The homeotic transformation of tails into limbs in Rana temporaria by retinoids

Retinoic Acid-Induced Limb Duplications

Retinoic acid (RA) and the family of molecules based on vitamin A known as retinoids have remarkable effects on limb regeneration in salamanders and newts and cause whole limb duplications in a concentration-dependent manner. They respecify all three axes of the limb-the proximodistal, the anteroposterior, and the dorsoventral axis. As a result, complete limbs can be induced to regenerate from distal amputation planes producing two limbs in tandem. Here, we describe the basic methods for undertaking these experiments as well as the use of new synthetic retinoids which have retinoic acid receptor-selective actions. These will be valuable tools in future studies on the molecular basis of limb duplications and thus our understanding of the nature of positional information in the regenerating salamander limb.

Schmidtea happens: Re-establishing the planarian as a model for studying the mechanisms of regeneration

Understanding the remarkable regenerative abilities of freshwater planarians was a classic problem of developmental biology. These animals were widely studied until the late 1960s, when their use as experimental subjects declined precipitously after some infamous experiments on memory transfer. By the mid-1990s, only a handful of laboratories worldwide were investigating the mechanisms of planarian regeneration. Here, we provide the personal stories behind our work to reinvigorate studies of these fascinating animals. We recount many of the challenges that had to be overcome and reflect on some of the fortuitous events that helped launch the planarian Schmidtea mediterranea as a model organism for studying the molecular basis of regeneration.

Hif1α and Wnt are required for posterior gene expression during Xenopus tropicalis tail regeneration

Regeneration of complex tissues is initiated by an injury-induced stress response, eventually leading to activation of developmental signaling pathways such as Wnt signaling. How early injury cues are interpreted and coupled to activation of these developmental signals and their targets is not well understood. Here, we show that Hif1α, a stress induced transcription factor, is required for tail regeneration in Xenopus tropicalis. We find that Hif1α is required for regeneration of differentiated axial tissues, including axons and muscle. Using RNA-sequencing, we find that Hif1α and Wnt converge on a broad set of genes required for posterior specification and differentiation, including the posterior hox genes. We further show that Hif1α is required for transcription via a Wnt-responsive element, a function that is conserved in both regeneration and early neural patterning. Our findings indicate that Hif1α has regulatory roles in Wnt target gene expression across multiple tissue contexts.

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.

RA Signaling in Limb Development and Regeneration in Different Species

This chapter brings together data on the role of retinoic acid (RA) in the embryonic development of fins in zebrafish , limbs in amphibians , chicks , and mice, and regeneration of the amphibian limb . The intention is to determine whether there is a common set of principles by which we can understand the mode of action of RA in both embryos and adults. What emerges from this synthesis is that there are indeed commonalities in the involvement of RA in processes that ventralize, posteriorize, and proximalize the developing and regenerating limb . Different axes of the limb have historically been studied independently; as for example, the embryonic development of the anteroposterior (AP) axis of the chick limb bud versus the regeneration of the limb bud proximodistal (PD) axis . But when we take a broader view, a unifying principle emerges that explains why RA administration to embryos and regenerating limbs results in the development of multiple limbs in both cases. As might be expected, different molecular pathways govern the development of different systems and model organisms, but despite these differences, the pathways involve similar RA signaling genes, such as tbx5, meis, shh, fgfs and hox genes. Studies of developing and regenerating systems have highlighted that RA acts by being synthesized in one embryonic location while acting in another one, exactly as embryonic morphogens do, although there is no evidence for the presence of an RA gradient within the limb . What also emerges is that there is a paucity of information on the involvement of RA in development of the dorsoventral (DV) axis . A molecular explanation as to how RA establishes and alters positional information in all three axes is the most important area of study for the future.

Vitamin A administration in lizards during tail regeneration determines epithelial mucogenesis and delays muscle and cartilage differentiation

Regenerating epidermis and spinal cord is essential to maintain tail regeneration in lizards. The effects of vitamin A, an inhibitor of epithelial cornification, have been studied in lizards during tail regeneration. The injection of high doses of vitamin A induces regeneration of a thinner tail with gummy consistency and suppression of the formation of a normal cartilaginous axial skeleton. Microscopic analysis reveals that all epithelia increase the secretion of glycoprotein-mucus. During the analyzed period the epidermis does not form scales and keratinocytes limit or stop the production of bundles of intermediate filament keratins and packets of corneous beta-proteins (β-keratins). Differentiation of oberhautchen and β-layers is much reduced or inhibited while α-keratinization and the formation of a corneous layer are affected as well. The effects of vitamin A are dramatic also on mesoderm cells since the treatment stimulates an invasion of blood cells likely due to the disruption of the wall of blood vessels, mesenchymal cell death (pycnosis), and diffuse phagocytosis by immune cells. A delay of cartilage differentiation and cartilage degradation due to an increase of lysosomes in these cells or released by white blood cells explains the lack of stiffness of the regenerating tail after vitamin A treatment. Regenerating muscles are variably affected, ranging from a variable necrotic effect with partial degradation of internal organelles and myofilaments to a massive or complete loss of myofibrils that do not organize in sarcomeres. In general hypervitaminosis A appears to delay epithelial but also mesodermal cell differentiation and maintains the regenerating tail in an immature condition.

