Retinoic acid stage-dependently alters the migration pattern and identity of hindbrain neural crest cells

Branchiomeric Muscle Development Requires Proper Retinoic Acid Signaling

The first and second branchiomeric (branchial arch) muscles are craniofacial muscles that derive from branchial arch mesoderm. In mammals, this set of muscles is indispensable for jaw movement and facial expression. Defects during embryonic development that result in congenital partial absence of these muscles can have significant impact on patients’ quality of life. However, the detailed molecular and cellular mechanisms that regulate branchiomeric muscle development remains poorly understood. Herein we investigated the role of retinoic acid (RA) signaling in developing branchiomeric muscles using mice as a model. We administered all-trans RA (25 mg/kg body weight) to Institute of Cancer Research (ICR) pregnant mice by gastric intubation from E8.5 to E10.5. In their embryos at E13.5, we found that muscles derived from the first branchial arch (temporalis, masseter) and second branchial arch (frontalis, orbicularis oculi) were severely affected or undetectable, while other craniofacial muscles were hypoplastic. We detected elevated cell death in the branchial arch mesoderm cells in RA-treated embryos, suggesting that excessive RA signaling reduces the survival of precursor cells of branchiomeric muscles, resulting in the development of hypoplastic craniofacial muscles. In order to uncover the signaling pathway(s) underlying this etiology, we focused on Pitx2, Tbx1, and MyoD1, which are critical for cranial muscle development. Noticeably reduced expression of all these genes was detected in the first and second branchial arch of RA-treated embryos. Moreover, elevated RA signaling resulted in a reduction in Dlx5 and Dlx6 expression in cranial neural crest cells (CNCCs), which disturbed their interactions with branchiomeric mesoderm cells. Altogether, we discovered that embryonic craniofacial muscle defects caused by excessive RA signaling were associated with the downregulation of Pitx2, Tbx1, MyoD1, and Dlx5/6, and reduced survival of cranial myogenic precursor cells.

An adverse outcome pathway on the disruption of retinoic acid metabolism leading to developmental craniofacial defects

Adverse outcome pathway (AOP) is a conceptual framework that links a molecular initiating event (MIE) via intermediate key events (KEs) with adverse effects (adverse outcomes, AO) relevant for risk assessment, through defined KE relationships (KERs). The aim of the present work is to describe a linear AOP, supported by experimental data, for skeletal craniofacial defects as the AO. This AO was selected in view of its relative high incidence in humans and the suspected relation to chemical exposure. We focused on inhibition of CYP26, a retinoic acid (RA) metabolizing enzyme, as MIE, based on robust previously published data. Conazoles were selected as representative stressors. Intermediate KEs are RA disbalance, aberrant HOX gene expression, disrupted specification, migration, and differentiation of neural crest cells, and branchial arch dysmorphology. We described the biological basis of the postulated events and conducted weight of evidence (WoE) assessments. The biological plausibility and the overall empirical evidence were assessed as high and moderate, respectively, the latter taking into consideration the moderate evidence for concordance of dose-response and temporal relationships. Finally, the essentiality assessment of the KEs, considered as high, supported the robustness of the presented AOP. This AOP, which appears of relevance to humans, thus contributes to mechanistic underpinning of selected test methods, thereby supporting their application in integrated new approach test methodologies and strategies and application in a regulatory context.

What's retinoic acid got to do with it? Retinoic acid regulation of the neural crest in craniofacial and ocular development

Retinoic acid (RA), the active derivative of vitamin A (retinol), is an essential morphogen signaling molecule and major regulator of embryonic development. The dysregulation of RA levels during embryogenesis has been associated with numerous congenital anomalies, including craniofacial, auditory, and ocular defects. These anomalies result from disruptions in the cranial neural crest, a vertebrate-specific transient population of stem cells that contribute to the formation of diverse cell lineages and embryonic structures during development. In this review, we summarize our current knowledge of the RA-mediated regulation of cranial neural crest induction at the edge of the neural tube and the migration of these cells into the craniofacial region. Further, we discuss the role of RA in the regulation of cranial neural crest cells found within the frontonasal process, periocular mesenchyme, and pharyngeal arches, which eventually form the bones and connective tissues of the head and neck and contribute to structures in the anterior segment of the eye. We then review our understanding of the mechanisms underlying congenital craniofacial and ocular diseases caused by either the genetic or toxic disruption of RA signaling. Finally, we discuss the role of RA in maintaining neural crest-derived structures in postembryonic tissues and the implications of these studies in creating new treatments for degenerative craniofacial and ocular diseases.

Generating retinoic acid gradients by local degradation during craniofacial development: One cell's cue is another cell's poison

Retinoic acid (RA) is a vital morphogen for early patterning and organogenesis in the developing embryo. RA is a diffusible, lipophilic molecule that signals via nuclear RA receptor heterodimeric units that regulate gene expression by interacting with RA response elements in promoters of a significant number of genes. For precise RA signaling, a robust gradient of the morphogen is required. The developing embryo contains regions that produce RA, and specific intracellular concentrations of RA are created through local degradation mediated by Cyp26 enzymes. In order to elucidate the mechanisms by which RA executes precise developmental programs, the kinetics of RA metabolism must be clearly understood. Recent advances in techniques for endogenous RA detection and quantification have paved the way for mechanistic studies to shed light on downstream gene expression regulation coordinated by RA. It is increasingly coming to light that RA signaling operates not only at precise concentrations but also employs mechanisms of degradation and feedback inhibition to self-regulate its levels. A global gradient of RA throughout the embryo is often found concurrently with several local gradients, created by juxtaposed domains of RA synthesis and degradation. The existence of such local gradients has been found especially critical for the proper development of craniofacial structures that arise from the neural crest and the cranial placode populations. In this review, we summarize the current understanding of how local gradients of RA are established in the embryo and their impact on craniofacial development.

