Synaptic Plasticity

Synaptic plasticity is the capacity of a synapse to change its strength — the cellular substrate of learning, and the property whose loss corresponds most closely to dementia. In this corpus it is rarely treated as a thing that simply fails. It is treated as a regulated process that can fail in two opposite directions, and the more interesting arguments here are about the second direction.

Two prize submissions bring the concept in from different sides, and they agree on the substrate while disagreeing about the failure mode.

The membrane is the machine

Both accounts locate plasticity not in the synapse abstractly but in the lipid environment that organises its receptors. Membrane lipid rafts — cholesterol-rich microdomains — compartmentalise the signalling and adaptor proteins that synaptogenesis depends on. Plasticity is a property of that organisation, so anything that degrades the membrane degrades the capacity to learn before it kills anything.

Head (submission 13) makes the case interventionally. Neuron-targeted caveolin-1 over-expression (SynCav1) increased raft formation and raft-localised TrkB and NMDA-receptor subunits (GluN1, GluN2A, GluN2B), raised markers of plasticity — synaptobrevin, synaptophysin, syntaxin 1A, neurexin — enhanced hippocampal neuroplasticity, and improved hippocampal-dependent learning and memory. A one-time AAV9 delivery preserved hippocampal structure and cognition in an Alzheimer mouse model without removing plaques.

That last clause is the argument. The corpus reads this submission as a therapy defined by the pathology it leaves alone: if cognition can be preserved while amyloid burden is untouched, then amyloid removal is not the operative variable for cognition. Head's own framing is blunter — failed anti-amyloid trials suggest that removing toxic amyloid species alone will not restore neuronal and synaptic plasticity in the degenerating brain.

Rappoport (submission 57) makes the case mechanistically and inverts the failure mode. On his account the disease is driven by neural cholesterol deficiency — failure of astrocyte-to-neuron cholesterol transport via ApoE particles — which prevents the raft formation that synaptic stabilisation requires. The consequence is not too little plasticity but too much: neurons trapped in chronic positive plasticity, where Aβ and hyperphosphorylated tau act as persistent retraction signals.

The double-edged reading

Rappoport's inversion is the corpus's most distinctive claim about this concept. Plasticity is normally treated as the good thing that Alzheimer's destroys. Here it is a cycle with a forward leg and a return leg, and the lesion is on the return: the synapse can still be modified but can no longer be fixed in place, so modification never consolidates into memory.

This reading has a known weakness, and the corpus records it: a theory in which plasticity failing in either direction produces the disease is at risk of being unfalsifiable by symmetry. The evaluation of Rappoport's work in this corpus makes that its central charge.

Where it sits in the framework

Both submissions score heavily on the same nodes — Endosomal Nexus (Head 9, Rappoport 8) and APOE4 Hub (Head 7, Rappoport 8) — which is the quantitative form of the shared claim: plasticity fails through lipid trafficking, and ApoE is the trafficker. Rappoport additionally loads Cytoskeletal Collapse Node (6), where the retraction signal does its damage.

In the temporal architecture, plasticity loss is a Phase III readout — the symptomatic stage — but both accounts insist its substrate is degraded far earlier, which is why the corpus treats membrane and cholesterol biology as upstream rather than as late-stage collateral.

What else in the corpus bears on it

• Trophic support: BDNF/TrkB is foundational to the plasticity Alzheimer's destroys, and the strong form of that claim survives only where BDNF has been experimentally restored.
• Matrix: perineuronal nets are the structure that closes plasticity at the end of a critical period; their remodelling in cognitive resilience is a claim about plasticity being reopened, not merely preserved.
• The reelin series treats plasticity as something actively guarded, with reelin staged in the matrix as the brake on tau.

Advanced by

Submission 13, Brian Head — caveolin-1 gene therapy preserves hippocampal function independently of amyloid. Submission 57, Ari Rappoport — cholesterol deficiency, lipid rafts, and chronic positive plasticity. (Rappoport's theory is final; he died in June 2025.)

Related

Membrane Biophysics · Cholesterol Transport · Lipid Transport · Perineuronal Nets · cognitive resilience · Tau Pathology · Endosomal Nexus · APOE4 Hub