Transcriptional repression coordinates the temporal switch from motor to serotonergic neurogenesis

Uncovering the role of FOXA2 in the Development of Human Serotonin Neurons

Directed differentiation of serotonin neurons (SNs) from human pluripotent stem cells (hPSCs) provides a valuable tool for uncovering the mechanism of human SN development and the associated neuropsychiatric disorders. Previous studies report that FOXA2 is expressed by serotonergic progenitors (SNPs) and functioned as a serotonergic fate determinant in mouse. However, in the routine differentiation experiments, it is accidentally found that less SNs and more non-neuronal cells are obtained from SNP stage with higher percentage of FOXA2-positive cells. This phenomenon prompted them to question the role of FOXA2 as an intrinsic fate determinant for human SN differentiation. Herein, by direct differentiation of engineered hPSCs into SNs, it is found that the SNs are not derived from FOXA2-lineage cells; FOXA2-knockout hPSCs can still differentiate into mature and functional SNs with typical serotonergic identity; FOXA2 overexpression suppresses the SN differentiation, indicating that FOXA2 is not intrinsically required for human SN differentiation. Furthermore, repressing FOXA2 expression by retinoic acid (RA) and dynamically modulating Sonic Hedgehog (SHH) signaling pathway promotes human SN differentiation. This study uncovers the role of FOXA2 in human SN development and improves the differentiation efficiency of hPSCs into SNs by repressing FOXA2 expression.

Foxa2 and Pet1 Direct and Indirect Synergy Drive Serotonergic Neuronal Differentiation

Neuronal programming by forced expression of transcription factors (TFs) holds promise for clinical applications of regenerative medicine. However, the mechanisms by which TFs coordinate their activities on the genome and control distinct neuronal fates remain obscure. Using direct neuronal programming of embryonic stem cells, we dissected the contribution of a series of TFs to specific neuronal regulatory programs. We deconstructed the Ascl1-Lmx1b-Foxa2-Pet1 TF combination that has been shown to generate serotonergic neurons and found that stepwise addition of TFs to Ascl1 canalizes the neuronal fate into a diffuse monoaminergic fate. The addition of pioneer factor Foxa2 represses Phox2b to induce serotonergic fate, similar to in vivo regulatory networks. Foxa2 and Pet1 appear to act synergistically to upregulate serotonergic fate. Foxa2 and Pet1 co-bind to a small fraction of genomic regions but mostly bind to different regulatory sites. In contrast to the combinatorial binding activities of other programming TFs, Pet1 does not strictly follow the Foxa2 pioneer. These findings highlight the challenges in formulating generalizable rules for describing the behavior of TF combinations that program distinct neuronal subtypes.

Tumor Suppressor Role and Clinical Significance of the FEV Gene in Prostate Cancer

Background

In our previous research, we developed a 32-gene risk index model that may be utilized as a robust prognostic method for predicting prostate cancer (PCa) recurrence after surgery. Among the 32 genes, the Fifth Ewing Variant (FEV) gene was one of the top downregulated genes in relapsed PCa. However, current understanding of the FEV gene and its involvement in PCa is limited.

Methods

FEV mRNA expression was analyzed and correlated to clinical outcomes in PCa patients who underwent prostatectomy at the Massachusetts General Hospital. Specimens from tissue microarray (TMA) including 102 prostate cancer patients were analysis for the expression of FEV. Meanwhile, FEV expression profiles were also assessed in PCa cell lines and in BPH-1 prostate epithelial cells using western blotting and quantitative reverse transcription-PCR (qRT-PCR). Furthermore, we transfected LNCaP and PC-3 cells with either an empty vector or full-length FEV gene and performed in vitro cell functional assays. The part FEV plays in tumor xenograft growth was also assessed in vivo.

Results

Of the 191 patients included in this study base on the DASL dataset, 77 (40.3%) and 24 (13.6%), respectively, developed prostate-specific antigen (PSA) relapse and metastasis postradical prostatectomy. Significant FEV downregulation was observed in PCa patients showing PSA failure and metastasis. The protein expression of FEV was significantly negatively correlated with the Gleason score and pathological stage in prostate cancer tissues. Similarly, FEV expression significantly decreased in all PCa cell lines relative to BPH-1 (all P < 0.05). Functional assays revealed that FEV expression markedly inhibited PCa cell growth, migration, and invasion, which in turn significantly repressed the growth of tumor xenografts in vivo.

Conclusion

The results of this study suggest an association between downregulated FEV expression and PSA relapse in PCa patients. In addition, FEV may act as a tumor suppressor in PCa.

Generation of caudal-type serotonin neurons and hindbrain-fate organoids from hPSCs

Serotonin (5-HT) neurons, the major components of the raphe nuclei, arise from ventral hindbrain progenitors. Based on anatomical location and axonal projection, 5-HT neurons are coarsely divided into rostral and caudal groups. Here, we propose a novel strategy to generate hindbrain 5-HT neurons from human pluripotent stem cells (hPSCs), which involves the formation of ventral-type neural progenitor cells and stimulation of the hindbrain 5-HT neural development. A caudalizing agent, retinoid acid, was used to direct the cells into the hindbrain cell fate. Approximately 30%–40% of hPSCs successfully developed into 5-HT-expressing neurons using our protocol, with the majority acquiring a caudal rhombomere identity (r5–8). We further modified our monolayer differentiation system to generate 5-HT neuron-enriched hindbrain-like organoids. We also suggest downstream applications of our 5-HT monolayer and organoid cultures to study neuronal response to gut microbiota. Our methodology could become a powerful tool for future studies related to 5-HT neurotransmission.

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Activation of SHH and RA signaling induces 5-HT neuronal fate from hPSCs

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The generated 5-HT neurons have caudal hindbrain characteristics

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Hindbrain-like organoids may form from hPSCs by activation of SHH and RA signaling

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5-HT neurons in monolayer and organoid culture can be used as a screening platform

The 5-HTTLPR-rs25531 S-A-S-A Haplotype and Chronic Stress Moderate the Association Between Acute Stress and Internalizing Mental Disorders Among HIV+ Children and Adolescents in Uganda

Background: Internalizing mental disorders (IMDs) among HIV-positive (HIV+) children and adolescents are associated with poor disease outcomes, such as faster HIV disease progression. Although it has been suggested that the development of IMDs is moderated by interaction of stressful life events and vulnerability factors, the underlying etiology is largely unknown. Serotonin transporter gene [solute carrier family 6 member A4 (SLC6A4)] and human tryptophan hydroxylase 2 gene (TPH2) polymorphisms have been implicated in the development of IMDs. This study investigated the association between acute stress and IMDs, and moderation by chronic stress and genetic variants in SLC6A4 and TPH2. Hypothesis: Acute stress acts through genetic and environmental vulnerability factors to increase the risk of developing IMDs. Methods: Polymorphisms in SLC6A4 (5-HTTLPR, rs25531, 5-HTTLPR-rs25531, and STin2 VNTR) and TPH2 (rs1843809, rs1386494, rs4570625, and rs34517220) were genotyped in 368 HIV+ children and adolescents (aged 5-17 years) with any internalizing mental disorder (depression, anxiety disorders, or posttraumatic stress disorder), and 368 age- and sex-matched controls, who were also HIV+. Chronic and acute stress categories were derived by hierarchical cluster analysis. Logistic regression analysis was used to assess the independent moderating effect of chronic stress and each selected polymorphism on the association between acute stress and IMDs. Results: We observed a statistically significant association between severe acute stress and IMDs (p = 0.001). Children and adolescents who experienced severe acute stress were twice as likely to develop IMDs, compared to children and adolescents who experienced mild acute stress (p = 0.001). Chronic stress interacted with severe acute stress to increase the risk of IMDs (p = 0.033). Acute stress was found to interact with 5-HTTLPR-rs25531 S-A-S-A haplotype to increase the risk for IMDs among Ugandan HIV+ children and adolescents (p = 0.049). We found no evidence for a combined interaction of acute stress, chronic stress, and 5-HTTLPR-rs25531 on IMDs. Conclusion: The odds of having an internalizing mental disorder (IMD) were higher among HIV+ children and adolescents who experienced severe acute stress compared to HIV+ children and adolescents who experienced mild acute stress. Chronic stress and 5-HTTLPR-rs25531 independently moderated the association between acute stress and IMDs.

