The bacterium Streptococcus pneumoniae (the pneumococcus) is carried in the nasopharynx of most children and some adults without causing disease. However, under some circumstances the bacterium becomes invasive and causes diseases such as pneumonia, meningitis, sepsis and otitis media (reviewed in (1)). The ability of the bacterium to colonise the nasopharynx and in some cases disease is related to the expression of a polysaccharide capsule that protects the organism from the immune system and a range of protein virulence factors. The pneumococcus produces a range of colonization and virulence factors including surface proteins, the polysaccharide capsule and a pore forming protein, pneumolysin (Ply). One of the surface proteins important in the ability of the pneumococcus to bind to human cells is pneumococcal surface antigen A (PsaA)(2). PsaA binds to host E-cadherin, a protein present in cell/cell junctions and this promotes invasion of the pneumococcus across the epithelial cells of the lung to cause sepsis or endothelial cells of the blood brain barrier to cause meningitis. These invasion processes can be studied in vitro using cell monolayer models using A549 cells for respiratory epithelium and Human Brain Microvascular Endothelial Cells (HBMEC) for blood brain barrier. Integrity of the cell barriers can be monitored by measuring transcellular electrical resistance. Ply plays a major role in the virulence of the organism and the pathogenesis of infection as demonstrated in animal models of pneumonia and meningitis. It is known that manipulation of levels of expression of Ply by a strain of pneumococcus affects the invasive ability of the strain. Expression of high levels of Ply in a cell vacuole leads to recruitment of cytosolic “eat me’ signals galectin-8 and ubiquitin and targets the pneumococcus for autophagic clearance while a low Ply producing subset halts autophagosomal maturation and evades all intracellular defence mechanisms, promoting its prolonged survival and successful transcytosis across BBB, both in vitro and in vivo (3).
The Ply protein forms large pores (25nm diameter) in cell membranes and alters a range of host cell functions including immune response, macrophage function and the permeability of epithelial and endothelial cell barriers. These later effects are important in the ability of the bacterium to cross from lung into blood and from blood into brain (4, 5). The human pathogen Staphylococcus aureus also makes a pore forming protein termed alpha-haemolysin which forms small pores (2nm diameter) in eukaryotic cell membranes. PLY and Staphylococcus haemolysin will be used to evaluate the effects of membrane pore size (large vs small) on the cell biology of both epithelial and endothelial cell barriers. Both Ply and staphylococcal alpha toxin alter the permeability of epithelial and endothelial cell barriers by activation of disintegrin and metalloprotease-10 (ADAM-10). ADAM-10 degrades cell junction proteins such as E-Cadherin and leads to protein ‘shedding’ from the cells. The activity of ADAM-10 is controlled by membrane bound tetraspanin-15 (6, 7). The hypothesis is that pore forming proteins produced by these bacterial pathogens activate Tspan15/ADAM-10 and lead to changes in barrier permeability, shedding of cell surface markers and bacterial invasion of the lung and brain during infection. As one of the proteins shed by the epithelium due to activation of ADAM-10 is E-Cadherin which is also a receptor for a cell surface protein of Streptococcus pneumoniae (2), it is possible that cleavage will affect bacterial binding to cells and may lead to invading bacteria being coated with cleaved E-Cadherin. It is also possible that production of pore forming toxin by one species of bacteria reduces the ability of the other species to bind due to receptor shedding.
Applicants should have a strong background in molecular microbiology and eukaryotic cell biology They should have a commitment to infectious disease research and hold or realistically expect to obtain at least an Upper Second Class Honours Degree in microbiology or biochemistry.
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