Reassessing the Dlx code: the genetic regulation of branchial arch skeletal pattern and development

Mutant DLX 3 disrupts odontoblast polarization and dentin formation

Tricho-dento-osseous (TDO) syndrome is an autosomal dominant disorder characterized by abnormalities in the thickness and density of bones and teeth. A 4-bp deletion mutation in the Distal-Less 3 (DLX3) gene is etiologic for most cases of TDO. To investigate the in vivo role of mutant DLX3 (MT-DLX3) on dentin development, we generated transgenic (TG) mice expressing MT-DLX3 driven by a mouse 2.3 Col1A1 promoter. Dentin defects were radiographically evident in all teeth and the size of the nonmineralized pulp was enlarged in TG mice, consistent with clinical characteristics in patients with TDO. High-resolution radiography, microcomputed tomography, and SEM revealed a reduced zone of mineralized dentin with anomalies in the number and organization of dentinal tubules in MT-DLX3 TG mice. Histological and immunohistochemical studies demonstrated that the decreased dentin was accompanied by altered odontoblast cytology that included disruption of odontoblast polarization and reduced numbers of odontoblasts. TUNEL assays indicated enhanced odontoblast apoptosis. Expression levels of the apoptotic marker caspase-3 were increased in odontoblasts in TG mice as well as in odontoblastic-like MDPC-23 cells transfected with MT-DLX3 cDNA. Expression of Runx2, Wnt 10A, and TBC1D19 colocalized with DLX3 expression in odontoblasts, and MT-DLX3 significantly reduced expression of all three genes. TBC1D19 functions in cell polarity and decreased TBC1D19 expression may contribute to the observed disruption of odontoblast polarity and apoptosis. These data indicate that MT-DLX3 acts to disrupt odontoblast cytodifferentiation leading to odontoblast apoptosis, and aberrations of dentin tubule formation and dentin matrix production, resulting in decreased dentin and taurodontism. In summary, this TG model demonstrates that MT-DLX3 has differential effects on matrix production and mineralization in dentin and bone and provides a novel tool for the investigation of odontoblast biology.

Whole genome microarray analysis of chicken embryo facial prominences

The face is one of the three regions most frequently affected by congenital defects in humans. To understand the molecular mechanisms involved, it is necessary to have a more complete picture of gene expression in the embryo. Here, we use microarrays to profile expression in chicken facial prominences, post neural crest migration and before differentiation of mesenchymal cells. Chip-wide analysis revealed that maxillary and mandibular prominences had similar expression profiles while the frontonasal mass chips were distinct. Of the 3094 genes that were differentially expressed in one or more regions of the face, a group of 56 genes was subsequently validated with quantitative polymerase chain reaction (QPCR) and a subset examined with in situ hybridization. Microarrays trends were consistent with the QPCR data for the majority of genes (81%). On the basis of QPCR and microarray data, groups of genes that characterize each of the facial prominences can be determined.

Dlx homeobox gene family expression in osteoclasts

Skeletal growth and homeostasis require the finely orchestrated secretion of mineralized tissue matrices by highly specialized cells, balanced with their degradation by osteoclasts. Time- and site-specific expression of Dlx and Msx homeobox genes in the cells secreting these matrices have been identified as important elements in the regulation of skeletal morphology. Such specific expression patterns have also been reported in osteoclasts for Msx genes. The aim of the present study was to establish the expression patterns of Dlx genes in osteoclasts and identify their function in regulating skeletal morphology. The expression patterns of all Dlx genes were examined during the whole osteoclastogenesis using different in vitro models. The results revealed that Dlx1 and Dlx2 are the only Dlx family members with a possible function in osteoclastogenesis as well as in mature osteoclasts. Dlx5 and Dlx6 were detected in the cultures but appear to be markers of monocytes and their derivatives. In vivo, Dlx2 expression in osteoclasts was examined using a Dlx2/LacZ transgenic mouse. Dlx2 is expressed in a subpopulation of osteoclasts in association with tooth, brain, nerve, and bone marrow volumetric growths. Altogether the present data suggest a role for Dlx2 in regulation of skeletal morphogenesis via functions within osteoclasts.

Current knowledge of tooth development: patterning and mineralization of the murine dentition

The integument forms a number of different types of mineralized element, including dermal denticles, scutes, ganoid scales, elasmoid scales, fin rays and osteoderms found in certain fish, reptiles, amphibians and xenarthran mammals. To this list can be added teeth, which are far more widely represented and studied than any of the other mineralized elements mentioned above, and as such can be thought of as a model mineralized system. In recent years the focus for studies on tooth development has been the mouse, with a wealth of genetic information accrued and the availability of cutting edge techniques. It is the mouse dentition that this review will concentrate on. The development of the tooth will be followed, looking at what controls the shape of the tooth and how signals from the mesenchyme and epithelium interact to lead to formation of a molar or incisor. The number of teeth generated will then be investigated, looking at how tooth germ number can be reduced or increased by apoptosis, fusion of tooth germs, creation of new tooth germs, and the generation of additional teeth from existing tooth germs. The development of mineralized tissue will then be detailed, looking at how the asymmetrical deposition of enamel is controlled in the mouse incisor. The continued importance of epithelial-mesenchymal interactions at these later stages of tooth development will also be discussed. Tooth anomalies and human disorders have been well covered by recent reviews, therefore in this paper we wish to present a classical review of current knowledge of tooth development, fitting together data from a large number of recent research papers to draw general conclusions about tooth development.

Ectodermal dysplasia-like syndrome with mental retardation due to contiguous gene deletion: further clinical and molecular delineation of del(2q32) syndrome

We report on a patient with an interstitial deletion of the long arm of chromosome 2 at 2q31.2q33.2. She had prenatal and postnatal growth retardation, microcephaly, facial dysmorphism, cleft palate, camptodactyly, bilateral talipes equinovarus, severe intellectual disability, and ectodermal anomalies. She showed thin, atrophic skin, sparse, brittle, slowly growing hair, oligodontia with abnormally shaped teeth, normal sweating, and normal fingernails, consistent with a diagnosis of ectodermal dysplasia. Array CGH analysis (Agilent 44K) showed the deletion to span 26 Mb, between cytogenetic bands 2q31.2 and 2q33. The deletion leads to hemizygosity for the HOXD cluster and its regulatory elements, COL3A1/COL5A2, GTF3C3, CASP8, CASP10, and SABT2 could perhaps interfere with long range control of DLX1 and DLX2 expression. This girl confirms the existence of a clinically recognizable 2q32 microdeletion syndrome, as recently delineated by Van Buggenhout et al. and confirms a novel putative locus for ectodermal dysplasia on chromosome 2q31q33. We recommend considering cytogenetic and/or molecular screening for del(2q32) in patients with developmental disability and ectodermal dysplasia-like phenotype, including thin skin, oligodontia, dysplastic teeth, and sparse hair.

