Programmed cell death during amphibian metamorphosis

Marking transgenic Xenopus froglets with passive micro transponders

Standard methods to mark Xenopus laevis individuals like tattooing or clipping toenails are inappropriate for the fast growing and regenerating small froglets and the previously used transponders are too large. In this study we successfully adapted micro transponders to tag these animals. Using these new transponders one can now tag small froglets directly after metamorphosis, which has not been possible previously. This new technique makes the breeding of transgenic frogs most efficient, because the frogs do not have to be kept separately and they grow much faster when kept together in large containers.

The role of mitochondrial function in the oocyte and embryo

Mitochondria have long been known to be the powerhouses of the cell but they also contribute to redox and Ca2+ homeostasis, provide intermediary metabolites and store proapoptotic factors. Mitochondria have a unique behavior during development. They are maternally transmitted with little (if any) paternal contribution, and they originate from a restricted founder population, which is amplified during oogenesis. Then, having established the full complement of mitochondria in the fully grown oocyte, there is no further increase of the mitochondrial population during early development. The localization of mitochondria in the egg during maturation and their segregation to blastomeres in the cleaving embryo are strictly regulated. Gradients in the distribution of mitochondria present in the egg have the potential to give rise to blastomeres receiving different numbers of mitochondria. Such maternally inherited differences in mitochondrial distribution are thought to play roles in defining the long-term viability of the blastomere in some cases and embryonic axes and patterning in others. Mitochondria may also regulate development by a number of other means, including modulating Ca2+ signaling, and the production of ATP, reactive oxygen species, and intermediary metabolites. If the participation of mitochondria in the regulation of sperm-triggered Ca2+ oscillations is now well established, the role of other properties of mitochondrial function during development remain largely unexplored probably due to the difficulty of accessing the mitochondrial compartment in an embryo. Maintaining a functional complement of maternally derived mitochondria is vital for the early embryo. Mitochondrial dysfunction may not only compromise developmental processes but also trigger apoptosis in the embryo. This dual role for mitochondria (to maintain life or to commit to cell death) may well represent a quality control system in the early embryo that will determine whether the embryo proceeds further into development or is quickly eliminated.

One of the duplicated matrix metalloproteinase-9 genes is expressed in regressing tail during anuran metamorphosis

The drastic morphological changes of the tadpole are induced during the climax of anuran metamorphosis, when the concentration of endogenous thyroid hormone is maximal. The tadpole tail, which is twice as long as the body, shortens rapidly and disappears completely in several days. We isolated a cDNA clone, designated as Xl MMP-9TH, similar to the previously reported Xenopus laevis MMP-9 gene, and showed that their Xenopus tropicalis counterparts are located tandemly about 9 kb apart from each other in the genome. The Xenopus MMP-9TH gene was expressed in the regressing tail and gills and the remodeling intestine and central nervous system, and induced in thyroid hormone-treated tail-derived myoblastic cultured cells, while MMP-9 mRNA was detected in embryos. Three thyroid hormone response elements in the distal promoter and the first intron were involved in the upregulation of the Xl MMP-9TH gene by thyroid hormone in transient expression assays, and their relative positions are conserved between X. laevis and X. tropicalis promoters. These data strongly suggest that the MMP-9 gene was duplicated, and differentiated into two genes, one of which was specialized in a common ancestor of X. laevis and X. tropicalis to be expressed in degenerating and remodeling organs as a response to thyroid hormone during metamorphosis.

Corticotropin-releasing factor is cytoprotective in Xenopus tadpole tail: coordination of ligand, receptor, and binding protein in tail muscle cell survival

Upon metamorphosis, amphibian tadpoles lose their tails through programmed cell death induced by thyroid hormone (T3). Before transformation, the tail functions as an essential locomotory organ. The binding protein for the stress neuropeptide corticotropin-releasing factor (CRF; CRF-BP) is strongly up-regulated in the tail of Xenopus tadpoles during spontaneous or T3-induced metamorphosis. This finding led us to investigate physiological roles for CRF and CRF-BP in tadpole tail. We found CRF, CRF-BP, and functional CRF1 receptor in tail and CRF and functional CRF1 receptors, but not CRF-BP, in the tail muscle-derived cell line XLT-15. CRF, acting via the CRF1 receptor, slowed spontaneous tail regression in explant culture and caused a reduction in caspase 3/7 activity. CRF increased, but stable CRF-BP overexpression decreased, [3H]thymidine incorporation in XLT-15 cells. Overexpression of CRF-BP in vivo accelerated the loss of tail muscle cells during spontaneous metamorphosis. Lastly, exposure of tail explants to hypoxia increased CRF and urocortin 1 but strongly decreased CRF-BP mRNA expression. We show that CRF is expressed in tadpole tail, is up-regulated by environmental stressors, and is cytoprotective. The inhibitory binding protein for CRF is regulated by hormones or by environmental stressors and can modulate CRF bioactivity.

Molecular and developmental analyses of thyroid hormone receptor function in Xenopus laevis, the African clawed frog

The current review focuses on the molecular mechanisms and developmental roles of thyroid hormone receptors (TRs) in gene regulation and metamorphosis in Xenopus laevis and discusses implications for TR function in vertebrate development and diversity. Questions addressed are: (1) what are the molecular mechanisms of gene regulation by TR, (2) what are the developmental roles of TR in mediating the thyroid hormone (TH) signal, (3) what are the roles of the different TR isoforms, and (4) how do changes in these molecular and developmental mechanisms affect evolution? Even though detailed knowledge of molecular mechanisms of TR-mediated gene regulation is available from in vitro studies, relatively little is known about how TR functions in development in vivo. Studies on TR function during frog metamorphosis are leading the way toward bridging the gap between in vitro and in vivo studies. In particular, a dual function model for the role of TR in metamorphosis has been proposed and investigated. In this model, TRs repress genes allowing tadpole growth in the absence of TH during premetamorphosis and activate genes important for metamorphosis when TH is present. Despite the lack of metamorphosis in most other vertebrates, TR has important functions in development across vertebrates. The underlying molecular mechanisms of TR in gene regulation are conserved through evolution, so other mechanisms involving TH-target genes and TH tissue-sensitivity and dependence underlie differences in role of TR across vertebrates. Continued analysis of molecular and developmental roles of TR in X. laevis will provide the basis for understanding how TR functions in gene regulation in vivo across vertebrates and how TR is involved in the generation of evolutionary diversity.

[To live or to die, an embryonic dilemma]

The development and formation of an organ needs both cell proliferation and cell death. It is commonly possible to decipher three types of basic cell death: 1) the so-called morphogenetic cell death (an example is the formation of the digits); 2) the histogenetic cell death like the apoptotic processes taking place in the spinal cord and allowing to regulate the number of motoneurons according to their targets; and 3) the phylogenetic cell death in which vestigial structures are selectively removed (e.g. destruction of the pronephros or removal of the tail in the anuran larva).

Mechanisms of steroid-triggered programmed cell death in Drosophila

Studies in Drosophila have provided a detailed understanding of how programmed cell death is regulated by steroid hormones during development. This work has defined a two-step hormone-triggered regulatory cascade that results in the coordinate induction of central players in the death pathway, including the reaper and hid death activators, the Apaf-1 ortholog dark, and the dronc apical caspase gene. Recent transcriptional profiling studies have identified many new players in this pathway. In addition, genetic studies are providing new insights into the control of autophagic cell death and revealing how this response is related to, but distinct from, apoptosis.