New Insights Into the Roles of Retinoic Acid Signaling in Nervous System Development and the Establishment of Neurotransmitter Systems

Secreted chiefly from the underlying mesoderm, the morphogen retinoic acid (RA) is well known to contribute to the specification, patterning, and differentiation of neural progenitors in the developing vertebrate nervous system. Furthermore, RA influences the subtype identity and neurotransmitter phenotype of subsets of maturing neurons, although relatively little is known about how these functions are mediated. This review provides a comprehensive overview of the roles played by RA signaling during the formation of the central and peripheral nervous systems of vertebrates and highlights its effects on the differentiation of several neurotransmitter systems. In addition, the evolutionary history of the RA signaling system is discussed, revealing both conserved properties and alternate modes of RA action. It is proposed that comparative approaches should be employed systematically to expand our knowledge of the context-dependent cellular mechanisms controlled by the multifunctional signaling molecule RA.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Phenotypic chemical screening using a zebrafish neural crest EMT reporter identifies retinoic acid as an inhibitor of epithelial morphogenesis

The epithelial-to-mesenchymal transition (EMT) is a highly conserved morphogenetic program essential for embryogenesis, regeneration and cancer metastasis. In cancer cells, EMT also triggers cellular reprogramming and chemoresistance, which underlie disease relapse and decreased survival. Hence, identifying compounds that block EMT is essential to prevent or eradicate disseminated tumor cells. Here, we establish a whole-animal-based EMT reporter in zebrafish for rapid drug screening, called Tg(snai1b:GFP), which labels epithelial cells undergoing EMT to produce sox10-positive neural crest (NC) cells. Time-lapse and lineage analysis of Tg(snai1b:GFP) embryos reveal that cranial NC cells delaminate from two regions: an early population delaminates adjacent to the neural plate, whereas a later population delaminates from within the dorsal neural tube. Treating Tg(snai1b:GFP) embryos with candidate small-molecule EMT-inhibiting compounds identified TP-0903, a multi-kinase inhibitor that blocked cranial NC cell delamination in both the lateral and medial populations. RNA sequencing (RNA-Seq) analysis and chemical rescue experiments show that TP-0903 acts through stimulating retinoic acid (RA) biosynthesis and RA-dependent transcription. These studies identify TP-0903 as a new therapeutic for activating RA in vivo and raise the possibility that RA-dependent inhibition of EMT contributes to its prior success in eliminating disseminated cancer cells.

Editors' choice: Generation and characterization of a novel neural crest EMT reporter for rapid in vivo drug screening in zebrafish that identifies a small-molecule EMT inhibitor that blocks this process by activating retinoic acid signaling.

Development and evolution of the pharyngeal apparatus

The oral or pharyngeal apparatus facilitates the dual functions of respiration and feeding. It develops during embryogenesis from transient structures called pharyngeal arches (PAs), which comprise a reiterated series of outgrowths on the lateral side of the head. The PAs and their segmental arrangement are highly conserved throughout evolution from invertebrate chordates such as amphioxus, through to vertebrate agnathans including avians, squamates, and mammals. The structural organization of the PAs is also highly conserved and involves contributions from each of the three primary endoderm, mesoderm, and ectoderm germ layers. The endoderm is particularly important for PA formation and segmentation and also plays a critical role in tissue-specific differentiation. The ectoderm gives rise to neural crest cells (NCC) which provide an additional layer of complexity to PA development and differentiation in vertebrates compared to invertebrate chordates that do not possess NCC. Collectively, the PAs give rise to much of the neurovasculature and musculoskeletal systems in the head and neck. The complexity of development renders the pharyngeal apparatus prone to perturbation and subsequently the pathogenesis of birth defects. Hence it is important to understand the signals and mechanisms that govern the development and evolution of the pharyngeal complex.

Retinoic acid upregulates ret and induces chain migration and population expansion in vagal neural crest cells to colonise the embryonic gut

Vagal neural crest cells (VNCCs) arise in the hindbrain, and at (avian) embryonic day (E) 1.5 commence migration through paraxial tissues to reach the foregut as chains of cells 1–2 days later. They then colonise the rest of the gut in a rostrocaudal wave. The chains of migrating cells later resolve into the ganglia of the enteric nervous system. In organ culture, E4.5 VNCCs resident in the gut (termed enteric or ENCC) which have previously encountered vagal paraxial tissues, rapidly colonised aneural gut tissue in large numbers as chains of cells. Within the same timeframe, E1.5 VNCCs not previously exposed to paraxial tissues provided very few cells that entered the gut mesenchyme, and these never formed chains, despite their ability to migrate in paraxial tissue and in conventional cell culture. Exposing VNCCs in vitro to paraxial tissue normally encountered en route to the foregut conferred enteric migratory ability. VNCC after passage through paraxial tissue developed elements of retinoic acid signalling such as Retinoic Acid Binding Protein 1 expression. The paraxial tissue's ability to promote gut colonisation was reproduced by the addition of retinoic acid, or the synthetic retinoid Am80, to VNCCs (but not to trunk NCCs) in organ culture. The retinoic acid receptor antagonist CD 2665 strongly reduced enteric colonisation by E1.5 VNCC and E4.5 ENCCs, at a concentration suggesting RARα signalling. By FACS analysis, retinoic acid application to vagal neural tube and NCCs in vitro upregulated Ret; a Glial-derived-neurotrophic-factor receptor expressed by ENCCs which is necessary for normal enteric colonisation. This shows that early VNCC, although migratory, are incapable of migrating in appropriate chains in gut mesenchyme, but can be primed for this by retinoic acid. This is the first instance of the characteristic form of NCC migration, chain migration, being attributed to the application of a morphogen.

Prevention of Retinoic Acid-Induced Early Craniofacial Abnormalities by Vitamin B12 in Mice

Objective

The purpose of the present study was to identify the potential effect of prenatal vitamin B12 administration on retinoic acid (RA)-induced early craniofacial abnormalities in mice and to investigate the possible mechanisms by which vitamin B12 reduces malformations.

Design

In our study, whole embryo culture was used to explore the effect of vitamin B12 on mouse embryos during the critical period of organogenesis. All embryos were exposed to 0.4 μM RA and different concentrations of vitamin B12 and scored for their growth in the branchial region at the end of a 48-hour culture period. The endothelin-1 (ET-1)/dHAND protein expression levels in the first branchial arch were investigated using an immunohistochemical method.

Results

In the whole embryo culture, 100 and 10 μM vitamin B12 dose-dependently prevented branchial region malformations and decreased craniofacial defects by 90.5% and 77.3%, respectively. ET-1 and dHAND protein levels were significantly increased in vitamin B12-supplemented embryos compared to the RA-exposed group in embryonic branchial region.

Conclusions

These results suggest that vitamin B12 may prevent RA-induced craniofacial abnormalities via prevention of an RA-induced decrease of ET-1 and dHAND protein levels in the branchial region during the organogenic period. This study may shed new light on preventing craniofacial abnormalities.

