Coordinated temporal and spatial control of motor neuron and serotonergic neuron generation from a common pool of CNS progenitors

Insm1 (IA-1) is an essential component of the regulatory network that specifies monoaminergic neuronal phenotypes in the vertebrate hindbrain

Monoaminergic neurons include the physiologically important central serotonergic and noradrenergic subtypes. Here, we identify the zinc-finger transcription factor, Insm1, as a crucial mediator of the differentiation of both subtypes, and in particular the acquisition of their neurotransmitter phenotype. Insm1 is expressed in hindbrain progenitors of monoaminergic neurons as they exit the cell cycle, in a pattern that partially overlaps with the expression of the proneural factor Ascl1. Consistent with this, a conserved cis-regulatory sequence associated with Insm1 is bound by Ascl1 in the hindbrain, and Ascl1 is essential for the expression of Insm1 in the ventral hindbrain. In Insm1-null mutant mice, the expression of the serotonergic fate determinants Pet1, Lmx1b and Gata2 is markedly downregulated. Nevertheless, serotonergic precursors begin to differentiate in Insm1 mutants, but fail to produce serotonin because of a failure to activate expression of tryptophan hydroxylase 2 (Tph2), the key enzyme of serotonin biosynthesis. We find that both Insm1 and Ascl1 coordinately specify Tph2 expression. In brainstem noradrenergic centres of Insm1 mutants, expression of tyrosine hydroxylase is delayed in the locus coeruleus and is markedly deficient in the medullary noradrenergic nuclei. However, Insm1 is dispensable for the expression of a second key noradrenergic biosynthetic enzyme, dopamine beta-hydroxylase, which is instead regulated by Ascl1. Thus, Insm1 regulates the synthesis of distinct monoaminergic neurotransmitters by acting combinatorially with, or independently of, Ascl1 in specific monoaminergic populations.

Congenital central hypoventilation syndrome (CCHS) and sudden infant death syndrome (SIDS): kindred disorders of autonomic regulation

Congenital central hypoventilation syndrome (CCHS) and sudden infant death syndrome (SIDS) were long considered rare disorders of respiratory control and more recently have been highlighted as part of a growing spectrum of disorders within the rubric of autonomic nervous system (ANS) dysregulation (ANSD). CCHS typically presents in the newborn period with a phenotype including alveolar hypoventilation, symptoms of ANSD and, in a subset of cases, Hirschsprung disease and later tumors of neural crest origin. Study of genes related to autonomic dysregulation and the embryologic origin of the neural crest led to the discovery of PHOX2B as the disease-defining gene for CCHS. Like CCHS, SIDS is thought to result from central deficits in control of breathing and ANSD, although SIDS risk is most likely defined by complex multifactorial genetic and environmental interactions. Some early genetic and neuropathological evidence is emerging to implicate serotonin systems in SIDS risk. The purpose of this article is to review the current understanding of the genetic basis for CCHS and SIDS, and discuss the impact of this information on clinical practice and future research directions.

Intraflagellar transport/Hedgehog-related signaling components couple sensory cilium morphology and serotonin biosynthesis in Caenorhabditis elegans

Intraflagellar transport in cilia has been proposed as a crucial mediator of Hedgehog signal transduction during embryonic pattern formation in both vertebrates and invertebrates. Here, we show that the Hh receptor Patched-related factor DAF-6 and intraflagellar transport modulate serotonin production in Caenorhabditis elegans animals, by remodeling the architecture of dendritic cilia of a pair of ADF serotonergic chemosensory neurons. Wild-type animals under aversive environment drastically reduce DAF-6 expression in glia-like cells surrounding the cilia of chemosensory neurons, resulting in cilium structural remodeling and upregulation of the serotonin-biosynthesis enzyme tryptophan hydroxylase tph-1 in the ADF neurons. These cellular and molecular modifications are reversed when the environment improves. Mutants of daf-6 or intraflagellar transport constitutively upregulate tph-1 expression. Epistasis analyses indicate that DAF-6/intraflagellar transport and the OCR-2/OSM-9 TRPV channel act in concert, regulating two layers of activation of tph-1 in the ADF neurons. The TRPV signaling turns on tph-1 expression under favorable and aversive conditions, whereas inactivation of DAF-6 by stress results in further upregulation of tph-1 independently of OCR-2/OSM-9 activity. Behavioral analyses suggest that serotonin facilitates larval animals resuming development when the environment improves. Our study revealed the cilium structure of serotonergic neurons as a trigger of regulated serotonin production, and demonstrated that a Hedgehog-related signaling component is dynamically regulated by environment and underscores neuroplasticity of serotonergic neurons in C. elegans under stress and stress recovery.

Characterization of primary neurospheres generated from mouse ventral rostral hindbrain

Serotonergic (5-HT) neurons of the reticular formation play a key role in the modulation of behavior, and their dysfunction is associated with severe neurological and psychiatric disorders, such as depression and schizophrenia. However, the molecular mechanisms underlying the differentiation of the progenitor cells and the specification of the 5-HT phenotype are not fully understood. A primary neurosphere cell-culture system from mouse ventral rostral hindbrain at embryonic day 12 was therefore established. The generated primary neurospheres comprised progenitor cells and fully differentiated neurons. Bromodeoxyuridine incorporation experiments in combination with immunocytochemistry for neural markers revealed the proliferation capacity of the neural multipotent hindbrain progenitors within neurospheres and their ability to differentiate toward the neuronal lineage and serotonergic phenotype. Gene expression analysis by reverse transcription with the polymerase chain reaction showed that the neurospheres were regionally specified, as reflected by the expression of the transcription factors Gata2 and Pet1. Treatment of dissociated primary neurospheres with exogenous Shh significantly increased the number of 5-HT-immunopositive cells compared with controls, whereas neutralization of endogenous Shh significantly decreased the number of 5-HT neurons. Thus, the primary neurosphere culture system presented here allows the expansion of hindbrain progenitor cells and the experimental control of their differentiation toward the serotonergic phenotype. This culture system is therefore a useful model for in vitro studies dealing with the development of 5-HT neurons.

The Drosophila Nkx6 homeodomain protein has both activation and repression domains and can activate target gene expression

Consistent with the common role of Nkx6 family members in specifying motor neuron identity, we show that over-expression of Drosophila Nkx6 results in an increase in the number of Fasiclin II expressing motor neurons in the intersegmental nerve B branch. Our dissection of the regulatory domains of Nkx6 using chimeric cell culture assays revealed the presence of two repression domains and a single activation domain within this transcription factor. As well as its conserved homeodomain, Nkx6 also has a candidate Engrailed homology 1 (Eh1) domain that is conserved amongst all NKx6 family members, through which vertebrate NKx6-type proteins bind the co-repressor, Groucho (Muhr, J., et al., 2001. Groucho-mediated transcriptional repression establishes progenitor cell pattern and neuronal fate in the ventral neural tube. Cell 104, 861-73). Paralleling our previous reports that the Eh1 domain of Vnd and Ind are ineffective in Gal4 chimeric assays (Von Ohlen, T., Syu, L.J., Mellerick, D.M., 2007. Conserved properties of the Drosophila homeodomain protein. Ind. Mech. Dev. 124, 925-934; Yu, Z., et al., 2005. Contextual interactions determine whether the Drosophila homeodomain protein, Vnd, acts as a repressor or activator. Nucleic Acids Res. 33, 1-12), we found that the Eh1 domain of Nkx6 did not significantly enhance repression in Gal4 chimeric assays. However, when we performed co-immunoprecipitation analyses, we found that Nkx6 can bind Groucho and that binding of Nkx6 to this co-repressor is modulated intra-molecularly. Full length Nkx6 interacted with Groucho poorly, because sequences at the carboxyl terminal of NKx6 interfere with Groucho binding, despite the presence of the Eh1 domain. In contrast, a carboxyl terminal Nkx6 deletion bound Groucho strongly. In keeping with the presence of an activation domain within Nkx6, we also report that Nkx6 can activate reporter expression driven by an Nkx6.1 enhancer that mediates auto-activation in transient transfection assays. The presence of multiple repression domains in Nkx6 supports Nkx6's role as a repressor, potentially using both Groucho-dependent and independent mechanisms. Thus, Nkx6 likely functions as a dual regulator in embryos.

