Transcription factor TLX1 controls retinoic acid signaling to ensure spleen development

Identification of a novel homozygous NR5A1 variant in a patient with a 46,XY disorders of sex development

OBJECTIVES

Nuclear receptor subfamily 5 group A member 1 (NR5A1) is a transcription factor critical for the development of various organs. Pathogenic variants in NR5A1 are associated with a spectrum of disorders of sex development (DSD).

CASE PRESENTATION

A 15-month-old baby, raised as a girl, was referred for genital swelling and ambiguous genitalia. Born to healthy consanguineous parents, the baby had a phallus, perineal hypospadias, labial fusion, and a hypoplastic scrotum. Hormonal evaluation showed normal levels, and ultrasonography revealed small gonads and absence of Müllerian derivatives. Post-human chorionic gonadotropin (hCG) testing indicated an adequate testosterone response. The karyotype was 46,XY, and in it was found a homozygous NR5A1 variant (c.307 C>T, p.Arg103Trp) in a custom 46 XY DSD gene panel. Notably, the patient exhibited complete sex reversal, hyposplenia, and no adrenal insufficiency.

CONCLUSIONS

Previously, NR5A1 pathogenic variants were considered to be dominantly inherited, and homozygous cases were thought to be associated with adrenal insufficiency. Despite the homozygous pathogenic variant, our patient showed hyposplenism with normal adrenal function; this highlights the complexity of NR5A1 genotype-phenotype correlations. These patients should be monitored for adrenal insufficiency and DSD as well as splenic function.

Spleen function is reduced in individuals with NR5A1 variants with or without a difference of sex development: a cross-sectional study

OBJECTIVE

NR5A1 is a key regulator of sex differentiation and has been implicated in spleen development through transcription activation of TLX1. Concerns exist about hypo- or asplenism in individuals who have a difference of sex development (DSD) due to an NR5A1 disease-causing variant. We aimed to assess spleen anatomy and function in a clinical cohort of such individuals and in their asymptomatic family member carriers.

DESIGN

Cross-sectional assessment in 22 patients with a DSD or primary ovarian insufficiency and 5 asymptomatic carriers from 18 families, harboring 14 different NR5A1 variants.

METHODS

Spleen anatomy was assessed by ultrasound, spleen function by peripheral blood cell count, white blood cell differentiation, percentage of nonswitched memory B cells, specific pneumococcal antibody response, % pitted red blood cells, and Howell-Jolly bodies.

RESULTS

Patients and asymptomatic heterozygous individuals had significantly decreased nonswitched memory B cells compared to healthy controls, but higher than asplenic patients. Thrombocytosis and spleen hypoplasia were present in 50% of heterozygous individuals. Four out of 5 individuals homozygous for the previously described p.(Arg103Gln) variant had asplenia.

CONCLUSIONS

Individuals harboring a heterozygous NR5A1 variant that may cause DSD have a considerable risk for functional hyposplenism, irrespective of their gonadal phenotype. Splenic function should be assessed in these individuals, and if affected or unknown, prophylaxis is recommended to prevent invasive encapsulated bacterial infections. The splenic phenotype associated with NR5A1 variants is more severe in homozygous individuals and is, at least for the p.(Arg103Gln) variant, associated with asplenism.

Foxn1 is not essential for T-cell development in teleosts

In mammals, T-cell development depends on the activity of the Foxn1 transcription factor in the thymic epithelium; mutations in the vertebrate-specific Foxn1 gene are associated with profound T-cell lymphopenia and fatal immunodeficiency. Here, we examined the extent of T-cell development in teleosts lacking a functional foxn1 gene. In zebrafish carrying a deleterious internal deletion of foxn1, reduced but robust lymphopoietic activity is maintained in the mutant thymus. Moreover, pseudogenization or loss of foxn1 in the genomes of deep-sea anglerfishes is independent of the presence or absence of the canonical signatures of the T-cell lineage. Thus, in contrast to the situation in mammals, the teleost thymus can support foxn1-independent lymphopoiesis, most likely through the activity of the Foxn4, an ancient metazoan paralog of Foxn1. Our results imply that during the early stages of vertebrate evolution, genetic control of thymopoiesis was functionally redundant and thus robust; in mammals, the genetic network was reorganized to become uniquely dependent on the FOXN1 transcription factor.

