Rôle du transporteur neuronal Potassium/Chlore KCC2 dans la plasticité des synapses glutamatergiques

The polarity and efficacy of GABAergic synaptic transmission are both influenced by the intra-neuronal chloride concentration. In mature neurons, chloride extrusion through the neuronal K/Cl cotransporter KCC2 allows an inhibitory influx of chloride upon activation of GABAA receptors. Nevertheless, KCC2 is enriched in the vicinity of excitatory synapses within the dendritic spines that are actin-rich protrusions emerging from dendritic shafts. While it has become clear that KCC2 suppression alters chloride homeostasis and GABA signaling, little is known on its impact on glutamatergic transmission. In the laboratory, we have previously demonstrated that KCC2 suppression in mature neurons leads to decreased glutamatergic transmission efficacy through an ion-transport independent function of KCC2. During my PhD, I have explored how KCC2 may also impact LTP of glutamatergic synapses. My work reveals that KCC2 suppression compromises both functional and structural LTP at these synapses. This effect is associated with inhibition of the actin-severing protein cofilin and enhanced mobilization of F-actin in dendritic spines. Since LTP can be rescued by preventing cofilin inhibition upon KCC2 suppression, I suggest KCC2 might influence LTP through altered actin cytoskeleton dynamics. My results demonstrate that KCC2 function extends beyond the mere control of neuronal chloride homoeostasis and suggest regulation of KCC2 membrane stability may act as a metaplastic switch to gate long term plasticity at excitatory synapses in cortical neurons.L'efficacité de la transmission synaptique GABAergique est influencée par la concentration intracellulaire en ions chlorure. Dans les neurones matures, l'extrusion de ces ions par le transporteur neuronal potassium chlore de type 2 (KCC2) permet l'influx d'ions chlorure lors de l'activation des récepteurs du GABA de type A. Néanmoins, KCC2 est aussi enrichi à proximité des synapses excitatrices portées par les épines dendritiques qui correspondent à des protrusions dendritiques enrichies en actine. Alors que l'effet d'une suppression de KCC2 sur l'homéostasie des ions chlorure et la transmission GABAergique est largement documenté, peu de choses sont connues sur l'impact qu'une telle suppression peut avoir sur la transmission glutamatergique. Lors de ma thèse, j'ai exploré le rôle de KCC2 dans la potentialisation à long terme (LTP) de la transmission glutamatergique à l'origine des phénomènes d'apprentissage et de mémorisation. Ce travail a révélé que la suppression de KCC2 compromet les modifications fonctionnelles et structurales sous-tendant la LTP. Cet effet est associé à une inhibition de la cofilin, protéine responsable de la dépolymérisation de l'actine, qui corrèle avec une augmentation de la quantité d'actine filamenteuse dans les épines dendritiques. En empêchant l'inhibition de la cofilin liée à l'absence de KCC2, il m'a alors été possible de restaurer la LTP suggérant que KCC2 pourrait influencer cette forme de plasticité en régulant la dynamique de polymérisation du cytosquelette d'actine. Mes résultats démontrent que la fonction de KCC2 va au-delà du contrôle de l'homéostasie des ions chlorure et influence les mécanismes de plasticité de la synapse excitatrice

Role of the neuronal K-Cl co-transporter KCC2 in inhibitory and excitatory neurotransmission

The K-Cl co-transporter KCC2 plays multiple roles in the physiology of central neurons and alterations of its function and/or expression are associated with several neurological conditions. By regulating intraneuronal chloride homeostasis, KCC2 strongly influences the efficacy and polarity of the chloride-permeable γ-aminobutyric acid (GABA) type A and glycine receptor (GlyR) mediated synaptic transmission. This appears particularly critical for the development of neuronal circuits as well as for the dynamic control of GABA and glycine signaling in mature networks. The activity of the transporter is also associated with transmembrane water fluxes which compensate solute fluxes associated with synaptic activity. Finally, KCC2 interaction with the actin cytoskeleton appears critical both for dendritic spine morphogenesis and the maintenance of glutamatergic synapses. In light of the pivotal role of KCC2 in the maturation and function of central synapses, it is of particular importance to understand the cellular and molecular mechanisms underlying its regulation. These include development and activity-dependent modifications both at the transcriptional and post-translational levels. We emphasize the importance of post-translational mechanisms such as phosphorylation and dephosphorylation, oligomerization, cell surface stability, clustering and membrane diffusion for the rapid and dynamic regulation of KCC2 function

Cortex-wide response mode of VIP-expressing inhibitory neurons by reward and punishment

