Dlx2 over-expression regulates cell adhesion and mesenchymal condensation in ectomesenchyme

Mesenchymal condensation in tooth development and regeneration: a focus on translational aspects of organogenesis

The teeth are vertebrate-specific, highly specialized organs performing fundamental functions of mastication and speech, the maintenance of which is crucial for orofacial homeostasis and is further linked to systemic health and human psychosocial well-being. However, with limited ability for self-repair, the teeth can often be impaired by traumatic, inflammatory, and progressive insults, leading to high prevalence of tooth loss and defects worldwide. Regenerative medicine holds the promise to achieve physiological restoration of lost or damaged organs, and in particular an evolving framework of developmental engineering has pioneered functional tooth regeneration by harnessing the odontogenic program. As a key event of tooth morphogenesis, mesenchymal condensation dictates dental tissue formation and patterning through cellular self-organization and signaling interaction with the epithelium, which provides a representative to decipher organogenetic mechanisms and can be leveraged for regenerative purposes. In this review, we summarize how mesenchymal condensation spatiotemporally assembles from dental stem cells (DSCs) and sequentially mediates tooth development. We highlight condensation-mimetic engineering efforts and mechanisms based on ex vivo aggregation of DSCs, which have achieved functionally robust and physiologically relevant tooth regeneration after implantation in animals and in humans. The discussion of this aspect will add to the knowledge of development-inspired tissue engineering strategies and will offer benefits to propel clinical organ regeneration.

A Neural Crest-specific Overexpression Mouse Model Reveals the Transcriptional Regulatory Effects of Dlx2 During Maxillary Process Development

Craniofacial morphogenesis is a complex process that requires precise regulation of cell proliferation, migration, and differentiation. Perturbations of this process cause a series of craniofacial deformities. Dlx2 is a critical transcription factor that regulates the development of the first branchial arch. However, the transcriptional regulatory functions of Dlx2 during craniofacial development have been poorly understood due to the lack of animal models in which the Dlx2 level can be precisely modulated. In this study, we constructed a Rosa26 site-directed Dlx2 gene knock-in mouse model Rosa26 ^{CAG-LSL-Dlx2-3xFlag} for conditionally overexpressing Dlx2. By breeding with wnt1 ^cre mice, we obtained wnt1 ^cre ; Rosa26 ^Dlx2/- mice, in which Dlx2 is overexpressed in neural crest lineage at approximately three times the endogenous level. The wnt1 ^cre ; Rosa26 ^Dlx2/- mice exhibited consistent phenotypes that include cleft palate across generations and individual animals. Using this model, we demonstrated that Dlx2 caused cleft palate by affecting maxillary growth and uplift in the early-stage development of maxillary prominences. By performing bulk RNA-sequencing, we demonstrated that Dlx2 overexpression induced significant changes in many genes associated with critical developmental pathways. In summary, our novel mouse model provides a reliable and consistent system for investigating Dlx2 functions during development and for elucidating the gene regulatory networks underlying craniofacial development.

Gene regulatory network from cranial neural crest cells to osteoblast differentiation and calvarial bone development

Calvarial bone is one of the most complex sequences of developmental events in embryology, featuring a uniquely transient, pluripotent stem cell-like population known as the cranial neural crest (CNC). The skull is formed through intramembranous ossification with distinct tissue lineages (e.g. neural crest derived frontal bone and mesoderm derived parietal bone). Due to CNC's vast cell fate potential, in response to a series of inductive secreted cues including BMP/TGF-β, Wnt, FGF, Notch, Hedgehog, Hippo and PDGF signaling, CNC enables generations of a diverse spectrum of differentiated cell types in vivo such as osteoblasts and chondrocytes at the craniofacial level. In recent years, since the studies from a genetic mouse model and single-cell sequencing, new discoveries are uncovered upon CNC patterning, differentiation, and the contribution to the development of cranial bones. In this review, we summarized the differences upon the potential gene regulatory network to regulate CNC derived osteogenic potential in mouse and human, and highlighted specific functions of genetic molecules from multiple signaling pathways and the crosstalk, transcription factors and epigenetic factors in orchestrating CNC commitment and differentiation into osteogenic mesenchyme and bone formation. Disorders in gene regulatory network in CNC patterning indicate highly close relevance to clinical birth defects and diseases, providing valuable transgenic mouse models for subsequent discoveries in delineating the underlying molecular mechanisms. We also emphasized the potential regenerative alternative through scientific discoveries from CNC patterning and genetic molecules in interfering with or alleviating clinical disorders or diseases, which will be beneficial for the molecular targets to be integrated for novel therapeutic strategies in the clinic.

