Altered Fast Synaptic Transmission in an in vitro Mouse Model of Epileptic Encephalopathy

Epileptic encephalopathies (EEs) are severe seizure disorders that occur in infants and young children, characterized by developmental delay, and cognitive decline. Recent efforts have identified a wide variety of genetic variants that cause EEs. Among these, variants in the DNM1 gene have emerged as definitive causes of EEs, including Infantile Spasms and Lennox-Gastaut syndrome. A mouse model of Dnm1-associated EE, known as “Fitful” (Dnm1Ftfl), recapitulates key features of the disease, including spontaneous seizures, developmental delay, and neuronal degeneration. Previous work showed that Dnm1 is a key regulator of synaptic vesicle (SV) endocytosis and synaptic transmission, and suggested that inhibitory neurotransmission is more reliant on Dnm1 function than excitatory transmission. The Dnm1Ftfl variant is thought to encode a dominant negative Dnm1 protein, however, the effects of the Dnm1Ftfl variant on synaptic transmission are largely unknown. To better understand these synaptic effects, we recorded from pairs of cultured mouse cortical neurons and characterized all four major connection types (E-E, E-I, I-E, I-I). Miniature and spontaneous EPSCs and IPSCs were larger, but less frequent, at all Dnm1Ftfl synaptic types, and Dnm1Ftfl/Ftfl neurons had reduced expression of excitatory and inhibitory SV markers. Baseline evoked transmission, however, was reduced only at inhibitory synapses onto excitatory neurons, due to a smaller pool of releasable SVs. In addition to these synaptic alterations, Dnm1Ftfl neurons degenerated after two weeks in vitro, even though their activity levels were not elevated, suggesting that Dnm1Ftfl may impair synaptic transmission and neuronal health through distinct mechanisms