Positional apoptosis during vertebrate CNS development in the absence of endogenous retinoids

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.

Human plasma lipocalins and serum albumin: Plasma alternative carriers?

Lipocalins are an evolutionarily conserved family of proteins that bind and transport a variety of exogenous and endogenous ligands. Lipocalins share a conserved eight anti-parallel β-sheet structure. Among the different lipocalins identified in humans, α-1-acid glycoprotein (AGP), apolipoprotein D (apoD), apolipoprotein M (apoM), α1-microglobulin (α1-m) and retinol-binding protein (RBP) are plasma proteins. In particular, AGP is the most important transporter for basic and neutral drugs, apoD, apoM, and RBP mainly bind endogenous molecules such as progesterone, pregnenolone, bilirubin, sphingosine-1-phosphate, and retinol, while α1-m binds the heme. Human serum albumin (HSA) is a monomeric all-α protein that binds endogenous and exogenous molecules like fatty acids, heme, and acidic drugs. Changes in the plasmatic levels of lipocalins and HSA are responsible for the onset of pathological conditions associated with an altered drug transport and delivery. This, however, does not necessary result in potential adverse effects in patients because many drugs can bind both HSA and lipocalins, and therefore mutual compensatory binding mechanisms can be hypothesized. Here, molecular and clinical aspects of ligand transport by plasma lipocalins and HSA are reviewed, with special attention to their role as alterative carriers in health and disease.

Retinoic acid-dependent signaling pathways and lineage events in the developing mouse spinal cord

Studies in avian models have demonstrated an involvement of retinoid signaling in early neural tube patterning. The roles of this signaling pathway at later stages of spinal cord development are only partly characterized. Here we use Raldh2-null mouse mutants rescued from early embryonic lethality to study the consequences of lack of endogenous retinoic acid (RA) in the differentiating spinal cord. Mid-gestation RA deficiency produces prominent structural and molecular deficiencies in dorsal regions of the spinal cord. While targets of Wnt signaling in the dorsal neuronal lineage are unaltered, reductions in Fibroblast Growth Factor (FGF) and Notch signaling are clearly observed. We further provide evidence that endogenous RA is capable of driving stem cell differentiation. Raldh2 deficiency results in a decreased number of spinal cord derived neurospheres, which exhibit a reduced differentiation potential. Raldh2-null neurospheres have a decreased number of cells expressing the neuronal marker β-III-tubulin, while the nestin-positive cell population is increased. Hence, in vivo retinoid deficiency impaired neural stem cell growth. We propose that RA has separable functions in the developing spinal cord to (i) maintain high levels of FGF and Notch signaling and (ii) drive stem cell differentiation, thus restricting both the numbers and the pluripotent character of neural stem cells.

Vitamin A-not for your eyes only: requirement for heart formation begins early in embryogenesis

Vitamin A insufficiency has profound adverse effects on embryonic development. Major advances in understanding the role of vitamin A in vertebrate heart formation have been made since the discovery that the vitamin A active form, all-trans-retinoic acid, regulates many genes, including developmental genes. Among the experimental models used, the vitamin A-deficient avian embryo has been an important tool to study the function of vitamin A during early heart formation. A cluster of retinoic acid-regulated developmental genes have been identified that participate in building the heart. In the absence of retinoic acid the embryonic heart develops abnormally leading to embryolethality.

Survival of Hoxa13 homozygous mutants reveals a novel role in digit patterning and appendicular skeletal development

The loss of HOXA13 function severely disrupts embryonic limb development. However, because embryos lacking HOXA13 die by mid-gestation, the defects present in the mutant limb could arise as a secondary consequence of failing embryonic health. In our analysis of the mutant Hoxa13(GFP) allele, we identified a surviving cohort of homozygous mutants exhibiting severe limb defects including: missing phalanx elements, fusions of the carpal/tarsal elements, and significant reductions in metacarpal/metatarsal length. Characterization of prochondrogenic genes in the affected carpal/tarsal regions revealed significant reduction in Gdf5 expression, whereas Bmp2 expression was significantly elevated. Analysis of Gdf5 mRNA localization also revealed diffuse expression in the carpal/tarsal anlagen, suggesting a role for HOXA13 in the organization of the cells necessary to delineate individual carpal/tarsal elements. Together these results identify Gdf5 as a potential target gene of HOXA13 target gene and confirm a specific role for HOXA13 during appendicular skeletal development.

Prenatal nutritional deficiency and risk of adult schizophrenia

Converging evidence suggests that a neurodevelopmental disruption plays a role in the vulnerability to schizophrenia. The authors review evidence supporting in utero exposure to nutritional deficiency as a determinant of schizophrenia. We first describe studies demonstrating that early gestational exposure to the Dutch Hunger Winter of 1944--1945 and to a severe famine in China are each associated with an increased risk of schizophrenia in offspring. The plausibility of several candidate micronutrients as potential risk factors for schizophrenia and the biological mechanisms that may underlie these associations are then reviewed. These nutrients include folate, essential fatty acids, retinoids, vitamin D, and iron. Following this discussion, we describe the methodology and results of an epidemiologic study based on a large birth cohort that has tested the association between prenatal homocysteine, an indicator of serum folate, and schizophrenia risk. The study capitalized on the use of archived prenatal serum specimens that make it possible to obtain direct, prospective biomarkers of prenatal insults, including levels of various nutrients during pregnancy. Finally, we discuss several strategies for subjecting the prenatal nutritional hypothesis of schizophrenia to further testing. These approaches include direct assessment of additional prenatal nutritional biomarkers in relation to schizophrenia in large birth cohorts, studies of epigenetic effects of prenatal starvation, association studies of genes relevant to folate and other micronutrient deficiencies, and animal models. Given the relatively high prevalence of nutritional deficiencies during pregnancy, this work has the potential to offer substantial benefits for the prevention of schizophrenia in the population.

Correlation of Hsp110 expression with caspase-3 and -9 during apoptosis induced by in vivo embryonic exposition to retinoic acid or irradiation in early mouse craniofacial development

OBJECTIVE

To analyze the expression and role of three proteins (HSP110, caspase-3 and caspase-9) during craniofacial development.

DESIGN

Seven pregnant C57Bl/6J mice received, by force-feeding at gestation day 9 (E9), 80 mg/kg of all-trans retinoic acid mixed to sesame oil. Seven pregnant NMRI mice received two grays irradiation at the same gestation day. Control mice of both strains (seven mice for each strain) were not submitted to any treatment. Embryos were obtained at various stages after exposition (3, 6, 12 and 24 h), fixed, dehydrated and embedded. Coronal sections (5 microm) were made. Slide staining occurred alternatively using anti-Hsp110, anti-caspase-3 and anti-caspase-9 immunohistochemistry.

