Nontranscriptional modulation of intracellular Ca2 signaling by ligan

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1 Department of Cellular and Structural Biology, University of Texas Health Science Center at San Antonio, San Antonio, TX 78229; H- E8 [/ h9 E% H+ F
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2 Department of Physiology, University of Texas Health Science Center at San Antonio, San Antonio, TX 78229
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Correspondence to James D. Lechleiter: lechleiter@uthscsa.edu9 d" H9 m! n% S- Y! S. P$ V4 n

Abstract8 M% S: x/ V/ d! L
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Thyroid hormone 3,5,3'-tri-iodothyronine (T3) binds and activates thyroid hormone receptors (TRs). Here, we present evidence for a nontranscriptional regulation of Ca2  signaling by T3-bound TRs. Treatment of Xenopus thyroid hormone receptor beta subtype A1 (xTR?A1) expressing oocytes with T3 for 10 min increased inositol 1,4,5-trisphosphate (IP3)-mediated Ca2  wave periodicity. Coexpression of TR?A1 with retinoid X receptor did not enhance regulation. Deletion of the DNA binding domain and the nuclear localization signal of the TR?A1 eliminated transcriptional activity but did not affect the ability to regulate Ca2  signaling. T3-bound TR?A1 regulation of Ca2  signaling could be inhibited by ruthenium red treatment, suggesting that mitochondrial Ca2  uptake was required for the mechanism of action. Both xTR?A1 and the homologous shortened form of rat TR1 (rTRF1) localized to the mitochondria and increased O2 consumption, whereas the full-length rat TR1 did neither. Furthermore, only T3-bound xTR?A1 and rTRF1 affected Ca2  wave activity. We conclude that T3-bound mitochondrial targeted TRs acutely modulate IP3-mediated Ca2  signaling by increasing mitochondrial metabolism independently of transcriptional activity.

N. Saelim's current address is: Dept. of Pharmacy Practice, Faculty of Pharmaceutical Sciences, Naresuan University, Pitsanulok, Thailand, 65000.; B1 H1 h# z& Y3 y

L.M. John's current address is: Pfizer, Inc., CVMD Biology, Groton, CT 06340.

Abbreviations used in this paper: ANT, adenine nucleotide translocator; DBD, DNA binding domain; , mitochondrial membrane potential; IP3, inositol 1,4,5-trisphosphate; MBS, modified barth's solution; O2, oxygen; pBOX, three amino acid sequence within the DNA binding domain that recognizes specific DNA binding sequences; RA, 9-cis retinoic acid; rTR1, rat thyroid hormone receptor alpha subtype 1; rTRF1, shortened form of rat TR1; Ru360, ruthenium 360; RXR, retinoid X receptor; SEAP, secreted placental alkaline phosphatase; T3, 3,5,3'-tri-iodothyronine; TMRE, tetramethylrhodamine ethyl ester; TR, thyroid receptor; TRE, thyroid hormone response element; xTR?A1, Xenopus thyroid hormone receptor beta subtype A1.
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Introduction! U( }, b* ~& b4 C$ k2 f

Thyroid hormones are lipophilic ligands composed of two iodinated tyrosine residues that regulate cellular differentiation and development, cardiac function, and basal metabolism (Abbaticchio et al., 1981; Oppenheimer et al., 1987, 1994; Nagai et al., 1989; Kawahara et al., 1991; Soboll, 1993a; Ichikawa and Hashizume, 1995). Thyroid receptors (TRs) are classified as steroid hormone receptors and have genomic effects similar to other nuclear receptors such as glucocorticoid, estrogen, and androgen receptors. Two separate genes encode thyroid hormone receptors  (TR) and ? (TR?). Alternative splicing or the use of different promoters generates multiple isoforms including the  (TR1, TR2) and ? (TR?1, TR?2) subtypes (Lazar, 1993). Thyroid hormones have been shown to increase the number of mitochondria and to induce the expression of mitochondrial proteins encoded by both nuclear and mitochondrial genes (Das and Harris, 1991; Soboll, 1993a; Iglesias et al., 1995; Wrutniak et al., 1995; Meehan and Kennedy, 1997; Schonfeld et al., 1997). Isolated mitochondria from hyperthyroid cells exhibit enhanced substrate cycling and increased oxygen (O2) consumption (Soboll, 1993b). Thyroid hormone also affects the mitochondrial membrane potential () through the expression of mitochondrial proteins (Soboll, 1993a,b). Collectively, these long-term effects take days or weeks to manifest and are thought to be mediated by nuclear and mitochondrial transcriptional regulation.; g/ a9 Z# I) A) h; t
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Increasing evidence suggests that thyroid hormone exerts nontranscriptional effects on mitochondrial metabolism. Initial studies demonstrated that treatment of cells with 3,5,3'-tri-iodothyronine (T3) results in a rapid increase in O2 consumption and ATP production in rat liver mitochondria (Sterling, 1980). These effects persisted in the presence of protein synthesis inhibitors, suggesting that the mechanism of action was nontranscriptional. Sterling and coworkers (Sterling, 1980; Sterling and Brenner, 1995) additionally demonstrated that exposure of mitochondria to T3, isolated from rat hepatocytes, increased both ATP production and O2 consumption. Acute exposure of isolated mitochondria to thyroid hormone has also been reported to increase pH and to increase mitochondrial Ca2  efflux (Sterling et al., 1980; Crespo-Armas and Mowbray, 1987; Soboll, 1993a). Mitochondrial localization of TRs was originally reported by Sterling and coworkers (Sterling, 1991). Later, Ardail et al. (1993) identified two high affinity T3 binding proteins in rat liver mitochondria. Wrutniak et al. (1995) and Casas et al. (1999) reported the presence of a high affinity (43 kD) T3 binding protein in rat liver mitochondrial matrix extracts, which was identified as an NH2 terminus shortened form of rat TR1 (rTRF1). The full-length form of the rat thyroid hormone receptor alpha subtype 1 (rTR1) is predominantly localized to the nucleus where it binds to DNA response elements and regulates transcriptional events (Wrutniak et al., 1995). Wrutniak (Wrutniak et al., 1995) suggested that the mitochondrial form of the rTR may be involved in mitochondrial transcriptional activity.1 w2 H# W- e+ X% e
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Intracellular Ca2  signaling has been intimately linked to mitochondrial metabolism. Several dehydrogenases within the citric acid cycle are Ca2  dependent (McCormack and Denton, 1989). Ca2  uptake into the mitochondria is a passive process driven by the mitochondrial  and occurs via the Ca2  uniporter. Because of the low Ca2  affinity of the uniporter, high cytosolic Ca2  concentrations are required to cause significant mitochondrial Ca2  uptake. Under physiological conditions, these concentrations only occur near an open ion channel pore. Consequently, close physical proximity between the ER and mitochondria is required for significant mitochondrial Ca2  uptake (Rizzuto et al., 1998, 1999). Work from our laboratory also demonstrated that mitochondrial Ca2  uptake itself modulated inositol 1,4,5-trisphosphate (IP3)-Ca2  release (Jouaville et al., 1995). Subsequently, Hajnoczky et al. (1995) demonstrated that IP3-mediated Ca2  oscillations efficiently stimulated mitochondrial metabolism. The local Ca2  signaling between the ER and mitochondria has now been supported by many other investigators (Simpson and Russell, 1996; Hajnoczky et al., 1999; Szalai et al., 2000). Control of mitochondrial metabolism by matrix Ca2  appears to be a fundamental mechanism whereby cells meet their energy requirements.3 ?6 T, C: }) E1 \* ?: f/ N' |

Xenopus laevis oocytes do not express detectable levels of endogenous TRs (Banker et al., 1991; Kawahara et al., 1991; Eliceiri and Brown, 1994). Induction of TR expression in Xenopus laevis occurs during the embryonic stages of development (Yaoita and Brown, 1990; Banker et al., 1991; Kawahara et al., 1991; Eliceiri and Brown, 1994). Consequently, Xenopus oocytes offer a unique model system to study the effects of thyroid hormones and their receptors on intracellular Ca2  signaling and mitochondrial metabolism.

