Two enhancer regions in the mouse En-2 locus direct expression to the mid/hindbrain region and mandibular myoblasts

A novel transgenic Cre allele to label mouse cardiac conduction system

The cardiac conduction system is a network of heterogeneous cell population that initiates and propagates electric excitations in the myocardium. Purkinje fibers, a network of specialized myocardial cells, comprise the distal end of the conduction system in the ventricles. The developmental origins of Purkinje fibers and their roles during cardiac physiology and arrhythmia have been reported. However, it is not clear if they play a role during ischemic injury and heart regeneration. Here we introduce a novel tamoxifen-inducible Cre allele that specifically labels a broad range of components in the cardiac conduction system while excludes other cardiac cell types and vital organs. Using this new allele, we investigated the cellular and molecular response of Purkinje fibers to myocardial injury. In a neonatal mouse myocardial infarction model, we observed significant increase in Purkinje cell number in regenerating myocardium. RNA-Seq analysis using laser-captured Purkinje fibers showed a unique transcriptomic response to myocardial infarction. Our finds suggest a novel role of cardiac Purkinje fibers in heart injury.

Neural cell adhesion molecule is required for ventricular conduction system development

The most distal portion of the ventricular conduction system (VCS) contains cardiac Purkinje cells (PCs), which are essential for synchronous activation of the ventricular myocardium. Contactin-2 (CNTN2), a member of the immunoglobulin superfamily of cell adhesion molecules (IgSF-CAMs), was previously identified as a marker of the VCS. Through differential transcriptional profiling, we discovered two additional highly enriched IgSF-CAMs in the VCS: NCAM-1 and ALCAM. Immunofluorescence staining showed dynamic expression patterns for each IgSF-CAM during embryonic and early postnatal stages, but ultimately all three proteins became highly enriched in mature PCs. Mice deficient in NCAM-1, but not CNTN2 or ALCAM, exhibited defects in PC gene expression and VCS patterning, as well as cardiac conduction disease. Moreover, using ST8sia2 and ST8sia4 knockout mice, we show that inhibition of post-translational modification of NCAM-1 by polysialic acid leads to disrupted trafficking of sarcolemmal intercalated disc proteins to junctional membranes and abnormal expansion of the extracellular space between apposing PCs. Taken together, our data provide insights into the complex developmental biology of the ventricular conduction system.

Summary: The cell adhesion molecule NCAM-1 and its post-translational modification by polysialylation are required for normal formation and function of the specialized ventricular conduction system.

Neural crest and the patterning of vertebrate craniofacial muscles

Patterning of craniofacial muscles overtly begins with the activation of lineage-specific markers at precise, evolutionarily conserved locations within prechordal, lateral, and both unsegmented and somitic paraxial mesoderm populations. Although these initial programming events occur without influence of neural crest cells, the subsequent movements and differentiation stages of most head muscles are neural crest-dependent. Incorporating both descriptive and experimental studies, this review examines each stage of myogenesis up through the formation of attachments to their skeletal partners. We present the similarities among developing muscle groups, including comparisons with trunk myogenesis, but emphasize the morphogenetic processes that are unique to each group and sometimes subsets of muscles within a group. These groups include branchial (pharyngeal) arches, which encompass both those with clear homologues in all vertebrate classes and those unique to one, for example, mammalian facial muscles, and also extraocular, laryngeal, tongue, and neck muscles. The presence of several distinct processes underlying neural crest:myoblast/myocyte interactions and behaviors is not surprising, given the wide range of both quantitative and qualitative variations in craniofacial muscle organization achieved during vertebrate evolution.

Learning about mammalian gene regulation from functional enhancer assays in the mouse

Enhancer biology is emerging as a critical area of research that informs studies of evolution, development, and disease. From early experiments that defined and mapped the first enhancers to recent enhancer models of human disease, functional experiments in the mouse have played a central role in revealing enhancer biology. Three decades of in vivo enhancer studies in mouse have laid the groundwork for the current understanding of mammalian enhancers, demonstrating the developmental and tissue-specific activity of enhancers and illuminating general features of enhancer evolution and function. Recent studies offer an emerging perspective on the importance of chromosomal context, combinatorial enhancer activity, and the functional consequences of enhancer sequence variation. This review describes the basic principles of functional testing in mouse, summarizes the contributions these studies have made to our understanding of enhancer biology, and describes limitations and future outlook of in vivo mouse enhancer studies.

Development of the head and trunk mesoderm in the dogfish, Scyliorhinus torazame: II. Comparison of gene expression between the head mesoderm and somites with reference to the origin of the vertebrate head

The vertebrate mesoderm differs distinctly between the head and trunk, and the evolutionary origin of the head mesoderm remains enigmatic. Although the presence of somite-like segmentation in the head mesoderm of model animals is generally denied at molecular developmental levels, the appearance of head cavities in elasmobranch embryos has not been explained, and the possibility that they may represent vestigial head somites once present in an amphioxus-like ancestor has not been ruled out entirely. To examine whether the head cavities in the shark embryo exhibit any molecular signatures reminiscent of trunk somites, we isolated several developmentally key genes, including Pax1, Pax3, Pax7, Pax9, Myf5, Sonic hedgehog, and Patched2, which are involved in myogenic and chondrogenic differentiation in somites, and Pitx2, Tbx1, and Engrailed2, which are related to the patterning of the head mesoderm, from an elasmobranch species, Scyliorhinus torazame. Observation of the expression patterns of these genes revealed that most were expressed in patterns that resembled those found in amniote embryos. In addition, the head cavities did not exhibit an overt similarity to somites; that is, the similarity was no greater than that of the unsegmented head mesoderm in other vertebrates. Moreover, the shark head mesoderm showed an amniote-like somatic/visceral distinction according to the expression of Pitx2, Tbx1, and Engrailed2. We conclude that the head cavities do not represent a manifestation of ancestral head somites; rather, they are more likely to represent a derived trait obtained in the lineage of gnathostomes.

