Expression of the homeo box-containing gene En-2 delineates a specific region of the developing mouse brain

Leveraging neighborhood representations of single-cell data to achieve sensitive DE testing with miloDE

Single-cell RNA-sequencing enables testing for differential expression (DE) between conditions at a cell type level. While powerful, one of the limitations of such approaches is that the sensitivity of DE testing is dictated by the sensitivity of clustering, which is often suboptimal. To overcome this, we present miloDE-a cluster-free framework for DE testing (available as an open-source R package). We illustrate the performance of miloDE on both simulated and real data. Using miloDE, we identify a transient hemogenic endothelia-like state in mouse embryos lacking Tal1 and detect distinct programs during macrophage activation in idiopathic pulmonary fibrosis.

Engrailed: Pathological and physiological effects of a multifunctional developmental gene

Engrailed-1 (EN1) is a developmental gene that encodes En1, a highly conserved transcription factor involved in regionalization during early embryogenesis and in the later maintenance of normal neurons. After birth, EN1 still plays a role in the development and physiology of the body; for example, it exerts a protective effect on midbrain dopaminergic (mDA) neurons, and loss of EN1 causes mDA neurons in the ventral midbrain to gradually die approximately 6 weeks after birth, resulting in motor and nonmotor symptoms similar to those observed in Parkinson's disease. Notably, EN1 has been identified as a possible susceptibility gene for idiopathic Parkinson's disease in humans. EN1 is involved in the processes of wound-healing scar production and tissue and organ fibrosis. Additionally, EN1 can lead to tumorigenesis and thus provides a target for the treatment of some tumors. In this review, we summarize the effects of EN1 on embryonic organ development, describe the consequences of the deletion or overexpression of the EN1 gene, and discuss the pathways in which EN1 is involved. We hope to clarify the role of EN1 as a developmental gene and present potential therapeutic targets for diseases involving the EN1 gene.

One-carbon metabolism is required for epigenetic stability in the mouse placenta

One-carbon metabolism, including the folate cycle, has a crucial role in fetal development though its molecular function is complex and unclear. The hypomorphic Mtrr ^gt allele is known to disrupt one-carbon metabolism, and thus methyl group availability, leading to several developmental phenotypes (e.g., neural tube closure defects, fetal growth anomalies). Remarkably, previous studies showed that some of the phenotypes were transgenerationally inherited. Here, we explored the genome-wide epigenetic impact of one-carbon metabolism in placentas associated with fetal growth phenotypes and determined whether specific DNA methylation changes were inherited. Firstly, methylome analysis of Mtrr ^gt/gt homozygous placentas revealed genome-wide epigenetic instability. Several differentially methylated regions (DMRs) were identified including at the Cxcl1 gene promoter and at the En2 gene locus, which may have phenotypic implications. Importantly, we discovered hypomethylation and ectopic expression of a subset of ERV elements throughout the genome of Mtrr ^gt/gt placentas with broad implications for genomic stability. Next, we determined that known spermatozoan DMRs in Mtrr ^gt/gt males were reprogrammed in the placenta with little evidence of direct or transgenerational germline DMR inheritance. However, some spermatozoan DMRs were associated with placental gene misexpression despite normalisation of DNA methylation, suggesting the inheritance of an alternative epigenetic mechanism. Integration of published wildtype histone ChIP-seq datasets with Mtrr ^gt/gt spermatozoan methylome and placental transcriptome datasets point towards H3K4me3 deposition at key loci. These data suggest that histone modifications might play a role in epigenetic inheritance in this context. Overall, this study sheds light on the mechanistic complexities of one-carbon metabolism in development and epigenetic inheritance.

Integration of spatial and single-cell transcriptomic data elucidates mouse organogenesis

Molecular profiling of single cells has advanced our knowledge of the molecular basis of development. However, current approaches mostly rely on dissociating cells from tissues, thereby losing the crucial spatial context of regulatory processes. Here, we apply an image-based single-cell transcriptomics method, sequential fluorescence in situ hybridization (seqFISH), to detect mRNAs for 387 target genes in tissue sections of mouse embryos at the 8-12 somite stage. By integrating spatial context and multiplexed transcriptional measurements with two single-cell transcriptome atlases, we characterize cell types across the embryo and demonstrate that spatially resolved expression of genes not profiled by seqFISH can be imputed. We use this high-resolution spatial map to characterize fundamental steps in the patterning of the midbrain-hindbrain boundary (MHB) and the developing gut tube. We uncover axes of cell differentiation that are not apparent from single-cell RNA-sequencing (scRNA-seq) data, such as early dorsal-ventral separation of esophageal and tracheal progenitor populations in the gut tube. Our method provides an approach for studying cell fate decisions in complex tissues and development.

Dissection of the Fgf8 regulatory landscape by in vivo CRISPR-editing reveals extensive intra- and inter-enhancer redundancy

Developmental genes are often regulated by multiple elements with overlapping activity. Yet, in most cases, the relative function of those elements and their contribution to endogenous gene expression remain poorly characterized. An example of this phenomenon is that distinct sets of enhancers have been proposed to direct Fgf8 in the limb apical ectodermal ridge and the midbrain-hindbrain boundary. Using in vivo CRISPR/Cas9 genome engineering, we functionally dissect this complex regulatory ensemble and demonstrate two distinct regulatory logics. In the apical ectodermal ridge, the control of Fgf8 expression appears distributed between different enhancers. In contrast, we find that in the midbrain-hindbrain boundary, one of the three active enhancers is essential while the other two are dispensable. We further dissect the essential midbrain-hindbrain boundary enhancer to reveal that it is also composed by a mixture of essential and dispensable modules. Cross-species transgenic analysis of this enhancer suggests that its composition may have changed in the vertebrate lineage.

Engrailed-2 (En2) deletion produces multiple neurodevelopmental defects in monoamine systems, forebrain structures and neurogenesis and behavior

Many genes involved in brain development have been associated with human neurodevelopmental disorders, but underlying pathophysiological mechanisms remain undefined. Human genetic and mouse behavioral analyses suggest that ENGRAILED-2 (EN2) contributes to neurodevelopmental disorders, especially autism spectrum disorder. In mouse, En2 exhibits dynamic spatiotemporal expression in embryonic mid-hindbrain regions where monoamine neurons emerge. Considering their importance in neuropsychiatric disorders, we characterized monoamine systems in relation to forebrain neurogenesis in En2-knockout (En2-KO) mice. Transmitter levels of serotonin, dopamine and norepinephrine (NE) were dysregulated from Postnatal day 7 (P7) to P21 in En2-KO, though NE exhibited the greatest abnormalities. While NE levels were reduced ∼35% in forebrain, they were increased 40 -: 75% in hindbrain and cerebellum, and these patterns paralleled changes in locus coeruleus (LC) fiber innervation, respectively. Although En2 promoter was active in Embryonic day 14.5 -: 15.5 LC neurons, expression diminished thereafter and gene deletion did not alter brainstem NE neuron numbers. Significantly, in parallel with reduced NE levels, En2-KO forebrain regions exhibited reduced growth, particularly hippocampus, where P21 dentate gyrus granule neurons were decreased 16%, suggesting abnormal neurogenesis. Indeed, hippocampal neurogenic regions showed increased cell death (+77%) and unexpectedly, increased proliferation. Excess proliferation was restricted to early Sox2/Tbr2 progenitors whereas increased apoptosis occurred in differentiating (Dcx) neuroblasts, accompanied by reduced newborn neuron survival. Abnormal neurogenesis may reflect NE deficits because intra-hippocampal injections of β-adrenergic agonists reversed cell death. These studies suggest that disruption of hindbrain patterning genes can alter monoamine system development and thereby produce forebrain defects that are relevant to human neurodevelopmental disorders.

