When you think of Chlamydia you usually think of the human sexually transmitted disease. Well, other types of Chlamydia can infect humans and animals, which are not sexually transmitted. Chlamydia psittaci is one such bug that mainly infects poultry where it causes respiratory and digestive tract infections. In other animals, like cattle and sheep, it can cause abortion and kill young animals. This can cause widespread issues for agriculture. If you like your steaks, lamb, or even wool, you should start getting concerned at this point. Oh wait, there’s more! In agriculture, people who take care of and process these animals can inhale the C. psittaci when in close contact with the feces and urine of the infected animals, showing that this bacteria can transfer between species (zoonotic). Once infected, they can develop respiratory infections and bacteremia. It doesn’t end here though because C. psittaci can enter phagocytic macrophages and can survive the intracellular environment, leading to persistent infections.
C. psittaci has two developmental stages, the extracellular elementary body (EB) and the intracellular reticulate body (RB). Not only do macrophages, and other phagocytic cells, recognize and take up the bacteria to attempt to clear the infection, but also the EB can bind to the surface of the cell and is then internalized. Once inside it turns into the RB that can replicate and survive inside the cell. It can also switch back and forth between the two forms. They continue to replicate inside a vacuole called an inclusion until they are released by cell lysis or extrusion. They are then free to circulate to more cells and repeat the process.
You now might be wondering, how do they survive inside the cells and are not broken down and degraded? They can avoid fusing with lysosomes, where they would be degraded, and they move around microtubules to ensure survival and growth, which then leads to the aforementioned cell lysis or extrusion. Inclusion membrane proteins maintain the stability of these vacuoles which protect the bacteria and also prevent apoptosis (programmed cell death) and autophagy (a survival strategy that will degrade damaged cell parts or intracellular bacteria). The exact mechanism of action for how Chlamydia can prevent lysosomal fusion is still unknown. Hence, Huang and colleagues set out to determine the role of a well-conserved inclusion membrane protein, CPSIT_0842, in macrophage autophagy and apoptosis.
First, they expressed and purified the CPSIT_0842 for further use. They were then able to stimulate macrophages with the protein and showed that different autophagy genes were upregulated after exposure in a dose-dependent manner. They were able to see using immunofluorescence and transmission electron microscopy that not only was autophagy initiated, but also double membrane, onion-like, autophagic vacuoles appeared in the protein-stimulated cells, but not the unstimulated ones. At this point it seemed like the process was going as it should and that the presence of the bacteria inside the cells was actually not inhibiting the autophagy and apoptosis that would help prevent the development of a persistent infection.
So now that we have more autophagosomes we need to see if the autophagosome and lysosome will fuse properly in macrophages, which would allow for the proper degradation of the sick parts of the cell. The Rap (autophagy enhancer) treated groups showed normal autophagosome-lysosome fusion, while the BafA1 (autophagy inhibitor) and CPSIT_0842 treated groups showed inhibition of this function. It is known that CPSIT_0842 can synergistically block autophagic flux by working with BafA1, an inhibitor of vacuolar H+-ATPase, which can block the formation of autolysosomes. These experiments determined that CPSIT_0842 induces autophagosome accumulation, the induction of autophagosomes, and the inhibition of autophagic flux. Another piece of the puzzle comes into place now that we know that the decrease in autophagic flux was due to the poor fusion of the lysosome and the autophagosome: what is the deal with that incomplete autophagy?
Multiple signaling pathways involving mTOR, mitogen-activated protein kinase (MAPK), and phosphatidylinositol 3-kinase (PI3K) regulate autophagy. Through testing the levels of phosphorylated and non-phosphorylated versions of different parts of the signaling pathways, they found that MAPK/ERK/mTOR are all pathways involved in the incomplete autophagy caused by CPSIT_0842. I think with this we have heard enough about this autophagy; what is happening with the apoptosis?
Some anti- and pro-apoptosis markers can be tested to understand how CPSIT_0842 is actually affecting apoptosis. The anti-apoptosis marker, Bcl-2, was inhibited with the treatment, and the pro-apoptosis markers Bax and activated caspase-3 were upregulated, confirming that CPSIT_0842 induces apoptosis. Is CPSIT_0842 the only factor affecting apoptosis or does the autophagy effect have anything to do with it, too? The autophagosome accumulation was shown to also promote macrophage apoptosis, showing that CPSIT_0842 affects apoptosis in many ways. Overall, CPSIT_0842 promotes incomplete autophagy, disrupts autophagic flux, and enhances macrophage apoptosis, which may be regulated by MAPK/ERK activation. So, is Chlamydia killing you from the inside out? It is definitely causing death to your macrophages!Overall, we now have more insight into the manipulation of the host cells by C. psittaci leading to persistent systemic infections in humans and agriculturally-produced livestock animals. There is currently no effective vaccine available. The understanding of the inclusion membrane protein, CPSIT_0842, in C. psittaci, can aid in the treatment and prevention of this disease and can potentially help develop treatments for other persistent bacterial infections.
Huang Y, Li S, He S, Li Y, He Q, Wu Y. Chlamydia psittaci inclusion membrane protein CPSIT_0842 induces macrophage apoptosis through MAPK/ERK-mediated autophagy. Int J Biochem Cell Biol. 2023 Apr;157:106376. doi: 10.1016/j.biocel.2023.106376.
Article authors: Autumn Dove. Autumn is a Ph.D. candidate at the University of Florida. Her research is focused on finding alternative treatments for antimicrobial-resistant infections.
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