Dlx and other homeobox genes in the morphological development of the dentition

Single-cell RNA analysis of chromodomain-encoding genes in mesenchymal stromal cells of the mouse dental pulp

The chromodomain helicase DNA-binding (CHD) and chromobox (CBX) families of proteins play crucial roles in cell fate decisions, differentiation, and cell proliferation in a broad variety of tissues and cell types. CHD proteins are ATP-dependent epigenetic enzymes actively engaged in transcriptional regulation, DNA replication, and DNA damage repair, whereas CBX proteins are transcriptional repressors mainly involved in the formation of heterochromatin. The pleiotropic effects of CHD and CBX proteins are largely dependent on their versatility to interact with other key components of the epigenetic and transcriptional machinery. Although the function and regulatory modes of CHD and CBX factors are well established in many cell types, little is known about their roles during osteogenic differentiation. A single-cell RNA-sequencing analysis of the mouse incisor dental pulp revealed distinct spatiotemporal expression patterns of CHD- and CBX-encoding genes within different clusters of mesenchymal stromal cells (MSCs) representing various stages of osteogenic differentiation. Additionally, genes encoding interaction partners of CHD and CBX proteins, such as subunits of the trithorax-COMPASS and polycomb chromatin remodeling complexes, exhibited differential co-expression behaviors within MSC subpopulations. Thus, CHD- and CBX-encoding genes show partially overlapping but distinct expression patterns in MSCs, suggesting their differential roles in osteogenic cell fate decisions.

Differential DNA Methylation Landscape in Skin Fibroblasts from African Americans with Systemic Sclerosis

The etiology and reasons underlying the ethnic disparities in systemic sclerosis (SSc) remain unknown. African Americans are disproportionally affected by SSc and yet are underrepresented in research. The aim of this study was to comprehensively investigate the association of DNA methylation levels with SSc in dermal fibroblasts from patients of African ancestry. Reduced representation bisulfite sequencing (RRBS) was performed on primary dermal fibroblasts from 15 SSc patients and 15 controls of African ancestry, and over 3.8 million CpG sites were tested for differential methylation patterns between cases and controls. The dermal fibroblasts from African American patients exhibited widespread reduced DNA methylation. Differentially methylated CpG sites were most enriched in introns and intergenic regions while depleted in 5' UTR, promoters, and CpG islands. Seventeen genes and eleven promoters showed significant differential methylation, mostly in non-coding RNA genes and pseudogenes. Gene set enrichment analysis (GSEA) and gene ontology (GO) analyses revealed an enrichment of pathways related to interferon signaling and mesenchymal differentiation. The hypomethylation of DLX5 and TMEM140 was accompanied by these genes' overexpression in patients but underexpression for lncRNA MGC12916. These data show that differential methylation occurs in dermal fibroblasts from African American patients with SSc and identifies novel coding and non-coding genes.

Distal-less homeobox 5 is a master regulator of the osteogenesis of human mesenchymal stem cells

Mesenchymal stem cells (MSCs) differentiate into multiple lineages and are a promising source of cells for clinical use. Previously, we found that the gene distal-less homeobox 5 (DLX5) is specifically expressed in MSCs with osteogenic potential. Understanding the mechanism of osteogenesis is necessary for successful bone regeneration using MSCs. The aim of this study was to examine the function of the DLX5 gene in MSCs during osteogenesis (bone development). We analyzed the possible association between DLX5 expression and osteogenesis-, chondrogenesis- and adipogenesis-related gene expression in different cells isolated from bone marrow and cord blood. Differentiation capacity was assessed by observing morphological changes, monitoring gene expression patterns, and staining with Von Kossa, safranin O, and Oil Red O. Suppression of DLX5 expression by means of a small interfering RNA (siRNA) downregulated osteogenic markers and reduced the signs of calcium mineralization. Tanshinone IIA is a known small molecule activator of bone morphogenetic protein (BMP) signaling. Here, we report that induction of DLX5 by tanshinone IIA in MSCs enhanced osteogenic differentiation. In addition, we showed that tanshinone IIA (as a mediator of BMP2 signaling) activates runt-related transcription factor 2 (RUNX2) in MSCs and initiates calcium mineralization during osteogenesis. Taken together, these findings indicate that, in MSCs, DLX5 is a master regulator of osteogenesis. Furthermore, tanshinone IIA may be valuable for stem cell-based therapies of certain bone diseases.

