Immunology:
Mobilizing Cytokine Receptor Key Step in Defense Coordination
Psychiatry:
Studies Give Boost to Therapies for Depression
Cell Biology:
Chemical Genetics Identifies New Way of Disrupting Cell's Protein Recycling System
Awards
Systems Bio Recruit Takes MacArthur Award
New Books:
The Fall Bookshelf
Structure Reveals Binding of Platelet Integrin
Eosinophils Play Role in Chronic Allergic Asthma
Complement Linked to Tissue Damage in Diabetes
Cell Death Proteins Counter Chemo Resistance
Commission Reports Racial and Ethnic Disparities in Health Professions
Applications Requested for 2005 Alzheimer's Research Pilot Grants
Science in the News Opens Fall Series
Fourth Annual Albright Symposium
Appointments to Full and Named Professorships
Honors and Advances
In Memoriam:
George Thorn
John Badwey
Howard Frank
Margaret Brenman-Gibson
Kenneth Herman
John Richard Gaintner
HMI and International Partners Combat HIV/AIDS Through Education
Structure Reveals Binding of Platelet Integrin
Platelets may look like cellular lightweights--snippets of cytoplasm lacking a nucleus. But they play a yeoman's role in the body. One of their main roles is to patch up blood vessel tears and to promote the formation of clots, which they do by binding the protein fibrinogen. Researchers know that this binding occurs by means of a specialized transmembrane protein, or integrin. Yet they have never actually seen how this binding occurs, in part because fibrinogen is too large to afford a focused view.
This crystal structure of the platelet integrin shows the headpiece of the beta subunit (magenta) touching the fibrinogen antagonist tirofiban hydrochloride (Aggrastat). The alpha subunit is shown in green. (Image courtesy of Tsan Xiao)
A team of HMS researchers has produced a high-resolution image of a portion of alpha IIb beta 3 integrin as it might appear in its activated state when binding fibrinogen. They did this by crystallizing the integrin with two small molecules that act as fibrinogen competitors, molecules that are actually used to prevent clotting in heart attack and stroke patients. The crystal structures by Tsan Xiao, HMS researcher in pathology, Tim Springer, the Latham Family professor of pathology, and colleagues appeared online in Nature on Sept. 19 (doi: 10.1038/nature02976). The work could lead to a better understanding of how fibrinogen and platelet integrins bind and how that binding might be more effectively disrupted for therapeutic purposes.
Platelet integrins consist of an alpha and beta subunit. The extracellular portion of each one consists of a headpiece, which binds fibrinogen, and a tailpiece, which connects to the plasma membrane. In the inactive state, these two domains are folded down onto each other like a closed switchblade, making contact with fibrinogen impossible. What was not clear is how this closed state opens to allow fibrinogen binding. As it turns out, the headpiece of the beta subunit consists of two domains joined by a hinge. Xiao and Springer, who are both at the CBR Institute for Biomedical Research, and their colleagues theorized that once the integrin is activated, these two domains open up, swinging out, which triggers the opening of the headpiece- tailpiece switchblade. The headpiece would then be free to bind the clot-promoting fibrinogen. Xiao and his colleagues crystallized the beta headpiece with two fibrinogen antagonists, eptifibatide (Integrilin) and tirofiban hydrochloride (Aggrastat), and observed their structures with X-ray crystallography. "The swinging out we see in the headpiece strongly suggests a switchblade model for the whole integrin," said Xiao.
Though the two drugs are used in patients to prevent clots, they can have deleterious side effects. "By visualizing the structure of the drugs binding to the integrins, we can find ways to improve them," Xiao said.
--Misia Landau
Eosinophils Play Role in Chronic Allergic Asthma
Ten million Americans--including three million children--suffer from allergic asthma. A recent study indicates that certain white blood cells, the eosinophils, play a major role in the chronic form of the disease by contributing to the destructive airway remodeling found in patients. The research, by co-first author Alison Humbles, HMS instructor in pediatrics, and senior author Craig Gerard, HMS professor of pediatrics, both at Children's Hospital Boston, is reported in the Sept. 17 Science.Normally rare in the bloodstream, eosinophils can accumulate in large numbers during a parasitic infection or an allergic asthma response. In the past, researchers viewed the eosinophil as a likely drug target in allergic asthma, but the approach was abandoned when a study in humans showed that reducing the number of eosinophils had no effect on the acute allergic asthma response. "What we normally test in clinical trials is lung function and the acute response," said Gerard. "On the other hand, there's another part of asthma that no one quite knows how to study: airway remodeling." This is a long-term effect of chronic allergic asthma that is characterized by scar tissue and muscle thickening in the lining of the lungs.