Splenic stromal niches in homeostasis and immunity

The spleen is a gatekeeper of systemic immunity where immune responses against blood-borne pathogens are initiated and sustained. Non-haematopoietic stromal cells construct microanatomical niches in the spleen that make diverse contributions to physiological spleen functions and regulate the homeostasis of immune cells. Additional signals from spleen autonomic nerves also modify immune responses. Recent insight into the diversity of the splenic fibroblastic stromal cells has revised our understanding of how these cells help to orchestrate splenic responses to infection and contribute to immune responses. In this Review, we examine our current understanding of how stromal niches and neuroimmune circuits direct the immunological functions of the spleen, with a focus on T cell immunity.

Coevolution of the Tlx homeobox gene with medusa development (Cnidaria: Medusozoa)

Cnidarians display a wide diversity of life cycles. Among the main cnidarian clades, only Medusozoa possesses a swimming life cycle stage called the medusa, alternating with a benthic polyp stage. The medusa stage was repeatedly lost during medusozoan evolution, notably in the most diverse medusozoan class, Hydrozoa. Here, we show that the presence of the homeobox gene Tlx in Cnidaria is correlated with the presence of the medusa stage, the gene having been lost in clades that ancestrally lack a medusa (anthozoans, endocnidozoans) and in medusozoans that secondarily lost the medusa stage. Our characterization of Tlx expression indicate an upregulation of Tlx during medusa development in three distantly related medusozoans, and spatially restricted expression patterns in developing medusae in two distantly related species, the hydrozoan Podocoryna carnea and the scyphozoan Pelagia noctiluca. These results suggest that Tlx plays a key role in medusa development and that the loss of this gene is likely linked to the repeated loss of the medusa life cycle stage in the evolution of Hydrozoa.

Seahorse Male Pregnancy as a Model System to Study Pregnancy, Immune Adaptations, and Environmental Effects

Seahorses, together with sea dragons and pipefishes, belong to the Syngnathidae family of teleost fishes. Seahorses and other Syngnathidae species have a very peculiar feature: male pregnancy. Among different species, there is a gradation of paternal involvement in carrying for the offspring, from a simple attachment of the eggs to the skin surface, through various degrees of egg coverage by skin flaps, to the internal pregnancy within a brood pouch, which resembles mammalian uterus with the placenta. Because of the gradation of parental involvement and similarities to mammalian pregnancy, seahorses are a great model to study the evolution of pregnancy and the immunologic, metabolic, cellular, and molecular processes of pregnancy and embryo development. Seahorses are also very useful for studying the effects of pollutants and environmental changes on pregnancy, embryo development, and offspring fitness. We describe here the characteristics of seahorse male pregnancy, its regulatory mechanisms, the development of immune tolerance of the parent toward the allogeneic embryos, and the effects of environmental pollutants on pregnancy and embryo development.

Absence of Nkx2-3 induces ectopic lymphatic endothelial differentiation associated with impaired extramedullary stress hematopoiesis in the spleen

