Altered Ca2+ sparks in aging skeletal and cardiac muscle

Ca2+/calmodulin signaling in organismal aging and cellular senescence: Impact on human diseases

Molecular mechanisms of aging processes at the level of organisms and cells are in the focus of a large number of research laboratories. This research culminated in recent breakthroughs, which contributed to the better understanding of the natural aging process and aging associated malfunctions leading to age-related diseases. Ca2+ in connection with its master intracellular sensor protein calmodulin (CaM) regulates a plethora of crucial cellular processes orchestrating a wide range of signaling processes. This review focuses on the involvement of Ca2+/CaM in cellular mechanisms, which are associated with normal aging, as well as playing a role in the development of diseases connected with signaling processes during aging. We specifically highlight processes that involve inactivation of proteins, which take part in Ca2+/CaM regulatory systems by oxygen or nitrogen free radical species, during organismal aging and cellular senescence. As examples of organs where aging processes have recently been investigated, we chose to review the literature on molecular aging processes with involvement of Ca2+/CaM in heart and neuronal diseases, as well as in cancer and metabolic diseases, all deeply affected by aging. In addition, this article focuses on cellular senescence, a mechanism that may contribute to aging processes and therefore has been proposed as a target to interfere with the progression of age-associated diseases.

Alleviating effect of Lactobacillus rhamnosus SDSP202418 on exercise-induced fatigue in mice

In this study, the effects of Lactobacillus rhamnosus SDSP202418 isolated from shrimp paste on the exercise performance of fatigued mice were analyzed, and the potential action mechanism was revealed. L. rhamnosus SDSP202418 significantly improved the exhaustion time of the mice and regulated the biochemical indices (lactate dehydrogenase, nitrogen, and uric acid) of the fatigued mice to resist fatigue. L. rhamnosus SDSP202418 also upregulated the mRNA expression of slow muscle fibers and downregulated the mRNA expression of fast muscle fibers in the exercise mice by activating the AMPK/PGC-1α pathway in the fatigued mice. It also increased the contents of antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT), and glutathione (GSH)) in the liver and muscle. These enzymes removed and repaired oxidative free radicals to achieve antifatigue. In addition, L. rhamnosus SDSP202418 can change the gut microbial structure and modulate the abundance and balance of fatigue-related gut microbiota, which in turn exerts antifatigue effects. L. rhamnosus SDSP202418 is a functional food component that relieves fatigue after exercise.

Ablation of skeletal muscle estrogen receptor alpha impairs contractility in male mice

Estradiol and estrogen receptor α (ERα) have been shown to be important for the maintenance of skeletal muscle strength in females; however, little is known about the roles of estradiol and ERα in male muscle. The purpose of this study was to determine if skeletal muscle ERα is required for optimal contractility in male mice. We hypothesize that reduced ERα in skeletal muscle impairs contractility in male mice. Skeletal muscle-specific knockout (skmERαKO) male mice exhibited reduced strength across multiple muscles and several contractile parameters related to force generation and kinetics compared with wild-type littermates (skmERαWT). Isolated EDL muscle-specific isometric tetanic force, peak twitch force, peak concentric and peak eccentric forces, as well as the maximal rates of force development and relaxation were 11%-21% lower in skmERαKO compared with skmERαWT mice. In contrast, isolated soleus muscles from skmERαKO mice were not affected. In vivo peak torque of the anterior crural muscles was 20% lower in skmERαKO compared with skmERαWT mice. Muscle masses, contractile protein contents, fiber types, phosphorylation of the myosin regulatory light chain, and caffeine-elicited force did not differ between muscles of skmERαKO and skmERαWT mice, suggesting that strength deficits were not due to size, composition, or calcium release components of muscle contraction. These results indicate that in male mice, reduced skeletal muscle ERα blunts contractility to a magnitude similar to that previously reported in females; however, the mechanism may be sexually dimorphic.NEW & NOTEWORTHY We comprehensively measured in vitro and in vivo contractility of leg muscles with reduced estrogen receptor α (ERα) in male mice and reported that force generation and contraction kinetics are impaired. In contrast to findings in females, phosphorylation of myosin regulatory light chain cannot account for low force production in male skeletal muscle ERα knockout mice. These results indicate that ERα is required for optimal contractility in males and females but via sexually dimorphic means.

