Hmx4 regulates Sonic hedgehog signaling through control of retinoic acid synthesis during forebrain patterning

The retinoic acid family-like nuclear receptor SmRAR identified by single-cell transcriptomics of ovarian cells controls oocyte differentiation in Schistosoma mansoni

Studies on transcription regulation in platyhelminth development are scarce, especially for parasitic flatworms. Here, we employed single-cell transcriptomics to identify genes involved in reproductive development in the trematode model Schistosoma mansoni. This parasite causes schistosomiasis, a major neglected infectious disease affecting >240 million people worldwide. The pathology of schistosomiasis is closely associated with schistosome eggs deposited in host organs including the liver. Unlike other trematodes, schistosomes exhibit distinct sexes, with egg production reliant on the pairing-dependent maturation of female reproductive organs. Despite this significance, the molecular mechanisms underlying ovary development and oocyte differentiation remain largely unexplored. Utilizing an organ isolation approach for S. mansoni, we extracted ovaries of paired females followed by single-cell RNA sequencing (RNA-seq) with disassociated oocytes. A total of 1967 oocytes expressing 7872 genes passed quality control (QC) filtering. Unsupervised clustering revealed four distinct cell clusters: somatic, germ cells and progeny, intermediate and late germ cells. Among distinct marker genes for each cluster, we identified a hitherto uncharacterized transcription factor of the retinoic acid receptor family, SmRAR. Functional analyses of SmRAR and associated genes like Smmeiob (meiosis-specific, oligonucleotide/oligosaccharide binding motif (OB) domain-containing) demonstrated their pairing-dependent and ovary-preferential expression and their decisive roles in oocyte differentiation of S. mansoni.

Haploinsufficiency of intraflagellar transport protein 172 causes autism-like behavioral phenotypes in mice through BDNF

INTRODUCTION

Primary cilia are hair-like solitary organelles growing on most mammalian cells that play fundamental roles in embryonic patterning and organogenesis. Defective cilia often cause a suite of inherited diseases called ciliopathies with multifaceted manifestations. Intraflagellar transport (IFT), a bidirectional protein trafficking along the cilium, actively facilitates the formation and absorption of primary cilia. IFT172 is the largest component of the IFT-B complex, and its roles in Bardet-Biedl Syndrome (BBS) have been appreciated with unclear mechanisms.

OBJECTIVES

We performed a battery of behavioral tests with Ift172 haploinsufficiency (Ift172+/-) and WT littermates. We use RNA sequencing to identify the genes and signaling pathways that are differentially expressed and enriched in the hippocampus of Ift172+/- mice. Using AAV-mediated sparse labeling, electron microscopic examination, patch clamp and local field potential recording, western blot, luciferase reporter assay, chromatin immunoprecipitation, and neuropharmacological approach, we investigated the underlying mechanisms for the aberrant phenotypes presented by Ift172+/- mice.

RESULTS

Ift172+/- mice displayed excessive self-grooming, elevated anxiety, and impaired cognition. RNA sequencing revealed enrichment of differentially expressed genes in pathways relevant to axonogenesis and synaptic plasticity, which were further confirmed by less spine density and synaptic number. Ift172+/- mice demonstrated fewer parvalbumin-expressing neurons, decreased inhibitory synaptic transmission, augmented theta oscillation, and sharp-wave ripples in the CA1 region. Moreover, Ift172 haploinsufficiency caused less BDNF production and less activated BDNF-TrkB signaling pathway through transcription factor Gli3. Application of 7,8-Dihydroxyflavone, a potent small molecular TrkB agonist, fully restored BDNF-TrkB signaling activity and abnormal behavioral phenotypes presented by Ift172+/- mice. With luciferase and chip assays, we provided further evidence that Gli3 may physically interact with BDNF promoter I and regulate BDNF expression.

CONCLUSIONS

Our data suggest that Ift172 per se drives neurotrophic effects and, when defective, could cause neurodevelopmental disorders reminiscent of autism-like disorders.

