ALTERATION OF THE PRESYNAPTIC PHOSPHATIDYLINOSITOL 4,5-BISPHOSPHATE DISTRIBUTION AND ITS IMPACT ON SYNAPTIC TRANSMISSION IN A DOWN SYNDROME MOUSE MODEL

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) plays a crucial role in neurotransmitter release and synaptic vesicle endocytosis at active zones (AZs) of presynaptic terminals by regulating key proteins involved in exocytosis and endocytosis. In Down syndrome (DS), a trisomy of human chromosome 21, orthologous to mouse chromosome 16, leads to the overexpression of numerous genes, including several involved in synaptic function, such as synaptojanin-1, a phosphoinositide phosphatase with both 5- and 4-phosphatase activities. This suggests that the levels of PI(4,5)P₂ and its precursor PI(4)P are reduced in the brain due to accelerated turnover.
Here, we investigated the nanoscale distribution of PI(4,5)P₂ in the presynaptic membranes of cerebellar parallel fiber (PF) terminals in the DS mouse model Ts65Dn, using the SDS-digested freeze-fracture replica labeling (SDS-FRL) method with a specific PI(4,5)P₂ probe, the recombinant Pleckstrin-homology (PH) domain of phospholipase δ1. Quantitative analysis of the PI(4,5)P₂-labeled gold particle distribution pattern revealed a significant reduction in PI(4,5)P₂ density specifically within the AZs of Ts65Dn mice compared with euploid controls. In parallel, patch-clamp recordings from Purkinje cells showed a decreased paired-pulse ratio (PPR) of excitatory postsynaptic currents (EPSCs) evoked by electrical stimulation of PFs, suggesting increased presynaptic release probability in Ts65Dn mice. The reduced PPR was rescued by normalizing Synj1 expression through crossing with Synj1 knock-out mice, indicating that Synj1 overexpression increases presynaptic release probability. These findings demonstrate a selective reduction of PI(4,5)P₂ at AZs in DS model mice and support the hypothesis that altered phosphoinositide organization contributes to presynaptic dysfunction in Down syndrome.