p75 neurotrophin receptor regulates differential mineralization of rat ectomesenchymal stem cells

OBJECTIVES

The aim of this study was to investigate whether p75NTR (p75 neurotrophin receptor) regulates differential mineralization capacity of rEMSCs (rat ectomesenchymal stem cells) and underlying mechanisms associated with Mage-D1 (melanoma-associated antigens-D1).

MATERIALS AND METHODS

Immunohistochemical staining of p75NTR in developing tooth germs was performed on E12.5d (embryonic 12.5 days) and E19.5d (embryonic 19.5 days). E12.5d EMSCs and E19.5d EMSCs were isolated in the same pregnant Sprague-Dawley rats from embryonic maxillofacial processes and tooth germs. p75NTR small-interfering RNA, p75NTR overexpression plasmid, Mage-D1 small-interfering RNA and recombined rat NGF were used to transfect cells.

RESULTS

p75NTR was expressed in epithelial-mesenchymal interaction areas at E12.5d and E19.5d tooth germ development stages. E19.5d EMSCs had higher p75NTR expression levels and differential mineralization capacity but lower levels of cell proliferation. Under induction by mineralized culture medium, the potential of differential mineralization had identical trends in regulation of p75NTR in EMSCs; Mage-D1 did not fluctuate and TrkA was not expressed. Binding of p75NTR and Mage-D1 were detected. Mage-D1 knockdown significantly down-regulated expression of related genes, which NGF could not rescue.

CONCLUSION

p75NTR participated in tooth germ development stages and mediated differential mineralization of EMSCs. p75NTR played a critical role in regulating the potential of differential mineralization of EMSCs. Mage-D1 seemed to act as a bridge in the underlying mechanism of effects of p75NTR.

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.

The Prx1 homeobox gene is critical for molar tooth morphogenesis

The paired-related homeobox genes, Prx1 and Prx2, encode transcription factors critical for orofacial development. Prx1(-/-)/Prx2(-/-) neonates have mandibular hypoplasia and malformed mandibular incisors. Although the mandibular incisor phenotype has been briefly described (ten Berge et al., 1998, 2001; Lu et al., 1999), very little is known about the role of Prx proteins during tooth morphogenesis. Since the posterior mandibular region was relatively normal, we examined molar tooth development in Prx1(-/-)/Prx2(-/-) embryos to determine whether the tooth malformation is primary to the loss of Prx protein or secondary to defects in surrounding tissues. Three-dimensional (3D) morphological reconstructions demonstrated that Prx1(-/-)/Prx2(-/-) embryos had molar malformations, including cuspal changes and ectopic epithelial projections. Although we demonstrate that Prx1 protein is expressed only mesenchymally, 3D reconstructions showed important morphological defects in epithelial tissues at the cap and bell stages. Analysis of these data suggests that the Prx homeoproteins are critical for mesenchymal-epithelial signaling during tooth morphogenesis.

Expression of Dlx genes during the development of the zebrafish pharyngeal dentition: evolutionary implications

In order to investigate similarities and differences in genetic control of development among teeth within and between species, we determined the expression pattern of all eight Dlx genes of the zebrafish during development of the pharyngeal dentition and compared these data with that reported for mouse molar tooth development. We found that (i) dlx1a and dlx6a are not expressed in teeth, in contrast to their murine orthologs, Dlx1 and Dlx6; (ii) the expression of the six other zebrafish Dlx genes overlaps in time and space, particularly during early morphogenesis; (iii) teeth in different locations and generations within the zebrafish dentition differ in the number of genes expressed; (iv) expression similarities and differences between zebrafish Dlx genes do not clearly follow phylogenetic and linkage relationships; and (v) similarities and differences exist in the expression of zebrafish and mouse Dlx orthologs. Taken together, these results indicate that the Dlx gene family, despite having been involved in vertebrate tooth development for over 400 million years, has undergone extensive diversification of expression of individual genes both within and between dentitions. The latter type of difference may reflect the highly specialized dentition of the mouse relative to that of the zebrafish, and/or genome duplication in the zebrafish lineage facilitating a redistribution of Dlx gene function during odontogenesis.

