Sphingolipids are important mediators and regulators of cell signaling pathways. Our studies have focused on the actions of two classes of sphingolipids represented by glycosphingolipids (GSLs) and sphingosine-1-phosphate. Our work is aimed at defining the normal functions of these sphingolipids and understanding their roles in disease processes. GSLs are found in the outer leaflet of the plasma membrane and are concentrated in specialized signaling structures. They are particularly abundant in neuronal cells in the form of gangliosides (sialic acid containing GSLs). Through genetic disruption of genes that encode synthetic enzymes for GSLs, we have created a series of mice that express limited glycosphingolipid structures. We are using these mice to discover the functions of GSLs. When the cellular machinery responsible for GSL degradation is defective, GSL storage diseases result in which profound neurodegeneration occurs. Examples are Tay-Sachs, Sandhoff and Gaucher diseases. We are attempting to understand how the accumulation of GSLs cause neurodegeneration through the construction of animal models of the diseases. Our major accomplishments this year include a demonstration the expanded macrophage/microglial population in the brain of Sandhoff disease mice is compounded by the infiltration of cells from the periphery. Coincident with the cellular infiltration was an increased expression of macrophage-inflammatory protein 1alpha (MIP-1alpha), a leukocyte chemokine, in astrocytes. Deletion of MIP-1alpha expression resulted in a substantial decrease in infiltration and macrophage/microglial-associated pathology together with neuronal apoptosis in Sandhoff disease mice. These mice without MIP-1alpha showed improved neurologic status and a longer lifespan. The results indicate that the pathogenesis of Sandhoff disease involves an increase in MIP-1alpha that induces monocytes to infiltrate the brain, expand the activated macrophage/microglial population, and trigger apoptosis of neurons, resulting in a rapid neurodegenerative course We also continued our studies on the G-protein coupled receptor for sphingosine-1-phosphate, S1P1. To determine the precise role of the S1P1 receptor on T-cells, we established a T-cell-specific S1P1 knock-out mouse. The mutant mice showed a block in the egress of mature T-cells into the periphery. The expression of the S1P1 receptor was up-regulated in mature thymocytes, and its deletion altered the chemotactic responses of thymocytes to sphingosine 1-phosphate. The results indicated that the expression of the S1P1 receptor on T-cells controls their exit from the thymus and entry into the blood and, thus, has a central role in regulating the numbers of peripheral T-cells..
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