Expression profile of differentiating serotonin neurons derived from rhesus embryonic stem cells and comparison to adult serotonin neurons

The rhesus monkey embryonic stem cell line 366.4 differentiates into serotonin neurons. We examined the genetic cascade during differentiation and compared ESC-derived serotonin neurons to adult monkey serotonin neurons. RNA was extracted from ESC colonies, embryoid bodies (EBs), neurospheres in selection (N1) and proliferation stages (N2), differentiated serotonin neurons (N3) and from laser captured (LC) serotonin neurons of spayed female macaques treated with placebo, estrogen (E), progesterone (P) or E+P. The RNA was labeled and hybridized to Rhesus Monkey Affymetrix Gene Chips (n=1 per stage and 2 per animal treatment). Gene expression was examined with GeneSifter software. 545 genes that were related to developmental processes showed a threefold or greater change between stages. TGFb, Wnt, VEGF and Hedgehog signaling pathways showed the highest percent of probe set changes during differentiation. Genes in the categories (a) homeobox binding and transcription factors, (b) growth factors and receptors, (c) brain and neural specific factors and (d) serotonin specific factors are reported. Pivotal genes were confirmed with quantitative RT-PCR. In the serotonin developmental cascade, FGFR2 was robustly expressed at each stage. GATA3 was robustly expressed in EBs. Sonic hedgehog (Shh), PTCH (Shh-R) and Fev1 transcription factor expression coincided with the induction of serotonin specific marker genes during N1-selection. A majority of the examined genes were expressed in adult serotonin neurons. However, in the ESC-derived neurons, there was significant over-representation of probe sets related to cell cycle, axon guidance & dorso-ventral axis formation. This analysis suggests that the 366.4 cell line possesses cues for serotonin differentiation at early stages of differentiation, but that ESC-derived serotonin neurons are still immature.

Axonal projections originating from raphe serotonergic neurons in the developing and adult zebrafish, Danio rerio, using transgenics to visualize raphe-specific pet1 expression

Serotonin is a major central nervous modulator of physiology and behavior and plays fundamental roles during development and plasticity of the vertebrate central nervous system (CNS). Understanding the developmental control and functions of serotonergic neurons is therefore an important task. In all vertebrates, prominent serotonergic neurons are found in the superior and inferior raphe nuclei in the hindbrain innervating most CNS regions. In addition, all vertebrates except for mammals harbor other serotonergic centers, including several populations in the diencephalon. This, in combination with the intricate and wide distribution of serotonergic fibers, makes it difficult to sort out serotonergic innervation originating from the raphe from that of other serotonergic cell populations. To resolve this issue, we isolated the regulatory elements of the zebrafish raphe-specific gene pet1 and used them to drive expression of an eGFP transgene in the raphe population of serotonergic neurons. With this approach together with retrograde tracing we 1) describe in detail the development, anatomical organization, and projection pattern of zebrafish pet1-positive neurons compared with their mammalian counterparts, 2) identify a new serotonergic population in the ventrolateral zebrafish hindbrain, and 3) reveal some extent of functional subdivisions within the zebrafish superior raphe complex. Together, our results reveal for the first time the specific innervation pattern of the zebrafish raphe and, thus, provide a new model and various tools to investigate further the role of raphe serotonergic neurons in vertebrates.

Hox genes and segmentation of the vertebrate hindbrain

In the vertebrate central nervous system, the hindbrain is an important center for coordinating motor activity, posture, equilibrium, sleep patterns, and essential unconscious functions, such as breathing rhythms and blood circulation. During development, the vertebrate hindbrain depends upon the process of segmentation or compartmentalization to create and organize regional properties essential for orchestrating its highly conserved functional roles. The process of segmentation in the hindbrain differs from that which functions in the paraxial mesoderm to generate somites and the axial skeleton. In the prospective hindbrain, cells in the neural epithelia transiently alter their ability to interact with their neighbors, resulting in the formation of seven lineage-restricted cellular compartments. These different segments or rhombomeres each go on to adopt unique characters in response to environmental signals. The Hox family of transcription factors is coupled to this process. Overlapping or nested patterns of Hox gene expression correlate with segmental domains and provide a combinatorial code and molecular framework for specifying the unique identities of hindbrain segments. The segmental organization and patterns of Hox expression and function are highly conserved among vertebrates and, as a consequence, comparative studies between different species have greatly enhanced our ability to build a picture of the regulatory cascades that control early hindbrain development. The purpose of this chapter is to review what is known about the regulatory mechanisms which establish and maintain Hox gene expression and function in hindbrain development.

Hox genes in neural patterning and circuit formation in the mouse hindbrain

The mammalian hindbrain is the seat of regulation of several vital functions that involve many of the organ systems of the body. Such functions are controlled through the activity of intricate arrays of neuronal circuits and connections. The establishment of ordered patterns of neuronal specification, migration, and axonal topographic connectivity during development is crucial to build such a complex network of circuits and functional connectivity in the mature hindbrain. The early development of the vertebrate hindbrain proceeds according to a fundamental metameric partitioning along the anteroposterior axis into cellular compartments known as rhombomeres. Such an organization has been highly conserved in vertebrate evolution and has a fundamental impact on the hindbrain adult structure, nuclear organization, and connectivity. Here, we review the cellular and molecular mechanisms underlying hindbrain neuronal circuitry in the mouse, with a specific focus on the role of the homeodomain transcription factors of the Hox gene family. The Hox genes are crucial determinants of rhombomere segmental identity and anteroposterior patterning. However, recent findings suggest that, in addition to their well-known roles at early embryonic stages, the Hox genes may play important roles also in later aspect of neuronal circuit development, including stereotypic neuronal migration, axon pathfinding, and topographic mapping of connectivity.

Role of chemoreceptors in mediating dyspnea

Dyspnea, or the uncomfortable awareness of respiratory distress, is a common symptom experienced by most people at some point during their lifetime. It is commonly encountered in individuals with pulmonary disease, such as chronic obstructive pulmonary disease (COPD), but can also be seen in healthy individuals after strenuous exercise, at altitude or in response to psychological stress. Dyspnea is a multifactorial sensation involving the brainstem, cortex, and limbic system, as well as mechanoreceptors, irritant receptors and chemoreceptors. Chemoreceptors appear to contribute to the sensation of dyspnea in two ways. They stimulate the respiratory control system in response to hypoxia and/or hypercapnia, and the resultant increase respiratory motor output can be consciously perceived as unpleasant. They also can induce the sensation of dyspnea through an as yet undetermined mechanism-potentially via direct ascending connections to the limbic system and cortex. The goal of this article is to briefly review how changes in blood gases reach conscious awareness and how chemoreceptors are involved in dyspnea.

Serotonergic transcription of human FEV reveals direct GATA factor interactions and fate of Pet-1-deficient serotonin neuron precursors

Altered expression of the human FEV (fifth Ewing variant) ETS transcription factor gene impacts the level of CNS serotonin (5-HT) neuron gene expression and maternal nurturing. However, the regulatory mechanisms that determine FEV expression are poorly understood. Here, we investigated the cis-regulatory control of FEV to begin to identify the upstream transcription factors that restrict FEV expression to 5-HT neurons. We find that sequences extending only 275 bp upstream of the FEV 5' untranslated region are sufficient to direct FEV transgene expression to embryonic 5-HT neurons, although sequences farther upstream are required for maintenance in adult 5-HT neurons. Two highly conserved consensus GATA factor binding sites within the 275 bp region interact with GATA factors in vitro. Chromatin immunoprecipitations with embryonic hindbrain demonstrated Gata-2 interactions with the orthologous mouse Pet-1 ETS cis-regulatory region. Mutagenesis of GATA sites revealed that one or the other site is required for serotonergic FEV transgene expression. Unexpectedly, FEV-LacZ transgenes enabled determination of 5-HT neuron precursor fate in the adult Pet-1(-/-) dorsal and median raphe nuclei and thus provided additional insight into FEV/Pet-1 function. Comparable numbers of FEV-LacZ-positive cells were detected in Pet-1(+/-) and Pet-1(-/-) adult dorsal raphe nuclei, indicating that the majority of mutant serotonergic precursors are not fated to apoptosis. However, B7 dorsal raphe cells were aberrantly distributed, suggesting a role for FEV/Pet-1 in their midline organization. Our findings identify a direct transcriptional interaction between Gata-2 and FEV and a unique marker for new insight into FEV/Pet-1 function in 5-HT neuron development.

