Vitamin A deficiency results in the dose-dependent acquisition of anterior character and shortening of the caudal hindbrain of the rat embryo

The multifaceted roles of retinoids in eye development, vision, and retinal degenerative diseases

Vitamin A (all-trans-retinol; at-Rol) and its derivatives, known as retinoids, have been adopted by vertebrates to serve as visual chromophores and signaling molecules, particularly in the eye/retina. Few tissues rely on retinoids as heavily as the retina, and the study of genetically modified mouse models with deficiencies in specific retinoid-metabolizing proteins has allowed us to gain insight into the unique or redundant roles of these proteins in at-Rol uptake and storage, or their downstream roles in retinal development and function. These processes occur during embryogenesis and continue throughout life. This review delves into the role of these genes in supporting retinal function and maps the impact that genetically modified mouse models have had in studying retinoid-related genes. These models display distinct perturbations in retinoid biochemistry, physiology, and metabolic flux, mirroring human ocular diseases.

Effects of microbiota-testis interactions on the reproductive health of male ruminants: A review

Ruminants are one of the world's economically important species, and their reproductive health is critical to the economic development of the livestock industry. In recent years, research on the relationship between microbiota and reproductive health has received much attention. Microbiota disruption affects the developmental health of the testes and epididymis, the male reproductive organs of the host, which in turn is related to sperm quality. Maintaining a stable microbiota protects the host from pathogens and increases breeding performance, which in turn promotes the economic development of animal husbandry. In addition, the effects and mechanisms of microbiota on reproduction were further explored. These findings support new approaches to improving and managing reproductive health in ruminants through the microbiota and facilitate further systematic exploration of microbiota-mediated reproductive impacts.

Effect of vitamin A on maternal, fetal, and neonatal outcomes: An overview of deficiency, excessive intake, and intake recommendations

Vitamin A imbalance during pregnancy and lactation is a global public health concern with potentially negative consequences for fetuses and neonates. Inadequate vitamin A intake during this critical period can lead to anemia, weakened immune function, night blindness, and increased susceptibility to infections. Conversely, excessive intake of vitamin A can result in birth defects, hypercalcemia, and psychiatric symptoms. This review aims to identify risk factors contributing to vitamin A deficiency in pregnant women and its impact on maternal, fetal, and neonatal outcomes. It also examines the effects of high-dose vitamin A supplementation during pregnancy on offspring health. By analyzing existing literature and recommendations, the review emphasizes the significance of vitamin A in the development of various body systems and organs. It provides a comprehensive overview of the effects of vitamin A during pregnancy and lactation, encompassing deficiencies, excessive intake, and supplementation guidelines. The need for further research in this field is highlighted. In conclusion, maintaining a balanced vitamin A status is crucial during pregnancy to promote better outcomes for fetuses and newborns. Effective monitoring and intervention strategies are essential to address vitamin A deficiency and excess in pregnant women, thereby improving fetal and neonatal health.

Non-canonical retinoid signaling in neural development, regeneration and synaptic function

Canonical retinoid signaling via nuclear receptors and gene regulation is critical for the initiation of developmental processes such as cellular differentiation, patterning and neurite outgrowth, but also mediates nerve regeneration and synaptic functions in adult nervous systems. In addition to canonical transcriptional regulation, retinoids also exert rapid effects, and there are now multiple lines of evidence supporting non-canonical retinoid actions outside of the nucleus, including in dendrites and axons. Together, canonical and non-canonical retinoid signaling provide the precise temporal and spatial control necessary to achieve the fine cellular coordination required for proper nervous system function. Here, we examine and discuss the evidence supporting non-canonical actions of retinoids in neural development and regeneration as well as synaptic function, including a review of the proposed molecular mechanisms involved.

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.

Retinoic acid signaling regulates late stages of semicircular canal morphogenesis and otolith maintenance in the zebrafish inner ear

BACKGROUND

The vitamin A derivative all-trans retinoic acid (RA) regulates early stages of inner ear development. As the early disruption of the RA pathway results in profound mispatterning of the developing inner ear, this confounds analyses of specific roles in later stages. Therefore, we used the temporal-specific exposure of all-trans RA or diethylaminobenzaldehyde to evaluate RA functions in late otic development.

RESULTS

Perturbing late RA signaling causes behavioral defects analogous to those expected in larvae suffering from vestibular dysfunction. These larvae also demonstrate malformations of the semi-circular canals, as visualized through (a) use of the transgenic strain nkhspdmc12a, a fluorescent reporter expressed in otic epithelium; and (b) injection of the fluorescent lipophilic dye DiI. We also noted the altered expression of genes encoding ECM proteins or modifying enzymes. Other malformations of the inner ear observed in our work include the loss or reduced size of the utricular and saccular otoliths, suggesting a role for RA in otolith maintenance.

CONCLUSION

Our work has identified a previously undescribed late phase of RA activity in otic development, demonstrating that vestibular defects observed in human patients in relation to perturbed RA signaling are not solely due to its early disruption in otic development.

Impact of endocrine disrupting chemicals on neurodevelopment: the need for better testing strategies for endocrine disruption-induced developmental neurotoxicity

INTRODUCTION

Brain development is highly dependent on hormonal regulation. Exposure to chemicals disrupting endocrine signaling has been associated with neurodevelopmental impairment. This raises concern about exposure to the suspected thousands of endocrine disruptors, and has resulted in efforts to improve regulation of these chemicals. Yet, the causal links between endocrine disruption and developmental neurotoxicity, which would be required for regulatory action, are still largely missing.

AREAS COVERED

In this review, we illustrate the importance of two endocrine systems, thyroid hormone and retinoic acid pathways, for neurodevelopment. We place special emphasis on TH and RA synthesis, metabolism, and how endocrine disrupting chemicals known or suspected to affect these systems are associated with developmental neurotoxicity.

