Characterization of mitochondrial respiratory complexes involved in the regulation of myoblast differentiation.

During myoblast differentiation, mitochondria undergo numerous changes that are necessary for the progression of the myogenic program. Notably, we previously showed that alteration in mitochondrial activity was able to control the expression of keys regulator of cell cycle withdrawal and terminal differentiation. Here, we assessed whether inhibition of one of the respiratory complexes was a key factor in the regulation of myogenic differentiation in C2C12 cells, and was associated with alteration in reactive oxygen species (ROS) production. C2C12 cells were treated from proliferation to differentiation with specific inhibitors of mitochondrial complexes at a concentration that were inhibiting respiration but not altering cell morphology. Proliferation was significantly repressed with inhibition of complexes I, II, and III, or mitochondrial protein synthesis (using Chloramphenicol treatment), while complex IV inhibition did not alter myoblast proliferation compared to control cells. Moreover, inhibition of complexes I and II altered cell cycle regulators, with p21 protein expression upregulated since proliferation and p27 protein expression reduced at differentiation. Myotubes formation and myogenin expression were blunted with complexes I and II inhibitors while MyoD protein expression was maintained, suggesting an alteration in its transcriptional activity. Finally, a decrease in overall ROS production was observed with continuous inhibition of mitochondrial complexes I-IV. In summary, our data provide evidence that complexes I and II may be the primary regulators of C2C12 myogenic differentiation. This occurs through specific regulation of myogenic rather than cell cycle regulators expression and ROS production at mitochondrial rather than cell level.

Regulation of mitochondrial activity controls the duration of skeletal muscle regeneration in response to injury.

Thyroid hormone is a major regulator of skeletal muscle development and repair, and also a key regulator of mitochondrial activity. We have previously identified a 43 kDa truncated form of the nuclear T3 receptor TRα1 (p43) which stimulates mitochondrial activity and regulates skeletal muscle features. However, its role in skeletal muscle regeneration remains to be addressed. To this end, we performed acute muscle injury induced by cardiotoxin in mouse tibialis in two mouse models where p43 is overexpressed in or depleted from skeletal muscle. The measurement of muscle fiber size distribution at different time point (up to 70 days) upon injury lead us to unravel requirement of the p43 signaling pathway for satellite cells dependent muscle regeneration; strongly delayed in the absence of p43; whereas the overexpression of the receptor enhances of the regeneration process. In addition, we found that satellite cells derived from p43-Tg mice display higher proliferation rates when cultured in vitro when compared to control myoblasts, whereas p43-/- satellites shows reduced proliferation capacity. These finding strongly support that p43 plays an important role in vivo by controling the duration of skeletal muscle regeneration after acute injury, possibly through the regulation of mitochondrial activity and myoblasts proliferation.

Thyroid Hormone Action: The p43 Mitochondrial Pathway.

The possibility that several pathways are involved in the multiplicity of thyroid hormone physiological influences led to searches for the occurrence of T3 extra nuclear receptors. The existence of a direct T3 mitochondrial pathway is now well established. The demonstration that TRα1 mRNA encodes not only a nuclear thyroid hormone receptor but also two proteins imported into mitochondria with molecular masses of 43 and 28 kDa has provided new clues to understand the pleiotropic influence of iodinated hormones.The use of a T3 photo affinity label derivative (T3-PAL) allowed detecting two mitochondrial T3 binding proteins. In association with western blots using antibodies raised against the T3 nuclear receptor TRα1, mitochondrial T3 receptors were identified as truncated TRα1 forms. Import and in organello transcription experiments performed in isolated mitochondria led to the conclusion that p43 is a transcription factor of the mitochondrial genome, inducing changes in the mitochondrial/nuclear crosstalk. In vitro experiments indicated that this T3 mitochondrial pathway affects cell differentiation, apoptosis, and transformation. Generation of transgenic mice demonstrated the involvement of this mitochondrial pathway in the determination of muscle phenotype, glucose metabolism, and thermogenesis.

