Structural and functional analysis of endosomal compartments in epithelial cells

Abstract

Epithelial cells display segregated early endosomal compartments, termed apical sorting endosomes and basolateral sorting endosomes, that converge into a common late endosomal-lysosomal degradative compartment and common recycling endosomes (CREs). Unlike recycling endosomes of nonpolarized cells, CREs have the ability to sort apical and basolateral plasma membrane proteins into distinct apical and basolateral recycling routes, utilizing mechanisms similar to those employed by the trans Golgi network in the biosynthetic pathway. The apical recycling route includes an additional compartment, the apical recycling endosomes, consisting of multiple vesicles bundled around the basal body. Recent evidence indicates that, in addition to their role in internalizing ligands and recycling their receptors back to the cell surface, endosomal compartments act as intermediate stations in the biosynthetic routes to the plasma membrane. Here we review methods employed by our laboratory to study the endosomal compartments of epithelial cells and their multiple trafficking roles.

Keywords: Endocytosis; Endosomes; Epithelial cells; Recycling; Transcytosis; Transferrin receptor.

FIGURE 1. Model

This cartoon displays the various endosomal compartments and endosomal trafficking pathways in nonepithelial (A) and polarized epithelial (B) cells. The basolateral pathways and mechanisms of polarized epithelial cells are shown in red and the apical in blue. (See color plate)

FIGURE 2

(A) ASE and ARE^Rab11 labeling. Polarized WT MDCK cells were incubated for 5 min with Alexa488-WGA (green) from the apical surface, washed with N-methyl-D-glucosamine (NMDG) and immunostained for Rab11. (B) BSE and CRE^TfR labeling. Polarized WT MDCK cells were incubated for 15 min from the basolateral surface with Alexa633-Tf (green) followed by a 5 min chase, with CF594-Tf (red). Orthogonal view (top) and two confocal sections at the supranuclear (middle) and nuclear (bottom) regions are shown. Scale: 10 μm. (See color plate)

FIGURE 3. Trafficking of basolaterally internalized TfR to ARE^Rab11

Polarized WTand AP-1B KD MDCK cells were incubated for 10 min with CF594-Tf (red) from the basolateral surface and immunostained with anti Rab11 antibody (green). Orthogonal view (top) and two confocal sections at the supranuclear (middle) and nuclear (bottom) regions are shown. Scale: 10 μm. (See color plate)