A contribution of V ATPase subunits specifically to phagosom

A contribution of V-ATPase subunits specifically to phagosome-lysosome fusion was uncovered by Peri and Nüsslein-Volhard who described that depletion of the VO subunit a1 in zebrafish interferes with phagosome maturation in microglial Phos-tag Biotin (Peri and Nusslein-Volhard, 2008). However, loss of the VO subunit a3 did not block phagolysosome formation in mouse macrophages (Sun-Wada et al., 2009). Similarly, we have recently shown that phagosome-lysosome fusion was unaffected by either lack of VO a3 or strongly decreased levels of total VO sectors in Atp6ap2 knockout cells (Kissing et al., 2015). Therefore, the link between V-ATPases and catalysis of (phagosome) membrane fusion is still being debated and the discussion is apparently complicated by cell type-specific differences.
Acquisition of V-ATPase by phagosomes V-ATPase VO complexes span the entire phagosome membrane and, hence, must be acquired from a membrane source. The V-ATPase transmembrane complexes are assembled in the ER and are then transported through the Golgi complex. Few details are known about the exact assembly states and trafficking route of V-ATPase from the ER to the endocytic and phagocytic systems in mammalian cells. Most insights are extrapolated from studies in baker’s yeast (Forgac, 2007). It is often assumed that phagosomal V-ATPase stems largely from the fusion between phagosomes and lysosomes that are rich in V-ATPase (Sun-Wada et al., 2009). On the other hand, some data speak for V-ATPase being trafficked to the phagosome from the trans-Golgi (Fratti et al., 2003), from early endosomes (Maxson and Grinstein, 2014) or from the plasma membrane (Kinchen and Ravichandran, 2008). We have recently investigated this question ourselves and we have revealed multiple sources of V-ATPases which feed into phagosomes one after another (Bunge and Haas, in preparation). Whereas the VO subunits must be recruited to the phagosome from a membrane source, the V1 subunits can stem from a cytosolic pool from which they can be recruited onto VO complexes when proton translocation is needed (Forgac, 2007). Further studies will be needed to dissect the exact sources and the kinetics of V-ATPase recruitment by phagosomes in professional and non-professional phagocytes. It should be noted that the timing of V-ATPase incorporation into phagosomes can also depend on phagocyte activities other than killing, particularly on antigen presentation. Immature dendritic cells contain near-neutral pH phagosomes in which internalized antigens are slowly degraded leading to reduced peptide loading on major histocompatibility complex class II (MHC II) for antigen presentation. Once dendritic cells mature their phagosome-resident V-ATPase becomes more active, as determined in cell-free lysosome acidification assays, and proteolysis is enhanced, together resulting in more class II-antigen complexes (Trombetta et al., 2003). As the purpose of tissue macrophages is to quickly remove apoptotic blebs and intruded microorganisms, acidification occurs quite rapidly and parallels fast proteolysis (Yates et al., 2005).
Non-oxidative microbe killing and V-ATPase: the highway to hell Lysosomes are the default end station for all ingested extracellular material unless this material is bound to recyclable receptors or is a receptor itself. For recycling, cytosolic rescue signals take care that the cargo is sorted away before reaching the lysosome. Ingested material is degraded in the (phago)lysosome at a pH of around 4.5–5.5 by an arsenal of some 50 different hydrolases (Saftig and Klumperman, 2009). ‘Death by lysosome’ rather than by other microbicidal means is probably the #1 ancient killing-and-digestion mechanism of professional phagocytes (Fig. 3). Supporting this notion, there is no oxidative burst in the amoeba Dictyostelium discoideum which is a professional phagocyte that feeds on microorganisms (Cosson and Lima, 2014).