Adenovirus-mediated ectopic expression of Msx2 in even-numbered rhombomeres induces apoptotic elimination of cranial neural crest cells in ovo

Msx genes delineate a novel molecular map of the developing cerebellar neuroepithelium

In the early cerebellar primordium, there are two progenitor zones, the ventricular zone (VZ) residing atop the IVth ventricle and the rhombic lip (RL) at the lateral edges of the developing cerebellum. These zones give rise to the several cell types that form the GABAergic and glutamatergic populations of the adult cerebellum, respectively. Recently, an understanding of the molecular compartmentation of these zones has emerged. To add to this knowledge base, we report on the Msx genes, a family of three transcription factors, that are expressed downstream of Bone Morphogenetic Protein (BMP) signaling in these zones. Using fluorescent RNA in situ hybridization, we have characterized the Msx (Msh Homeobox) genes and demonstrated that their spatiotemporal pattern segregates specific regions within the progenitor zones. Msx1 and Msx2 are compartmentalized within the rhombic lip (RL), while Msx3 is localized within the ventricular zone (VZ). The relationship of the Msx genes with an early marker of the glutamatergic lineage, Atoh1, was examined in Atoh1-null mice and it was found that the expression of Msx genes persisted. Importantly, the spatial expression of Msx1 and Msx3 altered in response to the elimination of Atoh1. These results point to the Msx genes as novel early markers of cerebellar progenitor zones and more importantly to an updated view of the molecular parcellation of the RL with respect to the canonical marker of the RL, Atoh1.

Advances in tooth agenesis and tooth regeneration

The lack of treatment options for congenital (0.1%) and partial (10%) tooth anomalies highlights the need to develop innovative strategies. Over two decades of dedicated research have led to breakthroughs in the treatment of congenital and acquired tooth loss. We revealed that by inactivating USAG-1, congenital tooth agenesis can be successfully ameliorated during early tooth development and that the inactivation promotes late-stage tooth morphogenesis in double knockout mice. Furthermore, Anti- USAG-1 antibody treatment in mice is effective in tooth regeneration and can be a breakthrough in treating tooth anomalies in humans. With approximately 0.1% of the population suffering from congenital tooth agenesis and 10% of children worldwide suffering from partial tooth loss, early diagnosis will improve outcomes and the quality of life of patients. Understanding the role of pathogenic USAG-1 variants, their interacting gene partners, and their protein functions will help develop critical biomarkers. Advances in next-generation sequencing, mass spectrometry, and imaging technologies will assist in developing companion and predictive biomarkers to help identify patients who will benefit from tooth regeneration.

MSX2 represses tumor stem cell phenotypes within oral squamous cell carcinomas via SOX2 degradation

Oral squamous cell carcinoma (OSCC) is the sixth malignancy in the world with high incidence. The MSX2 (muscle segment homeobox 2)-Sry-related high-mobility box 2 (SOX2) signaling pathway plays a significant role in maintaining cancer stem cells, which are the origin of malignancy, leading to unfavorable outcomes in several carcinomas. This study aims to elucidate the mechanisms through which the MSX2-SOX2 pathway controls the cancer stem cell-like characterization in OSCC. The results showed that MSX2 was remarkably downregulated in OSCC and that the MSX2 expression level was related to unfavorable outcomes in patients with OSCC. Meanwhile, the MSX2 expression level was lower in the CD44+/CD24- population than in the other populations of OSCC cells. The OSCC2 cells exhibited decreased percentage of CD44+/CD24- cells, owing to MSX2 overexpression but increased owing to MSX2 knockdown. Moreover, a negative correlation was observed between MSX2 expression and is SOX2 transcriptional levels in different populations within the OSCC cell lines. Regarding the loss and gain of function, cancer stem cell phenotypes such as tumor globular formation, CD44+ subpopulation cells, and stem cell-associated gene expression were enhanced by MSX2 knockdown in OSCC CD44+/CD24- cells but decreased by MSX2 overexpression in other OSCC populations. However, these events were counteracted by the co-knockdown or SOX2 overexpression. Cells with MSX2 overexpression or knockdown formed smaller or bigger cancers in vivo, thereby showing a lower or a higher tumor incidence, respectively. Thus, our results confirm that MSX2 has a tumor suppression effect on the cancer stem cell phenotypes of OSCC and indicate that the MSX2-SOX2 signaling pathway could be a useful target for OSCC treatment.

MSX2 and BCL2 expressions in the development of anorectal malformations in ethylenethiourea-induced rat embryos

BACKGROUND

This study aimed to determine Msh homeobox 2 (MSX2) and B cell lymphoma-2 (BCL2) expression patterns during anorectal development in anorectal malformations (ARM) and normal rat embryos, with the goals of determining the role of MSX2 and BCL2 in ARM pathogenesis.

METHODS

ARM was induced in rat embryos with ethylenethiourea administered to dams on gestational day 10 (GD10). Embryos were harvested by cesarean deliveries from GD14 to GD16. MSX2 and BCL2 expression was evaluated via immunohistochemical staining, immunofluorescence, western blotting and quantitative real-time polymerase chain reaction (qRT-PCR).

