Mouse Alx3: an aristaless-like homeobox gene expressed during embryogenesis in ectomesenchyme and lateral plate mesoderm

Developmental origin of the mammalian premaxilla

The evolution of jaws has played a major role in the success of vertebrate expansion into a wide variety of ecological niches. A fundamental, yet unresolved, question in craniofacial biology is about the origin of the premaxilla, the most distal bone present in the upper jaw of all amniotes. Recent reports have suggested that the mammalian premaxilla is derived from embryonic maxillary prominences rather than the frontonasal ectomesenchyme as previously shown in studies of chicken embryos. However, whether mammalian embryonic frontonasal ectomesenchyme contributes to the premaxillary bone has not been investigated and a tool to trace the contributions of the frontonasal ectomesenchyme to facial structures in mammals is lacking. The expression of the Alx3 gene is activated highly specifically in the frontonasal ectomesenchyme, but not in the maxillary mesenchyme, from the beginning of facial morphogenesis in mice. Here, we report the generation and characterization of a novel Alx3CreERT2 knock-in mouse line that express tamoxifen-inducible Cre DNA recombinase from the Alx3 locus. Tamoxifen treatment of Alx3CreERT2/+;Rosa26mTmG/+ embryos at E7.5, E8.5, E9.5, and E10.5, each induced specific labeling of the embryonic medial nasal and lateral nasal mesenchyme but not the maxillary mesenchyme. Lineage tracing of Alx3CreERT2-labeled frontonasal mesenchyme from E9.5 to E16.5 clearly showed that the frontonasal mesenchyme cells give rise to the osteoblasts generating the premaxillary bone. Furthermore, we characterize a Dlx1-Cre BAC transgenic mouse line that expresses Cre activity in the embryonic maxillary but not the frontonasal mesenchyme and show that the Dlx1-Cre labeled embryonic maxillary mesenchyme cells contribute to the maxillary bone as well as the soft tissues lateral to both the premaxillary and maxillary bones but not to the premaxillary bone. These results clearly demonstrate the developmental origin of the premaxillary bone from embryonic frontonasal ectomesenchyme cells in mice and confirm the evolutionary homology of the premaxilla across amniotes.

Zebrafish models of alx-linked frontonasal dysplasia reveal a role for Alx1 and Alx3 in the anterior segment and vasculature of the developing eye

The cellular and genetic mechanisms that coordinate formation of facial sensory structures with surrounding skeletal and soft tissue elements remain poorly understood. Alx1, a homeobox transcription factor, is a key regulator of midfacial morphogenesis. ALX1 mutations in humans are linked to severe congenital anomalies of the facial skeleton (frontonasal dysplasia, FND) with malformation or absence of eyes and orbital contents (micro- and anophthalmia). Zebrafish with loss-of-function alx1 mutations develop with craniofacial and ocular defects of variable penetrance, likely due to compensatory upregulation in expression of a paralogous gene, alx3. Here we show that zebrafish alx1;alx3 mutants develop with highly penetrant cranial and ocular defects that resemble human ALX1-linked FND. alx1 and alx3 are expressed in anterior cranial neural crest (aCNC), which gives rise to the anterior neurocranium (ANC), anterior segment structures of the eye and vascular pericytes. Consistent with a functional requirement for alx genes in aCNC, alx1; alx3 mutants develop with nearly absent ANC and grossly aberrant hyaloid vasculature and ocular anterior segment, but normal retina. In vivo lineage labeling identified a requirement for alx1 and alx3 during aCNC migration, and transcriptomic analysis suggested oxidative stress response as a key target mechanism of this function. Oxidative stress is a hallmark of fetal alcohol toxicity, and we found increased penetrance of facial and ocular malformations in alx1 mutants exposed to ethanol, consistent with a protective role for alx1 against ethanol toxicity. Collectively, these data demonstrate a conserved role for zebrafish alx genes in controlling ocular and facial development, and a novel role in protecting these key midfacial structures from ethanol toxicity during embryogenesis. These data also reveal novel roles for alx genes in ocular anterior segment formation and vascular development and suggest that retinal deficits in alx mutants may be secondary to aberrant ocular vascularization and anterior segment defects. This study establishes robust zebrafish models for interrogating conserved genetic mechanisms that coordinate facial and ocular development, and for exploring gene--environment interactions relevant to fetal alcohol syndrome.

FGF signalling plays similar roles in development and regeneration of the skeleton in the brittle star Amphiura filiformis

Regeneration as an adult developmental process is in many aspects similar to embryonic development. Although many studies point out similarities and differences, no large-scale, direct and functional comparative analyses between development and regeneration of a specific cell type or structure in one animal exist. Here, we use the brittle star Amphiura filiformis to characterise the role of the FGF signalling pathway during skeletal development in embryos and arm regeneration. In both processes, we find ligands expressed in ectodermal cells that flank underlying skeletal mesenchymal cells, which express the receptors. Perturbation of FGF signalling showed inhibited skeleton formation in both embryogenesis and regeneration, without affecting other key developmental processes. Differential transcriptome analysis finds mostly differentiation genes rather than transcription factors to be downregulated in both contexts. Moreover, comparative gene analysis allowed us to discover brittle star-specific differentiation genes. In conclusion, our results show that the FGF pathway is crucial for skeletogenesis in the brittle star, as in other deuterostomes, and provide evidence for the re-deployment of a developmental gene regulatory module during regeneration.

Transcription Factors of the Alx Family: Evolutionarily Conserved Regulators of Deuterostome Skeletogenesis

Members of the alx gene family encode transcription factors that contain a highly conserved Paired-class, DNA-binding homeodomain, and a C-terminal OAR/Aristaless domain. Phylogenetic and comparative genomic studies have revealed complex patterns of alx gene duplications during deuterostome evolution. Remarkably, alx genes have been implicated in skeletogenesis in both echinoderms and vertebrates. In this review, we provide an overview of current knowledge concerning alx genes in deuterostomes. We highlight their evolutionarily conserved role in skeletogenesis and draw parallels and distinctions between the skeletogenic gene regulatory circuitries of diverse groups within the superphylum.

ALX1-related frontonasal dysplasia results from defective neural crest cell development and migration

A pedigree of subjects presented with frontonasal dysplasia (FND). Genome sequencing and analysis identified a p.L165F missense variant in the homeodomain of the transcription factor ALX1 which was imputed to be pathogenic. Induced pluripotent stem cells (iPSC) were derived from the subjects and differentiated to neural crest cells (NCC). NCC derived from ALX1L165F/L165F iPSC were more sensitive to apoptosis, showed an elevated expression of several neural crest progenitor state markers, and exhibited impaired migration compared to wild-type controls. NCC migration was evaluated in vivo using lineage tracing in a zebrafish model, which revealed defective migration of the anterior NCC stream that contributes to the median portion of the anterior neurocranium, phenocopying the clinical presentation. Analysis of human NCC culture media revealed a change in the level of bone morphogenic proteins (BMP), with a low level of BMP2 and a high level of BMP9. Soluble BMP2 and BMP9 antagonist treatments were able to rescue the defective migration phenotype. Taken together, these results demonstrate a mechanistic requirement of ALX1 in NCC development and migration.

Insights into olfactory ensheathing cell development from a laser-microdissection and transcriptome-profiling approach

Olfactory ensheathing cells (OECs) are neural crest-derived glia that ensheath bundles of olfactory axons from their peripheral origins in the olfactory epithelium to their central targets in the olfactory bulb. We took an unbiased laser microdissection and differential RNA-seq approach, validated by in situ hybridization, to identify candidate molecular mechanisms underlying mouse OEC development and differences with the neural crest-derived Schwann cells developing on other peripheral nerves. We identified 25 novel markers for developing OECs in the olfactory mucosa and/or the olfactory nerve layer surrounding the olfactory bulb, of which 15 were OEC-specific (that is, not expressed by Schwann cells). One pan-OEC-specific gene, Ptprz1, encodes a receptor-like tyrosine phosphatase that blocks oligodendrocyte differentiation. Mutant analysis suggests Ptprz1 may also act as a brake on OEC differentiation, and that its loss disrupts olfactory axon targeting. Overall, our results provide new insights into OEC development and the diversification of neural crest-derived glia.