Making and shaping endochondral and intramembranous bones

Skeletal elements have a diverse range of shapes and sizes specialized to their various roles including protecting internal organs, locomotion, feeding, hearing, and vocalization. The precise positioning, size, and shape of skeletal elements is therefore critical for their function. During embryonic development, bone forms by endochondral or intramembranous ossification and can arise from the paraxial and lateral plate mesoderm or neural crest. This review describes inductive mechanisms to position and pattern bones within the developing embryo, compares and contrasts the intrinsic vs extrinsic mechanisms of endochondral and intramembranous skeletal development, and details known cellular processes that precisely determine skeletal shape and size. Key cellular mechanisms are employed at distinct stages of ossification, many of which occur in response to mechanical cues (eg, joint formation) or preempting future load‐bearing requirements. Rapid shape changes occur during cellular condensation and template establishment. Specialized cellular behaviors, such as chondrocyte hypertrophy in endochondral bone and secondary cartilage on intramembranous bones, also dramatically change template shape. Once ossification is complete, bone shape undergoes functional adaptation through (re)modeling. We also highlight how alterations in these cellular processes contribute to evolutionary change and how differences in the embryonic origin of bones can influence postnatal bone repair.

Compares and contrasts Endochondral and intramembranous bone development

Reviews embryonic origins of different bones

Describes the cellular and molecular mechanisms of positioning skeletal elements.

Describes mechanisms of skeletal growth with a focus on the generation of skeletal shape

Formation and Developmental Specification of the Odontogenic and Osteogenic Mesenchymes

Within the mandible, the odontogenic and osteogenic mesenchymes develop in a close proximity and form at about the same time. They both originate from the cranial neural crest. These two condensing ecto-mesenchymes are soon separated from each other by a very loose interstitial mesenchyme, whose cells do not express markers suggesting a neural crest origin. The two condensations give rise to mineralized tissues while the loose interstitial mesenchyme, remains as a soft tissue. This is crucial for proper anchorage of mammalian teeth. The situation in all three regions of the mesenchyme was compared with regard to cell heterogeneity. As the development progresses, the early phenotypic differences and the complexity in cell heterogeneity increases. The differences reported here and their evolution during development progressively specifies each of the three compartments. The aim of this review was to discuss the mechanisms underlying condensation in both the odontogenic and osteogenic compartments as well as the progressive differentiation of all three mesenchymes during development. Very early, they show physical and structural differences including cell density, shape and organization as well as the secretion of three distinct matrices, two of which will mineralize. Based on these data, this review highlights the consecutive differences in cell-cell and cell-matrix interactions, which support the cohesion as well as mechanosensing and mechanotransduction. These are involved in the conversion of mechanical energy into biochemical signals, cytoskeletal rearrangements cell differentiation, or collective cell behavior.