RESULTS

Expression of HSP110, caspase-3 and caspase-9 was found in cells of well-known locations of programmed cell death. After retinoic acid exposure, expressions were increased especially in neural crest cells of mandibular and hyoid arches. Quantification of positive cells shows that caspase-9 and Hsp110 were expressed before caspase-3. After irradiation, the expression of the three proteins quickly increased with a maximum 3 h after irradiation. For all three models of apoptosis (physiological, retinoic-induced and irradiation-induced) HSP110 positive cells were more numerous than caspase-3 positive cells. Caspase-3 positive cells were more numerous than caspase-9 positive cells especially in mesectodermal irradiation-induced apoptotic cells.

CONCLUSION

The findings show a potential function of HSP110 in apoptosis during embryo development. Caspase-3-expressing cells are more numerous than cells expressing caspase-9, especially irradiation-induced apoptotic neural crest cells. This suggests that other caspases, still to be identified, may activate caspase-3 in this model.

Distinct roles for hindbrain and paraxial mesoderm in the induction and patterning of the inner ear revealed by a study of vitamin-A-deficient quail

The hindbrain and cranial paraxial mesoderm have been implicated in the induction and patterning of the inner ear, but the precise role of the two tissues in these processes is still not clear. We have addressed these questions using the vitamin-A-deficient (VAD) quail model, in which VAD embryos lack the posterior half of the hindbrain that normally lies next to the inner ear. Using a battery of molecular markers, we show that the anlagen of the inner ear, the otic placode, is induced in VAD embryos in the absence of the posterior hindbrain. By performing grafting and ablation experiments in chick embryos, we also show that cranial paraxial mesoderm which normally lies beneath the presumptive otic placode is necessary for otic placode induction and that paraxial mesoderm from other locations cannot induce the otic placode. Two members of the fibroblast growth factor family, FGF3 and FGF19, continue to be expressed in this mesodermal population in VAD embryos, and these may be responsible for otic placode induction in the absence of the posterior hindbrain. Although the posterior hindbrain is not required for otic placode induction in VAD embryos, the subsequent patterning of the inner ear is severely disrupted. Several regional markers of the inner ear, such as Pax2, EphA4, SOHo1 and Wnt3a, are incorrectly expressed in VAD otocysts, and the sensory patches and vestibulo-acoustic ganglia are either greatly reduced or absent. Exogenous application of retinoic acid prior to 30 h of development is able rescue the VAD phenotype. By performing such rescue experiments before and after 30 h of development, we show that the inner ear defects of VAD embryos correlate with the absence of the posterior hindbrain. These results show that induction and patterning of the inner ear are governed by separate developmental processes that can be experimentally uncoupled from each other.

[Expression of caspase 3 and p53 during physiological apoptosis and apoptosis induced by three teratologic agents during early craniofacial development of the mouse embryo]

The neural crest-derived mesectoderm gives rise to physiologic apoptosis areas in early vertebrate embryos. Certain teratologic agents increase this phenomenon. The purpose of this work was to detect caspase 3 (which is associated with the apoptosis cascade) and p53 in cell death areas, both during physiological apoptosis and during apoptosis induced by three agents (retinoic acid, methyl-triazene, irradiation). Antibody revelation was performed using the aBC peroxidase kit. Quantifications were also performed on histological sections. We observed caspase 3 uptake on some apoptotic and preapoptotic cells in control embryos, and in the embryos exposed to the three teratogens. Immunoreactivity generally preceded the development of cytological features of apoptosis. However, p53 was expressed only in the embryos exposed to ionizing radiation and methyl-triazene (an alkylating agent), but not significantly in embryos exposed to retinoic acid. The present results throw some light on apoptosis mechanisms in several teratologic conditions.

Critical role for retinol in the generation/differentiation of angioblasts required for embryonic blood vessel formation

Numerous studies demonstrate that vitamin A (retinol) deficiency causes abnormal cardiovascular morphogenesis. We evaluated the impact of retinol deficiency on the regulation of the numbers of endothelial cells and angioblasts (endothelial progenitors) produced during embryonic quail development. At the one-somite stage, there were no discernible differences in the mean number of endothelial cells or angioblasts in normal and retinol-deficient embryos. However, retinol-deficient embryos at the three-somite stage had an increase in the mean number of endothelial cells but no difference in the mean number of angioblasts. By contrast, retinol-deficient embryos at the five-somite stage have 61% of the normal number of endothelial cells and 12% of the normal number of angioblasts. Similarly, retinol-deficient embryos at the 10-somite stage had 71% and 60% of normal numbers of endothelial cells in capillary-like networks and the sinuses venosus, respectively. Furthermore, we show that retinol deficiency did not elicit a global reduction in mesodermal cell numbers but was specific to cells of the endothelial lineage. Taken together, our findings suggest that vascular abnormalities observed under conditions of retinol deficiency are due to reduction in the number of angioblasts and consequently an insufficiency in the number of endothelial cells required to build complex vascular networks.

Msx1 disruption leads to diencephalon defects and hydrocephalus

We have analyzed the expression of the Msx1 gene in the developing mouse brain and examined the brain phenotype in homozygotes. Msx1 is expressed in every cerebral vesicle throughout development, particularly in neuroepithelia, such as those of the fimbria and the medulla. Timing analysis suggests that Msx1(nLacZ) cells delaminate and migrate radially from these epithelia, mainly at embryonic days 14-16, while immunohistochemistry studies reveal that some of the beta-galactosidase migrating cells are oligodendrocytes or astrocytes. Our results suggest that the Msx1 neuroepithelia of fimbria and medulla may be a source of glial precursors. The Msx1 mutants display severe hydrocephalus at birth, while the subcommissural organ, the habenula, and the posterior commissure fail to develop correctly. No label was detected in the mutant subcommissural organ using a specific antibody against Reissner's fiber. Besides, the fasciculus retroflexus deviates close to the subcommissural organ, while the paraventricular thalamic nucleus shows histological disorganization. Our results implicate the Msx1 gene in the differentiation of the subcommissural organ cells and posterior commissure and that Msx1 protein may play a role in the pathfinding and bundling of the fasciculus retroflexus and in the structural arrangement of the paraventricular thalamic nucleus.

Vitamin a requirement for early cardiovascular morphogenesis specification in the vertebrate embryo: insights from the avian embryo

Vitamin A is required throughout the life cycle, including crucial stages of embryonic and fetal development. With the identification of retinoic acid-specific nuclear transcription factors, the retinoid receptors, considerable advances have been made in understanding the molecular function of vitamin A. The requirement for vitamin A during early embryogenesis has successfully been examined in the vitamin A-deficient avian embryo during neurulation, when in the vertebrates crucial developmental decisions take place. These studies revealed that retinoic acid is essential during these early stages of embryogenesis for the initiation of organogenesis (i.e., formation of the heart). If retinoic acid is not present at this time, abnormal development ensues, leading to early embryonic death. Though the initial insult of the absence of vitamin A appears to be on the specification of cardiovascular tissues, subsequently all development is adversely affected and the embryo dies. Molecular and functional studies revealed that retinoic acid regulates the expression of the cardiogenic transcription factor GATA-4 and several heart asymmetry genes, which explains why the heart position is random in vitamin A-deficient quail embryos. During the crucial retinoic acid-requiring developmental window, retinoic acid transduces its signals to genes for heart morphogenesis via the receptors RARalpha2, RARgamma, and RXRalpha. Elucidation of the function of vitamin A during early embryonic development may lead to a better understanding of the cardiovascular birth defects prevalent in the Western world.

