Expression and role of Lhx8 in murine tooth development

LIM homeobox 8 reduced apoptosis and promoted periodontal tissue regeneration function of dental pulp stem cells

Stem cell-mediated tissue regeneration is a promising strategy for repairing tissue defects and functional reconstruction in periodontitis, a common disease that leads to the loss of alveolar bone and teeth. However, stem cell apoptosis, widely observed during tissue regeneration, impairs its efficiency. Therefore, the regulation of stem cell apoptosis is critical for improving regeneration efficiency. The LIM homeobox 8 gene LHX8, belongs to the LIM homeobox family, which was involved in tooth morphogenesis. Here, we found that LHX8 was significantly expressed in dental pulp. LHX8 knockdown significantly increased dental pulp mesenchymal stem cells (DPSCs) apoptosis, as confirmed by RT-PCR, western blotting, flow cytometry, and transmission electron microscopy. Additionally, LHX8 overexpression inhibited apoptosis and enhanced the osteo/odontogenic differentiation potential of hDPSCs in vitro. Furthermore, LHX8-overexpression could enhance the periodontal tissue regeneration efficiency of hDPSCs in mice with periodontitis. In conclusion, the present study indicates that LHX8 inhibits stem cell apoptosis and promotes functional tissue formation in stem cell-based tissue regeneration engineering, suggesting a new therapeutic target to increase the efficacy of periodontal tissue regeneration.

Homeobox Genes in Odontogenic Lesions: A Scoping Review

BACKGROUND

Homeobox genes play crucial roles in tooth morphogenesis and development and thus mutations in homeobox genes cause developmental disorders such as odontogenic lesions. The aim of this scoping review is to identify and compile available data from the literatures on the topic of homeobox gene expression in odontogenic lesions.

METHOD

An electronic search to collate all the information on studies on homeobox gene expression in odontogenic lesions was carried out in four databases (PubMed, EBSCO host, Web of Science and Cochrane Library) with selected keywords. All papers which reported expression of homeobox genes in odontogenic lesions were considered.

RESULTS

A total of eleven (11) papers describing expression of homeobox genes in odontogenic lesions were identified. Methods of studies included next generation sequencing, microarray analysis, RT-PCR, Western blotting, in situ hybridization, and immunohistochemistry. The homeobox reported in odontogenic lesions includes LHX8 and DLX3 in odontoma; PITX2, MSX1, MSX2, DLX, DLX2, DLX3, DLX4, DLX5, DLX6, ISL1, OCT4 and HOX C in ameloblastoma; OCT4 in adenomatoid odontogenic tumour; PITX2 and MSX2 in primordial odontogenic tumour; PAX9 and BARX1 in odontogenic keratocyst; PITX2, ZEB1 and MEIS2 in ameloblastic carcinoma while there is absence of DLX2, DLX3 and MSX2 in clear cell odontogenic carcinoma.

CONCLUSIONS

This paper summarized and reviews the possible link between homeobox gene expression in odontogenic lesions. Based on the current available data, there are insufficient evidence to support any definite role of homeobox gene in odontogenic lesions.

FGF8 and BMP2 mediated dynamic regulation of dental mesenchyme proliferation and differentiation via Lhx8/Suv39h1 complex

The homeobox gene, LIM-homeobox 8 (Lhx8), has previously been identified as an essential transcription factor for dental mesenchymal development. However, how Lhx8 itself is regulated and regulates odontogenesis remains poorly understood. In this study, we employed an RNAscope assay to detect the co-expression pattern of Lhx8 and Suv39h1 in the dental mesenchyme, which coincided with the dynamic expression profiles of the early epithelium signal of Fibroblast Growth Factor 8 (FGF8) and the later mesenchymal signal Bone Morphogenetic Protein 2 (BMP2). Moreover, FGF8 activated Lhx8, whereas BMP2 repressed Lhx8 expression at the transcriptional level. The high expression of Lhx8 in the early dental mesenchyme maintained the cell fate in an undifferentiated status by interacting with Suv39h1, a histone-lysine N-methyltransferase constitutively expressed in the dental mesenchyme. Further in the ex vivo organ culture model, the knockdown of Suv39h1 significantly blocked the function of Lhx8 and FGF8. Mechanistically, Lhx8/Suv39h1 recognized the odontoblast differentiation-related genes and repressed gene expression via methylating H3K9 on their promoters. Taken together, our data here suggest that Lhx8/Suv39h1 complex is inversely regulated by epithelium-mesenchymal signals, balancing the differentiation and proliferation of dental mesenchyme via H3K9 methylation.

