How new communication behaviors evolve: Androgens as modifiers of neuromotor structure and function in foot-flagging frogs

How diverse animal communication signals have arisen is a question that has fascinated many. Xenopus frogs have been a model system used for three decades to reveal insights into the neuroendocrine mechanisms and evolution of vocal diversity. Due to the ease of studying central nervous system control of the laryngeal muscles in vitro, Xenopus has helped us understand how variation in vocal communication signals between sexes and between species is produced at the molecular, cellular, and systems levels. Yet, it is becoming easier to make similar advances in non-model organisms. In this paper, we summarize our research on a group of frog species that have evolved a novel hind limb signal known as 'foot flagging.' We have previously shown that foot flagging is androgen dependent and that the evolution of foot flagging in multiple unrelated species is accompanied by the evolution of higher androgen hormone sensitivity in the leg muscles. Here, we present new preliminary data that compare patterns of androgen receptor expression and neuronal cell density in the lumbar spinal cord - the neuromotor system that controls the hind limb - between foot-flagging and non-foot-flagging frog species. We then relate our work to prior findings in Xenopus, highlighting which patterns of hormone sensitivity and neuroanatomical structure are shared between the neuromotor systems underlying Xenopus vocalizations and foot-flagging frogs' limb movement and which appear to be species-specific. Overall, we aim to illustrate the power of drawing inspiration from experiments in model organisms, in which the mechanistic details have been worked out, and then applying these ideas to a non-model species to reveal new details, further complexities, and fresh hypotheses.

The fast block to polyspermy breaks with convention

JGP study (Komondor et al. 2023. J. Gen. Physiol. https://doi.org/10.1085/jgp.202213258) reveals that conventional PLC activation pathways are not required for the fertilization-induced depolarization of Xenopus eggs that prevents the entry of additional sperm.

Picroscope: low-cost system for simultaneous longitudinal biological imaging

Simultaneous longitudinal imaging across multiple conditions and replicates has been crucial for scientific studies aiming to understand biological processes and disease. Yet, imaging systems capable of accomplishing these tasks are economically unattainable for most academic and teaching laboratories around the world. Here, we propose the Picroscope, which is the first low-cost system for simultaneous longitudinal biological imaging made primarily using off-the-shelf and 3D-printed materials. The Picroscope is compatible with standard 24-well cell culture plates and captures 3D z-stack image data. The Picroscope can be controlled remotely, allowing for automatic imaging with minimal intervention from the investigator. Here, we use this system in a range of applications. We gathered longitudinal whole organism image data for frogs, zebrafish, and planaria worms. We also gathered image data inside an incubator to observe 2D monolayers and 3D mammalian tissue culture models. Using this tool, we can measure the behavior of entire organisms or individual cells over long-time periods.

Corticosterone Is Essential for Survival Through Frog Metamorphosis

Thyroid hormone (TH) is required for frog metamorphosis, and corticosterone (CORT) increases TH signaling to accelerate metamorphic progression. However, a requirement for CORT in metamorphosis has been difficult to assess prior to the recent development of gene-editing technologies. We addressed this long-standing question using transcription activator-like effector nuclease (TALEN) gene disruption to knock out proopiomelanocortin (pomc) and disrupt CORT production in Xenopus tropicalis. As expected, mutant tadpoles had a reduced peak of plasma CORT at metamorphosis with correspondingly reduced expression of the CORT-response gene Usher syndrome type-1G (ush1g). Mutants had reduced rates of growth and development and exhibited lower expression levels of 2 TH response genes, Krüppel-like factor 9 (klf9) and TH receptor β (thrb). In response to exogenous TH, mutants had reduced TH response gene induction and slower morphological change. Importantly, death invariably occurred during tail resorption, unless rescued by exogenous CORT and, remarkably, by exogenous TH. The ability of exogenous TH by itself to overcome death in pomc mutants indicates that the CORT-dependent increase in TH signaling may ensure functional organ transformation required for survival through metamorphosis and/or may shorten the nonfeeding metamorphic transition to avoid lethal inanition.

A laboratory investigation into features of morphology and physiology for their potential to predict reproductive success in male frogs

Amphibian populations are declining globally, however, the contribution of reduced reproduction to declines is unknown. We investigated associations between morphological (weight/snout-vent length, nuptial pad colour/size, forelimb width/size) and physiological (nuptial pad/testis histomorphology, plasma hormones, gene expression) features with reproductive success in males as measured by amplexus success and fertility rate (% eggs fertilised) in laboratory maintained Silurana/Xenopus tropicalis. We explored the robustness of these features to predict amplexus success/fertility rate by investigating these associations within a sub-set of frogs exposed to anti-androgens (flutamide (50 μg/L)/linuron (9 or 45 μg/L)). In unexposed males, nuptial pad features (size/colour/number of hooks/androgen receptor mRNA) were positively associated with amplexus success, but not with fertility rate. In exposed males, many of the associations with amplexus success differed from untreated animals (they were either reversed or absent). In the exposed males forelimb width/nuptial pad morphology were also associated with fertility rate. However, a more darkly coloured nuptial pad was positively associated with amplexus success across all groups and was indicative of androgen status. Our findings demonstrate the central role for nuptial pad morphology in reproductive success in S. tropicalis, however, the lack of concordance between unexposed/exposed frogs complicates understanding of the utility of features of nuptial pad morphology as biomarkers in wild populations. In conclusion, our work has indicated that nuptial pad and forelimb morphology have potential for development as biomarkers of reproductive health in wild anurans, however, further research is needed to establish this.

The Influence of Behavioral, Social, and Environmental Factors on Reproducibility and Replicability in Aquatic Animal Models

The publication of reproducible, replicable, and translatable data in studies utilizing animal models is a scientific, practical, and ethical necessity. This requires careful planning and execution of experiments and accurate reporting of results. Recognition that numerous developmental, environmental, and test-related factors can affect experimental outcomes is essential for a quality study design. Factors commonly considered when designing studies utilizing aquatic animal species include strain, sex, or age of the animal; water quality; temperature; and acoustic and light conditions. However, in the aquatic environment, it is equally important to consider normal species behavior, group dynamics, stocking density, and environmental complexity, including tank design and structural enrichment. Here, we will outline normal species and social behavior of 2 commonly used aquatic species: zebrafish (Danio rerio) and Xenopus (X. laevis and X. tropicalis). We also provide examples as to how these behaviors and the complexity of the tank environment can influence research results and provide general recommendations to assist with improvement of reproducibility and replicability, particularly as it pertains to behavior and environmental complexity, when utilizing these popular aquatic models.

Accumulation and toxicity of multi-walled carbon nanotubes in Xenopus tropicalis tadpoles

Multi-walled carbon nanotubes (MWCNTs), a common nanomaterial widely used and discharged in environment, might exert toxic effects on aquatic animals. In this paper, filter-feeding tadpole of Xenopus tropicalis was selected as bioindicator to study the exposure effects of MWCNTs suspensions of 0.5, 1, 2, 4 and 8 mg/L for 72 h. The results showed that the tadpoles could remain high survival rate of over 96.7% after 24 h's exposure to MWCNTs, but then decrease considerably, showing a significant time-dependent relationship. The LC₅₀ was 2.53 mg/L for tadpoles exposed to MWCNTs for 72 h, when MWCNTs accumulated in their gills and digestive tracts. Moreover, the enrichment degree of MWCNTs in tadpole was related to exposure density than time. When MWCNTs suspension concentration was not over 1 mg/L, the heart rates increased significantly and then decreased continuously. The survivors from the toxicity test were transferred to fresh filtered water for recovery, but MWCNTs accumulated in the tadpoles' body didn't decrease obviously after 4 days. Although the maximum tadpoles survival rate of 80% was recorded in the exposure group of 0.5 mg/L MWCNTs, only 43.3% of the survivors could recover. Therefore, the final survival rate was negative related to the exposure densities of MWCNTs but positive related to the accumulating degree in tadpoles' body. The results demonstrated that MWCNTs exposure posed potential health risks to filter-feeding organisms by intake and accumulation in organs, which could provide useful information for the reasonable evaluation and scientific management of nanomaterials.