Skeletal muscle overexpression of short isoform Sirt3 altered mitochondrial cardiolipin content and fatty acid composition.

Cardiolipin (CL) is a phospholipid at the heart of mitochondrial metabolism, which plays a key role in mitochondrial function and bioenergetics. Among mitochondrial activity regulators, SIRT3 plays a crucial role in controlling the acetylation status of many enzymes participating in the energy metabolism in particular concerning lipid metabolism and fatty acid oxidation. Data suggest that possible connection may exist between SIRT3 and CL status that has not been evaluated in skeletal muscle. In the present study, we have characterized skeletal muscle lipids as well as mitochondrial lipids composition in mice overexpressing long (SIRT3-M1) and short (SIRT3-M3) isoforms of SIRT3. Particular attention has been paid for CL. We reported no alteration in muscle lipids content and fatty acids composition between the two mice SIRT3 strains and the control mice. However, mitochondrial CL content was significantly decreased in SIRT3-M3 mice and associated to an upregulation of tafazzin gene expression. In addition, mitochondrial phospholipids and fatty acids composition was altered with an increase in the PC/PE ratio and arachidonic acid content and a reduction in the MUFA/SFA ratio. These modifications in mitochondrial membrane composition are associated with a reduction in the enzymatic activities of mitochondrial respiratory chain complexes I and IV. In spite of these mitochondrial enzymatic alterations, skeletal muscle mitochondrial respiration remained similar in SIRT3-M3 and control mice. Surprisingly, none of those metabolic alterations were detected in mitochondria from SIRT3-M1 mice. In conclusion, our data indicate a specific action of the shorter SIRT3 isoform on lipid mitochondrial membrane biosynthesis and functioning.

Skeletal muscle expression of p43, a truncated thyroid hormone receptor α, affects lipid composition and metabolism.

Thyroid hormone is a major regulator of metabolism and mitochondrial function. Thyroid hormone also affects reactions in almost all pathways of lipids metabolism and as such is considered as the main hormonal regulator of lipid biogenesis. The aim of this study was to explore the possible involvement of p43, a 43 Kda truncated form of the nuclear thyroid hormone receptor TRα1 which stimulates mitochondrial activity. Therefore, using mouse models overexpressing p43 in skeletal muscle (p43-Tg) or lacking p43 (p43-/-), we have investigated the lipid composition in quadriceps muscle and in mitochondria. Here, we reported in the quadriceps muscle of p43-/- mice, a fall in triglycerides, an inhibition of monounsaturated fatty acids (MUFA) synthesis, an increase in elongase index and an decrease in desaturase index. However, in mitochondria from p43-/- mice, fatty acid profile was barely modified. In the quadriceps muscle of p43-Tg mice, MUFA content was decreased whereas the unsaturation index was increased. In addition, in quadriceps mitochondria of p43-Tg mice, we found an increase of linoleic acid level and unsaturation index. Last, we showed that cardiolipin content, a key phospholipid for mitochondrial function, remained unchanged both in quadriceps muscle and in its mitochondria whatever the mice genotype. In conclusion, this study shows that muscle lipid content and fatty acid profile are strongly affected in skeletal muscle by p43 levels. We also demonstrate that regulation of cardiolipin biosynthesis by the thyroid hormone does not imply p43.

Mitochondrial T3 receptor and targets.

The demonstration that TRα1 mRNA encodes a nuclear thyroid hormone receptor and two proteins imported into mitochondria with molecular masses of 43 and 28 kDa has brought new clues to better understand the pleiotropic influence of iodinated hormones. If p28 activity remains unknown, p43 binds to T3 responsive elements occurring in the organelle genome, and, in the T3 presence, stimulates mitochondrial transcription and the subsequent synthesis of mitochondrial encoded proteins. This influence increases mitochondrial activity and through changes in the mitochondrial/nuclear cross talk affects important nuclear target genes regulating cell proliferation and differentiation, oncogenesis, or apoptosis. In addition, this pathway influences muscle metabolic and contractile phenotype, as well as glycaemia regulation. Interestingly, according to the process considered, p43 exerts opposite or cooperative effects with the well-known T3 pathway, thus allowing a fine tuning of the physiological influence of this hormone.

