Lipid sorting and multivesicular endosome biogenesis

Abstract

Intracellular organelles, including endosomes, show differences not only in protein but also in lipid composition. It is becoming clear from the work of many laboratories that the mechanisms necessary to achieve such lipid segregation can operate at very different levels, including the membrane biophysical properties, the interactions with other lipids and proteins, and the turnover rates or distribution of metabolic enzymes. In turn, lipids can directly influence the organelle membrane properties by changing biophysical parameters and by recruiting partner effector proteins involved in protein sorting and membrane dynamics. In this review, we will discuss how lipids are sorted in endosomal membranes and how they impact on endosome functions.

Figure 1.

Organization of the endosomal pathway. In mammalian cells but perhaps not in yeast, the endocytic pathway can be divided into two functional territories, recycling/reutilization vs. degradation, which are linked by membrane transport. The plasma membrane and early endosomal elements belong to the recycling/reutilization territory through which large amounts of fluid or membrane are trafficked, ∼30% of the cell volume and the surface area of the plasma membrane per hour (Steinman et al. 1976; Besterman and Low 1983; Steinman et al. 1983), or perhaps more (Howes et al. 2010a). In early endosomes, cargo is sorted for recycling/reutilization or degradation. In the latter case, transport to late endosomes occurs via endosomal carrier vesicles/multivesicular bodies (ECVs/MVBs), where RAB5-to-RAB7 conversion may occur (Rink et al. 2005). This transport step ensures that recycling and degradation pathways remain well segregated. Importantly, the reutilization/recycling and degradation compartments not only fulfill distinct functions but also show different protein and lipid compositions. Regulatory short-lived phosphoinositides mediate dynamics of endosomal elements in the reutilization/recycling pathway. Less is known about the role of regulatory phosphoinositides at late stages of the degradation pathway, where degradation-resistant LBPA (lysobisphosphatidic acid) membranes may mediate intraendosomal dynamics and cholesterol export. Importantly, the endosomal pathway also serves as an input or output for other membrane trafficking pathways, as indicated.

Figure 2.

Dynamics of PI(3)P- and LBPA-containing endosomes in live cells. (A) HeLa cells were transfected with tandem FYVE-GFP and incubated overnight with mouse anti-LBPA and antimouse Cy3 antibodies. The concentration of anti-LBPA antibody (2.5 µg/mL in medium) and the levels of tandem FYVE-GFP expression were low to avoid possible interference with endosome dynamics. The cells were imaged in vivo and movies were taken at a frame rate of 0.6 sec by time-lapse confocal microscopy. Panel A shows in a still corresponding to the first frame that both cells contain endocytosed anti-LBPA antibodies but only the left cell expresses tandem FYVE-GFP. (B) Selected frames of the boxed magnified region. Arrowheads are showing movements of LBPA-positive endosomes. Scale bar, 10 µm. (From Bissig et al. 2012; reprinted, with permission, from Elsevier.)

Figure 3.

Phospholipid-binding domains and proteins involved in endosomal dynamics. (A) A schematic illustration of key endosomal lipids and their phospholipid-binding domains is shown, including kinases and phosphatases involved in endosomal phosphoinositide generation and turnover. However, other pathways may also exist for the generation of PI(5)P. Enzymes involved in the metabolism of phosphatidylserine (PS), which is synthesized in the ER like most phospholipids, are not shown. The enzymes involved in LBPA synthesis and degradation are not known. (B) Key endosomal lipids and their localization (light gray) are shown with the corresponding phospholipid-binding proteins and their endosomal functions (dark gray). ^aMobius et al. 2003; ^bSimonsen et al. 1998; McBride et al. 1999; Pankiv et al. 2010; ^cCarlton et al. 2004; Bonifacino and Hurley 2008; ^dPons et al. 2008; Harterink et al. 2011; Chen et al. 2013; ^eLloyd et al. 2002; Bache et al. 2003; Pons et al. 2008; ^fSimonsen et al. 2004; Filimonenko et al. 2010; ^gSbrissa et al. 2002a; ^hCabezas et al. 2006; Ikonomov et al. 2006; Rutherford et al. 2006; ⁱsee Tooze and Elazar 2013; and references in Mayinger 2012; ^jRamel et al. 2011; Zolov et al. 2012; Oppelt et al. 2013; ^kGagescu et al. 2000; Leventis and Grinstein 2010; Fairn et al. 2011; ^lUchida et al. 2011; ^mMobius et al. 2003; Ikonen 2008; Maxfield and van Meer 2010; ⁿVanier and Millat 2003; Ohgami et al. 2004; Xu et al. 2007; Kwon et al. 2009; ^oKobayashi et al. 1999, ; ^pMatsuo et al. 2004; Kirkegaard et al. 2010; Bissig et al. 2013.

Figure 4.

Chemical structures of the 2,2′- and 3,3′-dioleoyl LBPA isoforms. The major isoform present in late endosomes of BHK cells is 2,2′-dioleoyl LBPA. This isoform is thermodynamically unstable and the fatty acid chains can migrate to the 3,3′ positions of the glycerol backbone to form the 3,3′-LBPA isoform.