Chances are you've never even heard of the enzyme heme oxygenase. You might be surprised that a web search engine references 858,000 articles about it -- including international conferences devoted solely to its research and even a heme oxygenase fan club. Why all the fuss? One of its devotees, Mayo Clinic kidney specialist and researcher, Karl Nath, M.D., is part of the reason. In 1992, while working at the University of Minnesota, Dr. Nath's research team discovered the capacity of heme oxygenase (HO) to protect the body's tissues from injury. Since then it has attracted giants in the field of tissue injury and the number of HO-related studies has exploded.

In 1996, Dr. Nath came to Mayo Clinic from the University of Minnesota. Inspired by the courage of his patients who, for four hours, three times of every week, are tethered to a dialysis machine, his research is contributing to the fulfillment of his dream -- to see heme oxygenase developed into treatments that can spare people from the staggering challenges of kidney failure.

"My laboratory seeks to answer questions about the major mechanisms by which tissues are destroyed and, in particular, those mechanisms that apply to kidney disease and to the vascular channel." says Dr. Nath. "Our studies include model systems that are based on either diseased models in experimental animals, or genetic models in mice, that simulate the basic features of the human condition."

Two collaborators, vascular biologist, Zvonimir Katusic, M.D., Ph.D., and kidney pathologist, Joseph Grande, M.D., Ph.D. are vital partners in Dr. Nath's research effort.

"Dr. Katusic is one of the world's leading experts in the mechanisms of vascular pathobiology," says Dr. Nath. "And Dr. Grande is a nationally-recognized experimental kidney pathologist."

HO-1: Protector of the Cell

As Dr. Nath delved deeper into HO biology, he became particularly interested in the isozyme, heme oxygenase-1 (HO-1). You can think of an isozyme as a sort of sub-specialty enzyme that has evolved from a gene mutation and survived because its modification serves a useful function. HO-1 is induced by stressors such as lack of oxygen, poisons, and other chemicals that cause blood vessels to constrict or tissues to become inflamed.

The research community is interested in HO-1 because it causes a reaction that breaks down heme into its component parts, each of which has protective actions. The components are iron, carbon monoxide and biliverdin. Biliverdin is subsequently converted into another bile pigment, bilirubin. You have undoubtedly observed the reaction many times as you watched a bruise change its colors from red (heme) to green (biliverdin) to yellow (bilirubin). Heme is contained in many important proteins. The hemoglobin protein, for example, uses heme to transport oxygen to all the body's tissues.

HO-1 prevents toxic reactions from occurring as red blood cells age and break down. Free iron is toxic to cells. When HO-1 is produced, a protein called ferritin is simultaneously synthesized. Ferritin binds to iron, allowing it to be stored harmlessly until needed to make more hemoglobin or other proteins.

Carbon monoxide acts as an anti-inflammatory agent, relaxes blood vessels, and prevents cell death. Biliverdin and bilirubin are anti-inflammatory agents as well as antioxidants -- chemicals that reduce the rate of oxidation, a chemical reaction that can injure or kill cells if its rate is not controlled.

Research on injury and its adaptation to heme proteins has revealed many protective cellular pathways originating from HO-1.

"Once HO-1 is recruited into a cell, it's like an octopus," explains Dr. Nath. "It has tentacles that tap into an extensive network of cellular metabolism and signaling."

A Surprising Discovery: Protector Turns Poisonous

In 2006, Dr. Nath's group published a paper on sickle cell disease, which overturned the accepted view that the activity of HO-1 always protects the cell (Am J Pathol 21 Jul 2006, 169:21-31).

"Every time we did the experiment, the opposite evolved," says Dr. Nath. "It can be hard to stomach having your biases overturned, but the studies which suggest the opposite of what you predict often turn out to be the most important."

The group demonstrated that, in large amounts, the products of HO-1 induction may become toxic. They hypothesize that some mechanism of sickle cell disease may stimulate overproduction of HO-1, which generates toxic amounts of its byproducts.

The experiment involved administering mice with tin protoporphyrin (SnPP), a chemical known to inhibit HO-1 activity. By examining two groups of mice in the presence or absence of the chemical, the group also uncovered an experimental strategy that reduced kidney injury in a mouse model of sickle cell disease.