Self-organization of polarized cerebellar tissue in 3D culture of human pluripotent stem cells

During cerebellar development, the main portion of the cerebellar plate neuroepithelium gives birth to Purkinje cells and interneurons, whereas the rhombic lip, the germinal zone at its dorsal edge, generates granule cells and cerebellar nuclei neurons. However, it remains elusive how these components cooperate to form the intricate cerebellar structure. Here, we found that a polarized cerebellar structure self-organizes in 3D human embryonic stem cell (ESC) culture. The self-organized neuroepithelium differentiates into electrophysiologically functional Purkinje cells. The addition of fibroblast growth factor 19 (FGF19) promotes spontaneous generation of dorsoventrally polarized neural-tube-like structures at the level of the cerebellum. Furthermore, addition of SDF1 and FGF19 promotes the generation of a continuous cerebellar plate neuroepithelium with rhombic-lip-like structure at one end and a three-layer cytoarchitecture similar to the embryonic cerebellum. Thus, human-ESC-derived cerebellar progenitors exhibit substantial self-organizing potential for generating a polarized structure reminiscent of the early human cerebellum at the first trimester.

Mapping the Shh long-range regulatory domain

Coordinated gene expression controlled by long-distance enhancers is orchestrated by DNA regulatory sequences involving transcription factors and layers of control mechanisms. The Shh gene and well-established regulators are an example of genomic composition in which enhancers reside in a large desert extending into neighbouring genes to control the spatiotemporal pattern of expression. Exploiting the local hopping activity of the Sleeping Beauty transposon, the lacZ reporter gene was dispersed throughout the Shh region to systematically map the genomic features responsible for expression activity. We found that enhancer activities are retained inside a genomic region that corresponds to the topological associated domain (TAD) defined by Hi-C. This domain of approximately 900 kb is in an open conformation over its length and is generally susceptible to all Shh enhancers. Similar to the distal enhancers, an enhancer residing within the Shh second intron activates the reporter gene located at distances of hundreds of kilobases away, suggesting that both proximal and distal enhancers have the capacity to survey the Shh topological domain to recognise potential promoters. The widely expressed Rnf32 gene lying within the Shh domain evades enhancer activities by a process that may be common among other housekeeping genes that reside in large regulatory domains. Finally, the boundaries of the Shh TAD do not represent the absolute expression limits of enhancer activity, as expression activity is lost stepwise at a number of genomic positions at the verges of these domains.

The double inhibition of endogenously produced BMP and Wnt factors synergistically triggers dorsal telencephalic differentiation of mouse ES cells

Embryonic stem (ES) cells are becoming a popular model of in vitro neurogenesis, as they display intrinsic capability to generate neural progenitors that undergo the known steps of in vivo neural development. These include the acquisition of distinct regional fates, which depend on growth factors and signals that are present in the culture medium. The control of the intracellular signaling that is active at different steps of ES cell neuralization, even when cells are cultured in chemically defined medium, is complicated by the endogenous production of growth factors. However, this endogenous production has been poorly investigated so far. To address this point, we performed a high-throughput analysis of the expression of morphogens during mouse ES cell neuralization in minimal medium. We found that during their neuralization, ES cells increased the expression of members of Wnt, Fibroblast Growth Factor (FGF), and BMP families. Conversely, the expression of Activin/Nodal and Shh ligands was low in early steps of neuralization. In this experimental condition, neural progenitors and neurons generated by ES cells expressed a gene expression profile that was consistent with a midbrain identity. We found that endogenous BMP and Wnt signaling, but not FGF signaling, synergistically affected ES cell neural patterning, by turning off a profile of dorsal/telencephalic gene expression. Double BMP and Wnt inhibition allowed neuralized ES cells to sequentially activate key genes of cortical differentiation. Our findings are consistent with a novel synergistic effect of Wnt and BMP endogenous signaling of ES cells in inhibiting a cortical differentiation program.

Lineage-specific function of Engrailed-2 in the progression of chronic myelogenous leukemia to T-cell blast crisis

In hematopoietic malignancies, oncogenic alterations interfere with cellular differentiation and lead to tumoral development. Identification of the proteins regulating differentiation is essential to understand how they are altered in malignancies. Chronic myelogenous leukemia (CML) is a biphasic disease initiated by an alteration taking place in hematopoietic stem cells. CML progresses to a blast crisis (BC) due to a secondary differentiation block in any of the hematopoietic lineages. However, the molecular mechanisms of CML evolution to T-cell BC remain unclear. Here, we have profiled the changes in DNA methylation patterns in human samples from BC-CML, in order to identify genes whose expression is epigenetically silenced during progression to T-cell lineage-specific BC. We have found that the CpG-island of the ENGRAILED-2 (EN2) gene becomes methylated in this progression. Afterwards, we demonstrate that En2 is expressed during T-cell development in mice and humans. Finally, we further show that genetic deletion of En2 in a CML transgenic mouse model induces a T-cell lineage BC that recapitulates human disease. These results identify En2 as a new regulator of T-cell differentiation whose disruption induces a malignant T-cell fate in CML progression, and validate the strategy used to identify new developmental regulators of hematopoiesis.

Engrailed2 modulates cerebellar granule neuron precursor proliferation, differentiation and insulin-like growth factor 1 signaling during postnatal development

Background

The homeobox transcription factor Engrailed2 (En2) has been studied extensively in neurodevelopment, particularly in the midbrain/hindbrain region and cerebellum, where it exhibits dynamic patterns of expression and regulates cell patterning and morphogenesis. Because of its roles in regulating cerebellar development and evidence of cerebellar pathology in autism spectrum disorder (ASD), we previously examined an ENGRAILED2 association and found evidence to support EN2 as a susceptibility gene, a finding replicated by several other investigators. However, its functions at the cell biological level remain undefined. In the mouse, En2 gene is expressed in granule neuron precursors (GNPs) just as they exit the cell cycle and begin to differentiate, raising the possibility that En2 may modulate these developmental processes.

Methods

To define En2 functions, we examined proliferation, differentiation and signaling pathway activation in En2 knockout (KO) and wild-type (WT) GNPs in response to a variety of extracellular growth factors and following En2 cDNA overexpression in cell culture. In vivo analyses of cerebellar GNP proliferation as well as responses to insulin-like growth factor-1 (IGF1) treatment were also conducted.

Results

Proliferation markers were increased in KO GNPs in vivo and in 24-h cultures, suggesting En2 normally serves to promote cell cycle exit. Significantly, IGF1 stimulated greater DNA synthesis in KO than WT cells in culture, a finding associated with markedly increased phospho-S6 kinase activation. Similarly, there was three-fold greater DNA synthesis in the KO cerebellum in response to IGF1 in vivo. On the other hand, KO GNPs exhibited reduced neurite outgrowth and differentiation. Conversely, En2 overexpression increased cell cycle exit and promoted neuronal differentiation.

Conclusions

In aggregate, our observations suggest that the ASD-associated gene En2 promotes GNP cell cycle exit and differentiation, and modulates IGF1 activity during postnatal cerebellar development. Thus, genetic/epigenetic alterations of EN2 expression may impact proliferation, differentiation and IGF1 signaling as possible mechanisms that may contribute to ASD pathogenesis.

