f e a t u r e
TB’s deadly mysteries the director of the Molecular Foundry, a nanoscience institute at the Lawrence Berkeley National Laboratory. She has won numerous awards, including several campus teaching awards and a MacArthur Foundation “genius” award. The inventor Thomas Edison once remarked, “Genius is 1 percent inspiration and 99 percent perspiration.” Edison would have appreciated Bertozzi’s work ethic. She is a perpetual motion machine, some weeks spending almost as much time in the air flying to and from meetings as she does on the ground. Yet she still holds together a research group of almost 50 people, including undergraduates. The Bertozzi group’s progress in unraveling the metabolic mysteries of the TB bacterium has been due to painstaking research over the course of several years. “Back in 1999,” says Bertozzi, “we knew that sulfated glycans, or sugar-based molecules that contain sulfur, play important roles in cell-to-cell communication in higher animals. But we didn’t know much about the functions of these sulfated glycans in bacteria in general, much less in TB specifically.” TB is a uniquely human disease, and the organism that produces it has been co-evolving with humans for thousands of years. The TB bacterium may have adapted to its host by mimicking human cell-to-cell signaling pathways. “If we could learn how TB manipulates the immune system,” says Bertozzi, “we might be able to identify new anti-TB drug candidates.” Prior to the Bertozzi group’s work, only one TB sulfated glycolipid was known — sulfolipid-1 (SL-1). Its biosynthetic machinery had remained a mystery for over 40 years. After the group unraveled many of the SL-1 biosynthetic pathways, Bertozzi, Mougous, and collaborators in Berkeley’s molecular and cell biology and public health departments began to look for ways to block the production of SL-1 and other sulfated molecules. The sugar trehalose seemed like a good place to begin. It is one of the fundamental building blocks of TB glycolipids like SL-1. Starting from trehalose, the TB bacterium synthesizes SL-1 by adding long lipid chains and a sulfate (SO42-) group. Trehalose is a disaccharide, composed of two glucose molecules. In appearance, it is not that different from sucrose, the sugar we sprinkle on our cereal and put in our coffee. Sucrose is also a disaccharide, but it is composed of one molecule of glucose and one of fructose.
“Most bacteria, plants, and insects synthesize trehalose,” says Bertozzi, “but higher order vertebrates, like humans, don’t. We thought that if we found a drug candidate that interfered with trehalose biosynthesis, it could reduce the virulence of TB without having much of an effect on the person with TB who was taking it.” Bertozzi’s original hunch was correct — too correct. Trehalose is so important to the TB bacterium that it has evolved three redundant pathways to produce the sugar. Finding a drug that would block all three pathways of the trehalose biosynthesis system would be a very complex task. So Bertozzi set aside trehalose as a target and kept looking for other ways to block the production of sulfated glycolipids. Bertozzi and colleagues have had more success by targeting another way the TB bacterium uses sulfur-containing metabolites for its self-defense. TB bacteria can survive for decades in a dormant state inside macrophages, our immune system’s killer cells. Once they engulf bacteria, macrophages launch a barrage of potent oxidants, including nitric oxide, to kill the invaders. Says Bertozzi, “We know sulfur-containing metabolites help protect other organisms against oxidants, and we speculated that the enzymes involved in sulfur assimilation might be vital for survival of TB during its latent phase inside macrophages.” CysH is an enzyme essential for the TB bacterium to produce sulfur-containing metabolites. Bertozzi had discovered that disruption of CysH synthesis renders the TB bacterium incapable of producing cysteine and methionine, two sulfur-containing amino acids, as well as a cofactor called mycothiol that the bacteria use to protect themselves from oxidants. Disruption of CysH, Bertozzi found, also reduces the virulence of TB in mice. “CysH helps protect the TB bacterium,” say Bertozzi, “so that makes it an interesting target. In the last several years, working with Professor Lee Riley’s group in public health, we’ve been able to produce a genetically altered ‘knock-out’ strain of TB that lacks the CysH altogether, and we’ve found that this weakened version has potential as a vaccine, at least in the mouse model of TB.” The current vaccine for TB, the Bacillus Calmette-Guérin (BCG) vaccine, was developed in the 1930s in Europe. It is based on an attenuated strain of Mycobacterium bovis, the relative of the TB pathogen that usually infects cattle. The BCG vaccine is used mostly
Fall 2007 Catalyst
13