Keratinocyte overexpression of IL-17C promotes psoriasiform skin inflammation

IL-17C is a functionally distinct member of the IL-17 family that binds IL-17 receptor E/A to promote innate defense in epithelial cells and regulate Th17 cell differentiation. We demonstrate that IL-17C (not IL-17A) is the most abundant IL-17 isoform in lesional psoriasis skin (1058 versus 8 pg/ml; p < 0.006) and localizes to keratinocytes (KCs), endothelial cells (ECs), and leukocytes. ECs stimulated with IL-17C produce increased TNF-α and KCs stimulated with IL-17C/TNF-α produce similar inflammatory gene response patterns as those elicited by IL-17A/TNF-α, including increases in IL-17C, TNF-α, IL-8, IL-1α/β, IL-1F5, IL-1F9, IL-6, IL-19, CCL20, S100A7/A8/A9, DEFB4, lipocalin 2, and peptidase inhibitor 3 (p < 0.05), indicating a positive proinflammatory feedback loop between the epidermis and ECs. Psoriasis patients treated with etanercept rapidly decrease cutaneous IL-17C levels, suggesting IL-17C/TNF-α-mediated inflammatory signaling is critical for psoriasis pathogenesis. Mice genetically engineered to overexpress IL-17C in KCs develop well-demarcated areas of erythematous, flakey involved skin adjacent to areas of normal-appearing uninvolved skin despite increased IL-17C expression in both areas (p < 0.05). Uninvolved skin displays increased angiogenesis and elevated S100A8/A9 expression (p < 0.05) but no epidermal hyperplasia, whereas involved skin exhibits robust epidermal hyperplasia, increased angiogenesis and leukocyte infiltration, and upregulated TNF-α, IL-1α/β, IL-17A/F, IL-23p19, vascular endothelial growth factor, IL-6, and CCL20 (p < 0.05), suggesting that IL-17C, when coupled with other proinflammatory signals, initiates the development of psoriasiform dermatitis. This skin phenotype was significantly improved following 8 wk of TNF-α inhibition. These findings identify a role for IL-17C in skin inflammation and suggest a pathogenic function for the elevated IL-17C observed in lesional psoriasis skin.

Integration of Hedgehog and BMP signalling by the engrailed2a gene in the zebrafish myotome

Different levels and timing of Hedgehog (Hh) signalling activity have been proposed to specify three distinct cell types in the zebrafish myotome. Two of these, the medial fast-twitch fibres (MFFs) and the slow-twitch muscle pioneers (MPs) are characterised by expression of eng1a, -1b and -2a and require the highest levels of Hh for their specification. We have defined a minimal eng2a element sufficient to drive reporter expression specifically in MPs and MFFs. This element binds both Gli2a, a mediator of Hh signalling, and activated Smads (pSmads), mediators of bone morphogenic protein (BMP) signalling, in vivo. We found a strict negative correlation between nuclear accumulation of pSmad, and eng2a expression in myotomal cells and show that abrogation of pSmad accumulation results in activation of eng2a, even when Hh signalling is attenuated. Conversely, driving nuclear accumulation of pSmad suppresses the induction of eng expression even when Hh pathway activity is maximal. Nuclear accumulation of pSmads is depleted by maximal Hh pathway activation. We show that a synthetic form of the Gli2 repressor interacts with Smad1 specifically in the nuclei of myotomal cells in the developing embryo and that this interaction depends upon BMP signalling activity. Our results demonstrate that the eng2a promoter integrates repressive and activating signals from the BMP and Hh pathways, respectively, to limit its expression to MPs and MFFs. We suggest a novel basis for crosstalk between the Hh and BMP pathways, whereby BMP-mediated repression of Hh target genes is promoted by a direct interaction between Smads and truncated Glis, an interaction that is abrogated by Hh induced depletion of the latter.

Contactin-2 expression in the cardiac Purkinje fiber network

BACKGROUND

Purkinje cells (PCs) comprise the most distal component of the cardiac conduction system, and their unique electrophysiological properties and the anatomic complexity of the Purkinje fiber network may account for the prominent role these cells play in the genesis of various arrhythmic syndromes.

METHODS AND RESULTS

Differential transcriptional profiling of murine Purkinje fibers and working ventricular myocytes was performed to identify novel genes expressed in PCs. The most highly enriched transcript in Purkinje fibers encoded Contactin-2 (Cntn2), a cell adhesion molecule critical for neuronal patterning and ion channel clustering. Endogenous expression of Cntn2 in the murine ventricle was restricted to a subendocardial network of myocytes that also express beta-galactosidase in CCS-lacZ transgenic mice and the connexin40 gap junction protein. Both Cntn2-lacZ knockin mice and Cntn2-EGFP BAC transgenic reporter mice confirmed expression of Cntn2 in the Purkinje fiber network, as did immunohistochemical staining of single canine Purkinje fibers. Whole-cell patch-clamp recordings and measurements of Ca(2+) transients in Cntn2-EGFP(+) cells revealed electrophysiological properties indicative of PCs and distinctive from those of cardiac myocytes, including prolonged action potentials and frequent afterdepolarizations.

CONCLUSIONS

Cntn2 is a novel marker of the specialized cardiac conduction system. Endogenous expression of Cntn2 as well as Cntn2-dependent transcriptional reporters provides a new tool through which Purkinje cell biology and pathophysiology can now more readily be deciphered. Expression of a contactin family member within the CCS may provide a mechanistic basis for patterning of the conduction system network and the organization of ion channels within Purkinje cells.

Autism-associated haplotype affects the regulation of the homeobox gene, ENGRAILED 2

BACKGROUND

Association analysis identified the homeobox transcription factor, ENGRAILED 2 (EN2), as a possible autism spectrum disorder (ASD) susceptibility gene (ASD [MIM 608636]; EN2 [MIM 131310]). The common alleles (underlined) of two intronic single nucleotide polymorphisms (SNPs), rs1861972 (A/G) and rs1861973 (C/T), are over-transmitted to affected individuals both singly and as a haplotype in three separate datasets (518 families total, haplotype p = .00000035).

METHODS

Further support that EN2 is a possible ASD susceptibility gene requires the identification of a risk allele, a DNA variant that is consistently associated with ASD but is also functional. To identify possible risk alleles, additional association analysis and linkage disequilibrium (LD) mapping were performed. Candidate polymorphisms were then tested for functional differences by luciferase (Luc) reporter transfections and electrophoretic mobility shift assays (EMSAs).

RESULTS

Association analysis of additional EN2 polymorphisms and LD mapping with Hapmap SNPs identified the rs1861972-rs1861973 haplotype as the most appropriate candidate to test for functional differences. Luciferase reporters for the two common rs1861972-rs1861973 haplotypes (A-C and G-T) were then transfected into human and rat cell lines as well as primary mouse neuronal cultures. In all cases the A-C haplotype resulted in a significant increase in Luc levels (p < .005). The EMSAs were then performed, and nuclear factors were bound specifically to the A and C alleles of both SNPs.

CONCLUSIONS

These data indicate that the A-C haplotype is functional and, together with the association and LD mapping results, supports EN2 as a likely ASD susceptibility gene and the A-C haplotype as a possible risk allele.