Epidermal Growth Factor in the CNS: A Beguiling Journey from Integrated Cell Biology to Multiple Sclerosis. An Extensive Translational Overview

This article reviews the wealth of papers dealing with the different effects of epidermal growth factor (EGF) on oligodendrocytes, astrocytes, neurons, and neural stem cells (NSCs). EGF induces the in vitro and in vivo proliferation of NSCs, their migration, and their differentiation towards the neuroglial cell line. It interacts with extracellular matrix components. NSCs are distributed in different CNS areas, serve as a reservoir of multipotent cells, and may be increased during CNS demyelinating diseases. EGF has pleiotropic differentiative and proliferative effects on the main CNS cell types, particularly oligodendrocytes and their precursors, and astrocytes. EGF mediates the in vivo myelinotrophic effect of cobalamin on the CNS, and modulates the synthesis and levels of CNS normal prions (PrP^Cs), both of which are indispensable for myelinogenesis and myelin maintenance. EGF levels are significantly lower in the cerebrospinal fluid and spinal cord of patients with multiple sclerosis (MS), which probably explains remyelination failure, also because of the EGF marginal role in immunology. When repeatedly administered, EGF protects mouse spinal cord from demyelination in various experimental models of autoimmune encephalomyelitis. It would be worth further investigating the role of EGF in the pathogenesis of MS because of its multifarious effects.

A Shh/Gli-driven three-node timer motif controls temporal identity and fate of neural stem cells

How time is measured by neural stem cells during temporal neurogenesis has remained unresolved. By combining experiments and computational modeling, we define a Shh/Gli-driven three-node timer underlying the sequential generation of motor neurons (MNs) and serotonergic neurons in the brainstem. The timer is founded on temporal decline of Gli-activator and Gli-repressor activities established through down-regulation of Gli transcription. The circuitry conforms an incoherent feed-forward loop, whereby Gli proteins not only promote expression of Phox2b and thereby MN-fate but also account for a delayed activation of a self-promoting transforming growth factor-β (Tgfβ) node triggering a fate switch by repressing Phox2b. Hysteresis and spatial averaging by diffusion of Tgfβ counteract noise and increase temporal accuracy at the population level, providing a functional rationale for the intrinsically programmed activation of extrinsic switch signals in temporal patterning. Our study defines how time is reliably encoded during the sequential specification of neurons.

Development and Differentiation of Midbrain Dopaminergic Neuron: From Bench to Bedside

Parkinson’s Disease (PD) is a neurodegenerative disorder affecting the motor system. It is primarily due to substantial loss of midbrain dopamine (mDA) neurons in the substantia nigra pars compacta and to decreased innervation to the striatum. Although existing drug therapy available can relieve the symptoms in early-stage PD patients, it cannot reverse the pathogenic progression of PD. Thus, regenerating functional mDA neurons in PD patients may be a cure to the disease. The proof-of-principle clinical trials showed that human fetal graft-derived mDA neurons could restore the release of dopamine neurotransmitters, could reinnervate the striatum, and could alleviate clinical symptoms in PD patients. The invention of human-induced pluripotent stem cells (hiPSCs), autologous source of neural progenitors with less ethical consideration, and risk of graft rejection can now be generated in vitro. This advancement also prompts extensive research to decipher important developmental signaling in differentiation, which is key to successful in vitro production of functional mDA neurons and the enabler of mass manufacturing of the cells required for clinical applications. In this review, we summarize the biology and signaling involved in the development of mDA neurons and the current progress and methodology in driving efficient mDA neuron differentiation from pluripotent stem cells.

Negative Regulation of mTOR Signaling Restricts Cell Proliferation in the Floor Plate

The neural tube is composed of a number of neural progenitors and postmitotic neurons distributed in a quantitatively and spatially precise manner. The floor plate, located in the ventral-most region of the neural tube, has a lot of unique characteristics, including a low cell proliferation rate. The mechanisms by which this region-specific proliferation rate is regulated remain elusive. Here we show that the activity of the mTOR signaling pathway, which regulates the proliferation of the neural progenitor cells, is significantly lower in the floor plate than in other domains of the embryonic neural tube. We identified the forkhead-type transcription factor FoxA2 as a negative regulator of mTOR signaling in the floor plate, and showed that FoxA2 transcriptionally induces the expression of the E3 ubiquitin ligase RNF152, which together with its substrate RagA, regulates cell proliferation via the mTOR pathway. Silencing of RNF152 led to the aberrant upregulation of the mTOR signal and aberrant cell division in the floor plate. Taken together, the present findings suggest that floor plate cell number is controlled by the negative regulation of mTOR signaling through the activity of FoxA2 and its downstream effector RNF152.

Lmx1b is required at multiple stages to build expansive serotonergic axon architectures

Formation of long-range axons occurs over multiple stages of morphological maturation. However, the intrinsic transcriptional mechanisms that temporally control different stages of axon projection development are unknown. Here, we addressed this question by studying the formation of mouse serotonin (5-HT) axons, the exemplar of long-range profusely arborized axon architectures. We report that LIM homeodomain factor 1b (Lmx1b)-deficient 5-HT neurons fail to generate axonal projections to the forebrain and spinal cord. Stage-specific targeting demonstrates that Lmx1b is required at successive stages to control 5-HT axon primary outgrowth, selective routing, and terminal arborization. We show a Lmx1b→Pet1 regulatory cascade is temporally required for 5-HT arborization and upregulation of the 5-HT axon arborization gene, Protocadherin-alphac2, during postnatal development of forebrain 5-HT axons. Our findings identify a temporal regulatory mechanism in which a single continuously expressed transcription factor functions at successive stages to orchestrate the progressive development of long-range axon architectures enabling expansive neuromodulation.