Function of RARgamma and RARalpha2 at the initiation of retinoid signaling is essential for avian embryo survival and for distinct events in cardiac morphogenesis

Avian embryogenesis requires retinoid receptor activation by the vitamin A active form, retinoic acid (RA), during neurulation. We conducted loss-of-function analysis in quail embryos by nutritional deprivation of RA and by blocking generation of retinoid receptors. Here we identify a distinct role for RARalpha2 in cardiac inflow tract morphogenesis and for RARgamma in cardiac left/right orientation and looping morphogenesis. Blocking normal embryos with antisense oligonucleotides to RARalpha2 or RXRalpha diminishes GATA-4 transcripts, while blocking RARgamma or RXRalpha diminishes nodal and Pitx2 transcripts; the expression of these genes in the heart forming region resembles that of the vitamin A-deficient embryo. Blocking the function of RARgamma, RARalpha2, and RXRalpha recapitulates the complete vitamin A-deficient phenotype. RARgamma is the most potent mediator of the retinoid signal at this time of development. Our studies provide strong evidence that critical RA-requiring developmental events in the early avian embryo are regulated by means of distinct retinoid receptor signaling pathways.

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.

Retinoid receptors and vitamin A deficiency: differential patterns of transcription during early avian development and the rapid induction of RARs by retinoic acid

The functional links of specific retinoid receptors to early developmental events in the avian embryo are not known. Before such studies are undertaken, knowledge is required of the spatiotemporal expression patterns of the receptor genes and their regulation by endogenous retinoic acid levels during the early stages of development. Here, we report the expression patterns of mRNAs for RARalpha, RARalpha2, RARbeta2, RARgamma, RARgamma2, RXRalpha, and RARgamma from neurulation to HH10 in the normal and vitamin A-deficient (VAD) quail embryo. The transcripts for all retinoid receptors are detectable at HH5, except for RXRgamma, which is detected at the beginning of HH6. At the 4/5 somite stage of HH8, when retinoid signaling is initiated in the avian embryo, mRNAs of all receptors are present, with very strong and ubiquitous expression patterns for RARalpha, RARalpha2, RARgamma, RARgamma2, and RXRalpha, a persistent expression of RARgamma in the neural tissues, a strong expression of RARbeta2 in lateral plate mesoderm and somites, and an anterior expression of RXRgamma. All retinoid receptors are expressed in the heart primordia. In the VAD quail embryo, the general pattern of retinoid receptor transcript localization is similar to that of the normal, except that the expression of RARalpha2 and RARbeta2 is severely diminished. Administration of retinol or retinoic acid to VAD embryos at or before the 4/5 somite stage rescues the expression of RARalpha2 and RARbeta2 within approximately 45 min and restores normal development. RARbeta2 expression requires the expression of RARalpha2. After neurulation, the expression of all retinoid receptors in the VAD quail embryo becomes independent of vitamin A status and is similar to that of the normal. The mRNA levels and sites of expression of the key enzyme for retinoic acid biosynthesis, Raldh-2, are not affected by vitamin A status; the expression pattern is restricted and does not correspond to that of retinoid receptors at all sites. The general patterns and intensity of retinoid receptor gene expression during early quail development are comparable to those of the mammalian and thus validate the application of results from retinoid-regulated avian development studies to those of the mammalian.

Vitamin A and infancy. Biochemical, functional, and clinical aspects

Vitamin A is a very intriguing natural compound. The molecule not only has a complex array of physiological functions, but also represents the precursor of promising and powerful new pharmacological agents. Although several aspects of human retinol metabolism, including absorption and tissue delivery, have been clarified, the type and amounts of vitamin A derivatives that are intracellularly produced remain quite elusive. In addition, their precise function and targets still need to be identified. Retinoic acids, undoubtedly, play a major role in explaining activities of retinol, but, recently, a large number of physiological functions have been attributed to different retinoids and to vitamin A itself. One of the primary roles this vitamin plays is in embryogenesis. Almost all steps in organogenesis are controlled by retinoic acids, thus suggesting that retinol is necessary for proper development of embryonic tissues. These considerations point to the dramatic importance of a sufficient intake of vitamin A and explain the consequences if intake of retinol is deficient. However, hypervitaminosis A also has a number of remarkable negative consequences, which, in same cases, could be fatal. Thus, the use of large doses of retinol in the treatment of some human diseases and the use of megavitamin therapy for certain chronic disorders as well as the growing tendency toward vitamin faddism should alert physicians to the possibility of vitamin overdose.

Retinoic acid signaling is required for a critical early step in zebrafish pancreatic development

The mechanisms that subdivide the endoderm into the discrete primordia that give rise to organs such as the pancreas and liver are not well understood. However, it is known that retinoic acid (RA) signaling is critical for regionalization of the vertebrate embryo: when RA signaling is either prevented or augmented, anteroposterior (AP) patterning of the CNS and mesoderm is altered and major developmental defects occur. We have investigated the role of RA signaling in regionalization of the zebrafish endoderm. Using a mutant that prevents RA synthesis and an antagonist of the RA receptors, we show that specification of both the pancreas and liver requires RA signaling. By contrast, RA signaling is not required for the formation of the endodermal germ layer or for differentiation of other endodermal organs. Timed antagonist and RA treatments show that the RA-dependent step in pancreatic specification occurs at the end of gastrulation, significantly earlier than the expression of known markers of pancreatic progenitors. In addition to being required for pancreatic specification, RA has the capacity to transfate anterior endoderm to a pancreatic fate.

Programmed cell death in the neurulating embryo is prevented by the chaperone heat shock cognate 70

Neuronal cell death is a genuine developmental process, with precise regulation and defined roles. In striking contrast, characterization of cell death that occurs at early stages of neural development is very limited. We previously showed that embryonic proinsulin increases the level of the chaperone heat shock cognate 70 (Hsc70) and reduces the incidence of apoptosis in the neurulating chick embryo [de la Rosa, et al. (1998), Proc. Natl. Acad. Sci. USA, 95, 9950]. We now demonstrate that Hsc70 is directly involved in cell survival during neurulation, as specific downregulation of endogenous Hsc70 by antisense oligodeoxynucleotide interference provoked an increase in apoptosis both in vitro and in ovo. In parallel, activation of caspase-3 was increased after hsc70 antisense oligodeoxynucleotide treatment. Dead cells were located mostly in the developing nervous system, distributed in areas where the incidence of cell death was high. These areas coincided both in vivo and under different death-inducing conditions, including antisense interference and growth factor deprivation. Hsc70 immunostaining was strong in at least some areas of high cell death. Apoptotic cells within these areas presented undetectable Hsc70 levels, however, suggesting that this protein acts as an intrinsic protector of neuroepithelial and neural precursor cells.