NAD+: An Old but Promising Therapeutic Agent for Skeletal Muscle Ageing

More than a century after the discovery of nicotinamide adenine dinucleotide (NAD+), our understanding of the molecule's role in the biology of ageing continues to evolve. As a coenzyme or substrate for many enzymes, NAD+ governs a wide range of biological processes, including energy metabolism, genomic stability, signal transduction, and cell fate. NAD+ deficiency has been recognised as a bona fide hallmark of tissue degeneration, and restoring NAD+ homeostasis helps to rejuvenate multiple mechanisms associated with tissue ageing. The progressive loss of skeletal muscle homeostasis with age is directly associated with high morbidity, disability and mortality. The aetiology of skeletal muscle ageing is complex, involving mitochondrial dysfunction, senescence and stem cell depletion, autophagy defects, chronic cellular stress, intracellular ion overload, immune cell dysfunction, circadian clock disruption, microcirculation disorders, persistent denervation, and gut microbiota dysbiosis. This review focuses on the therapeutic potential of NAD+ restoration to alleviate the above pathological factors and discusses the effects of in vivo administration of different NAD+ boosting strategies on skeletal muscle homeostasis, aiming to provide a reference for combating skeletal muscle ageing.

Skeletal muscle atrophy, regeneration, and dysfunction in heart failure: Impact of exercise training

This review highlights some established and some more contemporary mechanisms responsible for heart failure (HF)-induced skeletal muscle wasting and weakness. We first describe the effects of HF on the relationship between protein synthesis and degradation rates, which determine muscle mass, the involvement of the satellite cells for continual muscle regeneration, and changes in myofiber calcium homeostasis linked to contractile dysfunction. We then highlight key mechanistic effects of both aerobic and resistance exercise training on skeletal muscle in HF and outline its application as a beneficial treatment. Overall, HF causes multiple impairments related to autophagy, anabolic-catabolic signaling, satellite cell proliferation, and calcium homeostasis, which together promote fiber atrophy, contractile dysfunction, and impaired regeneration. Although both wasting and weakness are partly rescued by aerobic and resistance exercise training in HF, the effects of satellite cell dynamics remain poorly explored.

Natural aging and ovariectomy induces parallel phosphoproteomic alterations in skeletal muscle of female mice

The loss of skeletal muscle strength mid-life in females is associated with the decline of estrogen. Here, we questioned how estrogen deficiency might impact the overall skeletal muscle phosphoproteome after contraction, as force production induces phosphorylation of several muscle proteins. Phosphoproteomic analyses of the tibialis anterior muscle after contraction in two mouse models of estrogen deficiency, ovariectomy (Ovariectomized (Ovx) vs. Sham) and natural aging-induced ovarian senescence (Older Adult (OA) vs. Young Adult (YA)), identified a total of 2,593 and 3,507 phosphopeptides in Ovx/Sham and OA/YA datasets, respectively. Further analysis of estrogen deficiency-associated proteins and phosphosites identified 66 proteins and 21 phosphosites from both datasets. Of these, 4 estrogen deficiency-associated proteins and 4 estrogen deficiency-associated phosphosites were significant and differentially phosphorylated or regulated, respectively. Comparative analyses between Ovx/Sham and OA/YA using Ingenuity Pathway Analysis (IPA) found parallel patterns of inhibition and activation across IPA-defined canonical signaling pathways and physiological functional analysis, which were similarly observed in downstream GO, KEGG, and Reactome pathway overrepresentation analysis pertaining to muscle structural integrity and contraction, including AMPK and calcium signaling. IPA Upstream regulator analysis identified MAPK1 and PRKACA as candidate kinases and calcineurin as a candidate phosphatase sensitive to estrogen. Our findings highlight key molecular signatures and pathways in contracted muscle suggesting that the similarities identified across both datasets could elucidate molecular mechanisms that may contribute to skeletal muscle strength loss due to estrogen deficiency.

