Membrane depolarization induces calcium-dependent upregulation of Hsp70 and Hmox-1 in skeletal muscle cells

Network model of skeletal muscle cell signalling predicts differential responses to endurance and resistance exercise training

Exercise-induced muscle adaptations vary based on exercise modality and intensity. We constructed a signalling network model from 87 published studies of human or rodent skeletal muscle cell responses to endurance or resistance exercise in vivo or simulated exercise in vitro. The network comprises 259 signalling interactions between 120 nodes, representing eight membrane receptors and eight canonical signalling pathways regulating 14 transcriptional regulators, 28 target genes and 12 exercise-induced phenotypes. Using this network, we formulated a logic-based ordinary differential equation model predicting time-dependent molecular and phenotypic alterations following acute endurance and resistance exercises. Compared with nine independent studies, the model accurately predicted 18/21 (85%) acute responses to resistance exercise and 12/16 (75%) acute responses to endurance exercise. Detailed sensitivity analysis of differential phenotypic responses to resistance and endurance training showed that, in the model, exercise regulates cell growth and protein synthesis primarily by signalling via mechanistic target of rapamycin, which is activated by Akt and inhibited in endurance exercise by AMP-activated protein kinase. Endurance exercise preferentially activates inflammation via reactive oxygen species and nuclear factor κB signalling. Furthermore, the expected preferential activation of mitochondrial biogenesis by endurance exercise was counterbalanced in the model by protein kinase C in response to resistance training. This model provides a new tool for investigating cross-talk between skeletal muscle signalling pathways activated by endurance and resistance exercise, and the mechanisms of interactions such as the interference effects of endurance training on resistance exercise outcomes.

Dissecting the interaction between HSP70 and vascular contraction: role of [Formula: see text] handling mechanisms

Heat-shock protein 70 (HSP70) is a ubiquitously expressed molecular chaperone with various biological functions. Recently, we demonstrated that HSP70 is key for adequate vascular reactivity. However, the specific mechanisms targeted by HSP70 to assist in this process remain elusive. Since there is a wealth of evidence connecting HSP70 to calcium ([Formula: see text]), a master regulator of contraction, we designed this study to investigate whether blockade of HSP70 disrupts vascular contraction via impairment of [Formula: see text] handling mechanisms. We performed functional studies in aortas isolated from male Sprague Dawley rats in the presence or absence of exogenous [Formula: see text], and we determined the effects of VER155008, an inhibitor of HSP70, on [Formula: see text] handling as well as key mechanisms that regulate vascular contraction. Changes in the intracellular concentration of [Formula: see text] were measured with a biochemical assay kit. We report that blockade of HSP70 leads to [Formula: see text] mishandling in aorta stimulated with phenylephrine, decreasing both phasic and tonic contractions. Importantly, in [Formula: see text] free Krebs' solution, inhibition of HSP70 only reduced the [Formula: see text] of the phasic contraction if the protein was blocked before IP3r-mediated [Formula: see text] release, suggesting that HSP70 has a positive effect towards this receptor. Corroborating this statement, VER155008 did not potentiate an IP3r inhibitor's outcomes, even with partial blockade. In another set of experiments, the inhibition of HSP70 attenuated the amplitude of the tonic contraction independently of the moment VER155008 was added to the chamber (i.e., whether it was before or after IP3r-mediated phasic contraction). More compelling, following re-addition of [Formula: see text], VER155008 amplified the inhibitory effects of a voltage-dependent [Formula: see text] channel blocker, but not of a voltage-independent [Formula: see text] channel inhibitor, indicating that HSP70 has a positive impact on the latter. Lastly, the mechanism by which HSP70 modulates vascular contraction does not involve the [Formula: see text] sensitizer protein, Rho-kinase, nor the SERCA pump, as blockade of these proteins in the presence of VER155008 almost abolished contraction. In summary, our findings shed light on the processes targeted by HSP70 during vascular contraction and open research avenues for potential new mechanisms in vascular diseases.

An additional physiological role for HSP70: Assistance of vascular reactivity

AIMS

HSP70, a molecular chaperone, helps to maintain proteostasis. In muscle biology, however, evidence suggests HSP70 to have a more versatile range of functions, as genetic deletion of its inducible genes impairs Ca²⁺ handling, and consequently, cardiac and skeletal muscle contractility. Still, it is unknown whether HSP70 is involved in vascular reactivity, an intrinsic physiological mechanism of blood vessels. Therefore, we designed this study to test the hypothesis that proper vascular reactivity requires the assistance of HSP70.