The evolution of cell types in animals: emerging principles from molecular studies

Cell types are fundamental units of multicellular life but their evolution is obscure. How did the first cell types emerge and become distinct in animal evolution? What were the sets of cell types that existed at important evolutionary nodes that represent eumetazoan or bilaterian ancestors? How did these ancient cell types diversify further during the evolution of organ systems in the descending evolutionary lines? The recent advent of cell type molecular fingerprinting has yielded initial insights into the evolutionary interrelationships of cell types between remote animal phyla and has allowed us to define some first principles of cell type diversification in animal evolution.

Temporal control of neuronal diversity: common regulatory principles in insects and vertebrates?

It is well established in species as diverse as insects and mammals that different neuronal and glial subtypes are born at distinct times during central nervous system development. In Drosophila, there is now compelling evidence that individual multipotent neuroblasts express a sequence of progenitor transcription factors which, in turn, regulates the postmitotic transcription factors that specify neuronal/glial temporal identities. Here, we examine the hypothesis that the regulatory principles underlying this mode of temporal specification are shared between insects and mammals, even if some of the factors themselves are not. We also propose a general model for birth-order-dependent neural specification and suggest some experiments to test its validity.

Diffusion tensor imaging demonstrates brainstem and cerebellar abnormalities in congenital central hypoventilation syndrome

Congenital central hypoventilation syndrome (CCHS) patients show reduced breathing drive during sleep, decreased hypoxic and hypercapnic ventilatory responses, and autonomic and affective deficits, suggesting both brainstem and forebrain injuries. Forebrain damage was previously described in CCHS, but methodological limitations precluded detection of brainstem injury, a concern because genetic mutations in CCHS target brainstem autonomic nuclei. To assess brainstem and cerebellar areas, we used diffusion tensor imaging-based measures, namely axial diffusivity, reflecting water diffusion parallel to fibers, and sensitive to axonal injury, and radial diffusivity, measuring diffusion perpendicular to fibers, and indicative of myelin injury. Diffusion tensor imaging was performed in 12 CCHS and 26 controls, and axial and radial diffusivity maps were compared between groups using analysis of covariance (covariates; age and gender). Increased axial diffusivity in CCHS appeared within the lateral medulla and clusters with injury extended from the dorsal midbrain through the periaqueductal gray, raphé, and superior cerebellar decussation, ventrally to the basal-pons. Cerebellar cortex and deep nuclei, and the superior and inferior cerebellar peduncles showed increased radial diffusivity. Midbrain, pontine, and lateral medullary structures, and the cerebellum and its fiber systems are injured in CCHS, likely contributing to the characteristics found in the syndrome.

Identification of Phox2b-regulated genes by expression profiling of cranial motoneuron precursors

Background

Branchiomotor neurons comprise an important class of cranial motor neurons that innervate the branchial-arch-derived muscles of the face, jaw and neck. They arise in the ventralmost progenitor domain of the rhombencephalon characterized by expression of the homeodomain transcription factors Nkx2.2 and Phox2b. Phox2b in particular plays a key role in the specification of branchiomotor neurons. In its absence, generic neuronal differentiation is defective in the progenitor domain and no branchiomotor neurons are produced. Conversely, ectopic expression of Phox2b in spinal regions of the neural tube promotes cell cycle exit and neuronal differentiation and, at the same time, induces genes and an axonal phenotype characteristic for branchiomotor neurons. How Phox2b exerts its pleiotropic functions, both as a proneural gene and a neuronal subtype determinant, has remained unknown.

Results

To gain further insights into the genetic program downstream of Phox2b, we searched for novel Phox2b-regulated genes by cDNA microarray analysis of facial branchiomotor neuron precursors from heterozygous and homozygous Phox2b mutant embryos. We selected for functional studies the genes encoding the axonal growth promoter Gap43, the Wnt antagonist Sfrp1 and the transcriptional regulator Sox13, which were not previously suspected to play roles downstream of Phox2b and whose expression was affected by Phox2b misexpression in the spinal cord. While Gap43 did not produce an obvious phenotype when overexpressed in the neural tube, Sfrp1 induced the interneuron marker Lhx1,5 and Sox13 inhibited neuronal differentiation. We then tested whether Sfrp1 and Sox13, which are down-regulated by Phox2b in the facial neuron precursors, would antagonize some aspects of Phox2b activity. Co-expression of Sfrp1 prevented Phox2b from repressing Lhx1,5 and alleviated the commissural axonal phenotype. When expressed together with Sox13, Phox2b was still able to promote cell cycle exit and neuronal differentiation, but the cells failed to relocate to the mantle layer and to extinguish the neural stem cell marker Sox2.

Conclusion

Our results suggest novel roles for Sfrp1 and Sox13 in neuronal subtype specification and generic neuronal differentiation, respectively, and indicate that down-regulation of Sfrp1 and Sox13 are essential aspects of the genetic program controlled by Phox2b in cranial motoneurons.

Signals from the neural crest regulate beta-cell mass in the pancreas

Pancreatic islet cells and neurons share common functions and similar ontogenies, but originate in different germ layers. To determine whether ectoderm-derived cells contribute instructive signals to the developing endoderm-derived pancreas, we defined the chronology of migration and differentiation of neural crest cells in the pancreas, and tested their role in the development of the islets. The homeodomain transcription factor Phox2b marks the neural precursors from the neural crest that colonize the gut to form the enteric nervous system. In the embryonic mouse pancreas, we found Phox2b expressed briefly together with Sox10 along the epithelial-mesenchymal border at E12.5 in cells derived from the neural crest. Downregulation of Phox2b shortly thereafter was dependent upon Nkx2.2 expressed in the adjacent pancreatic epithelium. In Phox2b-/- embryos, neurons and glia did not develop in the pancreas, and Nkx2.2 expression was markedly upregulated in the epithelium. In addition, the number and replication rate of insulin-expressing beta-cells increased in the Phox2b-/-mice. We conclude that, during pancreatic development, Phox2b and Nkx2.2 form a non-cell-autonomous feedback loop that links the neural crest with the pancreatic epithelium, regulates the size of the beta-cell population, and thereby impacts insulin-secretory capacity and energy homeostasis.

Hoxb1 controls cell fate specification and proliferative capacity of neural stem and progenitor cells

The directed differentiation of embryonic stem cells (ESCs) into neural stem cells (NSCs) of specific identities and the identification of endogenous pathways that may mediate expansion of NSCs are fundamental goals for the treatment of degenerative disorders and trauma of the nervous system. We report that timely induction of a Hoxb1 transgene in ESC-derived NSCs resulted in the specification of NSCs toward a hindbrain-specific identity through the activation of a rhombomere 4-specific genetic program and the repression of anterior neural identity. This change was accompanied by changes in signaling pathways that pattern the dorsoventral (DV) axis of the nervous system and concomitant changes in the expression of DV neural progenitor markers. Furthermore, Hoxb1 mediated the maintenance and expansion of posterior neural progenitor cells. Hoxb1(+) cells kept proliferating upon mitogen withdrawal and became transiently amplifying progenitors instead of terminally differentiating. This was partially attributed to Hoxb1-dependent activation of the Notch signaling pathway and Notch-dependent STAT3 phosphorylation at Ser 727, thus linking Hox gene function with maintenance of active Notch signaling and the JAK/STAT pathway. Thus, timely expression of specific Hox genes could be used to establish NSCs and neural progenitors of distinct posterior identities. ESC-derived NSCs have a mixed DV identity that is subject to regulation by Hox genes. Finally, these findings set the stage for the elucidation of molecular pathways involved in the expansion of posterior NSCs and neural progenitors. Disclosure of potential conflicts of interest is found at the end of this article.

