Recycling endosomes

The endocytic and exocytic system is important for cells to communicate with their surroundings. For instance, endocytosis allows the regulated internalisation of receptors (which can be ligand bound or not) into peripheral early endosomes and can thus modulate responses to external stimuli. Internalised molecules can be degraded after entering the late-endosomal/lysosomal pathway or be recycled to the cell surface (Maxfield and McGraw, 2004). Recycling to the cell surface can occur directly from peripheral early endosomes. However, many cells display a distinct subpopulation of endosomes that have a slightly higher pH of ~6.4 and also recycle membrane components. These are typically located deeper in the cell and centered around the microtubule-organising centre (MTOC) (Perret et al., 2005). These so-called recycling endosomes (REs) display a heterogeneous tubular-vesicular morphology, which suggests dynamic and intense trafficking activity, and connect the endocytic pathway to the exocytic pathway (Ang et al., 2004; Lock and Stow, 2005; Murray et al., 2005). The most prominent RE marker to date is the small GTPase Rab11. Studies of the function of Rab11 and the proteins with which it interacts in various experimental systems and organisms suggest that cells use REs for the delivery of membranes to regions of their surface that are subject to dynamic reorganisation, probably through regulated interactions with the exocyst, a multiprotein complex containing the Sec5, Sec6, Sec8, Sec10, Sec15 and Exo70 proteins that is thought to recruit material to areas of membrane growth. Consequently, REs are implicated in the regulation of a variety of cellular processes that depend on such trafficking. Several of these are highlighted in the poster and discussed briefly below. Epithelial cell-cell adhesion E-cadherin-mediated cell-cell adhesion controls epithelial cell polarisation, 1679Cell Science at a Glance (See poster insert