Researchers at Georgetown University Medical Center have a new theory as to why people with Alzheimer's disease have trouble performing even the simplest memory tasks.
That's because they discovered a physical link between apolipoprotein E ( APOE ), the transport molecules known to play a role in development of the disease, and glutamate, a brain chemical necessary for establishing human memory.

In a study published in the Journal of Biological Chemistry, the research team specifically found that receptors on the outside of brain nerve cells that bind on to APOE and glutamate are connected on the surface of neurons, separated from each other by only a small protein.

While the researchers don't know why these receptors are linked together, they say inefficient or higher-than-average levels of APOE in the brain could possibly be clogging these binding sites, preventing glutamate from activating the processes necessary to form memories.

" We have found out that two receptors previously thought to have nothing to do with each other do, in fact, interact, leading us to conclude that APOE affects the NMDA glutamate channel that is important in memory," says the study's senior author, G. William Rebeck, at Georgetown's Biomedical Graduate Research Organization.

The researchers also hypothesize that this interaction might have something to do with development of Alzheimer's disease, although they stress that at this early stage of research, this is impossible to prove.

Rebeck and first author Hyang-Sook Hoe, also of Georgetown, say that laboratory work now underway is attempting to unravel the relationship between APOE and NMDA receptors.

APOE is a protein that helps shuttle cholesterol and other non-soluble lipid particles around the body, moving these substances to where they are needed. All cells have receptors that bind on to APOE so that they can use lipids as needed, such as for quick energy, to store as fat for later use, or to repair wounds.

But researchers now know that APOE does more than distribute lipids, especially in the brain. About a decade ago, scientists linked APOE4, one of the three common forms of APOE, to development of Alzheimer's disease, although the biological link between the protein and neurodegenerative diseases such as Alzheimer's is not clear.

Based on recent research, Rebeck and others suspect that, in the brain, APOE also acts as a transporter, picking up lipids and perhaps other material that result from normal brain tissue wear and tear, or from trauma, and moving it to where it can be used or can be cleared away from the brain. Work in Rebeck's lab found that APOE receptor 2 ( ApoEr2 ), one of the eight different APOE receptor types, is crucial to both the development and operation of a normal brain.

Glutamate increases the strength of a synaptic response following stimulation. The NMDA glutamate receptor binds on to the drug NMDA, and also on to glutamate, an excitatory neurotransmitter that also stimulates nerve cell activity. Researchers know that the NMDA receptor is needed to produce the long-lasting synaptic response that is necessary in order to establish, or "lay down," memory, Rebeck says. " The molecular basis of memory depends on NMDA receptor. "

In work leading up to this study, Rebeck and the research team found that adding APOE to neurons in laboratory culture blocked NMDA receptors.
In this study, they confirmed through a series of experiments that the receptors for APOE and NMDA interacted, and that the protein that linked the two was PSD95, often found in neural synaptic junctions. Together, they form a multiprotein complex that could presumably be activated by either APOE, NMDA or glutamate.

Rebeck suspects that the APOE4 variant -- the one linked to Alzheimer's disease -- is less efficient at removing lipid debris in the brain than is APOE2 or APOE3, and because of this, brain cells secrete more of the faulty protein to do the job. If too much APOE ends up binding to the APOE/NMDA receptor, one of two things could possibly happen, Rebeck says. In one scenario, the receptor becomes over-stimulated due to the accumulating presence of APOE, which could trigger a process called excitotoxicity that results in death of the neruons. Or, in the presence of damage and secreted APOE, the receptor "turns down" its activity -- thus, hampering memory formation -- until the brain is repaired. " Having damage around tells the brain not to think too much for awhile, " Rebeck says. But if APOE4 cannot clear up accumulating damage, the ability to make new memories, and use old ones, may be increasingly lost.

" This is, of course, speculation, but now we have new avenues in which we can explore the molecular basis of memory and possibly Alzheimer's disease," Rebeck says.

Source: Georgetown University Medical Center, 2006


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