Spatial and temporal analysis of gene expression during growth and fusion of the mouse facial prominences

Orofacial malformations resulting from genetic and/or environmental causes are frequent human birth defects yet their etiology is often unclear because of insufficient information concerning the molecular, cellular and morphogenetic processes responsible for normal facial development. We have, therefore, derived a comprehensive expression dataset for mouse orofacial development, interrogating three distinct regions – the mandibular, maxillary and frontonasal prominences. To capture the dynamic changes in the transcriptome during face formation, we sampled five time points between E10.5–E12.5, spanning the developmental period from establishment of the prominences to their fusion to form the mature facial platform. Seven independent biological replicates were used for each sample ensuring robustness and quality of the dataset. Here, we provide a general overview of the dataset, characterizing aspects of gene expression changes at both the spatial and temporal level. Considerable coordinate regulation occurs across the three prominences during this period of facial growth and morphogenesis, with a switch from expression of genes involved in cell proliferation to those associated with differentiation. An accompanying shift in the expression of polycomb and trithorax genes presumably maintains appropriate patterns of gene expression in precursor or differentiated cells, respectively. Superimposed on the many coordinated changes are prominence-specific differences in the expression of genes encoding transcription factors, extracellular matrix components, and signaling molecules. Thus, the elaboration of each prominence will be driven by particular combinations of transcription factors coupled with specific cell:cell and cell:matrix interactions. The dataset also reveals several prominence-specific genes not previously associated with orofacial development, a subset of which we externally validate. Several of these latter genes are components of bidirectional transcription units that likely share cis-acting sequences with well-characterized genes. Overall, our studies provide a valuable resource for probing orofacial development and a robust dataset for bioinformatic analysis of spatial and temporal gene expression changes during embryogenesis.

New directions in craniofacial morphogenesis

The vertebrate head is an extremely complicated structure: development of the head requires tissue-tissue interactions between derivates of all the germ layers and coordinated morphogenetic movements in three dimensions. In this review, we highlight a number of recent embryological studies, using chicken, frog, zebrafish and mouse, which have identified crucial signaling centers in the embryonic face. These studies demonstrate how small variations in growth factor signaling can lead to a diversity of phenotypic outcomes. We also discuss novel genetic studies, in human, mouse and zebrafish, which describe cell biological mechanisms fundamental to the growth and morphogenesis of the craniofacial skeleton. Together, these findings underscore the complex interactions leading to species-specific morphology. These and future studies will improve our understanding of the genetic and environmental influences underlying human craniofacial anomalies.

Morphological integration of the skull in craniofacial anomalies

OBJECTIVES - To understand how surgical interventions impact the organization and internal integration of the major components of the skull, we address the functional and developmental relationships during perinatal development. METHODS - A number of methods for quantifying modularity and integration of morphological data are available. Here, measures derived from three-dimensional computed tomographic (CT) images are used to investigate the statistical relationships among measures of the cranial vault, face and cranial base. First, we establish the pattern of associations among quantitative measures in a sample of children unaffected by a craniofacial anomaly. We statistically compare these normative patterns of cranial integration to those of a sample of children with a facial anomaly (complete unilateral complete cleft lip and palate), and to children with a neurocranial anomaly (isolated sagittal synostosis). Finally, we test whether surgery affects the strength and pattern of associations among measures within the cranial base in the affected children. RESULTS - Our analyses reveal strong internal integration of the cranial base in unaffected children and in our samples of unoperated cleft lip and palate, and sagittal synostosis. Post-operatively, the magnitude of integration of the cranial base is reduced relative to the pre-operative condition in both samples of children with craniofacial anomalies. CONCLUSION - Our results show how the cranial base adjusts to its broader structural context, and provides added support for the developmental and structural integration of cranial base with both cranial vault and face.

Neuroembryology and functional anatomy of craniofacial clefts

The master plan of all vertebrate embryos is based on neuroanatomy. The embryo can be anatomically divided into discrete units called neuromeres so that each carries unique genetic traits. Embryonic neural crest cells arising from each neuromere induce development of nerves and concomitant arteries and support the development of specific craniofacial tissues or developmental fields. Fields are assembled upon each other in a programmed spatiotemporal order. Abnormalities in one field can affect the shape and position of developing adjacent fields. Craniofacial clefts represent states of excess or deficiency within and between specific developmental fields. The neuromeric organization of the embryo is the common denominator for understanding normal anatomy and pathology of the head and neck. Tessier's observational cleft classification system can be redefined using neuroanatomic embryology. Reassessment of Tessier's empiric observations demonstrates a more rational rearrangement of cleft zones, particularly near the midline. Neuromeric theory is also a means to understand and define other common craniofacial problems. Cleft palate, encephaloceles, craniosynostosis and cranial base defects may be analyzed in the same way.

Nasopharyngeal cysts

INTRODUCTION

Nasopharyngeal cysts are uncommon, and are mostly asymptomatic. However, these lesions are infrequently found during routine endoscopies and imaging studies. In even more rare cases, they may be the source for unexplained sinonasal symptoms, such as CSF rhinorrhea, visual disturbances and nasal obstruction.

PURPOSE OF REVIEW

This presentation systematically reviews the different nasopharyngeal cysts encountered in children, emphasizing the current knowledge on pathophysiology, recent advances in molecular biology and prenatal diagnosis, clinical presentation, imaging and treatment options.

SUMMARY

With the advent of flexible and rigid fiber-optic technology and modern imaging techniques, and in particularly prenatal diagnostic techniques, nasopharyngeal cysts recognition is more common than previous times and requires an appropriate consideration. Familiarity with these lesions is essential for the pediatric otolaryngologist.

A novel de novo 1.1 Mb duplication of 17q21.33 associated with cognitive impairment and other anomalies

We report on a 14-year-old girl with mild cognitive impairment, deafness, and an unusual pattern of anomalies associated with a previously unreported de novo duplication of chromosome 17q21.33. The 1.1 Mb duplication was detected by Affymetrix 100K GeneChip array genome hybridization and involves the genomic region between 45,093,544 and 46,196,038 base pairs on chromosome 17 (NCBI build 36.1). The patient has microcephaly, unusual cup-shaped ears, scoliosis and other skeletal defects. Two genes involved in the duplicated region, PPP1R9B and COL1A1, are strong candidates for producing her phenotype.

Chondrogenesis and homology of the visceral skeleton in the little skate, Leucoraja erinacea (Chondrichthyes: Batoidea)

Chondrichthyan fishes possess visceral skeletons that differ considerably, morphologically, from those of their sister taxon, the osteichthyans. Here, we use histological techniques and whole-mount skeletal preparations to visualize and describe the sequence of visceral skeletal condensation and chondrogenesis in a chondrichthyan, the little skate (Leucoraja erinacea). We demonstrate that visceral skeletal condensation begins rostrally, with the mandibular arch, and progresses caudally with the hyoid arch and posterior branchial arches condensing soon after. We provide a detailed account of the condensation and chondrogenesis of all major components of the L. erinacea visceral skeleton and discuss these data in the context of what is known from classical descriptions of chondrichthyan visceral skeletal development. Significant differences exist between the hypobranchial and basibranchial skeleton of L. erinacea and other chondrichthyan species, and the possible evolutionary and developmental significance of this is considered. We discuss the homology of the chondrichthyan hyoid arch and, based on patterns of mesenchymal condensation, we propose a model of condensation splitting and diversification that may account for the morphological diversification of gnathostome branchial arch derivatives. Finally, we suggest that the unique presence of certain visceral skeletal elements in chondrichthyans make oviparous chondrichthyans an ideal system for addressing questions of endoskeletal axial patterning during development.

Expression of Msx1 and Dlx1 during Dumbo rat head development: Correlation with morphological features

The Dumbo rat possesses some characteristics that evoke several human syndromes, such as Treacher-Collins: shortness of the maxillary, zygomatic and mandibular bones, and low position of the ears. Knowing that many homeobox genes are candidates in craniofacial development, we investigated the involvement of the Msx1 and Dlx1 genes in the Dumbo phenotype with the aim of understanding their possible role in abnormal craniofacial morphogenesis and examining the possibility of using Dumbo rat as an experimental model for understanding abnormal craniofacial development. We studied the expression of these genes during craniofacial morphogenesis by RT-PCR method. We used Dumbo embryos at E12 and E14 and included the Wistar strain as a control. Semi-quantitative PCR analysis demonstrated that Msx1 and Dlx1 are expressed differently between Dumbo and Wistar rats, indicating that their low expression may underly the Dumbo phenotype.

