Date Published: January 1, 2010
Publisher: Public Library of Science
Author(s): Corinne Beurrier, Mathilde Faideau, Khaled-Ezaheir Bennouar, Carole Escartin, Lydia Kerkerian-Le Goff, Gilles Bonvento, Paolo Gubellini, Colin Combs. http://doi.org/10.1371/journal.pone.0008550
Abstract: Ciliary neurotrophic factor (CNTF) is a potent neuroprotective cytokine in different animal models of glutamate-induced excitotoxicity, although its action mechanisms are still poorly characterized. We tested the hypothesis that an increased function of glial glutamate transporters (GTs) could underlie CNTF-mediated neuroprotection. We show that neuronal loss induced by in vivo striatal injection of the excitotoxin quinolinic acid (QA) was significantly reduced (by ∼75%) in CNTF-treated animals. In striatal slices, acute QA application dramatically inhibited corticostriatal field potentials (FPs), whose recovery was significantly higher in CNTF rats compared to controls (∼40% vs. ∼7%), confirming an enhanced resistance to excitotoxicity. The GT inhibitor dl-threo-β-benzyloxyaspartate greatly reduced FP recovery in CNTF rats, supporting the role of GT in CNTF-mediated neuroprotection. Whole-cell patch-clamp recordings from striatal medium spiny neurons showed no alteration of basic properties of striatal glutamatergic transmission in CNTF animals, but the increased effect of a low-affinity competitive glutamate receptor antagonist (γ-d-glutamylglycine) also suggested an enhanced GT function. These data strongly support our hypothesis that CNTF is neuroprotective via an increased function of glial GTs, and further confirms the therapeutic potential of CNTF for the clinical treatment of progressive neurodegenerative diseases involving glutamate overflow.
Partial Text: Ciliary neurotrophic factor (CNTF) is a neurotrophic cytokine belonging to the interleukin-6 type family. In the CNS, CNTF is released by astrocytes and stimulates the survival of developing neurons. CNTF is also neuroprotective in various models of acute neuronal death and neurodegenerative diseases , and it has been proposed as a neuroprotective agent for Huntington’s disease (HD) . In HD, glutamate receptor-mediated excitotoxicity is involved in the preferential loss of striatal medium-sized spiny neurons (MSNs). Neurons expressing high levels of NMDA receptors are lost early from the striatum of individuals affected with HD, and injection of NMDA receptor agonists such as quinolinic acid (QA) into the striatum of rodents or non-human primates mimics the pattern of neuronal damage observed in HD , . CNTF administration in the striatum protects MSNs against QA in rodents and primates –. A phase I clinical trial confirmed the safety of local brain administration of encapsulated cells genetically engineered to produce CNTF and reported a recovery of somatosensory evoked potentials in patient implanted with capsules releasing the largest amount of CNTF . Despite these encouraging results, the mechanisms mediating CNTF neuroprotective effect are still unclear. The change in astrocyte phenotype triggered by CNTF in the adult brain suggests that this cytokine may have an indirect neuroprotective effect through activated astrocytes –. Indeed, we have recently shown that CNTF-activated astrocytes display marked phenotypic and molecular changes associated with an improved handling of extracellular glutamate in the rat striatum . We suggested that such effect could be mediated by an increased function of astrocyte glutamate transporters (GTs), GLAST and GLT-1. These two GTs uptake the bulk of extracellular glutamate  and this function is crucial to prevent accumulation of glutamate to excitotoxic levels. Using lentivirus-mediated CNTF overexpression in the rat striatum, whole-cell patch-clamp and extracellular electrophysiological recordings on corticostriatal slices, we provide evidence that CNTF neuroprotective effects against QA are mediated through an enhanced glutamate uptake by activated astrocytes.
In this study, we show that CNTF neuroprotective effects against glutamate excitotoxicity do not rely on a direct action on striatal MSNs and glutamate transmission, but are mediated by an enhanced glutamate uptake in activated astrocytes.