One signal molecule, many signaling roles
How does a single-celled organism ‘know’ how to respond to its environmental conditions? Understanding microbial cell signaling is one way to determine how bacteria will react in a particular setting. In the past decade, researchers have revealed a significant role for cyclic-di-GMP in bacterial signaling – so much so that the Journal of Bacteriology has devoted its first issue of the year to summarizing a recent conference on cyclic-di-GMP discoveries.
Cyclic-di-GMP is a signaling molecule largely specific to Gram-negative bacteria. First discovered in the 1980s as part of the regulation of cellulose synthesis, the molecule has since been found to regulate a number of different bacterial processes. “When I started my lab fifteen years ago, basically nobody talked about cyclic-di-GMP,” says Journal of Bacteriology Editor and academic researcher, Dr. George O’Toole. “In the past ten years, it’s really exploded on the scene as a central signaling molecule for regulating biofilm formation, as well as other bacterial characteristics, like virulence.”
What is it about this molecule that has grabbed scientists’ attention? One aspect is the role played by the molecule in regulating biofilm formation, as discussed previously on mBiosphere. “The general model is that when cyclic-di-GMP concentrations are low, cells tend to be motile, swimming or floating around. When cyclic-di-GMP levels are high, bacteria switch to a sessile or nonmoving lifestyle: a biofilm lifestyle,” explains O’Toole. “That central role in regulation has generated quite a bit of interest.”
Biofilm formation is a broad behavior that results from a number of specific cellular changes, such as motility, exopolysaccharide secretion, and cell surface adhesion production. Cyclic-di-GMP plays a role in a number of these different processes, and scientists have been elucidating the exact role for each organism. For example, research from Dr. Lotte Søgaard-Andersen’s group demonstrated cyclic-di-GMP regulation of Myxococcus xanthus motility via transcriptional control of pilA, a major component of its type 4 pilus. In Pseudomonas aeruginosa, work from Dr. Holger Sondermann’s lab (in collaboration with O'Toole) showed that cyclic-di-GMP regulates posttranscriptional modification (as well as transcript levels) of the adhesin CdrA. These species-specific mechanisms of cyclic-di-GMP regulation are an important continuing focus of many research labs.
Another focus of cyclic-di-GMP research is the complexity of signal production and detection. In Pseudomonas aeruginosa, a commonly studied opportunistic pathogen, “there are on the order of fifty different proteins that make, break, or bind cyclic-di-GMP,” says O’Toole. “Thinking about how the organism coordinates the activity of all of those proteins to give specific outputs is a really challenging question, and something we really don’t understand very well.” This is the problem being addressed by Dr. Susanne Häussler’s group, which published data on synthetic peptide arrays used to find cyclic-di-GMP binding motifs. These arrays will help determine exactly how protein receivers interact with the cyclic-di-GMP signal.
Do physical interactions solve complexity issue?
O’Toole’s research lab has also focused on the complexity of cyclic-di-GMP signaling, with a recent publication in another ASM journal, mBio. First author Kurt Dahlstrom wanted to investigate the role of diguanylate cyclase (DGC) enzymes – the enzymes that make cyclic-di-GMP – in biofilms formed by Pseudomonas fluorescens. By deleting each DGC in turn from the cell (there are about 30 of them), the research team found only four mutants had an effect in the tested conditions. “For one of these proteins, signaling specificity occurs at least in part through the ability of the DGC enzyme to physically interact with the protein that binds cyclic-di-GMP,” explains O’Toole.
Using genetic, biochemical, and structural approaches (in collaboration with the Sondermann lab), Dahlstrom and collaborators identified an alpha helix on the surface of the DGC enzyme that interacts with an alpha helix on the receptor protein. Understanding this interaction allowed the team to find interacting pairs in another organism, based on the P. fluorescens results. “We think this is one mechanism by which signaling specificity can be conferred – that the producer and the receptor proteins directly interact with each other,” O’Toole concludes.
Special Issues: More to come
These special journal issues are “a really wonderful way to highlight the current state of the field,” says O’Toole. They are often published in conjunction with a conference or meeting as a way to quickly publish leading-edge techniques and discoveries. A conference overview, several minireviews, and research articles are all included in the special issue. The articles are submitted within a month of the meeting, after which they undergo a rigorous peer review and are bundled together in a single issue. “For those who can’t make the meeting, you can sit down and read that issue of Journal of Bacteriology and really have a good flavor for the recent work going on in the field.”
Stay tuned to the Journal of Bacteriology and mBiosphere for more special issues to come in the near future. The 2015 ASM Biofilms meeting will be an upcoming focus, with its own issue in the works for publication in the next couple of months.
-- Julie Wolf