Early embryonic lethality in Bmp5;Bmp7 double mutant mice suggests functional redundancy within the 60A subgroup

Twisted gastrulation limits apoptosis in the distal region of the mandibular arch in mice

The mandibular arch (BA1) is critical for craniofacial development. The distal region of BA1, which gives rise to most of the mandible, is dependent upon an optimal level of bone morphogenetic protein (BMP) signaling. BMP activity is modulated in the extracellular space by BMP-binding proteins such as Twisted gastrulation (TWSG1). Twsg1(-/-) mice have a spectrum of craniofacial phenotypes, including mandibular defects that range from micrognathia to agnathia. At E9.5, the distal region of the mutant BA1 was prematurely and variably fused with loss of distal markers eHand and Msx1. Expression of proximal markers Fgf8 and Barx1 was expanded across the fused BA1. The expression of Bmp4 and Msx2 was preserved in the distal region, but shifted ventrally. While wild type embryos showed a gradient of BMP signaling with higher activity in the distal region of BA1, this gradient was disrupted and shifted ventrally in the mutants. Thus, loss of TWSG1 results in disruption of the BMP4 gradient at the level of signaling activity as well as mRNA expression. Altered distribution of BMP signaling leads to a shift in gene expression and increase in apoptosis. The extent of apoptosis may account for the variable degree of mandibular defects in Twsg1 mutants.

The double-histone-acetyltransferase complex ATAC is essential for mammalian development

Acetylation of the histone tails, catalyzed by histone acetyltransferases (HATs), is a well-studied process that contributes to transcriptionally active chromatin states. Here we report the characterization of a novel mammalian HAT complex, which contains the two acetyltransferases GCN5 and ATAC2 as well as other proteins linked to chromatin metabolism. This multisubunit complex has a similar but distinct subunit composition to that of the Drosophila ADA2A-containing complex (ATAC). Recombinant ATAC2 has a weak HAT activity directed to histone H4. Moreover, depletion of ATAC2 results in the disassembly of the complex, indicating that ATAC2 not only carries out an enzymatic function but also plays an architectural role in the stability of mammalian ATAC. By targeted disruption of the Atac2 locus in mice, we demonstrate for the first time the essential role of the ATAC complex in mammalian development, histone acetylation, cell cycle progression, and prevention of apoptosis during embryogenesis.

Germline translocations in mice: unique tools for analyzing gene function and long-distance regulatory mechanisms

Translocations have provided invaluable tools for identifying both cancer-linked genes and loci associated with heritable human diseases, but heritable human translocations are rare and few mouse models exist. Here we report progress on analysis of a collection of heritable translocations generated by treatment of mice with specific chemicals or radiation during late spermatogenic stages. The translocation mutants exhibit a range of visible phenotypes reflecting the disruption of coding sequences or the separation of genes from essential regulatory elements. The breakpoints of both radiation-induced and chemically induced mutations in these mice are remarkably clean, with very short deletions, duplications, or inversions in some cases, and ligation mediated by microhomology, suggesting nonhomologous end joining as the major path of repair. These mutations provide new tools for the discovery of novel genes and regulatory elements linked to human developmental disorders and new clues to the molecular basis of human genetic disease.

An in vivo reporter of BMP signaling in organogenesis reveals targets in the developing kidney

Background

Bone morphogenetic proteins (BMPs) regulate essential processes during organogenesis, and a functional understanding of these secreted proteins depends on identification of their target cells. In this study, we generate a transgenic reporter for organogenesis studies that we use to define BMP pathway activation in the developing kidney.

Results

Mouse strains reporting on BMP pathway activation were generated by transgenically expressing β-galactosidase under the control of BMP responsive elements from Id1. Reporter expression corresponds well with immunoassays for pathway activation in all organs studied, validating the model. Using these reporters we have generated a detailed map of cellular targets of BMP signaling in the developing kidney. We find that SMAD dependent BMP signaling is active in collecting duct trunks, but not tips. Furthermore, glomerular endothelial cells, and proximal nephron tubules from the renal vesicle stage onward show pathway activation. Surprisingly, little activation is detected in the nephrogenic zone of the kidney, and in organ culture BMP treatment fails to activate SMAD dependent BMP signaling in nephron progenitor cells. In contrast, signaling is efficiently induced in collecting duct tips.

Conclusion

Transgenic reporters driven by control elements from BMP responsive genes such as Id1 offer significant advantages in sensitivity and consistency over immunostaining for studies of BMP pathway activation. They also provide opportunities for analysis of BMP signaling in organ and primary cell cultures subjected to experimental manipulation. Using such a reporter, we made the surprising finding that SMAD dependent BMP signaling is inactive in nephron progenitors, and that these cells are refractory to activation by applied growth factors. Furthermore, we find that the BMP pathway is not normally active in collecting duct tips, but that it can be ectopically activated by BMP treatment, offering a possible explanation for the inhibitory effects of BMP treatment on collecting duct growth and branching.

Embryonic development in the reduced folate carrier knockout mouse is modulated by maternal folate supplementation

BACKGROUND

The reduced folate carrier (RFC1) is a ubiquitously expressed integral membrane protein that mediates delivery of 5-methyltetrahydrofolate into mammalian cells. In this study, embryonic/fetal development is characterized in an RFC1 knockout mouse model in which pregnant dams receive different levels of folate supplementation.

METHODS

RFC1(+/-) males were mated to RFC1(+/-) females, and pregnant dams were treated with vehicle (control) or folic acid (25 or 50 mg/kg) by daily subcutaneous injection (0.1 mL/10 g bwt), beginning on E0.5 and continuing throughout gestation until the time of sacrifice.

RESULTS

Without maternal folate supplementation, RFC1 nullizygous embryos die shortly postimplantation. Supplementation of pregnant dams with 25 mg/kg/day folic acid prolongs survival of mutant embryos until E9.5-E10.5, but they are developmentally delayed relative to wild-type littermates, display a marked absence of erythropoiesis, severe neural tube and limb bud defects, and failure of chorioallantoic fusion. Fgfr2 protein levels are significantly reduced or absent in the extraembryonic membranes of RFC1 nullizygous embryos. Maternal folate supplementation with 50 mg/kg/day results in survival of 22% of RFC1 mutants to E18.5, but they develop with multiple malformations of the eyelids, lungs, heart, and skin.

CONCLUSIONS

High doses of daily maternal folate supplementation during embryonic/fetal development are necessary for early postimplantation embryonic viability of RFC1 nullizygous embryos, and play a critical role in chorioallantoic fusion, erythropoiesis, and proper development of the neural tube, limbs, lungs, heart, and skin.

