Shumin Tan, PhD

Tuberculosis remains one of the top causes of mortality from an infectious disease globally. The bacterium Mycobacterium tuberculosis (Mtb) is the causative agent of tuberculosis, and is highly adapted to surviving in its host for decades. The growth state of the bacterium in the host during infection is known to be non-uniform, and this has significant implications for successful treatment, as some drugs are more effective against actively growing Mtb. However, much remains unknown regarding the processes that “decide” whether Mtb grows or not during infection. Our project is focused on shedding light on this fundamental aspect of Mtb biology, through the study of an essential Mtb regulator, PrrA, which we discovered affects the bacterial response to multiple critical environmental cues. Importantly, these cues include nitric oxide (NO, a key immune modulator) and hypoxia, two signals known to be able to drive Mtb into a non-growing state. In the past year, we have found using global analyses of gene expression in Mtb that PrrA markedly and broadly affects the bacterial response to NO, with almost half of the genes differentially expressed upon NO exposure exhibiting an altered induction profile when prrA is overexpressed. Strikingly, prrA perturbation affected expression of NO and hypoxia-responsive genes controlled not just by the previously well-characterized and intensely studied DosR regulator, but also those that were not. PrrA appeared to act as a rheostat, changing the amplitude of the gene expression changes triggered by NO and hypoxia, rather than an on/off switch, as is the case with DosR control. We have also found that phosphorylation (transfer of a phosphate group) of PrrA by a class of proteins called serine/threonine protein kinases (STPKs) plays an important role in PrrA function. In particular, preventing STPKs from being able to phosphorylate PrrA dampened both NO and hypoxia-mediated induction of Mb gene expression, and resulted in the bacteria failing to enter an adaptive state of growth arrest upon NO exposure. Our novel research findings reveal PrrA as a global regulator of Mtb response to NO and hypoxia, two critical environmental signals that impact on Mtb growth state, and shed light on how PrrA activity may be controlled. They set the stage for further studies to understand the role of PrrA-driven growth regulation in response to additional environmental cues, such as acidic pH, and analysis of environment-dependent PrrA function during infection. Given the importance of linking environmental response with growth control for bacterial survival in the host, these studies hold significant potential for revealing new Achilles’s heels that can be targeted for therapeutic purposes.

Update: In the past year, we have found that the essential Mtb regulator (PrrA) broadly affects the bacterial response to nitric oxide (NO) and hypoxia, two environmental cues that impact on Mtb growth state. PrrA acts as a rheostat, changing the amplitude of the gene expression changes triggered by NO and hypoxia, rather than an on/off switch. Strikingly, preventing phosphorylation of PrrA by serine/threonine protein kinases significantly affected its function, resulting in failure of the bacteria to enter an adaptive state of growth arrest upon NO exposure. These results illustrate the key role that PrrA plays in linking Mtb environmental response with growth control.

How Does Mycobacterium Tuberculosis Adapt to Life in Humans? (2017-2018)

Tuberculosis is the leading cause of death worldwide from infectious diseases. To survive in the human host for decades, Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, needs to be able to "see" where it is in the body, and sense and respond appropriately to environmental signals in its immediate surroundings. We will study how Mtb may use potassium as one such critical environmental signal during infection. Potassium is the most abundant positively-charged ion within cells in our body and also in bacterial cells. Understanding how Mtb responds to potassium will lay the foundation for seeking ways to disrupt this signaling, thereby decreasing the bacterium's ability to colonize and cause disease.

Update: In the past year, we have discovered that Mtb has a unique gene expression response to potassium; at the same time, perturbation of potassium uptake by Mtb alters the bacterium's response to other key environmental signals, highlighting the importance of signal integration by Mtb. We find that potassium concentration changes in the host cell compartment in which the bacteria reside, and that maintenance of potassium homeostasis impacts on the ability of Mtb to grow in host cells. These results illustrate the important role that potassium can play in tuberculosis disease.