Inherited Eye Diseases with Retinal Manifestations through the Eyes of Homeobox Genes

Retinal development is under the coordinated control of overlapping networks of signaling pathways and transcription factors. The paper was conceived as a review of the data and ideas that have been formed to date on homeobox genes mutations that lead to the disruption of eye organogenesis and result in inherited eye/retinal diseases. Many of these diseases are part of the same clinical spectrum and have high genetic heterogeneity with already identified associated genes. We summarize the known key regulators of eye development, with a focus on the homeobox genes associated with monogenic eye diseases showing retinal manifestations. Recent advances in the field of genetics and high-throughput next-generation sequencing technologies, including single-cell transcriptome analysis have allowed for deepening of knowledge of the genetic basis of inherited retinal diseases (IRDs), as well as improve their diagnostics. We highlight some promising avenues of research involving molecular-genetic and cell-technology approaches that can be effective for IRDs therapy. The most promising neuroprotective strategies are aimed at mobilizing the endogenous cellular reserve of the retina.

MITF-the first 25 years

In this review, Goding and Arnheiter present the current understanding of MITF's role and regulation in development and disease and highlight key areas where our knowledge of MITF regulation and function is limited.

All transcription factors are equal, but some are more equal than others. In the 25 yr since the gene encoding the microphthalmia-associated transcription factor (MITF) was first isolated, MITF has emerged as a key coordinator of many aspects of melanocyte and melanoma biology. Like all transcription factors, MITF binds to specific DNA sequences and up-regulates or down-regulates its target genes. What marks MITF as being remarkable among its peers is the sheer range of biological processes that it appears to coordinate. These include cell survival, differentiation, proliferation, invasion, senescence, metabolism, and DNA damage repair. In this article we present our current understanding of MITF's role and regulation in development and disease, as well as those of the MITF-related factors TFEB and TFE3, and highlight key areas where our knowledge of MITF regulation and function is limited.

Parenchymal and stromal tissue regeneration of tooth organ by pivotal signals reinstated in decellularized matrix

Cells are transplanted to regenerate an organs' parenchyma, but how transplanted parenchymal cells induce stromal regeneration is elusive. Despite the common use of a decellularized matrix, little is known as to the pivotal signals that must be restored for tissue or organ regeneration. We report that Alx3, a developmentally important gene, orchestrated adult parenchymal and stromal regeneration by directly transactivating Wnt3a and vascular endothelial growth factor. In contrast to the modest parenchyma formed by native adult progenitors, Alx3-restored cells in decellularized scaffolds not only produced vascularized stroma that involved vascular endothelial growth factor signalling, but also parenchymal dentin via the Wnt/β-catenin pathway. In an orthotopic large-animal model following parenchyma and stroma ablation, Wnt3a-recruited endogenous cells regenerated neurovascular stroma and differentiated into parenchymal odontoblast-like cells that extended the processes into newly formed dentin with a structure-mechanical equivalency to native dentin. Thus, the Alx3-Wnt3a axis enables postnatal progenitors with a modest innate regenerative capacity to regenerate adult tissues. Depleted signals in the decellularized matrix may be reinstated by a developmentally pivotal gene or corresponding protein.

Ectomesoderm and epithelial-mesenchymal transition-related genes in spiralian development

BACKGROUND

Spiralians (e.g., annelids, molluscs, and flatworms) possess two sources of mesoderm. One is from endodermal precursors (endomesoderm), which is considered to be the ancestral source in metazoans. The second is from ectoderm (ectomesoderm) and may represent a novel cell type in the Spiralia. In the mollusc Crepidula fornicata, ectomesoderm is derived from micromere daughters within the A and B cell quadrants. Their progeny lie along the anterolateral edges of the blastopore. There they undergo epithelial-mesenchymal transition (EMT), become rounded and undergo delamination/ingression. Subsequently, they assume the mesenchymal phenotype, and migrate beneath the surface ectoderm to differentiate various cell types, including muscles and pigment cells.

RESULTS

We examined expression of several genes whose homologs are known to regulate Type 1 EMT in other metazoans. Most of these genes were expressed within spiralian ectomesoderm during EMT.

CONCLUSIONS

We propose that spiralian ectomesoderm, which exhibits analogous cellular behaviors to other populations of mesenchymal cells, may be controlled by the same genes that drive EMT in other metazoans. Perhaps these genes comprise a conserved metazoan EMT gene regulatory network (GRN). This study represents the first step in elucidating the GRN controlling the development of a novel spiralian cell type (ectomesoderm). Developmental Dynamics 247:1097-1120, 2018. © 2018 Wiley Periodicals, Inc.

Insights into the Etiology of Mammalian Neural Tube Closure Defects from Developmental, Genetic and Evolutionary Studies

The human neural tube defects (NTD), anencephaly, spina bifida and craniorachischisis, originate from a failure of the embryonic neural tube to close. Human NTD are relatively common and both complex and heterogeneous in genetic origin, but the genetic variants and developmental mechanisms are largely unknown. Here we review the numerous studies, mainly in mice, of normal neural tube closure, the mechanisms of failure caused by specific gene mutations, and the evolution of the vertebrate cranial neural tube and its genetic processes, seeking insights into the etiology of human NTD. We find evidence of many regions along the anterior–posterior axis each differing in some aspect of neural tube closure—morphology, cell behavior, specific genes required—and conclude that the etiology of NTD is likely to be partly specific to the anterior–posterior location of the defect and also genetically heterogeneous. We revisit the hypotheses explaining the excess of females among cranial NTD cases in mice and humans and new developments in understanding the role of the folate pathway in NTD. Finally, we demonstrate that evidence from mouse mutants strongly supports the search for digenic or oligogenic etiology in human NTD of all types.

Location, Location, Location: Signals in Muscle Specification

Muscles control body movement and locomotion, posture and body position and soft tissue support. Mesoderm derived cells gives rise to 700 unique muscles in humans as a result of well-orchestrated signaling and transcriptional networks in specific time and space. Although the anatomical structure of skeletal muscles is similar, their functions and locations are specialized. This is the result of specific signaling as the embryo grows and cells migrate to form different structures and organs. As cells progress to their next state, they suppress current sequence specific transcription factors (SSTF) and construct new networks to establish new myogenic features. In this review, we provide an overview of signaling pathways and gene regulatory networks during formation of the craniofacial, cardiac, vascular, trunk, and limb skeletal muscles.

Exome sequencing revealed a novel nonsense variant in ALX3 gene underlying frontorhiny

Frontorhiny is one of the two forms of mid-facial malformations characterized by ocular hypertelorism, wide and short nasal ridge, bifid nasal tip, broad columella, widely separated nares, long and wide philtrum and V-shaped hairline. Sometimes these phenotypes are associated with ptosis and midline dermoid cysts. Frontorhiny inherits in an autosomal recessive pattern. Sequence variants in the Aristaless-like homeobox 3 (ALX3) gene underlying frontorhiny have been reported previously. Here, in the present study, we have investigated four patients in a consanguineous family of Pakistani origin segregating frontorhiny in autosomal recessive manner. Genome scan using 250k Nsp1 array followed by exome and Sanger sequence analysis revealed a novel homozygous nonsense variant (c.604C>T, p.Gln202*) in the ALX3 gene resulting in frontorhiny in the family. This is the first mutation in the ALX3 gene, underlying frontorhiny, in Pakistani population.