Single-cell Transcriptional Landscape of Temporal Neutrophil Response to Burn Wound in Larval Zebrafish

Neutrophils accumulate early in tissue injury. However, the cellular and functional heterogeneity of neutrophils during homeostasis and in response to tissue damage remains unclear. In this study, we use larval zebrafish to understand neutrophil responses to thermal injury. Single-cell transcriptional mapping of myeloid cells during a 3-d time course in burn and control larvae revealed distinct neutrophil subsets and their cell-cell interactions with macrophages across time and conditions. The trajectory formed by three zebrafish neutrophil subsets resembles human neutrophil maturation, with varying transition patterns between conditions. Through ligand-receptor cell-cell interaction analysis, we found that neutrophils communicate more in burns in a pathway and temporal manner. Finally, we identified the correlation between zebrafish myeloid signatures and human burn severity, establishing GPR84+ neutrophils as a potential marker of early innate immune response in burns. This work builds a comparative single-cell transcriptomic framework to identify neutrophil markers of tissue damage using model organisms.

Single-cell transcriptional landscape of temporal neutrophil response to burn wound in larval zebrafish

Neutrophils accumulate early in tissue injury. However, the cellular and functional heterogeneity of neutrophils during homeostasis and in response to tissue damage remains unclear. Here, we use larval zebrafish to understand neutrophil responses to thermal injury. Single-cell transcriptional mapping of myeloid cells during a 3-day time course in burn and control larvae revealed distinct neutrophil subsets and their cell-cell interactions with macrophages across time and conditions. The trajectory formed by three zebrafish neutrophil subsets resembles human neutrophil maturation, with varying transition patterns between conditions. Through ligand-receptor cell-cell interaction analysis, we found neutrophils communicate more in burns in a pathway and temporal manner. Finally, we identified the correlation between zebrafish myeloid signatures and human burn severity, establishing GPR84+ neutrophils as a potential marker of early innate immune response in burns. This work builds the molecular foundation and a comparative single-cell genomic framework to identify neutrophil markers of tissue damage using model organisms.

Zebrafish as a Model to Study Retinoic Acid Signaling in Development and Disease

Retinoic acid (RA) is a metabolite of vitamin A (retinol) that plays various roles in development to influence differentiation, patterning, and organogenesis. RA also serves as a crucial homeostatic regulator in adult tissues. The role of RA and its associated pathways are well conserved from zebrafish to humans in both development and disease. This makes the zebrafish a natural model for further interrogation into the functions of RA and RA-associated maladies for the sake of basic research, as well as human health. In this review, we explore both foundational and recent studies using zebrafish as a translational model for investigating RA from the molecular to the organismal scale.

Single Cell/Nucleus Transcriptomics Comparison in Zebrafish and Humans Reveals Common and Distinct Molecular Responses to Alzheimer’s Disease

Neurogenesis is significantly reduced in Alzheimer’s disease (AD) and is a potential therapeutic target. Contrary to humans, a zebrafish can regenerate its diseased brain, and thus is ideal for studying neurogenesis. To compare the AD-related molecular pathways between humans and zebrafish, we compared single cell or nuclear transcriptomic data from a zebrafish amyloid toxicity model and its controls (N = 12) with the datasets of two human adult brains (N = 10 and N = 48 (Microglia)), and one fetal brain (N = 10). Approximately 95.4% of the human and zebrafish cells co-clustered. Within each cell type, we identified differentially expressed genes (DEGs), enriched KEGG pathways, and gene ontology terms. We studied synergistic and non-synergistic DEGs to point at either common or uniquely altered mechanisms across species. Using the top DEGs, a high concordance in gene expression changes between species was observed in neuronal clusters. On the other hand, the molecular pathways affected by AD in zebrafish astroglia differed from humans in favor of the neurogenic pathways. The integration of zebrafish and human transcriptomes shows that the zebrafish can be used as a tool to study the cellular response to amyloid proteinopathies. Uniquely altered pathways in zebrafish could highlight the specific mechanisms underlying neurogenesis, which are absent in humans, and could serve as potential candidates for therapeutic developments.

