Almost everything that happens inside a cell, including autophagy, is tightly regulated on a biochemical level. Like that, the cell makes sure that processes only take place when they are needed and that they are shut off when the need has expired. "Inside the cell, there exists a network of molecules. Between them, information is constantly being exchanged," says Schmitz, head of the research group "Systems-oriented Immunology and Inflammation Research" at HZI, who also holds a chair at the Otto von Guericke University in Magdeburg. "In a way, it looks like a big city subway map." However, only the starting point and the destination of a given "cellular subway line" are relatively easy to study. To explore the different stops along the way, is more difficult. But because other lines intersect and interact with each other at these points, it is very exciting for researchers to decode all molecules involved in these signal transduction processes. It also helps them better understand diseases caused by defects in these information highways.
What exactly happens on a molecular level during the later stages of autophagy was largely unknown - until now. Schmitz and his team, along with researchers from the Otto von Guericke University Magdeburg, the Heinrich Heine University D-sseldorf, the T-bingen University, and the Temple University School of Medicine in Philadelphia, USA, have decoded one part of the molecular subway map.
Under the microscope, researchers can observe how larger-sized cellular components destined for degradation and recycling are enclosed within a small bubble, the so-called autophagosome. This structure then fuses with yet another little bubble, which digests the autophagosome's contents. "Autophagy is a survival mechanism to ensure that the cell is able to obtain the necessary nutrients during times of starvation," explains Schmitz.
For their studies, the scientists stained certain molecules and autophagosomes inside cells. This allowed them to observe microscopically which molecules are in charge of regulating the formation of the little digestive bubbles. To prompt self-digestion, they either starved the cells or simulated an infection. In the process, they discovered that the cells simultaneously also turned on autophagy-inhibiting molecules - "like some kind of emergency brake that ensures autophagy doesn't get out of control." Such negative feedback loops are not unusual for cells, they frequently help prevent overshooting reactions.