Chromatin accessibility dynamics and single cell RNA-Seq reveal new regulators of regeneration in neural progenitors

Vertebrate appendage regeneration requires precisely coordinated remodeling of the transcriptional landscape to enable the growth and differentiation of new tissue, a process executed over multiple days and across dozens of cell types. The heterogeneity of tissues and temporally-sensitive fate decisions involved has made it difficult to articulate the gene regulatory programs enabling regeneration of individual cell types. To better understand how a regenerative program is fulfilled by neural progenitor cells (NPCs) of the spinal cord, we analyzed pax6-expressing NPCs isolated from regenerating Xenopus tropicalis tails. By intersecting chromatin accessibility data with single-cell transcriptomics, we find that NPCs place an early priority on neuronal differentiation. Late in regeneration, the priority returns to proliferation. Our analyses identify Pbx3 and Meis1 as critical regulators of tail regeneration and axon organization. Overall, we use transcriptional regulatory dynamics to present a new model for cell fate decisions and their regulators in NPCs during regeneration.

Methods for Examining Lens Regeneration in Xenopus

Some vertebrates are able to regenerate the lens following its removal. This includes species in the genus Xenopus (i.e., X. laevis, X. tropicalis, and X. borealis), the only anurans known to undergo lens regeneration. In Xenopus the regenerated lens is derived de novo from cells located within the basal-most layer of the larval corneal epithelium, and is triggered by factors provided by the neural retina. In larval frogs the corneal epithelium is underlain by an endothelium separated from the corneal epithelium except for a small central attachment (i.e., the "stromal-attracting center"). This connection grows larger as the stroma forms and the frogs approach metamorphosis. Here we provide instructions for performing lentectomies (removal of the original lens) to study lens regeneration.

The return to water in ancestral Xenopus was accompanied by a novel mechanism for producing and shaping vocal signals

Listeners locate potential mates using species-specific vocal signals. As tetrapods transitioned from water to land, lungs replaced gills, allowing expiration to drive sound production. Some frogs then returned to water. Here we explore how air-driven sound production changed upon re-entry to preserve essential acoustic information on species identity in the secondarily aquatic frog genus Xenopus. We filmed movements of cartilage and muscles during evoked sound production in isolated larynges. Results refute the current theory for Xenopus vocalization, cavitation, and favor instead sound production by mechanical excitation of laryngeal resonance modes following rapid separation of laryngeal arytenoid discs. Resulting frequency resonance modes (dyads) are intrinsic to the larynx rather than due to neuromuscular control. Dyads are a distinctive acoustic signature. While their component frequencies overlap across species, their ratio is shared within each Xenopus clade providing information on species identity that could facilitate both conspecific localization and ancient species divergence.

Editorial note

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Linking organochlorine exposure to biomarker response patterns in Anurans: a case study of Müller's clawed frog (Xenopus muelleri) from a tropical malaria vector control region

Organochlorine pesticides are highly persistent in aquatic ecosystems. Amphibians, specifically anurans, play an intricate part in the aquatic food web, and have very permeable skin which makes them prone to bioaccumulation of persistent pollutants. In this study the bioaccumulation of various legacy organochlorine pesticides (OCPs)-including dichlorodiphenyltrichloroethane (DDT), currently used for malaria vector control (MVC)-was assessed along with a set of biomarker responses in Müller's clawed frog Xenopus muelleri collected from the lower Phongolo River floodplain in South Africa. Possible relationships between bioaccumulation and biomarkers (of exposure, oxidative stress biomarkers, and cellular energy allocation) alongside their temporal changes were investigated. The OCP concentrations showed a significant increase over time for the duration of the study. The increase correlated negatively with rainfall from the region. DDT levels were well below expected effects levels with p,p-DDE being the main contributing metabolite. The results of this study indicate OCPs actively accumulate at sub-lethal levels in aquatic frogs from the study area, while showing possible relations towards some of the biochemical stress responses measured. Most notable were negative relationships indicated between p,p-DDE and acetylcholinesterase, malondialdehyde, and carbohydrates and protein energy availability. Levels of DDT were not found to be significantly higher than other legacy pesticides in the frog tissue, although evidence of newly introduced DDT in the frog tissue was found. Further investigation about sub-lethal effects of these pesticides on anurans is required to gain better insight into their full impact on animal livelihood.

The left-right asymmetry of liver lobation is generated by Pitx2c-mediated asymmetries in the hepatic diverticulum

Internal organs exhibit left-right asymmetric sizes, shapes and anatomical positions, but how these different lateralities develop is poorly understood. Here we use the experimentally tractable Xenopus model to uncover the morphogenetic events that drive the left-right asymmetrical lobation of the liver. On the right side of the early hepatic diverticulum, endoderm cells become columnar and apically constricted, forming an expanded epithelial surface and, ultimately, an enlarged right liver lobe. In contrast, the cells on the left side become rounder, and rearrange into a compact, stratified architecture that produces a smaller left lobe. Side-specific gain- and loss-of-function studies reveal that asymmetric expression of the left-right determinant Pitx2c elicits distinct epithelial morphogenesis events in the left side of the diverticulum. Surprisingly, the cellular events induced by Pitx2c during liver development are opposite those induced in other digestive organs, suggesting divergent cellular mechanisms underlie the formation of different lateralities.

Lifecycle exposure to perchlorate differentially alters morphology, biochemistry, and transcription as well as sperm motility in Silurana tropicalis frogs

Perchlorate (ClO₄^-) contamination has been reported in ground and surface waters across North America. However, few studies have examined the effects of prolonged exposure to this thyroid hormone disrupting chemical, particularly at environmentally relevant concentrations in lower vertebrates, such as amphibians. The aim of this study was to examine the effects of a yearlong chronic exposure to ClO₄^- in adult male and female Western clawed frogs (Silurana tropicalis). Frogs were spawned and raised from fertilized embryo until sexual maturity in potassium perchlorate (KClO₄)-treated water at different concentrations (0, 20, 53, and 107 μg/L). Developmental and reproductive indices - including adult morphology, androgen plasma levels, gonadal thyroid hormone- and sex steroid-related transcript levels, and sperm motility - were evaluated in male and female adult frogs. Female growth (e.g., body mass, snout-vent length, and hind limb length) was significantly reduced following chronic exposure to environmentally relevant concentrations of KClO₄ resulting in females with morphometric indices similar to those of control males - indicating potential sex-specific sensitivities to KClO₄. Changes to reproductive indices (i.e., plasma androgen levels, gonadal thyroid hormone- and sex steroid-related transcript levels, and sperm motility) were also observed in both sexes and suggest that KClO₄ exposure may also have indirect secondary effects on the reproductive axes in male and female adult frogs. These effects were observed at concentrations at or below those reported in surface waters contaminated with ClO₄^- suggesting that this contaminant may have developmental and reproductive effects post-metamorphosis in natural amphibian populations.