EXPERT OPINION

While it is clear that neurodevelopment is dependent on proper hormonal functioning, and evidence is increasing for developmental neurotoxicity induced by endocrine disrupting chemicals, this is not grasped by current chemical testing. Thus, there is an urgent need to develop test methods detecting endocrine disruption in the context of neurodevelopment. Key to this development is further mechanistic insights on the involvement of endocrine signaling in neurodevelopment as well as increased support to develop and validate new test methods for the regulatory context.

Disrupted spermatogenesis in a metabolic syndrome model: the role of vitamin A metabolism in the gut-testis axis

OBJECTIVE

Effects of the diet-induced gut microbiota dysbiosis reach far beyond the gut. We aim to uncover the direct evidence involving the gut-testis axis in the aetiology of impaired spermatogenesis.

DESIGN

An excessive-energy diet-induced metabolic syndrome (MetS) sheep model was established. The testicular samples, host metabolomes and gut microbiome were analysed. Faecal microbiota transplantation (FMT) confirmed the linkage between gut microbiota and spermatogenesis.

RESULTS

We demonstrated that the number of arrested spermatogonia was markedly elevated by using 10× single-cell RNA-seq in the MetS model. Furthermore, through using metabolomics profiling and 16S rDNA-seq, we discovered that the absorption of vitamin A in the gut was abolished due to a notable reduction of bile acid levels, which was significantly associated with reduced abundance of Ruminococcaceae_NK4A214_group. Notably, the abnormal metabolic effects of vitamin A were transferable to the testicular cells through the circulating blood, which contributed to abnormal spermatogenesis, as confirmed by FMT.

CONCLUSION

These findings define a starting point for linking the testicular function and regulation of gut microbiota via host metabolomes and will be of potential value for the treatment of male infertility in MetS.

Effects of intrauterine devices on proteins in the uterine lavage fluid of mares

Intrauterine devices block luteolysis in cyclic mares, but the underlying mechanism is unknown. To clarify the mechanisms, the protein profile of the endometrial secretome was analyzed using two-dimensional difference gel electrophoresis (2D-DIGE). Twenty-seven mares were classified according to whether they were inseminated (AI) or had an intrauterine device (IUD), a water-filled plastic sphere, inserted into the uterus on Day 3 after ovulation. Uterine lavage fluids were collected on Day 15 from pregnant inseminated mares (AI-P; n = 8), non-pregnant inseminated mares (AI-N; n = 4), and mares with IUD (n = 15). The IUD group was further divided into prolonged (IUD-P; n = 7) and normal luteal phase (IUD-N; n = 8) groups on the basis of ultrasound examinations, serum levels of progesterone and PGFM on Days 14 and 15, and COX-2 results on Day 15. Four mares from each group were selected for the 2D-DIGE analyses. Ten proteins had significantly different abundance among the groups, nine of the proteins were identified. Malate dehydrogenase 1, increased sodium tolerance 1, aldehyde dehydrogenase 1A1, prostaglandin reductase 1, albumin and hemoglobin were highest in pregnant mares; T-complex protein 1 was highest in non-pregnant mares; and annexin A1 and 6-phosphogluconolactonase were highest in IUD mares. The results suggest that the mechanism behind the intrauterine devices is likely related to inflammation.

Retinoid signaling in skeletal development: Scoping the system for predictive toxicology

All-trans retinoic acid (ATRA), the biologically active form of vitamin A, is instrumental in regulating the patterning and specification of the vertebrate embryo. Various animal models demonstrate adverse developmental phenotypes following experimental retinoid depletion or excess during pregnancy. Windows of vulnerability for altered skeletal patterning coincide with early specification of the body plan (gastrulation) and regional specification of precursor cell populations forming the facial skeleton (cranial neural crest), vertebral column (somites), and limbs (lateral plate mesoderm) during organogenesis. A common theme in physiological roles of ATRA signaling is mutual antagonism with FGF signaling. Consequences of genetic errors or environmental disruption of retinoid signaling include stage- and region-specific homeotic transformations to severe deficiencies for various skeletal elements. This review derives from an annex in Detailed Review Paper (DRP) of the OECD Test Guidelines Programme (Project 4.97) to support recommendations regarding assay development for the retinoid system and the use of resulting data in a regulatory context for developmental and reproductive toxicity (DART) testing.

Neurogenesis From Neural Crest Cells: Molecular Mechanisms in the Formation of Cranial Nerves and Ganglia

The neural crest (NC) is a transient multipotent cell population that originates in the dorsal neural tube. Cells of the NC are highly migratory, as they travel considerable distances through the body to reach their final sites. Derivatives of the NC are neurons and glia of the peripheral nervous system (PNS) and the enteric nervous system as well as non-neural cells. Different signaling pathways triggered by Bone Morphogenetic Proteins (BMPs), Fibroblast Growth Factors (FGFs), Wnt proteins, Notch ligands, retinoic acid (RA), and Receptor Tyrosine Kinases (RTKs) participate in the processes of induction, specification, cell migration and neural differentiation of the NC. A specific set of signaling pathways and transcription factors are initially expressed in the neural plate border and then in the NC cell precursors to the formation of cranial nerves. The molecular mechanisms of control during embryonic development have been gradually elucidated, pointing to an important role of transcriptional regulators when neural differentiation occurs. However, some of these proteins have an important participation in malformations of the cranial portion and their mutation results in aberrant neurogenesis. This review aims to give an overview of the role of cell signaling and of the function of transcription factors involved in the specification of ganglia precursors and neurogenesis to form the NC-derived cranial nerves during organogenesis.