Homeotic transformation of tails into limbs in anurans

Anuran tadpoles can regenerate their tails after amputation. However, they occasionally form ectopic limbs instead of the lost tail part after vitamin A treatment. This is regarded as an example of a homeotic transformation. In this phenomenon, the developmental fate of the tail blastema is apparently altered from that of a tail to that of limbs, indicating a realignment of positional information in the blastema. Morphological observations and analyses of the development of skeletal elements during the process suggest that positional information in the blastema is rewritten from tail to trunk specification under the influence of vitamin A, resulting in limb formation. Despite the extensive information gained from morphological observations, a comprehensive understanding of this phenomenon also requires molecular data. We review previous studies related to anuran homeotic transformation. The findings of these studies provide a basis for evaluating major hypotheses and identifying molecular data that should be prioritized in future studies. Finally, we argue that positional information for the tail blastema changes to that for a part of the trunk, leading to homeotic transformations. To suggest this hypothesis, we present published data that favor the rewriting of positional information.

Vitamin A induced homeotic hindlimb formation on dorsal and ventral sides of regenerating tissue of amputated tails of Japanese brown frog tadpoles

When anuran tadpoles are treated with vitamin A after tail amputation, hindlimb-like structures can be generated instead of the lost tail part at the amputation site. This homeotic transformation was initially expected to be a key to understanding the body plan of vertebrates. Unfortunately, homeotic limb formation has been reproduced in only some Indian frog species and a European species, but not in experimental anurans such as Xenopus laevis or Rana catesbeiana. Consequently, this fascinating phenomenon has not been well analyzed, especially at the molecular level. In addition, the initial processes of ectopic limb development are also unclear because morphological changes in the early phases have not been analyzed in detail. In this study, we report the induction of homeotic transformation using Japanese brown frogs and present a detailed morphological analysis. Unexpectedly, the ectopic limbs developed not only at the ventral sites, but also at the dorsal sites of the tail regenerates of vitamin A-treated tadpoles. The relationship between position and axial orientation of ectopic limbs suggested the double duplication of positional value order along the rostral-caudal axis and the dorsal-ventral axis of the tail regenerates.

Formation of a new limb bud at the boundary between a transplanted limb bud and the tail surface of Xenopus tadpoles

Through transplantation experiments with Xenopus laevis tadpoles, we found a new morphogenetic phenomenon consisting of limb bud formation at the boundary between transplanted whole limb buds and the tail surface. This phenomenon occurs without limb-limb stump interaction and has a number of unique features: (1) Only one extra limb bud was formed per transplant and the new limb and the transplanted limb were bilaterally symmetrical, forming a pair of limb girdles. (2) Extra new limb bud formation occurred not only at the tail but also at other non-limb regions, including abdominal and head surfaces. (3) Successful limb formation required the presence of basal 1/4 region (presumptive limb girdle) of a limb bud explant. (4) New limb formation was host-stage-dependent: before metamorphosis, limb bud formation ratio was high (> 90%), but as the host tadpole entered metamorphosis, this potential declined and morphological abnormalities of new limbs increased. (5) Cell lineage analysis showed that epidermis of the new limb bud always contained many (about 60%) host-derived cells, while new limb cartilage cells were completely graft-derived. These results suggest that heterotopic new limb formation occurs through interaction between graft mesenchyme and host epidermis. Thus, the present study has clarified the two important aspects of limb ontogeny: the importance of presumptive limb girdle for the limb bud initiation and the relationship between limb bud formation potential and metamorphic tissue remodeling. The present experimental system may help to improve our understanding of epithelial-mesenchymal interaction during limb bud initiation and subsequent limb cell differentiation during metamorphosis.

Chemical activation of RARβ induces post-embryonically bilateral limb duplication during Xenopus limb regeneration

The anuran amphibian Xenopus laevis can regenerate its limbs for a limited time during the larval stage, while limbs are still developing. Using this regeneration model, we evaluated the proximo-distal blastema cell identity when endogenous retinoids were increased by CYP26 inhibition or when RAR-specific agonists altered RA signaling. Simultaneous proximo-distal and antero-posterior limb duplications were generated, and the RAR-specific agonist can modify blastema identity after amputation, because chemical activation of RARβ produced bilateral hindlimb duplications that resulted in a drastic duplication phenotype of regenerating limbs.

