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STUDY UNCOVERS NEW CONTRIBUTOR TO BRAIN DAMAGE, SUGGESTING NOVEL DRUG TARGET
The results appear in the March issue of Nature Medicine. The first author is Byung Hee Han, M.D., research associate in neurology at Washington University School of Medicine in St. Louis. David M. Holtzman, M.D., an associate professor of neurology and of molecular biology and pharmacology, led the research team. Blockage of blood flow to the brain before or during birth can temporarily deprive the brain of oxygen and other nutrients, as happens when an adult has a stroke. Without a continuous supply of oxygen, some brain cells die or are damaged, causing hypoxic-ischemic injury. The outcome may be long-term cognitive difficulties, seizures and motor problems, as in cerebral palsy. Scientists know of two main types of cell death, each triggered by a different cascade of biological events. In apoptosis, cells shrink and die. In necrosis, cells swell and burst. Apoptosis accounts for about half of the cell death that occurs after H-I injury to the developing brain. Researchers have identified several ways to block apoptosis and thereby lessen brain deterioration. Necrosis might account for the other half of brain injury after prenatal or perinatal brain trauma. It also might account for most of the cell death after adult brain injury. But because scientists have not identified all the key components of the necrosis pathway, it has been difficult to intervene to prevent it from killing cells. To identify the main factors that contribute to the aftereffects of decreased oxygen and blood flow to the brain in newborns, Holtzman’s team first examined a mouse model of cerebral palsy that mimics the effects of H-I injury. They found that an enzyme called caspase-3 became highly active in the brain cells of these mice, indicating the onset of apoptosis. But some of the dying brain cells had large amounts of a protein called clusterin. Clusterin was known to accumulate during cell death in several organs, including the brain. But its role in nerve-cell damage was unclear. So the researchers determined how mice lacking clusterin react to brain injury. The mice suffered roughly half as much brain injury as mice that were able to make clusterin. "We thought clusterin might help protect cells against injury. But apparently, it actually contributes to cell death," Holtzman says. Two observations made the researchers conclude that clusterin is not involved in apoptosis. First, the lack of clusterin in the genetically altered mice failed to affect caspase-3 activity. Second, the brain cells that accumulated clusterin and the cells with activated caspase-3 did not overlap, cells had clusterin or caspase-3 but not both. "This is exciting because very few molecules are known to influence the necrotic component of cell death," says Holtzman. "Either clusterin contributes to a new pathway that’s neither necrotic nor apoptotic or it actually contributes to necrosis." Having shown that clusterin plays a critical role in neonatal brain injury, Holtzman and his colleagues hope to determine whether the protein also becomes involved after other forms of acute nervous system injury such as stroke and spinal cord injury as well as in neurodegenerative diseases such as Alzheimer’s disease. "If the implications of this paper are correct, we predict that clusterin will play a role in brain injury models that include some form of caspase-independent injury, such as necrosis," Holtzman says. "We also think that if you block both clusterin and caspase-3 effects, you will have an additive effect. Modifying the clusterin pathway may therefore be a new target for developing therapies."
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