Hypoxia reprograms calcium signaling and regulates myoglobin expression

Abstract

Myoglobin is an oxygen storage molecule that is selectively expressed in cardiac and slow-twitch skeletal muscles that have a high oxygen demand. Numerous studies have implicated hypoxia in the regulation of myoglobin expression as an adaptive response to hypoxic stress. However, the details of this relationship remain undefined. In the present study, adult mice exposed to 10% oxygen for periods up to 3 wk exhibited increased myoglobin expression only in the working heart, whereas myoglobin was either diminished or unchanged in skeletal muscle groups. In vitro and in vivo studies revealed that hypoxia in the presence or absence of exercise-induced stimuli reprograms calcium signaling and modulates myoglobin gene expression. Hypoxia alone significantly altered calcium influx in response to cell depolarization or depletion of endoplasmic reticulum calcium stores, which inhibited the expression of myoglobin. In contrast, our whole animal and transcriptional studies indicate that hypoxia in combination with exercise enhanced the release of calcium from the sarcoplasmic reticulum via the ryanodine receptors triggered by caffeine, which increased the translocation of nuclear factor of activated T-cells into the nucleus to transcriptionally activate myoglobin expression. The present study unveils a previously unrecognized mechanism where the hypoxia-mediated regulation of calcium transients from different intracellular pools modulates myoglobin gene expression. In addition, we observed that changes in myoglobin expression, in response to hypoxia, are not dependent on hypoxia-inducible factor-1 or changes in skeletal muscle fiber type. These studies enhance our understanding of hypoxia-mediated gene regulation and will have broad applications for the treatment of myopathic diseases. nuclear factor of activated T cells; calcineurin; skeletal muscle MYOGLOBIN IS A CYTOPLASMIC hemoprotein that is abundantly expressed in heart and oxidative skeletal myofibers. Elegant studies using physiological, biochemical, and spectroscopic analyses support an important role for myoglobin in facilitated oxygen transport, as a reservoir for oxygen and as a scavenger of reactive oxygen species in the mammalian heart and skeletal muscle (6,16,20,34,35,39). Detailed transcriptional analyses have been undertaken to define upstream activation motifs including a CCAC box, A/T element, nuclear factor of activated T cells (NFAT) response element, and E boxes that are necessary for muscle-specific transcription of the myoglobin gene (4,5,21,22). Following differentiation, myoglobin expression is coordinately regulated by neural and muscular activities that stimulate calcium signaling within the cell. Stimuli that enhance intracellular calcium levels increase activity and gene expression of calcineurin, a Ca 2ϩ /calmodulindependent serine phosphatase (13,47,48). Upon activation, calcineurin dephosphorylates the transcription factor NFAT, which translocates to the nucleus and combinatorially interacts with other transcription factors to regulate myo...

Neuroglobin, cytoglobin, and myoglobin contribute to hypoxia adaptation of the subterranean mole rat Spalax

“…cies is adapted to deliver an acute stress response, in agreement with a recent study on the molecular pathways of Mb hypoxia regulation in mouse (53).…”

Both hypoxia and work are required to enhance expression of myoglobin in skeletal muscle. Focus on “Hypoxia reprograms calcium signaling and regulates myoglobin expression”

“…For a long time, the import of Reynafarje's findings was obscured by apparently contradictory studies showing that low oxygen pressure alone is not a sufficient stimulus for myoglobin expression. Now, Kanatous et al (5), in the article under discussion, resolve this issue by showing that during hypoxia, myoglobin is elevated in the working heart, but not in the relatively idle skeletal musculature, of mice exposed to prolonged hypoxia. They conclude from their studies that hypoxia alone does not stimulate myoglobin expression, but that hypoxia in consort with repetitive contraction, exactly the conditions obtaining in the muscles of people residing at high altitude, does stimulate myoglobin genesis.…”

“…This sets the stage for their study of the regulation of myoglobin expression in muscle. They determine the effects of hypoxia on transcriptional regulation and expression of myoglobin both in the skeletal muscle of mice following a 3-wk acclimatization to hypoxia (10% oxygen) and in a cell culture system using myotubes, tubules of differentiated muscle cells (5).…”

“…Now Kanatous et al (5) show that, in skeletal muscle, myoglobin content is increased by the modulation of myoglobin gene expression regulated by changes in intracellular calcium pools induced by muscular contraction and concomitant hypoxia.…”

Rapid postnatal development of myoglobin from large liver iron stores in hooded seals

“…The seal fetus is repeatedly exposed to hypoxia in utero, when the mother is diving (Elsner et al, 1969;Liggins et al, 1980), and this most likely triggers the expression of HIF-1 and the development of blood O 2 stores before birth. Mb expression, however, has been shown to be regulated by the calcium-calcineurin-NFAT (nuclear factor of activated T-cells) pathway (Bassel-Duby et al, 1993; Kanatous et al, 2009), which in turn is also partly triggered by hypoxia, but apparently only in combination with muscular activity, at least in mice (Kanatous et al, 2009;Wittenberg, 2009). A recent study using cultured myocytes from Weddell seals (Leptonychotes weddellii) showed that control of Mb development in these animals differs somewhat from that outlined above, in that Mb levels were found to increase in response to hypoxia (and also after lipid supplementation), even in the absence of contraction (De Miranda et al, 2012).…”