Dynamics of tyrosine hydroxylase promoter activity during midbrain dopaminergic neuron development

Transcriptional regulation of tyrosine hydroxylase by estrogen: opposite effects with estrogen receptors alpha and beta and interactions with cyclic AMP

Reported effects of estrogen administration on tyrosine hydroxylase (TH) gene expression are confusing. Therefore, we studied the mechanism of regulation of TH transcription by estrogen with different estradiol receptor (ER) subtypes. PC12 cells, transiently co-transfected with expression vector for ERalpha or ERbeta, and luciferase gene under control of the TH promoter, were treated with 17 beta-estradiol (E2). E2 doubled luciferase activity with ERalpha; however, it was decreased with ERbeta. Mapping the TH promoter showed that the putative half estrogen response element (ERE) motif at - 675, as well as the activation protein 1 motif at - 205, were not required for response to E2 with either ER. The specificity protein 1/early growth response gene 1 (Egr 1) motif was required for the E2-elicited response with ERbeta, but not with ERalpha. Deletion of the cyclic AMP/Ca2+ response element (CRE/CaRE) nearly abolished E2-triggered responses with either ER. Further analysis revealed an imperfect canonical putative ERE overlapping with CRE/CaRE and Nurr1 response element. Oligonucleotides spanning this ERE displayed binding to ER, Cyclic AMP Response Element Binding Protein (CREB) and other proteins. Moreover, E2 attenuated the increase in TH transcription seen with cyclic AMP analogs. Thus, TH is transcriptionally regulated by estradiol in opposite directions depending on ER subtype. The overlapping ERE and CRE/CaRE may integrate interactions elicited by various regulators of TH transcription including cAMP and estrogens.

Use of TH-EGFP transgenic mice as a source of identified dopaminergic neurons for physiological studies in postnatal cell culture

The physiological and pharmacological properties of dopaminergic neurons in the brain are of major interest. Although much has been learned from cell culture studies, the physiological properties of these neurons remain difficult to study in such models because they are usually in minority and are difficult to distinguish from other non-dopaminergic neurons. Here we have taken advantage of a recently engineered transgenic mouse model expressing enhanced green fluorescence protein (EGFP) under the control of the tyrosine hydroxylase promoter to establish a more effective dopaminergic neuron cell culture model. We first evaluated the specificity of the EGFP expression. Although ectopic expression of EGFP was found in cultures derived from postnatal day 0 pups, this decreased over time in culture such that after 2 weeks, approximately 70% of EGFP-expressing neurons were dopaminergic. We next sought to validate this dopaminergic neuron culture model. We evaluated whether EGFP-expressing dopaminergic neurons displayed some of the well-established properties of dopaminergic neurons. Autoreceptor stimulation inhibited the activity of dopaminergic neurons while neurotensin receptor activation produced the opposite effect. Confocal imaging of the synaptic vesicle optical tracer FM4-64 in EGFP-expressing dopaminergic neurons demonstrated the feasibility of high resolution monitoring of the activity of single terminals established by these neurons. Together, this work provides evidence that primary cultures of postnatal TH-EGFP mice currently represent an excellent model to study the properties of these cells in culture.