We present evidence demonstrating that thyroid hormone-activated TRs acutely regulate mitochondrial metabolism and, thereby, Ca2  wave activity. Only expression of the NH2 terminus–truncated forms of TR that target the mitochondria were effective at stimulating mitochondria. Transcriptionally inactive TRs were fully capable of modulating Ca2  wave activity. These observations suggest an acute nontranscriptional pathway for modulation of intracellular Ca2  signaling via thyroid hormone receptor-stimulated mitochondrial metabolism.
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Results

T3-stimulated TR?A1s modulate IP3-mediated Ca2  wave activity

Acute nongenomic effects of thyroid hormones occur within minutes of ligand treatment (Hummerich and Soboll, 1989). To examine the importance of TRs on the nongenomic modulation of intracellular Ca2  signaling, stage VI Xenopus oocytes were injected with mRNA encoding the Xenopus thyroid hormone receptor betz subtype A1 (xTR?A1) as described previously (Camacho and Lechleiter, 2000). Expression of xTR?A1 was confirmed by Western blot analysis, 2–3 d after mRNA injection (Fig. 1). The Ca2  indicator dye was injected into oocytes 30–45 min before confocal imaging. When oocytes were injected with IP3, we observed repetitive Ca2  wave activity with interwave periods of 6.62 ± 0.20 s (n = 70; Fig. 1). When xTR?A1 expressing oocytes were treated with T3 10 min before IP3 injection, the Ca2  wave periodicity increased significantly to 8.40 ± 0.30 s (Fig. 1, a and c; n = 24, P
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Figure 1. T3-bound TR?A1 increases IP3-induced Ca2  wave period. (a) Spatial-temporal stacks of IP3 (300 nM)-induced Ca2  wave activity in a representative control (water injected) oocyte, a T3-treated (100 nM) oocyte expressing TR?A1 and a T3 (100 nM) treated oocyte. Each image is 745 x 745 μm. (b) Western blot showing expression of TR?A1. Protein extracts from all groups were collected and loaded at 0.5 oocytes per lane onto 10% SDS-PAGE. The membrane was probed with a monoclonal mouse anti–human TRs antibody (MA1-215) and labeled with an HRP-conjugated secondary antibody. (c) Histogram of average interwave period for each group of oocytes. n values in parentheses represent the total number of oocytes pooled from at least two frogs. Error bars correspond to the mean ± SEM. The asterisks (**) denote a statistic significant difference (ANOVA single factor, P

Transcriptional activity of TR?A1 is undetectable in the absence of xRXR, z8 C% M: z3 N6 [# q

Classically, activated thyroid hormone receptors heterodimerize to initiate transcription responses. Retinoid X receptor (RXR) is the most common dimerization partner that binds to the thyroid hormone response element (TRE; Leid et al., 1992; Bhat et al., 1994; Wong and Shi, 1995). To investigate the transcriptional activity of xTR?A1, we coinjected oocytes with xTR?A1 mRNA and a plasmid reporting vector containing a TRE system with two direct repeats (DR4) upstream of the secreted placental alkaline phosphatase (SEAP) gene (p-TRE-SEAP; CLONTECH Laboratories, Inc.). If the hormone receptor dimerizes and binds to the TRE enhancer, the oocyte expresses SEAP, which is secreted into the medium. mRNA-injected oocytes were continuously bathed in T3 (100 nM) for 3 d and the presence of SEAP was subsequently quantified by Western blot analysis and used as a marker for transcriptional activity. Using this TRE-reporting system, we observed no transcriptional activity in oocytes expressing the xTR?A1 protein by itself (Fig. 2 a, lane 3). However, when we coexpressed xRXR with xTR?A1 and oocytes were incubated with T3 (100 nM) and 9-cis retinoic acid (RA; 100 nM) for 3 d, SEAP expression was significantly increased (Fig. 2 a, lane 5). Note that xTR?A1/xRXR-mediated transcription requires both ligands, T3 and RA (Fig. 2 a, lanes 4 and 5). These data indicate that stimulation of xTR?A1 by T3 does not initiate detectable transcription in Xenopus oocytes.5 V) c! y/ K: Y4 v

Figure 2. Transcriptional activity of TR?A1 requires xRXR and both cognate ligands. Transcriptional activity was monitored with the TRE-reporter vector, pSEAP (TRE). (a) Lanes 1 and 2 are negative (pSEAP(–ve)) and positive (pSEAP( ve)) vector controls. Oocytes expressing TR?A1 or TR?A1 plus xRXR were incubated with 100 nM T3 (lanes 3–5) plus 100 nM RA (lane 5) for 3 d. Cytosolic extracts from each group of oocytes was prepared and loaded onto a 10% SDS-PAGE at 2.5 oocytes equivalents per lane. SEAP was detected with the polyclonal rabbit anti–human SEAP antibody and an HRP-conjugated secondary antibody. The SP labeled arrow indicates SEAP immunoreactivity, which was present only in oocytes expressing TR?A1 and xRXR exposed to both T3 and RA. (b) Transcriptional activity of TR?A1 requires the pBOX within the DBD and the NLS. Oocytes expressing xTR?A1pBox-NLS and xRXR show no SEAP immunoreactivity when incubated with T3 (lane 6) or T3 plus RA (lane 7). Western blot analysis shows that xRXR, TR?A1, and xTR?A1pBox-NLS are expressed at comparable levels (Western blots below lanes 4–7). TR?A1 and xTR?A1pBox-NLS were detected with the monoclonal mouse anti–human TRs antibody (MA1-215). xRXR was detected with a polyclonal rabbit anti–human RXR antibody (Sc-774).

Acute modulation of Ca2  signaling does not require heterodimerization with RXR$ I* v, o" {1 Q& l5 s& v
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To test whether heterodimerization of xTR?A1 with xRXR affects the acute modulation of Ca2  activity, we coinjected oocytes with both xRXR and xTR?A1 mRNA and confirmed protein expression levels using Western analysis 2–3 d after injection of mRNA (Fig. 3 b). Oocytes were loaded with Ca2  indicator dye and confocally imaged. Oocytes coexpressing xTR?A1 and xRXR were initially exposed to both T3 (100 nM) and RA (100 nM) 10 min before injection with IP3 (300 nM). The average Ca2  interwave period for xTR?A1-expressing oocytes was 6.58 ± 0.26 s (n = 67), whereas that of xRXR/xTR?A1 coexpressing oocytes was 6.72 ± 0.31 s (n = 82; Fig. 3, a and c). These values were not significantly different from each other (P = 0.22), but were both significantly larger than values in the control oocytes that exhibited an average Ca2  interwave period of 5.90 ± 0.43 s (n = 55, ANOVA single factor, P
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Figure 3. Acute modulation of Ca2  signaling does not require heterodimerization of TR?A1 with xRXR. (a) Spatial-temporal stacks of IP3-induced Ca2  wave activity in control oocytes compared with oocytes expressing TR?A1 or TR?A1 with xRXR. T3 (100 nM) and RA (100 nM) were added as indicated 10–15 min before injection with IP3 (300 nM). Scale is the same as Fig. 1. (b) Western blots of oocytes expressing TR?A1 and xRXR. Primary and secondary antibodies were identical to those used in Figs. 1 and 2. (c). Histogram of average interwave period (second) of each group of oocytes. The asterisks (**) denote a statistic significance using ANOVA single factor (P
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The DNA binding domain and NLS of TR?A1 are not required for acute affects on Ca2  signaling$ ?+ b1 _; W0 C3 @

