Surface diffusion of the astrocytic glutamate transporter glt-1 shapes synaptic transmission

Defended on June 6, 2014

The extracellular concentration of glutamate, the main excitatory neurotransmitter in the central nervous system, is tightly regulated by transporters located on both astrocytes and neurons. Astrocytes are particularly well suited to this role as they have a very negative, stable membrane potential necessary for efficient glutamate uptake. As such, astrocytic glutamate transporters GLT-1 and GLAST have been reported to be the most important for regulating glutamate concentration in the brain

Although, the glutamate timecourse in the synaptic cleft is brief, 1-2 ms, the transport cycle of glutamate transporters is relatively slow (70 ms). Thus, it has been suggested that high numbers of transporters effectively bind/buffer glutamate, competing with glutamate receptors for this neurotransmitter, thereby curtailing synaptic transmission. Furthermore, it has been repeatedly demonstrated that blocking transporters results in a prolonged timecourse of synaptic glutamate, implicating transporters as the primary means by which glutamate is removed from the synapse.

The molecular mechanisms regulating glutamate transporter expression and trafficking to the membrane are well defined. However, it is completely unknown whether there exists a physiological relationship between trafficking of transporters on the cell surface and the timecourse glutamate at the synapse. Here, we used a combination of single nanoparticle tracking and electrophysiological approaches to assess the role of surface diffusion of the astrocytic glutamate transporter GLT-1 in shaping synaptic transmission. We observed that GLT-1 is highly dynamic on the surface of astrocytes and that this surface diffusion is activity-dependent, reacting to changes in both neuronal and glial cell activity. We demonstrate that GLT-1 is subject to strict regulation close to the synaptic cleft where it is transiently trapped before being ‘un-leashed’ following exposure to glutamate. Functionally, immobilisation of GLT-1 using an antibody-based cross-link revealed the physiological role of GLT-1 surface diffusion in controlling the timecourse of synaptic glutamate, as indicated by slowed kinetics of excitatory postsynaptic currents (EPSCs) both in vitro and in acute hippocampal slices (following in vivo X-link induction).

PhD committee

• Valentin Nagerl
  U. Bordeaux  – President
• Laurent Aniksztejn
  INMED U. Aix-Marseille – Reporter
• Josef Kittler
  UCL, UK  – Reporter
• Marko Kreft
  U. Ljubljana, Slovenia  – Examiner
• Christian Giaume
  College de France – Examiner

Thesis supervisors