Functional properties of dopaminergic neurones in the mouse olfactory bulb

The olfactory bulb of mammals contains a large population of dopaminergic interneurones within the glomerular layer. Dopamine has been shown both in vivo and in vitro to modulate several aspects of olfactory information processing, but the functional properties of dopaminergic neurones have never been described due to the inability to recognize these cells in living preparations. To overcome this difficulty, we used a transgenic mouse strain harbouring an eGFP (enhanced green fluorescent protein) reporter construct under the promoter of tyrosine hydroxylase, the rate-limiting enzyme for cathecolamine synthesis. As a result, we were able to identify dopaminergic neurones (TH-GFP cells) in living preparations and, for the first time, we could study the functional properties of such neurones in the olfactory bulb, in both slices and dissociated cells. The most prominent feature of these cells was the autorhythmicity. In these cells we identified five main voltage-dependent conductances: the two having largest amplitude were a fast transient Na(+) current and a delayed rectifier K(+) current. In addition, we observed three smaller inward currents, sustained by Na(+) ions (persistent type) and by Ca(2)(+) ions (LVA and HVA). Using pharmacological tools and ion substitution methods we showed that the pacemaking process is supported by the interplay of the persistent Na(+) current and of a T-type Ca(2)(+) current. We carried out a complete kinetical analysis of the five conductances present in these cells, and developed a Hodgkin-Huxley model of TH-GFP cells, capable of reproducing accurately the properties of living cells, including autorhytmicity, and allowing a precise understanding of the process.

In Vivo and In Vitro Tissue-Specific Expression of Green Fluorescent Protein Using the Cre-Lox System in Mouse Embryonic Stem Cells

Embryonic stem cells (ES) are pluripotent and may therefore serve as a source for the generation of specific cell types required for future therapies based on cell replacement. The isolation of defined cell populations from a certain lineage or tissue is a prerequisite for the analysis of the potential of such ES-derived cells in animal transplantation studies. Here, using the Cre/loxP system, we report the generation of murine ES cells conditionally expressing the hrGFP gene at the cell surface. Such ES cells can be differentiated in vitro into neurons displaying GFP activity in neurites. Transgenic mice derived from these ES cells permit the targeting of GFP-expression to specific tissues and provide material from the three germ layers suitable for molecular and biochemical analysis.

Transgenic zebrafish that express tyrosine hydroxylase promoter in inner retinal cells

We have generated a transgenic zebrafish line [Tg(Th:GFP)] that expresses green fluorescence proteins (GFP) driven by rat tyrosine hydroxylase (TH) promoter. In zebrafish, the transgene was expressed as early as 16 hr postfertilization (hpf). The first transgene expression was detected in the midbrain. Within a few hours of development, the expression spread to the forebrain and hindbrain. In the retina, the first transgene expression was detected at approximately 40 hpf, at which time a single GFP-positive cell was seen in the ventral-nasal patch of the retina. In late development, GFP spread across the inner retina. GFP was found in retinal cells that expressed TH or phenylethanolamine N-methyl-transferase (PNMT), the first and last enzymes for synthesis of catecholamine, respectively. This suggests that the transgene is expressed in catecholaminergic neurons. Of interest, GFP was also detected in some retinal cells that release gamma-aminobutyric acid. These latter data suggest that the transgene may also be expressed in noncatecholaminergic cells.

Turning the heterogeneous into homogeneous: studies on selectively isolated GABAergic interneuron subsets

The amazing morphological and electrophysiological diversity of cortical GABAergic interneurons subserves the broad diversity of processes these cells modulate in neuronal networks. Until recently, interneuron development and functions have been extensively studied in heterogeneous in vitro and in vivo systems containing both excitatory and inhibitory components. However, mechanisms of interneuron specification during development, key signaling mechanisms controlling the establishment of particular inhibitory neuron subsets, and the spatial and temporal regulation of their integration in neuronal microcircuits remain poorly understood. Selective isolation of particular interneuron subsets may significantly extend our knowledge on the scenario of neurochemical and electrophysiological specification of developing interneurons, identification of signaling cues directing their axon growth, and principles of their anterograde and retrograde synaptic communication with other cell types. Here, we show that selective isolation of perisomatic inhibitory cells containing either parvalbumin or cholecystokinin reveals major differences in the temporal dynamics of their functional differentiation, and their dependence on target-derived signals like brain-derived neurotrophic factor and endocannabinoids. In addition, we discuss therapeutic prospects of modulating increased excitatory output in the hippocampus and subthalamic nucleus by re-adjusting the inhibitory control of principal cells.