Expression pattern of Dlx3 during cell differentiation in mineralized tissues

The present study was designed to compare the expression pattern of Dlx3 in four different mineralized tissues because of: 1-its role in skeleton patterning, 2-its expression in dental epithelium and mesenchyme during morphogenesis, 3-the membranous and endochondral bone and tooth phenotype of tricho-dento-osseous syndrome related to Dlx3 gene mutation and 4-recently emerging knowledge on Dlx family members in the bone field. Ameloblasts, odontoblasts, osteoblasts and chondrocytes were analyzed in vitro and in vivo. Dlx3 transcripts were detected by RT-PCR in established model systems (microdissected dental epithelium and mesenchyme; primary cultures of rat chondrocytes), as recently performed in osteoblasts in vitro. A human 414-bp Dlx3 probe was generated. A 4.5-kb human Dlx3 sense RNA was identified in maxillo-facial samples by Northern blotting. Immunolabeling and in situ hybridization were performed in mice from Theiler stage E 14.5 until birth. In teeth, although Dlx3 was still expressed in differentiated ameloblasts, it was down regulated during odontoblast polarization. During endochondral bone formation, Dlx3 protein was detected in chondrocytes and was most strongly expressed in the prehypertrophic cartilage zone and in differentiating and differentiated osteoblasts of metaphyseal periosteum. In vitro, real-time PCR studies supported this upregulation in prehypertrophic chondrocytes, closely correlated with Ihh variations. In membranous bone, Dlx3 was present in preosteoblasts, osteoblasts and osteoid-osteocytes. The present data on Dlx3 and recently published functional studies show that this transcription factor may be instrumental during growth in the control of matrix deposition and biomineralization in the entire skeleton.

Msx1/Bmp4 genetic pathway regulates mammalian alveolar bone formation via induction of Dlx5 and Cbfa1

In the developing mammalian tooth, the cranial neural crest derived dental mesenchyme consists of the dental papilla and dental follicle. The dental papilla gives rise to odontoblasts and dental pulp and the dental follicle gives rise to the periodontium, including the osteoblasts that contribute to the alveolar process. The alveolar process is a specialized intramembranous bone that forms the primary support structure for the dentition. The Msx1 gene controls many aspects of craniofacial development, as evidenced by craniofacial abnormalities seen in Msx1(-/-) mice, including the arrest of tooth development and the absence of the alveolar bone. Previous studies demonstrated that ectopic expression of Bmp4, a downstream target of Msx1, in the Msx1(-/-) dental mesenchyme rescued alveolar bone formation. Here we confirm an early requirement of BMP activity for alveolar bone formation. We show that the expression of Cbfa1 and Dlx5, two genes encode transcription factors that are critical for bone differentiation, overlaps with that of Msx1 and Bmp4 in the developing tooth and alveolar process. We have demonstrated that Dlx5 and Cbfa1 expression is down-regulated in Msx1(-/-) dental mesenchyme and that Msx1 and Bmp4 expression are unaltered in Cbfa1(-/-) mice. These data place Dlx5 and Cbfa1 downstream from the Msx1/Bmp4 in the genetic pathway that regulates tooth development. Ectopic expression of Bmp4 in Msx1 mutants restores the expression of Dlx5, but not Cbfa1, in the dental mesenchyme, and rescues the expression of both Dlx5 and Cbfa1 in the developing alveolar bone. Therefore, the early expression of Cfba1 in the dental mesenchyme appears dispensable for the development of the alveolar bone. Taken together with in vitro gene induction studies, our results demonstrate that BMP4 controls Dlx5 expression in dental mesenchyme, and functions upstream to both Dlx5 and Cbfa1 to regulate alveolar bone formation during tooth development.

