SummaryExcess copper is highly toxic and forms part of the host innate immune system's antibacterial arsenal, accumulating at sites of infection and acting within macrophages to kill engulfed pathogens. We show for the first time that a novel, horizontally gene transferred copper resistance locus (copXL), uniquely associated with the SCCmec elements of the highly virulent, epidemic, community acquired methicillin resistant Staphylococcus aureus (CA‐MRSA) USA300, confers copper hyper‐resistance. These genes are additional to existing core genome copper resistance mechanisms, and are not found in typical S. aureus lineages, but are increasingly identified in emerging pathogenic isolates. Our data show that CopX, a putative P1B‐3‐ATPase efflux transporter, and CopL, a novel lipoprotein, confer copper hyper‐resistance compared to typical S. aureus strains. The copXL genes form an operon that is tightly repressed in low copper environments by the copper regulator CsoR. Significantly, CopX and CopL are important for S. aureus USA300 intracellular survival within macrophages. Therefore, the emergence of new S. aureus clones with the copXL locus has significant implications for public health because these genes confer increased resistance to antibacterial copper toxicity, enhancing bacterial fitness by altering S. aureus interaction with innate immunity.

Author contributions: Concept, coding, image collection, statistical analysis performed by JT. TH helped in creating the study protocol and edited the article.

Low-level electrical stimulation to the eye has been shown to be neuroprotective against retinal degeneration in both human and animal subjects, using approaches such as subretinal implants and transcorneal electrical stimulation. In this study, we investigated the benefits of whole-eye electrical stimulation (WES) in a rodent model of retinitis pigmentosa. Transgenic rats with a P23H-1 rhodopsin mutation were treated with 30 min of low-level electrical stimulation (4 μA at 5 Hz; n = 10) or sham stimulation (Sham group; n = 15), twice per week, from 4 to 24 weeks of age. Retinal and visual functions were assessed every 4 weeks using electroretinography and optokinetic tracking, respectively. At the final time point, eyes were enucleated and processed for histology. Separate cohorts were stimulated once for 30 min, and retinal tissue harvested at 1 h and 24 h post-stimulation for real-time PCR detection of growth factors and inflammatory and apoptotic markers. At all time-points after treatment, WES-treated rat eyes exhibited significantly higher spatial frequency thresholds than untreated eyes. Inner retinal function, as measured by ERG oscillatory potentials (OPs), showed significantly improved OP amplitudes at 8 and 12 weeks post-WES compared to Sham eyes. Additionally, while photoreceptor segment and nuclei thicknesses in P23H-1 rats did not change between treatment groups, WES-treated eyes had significantly greater numbers of retinal ganglion cell nuclei than Sham eyes at 20 weeks post-WES. Gene expression levels of brain-derived neurotrophic factor (BDNF), caspase 3, fibroblast growth factor 2 (FGF2), and glutamine synthetase (GS) were significantly higher at 1 h, but not 24 h after WES treatment. Our findings suggest that WES has a beneficial effect on visual function in a rat model of retinal degeneration and that post-receptoral neurons may be particularly responsive to electrical stimulation therapy.

The U.S. Environmental Protection Agency (U.S. EPA) has included an amphibian metamorphosis assay (AMA) to detect thyroid active chemicals in Tier 1 testing of their endocrine screening program. To understand the variability, specificity, and reliability of the key endpoints of this assay, two exposure studies with Xenopus laevis tadpoles were conducted with two known thyroid-active compounds, namely, methimazole or L-thyroxine, for a total of 21 d. In addition, various increased-flow-rate treatments were included in the exposures to evaluate the effects of physical stress on metamorphic development. The endpoints examined in the exposures were wet weight, snout-vent length, hind-limb length, developmental stage, and thyroid and gonadal histopathology. As expected, the results indicated that both methimazole and L-thyroxine were thyroid active in the AMA, hind-limb length and thyroid histopathology being the most sensitive endpoints of thyroid activity. Tadpoles that were exposed to the various physical stressors in these experiments showed no signs of altered metamorphic development, and exposure to the thyroid-active compounds had no effect on the developing gonad of X. laevis. Taken together, these results support the use of the AMA as a Tier 1 endocrine screen for detection of potential thyroid pathway activity; however, the lack of a true negative response (no-effect) during the validation process prevents a full evaluation of this assay's specificity at this time.

Large granular lymphocyte leukemia (LGLL) is a common fatal disease in aging F344 rats. The current understanding of rat LGLL and a search for mechanistic data/correlations to human leukemia were examined with the goal of improving evaluation of the LGLL endpoint in cancer bioassays as it relates to human cancer risk assessments. The exact cell of origin of the F344 rat LGLL is not fully resolved, although natural killer (NK) cell characteristics were demonstrated in most, if not all cases. Similarities between rat LGLL and a rare human NK-LGLL exist, invalidating claims of no human counterpart, although the underlying etiopathogenesis may be different. There is insufficient data to establish a mode of action of chemical-induced rat LGLL. Evaluation of the National Toxicology Program database revealed only 34 substances (out of over 500 studied) that were possibly associated with increased incidences of LGLL. Of these, only five produced definitive LGLL effects in both sexes; the remaining 29 produced single sex responses and/or only "equivocal" associations with LGLL. Trends of increasing background/variability in LGLL incidence and its modulation by extraneous factors (e.g., corn oil gavage) are key confounders in interpretation. Given that LGLL is a common tumor in control F344 rats, interpretations of bioassays can be improved by increasing the statistical stringency (e.g., p<0.01 over traditional p<0.05), as an indicator of possible carcinogenic effects, but that alone would be insufficient evidence for declaring treatment-related increases. Thus, it was concluded that the evaluation of possible chemically related increases in rat LGLL utilize a "weight-of-evidence" approach.