The cytoskeleton in Alzheimer disease.

The strongest physical correlate with the severity of dementia in Alzheimer's disease and its most rational cause are the loss of neocortical and hippocampal synapses. Evidence, showing that beta-amyloid causes that loss is weak despite the popularity of that hypothesis. Other changes can better explain that damaging phenomenon. Axonal terminals are dependent on axoplasmic flow, and that function requires intact microtubules and the motor proteins kinesin, dynein and dynamin. It has been known since the earliest electron microscopic studies of AD that neuronal microtubules are lessened in number. Tubules are normally in equilibrium with unpolymerized tubulin, and the stability of the formed elements is dependent on normal binding of tau to the tubule. But, as is well known, tau is abnormally hyperphosphorylated in AD leading to tangle formation and to dissolution of the tubules. Tangles are insufficient in number to account for the cortical loss of neurons and synapses, but hyperphosphorylated tau in the unpolymerized pre-tangle state undoubtedly plays a role. Abnormalities in the motor proteins are now being investigated (some have already been found) and these too would contribute to the loss of synapses in AD by way diminished axoplasmic flow.