Signals from the ventral midline and isthmus regulate the development of Brn3.0-expressing neurons in the midbrain

Retinoic acid signaling regulates proliferation and lamina formation in the developing chick optic tectum

The retinotectal system has been extensively studied for investigating the mechanism(s) for topographic map formation. The optic tectum, which is composed of multiple laminae, is the major retino recipient structure in the developing avian brain. Laminar development of the tectum results from cell proliferation, differentiation and migration, coordinated in strict temporal and spatial patterns. However, the molecular mechanisms that orchestrate these complex developmental events, have not been fully elucidated. In this study, we have identified the presence of differential retinoic acid (RA) signaling along the rostro-caudal and dorsoventral axis of the tectum. We show for the first time that loss of RA signaling in the anterior optic tectum, leads to an increase in cell proliferation and gross changes in the morphology manifested as defects in lamination. Detailed analysis points to delayed migration of cells as the plausible cause for the defects in lamina formation. Thus, we conclude that in the optic tectum, RA signaling is involved in maintaining cell proliferation and in regulating the formation of the tectal laminae.

Crosstalk of Intercellular Signaling Pathways in the Generation of Midbrain Dopaminergic Neurons In Vivo and from Stem Cells

Dopamine-synthesizing neurons located in the mammalian ventral midbrain are at the center stage of biomedical research due to their involvement in severe human neuropsychiatric and neurodegenerative disorders, most prominently Parkinson’s Disease (PD). The induction of midbrain dopaminergic (mDA) neurons depends on two important signaling centers of the mammalian embryo: the ventral midline or floor plate (FP) of the neural tube, and the isthmic organizer (IsO) at the mid-/hindbrain boundary (MHB). Cells located within and close to the FP secrete sonic hedgehog (SHH), and members of the wingless-type MMTV integration site family (WNT1/5A), as well as bone morphogenetic protein (BMP) family. The IsO cells secrete WNT1 and the fibroblast growth factor 8 (FGF8). Accordingly, the FGF8, SHH, WNT, and BMP signaling pathways play crucial roles during the development of the mDA neurons in the mammalian embryo. Moreover, these morphogens are essential for the generation of stem cell-derived mDA neurons, which are critical for the modeling, drug screening, and cell replacement therapy of PD. This review summarizes our current knowledge about the functions and crosstalk of these signaling pathways in mammalian mDA neuron development in vivo and their applications in stem cell-based paradigms for the efficient derivation of these neurons in vitro.

Spatio-temporal neural stem cell behavior leads to both perfect and imperfect structural brain regeneration in adult newts

Adult newts can regenerate large parts of their brain from adult neural stem cells (NSCs), but how adult NSCs reorganize brain structures during regeneration remains unclear. In development, elaborate brain structures are produced under broadly coordinated regulations of embryonic NSCs in the neural tube, whereas brain regeneration entails exquisite control of the re-establishment of certain brain parts, suggesting that a yet-unknown mechanism directs NSCs upon partial brain excision. Here we report that upon excision of a quarter of the adult newt (Pleurodeles waltl) mesencephalon, active participation of local NSCs around specific brain subregions' boundaries leads to some imperfect and some perfect brain regeneration along an individual's rostrocaudal axis. Regeneration phenotypes depend on how wound closing occurs using local NSCs, and perfect regeneration replicates development-like processes, but takes more than 1 year. Our findings indicate that newt brain regeneration is supported by modularity of boundary-domain NSCs with self-organizing ability in neighboring fields.This article has an associated First Person interview with the first author of the paper.

Shape analysis of the basioccipital bone in Pax7-deficient mice

We compared the cranial base of newborn Pax7-deficient and wildtype mice using a computational shape modeling technology called particle-based modeling (PBM). We found systematic differences in the morphology of the basiooccipital bone, including a broadening of the basioccipital bone and an antero-inferior inflection of its posterior edge in the Pax7-deficient mice. We show that the Pax7 cell lineage contributes to the basioccipital bone and that the location of the Pax7 lineage correlates with the morphology most effected by Pax7 deficiency. Our results suggest that the Pax7-deficient mouse may be a suitable model for investigating the genetic control of the location and orientation of the foramen magnum, and changes in the breadth of the basioccipital.

Morphogenetic and Histogenetic Roles of the Temporal-Spatial Organization of Cell Proliferation in the Vertebrate Corticogenesis as Revealed by Inter-specific Analyses of the Optic Tectum Cortex Development

The central nervous system areas displaying the highest structural and functional complexity correspond to the so called cortices, i.e., concentric alternating neuronal and fibrous layers. Corticogenesis, i.e., the development of the cortical organization, depends on the temporal-spatial organization of several developmental events: (a) the duration of the proliferative phase of the neuroepithelium, (b) the relative duration of symmetric (expansive) versus asymmetric (neuronogenic) sub phases, (c) the spatial organization of each kind of cell division, (e) the time of determination and cell cycle exit and (f) the time of onset of the post-mitotic neuronal migration and (g) the time of onset of the neuronal structural and functional differentiation. The first five events depend on molecular mechanisms that perform a fine tuning of the proliferative activity. Changes in any of them significantly influence the cortical size or volume (tangential expansion and radial thickness), morphology, architecture and also impact on neuritogenesis and synaptogenesis affecting the cortical wiring. This paper integrates information, obtained in several species, on the developmental roles of cell proliferation in the development of the optic tectum (OT) cortex, a multilayered associative area of the dorsal (alar) midbrain. The present review (1) compiles relevant information on the temporal and spatial organization of cell proliferation in different species (fish, amphibians, birds, and mammals), (2) revises the main molecular events involved in the isthmic organizer (IsO) determination and localization, (3) describes how the patterning installed by IsO is translated into spatially organized neural stem cell proliferation (i.e., by means of growth factors, receptors, transcription factors, signaling pathways, etc.) and (4) describes the morpho- and histogenetic effect of a spatially organized cell proliferation in the above mentioned species. A brief section on the OT evolution is also included. This section considers how the differential operation of cell proliferation could explain differences among species.

