Research ArticleFibrosis

Translocation of TRPV4-PI3Kγ complexes to the plasma membrane drives myofibroblast transdifferentiation

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Science Signaling  12 Nov 2019:
Vol. 12, Issue 607, eaau1533
DOI: 10.1126/scisignal.aau1533

Figures

  • Fig. 1 TRPV4 selectively mediates TGF-β–driven PI3Kγ activity.

    (A) Representative Western blot showing TRPV4, phosphorylated AKT [p-AKT (Ser473)], total AKT (t-AKT), and GAPDH (loading control) in lysates of human lung fibroblast (HLF) treated with TRPV4 or scrambled siRNA ± TGF-β as indicated. (B) Quantification of p-AKT (Ser473) relative to total AKT as in (A) by densitometry. *P < 0.05 difference between untreated and +TGF-β conditions. Band densities were measured with respect to the untreated scrambled siRNA condition. a.u., arbitrary units. (C) Representative Western blot showing p-AKT (Ser473) and total AKT (t-AKT) in lysates from HLFs treated with the TRPV4 antagonist RN-1734 ± TGF-β as indicated. (D) Quantification of p-AKT (Ser473) relative to total AKT as in (C). **P < 0.01 difference between untreated and +TGF-β conditions. Band densities were measured with respect to the untreated condition. (E) Lipid kinase activity of PI3Kγ and PI3Kα immunoprecipitated from plasma membrane fractions of HLFs treated with the TRPV4 antagonist RN-1734 ± TGF-β as indicated. PI3Kγ and PI3Kα were assayed for their capacity to generate PIP by thin-layer chromatography. The line between the untreated and +TGF-β conditions indicates noncontiguous lanes from the same blot. (F) Quantification of PIP densities as in (E). *P < 0.05 as indicated. PIP densities were measured with respect to the untreated condition for each PI3K isoform. All graphs show means ± SEM from n = 3 independent experiments (*P ≤ 0.05 and **P ≤ 0.01, using ANOVA followed by Student-Newman-Keuls multiple comparisons test).

  • Fig. 2 TRPV4 and PI3Kγ interact directly and translocate to the plasma membrane in response to TGF-β.

    (A) Binding of purified PI3Kγ to purified 6-His-TRPV4 coupled to Ni-NTA beads was assessed by immunoblotting with an antibody specific for PI3Kγ. The blots were stripped and reprobed with an antibody for TRPV4. Control indicates beads alone (no protein bound to beads). n = 3 independent experiments. (B) Direct interaction between immobilized purified 6-His-TRPV4 and purified 6-His-PI3Kγ was measured by SPR. n = 3 independent experiments. (C) Binding of purified 6-His-PI3Kγ coupled to Ni-NTA beads to endogenous TRPV4 from HLF lysates was assessed by Western blotting. Blots were stripped and reprobed with antibodies specific for PI3Kγ, GRK2 (positive control), and TRPV2 (negative control). Control indicates beads alone (no protein bound to beads). n = 3 independent experiments. (D) PI3Kγ immunoprecipitates (IP) from plasma membrane and cytosolic fractions of HLFs treated with TGF-β as indicated were immunoblotted for PI3Kγ and TRPV4. The line between the plasma membrane and cytosolic fractions indicates noncontiguous lanes from the same blot. Total whole-cell lysate of cells treated ± TGF-β (input) was immunoblotted for TRPV4 and PI3Kγ. GAPDH is a loading control. (E) Quantification of TRPV4 in plasma membrane and cytosolic fractions normalized to TRPV4 in whole-cell lysate as in (D). Data represent means ± SEM. n = 3 independent experiments. *P ≤ 0.05 between untreated and TGF-β conditions, using ANOVA followed by Student-Newman-Keuls multiple comparisons test.

  • Fig. 3 TRPV4 and PI3Kγ are mutually required for translocation to the plasma membrane.