Deficient Vitamin E Uptake During Development Impairs Neural Tube Closure in Mice Lacking Lipoprotein Receptor SR-BI

SR-BI is the main receptor for high density lipoproteins (HDL) and mediates the bidirectional transport of lipids, such as cholesterol and vitamin E, between these particles and cells. During early development, SR-BI is expressed in extraembryonic tissue, specifically in trophoblast giant cells in the parietal yolk sac. We previously showed that approximately 50% of SR-BI^-/- embryos fail to close the anterior neural tube and develop exencephaly, a perinatal lethal condition. Here, we evaluated the role of SR-BI in embryonic vitamin E uptake during murine neural tube closure. Our results showed that SR-BI^-/- embryos had a very low vitamin E content in comparison to SR-BI^+/+ embryos. Whereas SR-BI^-/- embryos with closed neural tubes (nSR-BI^-/-) had high levels of reactive oxygen species (ROS), intermediate ROS levels between SR-BI^+/+ and nSR-BI^-/- embryos were detected in SR-BI^-/- with NTD (NTD SR-BI^-/-). Reduced expression of Pax3, Alx1 and Alx3 genes was found in NTD SR-BI^-/- embryos. Maternal α-tocopherol dietary supplementation prevented NTD almost completely (from 54% to 2%, p < 0.001) in SR-BI^-/- embryos and normalized ROS and gene expression levels. In sum, our results suggest the involvement of SR-BI in the maternal provision of embryonic vitamin E to the mouse embryo during neural tube closure.

Embryonic defence mechanisms against glucose-dependent oxidative stress require enhanced expression of Alx3 to prevent malformations during diabetic pregnancy

Oxidative stress constitutes a major cause for increased risk of congenital malformations associated to severe hyperglycaemia during pregnancy. Mutations in the gene encoding the transcription factor ALX3 cause congenital craniofacial and neural tube defects. Since oxidative stress and lack of ALX3 favour excessive embryonic apoptosis, we investigated whether ALX3-deficiency further increases the risk of embryonic damage during gestational hyperglycaemia in mice. We found that congenital malformations associated to ALX3-deficiency are enhanced in diabetic pregnancies. Increased expression of genes encoding oxidative stress-scavenging enzymes in embryos from diabetic mothers was blunted in the absence of ALX3, leading to increased oxidative stress. Levels of ALX3 increased in response to glucose, but ALX3 did not activate oxidative stress defence genes directly. Instead, ALX3 stimulated the transcription of Foxo1, a master regulator of oxidative stress-scavenging genes, by binding to a newly identified binding site located in the Foxo1 promoter. Our data identify ALX3 as an important component of the defence mechanisms against the occurrence of developmental malformations during diabetic gestations, stimulating the expression of oxidative stress-scavenging genes in a glucose-dependent manner via Foxo1 activation. Thus, ALX3 deficiency provides a novel molecular mechanism for developmental defects arising from maternal hyperglycaemia.

Developmental mechanisms of stripe patterns in rodents

Mammalian colour patterns are among the most recognizable characteristics found in nature and can have a profound impact on fitness. However, little is known about the mechanisms underlying the formation and subsequent evolution of these patterns. Here we show that, in the African striped mouse (Rhabdomys pumilio), periodic dorsal stripes result from underlying differences in melanocyte maturation, which give rise to spatial variation in hair colour. We identify the transcription factor ALX3 as a regulator of this process. In embryonic dorsal skin, patterned expression of Alx3 precedes pigment stripes and acts to directly repress Mitf, a master regulator of melanocyte differentiation, thereby giving rise to light-coloured hair. Moreover, Alx3 is upregulated in the light stripes of chipmunks, which have independently evolved a similar dorsal pattern. Our results show a previously undescribed mechanism for modulating spatial variation in hair colour and provide insights into how phenotypic novelty evolves.

Exome sequencing revealed a novel splice site variant in the ALX1 gene underlying frontonasal dysplasia

Frontonasal dysplasia (FND) is a heterogeneous group of disorders characterized by hypertelorism, telecanthus, broad nasal root, wide prominent nasal bridge, short and wide nasal ridge, broad columella and smooth philtrum. To date one X-linked and three autosomal recessive forms of FND have been reported in different ethnic groups. We sought to identify the gene responsible for FND in a consanguineous Pakistani family segregating the disorder in autosomal recessive pattern. Genome-wide homozygosity mapping using 250KNsp array revealed five homozygous regions in the selected affected individuals. Exome sequencing found a novel splice acceptor site variant (c.661-1G>C: NM_006982.2) in ALX1. Sanger sequencing confirmed the correct segregation of the pathogenic variant in the whole family. Our study concludes that the splice site variant identified in the ALX1 gene causes mild form of FND.

Skeletal regeneration in the brittle star Amphiura filiformis

Background

Brittle stars regenerate their whole arms post-amputation. Amphiura filiformis can now be used for molecular characterization of arm regeneration due to the availability of transcriptomic data. Previous work showed that specific developmental transcription factors known to take part in echinoderm skeletogenesis are expressed during adult arm regeneration in A. filiformis; however, the process of skeleton formation remained poorly understood. Here, we present the results of an in-depth microscopic analysis of skeletal morphogenesis during regeneration, using calcein staining, EdU labeling and in situ hybridization.

Results

To better compare different samples, we propose a staging system for the early A. filiformis arm regeneration stages based on morphological landmarks identifiable in living animals and supported by histological analysis. We show that the calcified spicules forming the endoskeleton first appear very early during regeneration in the dermal layer of regenerates. These spicules then mature into complex skeletal elements of the differentiated arm during late regeneration. The mesenchymal cells in the dermal area express the skeletal marker genes Afi-c-lectin, Afi-p58b and Afi-p19; however, EdU labeling shows that these dermal cells do not proliferate.

Conclusions

A. filiformis arms regenerate through a consistent set of developmental stages using a distalization-intercalation mode, despite variability in regeneration rate. Skeletal elements form in a mesenchymal cell layer that does not proliferate and thus must be supplied from a different source. Our work provides the basis for future cellular and molecular studies of skeleton regeneration in brittle stars.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-016-0149-x) contains supplementary material, which is available to authorized users.

A new frontonasal dysplasia syndrome associated with deletion of the SIX2 gene

The frontonasal dysplasias are a group of craniofacial phenotypes characterized by hypertelorism, nasal clefting, frontal bossing, and abnormal hairline. These conditions are caused by recessive mutations in members of the aristaless gene family, resulting in abnormal cranial neural crest migration and differentiation. We report a family with a dominantly inherited craniofacial phenotype comprised of frontal bossing with high hairline, ptosis, hypertelorism, broad nasal tip, large anterior fontanelle, cranial base anomalies, and sagittal synostosis. Chromosomal microarray identified a heterozygous 108.3 kilobase deletion of chromosome 2p21 segregating with phenotype and limited to the sine oculis homeobox gene SIX2 and surrounding noncoding DNA. Similar to the human SIX2 deletion phenotype, one mouse model of frontonasal dysplasia, brachyrrhine, exhibits dominant inheritance and impaired cranial base chondrogenesis associated with reduced Six2 expression. We report the first human autosomal dominant frontonasal dysplasia syndrome associated with SIX2 deletion and with phenotypic similarities to murine models of Six2 Loss-of-function.

Mosaic small supernumerary marker chromosome 1 at amniocentesis: prenatal diagnosis, molecular genetic analysis and literature review

We present prenatal diagnosis and molecular cytogenetic analysis of mosaic small supernumerary marker chromosome 1 [sSMC(1)]. We review the literature of sSMC(1) at amniocentesis and chromosome 1p21.1-p12 duplication syndrome. We discuss the genotype-phenotype correlation of the involved genes of ALX3, RBM15, NTNG1, SLC25A24, GPSM2, TBX15 and NOTCH2 in this case.

