NIAID-Funded Researchers Probe Potential TB Weaknesses
Tuberculosis (TB) kills more people annually than any other infectious disease. According to the World Health Organization (WHO), 490,000 people developed multidrug-resistant TB in 2016 alone—and infection rates are rising. Developing ways to extend the lifespan of existing antibiotics, as well as new treatments that will work alongside existing TB therapies, will be critical in the years to come. However, traditional drug discovery can be a long and difficult process. In a newly published study, a group of NIAID-funded researchers describe an unusually detailed and fruitful exploration of a single protein pathway in Mycobacterium tuberculosis (Mtb), the bacterium that causes TB; the researchers hope to exploit this pathway’s weaknesses to find a new weapon against TB.
The study, described in the April 25th edition of Science Translational Medicine, focuses on one key step in the process by which Mtb constructs its cellular membrane. A protein called biotin protein ligase (BPL) binds a biotin molecule to another construction protein. This activates the construction protein allowing it to create the lipids which later make up the bulk of the Mtb cell membrane. By introducing a BPL inhibitor, called Bio-AMS, the researchers suspected that they could significantly weaken the bacterium’s ability to renovate its own cell membrane. This, in turn, might make the bacteria more vulnerable to existing antibiotics, or even halt its growth entirely.
To probe the role of BPL in Mtb survival, the researchers developed a set of experiments to test this pathway under a variety of conditions. They first applied Bio-AMS to Mtb in vitro—and confirmed that Bio-AMS was lethal to the bacteria. To see whether Mtb might eventually become resistant to Bio-AMS, the researchers also sequenced the genome of the few Bio-AMS-resistant bacteria they found, pinpointing a mutation in the rv3405c gene that grants immunity to Bio-AMS.
The researchers then wanted to prove that inhibiting the action of BPL impacts Mtb growth in the mice. Since small animals, such as mice, have a fast metabolism and quickly eliminate drugs, they could not measure the effects of Bio-AMS on BPL directly in the animals. To overcome this limitation, the researchers first went back to the test tube and used a hollow-fiber system to test the effect of precisely-measured, fluctuating concentrations of Bio-AMS on Mtb, mimicking what might occur in mice.
After observing that Bio-AMS was still effective under those conditions, the researchers then used a second strategy to study what happens when Mtb can no longer produce enough BPL to remodel its cell membrane. They infected mice with an engineered strain of Mtb in which the production of BPL could be turned down on command. When treated with human first-line TB drugs, once Mtb was deprived of the ability to make BPL, and thus unable to make a fully functional outer cell envelope, it became much more vulnerable to rifampin and ethambutol, but seemed to be better able to avoid killing by isoniazid – highlighting the role of the outer surface of Mtb in modulating drug efficacy.
In all, as the researchers concluded, BPL appears to be a promising target for future research. This approach is fairly new: Through the combined use of laboratory tests that mimic fluctuating concentrations of drugs, as they are encountered in animals, and Mtb mutants in which the production of key proteins can be regulated in animals, the researchers were quickly able to assess whether interfering with a particular protein in Mtb could have negative consequences for the pathogen. Bio-AMS may not be suitable as a viable new drug candidate, but with these studies in hand, the researchers have developed an unusually-nuanced view of how BPL helps Mtb survive, which may provide insight into how to proceed to come up with new drug molecules that have the same activity as Bio-AMS.
507 East Jefferson Street
Brooksville, FL 34601
7341 Spring Hill Drive, #3138
Spring Hill, FL 34606-3138