Retinoids and developmental neurotoxicity: Utilizing toxicogenomics to enhance adverse outcome pathways and testing strategies

The use of genomic approaches in toxicological studies has greatly increased our ability to define the molecular profiles of environmental chemicals associated with developmental neurotoxicity (DNT). Integration of these approaches with adverse outcome pathways (AOPs), a framework that translates environmental exposures to adverse developmental phenotypes, can potentially inform DNT testing strategies. Here, using retinoic acid (RA) as a case example, we demonstrate that the integration of toxicogenomic profiles into the AOP framework can be used to establish a paradigm for chemical testing. RA is a critical regulatory signaling molecule involved in multiple aspects of mammalian central nervous system (CNS) development, including hindbrain formation/patterning and neuronal differentiation, and imbalances in RA signaling pathways are linked with DNT. While the mechanisms remain unresolved, environmental chemicals can cause DNT by disrupting the RA signaling pathway. First, we reviewed literature evidence of RA and other retinoid exposures and DNT to define a provisional AOP related to imbalances in RA embryonic bioavailability and hindbrain development. Next, by integrating toxicogenomic datasets, we defined a relevant transcriptomic signature associated with RA-induced developmental neurotoxicity (RA-DNT) in human and rodent models that was tested against zebrafish model data, demonstrating potential for integration into an AOP framework. Finally, we demonstrated how these approaches may be systematically utilized to identify chemical hazards by testing the RA-DNT signature against azoles, a proposed class of compounds that alters RA-signaling. The provisional AOP from this study can be expanded in the future to better define DNT biomarkers relevant to RA signaling and toxicity.

How Dietary Deficiency Studies Have Illuminated the Many Roles of Vitamin A During Development and Postnatal Life

Vitamin A deficiency studies have been carried out since the early 1900s. Initially, these studies led to the identification of fat soluble A as a unique and essential component of the diet of rodents, birds, and humans. Continuing work established that vitamin A deficiency produces biochemical and physiological dysfunction in almost every vertebrate organ system from conception to death. This chapter begins with a review of representative historical and current studies that used the nutritional vitamin A deficiency research model to gain an understanding of the many roles vitamin A plays in prenatal and postnatal development and well-being. This is followed by a discussion of recent studies that show specific effects of vitamin A deficiency on prenatal development and postnatal maintenance of the olfactory epithelium, brain, and heart. Vitamin A deficiency studies have helped define the necessity of vitamin A for the health of all vertebrates, including farm animals, but the breadth of deficient states and their individual effects on health have not been fully determined. Future work is needed to develop tools to assess the complete vitamin A status of an organism and to define the levels of vitamin A that optimally support molecular and systems level processes during all ages and stages of life.

Association of concentrations of beta-carotene in plasma on pregnancy per artificial insemination and pregnancy loss in lactating Holstein cows

The objective of this study was to determine the association of beta-carotene concentration in plasma at the moment of artificial insemination (AI) on pregnancy/AI in lactating Holstein cows. A total of 399 events from 364 lactating Holstein cows were enrolled in the trial (143 primiparous and 221 multiparous). All cows were assigned to a timed AI protocol based on estradiol and progesterone. Blood samples were collected at the moment of AI and at 24 and 31d post-AI (samples on 31 d post-AI were collected only from cows that were diagnosed pregnant). The BCS were recorded at the time of AI. Plasma beta-carotene was quantified from blood samples taken at the time of AI using a single step denaturation and extraction into a solvent, followed by measurement using a portable spectrophotometer. Pregnancy-associated glycoproteins (PAG) were analyzed in blood samples taken at 24 and 31 d post-AI of pregnant cows. Milk production was collected for the entire experimental period. Pregnancy diagnosis was performed by ultrasound 31 and 60 d post-AI. Data was analyzed using the MIXED and GLIMMIX procedures of SAS. Cows classified as thin (<2.75) tended to have lower concentration of beta-carotene at AI when compared with those classified as Moderate (≥3.00; 3.8 ± 0.1 vs. 4.3 ± 0.1 μg/mL; P = 0.09). Concentration of beta-carotene were greater in multiparous compared with primiparous (P < 0.01). There was no correlation between concentration of beta-carotene and milk production (r = 0.04; P = 0.10). When plasma beta-carotene was categorized in quartiles, cows in the 1st quartile had lower pregnancy/AI and higher pregnancy losses when compared with cows that were in the 2nd, 3rd and 4th quartile (pregnancy/AI = 19.2 ± 4.5, 33.7 ± 4.7, 36.9 ± 5.0 and 39.8 ± 5.4%, respectively; P = 0.05; pregnancy losses = 41.9 ± 4.8, 20.4 ± 3.7, 22.1 ± 4.1, and 15.7 ± 4.2%, respectively; P < 0.05). There was no association between concentrations of beta-carotene at AI and PAG at 24 d post-AI (P = 0.60). Cows with greater concentrations of beta-carotene at AI were more likely to have greater concentrations of PAG at 31 d post-AI (P < 0.01). In conclusion, the concentration of beta-carotene at AI was affected by BCS and parity. Cows with higher concentrations of plasma beta-carotene at AI had greater pregnancy/AI, lower pregnancy losses and greater concentrations of PAG at d 31 post-AI, suggesting it may be associated with placental function in lactating dairy cows.

RDH10 function is necessary for spontaneous fetal mouth movement that facilitates palate shelf elevation

Cleft palate is a common birth defect, occurring in approximately 1 in 1000 live births worldwide. Known etiological mechanisms of cleft palate include defects within developing palate shelf tissues, defects in mandibular growth and defects in spontaneous fetal mouth movement. Until now, experimental studies directly documenting fetal mouth immobility as an underlying cause of cleft palate have been limited to models lacking neurotransmission. This study extends the range of anomalies directly demonstrated to have fetal mouth movement defects correlated with cleft palate. Here, we show that mouse embryos deficient in retinoic acid (RA) have mispatterned pharyngeal nerves and skeletal elements that block spontaneous fetal mouth movement in utero. Using X-ray microtomography, in utero ultrasound video, ex vivo culture and tissue staining, we demonstrate that proper retinoid signaling and pharyngeal patterning are crucial for the fetal mouth movement needed for palate formation. Embryos with deficient retinoid signaling were generated by stage-specific inactivation of retinol dehydrogenase 10 (Rdh10), a gene crucial for the production of RA during embryogenesis. The finding that cleft palate in retinoid deficiency results from a lack of fetal mouth movement might help elucidate cleft palate etiology and improve early diagnosis in human disorders involving defects of pharyngeal development.

Summary: Fetal mouth immobility and defects in pharyngeal patterning underlie cleft palate in retinoid-deficient Rdh10 mutant mouse embryos.

