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. 2018 Sep 18;24(12):3099-3107.e4.
doi: 10.1016/j.celrep.2018.08.040.

Spatial Separation of Mitochondrial Calcium Uptake and Extrusion for Energy-Efficient Mitochondrial Calcium Signaling in the Heart

Sergio De La Fuente  1 , Jonathan P Lambert  2 , Zuzana Nichtova  1 , Celia Fernandez Sanz  3 , John W Elrod  2 , Shey-Shing Sheu  4 , György Csordás  5
Affiliations

Spatial Separation of Mitochondrial Calcium Uptake and Extrusion for Energy-Efficient Mitochondrial Calcium Signaling in the Heart

Sergio De La Fuente et al. Cell Rep. .

Abstract

Mitochondrial Ca2+ elevations enhance ATP production, but uptake must be balanced by efflux to avoid overload. Uptake is mediated by the mitochondrial Ca2+ uniporter channel complex (MCUC), and extrusion is controlled largely by the Na+/Ca2+ exchanger (NCLX), both driven electrogenically by the inner membrane potential (ΔΨm). MCUC forms hotspots at the cardiac mitochondria-junctional SR (jSR) association to locally receive Ca2+ signals; however, the distribution of NCLX is unknown. Our fractionation-based assays reveal that extensively jSR-associated mitochondrial segments contain a minor portion of NCLX and lack Na+-dependent Ca2+ extrusion. This pattern is retained upon in vivo NCLX overexpression, suggesting extensive targeting to non-jSR-associated submitochondrial domains and functional relevance. In cells with non-polarized MCUC distribution, upon NCLX overexpression the same given increase in matrix Ca2+ expends more ΔΨm. Thus, cardiac mitochondrial Ca2+ uptake and extrusion are reciprocally polarized, likely to optimize the energy efficiency of local calcium signaling in the beating heart.

Keywords: Ca2+ mitochondria; NCLX distribution; calcium signaling; cardiac excitation-energetics coupling; cardiac muscle; mitochondria-sarcoplasmic reticulum contact sites; mitochondrial Ca2+ uniporter distribution.

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Conflict of interest statement

DECLARATION OF INTERESTS

The authors declare no competing interests.

