Bartzokis notes that myelination of the brain follows an inverted U-shaped trajectory, growing strongly until our 50s, when it very slowly begins to unravel as we age. The myelin that is deposited in adulthood ensheaths increasing numbers of axons with smaller axon diameters and so spreads itself thinner and thinner, Bartzokis said. As a result, it becomes more susceptible to the ravages of age in the form of environmental and genetic insults and slowly begins to break down faster than it can be repaired.
The exclusive targeting of the amyloid-beta peptide for many years is understandable because the same genes and enzymes involved in controlling myelination and myelin repair are, ironically, also involved in the production of amyloid-beta proteins. Bartzokis' point is that the amyloid beta may actually develop as a result of the natural process of the repair and maintenance of myelin.
"So the breakdown that leads to Alzheimer's and other age-related brain diseases, such as Parkinson's, may begin much earlier, before the formation of the protein deposits that are used to define these diseases," Bartzokis said.
Most drugs being developed for Alzheimer's are targeting amyloid beta, but little if any clinical improvement is being seen. This is, according to Bartzokis, "similar to cleaning up a house that's been flooded by water but never repairing the actual pipe that created the flood.
"For drug development then, the targets should be much further upstream, earlier in the process before the AB plaques even develop," he said.
Instead of focusing on reducing amyloid beta, Bartzokis argues, the myelin model suggests entirely different approaches to treatment and prevention of Alzheimer's disease that precede plaque formation. With modern brain imaging technology, clinician
|Contact: Mark Wheeler|
University of California - Los Angeles