BMP-2 Induced Expression of Alx3 That Is a Positive Regulator of Osteoblast Differentiation

Bone morphogenetic proteins (BMPs) regulate many aspects of skeletal development, including osteoblast and chondrocyte differentiation, cartilage and bone formation, and cranial and limb development. Among them, BMP-2, one of the most potent osteogenic signaling molecules, stimulates osteoblast differentiation, while it inhibits myogenic differentiation in C2C12 cells. To evaluate genes involved in BMP-2-induced osteoblast differentiation, we performed cDNA microarray analyses to compare BMP-2-treated and -untreated C2C12 cells. We focused on Alx3 (aristaless-like homeobox 3) which was clearly induced during osteoblast differentiation. Alx3, a homeobox gene related to the Drosophilaaristaless gene, has been linked to developmental functions in craniofacial structures and limb development. However, little is known about its direct relationship with bone formation. In the present study, we focused on the mechanisms of Alx3 gene expression and function during osteoblast differentiation induced by BMP-2. In C2C12 cells, BMP-2 induced increase of Alx3 gene expression in both time- and dose-dependent manners through the BMP receptors-mediated SMAD signaling pathway. In addition, silencing of Alx3 by siRNA inhibited osteoblast differentiation induced by BMP-2, as showed by the expressions of alkaline phosphatase (Alp), Osteocalcin, and Osterix, while over-expression of Alx3 enhanced osteoblast differentiation induced by BMP-2. These results indicate that Alx3 expression is enhanced by BMP-2 via the BMP receptors mediated-Smad signaling and that Alx3 is a positive regulator of osteoblast differentiation induced by BMP-2.

Differential configurations involving binding of USF transcription factors and Twist1 regulate Alx3 promoter activity in mesenchymal and pancreatic cells

During embryonic development, the aristaless-type homeodomain protein Alx3 is expressed in the forehead mesenchyme and contributes to the regulation of craniofacial development. In the adult, Alx3 is expressed in pancreatic islets where it participates in the control of glucose homoeostasis. In the present study, we investigated the transcriptional regulation of Alx3 gene expression in these two cell types. We found that the Alx3 promoter contains two E-box regulatory elements, named EB1 and EB2, that provide binding sites for the basic helix-loop-helix transcription factors Twist1, E47, USF (upstream stimulatory factor) 1 and USF2. In primary mouse embryonic mesenchymal cells isolated from the forehead, EB2 is bound by Twist1, whereas EB1 is bound by USF1 and USF2. Integrity of both EB1 and EB2 is required for Twist1-mediated transactivation of the Alx3 promoter, even though Twist1 does not bind to EB1, indicating that binding of USF1 and USF2 to this element is required for Twist1-dependent Alx3 promoter activity. In contrast, in pancreatic islet insulin-producing cells, the integrity of EB2 is not required for proximal promoter activity. The results of the present study indicate that USF1 and USF2 are important regulatory factors for Alx3 gene expression in different cell types, whereas Twist1 contributes to transcriptional transactivation in mesenchymal, but not in pancreatic, cells.

Defective neural crest migration revealed by a Zebrafish model of Alx1-related frontonasal dysplasia

Frontonasal dysplasia (FND) refers to a class of midline facial malformations caused by abnormal development of the facial primordia. The term encompasses a spectrum of severities but characteristic features include combinations of ocular hypertelorism, malformations of the nose and forehead and clefting of the facial midline. Several recent studies have drawn attention to the importance of Alx homeobox transcription factors during craniofacial development. Most notably, loss of Alx1 has devastating consequences resulting in severe orofacial clefting and extreme microphthalmia. In contrast, mutations of Alx3 or Alx4 cause milder forms of FND. Whilst Alx1, Alx3 and Alx4 are all known to be expressed in the facial mesenchyme of vertebrate embryos, little is known about the function of these proteins during development. Here, we report the establishment of a zebrafish model of Alx-related FND. Morpholino knock-down of zebrafish alx1 expression causes a profound craniofacial phenotype including loss of the facial cartilages and defective ocular development. We demonstrate for the first time that Alx1 plays a crucial role in regulating the migration of cranial neural crest (CNC) cells into the frontonasal primordia. Abnormal neural crest migration is coincident with aberrant expression of foxd3 and sox10, two genes previously suggested to play key roles during neural crest development, including migration, differentiation and the maintenance of progenitor cells. This novel function is specific to Alx1, and likely explains the marked clinical severity of Alx1 mutation within the spectrum of Alx-related FND.

Evolution of the Alx homeobox gene family: parallel retention and independent loss of the vertebrate Alx3 gene

SUMMARY

The Alx gene family is implicated in craniofacial development and comprises two to four homeobox genes in each vertebrate genome analyzed. Using phylogenetics and comparative genomics, we show that the common ancestor of jawed vertebrates had three Alx genes descendent from the two-round genome duplications (Alx1, Alx3, Alx4), compared with a single amphioxus gene. Later in evolution one of the paralogues, Alx3, was lost independently from at least three different vertebrate lineages, whereas Alx1 and Alx4 were consistently retained. Comparison of spatial gene expression patterns reveals that the three mouse genes have equivalent craniofacial expression to the two chick and frog genes, suggesting that redundancy compensated for gene loss. We suggest that multiple independent loss of one Alx gene was predisposed by extensive and persistent overlap in gene expression between Alx paralogues. Even so, it is unclear whether it was coincidence or evolutionary bias that resulted in the same Alx gene being lost on each occasion, rather than different members of the gene family.

A new mechanistic scenario for the origin and evolution of vertebrate cartilage

The appearance of cellular cartilage was a defining event in vertebrate evolution because it made possible the physical expansion of the vertebrate "new head". Despite its central role in vertebrate evolution, the origin of cellular cartilage has been difficult to understand. This is largely due to a lack of informative evolutionary intermediates linking vertebrate cellular cartilage to the acellular cartilage of invertebrate chordates. The basal jawless vertebrate, lamprey, has long been considered key to understanding the evolution of vertebrate cartilage. However, histological analyses of the lamprey head skeleton suggest it is composed of modern cellular cartilage and a putatively unrelated connective tissue called mucocartilage, with no obvious transitional tissue. Here we take a molecular approach to better understand the evolutionary relationships between lamprey cellular cartilage, gnathostome cellular cartilage, and lamprey mucocartilage. We find that despite overt histological similarity, lamprey and gnathostome cellular cartilage utilize divergent gene regulatory networks (GRNs). While the gnathostome cellular cartilage GRN broadly incorporates Runx, Barx, and Alx transcription factors, lamprey cellular cartilage does not express Runx or Barx, and only deploys Alx genes in certain regions. Furthermore, we find that lamprey mucocartilage, despite its distinctive mesenchymal morphology, deploys every component of the gnathostome cartilage GRN, albeit in different domains. Based on these findings, and previous work, we propose a stepwise model for the evolution of vertebrate cellular cartilage in which the appearance of a generic neural crest-derived skeletal tissue was followed by a phase of skeletal tissue diversification in early agnathans. In the gnathostome lineage, a single type of rigid cellular cartilage became dominant, replacing other skeletal tissues and evolving via gene cooption to become the definitive cellular cartilage of modern jawed vertebrates.

Alx3-deficient mice exhibit decreased insulin in beta cells, altered glucose homeostasis and increased apoptosis in pancreatic islets

AIMS/HYPOTHESIS

Homeodomain transcription factors play an important role in the regulation of pancreatic islet function. In previous studies we determined that aristaless-like homeobox 3 (ALX3) is produced in islet cells, binds to the promoter of the insulin gene and regulates its expression. The purpose of the present study was to investigate the functional role of ALX3 in pancreatic islets and its possible involvement in the regulation of glucose homeostasis in vivo.

