Stabilizing the retromer complex rescues synaptic dysfunction and endosomal trafficking deficits in an Alzheimer's disease mouse model.

Disruptions in synaptic transmission and plasticity are early hallmarks of Alzheimer's disease (AD). Endosomal trafficking, mediated by the retromer complex, is essential for intracellular protein sorting, including the regulation of amyloid precursor protein (APP) processing. The VPS35 subunit, a key cargo-recognition component of the retromer, has been implicated in neurodegenerative diseases, with mutations such as L625P linked to early-onset AD. Despite growing evidence for retromer dysfunction in AD, its role in synaptic pathology and neuroinflammation remains incompletely understood. Here, we investigate the acute molecular effects of retromer stabilization in the 5xFAD mouse model of AD using the pharmacological chaperones R55 and R33, previously identified to enhance VPS35 stability. Following intracranial stereotaxic injections, we performed transcriptomic profiling, quantitative histology, and immunohistochemistry to assess synaptic function, neuroinflammation, and endosomal trafficking. Our findings reveal that retromer stabilization reverses multiple AD-associated molecular changes. R55 treatment significantly reduced Aβ-related pathology, normalized synaptic gene expression, and restored long-term potentiation (LTP)-associated pathways, including Gria1 (AMPA receptors), Grip1, and semaphorin/plexin signaling. Additionally, retromer stabilization counteracted dysregulated calcium signaling by modulating Ryr2 and L-type calcium channel expression. Beyond synaptic effects, we observed broad transcriptional and structural changes in the endosomal system. Notably, R55 treatment decreased VPS13 family gene expression, implicated in membrane contact site regulation, while increasing RAB7 levels, suggesting enhanced late-endosomal recycling. VPS35-positive vesicles were redistributed away from the nucleus, indicating restored intracellular trafficking dynamics. In the neuroinflammatory domain, retromer stabilization modulated microglial activation, shifting towards a profile characterized by balanced pro-inflammatory (Il1, Nfkb2) and anti-inflammatory (Il4r, Il13ra1, Stat6) markers, consistent with disease-associated microglia (DAM) phenotypes. Together, these findings demonstrate that retromer dysfunction contributes to key AD pathologies, including synaptic dysfunction and neuroinflammation, and that pharmacological retromer stabilization can restore cellular homeostasis. Given that 5xFAD mice lack direct VPS35 mutations, our results suggest that retromer-targeting strategies may be applicable to both familial and sporadic AD, offering a promising therapeutic avenue for modifying disease progression.