Concerted action of twodlxparalogs in sensory placode formation

Sensory placodes are ectodermal thickenings that give rise to elements of the vertebrate cranial sensory nervous system, including the inner ear and nose. Although mutations have been described in humans, mice and zebrafish that perturb ear and nose development, no mutation is known to prevent sensory placode formation. Thus, it has been postulated that a functional redundancy exists in the genetic mechanisms that govern sensory placode development. We describe a zebrafish deletion mutation, b380, which results in a lack of both otic and olfactory placodes.

The b380 deletion removes several known genes and expressed sequence tags, including dlx3 and dlx7, two transcription factors that share a homoeobox domain similar in sequence to the Drosophila Distal-less gene. dlx3 and dlx7 are expressed in an overlapping pattern in the regions that produce the otic and olfactory placodes in zebrafish. We present evidence suggesting that it is specifically the removal of these two genes that leads to the otic and olfactory phenotype of b380 mutants. Using morpholinos, antisense oligonucleotides that effectively block translation of target genes, we find that functional reduction of both dlx genes contributes to placode loss. Expression patterns of the otic marker pax2.1, olfactory marker anxV and eya1, a marker of both placodes, in morpholino-injected embryos recapitulate the reduced expression of these genes seen in b380 mutants. We also examine expression of dlx3 and dlx7 in the morpholino-injected embryos and present evidence for existence of auto- and cross-regulatory control of expression among these genes.

We demonstrate that dlx3 is necessary and sufficient for proper otic and olfactory placode development. However, our results indicate that dlx3 and dlx7 act in concert and their importance in placode formation is only revealed by inactivating both paralogs.

Genomic structure and functional control of the Dlx3-7 bigene cluster

The Dlx genes are involved in early vertebrate morphogenesis, notably of the head. The six Dlx genes of mammals are arranged in three convergently transcribed bigene clusters. In this study, we examine the regulation of the Dlx3-7 cluster of the mouse. We obtained and sequenced human and mouse P1 clones covering the entire Dlx3-7 cluster. Comparative analysis of the human and mouse sequences revealed several highly conserved noncoding regions within 30 kb of the Dlx3-7-coding regions. These conserved elements were located both 5' of the coding exons of each gene and in the intergenic region 3' of the exons, suggesting that some enhancers might be shared between genes. We also found that the protein sequence of Dlx7 is evolving more rapidly than that of Dlx3. We conducted a functional study of the 79-kb mouse genomic clone to locate cis-element activity able to reproduce the endogenous expression pattern by using transgenic mice. We inserted a lacZ reporter gene into the first exon of the Dlx3 gene by using homologous recombination in yeast. Strong lacZ expression in embryonic (E) stage E9.5 and E10.5 mouse embryos was found in the limb buds and first and second visceral arches, consistent with the endogenous Dlx3 expression pattern. This result shows that the 79-kb region contains the major cis-elements required to direct the endogenous expression of Dlx3 at stage E10.5. To test for enhancer location, we divided the construct in the mid-intergenic region and injected the Dlx3 gene portion. This shortened fragment lacking Dlx7-flanking sequences is able to drive expression in the limb buds but not in the visceral arches. This observation is consistent with a cis-regulatory enhancer-sharing model within the Dlx bigene cluster.

