ROS-triggered phosphorylation of complex II by Fgr kinase regulates cellular adaptation to fuel use

Abstract

Electron flux in the mitochondrial electron transport chain is determined by the superassembly of mitochondrial respiratory complexes. Different superassemblies are dedicated to receive electrons derived from NADH or FADH2, allowing cells to adapt to the particular NADH/FADH2 ratio generated from available fuel sources. When several fuels are available, cells adapt to the fuel best suited to their type or functional status (e.g., quiescent versus proliferative). We show that an appropriate proportion of superassemblies can be achieved by increasing CII activity through phosphorylation of the complex II catalytic subunit FpSDH. This phosphorylation is mediated by the tyrosine-kinase Fgr, which is activated by hydrogen peroxide. Ablation of Fgr or mutation of the FpSDH target tyrosine abolishes the capacity of mitochondria to adjust metabolism upon nutrient restriction, hypoxia/reoxygenation, and T cell activation, demonstrating the physiological relevance of this adaptive response.

Figure 1. Phosphorylation of FpSDH Increases CII Activity in CI-Deficient Cells

(A) Fibroblast lines with mutations in ND6 show decreased CI activity in proportion to the mutation load. Activity is expressed as the percentage of activity in FBalb/cJ cells (WT, no mutation; n = 12): EB2615 (70%; n = 6); E23 (34%; n = 6); E12 (20%; n = 9); FG12-1 (5%; n = 9); and FG23-1 (2%; n = 9). (B) CII activity (CoQ reduction) in cell lines with high loads of the iC13887 ND6 mutation, expressed as the percentage of activity in FBalb/cJ cells (n = 18): EB2615 (n = 9); E23 (9 n = 9); E12 (n = 12); FG12-1 (n = 9); and FG23-1 (n = 20). (C) Western blot after BNGE, detecting subunits of CI (NDUFA9), CII (FpSDH), and CIV (COX I). (D) G3PDH activity in FG23-1 and FBalb/cJ (n = 6). (E) Activities of CI (left) and CII (right) in a time course rotenone treatment of FBalb/cJ (200 nM) as a percentage of activity in untreated cells (n ≥ 6). Short: 2 and 6 hr treatment; long: 12 hr and 1.5 day treatment. (F) FpSDH immunoblot after 2D IEF/SDS-PAGE (IEF strip pH 3–10) of control and calf intestine phosphatase (CIP)-treated mitochondrial protein (100 μg) from FBalb/cJ and FG23-1 cells. The acidic shift of FpSDH spots in FG23-1 cells is blocked by CIP. The CI subunit NDUFS3 was used to align and compare the blots. (G) CII activity in CIP-treated mitochondria from FBalb/cJ and FG23-1 cells (n = 6). Data are presented as the percentage of activity in untreated FBalb/cJ. **p < 0.001; ***p < 0.0001. Enzyme activities were determined spectrophotometrically. All data are presented as mean ± SD. See also Figure S1.

Figure 2. FpSDH Phosphorylation and CII Activity Are ROS Dependent

(A) ROS (H₂O₂) levels measured in nontreated (n.t.) FG23-1 and FBalb/cJ cells and in cells treated with NAC (5 mM, 7 days) (control, n = 35; NAC, n = 12). (B) Effect of NAC treatment on CII activity in FBalb/cJ and in FG23-1 cells (untreated, n = 15; NAC, n = 4). Data in (A) and (B) are presented as the percentage of nontreated FBalb/cJ cells. (C) CII activity in isolated mouse liver mitochondria treated for 10 min with H₂O₂ (50 μM), catalase (25 U/ml), or both (n ≥ 4). (D) 2D IEF/SDS-PAGE (IEF strips pH 4–7) Western analysis of FpSDH from H₂O₂-treated mouse liver mitochondria. (E) CII activity in mitochondria isolated from SCAFI– (left) and SCAFI+ (right) fibroblasts treated with the ROS generator xanthine oxidase (XO) (n = 4). Data are presented as the percentage of activity in nontreated fibroblasts. (F) H₂O₂ production in FG23-1 and rotenone-treated FBalb/cJ fibroblasts is prevented by expression of mitochondrially targeted catalase (mt-cat, n = 5). (G) Elevated CII activity in FG23-1 and rotenone-treated FBalb/cJ cells is prevented by expression of mt-cat but not MnSOD (n = 3). *p < 0.01; **p < 0.001; ***p < 0.0001. All data are presented as mean ± SD. See also Figure S2.