Irreversible effects of retinoic acid pulse on Xenopus jaw morphogenesis: new insight into cranial neural crest specification

Jaws are formed by cephalic neural crest (CNCCs) and mesodermal cells migrating to the first pharyngeal arch (PA1). A complex signaling network involving different PA1 components then establishes the jaw morphogenetic program. To gather insight on this developmental process, in this study, we analyze the teratogenic effects of brief (1-15 min) pulses of low doses of retinoic acid (RA: 0.25-2 µM) or RA agonists administered to early Xenopus laevis (X.l.) embryos. We show that these brief pulses of RA cause permanent craniofacial defects specifically when treatments are performed during a 6-hr window (developmental stages NF15-NF23) that covers the period of CNCCs maintenance, migration, and specification. Earlier or later treatments have no effect. Similar treatments performed at slightly different developmental stages within this temporal window give rise to different spectra of malformations. The RA-dependent teratogenic effects observed in Xenopus can be partially rescued by folinic acid. We provide evidence suggesting that in Xenopus, as in the mouse, RA causes craniofacial malformations by perturbing signaling to CNCCs. Differently from the mouse, where RA affects CNCCs only at the end of their migration, in Xenopus, RA has an effect on CNCCs during all the period ranging from their exit from the neural tube until their arrival in the PA1. Our findings provide a conceptual framework to understand the origin of individual facial features and the evolution of different craniofacial morphotypes.

Cranial neural crest migration: new rules for an old road

The neural crest serve as an excellent model to better understand mechanisms of embryonic cell migration. Cell tracing studies have shown that cranial neural crest cells (CNCCs) emerge from the dorsal neural tube in a rostrocaudal manner and are spatially distributed along stereotypical, long distance migratory routes to precise targets in the head and branchial arches. Although the CNCC migratory pattern is a beautifully choreographed and programmed invasion, the underlying orchestration of molecular events is not well known. For example, it is still unclear how single CNCCs react to signals that direct their choice of direction and how groups of CNCCs coordinate their interactions to arrive at a target in an ordered manner. In this review, we discuss recent cellular and molecular discoveries of the CNCC migratory pattern. We focus on events from the time when CNCCs encounter the tissue adjacent to the neural tube and their travel through different microenvironments and into the branchial arches. We describe the patterning of discrete cell migratory streams that emerge from the hindbrain, rhombomere (r) segments r1-r7, and the signals that coordinate directed migration. We propose a model that attempts to unify many complex events that establish the CNCC migratory pattern, and based on this model we integrate information between cranial and trunk neural crest development.

Contamination with retinoic acid receptor agonists in two rivers in the Kinki region of Japan

This study was conducted to investigate the agonistic activity against human retinoic acid receptor (RAR) alpha in the Lake Biwa-Yodo River and the Ina River in the Kinki region of Japan. To accomplish this, a yeast two-hybrid assay was used to elucidate the spatial and temporal variations and potential sources of RARalpha agonist contamination in the river basins. RARalpha agonistic activity was commonly detected in the surface water samples collected along two rivers at different periods, with maximum all-trans retinoic acid (atRA) equivalents of 47.6 ng-atRA/L and 23.5 ng-atRA/L being observed in Lake Biwa-Yodo River and Ina River, respectively. The results indicated that RARalpha agonists are always present and widespread in the rivers. Comparative investigation of RARalpha and estrogen receptor alpha agonistic activities at 20 stations along each river revealed that the spatial variation pattern of RARalpha agonist contamination was entirely different from that of the estrogenic compound contamination. This suggests that the effluent from municipal wastewater treatment plants, a primary source of estrogenic compounds, seemed not to be the cause of RARalpha agonist contamination in the rivers. Fractionation using high performance liquid chromatography (HPLC) directed by the bioassay found two bioactive fractions from river water samples, suggesting the presence of at least two RARalpha agonists in the rivers. Although a trial conducted to identify RARalpha agonists in the major bioactive fraction was not completed as part of this study, comparison of retention times in HPLC analysis and quantification with liquid chromatography-mass spectrometry analysis revealed that the major causative contaminants responsible for the RARalpha agonistic activity were not RAs (natural RAR ligands) and 4-oxo-RAs, while 4-oxo-RAs were identified as the major RAR agonists in sewage in Beijing, China. These findings suggest that there are unknown RARalpha agonists with high activity in the rivers.

Downstream genes of Pax6 revealed by comprehensive transcriptome profiling in the developing rat hindbrain

BACKGROUND

The transcription factor Pax6 is essential for the development of the central nervous system and it exerts its multiple functions by regulating the expression of downstream target molecules. To screen for genes downstream of Pax6, we performed comprehensive transcriptome profiling analyses in the early hindbrain of Pax6 homozygous mutant and wild-type rats using microarrays.

RESULTS

Comparison of quadruplicate microarray experiments using two computational methods allowed us to identify differentially expressed genes that have relatively small fold changes or low expression levels. Gene ontology analyses of the differentially expressed molecules demonstrated that Pax6 is involved in various signal transduction pathways where it regulates the expression of many receptors, signaling molecules, transporters and transcription factors. The up- or down-regulation of these genes was further confirmed by quantitative RT-PCR. In situ staining of Fabp7, Dbx1, Unc5h1 and Cyp26b1 mRNAs showed that expression of these transcripts not only overlapped with that of Pax6 in the hindbrain of wild-type and Pax6 heterozygous mutants, but also was clearly reduced in the hindbrain of the Pax6 homozygous mutant. In addition, the Pax6 homozygous mutant hindbrain showed that Cyp26b1 expression was lacked in the dorsal and ventrolateral regions of rhombomeres 5 and 6, and that the size of rhombomere 5 expanded rostrocaudally.

CONCLUSIONS

These results indicate that Unc5h1 and Cyp26b1 are novel candidates for target genes transactivated by Pax6. Furthermore, our results suggest the interesting possibility that Pax6 regulates anterior-posterior patterning of the hindbrain via activation of Cyp26b1, an enzyme that metabolizes retinoic acid.

Retinoic acid affects craniofacial patterning by changing Fgf8 expression in the pharyngeal ectoderm

Retinoic acid signaling plays important roles in establishing normal patterning and cellular differentiation during embryonic development. In this study, we show that single administration of retinoic acid at embryonic day 8.5 causes homeotic transformation of the lower jaw into upper jaw-like structures. This homeosis was preceded by downregulation of Fgf8 and Sprouty expression in the proximal domain of the first pharyngeal arch. Downregulation of mesenchymal genes such as Dlx5, Hand2, Tbx1 and Pitx2 was also observed. The oropharynx in retinoic acid-treated embryos was severely constricted. Consistent with this observation, Patched expression in the arch endoderm and mesenchyme was downregulated. Thus, retinoic acid affects the expression of subsets of epithelial and mesenchymal genes, possibly disrupting the regional identity of the pharyngeal arch.

