The UPEC pore-forming toxin α-hemolysin triggers proteolysis of host proteins to disrupt cell adhesion, inflammatory, and survival pathways

Abstract

Uropathogenic Escherichia coli (UPEC), which are the leading cause of both acute and chronic urinary tract infections, often secrete a labile pore-forming toxin known as α-hemolysin (HlyA). We show that stable insertion of HlyA into epithelial cell and macrophage membranes triggers degradation of the cytoskeletal scaffolding protein paxillin and other host regulatory proteins, as well as components of the proinflammatory NFκB signaling cascade. Proteolysis of these factors requires host serine proteases, and paxillin degradation specifically involves the serine protease mesotrypsin. The induced activation of mesotrypsin by HlyA is preceded by redistribution of mesotrypsin precursors from the cytosol into foci along microtubules and within nuclei. HlyA intoxication also stimulated caspase activation, which occurred independently of effects on host serine proteases. HlyA-induced proteolysis of host proteins likely allows UPEC to not only modulate epithelial cell functions, but also disable macrophages and suppress inflammatory responses.

Figure 1. Paxillin is degraded in UPEC-infected bladder cells

(A) Lysates from uninfected (UI) BECs and BECs that were infected with UTI89 for the indicated time periods were probed by Western blot using antibodies specific for paxillin. Staining for β-actin levels in each sample served as a loading control. (B) Lysates from BECs transfected with the plasmids pEGFP or pEGFP-paxillin and then infected with UTI89 for 0, 2 or 4 h were analyzed by Western blot using antibodies specific for GFP (top), paxillin (middle), or actin (bottom). See also Figure S1.

Figure 2. HlyA triggers paxillin degradation in BECs along with the disruption of host actin and microtubule networks

(A) Western blots show levels of paxillin and actin in 5637 BECs following 4 h infections with UPEC isolates, non-UPEC E. coli strains, or non-E. coli uropathogenic bacteria. Uninfected (UI) control cells were treated with M9 medium without bacteria. Bacterial strains were scored as hemolytic (+) based on their ability to lyse erythrocytes on blood agar plates. (B - D) Western blots show paxillin and actin levels present in BECs at the indicated time points after infection with wild type UTI89, UTI89ΔhlyA, UTI89ΔhlyA/pSF4000, UTI89Δcnf1, UTI89Δcnf1/pHLK102, WAM783 (+hlyABD ΔhlyC), or WAM582 (+hlyCABD). (E) Western blots show paxillin and actin levels in 5637 BECs following 0 to 4 h incubations with preparations of crude, bacteria-free non-acylated HlyA (HlyA⁷⁸³) or wild type HlyA (HlyA^783/pHlyC). (F) Phase contrast microscopy of BECs infected with UTI89 or UTI89ΔhlyA for the indicated times. Scale bar, 50 μm. (G) Confocal images of BECs infected for 4 h with UTI89 or UTI89ΔhlyA and then stained to visualize microtubules or F-actin. Scale bar, 20 μm. (H - I) BECs were infected with UTI89 for the indicated times in the presence of MβCD (5 mM), (B) EGTA (4 mM), the intracellular Ca²⁺ chelator BAPTA-AM (10 μM), or vehicle alone (DMSO, control). Paxillin and actin levels were assessed by Western blots. See also Figure S2 and Table S1.