The serotonergic system and the control of breathing during development

Serotonin (5-hydroxytryptamine 5-HT) was first discovered in the late 1940's as an endogenous bioactive amine capable of inducing vasoconstriction, and in the mid-1950's was found in the brain. It was in these early years that some of the first demonstrations were made regarding a role for brain 5-HT in neurological function and behavior, including data implicating reduced brain levels of 5-HT in clinical depression. Since that time, advances in molecular biology and physiological approaches in basic science research have intensely focused on 5-HT in the brain, and the many facets of its role during embryonic development, post-natal maturation, and neural function in adulthood continues to be established. This review focuses on what is known about the developmental roles for the 5-HT system, which we define as the neurons producing 5-HT along with pre-and post-synaptic receptors, in a vital homeostatic motor behavior - the control of breathing. We will cover what is known about the embryonic origins and fate specification of 5-HT neurons, and how the 5-HT system influences pre- and post-natal maturation of the ventilatory control system. In addition, we will focus on the role of the 5-HT system in specific respiratory behaviors during fetal, neonatal and postnatal development, and the relevance of dysfunction in this system in respiratory-related human pathologies including Sudden Infant Death Syndrome (SIDS).

Altered enteric expression of the homeobox transcription factor Phox2b in patients with diverticular disease

Background

Diverticular disease, a major gastrointestinal disorder, is associated with modifications of the enteric nervous system, encompassing alterations of neurochemical coding and of the tyrosine receptor kinase Ret/GDNF pathway. However, molecular factors underlying these changes remain to be determined.

Objectives

We aimed to characterise the expression of Phox2b, an essential regulator of Ret and of neuronal subtype development, in the adult human enteric nervous system, and to evaluate its potential involvement in acute diverticulitis.

Methods

Site-specific gene expression of Phox2b in the adult colon was analysed by quantitative polymerase chain reaction. Colonic specimens of adult controls and patients with diverticulitis were subjected to quantitative polymerase chain reaction for Phox2b and dual-label immunochemistry for Phox2b and the neuronal markers RET and tyrosine hydroxylase or the glial marker S100β.

Results

The results indicate that Phox2b is physiologically expressed in myenteric neuronal and glial subpopulations in the adult enteric nervous system. Messenger RNA expression of Phox2b was increased in patients with diverticulitis and both neuronal, and glial protein expression of Phox2b were altered in these patients.

Conclusions

Alterations of Phox2b expression may contribute to the enteric neuropathy observed in diverticular disease. Future studies are required to characterise the functions of Phox2b in the adult enteric nervous system and to determine its potential as a therapeutic target in gastrointestinal disorders.

Talking back: Development of the olivocochlear efferent system

Developing sensory systems must coordinate the growth of neural circuitry spanning from receptors in the peripheral nervous system (PNS) to multilayered networks within the central nervous system (CNS). This breadth presents particular challenges, as nascent processes must navigate across the CNS-PNS boundary and coalesce into a tightly intermingled wiring pattern, thereby enabling reliable integration from the PNS to the CNS and back. In the auditory system, feedforward spiral ganglion neurons (SGNs) from the periphery collect sound information via tonotopically organized connections in the cochlea and transmit this information to the brainstem for processing via the VIII cranial nerve. In turn, feedback olivocochlear neurons (OCNs) housed in the auditory brainstem send projections into the periphery, also through the VIII nerve. OCNs are motor neuron-like efferent cells that influence auditory processing within the cochlea and protect against noise damage in adult animals. These aligned feedforward and feedback systems develop in parallel, with SGN central axons reaching the developing auditory brainstem around the same time that the OCN axons extend out toward the developing inner ear. Recent findings have begun to unravel the genetic and molecular mechanisms that guide OCN development, from their origins in a generic pool of motor neuron precursors to their specialized roles as modulators of cochlear activity. One recurrent theme is the importance of efferent-afferent interactions, as afferent SGNs guide OCNs to their final locations within the sensory epithelium, and efferent OCNs shape the activity of the developing auditory system. This article is categorized under: Nervous System Development > Vertebrates: Regional Development.

Serotonin neuron development: shaping molecular and structural identities

The continuing fascination with serotonin (5-hydroxytryptamine, 5-HT) as a nervous system chemical messenger began with its discovery in the brains of mammals in 1953. Among the many reasons for this decades-long interest is that the small numbers of neurons that make 5-HT influence the excitability of neural circuits in nearly every region of the brain and spinal cord. A further reason is that 5-HT dysfunction has been linked to a range of psychiatric and neurological disorders many of which have a neurodevelopmental component. This has led to intense interest in understanding 5-HT neuron development with the aim of determining whether early alterations in their generation lead to brain disease susceptibility. Here, we present an overview of the neuroanatomical organization of vertebrate 5-HT neurons, their neurogenesis, and prodigious axonal architectures, which enables the expansive reach of 5-HT neuromodulation in the central nervous system. We review recent findings that have revealed the molecular basis for the tremendous diversity of 5-HT neuron subtypes, the impact of environmental factors on 5-HT neuron development, and how 5-HT axons are topographically organized through disparate signaling pathways. We summarize studies of the gene regulatory networks that control the differentiation, maturation, and maintenance of 5-HT neurons. These studies show that the regulatory factors controlling acquisition of 5-HT-type transmitter identity continue to play critical roles in the functional maturation and the maintenance of 5-HT neurons. New insights are presented into how continuously expressed 5-HT regulatory factors control 5-HT neurons at different stages of life and how the regulatory networks themselves are maintained. WIREs Dev Biol 2018, 7:e301. doi: 10.1002/wdev.301 This article is categorized under: Nervous System Development > Vertebrates: General Principles Gene Expression and Transcriptional Hierarchies > Gene Networks and Genomics Gene Expression and Transcriptional Hierarchies > Cellular Differentiation Nervous System Development > Secondary: Vertebrates: Regional Development.

Serotonin in psychiatry: in vitro disease modeling using patient-derived neurons

Several lines of evidence implicate serotonin in the etiology of multiple psychiatric disorders, especially mood disorders, such as major depressive disorder (MDD) and bipolar disorder (BD). Much of our current understanding of biological mechanisms underlying serotonergic alterations in mood disorders comes from animal studies. Innovation in induced pluripotent stem cell and transdifferentiation technologies for deriving neurons from adult humans has enabled the study of disease-relevant cellular phenotypes in vitro. In this context, human serotonergic neurons can now be generated using three recently published methodologies. In this mini-review, we broadly discuss evidence linking altered serotonergic neurotransmission in MDD and BD and focus on recently published methods for generating human serotonergic neurons in vitro.

Association of regulatory TPH2 polymorphisms with higher reduction in depressive symptoms in children and adolescents treated with fluoxetine

Genetic variability related to the brain serotonergic system has a significant impact on both the susceptibility to psychiatric disorders, such as major depressive disorder (MDD), and the response to antidepressant drugs, such as fluoxetine. TPH2 is one of the most important serotonergic candidate genes in selective serotonin reuptake inhibitors (SSRIs) pharmacogenetic studies. The aim of the present study was to evaluate the influence of regulatory polymorphisms that are specifically located in human TPH2 transcription factor binding sites (TFBSs), and therefore could be functional by altering gene expression, on clinical improvement in children and adolescents treated with fluoxetine. The selection of SNPs was also based on their linkage disequilibrium with TPH2 rs4570625, a genetic variant with questionable functionality, which was previously associated with clinical response in our pediatric population. A total of 83 children and adolescents were clinically evaluated 12weeks after initiating antidepressant treatment with fluoxetine for the first time. Clinical improvement was assessed by reductions in depressive symptoms measured using the Children's Depression Inventory (CDI) scale. The polymorphisms rs11179002, rs60032326 and rs34517220 were, for the first time in the literature, significantly associated with higher clinical improvement. The strongest association was found for rs34517220. In particular, minor allele homozygotes showed higher score reductions on the CDI scale compared with the major allele carriers. Interestingly, this polymorphism is located in a human TPH2 TFBS for two relevant transcription factors in the serotoninergic neurons, Foxa1 and Foxa2, which together with the high level of significance found for this SNP, could indicate that rs34517220 is in fact the crucial functional genetic variant related to the fluoxetine response. These results provide new evidence for the role of regulatory genetic variants that could modulate human TPH2 expression in the SSRI antidepressant response.