The red and white pulps as two main parts of the spleen are arranged around distinct types of vasculature, and perform significantly different functions in both humans and mice. Previous observations indicated a profound alteration of the local vessel specialization in mice lacking Nkx2-3 homeodomain transcription factor, including contradictory results suggesting presence of an ectopic lymphatic vascular structure. Furthermore, how the absence of Nkx2-3 and the consequential changes in endothelial components affect the extramedullary hematopoietic activity restricted to the splenic red pulp is unknown. In this work, we investigated the role of Nkx2-3 homeodomain transcription factor as a major morphogenic determinant for vascular specification, and its effect in the extramedullary hematopoiesis following acute blood loss and pharmacological stimulation of megakaryocyte differentiation after treatment with thrombopoietin-receptor mimetic Romiplostim. We found that, in mice lacking Nkx2-3, Prox1-positive lymphatic capillaries containing gp38/CD31 double positive lymphatic endothelial cells develop, arranged into an extensive meshwork, while the Clever1-positive venous segments of red pulp blood vasculature are absent. This lymphatic endothelial shift is coupled with a severely compromised splenic erythropoiesis and a significantly reduced splenic megakaryocyte colony formation following Romiplostim treatment in mice lacking Nkx2-3. These findings indicate that the shift of microvascular patterning in the absence of Nkx2-3 includes the emergence of ectopic Prox1-positive lymphatic vessels, and that this pivoting towards lymph node-like vascular patterning is associated with an impaired reserve hematopoietic capacity of the splenic red pulp.

Retinoid metabolism: new insights

Vitamin A (retinol) is a critical micronutrient required for the control of stem cell functions, cell differentiation, and cell metabolism in many different cell types, both during embryogenesis and in the adult organism. However, we must obtain vitamin A from food sources. Thus, the uptake and metabolism of vitamin A by intestinal epithelial cells, the storage of vitamin A in the liver, and the metabolism of vitamin A in target cells to more biologically active metabolites, such as retinoic acid (RA) and 4-oxo-RA, must be precisely regulated. Here, I will discuss the enzymes that metabolize vitamin A to RA and the cytochrome P450 Cyp26 family of enzymes that further oxidize RA. Because much progress has been made in understanding the regulation of ALDH1a2 (RALDH2) actions in the intestine, one focus of this review is on the metabolism of vitamin A in intestinal epithelial cells and dendritic cells. Another focus is on recent data that 4-oxo-RA is a ligand required for the maintenance of hematopoietic stem cell dormancy and the important role of RARβ (RARB) in these stem cells. Despite this progress, many questions remain in this research area, which links vitamin A metabolism to nutrition, immune functions, developmental biology, and nuclear receptor pharmacology.

Expression characteristics and interaction networks of microRNAs in spleen tissues of grass carp (Ctenopharyngodon idella)

The spleen is an important immune organ in fish. MicroRNAs (miRNAs) have been shown to play an important role in the regulation of immune function. However, miRNA expression profiles and their interaction networks associated with the postnatal late development of spleen tissue are still poorly understood in fish. The grass carp (Ctenopharyngodon idella) is an important economic aquaculture species in China. Here, two small RNA libraries were constructed from the spleen tissue of healthy grass carp at one-year-old and three-year-old. A total of 324 known conserved miRNAs and 9 novel miRNAs were identified by using bioinformatic analysis. Family analysis showed that 23 families such as let-7, mir-1, mir-10, mir-124, mir-8, mir-7, mir-9, and mir-153 were highly conserved between vertebrates and invertebrates. In addition, 14 families such as mir-459, mir-430, mir-462, mir-7147, mir-2187, and mir-722 were present only in fish. Expression analysis showed that the expression patterns of miRNAs in the spleen of one-year-old and three-year-old grass carp were highly consistent, and the percentage of miRNAs with TPM > 100 was above 39%. Twenty significant differentially expressed (SDE) miRNAs were identified. Gene ontology (GO) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis showed that these SDE miRNAs were primarily involved in erythrocyte differentiation, lymphoid organ development, immune response, lipid metabolic process, the B cell receptor signaling pathway, the T cell receptor signaling pathway, and the PPAR signaling pathway. In addition, the following miRNA-mRNA interaction networks were constructed: immune and hematopoietic, cell proliferation and differentiation, and lipid metabolism. This study determined the miRNA transcriptome as well as miRNA-mRNA interaction networks in normal spleen tissue during the late development stages of grass carp. The results expand the number of known miRNAs in grass carp and are a valuable resource for better understanding the molecular biology of the spleen development in grass carp.