Out FOXing Parkinson disease: where development meets neurodegeneration

The central survival role of FOX proteins may allow a unified view of the genetic and environmental factors that cause Parkinson disease.

Zebrafish homologue irx1a is required for the differentiation of serotonergic neurons

Serotonergic (5HT) neurons produce neurotransmitter serotonin, which modulates various neuronal circuits. The specification and differentiation of 5HT neurons require both extrinsic signals such as Shh and Fgf, as well as intrinsic transcription factors such as nkx2.2, mash1, phox2b, Gata2, and pet1. In this study, we show that iroquois homeodomain factor irx1a, but not irx1b, is expressed in the 5HT neurons. Knockdown of irx1a by antisense morpholino nucleotides reveals that it is a critical determinant for the differentiation of 5HT neurons in the hindbrain. However, irx1a morphants do not show a reduction of the progenitors of 5HT neurons. Hence, irx1a is not required for the initial specification but it is required for the complete differentiation of 5HT neurons.

Neurospheres derived from human embryoid bodies treated with retinoic Acid show an increase in nestin and ngn2 expression that correlates with the proportion of tyrosine hydroxylase-positive cells

In the central nervous system (CNS), generation of phenotypic diversity within the neuronal lineage is precisely regulated in a spatial and temporal fashion. Neural basic helix-loop-helix (bHLH) transcription factors are cell intrinsic factors that control commitment to neuronal lineage and play an important role in neuronal cell type specification. The ability to differentiate human embryonic stem (hES) cells into neurons provides a good model system to address human neuronal specification. Previous studies have shown neurogenin-2 (Ngn2) to be involved in the development of mesencephalic dopaminergic neurons. Toward the goal of correlating neuronal phenotype with early gene expression pattern, we have characterized the expression of Ngn2 during hES cell differentiation. Our results show that treatment of embryoid bodies (EBs) with retinoic acid (RA) leads to the greatest proportion of tyrosine hydroxylase (TH)-positive cells followed by vasoactive intestinal peptide (VIP)-treated EBs as compared to untreated EBs. This increase in the proportion of TH-positive neurons was correlated with the unique morphology of RA-treated aggregates and the spatial delocalization of the expression of Ngn2 within the EB. Neurospheres derived from RA-treated EBs contained many nestin-positive cells within regions that expressed Ngn2. We show that the extent of nestin-positive cells that arise from the region of Ngn2 expression is correlated with the appearance of TH-positive neurons. Our results show for the first time the expression of Ngn2 during the differentiation of hES cells.

Patterning and axon guidance of cranial motor neurons

The cranial motor nerves control muscles involved in eye, head and neck movements, feeding, speech and facial expression. The generic and specific properties of cranial motor neurons depend on a matrix of rostrocaudal and dorsoventral patterning information. Repertoires of transcription factors, including Hox genes, confer generic and specific properties on motor neurons, and endow subpopulations at various axial levels with the ability to navigate to their targets. Cranial motor axon projections are guided by diffusible cues and aided by guideposts, such as nerve exit points, glial cells and muscle primordia. The recent identification of genes that are mutated in human cranial dysinnervation disorders is now shedding light on the functional consequences of perturbations of cranial motor neuron development.

Transcriptional repression coordinates the temporal switch from motor to serotonergic neurogenesis

In many regions of the developing CNS, distinct cell types are born at different times. The means by which discrete and stereotyped temporal switches in cellular identities are acquired remains poorly understood. To address this, we have examined how visceral motor neurons (VMNs) and serotonergic neurons, two neuronal subtypes, are sequentially generated from a common progenitor pool in the vertebrate hindbrain. We found that the forkhead transcription factor Foxa2, acting in progenitors, is essential for the transition from VMN to serotonergic neurogenesis. Loss-of-function and gain-of-function experiments indicated that Foxa2 activates the switch through a temporal cross-repressive interaction with paired-like homeobox 2b (Phox2b), the VMN progenitor determinant. This mechanism bears a marked resemblance to the cross-repression between neighboring domains of transcription factors that establish discrete progenitor identities along the spatial axes. Moreover, the subsequent differentiation of central serotonergic neurons required both the suppression of VMN neurogenesis and the induction of downstream intrinsic determinants of serotonergic identity by Foxa2.

Central nervous system distribution of the transcription factor Phox2b in the adult rat

Phox2b is required for development of the peripheral autonomic nervous system and a subset of cranial nerves and lower brainstem nuclei. Phox2b mutations in man cause diffuse autonomic dysfunction and deficits in the automatic control of breathing. Here we study the distribution of Phox2b in the adult rat hindbrain to determine whether this protein is selectively expressed by neurons involved in respiratory and autonomic control. In the medulla oblongata, Phox2b-immunoreactive nuclei were present in the dorsal vagal complex, intermediate reticular nucleus, dorsomedial spinal trigeminal nucleus, nucleus ambiguus, catecholaminergic neurons, and retrotrapezoid nucleus (RTN). Phox2b was expressed by both central excitatory relays of the sympathetic baroreflex (nucleus of the solitary tract and C1 neurons) but not by the inhibitory relay of this reflex. Phox2b was absent from the ventral respiratory column (VRC) caudal to RTN and rare within the parabrachial nuclei. In the pons, Phox2b was confined to cholinergic efferent neurons (salivary, vestibulocochlear) and noncholinergic peritrigeminal neurons. Rostral to the pons, Phox2b was detected only in the oculomotor complex. In adult rats, Phox2b is neither a comprehensive nor a selective marker of hindbrain autonomic pathways. This marker identifies a subset of hindbrain neurons that control orofacial movements (dorsomedial spinal trigeminal nucleus, pontine peritrigeminal neurons), balance and auditory function (vestibulocochlear efferents), the eyes, and both divisions of the autonomic efferent system. Phox2b is virtually absent from the respiratory rhythm and pattern generator (VRC and dorsolateral pons) but is highly expressed by neurons involved in the chemical drive and reflex regulation of this oscillator.

Fate-mapping the mammalian hindbrain: segmental origins of vestibular projection neurons assessed using rhombomere-specific Hoxa2 enhancer elements in the mouse embryo

As a step toward generating a fate map of identified neuron populations in the mammalian hindbrain, we assessed the contributions of individual rhombomeres to the vestibular nuclear complex, a major sensorimotor area that spans the entire rhombencephalon. Transgenic mice harboring either the lacZ or the enhanced green fluorescent protein reporter genes under the transcriptional control of rhombomere-specific Hoxa2 enhancer elements were used to visualize rhombomere-derived domains. We labeled functionally identifiable vestibular projection neuron groups retrogradely with conjugated dextran-amines at successive embryonic stages and obtained developmental fate maps through direct comparison with the rhombomere-derived domains in the same embryos. The fate maps show that each vestibular neuron group derives from a unique rostrocaudal domain that is relatively stable developmentally, suggesting that anteroposterior migration is not a major contributor to the rostrocaudal patterning of the vestibular system. Most of the groups are multisegmental in origin, and each rhombomere is fated to give rise to two or more vestibular projection neuron types, in a complex pattern that is not segmentally iterated. Comparison with studies in the chicken embryo shows that the rostrocaudal patterning of identified vestibular projection neuron groups is generally well conserved between avians and mammalians but that significant species-specific differences exist in the rostrocaudal limits of particular groups. This mammalian hindbrain fate map can be used as the basis for targeting genetic manipulation to specific subpopulations of vestibular projection neurons.

