A ganglioside-specific sialyltransferase localizes to axons and non-Golgi structures in neurons

Abstract

To investigate the tissue distribution and subcellular localization of ST3GaIV (CMP-NeuAc:lactosylceramide α2,3 sialyltransferase/GM3 synthase) in the adult mouse, we generated two antisera against mouse ST3GaIV that were designated CS2 (directed against amino acids K227-I272) and CS14 (directed against amino acids D308-H359). We previously reported that CS2 antiserum stains medial and trans-Golgi cisternae in all cell types investigated. In neural tissue, however, CS14 antiserum reveals a subpopulation of ST3GaIV with a subcellular distribution complementary to CS2 antiserum. CS14 antiserum strongly stains axons in cortical, cerebellar, brainstem, and spinal cord tissue sections. The subcellular localization of neuronal ST3GaIV is maintained in primary cultures of rat hippocampal neurons and in PC12 cells. In PC12 cells, ST3GaIV localization evolves during NGF-induced differentiation such that a pool of enzyme leaves the Golgi for a distal compartment in conjunction with neurite outgrowth. In PC12 cells transfected with an epitope-tagged form of ST3GaIV, staining for the epitope tag coincides with expression of endogenous enzyme. The non-Golgi pool of ST3GaIV does not colocalize with markers for the trans-Golgi network, endosome, or synaptic vesicles, nor is it detected on the cell surface. Distinct subpopulations of ST3GaIV imply that ganglioside synthesis can occur outside of the Golgi or, alternatively, that a portion of the total ST3GaIV pool subserves a nonenzymatic function. Significantly fewer transfected cells were found in PC12 cultures treated with plasmid encoding ST3GaIV than in cultures treated with control plasmid, indicating that the expression of ST3GaIV in excess of endogenous levels results in either cell death or a decreased rate of cell division.