Mechanical and signaling roles for keratin intermediate filaments in the assembly and morphogenesis of Xenopus mesendoderm tissue at gastrulation

The coordination of individual cell behaviors is a crucial step in the assembly and morphogenesis of tissues. Xenopus mesendoderm cells migrate collectively along a fibronectin (FN) substrate at gastrulation, but how the adhesive and mechanical forces required for these movements are generated and transmitted is unclear. Traction force microscopy (TFM) was used to establish that traction stresses are limited primarily to leading edge cells in mesendoderm explants, and that these forces are balanced by intercellular stresses in follower rows. This is further reflected in the morphology of these cells, with broad lamellipodial protrusions, mature focal adhesions and a gradient of activated Rac1 evident at the leading edge, while small protrusions, rapid turnover of immature focal adhesions and lack of a Rac1 activity gradient characterize cells in following rows. Depletion of keratin (krt8) with antisense morpholinos results in high traction stresses in follower row cells, misdirected protrusions and the formation of actin stress fibers anchored in streak-like focal adhesions. We propose that maintenance of mechanical integrity in the mesendoderm by keratin intermediate filaments is required to balance stresses within the tissue to regulate collective cell movements.

Growth, Development, and Intestinal Remodeling Occurs in the Absence of Thyroid Hormone Receptor α in Tadpoles of Xenopus tropicalis

During development in all vertebrates, thyroid hormone receptors (TRs) are expressed before as well as during and after the peak in plasma thyroid hormone (TH) levels. Previously, we established a role for unliganded TRα in gene repression and developmental timing using tadpoles of TRα knockout (TRαKO) frogs. Here, we examined the role of liganded TRα on growth, development, and intestinal remodeling during natural and TH-induced metamorphosis. Disrupted TRα had little effect on growth during the larval period, but after metamorphosis, TRαKO juveniles grew more slowly than wild-type (WT) juveniles. TRαKO tadpoles developed faster throughout premetamorphosis when TH was low or absent, and despite their decreased responsivity to exogenous TH, TRαKO tadpoles not only were able to complete TH-dependent metamorphosis but also did so earlier than WT tadpoles. In contrast to external morphology, larval epithelial cell apoptosis and adult cell proliferation of intestinal remodeling were delayed in TRαKO tadpoles. Also, TRαKO intestines did not shrink in length to the full extent, and fewer intestinal folds into the lumen were present in TRαKO compared with WT juveniles. Such delayed remodeling occurred despite higher premetamorphic expression levels of TH target genes important for metamorphic progression-namely, TRβ, Klf9, and ST3. Furthermore, the decreased TH-dependent intestinal shrinkage was consistent with reduced TH response gene expression during natural and TH-induced metamorphosis. As in the TRα null mouse model, TRαKO frogs had statistically significant but surprisingly mild growth and development phenotypes with normal survival and fertility.

Chromosome divergence during evolution of the tetraploid clawed frogs, Xenopus mellotropicalis and Xenopus epitropicalis as revealed by Zoo-FISH

Whole genome duplication (WGD) generates new species and genomic redundancy. In African clawed frogs of the genus Xenopus, this phenomenon has been especially important in that (i) all but one extant species are polyploid and (ii) whole genome sequences of some species provide an evidence for genomic rearrangements prior to or after WGD. Within Xenopus in the subgenus Silurana, at least one allotetraploidization event gave rise to three extant tetraploid (2n = 4x = 40) species–Xenopus mellotropicalis, X. epitropicalis, and X. calcaratus–but it is not yet clear the degree to which these tetraploid genomes experienced rearrangements prior to or after allotetraploidization. To explore genome evolution during diversification of these species, we performed cytogenetic analyses of X. mellotropicalis, including assessment of the localization of nucleolar organizer region, chromosome banding, and determination of the p/q arm ratios for each chromosome pair. We compared these data to a previously characterized karyotype of X. epitropicalis. Morphometric, C-banding and Zoo-FISH data support a previously hypothesized common allotetraploid predecessor of these species. Zoo-FISH with whole chromosome painting (WCP) probes derived from the closely related diploid species X. tropicalis confirmed the existence of ten chromosomal quartets in X. mellotropicalis somatic cells, as expected by its ploidy level and tetraploid ancestry. The p/q arm ratio of chromosome 2a was found to be substantially different between X. mellotropicalis (0.81) and X. epitropicalis (0.67), but no substantial difference between these two species was detected in this ratio for the homoeologous chromosome pair 2b, or for other chromosome pairs. Additionally, we identified variation between these two species in the locations of a heterochromatic block on chromosome pair 2a. These results are consistent with a dynamic history of genomic rearrangements before and/or after genome duplication, a surprising finding given the otherwise relatively conserved genomic structure of most frogs.

Xenopus: leaping forward in kidney organogenesis

While kidney donations stagnate, the number of people in need of kidney transplants continues to grow. Although transplanting culture-grown organs is years away, pursuing the engineering of the kidney de novo is a valid means of closing the gap between the supply and demand of kidneys for transplantation. The structural organization of a mouse kidney is similar to that of humans. Therefore, mice have traditionally served as the primary model system for the study of kidney development. The mouse is an ideal model organism for understanding the complexity of the human kidney. Nonetheless, the elaborate structure of the mammalian kidney makes the discovery of new therapies based on de novo engineered kidneys more challenging. In contrast to mammals, amphibians have a kidney that is anatomically less complex and develops faster. Given that analogous genetic networks regulate the development of mammalian and amphibian nephric organs, using embryonic kidneys of Xenopus laevis (African clawed frog) to analyze inductive cell signaling events and morphogenesis has many advantages. Pioneering work that led to the ability to generate kidney organoids from embryonic cells was carried out in Xenopus. In this review, we discuss how Xenopus can be utilized to compliment the work performed in mammalian systems to understand kidney development.

Ultrasound modulates ion channel currents

Transcranial focused ultrasound (US) has been demonstrated to stimulate neurons in animals and humans, but the mechanism of this effect is unknown. It has been hypothesized that US, a mechanical stimulus, may mediate cellular discharge by activating mechanosensitive ion channels embedded within cellular membranes. To test this hypothesis, we expressed potassium and sodium mechanosensitive ion channels (channels of the two-pore-domain potassium family (K2P) including TREK-1, TREK-2, TRAAK; NaV1.5) in the Xenopus oocyte system. Focused US (10 MHz, 0.3-4.9 W/cm(2)) modulated the currents flowing through the ion channels on average by up to 23%, depending on channel and stimulus intensity. The effects were reversible upon repeated stimulation and were abolished when a channel blocker (ranolazine to block NaV1.5, BaCl2 to block K2P channels) was applied to the solution. These data reveal at the single cell level that focused US modulates the activity of specific ion channels to mediate transmembrane currents. These findings open doors to investigations of the effects of  US on ion channels expressed in neurons, retinal cells, or cardiac cells, which may lead to important medical applications. The findings may also pave the way to the development of sonogenetics: a non-invasive, US-based analogue of optogenetics.