Optogenetic control of mesenchymal cell fate towards precise bone regeneration

Rationale: Spatial-temporal control of cell fate in vivo is of great importance for regenerative medicine. Currently, there remain no practical strategies to tune cell-fate spatial-temporally. Optogenetics is a biological technique that widely used to control cell activity in genetically defined neurons in a spatiotemporal-specific manner by light. In this study, optogenetics was repurposed for precise bone tissue regeneration.

Methods: Lhx8 and BMP2 genes, which are considered as the master genes for mesenchymal stem cell proliferation and differentiation respectively, were recombined into a customized optogenetic control system. In the system, Lhx8 was constitutively expressed, while BMP2 together with shLhx8 expression was driven by blue light.

Results: As expected, blue light induced BMP2 expression and inactivated Lhx8 expression in cells infected with the optogenetic control system. Optogenetic control of BMP2 and Lhx8 expression inversely regulates MSC fate in vitro. By animal study, we found that blue light could fine-tune the regeneration in vivo. Blue light illumination significantly promotes bone regeneration when the scaffold was loaded with MSCs infected with adeno-Lhx8, GI-Gal4DBD, LOV-VP16, and BMP2-shLhx8.

Conclusions: Together, our study revealed that optogenetic control of the master genes for mesenchymal stem cell proliferation and differentiation would be such a candidate strategy for precise regenerative medicine.

Long non-coding RNA GAS5 promotes PC12 cells differentiation into Tuj1-positive neuron-like cells and induces cell cycle arrest

Growth arrest-specific 5 (GAS5) is an anti-oncogene that has been extensively studied in tumors. However, research on GAS5 in the context of nervous system disease is rare at present. This study aimed to investigate the role of the long non-coding RNA GAS5 in rat pheochromocytoma cells (PC12 cells). GAS5-overexpressing lentivirus was transfected into PC12 cells, and expression levels of GAS5 and C-myc were detected by real-time PCR. Ratios of cells in S phase were detected by 5-ethynyl-2′-deoxyuridine. Immunohistochemical staining was used to detect the immunoreactivity of neuron microtubule markers Tuj1, doublecortin, and microtubule-associated protein 2. Apoptosis was detected by flow cytometry, while expression of acetylcholine in cells was detected by western blot assay. We found that GAS5 can promote PC12 cells to differentiate into Tuj1-positive neuron-like cells with longer processes. In addition, cell proliferation and cell cycle were significantly suppressed by GAS5, whereas it had no effect on apoptosis of PC12 cells. Our results indicate that GAS5 could increase the expression of choline acetyltransferase and acetylcholine release. Thus, we speculate that GAS5 is beneficial to the recovery of neurons and the cholinergic nervous system.

Beadex, a Drosophila LIM domain only protein, function in follicle cells is essential for egg development and fertility

LIM domain, constituted by two tandem C2H2 zinc finger motif, proteins regulate several biological processes. They are usually found associated with various functional domains like Homeodomain, kinase domain and other protein binding domains. LIM proteins that are devoid of other domains are called LIM only proteins (LMO). LMO proteins were first identified in humans and are implicated in development and oncogenesis. They regulate various cell specifications by regulating the activity of respective transcriptional complexes. The Drosophila LMO protein (dLMO), Beadex (Bx), regulates various developmental processes like wing margin development and bristle development. It also regulates Drosophila behavior in response to cocaine and ethanol. We have previously generated Bx null flies and shown its essential function in neurons for multiple aspects of female reproduction. However, it was not known whether Bx affects reproduction through its independent function in ovaries. In this paper we show that female flies null for Bx lay eggs with multiple defects. Further, through knock down studies we demonstrate that function of Bx in follicle cells is required for normal egg development. We also show that function of Bx is particularly required in border cells for Drosophila fertility.