miR-222-3p is involved in neural tube closure by directly targeting Ddit4 in RA induced NTDs mouse model

Previously our results showed miR-222-3p was significantly downregulated in retinoic acid-induced neural tube defect (NTD) mouse model through transcriptome. Down-regulation of miR-222-3p may be a causative biomarker in NTDs. In this study, RNA was extracted from mouse embryos at E8.5, E9.5 and E10.5, and the expression level of miR-222-3p was measured by quantitative real-time PCR analysis. The preliminary mechanism of miR-222-3p in NTDs involved in cell proliferation, apoptosis and migration was investigated in mouse HT-22 cell line. The expression of miR-222-3p was significantly decreased at E8.5, E9.5 and E10.5 developed in mouse embryos which were consistent with our transcriptome sequencing. Suppression of miR-222-3p in HT-22 cells resulted in the inhibition of cell proliferation and migration, cell cycle and apoptosis. Moreover, DNA damage transcript 4 (Ddit4) was identified as a direct and functional target of miR-222-3p. miR-222-3p is negatively regulated by Ddit4. The mutation of binding site of Ddit4 3'UTR abrogated the responsiveness of luciferase reporters to miR-222-3p and showed that Ddit4 expression partially attenuated the function of miR-222-3p. We preliminatively confirmed that low expression of miR-222-3p has reduced the expression of β-catenin, TCF4 and other related genes in the Wnt/β-catenin signaling pathway.Collectively, these results demonstrated that miR-222-3p regulates the Wnt/β-catenin signaling pathway through Ddit4 inhibition in HT-22 cells, resulted in cell proliferation and apoptosis imbalance, and thus led to neural tube defects.

Single cell transcriptomics of the developing zebrafish lens and identification of putative controllers of lens development

The vertebrate lens is a valuable model system for investigating the gene expression changes that coordinate tissue differentiation due to its inclusion of two spatially separated cell types, the outer epithelial cells and the deeper denucleated fiber cells that they support. Zebrafish are a useful model system for studying lens development given the organ's rapid development in the first several days of life in an accessible, transparent embryo. While we have strong foundational knowledge of the diverse lens crystallin proteins and the basic gene regulatory networks controlling lens development, no study has detailed gene expression in a vertebrate lens at single cell resolution. Here we report an atlas of lens gene expression in zebrafish embryos and larvae at single cell resolution through five days of development, identifying a number of novel putative regulators of lens development. Our data address open questions about the temperospatial expression of α-crystallins during lens development that will support future studies of their function and provide the first detailed view of β- and γ-crystallin expression in and outside the lens. We describe divergent expression in transcription factor genes that occur as paralog pairs in the zebrafish. Finally, we examine the expression dynamics of cytoskeletal, membrane associated, RNA-binding, and transcription factor genes, identifying a number of novel patterns. Overall these data provide a foundation for identifying and characterizing lens developmental regulatory mechanisms and revealing targets for future functional studies with potential therapeutic impact.

Fish models for investigating nutritional regulation of embryonic development

In recent decades, biologist have focused on the spatiotemporal regulation and function of genes to understand embryogenesis. It is clear that maternal diet impacts fetal development but how nutrients, like lipids and vitamins, modify developmental programs is not completely understood. Fish are useful research organisms for such investigations. Most species of fish produce eggs that develop outside the mother, dependent on a finite amount of yolk to form and grow. The developing embryo is a closed system that can be readily biochemically analyzed, easily visualized, and manipulated to understand the role of nutrients in tissue specification, organogenesis, and growth. Natural variation in yolk composition observed across fish species may be related to unique developmental strategies. In this review, we discuss the reasons that teleost fishes are powerful models to understand nutritional control of development and highlight three species that are particularly valuable for future investigations: the zebrafish, Danio rerio, the African Killifish, Nothobranchius furzeri, and the Mexican tetra, Astyanax mexicanus. This review is a part of a special issue on nutritional, hormonal, and metabolic drivers of development.

