Induction of midbrain dopaminergic neurons by Sonic hedgehog

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.

THE HEDGEHOG PATHWAY AND NEUROLOGICAL DISORDERS

The hedgehog pathway is a major regulator of embryonic development, and mutations that decrease its activity are known to be associated with severe defects in nervous system development. Recent evidence suggests hedgehog continues to function in adult tissue, normal as well as diseased, by regulating both cell proliferation and the production of growth and angiogenic factors. In the adult nervous system, this dual ability is especially important in regulating the behavior of neural stem and progenitor cells. This review summarizes information connecting hedgehog signaling and neural diseases, including neurodegenerative disorders and brain tumors, particularly medulloblastoma. We also describe the discovery and utility of small molecule agonists and antagonists of this pathway and their potential as novel types of therapeutics.

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

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

Differential regulation of tyrosine hydroxylase expression by sonic hedgehog

Sonic hedgehog functions to induce floor plate in early stages, and spinal motor neurons and midbrain dopaminergic neurons in later stages of development. Here, we investigated the effects of sonic hedgehog on tyrosine hydroxylase expression in three cell lines that correspond to different stages of neural development. Sonic hedgehog increased the tyrosine hydroxylase gene expression in pluripotent P19 cells but repressed it in tyrosine hydroxylase-producing PC12 cells. Promoter analysis in mouse neural stem cells indicated that the N-terminal of sonic hedgehog repressed both the basal and cAMP-dependent protein kinase A-mediated tyrosine hydroxylase activity. These results suggest that the N-terminal of sonic hedgehog increases tyrosine hydroxylase gene expression in cells to acquire dopaminergic phenotypes, but decreases expression in late born neurons by antagonizing the protein kinase A cAMP-responsive element binding protein pathway.

Determination of cell fate along the anteroposterior axis of the Drosophila ventral midline

The Drosophila ventral midline has proven to be a useful model for understanding the function of central organizers during neurogenesis. The midline is similar to the vertebrate floor plate, in that it plays an essential role in cell fate determination in the lateral CNS and also, later,in axon pathfinding. Despite the importance of the midline, the specification of midline cell fates is still not well understood. Here, we show that most midline cells are determined not at the precursor cell stage, but as daughter cells. After the precursors divide, a combination of repression by Wingless and activation by Hedgehog induces expression of the proneural gene lethal of scute in the most anterior midline daughter cells of the neighbouring posterior segment. Hedgehog and Lethal of scute activate Engrailed in these anterior cells. Engrailed-positive midline cells develop into ventral unpaired median (VUM) neurons and the median neuroblast (MNB). Engrailed-negative midline cells develop into unpaired median interneurons (UMI), MP1 interneurons and midline glia.

Identification of intrinsic determinants of midbrain dopamine neurons

The prospect of using cell replacement therapies has raised the key issue of whether elucidation of developmental pathways can facilitate the generation of therapeutically important cell types from stem cells. Here we show that the homeodomain proteins Lmx1a and Msx1 function as determinants of midbrain dopamine neurons, cells that degenerate in patients with Parkinson's disease. Lmx1a is sufficient and required to trigger dopamine cell differentiation. An early activity of Lmx1a is to induce the expression of Msx1, which complements Lmx1a by inducing the proneural protein Ngn2 and neuronal differentiation. Importantly, expression of Lmx1a in embryonic stem cells results in a robust generation of dopamine neurons with a "correct" midbrain identity. These data establish that Lmx1a and Msx1 are critical intrinsic dopamine-neuron determinants in vivo and suggest that they may be essential tools in cell replacement strategies in Parkinson's disease.

Adult rat bone marrow stromal cells express genes associated with dopamine neurons

An intensive search is underway to identify candidates to replace the cells that degenerate in Parkinson's disease (PD). To date, no suitable substitute has been found. We have recently found that adult rat bone marrow stromal cells (MSCs) can be induced to assume a neuronal phenotype in vitro. These findings may have particular relevance to the treatment of PD. We now report that adult MSCs express multiple dopaminergic genes, suggesting that they are potential candidates for cell therapy. Using RT-PCR, we have examined families of genes that are associated with the development and/or survival of dopaminergic neurons. MSCs transcribe a variety of dopaminergic genes including patched and smoothened (components of the Shh receptor), Gli-1 (downstream mediator of Shh), and Otx-1, a gene associated with formation of the mesencephalon during development. Furthermore, Shh treatment elicits a 1.5-fold increase in DNA synthesis in cultured MSCs, suggesting the presence of a functional Shh receptor complex. We have also found that MSCs transcribe and translate Nurr-1, a nuclear receptor essential for the development of dopamine neurons. In addition, MSCs express a variety of growth factor receptors including the glycosyl-phosphatidylinositol-anchored ligand-binding subunit of the GDNF receptor, GFRalpha1, as well as fibroblast growth factor receptors one and four. The expression of genes that are associated with the development and survival of dopamine neurons suggests a potential role for these cells in the treatment of Parkinson's disease.

Directed differentiation of neural cells to hypothalamic dopaminergic neurons

Hypothalamic neurons play a key role in homeostasis, yet little is known about their differentiation. Here, we demonstrate that Shh and Bmp7 from the adjacent prechordal mesoderm govern hypothalamic neural fate, their sequential action controlling hypothalamic dopaminergic neuron generation in a Six3-dependent manner. Our data suggest a temporal distinction in the requirement for the two signals. Shh acts early to specify dopaminergic neurotransmitter phenotype. Subsequently, Bmp7 acts on cells that are ventralised by Shh, establishing aspects of hypothalamic regional identity in late-differentiating/postmitotic cells. The concerted actions of Shh and Bmp7 can direct mouse embryonic stem cell-derived neural progenitor cells to a hypothalamic dopaminergic fate ex vivo.