The ability of T3-bound xTR?A1 to rapidly modulate Ca2  activity suggested a nontranscriptional mechanism of action. Our strategy to test this hypothesis was to delete the DNA binding domain (DBD) and mutate the NLS from the thyroid hormone receptor and test whether the mutant receptors were (a) transcriptionally inactive and (b) still effective at modulating Ca2  signaling. Oocytes were injected with the p-TRE-SEAP plasmid reporting vector. This reporting system requires heterodimerization of xRXR and xTR?A1 to transactivate the reporter gene (Fig. 2 a). Consequently, test oocytes were coinjected with xRXR mRNA with mRNA encoding either wild-type xTR?A1 (control), mutant xTR?A1 lacking the NLS (xTR?A1-NLS), or the mutant lacking both the NLS and the pBOX (xTR?A1pBox-NLS). Once injected, oocytes were continuously bathed in T3 (100 nM) and RA (100 nM) for 3 d. Expression levels of xTR?A1 mutants and xRXR groups were comparable to xTR?A1 and xRXR groups (Fig. 2 b, bottom). Using the expression of SEAP as a marker for transcriptional activity, we confirmed that oocytes expressing the xTR?A1 mutants and xRXR proteins were transcriptionally inactive, whereas oocytes expressing wild-type xTR?A1 and xRXR proteins exhibited strong transcriptional activity (Fig. 2, lanes 7 and 5).
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Subsequently, we tested whether the transcriptionally inactive xTR?A1 mutants could still acutely regulate Ca2  signaling. Oocytes were injected with xTR?A1 mRNA or its mutants and protein expression levels were confirmed using Western analysis 2–3 d after injection (Fig. 4 c). Oocytes expressing xTR?A1 or the mutants were exposed to T3 (100 nM) 10 min before injection with IP3 (300 nM). Ca2  activity was confocally imaged, as described above. The average Ca2  interwave period for the control group (water-injected oocytes) was 6.6 ± 0.20 s (n = 70), which was significantly shorter than that in the xTR?A1 expressing oocytes (8.40 ± 0.30 s, n = 40; ANOVA single factor, P 8 {7 O. y/ O2 D! x# S
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Figure 4. The pBOX and NLSs of TR?A1 are not required for the acute regulation of Ca2  signaling. (a) Schematic figure depicting the position of the pBOX deletion in the DBD and the NLS modification within TR?A1. (b) Spatial-temporal stack of IP3-induced Ca2  wave activity in control oocytes compared with oocytes expressing TR mutants pBox-NLS and NLS. Oocytes expressing the TR mutants were incubated with T3 (100 nM) 10–15 min before IP3 (300 nM) injections. (c) Western blot analysis confirming comparable levels of protein expression for both wild-type and mutant TR?A1. (d) Histogram of the average Ca2  wave periods for each group of oocytes (n values are in parentheses). Statistic significance over control oocytes is indicated by the asterisks (**; ANOVA single factor, P 3 k' q  F* ?% k/ }* w. k9 q- W1 h

T3-bound TR?A1 appears to regulate Ca2  signaling by increasing mitochondrial respiration8 I) ~6 d. m. E1 q; s- q

We reported previously that pyruvate/malate-energized mitochondria increase the amplitude and interwave period of IP3-induced Ca2  waves in Xenopus oocytes (Jouaville et al., 1995). These effects on Ca2  wave activity were similar to those observed in TR?A1 overexpressing oocytes with acute T3 incubation (Fig. 1). Sterling and colleagues (Sterling, 1980) initially reported that T3 increases mitochondrial metabolism, particularly oxidative phosphorylation, in less than 30 min. Consequently, we hypothesized that the regulation of Ca2  signaling by T3-activated xTR?A1 was mediated by its acute modulation of mitochondrial metabolism, which, in turn, increased mitochondrial Ca2  uptake. Our strategy to test this hypothesis was threefold. First, we examined the effect of T3 on  in TR?A1 expressing oocytes using the potential sensitive dye tetramethylrhodamine ethyl ester (TMRE). Oocytes were bathed in 200 nM TMRE for 5 min before imaging fluorescence with two-photon excitation (800 nm).  was estimated by monitoring Log(Fmito/Fcyto) where Fmito is the fluorescence intensity of individual mitochondria and Fcyto is the cytosolic fluorescence (Farkas et al., 1989). We found that T3 significantly increased  from a resting value of 0.33 ± 0.01 (n = 75) to 0.44 ± 0.01 (n = 92, P   J. \- e& z3 w7 f6 Z

Figure 5. T3 stimulation of oocytes expressing TR?A1 increases the . (a) Images of mitochondria labeled with the potential sensitive dye TMRE. The oocytes are expressing TR?A1 and have been exposed to T3 for the indicated amount of time. Images are 50 x 100 μm. (b) Histogram of the log of mitochondrial TMRE fluorescence (Fmito) divided by the cytosolic fluorescence (Fcyto) at the indicated times of T3 exposure. Values in parentheses refers to the number of mitochondrion analyzed. Statistical significance is indicated by the asterisks (**; ANOVA single factor, P

Figure 6. Ru360 blocks T3-bound TR?A1 increases in IP3-induced Ca2  wave period. (a) Spatial-temporal stacks of the effect of Ru360 treatment on Ca2  wave activity in control oocytes compared with oocytes expressing TR?A1as labeled. (b) Histogram of average interwave period (seconds) of each group of oocytes shown in a. The asterisk (*) denotes a statistic significance using ANOVA single factor (P

Third, we directly test whether thyroid hormone receptor together with T3 stimulates mitochondrial respiration. Xenopus oocytes were injected with TR?A1 mRNA or water and incubated for 3 d. The rate of O2 consumption was measured as an indicator of respiration. 200 oocytes in each group were loaded into a 2-ml O2 probe chamber filled with modified barth's solution (MBS) solution. After 15 min of stabilization, the medium was exchanged with fresh MBS and O2 consumption was monitored for 30 min. The medium was exchanged a third time with MBS containing 100 nM T3 and O2 consumption was followed for another 30 min (Fig. 7, a and b). After this protocol, the rate of O2 consumption in water-injected oocytes after T3 exposure was 0.42 ± 0.25 nmol/min (n = 8). In contrast, the rate of O2 consumption in TR?A1-injected oocytes after T3 exposure was significantly increased to 1.68 ± 0.52 nmol/min (n = 4, P

Figure 7. T3 stimulation of oocytes expressing TR?A1 increases O2 consumption. (a) Plots of O2 levels in oocytes as labeled (n = 200 oocytes per group). (b) Histogram represents average change of O2 consumption rates (before and after T3 exposure) in control and xTR?A1 groups. Statistical significance is indicated by the asterisk (*; t test, P + S3 h( l* z3 f2 f( k$ ~

TRs targeted to the mitochondria are required for a T3-stimulated increase in respiration and the regulation of Ca2  signaling9 u* y. Z2 }& y$ u

T3 treatment has previously been reported to increase mitochondrial metabolism (Sterling et al., 1980; Soboll, 1993a). Our data suggest that the acute effects of T3 on mitochondrial metabolism are likely to be mediated by T3-activated thyroid hormone receptors. A truncated form of rat TR1 (rTR1F) has been shown to localize to mitochondria matrix (Ardail et al., 1993; Wrutniak et al., 1995; Casas et al., 1999). Furthermore, the NH2 terminus of the Xenopus TR?A1 that we used throughout this work has a high homology to the NH2 terminus of rTR1F (Fig. 8 a). Our strategy in this experiment was to test whether mitochondrial targeting of TRs was necessary to modulate Ca2  signaling. First, we examined the cellular targeting of xTR?A1, rTR1, and rTR1F by injecting Xenopus oocytes with their respective mRNAs. After 3 d of expression, mitochondria were isolated by centrifugation. Whole oocyte extract (minus mitochondria) and mitochondrial extract from each group were subjected to immunoprecipitation using a TR antibody (MA1-215; Affinity BioReagents, Inc.). The immunocomplexes (TRs/MA1-215) were loaded onto a 10% SDS-PAGE gel for Western blot analysis. As shown in Fig. 8 c, only xTR?A1 and rTR1F were detected in the mitochondria extracts. Full-length rTR1 did not localize to mitochondria. These results are consistent with previous reports (Ardail et al., 1993; Wrutniak et al., 1995; Casas et al., 1999). Our next step was to compare the rate of O2 consumption for oocytes expressing either rTR1 or rTR1F (Fig. 9). Consistent with its mitochondrial targeting, the rate of O2 consumption in rTR1F expressing oocytes after T3 exposure was significantly increased 1.88 ± 0.35 nmol/min (n = 3, P 0 |. S" R1 O4 V$ M, V' C