Zebrafish Pax1a and Pax1b are required for pharyngeal pouch morphogenesis and ceratobranchial cartilage development

Pharyngeal arches are derived from all three germ layers and molecular interactions among the tissue types are required for proper development of subsequent pharyngeal cartilages; however, the mechanisms underlying this process are not fully described. Here we report that in zebrafish, Pax1a and Pax1b have overlapping and essential functions in pharyngeal pouch morphogenesis and subsequent ceratobranchial cartilage development. Both pax1a and pax1b are co-expressed in pharyngeal pouches, and time-lapse imaging of a novel Tg(pax1b:eGFP) enhancer trap line further revealed the sequential segmental development of pharyngeal pouches. Zebrafish pax1a^-/-; pax1b^-/- double mutant embryos generated by CRISPR-Cas9 mutagenesis exhibit unsegmented pharyngeal pouches 2-5 with small outpocketings. Endodermal expression of fgf3, tbx1 and edn1 is also absent in pharyngeal pouches 2-5 at 36 h post fertilization (hpf). Loss of ceratobranchial cartilage 1-4 and reduced or absent expression of dlx2a and hand2 in the pharyngeal arches 3-6 are observed in CRISPR mutant and morphant embryos that are deficient in both zebrafish pax1a and pax1b at 96 or 36 hpf. These results suggest that zebrafish Pax1a and Pax1b both regulate pharyngeal pouch morphogenesis by modulating expression of fgf3 and tbx1. Furthermore, our data support a model wherein endodermal Pax1a and Pax1b act through Fgf3 and Tbx-Edn1 signaling to non-autonomously regulate the development of ceratobranchial cartilage via expression of dlx2a and hand2.

Partial duplication of the jaw: case reports and review of relevant publications

Craniofacial duplication is a rare congenital malformation with a wide phenotypic range. The signs and symptoms range from partial craniofacial duplication to bicephalus. We describe two cases of partial duplication of jaw: a girl with a duplication of the maxilla, and a boy with duplication of the mandible. We review the relevant publications and discuss the pathogenesis.

Dental and periodontal phenotypes of Dlx2 overexpression in mice

Distal-less homeobox 2 (Dlx2) is a member of the homeodomain family of transcription factors and is important for the development of cranial neural crest cells (CNCCs)-derived craniofacial tissues. Previous studies revealed that Dlx2 was expressed in the cementum and a targeted null mutation disrupted tooth development in mice. However, whether Dlx2 overexpression may impair in vivo tooth morphogenesis remains to be elucidated. The present study used a transgenic mouse model to specifically overexpress Dlx2 in neural crest cells in order to identify the dental phenotypes in mice by observation, micro-computed tomography and histological examination. The Dlx2-overexpressed mice exhibited tooth abnormalities including incisor cross-bite, shortened tooth roots, increased cementum deposition, periodontal ligament disorganization and osteoporotic alveolar bone. Therefore, Dlx2 overexpression may alter the alveolar bone, cementum and periodontal ligament (PDL) phenotypes in mice.

Separate development of the maxilla and mandible is controlled by regional signaling of the maxillomandibular junction during avian development

BACKGROUND

Syngnathia is a congenital craniofacial disorder characterized by bony or soft tissue fusion of upper and lower jaws. Previous studies suggested some causative signals, such as Foxc1 or Bmp4, cause the disruption of maxillomandibular identity, but their location and the interactive signals involved remain unexplored. We wanted to examine the embryonic origin of syngnathia based on the assumption that it may be located at the separation between the maxillary and mandibular processes. This region, known as the maxillomandibular junction (MMJ), is involved in segregation of cranial neural crest-derived mesenchyme into the presumptive upper and lower jaws.

RESULTS

Here we investigated the role of Fgf, Bmp, and retinoid signaling during development of MMJ in chicken embryos. By changing the levels of these signals with bead implants, we induced syngnathia with microstomia on the treated side, which showed increased Barx1 and neural cell adhesion molecule (NCAM) expression. Redistribution of proliferating cells was also observed at the proximal region to maxillary and mandibular arch around MMJ.