Neocortex is classically divided into distinct areas, each specializing in different function, but all could benefit from reinforcement feedback to inform and update local processing. Yet it remains elusive how global signals like reward and punishment are represented in local cortical computations. Previously, we identified a cortical neuron type, vasoactive intestinal polypeptide (VIP)-expressing interneurons, in auditory cortex that is recruited by behavioral reinforcers and mediates disinhibitory control by inhibiting other inhibitory neurons. As the same disinhibitory cortical circuit is present virtually throughout cortex, we wondered whether VIP neurons are likewise recruited by reinforcers throughout cortex. We monitored VIP neural activity in dozens of cortical regions using three-dimensional random access two-photon microscopy and fiber photometry while mice learned an auditory discrimination task. We found that reward and punishment during initial learning produce rapid, cortex-wide activation of most VIP interneurons. This global recruitment mode showed variations in temporal dynamics in individual neurons and across areas. Neither the weak sensory tuning of VIP interneurons in visual cortex nor their arousal state modulation was fully predictive of reinforcer responses. We suggest that the global response mode of cortical VIP interneurons supports a cell-type-specific circuit mechanism by which organism-level information about reinforcers regulates local circuit processing and plasticity

Pyk2 modulates hippocampal excitatory synapses and contributes to cognitive deficits in a Huntington's disease model.

The structure and function of spines and excitatory synapses are under the dynamic control of multiple signalling networks. Although tyrosine phosphorylation is involved, its regulation and importance are not well understood. Here we study the role of Pyk2, a non-receptor calcium-dependent protein-tyrosine kinase highly expressed in the hippocampus. Hippocampal-related learning and CA1 long-term potentiation are severely impaired in Pyk2-deficient mice and are associated with alterations in NMDA receptors, PSD-95 and dendritic spines. In cultured hippocampal neurons, Pyk2 has autophosphorylation-dependent and -independent roles in determining PSD-95 enrichment and spines density. Pyk2 levels are decreased in the hippocampus of individuals with Huntington and in the R6/1 mouse model of the disease. Normalizing Pyk2 levels in the hippocampus of R6/1 mice rescues memory deficits, spines pathology and PSD-95 localization. Our results reveal a role for Pyk2 in spine structure and synaptic function, and suggest that its deficit contributes to Huntington's disease cognitive impairments

Cortex-wide response mode of VIP-expressing inhibitory neurons by reward and punishment

Neocortex is classically divided into distinct areas, each specializing in different function, but all could benefit from reinforcement feedback to inform and update local processing. Yet it remains elusive how global signals like reward and punishment are represented in local cortical computations. Previously, we identified a cortical neuron type, vasoactive intestinal polypeptide (VIP)-expressing interneurons, in auditory cortex that is recruited by behavioral reinforcers and mediates disinhibitory control by inhibiting other inhibitory neurons. As the same disinhibitory cortical circuit is present virtually throughout cortex, we wondered whether VIP neurons are likewise recruited by reinforcers throughout cortex. We monitored VIP neural activity in dozens of cortical regions using three-dimensional random access two-photon microscopy and fiber photometry while mice learned an auditory discrimination task. We found that reward and punishment during initial learning produce rapid, cortex-wide activation of most VIP interneurons. This global recruitment mode showed variations in temporal dynamics in individual neurons and across areas. Neither the weak sensory tuning of VIP interneurons in visual cortex nor their arousal state modulation was fully predictive of reinforcer responses. We suggest that the global response mode of cortical VIP interneurons supports a cell-type-specific circuit mechanism by which organism-level information about reinforcers regulates local circuit processing and plasticity

Involvement of the neuronal K/Cl cotransporter KCC2 in the plasticity of glutamatergic synapses

L'efficacité de la transmission synaptique GABAergique est influencée par la concentration intracellulaire en ions chlorure. Dans les neurones matures, l'extrusion de ces ions par le transporteur neuronal potassium chlore de type 2 (KCC2) permet l'influx d'ions chlorure lors de l'activation des récepteurs du GABA de type A. Néanmoins, KCC2 est aussi enrichi à proximité des synapses excitatrices portées par les épines dendritiques qui correspondent à des protrusions dendritiques enrichies en actine. Alors que l'effet d'une suppression de KCC2 sur l'homéostasie des ions chlorure et la transmission GABAergique est largement documenté, peu de choses sont connues sur l'impact qu'une telle suppression peut avoir sur la transmission glutamatergique. Lors de ma thèse, j'ai exploré le rôle de KCC2 dans la potentialisation à long terme (LTP) de la transmission glutamatergique à l'origine des phénomènes d'apprentissage et de mémorisation. Ce travail a révélé que la suppression de KCC2 compromet les modifications fonctionnelles et structurales sous-tendant la LTP. Cet effet est associé à une inhibition de la cofilin, protéine responsable de la dépolymérisation de l'actine, qui corrèle avec une augmentation de la quantité d'actine filamenteuse dans les épines dendritiques. En empêchant l'inhibition de la cofilin liée à l'absence de KCC2, il m'a alors été possible de restaurer la LTP suggérant que KCC2 pourrait influencer cette forme de plasticité en régulant la dynamique de polymérisation du cytosquelette d'actine. Mes résultats démontrent que la fonction de KCC2 va au-delà du contrôle de l'homéostasie des ions chlorure et influence les mécanismes de plasticité de la synapse excitatrice.The polarity and efficacy of GABAergic synaptic transmission are both influenced by the intra-neuronal chloride concentration. In mature neurons, chloride extrusion through the neuronal K/Cl cotransporter KCC2 allows an inhibitory influx of chloride upon activation of GABAA receptors. Nevertheless, KCC2 is enriched in the vicinity of excitatory synapses within the dendritic spines that are actin-rich protrusions emerging from dendritic shafts. While it has become clear that KCC2 suppression alters chloride homeostasis and GABA signaling, little is known on its impact on glutamatergic transmission. In the laboratory, we have previously demonstrated that KCC2 suppression in mature neurons leads to decreased glutamatergic transmission efficacy through an ion-transport independent function of KCC2. During my PhD, I have explored how KCC2 may also impact LTP of glutamatergic synapses. My work reveals that KCC2 suppression compromises both functional and structural LTP at these synapses. This effect is associated with inhibition of the actin-severing protein cofilin and enhanced mobilization of F-actin in dendritic spines. Since LTP can be rescued by preventing cofilin inhibition upon KCC2 suppression, I suggest KCC2 might influence LTP through altered actin cytoskeleton dynamics. My results demonstrate that KCC2 function extends beyond the mere control of neuronal chloride homoeostasis and suggest regulation of KCC2 membrane stability may act as a metaplastic switch to gate long term plasticity at excitatory synapses in cortical neurons