Embryonic development of selectively vulnerable neurons in Parkinson's disease

A specific set of brainstem nuclei are susceptible to degeneration in Parkinson's disease. We hypothesise that neuronal vulnerability reflects shared phenotypic characteristics that confer selective vulnerability to degeneration. Neuronal phenotypic specification is mainly the cumulative result of a transcriptional regulatory program that is active during the development. By manual curation of the developmental biology literature, we comprehensively reconstructed an anatomically resolved cellular developmental lineage for the adult neurons in five brainstem regions that are selectively vulnerable to degeneration in prodromal or early Parkinson's disease. We synthesised the literature on transcription factors that are required to be active, or required to be inactive, in the development of each of these five brainstem regions, and at least two differentially vulnerable nuclei within each region. Certain transcription factors, e.g., Ascl1 and Lmx1b, seem to be required for specification of many brainstem regions that are susceptible to degeneration in early Parkinson's disease. Some transcription factors can even distinguish between differentially vulnerable nuclei within the same brain region, e.g., Pitx3 is required for specification of the substantia nigra pars compacta, but not the ventral tegmental area. We do not suggest that Parkinson's disease is a developmental disorder. In contrast, we consider identification of shared developmental trajectories as part of a broader effort to identify the molecular mechanisms that underlie the phenotypic features that are shared by selectively vulnerable neurons. Systematic in vivo assessment of fate determining transcription factors should be completed for all neuronal populations vulnerable to degeneration in early Parkinson's disease.

Cellular and Molecular Underpinnings of Neuronal Assembly in the Central Auditory System during Mouse Development

During development, the organization of the auditory system into distinct functional subcircuits depends on the spatially and temporally ordered sequence of neuronal specification, differentiation, migration and connectivity. Regional patterning along the antero-posterior axis and neuronal subtype specification along the dorso-ventral axis intersect to determine proper neuronal fate and assembly of rhombomere-specific auditory subcircuits. By taking advantage of the increasing number of transgenic mouse lines, recent studies have expanded the knowledge of developmental mechanisms involved in the formation and refinement of the auditory system. Here, we summarize several findings dealing with the molecular and cellular mechanisms that underlie the assembly of central auditory subcircuits during mouse development, focusing primarily on the rhombomeric and dorso-ventral origin of auditory nuclei and their associated molecular genetic pathways.

Gata2 and Gata3 regulate the differentiation of serotonergic and glutamatergic neuron subtypes of the dorsal raphe

Serotonergic and glutamatergic neurons of the dorsal raphe regulate many brain functions and are important for mental health. Their functional diversity is based on molecularly distinct subtypes; however, the development of this heterogeneity is poorly understood. We show that the ventral neuroepithelium of mouse anterior hindbrain is divided into specific subdomains giving rise to serotonergic neurons as well as other types of neurons and glia. The newly born serotonergic precursors are segregated into distinct subpopulations expressing vesicular glutamate transporter 3 (Vglut3) or serotonin transporter (Sert). These populations differ in their requirements for transcription factors Gata2 and Gata3, which are activated in the post-mitotic precursors. Gata2 operates upstream of Gata3 as a cell fate selector in both populations, whereas Gata3 is important for the differentiation of the Sert⁺ precursors and for the serotonergic identity of the Vglut3⁺ precursors. Similar to the serotonergic neurons, the Vglut3-expressing glutamatergic neurons, located in the central dorsal raphe, are derived from neural progenitors in the ventral hindbrain and express Pet1 Furthermore, both Gata2 and Gata3 are redundantly required for their differentiation. Our study demonstrates lineage relationships of the dorsal raphe neurons and suggests that functionally significant heterogeneity of these neurons is established early during their differentiation.

The dorsal spinal cord and hindbrain: From developmental mechanisms to functional circuits

Neurons of the dorsal hindbrain and spinal cord are central in receiving, processing and relaying sensory perception and participate in the coordination of sensory-motor output. Numerous cellular and molecular mechanisms that underlie neuronal development in both regions of the nervous system are shared. We discuss here the mechanisms that generate neuronal diversity in the dorsal spinal cord and hindbrain, and emphasize similarities in patterning and neuronal specification. Insight into the developmental mechanisms has provided tools that can help to assign functions to small subpopulations of neurons. Hence, novel information on how mechanosensory or pain sensation is encoded under normal and neuropathic conditions has already emerged. Such studies show that the complex neuronal circuits that control perception of somatosensory and viscerosensory stimuli are becoming amenable to investigations.

Generating human serotonergic neurons in vitro: Methodological advances

Technologies for deriving human neurons in vitro have transformed our ability to study cellular and molecular components of human neurotransmission. Three groups, including our own, have recently published methods for efficiently generating human serotonergic neurons in vitro. Remarkably, serotonergic neurons derived from each method robustly produce serotonin, express raphe genes, are electrically active, and respond to selective serotonin reuptake inhibitors in vitro. Two of the methods utilize transdifferentiation technology by overexpressing key serotonergic transcription factors. The third and most recent method involves differentiating induced pluripotent stem cells (iPSCs) to serotonergic neurons using developmental patterning cues. In this mini-review, we briefly describe the developmental programs governing serotonergic specification in vivo and how they have been harnessed to achieve serotonergic differentiation in vitro. We discuss the distinct and overlapping features of the recently published methodologies and their value in the context of in vitro disease modeling. Also see the video abstract here.

Developmental Pattern Formation in Phases

Cells in developing organs undergo a series of changes in their transcriptional state until a complete repertoire of cell types is specified. These changes in cell identity, together with the control of tissue growth, determine the pattern of gene expression in the tissue. Recent studies explore the dynamics of pattern formation during development and provide new insights into the control mechanisms. Changes in morphogen signalling and transcriptional networks control the specification of cell types. This is often followed by a distinct second phase, where pattern is elaborated by tissue growth. Here, we discuss the transitions between distinct phases in pattern formation. We consider the implications of the underlying mechanisms for understanding how reproducible patterns form during development.

Direct conversion of human fibroblasts to induced serotonergic neurons

Serotonergic (5HT) neurons exert diverse and widespread functions in the brain. Dysfunction of the serotonergic system gives rise to a variety of mental illnesses including depression, anxiety, obsessive compulsive disorder, autism and eating disorders. Here we show that human primary fibroblasts were directly converted to induced serotonergic (i5HT) neurons by the expression of Ascl1, Foxa2, Lmx1b and FEV. The transdifferentiation was enhanced by p53 knockdown and appropriate culture conditions including hypoxia. The i5HT neurons expressed markers for mature serotonergic neurons, had Ca(2+)-dependent 5HT release and selective 5HT uptake, exhibited spontaneous action potentials and spontaneous excitatory postsynaptic currents. Application of serotonin significantly increased the firing rate of spontaneous action potentials, demonstrating the functional utility of i5HT neurons for studying serotonergic neurotransmission. The availability of human i5HT neurons will be very useful for research and drug discovery on many serotonin-related mental disorders.