Global phosphoproteomic profiling of skeletal muscle in ovarian hormone-deficient mice

Protein phosphorylation is important in skeletal muscle development, growth, regeneration, and contractile function. Alterations in the skeletal muscle phosphoproteome due to aging have been reported in males; however, studies in females are lacking. We have demonstrated that estrogen deficiency decreases muscle force, which correlates with decreased myosin regulatory light chain phosphorylation. Thus, we questioned whether the decline of estrogen in females that occurs with aging might alter the skeletal muscle phosphoproteome. C57BL/6J female mice (6 mo) were randomly assigned to a sham-operated (Sham) or ovariectomy (Ovx) group to investigate the effects of estrogen deficiency on skeletal muscle protein phosphorylation in a resting, noncontracting condition. After 16 wk of estrogen deficiency, the tibialis anterior muscle was dissected and prepped for label-free nano-liquid chromatography-tandem mass spectrometry phosphoproteomic analysis. We identified 4,780 phosphopeptides in tibialis anterior muscles of ovariectomized (Ovx) and Sham-operated (Sham) control mice. Further analysis revealed 647 differentially regulated phosphopeptides (Benjamini-Hochberg adjusted P value < 0.05 and 1.5-fold change ratio) that corresponded to 130 proteins with 22 proteins differentially phosphorylated (3 unique to Ovx, 2 unique to Sham, 6 upregulated, and 11 downregulated). Differentially phosphorylated proteins associated with the sarcomere, cytoplasm, and metabolic and calcium signaling pathways were identified. Our work provides the first global phosphoproteomic analysis in females and how estrogen deficiency impacts the skeletal muscle phosphoproteome.

Altered Ca2+ Handling and Oxidative Stress Underlie Mitochondrial Damage and Skeletal Muscle Dysfunction in Aging and Disease

Skeletal muscle contraction relies on both high-fidelity calcium (Ca²⁺) signals and robust capacity for adenosine triphosphate (ATP) generation. Ca²⁺ release units (CRUs) are highly organized junctions between the terminal cisternae of the sarcoplasmic reticulum (SR) and the transverse tubule (T-tubule). CRUs provide the structural framework for rapid elevations in myoplasmic Ca²⁺ during excitation-contraction (EC) coupling, the process whereby depolarization of the T-tubule membrane triggers SR Ca²⁺ release through ryanodine receptor-1 (RyR1) channels. Under conditions of local or global depletion of SR Ca²⁺ stores, store-operated Ca²⁺ entry (SOCE) provides an additional source of Ca²⁺ that originates from the extracellular space. In addition to Ca²⁺, skeletal muscle also requires ATP to both produce force and to replenish SR Ca²⁺ stores. Mitochondria are the principal intracellular organelles responsible for ATP production via aerobic respiration. This review provides a broad overview of the literature supporting a role for impaired Ca²⁺ handling, dysfunctional Ca²⁺-dependent production of reactive oxygen/nitrogen species (ROS/RNS), and structural/functional alterations in CRUs and mitochondria in the loss of muscle mass, reduction in muscle contractility, and increase in muscle damage in sarcopenia and a wide range of muscle disorders including muscular dystrophy, rhabdomyolysis, central core disease, and disuse atrophy. Understanding the impact of these processes on normal muscle function will provide important insights into potential therapeutic targets designed to prevent or reverse muscle dysfunction during aging and disease.