MAIN METHODS

We performed functional studies in a wire-myograph using thoracic aorta isolated from male Sprague Dawley rats. Experiments were conducted with and without an HSP70 inhibitor as well as in heat-stressed vessels. The expression levels of HSP70 were evaluated with Western blotting. NO and ROS levels were assessed with fluorescence microscopy.

KEY FINDINGS

We report that blockade of HSP70 weakens contraction in response to phenylephrine (dose-response) in the aorta. Additionally, we demonstrated that inhibition of HSP70 affects the amplitude of the fast and of the slow components of the time-force curve. Corroborating these findings, we found that inhibition of HSP70, in vessels over-expressing this protein, partly rescues the contractile phenotype of aortic rings. Furthermore, we show that blockade of HSP70 facilitates relaxation in response to acetylcholine and clonidine without affecting the basal levels of NO and ROS.

SIGNIFICANCE

Our work introduces an additional physiological role for HSP70, the assistance of vascular reactivity, which highlights this protein as a new player in vascular physiology, and therefore, uncovers a promising research avenue for vascular diseases.

Sarcoplasmic reticulum and calcium signaling in muscle cells: Homeostasis and disease

The sarco/endoplasmic reticulum is an extensive, dynamic and heterogeneous membranous network that fulfills multiple homeostatic functions. Among them, it compartmentalizes, stores and releases calcium within the intracellular space. In the case of muscle cells, calcium released from the sarco/endoplasmic reticulum in the vicinity of the contractile machinery induces cell contraction. Furthermore, sarco/endoplasmic reticulum-derived calcium also regulates gene transcription in the nucleus, energy metabolism in mitochondria and cytosolic signaling pathways. These diverse and overlapping processes require a highly complex fine-tuning that the sarco/endoplasmic reticulum provides by means of its numerous tubules and cisternae, specialized domains and contacts with other organelles. The sarco/endoplasmic reticulum also possesses a rich calcium-handling machinery, functionally coupled to both contraction-inducing stimuli and the contractile apparatus. Such is the importance of the sarco/endoplasmic reticulum for muscle cell physiology, that alterations in its structure, function or its calcium-handling machinery are intimately associated with the development of cardiometabolic diseases. Cardiac hypertrophy, insulin resistance and arterial hypertension are age-related pathologies with a common mechanism at the muscle cell level: the accumulation of damaged proteins at the sarco/endoplasmic reticulum induces a stress response condition termed endoplasmic reticulum stress, which impairs proper organelle function, ultimately leading to pathogenesis.

Role of heat shock proteins 70/90 in exercise physiology and exercise immunology and their diagnostic potential in sports

Heat shock proteins (HSPs) are molecular chaperones facilitating the unfolding or folding of secondary structures of proteins, their client proteins, in cellular stress situations. Various internal and external physiological and mechanical stress factors induce a homeostatic imbalance, followed by an increased expression of HSP70 and HSP90. Exercise is a stress factor, too, and its cumulative physiological perturbation manifests at a cellular level by threatening the protein homeostasis of various cell types. Consequently, an increase of HSP70/90 was described in plasma and mononuclear cells and various organs and tissues, such as muscle, liver, cardiac tissue, and brain, after an acute bout of exercise. The specific response of HSP70/90 seems to be strongly related to the modality of exercise, with several dependent factors such as duration, intensity, exercise type, subjects’ training status, and environmental factors, e.g., temperature. It is suggested that HSP70/90 play a major role in immune regulation and cell protection during exercise and in the efficiency of regeneration and reparation processes. During long-term training, HSP70/90 are involved in preconditioning and adaptation processes that might also be important for disease prevention and therapy. With regard to their highly sensitive and individual response to specific exercise and training modalities, this review discusses whether and how HSP70 and HSP90 can be applied as biomarkers for monitoring exercise and training.