    (A) Representative confocal images showing TRPV4 in wild-type (WT) and PI3Kγ knockout (KO) murine lung fibroblasts (MLFs) ± TGF-β as indicated and the corresponding plot profiles of the TRPV4 immunofluorescence. Orange lines indicate regions where plot profiles were obtained. White boxes indicate areas shown in higher-magnification insets. (B) Quantification of plasma membrane:cytoplasm fluorescence for experiments in (A). n = 3 independent experiments with at least 30 cells per condition. ***P < 0.005 compared with untreated WT MLF or with PI3Kγ KO MLF ± TGF-β. (C) Representative confocal images showing PI3Kγ in WT and TRPV4 KO MLFs ± TGF-β and the corresponding plot profiles of the PI3Kγ immunofluorescence. (D) Quantification of results in (C). n = 3 independent experiments with at least 30 cells per condition. ***P < 0.005 compared with untreated WT MLF or with TRPV4 KO MLF ± TGF-β. (E) Representative immunoblots for TRPV4 and β1 integrin (loading control) from a surface biotinylation assay in WT and PI3Kγ KO MLF ± TGF-β. (F) Quantification of results from (E). n = 3 independent experiments. **P < 0.01 compared with untreated WT MLF or with PI3Kγ KO MLF ± TGF-β. (G) Representative immunoblots for PI3Kγ and flotillin-1 (loading control) from the plasma membrane fractions of WT and TRPV4 KO MLF ± TGF-β. The line between WT and TRPV4 KO MLF conditions indicates noncontiguous lanes from the same blot. (H) Quantification of results from (G). n = 3 independent experiments. ***P < 0.005 compared with untreated WT MLF or with TRPV4 KO ± TGF-β. All graphs show means ± SEM from n = 3 independent experiments. **P ≤ 0.01 and ***P ≤ 0.005, using ANOVA followed by Student-Newman-Keuls multiple comparisons test. Scale bars, 50 μm.

  • Fig. 4 PI3Kγ is required for TRPV4 ion channel function.

    (A) Representative plots showing the effects of scrambled, PI3Kα, and PI3Kγ siRNA on Ca2+ influx induced by the TRPV4 agonist GSK1016790A (GSK) in HLFs. Ca2+ influx was measured in relative fluorescence units (RFU) using calcium 5 dye on intact HLF (19Lu) monolayers treated with scrambled, PI3Kα, or PI3Kγ siRNA. (B) Quantification of experiments in (A). ***P < 0.005 compared with PI3Kα siRNA or scrambled siRNA. (C) Representative immunoblots showing PI3Kα and PI3Kγ in 19Lu cells treated with scrambled, PI3Kα, or PI3Kγ siRNA. GAPDH is a loading control. (D) Quantification of PI3Kα and PI3Kγ band density relative to GAPDH in (C). ***P < 0.005 as indicated. (E) Representative plot showing GSK1016790A-induced Ca2+ influx in WT and PI3Kγ KO MLFs treated ± TGF-β as indicated. (F) Quantification of RFU in 300 nM GSK1016790A conditions in (E). *P < 0.05 as indicated. (G) Representative immunoblot showing TRPV4 in WT and PI3Kγ KO MLFs treated ± TGF-β. (H) Quantification of TRPV4 band density relative to GAPDH in (G). All graphs show means ± SEM from n = 3 independent experiments (*P ≤ 0.05 and ***P ≤ 0.005, using ANOVA followed by Student-Newman-Keuls multiple comparisons test).

  • Fig. 5 Both TRPV4 and PI3Kγ are necessary for profibrotic responses of myofibroblasts.

    (A) Representative immunoblots showing collagen-1 in WT, TRPV4 KO, and PI3Kγ KO MLFs treated ± TGF-β. Black lines indicate noncontiguous lanes from the same blot. GAPDH is a loading control. (B) Quantification of collagen-1:GAPDH band density ratios (means ± SEM) from (A). n = 3 independent experiments. *P < 0.05 compared with untreated WT MLF, TRPV4 KO MLF ± TGF-β, or PI3Kγ KO MLF ± TGF-β. (C) Representative traction force intensity images and corresponding phase images with the region of interest outlined in red for WT, TRPV4 KO, and PI3Kγ KO MLFs treated ± TGF-β. Red and orange, areas of high contraction; blue, areas of low contraction. Scale bar, 50 μm. (D) Quantification of root mean square (RMS) traction force (means ± SEM) results from (C). n = 3 independent experiments with at least 30 cells per condition. ***P < 0.005 compared with untreated WT MLF, TRPV4 KO MLF ± TGF-β, or PI3Kγ KO MLF ± TGF-β. (E) Quantification of WT and PI3Kγ KO MLFs that differentiated into myofibroblasts when plated on polyacrylamide gels of varying stiffness and treated with TGF-β, as measured by α-smooth muscle actin (α-SMA) in stress fibers. Data are shown as % of WT MLF on glass. n = 3 independent experiments with at least 30 cells per condition *P < 0.05 between WT and PI3Kγ KO MLF at 8 kPa, ***P < 0.005 between WT and PI3Kγ KO MLF at 25 kPa and glass. (F) Quantification of WT and PI3Kγ KO MLFs plated on polyacrylamide gels of varying stiffness and treated with TGF-β that showed a TRPV4 plasma membrane:cytoplasm ratio of >1 as measured by TRPV4 immunofluorescence. Data are shown as % of WT MLF on glass. n = 3 independent experiments with at least 30 cells per condition. ***P < 0.005 between WT and PI3Kγ KO MLF at 8 kPa, 25 kPa, and glass. All graphs show means ± SEM from n = 3 independent experiments (*P ≤ 0.05 and ***P ≤ 0.005, using ANOVA followed by Student-Newman-Keuls multiple comparisons test).