Competence, specification and commitment to an olfactory placode fate

The nasal placode shares a common origin with other sensory placodes within a pre-placodal domain at the cranial neural plate border. However, little is known about early events in nasal placode development as it segregates from prospective lens, neural tube and epidermis. Here, Dlx3, Dlx5, Pax6 and the pan-neuronal marker Hu serve as molecular labels to follow the maturation of olfactory precursors over time. When competence to form olfactory placode was tested by grafting ectoderm from different axial levels to the anterior neural fold, we found that competence is initially broad for head, but not trunk, ectoderm and declines rapidly with time. Isolated olfactory precursors are specified by HH10, concomitant with their complete segregation from other placodal, epidermal and neural progenitors. Heterotopic transplantation of olfactory progenitors reveals they are capable of autonomous differentiation only 12 hours later, shortly before overt placode invagination at HH14. Taken together, these results show that olfactory placode development is a step-wise process whereby signals from adjacent tissues specify competent ectoderm at or before HH10, followed by gradual commitment just prior to morphological differentiation.

MicroRNAs and micromanaging the skeleton in disease, development and evolution

MicroRNAs (miRNAs) are short, non-protein-encoding RNAs that effect post-transcriptional gene regulation by targeting messenger RNAs. miRNAs are associated with specific human diseases and help regulate development. Here we review recent advances in understanding the roles of miRNAs in skeletal malformations, including cleft palate, and in the evolution of skeletal morphologies. We propose the hypothesis that evolutionary variation in miRNA expression patterns or structural variation in miRNA binding sites in messenger RNAs can help explain the evolution of craniofacial variation among species, the development of human craniofacial disease and physiological changes leading to osteopenia that increases with ageing.

Retinoic acid affects craniofacial patterning by changing Fgf8 expression in the pharyngeal ectoderm

Retinoic acid signaling plays important roles in establishing normal patterning and cellular differentiation during embryonic development. In this study, we show that single administration of retinoic acid at embryonic day 8.5 causes homeotic transformation of the lower jaw into upper jaw-like structures. This homeosis was preceded by downregulation of Fgf8 and Sprouty expression in the proximal domain of the first pharyngeal arch. Downregulation of mesenchymal genes such as Dlx5, Hand2, Tbx1 and Pitx2 was also observed. The oropharynx in retinoic acid-treated embryos was severely constricted. Consistent with this observation, Patched expression in the arch endoderm and mesenchyme was downregulated. Thus, retinoic acid affects the expression of subsets of epithelial and mesenchymal genes, possibly disrupting the regional identity of the pharyngeal arch.

A SHH-responsive signaling center in the forebrain regulates craniofacial morphogenesis via the facial ectoderm

Interactions among the forebrain, neural crest and facial ectoderm regulate development of the upper jaw. To examine these interactions, we activated the Sonic hedgehog (SHH) pathway in the brain. Beginning 72 hours after activation of the SHH pathway, growth within the avian frontonasal process (FNP) was exaggerated in lateral regions and impaired in medial regions. This growth pattern is similar to that in mice and superimposed a mammalian-like morphology on the upper jaw. Jaw growth is controlled by signals from the frontonasal ectodermal zone (FEZ), and the divergent morphologies that characterize birds and mammals are accompanied by changes in the FEZ. In chicks there is a single FEZ spanning the FNP, but in mice both median nasal processes have a FEZ. In treated chicks, the FEZ was split into right and left domains that resembled the pattern present in mice. Additionally, we observed that, in the brain, fibroblast growth factor 8 (Fgf8) was downregulated, and signals in or near the nasal pit were altered. Raldh2 expression was expanded, whereas Fgf8, Wnt4, Wnt6 and Zfhx1b were downregulated. However, Wnt9b, and activation of the canonical WNT pathway, were unaltered in treated embryos. At later time points the upper beak was shortened owing to hypoplasia of the skeleton, and this phenotype was reproduced when we blocked the FGF pathway. Thus, the brain establishes multiple signaling centers within the developing upper jaw. Changes in organization of the brain that occur during evolution or as a result of disease can alter these centers and thereby generate morphological variation.

An endothelin-1 switch specifies maxillomandibular identity

Articulated jaws are highly conserved structures characteristic of gnathostome evolution. Epithelial-mesenchymal interactions within the first pharyngeal arch (PA1) instruct cephalic neural crest cells (CNCCs) to form the different skeletal elements of the jaws. The endothelin-1 (Edn1)/endothelin receptor type-A (Ednra)-->Dlx5/6-->Hand2 signaling pathway is necessary for lower jaw formation. Here, we show that the Edn1 signaling is sufficient for the conversion of the maxillary arch to mandibular identity. Constitutive activation of Ednra induced the transformation of upper jaw, maxillary, structures into lower jaw, mandibular, structures with duplicated Meckel's cartilage and dermatocranial jaws constituted by 4 dentary bones. Misexpression of Hand2 in the Ednra domain caused a similar transformation. Skeletal transformations are accompanied by neuromuscular remodeling. Ednra is expressed by most CNCCs, but its constitutive activation affects predominantly PA1. We conclude that after migration CNCCs are not all equivalent, suggesting that their specification occurs in sequential steps. Also, we show that, within PA1, CNCCs are competent to form both mandibular and maxillary structures and that an Edn1 switch is responsible for the choice of either morphogenetic program.

Dlx genes pattern mammalian jaw primordium by regulating both lower jaw-specific and upper jaw-specific genetic programs

Dlx transcription factors are implicated in patterning the mammalian jaw, based on their nested expression patterns in the first branchial arch (primordium for jaw) and mutant phenotypes; inactivation of Dlx1 and Dlx2 (Dlx1/2-/-) causes defects in the upper jaw, whereas Dlx5/6(-/-) results in homeotic transformation of the lower jaw into upper jaw. Therefore, the 'Dlx codes' appear to regionalize the jaw primordium such that Dlx1/2 regulate upper jaw development, while Dlx5/6 confer the lower jaw fate. Towards identifying the genetic pathways downstream of Dlx5/6, we compared the gene expression profiles of the wild-type and Dlx5/6(-/-) mouse mandibular arch (prospective lower jaw). We identified 20 previously unrecognized Dlx5/6-downstream genes, of which 12 were downregulated and 8 upregulated in the mutant. We found a Dlx-regulated transcriptional enhancer in close proximity to Gbx2, one of the Dlx5/6-downstream genes, strongly suggesting that Gbx2 is a direct target of Dlx5/6. We also showed that Pou3f3 is normally expressed in the maxillary (prospective upper jaw) but not mandibular arch, is upregulated in the mandibular arch of Dlx5/6(-/-), and is essential for formation of some of the maxillary arch-derived skeleton. A comparative analysis of the morphological and molecular phenotypes of various Dlx single and double mutants revealed that Dlx1, 2, 5 and 6 act both partially redundantly and antagonistically to direct differential expression of downstream genes in each domain of the first branchial arch. We propose a new model for Dlx-mediated mammalian jaw patterning.

