A new role for “good” microorganisms

Findings at Yale question the consensus on how the immune system interacts with bacteria.

The human immune system is a finely honed defense mechanism that quickly detects and destroys bacterial and viral invaders. But so-called “good” bacteria—which perform useful physiological roles—are a puzzling exception: they somehow slip under the immunological radar and form flourishing colonies on or in our bodies. Such “commensal microflora” are particularly abundant in the colon, which teems with some 10 trillion bacteria that help to metabolize nutrients and guide normal tissue development.

The scientific consensus has been that the immune system overlooks the colon’s commensal microbes because they remain “sequestered” within a layer of epithelial cells. However, the immune system does sometimes attack the colon’s commensal bacteria, causing inflammatory bowel diseases such as Crohn’s disease and ulcerative colitis.

In 1997, in collaboration with the late Charles A. Janeway Jr., M.D., Ruslan M. Medzhitov, Ph.D., professor of immunobiology, discovered toll-like receptors (TLRs), a new class of molecules in the innate immune system [“The Toll Road,” Spring 2002], and he suspected that they might be involved in inflammatory bowel diseases. To find out, he and his colleagues injected DSS, a substance that kills colonic epithelial cells, into both normal mice and mice with nonfunctional TLRs. Because of the TLRs’ assumed role in inflammatory bowel diseases as attackers of good bacteria, the researchers fully expected that there would be no inflammation in the mice with the disabled TLRs.

But to Medzhitov’s amazement, the opposite occurred. As reported last July in the journal Cell, even the smallest doses of DSS had no effect on normal mice, but in the mice with compromised TLRs they caused marked weight loss, severe colonic bleeding and death. Moreover, tissue samples from the colons of these mice showed that the normal cell cycle was profoundly disrupted. In short, TLRs looked more like defenders than attackers.

These startling findings led Medzhitov to question the long-held view that commensal microbes are fully shielded from the immune system. Instead he surmised that commensal bacteria may be only partially sequestered, and that recognition of these bacteria by TLRs triggers the production of molecules that protect the colon. In the case of acute epithelial injury, a TLR response to exposed microflora might efficiently recruit and direct healing molecules.

To test these wholly new ideas, Medzhitov and his colleagues turned their original procedure on its head. Instead of knocking out TLRs genetically, they used antibiotics to eliminate good bacteria from mice with normal TLR activity. Doses of DSS caused the same profuse intestinal bleeding and high mortality rate found in mice with faulty TLRs. But DSS caused no ill effects in mice that had received bacterial fragments known to activate TLRs.

“Because this all was so unexpected, it brought up a lot of questions that we’re following up now and finding a lot of interesting and surprising results,” said Medzhitov. But he believes that the studies have one immediate implication for clinical practice. Patients at risk for opportunistic infections, including those undergoing treatment with radiation or chemotherapy, routinely receive powerful antibiotics that kill off commensal intestinal flora. By combining antibiotics with TLR-activating compounds such as the bacterial fragments used in his mouse studies, Medzhitov says, it may be possible to prevent infection while triggering tissue protective responses needed to repair damage induced by radiation and chemotherapy.

“This idea could be directly relevant, and could be tested in clinical trials and used in patients—if not the precise regimen, the basic approach,” Medzhitov said. “That would be very exciting.”

—Peter Farley

Lipid found to play key role in transmitting information between synapses

Yale researchers have found that a membrane lipid plays a crucial role in communicating information between synapses in the brain, according to a study published in Nature in September.

“This study is the first to show that lowering the levels of this lipid in nerve terminals affects the efficiency of neurotransmission,” said senior author Pietro De Camilli, M.D., FW ’79, the Eugene Higgins Professor of Cell Biology and a Howard Hughes Medical Institute investigator.

De Camilli and his team started by genetically engineering laboratory mice that lacked an enzyme, PIPK1-gamma, which in turn plays a role in synthesizing the lipid under investigation, phosphatidylinositol-4,5-bisphosphate—or PtdIns(4,5)P2—a member of a class of lipids called phosphoinositides. The mice born without PIPK1-gamma were apparently normal, but they were unable to feed and died quickly. Studies of their nervous systems revealed lower levels of PtdIns(4,5)P2 and a partial impairment both of the process of fusion of synaptic vesicles and of their recycling.

De Camilli’s laboratory has studied extensively the mechanism underlying cycling of synaptic vesicles, small sacs that contain neurotransmitters that exchange information between neurons. Synaptic vesicles release their contents at junctions between nerve terminals by fusing with the plasma membrane, where they rapidly reinternalize, reload with neurotransmitter and are reused.

These studies not only provide new insight into basic mechanisms in synaptic transmission, said De Camilli, a member of the Kavli Institute for Neuroscience at Yale, but also have implications for medicine. For example, Down syndrome patients have an extra copy of the gene encoding the enzyme synaptojanin 1, which degrades PtdIns(4,5)P2 in the brain. Patients with Lowe syndrome, who also have mental retardation, lack another PtdIns(4,5)P2-degrading enzyme. Cancer and diabetes also can result from abnormal metabolism of phosphoinositides, De Camilli said.

“Typically, studies of synaptic transmission have focused on membrane proteins,” he said. “Only recently has the importance of the chemistry of membrane lipids and of their metabolism started to be fully appreciated. The field is still in its infancy, but rapid advancements in the methodology for the analysis of lipids promise major progress in the field and the possibility of identifying new targets for therapeutic interventions in human diseases.”

—Jacqueline Weaver

Et Cetera

Alzheimer’s protein solved

Using X-ray crystallography, Yale scientists have discerned, for the first time, the atomic structure of a protein that is linked to Alzheimer’s disease.

Ya Ha, Ph.D., assistant professor of pharmacology, reported in August in Molecular Cell that he had observed an unusual feature of human amyloid precursor protein (APP). Rare mutations of APP cause Alzheimer’s at an early age in a small number of people. Researchers have been trying to determine what app does and how it converts to a smaller protein, amyloid beta-peptide, which forms neuronal and vascular amyloid deposits typical of Alzheimer’s disease.

Ha and Yongcheng Wang, Ph.D., a postdoc in his laboratory, found that APP consists of two long rodlike molecules that form a tight complex, with the head of one molecule touching the tail of the other.

“That observation suggested a novel possibility,” said Ha, “that app may function to mediate cell-to-cell contact by interacting with itself.”

—John Curtis

Growth factor linked to asthma

Yale scientists have found that a molecule normally associated with the growth of new blood vessels in the lungs probably plays a role in asthma, raising the possibility of developing drugs that block the molecule’s receptors and signaling pathways.

The molecule, vascular endothelial growth factor (VEGF), induced asthmalike abnormalities when it was expressed in the lungs of transgenic mice, according to a report published in the journal Nature Medicine in September.

“To our surprise, in addition to growing new blood vessels, many features of asthma were also seen in these mice,” said principal investigator Jack A. Elias, M.D., the Waldemar Von Zedtwitz Professor of Medicine. “We saw mucus formation, airway fibrosis and asthmalike pulmonary function abnormalities. We also found that if you block VEGF, you block the asthmalike manifestations in other mouse asthma models.”

Elias and his team are currently examining how VEGF works at the cellular and molecular levels.

—Karen Peart


			Originally published in Yale Medicine, Spring 2005. Copyright © 2005 Yale University School of Medicine. All rights reserved.