Enzymatic key opens door to quicker therapy
Researcher uncovered clues in 1950s-era studies
Some researchers use the word "persistence" to describe how the TB mycobacterium seemingly goes into hiding. Not quite the same as "clinical latency," the word refers to the bug’s maddening ability to duck pharmaceutical bullets, a trick that prolongs treatment to months, even years.
Practically speaking, if a way could be found to shut down the machinery that powers persistence, TB therapy could be shortened dramatically, experts believe — conceivably to just a few weeks.
All this helps explains the enormous fuss that greeted the article in the Aug. 17 Nature that describes a primitive metabolic pathway that’s kicked off by an exotic-sounding enzyme called isocitrate lyase, or ICL.
By genetically switching off ICL, a team of researchers from Cornell University and Rockefeller University in New York City found they could effectively shut down the persistence shop. Bugs without the ability to produce the ICL enzyme did just fine during the acute stage of infection but couldn’t survive during the chronic stage. Mice infected with bugs lacking the gene to make ICL, in fact, are still scrambling about in their cages and look as if they will enjoy normal mouse life spans.
"It’s very exciting — like the first chink in the wall that lets us peek through and take a look at what goes on during persistence," says John McKinney, PhD, assistant professor at Rockefeller and head of the university’s laboratory for infection biology.
The shift that ICL permits in the TB bug’s metabolism is so odd that references to it caught McKinney’s eye years ago when he was poking through dusty library stacks, grubbing around in 1950s literature to see what nuggets he might glean from the pre-Medline era.
"I think it was James Watson who said, A year in the lab is worth a day in the library,’" chuckles McKinney. "You know, in the preantibiotic era, TB engaged some of the greatest minds of the time. These people didn’t have a lot of fancy technology, but they certainly spent a lot of time thinking about how things work."
Obscure researchers and dusty clues
What caught McKinney’s attention was a series of papers published starting in 1956 by a now-obscure pair of researchers named Segal and Bloch. Setting out to elucidate the differences between in vitro and in vivo TB bugs, the two compared mycobacteria taken from the lungs of infected mice with bugs grown in standard lab culture. Then they began playing around, offering the two sets of bugs various kinds of "food" in the form of carbon-substrate energy sources.
To the surprise of Bloch and Segal — and, decades later, McKinney as well — the in vivo bugs harvested from mice showed an astonishing dining preference. They completely ignored everything but fatty acids, roughly the gastronomic equivalent of a hungry man in a gourmet pastry shop bypassing the crème horns and tarts and heading for a bag of week-old, rock-hard bagels.
"It was weird, because fatty acids are not a preferred [energy] source," says McKinney. "If the bugs were using it, it was because they had to."
Two labs combine forces
That wasn’t the only thing Bloch and Segal turned up that caught McKinney’s eye. "They observed in another paper that these in vivo bacteria expressed enzymes in two pathways specifically required for use of fatty acids — the beta-oxidation pathway, which we now study in my new lab, and the glyoxylate shunt." Because it had largely disappeared during evolution, that second pathway is where McKinney decided to begin his own exploration.
By coincidence, McKinney found he wasn’t the only one looking at the pathway and at ICL, the enzyme that switches it on.
"[McKinney’s lab was] taking the genetic approach, just knocking the gene out, but in my lab, we were looking at what proteins were up-regulated during the shift," says David Russell, PhD, chair of immunology and microbiology at Cornell University’s College of Veterinary Medicine. "So we decided to combine forces."
Sure enough, Russell and McKinney found that TB bugs lacking the enzyme for ICL couldn’t shift into the primitive, carbon-sparing metabolic pathway. But what was prompting the metabolic shift in the first place?
"We felt it represented some sort of transition between the naive host and the immune host," says Russell. "We knew that in the immune host, there are activated macrophages. This seemed to suggest that where there are activated macrophages, the bacterium, in order to maintain an infection, has to metabolize carbon from a different source" by cranking up the glyoxylate shunt pathway.
Same enzyme in other pathogens
What exactly the switched-on macrophage does to make this happen remains to be seen, says McKinney. "Maybe the host immune response is withholding carbon sources, or maybe it’s putting the bug into a compartment where it can’t get at other energy sources," he theorizes. "What we can say is that the shift is brought about by the host immune response, and that, as far as I can tell, is an entirely novel finding."
Drug maker Glaxo-Wellcome, which has helped support McKinney and Russell’s work, already has found several compounds that inhibit production of the ICL enzyme, at least in vitro. McKinney says he’s cautiously optimistic that animal testing of some of these promising compounds will be under way by the end of the year.
There’s an exciting postscript to the story, as well. "Recently, we’ve seen some unpublished reports in which it’s been shown ICL is up-regulated in a number of other pathogens when they enter macrophages," McKinney says. Among them are Cryptococcus neoformans and Candida albicans, two fungal pathogens; and Toxoplasma gondii, a parasite.
Could it be the same host immunity response McKinney and Russell uncovered is driving a metabolic switch in these other pathogens, too? Such a finding would be more than just a chink in the wall. Indeed, it could open up enormous new vistas in research, McKinney believes. "We may have hit on a general theme here," he says. "Anyway, that’s our hope."
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