Figure 3. The Src-Type Kinase Fgr Phosphorylates CII

(A) Immunoblot phoshoprotein analysis in FpSDH immunoprecipitates from FBalb/cJ and FG23-1 cells. (B) Complex II activity in mouse liver mitochondria incubated with the Tyr kinase inhibitor PP2, the PKA agonist 8Br-cAMP, or the PKA antagonist H89 (n = 5). (C) 2D IEF/SDS-PAGE (3–10 IEF strips) Western analysis of mouse mitochondria treated with PP2, 8Br-cAMP, or H89. PP2 induces a basic shift in FpSDH compared with nontreated mitochondria (n.t.). Modulation of PKA activity had no effect on FpSDH phosphorylation. NDUFS3 (CI) was detected to align and compare blots. (D) H₂O₂-induced activation of complex II is blocked by the Tyr kinase inhibitor PP2, but not by the Tyr phosphatase inhibitor orthovanadate (Ov) (n = 6). (E) Intact mouse mitochondria (mt) protect a portion of Fgr, Csk, Src, Lyn, and PKA kinases from digestion by proteinase K: hom, cell homogenate; n.t., non-treated; PK, proteinase K; T+PK, proteinase K treatment of mitochondria solubilized with Triton X-100. Tim23 was used as a marker of intact mitochondria. (F) Anti-FpSDH specifically coimmunoprecipitates Fgr kinase from mitochondria solubilized with digitonin (DIG), which preserves intact supercomplexes, but not dodecyl maltoside (DDM), which resolves individual respiratory complexes. mt, fully solubilized mitochondria. (G) 2D BNGE/SDS-PAGE of DDM- and DIG-solubilized mitochondria, showing comigration of Fgr and FpSDH in DIG-treated mitochondria. Note comigration of Csk and Src in both preparations. (H) Mitochondrial Fgr is located in the matrix. Fractionation experiment showing protection of Fgr from PK digestion in intact mitochondria (mt) and mitoplasts (Mp) but not in either fraction solubilized with Triton X-100 (T+PK). Fgr was present in the supernatant fraction of Triton X-100-solubilized mitoplasts (Mp+T-SN). P-Mp, post-mitoplast fraction. Markers used: hsp60 and Grp75, mitochondrial matrix; FpSDH, association with inner membrane; COXI, inner membrane; cyt c, intermembrane space; β-actin, cytoplasm. All data are presented as mean ± SD.

Figure 4. CII Is Regulated by Fgr Phosphorylation at Tyr604

(A) SDS-PAGE western analysis of Fgr tyrosine kinase in liver mitochondrial protein preparations from control or Fgr^−/− mice. (B) CII activity in isolated liver mitochondria from WT or Fgr^−/− mice in the presence of H₂O₂, PP2, or both. Data are presented as mean ± SD. (C) 2D IEF/SDS-PAGE (IEF strips pH 4–7) Western analysis of FpSDH from WT liver mitochondria and from nontreated and H₂O₂- or PP2-treated Fgr^−/− liver mitochondria. NDUFS3 (CI) is used to align and compare blots. (D) Complex II activity in liver mitochondria from different Tyr kinase null mice. (E) Complex II activity in tissue homogenates from Fgr^+/− and Fgr^−/− null mice (n = 4). (F and G) Individual and combined mitochondrial complex activities in isolated mitochondria from liver (F) and heart (G) (n ≥ 4). In(D)–(G), lines extending from the boxes indicate the variability outside the upper and lower quartiles. (H) CII activity in FBalb/cJ cells silenced for endogenous FpSDH and re-expressing WT and mutant FpSDH variants (n = 4). ROT, rotenone (200 nM); shRNA, silencing of endogenous FpSDH. Data are presented as the percentage of activity in nontreated, mock-infected FBalb/cJ (mean ± SD). Statistical significance versus nontreated: **p < 0.001; ***p < 0.0001. Statistical significance versus shRNA: #, p < 0.01; ###, p < 0.0001. (I) Fgr in vitro phosphorylation (bottom) and FpSDH immunodetection (top) of immunocaptured complex II from FBalb/cJ cells expressing WT FpSDH (left line) or the Y604F mutant (right line). See also Figure S3.

Figure 5. T Cell Activation and CII Activity Response

(A) Flow cytometry analysis of T cell activation. Naive T cells from Fgr^+/− and Fgr^−/− mice were activated with anti-CD3/CD28, and activation was assessed by detection of CD69. Gray line: negative control. The chart shows data from one representative experiment of four. (B) Flow cytometry analysis of ROS production (2,7-DCFH₂-DA) in resting T cells and upon CD3/CD28 or ConA activation (n = 3). Data are presented as mean ± SD. (C) CII activity measured in resting and activated T cells (n = 4). Data are presented as the percentage of activity compared to resting T cells. Lines extending from the boxes indicate the variability outside the upper and lower quartiles.