Molecular and histological endpoints for developmental reproductive toxicity in Xenopus tropicalis: Levonorgestrel perturbs anti-Müllerian hormone and progesterone receptor expression

There is an increasing concern regarding the risks associated with developmental exposure to endocrine disrupting chemicals and the consequences for reproductive capability. The present study aimed to refine the Xenopus (Silurana) tropicalis test system for developmental reproductive toxicity by characterising molecular and histological features of sexual development, and to explore effects of exposure to the progestagen levonorgestrel (LNG). Larvae were exposed to LNG (0, 3, 30, 300 ng/L) over the first three weeks of development, encompassing the beginning of gonadal differentiation. mRNA levels of amh (anti-Müllerian hormone), amhr2 (amh receptor 2), ipgr (intracellular progesterone receptor), mpgr beta (membrane progesterone receptor beta), and cyp19a1 (cytochrome p450 19a1) were quantified in larvae and juveniles (4 weeks post-metamorphosis). Relative cyp19a1 and amh expression was used as a molecular marker for phenotypic sex of larvae. Gonadal and Müllerian duct development were characterised histologically in juveniles. Compared to controls, LNG exposure increased the expression of amh and ipgr in male larvae. In juveniles, mpgr beta expression was increased in both sexes and amhr2 expression was decreased in males, implying persistent effects of developmental progestagen exposure on amh and pgr expression signalling. No effects of LNG on the gonadal or Müllerian duct development were found, implying that the exposure window was not critical with regard to these endpoints. In juveniles, folliculogenesis had initiated and the Müllerian ducts were larger in females than in males. This new knowledge on sexual development in X. tropicalis is useful in the development of early life-stage endpoints for developmental reproductive toxicity.

Sex differences and endocrine regulation of auditory-evoked, neural responses in African clawed frogs (Xenopus)

Mating depends on the accurate detection of signals that convey species identity and reproductive state. In African clawed frogs, Xenopus, this information is conveyed by vocal signals that differ in temporal patterns and spectral features between sexes and across species. We characterized spectral sensitivity using auditory-evoked potentials (AEPs), commonly known as the auditory brainstem response, in males and females of four Xenopus species. In female X. amieti, X. petersii, and X. laevis, peripheral auditory sensitivity to their species own dyad-two, species-specific dominant frequencies in the male advertisement call-is enhanced relative to males. Males were most sensitive to lower frequencies including those in the male-directed release calls. Frequency sensitivity was influenced by endocrine state; ovariectomized females had male-like auditory tuning while dihydrotestosterone-treated, ovariectomized females maintained female-like tuning. Thus, adult, female Xenopus demonstrate an endocrine-dependent sensitivity to the spectral features of conspecific male advertisement calls that could facilitate mating. Xenopus AEPs resemble those of other species in stimulus and level dependence, and in sensitivity to anesthetic (MS222). AEPs were correlated with body size and sex within some species. A frequency following response, probably encoded by the amphibian papilla, might facilitate dyad source localization via interaural time differences.

Regeneration inducers in limb regeneration

Limb regeneration ability, which can be observed in amphibians, has been investigated as a representative phenomenon of organ regeneration. Recently, an alternative experimental system called the accessory limb model was developed to investigate early regulation of amphibian limb regeneration. The accessory limb model contributed to identification of limb regeneration inducers in urodele amphibians. Furthermore, the accessory limb model may be applied to other species to explore universality of regeneration mechanisms. This review aims to connect the insights recently gained to emboss universality of regeneration mechanisms among species. The defined molecules (BMP7 (or2) + FGF2 + FGF8) can transform skin wound healing to organ (limb) regeneration responses. The same molecules can initiate regeneration responses in some species.

Transmembrane signal transduction in oocyte maturation and fertilization: focusing on Xenopus laevis as a model animal

Fertilization is a cell biological phenomenon of crucial importance for the birth of new life in a variety of multicellular and sexual reproduction species such as algae, animal and plants. Fertilization involves a sequence of events, in which the female gamete "egg" and the male gamete "spermatozoon (sperm)" develop, acquire their functions, meet and fuse with each other, to initiate embryonic and zygotic development. Here, it will be briefly reviewed how oocyte cytoplasmic components are orchestrated to undergo hormone-induced oocyte maturation and sperm-induced activation of development. I then review how sperm-egg membrane interaction/fusion and activation of development in the fertilized egg are accomplished and regulated through egg coat- or egg plasma membrane-associated components, highlighting recent findings and future directions in the studies using Xenopus laevis as a model experimental animal.

Crosstalk between the thyroid hormone and androgen axes during reproductive development in Silurana tropicalis

The objectives of this study were to examine the effects of potassium perchlorate (KClO4) treatment on androgen- and thyroid hormone (TH)-related transcript levels during gonadogenesis in the frog Silurana tropicalis. Androgen- and TH-related gene expression was examined in gonad-mesonephros complex (GMC) and liver tissues at stage NF 56 and stage NF 60. These stages of development coincide with the period of sexual differentiation. Real-time RT-PCR analyses revealed that androgen- and TH-related transcript levels in the GMC and liver of stage NF 56 and NF 60 frogs are responsive to KClO4 exposure during prometamorphosis. An increase in srd5α2 mRNA levels in hepatic tissues of KClO4-treated NF 56 tadpoles suggests an important role for hepatic tissues in androgen metabolism. Gene transcript differences highlight possible stage- and tissue-specific sensitivities to KClO4. A greater number of TH- and androgen-related transcriptional changes were discerned in the hepatic tissues compared to the gonads, and overall fewer transcriptional changes were observed in stage NF 60 tadpoles compared to stage NF 56 larvae. Perchlorate suppressed somatic and hind-limb development during the 96-d exposure period. Treatment with KClO4 had no significant effect on sex ratios, however a notable reduction in the percentage of males (33.3% M: 66.7% F) in the highest KClO4 concentration (107 μg/L) was observed. Overall, these findings suggest that KClO4 has secondary androgenic disrupting properties in addition to its known primary thyroid hormone-disrupting role.

Individual variation in thermal performance curves: swimming burst speed and jumping endurance in wild-caught tropical clawed frogs

The importance of studying individual variation in locomotor performance has long been recognized as it may determine the ability of an organism to escape from predators, catch prey or disperse. In ectotherms, locomotor performance is highly influenced by ambient temperature (Ta), yet several studies have showed that individual differences are usually retained across a Ta gradient. Less is known, however, about individual differences in thermal sensitivity of performance, despite the fact that it could represent adaptive sources of phenotypic variation and/or additional substrate for selection to act upon. We quantified swimming and jumping performance in 18 wild-caught tropical clawed frogs (Xenopus tropicalis) across a Ta gradient. Maximum swimming velocity and acceleration were not repeatable and individuals did not differ in how their swimming performance varied across Ta. By contrast, time and distance jumped until exhaustion were repeatable across the Ta gradient, indicating that individuals that perform best at a given Ta also perform best at another Ta. Moreover, thermal sensitivity of jumping endurance significantly differed among individuals, with individuals of high performance at low Ta displaying the highest sensitivity to Ta. Individual differences in terrestrial performance increased with decreasing Ta, which is opposite to results obtained in lizards at the inter-specific and among-individual levels. To verify the generality of these patterns, we need more studies on individual variation in thermal reaction norms for locomotor performance in lizards and frogs.