Role and distribution of retinoic acid during CNS development

Retinoic acid (RA), the biologically active derivative of vitamin A, induces a variety of embryonal carcinoma and neuroblastoma cell lines to differentiate into neurons. The molecular events underlying this process are reviewed with a view to determining whether these data can lead to a better understanding of the normal process of neuronal differentiation during development. Several transcription factors, intracellular signaling molecules, cytoplasmic proteins, and extracellular molecules are shown to be necessary and sufficient for RA-induced differentiation. The evidence that RA is an endogenous component of the developing central nervous system (CNS) is then reviewed, data which include high-pressure liquid chromotography (HPLC) measurements, reporter systems and the distribution of the enzymes that synthesize RA. The latter is particularly relevant to whether RA signals in a paracrine fashion on adjacent tissues or whether it acts in an autocrine manner on cells that synthesize it. It seems that a paracrine system may operate to begin early patterning events within the developing CNS from adjacent somites and later within the CNS itself to induce subsets of neurons. The distribution of retinoid-binding proteins, retinoid receptors, and RA-synthesizing enzymes is described as well as the effects of knockouts of these genes. Finally, the effects of a deficiency and an excess of RA on the developing CNS are described from the point of view of patterning the CNS, where it seems that the hindbrain is the most susceptible part of the CNS to altered levels of RA or RA receptors and also from the point of view of neuronal differentiation where, as in the case of embryonal carcinoma (EC) cells, RA promotes neuronal differentiation. The crucial roles played by certain genes, particularly the Hox genes in RA-induced patterning processes, are also emphasized.

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.

Regulation of mandibular growth and morphogenesis

The development of the vertebrate face is a dynamic process that starts with the formation of facial processes/prominences. Facial processes are small buds made up of mesenchymal masses enclosed by an epithelial layer that surround the primitive mouth. The 2 maxillary processes, the 2 lateral nasal processes, and the frontonasal processes form the upper jaw. The lower jaw is formed by the 2 mandibular processes. Although the question of the embryonic origin of facial structures has received considerable attention, the mechanisms that control differential growth of the facial processes and patterning of skeletal tissues within these structures have been difficult to study and still are not well-understood. This has been partially due to the lack of readily identifiable morphologically discrete regions in the developing face that regulate patterning of the face. Nonetheless, in recent years there has been significant progress in the understanding of the signaling network controlling the patterning and development of the face (for review, see Richman et al., 1991; Francis-West et al., 1998). This review focuses on current understanding of the processes and signaling molecules that are involved in the formation of the mandibular arch.

Magnetic resonance imaging of spinal dysraphism

Spinal cord development occurs through three consecutive periods. Gastrulation (weeks 2-3) is characterized by conversion of the embryonic disk from a bilaminar to a trilaminar arrangement and establishment of a notochord. Primary neurulation (weeks 3-4) produces the uppermost nine tenths of the spinal cord. Secondary neurulation and retrogressive differentiation (weeks 5-6) result in formation of the conus tip and filum terminale. Defects in these early embryonic stages produce spinal dysraphisms, which are characterized by anomalous differentiation and fusion of dorsal midline structures. Spinal dysraphisms may be categorized clinically into two subsets. In open spinal dysraphisms, the placode (non-neurulated neural tissue) is exposed to the environment. These disorders include myelomeningocele, myeloschisis, hemimyelomeningocele, and hemimyelocele, and are always associated with a Chiari II malformation. Closed spinal dysraphisms are covered by intact skin, although cutaneous stigmata usually indicate their presence. Two subsets may be identified based on whether a subcutaneous mass is present in the low back. Closed spinal dysraphisms with mass comprise lipomyeloschisis, lipomyelomeningocele, meningocele, and myelocystocele. Closed spinal dysraphisms without mass comprise complex dysraphic states (ranging from complete dorsal enteric fistula to neurenteric cysts, split cord malformations, dermal sinuses, caudal regression, and spinal segmental dysgenesis), bony spina bifida, tight filum terminale, filar and intradural lipomas, and persistent terminal ventricle. Magnetic resonance imaging is the imaging method of choice for investigation of this complex group of disorders.

Quantitative axial profiles of retinoic acid in the embryonic mouse spinal cord: 9-cis retinoic acid only detected after all-trans-retinoic acid levels are super-elevated experimentally

Studies using bioassays in normal mice and gene activation in transgenic reporter mice have demonstrated peaks of retinoic acid receptor (RAR) signaling in the brachial and lumbar regions of the spinal cord. Recently, Solomin et al. (Solomin et al. [1998] Nature 395:398-402) detected a retinoid X receptor (RXR) signal in the same region of the developing spinal cord at a slightly later stage than the RAR signal. This finding raises the question of which retinoid ligands underlie RAR and RXR signaling in this part of the embryo. Quantitative measurements of regional differences in retinoid profiles have not been reported previously due to limitation in the sensitivity and specificity of available retinoid detection methods. Here, by using a recently developed ultrasensitive HPLC technique (Sakhi et al. [1998] J. Chromatogr. A 828:451-460), we address this question in an attempt to identify definitively the endogenous retinoids present in different regions of the spinal cord at the stages when regional differences in RAR and RXR signaling have been reported. We find a bimodal distribution of all-trans retinoic acid (at-RA), the ligand for RARs, and relate this to the expression of several retinoid-synthesizing enzymes. However, we do not detect 9-cis-retinoic acid (9-cis-RA), the putative RXR ligand, in any region of the spinal cord unless retinoid levels are massively increased experimentally by gavage feeding pregnant mice with teratogenic doses of at-RA. This study provides for the first time quantitative profiles of endogenous retinoids along the axis of the developing spinal cord, thereby establishing a foundation for more definitive studies of retinoid function in the future. It sets definite limits on how much 9-cis-RA potentially is present and demonstrates that at-RA predominates over 9-cis-RA by at least 30- to 180-fold in different spinal cord regions.

A screen for mutations in human homologues of mice exencephaly genes Tfap2alpha and Msx2 in patients with neural tube defects

BACKGROUND

Very little is known about the identity of genetic factors involved in the complex etiology of nonsyndromic neural tube defects (NTD). Potential susceptibility genes have emerged from the vast number of mutant mouse strains displaying NTD. Reasonable candidates are the human homologues of mice exencephaly genes Tfap2alpha and Msx2, which are expressed in the developing neural tube.

METHODS

A single-strand conformation analysis (SSCA) mutation screen of the coding sequences of TFAP2alpha and MSX2 was performed for 204 nonsyndromic NTD patients including cases of anencephaly (n = 10), encephalocele (n = 8), and spina bifida aperta, SBA (n = 183). A selected number of SBA patients was additionally tested for specific mutations in MTHFD, FRalpha, and PAX1 already shown to be related to NTD.

