Abstracts

Rationale: AMPA receptors (AMPARs) mediate excitatory synaptic transmission in the central nervous system and are involved in diverse forms of synaptic plasticity. They are heterotetrameric receptors composed of four pore-forming subunits (GluA1/4). GluA2 subunits are the most abundant, and present in virtually all AMPAR in hippocampal pyramidal neuron and other cell types. While GluA1, GluA3, and GluA4 subunits remain unedited, the GluA2 subunit undergoes Q/R editing in adult mammalian neurons. Edited GluA2-containing AMPARs are calcium-impermeable (CI-AMPARs), whereas edited GluA2-lacking AMPARs are calcium-permeable (CP-AMPARs). These characteristics change the electrophysiological properties of the receptors, making them functionally distinct in neuronal circuitry. The balance between CP-AMPARs and CI-AMPARs is crucial, as de novo mutation in Gria2, which encodes GluA2, result in neurodevelopmental disorders frequently accompanied by seizures in human (Salpietro et al., 2019). Furthermore, various mouse models have been developed to study the importance of CI AMPARs in brain alterations. Loss of edited GluA2 subunit function in mutant mice induces a decrease in weight and size, as well as premature mortality. These mice also exhibit cognitive impairment in memory-related behavioral tests as well as impairment in motor coordination (Jia et al., 1996). Some authors have also observed spontaneous epileptic seizures and increased susceptibility to kainate-induced seizures (Konen et al., 2020).

The cell types and the mechanisms associated with CI AMPARs that lead to seizures must be clarified to better understand these pathologies and develop successful therapeutic strategies. Knowing that AMPAR subunit composition differs in different cell types, we studied GluA2 deletion in GABAergic neurons and principal neurons separately.


Methods: We used the Cre-Lox system, with a floxed Gria2 gene under the control of the CamKII promoter to study GluA2 deletion in principal neurons and the GAD2 promoter for studying GluA2 deletion in GABAergic neurons. We performed depth electrode recording to monitor epileptic activity, and motor coordination behavioral tests used for cerebellar ataxia evaluations (Guyenet et al., 2010).


Results: Compared to control mice and floxed GluA2 CamKII-cre mice, floxed GluA2 GAD2-cre mice are smaller and have decreased weight. Interestingly, we found that GAD2-cre, but not CamKII-cre mice, show spontaneous seizures. The depth electrode recordings confirmed that floxed GluA2 GAD2-cre mice have abnormal neuronal activity. Additionally, behavioral tests show that GABAergic cell-specific GluA2 deletion led to motor coordination deficits.


Conclusions: AMPAR GluA2 subunit expression in GABAergic neurons but not in principal neurons is essential for a healthy brain. Ongoing analyses and whole-cell patch-clamp experiment will characterize the pathological electrophysiological activity underlying increased seizure susceptibility in GluA2 GAD2-cre mice.


Funding: NIH / NIMH R00 MH118425 to JDA and Brain and Behavior Research Foundation 30264 to JDA.