Evaluation of the ROS Inhibiting Activity and Mitochondrial Targeting of Phenolic Compounds in Fibroblast Cells Model System and Enhancement of Efficiency by Natural Deep Eutectic Solvent (NADES) Formulation.

PURPOSE: Many phenolics have already been tested for their antioxidant activities using in vitro methods. However, such assays do not consider the complexity of real cellular systems, and most of the phenolics characterized with such assays shows disappointing results when evaluated in cells. Accordingly, there is a need to develop effective screening methods. METHODS: Antioxidants were first evaluated by CAT assay and then, evaluated for their ability (i) to reduce the level of ROS using fluorescent probe, (ii) to cross fibroblast cell membranes using confocal microscopy, and (iii) to target mitochondria. Antioxidants were also formulated in NADES. RESULTS: Correlation was obtained when comparing CAT results with short term inhibition (2 h) in the fibroblast cells. On the contrary, it was difficult to anticipate ROS inhibiting efficiency at long term (24 h) from both the CAT assay and the short term inhibition measurements. Indeed, some molecules displayed activity rapidly but lost it over time. In contrast, other molecules were better for long term. The comparable efficiency at long term of Bis-Ethylhexyl Hydroxydimethoxy Benzylmalonate (Bis-EHBm) and decyl rosmarinate, prompted us to further investigate the potential mitochondrial targeting of the former. Using mitochondrial probes, our results confirmed its mitochondrial location. Finally, the formulation of antioxidants in NADES could greatly improve their activity. CONCLUSIONS: Combinations of fast acting and slow acting molecules could be promising strategies to identify a performant antioxidant system. Bis-EHBm behaves as decyl rosmarinate with a confirmed mitochondrial location. Finally, the formulation of antioxidants in NADES could greatly improve their activity for ROS inhibition.

Protective Activity of Total Polyphenols from Genista quadriflora Munby and Teucrium polium geyrii Maire in Acetaminophen-Induced Hepatotoxicity in Rats.

Oxidative stress is a major cause of drug-induced hepatic diseases and several studies have demonstrated that diet supplementation with plants rich in antioxidant compounds provides a variety of health benefits in these circumstances. Genista quadriflora Munby (Gq) and Teucrium polium geyrii Maire (Tp) are known to possess antioxidant and numerous biological properties and these endemic plants are often used for dietary or medicinal applications. Herein, we evaluated the beneficial effect of rich-polyphenol fractions of Gq and Tp to prevent Acetaminophen-induced liver injury and investigated the mechanisms involved in this protective action. Rats were orally administered polyphenolic extracts from Gq or Tp (300 mg/kg) or N-acetylcysteine (NAC: 200 mg/kg) once daily for ten days prior to the single oral administration of Acetaminophen (APAP: 1 g/kg). The results show that preventive administration of polyphenolic extracts from Gq or Tp exerts a hepatoprotective influence during APAP treatment by improving transaminases leakage and liver histology and stimulating antioxidant defenses. Besides, suppression of liver CYP2E1, GSTpi and TNF-α mRNA levels, with enhancement of mitochondrial bioenergetics may contribute to the observed hepatoprotection induced by Gq and Tp extracts. The effect of Tp extract is significantly higher (1.5-2 fold) than that of Gq extract and NAC regarding the enhancement of mitochondrial functionality. Overall, this study brings the first evidence that pretreatment with these natural extracts display in vivo protective activity against APAP hepatotoxicity through improving mitochondrial bioenergetics, oxidant status, phase I and II enzymes expression and inflammatory processes probably by virtue of their high total polyphenols content.