"When we administered SnPP over longer times, it caused inflammation and scarring in normal mice but not in those with sickle cell disease," says Dr. Nath. "We are now examining the mechanisms by which SnPP protects the cell in sickle cell disease.

Sickle Cell Disease

Sickle cell disease is an inherited condition in which red blood cells become rigid and sticky and are shaped like sickles or crescent moons. When sickle cells die prematurely, they cause a chronic shortage of red blood cells. They can also reduce or block blood flow and oxygen to vital organs by sticking to the walls of small blood vessels. Bone marrow transplant offers the only potential cure for sickle cell disease. But very few people have a suitable donor for transplant.

Fruitful Collaborations

For 13 years, Dr. Nath's group, in collaboration with the University of Minnesota, has been funded by the National Institutes of Health (NIH) to study injury and adaptation in the kidney and vasculature in sickle cell disease. In 2006, Dr. Nath received the Alumnus Distinguished Lifetime Achievement Award from the University of Minnesota for his work on sickle cell disease.

Meanwhile, Dr. Katusic has been studying the function of endothelial cells -- the cells that line the vessel walls -- because of their importance in the regulation of blood flow. He hopes to gain better insight into the mechanisms involved leading to improved methods that promote smooth muscle relaxation and vasodilation. He is now helping Dr. Nath to home in on the function of sickle endothelial cells.

"We have known that nitroglycerine relieves angina for 100 years," says Dr. Katusic. "But we didn't know why until quite recently when scientists showed that the gas, nitric oxide (NO) sends a signal to the nervous system to relax and widen the blood vessels when the need for oxygen increases. It turns out that every endothelial cell generates a variety of substances that are responsible for relaxing blood vessel walls, the most important of which is NO."

Dr. Katusic's goal is to supplement this self-generated cellular pharmacy with gene therapy.

"Gene transfer technology is proving a powerful tool to address key issues in biology of blood vessel walls," says Dr. Nath. "Dr. Katusic has developed methods to deliver HO-1 to endothelial cells using the adenovirus as a vector."

Although preliminary animal studies indicate that the enzyme causes blood vessels to relax significantly, many puzzles remain to be solved.

"We have shown that the basic principle of being able to increase cellular production of vasodilators is correct," says Dr. Katusic. "Now we are working on better methods of controlling the amounts released and finding ways to prevent the tendency of the viral vectors to induce inflammation."

Having access to the skills and knowledge of an experimental pathologist is another important aspect of Dr. Nath's research. Besides accurate assessment of pathology samples generated by the studies, a pathologist who works in research understands scientific language and can think mechanistically. For this he relies on Dr. Grande.

"My lab focuses on the mechanisms of tissue injury in kidney disease so it's a natural fit," says Dr. Grande. "Dr. Nath and I shared lab space when he first came to Mayo and we have had a very fruitful collaboration ever since."

Dr. Grande's major focus is signaling in progressive kidney disease. The two have co-authored 20 papers.

A Rich Scientific Community

Sickle cell disease is predominantly a disease of African Americans, a group whose small numbers in Rochester is a potentially limiting factor when competing for NIH funding.

"I credit our success to the richness of the basic science community and Mayo's world class clinicians who interact with ease," says Dr. Nath. "Such collaborations keep research centered on answering a clinical question. And Mayo has the infrastructure to translate developments expediently into a clinical setting."

Dr. Nath's research on HO-1 has the potential to relieve human suffering caused by kidney disease in three ways: by preventing kidney disease and developing better treatments for it; by preventing and treating kidney damage in sickle cell disease; and by prolonging the lifetime of the external shunt through which blood is exchanged during dialysis. All three areas of interest are funded by NIH grants.

Research satisfies Dr. Nath's passion for knowing something new. Another passion has its roots in his childhood. Imbued with a sense of reverence for the English language as a young schoolboy on the Caribbean island of Trinidad, he reads voraciously, always in search of an elegantly turned phrase or the brilliant cadence of a well-crafted paragraph. He often spices his scientific papers with ideas culled from non-medical books.

"I have the illusion that, some day, I might write some piece of non-fiction that is reasonably good," says Dr. Nath. "In the meantime, I am content to try to be creative with language within the scientific context."

- Yvonne Hubmayr