Figure 3. Activation of caspases and TLCK-sensitive serine proteases by HlyA

(A) Caspase-3/7 activation over time in 5637 BECs infected with UTI89 or UTI89ΔhlyA, as determined by fluorometry of FLICA-treated samples. Data are expressed as the mean ± SEM of three independent experiments carried out in duplicate or triplicate. *p ≤ 0.0001, as determined by Student’s t-test. (B) Effects of suramin (1 mM) and TLCK (100 μM) on caspase-3/7 activation in BECs infected for 4 h with UTI89, as quantified by fluorometry of FLICA-treated samples. Control UTI89-infected samples were treated with vehicle alone. Data are expressed as the mean ± SEM of four independent experiments performed in triplicate. *p ≤ 0.0001, versus control values, as determined by Student’s t-test. (C) Western blots showing levels of paxillin and actin in BECs infected with UTI89 for the indicated times in the presence of 100 μM of the CASP3/7-specific inhibitor Z-DEVD-FMK, the pan-caspase inhibitor Z-VAD-FMK, the CASP1/4 inhibitor Z-YVAD-FMK, or the negative control Z-FA-FMK. (D) Paxillin and actin levels in BECs infected with UTI89 for the indicated times in the presence or absence of TLCK (100 μM), as determined by Western blots. (E) Phase contrast (left) and corresponding fluorescent images (right) of BECs infected with UTI89 or UTI89ΔhlyA for 4 h in presence of the fluorescent probe FSLCK (5 μM). Scale bar, 20 μm. (F) Fluorometric quantification of the levels of FSLCK-labeled, active serine proteases over time in BECs infected with UTI89 or UTI89ΔhlyA. Data are expressed as the mean ± SEM of three independent experiments carried out in triplicate. *p ≤ 0.0001, as determined by Student’s t-test. (G) Immunoblots probed using anti-FITC antibody to detect FSLCK-tagged proteins recovered from uninfected BECs, BECs infected with UTI89ΔhlyA, or BECs infected with UTI89 ± 1 mM suramin. (H) Levels of activated FSLCK-tagged serine proteases present in the bladder mucosa of mice 5 h after infection with UTI89 or UTI89ΔhlyA were quantified by fluorometry. Data are expressed as the mean ± SEM from at least 5 mice. *p = 0.0017, as determined by Student’s t-test. (I) Activated serine proteases present in the bladder mucosa of uninfected mice or mice infected with UTI89 or UTI89ΔhlyA for 5 h were detected, following treatment with FSLCK, by Western blots probed with anti-FITC antibody. Further analysis by mass spectroscopy identified the 23-kDa band (arrow) as mesotrypsin. See also Figure S3.

Figure 4. HlyA induced paxillin degradation is facilitated by mesotrypsin

(A) 5637 BECs express enteropeptdase and trypsinogen 4, but not other trypsinogens, as determined by RT-PCR. (B) Semi-quantitative RT-PCR shows levels of PRSS3 (encoding trypsinogen 4) and GAPDH transcripts in BECS 72 h after transfection of 5637 BECs with either control nonspecific siRNA or PRSS3-specific siRNA detected. (C) Western blots show levels of paxillin and actin present in lysates recovered from BECs treated with control, nonspecific siRNA or PRSS3-specific siRNA and infected with UTI89 for the indicated times. (D) Confocal micrographs of uninfected BECs or BECs that were infected with UTI89 or UTI89ΔhlyA for the indicated times and then stained to visualize trypsinogens (using ECS antibody, green), LAMP1 (red), and nuclei (blue). The boxed area in each micrograph is enlarged and placed directly below the corresponding image to show details. Scale bars, 20 μm (first and third rows) and 5 μm (second and fourth rows). See also Figure S4.

Figure 5. Widespread effects of HlyA on host protein stability and NFκB signaling

(A - B) Levels of the host proteins indicated were assessed by Western blot using lysates from 5637 BECs infected for 0 to 4 h with UTI89ΔhlyA or wild type UTI89 ± TLCK (100 μM). (C) 5637 BECs were infected with UTI89ΔhlyA or wild type UTI89 in the presence of LPS (5 μg/ml) or incubated with LPS alone, and 4 h later IL-6 levels in the cell culture supernatants were measured by ELISA. Data are expressed as the mean ± SEM of results from at least three independent experiments performed in triplicate; p values determined by Student’s t-test.

Figure 6. HlyA triggers the activation of serine proteases and degradation of host proteins within macrophages

(A- B) Western blots showing levels of paxillin and actin in either RAW264.7 or mouse peritoneal macrophages following infection with either UTI89 or UTI89ΔhlyA for the indicated times. (C) Western blots showing levels of IκBα, RelA, and actin in RAW264.7 cells following infection with UTI89 or UTI89ΔhlyA for the times indicated. (D) Phase contrast (left) and corresponding fluorescent images (right) of peritoneal macrophages infected with UTI89 or UTI89ΔhlyA for 3 h in presence FSLCK (5 μM) to label activated serine proteases. Scale bar, 50 μm.