Overexpression of a Mutant B Subunit in Toxigenic Vibrio cholerae Diminishes Production of Active Cholera Toxin In Vivo

A mutant cholera toxin B subunit containing a G33E substitution was constructed and expressed in V. cholerae. The G33E amino acid substitution did not affect the amount of recombinant CTB secreted to the culture medium. The overexpression of the mutant B subunits in wild-type toxigenic cholera vibrios led to an 80% decrease in production of active cholera toxin in vitro and in vivo. Overexpression of B^G33E subunits could be instrumental in the increase of the biosafety of live attenuated cholera candidate vaccine strains. Received: 29 January 1998 / Accepted: 30 April 1998     

Effect of streptozotocin-diabetes and insulin treatment on regulation of Leydig cell function in the rat

The present study was performed to further clarify the possible role played by insulin deficiency on the steroidogenic capacity of the rat testis. Sprague-Dawley rats weighing 250-300 g were used in all experiments. Diabetes was induced by i.p. injection (40 mg/kg b.w.) of streptozotocin and was monitored at 2-day intervals by measuring body weight and serum glucose, glucosuria and ketonuria levels. The effect of insulin therapy on pituitary LH content and plasma LH concentrations, as well as on the cyclic AMP level in interstitial cell incubation medium and plasma testosterone concentrations, was measured 30 days after the induction of diabetes by radioimmunoassay. Streptozotocin-induced diabetes resulted in significantly reduced pituitary LH (16%, P less than 0.025) and plasma LH (34%, P less than 0.02); insulin treatment completely restored these levels. Similarly, the cyclic AMP content of interstitial cell incubation medium and the plasma testosterone concentrations were dramatically decreased in the diabetic state (50%, P less than 0.005 and 63%, P less than 0.025, respectively) and combined treatment with insulin plus hCG appeared slightly more effective than treatment with either of these hormones alone, suggesting a possible synergistic action. It is concluded that decreased testicular steroidogenesis in the diabetic rat may represent, at least in part, a direct consequence of insulin deficiency at the hypothalamic and/or pituitary levels. However, our findings would also be consistent with other reports suggesting that insulin may play a direct role in the rat testis.     

Heteromorphic Change of Adult Fibroblasts by Ribonucleoproteins

Cultures of subcutaneous areolar fibroblasts from adult rats, when confronted in vitro with microsomes from rat liver or kidney, are changed heteromorphically so as to resemble cultures of nervous tissue. Similar effects follow exposure to the deoxycholate-insoluble fraction of microsomes, and to purified RNA from rat liver or from yeast. An equivalent ribonucleoside mixture has no heteromorphic effect. The degree of heteromorphosis can be related quantitatively to the amount of RNA administered, up to a maximum of 150 γ per slide, above which toxicity intervenes. Ribonuclease destroys in considerable degree the effectiveness of the active agents. Heteromorphosis cannot be induced in this adult tissue by a short exposure (1 to 3 hours) followed by removal to normal medium. A 24 hour exposure to microsome suspensions, however, is followed by partial change lasting for at least several days. Results are most clear cut when cultures of the explant type are maintained continuously in contact with the RNA-containing agents; nevertheless, cell suspensions exposed for 2 to 3 days to heteromorphic agents in suitable concentration appear to be permanently changed. Interspecies experiments between rat and mouse indicate that rat fibroblasts are more labile than mouse, and/or rat microsomes are more potent as agents of heteromorphosis. Mouse liver microsomes have no morphogenetic effect on homologous fibroblasts, but exert a slight action on rat fibroblasts. Rat microsomes have a growth-stimulating effect, but no heteromorphic action, on mouse fibroblasts. Purified protein from snake venom, which is highly active as a growth factor for avian nervous tissue, is growth-stimulating to rat fibroblasts but has no heteromorphic action on this material.     

Influences on koala habitat selection across four local government areas on the far north coast of NSW

Conserving habitats crucial for threatened koala (Phascolarctos cinereus) populations requires rating habitat quality from a fine spatial scale to patches, landscapes and then regions. The koala has a specialized diet focused on the leaves of a suite of Eucalyptus species. We asked: what are the key regional influences on habitat selection by koalas in the far north coast of New South Wales? We addressed this question by investigating the multi-scale factors, and within-scale and cross-scale interactions, that influence koala habitat selection and distribution across four local government areas on the far north coast of New South Wales. We assembled and analysed a large data set of tree selection, identified by the presence of scats, in a wide range of randomly selected 5 × 5 km grids across the region. This resulted in more than 9000 trees surveyed for evidence of koala use from 302 field sites, together with associated biophysical site features. The dominant factor influencing habitat use and koala occurrence was the distribution of five Eucalyptus species. Koalas were more likely to use medium-sized trees of these species where they occurred on soils with high levels of Colwell phosphorous. We also identified new interactions among the distribution of preferred tree species and soil phosphorous, and their distribution with the amount of suitable habitat in the surrounding landscape. Our study confirmed that non-preferred species of eucalypts and non-eucalypts are extensively used by koalas and form important components of koala habitat. This finding lends support to restoring a mosaic of koala-preferred tree species and other species recognized for their value as shelter. Our study has provided the ecological foundation for developing a novel regional-scale approach to the conservation of koalas, with adaptability to other wildlife species.