Cellular programming and reprogramming: sculpting cell fate for the production of dopamine neurons for cell therapy

Pluripotent stem cells are regarded as a promising cell source to obtain human dopamine neurons in sufficient amounts and purity for cell replacement therapy. Importantly, the success of clinical applications depends on our ability to steer pluripotent stem cells towards the right neuronal identity. In Parkinson disease, the loss of dopamine neurons is more pronounced in the ventrolateral population that projects to the sensorimotor striatum. Because synapses are highly specific, only neurons with this precise identity will contribute, upon transplantation, to the synaptic reconstruction of the dorsal striatum. Thus, understanding the developmental cell program of the mesostriatal dopamine neurons is critical for the identification of the extrinsic signals and cell-intrinsic factors that instruct and, ultimately, determine cell identity. Here, we review how extrinsic signals and transcription factors act together during development to shape midbrain cell fates. Further, we discuss how these same factors can be applied in vitro to induce, select, and reprogram cells to the mesostriatal dopamine fate.

Autism-relevant social abnormalities and cognitive deficits in engrailed-2 knockout mice

ENGRAILED 2 (En2), a homeobox transcription factor, functions as a patterning gene in the early development and connectivity of rodent hindbrain and cerebellum, and regulates neurogenesis and development of monoaminergic pathways. To further understand the neurobiological functions of En2, we conducted neuroanatomical expression profiling of En2 wildtype mice. RTQPCR assays demonstrated that En2 is expressed in adult brain structures including the somatosensory cortex, hippocampus, striatum, thalamus, hypothalamus and brainstem. Human genetic studies indicate that EN2 is associated with autism. To determine the consequences of En2 mutations on mouse behaviors, including outcomes potentially relevant to autism, we conducted comprehensive phenotyping of social, communication, repetitive, and cognitive behaviors. En2 null mutants exhibited robust deficits in reciprocal social interactions as juveniles and adults, and absence of sociability in adults, replicated in two independent cohorts. Fear conditioning and water maze learning were impaired in En2 null mutants. High immobility in the forced swim test, reduced prepulse inhibition, mild motor coordination impairments and reduced grip strength were detected in En2 null mutants. No genotype differences were found on measures of ultrasonic vocalizations in social contexts, and no stereotyped or repetitive behaviors were observed. Developmental milestones, general health, olfactory abilities, exploratory locomotor activity, anxiety-like behaviors and pain responses did not differ across genotypes, indicating that the behavioral abnormalities detected in En2 null mutants were not attributable to physical or procedural confounds. Our findings provide new insight into the role of En2 in complex behaviors and suggest that disturbances in En2 signaling may contribute to neuropsychiatric disorders marked by social and cognitive deficits, including autism spectrum disorders.

The engrailed homeobox genes are required in multiple cell lineages to coordinate sequential formation of fissures and growth of the cerebellum

The layered cortex of the cerebellum is folded along the anterior-posterior axis into lobules separated by fissures, allowing the large number of cells needed for advanced cerebellar functions to be packed into a small volume. During development, the cerebellum begins as a smooth ovoid structure with two progenitor zones, the ventricular zone and upper rhombic lip, which give rise to distinct cell types in the mature cerebellum. Initially, the cerebellar primordium is divided into five cardinal lobes, which are subsequently further subdivided by fissures. The cellular processes and genes that regulate the formation of a normal pattern of fissures are poorly understood. The engrailed genes (En1 and En2) are expressed in all cerebellar cell types and are critical for regulating formation of specific fissures. However, the cerebellar cell types that En1 and En2 act in to control growth and/or patterning of fissures has not been determined. We conditionally eliminated En2 or En1 and En2 either in both progenitor zones and their descendents or in the two complementary sets of cells derived from each progenitor zone. En2 was found to be required only transiently in the progenitor zones and their immediate descendents to regulate formation of three fissures and for general growth of the cerebellum. In contrast, En1 and En2 have overlapping functions in the cells derived from each progenitor zone in regulating formation of additional fissures and for extensive cerebellar growth. Furthermore, En1/2 function in ventricular zone-derived cells plays a more significant role in determining the timing of initiation and positioning of fissures, whereas in upper rhombic lip-derived cells the genes are more important in regulating cerebellar growth. Our studies reveal the complex manner in which the En genes control cerebellar growth and foliation in distinct cell types.

Geminin is required for epithelial to mesenchymal transition at gastrulation

Geminin is a multifunctional protein previously suggested to both maintain the bone morphogenetic protein inhibition required for neural induction and to control cell-cycle progression and cell fate in the early embryo. Since Geminin is required in the blastocyst on E3.5, we employed shRNA to examine its role during postimplantation development. Geminin knockdown inhibited the epithelial to mesenchymal transition (EMT) required at gastrulation and neural crest delamination, resulting in anterior-posterior axis and patterning defects, while overexpression promoted EMT at both locations. Geminin was negatively correlated with expression of E-cadherin, which is critically involved in controlling epithelial architecture. In addition, Geminin expression level was correlated with Wnt signaling and expression of the Wnt target gene Axin2 and with Msx2, and negatively correlated with the expression of Bmp4 and Neurog1 in quantitative reverse transcriptase-polymerase chain reaction analysis of RNAs from individual embryos. These results suggest that in addition to patterning the early embryo, Geminin plays a previously unrecognized role in EMT via its ability to affect Wnt signaling and E-cadherin expression.

Cut-like homeobox 1 and nuclear factor I/B mediate ENGRAILED2 autism spectrum disorder-associated haplotype function

Both common and rare variants contribute to autism spectrum disorder (ASD) risk, but few variants have been established as functional. Previously we demonstrated that an intronic haplotype (rs1861972-rs1861973 A-C) in the homeobox transcription factor ENGRAILED2 (EN2) is significantly associated with ASD. Positive association has also been reported in six additional data sets, suggesting EN2 is an ASD susceptibility gene. Additional support for this possibility requires identification of functional variants that affect EN2 regulation or activity. In this study, we demonstrate that the A-C haplotype is a transcriptional activator. Luciferase (luc) assays in mouse neuronal cultures determined that the A-C haplotype increases expression levels (50%, P < 0.01, 24 h; 250%, P < 0.0001, 72 h). Mutational analysis indicates that the A-C haplotype activator function requires both associated A and C alleles. A minimal 202-bp element is sufficient for function and also specifically binds a protein complex. Mass spectrometry identified these proteins as the transcription factors, Cut-like homeobox 1 (Cux1) and nuclear factor I/B (Nfib). Subsequent antibody supershifts and chromatin immunoprecipitations demonstrated that human CUX1 and NFIB bind the A-C haplotype. Co-transfection and knock-down experiments determined that both CUX1 and NFIB are required for the A-C haplotype activator function. These data demonstrate that the ASD-associated A-C haplotype is a transcriptional activator, and both CUX1 and NFIB mediate this activity. These results provide biochemical evidence that the ASD-associated A-C haplotype is functional, further supporting EN2 as an ASD susceptibility gene.

