The role of Drosophila TNF alpha in immune cell invasion

Immune cells protect humans against infection. Many immune cells travel throughout the body by flowing with the blood in blood vessels. To reach invaders, immune cells respond to signals indicating an infection by sticking to and then pushing their way between the cells that make up the blood vessel wall. One of these signals of infection is called Tumor necrosis factor (TNF) . It causes the cells of the blood vessel to increase their production of proteins that the immune cells adhere to, as well as to decrease how much the blood vessel cells stick to each other. One relatively unexplored aspect of TNF is its ability to change the shape of the blood vessel cells, as well as whether TNF signaling alters the tissues to decrease the amount of force that the immune cells need to exert to penetrate the barrier. We propose to study these questions in the fruit fly, Drosophila melanogaster. The use of Drosophila facilitates the quick and cheap manipulation of genes, the instruction manuals needed to produce proteins. In Drosophila genes can be turned on and off at specific times and in specific regions of the body to investigate their role in a particular process. Drosophila also permits the live imaging of cell or protein movement in the intact embryo, by the use of fluorescent tags. Our lab has shown previously that the embryonic migration of Drosophila immune cells involves a step in which these cells penetrate a tissue barrier in a manner very similar to the penetration of the vasculature by immune cells in man. Intriguingly, we have shown that the Drosophila version of the TNF signal and the proteins that receive this signal are all required for the barrier penetration by immune cells in the fly. This provides us a unique opportunity to investigate TNF role in regulating the cellular and biophysical properties in a live animal, using the most modern techniques to assess cellular tension. We will test our hypothesis that TNF induces changes in the stiffness and adhesion of the cells the immune cells move between by altering the localization and functionality of a protein called myosin. Myosin acts as a molecular motor, pulling stiff protein rods called actin towards another. We will also investigate if this mechanism occurs in higher organisms, as well as the importance of this signaling in the development of fruit flies, and vertebrates.

Like tiny building blocks, billions of cells form our bodies. But unlike building blocks, some cells are able to move around the body. The most notorious cell movement occurs during metastasis, when cancer cells spread from a primary tumor. Cell movement can also have beneficial effects, for example, when immune cells need to get out of the blood vessel to reach damaged or inflamed tissue. These movements require both cancer or immune cells to move through tissue barriers where cells that sit closely together-like a tight wall-make it hard for migrating cells to get through. In their study, the research group of Daria Siekhaus from the Institute of Science and Technology Austria (IST Austria) showed that in the fruit fly, a certain type of immune cells called macrophages has an easier time squeezing through tissue barriers when a signal is sent to change the tension of the barrier cells. Macrophages, which play an important role in development, migrate through the developing fruit fly embryo. Along their way, they need to get through a tissue called the germband. The researchers found that migrating macrophages stall when they reach this barrier. They spend time trying to push their way in, a task that is made easier by a signal sent to the cells of the barrier that reduces their tension. This change in tension makes the barrier cells less stiff and more deformable, allowing macrophages to more easily squeeze through between them.