Development of the mesencephalic dopaminergic neuron system is compromised in the absence of neurogenin 2

Neurogenin 2 (Ngn2) is a proneural gene involved in neuronal differentiation and subtype specification in various regions of the nervous system. In the ventral midbrain, Ngn2 is expressed in a spatiotemporal pattern that correlates with the generation of mesencephalic dopaminergic (mesDA)neurons. We show here that lack of Ngn2 impairs the development of mesDA neurons, such that less than half of the normal mesDA neuron number remain in Ngn2 mutant mice at postnatal stages. Analysis of Ngn2mutant mice during mesDA neurogenesis show that medially located precursors are formed but are arrested in their differentiation at a stage when they have not yet acquired the characteristics of mesDA neuron precursors. Loss of Ngn2 function appears to specifically affect the generation of DA neurons, as the development of other types of neurons within the ventral midbrain is unaltered. Ngn2 is the first example of a gene expressed in progenitors in the ventricular zone of the mesDA neuron domain that is essential for proper mesDA neuron differentiation, and whose loss of function causes impaired mesDA neurogenesis without other major abnormalities in the ventral midbrain.

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.

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.

Cardiac and CNS defects in a mouse with targeted disruption of suppressor of fused

The hedgehog (Hh) pathway is conserved from Drosophila to humans and plays a key role in embryonic development. In addition, activation of the pathway in somatic cells contributes to cancer development in several tissues. Suppressor of fused is a negative regulator of Hh signaling. Targeted disruption of the murine suppressor of fused gene (Sufu) led to a phenotype that included neural tube defects and lethality at mid-gestation (9.0-10.5 dpc). This phenotype resembled that caused by loss of patched (Ptch1), another negative regulator of the Hh pathway. Consistent with this finding, Ptch1 and Sufu mutants displayed excess Hh signaling and resultant altered dorsoventral patterning of the neural tube. Sufu mutants also had abnormal cardiac looping, indicating a defect in the determination of left-right asymmetry. Marked expansion of nodal expression in 7.5 dpc embryos and variable degrees of node dysmorphology in 7.75 dpc embryos suggested that the pathogenesis of the cardiac developmental abnormalities was related to node development. Other mutants of the Hh pathway, such as Shh, Smo and Shh/Ihh compound mutants, also have laterality defects. In contrast to Ptch1 heterozygous mice, Sufu heterozygotes had no developmental defects and no apparent tumor predisposition. The resemblance of Sufu homozygotes to Ptch1 homozygotes is consistent with mouse Sufu being a conserved negative modulator of Hh signaling.

Delivery of sonic hedgehog or glial derived neurotrophic factor to dopamine-rich grafts in a rat model of Parkinson's disease using adenoviral vectors Increased yield of dopamine cells is dependent on embryonic donor age

The poor survival of dopamine grafts in Parkinson's disease is one of the main obstacles to the widespread application of this therapy. One hypothesis is that implanted neurons, once removed from the embryonic environment, lack the differentiation factors needed to develop the dopaminergic phenotype. In an effort to improve the numbers of dopamine neurons surviving in the grafts, we have investigated the potential of adenoviral vectors to deliver the differentiation factor sonic hedgehog or the glial cell line-derived neurotrophic factor GDNF to dopamine-rich grafts in a rat model of Parkinson's disease. Adenoviral vectors containing sonic hedgehog, GDNF, or the marker gene LacZ were injected into the dopamine depleted striatum of hemiparkinsonian rats. Two weeks later, ventral mesencephalic cell suspensions were prepared from embryos of donor ages E12, E13, E14 or E15 and implanted into the vector-transduced striatum. Pre-treatment with the sonic hedgehog vector produced a three-fold increase in the numbers of tyrosine hydroxylase-positive (presumed dopaminergic) cells in grafts derived from E12 donors, but had no effect on E13-E15 grafts. By contrast, pre-treatment with the GDNF vector increased yields of dopamine cells in grafts derived from E14 and E15 donors but had no effect on grafts from younger donors. The results indicate that provision of both trophic and differentiation factors can enhance the yields of dopamine neurons in ventral mesencephalic grafts, but that the two factors differ in the age and stage of embryonic development at which they have maximal effects.

Differential expression of cell fate determinants in neurons and glial cells of adult mouse spinal cord after compression injury

Cellular responses after spinal cord injury include activation of astrocytes, degeneration of neurons and oligodendrocytes, and reactions of the ependymal layer and meningeal cells. Because it has been suggested that tissue repair partially recapitulates morphogenesis, we have investigated the expression of several developmentally prominent molecules after spinal cord injury of adult mice where neurogenesis does not occur after injury. Cell fate determinants Numb, Notch-1, Shh and BMPs are abundantly expressed during development but mostly decline in the adult. In the present study, we investigated whether these genes are triggered by spinal cord injury as a sign of attempted recapitulation of development. Expression of Numb, Notch, Shh, BMP2/4 and Msx1/2 was analysed in the adult mouse spinal cord after compression injury by in situ hybridization up to 1 month after injury. The mRNA expression levels of Notch-1, Numb, Shh, BMP4 and Msx2 increased in the grey matter and/or white matter and in the ependyma rostral and caudal to the lesion site after injury. However, BMP2 and Msx1 were not up-regulated. Combining immunohistochemistry of cell type-specific markers with in situ hybridization we found that all the up-regulated genes were expressed in neurons. Moreover, Numb, BMP4 and Msx2 were also expressed by GFAP-positive astrocytes, while Shh was expressed by MBP-positive oligodendrocytes. In conclusion, the cell fate determinants Notch-1, Numb, Shh, BMP4 and Msx2 are expressed in neurons and/or glial cells after injury in a time-dependent manner, suggesting that these genes reflect to some extent an endogenous self-repair potential by recapitulating some features of development.

