ANNEXIN-VI DEFINES AN APICAL ENDOCYTIC COMPARTMENT IN RAT-LIVER HEPATOCYTES

Annexin VI has been demonstrated previously to be a marker for hepatic endosomes, By western blotting with an affinity purified anti-annexin VI antibody it was shown that annexin VI was present in the three mor phologically and functionally different endosomal fractions from rat l iver. We have quantified the gold-labeled endosomes by immunoelectron microscopy in ultrathin Lowicryl sections of rat liver and now demonst rate that 80% of the total labeling with anti-annexin VI was associate d with endocytic structures surrounding the bile canaliculus, the apic al domain of hepatocytes, whereas only 20% was found in the subsinusoi dal endosomes. In double immune-gold labeling experiments 80% of the R ab5 positive apical endosomes were also labeled with anti-annexin VI a ntibodies, However, there was no significant colocalization with antib odies to the polymeric immunoglobulin receptor, Finally, we demonstrat e that 50% of endosomes containing internalized gold-labeled transferr in were double labeled with anti-annexin VI antibodies. Thus, annexin VI becomes the first known structural protein at the apical 'early' en docytic compartment of the hepatocyte that may be involved in the rece ptor recycling and transport to late endocytic/lysosomal compartment p athways.