Assessment of gene regulation by bone morphogenetic protein 2 in human marrow stromal cells using gene array technology

Marrow stromal cells can differentiate into osteoblasts, adipocytes, myoblasts, and chondrocytes. Bone morphogenetic protein 2 (BMP-2) is a potent stimulator of osteoblastic differentiation, and identification of the genes regulated by BMP-2 in these cells should provide insight into the mechanism(s) of osteoblastic differentiation. Thus, we used a conditionally immortalized human marrow stromal cell line (hMS) and a gene expression microarray containing probes for a total of 6800 genes to compare gene expression in control and BMP-2-treated cultures. A total of 51 genes showed a consistent change in messenger RNA (mRNA) frequency between two repeat experiments. Seventeen of these genes showed a change in expression of at least 3-fold in BMP-2-treated cultures over control cultures. These included nuclear binding factors (10 genes), signal transduction pathway genes (2 genes), molecular transport (1 gene), cell surface proteins (2 genes) and growth factors (2 genes). Of particular interest were four of the nuclear binding factor genes ID-1, ID-2, ID-3, and ID-4. These encode dominant negative helix-loop-helix (dnHLH) proteins that lack the nuclear binding domain of the basic HLH proteins and thus have no transcriptional activity. They have been implicated in blocking both myogenesis and adipogenesis. Other transcription factors up-regulated at least 3-fold by BMP-2 included Dlx-2, HES-1, STAT1, and JunB. The changes in these nuclear binding factor mRNA levels were confirmed by real-time reverse-transcriptase-polymerase chain reaction (RT-PCR). A further three transcription factors, core binding factor beta (CBFbeta), AREB6, and SOX4, showed changes in expression of between 2- and 3-fold with BMP-2 treatment. In summary, we have used a gene chip microarray to identify a number of BMP-2 responsive genes in hMS cells. Thus, these studies provide potential candidate genes that may induce osteoblastic differentiation or, in the case of the ID proteins, block differentiation along alternate pathways.

Lamprey Dlx genes and early vertebrate evolution

Gnathostome vertebrates have multiple members of the Dlx family of transcription factors that are expressed during the development of several tissues considered to be vertebrate synapomorphies, including the forebrain, cranial neural crest, placodes, and pharyngeal arches. The Dlx gene family thus presents an ideal system in which to examine the relationship between gene duplication and morphological innovation during vertebrate evolution. Toward this end, we have cloned Dlx genes from the lamprey Petromyzon marinus, an agnathan vertebrate that occupies a critical phylogenetic position between cephalochordates and gnathostomes. We have identified four Dlx genes in P. marinus, whose orthology with gnathostome Dlx genes provides a model for how this gene family evolved in the vertebrate lineage. Differential expression of these lamprey Dlx genes in the forebrain, cranial neural crest, pharyngeal arches, and sensory placodes of lamprey embryos provides insight into the developmental evolution of these structures as well as a model of regulatory evolution after Dlx gene duplication events.

Distal-less-related homeobox genes of vertebrates: Evolution, function, and regulation

Homeobox genes of the Distal-less family have been identified in virtually all metazoan groups where they play roles in the ontogeny of these animals. The vertebrate Distal-less related genes (Dlx genes) are thought to have arisen as a result of a tandem gene duplication event followed by a number of larger genomic scale duplications and thus represent an interesting model with which to study the evolution of clustered gene families. Dlx genes are involved in the development of the forebrain, branchial arches, sensory organs, and limbs. Here we describe the current state of knowledge of the Dlx genes in terms of their developmental expression, how this expression is regulated and how the products of these genes function, once expressed. We highlight a number of recent studies that have shed light on the transcriptional regulation of this gene family. These findings have not only contributed to our understanding of the selective pressures involved in the maintenance of familial gene clustering in genomes, but also to our understanding of how genes may diverge in function during the course of evolution as a result of divergence of regulatory mechanisms.Key words: genome, homeodomain, inner ear, olfactory placode, transcription.

Roles for Msx and Dlx homeoproteins in vertebrate development

This review provides a comparative analysis of the expression patterns, functions, and biochemical properties of Msx and Dlx homeobox genes. These comprise multi-gene families that are closely related with respect to sequence features as well as expression patterns during vertebrate development. Thus, members of the Msx and Dlx families are expressed in overlapping, but distinct, patterns and display complementary or antagonistic functions, depending upon the context. A common theme shared among Msx and Dlx genes is that they are required during early, middle, and late phases of development where their differential expression mediates patterning, morphogenesis, and histogenesis of tissues in which they are expressed. With respect to their biochemical properties, Msx proteins function as transcriptional repressors, while Dlx proteins are transcriptional activators. Moreover, their ability to oppose each other's transcriptional actions implies a mechanism underlying their complementary or antagonistic functions during development.