When Stuart Orkin, the David G. Nathan professor of pediatrics at Dana-Farber Cancer Institute and Children's, made a mouse that completely lacks eosinophils, Humbles and Gerard saw their chance to retest the eosinophil-asthma connection. Humbles immunized normal mice and the eosinophil-lacking mice against an antigen and then administered the same antigen to the animals in mist form. As predicted, the normal mice had an acute allergic asthma response, but so did the eosinophil-lacking mice. "We didn't see any changes in airway function," said Humbles, "but we needed to look later." When the researchers examined the mice after 55 days, they found that the eosinophil-lacking mice had less collagen build-up and less smooth- muscle thickening, indicating that eosinophils play a major role in airway remodeling.
"This is exciting because our data mirrors the human data," explained Gerard. "This study has now brought eosinophils back to the mix in asthma for both research and drug discovery."
--Jillian Lokere
Complement Linked to Tissue Damage in Diabetes
The high blood sugar of diabetes may leave tissues prey to the complement arm of the immune system. The first evidence in humans that makes this link--between complement and the vascular complications of the disease--appears in the Sept. 24 Diabetes.Jose Halperin, HMS associate professor of medicine, and his colleagues have been gradually gathering evidence to explain why chronically high levels of glucose in the blood of diabetics often result in extensive kidney, nerve, retina, and vascular damage. The link they now report is a side effect of high levels of blood sugar shutting off the cells' ability to defend themselves against complement, a first line of defense against foreign invaders.
Several complement proteins band together to form the membrane attack complex (MAC), which drills pores directly into the cells of bacteria, foreign cells, and other invaders. Host cells have to protect themselves against these constantly roaming attackers with a fortress of protective proteins. Halperin and colleagues have shown that high levels of glucose can chemically alter one of these protective proteins, CD59, rendering it inactive. With defenses down, cells can be breached by MAC, allowing growth factors and other signals to flow out and leading to the abnormal proliferation of cells found in the blood vessels of diabetes patients.
Though the hypothesis makes a compelling case, evidence in humans has been missing. In their latest study, the researchers painstakingly constructed an antibody that would bind only to the glycated form of CD59. This probe identified glycated CD59 in about 60 percent of biopsy samples of kidney and nerve tissue from diabetes patients, but in none of the samples from nondiabetics. The team also found MAC deposits in the same vicinity as the altered CD59 protein.
Other mechanisms undoubtedly work together with CD59 inhibition to cause vascular complications of diabetes, Halperin said, but no single mechanism explains why animal models of diabetes do not reproduce these complications in the combination and intensity seen in humans, the only species having a glycation site on CD59. "This could be the missing link," Halperin said.
--Courtney Humphries
Cell Death Proteins Counter Chemo Resistance
Cancer treatment is often complicated by the resistance of some cancer cells to chemotherapeutic drugs. Now, a study led by Beth Israel Deaconess faculty members Luiz Zerbini, HMS research fellow in medicine, and Towia Libermann, HMS associate professor of medicine, has uncovered two potential targets for emerging cancer treatments. The researchers found that the expression of two key pro-apoptotic proteins, GADD45-alpha and GADD45-gamma, is often suppressed in cancer cells and that restoring their expression causes the cells to commit suicide. The study is published in the Sept. 14 Proceedings of the National Academy of Sciences.Previous work had indicated that the transcription factor NF-kappa B activates several anti-apoptotic genes. Many cancer cells express active NF-kappa B and so are able to resist the cell death-inducing properties of chemotherapy. When Zerbini blocked NF-kappa B in various cancer cell lines, he found that the cells underwent apoptosis. The dying cells had a strong upregulation of GADD45-alpha and GADD45-gamma, two genes that are downstream of NF-kappa B in the signaling pathway. "Although we knew GADD45-alpha and -gamma could induce cell growth arrest, we had no previous evidence for a correlation with apoptosis," said Zerbini.
To test the connection, Zerbini overexpressed GADD45-alpha and -gamma in prostate cancer cells and found the cells were induced to self-destruct. RNA interference against GADD45-alpha and -gamma had the opposite effect, allowing the cells to strongly resist pro-apoptotic signals. "GADD45-alpha and -gamma might be master switches in apoptosis because we see a lot of other pathways that also lead to the GADD45 family," Libermann said. The discovery that GADD45-alpha and -gamma can counteract the effects of constitutively activated NF-kappa B means that researchers could design drugs to specifically activate GADD45-alpha and -gamma, thereby sending cancer cells to their death.
"A lot of chemotherapeutic agents actually lead to activation of NF-kappa B, which is counterproductive and will lead to resistance to cell death," said Libermann. "Ideally, you would like to be able to combine a therapeutic approach that activates pro-apoptotic genes downstream of NF-kappa B with those chemotherapeutic agents."
--Jillian Lokere