Mouse Shh is required for prechordal plate maintenance during brain and craniofacial morphogenesis

In humans, holoprosencephaly (HPE) is a common birth defect characterized by the absence of midline cells from brain, facial, and oral structures. To understand the pathoetiology of HPE, we investigated the involvement of mammalian prechordal plate (PrCP) cells in HPE pathogenesis and the requirement of the secreted protein sonic hedgehog (Shh) in PrCP development. We show using rat PrCP lesion experiments and DiI labeling that PrCP cells are essential for midline development of the forebrain, foregut endoderm, and ventral cranial mesoderm in mammals. We demonstrate that PrCP cells do not develop into ventral cranial mesoderm in Shh(-/-) embryos. Using Shh(-/-) and chimeric embryos we show that Shh signal is required for the maintenance of PrCP cells in a non-cell autonomous manner. In addition, the hedgehog (HH)-responding cells that normally appear during PrCP development to contribute to midline tissues, do not develop in the absence of Shh signaling. This suggests that Shh protein secreted from PrCP cells induces the differentiation of HH-responding cells into midline cells. In the present study, we show that the maintenance of a viable population of PrCP cells by Shh signal is an essential process in development of the midline of the brain and craniofacial structures. These findings provide new insight into the mechanism underlying HPE pathoetiology during dynamic brain and craniofacial morphogenesis.

Dynamic expression of Dab2 in the mouse embryonic central nervous system

BACKGROUND

Dab2, one of two mammalian orthologs of Drosophila Disabled, has been shown to be involved in cell positioning and formation of visceral endoderm during mouse embryogenesis, but its role in neuronal development is not yet fully understood. In this report, we have examined the localization of the Dab2 protein in the mouse embryonic central nervous system (CNS) at different developmental stages.

RESULTS

Dab2 protein was transiently expressed in rhombomeres 5 and 6 of the developing hindbrain between E8.5 and E11.5, and in the floor plate of the neural tube from E9.5 to E12.5, following which it was no longer detectable within these regions. Dab2 protein was also identified within circumventricular organs including the choroid plexus, subcommissural organ and pineal gland during their early development. While Dab2 was still strongly expressed in the adult choroid plexus, immunoreactivity within the subcommissural organ and pineal gland was lost after birth. In addition, Dab2 was transiently expressed within a subpopulation of Iba1-positive mononuclear phagocytes (including presumed microglial progenitors) within the neural tube from E10.0 and was lost by E14.5. Dab2 was separately localized to Iba1 positive cells from E9.5 and subsequently to F4/80 positive cells (mature macrophage/myeloid-derived dendritic cells) positioned outside the neural tube from E12.5 onwards, implicating Dab2 expression in early cells of the mononuclear phagocyte lineage. Dab2 did not co-localize with the pan-neuronal marker PGP9.5 at any developmental stage, suggesting that Dab2 positive cells in the developing CNS are unlikely to be differentiating neurons.

CONCLUSION

This is the first study to demonstrate the dynamic spatiotemporal expression of Dab2 protein within the CNS during development.

Craniofacial malformations: intrinsic vs extrinsic neural crest cell defects in Treacher Collins and 22q11 deletion syndromes

The craniofacial complex is anatomically the most sophisticated part of the body. It houses all the major sensory organ systems and its origins are synonymous with vertebrate evolution. Of fundamental importance to craniofacial development is a specialized population of stem and progenitor cells, known as the neural crest, which generate the majority of the bone, cartilage, connective and peripheral nerve tissue in the head. Approximately one third of all congenital abnormalities exhibit craniofacial malformations and consequently, most craniofacial anomalies are considered to arise through primary defects in neural crest cell development. Recent advances however, have challenged this classical dogma, underscoring the influence of tissues with which the neural crest cells interact as the primary origin of patterning defects in craniofacial morphogenesis. In this review we discuss these neural crest cell interactions with mesoderm, endoderm and ectoderm in the head in the context of a better understanding of craniofacial malformations such as in Treacher Collins and 22q11 deletion syndromes.

Molecular dynamics of retinoic acid-induced craniofacial malformations: implications for the origin of gnathostome jaws

Background

Intake of retinoic acid (RA) or of its precursor, vitamin A, during early pregnancy is associated with increased incidence of craniofacial lesions. The origin of these teratogenic effects remains enigmatic as in cranial neural crest cells (CNCCs), which largely contribute to craniofacial structures, the RA-transduction pathway is not active. Recent results suggest that RA could act on the endoderm of the first pharyngeal arch (1stPA), through a RARß-dependent mechanism.

Methodology/Principal Findings

Here we show that RA provokes dramatically different craniofacial malformations when administered at slightly different developmental times within a narrow temporal interval corresponding to the colonization of the 1^st PA by CNCCs. We provide evidence showing that RA acts on the signalling epithelium of the 1^st PA, gradually reducing the expression of endothelin-1 and Fgf8. These two molecular signals are instrumental in activating Dlx genes in incoming CNCCs, thereby triggering the morphogenetic programs, which specify different jaw elements.

Conclusions/Significance

The anatomical series induced by RA-treatments at different developmental times parallels, at least in some instances, the supposed origin of modern jaws (e.g., the fate of the incus). Our results might provide a conceptual framework for the rise of jaw morphotypes characteristic of gnathostomes.

Platelet-derived growth factor C plays a role in the branchial arch malformations induced by retinoic acid

BACKGROUND

All-trans-retinoic acid (RA) can produce branchial arch abnormalities in postimplantation rodent embryos cultured in vitro. Platelet-derived growth factor C (PDGF-C) was recently identified as a member of the PDGF ligand family. Many members of the PDGF family are essential for branchial arch morphogenesis and can be regulated by RA. The roles of PDGF-C in branchial arch malformations induced by RA and possible mechanisms were investigated.

METHODS

In whole embryo culture (WEC), mouse embryos were exposed to RA at 0, 0.1, 0.4, 1.0, or 10.0 microM, PDGF-C at 25, 50, or 75 ng/mL, or PDGF-C at 25, 50, or 75 ng/mL containing 0.4 microM RA. After 48 h of culture, mouse embryos were examined for dysmorphogenesis, and whole-mount immunohistochemistry was applied to PDGF-C. In explant cultures, explants were exposed to the same doses of RA and PDGF-C as WEC. Semiquantitative RT-PCR, zymography, and reverse zymography were used to evaluate the expressions and activities of matrix metalloproteinase (MMP)-2, MMP-14, and tissue inhibitor of metalloproteinase (TIMP)-2.