Brain as a paradigm of organ growth: Hedgehog-Gli signaling in neural stem cells and brain tumors

The Hedgehog-Gli (Hh-Gli) signaling pathway is essential for numerous events during the development of many animal cell types and organs. In particular, it controls neural cell precursor proliferation in dorsal brain structures and regulates the number of neural stem cells in distinct embryonic, perinatal, and adult niches, such as the developing neocortex, the subventricular zone of the lateral ventricle of the forebrain, and the hippocampus. We have proposed that Hh-Gli signaling regulates dorsal brain growth during ontogeny and that its differential regulation underlays evolutionary change in the morphology (size and shape) of dorsal brain structures. It is also critically involved in sporadic brain tumorigenesis--as well as several other human cancer--suggesting that tumors derive from stem cells or progenitors maintaining an inappropriate active Hh-Gli pathway. Importantly, we and others have demonstrated that human sporadic tumors from the brain and other organs require sustained HH-GLI signaling for sustained growth and survival. Modulating HH-GLI signaling thus represents a novel rational avenue to treat, on one hand, brain degeneration and injury by inducing controlled HH-GLI-mediated regeneration and growth, and on the other hand, to combat cancer by blocking its abnormal activity in tumor cells.

Morphing into cancer: the role of developmental signaling pathways in brain tumor formation

Morphogens play a critical role in most aspects of development, including expansion and patterning of the central nervous system. Activating germline mutations in components of the Hedgehog and Wnt pathways have provided evidence for the important roles morphogens play in the genesis of brain tumors such as cerebellar medulloblastoma. In addition, aberrant expression of transforming growth factor-beta (TGF-beta) superfamily members has been demonstrated to contribute to progression of malignant gliomas. This review summarizes our current knowledge about the roles of morphogens in central nervous system tumorigenesis.

Loss of the serine/threonine kinase fused results in postnatal growth defects and lethality due to progressive hydrocephalus

The Drosophila Fused (Fu) kinase is an integral component of the Hedgehog (Hh) pathway that helps promote Hh-dependent gene transcription. Vertebrate homologues of Fu function in the Hh pathway in vitro, suggesting that Fu is evolutionarily conserved. We have generated fused (stk36) knockout mice to address the in vivo function of the mouse Fu (mFu) homologue. fused knockouts develop normally, being born in Mendelian ratios, but fail to thrive within 2 weeks, displaying profound growth retardation with communicating hydrocephalus and early mortality. The fused gene is expressed highly in ependymal cells and the choroid plexus, tissues involved in the production and circulation of cerebral spinal fluid (CSF), suggesting that loss of mFu disrupts CSF homeostasis. Similarly, fused is highly expressed in the nasal epithelium, where fused knockouts display bilateral suppurative rhinitis. No obvious defects were observed in the development of organs where Hh signaling is required (limbs, face, bones, etc.). Specification of neuronal cell fates by Hh in the neural tube was normal in fused knockouts, and induction of Hh target genes in numerous tissues is not affected by the loss of mFu. Furthermore, stimulation of fused knockout cerebellar granule cells to proliferate with Sonic Hh revealed no defect in Hh signal transmission. These results show that the mFu homologue is not required for Hh signaling during embryonic development but is required for proper postnatal development, possibly by regulating the CSF homeostasis or ciliary function.

Netrin-1 and slit-2 regulate and direct neurite growth of ventral midbrain dopaminergic neurons

We investigated the roles of netrin-1 and slit-2 in regulation and navigation of dopamine (DA) axon growth using an explant culture preparation of embryonic ventral midbrain (embryonic day 14) and a co-culture system. We found that netrin-1 protein significantly enhanced DA axonal outgrowth and promoted DA axonal outgrowth in a co-culture system of netrin-1 expressing cells. Such effects were mediated by the receptor DCC as demonstrated by antibody perturbation of the DCC receptor. In contrast, slit-2 inhibited DA neuron extensions and repelled DA neurite growth. These slit-2 activities required robo receptors since the reduced neurite extension was abolished by addition of excess robo receptors. In this system, netrin-1 stimulated and slit-2 opposed DA neurite growth. Such regulation may be important for DA axonal maintenance, regeneration, and phenotypic target recognition.

Caffeine induces sonic hedgehog gene expression in cultured astrocytes and neurons

Caffeine affects early in vivo murine brain development by accelerating the evagination of the primitive neuroepithelium into telencephalic vesicles. In this model, caffeine induces the expression of the regulatory subunit alpha of protein kinase A (PKA RI alpha) and of Sonic hedgehog (Shh). The understanding of the molecular mechanisms linking caffeine and neural gene expression would benefit from a reproducible in vitro model. Accordingly, the present study aimed to determine whether caffeine modulated the expression of these genes in primary neuronal and astroglial cultures derived from developing murine neocortex. Using real-time PCR, the results showed that caffeine induced robust overexpression of Shh mRNA in both cell types without significantly modifying PKA RI alpha gene expression.