Transforming growth factor-beta (TGF- 1) down-regulates Notch3 in fibroblasts to promote smooth muscle gene expression

Select signaling pathways have emerged as key players in regulating smooth muscle gene expression during myofibroblast and smooth muscle differentiation, an event that is important for wound healing and vascular remodeling. These include the transforming growth factor-beta (TGF-beta1) signaling cascade, which has been assigned multiple roles in these cells, and the Notch pathway. Notch family members have been implicated in governing cell fate in a variety of cells; however, the mechanisms are not well understood. We sought to explore how these prominent signaling mediators regulate differentiation, and in particular, how they might converge to control the transcription of smooth muscle genes. Using TGF-beta1 to induce the differentiation of 10T1/2 fibroblasts, we investigated the specific function of Notch3. Overexpression of activated Notch3 caused repression of TGF-beta1-induced smooth muscle-specific genes, whereas knockdown of Notch3 by small interfering RNA did not convincingly alter their expression. Surprisingly, the addition of TGF-beta1 caused a significant decrease in Notch3 RNA and protein and a reciprocal increase in Hes1 gene transcription. The repression of Notch3 was mediated by SMAD activity and p38 mitogen-activated protein (MAP) kinase, whereas analysis of the Hes1 promoter revealed direct activation by Smad2 but not Smad3. Furthermore, the Hes1 repressor protein augmented Smad3 transactivation of the SM22alpha promoter. These results offer a novel mechanism by which TGF-beta1 promotes the expression of smooth muscle differentiation genes through the inhibition of Notch3 and activation of Hes1.

Cis-regulation of the amphioxus engrailed gene: Insights into evolution of a muscle-specific enhancer

To gain insights into the relation between evolution of cis-regulatory DNA and evolution of gene function, we identified tissue-specific enhancers of the engrailed gene of the basal chordate amphioxus (Branchiostoma floridae) and compared their ability to direct expression in both amphioxus and its nearest chordate relative, the tunicate Ciona intestinalis. In amphioxus embryos, the native engrailed gene is expressed in three domains - the eight most anterior somites, a few cells in the central nervous system (CNS) and a few ectodermal cells. In contrast, in C. intestinalis, in which muscle development is highly divergent, engrailed expression is limited to the CNS. To characterize the tissue-specific enhancers of amphioxus engrailed, we first showed that 7.8kb of upstream DNA of amphioxus engrailed directs expression to all three domains in amphioxus that express the native gene. We then identified the amphioxus engrailed muscle-specific enhancer as the 1.2kb region of upstream DNA with the highest sequence identity to the mouse en-2 jaw muscle enhancer. This amphioxus enhancer directed expression to both the somites in amphioxus and to the larval muscles in C. intestinalis. These results show that even though expression of the native engrailed has apparently been lost in developing C. intestinalis muscles, they express the transcription factors necessary to activate transcription from the amphioxus engrailed enhancer, suggesting that gene networks may not be completely disrupted if an individual component is lost.

Complex genomic rearrangement in CCS-LacZ transgenic mice

The cardiac conduction system (CCS)-lacZ insertional mouse mutant strain genetically labels the developing and mature CCS. This pattern of expression is presumed to reflect the site of transgene integration rather than regulatory elements within the transgene proper. We sought to characterize the genomic structure of the integration locus and identify nearby gene(s) that might potentially confer the observed CCS-specific transcription. We found rearrangement of chromosome 7 between regions D1 and E1 with altered transcription of multiple genes in the D1 region. Several lines of evidence suggested that regulatory elements from at least one gene, Slco3A1, influenced CCS-restricted reporter gene expression. In embryonic hearts, Slco3A1 was expressed in a spatial pattern similar to the CCS-lacZ transgene and was similarly neuregulin-responsive. At later stages, however, expression patterns of the transgene and Slco3A1 diverged, suggesting that the Slco3A1 locus may be necessary, but not sufficient to confer CCS-specific transgene expression in the CCS-lacZ line.

Characterization of sinoatrial node in four conduction system marker mice

The specialized cardiac conduction system (CCS) consists of the sinoatrial node (SAN) and the atrioventricular (AV) conduction system (AVCS), which includes proximal (AV node, bundle of His and bundle branches) and distal (Purkinje fibers) components. In four CCS marker mice [two transgenic (cGATA6|lacZ, CCS|lacZ) and two targeted gene knock-in (minK|lacZ, Hop|lacZ)] the expression of the lacZ gene (beta-gal) has been reported to mark portions of the proximal and distal AVCS; the expression of this marker in the adult SAN is unknown. The primary objective of this study was to analyze the utility of these marker mice in the identification of the SAN. Intercaval and interventricular septal regions, containing all the components of the CCS, were freshly dissected from adult mice based on the anatomical landmarks and sectioned. Immunohistochemical characterization was performed with SAN markers (Cx45, HCN4), compared to the reporter expression (beta-gal) and markers of the working myocardium (Cx40 and Cx43). In all four of the CCS marker mice, we found that beta-gal expression is consistently observed in the proximal and distal AVCS. However, the presence of lacZ gene expression in the working myocardium outside the CCS and/or the absence of this reporter expression in the SAN prevent the effective use of these mice to identify the SAN, leading us to conclude that none of the four CCS marker mice we studied specifically mark the SAN.

A unique subpopulation of Tbr1-expressing deep layer neurons in the developing cerebral cortex

Cells of the subplate (SP) and deep cortical plate (CP) are among the pioneer neurons of the developing cerebral cortex, an important group of early-born cells that impact cortical organization and function. Similarities between pioneer neurons in different cortical positions and heterogeneities in pioneer cells in the same cortical location, however, have made it difficult to appreciate the characteristics and functions of particular sets of these cells. Here, we provide a tool to illuminate a unique subset of SP and deep CP neurons: expression of a Tbrain-1 (Tbr1)-driven transgene. Transgene-expressing cells were consistently positive for neuronal but not glial markers, were born early in corticogenesis, representing just a subset of SP and deep CP neurons, were morphologically complex during the formation of the cortex, and were maintained into maturity. This analysis reveals a novel group of pioneer neurons and demonstrates unrecognized diversity within this cortical population. In the future, this information will help to uncover the roles of discrete pioneer populations in cortical development.

A unique subpopulation of Tbr1-expressing deep layer neurons in the developing cerebral cortex

Cells of the subplate (SP) and deep cortical plate (CP) are among the pioneer neurons of the developing cerebral cortex, an important group of early-born cells that impact cortical organization and function. Similarities between pioneer neurons in different cortical positions and heterogeneities in pioneer cells in the same cortical location, however, have made it difficult to appreciate the characteristics and functions of particular sets of these cells. Here, we provide a tool to illuminate a unique subset of SP and deep CP neurons: expression of a Tbrain-1 (Tbr1)-driven transgene. Transgene-expressing cells were consistently positive for neuronal but not glial markers, were born early in corticogenesis, representing just a subset of SP and deep CP neurons, were morphologically complex during the formation of the cortex, and were maintained into maturity. This analysis reveals a novel group of pioneer neurons and demonstrates unrecognized diversity within this cortical population. In the future, this information will help to uncover the roles of discrete pioneer populations in cortical development.