Neurogenin3 restricts serotonergic neuron differentiation to the hindbrain

The development of the nervous system is critically dependent on the production of functionally diverse neuronal cell types at their correct locations. In the embryonic neural tube, dorsoventral signaling has emerged as a fundamental mechanism for generating neuronal diversity. In contrast, far less is known about how different neuronal cell types are organized along the rostrocaudal axis. In the developing mouse and chick neural tube, hindbrain serotonergic neurons and spinal glutamatergic V3 interneurons are produced from ventral p3 progenitors, which possess a common transcriptional identity but are confined to distinct anterior-posterior territories. In this study, we show that the expression of the transcription factor Neurogenin3 (Neurog3) in the spinal cord controls the correct specification of p3-derived neurons. Gain- and loss-of-function manipulations in the chick and mouse embryo show that Neurog3 switches ventral progenitors from a serotonergic to V3 differentiation program by repressing Ascl1 in spinal p3 progenitors through a mechanism dependent on Hes proteins. In this way, Neurog3 establishes the posterior boundary of the serotonergic system by actively suppressing serotonergic specification in the spinal cord. These results explain how equivalent p3 progenitors within the hindbrain and the spinal cord produce functionally distinct neuron cell types.

Nato3 plays an integral role in dorsoventral patterning of the spinal cord by segregating floor plate/p3 fates via Nkx2.2 suppression and Foxa2 maintenance

During embryogenesis, the dorsal roof plate and the ventral floor plate (FP) act as organizing centers to pattern the developing neural tube. Organizer-secreted morphogens provide signals that are interpreted via the graded expression of transcription factors. These factors establish a combinatorial code, which subsequently determines the fate of neuronal progenitors along the dorsoventral axis. To further separate the fates and promote distinct identities of the neural progenitors, mutual repression takes place among transcription factors expressed in progenitors situated along the dorsoventral axis. The molecular mechanisms acting in the developing spinal cord and underlying the segregation of the progenitor pool containing cells with a mixed FP/p3 fate into separate FP cells and V3 neurons are not fully understood. Using in vivo ectopic expression in chick, we found that Nato3 induces ectopic Foxa2-positive cells and indirectly downregulates Nkx2.2 expression. To examine the role of Nato3 in the FP, Foxa2-Nato3 signaling was blocked in Nato3 null mice and to a greater extent in Nato3 null/Foxa2 heterozygous bigenic mutants. Complementary to the findings obtained by gain of function in chick, the loss of function in mouse indicated that the segregation of the FP/p3 population into its derivatives was interrupted. Together, the data suggest that Nato3 is a novel determinant factor regulating the segregation of the FP and p3 identities, which is an essential step for establishing a definitive FP fate in the embryonic spinal cord.

Hox genes and region-specific sensorimotor circuit formation in the hindbrain and spinal cord

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. In vertebrates, 39 Hox genes have been identified and like their Drosophila counterparts they are organized within chromosomal clusters. Hox genes interact with various cofactors, such as the TALE homeodomain proteins, in recognition of consensus sequences within regulatory elements of their target genes. In vertebrates, Hox genes display spatially restricted patterns of expression within the developing hindbrain and spinal cord, and are considered crucial determinants of segmental identity and cell specification along the anterioposterior and dorsoventral axes of the embryo. Here, we review their later roles in the assembly of neuronal circuitry, in stereotypic neuronal migration, axon pathfinding, and topographic connectivity. Importantly, we will put some emphasis on how their early-segmented expression patterns can influence the formation of complex vital hindbrain and spinal cord circuitries.

Subset specification of central serotonergic neurons

The last decade the serotonin (5-hydroxytryptamine; 5-HT) system has received enormous attention due to its role in regulation of behavior, exemplified by the discovery that increased 5-HT tone in the central nervous system is able to alleviate affective disorders. Here, we review the developmental processes, with a special emphasis on subset specification, leading to the formation of the 5-HT system in the brain. Molecular classification of 5-HT neuronal groups leads to the definition of two independent rostral groups positioned in rhombomere 1 and 2/3 and a caudal group in rhombomere 5-8. In addition, more disperse refinement of these subsets is present as shown by the selective expression of the 5-HT1A autoreceptor, indicating functional diversity between 5-HT subsets. The functional significance of the molecular coding differences is not well known and the molecular basis of described specific connectivity patterns remain to be elucidated. Recent developments in genetic lineage tracing models will provide these data and form a major step-up toward the full understanding of the importance of developmental programming and function of 5-HT neuronal subsets.

Valproic acid silencing of ascl1b/Ascl1 results in the failure of serotonergic differentiation in a zebrafish model of fetal valproate syndrome

Fetal valproate syndrome (FVS) is caused by in utero exposure to the drug sodium valproate. Valproate is used worldwide for the treatment of epilepsy, as a mood stabiliser and for its pain-relieving properties. In addition to birth defects, FVS is associated with an increased risk of autism spectrum disorder (ASD), which is characterised by abnormal behaviours. Valproate perturbs multiple biochemical pathways and alters gene expression through its inhibition of histone deacetylases. Which, if any, of these mechanisms is relevant to the genesis of its behavioural side effects is unclear. Neuroanatomical changes associated with FVS have been reported and, among these, altered serotonergic neuronal differentiation is a consistent finding. Altered serotonin homeostasis is also associated with autism. Here we have used a chemical-genetics approach to investigate the underlying molecular defect in a zebrafish FVS model. Valproate causes the selective failure of zebrafish central serotonin expression. It does so by downregulating the proneural gene ascl1b, an ortholog of mammalian Ascl1, which is a known determinant of serotonergic identity in the mammalian brainstem. ascl1b is sufficient to rescue serotonin expression in valproate-treated embryos. Chemical and genetic blockade of the histone deacetylase Hdac1 downregulates ascl1b, consistent with the Hdac1-mediated silencing of ascl1b expression by valproate. Moreover, tonic Notch signalling is crucial for ascl1b repression by valproate. Concomitant blockade of Notch signalling restores ascl1b expression and serotonin expression in both valproate-exposed and hdac1 mutant embryos. Together, these data provide a molecular explanation for serotonergic defects in FVS and highlight an epigenetic mechanism for genome-environment interaction in disease.

