Immunocytochemical localization of ecto-5'-nucleotidase in cultures of cerebellar granule cells

The distribution of ecto-5'-nucleotidase, a glycosyl phosphatidylinositol anchored membrane protein capable of hydrolysing extracellular nucleoside monophosphates, was investigated by immunocytochemistry in cultures of rat cerebellar cells obtained at postnatal days 6 and 8. The enzyme was expressed at the surface of granule cells including their neurites as well as on other neurons in the culture. The distribution of 5'-nucleotidase matched that of the synaptic vesicle protein 2. Oligodendroglial cells were identified by their immunoreactivity for 2',3'-cyclic nucleotide 3'-phosphodiesterase. Their entire surface was labelled for 5'-nucleotidase. In contrast, only a subset of astrocytes immunopositive for the glial fibrillary acidic protein revealed surface-located immunoreactivity for 5'-nucleotidase. Antibody-binding of the labelled-astrocytes was enhanced at restricted surface domains. Endothelial cells that avidly bind Lycopersicon esculentum lectin were the most intensely anti-5'-nucleotidase-labelled cell type of the culture. Double labelling revealed an exact match of surface-located antibody binding sites for 5'-nucleotidase and laminin. Immunoreactivity for 5'-nucleotidase was essentially absent from fibroblasts that could be identified by their immunoreactivity for fibronectin. All cell types that carried surface-bound 5'-nucleotidase also revealed a cytoplasmic pool of the enzyme. Our results provide the first immunocytochemical demonstration of the surface-location of 5'-nucleotidase in neurons. They suggest that the broad distribution of the enzyme at the surface of neurons and other cells types from neonatal brain reflects its functional importance in the differentiation of the nervous system.