RESULTS

Immunohistochemical staining of ARM embryos revealed that MSX2 was mainly expressed in the epithelium of the hindgut and urorectal septum (URS) on GD14. On GD15 and GD16, MSX2-immunolabeled cells were noted in the epithelium of the rectum, fistula and URS. However, in normal embryos, faint immunopositivity for MSX2 was demonstrated in the epithelium of the rectum and URS from GD14 to GD16. As for BCL2, in normal embryos, BCL2-immunopositive cells were extensively expressed in the epithelium of the hindgut and URS on GD14 and GD15. In ARM embryos, weak immunopositivity for BCL2 was detected in the epithelium of hindgut and URS on GD14 and GD15. Immunofluorescence revealed that MSX2 and BCL2 colocalized in the hindgut. In ARM embryos, we observed more MSX2-positive than BCL2-positive cells on GD14; the normal embryos had the opposite pattern. Analyses by western blot and qRT-PCR showed that MSX2 protein and mRNA expression was significantly increased in ARM embryos compared with the normal embryos on GD15 and GD16 (p < 0.05). However, BCL2 protein and mRNA expression was significantly decreased in ARM embryos compared with the normal embryos on GD14 (p < 0.05). The MSX2/BCL2 ratio of protein and mRNA expression level in the ARM group was the highest on GD15.

CONCLUSION

These results indicate that upregulation of MSX2 and downregulation of BCL2 during cloacal development into the rectum and urethra might be related to the ARM development, and MSX2 promoted apoptosis through reduction of BCL2 expression during the development of anorectal development in ARM.

Possibilities and limitations of three-dimensional reconstruction and simulation techniques to identify patterns, rhythms and functions of apoptosis in the early developing neural tube

The now classical idea that programmed cell death (apoptosis) contributes to a plethora of developmental processes still has lost nothing of its impact. It is, therefore, important to establish effective three-dimensional (3D) reconstruction as well as simulation techniques to decipher the exact patterns and functions of such apoptotic events. The present study focuses on the question whether and how apoptosis promotes neurulation-associated processes in the spinal cord of Tupaia belangeri (Tupaiidae, Scandentia, Mammalia). Our 3D reconstructions demonstrate that at least two craniocaudal waves of apoptosis consecutively pass through the dorsal spinal cord. The first wave appears to be involved in neural fold fusion and/or in selection processes among premigratory neural crest cells. The second one seems to assist in establishing the dorsal signaling center known as the roof plate. In the hindbrain, in contrast, apoptosis among premigratory neural crest cells progresses craniocaudally but discontinuously, in a segment-specific manner. Unlike apoptosis in the spinal cord, these segment-specific apoptotic events, however, precede later ones that seemingly support neural fold fusion and/or postfusion remodeling. Arguing with Whitehead that biological patterns and rhythms differ in that biological rhythms depend "upon the differences involved in each exhibition of the pattern" (Whitehead in An enquiry concerning the principles of natural knowledge. Cambridge University Press, London, 1919, p. 198) we show that 3D reconstruction and simulation techniques can contribute to distinguish between (static) patterns and (dynamic) rhythms of apoptosis. By deciphering novel patterns and rhythms of developmental apoptosis, our reconstructions help to reconcile seemingly inconsistent earlier findings in chick and mouse embryos, and to create rules for computer simulations.

Lens-specific deletion of the Msx2 gene increased apoptosis by enhancing the caspase-3/caspase-8 signaling pathway

Objective To investigate the influence of Msx2 conditional gene knockout during lens development in mice. Methods Lens-specific Msx2 knockout mice were generated using the Cre-loxP system. The eyes of Msx2 conditional knockout ( Msx2CKO) and wild-type ( Msx2WT) mice were examined during embryonic and early postnatal periods using histological, immunofluorescence, in situ hybridization, cell proliferation, apoptosis, and mRNA microarray analyses. Results Msx2CKO mice exhibited small lens formation and microphthalmia after birth, while Msx2CKO embryos exhibited a persistent lens stalk, small lens formation, and microphthalmia. Conditional deletion of Msx2 also led to an increased apoptosis rate, a significant reduction in FoxE3 expression, and an upregulation of Prox1 expression in the lens vesicle during the early embryonic period. Microarray comparison of Msx2CKO and Msx2WT lens transcriptomes identified a large number of differentially expressed genes. Real-time PCR showed that Casp8 and Casp3 expression was upregulated in Msx2CKO mice at post-natal day 1. Conclusion The activation of apoptosis through the caspase-8/caspase-3 signaling pathway, together with the downregulation of FoxE3 expression, appeared to account for the smaller lens formation in Msx2CKO mice.

Advances and perspectives in tooth tissue engineering

Bio-engineered teeth that can grow and remodel in a manner similar to that of natural teeth have the potential to serve as permanent replacements to the currently used prosthetic teeth, such as dental implants. A major challenge in designing functional bio-engineered teeth is to mimic both the structural and anisotropic mechanical characteristics of the native tooth. Therefore, the field of dental and whole tooth regeneration has advanced towards the molecular and nanoscale design of bio-active, biomimetic systems, using biomaterials, drug delivery systems and stem cells. The focus of this review is to discuss recent advances in tooth tissue engineering, using biomimetic scaffolds that provide proper architectural cues, exhibit the capacity to support dental stem cell proliferation and differentiation and sequester and release bio-active agents, such as growth factors and nucleic acids, in a spatiotemporal controlled manner. Although many in vitro and in vivo studies on tooth regeneration appear promising, before tooth tissue engineering becomes a reality for humans, additional research is needed to perfect methods that use adult human dental stem cells, as opposed to embryonic dental stem cells, and to devise the means to generate bio-engineered teeth of predetermined size and shape. Copyright © 2016 John Wiley & Sons, Ltd.

