Phosphoinositides in the mammalian endo-lysosomal network

Abstract

The endo-lysosomal system is an interconnected tubulo-vesicular network that acts as a sorting station to process and distribute internalised cargo. This network accepts cargoes from both the plasma membrane and the biosynthetic pathway, and directs these cargos either towards the lysosome for degradation, the peri-nuclear recycling endosome for return to the cell surface, or to the trans-Golgi network. These intracellular membranes are variously enriched in different phosphoinositides that help to shape compartmental identity. These lipids act to localise a number of phosphoinositide-binding proteins that function as sorting machineries to regulate endosomal cargo sorting. Herein we discuss regulation of these machineries by phosphoinositides and explore how phosphoinositide-switching contributes toward sorting decisions made at this platform.

Figure 1. The endo-lysosomal network

A. The interconnected membranous network that constitutes the endo-lysosomal network. B. Steady state distribution of phosphoinositide isomers within the network: the ‘phosphoinositide map’.

Figure 2. Clathrin-mediated endocytosis

A. Schematic depicting stages of clathrin-coated vesicle (CCV) formation. B. Phosphoinositide kinases and phosphatases involved in shaping the phosphoinositide profile of the nascent CCV and predicted PtdInd(4)P – PtdIns(4,5)P₂ transition during this process.

Figure 3. Mechanisms of enhancing the strength of interaction between phosphoinositides and membranes

A. Oligomerisation of dynamin’s PH domain strengthens its interaction with membranes through avidity-mediated enhancement of affinity. B. The PX-domain of p47phox binds to both PtdIns(3)P and acidic phospholipids such as PtdSer/PtdOH. Ligation of both classes of lipid enhances membrane association. C. Sorting nexins contain tandem membrane-binding PX and BAR domains. Membrane association requires co-incident recognition of both PtdIns(3)P and curved membranes.

Figure 4. Phosphoinositide-dependent regulation of ESCRT activity

A. Topological equivalence of sites of ESCRT-activity. The ESCRT-machinery regulates membrane fission events leading to MVB biogenesis, cytokinesis and viral release, indicated by the red asterisks. Note topological equivalence of these events and contrast with topology of internalisation. B. Known phosphoinositide-binding activities within discrete ESCRT-complexes. Yeast nomenclature used for simplicity (barring mVps24). It is unknown whether yeast Vps24 shares PtdIns(3,5)P₂ binding specificity

Figure 5. Retromer-mediated retrograde endosome-to-TGN transport

A. Retrieval pathways in mammalian cells. TGN resident cargos (CI-MPR) are retrieved from endosomes through the actions of the retromer complex. WASH and EHD-1 assist this process. F-actin polymerisation may drive carrier biogenesis, Directed movement to the TGN is accomplished through dynein-mediated transport to the perinuclear region where Rab6IP1 acts to tether incident cargo and dephosphorylation ot PtdIns(3)P is thought to allow removal and recycling of retromer components. B. Schematic depiction of retromer assembly. Retromer comprises 2 subcomplexes – a membrane deforming SNX-BAR protein containing subcomplex and a cargo-binding VPS26:VPS29:VPS35 subcomplex. Rab7-GTP is thought to help localise the cargo-binding subcoplex. 3′-phosphoinositides are thought to localise the SNX-BAR subcomplex.