The serotonergic phenotype is acquired by converging genetic mechanisms within the zebrafish central nervous system

To gain knowledge about the developmental origin of serotonergic precursors and the regulatory cascades of serotonergic differentiation in vertebrates, we determined the spatiotemporal expression profile of the Ets-domain transcription factor-encoding gene pet1 in developing and adult zebrafish. We show that it is an early, specific marker of raphe serotonergic neurons, but not of other serotonergic populations. We then use pet1 expression together with tracing techniques to demonstrate that serotonergic neurons of rhombomeres (r) 1-2 largely originate from a progenitor pool at the midbrain-hindbrain boundary. Furthermore, by combining expression analyses of pet1 and the raphe tryptophan hydroxylase (Tph2) with rhombomere identity markers, we show that anterior and posterior hindbrain clusters of serotonergic precursors are separated by r3, rather than r4 as in other vertebrates. Our findings establish the origin of r1-2 serotonergic precursors, and strengthen the evidence for molecular, ontogenic and phylogenic heterogeneities among the vertebrate brain serotonergic cell populations.

Sudden Infant Death Syndrome: review of implicated genetic factors

Genetic studies in Sudden Infant Death Syndrome (SIDS) have been motivated by clinical, epidemiological, and/or neuropathological observations in SIDS victims, with subsequent pursuit of candidate genes in five categories: (1) genes for ion channel proteins based on electrocardiographic evidence of prolonged QT intervals in SIDS victims, (2) gene for serotonin transporter based on decreased serotonergic receptor binding in brainstems of SIDS victims, (3) genes pertinent to the early embryology of the autonomic nervous system (ANS) (and with a link to the 5-HT system) based on reports of ANS dysregulation in SIDS victims, (4) genes for nicotine metabolizing enzymes based on evidence of cigarette smoking as a modifiable risk factor for SIDS, and (5) genes regulating inflammation, energy production, hypoglycemia, and thermal regulation based on reports of postnatal infection, low birth weight, and/or overheating in SIDS victims. Evidence for each of these classes of candidate genes is reviewed in detail. As this review indicates, a number of genetically controlled pathways appear to be involved in at least some cases of SIDS. Given the diversity of results to date, genetic studies support the clinical impression that SIDS is heterogeneous with more than one entity and with more than one possible genetic etiology. Future studies should consider expanded phenotypic features that might help clarify the heterogeneity and improve the predictive value of the identified genetic factors. Such features should be evaluated to the extent possible in both SIDS victims and their family members. With 2,162 infants dying from SIDS in 2003 in the U.S. alone, and improved but still imperfect parent and caretaker compliance with known modifiable risk factors for SIDS, it behooves clinicians, researchers, and parents to combine efforts to reach a common goal. The message of the "Back to Sleep" campaign needs to be re-introduced/re-engineered to reach families and caretakers of all ethnic groups. Clinicians and researchers need to gently inform new SIDS parents about the opportunity to contribute tissue to the NICHD-funded University of Maryland Brain and Tissue Bank. By expanding the network of clinicians, scientists, and families working together, and by combined efforts in a collaborative multi-center study of candidate genes and/or genomics, the discovery of the genetic profile of the infant at risk for SIDS can ultimately be determined.

Molecular architecture of annelid nerve cord supports common origin of nervous system centralization in bilateria

To elucidate the evolutionary origin of nervous system centralization, we investigated the molecular architecture of the trunk nervous system in the annelid Platynereis dumerilii. Annelids belong to Bilateria, an evolutionary lineage of bilateral animals that also includes vertebrates and insects. Comparing nervous system development in annelids to that of other bilaterians could provide valuable information about the common ancestor of all Bilateria. We find that the Platynereis neuroectoderm is subdivided into longitudinal progenitor domains by partially overlapping expression regions of nk and pax genes. These domains match corresponding domains in the vertebrate neural tube and give rise to conserved neural cell types. As in vertebrates, neural patterning genes are sensitive to Bmp signaling. Our data indicate that this mediolateral architecture was present in the last common bilaterian ancestor and thus support a common origin of nervous system centralization in Bilateria.

Specification and differentiation of serotonergic neurons

Serotonin is an important neurotransmitter with multiple functions in the whole central nervous system. Its synthesis, however, is restricted to a very limited number of cells in the brainstem raphe nuclei with a vast axonal network. These cells express markers of the serotonin lineage such as the rate-limiting enzyme in serotonin synthesis, tryptophan hydroxylase 2, the serotonin transporter, and the transcription factor Pet1. Pet1 together with Lmx1b, Nkx2.2, Mash1, Gata2, Gata3, and Phox2b form a transcriptional network, which specifies the differentiation of serotonergic neurons around embryonic day 11 in the mouse. These cells are generated in rhombomeres r1-r3 and r5-r7 caudal to the midbrain- hindbrain organizer under the control of the fibroblast growth factors 4 and 8 and sonic hedgehog (SHH) from precursors, which have produced motoneurons before. Because serotonin is a relevant pathophysiological factor in several neurological diseases such as bipolar disorder and depression tools to generate or maintain serotonergic neurons might be of therapeutic value. Such tools can be assessed in embryonic stem cells, which can be differentiated in vitro to produce serotonergic neurons. Culture systems for these cells including embryoid bodies based and monolayer differentiation have been established, which allows the generation of up to 50% serotonergic neurons in all neurons developed.

Pre-/post-otic rhombomeric interactions control the emergence of a fetal-like respiratory rhythm in the mouse embryo

How regional patterning of the neural tube in vertebrate embryos may influence the emergence and the function of neural networks remains elusive. We have begun to address this issue in the embryonic mouse hindbrain by studying rhythmogenic properties of different neural tube segments. We have isolated pre- and post-otic hindbrain segments and spinal segments of the mouse neural tube, when they form at embryonic day (E) 9, and grafted them into the same positions in stage-matched chick hosts. Three days after grafting, in vitro recordings of the activity in the cranial nerves exiting the grafts indicate that a high frequency (HF) rhythm (order: 10 bursts/min) is generated in post-otic segments while more anterior pre-otic and more posterior spinal territories generate a low frequency (LF) rhythm (order: 1 burst/min). Comparison with homo-specific grafting of corresponding chick segments points to conservation in mouse and chick of the link between the patterning of activities and the axial origin of the hindbrain segment. This HF rhythm is reminiscent of the respiratory rhythm known to appear at E15 in mice. We also report on pre-/post-otic interactions. The pre-otic rhombomere 5 prevents the emergence of the HF rhythm at E12. Although the nature of the interaction with r5 remains obscure, we propose that ontogeny of fetal-like respiratory circuits relies on: (i) a selective developmental program enforcing HF rhythm generation, already set at E9 in post-otic segments, and (ii) trans-segmental interactions with pre-otic territories that may control the time when this rhythm appears.

The columnar gene vnd is required for tritocerebral neuromere formation during embryonic brain development of Drosophila

In Drosophila, evolutionarily conserved transcription factors are required for the specification of neural lineages along the anteroposterior and dorsoventral axes, such as Hox genes for anteroposterior and columnar genes for dorsoventral patterning. In this report, we analyse the role of the columnar patterning gene ventral nervous system defective (vnd) in embryonic brain development. Expression of vnd is observed in specific subsets of cells in all brain neuromeres. Loss-of-function analysis focussed on the tritocerebrum shows that inactivation of vnd results in regionalized axonal patterning defects, which are comparable with the brain phenotype caused by mutation of the Hox gene labial (lab). However, in contrast to lab activity in specifying tritocerebral neuronal identity, vnd is required for the formation and specification of tritocerebral neural lineages. Thus, in early vnd mutant embryos, the Tv1-Tv5 neuroblasts, which normally express lab, do not form. Later in embryogenesis, vnd mutants show an extensive loss of lab-expressing cells because of increased apoptotic activity, resulting in a gap-like brain phenotype that is characterized by an almost complete absence of the tritocerebral neuromere. Correspondingly, genetic block of apoptosis in vnd mutant embryos partially restores tritocerebral cells as well as axon tracts. Taken together, our results indicate that vnd is required for the genesis and proper identity specification of tritocerebral neural lineages during embryonic brain development of Drosophila.

