When cells are attacked by bacteria they use all means at their disposal to defend themselves. But cellular defence systems can damage the cells themselves and so need to be kept tightly in check. Recent results help us to understand how this is done and give pointers to new ways of combating disease. Matthias Farlik in the group of Thomas Decker at the Centre for Molecular Biology of the University of Vienna (Max F. Perutz Laboratories) and Mathias Müller of the University of Veterinary Medicine, Vienna have published these findings in the current issue of the journal "Immunity". The work was funded by the Austrian Science Fund (FWF) in the framework of the Special Research Programme (SFB) "Jak-Stat-Signalling from Basics to Disease".
Cells respond to their environments in a number of different ways. In some cases they modify existing proteins but often the response entails the production of a new protein or group of proteins. A classic case of a response to an environmental cue is provided by the generation of the gas nitric oxide when cells are invaded by bacteria. Nitric oxide (NO) has a general antimicrobial activity and so represents one of the cell's first lines of defence against attack. Recent studies have shed intriguing light on the process by which NO is generated and have at the same time uncovered a new mechanism for regulating gene transcription. The results of Matthias Farlik in the group of Thomas Decker at the Centre for Molecular Biology of the University of Vienna (Max F. Perutz Laboratories) and Mathias Müller of the University of Veterinary Medicine, Vienna represent an extremely important contribution to our understanding of the molecular mechanism of transcriptional initiation. The work could potentially open up new avenues for the treatment of infections.
Cell checks two signals before starting its attack
Farlik and his colleagues have been investigating the way various signals are integrated within cells. When cells are infected by microorganisms, a number of different pathways are activated and these can combine, for example to cause the switching on of particular genes that are important for fighting the infection. Farlik has been studying the bacterial pathogen "Listeria monocytogenes", one of the most virulent foodborne pathogens and the causative agent of listeriosis, which has a fatality rate of about 30%. Listeria infection causes transcription of "inducible nitric oxide synthase" (iNOS), the enzyme that produces NO. Synthesis of the enzyme requires the interaction of two distinct signals, one mediated by a type of interferon (so called because it interferes with pathogens) and the other involving transcription factors that are activated by certain patterns associated with microbial pathogens. The requirement for two distinct pathways for activation makes sense to ensure that NO is not generated when it is not needed: the last thing a cell wants is to produce large quantities of a toxic gas under inappropriate circumstances, especially as NO is known to be associated with various cancers and inflammatory conditions.
As Thomas Decker puts it, "Cells must ensure that they have enough information to decide whether NO is really needed and they get this by checking the status of several different signalling pathways. The interesting question is how this happens."
What happens when the signals don't arrive at the same time?
Farlik and his colleagues have used a clever genetic trick to separate the two immunological signals for iNOS activation, enabling them to be investigated independently. It is known that gene transcription requires the assembly of a number of proteins on the so-called promoter region, the part of a gene that controls whether it is on or off. Farlik and colleagues have shown that each of the signals performs only part of the process and that both pathways must be active to form the entire complex and thus to switch on the gene. The problem is that the two signals do not always arrive at the same time. Cells solve this by an ingenious method: each of the pathways can form part of the initiation complex independently and the part-complex remains on the promoter as a sort of molecular memory. If the missing information arrives in time the gene is switched on: if not, the part-complex is removed, the initial signal is "forgotten" and the gene again cannot be switched on unless both signals are provided.