Lhx6 and Lhx8: cell fate regulators and beyond

As transcription factors of the lines (LIN)-11/Islet (Isl)-1/mitosis entry checkpoint (MEC)-3 (LIM)-homeobox subfamily, LIM homeobox (Lhx)6 and -8 are remarkably conserved and involved in the morphogenesis of multiple organ systems. Lhx6 and -8 play overlapping and distinctive roles, but in general act as cell fate mediators and in turn are regulated by several transcriptional factors, such as sonic hedgehog, fibroblast growth factors, and wingless-int (Wnt)/β-catenin. In this review, we first summarize Lhx6 and -8 distributions in development and then explore how Lhx6 and -8 act as transcription factors and coregulators of cell lineage specification. Known Lhx6 and -8 functions and targets are outlined in neurogenesis, craniofacial development, and germ cell differentiation. The underlying mechanisms of Lhx6 and -8 in regulating cell fate remain elusive. Whether Lhx6 and -8 affect functions in tissues and organs other than neural, craniofacial, oocytes, and germ cells is largely unexplored. Taken together, Lhx6 and -8 are important regulators of cell lineage specification and may act as one of the pivotal mediators of stem cell fate. Undoubtedly, future investigations of Lhx6 and -8 biology will continue to yield fascinating insights into tissue development and homeostasis, in addition to their putative roles in tissue regeneration and ageing.

Lhx8 mediated Wnt and TGFβ pathways in tooth development and regeneration

LIM homeobox 8 (Lhx8) is a highly conserved transcriptional factor with recently illustrated roles in cholinergic and GABAergic differentiation, and is expressed in neural crest derived craniofacial tissues during development. However, Lhx8 functions and signaling pathways are largely elusive. Here we showed that Lhx8 regulates dental mesenchyme differentiation and function via Wnt and TGFβ pathways. Lhx8 expression was restricted to dental mesenchyme from E11.5 to a peak at E14.5, and absent in dental epithelium. By reconstituting dental epithelium and mesenchyme in an E16.5 tooth organ, Lhx8 knockdown accelerated dental mesenchyme differentiation; conversely, Lhx8 overexpression attenuated dentin formation. Lhx8 overexpressed adult human dental pulp stem/progenitor cells in β-tricalcium phosphate cubes attenuated mineralized matrix production in vivo. Gene profiling revealed that postnatal dental pulp stem/progenitor cells upon Lhx8 overexpression modified matrix related gene expression including Dspp, Cola1 and osteocalcin. Lhx8 transcriptionally activated Wnt and TGFβ pathways, and its attenuation upregulated multiple dentinogenesis genes. Together, Lhx8 regulates dentin development and regeneration by fine-turning Wnt and TGFβ signaling.

Odontomas and supernumerary teeth: is there a common origin?

The aim of the present work is to analyze all scientific evidence to verify whether similarities supporting a unified explanation for odontomas and supernumerary teeth exist. A literature search was first conducted for epidemiologic studies indexed by PubMed, to verify their worldwide incidence. The analysis of the literature data shows some interesting similarities between odontomas and supernumerary teeth concerning their topographic distribution and pathologic manifestations. There is also some indication of common genetic and immuno-histochemical factors. Although from a nosological point of view, odontomas and supernumeraries are classified as distinct entities, they seem to be the expression of the same pathologic process, either malformative or hamartomatous.

Inductive ability of human developing and differentiated dental mesenchyme

The development of cell-based therapeutic strategies to bioengineer tooth tissue is a promising approach for the treatment of lost or damaged tooth tissue. The lack of a readily available cell source for human dental epithelial cells (ECs) severely constrains the progress of tooth bioengineering. Previous studies in model organisms have demonstrated that developing dental mesenchyme can instruct nondental epithelium to differentiate into enamel-forming epithelium. In this study, we characterized the ability of fetal and adult human dental mesenchyme to promote differentiation of human embryonic stem cell (hESC)-derived ECs (ES-ECs) into ameloblast-lineage cells. ES-ECs were co-cultured either with human fetal dental mesenchymal cells (FDMCs) or with adult dental mesenchymal cells (ADMCs) in either a three-dimensional culture system, or in the renal capsules of SCID mice. When co-cultured with FDMCs in vitro, ES-ECs polarized and expressed amelogenin. Tooth organ-like structures assembled with epithelium and encased mesenchyme and developing enamel-like structures could be detected in the complexes resulting from in vitro and ex vivo co-culture of ES-ECs and FDMCs. In contrast, co-cultured ES-ECs and ADMCs formed amorphous spherical structures and occasionally formed hair. Transcription factors were significantly upregulated in FDMCs compared to ADMCs including MSX1, GLI1, LHX6, LHX8,LEF1 and TBX1. In summary, FDMCs but not ADMCs had the capacity to induce differentiation of ES-ECs into ameloblast lineage cells. Further characterization of the functional differences between these two types of dental mesenchyme could enable reprogramming of ADMCs to enhance their odontogenic inductive competence.