En1 and Wnt signaling in midbrain dopaminergic neuronal development

Dopaminergic neurons of the ventral mesodiencephalon are affected in significant health disorders such as Parkinson's disease, schizophrenia, and addiction. The ultimate goal of current research endeavors is to improve the clinical treatment of such disorders, such as providing a protocol for cell replacement therapy in Parkinson's disease that will successfully promote the specific differentiation of a stem cell into a dopaminergic neuronal phenotype. Decades of research on the developmental mechanisms of the mesodiencephalic dopaminergic (mdDA) system have led to the identification of many signaling pathways and transcription factors critical in its development. The unraveling of these pathways will help fill in the pieces of the puzzle that today dominates neurodevelopment research: how to make and maintain a mdDA neuron. In the present review, we provide an overview of the mdDA system, the processes and signaling molecules involved in its genesis, with a focus on the transcription factor En1 and the canonical Wnt pathway, highlighting recent findings on their relevance--and interplay--in the development and maintenance of the mdDA system.

The Engrailed homeobox genes determine the different foliation patterns in the vermis and hemispheres of the mammalian cerebellum

Little is known about the genetic pathways and cellular processes responsible for regional differences in cerebellum foliation, which interestingly are accompanied by regionally distinct afferent circuitry. We have identified the Engrailed (En) homeobox genes as being crucial to producing the distinct medial vermis and lateral hemisphere foliation patterns in mammalian cerebella. By producing a series of temporal conditional mutants in En1 and/or En2, we demonstrate that both En genes are required to ensure that folia exclusive to the vermis or hemispheres form in the appropriate mediolateral position. Furthermore, En1/En2 continue to regulate foliation after embryonic day 14, at which time Fgf8 isthmic organizer activity is complete and the major output cells of the cerebellar cortex have been specified. Changes in spatially restricted gene expression occur prior to foliation in mutants, and foliation is altered from the onset and is accompanied by changes in the thickness of the layer of proliferating granule cell precursors. In addition, the positioning and timing of fissure formation are altered. Thus, the En genes represent a new class of genes that are fundamental to patterning cerebellum foliation throughout the mediolateral axis and that act late in development.

Spatial analysis of expression patterns predicts genetic interactions at the mid-hindbrain boundary

The isthmic organizer mediating differentiation of mid- and hindbrain during vertebrate development is characterized by a well-defined pattern of locally restricted gene expression domains around the mid-hindbrain boundary (MHB). This pattern is established and maintained by a regulatory network between several transcription and secreted factors that is not yet understood in full detail. In this contribution we show that a Boolean analysis of the characteristic spatial gene expression patterns at the murine MHB reveals key regulatory interactions in this network. Our analysis employs techniques from computational logic for the minimization of Boolean functions. This approach allows us to predict also the interplay of the various regulatory interactions. In particular, we predict a maintaining, rather than inducing, effect of Fgf8 on Wnt1 expression, an issue that remained unclear from published data. Using mouse anterior neural plate/tube explant cultures, we provide experimental evidence that Fgf8 in fact only maintains but does not induce ectopic Wnt1 expression in these explants. In combination with previously validated interactions, this finding allows for the construction of a regulatory network between key transcription and secreted factors at the MHB. Analyses of Boolean, differential equation and reaction-diffusion models of this network confirm that it is indeed able to explain the stable maintenance of the MHB as well as time-courses of expression patterns both under wild-type and various knock-out conditions. In conclusion, we demonstrate that similar to temporal also spatial expression patterns can be used to gain information about the structure of regulatory networks. We show, in particular, that the spatial gene expression patterns around the MHB help us to understand the maintenance of this boundary on a systems level.

Understanding brain formation during development is a tantalizing challenge. It is also essential for the fight against neurodegenerative diseases. In vertebrates, the central nervous system arises from a structure called the neural plate. This tissue is divided into four regions, which continue to develop into forebrain, midbrain, hindbrain and spinal cord. Interactions between locally expressed genes and signaling molecules are responsible for this patterning. Two key signaling molecules in this process are Fgf8 and Wnt1 proteins. They are secreted from a signaling center located at the boundary between prospective mid- and hindbrain (mid-hindbrain boundary, MHB) and mediate development of these two brain regions. Here, we logically analyze the spatial gene expression patterns at the MHB and predict interactions involved in the differentiation of mid- and hindbrain. In particular, our analysis indicates that Wnt1 depends on Fgf8 for stable maintenance. A time-course analysis of Wnt1 expression after implantation of Fgf8-coated beads in mouse neural plate/tube explants experimentally validates our prediction about the interactions between these two key patterning molecules. Subsequently, we demonstrate that available data allows construction of a mathematical model able to explain the maintenance of the signaling center at the MHB. We begin to understand this small aspect of brain formation on a systems level.

Engrailed homeobox genes determine the organization of Purkinje cell sagittal stripe gene expression in the adult cerebellum

Underlying the seemingly uniform cellular composition of the adult mammalian cerebellum (Cb) are striking parasagittal stripes of gene expression along the medial-lateral (ML) axis that are organized with respect to the lobules that divide the Cb along the anterior-posterior (AP) axis. Although there is a clear correlation between the organization of gene expression stripes and Cb activity patterns, little is known about the genetic pathways that determine the intrinsic stripe molecular code. Here we establish that ML molecular code patterning is highly dependent on two homeobox transcription factors, Engrailed1 (En1) and En2, both of which are also required for patterning the lobules. Gene expression analysis of an allelic series of En1/2 mutant mice that have an intact Purkinje cell layer revealed severe patterning defects using three known components of the ML molecular code and a new marker of Hsp25 negative stripes (Neurofilament heavy chain, Nfh). Importantly, the complementary expression of ZebrinII/PhospholipaseC beta4 and Hsp25/Nfh changes in unison in each mutant. Furthermore, each En gene has unique as well as overlapping functions in patterning the ML molecular code and each En protein has dominant functions in different AP domains (subsets of lobules). Remarkably, in En1/2 mutants with almost normal foliation, ML molecular code patterning is severely disrupted. Thus, independent mechanisms that use En1/2 must pattern foliation and spatial gene expression separately. Our studies reveal that En1/2 are fundamental components of the genetic pathways that pattern the two intersecting coordinate systems of the Cb, morphological divisions and the molecular code.

FGF8 signaling regulates growth of midbrain dopaminergic axons by inducing semaphorin 3F

Accumulating evidence indicates that signaling centers controlling the dorsoventral (DV) polarization of the neural tube, the roof plate and the floor plate, play crucial roles in axon guidance along the DV axis. However, the role of signaling centers regulating the rostrocaudal (RC) polarization of the neural tube in axon guidance along the RC axis remains unknown. Here, we show that a signaling center located at the midbrain-hindbrain boundary (MHB) regulates the rostrally directed growth of axons from midbrain dopaminergic neurons (mDANs). We found that beads soaked with fibroblast growth factor 8 (FGF8), a signaling molecule that mediates patterning activities of the MHB, repelled mDAN axons that extended through the diencephalon. This repulsion may be mediated by semaphorin 3F (sema3F) because (1) FGF8-soaked beads induced an increase in expression of sema3F, (2) sema3F expression in the midbrain was essentially abolished by the application of an FGF receptor tyrosine kinase inhibitor, and (3) mDAN axonal growth was also inhibited by sema3F. Furthermore, mDAN axons expressed a sema3F receptor, neuropilin-2 (nrp2), and the removal of nrp-2 by gene targeting caused caudal growth of mDAN axons. These results indicate that the MHB signaling center regulates the growth polarity of mDAN axons along the RC axis by inducing sema3F.

