Endocytosis: The APPL of my PI3P eye

Lipidomics Gateway (1 May 2009) [doi:10.1038/lipidmaps.2009.2]

The acquisition of PtdIns(3)P by endocytic vesicles coincides with attenuation of signaling, implicating this lipid in the control of intracellular signaling.

Pathways of endocytosis: endocytosed vesicles develop into, or fuse with, early endosomes where cargo is sorted and targeted for recycling or degradation. For full description see Nature Reviews Molecular Cell Biology 10, 287–292 (April 2009) | doi:10.1038/nrm2652

The composition of cellular vesicles changes dynamically as they move along the endocytic pathway. Progressive changes in vesicular identity occur either through fusion with preexisting compartments or through maturation of the vesicle itself. Early endosomes are an intermediate compartment thought to act as molecular sorting stations for endocytosed material, sending cargo to its appropriate destination via vesicular traffic. Canonical early endosomes are characterized by the presence of the small GTPase Rab5, its adaptor molecule EEA1 and the lipid phosphatidylinositol-3-phosphate (PtdIns(3)P). Growing evidence is revealing heterogeneity of early endosomes, including identification of a distinct endosomal compartment characterized by the presence of other adaptors, APPL1 and APPL2. These APPL endosomes may function as platforms for the assembly of intracellular signaling complexes, like that for EGF, but their relationship with EEA1 endosomes is unclear. In a recent issue of Cell, Zoncu et al. show that APPL endosomes are transient precursors to PtdIns(3)P-positive early endosomes and that PtdIns(3)P acts as a switch to turn off growth factor signaling.

The authors first monitored the dynamics of APPL1, Rab5, EEA1, and phosphoinositides during macropinocytosis, a form of bulk uptake of fluid and protein cargo stimulated by activation of Ras signaling. Using fluorescence microscopy to monitor individual vesicles as they progress towards the middle of the cell, they found that APPL1 is present only at very early stages and that its loss correlates with the appearance of PtdIns(3)P and with recruitment of EEA1. Time-lapse TIRF microscopy confirmed that the APPL1 compartments are a very early compartment, formed immediately after fission of endocytic vesicles from the plasma membrane. This places APPL endosomes directly downstream of both clathrin-coated pits and macropinosomes involved in internalization of growth factors, and upstream of EEA1 compartments.

To understand the role of PtdIns(3)P in the identity and function of APPL endosomes, the authors used two different strategies to deplete it. This depletion caused a 'back conversion' of EEA1-stage vesicles to APPL1-stage vesicles and an enlargement of the latter vesicles. These results suggest that PtdIns(3)P production on endosomes acts as a switch that causes APPL1 loss from the membrane and recruitment of effectors. The same PtdIns(3)P depletion that produced the enlarged APPL endosomes also caused an APPL1-dependent increase in EGF-mediated signaling by prolonged residency of signaling components in the blocked compartment, supporting the idea that the residence time of growth factor receptors in the APPL-positive compartment affects the strength of the signal that they generate.

These results suggest a model where APPL endosomes are involved in control of signaling processes and where the formation of APPL endosomes depends on the cargo that is present. Although the exact mechanism is not yet known, because this maturation is coincident with the appearance of PtdIns(3)P on vesicles, this lipid is directly implicated in the conversion.

Mirella Bucci