Figure 8. Xenopus TR?A1 and NH2-terminal truncated rat TR1 (rTR1F) localize to mitochondria. (a) Schematic diagram of TRs showing that rTR1F and xTR?A1 have a similar NH2 terminus. (b and c) Western blots of TR1, rTR1F, and xTR?A1 expression in whole oocytes and mitochondrial extracts respectively. FL, full-length receptor; SH, shortened form of the receptor. Extracts were prepared from 300 oocytes in each group. All oocytes were exposed to 100 nM T3 for at least 15 min before organelle extraction. TRs were immunoprecipitated with a monoclonal mouse anti–human TRs antibody (MA1-215), captured with immobilized protein G, concentrated, and loaded onto a 10% SDS-PAGE. An HRP-conjugated secondary antibody was used for visualization.- Z1 b$ x( h& d6 I" E5 @6 u" m

Figure 9. NH2-terminal truncated rat TR1 (rTR1F) stimulates O2 consumption. (a) Plots of O2 levels for oocytes expressing full-length rTR1 with and without T3 compared with oocytes expressing the NH2-terminal truncated rTR1F with or without T3. Protocols used were identical to those described in Fig. 7. (b) Histogram of the average change of the O2 consumption rates after T3 exposure in rTR1 versus rTR1F groups. The asterisk (*) indicates statistical significance (t test, P

Finally, we tested whether targeting of TRs to mitochondria was required to regulate Ca2  signaling. As before, oocytes were injected with either full-length rTR1 or NH2 terminus–truncated rTR1F mRNAs. Protein expression levels were measured 2–3 d after injection (Fig. 10 b). Ca2  activity was confocally imaged 10 min after treatment with T3 (100 nM). We found that the average Ca2  interwave period for rTR1F-injected oocytes was 8.8 ± 0.26 s (n = 24), which was significantly higher (ANOVA single factor, P $ l, B  \% I. w" n# \6 m
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Figure 10. The truncated rTR1F regulates intracellular Ca2  release. (a) Spatio-temporal stacks of IP3-induced Ca2  wave activity in control oocytes compared with oocytes expressing rTR1F or rTR1F. TR expressing oocytes were treated with 100 nM T3 10–15 min before IP3 (300 nM) injections and confocal imaging. (b) Western blots of rTR1 and rTR1F expression levels in experimental oocytes. (c) Histogram of the average interwave periods for each group (n values in parentheses). Note that rTR1F has significantly longer periods even though its expression levels are lower than those of full-length rTR1. The asterisks (**) indicate statistical significance with P

Discussion5 g* q4 u4 h/ p% v5 J) d1 h

In this work, we report that the acute exposure of oocytes expressing mitochondrially targeted TR to T3 regulates IP3-mediated Ca2  wave activity. We observed a T3-bound TR induced increase in the Ca2  wave period and amplitude. These changes in Ca2  activity were similar to those observed in Xenopus oocytes when mitochondria were energized with respiratory chain substrates (Jouaville et al., 1995). In that report, the modulation of IP3-mediated Ca2  release was due to an increase in mitochondrial Ca2  uptake via an increase in the . Our current work is consistent with this model because we could inhibit the effects of T3-bound TRs by inhibiting mitochondrial Ca2  uptake with Ru360. We also directly demonstrated that T3 exposure increased  in oocytes expressing TRs. An increase in  could be attributed to either a direct effect on electron transport or to a decrease in proton leak (Gunter and Pfeiffer, 1990; Gunter and Gunter, 1994; Gunter et al., 1998). The application of T3 to mitochondria has been reported to decrease proton leak in several preparations (Crespo-Armas and Mowbray, 1987; Soboll, 1993a). However, we found that T3 exposure increases O2 consumption in TR-expressing oocytes. An increase in the rate of O2 consumption is not consistent with a decrease in proton leak. Together, our data favors the conclusion that T3-bound TR regulates Ca2  activity by increasing  via an increase in proton pumping by the respiratory chain.- j  B" T9 z( ], q3 P
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Application of thyroid hormones to mitochondria has long been known to increase metabolism (Sterling, 1980). Mitochondria were also known to be target organelles of T3 accumulation in cells (Sterling et al., 1984; Morel et al., 1996). However, a mitochondrial hormone receptor that mediated these effects has never been conclusively identified. Sterling (1986)(1991) initially suggested that the adenine nucleotide translocator (ANT) bound to T3 with high affinity. Romani et al. (1996) also suggested that thyroid hormone had its specific mitochondrial target site at the matrix side of ANT. They found that bongkrekic acid, a membrane-permeant inhibitor of ANT, blocked a thyroid hormone-induced release of Mg2  from mitochondria. On the other hand, Wrutniak and coworkers (Wrutniak-Cabello et al., 2001) found no evidence demonstrating a direct interaction between ANT and T3. Our data indicate that ANT alone is not the thyroid hormone receptor that mediates the regulation of mitochondrial metabolism. Rather, our data reveal that a mitochondrial targeted TR is a required element of acute thyroid hormone regulation of metabolism. The use of Xenopus oocytes in these experiments was crucial in this determination because oocytes do not express endogenous TRs (Yaoita and Brown, 1990; Kawahara et al., 1991). The ubiquitous expression of endogenous TRs would have hidden this finding in earlier studies.& y; B% ]( M0 G5 B  U
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The ability of specific thyroid hormone receptors to target mitochondria has been demonstrated by other investigators. A truncated form of rat TR1 (rTR1F) and not its full-length form, localized to the matrix of mitochondria (Ardail et al., 1993; Wrutniak et al., 1995; Casas et al., 1999). Our work corroborated these reports and further demonstrated that the xTR?A1, which is highly similar to rTR1F, targeted the mitochondria. Casas and coworkers (Casas et al., 1999) reported that mitochondrial activity was stimulated by overexpression of p43 (mitochondria-targeted, truncated-TR), which in turn, stimulated mitochondrial genome transcription of some enzyme units that played a role in the respiratory chain. The p43 protein had the same affinity to T3 as the full-length TR to bind to the D-loop of two mt-TREs in the mitochondria, leading to mitochondrial protein synthesis (Casas et al., 1999). Their data suggested that p43 bound to mt-TREs as a homodimer because no RXR-isoform in the mitochondrial extract was detected (Casas et al., 1999). Hadzic suggested that the NH2 terminus of TRs plays a role in TR-homodimerization in mitochondria (Hadzic et al., 1998). Together, these studies demonstrated that mitochondrial-targeted TRs could regulate mitochondrial metabolism by initiating transcription. However, our results cannot be accounted for by this mechanism of action. Specifically, transcriptionally inactive TR mutants modulated Ca2  wave activity with the same efficacy as the wild type, xTR?A1. We confirmed that xRXR was required for xTR?A1 to transactivate a reporter gene in our system, but more importantly, the presence of xRXR did not affect the ability of xTR?A1 to modulate Ca2  activity. Thus, we concluded that the mechanism by which T3-activated TRs regulate Ca2  signaling cannot be attributed to transcription." T. F2 _6 i( `% x8 r. {8 P9 O4 ]
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Nongenomic effects of various steroid receptors have been reported for mineralocorticoids (Moura and Worcel, 1984; Zhou and Bubien, 2001), glucocorticoids (Borski, 2000; Borski et al., 2002), gonadal steroids (Pietras and Szego, 1975; Wasserman et al., 1980; Lieberherr and Grosse, 1994; Guo et al., 2002a,b; Minshall et al., 2002), vitamin D3 (Sergeev and Rhoten, 1995), and thyroid hormone (Hummerich and Soboll, 1989; Davis and Davis, 1996, 2002; Rojas et al., 2003). Most of these studies proposed the presence of specific membrane-bound receptors for nongenomic effects; however, specific receptors were not cloned or identified. For thyroid hormones in particular, Davis and Davis (2002) suggested that the mechanism of the nongenomic effects of thyroid hormone may not require TRs, and could involve actions of the hormone itself on signal transduction pathway via specific G protein–coupled protein. Recent work by Scanlan et al. (2004) identified an endogenous, rapid-acting derivative of thyroid hormone that is a potent agonist of the G protein-coupled trace amine receptor (TAR1). Activation of TAR1 increased cAMP production, which in turn, would active protein kinase A and phosphorylation of multiple proteins in cells. Our results do not exclude a potential role of second messenger systems in the mechanism of action of T3 on mitochondria. Rather, they demonstrate that classic TRs, those that have long been known to regulate gene transcription, will also acutely regulate mitochondrial activity when bound with T3. Stimulation is dependent on mitochondrial targeting of the TR, but not on its ability to initiate transcription. Together, these observations reveal a nontranscriptional pathway for modulation of intracellular Ca2  signaling via T3/TR-stimulated mitochondrial metabolism.
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The discovery of T3/TR-regulated Ca2  signaling is potentially important for several reasons. First, any process that acutely regulates intracellular Ca2  release will impact the multitude of Ca2 -sensitive cellular processes ranging from contractility and secretion to proteolysis and cell death. Second, the ability of a steroid hormone to increase proton pumping provides a rapid method to increase metabolism in response to short-term energy requirements; for example, during increased neuronal activity or during a transient increase in muscle activity. Third, and potentially more importantly, a rapid increase in mitochondrial Ca2  uptake could protect cells under conditions of stress. Mitochondria have long been recognized for their capacity to sequester large Ca2  concentrations under pathological conditions (Gunter et al., 1994). The ability to transiently remove Ca2  from the cytosol could be used to minimize tissue damage after stroke in neuronal tissue or to reduce the instability of cardiac cells after periods of hypoxia. Clearly, the identification of a mitochondrial receptor for thyroid hormone-induced increases in metabolism offers a new pharmacological target from which it will be possible to regulate a broad range of physiological and pathological processes.# p, I3 H/ _6 Z; Z! ]' O3 \
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Materials and methods