Temperature homeostasis in mice lacking the p43 mitochondrial T3 receptor.

Thyroid hormones and Thra gene play a key role in energy expenditure regulation, temperature homeostasis, and mitochondrial function. To decipher the function of the mitochondrial TRα receptor in these phenomena, we used mice lacking specifically the p43 mitochondrial T3 receptor. We found that these animals were hypermetabolic, hyperphagic, and displayed a down setting of the core body temperature. However, p43-/- animals do not present cold intolerance or defect of facultative thermogenesis. In addition, the mitochondrial function of BAT is slightly affected in the absence of p43. Our study, therefore, suggests a complementarity of action between the mitochondrial receptor and other proteins encoded by the Thra gene in the control of basal metabolism, facultative thermogenesis, and determination of the set point of temperature regulation.

SIRT3, a mitochondrial NAD⁺-dependent deacetylase, is involved in the regulation of myoblast differentiation.

Sirtuin 3 (SIRT3), one of the seven mammalian sirtuins, is a mitochondrial NAD+-dependent deacetylase known to control key metabolic pathways. SIRT3 deacetylases and activates a large number of mitochondrial enzymes involved in the respiratory chain, in ATP production, and in both the citric acid and urea cycles. We have previously shown that the regulation of myoblast differentiation is tightly linked to mitochondrial activity. Since SIRT3 modulates mitochondrial activity, we decide to address its role during myoblast differentiation. For this purpose, we first investigated the expression of endogenous SIRT3 during C2C12 myoblast differentiation. We further studied the impact of SIRT3 silencing on both the myogenic potential and the mitochondrial activity of C2C12 cells. We showed that SIRT3 protein expression peaked at the onset of myoblast differentiation. The inhibition of SIRT3 expression mediated by the stable integration of SIRT3 short inhibitory RNA (SIRT3shRNA) in C2C12 myoblasts, resulted in: 1) abrogation of terminal differentiation - as evidenced by a marked decrease in the myoblast fusion index and a significant reduction of Myogenin, MyoD, Sirtuin 1 and Troponin T protein expression - restored upon MyoD overexpression; 2) a decrease in peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and citrate synthase protein expression reflecting an alteration of mitochondrial density; and 3) an increased production of reactive oxygen species (ROS) mirrored by the decreased activity of manganese superoxide dismutase (MnSOD). Altogether our data demonstrate that SIRT3 mainly regulates myoblast differentiation via its influence on mitochondrial activity.

p28, a truncated form of TRα1 regulates mitochondrial physiology.

We have previously identified in mitochondria two truncated forms of the T3 nuclear receptor TRα1, with molecular weights of 43kDa (p43) and 28kDa (p28) respectively located in the matrix and in the inner membrane. Previously, we have demonstrated that p43 stimulates mitochondrial transcription and protein synthesis in the presence of T3. Here we report that p28 is targeted into the organelle in a T3-dependent manner and displays an affinity for T3 higher than the nuclear receptor. We tried to generate mice overexpressing p28 using the human α-skeletal actin promoter, however we found an early embryonic lethality that was probably linked to a transient expression of p28 in trophoblast giant cells. This could be partly explained by the observation that overexpression of p28 in human fibroblasts induced alterations of mitochondrial physiology.

Regulation of skeletal muscle oxidative capacity and muscle mass by SIRT3.