Interactions between BMP-7 and USAG-1 (uterine sensitization-associated gene-1) regulate supernumerary organ formations

Bone morphogenetic proteins (BMPs) are highly conserved signaling molecules that are part of the transforming growth factor (TGF)-beta superfamily, and function in the patterning and morphogenesis of many organs including development of the dentition. The functions of the BMPs are controlled by certain classes of molecules that are recognized as BMP antagonists that inhibit BMP binding to their cognate receptors. In this study we tested the hypothesis that USAG-1 (uterine sensitization-associated gene-1) suppresses deciduous incisors by inhibition of BMP-7 function. We learned that USAG-1 and BMP-7 were expressed within odontogenic epithelium as well as mesenchyme during the late bud and early cap stages of tooth development. USAG-1 is a BMP antagonist, and also modulates Wnt signaling. USAG-1 abrogation rescued apoptotic elimination of odontogenic mesenchymal cells. BMP signaling in the rudimentary maxillary incisor, assessed by expressions of Msx1 and Dlx2 and the phosphorylation of Smad protein, was significantly enhanced. Using explant culture and subsequent subrenal capsule transplantation of E15 USAG-1 mutant maxillary incisor tooth primordia supplemented with BMP-7 demonstrated in USAG-1+/- as well as USAG-1-/- rescue and supernumerary tooth development. Based upon these results, we conclude that USAG-1 functions as an antagonist of BMP-7 in this model system. These results further suggest that the phenotypes of USAG-1 and BMP-7 mutant mice reported provide opportunities for regenerative medicine and dentistry.

Id2 controls chondrogenesis acting downstream of BMP signaling during maxillary morphogenesis

Maxillofacial dysmorphogenesis is found in 5% of the population. To begin to understand the mechanisms required for maxillofacial morphogenesis, we employed the inhibitors of the differentiation 2 (Id2) knock-out mouse model, in which Id proteins, members of the regulator of basic helix-loop-helix (bHLH) transcription factors, modulate cell proliferation, apoptosis, and differentiation. We now report that spatially-restricted growth defects are localized at the skull base of Id2 KO mice. Curiously, at birth, neither the mutant Id2 KO nor wild-type (WT) mice differed, based upon cephalometric and histological analyses of cranial base synchondroses. In postnatal week 2, a narrower hypertrophic zone and an inhibited proliferative zone in presphenoid synchondrosis (PSS) and spheno-occipital synchondrosis (SOS) with maxillary hypoplasia were identified in the Id2 mutant mice. Complementary studies revealed that exogenous bone morphogenetic proteins (BMPs) enhanced cartilage growth, matrix deposition, and chondrocyte proliferation in the WT but not in the mutant model. Id2-deficient chondrocytes expressed more Smad7 transcripts. Based on our results, we assert that Id2 plays an essential role, acting downstream of BMP signaling, to regulate cartilage formation at the postnatal stage by enhancing BMP signals through inhibiting Smad7 expression. As a consequence, abnormal endochondral ossification was observed in cranial base synchondroses during the postnatal growth period, resulting in the clinical phenotype of maxillofacial dysmorphogenesis.

Functional and prognostic relevance of the homeobox protein MSX2 in malignant melanoma

Background:

The homeobox containing transcription factor MSX2 is a key regulator of embryonic development and has been implicated to have a role in breast and pancreatic cancer.

Methods:

Using a selection of two- and three-dimensional in vitro assays and tissue microarrays (TMAs), the clinical and functional relevance of MSX2 in malignant melanoma was explored. A doxycyline-inducible over-expression system was applied to study the relevance of MSX2 in vitro. For TMA construction, tumour material from 218 melanoma patients was used.

Results:

Ectopic expression of MSX2 resulted in the induction of apoptosis and reduced the invasive capacity of melanoma cells in three-dimensional culture. MSX2 over-expression was shown to affect several signalling pathways associated with cell invasion and survival. Downregulation of N-Cadherin, induction of p21 and inhibition of both BCL2 and Survivin were observed. Cytoplasmic MSX2 expression was found to correlate significantly with increased recurrence-free survival (P=0.008). Nuclear expression of MSX2 did not result in significant survival correlations, suggesting that the beneficial effect of MSX2 may be independent of its DNA binding activity.

Conclusions:

MSX2 may be an important regulator of melanoma cell invasion and survival. Cytoplasmic expression of the protein was identified as biomarker for good prognosis in malignant melanoma patients.

Homeobox transcription factor muscle segment homeobox 2 (Msx2) correlates with good prognosis in breast cancer patients and induces apoptosis in vitro

Introduction

The homeobox-containing transcription factor muscle segment homeobox 2 (Msx2) plays an important role in mammary gland development. However, the clinical implications of Msx2 expression in breast cancer are unclear. The aims of this study were to investigate the potential clinical value of Msx2 as a breast cancer biomarker and to clarify its functional role in vitro.

Methods

Msx2 gene expression was first examined in a well-validated breast cancer transcriptomic dataset of 295 patients. Msx2 protein expression was then evaluated by immunohistochemistry in a tissue microarray (TMA) containing 281 invasive breast tumours. Finally, to assess the functional role of Msx2 in vitro, Msx2 was ectopically expressed in a highly invasive breast tumour cell line (MDA-MB-231) and an immortalised breast cell line (MCF10a), and these cell lines were examined for changes in growth rate, cell death and cell signalling.