Expression vector construction

The coding fragments of rat TR1 cDNA, Xenopus RXR (a gift from R.M. Evans, The Howard Hughes Medical Institute, Chevy Chase, MD, and The Salk Institute for Biological Studies, La Jolla, CA) were amplified by PCR with a forward primer 5'-acgtggatccatggaacagaagccaagcaaggtg-3' (for rTR1), 5'-acgtggatccatgagttcagcagccatggacaccaaacat-3' (for xRXR) containing a BamHI site, and a reverse primer, 5'-atcgaagcttttagacttcctgatcctcaaagacctc-3' (for rTR1), 5'-atcgaagcttttaagtcatttggtgaggggcttccagcat-3' (for xRXR) containing a HinDIII site. The PCR products were then subcloned into the Xenopus oocyte expression vector, pGEM-HeNot between the BamHI and HinDIII sites (Camacho and Lechleiter, 1995). All restriction enzymes were purchased from Life Technologies. The truncated form of rTR ﹞ (rTR1F) was generated by PCR at the second ORF of rTR1 cDNA using a forward primer, 5'-acgtggatccatgtcagggtatatccctagttacc-3' containing a BamHI site, and a reverse primer, 5'-atcgaagcttttagacttcctgatcctcaaagacctc-3' containing a HinDIII site. The PCR product was placed into the pGEM-HeNot vector between the BamHI and HinDIII sites.
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Xenopus TR?A1 was amplified by PCR with primers 5'-gctaggatccatggaagggtatatacccagctacttgg-3' and 5'-atcgaagcttctagtcctcaaacacttccaagaacagtggggg-3' and subcloned into vector pGEM-HeNot between the BamHI and HinDIII sites to create pGEM-HeNot-xTR?A1. Xenopus mutant xTR?A1-NLS had its NLS removed by modifying the sequence from KR to AA. xTR?A1pBox-NLS had the same NLS modification as well as the pBOX deletion of CEGCK within the DBD. Both mutants were generated by QuikChange site-directed mutagenesis (Stratagene) using pGEM-HeNot-xTR?A1 as a template. The forward primer for xTR?A1NLS was 5'-ggttttggatgacaacgcagctttggcaaaaagaaagc-3' and the reverse complement primer was 5'-gctttctttttgccaaagctgcgttgtcatccaaaacc-3'. For the double mutation, pGEM-HeNot-xTR?A1NLS was used as a template. The forward primer for xTR?A1pBox-NLS was 5'-gggtatcattatagatgtatcaccggcttttttagaagaactattcag-3' and the reverse compliment primer was 5'-ctgaatagttcttctaaaaaagccggtgatacatctataatgataccc-3'. All mutations were confirmed by nucleotide sequencing (UTHSCSA DNA core facility).( w/ E6 i4 O3 v% g% X

In vitro transcriptions and oocyte protocols

Synthetic mRNA was prepared as described previously (Camacho and Lechleiter, 1995). In brief, the pGEM-HeNot vector containing cDNA template was linearized by a NotI restriction enzyme. From the linearized templates, mRNA was generated using the T7 promoter (MEGAscript; Ambion). Cap analogue, m7G(5')ppp(5") (Ambion) was added to the reaction. The mRNA products were quantified by 1% agarose gel and spectrophotometry. RNase-free synthetic RNAs were resuspended at a concentration of 1.5–2.0 μg/μl and stored in aliquots of 3 μl at –80°C.( `" ?, F4 E: |* K2 L: h8 p
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Stage VI oocytes were obtained from adult female Xenopus laevis. After defolliculation, oocytes were incubated in MBS (in mM: 88 NaCl, 1 KCl, 0.41 CaCl2, 0.33 Ca(NO3)2, 0.82 MgSO4, 2.40 NaHCO3, 10 Hepes, pH 7.5) at 18°C. mRNA was injected into the oocytes by a 50-nL bolus using a positive pressure injector (Nanoject; Drummond Scientific Co.). Control oocytes were injected with diethyl pyrocarbonate–treated water. Oocytes were incubated at 18°C for 2–3 d to allow full expression of proteins in MBS supplemented with antibiotics streptomycin, penicillin, and fungizone (GIBCO-BRL). Media was changed daily. Unhealthy oocytes were discarded daily.8 h9 T9 p- g) T& a* w. s

Imaging acquisition and analysis
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Ca2  wave activity was imaged as described previously (Camacho and Lechleiter, 1995). In brief, oocytes were injected with 50 nl of a fluorescent Ca2  sensitive dye (0.25 mM, Oregon green BAPTA2-cell impermeant; Molecular Probes) and incubated for 30–60 min before the experiment. Images were acquired with a confocal laser-scanning microscope (model PCM2000; Nikon) attached to an inverted microscope (model TE200; Nikon) at the rate of 1.5 images/s. We used a 10x 0.45 NA objective (UVFLUOR; Nikon). Each group of mRNA-injected oocytes was randomly assigned into two subgroups, one was exposed to 100 nM of T3 for 10 min and the other was untreated with T3. Ca2  wave activity was initiated by injecting a 50-nl bolus of 6 μM IP3. The Ca2  waves were analyzed with ANALYZE software (The Mayo Foundation, Rochester, MN). Statistical significance was calculated by either one-factor ANOVA or a t test as indicated.

