Distal-less and other homeobox genes in the development of the dentition

BMP Signaling Pathway in Dentin Development and Diseases

BMP signaling plays an important role in dentin development. BMPs and antagonists regulate odontoblast differentiation and downstream gene expression via canonical Smad and non-canonical Smad signaling pathways. The interaction of BMPs with their receptors leads to the formation of complexes and the transduction of signals to the canonical Smad signaling pathway (for example, BMP ligands, receptors, and Smads) and the non-canonical Smad signaling pathway (for example, MAPKs, p38, Erk, JNK, and PI3K/Akt) to regulate dental mesenchymal stem cell/progenitor proliferation and differentiation during dentin development and homeostasis. Both the canonical Smad and non-canonical Smad signaling pathways converge at transcription factors, such as Dlx3, Osx, Runx2, and others, to promote the differentiation of dental pulp mesenchymal cells into odontoblasts and downregulated gene expressions, such as those of DSPP and DMP1. Dysregulated BMP signaling causes a number of tooth disorders in humans. Mutation or knockout of BMP signaling-associated genes in mice results in dentin defects which enable a better understanding of the BMP signaling networks underlying odontoblast differentiation and dentin formation. This review summarizes the recent advances in our understanding of BMP signaling in odontoblast differentiation and dentin formation. It includes discussion of the expression of BMPs, their receptors, and the implicated downstream genes during dentinogenesis. In addition, the structures of BMPs, BMP receptors, antagonists, and dysregulation of BMP signaling pathways associated with dentin defects are described.

Msx and dlx homeogene expression in epithelial odontogenic tumors

Epithelial odontogenic tumors are rare jaw pathologies that raise clinical diagnosis and prognosis dilemmas notably between ameloblastomas and clear cell odontogenic carcinomas (CCOCs). In line with previous studies, the molecular determinants of tooth development-amelogenin, Msx1, Msx2, Dlx2, Dlx3, Bmp2, and Bmp4-were analyzed by RT-PCR, ISH, and immunolabeling in 12 recurrent ameloblastomas and in one case of CCOC. Although Msx1 expression imitates normal cell differentiation in these tumors, other genes showed a distinct pattern depending on the type of tumor and the tissue involved. In benign ameloblastomas, ISH localized Dlx3 transcripts and inconstantly detected Msx2 transcripts in epithelial cells. In the CCOC, ISH established a lack of both Dlx3 and Msx2 transcripts but allowed identification of the antisense transcript of Msx1, which imitates the same scheme of distribution between mesenchyme and epithelium as in the cup stage of tooth development. Furthermore, while exploring the expression pattern of signal molecules by RT-PCR, Bmp2 was shown to be completely inactivated in the CCOC and irregularly noticeable in ameloblastomas. Bmp4 was always expressed in all the tumors. Based on the established roles of Msx and Dlx transcription factors in dental cell fates, these data suggest that their altered expression is a proposed trail to explain the genesis and/or the progression of odontogenic tumors.

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.

DLX3 c.561_562delCT mutation causes attenuated phenotype of tricho-dento-osseous syndrome

The distal-less homeobox gene DLX3 is expressed in a variety of tissues including placenta, skin, hair, teeth, and bone. Mutation of DLX3 (c.571_574delGGGG) causes the tricho-dento-osseous syndrome (TDO), characterized by abnormal hair, teeth, and bone. Evaluation of a kindred segregating the DLX3 c.561_562delCT mutation revealed distinct changes in the hair, teeth, and bones as has been observed with the DLX3 c.571_574delGGGG mutation. Previously, the DLX3 c.561_562delCT mutation was associated with autosomal dominant amelogenesis imperfecta with taurodontism. The present study shows that the DLX3 c.560_561delCT mutation causes an attenuated TDO phenotype with less severe hair, tooth, and bone manifestations compared with individuals having the DLX3 c.571_574delGGGG mutation. Careful phenotyping of individuals with allelic DLX3 mutations reveals marked differences in phenotypic severity indicating that the carboxy-terminus of the DLX3 protein is critical in determining its function during development in these different tissues.