Hmx1 regulates urfh1 expression in the craniofacial region in zebrafish

H6 family homeobox 1 (HMX1) regulates multiple aspects of craniofacial development as it is widely expressed in the eye, peripheral ganglia and branchial arches. Mutations in HMX1 are linked to an ocular defect termed Oculo-auricular syndrome of Schorderet-Munier-Franceschetti (MIM #612109). We identified UHRF1 as a target of HMX1 during development. UHRF1 and its partner proteins actively regulate chromatin modifications and cellular proliferation. Luciferase assays and in situ hybridization analyses showed that HMX1 exerts a transcriptional inhibitory effect on UHRF1 and a modification of its expression pattern. Overexpression of hmx1 in hsp70-hmx1 zebrafish increased uhrf1 expression in the cranial region, while mutations in the hmx1 dimerization domains reduced uhrf1 expression. Moreover, the expression level of uhrf1 and its partner dnmt1 was increased in the eye field in response to hmx1 overexpression. These results indicate that hmx1 regulates uhrf1 expression and, potentially through regulating the expression of factors involved in DNA methylation, contribute to the development of the craniofacial region of zebrafish.

Hmx3a Has Essential Functions in Zebrafish Spinal Cord, Ear and Lateral Line Development

Transcription factors that contain a homeodomain DNA-binding domain have crucial functions in most aspects of cellular function and embryonic development in both animals and plants. Hmx proteins are a subfamily of NK homeodomain-containing proteins that have fundamental roles in development of sensory structures such as the eye and the ear. However, Hmx functions in spinal cord development have not been analyzed. Here, we show that zebrafish (Danio rerio) hmx2 and hmx3a are coexpressed in spinal dI2 and V1 interneurons, whereas hmx3b, hmx1, and hmx4 are not expressed in spinal cord. Using mutational analyses, we demonstrate that, in addition to its previously reported role in ear development, hmx3a is required for correct specification of a subset of spinal interneuron neurotransmitter phenotypes, as well as correct lateral line progression and survival to adulthood. Surprisingly, despite similar expression patterns of hmx2 and hmx3a during embryonic development, zebrafish hmx2 mutants are viable and have no obviously abnormal phenotypes in sensory structures or neurons that require hmx3a In addition, embryos homozygous for deletions of both hmx2 and hmx3a have identical phenotypes to severe hmx3a single mutants. However, mutating hmx2 in hypomorphic hmx3a mutants that usually develop normally, results in abnormal ear and lateral line phenotypes. This suggests that while hmx2 cannot compensate for loss of hmx3a, it does function in these developmental processes, although to a much lesser extent than hmx3a More surprisingly, our mutational analyses suggest that Hmx3a may not require its homeodomain DNA-binding domain for its roles in viability or embryonic development.

Hypothesis: Wound-induced TLR3 activation stimulates endogenous retinoic acid synthesis and signalling during regeneration

Although the mechanism is unclear, it has been shown that genetically normal adult mice with a large wound form de novo morphogenesis of hair follicles in wound-induced hair neogenesis (WIHN)(1). We focused on how tissues recognize damage signals and identified that double-stranded RNA (dsRNA)-mediated toll-like receptor 3 (TLR3) activation stimulates WIHN. Here, we propose a hypothesis that TLR3 stimulates retinoic acid synthesis and signalling to allow for regeneration, suggesting that common clinical methods of facial rejuvenation in human subjects through damage (such as lasers or dermabrasion), and the use of topical retinoids reflect the same biologic pathway.