Msx1 and Msx2 are required for endothelial-mesenchymal transformation of the atrioventricular cushions and patterning of the atrioventricular myocardium

Background

Msx1 and Msx2, which belong to the highly conserved Nk family of homeobox genes, display overlapping expression patterns and redundant functions in multiple tissues and organs during vertebrate development. Msx1 and Msx2 have well-documented roles in mediating epithelial-mesenchymal interactions during organogenesis. Given that both Msx1 and Msx2 are crucial downstream effectors of Bmp signaling, we investigated whether Msx1 and Msx2 are required for the Bmp-induced endothelial-mesenchymal transformation (EMT) during atrioventricular (AV) valve formation.

Results

While both Msx1-/- and Msx2-/- single homozygous mutant mice exhibited normal valve formation, we observed hypoplastic AV cushions and malformed AV valves in Msx1-/-; Msx2-/- mutants, indicating redundant functions of Msx1 and Msx2 during AV valve morphogenesis. In Msx1/2 null mutant AV cushions, we found decreased Bmp2/4 and Notch1 signaling as well as reduced expression of Has2, NFATc1 and Notch1, demonstrating impaired endocardial activation and EMT. Moreover, perturbed expression of chamber-specific genes Anf, Tbx2, Hand1 and Hand2 reveals mispatterning of the Msx1/2 double mutant myocardium and suggests functions of Msx1 and Msx2 in regulating myocardial signals required for remodelling AV valves and maintaining an undifferentiated state of the AV myocardium.

Conclusion

Our findings demonstrate redundant roles of Msx1 and Msx2 in regulating signals required for development of the AV myocardium and formation of the AV valves.

Dominant negative Bmp5 mutation reveals key role of BMPs in skeletal response to mechanical stimulation

Background

Over a hundred years ago, Wolff originally observed that bone growth and remodeling are exquisitely sensitive to mechanical forces acting on the skeleton. Clinical studies have noted that the size and the strength of bone increase with weight bearing and muscular activity and decrease with bed rest and disuse. Although the processes of mechanotransduction and functional response of bone to mechanical strain have been extensively studied, the molecular signaling mechanisms that mediate the response of bone cells to mechanical stimulation remain unclear.

Results

Here, we identify a novel germline mutation at the mouse Bone morphogenetic protein 5 (Bmp5) locus. Genetic analysis shows that the mutation occurs at a site encoding the proteolytic processing sequence of the BMP5 protein and blocks proper processing of BMP5. Anatomic studies reveal that this mutation affects the formation of multiple skeletal features including several muscle-induced skeletal sites in vivo. Biomechanical studies of osteoblasts from these anatomic sites show that the mutation inhibits the proper response of bone cells to mechanical stimulation.

Conclusion

The results from these genetic, biochemical, and biomechanical studies suggest that BMPs are required not only for skeletal patterning during embryonic development, but also for bone response and remodeling to mechanical stimulation at specific anatomic sites in the skeleton.

Mouse models of congenital cardiovascular disease

Congenital heart defects occur in nearly 1% of human live births and many are lethal if not surgically repaired. In addition, the genetic contribution to congenital or acquired cardiovascular diseases that are silent at birth, but progress to cause significant disease in later life is being increasingly appreciated. Heart development and structure are highly conserved between mouse and human. The discoveries that are being made in this model system are highly relevant to understanding the pathogenesis of human heart defects whether they occus in isolation, or in the context of a syndrome. Many of the genes required for cardiovascular development were discovered fortuitously when early lethality or structural defects were observed in mouse mutants generated for other purposes, and relevant genes continue to be defined in this manner. Candidate genes for this process are being identified by their roles other species, or by their expression in pertinent tissues in mice. In this review, I will briefly summarize heart development as currently understood in the mouse, and then discuss how complementary studies in mouse and human have identified genes and pathways that are critical for normal cardiovascular development, and for maintaining the structure and function of this organ system throughout life.

The etiopathogenesis of cleft lip and cleft palate: usefulness and caveats of mouse models

Cleft lip and cleft palate are frequent human congenital malformations with a complex multifactorial etiology. These orofacial clefts can occur as part of a syndrome involving multiple organs or as isolated clefts without other detectable defects. Both forms of clefting constitute a heavy burden to the affected individuals and their next of kin. Human and mouse facial traits are utterly dissimilar. However, embryonic development of the lip and palate are strikingly similar in both species, making the mouse a model of choice to study their normal and abnormal development. Human epidemiological and genetic studies are clearly important for understanding the etiology of lip and palate clefting. However, our current knowledge about the etiopathogenesis of these malformations has mainly been gathered throughout the years from mouse models, including those with mutagen-, teratogen- and targeted mutation-induced clefts as well as from mice with spontaneous clefts. This review provides a comprehensive description of the numerous mouse models for cleft lip and/or cleft palate. Despite a few weak points, these models have revealed a high order of molecular complexity as well as the stringent spatiotemporal regulations and interactions between key factors which govern the development of these orofacial structures.

Essential role of Smad4 in maintaining cardiomyocyte proliferation during murine embryonic heart development

Transforming growth factor-beta/bone morphogenetic protein (TGF-beta/BMP) signaling pathway is essential for embryonic and postnatal heart development and remodeling. The intracellular factor Smad4 plays a pivotal role in mediating TGF-beta/BMP signal transduction in the nucleus. To examine the function of Smad4 in embryonic cardiac development during mid-gestation, we specifically deleted the Smad4 gene in embryonic cardiomyocytes using the Cre-LoxP system. Deletion of Smad4 as early as E9.5, led to embryonic lethality between E12.5 and E15.5, and embryos exhibited severe morphological defects in the heart, including a thin compact layer, disorganized trabeculae, and ventricular septum defects (VSD). Smad4 deletion also led to a dramatic decrease in cardiomyocyte proliferation accompanied by downregulation of contractile protein-encoding genes such as alpha-myosin heavy chain, beta-myosin heavy chain, ventricular myosin light chain 2, and alpha-cardiac actin. In addition, deletion of Smad4 resulted in perturbation of TGF-beta/BMP ligand expression and signaling, and defects in expression of several cardiac transcription factor genes such as Nkx2.5, GATA4, and MEF2c. These results provide direct genetic evidences that Smad4 is essential for regulating cardiomyocyte proliferation and differentiation during murine cardiogenesis, and provides new insights into potential causes of congenital heart disease.