Liver X receptors and cholesterol metabolism: role in ventral midbrain development and neurodegeneration

The development of the ventral midbrain is orchestrated by a number of cell-extrinsic and -intrinsic factors that control critical processes, such as the patterning of the neural tube along the main body axis and the specification of diverse neuronal cell types in distinct positions of the neural tube. Subsequently, the regulation of neurogenesis and survival— acquire particular relevance in order to define the final size of diverse neuronal populations. In a series of studies during the last few years, we have identified liver X receptors (LXRs) as critical regulators of ventral midbrain development. Moreover, specific cholesterol derivatives present in the midbrain or in the cerebrospinal fluid were identified as LXR ligands, capable of specifically and selectively regulating neurogenesis and the survival of distinct neuronal populations, including midbrain dopamine neurons. These studies have shown that cholesterol derivatives are an entirely new class of factors capable of regulating both neuronal survival and neurogenesis, thus providing a direct link between cholesterol metabolism and brain development. In addition, LXRs and cholesterol metabolism were found to play a critical role in regulating the balance between neuronal survival and death in diverse midbrain neuronal populations. In this review, we will focus on these two aspects and on the possible role of cholesterol metabolism and LXRs in neurodegeneration.

Genetic control of midbrain dopaminergic neuron development

Midbrain dopaminergic neurons are involved in regulating motor control, reward behavior, and cognition. Degeneration or dysfunction of midbrain dopaminergic neurons is implicated in several neuropsychiatric disorders such as Parkinson's disease, substance use disorders, depression, and schizophrenia. Understanding the developmental processes that generate midbrain dopaminergic neurons will facilitate the generation of dopaminergic neurons from stem cells for cell replacement therapies to substitute degenerating cells in Parkinson's disease patients and will forward our understanding on how functional diversity of dopaminergic neurons in the adult brain is established. Midbrain dopaminergic neurons develop in a multistep process. Following the induction of the ventral midbrain, a distinct dopaminergic progenitor domain is specified and dopaminergic progenitors undergo proliferation, neurogenesis, and differentiation. Subsequently, midbrain dopaminergic neurons acquire a mature dopaminergic phenotype, migrate to their final position and establish projections and connections to their forebrain targets. This review will discuss insights gained on the signaling network of secreted molecules, cell surface receptors, and transcription factors that regulate specification and differentiation of midbrain dopaminergic progenitors and neurons, from the induction of the ventral midbrain to the migration of dopaminergic neurons. For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

Specification of sensory neurons occurs through diverse developmental programs functioning in the brain and spinal cord

BACKGROUND

Vertebrates possess two populations of sensory neurons located within the central nervous system: Rohon-Beard (RB) and mesencephalic trigeminal nucleus (MTN) neurons. RB neurons are transient spinal cord neurons whilst MTN neurons are the proprioceptive cells that innervate the jaw muscles. It has been suggested that MTN and RB neurons share similarities and may have a common developmental program, but it is unclear how similar or different their development is.

RESULTS

We have dissected RB and MTN neuronal specification in zebrafish. We find that RB and MTN neurons express a core set of genes indicative of sensory neurons, but find these are also expressed by adjacent diencephalic neurons. Unlike RB neurons, our evidence argues against a role for the neural crest during MTN development. We additionally find that neurogenin1 function is dispensable for MTN differentiation, unlike RB cells and all other sensory neurons. Finally, we demonstrate that, although Notch signalling is involved in RB development, it is not involved in the generation of MTN cells.

CONCLUSIONS

Our work reveals fundamental differences between the development of MTN and RB neurons and suggests that these populations are non-homologous and thus have distinct developmental and, probably, evolutionary origins.

Fuz mutant mice reveal shared mechanisms between ciliopathies and FGF-related syndromes

Ciliopathies are a broad class of human disorders with craniofacial dysmorphology as a common feature. Among these is high arched palate, a condition that affects speech and quality of life. Using the ciliopathic Fuz mutant mouse, we find that high arched palate does not, as commonly suggested, arise from midface hypoplasia. Rather, increased neural crest expands the maxillary primordia. In Fuz mutants, this phenotype stems from dysregulated Gli processing, which in turn results in excessive craniofacial Fgf8 gene expression. Accordingly, genetic reduction of Fgf8 ameliorates the maxillary phenotypes. Similar phenotypes result from mutation of oral-facial-digital syndrome 1 (Ofd1), suggesting that aberrant transcription of Fgf8 is a common feature of ciliopathies. High arched palate is also a prevalent feature of fibroblast growth factor (FGF) hyperactivation syndromes. Thus, our findings elucidate the etiology for a common craniofacial anomaly and identify links between two classes of human disease: FGF-hyperactivation syndromes and ciliopathies.