Comparative study of MSX-2, DLX-5, and DLX-7 gene expression during early human tooth development

Msx and Dlx family transcription factors are key elements of craniofacial development and act in specific combinations with growth factors to control the position and shape of various skeletal structures in mice. In humans, the mutations of MSX and DLX genes are associated with specific syndromes, such as tooth agenesis, craniosynostosis, and tricho-dento-osseous syndrome. To establish some relationships between those reported human syndromes, previous experimental data in mice, and the expression patterns of MSX and DLX homeogenes in the human dentition, we investigated MSX-2, DLX-5, and DLX-7 expression patterns and compared them in orofacial tissues of 7.5- to 9-wk-old human embryos by using in situ hybridization. Our data showed that MSX-2 was strongly expressed in the progenitor cells of human orofacial skeletal structures, including mandible and maxilla bones, Meckel's cartilage, and tooth germs, as shown for DLX-5. DLX-7 expression was restricted to the vestibular lamina and, later on, to the vestibular part of dental epithelium. The comparison of MSX-2, DLX-5, and DLX-7 expression patterns during the early stages of development of different human tooth types showed the existence of spatially ordered sequences of homeogene expression along the vestibular/lingual axis of dental epithelium. The expression of MSX-2 in enamel knot, as well as the coincident expression of MSX-2, DLX-5, and DLX-7 in a restricted vestibular area of dental epithelium, suggests the existence of various organizing centers involved in the control of human tooth morphogenesis.

Dlx5 regulates regional development of the branchial arches and sensory capsules

We report the generation and analysis of mice homozygous for a targeted deletion of the Dlx5 homeobox gene. Dlx5 mutant mice have multiple defects in craniofacial structures, including their ears, noses, mandibles and calvaria, and die shortly after birth. A subset (28%) exhibit exencephaly. Ectodermal expression of Dlx5 is required for the development of olfactory and otic placode-derived epithelia and surrounding capsules. The nasal capsules are hypoplastic (e.g. lacking turbinates) and, in most cases, the right side is more severely affected than the left. Dorsal otic vesicle derivatives (e. g. semicircular canals and endolymphatic duct) and the surrounding capsule, are more severely affected than ventral (cochlear) structures. Dlx5 is also required in mandibular arch ectomesenchyme, as the proximal mandibular arch skeleton is dysmorphic. Dlx5 may control craniofacial development in part through the regulation of the goosecoid homeobox gene. goosecoid expression is greatly reduced in Dlx5 mutants, and both goosecoid and Dlx5 mutants share a number of similar craniofacial malformations. Dlx5 may perform a general role in skeletal differentiation, as exemplified by hypomineralization within the calvaria. The distinct focal defects within the branchial arches of the Dlx1, Dlx2 and Dlx5 mutants, along with the nested expression of their RNAs, support a model in which these genes have both redundant and unique functions in the regulation of regional patterning of the craniofacial ectomesenchyme.

Tricho-dento-osseous syndrome and amelogenesis imperfecta with taurodontism are genetically distinct conditions