Results

Examination of Msx2 mRNA expression in a breast cancer transcriptomic dataset demonstrated that increased levels of Msx2 were associated with good prognosis (P = 0.011). Evaluation of Msx2 protein expression on a TMA revealed that Msx2 was detectable in both tumour cell nuclei and cytoplasm. Cytoplasmic Msx2 expression was associated with low grade tumours (P = 0.012) and Ki67 negativity (P = 0.018). Nuclear Msx2 correlated with low-grade tumours (P = 0.015), estrogen receptor positivity (P = 0.038), low Ki67 (P = 0.005) and high cyclin D1 expression (P = 0.037). Increased cytoplasmic Msx2 expression was associated with a prolonged breast cancer-specific survival (P = 0.049), recurrence-free survival (P = 0.029) and overall survival (P = 0.019). Ectopic expression of Msx2 in breast cell lines resulted in radically decreased cell viability mediated by induction of cell death via apoptosis. Further analysis of Msx2-expressing cells revealed increased levels of p21 and phosphorylated extracellular signal-regulated kinase (ERK) and decreased levels of Survivin and the 'split ends' (SPEN) protein family member RBM15.

Conclusions

We conclude that increased Msx2 expression results in improved outcome for breast cancer patients, possibly by increasing the likelihood of tumour cell death by apoptosis.

BMP signaling in the human fetal ovary is developmentally regulated and promotes primordial germ cell apoptosis

Primordial germ cells (PGCs) are the embryonic precursors of gametes in the adult organism, and their development, differentiation, and survival are regulated by a combination of growth factors collectively known as the germ cell niche. Although many candidate niche components have been identified through studies on mouse PGCs, the growth factor composition of the human PGC niche has not been studied extensively. Here we report a detailed analysis of the expression of components of the bone morphogenetic protein (BMP) signaling apparatus in the human fetal ovary, from postmigratory PGC proliferation to the onset of primordial follicle formation. We find developmentally regulated and reciprocal patterns of expression of BMP2 and BMP4 and identify germ cells to be the exclusive targets of ovarian BMP signaling. By establishing long-term cultures of human fetal ovaries in which PGCs are retained within their physiological niche, we find that BMP4 negatively regulates postmigratory PGC numbers in the human fetal ovary by promoting PGC apoptosis. Finally, we report expression of both muscle segment homeobox (MSX)1 and MSX2 in the human fetal ovary and reveal a selective upregulation of MSX2 expression in human fetal ovary in response to BMP4, suggesting this gene may act as a downstream effector of BMP-induced apoptosis in the ovary, as in other systems. These data reveal for the first time growth factor regulation of human PGC development in a physiologically relevant context and have significant implications for the development of cultures systems for the in vitro maturation of germ cells, and their derivation from pluripotent stem cells.

Msx genes are important apoptosis effectors downstream of the Shh/Gli3 pathway in the limb

In tetrapods, the anteroposterior (AP) patterning of the limb is under the control of the antagonistic activities of the secreted factor Sonic hedgehog (Shh) and Gli3R, the truncated repressor form of the transcription factor Gli3. In this report, we show that Msx1 and Msx2 are targets and downstream effectors of Gli3R. Consequently, in Shh null mutants, Msx genes are overexpressed and, furthermore, partially responsible for the limb phenotype. This is exemplified by the fact that reducing Msx activity in Shh mutants partially restores a normal limb development. Finally, we show that the main action of the Msx genes, in both normal and Shh(-/-) limb development, is to control cell death in the mesenchyme. We propose that, in the limb, Msx genes act downstream of the Shh/Gli3 pathway by transducing BMP signaling and that, in the absence of Shh signaling, their deregulation contributes to the extensive apoptosis that impairs limb development.

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.

Reiterated Wnt and BMP signals in neural crest development

Common signaling pathways such as those for Wnts and BMPs are used many times during embryogenesis. During the development of the neural crest, Wnt and BMP signals are used repeatedly at different stages to influence initial induction, segregation from the neuroepithelium and cell fate determination. This review considers how specificity is generated within the neural crest for these reiterated signals, discussing examples of how the outcomes of signaling events are modulated by context.

msh/Msx gene family in neural development

The involvement of Msx homeobox genes in skull and tooth formation has received a great deal of attention. Recent studies also indicate a role for the msh/Msx gene family in development of the nervous system. In this article, we discuss the functions of these transcription factors in neural-tissue organogenesis. We will deal mainly with the interactions of the Drosophila muscle segment homeobox (msh) gene with other homeobox genes and the repressive cascade that leads to neuroectoderm patterning; the role of Msx genes in neural-crest induction, focusing especially on the differences between lower and higher vertebrates; their implication in patterning of the vertebrate neural tube, particularly in diencephalon midline formation. Finally, we will examine the distinct activities of Msx1, Msx2 and Msx3 genes during neurogenesis, taking into account their relationships with signalling molecules such as BMP.

Somatic transgenesis in the avian model system

The chick embryo is a versatile model system, in which classical embryology can be combined with modern molecular approaches. In the last two decades, several efficient methods have been developed to introduce exogenous genes into the chick embryo. These techniques allow alteration of gene expression levels in a spatially and temporally restricted manner, thereby circumventing embryonic lethality and/or eliminating secondary effects in other tissues. Here, we present the current status of avian somatic transgenic techniques, focusing on electroporation and retrovirus-mediated gene transfer. Electroporation allows quick and efficient gain-of-function studies based on transient misexpression of genes. Retroviral vectors, which are capable of integrating exogenous genes into the host chromosome, permit analysis of long-term effects of gene misexpression. The variety of methods available for somatic transgenesis, along with the recent completion of the chicken genome, are transforming the chick embryo into one of the most attractive model systems to examine function of genes that are important for embryonic development.

