Limb Development: The Rise and Fall of Retinoic Acid

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.

FGF2 and FGF10 expression patterns in the epidermis and mesenchyme during homeotic transformation of tail into hindlimbs in frog tadpoles

Limbs are trunk quintessential in tetrapods. Their development relies on the Retinoic acid (RA) gradient in association with the Fibroblast Growth Factors (FGFs). The role of various FGFs have been probed extensively and confirmed during the induction of ectopic limbs in vertebrates. On such factual backdrops, we studied the expression patterns of FGF2 and FGF10 in the epidermis and mesenchyme by immunohistochemical localization in the regenerating tails of tadpoles of the Indian tree frog, Polypedates maculatus. These tadpoles are known to exhibit a kind of homeotic transformation of tail to limbs during regeneration, whose exact mechanism is still to be established by scientific investigations. Here in this study, we provide the first evidence of the putative involvement of FGF2 and FGF10 during such ectopic appendage development.

Role of Retinoic Acid Signaling, FGF Signaling and Meis Genes in Control of Limb Development

The function of retinoic acid (RA) during limb development is still debated, as loss and gain of function studies led to opposite conclusions. With regard to limb initiation, genetic studies demonstrated that activation of FGF10 signaling is required for the emergence of limb buds from the trunk, with Tbx5 and RA signaling acting upstream in the forelimb field, whereas Tbx4 and Pitx1 act upstream in the hindlimb field. Early studies in chick embryos suggested that RA as well as Meis1 and Meis2 (Meis1/2) are required for subsequent proximodistal patterning of both forelimbs and hindlimbs, with RA diffusing from the trunk, functioning to activate Meis1/2 specifically in the proximal limb bud mesoderm. However, genetic loss of RA signaling does not result in loss of limb Meis1/2 expression and limb patterning is normal, although Meis1/2 expression is reduced in trunk somitic mesoderm. More recent studies demonstrated that global genetic loss of Meis1/2 results in a somite defect and failure of limb bud initiation. Other new studies reported that conditional genetic loss of Meis1/2 in the limb results in proximodistal patterning defects, and distal FGF8 signaling represses Meis1/2 to constrain its expression to the proximal limb. In this review, we hypothesize that RA and Meis1/2 both function in the trunk to initiate forelimb bud initiation, but that limb Meis1/2 expression is activated proximally by a factor other than RA and repressed distally by FGF8 to generate proximodistal patterning.

The evolutionary origins and diversity of the neuromuscular system of paired appendages in batoids

Appendage patterning and evolution have been active areas of inquiry for the past two centuries. While most work has centred on the skeleton, particularly that of amniotes, the evolutionary origins and molecular underpinnings of the neuromuscular diversity of fish appendages have remained enigmatic. The fundamental pattern of segmentation in amniotes, for example, is that all muscle precursors and spinal nerves enter either the paired appendages or body wall at the same spinal level. The condition in finned vertebrates is not understood. To address this gap in knowledge, we investigated the development of muscles and nerves in unpaired and paired fins of skates and compared them to those of chain catsharks. During skate and shark embryogenesis, cell populations of muscle precursors and associated spinal nerves at the same axial level contribute to both appendages and body wall, perhaps representing an ancestral condition of gnathostome appendicular neuromuscular systems. Remarkably in skates, this neuromuscular bifurcation as well as colinear Hox expression extend posteriorly to pattern a broad paired fin domain. In addition, we identified migratory muscle precursors (MMPs), which are known to develop into paired appendage muscles with Pax3 and Lbx1 gene expression, in the dorsal fins of skates. Our results suggest that muscles of paired fins have evolved via redeployment of the genetic programme of MMPs that were already involved in dorsal fin development. Appendicular neuromuscular systems most likely have emerged as side branches of body wall neuromusculature and have been modified to adapt to distinct aquatic and terrestrial habitats.