Induction of mirror-image supernumerary jaws in chicken mandibular mesenchyme by Sonic Hedgehog-producing cells

Previous studies have shown that Sonic Hedgehog (Shh) signaling is crucial for the development of the first branchial arch (BA1) into a lower-jaw in avian and mammalian embryos. We have already shown that if Shh expression is precociously inhibited in pharyngeal endoderm, neural crest cells migrate to BA1 but fail to survive, and Meckel's cartilage and associated structures do not develop. This phenotype can be rescued by addition of an exogenous source of Shh. To decipher the role of Shh, we explored the consequences of providing an extra source of Shh to the presumptive BA1 territory. Grafting quail fibroblasts engineered to produce Shh (QT6-Shh), at the 5- to 8-somite stage, resulted in the induction of mirror-image extra lower jaws, caudolateral to the normal one. It turns out that the oral opening epithelium, in which Shh, Fgf8 and Bmp4 are expressed in a definite pattern, functions as an organizing center for lower-jaw development. In our experimental design, the extra source of Shh activates Fgf8, Bmp4 and Shh genes in caudal BA1 ectoderm in a spatial pattern similar to that of the oral epithelium, and regularly leads to the formation of two extra lower-jaw-organizing centers with opposite rostrocaudal polarities. These results emphasize the similarities between the developmental processes of the limb and mandibular buds, and show that in both cases Shh-producing cells create a zone of polarizing activity for the structures deriving from them.

Physiological implications of DLX homeoproteins in enamel formation

Tooth development is a complex process including successive stages of initiation, morphogenesis, and histogenesis. The role of the Dlx family of homeobox genes during the early stages of tooth development has been widely analyzed, while little data has been reported on their role in dental histogenesis. The expression pattern of Dlx2 has been described in the mouse incisor; an inverse linear relationship exists between the level of Dlx2 expression and enamel thickness, suggesting a role for Dlx2 in regulation of ameloblast differentiation and activity. In vitro data have revealed that DLX homeoproteins are able to regulate the expression of matrix proteins such as osteocalcin. The aim of the present study was to analyze the expression and function of Dlx genes during amelogenesis. Analysis of Dlx2/LacZ transgenic reporter mice, Dlx2 and Dlx1/Dlx2 null mutant mice, identified spatial variations in Dlx2 expression within molar tooth germs and suggests a role for Dlx2 in the organization of preameloblastic cells as a palisade in the labial region of molars. Later, during the secretory and maturation stages of amelogenesis, the expression pattern in molars was found to be similar to that described in incisors. The expression patterns of the other Dlx genes were examined in incisors and compared to Dlx2. Within the ameloblasts Dlx3 and Dlx6 are expressed constantly throughout presecretory, secretory, and maturation stages; during the secretory phase when Dlx2 is transitorily switched off, Dlx1 expression is upregulated. These data suggest a role for DLX homeoproteins in the morphological control of enamel. Sequence analysis of the amelogenin gene promoter revealed five potential responsive elements for DLX proteins that are shown to be functional for DLX2. Regulation of amelogenin in ameloblasts may be one method by which DLX homeoproteins may control enamel formation. To conclude, this study establishes supplementary functions of Dlx family members during tooth development: the participation in establishment of dental epithelial functional organization and the control of enamel morphogenesis via regulation of amelogenin expression.

Molecular switches involving homeodomain proteins, HOXA10 and RUNX2 regulate osteoblastogenesis

The osteoinductive BMP2 signal facilitates commitment to the osteoblast phenotype by inducing several classes of early response genes. Among these are bone-related HOX factors, homeodomain, RUNX and OSTERIX proteins. Here we demonstrate molecular events among BMP2-induced transcription factors that constitute a network of molecular switches on promoters of bone-related genes to coordinate their temporal expression during cellular differentiation. Our studies provide evidence for (1) selective association of HOXA10, MSX2, DLX3 and DLX5 homeodomain transcription factors on Runx2 and OC genes at stages of osteoblast maturation as well as (2) participation of these factors with RUNX2 in chromatin remodeling of bone-specific genes for repression, activation and attenuation of transcription. These findings reveal the requirement for multiple levels of control for the appropriate timing of osteoblast-related gene expression.

Tweaking the hinge and caps: testing a model of the organization of jaws

Historically, examinations of gnathostome skulls have indicated that for essentially the entirety of their existence, jaws have been characterized by a high degree of fidelity to an initial basic structural design that will then go on to manifest an amazing array of end-point phenotypes. These two traits-bauplan fidelity and elaboration of design-are inter-connected and striking, and beg a number of questions, including: Are all jaws made in the same manner and if not how not? To begin to tackle such questions, we herein operationally define jaws as two appositional, hinged cranial units for which polarity and potential modularity are characteristics, and then address what is necessary for them to form, including delineating both the sources of cells and tissues that will formally yield the jaws as well as what informs their ontogeny (e.g., sources of positional information and factors directing the interpretation of developmental cues). Following on this, we briefly describe a predictive, testable model of jaw development (the "Hinge and Caps" model) and present evidence that the Satb2+cell population in the developing jaw primordia of mice defines a developmentally and evolutionarily significant jaw module such as would be predicted by the model.

Distal-less homeobox transcription factors regulate development and maturation of natural killer cells

Natural killer (NK) cells constitute a subpopulation of lymphocytes that develop from precursors in the bone marrow (BM), but the transcriptional regulation of their development and maturation is only beginning to be understood, in part due to their relatively rare abundance, especially of developmental subsets. Using a mouse model in which NK cells are arrested at an immature stage of development, and a gene expression profiling approach, we uncovered transient normal NK cell expression of a homeobox transcription factor (TF) family, called Distal-less (Dlx), which had been primarily implicated in murine CNS, craniofacial, limb, and skin development. Our studies demonstrate that Dlx1, Dlx2, and Dlx3 are transiently expressed in immature Mac-1(lo) NK cells within the BM, with Dlx3 being the predominantly expressed member. These genes are expressed in a temporally regulated pattern with overlapping waves of expression, and they display functional redundancy. Expression is extinguished in fully mature splenic NK cells, and persistent expression of Dlx genes leads to functionally immature NK cells arrested at the Mac-1(lo) stage. Whereas conventional splenic NK cells develop but are arrested at an immature stage, there appears to be a complete failure to develop CD127(+) thymic NK cells when Dlx genes are persistently expressed. We also observed that T and B cells fail to develop in the context of persistent Dlx1 expression. Thus, these studies indicate that Dlx TFs play a functional role in lymphocyte development.

Molecular consequences of a frameshifted DLX3 mutant leading to Tricho-Dento-Osseous syndrome

The homeodomain protein Distal-less-3 (Dlx3) plays a crucial role during embryonic development. This transcription factor is known to be essential for placental formation and to be involved in skin and skeletal organogenesis. In humans, a frameshift mutation in the coding sequence of the DLX3 gene results in an ectodermal dysplasia called Tricho-Dento-Osseous syndrome (TDO). The main features of this autosomal dominant disorder are defects in hair, teeth, and bone. To investigate the functional alterations caused by the mutated DLX3(TDO) isoform ex vivo, we used tetracycline-inducible osteoblastic and keratinocyte cell lines and calvarial derived osteoblasts in which the expression of DLX3(WT) and/or DLX3(TDO) could be regulated and monitored. Immunocytochemical analysis revealed that both DLX3(WT) and DLX3(TDO) recombinant proteins are targeted to the nucleus. However, as demonstrated by electrophoresis mobility shift assay, DLX3(TDO) is not able to bind to the canonical Dlx3 binding site. Furthermore, we demonstrate that the frameshifted C-terminal domain in DLX3(TDO) is accountable for the loss of DNA binding activity because the C-terminal domain in DLX3(WT) is not required for DNA binding activity. Although DLX3(TDO) alone cannot bind to a Dlx3 responsive element, when DLX3(WT) and DLX3(TDO) are co-expressed they form a complex that can bind DNA. Concomitant with the inability to bind DNA, DLX3(TDO) has a defective transcriptional activity. Moreover, the transcriptional activity of DLX3(WT) is significantly reduced in the presence of the mutated isoform, indicating that DLX3(TDO) has a dominant negative effect on DLX3(WT) transcriptional activity.