Ectopic eyes outside the head in Xenopus tadpoles provide sensory data for light-mediated learning

A major roadblock in the biomedical treatment of human sensory disorders, including blindness, has been an incomplete understanding of the nervous system and its ability to adapt to changes in sensory modality. Likewise, fundamental insight into the evolvability of complex functional anatomies requires understanding brain plasticity and the interaction between the nervous system and body architecture. While advances have been made in the generation of artificial and biological replacement components, the brain's ability to interpret sensory information arising from ectopic locations is not well understood. We report the use of eye primordia grafts to create ectopic eyes along the body axis of Xenopus tadpoles. These eyes are morphologically identical to native eyes and can be induced at caudal locations. Cell labeling studies reveal that eyes created in the tail send projections to the stomach and trunk. To assess function we performed light-mediated learning assays using an automated machine vision and environmental control system. The results demonstrate that ectopic eyes in the tail of Xenopus tadpoles could confer vision to the host. Thus ectopic visual organs were functional even when present at posterior locations. These data and protocols demonstrate the ability of vertebrate brains to interpret sensory input from ectopic structures and incorporate them into adaptive behavioral programs. This tractable new model for understanding the robust plasticity of the central nervous system has significant implications for regenerative medicine and sensory augmentation technology.

Cyanobacteria blooms produce teratogenic retinoic acids

Deformed amphibians have been observed in eutrophic habitats, and some clues point to the retinoic acids (RAs) or RA mimics. However, RAs are generally thought of as vertebrate-specific hormones, and there was no evidence that RAs exist in cyanobacteria or algae blooms. By analyzing RAs and their analogs 4-oxo-RAs in natural cyanobacteria blooms and cultures of cyanobacteria and algae, we showed that cyanobacteria blooms could produce RAs, which were powerful animal teratogens. Intracellular RAs and 4-oxo-RAs with concentrations between 0.4 and 4.2 × 10(2) ng/L were detected in all bloom materials, and extracellular concentrations measured in water from Taihu Lake, China, were as great as 2.0 × 10 ng/L, which might pose a risk to wildlife through chronic exposure. Further examination of 39 cyanobacteria and algae species revealed that 32 species could produce RAs and 4-oxo-RAs (1.6-1.4 × 10(3) ng/g dry weight), and the dominant cyanobacteria species in Taihu Lake, Microcystis flos-aquae and Microcystis aeruginosa, produced high amounts of RAs and 4-oxo-RAs with concentrations of 1.4 × 10(3) and 3.7 × 10(2) ng/g dry weight, respectively. Most genera of cyanobacteria that could produce RAs and 4-oxo-RAs, such as Microcystis, Anabaena, and Aphanizomenon, often occur dominantly in blooms. Production of RAs and 4-oxo-RAs by cyanobacteria was associated with species, origin location, and growth stage. These results represent a conclusive demonstration of endogenous production of RAs in freshwater cyanobacteria blooms. The observation of teratogenic RAs in cyanobacteria is evolutionarily and ecologically significant because RAs are vertebrate-specific hormones, and cyanobacteria form extensive and highly visible blooms in many aquatic ecosystems.

All-trans retinoic acid-induced ectopic limb and caudal structures: murine strain sensitivities and pathogenesis

Treatment of pregnant mice at the egg cylinder stage with retinoic acid (RA) has caused ectopic hindlimbs in the offspring. Proposed causes of ectopic hindlimbs include homeotic transformation or multiple axis formation. Two mouse strains were determined to be divergent in susceptibility to this malformation (C57BL/6N, highly sensitive; SWV/Fnn, less sensitive). Ectopic limbs were hindlimbs (expressing Pitx1 and Tbx4 but not Tbx5), yet they also expressed the predominantly forelimb Hoxb8. Ectopic body axis formation was indicated by gene expression for ectopic primitive streaks, notochords, and nodes, as well as inhibition of anterior visceral endoderm and mesodermal migration. The earlier in development that embryos were examined, the higher the rate of ectopic hindlimb development and axis formation. Ectopic axis formation and cell migration inhibition had the same strain susceptibility as the dysmorphogenesis. We propose that all extra hindlimbs were derived from ectopic axis formation, perturbation of which is genetic background dependent.

Effects of retinoids and juvenoids on moult and on phenoloxidase activity in the blood-sucking insect Rhodnius prolixus

Retinoic acid and insect juvenile hormone (JH) are structurally related terpenoids which are widespread in nature and are involved in many biological events such as morphogenesis, embryogenesis and cellular differentiation. Here, we investigated the effects of the retinoids 9-cis retinoic acid (9cisRA), all trans retinol (atROH), all trans retinoic acid (atRA) and the juvenoids methoprene (Met) and JH injection on moult and on phenoloxidase activity in the blood-sucking insect Rhodnius prolixus. Overall, we observed that injection of retinoids or juvenoids (120 pmols) in the hemocoel of 4th instar nymphs reduced the percentage of insects which appeared normal in morphology upon moult. Noteworthy, insects exposed to 9cisRA or JH underwent profound morphological changes upon moult, generating abnormal 5th instar nymphs and also markedly increased the death of insects during the moulting process. In addition, reduction in the percentage of insects that moult without any morphological alteration, induced by retinoids or juvenoids treatment, was negatively correlated with insects that both display abnormal moult and those that die during moult. Hemolymphatic phenoloxidase activity in adult male insects injected with 9cisRA, Met and JH were significantly reduced after a bacterial challenge. Together, these results indicate that not only juvenoids but also retinoids play an important role on morphogenesis and on immune response in R. prolixus, suggesting that the molecular mechanisms involved in these events recognize the terpenoid backbone as an important structural determinant in insects.