Protein phosphatase 2A as a new target for morphogenetic studies in the chick limb

The family of ser/thr protein phosphatases 2A (PP2A) is a major regulator of cell proliferation and cell death and is critically involved in the maintenance of homeostasis. In order to analyse the importance of PP2A proteins in apoptotic and developmental processes, this review focuses on previous studies concerning the role of PP2A in morphogenesis. We first analyse wing formation in Drosophila, a model for invertebrates, then chick limb bud, a model for vertebrates. We also present a pioneer experiment to illustrate the potential relevance of PP2A studies in BMP signalling during chicken development and we finally discuss the BMP downstream signalling pathways.

Msx homeobox gene family and craniofacial development

Vertebrate Msx genes are unlinked, homeobox-containing genes that bear homology to the Drosophila muscle segment homeobox gene. These genes are expressed at multiple sites of tissue-tissue interactions during vertebrate embryonic development. Inductive interactions mediated by the Msx genes are essential for normal craniofacial, limb and ectodermal organ morphogenesis, and are also essential to survival in mice, as manifested by the phenotypic abnormalities shown in knockout mice and in humans. This review summarizes studies on the expression, regulation, and functional analysis of Msx genes that bear relevance to craniofacial development in humans and mice. Key words: Msx genes, craniofacial, tooth, cleft palate, suture, development, transcription factor, signaling molecule.

Comparative analysis of neural crest cell death, migration, and function during vertebrate embryogenesis

Cranial neural crest cells are a multipotent, migratory population that generates most of the cartilage, bone, connective tissue and peripheral nervous system in the vertebrate head. Proper neural crest cell patterning is essential for normal craniofacial morphogenesis and is highly conserved among vertebrates. Neural crest cell patterning is intimately connected to the early segmentation of the neural tube, such that neural crest cells migrate in discrete segregated streams. Recent advances in live embryo imaging have begun to reveal the complex behaviour of neural crest cells which involve intricate cell-cell and cell-environment interactions. Despite the overall similarity in neural crest cell migration between distinct vertebrates species there are important mechanistic differences. Apoptosis for example, is important for neural crest cell patterning in chick embryos but not in mouse, frog or fish embryos. In this paper we highlight the potential evolutionary significance of such interspecies differences in jaw development and evolution. Developmental Dynamics 229:14-29, 2004.

Somatic transgenesis using retroviral vectors in the chicken embryo

The avian embryo is an excellent model system for experimental studies because of its accessibility and ease of microsurgical manipulations. While the complete chicken genome sequence will soon be determined, a comprehensive germ cell transmission-based genetic approach is not available for this animal model. Several techniques of somatic cell transgenesis have been developed in the past decade. Of these, the retroviral shuttle vector system provides both (1) stable integration of exogenous genes into the host cell genome, and (2) constant expression levels in a target cell population over the course of development. This review summarizes retroviral vectors available for the avian model and outlines the uses of retroviral-mediated gene transfer for cell lineage analysis as well as functional studies of genes and proteins in the chick embryo.

Smad4 and beta-catenin co-activators functionally interact with lymphoid-enhancing factor to regulate graded expression of Msx2

Recent in vivo evidence suggests that Wnt signaling plays a central role in determining the fate of stem cells in the ectoderm and in the neural crest by modulating bone morphogenetic protein (BMP) levels, which, in turn, influence Msx gene expression. However, the molecular mechanism regulating the expression of the Msx genes as key regulators of cell fate has not been elucidated. Here we show in murine embryonic stem cells that BMP-dependent activation of Msx2 is mediated via the cooperative binding of Smad4 at two Smad binding elements and of lymphoid enhancing factor (Lef1) at two Lef1/TCF binding sites. Lef1 can synergize with Smad4 and Smad1 to activate Msx2 promoter, and this transcriptional complex is assembled on the endogenous promoter in response to BMP2. The Wnt/beta-catenin signaling pathway can activate Msx2 via the binding of Lef1 to its promoter and synergizes with BMP2 to activate Msx2 expression, possibly via enhanced recruitment of the p300/cAMP-response element-binding protein-binding protein co-factor. Interestingly, the Wnt/beta-catenin-dependent activation of Msx2 was defective in Smad4-deficient embryonic stem cells or when Smad binding elements were mutated but persisted in the presence of various BMP antagonists, indicating that Smad4 was involved in transducing the Wnt/beta-catenin signals in the absence of a BMP autocrine loop. A chromatin immunoprecipitation analysis revealed that endogenous Smad4, but not Smad1, was part of the Lef1 transcriptional complex in response to beta-catenin activation, dismissing any implication of BMP signaling in this response. We propose that Wnt signaling pathway could dictate cell fate not only by modulating BMP levels but also by directly regulating cooperatively BMP-target genes.

