Krox20 and kreisler co-operate in the transcriptional control of segmental expression of Hoxb3 in the developing hindbrain

Transcription factors regulating the specification of brainstem respiratory neurons

Breathing (or respiration) is an unconscious and complex motor behavior which neuronal drive emerges from the brainstem. In simplistic terms, respiratory motor activity comprises two phases, inspiration (uptake of oxygen, O2) and expiration (release of carbon dioxide, CO2). Breathing is not rigid, but instead highly adaptable to external and internal physiological demands of the organism. The neurons that generate, monitor, and adjust breathing patterns locate to two major brainstem structures, the pons and medulla oblongata. Extensive research over the last three decades has begun to identify the developmental origins of most brainstem neurons that control different aspects of breathing. This research has also elucidated the transcriptional control that secures the specification of brainstem respiratory neurons. In this review, we aim to summarize our current knowledge on the transcriptional regulation that operates during the specification of respiratory neurons, and we will highlight the cell lineages that contribute to the central respiratory circuit. Lastly, we will discuss on genetic disturbances altering transcription factor regulation and their impact in hypoventilation disorders in humans.

Identification of a neural development gene expression signature in colon cancer stem cells reveals a role for EGR2 in tumorigenesis

Recent evidence demonstrates that colon cancer stem cells (CSCs) can generate neurons that synapse with tumor innervating fibers required for tumorigenesis and disease progression. Greater understanding of the mechanisms that regulate CSC driven tumor neurogenesis may therefore lead to more effective treatments. RNA-sequencing analyses of ALDHPositive CSCs from colon cancer patient-derived organoids (PDOs) and xenografts (PDXs) showed CSCs to be enriched for neural development genes. Functional analyses of genes differentially expressed in CSCs from PDO and PDX models demonstrated the neural crest stem cell (NCSC) regulator EGR2 to be required for tumor growth and to control expression of homebox superfamily embryonic master transcriptional regulator HOX genes and the neural stem cell and master cell fate regulator SOX2. These data support CSCs as the source of tumor neurogenesis and suggest that targeting EGR2 may provide a therapeutic differentiation strategy to eliminate CSCs and block nervous system driven disease progression.

Early development of the breathing network

Breathing (or respiration) is a complex motor behavior that originates in the brainstem. In minimalistic terms, breathing can be divided into two phases: inspiration (uptake of oxygen, O₂) and expiration (release of carbon dioxide, CO₂). The neurons that discharge in synchrony with these phases are arranged in three major groups along the brainstem: (i) pontine, (ii) dorsal medullary, and (iii) ventral medullary. These groups are formed by diverse neuron types that coalesce into heterogeneous nuclei or complexes, among which the preBötzinger complex in the ventral medullary group contains cells that generate the respiratory rhythm (Chapter 1). The respiratory rhythm is not rigid, but instead highly adaptable to the physic demands of the organism. In order to generate the appropriate respiratory rhythm, the preBötzinger complex receives direct and indirect chemosensory information from other brainstem respiratory nuclei (Chapter 2) and peripheral organs (Chapter 3). Even though breathing is a hard-wired unconscious behavior, it can be temporarily altered at will by other higher-order brain structures (Chapter 6), and by emotional states (Chapter 7). In this chapter, we focus on the development of brainstem respiratory groups and highlight the cell lineages that contribute to central and peripheral chemoreflexes.

Molecular Segmentation of the Spinal Trigeminal Nucleus in the Adult Mouse Brain

The trigeminal column is a hindbrain structure formed by second order sensory neurons that receive afferences from trigeminal primary (ganglionic) nerve fibers. Classical studies subdivide it into the principal sensory trigeminal nucleus located next to the pontine nerve root, and the spinal trigeminal nucleus which in turn consists of oral, interpolar and caudal subnuclei. On the other hand, according to the prosomeric model, this column would be subdivided into segmental units derived from respective rhombomeres. Experimental studies have mapped the principal sensory trigeminal nucleus to pontine rhombomeres (r) r2-r3 in the mouse. The spinal trigeminal nucleus emerges as a plurisegmental formation covering several rhombomeres (r4 to r11 in mice) across pontine, retropontine and medullary hindbrain regions. In the present work we reexamined the issue of rhombomeric vs. classical subdivisions of this column. To this end, we analyzed its subdivisions in an AZIN2-lacZ transgenic mouse, known as a reference model for hindbrain topography, together with transgenic reporter lines for trigeminal fibers. We screened as well for genes differentially expressed along the axial dimension of this structure in the adult and juvenile mouse brain. This analysis yielded genes from multiple functional families that display transverse domains fitting the mentioned rhombomeric map. The spinal trigeminal nucleus thus represents a plurisegmental structure with a series of distinct neuromeric units having unique combinatorial molecular profiles.

New and sex-specific migraine susceptibility loci identified from a multiethnic genome-wide meta-analysis

Migraine is a common disabling primary headache disorder that is ranked as the most common neurological cause of disability worldwide. Women present with migraine much more frequently than men, but the reasons for this difference are unknown. Migraine heritability is estimated to up to 57%, yet much of the genetic risk remains unaccounted for, especially in non-European ancestry populations. To elucidate the etiology of this common disorder, we conduct a multiethnic genome-wide association meta-analysis of migraine, combining results from the GERA and UK Biobank cohorts, followed by a European-ancestry meta-analysis using public summary statistics. We report 79 loci associated with migraine, of which 45 were novel. Sex-stratified analyses identify three additional novel loci (CPS1, PBRM1, and SLC25A21) specific to women. This large multiethnic migraine study provides important information that may substantially improve our understanding of the etiology of migraine susceptibility.

The Hox protein conundrum: The "specifics" of DNA binding for Hox proteins and their partners

Homeotic genes (Hox genes) are homeodomain-transcription factors involved in conferring segmental identity along the anterior-posterior body axis. Molecular characterization of HOX protein function raises some interesting questions regarding the source of the binding specificity of the HOX proteins. How do HOX proteins regulate common and unique target specificity across space and time? This review attempts to summarize and interpret findings in this area, largely focused on results from in vitro and in vivo studies in Drosophila and mouse systems. Recent studies related to HOX protein binding specificity compel us to reconsider some of our current models for transcription factor-DNA interactions. It is crucial to study transcription factor binding by incorporating components of more complex, multi-protein interactions in concert with small changes in binding motifs that can significantly impact DNA binding specificity and subsequent alterations in gene expression. To incorporate the multiple elements that can determine HOX protein binding specificity, we propose a more integrative Cooperative Binding model.

Senescence and Apoptosis: Architects of Mammalian Development

Mammalian development involves an exquisite choreography of cell division, differentiation, locomotion, programmed cell death, and senescence that directs the transformation of a single cell zygote to a mature organism containing on the order of 40 trillion cells in humans. How a single totipotent zygote undergoes the rapid stages of embryonic development to form over 200 different cell types is complex in the extreme and remains the focus of active research. Processes such as programmed cell death or apoptosis has long been known to occur during development to help sculpt organs and tissue systems. Other processes such as cellular senescence, long thought to only occur in pathologic states such as aging and tumorigenesis have been recently reported to play a vital role in development. In this review, we focus on apoptosis and senescence; the former as an integral mechanism that plays a critical role not only in mature organisms, but that is also essential in shaping mammalian development. The latter as a well-defined feature of aging for which some reports indicate a function in development. We will dissect the dual roles of major gene families, pathways such as Hox, Rb, p53, and epigenetic regulators such as the ING proteins in both early and the late stages and how they play antagonistic roles by increasing fitness and decreasing mortality early in life but contribute to deleterious effects and pathologies later in life.