Inhibition of NRF2 signaling and increased reactive oxygen species during embryogenesis in a rat model of retinoic acid-induced neural tube defects

Exposure to retinoic acid (RA) during pregnancy increases the risk of serious neural tube defects (NTDs) in the developing fetus. The precise molecular mechanism for this process is unclear; however, RA is associated with oxidative stress mediated by reactive oxygen species. Nuclear factor erythroid 2-related factor 2 (NRF2) is a master regulator of oxidative stress that directs the expression of antioxidant genes and detoxifying proteins to maintain redox homeostasis. We established a rat model of NTDs in which pregnant dams were administered all-trans (at)RA on gestational day 10, and oxidative stress levels and the spatiotemporal expression of NRF2 and its downstream targets were examined in the resulting embryos and in maternal blood. In the NTD group, total antioxidative capacity decreased and 8-hydroxy-2'-deoxyguanosine increased in maternal serum and fetal spinal cord tissues. Plasma GSH content, the GSH/GSSG ratio, and glutathione peroxidase activity in fetal spinal cords were lower in the NTD group relative to controls. We detected NRF2 protein reduction and concomitant upregulation of Kelch-like ECH-associated protein 1 (KEAP1) - a cytoplasmic inhibitor of NRF2 - in the NTD group. The mRNA and protein levels of downstream targets of NRF2 were downregulated in the spinal cords of NTD embryos. These data demonstrate substantial oxidative stress and NRF2 signaling pathway disruption in a model of NTDs induced by atRA. The inhibitory effects of atRA on NRF2 signaling may lower cellular defenses against RA-induced oxidative stress and could play important roles in NTD occurrence during embryonic development.

Somite-Derived Retinoic Acid Regulates Zebrafish Hematopoietic Stem Cell Formation

Hematopoietic stem cells (HSCs) are multipotent progenitors that generate all vertebrate adult blood lineages. Recent analyses have highlighted the importance of somite-derived signaling factors in regulating HSC specification and emergence from dorsal aorta hemogenic endothelium. However, these factors remain largely uncharacterized. We provide evidence that the vitamin A derivative retinoic acid (RA) functions as an essential regulator of zebrafish HSC formation. Temporal analyses indicate that RA is required for HSC gene expression prior to dorsal aorta formation, at a time when the predominant RA synthesis enzyme, aldh1a2, is strongly expressed within the paraxial mesoderm and somites. Previous research implicated the Cxcl12 chemokine and Notch signaling pathways in HSC formation. Consequently, to understand how RA regulates HSC gene expression, we surveyed the expression of components of these pathways in RA-depleted zebrafish embryos. During somitogenesis, RA-depleted embryos exhibit altered expression of jam1a and jam2a, which potentiate Notch signaling within nascent endothelial cells. RA-depleted embryos also exhibit a severe reduction in the expression of cxcr4a, the predominant Cxcl12b receptor. Furthermore, pharmacological inhibitors of RA synthesis and Cxcr4 signaling act in concert to reduce HSC formation. Our analyses demonstrate that somite-derived RA functions to regulate components of the Notch and Cxcl12 chemokine signaling pathways during HSC formation.

Genes and signaling networks regulated during zebrafish optic vesicle morphogenesis

BACKGROUND

The genetic cascades underpinning vertebrate early eye morphogenesis are poorly understood. One gene family essential for eye morphogenesis encodes the retinal homeobox (Rx) transcription factors. Mutations in the human retinal homeobox gene (RAX) can lead to gross morphological phenotypes ranging from microphthalmia to anophthalmia. Zebrafish rx3 null mutants produce a similar striking eyeless phenotype with an associated expanded forebrain. Thus, we used zebrafish rx3-/- mutants as a model to uncover an Rx3-regulated gene network during early eye morphogenesis.