An illustrated review of early pancreas development in the mouse

Pancreas morphogenesis and cell differentiation are highly conserved among vertebrates during fetal development. The pancreas develops through simple budlike structures on the primitive gut tube to a highly branched organ containing many specialized cell types. This review presents an overview of key molecular components and important signaling sources illustrated by an extensive three-dimensional (3D) imaging of the developing mouse pancreas at single cell resolution. The 3D documentation covers the time window between embryonic days 8.5 and 14.5 in which all the pancreatic cell types become specified and therefore includes gene expression patterns of pancreatic endocrine hormones, exocrine gene products, and essential transcription factors. The 3D perspective provides valuable insight into how a complex organ like the pancreas is formed and a perception of ventral and dorsal pancreatic growth that is otherwise difficult to uncover. We further discuss how this global analysis of the developing pancreas confirms and extends previous studies, and we envisage that this type of analysis can be instrumental for evaluating mutant phenotypes in the future.

Regulation of cardiomyocyte differentiation of embryonic stem cells by extracellular signalling

Investigating the signalling pathways that regulate heart development is essential if stem cells are to become an effective source of cardiomyocytes that can be used for studying cardiac physiology and pharmacology and eventually developing cell-based therapies for heart repair. Here, we briefly describe current understanding of heart development in vertebrates and review the signalling pathways thought to be involved in cardiomyogenesis in multiple species. We discuss how this might be applied to stem cells currently thought to have cardiomyogenic potential by considering the factors relevant for each differentiation step from the undifferentiated cell to nascent mesoderm, cardiac progenitors and finally a fully determined cardiomyocyte. We focus particularly on how this is being applied to human embryonic stem cells and provide recent examples from both our own work and that of others.

Factors that regulate embryonic gustatory development

Numerous molecular factors orchestrate the development of the peripheral taste system. The unique anatomy/function of the taste system makes this system ideal for understanding the mechanisms by which these factors function; yet the taste system is underutilized for this role. This review focuses on some of the many factors that are known to regulate gustatory development, and discusses a few topics where more work is needed. Some attention is given to factors that regulate epibranchial placode formation, since gustatory neurons are thought to be primarily derived from this region. Epibranchial placodes appear to arise from a pan-placodal region and a number of regulatory factors control the differentiation of individual placodes. Gustatory neuron differentiation is regulated by a series of transcription factors and perhaps bone morphongenic proteins (BMP). As neurons differentiate, they also proliferate such that their numbers exceed those in the adult, and this is followed by developmental death. Some of these cell-cycling events are regulated by neurotrophins. After gustatory neurons become post-mitotic, axon outgrowth occurs. Axons are guided by multiple chemoattractive and chemorepulsive factors, including semaphorins, to the tongue epithelium. Brain derived neurotrophic factor (BDNF), functions as a targeting factor in the final stages of axon guidance and is required for gustatory axons to find and innervate taste epithelium. Numerous factors are involved in the development of gustatory papillae including Sox-2, Sonic hedge hog and Wnt-β-catenin signaling. It is likely that just as many factors regulate taste bud differentiation; however, these factors have not yet been identified. Studies examining the molecular factors that regulate terminal field formation in the nucleus of the solitary tract are also lacking. However, it is possible that some of the factors that regulate geniculate ganglion development, outgrowth, guidance and targeting of peripheral axons may have the same functions in the gustatory CNS.

Control of kidney, eye and limb expression of Bmp7 by an enhancer element highly conserved between species

Bmp7 is expressed in numerous tissues throughout development and is required for morphogenesis of the eye, hindlimb and kidney. In this study we show that the majority if not all of the cis-regulatory sequence governing expression at these anatomical sites during development is present in approximately 20 kb surrounding exon 1. In eye, limb and kidney, multiple distinct enhancer elements drive Bmp7 expression within each organ. In the eye, the elements driving expression in the pigmented epithelium and iris are spatially separated. In the kidney, Bmp7 expression in collecting ducts and nephron progenitors is driven by separate enhancer elements. Similarly, limb mesenchyme and apical ectodermal ridge expression are governed by separate elements. Although enhancers for pigmented epithelium, nephrogenic mesenchyme and apical ectodermal ridge are distributed across the approximately 20 kb region, an element of approximately 480 base pairs within intron 1 governs expression within the developing iris, collecting duct system of the kidney and limb mesenchyme. This element is remarkably conserved both in sequence and position in the Bmp7 locus between different vertebrates, ranging from Xenopus tropicalis to Homo sapiens, demonstrating that there is strong selective pressure for Bmp7 expression at these tissue sites. Furthermore, we show that the frog enhancer functions appropriately in transgenic mice. Interestingly, the intron 1 element cannot be found in the Bmp7 genes of vertebrates such as Danio rerio and Takifugu rubripes indicating that this modification of the Bmp7 gene might have arisen during the adaptation from aquatic to terrestrial life. Mutational analysis demonstrates that the enhancer activity of the intron 1 element is entirely dependent on the presence of a 10 base pair site within the intron 1 enhancer containing a predicted binding site for the FOXD3 transcription factor.

Neural crest inducing signals

The formation of the neural crest has been traditionally considered a classic example of secondary induction, where signals form one tissue elicit a response in a competent responding tissue. Interactions of the neural plate with paraxial mesoderm or nonneural ectoderm can generate neural crest. Several signaling pathways converge at the border between neural and nonneural ectoderm where the neural crest will form. Among the molecules identified in this process are members of the BMP, Wnt, FGF and Notch signaling pathways. The concerted action of these signals and their downstream targets will define the identity of the neural crest.

The murine allantois: emerging paradigms in development of the mammalian umbilical cord and its relation to the fetus

The fertilized egg of the mammal gives rise to the embryo and its extraembryonic structures, all of which develop in intimate relation with each other. Yet, whilst the past several decades have witnessed a vast number of studies on the embryonic component of the conceptus, study of the extraembryonic tissues and their relation to the fetus have been largely ignored. The allantois, precursor tissue of the mature umbilical cord, is a universal feature of all placental mammals that establishes the vital vascular bridge between the fetus and its mother. The allantois differentiates into the umbilical blood vessels, which become secured onto the chorionic component of the placenta at one end and onto the fetus at the other. In this way, fetal blood is channeled through the umbilical cord for exchange with the mother. Despite the importance of this vascular bridge, little is known about how it is made. The aim of this review is to address current understanding of the biology of the allantois in the mouse and genetic control of its features and functions, and to highlight new paradigms concerning the developmental relationship between the fetus and its umbilical cord.

Mouse mutants with neural tube closure defects and their role in understanding human neural tube defects

BACKGROUND

The number of mouse mutants and strains with neural tube closure defects (NTDs) now exceeds 190, including 155 involving known genes, 33 with unidentified genes, and eight "multifactorial" strains.

METHODS

The emerging patterns of mouse NTDs are considered in relation to the unknown genetics of the common human NTDs, anencephaly, and spina bifida aperta.