Retinoids and developmental neurotoxicity: Utilizing toxicogenomics to enhance adverse outcome pathways and testing strategies

The use of genomic approaches in toxicological studies has greatly increased our ability to define the molecular profiles of environmental chemicals associated with developmental neurotoxicity (DNT). Integration of these approaches with adverse outcome pathways (AOPs), a framework that translates environmental exposures to adverse developmental phenotypes, can potentially inform DNT testing strategies. Here, using retinoic acid (RA) as a case example, we demonstrate that the integration of toxicogenomic profiles into the AOP framework can be used to establish a paradigm for chemical testing. RA is a critical regulatory signaling molecule involved in multiple aspects of mammalian central nervous system (CNS) development, including hindbrain formation/patterning and neuronal differentiation, and imbalances in RA signaling pathways are linked with DNT. While the mechanisms remain unresolved, environmental chemicals can cause DNT by disrupting the RA signaling pathway. First, we reviewed literature evidence of RA and other retinoid exposures and DNT to define a provisional AOP related to imbalances in RA embryonic bioavailability and hindbrain development. Next, by integrating toxicogenomic datasets, we defined a relevant transcriptomic signature associated with RA-induced developmental neurotoxicity (RA-DNT) in human and rodent models that was tested against zebrafish model data, demonstrating potential for integration into an AOP framework. Finally, we demonstrated how these approaches may be systematically utilized to identify chemical hazards by testing the RA-DNT signature against azoles, a proposed class of compounds that alters RA-signaling. The provisional AOP from this study can be expanded in the future to better define DNT biomarkers relevant to RA signaling and toxicity.

Hindbrain induction and patterning during early vertebrate development

The hindbrain is a key relay hub of the central nervous system (CNS), linking the bilaterally symmetric half-sides of lower and upper CNS centers via an extensive network of neural pathways. Dedicated neural assemblies within the hindbrain control many physiological processes, including respiration, blood pressure, motor coordination and different sensations. During early development, the hindbrain forms metameric segmented units known as rhombomeres along the antero-posterior (AP) axis of the nervous system. These compartmentalized units are highly conserved during vertebrate evolution and act as the template for adult brainstem structure and function. TALE and HOX homeodomain family transcription factors play a key role in the initial induction of the hindbrain and its specification into rhombomeric cell fate identities along the AP axis. Signaling pathways, such as canonical-Wnt, FGF and retinoic acid, play multiple roles to initially induce the hindbrain and regulate Hox gene-family expression to control rhombomeric identity. Additional transcription factors including Krox20, Kreisler and others act both upstream and downstream to Hox genes, modulating their expression and protein activity. In this review, we will examine the earliest embryonic signaling pathways that induce the hindbrain and subsequent rhombomeric segmentation via Hox and other gene expression. We will examine how these signaling pathways and transcription factors interact to activate downstream targets that organize the segmented AP pattern of the embryonic vertebrate hindbrain.

Segmental arithmetic: summing up the Hox gene regulatory network for hindbrain development in chordates

Organization and development of the early vertebrate hindbrain are controlled by a cascade of regulatory interactions that govern the process of segmentation and patterning along the anterior-posterior axis via Hox genes. These interactions can be assembled into a gene regulatory network that provides a framework to interpret experimental data, generate hypotheses, and identify gaps in our understanding of the progressive process of hindbrain segmentation. The network can be broadly separated into a series of interconnected programs that govern early signaling, segmental subdivision, secondary signaling, segmentation, and ultimately specification of segmental identity. Hox genes play crucial roles in multiple programs within this network. Furthermore, the network reveals properties and principles that are likely to be general to other complex developmental systems. Data from vertebrate and invertebrate chordate models are shedding light on the origin and diversification of the network. Comprehensive cis-regulatory analyses of vertebrate Hox gene regulation have enabled powerful cross-species gene regulatory comparisons. Such an approach in the sea lamprey has revealed that the network mediating segmental Hox expression was present in ancestral vertebrates and has been maintained across diverse vertebrate lineages. Invertebrate chordates lack hindbrain segmentation but exhibit conservation of some aspects of the network, such as a role for retinoic acid in establishing nested Hox expression domains. These comparisons lead to a model in which early vertebrates underwent an elaboration of the network between anterior-posterior patterning and Hox gene expression, leading to the gene-regulatory programs for segmental subdivision and rhombomeric segmentation. WIREs Dev Biol 2017, 6:e286. doi: 10.1002/wdev.286 For further resources related to this article, please visit the WIREs website.

A distal intergenic region controls pancreatic endocrine differentiation by acting as a transcriptional enhancer and as a polycomb response element

Lineage-selective expression of developmental genes is dependent on the interplay between activating and repressive mechanisms. Gene activation is dependent on cell-specific transcription factors that recognize transcriptional enhancer sequences. Gene repression often depends on the recruitment of Polycomb group (PcG) proteins, although the sequences that underlie the recruitment of PcG proteins, also known as Polycomb response elements (PREs), remain poorly understood in vertebrates. While distal PREs have been identified in mammals, a role for positive-acting enhancers in PcG-mediated repression has not been described. Here we have used a highly efficient procedure based on lentiviral-mediated transgenesis to carry out in vivo fine-mapping of, cis-regulatory sequences that control lineage-specific activation of Neurog3, a master regulator of pancreatic endocrine differentiation. Our findings reveal an enhancer region that is sufficient to drive correct spacio-temporal expression of Neurog3 and demonstrate that this same region serves as a PRE in alternative lineages where Neurog3 is inactive.

The vertebrate Hox gene regulatory network for hindbrain segmentation: Evolution and diversification: Coupling of a Hox gene regulatory network to hindbrain segmentation is an ancient trait originating at the base of vertebrates

Hindbrain development is orchestrated by a vertebrate gene regulatory network that generates segmental patterning along the anterior-posterior axis via Hox genes. Here, we review analyses of vertebrate and invertebrate chordate models that inform upon the evolutionary origin and diversification of this network. Evidence from the sea lamprey reveals that the hindbrain regulatory network generates rhombomeric compartments with segmental Hox expression and an underlying Hox code. We infer that this basal feature was present in ancestral vertebrates and, as an evolutionarily constrained developmental state, is fundamentally important for patterning of the vertebrate hindbrain across diverse lineages. Despite the common ground plan, vertebrates exhibit neuroanatomical diversity in lineage-specific patterns, with different vertebrates revealing variations of Hox expression in the hindbrain that could underlie this diversification. Invertebrate chordates lack hindbrain segmentation but exhibit some conserved aspects of this network, with retinoic acid signaling playing a role in establishing nested domains of Hox expression.