Retinoid acid specifies neuronal identity through graded expression of Ascl1

Cell diversity and organization in the neural tube depend on the integration of extrinsic signals acting along orthogonal axes. These are believed to specify distinct cellular identities by triggering all-or-none changes in expression of combinations of transcription factors [1]. Under the influence of a common dorsoventral signal, sonic hedgehog, and distinct anterior-posterior (A-P) inductive signals [2, 3], two topographically related progenitor pools that share a common transcriptional code produce serotonergic and V3 neurons in the hindbrain and spinal cord, respectively [4–7]. These neurons have different physiological properties, functions, and connectivity [8, 9]. Serotonergic involvement in neuropsychiatric diseases has prompted greater characterization of their postmitotic repertoire of fate determinants, which include Gata2, Lmx1b, and Pet1 [10], whereas V3 neurons express Sim1 [4]. How distinct serotonergic and V3 neuronal identities emerge from progenitors that share a common transcriptional code is not understood. Here, we show that changes in retinoid activity in these two progenitor pools determine their fates. Retinoids, via Notch signaling, control the expression level in progenitors of the transcription factor Ascl1, which selects serotonergic and V3 neuronal identities in a dose-dependent manner. Therefore, quantitative differences in the expression of a single component of a transcriptional code can select distinct cell fates.

Development of the serotonergic cells in murine raphe nuclei and their relations with rhombomeric domains

The raphe nuclei represent the origin of central serotonergic projections. The literature distinguishes seven nuclei grouped into rostral and caudal clusters relative to the pons. The boundaries of these nuclei have not been defined precisely enough, particularly with regard to developmental units, notably hindbrain rhombomeres. We hold that a developmental point of view considering rhombomeres may explain observed differences in connectivity and function. There are twelve rhombomeres characterized by particular genetic profiles, and each develops between one and four distinct serotonergic populations. We have studied the distribution of the conventional seven raphe nuclei among these twelve units. To this aim, we correlated 5-HT-immunoreacted neurons with rhombomeric boundary landmarks in sagittal mouse brain sections at different developmental stages. Furthermore, we performed a partial genoarchitectonic analysis of the developing raphe nuclei, mapping all known serotonergic differentiation markers, and compared these results, jointly with others found in the literature, with our map of serotonin-containing populations, in order to examine regional variations in correspondence. Examples of regionally selective gene patterns were identified. As a result, we produced a rhombomeric classification of some 45 serotonergic populations, and suggested a corresponding modified terminology. Only a minor rostral part of the dorsal raphe nucleus lies in the midbrain. Some serotonergic neurons were found in rhombomere 4, contrary to the conventional assumption that it lacks such neurons. We expect that our reclassification of raphe nuclei may be useful for causal analysis of their differential molecular specification, as well as for studies of differential connectivity and function.

Neuroimaging and genetics: exploring, searching, and finding

This issue on the genetics of brain imaging phenotypes is a celebration of the happy marriage between two of science's highly interesting fields: neuroscience and genetics. The articles collected here are ample evidence that a good deal of synergy exists in this marriage. A wide selection of papers is presented that provide many different perspectives on how genes cause variation in brain structure and function, which in turn influence behavioral phenotypes (including psychopathology). They are examples of the many different methodologies in contemporary genetics and neuroscience research. Genetic methodology includes genome-wide association (GWA), candidate-gene association, and twin studies. Sources of data on brain phenotypes include cortical gray matter (GM) structural/volumetric measures from magnetic resonance imaging (MRI); white matter (WM) measures from diffusion tensor imaging (DTI), such as fractional anisotropy; functional- (activity-) based measures from electroencephalography (EEG), and functional MRI (fMRI). Together, they reflect a combination of scientific fields that have seen great technological advances, whether it is the single-nucleotide polymorphism (SNP) array in genetics, the increasingly high-resolution MRI imaging, or high angular resolution diffusion imaging technique for measuring WM connective properties.

Engrailed is required in maturing serotonin neurons to regulate the cytoarchitecture and survival of the dorsal raphe nucleus

Analysis of constitutive Engrailed (En) null mice previously implicated the two En homeobox paralogs in the development of serotonin (5-HT) neurons. An unresolved question is whether En plays intrinsic roles in these neurons. Here, we show that En1 and En2 are expressed in maturing 5-HT neurons that will form the dorsal raphe nucleus (DRN) and part of the median raphe nucleus. Although En1 expression in 5-HT neurons persists postnatally, En2 expression is extinguished by embryonic day 17.5. To investigate intrinsic serotonergic functions for En1/2, we generated compound conditional En mutants with floxed alleles and a cre recombinase line that becomes active in postmitotic fetal 5-HT neurons. We present evidence in support of a requirement for En1/2 in the maturation of DRN cytoarchitecture. The disruption of DRN cytoarchitecture appears to result from a defect in secondary migration of serotonergic cell bodies toward the midline rather than disruption of their primary ventral migration away from the ventricular zone. Furthermore, En1/2 are required for perinatal maintenance of serotonergic identity and postnatal forebrain 5-HT levels. Increased numbers of caspase-3-expressing cells and loss of significant numbers of 5-HT neuron cell bodies, indicative of apoptosis, occurred after loss of serotonergic identity. Analysis of an allelic series of conditional mutants showed that En1 is the predominant functional En paralog in maturing 5-HT neurons, although a small contribution from En2 was reproducibly detected. Together, our findings reveal complex intrinsic functions for En in maturing 5-HT neurons, hence necessitating a reinterpretation of their roles in 5-HT system development.

A novel role for FOXA2 and SHH in organizing midbrain signaling centers

The floor plate (FP) is a midline signaling center, known to direct ventral cell fates and axon guidance in the neural tube. The recent identification of midbrain FP as a source of dopaminergic neurons has renewed interest in its specification and organization, which remain poorly understood. In this study, we have examined the chick midbrain and spinal FP and show that both can be partitioned into medial (MFP) and lateral (LFP) subdivisions. Although Hedgehog (HH) signaling is necessary and sufficient for LFP specification, it is not sufficient for MFP induction. By contrast, the transcription factor FOXA2 can execute the full midbrain and spinal cord FP program via HH-independent and dependent mechanisms. Interestingly, although HH-independent FOXA2 activity is necessary and sufficient for inducing MFP-specific gene expression (e.g., LMX1B, BMP7), it cannot confer ventral identity to midline cells without also turning on Sonic hedgehog (SHH). We also note that the signaling centers of the midbrain, the FP, roof plate (RP) and the midbrain-hindbrain boundary (MHB) are physically contiguous, with each expressing LMX1B and BMP7. Possibly as a result, SHH or FOXA2 misexpression can transform the MHB into FP and also suppress RP induction. Conversely, HH or FOXA2 knockdown expands the endogenous RP and transforms the MFP into a RP and/or MHB fate. Finally, combined HH blockade and FOXA2 misexpression in ventral midbrain induces LMX1B expression, which triggers the specification of the RP, rather than the MFP. Thus we identify HH-independent and dependent roles for FOXA2 in specifying the FP. In addition, we elucidate for the first time, a novel role for SHH in determining whether a midbrain signaling center will become the FP, MHB or RP.