TGF-beta mediated Dlx5 signaling plays a crucial role in osteo-chondroprogenitor cell lineage determination during mandible development

Transforming growth factor-beta (TGF-beta) signaling is crucial for mandible development. During its development, the majority of the mandible is formed through intramembranous ossification whereas the proximal region of the mandible undergoes endochondral ossification. Our previous work has shown that TGF-beta signaling is required for the proliferation of cranial neural crest (CNC)-derived ectomesenchyme in the mandibular primordium where intramembranous ossification takes place. Here we show that conditional inactivation of Tgfbr2 in CNC cells results in accelerated osteoprogenitor differentiation and perturbed chondrogenesis in the proximal region of the mandible. Specifically, the appearance of chondrocytes in Tgfbr2(fl/fl);Wnt1-Cre mice is delayed and they are smaller in size in the condylar process and completely missing in the angular process. TGF-beta signaling controls Sox9 expression in the proximal region, because Sox9 expression is delayed in condylar processes and missing in angular process in Tgfbr2(fl/fl);Wnt1-Cre mice. Moreover, exogenous TGF-beta can induce Sox9 expression in the mandibular arch. In the angular processes of Tgfbr2(fl/fl);Wnt1-Cre mice, osteoblast differentiation is accelerated and Dlx5 expression is elevated. Significantly, deletion of Dlx5 in Tgfbr2(fl/fl);Wnt1-Cre mice results in the rescue of cartilage formation in the angular processes. Finally, TGF-beta signaling-mediated Scleraxis expression is required for tendonogenesis in the developing skeletal muscle. Thus, CNC-derived cells in the proximal region of mandible have a cell intrinsic requirement for TGF-beta signaling.

Compressive force stimulates the expression of osteogenesis-related transcription factors in ROS 17/2.8 cells

OBJECTIVE

To determine how compressive force affects the expression of osteogenesis-related transcription factors in osteoblasts.

DESIGN

Cells of ROS 17/2.8, a typical osteoblastic cell line, were cultured with or without continuous compressive force (0.5-2.0 g/cm(2)). Expression of mRNA encoding the osteogenesis-related transcription factors Runx2, Osterix, Msx2, Dlx5 and AJ18 was measured using real-time polymerase chain reaction. Protein expression of these transcription factors was determined by Western blotting.

RESULTS

A compressive force of 1.0 g/cm(2) significantly increased mRNA and protein expression of Runx2, Osterix, Msx2 and Dlx5, which are critical for osteoblast differentiation. In contrast, mRNA and protein expression of AJ18, which downregulates osteoblast differentiation, were decreased with 1.0 g/cm(2) of compressive force.

CONCLUSIONS

A compressive force of 1.0 g/cm(2), which was considered optimal for bone formation under the present experimental conditions, stimulates osteoblastic differentiation via the modulation of osteogenesis-related transcription factors.

GAD isoforms exhibit distinct spatiotemporal expression patterns in the developing mouse lens: correlation with Dlx2 and Dlx5

Gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter of the adult nervous system and its biosynthetic enzyme glutamic acid decarboxylase (GAD) are abundantly expressed in the embryonic nervous system and are involved in the modulation of cell proliferation, migration, and differentiation. Here we describe for the first time the expression of GABA and embryonic and adult GAD isoforms in the developing mouse lens. We show that the GAD isoforms are sequentially induced with specific spatiotemporal profiles: GAD65 and embryonic GAD isoforms prevail in primary fibers, while GAD67 is the predominant GAD expressed in the postnatal secondary fibers. This pattern correlates well with the expression of Dlx2 and Dlx5, known as upstream regulators of GAD. GABA and GAD are most abundant at the tips of elongating fibers and are absent from organelle-free cells, suggesting their involvement is primarily in shaping of the cytoskeleton during fiber elongation stages.

Recombinase-mediated cassette exchange reveals the selective use of Gq/G11-dependent and -independent endothelin 1/endothelin type A receptor signaling in pharyngeal arch development

The endothelin (Edn) system comprises three ligands (Edn1, Edn2 and Edn3) and their G-protein-coupled type A (Ednra) and type B (Ednrb) receptors. During embryogenesis, the Edn1/Ednra signaling is thought to regulate the dorsoventral axis patterning of pharyngeal arches via Dlx5/Dlx6 upregulation. To further clarify the underlying mechanism, we have established mice in which gene cassettes can be efficiently knocked-in into the Ednra locus using recombinase-mediated cassette exchange (RMCE) based on the Cre-lox system. The first homologous recombination introducing mutant lox-flanked Neo resulted in homeotic transformation of the lower jaw to an upper jaw, as expected. Subsequent RMCE-mediated knock-in of lacZ targeted its expression to the cranial/cardiac neural crest derivatives as well as in mesoderm-derived head mesenchyme. Knock-in of Ednra cDNA resulted in a complete rescue of craniofacial defects of Ednra-null mutants. By contrast, Ednrb cDNA could not rescue them except for the most distal pharyngeal structures. At early stages, the expression of Dlx5, Dlx6 and their downstream genes was downregulated and apoptotic cells distributed distally in the mandible of Ednrb-knock-in embryos. These results, together with similarity in craniofacial defects between Ednrb-knock-in mice and neural-crest-specific Galpha(q)/Galpha(11)-deficient mice, indicate that the dorsoventral axis patterning of pharyngeal arches is regulated by the Ednra-selective, G(q)/G(11)-dependent signaling, while the formation of the distal pharyngeal region is under the control of a G(q)/G(11)-independent signaling, which can be substituted by Ednrb. This RMCE-mediated knock-in system can serve as a useful tool for studies on gene functions in craniofacial development.

BMP-2 induces Osterix expression through up-regulation of Dlx5 and its phosphorylation by p38

Osterix, a zinc-finger transcription factor, is specifically expressed in osteoblasts and osteocytes of all developing bones. Because no bone formation occurs in Osterix null mice, Osterix is thought to be an essential regulator of osteoblast differentiation. We report that bone morphogenetic protein-2 (BMP-2) induces an increase in Osterix expression, which is mediated through a homeodomain sequence located in the proximal region of the Osterix promoter. Our results demonstrate that induction of Dlx5 by BMP-2 mediates Osterix transcriptional activation. First, BMP-2 induction of Dlx5 precedes the induction of Osterix. Second, Dlx5 binds to the BMP-responsive homeodomain sequences both in vitro and in vivo. Third, Dlx5 overexpression and knock-down assays demonstrate its role in activating Osterix expression in response to BMP-2. Furthermore, we show that Dlx5 is a novel substrate for p38 MAPK in vitro and in vivo and that Ser-34 and Ser-217 are the sites phosphorylated by p38. Phosphorylation at Ser-34/217 increases the transactivation potential of Dlx5. Thus, we propose that BMP activates expression of Osterix through the induction of Dlx5 and its further transcriptional activation by p38-mediated phosphorylation.