YAP regulates the expression of Hoxa1 and Hoxc13 in mouse and human oral and skin epithelial tissues

Yes-associated protein (YAP) is a Hippo signaling transcriptional coactivator that plays pivotal roles in stem cell proliferation, organ size control, and tumor development. The downstream targets of YAP have been shown to be highly context dependent. In this study, we used the embryonic mouse tooth germ as a tool to search for the downstream targets of YAP in ectoderm-derived tissues. Yap deficiency in the dental epithelium resulted in a small tooth germ with reduced epithelial cell proliferation. We compared the gene expression profiles of embryonic day 14.5 (E14.5) Yap conditional knockout and YAP transgenic mouse tooth germs using transcriptome sequencing (RNA-Seq) and further confirmed the differentially expressed genes using real-time PCR and in situ hybridization. We found that YAP regulates the expression of Hoxa1 and Hoxc13 in oral and dental epithelial tissues as well as in the epidermis of skin during embryonic and adult stages. Sphere formation assay suggested that Hoxa1 and Hoxc13 are functionally involved in YAP-regulated epithelial progenitor cell proliferation, and chromatin immunoprecipitation (ChIP) assay implies that YAP may regulate Hoxa1 and Hoxc13 expression through TEAD transcription factors. These results provide mechanistic insights into abnormal YAP activities in mice and humans.

The gene regulatory networks underlying formation of the auditory hindbrain

Development and evolution of auditory hindbrain nuclei are two major unsolved issues in hearing research. Recent characterization of transgenic mice identified the rhombomeric origins of mammalian auditory nuclei and unraveled genes involved in their formation. Here, we provide an overview on these data by assembling them into rhombomere-specific gene regulatory networks (GRNs), as they underlie developmental and evolutionary processes. To explore evolutionary mechanisms, we compare the GRNs operating in the mammalian auditory hindbrain with data available from the inner ear and other vertebrate groups. Finally, we propose that the availability of genomic sequences from all major vertebrate taxa and novel genetic techniques for non-model organisms provide an unprecedented opportunity to investigate development and evolution of the auditory hindbrain by comparative molecular approaches. The dissection of the molecular mechanisms leading to auditory structures will also provide an important framework for auditory processing disorders, a clinical problem difficult to tackle so far. These data will, therefore, foster basic and clinical hearing research alike.

A novel function for Egr4 in posterior hindbrain development

Segmentation of the vertebrate hindbrain is an evolutionarily conserved process. Here, we identify the transcription factor early growth response 4 (egr4) as a novel regulator of posterior hindbrain development in Xenopus. egr4 is specifically and transiently expressed in rhombomeres 5 and 6 (r5/r6), and Egr4 knockdown causes a loss of mafb/kreisler and krox20/egr2 expression in r5/r6 and r5, respectively. This phenotype can be fully rescued by injection of frog or mouse Egr4 mRNA. Moreover Egr4-depleted embryos exhibit a specific loss of the neural crest stream adjacent to r5, and have inner ear defects. While the homeodomain protein vHnf1/Hnf1b directly activates Mafb and Krox20 expression in the mouse hindbrain to specify r5, we show that in Xenopus this process is indirect through the activation of Egr4. We provide evidence that rearrangements in the regulatory sequences around egr4 and mafb genes may account for this difference.

A Hox regulatory network of hindbrain segmentation is conserved to the base of vertebrates

A defining feature governing head patterning of jawed vertebrates is a highly conserved gene regulatory network that integrates hindbrain segmentation with segmentally restricted domains of Hox gene expression. Although non-vertebrate chordates display nested domains of axial Hox expression, they lack hindbrain segmentation. The sea lamprey, a jawless fish, can provide unique insights into vertebrate origins owing to its phylogenetic position at the base of the vertebrate tree. It has been suggested that lamprey may represent an intermediate state where nested Hox expression has not been coupled to the process of hindbrain segmentation. However, little is known about the regulatory network underlying Hox expression in lamprey or its relationship to hindbrain segmentation. Here, using a novel tool that allows cross-species comparisons of regulatory elements between jawed and jawless vertebrates, we report deep conservation of both upstream regulators and segmental activity of enhancer elements across these distant species. Regulatory regions from diverse gnathostomes drive segmental reporter expression in the lamprey hindbrain and require the same transcriptional inputs (for example, Kreisler (also known as Mafba), Krox20 (also known as Egr2a)) in both lamprey and zebrafish. We find that lamprey hox genes display dynamic segmentally restricted domains of expression; we also isolated a conserved exonic hox2 enhancer from lamprey that drives segmental expression in rhombomeres 2 and 4. Our results show that coupling of Hox gene expression to segmentation of the hindbrain is an ancient trait with origin at the base of vertebrates that probably led to the formation of rhombomeric compartments with an underlying Hox code.

Transcription factors define the neuroanatomical organization of the medullary reticular formation

The medullary reticular formation contains large populations of inadequately described, excitatory interneurons that have been implicated in multiple homeostatic behaviors including breathing, viserosensory processing, vascular tone, and pain. Many hindbrain nuclei show a highly stereotyped pattern of localization across vertebrates suggesting a strong underlying genetic organization. Whether this is true for neurons within the reticular regions of hindbrain is unknown. Hindbrain neurons are derived from distinct developmental progenitor domains each of which expresses distinct patterns of transcription factors (TFs). These neuronal populations have distinct characteristics such as transmitter identity, migration, and connectivity suggesting developmentally expressed TFs might identify unique subpopulations of neurons within the reticular formation. A fate-mapping strategy using perinatal expression of reporter genes within Atoh1, Dbx1, Lmx1b, and Ptf1a transgenic mice coupled with immunohistochemistry (IHC) and in situ hybridization (ISH) were used to address the developmental organization of a large subset of reticular formation glutamatergic neurons. All hindbrain lineages have relatively large populations that extend the entire length of the hindbrain. Importantly, the location of neurons within each lineage was highly constrained. Lmx1b- and Dbx1- derived populations were both present in partially overlapping stripes within the reticular formation extending from dorsal to ventral brain. Within each lineage, distinct patterns of gene expression and organization were localized to specific hindbrain regions. Rostro-caudally sub-populations differ sequentially corresponding to proposed pseudo-rhombomereic boundaries. Dorsal-ventrally, sub-populations correspond to specific migratory positions. Together these data suggests the reticular formation is organized by a highly stereotyped developmental logic.

A novel role for Pax6 in the segmental organization of the hindbrain

Complex patterns and networks of genes coordinate rhombomeric identities, hindbrain segmentation and neuronal differentiation and are responsible for later brainstem functions. Pax6 is a highly conserved transcription factor crucial for neuronal development, yet little is known regarding its early roles during hindbrain segmentation. We show that Pax6 expression is highly dynamic in rhombomeres, suggesting an early function in the hindbrain. Utilization of multiple gain- and loss-of-function approaches in chick and mice revealed that loss of Pax6 disrupts the sharp expression borders of Krox20, Kreisler, Hoxa2, Hoxb1 and EphA and leads to their expansion into adjacent territories, whereas excess Pax6 reduces these expression domains. A mutual negative cross-talk between Pax6 and Krox20 allows these genes to be co-expressed in the hindbrain through regulation of the Krox20-repressor gene Nab1 by Pax6. Rhombomere boundaries are also distorted upon Pax6 manipulations, suggesting a mechanism by which Pax6 acts to set hindbrain segmentation. Finally, FGF signaling acts upstream of the Pax6-Krox20 network to regulate Pax6 segmental expression. This study unravels a novel role for Pax6 in the segmental organization of the early hindbrain and provides new evidence for its significance in regional organization along the central nervous system.