RESULTS

Of the 150 mouse mutants that survive past midgestation, 20% have risk of either exencephaly and spina bifida aperta or both, parallel to the majority of human NTDs, whereas 70% have only exencephaly, 5% have only spina bifida, and 5% have craniorachischisis. The primary defect in most mouse NTDs is failure of neural fold elevation. Most null mutations (>90%) produce syndromes of multiple affected structures with high penetrance in homozygotes, whereas the "multifactorial" strains and several null-mutant heterozygotes and mutants with partial gene function (hypomorphs) have low-penetrance nonsyndromic NTDs, like the majority of human NTDs. The normal functions of the mutated genes are diverse, with clusters in pathways of actin function, apoptosis, and chromatin methylation and structure. The female excess observed in human anencephaly is found in all mouse exencephaly mutants for which gender has been studied. Maternal agents, including folate, methionine, inositol, or alternative commercial diets, have specific preventative effects in eight mutants and strains.

CONCLUSIONS

If the human homologs of the mouse NTD mutants contribute to risk of common human NTDs, it seems likely to be in multifactorial combinations of hypomorphs and low-penetrance heterozygotes, as exemplified by mouse digenic mutants and the oligogenic SELH/Bc strain.

Apoptosis in the medaka embryo in the early developmental stage

Apoptosis is an important event of the development of various organs. In this study, we used in situ terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) to visualize the temporal and spatial distribution of apoptosis in the developing medaka embryo, which is a useful model for developmental biology and genetics. Most of the apoptotic cells were distributed in the central nervous system and tailbud. In the brain and retina, most of the apoptosis occurred in the restricted period. In situ hybridization against caspase 3A and caspase 3B showed that these were distributed in the tailbud and the head, respectively. These results suggested that two types of caspase 3 were involved in apoptosis in different areas.

Combinatorial signaling in the heart orchestrates cardiac induction, lineage specification and chamber formation

The complexity of mammalian cardiogenesis is compounded, as the heart must function in the embryo whilst it is still being formed. Great advances have been made recently as additional cardiac progenitor cell populations have been identified. The induction and maintenance of these progenitors, and their deployment to the developing heart relies on combinatorial molecular signalling, a feature also of cardiac chamber formation. Many forms of congenital heart disease in humans are likely to arise from defects in the early stages of heart development; therefore it is important to understand the molecular pathways that underlie some of the key events that shape the heart during the early stages of it development.

The bone morphogenetic protein antagonist noggin regulates mammalian cardiac morphogenesis

Bone morphogenetic proteins (BMPs) play many roles in mammalian cardiac development. Here we address the functions of Noggin, a dedicated BMP antagonist, in the developing mouse heart. In early cardiac tissues, the Noggin gene is mainly expressed in the myocardial cells of the outflow tract, atrioventricular canal, and future right ventricle. The major heart phenotypes of Noggin mutant embryos are thicker myocardium and larger endocardial cushions. Both defects result from increased cell number. Cell proliferation is increased and cell cycle exit is decreased in the myocardium. Although we find evidence of increased BMP signal transduction in the myocardium and endocardium, we show that the cardiac defects of Noggin mutants are rescued by halving the gene dosage of Bmp4. In culture, BMP increases the epithelial-to-mesenchymal transformation (EMT) of endocardial explant cells. Increased EMT likely accounts for the enlarged atrioventricular cushion. In the outflow tract cushion, we observed an increased contribution of cardiac neural crest cells to the mutant cushion mesenchyme, although many cells of the cushion were not derived from neural crest. Thus the enlarged outflow tract cushion of Noggin mutants likely arises by increased contributions both of endocardial cells that have undergone EMT as well as cells that have migrated from the neural crest. These data indicate that antagonism of BMP signaling by Noggin plays a critical role in ensuring proper levels of cell proliferation and EMT during cardiac morphogenesis in the mouse.

Deconstructing digit chondrogenesis

Chondrogenesis is a key process in skeletogenesis since endochondral ossification requires the formation of a cartilaginous template. Knowledge of molecular mechanisms regulating chondrogenesis is extremely valuable not only to understand many human disorders but also in regenerative medicine. Embryonic skeletogenesis is an excellent model to study this mechanism. Most cartilages share the cellular basis underlying chondrogenesis but the high heterogeneity in morphologies of the different skeletal elements appears to be generated by differential participation of a variety of chondrogenic signals. Here we overview the regulatory factors responsible for chondrogenesis concluding that early chondrogenic signals for the digit cartilages differ from those implicated in the formation of other axial and appendicular skeletal components.

Bmp2 is required for migration but not for induction of neural crest cells in the mouse

Bone morphogenetic protein (BMP) signaling is essential for neural crest development in several vertebrates. Genetic experiments in the mouse have shown that Bmp2 is essential for the genesis of migratory neural crest cells. Using several markers and a transgenic reporter approach, we now show that neural crest cells are induced in Bmp2 null mutant embryos, but that these cells fail to migrate out of the neural tube. The absence of migratory neural crest cells in these mutants is not due to their elimination by cell death. The neuroectoderm of Bmp2-/- embryos fail to close and create abnormal folds both along the anterior-posterior and medio-lateral axes, which are associated with an apparent medio-lateral expansion of the neural tube. Finally, our data suggest that the molecular cascade downstream of BMP signaling in early neural crest development may be different in mouse and avian embryos.

BMPs, FGF8 and Wnts regulate the differentiation of locus coeruleus noradrenergic neuronal precursors

In the present study, we investigated the involvement of rhombomere 1 patterning proteins in the regulation of the major noradrenergic centre of the brain, the locus coeruleus. Primary cultures of rat embryonic day 13.5 locus coeruleus were treated with fibroblast growth factor-8, noggin and members of the bone morphogenetic and Wnt protein families. We show that bone morphogenetic proteins 2, 5 and 7 increase and noggin decreases the number of tyrosine hydroxylase-positive locus coeruleus neurons. Interestingly, from all Wnts expressed in the first rhombomere by embryonic day 12.5 in the mice, we only found expression of wnt5a mRNA in the vicinity of the locus coeruleus. In agreement with this finding, from all Wnts studied in vitro, only Wnt5a increased the number of tyrosine hydroxylase-positive neurons in locus coeruleus cultures. Finally, we also found that fibroblast growth factor-8 increased the number of tyrosine hydroxylase-positive cells in locus coeruleus cultures. Neither of the identified factors affected the survival of tyrosine hydroxylase-positive locus coeruleus noradrenergic neurons or the proliferation of their progenitors or neurogenesis. Instead, our results suggest that these patterning signals of rhombomere 1 may work to promote the differentiation of noradrenergic progenitors at later stages of development.