The mouse Engrailed genes: a window into autism

The complex behavioral symptoms and neuroanatomical abnormalities observed in autistic individuals strongly suggest a multi-factorial basis for this perplexing disease. Although not the perfect model, we believe the Engrailed genes provide an invaluable "window" into the elusive etiology of autism spectrum disorder. The Engrailed-2 gene has been associated with autism in genetic linkage studies. The En2 knock-out mouse harbors cerebellar abnormalities that are similar to those found in autistic individuals and, as we report here, has a distinct anterior shift in the position of the amygdala in the cerebral cortex. Our initial analysis of background effects in the En1 mouse knock-out provides insight as to possible molecular mechanisms and gender differences associated with autism. These findings further the connection between Engrailed and autism and provide new avenues to explore in the ongoing study of the biological basis of this multifaceted disease.

Genetic basis of Joubert syndrome and related disorders of cerebellar development

Over three decades have passed since Marie Joubert described the original proband for Joubert syndrome, a rare neurological disorder featuring absence of the cerebellar vermis (i.e. midline). Efforts at deciphering the molecular basis for this disease have been complicated by the clinical and genetic heterogeneity as well as extensive phenotypic overlap with other syndromes. However, progress has been made in recent years with the mapping of three genetic loci and the identification of mutations in two genes, AHI1 and NPHP1. These genes encode proteins with some shared functional domains, but their role in brain development is unclear. Clues may come from studies of related syndromes, including Bardet-Biedl syndrome and nephronophthisis, for which all of the encoded proteins localize to primary cilia. The data suggest a tantalizing connection between intraflagellar transport in cilia and brain development.

Adverse effects of prenatal tobacco smoke exposure on biological parameters of the developing brainstem

We aimed to study the consequences of chronic exposure to tobacco smoke in utero on the morphological and functional maturation of the brainstem by comparing stillbirths of smoker mothers versus nonsmoker mothers. A total of 42 stillbirths, aged 25-40 gestational weeks, underwent autopsy according to our guidelines (). The brainstem was studied on serial sections and by immunohistochemistry to assay the expression of the EN2 gene, somatostatin (SS) and the tyrosine hydroxylase enzyme (TH). We observed a significant correlation between maternal smoking and sudden intrauterine unexplained death (SIUD), hypoplasia of the ArcN, no immunostaining of the EN2 in the arcuate nucleus (ArcN), and of TH in the locus coeruleus (LC) (P < 0.05). An increased incidence of maternal smoking was also observed in fetuses with SS negativity in the hypoglossus nucleus (HypoglN). Exposure in utero to maternal smoking may strongly interfere with brain biological parameters, giving rise not only to structural developmental abnormalities of the arcuate nucleus, but also to a decrease of noradrenergic activity in the LC, of EN2 gene expression in the ArcN and of SS in the HypoglN.

Pluripotent neural crest stem cells in the adult hair follicle

We report the presence of pluripotent neural crest stem cells in the adult mammalian hair follicle. Numerous neural crest cells reside in the outer root sheath from the bulge to the matrix at the base of the follicle. Bulge explants from adult mouse whisker follicles yield migratory neural crest cells, which in clonal culture form colonies consisting of over a thousand cells. Clones contain neurons, smooth muscle cells, rare Schwann cells and melanocytes, demonstrating pluripotency of the clone-forming cell. Targeted differentiation into Schwann cells and chondrocytes was achieved with neuregulin-1 and bone morphogenetic protein-2, respectively. Serial cloning in vitro demonstrated self-renewal capability. Together, the data show that the adult mouse whisker follicle contains pluripotent neural crest stem cells, termed epidermal neural crest cells (eNCSC). eNCSC are promising candidates for diverse cell therapy paradigms because of their high degree of inherent plasticity and due to their easy accessibility in the skin.

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

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

Engrailed genes control developmental fate of serotonergic and noradrenergic neurons in mid- and hindbrain in a gene dose-dependent manner

In vertebrates and insects, the homeobox transcription factors of the engrailed family have a dual function. They take part in regionalization during early embryogenesis and later in neuronal specification. In mammals, two engrailed homologues exist, engrailed-1 and engrailed-2, which are expressed in a broad band around the isthmus at an age when the serotonergic and noradrenergic neurons in mid/hindbrain are generated. The analysis of engrailed-1 and -2 double mutant mice revealed a specific, redundant, and gene dose-dependent requirement of the two transcription factors for the development of the serotonergic dorsal raphe nucleus and the noradrenergic locus caeruleus. Both nuclei are lost in engrailed double mutant mice; however, directly adjacent nuclei of the same neurotransmitter phenotype are not affected. An almost identical phenotype is found in mutant mice null for Wnt1, indicating that the engrailed genes provide essential positional information for the development of the two nuclei during early embryogenesis.

Identification of Pax2-regulated genes by expression profiling of the mid-hindbrain organizer region

The paired domain transcription factor Pax2 is required for the formation of the isthmic organizer (IsO) at the midbrain-hindbrain boundary, where it initiates expression of the IsO signal Fgf8. To gain further insight into the role of Pax2 in mid-hindbrain patterning, we searched for novel Pax2-regulated genes by cDNA microarray analysis of FACS-sorted GFP+ mid-hindbrain cells from wild-type and Pax2-/- embryos carrying a Pax2(GFP) BAC transgene. Here, we report the identification of five genes that depend on Pax2 function for their expression in the mid-hindbrain boundary region. These genes code for the transcription factors En2 and Brn1 (Pou3f3), the intracellular signaling modifiers Sef and Tapp1, and the non-coding RNA Ncrms. The Brn1 gene was further identified as a direct target of Pax2, as two functional Pax2-binding sites in the promoter and in an upstream regulatory element of Brn1 were essential for lacZ transgene expression at the mid-hindbrain boundary. Moreover, ectopic expression of a dominant-negative Brn1 protein in chick embryos implicated Brn1 in Fgf8 gene regulation. Together, these data defined novel functions of Pax2 in the establishment of distinct transcriptional programs and in the control of intracellular signaling during mid-hindbrain development.

The evolutionary origins of vertebrate midbrain and MHB: insights from mouse, amphioxus and ascidian Dmbx homeobox genes

Comparative studies on developmental gene expression suggest that the ancestral chordate central nervous system comprised anterior, midbrain-hindbrain boundary (MHB) and posterior regions. The most anterior region consists of both forebrain and midbrain in vertebrates. It remains, however, unresolved when or how the vertebrate midbrain was established from this anterior zone. I previously reported a mouse PRD-class homeobox gene, Dmbx1, expressed in the presumptive midbrain at early developmental stages, and in the hindbrain at later stages, with exclusion from the MHB. To investigate the evolution of midbrain development, I have cloned Dmbx genes from amphioxus and from Ciona, representing the two most closely related lineages to the vertebrates, and examined embryonic Dmbx expression in these species. In amphioxus, no Dmbx expression is observed in the neural tube, supporting previous arguments that the MHB equivalent region has been secondarily lost in evolution. In Ciona, the CiDmbx gene is detected in neural cells posterior to Pax-2/5/8-positive cells (MHB homologue), but not in any cells anterior to them. These results support the lack of a midbrain homologue in Ciona, and suggest that midbrain development is a vertebrate innovation. Here, I report the full sequences of these genes and discuss the evolution of midbrain development in relation to the tripartite neural ground plan and the origin of the MHB organizer.