METHODS

Alx3-knockout mice were used. Glucose and insulin tolerance tests were carried out, and serum insulin concentrations were determined. Isolated islets were used to test insulin secretion and gene expression. The pancreatic islets were also studied using both confocal and conventional microscopy.

RESULTS

ALX3 deficiency resulted in increased blood glucose levels and impaired glucose tolerance in the presence of normal serum insulin concentrations. Insulin, glucagon and glucokinase expression were reduced in Alx3-null pancreatic islets. Reduced insulin content was reflected by decreased insulin secretion from isolated islets. Alx3-deficient islets also showed increased apoptosis, and morphometric analyses indicated that they were, on average, of smaller size than islets from control mice. ALX3 deficiency resulted in reduced beta cell mass. Finally, mature Alx3-null mice developed age-dependent insulin resistance due to impaired peripheral insulin receptor signalling.

CONCLUSIONS/INTERPRETATION

ALX3 participates in the regulation of the expression of essential genes for the function of pancreatic islets, and its deficiency alters the regulation of glucose homeostasis in vivo. We suggest that ALX3 constitutes a potential candidate to consider in the aetiopathogenesis of diabetes mellitus.

Alx3-deficient mice exhibit folic acid-resistant craniofacial midline and neural tube closure defects

Neural tube closure defects are among the most frequent congenital malformations in humans. Supplemental maternal intake of folic acid before and during pregnancy reduces their incidence significantly, but the mechanism underlying this preventive effect is unknown. As a number of genes that cause neural tube closure defects encode transcriptional regulators in mice, one possibility is that folic acid could induce the expression of transcription factors to compensate for the primary genetic defect. We report that folic acid is required in mouse embryos for the specific expression of the homeodomain gene Alx3 in the head mesenchyme, an important tissue for cranial neural tube closure. Alx3-deficient mice exhibit increased failure of cranial neural tube closure and increased cell death in the craniofacial region, two effects that are also observed in wild type embryos developing in the absence of folic acid. Folic acid cannot prevent these defects in Alx3-deficient embryos, indicating that one mechanism of folic acid action is through induced expression of Alx3. Thus, Alx3 emerges as a candidate gene for human neural tube defects and reveals the existence of induced transcription factor gene expression as a previously unknown mechanism by which folic acid prevents neural tube closure defects.

aGEM: an integrative system for analyzing spatial-temporal gene-expression information

Motivation: The work presented here describes the ‘anatomical Gene-Expression Mapping (aGEM)’ Platform, a development conceived to integrate phenotypic information with the spatial and temporal distributions of genes expressed in the mouse. The aGEM Platform has been built by extending the Distributed Annotation System (DAS) protocol, which was originally designed to share genome annotations over the WWW. DAS is a client-server system in which a single client integrates information from multiple distributed servers.

Results: The aGEM Platform provides information to answer three main questions. (i) Which genes are expressed in a given mouse anatomical component? (ii) In which mouse anatomical structures are a given gene or set of genes expressed? And (iii) is there any correlation among these findings? Currently, this Platform includes several well-known mouse resources (EMAGE, GXD and GENSAT), hosting gene-expression data mostly obtained from in situ techniques together with a broad set of image-derived annotations.

Availability: The Platform is optimized for Firefox 3.0 and it is accessed through a friendly and intuitive display: http://agem.cnb.csic.es

Contact:natalia@cnb.csic.es

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Frontorhiny, a distinctive presentation of frontonasal dysplasia caused by recessive mutations in the ALX3 homeobox gene

We describe a recessively inherited frontonasal malformation characterized by a distinctive facial appearance, with hypertelorism, wide nasal bridge, short nasal ridge, bifid nasal tip, broad columella, widely separated slit-like nares, long philtrum with prominent bilateral swellings, and midline notch in the upper lip and alveolus. Additional recurrent features present in a minority of individuals have been upper eyelid ptosis and midline dermoid cysts of craniofacial structures. Assuming recessive inheritance, we mapped the locus in three families to chromosome 1 and identified mutations in ALX3, which is located at band 1p13.3 and encodes the aristaless-related ALX homeobox 3 transcription factor. In total, we identified seven different homozygous pathogenic mutations in seven families. These mutations comprise missense substitutions at critical positions within the conserved homeodomain as well as nonsense, frameshift, and splice-site mutations, all predicting severe or complete loss of function. Our findings contrast with previous studies of the orthologous murine gene, which showed no phenotype in Alx3(-/-) homozygotes, apparently as a result of functional redundancy with the paralogous Alx4 gene. We conclude that ALX3 is essential for normal facial development in humans and that deficiency causes a clinically recognizable phenotype, which we term frontorhiny.

Genetic determinants of facial clefting: analysis of 357 candidate genes using two national cleft studies from Scandinavia

Background

Facial clefts are common birth defects with a strong genetic component. To identify fetal genetic risk factors for clefting, 1536 SNPs in 357 candidate genes were genotyped in two population-based samples from Scandinavia (Norway: 562 case-parent and 592 control-parent triads; Denmark: 235 case-parent triads).

Methodology/Principal Findings

We used two complementary statistical methods, TRIMM and HAPLIN, to look for associations across these two national samples. TRIMM tests for association in each gene by using multi-SNP genotypes from case-parent triads directly without the need to infer haplotypes. HAPLIN on the other hand estimates the full haplotype distribution over a set of SNPs and estimates relative risks associated with each haplotype. For isolated cleft lip with or without cleft palate (I-CL/P), TRIMM and HAPLIN both identified significant associations with IRF6 and ADH1C in both populations, but only HAPLIN found an association with FGF12. For isolated cleft palate (I-CP), TRIMM found associations with ALX3, MKX, and PDGFC in both populations, but only the association with PDGFC was identified by HAPLIN. In addition, HAPLIN identified an association with ETV5 that was not detected by TRIMM.

Conclusion/Significance

Strong associations with seven genes were replicated in the Scandinavian samples and our approach effectively replicated the strongest previously known association in clefting—with IRF6. Based on two national cleft cohorts of similar ancestry, two robust statistical methods and a large panel of SNPs in the most promising cleft candidate genes to date, this study identified a previously unknown association with clefting for ADH1C and provides additional candidates and analytic approaches to advance the field.

Insights from amphioxus into the evolution of vertebrate cartilage

Central to the story of vertebrate evolution is the origin of the vertebrate head, a problem difficult to approach using paleontology and comparative morphology due to a lack of unambiguous intermediate forms. Embryologically, much of the vertebrate head is derived from two ectodermal tissues, the neural crest and cranial placodes. Recent work in protochordates suggests the first chordates possessed migratory neural tube cells with some features of neural crest cells. However, it is unclear how and when these cells acquired the ability to form cellular cartilage, a cell type unique to vertebrates. It has been variously proposed that the neural crest acquired chondrogenic ability by recruiting proto-chondrogenic gene programs deployed in the neural tube, pharynx, and notochord. To test these hypotheses we examined the expression of 11 amphioxus orthologs of genes involved in neural crest chondrogenesis. Consistent with cellular cartilage as a vertebrate novelty, we find that no single amphioxus tissue co-expresses all or most of these genes. However, most are variously co-expressed in mesodermal derivatives. Our results suggest that neural crest-derived cartilage evolved by serial cooption of genes which functioned primitively in mesoderm.

