Ubiquitination of the GTPase Rap1B by the ubiquitin ligase Smurf2 is required for the establishment of neuronal polarity

The development of a polarised morphology with multiple dendrites and a single axon is an essential step in the differentiation of neurons. The establishment of neuronal polarity is directed by the sequential activity of the GTPases Rap1B and Cdc42. Rap1B is initially present in all neurites of unpolarised neurons, but becomes restricted to the tip of a single process during the establishment of neuronal polarity where it specifies axonal identity. Here, we show that the ubiquitin ligases Smad ubiquitination regulatory factor-1 (Smurf1) and Smurf2 are essential for neurite growth and neuronal polarity, respectively, and regulate the GTPases Rho and Rap1B in hippocampal neurons. Smurf2 is required for the restriction of Rap1B to a single neurite. Smurf2 ubiquitinates inactive Rap1B and initiates its degradation through the ubiquitin/proteasome pathway (UPS). Degradation of Rap1B restricts it to a single neurite and thereby ensures that neurons extend a single axon.     

Functional interactions between coexisting toxin-antitoxin systems of the ccd family in Escherichia coli O157:H7

Toxin-antitoxin (TA) systems are widely represented on mobile genetic elements as well as in bacterial chromosomes. TA systems encode a toxin and an antitoxin neutralizing it. We have characterized a homolog of the ccd TA system of the F plasmid (ccd(F)) located in the chromosomal backbone of the pathogenic O157:H7 Escherichia coli strain (ccd(O157)). The ccd(F) and the ccd(O157) systems coexist in O157:H7 isolates, as these pathogenic strains contain an F-related virulence plasmid carrying the ccd(F) system. We have shown that the chromosomal ccd(O157) system encodes functional toxin and antitoxin proteins that share properties with their plasmidic homologs: the CcdB(O157) toxin targets the DNA gyrase, and the CcdA(O157) antitoxin is degraded by the Lon protease. The ccd(O157) chromosomal system is expressed in its natural context, although promoter activity analyses revealed that its expression is weaker than that of ccd(F). ccd(O157) is unable to mediate postsegregational killing when cloned in an unstable plasmid, supporting the idea that chromosomal TA systems play a role(s) other than stabilization in bacterial physiology. Our cross-interaction experiments revealed that the chromosomal toxin is neutralized by the plasmidic antitoxin while the plasmidic toxin is not neutralized by the chromosomal antitoxin, whether expressed ectopically or from its natural context. Moreover, the ccd(F) system is able to mediate postsegregational killing in an E. coli strain harboring the ccd(O157) system in its chromosome. This shows that the plasmidic ccd(F) system is functional in the presence of its chromosomal counterpart.     

Na+ -D-glucose cotransporter in the kidney of Leucoraja erinacea: molecular identification and intrarenal distribution

Studies on membrane vesicles from the kidney of Leucoraja erinacea suggested the sole presence of a sodium-D-glucose cotransporter type 1 involved in renal D-glucose reabsorption. For molecular characterization of this transport system, an mRNA library was screened with primers directed against conserved regions of human sglt1. A cDNA was cloned whose nucleotide and derived amino acid sequence revealed high homology to sodium glucose cotransporter 1 (SGLT1). Xenopus laevis oocytes injected with the respective cRNA showed sodium-dependent high-affinity uptake of D-glucose. Many positions considered functionally essential for sodium glucose cotransporter 1 (SGLT1) are also found in the skate protein. High conservation preferentially in transmembrane helices and small linking loops suggests early appearance and continued preservation of these regions. Larger loops, especially loop 13, which is associated with phlorizin binding, were more variable, as is the interaction with the specific inhibitor in various species. To study the intrarenal distribution of the transporter, a skate SGLT1-specific antibody was generated. In cryosections of skate kidney, various nephron segments could be differentiated by lectin staining. Immunoreaction with the antibody was observed in the proximal tubule segments PIa and PIIa, the early distal tubule, and the collecting tubule. Thus Leucoraja, in contrast to the mammalian kidney, employs only SGLT1 to reabsorb d-glucose in the early, as well as in the late segments of the proximal tubule and probably also in the late distal tubule (LDT). Thereby, it differs also partly from the kidney of the close relative Squalus acanthias, which uses SGLT2 in more distal proximal tubule segments but shows also expression in the later nephron parts.     