Involvement of the EN-2 gene in normal and abnormal development of the human arcuate nucleus

In this study, we wanted to evaluate whether the engrailed EN-2 gene, a homeobox gene with an essential role in the development of the rhombic lip derivatives in different species, is (1) expressed also in man in the differentiation process of the medullary arcuate nucleus (ArcN) and (2) involved in sudden unexplained perinatal and infant death, frequently related to developmental defects of the ArcN. We evaluated by means of the monoclonal antibody 4D9, exclusively recognizing engrailed-2 protein, the expression of the EN-2 gene in the ArcN on histological sections of the brainstems of 30 subjects aged from 17 gestational weeks to 10 postnatal months, who had died of known (17 cases) and unknown causes (13 cases). We observed in the greater number of the cases that the expression of the EN-2 gene is very high in the ArcN neurons from the 17th to the 22nd gestational week, then decreases up to the first days after birth and later disappears. Moreover, in eight of the 13 sudden deaths (61%), a hypoplasia of the ArcN was present. In almost all of these cases, EN-2 expression was negative. In conclusion, we support the role of the EN-2 gene in the normal neuronal development and in the anatomic organization of the human ArcN as well as the possible existence of EN-2 mutations related to hypoplasia of this nucleus.

Morphogenetic and cellular movements that shape the mouse cerebellum; insights from genetic fate mapping

We used the cerebellum as a model to study the morphogenetic and cellular processes underlying the formation of elaborate brain structures from a simple neural tube, using an inducible genetic fate mapping approach in mouse. We demonstrate how a 90 degrees rotation between embryonic days 9 and 12 converts the rostral-caudal axis of dorsal rhombomere 1 into the medial-lateral axis of the wing-like bilateral cerebellar primordium. With the appropriate use of promoters, we marked specific medial-lateral domains of the cerebellar primordium and derived a positional fate map of the murine cerebellum. We show that the adult medial cerebellum is produced by expansion, rather than fusion, of the thin medial primordium. Furthermore, ventricular-derived cells maintain their original medial-lateral coordinates into the adult, whereas rhombic lip-derived granule cells undergo lateral to medial posterior transverse migrations during foliation. Thus, we show that progressive changes in the axes of the cerebellum underlie its genesis.

Expression patterns of engrailed-like proteins in the chick embryo

The protein products of both of the identified chick engrailed-like (En) genes, chick En-1 and chick En-2, are localized in cells of the developing brain, mandibular arch, spinal cord, dermatome, and ventral limb bud ectoderm, as demonstrated by labeling with the polyclonal antiserum alpha Enhb-1 developed by Davis et al. (Development 111:281-298, 1991). A subpopulation of cephalic neural crest cells is also En-protein-positive. The monoclonal antibody 4D9 recognizes the chick En-2 gene product exclusively (Patel et al.: Cell 58:955-968, 1989; Davis et al., 1991) and colocalizes with chick En-2 mRNA in the developing head region of the chick embryo as shown by in situ hybridization (Gardner et al.: J. Neurosci. Res. 21:426-437, 1988). In the present study we examine the pattern of alpha Enhb-1 and 4D9 localization throughout the chick embryo from the first appearance of antibody (Ab)-positive cells at stage 8 (Hamburger and Hamilton: J. Morphol. 88:49-92, 1951) through stage 28 (1-5.5 days). We compare the localization patterns of the two Abs to each other, as well as to the localization of the monoclonal Ab, HNK-1, which recognizes many neural crest cells, using double- and triple-label fluorescence immunohistochemistry. Most En protein-positive cells in the path of neural crest cell migration are not HNK-1 positive. In detailed examination of alpha Enhb-1 and 4D9 localization, we find previously undetected patterns of En protein localization in the prechordal plate, hindbrain, myotome, ventral body-wall mesoderm, and extraembryonic membranes. Based upon these observations we propose: 1) that En expression in the mesoderm may be induced through interaction with En expressing cells in the neuroectoderm; 2) that En expression in the head mesenchyme is associated with somitomere 4; and 3) that En expression may be involved in epithelial-mesenchymal cell transformations.

The adult hair follicle: cradle for pluripotent neural crest stem cells

This review focuses on the recent identification of two novel neural crest-derived cells in the adult mammalian hair follicle, pluripotent stem cells, and Merkel cells. Wnt1-cre/R26R compound transgenic mice, which in the periphery express beta-galactosidase in a neural crest-specific manner, were used to trace neural crest cells. Neural crest cells invade the facial epidermis as early as embryonic day 9.5. Neural crest-derived cells are present along the entire extent of the whisker follicle. This includes the bulge area, an epidermal niche for keratinocyte stem cells, as well as the matrix at the base of the hair follicle. We have determined by in vitro clonal analysis that the bulge area of the adult whisker follicle contains pluripotent neural crest stem cells. In culture, beta-galactosidase-positive cells emigrate from bulge explants, identifying them as neural crest-derived cells. When these cells are resuspended and grown in clonal culture, they give rise to colonies that contain multiple differentiated cell types, including neurons, Schwann cells, smooth muscle cells, pigment cells, chondrocytes, and possibly other types of cells. This result provides evidence for the pluripotentiality of the clone-forming cell. Serial cloning showed that bulge-derived neural crest cells undergo self-renewal, which identifies them as stem cells. Pluripotent neural crest cells are also localized in the back skin hair of adult mice. The bulge area of the whisker follicle is surrounded by numerous Merkel cells, which together with innervating nerve endings form slowly adapting mechanoreceptors that transduce steady skin indentation. Merkel cells express beta-galactosidase in double transgenic mice, which confirms their neural crest origin. Taken together, our data indicate that the epidermis of the adult hair follicle contains pluripotent neural crest stem cells, termed epidermal neural crest stem cells (eNCSCs), and one newly identified neural crest derivative, the Merkel cell. The intrinsic high degree of plasticity of eNCSCs and the fact that they are easily accessible in the skin make them attractive candidates for diverse autologous cell therapy strategies.

Midbrain dopaminergic neurons: control of their cell fate by the engrailed transcription factors

As for any other cell population, the development, cell fate, and properties of mesencephalic dopaminergic (mesDA) neurons are ultimately controlled at the transcriptional level. The genes for two transcription factors Engrailed-1 ( En1) and Engrailed-2 ( En2) play an essential role in the development and maintenance of these cells. They belong to a family of genes that have been investigated in Drosophila for more than half a century. The products of these genes are all characterized by homeotic tissue transformation and a highly conserved protein sequence, the homeobox. En1 and En2 act upon at least two steps of the differentiation of mesDA neurons. They take part in the regionalization event, which gives rise to the neuroepithelium that provides the precursor cells in the ventral midbrain with the fibroblast growth factor 8 signal necessary for their induction. Additionally, these genes are required in postmitotic mesDA neurons in which they are expressed from embryonic day 12 continuously into adulthood. In mutant mice homozygous null for En1 and En2, the neurons are generated in the ventral midbrain, become postmitotic, and begin to express their neurotransmitter phenotype. However, thereafter, they rapidly die by apoptosis. Cell mixing experiments in vitro and in vivo have demonstrated that the engrailed requirement for the survival of mesDA neurons is cell-autonomous. The inactivation of engrailed by RNA interference induces apoptosis in less than 24 h. These data suggest that the engrailed genes control an essential mechanism for the survival of mesDA neurons.

