GFAP is expressed as a major soluble pool associated with glucagon secretory granules in A-cells of mouse pancreas

To elucidate the role of intermediate filament proteins in endocrine cells, we investigated the expression and subcellular distribution of GFAP in mou se islets of Langerhans. For this purpose, combined immunocytochemical and biochemical analysis with a panel of antibodies was carried out to identify GFAP-immunoreactive cells in mouse endocrine pancreas. Cell fractionation into NP-40-soluble and detergent/high salt-insoluble components was perform ed to assess whether GFAP was located in the cytosolic and/or cytoskeletal compartments of immunoreactive cells. Immunoelectron microscopic analysis w as carried out to determine the subcellular distribution of the protein. Pe ripheral islet cells were stained with anti-GFAP antiserum. These cells wer e identified as glucagon-secreting cells by immunocytochemical staining of consecutive sections with anti-somatostatin, anti-GFAP, and anti-glucagon a ntisera. Western blotting analysis of both NP-40-soluble and detergent/high -salt insoluble fractions of isolated islets of Langerhans allowed detectio n of GFAP in both cytosolic and cytoskeletal compartments. Interestingly, h owever, the former location was highly predominant. In addition, immunoelec tron microscopy localized GFAP associated with the periphery of secretory g ranules. On the basis of these results, an intriguing role for GFAP in secr etory events should be strongly suspected.