Zeitgebers of skeletal muscle and implications for metabolic health

Metabolic health is a crucial area of current research, and is an outcome of innate physiology, and interactions with the environment. Environmental cues, such as the Earth's day-night rhythm, partly regulate diurnal hormones and metabolites. Circadian physiology consists of highly conserved biological processes over ∼24-h cycles, which are influenced by external cues (Zeitgebers - 'time-keepers'). Skeletal muscle has diurnal variations of a large magnitude, owing in part to the strong nature of physical activity throughout the day and other external Zeitgebers. The orchestration of whole-body and skeletal muscle metabolism is a complex, finely tuned process, and molecular diurnal variations are regulated by a transcription-translation feedback loop controlled by the molecular clock, as well as non-transcriptional metabolic processes. The mitochondrion may play an important role in regulating diurnal metabolites within skeletal muscle, given its central role in the regulation of NAD⁺ /NADH, O₂ , reactive oxygen species and redox metabolism. These molecular pathways display diurnal variation and illustrate the complex orchestration of circadian metabolism in skeletal muscle. Probably the most robust Zeitgeber of skeletal muscle is exercise, which alters glucose metabolism and flux, in addition to a range of other diurnal metabolic pathways. Indeed, performing exercise at different times of the day may alter metabolism and health outcomes in some cohorts. The objective of this Symposium Review is to briefly cover the current literature, and to speculate regarding future areas of research. Thus, we postulate that metabolic health may be optimized by altering the timing of external cues such as diet and exercise.

Paxilline Prevents the Onset of Myotonic Stiffness in Pharmacologically Induced Myotonia: A Preclinical Investigation

Reduced Cl⁻ conductance causes inhibited muscle relaxation after forceful voluntary contraction due to muscle membrane hyperexcitability. This represents the pathomechanism of myotonia congenita. Due to the prevailing data suggesting that an increased potassium level is a main contributor, we studied the effect of a modulator of a big conductance Ca²⁺- and voltage-activated K⁺ channels (BK) modulator on contraction and relaxation of slow- and high-twitch muscle specimen before and after the pharmacological induction of myotonia. Human and murine muscle specimens (wild-type and BK^−/−) were exposed to anthracene-9-carboxylic acid (9-AC) to inhibit CLC-1 chloride channels and to induce myotonia in-vitro. Functional effects of BK-channel activation and blockade were investigated by exposing slow-twitch (soleus) and fast-twitch (extensor digitorum longus) murine muscle specimens or human musculus vastus lateralis to an activator (NS1608) and a blocker (Paxilline), respectively. Muscle-twitch force and relaxation times (T_90/10) were monitored. Compared to wild type, fast-twitch muscle specimen of BK^−/− mice resulted in a significantly decreased T_90/10 in presence of 9-AC. Paxilline significantly shortened T_90/10 of murine slow- and fast-twitch muscles as well as human vastus lateralis muscle. Moreover, twitch force was significantly reduced after application of Paxilline in myotonic muscle. NS1608 had opposite effects to Paxilline and aggravated the onset of myotonic activity by prolongation of T_90/10. The currently used standard therapy for myotonia is, in some individuals, not very effective. This in vitro study demonstrated that a BK channel blocker lowers myotonic stiffness and thus highlights its potential therapeutic option in myotonia congenital (MC).

Novel biomarkers for the evaluation of aging-induced proteinopathies

Proteinopathies are characterized by aging related accumulation of misfolded protein aggregates. Irreversible covalent modifications of aging proteins may significantly affect the native three dimentional conformation of proteins, alter their function and lead to accumulation of misfolded protein as dysfunctional aggregates. Protein misfolding and accumulation of aberrant proteins are known to be associated with aging-induced proteinopathies such as amyloid ß and tau proteins in Alzheimer's disease, α-synuclein in Parkinson's disease and islet amyloid polypeptides in Type 2 diabetes mellitus. Protein oxidation processes such as S-nitrosylation, dityrosine formation and some of the newly elucidated processes such as carbamylation and citrullination recently drew the attention of researchers in the field of Gerontology. Studying over these processes and illuminating their relations between proteinopathies may help to diagnose early and even to treat age related disorders. Therefore, we have chosen to concentrate on aging-induced proteinopathic nature of these novel protein modifications in this review.

Dynamics expression of DmFKBP12/Calstabin during embryonic early development of Drosophila melanogaster

Background

Calcium signaling are conserved from invertebrates to vertebrates and plays critical roles in many molecular mechanisms of embryogenesis and postnatal development. As a critical component of the signaling pathway, the RyR medicated calcium-induced calcium release signaling system, has been well studied along with their regulator FK506-binding protein 12 (FKBP12/Calstabin). Lack of FKBP12 is known to result in lethal cardiac dysfunction in mouse. However, precisely how FKBP12 is regulated and effects calcium signaling in Drosophila melanogaster remains largely unknown.