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A genetic model for high arched palate, commonly seen in human craniofacial syndromes

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In ciliopathic mice, Fgf8 overexpression leads to cranial neural crest hyperplasia

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Enlargement of the maxillary primordia underlies high arched palate in Fuz mutants

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An etiological link between ciliopathies and FGF-hyperactivation syndromes

Fgf22 regulated by Fgf3/Fgf8 signaling is required for zebrafish midbrain development

Fibroblast growth factor (Fgf) signaling plays important roles in various developmental processes including brain development. Here, we identified zebrafish fgf22 predominantly expressed in the posterior midbrain and anterior midbrain-hindbrain boundary (MHB) primordia during early embryonic brain development. To examine roles of Fgf22 in midbrain development, we analyzed fgf22 knockdown embryos. The fgf22 morphants were defective in proper formation of the MHB constriction and the midbrain. The knockdown of fgf22 caused decreased cell proliferation in the midbrain, expanded expression of roof plate and tegmental marker genes, and decreased expression of tectal marker genes, indicating that Fgf22 is required for cell proliferation, roof plate formation, and tectum specification in the midbrain. Fgf receptor 2b (Fgfr2b), a potential receptor for Fgf22, was also required, indicating that Fgf22 signaling is mediated through Fgfr2b. The floor plate and the MHB are crucial for the dorsoventral patterning of the midbrain through Hedgehog (Hh) and Fgf signaling, respectively. The fgf3/fgf8 double morphant phenotype was essentially similar to that of fgf22 morphants, whereas the phenotype caused by inhibition of Hh signaling was not. fgf3 and fgf8 were expressed earlier than fgf22 in the MHB primordium and Fgf3/Fgf8 signaling was required for fgf22 expression in the posterior midbrain. Furthermore, fgf22 partially rescued the fgf3/fgf8 double morphant phenotype. The present results indicate Fgf22 to be involved in midbrain development downstream of Fgf3 and Fgf8 in the MHB but not of Hh in the floor plate.

Expression of Neurogenin 1 in mouse embryonic stem cells directs the differentiation of neuronal precursors and identifies unique patterns of down-stream gene expression

BACKGROUND

Delineating the cascades of growth and transcription factor expression that shape the developing nervous system will improve our understanding of its molecular histogenesis and suggest strategies for cell replacement therapies. In the current investigation, we examined the ability of the proneural gene, Neurogenin1 (Neurog1; also Ngn1, Neurod3), to drive differentiation of pluripotent embryonic stem cells (ESC).

RESULTS

Transient expression of Neurog1 in ESC was sufficient to initiate neuronal differentiation, and produced neuronal subtypes reflecting its expression pattern in vivo. To begin to address the molecular mechanisms involved, we used microarray analysis to identify potential down-stream targets of Neurog1 expressed at sequential stages of neuronal differentiation.

CONCLUSIONS

ESC expressing Neurogenin1 begin to withdraw from cycle and form precursors that differentiate exclusively into neurons. This work identifies unique patterns of gene expression following expression of Neurog1, including genes and signaling pathways involved in process outgrowth and cell migration, regional differentiation of the nervous system, and cell cycle.

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

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

Intrinsic properties guide proximal abducens and oculomotor nerve outgrowth in avian embryos

Proper movement of the vertebrate eye requires the formation of precisely patterned axonal connections linking cranial somatic motoneurons, located at defined positions in the ventral midbrain and hindbrain, with extraocular muscles. The aim of this research was to assess the relative contributions of intrinsic, population-specific properties and extrinsic, outgrowth site-specific cues during the early stages of abducens and oculomotor nerve development in avian embryos. This was accomplished by surgically transposing midbrain and caudal hindbrain segments, which had been pre-labeled by electroporation with an EGFP construct. Graft-derived EGFP+ oculomotor axons entering a hindbrain microenvironment often mimicked an abducens initial pathway and coursed cranially. Similarly, some EGFP+ abducens axons entering a midbrain microenvironment mimicked an oculomotor initial pathway and coursed ventrally. Many but not all of these axons subsequently projected to extraocular muscles that they would not normally innervate. Strikingly, EGFP+ axons also took initial paths atypical for their new location. Upon exiting from a hindbrain position, most EGFP+ oculomotor axons actually coursed ventrally and joined host branchiomotor nerves, whose neurons share molecular features with oculomotor neurons. Similarly, upon exiting from a midbrain position, some EGFP+ abducens axons turned caudally, elongated parallel to the brainstem, and contacted the lateral rectus muscle, their originally correct target. These data reveal an interplay between intrinsic properties that are unique to oculomotor and abducens populations and shared ability to recognize and respond to extrinsic directional cues. The former play a prominent role in initial pathway choices, whereas the latter appear more instructive during subsequent directional choices.