Characterization of a multiprotein complex involved in excitation-transcription coupling of skeletal muscle

BACKGROUND

Electrical activity regulates the expression of skeletal muscle genes by a process known as "excitation-transcription" (E-T) coupling. We have demonstrated that release of adenosine 5'-triphosphate (ATP) during depolarization activates membrane P2X/P2Y receptors, being the fundamental mediators between electrical stimulation, slow intracellular calcium transients, and gene expression. We propose that this signaling pathway would require the proper coordination between the voltage sensor (dihydropyridine receptor, DHPR), pannexin 1 channels (Panx1, ATP release conduit), nucleotide receptors, and other signaling molecules. The goal of this study was to assess protein-protein interactions within the E-T machinery and to look for novel constituents in order to characterize the signaling complex.

METHODS

Newborn derived myotubes, adult fibers, or triad fractions from rat or mouse skeletal muscles were used. Co-immunoprecipitation, 2D blue native SDS/PAGE, confocal microscopy z-axis reconstruction, and proximity ligation assays were combined to assess the physical proximity of the putative complex interactors. An L6 cell line overexpressing Panx1 (L6-Panx1) was developed to study the influence of some of the complex interactors in modulation of gene expression.

RESULTS

Panx1, DHPR, P2Y2 receptor (P2Y2R), and dystrophin co-immunoprecipitated in the different preparations assessed. 2D blue native SDS/PAGE showed that DHPR, Panx1, P2Y2R and caveolin-3 (Cav3) belong to the same multiprotein complex. We observed co-localization and protein-protein proximity between DHPR, Panx1, P2Y2R, and Cav3 in adult fibers and in the L6-Panx1 cell line. We found a very restricted location of Panx1 and Cav3 in a putative T-tubule zone near the sarcolemma, while DHPR was highly expressed all along the transverse (T)-tubule. By Panx1 overexpression, extracellular ATP levels were increased both at rest and after electrical stimulation. Basal mRNA levels of the early gene cfos and the oxidative metabolism markers citrate synthase and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α) were significantly increased by Panx1 overexpression. Interleukin 6 expression evoked by 20-Hz electrical stimulation (270 pulses, 0.3 ms each) was also significantly upregulated in L6-Panx1 cells.

CONCLUSIONS

We propose the existence of a relevant multiprotein complex that coordinates events involved in E-T coupling. Unveiling the molecular actors involved in the regulation of gene expression will contribute to the understanding and treatment of skeletal muscle disorders due to wrong-expressed proteins, as well as to improve skeletal muscle performance.

Cytosolic calcium transients are a determinant of contraction-induced HSP72 transcription in single skeletal muscle fibers

The intrinsic activating factors that induce transcription of heat shock protein 72 (HSP72) in skeletal muscle following exercise remain unclear. We hypothesized that the cytosolic Ca(2+) transient that occurs with depolarization is a determinant. We utilized intact, single skeletal muscle fibers from Xenopus laevis to test the role of the cytosolic Ca(2+) transient and several other exercise-related factors (fatigue, hypoxia, AMP kinase, and cross-bridge cycling) on the activation of HSP72 transcription. HSP72 and HSP60 mRNA levels were assessed with real-time quantitative PCR; cytosolic Ca(2+) concentration ([Ca(2+)]cyt) was assessed with fura-2. Both fatiguing and nonfatiguing contractions resulted in a significant increase in HSP72 mRNA. As expected, peak [Ca(2+)]cyt remained tightly coupled with peak developed tension in contracting fibers. Pretreatment with N-benzyl-p-toluene sulfonamide (BTS) resulted in depressed peak developed tension with stimulation, while peak [Ca(2+)]cyt remained largely unchanged from control values. Despite excitation-contraction uncoupling, BTS-treated fibers displayed a significant increase in HSP72 mRNA. Treatment of fibers with hypoxia (Po2: <3 mmHg) or AMP kinase activation had no effect on HSP72 mRNA levels. These results suggest that the intermittent cytosolic Ca(2+) transient that occurs with skeletal muscle depolarization provides a sufficient activating stimulus for HSP72 transcription. Metabolic or mechanical factors associated with fatigue development and cross-bridge cycling likely play a more limited role.

