The clonal origin of myocardial cells in different regions of the embryonic mouse heart

When and how cells form and pattern the myocardium is a central issue for heart morphogenesis. Many genes are differentially expressed and function in subsets of myocardial cells. However, the lineage relationships between these cells remain poorly understood. To examine this, we have adopted a retrospective approach in the mouse embryo, based on the use of the laacZ reporter gene, targeted to the alpha-cardiac actin locus. This clonal analysis demonstrates the existence of two lineages that segregate early from a common precursor. The primitive left ventricle and the presumptive outflow tract are derived exclusively from a single lineage. Unexpectedly, all other regions of the heart, including the primitive atria, are colonized by both lineages. These results are not consistent with the prespecification of the cardiac tube as a segmented structure. They are discussed in the context of different heart fields and of the evolution of the heart.     

The Extracellular Domain of Lrp5/6 Inhibits Noncanonical Wnt Signaling In Vivo

Lrp5/6 are crucial coreceptors for Wnt/β-catenin signaling, a pathway biochemically distinct from noncanonical Wnt signaling pathways. Here, we examined the possible participation of Lrp5/6 in noncanonical Wnt signaling. We found that Lrp6 physically interacts with Wnt5a, but that this does not lead to phosphorylation of Lrp6 or activation of the Wnt/β-catenin pathway. Overexpression of Lrp6 blocks activation of the Wnt5a downstream target Rac1, and this effect is dependent on intact Lrp6 extracellular domains. These results suggested that the extracellular domain of Lrp6 inhibits noncanonical Wnt signaling in vitro. In vivo, Lrp6−/− mice exhibited exencephaly and a heart phenotype. Surprisingly, these defects were rescued by deletion of Wnt5a, indicating that the phenotypes resulted from noncanonical Wnt gain-of-function. Similarly, Lrp5 and Lrp6 antisense morpholino-treated Xenopus embryos exhibited convergent extension and heart phenotypes that were rescued by knockdown of noncanonical XWnt5a and XWnt11. Thus, we provide evidence that the extracellular domains of Lrp5/6 behave as physiologically relevant inhibitors of noncanonical Wnt signaling during Xenopus and mouse development in vivo.     

Habitats of Pleistocene megaherbivores reconstructed from the frozen fauna remains

The Late Pleistocene landscape in northern Eurasia and North America was inhabited by a specific megafaunal complex, which largely disappeared during the Pleistocene/Holocene transition. Vegetation changes are considered as one of the factors responsible for these extinctions, but the structure and composition of the Pleistocene vegetation are still poorly known. Here we complement previous studies by comparing the taxonomic composition of the plant remains found in the gastrointestinal tracts of the frozen carcasses of Pleistocene megaherbivores with the species composition of the current Siberian vegetation. We compiled a dataset of palaeobotanical records from frozen individuals of Pleistocene megaherbivores found in northern Siberia and Beringia and dated to the period from more than 50 kyr BP to 9 kyr BP. We also compiled a dataset of vegetation plots from several regions in Siberia. We analysed the similarity in taxonomic composition of plants between these two datasets using a novel method that accounts for variable taxonomic resolution in palaeobotanical data. For most megaherbivore individuals, plant remains in their gastrointestinal tracts corresponded to tundra, forest and mire vegetation, while they showed low similarity to steppe. This pattern was relatively constant over time, showing no remarkable differences between the Last Glacial Maximum and the periods before and afterwards. This suggests that during the Upper Pleistocene, a mosaic of mesic and wet vegetation types such as tundra with patches of forests and mires was common in northern Siberia and Beringia. In contrast, the steppe was rare to absent in the landscape or underused by the megaherbivores as a pasture since they found enough food in the widespread mesic and wet habitats with more productive vegetation.     

UvrB domain 4, an autoinhibitory gate for regulation of DNA binding and ATPase activity

UvrB, a central DNA damage recognition protein in bacterial nucleotide excision repair, has weak affinity for DNA, and its ATPase activity is activated by UvrA and damaged DNA. Regulation of DNA binding and ATP hydrolysis by UvrB is poorly understood. Using atomic force microscopy and biochemical assays, we found that truncation of domain 4 of Bacillus caldotenax UvrB (UvrBDelta4) leads to multiple changes in protein function. Protein dimerization decreases with an approximately 8-fold increase of the equilibrium dissociation constant and an increase in DNA binding. Loss of domain 4 causes the DNA binding mode of UvrB to change from dimer to monomer, and affinity increases with the apparent dissociation constants on nondamaged and damaged single-stranded DNA decreasing 22- and 14-fold, respectively. ATPase activity by UvrBDelta4 increases 14- and 9-fold with and without single-stranded DNA, respectively, and UvrBDelta4 supports UvrA-independent damage-specific incision by Cho on a bubble DNA substrate. We propose that other than its previously discovered role in regulating protein-protein interactions, domain 4 is an autoinhibitory domain regulating the DNA binding and ATPase activities of UvrB.     