Expression of Hoxa2 in rhombomere 4 is regulated by a conserved cross-regulatory mechanism dependent upon Hoxb1

The Hoxa2 gene is an important component of regulatory events during hindbrain segmentation and head development in vertebrates. In this study we have used sequenced comparisons of the Hoxa2 locus from 12 vertebrate species in combination with detailed regulatory analyses in mouse and chicken embryos to characterize the mechanistic basis for the regulation of Hoxa2 in rhombomere (r) 4. A highly conserved region in the Hoxa2 intron functions as an r4 enhancer. In vitro binding studies demonstrate that within the conserved region three bipartite Hox/Pbx binding sites (PH1-PH3) in combination with a single binding site for Pbx-Prep/Meis (PM) heterodimers co-operate to regulate enhancer activity in r4. Mutational analysis reveals that these sites are required for activity of the enhancer, suggesting that the r4 enhancer from Hoxa2 functions in vivo as a Hox-response module in combination with the Hox cofactors, Pbx and Prep/Meis. Furthermore, this r4 enhancer is capable of mediating a response to ectopic HOXB1 expression in the hindbrain. These findings reveal that Hoxa2 is a target gene of Hoxb1 and permit us to develop a gene regulatory network for r4, whereby Hoxa2, along with Hoxb1, Hoxb2 and Hoxa1, is integrated into a series of auto- and cross-regulatory loops between Hox genes. These data highlight the important role played by direct cross-talk between Hox genes in regulating hindbrain patterning.

Reversal of Hox1 gene subfunctionalization in the mouse

In vertebrates, paralogous Hox genes play diverse biological roles. We examined the interchangeability of Hoxa1 and Hoxb1 in mouse development by swapping their protein-coding regions. Remarkably, the mice expressing the Hox-B1 protein from the Hoxa1 locus, and vice versa, are essentially normal. We noted, nonetheless, a specific facial nerve hypomorphism in hemizygous Hoxb1(A1/-) mice and decreased viability in homozygous Hoxa1(B1/B1) embryos. Further, we established a mouse line in which we have inserted the 107 bp Hoxb1 autoregulatory enhancer into the Hoxa1 promoter. Strikingly, the newly generated autoregulatory Hoxa1 gene can deliver the functionality of both paralogs in these mice, providing normal viability as well as proper facial nerve formation even in the Hoxb1 mutant background. This study affirms that subfunctionalization of the transcriptional regulatory elements has a principal role in the diversification of paralogous Hox genes. Moreover, we show that the ancestral vertebrate Hox1 gene can still be experimentally reconstructed.

Expression of Phox2b by brainstem neurons involved in chemosensory integration in the adult rat

Central congenital hypoventilation syndrome is caused by mutations of the gene that encodes the transcription factor Phox2b. The syndrome is characterized by a severe form of sleep apnea attributed to greatly compromised central and peripheral chemoreflexes. In this study, we analyze whether Phox2b expression in the brainstem respiratory network is preferentially associated with neurons involved in chemosensory integration in rats. At the very rostral end of the ventral respiratory column (VRC), Phox2b was present in many VGlut2 (vesicular glutamate transporter 2) mRNA-containing neurons. These neurons were functionally identified as the respiratory chemoreceptors of the retrotrapezoid nucleus (RTN). More caudally in the VRC, many fewer neurons expressed Phox2b. These cells were not part of the central respiratory pattern generator (CPG), because they were typically cholinergic visceral motor neurons or catecholaminergic neurons (presumed C1 neurons). Phox2b was not detected in serotonergic neurons, in the A5, A6, and A7 noradrenergic cell groups nor within the main cardiorespiratory centers of the dorsolateral pons. Phox2b was expressed by many solitary tract nucleus (NTS) neurons including those that relay peripheral chemoreceptor information to the RTN. These and previous observations by others suggest that Phox2b is expressed by an uninterrupted chain of neurons involved in the integration of peripheral and central chemoreception (carotid bodies, chemoreceptor afferents, chemoresponsive NTS neurons projecting to VRC, RTN chemoreceptors). The presence of Phox2b in this circuit and its apparent absence from the respiratory CPG could explain why Phox2b mutations disrupt breathing automaticity during sleep without causing major impairment of respiration during waking.

Stereospecificity and PAX6 function direct Hoxd4 neural enhancer activity along the antero-posterior axis

The antero-posterior (AP) and dorso-ventral (DV) patterning of the neural tube is controlled in part by HOX and PAX transcription factors, respectively. We have reported on a neural enhancer of Hoxd4 that directs expression in the CNS with the correct anterior border in the hindbrain. Comparison to the orthologous enhancer of zebrafish revealed seven conserved footprints including an obligatory retinoic acid response element (RARE), and adjacent sites D, E and F. Whereas enhancer function in the embryonic CNS is destroyed by separation of the RARE from sites D-E-F by a half turn of DNA, it is rescued by one full turn, suggesting stereospecific constraints between DNA-bound retinoid receptors and the factor(s) recognizing sites D-E-F. Alterations in the DV trajectory of the Hoxd4 anterior expression border following mutation of site D or E implicated transcriptional regulators active across the DV axis. We show that PAX6 specifically binds sites D and E in vitro, and use chromatin immunoprecipitation to demonstrate recruitment of PAX6 to the Hoxd4 neural enhancer in mouse embryos. Hoxd4 expression throughout the CNS is reduced in Pax6 mutant Sey(Neu) animals on embryonic day 8. Additionally, stage-matched zebrafish embryos having decreased pax6a and/or pax6b activity display malformed rhombomere boundaries and an anteriorized hoxd4a expression border. These results reveal an evolutionarily conserved role for Pax6 in AP-restricted expression of vertebrate Hoxd4 orthologs.

Gli2 and Gli3 play distinct roles in the dorsoventral patterning of the mouse hindbrain

Sonic Hedgehog (Shh) signaling plays a critical role during dorsoventral (DV) patterning of the developing neural tube by modulating the expression of neural patterning genes. Overlapping activator functions of Gli2 and Gli3 have been shown to be required for motoneuron development and correct neural patterning in the ventral spinal cord. However, the role of Gli2 and Gli3 in ventral hindbrain development is unclear. In this paper, we have examined DV patterning of the hindbrain of Shh(-/-), Gli2(-/-) and Gli3(-/-) embryos, and found that the respective role of Gli2 and Gli3 is not only different between the hindbrain and spinal cord, but also at distinct rostrocaudal levels of the hindbrain. Remarkably, the anterior hindbrain of Gli2(-/-) embryos displays ventral patterning defects as severe as those observed in Shh(-/-) embryos suggesting that, unlike in the spinal cord and posterior hindbrain, Gli3 cannot compensate for the loss of Gli2 activator function in Shh-dependent ventral patterning of the anterior hindbrain. Loss of Gli3 also results in a distinct patterning defect in the anterior hindbrain, including dorsal expansion of Nkx6.1 expression. Furthermore, we demonstrate that ventral patterning of rhombomere 4 is less affected by loss of Gli2 function revealing a different requirement for Gli proteins in this rhombomere. Taken together, these observations indicate that Gli2 and Gli3 perform rhombomere-specific function during DV patterning of the hindbrain.

Hoxa2 knockdown in Xenopus results in hyoid to mandibular homeosis

The skeletal structures of the face and throat are derived from cranial neural crest cells (NCCs) that migrate from the embryonic neural tube into a series of branchial arches (BAs). The first arch (BA1) gives rise to the upper and lower jaw cartilages, whereas hyoid structures are generated from the second arch (BA2). The Hox paralogue group 2 (PG2) genes, Hoxa2 and Hoxb2, show distinct roles for hyoid patterning in tetrapods and fishes. In the mouse, Hoxa2 acts as a selector of hyoid identity, while its paralogue Hoxb2 is not required. On the contrary, in zebrafish Hoxa2 and Hoxb2 are functionally redundant for hyoid arch patterning. Here, we show that in Xenopus embryos morpholino-induced functional knockdown of Hoxa2 is sufficient to induce homeotic changes of the second arch cartilage. Moreover, Hoxb2 is downregulated in the BA2 of Xenopus embryos, even though initially expressed in second arch NCCs, similar to mouse and unlike in zebrafish. Finally, Xbap, a gene involved in jaw joint formation, is selectively upregulated in the BA2 of Hoxa2 knocked-down frog embryos, supporting a hyoid to mandibular change of NCC identity. Thus, in Xenopus Hoxa2 does not act redundantly with Hoxb2 for BA2 patterning, similar to mouse and unlike in fish. These data bring novel insights into the regulation of Hox PG2 genes and hyoid patterning in vertebrate evolution and suggest that Hoxa2 function is required at late stages of BA2 development.