Cell-autonomous FGF signaling regulates anteroposterior patterning and neuronal differentiation in the mesodiencephalic dopaminergic progenitor domain

The structure and projection patterns of adult mesodiencephalic dopaminergic (DA) neurons are one of the best characterized systems in the vertebrate brain. However, the early organization and development of these nuclei remain poorly understood. The induction of midbrain DA neurons requires sonic hedgehog (Shh) from the floor plate and fibroblast growth factor 8 (FGF8) from the isthmic organizer, but the way in which FGF8 regulates DA neuron development is unclear. We show that, during early embryogenesis, mesodiencephalic neurons consist of two distinct populations: a diencephalic domain, which is probably independent of isthmic FGFs; and a midbrain domain, which is dependent on FGFs. Within these domains, DA progenitors and precursors use partly different genetic programs. Furthermore, the diencephalic DA domain forms a distinct cell population, which also contains non-DA Pou4f1(+) cells. FGF signaling operates in proliferative midbrain DA progenitors, but is absent in postmitotic DA precursors. The loss of FGFR1/2-mediated signaling results in a maturation failure of the midbrain DA neurons and altered patterning of the midbrain floor. In FGFR mutants, the DA domain adopts characteristics that are typical for embryonic diencephalon, including the presence of Pou4f1(+) cells among TH(+) cells, and downregulation of genes typical of midbrain DA precursors. Finally, analyses of chimeric embryos indicate that FGF signaling regulates the development of the ventral midbrain cell autonomously.

Temporal-spatial changes in Sonic Hedgehog expression and signaling reveal different potentials of ventral mesencephalic progenitors to populate distinct ventral midbrain nuclei

Background

The ventral midbrain contains a diverse array of neurons, including dopaminergic neurons of the ventral tegmental area (VTA) and substantia nigra (SN) and neurons of the red nucleus (RN). Dopaminergic and RN neurons have been shown to arise from ventral mesencephalic precursors that express Sonic Hedgehog (Shh). However, Shh expression, which is initially confined to the mesencephalic ventral midline, expands laterally and is then downregulated in the ventral midline. In contrast, expression of the Hedgehog target gene Gli1 initiates in the ventral midline prior to Shh expression, but after the onset of Shh expression it is expressed in precursors lateral to Shh-positive cells. Given these dynamic gene expression patterns, Shh and Gli1 expression could delineate different progenitor populations at distinct embryonic time points.

Results

We employed genetic inducible fate mapping (GIFM) to investigate whether precursors that express Shh (Shh-GIFM) or transduce Shh signaling (Gli1-GIFM) at different time points give rise to different ventral midbrain cell types. We find that precursors restricted to the ventral midline are labeled at embryonic day (E)7.5 with Gli1-GIFM, and with Shh-GIFM at E8.5. These precursors give rise to all subtypes of midbrain dopaminergic neurons and the anterior RN. A broader domain of progenitors that includes the ventral midline is marked with Gli1-GIFM at E8.5 and with Shh-GIFM at E9.5; these fate-mapped cells also contribute to all midbrain dopaminergic subtypes and to the entire RN. In contrast, a lateral progenitor domain that is labeled with Gli1-GIFM at E9.5 and with Shh-GIFM at E11.5 has a markedly reduced potential to give rise to the RN and to SN dopaminergic neurons, and preferentially gives rise to the ventral-medial VTA. In addition, cells derived from Shh- and Gli1-expressing progenitors located outside of the ventral midline give rise to astrocytes.

Conclusions

We define a ventral midbrain precursor map based on the timing of Gli1 and Shh expression, and suggest that the diversity of midbrain dopaminergic neurons is at least partially determined during their precursor stage when their medial-lateral position, differential gene expression and the time when they leave the ventricular zone influence their fate decisions.

Lmx1a and Lmx1b cooperate with Foxa2 to coordinate the specification of dopaminergic neurons and control of floor plate cell differentiation in the developing mesencephalon

Mesencephalic dopaminergic (mesDA) neurons control movement and behavior, and their loss causes severe neurological disorders, such as Parkinson's disease. Recent studies have revealed that mesDA neurons originate from mesencephalic floor plate (FP) cells, which had been thought of as non-neurogenic organizer cells regulating regional patterning and axonal projections. Otx2 and its FP-specific downstream factor Lmx1a have been shown to be sufficient to confer neurogenic activity on FP cells and determine a mesDA fate. However, the mechanism underlying how these factors control mesDA development and how FP cells and mesDA neurons are coordinately specified are still largely unknown. In the present study, we obtained evidence that Lmx1a and Lmx1b cooperate with Foxa2 to specify mesDA neuron identity by gain-of-function approaches using transgenic mice. Lmx1a/b appeared to select a mesDA fate by suppressing red nucleus fate in the context of Foxa2-positive progenitors, at least in part, through repressing the Sim1-Lhx1 and Ngn1 pathways that inhibit proper mesDA differentiation. We also found that, in the mesencephalon, FP cell fate is primarily determined by Foxa2 with a supportive action of Lmx1a/b through repressing Nkx6.1, which inhibits FP cell differentiation. Thus, FP and mesDA identities are determined by distinct specification pathways, both of which are controlled by the same combination of transcription factors, Lmx1a/b and Foxa2, and, as a consequence, mesDA neurons are generated from mesencephalic FP cells.