RESULTS

PDGF-C was reduced by RA, and exogenous PDGF-C rescued the branchial arch malformations induced by RA. Moreover, PDGF-C prevented RA-induced inhibition of the migratory ability of mesenchymal cells in the first branchial arch, by regulating the expressions of MMP-2, MMP-14, and TIPM-2.

CONCLUSIONS

Our results suggest that RA exposure reduces the expression of PDGF-C. The branchial arch malformations resulting from fetal RA exposure are caused at least partially by loss of PDGF-C and subsequent misregulations of the expressions of MMP-2, MMP-14, and TIMP-2.

Rhabdomyomatous hamartomata of the pharyngeal region with bilateral microtia and aural atresia: A new association?

BACKGROUND

Bilateral microtia with aural atresia is rare. Rhabdomyomatous hamartomata containing salivary tissue, situated bilaterally and symmetrically simulating the palatine (faucial) tonsils, has apparently not been reported. We present the combination of these findings in two unrelated patients.

CASES

In the first case, the patient was exposed prenatally to 13-cis-retinoic acid (isotretinoin) and has typical features of this exposure, including bilateral microtia with aural atresia and bilateral 7th nerve palsies. Due to symptoms of obstructive sleep apnea, patient 1 had a "tonsillectomy" and adenoidectomy. Histopathologic studies demonstrated rhabdomyomatous hamartomata containing salivary and striated muscle tissue in place of the palatine tonsils. In the second case, the patient had been prenatally exposed to alcohol, cocaine, and marijuana. He has been noted to have developmental delay and behavioral issues in addition to bilateral microtia with aural atresia. "Tonsillectomy" and adenoidectomy were performed to alleviate chronic upper respiratory infections and snoring. Again, histopathologic studies of the tissue submitted as "tonsil" demonstrated rhabdomyomatous hamartomata containing salivary and muscle tissue. Although an extended banded karyotype and subtelomere panel were normal, a genetic etiology for the second patient's features cannot be excluded.

CONCLUSIONS

We hypothesize that the findings of bilateral microtia with aural atresia and rhabdomyomatous hamartomata containing salivary and muscle tissue in the area of the palatine tonsils may represent a newly recognized association, which may have a teratogenic and/or genetic etiology.

PDGF-C participates in branchial arch morphogenesis and is down-regulated by retinoic acid

Retinoic acid (RA) is a teratogen that induces a variety of craniofacial abnormalities, including branchial arch deformities and cleft palate. Platelet-derived growth factor C (PDGF-C) is a recently identified member of the PDGF family. PDGF-C contributes to normal development of the heart, central nervous system, kidney and palatogenesis. But the roles of PDGF-C in branchial arches development and the relationship between PDGF-C and RA-induced branchial arches abnormalities are poorly understood. We examined the effects of RA on PDGF-C and its receptor PDGFR-alpha expressions. We demonstrated that administration of RA to mouse embryos resulted in dramatic losses of PDGF-C and its receptor PDGFR-alpha. Furthermore, we confirmed that blocking PDGF-C signaling by anti-PDGF-C neutralization antibody led to branchial arch malformations similar to that of RA induced, both hypoplastic branchial arches and FBA. These findings suggest the down-regulation of PDGF-C may be one of mechanisms of branchial arch abnormalities induced by RA and PDGF-C signaling is required for branchial arch morphogenesis.

CYP26A1 and CYP26C1 cooperatively regulate anterior-posterior patterning of the developing brain and the production of migratory cranial neural crest cells in the mouse

The appropriate regulation of retinoic acid signaling is indispensable for patterning of the vertebrate central nervous system along the anteroposterior (A-P) axis. Although both CYP26A1 and CYP26C1, retinoic acid-degrading enzymes that are expressed at the anterior end of the gastrulating mouse embryo, have been thought to play an important role in central nervous system patterning, the detailed mechanism of their contribution has remained largely unknown. We have now analyzed CYP26A1 and CYP26C1 function by generating knockout mice. Loss of CYP26C1 did not appear to affect embryonic development, suggesting that CYP26A1 and CYP26C1 are functionally redundant. In contrast, mice lacking both CYP26A1 and CYP26C1 were found to manifest a pronounced anterior truncation of the brain associated with A-P patterning defects that reflect expansion of posterior identity at the expense of anterior identity. Furthermore, Cyp26a1-/-Cyp26c1-/- mice fail to produce migratory cranial neural crest cells in the forebrain and midbrain. These observations, together with a reevaluation of Cyp26a1 mutant mice, suggest that the activity of CYP26A1 and CYP26C1 is required for correct A-P patterning and production of migratory cranial neural crest cells in the developing mammalian brain.

Postulated pathogenic pathway in triazole fungicide induced dysmorphogenic effects

Triazole fungicides are used in medicine as well as in agricultural treatment of mycoses. The pharmacological mechanism is related to the inhibition of CYP enzymes involved in the formation of the fungal walls. A similar inhibition of human CYP enzymes has been suggested as the cause of triazole side effects in humans. An important role of some CYP isoforms (CYP26 isoforms) expressed during mammalian development is the catabolism of retinoic acid, a known morphogen in vertebrates and invertebrates. The adverse effects on morphogenesis, observed after exposure of mammalian, amphibian and ascidiacea, are compared to the reported effects of triazole in humans. The possible pathogenic pathway in triazole-related teratogenesis is discussed on the basis of different experimental approaches.

Function of retinoid nuclear receptors: lessons from genetic and pharmacological dissections of the retinoic acid signaling pathway during mouse embryogenesis

Retinoic acid (RA) is involved in vertebrate morphogenesis, growth, cellular differentiation, and tissue homeostasis. The use of in vitro systems initially led to the identification of nuclear receptor RXR/RAR heterodimers as possible transducers of the RA signal. To unveil the physiological functions of RARs and RXRs, genetic and pharmacological studies have been performed in the mouse. Together, their results demonstrate that (a) RXR/RAR heterodimers in which RXR is either transcriptionally active or silent are involved in the transduction of the RA signal during prenatal development, (b) specific RXRalpha/RAR heterodimers are required at many distinct stages during early embryogenesis and organogenesis, (c) the physiological role of RA and its receptors cannot be extrapolated from teratogenesis studies using retinoids in excess. Additional cell type-restricted and temporally controlled somatic mutagenesis is required to determine the functions of RARs and RXRs during postnatal life.