RESULTS

Two TFAP2alpha point mutations in individual SBA patients were silent on the amino acid level (C308C, T396T). On nucleic acid level, these mutations change evolutionary conserved codons and thus may influence mRNA processing and translation efficiency. One SBA patient displayed an exonic 9-bp deletion in MSX2 leading to a shortened and possibly less functional protein. None of these mutations was found in 222 controls. Seven polymorphisms detected in TFAP2alpha and MSX2 were equally distributed in patients and controls. Patients with combined heterozygosity of an exonic MSX2 and an intronic TFAP2alpha polymorphism were at a slightly increased risk of NTD (OR 1.71; 95% CI 0.57-5.39).

CONCLUSIONS

Although several new genetic variants were found in TFAP2 and MSX2, no statistically significant association was found between NTD cases and the new alleles or their combinations. Further studies are necessary to finally decide if these gene variants may have acted as susceptibility factors in our individual cases.

Developmental mechanisms in the pathogenesis of neurodegenerative diseases

Cellular genes that are mutated in neurodegenerative diseases code for proteins that are expressed throughout neural development. Genetic analysis suggests that these genes are essential for a broad range of normal neurodevelopmental processes. The proteins they code for interact with numerous other cellular proteins that are components of signaling pathways involved in patterning of the neural tube and in regional specification of neuronal subtypes. Further, pathogenetic mutations of these genes can cause progressive, sublethal alterations in the cellular homeostasis of evolving regional neuronal subpopulations, culminating in late-onset cell death. Therefore, as a consequence of the disease mutations, targeted cell populations may retain molecular traces of abnormal interactions with disease-associated proteins by exhibiting changes in a spectrum of normal cellular functions and enhanced vulnerability to a host of environmental stressors. These observations suggest that the normal functions of these disease-associated proteins are to ensure the fidelity and integration of developmental events associated with the progressive elaboration of neuronal subtypes as well as the maintenance of mature neuronal populations during adult life. The ability to identify alterations within vulnerable neuronal precursors present in pre-symptomatic individuals prior to the onset of irrevocable cellular injury may help foster the development of effective therapeutic interventions using evolving pharmacologic, gene and stem cell technologies.

Patterning the pharyngeal arches

The presence of a muscularised pharynx with skeletal support is a fundamental vertebrate characteristic. Developmentally, the pharynx arises from the pharyngeal arches on either side of the head of vertebrate embryos. The development of the pharyngeal arches is complex involving a number of disparate embryonic populations, ectoderm, endoderm, neural crest and mesoderm, which must be co-ordinated to generate the components and overall identity of each of the arches. Previous studies suggested that it is the neural crest that plays a pivotal role in patterning the pharyngeal arches. It is now also becoming clear, however, that there are crest-independent patterning mechanisms. Therefore, pharyngeal arch patterning is more complex than was previously believed and there must be an integration of crest-dependent and -independent patterning mechanisms. BioEssays 23:54-61, 2001.

Retinoic acid-mediated gene expression in transgenic reporter zebrafish

Retinoic acid-mediated gene activation is important for normal vertebrate development. The size and nature of retinoic acid make it difficult to identify the precise cellular location of this signaling molecule throughout an embryo. Additionally, retinoic acid (RA) signaling is regulated by a complex combination of receptors, coactivators, and antagonizing proteins. Thus, in order to integrate these signals and identify regions within a whole developing embryo where cells can respond transcriptionally to retinoic acid, we have used a reporter transgenic approach. We have generated several stable lines of transgenic zebrafish which use retinoic acid response elements to drive fluorescent protein expression. In these zebrafish lines, transgene expression is localized to regions of the neural tube, retina, notochord, somites, heart, pronephric ducts, branchial arches, and jaw muscles in embryos and larvae. Transgene expression can be induced in additional regions of the neural tube and retina as well as the immature notochord, hatching gland, enveloping cell layer, and fin by exposing embryos to retinoic acid. Treatment with retinoic acid synthase inhibitors, citral and diethylaminobenzaldehyde (DEAB), during neurulation, greatly reduces transgene expression. DEAB treatment of embryos at gastrulation phenocopies the embryonic effects of vitamin A deprivation or targeted disruption of the RA synthase retinaldehyde dehydrogenase-2 in other vertebrates. Together these data suggest that the reporter expression we see in zebrafish is dependent upon conserved vertebrate pathways of RA synthesis.

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.

Retinoid signalling and hindbrain patterning

Retinoid signalling has been implicated in regulating a wide variety of processes in vertebrate development. Recent advances from analyses on the synthesis, degradation and distribution of retinoids in combination with functional analysis of signalling components have provided important insights into the regulation of patterning the nervous system and the hindbrain in particular.

Retinoids in embryonal development

The key role of vitamin A in embryonal development is reviewed. Special emphasis is given to the physiological action of retinoids, as evident from the retinoid ligand knockout models. Retinoid metabolism in embryonic tissues and teratogenic consequences of retinoid administration at high doses are presented. Physiological and pharmacological actions of retinoids are outlined and explained on the basis of their interactions as ligands of the nuclear retinoid receptors. Immediate target genes and the retinoid response elements of their promoters are summarized. The fundamental role of homeobox genes in embryonal development and the actions of retinoids on their expression are discussed. The similarity of the effects of retinoid ligand knockouts to effects of compound retinoid receptor knockouts on embryogenesis is presented. Although much remains to be clarified, the emerging landscape offers exciting views for future research.

Locally released retinoic acid repatterns the first branchial arch cartilages in vivo

The fates of cranial neural crest cells are unique compared to trunk neural crest. Cranial neural crest cells form bone and cartilage and ultimately these cells make up the entire facial skeleton. Previous studies had established that exogenous retinoic acid has effects on neurogenic derivatives of cranial neural crest cells and on segmentation of the hindbrain. In the present study we investigated the role of retinoic acid on the skeletal derivatives of migrating cranial neural crest cells. We wanted to test whether low doses of locally applied retinoic acid could respecify the neural crest-derived, skeletal components of the beak in a reproducible manner. Retinoic acid-soaked beads were positioned at the presumptive mid-hindbrain junction in stage 9 chicken embryos. Two ectopic cartilage elements were induced, the first a sheet of cartilage ventral and lateral to the quadrate and the second an accessory cartilage rod branching from Meckel's cartilage. The accessory rod resembled a retroarticular process that had formed within the first branchial arch domain. In addition the quadrate was often displaced laterally and fused to the retroarticular process. The next day following bead implantation, expression domains of Hoxa2 and Hoxb1 were shifted in an anterior direction up to the mesencephalon and Msx-2 was slightly down-regulated in the hindbrain. Despite down-regulation in neural crest cells, the onset of Msx-2 expression in the facial prominences at stage 18-20 was normal. This correlates with normal distal beak morphology. Focal labeling of neural crest with DiI showed that instead of migrating in a neat group toward the second branchial arch, a cohort of labeled cells from r4 spread anteriorly toward the proximal first arch region. AP-2 expression data confirmed the uninterrupted presence of AP-2-expressing cells from the anterior mesencephalon to r4. The morphological changes can be explained by mismigration of r4 neural crest into the first arch, but at the same time maintenance of their identity. Up-regulation of the Hoxa2 gene in the first branchial arch may have encouraged r4 cells to move in the anterior direction. This combination of events leads to the first branchial arch assuming some of the characteristics of the second branchial arch.