Endogenous bone morphogenetic protein antagonists regulate mammalian neural crest generation and survival

We demonstrate here that Chordin and Noggin function as bone morphogenetic protein (BMP) antagonists in vivo to promote mammalian neural crest development. Using Chrd and Nog single and compound mutants, we find that Noggin has a major role in promoting neural crest formation, in which Chordin is partially redundant. BMP signaling is increased in dorsal tissues lacking Noggin and is further increased when Chordin is also absent. The early neural crest domain is expanded with decreased BMP antagonism in vivo. Noggin and Chordin also regulate subsequent neural crest cell emigration from the neural tube. However, reduced levels of these BMP antagonists ultimately result in perturbation of neural crest cell derived peripheral nervous system and craniofacial skeletal elements. Such defects reflect, at least in part, a function to limit apoptosis in neural crest cells. Noggin and Chordin, therefore, function together to regulate both the generation and survival of neural crest cells in mammalian development.

BMP signaling regulates PGC numbers and motility in organ culture

Members of the bone morphogenetic protein (BMP) family play diverse roles in multiple developmental processes. However, in the mouse, mutations in many BMPs, BMP receptors and signaling components result in early embryonic lethality making it difficult to analyze the role of these factors during organogenesis or tissue homeostasis in the adult. To bypass this early lethality, we used an organ culture system to study the role of BMPs during primordial germ cell (PGC) migration. PGCs are the embryonic precursors of the sperm and eggs. BMPs induce formation of primordial germ cells within the proximal epiblast of embryonic day 7.5 (E7.5) mouse embryos. PGCs then migrate via the gut to arrive at the developing gonads by E10.5. Addition of BMP4 or the BMP-antagonist Noggin to transverse slices dissected from E9.5 embryos elevated PGC numbers or reduced PGC numbers, respectively. Noggin treatment also slowed and randomized PGC movements, resulting in a failure of PGCs to colonize the urogenital ridges (UGRs). Based on p-Smad1/5/8 staining, migratory PGCs do not respond to endogenous BMPs. Instead, the somatic cells of the urogenital ridges exhibit elevated p-Smad1/5/8 staining revealing active BMP signaling within the UGRs. Noggin treatment abrogated p-Smad staining within the UGRs and blocked localized expression of Kitl, a cytokine known to regulate the survival and motility of PGCs and Id1, a transcription factor expressed within the UGRs. We propose that BMP signaling regulates PGC migration by controlling gene expression within the somatic cells along the migration route and within the genital ridges.

Expression pattern of BMPs during chick limb development

In vertebrates, BMPs (bone morphogenic proteins) play critical roles in establishing the basic embryonic body plan and are involved in the development of a large variety of organs and tissues. Here, we analyzed the expression pattern of various BMPs (2, 4, 5 and 7) by whole mount in situ hybridization during chick limb development. In limb, expression of BMPs suggests evolutionary conserved mechanisms of BMP-dependent differentiation between lower and higher vertebrates. During the early developmental stages, BMP-2 and BMP-7 are expressed in the posterior distal mesenchyme leaving a less prominent expression anteriorly. BMP-4 is initially expressed in the anterior mesenchyme and spreads later to the whole mesenchyme leaving a stronger expression at the anterior side. From HH-stage 25, expression of BMP-4 is observed in the anterior-posterior margins of the limb bud. The BMPs 2, 4 and 7 are expressed strongly in the AER, whereas BMP-5 is expressed as a weak signal in the distal mesoderm during the early stages of limb development. Later from HH-stage 25 onwards, BMP-5 is expressed in the dorsal and ventral muscular mass of the developing limb. As digits become identifiable, expression of BMPs are observed in the interdigital mesenchyme and can also be detected along the contours of the developing phalanges and at the distal tips of the digits. All these BMPs are found to be expressed in the developing feather buds from day 8 onwards.

Dose-dependent Smad1, Smad5 and Smad8 signaling in the early mouse embryo

Three closely related mammalian R-Smads, namely Smad1, Smad5 and Smad8, are activated by BMP receptors. Here we have taken a genetic approach to further dissect their possibly unique and/or shared roles during early mouse development. A Smad8.LacZ reporter allele was created to visualize Smad8 expression domains. Smad8 is initially expressed only in the visceral yolk sac (VYS) endoderm and shows a highly restricted pattern of expression in the embryo proper at later stages. In addition, Smad8 conditional and null alleles were engineered. All alleles clearly demonstrate that adult Smad8 homozygous mutants are viable and fertile. To elucidate gene dosage effects, we manipulated expression ratios of the three BMP R-Smads. Smad8 homozygotes also lacking one copy of Smad1 or Smad5 did not exhibit overt phenotypes, and the tissue disturbances seen in Smad1 or Smad5 null embryos were not exacerbated in the absence of Smad8. However, we discovered a profound genetic interaction between Smad1 and Smad5. Thus, as for Smad1 and Smad5 mutant embryos, Smad1+/-:Smad5+/- double heterozygotes die by E10.5 and display defects in allantois morphogenesis, cardiac looping and primordial germ cell (PGC) specification. These experiments demonstrate for the first time that Smad1 and Smad5 function cooperatively to govern BMP target gene expression in the early mammalian embryo.

Branching morphogenesis during development of placental villi

The placenta forms a complex interface between the mother and fetus during development that is designed for efficient nutrient exchange. A large surface area is created by extensive branching morphogenesis of the trophoblast-derived epithelium to create a villous network, called the labyrinth in rodents. These villi are subsequently vascularized with an elaborate capillary network. Morphogenesis begins with selection of a subset of trophoblast cells in the basal layer of the chorion that express the Gcm1 transcription factor. These cells leave the cell cycle and undergo cell shape changes that initiate a process of involution to create primary villi into which fetal blood vessels grow. Much less is known about the regulation of subsequent events in branching, certainly compared with other organs. However, over 60 different mouse mutants have defects during later labyrinth development. Some of these mutant genes encode components of signaling pathways such as the fibroblast growth factor and Wnt pathways that play evolutionarily conserved roles in other branched organs, These mutants represent a still largely untapped resource as most of them have not been studied in detail in relation to placental morphogenesis.

R-spondin3 is required for mouse placental development

Mouse R-spondin3 (Rspo3) is a member of the R-spondin protein family, which is characterized by furin-like cysteine-rich domains and a thrombospondin type 1 repeat. Rspo3 has been proposed to function as a secretory molecule that promotes the Wnt/beta-catenin signaling pathway. We generated mice bearing a mutant Rspo3 allele in which a lacZ-coding region replaced the coding region of the first exon. The homozygous mutant mice died at about embryonic day 10, due to impaired formation of the labyrinthine layer of the placenta. Rspo3 was expressed in the allantoic component of the labyrinth. In the homozygous mutant placentas, fetal blood vessels did not penetrate into the chorion, and expression of Gcm1, encoding the transcription factor glial cells missing-1 (Gcm1), was dramatically reduced in the chorionic trophoblast cells. These findings suggest a critical role for Rspo3 in the interaction between chorion and allantois in labyrinthine development.