Aphidicolin induces myogenic differentiation in the human rhabdomyosarcoma cell line KFR.

The effects of aphidicolin, a specific inhibitor of DNA polymerase α, on cell growth, DNA synthesis and myogenic differentiation in the human alveolar rhabdomyosarcoma cell line KFR were studied. The treatment with aphidicolin at 5 × 10⁻⁶ M concentration, which completely inhibited DNA synthesis and cell growth, induced morphological differentiation of small mononuclear cells to elongated, multinucleated (myotube-like) structures. The morphological differentiation was accompanied by the expression of skeletal muscle myosin; about 30% myosin-positive cells were observed after 14 days of treatment, compared to 2.3% in untreated cultures. The results showed that aphidicolin induces differentiation of human rhabdomyosarcoma cells and that multinucleated myotube-like elements may develop simply by cell fusion without cell division and DNA synthesis.     

Über die Induktion der sexuellen Vermehrung und die genotypische Determination der Wuchsform bei Microsporon gypseum (Bodin) Guiart & Grigoraki

Zusammenfassung Die Autoren beschreiben die Technik der Induktion der Cleistothecienentwicklung auf Agarnährböden und fassen die Ergebnisse der Hybridisation von zwei morphologisch verschiedenen Wuchsformen desMicrosporon gypseum (Bodin)Guiart &Grigoraki 1928 sensu lato zusammen. Ein Stamm (phänotypisch incurvata) wurde aus einer mykotischen Läsion des Menschen isoliert und ist mit dem ConidienstadiumNannizzia incurvataStockdale 1961 identisch. Der zweite Stamm (phänotypisch lanogypsea) wurde aus der Erde isoliert und entspricht morphologisch weder dem ConidienstadiumN. incurvataStockd. 1961,N. gypseaStockd. 1963 nochN. fulvaStockd. 1963. Da beide Stämme kompatibel sind, halten die Autoren die Wuchsform lanogypsea für eine morphologische Mutante der ArtN. incurvata.In sieben unabhängigen Hybridisationen wurden insgesamt 3951 Kulturen verfolgt. Mit Hilfe der Mikromanipulationstechnik erhielt man darunter 486 monoascosporische Kulturen. Die Spaltungsverhältnisse wurden durch den X² Test beglaubigt.Die Wuchsform incurvata und lanogypsea wird durch den Mechanismus der Kernvererbung gelenkt. Die Erblichkeit der studierten Wuchsform wird durch die Voraussetzung monofaktorialer Anlage vom Typ der asexuellen Sporulation erklärt. Diese Voraussetzung bestätigten die durchgeführten Kreuzungsversuche.Die spontane Pleomorphie der monoascosporischen Kulturen ohne Segregationserscheinungen bezeugt, daß sie nicht durch den Zustand einfacher Heterokaryose bewirkt wird.

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Summary The authors of the present paper describe the technique of the induction of production of cleistoteciae in the agar medium and they summarize the results of hybridization of two morphologically different growth forms ofMicrosporon gypseum (Bodin)Guiart &Grigoraki 1928 sensu lato. One strain (phenotype incurvata) has been isolated from a mycotic lesion of man and is identic with the conidial stage ofNannizzia incurvataStockdale 1961. The other strain (phenotype lanogypsea) has been isolated from soil and it does correspond neither morphologically to the conidial stage ofN. incurvataStockdale 1961, nor toN. gypseaStockdale 1963, nor toN. fulvaStockdale 1963. As both strains are compatible, the authors take the growth form lanogypsea for a morphological mutant of the speciesN. incurvata.On the whole 3951 cultures were followed in seven independent hybridisations. Out of this number 486 cultures were monoascosporic and were obtained by means of the micromanipulation technique. The segregation ratio was verified by the X² test.The growth form incurvata and lanogypsea is controlled by the mechanism of nuclear heredity. The inheritance of the growth form is explained by the hypothetic monofactorial allelic basis of the type of asexual sporulation. This presumption has been proved by the hybridisations which had been effectuated.The spontaneous pleomorphism of monoascosporic cultures gives evidence of the fact that pleomorphism is not conditional on the state of simple heterokaryosis, but by mutation.