Postnatal cerebellar defects in mice deficient in methylenetetrahydrofolate reductase

Patients with severe deficiency of methylenetetrahydrofolate reductase (MTHFR) suffer from a wide variety of neurological problems, which can begin in the neonatal period. MTHFR is a critical enzyme in folate metabolism; the product of the MTHFR reaction, 5-methyltetrahydrofolate, is required for homocysteine remethylation to methionine and synthesis of S-adenosylmethionine (SAM). To understand the mechanisms by which MTHFR deficiency leads to significant neuropathology, we examined early postnatal brain development in mice with a homozygous knockout of the Mthfr gene. These mice displayed a dramatically reduced size of the cerebellum and cerebral cortex, with enlarged lateral ventricles. Mthfr deficiency affected granule cell maturation, but not neurogenesis. Depletion of external granule cells and disorganization of Purkinje cells were mainly confined to the anterior lobules of mutant cerebella. Decreased cellular proliferation and increased cell death contributed to the granule cell loss. Reduced expression of Engrailed-2 (En2), Reelin (Reln) and inositol 1,4,5-triphosphate receptor type 1 (Itpr1) genes was observed in the cerebellum. Supplementation of Mthfr(+/-) dams with an alternate methyl donor, betaine, reduced cerebellar abnormalities in the Mthfr(-/-) pups. Our findings suggest that MTHFR plays a role in cerebellar patterning, possibly through effects on proliferation or apoptosis.

Functional imaging of the embryonic pacemaking and cardiac conduction system over the past 150 years: Technologies to overcome the challenges

Early analyses of cardiac pacemaking and conduction system (CPCS) development relied on classic histology and visual inspection of the beating heart. Current techniques that facilitate delineation of the CPCS include the use of specific antibody markers and transgenic mouse lines specifically expressing reporter genes. Assaying the function of tiny embryonic hearts required an increase in the level of spatial and temporal resolution. Current methods for such analyses include the use of intracellular and extracellular microelectrodes, echocardiography, rapid optical imaging using fluorescent dyes, and most recently optical coherence tomography. This review will focus on methods developed to investigate the functional emergence of the embryonic cardiac conduction system. Where appropriate, the methods used to delineate the anatomic pathways will also be discussed. The combination of techniques to capture both morphological and functional data from the CPCS will further improve with continued interdisciplinary collaboration. The Supplementary Material referred to in this article can be found at the Anatomical Record website (http://www.interscience.wiley.com/jpages/0003-276X/suppmat).

Functional inhibition of secreted angiopoietin: a novel role for angiopoietin 1 in coronary vessel patterning

The angiopoietins are a family of growth factors critical for development and maintenance of the vasculature. The primary amino acid sequence of the angiopoietins predicts that they are comprised of a coiled-coiled and a fibrinogen-like domain. The coiled-coiled domain mediates ligand multimerization, whereas the fibrinogen domain engages the receptor. This multimerization is required to elicit a ligand-mediated biological effect via activation of their receptor Tie2. In vitro and in vivo knockout studies have suggested that the angiopoietins are chemotactic for endothelial cells. We were interested in ascertaining whether the angiopoietins have this activity within the animal proper. To accomplish this we engineered a dominant-interfering form of angiopoietin (Ang) 1, called Ang1cc. Ang1cc contains the coiled-coiled domain, which can heterodimerize with other angiopoietins produced in the same cell. We show that Ang1cc can inhibit Tie2 activation and can inhibit Ang1 activity in vitro and in vivo.

Regulatory Elements Directing Gut Expression of the GATA6 Gene during Mouse Early Development

The GATA6 transcription factor, as well as GATA4 and GATA5, is expressed in a variety of mammalian tissues including the precardiac mesoderm and endoderm, and gut-related organs. Genetic studies have also implicated GATA factors as important regulators of gut endoderm development. Previously, we identified the promoter and a cardiac-specific enhancer of mouse GATA6 [Sun-Wada et al. (2000) J. Biochem. 127, 703-709], however, little is known about the regulatory elements that govern GATA6 expression in the primitive gut. Here, we identified a distal enhancer of the GATA6 gene directing expression in the gut by creating transgenic mice. A sequence of approximately 200 bp between -8.0 kb and -7.8 kb contains element(s) that enhance transcription in the gut during embryonic development, when linked to the hsp68 promoter/lacZ fusion gene. Our results also show that GATA6 expression is controlled by multiple regulatory regions including cardiac-specific and gut-specific enhancers.

Identification of a short PIASx gene promoter that directs male germ cell-specific transcription in vivo

PIASx gene encodes two SUMO E3 ligases that are highly expressed in the testis. We have isolated and analyzed the promoter of the murine PIASx gene. Electrophoretic mobility shift assays with testicular nuclear extracts showed that the proximal promoter forms a major DNA-protein complex containing Sp1, Sp2, and Sp3 transcription factors. Reporter gene assays in cultured cells indicated that a fragment comprising nucleotides from -168 to +76 relative to transcription start site is sufficient for basal promoter activity in cultured cells, but these in vitro assays failed to reveal clear differences in promoter activity between testis- and non-testis-derived cell lines. Interestingly, however, the proximal promoter encompasses the elements necessary for a testis-specific transcription in vivo, as it directed beta-galactosidase expression exclusively to male germ cells in transgenic mice. In conclusion, we have characterized the minimal PIASx promoter that can be used for highly specific targeting of transgene expression to male germ cells.

A novel gene, GliH1, with homology to the Gli zinc finger domain not required for mouse development

The Sonic hedgehog (Shh)-Gli signaling pathway regulates development of many organs, including teeth. We cloned a novel gene encoding a transcription factor that contains a zinc finger domain with highest homology to the Gli family of proteins (61-64% amino acid sequence identity) from incisor pulp. Consistent with this sequence conservation, gel mobility shift assays demonstrated that this new Gli homologous protein, GliH1, could bind previously characterized Gli DNA binding sites. Furthermore, transfection assays in dental pulp cells showed that whereas Gli1 induces a nearly 50-fold increase in activity of a luciferase reporter containing Gli DNA binding sites, coexpression of Gli1 with Gli3 and/or GliH1 results in inhibition of the Gli1-stimulated luciferase activity. In situ hybridization analysis of mouse embryos demonstrated that GliH1 expression is initiated later than the three Gli genes and has a more restricted expression pattern. GliH1 is first detected diffusely in the limb buds at 10.0 days post coitus and later is expressed in the branchial arches, craniofacial interface, ventral part of the tail, whisker follicles and hair, intervertebral discs, teeth, eyes and kidney. LacZ was inserted into the GliH1 allele in embryonic stem cells to produce mice lacking GliH1 function. While this produced indicator mice for GliH1-expression, analysis of mutant mice revealed no discernible phenotype or required function for GliH1. A search of the Celera Genomics and associated databases identified possible gene sequences encoding a zinc finger domain with approximately 90% homology to that of GliH1, indicating there is a family of GliH genes and raising the possibility of overlapping functions during development.

