SNX3 drives maturation of Borrelia phagosomes by forming a hub for PI(3)P, Rab5a, and galectin-9

Abstract

The spirochete Borrelia burgdorferi, the causative agent of Lyme disease, is internalized by macrophages and processed in phagolysosomes. Phagosomal compaction, a crucial step in phagolysosome maturation, is driven by contact of Rab5a-positive vesicles with the phagosomal coat. We show that the sorting nexin SNX3 is transported with Rab5a vesicles and that its PX domain enables vesicle–phagosome contact by binding to PI(3)P in the phagosomal coat. Moreover, the C-terminal region of SNX3 recruits galectin-9, a lectin implicated in protein and membrane recycling, which we identify as a further regulator of phagosome compaction. SNX3 thus forms a hub for two distinct vesicle populations, constituting a convergence point for the endosomal recycling machinery, to contribute to phagosome maturation and intracellular processing of borreliae. These data also suggest that the helical shape of B. burgdorferi itself, providing sites of high curvature and thus local PI(3)P enrichment at phagosomes, may be one of the driving elements underlying the efficient elimination of spirochetes by immune cells.

SNX17 regulates antigen internalization and phagosomal maturation by dendritic cells

“…41 Moreover, it was shown in macrophages that SNX3 is transported with Rab5a positive vesicles towards Borrelia burgdorferi-containing phagosomes to promote maturation of these organelles and the spirochete compaction. 42 Also in macrophages, SNX5 plays an important role during macropinosome biogenesis controlling the uptake and processing of soluble exogenous antigens. 43 We still do not know whether the difference of phagosomal maturation observed in SNX17 KD DCs enhances or partially restores the phenotype of cross-presentation found in these cells.…”

Molecular Mechanisms of Borrelia burgdorferi Phagocytosis and Intracellular Processing by Human Macrophages

“…This reduction of the membrane surface area is likely due to the extrusion of membrane tubules, which could be observed frequently, especially at sites of curvature discontinuity [21,56]. Multiple proteins have been identified which play crucial roles in the regulation of this process, including RabGTPases, sorting nexin 3 (SNX3) and galectin-9 [60]. RabGTPases are regulators of vesicle trafficking and vesicle fusion with and fission from membranes [61]; sorting nexins are a family of proteins involved in intracellular trafficking and possess a phospholipid binding PX domain [62]; the galectin family is involved in carbohydrate binding on the cell surface, but also in the regulation of intracellular trafficking pathways [63].…”

“…Initially, Borrelia are internalized in a Rab22a-positive phagosome, which is in turn fused with vesicles that are positive for both Rab5a and SNX3 (Figure 3). siRNA-mediated depletion of either of those proteins significantly decreased the proportion of compacted Borrelia [21,60]. Interestingly, these points of contact between phagosome and vesicles are especially found at sites of altered membrane curvature on the phagosomal membrane, which also constitute the sites of membrane tubule formation and abscission [18,21,60].…”

“…siRNA-mediated depletion of either of those proteins significantly decreased the proportion of compacted Borrelia [21,60]. Interestingly, these points of contact between phagosome and vesicles are especially found at sites of altered membrane curvature on the phagosomal membrane, which also constitute the sites of membrane tubule formation and abscission [18,21,60]. It has been shown that Rab5a vesicles contact the phagosomal coat through binding of vesicle-localized SNX3 with the phagosomal phospholipid PI(3)P (Phosphatidylinositol 3-phosphate) [60].…”

Galectins in Intra- and Extracellular Vesicles

“…Further work has demonstrated that apart from galectin-3 and galectin-8, other galectins, including galectin-1, galectin-4, and galectin-9, are targeted towards impaired endocytic vesicles [ 23 , 24 ]. Remarkably, the timing of galectin-recruitment seems to differ between distinct galectin variants.…”