Dr. Kanchan Jaswal’s interview with Bio Patrika hosting “Vigyan Patrika”, a series of author interviews. Jaswal has done Ph.D. with Dr. Rachna Chaba at the Indian Institute of Science Education and Research (IISER) Mohali. In her PhD, she investigated the interconnection between carbon metabolism, electron transport chain and envelope redox homeostasis in Escherichia coli. In future she would like to address key questions in the area of host-gut microbiome interactions. As part of her PhD, Kanchan published a paper entitled “Metabolism of long-chain fatty acids affects disulfide bond formation in Escherichia coli and activates envelope stress response pathways as a combat strategy” as a first author in PLOS Genetics (2020).
How would you explain your paper’s key results to the non-scientific community?
The human host provides a nutrient-rich environment for the survival of various microorganisms, including disease-causing bacteria (termed as pathogens). Long-chain fatty acids, fatty acids with ≥ 12 carbon atoms (abbreviated here as LCFAs), are obtained from host tissues and used as an energy-rich nutrient source by several bacterial pathogens, such as Vibrio cholerae (causes cholera), Salmonella typhimurium (causative agent of diarrhea), Mycobacteriumtuberculosis (causes tuberculosis) and Pseudomonas aeruginosa (causative agent of chronic lung infections). Using Escherichia coli, a gut bacterium, as a model, we are trying to understand how the use of LCFAs as a nutrient source, i.e., LCFA metabolism affects bacterial physiology.
E. coli has two cellular compartments: the cytoplasm and the envelope (Figure 1). The envelope surrounds the cytoplasm and is in direct contact with the external environment. It protects the cell from environmental stresses and is the site for a myriad of cellular functions. Many proteins that reside in the envelope require the formation of disulfide bonds (bonds formed between two sulfur atoms) for their function. The proteins, DsbA and DsbB are the components of the disulfide bond-forming machinery, which form disulfide bonds in envelope proteins. In this process, DsbA-DsbB takes up electrons from substrate proteins and become reduced. Reduced DsbA-DsbB then transfers electrons to ubiquinone, a lipid-soluble component of the electron transport chain, thereby becoming oxidized again to carry out another round of disulfide bond formation. Besides transferring electrons from the disulfide bond-forming machinery, ubiquinone also transfers electrons derived from metabolism of nutrient sources. From ubiquinone, the electrons are finally transferred to molecular oxygen by terminal oxidases (Figure 1).
In this study, we showed that LCFA degradation increases electron flow towards ubiquinone, which renders ubiquinone unavailable to take up electrons from DsbA-DsbB machinery, compromising disulfide bond formation (Figure 2). Because LCFA metabolism increases the infectivity of several pathogens and many virulence factors (proteins required for a bacterium to cause disease) require disulfide bond formation for their activity, we suggested that bacteria must be employing clever strategies to deal with issues in disulfide bond formation. In this direction, we showed that problems in disulfide bond formation are sensed by a signaling pathway (the Cpx system), which increases the levels of ubiquinone in the cell; this feedback likely helps restore disulfide bonds in proteins (Figure 2).
What are the possible consequences of these findings for your research area?
LCFA metabolism has been studied in bacteria for the last several decades and its role in promoting the infectivity of disease-causing bacteria is well-appreciated. However, how LCFA metabolism is interconnected with other cellular processes is still unexplored. Our work presents the foremost instance of the integration of LCFA metabolism with cellular pathways that serve to relieve LCFA-induced problems in disulfide bond formation. Our research also provides the basis for examining the interconnection between LCFA metabolism, disulfide bond formation, and signaling pathways in other disease-causing bacteria. Such studies are needed to determine whether pathways that maintain disulfide bond formation can be targeted by drugs to control LCFA-utilizing pathogenic bacteria.
What was the exciting moment (eureka moment) during your research?
An earlier study reported that ubiquinone levels are ~15 to 20-fold higher than other electron transport chain components (Cox et al., 1970). Since then, ubiquinone has been considered to be non-limiting for its electron transfer function. However, during the course of this research, I observed that the disulfide bond-forming machinery accumulates in a completely reduced form when E. coli is grown in LCFAs, and this could be prevented by exogenously supplementing cells with ubiquinone. The above experiment convincingly established that ubiquinone is limiting for its electron transfer function when E. coli is grown in nutrient sources that increase electron flow in the electron transport chain. This was an exhilarating moment as it overturned the existing paradigm about the sufficiency of ubiquinone.
What do you hope to do next?
My research has opened up several new areas of investigation. The immediate next step is to identify the molecular signal that leads to Cpx activation during LCFA metabolism and to understand the mechanism by which the Cpx pathway restores disulfide bond formation in proteins.
Where do you seek scientific inspiration?
I always get amazed at how bacteria, the single-celled organisms, employ sophisticated strategies that enable them to survive in diverse environmental conditions. My curiosity to understand the intricate design features in bacteria drives me to plan elegant experiments and test my hypothesis. I draw inspiration from my Ph.D. supervisor, who has not only guided me over the years of my Ph.D. but has also instilled scientific aptitude and rigor in me.
How do you intend to help Indian science improve?
I would like to contribute to Indian science by inculcating scientific temperament in young minds and propagating the culture of rational thinking amongst the general public. In addition, I strongly believe in mainstreaming gender. For the balanced development of any field, the representation of women is essential. The fraction of women scientists in India is very small compared to women with Ph.Ds., irrespective of discipline. I am fortunate to have a woman faculty as my Ph.D. supervisor. It made me realize that it is possible to pursue one’s professional dreams while taking care of the family. This experience makes me believe that it is critical to have women support groups for female research enthusiasts. I would like to actively initiate and engage with such support groups and contribute towards fixing ‘the leaky pipeline.’
Jaswal K, Shrivastava M, Roy D, Agrawal S and Chaba R (2020) Metabolism of long-chain fatty acids affects disulfide bond formation in Escherichia coli and activates envelope stress response pathways as a combat strategy. PLOS Genetics 16(10): e1009081.
Learn more about Rachna Chaba’s lab research interest here: https://web.iisermohali.ac.in/dept/dbs/?iiser-faculty=rachna-chaba