Anterior endoderm is sufficient to rescue foregut apoptosis and heart tube morphogenesis in an embryo lacking retinoic acid

The vitamin A deficient (VAD) quail embryo lacks active retinoids, fails to express normally GATA-4, and develops a nonlooping heart tube morphogenetic defect that is a model for congenital cardiomyopathy. VAD quail embryos, or chick embryos depleted specifically for GATA factors, show in addition abnormal foregut development, characterized by apoptosis of the endoderm cells associated with presumptive myocardium during the process of heart tube formation. Exogenous retinoic acid or transplantation of normal chick embryo anterior endoderm is sufficient to rescue apoptosis as well as GATA-4 expression and results in normal development and heart tube morphogenesis. Normal posterior endoderm also contains retinoids but is unable to rescue the VAD defect. Our results indicate that a retinoid-dependent transcriptional program, mediated at least in part by GATA factors, is critical in presumptive foregut endoderm for normal heart tube morphogenesis.

Retinoic acid synthesis and hindbrain patterning in the mouse embryo

Targeted disruption of the murine retinaldehyde dehydrogenase 2 (Raldh2) gene precludes embryonic retinoic acid (RA) synthesis, leading to midgestational lethality (Niederreither, K., Subbarayan, V., Dolle, P. and Chambon, P. (1999). Nature Genet. 21, 444–448). We describe here the effects of this RA deficiency on the development of the hindbrain and associated neural crest. Morphological segmentation is impaired throughout the hindbrain of Raldh2−/− embryos, but its caudal portion becomes preferentially reduced in size during development. Specification of the midbrain region and of the rostralmost rhombomeres is apparently normal in the absence of RA synthesis. In contrast, marked alterations are seen throughout the caudal hindbrain of mutant embryos. Instead of being expressed in two alternate rhombomeres (r3 and r5), Krox20 is expressed in a single broad domain, correlating with an abnormal expansion of the r2-r3 marker Meis2. Instead of forming a defined r4, Hoxb1- and Wnt8A-expressing cells are scattered throughout the caudal hindbrain, whereas r5/r8 markers such as kreisler or group 3/4 Hox genes are undetectable or markedly downregulated. Lack of alternate Eph receptor gene expression could explain the failure to establish rhombomere boundaries. Increased apoptosis and altered migratory pathways of the posterior rhombencephalic neural crest cells are associated with impaired branchial arch morphogenesis in mutant embryos. We conclude that RA produced by the embryo is required to generate posterior cell fates in the developing mouse hindbrain, its absence leading to an abnormal r3 (and, to a lesser extent, r4) identity of the caudal hindbrain cells.

Hindbrain respecification in the retinoid-deficient quail

We report here the development and rescue of the truncated hindbrain of retinoid-deprived quail embryos. The embryo is completely rescued by an injection of retinol into the egg; this confirms retinol, or a related retinoid, as a required molecule in hindbrain development. Staging the retinoid replacement enabled us to determine that the 3-4 somite stage is the period when retinoids are required for normal development. Analysis of the development of the retinoid-deprived hindbrain phenotype through somitogenesis has revealed a pathway of retinoid action in early hindbrain regionalization. The hindbrain of the retinoid-deprived embryo is normal in size, during early somitogenesis, but has a respecified pattern of Krox-20 expression. From the earliest expression of Krox-20, at the 5 somite stage, the rhombomere 3 stripe fills the caudal third of the developing hindbrain to the level of the first somite. Morphologically only 2, instead of the normal 5, rhombomere bulges form. These 2 bulges express genes and, later, develop morphology characteristic of rhombomeres 1 and 2 and rhombomere 3. Posterior hindbrain specific genes, Hoxb-1, Fgf3, MafB, and the rhombomere 5 stripe of Krox-20 are never expressed in the head neuroepithelium of these embryos. From the initial formation of the neural plate, there is no evidence of rhombomere 4-7 specific characteristics. These results indicate the specification of the posterior hindbrain is lost and its cells participate in the formation of an enlarged anterior hindbrain. In our previous study, we reported the absence of the posterior hindbrain in retinoid-deprived quails (Maden, M., Gale, E., Kostetskii, I., Zile, M., 1996. Vitamin A-deficient quail embryos have half a hindbrain and other neural defects. Curr. Biol. 6, 417-426). Here, we show this phenotype to be the result of respecification of the hindbrain cells. This provides evidence for a region specific response to a single stimulus, retinol, which suggests a pre-rhombomeric regionalization of the hindbrain.

Mice mutant for both Hoxa1 and Hoxb1 show extensive remodeling of the hindbrain and defects in craniofacial development

The analysis of mice mutant for both Hoxa1 and Hoxb1 suggests that these two genes function together to pattern the hindbrain. Separately, mutations in Hoxa1 and Hoxb1 have profoundly different effects on hindbrain development. Hoxa1 mutations disrupt the rhombomeric organization of the hindbrain, whereas Hoxb1 mutations do not alter the rhombomeric pattern, but instead influence the fate of cells originating in rhombomere 4. We suggest that these differences are not the consequences of different functional roles for these gene products, but rather reflect differences in the kinetics of Hoxa1 and Hoxb1 gene expression. In strong support of the idea that Hoxa1 and Hoxb1 have overlapping functions, Hoxa1/Hoxb1 double mutant homozygotes exhibit a plethora of defects either not seen, or seen only in a very mild form, in mice mutant for only Hoxa1 or Hoxb1. Examples include: the loss of both rhombomeres 4 and 5, the selective loss of the 2(nd) branchial arch, and the loss of most, but not all, 2(nd) branchial arch-derived tissues. We suggest that the early role for both of these genes in hindbrain development is specification of rhombomere identities and that the aberrant development of the hindbrain in Hoxa1/Hoxb1 double mutants proceeds through two phases, the misspecification of rhombomeres within the hindbrain, followed subsequently by size regulation of the misspecified hindbrain through induction of apoptosis.