A morphologist's perspective on terminal growth and segmentation

When approaching the study of terminal growth and segmentation, comparative morphology provides an important guide to formulate questions. There are often problems in unambiguously identifying the axis along which we wish to study terminal (often, actually, subterminal) growth, especially when the trunk axis is posteriorly prolonged in an appendage (as with the tail of vertebrates), or when the polarity of the "external animal" is other than the polarity of the "internal animal," as in polypoid bilaterians. We cannot ignore that the rear end of the main body axis is possibly defined very early in development in some groups, for example, arthropods, whereas in others, vertebrates, for example, it is defined much later. We cannot think of segmentation as always corresponding to the sequential posterior addition of new units, thus ignoring the widespread occurrence of double segmentation. A more subtle problem is represented by the overlapping of different processes, all of them contributing to elongating the body, such as segmentation, cell proliferation, cell rearrangement, and cell growth. Within a segmented trunk, cell proliferation and differentiation may go on in parallel from as many growth points as there are groups of regularly spaced cells. The main consequence, however, is not so much to expedite elongation as to reduce the disparity of metabolic conditions, gene expression patterns and "relative age" of different body districts, otherwise possibly troublesome within the limited space of the embryo.

There's more than one frog in the pond: a survey of the Amphibia and their contributions to developmental biology

The study of developmental biology has benefited greatly from the insights gained using amphibians as experimental models. Although Xenopus is currently the predominant model, much of our embryological knowledge derives from research on other amphibians. I will review some of these discoveries, made through astute choice of model organism, and I will examine the reasons behind the adoption of Xenopus as the standard for amphibian research. Additionally, I will discuss the diversity in developmental and reproductive strategies that exists within the Amphibia, and consider some of the recent advances in our understanding of the mechanisms underlying this developmental diversity.

Retinoic acid, a regeneration-inducing molecule

Retinoic acid (RA) is the biologically active metabolite of vitamin A. It is a low molecular weight, lipophilic molecule that acts on the nucleus to induce gene transcription. In amphibians and mammals, it induces the regeneration of several tissues and organs and these examples are reviewed here. RA induces the "super-regeneration" of organs that can already regenerate such as the urodele amphibian limb by respecifying positional information in the limb. In organs that cannot normally regenerate such as the adult mammalian lung, RA induces the complete regeneration of alveoli that have been destroyed by various noxious treatments. In the mammalian central nervous system (CNS), which is another tissue that cannot regenerate, RA does not induce neurite outgrowth as it does in the embryonic CNS, because one of the retinoic acid receptors, RAR beta 2, is not up-regulated. When RAR beta 2 is transfected into the adult spinal cord in vitro, then neurite outgrowth is stimulated. In all these cases, RA is required for the development of the organ, in the first place suggesting that the same gene pathways are likely to be used for both development and regeneration. This suggestion, therefore, might serve as a strategy for identifying potential tissue or organ targets that have the capacity to be stimulated to regenerate.

Oxidative stress during vitamin A-induced abnormal tail regeneration in the tadpoles of Polypedates maculatus

Vitamin A and its derivatives inhibit normal tail regeneration in amphibians. The most remarkable effect is the development of limbs at the cut end of the tail in anurans. Prior to ectopic limb development, there is an abnormal tail regeneration in the treated tadpoles. The purpose of the present study was to compare oxidative stress condition in the regenerated tail of normal and vitamin A (10I U/ml, 72 h) treated tadpoles. The present findings show a hyper-oxidative stress condition in the regenerated tail of the vitamin A-treated tadpoles of the Indian jumping frog, Polypedates maculatus (Anura: Rhacophoridae).

Lens regeneration in mice under the influence of vitamin A

The effect of vitamin A has been studied on lens regeneration in young (7 days old) as well as adult mice. A longitudinal slit was made under local anesthesia in the cornea over the lens. The lens was extracted intact through the incision. Intraperitonial injection of vitamin A (0.05 ml of 30 IU/ml in young and 0.05 ml of 50 IU/ml in adult) was given to the operated animals. Vitamin A was found to induce lens regeneration in not only young but also in adult mice. Regenerated lenses were similar in shape, size, transparency and histological features to normal intact lenses.

Homeotic regeneration of eye in amphibian tadpoles and its enhancement by vitamin A

After removal of both the lateral eyes of external gill stage tadpoles of the toad Bufo melanostictus, the pineal organ gets transformed into a median eye. This type of transformation occurs in tadpoles of both control and vitamin A treated groups. However, vitamin A increases the likelihood of homeotic regeneration (57% in the control group and 71% in the vitamin A treated group). Histological studies showed that the newly transformed median eye developed from the pineal organ. The pineal eye so developed possessed all components of a normal eye such as a retina, sensory cells and lens.