A genetic approach to the transcriptional regulation of Hox gene clusters

The evolution of vertebrate genomes was accompanied by an astounding increase in the complexity of their regulatory modalities. Genetic redundancy resulting from large-scale genome duplications at the base of the chordate tree was repeatedly exploited by the functional redeployment of paralogous genes via innovations in their regulatory circuits. As a paradigm of such regulatory evolution, we have extensively studied those control mechanisms at work in-cis over vertebrate Hox gene clusters. Here, we review the portfolio of genetic strategies that have been developed to tackle the intricate relationship between genomic topography and the transcriptional activities in this gene family, and we describe some of the mechanistic insights we gained by using the HoxD cluster as an example. We discuss the high heuristic value of this system in our general understanding of how changes in transcriptional regulation can diversify gene function and thereby fuel morphological evolution.

Evolution of anterior Hox regulatory elements among chordates

Background

The Hox family of transcription factors has a fundamental role in segmentation pathways and axial patterning of embryonic development and their clustered organization is linked with the regulatory mechanisms governing their coordinated expression along embryonic axes. Among chordates, of particular interest are the Hox paralogous genes in groups 1-4 since their expression is coupled to the control of regional identity in the anterior nervous system, where the highest structural diversity is observed.

Results

To investigate the degree of conservation in cis-regulatory components that form the basis of Hox expression in the anterior nervous system, we have used assays for transcriptional activity in ascidians and vertebrates to compare and contrast regulatory potential. We identified four regulatory sequences located near the CiHox1, CiHox2 and CiHox4 genes of the ascidian Ciona intestinalis which direct neural specific domains of expression. Using functional assays in Ciona and vertebrate embryos in combination with sequence analyses of enhancer fragments located in similar positions adjacent to Hox paralogy group genes, we compared the activity of these four Ciona cis-elements with a series of neural specific enhancers from the amphioxus Hox1-3 genes and from mouse Hox paralogous groups 1-4.

Conclusions

This analysis revealed that Kreisler and Krox20 dependent enhancers critical in segmental regulation of the hindbrain appear to be specific for the vertebrate lineage. In contrast, neural enhancers that function as Hox response elements through the action of Hox/Pbx binding motifs have been conserved during chordate evolution. The functional assays reveal that these Hox response cis-elements are recognized by the regulatory components of different and extant species. Together, our results indicate that during chordate evolution, cis-elements dependent upon Hox/Pbx regulatory complexes, are responsible for key aspects of segmental Hox expression in neural tissue and appeared with urochordates after cephalochordate divergence.

xCITED2 Induces Neural Genes in Animal Cap Explants of Xenopus Embryos

Neural tissue is arisen from presumptive ectoderm via inhibition of bone morphogenetic protein (BMP) signaling during Xenopus early development. Previous studies demonstrate that ectopic expression of dominant negative BMP4 receptor (DNBR) produces neural tissue in animal cap explants (AC) and also increases the expression level of various genes involved in neurogenesis. To investigate detail mechanism of neurogenesis in transcriptional level, we analyzed RNAs increased by DNBR using total RNA sequencing analysis and identified several candidate genes. Among them, xCITED2 (Xenopus CBP/p300-interacting transcription activator) was induced 4.6 fold by DNBR and preferentially expressed in neural tissues at tadpole stage. Ectopic expression of xCITED2 induced anterior neural genes without mesoderm induction and reduced BMP downstream genes, an eye specific marker and posterior neural marker. Taken together, these results suggest that xCITED2 may have a role in the differentiation of anterior neural tissue during Xenopus early development.

Hindbrain patterning requires fine-tuning of early krox20 transcription by Sprouty 4

Vertebrate hindbrain segmentation is an evolutionarily conserved process that involves a complex interplay of transcription factors and signalling pathways. Fibroblast growth factor (FGF) signalling plays a major role, notably by controlling the expression of the transcription factor Krox20 (Egr2), which is required for the formation and specification of two segmental units: rhombomeres (r) 3 and 5. Here, we explore the molecular mechanisms downstream of FGF signalling and the function of Sprouty 4 (Spry4), a negative-feedback regulator of this pathway, in zebrafish. We show that precise modulation of FGF signalling by Spry4 is required to determine the appropriate onset of krox20 transcription in r3 and r5 and, ultimately, rhombomere size in the r3-r5 region. FGF signalling acts by modulating the activity of krox20 initiator enhancer elements B and C; in r5, we show that this regulation is mediated by direct binding of the transcription factor MafB to element B. By contrast, FGF signalling does not control the krox20 autoregulatory element A, which is responsible for amplification and maintenance of krox20 expression. Therefore, early krox20 transcription sets the blueprint for r3-r5 patterning. This work illustrates the necessity for fine-tuning in a common and fundamental patterning process, based on a bistable cell-fate choice involving the coupling of an extracellular gradient with a positive-feedback loop. In this mode of patterning, precision and robustness can be achieved by the introduction of a negative-feedback loop, which, in the hindbrain, is mediated by Spry4.

Analysis of expression and function of FGF-MAPK signaling components in the hindbrain reveals a central role for FGF3 in the regulation of Krox20, mediated by Pea3

The development of the vertebrate hindbrain requires multiple coordinated signals which act via several pathways. One such signal is Fibroblast Growth Factor (FGF), which is necessary for the patterning of a major transcription factor in the hindbrain, Krox20. However, in the chick, it is still not known which specific FGF ligand is responsible for the regulation of Krox20 and how the signal is dispatched. The most characterized signaling pathway which FGF acts through in the nervous system is the MAPK/Erk1/2 pathway. Nevertheless, a detailed analysis of the hindbrain distribution of various components of this pathway has not been fully described. In this study we present a comprehensive atlas of the FGF ligands, receptors and members of the MAPK/Erk1/2 signaling components in subsequent stages of avian hindbrain development. Moreover, we show that FGF is a major signaling pathway that contributes to the activation of ERK1/2 and expression of the downstream targets Pea3 and Erm. Central to this study, we provide multiple evidence that FGF3 is required for the upregulation of Pea3 that in turn is necessary for Krox20 distribution in rhombomeres 3 and 5. These results show for the first time that Pea3 mediates the FGF3 signal to regulate the hindbrain expression of Krox20.

