Published in the July 17 issue of the Proceedings of the National Academy of Sciences, the study shows how apoE4 could cause the formation of neurofibrillary tangles, one of two pathological hallmarks of Alzheimer's that are presumed to contribute to the degeneration of neurons in this disease. The tangles represent an abnormal buildup of various nerve cell cytoskeletal proteins, which may contribute to brain cell damage.
ApoE4 is formally called apolipoprotein E4 and is one of three forms of a group of apolipoproteins. The other two are called apoE2 and apoE3 and are not risk factors for Alzheimer's. Anywhere from 40 to 60 percent of patients with Alzheimer's carry apoE4.
The present study demonstrates that the apoE4 taken up by or made specifically by nerve cells is cut by an unknown enzyme, which activates the apoE fragment in such a way that it interacts with the intracellular cytoskeletal proteins, called tau and neurofilaments, to form neurofibrillary tangle-like structures.
"This is a significant step that suggests how apoE4 might disrupt the ability of brain cells to interact and interconnect," said co-author Robert W. Mahley, MD, PhD, senior Gladstone investigator and UCSF professor of pathology and medicine. "The cleavage of apoE4 to create an active fragment provides a mechanism whereby apoE can alter the structure and function of the cytoskeleton, precipitate the cytoskeletal proteins to form the tangle-like structures, and impair normal nerve cell function."
Previously, Gladstone scientists have shown that apoE4 interferes with the ability of nerve cells to send out the long cellular processes that connect one cell to another, whereas apoE3 and apoE2 promote the formation of these connections. A normal cytoskeleton within nerve cells is required for the cells in the nervous system to establish their connections and to transmit impulses or messages important in memory, learning, and other neurological functions.
Identifying the enzyme that clips and activates the apoE could lead to the development of an Alzheimer's disease treatment, said lead author Yadong Huang, MD, PhD, Gladstone investigator and UCSF assistant professor of pathology.
"When we find the enzyme that cleaves apoE4, then we can design an inhibitor to block its action and, possibly, alter the progress of the disease," he said.
The researchers first examined the brains of deceased Alzheimer's patients and found that the neurofibrillary tangles were composed of truncated versions of apoE4, along with cytoskeletal proteins. The next piece of evidence came from studies of cells grown in culture. Examining those that express apoE4 and apoE3 showed that the unknown enzyme was more likely to cut apoE4 than apoE3 and that the truncated form of apoE4 was more active in forming the tangle-like structures in the nerve cells. Cells from peripheral organs that were made to express apoE4 and apoE3 did not form any tangle-like structures.
The researchers don't know exactly why apoE4 is so much more susceptible to being cut to form the active fragment, but Huang speculated that the reason could lie in the three-dimensional structure of the protein.
The cells expressing apoE4 were also more likely to contain clumps that resemble neurofibrillary tangles. About 78 percent of nerve cells that expressed truncated apoE4 contained neurofibrillary tangle-like structures. These tangles were only present in 32 percent of nerve cells expressing truncated apoE3.
Co-investigators of this study include Xiao Qin Liu, MD, research associate, Walter J. Brecht, senior research associate, and David A. Sanan, PhD, staff research scientist, all of the Gladstone Institute for Cardiovascular Disease; and Tony Wyss-Coray, PhD, staff research investigator at the Gladstone Institute of Neurological Disease and UCSF assistant professor of neurology.
This study was funded by the J. David Gladstone Institutes.
The Gladstone Institute of Neurological Disease is one of three research institutes that compose The J. David Gladstone Institutes, a private biomedical research institution affiliated with UCSF. The institution is named for a prominent real estate developer who died in 1971. His will created a testamentary trust that reflects his long-standing personal interest in medical education and research.