Proliferation and pluripotency potential of ectomesenchymal cells derived from first branchial arch

Cranial neural crest-derived ectomesenchymal cells are multipotential progenitors that contribute to various tissue types during embryogenesis. Their potential to be expanded in culture as a monolayer and to be induced into different cell lineages in vitro has not been previously reported in detail. In this study, the ectomesenchymal cells in the first branchial arch were enzymatically isolated from the mandibular processes of BALB/c mice and were maintained in an intact state in a medium containing leukaemia inhibitory factor. Here, we first evaluated the proliferative activity of the cells after the third passage, using bromodeoxyuridine labelling and in situ hybridization of telomerase mRNA. Positive staining for expression of HNK-1, S-100 and vimentin confirmed that the population of stem cells originated from the ectomesenchyme, which did not express cytokeratin. Then we investigated the molecular and cellular characteristics of the ectomesenchymal cells during their differentiation towards neurogenic, endothelial, myogenic and odontogenic lineages. Expression of multiple lineage-specific genes and proteins was detected by utilizing a range of molecular and biochemical approaches when the cells were transferred to inductive medium. Histological and immunohistochemical analysis of the induced cells at various intervals indicated obvious phenotypic alteration and presence of specific proteins for the differentiated lineages, for example nestin, factor VIII, alpha-SMA and dentin sialophosphoprotein (DSPP), respectively. Correlatively, results of reverse transcription-PCR corroborated at mRNA level the expression of the characteristic molecules during differentiation. Therefore, it is suggested that the ectomesenchymal cells derived from the first branchial arch may represent a novel source of multipotential stem cells capable of undergoing expansion and variant differentiation in vitro.

Dysmorphogenic effects of some fungicides derived from the imidazole on rat embryos cultured in vitro

Like triazole-derivatives, imidazole-derivatives exert their antifungal and toxicological properties by inhibiting P450 enzymes (Cyps). At the embryonic level, Cyp enzymes are involved also in the catabolism of the retinoic acid. Specific effects of triazole-derivatives have been reported on developing rodent embryos, and were correlated to an imbalance of the retinoid homeostasis. The aim of this work was to investigate if imidazole-derivatives are able to induce specific malformations similar to those observed after triazole-derivative exposure. Post implantation rat embryos were exposed in vitro to 1,000 microM Imidazole and to 5-100 microM of the imidazole-derivatives Ketoconazole and Enilconazole. After 48 h in culture, the embryos exposed to the imidazole-derivatives showed specific malformations, quite similar to those observed after triazole-derivative exposure. The common dysmorphogenic effects of the azole-derivatives of the two classes could be due to the inhibition of retinoid catabolism. From this point of view, the contemporaneous exposure to these substances or their therapeutic use could be considered as potentially dangerous for human conceptuses.

Role of morphogens in neural crest cell determination

The neural crest is a transient, migratory cell population found in all vertebrate embryos that generate a diverse range of cell and tissue derivatives including, but not limited, to the neurons and glia of the peripheral nervous system, smooth muscle, connective tissue, melanocytes, craniofacial cartilage, and bone. Over the past few years, many studies have provided tremendous insights into understanding the mechanisms regulating the induction and migration of neural crest cell development. This review highlights the surprising and perhaps unexpected roles for morphogens in these distinct processes. A comparison of studies performed in several different vertebrates emphasizes the requirement for coordination between multiple signaling pathways in the induction and migration of neural crest cells in the developing embryo.

Study on the common teratogenic pathway elicited by the fungicides triazole-derivatives

Triazole-derivatives alter the pharyngeal apparatus morphogenesis of rodent embryos cultured in vitro. The hindbrain segmentation and the rhombencephalic neural crest cell (NCCs) migration are altered by Fluconazole exposure in vitro. The aim of the present work is to identify if a common pathogenic pathway is detectable also for other molecules of this class of compounds. 9.5 days post coitum (d.p.c.) old rat embryos were exposed in vitro to the teratogenic concentrations of Flusilazole, Triadimefon and Triadimenol and cultured for 24, 48 or 60 h. The expression and localisation of Hox-b1 and Krox-20 proteins (used as markers for hindbrain segmentation) were evaluated after 24 h of culture. The localisation and distribution of NCC was evaluated after 24, 30 and 48 h of culture. The morphology of the embryos was analysed after 48 h, while the branchial nerve structures were evaluated after 60 h of culture. Hindbrain segmentation and NCC migration alteration as well as pharyngeal arch and cranial nerve abnormalities were detected after exposure of the tested molecules. A common severe teratogenic intrinsic property for the tested molecules of this chemical class has been found, acting through alteration of the normal hindbrain developmental pattern.

Semaphorin/neuropilin signaling influences the positioning of migratory neural crest cells within the hindbrain region of the chick

Within the hindbrain region, neural crest cell migration is organized into three streams that follow the segmentation of the neuroepithelium into distinct rhombomeric compartments. Although the streaming of neural crest cells is known to involve signals derived from the neuroepithelium, the molecular properties underlying this process are poorly understood. Here, we have mapped the expression of the signaling component of two secreted class III Semaphorins, Semaphorin (Sema) 3A and Sema 3F, at time points that correspond to neural crest cell migration within the hindbrain region of the chick. Both Semaphorins are expressed within rhombomeres at levels adjacent to crest-free mesenchyme and expression of the receptor components essential for Semaphorin activity by neural crest cells suggests a function in restricting neural crest cell migration. By using bead implantation and electroporation in ovo, we define a role for both Semaphorins in the maintenance of neural crest cell streams in proximity to the neural tube. Attenuation of Semaphorin signaling by expression of soluble Neuropilin-Fc resulted in neural crest cells invading adjacent mesenchymal territories that are normally crest-free. The loss or misguidance of specific neural crest cell populations after changes in Semaphorin signaling also affects the integration of the cranial sensory ganglia. Thus, Sema 3A and 3F, expressed and secreted by the hindbrain neuroepithelium contributes to the appropriate positioning of neural crest cells in proximity to the neural tube, a process crucial for the subsequent establishment of neuronal connectivity within the hindbrain region.