Cis-regulatory characterization of sequence conservation surrounding the Hox4 genes

Hox genes are key regulators of anterior-posterior axis patterning and have a major role in hindbrain development. The zebrafish Hox4 paralogs have strong overlapping activities in hindbrain rhombomeres 7 and 8, in the spinal cord and in the pharyngeal arches. With the aim to predict enhancers that act on the hoxa4a, hoxb4a, hoxc4a and hoxd4a genes, we used sequence conservation around the Hox4 genes to analyze all fish:human conserved non-coding sequences by reporter assays in stable zebrafish transgenesis. Thirty-four elements were functionally tested in GFP reporter gene constructs and more than 100 F1 lines were analyzed to establish a correlation between sequence conservation and cis-regulatory function, constituting a catalog of Hox4 CNEs. Sixteen tissue-specific enhancers could be identified. Multiple alignments of the CNEs revealed paralogous cis-regulatory sequences, however, the CNE sequence similarities were found not to correlate with tissue specificity. To identify ancestral enhancers that direct Hox4 gene activity, genome sequence alignments of mammals, teleosts, horn shark and the cephalochordate amphioxus, which is the most basal extant chordate possessing a single prototypical Hox cluster, were performed. Three elements were identified and two of them exhibited regulatory activity in transgenic zebrafish, however revealing no specificity. Our data show that the approach to identify cis-regulatory sequences by genome sequence alignments and subsequent testing in zebrafish transgenesis can be used to define enhancers within the Hox clusters and that these have significantly diverged in their function during evolution.

Hox genes and segmentation of the hindbrain and axial skeleton

Segmentation is an important process that is frequently used during development to segregate groups of cells with distinct features. Segmental compartments provide a mechanism for generating and organizing regional properties along an embryonic axis and within tissues. In vertebrates the development of two major systems, the hindbrain and the paraxial mesoderm, displays overt signs of compartmentalization and depends on the process of segmentation for their functional organization. The hindbrain plays a key role in regulating head development, and it is a complex coordination center for motor activity, breathing rhythms, and many unconscious functions. The paraxial mesoderm generates somites, which give rise to the axial skeleton. The cellular processes of segmentation in these two systems depend on ordered patterns of Hox gene expression as a mechanism for generating a combinatorial code that specifies unique identities of the segments and their derivatives. In this review, we compare and contrast the signaling inputs and transcriptional mechanisms by which Hox gene regulatory networks are established during segmentation in these two different systems.

Krox20 controls the transcription of its various targets in the developing hindbrain according to multiple modes

The zinc finger transcription factor Krox20 plays an essential role in the vertebrate hindbrain segmentation process. It positively or negatively controls a large variety of other regulatory genes, coordinating delimitation of segmental territories, specification of their identity, and maintenance of their integrity. We have investigated the molecular mechanisms of Krox20 transcriptional control by performing a detailed structure-function analysis of the protein in the developing chick hindbrain. This revealed an unsuspected diversity in the modes of action of a transcription factor in a single tissue, since regulation of each of the five tested target genes requires different parts of the protein and/or presumably different co-factors. The multiplicity of Krox20 functions might rely on this diversity. Investigation of known Krox20 co-factors was initiated in relation to this analysis. Nab was shown to act as a negative feedback modulator of the different Krox20 activating functions in the hindbrain. HCF-1 was found to bind to a Krox20 N-terminal region, which was shown to rely on multiple elements, including acidic domains, to convey Nab activation and Krox20 autoregulation.

Rhombomere-specific analysis reveals the repertoire of genetic cues expressed across the developing hindbrain

BACKGROUND

The Hox family of homeodomain transcription factors comprises pivotal regulators of cell specification and identity during animal development. However, despite their well-defined roles in the establishment of anteroposterior pattern and considerable research into their mechanism of action, relatively few target genes have been identified in the downstream regulatory network. We have sought to investigate this issue, focussing on the developing hindbrain and the cranial motor neurons that arise from this region. The reiterated anteroposterior compartments of the developing hindbrain (rhombomeres (r)) are normally patterned by the combinatorial action of distinct Hox genes. Alteration in the normal pattern of Hox cues in this region results in a transformation of cellular identity to match the remaining Hox profile, similar to that observed in Drosophila homeotic transformations.

RESULTS

To define the repertoire of genes regulated in each rhombomere, we have analysed the transcriptome of each rhombomere from wild-type mouse embryos and not those where pattern is perturbed by gain or loss of Hox gene function. Using microarray and bioinformatic methodologies in conjunction with other confirmatory techniques, we report here a detailed and comprehensive set of potential Hox target genes in r2, r3, r4 and r5. We have demonstrated that the data produced are both fully reflective and predictive of rhombomere identity and, thus, may represent some the of Hox targets. These data have been interrogated to generate a list of candidate genes whose function may contribute to the generation of neuronal subtypes characteristic of each rhombomere. Interestingly, the data can also be classified into genetic motifs that are predicted by the specific combinations of Hox genes and other regulators of hindbrain anteroposterior identity. The sets of genes described in each or combinations of rhombomeres span a wide functional range and suggest that the Hox genes, as well as other regulatory inputs, exert their influence across the full spectrum of molecular machinery.

CONCLUSION

We have performed a systematic survey of the transcriptional status of individual segments of the developing mouse hindbrain and identified hundreds of previously undescribed genes expressed in this region. The functional range of the potential candidate effectors or upstream modulators of Hox activity suggest multiple unexplored mechanisms. In particular, we present evidence of a potential new retinoic acid signalling system in ventral r4 and propose a model for the refinement of identity in this region. Furthermore, the rhombomeres demonstrate a molecular relationship to each other that is consistent with known observations about neurogenesis in the hindbrain. These findings give the first genome-wide insight into the complexity of gene expression during patterning of the developing hindbrain.

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.

Altered brain gene expression profiles associated with the pathogenesis of phenylketonuria in a mouse model

BACKGROUND

Phenylketonuria (PKU) is an autosomal recessive disorder caused by a deficiency of phenylalanine hydroxylase (PAH), which catalyzes the conversion of phenylalanine to tyrosine. The resultant hyperphenylalaninemia causes mental retardation, seizure, and abnormalities in behavior and movement.

METHODS

We analyzed gene expression profiles in brain tissues of Pah(enu2) mice to elucidate the mechanisms involved in phenylalanine-induced neurological damage. The altered gene expression was confirmed by real-time PCR and Western blotting. To identify markers associated with neurological damage, we examined TTR expression in serum by Western blotting.

RESULTS

Gene expression profiling of brain tissue from a mouse model of PKU revealed overexpression of transthyretin (Ttr), sclerostin domain containing 1 (Sostdc1), alpha-Klotho (Kl), prolactin receptor (Prlr), and early growth response 2 (Egr2). In contrast to its overexpression in the brain, TTR expression was low in the sera of PKU mice offered unrestricted access to a diet containing phenylalanine. Expression of TTR decreased in a time-dependent manner in phenylalanine-treated HepG2 cells.

CONCLUSIONS

These findings indicate that Ttr, Sostdc1, Kl, Prlr, and Egr2 can be involved in the pathogenesis of PKU and that phenylalanine might have a direct effect on the level of TTR in serum.