Fate mapping and scRNA sequencing reveal origin and diversity of lymph node stromal precursors

Lymph node (LN) stromal cells play a crucial role in LN development and in supporting adaptive immune responses. However, their origin, differentiation pathways, and transcriptional programs are still elusive. Here, we used lineage-tracing approaches and single-cell transcriptome analyses to determine origin, transcriptional profile, and composition of LN stromal and endothelial progenitors. Our results showed that all major stromal cell subsets and a large proportion of blood endothelial cells originate from embryonic Hoxb6⁺ progenitors of the lateral plate mesoderm (LPM), whereas lymphatic endothelial cells arise from Pax3⁺ progenitors of the paraxial mesoderm (PXM). Single-cell RNA sequencing revealed the existence of different Cd34⁺ and Cxcl13⁺ stromal cell subsets and showed that embryonic LNs contain proliferating progenitors possibly representing the amplifying populations for terminally differentiated cells. Taken together, our work identifies the earliest embryonic sources of LN stromal and endothelial cells and demonstrates that stromal diversity begins already during LN development.

A diverse fibroblastic stromal cell landscape in the spleen directs tissue homeostasis and immunity

[Figure: see text].

Identification of key genes involved in the recurrence of glioblastoma multiforme using weighted gene co-expression network analysis and differential expression analysis

Glioblastoma multiforme (GBM) is the most fatal malignancy, and despite extensive treatment, tumors inevitably recur. This study aimed to identify recurrence-associated molecules in GBM. The gene expression profile GSE139533, containing 70 primary and 47 recurrent GBM tissues and their corresponding clinical traits, was downloaded from the Gene Expression Omnibus (GEO) database and used for weighted gene co-expression network analysis (WGCNA) and differentially expressed gene (DEG) analysis. After identifying the hub genes which differentially expressed in recurrent GBM tissues and in the gene modules correlated with recurrence, data from the Chinese Glioma Genome Atlas (CCGA) and The Cancer Genome Atlas (TCGA) databases were analyzed with GSE43378 to determine the relationship between hub genes and patient prognosis. The diagnostic value of the identified hub genes was verified using 52 GBM tissues. Three gene modules were correlated with recurrence and 2623 genes were clustered in these clinically significant modules. Among these, 13 genes - EHF, TRPM1, FXYD4, CDH15, LHX5, TP73, FBN3, TLX1, C1QL4, COL2A, SEC61G, NEUROD4 and GPR139 - were differentially expressed in recurrent GBM samples; low LHX5 and TLX1 expression predicted poor outcomes. LHX5 and TLX1 expression showed weak positive relationships with Karnofsky performance scale scores. Additionally, LHX5 and TLX1 expression was found to be decreased in our recurrent GBM samples compared with that in primary samples; these genes exhibited high diagnostic value in distinguishing recurrent samples from primary samples. Our findings indicate that LHX5 and TLX1 might be involved in GBM recurrence and act as potential biomarkers for this condition.

Altered Expression of Candidate Genes in Mayer-Rokitansky-Küster-Hauser Syndrome May Influence Vaginal Keratinocytes Biology: A Focus on Protein Kinase X

Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome is a rare and complex disease defined by congenital aplasia of the vagina and uterus in 46,XX women, often associated with kidney and urinary tract anomalies. The aetiopathogenesis of MRKH syndrome is still largely unknown. Herein, we investigated the role of selected candidate genes in the aetiopathogenesis of MRKH syndrome, with a focus on PRKX, which encodes for protein kinase X. Through RT-qPCR analyses performed on vaginal dimple samples from patients, and principal component analysis (PCA), we highlighted a phenotype-related expression pattern of PRKX, MUC1, HOXC8 and GREB1L in MRKH patients. By using an in vitro approach, we proved that PRKX ectopic overexpression in a cell model of vaginal keratinocytes promotes cell motility through epithelial-to-mesenchymal transition (EMT) activation, a fundamental process in urogenital tract morphogenesis. Moreover, our findings showed that PRKX upregulation in vaginal keratinocytes is able to affect transcriptional levels of HOX genes, implicated in urinary and genital tract development. Our study identified the dysregulation of PRKX expression as a possible molecular cause for MRKH syndrome. Moreover, we propose the specific role of PRKX in vaginal keratinocyte biology as one of the possible mechanisms underlying this complex disease.