Figure 6. Mitochondria Lacking Fgr or Expressing Y604F FpSDH Respond Abnormally to Starvation and Serum Deprivation

(A) Succinate-driven OXPHOS function in mouse liver mitochondria from Fgr^+/− and Fgr^−/− mice fed a normal diet (well fed) or starved overnight (ON st, n ≥ 4). CII activity (left), succinate-driven respiration (center left), succinate-driven ATP synthesis (center right), and citrate synthase activity (CS, right). (B) NADH-driven OXPHOS function in mouse liver mitochondria from Fgr^+/− and Fgr^−/− mice fed a normal diet or starved overnight (n ≥ 4). CI activity (left), glutamate-driven respiration (center left), pyruvate-driven respiration (center), glutamate-driven ATP synthesis (center right), and pyruvate-driven ATP synthesis (right) are shown. (C) Rate of use of NADH/FADH reducing equivalents in mouse liver mitochondria from Fgr^+/− and Fgr^−/− mice (n ≥ 4). Data in (A)–(C) are presented as the percentage of values obtained in well-fed Fgr^+/− mice. Lines extending from the boxes indicate the variability outside the upper and lower quartiles. (D) Immunostaining of mitochondria (Tom20, green) in Fgr^+/− and Fgr^−/− fibroblasts cultured with serum or without serum overnight (No FBS ON). (E) Immunoblot showing the migration of OPA1 in lysates from control and serum-deprived Fgr^+/− and Fgr^−/− cells (left) or control and serum-deprived FpSDH-silenced fibroblasts re-expressing WT or Y604F FpSDH (right). (F)CII activity in Fgr^+/− and Fgr^−/− fibroblasts grown in 10 mM glucose in the indicated conditions (n ≥ 5). Data are presented as the percentage activity in Fgr^+/− cells grown in normoxia; lines extending from the boxes indicate the variability outside the upper and lower quartiles. (G)CII activity in FpSDH-silenced fibroblasts re-expressing WT or Y604F FpSDH, cultured at 5 mM glucose under the indicated oxygenation conditions (n ≥ 5). Data are presented as the percentage of activity in WT-expressing cells grown in normoxia; lines extending from the boxes indicate the variability outside the upper and lower quartiles. (H) Apoptotic events in FpSDH-silenced fibroblasts re-expressing WT or Y604F FpSDH and grown with or without serum. Apoptosis was determined by flow cytometry as the percentage of cells positive for annexin V and propidium iodide (n = 3). Data are presented as mean ± SD. *p < 0.01; **p < 0.001; ***p< 0.0001. See also Figures S4 and S5.

Figure 7. Mitochondria Lacking Fgr or Expressing Y604F FpSDH Respond Abnormally to Hypoxia-Reoxygenation

(A) Immunostaining of mitochondria (Tom20, green) in Fgr^+/− and Fgr^−/− fibroblasts cultured in normoxia (21% O₂) or hypoxia (1 % O₂)for48 hr followed by 48 hr normoxia (Reoxy 48 hr). (B)Left: Immunoblot analysis of Fgr^+/− and Fgr^−/− cells cultured under normoxia (Nx), hypoxia for 48 hr(Hyp 48 hr), or hypoxia followed by normoxia (Reoxy 48 hr). Right: Immunoblot analysis of FpSDH-silenced fibroblasts re-expressing WT or Y604F FpSDH and cultured under the indicated oxygenation conditions. Numbers beneath blots show FpSDH (Fp):actin and Tom20 (T20):actin ratios (n = 4). (C) Top: CII activity in Fgr^+/− and Fgr^−/− fibroblasts grown in 10 mM glucose under the indicated conditions (n ≥ 5). Bottom: FpSDH-silenced fibroblasts re-expressing WT or Y604F FpSDH, cultured with 5 mM glucose under the indicated oxygenation conditions (n ≥ 5). Data are presented as the percentage activity in Fgr^+/− cells or WT-FpSDH-re-expressing cells grown in normoxia; lines extending from the boxes indicate the variability outside the upper and lower quartiles. (D) Top: Relative activities of CII (CoQ reduction) and SDH in WT- and Y604F-re-expressing FpSDH-silenced cells grown in normoxia or through a hypoxia/reoxygenation cycle. For each cell line and condition, 100% = the sum of CII and SDH activities; absolute SDH activity did not differ between cell lines and conditions. Bottom: Apoptotic events in FpSDH-silenced fibroblasts re-expressing WT or Y604F FpSDH and grown in normoxia or through a hypoxia/reoxygenation cycle. Apoptosis was determined by flow cytometry as the percentage of annexin V- and PI-positive cells (n = 3). Data are presented as mean ± SD. (E) BNGE of FpSDH-silenced fibroblasts re-expressing WT or Y604F FpSDH cultured at 5 mM glucose under the indicated oxygenation conditions; the blot reveals the distribution of CIII (anti-core 1 immunodetection) among the different forms of free complex and supercomplexes. The outer membrane protein Tom20 is used as a mitochondrial protein loading control. Upper and lower panels are taken from two independent experiments. Note that upon reoxygenation the amount of CIII super assembled with CI is abnormally elevated in the Y604F mutant. See also Figure S5.