Function and regulation of Alx4 in limb development: complex genetic interactions with Gli3 and Shh

The role of the aristaless-related homeobox gene Alx4 in antero-posterior (AP-) patterning of the developing vertebrate limb has remained somewhat elusive. Polydactyly of Alx4 mutant mice is known to be accompanied by ectopic anterior expression of genes like Shh, Fgf4 and 5'Hoxd. We reported previously that polydactyly in Alx4 mutant mice requires SHH signaling, but we now show that in early Alx4-/- limb buds the anterior ectopic expression of Fgf4 and Hoxd13, and therefore disruption of AP-patterning, occurs independently of SHH signaling. To better understand how Alx4 functions in the pathways that regulate AP-patterning, we also studied genomic regulatory sequences that are capable of directing expression of a reporter gene in a pattern corresponding to endogenous Alx4 expression in anterior limb bud mesenchyme. We observed, as expected for authentic Alx4 expression, expansion of reporter construct expression in a Shh-/- background. Total lack of reporter expression in a Gli3-/- background confirms the existence of Gli3-dependent and -independent Alx4 expression in the limb bud. Apparently, these two modules of Alx4 expression are linked to dissimilar functions.

The T-box transcription factor Tbx15 is required for skeletal development

During early limb development several signaling centers coordinate limb bud outgrowth as well as patterning. Members of the T-box gene family of transcriptional regulators are crucial players in these processes by activating and interpreting these signaling pathways. Here, we show that Tbx15, a member of this gene family, is expressed during limb development, first in the mesenchyme of the early limb bud, then during early endochondral bone development in prehypertrophic chondrocytes of cartilaginous templates. Expression is also found in mesenchymal precursor cells and prehypertrophic chondrocytes, respectively, during development of skeletal elements of the vertebral column and the head. Analysis of Tbx15 null mutant mice indicates a role of Tbx15 in the development of skeletal elements throughout the body. Mutants display a general reduction of bone size and changes of bone shape. In the forelimb skeleton, the scapula lacks the central region of the blade. Cartilaginous templates are already reduced in size and show a transient delay in ossification in mutant embryos. Mutants show a significantly reduced proliferation of prehypertrophic chondrocytes as well as of mesenchymal precursor cells. These data suggest that Tbx15 plays an important role in the development of the skeleton of the limb, vertebral column and head by controlling the number of mesenchymal precursor cells and chondrocytes.

Genetics of shoulder girdle formation: roles of Tbx15 and aristaless-like genes

The diverse cellular contributions to the skeletal elements of the vertebrate shoulder and pelvic girdles during embryonic development complicate the study of their patterning. Research in avian embryos has recently clarified part of the embryological basis of shoulder formation. Although dermomyotomal cells provide the progenitors of the scapular blade, local signals appear to have an essential guiding role in this process. These signals differ from those that are known to pattern the more distal appendicular skeleton. We have studied the impact of Tbx15, Gli3, Alx4 and related genes on formation of the skeletal elements of the mouse shoulder and pelvic girdles. We observed severe reduction of the scapula in double and triple mutants of these genes. Analyses of a range of complex genotypes revealed aspects of their genetic relationship, as well as functions that had been previously masked due to functional redundancy. Tbx15 and Gli3 appear to have synergistic functions in formation of the scapular blade. Scapular truncation in triple mutants of Tbx15, Alx4 and Cart1 indicates essential functions for Alx4 and Cart1 in the anterior part of the scapula, as opposed to Gli3 function being linked to the posterior part. Especially in Alx4/Cart1 mutants, the expression of markers such as Pax1, Pax3 and Scleraxis is altered prior to stages when anatomical aberrations are visible in the shoulder region. This suggests a disorganization of the proximal limb bud and adjacent flank mesoderm, and is likely to reflect the disruption of a mechanism providing positional cues to guide progenitor cells to their destination in the pectoral girdle.

Opposing roles of two isoforms of the Prx1 homeobox gene in chondrogenesis

The Prx1 homeobox gene is critical for cartilage and bone development as suggested by previous expression studies and demonstrated by gene targeting. However, neither approach assessed the individual roles of the two isoforms Prx1a and Prx1b. In this study, Western blot analysis demonstrates that, in the early stages of chondrogenesis, during mesenchymal condensation, only Prx1a is expressed. Higher level Prx1b expression is concomitant with the formation of a defined perichondrium. Prx1a overexpression in limb micro mass cultures results in an increase in the number of prechondrogenic condensations and cartilage nodules, whereas overexpression of Prx1b results in a decrease. Prx1a increases the percentage of proliferating cells in micro mass cultures and decreases apoptosis. The Prx1b isoform does not alter proliferation, but it does increase apoptosis, which is opposite of Prx1a. These results suggest that the Prx1a:Prx1b ratio and the alternative splicing mechanism that generates these two isoforms are critical in controlling chondrogenesis.

Requirement for Mab21l2 during development of murine retina and ventral body wall

The mab-21 gene was first identified because of its requirement for ray identity specification in Caenorhabditis elegans. It is now known to constitute a family of genes that are highly conserved from vertebrates to invertebrates, and two homologues Mab21l1 and Mab21l2 have been identified in many species. Here we describe the generation of Mab21l2-deficient mice, which have defects in eye and body wall formation. The mutant mouse eye has a rudimentary retina, as a result of insufficient invagination of the optic vesicle due to deficient proliferation, causing the absence of lens. The defects in optic vesicle development correlate with reduced expression of Chx10, which is also required for retina development; Rx, Lhx2, and Pax6 expression is not significantly affected. We conclude that Mab21l2 expression is essential for optic vesicle growth and formation of the optic cup, its absence causing reduced expression of Chx10. Mutant mice also display abnormal extrusion of abdominal organs, defects in ventral body wall formation, resulting in death in utero at mid-gestational stage. Our results reveal that Mab21l2 plays crucial roles in retina and in ventral body wall formation.

Tibial aplasia, lower extremity mirror image polydactyly, brachyphalangy, craniofacial dysmorphism and genital hypoplasia: further delineation and mutational analysis

Congenital anomalies involving tibial aplasia are rare. Recently, four children with an unusual combination of limb anomalies, facial dysmorphism and genital hypoplasia have been reported. All affected children reported were male. One case noted father to son transmission, implying autosomal dominant inheritance. We report the first female patient with this syndrome. The patient had tibial aplasia, mirror image preaxial polydactyly involving her feet, brachyphalangy, genital hypoplasia as well as facial dysmorphism including telecanthus, blepharophimosis, a flat nasal bridge with a small nose and a small mouth. Consistent with reports in males of a micropenis and hypoplastic scrotum, our patient had absent labia minora and a very small clitoris. Her father had very minor anomalies suggestive of somatic mosaicism or marked variability. Mouse models affecting limb development are powerful tools in the study of human syndromes. The clinical phenotype of patients with this syndrome is reminiscent of some luxoid mouse mutants suggesting Alx4 and related members of the paired homeodomain class as candidate genes. ALX4 haploinsufficiency in humans causes parietal foramina, which one patient with this syndrome was reported to have. Sequencing of coding exons of ALX4 and its related homologue, ALX3, in the proband failed to reveal coding sequence alterations. Our father/daughter pair is the second family reported, supporting a dominant mode of inheritance. Moreover, the very mild phenotype in the father suggests the need for very careful attention to parental examination in such cases.