The distribution of some homeobox proteins in the bovine placenta during gestation

Homeobox proteins are transcription factors known to be involved in the molecular basics of body model formation and transformation. Some homeobox proteins are known to play critical roles in the control of the extraembryonic development of the placenta during gestation and in the regulation of uterine and placental physiology in adults. The gravid uteruses and placentas of 27 Holsteins cows, obtained from private slaughterhouses, were used in this study. The tissues were assigned to three groups as belonging to the first, second and third timesters of gestation, based on the fetal ages determined. Subsequently, the tissues were subjected to immunohistochemical staining using antibodies specific to the proteins investigated in the study. The evaluation of the immunohistochemical findings obtained in this study, demonstrated the presence of trimester-dependent varying intensities of immunoreactions in the uterus and placenta. Immunoreactivity was observed particularly in the luminal and glandular epithelial cells of the uterus, as well as in stromal and some endothelial cells. Furthermore, immunoreactivity for the proteins HOXA10, HOXB6, HOXC6 and Dlx-5 was determined in the smooth muscle cells. Moreover, immunoreactivity was also detected in the maternal epithelium and fetal trophoblasts found in the structure of the placenta. The results suggest that the homeobox proteins investigated may have critical roles in the regulation of endometrial functions in cows, and the proliferation and differentiation of endometrial and placental cells. It is concluded that these proteins may have physiological roles in the formation and development of the placenta, as well as in the maintenance of pregnancy.

Long Non-Coding RNA BCAR4 Binds to miR-644a and Targets TLX1 to Promote the Progression of Bladder Cancer

Background

Bladder cancer is a serious threat to human health. It is meaningful to study the pathogenesis of bladder cancer. Long non-coding RNAs (lncRNAs) are reported to promote or inhibit bladder cancer development. However, the role of lncRNA BCAR4 in the regulation of bladder cancer remains unclear.

Purpose

This study was to explore the role of lncRNA BCAR4 in the progression of bladder cancer cell.

Methods

RT-PCR was used to examine the expression of BCAR4 and miR-644a. CCK8 assay, colony formation assay, Transwell assay were used to detect the progression of bladder cancer cells after transfecting of indicated plasmids.

Results

The expression of BCAR4 was higher in bladder cancer cell lines than normal urothelial cell line. Moreover, the expression of BCAR4 was associated with the advanced stage and metastasis of bladder cancer. Through knockdown of BCAR4, we discovered that knockdown of BCAR4 significantly decreased the proliferation, migration and invasive abilities of bladder cancer cells. Mechanically, we showed that BCAR4 can bind to miR-644a directly and targets TLX1. Moreover, we also showed that miR-644a was also highly expressed in bladder cancer cells and inhibition of miR-644a or overexpression of TLX1 can increased the migration abilities of bladder cancer caused by knockdown of BCAR4.

Conclusion

We showed that BCAR4 sponged miR-644a to modulate the expression of TLX1 and promote bladder cancer development.

Regulating Retinoic Acid Availability during Development and Regeneration: The Role of the CYP26 Enzymes

This review focuses on the role of the Cytochrome p450 subfamily 26 (CYP26) retinoic acid (RA) degrading enzymes during development and regeneration. Cyp26 enzymes, along with retinoic acid synthesising enzymes, are absolutely required for RA homeostasis in these processes by regulating availability of RA for receptor binding and signalling. Cyp26 enzymes are necessary to generate RA gradients and to protect specific tissues from RA signalling. Disruption of RA homeostasis leads to a wide variety of embryonic defects affecting many tissues. Here, the function of CYP26 enzymes is discussed in the context of the RA signalling pathway, enzymatic structure and biochemistry, human genetic disease, and function in development and regeneration as elucidated from animal model studies.

