Loss of Bladder Epithelium Induced by Cytolytic Mast Cell Granules

Abstract

Programmed death and shedding of epithelial cells is a powerful defense mechanism to reduce bacterial burden during infection but this activity cannot be indiscriminate because of the critical barrier function of the epithelium. We report that during cystitis, shedding of infected bladder epithelial cells (BECs) was preceded by the recruitment of mast cells (MCs) directly underneath the superficial epithelium where they docked and extruded their granules. MCs were responding to interleukin-1β (IL-1β) secreted by BECs after inflammasome and caspase-1 signaling. Upon uptake of granule-associated chymase (mouse MC protease 4 [mMCPT4]), BECs underwent caspase-1-associated cytolysis and exfoliation. Thus, infected epithelial cells require a specific cue for cytolysis from recruited sentinel inflammatory cells before shedding.

Keywords: IL-1β; bladder epithelial cells; caspase 1; chymase; cytolysis; cytolytic granules; exfoliation; inflammasome; mast cells; uropathogenic E. coli.

Figure 1. Shedding of mice superficial BECs during bladder infections reduces bacterial load

(A, B, C) Significant superficial BECs shedding occurs after 6 h UPEC infection. Bladders of saline or UPEC CI5 infected C57BL/6 mice were examined at various time points post-infection (p.i.). (A) Frozen sections of bladders were stained with α-E-cadherin antibody (intermediate epithelium, red) and wheat germ agglutinin (WGA) (superficial epithelium, green). Representative image from the bladders of 3–5 mice per group. “L” indicates lumen. (B) Significantly fewer superficial BECs were observed at 12 h UPEC p.i. compared to saline controls. Bladder whole mounts were stained with α-ZO-1 antibody (tight junction) and WGA. Average number of superficial BECs from ten randomly chosen bladder fields were examined. n = 3–5 mice per group. (C) Mice infected for 12 and 24 h had significantly fewer bacteria per bladder compared to 6 h infected mice. No significant difference in CFUs were observed following 3, 6 and 9 h of infection. (D) Sedimented BECs from urines of UPEC infected mice were stained with crystal violet to show UPEC infected (left) or sparsely infected (right) BEC. (E) Relative proportion of live and dead or dying BECs shed in the urine of UPEC-infected (left) or dispase II-treated (right) mice. Dead or dying cells as indicated by positive ethidium homodimer-2 staining were quantified from randomly selected fields. n = 4–5 mice per group. Data represent 3 independent experiments. Scale bar: (A) 100 μm, (D) 50 μm. *p<0.01, **p<0.05, N.S., not significant., See also Figure S1.

Figure 2. Shedding of UPEC infected BEC is dependent on MCs

(A, B) UPEC infections to triggers secretion of IL-1β but fails to evoke lytic cell death of 5637 BECs in vitro. BECs were exposed to UPEC or Salmonella at different MOIs. After 12 h, culture supernatants were analyzed (A) for LDH release and (B) for IL-1β secretion. (C, D) MC migration into the bladder epithelium following infection. Uninfected and infected mouse bladders were harvested at 6 h and (C) 12 h p.i., whole mount stained with α-ZO-1 antibody (urothelium, red) and avidin (MCs, green). In-depth Z stack imaging of whole mounted bladders were displayed as indicated axis. Representative images of n = 3 mice per treatment. (D) Whole mounts were stained for intermediate epithelia (red), basal membrane (blue), and MCs (green). Images were processed as described in figure 2C. (Right) 4–5 random fields per bladder were Z stack-imaged, and the number of MCs that crossed basement membrane (B.M.) per field enumerated. (E) Shedding of BECs in infected MC sufficient but not MC-deficient mice. WT, MC-deficient (Wsh), and MC-repleted Wsh mice (Wsh+ BMMC) were infected with UPEC for 12 h. Frozen sections of bladders were stained for superficial (green) and intermediate (red) epithelia. (F) Whole mount quantification of superficial BECs reveal Wsh mice retain significantly more cells than WT and Wsh+BMMC mice. n = 3–5 mice per group. (G) Wsh mice have significantly higher bladder bacterial counts than WT and Wsh+BMMC mice. (H, I) Failure to exfoliate and increased bacterial burden in bladders of conditional MC-deficient mice. WT or Mcpt5-Cre⁺iDTR⁺ mice were inoculated with UPEC after MC depletion with DT. Collapsed bladders (12 h p.i.) from each group were stained and viewed (E). (I) Bladder bacterial counts presented as CFUs. Data represent 3–4 independent experiments. “L” indicates lumen. Scale bar: 100 μm. *p<0.05, **p<0.01, ***p<0.001, N.S.: Not Significant., See also Figure S2.

