Plasma growth-associated protein 43 correlates with synaptic loss in Alzheimer’s disease.

Synaptic loss is a hallmark of Alzheimer's disease (AD) but lacks robust blood-based biomarkers. We investigate growth-associated protein 43 (GAP-43), previously identified as a synaptic candidate in the cerebrospinal fluid (CSF). Postmortem proteomic profiling of brain-derived extracellular vesicles (n = 21) highlights GAP-43 as a central hub within synaptic protein networks co-depleted in AD and closely linked with proteins enriched in immune-, metabolic-, and synaptic-related modules. In two well-characterized Chinese AD cohorts (n = 785), we measure plasma GAP-43, including subgroups with CSF biomarkers (n = 72), SV2A-PET (positron emission tomography) (n = 85), tau-PET (n = 280), and magnetic resonance imaging (MRI) (n = 595). Plasma GAP-43 correlates with CSF GAP-43, neurofilament light, and both baseline and longitudinal synaptic PET. Elevated plasma GAP-43 is associated with greater tau aggregation, faster brain atrophy, and accelerated cognitive decline, particularly among cognitively unimpaired individuals. These findings support plasma GAP-43 as a promising biomarker of early synaptic degeneration and a potential tool for identifying individuals at risk of AD progression.