An essential role for the nuclear protein Akirin2 in mouse limb interdigital tissue regression

The regulation of interdigital tissue regression requires the interplay of multiple spatiotemporally-controlled morphogen gradients to ensure proper limb formation and release of individual digits. Disruption to this process can lead to a number of limb abnormalities, including syndactyly. Akirins are highly conserved nuclear proteins that are known to interact with chromatin remodelling machinery at gene enhancers. In mammals, the analogue Akirin2 is essential for embryonic development and critical for a wide variety of roles in immune function, meiosis, myogenesis and brain development. Here we report a critical role for Akirin2 in the regulation of interdigital tissue regression in the mouse limb. Knockout of Akirin2 in limb epithelium leads to a loss of interdigital cell death and an increase in cell proliferation, resulting in retention of the interdigital web and soft-tissue syndactyly. This is associated with perdurance of Fgf8 expression in the ectoderm overlying the interdigital space. Our study supports a mechanism whereby Akirin2 is required for the downregulation of Fgf8 from the apical ectodermal ridge (AER) during limb development, and implies its requirement in signalling between interdigital mesenchymal cells and the AER.

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.

Negative effects of retinoic acid on stem cell niche of mouse incisor

The continuous growth of mouse incisors depends on epithelial stem cells (SCs) residing in the SC niche, called labial cervical loop (LaCL). The homeostasis of the SCs is subtly regulated by complex signaling networks. In this study, we focus on retinoic acid (RA), a derivative of Vitamin A and a known pivotal signaling molecule in controlling the functions of stem cells (SCs). We analyzed the expression profiles of several key molecules of the RA signaling pathway in cultured incisor explants upon exogenous RA treatment. The expression patterns of these molecules suggested a negative feedback regulation of RA signaling in the developing incisor. We demonstrated that exogenous RA had negative effects on incisor SCs and that this was accompanied by downregulation of Fgf10, a mesenchymally expressed SC survival factor in the mouse incisor. Supplement of Fgf10 in incisor cultures completely blocked RA effects by antagonizing apoptosis and increasing proliferation in LaCL epithelial SCs. In addition, Fgf10 obviously antagonized RA-induced downregulation of the SC marker Sox2 in incisor epithelial SCs. Our findings suggest that the negative effects of RA on incisor SCs result from inhibition of mesenchymal Fgf10.

Vitamin A (retinoid) metabolism and actions: What we know and what we need to know about amphibians

Vitamin A status is an important consideration in the health of both wild and captive amphibians. Data concerning whole body vitamin A homeostasis in amphibians are scarce, although these animals have been used as experimental models to study the actions of vitamin A in vision, limb regeneration and embryogenesis. The available data suggest that many aspects of vitamin A biology in amphibians are similar to the canonical characteristics of vitamin A metabolism and actions established in mammals. This is consistent with the evolutionary conservation of these important biological processes. Amphibians must obtain vitamin A in their diet, with captive animals being prone to vitamin A deficiency. There is still much to be learned about vitamin A biology in amphibians that can only be achieved through rigorous scientific research. Improved understanding of amphibian vitamin A biology will aid the conservation of endangered amphibians in the wild, as well as the successful maintenance of ex situ populations.

Retinoic acid signaling pathways in development and diseases

Retinoids comprise a group of compounds each composed of three basic parts: a trimethylated cyclohexene ring that is a bulky hydrophobic group, a conjugated tetraene side chain that functions as a linker unit, and a polar carbon-oxygen functional group. Biochemical conversion of carotenoid or other retinoids to retinoic acid (RA) is essential for normal regulation of a wide range of biological processes including development, differentiation, proliferation, and apoptosis. Retinoids regulate various physiological outputs by binding to nuclear receptors called retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which themselves are DNA-binding transcriptional regulators. The functional response of RA and their receptors are modulated by a host of coactivators and corepressors. Retinoids are essential in the development and function of several organ systems; however, deregulated retinoid signaling can contribute to serious diseases. Several natural and synthetic retinoids are in clinical use or undergoing trials for treating specific diseases including cancer. In this review, we provide a broad overview on the importance of retinoids in development and various diseases, highlighting various retinoids in the drug discovery process, ranging all the way from retinoid chemistry to clinical uses and imaging.