A knockdown with smoke model reveals FHIT as a repressor of Heme oxygenase 1

Fragile histidine triad (FHIT) gene deletions are among the earliest and most frequent events in carcinogenesis, particularly in carcinogen-exposed tissues. Though FHIT has been established as an authentic tumor suppressor, the mechanism underlying tumor suppression remains opaque. Most experiments designed to clarify FHIT function have analyzed the consequence of re-expressing FHIT in FHIT-negative cells. However, carcinogenesis occurs in cells that transition from FHIT-positive to FHIT-negative. To better understand cancer development, we induced FHIT loss in human bronchial epithelial cells with RNA interference. Because FHIT is a demonstrated target of carcinogens in cigarette smoke, we combined FHIT silencing with cigarette smoke extract (CSE) exposure and measured gene expression consequences by RNA microarray. The data indicate that FHIT loss enhances the expression of a set of oxidative stress response genes after exposure to CSE, including the cytoprotective enzyme heme oxygenase 1 (HMOX1) at the RNA and protein levels. Data are consistent with a mechanism in which Fhit protein is required for accumulation of the transcriptional repressor of HMOX1, Bach1 protein. We posit that by allowing superinduction of oxidative stress response genes, loss of FHIT creates a survival advantage that promotes carcinogenesis.

ROS Production via P2Y1-PKC-NOX2 Is Triggered by Extracellular ATP after Electrical Stimulation of Skeletal Muscle Cells

During exercise, skeletal muscle produces reactive oxygen species (ROS) via NADPH oxidase (NOX2) while inducing cellular adaptations associated with contractile activity. The signals involved in this mechanism are still a matter of study. ATP is released from skeletal muscle during electrical stimulation and can autocrinely signal through purinergic receptors; we searched for an influence of this signal in ROS production. The aim of this work was to characterize ROS production induced by electrical stimulation and extracellular ATP. ROS production was measured using two alternative probes; chloromethyl-2,7- dichlorodihydrofluorescein diacetate or electroporation to express the hydrogen peroxide-sensitive protein Hyper. Electrical stimulation (ES) triggered a transient ROS increase in muscle fibers which was mimicked by extracellular ATP and was prevented by both carbenoxolone and suramin; antagonists of pannexin channel and purinergic receptors respectively. In addition, transient ROS increase was prevented by apyrase, an ecto-nucleotidase. MRS2365, a P2Y₁ receptor agonist, induced a large signal while UTPyS (P2Y₂ agonist) elicited a much smaller signal, similar to the one seen when using ATP plus MRS2179, an antagonist of P2Y₁. Protein kinase C (PKC) inhibitors also blocked ES-induced ROS production. Our results indicate that physiological levels of electrical stimulation induce ROS production in skeletal muscle cells through release of extracellular ATP and activation of P2Y1 receptors. Use of selective NOX2 and PKC inhibitors suggests that ROS production induced by ES or extracellular ATP is mediated by NOX2 activated by PKC.

Membrane lipid therapy: Modulation of the cell membrane composition and structure as a molecular base for drug discovery and new disease treatment

Nowadays we understand cell membranes not as a simple double lipid layer but as a collection of complex and dynamic protein-lipid structures and microdomains that serve as functional platforms for interacting signaling lipids and proteins. Membrane lipids and lipid structures participate directly as messengers or regulators of signal transduction. In addition, protein-lipid interactions participate in the localization of signaling protein partners to specific membrane microdomains. Thus, lipid alterations change cell signaling that are associated with a variety of diseases including cancer, obesity, neurodegenerative disorders, cardiovascular pathologies, etc. This article reviews the newly emerging field of membrane lipid therapy which involves the pharmacological regulation of membrane lipid composition and structure for the treatment of diseases. Membrane lipid therapy proposes the use of new molecules specifically designed to modify membrane lipid structures and microdomains as pharmaceutical disease-modifying agents by reversing the malfunction or altering the expression of disease-specific protein or lipid signal cascades. Here, we provide an in-depth analysis of this emerging field, especially its molecular bases and its relevance to the development of innovative therapeutic approaches.