Associations Between Variants Near a Monoaminergic Pathways Gene (PHOX2B) and Amygdala Reactivity: A Genome-Wide Functional Imaging Study

As the amygdala is part of the phylogenetic old brain, and its anatomical and functional properties are conserved across species, it is reasonable to assume genetic influence on its activity. A large corpus of candidate gene studies indicate that individual differences in amygdala activity may be caused by genetic variants within monoaminergic signaling pathways such as dopamine, serotonin, and norepinephrine. However, to our knowledge, the use of genome-wide data to discover genetic variants underlying individual differences in adult amygdala activity is novel. In the present study, the combination of genome-wide data and functional imaging phenotypes from an emotional faces task yielded a significant association between rs10014254 and the amygdala using a region of interest approach. This single nucleotide polymorphism is located in a regulatory region upstream of the Paired-like homeobox 2b (PHOX2B) gene; therefore it could affect the expression of this gene. PHOX2B regulates the expression of enzymes necessary for the synthesis of several monoamines and is essential for the development of the autonomic nervous system. However, an attempt to replicate the finding in an independent sample from North America did not succeed. The synthesis of functional magnetic resonance imaging (fMRI) and genome-wide data takes a hypothesis-free approach as to which genetic variants are of interest. Therefore, we believe that an undirected finding within such a plausible region is of interest, and that our results add further support to the hypothesis that monoaminergic signaling pathways play a central role in regulating amygdala activity.

Genome-wide characterization of Foxa2 targets reveals upregulation of floor plate genes and repression of ventrolateral genes in midbrain dopaminergic progenitors

The transcription factors Foxa1 and Foxa2 promote the specification of midbrain dopaminergic (mDA) neurons and the floor plate. Whether their role is direct has remained unclear as they also regulate the expression of Shh, which has similar roles. We characterized the Foxa2 cis-regulatory network by chromatin immunoprecipitation followed by high-throughput sequencing of mDA progenitors. This identified 9160 high-quality Foxa2 binding sites associated with 5409 genes, providing mechanistic insights into Foxa2-mediated positive and negative regulatory events. Foxa2 regulates directly and positively key determinants of mDA neurons, including Lmx1a, Lmx1b, Msx1 and Ferd3l, while negatively inhibiting transcription factors expressed in ventrolateral midbrain such as Helt, Tle4, Otx1, Sox1 and Tal2. Furthermore, Foxa2 negatively regulates extrinsic and intrinsic components of the Shh signaling pathway, possibly by binding to the same enhancer regions of co-regulated genes as Gli1. Foxa2 also regulates the expression of floor plate factors that control axon trajectories around the midline of the embryo, thereby contributing to the axon guidance function of the floor plate. Finally, this study identified multiple Foxa2-regulated enhancers that are active in the floor plate of the midbrain or along the length of the embryo in mouse and chick. This work represents the first comprehensive characterization of Foxa2 targets in mDA progenitors and provides a framework for elaborating gene regulatory networks in a functionally important progenitor population.

Serotonergic transcriptional networks and potential importance to mental health

Transcription regulatory networks governing the genesis, maturation and maintenance of vertebrate brain serotonin (5-HT) neurons determine the level of serotonergic gene expression and signaling throughout an animal's lifespan. Recent studies suggest that alterations in these networks can cause behavioral and physiological pathogenesis in mice. Here, we synthesize findings from vertebrate loss-of-function and gain-of-function studies to build a new model of the transcriptional regulatory networks that specify 5-HT neurons during fetal life, integrate them into CNS circuitry in early postnatal life and maintain them in adulthood. We then describe findings from animal and human genetic studies that support possible alterations in the activity of serotonergic regulatory networks in the etiology of mental illness. We conclude with a discussion of the potential utility of our model, as an experimentally well-defined molecular pathway, to predict and interpret the biological effect of genetic variation that may be discovered in the orthologous human network.

Impaired ventilatory and thermoregulatory responses to hypoxic stress in newborn phox2b heterozygous knock-out mice

The Phox2b genesis necessary for the development of the autonomic nervous system, and especially, of respiratory neuronal circuits. In the present study, we examined the role of Phox2b in ventilatory and thermoregulatory responses to hypoxic stress, which are closely related in the postnatal period. Hypoxic stress was generated by strong thermal stimulus, combined or not with reduced inspired O(2). To this end, we exposed 6-day-old Phox2b(+/-) pups and their wild-type littermates (Phox2b(+/+)) to hypoxia (10% O(2)) or hypercapnia (8% CO(2)) under thermoneutral (33°C) or cold (26°C) conditions. We found that Phox2b(+/-) pups showed less normoxic ventilation (V(E)) in the cold than Phox2b(+/+) pups. Phox2b(+/-) pups also showed lower oxygen consumption (VO(2)) in the cold, reflecting reduced thermogenesis and a lower body temperature. Furthermore, while the cold depressed ventilatory responses to hypoxia and hypercapnia in both genotype groups, this effect was less pronounced in Phox2b(+/-) pups. Finally, because serotonin (5-HT) neurons are pivotal to respiratory and thermoregulatory circuits and depend on Phox2b for their differentiation, we studied 5-HT metabolism using high pressure liquid chromatography, and found that it was altered in Phox2b(+/-) pups. We conclude that Phox2b haploinsufficiency alters the ability of newborns to cope with metabolic challenges, possibly due to 5-HT signaling impairments.

Directed expression of Gata2, Mash1, and Foxa2 synergize to induce the serotonergic neuron phenotype during in vitro differentiation of embryonic stem cells

Investigation of serotonergic neuronal activity and its relationship to disease has been limited by a lack of physiologically relevant in vitro cell models. Serotonergic neurons derived from embryonic stem cells (ESCs) could provide a platform for such studies and provide models for use in drug discovery. Here, we report enhancement of serotonergic differentiation using a genetic approach. Expression of Gata2 increased the yield of serotonergic neurons. Enhancement was only achieved when Gata2 was expressed under the control of the tissue-specific promoter of the transcription factor Nkx6.1. High levels of Gata2 expression in ESCs compromised pluripotency and induced non-neuronal differentiation. Combined directed expression of Gata2, proneural gene Mash1, and forkhead transcription factor Foxa2 further enhanced serotonergic neural differentiation, resulting in a 10-fold increase in serotonin content. These neurons were also capable of depolarization (KCl, 30 mM)-induced elevations of intracellular Ca(2+) . The presence of sonic hedgehog during differentiation produced a further modest increase in numbers (1.5-fold). Transgene expression did not influence the number of tyrosine hydroxylase positive neurons in the cultures after 20 days, implying that Gata2, Mash1, and Foxa2 modulate in vitro differentiation at a time beyond the decision-point for dopaminergic or nondopaminergic commitment. This study demonstrates that the directed expression of specific transcription factors enhances serotonergic neuron differentiation in vitro and highlights the importance of transgene expression at the right stage of ESC differentiation to effect the generation of a desired neural subtype.

Molecular genetics of mouse serotonin neurons across the lifespan

New molecular genetics approaches have been developed over the past several years to study brain serotonin (5-HT) neuron development and the roles of 5-HT neurons in behavior and physiology. These approaches were enabled by manipulation of the gene encoding the Pet-1 ETS transcription factor whose expression in the brain is restricted to developing and adult 5-HT neurons. Targeting of the Pet-1 gene led to the development of a mouse line with a severe and stable deficiency of embryonic 5-HT-synthesizing neurons. The Pet-1 transcription regulatory region has been used to create several new 5-HT neuron-type transgenic tools that have greatly increased the experimental accessibility of the small number of brain 5-HT neurons. Permanent and specific marking of 5-HT neurons with Pet-1-based transgenic tools have now been used for flow cytometry, whole cell electrophysiological recordings, progenitor fate mapping, and live time lapse imaging of these neurons. Additional tools provide multiple strategies for conditional temporal targeting of gene expression in 5-HT neurons at different stages of life. Pet-1-based approaches have led to advances in understanding the role of 5-HT neurons in respiration, thermoregulation, emotional behaviors, maternal behavior, and the mechanism of antipsychotic drug actions. In addition, these approaches have begun to reveal the molecular basis of 5-HT neuron heterogeneity and the transcriptional mechanisms that direct 5-HT neuron-type identity, maturation, and maintenance.