Characterization and chondrocyte differentiation stage-specific expression of KRAB zinc-finger protein gene ZNF470

As part of a study to identify novel transcriptional regulators of chondrogenesis-related gene expression, we have cloned and characterized cDNA for zinc-finger protein 470 (ZNF470), the human ortholog of which encodes a 717 amino acid residue protein containing 17 Cys(2)His(2) zinc-finger domains, as well as KRAB-A and KRAB-B motifs. The cDNA library used to isolate the initial ZNF470 clone was prepared from human bone marrow-derived mesenchymal progenitor cells at an intermediate stage of chondrogenic differentiation. We have determined the intron-exon structure of the human ZNF470 gene, which has been mapped to a zinc-finger cluster in a known imprinted region of human chromosome 19q13.4. ZNF470 is expressed at high levels in human testis and is expressed at low or undetectible levels in other adult tissues. Human ZNF470 expressed in mammalian cells as an EGFP fusion protein localizes predominantly to the nucleus, consistent with a role in transcriptional regulation. ZNF470, analyzed by quantitative real time PCR, was transiently expressed before the maximal expression of COL2A1 during chondrogenic differentiation in vitro. We have also characterized the bovine ortholog of human ZNF470, which encodes a 508 amino acid residue protein having 10 zinc-finger domains. A bovine ZNF470 cDNA clone was used to examine expression of ZNF470 in bovine articular chondrocytes treated with retinoic acid to stimulate dedifferentiation. Bovine ZNF470 expression was undetectable in freshly isolated bovine articular chondrocytes, but was dramatically upregulated in dedifferentiated retinoic acid-treated chondrocytes. These results, in two model systems, suggest a possible role for ZNF470 in the regulation of chondrogenesis-specific gene expression.

The homeoprotein Alx3 contains discrete functional domains and exhibits cell-specific and selective monomeric binding and transactivation

Alx3 is a paired class aristaless-like homeoprotein expressed during embryonic development. Transcriptional transactivation by aristaless-like proteins has been associated with cooperative dimerization upon binding to artificially generated DNA consensus sequences known as P3 sites, but natural target sites in genes regulated by Alx3 are unknown. We report the cloning of a cDNA encoding the rat homolog of Alx3, and we characterize the protein domains that are important for transactivation, dimerization, and binding to DNA. Two proline-rich domains located amino-terminal to the homeodomain (Pro1 and Pro2) are necessary for Alx3-dependent transactivation, whereas another one (Pro3) located in the carboxyl terminus is dispensable but contributes to enhance the magnitude of the response. We confirmed that transcriptional activity of Alx3 from a P3 site correlates with cooperative dimerization upon binding to DNA. However, Alx3 was found to bind selectively to non-P3-related TAAT-containing sites present in the promoter of the somatostatin gene in a specific manner that depends on the nuclear protein environment. Cell-specific transactivation elicited by Alx3 from these sites could not be predicted from in vitro DNA-binding experiments by using recombinant Alx3. In addition, transactivation did not depend on cooperative dimerization upon binding to cognate somatostatin DNA sites. Our data indicate that the paradigm according to which Alx3 must act homodimerically via cooperative binding to P3-like sites is insufficient to explain the mechanism of action of this homeoprotein to regulate transcription of natural target genes. Instead, Alx3 undergoes restrictive or permissive interactions with nuclear proteins that determine its binding to and transactivation from TAAT target sites selected in a cell-specific manner.

Dorsoventral patterning of the mouse coat by Tbx15

Many members of the animal kingdom display coat or skin color differences along their dorsoventral axis. To determine the mechanisms that control regional differences in pigmentation, we have studied how a classical mouse mutation, droopy ear (de^H), affects dorsoventral skin characteristics, especially those under control of the Agouti gene. Mice carrying the Agouti allele black-and-tan (a^t) normally have a sharp boundary between dorsal black hair and yellow ventral hair; the de^H mutation raises the pigmentation boundary, producing an apparent dorsal-to-ventral transformation. We identify a 216 kb deletion in de^H that removes all but the first exon of the Tbx15 gene, whose embryonic expression in developing mesenchyme correlates with pigmentary and skeletal malformations observed in de^H/de^H animals. Construction of a targeted allele of Tbx15 confirmed that the de^H phenotype was caused by Tbx15 loss of function. Early embryonic expression of Tbx15 in dorsal mesenchyme is complementary to Agouti expression in ventral mesenchyme; in the absence of Tbx15, expression of Agouti in both embryos and postnatal animals is displaced dorsally. Transplantation experiments demonstrate that positional identity of the skin with regard to dorsoventral pigmentation differences is acquired by E12.5, which is shortly after early embryonic expression of Tbx15. Fate-mapping studies show that the dorsoventral pigmentation boundary is not in register with a previously identified dermal cell lineage boundary, but rather with the limb dorsoventral boundary. Embryonic expression of Tbx15 in dorsolateral mesenchyme provides an instructional cue required to establish the future positional identity of dorsal dermis. These findings represent a novel role for T-box gene action in embryonic development, identify a previously unappreciated aspect of dorsoventral patterning that is widely represented in furred mammals, and provide insight into the mechanisms that underlie region-specific differences in body morphology.

Greg Barsh and colleagues show that a member of the well-known family of T-box genes helps to form an important pigmentation boundary in mice

The clustered olfactory receptor gene family 262: genomic organization, promotor elements, and interacting transcription factors

For six mouse olfactory receptor genes from family 262 which are expressed in clustered populations of olfactory sensory neurons, the genomic as well as cDNA structures were deciphered. All genes contained several exons which in some cases were alternatively spliced. Immediately upstream of the transcription start sites, sequence motif blocks were identified that are highly conserved among olfactory receptor (OR) genes which are expressed in clustered neuronal populations. By means of electrophoretic mobility shift assays, it was demonstrated that segments of the motif block region interact with proteins extracted from nuclear fractions of the olfactory epithelium. Yeast one-hybrid screenings of an olfactory cDNA library led to the identification of a set of transcription factors that specifically bind to particular elements of the motif block region. The identified factors can be categorized into two types: One group is known to be involved in transcriptional initiation, and the second group represents factors involved in pattern formations. The identified components may contribute to govern the precise topographic expression pattern of olfactory receptor genes.

Alx1, a member of the Cart1/Alx3/Alx4 subfamily of Paired-class homeodomain proteins, is an essential component of the gene network controlling skeletogenic fate specification in the sea urchin embryo

In the sea urchin embryo, the large micromeres and their progeny function as a critical signaling center and execute a complex morphogenetic program. We have identified a new and essential component of the gene network that controls large micromere specification, the homeodomain protein Alx1. Alx1 is expressed exclusively by cells of the large micromere lineage beginning in the first interphase after the large micromeres are born. Morpholino studies demonstrate that Alx1 is essential at an early stage of specification and controls downstream genes required for epithelial-mesenchymal transition and biomineralization. Expression of Alx1 is cell autonomous and regulated maternally through beta-catenin and its downstream effector, Pmar1. Alx1 expression can be activated in other cell lineages at much later stages of development, however, through a regulative pathway of skeletogenesis that is responsive to cell signaling. The Alx1 protein is highly conserved among euechinoid sea urchins and is closely related to the Cart1/Alx3/Alx4 family of vertebrate homeodomain proteins. In vertebrates, these proteins regulate the formation of skeletal elements of the limbs, face and neck. Our findings suggest that the ancestral deuterostome had a population of biomineral-forming mesenchyme cells that expressed an Alx1-like protein.

Regulation of Dlx3 gene expression in visceral arches by evolutionarily conserved enhancer elements

The mammalian Distal-less (Dlx) clusters (Dlx1-2, Dlx5-6, and Dlx3-7) have a nested expression pattern in developing visceral (branchial) arches. Genetic regulatory mechanisms controlling Dlx spatial expression within the visceral arches have not yet been defined. Here we show that an enhancer in the Dlx3-7 cluster can regulate the visceral arch specific expression pattern of the Dlx3 gene. We have used a 79-kb transgene construct containing the entire Dlx3-7 bigene cluster with a LacZ reporter inserted in frame in the first exon of the Dlx3 gene. Visceral arch expression is absent when a 4-kb element located within the Dlx3-7 intergenic region is deleted. A 245-bp element (I37-2) whose DNA sequence is highly conserved between human and mouse located within the 4kb-deleted region can drive visceral arch expression when fused to a hsp68-lacZ reporter transgene construct. Reporter expression is detected in 9.5 and 10.5 days postcoitum transgenic embryos in a manner consistent with the endogenous Dlx3 expression pattern in the mesenchyme of the first and second visceral arches. Thus the I37-2 element is both necessary and sufficient for Dlx3 expression. The I37-2 element contains several putative binding sites for several transcription factors including Dlx and other homeodomain proteins within the evolutionarily conserved region. Significantly, the I37-2 element shows a sequence-match including a Dlx binding site to a cis-element in the Dlx5-6 intermediate region designated mI56i [Zerucha, T., Stuhmer, T., Hatch, G., Park, B. K., Long, Q., Yu, G., Gambarotta, A., Schultz, J. R., Rubenstein, J. L. & Ekker, M. (2000) J. Neurosci. 20, 709-721], despite distant phylogenetic relationship between these clusters. Our results provide evidence for a concerted role for DLX auto- and cross-regulation in the establishment of a nested expression pattern for Dlx3-7 and Dlx5-6 clusters within the visceral arches.