A homozygous 237-kb deletion at 1p31 identified as the locus for midline cleft of the upper and lower lip in a consanguineous family

Orofacial clefts are congenital defects that vary widely in type and severity, and can occur in isolation or in association with a variety of other defects. Herein, we describe a consanguineous family afflicted with a unique form of orofacial clefting manifesting as a facial midline defect that also involves mandibular and maxillary structures. All four affected sibs had median clefts of the upper and lower lips, tooth misalignment, and poor oral hygiene. Linkage analysis of 17 family members identified a 15.3-Mb pair recessive locus at 1p31 with a LOD score of 3.63. To the best of our knowledge, this is, to date, the first locus reported for facial midline clefting and the first recessive locus for an isolated orofacial defect. The locus harboured a novel intergenic deletion of 273 164 bp, for which all fully affected sibs were homozygous. We did not note any potentially pathogenic gene variant at the 1p31 locus via exome-sequencing analysis. The identified deletion could be harbouring a regulatory element for the gene associated with the orofacial defect. The best candidate for the putative target gene is LHX8, located 49 149 bp upstream of the deletion. The gene is known to be associated with facial development in several animals. Four other family members had a subclinical phenotype--a simple notch in the lower lip or an increase in the interdental distance between the lower incisors--indicative of very low-level expression of the trait.

Molars and incisors: show your microarray IDs

Background

One of the key questions in developmental biology is how, from a relatively small number of conserved signaling pathways, is it possible to generate organs displaying a wide range of shapes, tissue organization, and function. The dentition and its distinct specific tooth types represent a valuable system to address the issues of differential molecular signatures. To identify such signatures, we performed a comparative transcriptomic analysis of developing murine lower incisors, mandibular molars and maxillary molars at the developmental cap stage (E14.5).

Results

231 genes were identified as being differentially expressed between mandibular incisors and molars, with a fold change higher than 2 and a false discovery rate lower than 0.1, whereas only 96 genes were discovered as being differentially expressed between mandibular and maxillary molars. Numerous genes belonging to specific signaling pathways (the Hedgehog, Notch, Wnt, FGF, TGFβ/BMP, and retinoic acid pathways), and/or to the homeobox gene superfamily, were also uncovered when a less stringent fold change threshold was used. Differential expressions for 10 out of 12 (mandibular incisors versus molars) and 9 out of 10 selected genes were confirmed by quantitative reverse transcription-PCR (qRT-PCR). A bioinformatics tool (Ingenuity Pathway Analysis) used to analyze biological functions and pathways on the group of incisor versus molar differentially expressed genes revealed that 143 genes belonged to 9 networks with intermolecular connections. Networks with the highest significance scores were centered on the TNF/NFκB complex and the ERK1/2 kinases. Two networks ERK1/2 kinases and tretinoin were involved in differential molar morphogenesis.

Conclusion

These data allowed us to build several regulatory networks that may distinguish incisor versus molar identity, and may be useful for further investigations of these tooth-specific ontogenetic programs. These programs may be dysregulated in transgenic animal models and related human diseases leading to dental anomalies.

WNK signaling is involved in neural development via Lhx8/Awh expression

WNK kinase family is conserved among many species and regulates SPAK/OSR1 and ion co-transporters. Some mutations in human WNK1 or WNK4 are associated with Pseudohypoaldosteronism type II, a form of hypertension. WNK is also involved in developmental and cellular processes, but the molecular mechanisms underlying its regulation in these processes remain unknown. Here, we identify a new target gene in WNK signaling, Arrowhead and Lhx8, which is a mammalian homologue of Drosophila Arrowhead. In Drosophila, WNK was shown to genetically interact with Arrowhead. In Wnk1 knockout mice, levels of Lhx8 expression were reduced. Ectopic expression of WNK1, WNK4 or Osr1 in mammalian cells induced the expression of the Lhx8. Moreover, neural specification was inhibited by the knockdown of both Wnk1 and Wnk4 or Lhx8. Drosophila WNK mutant caused defects in axon guidance during embryogenesis. These results suggest that WNK signaling is involved in the morphological and neural development via Lhx8/Arrowhead.

