Tissue-plasminogen activator (t-PA) is now available for the treatment of thrombo-embolic stroke but adverse effects have been reported in some patients, particularly hemorrhaging. In contrast, the results of animal studies have indicated that t-PA could increase neuronal damage after focal cerebral ischemia. Here we report for the first time that t-PA potentiates signaling mediated by glutamatergic receptors by modifying the properties of the N-methyl-D-aspartate (NMDA) receptor. When depolarized, cortical neurons release bio-active t-PA that interacts with and cleaves the NR1 subunit of the NMDA receptor. Moreover, the treatment with recombinant t-PA leads to a 37% increase in NMDA-stimulated fura-2 fluorescence, which may reflect an increased NMDA-receptor function. These results were confirmed in vivo by the intrastriatal injection of recombinant-PA, which potentiated the excitotoxic lesions induced by NMDA. These data provide insight into the regulation of NMDA-receptor-mediated signaling and could initiate therapeutic strategies to improve the efficacy of t-PA treatment in man.

Tau is a microtubule-associated protein well known for its stabilization of microtubules in axons. Recently, it has emerged that tau participates in synaptic function as part of the molecular pathway leading to amyloid-beta (A␤)-driven synaptotoxicity in the context of Alzheimer's disease. Here, we report the implication of tau in the profound functional synaptic modification associated with synaptic plasticity. By exposing murine cultured cortical neurons to a pharmacological synaptic activation, we induced translocation of endogenous tau from the dendritic to the postsynaptic compartment. We observed similar tau translocation to the postsynaptic fraction in acute hippocampal slices subjected to long-term potentiation. When we performed live confocal microscopy on cortical neurons transfected with human-tau-GFP, we visualized an activity-dependent accumulation of tau in the postsynaptic density. Coprecipitation using phalloidin revealed that tau interacts with the most predominant cytoskeletal component present, filamentous actin. Finally, when we exposed cortical cultures to 100 nM human synthetic A␤ oligomers (A␤o's) for 15 min, we induced mislocalization of tau into the spines under resting conditions and abrogated subsequent activity-dependent synaptic tau translocation. These changes in synaptic tau dynamics may rely on a difference between physiological and pathological phosphorylation of tau. Together, these results suggest that intense synaptic activity drives tau to the postsynaptic density of excitatory synapses and that A␤o-driven tau translocation to the spine deserves further investigation as a key event toward synaptotoxicity in neurodegenerative diseases.

N-methyl-D-aspartate receptors (NMDARs) are critical for synaptic plasticity that underlies learning and memory. But, they have also been described as a common source of neuronal damage during stroke and neurodegenerative diseases. Several studies have suggested that cellular location of NMDARs (synaptic or extrasynaptic) is a key parameter controlling their effect on neuronal viability. The aim of the study was to understand the relation between these two pools of receptors and to determine their implication in both beneficial and/or deleterious events related to NMDAR activation. We demonstrated that selective extrasynaptic NMDAR activation, as well as NMDA bath application, does not activate extracellular signal-regulated kinase (ERK) pathways, but induces mitochondrial membrane potential breakdown and triggers cell body and dendrite damages, whereas synaptic NMDAR activation is innocuous and induces a sustained ERK activation. The functional dichotomy between these two NMDAR pools is tightly controlled by glutamate uptake systems. Finally, we demonstrated that the only clinically approved NMDAR antagonist, memantine, preferentially antagonizes extrasynaptic NMDARs. Together, these results suggest that extrasynaptic NMDAR activation contributes to excitotoxicity and that a selective targeting of the extrasynaptic NMDARs represents a promising therapeutic strategy for brain injuries.