Absorptive cells of the intestinal epithelium endocytose proteins from both apical and basolateral membrane domains. In absorptive cells of suckling rat ileum, luminal protein tracers first enter an apical tubulovesicular endosomal system, then enter larger apical endosomal vesicles and multivesicular bodies (MVB), and finally are delivered to a giant supranuclear lysosomal vacuole. To determine whether proteins endocytosed from the basolateral domain in vivo enter the same endosomal or lysosomal compartments as those taken up from the apical side, we simultaneously applied cationized ferritin (CF) apically (by intra-luminal injection) and horseradish peroxidase (HRP) basally (by intravenous injection), and examined absorptive cells after 3 min to 60 min using light, electron and fluorescence microscopy. At early times, CF and HRP entered separate endosomal compartments at apical and basolateral poles. At no time did HRP enter the apical tubulovesicular system, and CF never entered early basolateral endosomes. After 15 min, however, both tracers appeared together in large late endosomes and MVB located apically, above the giant vacuole. From 15 to 60 min both tracers accumulated in the giant vacuole. Membranes of some apical late endosomes, all apical MVB, the giant vacuole, and occasional sub-nuclear vesicles contained immunoreactive Igp120, a glycoprotein specific to late compartments of the endosome-lysosome system. These results show that highly polarized intestinal epithelial cells have separate apical and basolateral early endosomal compartments, presumably to maintain distinct membrane domains while allowing endocytosis and recycling of membrane from both surfaces. Apical and basolateral endocytic pathways, and presumably vesicles delivering hydrolytic enzymes and lysosomal membrane components, converge at the apical late endosome.
Defective Slc7a7 transport reduces systemic arginine availability compromising erythropoiesis and iron homeostasis