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.

Identification of a CArG box-dependent enhancer within the cysteine-rich protein 1 gene that directs expression in arterial but not venous or visceral smooth muscle cells

Smooth muscle cells (SMCs) are heterogeneous with respect to their contractile, synthetic, and proliferative properties, though the regulatory factors responsible for their phenotypic diversity remain largely unknown. To further our understanding of smooth muscle gene regulation, we characterized the cis-regulatory elements of the murine cysteine-rich protein 1 gene (CRP1/Csrp1). CRP1 is expressed in all muscle cell types during embryogenesis and predominates in vascular and visceral SMCs in the adult. We identified a 5-kb enhancer within the CRP1 gene that is sufficient to drive expression in arterial but not venous or visceral SMCs in transgenic mice. This enhancer also exhibits region-specific activity in the outflow tract of the heart and the somites. Within the 5-kb CRP1 enhancer, we found a single CArG box that binds serum response factor (SRF), and by mutational analysis, demonstrate that the activity of the enhancer is dependent on this CArG element. Our findings provide further evidence for the existence of distinct regulatory programs within SMCs and suggest a role for SRF in the activation of the CRP1 gene.

Sequential expression of annexin A5 in the vasculature and skeletal elements during mouse development

Annexin A5 (annexin V, anchorin CII) represents the prototype member of the large annexin family, characterized by its ability to interact with phospholipids in a calcium-dependent manner and to form calcium-specific ion channels. Despite intense biochemical analysis, the in vivo expression and function of this annexin during mouse development, still remains unclear. Immunohistochemistry, in situ hybridization and reporter gene expression were used to define expression of annexin A5 during early mouse development. First, annexin A5 expression is associated with the developing vascular system. Later, expression is detected within the notochord and found in parallel to the differentiation of cartilage and bone. Therefore, expression of the Anxa5 gene may represent a novel marker characterizing cell lineages involved in the development of the vascular as well as the skeletal system.

Early anterior/posterior patterning of the midbrain and cerebellum

Transplantation studies performed in chicken embryos indicated that early anterior/posterior patterning of the vertebrate midbrain and cerebellum might be regulated by an organizing center at the junction between the midbrain and hindbrain. More than a decade of molecular and genetic studies have shown that such an organizer is indeed central to development of the midbrain and anterior hindbrain. Furthermore, a complicated molecular network that includes multiple positive and negative feedback loops underlies the establishment and refinement of a mid/hindbrain organizer, as well as the subsequent function of the organizer. In this review, we first introduce the expression patterns of the genes known to be involved in this patterning process and the quail-chick transplantation experiments that have provided the foundation for understanding the genetic pathways regulating mid/hindbrain patterning. Subsequently, we discuss the molecular genetic studies that have revealed the roles for many genes in normal early patterning of this region. Finally, some of the remaining questions and future directions are discussed.

Overexpression of Pax5 is not sufficient for neoplastic transformation of mouse neuroectoderm

The developmental control genes of the Pax family are essential for brain development. Several Pax genes are also involved in chromosomal translocations causing malignancies in humans, and Pax5 expression is deregulated in medulloblastomas. We have investigated whether Pax5 can induce tumors in the developing mouse brain. Primary mouse embryonic neuroectodermal cells were retrovirally transduced with mouse Pax5 and transplanted into the brain of syngeneic host mice. No tumors developed in 36 transplants after one year, and there were no alterations in the differentiation pattern of the neural transplants. We then generated transgenic mice expressing human Pax5 under control of the Engrailed-2 promoter, which is expressed in the cerebellar external granule cell layer and in medulloblastomas. Sustained expression was achieved in the cerebellum of transgenic animals throughout lifetime. Expression levels were similar to those observed in human medulloblastomas. Again, cerebellar morphogenesis was undisturbed, and no tumors arose. These results strongly argue against a dominant transforming activity of PAX5 in NEC and in cerebellar granule cell precursors of mice, and underline the restricted tissue-specificity of PAX5 related oncogenesis.

Visualization and functional characterization of the developing murine cardiac conduction system

The cardiac conduction system is a complex network of cells that together orchestrate the rhythmic and coordinated depolarization of the heart. The molecular mechanisms regulating the specification and patterning of cells that form this conductive network are largely unknown. Studies in avian models have suggested that components of the cardiac conduction system arise from progressive recruitment of cardiomyogenic progenitors, potentially influenced by inductive effects from the neighboring coronary vasculature. However, relatively little is known about the process of conduction system development in mammalian species, especially in the mouse, where even the histological identification of the conductive network remains problematic. We have identified a line of transgenic mice where lacZ reporter gene expression delineates the developing and mature murine cardiac conduction system, extending proximally from the sinoatrial node to the distal Purkinje fibers. Optical mapping of cardiac electrical activity using a voltage-sensitive dye confirms that cells identified by the lacZ reporter gene are indeed components of the specialized conduction system. Analysis of lacZ expression during sequential stages of cardiogenesis provides a detailed view of the maturation of the conductive network and demonstrates that patterning occurs surprisingly early in embryogenesis. Moreover, optical mapping studies of embryonic hearts demonstrate that a murine His-Purkinje system is functioning well before septation has completed. Thus, these studies describe a novel marker of the murine cardiac conduction system that identifies this specialized network of cells throughout cardiac development. Analysis of lacZ expression and optical mapping data highlight important differences between murine and avian conduction system development. Finally, this line of transgenic mice provides a novel tool for exploring the molecular circuitry controlling mammalian conduction system development and should be invaluable in studies of developmental mutants with potential structural or functional conduction system defects.

Bone-directed expression of Col1a1 promoter-driven self-inactivating retroviral vector in bone marrow cells and transgenic mice

Gene therapy of bone would benefit from the availability of vectors that provide stable, osteoblast-specific expression. This would allow bone-specific expression of Col1a1 cDNAs for treatment of osteogenesis imperfecta. In addition, such a vector would restrict expression of secreted therapeutic proteins to the bone-synthesizing regions of the bone marrow after ex vivo transduction of marrow stromal cells and reintroduction of the cells into patients. Retrovirus vectors stably integrate into target cell genomes; however, long-term regulated expression from internal cellular promoters has not been consistently achieved. In some cases this is due to a stem cell-specific mechanism for transcriptional repression of retroviruses. We evaluated the ability of self-inactivating ROSA-derived vectors containing a bone-directed 2.3-kb rat Col1a1 promoter to display osteoblast-specific expression. In vitro expression was examined in bone marrow stromal cell cultures induced to undergo osteoblastic differentiation. In vivo expression was evaluated in chimeric mice derived from transduced embryonic stem cells. The results indicate that self-inactivating retrovirus vectors containing the Col1a1 promoter are not permanently inactivated in embryonic stem cells and are specifically expressed in osteoblasts in vivo and in vitro. Thus these vectors should be useful for bone-directed gene therapy.