CONCLUSIONS

We propose that interactive molecular signaling by Fgfs, Bmps, and retinoids around MMJ is required for normal separation of the maxilla and mandible, as well as the proper positioning of beak commissure during early facial morphogenesis. Developmental Dynamics 246:28-40, 2017. © 2016 Wiley Periodicals, Inc.

Overexpression of Dlx2 leads to postnatal condyle degradation

Distal-less homeobox 2 (Dlx2), a member of the Dlx family of transcription factors, is important for the development of craniofacial tissues. Previous studies based on knock-out mutant mice revealed that Dlx2 primarily disturbed the development of tissues from maxillary arch. The present study used a transgenic mouse model to specifically overexpress Dlx2 in neural crest cells in order to investigate the role of Dlx2 overexpression in post-natal condyle in mice. The model was constructed and the phenotype observed using gross observation, micro-CT scan and histological examination. The model determined that overexpression of Dlx2 may lead to postnatal condyle malformation, subchondral bone degradation and irregular histological structure of the condylar cartilage. In addition, the expression of osteocalcin in the condyle region was markedly downregulated, whereas expression of msh homeobox 2 was upregulated. The results of the present study suggest that Dlx2 overexpression in cranial neural crest cells would disrupt the development of post-natal condyle, which demonstrates that the expression level and the spatiotemporal expression patterns of Dlx2 may be important in regulating the development of post-natal condyle in mice, and also offered a possible temporal-mandibular joint osteoarthritis model animal for future studies.

Osteogenic transcription factors and proto-oncogene regulate bone sialoprotein gene transcription

Runt homeodomain protein 2 (Runx2), distalless 5 (Dlx5) and Smad1 are transcription factors that play critical roles in controlling the differentiation of osteoblasts and mineralization of bone. Proto-oncogene tyrosine-protein kinase, Src, is an enzyme encoded by the Src gene. The normal cellular gene is called cellular-Src (c-Src). Bone sialoprotein (BSP), a protein implicated in the initial mineralization of newly formed bone, is an early phenotypic marker of differentiated osteoblasts. In this study, we used overexpression plasmids with Runx2, Dlx5, Smad1 or c-Src inserts to search for the effects of these transcription factors and proto-oncogene on BSP gene expression using rat osteoblast-like ROS 17/2.8. When we used Runx2, Dlx5 or c-Src overexpression plasmids for the transfection, BSP and Runx2 mRNA levels were increased in ROS 17/2.8 cells. However, overexpression of Smad1 did not induce BSP and Runx2 mRNA. Transient transfection analyses were performed using chimeric constructs of the rat BSP gene promoter linked to a luciferase reporter gene. Transfection of ROS 17/2.8 cells with Runx2, Dlx5 or c-Src overexpression plasmid increased the luciferase activities of the constructs, pLUC3 (-116 to +60), pLUC4 (-425 to +60) and pLUC5 (-801 to +60). However, Smad1 overexpression had no effect on the luciferase activities. These results demonstrate that overexpression of Runx2, Dlx5 or c-Src stimulates BSP transcription, and suggest that Runx2, Dlx5 and c-Src might be crucial transcriptional regulators of mineralization and bone formation.

Pax3 and Pax7 play essential safeguard functions against environmental stress-induced birth defects

Exposure to environmental teratogenic pollutant leads to severe birth defects. However, the biological events underlying these developmental abnormalities remain undefined. Here, we report a molecular link between an environmental stress response pathway and key developmental genes during craniofacial development. Strikingly, mutant mice with impaired Pax3/7 function display severe craniofacial defects. We show that these are associated with an upregulation of the signaling pathway mediated by the Aryl hydrocarbon receptor (AHR), the receptor to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), revealing a genetic interaction between Pax3 and AHR signaling. Activation of AHR signaling in Pax3-deficient embryos drives facial mesenchymal cells out of the cell cycle through the upregulation of p21 expression. Accordingly, inhibiting AHR activity rescues the cycling status of these cells and the facial closure of Pax3/7 mutants. Together, our findings demonstrate that the regulation of AHR signaling by Pax3/7 is required to protect against TCDD/AHR-mediated teratogenesis during craniofacial development.