  • Fig. 6 Both PI3Kγ and TRPV4 are required for myofibroblast transdifferentiation.

    (A) Representative fluorescence images showing α-SMA (green) and F-actin (red) in TRPV4 KO and PI3Kγ KO MLFs treated with empty vector (control) lentivirus (LV), TRPV4 LV, or PI3Kγ LV and then stimulated with TGF-β. Non-transfected WT MLFs were used as a positive control. Scale bar, 100 μm. (B) Quantification of cells with α-SMA–positive stress fibers in (A). Data are presented as % myofibroblasts (means ± SEM). n = 3 independent experiments with at least 30 cells per condition. *P < 0.05 and ***P < 0.005 as indicated. (C) Representative Western blot showing TRPV4, PI3Kγ, and α-SMA in TRPV4 KO MLFs transfected with PI3Kγ LV or TRPV4 LV and then stimulated with TGF-β. (D) Quantification of α-SMA:GAPDH band density ratios (means ± SEM) in (C). n = 3 independent experiments. **P < 0.01 difference between untreated and +TGF-β conditions with TRPV4 LV. (E) Representative Western blot showing TRPV4, PI3Kγ, and α-SMA in PI3Kγ KO MLFs transfected with PI3Kγ LV or TRPV4 LV and then stimulated with TGF-β. GAPDH is a loading control. (F) Quantification of α-SMA:GAPDH band density ratios (means ± SEM) in (E). n = 3 independent experiments. *P < 0.05 difference between untreated and +TGF-β conditions with PI3Kγ LV. All graphs show means ± SEM from n = 3 independent experiments (*P ≤ 0.05, **P ≤ 0.01, and ***P ≤ 0.005, using ANOVA followed by Student-Newman-Keuls multiple comparisons test).

  • Fig. 7 The noncatalytic, N-terminal domain of PI3Kγ is necessary and sufficient for TRPV4 translocation and myofibroblast transdifferentiation.

    (A) Domain structure of WT and mutant PI3Kγ constructs consisting of only the noncatalytic N-terminal domain (N-term) or lacking both the N-terminal domain and the ATP binding site in the catalytic domain (N-del). (B) Immunoblotting of His-tagged N-del or N-term forms of PI3Kγ coupled to Ni-NTA beads incubated with lysates of HLFs. Blots were probed with an antibody recognizing TRPV4 and then stripped and reprobed for PI3Kγ. n = 3 independent experiments. (C) Direct interaction between immobilized purified 6-His-TRPV4 and purified 6-His-N-term-PI3Kγ was measured by SPR. n = 3 independent experiments. (D) Representative fluorescence images showing TRPV4 in PI3Kγ KO MLFs transfected with N-del or N-term PI3Kγ LV and treated with TGF-β and the corresponding plot profiles of TRPV4 immunofluorescence. White arrowheads indicate TRPV4 at the plasma membrane; orange lines indicate regions where plot profiles were obtained; white boxes indicate higher-magnification insets. Scale bar, 50 μm. (E) Quantification of results from (D). n = 3 independent experiments with at least 30 cells per condition. ***P < 0.005 compared with PI3Kγ KO MLF + N-del PI3Kγ LV by Student’s t test. (F) Western blotting of plasma membrane fractions of PI3Kγ KO MLFs transfected with lentivirus encoding WT PI3Kγ, N-term PI3Kγ, or N-del PI3Kγ and then treated with TGF-β as indicated. Blots were probed for TRPV4, PI3Kγ, and flotillin-1 (loading control). n = 3 independent experiments. (G) Representative fluorescence images showing α-SMA in PI3Kγ KO MLFs transfected with control LV (empty vector), WT PI3Kγ LV, N-term PI3Kγ LV, or N-del PI3Kγ LV and then stimulated with TGF-β. White boxes indicate areas enlarged in insets. Scale bar, 100 μm. (H) Quantification of results from (G). n = 3 independent experiments with at least 30 cells per condition. ***P < 0.005 untreated versus +TGF-β conditions by ANOVA followed by Student-Newman-Keuls multiple comparisons test. All graphs show means ± SEM from n = 3 independent experiments.

  • Fig. 8 Schematic working model showing TGF-β–stimulated activation of TRPV4-PI3Kγ complexes in myofibroblast transdifferentiation.

    Our data suggest that TRPV4 binds to the noncatalytic, N-terminal domain of PI3Kγ, forming a TRPV4-PI3Kγ complex. Upon TGF-β stimulation, the N terminus of PI3Kγ acts as a scaffold, recruiting TRPV4-PI3Kγ complexes from the cytoplasm to the plasma membrane. This translocation leads to an increase in TRPV4-mediated Ca2+ influx, which promotes myofibroblast transdifferentiation.

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