[Msx1 and its influence on craniofacial growth]

Many genes that interact in a complex and interdependent manner participate in the development of the craniofacial complex. One of them, the Msxl homeobox gene, a transcription factor, is expressed from early developmental stages to adulthood in accordance with specific spatio-temporal patterns. When it is suppressed, transgenic mice exhibit craniofacial abnormalities that demonstrate what is its function in normal growth, just as it has been shown that certain Msxl mutations in humans are commonly associated with tooth agenesis.

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 of thedlx gene family during formation of the cranial bones in the zebrafish (Danio rerio): Differential involvement in the visceral skeleton and braincase

We have used dlx genes to test the hypothesis of a separate developmental program for dermal and cartilage bones within the neuro- and splanchnocranium by comparing expression patterns of all eight dlx genes during cranial bone formation in zebrafish from 1 day postfertilization (dPF) to 15 dPF. dlx genes are expressed in the visceral skeleton but not during the formation of dermal or cartilage bones of the braincase. The spatiotemporal expression pattern of all the members of the dlx gene family, support the view that dlx genes impart cellular identity to the different arches, required to make arch-specific dermal bones. Expression patterns seemingly associated with cartilage (perichondral) bones of the arches, in contrast, are probably related to ongoing differentiation of the underlying cartilage rather than with differentiation of perichondral bones themselves. Whether dlx genes originally functioned in the visceral skeleton only, and whether their involvement in the formation of neurocranial bones (as in mammals) is secondary, awaits clarification.

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.

Increased bone density associated with DLX3 mutation in the tricho-dento-osseous syndrome

Tricho-dento-osseous syndrome (TDO) (OMIM #190320) is an autosomal dominant disorder characterized and named for the three most commonly affected tissues hair, teeth, and bones. Common to all individuals with TDO studied to date is a four base-pair deletion in the DLX3 gene on chromosome 17q21. This mutation is associated with a variable bone phenotype that includes alteration in intramembranous bone formation in the skull. The purpose of this study was to characterize and compare endochondral bone phenotypes and variability at central and peripheral locations of the skeleton by evaluating bone density in individuals having the same DLX3, 4 bp DEL,NT3198 mutation (OMIM 600525) and non-affected family members using dual-energy x-ray absorptiometry (DEXA). Thirty-four individuals (20 TDO-affected and 14 non-affected) participated in this prospective study. All participants were evaluated for the DLX3 mutation associated with TDO. All subjects received DEXA scans at common, literature-supported osteoporotic test regions including: (1) non-dominant distal radius/ulna, (2) femoral neck, and (3) lumbar spine L2-4. There was a significant increase (P < 0.05) in bone mineral density in TDO-affected individuals compared with control individuals at each test region. The markedly increased bone density in individuals having the DLX3, 4 bp DEL,NT3198 mutation shows that this alteration affects both endochondral and intramembranous bone formation and suggests that the DLX3 gene is important in bone formation and/or homeostasis of the appendicular 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.

Dlx genes integrate positive and negative signals during feather bud development

In the embryonic chicken skin, feather buds and the intervening interbud tissue form in a reiterated and sequential pattern that is dependent on interactions between the epidermis and dermis. Feather promoting and inhibiting signals such as fibroblast growth factors (FGF) and bone morphogenetic proteins (BMP), respectively, direct the formation of this periodic pattern. However, the transcription factors that mediate the response to these signals and transmit this information to downstream effector genes are largely unknown. Here we have explored the DLX transcription factors as candidate transcriptional mediators downstream of the described feather patterning signals. We show that several Dlx members are expressed in the dermis and epidermis of the developing feather buds and their expression is induced in embryonic chick skin by the ectopic activation of BMP and FGF signaling. Misexpression of Dlx in the chick skin leads to both feather loss and feather bud fusions, suggesting that DLX proteins play a negative as well as a positive role in feather development. Moreover, DLX regulates the expression of NCAM and tenascin, molecules that are important for feather bud initiation as well as bud outgrowth and morphogenesis. Our results suggest that DLX transcription factors serve to integrate and transduce feather patterning signals to downstream effector molecules.

