Recycling: It Can Save Your Life

...assuming you're a lung cell.  I recently read an article published in the Public Library of Science (link) about how Pseudomonas aeruginosa infects people.  The bacterium can infect the lungs of people with other preexisting lung conditions, including pneumonia and chronic obstructive pulmonary disease (COPD).

Pseudomonas aeruginosa is an interesting infection, mostly because it requires a bit of sophistication on the part of the bacterium.  In the lungs, there is a protective mucous membrane that coats the outer layer of cells.  This outer layer of cells is known as the epithelium.  This mucous prevents most everything that is foreign to the lungs from directly contacting the epithelium, which can prevent many kinds of damage and infection.  Pseudomonas aeruginosa can't break through this layer, so it devises a strategy: send specially manufactured vesicles that can.  These vesicles have proteins on the surface that allow them to bind and fuse with cells in the epithelium, and they contain proteins that cause cellular change.  They are somewhat analogous to so called "bunker buster" bombs, which are able to penetrate a formidable outer shell and deliver a payload to the inside of the structure.  Only these are released with little guidance.


As for the payload, Pseudomonas aeruginosa causes a slight but severe change in infected cells.  In healthy cells there is a protein, namely CFTR, that regulates the amount of mucous there is in the lungs.  The protein must reside on the surface of cells to have any effect.  As part of normal cellular activities, this protein is occasionally ubiquitinated, meaning a ubiquitin group is bound to it.  This triggers a pipeline of events to occur.  The ubiquitinated protein is first sequestered from the cell membrane.  It then can follow one of two paths.  In one path, the ubiquitin group is removed, and the protein returns to the cell membrane.  In the other path, the ubiquitin group remains bound, and the protein is eventually degraded.  In healthy cells, these two paths run in tandem.  This is necessary to remove CFTR proteins from the membrane that no longer function, and are essentially just wasting space on the membrane.

What was previously known is that Pseudomonas aeruginosa infection somehow selectively shuts down the path that causes CFTR to return to the cell membrane.  As such, all the sequestered CFTR ends up being degraded.  The cell ends up degrading more CFTR than it can spare, and proper function is lost.  Without CFTR to regulate mucous properly, mucous builds up.  This is beneficial to Pseudomonas aeruginosa, as the once protective mucous ends up being its home, but this is at the detriment of its victim.  This mucous buildup is how people can literally drown in their own lung fluids, not to mention that it makes for a friendly environment for other opportunistic pathogens.

This paper investigated exactly how Pseudomonas aeruginosa is able to shut down the recycling pathway, forcing all ubiquitinated CFTR to be degraded.  The authors found that the vesicle payload contains a protein called Cif.  Although they were unable to determine exactly how, they found that Cif prevents the enzyme that deubiquitinates CFTR from functioning properly.  The reason why is somewhat complicated.  There is another protein, namely G3BP1 that normally inhibits the deubiquitination enzyme from function.  This protein is naturally occurring in lung cells, and it is presumably necessary for normal function.  G3BP1 can bind to the deubiquitination enzyme, temporarily preventing it from functioning.  In healthy cells, G3BP1 does not bind with very high affinity, presumably without other naturally occurring factors to help it along, so the net effect on the deubiquitination enzyme is minimal.

This is where Cif comes in for infected lung cells.  Cif stabilizes the interaction between G3BP1 and the deubiquitination enzyme, preventing the enzyme from functioning for much longer than with G3BP1 alone.  The effect is that the overwhelming majority of ubiquitinated CFTR never ends up getting deubiquitinated, as the deubiquitination enzyme has been inhibited by the interaction between G3BP1 and Cif.

I have a few questions regarding this mechanism, which could make for good future work.  For one, I suspect that some people are naturally immune to Pseudomonas aeruginosa infection, simply because they have mutations in either G3BP1 or the deubiquitination enzyme that prevent Cif from binding well.  It should be possible to conduct a clinical study on people with preexisting lung disorders, looking for those who for some reason never develop Pseudomonas aeruginosa infections, despite the significantly likelihood.

I also think that knowledge of this mechanism could lead to a novel drug treatment that prevents Cif from working properly.  Such a drug would somehow induce a conformational change in Cif that would prevent its proper binding to G3BP1.

The overall infection mechanism could be exploited for other purposes, as well.  Classically, specific drug delivery is a problem.  However, Pseudomonas aeruginosa is able to release vesicles that seem specific to lung tissues and contain specific payloads for said tissues.  With genetic engineering, it should be possible to change the payload to be whatever is necessary at the moment.  The result would be a targeted drug delivery system, injecting a specific drug into a specific tissue at a (below) microscopic level.  Perhaps we could even deliver an anti-Cif drug via the same mechanism used to inject Cif in the first place, of all ironic things.