by Brooke Napier
Chlamydia, everyone has heard of it, very few people understand it.
This might seem like a surprise since Chlamydia subspecies (spp.) are responsible for one of the most common sexually transmitted infections worldwide. To put it in perspective, there are 2.8 million new cases of Chlamydia in the United States each year. That’s 10% of the population that is suffering from this disease a year, in one of the richest countries in the world. If you’d like the gritty details of the symptoms go here.
So, why do we know so little about the culprit behind this epidemiological mess?
Chlamydia spp. are tricky little guys.
First, these 3 subspecies are obligate intracellular human pathogens, or human pathogens that are only capable of growing and reproducing inside of cells – think parasites, but the bacterial version. Therefore, you cannot grow Chlamydia spp. in broth, which makes working with this bacterium especially challenging. Additionally, Chlamydia spp. has two distinct parts of its life cycle, 1) elementary body (EB) and 2) reticulate body (RB). See the figure below. The infectious agent is the EB, which binds and enters the host cell. Once in the host cell it resides within a protected membrane-bound inclusion, where it differentiates into a metabolically active reticulate body (replication time!).
Because of these attributes, there is no straightforward genetic system with which to manipulate the Chlamydia genome. Therefore, scientists who work with Chlamydia cannot easily knock out, mutate, alter, or add to the Chlamydia genome – this renders us scientists pretty much useless.
Why does this make scientists useless?
In order to study host-pathogen interactions we need to know the proteins in the bacteria that are required for infection. If we can identify these proteins we can more easily understand why and how the bacteria is causing disease in the patient. Without genetic manipulation it is very difficult to study bacterial proteins.
Then why are we talking about it?
Great question. I brought this up because Rosmarin et al. published this week that they have found a way to identify genes required for infection of Chlamydia.
How? By genetic manipulation of the host cell.
Briefly, they performed a loss-of-function genetic screen in human haploid cells to identify mutated host cells that were now resistant to Chlamydia. This means that they knocked out (ideally) one host protein at a time and looked to see if Chlamydia could still infect this line of host cells. Therefore, that one gene has lost its function.
Host cell lines that were resistant to Chlamydia identified genes that the host requires for Chlamydia infection. By identifying required host proteins for Chlamydia infection, you begin to understand what host proteins Chlamydia manipulates to infect and cause disease in humans.
What host proteins did they find that are required for infection?
With this screen, they found 3 host genes (B3GAT3, B4GALT7, and SLC35B2) that are involved in the sulfation of host proteins. Sulfation is important for multiple host processes including (ex: viability and neural development in C. elegans, cartilage homeostatis, etc). Interestingly, sulfation is also important for binding and entry of HIV and Chlamydia.
Although the amount of heparan sulfate on the host cell surface has been previously shown to be involved in attachment of Chlamydia to the host cell surface, they found that the degree of Chlamydia attachment strongly correlates with the amount of sulfation on the host cell beyond heparan sulfate. These data suggest that sulfation may play a more important role in Chlamydia attachment and infection than originally thought. Additionally, this indicates that unidentified host cell sulfated macromolecules must contribute to infection.
An interesting tidbit. They measured "attachment" by immunofluorescence microscopy. Above are two images, the first is wild-type (WT) human cells (purple) infected with Chlamydia (green). You can see there is a lot of attachment of the bacteria to the human cells. However, in one of the knock-outs created in this screen (B3GAT3-/-) you can see that Chlamydia can no longer attach to the outside of the host cell.
Though this paper did not identified the most novel host cell proteins required for Chlamydia infection, the tool that they have developed can be used to further investigate host-pathogen interactions with a very difficult bacteria to understand/study. As they say in closing, “genes identified in this screen […] may find application in other areas of biology as well”.
I'm excited and intrigued to read any follow-up research being done on host genes identified in this screen. Future posts!
Rosmarin DM, Carette JE, Olive AJ, Starnbach MN, Brummelkamp TR, & Ploegh HL (2012). Attachment of Chlamydia trachomatis L2 to host cells requires sulfation. Proceedings of the National Academy of Sciences of the United States of America PMID: 22675117