was estimated as described previously (Lin and Lechleiter, 2002). In brief, 200 nM TMRE (Molecular Probes) was added to the bath and images were acquired with a 60x 1.4 NA objective on the Nikon PCM2000 custom adapted for two-photon imaging. TMRE was excited at 800 nm using a Ti-sapphire Coherent Mira 900 Laser pumped with a 5W Verdi laser (Coherent Inc.). Laser intensity was attenuated with a neutral-density filter wheel such that no detectable photobleaching of TMRE was observed.% a1 c6 D6 W) B5 n0 `4 |
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Transcriptional activity assay7 T) w3 r$ C( ]
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The transcriptional activity of TR and mutants were confirmed by using a reporting vector with the thyroid response element (TRE) as a cis-acting enhancer for the SEAP gene (Mercury Pathway Profiling SEAP System2; CLONTECH Laboratories, Inc.). The negative control vector (pSEAP(–ve)) lacks the enhancer element, but contains a promoter and SEAP reporter gene. Oocytes in each group were injected with mRNA (0.5 μg) and vector (0.5 μg) as designated, and incubated in 1 ml MBS with 100 nM T3, and/or RA for 3 d. Media was collected and replaced every 24 h for 3 d. Collected media from each group was pooled, concentrated (Amicon ultra 10000 MWCO; Millipore) to 40 μl and run on a 10% SDS-PAGE. Oocyte cytosolic extract from each group was prepared and loaded onto 10% SDS-PAGE at amounts equivalent to 2.5 oocytes per lane. SEAP was detected with polyclonal rabbit anti–human SEAP antibody (Zymed Laboratories, Inc.). HRP-conjugated secondary antibody (Jackson ImmunoResearch Laboratories Inc.) was used and visualized by chemiluminescence (PerkinElmer).
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Western blot analysis
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Oocytes were washed twice times in homogenization buffer (in mM: 15 Tris-HCl, 140 NaCl, 250 sucrose, 1% Triton X-100, Complete protease inhibitor cocktail) at a concentration of 40 μl/oocyte. Washed oocytes were homogenized and centrifuged at 4,500 g for 15 min at 4°C. The supernatant was collected and loaded at 0.5 oocytes per lane onto 10% SDS-PAGE. TRs and mutants were detected with monoclonal mouse anti–human TRs antibody (MA1-215; Affinity BioReagents, Inc.). xRXR was detected with polyclonal rabbit anti–human RXRs antibody (Sc-774; Santa Cruz Biotechnology, Inc.). HRP-conjugated secondary antibody (Jackson ImmunoResearch Laboratories Inc.) was used and visualized by chemiluminescence (PerkinElmer).( u$ J3 i, `! z$ G

Cytosolic and mitochondrial extract preparations4 |7 B- G* q$ t& s- [4 S

300 oocytes in each group (water, xTR?A1, rTR1, rTR1F) were allowed to express for 3 d and then treated with 100 nM T3 for 15 min at RT. Oocytes were washed twice times with buffer A (in mM: 190 sorbitol, 1 CaCl2, 10 TES, pH 7.4) and resuspended in buffer A at a final volume of 500 μl. Oocytes were sequentially homogenized with a hand-held homogenizer and centrifuged at 1,000 g for 5 min at 4°C. The supernatant was transferred to new tube and centrifuged at 14,000 g for 15 min at 4°C. Supernatant and pellet were collected separately. The pellet, which contained mitochondria, was washed several times with buffer B (in mM: 195 sorbitol, 5 EDTA, 5 TES, pH 7.4) and spun at 1,000 g for 5 min at 4°C to eliminate contaminants. The mitochondrial portion was finally obtained by centrifugation at 14,000 g for 15 min at 4°C. Mitochondria in each group were washed twice by resuspending in buffer B, centrifuged again at 14,000 g for 15 min at 4°C and lysed in the presence of 1% Triton X-100. The cytosolic fraction was centrifuged at 100,000 g for 15 min at 4°C to eliminate contaminating membranes.0 T- G- D8 E- `" o. K2 S
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O2 consumption assay+ I  N3 ^" X+ _
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A biological O2 monitor (model 5300; YSI Inc.) was used to measure O2 consumption. 200 oocytes in each group were loaded into a 2-ml O2 probe chamber avoiding contact of the oocytes with the O2 probe. 1.5 ml of MBS was added to the chamber and the system was allowed to stabilize for 15 min. The medium was subsequently exchanged with 1.25 ml of fresh MBS solution and O2 consumption was monitored for 30 min. The media was exchanged again with MBS containing 100 nM T3 and O2 consumption was followed for the next 30 min. The slope of O2 levels was calculated before and after the addition of T3.8 Z8 v, {- \! W9 G( s1 C, i5 K- G
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Acknowledgments! Y' l) f/ p% b$ C4 M

We wish to thank Elizabeth Muller and Julie Etzler for their helpful and thoughtful critiques of the manuscript.
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This work was supported by National Institutes of Health grants R01 GM48451 and PO1 AG19316.' k8 z5 W& F5 l; i+ _/ M: n& G6 X

References

Abbaticchio, G., R. Giorgino, F.M. Gentile, A. Cassano, F. Gattuccio, G. Orlando, and A. Ianni. 1981. Hormones in the seminal fluid. The transport proteins of the thyroid hormones. Acta Eur. Fertil. 12:307–311.
0 |- J4 j3 `4 Q/ T' G  ]7 X7 B( \
Ardail, D., F. Lerme, J. Puymirat, and G. Morel. 1993. Evidence for the presence of  and ?-related T3 receptors in rat liver mitochondria. Eur. J. Cell Biol. 62:105–113.- T( O' u, g# A; k" n% ]

Banker, D.E., J. Bigler, and R.N. Eisenman. 1991. The thyroid hormone receptor gene (c-erbA alpha) is expressed in advance of thyroid gland maturation during the early embryonic development of Xenopus laevis. Mol. Cell. Biol. 11:5079–5089.
3 D" _9 T8 Q2 I- F2 m6 P- Y! E
Bhat, M.K., K. Ashizawa, and S.Y. Cheng. 1994. Phosphorylation enhances the target gene sequence-dependent dimerization of thyroid hormone receptor with retinoid X receptor. Proc. Natl. Acad. Sci. USA. 91:7927–7931.
- \7 }: y9 }0 Z8 k4 ]8 s8 h5 a
Borski, R.J. 2000. Nongenomic membrane actions of glucocorticoids in vertebrates. Trends Endocrinol. Metab. 11:427–436.

Borski, R.J., G.N. Hyde, and S. Fruchtman. 2002. Signal transduction mechanisms mediating rapid, nongenomic effects of cortisol on prolactin release. Steroids. 67:539–548." _* u6 T$ b) t

Camacho, P., and J.D. Lechleiter. 1995. Calreticulin inhibits repetitive intracellular Ca2  waves. Cell. 82:765–771.0 ^6 H: R8 P" j( h- t
9 m. N: d/ l" ~
Camacho, P., and J.D. Lechleiter. 2000. Xenopus oocytes as a tool in calcium signaling research. Methods in Calcium Signaling Research. J. Putney, editor. CRC Press, Boca Raton, FL. 157–181.5 L. X# l: u/ o# ~
: \  C( Z# f# Q* Q0 B
Casas, F., P. Rochard, A. Rodier, I. Cassar-Malek, S. Marchal-Victorion, R.J. Wiesner, G. Cabello, and C. Wrutniak. 1999. A variant form of the nuclear triiodothyronine receptor c-ErbAalpha1 plays a direct role in regulation of mitochondrial RNA synthesis. Mol. Cell. Biol. 19:7913–7924.; `8 w8 K) ]2 d% B/ A

Crespo-Armas, A., and J. Mowbray. 1987. The rapid alteration by tri-iodo-L-thyronine in vivo of both theADP/O ratio and the apparent H /O ratio in hypothyroid-rat liver mitochondria. Biochem. J. 241:657–661.
$ O" q2 H+ \3 ~# k# t$ {# c: ~8 v
Das, A.M., and D.A. Harris. 1991. Control of mitochondrial ATP synthase in rat cardiomyocytes: effects of thyroid hormone. Biochim. Biophys. Acta. 1096:284–290.; k; |/ D& R$ b( P9 i' ~. F( Z; W$ o

Davis, P.J., and F.B. Davis. 1996. Nongenomic actions of thyroid hormone. Thyroid. 6:497–504.1 q2 L1 h+ T' n8 z. p% Z) K
' ?* [% ~% S9 P6 S. W; z- v3 B+ m& u
Davis, P.J., and F.B. Davis. 2002. Nongenomic actions of thyroid hormone on the heart. Thyroid. 12:459–466.