Morphogenesis and dysmorphogenesis of the appendicular skeleton

Cartilage patterning and differentiation are prerequisites for skeletal development through endochondral ossification (EO). Multipotential mesenchymal cells undergo a complex process of cell fate determination to become chondroprogenitors and eventually differentiate into chondrocytes. These developmental processes require the orchestration of cell-cell and cell-matrix interactions. In this review, we present limb bud development as a model for cartilage patterning and differentiation. We summarize the molecular and cellular events and signaling pathways for axis patterning, cell condensation, cell fate determination, digit formation, interdigital apoptosis, EO, and joint formation. The interconnected nature of these pathways underscores the effects of genetic and teratogenic perturbations that result in skeletal birth defects. The topics reviewed also include limb dysmorphogenesis as a result of genetic disorders and environmental factors, including FGFR, GLI3, GDF5/CDMP1, Sox9, and Cbfa1 mutations, as well as thalidomide- and alcohol-induced malformations. Understanding the complex interactions involved in cartilage development and EO provides insight into mechanisms underlying the biology of normal cartilage, congenital disorders, and pathologic adult cartilage.

Prospects for tooth regeneration in the 21st century: a perspective

The prospects for tooth regeneration in the 21st century are compelling. Using the foundations of experimental embryology, developmental and molecular biology, the principles of biomimetics (the mimicking of biological processes), tooth regeneration is becoming a realistic possibility within the next few decades. The cellular, molecular, and developmental "rules" for tooth morphogenesis are rapidly being discovered. The knowledge gained from adult stem cell biology, especially associated with dentin, cartilage, and bone tissue regeneration, provides additional opportunities for eventual tooth organogenesis. The centuries of tooth development using xenotransplantation, allotransplantation, and autotransplantation have resulted in many important insights that can enhance tooth regeneration. In considering the future, several lines of evidence need to be considered: (1) enamel organ epithelia and dental papilla mesenchyme tissues contain stem cells during postnatal stages of life; (2) late cap stage and bell stage tooth organs contain stem cells; (3) odontogenic adult stem cells respond to mechanical as well as chemical "signals"; (4) presumably adult bone marrow as well as dental pulp tissues contain "odontogenic" stem cells; and (5) epithelial-mesenchymal interactions are pre-requisite for tooth regeneration. The authors express "guarded enthusiasm," yet there should be little doubt that adult stem cell-mediated tooth regeneration will be realized in the not too distant future. The prospects for tooth regeneration could be realized in the next few decades and could be rapidly utilized to improve the quality of human life in many nations around the world.

Region- and stage-specific effects of FGFs and BMPs in chick mandibular morphogenesis

The mandibular processes are specified as at least two independent functional regions: two large lateral regions where morphogenesis is dependent on fibroblast growth factor (FGF)-8 signaling, and a small medial region where morphogenesis is independent of FGF-8 signaling. To gain insight into signaling pathways that may be involved in morphogenesis of the medial region, we have examined the roles of pathways regulated by FGFs and bone morphogenetic proteins (BMPs) in morphogenesis of the medial and lateral regions of the developing chick mandible. Our results show that, unlike in the lateral region, the proliferation and growth of the mesenchyme in the medial region is dependent on signals derived from the overlying epithelium. We also show that medial and lateral mandibular mesenchyme respond differently to exogenous FGFs and BMPs. FGF-2 and FGF-4 can mimic many of the effects of mandibular epithelium from the medial region, including supporting the expression of Msx genes, outgrowth of the mandibular processes and elongation of Meckel's cartilage. On the other hand, laterally placed FGF beads did not induce ectopic expression of Msx genes and did not affect the growth of the mandibular processes. These functional studies, together with our tissue distribution studies, suggest that FGF-mediated signaling (other than FGF-8), through interactions with FGF receptor-2 and downstream target genes including Msx genes, is part of the signaling pathway that mediates the growth-promoting interactions in the medial region of the developing mandible. Our observations also suggest that BMPs play multiple stage- and region-specific roles in mandibular morphogenesis. In this study, we show that exogenous BMP-7 applied to the lateral region at early stages of development (stage 20) caused apoptosis, ectopic expression of Msx genes, and inhibited outgrowth of the mandibular processes and the formation of Meckel's cartilage. Our additional experiments suggest that the differences between the effects of BMP-7 on lateral mandibular mesenchyme at stage 20 and previously reported results at stage 23 (Wang et al., [1999] Dev. Dyn. 216:320-335) are related to differences in stages of differentiation in that BMP-7 promotes apoptosis in undifferentiated lateral mandibular mesenchyme, whereas it promotes chondrogenesis at later stages of development. We also showed that, unlike mandibular epithelium and medially placed FGF beads, medially placed BMP-7 did not support outgrowth of the isolated mesenchyme and at stage 20 induced the formation of a duplicated rod of cartilage extending from the body of Meckel's cartilage. These observations suggest that BMPs do not play essential roles in growth-promoting interactions in the medial region of the developing mandible. However, BMP-mediated signaling is a part of the signaling pathways regulating chondrogenesis of the mandibular mesenchyme.

Retinal ablation and altered lens differentiation induced by ocular overexpression of BMP7

The alphaA-crystallin promoter was used to target expression of bone morphogenetic protein 7 (BMP7) to lens fiber cells in transgenic mice. Surprisingly, lens-specific expression of BMP7 induced widespread apoptosis and rapid ablation of the neural retina in multiple families. Subsequent to retinal ablation, the lens bow region shifted posteriorly until lens epithelial cells completely enveloped the lens. Lens-specific expression of FGF3 was found to rescue the loss of fiber cell differentiation. Our results show that elevated BMP7 levels can induce rapid retinal degeneration accompanied by disruption of the endogenous ocular system for fiber cell induction.