Application of an amphibian (Silurana tropicalis) metamorphosis assay to the testing of the chronic toxicity of three rice paddy herbicides: simetryn, mefenacet, and thiobencarb

We examined the chronic toxicity of three rice paddy herbicides (simetryn, mefenacet, and thiobencarb) using an amphibian (Silurana tropicalis) metamorphosis assay (a 28-day semistatic test under an individual-separated exposure system). Each herbicide was tested at two concentrations (1/100 and 1/10 of the 96-h LC50 value reported previously) with morphometric, gravimetric, and thyroid-histological endpoints. Simetryn caused significant retardation in growth and development at both test concentrations (0.04 and 0.40mg/L), as indicated by significantly shorter total body lengths and hind limb lengths, smaller wet body masses, and delayed developmental stages compared to those observed in the control tadpoles. However, no clear histopathology was observed in the thyroid glands of the tadpoles exposed to simetryn. These results suggest that simetryn can act as a chemical stressor retarding tadpole growth and development without disrupting thyroid functions, even at 1/100 of the 96-h LC50 value. In addition, scoliosis near the tail base was observed in the tadpoles exposed to 0.40mg/L of simetryn at a significantly high incidence (7/30=23.3%). Therefore, simetryn can also act as a teratogen inducing axial malformations at 1/10 of the 96-h LC50 value. During the 28 days of exposure, neither mefenacet (0.03 and 0.30mg/L) nor thiobencarb (0.008 and 0.080mg/L) induced any abnormalities, although the test concentrations measured immediately before the solution renewals decreased to nearly 50 percent of the nominal concentrations since day 14. Because the concentrations tested for simetryn are likely to occur in paddy water, wild anuran tadpoles in paddy water may therefore be adversely impacted by simetryn.

sfrp1 promotes cardiomyocyte differentiation in Xenopus via negative-feedback regulation of Wnt signalling

Wnt signalling is a key regulator of vertebrate heart development, yet it is unclear which specific Wnt signalling components are required to regulate which aspect of cardiogenesis. Previously, we identified Wnt6 as an endogenous Wnt ligand required for controlling heart muscle differentiation via canonical Wnt/β-catenin signalling. Here we show for the first time a requirement for an endogenous Wnt signalling inhibitor for normal heart muscle differentiation. Expression of sfrp1 is strongly induced in differentiating heart muscle. We show that sfrp1 is not only able to promote heart muscle differentiation but is also required for the formation of normal size heart muscle in the embryo. sfrp1 is functionally able to inhibit Wnt6 signalling and its requirement during heart development relates to relieving the cardiogenesis-restricting function of endogenous wnt6. In turn, we discover that sfrp1 expression in the heart is regulated by Wnt6 signalling, which for the first time indicates that sfrp genes can function as part of a Wnt negative-feedback regulatory loop. Our experiments indicate that sfrp1 controls the size of the differentiating heart muscle primarily by regulating cell fate within the cardiac mesoderm between muscular and non-muscular cell lineages. The cardiac mesoderm is therefore not passively patterned by signals from the surrounding tissue, but regulates its differentiation into muscular and non-muscular tissue using positional information from the surrounding tissue. This regulatory network might ensure that Wnt activation enables expansion and migration of cardiac progenitors, followed by Wnt inhibition permitting cardiomyocyte differentiation.

Examination of an amphibian metamorphosis assay under an individual-separated exposure system using Silurana tropicalis tadpoles

We examined the validity of an amphibian (Silurana tropicalis) metamorphosis assay (a 28-day semistatic test) under an individual-separated exposure system, where tadpoles were individually held in small glass beakers. We first conducted a comparative rearing experiment for 28 days between this exposure system and the traditional individual-grouped exposure system, both of which held 30 tadpoles (stages 49 and 50) in dechlorinated tap water (a control solution). The former system served to reduce interindividual variability in regard to three morphological measures (developmental stage, hind limb length, and total body length). Under this system, we tested thyroxine (T4, 1μg/L) and propylthiouracil (PTU, 75mg/L) for 28 days of exposure. The morphological data collected at 7-day intervals indicated that significant metamorphic acceleration and retardation were consistently induced in the tadpoles exposed to T4 and PTU, respectively. In addition, the thyroid glands of the tadpoles exposed to T4 and PTU clearly exhibited atrophy and hypertrophy accompanied with severe follicular cell hyperplasia, respectively. Our results are in agreement with the historical data generated from previous studies employing the traditional exposure system, thus indicating the validity of our alternative testing protocol.

Katanin contributes to interspecies spindle length scaling in Xenopus

Bipolar spindles must separate chromosomes by the appropriate distance during cell division, but mechanisms determining spindle length are poorly understood. Based on a 2D model of meiotic spindle assembly, we predicted that higher localized microtubule (MT) depolymerization rates could generate the shorter spindles observed in egg extracts of X. tropicalis compared to X. laevis. We found that katanin-dependent MT severing was increased in X. tropicalis, which, unlike X. laevis, lacks an inhibitory phosphorylation site in the katanin p60 catalytic subunit. Katanin inhibition lengthened spindles in both species. In X. tropicalis, k-fiber MT bundles that connect to chromosomes at their kinetochores extended through spindle poles, disrupting them. In both X. tropicalis extracts and the spindle simulation, a balance between k-fiber number and MT depolymerization is required to maintain spindle morphology. Thus, mechanisms have evolved in different species to scale spindle size and coordinate regulation of multiple MT populations in order to generate a robust steady-state structure.

Husbandry of Xenopus tropicalis

Xenopus tropicalis combine the advantages of X. laevis, for example using explants and targeted gain of function, with the ability to take classical genetics approaches to answering cell and developmental biology questions making it arguably the most versatile of the model organisms. Against this background, husbandry of X. tropicalis is less well developed than for its larger, more robust relative. Here we describe the methods used to keep and breed these frogs successfully.

Effects of 4-tert-octylphenol on Xenopus tropicalis in a long term exposure

Endocrine disrupting chemicals that activate the estrogen receptor are routinely detected in the environment and are a concern for the health of both exposed humans and indigenous wildlife. We exposed the western clawed frog (Xenopus tropicalis) to the weak estrogen octylphenol from Nieuwkoop-Faber (NF) stage 46 tadpoles through adulthood in order to document the effects of a weak estrogen on the life history of an amphibian species. Frogs were exposed to 1, 3.3, 11 and 36 μg/L octylphenol in a continuous flow-through water system. Just prior to completion of metamorphosis (NF 65), a random subsample of froglets was collected and assessed, while the remaining frogs received continued exposure through 31 weeks of exposure when the remaining animals were sampled. Significant induction of the female egg yolk protein precursor vitellogenin was observed in the high treatment at the larval subsampling for both males and females, but not at the final sampling for either sex. No significant deviation from the control sex ratio was observed for either sampling period, suggesting minimal to no effect of octylphenol exposure on gonad differentiation. No effects in the adult frogs were observed for mortality, body mass and size, liver somatic index, estradiol and testosterone serum levels, sperm counts, or oocyte counts. The development and growth of oviducts, a female-specific secondary sex characteristic, was observed in males exposed to octylphenol. These results indicate that octylphenol exposure can induce vitellogenin in immature froglets and the development of oviducts in male adult frogs. The lack of effect observed on the developing gonads suggests that in amphibians, secondary sex characteristics are more susceptible to impact from estrogenic compounds than the developing gonads.

Early life progestin exposure causes arrested oocyte development, oviductal agenesis and sterility in adult Xenopus tropicalis frogs

Levonorgestrel (LNG) is a commonly used pharmaceutical progestin found in the environment. Information on the long-term toxicity of progestins following early life exposure is scant. We investigated the effects of developmental LNG exposure on sex differentiation, reproductive organ development and fertility in the model frog Xenopus tropicalis. Tadpoles were exposed to 0, 0.06 or 0.5nM LNG via the water from hatching until metamorphosis. At metamorphosis effects on gonadal differentiation were evaluated using a subsample of frogs. Remaining animals were held unexposed for nine months, at which time reproductive organ structure, function and fertility were determined. LNG exposure severely impaired oviduct and ovary development and fertility. All adult females in the 0.5nM group (n=10) completely lacked oviducts. They also displayed a significantly larger fraction of immature oocytes, arrested in meiotic prophase, than control females. Upon mating with unexposed males, only one of 11 LNG-exposed females laid eggs, whereas all control females did. No effects on testicular development, sperm count or male fertility were observed. At metamorphosis, no effects on sex ratio or gonadal histology were evident. The effects on ovarian and oviductal development were detected at adult age but not at metamorphosis, emphasising the importance of investigating the long-term consequences of developmental exposure. This is the first developmental reproductive toxicity study of a progestin in an aquatic vertebrate. Considering that several progestins are present in contaminated surface waters, further investigation into the sensitivity of frogs to progestins is warranted to understand the risk such compounds may pose to wild frog populations.

