Anne Picard

Foxo Transcription Factors Induce the Atrophy-Related Ubiquitin Ligase Atrogin-1 and Cause Skeletal Muscle Atrophy

Skeletal muscle atrophy is a debilitating response to fasting, disuse, cancer, and other systemic diseases. In atrophying muscles, the ubiquitin ligase, atrogin-1 (MAFbx), is dramatically induced, and this response is necessary for rapid atrophy. Here, we show that in cultured myotubes undergoing atrophy, the activity of the PI3K/AKT pathway decreases, leading to activation of Foxo transcription factors and atrogin-1 induction. IGF-1 treatment or AKT overexpression inhibits Foxo and atrogin-1 expression. Moreover, constitutively active Foxo3 acts on the atrogin-1 promoter to cause atrogin-1 transcription and dramatic atrophy of myotubes and muscle fibers. When Foxo activation is blocked by a dominant-negative construct in myotubes or by RNAi in mouse muscles in vivo, atrogin-1 induction during starvation and atrophy of myotubes induced by glucocorticoids are prevented. Thus, forkhead factor(s) play a critical role in the development of muscle atrophy, and inhibition of Foxo factors is an attractive approach to combat muscle wasting.

The mitochondrial calcium uniporter is a multimer that can include a dominant‐negative pore‐forming subunit

Mitochondrial calcium uniporter (MCU) channel is responsible for Ruthenium Red‐sensitive mitochondrial calcium uptake. Here, we demonstrate MCU oligomerization by immunoprecipitation and Förster resonance energy transfer (FRET) and characterize a novel protein (MCUb) with two predicted transmembrane domains, 50% sequence similarity and a different expression profile from MCU. Based on computational modelling, MCUb includes critical amino‐acid substitutions in the pore region and indeed MCUb does not form a calcium‐permeable channel in planar lipid bilayers. In HeLa cells, MCUb is inserted into the oligomer and exerts a dominant‐negative effect, reducing the [Ca2+]mt increases evoked by agonist stimulation. Accordingly, in vitro co‐expression of MCUb with MCU drastically reduces the probability of observing channel activity in planar lipid bilayer experiments. These data unveil the structural complexity of MCU and demonstrate a novel regulatory mechanism, based on the inclusion of dominant‐negative subunits in a multimeric channel, that underlies the fine control of the physiologically and pathologically relevant process of mitochondrial calcium homeostasis.

NFAT isoforms control activity-dependent muscle fiber type specification

The intracellular signals that convert fast and slow motor neuron activity into muscle fiber type specific transcriptional programs have only been partially defined. The calcium/calmodulin-dependent phosphatase calcineurin (Cn) has been shown to mediate the transcriptional effects of motor neuron activity, but precisely how 4 distinct muscle fiber types are composed and maintained in response to activity is largely unknown. Here, we show that 4 nuclear factor of activated T cell (NFAT) family members act coordinately downstream of Cn in the specification of muscle fiber types. We analyzed the role of NFAT family members in vivo by transient transfection in skeletal muscle using a loss-of-function approach by RNAi. Our results show that, depending on the applied activity pattern, different combinations of NFAT family members translocate to the nucleus contributing to the transcription of fiber type specific genes. We provide evidence that the transcription of slow and fast myosin heavy chain (MyHC) genes uses different combinations of NFAT family members, ranging from MyHC-slow, which uses all 4 NFAT isoforms, to MyHC-2B, which only uses NFATc4. Our data contribute to the elucidation of the mechanisms whereby activity can modulate the phenotype and performance of skeletal muscle.

Adaptation of Mouse Skeletal Muscle to Long-Term Microgravity in the MDS Mission

The effect of microgravity on skeletal muscles has so far been examined in rat and mice only after short-term (5–20 day) spaceflights. The mice drawer system (MDS) program, sponsored by Italian Space Agency, for the first time aimed to investigate the consequences of long-term (91 days) exposure to microgravity in mice within the International Space Station. Muscle atrophy was present indistinctly in all fiber types of the slow-twitch soleus muscle, but was only slightly greater than that observed after 20 days of spaceflight. Myosin heavy chain analysis indicated a concomitant slow-to-fast transition of soleus. In addition, spaceflight induced translocation of sarcolemmal nitric oxide synthase-1 (NOS1) into the cytosol in soleus but not in the fast-twitch extensor digitorum longus (EDL) muscle. Most of the sarcolemmal ion channel subunits were up-regulated, more in soleus than EDL, whereas Ca2+-activated K+ channels were down-regulated, consistent with the phenotype transition. Gene expression of the atrophy-related ubiquitin-ligases was up-regulated in both spaceflown soleus and EDL muscles, whereas autophagy genes were in the control range. Muscle-specific IGF-1 and interleukin-6 were down-regulated in soleus but up-regulated in EDL. Also, various stress-related genes were up-regulated in spaceflown EDL, not in soleus. Altogether, these results suggest that EDL muscle may resist to microgravity-induced atrophy by activating compensatory and protective pathways. Our study shows the extended sensitivity of antigravity soleus muscle after prolonged exposition to microgravity, suggests possible mechanisms accounting for the resistance of EDL, and individuates some molecular targets for the development of countermeasures.

