Immunogold localization of actin in the testis and exocrine pancreas: spatial relationship with tight junctional strands.

The fracture-label technique was used in conjunction with a monoclonal antibody to actin and the phospholipase A2-colloidal gold (PLA2-CG) method to examine the spatial distribution of actin filaments in relation to the three-dimensional arrangement of tight junctional strands in rat testes and exocrine pancreatic acinar cells. The intimate association of actin filaments with tight junctional strands in the pancreas and testis was also illustrated by a double-labeling experiment in which freeze-fractured pancreas or testis was labeled with monoclonal antibody-protein A-gold (30 nm gold size) followed by incubation with a PLA2-CG complex (11 nm gold size). Freeze-fracture-exposed tight junctional strands in both testicular and exocrine pancreatic cells labeled by PLA2-CG complex, indicated the presence of phospholipids in these cylindrical membranous structures. Immunolabeling of freeze-fractured testes with a monoclonal antibody to actin revealed a narrow band of gold particles juxtaposed to the cytoplasmic aspect of the protoplasmic membrane halves decorated with parallel linear arrays of cylindrical tight junctional strands. Many of the gold particles representing actin antigenic sites were in direct contact with the cross-fractured tight junctional strands. Fracture-label preparations of exocrine pancreas labeled with the monoclonal anti-actin antibody also exhibited a similar labeling pattern at the apex of acinars cells where the tight junction complex is located. Double-labeling experiments revealed the simultaneous labeling of actin and phospholipids in the same fracture-label preparations. Digestion of testicular and pancreatic tissue samples in a free PLA2 solution prior to labeling with the monoclonal antibody or PLA2-CG complex removed not only the gold labeling previously seen over the tight junctional strands but also reduced drastically the immunolabeling for actin that was previously seen associated with the tight junction complex. Taken together, results of the present study showed that actin filaments are structural components of the tight junction strands and are connected to the cytoplasmic aspect of the latter structures. The interaction between this particular cytoskeletal element and the tight junction may be through the binding of a special domain of the actin filament to the phospholipids that partially make up the tight junctional complex.