A Small-Molecule TrkB Agonist Promotes Synaptic Resilience Against Aβ and Tau Oligomers and Mitigates Alterations in TrkB-Akt and Fyn Signaling

BACKGROUND: Pathogenic amyloid-beta and tau oligomers (oAβ, oTau) drive synaptic degeneration and cognitive decline in Alzheimer's disease (AD). Tropomyosin receptor kinase B (TrkB), expressed in neurons and glia, supports synaptic integrity in AD-affected regions. We tested whether PTX-BD4-3, a small-molecule TrkB partial agonist, promotes neuroprotective signaling, reduces degenerative signaling, and prevents synaptic loss in an AD mouse model and hippocampal neurons exposed to oAβ or oTau. METHODS: Cultured hippocampal neurons were treated with oAβ or oTau ± PTX-BD4-3. Effects on synaptic proteins (±TrkB-blocking antibody), spine density, and Fyn, NR2B, and tau phosphorylation as well as tau oligomerization were assessed. Two cohorts of APPL/S mice, 5 and 12 months of age, received intraperitoneal PTX-BD4-3 or vehicle for 3 months, followed by assessment of synaptic proteins, dendritic spines, TrkB signaling, and Y-maze analysis. RESULTS: In vitro, PTX-BD4-3 prevented oAβ-induced synaptic marker loss in a TrkB-dependent manner, reduced phosphorylation of Fyn, NR2B, and tau, suppressed oligomeric tau formation, and attenuated oTau-induced spine loss. In APPL/S mice, PTX-BD4-3 mitigated deficient TrkB/Akt signaling, reduced phosphorylation of Fyn and tau, preserved synaptic markers and dendritic spines, and improved Y-maze performance. CONCLUSIONS: Small-molecule TrkB partial agonists may normalize TrkB signaling and promote synaptic resilience, supporting their potential as AD therapeutics.