Work by Weill Cornell Scientists Points to Possible New Target for Drugs To Treat Tuberculosis
Jan 25, 2002
One of the challenges posed by the tubercle bacillus, which causes tuberculosis (TB), is to understand how the bacillus, once it infects tissue, persists for a person's entire lifetime despite the attack of the body's immune system. Mycobacterium tuberculosis (Mtb) persists despite prolonged oxidative and nitrosative stress—forces that the immune system uses to kill many other invading pathogens. Scientists at Weill Cornell Medical College, led by Dr. Carl Nathan, have now found that Mtb defends itself against oxidative stress by using a "bucket brigade" of proteins—including two proteins that have been widely known as being involved in essential metabolism.
"This may be the first known instance in which essential metabolic enzymes also support antioxidant defenses," the authors write in their article in the January 17 web-based advance issue of Science. (The hard copy is scheduled for the issue of February 8.) They also say, "one or more [of these proteins] may hold interest as a drug target for tuberculosis."
Mtb infects about one-third of the people in the world. Five to ten percent of them will at some point come down with the disease, and until 1952, when the first effective drug was introduced, about half of these patients would die of it. Worldwide, tuberculosis is still the single leading cause of death from bacterial infection, with two-and-a-half to three million dying of it each year. The recent spread of the AIDS virus, HIV, makes TB worse, and TB makes HIV infection worse. On top of that is the recent appearance of Mtb strains that resist existing drugs.
The new article explains the authors' discovery of new functions for three proteins, which, together with a fourth protein, act in what Dr. Nathan calls a "bucket brigade" to disarm the peroxide and peroxynitrite produced by the immune system's macrophage cells. The four proteins are dihydrolipoamide dehydrogenase (Lpd), dihydrolipoamide succinyltransferase (SucB, pronounced "suck-B"), alkyl hydroperoxide reductase (AhpC), and AhpD, which was given its name because its gene is next to and just downstream from AhpC.
It was already known that AhpC disarms peroxides. Dr. Nathan and his postdoctoral associate Dr. Ruslana Bryk, now an Instructor in the Department of Microbiology and Immunology, reported in Nature just over a year ago that AhpC also disarms peroxynitrite. To provide this protection again and again, AhpC has to be, in a sense, recharged—or, actually, reduced, since it has been oxidized—and the protein that does that in Mtb, the authors found, is AhpD. For AhpD to be reduced, Mtb uses SucB. And for SucB to be reduced, the only protein that can do that is Lpd. Lpd is reduced by drawing from a source of electrons, called NADH, which arises from energy metabolism. "What's basically happening is, an electron is coming from NADH and inactivating, or reducing, peroxides and peroxynitrite, but it takes four proteins to do that," Dr. Nathan says.
Lpd and SucB have been better-known as parts of the metabolic "factory" of any cell, producing energy and building blocks to make up other proteins. Lpd and SucB take part in the essential process in biochemistry called the Krebs cycle. "The idea that they participate in defending against oxidative stress or resisting the immune system is quite new," Dr. Nathan says.
Other scientists have found that although the genome of Mtb suggests that there are three versions of Lpd, there is actually only one working version of it in Mtb. Likewise, the experiments of one of the lead authors, Dr. Bryk, suggest that only one version of SucB is made as a fully functioning protein. These facts suggest that if medicine could inhibit selectively the Mtb versions of Lpd or SucB, it could have an impact on the energy metabolism and biosynthesis capacity of the pathogen, while not targeting the body's own cells, where the human versions of Lpd and SucB play a vital role. Dr. Nathan also suggests that interfering with these enzymes might cripple Mtb's ability to build its unusually thick and hard-to-penetrate cell wall, thus further exposing it to medicines and to the immune system's "killer" cells.
AhpD, which the authors studied through x-ray crystallography, is a small protein that is found in Mtb and only a few other species. AhpD is not otherwise found in the human body. Thus there is the possibility that a specific AhpD inhibitor might fight Mtb infection without cross-reactions with homologous structures.
Dr. Bryk agrees with Dr. Nathan's caution that this is only a preliminary study and that much work remains to be done before we can speak of candidate targets for new drugs. Right now, one of the studies that is going on in the laboratory is to eliminate the genes that produce AhpD, SucB, and Lpd in Mtb and then to examine the viability of the Mtb.
Dr. Nathan is the R. A. Rees Pritchett Professor of Microbiology and Chairman of the Department of Microbiology and Immunology at Weill Cornell. He also serves as co-Chair of the Program in Immunology at the Weill Graduate School of Biomedical Sciences. Joining Dr. Bryk as lead co-author is Assistant Professor of Biochemistry Dr. Christopher D. Lima of Weill Cornell, and the other authors, Drs. Hediye Erdjument-Bromage and Paul Tempst of the Protein Center of Memorial Sloan-Kettering Cancer Center.