T H E   N I H    C A T A L Y S T     J A N U A R Y   –  F E B R U A R Y   2004


text and photo
by Peter Kozel
Tom Leto

Thomas Leto has his finger on the button.

The head of NIAID’s Molecular Defenses section, Leto studies genes that control the production of one of the body’s most potent weapons for innate immunity—reactive oxygen species (ROS). 

Produced by enzymes called oxidases, ROS are energetic compounds that react with just about anything. "We think that the production of reactive oxidants is an aspect of innate immunity that’s very ancient, going back before the split of plant and animal kingdoms," Leto says. The genes that produce, and systems that regulate, oxidant production also seem to be conserved across kingdoms. 

The Double-Edged Sword

But ROS are a double-edged sword. "The good side to oxidases is that they can kill microbes; the downside is that you can damage your own tissues, and sometimes you can have uncontrolled inflammatory processes," Leto said in a recent interview with The NIH Catalyst.

Leto began his work on oxidases when he joined the NIAID Laboratory of Host Defenses in 1988. Harry Malech and John Gallin, now chief of that lab and CC director, respectively, had been working with patients with chronic granulomatous disease (CGD) for more than a decade. These patients, Leto noted, "present with certain problems with low-grade pathogens that don’t affect the uncompromised host." http://www.cc.nih.gov/ccc/director/welcome.html

The team identified two of the mutated genes associated with different CGD types that figured in about one-third of cases. These mutations block the synthesis of ROS by neutrophils and other circulating phagocytes, impairing the ability of CGD patients to clear bacterial and fungal infections.

"These diseases are rare, affecting only five people out of a million," Leto said. "It’s only because we have such a large cohort of these rare patients congregated here that we’ve made such headway. One of the phagocyte oxidase lesions is a component that affects only five percent of these patients. We have one patient with that lesion. He was the missing link. Using biochemical reconstitution, we put the system together and recognized that it is a multicomponent system."

After working out much of the molecular biochemistry of CGD, Leto’s lab wanted to apply the knowledge to other tissues. "The new direction we’re taking now is a byproduct of the Human Genome Project," he observed. Using the phagocyte oxidases as a starting point, Leto’s lab mined databases to identify related new genes that produce oxidants in specific tissues throughout the body. The team found new genes and identified additional tissues where ROS are produced—and the new genes pointed to new molecular targets in other diseases, he said.

For example, excessive activity of another oxidase gene, Nox1, could lead to inflammatory conditions in the colon, such as inflammatory bowel disease. "It would be very interesting to identify whether Nox genes are affected in [IBD] patients," Leto noted.

Perhaps the most exciting finding is the Duox gene family and its connection to lactoperoxidase (LPO), Leto says. LPO is a potent antimicrobial enzyme found in milk, saliva, and the mucosal layer of the airways. Scientists have long wondered what the purpose of LPO was in those locations since the source of hydrogen peroxide needed for LPO activity remained unknown. Recently, Leto’s lab demonstrated that Duox genes are expressed in secretory and salivary glands, trachea, and bronchium—precisely the locations to synergize with LPO.

Clinical studies of severe hypothyroid patients demonstrated that Duox also plays a "critical role" in thyroxine synthesis. Low levels of oxidative output could lead to susceptibility to lung infections, whereas abnormally high oxidative output could contribute to inflammatory diseases of the airway, including asthma, chronic obstructive pulmonary disease, or even acute allergies. 

"Who knows, it may represent an important antiviral system in airways," Leto speculates. "It could relate to control of respiratory viruses such as SARS—it might not be a stretch!"

It’s unlikely every oxidase serves identically. Kidney tubule epithelium cells, for example, produce a renal-specific oxidase, Nox4. This protein may not play a role in host defense at all.

Noting that the kidney is "a key organ for sensing anemia" and is the source of red blood cell–promoting erythropoietin, Leto and his colleagues suggested early on that Nox4 is an oxygen sensor. Because of difficulty getting human kidney tissue, the lab is studying a mouse model deficient in Nox4.

Although they may be serving different tissue-specific roles, all of the oxidases share an important feature: tight regulation. "Phagocytic cells work very hard in containing the sites at which these toxic, indiscriminate molecules are generated," Leto said. "It’s compartmentalized within a phagosome where microbes are being engulfed."


Leto says location is one of the most important aspects of oxidase-based defense mechanisms "because it’s right at the interface where you would want it." He has shown that all of these new genes are expressed predominantly on epithelial surfaces, "aimed away from the host."

Peroxidases in the mucosa, such as LPO, "seem to be tailored to make milder, less destructive oxidants"—an additional layer of control. "Oxidative capacity comes about very late in differentiation, only when cells get out there on the surface," Leto said, noting that this prevents damaging compounds from being created deep within tissues.

Tight control over the quality and location of oxidase activity gives ROS their utility beyond defense. "The diversity of the tissue-specific isozymes is nicely matched with the diversity of functions of reactive oxidants that have already been recognized," Leto observed.

ROS in cells with high oxidative output typically act as potent weapons, while "cells with lower oxidative capabilities can use the oxidants as signals such as in growth-factor signaling, regulation of proliferation, cellular senescence, apoptosis, and vasoconstriction," he said.http://dir.nhlbi.nih.gov/labs/cb/index.asp

Perched high on the top floor of the Clinical Center, Leto’s lab looks out over the new Mark O. Hatfield Center, where the clinical sections of the Laboratory of Host Defenses will move. The future may also reveal new functions of oxidases and new diseases to study, as well as more peaceful but no less exciting cellular processes.


A review article, "Oxidant Signals and Oxidative Stress," by Toren Finkel, senior investigator, NHLBI Cardiovascular Branch, appears in Current Opinion in Cell Biology (15:247–254, 2003) and cites 54 recent articles by NIH and other investigators.

Finkel’s abstract reads: "Although oxidants clearly possess the capacity to behave in a random and destructive fashion, growing evidence suggests that in many instances the production of reactive oxygen species is tightly regulated and their downstream targets exquisitely specific. This past year, several notable advances have been made in defining the specific redox-dependent targets of intracellular oxidants, as well as the myriad pathways that appear to employ oxidants as effector molecules. These new studies have significantly altered our understanding of how reactive oxygen species participate in diverse processes from tumourigenesis to ageing."




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