Differentiation of the dopaminergic phenotype in the olfactory system of neonatal and adult mice

Olfactory bulb (OB) interneurons are derived primarily postnatally from progenitors in the anterior subventricular zone (SVZa) and migrate to the OB in the rostral migratory stream (RMS). Progenitors differentiate into phenotypically diverse granule and periglomerular cells by as yet undefined mechanisms. To visualize spatiotemporal aspects of periglomerular dopamine (DA) neuron differentiation, two independently derived transgenic mouse lines were analyzed with a 9-kb tyrosine hydroxylase (TH) promoter to drive either a LacZ or an enhanced green fluorescent protein (EGFP) reporter gene. Both reporters showed similar neonatal expression that varied from low levels in RMS, to moderate in the superficial granule cell layer, to strong in relatively large cells, possibly external tufted cells, in the periglomerular region. TH mRNA and protein were not detected in the RMS but were colocalized with the transgenes in neonatal superficial granule and periglomerular cells. By comparison, TH protein in adults was further limited to periglomerular cells. To demonstrate that transcriptional regulation was the same for EGFP and TH, expression was shown to decline similarly in the OB ipsilateral to odor deprivation produced by adult unilateral naris closure. Of two genes previously hypothesized to regulate OB DA expression, only regulated expression of the orphan receptor Nurr1, but not the homeobox-containing genes Dlx-1 and -2, was consistent with a role in regulation of the DA phenotype. These data demonstrate for the first time that DA phenotypic differentiation in neonates begins with low-level transcription in migrating progenitors in the RMS and culminates with activity-dependent protein expression in periglomerular cells innervated by olfactory receptor cells.

Survival of developing motor neurons mediated by Rho GTPase signaling pathway through Rho-kinase

A variety of neurons generated during embryonic development survive or undergo programmed cell death (PCD) at later developmental stages. Survival or death of developing neurons is generally considered to depend on trophic support from various target tissues. The small GTPase Rho regulates diverse cellular processes such as cell morphology, cell adhesion, cell motility, and apoptosis. Rho-dependent serine-threonine protein kinase (Rho-kinase-ROK-ROCK), one of the effector proteins, transmits signals for some Rho-mediated processes. Here, we report the in vivo role of the Rho signaling pathway through Rho-kinase during development of motor neurons (MNs) in the spinal cord. We performed conditional expression of a dominant-negative form for RhoA (RhoA DN) or for Rho-kinase (Rho-K DN) in transgenic mice by using the Cre-loxP system to suppress the activity of these signaling molecules in developing MNs. Expression of RhoA DN reduced the number of MNs in the spinal cord because of increased apoptosis while preserving the gross patterning of motor axons. Expression of Rho-K DN produced developmental defects similar to those observed in RhoA DN expression. In addition, analysis of transgenic mice expressing Rho-K DN showed that the increased apoptosis of MNs was induced at the early embryonic stages before the initiation of PCD, and that MN death at the late embryonic stages corresponding to the period of PCD was moderately enhanced in the transgenic mice. These findings indicate that the Rho signaling pathway, primarily through Rho-kinase, plays a crucial role in survival of spinal MNs during embryogenesis, particularly at the early developmental stages.

Isolation and transplantation of dopaminergic neurons generated from mouse embryonic stem cells

Embryonic stem (ES) cells differentiate into dopamine (DA)-producing neurons when co-cultured with PA6 stromal cells, but the resulting cultures contain a variety of unidentified cells. In order to label live DA neurons in mixed populations, we introduced a GFP reporter under the control of the tyrosine hydroxylase (TH) gene promoter into ES cells. GFP expression was observed in TH-immunoreactive cells that differentiated from the ES cells that carried the TH-GFP reporter gene. DA neurons expressing GFP were sorted from the mixed cell population by fluorescence-activated cell sorting of cells exhibiting GFP fluorescence, and the sorted GFP(+) cells obtained were transplanted into a rat model of Parkinson's disease. Some of these cells survived and innervated the host striatum, resulting in a partial recovery from parkinsonian behavioral defects. This strategy of isolation and transplantation of ES-cell-derived DA neurons should be useful for cellular and molecular studies of DA neurons and for clinical application in the treatment of Parkinson's disease.