Factors involved in the determination of the neurotransmitter phenotype of developing neurons of the CNS: Applications in cell replacement treatment for Parkinson's disease

The developmental stages involved in the conversion of stem cells to fully functional neurons of specific neurotransmitter phenotype are complex and not fully understood. Over the past decade many studies have been published that demonstrate that in vitro manipulation of the epigenetic environment of the stem cells allows experimental control of final neuronal phenotypic choice. This review presents the evidence for the involvement of a number of endogenous neurobiochemicals, which have been reported to potently influence DAergic (and other neurotransmitter) phenotype expression in vitro. They act at different stages on the pathway to neurotransmitter phenotype determination, and in different ways. Many are better known for their involvement in other aspects of development, and in other biochemical roles. Their proper place, and precise roles, in neurotransmitter phenotype determination in vivo will no doubt be determined in the future. Meanwhile, considerable medical benefits are offered from producing large, long-term, viable cryostores of self-regenerating multipotential neural precursor cells (i.e., brain stem cells), which can be used for cell replacement therapies in the treatment of degenerative brain diseases, such as Parkinson's disease.

Development of the dopaminergic neurons in the rodent brainstem

The loss of dopaminergic (DA) neurons in the ventral midbrain is the principal cause of Parkinson's disease. The search for candidate molecules that promote the genesis and survival capacities of DA neurons is a major area of investigation and hope. A better characterization of the developmental pathways that govern the specification, differentiation, and survival of these neurons will be essential in devising therapies aimed to rescue or replace midbrain DA neurons in Parkinson's patients. In this brief review, we will discuss the major steps in the normal development of midbrain DA neurons.

Monoaminergic neuronal development is not affected in PACAP-gene-deficient mice

Pituitary adenylate cyclase-activating polypeptide (PACAP) has been implicated in several physiological functions. Several lines of evidence from in vitro studies have shown that PACAP plays some important roles in development of nervous system such as neural proliferation and differentiation. Recently, mice lacking PACAP have been reported to show a higher mortality shortly after birth, impaired thermal adaptation, and altered psychomotor behaviors. Inasmuch as monoaminergic nervous systems are implicated in these phenotypes and a quite few data have been reported on the role of this peptide in nervous development in vitro, we studied early development [embryonic days 10.5 (E10.5) and 12.5 (E12.5)] of monoaminergic nervous systems in mice lacking PACAP. The fetuses lacking PACAP showed immunoreactivities (IRs) for tyrosine hydroxylase (TH) and serotonin (5-HT) similarly to the wild type. We observed TH-IR in the forebrain [striatal differentiating zone (dz) and hypothalamic dz], midbrain, hindbrain, neural-crest-derived sympathetic ganglionic primordia, ventral spinal cord dz, and bowel at E10.5 in both PACAP null and wild type with no difference. At E12.5, in the wild-type- and PACAP-gene-deficient mice, no differences of 5-HT- and TH-IRs were observed in several brain regions, including brainstem (midbrain and pons). Thus, the depletion of PACAP does not affect monoaminergic nervous systems in the early development.

The floor plate: multiple cells, multiple signals

One of the key organizers in the CNS is the floor plate - a group of cells that is responsible for instructing neural cells to acquire distinctive fates, and that has an important role in establishing the elaborate neuronal networks that underlie the function of the brain and spinal cord. In recent years, considerable controversy has arisen over the mechanism by which floor plate cells form. Here, we describe recent evidence that indicates that discrete populations of floor plate cells, with characteristic molecular properties, form in different regions of the neuraxis, and we discuss data that imply that the mode of floor plate induction varies along the anteroposterior axis.

Neuronal expression of the transcription factor Gli1 using the Talpha1 alpha-tubulin promoter is neuroprotective in an experimental model of Parkinson's disease

Nigrostriatal neurons degenerate during Parkinson's disease. Experimentally, neurotoxins such as 6-hydroxydopamine (6-OHDA) in rodents, and MPTP in mice and non-human primates, are used to model the disease-induced degeneration of midbrain dopaminergic neurons. Glial-cell-derived neurotrophic factor (GDNF) is a very powerful neuroprotector of dopaminergic neurons in all species examined. However, recent reports have indicated the possibility that GDNF may, in the long term and if expressed in an unregulated manner, exert untoward effects on midbrain dopaminergic neuronal structure and function. Although GDNF remains a powerful neurotrophin, the search for alternative therapies based on alternative and complementary mechanisms of action to GDNF is warranted. Recently, recombinant adenovirus-derived vectors encoding the differentiation factor Sonic Hedgehog (Shh) and its downstream transcriptional activator (Gli1) were shown to protect dopaminergic neurons in the substantia nigra pars compacta from 6-OHDA-induced neurotoxicity in rats in vivo. A pancellular human CMV (hCMV) promoter was used to drive the expression of both Shh and Gli1. Since Gli1 is a transcription factor and therefore exerts its actions intracellularly, we decided to test whether expression of Gli1 within neurons would be effective for neuroprotection. We demonstrate that neuronal-specific expression of Gli1 using the neuron-specific Talpha1 alpha-tubulin (Talpha1) promoter was neuroprotective, and its efficiency was comparable to the pancellular strong viral hCMV promoter. These results suggest that expression of the transcription factor Gli1 solely within neurons is neuroprotective for dopaminergic neurons in vivo and, furthermore, that neuronal-specific promoters are effective within the context of adenovirus-mediated gene therapy-induced neuroprotection of dopaminergic midbrain neurons. Since cell-type specific promoters are known to be weaker than the viral hCMV promoter, our data demonstrate that neuronal-specific expression of transcription factors is an effective, specific, and sufficient targeted approach for neurological gene therapy applications, potentially minimizing side effects due to unrestricted promiscuous gene expression within target tissues.