Nkx6-1 controls the identity and fate of red nucleus and oculomotor neurons in the mouse midbrain

Little is known about the cues controlling the generation of motoneuron populations in the mammalian ventral midbrain. We show that Otx2 provides the crucial anterior-posterior positional information for the generation of red nucleus neurons in the murine midbrain. Moreover, the homeodomain transcription factor Nkx6-1 controls the proper development of the red nucleus and of the oculomotor and trochlear nucleus neurons. Nkx6-1 is expressed in ventral midbrain progenitors and acts as a fate determinant of the Brn3a(+) (also known as Pou4f1) red nucleus neurons. These progenitors are partially dorsalized in the absence of Nkx6-1, and a fraction of their postmitotic offspring adopts an alternative cell fate, as revealed by the activation of Dbx1 and Otx2 in these cells. Nkx6-1 is also expressed in postmitotic Isl1(+) oculomotor and trochlear neurons. Similar to hindbrain visceral (branchio-) motoneurons, Nkx6-1 controls the proper migration and axon outgrowth of these neurons by regulating the expression of at least three axon guidance/neuronal migration molecules. Based on these findings, we provide additional evidence that the developmental mechanism of the oculomotor and trochlear neurons exhibits more similarity with that of special visceral motoneurons than with that controlling the generation of somatic motoneurons located in the murine caudal hindbrain and spinal cord.

Ventral specification and perturbed boundary formation in the mouse midbrain in the absence of Hedgehog signaling

Although Hedgehog (HH) signaling plays a critical role in patterning the ventral midbrain, its role in early midbrain specification is not known. We examined the midbrains of sonic hedgehog (Shh) and smoothened (Smo) mutant mice where HH signaling is respectively attenuated and eliminated. We show that some ventral (Evx1+) cell fates are specified in the Shh-/- mouse in a Ptc1- and Gli1-independent manner. HH-independent ventral midbrain induction was further confirmed by the presence of a Pax7-negative ventral midbrain territory in both Shh-/- and Smo-/- mice at and before embryonic day (E) 8.5. Midbrain signaling centers are severely disrupted in the Shh-/- mutant. Interestingly, dorsal markers are up-regulated (Wnt1, Gdf7, Pax7), down-regulated (Lfng), or otherwise altered (Zic1) in the Shh-/- midbrain. Together with the increased cell death seen specifically in Shh-/- dorsal midbrains (E8.5-E9), our results suggest specific regulation of dorsal patterning by SHH, rather than a simple deregulation due to its absence.

The embryonic midbrain directs neuronal specification of embryonic stem cells at early stages of differentiation

Specific neuronal differentiation of Embryonic Stem Cells (ESCs) depends on their capacity to interpret environmental cues. At present, it is not clear at which stage of differentiation ESCs become competent to produce multiple neuronal lineages in response to the niche of the embryonic brain. To unfold the developmental potential of ESC-derived precursors, we transplanted these cells into the embryonic midbrain explants, where neurogenesis occurs as in normal midbrain development. Using this experimental design, we show that the transition from ESCs to Embryoid Body (EB) precursors is necessary to differentiate into Lmx1a(+)/Ptx3(+)/TH(+) dopaminergic neurons around the ventral midline of the midbrain. In addition, EB cells placed at other dorsal-ventral levels of the midbrain give rise to Nkx6.1(+) red nucleus (RN) neurons, Nkx2.2(+) ventral interneurons and Pax7(+) dorsal neurons at the correct positions. Notably, differentiation of ESCs into Neural Precursor Cells (NPCs) prior to transplantation markedly reduces specification at the Lmx1a, Nkx6.1 and Pax7 expression domains, without affecting neuronal differentiation. Finally, exposure to Fgf8 and Shh in vitro promotes commitment of some ESC-derived NPCs to differentiate into putative Lmx1a(+) dopaminergic neurons in the midbrain. Our data demonstrate intrinsic developmental potential differences among ESC-derived precursor populations.

Cardiac expression of Brn-3a and Brn-3b POU transcription factors and regulation of Hsp27 gene expression

The Brn-3 family of transcription factors play a critical role in regulating expression of genes that control cell fate, including the small heat shock protein Hsp27. The aim of this study was to investigate the relationship between Brn-3a and Brn-3b and Hsp27 expression in the developing rodent heart. Brn-3a and Brn-3b were detected from embryonic days 9.5-10.5 (E9.5-E10.5) in the mouse heart, with significant increases seen later during development. Two isoforms (long and short) of each protein were detected during embryogenesis and postnatally. Brn-3a messenger RNA (mRNA) and protein were localized by E13.0 to the atrio-ventricular (AV) valve cushions and leaflets, outflow tract (OFT), epicardium and cardiac ganglia. By E14.5, Brn-3a was also localised to the septa and compact ventricular myocardium. An increase in expression of the long Brn-3a(l) isoform between E17 and adult coincided with a decrease in expression of Brn-3b(l) and a marked increase in expression of Hsp27. Hearts from Brn-3a-/- mice displayed a partially penetrant phenotype marked by thickening of the endocardial cushions and AV valve leaflets and hypoplastic ventricular myocardium. Loss of Brn-3a was correlated with a compensatory increase in Brn-3b and GATA3 mRNA but no change in Hsp27 mRNA. Reporter assays in isolated cardiomyocytes demonstrated that both Brn-3a and Brn-3b activate the hsp27 promoter via a consensus Brn-3-binding site. Therefore, Brn-3 POU factors may play an important role in the development and maintenance of critical cell types and structures within the heart, in part via developmental regulation of myocardial Hsp27 expression. Furthermore, Brn-3a may be necessary for correct valve and myocardial remodelling and maturation.