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.

Hindbrain induction and patterning during early vertebrate development

The hindbrain is a key relay hub of the central nervous system (CNS), linking the bilaterally symmetric half-sides of lower and upper CNS centers via an extensive network of neural pathways. Dedicated neural assemblies within the hindbrain control many physiological processes, including respiration, blood pressure, motor coordination and different sensations. During early development, the hindbrain forms metameric segmented units known as rhombomeres along the antero-posterior (AP) axis of the nervous system. These compartmentalized units are highly conserved during vertebrate evolution and act as the template for adult brainstem structure and function. TALE and HOX homeodomain family transcription factors play a key role in the initial induction of the hindbrain and its specification into rhombomeric cell fate identities along the AP axis. Signaling pathways, such as canonical-Wnt, FGF and retinoic acid, play multiple roles to initially induce the hindbrain and regulate Hox gene-family expression to control rhombomeric identity. Additional transcription factors including Krox20, Kreisler and others act both upstream and downstream to Hox genes, modulating their expression and protein activity. In this review, we will examine the earliest embryonic signaling pathways that induce the hindbrain and subsequent rhombomeric segmentation via Hox and other gene expression. We will examine how these signaling pathways and transcription factors interact to activate downstream targets that organize the segmented AP pattern of the embryonic vertebrate hindbrain.

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.

Prenatal Vitamin a Deficiency Causes Laryngeal Malformation in Rats

Objectives:

Our previous research demonstrated that vitamin A might be related to vocal fold development. The purpose of this study was to determine whether vitamin A deficiency affects prenatal laryngeal development in rats.

Methods:

Two considerations were necessary in designing a study using a rat model: For embryonic survival, vitamin A is necessary through day 10 of gestation, and laryngeal formation occurs primarily after day 11. Thus, we created a rat model that developed vitamin A deficiency after embryonic day 11. Ten pregnant rats (5 vitamin A-deficient rats and 5 control rats) were studied. Embryos were collected at embryonic day 18.5 and analyzed histologically.

Results:

Eighteen percent of the vitamin A-deficient embryos were alive and demonstrated laryngotracheal cartilage malformation, incomplete separation of the glottis, and/or laryngoesophageal clefts.

Conclusions:

These results document the important role played by vitamin A in laryngeal development.

Retinoic-acid-mediated HRas stabilization induces neuronal differentiation of neural stem cells during brain development

Ras signaling is tightly regulated during neural stem cell (NSC) differentiation, and defects in this pathway result in aberrant brain development. However, the mechanism regulating Ras signaling during NSC differentiation was unknown. Here, we show that stabilized HRas specifically induces neuronal differentiation of NSCs. Lentivirus-mediated HRas overexpression and knockdown resulted in stimulation and inhibition, respectively, of NSC differentiation into neuron in the ex vivo embryo. Retinoic acid, an active metabolite of vitamin A, promoted neuronal differentiation of NSCs by stabilizing HRas, and HRas knockdown blocked the retinoic acid effect. Vitamin-A-deficient mice displayed abnormal brain development with reduced HRas levels and a reduced thickness of the postmitotic region containing differentiated neurons. All of these abnormal phenotypes were rescued with the restoration of HRas protein levels achieved upon feeding with a retinoic-acid-supplemented diet. In summary, this study shows that retinoic acid stabilizes HRas protein during neurogenesis, and that this is required for NSC differentiation into neurons and murine brain development.

A cellular and molecular mosaic establishes growth and differentiation states for cranial sensory neurons

We compared apparent origins, cellular diversity and regulation of initial axon growth for differentiating cranial sensory neurons. We assessed the molecular and cellular composition of the developing olfactory and otic placodes, and cranial sensory ganglia to evaluate contributions of ectodermal placode versus neural crest at each site. Special sensory neuron populations-the olfactory and otic placodes, as well as those in vestibulo-acoustic ganglion- are entirely populated with cells expressing cranial placode-associated, rather than neural crest-associated markers. The remaining cranial sensory ganglia are a mosaic of cells that express placode-associated as well as neural crest-associated markers. We found two distinct populations of neural crest in the cranial ganglia: the first, as expected, is labeled by Wnt1:Cre mediated recombination. The second is not labeled by Wnt1:Cre recombination, and expresses both Sox10 and FoxD3. These populations-Wnt1:Cre recombined, and Sox10/Foxd3-expressing- are proliferatively distinct from one another. Together, the two neural crest-associated populations are substantially more proliferative than their placode-associated counterparts. Nevertheless, the apparently placode- and neural crest-associated populations are similarly sensitive to altered signaling that compromises cranial morphogenesis and differentiation. Acute disruption of either Fibroblast growth factor (Fgf) or Retinoic acid (RA) signaling alters axon growth and cell death, but does not preferentially target any of the three distinct populations. Apparently, mosaic derivation and diversity of precursors and early differentiating neurons, modulated uniformly by local signals, supports early cranial sensory neuron differentiation and growth.

Origin and function of embryonic Sertoli cells

In the adult testis, Sertoli cells (SCs) are the epithelial supporting cells of the seminiferous tubules that provide germ cells (GCs) with the required nutrients and structural and regulatory support to complete spermatogenesis. SCs also form the blood-testis barrier, phagocytose apoptotic spermatocytes and cell debris derived from spermiogenesis, and produce and secrete numerous paracrine and endocrine signals involved in different regulatory processes. In addition to their essential functions in the adult testis, SCs play a pivotal role during testis development. They are the first cells to differentiate in the embryonic XY gonadal primordium and are involved in the regulation of testis-specific differentiation processes, such as prevention of GC entry into meiosis, Leydig and peritubular myoid cell differentiation, and regression of the Müllerian duct, the anlagen of the uterus, oviducts, and the upper part of the vagina. Expression of the Y-linked gene SRY in pre-SCs initiates a genetic cascade that leads to SC differentiation and subsequently to testis development. Since the identification of the SRY gene, many Sertoli-specific transcription factors and signals underlying the molecular mechanisms of early testis differentiation have been identified. Here, we review the state of the art of the molecular interactions that commit the supporting cell lineage of the gonadal primordium to differentiate as SCs and the subsequent Sertoli-specific signaling pathways involved in early testis differentiation.