Noggin and basic FGF were implicated in forebrain fate and caudal fate, respectively, of the neural tube-like structures emerging in mouse ES cell culture

We developed neural tube-like structures accompanying neural crest-like cells by treating embryonic stem (ES) cells with retinoic acid. The structures contained pseudostratified Nestin+Vimentin+ neuroepithelial cells surrounded by Masson staining+ basement membrane. betaIIItubulin+Synaptophysin+ mature neurons and glial fibrillary acidic protein (GFAP)+ glial cells dispersed outside of the membrane. Addition of Noggin to the culture induced prominent proliferation of the neuroepithelial cells, leading to epithelial hyperstratification of the structures. mRNAs of transcription factors essential for forebrain development such as Emx1/2 and Pax6 were specifically expressed and Islet1+Lim1/2- motoneurons appeared by the addition of Noggin. In contrast, basic fibroblast growth factor (bFGF) promoted enlargement of central lumen and elongation of the structures. mRNAs of caudal markers, Gbx2, Cdx2 and Hoxb4/9 were expressed and Lim1/2+ spinal motoneurons appeared by the addition of bFGF. Addition of BMP-4 similarly brought about mild enlargement of central lumen of the structures. Interestingly, the addition of BMP-4 induced Slug+ neural crest-like cells surrounding the tube-like structures. mRNAs of Snail and dHand, other markers for neural crest cells, were also expressed by the addition of BMP-4. These results suggest that Noggin lead the neural-tube like structures to forebrain fate, whereas bFGF was involved in the caudalization. BMP-4 was implicated in emergence of the neural crest-like cells. Differentiation of ES cells by the present methods may mimic neurulation and subsequent neural development of early embryos, and elucidates the opposite effects of Noggin and bFGF for the neural tube development.

Monorail/Foxa2 regulates floorplate differentiation and specification of oligodendrocytes, serotonergic raphé neurones and cranial motoneurones

In this study, we elucidate the roles of the winged-helix transcription factor Foxa2 in ventral CNS development in zebrafish. Through cloning of monorail (mol), which we find encodes the transcription factor Foxa2, and phenotypic analysis of mol-/- embryos, we show that floorplate is induced in the absence of Foxa2 function but fails to further differentiate. In mol-/- mutants, expression of Foxa and Hh family genes is not maintained in floorplate cells and lateral expansion of the floorplate fails to occur. Our results suggest that this is due to defects both in the regulation of Hh activity in medial floorplate cells as well as cell-autonomous requirements for Foxa2 in the prospective laterally positioned floorplate cells themselves. Foxa2 is also required for induction and/or patterning of several distinct cell types in the ventral CNS. Serotonergic neurones of the raphenucleus and the trochlear motor nucleus are absent in mol-/- embryos, and oculomotor and facial motoneurones ectopically occupy ventral CNS midline positions in the midbrain and hindbrain. There is also a severe reduction of prospective oligodendrocytes in the midbrain and hindbrain. Finally, in the absence of Foxa2, at least two likely Hh pathway target genes are ectopically expressed in more dorsal regions of the midbrain and hindbrain ventricular neuroepithelium, raising the possibility that Foxa2 activity may normally be required to limit the range of action of secreted Hh proteins.

Development of putative GABAergic neurons in the appendicularian urochordateOikopleura dioica

Studying the developing brain of urochordates can increase our understanding of brain evolution in the chordate lineage. To begin addressing regional patterns of neuronal differentiation in appendicularian urochordates, we examined the development of putative GABAergic neurons in Oikopleura dioica using GABA immunohistochemistry and in situ hybridization for the GABA-synthesizing enzyme GAD. First, we assessed the developmental dynamics of neuron number and organization in the cerebral and caudal ganglia. We then identified and mapped the positions of putative GABAergic neurons using confocal microscopy. We found GAD mRNA-positive and GABA-immunopositive neurons in the first brain nerves and the cerebral and caudal ganglia, but not in the caudal nerve cord. In both ganglia GAD mRNA-positive and GABA-immunopositive neurons are found in the same characteristic intraganglionic locations. The differentiation of these GABAergic markers occurs first in the first brain nerves and the cerebral ganglion and then with a several-hour delay in the caudal ganglion. In all three structures GAD mRNA expression appears 2-3 hours prior to GABA expression. In general, GABA is expressed by the same number of neurons as express GAD. Several discrepancies suggest differential regulation of the GABAergic phenotype in different neurons, however. Our results show that the GABAergic phenotype has a stereotyped pattern of expression along the anteroposterior axis of the CNS. Given recent genome sequencing and developmental patterning gene studies in this species, the GABAergic neurons in O. dioica provide a good model for assessing, at the invertebrate-vertebrate transition, the molecular mechanisms that specify the GABAergic phenotype.

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

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

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.