Zebrafish dlx2a contributes to hindbrain neural crest survival, is necessary for differentiation of sensory ganglia and functions with dlx1a in maturation of the arch cartilage elements

The Dlx genes are expressed in a coordinate manner, establishing proximal-distal polarity within the pharyngeal arches. In zebrafish, dlx2a is expressed in the migrating cranial neural crest that contributes to the pharyngeal arches. Expression of dlx2a in the arches is subsequently followed by overlapping expression of the physically linked dlx1a gene, and of other paralogues that include dlx5a/dlx6a and dlx3b/dlx4b. To investigate the patterning and establishment of arch proximodistal polarity in zebrafish, we characterized the function of dlx2a and dlx1a, using antisense morpholino oligonucleotides (MOs). We show that embryos injected with dlx1a and dlx2a MOs exhibit reduced and dysmorphic arch cartilage elements. The combined loss of dlx1a and dlx2a causes severe arch cartilage dysmorphology, revealing a role for these genes in maturation and patterning of arch chondrogenesis. Knockdown of dlx2a affects migrating neural crest cells as evidenced by reduced expression of crestin, and sox9a transcripts, in addition to increased levels of apoptosis. During pharyngogenesis, loss of dlx2a results in aberrant barx1 expression and the absence of goosecoid transcripts in the dorsal region of the ceratohyal arch. Defects in the differentiation of ectomesenchymal derivatives, including sensory ganglia and cartilage elements, indicate a role for dlx2a in specification and maintenance of cranial neural crest.

Dlx transcription factors promote migration through repression of axon and dendrite growth

In the mouse telencephalon, Dlx homeobox transcription factors are essential for the tangential migration of subpallial-derived GABAergic interneurons to neocortex. However, the mechanisms underlying this process are poorly understood. Here, we demonstrate that Dlx1/2 has a central role in restraining neurite growth of subpallial-derived immature interneurons at a stage when they migrate tangentially to cortex. In Dlx1-/-;Dlx2-/- mutants, neurite length is increased and cells fail to migrate. In Dlx1-/-;Dlx2+/- mutants, while the tangential migration of immature interneurons appears normal, they develop dendritic and axonal processes with increased length and decreased branching, and have deficits in their neocortical laminar positions. Thus, Dlx1/2 is required for coordinating programs of neurite maturation and migration. In this regard, we provide genetic evidence that in immature interneurons Dlx1/2 repression of the p21-activated serine/threonine kinase PAK3, a downstream effector of the Rho family of GTPases, is critical in restraining neurite growth and promoting tangential migration.

Molecular dynamics of retinoic acid-induced craniofacial malformations: implications for the origin of gnathostome jaws

Background

Intake of retinoic acid (RA) or of its precursor, vitamin A, during early pregnancy is associated with increased incidence of craniofacial lesions. The origin of these teratogenic effects remains enigmatic as in cranial neural crest cells (CNCCs), which largely contribute to craniofacial structures, the RA-transduction pathway is not active. Recent results suggest that RA could act on the endoderm of the first pharyngeal arch (1stPA), through a RARß-dependent mechanism.

Methodology/Principal Findings

Here we show that RA provokes dramatically different craniofacial malformations when administered at slightly different developmental times within a narrow temporal interval corresponding to the colonization of the 1^st PA by CNCCs. We provide evidence showing that RA acts on the signalling epithelium of the 1^st PA, gradually reducing the expression of endothelin-1 and Fgf8. These two molecular signals are instrumental in activating Dlx genes in incoming CNCCs, thereby triggering the morphogenetic programs, which specify different jaw elements.

Conclusions/Significance

The anatomical series induced by RA-treatments at different developmental times parallels, at least in some instances, the supposed origin of modern jaws (e.g., the fate of the incus). Our results might provide a conceptual framework for the rise of jaw morphotypes characteristic of gnathostomes.

mef2ca is required in cranial neural crest to effect Endothelin1 signaling in zebrafish

Mef2 genes encode highly conserved transcription factors involved in somitic and cardiac mesoderm development in diverse bilaterians. Vertebrates have multiple mef2 genes. In mice, mef2c is required for heart and vascular development. We show that a zebrafish mef2c gene (mef2ca) is required in cranial neural crest (CNC) for proper head skeletal patterning. mef2ca mutants have head skeletal phenotypes resembling those seen upon partial loss-of-function of endothelin1 (edn1). Furthermore, mef2ca interacts genetically with edn1, arguing that mef2ca functions within the edn1 pathway. mef2ca is expressed in CNC and this expression does not require edn1 signaling. Mosaic analyses reveal that mef2ca is required in CNC for pharyngeal skeletal morphogenesis. Proper expression of many edn1-dependent target genes including hand2, bapx1, and gsc, depends upon mef2ca function. mef2ca plays a critical role in establishing the proper nested expression patterns of dlx genes. dlx5a and dlx6a, known Edn1 targets, are downregulated in mef2ca mutant pharyngeal arch CNC. Surprisingly, dlx4b and dlx3b are oppositely affected in mef2ca mutants. dlx4b expression is abolished while the edn1-dependent dlx3b is ectopically expressed in more dorsal CNC. Together our results support a model in which CNC cells require mef2ca downstream of edn1 signaling for proper craniofacial development.

The transcription factor MEF2C is required for craniofacial development

MEF2 transcription factors are well-established regulators of muscle development. We have discovered an unanticipated role for MEF2C in the neural crest, where tissue-specific inactivation results in neonatal lethality due to severe craniofacial defects. We show that MEF2C is required for expression of the Dlx5, Dlx6, and Hand2 transcription factor genes in the branchial arches, and we identify a branchial arch-specific enhancer in the Dlx5/6 locus, which is activated synergistically by MEF2C and Dlx5, demonstrating that these factors interact to induce transcription. Mef2c and Dlx5/6 also interact genetically. Mice heterozygous for either Dlx5/6 or Mef2c are normal at birth and survive to weaning. By contrast, heterozygosity for both Mef2c and Dlx5/6 results in defective palate development and neonatal lethality. Taken together, the studies presented here define a feed-forward transcriptional circuit between the MADS-box transcription factor MEF2C and the homeodomain transcription factors Dlx5 and Dlx6 in craniofacial development.

HOXA10 controls osteoblastogenesis by directly activating bone regulatory and phenotypic genes

HOXA10 is necessary for embryonic patterning of skeletal elements, but its function in bone formation beyond this early developmental stage is unknown. Here we show that HOXA10 contributes to osteogenic lineage determination through activation of Runx2 and directly regulates osteoblastic phenotypic genes. In response to bone morphogenic protein BMP2, Hoxa10 is rapidly induced and functions to activate the Runx2 transcription factor essential for bone formation. A functional element with the Hox core motif was characterized for the bone-related Runx2 P1 promoter. HOXA10 also activates other osteogenic genes, including the alkaline phosphatase, osteocalcin, and bone sialoprotein genes, and temporally associates with these target gene promoters during stages of osteoblast differentiation prior to the recruitment of RUNX2. Exogenous expression and small interfering RNA knockdown studies establish that HOXA10 mediates chromatin hyperacetylation and trimethyl histone K4 (H3K4) methylation of these genes, correlating to active transcription. HOXA10 therefore contributes to early expression of osteogenic genes through chromatin remodeling. Importantly, HOXA10 can induce osteoblast genes in Runx2 null cells, providing evidence for a direct role in mediating osteoblast differentiation independent of RUNX2. We propose that HOXA10 activates RUNX2 in mesenchymal cells, contributing to the onset of osteogenesis, and that HOXA10 subsequently supports bone formation by direct regulation of osteoblast phenotypic genes.

