Reciprocal Regulation of KCC2 Trafficking and Synaptic Activity

The main inhibitory neurotransmitter receptors in the adult central nervous system (CNS) are type A γ-aminobutyric acid receptors (GABAARs) and glycine receptors (GlyRs). Synaptic responses mediated by GlyR and GABAAR display a hyperpolarizing shift during development. This shift relies mainly on the developmental up-regulation of the K+-Cl− co-transporter KCC2 responsible for the extrusion of Cl−. In mature neurons, altered KCC2 function—mainly through increased endocytosis—leads to the re-emergence of depolarizing GABAergic and glycinergic signaling, which promotes hyperexcitability and pathological activities. Identifying signaling pathways and molecular partners that control KCC2 surface stability thus represents a key step in the development of novel therapeutic strategies. Here, we present our current knowledge on the cellular and molecular mechanisms governing the plasma membrane turnover rate of the transporter under resting conditions and in response to synaptic activity. We also discuss the notion that KCC2 lateral diffusion is one of the first parameters modulating the transporter membrane stability, allowing for rapid adaptation of Cl− transport to changes in neuronal activity

Rapid regulation of the neuronal K/Cl co-transporter KCC2 by excitation and inhibition in mature neurons

La polarité et l'efficacité de la transmission GABAergique dépendent de la concentration intra-neuronale en chlore. Dans les neurones matures, le co-transporteur K+/Cl- KCC2 maintient la concentration intracellulaire en chlore à un niveau bas, permettant ainsi une réponse inhibitrice du GABA. En plus de son rôle dans la transmission GABAergique, KCC2 régule aussi l'efficacité de la transmission glutamatergique en contrôlant la spinogenèse, l'exocytose et la dynamique membranaire des récepteurs AMPA. Du fait de son importance aux synapses excitatrices et inhibitrices, il est crucial de comprendre les mécanismes qui régulent l'expression membranaire et la fonction de KCC2. La régulation de KCC2 par l'activité glutamatergique excitatrice ayant été bien caractérisée, il reste à déterminer si l'expression et la fonction de KCC2 sont régulées par l'activité inhibitrice GABAergique. Pendant ma thèse, j'ai montré que KCC2 est en effet directement régulé par la transmission GABAergique. J'ai trouvé que l'activation aigue des RGABAA confine KCC2 dans la membrane alors que le blocage des RGABAA augmente la dynamique membranaire et l'internalisation du transporteur. Les mécanismes moléculaires impliquent le chlore comme messager secondaire, la kinase WNK1 et la phosphorylation de KCC2 sur des résidus thréonines clés. J'ai ensuite pu montrer que cette régulation à un impact aux synapses inhibitrice et excitatrice. Mon travail propose un mécanisme nouveau de la régulation de l'homéostasie du chlore par l'inhibition GABAergique. Ainsi les neurones peuvent compenser une augmentation ou une diminution en chlore neuronale par une adaptation rapide de KCC2 à la surface cellulaire.The polarity and efficacy of GABAergic neurotransmission depends on the intraneuronal chloride concentration. In mature neurons chloride extrusion by the K+/Cl- co-transporter KCC2 permits an inhibitory influx upon activation of GABAA receptors. In addition to its role in GABAergic transmission, KCC2 regulates also glutamatergic transmission in an ion-independent manner by controlling spinogenesis and AMPAR exocytosis and membrane diffusion in dendritic spines. Knowing its pivotal role at central synapses, it is of particular importance to understand the cellular and molecular mechanisms underlying its regulation. While regulation of KCC2 by neuronal excitation is well documented, it is still unknown whether neuronal inhibition itself can regulate the transporter’s membrane expression and/or activity. During my PhD I was able to demonstrate a direct regulation of KCC2 membrane diffusion and stability by GABAA receptor-mediated inhibition and I characterized the underlying signaling cascade. I found that activation of GABAAR decreased KCC2 lateral diffusion while GABAAR blockade led to increased membrane dynamics and internalization of the transporter. I could show that KCC2 regulation by neuronal inhibition requires chloride as second intracellular messenger and chloride-sensing WNK1 kinase that directly phosphorylate KCC2 on key Threonine residues. This regulation has a functional impact at both excitatory and inhibitory synapses. My work reports a novel and rapid mechanism of control of chloride homeostasis by GABAA receptor-mediated inhibition that allows maintaining the polarity and activity of GABAA receptors constant

Reciprocal Regulation of KCC2 Trafficking and Synaptic Activity

International audienceThe main inhibitory neurotransmitter receptors in the adult central nervous system (CNS) are type A γ-aminobutyric acid receptors (GABA A Rs) and glycine receptors (GlyRs). Synaptic responses mediated by GlyR and GABA A R display a hyperpolarizing shift during development. This shift relies mainly on the developmental up-regulation of the K +-Cl − co-transporter KCC2 responsible for the extrusion of Cl −. In mature neurons, altered KCC2 function-mainly through increased endocytosis-leads to the re-emergence of depolarizing GABAergic and glycinergic signaling, which promotes hyperexcitability and pathological activities. Identifying signaling pathways and molecular partners that control KCC2 surface stability thus represents a key step in the development of novel therapeutic strategies. Here, we present our current knowledge on the cellular and molecular mechanisms governing the plasma membrane turnover rate of the transporter under resting conditions and in response to synaptic activity. We also discuss the notion that KCC2 lateral diffusion is one of the first parameters modulating the transporter membrane stability, allowing for rapid adaptation of Cl − transport to changes in neuronal activity

Evolutionary change from induced to constitutive expression of an indirect plant resistance

Induced plant resistance traits are expressed in response to attack and occur throughout the plant kingdom1,2. Despite their general occurrence, the evolution of such resistances has rarely been investigated3. Here we report that extrafloral nectar, a usually inducible trait, is constitutively secreted by Central American Acacia species that are obligately inhabited by ants. Extrafloral nectar is secreted as an indirect resistance4, attracting ants that defend plants against herbivores5. Leaf damage induces extrafloral nectar secretion in several plant species6-8; among these are various Acacia species and other Fabaceae investigated here. In contrast, Acacia species obligately inhabited by symbiotic ants9 nourish these ants by secreting extrafloral nectar constitutively at high rates that are not affected by leaf damage. The phylogeny of the genus Acacia and closely related genera indicate that the inducibility of extrafloral nectar is the plesiomorphic or 'original' state, whereas the constitutive extrafloral nectar flow is derived within Acacia. A constitutive resistance trait has evolved from an inducible one, obviously in response to particular functional demands