Targeted deletion of a branchial arch-specific enhancer reveals a role of dHAND in craniofacial development

The basic helix-loop-helix transcription factor dHAND is expressed in the mesenchyme of branchial arches and the developing heart. Mice homozygous for a dHAND (Hand2) null mutation die early in embryogenesis from cardiac abnormalities, precluding analysis of the potential role of dHAND in branchial arch development. Two independent enhancers control expression of dHAND in the heart and branchial arches. Endothelin-1 (ET-1) signaling regulates the branchial arch enhancer and is required for dHAND expression in the branchial arches. To determine the potential role of dHAND in branchial arch development and to assess the role of the ET-1-dependent enhancer in dHAND regulation in vivo, we deleted this enhancer by homologous recombination. Mice lacking the dHAND branchial arch enhancer died perinatally and exhibited a spectrum of craniofacial defects that included cleft palate, mandibular hypoplasia and cartilage malformations. Expression of dHAND was abolished in the ventolateral regions of the first and second branchial arches in these mutant mice, but expression was retained in a ventral domain where the related transcription factor eHAND is expressed. We conclude that dHAND plays an essential role in patterning and development of skeletal elements derived from the first and second branchial arches and that there are heterogeneous populations of cells in the branchial arches that rely on different cis-regulatory elements for activation of dHAND transcription.

The OAR/aristaless domain of the homeodomain protein Cart1 has an attenuating role in vivo

Aristaless-related genes encode a structurally defined group of homeoproteins that share a C-terminal stretch of amino acids known as the OAR- or aristaless domain. Many aristaless-related genes have been linked to major developmental functions, but the function of the aristaless domain itself is poorly understood. Expression and functional studies have shown that a subgroup of these genes, including Prx1, Prx2, Alx3, Alx4 and Cart1, is essential for correct morphogenesis of the limbs and cranium. We now demonstrate the function of the aristaless domain in vivo by ectopically expressing normal and mutated forms of Cart1 and Alx3. Ectopic expression of Cart1 in transgenic mice does not disturb development, whereas expression of a Cart1 form from which the aristaless domain has been deleted results in severe cranial and vertebral malformations. The Alx3 protein contains a divergent aristaless domain that appears not to be functional, as ectopic expression of Alx3 results in an altered phenotype irrespective of the presence of this aristaless domain. Linking the Cart1 aristaless domain to Alx3 extinguishes teratogenicity. We show that, at the molecular level, the most important consequence of deleting the aristaless domain is increased DNA binding to its palindromic target sequence. This demonstrates that the aristaless domain functions as an intra-molecular switch to contain the activity of the transcription factor that it is part of.

Genetic modifiers of otocephalic phenotypes inOtx2heterozygous mutant mice

Mice heterozygous for the Otx2 mutation display a craniofacial malformation, known as otocephaly or agnathia-holoprosencephaly complex. The severity of the phenotype is dependent on the genetic background of a C57BL/6 (B6) strain; most of the offspring of Otx2 knock-out chimeras, which are equivalent to the F1 of CBA and B6 strains, backcrossed with B6 females display reduction or loss of mandible, whereas those backcrossed with CBA females do not show noticeable phenotype at birth. The availability of phenotypically disparate strains renders identification of Otx2 modifier loci possible. In this study, a backcross of chimera with B6 was generated and genome-wide scans were conducted with polymorphic markers for non-mendelian distribution of alleles in Otx2 heterozygous mutant mice displaying abnormalities in the lower jaw. We identified one significant locus, Otmf18, between D18Mit68 and D18Mit120 on chromosomes 18, linked to the mandibular phenotype (LOD score 3.33). A similar replication experiment using a second backcross (N3) mouse demonstrated the presence of another significant locus, Otmf2 between D2Mit164 and D2Mit282 on chromosome 2, linked to the mandibular phenotype (LOD score 3.93). These two modifiers account for the distribution of the craniofacial malformations by the genetic effect between B6 and CBA strains. Moreover, Otmf2 contain a candidate gene for several diseases in mice and humans. These genetic studies involving an otocephalic mouse model appear to provide new insights into mechanistic pathways of craniofacial development. Furthermore, these experiments offer a powerful approach with respect to identification and characterization of candidate genes that may contribute to human agnathia-holoprosencephaly complex diseases.

Twist plays an essential role in FGF and SHH signal transduction during mouse limb development

Loss of Twist gene function arrests the growth of the limb bud shortly after its formation. In the Twist(-/-) forelimb bud, Fgf10 expression is reduced, Fgf4 is not expressed, and the domain of Fgf8 and Fgfr2 expression is altered. This is accompanied by disruption of the expression of genes (Shh, Gli1, Gli2, Gli3, and Ptch) associated with SHH signalling in the limb bud mesenchyme, the down-regulation of Bmp4 in the apical ectoderm, the absence of Alx3, Alx4, Pax1, and Pax3 activity in the mesenchyme, and a reduced potency of the limb bud tissues to differentiate into osteogenic and myogenic tissues. Development of the hindlimb buds in Twist(-/-) embryos is also retarded. The overall activity of genes involved in SHH signalling is reduced.Fgf4 and Fgf8 expression is lost or reduced in the apical ectoderm, but other genes (Fgf10, Fgfr2) involved with FGF signalling are expressed in normal patterns. Twist(+/-);Gli3(+/XtJ) mice display more severe polydactyly than that seen in either Twist(+/-) or Gli3(+/XtJ) mice, suggesting that there is genetic interaction between Twist and Gli3 activity. Twist activity is therefore essential for the growth and differentiation of the limb bud tissues as well as regulation of tissue patterning via the modulation of SHH and FGF signal transduction.

The homeobox gene mbx is involved in eye and tectum development

The homeobox gene mbx is first activated at the end of gastrulation in zebrafish in the presumptive forebrain and midbrain region. During somitogenesis stages, the anterior expression of mbx, which partly overlaps the future eye field, gradually decreases, while midbrain expression intensifies and becomes restricted to the presumptive tectum. Knockdown of mbx expression by morpholino antisense oligonucleotides (mbx-MO) leads to a reduction in the size of the eyes and tectum. Expression domains of rx1 and pax6 in the eye field and of mab21l2 in the eye field and tectum anlage were reduced in size in mbx-MO-injected embryos by somitogenesis stages. Further, induction of islet1 and lim3 expression in the eye at 2 days postfertilization (dpf) was suppressed in mbx-MO-injected embryos. In mbx-MO-injected embryos at 2-5 dpf, the lamination of the eye was disorganized and the number of retinal axons was substantially reduced, but the few remaining axons navigated appropriately to the contralateral tectum. A chimeric protein composed of the Mbx DNA-binding domain and the VP16 activation domain affected eye and tectum development similarly to mbx-MO knockdown, suggesting that Mbx acts as a transcriptional repressor in the zebrafish embryo. Based on these data, we propose that the mbx homeobox gene is required for the development of the eyes and tectum.