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.

EN and GBX2 play essential roles downstream of FGF8 in patterning the mouse mid/hindbrain region

Fgf8, which is expressed at the embryonic mid/hindbrain junction, is required for and sufficient to induce the formation of midbrain and cerebellar structures. To address through what genetic pathways FGF8 acts, we examined the epistatic relationships of mid/hindbrain genes that respond to FGF8, using a novel mouse brain explant culture system. We found that En2 and Gbx2 are the first genes to be induced by FGF8 in wild-type E9.5 diencephalic and midbrain explants treated with FGF8-soaked beads. By examining gene expression in En1/2 double mutant mouse embryos, we found that Fgf8, Wnt1 and Pax5 do not require the En genes for initiation of expression, but do for their maintenance, and Pax6 expression is expanded caudally into the midbrain in the absence of EN function. Since E9.5 En1/2 double mutants lack the mid/hindbrain region, forebrain mutant explants were treated with FGF8 and, significantly, the EN transcription factors were found to be required for induction of Pax5. Thus, FGF8-regulated expression of Pax5 is dependent on EN proteins, and a factor other than FGF8 could be involved in initiating normal Pax5 expression in the mesencephalon/metencephalon. The En genes also play an important, but not absolute, role in repression of Pax6 in forebrain explants by FGF8. Previous Gbx2 gain-of-function studies have shown that misexpression of Gbx2 in the midbrain can lead to repression of Otx2. However, in the absence of Gbx2, FGF8 can nevertheless repress Otx2 expression in midbrain explants. In contrast, Wnt1 is initially broadly induced in Gbx2 mutant explants, as in wild-type explants, but not subsequently repressed in cells near FGF8 that normally express Gbx2. Thus GBX2 acts upstream of, or parallel to, FGF8 in repressing Otx2, and acts downstream of FGF8 in repression of Wnt1. This is the first such epistatic study performed in mouse that combines gain-of-function and loss-of-function approaches to reveal aspects of mouse gene regulation in the mesencephalon/metencephalon that have been difficult to address using either approach alone.

Two Pax2/5/8-binding sites in Engrailed2 are required for proper initiation of endogenous mid-hindbrain expression

During early brain development mouse Engrailed2 (En2) is expressed in a broad band across most of the mid-hindbrain region. Evidence from gene expression data, promoter analysis in transgenic mice and mutant phenotype analysis in mice and zebrafish has suggested that Pax2, 5 and 8 play a critical role in regulating En2 mid-hindbrain expression. Previously, we identified two Pax2/5/8-binding sites in a 1.0 kb En2 enhancer fragment that is sufficient to directed reporter gene expression to the early mid-hindbrain region and showed that the two Pax2/5/8-binding sites are essential for the mid-hindbrain expression in transgenic mice. In the present study we have examined the functional requirements of these two Pax2/5/8-binding sites in the context of the endogenous En2 gene for directing mid-hindbrain expression. The two Pax2/5/8-binding sites were deleted from the En2 locus and replaced with the bacterial neo gene by homologous recombination in mouse embryonic stem cells. After transmitting the mutation into mice, the neo gene was removed by breeding with transgenic mice expressing cre from a CMV promoter. Embryos homozygous for this En2 Pax2/5/8-binding site deletion mutation had a mild reduction in En2 expression in the presumptive mid-hindbrain region at the 5-7 somite stage, when En2 expression is normally initiated. However, from embryonic day 9.0 onwards, the mutant embryos showed En2 expression indistinguishable from that seen in wild type embryos. Furthermore, the mutants did not show the cerebellar defect seen in mice with a null mutation in En2. This result demonstrates that the two Pax2/5/8-binding sites that were deleted, while being required for mid-hindbrain expression in the context of a 1.0 kb En2 enhancer, are only required for proper initiation of expression of the endogenous En2 gene. Interestingly, a comparison of the lacZ RNA and protein expression patterns directed by the 1.0 kb enhancer fragment revealed that lacZ protein was acting as a lineage marker in the mid-hindbrain region by persisting longer than the mRNA. The transgene expression directed by the 1.0 kb enhancer fragment therefore does not mimic the entire broad domain of En2 expression. Taken together, these two studies demonstrate that DNA binding sites in addition to the two Pax2/5/8-binding sites must be necessary for En2 mid-hindbrain expression.

Neural induction and patterning in the mouse in the absence of the node and its derivatives

The signals which induce vertebrate neural tissue and pattern it along the anterior-posterior (A-P) axis have been proposed to emanate from Spemann's organizer, which in mammals is a structure termed the node. However, mouse embryos mutant for HNF3 beta lack a morphological node and node derivatives yet undergo neural induction. Gene expression domains occur at their normal A-P axial positions along the mutant neural tubes in an apparently normal temporal manner, including the most anterior and posterior markers. This neural patterning occurs in the absence of expression of known organizer genes, including the neural inducers chordin and noggin. Other potential signaling centers in gastrulating mutant embryos appear to express their normal constellation of putative secreted factors, consistent with the possibility that neural-inducing and -patterning signals emanate from elsewhere or at an earlier time. Nevertheless, we find that the node and the anterior primitive streak, from which the node derives, are direct sources of neural-inducing signals, as judged by expression of the early midbrain marker Engrailed, in explant-recombination experiments. Similar experiments showed the neural-inducing activity in HNF3 beta mutants to be diffusely distributed. Our results indicate that the mammalian organizer is capable of neural induction and patterning of the neural plate, but that maintenance of an organizer-like signaling center is not necessary for either process.

Granule cell dispersion is restricted across transverse boundaries in mouse chimeras

The granular layer of the developing and adult cerebellum is marked by the presence of several transverse boundaries, revealed in gene expression patterns or as a consequence of genetic mutations. It is unclear whether these boundaries represent fundamental differences between granule cell populations or if they are a secondary response to regional differences in the underlying Purkinje cells. One possibility is that boundaries mark different spatial domains of granule cells in a lineage-dependent fashion. To test this hypothesis, we have analysed a series of murine embryonic stem cell chimeras marked by the constitutive expression of beta-galactosidase in donor granule cells. The chimeras show a consistent spatial restriction boundary, located in the granular layer of lobule VI in the vermis and extending laterally into crus I of the hemispheres. A second boundary was found separating lobules IX and X in the vermis. No correlation was found between the genotypes of molecular layer interneurons and the underlying granule cells, suggesting that they arise independently. No transverse boundaries were observed for the molecular layer interneurons, consistent with the hypothesis that they are not generated from precursors in the external granular layer. These results indicate that the granular layer of the cerebellum comprises cellular domains with different histories separated by consistent spatial restriction boundaries.