Gli3 coordinates three-dimensional patterning and growth of the tectum and cerebellum by integrating Shh and Fgf8 signaling

The coordination of anterior-posterior (AP) and dorsal-ventral (DV) patterning of the mesencephalon (mes) and rhombomere 1 (r1) is instrumental for the development of three distinct brain structures: the tectum and cerebellum dorsally and the tegmentum ventrally. Patterning of the mes/r1 is primarily mediated by signaling molecules secreted from two organizers: sonic hedgehog (Shh) from the floor plate (DV) and Fgf8 from the isthmus (AP). Gli3, a zinc-finger transcription factor in the Shh signaling pathway, has been implicated in regulating Fgf8 expression and is therefore a potential candidate for coordinating the action of the two organizers. By inactivating mouse Gli3 at successive embryonic time points in vivo, we uncovered the extent and the underlying mechanism of Gli3 function in the mes/r1. We demonstrate that before E9.0, Gli3 is required for establishing a distinct posterior tectum, isthmus and cerebellum, but does not play a role in the development of the tegmentum. Between E9.0 and E11.0, Gli3 continues to be required for isthmus and cerebellum development, but primarily for defining the cerebellar foliation pattern. We show that Gli3 regulates patterning of the isthmus and cerebellar anlage by confining Fgf8 expression to the isthmus, and attenuates growth of dorsal r1 (before E11.0) and the dorsal mes and isthmus (beyond E11.0) through regulation of cell proliferation and viability. In conclusion, our results show that Gli3 is essential for the coordinated three-dimensional patterning and growth of the dorsal mes/r1.

Regulation of the development of tectal neurons and their projections by transcription factors Brn3a and Pax7

The rostral part of the dorsal midbrain, known as the superior colliculus in mammals or the optic tectum in birds, receives a substantial retinal input and plays a diverse and important role in sensorimotor integration. However, little is known about the development of specific subtypes of neurons in the tectum, particularly those which contribute tectofugal projections to the thalamus, isthmic region, and hindbrain. Here we show that two homeodomain transcription factors, Brn3a and Pax7, are expressed in mutually exclusive patterns in the developing and mature avian midbrain. Neurons expressing these factors are generated at characteristic developmental times, and have specific laminar fates within the tectum. In mice expressing betagalactosidase targeted to the Pou4f1 (Brn3a) locus, Brn3a-expressing neurons contribute to the ipsilateral but not the contralateral tectofugal projections to the hindbrain. Using misexpression of Brn3a and Pax7 by electroporation in the chick tectum, combined with GFP reporters, we show that Brn3a determines the laminar fate of subsets of tectal neurons. Furthermore, Brn3a regulates the development of neurons contributing to specific ascending and descending tectofugal pathways, while Pax7 globally represses the development of tectofugal projections to nearly all brain structures.

Molecular physiology of pituitary development: signaling and transcriptional networks

The pituitary gland is a central endocrine organ regulating basic physiological functions, including growth, the stress response, reproduction, metabolic homeostasis, and lactation. Distinct hormone-producing cell types in the anterior pituitary arise from a common ectodermal primordium during development by extrinsic and intrinsic mechanisms, providing a powerful model system for elucidating general principles in mammalian organogenesis. The central purpose of this review is to inspect the integrated signaling and transcriptional events that affect precursor proliferation, cell lineage commitment, terminal differentiation, and physiological regulation by hypothalamic tropic factors.

Differences in neurogenic potential in floor plate cells along an anteroposterior location: midbrain dopaminergic neurons originate from mesencephalic floor plate cells

Directed differentiation and purification of mesencephalic dopaminergic(mesDA) neurons from stem cells are crucial issues for realizing safe and efficient cell transplantation therapies for Parkinson's disease. Although recent studies have identified the factors that regulate mesDA neuron development, the mechanisms underlying mesDA neuron specification are not fully understood. Recently, it has been suggested that mesencephalic floor plate (FP) cells acquire neural progenitor characteristics to generate mesDA neurons. Here, we directly examined this in a fate mapping experiment using fluorescence-activated cell sorting (FACS) with an FP cell-specific surface marker, and demonstrate that mesencephalic FP cells have neurogenic activity and generate mesDA neurons in vitro. By contrast, sorted caudal FP cells have no neurogenic potential, as previously thought. Analysis of dreher mutant mice carrying a mutation in the Lmx1a locus and transgenic mice ectopically expressing Otx2 in caudal FP cells demonstrated that Otx2 determines anterior identity that confers neurogenic activity to FP cells and specifies a mesDA fate, at least in part through the induction of Lmx1a. We further show that FACS can isolate mesDA progenitors, a suitable transplantation material, from embryonic stem cell-derived neural cells. Our data provide insights into the mechanisms of specification and generation of mesDA neurons, and illustrate a useful cell replacement approach for Parkinson's disease.