Amelogenesis imperfecta of the hypomaturation-hypoplasia type with taurodontism (AIHHT) is inherited as a highly penetrant autosomal dominant trait. These dental findings are similar to those of another autosomal dominant condition, the tricho-dento-osseous syndrome (TDO), from which AIHHT differs primarily by lack of changes in the hair and bones. TDO is characterized by a highly variable clinical phenotype. While enamel hypoplasia and taurodontism appear to be present in all TDO cases, non-dental features may be absent, with approximately half of TDO cases losing the kinky/curly hair phenotype seen in infancy by adolescence, and in almost 20% of cases, osseous changes are not evident. The genetic basis for AIHHT is unknown and it has been questioned whether AIHHT and TDO are separate conditions or a spectrum of disease. The genetic basis for TDO has recently been identified as a deletion mutation in the distal-less 3 (DLX3) transcription factor gene. To determine if AIHHT and TDO represent variable expression of a common DLX3 gene mutation, allelic mutations of the DLX3 gene, or mutations in DLX7 (the linked paralogue to DLX3 on chromosome 17), we have performed mutational analysis and sequencing studies of the DLX3 and DLX7 genes in three individuals (two affected and one unaffected) from a family with AIHHT. Results of the analysis demonstrate that AIHHT and TDO are not due to a common DLX3 gene mutation. Sequence analyses of the DLX3 and DLX7 genes suggest AIHHT is not due to genetic mutations or polymorphisms in the exons of these genes. These results suggest that AI-HHT and TDO are two genetically distinct conditions.

Msx1 is required for the induction of Patched by Sonic hedgehog in the mammalian tooth germ

We have used the mouse developing tooth germ as a model system to explore the transmission of Sonic hedgehog (Shh) signal in the induction of Patched (Ptc). In the early developing molar tooth germ, Shh is expressed in the dental epithelium, and the transcripts of Shh downstream target genes Ptc and Gli1 are expressed in dental epithelium as well as adjacent mesenchymal tissue. The homeobox gene Msx1 is also expressed in the dental mesenchyme of the molar tooth germ at this time. We show here that the expression of Ptc, but not Gli1, was downregulated in the dental mesenchyme of Msx1 mutants. In wild-type E11.0 molar tooth mesenchyme SHH-soaked beads induced the expression of Ptc and Gli1. However, in Msx1 mutant dental mesenchyme SHH-soaked beads were able to induce Gli1 but failed to induce Ptc expression, indicating a requirement for Msx1 in the induction of Ptc by SHH. Moreover, we show that another signaling molecule, BMP4, was able to induce Ptc expression in wild-type dental mesenchyme, but induced a distinct expression pattern of Ptc in the Msx1 mutant molar mesenchyme. We conclude that in the context of the tooth germ Msx1 is a component of the Shh signaling pathway that leads to Ptc induction. Our results also suggest that the precise pattern of Ptc expression in the prospective tooth-forming region is controlled and coordinated by at least two inductive signaling pathways.

Odontoblast commitment and differentiation

Histological and cytological organization confer specificity to the odontoblasts. These postmitotic, neural crest derived, polarized cells are aligned in a single layer at the periphery of the dental pulp and secrete the organic components of predentin-dentin. The developmental history of these cells demands a cascade of epigenetic signalling events comprising the acquisition of odontogenic potential by neural crest cells, their patterning in the developing jaws, the initiation of odontogenesis through interaction with the oral epithelium, commitment, and tooth-specific spatial distribution of competent preodontoblasts able to overtly differentiate. Recent experimental investigations are critically summarized, many open questions are stressed, and current hypotheses concerning the control of terminal odontoblast differentiation are outlined.Key words: odontoblast, neural crest, oral ectoderm, differentiation.

Relationship between the genomic organization and the overlapping embryonic expression patterns of the zebrafish dlx genes

To understand the relationship between the expression and the genomic organization of the zebrafish dlx genes, we have determined the genomic structure of the dlx2 and dlx4 loci. This led to the identification of the zebrafish dlx1 and dlx6 genes, which are closely linked to dlx2 and dlx4, respectively. Therefore, the inverted convergent configuration of Dlx genes is conserved among vertebrates. Analysis of the expression patterns of dlx1 and dlx6 showed striking similarities to those of dlx2 and dlx4, respectively, the genes to which they are linked. Furthermore, the expression patterns of dlx3 and dlx7, which likely constitute a third pair of convergently transcribed genes, are indistinguishable. Thus, the overlapping expression patterns of linked Dlx genes during embryonic development suggest that they share cis-acting sequences that control their spatiotemporal expression. The evolutionary conservation of the genomic organization and combinatorial expression of Dlx genes in distantly related vertebrates suggest tight control mechanisms that are essential for their function during development.