Figure 3. Inflammasome mediated IL-1β recruits MCs to the site of BEC infection

(A, B, C) IL-1β is a potent chemoattractant for MCs in vitro. (A, C) WT or Il1r^−/− BMMCs were cultured on the apical side of trans-wells and rmSCF or rmIL-1β was placed on the basolateral side of the wells and migration of MCs into the basolateral side was assessed after 4 h of incubation. (B) Same as above except that rmIL-1β was pre-incubated with increasing doses of α-IL-1β antibody or vehicle (D) In vivo MC recruitment by IL-1β. WT or Il1r^−/− mice were intravesicularly administered with rmIL-1β (200 ng). 4h post-treatment, whole mounted bladders were assessed for recruited MCs in direct contact with the basement membrane of WT and Il1r^−/− mice. Data is presented as relative values. (E–G) Inflammasome components: Nlrp3, Asc and Casp1Casp11 are required for bladder exfoliation. 12 h p.i. with UPEC, infected bladders from (E) Nlrp3^−/−, (F) Asc^−/− or (G) Casp1^−/−Casp11^−/− mice were processed and examined. (E–G) are frozen sections of collapsed bladders stained for superficial (green), and intermediate (red) epithelia and UPEC (blue). Right panel are bladder bacterial counts presented as CFUs. (H, I, J) Defects in inflammasome reduces MC migration into infected bladder epithelium. Whole mount bladders from (H) WT and Casp1^−/−Casp11^−/− mice (I) WT and Il1r^−/− mice, were stained for intermediate epithelia (red), basal membrane (blue), MCs (green). In-depth Z stacks were displayed as X–Y and X–Z axis. (H, I) Right panel depict MC counts above the basement membrane. (J) MC counts above the basement membrane 6 h p.i. in Wsh + WT BMMCs and Wsh + Il1r^−/− BMMCs (presented as relative counts). (D, H, I, J) Quantification of MCs that had crossed the B.M. 4–5 random fields per bladder were Z stack-imaged, and the number of MCs that crossed B.M. or reached B.M. per field was enumerated. Representative images are shown. Scale bar: 100 μm. “L” indicates lumen. *p<0.05, **P<0.01, N.S.: Not Significant., See also Figure S3.

Figure 4. UPEC-infected BEC undergo lytic cell death following endocytosis of MC granules

(A, B) Cell lysis and intracellular cytotoxicity induced by UPEC+granules or Salmonella. Infected 5637 BECs were exposed to vehicle, MC granules or Salmonella. (A) 1 h post-exposure, media were replaced with propidium iodide and time lapse imaging performed. Arrowheads depict sites of release of cellular contents. (B) 5637 BECs were exposed to vehicle, the apoptosis inducer staurosporin (1 μM), UPEC+granules or Salmonella for 16 h. Thereafter, cell cross sections were observed by TEM. Arrow heads depict localized membrane perturbations. (C, D, E) Endocytosis of MC granules induce BEC detachment and death of 5637 BECs in vitro. (C) Quantification of propidium iodide positive BECs following exposure to vehicle, UPEC+granules or granules. (D) LDH release from BECs following exposure to vehicle, UPEC, UPEC+granules or granules (E) LDH release from BECs following pretreatment with vehicle or dynasore (30 min 100 μM), followed by UPEC+granules. Data represent 3 independent experiments. Scale bar: 2 μm. *p<0.05, **P<0.001, See also Movies S1, S2, S3 and Figure S4.