Electrical stimulation induces IL-6 in skeletal muscle through extracellular ATP by activating Ca(2+) signals and an IL-6 autocrine loop

Interleukin-6 (IL-6) is an important myokine that is highly expressed in skeletal muscle cells upon exercise. We assessed IL-6 expression in response to electrical stimulation (ES) or extracellular ATP as a known mediator of the excitation-transcription mechanism in skeletal muscle. We examined whether the canonical signaling cascade downstream of IL-6 (IL-6/JAK2/STAT3) also responds to muscle cell excitation, concluding that IL-6 influences its own expression through a positive loop. Either ES or exogenous ATP (100 μM) increased both IL-6 expression and p-STAT3 levels in rat myotubes, a process inhibited by 100 μM suramin and 2 U/ml apyrase. ATP also evoked IL-6 expression in both isolated skeletal fibers and extracts derived from whole FDB muscles. ATP increased IL-6 release up to 10-fold. STAT3 activation evoked by ATP was abolished by the JAK2 inhibitor HBC. Blockade of secreted IL-6 with a neutralizing antibody or preincubation with the STAT3 inhibitor VIII reduced STAT3 activation evoked by extracellular ATP by 70%. Inhibitor VIII also reduced by 70% IL-6 expression evoked by ATP, suggesting a positive IL-6 loop. In addition, ATP increased up to 60% the protein levels of SOCS3, a negative regulator of the IL-6 signaling pathway. On the other hand, intracellular calcium chelation or blockade of IP3-dependent calcium signals abolished STAT3 phosphorylation evoked by either extracellular ATP or ES. These results suggest that expression of IL-6 in stimulated skeletal muscle cells is mediated by extracellular ATP and nucleotide receptors, involving IP3-dependent calcium signals as an early step that triggers a positive IL-6 autocrine loop.

Time course-dependent changes in the transcriptome of human skeletal muscle during recovery from endurance exercise: from inflammation to adaptive remodeling

Reprogramming of gene expression is fundamental for skeletal muscle adaptations in response to endurance exercise. This study investigated the time course-dependent changes in the muscular transcriptome after an endurance exercise trial consisting of 1 h of intense cycling immediately followed by 1 h of intense running. Skeletal muscle samples were taken at baseline, 3 h, 48 h, and 96 h postexercise from eight healthy, endurance-trained men. RNA was extracted from muscle. Differential gene expression was evaluated using Illumina microarrays and validated with qPCR. Gene set enrichment analysis identified enriched molecular signatures chosen from the Molecular Signatures Database. Three hours postexercise, 102 gene sets were upregulated [family wise error rate (FWER), P < 0.05], including groups of genes related with leukocyte migration, immune and chaperone activation, and cyclic AMP responsive element binding protein (CREB) 1 signaling. Forty-eight hours postexercise, among 19 enriched gene sets (FWER, P < 0.05), two gene sets related to actin cytoskeleton remodeling were upregulated. Ninety-six hours postexercise, 83 gene sets were enriched (FWER, P < 0.05), 80 of which were upregulated, including gene groups related to chemokine signaling, cell stress management, and extracellular matrix remodeling. These data provide comprehensive insights into the molecular pathways involved in acute stress, recovery, and adaptive muscular responses to endurance exercise. The novel 96 h postexercise transcriptome indicates substantial transcriptional activity potentially associated with the prolonged presence of leukocytes in the muscles. This suggests that muscular recovery, from a transcriptional perspective, is incomplete 96 h after endurance exercise involving muscle damage.

Neuronal reprograming of protein homeostasis by calcium-dependent regulation of the heat shock response

Protein quality control requires constant surveillance to prevent misfolding, aggregation, and loss of cellular function. There is increasing evidence in metazoans that communication between cells has an important role to ensure organismal health and to prevent stressed cells and tissues from compromising lifespan. Here, we show in C. elegans that a moderate increase in physiological cholinergic signaling at the neuromuscular junction (NMJ) induces the calcium (Ca(2+))-dependent activation of HSF-1 in post-synaptic muscle cells, resulting in suppression of protein misfolding. This protective effect on muscle cell protein homeostasis was identified in an unbiased genome-wide screening for modifiers of protein aggregation, and is triggered by downregulation of gei-11, a Myb-family factor and proposed regulator of the L-type acetylcholine receptor (AChR). This, in-turn, activates the voltage-gated Ca(2+) channel, EGL-19, and the sarcoplasmic reticulum ryanodine receptor in response to acetylcholine signaling. The release of calcium into the cytoplasm of muscle cells activates Ca(2+)-dependent kinases and induces HSF-1-dependent expression of cytoplasmic chaperones, which suppress misfolding of metastable proteins and stabilize the folding environment of muscle cells. This demonstrates that the heat shock response (HSR) can be activated in muscle cells by neuronal signaling across the NMJ to protect proteome health.