Cyp26b1 is an essential regulator of distal airway epithelial differentiation during lung development

Proper organ development depends on coordinated communication between multiple cell types. Retinoic acid (RA) is an autocrine and paracrine signaling molecule essential for the development of most organs, including the lung. Despite extensive work detailing effects of RA deficiency in early lung morphogenesis, little is known about how RA regulates late gestational lung maturation. Here, we investigate the role of the RA catabolizing protein Cyp26b1 in the lung. Cyp26b1 is highly enriched in lung endothelial cells (ECs) throughout development. We find that loss of Cyp26b1 leads to reduction of alveolar type 1 cells, failure of alveolar inflation and early postnatal lethality in mouse. Furthermore, we observe expansion of distal epithelial progenitors, but no appreciable changes in proximal airways, ECs or stromal populations. Exogenous administration of RA during late gestation partially mimics these defects; however, transcriptional analyses comparing Cyp26b1-/- with RA-treated lungs reveal overlapping, but distinct, responses. These data suggest that defects observed in Cyp26b1-/- lungs are caused by both RA-dependent and RA-independent mechanisms. This work reports crucial cellular crosstalk during lung development involving Cyp26b1-expressing endothelium and identifies a novel RA modulator in lung development.

RDH10-mediated retinol metabolism and RARα-mediated retinoic acid signaling are required for submandibular salivary gland initiation

In mammals, the epithelial tissues of major salivary glands generate saliva and drain it into the oral cavity. For submandibular salivary glands (SMGs), the epithelial tissues arise during embryogenesis from naïve oral ectoderm adjacent to the base of the tongue, which begins to thicken, express SOX9 and invaginate into underlying mesenchyme. The developmental mechanisms initiating salivary gland development remain unexplored. In this study, we show that retinoic acid (RA) signaling activity at the site of gland initiation is colocalized with expression of retinol metabolic genes Rdh10 and Aldh1a2 in the underlying SMG mesenchyme. Utilizing a novel ex vivo assay for SMG initiation developed for this study, we show that RDH10 and RA are required for salivary gland initiation. Moreover, we show that the requirement for RA in gland initiation involves canonical signaling through retinoic acid receptors (RAR). Finally, we show that RA signaling essential for gland initiation is transduced specifically through RARα, with no contribution from other RAR isoforms. This is the first study to identify a molecular signal regulating mammalian salivary gland initiation.

Origin and specialization of splenic macrophages

Macrophage heterogeneity in the spleen has been long documented, with four subsets populating the different splenic compartments. The diverse environments on the splenic compartments determine their varied phenotype and functions. In the white pulp, highly phagocytic macrophages contribute to the generation of the immune response. The marginal zone contains two populations of macrophages, which also contribute to the immune response. Their strategic position in the bloodstream and their unique phenotype confer them a crucial role in the defense against blood borne pathogens, placing them at the crossroad between innate and adaptive immune responses. Macrophages in the red pulp are classically linked to homeostatic and metabolic functions in erythrocyte phagocytosis and iron recycling. We review here recent advances demonstrating the importance of macrophage ontogeny and organ development in determining the phenotype, transcriptional profile and, ultimately, the functions of the populations of splenic macrophages.

A retinoic acid-dependent stroma-leukemia crosstalk promotes chronic lymphocytic leukemia progression

In chronic lymphocytic leukemia (CLL), the non-hematopoietic stromal microenvironment plays a critical role in promoting tumor cell recruitment, activation, survival, and expansion. However, the nature of the stromal cells and molecular pathways involved remain largely unknown. Here, we demonstrate that leukemic B lymphocytes induce the activation of retinoid acid synthesis and signaling in the microenvironment. Inhibition of RA-signaling in stromal cells causes deregulation of genes associated with adhesion, tissue organization and chemokine secretion including the B-cell chemokine CXCL13. Notably, reducing retinoic acid precursors from the diet or inhibiting RA-signaling through retinoid-antagonist therapy prolong survival by preventing dissemination of leukemia cells into lymphoid tissues. Furthermore, mouse and human leukemia cells could be distinguished from normal B-cells by their increased expression of Rarγ2 and RXRα, respectively. These findings establish a role for retinoids in murine CLL pathogenesis, and provide new therapeutic strategies to target the microenvironment and to control disease progression.

