Structure Casts Light on Inflammatory Step in Asthma

A recent paper by HMS researchers and their colleagues reports the crystal structure of a pivotal enzyme bound to its substrate protein, revealing a specific interaction in the inflammatory cascade of bronchial asthma. This biological snapshot could facilitate further research toward improved asthma therapies.

Courtesy Yoshihide Kanaoka

Matchmaker. The enzyme LTC4 synthase makes asthma-causing cysteinyl leukotrienes by uniting LTA4, an elusive lipid produced by immune cells, with glutathione molecules that sit in the active site between two of the enzyme’s monomers. Since it has three monomers, three active sites are available for glutathione to reside in, making LTC4 synthase an unusually industrious enzyme.

Appearing in the Aug. 2 issue of Nature, the work was conducted by K. Frank Austen, the AstraZeneca professor of respiratory and inflammatory diseases at HMS and Brigham and Women’s Hospital, and Yoshihide Kanaoka, HMS assistant professor of medicine at BWH; they collaborated with the Miyano laboratory at RIKEN in Japan. Using X-ray crystallography, the researchers delineated the structure of the enzyme LTC4 synthase, which manufactures asthma-causing cysteinyl leukotrienes (cys-LTs).

“The exciting thing is, the crystal structure really does explain how the enzyme works,” said Austen.

The enzyme’s job is to unite LTA4, an elusive lipid produced by immune cells, with the tripeptide glutathione. When these two compounds are joined, LTA4 becomes the inflammatory compound LTC4.

This action is standard enzyme behavior. What sets LTC4 synthase apart from other glutathione transferases is its remarkable specificity. The enzyme is composed of three identical monomers, with the metabolically abundant glutathione wedged between adjacent units, ready to be attached to the LTA4 when it arrives. An arginine residue of each monomer activates the glutathione for this interaction, a step the researchers did not expect.

The arrangement, said Austen “is not only optimal, it’s astonishing,” for its capacity to synthesize LTC4 efficiently. The LTA4 is trapped in a somewhat hydrophobic area to provide stability during the conjugation with glutathione. The two adjacent monomers play an active role in uniting LTA4 and glutathione, so the enzyme requires a minimum of two monomers to do its job.

But why bother having three monomers when all you need is two? “What the trimer gives you is three effective sites,” said Austen, “because every monomer works on both of its sides.” Think of it as a troupe of three jugglers versus two jugglers. The two jugglers can only get one volley of balls going between them—three jugglers could (theoretically) get three volleys going.

Once LTC4 is produced, it proceeds to interact with specific smooth muscle receptors and induces the inflammation and constriction associated with bronchial asthma. These receptors are also found in several other areas of the body, such as leukocytes, vasculature, heart, spleen, and central nervous system. Thus, while “cys-LTs started with asthma, they may have more implications,” Kanaoka said.

Now that the structure of the zealous enzyme has been exposed, drugs might be created to block its activity, and as a result, diminish the entire constellation of health problems tied to it, including asthma, atherosclerosis, and pulmonary fibrosis.

Meanwhile, Austen and Kanaoka are preparing to determine an even clearer depiction of the enzyme’s structure. “We want to know how it looks in the nuclear membrane,” Kanaoka said.

—Lauren Cahoon

Telescopic Implant Brings Sight to Diseased Eyes

Restoring vision to the blind is an age-old dream that may be within reach for patients who have the most severe form of macular degeneration, according to a study in the August Archives of Ophthalmology. But doctors have to be willing to make an unusually large incision in the eyes of their patients.

Macular degeneration eats away at the receptor-rich center of the retina, leaving a black hole, or scotoma, in the middle of the field of vision. In the most severe cases, the disease affects both eyes and is accompanied by an overgrowth of new blood vessels that leak, expanding the dark crater.

Several years ago, an Israeli researcher, Isaac Lipshitz, had the idea to create a miniature implantable telescope that would enlarge visual images so they would project onto healthy cells around the periphery of the damaged retinal center, reducing the impact of the scotoma. Ophthalmologists in the United States conducted a clinical trial of the device in 206 patients with end-stage bilateral macular degeneration. They found it improved vision and quality of life in the vast majority of patients.

Yet there was a cost. Cells lining the cornea were damaged in many patients—as many as 25 percent of cells, which exceeds the FDA’s limit of 17 percent cell death in corneal procedures. HMS assistant professor of ophthalmology Kathryn Colby, working with colleagues at several institutions, analyzed the trial results and found that much of the cell loss was due to the initial surgery, which is not surprising. “This is a very large device compared to the size of the eye,” said Colby, director of the Joint Clinical Research Center at the Massachusetts Eye and Ear Infirmary. To replace a lens in a cataract patient, doctors typically make an incision 3 mm long. To insert the miniature telescope properly, without scraping cells off the cornea, requires a 12 mm incision.

Of the 32 eye surgeons who participated in the clinical trial, the cornea specialists had the best outcomes. “We’d have made the benchmark,” said Colby, who is a cornea surgeon. “We’re used to making big wounds and then sewing them up. And we’re used to being very careful with the corneal endothelium.”

“You don’t have to be a cornea specialist to do this,” she added. “But people who are not used to making large incisions in the eye have to get used to it. That is the real take-home message—you can’t skimp on wound size.”

The device is currently awaiting FDA approval.

—Misia Landau

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