The LIM homeodomain transcription factor LHX6: a transcriptional repressor that interacts with pituitary homeobox 2 (PITX2) to regulate odontogenesis

LHX6 is a LIM-homeobox transcription factor expressed during embryogenesis; however, the molecular mechanisms regulating LHX6 transcriptional activities are unknown. LHX6 and the PITX2 homeodomain transcription factor have overlapping expression patterns during tooth and craniofacial development, and in this report, we demonstrate new transcriptional mechanisms for these factors. PITX2 and LHX6 are co-expressed in the oral and dental epithelium and epithelial cell lines. Lhx6 expression is increased in Pitx2c transgenic mice and decreased in Pitx2 null mice. PITX2 activates endogenous Lhx6 expression and the Lhx6 promoter, whereas LHX6 represses its promoter activity. Chromatin immunoprecipitation experiments reveal endogenous PITX2 binding to the Lhx6 promoter. LHX6 directly interacts with PITX2 to inhibit PITX2 transcriptional activities and activation of multiple promoters. Bimolecular fluorescence complementation assays reveal an LHX6·PITX2 nuclear interaction in living cells. LHX6 has a dominant repressive effect on the PITX2 synergistic activation with LEF-1 and β-catenin co-factors. Thus, LHX6 acts as a transcriptional repressor and represses the expression of several genes involved in odontogenesis. We have identified specific defects in incisor, molar, mandible, bone, and root development and late stage enamel formation in Lhx6 null mice. Amelogenin and ameloblastin expression is reduced and/or delayed in the Lhx6 null mice, potentially resulting from defects in dentin deposition and ameloblast differentiation. Our results demonstrate that LHX6 regulates cell proliferation in the cervical loop and promotes cell differentiation in the anterior region of the incisor. We demonstrate new molecular mechanisms for LHX6 and an interaction with PITX2 for normal craniofacial and tooth development.

Characteristics and osteogenic differentiation of stem/progenitor cells in the human dental follicle analyzed by gene expression profiling

The dental follicle is an ectomesenchymal tissue that surrounds developing tooth germ and that contains osteoblastic-lineage-committed stem/progenitor cells. We examined the osteogenic potential of human dental follicle cells (hDFC) by microarray analysis. We first compared the characteristics of hDFC with those of human bone marrow mesenchymal stem cells (hMSC). Like hMSC, hDFC expressed stem cell markers such as STRO-1 and Notch-1 and differentiated not only into the osteoblastic lineage, but also into the adipogenic lineage. We analyzed the gene expression profiles of hDFC and hMSC that were not differentiated toward the osteogenic lineage. The expression of cell markers and growth factor receptors by hDFC and hMSC was similar, whereas the expression pattern of homeobox genes differed between hDFC and hMSC. Next, we investigated gene expression in hDFC during osteogenic differentiation. Gene expression profiles were analyzed in hDFC cultured in osteogenic induction medium (OIM) or in growth medium (GM) for 3 and 10 days. Many genes whose expression was regulated under these conditions were functionally categorized as "transcription" genes. Osteogenic markers were up-regulated in hDFC during osteogenic differentiation, whereas neurogenic markers were down-regulated. The genes whose expression was regulated in hDFC during osteogenic differentiation were further analyzed by ingenuity pathway analysis and real-time polymerase chain reaction. Bone morphogenetic protein and transforming growth factor-β signaling pathways were activated in hDFC cultured in OIM for 3 days. This study indicates that the dental follicle contains stem cells and/or osteoblastic progenitor cells and is a potential cellular resource for bone regeneration therapy.

Identification of novel transcription-regulating genes expressed during murine molar development

BACKGROUND

The mechanism of tooth development is a complex process regulated by numerous genes including transcription factors, growth factors, and other intra- and extracellular molecules. Especially, transcription factors play a central role in gene expression, regulating a wide spectrum of biological processes including organogenesis. Substantial evidence has been demonstrated by a number of studies using genetically engineered animal models. However, detailed molecular mechanisms of tooth development have not been completely elucidated, partially because numerous genes that play essential roles in tooth development remain unidentified.