Screening from a subtracted embryonic chick hindbrain cDNA library: identification of genes expressed during hindbrain, midbrain and cranial neural crest development

The vertebrate hindbrain is segmented into a series of transient structures called rhombomeres. Despite knowing several factors that are responsible for the segmentation and maintenance of the rhombomeres, there are still large gaps in understanding the genetic pathways that govern their development. To find previously unknown genes that are expressed within the embryonic hindbrain, a subtracted chick hindbrain cDNA library has been made and 445 randomly picked clones from this library have been analysed using whole mount in situ hybridisation. Thirty-six of these clones (8%) display restricted expression patterns within the hindbrain, midbrain or cranial neural crest and of these, twenty-two are novel and eleven encode peptides that correspond to or are highly related to proteins with previously uncharacterised roles during early neural development. The large proportion of genes with restricted expression patterns and previously unknown functions in the embryonic brain identified during this screen provides insights into the different types of molecules that have spatially regulated expression patterns in cranial neural tissue.

Transient modulation of gene expression in the neurulation staged mouse embryo

Transient modulation of gene expression in the embryo during early organogenesis will allow studies to be conducted that determine tissue- and stage-specific function(s) of genes. To achieve this goal, viral vectors and antisense oligodeoxynucleotides have been used to produce gain-of-function and loss-of-function models. Adenoviral transduction of whole embryos, embryonic heart and vasculature, and primary neural crest cell culture has been reported. The morphological consequences of overexpression or decreasing expression of selected genes have been evaluated using these tools. Gene-teratogen interaction studies have also been performed. The viral vectors appear to be important tools for modulating gene expression and hold great promise for future research.

The WNT antagonist cSFRP2 modulates programmed cell death in the developing hindbrain

In the avian hindbrain, the loss of premigratory neural crest cells from rhombomeres 3 and 5 (r3, r5) through programmed cell death contributes to the patterning of emigrant crest cells into three discrete streams. Programmed cell death is induced by the upregulation of Bmp4 and Msx2 in r3 and r5. We show that cSFRP2, a WNT antagonist, is expressed in the even-numbered rhombomeres and that over-expression of cSfrp2 inhibits Bmp4 expression in r3 and r5, preventing programmed cell death. By contrast, depleting cSFRP2 function in r4 results in elevated levels of Msx2 expression and ectopic programmed cell death, as does overexpression of Wnt1. We propose that programmed cell death in the rhombencephalic neural crest is modulated by pre-patterned cSfrp2 expression and a WNT-BMP signalling loop.

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.

Positionally-dependent chondrogenesis induced by BMP4 is co-regulated by Sox9 and Msx2

Cranial neural crest cells emigrate from the posterior midbrain and anterior hindbrain to populate the first branchial arch and eventually differentiate into multiple cell lineages in the maxilla and mandible during craniofacial morphogenesis. In the developing mouse mandibular process, the expression profiles of BMP4, Msx2, Sox9, and type II collagen demonstrate temporally and spatially restrictive localization patterns suggestive of their functions in the patterning and differentiation of cartilage. Under serumless culture conditions, beads soaked in BMP4 and implanted into embryonic day 10 (E10) mouse mandibular explants induced ectopic cartilage formation in the proximal position of the explant. However, BMP4-soaked beads implanted at the rostral position did not have an inductive effect. Ectopic chondrogenesis was associated with the up-regulation of Sox9 and Msx2 expression in the immediate vicinity of the BMP4 beads 24 hours after implantation. Control beads had no effect on cartilage induction or Msx2 and Sox9 expression. Sox9 was induced at all sites of BMP4 bead implantation. In contrast, Msx2 expression was induced more intensely at the rostral position when compared with the proximal position, and suggested that Msx2 expression was inhibitory to chondrogenesis. To test the hypothesis that over-expression of Msx2 inhibits chondrogenesis, we ectopically expressed Msx2 in the mandibular process organ culture system using adenovirus gene delivery strategy. Microinjection of the Msx2-adenovirus to the proximal position inhibited BMP4-induced chondrogenesis. Over-expression of Msx2 also resulted in the abrogation of endogenous cartilage and the down-regulation of type II collagen expression. Taken together, these results suggest that BMP4 induces chondrogenesis, the pattern of which is positively regulated by Sox9 and negatively by Msx2. Chondrogenesis only occurs at sites where Sox9 expression is high relative to that of Msx2. The combinatorial action of these transcription factors appear to establish a threshold for Sox9 function and thereby restricts the position of chondrogenesis.

Ectopic expression of Msx2 in chick retinal pigmented epithelium cultures suggests a role in patterning the optic vesicle

During the initial stages of vertebrate retinogenesis, cells of the optic vesicle adopt one of two alternate cell fates. Cells in the distal-most part of the vesicle, immediately beneath the surface ectoderm, undergo neural differentiation; cells in the proximal part differentiate into retinal pigmented epithelial cells. The mechanisms that establish this pattern of differentiation are poorly understood. In the mouse embryo, Msx2, a homeobox-containing transcription factor, is expressed in cells of the optic vesicle that will form the neural retina, whilst the developing retinal pigmented epithelium (RPE) does not express this gene. Msx2 could therefore be involved in patterning the optic vesicle into neural and pigmented domains. To explore this possibility we ectopically expressed mouse Msx2 in cultures of chick RPE cells. Compared with cultures transfected with a control construct, Msx2-transfected cultures contained fewer cells expressing the RPE marker, Mitf, and more cells expressing class III beta-tubulin, a neuronal marker. In addition a small proportion of Msx2-transfected cells acquired a neural-like morphology. These results show that Msx2 can suppress the differentiated state of RPE cells and promote their differentiation into neural cell types. We suggest that Msx2 may pattern the optic vesicle into neural and pigmented domains by affecting the balance between RPE and neural retina differentiation.