Leaping lopsided: a review of the current hypotheses regarding etiologies of limb malformations in frogs

Recent progress in the investigation of limb malformations in free-living frogs has underlined the wide range in the types of limb malformations and the apparent spatiotemporal clustering of their occurrence. Here, we review the current understanding of normal and abnormal vertebrate limb development and regeneration and discuss some of the molecular events that may bring about limb malformation. Consideration of the differences between limb development and regeneration in amphibians has led us to the hypothesis that some of the observed limb malformations come about through misdirected regeneration. We report the results of a pilot study that supports this hypothesis. In this study, the distal aspect of the right hindlimb buds of X. laevis tadpoles was amputated at the pre-foot paddle stage. The tadpoles were raised in water from a pond in Minnesota at which 7% of surveyed newly metamorphosed feral frogs had malformations. Six percent (6 of 100) of the right limbs of the tadpoles raised in pond water developed abnormally. One truncated right limb was the only malformation in the control group, which was raised in dechlorinated municipal water. All unamputated limbs developed normally in both groups. Three major factors under consideration for effecting the limb malformations are discussed. These factors include environmental chemicals (primarily agrichemicals), encysted larvae (metacercariae) of trematode parasites, and increased levels of ultraviolet light. Emphasis is placed on the necessary intersection of environmental stressors and developmental events to bring about the specific malformations that are observed in free-living frog populations.

Expression of Hoxb13 and Hoxc10 in developing and regenerating Axolotl limbs and tails

The expression of Hox complex genes in correct spatial and temporal order is critical to patterning of the body axis and limbs during embryonic development. In order to understand the role such genes play in appendage regeneration, we have compared the expression of two 5' Hox complex genes: Hoxb13 and Hoxc10 during development and regeneration of the body axis and the limbs of axolotls. In contrast to higher vertebrates, Hoxb13 is expressed not only in the tip of the developing tail, but also in the distal mesenchyme of developing hind limbs, and at low levels in developing forelimbs. Hoxc10 is expressed as two transcripts during both development and regeneration. The short transcript (Hoxc10S) is expressed in the tip of the developing tail, in developing hind limbs, and at low levels in developing forelimbs. The long transcript (Hoxc10L) is expressed in a similar pattern, with the exception that no expression in developing forelimbs could be detected. Hoxb13 and both transcripts of Hoxc10 are expressed at high levels in the regenerating spinal cord during tail regeneration, and in both regenerating hind limbs and forelimbs. The up-regulation of expression of these genes during forelimb regeneration, relative to the very low levels of expression during forelimb development, suggests that they play a critical and perhaps unique role in regeneration. This is particularly true for Hoxc10L, which is not expressed during forelimb development, but is expressed during forelimb regeneration; thus making it the first truly "regeneration-specific" gene transcript identified to date.

Opposing RA and FGF signals control proximodistal vertebrate limb development through regulation of Meis genes

Vertebrate limbs develop in a temporal proximodistal sequence, with proximal regions specified and generated earlier than distal ones. Whereas considerable information is available on the mechanisms promoting limb growth, those involved in determining the proximodistal identity of limb parts remain largely unknown. We show here that retinoic acid (RA) is an upstream activator of the proximal determinant genes Meis1 and Meis2. RA promotes proximalization of limb cells and endogenous RA signaling is required to maintain the proximal Meis domain in the limb. RA synthesis and signaling range, which initially span the entire lateral plate mesoderm, become restricted to proximal limb domains by the apical ectodermal ridge (AER) activity following limb initiation. We identify fibroblast growth factor (FGF) as the main molecule responsible for this AER activity and propose a model integrating the role of FGF in limb cell proliferation, with a specific function in promoting distalization through inhibition of RA production and signaling.

Limbs and tail as evolutionarily diverging duplicates of the main body axis

Contrasting hypotheses have been proposed to explain the pervasive parallels in the patterning of arthropod and vertebrate appendages. These hypotheses either call for a common ancestor already provided with patterned appendages or body outgrowths, or for the recruitment in limb patterning of single genes or genetic cassettes originally used for purposes other than axis patterning. I suggest instead that body appendages such as arthropod and vertebrate limbs and chordate tails are evolutionarily divergent duplicates (paramorphs) of the main body axis, that is, its duplicates, albeit devoid of endodermal component. Thus, vertebrate limbs and arthropod limbs are not historical homologs, but homoplastic features only transitively related to real historical homologs. Thus, the main body axis and the axis of the appendages have distinct but not independent evolutionary histories and may be involved in processes of homeotic co-option producing effects of morphological assimilation. For instance, chordate segmentation may have originated in the posterior appendage (tail) and subsequently extended to the trunk.