Gene expression analysis of the hedgehog signaling cascade in the chick midbrain and spinal cord

The signaling molecule Sonic Hedgehog (SHH) plays a critical role in patterning the ventral midbrain of vertebrates. Our recent studies have established that the requirement for Hedgehog (HH) signaling in the chick midbrain is modulated spatially and temporally in a complex manner across the midbrain anlage. Unfortunately, the patterns of expression of downstream regulators that might modulate the HH signal in the midbrain are not currently known. To fill this gap, we have examined across time, the expression pattern of 14 genes that function in the HH signaling cascade in the midbrain and spinal cord. Our results suggest that SHH expression in the axial mesendoderm begins before the expression of known HH receptors/HH-binding proteins (e.g., PTC1, PTC2, HHIP, BOC, MEGALIN). In the midbrain, PTC and GLI genes are expressed and then eliminated very early from the ventral midline. However, they exhibit high and persistent expression in the midbrain region circumscribing the SHH source. Intriguingly, multiple HH-binding proteins (BOC, MEGALIN) and HH effectors (GLI1-3, SMO, SUFU, DZIP) are expressed in the dorsal midbrain and the midbrain-hindbrain boundary. Finally, we report for the first time that IHH is expressed in intermediate regions of the spinal cord, where its expression does not overlap with that of SHH.

Genetic control of basal midbrain development

Recent breakthroughs in the understanding of the genetic program that controls specification of ventral cell fate in the spinal cord and hindbrain have produced useful tools for the study of similar genetic networks in the more complex rostral regions of the central nervous system. Several research groups have elucidated key factors in the potential signaling processes, as well as transcription factors necessary for differentiation of various basal midbrain nuclei. Importantly, there has been substantial progress in understanding the genetic cascade involved in specifying dopaminergic neurons. This knowledge will be crucial in the understanding and possible treatment of Parkinson's disease.

Morphogen to mitogen: the multiple roles of hedgehog signalling in vertebrate neural development

Sonic hedgehog has received an enormous amount of attention since its role as a morphogen that directs ventral patterning in the spinal cord was discovered a decade ago. Since that time, a bewildering array of information has been generated concerning both the components of the hedgehog signalling pathway and the remarkable number of contexts in which it functions. Nowhere is this more evident than in the nervous system, where hedgehog signalling has been implicated in events as disparate as axonal guidance and stem cell maintenance. Here we review our present knowledge of the hedgehog signalling pathway and speculate about areas in which further insights into this versatile pathway might be forthcoming.

Columnar projections from the cholinergic nucleus isthmi to the optic tectum in chicks (Gallus gallus): a possible substrate for synchronizing tectal channels

The cholinergic division of the avian nucleus isthmi, the homolog of the mammalian nucleus parabigeminalis, is composed of the pars parvocellularis (Ipc) and pars semilunaris (SLu). Ipc and SLu were studied with in vivo and in vitro tracing and intracellular filling methods. 1) Both nuclei have reciprocal homotopic connections with the ipsilateral optic tectum. The SLu connection is more diffuse than that of Ipc. 2) Tectal inputs to Ipc and SLu are Brn3a-immunoreactive neurons in the inner sublayer of layer 10. Tectal neurons projecting on Ipc possess "shepherd's crook" axons and radial dendritic fields in layers 2-13. 3) Neurons in the mid-portion of Ipc possess a columnar spiny dendritic field. SLu neurons have a large, nonoriented spiny dendritic field. 4) Ipc terminals form a cylindrical brush-like arborization (35-50 microm wide) in layers 2-10, with extremely dense boutons in layers 3-6, and a diffuse arborization in layers 11-13. SLu neurons terminate in a wider column (120-180 microm wide) lacking the dust-like boutonal features of Ipc and extend in layers 4c-13 with dense arborizations in layers 4c, 6, and 9-13. 5) Ipc and SLu contain specialized fast potassium ion channels. We propose that dense arborizations of Ipc axons may be directed to the distal dendritic bottlebrushes of motion detecting tectal ganglion cells (TGCs). They may provide synchronous activation of a group of adjacent bottlebrushes of different TGCs of the same type via their intralaminar processes, and cross channel activation of different types of TGCs within the same column of visual space.

Classical Embryological Studies and Modern Genetic Analysis of Midbrain and Cerebellum Development

The brain is a remarkably complex anatomical structure that contains a diverse array of subdivisions, cell types, and synaptic connections. It is equally extraordinary in its physiological properties, as it constantly evaluates and integrates external stimuli as well as controls a complicated internal environment. The brain can be divided into three primary broad regions: the forebrain, midbrain (Mb), and hindbrain (Hb), each of which contain further subdivisions. The regions considered in this chapter are the Mb and most-anterior Hb (Mb/aHb), which are derived from the mesencephalon (mes) and rhombomere 1 (r1), respectively. The dorsal Mb consists of the laminated superior colliculus and the globular inferior colliculus (Fig. 1A and B), which modulate visual and auditory stimuli, respectively. The dorsal component of the aHb is the highly foliated cerebellum (Cb), which is primarily attributed to controlling motor skills (Fig. 1A and B). In contrast, the ventral Mb/aHb (Fig. 1B) consists of distinct clusters of neurons that together comprise a network of nuclei and projections-notably, the Mb dopaminergic and Hb serotonergic and Mb/aHb cholinergic neurons (Fig. 1G and H), which modulate a collection of behaviors, including movement, arousal, feeding, wakefulness, and emotion. Historically, the dorsal Mb and Cb have been studied using the chick as a model system because of the ease of performing both cell labeling and tissue transplants in the embryo in ovo; currently DNA electroporation techniques are also used. More recently the mouse has emerged as a powerful genetic system with numerous advantages to study events underpinning Mb/aHb development. There is a diverse array of spontaneous mutants with both Mb- and Cb-related phenotypes. In addition, numerous gene functions have been enumerated in mouse, gene expression is similar across vertebrates, and powerful genetic tools have been developed. Finally, additional insight into Mb/aHb function has been gained from studies of genetic diseases, such as Parkinson's disease, schizophrenia, cancer, and Dandy Walker syndrome, that afflict the Mb/aHb in humans and have genetic counterparts in mouse. Accordingly, this chapter discusses a spectrum of experiments, including classic embryology, in vitro assays, sophisticated genetic methods, and human diseases. We begin with an overview of Mb and aHb anatomy and physiology and mes/r1 gene expression patterns. We then provide a summary of fate-mapping studies that collectively demonstrate the complex cell behaviors that occur while the Mb and aHb primordia are established during embryogenesis and discuss the integration of both anterior-posterior (A-P) and dorsal-ventral (D-V) patterning. Finally, we describe some aspects of postnatal development and some of the insights gained from human diseases.