Developmental functions of theDistal-less/Dlx homeobox genes

Distal-less is the earliest known gene specifically expressed in developing insect limbs; its expression is maintained throughout limb development. The homeodomain transcription factor encoded by Distal-less is required for the elaboration of proximodistal pattern elements in Drosophila limbs and can initiate proximodistal axis formation when expressed ectopically. Distal-less homologs, the Dlx genes, are expressed in developing appendages in at least six phyla, including chordates, consistent with requirements for Dlx function in normal appendage development across the animal kingdom. Recent work implicates the Dlx genes of vertebrates in a variety of other developmental processes ranging from neurogenesis to hematopoiesis. We review what is known about the invertebrate and vertebrate Dll/Dlx genes and their varied roles during development. We propose revising the vertebrate nomenclature to reflect phylogenetic relationships among the Dlx genes.

Cranial neural crest-cell migration in the direct-developing frog, Eleutherodactylus coqui: molecular heterogeneity within and among migratory streams

Direct development is a specialized reproductive mode that has evolved repeatedly in many different lineages of amphibians, especially anurans. A fully formed, albeit miniature adult hatches directly from the egg; there is no free-living larva. In many groups, the evolution of direct development has had profound consequences for cranial development and morphology, including many components that are derived from the embryonic neural crest. Yet, the developmental bases of these effects remain poorly known. In order to more fully characterize these changes, we used three molecular markers to analyze cranial neural crest-cell emergence and migration in the direct-developing frog, Eleutherodactylus coqui: HNK-1 immunoreactivity, Dlx protein expression, and cholinesterase activity. Our study validates and extends earlier results showing that the comprehensive changes in embryonic cranial patterning, differentiation, and developmental timing that are associated with direct development in Eleutherodactylus have not affected gross features of cranial neural crest biology: the relative timing of crest emergence and the number, configuration and identity of the principal migratory streams closely resemble those seen in metamorphic anurans. The three markers are variably expressed within and among neural crest-cell populations. This variation suggests that determination of cranial neural crest-cells may already have begun at or soon after the onset of migration, when the cells emerge from the neural tube. It is not known how or even if this variation correlates with differential cell lineage or fate. Finally, although HNK-1 expression is widely used to study neural crest migration in teleost fishes and amniotes, E. coqui is the only amphibian known in which it effectively labels migrating neural crest-cells. There are not enough comparative data to determine whether this feature is functionally associated with direct development or is instead unrelated to reproductive mode.

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.

Key regulatory transcription factors involved in placental trophoblast development--a review

Specification of the trophoblast cell lineage comprising the outermost epithelial cell layer of the blastocyst occurs early in development and is a prerequisite for implantation of the embryo and subsequent formation of the placenta, a multifunctional organ which is indispensable for the proper development of the fetus. Trophoblast stem cells of the placenta give rise to distinct highly differentiated trophoblast subtypes which build the functional units of the organ. These specialized cells assure anchorage of the embryo to the mother, establishing a vascular connection transporting nutrients and gases and expression of hormones that are required for the successful progression of pregnancy. Developmental processes of the trophoblast occur in a spatially and temporally highly organized manner. Despite these facts, little is known on the key regulatory factors which commit and differentiate trophoblast cells in humans. Recent studies in mice, however, provided evidence that various cell-type specific transcription factors play crucial roles in the developmental programme of the trophoblast. In this review we will focus on the function of these major regulatory factors in murine trophoblast/placental development and discuss the potential role of their homologues in the human system.

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.

Biomineralization, life-time of odontogenic cells and differential expression of the two homeobox genes MSX-1 and DLX-2 in transgenic mice

Msx and Dlx homeobox genes encode for transcription factors that control early morphogenesis. More specifically, Msx-1, Msx-2, and Dlx-2 homeobox genes contribute to the initial patterning of the dentition. The present study is devoted to the potential role of those homeobox genes during the late formation of mineralized tissues, using the rodent incisor as an experimental system. The continuously erupting mandibular incisor allows (1) the coinvestigation of the whole sequences of amelogenesis and dentinogenesis, aligned along the main dental axis in a single sample in situ and (2) the differential characterization of transcripts generated by epithelial and ectomesenchymal odontogenic cells. Northern blot experiments on microdissected cells showed the continuing expression of Msx-2 and Dlx-2 in the later stages of dental biomineralization, differentially in epithelial and ectomesenchymal compartments. Transgenic mice produced with LacZ reporter constructs for Dlx-2 and Msx-1 were used to detect different components of the gene expression patterns with the sensitive beta-galactosidase histoenzymology. The results show a prominent epithelial involvement of Dlx-2, with stage-specific variations in the cells involved in enamel formation. Quantitative analyses identified specific modulations of Dlx-2 expression in ameloblasts depending on the anatomical sites of the incisor, showing more specifically an inverse linear relationship between the Dlx-2 promoter activity level and enamel thickness. This investigation extends the role of homeoproteins to postmitotic stages, which would control secretory cell activity, in a site-specific manner as shown here for Dlx-2.