Environmental aspects of congenital scoliosis

Growing evidence has proved that many aspects of our lifestyle and the environment contribute to the development of congenital disease. Congenital spinal deformities are due to anomalous development of the vertebrae including failure of formation and segmentation during embryogenesis. The causes of congenital scoliosis have not been fully identified. A variety of factors are implicated in the development of vertebral abnormalities. Previous studies have demonstrated that both genetics and environmental factors are implicated in the development of vertebral abnormalities. However, no specific cause for congenital scoliosis has been identified. In our review, we focus on the environmental factors for the development of congenital scoliosis. Various maternal exposures during pregnancy including hypoxia, alcohol use, vitamin deficiency, valproic acid, boric acid, and hyperthermia have been observed to be associated with the occurrence of congenital scoliosis. This review describes the major environmental contributors of congenital scoliosis with an emphasis on treatment aspects associated with environmental disposition in congenital scoliosis.

Mechanisms of retinoic acid signalling and its roles in organ and limb development

Retinoic acid (RA) signalling has a central role during vertebrate development. RA synthesized in specific locations regulates transcription by interacting with nuclear RA receptors (RARs) bound to RA response elements (RAREs) near target genes. RA was first implicated in signalling on the basis of its teratogenic effects on limb development. Genetic studies later revealed that endogenous RA promotes forelimb initiation by repressing fibroblast growth factor 8 (Fgf8). Insights into RA function in the limb serve as a paradigm for understanding how RA regulates other developmental processes. In vivo studies have identified RAREs that control repression of Fgf8 during body axis extension or activation of homeobox (Hox) genes and other key regulators during neuronal differentiation and organogenesis.

A complex RARE is required for the majority of Nedd9 embryonic expression

Neural precursor cell expressed, developmentally down-regulated 9 (Nedd9, Casl, Hef1, p105cas, Ef1) is a scaffolding protein that assembles complexes involved in regulating cell adhesion, migration, division, and survival. Nedd9 is found very early in the developing embryonic nervous system. A highly conserved complex retinoic acid response element (RARE) is located 485 base pairs (bp) upstream of exon 2B in the promoter of the Nedd9 gene. Mice transgenic for a 5.2 kilobase (kb) region of the 2B Nedd9 promoter containing the RARE upstream of a lacZ reporter gene [Nedd9(RARE)-lacZ] show a large subset of the normal endogenous Nedd9 expression including that in the caudal hindbrain neuroepithelium, spinal cord, dorsal root ganglia (drg) and migrating neural crest (ncc). However, the transgenic mice do not recapitulate the native Nedd9 expression pattern in presumptive rhombomeres (pr) 3 and 5 of the early hindbrain, the base of the neuroepithelium in the midbrain, nor the forebrain telencephalon. Thus, the 5.2 kb region containing the intact RARE drives a large subset of Nedd9 expression, with additional sequences outside of this region needed to define the full complement of expression. When the 5.2 kb construct is modified (eight point mutations) to eliminate responsiveness of the RARE to all-trans retinoic acid (atRA) [Nedd9(mutRARE)-lacZ], virtually all β-galactosidase (β-gal, lacZ) expression is lost. Exposure of Nedd9(RARE)-lacZ transgenic embryos to excess atRA at embryonic day 8.0 (E8.0) leads to rostral ectopic transgene expression within 6 h whereas the Nedd9(mutRARE)-lacZ mutant does not show this effect. Thus the RARE upstream of the Nedd9 2B promoter is necessary for much of the endogenous gene expression during early development as well as ectopic expression in response to atRA.

Retinoids and glomerular regeneration

Retinoids are essential in the development and function of several organs, exerting potent effects on stem cell systems. All-trans retinoic acid, through binding to the retinoic acid response elements, alters transcription of numerous genes in stem cells, leading to an exit from the self-renewing state and promoting differentiation. In the kidney, retinoids protect against injury and ameliorate function in multiple experimental models of disease. Recent evidence suggests that retinoids act on renal progenitors by promoting their differentiation into mature podocytes and retinoic acid-induced podocyte differentiation is impaired by proteinuria because of sequestration of retinoic acid by albumin. However, retinoic acid administration can revert renal progenitor differentiation and promote podocyte regeneration. A more complete understanding of retinoid-dependent renal progenitor differentiation into podocytes should reward us with new insights into the mechanisms of progression toward glomerulosclerosis.

Gestational vitamin A deficiency: a novel cause of sensorineural hearing loss in the developing world?

Hearing loss is a substantial public health problem with profound social and economic consequences in the developing world. The World Health Organization (WHO) estimates that there are 360 million people living with disabling hearing loss globally, and 80% of these individuals are from low- and middle-income countries. The epidemiology of hearing impairment remains poorly defined in most impoverished societies. Middle ear infections in childhood are a key determinant; however, congenital anomalies may also comprise an important etiology and may arise from gestational malnutrition. While evidence exists that preventable vitamin A deficiency exacerbates the severity of ear infections and, consequently, hearing loss, antenatal vitamin A deficiency during sensitive periods of fetal development may represent an etiologically distinct and virtually unexplored causal pathway. Evidence from multiple animal systems clearly shows that fetal inner ear development requires adequate vitamin A nutriture to proceed normally. Inner ear malformations occur in experimentally imposed maternal vitamin A deficiency in multiple species in a dose-response manner. These anomalies are likely due to the loss of retinoic acid-dependent regulation of both hindbrain development and otic morphogenic processes. Based on in vivo evidence in experimental animals, we hypothesize that preventable gestational vitamin A deficiency, especially during early stages of fetal development, may predispose offspring to inner ear malformations and sensorineural hearing loss. As vitamin A deficiency affects an estimated 20 million pregnant women globally, we hypothesize that, in undernourished settings, routine provision of supplemental vitamin A at the recommended allowance throughout pregnancy may promote normal inner ear development and reduce risk of an as yet unknown fraction of sensorineural hearing loss. If our hypothesis proves correct, gestational vitamin A deficiency would represent a potentially preventable etiology of sensorineural hearing loss of substantial public health significance.

