Active extrusion of Cl- from the neuronal cytoplasm by the neuron-specific K-Cl co-transporter isoform KCC2 is necessary for the hyperpolarizing inhibitory Cl- currents mediated by the GABA receptors (GABAARs). Early in development and following cellular trauma or seizures, GABAAR-mediated signaling is often depolarizing and may even, in contrast to its classical inhibitory action, promote action potential firing. Developmental up-regulation of KCC2 is largely responsible for the shift from depolarizing to hyperpolarizing GABAAR-mediated signaling, and conditions associated with brain pathology often lead to loss of KCC2 and re-emergence of depolarizing GABAAR responses. The molecular mechanisms responsible for the up-regulation of KCC2 during development and those mediating its down-regulation, however, remain elusive.
The present Thesis demonstrates that the low level of KCC2 protein in immature neurons is not a limiting factor for its functional activation. A single seizure episode induced with kainate triggers a fast transient enhancement of neuronal Cl- extrusion capacity paralleled by a large increase in surface-expressed but not total KCC2 protein in the hippocampus of neonatal rodents. This post-translational activation of KCC2 appears to be mediated by BDNF-TrkB signaling, as evidenced by its sensitivity to Trk inhibition and its absence in BDNF knockout mice. In contrast to these fast changes in functional expression of KCC2, no requirement for endogenous BDNF was observed for the developmental up-regulation of KCC2 protein. Another key finding of this work is that down-regulation and inactivation of KCC2 following intense NMDA receptor (NMDAR) activation is mediated via cleavage and truncation of KCC2 by the calcium-activated protease calpain. Importantly, the data obtained using inhibitors of protein degradation and protein synthesis indicate that the basal turn-over of KCC2 protein is slow and, consequently, down-regulation under pathological conditions is likely to result from enhanced degradation rather than from reduced de novo KCC2 synthesis. Together, the present findings highlight post-translational regulation as an important mediator of changes in the functional expression of KCC2 in response to conditions of enhanced neuronal activity, such as epileptic seizures.
KCC2 has been traditionally regarded to have the most clearly defined physio-logical role of all the K-Cl cotransporters, as it is uniquely expressed in central neurons, and determines the neuronal response to activation of GABAA and glycine receptors. However, such a view has changed drastically following the unexpected observation that KCC2 has also a structural role in the morphological maintenance of dendritic spines, one that is independent of its ability to transport ions. The intimate temporal coincidence between the developmental onset of KCC2 expression and the most intense phase of synaptogenesis during the brain growth spurt points to a possible role for this protein in synapse formation. Importantly, whether KCC2 plays a role in spinogenesis i.e. in induction of spines during the brain growth spurt has not been investigated so far. The results of the present work demonstrate that expression of KCC2 is not only a necessary but also a sufficient condition for the induction of functional glutamatergic spines during the brain growth spurt. The results of this work support the idea of KCC2 as an important synchronizing factor in the functional development of glutamatergic and GABAergic signaling.KCC2 on solukalvolla sijaitseva kalium-kloridi-kuljettaja, joka yhdessä mm. natrium-kalium-kloridi-kuljettajan (NKCC1) kanssa säätelee neuronien solunsisäistä kloridipitoisuutta. Kloridipitoisuuden muutokset puolestaan määräävät ovatko hermosolun GABA- tai glysiinireseptoreiden vasteet solukalvoa depolaroivia vai hyperpolaroivia. Aivojen varhaiskehityksen aikana nämä vasteet ovat usein depolaroivia ja aivojen kehittyessä KCC2:n toiminnallisen ilmentymisen kasvun seurauksena nämä vasteet muuttuvat hyperpolaroiviksi. KCC2:lla on myös toinen, ioninkuljetustoiminnoista riippumaton rakenteellinen rooli hermosolujen synapsien ylläpitämisessä. Ei ole kuitenkaan tiedossa mikä on KCC2:n merkitys uusien synapsien synnyssä.
Aivotrauman tai epileptisen hermoaktiivisuuden seurauksena GABA- tai glysiinireseptoreiden vasteet muuttuvat usein depolaroiviksi aikuisaivoissa. Syyt tähän ovat paljolti tuntemattomia, mutta seurauksena voi olla mm. epileptisen aktiivisuuden leviämistä estävien aivomekanismien eroosio sekä hyperpolarisoivia GABA vasteita hyödyntävien antiepileptisten lääkkeiden tehon heikkeneminen.
Tässä väitöskirjatyössä tutkitaan epileptisen aktiivisuuden laadullisesti erilaista vaikutusta KCC2:n kloridinkuljetustoiminnon ilmentymiseen aivojen varhaiskehityksen ja myöhemmän kehityksen välillä. Työn tulokset osoittavat, että kalpaiini-niminen entsyymi vastaa KCC2:n toiminnallisesta alasajosta epileptisen aktiivisuuden seurauksena. Tämän väitöskirjatyön toinen tärkeä löydös on, että KCC2:n geneettisesti ohjattu ylituotanto (yliekspressio) johtaa uusien synapsirakenteiden syntyyn