Specification of midbrain territory

The vertebrate neural plate is subdivided into four distinct territories comprising the presumptive forebrain, midbrain, hindbrain, and the spinal cord, shortly after gastrulation. Initially, this subdivision relies on a defined pattern of expression of distinct transcription and secreted factors within the newly formed neuroectoderm, even before morphological partitioning is evident. Subdivision of the neural plate into distinct territories is a complex process, which is also known as patterning or regionalisation, and involves both planar and vertical signals coming from within the neuroectoderm and from neighbouring non-neural tissues. During the course of embryogenesis, this gross subdivision of the neural plate is progressively refined by a variety of mechanisms, leading to the establishment of various subdomains that ultimately give rise to specific cell populations characteristic for the corresponding brain and spinal cord regions. Once again, a prominent feature of these later processes is the defined expression of specific genes within the developing neural tube. In the present review, we will concentrate on the genes active in the progressive refinement of the midbrain territory as a distinct subdivision of the brain. We will also give an outlook on genes that are active during early induction of the anterior neural plate and genetic mechanisms that control the generation of specific cell populations of the ventral midbrain, with special focus on the mesencephalic dopaminergic neurons.

Protein kinase A phosphorylates serine 267 in the homeodomain of engrailed-2 leading to decreased DNA binding

Engrailed-2 (En-2) belongs to an evolutionarily conserved family of DNA binding homeodomain-containing proteins that are expressed in mammalian brain during development. Here, we demonstrate that serine 267 in the homeodomain of En-2 is phosphorylated by protein kinase A (PKA) in forskolin-treated COS-7 cells. Furthermore, we analyze the physiological function of En-2 phosphorylation by PKA. The nuclear localization of En-2 is not influenced by the phosphorylation of serine 267. However, substitution of serine 267 with alanine resulted in increased binding of En-2 to DNA, while replacing serine 267 with glutamic acid resulted in decreased En-2 DNA binding. These results suggest that the transcriptional activity of En-2 is regulated by PKA.

Engrailed-2 negatively regulates the onset of perinatal Purkinje cell differentiation

The transcription factor Engrailed-2 is expressed in cerebellar Purkinje cells (PCs) throughout embryonic development but is downregulated in PCs after birth. Since the onset of PC differentiation coincides with this change of gene expression, we asked whether downregulation of Engrailed-2 is necessary for proper timing of PC differentiation. To investigate this, we used an L7En-2 transgenic mouse model in which Engrailed-2 expression in PCs is maintained beyond the day of birth. In these L7En-2 mice the onset of parvalbumin expression was delayed in all PCs by about 3 days; the spatial expression pattern, however, remained comparable to wildtype cerebella. Furthermore, parvalbumin expression resembled the known pattern of normal PC maturation, suggesting a direct link between parvalbumin expression and PC differentiation. Consistent with a delay of PC differentiation, we found that PCs of L7En-2 cerebella displayed a reduced tendency to align in the typical monolayer. The average size of L7En-2 PCs was reduced and the dendritic arbor developed more slowly than in wildtype PCs. In contrast, major morphological features of PCs were comparable in L7En-2 and wildtype cerebella after postnatal day 11. In addition, we observed a transient reduction of PC survival in organotypic slice cultures of L7En-2 cerebella in comparison with wildtype slice cultures. Since PC survival parallels PC differentiation in vitro, we propose that the observed delay in PC differentiation upon Engrailed-2 overexpression is an intrinsic property of Engrailed-2 activity, and that downregulation of Engrailed-2 in wildtype PCs around the day of birth is critical for the timing of distinct steps of PC differentiation.

Factors in the genetic background suppress the engrailed-1 cerebellar phenotype

The mouse homeodomain protein, Engrailed-1, is generally viewed as an essential player in the early establishment and maintenance of the midbrain/hindbrain region that gives rise to the cerebellum and midbrain. In keeping with this, engineered null mutations at this locus have been reported to lead to perinatal lethality accompanied by near-total absence of cerebellar and caudal midbrain structures. We report here that these cerebellar phenotypes are nearly completely suppressed on a C57BL/6J genetic background. All cell types are present and arranged properly in both the cortex and the deep nuclei, and cell counts reveal no significant absence of cerebellar Purkinje cells. Folial patterns are nearly normal, although an apparent fusion of lobules IV and V is consistently noted. Significantly, no change in the Engrailed-2 mutant phenotype occurs after a similar background switch, and whole-mount in situ hybridization reveals identical En2 expression patterns in wild-type C57BL/6J and 129/Sv mice. One likely mechanism for the En1-/- phenotype suppression is a temporal and/or spatial change in the pattern of Engrailed-2 expression apparent only in the absence of Engrailed-1. In support of this, C57BL/6-En1-/- embryos that are also En2+/- lack a cerebellum and caudal midbrain: a phenotype identical to 129/Sv-En1-/- mice.

Neural and orofacial defects in Folp1 knockout mice [corrected]

BACKGROUND

Folic acid is essential for the development of the nervous system and other associated structures. Mice deficient in the folic acid-binding protein one (Folbp1) gene display multiple developmental abnormalities, including neural and craniofacial defects. To better understand potential interactions between Folbp1 gene and selected genes involved in neural and craniofacial morphogenesis, we evaluated the expression patterns of a panel of crucial differentiation markers (Pax-3, En-2, Hox-a1, Shh, Bmp-4, Wnt-1, and Pax-1).

METHODS

Folbp1 mice were supplemented with low dosages of folinic add to rescue nullizygotes from dying in utero before gestational day 10. The gene marker analyses were carried out by in situ hybridization.

RESULTS

In nullizygote embryos with open cranial neural tube defects, the downregulation of Pax-3 and En-2 in the impaired midbrain, along with an observed upregulation of the ventralizing marker Shh in the expanded floor plate, suggested an important regulatory interaction among these three genes. Moreover, the nullizygotes also exhibit craniofacial abnormalities, such as cleft lip and palate. Pax-3 signals in the impaired medial nasal primordia were significantly increased, whereas Pax-1 showed no expression in the undeveloped lateral nasal processes. Although Shh was downregulated, Bmp-4 was strongly expressed in the medial and lateral nasal processes, highlighting the antagonistic activities of these molecules.

CONCLUSIONS

Impairment of Folbp1 gene function adversely impacts the expression of several critical signaling molecules. Mis-expression of these molecules, perhaps mediated by Shh, may potentially contribute to the observed failure of neural tube closure and the development of craniofacial defects in the mutant mice.

Neural crest origin of mammalian Merkel cells

Here, we provide evidence for the neural crest origin of mammalian Merkel cells. Together with nerve terminals, Merkel cells form slowly adapting cutaneous mechanoreceptors that transduce steady indentation in hairy and glabrous skin. We have determined the ontogenetic origin of Merkel cells in Wnt1-cre/R26R compound transgenic mice, in which neural crest cells are marked indelibly. Merkel cells in whiskers and interfollicular locations express the transgene, beta-galactosidase, identifying them as neural crest descendants. We thus conclude that murine Merkel cells originate from the neural crest.

Cellular and cis-regulation of En-2 expression in the mandibular arch

Investigations into early muscle development have focused primarily on somite derived cells. Cranial mesoderm does not undergo somitogenesis, and muscle formation in this region is less well understood. In the present study, we have focused upon the expression of engrailed in mandibular arch myoblasts. We demonstrate that En-2 is expressed in mandibular arch myoblasts of the mouse. The activity of the En-2 enhancer is maintained in several functionally related muscles that arise from the first arch. Through the use of reporter transgenics, we demonstrate that local cell-cell interactions are important in maintaining En-2 expression in the mandibular arch cells. En-2 enhancer activity in the first arch requires a combination of cis-acting sequences that includes a motif which is identical to one found in the Otx2 enhancer and which is sufficient to direct expression in the first arch. These data support the notion that cranial muscle development is regulated by local cell-cell interactions which distinguish distinct anatomical and functional muscle groups.