Regulation of cadherin expression in nervous system development

This review addresses our current understanding of the regulatory mechanisms for classical cadherin expression during development of the vertebrate nervous system. The complexity of the spatial and temporal expression patterns is linked to morphogenic and functional roles in the developing nervous system. While the regulatory networks controlling cadherin expression are not well understood, it is likely that the multiple signaling pathways active in the development of particular domains also regulate the specific cadherins expressed at that time and location. With the growing understanding of the broader roles of cadherins in cell-cell adhesion and non-adhesion processes, it is important to understand both the upstream regulation of cadherin expression and the downstream effects of specific cadherins within their cellular context.

Cleft palate defect of Dlx1/2-/- mutant mice is caused by lack of vertical outgrowth in the posterior palate

BACKGROUND

Mice lacking the activities of Dlx1 and Dlx2 (Dlx1/2-/-) exhibit cleft palate, one of the most common human congenital defects, but the etiology behind this phenotype has been unknown. Therefore, we analyzed the morphological, cellular, and molecular changes caused by inactivation of Dlx1 and Dlx2 as related to palate development.

RESULTS

Dlx1/2-/- mutants exhibited lack of vertical growth in the posterior palate during the earliest stage of palatogenesis. We attributed this growth deficiency to reduced cell proliferation. Expression of a cell cycle regulator Ccnd1 was specifically down-regulated in the same region. Previous studies established that the epithelial-mesenchymal signaling loop involving Shh, Bmp4, and Fgf10 is important for cell proliferation and tissue growth during palate development. This signaling loop was disrupted in Dlx1/2-/- palate. Interestingly, however, the decreases in Ccnd1 expression and mitosis in Dlx1/2-/- mutants were independent of this signaling loop. Finally, Dlx1/2 activity was required for normal expression of several transcription factor genes whose mutation results in palate defects.

CONCLUSIONS

The functions of Dlx1 and Dlx2 are crucial for the initial formation of the posterior palatal shelves, and that the Dlx genes lie upstream of multiple signaling molecules and transcription factors important for later stages of palatogenesis.

Altered gene expression in rat cranial neural crest cells exposed to a teratogenic glucose concentration in vitro: paradoxical downregulation of antioxidative defense genes

BACKGROUND

Diabetic pregnancy is associated with increased risk of malformation in the infant. Diabetes-induced anomalies of the face and heart are strongly correlated with neural crest cell (NCC) maldevelopment. We aimed to study glucose-induced alterations of mRNA levels in cranial and trunk NCCs isolated from rat embryos with increased risk of developing mandibular and cardiac malformations in diabetic pregnancy.

METHODS

Inbred Sprague-Dawley rat embryos were used for NCC isolation from neural tube explants. The migrating cells were exposed to 5.5 or 30 mmol/l glucose concentration for 48 hr, harvested, and prepared for gene expression measurement by RT-PCR or immunostaining with either distal-less (Dlx) or AP-2-α antibodies.

RESULTS

Evaluation of the immunostained slides showed that approximately 75% of the cells were of NCC origin. Exposure to 30 mM glucose decreased mRNA levels of Copper-Zinc superoxide dismutase, manganese superoxide dismutase, extracellular superoxide dismutase, Catalase, Gpx-1, Nrf2, poly-ADP ribose polymerase, B-cell leukemia/lymphoma protein 2, and β-Catenin genes in cranial neural crest explant cultures. In addition, Pax-3, Pax-6, Wnt3a, and Apc mRNA levels were decreased by high glucose exposure in both cranial and trunk neural crest explant cultures.

