β-Amyloid induces microglial expression of GPC4 and APOE leading to increased neuronal tau pathology and toxicity.

To define how Aβ pathology alters microglia function in Alzheimer’s disease, we profiled the microglia surfaceome following treatment with Aβ fibrils. Our findings reveal that Aβ-associated human microglia upregulate Glypican 4 (GPC4), a GPI-anchored heparan sulfate proteoglycan (HSPG). Glial GPC4 expression exacerbates motor deficits and reduces lifespan in a Drosophila amyloidosis model, implicating GPC4 in a toxic neurodegenerative program. In cell culture, GPC4 enhances microglia phagocytosis of tau aggregates, and shed GPC4 can act in trans to facilitate tau aggregate uptake and seeding in neurons. Additionally, our data demonstrate that GPC4-mediated effects are amplified in the presence of APOE. In human Alzheimer’s disease brain, microglial GPC4 expression surrounding Aβ plaques correlates with neuritic tau pathology, supporting a pathological link between amyloid, GPC4, and tau. These studies define a mechanistic pathway by which Aβ primes microglia to promote tau pathology via HSPGs and APOE.