and/or heal endogenous MDNs, (ii) replace the affected tissue with exogenous MDNs from in vitro cultivated stem cells and (iii) restore normal connectivity. Recent developmental biology studies show great promise in understanding how MDNs develop both in vivo and in vitro. This information has great therapeutic value and may provide insight into how environmental and genetic factors increase vulnerability to addiction.