Sonic hedgehog regulates proliferation of the retinal ciliary marginal zone in posthatch chicks

The ciliary marginal zone (CMZ) has long been known to be a source of postembryonic neuronal production in the retinas of fish and amphibians, and more recently, birds. However, there is little known about the factors that are required for the maintenance of this neural stem cell zone. The cells of the CMZ respond to mitogens such as endothelial growth factor, insulin-like growth factor-1, and insulin, factors that are also mitogenic for embryonic retinal progenitors, suggesting that the continued expression of embryonic mitogenic factors might be required to maintain the postembryonic proliferative potential of the CMZ. To test this hypothesis, we examined the expression and functional role of a critical embryonic retinal progenitor mitogen, Sonic hedgehog (Shh) in the regulation of proliferation of the cells of the CMZ. We have found that Shh is concentrated at the retinal margin of postembryonic chicks. Moreover, we report that intraocular injection of Shh stimulates proliferation of the CMZ cells, whereas cyclopamine, an inhibitor of the Shh pathway, inhibits CMZ proliferation. We conclude that Shh signaling is an important factor in the maintenance of postembryonic retinal neurogenesis.

The sonic hedgehog signaling system as a bistable genetic switch

Sonic hedgehog (Shh) controls critical cellular decisions between distinct fates in many systems, particularly in stem cells. The Shh network functions as a genetic switch, and we have theoretically and computationally analyzed how its structure can endow it with the ability to switch fate choices at a threshold Shh concentration. The network is composed of a positive transcriptional feedback loop embedded within a negative signaling feedback loop. Specifically, positive feedback by the transcription factor Gli, which upregulates its own expression, leads to a switch that can adopt two distinct states as a function of Shh. However, Gli also upregulates the signaling repressor Patched, negative feedback that reins in the strong Gli autoregulatory loop. Mutations that have been associated with cancer are predicted to yield an irreversible switch to a high Gli state. Finally, stochastic simulation reveals the negative Patched feedback loop serves a critical function of dampening Gli fluctuations to reduce spontaneous state switching and preserve the network's robust, switch-like behavior. Tightly linked positive and negative feedback loops are present in many signaling systems, and the Shh system is therefore likely representative of a large set of gene regulation networks that control stem cell fate throughout development and into adulthood.

Novel lipid modifications of secreted protein signals

Secreted signaling proteins function in a diverse array of essential patterning events during metazoan development, ranging from embryonic segmentation in insects to neural tube differentiation in vertebrates. These proteins generally are expressed in a localized manner, and they may elicit distinct concentration-dependent responses in the cells of surrounding tissues and structures, thus functioning as morphogens that specify the pattern of cellular responses by their tissue distribution. Given the importance of signal distribution, it is notable that the Hedgehog (Hh) and Wnt proteins, two of the most important families of such signals, are known to be covalently modified by lipid moieties, the membrane-anchoring properties of which are not consistent with passive models of protein mobilization within tissues. This review focuses on the mechanisms underlying biogenesis of the mature Hh proteins, which are dually modified by cholesteryl and palmitoyl adducts, as well as on the relationship between Hh proteins and the self-splicing proteins (i.e., proteins containing inteins) and the Hh-like proteins of nematodes. We further discuss the cellular mechanisms that have evolved to handle lipidated Hh proteins in the spatial deployment of the signal in developing tissues and the more recent findings that implicate palmitate modification as an important feature of Wnt signaling proteins.

Mutation analysis of the Sonic hedgehog promoter and putative enhancer elements in Parkinson's disease patients

Sonic hedgehog (SHH) is involved in the induction and differentiation of nigrostriatale dopaminergic neurons. We have investigated the promoter, two putative enhancer elements and the coding region of SHH for mutations in patients with Parkinson's disease (PD). None of the identified sequence variations were present at a significantly different frequency in PD patients compared to healthy individuals, suggesting that they are not involved in the pathogenesis of PD.

Differentiation and transcription factor gene therapy in experimental parkinson's disease: sonic hedgehog and Gli-1, but not Nurr-1, protect nigrostriatal cell bodies from 6-OHDA-induced neurodegeneration

We tested the activity of the dopaminergic neuron differentiation factor sonic hedgehog, its downstream transcription factor target Gli-1, and an orphan nuclear receptor, Nurr-1, necessary for the induction of the dopaminergic phenotype of nigrostriatal neurons, in an in vivo model of nigrostriatal neurodegeneration. Our preliminary experiments demonstrated that all three constructs expressed the proper molecules and that these had the predicted biological activities in vitro. We expressed the N-terminal of sonic hedgehog (ShhN) and the Gli-1 and Nurr-1 entire coding regions from highly purified, and quality controlled, replication-defective adenoviral vectors injected into the brains of rats and used the dopaminergic growth factor GDNF as a positive control. The neurotoxin 6-hydroxydopamine was used to lesion the nigrostriatal dopaminergic innervation; RAd-ShhN and RAd-Gli-1 protected dopaminergic neuronal cell bodies in the substantia nigra, but not axonal terminals in the striatum, from 6-OHDA-induced cell death, while RAd-Nurr-1 was ineffective in protecting either cell bodies or axons. RAd-GDNF was able to protect both the dopaminergic cell bodies and the striatal axon terminals. Our results establish for the first time, to the best of our knowledge, that gene transfer of ShhN and one of its target transcription factors can selectively protect dopaminergic nigrostriatal neuronal cell bodies from a specific neurotoxic insult. Selective protection of nigrostriatal dopaminergic cell bodies by the differentiation factor ShhN and the transcription factor Gli-1 was achieved in a neurotoxic model that eliminates more than 70% of the nigral neurons under consideration. Differentiation and transcription factors can thus be used for the treatment of neurodegeneration by gene therapy.