Figures

Figure 1.
Figure 1.. NCLX Levels Are Very Low in the jSR-Associated Mitochondrial Segments
(A) Schematics illustrating how mechanical homogenization of ventricular muscle yields mitochondria and mitochondrial fragments with different extents of jSR association in the mitochondrial (Mt) and jSR fractions (De La Fuente et al., 2016). (B) Western blot analysis of the abundance of the indicated proteins in the rat heart crude Mt, jSR, and Percoll-purified Mt (pMt) fractions. Representative membraneimages are on the left. The graph shows the cumulated band density (fold) differences between the Mt and jSR fractions. Bars representing proteins more abundant in the Mt or jSR fractions, respectively, point leftward and rightward. The red gridlines indicate the value of 1 (equal abundance). Means ± SEs, n = 4 rats. (C) Quantitation of the Percoll purification of the mitochondrial fraction. Mean band densities after purification (pMt) are normalized to the levels before purification (Mt). Means ± SEs, n = 4 rats. *p < 0.05, **p < 0.001. See also Figure S1.
Figure 2.
Figure 2.. NCE Is Robust in the Mitochondrial Fraction, but Not in the jSR Fraction
(A–D) 45Ca2+ retention assays assessing mitochondrial NCE based on how it counters MCUC-dependent Ca2+ uptake or (E and F) directly, based on the fractional loss in the sequestered 45Ca2+ after suspending the uptake pharmacologically. To isolate mitochondrial Ca2+ fluxes, the SR was pre-depleted and the SERCA pumps blocked (pretreatment with thapsigargin 10 μM and caffeine 10 mM in the presence of 50 μM EGTA/Tris). (A) MCUC-mediated (Ru360-sensitive) 45Ca2+ uptake in the Mt (black) and jSR (red) fractions in the presence (+) and absence (−) of Na+ (10 mM) at the indicated time points after increasing [Ca2+] in the assay buffer from virtually 0 to ~1.5 μM. The Na+ (no NCE) Ca2+ sequestration represents the “gross” unopposed Ca2+ uptake, while the +Na+ (NCE active) Ca2+ sequestration is the “net” uptake, and the difference between the two is counted as the Na+-dependent efflux (shaded areas in the graph). For clearer visualization of the proportions of gross and net uptake and the efflux in the jSR and Mt fractions, the corresponding graph segments are schematized on the right. Means ± SEs, n = 3–4 rats (triplicates). nmol/mg of protein*: to aΔΨust to the difference in mitochondrial content between the fractions, a correction was applied based on the relative citrate synthase activity as outlined by De La Fuente et al. (2016). (B) NCE from (A), expressed as the percentage of gross uptake. (C and D) Gross and net MCUC-mediated 45Ca2+ uptake and NCE determined and quantified as in (A) and (B), respectively, except that gross uptake is measured using the mitochondrial Na+/Ca2+ exchanger inhibitor CGP-37157 (CGP 20 μM) in the presence of Na+. (E) Following a 1-min period of 45Ca2+ sequestration, as in (C), MCUC was blocked by adding Ru360 (10 μM), and the Ca2+ retained in the Mt and jSR fractions was measured at the indicated time points. To better isolate the Ca2+ loss due to mitochondrial NCE, the experiments were conducted both in the presence and absence of CGP. (F) CGP-sensitive Ca2+ extrusion (difference between ± CGP) from (E), expressed as the percentage of (gross) Ca2+ retention at the time of Ru360 addition. Note that at time 0 there is already significant extrusion (as the gross minus net uptake difference). Means ± SEs, n = 3 rats (triplicates). (G) MCUC-independent (Ru360-insensitive) Ca2+ retention in the Mt and jSR fractions determined from the 30-s points in (A) in the presence of Ru360. See also Figure S2.
Figure 3.
Figure 3.. Transgenic Overexpression of NCLX Increases NCE in the Mitochondrial Fraction, but Not in the jSR Fraction
NCE in the cardiac mitochondrial and jSR fraction of control (Ctr) mice and those with adult cardiomyocyte-specific NCLX overexpression (OE) was determined by means of identical 45Ca2+ retention assays, as in Figures 2A and 2B. (A) Gross (-Na+) mitochondrial Ca2+ uptakes at 30 s. Net (+Na+) uptake levels are indicated with dashed lines. (B) Na+-dependent efflux (NCE) as the difference between gross and net uptakes. (C) Na+-dependent efflux as the percentage of the gross uptake. Note the significant increase in the fractional efflux in the mitochondrial (black) but not in the jSR (red) fraction. Bar charts are means ± SEs, n = 3 Ctr and 3 OE mice (triplicates for each). *p < 0.05, **p < 0.001. See also Figures S2, S3, and S5F.
Figure 4.
Figure 4.. NCLX, When Not Separated from MCUC, Increases the Energy Cost of Mitochondrial Ca2+ Signal Generation
H9c2 myoblasts (with non-polarized MCU distribution; Figure S4), transiently transfected with NCLX-FLAG (NCLX-OE, OE) or control vector (Ctr), were loaded with rhod-2/AM or TMRM to record [Ca2+]m and ΔΨm, respectively, then gently permeabilized. Fluorescence time courses were taken using a wide-field microscope fitted with an EMCCD camera. To isolate mitochondrial Ca2+ handling, the ER was pre-depleted with thapsigargin (2 μM). After 1-min baseline recording in virtually Ca2+-free intracellular buffer (supplemented with 10 μM EGTA), [Ca2+] in the buffer ([Ca2+]c) was raised to ~5 μM(determined by Fura-LoAff, not shown by a CaCl2 bolus (20 μM) for a period of 2–2.5 min (2 min in A). In turn, [Ca2+]c was chelated down to the low nanomolar range by adding 100 μM EGTA/Tris (pH 7.4) to suspend further mitochondrial Ca2+ uptake and facilitate extrusion. Three minutes later, the mitochondrial uncoupler FCCP and the Ca2+ ionophore ionomycin were added to dissipate ΔΨm and release the Ca2+ from the matrix. (A) Representative time courses of [Ca2+]m (rhod-2 fluorescence normalized to the range Fmax–Fmin) recorded in control and NCLX-OE cells in Na+-containing medium. (B) Bar chart showing the time to peak [Ca2+]m in the presence and absence of Na+. (C) Fractional recovery of the [Ca2+]m rise 30 s after EGTA addition. (D) Representative time courses of ΔΨm (FTMRM normalized to the range Fmax–Fmin) recorded in Ctr and NCLX-OE cells in Na+-containing medium. (E) Left bar chart shows theΔΨm loss (from D) 60 s after the CaCl2 addition in the presence and absence of Na+. The fold enhancement of the depolarization by the NCLX OE relative to Ctr is shown on the right. (F) Residual depolarization after returning [Ca2+]c to the low nanomolar range with EGTA in the experiments shown in (D). The values, as depicted in (D) by the red (OE) double-headed arrow and black opposing arrowheads (Ctr), are corrected to a baseline drift (shown in D by dashed linear fits to the initial period and to the post-EGTA quasi-steady-state period). Bar charts are means ± SEs, n = 3 independent experiments (1–3 technical replicates each). (B, E, and F) *p < 0.05, **p < 0.001. See also Figures S4 and S5.
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