THE PATHOGENIC KCC2 R857G VARIANT INDUCES ALTERED CORTICAL DEVELOPMENT AND IMPAIRED GABA SIGNALING IN A NOVEL EIMFS MOUSE MODEL

The potassium–chloride cotransporter KCC2, encoded by SLC12A5, is the primary chloride (Cl⁻) extruder in mature neurons and is essential for fast synaptic inhibition. Mutations in SLC12A5 are linked to epilepsy of infancy with migrating focal seizures (EIMFS), a severe developmental epileptic encephalopathy characterized by early-onset seizures and developmental regression. In this study, we characterize a de novo mouse model carrying the Slc12a5 c.2569C>G (R857G) mutation, which affects the same amino-acid residue as the pathogenic human R857L variant identified in an individual with EIMFS.
Our findings show that the KCC2^R857G mutation leads to reduced glycosylation, impaired membrane trafficking, and intracellular retention of the transporter. Heterozygous KCC2^R857G mice exhibit a pathological phenotype, including spontaneous tonic–clonic seizures, interictal spikes, and memory deficits in a novel object recognition task. At the cellular level, the mutation disrupts early neurodevelopment, as evidenced by a layer-specific reduction in cortical neuron density at birth, and induces brain region–specific alterations in dendritic spine morphology in adulthood, likely reflecting non-canonical KCC2 functions related to actin dynamics. At postnatal day 30, hippocampal CA1 pyramidal neurons display a shift in the polarity of GABAergic responses, consistent with altered intracellular chloride homeostasis and impaired inhibition.
Collectively, these results demonstrate that the KCC2^R857G mouse model recapitulates key cellular and network features relevant to EIMFS. This work provides mechanistic insight into how disrupted inhibition and developmental defects combine to drive the pathophysiology of severe childhood epilepsies and establishes a valuable platform for investigating disease mechanisms and therapeutic strategies.