Directed neural differentiation of mouse embryonic stem cells is a sensitive system for the identification of novel Hox gene effectors

The evolutionarily conserved Hox family of homeodomain transcription factors plays fundamental roles in regulating cell specification along the anterior posterior axis during development of all bilaterian animals by controlling cell fate choices in a highly localized, extracellular signal and cell context dependent manner. Some studies have established downstream target genes in specific systems but their identification is insufficient to explain either the ability of Hox genes to direct homeotic transformations or the breadth of their patterning potential. To begin delineating Hox gene function in neural development we used a mouse ES cell based system that combines efficient neural differentiation with inducible Hoxb1 expression. Gene expression profiling suggested that Hoxb1 acted as both activator and repressor in the short term but predominantly as a repressor in the long run. Activated and repressed genes segregated in distinct processes suggesting that, in the context examined, Hoxb1 blocked differentiation while activating genes related to early developmental processes, wnt and cell surface receptor linked signal transduction and cell-to-cell communication. To further elucidate aspects of Hoxb1 function we used loss and gain of function approaches in the mouse and chick embryos. We show that Hoxb1 acts as an activator to establish the full expression domain of CRABPI and II in rhombomere 4 and as a repressor to restrict expression of Lhx5 and Lhx9. Thus the Hoxb1 patterning activity includes the regulation of the cellular response to retinoic acid and the delay of the expression of genes that commit cells to neural differentiation. The results of this study show that ES neural differentiation and inducible Hox gene expression can be used as a sensitive model system to systematically identify Hox novel target genes, delineate their interactions with signaling pathways in dictating cell fate and define the extent of functional overlap among different Hox genes.

Doublesex and mab-3-related transcription factor 5 promotes midbrain dopaminergic identity in pluripotent stem cells by enforcing a ventral-medial progenitor fate

Understanding the control of cell-fate choices during embryonic stem cell (ESC) differentiation is crucial for harnessing strategies for efficient production of desired cell types for pharmaceutical drug screening and cell transplantation. Here we report the identification of the zinc finger-like doublesex and mab-3-related transcription factor 5 (Dmrt5) as a marker for mammalian ventral-medial mesencephalic neuroepithelium that give rise to dopamine neurons. Gain- and loss-of-function studies in ESC demonstrate that Dmrt5 is critically involved in the specification of ventral-medial neural progenitor cell fate and the subsequent generation of dopamine neurons expressing essential midbrain characteristics. Genome-wide analysis of Dmrt5-mediated transcriptome changes and expression profiling of ventral-medial and ventral-lateral mesencephalic neuroepithelium revealed suppressive and inductive regulatory roles for Dmrt5 in the transcription program associated with the ventral-medial neural progenitor fates. Together, these data identify Dmrt5 as an important player in ventral mesencephalic neural fate specification.

Hoxb3 negatively regulates Hoxb1 expression in mouse hindbrain patterning

The spatial regulation of combinatorial expression of Hox genes is critical for determining hindbrain rhombomere (r) identities. To address the cross-regulatory relationship between Hox genes in hindbrain neuronal specification, we have generated a gain-of-function transgenic mouse mutant Hoxb3(Tg) using the Hoxb2 r4-specific enhancer element. Interestingly, in r4 of the Hoxb3(Tg) mutant where Hoxb3 was ectopically expressed, the expression of Hoxb1 was specifically abolished. The hindbrain neuronal defects of the Hoxb3(Tg) mutant mice were similar to those of Hoxb1(-/-) mutants. Therefore, we hypothesized that Hoxb3 could directly suppress Hoxb1 expression. We first identified a novel Hoxb3 binding site S3 on the Hoxb1 locus and confirmed protein binding to this site by EMSA, and by in vivo ChIP analysis using P19 cells and hindbrain tissues from the Hoxb3(Tg) mutant. We further showed that Hoxb3 could suppress Hoxb1 transcriptional activity by chick in ovo luciferase reporter assay. Moreover, in E10.5 wildtype caudal hindbrain, where Hoxb1 is not expressed, we showed by in vivo ChIP that Hoxb3 was consistently bound to the S3 site on the Hoxb1 gene. This study reveals a novel negative regulatory mechanism by which Hoxb3 as a posterior gene serves to restrict Hoxb1 expression in r4 by direct transcriptional repression to maintain the rhombomere identity.

A homeodomain feedback circuit underlies step-function interpretation of a Shh morphogen gradient during ventral neural patterning

The deployment of morphogen gradients is a core strategy to establish cell diversity in developing tissues, but little is known about how small differences in the concentration of extracellular signals are translated into robust patterning output in responding cells. We have examined the activity of homeodomain proteins, which are presumed to operate downstream of graded Shh signaling in neural patterning, and describe a feedback circuit between the Shh pathway and homeodomain transcription factors that establishes non-graded regulation of Shh signaling activity. Nkx2 proteins intrinsically strengthen Shh responses in a feed-forward amplification and are required for ventral floor plate and p3 progenitor fates. Conversely, Pax6 has an opposing function to antagonize Shh signaling, which provides intrinsic resistance to Shh responses and is important to constrain the inductive capacity of the Shh gradient over time. Our data further suggest that patterning of floor plate cells and p3 progenitors is gated by a temporal switch in neuronal potential, rather than by different Shh concentrations. These data establish that dynamic, non-graded changes in responding cells are essential for Shh morphogen interpretation, and provide a rationale to explain mechanistically the phenomenon of cellular memory of morphogen exposure.

Foxj1 regulates floor plate cilia architecture and modifies the response of cells to sonic hedgehog signalling

Sonic hedgehog signalling is essential for the embryonic development of many tissues including the central nervous system, where it controls the pattern of cellular differentiation. A genome-wide screen of neural progenitor cells to evaluate the Shh signalling-regulated transcriptome identified the forkhead transcription factor Foxj1. In both chick and mouse Foxj1 is expressed in the ventral midline of the neural tube in cells that make up the floor plate. Consistent with the role of Foxj1 in the formation of long motile cilia, floor plate cells produce cilia that are longer than the primary cilia found elsewhere in the neural tube, and forced expression of Foxj1 in neuroepithelial cells is sufficient to increase cilia length. In addition, the expression of Foxj1 in the neural tube and in an Shh-responsive cell line attenuates intracellular signalling by decreasing the activity of Gli proteins, the transcriptional mediators of Shh signalling. We show that this function of Foxj1 depends on cilia. Nevertheless, floor plate identity and ciliogenesis are unaffected in mouse embryos lacking Foxj1 and we provide evidence that additional transcription factors expressed in the floor plate share overlapping functions with Foxj1. Together, these findings identify a novel mechanism that modifies the cellular response to Shh signalling and reveal morphological and functional features of the amniote floor plate that distinguish these cells from the rest of the neuroepithelium.