The role of retinoic acid in embryonic and post-embryonic development

Retinoic acid (RA) is the bioactive metabolite of vitamin A (retinol) which acts on cells to establish or change the pattern of gene activity. Retinol is converted to RA by the action of two types of enzyme, retinol dehydrogenases and retinal dehydrogenases. In the nucleus RA acts as a ligand to activate two families of transcription factors, the RA receptors (RAR) and the retinoid X receptors (RXR) which heterodimerize and bind to the upstream sequences of RA-responsive genes. Thus, in addition to the well-established experimental paradigm of depriving animals of vitamin A to determine the role of RA in embryonic and post-embryonic development, molecular biology has provided us with two additional methodologies: knockout the enzymes or the RAR and RXR in the mouse embryo. The distribution of the enzymes and receptors, and recent experiments to determine the endogenous distribution of RA in the embryo are described here, as well as the effects on the embryo of knocking out the enzymes and receptors. In addition, recent studies using the classical vitamin A-deprivation technique are described, as they have provided novel insights into the regions of the embryo which crucially require RA, and the gene pathways involved in their development. Finally, the post-embryonic or regenerating systems in which RA plays a part are described, i.e. the regenerating limb, lung regeneration, hair cell regeneration in the ear and spinal cord regeneration in the adult.

Strategies for assessing the implications of malformed frogs for environmental health

The recent increase in the incidence of deformities among natural frog populations has raised concern about the state of the environment and the possible impact of unidentified causative agents on the health of wildlife and human populations. An open workshop on Strategies for Assessing the Implications of Malformed Frogs for Environmental Health was convened on 4-5 December 1997 at the National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina. The purpose of the workshop was to share information among a multidisciplinary group with scientific interest and responsibility for human and environmental health at the federal and state level. Discussions highlighted possible causes and recent findings directly related to frog deformities and provided insight into problems and strategies applicable to continuing investigation in several areas. Possible causes of the deformities were evaluated in terms of diagnostics performed on field amphibians, biologic mechanisms that can lead to the types of malformations observed, and parallel laboratory and field studies. Hydrogeochemistry must be more integrated into environmental toxicology because of the pivotal role of the aquatic environment and the importance of fates and transport relative to any potential exposure. There is no indication of whether there may be a human health factor associated with the deformities. However, the possibility that causal agents may be waterborne indicates a need to identify the relevant factors and establish the relationship between environmental and human health in terms of hazard assessment.

Morphological clues from multilegged frogs: are retinoids to blame?

Morphological analysis was performed on multilegged deformed frogs representing five species from 12 different localities in California, Oregon, Arizona, and New York. The pattern of duplicated limbs was consistent with mechanical perturbation by trematode infestation but not with the effects of retinoids.

Cell determination and transdetermination in Drosophila imaginal discs

Drosophila imaginal discs, the precursors of the adult fly appendages, are an important system for studying mechanisms of cell determination. How the different imaginal discs acquire and maintain their appendage-specific determined states are problems that have been addressed using experimental embryology as well as genetic and molecular approaches. Here we discuss the concept of cell determination and describe what is known about how determination is established and maintained in imaginal disc cells. The phenomenon of imaginal disc transdetermination, originally discovered in the 1960s, has remained an intriguing problem for understanding imaginal disc cell determination. We review the topic of imaginal disc transdetermination and describe how recent results from molecular genetic approaches have provided new insights into imaginal disc transdetermination and determination. We also discuss how an understanding of imaginal disc transdetermination can aid our understanding of parallel phenomena in other organisms, including human metaplasias.

Retinoic acid and thyroid hormone may function through similar and competitive pathways in regenerating axolotls

The objective of this study was to determine whether thyroid hormone (TH) would interfere with retinoic acid (RA), which proximalizes axolotl larvae regenerate limb pattern. RA and TH are ligands for members of the steroid hormone thyroid hormone nuclear binding protein superfamily which form functional homodimers, but may also form stable heterodimers with the RXR protein and may recognize identical DNA sequences. TH alone does not affect limb pattern but induces metamorphosis in regenerating animals. Coinjected animals do not metamorphose, and when compared to RA controls regenerate more proximal and in some cases anteroposterior (AP) and dorsoventral (DV) duplicate limb structures. In addition, the tissues that are normally lost or changed during metamorphosis appear to be sensitized resulting in the formation of (1) new dorsal gill lamellae accompanied by bifurcation and broadening of the original gill lamellae, (2) partial resorption of the tail fin, and (3) changes in eye position and snout morphology. Bifurcation of gill lamellae tips, but not the formation of supernumerary gills, is also observed in animals treated with RA alone. These results indicate that the molecular mechanism of RA and TH function through similar and perhaps competitive pathways.