Cross-interactions between two members of the Dlx family of homeobox-containing genes during zebrafish development

The Dlx homeobox genes of vertebrates are transcribed in multiple cells of the embryo with overlapping patterns but often with different onsets of expression. Here we describe the interaction between two dlx genes, dlx3 and dlx4, during zebrafish development. The observation that dlx3 expression precedes that of dlx4 in the otic vesicle led us to investigate whether dlx3 had the ability to control expression of dlx4. Truncated versions of dlx3 were overexpressed in zebrafish embryos and the expression patterns of dlx4 were examined later in development. Overexpression of truncated forms of Dlx3 or of a Dlx3-Dlx2 chimera was found to result in perturbations in dlx4 expression. In addition, cotransfection experiments indicated the ability of Dlx3 to activate transcription through a 1.7-kb fragment of the 5 prime flanking region of dlx4. These results suggest that dlx4 is one of the target genes of dlx3 in embryos and that cross-regulatory interactions between Dlx genes may be one of the mechanisms responsible for their overlapping expression.

Spatiotemporal expression of the homeobox gene S8 during mouse tooth development

The murine S8 gene encodes a nuclear homeodomain containing transcription factor that is expressed at sites of epithelial-mesenchymal interactions, including those in cranofacial tissues. The spatiotemporal expression of S8 mRNA was examined in tooth primordia by in situ hybridization. S8 transcripts were found in all stages of tooth development in 13- to 16.5-day-old mouse embryos (E13-E16.5), covering the early bud stage up to the late bell stage. S8 mRNA was found exclusively in the ectomesenchyme and its derivatives that originate from the neural crest: future pulp cells, odontoblast precursors and dental follicle cells. Expression was highest at the late cap and early bud stages and declined at the mid-bell stage, in both first molar and incisor primordia. In E13 jaw explants grown in organ culture for 48 h, S8 mRNA was still present in first and second molar primordia after culture. At E15.5, S8 mRNA was also transiently present in the surrounding osteogenic tissue. It is concluded that the distribution pattern of S8 mRNA during tooth development indicates a role for the gene in defining the identity of dental papilla and follicle cells. It is speculated that the time-restricted expression of S8 in tooth primordia involves establishing the definitive form of the tooth organ.

Patterning of the mammalian dentition in development and evolution

The mammalian dentition is a segmented organ system with shape differences among its serially homologous elements (individual teeth). It is believed to have evolved from simpler precursors with greater similarities in shape among teeth, and a wealth of descriptive data exist on changes to the dentition that have occurred within mammals. Recent progress has been made in determining the genetic basis of the processes that form an individual tooth, but patterning of the dentition as a whole (i.e. the number, location and shape of the teeth) is less well understood. In contrast to similarly organized systems, such as the vertebral column and limb, Hox genes are not involved in specifying differences among elements. Nevertheless, recent work on a variety of systems is providing clues to the transcription factors and extracellular signalling molecules involved.

The evolution of the vertebrate Dlx gene family

The vertebrate Dlx gene family consists of homeobox-containing transcription factors distributed in pairs on the same chromosomes as the Hox genes. To investigate the evolutionary history of Dlx genes, we have cloned five new zebrafish family members and have provided additional sequence information for two mouse genes. Phylogenetic analyses of Dlx gene sequences considered in the context of their chromosomal arrangements suggest that an initial tandem duplication produced a linked pair of Dlx genes after the divergence of chordates and arthropods but prior to the divergence of tunicates and vertebrates. This pair of Dlx genes was then duplicated in the chromosomal events that led to the four clusters of Hox genes characteristic of bony fish and tetrapods. It is possible that a pair of Dlx genes linked to the Hoxc cluster has been lost from mammals. We were unable to distinguish between independent duplication and retention of the ancestral state of bony vertebrates to explain the presence of a greater number of Dlx genes in zebrafish than mammals. Determination of the linkage relationship of these additional zebrafish Dlx genes to Hox clusters should help resolve this issue.