TGF-β superfamily expression and actions in the endometrium and placenta

Transforming growth factor β (TGFβ) superfamily members are closely associated with tissue remodelling events and reproductive processes. This review summarises the current state of knowledge regarding the expression and actions of TGFβ superfamily members in the uterus, during the menstrual cycle and establishment of pregnancy. TGFβs and activin β subunits are abundantly expressed in the endometrium, where roles in preparation events for implantation have been delineated, particularly in promoting decidualisation of endometrial stroma. These growth factors are also expressed by epithelial glands and secreted into uterine fluid, where interactions with preimplantation embryos are anticipated. Knockout models and embryo culture experiments implicate activins, TGFβs, nodal and bone morphogenetic proteins (BMPs) in promoting pre- and post-implantation embryo development. TGFβ superfamily members may therefore be important in the maternal support of embryo development. Following implantation, invasion of the decidua by fetal trophoblasts is tightly modulated. Activin promotes, whilst TGFβ and macrophage inhibitory cytokine-1 (MIC-1) inhibit, trophoblast migration in vitro, suggesting the relative balance of TGFβ superfamily members participate in modulating the extent of decidual invasion. Activins and TGFβs have similar opposing actions in regulating placental hormone production. Inhibins and activins are produced by the placenta throughout pregnancy, and have explored as a potential markers in maternal serum for pregnancy and placental pathologies, including miscarriage, Down’s syndrome and pre-eclampsia. Finally, additional roles in immunomodulation at the materno-fetal interface, and in endometrial inflammatory events associated with menstruation and repair, are discussed.

Noggin regulation of bone morphogenetic protein (BMP) 2/7 heterodimer activity in vitro

Bone morphogenic proteins (BMPs) are growth factors important for skeletal development and bone growth. Noggin, one of the soluble BMP antagonists, regulates the action of BMPs on mesenchymal precursor cells, partially through a feedback type of inhibition. In this study, we constructed a novel BMP2/7 'fusion gene' that encodes both BMP2 and BMP7 genes in tandem by a linker. Polymerase chain reaction (PCR) and Western blotting showed that the BMP2/7 fusion gene construct led to the production of BMP2/7 heterodimers in A549 'producer' cells. When applied to C2C12 myoblastic cells, BMP2/7 heterodimers increased alkaline phosphatase (ALP) activity and osteocalcin (OCN) expression (markers of osteoblastic differentiation) more effectively than either BMP2 or BMP7 homodimers. Moreover, this heterodimer induced significantly lower levels of Noggin expression in C2C12 cells than respective homodimers at similar doses. The addition of Noggin did not affect the heterodimer's activities in increasing osteoblastic differentiation in C2C12 cells. In contrast, BMP2 and BMP7 homodimers were largely inhibited by Noggin. Our finding suggests that the 'fusion gene' construct led to the production of bioactive BMP2/7 heterodimers, which were not antagonized by Noggin as effectively as it to BMP homodimers. The weaker Noggin antagonism on BMP heterodimers compared to homodimers may contribute to increased osteogenic potency of heterodimers in vitro and in vivo.

The transcription factors Emx1 and Emx2 suppress choroid plexus development and promote neuroepithelial cell fate

The transcription factors Emx1 and Emx2 exert important functions during development of the cerebral cortex, including its arealization. Here, we addressed their role in development of the derivatives of the midline region in the telencephalon. The center of the midline region differentiates into the choroid plexus, but little is known about its molecular specification. As we noted a lack of Emx1 or 2 expression in the midline region early in development, we interfered by misexpressing Emx1 and/or Emx2 in this region of the chick telencephalon. Ectopic expression of either Emx1 or Emx2 prior to HH 13 instructed a neuroepithelial identity in the previous midline region instead of a choroidal fate. Thus, Gli3 and Lhx2 normally restricted to the neuroepithelium expanded into the Emx misexpressing region. This was accompanied by down-regulation of Otx2 and BMP7, which implicates that these factors are essential for choroid plexus specification and differentiation. Interestingly, the region next to the ectopic Emx-misexpression then acquired a hybrid identity with some choroidal features such as Bmp7, Otx2 and Ttr gene expression, as well as some neuroepithelial features. These observations indicate that the expression levels of Emx1 and/or Emx2 restrict the prospective choroid plexus territory, a novel role of these transcription factors.

The BMP antagonist Noggin promotes cranial and spinal neurulation by distinct mechanisms

Here we characterize the consequences of elevated bone morphogenetic protein (BMP) signaling on neural tube morphogenesis by analyzing mice lacking the BMP antagonist, Noggin. Noggin is expressed dorsally in the closing neural folds and ventrally in the notochord and somites. All Noggin-/- pups are born with lumbar spina bifida; depending on genetic background, they may also have exencephaly. The exencephaly is due to a primary failure of neurulation, resulting from a lack of mid/hindbrain dorsolateral hinge point (DLHP) formation. Thus, as previously shown for Shh signaling at spinal levels, BMP activity may inhibit cranial DLHP morphogenesis. However, the increased BMP signaling observed in the Noggin-/- dorsal neural tube is not sufficient to cause exencephaly; it appears to also depend on the action of a genetic modifier, which may act to increase dorsal Shh signaling. The spinal neural tube defect results from a different mechanism: increased BMP signaling in the mesoderm between the limb buds leads to abnormal somite differentiation and axial skeletal malformation. The resulting lack of mechanical support for the neural tube causes spina bifida. We show that this defect is due to elevated BMP4 signaling. Thus, Noggin is required for mammalian neurulation in two contexts, dependent on position along the rostrocaudal axis.

Congenital heart diseases in small animals: part I. Genetic pathways and potential candidate genes

Proper cardiac morphogenesis requires a series of specific cell and tissue interactions driven by several cardiac transcription factors and downstream cardiac genes. To date, a number of genetic aetiologies responsible for human congenital heart defects (CHDs) have been identified, although none has been found for CHDs in small animals. Most gene mutations responsible for human CHDs exist in genetic pathways associated with cardiomorphogenesis. Insights into cardiomorphogenesis from human and mouse genetic studies will help us to identify potential genetic aetiologies in CHDs in small animals. Therefore, in this first part of a two-part review, the major genetic pathways for cardiomorphogenesis and important candidate genes for CHDs, based on mouse knock-out and human genetic studies are discussed.