The homeobox genes MSX2 and MOX2 are candidates for regulating epithelial-mesenchymal cell interactions in the human placenta

Homeobox genes of the Msx and Mox families are coexpressed in the vertebrate embryo in regions of epithelial-mesenchymal interactions. Here we show that a member of each family is expressed in extra-embryonic structures where epithelial and mesenchymal cell layers contact. In situ hybridization studies on first trimester human placental sections reveal that MSX2 and MOX2 are expressed predominantly in the cytotrophoblast cell layer. In term placenta, MSX2 and MOX2 are expressed in the syncytiotrophoblast. This is the first study to describe the expression of MOX2 in human tissues and to show that members of the Msx and Mox families of homeobox genes are expressed where epithelial and mesenchymal cell layers contact in the human placenta. A combinatorial code of homeobox genes that includes members of the Msx, Mox and Dlx families has been predicted to regulate epithelial-mesenchymal cell interactions in the vertebrate embryo. We have shown that MSX2, MOX2, DLX4 and the HB24 homeobox gene are expressed in the epithelial and mesenchymal cell types that form the placenta. We predict that this combination of homeobox genes is involved in regulating epithelial-mesenchymal cell interactions in extraembryonic tissues.

Epithelial Dlx-2 homeogene expression and cementogenesis

The Dlx-2 (distal-less gene) homeoprotein transcription factor controls early tooth development but has not been studied during the late stages of biomineralization. Transgenic mice containing a Dlx-2/LacZ reporter construct were used to map the Dlx-2 expression pattern in cementoblasts, the dental cells most closely related to bone cells and therefore suggested to be uniquely positioned osteoblasts. During initial root formation, marked expression of Dlx-2 was evident in molar and incisor root epithelium, whereas dental papilla and follicle were negative. Dlx-2 was expressed in this epithelium from the apical loop to the area of its disruption. During acellular cementum formation in both incisors and molars, Dlx-2 expression was observed in the majority of differentiated cementoblasts from the apical region to the erupting zones. During cellular cementum formation, the presence of which characterizes growth-limited molars, Dlx-2 expression was restricted to the innermost cementoblasts and entrapped cementocytes. These data further support the hypothesis of a complex origin and fate of cementum-forming cells, as previously suggested by the expression patterns of a set of mesenchymal and epithelial markers, notably ameloblastin as shown here. Dlx-2 expression might constitute a landmark of cementoblast subpopulations of epithelial origin. (J Histochem Cytochem 48:277-283, 2000)

Expression of Distal-less in molluscan eggs, embryos, and larvae

Distal-less (Dll) is best known as a transcription factor involved with "limb patterning" in Drosophila melanogaster. Observations of both deuterostome and protostome phyla have led to the suggestion that some aspect of this gene's function in "appendage" or proximal-distal "outgrowth" development is conserved. Here we explore the possibility of other conserved roles operating earlier in development. We examine the expression of DLL protein during the early development of two molluscan classes, Polyplacophora (chiton) and Gastropoda (snail). Using an antibody approach, we find DLL expression in the oocytes of a chiton (Mopalia muscosa) and in the pregastrulae through early veliger larvae of a marine snail (Kelletia kelletii). We observe antibody localization in the oocyte, nuclear expression in all cells of the pregastrulae, and predominant expression in the ectoderm of postgastrulae and early veliger larvae. Comparison of our observations on spiralian taxa, thought to have conservative development with previous work, primarily on deuterostomes, suggests the possibility of an ancient role(s) for DLL in early development. Possible functions appear to include maternal and zygotic involvement in the establishment of embryonic polarity, involvement in the process of germ layer formation, and a role in the specification and/or differentiation of ectoderm/epithelia. We note that the exploration of conserved gene function in early development may be clarified by examining taxa whose early development has putatively not been subject to dramatic evolutionary change.