Regulation of Gli2 and Gli3 activities by an amino-terminal repression domain: implication of Gli2 and Gli3 as primary mediators of Shh signaling

Gli family zinc finger proteins are mediators of Sonic hedgehog (Shh) signaling in vertebrates. The question remains unanswered, however, as to how these Gli proteins participate in the Shh signaling pathway. In this study, regulatory activities associated with the Gli2 protein were investigated in relation to the Shh signaling. Although Gli2 acts as a weak transcriptional activator, it is in fact a composite of positive and negative regulatory domains. In cultured cells, truncation of the activation domain in the C-terminal half results in a protein with repressor activity, while removal of the repression domain at the N terminus converts Gli2 into a strong activator. In transgenic mouse embryos, N-terminally truncated Gli2, unlike the full length protein, activates a Shh target gene, HNF3beta, in the dorsal neural tube, thus mimicking the effect of Shh signal. This suggests that unmasking of the strong activation potential of Gli2 through modulation of the N-terminal repression domain is one of the key mechanisms of the Shh signaling. A similar regulatory mechanism involving the N-terminal region was also found for Gli3, but not for Gli1. When the Shh signal derived from the notochord is received by the neural plate, the widely expressed Gli2 and Gli3 proteins are presumably converted to their active forms in the ventral cells, leading to activation of transcription of their target genes, including Gli1.

Direct regulation of the Xenopus engrailed-2 promoter by the Wnt signaling pathway, and a molecular screen for Wnt-responsive genes, confirm a role for Wnt signaling during neural patterning in Xenopus

The co-activation of Wnt signaling and concomitant inhibition of BMP signaling has previously been implicated in vertebrate neural patterning, as evidenced by the combinatorial induction of engrailed-2 and krox-20 in Xenopus. However, screens have not previously been conducted to identify additional potential target genes. Using a PCR-based screening method we determined that XA-1, xCRISP, UVS.2, two UVS.2-related genes, and xONR1 are induced in response to Xwnt-3a and a BMP-antagonist, noggin. Two additional genes, connexin 30 and retinoic acid receptor gamma were induced by Xwnt-3a alone. To determine whether any of the induced genes are direct targets of Wnt signaling, we focussed on engrailed-2. In the present study we show that the Xenopus engrailed-2 promoter contains three consensus binding sites for LEF/TCF, which are HMG box transcription factors which bind to beta-catenin in response to activation of the Wnt- 1 signaling pathway. An engrailed-2 promoter luciferase reporter construct containing these LEF/TCF sites is induced in embryo explant assays by the combination of Xwnt-3a or beta-catenin and noggin. These LEF/TCF sites are required for expression of engrailed-2, as a dominant negative Xtcf-3 blocks expression of endogenous engrailed-2 as well as expression of the reporter construct. Moreover, mutation of these three LEF/TCF sites abrogates expression of the reporter construct in response to noggin and Xwnt-3a or beta-catenin. We conclude that the engrailed-2 gene is a direct target of the Wnt signaling pathway, and that Wnt signaling works with BMP antagonists to regulate gene expression during patterning of the developing nervous system of Xenopus.

Ectopic Msx2 overexpression inhibits and Msx2 antisense stimulates calvarial osteoblast differentiation

Msx2 is believed to play a role in regulating bone development, particularly in sutures of cranial bone. In this study we investigated the effects of retroviral-mediated overexpression of Msx2 mRNA, in both sense and antisense orientations, on primary cultured chick calvarial osteoblasts. Unregulated overexpression of sense mRNA produced high levels of Msx2 protein throughout the culture period, preventing the expected fall as the cells differentiate. The continued high expression of Msx2 prevented osteoblastic differentiation and mineralization of the extracellular matrix. In contrast, expression of antisense Msx2 RNA decreased proliferation and accelerated differentiation. In other studies, we showed that the Msx2 promoter was widely expressed during the proliferative phase of mouse calvarial osteoblast cultures but was preferentially downregulated in osteoblastic nodules. These results support a model in which Msx2 prevents differentiation and stimulates proliferation of cells at the extreme ends of the osteogenic fronts of the calvariae, facilitating expansion of the skull and closure of the suture.

Engrailed negatively regulates the expression of cell adhesion molecules connectin and neuroglian in embryonic Drosophila nervous system

Engrailed is expressed in subsets of interneurons that do not express Connectin or appreciable Neuroglian, whereas other neurons that are Engrailed negative strongly express these adhesion molecules. Connectin and Neuroglian expression are virtually eliminated in interneurons when engrailed expression is driven ubiquitously in neurons, and greatly increased when engrailed genes are lacking in mutant embryos. The data suggest that Engrailed is normally a negative regulator of Connectin and neuroglian. These are the first two "effector" genes identified in the nervous system of Drosophila as regulatory targets for Engrailed. We argue that differential Engrailed expression is crucial in determining the pattern of expression of cell adhesion molecules and thus constitutes an important determinant of neuronal shape and perhaps connectivity.

Alteration of limb and brain patterning in early mouse embryos by ultrasound-guided injection of Shh-expressing cells

A basic limitation of the study of development in the mouse is the inaccessibility of the embryos, which are encased in the maternal uterus. We demonstrate the first use of ultrasound backscatter microscopy for guiding injections of cells and other agents into early stage mouse embryos. Cells were injected into the mouse neural tube cavity as early as 9.5 days post coitus (E9.5), and into the developing limb buds as early as E10.5. Furthermore, a cell-line engineered to express the secreted factor Sonic Hedgehog (Shh) was injected into early developing mouse brains or limbs. The Shh-expressing cells were found to induce ectopic expression of the Shh target genes Patched and Hnf3beta in the dorsal brain, and to alter digit patterning in the anterior limb bud. These results show that gene misexpression studies can be performed in mouse embryos using ultrasound-guided injection of transfected cells or retroviruses. In combination with the many available mouse mutants, this method offers a new approach for analyzing genetic interactions through gain-of-function studies performed in mutant mouse backgrounds.

Signalling interactions during facial development

The development of the vertebrate face is a dynamic multi-step process which starts with the formation of neural crest cells in the developing brain and their subsequent migration to form, together with mesodermal cells, the facial primordia. Signalling interactions co-ordinate the outgrowth of the facial primordia from buds of undifferentiated mesenchyme into the intricate series of bones and cartilage structures that, together with muscle and other tissues, form the adult face. Some of the molecules that are thought to be involved have been identified through the use of mouse mutants, data from human craniofacial syndromes and by expression studies of signalling molecules during facial development. However, the way that these molecules control the epithelial-mesenchymal interactions which mediate facial outgrowth and morphogenesis is unclear. The role of neural crest cells in these processes has also not yet been well defined. In this review we discuss the complex interaction of all these processes during face development and describe the candidate signalling molecules and their possible target genes.