Morphing the hyomandibular skeleton in development and evolution

How might changes in developmental regulatory pathways underlie evolutionary changes in morphology? Here we focus on a particular pathway regulated by a secreted, signaling peptide, Endothelin1 (Edn1). Developmental genetic analyses show the Edn1-pathway to be crucial for hyomandibular patterning, and we discuss our work with zebrafish suggesting how the signal may function in regulating numbers of skeletal elements, their sizes and their shapes. We then review a broader collection of comparative studies that examine morphological evolution of a subset of the same skeletal elements-the opercular-branchiostegal series of bones of the hyoid arch. We find that phenotypic changes in zebrafish mutants copy evolutionary changes that recur along many actinopterygian lineages. Hence the developmental genetic studies are informative for providing candidate pathways for macroevolution of facial morphology, as well as for our understanding of how these pathways work.

Gene expression analysis of TFII-I modulated genes in mouse embryonic fibroblasts

TFII-I is a founding member of a family of helix-loop-helix transcription factors involved in modulation of genes through interaction with various nuclear factors and chromatin remodeling complexes. Recent studies indicate that TFII-I performs important function in cell physiology and mouse embryogenesis. In order to understand its molecular role, TFII-I was overexpressed in primary mouse embryonic fibroblasts (MEFs) and alterations in gene expression were monitored with a mouse 16 K oligonucleotide microarray. These studies allowed us to identify genes that lie downstream of TFII-I-dependent pathways. Among the modulated candidates were genes involved in the immunity response, catalytic activity, signaling pathways and transcriptional regulation. Expression of several candidates including those for the interferon-stimulated protein (G1p2), small inducible cytokine A7 (Ccl7), ubiquitin-conjugating enzyme 8 (Ube2l6), cysteine-rich protein (Csrp2) and Drosophila delta-like 1 homolog (Dlk1) were confirmed by real-time PCR. The obtained results suggest that TFII-I participates in multiple signaling and regulatory pathways in MEFs.

2q24–q31 Deletion: Report of a case and review of the literature

We report a patient with a de novo interstitial deletion of the long arm of chromosome 2 involving bands 2q24.3-q31.1. The patient shows postnatal growth retardation, microcephaly, ptosis, down-slanting palpebral fissures, long eyelashes and micrognathia. Halluces are long, broad and medially deviated, while the other toes are laterally deviated and remarkably short with hypoplastic phalanges. She also showed developmental delay, seizures, lack of eye contact, stereotypic and repetitive hand movements and sleep disturbances with breath holding. Prenatal and three independent postnatal karyotypes were normal. Array-CGH analysis allowed us to identify and characterize a "de novo" 2q interstitial deletion of about 10.4Mb, involving segment between cytogenetic bands 2q24.3 and 2q31.1. The deletion was confirmed by quantitative PCR. About 30 children with 2q interstitial deletion have been reported. The deletion described here is overlapping with 15 of these cases. We have attempted to compare the clinical features of our patient with 15 overlapping cases. The emerging phenotypes include low birth weight, postnatal growth retardation, mental retardation and developmental delay, microcephaly, and peculiar facial dysmorphisms. Peculiar long and broad halluces with an increased distance between the first and the second toe are ("sandal gap" sign) present in most of the described patients. The gene content analysis of the deleted region revealed the presence of some genes that may be indicated as good candidates in generating both neurological and dysmorphic phenotype in the patient. In particular, a cluster of SCNA genes is located within the deleted region and it is known that loss of function mutations in SCNA1 gene cause a severe form of epilepsy.

21st century neontology and the comparative development of the vertebrate skull

Classic neontology (comparative embryology and anatomy), through the application of the concept of homology, has demonstrated that the development of the gnathostome (jawed vertebrate) skull is characterized both by a fidelity to the gnathostome bauplan and the exquisite elaboration of final structural design. Just as homology is an old concept amended for modern purposes, so are many of the questions regarding the development of the skull. With due deference to Geoffroy-St. Hilaire, Cuvier, Owen, Lankester et al., we are still asking: How are bauplan fidelity and elaboration of design maintained, coordinated, and modified to generate the amazing diversity seen in cranial morphologies? What establishes and maintains pattern in the skull? Are there universal developmental mechanisms underlying gnathostome autapomorphic structural traits? Can we detect and identify the etiologies of heterotopic (change in the topology of a developmental event), heterochronic (change in the timing of a developmental event), and heterofacient (change in the active capacetence, or the elaboration of capacity, of a developmental event) changes in craniofacial development within and between taxa? To address whether jaws are all made in a like manner (and if not, then how not), one needs a starting point for the sake of comparison. To this end, we present here a "hinge and caps" model that places the articulation, and subsequently the polarity and modularity, of the upper and lower jaws in the context of cranial neural crest competence to respond to positionally located epithelial signals. This model expands on an evolving model of polarity within the mandibular arch and seeks to explain a developmental patterning system that apparently keeps gnathostome jaws in functional registration yet tractable to potential changes in functional demands over time. It relies upon a system for the establishment of positional information where pattern and placement of the "hinge" is driven by factors common to the junction of the maxillary and mandibular branches of the first arch and of the "caps" by the signals emanating from the distal-most first arch midline and the lamboidal junction (where the maxillary branch meets the frontonasal processes). In this particular model, the functional registration of jaws is achieved by the integration of "hinge" and "caps" signaling, with the "caps" sharing at some critical level a developmental history that potentiates their own coordination. We examine the evidential foundation for this model in mice, examine the robustness with which it can be applied to other taxa, and examine potential proximate sources of the signaling centers. Lastly, as developmental biologists have long held that the anterior-most mesendoderm (anterior archenteron roof or prechordal plate) is in some way integral to the normal formation of the head, including the cranial skeletal midlines, we review evidence that the seminal patterning influences on the early anterior ectoderm extend well beyond the neural plate and are just as important to establishing pattern within the cephalic ectoderm, in particular for the "caps" that will yield medial signaling centers known to coordinate jaw development.

Transgenic analysis of Dlx regulation in fish tooth development reveals evolutionary retention of enhancer function despite organ loss

It has been considered a "law" that a lost structure cannot reappear in evolution. The common explanation, that genes required for the development of the lost structure degrade by mutation, remains largely theoretical, however. Additionally, the extent to which this mechanism applies to systems of repeated parts, where individual modules are likely to exhibit few unique aspects of genetic control, is unclear. We investigated reversibility of evolution in one such system, the vertebrate dentition, using as a model loss of oral teeth in cypriniform fishes, which include the zebrafish. This evolutionary event, which occurred > 50 million years ago, has not been reversed despite subsequent diversification of feeding modes and retention of pharyngeal teeth. We asked whether the cis-regulatory region of a gene whose expression loss parallels cypriniform tooth loss, Dlx2b, retains the capacity for expression in oral teeth. We first created a zebrafish reporter transgenic line that recapitulates endogenous dlx2b expression. We then showed that this zebrafish construct drives reporter expression in oral teeth of the related characiform Astyanax mexicanus. This result, along with our finding that Dlx genes are required for normal tooth development, suggests that changes in trans-acting regulators of these genes were responsible for loss of cypriniform oral teeth. Preservation of oral enhancer function unused for > 50 million years could be the result of pleiotropic function in the pharyngeal dentition. If enhancers of other genes in the tooth developmental pathway are similarly preserved, teeth lost from specific regions may be relatively easy to reacquire in evolution.

