Title

Polysaccharide modulation of AMPA-type glutamate receptors: Importance to synaptic function and neuronal maintenance

Date of Completion

January 2003

Keywords

Health Sciences, Pharmacology

Degree

Ph.D.

Abstract

Endogenous regulators, such as polysaccharides, are found in the brain surrounding neurons at the synapse. Within the synapse, polysaccharides interact with cell surface receptors and other extracellular molecules to promote protein-protein or protein-carbohydrate interactions. Through these interactions, polysaccharides modulate synaptic activity to promote cell adhesion, cell-to-cell signaling, and neuronal communication. Neuronal communication through glutamate receptors is important for, synaptic efficacy, cellular maintenance, and cell survival. The modulation of AMPA-type glutamate receptor functions by endogenous regulators like synaptic polysaccharides, leads to changes in receptor properties and intracellular signaling cascades that may promote neuroprotection and neuronal maintenance. Here, the modulation of AMPA receptors was tested using dextran sulfate to see if interactions with the polysaccharide result in altered AMPA receptor properties. The experiments described in the present work show that dextran sulfate decreases AMPA receptor binding affinity, increases the single channel opening events to prolong the open state of activated receptors, and enhances the calcium permeability of AMPA receptors, all in a sulfate- and size-dependent manner. The enhancement of calcium influx was evident in neuronal cell bodies and dendritic processes, and was correlated with cell survival in primary hippocampal cells. To test for a link between neuroprotection and dextran sulfate-mediated effects on AMPA receptors, hippocampal slice cultures were used. In these studies, dextran sulfate treatment following excitotoxic insult promoted the maintenance of cytoskeletal structure and synaptic integrity. A link to survival signaling was also determined in experiments where hippocampal slices were treated with dextran sulfate for I hour, resulting in the stimulation of the mitogen-activated protein kinase (MAPK) survival pathway through the upstream enzyme MEK. Interestingly, the polysaccharide-mediated MAPK activation was blocked with the AMPA receptor antagonist CNQX. In sum, the data presented here suggest that polysaccharide modulation of AMPA-type glutamate receptors results in changes in receptor function, to promote plasticity mechanisms and cell survival. Finally, this study provides important insight into the alterations in synaptic function and cellular signaling that occur due to polysaccharide interactions with cell surface receptors at the synapse. ^