CONCLUSION

Cranial NCCs diminish their mRNA levels of antioxidative enzymes and the Nrf2 response factor, as well as the antiapoptotic B-cell leukemia/lymphoma protein 2 gene, in response to increased ambient glucose concentration. Furthermore, both cranial and trunk NCC decrease the mRNA levels of the transcription factors Pax-3 and Pax-6, as well as key components of the Wnt pathway. These patterns of glucose-altered gene expression in a developmentally important cell population may be of etiological importance for NCC-associated malformations in diabetic pregnancy.

Role of Dlx genes in craniofacial morphogenesis: Dlx2 influences skeletal patterning by inducing ectomesenchymal aggregation in ovo

Dlx homeodomain transcription factors are expressed in neural crest-derived mesenchyme of the pharyngeal arches and are required for patterning of the craniofacial skeleton. However, the cellular and molecular mechanisms by which Dlx factors control skeletogenesis in the facial primordia are unclear. We have investigated the function of Dlx2 and Dlx5 by sustained misexpression in ovo. We find that RCAS-Dlx2- and RCAS-Dlx5-infected avian embryos exhibit very similar patterns of local, stereotypical changes in skeletal development in the upper jaw. The changes include ectopic dermal bone along the jugal arch, and ectopic cartilages that develop between the quadrate and the trabecula. The ectopic cartilage associated with the trabecula is reminiscent of a normally occurring element in this region in some bird taxa. Analysis of the distribution of RCAS-Dlx2-infected cells suggests that Dlx2 induces aggregation of undifferentiated mesenchyme, which subsequently develops into the ectopic skeletal elements. Comparison of infected embryos with restricted or widespread misexpression, and of embryos in which Dlx genes were delivered to migratory or postmigratory neural crest, indicate that there are limited regions of competence in which the ectopic elements can arise. The site-specific differentiation program that the aggregates undergo may be dependent on local environmental signals. Our results suggest that Dlx factors mediate localization of ectomesenchymal subpopulations within the pharyngeal arches and in doing so define where skeletogenic condensations will arise. Consequently, variation in Dlx expression or activity may have influenced the morphology of jaw elements during vertebrate evolution.

Zebrafish wnt9a is expressed in pharyngeal ectoderm and is required for palate and lower jaw development

Wnt activity is critical in craniofacial morphogenesis. Dysregulation of Wnt/β-catenin signaling results in significant alterations in the facial form, and has been implicated in cleft palate phenotypes in mouse and man. In zebrafish, we show that wnt9a is expressed in the pharyngeal arch, oropharyngeal epithelium that circumscribes the ethmoid plate, and ectodermal cells superficial to the lower jaw structures. Alcian blue staining of morpholino-mediated knockdown of wnt9a results in loss of the ethmoid plate, absence of lateral and posterior parachordals, and significant abrogation of the lower jaw structures. Analysis of cranial neural crest cells in the sox10:eGFP transgenic demonstrates that the wnt9a is required early during pharyngeal development, and confirms that the absence of Alcian blue staining is due to absence of neural crest derived chondrocytes. Molecular analysis of genes regulating cranial neural crest migration and chondrogenic differentiation suggest that wnt9a is dispensable for early cranial neural crest migration, but is required for chondrogenic development of major craniofacial structures. Taken together, these data corroborate the central role for Wnt signaling in vertebrate craniofacial development, and reveal that wnt9a provides the signal from the pharyngeal epithelium to support craniofacial chondrogenic morphogenesis in zebrafish.

Staurosporine augments EGF-mediated EMT in PMC42-LA cells through actin depolymerisation, focal contact size reduction and Snail1 induction - a model for cross-modulation

BACKGROUND

A feature of epithelial to mesenchymal transition (EMT) relevant to tumour dissemination is the reorganization of actin cytoskeleton/focal contacts, influencing cellular ECM adherence and motility. This is coupled with the transcriptional repression of E-cadherin, often mediated by Snail1, Snail2 and Zeb1/deltaEF1. These genes, overexpressed in breast carcinomas, are known targets of growth factor-initiated pathways, however it is less clear how alterations in ECM attachment cross-modulate to regulate these pathways. EGF induces EMT in the breast cancer cell line PMC42-LA and the kinase inhibitor staurosporine (ST) induces EMT in embryonic neural epithelial cells, with F-actin de-bundling and disruption of cell-cell adhesion, via inhibition of aPKC.