Potential role of Pax-2 in retinal axon navigation through the chick optic nerve stalk and optic chiasm

The degree of fiber decussation at the optic chiasm differs between species, ranging from complete crossing in lower vertebrates to highly complex patterns of intermingling of the fibers from the two eyes seen in mammals and birds. Understanding the genetic control of fiber guidance through the chiasm is therefore important to unravel the developmental mechanisms within the visual system. Here we first report on early stages of chiasm formation, with pioneering axons from the left eye consistently arriving earlier than their counterparts from the right eye. This initial left-right asymmetry is transient and no functional significance is assigned to it yet. Secondly, we examined formation of the chiasm in relation with the expression of the transcription factor Pax-2 along the ventral eye cup and optic nerve stalk. Finally, in order to examine causal involvement of Pax-2 in chiasm formation, the gene was overexpressed along the neuraxis and in the eye cup at embryonic stages preceding the exit of axons from the eye, and hence arrival of axons at the chiasm. When studied with neuroanatomical tracing, Pax-2 overexpression resulted in visibly anomalous decussation of axons at the chiasm. A likely consequence of this perturbation was erroneous arrival of axons at the tectum, as observed by anterograde staining from the retina. These data suggest that balanced expression of Pax-2 results in the correct formation of the chick chiasm at early stages by imposing accurate pathfinding within the optic stalk and the midline.

Helt, a Novel Basic-Helix-Loop-Helix Transcriptional Repressor Expressed in the Developing Central Nervous System

Neuronal differentiation is regulated by many basic-helix-loop-helix (bHLH) family transcriptional activators and repressors, and the balance of activity between these factors is important for the differentiation process. Here, we report the identification of a novel transcriptional repressor, designated Helt. Helt encoded a Hey-related bHLH protein containing the bHLH and Orange domains. Helt could homodimerize, and heterodimerize with Hes5 or Hey2. Both the bHLH and Orange domains were involved in the homodimerization. In contrast, only the bHLH domain was required for the heterodimerization with Hey2, whereas only the Orange domain mediated the interaction between Helt and Hes5. Thus, Helt has two dimerization domains, and these domains independently select a partner. Identification of preferred recognition sequences by CASTing experiments revealed that Helt bound to the E box, which was distinct from the Hes1 optimal sequence around the E box core. Not only the core sequence but also sequences flanking the E box were essential for the recognition by Helt and Hes1. Furthermore, Helt repressed transcription from an artificial promoter through binding to the optimal E box elements, as well as transcription from its own promoter. Using in situ hybridization and immunohistochemistry, Helt expression in embryos was investigated. Helt was mainly expressed in undifferentiated neural progenitors in some of the developing brain regions, including the mesencephalon and diencephalon, at the neurogenesis stage. These results suggest that Helt acts as a transcriptional repressor to regulate neuronal differentiation and/or identity.

Sonic hedgehog regulates the position of the trigeminal ganglia

The trigeminal ganglia differentiate in part from specialized ectodermal structures in the embryonic head termed the trigeminal placodes. However, the signals which govern the migration of trigeminal precursors and the final morphology of the ganglia are poorly defined. Here, we show that notochord or floor plate tissue can induce the formation of ectopic sensory ganglia adjacent to the developing dorsal mesencephalon. Neurons within these ganglia coexpress the transcription factors Brn3a and Islet, which together characterize primary sensory neurons throughout the developing embryo. The ectopic ganglia originate from Pax3-expressing regions of the surface ectoderm that normally contribute to the ophthalmic trigeminal (op5), and can only be induced at developmental stages during which op5 precursors are present in the mesencephalic region. The migration of trigeminal precursors is also blocked by a local source of recombinant Shh, while in mouse embryos lacking Shh, these cells continue to migrate until they fuse into a single ganglion at the ventral midline. Together, these results suggest that Shh acts to arrest the migration of sensory precursors rather than to induce sensory neurons de novo. Consistent with this hypothesis, Shh induces the expression of the proteoglycan PG-M/versican in the cranial mesoderm, which has been previously implicated in the regulation of the movement of sensory neural precursors.