Regulation of excitatory synapses is central to learning and memory, and their dysregulation leads to human cognitive diseases including intellectual disability. Glutamate is the major excitatory neurotransmitter and AMPA-type glutamate receptors (AMPARs) are the predominant mediators of excitatory neurotransmission. Accessory molecules exquisitely regulate AMPAR expression at the synapse to drive synaptic plasticity, learning and memory. We discovered unanticipated AMPAR regulatory molecules, gangliosides and the transmembrane protein Nicalin, the study of which are the basis for this project. Gangliosides, sialylated glycosphingolipids, are prominent structures on the surfaces of neurons, with four structures - GM1, GD1a, GD1b, and GT1b - comprising >95% of all gangliosides in the brains of mice and humans alike. Human disorders of ganglioside biosynthesis invariably result in cognitive disability, often accompanied by seizures, outcomes phenocopied in mutant mice. The link between gangliosides, learning, and excitatory neurotransmission had been unclear. Using an unbiased molecular screen we found that ganglioside GT1b associates with a molecular complex responsible for endocytosis of AMPARs, especially those containing the GluR2 subunit known to regulate neuronal excitability. We found that GluR2 binds to a different major brain ganglioside, GM1. We hypothesize that different gangliosides on nerve cells segregate GluR2-containing AMPARs from their endocytosis complex, enhancing their stability. Since GluR2-containing AMPARs limit seizures and are essential for long-term potentiation (LTP) and long-term depression (LTD), molecular correlates of learning and memory, our findings may mechanistically link ganglioside expression to learning, memory, and seizures. Individuals with altered ganglioside expression may suffer pathological changes in the steady state levels of GluR2-containing AMPARs resulting in seizures and diminished learning. This project tests the functional consequences of the interactions between brain gangliosides, AMPARs, and AMPAR endocytosis complexes. Pharmacological, enzymatic and genetic tools will be used to predictably alter the expression of gangliosides and the ganglioside-associated trafficking protein Nicalin on nerve cells and tissues. Specific alterations in gangliosides and Nicalin will be quantified analytically and immunocytochemically, then altered ganglioside and Nicalin expression on cultured hippocampal neurons will be correlated with changes in AMPAR trafficking at baseline and in response to chemically-induced LTP and LTD. Acute hippocampal slices from ganglioside-altered mutant mice will be used to test the role of gangliosides in stimulus-evoked LTP and LTD. Finally, reconstitution with exogenous gangliosides will be used to test the ability of specific ganglioside structures and ganglioside mimetics to modulate AMPAR trafficking, LTP and LTD. If successful, this exploratory project will open new areas of research on the functions of gangliosides and ganglioside binding proteins in regulating neurotransmitter receptor expression and function.
Learning and memory are encoded, in part, by excitatory neurotransmitter (glutamate) receptors on the surface of nerve cells whose expression and distribution are carefully regulated to ensure efficient information storage while avoiding over-excitation. Gangliosides, molecules common to the surfaces nerve cells, participate in glutamate receptor regulation, and inborn errors of ganglioside synthesis result in learning disabilities and seizures. This project explores the relationship between gangliosides and glutamate receptors and will help us understand how gangliosides contribute to learning and memory in health and disease.
|Naito-Matsui, Yuko; Davies, Leela R L; Takematsu, Hiromu et al. (2017) Physiological Exploration of the Long Term Evolutionary Selection against Expression of N-Glycolylneuraminic Acid in the Brain. J Biol Chem 292:2557-2570|