Emx2directs the development of diencephalon in cooperation withOtx2

The vertebrate brain is among the most complex biological structures of which the organization remains unclear. Increasing numbers of studies have accumulated on the molecular basis of midbrain/hindbrain development, yet relatively little is known about forebrain organization. Nested expression among Otx and Emx genes has implicated their roles in rostral brain regionalization, but single mutant phenotypes of these genes have not provided sufficient information. In order to genetically determine the interaction between Emx and Otx genes in forebrain development, we have examined Emx2−/−Otx2+/− double mutants and Emx2 knock-in mutants into the Otx2 locus (Otx2+/Emx2). Emx2−/−Otx2+/− double mutants did not develop diencephalic structures such as ventral thalamus, dorsal thalamus/epithalamus and anterior pretectum. The defects were attributed to the loss of the Emx2-positive region at the three- to four-somite stage, when its expression occurs in the laterocaudal forebrain primordia. Ventral structures such as the hypothalamus, mammillary region and tegmentum developed normally. Moreover, dorsally the posterior pretectum and posterior commissure were also present in the double mutants. In contrast, Otx2+/Emx2 knock-in mutants displayed the majority of these diencephalic structures; however, the posterior pretectum and posterior commissure were specifically absent. Consequently, development of the dorsal and ventral thalamus and anterior pretectum requires cooperation between Emx2 and Otx2, whereas Emx2 expression is incompatible with development of the commissural region of the pretectum.

The Wheels mutation in the mouse causes vascular, hindbrain, and inner ear defects

In a screen for mouse mutations with dominant behavioral anomalies, we identified Wheels, a mutation associated with circling and hyperactivity in heterozygotes and embryonic lethality in homozygotes. Mutant Wheels embryos die at E10.5-E11.5 and exhibit a host of morphological anomalies which include growth retardation and anomalies in vascular and hindbrain development. The latter includes perturbation of rhombomeric boundaries as detected by Krox20 and Hoxb1. PECAM-1 staining of embryos revealed normal formation of the primary vascular plexus. However, subsequent stages of branching and remodeling do not proceed normally in the yolk sac and in the embryo proper. To obtain insights into the circling behavior, we examined development of the inner ear by paint-filling of membranous labyrinths of Whl/+ embryos. This analysis revealed smaller posterior and lateral semicircular canal primordia and a delay in the canal fusion process at E12.5. By E13.5, the lateral canal was truncated and the posterior canal was small or absent altogether. Marker analysis revealed an early molecular phenotype in heterozygous embryos characterized by perturbed expression of Bmp4 and Msx1 in prospective lateral and posterior cristae at E11.5. We have constructed a genetic and radiation hybrid map of the centromeric portion of mouse Chromosome 4 across the Wheels region and refined the position of the Wheels locus to the approximately 1.1-cM region between D4Mit104 and D4Mit181. We have placed the locus encoding Epha7, in the Wheels candidate region; however, further analysis showed no mutations in the Epha7-coding region and no detectable changes in mRNA expression pattern. In summary, our findings indicate that Wheels, a gene which is essential for the survival of the embryo, may link diverse processes involved in vascular, hindbrain, and inner ear development.

The neuronal microtubule-associated protein 1B is under homeoprotein transcriptional control

To identify genes regulated by homeoprotein transcription factors in postnatal neurons, the DNA-binding domain (homeodomain) of Engrailed homeoprotein was internalized into rat cerebellum neurons. The internalized homeodomain (EnHD) acts as a competitive inhibitor of Engrailed and of several homeoproteins (Mainguy et al., 2000). Analysis by differential display revealed that microtubule-associated protein 1B (MAP1B) mRNA is upregulated by EnHD. This upregulation does not require protein synthesis, suggesting a direct effect of the homeodomain on MAP1B transcription. The promoter region of MAP1B was cut into several subdomains, and each subdomain was tested for its ability to bind Engrailed and EnHD and to associate with Engrailed-containing cerebellum nuclear extracts. In addition, the activity, and regulation by Engrailed, of each subdomain and of the entire promoter were evaluated in vivo by electroporation in the chick embryo neural tube. These experiments demonstrate that MAP1B promoter is regulated by Engrailed in vivo. Moreover, they show that one promoter domain that contains all ATTA homeoprotein cognate binding sites common to the rat and human genes is an essential element of this regulation. It is thus proposed that MAP1B, a cytoskeleton protein involved in neuronal growth and regeneration, is under homeoprotein transcriptional regulation.

Fate of midbrain dopaminergic neurons controlled by the engrailed genes

Deficiencies in neurotransmitter-specific cell groups in the midbrain result in prominent neural disorders, including Parkinson's disease, which is caused by the loss of dopaminergic neurons of the substantia nigra. We have investigated in mice the role of the engrailed homeodomain transcription factors, En-1 and En-2, in controlling the developmental fate of midbrain dopaminergic neurons. En-1 is highly expressed by essentially all dopaminergic neurons in the substantia nigra and ventral tegmentum, whereas En-2 is highly expressed by a subset of them. These neurons are generated and differentiate their dopaminergic phenotype in En-1/En-2 double null mutants, but disappear soon thereafter. Use of an En-1/tau-LacZ knock-in mouse as an autonomous marker for these neurons indicates that they are lost, rather than that they change their neurotransmitter phenotype. A single allele of En-1 on an En-2 null background is sufficient to produce a wild type-like substantia nigra and ventral tegmentum, whereas in contrast a single allele of En-2 on an En-1 null background results in the survival of only a small proportion of these dopaminergic neurons, a finding that relates to the differential expression of En-1 and En-2. Additional findings indicate that En-1 and En-2 regulate expression of alpha-synuclein, a gene that is genetically linked to Parkinson's disease. These findings show that the engrailed genes are expressed by midbrain dopaminergic neurons from their generation to adulthood but are not required for their specification. However, the engrailed genes control the survival of midbrain dopaminergic neurons in a gene dose-dependent manner. Our findings also suggest a link between engrailed and Parkinson's disease.

A role for Engrailed-2 in determination of skeletal muscle physiologic properties

The molecular basis underlying the establishment of the myogenic lineage, subsequent differentiation, and the establishment of specific fiber types (i.e., fast versus slow) is becoming well understood. In contrast, the regulation of the general properties of a specific anatomical muscle group (e.g., leg versus jaw muscles) and the regulation of muscle-fiber properties within a particular group are less well characterized. We have investigated the potential role of the homeobox-containing gene, Engrailed-2 (En-2), in the mouse, which is specifically expressed in myoblasts in the first arch and maintained in the muscles of mastication in the adult. We have generated mice that ectopically express En-2 in all muscles during early development and primarily in fast muscles in the adult. Ectopic En-2 in nonjaw muscles leads to a decrease in fiber size, whereas overexpression in the jaw muscles leads to a shift in fiber metabolic properties as well as a decrease in fiber size. In contrast, loss of En-2 in the jaw leads to a shift in fiber metabolic properties in the jaw of female mice only. Jaw muscles are sexually dimorphic, and we propose that the function of En-2 and mechanisms guiding sexual dimorphism of the jaw muscles are integrated. We conclude that the specific expression of En-2 in the jaw therefore plays a role in specifying muscle-fiber characteristics that contribute to the physiologic properties of specific muscle groups.