Expression of regeneration-associated cytoskeletal proteins reveals differences and similarities between regenerating organs

The unique events which allow regeneration of an entire organ to occur are formation of a specialized wound epidermis and accumulation of progenitor cells (blastemal cells) at the amputated surface to form a blastema. In order to identify some of the molecular events underlying the early stages of the regenerative process which are either common to different systems or specific to one of them, we have investigated whether molecules which are induced in limb blastemas are also expressed in skin repair and during regeneration of other complex body structures (lower jaws, upper jaws, and tails). In addition, we have addressed the issue of the identity of progenitor cells during jaw development and regeneration by analyzing the expression of limb blastemal markers in the developing head and face. We have focused on cytoskeletal components, and particularly on the epidermal keratin NvKII, the simple epithelial keratins 8 and 18 and 22/18, because they are among the few molecules which have been shown to be associated with regeneration in the limb and may play significant roles in various developmental processes. Some important findings emerge from this study: 1) Expression of the epidermal keratin NvKII, unlike that of its mammalian homologue K6, is not simply induced in response to wounding, but is associated with regeneration of specific organs. In fact, NvKII is expressed in regenerating limbs and tails, but not in upper or in lower jaw regenerates, demonstrating the existence of molecular differences in the composition of the wound epidermis in these systems. This, together with the fact that NvKII mRNA is regulated by retinoic acid, which differentially affects patterning of limbs and jaws, argues for distinct inductive abilities of the wound epidermis in different organs. 2) In contrast to the differential expression of the epidermal keratin NvKII, the regeneration-associated cytoskeletal molecules identified in limb blastemal cells are expressed in a similar fashion in jaw and tail blastemas. Therefore, it appears that similar cellular events lead to the establishment of an actively proliferating population of progenitor cells from the stump of different organs. Finally, the mesenchyme of the facial rudiments, unlike that of developing limb buds, expresses simple epithelial keratins. Thus, it appears that mesenchymal progenitor cells of developing and regenerating jaws are alike in regard to their intermediate filament content, and this may be related to nerve-dependent growth control of progenitor cells in different developing and regenerating systems.

Expression of contact, a new zebrafish DVR member, marks mesenchymal cell lineages in the developing pectoral fins and head and is regulated by retinoic acid

Contact, a new zebrafish transforming growth factor-beta (TGF-beta) member is most closely related to mouse GDF5 and to human CDMP-1 responsible, when mutated, for limb brachypodism phenotype and Hunter-Thompson syndrome, respectively. Contact exhibits a dynamic spatial expression pattern in the pharyngeal arches and the pectoral fin buds that much prefigures cartilage formation. Within the fin buds, contact expression is detected in the proximal mesenchyme from which the endoskeleton will develop. Exogeneously applied retinoic acid (RA) induces duplication of the pectoral fin rudiment in zebrafish embryos as well as contact expression along the proximal margin of the fin mesenchyme showing that both endoskeleton and exoskeleton can be duplicated.

Amphibian limb regeneration: rebuilding a complex structure

The ability to regenerate complex structures is widespread in metazoan phylogeny, but among vertebrates the urodele amphibians are exceptional. Adult urodeles can regenerate their limbs by local formation of a mesenchymal growth zone or blastema. The generation of blastemal cells depends not only on the local extracellular environment after amputation or wounding but also on the ability to reenter the cell cycle from the differentiated state. The blastema replaces structures appropriate to its proximodistal position. Axial identity is probably encoded as a graded property that controls cellular growth and movement through local cell interactions. The molecular basis is not understood, but proximodistal identity in newt blastemal cells may be respecified by signaling through a retinoic acid receptor isoform. The possibility of inducing a blastema on a mammalian limb cannot be discounted, although the molecular constraints are becoming clearer as we understand more about the mechanisms of urodele regeneration.

Two novel chick T-box genes related to mouse Brachyury are expressed in different, non-overlapping mesodermal domains during gastrulation

The mouse Brachyury (T) gene plays critical roles in the genesis of normal mesoderm during gastrulation and in the maintenance of a functioning notochord. Abrogation of Brachyury (T) expression within the chordamesoderm of homozygous null mutants nevertheless spares anterior axis formation. An intriguing possibility to explain the preservation of anterior axis formation in these mutants would be the existence of other genes compensating for the loss of Brachyury. This compensation and the recent demonstration that Brachyury is the prototype for an evolutionarily conserved family, prompted a search for other T-box genes participating in axis formation. The chick Brachyury orthologue and two related chick T-box genes that are expressed at the onset of gastrulation have been isolated. One of these novel genes (Ch-TbxT) becomes restricted to the axial mesoderm lineage and is a potential candidate for complementing or extending Brachyury function in the anterior axis (formation of the head process, prechordal plate). The other gene (Ch-Tbx6L), together with chick T, appears to mark primitive streak progenitors before gastrulation. As cells leave the primitive streak, Ch-Tbx6L becomes restricted to the early paraxial mesoderm lineage and could play a role in regulating somitogenesis.

Retinoic acid is required for the initiation of outgrowth in the chick limb bud

BACKGROUND

Retinoic acid (RA) is present in the chick limb bud, and excess RA induces limb duplications. Here, we have investigated the role of endogenous RA during chick limb development by preventing the synthesis of RA and testing the effect on various genes expressed during limb initiation and outgrowth.