Rostral hindbrain patterning involves the direct activation of a Krox20 transcriptional enhancer by Hox/Pbx and Meis factors

The morphogenesis of the vertebrate hindbrain involves the generation of metameric units called rhombomeres (r), and Krox20 encodes a transcription factor that is expressed in r3 and r5 and plays a major role in this segmentation process. Our knowledge of the basis of Krox20 regulation in r3 is rather confusing, especially concerning the involvement of Hox factors. To investigate this issue, we studied one of the Krox20 hindbrain cis-regulatory sequences, element C, which is active in r3-r5 and which is the only initiator element in r3. We show that element C contains multiple binding sites for Meis and Hox/Pbx factors and that these proteins synergize to activate the enhancer. Mutation of these binding sites allowed us to establish that Krox20 is under the direct transcriptional control of both Meis (presumably Meis2) and Hox/Pbx factors in r3. Furthermore, our data indicate that element C functions according to multiple modes, in Meis-independent or -dependent manners and with different Hox proteins, in r3 and r5. Finally, we show that the Hoxb1 and Krox20 expression domains transiently overlap in prospective r3, and that Hoxb1 binds to element C in vivo, supporting a cell-autonomous involvement of Hox paralogous group 1 proteins in Krox20 regulation. Altogether, our data clarify the molecular mechanisms of an essential step in hindbrain patterning. We propose a model for the complex regulation of Krox20, involving a novel mode of initiation, positive and negative controls by Hox proteins, and multiple direct and indirect autoregulatory loops.

hnf1b genes in zebrafish hindbrain development

The Hnf1b transcription factor acts during formation of rhombomeres (r) 5 and 6 in the hindbrain. To determine if hnf1b is absolutely required in r5/r6, we examined the hnf1b(hi2169) and hnf1b(hi1843) retroviral insertion alleles. Hnf1b(hi2169) shows highly variable residual expression of several genes in r5/r6, but this is not due to full-length hnf1b transcripts persisting in hnf1b(hi2169) embryos, nor to hnf1bl, a novel hnf1 family member expressed in r5 that we identified. Instead, we find evidence for a virus-hnf1b fusion transcript in hnf1b(hi2169) embryos and demonstrate that morpholino-mediated knockdown of this transcript leads to near-undetectable r5 gene expression. The hnf1b(hi1843) allele has a more severe phenotype with near-undetectable expression of r5/r6 genes. We next examined if hoxb1b, which functions upstream of hnf1b in r5/r6 formation, can induce expression of r5/r6 genes in hnf1b mutants. We find that microinjected hoxb1b mRNA induces ectopic gene expression anterior to the hindbrain in hnf1b(hi2169) and hnf1b(hi1843) embryos, but cannot restore gene expression in r5/r6 of the mutants. We conclude that hnf1b(hi2169) is hypomorphic to hnf1b(hi1843) and that, while hnf1b is required for r5/r6 gene expression in the hindbrain, r5/r6 gene expression can be experimentally induced independently of hnf1b anterior to the hindbrain.

Distinct functions of egr gene family members in cognitive processes

The different gene members of the Egr family of transcriptional regulators have often been considered to have related functions in brain, based on their co-expression in many cell-types and structures, the relatively high homology of the translated proteins and their ability to bind to the same consensus DNA binding sequence. Recent research, however, suggest this might not be the case. In this review, we focus on the current understanding of the functional roles of the different Egr family members in learning and memory. We briefly outline evidence from mutant mice that Egr1 is required specifically for the consolidation of long-term memory, while Egr3 is primarily essential for short-term memory. We also review our own recent findings from newly generated forebrain-specific conditional Egr2 mutant mice, which revealed that Egr2, as opposed to Egr1 and Egr3, is dispensable for several forms of learning and memory and on the contrary can act as an inhibitory constraint for certain cognitive functions. The studies reviewed here highlight the fact that Egr family members may have different, and in certain circumstances antagonistic functions in the adult brain.

An EGR2/CITED1 transcription factor complex and the 14-3-3sigma tumor suppressor are involved in regulating ErbB2 expression in a transgenic-mouse model of human breast cancer

Amplification and elevated expression of the ErbB2 receptor tyrosine kinase occurs in 20% of human breast cancers and is associated with a poor prognosis. We have previously demonstrated that mammary tissue-specific expression of activated ErbB2 under the control of its endogenous promoter results in mammary tumor formation. Tumor development was associated with amplification and overexpression of ErbB2 at both the transcript and protein levels. Here we demonstrate that the EGR2/Krox20 transcription factor and its coactivator CITED1 are coordinately upregulated during ErbB2 tumor induction. We have identified an EGR2 binding site in the erbB2 promoter and demonstrated by chromatin immunoprecipitation assays that EGR2 and CITED1 associate specifically with this region of the promoter. EGR2 and CITED1 were shown to associate, and expression from an erbB2 promoter-reporter construct was stimulated by EGR2 and was further enhanced by CITED1 coexpression. Furthermore, expression of the 14-3-3sigma tumor suppressor led to downregulation of ErbB2 protein levels and relocalization of EGR2 from the nucleus to the cytoplasm. Taken together, these observations suggest that, in addition to an increased gene copy number and upregulation of EGR2 and CITED1, an elevated erbB2 transcript level involves the loss of 14-3-3sigma, which sequesters a key transcriptional regulator of the erbB2 promoter.

Direct regulation of vHnf1 by retinoic acid signaling and MAF-related factors in the neural tube

The homeodomain transcription factor vHNF1 plays an essential role in the patterning of the caudal segmented hindbrain, where it participates in the definition of the boundary between rhombomeres (r) 4 and 5 and in the specification of the identity of r5 and r6. Understanding the molecular basis of vHnf1 own expression therefore constitutes an important issue to decipher the regulatory network governing hindbrain patterning. We have identified a highly conserved 800-bp enhancer element located in the fourth intron of vHnf1 and whose activity recapitulates vHnf1 neural expression in transgenic mice. Functional analysis of this enhancer revealed that it contains two types of essential motifs, a retinoic acid response element and two half T-MARE sites, indicating that it integrates direct inputs from the retinoic acid signaling cascade and MAF-related factors. Our data suggest that MAFB, which is itself regulated by vHNF1, acts as a positive modulator of vHnf1 in r5 and r6, whereas another MAF-related factor is absolutely required for the expression of vHnf1 in both the hindbrain and the spinal cord. We propose a model accounting for the initiation and maintenance phases of vHnf1 expression and for the establishment of the r4/r5 boundary, based on cooperative contributions of Maf factors and retinoic acid signaling.

PIASxbeta acts as an activator of Hoxb1 and is antagonized by Krox20 during hindbrain segmentation

The zinc-finger transcription factor Krox20 constitutes a key regulator of hindbrain development, essential for the formation and specification of rhombomeres (r) 3 and 5. It is in particular responsible for the respective activation and repression of odd- and even-numbered rhombomere-specific genes, which include Hox genes. In this study, we have identified PIASxbeta as a novel direct interactor of Krox20. In addition, we found that PIASxbeta is able to activate the r4-specific gene Hoxb1. Binding of Krox20 prevents this activation, providing a molecular basis for the repression of Hoxb1 by Krox20. The same domain in the Krox20 protein, the zinc-fingers, is involved in DNA binding for transcriptional activation and in interaction with PIASxbeta for transcriptional repression, although the actual precise contacts are different. Our findings add an additional level in the complexity of Hox gene regulation and provide an example of how a single regulator can coordinate the activation and repression of a set of genes by very different mechanisms, acting as a molecular switch to specify cell identity and fate.