Results

In this study, we identified both temporal and localization changes in expression of DmFKBP12, a translational and transcriptional regulator of Drosophila RyR (DmRyR) and FKBP12, through embryonic development. DmFKBP12 is first expressed at the syncytial blastoderm stage and undergoes increased expression during the cellular blastoderm and early gastrulation stages. At late gastrulation, DmFKBP12 expression begins to decline until it reaches homeostasis, which it then maintains throughout the rest of development. Throughout these described changes in expression, DmFKBP12 mRNA remain stable, which indicates that protein dynamics are attributed to regulation at the mRNA to protein translation level. In addition to temporal changes in expression, dynamic expression profiles during Drosophila development also revealed DmFKBP12 localization. Although DmFKBP12 is distributed evenly between the anterior to posterior poles of the blastoderm egg, the protein is expressed more strongly in the cortex of the early Drosophila gastrula with the highest concentration found in the basement membrane of the cellular blastoderm. Fertilized egg, through the profile as under-membrane cortex distribution concentering onto basement at cellular blastoderm, to the profile as three-gem layer localization in primitive neuronal and digestion architecture of early Drosophila gastrula. By late gastrulation, DmFKBP12 is no longer identified in the yolk or lumen of duct structures and has relocated to the future brain (suboesophageal and supraesophageal ganglions), ventral nervous system, and muscular system. Throughout these changes in distribution, in situ DmFKBP12 mRNA monitoring detected equal distribution of DmFKBP12 mRNA, once again indicating that regulation of DmFKBP12 occurs at the translational level in Drosophila development.

Conclusion

As a critical regulator of the DmRyR-FKBP complex, DmFKBP12 expression in Drosophila fluctuates temporally and geographically with the formation of organ systems. These finding indicate that DmFKBP12 and RyR associated calcium signaling plays an essential role in the successful development of Drosophila melanogaster. Further study on the differences between mammalian RyR-FKBP12 and Drosophila DmRyR-FKBP12 can be exploited to develop safe pesticides.

Electronic supplementary material

The online version of this article (10.1186/s13578-019-0270-6) contains supplementary material, which is available to authorized users.

NOX Inhibition Improves β-Adrenergic Stimulated Contractility and Intracellular Calcium Handling in the Aged Rat Heart

Cardiac aging is characterized by alterations in contractility and intracellular calcium ([Ca²⁺]_i) homeostasis. It has been suggested that oxidative stress may be involved in this process. We and others have reported that in cardiomyopathies the NADPH oxidase (NOX)-derived superoxide is increased, with a negative impact on [Ca²⁺]_i and contractility. We tested the hypothesis that in the aged heart, [Ca²⁺]_i handling and contractility are disturbed by NOX-derived superoxide. For this we used adults (≈5 month-old) and aged (20–24 month-old) rats. Contractility was evaluated in isolated hearts, challenged with isoproterenol. To assess [Ca²⁺]_i, isolated cardiac myocytes were field-stimulated and [Ca²⁺]_i was monitored with fura-2. Cardiac concentration-response to isoproterenol was depressed in aged compared to adults hearts (p < 0.005), but was restored by NOX inhibitors apocynin and VAS2870. In isolated cardiomyocytes, apocynin increased the amplitude of [Ca²⁺]_i in aged myocytes (p < 0.05). Time-50 [Ca²⁺]_i decay was increased in aged myocytes (p < 0.05) and reduced towards normal by NOX inhibition. In addition, we found that myofilaments Ca²⁺ sensitivity was reduced in aged myocytes (p < 0.05), and was further reduced by apocynin. NOX2 expression along with NADPH oxidase activity was increased in aged hearts. Phospholamban phosphorylation (Ser16/Thr17) after isoproterenol treatment was reduced in aged hearts compared to adults and was restored by apocynin treatment (p < 0.05). In conclusion, β-adrenergic-induced contractility was depressed in aged hearts, and NOX inhibition restored back to normal. Moreover, altered Ca²⁺ handling in aged myocytes was also improved by NOX inhibition. These results suggest a NOX-dependent effect in aged myocytes at the level of Ca²⁺ handling proteins and myofilaments.