The stromal microenvironment plays a key role in the expansion of chronic lymphocytic leukemia. Here, the authors use the Eµ-TCL1 mouse model to show that leukemic B-cells induce the activation of retinoic acid synthesis in stromal cells of the lymphoid microenvironment, and that impacting on retinoic acid signalling via diet or chemical inhibition prolonged survival by preventing leukemia dissemination and accumulation in lymphoid tissues.

Origin and Immunological Functions of Spleen Stromal Cells

The mammalian spleen is a peripheral lymphoid organ that plays a central role in host defense. Consequently, the lack of spleen is often associated with immunodeficiency and increased risk of overwhelming infections. Growing evidence suggests that non-hematopoietic stromal cells are central players in spleen development, organization, and immune functions. In addition to its immunological role, the spleen also provides a site for extramedullary hematopoiesis (EMH) in response to injuries. A deeper understanding of the biology of stromal cells is therefore essential to fully comprehend how these cells modulate the immune system during normal and pathological conditions. Here, we review the specificities of the different mouse spleen stromal cell subsets and complement the murine studies with human data when available.

Rdh10 loss-of-function and perturbed retinoid signaling underlies the etiology of choanal atresia

Craniofacial development is a complex process that involves sequential growth and fusion of the facial prominences. When these processes fail, congenital craniofacial anomalies can occur. For example, choanal atresia (CA) is a congenital craniofacial anomaly in which the connection between the nasal airway and nasopharynx is completely blocked. CA occurs in approximately 1/5000 live births and is a frequent component of congenital disorders such as CHARGE, Treacher Collins, Crouzon and Pfeiffer syndromes. However, the detailed cellular and molecular mechanisms underpinning the etiology and pathogenesis of CA remain elusive. In this study, we discovered that mice with mutations in retinol dehydrogenase 10 (Rdh10), which perturbs Vitamin A metabolism and retinoid signaling, exhibit fully penetrant CA. Interestingly, we demonstrate Rdh10 is specifically required in non-neural crest cells prior to E10.5 for proper choanae formation, and that in the absence of Rdh10, Fgf8 is ectopically expressed in the nasal fin. Furthermore, we found that defects in choanae development are associated with decreased cell proliferation and increased cell death in the epithelium of the developing nasal cavity, which retards invagination of the nasal cavity, and thus appears to contribute to the pathogenesis of CA. Taken together, our findings demonstrate that RDH10 is essential during the early stages of facial morphogenesis for the formation of a functional nasal airway, and furthermore establish Rdh10 mutant mice as an important model system to study CA.

Enzymatic Metabolism of Vitamin A in Developing Vertebrate Embryos

Embryonic development is orchestrated by a small number of signaling pathways, one of which is the retinoic acid (RA) signaling pathway. Vitamin A is essential for vertebrate embryonic development because it is the molecular precursor of the essential signaling molecule RA. The level and distribution of RA signaling within a developing embryo must be tightly regulated; too much, or too little, or abnormal distribution, all disrupt embryonic development. Precise regulation of RA signaling during embryogenesis is achieved by proteins involved in vitamin A metabolism, retinoid transport, nuclear signaling, and RA catabolism. The reversible first step in conversion of the precursor vitamin A to the active retinoid RA is mediated by retinol dehydrogenase 10 (RDH10) and dehydrogenase/reductase (SDR family) member 3 (DHRS3), two related membrane-bound proteins that functionally activate each other to mediate the interconversion of retinol and retinal. Alcohol dehydrogenase (ADH) enzymes do not contribute to RA production under normal conditions during embryogenesis. Genes involved in vitamin A metabolism and RA catabolism are expressed in tissue-specific patterns and are subject to feedback regulation. Mutations in genes encoding these proteins disrupt morphogenesis of many systems in a developing embryo. Together these observations demonstrate the importance of vitamin A metabolism in regulating RA signaling during embryonic development in vertebrates.