Recent advances in the molecular mechanism of mitochondrial calcium uptake [version 1; referees: 4 approved]

In the last few decades, a large body of experimental evidence has highlighted the complex role for mitochondria in eukaryotic cells: they are not only the site of aerobic metabolism (thus providing most of the ATP supply for endergonic processes) but also a crucial checkpoint of cell death processes (both necrosis and apoptosis) and autophagy. For this purpose, mitochondria must receive and decode the wide variety of physiological and pathological stimuli impacting on the cell. The “old” notion that mitochondria possess a sophisticated machinery for accumulating and releasing Ca2+, the most common and versatile second messenger of eukaryotic cells, is thus no surprise. What may be surprising is that the identification of the molecules involved in mitochondrial Ca2+ transport occurred only in the last decade for both the influx (the mitochondrial Ca2+ uniporter, MCU) and the efflux (the sodium calcium exchanger, NCX) pathways. In this review, we will focus on the description of the amazing molecular complexity of the MCU complex, highlighting the numerous functional implications of the tissue-specific expression of the variants of the channel pore components (MCU/MCUb) and of the associated proteins (MICU 1, 2, and 3, EMRE, and MCUR1)

The calcineurin-NFAT pathway controls activity-dependent circadian gene expression in slow skeletal muscle

OBJECTIVE: Physical activity and circadian rhythms are well-established determinants of human health and disease, but the relationship between muscle activity and the circadian regulation of muscle genes is a relatively new area of research. It is unknown whether muscle activity and muscle clock rhythms are coupled together, nor whether activity rhythms can drive circadian gene expression in skeletal muscle. METHODS: We compared the circadian transcriptomes of two mouse hindlimb muscles with vastly different circadian activity patterns, the continuously active slow soleus and the sporadically active fast tibialis anterior, in the presence or absence of a functional skeletal muscle clock (skeletal muscle-specific Bmal1 KO). In addition, we compared the effect of denervation on muscle circadian gene expression. RESULTS:We found that different skeletal muscles exhibit major differences in their circadian transcriptomes, yet core clock gene oscillations were essentially identical in fast and slow muscles. Furthermore, denervation caused relatively minor changes in circadian expression of most core clock genes, yet major differences in expression level, phase and amplitude of many muscle circadian genes. CONCLUSIONS: We report that activity controls the oscillation of around 15% of skeletal muscle circadian genes independently of the core muscle clock, and we have identified the Ca2+-dependent calcineurin-NFAT pathway as an important mediator of activity-dependent circadian gene expression, showing that circadian locomotor activity rhythms drive circadian rhythms of NFAT nuclear translocation and target gene expression

A protein kinase B-dependent and rapamycin-sensitive pathway controls skeletal muscle growth but not fiber type specification

Nerve activity controls fiber size and fiber type in skeletal muscle, but the underlying molecular mechanisms remain largely unknown. We have previously shown that Ras–mitogen-activated protein kinase and calcineurin control fiber type but not fiber size in regenerating rat skeletal muscle. Here we report that constitutively active protein kinase B (PKB), also known as Akt, increases fiber size and prevents denervation atrophy in regenerating and adult rat muscles but does not affect fiber type profile. The coexistence of hypertrophic muscle fibers overexpressing activated PKB with normal-size untransfected fibers within the same muscle points to a cell-autonomous control of muscle growth by PKB. The physiological role of this pathway is confirmed by the finding that PKB kinase activity and phosphorylation status are significantly increased in innervated compared with denervated regenerating muscles in parallel with muscle growth. Muscle fiber hypertrophy induced by activated PKB and by a Ras double mutant (RasV12C40) that activates selectively the phosphoinositide 3-kinase–PKB pathway is completely blocked by rapamycin, showing that the mammalian target of rapamycin kinase is the major downstream effector of this pathway in the control of muscle fiber size. On the other hand, nerve activity-dependent growth of regenerating muscle is only partially inhibited by dominant negative PKB and rapamycin, suggesting that other nerve-dependent signaling pathways are involved in muscle growth. The present results support the notion that fiber size and fiber type are regulated by nerve activity through different mechanisms

An adult tissue-specific stem cell in its niche: a gene profiling analysis of in vivo quiescent and activated muscle satellite cells.

International audienceThe satellite cell of skeletal muscle provides a paradigm for quiescent and activated tissue stem cell states. We have carried out transcriptome analyses on satellite cells purified by flow cytometry from Pax3(GFP/+) mice. We compared samples from adult skeletal muscles where satellite cells are mainly quiescent, with samples from growing muscles or regenerating (mdx) muscles, where they are activated. Analysis of regulation that is shared by both activated states avoids other effects due to immature or pathological conditions. This in vivo profile differs from that of previously analyzed satellite cells activated after cell culture. It reveals how the satellite cell protects itself from damage and maintains quiescence, while being primed for activation on receipt of the appropriate signal. This is illustrated by manipulation of the corepressor Dach1, and by the demonstration that quiescent satellite cells are better protected from oxidative stress than those from mdx or 1-week-old muscles. The quiescent versus in vivo activated comparison also gives new insights into how the satellite cell controls its niche on the muscle fiber through cell adhesion and matrix remodeling. The latter also potentiates growth factor activity through proteoglycan modification. Dismantling the extracellular matrix is important for satellite cell activation when the expression of proteinases is up-regulated, whereas transcripts for their inhibitors are high in quiescent cells. In keeping with this, we demonstrate that metalloproteinase function is required for efficient regeneration in vivo

Characterization of Pax3-expressing cells from adult blood vessels.