SERCA1 attenuates diaphragm relaxation and uptake rate of SERCA in rats with acute sepsis

The present study aimed to investigate the effects of acute sepsis on diaphragm contractility and relaxation, via examining the Ca2+‑uptake function of sarco/endoplasmic reticulum Ca2+ adenosine triphosphatase (SERCA), and the protein levels of SERCA1, SERCA2 and the ryanodine receptor (RyR) of the sarcoplasmic reticulum (SR). A sepsis rat model was established through cecal ligation and puncture (CLP). A total of 6 and 12 h following CLP, the isometric contractile and relaxation parameters of the diaphragm were measured. In addition, Ca2+ uptake and release from the SR, and the protein expression levels of SERCA1, SERCA2 and RyR in diaphragm muscle tissue were investigated. At 6 and 12 h post‑CLP, the diaphragm half‑relaxation time was prolonged and the maximum rate of tension decline was decreased and the Ca2+‑uptake function of SERCA was markedly reduced. The maximum rate of twitch force development, the maximal twitch and tetanic tension, and the release function of SR were decreased at 12 h post‑CLP. A total of 12 h following CLP, the protein expression levels of SERCA1 were significantly downregulated, and its activity was significantly reduced; conversely, the protein levels of SERCA2 remained unaltered. The present findings indicated that at the acute stage of sepsis induced by CLP the contractile and relaxation functions of the diaphragm were significantly compromised. The impairments in relaxation may be a result of the impaired uptake function of the SR and the downregulation in SERCA1 protein expression. Conversely, the compromised contractility may be a result of the impaired release function of the SR and the downregulation in RyR protein levels. This could provide some new insights into the treatment of sepsis. In acute stages of sepsis, the improvement of SERCA function could reduce the disequilibrium of calcium homeostasis to improve the critical illness myopathy and respiratory failure.

Effect of acute peritonitis on rocuronium-induced intraperitoneal pressure reduction and the uptake function of the sarcoplasmic reticulum

Previous studies have reported the incomplete relaxation effect of neuromuscular blockers on skeletal muscles in acute peritonitis (AP) and other inflammatory processes; however, the underlying mechanisms responsible for this effect have not yet been satisfactorily identified. The impaired removal of cytosolic Ca²⁺ through sarcoendoplasmic Ca²⁺-ATPase (SERCA) and defects in sarcoplasmic reticulum (SR) Ca²⁺ uptake are the major contributing factors to diastolic dysfunction. Previous studies on the effects of neuromuscular blockers have primarily focused on neuromuscular transmission. Because of the reduced calcium uptake in the SR itself, even when neuromuscular transmission is fully blocked, the muscle is not able to relax effectively. In the present study, the impact of AP on rocuronium-induced intraperitoneal pressure reduction and rectus abdominal muscle relaxation, and SERCA uptake function was investigated. AP was induced via gastric perforation and changes in the intraperitoneal pressure before and after the administration of rocuronium were recorded. Muscle contractile properties, uptake and release functions and SERCA activity in the rectus abdominal muscles of AP model rats were measured. The half-relaxation time in the AP group was significantly prolonged compared with that in the control group (P<0.01). The peak rate of SR Ca²⁺ uptake for whole muscle homogenates was significantly reduced (P<0.05) in AP model rats without reduction of the rate of Ca²⁺ release evoked through AgNO₃. In conclusion, gastric perforation-induced AP attenuates the intraperitoneal pressure-reducing effect of rocuronium, and AP induces diastolic dysfunction of the rectus abdominal muscle. The SR Ca²⁺-ATPase uptake rate was also reduced by AP.

