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12
OM&P
Section CELLULAR NEUROSCIENCE
Using three independent super-resolution imaging methods, on both genetically tagged and endogenous receptors, we have demonstrated that, in live hippocampal neurons, AMPAR are highly concentrated inside synapses into a few clusters of around seventy nanometers. AMPAR are stabilized reversibly in these domains and diffuse freely outside them. Nanodomains are themselves dynamic in their shape and position within synapses as they can form and disappear within minutes, although they are for the most part stable for at least up to an hour. These results open the new possibility that glutamatergic synaptic transmission is controlled by the regulation at the nanometer scale of the position and composition of these highly concentrated nanodomains. In support of this hypothesis, we recently demonstrated that AMPAR conformation strongly impacts their mobility, indicating that desensitized AMPAR can escape synapses. This finding provides a functional support to our hypothesis that fast AMPAR surface diffusion can tune short term plasticity by allowing fast replacement of desensitized AMPAR by naive ones during high frequency stimulation.
Mechanisms and Consequences of Neuronal Protein SUMOylation in Health and Disease
Jeremy Henley*
School of Biochemistry, Centre for Synaptic Plasticity, Biomedical Sciences Building, University of Bristol, Bristol BS8 1TD, UK. * Presenting e-mail: j.m.henley@bristol.ac.uk
Protein SUMOylation is a critically important post-translational protein modification that participates in nearly all aspects of cellular physiology. In neurons, SUMOylation participates in cellular processes, ranging from neuronal differentiation and control of synapse formation to regulation of synaptic transmission and mitochondrial function. SUMOylation is a highly dynamic and usually transient modification that enhances or hinders interactions between proteins and its consequences are extremely diverse. Hundreds of different proteins are SUMO substrates and the mechanisms and protein targets of SUMOylation are activity-dependently controlled and highly sensitive to cell stress. Unsurprisingly, dysfunction of protein SUMOylation is strongly implicated in a many different diseases. I will outline the SUMO system and discuss recent discoveries from our lab that illustrate some of the roles of SUMOylation in healthy and diseased neurons.
Molecular Mechanisms Regulating Synaptic Expression of NMDA Receptors
Katherine Roche*
The National Institutes of Health (NIH),USA. * Presenting e-mail: rochek@ninds.nih.gov
NMDA receptors are critical for neuronal development and synaptic plasticity. Functional NMDA receptors are tetramers most often composed of two GluNl subunits and two GluN2 subunits (GluN2A-D). Although synaptic NMDA receptors are tightly anchored to the postsynaptic membrane via the postsynaptic density (PSD), they are also dynamic at the cell surface. Indeed, we find that phosphorylation and dephosphorylation regulates synaptic expression and endocytosis in a subunit-specific manner. We find that CK2 phosphorylation of GluN2B on S1480 disrupts PSD-95 binding, drives GluN2B endocytosis thus removing GluN2B from synapses resulting in an increase in synaptic GluN2A expression. Furthermore, there is an interplay between S1480 phosphorylation and the nearby GluN2B tyrosine-based endocytic motif (YEKL), providing a molecular mechanism for the observed effects on NMDA receptor trafficking. Namely, there is coordinated phosphorylation and dephosphorylation of two different residues on GluN2B, S1480 within the PDZ domain binding domain and Y1472 within the tyrosine-based YEKL endocytic motif, which play opposing roles in the regulation of NMDA receptor endocytosis and ultimately the surface expression of GluN2B . Both the tyrosine kinase Fyn and the tyrosine phosphatase STEP are known to target GluN2B on Y1472. However, nothing is known about a potential functional interaction between STEP and PSD-95. We find that STEP binds to PSD-95, but not other PSD-95 family members and that PSD-95 drives degradation of STEP. These findings support an unexpected dual role for PSD-95 to stabilize NMDARs by binding directly to GluN2B, but also by promoting degradation of the negative regulator STEP.
4 Opera Med Physiol 2016 Vol. 2 (S1)
