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Scientists elucidate the mechanism behind Mycobacterium hijacking Macrophages for its own survival

  • Signalling casacade in an eukaryotic cell

Photo: Siddharth Kankaria

In the movie “Terminator: The Rise of Machines”, the character Terminatrix manipulates the Cyborgs tweaking them to work against humans and to her own advantage. Now, scientists have discovered that some strains of bacteria could do the same to some of our cells. Mycobacterium tuberculosis, the bacterium that causes tuberculosis, is one such. It manipulates the macrophages, a type of white blood cell that hunts and engulfs invading pathogens, to act as bacterial reservoirs and provide a survival niche. This niche not only provides the bacteria with nutrients, but also helps evade the normal immune response. In a recent study, a team of scientists from the Indian Institute of Science, Bangalore, has explored the mechanism behind the manipulation of macrophages by this bacteria.

Macrophages scout for foreign pathogens and engulf them, providing us with innate immunity. However, when specific strains of Mycobacterium infect these macrophages, they reprogram these cells to act as safe havens and to obtain nourishment from them. These modified macrophages are called Foamy Macrophages (FM) and are found in the granulomas of the lungs of infected individuals. “ Unlike other bacteria, Mycobacterium tuberculosis is not explicitly pathogenic, but it can lie dormant. Even today, it is one of the most dreaded pathogens with 6.1 million individuals newly infected with TB and 1.4 million dead due to the disease in 2015 alone. Thus, it is not surprising that this bacterium uses all means available at its disposal to manipulate the host environment”, explains Ms. Kasturi Mahadik, a research scholar at the Department of Microbiology and Cell Biology, IISc.

The researchers have worked out the molecular mechanism involved in the generation of Foamy Macrophages. The elucidation of this pathway could generate new and effective targets for drug development to cure tuberculosis. The study found that the bacterium interacts with a receptor found on the membrane of macrophages called TLR2 (Toll-like receptor). These receptors recognize foreign substances and pass on appropriate signals to the cells of the immune system through one of the signaling pathways called NOTCH1signaling pathway. “While viruses are legendary host modifiers, bacteria have been known to manipulate the host epigenetic machinery - host genes involved in cell cycle progression, cell ageing, survival, inflammation and immunity being important targets for such epigenetic control. Study of mycobacteria modulating the host epigenome is now gaining increasing recognition”, remarks Ms, Mahadik on the importance of the study.

Signaling pathways govern basic activities of cells and co-ordinate cell actions by allowing them to sense or perceive changes in their microenvironment and bring about necessary changes in activities like gene expression. The signal from outside of the cells is transmitted via a chain of mediators, the end point of which may culminate in the expression of certain genes, whose products are required to respond to the changes in the microenvironment.

Foamy Macrophages contain lipid bodies that provide nutrients and anti-inflammatory mediators to help the bacteria evade our normal immune response. Genes involved in lipid biosynthesis and lipid droplet synthesis are “regulated” or “turned on” for the purpose. Conventionally, gene regulation is brought about by a protein acting as an activator or a repressor of gene function.

The study found that either condensing or relaxing a chromosome regulates the genes required for the generation of Foamy Macrophages. Genes reside on chromosomes, and when chromosomes tightly coil up, they are not free to be transcribed and are therefore switched off. On the other hand, a relaxed and free chromosome can be bound by positive activators, which assist in gene expression and can be switched on. The DNA in the chromosome is wound around proteins called histones. Action of methylase enzyme at certain places on histones leads to tightening of chromosomes and silencing of the genes, while actions of a demethylase enzyme can remove the methyl groups on the histones and relax the chromosome for gene expression.

Another important aspect that the study found is the role of a demethylase enzyme namely JMJD3 (Jumonji Domain containing protein) that is responsible for demethylating histones resulting in the expression of the above mentioned genes. JMJD3 is repressed by a complex of proteins and one of them called MINT/spen was not found to be produced in Foamy Macrophages, thus allowing JMJD3 to express the genes involved in Foamy Macrophage development. Also, the gene to produce MINT/spen is inhibited by the action of MUSHASI - an RNA binding protein, which binds mRNA at a specific site. MUSASHI restrains MINT/spen from getting translated and the absence of MINT/spen leaves JMJD3 free to demethylate histones. Thus, the chromosome gets relaxed at the genes required for FM generation and they get expressed. The lipid droplets that provide a survival niche to the bacterium and help evade immune responses are produced. And finally, MUSASHI is NOTCH1 responsive - meaning that it could be expressed by NOTCH -1 signaling, which enables the bacterium to manipulate.

Though it is too early to claim any therapeutic intervention based on epigenetic modifications of the host cells, Ms. Mahadik opines - “Histone marks, DNA methylation, that are marks of epigenetic modification could serve as biomarkers for the disease. These marks are going to remain in the host genome and thus could be used as epigenetic memory in the design of vaccines.”

This research is a key insight that could drive the direction for drug development against TB. “Study of factors leading to reactivation from latent TB into active TB could be vital to completely eradicate the disease. Knowledge of mycobacteria governed epigenomics would also aid the understanding of pathologies associated with the disease progression. Further, mycobacterial infection results in a predisposition to several other infections and it would be fascinating to note the impact of epigenetic factors during co-infections”, signs off Ms. Mahadik stressing the importance of such studies.