RESULTS

We demonstrate that the stage 20/21 limb bud synthesizes didehydroretinoic acid (ddRA), and that the posterior half of the limb bud synthesizes ddRA at a higher rate than the anterior half. Disulphiram inhibits this synthesis at micromolar concentrations. Administering disulphiram to embryos prior to limb bud outgrowth (stages 12-18) abolishes outgrowth, and no limb develops in the majority of cases. Disulphiram treatment also prevents the expression of Sonic hedgehog (Shh), but the expression of the fibroblast growth factor-8 gene (Fgf-8) appears as normal in the ectoderm over the prospective limb bud. The application of a bead soaked in RA can rescue Shh expression. Disulphiram treatment of later limb buds (stages 20-23) similarly down-regulates Shh, and also Fgf-4, expression, whereas the expression of Fgf-8, as at earlier stages, is initially unaffected. Again, RA can rescue the expression of Shh in these limb buds.

CONCLUSIONS

RA, in conjunction with Fgf-8, may be needed for the induction of the chick limb bud and the induction of Shh and Fgf-4 expression. The expression of Shh and Fgf-4 remains dependent upon the continued synthesis of RA within the limb bud. Didehydroretinoic acid is the major active retinoid in the stage 20 chick limb bud.

Retinoids in patterning: chimeras win by a knockout

Recent studies on the regenerating newt limb, using cells transfected with chimeric retinoic acid receptors that can be activated by thyroid hormone, have provided unique insights into the function of specific retinoic acid receptor isoforms.

Endogenous retinoids in the zebrafish embryo and adult

Retinoic acid and its isoforms are considered to be endogenous compounds which regulate embryonic development. In the work reported here we have determined which retinoids are present in zebrafish embryos and how their levels change throughout development and into adulthood. All-trans-RA is present and its level does not change significantly during embryogenesis. We failed to detect other retinoic acid isomers such as 9-cis-RA and 4-oxo-RA, but we did observe a rapid rise in the level of didehydroretinol after gastrulation. The most striking result is that the zebrafish embryo, like Xenopus and tunicates, contains a vast excess of t-retinal whereas the embryos of higher vertebrates have an excess of t-retinol. However, as the zebrafish grows, the levels of t-retinol rise so that by adulthood t-retinol and t-retinal concentrations are more equivalent, indicating a changing pattern of retinoid metabolism with growth. To examine the significance of the use of t-retinal as a precursor of t-RA we treated embryos with disulphiram, an inhibitor of retinaldehyde dehydrogenase. This resulted in embryos with an undulating notochord and correspondingly abnormal somites and ventral floor plate. In contrast to this effect, 4-methylpyrazole, which inhibits alcohol dehydrogenases, had no effect on development. This effect of disulphiram suggests that t-RA may be involved in the establishment of the anteroposterior axis of the embryo.

Role of thyroid hormone and retinoid receptors in the homeotic transformation of tails into limbs in frogs

We provide here further data on the dramatic homeotic transformation of tails into limbs which is induced by retinoids during frog tadpole tail regeneration. The effect can still be produced up to nine days after tail amputation by which time tail regeneration has essentially been completed. Complete tail amputation is needed for the effects to be manifest, partial damage of various sorts to the tail is not enough. We show that as well as retinyl palmitate, other retinoids such as all-trans-retinoic acid and TTNPB, which is a RAR specific retinoid, can induce the homeotic transformation. TTNPB has a 300 x greater potency than retinoic acid. Prolactin, which inhibits thyroid hormone production, prevents the appearance of limbs on the tail from which we conclude that thyroid hormone is needed. We present preliminary evidence from RT-PCR that all six retinoid receptors, the three retinoic acid receptors (RARs), and the three retinoid X receptors (RXRs), are present in the normal tail blastema and that after retinoid treatment RAR alpha, RXR alpha, and RXR beta may be up-regulated. Finally, we show that when RA synthesis is inhibited, normal tail regeneration is inhibited. We conclude that tail regeneration depends upon a particular endogenous level of RA, but that when this level is raised by external administration and thyroid hormone receptors are present the up-regulation of certain retinoid receptors allows novel nuclear receptor interactions which results in the induction of limb-specific genes leading to the appearance of limbs on the tail.

Wingless induces transdetermination in developing Drosophila imaginal discs

Drosophila imaginal discs, the precursors of the adult fly appendages, have been the subject of intensive developmental studies, particularly on cell determination. Cultured disc fragments are recognized not only for the ability to maintain their determined state through extra cell divisions but also for the ability to transdetermine, or switch to the determined state of a different disc. An understanding of transdetermination at a molecular level will provide further insight into the requirements for maintaining cell determination. We find that ectopic expression of the Drosophila gene wingless induces transdetermination of foreleg imaginal disc cells to wing cells. This transdetermination occurs in foreleg discs of developing larvae without disc fragmentation. The in situ-transdetermining cells localize to the dorsal region of the foreleg disc. This wingless-induced transdetermination event is remarkably similar to the leg-to-wing switch that occurs after leg disc culture. Thus we have identified a new approach to a molecular dissection of transdetermination.