Gene, phenotype and function: GLABROUS1 and resistance to herbivory in natural populations of Arabidopsis lyrata

The molecular genetic basis of adaptive variation is of fundamental importance for evolutionary dynamics, but is still poorly known. Only in very few cases has the relationship between genetic variation at the molecular level, phenotype and function been established in natural populations. We examined the functional significance and genetic basis of a polymorphism in production of leaf hairs, trichomes, in the perennial herb Arabidopsis lyrata. Earlier studies suggested that trichome production is subject to divergent selection. Here we show that the production of trichomes is correlated with reduced damage from insect herbivores in natural populations, and using statistical methods developed for medical genetics we document an association between loss of trichome production and mutations in the regulatory gene GLABROUS1. Sequence data suggest that independent mutations in this regulatory gene have provided the basis for parallel evolution of reduced resistance to insect herbivores in different populations of A. lyrata and in the closely related Arabidopsis thaliana. The results show that candidate genes identified in model organisms provide a valuable starting point for analysis of the genetic basis of phenotypic variation in natural populations.     

Phosphorylation of HuR by Chk2 regulates SIRT1 expression

The RNA binding protein HuR regulates the stability of many target mRNAs. Here, we report that HuR associated with the 3' untranslated region of the mRNA encoding the longevity and stress-response protein SIRT1, stabilized the SIRT1 mRNA, and increased SIRT1 expression levels. Unexpectedly, oxidative stress triggered the dissociation of the [HuR-SIRT1 mRNA] complex, in turn promoting SIRT1 mRNA decay, reducing SIRT1 abundance, and lowering cell survival. The cell cycle checkpoint kinase Chk2 was activated by H(2)O(2), interacted with HuR, and was predicted to phosphorylate HuR at residues S88, S100, and T118. Mutation of these residues revealed a complex pattern of HuR binding, with S100 appearing to be important for [HuR-SIRT1 mRNA] dissociation after H(2)O(2). Our findings demonstrate that HuR regulates SIRT1 expression, underscore functional links between the two stress-response proteins, and implicate Chk2 in these processes.     

Diversity of Wolbachia isolated from the Cubitermes sp. affinis subarquatus complex of species (Termitidae), revealed by multigene phylogenies

Wolbachia are endosymbiotic bacteria that may alter the reproductive mechanisms of arthropod hosts. Eusocial termites provide considerable scope for Wolbachia studies owing to their ancient origin, their great diversity and their considerable ecological, biological and behavioral plasticity. This article describes the phylogenetic distribution of Wolbachia infecting termites of the Cubitermes genus, which are particularly abundant soil-feeders in equatorial Africa. Fourteen colonies of the Cubitermes sp. affinis subarquatus complex of species were screened using five bacterial genes (wsp, ftsZ, coxA, fbpA and 16S rRNA genes) and a striking diversity of Wolbachia strains was identified within these closely related species. In the host complex, three Wolbachia variants were found that were not in the super groups usually reported for termites (F and H), each infecting one or two Cubitermes species.     

Complex life cycles of multicellular eukaryotes: new approaches based on the use of model organisms

A wide variety of life cycles can be found in the different groups of multicellular eukaryotes. Here we provide an overview of this variety, and review some of the theoretical arguments that have been put forward to explain the evolutionary stability of different life cycle strategies. We also describe recent progress in the analysis of the haploid-diploid life cycle of the model angiosperm Arabidopsis thaliana and show how new molecular data are providing a means to test some of the theoretical predictions. Finally, we describe an emerging model organism from the brown algae, Ectocarpus siliculosus, and highlight the potential of this system for the investigation of the mechanisms that regulate complex life cycles.     

Understanding the regulation and function of adult neurogenesis: contribution from an insect model, the house cricket

Since the discovery of adult neurogenesis, a major issue is the role of newborn neurons and the function-dependent regulation of adult neurogenesis. We decided to use an animal model with a relatively simple brain to address these questions. In the adult cricket brain as in mammals, new neurons are produced throughout life. This neurogenesis occurs in the main integrative centers of the insect brain, the mushroom bodies (MBs), where the neuroblasts responsible for their formation persist after the imaginal molt. The rate of production of new neurons is controlled not only by internal cues such as morphogenetic hormones but also by external environmental cues. Adult crickets reared in an enriched sensory environment experienced an increase in neuroblast proliferation as compared with crickets reared in an impoverished environment. In addition, unilateral sensory deprivation led to reduced neurogenesis in the MB ipsilateral to the lesion. In search of a functional role for the new cells, we specifically ablated MB neuroblasts in young adults using brain-focused gamma ray irradiation. We developed a learning paradigm adapted to the cricket, which we call the "escape paradigm." Using this operant associative learning test, we showed that crickets lacking neurogenesis exhibited delayed learning and reduced memory retention of the task when olfactory cues were used. Our results suggest that environmental cues are able to influence adult neurogenesis and that, in turn, newly generated neurons participate in olfactory integration, optimizing learning abilities of the animal, and thus its adaptation to its environment. Nevertheless, odor learning in adult insects cannot always be attributed to newly born neurons because neurogenesis is completed earlier in development in many insect species. In addition, many of the irradiated crickets performed significantly better than chance on the operant learning task.