Aging asymmetry: systematic survey of changes in age-related biomarkers in the annual fish Nothobranchius guentheri

Aging asymmetry is the observation that different tissues age in different ways and at different rates. This has not been assessed in a single organism using multiple biomarkers of aging. Here we clearly demonstrated that the levels of protein oxidation and lipid peroxidation as well as CAT, SOD and GPX activities all showed a tissue-dependent change with advancing age; and DNA repair ability, as revealed by the expression of ercc1 and its protein levels, also exhibited a tissue-specific variation with age. We also found that protein oxidation and lipid peroxidation levels remained relatively stable in the liver, intestine, skin and testis as well as in the brain, eye and heart of young, adult and aged fishes; SOD and GPX activities displayed little variation in the intestine, eye and skin as well as in the brain and skin of young, adult and aged fishes; and low and stable expression of ercc1 was observed in the spleen, eye and heart of young, adult and aged fishes. Collectively, these results indicate that aging is tissue specific and asymmetric in N. guentheri. The observation of aging asymmetry may have practical implications for the application of non-intrusion intervention approaches to prolong lifespan.

Dual function of MG53 in membrane repair and insulin signaling

MG53 is a member of the TRIM-family protein that acts as a key component of the cell membrane repair machinery. MG53 is also an E3-ligase that ubiquinates insulin receptor substrate-1 and controls insulin signaling in skeletal muscle cells. Since its discovery in 2009, research efforts have been devoted to translate this basic discovery into clinical applications in human degenerative and metabolic diseases. This review article highlights the dual function of MG53 in cell membrane repair and insulin signaling, the mechanism that underlies the control of MG53 function, and the therapeutic value of targeting MG53 function in regenerative medicine. [BMB Reports 2016; 49(8): 414-423]

In vitro myogenesis induced by human recombinant elastin-like proteins

Mammalian adult skeletal muscle has a limited ability to regenerate after injury, usage or trauma. A promising strategy for successful regenerative technology is the engineering of bio interfaces that mimic the characteristics of the extracellular matrix. Human elastin-like polypeptides (HELPs) have been synthesized as biomimetic materials that maintain some peculiar properties of the native protein. We developed a novel Human Elastin Like Polypeptide obtained by fusing the elastin-like backbone to a domain present in the α2 chain of type IV collagen, containing two RGD motives. We employed this peptide as adhesion substrate for C2C12 myoblasts and compared its effects to those induced by two other polypeptides of the HELP series. Myoblast adhered to all HELPs coatings, where they assumed morphology and cytoarchitecture that depended on the polypeptide structure. Adhesion to HELPs stimulated at a different extent cell proliferation and differentiation, the expression of Myosin Heavy Chain and the fusion of aligned fibers into multinucleated myotubes. Adhesion substrates significantly altered myotubes stiffness, measured by Atomic Force Microscopy, and differently affected the cells Ca(2+) handling capacity and the maturation of excitation-contraction coupling machinery, evaluated by Ca(2+) imaging. Overall, our findings indicate that the properties of HELP biopolymers can be exploited for dissecting the molecular connections underlying myogenic differentiation and for designing novel substrates for skeletal muscle regeneration.

Interaction between mitsugumin 29 and TRPC3 participates in regulating Ca(2+) transients in skeletal muscle

Mitsugumin 29 (MG29) is related to the fatigue and aging processes of skeletal muscle. To examine the roles of MG29 in conjunction with its binding protein, the canonical-type transient receptor potential cation channel 3 (TRPC3), in skeletal muscle, the binding region of MG29 to TRPC3 was studied along with the functional relevance of the binding in mouse primary skeletal myotubes using co-immunoprecipitation assays and Ca(2+) imaging experiments. The N-terminus and the I-II loop of MG29 constitute the binding region for TRPC3. The myotubes that expressed the MG29 mutant missing the entire TRPC3-binding region showed a disrupted binding between endogenous MG29 and TRPC3 and a reduction in Ca(2+) transients in response to membrane depolarization without affecting ryanodine receptor 1 activity, the resting cytosolic Ca(2+) level, and the amount of releasable Ca(2+) from the sarcoplasmic reticulum. Among the proteins mediating Ca(2+) movements in skeletal muscle, TRPC4 expression was significantly decreased by the MG29 mutant. Therefore, MG29 could be a new factor for regulating Ca(2+) transients during skeletal muscle contraction possibly via a correlation with TRPC3 and TRPC4.

