The research appears as the "Paper of the Week" in the March 25 issue of the Journal of Biological Chemistry, an American Society for Biochemistry and Molecular Biology journal.
Listeria causes a variety of diseases, the most severe being meningoencephalitis, an inflammation of the brain and the membranes that envelop the brain and spinal cord. Infection begins when the bacterium binds to a receptor on the surface of a cell, causing the cell to ingest it. The bacterium multiplies inside the cell and then uses a cellular protein called ActA to stimulate the host cell's actin to form filaments at one end of the bacterium.
"As these filaments lengthen, they drive the bacterium through the cell until it reaches the peripheral or outer cell membrane," explains Dr. Frederick Southwick of the University of Florida College of Medicine. "Here the growing actin filaments push the bacterium against the membrane, forming long membrane projections called filopodia. These filopodia push into adjacent cells and are ingested by them. The bacteria then enter the new cell and begin the cycle anew. Essentially Listeria takes over or hijacks the host cell's actin cytoskeleton to move within cells, and to spread from cell to cell."
In most cells, two membrane lipids, PIP2 and PIP3, are associated with the formation of new actin filaments. PIP3 is synthesized from PIP2 by an enzyme called PI3-kinase. The lipids attract and modify the functions of proteins involved in regulating actin assembly. PIP2 and PIP3 also prevent capping proteins from binding to the ends of actin filaments, allowing new actin filament assembly.
Because Listeria is capable of stimulatin g actin assembly and PIP2 and PIP3 are known to localize to regions of new actin assembly, Dr. Southwick and his colleagues decided to explore the roles these lipids play in Listeria infection.
"We had expected to see PIP2 and PIP3 only at the very back of Listeria where new actin assembly was taking place," recalls Dr. Southwick. "To our surprise these lipids also localized to the front of the moving bacteria." The researchers also noticed that Listeria movement slowed down when the bacteria were treated with molecules that inhibited PI3-kinase, proving that Listeria depend on PI3-kinase to move.
"Our studies show that Listeria is capable of inside-out signaling," explains Dr. Southwick. "Most signals arise from molecules binding receptors on the outside of the cell. In the case of Listeria, we find that this intracellular pathogen can harness signals from the inner rather than the outer surface of the cell membrane.
"The most exciting and surprising finding is that an intracellular bacteria is able to attract host cell membrane lipids to its surface and these membrane lipids facilitate the ability of the bacterium to move within cells. This capability is unique to Listeria and is not found in another intracellular bacteria, Shigella. Our experiments show that Listeria is a simplified model system for studying how phosphoinositides regulate the actin cytoskeleton, and this model promises to yield additional insights into how these phospholipids control the cell's actin cytoskeleton. Our discoveries provide additional fundamental clues as how cells move."
These findings may also open the door to using PI3-kinase inhibitors or other agents that lower PIP2 and PIP3 levels to slow the spread of Listeria and control infection in patients who are not responding to antibiotics, although that application is a long way off, says Dr. Southwick.