Conserved co-regulation and promoter sharing of hoxb3a and hoxb4a in zebrafish

The expression of zebrafish hoxb3a and hoxb4a has been found to be mediated through five transcripts, hoxb3a transcripts I-III and hoxb4a transcripts I-II, driven by four promoters. A "master" promoter, located about 2 kb downstream of hoxb5a, controls transcription of a pre-mRNA comprising exon sequences of both genes. This unique gene structure is proposed to provide a novel mechanism to ensure overlapping, tissue-specific expression of both genes in the posterior hindbrain and spinal cord. Transgenic approaches were used to analyze the functions of zebrafish hoxb3a/hoxb4a promoters and enhancer sequences containing regions of homology that were previously identified by comparative genomics. Two neural enhancers were shown to establish specific anterior expression borders within the hindbrain and mediate expression in defined neuronal populations derived from hindbrain rhombomeres (r) 5 to 8, suggesting a late role of the genes in neuronal cell lineage specification. Species comparison showed that the zebrafish hoxb3a r5 and r6 enhancer corresponded to a sequence within the mouse HoxA cluster controlling activity of Hoxa3 in r5 and r6, whereas a homologous region within the HoxB cluster activated Hoxb3 expression but limited to r5. We conclude that the similarity of hoxb3a/Hoxa3 regulatory mechanisms reflect the shared descent of both genes from a single ancestral paralog group 3 gene.

Krox20hindbrain cis-regulatory landscape: interplay between multiple long-range initiation and autoregulatory elements

The vertebrate hindbrain is subject to a transient segmentation process leading to the formation of seven or eight metameric territories termed rhombomeres (r). This segmentation provides the basis for the subsequent establishment of hindbrain neuronal organization and participates in the patterning of the neural crest involved in craniofacial development. The zinc-finger gene Krox20 is expressed in r3 and r5, and encodes a transcription factor that plays a key role in hindbrain segmentation,coordinating segment formation, specification of odd- and even-numbered rhombomeres, and cell segregation between adjacent segments, through the regulation of numerous downstream genes. In order to further elucidate the genetic network underlying hindbrain segmentation, we have undertaken the analysis of the cis-regulatory sequences governing Krox20 expression. We have found that the control of Krox20 transcription relies on three very long-range (200 kb) enhancer elements (A, B and C) that are conserved between chick, mouse and human genomes. Elements B and C are activated at the earliest stage of Krox20 expression in r5 and r3-r5,respectively, and do not require the Krox20 protein. These elements are likely to function as initiators of Krox20 expression. Element B contains a binding site for the transcription factor vHNF1, the mutation of which abolishes its activity, suggesting that vHNF1 is a direct initiator of Krox20 expression in r5. Element A contains Krox20-binding sites,which are required, together with the Krox20 protein, for its activity. This element therefore allows the establishment of a direct positive autoregulatory loop, which takes the relay of the initiator elements and maintains Krox20 expression. Together, our studies provide a basis for a model of the molecular mechanisms controlling Krox20 expression in the developing hindbrain and neural crest.

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.

Evolution of the brain developmental plan: Insights from agnathans

In vertebrate evolution, the brain exhibits both conserved and unique morphological features in each animal group. Thus, the molecular program of nervous system development is expected to have experienced various changes through evolution. In this review, we discuss recent data from the agnathan lamprey (jawless vertebrate) together with available information from amphioxus and speculate the sequence of changes during chordate evolution that have been brought into the brain developmental plan to yield the current variety of the gnathostome (jawed vertebrate) brains.

vhnf1 integrates global RA patterning and local FGF signals to direct posterior hindbrain development in zebrafish

The vertebrate hindbrain is transiently divided along the anterior-posterior axis into seven morphologically and molecularly distinct segments, or rhombomeres, that correspond to Hox expression domains. The establishment of a proper 'hox code' is required for the development of unique rhombomere identities, including specification of neuronal fates. valentino (val), the zebrafish ortholog of mafB/Kreisler (Kr), encodes a bZip transcription factor that is required cell autonomously for the development of rhombomere (r) 5 and r6 and for activation of Hox group 3 gene expression. Recent work has demonstrated that the expression of val itself depends on three factors: retinoic acid (RA) signals from the paraxial mesoderm; fibroblast growth factor (Fgf) signals from r4; and variant hepatocyte nuclear factor 1 (vhnf1, also known as tcf2), a homeodomain transcription factor expressed posterior to the r4-5 boundary. We have investigated the interactions between these inputs onto val expression in the developing zebrafish hindbrain. We show that RA induces val expression via activation of vhnf1 expression in the hindbrain. Fgf signals from r4, acting through the MapK pathway, then cooperate with Vhnf1 to activate val expression and subsequent r5 and r6 development. Additionally, vhnf1 and val function as part of a multistep process required for the repression of r4 identity in the posterior hindbrain. vhnf1 acts largely independently of val to repress the r4 'hox code' posterior to the r4-5 boundary and therefore to block acquisition of r4-specific neuronal fates in the posterior hindbrain. However, vhnf1 is not able to repress all aspects of r4 identity equivalently. val is required downstream of vhnf1 to repress r4-like cell-surface properties, as determined by an 'Eph-ephrin code', by repressing ephrin-B2a expression in r5 and r6. The different requirements for vhnf1 and val to repress hoxb1a and ephrin-B2a, respectively, demonstrate that not all aspects of an individual rhombomere's identity are regulated coordinately.

Respiratory adaptations to lung morphological defects in adult mice lacking Hoxa5 gene function

The Hoxa5 mutation is associated with a high perinatal mortality rate caused by a severe obstruction of the laryngotracheal airways, pulmonary dysmorphogenesis, and a decreased production of surfactant proteins. Surviving Hoxa5(-/-) mutant mice also display lung anomalies with deficient alveolar septation and areas of collapsed tissue, thus demonstrating the importance of Hoxa5 throughout lung development and maturation. Here, we address the functional consequences of the Hoxa5 mutation on respiration and chemoreflexes by comparing the breathing pattern of Hoxa5(-/-) mice to that of wild-type animals under resting conditions and during exposure to moderate ventilatory stimuli such as hypoxia and hypercapnia. Resting Hoxa5(-/-) mice present a higher breathing frequency and overall minute ventilation that likely compensate for their reduced lung alveolar surface available for gas exchange and their increased upper airway resistance. When exposed to ventilatory stimuli, Hoxa5(-/-) mice maintain the higher minute ventilation by adapting the tidal volume and/or the breathing frequency. The minute ventilation increase seen during hypoxia was similar for both groups of mice; however, the dynamics of the frequency response was genotype-dependent. The hypercapnic ventilatory response did not differ between genotypes. These findings reveal the strategies allowing survival of Hoxa5(-/-) mice facing morphologic anomalies leading to a significant deficit in gas exchange capacity.

