The endoplasmic reticulum (ER) is a crucial organelle present in eukaryotic cells, including human cells. The ER plays a vital part in making sure proteins are properly constructed, and that they get to the right places in cells. The ER is a large and dynamic structure that spreads around the cell, and can enable communications between different cellular parts and organelles, helping to control many processes. Now scientists have shown that a bacterial pathogen can infect cells and interact extensively with the ER. This is the first known example of direct, interkingdom contact between a bacterial pathogen and eukaryotic organelle membrane, according to the researchers. The findings have been reported in the Journal of Cell Biology.
In this work, the investigators used the research model Rickettsia parkeri; it acts as a safe but similar substitute for the bacterial pathogen Rickettsia rickettsii, which causes Rocky Mountain Spotted Fever. This disease must be promptly treated or it can be fatal.
R. parkeri can spread from one cell to another through special cellular contact sites. Once R. parkeri is engulfed by a neighboring cell, it spreads the infection. First study author Yamilex Acevedo-Sánchez observed that high levels of R. parkeri moved close enough to the ER to allow for inteeoganelle communication. (This distance is known to be between ten and eighty nanometers.) Further work showed that the connection was not immune related. The stable links were surprising to see, because the ER constantly remodels in a dynamic way, forming and breaking connections continuously.
The investigators generated advanced images of the interactions between Rickettsia and the ER, and analyzed the images with machine learning. Three dimensional models were created, which revealed that less than 5 percent of R. parkeri was connected to the ER.
It is known, however, that R. parkeri has two states, one of which is motile, while another is not. The R. parkeri that is not motile, and lacks an actin tail, cannot move to another cell. But even at the height of infection the level of R. parkeri with tails does not exceed 15 percent.
Nonmotile R. parkeri is the only form that interacts with the ER.
Now, the researchers have to determine what this interaction means.
“It’s not just bacteria accidentally bumping into the ER. These interactions are extremely stable. The ER is clearly extensively wrapping around the bacterium, and is still connected to the ER network,” said senior study author Rebecca Lamason, an Associate Professor of Biology at MIT. “It seems like it has a purpose—what that purpose is remains a mystery.”
Sources: Massachusetts Institute of Technology (MIT), Journal of Cell Biology