Purification and properties of antimicrobial peptides from skin secretions of the Eritrea clawed frog Xenopus clivii (Pipidae)

Five peptides with antimicrobial activity were isolated from norepinephrine-stimulated skin secretions of the tetraploid frog Xenopus clivii Peracca, 1898 (Pipidae). Characterization of the peptides demonstrated that they are structurally similar to magainins (2 peptides), caerulein-precursor fragments, CPF (2 peptides), and xenopsin-precursor fragments, XPF (1 peptide) that have been previously isolated from other species of the genus Xenopus. The magainins and the XPF peptide were active only against the Gram-negative microorganism Escherichia coli whereas the CPF peptides were also active against the Gram-positive Staphylococcus aureus. The most abundant antimicrobial peptide in the secretions, CPF-C1 (GFGSLLGKALRLG ANVL.NH(2)) inhibited the growth of the Gram-negative bacteria Acinetobacter baumannii, Klebsiella pneumoniae, and Pseudomonas aeruginosa (MIC≤25μM) suggesting potential for development into an anti-infective agent for use against these emerging antibiotic-resistant pathogens.

Developmental genetics in Xenopus tropicalis

The diploid pipid frog Xenopus tropicalis has recently emerged as a powerful new model system for combining genetic and genomic analysis of tetrapod development with embryological and biochemical assays. Its early development closely resembles that of its well-understood tetraploid relative Xenopus laevis, from which techniques and reagents can be readily transferred, but its compact genome is highly syntenic with those of amniotes. Genetic approaches are facilitated by the large number of embryos produced and the ease of haploid genetics and gynogenesis.

Depolarization-induced Ca2+ entry preferentially evokes release of large quanta in the developing Xenopus neuromuscular junction

The amplitude histogram of spontaneously occurring miniature synaptic currents (mSCs) is skewed positively at developing Xenopus neuromuscular synapses formed in culture. To test whether the quantal size of nerve-evoked quanta (eSCs) distributes similarly, we compared the amplitude histogram of single quantum eSCs in low external Ca(2+) with that of mSCs and found that nerve stimulation preferentially released large quanta. Depolarization of presynaptic terminals by elevating [K(+)] in the external solution or by direct injection of current through a patch pipette increased the mSC frequency and preferentially, but not exclusively, evoked the release of large quanta, resulting in a second broad peak in the amplitude histogram. Formation of the second peak under these conditions was blocked by the N-type Ca(2+) channel blocker, ω-conotoxin GVIA. In contrast, when the mSC frequency was elevated by thapsigargin- or caffeine-induced mobilization of internal Ca(2+), formation of the second peak did not occur. We conclude that the second peak in the amplitude histogram is generated by Ca(2+) influx through N-type Ca(2+) channels, causing a local elevation of internal Ca(2+). The mSC amplitude in the positively skewed portion of the histogram varied over a wide range. A competitive blocker of acetylcholine (ACh) receptors, d-tubocurarine, reduced the amplitude of smaller mSCs in this range relatively more than that of larger mSCs, suggesting that this variation in the mSC amplitude is due to variable amounts of ACh released from synaptic vesicles. We suggest that Ca(2+) influx through N-type Ca(2+) channels preferentially induces release of vesicles with large ACh content.

Aging of Xenopus tropicalis eggs leads to deadenylation of a specific set of maternal mRNAs and loss of developmental potential

As first shown more than 100 years ago, fertilization of an aged (overripe) egg increases the rate of malformations and embryonic loss in several vertebrates, including possibly humans as well. Since the molecular events in aging eggs may be similar in these species, we established in the frog Xenopus tropicalis a defined protocol for delayed fertilization of eggs. A three-hour delayed fertilization led to a dramatic increase in malformation and mortality. Gene expression profiling revealed that 14% of the polyadenylated maternal transcripts were downregulated upon aging. These transcripts were not degraded, but rather deadenylated as shown for specific maternal mRNAs. The affected transcripts are characterized by a relatively short 3′UTR and a paucity of cytoplasmic polyadenylation elements (CPE) and polyadenylation signals (PAS). Furthermore, maternal mRNAs known to be deadenylated during egg maturation as well as after fertilization were preferentially deadenylated in aged eggs. Taken together our analysis of aging eggs reveals that unfertilized eggs are in a dynamic state that was previously not realized. On the one hand deadenylation of transcripts that are typically deadenylated during egg maturation continues and this implies overripeness of the aged egg in the truest sense of the word. On the other hand transcripts that normally are deadenylated after fertilization loose their poly(A) in the aged egg and this implies that the egg awaiting fertilization starts processes that are normally only observed after fertilization. Based on our novel finding we postulate that the imbalance of the polyadenylated maternal transcripts upon egg aging contributes to the loss of developmental potential. Based on this hypothesis the developmental consequences of downregulation of specific transcripts can be analyzed in future.

Truncated beta epithelial sodium channel (ENaC) subunits responsible for multi-system pseudohypoaldosteronism support partial activity of ENaC

Aldosterone regulated epithelial sodium channels (ENaC) are constructed of three homologous subunits. Mutations in the alpha-, beta- and gamma-ENaC subunit genes (SCNN1A, SCNN1B and SCNN1G) are associated with multi-system pseudohypoaldosteronism (PHA), and mutations in the PY motif of carboxy-terminal region of beta and gamma subunits are associated with Liddle syndrome of hereditary hypertension. In this study we identified two frameshift mutations in the SCNN1B alleles of a female infant diagnosed with multi-system PHA inherited from her parents. This is the first case of PHA in an Ashkenazi family in Israel. The p.Glu217fs (c.648dupA in exon 4) and p.Tyr306fs (c.915delC in exon 6) mutations produce shortened beta-ENaC subunits with 253 and 317 residues respectively instead of the 640 residues present in beta-ENaC subunit. Expression of cRNAs carrying these mutations in Xenopus oocytes showed that the mutations drastically reduce but do not eliminate ENaC activity. The findings reveal that truncated beta-ENaC subunits are capable of partially supporting intracellular transport of the other two subunits to the membrane and the final assembly of a weakly active channel together with normal alpha- and gamma-ENaC subunits. Moreover, these results enhance our understanding of the long-term consequences of these types of mutations in PHA patients.

Kir5.1 underlies long-lived subconductance levels in heteromeric Kir4.1/Kir5.1 channels from Xenopus tropicalis

The inwardly-rectifying potassium channel subunit Kir5.1 selectively co-assembles with members of the Kir4.0 subfamily to form novel pH-sensitive heteromeric channels with unique single channel properties. In this study, we have cloned orthologs of Kir4.1 and Kir5.1 from the genome of the amphibian, Xenopus tropicalis (Xt). Heteromeric XtKir4.1/XtKir5.1 channels exhibit similar macroscopic current properties to rat Kir4.1/Kir5.1 with a faster time-dependent rate of activation. However, single channel analysis of heteromeric XtKir4.1/XtKir5.1 channels reveals that they have markedly different long-lived, multi-level subconductance states. Furthermore, we demonstrate that the XtKir5.1 subunit is responsible for these prominent subconductance levels. These results are consistent with a model in which the slow transitions between sublevel states represent the movement of individual subunits. These novel channels now provide an excellent model system to determine the structural basis of subconductance levels and contribution of heteromeric pore architecture to this process.