RESULTS

In this study, we conducted an expression-based screening using gene expression database and in situ hybridization assays. Based on the gene expression database "EMBRYS," 207 out of 1,520 genes were expressed in the maxillary and/or mandibular processes and thus were selected for further analysis by section in situ hybridization. Among these candidates, 28 genes were newly identified as potential factors associated with tooth development by in situ hybridization assays with frontal sections of embryonic day 13.5 and 14.5 mouse embryos. The expression patterns were also examined at embryonic day 16.5 and 18.5.

CONCLUSIONS

These results will contribute to elucidating the mechanisms of tooth development and to improving the technology for regeneration of tooth.

A human embryonic stem cell-derived clonal progenitor cell line with chondrogenic potential and markers of craniofacial mesenchyme

AIMS

We screened 100 diverse human embryonic stem-derived progenitor cell lines to identify novel lines with chondrogenic potential.

MATERIALS & METHODS

The 4D20.8 cell line was compared with mesenchymal stem cells and dental pulp stem cells by assessing osteochondral markers using immunohistochemical methods, gene expression microarrays, quantitative real-time PCR and in vivo repair of rat articular condyles.

RESULTS

4D20.8 expressed the site-specific gene markers LHX8 and BARX1 and robustly upregulated chondrocyte markers upon differentiation. Differentiated 4D20.8 cells expressed relatively low levels of COL10A1 and lacked IHH and CD74 expression. Transplantation of 4D20.8 cells into experimentally induced defects in the femoral condyle of athymic rats resulted in cartilage and bone differentiation approximating that of the original tissue architecture. Relatively high COL2A1 and minimal COL10A1 expression occurred during differentiation in HyStem-C hydrogel with TGF-β3 and GDF-5.

CONCLUSION

Human embryonic stem cell-derived embryonic progenitor cell lines may provide a novel means of generating purified site-specific osteochondral progenitor cell lines that are useful in research and therapy.

Comparative study of LHX8 expression between odontoma and dental tissue-derived stem cells

BACKGROUND

LHX8 (LIM-homeobox gene 8) is known as an important regulating factor in tooth morphogenesis. Odontoma is a mixed odontogenic tumor where epithelium and mesenchyme differentiated together, resulting in anomalous tooth structures. In this study, gene and protein expressions of LHX8 were analyzed in human odontoma-derived mesenchymal cells (HODC) compared to adult dental mesenchymal stem cells (aDSC), as well as morphological and histological characteristics of odontoma were analyzed.

METHODS

aDSCs were isolated from normal teeth, and HODCs were isolated from surgically removed odontoma mass. Morphological and histological evaluations were performed to compare between compound odontomas and normal premolars. RT-PCR and real-time PCR were performed to identify LHX8 mRNA expression in the HODCs and aDSCs. LHX8 protein expression levels were observed by immunoblotting and immunofluorescent staining.

RESULTS

The compound odontoma was composed of multiple tooth-like structures, which contained disorganized but recognizable enamel matrix, dentin, pulp, and cementum. LHX8 mRNA and LHX8 protein expressions were all higher in HODCs compared to those in aDSCs examined by RT-PCR, immunoblot, and immunofluorescent staining. Especially, real-time PCR showed 2.77-fold higher LHX8 expression in HODCs than in normal periodontal ligament stem cells (PDLSCs), while alveolar bone marrow stem cells (ABMSCs) expressed 0.12-fold LHX8 than PDLSCs.

CONCLUSIONS

Based on these observations, LHX8 might play an important role in odontoma formation. This is the first report regarding the comparison of LHX8 expression between HODC and normal aDSCs and its overexpression in human samples. The specific mechanism of LHX8 in odontoma morphogenesis awaits further study.

Introduction of a three-dimensional and layered (TDL) culture, a novel primary co-culture method for ameloblasts and pulp-derived cells