Cortical and striatal expression of tyrosine hydroxylase mRNA in neonatal and adult mice

1. Elucidating the mechanisms underlying regulation of the dopamine (DA) phenotype during development and in adult animals was a major focus of many of the students and postdoctoral fellows in the Laboratory of Dr Donald Reis. In one series of studies, expression of tyrosine hydroxylase (TH), the first enzyme in the DA biosynthetic pathway, was induced in primary cultures prepared from the cortical anlage of embryonic day 13 (E13)-E17 rat embryos. On the basis of these data, the current studies investigated whether under appropriate conditions TH expression might occur in forebrain regions that do not normally contain DA neurons. 2. A transgenic mouse strain harboring a 9-kb TH promoter/EGFP (enhanced green fluorescent protein) reporter construct was analyzed as adults for coexpression of the fluorescent reporter and the endogenous gene, the latter using a sensitive nonradioactive in situ hybridization procedure. The latter procedure was also used to determine the development of neonatal cortical endogenous TH expression. 3. Cortical and striatal cells containing TH mRNA were observed at postnatal day 5 (P5), but not P2, increased in number at P7 and were found in adults. Many cells in the cortex and striatum coexpressed TH mRNA and EGFP, but TH protein was not detected in these brain regions indicating independent transcriptional and translational regulation of TH expression. Overlapping expression of the two transcriptional indicators and TH protein in olfactory bulb occurred only in those DA neurons that receive afferent stimulation from receptor cells. 4. These findings suggest that partial DAergic differentiation may occur in some cortical and striatal cells, but that full expression of the phenotype requires synaptic activation or activity-dependent release of an as-yet unidentified factor(s).

Identification of GABAA receptor subunit variants in midbrain dopaminergic neurons

Modulation of the activity of dopamine (DA)-producing neurons by GABA plays an important role in the control of DA-mediated brain functions. Ionotropic GABA(A) receptors exist as heteropentametric structures assembling different subunits composed of various subtypes. However, the expression pattern of these subunits in DA neurons in the ventral midbrain has not been fully defined. In the present study, we investigated the subunit composition of GABA(A) receptors in DA neurons in the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA). We isolated DA neurons from the ventral midbrain of transgenic mice that express green fluorescent protein under the control of the tyrosine hydroxylase (TH) gene promoter and analyzed expression of various GABA(A) receptor subunits in single cells by using the reverse transcription-polymerase chain reaction. This analysis showed the presence of the transcripts encoding alpha2, alpha3, alpha4, beta1, beta3 and gamma2 subunits in the isolated DA neurons. Double fluorescence in situ hybridization with probes for TH and GABA(A) receptor subunit mRNAs revealed the expression of these six subunits in the majority of DA neurons in the SNc and the VTA.

Modulation of nurr1 gene expression in mesencephalic dopaminergic neurones

The transcription factor/nuclear receptor Nurr1 is essential for the differentiation of midbrain dopaminergic neurones. Here we demonstrate that, during the ontogeny of rat ventral mesencephalon, nurr1 gene expression is developmentally regulated and its levels show a sharp peak between embryonic day E13 and E15, when most dopaminergic neurones differentiate. In addition, in primary cultures from embryonic rat mesencephalon, nurr1 gene follows a temporal pattern of expression comparable to that observed in vivo. We also report that exposure of embryonic mesencephalic cultures to depolarizing stimuli leads to a robust increase in nurr1 mRNA and protein. The depolarizing effect is also detected in mesencephalic cultures enriched in dopaminergic neurones by using a combination of bFGF and Sonic hedgehog. The latter further increases the number of dopaminergic neurones in these 'expanded' cultures, an effect abolished in the presence of anti-Sonic hedgehog antibodies. Our data show that nurr1 gene is highly expressed in midbrain dopaminergic neurones in a sharp temporal window and that its expression is plastic, both in vivo and in vitro. In addition we show that Sonic hedgehog can direct dopaminergic differentiation in proliferating dopaminergic neuroblasts in vitro.