We have previously reported that the expression of mitochondrial deacetylase SIRT3 is high in the slow oxidative muscle and that the expression of muscle SIRT3 level is increased by dietary restriction or exercise training. To explore the function of SIRT3 in skeletal muscle, we report here the establishment of a transgenic mouse model with muscle-specific expression of the murine SIRT3 short isoform (SIRT3M3). Calorimetry study revealed that the transgenic mice had increased energy expenditure and lower respiratory exchange rate (RER), indicating a shift towards lipid oxidation for fuel usage, compared to control mice. The transgenic mice exhibited better exercise performance on treadmills, running 45% further than control animals. Moreover, the transgenic mice displayed higher proportion of slow oxidative muscle fibers, with increased muscle AMPK activation and PPARδ expression, both of which are known regulators promoting type I muscle fiber specification. Surprisingly, transgenic expression of SIRT3M3 reduced muscle mass up to 30%, likely through an up-regulation of FOXO1 transcription factor and its downstream atrophy gene MuRF-1. In summary, these results suggest that SIRT3 regulates the formation of oxidative muscle fiber, improves muscle metabolic function, and reduces muscle mass, changes that mimic the effects of caloric restriction.

Chicoric acid is an antioxidant molecule that stimulates AMP kinase pathway in L6 myotubes and extends lifespan in Caenorhabditis elegans.

Chicoric acid (CA) is a caffeoyl derivative previously described as having potential anti-diabetic properties. As similarities in cellular mechanism similarities between diabetes and aging have been shown, we explored on L6 myotubes the effect of CA on the modulation of intracellular pathways involved in diabetes and aging. We also determined its influence on lifespan of Caenorhabditis elegans worm (C. elegans). In L6 myotubes, CA was a potent reactive oxygen species (ROS) scavenger, reducing ROS accumulation under basal as well as oxidative stress conditions. CA also stimulated the AMP-activated kinase (AMPK) pathway and displayed various features associated with AMPK activation: CA (a) enhanced oxidative enzymatic defences through increase in glutathion peroxidase (GPx) and superoxide dismutase (SOD) activities, (b) favoured mitochondria protection against oxidative damage through up-regulation of MnSOD protein expression, (c) increased mitochondrial biogenesis as suggested by increases in complex II and citrate synthase activities, along with up-regulation of PGC-1α mRNA expression and (d) inhibited the insulin/Akt/mTOR pathway. As AMPK stimulators (e.g. the anti-diabetic agent meformin or polyphenols such as epigallocatechingallate or quercetin) were shown to extend lifespan in C. elegans, we also determined the effect of CA on the same model. A concentration-dependant lifespan extension was observed with CA (5-100 µM). These data indicate that CA is a potent antioxidant compound activating the AMPK pathway in L6 myotubes. Similarly to other AMPK stimulators, CA is able to extend C. elegans lifespan, an effect measurable even at the micromolar range. Future studies will explore CA molecular targets and give new insights about its possible effects on metabolic and aging-related diseases.

Mice lacking the p43 mitochondrial T3 receptor become glucose intolerant and insulin resistant during aging.

Thyroid hormones (TH) play an important regulatory role in energy expenditure regulation and are key regulators of mitochondrial activity. We have previously identified a mitochondrial triiodothyronine (T3) receptor (p43) which acts as a mitochondrial transcription factor of the organelle genome, which leads in vitro and in vivo, to a stimulation of mitochondrial biogenesis. Recently, we generated mice carrying a specific p43 invalidation. At 2 months of age, we reported that p43 depletion in mice induced a major defect in insulin secretion both in vivo and in isolated pancreatic islets, and a loss of glucose-stimulated insulin secretion. The present study was designed to determine whether p43 invalidation influences life expectancy and modulates blood glucose and insulin levels as well as glucose tolerance or insulin sensitivity during aging. We report that from 4 months old onwards, mice lacking p43 are leaner than wild-type mice. p43-/- mice also have a moderate reduction of life expectancy compared to wild type. We found no difference in blood glucose levels, excepted at 24 months old where p43-/- mice showed a strong hyperglycemia in fasting conditions compared to controls animals. However, the loss of glucose-stimulated insulin secretion was maintained whatever the age of mice lacking p43. If up to 12 months old, glucose tolerance remained unchanged, beyond this age p43-/- mice became increasingly glucose intolerant. In addition, if up to 12 months old p43 deficient animals were more sensitive to insulin, after this age we observed a loss of this capacity, culminating in 24 months old mice with a decreased sensitivity to the hormone. In conclusion, we demonstrated that during aging the depletion of the mitochondrial T3 receptor p43 in mice progressively induced an increased glycemia in the fasted state, glucose intolerance and an insulin-resistance several features of type-2 diabetes.