The metabolic signaling of the nucleoredoxin-like 2 gene supports brain function

International audienceThe nucleoredoxin gene NXNL2 encodes for two products through alternative splicing, rod-derived cone viability factor-2 (RdCVF2) that mediates neuronal survival and the thioredoxin-related protein (RdCVF2L), an enzyme that regulates the phosphorylation of TAU. To investigate the link between NXNL2 and tauopathies, we studied the Nxnl2 knockout mouse (Nxnl2−/−). We established the expression pattern of the Nxnl2 gene in the brain using a Nxnl2 reporter mouse line, and characterized the behavior of the Nxnl2−/− mouse at 2 months of age. Additionally, long term potentiation and metabolomic from hippocampal specimens were collected at 2 months of age. We studied TAU oligomerization, phosphorylation and aggregation in Nxnl2−/− brain at 18 months of age. Finally, newborn Nxnl2−/− mice were treated with adeno-associated viral vectors encoding for RdCVF2, RdCVF2L or both and measured the effect of this therapy on long-term potential, glucose metabolism and late-onset tauopathy. Nxnl2−/− mice at 2 months of age showed severe behavioral deficiency in fear, pain sensitivity, coordination, learning and memory. The Nxnl2−/− also showed deficits in long-term potentiation, demonstrating that the Nxnl2 gene is involved in regulating brain functions. Dual delivery of RdCVF2 and RdCVF2L in newborn Nxnl2−/− mice fully correct long-term potentiation through their synergistic action. The expression pattern of the Nxnl2 gene in the brain shows a predominant expression in circumventricular organs, such as the area postrema. Glucose metabolism of the hippocampus of Nxnl2−/− mice at 2 months of age was reduced, and was not corrected by gene therapy. At 18-month-old Nxnl2−/− mice showed brain stigmas of tauopathy, such as oligomerization, phosphorylation and aggregation of TAU. This late-onset tauopathy can be prevented, albeit with modest efficacy, by recombinant AAVs administrated to newborn mice. The Nxnl2−/− mice have memory dysfunction at 2-months that resembles mild-cognitive impairment and at 18-months exhibit tauopathy, resembling to the progression of Alzheimer's disease. We propose the Nxnl2−/− mouse is a model to study multistage aged related neurodegenerative diseases. The NXNL2 metabolic and redox signaling is a new area of therapeutic research in neurodegenerative diseases

Pyk2 modulates hippocampal excitatory synapses and contributes to cognitive deficits in a Huntington's disease model.

The structure and function of spines and excitatory synapses are under the dynamic control of multiple signalling networks. Although tyrosine phosphorylation is involved, its regulation and importance are not well understood. Here we study the role of Pyk2, a non-receptor calcium-dependent protein-tyrosine kinase highly expressed in the hippocampus. Hippocampal-related learning and CA1 long-term potentiation are severely impaired in Pyk2-deficient mice and are associated with alterations in NMDA receptors, PSD-95 and dendritic spines. In cultured hippocampal neurons, Pyk2 has autophosphorylation-dependent and -independent roles in determining PSD-95 enrichment and spines density. Pyk2 levels are decreased in the hippocampus of individuals with Huntington and in the R6/1 mouse model of the disease. Normalizing Pyk2 levels in the hippocampus of R6/1 mice rescues memory deficits, spines pathology and PSD-95 localization. Our results reveal a role for Pyk2 in spine structure and synaptic function, and suggest that its deficit contributes to Huntington's disease cognitive impairments