METHODS

PMC42-LA cells were treated for 72 h with 10 ng/ml EGF, 40 nM ST, or both, and assessed for expression of E-cadherin repressor genes (Snail1, Snail2, Zeb1/deltaEF1) and EMT-related genes by QRT-PCR, multiplex tandem PCR (MT-PCR) and immunofluorescence +/- cycloheximide. Actin and focal contacts (paxillin) were visualized by confocal microscopy. A public database of human breast cancers was assessed for expression of Snail1 and Snail2 in relation to outcome.

RESULTS

When PMC42-LA were treated with EGF, Snail2 was the principal E-cadherin repressor induced. With ST or ST+EGF this shifted to Snail1, with more extreme EMT and Zeb1/deltaEF1 induction seen with ST+EGF. ST reduced stress fibres and focal contact size rapidly and independently of gene transcription. Gene expression analysis by MT-PCR indicated that ST repressed many genes which were induced by EGF (EGFR, CAV1, CTGF, CYR61, CD44, S100A4) and induced genes which alter the actin cytoskeleton (NLF1, NLF2, EPHB4). Examination of the public database of breast cancers revealed tumours exhibiting higher Snail1 expression have an increased risk of disease-recurrence. This was not seen for Snail2, and Zeb1/deltaEF1 showed a reverse correlation with lower expression values being predictive of increased risk.

CONCLUSION

ST in combination with EGF directed a greater EMT via actin depolymerisation and focal contact size reduction, resulting in a loosening of cell-ECM attachment along with Snail1-Zeb1/deltaEF1 induction. This appeared fundamentally different to the EGF-induced EMT, highlighting the multiple pathways which can regulate EMT. Our findings add support for a functional role for Snail1 in invasive breast cancer.

Development of satellite glia in mouse sympathetic ganglia: GDNF and GFR alpha 1 are not essential

The phenotypic development of satellite cells in mouse sympathetic ganglia was examined by localizing the transcription factors, Sox10 and Phox2b, the neuronal marker, tyrosine hydroxylase (TH), and brain-derived fatty acid binding protein (B-FABP), which identifies glial precursors and mature glia. In E10.5 mice, most cells in the sympathetic chain expressed both Sox10 and Phox2b, with a minority of cells expressing Sox10 only or Phox2b only. In E11.5 mice, the majority of cells expressed Sox10 only or Phox2b only. B-FABP was colocalized with Sox10 in satellite glial precursors, which were located on the periphery of the ganglion. There was no overlap between B-FABP and Phox2b or B-FABP and TH. During subsequent development, the number of B-FABP+ cells increased and they became more common deep within the ganglion. In E12.5 and E18.5 mice, there was no overlap between Sox10 and Phox2b, and 98% of Sox10 cells were also B-FABP+. Satellite glial precursors in E11.5-E15.5 mice also expressed the GDNF-binding molecule, GFRalpha1. B-FABP immunoreactive cells did not express Ret or NCAM, two potential signaling molecules for GDNF/GFRalpha1. In E12.5 and E18.5 mice lacking GFRalpha1 or GDNF, the development of B-FABP immunoreactive satellite cells was normal, and hence neither GDNF or GFRalpha1 are essential for the development of satellite glia in sympathetic ganglia.

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.

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.

