Science News
University of Pennsylvania Medical Center Department of Public Affairs
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Contact: Franklin Hoke
Phone: (215) 349-5659 or (215) 662-2560
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SCIENCE TIP SHEET: April 1997
Philadelphia, Pa. -- Below are three selected story ideas based on research projects in progress at the University of Pennsylvania Medical Center:
RECONNECTING AFTER SPINAL CORD DAMAGE: Young sea lampreys possess a remarkable capability that humans do not: "After a spinal cord injury, the lamprey becomes paralyzed just as humans do, but, within a few weeks, it recovers," says Michael E. Selzer, MD, PhD, a professor of neurology. "Unfortunately, this is something that mammals cannot do, and we would very much like to better understand the basis of the lamprey's regenerative abilities." Investigations by Selzer and his colleagues of the eel-like fish have shown that lamprey nerve fibers regenerate across the site of injury in a coordinated manner. "The nerve fibers grow in the correct direction, and when they cross the damaged area they create synapses with the appropriate target nerve cells to restore function. It is not a random process." Selzer's team is exploring the mechanisms underlying the lamprey's enviable powers. So far, experiments have shown that the tip of the regenerating lamprey nerve cell, known as the growth cone, is tightly packed with structures called neurofilaments. This differs dramatically from what is seen during neural growth in embryonic development -- the only time of life when most mammalian neurons grow. Embryonic neural growth cones lack neurofilament protein and advance with the help of structures known as filopodia, which contain a protein called actin. But filopodia are absent and actin is sparse in the growth cones of regenerating lamprey nerve cells. "Our hypothesis is that the transport of neurofilament into the growth cone pushes it forward from within, almost as though it was under pressure, as opposed to the embryonic mechanism, whereby the growth cone is pulled by the filopodia," says Selzer. In current experiments, the scientists are overexpressing the neurofilament protein in lamprey nerve cells to see whether the speed of regeneration can be increased. The lamprey neurofilament protein has mammalian analogs, and Selzer suggests the work might someday lead to new therapies in humans: "It is pure speculation at this point, but it may be that temporarily expressing lamprey neurofilament in people with central nervous system injuries would cause the nerve fibers to grow and move beyond the barriers to regeneration."
-- Dr. Michael E. Selzer can be reached at (215) 662-3396.
SERENDIPITOUSLY, HIV-RELATED ANTIBODY IN HAND: The big news from the HIV basic research front in 1996 was the discovery by Penn researchers and others of a group of cell-surface receptor molecules that, along with the long-known receptor CD4, must be present for the virus to enter and infect cells of the immune system. The usual role of these receptors is to offer binding sites for various chemokines, a family of molecules involved in mediating immune responses; different HIV strains, however, have evolved to use the receptors for their purposes, too. During the same period, James A. Hoxie, MD, an associate professor of medicine, and his coworkers developed an antibody against one of these important chemokine receptors, a molecule called CXCR4, or fusin, discovered at the National Institutes of Health -- although this fact was not immediately apparent to them. The Penn scientists had originally set out to produce monoclonal antibodies to SIV, or simian immunodeficiency virus, which causes an AIDS-like disease in monkeys. To do this, they immunized mice with SIV-infected cells and screened antibodies derived from the mice for the ability to block infection by the virus in cell lines. While they were successful in producing a number of antibodies against SIV in this way, they also inadvertently stumbled upon an antibody that was able to block infection but that reacted with the cell rather than with the virus. When this antibody, termed 12G5, was also shown to block some types of HIV from infecting cells, they redoubled their efforts to identify the molecule with which 12G5 was reacting. Following the lead that CXCR4 was required by some viruses to infect cells, they checked to see if 12G5 reacted with CXCR4 and were pleasantly surprised to find that it did. The new antibody represented the first anti-CXCR4 antibody and has since made many new studies involving this molecule possible. As a tool, the antibody has enormous value to HIV investigators, as evidenced by the fact that Hoxie has so far supplied it on request to about 250 other laboratories around the world. "Our discovery of this antibody was somewhat serendipitous, but now we're up and running with a unique reagent that has been extremely useful to many laboratories" says Hoxie. In the March 4 issue of the Proceedings of the National Academy of Sciences, for example, researchers at Harvard Medical School and LeukoSite Inc. used Hoxie's antibody and one against another chemokine receptor known as CCR5 to create a detailed hypothesis describing how HIV infection leads to the slow, progressive destruction of the immune system.
-- Dr. James A. Hoxie can be reached at (215) 898-0261.
MAGNETIC ATTRACTION -- TOWARDS A NEW ERA IN TREATING RH-FACTOR INCOMPATIBILITY: Physicians have known for decades that fetuses positive for a red blood cell antigen called Rh-factor D are at risk when their mothers are negative for the factor. If blood from the developing fetus mingles with the mother's blood, the mother will generate powerful antibodies capable of devastating attacks on the baby's red blood cells. Since the early 1960s, however, Rh immune globulin -- which can only be derived from Rh-negative individuals who have been exposed to Rh-positive blood -- has been given prophylactically to Rh-negative pregnant women. Rh immune globulin is itself composed of anti-Rh antibodies, yet when administered as a drug it prevents formation of the deadly antibodies against the fetus. This treatment has been very successful, but it has given rise to a new problem: As time passes, the available pool of Rh immune globulin donors exposed to Rh-positive blood prior to the 1960s dwindles. It is unethical to create new donors through intentional exposure, and no animal is able to generate the needed antibodies. Now, Donald L. Siegel, MD, PhD, an assistant professor of pathology and laboratory medicine, has developed a new technology that promises to address this need and that may have uses against cancer and other diseases, too. Siegel's approach extends a technique for producing large libraries of antibodies called phage display, in which molecular biologists engineer millions of bacteria to display a wide range of antibodies on their surfaces. Purified antigen -- the entity against which an antibody reacts -- is then attached to an inert laboratory surface to which the phage-displayed antibodies are exposed. The specific desired antibodies bind to the antigen and are thus isolated. This selection process, called panning, is repeated to achieve greater purity. "The limitation, however, is that there are many important antigens -- Rh factors, for example -- that we want antibodies against but that are bound in the membranes of cells and cannot be purified in a way that maintains their native properties," Siegel notes. To circumvent this difficulty, Siegel uses whole cells in a modified panning process. He first coats a small number of Rh-positive cells with magnetic beads and then adds a much larger number of Rh-negative cells. To this mix, he introduces a phage library developed from an Rh immune globulin donor. Most of the antibodies, against antigens of no current interest, bind to the excess of Rh-negative cells, and the desired anti-Rh antibodies bind to the Rh-positive cells. Siegel then uses a magnet to pull the target cells with their antibodies in tow away from the other cells. As with ordinary panning, repetition leads to refinement. With this strategy, Siegel isolated more than 50 different anti-Rh antibodies in one experiment, an unprecedented number. In addition to its effectiveness, the process is inexpensive and fast. Also, differences between cells within an individual could be exploited in this way to discover diagnostic or therapeutic antibodies, such as antibodies that might act against lung tumor cells but not against normal lung cells.
-- Dr. Donald L. Siegel can be reached at (215) 662-3942.
The University of Pennsylvania Medical Center's sponsored research ranks fifth in the United States, based on grant support from the National Institutes of Health, the primary funder of biomedical research in the nation -- $149 million in federal fiscal year 1996. In addition, for the second consecutive year, the institution posted the highest growth rate in its research activity -- 9.1 percent -- of the top ten U.S. academic medical centers during the same period.
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