International audienceWe report expression of Pax3, an important regulator of skeletal muscle stem cell behaviour, in the brachial and femoral arteries of adult mice. In these contractile arteries of the limb, but not in the elastic arteries of the trunk, bands of GFP-positive cells were observed in Pax3(GFP/+) mice. Histological and biochemical examination of the vessels, together with clonal analysis after purification of Pax3-GFP-positive cells by flow cytometry, established their vascular smooth muscle identity. These blood-vessel-derived cells do not respond to inducers of other mesodermal cell types, such as bone, however, they can contribute to muscle fibre formation when co-cultured with skeletal muscle cells. This myogenic conversion depends on the expression of Pax3, but is rare and non-cell autonomous as it requires cell fusion. Myocardin, which promotes acquisition of a mature smooth muscle phenotype in these Pax3-GFP-positive cells, antagonises their potential for skeletal muscle differentiation. Genetic manipulation shows that myocardin is, however, positively regulated by Pax3, unlike genes for other myocardin-related factors, MRTFA, MRTFB or SRF. Expression of Pax3 overlaps with that reported for Msx2, which is required for smooth muscle differentiation of blood vessel-derived multipotent mesoangioblasts. These observations are discussed with respect to the origin and function of Pax3-expressing cells in blood vessels, and more general questions of cell fate determination and adult cell plasticity and reprogramming

Aldosterone Biosynthesis Is Potently Stimulated by Perfluoroalkyl Acids: A Link between Common Environmental Pollutants and Arterial Hypertension

The large environmental contamination of drinking water by perfluoroalkyl substances (PFAS) markedly increased the plasma levels of pentadecafluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) in a Northern Italy population with a high prevalence of arterial hypertension and cardiovascular disease. As the link between PFAS and arterial hypertension is unknown, we investigated if they enhance the biosynthesis of the well-known pressor hormone aldosterone. We found that PFAS increased aldosterone synthase (CYP11B2) gene expression by three-fold and doubled aldosterone secretion and cell and mitochondria reactive oxygen species (ROS) production over controls (p < 0.01 for all) in human adrenocortical carcinoma cells HAC15. They also enhanced the effects of Ang II on CYP11B2 mRNA and aldosterone secretion (p < 0.01 for all). Moreover, when added 1 h before, the ROS scavenger tempol abolished the effect of PFAS on CYP11B2 gene expression. These results indicate that at concentrations mimicking those found in human plasma of exposed individuals, PFAS are potent disruptors of human adrenocortical cell function, and might act as causative factors of human arterial hypertension via increased aldosterone production

Heterogeneity of the endoplasmic reticulum Ca2+ store determines colocalization with mitochondria

: Contact sites between the endoplasmic reticulum (ER) and mitochondria play a pivotal role in cell signaling, and the interaction between these organelles is dynamic and finely regulated. We have studied the role of ER Ca2+ concentration ([Ca2+]ER) in modulating this association in HeLa and HEK293 cells and human fibroblasts. According to Manders' coefficient, ER-mitochondria colocalization varied depending on the ER marker; it was the highest with ER-Tracker and the lowest with ER Ca2+ indicators (Mag-Fluo-4, erGAP3, and G-CEPIA1er) in both HeLa cells and human fibroblasts. Only GEM-CEPIA1er displayed a high colocalization with elongated mitochondria in HeLa cells, this ER Ca2+ indicator reveals low Ca2+ regions because this ion quenches its fluorescence. On the contrary, the typical rounded and fragmented mitochondria of HEK293 cells colocalized with Mag-Fluo-4 and, to a lesser extent, with GEM-CEPIA1er. The ablation of the three IP3R isoforms in HEK293 cells increased mitochondria-GEM-CEPIA1er colocalization. This pattern of colocalization was inversely correlated with the rate of ER Ca2+ leak evoked by thapsigargin (Tg). Moreover, Tg and Histamine in the absence of external Ca2+ increased mitochondria-ER colocalization. On the contrary, in the presence of external Ca2+, both Bafilomycin A1 and Tg reduced the mitochondria-ER interaction. Notably, knocking down MCU decreased mitochondria-ER colocalization. Overall, our data suggest that the [Ca2+] is not homogenous within the ER lumen and that mitochondria-ER interaction is modulated by the ER Ca2+ leak and the [Ca2+]i