Alzheimer: how microtubules could be the bridge between amyloid and tau
The two best-known hallmarks of Alzheimer's disease, amyloid-β plaques and tau tangles, never appear at the same time. Amyloid comes first; tau follows years later. What connects the two is something nobody fully understands. A new hypothesis points to microtubules, the internal scaffolding of neurons.
The amyloid cascade hypothesis has been the dominant explanation for Alzheimer's disease for decades. It holds that an abnormal build-up of amyloid-β proteins sets the disease in motion. Tau aggregation only enters the picture later, forming tangles of tau proteins inside neurons themselves, and it is at that point that neurons begin dying in large numbers and cognitive decline takes hold. Yet how amyloid actually leads to tau pathology has never been satisfactorily explained.
One theory points to chronic neuroinflammation as the missing link: amyloid activates microglia and astrocytes, which in turn cause damage that destabilises tau. A more recent hypothesis, however, focuses on microtubules, the protein structures that run through neurons like rails, enabling the transport of nutrients and signalling molecules. Under normal conditions, tau acts as a stabiliser for microtubules. When tau loses that function or begins to aggregate, the entire transport system breaks down.
Amyloid undermines the rails
The emerging thinking suggests that amyloid-β directly or indirectly damages microtubules, even before tau becomes visibly pathological. Amyloid oligomers, small soluble clusters generally considered more toxic than the larger plaques, can disrupt the calcium balance inside neurons. Elevated calcium concentrations activate enzymes that break microtubules down. As those microtubular rails become less stable, tau has to work harder to keep them intact, and the system becomes overwhelmed.
This mechanism would explain why tau pathology appears later than amyloid: damage to microtubules accumulates slowly, until the point where tau can no longer function as a stabiliser and instead aggregates into a problem of its own. It would also explain why tau pathology does not spread uniformly across the brain, but starts in specific regions that are more vulnerable to this kind of stress.
Therapeutic possibilities, but plenty of open questions
If microtubules really are the missing link, that opens up new therapeutic angles. Compounds that stabilise microtubules already exist and are used in cancer treatment, taxol being one example. Applying them to neurological conditions is complicated, though: crossing the blood-brain barrier is notoriously difficult, and interfering with microtubular dynamics has wide-ranging effects throughout the body.
For now, this remains theoretical. The hypothesis is plausible and internally consistent, but direct experimental evidence for microtubules playing a specific connecting role between amyloid and tau in Alzheimer's disease is thin on the ground. The field needs more mechanistic hypotheses of this kind, but it also needs the patience to test them rigorously before presenting them as established explanations.