Figure 5. Protease from endocytosed granules induce cell death and exfoliation

(A) Breakdown of endocytosed MC granules and leakage of contents into BEC cytosol. BECs with intracellular bacteria and MC granules (arrows). Magnified image of granule intruding into cytosol (arrowheads). (B) Protease inhibitors block granule-induced cytoxicity in infected BECs. Granules were pre-incubated (30 min) with or without protease inhibitor cocktail and added to pre-infected 5637 BECs. Culture supernatants were analyzed for LDH release. (C) Endocytozed granules are encased in LAMP1⁺ lysosomes (arrows). Cells were stained for granules (avidin, red), actin (phalloidin, blue), and lysosome (α-LAMP1 antibody, green). Granule contents (stained red) appear diffusely in cytosol (arrowheads) (middle), indicating disruption of lysosome. Disruption of lysosome and leakage of granule contents is blocked in presence of protease inhibitor (right). (D) Inhibitors of endosome acidification block granule-induced cytotoxicity in infected 5637 BECs. Granules were added to pre-infected cells with or without pre-treatment (2 h) with NH₄Cl, followed by LDH release assay. (E) Instillation of MC granules, but not protease inhibitor cocktail-treated granules, into UPEC-infected bladders of Wsh mice triggers exfoliation of superficial cells. Detached superficial cells (WGA, red) containing granules (avidin, green) could be seen in the lumen (L) (left), with (arrowheads) or without (arrows) UPEC. Bladders were harvested after 6 h p.i. (F) Granule treatment but not granule+protease inhib reduced bacterial load in the bladders of Wsh mice. Scale bar: (A) 2 μm (left), 200 nm (right), (C) 10 μm, (E) 50 μm. Data represent 3 independent experiments. *p<0.05, **P<0.001, See also Figure S5.

Figure 6. mMCPT4 induces cytolysis and exfoliation of BEC

(A, B) Bladders of Mcpt4^−/− mice exfoliated less and had higher CFU than WT 12 h p.i. (A, D) Frozen sections of collapsed bladders stained for superficial epithelia (green), intermediate epithelia (red), and UPEC (blue). (C) rmMCPT4-TAT induce lytic cell death of 5637 BECs in vitro. Various doses of rmMCPT4-TAT were added to 5637 BECs, followed by LDH assay 12 h post-treatment. (D, E) mMCPT4-TAT induced exfoliation of infected bladder epithelium in MC deficient (Wsh) mice is associated with reduced bacterial burden. Bladders of Wsh mice were infected with UPEC for 1 h, followed by intravesicular instillation of rmMCPT4-TAT or vehicle. Bladders were examined 12 h later. (E) Bacterial burden in bladders. (F) Direct cleavage of procaspase 1 mediated by rmMCPT4-TAT. Increasing concentrations of rmMCPT4-TAT were incubated with recombinant procaspase 1 for 1h. Full-length and cleaved procaspase as well as rmMCPT4-TAT-His₆ were detected by α-caspase 1 antibody (p20 active or full length unit) and α-His₆ antibody, respectively. (G–I) Examination of BECs from urine samples (5 randomly selected patients with acute UTIs). (G) Crystal violet staining of sedimented superficial BECs showing highly UPEC infected (left) and sparsely infected (right) BECs. (H) Relative numbers of bacteria-infected and sparsely infected BECs in urine (n=565 cells). (I) MC granule remnants visualized inside urothelial cells shed in urine of UTI patients. A cluster of cells (left) and two isolated cells at a higher magnification (right). Sedimented BECs were stained for superficial BECs (WGA, red), UPEC (α-E. coli antibody, blue), and MC granules (avidin, green). Scale bar: (A, D) 100 μm, (G, I) 20 μm. “L” indicates lumen. Data represent 2–3 independent experiments. *p<0.05, **P<0.01, ***P<0.001, See also Figure S6.