Egg collection and in vitro fertilization of the western clawed frog Xenopus tropicalis

The eggs of Xenopus laevis have been widely used in studies investigating a variety of aspects of biology, such as control of the cell cycle, RNA processing, and the cytoskeleton. The Western clawed frog Xenopus tropicalis is likely to prove useful for such studies in the future, because of the potential to combine traditional experimental approaches with genetic analysis and the available genome sequence. The eggs of X. tropicalis are also a key starting material for transgenesis. Here, we describe a method for the routine collection of eggs from X. tropicalis, together with a method for in vitro fertilization. Very large numbers of eggs, often more than 2000, can be obtained from a single X. tropicalis female. In vitro fertilization is a valuable alternative to natural mating for generating embryos. It is particularly useful for microinjection experiments and when collecting embryos at a series of defined developmental stages (e.g., for studies of gene expression), because it produces embryos that develop synchronously during early embryonic development. The typical yield of embryos ranges from several hundred to more than 1000 per fertilization.

Two Piwi proteins, Xiwi and Xili, are expressed in the Xenopus female germline

The Argonaute superfamily is a large family of RNA-binding proteins involved in gene regulation mediated by small noncoding RNA and characterized by the presence of PAZ and PIWI domains. The family consists of two branches, the Ago and the Piwi clade. Piwi proteins bind to 21-30-nucleotide-long Piwi-interacting RNAs (piRNAs), which map primarily to transposons and repeated sequence elements. Piwi/piRNAs are important regulators of gametogenesis and have been proposed to play roles in transposon silencing, DNA methylation, transcriptional silencing, and/or post-transcriptional control of translation and RNA stability. Most reports to date have concentrated on the Piwi family members in the male germline. We have identified four Piwi proteins in Xenopus and demonstrate that two, namely, Xiwi1b and Xili, are expressed in the oocyte and early embryo. Xiwi1 and Xili are predominantly found in small, separate complexes, and we do not detect significant interaction of Piwi proteins with the cap-binding complex. Putative nuclear localization and export signals were identified in Xiwi1 and Xili, supporting our observation that Xiwi1, but not Xili, is a nucleo-cytoplasmic protein. Furthermore, by immunoprecipitation of small RNAs, we establish Xiwi1 as a bona fide Piwi protein. These results suggest that the Piwi/piRNA pathway is active in translationally repressed oocytes. This is a significant finding as the Xenopus model provides an excellent tool to study post-transcriptional mechanisms.

Sex reversal of the amphibian, Xenopus tropicalis, following larval exposure to an aromatase inhibitor

Aromatase is a steroidogenic enzyme that catalyzes the conversion of androgens to estrogens in vertebrates. Modulation of this enzyme's activity by xenobiotic exposure has been shown to adversely affect gonad differentiation in a number of diverse species. We hypothesized that exposure to the aromatase inhibitor, fadrozole, during the larval development of the tropical clawed frog, Xenopus tropicalis, would result in masculinization of the developing female gonad. Tadpoles were exposed to fadrozole at nominal concentrations from 1 to 64 microg/L in a flow-through system from < 24 h post-fertilization (Nieuwkoop Faber (NF) stage 15-20) to metamorphosis (NF stage 66). At metamorphosis, morphologically examined gonads indicated complete masculinization of all tadpoles at concentrations of 16 microg/L and above and a significant bias in sex ratio towards males at concentrations of 1 microg/L and above. No effects on time to metamorphosis, body mass, or body length were observed. A random subsample of frogs was raised to reproductive maturity (39 weeks post-fertilization) in control water. All frogs exposed as tadpoles to 16 microg/L fadrozole or greater possessed testes at sexual maturity. Intersexed gonads characterized by the presence of both testicular and ovarian tissue were observed in 12% of frogs in the 4 microg/L treatment. No differences in estradiol, testosterone, or vitellogenin plasma concentrations were observed in exposed males or females compared to controls. Females in the 4 microg/L treatment possessed a significantly greater percentage of pre-vitellogenic oocytes than controls and were significantly smaller in body mass. No differences in sperm counts were observed in exposed males compared to controls. Results from this study demonstrate that larval exposure to an aromatase inhibitor can result in the complete masculinization of female gonads. These masculinized females are phenotypically indistinguishable from normal males at adulthood. Lower levels of aromatase inhibition resulted in intersexed gonads and possible female reproductive impairment at adulthood. These results indicate that exposure of amphibians to xenobiotics capable of inhibiting aromatase would result in adverse reproductive consequences.

Xenopus tropicalis as a test system for developmental and reproductive toxicity

The usefulness of Xenopus tropicalis as a model species to investigate endocrine disruption and developmental reproductive toxicity was assessed. In our test system tadpoles were exposed to test substances from shortly after hatching until metamorphosis, including the period of gonadal differentiation. Effects on the sex hormone and thyroid hormone axes were evidenced as skewed sex ratios, malformations of reproductive organs, altered cytochrome (CYP19) (aromatase) activity, and gene expression in gonads and brain, as well as changed thyroid histology and time to metamorphosis. Reproductive toxicity was evaluated at sexual maturity. Male-to-female sex reversal was implied at concentrations as low as 6 pM (1.8 ng/L) ethynylestradiol (EE2), which is comparable to EE2 levels observed in the environment. EE2-exposed males that were not sex reversed had significantly reduced fertility and a reduced amount of spermatozoa in testes compared with control males. This indicates that reproduction in wild frogs might be impaired by estrogenic environmental pollutants. Aromatase activity in brain and testes of adult frogs was not affected by larval EE2 exposure. Preliminary results indicate that exposure to the environmentally relevant pharmaceutical clotrimazole modulated aromatase activity in brain and gonads during sex differentiation, which warrants further investigation. The susceptibility to estrogen-induced sex reversal of X. tropicalis was comparable to that of other frog species and fish. Similarities between the reproductive effects in X. tropicalis and those reported in fish, birds, and mammals after developmental exposure to estrogens make X. tropicalis promising model for research on endocrine disruption and developmental reproductive toxicity.

Skeletogenesis in Xenopus tropicalis: characteristic bone development in an anuran amphibian

In mammals and birds, most of the skeletal bones develop via endochondral ossification. Chondrocytes in the cartilaginous anlagen undergo processes of maturation such as hypertrophy, calcification and apoptosis. Concomitantly, osteoblasts are recruited to replace the cartilage scaffold gradually with bone matrix and become osteocytes in the trabecular bones. Throughout the successive development of bones, several gene products have been identified as being the components of the molecular mechanism regulating bone development. Transcription factor SOX9 plays essential roles during developmental steps from undifferentiated mesenchymal cells to proliferating chondrocytes, meanwhile, it inhibits transition of proliferating chondrocytes to hypertrophy. Other transcription factors RUNX2 and OSTERIX are critical in osteoblast differentiation, and RUNX2 is also essential for chondrocyte maturation such as hypertrophy and matrix mineralization. GDF5, a protein belonging to the transforming growth factor beta superfamily, is involved in joint formation and chondrogenesis. The limb skeleton of one of the ancestral tetrapod, anuran amphibians also develops through cartilaginous anlagen to bones, but their skeletogenesis has some unique characteristics compared with that of mammals and birds. Anuran amphibians develop and grow with less bone trabeculae and poor epiphyseal growth plates, and its endochondral ossification was found to be a delayed process. In order to address the characteristic skeletal development of anuran amphibians, we cloned Xenopus tropicalis RUNX2 (Xt-runx2), OSTERIX (Xt-osterix) and GDF5 (Xt-gdf5) homologue, and observed expression patterns together with Xt-sox9. In X. tropicalis limbs, histological observation and section in situ hybridization analysis suggest that Xt-SOX9 is involved in chondrogenesis, Xt-RUNX2 and Xt-OSTERIX are involved in osteogenesis, and Xt-GDF5 is involved in joint formation. In the cartilaginous anlagen, Xt-runx2 expression was found in perichondrium and immature chondrocytes as seen in other vertebrates. However, Xt-runx2 expression in enlarged chondrocytes was weak and dissimilar to common hypertrophic chondrocytes. These observations suggest that weak Xt-runx2 expression in maturing chondrocytes affects characteristic bone development in X. tropicalis long bones.