Large-scale reprogramming of cranial neural crest gene expression by retinoic acid exposure

Although retinoic acid (RA), the active form of vitamin A, is required for normal embryonic growth and development, it is also a powerful teratogen. Infants born to mothers exposed to retinoids during pregnancy have a 25-fold increased risk for malformations, nearly exclusively of cranial neural crest-derived tissues. To characterize neural crest cell responses to RA, we exposed murine crest cultures to teratogenic levels of RA and subjected their RNA to microarray-based gene expression profile analysis using Affymetrix MG-U74Av2 GeneChips. RNAs were isolated from independent cultures treated with 10(-6) M RA for 6, 12, 24, or 48 h. Statistical analyses of gene expression profile data facilitated identification of the 205 top-ranked differentially regulated genes whose expression was reproducibly changed by RA over time. Cluster analyses of these genes across the independently treated sample series revealed distinctive kinetic patterns of altered gene expression. The largest group was transiently affected within the first 6 h of exposure, representing early responding genes. Group 2 showed sustained induction by RA over all times, whereas group 3 was characterized by the suppression of a time-dependent expression increase normally seen in untreated cells. Additional patterns demonstrated time-dependent increased or decreased expression among genes not normally regulated to a significant extent. Gene function analysis revealed that more than one-third of all RA-regulated genes were associated with developmental regulation, including both canonical and noncanonical Wnt signaling pathways. Multiple genes associated with cell adhesion and cell cycle regulation, recognized targets for the biological effects of RA, were also affected. Taken together, these results support the hypothesis that the teratogenic effects of RA derive from reprogramming gene expression of a host of genes, which play critical roles during embryonic development regulating pathways that determine subsequent differentiation of cranial neural crest cells.

Retinoic acid induces CDK inhibitors and growth arrest specific (Gas) genes in neural crest cells

Retinoic acid (RA), the active metabolite of vitamin A, regulates cellular growth and differentiation during embryonic development. In excess, this vitamin is also highly teratogenic to animals and humans. The neural crest is particularly sensitive to RA, and high levels adversely affect migration, proliferation and cell death. We investigated potential gene targets of RA associated with neural crest proliferation by determining RA-mediated changes in gene expression over time, using microarrays. Statistical analysis of the top ranked RA-regulated genes identified modest changes in multiple genes previously associated with cell cycle control and proliferation including the cyclin-dependent kinase inhibitors Cdkn1a (p21), Cdkn2b (p15(INK4b)), and Gas3/PMP22. The expression of p21 and p15(INK4b) contribute to decreased proliferation by blocking cell cycle progression at G1-S. This checkpoint is pivotal to decisions regulating proliferation, apoptosis, or differentiation. We have also confirmed the overexpression of Gas3/PMP22 in RA-treated neural crests, which is associated with cytoskeletal changes and increased apoptosis. Our results suggest that increases in multiple components of diverse regulatory pathways have an overall cumulative effect on cellular decisions. This heterogeneity contributes to the pleiotropic effects of RA, specifically those affecting proliferation and cell death.

Teratogenic effects of two antifungal triazoles, triadimefon and triadimenol, on Xenopus laevis development: craniofacial defects

Triadimefon and triadimenol, fungicides used in agriculture, are suspected of producing craniofacial malformations. As the results of FETAX analysis showed that Triadimefon was highly teratogenic, we studied the action of these triazoles on the development of the branchial apparatus in Xenopus, using early molecular markers and analysis of the cartilaginous-muscular elements of tadpoles. Teratogenic effects were observed, after exposure at the neurula stage, at the level of cartilages and muscles of the 1st and 2nd branchial arches. By in situ hybridization, we observed that in exposed specimens the territories of the branchial arches are perturbed. From these results, we considered the craniofacial malformations related to the influence of triazoles on the differentiation of branchial arches. Comparing the anomalies caused by triazoles and by retinoic acid (RA) led us to the conclusion that triazoles can affect endogenous RA content, as has been shown for mammals.

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.

Transcriptional activities of retinoic acid receptors

Vitamin A derivatives plays a crucial role in embryonic development, as demonstrated by the teratogenic effect of either an excess or a deficiency in vitamin A. Retinoid effects extend however beyond embryonic development, and tissue homeostasis, lipid metabolism, cellular differentiation and proliferation are in part controlled through the retinoid signaling pathway. Retinoids are also therapeutically effective in the treatment of skin diseases (acne, psoriasis and photoaging) and of some cancers. Most of these effects are the consequences of retinoic acid receptors activation, which triggers transcriptional events leading either to transcriptional activation or repression of retinoid-controlled genes. Synthetic molecules are able to mimic part of the biological effects of the natural retinoic acid receptors, all-trans retinoic acid. Therefore, retinoic acid receptors are considered as highly valuable therapeutic targets and limiting unwanted secondary effects due to retinoid treatment requires a molecular knowledge of retinoic acid receptors biology. In this review, we will examine experimental evidence which provide a molecular basis for the pleiotropic effects of retinoids, and emphasize the crucial roles of coregulators of retinoic acid receptors, providing a conceptual framework to identify novel therapeutic targets.

Retinoic acid signalling in the development of branchial arches

Branchial arches develop through a complex sequence of interactions between migrating cells, derived from neural crest and mesoderm, and epithelia of ectodermal and endodermal origin, to yield a variety of derivatives, notably skeletal elements, arteries and glands. In all vertebrate species, dramatic malformations generated by experimental blocks or activations of retinoic acid signalling highlight key roles for this molecule in the endoderm for branchial arch formation and morphogenesis.

All-trans retinoic acid-responsive genes identified in the human SH-SY5Y neuroblastoma cell line and their regulated expression in the nervous system of early embryos

The vitamin A metabolite, all-trans retinoic acid (atRA), is required for embryonic development. atRA binds to the nuclear retinoic acid receptors and regulates the transcription of specific target genes. In order to identify atRA-induced genes that play a role in neural development, a subtractive library was created from SH-SY5Y neuroblastoma cells, a human cell line that exhibits changes in cell adhesion and neurite outgrowth after exposure to the vitamin A acid. We report here the identification of 14 genes that are rapidly induced by atRA (retinoic acid induced in neuroblastoma or RAINB), eight of which were previously not known to be atRA responsive (BTBD11, calmin, cyclin M2, ephrin B2, HOXD10, NEDD9, RAINB6 and tenascin R). mRNA regulation by atRA was confirmed in SH-SY5Y cells by Northern blotting, and gene regulation was studied in additional human cell lines using the quantitative polymerase chain reaction. The majority of the atRA-responsive clones revealed in this screen are highly expressed in the nervous system of developing rat embryos. Further, the expression of several of these genes is perturbed in developing rat embryos exposed to excess atRA or conversely, deprived of sufficient retinoid during early development. We propose that a subset of these genes lie downstream of atRA and its receptors in the regulation of neurite outgrowth and cell adhesion in both neural and non-neural tissues within the developing embryo.

