Role of mycobacterial histidine biosynthesis in murine tubercular infection and pathogenesis

Dr. Abhisek Dwivedy’s interview with Bio Patrika hosting “Vigyan Patrika”, a series of author interviews. Dr. Dwivedy is a native of Angul, Odisha. He was born and raised in Chakradharpur, a small railway settlement in Jharkhand. After completing his schooling, he pursued the 5-years Integrated Masters Programme in Biology at the National Institute of Science Education and Research, Bhubaneswar. He undertook his doctoral research in structural and infectious disease biology at the National Institute of Immunology, New Delhi, where he is currently working as a post-doctoral Research Associate. Here, Abhisek talks about his first author work on “De novo histidine biosynthesis protects Mycobacterium tuberculosis from host IFN-γ mediated histidine starvation” published in Communications Biology.

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How would you explain your paper’s key results to the non-scientific community?

One can visualize infectious diseases as a set of two events- a burglary followed by a tug of war. A pathogen enters the host’s body and attempts to utilize the host resources to support their growth and multiplication. The host on the other hand uses various cellular mechanisms to prevent this unauthorized usage of their resources and also mounts an immune response to neutralize the pathogen. Mycobacterium tuberculosis (Mtb), the bacteria that causes tuberculosis in humans, siphons up a number of key resources from its hosts such as amino acids, iron and lipids for supports its own growth. There are 20 canonical amino acids in the living world. Many bacteria, fungi and plants can generate all the amino acids ab initio, however, mammals including humans lack the cellular machinery for the generation of 9 amino acids- histidine, isoleucine, leucine, valine, methionine, phenylalanine, tryptophan, lysine and threonine. These amino acids are therefore necessary to be absorbed through dietary means to maintain a healthy living and thus are termed as essential amino acids.

In this context, using a murine model of tuberculosis, we investigated whether Mtb utilizes its own machinery for histidine generation or relies on the free available histidine within the host. Our study indicated that in the early phases of infection, Mtb imports the free available histidine in the host cells. However, after the host’s adaptive immunity kick-starts, an interferon gamma mediated molecular mechanism lowers the levels of free histidine within the host cells. This strategy is utilized by the host to starve Mtb of free available histidine and thus minimize the infection in the process. Mtb senses this decrease in histidine availability and switches on its own machinery to generate histidine ab initio in order to fulfill its metabolic requirements. Therapeutic interventions can be developed to impede the functioning of this histidine synthesizing machinery by utilizing small molecule inhibitors, so as to clear tubercular infection.

What are the possible consequences of these findings for your research area?

For the past decade, our laboratory has been elucidating the 3D structures of the mycobacterial enzymes involved in the biosynthesis of histidine. Of note, previous high-throughput and computational studies have predicted the essentiality of histidine biosynthesis enzymes (Agüero F. et al., Nat Rev Drug Dis, 2008). However, the importance of histidine as a crucial resource, important for the survival of Mtb and its pathogenesis remained unclear. Our study not only provides evidence for the importance of histidine towards Mtb’s survival within a host but also reports an uncharacterized mechanism of host immunity. This study augments the very idea that designing novel inhibitors against the histidine biosynthesis enzymes using a structure-guided approach may aid in developing novel anti-tuberculosis compounds.

This study augments the very idea that designing novel inhibitors against the histidine biosynthesis enzymes using a structure guided approach may aid in developing novel antituberculosis compounds.

What was the exciting moment (eureka moment) during your research?

This study is the culmination of four years of never ending experimentations and failures. Back in 2017-18, using immunoblotting, we observed an increase in the expression levels of histidine sequestering enzymes (Histidine ammonia lyase/HAL & Histidine decarboxylase/HDC) within mice lungs post 20 days of Mtb infection. The samples used for immunoblotting were whole lungs lysates. This raised a key question- is the observed increase in expression of HAL and HDC occurring throughout the lungs or is it specific to the lung lesions filled with Mtb? To answer this, we utilized immuno-histochemistry of lung sections and observed that the increased expressions of HAL and HDC are limited specifically to the lung lesions as evidenced by a co-localization with the expression of Mtb HspX (Figure 2C of the publication). While there are no specific “eureka” moment(s), this result in particular, has been instrumental in boosting our confidence.

What do you hope to do next?

I plan to continue as a post-doctoral researcher abroad in order to expand my scientific and technical skillset. As observed in our study and in a previous study (Zhang Y. J. et al., Cell, 2013), host depletes histidine and tryptophan, respectively, in order to starve Mtb. The end products of histidine and tryptophan catabolism- urocanate and kynurenine, respectively are known to induce anti-inflammatory responses by recruiting regulatory T cells. Regulatory T cells are known to dampen the initial pro-inflammatory responses, thus acting as a feedback inhibitor. I am interested in understanding the effects of similar immuno-metabolic crosstalk and the underlying immune regulation in pulmonary diseases caused by opportunistic pathogens, Pseudomonas aeruginosa in particular.

Where do you seek scientific inspiration?

While my scientific experience has diversified over the past decade, nothing has shaped my research interests more than my graduate education. As a student of the 5 years Integrated Masters program at the National Institute of Science Education and Research, I was privileged to learn the basics of immunology and infectious disease biology while carrying out a dissertation research that spanned over two years. The major inspiration towards pursuing this study was the constant support and encouragement by Dr. Bichitra, who despite being a structural biologist helped us design, troubleshoot and shape our study.

How do you intend to help Indian science improve?

Anti-microbial resistance is an emerging field of concern in human healthcare. It is estimated that by the mid of 21st century, antibiotic resistant and opportunistic pathogens will be the leading cause of deaths, primarily in tropical and third world countries. India witnessed 4.4 million deaths attributed to infectious diseases in 2019. I plan to establish myself as an independent researcher, where my primary focus would be to understand the metabolic cross-talk between host and opportunistic pathogens associated with hospital-acquired infections such as Pseudomonas and Chlamydia.

Reference

Dwivedy, A., Ashraf, A., Jha, B. et al. De novo histidine biosynthesis protects Mycobacterium tuberculosis from host IFN-γ mediated histidine starvation. Commun Biol 4, 410 (2021). https://doi.org/10.1038/s42003-021-01926-4

Dr. Bichitra Biswal lab: http://www.nii.res.in/research/bichitra-k-biswal

Edited by: Sukanya Madhwal and Anjali Mahilkar

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