A new study in mice suggests that Alzheimer's disease (AD) may be triggered when adult neurons try to divide. The finding helps researchers understand what goes wrong in the disease and may lead to new ways of treating it. The study was funded in part by the National Institute of Neurological Disorders and Stroke (NINDS), part of the National Institutes of Health, and appears in the January 18, 2006 issue of The Journal of Neuroscience*.
For unknown reasons, nerve cells (neurons) affected by AD and many other neurodegenerative diseases often start to divide before they die. The new study shows that, in animal models of AD, this abnormal cell division starts long before amyloid plaques or other other markers of the disease appear. Cell division occurs through a process called the cell cycle. “If you could stop cell cycling, you might be able to stop neurons from dying prematurely. This could be a fresh approach to therapy for Alzheimer's and other diseases, including stroke, amyotrophic lateral sclerosis [also known as Lou Gehrig's disease], and HIV dementia,” says Karl Herrup, Ph.D., of Case Western Reserve University in Cleveland, who led the study.
The researchers compared the brains of three different mouse models of AD to brains from normal mice, looking specifically for markers of cell cycling. They found that, in the AD mouse models, cell cycle-related proteins appeared in neurons 6 months before the first amyloid plaques or disease-related immune reactions developed in the brain. Many of the neurons also had increased numbers of chromosomes, which is typical of cells that have begun to divide. These changes were not seen in normal mice. The regions of the brain most affected by the neuronal cell cycling were the cortex and the hippocampus — the same regions most affected in AD. The cortex is important for thought and reasoning, while the hippocampus plays a key role in learning and memory. Some parts of the brainstem also showed evidence of cell cycling.
While the cell cycling appeared to be necessary for neurons to die, it was not an immediate cause of cell death in the mouse models of AD. Instead, the affected neurons appeared to live for many months in a near-functional state, with the mice showing only mild behavioral changes during that time. This suggests that another type of cellular problem, still unidentified, must damage the neurons in order for them to die.
The findings shed new light on the theory that the accumulation of amyloid beta in the brain causes the neuron death in AD. Because the abnormal cell cycling begins months before the formation of amyloid plaques, it is unlikely that the plaques themselves trigger the disease process. However, tiny clumps made up of several amyloid beta molecules (called micro-molecular aggregates) form before the plaques and may trigger the disease. Since the three mouse models tested in this study all had mutations in the gene that codes for amyloid precursor protein, the similarity between affected brain regions in these mice and in people with AD also supports the amyloid hypothesis.
*YangY, Varvel NH, Lamb BT, Herrup K. Ectopic cell cycle events link human Alzheimer’s disease and APP transgenic mouse models. The Journal of Neuroscience, January 18, 2005, Vol. 26, No. 3, pp. 775-784.
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