Retinoic acid signalling during development

Retinoic acid (RA) is a vitamin A-derived, non-peptidic, small lipophilic molecule that acts as ligand for nuclear RA receptors (RARs), converting them from transcriptional repressors to activators. The distribution and levels of RA in embryonic tissues are tightly controlled by regulated synthesis through the action of specific retinol and retinaldehyde dehydrogenases and by degradation via specific cytochrome P450s (CYP26s). Recent studies indicate that RA action involves an interplay between diffusion (morphogen-like) gradients and the establishment of signalling boundaries due to RA metabolism, thereby allowing RA to finely control the differentiation and patterning of various stem/progenitor cell populations. Here, we provide an overview of the RA biosynthesis, degradation and signalling pathways and review the main functions of this molecule during embryogenesis.

Maternal-fetal transfer and metabolism of vitamin A and its precursor β-carotene in the developing tissues

The requirement of the developing mammalian embryo for retinoic acid is well established. Retinoic acid, the active form of vitamin A, can be generated from retinol and retinyl ester obtained from food of animal origin, and from carotenoids, mainly β-carotene, from vegetables and fruits. The mammalian embryo relies on retinol, retinyl ester and β-carotene circulating in the maternal bloodstream for its supply of vitamin A. The maternal-fetal transfer of retinoids and carotenoids, as well as the metabolism of these compounds in the developing tissues are still poorly understood. The existing knowledge in this field has been summarized in this review in reference to our basic understanding of the transport and metabolism of retinoids and carotenoids in adult tissues. The need for future research on the metabolism of these essential lipophilic nutrients during development is highlighted. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

The all-trans retinoic acid (atRA)-regulated gene Calmin (Clmn) regulates cell cycle exit and neurite outgrowth in murine neuroblastoma (Neuro2a) cells

The vitamin A metabolite all-trans retinoic acid (atRA) functions in nervous system development and regulates cell proliferation and differentiation. Neuroblastoma cells (SH-SY5Y and Neuro2a or N2A) exposed to atRA undergo growth inhibition and neuronal differentiation, both of which are preceded by an increase in Clmn mRNA. Treatment of N2A cells with atRA produces a reduction in phosphohistone 3 immunostaining and BrdU incorporation, both indicators of a reduction in cell proliferation. These effects are nearly eliminated in atRA-treated shClmn knockdown cells. Loss of Clmn in the mouse N2A cell line also results in a significant reduction of atRA-mediated neurite outgrowth, a response that can be rescued by reintroduction of the Clmn sequence. In contrast, ectopic overexpression of Clmn produces an increase in the cyclin dependent kinase inhibitor, p21(Cip1), a decrease in cyclin D1 protein and an increase in hypophosphorylated Rb, showing that Clmn participates in G(1)/S arrest. Clmn overexpression alone is sufficient to inhibit N2A cell proliferation, whereas both Clmn and atRA must be present to induce neurite outgrowth. This study shows that the atRA-responsive gene Clmn promotes exit from the cell cycle, a requisite event for neuronal differentiation.

Vitamin A in reproduction and development

The requirement for vitamin A in reproduction was first recognized in the early 1900's, and its importance in the eyes of developing embryos was realized shortly after. A greater understanding of the large number of developmental processes that require vitamin A emerged first from nutritional deficiency studies in rat embryos, and later from genetic studies in mice. It is now generally believed that all-trans retinoic acid (RA) is the form of vitamin A that supports both male and female reproduction as well as embryonic development. This conclusion is based on the ability to reverse most reproductive and developmental blocks found in vitamin A deficiency induced either by nutritional or genetic means with RA, and the ability to recapitulate the majority of embryonic defects in retinoic acid receptor compound null mutants. The activity of the catabolic CYP26 enzymes in determining what tissues have access to RA has emerged as a key regulatory mechanism, and helps to explain why exogenous RA can rescue many vitamin A deficiency defects. In severely vitamin A-deficient (VAD) female rats, reproduction fails prior to implantation, whereas in VAD pregnant rats given small amounts of carotene or supported on limiting quantities of RA early in organogenesis, embryos form but show a collection of defects called the vitamin A deficiency syndrome or late vitamin A deficiency. Vitamin A is also essential for the maintenance of the male genital tract and spermatogenesis. Recent studies show that vitamin A participates in a signaling mechanism to initiate meiosis in the female gonad during embryogenesis, and in the male gonad postnatally. Both nutritional and genetic approaches are being used to elucidate the vitamin A-dependent pathways upon which these processes depend.

Her9 represses neurogenic fate downstream of Tbx1 and retinoic acid signaling in the inner ear

Proper spatial control of neurogenesis in the inner ear ensures the precise innervation of mechanotransducing cells and the propagation of auditory and equilibrium stimuli to the brain. Members of the Hairy and enhancer of split (Hes) gene family regulate neurogenesis by inhibiting neuronal differentiation and maintaining neural stem cell pools in non-neurogenic zones. Remarkably, their role in the spatial control of neurogenesis in the ear is unknown. In this study, we identify her9, a zebrafish ortholog of Hes1, as a key gene in regulating otic neurogenesis through the definition of the posterolateral non-neurogenic field. First, her9 emerges as a novel otic patterning gene that represses proneural function and regulates the extent of the neurogenic domain. Second, we place Her9 downstream of Tbx1, linking these two families of transcription factors for the first time in the inner ear and suggesting that the reported role of Tbx1 in repressing neurogenesis is in part mediated by the bHLH transcriptional repressor Her9. Third, we have identified retinoic acid (RA) signaling as the upstream patterning signal of otic posterolateral genes such as tbx1 and her9. Finally, we show that at the level of the cranial otic field, opposing RA and Hedgehog signaling position the boundary between the neurogenic and non-neurogenic compartments. These findings permit modeling of the complex genetic cascade that underlies neural patterning of the otic vesicle.