Eliceiri, B.P., and D. Brown. 1994. Quantitation of endogenous thyroid hormone receptors alpha and beta during embryogenesis and metamorphosis in Xenopus laevis. J. Biol. Chem. 269:24459–24465.
- |2 c1 p) g7 K: y; r4 q) |
Farkas, D.L., M. Wei, P. Febbroriello, J.H. Carson, and L.M. Loew. 1989. Simultaneous imaging of cell and mitochondrial membrane potential. Biophys. J. 56:1053–1069.

Gunter, K.K., and T.E. Gunter. 1994. Transport of calcium by mitochondria. J. Bioenerg. Biomembr. 26:471–485.
3 b; s7 I3 `; m
Gunter, T.E., and D.R. Pfeiffer. 1990. Mechanisms by which mitochondria transport calcium. Am. J. Physiol. 258:C755–C786.& q4 N' u' }% w+ m8 F1 U% |

Gunter, T.E., K.K. Gunter, S.S. Sheu, and C.E. Gavin. 1994. Mitochondrial Ca transport: physiological and pathological relevance. Am. J. Physiol. 267:C313–C339.8 }: @: {, z+ d$ D
) W$ J5 O% t) T& k4 Q9 `4 R1 o
Gunter, T.E., L. Buntinas, G.C. Sparagna, and K.K. Gunter. 1998. The Ca2  transport mechanisms of mitochondria and Ca2  uptake from physiological-type Ca2  transients. Biochim. Biophys. Acta. 1366:5–15.
$ k9 ]) E3 a; h% G# I. r
Guo, Z., W.P. Benten, J. Krucken, and F. Wunderlich. 2002a. Nongenomic testosterone calcium signaling. Genotropic actions in androgen receptor-free macrophages. J. Biol. Chem. 277:29600–29607.

Guo, Z., J. Krucken, W.P. Benten, and F. Wunderlich. 2002b. Estradiol-induced nongenomic calcium signaling regulates genotropic signaling in macrophages. J. Biol. Chem. 277:7044–7050.7 R/ }. \% n4 Q# H

Hadzic, E., I. Habeos, B.M. Raaka, and H.H. Samuels. 1998. A novel multifunctional motif in the amino-terminal A/B domain of T3Ralpha modulates DNA binding and receptor dimerization. J. Biol. Chem. 273:10270–10278.
4 u7 [3 g. o! M  g
Hajnoczky, G., L.D. Robb-Gaspers, M.B. Seitz, and A.P. Thomas. 1995. Decoding of cytosolic calcium oscillations in the mitochondria. Cell. 82:415–424.& H) I* c( k; f! \+ ]" A# H

Hajnoczky, G., R. Hager, and A.P. Thomas. 1999. Mitochondria suppress local feedback activation of inositol 1,4,5-trisphosphate receptors by Ca2 . J. Biol. Chem. 274:14157–14162.
+ U* s- K8 R$ f' [. w$ P, t
Hummerich, H., and S. Soboll. 1989. Rapid stimulation of calcium uptake into rat liver by L-tri-iodothyronine. Biochem. J. 258:363–367.5 r9 v! J! B3 g( D2 \
* B$ j7 Z( r/ T0 j9 y) U# G. \
Ichikawa, K., and K. Hashizume. 1995. Thyroid hormone action in the cell. Endocr. J. 42:131–140.
/ }2 n0 z/ `4 G1 Q$ i# {' ?
Iglesias, T., J. Caubin, A. Zaballos, J. Bernal, and A. Munoz. 1995. Identification of the mitochondrial NADH dehydrogenase subunit 3 (ND3) as a thyroid hormone regulated gene by whole genome PCR analysis. Biochem. Biophys. Res. Commun. 210:995–1000.
7 ^# w9 V$ d; c) ]) b$ ~  V
Jouaville, L.S., F. Ichas, E.L. Holmuhamedov, P. Camacho, and J.D. Lechleiter. 1995. Synchronization of calcium waves by mitochondrial substrates in Xenopus laevis oocytes. Nature. 377:438–441.! [3 t. I: C: ^" @6 j% N9 I% r, k
" Q9 Y9 [& d) Q
Kawahara, A., B.S. Baker, and J.R. Tata. 1991. Developmental and regional expression of thyroid hormone receptor genes during Xenopus metamorphosis. Development. 112:933–943.' k9 _! S' K4 p9 ]5 z4 V

Lazar, M.A. 1993. Thyroid hormone receptors: multiple forms, multiple possibilities. Endocr. Rev. 14:184–193.' T8 p7 ~" g5 V7 i  A0 n
8 W3 {; u1 z( I- A1 w
Leid, M., P. Kastner, R. Lyons, H. Nakshatri, M. Saunders, T. Zacharewski, J.Y. Chen, A. Staub, J.M. Garnier, S. Mader, et al. 1992. Purification, cloning, and RXR identity of the HeLa cell factor with which RAR or TR heterodimerizes to bind target sequences efficiently. Cell. 68:377–395.
5 ?2 X4 y% f' l
Lieberherr, M., and B. Grosse. 1994. Androgens increase intracellular calcium concentration and inositol 1,4,5-trisphosphate and diacylglycerol formation via a pertussis toxin-sensitive G-protein. J. Biol. Chem. 269:7217–7223.
6 j; D- t- m4 ]  T/ j9 S
Lin, D.T., and J.D. Lechleiter. 2002. Mitochondrial targeted cyclophilin D protects cells from cell death by peptidyl prolyl isomerization. J. Biol. Chem. 277:31134–31141.
$ x% |* v' P2 z3 k) L) [0 c0 O4 ?$ ?
McCormack, J.G., and R.M. Denton. 1989. The role of Ca2  ions in the regulation of intramitochondrial metabolism and energy production in the rat heart. Mol. Cell. Biochem. 89:121–125.- v9 b7 ^+ [5 `! O' q4 h

Meehan, J., and J.M. Kennedy. 1997. Influence of thyroid hormone on the tissue-specific expression of cytochrome c oxidase isoforms during cardiac development. Biochem. J. 327:155–160., g' C: G6 ~# a6 N
/ X3 b8 a. s6 `$ e& D9 [9 G
Minshall, R.D., D. Pavcnik, D.L. Browne, and K. Hermsmeyer. 2002. Nongenomic vasodilator action of progesterone on primate coronary arteries. J. Appl. Physiol. 92:701–708.# o1 e4 c3 i. W4 O) z$ ?4 X

Morel, G., S. Ricard-Blum, and D. Ardail. 1996. Kinetics of internalization and subcellular binding sites for T3 in mouse liver. Biol. Cell. 86:167–174.