A Wnt1-regulated genetic network controls the identity and fate of midbrain-dopaminergic progenitors in vivo

Midbrain neurons synthesizing the neurotransmitter dopamine play a central role in the modulation of different brain functions and are associated with major neurological and psychiatric disorders. Despite the importance of these cells, the molecular mechanisms controlling their development are still poorly understood. The secreted glycoprotein Wnt1 is expressed in close vicinity to developing midbrain dopaminergic neurons. Here, we show that Wnt1 regulates the genetic network, including Otx2 and Nkx2-2, that is required for the establishment of the midbrain dopaminergic progenitor domain during embryonic development. In addition, Wnt1 is required for the terminal differentiation of midbrain dopaminergic neurons at later stages of embryogenesis. These results identify Wnt1 as a key molecule in the development of midbrain dopaminergic neurons in vivo. They also suggest the Wnt1-controlled signaling pathway as a promising target for new therapeutic strategies in the treatment of Parkinson's disease.

A differentially autoregulated Pet-1 enhancer region is a critical target of the transcriptional cascade that governs serotonin neuron development

The Pet-1 [pheochromocytoma 12 ETS (E26 transformation-specific)] gene plays a critical role in the development of serotonin (5-HT)-modulated behaviors via its control of embryonic 5-HT neuron differentiation. Pet-1 transcription is induced exclusively in 5-HT neuron postmitotic precursors before the appearance of transmitter, and its restricted expression is maintained in the adult. However, the mechanisms that direct Pet-1 expression to this single CNS neuronal cell type are unknown. Here, we show, using transgenic methods, that genomic sequences upstream, but not downstream or within the Pet-1-coding region, are sufficient for 5-HT neuron-specific transgene expression. Enhancer sequences within a 40 kb upstream fragment directed position-independent lacZ (beta-D-galactosidase) transgene expression to the developing hindbrain before the appearance of 5-HT. Moreover, virtually all of the 5-HT neurons in the adult were lacZ positive in all of the lines examined. Transgene expression in 5-HT neurons was maintained when the 40 kb fragment was truncated on its 5' end to either 12 or 1.8 kb, although position independence was then lost. Analysis of transgene expression in Pet-1 null mice indicated that Pet-1 was required to maintain the activity of the Pet-1 enhancer region in a subset of 5-HT neurons. These findings suggest that a conserved 1.8 kb region immediately flanking the Pet-1-coding region is a critical genomic target of the transcriptional cascade that governs 5-HT neuron development and provide additional evidence for 5-HT neuron heterogeneity at the genetic level. We discuss the potential application of the Pet-1 transgenes reported here to the selective genetic manipulation of 5-HT neurons.

GABAergic specification in the basal forebrain is controlled by the LIM-hd factor Lhx7

We present evidence for a temporal control of GABAergic neurotransmitter specification in the basal forebrain orchestrated by the LIM-homeodomain factor Lhx7. In Xenopus, using in vivo overexpression experiments, we show that x-Lhx7 and x-Nkx2.1 inhibit GABAergic specification in the Dlx-expressing areas of the forebrain (subpallium and diencephalon). In addition, x-Lhx7 almost totally represses GABAergic differentiation at early but not late embryonic stages in subpallial mouse primary neurons in culture, indicating that x-Lhx7 is not able to withdraw the GABAergic phenotype once it is acquired. Moreover, anatomical data show striking correlations between x-Lhx7 expression and the GABAergic/cholinergic phenotypes. These functional and anatomical observations suggest a sequential role for x-Lhx7 in neurotransmitter specification. Thus, x-Lhx7 would first prevent a pool of cells to become GABAergic early in development and then promote cholinergic differentiation later on in this pool. We propose two distinct modulatory roles for a single LIM-hd factor, depending on the developmental time window.

Mash1 and Math3 are required for development of branchiomotor neurons and maintenance of neural progenitors

Basic helix-loop-helix (bHLH) transcription factors are known to play important roles in neuronal determination and differentiation. However, their exact roles in neural development still remain to be determined because of the functional redundancy. Here, we examined the roles of neural bHLH genes Mash1 and Math3 in the development of trigeminal and facial branchiomotor neurons, which derive from rhombomeres 2-4. In Math3-null mutant mice, facial branchiomotor neurons are misspecified, and both trigeminal and facial branchiomotor neurons adopt abnormal migratory pathways. In Mash1;Math3 double-mutant mice, trigeminal and facial branchiomotor neurons are severely reduced in number partly because of increased apoptosis. In addition, neurons with migratory defects are intermingled over the midline from either side of the neural tube. Furthermore, oligodendrocyte progenitors of rhombomere 4 are reduced in number. In the absence of Mash1 and Math3, expression of Notch signaling components is severely downregulated in rhombomere 4 and neural progenitors are not properly maintained, which may lead to intermingling of neurons and a decrease in oligodendrocyte progenitors. These results indicate that Mash1 and Math3 not only promote branchiomotor neuron development but also regulate the subsequent oligodendrocyte development and the cytoarchitecture by maintaining neural progenitors through Notch signaling.

Neural tube patterning by Krox20 and emergence of a respiratory control

Recent data begin to bridge the gap between developmental events controlling hindbrain neural tube regional patterning and the emergence of breathing behaviour in the fetus and its vital adaptive function after birth. In vertebrates, Hox paralogs and Hox-regulating genes orchestrate, in a conserved manner, the transient formation of developmental compartments in the hindbrain, the rhombomeres, in which rhythmic neuronal networks of the brainstem develop. Genetic inactivation of some of these genes in mice leads to pathological breathing at birth pointing to the vital importance of rhombomere 3 and 4 derived territories for maintenance of the breathing frequency. In chick embryo at E7, we investigated neuronal activities generated in neural tube islands deriving from combinations of rhombomeres isolated at embryonic day E1.5. Using a gain of function approach, we reveal a role of the transcription factor Krox20, specifying rhombomeres 3 and 5, in inducing a rhythm generator at the parafacial level of the hindbrain. The developmental genes selecting and regionally coordinating the fate of CNS progenitors may hold further clues to conserved aspects of neuronal network formation and function. However, the most immediate concern is to take advantage of early generated rhythmic activities in the hindbrain to pursue their downstream cellular and molecular targets, for it seems likely that it will be here that rhythmogenic properties will eventually take on a vital role at birth.

Cloning and analysis of Nkx6.3 during CNS and gastrointestinal development

Members of the Nkx family of homeodomain proteins are involved in a variety of developmental processes such as cell fate determination in the CNS and in the pancreas. Here we describe the cloning and developmental expression pattern of Nkx6.3, a new member of the Nkx6 subfamily of homeodomain proteins. Nkx6.3 is expressed in the developing CNS and gastro-intestinal tract. In contrast to Nkx6.1 and Nkx6.2 that are broadly expressed in ventral positions of the developing CNS, Nkx6.3 shows a remarkably selective expression in a subpopulation of differentiating V2 neurons at caudal hindbrain levels. The expression of Nkx6.3 at this level depends on the activity of other Nkx6 proteins. In the gut, Nkx6.3 is expressed in duodenal and glandular stomach endoderm and at the end of gestation Nkx6.3 became restricted to the base of the gastric units in the glandular stomach. The expression of Nkx6.3 overlapped with the expression of Nkx6.2 both in the CNS and in the gut. Transient Nkx6.2 expression was also detected in the developing pancreas. However, analysis of Nkx6.2(-/-) mice did not display any obvious aberrations of pancreatic or stomach development.

