Lithium as a therapeutic strategy for SNAP25 encephalopathies via modulation of homeostatic plasticity

Synaptic vesicle fusion is orchestrated by the soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) complex. The SNARE complex includes SNAP25, synaptobrevin-2, and syntaxin-1. This machinery drives action potential-evoked release, as well as the majority of spontaneous release. Mutations in components of the SNARE machinery can differentially affect spontaneous or evoked release and result in severe neurodevelopmental phenotypes, as exemplified by SNAP25 encephalopathies that arise from mutations in the SNAP25 gene. Among various pathogenic SNAP25 variants, L50S, V48F, and D166Y increase spontaneous glutamate release that destabilizes network activity. Patients who carry these variants develop recurrent seizures that are refractory to conventional antiepileptic drugs, which primarily act on components of evoked neurotransmission and leave spontaneous release abnormalities unaddressed. To surmount this limitation, we explored a strategy targeting homeostatic plasticity mechanisms with lithium, which has been shown to downscale postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR). In vitro, lithium corrected cellular phenotypes in neurons expressing SNAP25 variants. In vivo, we generated a knock-in mouse model recapitulating the SNAP25 L50S mutation to test these results. These mice displayed increased neonatal mortality, infertility, hypomobility, frequent seizure activity, anxiety-like behavior, social deficits, and impaired learning and memory. Chronic lithium treatment reduced seizure frequency and ameliorated long-term memory and locomotor activity. Taken together, lithium alleviated synaptic and behavioral disturbances driven by pathological spontaneous glutamate release by inducing homeostatic AMPAR downscaling. Thus, this work provides a mechanistically grounded therapeutic strategy for SNAP25 encephalopathies and other disorders characterized by elevated spontaneous glutamate release.