Fate mapping of the mouse midbrain-hindbrain constriction using a site-specific recombination system

The mouse midbrain-hindbrain constriction is centrally involved in patterning of the midbrain and anterior hindbrain (cerebellum), as revealed by recent genetic studies using mice and embryological studies in chick (reviewed in [1,2]). This region can act as an organizer region to induce midbrain and cerebellar development. Genes such as Engrailed-1, Pax-2 and Pax-5, which are expressed in the embryonic cells that will form the midbrain and the cerebellum, are required for development of these regions. Fate-mapping experiments at early somite stages in chick have revealed that the cerebellar primordium is located both anterior and posterior to the midbrain-hindbrain constriction, whereas midbrain precursors lie more anteriorly. Fate mapping in mice has been complicated by the inaccessibility of the postimplantation embryo. Here, we report the use of a new in vivo approach involving the Cre-IoxP site-specific recombination system [3] to map the fate of cells in the mouse midbrain-hindbrain constriction. We show that cells originating in the mouse dorsal midbrain-hindbrain constriction during embryonic days 9-12 contribute significantly to the medial cerebellum and colliculi. Our data demonstrate the feasibility of using a recombinase-based lineage-tracing system for fate mapping in the mouse.

Multiple elements regulate Mash1 expression in the developing CNS

Mash1, a transcription factor of the basic helix-loop-helix class, is expressed during embryogenesis in restricted regions of the nervous system. An essential role for Mash1 in neural development was demonstrated previously in mice carrying a targeted disruption of the Mash1 gene. Regulation of the precise temporal and spatial expression of Mash1 is thus likely to be important for proper neural development. In this study, sequences that regulate Mash1 expression in the central nervous system were characterized by assaying the expression of lacZ reporter genes in transgenic embryos. A 1158-bp enhancer localized approximately 7 kb upstream of the Mash1 coding region was identified. Deletions within this enhancer region reveal the presence of both positive and negative cis-acting elements. Analysis of multiple sequences within the enhancer demonstrate that different elements preferentially function in different regions within the Mash1-specific CNS expression domain. In addition, a role for sequences 3' of the Mash1 coding region is revealed, providing evidence for posttranscriptional control of Mash1 expression in multiple CNS domains.

The role of Engrailed in establishing the dorsoventral axis of the chick limb

Expression and mutation analyses in mice suggest that the homeobox-containing gene Engrailed (En) plays a role in dorsoventral patterning of the limb. During the initial stages of limb bud outgrowth, En-1 mRNA and protein are uniformly distributed throughout the ventral limb bud ectoderm. Limbs of En-1(−/−) mice display a double dorsal phenotype suggesting that normal expression of En-1 in the ventral ectoderm is required to establish and/or maintain ventral limb characteristics. Loss of En-1 function also results in ventral expansion of the apical ectodermal ridge (AER), suggesting that En-1 is also required for proper formation of the AER. To further investigate the role En plays in dorsoventral patterning and AER formation, we have used the replication competent retroviral vector, RCAS, to mis-express mouse En-1 in the early chick limb bud. We show that ectopic En-1 expression in dorsal ectoderm is sufficient to repress the endogenous expression of the dorsal ectodermal marker Wnt7a, with a resultant decrease in Lmx1 expression in underlying dorsal mesenchyme. Furthermore, the AER is disrupted morphologically and the expression patterns of the AER signalling molecules Fgf-8 and Fgf-4 are altered. Consistent with recent evidence that there is a reciprocal interaction between signalling molecules in the dorsal ectoderm, AER, and zone of polarising activity (ZPA), loss of Wnt7a, Fgf-8 and Fgf-4 expression leads to a decrease in expression of the signalling molecule Shh in the ZPA. These results strongly support the idea that, in its normal domain of expression, En-1 represses Wnt7a-mediated dorsal differentiation by limiting the expression of Wnt7a to the dorsal ectoderm. Furthermore, our results provide additional evidence that En-1 is involved in AER formation and suggest that En-1 may act to define ventral ectodermal identity.

Sequence and embryonic expression of the amphioxus engrailed gene (AmphiEn): the metameric pattern of transcription resembles that of its segment-polarity homolog in Drosophila

Vertebrate segmentation has been proposed as an evolutionary inheritance either from some metameric protostome or from a more closely related deuterostome. To address this question, we studied the developmental expression of AmphiEn, the engrailed gene of amphioxus, the closest living invertebrate relative of the vertebrates. In neurula embryos of amphioxus, AmphiEn is expressed along the anteroposterior axis as metameric stripes, each located in the posterior part of a nascent or newly formed segment. This pattern resembles the expression stripes of the segment-polarity gene engrailed, which has a key role in establishing and maintaining the metameres in embryos of Drosophila and other metameric protostomes. Later, amphioxus embryos express AmphiEn in non-metameric patterns - transiently in the embryonic ectoderm and dorsal nerve cord. Nerve cord expression occurs in a few cells approximately midway along the rostrocaudal axis and also in a conspicuous group of anterior cells in the cerebral vesicle at a level previously identified as corresponding to the vertebrate diencephalon. Compared to vertebrate engrailed expression at the midbrain/hindbrain boundary, AmphiEn expression in the cerebral vesicle is relatively late. Thus, it is uncertain whether the cerebral vesicle expression marks the rostral end of the amphioxus hindbrain; if it does, then amphioxus may have little or no homolog of the vertebrate midbrain. The segmental expression of AmphiEn in forming somites suggests that the functions of engrailed homologs in establishing and maintaining a metameric body plan may have arisen only once during animal evolution. If so, the protostomes and deuterostomes probably shared a common segmented ancestor.

The embryonic cerebellum contains topographic cues that guide developing inferior olivary axons

The formation of the olivocerebellar projection is supposed to be regulated by positional information shared between pre- and postsynaptic neurons. However, experimental evidence to support this hypothesis is missing. In the chick, caudal neurons in the inferior olive project to the anterior cerebellum and rostral ones to the posterior cerebellum. We here report in vitro experiments that strongly support the existence of anteroposterior polarity cues in the embryonic cerebellum. We developed an in vitro system that was easily accessible to experimental manipulations. Large hindbrain explants of E7.5-E8 chick embryos, containing the cerebellum and its attached brainstem, were plated and studied using axonal tracing methods. In these cultures, we have shown that the normal anteroposterior topography of the olivocerebellar projection was acquired, even when the cerebellar lamella was detached from the brainstem and placed again in its original position. We also found that, following various experimental rotations of the anteroposterior axis of the cerebellum, the rostromedian olivary neurons still project to the posterior vermis and the caudolateral neurons to the anterior vermis, that now have inverted locations. Thus, the rotation of the target region results in the rotation of the projection. In addition, we have shown that the formation of the projection map could be due to the inability of rostromedian inferior olivary axons to grow in the anterior cerebellum. All these experiments strongly indicate that olivocerebellar fibers recognize within their target region polarity cues that organize their anteroposterior topography, and we suggest that Purkinje cells might carry these cues.