Disparate Igf1 expression and growth in the fore- and hind limbs of a marsupial mammal (Monodelphis domestica)

Proper regulation of growth is essential to all stages of life, from development of the egg into an embryo to the maintenance of normal cell cycle progression in adults. However, despite growth's importance to basic biology and health, little is known about how mammalian growth is regulated. In this study, we investigated the molecular basis of the highly disparate growth of opossum fore- and hind limbs in utero. We first used a novel, opossum-specific microarray to identify several growth-related genes that are differentially expressed in opossum fore- and hind limbs of comparable developmental stages. These genes included Igf1. Given Igf1's role in the growth of other systems, we further investigated the role of Igf1 in opossum limb growth. Supporting the microarray results, RT-PCR indicated that Igf1 levels are approximately two times higher in opossum fore- than hind limbs. Consistent with this, while Igf1 transcripts were readily detectable in opossum forelimbs using whole-mount in situ hybridization, they were not detectable in opossum hind limbs. Furthermore, opossum limbs treated with exogenous Igf1 protein experienced significantly greater cellular proliferation and growth than control limbs in vitro. Taken together, results suggest that the differential expression of Igf1 in developing opossum limbs contributes to their divergent rate of growth, and the unique limb phenotype of opossum newborns. This study establishes the opossum limb as a new mammalian model system for study of organ growth.

Vitamin A: a multifunctional tool for development

Extensive research carried out over the last 100 years has established that the fat-soluble organic compound vitamin A plays crucial roles in early development, organogenesis, cell proliferation, differentiation and apoptosis as well as in tissue homeostasis. Given its importance during development, the delivery of vitamin A to the embryo is very tightly regulated with perturbations leading to severe malformations. This review discusses the roles of vitamin A during human development and the molecular mechanisms controlling its biological effects, hence bridging the gap between human development and molecular genetic work carried out in animal models. Vitamin A delivery during pregnancy and its developmental teratology in humans are thus discussed alongside work on model organisms, such as chicken or mice, revealing the molecular layout and functions of vitamin A metabolism and signaling. We conclude that, during development, vitamin A-derived signals are very tightly controlled in time and space and that this complex regulation is achieved by elaborate autoregulatory loops and by sophisticated interactions with other signaling cascades.

Presence of Ribeiroia ondatrae in the developing anuran limb disrupts retinoic acid levels

The widespread reports of malformed frogs have sparked interest worldwide to try and determine the causes of such malformations. Ribeiroia ondatrae is a digenetic trematode, which has been implicated as one such cause, as this parasite encysts within the developing tadpole hind limb bud and inguinal region causing dramatic limb malformations. Currently, the mechanisms involved in parasite-induced limb deformities remain unclear. We sought to investigate whether the level of retinoic acid (RA), a morphogenetic factor known to play a critical role in limb bud formation, is altered by the presence of R. ondatrae within the infected tadpole. Alteration of RA levels within the limb bud caused by the presence of the parasite may be achieved in three ways. First, metacercariae are actively secreting RA; second, cercariae, upon entering the limb/inguinal region, may release a large amount of RA; finally, the metacercariae may induce either an increase in the synthesis or a decrease in the degradation of the host's endogenous retinoic acid levels. Here, we show through high performance liquid chromatography and mass spectrometry that limb bud tissue of Lithobates sylvaticus, which has been parasitised, contains 70% more RA compared to the unparasitised control. Furthermore, parasites that have encysted within the limb buds appear to contain substantially less RA (56%) than the free swimming cercariae (defined as the infectious stage of the parasite). Taken together, these data illustrate for the first time that encystment of R. ondatrae leads to an increase in RA levels in the tadpole limb bud and may offer insight into the mechanisms involved in parasite-induced limb deformities.

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.

Proximal to distal patterning during limb development and regeneration: a review of converging disciplines

Regeneration of lost structures typically involves distinct events: wound healing at the damaged site, the accumulation of cells that will be used as future building blocks and, finally, the initiation of molecular signaling pathways that dictate the form and pattern of the regenerated structures. Amphibians and urodeles in particular, have long been known to have exceptional regenerative properties. For many years, these animals have been the model of choice for understanding limb regeneration, a complex process that involves reconstructing skin, muscle, bone, connective tissue and nerves into a functional 3D structure. It appears that this process of rebuilding an adult limb has many similarities with how the limb forms in the first place – for example, in the embryo, all the components of the limb need to be formed and this requires signaling mechanisms to specify the final pattern. Thus, both limb formation and limb regeneration are likely to employ the same molecular pathways. Given the available tools of molecular biology and genetics, this is an exciting time for both fields to share findings and make significant progress in understanding more about the events that dictate embryonic limb pattern and control limb regeneration. This article focuses particularly on what is known about the molecular control of patterning along the proximal–distal axis.