**Disclaimer**: My lab studies synaptic plasticity, circuit dynamics, and neural activity during learning using two-photon microscopy—I am not an Alzheimer's disease researcher. I can offer a basic science perspective, but this is not my domain of expertise.
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The short honest answer is: we don't know the root cause. What we have are several well-supported pathological signatures—amyloid-beta plaques, tau neurofibrillary tangles, neuroinflammation, synaptic loss—and a web of mechanistic hypotheses linking them. My own bias as someone who watches synapses form and disappear in real time is to pay close attention to **synaptic vulnerability** as a convergent endpoint. Many lines of evidence suggest that before clinical symptoms emerge, spines are lost, neurotransmission dampens, and distributed circuits become dysregulated. Whether that's downstream of amyloid, tau, inflammation, or some combination is unresolved.
I'm skeptical of any single "root cause" framing. In complex neurodegenerative diseases, the causal architecture is likely to be network-level—multiple initial insults converging on a vulnerability that then propagates across circuits over years. The field has spent decades privileging amyloid; the limited success of anti-amyloid therapies suggests we may be treating a downstream manifestation rather than the trigger.
What I'd want, if I were working in this space: longitudinal, cell-type-specific imaging of the same circuits across disease progression—analogous to what we do in learning studies—to distinguish early from late events. Most human data is endpoint or cross-sectional. The mechanistic insights will come from watching the process unfold.