Comparative transcriptional and biochemical studies in muscle of myotonic dystrophies (DM1 and DM2)

Myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (proximal muscular myopaty/DM2) are caused by similar dynamic mutations at two distinct genetic loci. The two diseases also lead to similar phenotypes but different clinical severity. Dysregulation of alternative splicing has been suggested as the common pathogenic mechanism. Here, we investigate the molecular differences between DM1 and DM2 using reverse transcriptase-polymerase chain reaction of troponin T (TnT) and the insulin receptor (IR), as well as immunoblotting of TnT in muscle biopsies from DM1 and DM2 patients. We found that: (a) slow TnT was encoded by two different transcripts in significantly different ratios in DM1 and DM2 muscles; (b) DM2 muscles exhibited a higher degree of alternative splicing dysregulation for fast TnT transcripts when compared to DM1 muscles; (c) the distribution of TnT proteins was significantly skewed towards higher molecular weight species in both diseases; (d) the RNA for the insulin-independent IR-A isoform was significantly increased and appeared related to the fibre-type composition in the majority of the cases examined. On the whole, these data should give a better insight on pathogenesis of DM1 and DM2.

GATA elements control repression of cardiac troponin I promoter activity in skeletal muscle cells

BackgroundWe reported previously that the cardiac troponin I (cTnI) promoter drives cardiac-specific expression of reporter genes in cardiac muscle cells and in transgenic mice, and that disruption of GATA elements inactivates the cTnI promoter in cultured cardiomyocytes. We have now examined the role of cTnI promoter GATA elements in skeletal muscle cells.ResultsMutation or deletion of GATA elements induces a strong transcriptional activation of the cTnI promoter in regenerating skeletal muscle and in cultured skeletal muscle cells. Electrophoretic mobility shift assays show that proteins present in nuclear extracts of C2C12 muscle cells bind the GATA motifs present in the cTnI promoter. However, GATA protein complex formation is neither reduced nor supershifted by antibodies specific for GATA-2, -3 and -4, the only GATA transcripts present in muscle cells.ConclusionThese findings indicate that the cTnI gene promoter is repressed in skeletal muscle cells by GATA-like factors and open the way to further studies aimed at identifying these factors.

Reg3G gene expression in regenerating skeletal muscle and corresponding nerve

Introduction: The Reg genes play a major role in the regeneration of various tissues; however, no reports have been published regarding expression of the Reg3G gene in skeletal muscle. In this study we investigated the expression of the Reg3G gene in regeneration of rat skeletal muscle and injured nerves. Methods: We used 3 experimental models of muscle and nerve injury. RT‐PCR and Western blot analysis were performed for detection of Reg3G in regenerating muscle and nerve. Results: We found transcriptional activation of the Reg3G gene in the soleus and extensor digitorum longus muscles and in their corresponding nerves after both muscle and nerve injury in different time periods, respectively. Conclusions: The results suggest that the Reg3G gene plays a major role in communication between injured axons and muscle and may play a significant role in skeletal muscle and peripheral nerve regeneration. Muscle Nerve 49: 61–68, 2014

Reg IV protein and mRNA expression in different rat organs.

The Reg IV gene has been documented in the human colon, small intestine, stomach and pancreas. Expression of the Reg IV in different cell types has been associated with regeneration, cell growth and cell survival, cell adhesion and resistance to apoptosis. Since the distribution of the Reg IV protein in normal rat tissues is unknown, the aim of this study was to reveal the expression of the Reg IV protein in structurally and functionally different rat organs. The expression of Reg IV gene was analyzed by Western blot and reverse transcription-polymerase chain reaction. Immunohistochemistry was used to localize Reg IV protein. Reg IV protein was expressed in pancreas, stomach, small intestine, colon, brain, spleen, kidney and urinary bladder in two-month-old male Wistar rats. In addition, the expression of Reg IV mRNA by reverse transcription-polymerase chain reaction was confirmed. Our study provides detailed information about the expression and localization of Reg IV protein in different rat organs. These findings provide an evidence of Reg IV expression in different rat organs, which may help elucidate a potential role in growth and proliferation of different cells like other members of the Reg family genes which act as growth factors in the different organs.

Heart morphogenesis is not affected by overexpression of the Sh3bgr gene mapping to the Down syndrome heart critical region

Congenital heart disease (CHD) is the most common birth defect in humans and is present in 40% of newborns affected by Down syndrome (DS). The SH3BGR gene maps to the DS-CHD region and is a potential candidate for the pathogenesis of CHD, since it is selectively expressed in cardiac and skeletal muscle. To determine whether overexpression of Sh3bgr in the murine heart may cause abnormal cardiac development, we have generated transgenic mice using a cardiac- and skeletal-muscle-specific promoter to drive the expression of a Sh3bgr transgene. We report here that heart morphogenesis is not affected by overexpression of Sh3bgr.