Retinoid signaling in inner ear development: A "Goldilocks" phenomenon

Retinoic acid (RA) is a biologically active derivative of vitamin A that is indispensable for inner ear development. The normal function of RA is achieved only at optimal homeostatic concentrations, with an excess or deficiency in RA leading to inner ear dysmorphogenesis. We present an overview of the role of RA in the developing mammalian inner ear, discussing both how and when RA may act to critically control a program of inner ear development. Molecular mechanisms of otic teratogenicity involving two members of the fibroblast growth factor family, FGF3 and FGF10, and their downstream targets, Dlx5 and Dlx6, are examined under conditions of both RA excess and deficiency. We term the effect of too little or too much RA on FGF/Dlx signaling a Goldilocks phenomenon. We demonstrate that in each case (RA excess, RA deficiency), RA can directly affect FGF3/FGF10 signaling within the otic epithelium, leading to downregulated expression of these essential signaling molecules, which in turn, leads to diminution in Dlx5/Dlx6 expression. Non-cell autonomous affects of the otic epithelium subsequently occur, altering transforming growth factor-beta (TGFβ) expression in the neighboring periotic mesenchyme and serving as a putative explanation for RA-mediated otic capsule defects. We conclude that RA coordinates inner ear morphogenesis by controlling an FGF/Dlx signaling cascade, whose perturbation by deviations in local retinoid concentrations can lead to inner ear dysmorphogenesis.

Retinoic acid metabolizing enzyme CYP26A1 is implicated in rat embryo implantation

BACKGROUND

The retinoic acid metabolizing enzyme Cyp26a1 plays a pivotal role in vertebrate embryo development. Cyp26a1 was characterized previously as a differentially expressed gene in peri-implantation rat uteri via suppressive subtracted hybridization analysis. However, the role of Cyp26a1 in rat embryo implantation remained elusive.

METHODS

The expression of Cyp26a1 in the uteri of early pregnancy, pseudopregnancy and artificial decidualization was detected by northern blotting, real time-PCR, in situ hybridization, western blotting and immunofluorescent staining. The effect of Cyp26a1 on apoptosis of endometrial stromal cells (ESCs) isolated from rat uteri was determined by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) and Hoechst staining. Apoptosis-related proteins in ESCs were detected by western blotting.

RESULTS

Cyp26a1 showed distinctive expression patterns in embryos and uteri during the peri-implantation period, with a remarkable increase (P < 0.01 versus Days 4-5) in mRNA and protein in the implantation phase (Days 5.5-6.5 of pregnancy). CYP26A1 was specifically localized in glandular epithelium, luminal epithelium and decidua basalis. The level of CYP26A1 protein was significantly increased in uteri of artificial decidualization (P < 0.01 versus control). Forced Cyp26a1 overexpression significantly reduced the sensitivity of ESCs to etoposide-induced apoptosis, with reductions in p53 (P < 0.01) and Fas (P < 0.05) proteins versus control, while in contrast, FasL (P < 0.01) and proliferating cell nuclear antigen (P < 0.05) proteins increased.

CONCLUSIONS

Cyp26a1 is spatiotemporally expressed in the uterus during embryo implantation and decidualization. Overexpression of Cyp26a1 attenuates the process of uterine stromal cell apoptosis, probably via down-regulating the expression of p53 and FasL.

Maternal diet modulates the risk for neural tube defects in a mouse model of diabetic pregnancy

Pregnancies complicated by maternal diabetes have long been known to carry a higher risk for congenital malformations, such as neural tube defects. Using the FVB inbred mouse strain and the Streptozotocin-induced diabetes model, we tested whether the incidence of neural tube defects in diabetic pregnancies can be modulated by maternal diet. In a comparison of two commercial mouse diets, which are considered nutritionally replete, we found that maternal consumption of the unfavorable diet was associated with a more than 3-fold higher rate of neural tube defects. Our results demonstrate that maternal diet can act as a modifier of the risk for abnormal development in high-risk pregnancies, and provide support for the possibility that neural tube defects in human diabetic pregnancies might be preventable by optimized maternal nutrition.

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

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

Nav2 is necessary for cranial nerve development and blood pressure regulation

BACKGROUND

All-trans retinoic acid (atRA) is required for nervous system development, including the developing hindbrain region. Neuron navigator 2 (Nav2) was first identified as an atRA-responsive gene in human neuroblastoma cells (retinoic acid-induced in neuroblastoma 1, Rainb1), and is required for atRA-mediated neurite outgrowth. In this paper, we explore the importance of Nav2 in nervous system development and function in vivo.

RESULTS

Nav2 hypomorphic homozygous mutants show decreased survival starting at birth. Nav2 mutant embryos show an overall reduction in nerve fiber density, as well as specific defects in cranial nerves IX (glossopharyngeal) and X (vagus). Nav2 hypomorphic mutant adult mice also display a blunted baroreceptor response compared to wild-type controls.

CONCLUSIONS

Nav2 functions in mammalian nervous system development, and is required for normal cranial nerve development and blood pressure regulation in the adult.

Expression profiling in peripheral blood reveals signature for penetrance in DYT1 dystonia

DYT1 dystonia is an autosomal-dominantly inherited movement disorder, which is usually caused by a GAG deletion in the TOR1A gene. Due to the reduced penetrance of approximately 30-40%, the determination of the mutation in a subject is of limited use with regard to actual manifestation of symptoms. In the present study, we used Affymetrix oligonucleotide microarrays to analyze global gene expression in blood samples of 15 manifesting and 15 non-manifesting mutation carriers in order to identify a susceptibility profile beyond the GAG deletion which is associated with the manifestation of symptoms in DYT1 dystonia. We identified a genetic signature which distinguished between asymptomatic mutation carriers and symptomatic DYT1 patients with 86.7% sensitivity and 100% specificity. This genetic signature could correctly predict the disease state in an independent test set with a sensitivity of 87.5% and a specificity of 85.7%. Conclusively, this genetic signature might provide a possibility to distinguish DYT1 patients from asymptomatic mutation carriers.