RESULTS

Rx3-regulated genes were identified using whole transcriptomic sequencing (RNA-seq) of rx3-/- mutants and morphologically wild-type siblings during optic vesicle morphogenesis. A gene co-expression network was then constructed for the Rx3-regulated genes, identifying gene cross-talk during early eye development. Genes highly connected in the network are hub genes, which tend to exhibit higher expression changes between rx3-/- mutants and normal phenotype siblings. Hub genes down-regulated in rx3-/- mutants encompass homeodomain transcription factors and mediators of retinoid-signaling, both associated with eye development and known human eye disorders. In contrast, genes up-regulated in rx3-/- mutants are centered on Wnt signaling pathways, associated with brain development and disorders. The temporal expression pattern of Rx3-regulated genes was further profiled during early development from maternal stage until visual function is fully mature. Rx3-regulated genes exhibited synchronized expression patterns, and a transition of gene expression during the early segmentation stage when Rx3 was highly expressed. Furthermore, most of these deregulated genes are enriched with multiple RAX-binding motif sequences on the gene promoter.

CONCLUSIONS

Here, we assembled a comprehensive model of Rx3-regulated genes during early eye morphogenesis. Rx3 promotes optic vesicle morphogenesis and represses brain development through a highly correlated and modulated network, exhibiting repression of genes mediating Wnt signaling and concomitant enhanced expression of homeodomain transcription factors and retinoid-signaling genes.

ALDH1A1 Deficiency in Gorlin Syndrome Suggests a Central Role for Retinoic Acid and ATM Deficits in Radiation Carcinogenesis

We hypothesize that aldehyde dehydrogenase 1A1 (ALDH1A1) deficiency will result in impaired ataxia-telangiectasia mutated (ATM) activation in a retinoic acid-sensitive fashion. Data supporting this hypothesis include (1) reduced ATM activation in irradiated primary dermal fibroblasts from ALDH1A1-deficient Gorlin syndrome patients (GDFs), relative to ALDH1A1-positive normal human dermal fibroblasts (NHDFs) and (2) increased ATM activation by X-radiation in GDFs pretreated with retinoic acid, however, the impact of donor variability on ATM activation in fibroblasts was not assessed and is a prudent consideration in future studies. Clonogenic survival of irradiated cells showed differential responses to retinoic acid as a function of treatment time. Long-term (5 Day) retinoic acid treatment functioned as a radiosensitizer and was associated with downregulation of ATM protein levels. Short-term (7 h) retinoic acid treatment showed a trend toward increased survival of irradiated cells and did not downregulate ATM protein levels. Using a newly developed IncubATR technology, which defines changes in bulk chemical bond patterns in live cells, we can discriminate between the NHDF and GDF phenotypes, but treatment of GDFs with retinoic acid does not induce reversion of bulk chemical bond patterns associated with GDFs toward the NHDF phenotype. Collectively, our preliminary investigation of the Gorlin phenotype has identified deficient ALDH1A1 expression associated with deficient ATM activation as a possible susceptibility factor that is consistent with the high incidence of spontaneous and radiation-induced carcinogenesis in these patients. The IncubATR technology exhibits sufficient sensitivity to detect phenotypic differences in live cells that may be relevant to radiation health effects.

Uracil-DNA glycosylase is involved in DNA demethylation and required for embryonic development in the zebrafish embryo

Uracil-DNA glycosylase (Ung) is a component of the base excision repair process and has the ability to remove uracil from U:G mispairs in DNA. However, its implications in development of vertebrate embryos are poorly understood. In this study, we found that zebrafish uracil-DNA glycosylase a (Unga) is maternally expressed at high levels and accumulated in nuclei during cleavage and blastulation periods. Knockdown of unga in zebrafish embryos causes an increase of the global DNA methylation level concomitantly with a reduction of overall transcriptional activity in the nucleus, ultimately resulting in embryonic lethality during segmentation period. Conversely, unga overexpression is sufficient to reduce the global DNA methylation level, to increase H3K4me3 and H3K27me3 marks, and to activate genome transcription. Furthermore, overexpression of unga(D132A) mRNA, encoding a mutant Unga without DNA glycosylase activity, does not affect global DNA methylation level, indicating that its involvement in DNA demethylation is dependent on its glycosylase activity. These results together suggest that Unga is implicated in postfertilization genomic DNA demethylation, zygotic gene transcription, and normal embryonic development in zebrafish.