Single-species microarrays and comparative transcriptomics

BACKGROUND

Prefabricated expression microarrays are currently available for only a few species but methods have been proposed to extend their application to comparisons between divergent genomes.

METHODOLOGY/PRINCIPAL FINDINGS

Here we demonstrate that the hybridization intensity of genomic DNA is a poor basis on which to select unbiased probes on Affymetrix expression arrays for studies of comparative transcriptomics, and that doing so produces spurious results. We used the Affymetrix Xenopus laevis microarray to evaluate expression divergence between X. laevis, X. borealis, and their F1 hybrids. When data are analyzed with probes that interrogate only sequences with confirmed identity in both species, we recover results that differ substantially analyses that use genomic DNA hybridizations to select probes.

CONCLUSIONS/SIGNIFICANCE

Our findings have implications for the experimental design of comparative expression studies that use single-species microarrays, and for our understanding of divergent expression in hybrid clawed frogs. These findings also highlight important limitations of single-species microarrays for studies of comparative transcriptomics of polyploid species.

TGF-beta signaling is required for multiple processes during Xenopus tail regeneration

Xenopus tadpoles can fully regenerate all major tissue types following tail amputation. TGF-beta signaling plays essential roles in growth, repair, specification, and differentiation of tissues throughout development and adulthood. We examined the localization of key components of the TGF-beta signaling pathway during regeneration and characterized the effects of loss of TGF-beta signaling on multiple regenerative events. Phosphorylated Smad2 (p-Smad2) is initially restricted to the p63+ basal layer of the regenerative epithelium shortly after amputation, and is later found in multiple tissue types in the regeneration bud. TGF-beta ligands are also upregulated throughout regeneration. Treatment of amputated tails with SB-431542, a specific and reversible inhibitor of TGF-beta signaling, blocks tail regeneration at multiple points. Inhibition of TGF-beta signaling immediately following tail amputation reversibly prevents formation of a wound epithelium over the future regeneration bud. Even brief inhibition immediately following amputation is sufficient, however, to irreversibly block the establishment of structures and cell types that characterize regenerating tissue and to prevent the proper activation of BMP and ERK signaling pathways. Inhibition of TGF-beta signaling after regeneration has already commenced blocks cell proliferation in the regeneration bud. These data reveal several spatially and temporally distinct roles for TGF-beta signaling during regeneration: (1) wound epithelium formation, (2) establishment of regeneration bud structures and signaling cascades, and (3) regulation of cell proliferation.

Preparation, functional characterization, and NMR studies of human KCNE1, a voltage-gated potassium channel accessory subunit associated with deafness and long QT syndrome

KCNE1, also known as minK, is a member of the KCNE family of membrane proteins that modulate the function of KCNQ1 and certain other voltage-gated potassium channels (KV). Mutations in human KCNE1 cause congenital deafness and congenital long QT syndrome, an inherited predisposition to potentially life-threatening cardiac arrhythmias. Although its modulation of KCNQ1 function has been extensively characterized, many questions remain regarding KCNE1's structure and location within the channel complex. In this study, KCNE1 was overexpressed in Escherichia coli and purified. Micellar solutions of the protein were then microinjected into Xenopus oocytes expressing KCNQ1 channels, followed by electrophysiological recordings aimed at testing whether recombinant KCNE1 can co-assemble with the channel. Nativelike modulation of channel properties was observed following injection of KCNE1 in lyso-myristoylphosphatidylglycerol (LMPG) micelles, indicating that KCNE1 is not irreversibly misfolded and that LMPG is able to act as a vehicle for delivering membrane proteins into the membranes of viable cells. 1H-15N TROSY NMR experiments indicated that LMPG micelles are well-suited for structural studies of KCNE1, leading to assignment of its backbone resonances and to relaxation studies. The chemical shift data confirmed that KCNE1's secondary structure includes several alpha-helices and demonstrated that its distal C-terminus is disordered. Surprisingly, for KCNE1 in LMPG micelles, there appears to be a break in alpha-helicity at sites 59-61, near the middle of the transmembrane segment, a feature that is accompanied by increased local backbone mobility. Given that this segment overlaps with sites 57-59, which are known to play a critical role in modulating KCNQ1 channel activation kinetics, this unusual structural feature likely has considerable functional relevance.

GATA-2 functions downstream of BMPs and CaM KIV in ectodermal cells during primitive hematopoiesis

In Xenopus, primitive blood originates from the mesoderm, but extrinsic signals from the ectoderm are required during gastrulation to enable these cells to differentiate as erythrocytes. The nature of these signals, and how they are transcriptionally regulated, is not well understood. We have previously shown that bone morphogenetic proteins (BMPs) are required to signal to ectodermal cells to generate secondary non-cell-autonomous signal(s) necessary for primitive erythropoiesis, and that calmodulin-dependent protein kinase IV (CaM KIV) antagonizes BMP signaling. The current studies demonstrate that Gata-2 functions downstream of BMP receptor activation in these same cells, and is a direct target for antagonism by CaM KIV. We show, using loss of function analysis in whole embryos and in explants, that ectodermal Gata-2 is required for primitive erythropoiesis, and that BMP signals cannot rescue blood defects caused by ectoderm removal or loss of ectodermal GATA-2. Furthermore, we provide evidence that acetylation of GATA-2 is required for its function in primitive blood formation in vivo. Our data support a model in which Gata-2 is a transcriptional target downstream of BMPs within ectodermal cells, while activation of the CaM KIV signaling pathway alters GATA-2 function posttranslationally, by inhibiting its acetylation.

Origin of excitatory drive to a spinal locomotor network

A long-standing hypotheses is that locomotion is turned on by descending excitatory synaptic drive. In young frog tadpoles, we show that prolonged swimming in response to a brief stimulus can be generated by a small region of caudal hindbrain and rostral spinal cord. Whole-cell patch recordings in this region identify hindbrain neurons that excite spinal neurons to drive swimming. Some of these hindbrain reticulospinal neurons excite each other. We consider how feedback excitation within the hindbrain may provide a mechanism to drive spinal locomotor networks.

Understanding the somitogenesis clock: what's missing?

The segmentation of vertebrate embryos depends on a complex genetic network that generates highly dynamic gene expression. Many of the elements of the network have been identified, but their interaction and their influence on segmentation remain poorly understood. A few mathematical models have been proposed to explain the dynamics of subsets of the network, but the mechanistic bases remain controversial. This review focuses on outstanding problems with the generation of somitogenesis clock oscillations, and the ways they could regulate segmentation. Proposals that oscillations are generated by a negative feedback loop formed by Lunatic fringe and Notch signaling are weighed against a model based on positive feedback, and the experimental basis for models of simple negative feedback involving Her/Hes genes or Wnt targets is evaluated. Differences are then made explicit between the many 'clock and wavefront' model variants that have been proposed to explain how the clock regulates segmentation. An understanding of the somitogenesis clock will require addressing experimentally the many questions that arise from the study of simple models.