Differential distribution of retinoic acid synthesis in the chicken embryo as determined by immunolocalization of the retinoic acid synthetic enzyme, RALDH-2

Retinaldehyde dehydrogenase type 2 (RALDH-2) is a major retinoic acid generating enzyme in the early embryo. Here we report the immunolocalization of this enzyme (RALDH-2-IR) in stage 6-29 chicken embryos; we also show that tissues that exhibit strong RALDH-2-IR in the embryo contain RALDH-2 and synthesize retinoic acid. RALDH-2-IR indicates dynamic and discrete patterns of retinoic acid synthesis in the embryo, particularly within the somitic mesoderm, lateral mesoderm, kidney, heart, and spinal motor neurons. Prior to somitogenesis, RALDH-2-IR is present in the paraxial mesoderm with a rostral boundary at the level of the presumptive first somite; as the somites form, they exhibit strong RALDH-2-IR. Cervical presomitic mesoderm exhibits RALDH-2-IR but thoracic presomitic mesoderm does not. Neural crest cells do not express detectable levels of RALDH-2, but migrating crest cells are associated with RALDH-2 expressing mesoderm. The developing limb mesoderm expresses little RALDH-2-IR; however, RALDH-2-IR is strongly expressed in tissues adjacent to the limb. The most lateral, earliest-projecting motor neurons at all levels of the spinal cord exhibit RALDH-2-IR. Subsequently, many additional motor neurons in the brachial and lumbar cord regions express RALDH-2-IR. Motor neuronal expression of RALDH-2-IR is present in the growing axons as they extend to the periphery, indicating a potential role of retinoic acid in nerve influences on peripheral differentiation. With the exception of a transient expression in the facial/vestibulocochlear nucleus, cranial motor neurons do not express detectable levels of RALDH-2-IR.

Modulation of early but not later stages of programmed cell death in embryonic avian spinal cord by sonic hedgehog

Sonic hedgehog (Shh) is a secreted glycoprotein expressed by the notochord and floor plate that is involved in the induction and specification of ventral phenotypes in the vertebrate neural tube. Recently, Shh has also been shown to promote the survival of cultured rat embryo ventral brain and spinal cord cells. We have examined whether Shh can promote the survival of chick embryo neurons in vivo or in vitro. In the chick, Shh is expressed in notochord, floor plate, and ventral neural tube/spinal cord at several stages at which programmed cell death (PCD) occurs. However, the administration of exogenous Shh to embryos in vivo or to motoneuron cultures at these stages failed to promote the survival of several different neuronal populations, including spinal motoneurons, spinal interneurons, sympathetic preganglionic neurons, sensory neurons, and neuronal precursor cells. Rather, at the earliest stage of PCD examined here (embryonic day 3) Shh selectively induced the death of ventral neuronal precursors and floor-plate cells, resulting in a net loss of cells in the neural tube. Altered concentrations of Shh induce aberrant phenotypes that are removed by PCD. Accordingly, normal PCD in the early neural tube may play a role in dorsal-ventral patterning.

Abnormal anteroposterior and dorsoventral patterning of the limb bud in the absence of retinoids

We describe here how the early limb bud of the quail embryo develops in the absence of retinoids, including retinoic acid. Retinoid-deficient embryos develop to about stage 20/21, thus allowing patterns of early gene activity in the limb bud to be readily examined. Genes representing different aspects of limb polarity were analysed. Concerning the anteroposterior axis, Hoxb-8 was up-regulated and its border was shifted anteriorly whereas shh and the mesodermal expression of bmp-2 were down-regulated in the absence of retinoids. Concerning the apical ectodermal genes, fgf-4 was down-regulated whereas fgf-8 and the ectodermal domain of bmp-2 were unaffected. Genes involved in dorsoventral polarity were all disrupted. Wnt-7a, normally confined to the dorsal ectoderm, was ectopically expressed in the ventral ectoderm and the corresponding dorsal mesodermal gene Lmx-1 spread into the ventral mesoderm. En-1 was partially or completely absent from the ventral ectoderm. These dorsoventral patterns of expression resemble those seen in En-1 knockout mouse limb buds. Overall, the patterns of gene expression are also similar to the Japanese limbless mutant. These experiments demonstrate that the retinoid-deficient embryo is a valuable tool for dissecting pathways of gene activity in the limb bud and reveal for the first time a role for retinoic acid in the organisation of the dorsoventral axis.

Retinoid signaling required for normal heart development regulates GATA-4 in a pathway distinct from cardiomyocyte differentiation

Vitamin A is essential for normal embryonic cardiogenesis. The vitamin A-deficient phenotype in the avian embryo includes an abnormal heart tube closed at the sinus venosus and the absence of large vessels that normally connect the embryonic heart to the developing circulatory system. In vitamin A-deficient embryos the expression of cardiomyocyte differentiation genes, including atrial-specific myosin heavy chain, ventricular-specific myosin, and sarcomeric myosins as well as the putative cardiomyocyte specification gene Nkx-2.5, is not altered. However, the expression of transcription factor GATA-4 is severely decreased in the heart-forming regions of vitamin A-deficient stage 7-10 embryos. Significantly, GATA-4 transcripts are completely lacking in the lateral mesoderm posterior to the heart, in the area of the developing cardiac inflow tract that later displays prominent morphological defects, including a closed nonseptated heart lacking a sinus venosus. The administration of retinol to the vitamin A-deficient embryo restores GATA-4 expression and completely rescues the vitamin A-deficient phenotype. Our results indicate that GATA-4 is a component of the retinoid-mediated cardiogenic pathway unlinked to cardiomyocyte differentiation, but involved in the morphogenesis of the posterior heart tube and the development of the cardiac inflow tract.

Differential expression of retinoid receptors in the adult mouse central nervous system

The immunocytochemical distribution of retinoid receptors has been analysed in the mouse adult central nervous system. All retinoic acid receptors (alpha, beta and gamma) and retinoid X receptors (alpha, beta and gamma) were detected and found to exhibit specific patterns of expression in various areas of the telencephalon, diencephalon and rhombencephalon. The protein localization of several retinoic acid receptors and retinoid X receptors did not correlate with the distribution of the corresponding RNA transcripts, as studied by in situ hybridization and RNase protection assays. This suggests that the expression of retinoid receptors could be post-transcriptionally regulated, which may contribute to their specific localization in the adult nervous system.

Endocrine, immune, and behavioral effects of aldicarb (carbamate), atrazine (triazine) and nitrate (fertilizer) mixtures at groundwater concentrations

This paper describes the results of 5 years of research on interactive effects of mixtures of aldicarb, atrazine, and nitrate on endocrine, immune, and nervous system function. The concentrations of chemicals used were the same order of magnitude as current maximum contaminant levels (MCLs) for all three compounds. Such levels occur in groundwater across the United States. Dosing was through voluntary consumption of drinking water. We used fractional and full factorial designs with center replicates to determine multifactor effects. We used chronic doses in experiments that varied in duration from 22 to 103 days. We tested for changes in thyroid hormone levels, ability to make antibodies to foreign proteins, and aggression in wild deer mice, Peromyscus maniculatus, and white outbred Swiss Webster mice, Mus musculus, ND4 strain. Endocrine, immune, and behavior changes occurred due to doses of mixtures, but rarely due to single compounds at the same concentrations. Immune assay data suggest the possibility of seasonal effects at low doses. We present a multiple-level model to help interpret the data in the context of human health and biological conservation concerns. We discuss six testing deficiencies of currently registered pesticides, and suggest areas of human health concerns if present trends in pesticide use continue.

