Recycling endosomes

Abstract

The endosomal membrane recycling system represents a dynamic conduit for sorting and re-exporting internalized membrane constituents. The recycling system is composed of multiple tubulovesicular recycling pathways that likely confer distinct trafficking pathways for individual cargoes. In addition, elements of the recycling system are responsible for assembly and maintenance of apical membrane specializations including primary cilia and apical microvilli. The existence of multiple intersecting and diverging recycling tracks likely accounts for specificity in plasma membrane recycling trafficking.

Figure 1

A complex scheme for endosomal pathways leading to membrane recycling. Membrane proteins are internalized by multiple mechanisms including clathrin-dependent endocytosis (CDE), non-clathrin-dependent endocytosis (NCDE) and macropinocytosis (MP). The early endosomes from these pathways are thought to traffic through a common sorting endosome (SE). Proteins can then be shunted for degradation to the lysosome, recycled to the plasma membrane using a rapid recycling pathway (thought to be mediated by Rab4) or processed into slower recycling endosome (RE) pathways (thought to be regulated by Rab11a and Rab8a). While both Rab11a and Rab8a can interact with MYO5B, specific proteins interacting with membranes containing either Rab11a (Rab11-FIPs and KIF13) or Rab8a (MICAL-L1, EHD1 and EHD3) may regulate specific trafficking pathways out of the recycling system.

Figure 2

Pericentriolar coordination of primary cilium construction by components of recycling endosomes. The mother centriole provides a point of coordination for trafficking through the pericentriolar region. Microtubule-based tracks lead towards and away from the pericentriolar region and provide paths for anterograde and retrograde trafficking to the plasma membrane surface. These trafficking processes are necessary for the assembly and maintenance of apical specializations such as the primary cilium. The mother centriole acts a coordinating point for the recruitment by Rab11a of the Rab8a GEF, Rabin8, which can in turn activate Rab8a. It is important to note that this coordinated activity in the pericentriolar region likely involves the juxtaposition of distinct endosomal membranes containing either Rab11a or Rab8a.