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.

Dynamic interactions and the evolutionary genetics of dental patterning

The mammalian dentition is a segmental, or periodically arranged, organ system whose components are arrayed in specific number and in regionally differentiated locations along the linear axes of the jaws. This arrangement evolved from simpler dentitions comprised of many single-cusp teeth of relatively indeterminate number. The different types of mammalian teeth have subsequently evolved as largely independent units. The experimentally documented developmental autonomy of dental primordia shows that the basic dental pattern is established early in embryogenesis. An understanding of how genetic patterning processes may work must be consistent with the different modes of development, and partially independent evolution, of the upper and lower dentition in mammals. The periodic nature of the location, number, and morphological structure of teeth suggests that processes involving the quantitative interaction of diffusible signaling factors may be involved. Several extracellular signaling molecules and their interactions have been identified that may be responsible for locating teeth along the jaws and for the formation of the incisor field. Similarly, the wavelike expression of signaling factors within developing teeth suggests that dynamic interactions among those factors may be responsible for crown patterns. These factors seem to be similar among different tooth types, but the extent to which crown differences can be explained strictly in terms of variation in the parameters of interactions among the same genes, as opposed to tooth-type-specific combinatorial codes of gene expression, is not yet known. There is evidence that combinatorial expression of intracellular transcription factors, including homeobox gene families, may establish domains within the jaws in which different tooth types are able to develop. An evolutionary perspective can be important for our understanding of dental patterning and the designing of appropriate experimental approaches, but dental patterns also raise basic unresolved questions about the nature of the evolutionary assumptions made in developmental genetics.

A distal-less class homeobox gene, DLX4, is a candidate for regulating epithelial-mesenchymal cell interactions in the human placenta

Homeobox genes of the Distal-less (Dlx) family are expressed in the vertebrate embryo in regions where epithelial cell layers contact adjacent mesenchymal cells. This study shows that the human Dlx family member, DLX4, is expressed in the placenta, primarily in regions where epithelial and mesenchymal cell layers contact. In situ hybridization studies at first trimester human placental sections revealed that DLX4 was expressed predominantly in the cytotrophoblast stem cell layer. In term placenta, DLX4 was expressed in the syncytiotrophoblast. Northern analysis revealed two DLX4 transcripts in first trimester placenta of 2.8 and 3.0 kb. Elevated levels of DLX4 mRNA were detected in a choriocarcinoma derived cell line when compared with a cytotrophoblast cell line and normal placenta. This is the first study to show that a member of the Dlx family of homeobox genes is expressed in regions of epithelial and mesenchymal cell layer contact in the human. Accumulated evidence from several studies suggest that a combinatorial code of homeobox genes is required to regulate epithelial-mesenchymal cell interactions in the vertebrate embryo. It is predicted that a similar combination of homeobox genes, that includes DLX4, is involved in regulating epithelial-mesenchymal cell interactions in extraembryonic tissues. DLX4 may also have a role in the regulation of the genes important for trophoblast invasion since the level of expression in trophoblast cell lines reflects invasive potential.

Cloning and mapping of the MEIS1 gene, the human homolog of a murine leukemogenic gene

The mammalian homeobox genes encode a family of transcription factors that are important in a wide range of cellular processes, including hematopoiesis. Aberrant expression of some homeobox genes is known to be oncogenic. We report the cloning and initial characterization of a human homeobox gene, MEIS1, identified in a survey of homeobox genes expressed in the human fetal liver. The complete cDNA sequence shows that MEIS1 is likely to be the human homolog of Meis1, a mouse gene that is known to be activated in myeloid leukemia by retroviral insertion. We found that the MEIS1 gene is expressed at low levels in normal immunohematopoietic tissues, including the fetal liver. However, consistent with its possible role in myeloid leukemogenesis, MEIS1 was expressed in a subset of myeloid leukemia cell lines, with the highest expression seen in those with a megakaryocytic-erythroid phenotype. The gene is also expressed at high levels in the cerebellum. The gene is located on human chromosome region 2p13-p14 and contains the previously identified anonymous markers D2S134 and NIB1519. Whether this gene, which is leukemogenic in mice, also plays a leukemogenic role in humans will require further study.