The RXRalpha ligand-dependent activation function 2 (AF-2) is important for mouse development

We have engineered a mouse mutation that specifically deletes the C-terminal 18 amino acid sequence of the RXRalpha protein. This deletion corresponds to the last helical alpha structure (H12) of the ligand-binding domain (LBD), and includes the core of the Activating Domain of the Activation Function 2 (AF-2 AD core) that is thought to be crucial in mediating ligand-dependent transactivation by RXRalpha. The homozygous mutants (RXRalpha af2(o)), which die during the late fetal period or at birth, exhibit a subset of the abnormalities previously observed in RXRalpha −/− mutants, often with incomplete penetrance. In marked contrast, RXRalpha af2(o)/RXRbeta −/− and RXRalpha af2(o)/RXRbeta −/− /RXRgamma −/− compound mutants display a large array of malformations, which nearly recapitulate the full spectrum of the defects that characterize the fetal vitamin A-deficiency (VAD) syndrome and were previously found in RAR single and compound mutants, as well as in RXRalpha/RAR(alpha, beta or gamma) compound mutants. Analysis of RXRalpha af2(o)/RAR(alpha, beta or gamma) compound mutants also revealed that they exhibit many of the defects observed in the corresponding RXR alpha/RAR compound mutants. Together, these results demonstrate the importance of the integrity of RXR AF-2 for the developmental functions mediated by RAR/RXR heterodimers, and hence suggest that RXR ligand-dependent transactivation is instrumental in retinoid signalling during development.

Initial retinoid requirement for early avian development coincides with retinoid receptor coexpression in the precardiac fields and induction of normal cardiovascular development

Vitamin A requirement for early embryonic development is clearly evident in the gross cardiovascular and central nervous system abnormalities and an early death of the vitamin A-deficient quail embryo. This retinoid knockout model system was used to examine the biological activity of various natural retinoids in early cardiovascular development. We demonstrate that all-trans-, 9-cis-, 4-oxo-, and didehydroretinoic acids, and didehydroretinol and all-trans-retinol induce and maintain normal cardiovascular development as well as induce expression of the retinoic acid receptor beta2 in the vitamin A-deficient quail embryo. The expression of RARbeta2 is at the same level and at the same sites where it is expressed in the normal embryo. Retinoids provided to the vitamin A-deficient embryo up to the 5-somite stage of development, but not later, completely rescue embryonic development, suggesting the 5-somite stage as a critical retinoid-sensitive time point during early avian embryogenesis. Retinoid receptors RARalpha, RARgamma, and RXRalpha are expressed in both the precardiac endoderm and mesoderm in the normal and the vitamin A-deficient quail embryo, while the expression of RXRgamma is restricted to precardiac endoderm. Vitamin A deficiency downregulates the expression of RARalpha and RARbeta. Our studies provide strong evidence for a narrow retinoid-requiring developmental window during early embryogenesis, in which the presence of bioactive retinoids and their receptors is essential for a subsequent normal embryonic development.

How to build a vertebrate hindbrain. Lessons from genetics

During vertebrate embryogenesis, the hindbrain is the site of a segmentation process which leads to the formation, along the anterior-posterior axis, of 7-8 metameres called rhombomeres. This phenomenon plays an essential role in early hindbrain regionalisation and in the specification of the pattern of developing structures in this region of the brain. Data accumulated during the last 10 years have also shown that rhombomeres are units of gene expression and of cell lineage. Hence, a number of regulatory genes are expressed according to segment-specific patterns in the hindbrain and have been implicated in the pattern formation process. In this review, we focus on the analysis of the function and regulation of these genes along the different steps of hindbrain segmentation, from segment delimitation to acquisition of positional identity. On this basis, we propose a model for the control of early hindbrain development.

Local induction of patterning and programmed cell death in the developing Drosophila retina

Local cell signaling can pattern the nervous system by directing cell fates, including programmed cell death. In the developing Drosophila retina, programmed cell death is used to remove excess cells between ommatidia. Cell ablation revealed the source and position of signals required for regulating the pattern of programmed cell death among these interommatidial cells. Two types of signals regulate this patterning event. Notch-mediated signals between interommatidial precursors result in removal of unneeded cells. Cone cells and primary pigment cells oppose this signal by supplying a ‘life’-promoting activity; evidence is provided that this signal occurs through localized activation of the EGF Receptor/Ras pathway. Together, these signals refine the highly regular pattern observed in the adult retina.

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

Distinct cranial neural crest-derived cell types (a number of neuronal as well as non-neuronal cell lineages) are generated at characteristic times and positions in the rhombomeres of the hindbrain in developing vertebrate embryos. To examine this developmental process, we developed a novel strategy designed to test the efficacy of gain-of-function Msx2 expression within rhombomeres in ovo prior to the emigration of cranial neural crest cells (CNCC). Previous studies indicate that CNCC from odd-numbered rhombomeres (r3 and r5) undergo apoptosis in response to exogenous BMP4. We provide evidence that targeted infection in ovo using adenovirus containing Msx2 and a reporter molecule indicative of translation can induce apoptosis in either even- or odd-numbered rhombomeres. Furthermore, infected lacZ-control explants indicated that CNCC emigrated, and that 20% of these cells were double positive for crest cell markers HNK-1 and beta-gal. In contrast, there were no HNK-1 and Msx2 double positive cells emigrating from Msx2 infected explants. These results support the hypothesis that apoptotic elimination of CNCC can be induced by ‘gain-of-function’ Msx2 expression in even-numbered rhombomeres. These inductive interactions involve qualitative, quantitative, positional and temporal differences in TGF-beta-related signals, Msx2 expression and other transcriptional control.

Involvement of thyroid hormone and its alpha receptor in avian neurulation

We have analyzed the expression pattern of c-erb A alpha and c-erb A beta which encode the thyroid hormone receptors (T3R alpha and T3R beta) during early chicken embryogenesis. Only c-erb A alpha expression was detected by RT-PCR and whole-mount in situ hybridization. c-erb A alpha transcripts were found to be already present at low level in embryos before egg incubation. During neurulation a marked increase was observed in neurectoderm. A reporter cell line was then constructed and used to demonstrate the release of significant amount of thyroid hormone (T3) from egg yolk by area opaca cells before gastrulation. During gastrulation T3 was found to be enriched in the primitive streak and Hensen's node. Introduction of excess T3 frequently resulted in abnormal development of anterior structures, mainly neural tube defects and anencephalia. These observations suggest that T3R alpha, like the closely related retinoic acid receptors, fulfills functions which are important for embryonic development well before the onset of thyroid gland function.

The role of vitamin A in the development of the central nervous system

We describe here the defects that arise in the central nervous system (CNS) of quail embryos when they develop in the absence of vitamin A. It has been assumed that because of the effects of excess vitamin A and its metabolites, particularly retinoic acid (RA), on the CNS they are involved in various aspects of CNS development. We show that this is indeed the case, because these deficient quail embryos have three defects in their CNS. First, the posterior hindbrain fails to develop because the cells fated to form this part of the CNS in the very early embryo die by apoptosis. Second, the neural tube fails to extend neurites into the periphery both in vivo and in vitro. Third, the neural crest cells throughout the embryo die by apoptosis. These results demonstrate a crucial requirement for vitamin A in CNS development.