Regulation of Synaptic Glutamate Receptors by MAGUK Scaffolding Proteins
The nervous system is tasked with quickly gathering, interpreting, and responding to information about the outside world. To carry out this task, it has signaling cells called neurons, their support cells, and most importantly, the connections between neurons which can be used in a combinatorial manner to generate a wide array of behaviors. These connections occur at locations termed synapses. Glutamatergic synapses, consisting of a post-synaptic specialization with clustered glutamate receptors opposite a pre-synaptic terminal, are the sites of fast excitatory neurotransmission in the brain. Communication between neurons via synaptic transmission enables the nervous system to respond quickly to stimuli. Changes in synaptic strength and connectivity change these responses, through processes commonly termed learning and memory. The mechanisms controlling synapse growth and maintenance are therefore critically important for learning and memory. Proper formation of the post-synaptic specialization requires that glutamate receptors localize to the synapse and associate with the complex network of signaling and scaffolding molecules known as the post-synaptic density (PSD). Multiple lines of evidence demonstrate that the PSD scaffolding proteins themselves play an instructive role in regulating the localization of synaptic glutamate receptors. The primary protein family implicated in synaptic glutamate receptor localization is the four-member membrane-associated guanylyl kinase (MAGUK) family: PSD-93, PSD-95, SAP97, and SAP102. Here, we examine the role of the MAGUKs in several contexts. We find that MAGUKs are specifically responsible for creating functional synapses after initial spine formation by filling functionally silent spines with glutamate receptors. Removal of the MAGUKs causes an initial reduction in synaptic strength, followed by activation of a compensatory program that consolidates weakened synapses to normalize synaptic strength. We next find that overexpression of the MAGUKs fills additional silent spines with glutamate receptors. Removal or overexpression of MAGUKs therefore does not affect synaptic strength, which appears to be tightly regulated. Combination of these two manipulations by simultaneous removal of endogenous MAGUKs and overexpression of recombinant MAGUK destroys existing PSDs, disrupts regulation of synaptic strength, and results in formation of very strong synapses. Finally, we examine the role of the MAGUKs in vivo, characterizing their role during development of the hippocampus.