A genetic approach to access serotonin neurons for in vivo and in vitro studies

Serotonin (5HT) is a critical modulator of neural circuits that support diverse behaviors and physiological processes, and multiple lines of evidence implicate abnormal serotonergic signaling in psychiatric pathogenesis. The significance of 5HT underscores the importance of elucidating the molecular pathways involved in serotonergic system development, function, and plasticity. However, these mechanisms remain poorly defined, owing largely to the difficulty of accessing 5HT neurons for experimental manipulation. To address this methodological deficiency, we present a transgenic route to selectively alter 5HT neuron gene expression. This approach is based on the ability of a Pet-1 enhancer region to direct reliable 5HT neuron-specific transgene expression in the CNS. Its versatility is illustrated with several transgenic mouse lines, each of which provides a tool for 5HT neuron studies. Two lines allow Cre-mediated recombination at different stages of 5HT neuron development. A third line in which 5HT neurons are marked with yellow fluorescent protein will have numerous applications, including their electrophysiological characterization. To demonstrate this application, we have characterized active and passive membrane properties of midbrain reticular 5HT neurons, which heretofore have not been reported to our knowledge. A fourth line in which Pet-1 loss of function is rescued by expression of a Pet-1 transgene demonstrates biologically relevant levels of transgene expression and offers a route for investigating serotonergic protein structure and function in a behaving animal. These findings establish a straightforward and reliable approach for developing an array of tools for in vivo and in vitro studies of 5HT neurons.

Molecular genetics of the early development of hindbrain serotonergic neurons

The serotonergic (5HT) system plays a key role in modulating behaviors, such as appetite and anxiety and has been implicated in many human disorders of mood and mind. Recent studies have begun to identify the signaling molecules and transcriptional cascades governing 5HT neuron development in the hindbrain. Already at early stages, local differences in requirements of 5HT neuron development have become apparent. These studies point toward cryptic heterogeneity amongst 5HT neurons and suggest that 5HT neuron determination and differentiation may be more flexible and less absolute biologic processes than might have been expected. Ultimately, the intrinsic heterogeneity and environmental sensitivity of 5HT neurons may help explain the variability observed in some human behavioral disorders, such as autism spectrum disorder, and the less predictable behavioral consequences of fetal alcohol syndrome.

Cell-type specific regulation of serotonergic identity by the C. elegans LIM-homeodomain factor LIM-4

How a common neurotransmitter phenotype specified in neurons of different origins is an outstanding issue in neuronal development and function. In C. elegans larvae, serotonin is synthesized in 2 pairs of neurons, the secretory neurons NSM and the chemosensory neurons ADF. In order to delineate the molecular mechanisms of serotonergic phenotype establishment, we have screened for neuron-specific serotonin deficient (nss) mutants. Our prior study showed that the POU-homeodomain factor UNC-86 is expressed in and required for the NSM neurons to adopt serotonergic phenotype and correct pathfinding, whereas ADF are unaffected in unc-86-null mutants. Here, we report that the LIM-homeodomain factor LIM-4 regulates ADF serotonergic phenotype. In lim-4 mutants, many aspects of ADF differentiation occur, however, they fail to express serotonin phenotype and exhibit aberrant cilia properties. LIM-4 expression rises in the neuroblast that produces two distinct neurons: ADF and the olfactory neuron AWB. We show that lim-4 is regulated by separable mechanisms to determine disparate subtype identities in these two neuronal types. In vivo promoter analyses reveal that cis-element(s) within introns are necessary and sufficient to direct lim-4 to specify serotonergic phenotype, whereas its 5'-upstream sequence directs lim-4 function in AWB. Thus, a transcription factor may act independently to specify distinct differentiation traits in two sister cells. We propose that serotonergic identity is specified in cell-specific contexts to coordinate the development and function.

HoxB2 binds mutant SOD1 and is altered in transgenic model of ALS

Mutations in Cu/Zn superoxide dismutase (SOD1) cause approximately 20% of familial amyotrophic lateral sclerosis by a toxic gain of function; however, the precise mechanisms remain unclear. Here, we report the identification of HoxB2, a homeodomain-containing transcription factor, as a G93A mutant SOD1 interactive protein in a yeast two-hybrid screen. We show that HoxB2 co-precipitates and co-localizes with mutant SOD1 in neuronal cell lines, as well as in brain and spinal cord of G93A mutant SOD1 transgenic mice. Mutagenesis further shows that this interaction is mediated by the central homeodomain of HoxB2. In motor neuron-like NSC-34 cells, overexpression of HoxB2 or its homeodomain decreases the insolubility of mutant SOD1 and inhibits G93A or G86R mutant SOD1-induced neuronal cell death. In human and mouse tissues, we show that expression of HoxB2 persists in adult spinal cord and is primarily localized in nuclei of motor neurons. In G93A transgenic mice, HoxB2 co-localizes with mutant SOD1 and is redistributed to perikarya and proximal neurites of motor neurons. In addition, there is progressive accumulation of HoxB2 and mutant SOD1 as punctate inclusions in the neuropil surrounding motor neurons. Taken together, our findings demonstrate that interaction of HoxB2 with mutant SOD1 occurs in motor neurons of G93A mutant SOD1 transgenic mice and suggest that this interaction may modulate the neurotoxicity of mutant SOD1.

Pediatric disorders with autonomic dysfunction: what role for PHOX2B?

Hirschsprung disease, neuroblastomas, and congenital central hypoventilation syndrome can occur in combination, and familial cases have been reported in all three conditions. This suggests variable expression of a single genetic abnormality as the common cause to these neural crest disorders. Because the PHOX2B gene is pivotal in the development of most relays of the autonomic nervous system, including all autonomic neural crest derivatives, it was considered a candidate gene for the above conditions. Recent studies have shown that 1) PHOX2B is the main disease-causing gene for congenital central hypoventilation syndrome, an autosomal dominant disorder with incomplete penetrance; 2) PHOX2B is the first gene for which germline mutations have been demonstrated to predispose to neuroblastoma; and 3) Hirschsprung disease was associated with an intronic single-nucleotide polymorphism of the PHOX2B gene in a case-control study. For clarifying the variable clinical expression of the autonomic nervous system dysfunction observed in neural crest disorders, international databases of clinical symptoms and molecular test results should be established. Furthermore, the development of genetic mouse models should help to improve our understanding of the molecular mechanisms underlying neural crest disorders.

Hox genes and their candidate downstream targets in the developing central nervous system

1. Homeobox (Hox) genes were originally discovered in the fruit fly Drosophila, where they function through a conserved homeodomain as transcriptional regulators to control embryonic morphogenesis. Since then over 1000 homeodomain proteins have been identified in several species. In vertebrates, 39 Hox genes have been identified as homologs of the original Drosophila complex, and like their Drosophila counterparts they are organized within chromosomal clusters. Vertebrate Hox genes have also been shown to play a critical role in embryonic development as transcriptional regulators. 2. Both the Drosophila and vertebrate Hox genes have been shown to interact with various cofactors, such as the TALE homeodomain proteins, in recognition of consensus sequences within regulatory elements of their target genes. These protein-protein interactions are believed to contribute to enhancing the specificity of target gene recognition in a cell-type or tissue- dependent manner. The regulatory activity of a particular Hox protein on a specific regulatory element is highly variable and dependent on its interacting partners within the transcriptional complex. 3. In vertebrates, Hox genes display spatially restricted patterns of expression within the developing CNS, both along the anterioposterior and dorsoventral axis of the embryo. Their restricted gene expression is suggestive of a regulatory role in patterning of the CNS, as well as in cell specification. Determining the precise function of individual Hox genes in CNS morphogenesis through classical mutational analyses is complicated due to functional redundancy between Hox genes. 4. Understanding the precise mechanisms through which Hox genes mediate embryonic morphogenesis requires the identification of their downstream target genes. Although Hox genes have been implicated in the regulation of several pathways, few target genes have been shown to be under their direct regulatory control. Development of methodologies used for the isolation of target genes and for the analysis of putative targets will be beneficial in establishing the genetic pathways controlled by Hox factors. 5. Within the developing CNS various cell adhesion molecules and signaling molecules have been identified as candidate downstream target genes of Hox proteins. These targets play a role in processes such as cell migration and differentiation, and are implicated in contributing to neuronal processes such as plasticity and/or specification. Hence, Hox genes not only play a role in patterning of the CNS during early development, but may also contribute to cell specification and identity.