BIN1 gain-of-function in the presynaptic compartment leads to isoform-specific synaptotoxicity

Alzheimer's disease (AD) is associated with strong genetic predisposition and early synaptic loss that correlates with cognitive decline. While genetic determinants are thought to contribute to synaptic vulnerability, their precise role in AD pathogenesis at the synaptic level remains unclear. BIN1, a major AD susceptibility gene, is expressed in multiple isoforms, but isoform-specific effects at synapses have not been well defined.

We investigated the impact of human BIN1 isoforms on synaptic structure and function using Drosophila and mammalian models. In flies, we overexpressed human BIN1 isoforms in retinal photoreceptor neurons and motoneurons, assessing synaptic function by electrophysiology and ultrastructural analyses. Morphological changes at neuromuscular junctions were also quantified. For both readouts, Rab11 modulation was tested as a potential rescue strategy. To determine conservation in mammals and distinguish presynaptic versus postsynaptic roles, we overexpressed BIN1 isoform 1 selectively in presynaptic or postsynaptic compartments of rat hippocampal neurons cultured in microfluidic devices. We assessed structural and functional connectivity using immunofluorescence and microelectrode arrays.

Gain-of-function of BIN1 isoform 1, but not isoforms 8 or 9, induced early loss of synaptic transmission in Drosophila photoreceptor neurons indicating BIN1iso1 synaptotoxicity. Structural analyses revealed accumulation of abnormally large vesicles in photoreceptor terminals, resembling BIN1-induced endosomal defects in cell bodies. Moreover, Rab11 gain-of-function prevented BIN1iso1 synaptotoxicity, suggesting that it originates from endosomal trafficking defects. In motoneurons, BIN1iso1 overexpression induced synapse remodelling, altering bouton morphology, including increased bouton number, reduced bouton size, and formation of satellite boutons. Rab11 modulation was additive to BIN1 isoform 1 effects, suggesting a distinct mechanism. In rat hippocampal neurons, BIN1 isoform 1 decreased synaptic connectivity only when overexpressed presynaptically, a finding confirmed by microelectrode array recordings.

Our findings demonstrate that BIN1iso1 exerts isoform-specific, presynaptic disruption during synapse development and maintenance that compromises synaptic integrity across species. BIN1iso1 synaptotoxicity may contribute to early synapse loss observed in AD and provides mechanistic evidence that genetic determinants such as BIN1 predispose synapses to failure. These results highlight BIN1iso1 as a potential target for therapeutic strategies aimed at preserving synaptic function in AD.