Nurr1, an orphan nuclear receptor with essential functions in developing dopamine cells

Nurr1 is a transcription factor that is expressed in the embryonic ventral midbrain and is critical for the development of dopamine (DA) neurons. It belongs to the conserved family of nuclear receptors but lacks an identified ligand and is therefore referred to as an orphan receptor. Recent structural studies have indicated that Nurr1 belongs to a class of ligand-independent nuclear receptors that are unable to bind cognate ligands. However, Nurr1 can promote signaling via its heterodimerization partner, the retinoid X receptor (RXR). RXR ligands can promote the survival of DA neurons via a process that depends on Nurr1-RXR heterodimers. In developing DA cells, Nurr1 is required for the expression of several genes important for DA synthesis and function. However, Nurr1 is probably also important for the maintenance of adult DA neurons and plays additional less-well-elucidated roles in other regions of the central nervous system and in peripheral tissues.

Induction and specification of midbrain dopaminergic cells: focus on SHH, FGF8, and TGF-?

Cell-fate decisions along the dorsoventral and anterior-posterior axis of the neural tube are dictated by factors from signaling and organizing centers. According to the prevailing notion, the formation of mesencephalic dopaminergic neurons is directed by diffusable signals from the notochord, floor plate, and isthmic organizer. Sonic hedgehog (Shh), secreted by the notochord and floor plate, and fibroblast growth factor (FGF) 8, secreted by the isthmus, are thought to be key molecules involved in the development of midbrain dopaminergic neurons. During the last decade, the introduction of elegant explant culture systems and the generation of transgenic and mutant mice have greatly contributed to a better understanding of the molecular signals that direct the induction and specification of midbrain dopaminergic neurons. In this context, experimental evidence has challenged the dominant roles of Shh and FGF8 in dopaminergic neuron development. Additional molecules have been identified as being required for the generation of mesencephalic dopaminergic neurons, particularly members of the transforming growth factor beta superfamily.

TGF-beta promotes survival on mesencephalic dopaminergic neurons in cooperation with Shh and FGF-8

Impaired neuronal survival is a key event in the development of degenerative diseases, such as Parkinson's disease (PD). Here we show that transforming growth factor beta (TGF-beta) acts directly on rat E14 midbrain dopaminergic neurons in vitro, its survival-promoting effect being not mediated by BDNF, NT-3, or GDNF. Treatment with TGF-beta, sonic hedgehog (Shh), or fibroblast growth factor-8 (FGF8) significantly increased number of tyrosine hydroxylase (TH)-immunoreactive neurons after 7 days, whereas application of these factors added together further increased number of TH-positive neurons, compared to single-factor treatments. Neutralization of endogenous TGF-beta, Shh, or FGF8 significantly reduced number of dopaminergic neurons. TGF-beta treatment decreased number of apoptotic cells, having no effect on cell proliferation. Neutralization of TGF-beta in vivo during chick E6-10 resulted in reduced number of midbrain dopaminergic neurons. The results suggest that TGF-beta is required for survival of mesencephalic dopaminergic neurons acting in cooperation with Shh and FGF8.

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.

Simultaneous absence of dopamine D1 and D2 receptor-mediated signaling is lethal in mice

Dopamine (DA) controls a wide variety of physiological functions in the central nervous system as well as in the neuroendocrine and gastrointestinal systems. DA signaling is mediated by five cloned receptors named D1-D5. Knockout mouse models for the five receptors have been generated, and, albeit impaired for some important DA-mediated functions, they are viable and can reproduce. D1 and D2 receptors are the most abundant and widely expressed DA receptors. Cooperative/synergistic effects mediated by these receptors have been suggested, in particular, in the control of motor behaviors. To analyze the extent of such interrelationship, we have generated double D1/D2 receptor mutants. Interestingly, in contrast to single knockouts, we found that concurrent ablation of the D1 and D2 receptors is lethal during the second or third week after birth. This dramatic phenotype is likely to be related to altered feeding behavior and dysfunction of the gastrointestinal system, especially because major anatomical changes were not identified in the brain. Similarly, in the absence of functional D1, heterozygous D2 mutants (D1r(-/-);D2r(+/-)) showed severe growth retardation and did not survive their postweaning period. The analysis of motor behavior in D1r/D2r compound mutants showed that loss of D2-mediated functions reduces motor abilities, whereas the effect of D1r ablation on locomotion strongly depends on the experimental paradigms used. These studies highlight the interrelationship between D1 and D2 receptor-mediated control of motor activity, food intake, and gastrointestinal functions, which has been elusive in the single-gene ablation studies.

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.

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.

