Trafficking Ion Transporters to the Apical Membrane of Polarized Intestinal Enterocytes

Abstract

Epithelial cells lining the gastrointestinal tract require distinct apical and basolateral domains to function properly. Trafficking and insertion of enzymes and transporters into the apical brush border of intestinal epithelial cells is essential for effective digestion and absorption of nutrients. Specific critical ion transporters are delivered to the apical brush border to facilitate fluid and electrolyte uptake. Maintenance of these apical transporters requires both targeted delivery and regulated membrane recycling. Examination of altered apical trafficking in patients with Microvillus Inclusion disease caused by inactivating mutations in MYO5B has led to insights into the regulation of apical trafficking by elements of the apical recycling system. Modeling of MYO5B loss in cell culture and animal models has led to recognition of Rab11a and Rab8a as critical regulators of apical brush border function. All of these studies show the importance of apical membrane trafficking dynamics in maintenance of polarized epithelial cell function.

Figure 1.

Structure of the apical brush border. The intestinal microvilli consist of cores of actin filaments bundled by villin and espin. Cross-linking of the membrane to the actin cytoskeleton is regulated by ezrin and myosin 1a. Protocadherin-24 and mucin-like protocadherin form a trans-heterophilic adhesion complex that links the distal ends of microvilli for optimal packing and uniformity of the microvilli.

Figure 2.

Trafficking to the apical brush border. On exit from the trans-Golgi network, proteins are targeted for trafficking to the apical, basolateral, or lateral membrane. Apically targeted proteins can be directed to the apical early endosome (AEE) or the apical recycling endosome (ARE) before being delivered to the plasma membrane. Proteins taken up by the cell from the apical membrane can be delivered to the ARE for recycling back to the apical surface or can be taken to the common recycling endosome and late endosome if targeted for degradation.The motor protein, myosin Vb, restricts Rab8a- and Rab11a-containing vesicles within subapical F-actin-containing domains. Cystic fibrosis transmembrane conductance regulator (CFTR), sodium hydrogen exchanger isoform 3 (NHE3), and down-regulated in adenoma (DRA) are transported to the apical membrane for insertion and are reported to interact directly and/or indirectly with ezrin. LE, Late endosome; lys, lysosome; BEE basolateral early endosome.

Figure 3.

Altered trafficking associated with deletion of MYO5B in intestinal enterocytes. Loss of myosin Vb (MYO5B) in MYO5B germline-null mice results in the formation of inclusions from the apical cell surface of intestinal enterocytes. Phalloidin staining shows the presence of intracellular inclusions positive for F-actin, showing that the inclusions originate from the brush border. Arrows indicate the formation of inclusions by invagination of the plasma membrane. F-actin staining at the apical membrane is diminished but still presents in short microvilli.