Craniofacial and axial skeletal defects induced by the fungicide triadimefon in the mouse

BACKGROUND

Triadimefon is an antifungal derived from triazole. In in vitro whole-rodent embryo cultures, triazole-derivatives showed specific teratogenic effects at the branchial apparatus. The aim of the present work was to test in vivo triadimefon (FON), in order to verify a relationship between triazole exposure, embryonic abnormalities, and/or fetal malformations.

METHODS

Pregnant CD-1 mice were treated with 0-300 mg/kg FON by gavage on day 8 post coitum (p.c.) at 10:00 AM, and sacrificed on day 8 p.c. at 1:00 PM, on day 9 p.c. at 10:00 AM, on day 10 p.c. at 10:00 AM, and at term of gestation (day 18 p.c.). At midgestation, the embryos were processed for specific immunostainings to visualize the hindbrain segmentation (day 8 p.c.) and the neural crest cell migration (days 8 and 9 p.c.). Fetuses explanted at term were all processed for skeletal examination after double-staining of osseous and cartilaginous tissues.

RESULTS

At midgestation, the immunostaining of rhombomeres 3 and 5 showed a light scattering of the immunostained areas; the neural crest cell migration was unaffected, but their localization at the branchial arch level was abnormal. At term, several severe malformations were observed at the craniofacial and at the axial skeletal level. Ectopic cartilage was observed at the upper jaw.

CONCLUSIONS

Triadimefon is teratogenic. The observed craniofacial malformations could be explained by an alteration of the rhombomeric organization and neural crest migration to the branchial arches; the axial abnormalities could be explained by the abnormal segmental identity specification.

Cellular anomalies underlying retinoid-induced phocomelia

The question of how alterations in cell behavior produced by retinoic acid (RA) influenced the development of skeletogenic mesenchyme of the limb bud was examined in this study. Our established model was employed, which involves treatment of pregnant mice with a teratogenic dose of RA (100 mg/kg) on 11 days postcoitum (dpc) resulting in a severe truncation of all long bones of the forelimbs in virtually every exposed fetus. It is shown that RA, administered at a stage to induce phocomelia in virtually all exposed embryos, resulted in immediate appearance of enhanced cell death within the mesenchyme in the central core of the limb bud, an area destined for chondrogenesis. The central core mesenchyme, which in the untreated limb buds experiences a sharp decline in cell proliferation heralding the onset of chondrogenesis, demonstrated a reversal of the process; this mesenchyme maintained a higher rate of cell proliferation upon RA exposure. These events resulted in a truncation and disorganization of the chondrogenic anlage, more pronounced in zeugopodal mesenchyme than in the autopod. We conclude that an inhibition of chondrogenesis was secondary to a disruption in cellular behavior caused by RA, a likely consequence of misregulation in the growth factor signaling cascade.

Gene–nutrient interactions: importance of folates and retinoids during early embryogenesis

The role that nutritional factors play in mammalian development has received renewed attention over the past two decades, as the scientific literature exploded with reports of retinoid compounds disrupting craniofacial development, and with other reports that folic acid supplementation in the periconceptional period can protect embryos from highly significant malformations. As was often the case, the situation became far more complicated, as the interaction between nutritional factors with selected genes was recognized. In this review, we attempt to summarize a complex clinical and experimental literature of nutritional factors, their biological transport mechanisms, and the impact that they have during early embryogenesis. Although not exhaustive, our goal was to provide an overview of important gene-nutrient interactions and a framework for their investigation.

NPDC-1, a novel regulator of neuronal proliferation, is degraded by the ubiquitin/proteasome system through a PEST degradation motif

Neural proliferation and differentiation control protein-1 (NPDC-1) is a protein expressed primarily in brain and lung and whose expression can be correlated with the regulation of cellular proliferation and differentiation. Embryonic differentiation in brain and lung has classically been linked to retinoid signaling, and we have recently characterized NPDC-1 as a regulator of retinoic acid-mediated events. Regulators of differentiation and development are themselves highly regulated and usually through multiple mechanisms. One such mechanism, protein degradation via the ubiquitin/proteasome degradation pathway, has been linked to the expression of a number of proteins involved in control of proliferation or differentiation, including cyclin D1 and E2F-1. The data presented here demonstrate that NPDC-1 is likewise degraded by the ubiquitin/proteasome system. MG-132, a proteasome inhibitor, stabilized the expression of NPDC-1 and allowed detection of ubiquitinated NPDC-1 in vivo. A PEST motif (rich in proline, glutamine, serine, and threonine) located in the carboxyl terminus of NPDC-1 was shown to target the protein for degradation. Deletion of the PEST motif increased NPDC-1 protein stability and NPDC-1 inhibitory effect on retinoic acid-mediated transcription. NPDC-1 was phosphorylated by several kinases, including extracellular signal-regulated kinase. Phosphorylation of NPDC-1 increased the in vitro rate of NPDC-1 ubiquitination. The MEK inhibitor, PD-98059, an inhibitor of extracellular signal-regulated activation, also inhibited the formation of ubiquitinated NPDC-1 in vivo. Together these results suggest that retinoic acid signaling can be modulated by the presence of NPDC-1 and that the protein level and activity of NPDC-1 can be regulated by phosphorylation-mediated proteasomal degradation.

Relationship between hindbrain segmentation, neural crest cell migration and branchial arch abnormalities in rat embryos exposed to fluconazole and retinoic acid in vitro

Fluconazole (FLUCO) and retinoic acid (RA) can perturb morphogenesis of the branchial apparatus in rodent embryos exposed in vitro. The aim of the present study was to compare the effects induced by in vitro exposure to FLUCO or to RA on rhombomere organisation, neural crest cell (NCC) migration and cranial nerve differentiation using specific antibodies. For this purpose 9.5 d.p.c. rat embryos were exposed to teratogenic concentrations of FLUCO or RA; another group was exposed to no-effect concentrations of both agents. Expression of Hox-b1 and Krox20 (markers of specific rhombomeres) was altered after FLUCO and RA exposure. Furthermore, FLUCO and RA showed a synergistic effect. These results suggest that the observed branchial abnormalities are due to anomalous NCC migration related to incorrect organisation of specific rhombomeres.