Extracellular signal-regulated kinase induces the megakaryocyte GPIIb/CD41 gene through MafB/Kreisler

Extracellular signal-regulated kinase (ERK) facilitates cell cycle progression in most mammalian cells, but in certain cell types prolonged signaling through this pathway promotes differentiation and lineage-specific gene expression through mechanisms that are poorly understood. Here, we characterize the transcriptional regulation of platelet GPIIb integrin (CD41) by ERK during megakaryocyte differentiation. ERK-dependent transactivation involves the proximal promoter of GPIIb within 114 bp upstream of the transcriptional start site. GATA, Ets, and Sp1 consensus sequences within this region are each necessary and function combinatorially in ERK-activated transcription. MafB/Kreisler is induced in response to ERK and synergizes with GATA and Ets to enhance transcription from the proximal promoter. The requirement for MafB in promoter regulation is demonstrated by inhibition of transactivation following dominant-negative or antisense suppression of MafB function. Thus, ERK promotes megakaryocyte differentiation by coordinate regulation of nuclear factors that synergize in GPIIb promoter regulation. These results establish a novel role for MafB as a regulator of ERK-induced gene expression.

Comparative genomic analysis of vertebrate Hox3 and Hox4 genes

We used a comparative genomic approach to identify putative cis-acting regulatory sequences of the zebrafish hoxb3a and hoxb4a genes. We aligned genomic sequences spanning the clustered Hoxb1 to Hoxb5 genes from pufferfish, mice, and humans with the zebrafish hoxba and hoxbb cluster sequences. We identified multiple blocks of conserved sequences in non-coding regions within and surrounding the Hoxb3/b4 gene locus; a subset of these blocks are conserved in the zebrafish hoxbb cluster, despite loss of hoxb3/b4 genes. Overall, we find that the architecture of the Hoxb3/b4 loci and of the conserved sequence elements is very similar in teleosts and mammals. Our analyses also revealed two alternative transcripts of the zebrafish hoxb3a gene and an exon sequence unusually located 10 kb upstream of adjacent hoxb4a; an equivalent murine Hoxb3 exon has not yet been confirmed. We show that many of the Hoxb3/b4 conserved non-coding sequences correlate with functional neural enhancers previously described in the mouse. Further, within the conserved non-coding sequences we have identified binding sites for transcription factors, including Kreisler/Valentino, Krox20, Hox, and Pbx, some of which had not been previously described for the mouse. Finally, we demonstrate that the regulatory sequences of zebrafish hoxa3a are divergent with respect to the mouse ortholog Hoxa3, or the paralog hoxb3a. Despite limited conservation of regulatory sequences, zebrafish hoxa3a and hoxb3a genes share very similar expression profiles.

Establishment of embryonic neuroepithelial cell lines exhibiting an epiplastic expression pattern of region specific markers

Neuroepithelial b2T cells were derived from the hindbrain and the spinal cord of mouse transgenic embryos, which expressed SV40 T antigen under the control of a Hoxb2 enhancer. Strikingly, b2T cell lines of either origin exhibit a very similar gene expression pattern, including markers of the hindbrain and the spinal cord, such as Hox genes, but not of more anterior cephalic regions. In addition, the broad expression pattern of b2T cells, probably linked to culture conditions, appeared to be appropriately modulated when the cells were reimplanted at different longitudinal levels into chick host embryos, suggesting that these cells are responsive to exogenous signalling mechanisms. Further support for these allegations was obtained by culturing b2T cells in defined medium and by assessing the expression of Krox20, an odd-numbered rhombomere marker, which appeared to be modulated by a complex interplay between FGF, retinoic acid (RA), and noggin. With respect to these as yet unique properties, b2T cells constitute an original alternative tool to in vivo models for the analysis of molecular pathways involved in the patterning of the neural tube.

Identification of novel roles of the cytochrome p450 system in early embryogenesis: effects on vasculogenesis and retinoic Acid homeostasis

The cytochrome P450-dependent monooxygenase system catalyzes the metabolism of xenobiotics and endogenous compounds, including hormones and retinoic acid. In order to establish the role of these enzymes in embryogenesis, we have inactivated the system through the deletion of the gene for the electron donor to all microsomal P450 proteins, cytochrome P450 reductase (Cpr). Mouse embryos homozygous for this deletion died in early to middle gestation (approximately 9.5 days postcoitum [dpc]) and exhibited a number of novel phenotypes, including the severe inhibition of vasculogenesis and hematopoiesis. In addition, defects in the brain, limbs, and cell types where CPR was shown to be expressed were observed. Some of the observed abnormalities have been associated with perturbations in retinoic acid homeostasis in later embryogenesis. Consistent with this possibility, embryos at 9.5 dpc had significantly elevated levels of retinoic acid and reduced levels of retinol. Further, some of the observed phenotypes could be either reversed or exacerbated by decreasing or increasing maternal retinoic acid exposure, respectively. Detailed analysis demonstrated a close relationship between the observed phenotype and the expression of genes controlling vasculogenesis. These data demonstrate that the cytochrome P450 system plays a key role in early embryonic development; this process appears to be, at least in part, controlled by regional concentrations of retinoic acid and has profound effects on blood vessel formation.

Retinal pigmented epithelium determination requires the redundant activities of Pax2 and Pax6

The transcription factors Pax2 and Pax6 are co-expressed in the entire optic vesicle (OV) prior and concomitant with the establishment of distinct neuroretinal, retinal, pigmented-epithelial and optic-stalk progenitor domains, suggesting redundant functions during retinal determination. Pax2; Pax6 compound mutants display a dose-dependent reduction in the expression of the melanocyte determinant Mitf, accompanied by transdifferentiation of retinal pigmented epithelium (RPE) into neuroretina (NR) in Pax2(-/-); Pax6(+/-) embryos, which strongly resembles the phenotype of Mitf-null mutants. In Pax2(-/-); Pax6(-/-) OVs Mitf fails to be expressed and NR markers occupy the area that usually represents the Mitf(+) RPE domain. Furthermore, both, Pax2 and Pax6 bind to and activate a MITF RPE-promoter element in vitro, whereas prolonged expression of Pax6 in the Pax2-positive optic stalk leads to ectopic Mitf expression and RPE differentiation in vivo. Together, these results demonstrate that the redundant activities of Pax2 and Pax6 direct the determination of RPE, potentially by directly controlling the expression of RPE determinants.

c-jun regulation and function in the developing hindbrain

Hindbrain development is a well-characterised segmentation process in vertebrates. The bZip transcription factor MafB/kreisler is specifically expressed in rhombomeres (r) 5 and 6 of the developing vertebrate hindbrain and is required for proper caudal hindbrain segmentation. Here, we provide evidence that the mouse protooncogene c-jun, which encodes a member of the bZip family, is coexpressed with MafB in prospective r5 and r6. Analysis of mouse mutants suggests that c-jun expression in these territories is dependent on MafB but independent of the zinc-finger transcription factor Krox20, another essential determinant of r5 development. Loss- and gain-of-function studies, performed in mouse and chick embryos, respectively, demonstrate that c-Jun participates, together with MafB and Krox20, in the transcriptional activation of the Hoxb3 gene in r5. The action of c-Jun is likely to be direct, since c-Jun homodimers and c-Jun/MafB heterodimers can bind to essential regulatory elements within the transcriptional enhancer responsible for Hoxb3 expression in r5. These data indicate that c-Jun acts both as a downstream effector and a cofactor of MafB and belongs to the complex network of factors governing hindbrain patterning.