Moura, A.M., and M. Worcel. 1984. Direct action of aldosterone on transmembrane 22Na efflux from arterial smooth muscle. Rapid and delayed effects. Hypertension. 6:425–430.
, x# a2 Y' }1 s$ X5 W; l+ O
Nagai, R., A. Zarain-Herzberg, C.J. Brandl, J. Fujii, M. Tada, D.H. MacLennan, N.R. Alpert, and M. Periasamy. 1989. Regulation of myocardial Ca2 -ATPase and phospholamban mRNA expression in response to pressure overload and thyroid hormone. Proc. Natl. Acad. Sci. USA. 86:2966–2970.

Oppenheimer, J.H., H.L. Schwartz, C.N. Mariash, W.B. Kinlaw, N.C. Wong, and H.C. Freake. 1987. Advances in our understanding of thyroid hormone action at the cellular level. Endocr. Rev. 8:288–308.
. h9 Z, q/ `1 g4 S0 u3 d
Oppenheimer, J.H., H.L. Schwartz, and K.A. Strait. 1994. Thyroid hormone action 1994: the plot thickens. Eur. J. Endocrinol. 130:15–24./ r8 X8 d8 U. E& |( |  w
8 R: a; ^6 M4 j; Q2 n8 B, }
Pietras, R.J., and C.M. Szego. 1975. Endometrial cell calcium and oestrogen action. Nature. 253:357–359.1 F# N. J9 X% B5 M; P
3 H- o. B3 u- _- N; v$ c  h
Rizzuto, R., P. Pinton, W. Carrington, F.S. Fay, K.E. Fogarty, L.M. Lifshitz, R.A. Tuft, and T. Pozzan. 1998. Close contacts with the endoplasmic reticulum as determinants of mitochondrial Ca2  responses. Science. 280:1763–1766.

Rizzuto, R., P. Pinton, M. Brini, A. Chiesa, L. Filippin, and T. Pozzan. 1999. Mitochondria as biosensors of calcium microdomains. Cell Calcium. 26:193–199.; Y5 G. _- M8 m6 s) D

Rojas, L.V., L. Bonilla, S. Baez, and J.A. Lasalde-Dominicci. 2003. Thyroid hormones regulate the frequency of miniature end-plate currents in pre- and prometamorphic stages of the tadpole tail. J. Neurosci. Res. 71:670–678.7 i. m3 A, Z: Q/ }: ^" G

Romani, A., C. Marfella, and M. Lakshmanan. 1996. Mobilization of Mg2  from rat heart and liver mitochondria following the interaction of thyroid hormone with the adenine nucleotide translocase. Thyroid. 6:513–519.8 f$ i$ p( t+ p" R* |

Scanlan, T.S., K.L. Suchland, M.E. Hart, G. Chiellini, Y. Huang, P.J. Kruzich, S. Frascarelli, D.A. Crossley, J.R. Bunzow, S. Ronca-Testoni, et al. 2004. 3-Iodothyronamine is an endogenous and rapid-acting derivative of thyroid hormone. Nat. Med. 10:638–642.& Z- N5 R; I. p9 J

Schonfeld, P., M.R. Wieckowski, and L. Wojtczak. 1997. Thyroid hormone-induced expression of the ADP/ATP carrier and its effect on fatty acid-induced uncoupling of oxidative phosphorylation. FEBS Lett. 416:19–22.
5 L  l0 j5 A: G1 ?+ q; y, R
Sergeev, I.N., and W.B. Rhoten. 1995. 1,25-Dihydroxyvitamin D3 evokes oscillations of intracellular calcium in a pancreatic beta-cell line. Endocrinology. 136:2852–2861.0 }& k* G. L$ {  w- q
/ x1 q; T" ~6 w5 z/ H
Simpson, P.B., and J.T. Russell. 1996. Mitochondrial support inositol 1,4,5-trisphosphate-mediated Ca2  waves in cultured oligodendrocytes. J. Biol. Chem. 271:33493–33501.

Soboll, S. 1993a. Long-term and short-term changes in mitochondrial parameters by thyroid hormones. Biochem. Soc. Trans. 21:799–803.. A0 Q. f4 D: ^6 s6 ]

Soboll, S. 1993b. Thyroid hormone action on mitochondrial energy transfer. Biochim. Biophys. Acta. 1144:1–16.

Sterling, K. 1980. Rapid effects of triiodothyronine on the mitochondrial pathway in rat liver in vivo. Science. 210:340–342.# `- }% n; G+ C
) f- v) K0 Y! K6 k0 w! z8 g
Sterling, K. 1986. Direct thyroid hormone activation of mitochondria: the role of adenine nucleotide translocase. Endocrinology. 119:292–295.: ^, i( T8 Z) c) _/ E; a

Sterling, K. 1991. Thyroid hormone action: identification of the mitochondrial thyroid hormone receptor as adenine nucleotide translocase. Thyroid. 1:167–171., V1 `" Q: e  j- W' u
( i2 h% }$ v, U7 ^9 b& u" u5 r
Sterling, K., and M.A. Brenner. 1995. Thyroid hormone action: effect of triiodothyronine on mitochondrial adenine nucleotide translocase in vivo and in vitro. Metabolism. 44:193–199.- b: i1 U' L+ z6 a5 a
8 k2 F5 |% Z1 J1 l* A
Sterling, K., M.A. Brenner, and T. Sakurada. 1980. Rapid effect of triiodothyronine on the mitochondrial pathway in rat liver in vivo. Science. 210:340–342.

Sterling, K., G.A. Campbell, G.S. Taliadouros, and E.A. Nunez. 1984. Mitochondrial binding of triiodothyronine (T3). Demonstration by electron-microscopic radioautography of dispersed liver cells. Cell Tissue Res. 236:321–325.
) ?1 k! C2 {$ w9 x" a+ P3 i6 Z7 U
Szalai, G., G. Csordas, B.M. Hantash, A.P. Thomas, and G. Hajnoczky. 2000. Calcium signal transmission between ryanodine receptors and mitochondria. J. Biol. Chem. 275:15305–15313.
% v$ i- P% i% J
Wasserman, W.J., L.H. Pinto, C.M. O'Connor, and L.D. Smith. 1980. Progesterone induces a rapid increase in  in of Xenopus laevis oocytes. Proc. Natl. Acad. Sci. USA. 77:1534–1536., ]3 M% r* |" V

Wong, J., and Y.B. Shi. 1995. Coordinated regulation of and transcriptional activation by Xenopus thyroid hormone and retinoid X receptors. J. Biol. Chem. 270:18479–18483.( J+ s1 ^4 Z3 m# n9 g; t

Wrutniak, C., I. Cassar-Malek, S. Marchal, A. Rascle, S. Heusser, J.M. Keller, J. Flechon, M. Dauca, J. Samarut, J. Ghysdael, et al. 1995. A 43-kDa protein related to c-Erb A alpha 1 is located in the mitochondrial matrix of rat liver. J. Biol. Chem. 270:16347–16354.8 x# K3 p2 m$ _+ E& C; q
; n0 H& l. U6 b- q, j' o- i
Wrutniak-Cabello, C., F. Casas, and G. Cabello. 2001. Thyroid hormone action in mitochondria. J. Mol. Endocrinol. 26:67–77.
* Q8 M  L, d: c& y6 m
Yaoita, Y., and D.D. Brown. 1990. A correlation of thyroid hormone receptor gene expression with amphibian metamorphosis. Genes Dev. 4:1917–1924.
1 l+ i1 g& D. M  V& ?
Ying, W.L., J. Emerson, M.J. Clarke, and D.R. Sanadi. 1991. Inhibition of mitochondrial calcium ion transport by an oxo-bridged dinuclear ruthenium ammine complex. Biochemistry. 30:4949–4952.7 T: D0 H! ~8 v* p; l- [. r; H. B

Zhou, Z.H., and J.K. Bubien. 2001. Nongenomic regulation of ENaC by aldosterone. Am. J. Physiol. Cell Physiol. 281:C1118–C1130.(Nuttawut Saelim1, Linu M. John2, Jun Wu1)