Spatial Distribution of the Pathways of Cholesterol Homeostasis in Human Retina

The retina is a light-sensitive tissue lining the inner surface of the eye and one of the few human organs whose cholesterol maintenance is still poorly understood. Challenges in studies of the retina include its complex multicellular and multilayered structure; unique cell types and functions; and specific physico-chemical environment.We isolated specimens of the neural retina (NR) and underlying retinal pigment epithelium (RPE)/choroid from six deceased human donors and evaluated them for expression of genes and proteins representing the major pathways of cholesterol input, output and regulation. Eighty-four genes were studied by PCR array, 16 genes were assessed by quantitative real time PCR, and 13 proteins were characterized by immunohistochemistry. Cholesterol distribution among different retinal layers was analyzed as well by histochemical staining with filipin. Our major findings pertain to two adjacent retinal layers: the photoreceptor outer segments of NR and the RPE. We demonstrate that in the photoreceptor outer segments, cholesterol biosynthesis, catabolism and regulation via LXR and SREBP are weak or absent and cholesterol content is the lowest of all retinal layers. Cholesterol maintenance in the RPE is different, yet the gene expression also does not appear to be regulated by the SREBPs and varies significantly among different individuals.This comprehensive investigation provides important insights into the relationship and spatial distribution of different pathways of cholesterol input, output and regulation in the NR-RPE region. The data obtained are important for deciphering the putative link between cholesterol and age-related macular degeneration, a major cause of irreversible vision loss in the elderly

Quorum-sensing control of antibiotic resistance stabilizes cooperation in Chromobacterium violaceum

Many Proteobacteria use quorum sensing to regulate production of public goods, such as antimicrobials and proteases, that are shared among members of a community. Public goods are vulnerable to exploitation by cheaters, such as quorum sensing-defective mutants. Quorum sensing- regulated private goods, goods that benefit only producing cells, can prevent the emergence of cheaters under certain growth conditions. Previously, we developed a laboratory co-culture model to investigate the importance of quorum-regulated antimicrobials during interspecies competition. In our model, Burkholderia thailandensis and Chromobacterium violaceum each use quorum sensing-controlled antimicrobials to inhibit the other species' growth. Here, we show that C. violaceum uses quorum sensing to increase resistance to bactobolin, a B. thailandensis antibiotic, by increasing transcription of a putative antibiotic efflux pump. We demonstrate conditions where C. violaceum quorum-defective cheaters emerge and show that in these conditions, bactobolin restrains cheaters. We also demonstrate that bactobolin restrains quorum-defective mutants in our co-culture model, and the increase in antimicrobial-producing cooperators drives the C. violaceum population to become more competitive. Our results describe a mechanism of cheater restraint involving quorum control of efflux pumps and demonstrate that interspecies competition can reinforce cooperative behaviors by placing constraints on quorum sensing-defective mutants