The role of Zic transcription factors in regulating hindbrain retinoic acid signaling

Background

The reiterated architecture of cranial motor neurons aligns with the segmented structure of the embryonic vertebrate hindbrain. Anterior-posterior identity of cranial motor neurons depends, in part, on retinoic acid signaling levels. The early vertebrate embryo maintains a balance between retinoic acid synthetic and degradative zones on the basis of reciprocal expression domains of the retinoic acid synthesis gene aldhehyde dehydrogenase 1a2 (aldh1a2) posteriorly and the oxidative gene cytochrome p450 type 26a1 (cyp26a1) in the forebrain, midbrain, and anterior hindbrain.

Results

This manuscript investigates the role of zinc finger of the cerebellum (zic) transcription factors in regulating levels of retinoic acid and differentiation of cranial motor neurons. Depletion of zebrafish Zic2a and Zic2b results in a strong downregulation of aldh1a2 expression and a concomitant reduction in activity of a retinoid-dependent transgene. The vagal motor neuron phenotype caused by loss of Zic2a/2b mimics a depletion of Aldh1a2 and is rescued by exogenously supplied retinoic acid.

Conclusion

Zic transcription factors function in patterning hindbrain motor neurons through their regulation of embryonic retinoic acid signaling.

Investigation of retinoic acid function during embryonic brain development using retinaldehyde-rescued Rdh10 knockout mice

BACKGROUND

Retinoic acid (RA) signaling controls patterning and neuronal differentiation within the hindbrain, but forebrain RA function remains controversial. RA is produced from metabolism of retinol to retinaldehyde by retinol dehydrogenase (RDH), followed by metabolism of retinaldehyde to RA by retinaldehyde dehydrogenase (RALDH). Previous studies on Raldh2-/- and Raldh3-/- mice demonstrated an RA requirement for γ-aminobutyric acid (GABA)ergic and dopaminergic differentiation in forebrain basal ganglia, but no RA requirement was observed during early forebrain patterning or subsequent forebrain cortical expansion. However, other studies suggested that RA controls forebrain patterning, and analysis of ethylnitrosourea-induced Rdh10 mutants suggested that RA synthesized in the meninges stimulates forebrain cortical expansion.

RESULTS

We generated Rdh10-/- mouse embryos that lack RA activity early in the head and later in the meninges. We observed defects in hindbrain patterning and eye RA signaling, but early forebrain patterning was unaffected. Retinaldehyde treatment of Rdh10-/- embryos from E7-E9 rescues a cranial skeletal defect, resulting in E14.5 embryos lacking meningeal RA activity but maintaining normal forebrain shape and cortical expansion.

CONCLUSIONS

Rdh10-/- embryos demonstrate that RA controls hindbrain but not early forebrain patterning, while studies on retinaldehyde-rescued Rdh10-/- embryos show that meningeal RA synthesis is unnecessary to stimulate forebrain cortical expansion.

The Nkx5/HMX homeodomain protein MLS-2 is required for proper tube cell shape in the C. elegans excretory system

Cells perform wide varieties of functions that are facilitated, in part, by adopting unique shapes. Many of the genes and pathways that promote cell fate specification have been elucidated. However, relatively few transcription factors have been identified that promote shape acquisition after fate specification. Here we show that the Nkx5/HMX homeodomain protein MLS-2 is required for cellular elongation and shape maintenance of two tubular epithelial cells in the C. elegans excretory system, the duct and pore cells. The Nkx5/HMX family is highly conserved from sea urchins to humans, with known roles in neuronal and glial development. MLS-2 is expressed in the duct and pore, and defects in mls-2 mutants first arise when the duct and pore normally adopt unique shapes. MLS-2 cooperates with the EGF-Ras-ERK pathway to turn on the LIN-48/Ovo transcription factor in the duct cell during morphogenesis. These results reveal a novel interaction between the Nkx5/HMX family and the EGF-Ras pathway and implicate a transcription factor, MLS-2, as a regulator of cell shape.