BMP7 inhibits branching morphogenesis in the prostate gland and interferes with Notch signaling

The mouse prostate gland develops by branching morphogenesis from the urogenital epithelium and mesenchyme. Androgens and developmental factors, including FGF10 and SHH, promote prostate growth (Berman, D.M., Desai, N., Wang, X., Karhadkar, S.S., Reynon, M., Abate-Shen, C., Beachy, P.A., Shen, M.M., 2004. Roles for Hedgehog signaling in androgen production and prostate ductal morphogenesis. Dev. Biol. 267, 387-398; Donjacour, A.A., Thomson, A.A., Cunha, G.R., 2003. FGF-10 plays an essential role in the growth of the fetal prostate. Dev. Biol. 261, 39-54), while BMP4 signaling from the mesenchyme has been shown to suppresses prostate branching (Lamm, M.L., Podlasek, C.A., Barnett, D.H., Lee, J., Clemens, J.Q., Hebner, C.M., Bushman, W., 2001. Mesenchymal factor bone morphogenetic protein 4 restricts ductal budding and branching morphogenesis in the developing prostate. Dev. Biol. 232, 301-314). Here, we show that Bone Morphogenetic Protein 7 (BMP7) restricts branching of the prostate epithelium. BMP7 is expressed in the periurethral urogenital mesenchyme prior to formation of the prostate buds and, subsequently, in the prostate epithelium. We show that BMP7(lacZ/lacZ) null prostates show a two-fold increase in prostate branching, while recombinant BMP7 inhibits prostate morphogenesis in organ culture in a concentration-dependent manner. We further explore the mechanisms by which the developmental signals may be interpreted in the urogenital epithelium to regulate branching morphogenesis. We show that Notch1 activity is associated with the formation of the prostate buds, and that Notch1 signaling is derepressed in BMP7 null urogenital epithelium. Based on our studies, we propose a model that BMP7 inhibits branching morphogenesis in the prostate and limits the number of domains with high Notch1/Hes1 activity.

N-Myc functions in transcription and development

N-Myc is a member of the Myc family of proteins, which are best known for their potent oncogenic activities and association with a large proportion of human cancers. Intense scrutiny of the oncogenic properties of Myc family proteins over the last several decades has revealed a great deal about their transcriptional and oncogenic activities. Myc proteins have broad effects on transcription and can stimulate a variety of cell behaviors that contribute to the malignant phenotype. N-Myc and c-Myc also play essential functions during embryonic development, and loss of these proteins has deleterious effects in most, if not all, tissues and organ systems. What remains to be fully unraveled is the relationship between the diverse activities associated with deregulated and overexpressed Myc and their normal roles during embryonic development and tissue homeostasis. In this review I summarize our understanding of the transcriptional activities of Myc family proteins and the roles of N-myc in morphogenesis, particularly as they relate to cellular proliferation and apoptosis.

Gene networks controlling early cerebral cortex arealization

Early thalamus-independent steps in the process of cortical arealization take place on the basis of information intrinsic to the cortical primordium, as proposed by Rakic in his classical protomap hypothesis [Rakic, P. (1988)Science, 241, 170-176]. These steps depend on a dense network of molecular interactions, involving genes encoding for diffusible ligands which are released around the borders of the cortical field, and transcription factor genes which are expressed in graded ways throughout this field. In recent years, several labs worldwide have put considerable effort into identifying members of this network and disentangling its topology. In this respect, a considerable amount of knowledge has accumulated and a first, provisional description of the network can be delineated. The aim of this review is to provide an organic synthesis of our current knowledge of molecular genetics of early cortical arealization, i.e. to summarise the mechanisms by which secreted ligands and graded transcription factor genes elaborate positional information and trigger the activation of distinctive area-specific morphogenetic programs.

Atrioventricular cushion transformation is mediated by ALK2 in the developing mouse heart

Developmental abnormalities in endocardial cushions frequently contribute to congenital heart malformations including septal and valvular defects. While compelling evidence has been presented to demonstrate that members of the TGF-beta superfamily are capable of inducing endothelial-to-mesenchymal transdifferentiation in the atrioventricular canal, and thus play a key role in formation of endocardial cushions, the detailed signaling mechanisms of this important developmental process, especially in vivo, are still poorly known. Several type I receptors (ALKs) for members of the TGF-beta superfamily are expressed in the myocardium and endocardium of the developing heart, including the atrioventricular canal. However, analysis of their functional role during mammalian development has been significantly complicated by the fact that deletion of the type I receptors in mouse embryos often leads to early embryonal lethality. Here, we used the Cre/loxP system for endothelial-specific deletion of the type I receptor Alk2 in mouse embryos. The endothelial-specific Alk2 mutant mice display defects in atrioventricular septa and valves, which result from a failure of endocardial cells to appropriately transdifferentiate into the mesenchyme in the AV canal. Endocardial cells deficient in Alk2 demonstrate decreased expression of Msx1 and Snail, and reduced phosphorylation of BMP and TGF-beta Smads. Moreover, we show that endocardial cells lacking Alk2 fail to delaminate from AV canal explants. Collectively, these results indicate that the BMP type I receptor ALK2 in endothelial cells plays a critical non-redundant role in early phases of endocardial cushion formation during cardiac morphogenesis.

Msx1 and Dlx5 act independently in development of craniofacial skeleton, but converge on the regulation of Bmp signaling in palate formation

Msx and Dlx homeoproteins control the morphogenesis and organization of craniofacial skeletal structures, specifically those derived from the pharyngeal arches. In vitro Msx and Dlx proteins have opposing transcriptional properties and form heterodimeric complexes via their homeodomain with reciprocal functional repression. In this report we examine the skeletal phenotype of Msx1; Dlx5 double knock-out (DKO) mice in relationship with their expression territories during craniofacial development. Co-expression of Dlx5 and Msx1 is only observed in embryonic tissues in which these genes have independent functions, and thus direct protein interactions are unlikely to control morphogenesis of the cranium. The DKO craniofacial phenotypes indicate a complex interplay between these genes, acting independently (mandible and middle ear), synergistically (deposition of bone tissue) or converging on the same morphogenetic process (palate growth and closure). In the latter case, the absence of Dlx5 rescues in part the Msx1-dependent defects in palate growth and elevation. At the basis of this effect, our data implicate the Bmp (Bmp7, Bmp4)/Bmp antagonist (Follistatin) signal: in the Dlx5(-/-) palate changes in the expression level of Bmp7 and Follistatin counteract the reduced Bmp4 expression. These results highlight the importance of precise spatial and temporal regulation of the Bmp/Bmp antagonist system during palate closure.