Boosting antioxidants by lipophilization: a strategy to increase cell uptake and target mitochondria.

PURPOSE: To explore the possibility to boost phenolic antioxidants through their structural modification by lipophilization and check the influence of such covalent modification on cellular uptake and mitochondria targeting. METHODS: Rosmarinic acid was lipophilized by various aliphatic chain lengths (butyl, octyl, decyl, dodecyl, hexadecyl, and octadecyl) to give rosmarinate alkyl esters which were then evaluated for their ability (i) to reduce the level of reactive oxygen species (ROS) using 2',7'-dichlorodihydrofluorescein diacetate probe, (ii) to cross fibroblast cell membranes using confocal microscopy, and (iii) to target mitochondria using MitoTracker® Red CMXRos. RESULTS: Increasing the chain length led to an improvement of the antioxidant activity until a threshold is reached for medium chain (10 carbon atoms) and beyond which lengthening resulted in a decrease of activity. This nonlinear phenomenon-also known as the cut-off effect-is discussed here in connection to the previously similar results observed in emulsified, liposomal, and cellular systems. Moreover, butyl, octyl, and decyl rosmarinates passed through the membranes in less than 15 min, whereas longer esters did not cross membranes and formed extracellular aggregates. Besides cell uptake, alkyl chain length also determined the subcellular localization of esters: mitochondria for medium chains esters, cytosol for short chains and extracellular media for longer chains. CONCLUSION: The localization of antioxidants within mitochondria, the major site and target of ROS, conferred an advantage to medium chain rosmarinates compared to both short and long chains. In conjunction with changes in cellular uptake, this result may explain the observed decrease of antioxidant activity when lengthening the lipid chain of esters. This brings a proof-of-concept that grafting medium chain allows the design of mitochondriotropic antioxidants.

How to boost antioxidants by lipophilization?

Covalent modification of antioxidants through lipophilization is an important field of research aiming at developing antioxidants with improved efficacy. However, due to insufficient knowledge on how hydrophobicity affects antioxidant activity, lipophilization strategies have been largely based on empirism. Often, the resulting lipophilized antioxidants were not optimal. Here we described how the body of knowledge regarding hydrophobicity has been dramatically redefined as unexpected results were recently published. Using a broad range of lipophilized antioxidants assessed in dispersed lipids models and cultured cells, it has been demonstrated that the antioxidant activity increases progressively with increasing chain length up to a critical point, beyond which the activity of the compounds dramatically decreases. Taking into account this nonlinear phenomenon, also known as cut-off effect, antioxidant drug designers now have to seek the critical chain length to synthesize the optimal drug in a rational manner. Here, we briefly presented three putative mechanisms of action to try to account for the cut-off effect.

Protein sequences involved in the mitochondrial import of the 3,5,3'-L-triiodothyronine receptor p43.

The major effect of T3 on mitochondrial activity has been partly explained by the discovery of p43, a T3-dependent transcription factor of the mitochondrial genome. P43 is imported into mitochondria in an atypical manner which is not yet fully understood. Our aim was to characterize the p43 sequences inducing its mitochondrial import, using in organello import experiments with wild-type or mutated proteins and validation in CV1 cells. We find that several sequences define the mitochondrial addressing. Two alpha helices in the C-terminal part of p43 are actual mitochondrial import sequences as fusion to a cytosolic protein induces its mitochondrial translocation. Helix 5 drives the atypical mitochondrial import process, whereas helices 10/11 induce a classical import process. However, despite its inability to drive a mitochondrial import, the N-terminal region of p43 also plays a permissive role as in the presence of the C-terminal import sequences different N-terminal regions determine whether the protein is imported or not. These results can be extrapolated to other mitochondrial proteins related to the nuclear receptor superfamily, devoid of classical mitochondrial import sequences.