Key role of lipids in heat stress management

Heat stress is a common and, therefore, an important environmental impact on cells and organisms. While much attention has been paid to severe heat stress, moderate temperature elevations are also important. Here we discuss temperature sensing and how responses to heat stress are not necessarily dependent on denatured proteins. Indeed, it is clear that membrane lipids have a pivotal function. Details of membrane lipid changes and the associated production of signalling metabolites are described and suggestions made as to how the interconnected signalling network could be modified for helpful intervention in disease.

Dysferlin and animal models for dysferlinopathy

Dysferlin (DYSF) is involved in the membrane-repair process, in the intracellular vesicle system and in T-tubule development in skeletal muscle. It interacts with mitsugumin 53, annexins, caveolin-3, AHNAK, affixin, S100A10, calpain-3, tubulin and dihydropyridine receptor. Limb-girdle muscular dystrophy 2B (LGMD2B) and Miyoshi myopathy (MM) are muscular dystrophies associated with recessively inherited mutations in the DYSF gene. The diseases are characterized by weakness and muscle atrophy that progress slowly and symmetrically in the proximal muscles of the limb girdles. LGMD2B and MM, which are collectively termed “dysferlinopathy”, both lead to abnormalities in vesicle traffic and membrane repair at the plasma membrane in muscle fibers. SJL/J (SJL) and A/J mice are naturally occurring animal models for dysferlinopathy. Since there has been no an approach to therapy for dysferlinopathy, the immediate development of a therapeutic method for this genetic disorder is desirable. The murine models are useful in verification experiments for new therapies and they are valuable tools for identifying factors that accelerate dystrophic changes in skeletal muscle. It could be possible that the genetic or immunological background in SJL or A/J mice could modify muscle damage in experiments involving these models, because SJL and A/J mice show differences in the progress and prevalent sites of skeletal muscle lesions as well as in the gene-expression profiles of their skeletal muscle. In this review, we provide up-to-date information on the function of dysferlin, the development of possible therapies for muscle dystrophies (including dysferlinopathy) and the detection of new therapeutic targets for dysferlinopathy by means of experiments using animal models for dysferlinopathy.

Measurement of calcium release due to inositol trisphosphate receptors in skeletal muscle

Calcium transients elicited by IP(3) receptors upon electrical stimulation of skeletal muscle cells (slow calcium signals) are often hard to visualize due to their relatively small amplitude compared to the large transient originated from ryanodine receptors associated to excitation-contraction coupling. The study of slow calcium transients, however, is relevant due to their function in regulation of muscle gene expression and in the process of excitation-transcription coupling. Discussed here are the procedures used to record slow calcium signals from both cultured mouse myotubes and from cultured adult skeletal muscle fibers.

Comparative Gene Expression Analysis in the Skeletal Muscles of Dysferlin-deficient SJL/J and A/J Mice

Quantitative real-time polymerase chain reaction (qRT-PCR) analysis was conducted to determine whether or not there are interstrain or site-dependent differences in the gene expression profiles of skeletal muscles in SJL/J and A/J mice as dysferlinopathy models. Upon analysis by qRT-PCR, SJL/J mice showed a trend of increased gene expression level of uncoupling protein 2 in the rectus femoris and longissimus lumborum at 30 weeks of age when dystrophic lesions became histopathologically pronounced. Heme oxygenase 1 and S100 calcium binding protein A4 were upregulated in the rectus femoris, longissimus lumborum and abdominal muscles, in which dystrophic lesions occur more commonly in SJL mice. The gene expression levels of heat shock protein 70 in most muscles of A/J mice were lower than those of BALB/c mice as control. SJL/J mice exhibited a marked lowering of decay-accelerating factor 1/CD55 gene expression level in all studied muscles except for the heart at all ages compared with that of BALB/c mice. This study showed that there were some interstrain differences in the gene expres sion profiles of skeletal muscles between SJL/J and A/J mice. Further investigation is required to reveal whether these alterations of the expression levels are the cause of dystrophic changes or occur subsequent to muscle damage.