Dlx5- and Dlx6-mediated chondrogenesis: Differential domain requirements for a conserved function

During endochondral ossification in the vertebrate limb, multipotent mesenchymal cells first differentiate into chondroblasts (chondrogenesis) that further differentiate (via chondrocyte hypertrophy) to a terminal cellular phenotype. Dlx5 and Dlx6 are functionally redundant regulators of chondrocyte hypertrophy. We now show that Dlx5 and Dlx6 also regulate the earlier step of chondrogenesis in the limb. Limb bud mesenchymal cells from Dlx5/6(-/-) embryos show reduced chondrogenesis compared to wild-type littermates, and expression of either Dlx5 or Dlx6 stimulated differentiation of limb bud mesenchymal cells to chondroblasts. The functional overlap between Dlx5 and Dlx6 occurs despite the fact that the amino- and carboxyl-terminal domains of the encoded proteins are dissimilar. In order to reconcile the disparity between the divergent structures of Dlx5 and Dlx6 with their overlapping biological functions, we investigated the domain requirements and transcriptional activities associated with Dlx5- and Dlx6-mediated chondrogenesis. We find distinct domain requirements for the chondrogenic function of these related homeoproteins, indicating divergent molecular mechanisms of action.

Induction and specification of cranial placodes

Cranial placodes are specialized regions of the ectoderm, which give rise to various sensory ganglia and contribute to the pituitary gland and sensory organs of the vertebrate head. They include the adenohypophyseal, olfactory, lens, trigeminal, and profundal placodes, a series of epibranchial placodes, an otic placode, and a series of lateral line placodes. After a long period of neglect, recent years have seen a resurgence of interest in placode induction and specification. There is increasing evidence that all placodes despite their different developmental fates originate from a common panplacodal primordium around the neural plate. This common primordium is defined by the expression of transcription factors of the Six1/2, Six4/5, and Eya families, which later continue to be expressed in all placodes and appear to promote generic placodal properties such as proliferation, the capacity for morphogenetic movements, and neuronal differentiation. A large number of other transcription factors are expressed in subdomains of the panplacodal primordium and appear to contribute to the specification of particular subsets of placodes. This review first provides a brief overview of different cranial placodes and then synthesizes evidence for the common origin of all placodes from a panplacodal primordium. The role of various transcription factors for the development of the different placodes is addressed next, and it is discussed how individual placodes may be specified and compartmentalized within the panplacodal primordium. Finally, tissues and signals involved in placode induction are summarized with a special focus on induction of the panplacodal primordium itself (generic placode induction) and its relation to neural induction and neural crest induction. Integrating current data, new models of generic placode induction and of combinatorial placode specification are presented.

Early Hedgehog signaling from neural to oral epithelium organizes anterior craniofacial development

Hedgehog (Hh) signaling plays multiple roles in the development of the anterior craniofacial skeleton. We show that the earliest function of Hh is indirect, regulating development of the stomodeum, or oral ectoderm. A subset of post-migratory neural crest cells, that gives rise to the cartilages of the anterior neurocranium and the pterygoid process of the palatoquadrate in the upper jaw, condenses upon the upper or roof layer of the stomodeal ectoderm in the first pharyngeal arch. We observe that in mutants for the Hh co-receptor smoothened (smo) the condensation of this specific subset of crest cells fails, and expression of several genes is lost in the stomodeal ectoderm. Genetic mosaic analyses with smo mutants show that for the crest cells to condense the crucial target tissue receiving the Hh signal is the stomodeum, not the crest. Blocking signaling with cyclopamine reveals that the crucial stage, for both crest condensation and stomodeal marker expression, is at the end of gastrulation--some eight to ten hours before crest cells migrate to associate with the stomodeum. Two Hh genes, shh and twhh, are expressed in midline tissue at this stage, and we show using mosaics that for condensation and skeletogenesis only the ventral brain primordium, and not the prechordal plate, is an important Hh source. Thus, we propose that Hh signaling from the brain primordium is required for proper specification of the stomodeum and the stomodeum, in turn, promotes condensation of a subset of neural crest cells that will form the anterior neurocranial and upper jaw cartilage.