Bone Morphogenetic Protein 5 Expression in the Rat Ovary: Biological Effects on Granulosa Cell Proliferation and Steroidogenesis1

Recently, the role of several elements of the bone morphogenetic protein (BMP) family has been studied in the ovary, some of them being crucial for ovarian function. In the present work, we have studied bone morphogenetic protein 5 (BMP5) expression and its biological role in the rat ovary. BMP5 is expressed by rat granulosa cells (GCs) and exerts specific biological effects on proliferation and steroidogenesis of these cells in an autocrine manner. These effects were shown to be associated with an increase in cyclin D2 protein level and a decrease in steroidogenic acute regulatory (StAR) protein expression in GCs in vitro. Ultimately, BMP5 actions were inhibited by follistatin. Overall, these data show that BMP5 is a novel element of the BMP family that might play a fully paracrine role in rodent ovarian folliculogenesis.

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.

Fluorosis: a new model and new insights

Fluoride is an effective agent for the prevention of dental caries. However, the mechanism of how excessive fluoride exposure causes fluorosis remains uncertain. Zebrafish (Danio rerio) exhibit periodic tooth replacement throughout their lives, thereby providing continuous access to teeth at developmental stages susceptible to fluoride exposure. Zebrafish teeth do not contain true enamel, but consist of a hard enameloid surface. Therefore, we asked whether zebrafish could be used as a model organism for the study of dental fluorosis. Scanning electron microscopy of fluoride-treated teeth demonstrated that the enameloid was pitted and rough, and FTIR analysis demonstrated that the teeth also contained a significantly higher organic content when compared with untreated controls. Furthermore, we demonstrate for the first time that decreased expression of an important signaling molecule (Alk8) in tooth development may contribute to the observed fluorotic phenotype, and that increased cell apoptosis may also play a role in the mechanism of fluorosis.

Targeted Disruption ofSmad4in Cardiomyocytes Results in Cardiac Hypertrophy and Heart Failure

Transforming growth factor-betas (TGF-betas) are pleiotropic cytokines involved in many physiological and pathological processes, including heart development and heart disease. Smad4 is the central intracellular mediator of TGF-beta signaling. To investigate the function of Smad4 in heart development further, we generated a strain of cardiomyocyte-specific Smad4 knockout mice using the Cre-loxP system. Unexpectedly, the deletion of Smad4 in cardiomyocytes resulted in cardiac hypertrophy characterized by an increase in the size of cardiac myocytes, age-associated fibrosis, and reexpression of certain fetal genes. Approximately 70% of the Smad4 mutant mice died spontaneously between 5 and 12 months of age. Echocardiography and an invasive hemodynamic study of the left ventricle revealed markedly decreased cardiac contractility in Smad4 mutant mice compared with littermate controls. Moreover, phosphorylated extracellular signal-regulated kinase (ERK) 1/2 and mitogen-activated protein kinase-ERK (MEK) 1 were increased in the Smad4 mutants, suggesting that an upregulation of MEK1-ERK1/2 signaling as a consequence of deletion of Smad4 underlies the impaired cardiac function. These results reveal an important function of Smad4 in cardiac remodeling and suggest that an altered cellular response to TGF-beta could be a mechanism by which cardiac myocytes undergo hypertrophy.

BMP4 substitutes for loss of BMP7 during kidney development

Functional inactivation of divergent bone morphogenetic proteins (BMPs) causes discrete disturbances during mouse development. BMP4-deficient embryos display mesodermal patterning defects at early post-implantation stages, whereas loss of BMP7 selectively disrupts kidney and eye morphogenesis. Whether these distinct phenotypes simply reflect differences in expression domains, or alternatively intrinsic differences in the signaling properties of these ligands remains unknown. To address this issue, we created embryos exclusively expressing BMP4 under control of the BMP7 locus. Surprisingly, this novel knock-in allele efficiently rescues kidney development. These results demonstrate unequivocally that these structurally divergent BMP family members, sharing only minimal sequence similarity can function interchangeably to activate all the essential signaling pathways for growth and morphogenesis of the kidney. Thus, we conclude that partially overlapping expression patterns of BMPs serve to modulate strength of BMP signaling rather than create discrete fields of ligands with intrinsically different signaling properties.

Does the isthmic organizer influence D/V patterning of the midbrain?

Early brain and spinal cord regionalization along the dorsoventral axis are thought to be governed by similar mechanisms. Subsequently, the size of the alar plate of the neural tube increases dramatically in the midbrain and anterior forebrain, compared to the spinal cord. This suggests that additional mechanisms may be required to refine A/P and D/V patterning in these structures. The isthmic organizer is a signaling center that controls both growth and patterning in the midbrain and anterior hindbrain through the production of several secreted molecules, in particular FGF8. Several studies have indicated that the isthmic organizer is involved in the positioning and development of the midbrain roof and floor plates, the two structures that respectively mark the dorsal and ventral axis of the neural tube. It remains unclear whether its influence on axis formation in the midbrain is a consequence of a more general function of the isthmic organizer/FGF8 as a modulator of DV patterning or if selection of an axis is a necessary and general by-product of its organizing function not directly related to D/V patterning. In this paper, we review the current data supporting each possibility.

Specification of neural crest cell formation and migration in mouse embryos

Of all the model organisms used to study human development, rodents such as mice most accurately reflect human craniofacial development. Collective advances in mouse embryology and mouse genetics continue to shape our understanding of neural crest cell development and by extrapolation the etiology of human congenital head and facial birth defects. The aim of this review is to highlight the considerable progress being made in our understanding of cranial neural crest cell patterning in mouse embryos.

Smad1 and Smad8 function similarly in mammalian central nervous system development

Smads 1, 5, and 8 are the intracellular mediators for the bone morphogenetic proteins (BMPs), which play crucial roles during mammalian development. Previous research has shown that Smad1 is important in the formation of the allantois, while Smad5 has been shown to be critical in the process of angiogenesis. To further analyze the BMP-responsive Smads, we disrupted the murine Smad8 gene utilizing the Cre/loxP system. A Smad8 hypomorphic allele (Smad8(Deltaexon3)) was constructed that contains an in-frame deletion of exon 3, removing one-third of the MH2 domain and a small portion of the linker region. Xenopus injection assays indicated that this Smad8 deletion allele is still functional but has reduced ventralizing capability compared to the wild type. Although Smad8(Deltaexon3/Deltaexon3) embryos are phenotypically normal, homozygotes of another hypomorphic allele of Smad8 (Smad8(3loxP)) containing a neomycin cassette within intron 3, phenocopy an embryonic brain defect observed in roughly 22% of Smad1(+/)(-) embryos analyzed at embryonic day 11.5. These observations suggest that BMP-responsive Smads have critical functions in the development of the mammalian central nervous system.