BioWorld International Correspondent
LONDON - When someone eats a chicken and mayonnaise sandwich that is crawling with Salmonella bacteria, the bacteria that manage to pass through the stomach then attempt to invade the cells that line the small intestine. Understanding how they do that, and therefore how to thwart them, could provide new targets for novel antibiotics.
Just how the bacteria are able to subvert the normal workings of the intestinal epithelial cells has been a topic of interest among microbiologists and cell biologists - and now new research has filled in some of the gaps in the process.
A study reported in the Feb. 27, 2004, issue of Molecular Cell describes how a protein deployed by Salmonella encourages intestinal cells to welcome the invaders with open arms. The paper, by Emma McGhie, Richard Hayward and Vassilis Koronakis from the department of pathology at the University of Cambridge in the UK, is titled "Control of Actin Turnover by a Salmonella Invasion Protein."
Koronakis, reader in molecular biology, told BioWorld International: "This study furthers the understanding of the mechanism by which Salmonella, a major human food-borne pathogen, induces alterations to the host cell's cytoskeleton, which is an essential early step in Salmonella pathogenesis. Greater insight into these processes might eventually allow the development of novel therapeutics, which will be important with the increasing incidence of antibiotic resistance."
Bacteria that target phagocytic host cells have an easy time of it. They simply stick to the cell and wait for it to engulf them. But many bacteria need to invade body cells that do not behave that way. Salmonella bacteria, for example, force their way into intestinal cells, which do not carry out phagocytosis.
To get the host cells to take them up, bacterial pathogens such as Salmonella have to stimulate the cells to rearrange their cytoskeletons and change shape. The cytoskeleton is made of a protein called actin. Single units of actin, called monomers, can polymerize to form filaments (F-actin) that together comprise the cytoskeleton.
However, filament polymerization is not enough to allow changes in cell shape. F-actin also must be actively depolymerized to ensure that enough actin monomers remain available. The cellular actin-binding proteins, ADF/cofilin and gelsolin, play an important role in slicing up F-actin during depolymerization.
The whole process of filament assembly and disassembly is called "actin turnover" or "actin treadmilling."
When bacteria invade host cells, the first stage in rearranging the cytoskeleton is the delivery into the host cell of a cocktail of proteins, including Salmonella invasion proteins, or Sips. Two of those, SipA and SipC, are actin-binding proteins.
Strangely, SipA and SipC share no similarity with each other or with any other known cellular actin-binding proteins. SipC is essential for Salmonella's entry into cells. SipA binds and stabilizes F-actin and augments the activity of SipC.
Ultimately, the rearrangements of the host cell's cytoskeleton, together with stimulation of host-cell signal transduction by some of the other proteins delivered into the target cell, result in the formation of membrane ruffles that envelop the bacteria, allowing them to be internalized into a membrane-bound intracellular vacuole, in which they survive and replicate.
McGhie, Hayward and Koronakis set out to find out more about SipA, using a system that mimicked the environment of the target cell.
Koronakis said: "We demonstrated that Salmonella SipA directly arrests the cellular mechanisms of actin turnover. SipA prevents ADF and cofilin from binding to F-actin, and it displaces these proteins from F-actin if they have already bound to it."
The team found that SipA also can protect F-actin from severing by gelsolin, and can re-anneal fragments of F-actin filaments that have been severed by gelsolin.
Koronakis said: "The data suggest that SipA focuses host actin rearrangements by locally inhibiting both ADF/cofilin and gelsolin-directed actin disassembly, while simultaneously stimulating pathogen-induced actin polymerization, and therefore has a fundamental role in controlling actin treadmilling."
He added: "We now intend to examine how SipA inhibits ADF/cofilin and gelsolin at a molecular level and to understand how SipA works together with the other proteins that Salmonella delivers to the host cell to efficiently manipulate the actin cytoskeleton."