It is established that necroptosis is closely linked to inflammation. All of the details of the exact causes of inflammation, however, are still not well understood. "What causes inflammation?" asked Manolis Pasparakis, Professor at the Institute for Genetics of the University of Cologne as well as the senior author of the report. "Can necroptosis be the trigger of inflammation in some cases and how is necroptosis regulated?"
RIPK1 is one kinase that is known to induce necroptosis. Pasparakis and colleagues engineered mice that did not express RIPK1 only in keratinocytes, a type of skin cell. "We expected that the lack of RIPK1 would prevent necroptosis - but we observed the opposite. Keratinocytes in these mice died by necroptosis, causing skin inflammation. This was puzzling: How could the removal of RIPK1 cause necroptosis?" commented another author of the work, Snehlata Kumari.
The researchers found that RIPK1 acts to halt the activity of another protein, ZIBP1, which causes necroptosis. If ZBP1 is taken out at the genetic level, the necroptosis and inflammation caused by a lack of RIPK1 are alleviated.
"ZBP1 was known as a sensor of DNA that contributes to immunity against some viruses, but so far it has not been implicated in inflammation," said Chun Kim, another author of the study.
To investigate how RIPK1 inhibits the activity of ZBP1, the scientists utilized CRISPR gene editing to alter a few amino acids in a specific part of the RIPK1 protein. That alteration should interfere with the ability of RIPK1 to interact with other proteins that influence necroptosis. In mice, if that alteration occurs in all cells of the animal, it will not survive after birth. When it occurs in only the skin cells of the animal, skin inflammation results. The researchers were able to demonstrate that the skin inflammation and embryonic death were due to the genetic alteration of the amino acids.
"This was a surprising result. These three amino acids of RIPK1 prevent ZBP1 from inducing necroptosis, and this is essential for mouse survival and the prevention of skin inflammation," said Juan Lin, one of the lead authors of the report.
"We made progress, but many pieces of the greater puzzle remain unclear," said Pasparakis. "ZBP1 has been known as a viral sensor, and now our results linked it to inflammation and disease. The triggers of chronic inflammation in humans are, in most cases, entirely unclear. Why does inflammation occur in a certain person at a certain moment? Bacterial and viral infections are discussed as possible triggers of chronic inflammation. In our work, we discovered the role of ZBP1 by experimentally altering RIPK1. Now we are wondering whether viruses or bacteria could activate ZBP1 to cause inflammation."
The investigators are currently working to reveal more about the process, including the exploration of a possible connection between ZBP1 and chronic inflammatory diseases that affect humans.
To learn more about necroptosis, watch the video above, a short webinar from Abcam featuring Assistant Professor Dr. Andrew Oberst of the University of Washington, or, read this Nature review on necroptosis and inflammation.
Source: AAAS/Eurekalert! via University of Cologne, Genes & Development, Nature