GNE is a bifunctional enzyme responsible for the first committed, rate limiting step in the synthesis of sialic acid, the most common terminal monosaccharide in cellular glycosylation. Mutations in GNE are responsible for two rare human disorders, Hereditary Inclusion Body Myopathy (HIBM) and Sialuria. The hyposialylation of glycoproteins has been a focus in HIBM research based on the assumption that mutations in GNE decrease synthesis of CMP-sialic acid thereby decreasing downstream sialylation. However, glycoproteins are only one class of prevalent, sialylated molecules in mammalian tissues. Analysis of glycosphingolipids revealed aberrant expression in both in vivo and in vitro models of HIBM. Surwisingly, in a mouse model of HIBM with knock-in founder mutation M712T, mutants (GNEM712T/M 12T) display a global increase in all expressed GSL species, including sialylated gangliosides. These findings were confirmed in primary human fibroblasts isolated from four H IBM patients with mutations throughout both enzymatic domains of GNE. All patient cell lines displayed an increase in absolute amounts of all glycosphingolipids when compared to control cell lines. Furthermore an HIBM like GSL phenotype could be induced in control fibroblasts after inhibition of GNE epimerase activity with a novel imino sugar inhibitor. HIBM fibroblasts grown in the presence of this inhibitor displayed dose dependent increases in glycosphingolipid expression above already elevated levels. Treatment with ManNAc and CMP-SA, metabolites downstream of GNE epimerase, had the opposite effect lowering total GSL expression in HIBM cell lines. These results link aberrant GSL expression in HIBM directly to GNE epimerase function. Fibroblasts from patients with COG la, containing mutated phosphmannomutase, also display a global increase in GSL expression. This up-regulation is thought to be a compensatory mechanism for loss of N-linked glycans in an effort to maintain net glycosylation on the cell surface. Similarly, global increases in total GSL expression seen in HIBM may be a compensatory mechanism for decreases in N-linked sialylation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:556113 |
| Date | January 2011 |
| Creators | Patzel, Katherine A. |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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