Association of AMP-activated Protein Kinase Subunits With Glycogen Particles as Revealed In Situ by Immunoelectron Microscopy

Abstract

Immunogold cytochemistry was applied to reveal the intracellular location of AMP-activated protein kinase (AMPK) subunits in liver tissue of normal rats fed ad libitum. AMPK α and β subunits were located both in the cytosol and in close association with rosettes of glycogen particles (α particles). To reveal their true in situ association with glycogen, particular tissue processing conditions that retain glycogen in the cells were required. These included fixation with a combination of glutaraldehyde and paraformaldehyde, followed by postfixation with osmium tetroxide and lead citrate and embedding in Epon. Processing by less-stringent fixation conditions and embedding in Lowicryl led to the extraction of the glycogen deposits, which in turn resulted in the absence of any labeling. This indicates that the loss of glycogen deposits leads to the loss of closely associated proteins. Labeling for the α1 and α2 subunits of AMPK was found to be about 2-fold greater over glycogen than over cytosol, whereas labeling for β1 was 8-fold higher over the glycogen particles than over the cytosol. Immunogold combined with morphometric analysis demonstrated that the β1 subunits are located at the periphery of the glycogen rosettes, consistent with a recent hypothesis developed via biochemical approaches.