Maternal and zygotic aldh1a2 activity is required for pancreas development in zebrafish

We have isolated and characterized a novel zebrafish pancreas mutant. Mutant embryos lack expression of isl1 and sst in the endocrine pancreas, but retain isl1 expression in the CNS. Non-endocrine endodermal gene expression is less affected in the mutant, with varying degrees of residual expression observed for pdx1, carbA, hhex, prox1, sid4, transferrin and ifabp. In addition, mutant embryos display a swollen pericardium and lack fin buds. Genetic mapping revealed a mutation resulting in a glycine to arginine change in the catalytic domain of the aldh1a2 gene, which is required for the production of retinoic acid from vitamin A. Comparison of our mutant (aldh1a2^um22) to neckless (aldh1a2ⁱ²⁶), a previously identified aldh1a2 mutant, revealed similarities in residual endodermal gene expression. In contrast, treatment with DEAB (diethylaminobenzaldehyde), a competitive reversible inhibitor of Aldh enzymes, produces a more severe phenotype with complete loss of endodermal gene expression, indicating that a source of Aldh activity persists in both mutants. We find that mRNA from the aldh1a2^um22 mutant allele is inactive, indicating that it represents a null allele. Instead, the residual Aldh activity is likely due to maternal aldh1a2, since we find that translation-blocking, but not splice-blocking, aldh1a2 morpholinos produce a phenotype similar to DEAB treatment. We conclude that Aldh1a2 is the primary Aldh acting during pancreas development and that maternal Aldh1a2 activity persists in aldh1a2^um22 and aldh1a2ⁱ²⁶ mutant embryos.

Cyp26b1 expression in murine Sertoli cells is required to maintain male germ cells in an undifferentiated state during embryogenesis

In mammals, germ cells within the developing gonad follow a sexually dimorphic pathway. Germ cells in the murine ovary enter meiotic prophase during embryogenesis, whereas germ cells in the embryonic testis arrest in G0 of mitotic cell cycle and do not enter meiosis until after birth. In mice, retinoic acid (RA) signaling has been implicated in controlling entry into meiosis in germ cells, as meiosis in male embryonic germ cells is blocked by the activity of a RA-catabolizing enzyme, CYP26B1. However, the mechanisms regulating mitotic arrest in male germ cells are not well understood. Cyp26b1 expression in the testes begins in somatic cells at embryonic day (E) 11.5, prior to mitotic arrest, and persists throughout fetal development. Here, we show that Sertoli cell-specific loss of CYP26B1 activity between E15.5 and E16.5, several days after germ cell sex determination, causes male germ cells to exit from G0, re-enter the mitotic cell cycle and initiate meiotic prophase. These results suggest that male germ cells retain the developmental potential to differentiate in meiosis until at least at E15.5. CYP26B1 in Sertoli cells acts as a masculinizing factor to arrest male germ cells in the G0 phase of the cell cycle and prevents them from entering meiosis, and thus is essential for the maintenance of the undifferentiated state of male germ cells during embryonic development.

Effect of vitamin A deficiency on retinol and retinyl esters contents in rat brain

In the present study, the pattern of vitamin A (retinol and retinyl esters) contents in discrete brain areas was investigated in Wistar rats (both sexes of 10-12 weeks old) fed on vitamin A deficient diet. The animals were placed on standard laboratory diet for the control animals and a vitamin A deficient diet for the experimental animals for 20 weeks. At the end of this period, brain retinol and retinyl esters contents from control and vitamin A deficient diet animals were measured by HPLC. Retinol was the predominant form of retinoids in male rat brains (77 to 92% of total retinol) and retinyl esters were the predominant form in female brain rats (4 to 44% of total retinol). The abundant ester in both sexes was the retinyl linoleate. Olfactory bulb and the midbrain contained the highest quantities of retinol and retinyl esters in both sexes. On the other hand, the vitamin A deficient diet significantly decreased the retinoid contents in male brain, in olfactory bulb (-30.7%), hindbrain (-46.2%) and increased it in forebrain (84.3%) and midbrain (2.2%). Total retinol was decreased in olfactory bulb (-38.7%), forebrain (-44.5%) and midbrain (-30.7%) and increased in hindbrain (23.4%) of vitamin A deficient female rats. In conclusion, retinol and retinyl esters were the brain compounds heterogeneously distributed throughout the brain areas in both the sexes and were significantly affected by vitamin A deficiency status as well.

Function of retinoic acid receptors during embryonic development

Retinoids, the active metabolites of vitamin A, regulate complex gene networks involved in vertebrate morphogenesis, growth, cellular differentiation and homeostasis. Studies performed in vitro, using either acellular systems or transfected cells, have shown that retinoid actions are mediated through heterodimers between the RAR and RXR nuclear receptors. However, in vitro studies indicate what is possible, but not necessarily what is actually occurring in vivo, because they are performed under non-physiological conditions. Therefore, genetic approaches in the animal have been be used to determine the physiological functions of retinoid receptors. Homologous recombination in embryonic stem cells has been used to generate germline null mutations of the RAR- and RXR-coding genes in the mouse. As reviewed here, the generation of such germline mutations, combined with pharmacological approaches to block the RA signalling pathway, has provided genetic evidence that RAR/RXR heterodimers are indeed the functional units transducing the RA signal during prenatal development. However, due to (i) the complexity in “hormonal” signalling through transduction by the multiple RARs and RXRs, (ii) the functional redundancies (possibly artefactually generated by the mutations) within receptor isotypes belonging to a given family, and (iii) in utero or early postnatal lethality of certain germline null mutations, these genetic studies have failed to reveal all the physiological functions of RARs and RXRs, notably in adults. Spatio-temporally-controlled somatic mutations generated in given cell types/tissues and at chosen times during postnatal life, will be required to reveal all the functions of RAR and RXR throughout the lifetime of the mouse.