The enamel organ engaged in enamel matrix formation in tooth germs comprises four different cell types: the ameloblasts, the cells of the stratum intermedium, stellate reticulum, and the outer enamel epithelium, each characterized by distinct structural features. In ordinary primary cultures of tooth-derived cells, these cells generally become flat in profile and hardly regain their original profiles comparable to those in vivo, even under conditions that can induce the expression of functional markers from these cells. To overcome this limitation inherent to the cell culture of tooth-derived cells, we introduced a novel co-culture method, a "three-dimensional and layered (TDL) culture", a three-dimensional (3D) culture of dental pulp-derived cells dispersed in type I collagen gel combined with a layered culture of enamel epithelial cells seeded on top of the gel to establish thereby a culture condition where the functional tooth-derived cells regain their original structures and spatial arrangements. We subjected the TDL gels thus prepared to floating cultures and found that, in the layered epithelial cells, those facing the 3D gel became cuboidal/short columnar in shape, showed cell polarity and well-developed intercellular junctions, had PAS positive material in their cytoplasm, and expressed a distinct immunoreactivity for cyotokeratin 14 and amelogenins. Pulpal cells in the gel displayed a strong ALP activity throughout the 3D gel. The current observations have clearly shown that the structural and functional features reminiscent of early secretory ameloblasts could be restored in the enamel organ-derived cells in a TDL culture.

Whole genome microarray analysis of chicken embryo facial prominences

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

The LIM homeodomain transcription factors Lhx6 and Lhx7 are key regulators of mammalian dentition

Genes encoding LIM homeodomain transcription factors are implicated in cell type specification and differentiation during embryogenesis. Two closely related members of this family, Lhx6 and Lhx7, are expressed in the ectomesenchyme of the maxillary and mandibular processes and have been suggested to control patterning of the first branchial arch (BA1) and odontogenesis. However, mice homozygous for single mutations either have no cranial defects (Lhx6) or show only cleft palate (Lhx7). To reveal the potential redundant activities of Lhx6 and Lhx7 in cranial morphogenesis, we generated mice with all combinations of wild-type and mutant alleles. Double homozygous mice have characteristic defects of the cranial skeleton and die shortly after birth, most likely because of cleft palate. In addition, Lhx6/7 deficient embryos lack molar teeth. The absence of molars in double mutants is not due to patterning defects of BA1 but results from failure of specification of the molar mesenchyme. Despite molar agenesis, Lhx6/7-deficient animals have normal incisors which, in the maxilla, are flanked by a supernumerary pair of incisor-like teeth. Our experiments demonstrate that the redundant activities of the LIM homeodomain proteins Lhx6 and Lhx7 are critical for craniofacial development and patterning of mammalian dentition.

LIM-homeodomain genes in mammalian development and human disease

The human and mouse genomes each contain at least 12 genes encoding LIM homeodomain (LIM-HD) transcription factors. These gene regulatory proteins feature two LIM domains in their amino termini and a characteristic DNA binding homeodomain. Studies of mouse models and human patients have established that the LIM-HD factors are critical for the development of specialized cells in multiple tissue types, including the nervous system, skeletal muscle, the heart, the kidneys, and endocrine organs such as the pituitary gland and the pancreas. In this article, we review the roles of the LIM-HD proteins in mammalian development and their involvement in human diseases.

Fgf signaling is required for zebrafish tooth development

We have investigated fibroblast growth factor (FGF) signaling during the development of the zebrafish pharyngeal dentition with the goal of uncovering novel roles for FGFs in tooth development as well as phylogenetic and topographic diversity in the tooth developmental pathway. We found that the tooth-related expression of several zebrafish genes is similar to that of their mouse orthologs, including both epithelial and mesenchymal markers. Additionally, significant differences in gene expression between zebrafish and mouse teeth are indicated by the apparent lack of fgf8 and pax9 expression in zebrafish tooth germs. FGF receptor inhibition with SU5402 at 32 h blocked dental epithelial morphogenesis and tooth mineralization. While the pharyngeal epithelium remained intact as judged by normal pitx2 expression, not only was the mesenchymal expression of lhx6 and lhx7 eliminated as expected from mouse studies, but the epithelial expression of dlx2a, dlx2b, fgf3, and fgf4 was as well. This latter result provides novel evidence that the dental epithelium is a target of FGF signaling. However, the failure of SU5402 to block localized expression of pitx2 suggests that the earliest steps of tooth initiation are FGF-independent. Investigations of specific FGF ligands with morpholino antisense oligonucleotides revealed only a mild tooth shape phenotype following fgf4 knockdown, while fgf8 inhibition revealed only a subtle down-regulation of dental dlx2b expression with no apparent effect on tooth morphology. Our results suggest redundant FGF signals target the dental epithelium and together are required for dental morphogenesis. Further work will be required to elucidate the nature of these signals, particularly with respect to their origins and whether they act through the mesenchyme.