M19 modulates skeletal muscle differentiation and insulin secretion in pancreatic ß-cells through modulation of respiratory chain activity.

Mitochondrial dysfunction due to nuclear or mitochondrial DNA alterations contributes to multiple diseases such as metabolic myopathies, neurodegenerative disorders, diabetes and cancer. Nevertheless, to date, only half of the estimated 1,500 mitochondrial proteins has been identified, and the function of most of these proteins remains to be determined. Here, we characterize the function of M19, a novel mitochondrial nucleoid protein, in muscle and pancreatic ß-cells. We have identified a 13-long amino acid sequence located at the N-terminus of M19 that targets the protein to mitochondria. Furthermore, using RNA interference and over-expression strategies, we demonstrate that M19 modulates mitochondrial oxygen consumption and ATP production, and could therefore regulate the respiratory chain activity. In an effort to determine whether M19 could play a role in the regulation of various cell activities, we show that this nucleoid protein, probably through its modulation of mitochondrial ATP production, acts on late muscle differentiation in myogenic C2C12 cells, and plays a permissive role on insulin secretion under basal glucose conditions in INS-1 pancreatic ß-cells. Our results are therefore establishing a functional link between a mitochondrial nucleoid protein and the modulation of respiratory chain activities leading to the regulation of major cellular processes such as myogenesis and insulin secretion.

Mitochondrial T3 receptor p43 regulates insulin secretion and glucose homeostasis.

Thyroid hormone is a major determinant of energy expenditure and a key regulator of mitochondrial activity. We have previously identified a mitochondrial triiodothyronine receptor (p43) that acts as a mitochondrial transcription factor of the organelle genome, which leads, in vitro and in vivo, to a stimulation of mitochondrial biogenesis. Here we generated mice specifically lacking p43 to address its physiological influence. We found that p43 is required for normal glucose homeostasis. The p43(-/-) mice had a major defect in insulin secretion both in vivo and in isolated pancreatic islets and a loss of glucose-stimulated insulin secretion. Moreover, a high-fat/high-sucrose diet elicited more severe glucose intolerance than that recorded in normal animals. In addition, we observed in p43(-/-) mice both a decrease in pancreatic islet density and in the activity of complexes of the respiratory chain in isolated pancreatic islets. These dysfunctions were associated with a down-regulation of the expression of the glucose transporter Glut2 and of Kir6.2, a key component of the K(ATP) channel. Our findings establish that p43 is an important regulator of glucose homeostasis and pancreatic ß-cell function and provide evidence for the first time of a physiological role for a mitochondrial endocrine receptor.

Depletion of the p43 mitochondrial T3 receptor in mice affects skeletal muscle development and activity.

In vertebrates, skeletal muscle myofibers display different contractile and metabolic properties associated with different mitochondrial content and activity. We have previously identified a mitochondrial triiodothyronine receptor (p43) regulating mitochondrial transcription and mitochondrial biogenesis. When overexpressed in skeletal muscle, it increases mitochondrial DNA content, stimulates mitochondrial respiration, and induces a shift in the metabolic and contractile features of muscle fibers toward a slower and more oxidative phenotype. Here we show that a p43 depletion in mice decreases mitochondrial DNA replication and respiratory chain activity in skeletal muscle in association with the induction of a more glycolytic muscle phenotype and a decrease of capillary density. In addition, p43(-/-) mice displayed a significant increase in muscle mass relative to control animals and had an improved ability to use lipids. Our findings establish that the p43 mitochondrial receptor strongly affects muscle mass and the metabolic and contractile features of myofibers and provides evidence that this receptor mediates, in part, the influence of thyroid hormone in skeletal muscle.