Abnormal serotonergic development in a mouse model for the Smith-Lemli-Opitz syndrome: implications for autism

The Smith-Lemli-Opitz syndrome (SLOS) is a malformation/mental retardation syndrome resulting from an inborn error in 3beta-hydroxysteroid Delta7-reductase (DHCR7), the terminal enzyme required for cholesterol biosynthesis. Using a targeting strategy designed to virtually eliminate Dhcr7 activity, we have created a SLOS mouse model that exhibits commissural deficiencies, hippocampal abnormalities, and hypermorphic development of serotonin (5-HT) neurons. The latter is of particular interest with respect to current evidence that serotonin plays a significant role in autism spectrum disorders and the recent clinical observation that 50% of SLOS patients present with autistic behavior. Immunohistochemical analyses have revealed a 306% increase in the area of 5-HT immunoreactivity (5-HT IR) in the hindbrains of mutant (Dhcr7-/-) mice as compared to age-matched wild type animals. Amount of 5-HT IR was measured as total area of IR per histological section. Additionally, a regional increase as high as 15-fold was observed for the most lateral sagittal hindbrain sections. In Dhcr7-/- mice, an expansion of 5-HT IR into the ventricular zone and floor plate region was observed. In addition, the rostral and caudal raphe groups exhibited a radial expansion in Dhcr7-/- mice, with 5-HT IR cells present in locations not seen in wild type mice. This increase in 5-HT IR appears to represent an increase in total number of 5-HT neurons and fibers. These observations may help explain the behavioral phenotype seen in SLOS, and provide clues for future therapeutic interventions that utilize pharmacological modulation of the serotonergic system.

Comprehensive transcriptome analysis of differentiation of embryonic stem cells into midbrain and hindbrain neurons

Neurogenesis is one of the most complex events in embryonic development. However, little information is available regarding the molecular events that occur during neurogenesis. To identify regulatory genes and underlying mechanisms involved in the differentiation of embryonic stem (ES) cells to neurons, gene expression profiling was performed using cDNA microarrays. In mouse ES cells, we compared the gene expression of each differentiated cell stage using a five-stage lineage selection method. Of 10,368 genes, 1633 (16%) known regulatory genes were differentially expressed at least 2-fold or greater at one or more stages. At stage 3, during which ES cells differentiate into neural stem cells, modulation of nearly 1000 genes was observed. Most of transcription factors (Otx2, Ebf-3, Ptx3, Sox4, 13, 18, engrailed, Irx2, Pax8, and Lim3), signaling molecules (Wnt, TGF, and Shh family members), and extracellular matrix/adhesion molecules (collagens, MAPs, and NCAM) were up-regulated. However, some genes which may play important roles in maintaining the pluripotency of ES cells (Kruppel-like factor 2, 4, 5, 9, myeloblast oncogene like2, ZFP 57, and Esg-1) were down-regulated. The many genes identified with this approach that are modulated during neurogenesis will facilitate studies of the mechanisms underlying ES cell differentiation, neural induction, and neurogenesis.

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.

p57(Kip2) cooperates with Nurr1 in developing dopamine cells

Cyclin-dependent kinase inhibitors of the Cip/Kip family play critical roles in regulating cell proliferation during embryogenesis. However, these proteins also influence cell differentiation by mechanisms that have remained unknown. Here we show that p57Kip2 is expressed in postmitotic differentiating midbrain dopamine cells. Induction of p57Kip2 expression depends on Nurr1, an orphan nuclear receptor that is essential for dopamine neuron development. Moreover, analyses of p57Kip2 gene-targeted mice revealed that p57Kip2 is required for the maturation of midbrain dopamine neuronal cells. Additional experiments in a dopaminergic cell line demonstrated that p57Kip2 can promote maturation by a mechanism that does not require p57Kip2-mediated inhibition of cyclin-dependent kinases. Instead, evidence indicates that p57Kip2 functions by a direct protein-protein interaction with Nurr1. Thus, in addition to its established function in control of proliferation, these results reveal a mechanism whereby p57Kip2 influences postmitotic differentiation of dopamine neurons.

Hedgehog signaling is directly required for the development of zebrafish dorsal root ganglia neurons

Hedgehog (Hh) signal transduction is directly required in zebrafish DRG precursors for proper development of DRG neurons. Zebrafish mutations in the Hh signaling pathway result in the absence of DRG neurons and the loss of expression of neurogenin1 (ngn1), a gene required for determination of DRG precursors. Cell transplantation experiments demonstrate that Hh acts directly on DRG neuron precursors. Blocking Hh pathway activation at later stages of embryogenesis with the steroidal alkaloid, cyclopamine, further reveals that the requirement for a Hh signal response in DRG precursors correlates with the onset of ngn1 expression. These results suggest that Hh signaling may normally promote DRG development by regulating expression of ngn1 in DRG precursors.

Dopamine phenotype and behaviour in animal models: in relation to attention deficit hyperactivity disorder

The phenotypic expression of behaviour is the outcome of interacting neuronal networks and is modulated by different subcortical systems. In the present paper the role of a major subcortical neurochemical system, dopamine (DA), is reviewed. In particular, knockout (KO) technology has given an overwhelming insight into the effects of specific component of the dopaminergic system. Therefore, the behavioural profile of dopamine transporter (DAT), tyrosine hydroxylase (TH), DA and cAMP-regulated phosphoprotein (DARPP 32), and D1, D2, D3, D4 and D5 dopamine receptors knockouts (and their combination) is reviewed.TH, D1, D2, D4 KO mice exhibit decreased locomotor activity, perhaps due to decreased motivational level. D3 KO and DAT KO mice show an increase in basal and novelty-induced activity respectively. It is possible that the increased dopamine levels in DAT KO mice enhance motivation. These observations support the hyperDA hypothesis in hyperactive phenotypes. Moreover, they suggest that the inhibitory effect of psychostimulant drugs, such as methylphenidate and amphetamines, in Attention Deficit Hyperactivity Disorder may be the outcome of an altered balance between auto- and hetero-receptors. However, since KO technology is hampered by blockade of the target at early stages of development, some alternatives have been proposed, such as inducible mutagenesis and inhibitory small RNAs conveyed to target by viral vectors in adulthood.