Impaired nuclear import of mammalian Dlx4 proteins as a consequence of rapid sequence divergence

Dlx genes encode a developmentally important family of transcription factors with a variety of functions and sites of action during vertebrate embryogenesis. The murine Dlx4 gene is an enigmatic member of the family; little is known about the normal developmental function(s) of Dlx4. Here, we show that Dlx4 is expressed in the murine placenta and in a trophoblast cell line where the protein localizes to both the nucleus and cytoplasm. Despite the presence of several leucine/valine-rich motifs that match known nuclear export sequences, cytoplasmic Dlx4 is not due to CRM-1-mediated nuclear export. Rather, nuclear import of Dlx4 is compromised by specific residues that flank the nuclear localization signal. One of these residues represents a novel conserved feature of the Dlx4 protein in placental mammals, and the second represents novel variation within mouse Dlx4 isoforms. Comparison of orthologous protein sequences reveals a particularly high rate of non-synonymous change in the coding regions of mammalian Dlx4 genes. Since impaired nuclear localization is unlikely to enhance the function of a nuclear transcription factor, these data point to reduced selection pressure as the basis for the rapid divergence of the Dlx4 gene within the mammalian clade.

BMP2 commitment to the osteogenic lineage involves activation of Runx2 by DLX3 and a homeodomain transcriptional network

Several homeodomain (HD) proteins are critical for skeletal patterning and respond directly to BMP2 as an early step in bone formation. RUNX2, the earliest transcription factor proven essential for commitment to osteoblastogenesis, is also expressed in response to BMP2. However, there is a gap in our knowledge of the regulatory cascade from BMP2 signaling to the onset of osteogenesis. Here we show that BMP2 induces DLX3, a homeodomain protein that activates Runx2 gene transcription. Small interfering RNA knockdown studies in osteoblasts validate that DLX3 is a potent regulator of Runx2. Furthermore in Runx2 null cells, DLX3 forced expression suffices to induce transcription of Runx2, osteocalcin, and alkaline phosphatase genes, thus defining DLX3 as an osteogenic regulator independent of RUNX2. Our studies further show regulation of the Runx2 gene by several homeodomain proteins: MSX2 and CDP/cut repress whereas DLX3 and DLX5 activate endogenous Runx2 expression and promoter activity in non-osseous cells and osteoblasts. These HD proteins exhibit distinct temporal expression profiles during osteoblast differentiation as well as selective association with Runx2 chromatin that is related to Runx2 transcriptional activity and recruitment of RNA polymerase II. Runx2 promoter mutagenesis shows that multiple HD elements control expression of Runx2 in relation to the stages of osteoblast maturation. Our studies establish mechanisms for commitment to the osteogenic lineage directly through BMP2 induction of HD proteins DLX3 and DLX5 that activate Runx2, thus delineating a transcriptional regulatory pathway mediating osteoblast differentiation. We propose that the three homeodomain proteins MSX2, DLX3, and DLX5 provide a key series of molecular switches that regulate expression of Runx2 throughout bone formation.

Severe hearing loss in Dlxl mutant mice

The Dlx homeobox gene family participates in regulating middle and inner ear development. A significant role for Dlxl, in particular,has been demonstrated in the development of the middle ear ossicles, but the functional consequences of Dlx.l gene mutation on hearing thresholds has not been assessed. The present study characterizes auditory brainstem responses to click and tonal stimuli in a non-lethal variant of a Dlxl gene knockout. We found that peripheral hearing thresholds for click and tonal stimuli were significantly elevated in homozygous Dlxl knockout (Dlxl-/ ) compared to both heterozygous (Dlxl+/ ) and wild type (Dlxl+/+) mice. Thus, abnormal mor-phogenesis of the incus and stapes that has been documented previously with histological measures is now known to result in a severe peripheral hearing deficit.

Cell dissociation experiments reveal that positional information operates in the chicken frontonasal mass

In this study we examined the role of cell-cell affinity in patterning the avian frontonasal mass-the facial prominence that forms the prenasal cartilage and premaxillary bone. Reconstituted cell pellets derived from undifferentiated, frontonasal mass mesenchyme were recombined with facial epithelium and grafted to host embryos to continue development. We determined that the cells reestablished a recognizable frontonasal mass pattern and were able to induce egg teeth in overlying ectoderm. Further analysis revealed there were region-specific differences in the cartilage patterns such that central recombinations were more likely to form a straight cartilage rod, whereas lateral mesenchyme pellets were more likely to form complex, branched cartilage patterns. The basis for the pattern differences was that central mesenchyme cells showed preferential clustering in the cartilage condensations in the center of the graft, whereas lateral cells were spread throughout as determined by dye labeling and quail chicken chimeras. The disruption of cell contacts temporarily delayed onset of gene expression but by 48 h both Msx2 and Dlx5 were expressed. Msx2, in particular, had very clear edges to the expression domains and often the pattern of expression correlated with type of cartilage morphology. Together, these data suggest that an important patterning mechanism in the face is the ability of mesenchymal cells to sort out according to position and that Msx2 may help repress chondrogenic potential in the lateral frontonasal mass.

Networks and hubs for the transcriptional control of osteoblastogenesis

We present an overview of the concepts of tissue-specific transcriptional control mechanisms essential for development of the bone cell phenotype. BMP2 induced transcription factors constitute a network of activities and molecular switches for bone development and osteoblast differentiation. Among these regulators are Runx2 (Cbfa1/AML3), the principal osteogenic master gene for bone formation, as well as homeodomain proteins and osterix. Runx2 has multiple regulatory activities, including activation or repression of gene expression, and integration of biological signals from developmental cues, such as BMP/TGFbeta, Wnt and Src signaling pathways. Runx2 provides a new paradigm for transcriptional control by functioning as a principal scaffolding protein in nuclear microenvironments to control gene expression in response to physiologic signals (growth factors, cytokines and hormones). The protein serves as a hub for the coordination of activities essential for the expansion and differentiation of osteogenic lineage cells through the formation of co-regulatory protein complexes organized in subnuclear domains. Mechanisms by which Runx2 supports commitment to osteogenesis and determines cell fate involve its retention on mitotic chromosomes. Disruption of a unique protein module, the subnuclear targeting signal of Runx2, has profound effects on osteoblast differentiation and metastasis of cancer cells in the bone microenvironment. Runx2 target genes include regulators of cell growth control, components of the bone extracellular matrix, angiogenesis, and signaling proteins for development of the osteoblast phenotype and bone turnover. The specificity of Runx2 regulatory activities provides a basis for novel therapeutic strategies to correct bone disorders.

Zebrafish furin mutants reveal intricacies in regulating Endothelin1 signaling in craniofacial patterning

Endothelin1 (Edn1) signaling promotes ventral character to the facial skeleton. In zebrafish edn1 mutants, the ventral jaw structures are severely reduced and fused to their dorsal counterparts, with a loss of joints that normally form at an intermediate dorsal-ventral position. Loss of function at another locus, sturgeon, also yields joint losses, but only mild reductions in the ventral jaw structures. We show that sturgeon encodes one of two orthologs of Furin present in zebrafish, and that both furin genes may function partially redundantly to activate Edn1 signaling. Supporting this hypothesis, early expression of edn1-dependent genes is downregulated in sturgeon (furinA) mutants. Later in development, expression of most of these genes recovers to near wild-type levels in furinA mutants but not in edn1 mutants. The recovery explains the less severe furinA mutant skeletal phenotype and suggests that late gene expression is dependent on a critical level of Edn1 signaling not present in the more severe edn1 mutants. However, expression defects in the intermediate joint-forming domains in both mutants persist, explaining the joint losses observed later in both mutants. We further show that in both mutants the arches fail to correctly undergo ventral elongation before skeletogenesis begins and propose a model in which this failure is largely responsible for the loss of an Edn1-dependent compartmentation of the arch into the intermediate and ventral domains.