Week 9: TB Membrane Madness!

In my post from week 2, I briefly outlined one of the most unique properties of Mycobacterium Tuberculosis (MTB for short): its nifty cell membrane. Ever since that post, I’ve really, really wanted to get back to that. Gather ’round my friends, for the time is nigh!

Hopefully you remember this SEM photo:

A scanning electron microscope image of a macrophage consuming Mycobacterium Tuberculosis. Poor phage – they’ve got him right where they want him.

Like a Trojan horse: MTB has an impressively complex mosaic of components across its mostly lipid based plasma membrane. It boasts multiple receptors on its surface that facilitate its interaction with macrophages; the sheer variation of receptors is unusual, allowing it to attract and be engulfed by phages easily. For many pathogens, this would be a selected-against trait, meaning bad for the organism’s survival. It’s like being the girl in the horror movie that goes down to the creepy basement and says, “Hello? Is anybody there? Helloooo…” However, MTB wants to be engulfed by a phage, because that’s where the magic happens (Todar, 2012). Sneaky!

Infiltrated by the enemy: While our macrophages (WBCs) have little trouble consuming the buggers, once inside its phagosomes, the bacteria are more or less impervious to the macrophage’s attempts to digest it. By manipulating the properties of the plasma membranes on the phagosome and disorienting any approaching lysosomes, MTB basically hijacks the cell. Conversely, it has no qualms about taking in vesicles containing smaller molecules for nutrition – allowing the TB bacteria to hide and multiply inside the macrophage (Todar, 2012).

Welp, this just sounds nuts. Macrophages have ONE job. (OK, a few more than one, but you know what I mean. They’re really good at immune stuff.) How are TB bacteria able to overcome such an efficient and powerful army of warriors? We need explore this subject a little deeper to find out.

Endoterrorism: Once the bacterium enters the phage, it immediately hops inside a phagosome and starts secreting a special eukaryotic-like phosphatase called SapM and a kinase called PknG that prevents fusion of the macrophage’s toxin-carrying lysosomes with the phagosome. It’s important to know that  phagosome-lysosome fusion is reliant on a host trafficking event that is regulated by phosphatidylinositol 3-phosphate (PI3P), which is generated locally on the phagosome’s membrane. If PI3P isn’t there, the lysosome will be totally confused. MTB’s SapM phosphatase works by dephosphorylating PIP3 on the phagosome’s membrane.  Like most mycobacterial kinases, PknG is a transmembrane molecule. The kinase domain actually sticks out into the cytosol. Interestingly, PknG is the only soluble kinase found in ALL pathogenic mycobacteria. As a kinase, it phosphorylates host molecules involved in phagosome-lysosome fusion, causing a conformational change, which strips them of the ability to do their job. For these reasons, there is a huge interest in developing an inhibitor for PknG. In MTB especially, functional PknG is required for its survival (Pieters, 2008).

Q: Hold up… how do we know that DNA coding for the kinase PknG is found in the genomes of ALL pathogenic mycobacterium?

A: Metagenomics 🙂 See my other posts about that!

Q: So wait… could an inhibitor for PknG be a solution for treating antibacterial resistant TB? Or even a solution that can be used for all mycobacteria?

A: Yes. OMG yes, it totally could. Antibiotics target MTB itself, allowing it to adapt over time. An inhibitor would target MTB’s product, PknG, which is released into the phagosomal cytosol – beyond MTB’s wicked membrane. Once outside the bacterial cell, it’s fair game (Pieters, 2008).

Q: What about SapM? Won’t that still be acting on PI3P?

A: Apparently, MTB needs PknG to be active in order to overwhelmingly sabotage phagosome-lysosome fusion. SapM alone will not do the job. With PknG unable to phosphorylate stuff (a dose of its own medicine if you ask me!) the normal fusion process will commence, thus destroying the bacterium (Pieters, 2008).

Q: Dude!! Why hasn’t this been found or artificially made yet?!

A: It kinda has. A very selective inhibitor for PknG is being tested. It is selective enough for use in eukaryotic cells, as it targets a domain on PknG which is not found in eukaryotic kinases (Scherr et al., 2007). It takes a long time to approve stuff for medical use; but it’s exciting to know that humanity is on the brink of a real breakthrough in the treatment of Tuberculosis.

References

1. Pieters J. Mycobacterium tuberculosis and the Macrophage: Maintaining a Balance. Cell Host & Microbe. 2008;3(6):399-407. doi:10.1016/j.chom.2008.05.006.

2. Scherr N, Honnappa S, Kunz G et al. Structural basis for the specific inhibition of protein kinase G, a virulence factor of Mycobacterium tuberculosis. Proceedings of the National Academy of Sciences. 2007;104(29):12151-12156. doi:10.1073/pnas.0702842104.

3. Todar, Kenneth (2012). “Mycobacterium tuberculosis”. Todar’s Online Textbook of Bacteriology. http://textbookofbacteriology.net/tuberculosis.html

Bonus video!!!!

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