Editors' ChoiceHost-Pathogen Interactions

Evicting bacteria by exosome

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Sci. Signal.  16 Jun 2015:
Vol. 8, Issue 381, pp. ec156
DOI: 10.1126/scisignal.aac7878

Cells can eliminate intracellular pathogens by sequestering them and then degrading them in autophagosomes that fuse with lysosomes. The activity of the various enzymes and antimicrobial peptides in lysosomes is enhanced by an acidic pH. Miao et al. (see also Sergeeva et al.) found that neutralization of lysosomal pH by uropathogenic Escherichia coli resulted in the expulsion of these bacteria in exosomes by bladder epithelial cells. Bacteria in the urine of infected mice were resistant to the antibiotic gentamycin unless simultaneously exposed to Triton-X100, a detergent that permeabilizes the host cell membranes and not the bacterial membrane, suggesting that bacteria were encapsulated in host cell-derived membranes. Uropathogenic E. coli in the extracellular medium of infected bladder epithelial cells colocalized with various exosome markers, and application of dimethyl amiloride (DMA), which inhibits exosome release, to the bladders of infected mice increased bacterial load and caused the formation of large bacterial aggregates in the uroepithelium. In bladder epithelial cells, uropathogenic E. coli were engulfed by single membrane vacuoles, then autophagosomes. Application of a peptide that increases autophagy to the bladders of infected mice reduced bacterial burden, whereas mice with an inducible and bladder epithelium-specific knockout of the autophagy component Atg3 had increased bacterial load. Autophagosomes containing uropathogenic E. coli were trafficked to intraluminal vesicles in multivesicular bodies. Exosome release in infected bladder epithelial cells was blocked by knockdown of various components required for the trafficking of cargo from multivesicular bodies to lysosomes or by knockdown of synaptotagmin 7, which is required for lysosome exocytosis. Although both bacterial strains localized to lysosomes, lysosomes were acidic in cells infected with an innocuous strain of E. coli, but not in cells infected with uropathogenic E. coli. Preventing lysosomal acidification with bafilomycin A increased exosome release from bladder epithelial cells. These results suggested that the neutralization of lysosomal pH triggered the release of bacteria-containing exosomes from lysosomes. The mucolipin TRP cation channels (TRPML) are located on lysosomes and conduct calcium, which is required for exocytosis. Knockdown of TRPML3 reduced bacterial expulsion and increased bacterial load in infected bladder epithelial cells. TRPML3 mediated calcium efflux from lysosomes at neutral pH, and in bladder epithelial cells with TRPML3 knockdown, bacterial expulsion was not rescued when the cells were reconstituted with pH insensitive or nonconducting pore mutants. Thus, TRPML3 detects when uropathogenic E. coli attempt to evade destruction by neutralizing lysosomal pH and triggers the expulsion of bacteria-containing exosomes.

Y. Miao, G. Li, X. Zhang, H. Xu, S. N. Abraham, A TRP channel senses lysosome neutralization by pathogens to trigger their expulsion. Cell 161, 1306–1319 (2015). [PubMed]

O. A. Sergeeva, F. G. van der Goot, Kicking out pathogens in exosomes. Cell 161, 1241–1242 (2015). [PubMed]