Electrical coupling and neuronal synchronization in the Mammalian brain

Certain neurons in the mammalian brain have long been known to be joined by gap junctions, which are the most common type of electrical synapse. More recently, cloning of neuron-specific connexins, increased capability of visualizing cells within brain tissue, labeling of cell types by transgenic methods, and generation of connexin knockouts have spurred a rapid increase in our knowledge of the role of gap junctions in neural activity. This article reviews the many subtleties of transmission mediated by gap junctions and the mechanisms whereby these junctions contribute to synchronous firing.

Postnatal lethality in mice lacking the Sax2 homeobox gene homologous to Drosophila S59/slouch: evidence for positive and negative autoregulation

Homeobox gene transcription factors direct multiple functions during development. They are involved in early patterning of the embryo as well as cell specification, cell differentiation, and organogenesis. Here we describe a previously uncharacterized murine homeobox gene, Sax2, that shows high similarity to the Drosophila S59/slouch and murine Sax1 genes. We show that Sax2 gene expression occurs early during embryogenesis in the midbrain, the midbrain-hindbrain boundary, the ventral neural tube, the developing eye, and the apical ectodermal ridge of the limb. To determine the role of Sax2 during development, we generated a knockout mouse line by replacing part of the Sax2 coding sequences with the lacZ gene. The Sax2 null allele mutants exhibit a strong phenotype indicated by growth retardation starting immediately after birth and leading to premature death within the first 3 weeks postnatal. Intriguingly, our studies also demonstrated a striking autoregulation of the Sax2 gene in both positive- and negative-feedback mechanisms depending on the specific cell type expressing Sax2.

Altered midbrain dopaminergic neurotransmission during development in an animal model of ADHD

To understand the onset and the molecular mechanisms triggering dopaminergic (DA) dysregulation in Attention-Deficit Hyperactivity Disorder (ADHD), we have used the Spontaneously Hypertensive Rats (SHR), the most widely studied animal model for this disease. We have studied the pattern of expression of specific genes involved in DA neuron differentiation, survival and function during postnatal (P) development of the ventral midbrain in SHR males. Our results show that tyrosine hydroxylase and DA transporter gene expression are significantly and transiently reduced in the SHR midbrain during the first month of postnatal development, although with a different kinetic. The other genes analyzed do not show significant variation between SHR and control rats. In addition, high-affinity DA uptake activity is significantly reduced in synaptosomes obtained from the striatum of 1-month-old SHR, when compared to controls. Our data suggest that down-regulation of DA neurotransmission occurs in the midbrain of SHR in a developmentally regulated temporal window during postnatal development, thus strengthening the hypodopaminergic hypothesis in the pathogenesis of ADHD.

Sonic hedgehog and FGF8 collaborate to induce dopaminergic phenotypes in the Nurr1-overexpressing neural stem cell

Neural stem cells are self-renewing cells capable of differentiating into all neural lineage cells in vivo and in vitro. In the present study, coordinated induction of midbrain dopaminergic phenotypes in an immortalized multipotent neural stem cell line can be achieved by both overexpression of nuclear receptor Nurr1, and fibroblast growth factor-8 (FGF-8), and sonic hedgehog (Shh) signals. Nurr1 overexpression induces neuronal differentiation and confers competence to respond to extrinsic signals such as Shh and FGF-8 that induce dopaminergic fate in a mouse neural stem cell line. Our findings suggest that immortalized NSCs can serve as an excellent model for understanding mechanisms that regulate specification of ventral midbrain DA neurons and as an unlimited source of DA progenitors for treating Parkinson disease patients by cell replacement.