Role of DLG-MAGUKs in surface NMDAR localization and its patho-physiological functions

by Tanmoy Samaddar
Doctoral thesis
Date of Examination:2014-05-12
Date of issue:2014-10-16
Advisor:Dr. Dr. Oliver Schlüter
Referee:Prof. Dr. Reinhard Jahn
Referee:Prof. Dr. Henning Urlaub
Persistent Address: http://dx.doi.org/10.53846/goediss-4742

 

 

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Abstract

English

The post-synaptic side of an excitatory synapse is characterized by an electron-dense, highly dynamic architecture of proteins, called the post-synaptic density or PSD. PSDs comprises of a diverse group of proteins, from glutamate receptors, scaffolding proteins, signaling molecules and cyto-skeletal elements. Amongst this protein en-semble, the MAGUK family of scaffolding proteins is well known for their roles in synaptic maturation, glutamatergic receptor organization and for serving as platform for post-translational modifications of other proteins in the PSD. PSD-93, one of the lesser-known members of this family of proteins had been reported to have contradic-tory functions in maintaining AMPA and NMDA receptor levels in different brain regions. In this study, using diverse biochemical approaches in a mouse model, I stud-ied the role of PSD-93 in maintaining surface level of glutamatergic receptors and its patho-physiological significance in glutamate induced neuronal toxicity. By using surface protein cross-linking approach, I could demonstrate that in the layer 2/3 visual cortex of a PSD-93 knockout mice, there is a reduction in the surface level of both NMDA receptor subunits GluN2A and GluN2B. Additionally, I could also demonstrate that this reduction in surface level of GluN2B is caused independent of the change in phosphorylation of the GluN2B-Y1472 site, possibly through other phosphorylation mechanism. Since, NMDA receptors are classically linked with glutamate-induced neurotoxicity during ischemic brain damage, I looked at the level of neurotoxicity in PSD-93KO cortical cultures. I established a NMDA challenge protocol that enabled neuronal apoptosis and I could show an almost 80% reduction in cortical cell death, when PSD-93 is absent. This form of NMDA neurotoxicity was largely mediated via the GluN2B subunit of NMDA receptors and the neuro-protection effect in PSD-93KO was medi-ated by perturbation of the P38 MAPK pathway of apoptosis induction. PSD-95 had been classically reported to link NMDA receptor activation with down-stream apoptotic signaling cascade. I could show that deletion of both PSD-93 and VI PSD-95 showed equal degrees of neuro-protection at lower doses of NMDA chal-lenge whereas at higher doses, only PSD-93 retained its neuro-protective effects. Overall, my data helps to understand the role of PSD-93 in maintaining surface NMDA receptors levels in cortical neurons and linking them to their patho-physiological functions.
Keywords: PSD93, PSD-95, MAGUK, neurotoxicity, ischemia
 
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