A zinc-finger transcription factor induced by TGF-beta promotes apoptotic cell death in epithelial Mv1Lu cells

Transforming growth factor-beta (TGF-beta) superfamily members constitute a group of multifunctional factors that are able to stimulate apoptotic cell death in a variety of cells. In this report, we show that a zinc-finger transcription factor (TIEG) is an immediate early gene transcriptionally induced by TGF-beta in the epithelial Mv1Lu cell line. We also demonstrate that, mimicking TGF-beta effects, ectopic overexpression of TIEG is sufficient to trigger the apoptotic cell program in these cells, which is preceded by a decrease of Bcl-2 protein levels. Finally, apoptotic events elicited by TIEG overexpression can be effectively prevented by ectopic co-expression of Bcl-2. On the basis of these results we suggest that induction of TIEG expression has a role in the pro-apoptotic properties of TGF-beta.

A paraxial exclusion zone creates patterned cranial neural crest cell outgrowth adjacent to rhombomeres 3 and 5

Cranial neural crest cell migration is patterned, with neural crest cell-free zones adjacent to rhombomere (R) 3 and R5. These zones have been suggested to result from death of premigratory neural crest cells via upregulation of BMP-4 and Msx-2 in R3 and R5, consequent to R2-, R4-, and R6-derived signals. We reinvestigated this model and found that cell death detected by acridine orange staining in avian embryos varied widely numerically and in pattern, but with a tendency for an elevated zone centered at the R2/3 boundary. In situ hybridization of BMP-4 mRNA resolved to centers at R3 and R5 but Msx-2 resolved to the R2/3 border with only a faint smear from R5 to R6. Outgrowth of neural crest cells was less in isolated R3 cultures than in R1+2, R2, and R4 cultures, but R3 showed neither a decrease in outgrowth of neural crest cells nor an increase in cell death when cocultured with R1+2, R2, or R4. In addition, in serum-free culture, exogenous BMP-4 strikingly reduced neural crest cell outgrowth from R1+2 and R4 as well as R3. Thus we cannot confirm the role of intraneural cell death in patterning rhombomeric neural crest outgrowth. However, grafting quail R2 or R4 adjacent to the chick hindbrain demonstrated a neural crest cell exclusion zone next to R3 and R5. We suggest that one important pattern determinant for rhombomeric neural crest cell migration involves the microenvironment next to the neural tube.

Ectopic bone morphogenetic proteins 5 and 4 in the chicken forebrain lead to cyclopia and holoprosencephaly

Proper dorsal-ventral patterning in the developing central nervous system requires signals from both the dorsal and ventral portions of the neural tube. Data from multiple studies have demonstrated that bone morphogenetic proteins (BMPs) and Sonic hedgehog protein are secreted factors that regulate dorsal and ventral specification, respectively, within the caudal neural tube. In the developing rostral central nervous system Sonic hedgehog protein also participates in ventral regionalization; however, the roles of BMPs in the developing brain are less clear. We hypothesized that BMPs also play a role in dorsal specification of the vertebrate forebrain. To test our hypothesis we implanted beads soaked in recombinant BMP5 or BMP4 into the neural tube of the chicken forebrain. Experimental embryos showed a loss of the basal telencephalon that resulted in holoprosencephaly (a single cerebral hemisphere), cyclopia (a single midline eye), and loss of ventral midline structures. In situ hybridization using a panel of probes to genes expressed in the dorsal and ventral forebrain revealed the loss of ventral markers with the maintenance of dorsal markers. Furthermore, we found that the loss of the basal telencephalon was the result of excessive cell death and not a change in cell fates. These data provide evidence that BMP signaling participates in dorsal-ventral patterning of the developing brain in vivo, and disturbances in dorsal-ventral signaling result in specific malformations of the forebrain.

Progress toward understanding craniofacial malformations

Significant advances in the study of the human face have revealed the genetic and gene-environment bases of numerous common and rare craniofacial disorders. Classification of craniofacial malformations based on clinical phenotypes is sometimes quite different from the genetic findings of patients. Different mutations in a single gene can cause distinct syndromes, and mutations in different genes can cause the same syndrome. The extracellular signaling molecule SHH, fibroblast growth factor receptors, and transcription factors GLI3, MSX2, and TWIST are discussed as examples of molecules involved in interrelated signal transduction networks regulating craniofacial development. Progress in the understanding of normal and abnormal craniofacial development, through the study of morphoregulatory signaling pathways, has benefited from multifactorial approaches recommended 40 years ago at the National Institute of Dental Research-sponsored landmark Gatlinburg Conference. The utilization of biochemistry, protein structure analyses, tissue culture, and animal model systems for developmental genetics has resulted in remarkable scientific advances. The evolutionary conservation of morphoregulatory pathways has revealed the homology of genes associated with human craniofacial malformations and their counterparts that regulate the morphogenesis of fruit flies. The continued investments in basic, translational, and patient-oriented research regarding normal and abnormal craniofacial development will translate into substantial improvements in the prevention, diagnosis, and treatment of craniofacial diseases and disorders.