Isolation and identification of homeobox genes from the human placenta including a novel member of the Distal-less family, DLX4

We have carried out a DNA binding site screen of a 32-week human placental cDNA library using a consensus homeodomain binding site as a probe. This study represents the first library screen carried out to isolate homeobox genes from the human placenta. We have shown that three homeobox genes known to be expressed in the embryo, HB24, GAX and MSX2 are also expressed in the placenta. We have also identified a novel homeobox gene, DLX4, that shows 85% sequence identity with the homeodomain encoded by the Drosophila Distal-less (Dll) gene. DLX4 therefore represents a new member of the Distal-less family of homeobox genes. This is the first evidence that members of the Distal-less family of homeobox genes are expressed in the placenta. Using fluorescence in situ hybridisation (FISH), DLX4 has been assigned to human chromosome 17q21-q22. This places DLX4 in the same region of chromosome 17 as another member of the Distal-less family, DLX3 (Scherer et al., 1995), and the HOX-B homeobox gene cluster (Acampora et al., 1989: Boncinelli et al., 1991). Members of the Distal-less family (DLX1 and DLX2; DLX5 and DLX6) are found as closely linked pairs on human chromosomes (Simeone et al., 1994). We predict that DLX3 and DLX4 are closely linked and have arisen through gene duplication and divergence from a common ancestral precursor.

Salivary glands: a paradigm for diversity of gland development

The major salivary glands of mammals are represented by three pairs of organs that cooperate functionally to produce saliva for the oral cavity. While each type of gland produces a signature secretion that complements the secretions from the other glands, there is also redundancy as evidenced by secretion of functionally similar and, in some cases, identical products in the three glands. This, along with their common late initiation of development, in fetal terms, their similarities in developmental pattern, and their proximate sites of origin, suggests that a common regulatory cascade may have been shared until shortly before the onset of overt gland development. Furthermore, occasional ectopic differentiation of individual mature secretory cells in the "wrong" gland suggests that control mechanisms responsible for the distinctive cellular composition of each gland also share many common steps, with only minor differences providing the impetus for diversification. To begin to address this area, we examine here the origins of the salivary glands by reviewing the expression patterns of several genes with known morphogenetic potential that may be involved based on developmental timing and location. The possibility that factors leading to determination of the sites of mammalian salivary gland development might be homologous to the regulatory cascade leading to salivary gland formation in Drosophila is also evaluated. In a subsequent section, cellular phenotypes of neonatal and adult glands are compared and evaluated for insights into the mechanisms and lineages leading to cellular diversification. Finally, the phenomena of proliferation, repair, and regeneration in adult salivary glands are reviewed, with emphasis on the extent to which the cellular diversity is reversible and which cell type other than stem cells has the ability to redifferentiate into other cell types.

Checklist: vertebrate homeobox genes

Up to now around 170 different homeobox genes have been cloned from vertebrate genomes. A compilation of the various isolates from mouse, chick, frog, fish and man is presented in the form of a concise checklist, including the designations from the original publications. Putative homologs from different species are aligned, and key characteristics of embryonic or adult expression domains, as well as mutant phenotypes are briefly indicated.

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

The dentition is a segmental system whose evolution and morphology bears analogy to the evolution of segmentation in the vertebral column and limb. Combinatorial expression of members of the large "Hox" class of homeobox regulatory genes has been shown to play an important role in positional specification in these skeletal systems. This raises the possibility that homeobox genes are also used for positional specification in the dentition, and several homeobox genes are known to be expressed in developing teeth. To identify additional dentally expressed homeobox genes, cDNA from from murine tooth germs at 9.5, 14.5, and 17.5 days gestational age was amplified by PCR using sets of degenerate primers to the homeodomains of 18 different classes of homeobox genes. Amplification products were cloned and sequenced and compared to known gene sequences. To date this approach has confirmed the presence of Msx1, Msx2, Dlx1, and Dlx2, and identified several other homeobox genes not previously known to be expressed in teeth: Dbx, MHox, and Mox2A, plus an a additional Dlx gene, Dlx7. The Msx and Dlx genes are the best current candidates for a combinatorial mechanism that controls the differentiation of structures within and between teeth, and perhaps also the evolution of those structures.