Scientists have published complementary studies in Nature Communications that have greatly advanced our understanding of a protein called MLKL. They learned that variants of the gene that makes the protein can influence a person's risk of developing an inflammatory disease, and have identified steps in MLKL activation, which is critical to a cell death process called necroptosis.
Cells have to be able to recognize pathology and trigger cell death if necessary; cells that are infected with bacteria or inflamed may kill themselves off to protect their neighbors. Necroptosis is one such cell death pathway, and it is tightly controlled; MLKL is important for that regulation.
"While MLKL and necroptosis protect our bodies from infections, excessive necroptosis has been linked with inflammatory conditions such as inflammatory bowel diseases," said Associate Professor James Murphy. "Our research team has taken several complementary approaches to better understand how MLKL functions - which could improve the understanding and treatment of diseases involving excessive necroptosis."
In one study, the researchers applied imaging tools to visualize MLKL while cells became necroptotic, and two checkpoints in the process were identified.
"We could see how MLKL changed its location as necroptosis occurred, clumping and migrating to different parts of the cell as the cell progressed towards death," said the leader of that report, Dr. Andre Samson. "Intriguingly, we could see activated MLKL gather at the junctions between neighboring cells - potentially suggesting a way for one dying cell to trigger necroptosis in surrounding cells, which could be a form of protection against infections."
In work led by Dr. Joanne Hildebrand and Dr. Maria Kauppi, the investigators identified an MLKL variant. When they expressed the variant in lab models, a lethal inflammatory disease occurred.
"We discovered this form of MLKL contained a single mutation in a particular region of the protein that made MLKL hyperactive, triggering necroptosis and inflammation," said Hildebrand.
"By searching genome databases, we discovered similar variants in the human MLKL gene are surprisingly common - around ten percent of human genomes from around the world carry altered forms of the MLKL gene that result in a more-easily activated, more inflammatory version of the protein."
The searched to see whether the inflammation-linked MLKL variant was connected to disease. "We looked more closely at databases of genomes of people with inflammatory diseases to understand the prevalence of MLKL variants. Indeed, people with an autoinflammatory condition chronic recurrent multifocal osteomyelitis (CRMO) were much more likely to carry two copies of a pro-inflammatory variant of the MLKL gene than people without an inflammatory disease. This is the first time changes in MLKL have been associated with a human inflammatory disease," noted Hildebrand.
In another study examining MLKL is vertebrate species, researchers found significant differences between the versions carried by different animals. Often, proteins that are really important to biology are very similar, or highly conserved, from one species to another, which was not the case here.
"To our surprise, the structures of MLKL were quite different between different vertebrate species - even between closely related species such as rats and mice. In fact, rat MLKL is so different from mouse MLKL that the rat protein cannot function in mouse cells - which is surprising as many proteins are interchangeable between these two species," said the study leader, Dr. Katherine Davies.
"We think that evolutionary pressures such as infections may have driven substantial changes in MLKL as vertebrates evolved. Animals with variant forms of MLKL may have been able to survive some pressures better than other animals, driving changes in MLKL to accumulate, much faster than for many other proteins," added Davies. "Together with the data for human variations in MLKL, this suggests MLKL is critical for cells to balance beneficial inflammation, which protects against infections, with harmful inflammation that causes inflammatory diseases."
Sources: AAAS/Eurekalert! via Walter and Eliza Hall Institute, Nature Communications Hildebrand, Kauppi et al, Nature Communications Samson et al, Nature Communications Davies et al