Loss of KCC2 in GABAergic Neurons Causes Seizures and an Imbalance of Cortical Interneurons

Abstract

K-Cl transporter KCC2 is an important regulator of neuronal development and neuronal function at maturity. Through its canonical transporter role, KCC2 maintains inhibitory responses mediated by γ-aminobutyric acid (GABA) type A receptors. During development, late onset of KCC2 transporter activity defines the period when depolarizing GABAergic signals promote a wealth of developmental processes. In addition to its transporter function, KCC2 directly interacts with a number of proteins to regulate dendritic spine formation, cell survival, synaptic plasticity, neuronal excitability, and other processes. Either overexpression or loss of KCC2 can lead to abnormal circuit formation, seizures, or even perinatal death. GABA has been reported to be especially important for driving migration and development of cortical interneurons (IN), and we hypothesized that properly timed onset of KCC2 expression is vital to this process. To test this hypothesis, we created a mouse with conditional knockout of KCC2 in Dlx5-lineage neurons (Dlx5 KCC2 cKO), which targets INs and other post-mitotic GABAergic neurons in the forebrain starting during embryonic development. While KCC2 was first expressed in the INs of layer 5 cortex, perinatal IN migrations and laminar localization appeared to be unaffected by the loss of KCC2. Nonetheless, the mice had early seizures, failure to thrive, and premature death in the second and third weeks of life. At this age, we found an underlying change in IN distribution, including an excess number of somatostatin neurons in layer 5 and a decrease in parvalbumin-expressing neurons in layer 2/3 and layer 6. Our research suggests that while KCC2 expression may not be entirely necessary for early IN migration, loss of KCC2 causes an imbalance in cortical interneuron subtypes, seizures, and early death. More work will be needed to define the specific cellular basis for these findings, including whether they are due to abnormal circuit formation versus the sequela of defective IN inhibition.

Layer-specific changes of KCC2 and NKCC1 in the mouse dentate gyrus after entorhinal denervation

“…However, compared to the ipsilateral iml and the contralateral oml/mml, the fluorescence intensity in the ipsilateral oml/mml was reduced, suggesting that KCC2 is primarily lost from the denervated dendritic segments of the granule cells but not from the denervated dendrites of interneurons. Interestingly, a recent study showed that a selective permanent loss of KCC2 in interneurons leads to cortical seizures due to alterations in the distribution of interneuron subtypes ( Zavalin et al, 2022 ). That suggests that a granule cell-specific, transient reduction of KCC2 in the denervated dentate gyrus with preserved KCC2 in interneurons would not cause such a dramatic excitation-inhibition imbalance.…”

Neuronal K+-Cl- cotransporter KCC2 as a promising drug target for epilepsy treatment

“… [ 150 ] KCC2 Mouse with conditional knockout of KCC2 in Dlx5-lineage neurons (Dlx5 KCC2 cKO) (in vivo) Electrophysiology Loss of KCC2 caused occasional spontaneous seizures. [ 159 ] KCC2 A mouse line with 2–3-fold KCC2 overexpression occurs in pyramidal neurons (in vivo) EEG d monitoring Adult Enhanced KCC2 expression increased diazepam’s efficacy in stopping EEG seizures. [ 160 ] Preventing KCC2- Thr906/Thr1007 phosphorylation Intraperitoneal injection of kainate in male KCC2-Thr906Ala/Thr1007Ala knock-in mice (in vivo) EEG monitoring Neonatal Prevention of Thr906 and Thr1007 phosphorylation delayed the onset and severity of kainate induced seizures.…”

“…Zavalin et al conducted a study where they conditionally knocked out KCC2 in Dlx5-lineage neurons (Dlx5 KCC2 cKO) in a mouse line, targeting cortical interneurons and post-mitotic GABAergic neurons in the forebrain during embryonic development. They found that the loss of KCC2 caused an imbalance in cortical interneuron subtypes, occasional spontaneous seizures, and early death [ 159 ]. Cheung et al established a mouse line with 2–3-fold KCC2 overexpression occurring in pyramidal neurons.…”

Rett and Rett-related disorders: Common mechanisms for shared symptoms?

“…122 Reduced expression of KCC2 disrupts the E/I balance causing brain dysfunction, including intractable epilepsy. [125][126][127][128][129] Indeed, KCC2 mutations that impair extrusion of Cl − humans result in severe seizures in mice and in humans [130][131][132] and are associated with schizophrenia and ASD. 130 Similarly, psychological stress and neuroinflammation during early prenatal development reduce KCC2 expression delaying the E/I GABA shift, which has been implicated in neurodevelopmental and psychiatric disorders.…”