Trimeric intracellular cation channels and sarcoplasmic/endoplasmic reticulum calcium homeostasis

Trimeric intracellular cation channels (TRIC) represents a novel class of trimeric intracellular cation channels. Two TRIC isoforms have been identified in both the human and the mouse genomes: TRIC-A, a subtype predominantly expressed in the sarcoplasmic reticulum (SR) of muscle cells, and TRIC-B, a ubiquitous subtype expressed in the endoplasmic reticulum (ER) of all tissues. Genetic ablation of either TRIC-A or TRIC-B leads to compromised K(+) permeation and Ca(2+) release across the SR/ER membrane, supporting the hypothesis that TRIC channels provide a counter balancing K(+) flux that reduces SR/ER membrane depolarization for maintenance of the electrochemical gradient that drives SR/ER Ca(2+) release. TRIC-A and TRIC-B seem to have differential functions in Ca(2+) signaling in excitable and nonexcitable cells. Tric-a(-/-) mice display defective Ca(2+) sparks and spontaneous transient outward currents in arterial smooth muscle and develop hypertension, in addition to skeletal muscle dysfunction. Knockout of TRIC-B results in abnormal IP3 receptor-mediated Ca(2+) release in airway epithelial cells, respiratory defects, and neonatal lethality. Double knockout mice lacking both TRIC-A and TRIC-B show embryonic lethality as a result of cardiac arrest. Such an aggravated lethality indicates that TRIC-A and TRIC-B share complementary physiological functions in Ca(2+) signaling in embryonic cardiomyocytes. Tric-a(-/-) and Tric-b(+/-) mice are viable and susceptible to stress-induced heart failure. Recent evidence suggests that TRIC-A directly modulates the function of the cardiac ryanodine receptor 2 Ca(2+) release channel, which in turn controls store-overload-induced Ca(2+) release from the SR. Thus, the TRIC channels, in addition to providing a countercurrent for SR/ER Ca(2+) release, may also function as accessory proteins that directly modulate the ryanodine receptor/IP3 receptor channel functions.

Assessment of calcium sparks in intact skeletal muscle fibers

Maintaining homeostatic Ca(2+) signaling is a fundamental physiological process in living cells. Ca(2+) sparks are the elementary units of Ca(2+) signaling in the striated muscle fibers that appear as highly localized Ca(2+) release events mediated by ryanodine receptor (RyR) Ca(2+) release channels on the sarcoplasmic reticulum (SR) membrane. Proper assessment of muscle Ca(2+) sparks could provide information on the intracellular Ca(2+) handling properties of healthy and diseased striated muscles. Although Ca(2+) sparks events are commonly seen in resting cardiomyocytes, they are rarely observed in resting skeletal muscle fibers; thus there is a need for methods to generate and analyze sparks in skeletal muscle fibers. Detailed here is an experimental protocol for measuring Ca(2+) sparks in isolated flexor digitorm brevis (FDB) muscle fibers using fluorescent Ca(2+) indictors and laser scanning confocal microscopy. In this approach, isolated FDB fibers are exposed to transient hypoosmotic stress followed by a return to isotonic physiological solution. Under these conditions, a robust Ca(2+) sparks response is detected adjacent to the sarcolemmal membrane in young healthy FDB muscle fibers. Altered Ca(2+) sparks response is detected in dystrophic or aged skeletal muscle fibers. This approach has recently demonstrated that membrane-delimited signaling involving cross-talk between inositol (1,4,5)-triphosphate receptor (IP3R) and RyR contributes to Ca(2+) spark activation in skeletal muscle. In summary, our studies using osmotic stress induced Ca(2+) sparks showed that this intracellular response reflects a muscle signaling mechanism in physiology and aging/disease states, including mouse models of muscle dystrophy (mdx mice) or amyotrophic lateral sclerosis (ALS model).

mTOR pathway and Ca²⁺ stores mobilization in aged smooth muscle cells

Aging is considered to be driven by the so called senescence pathways, especially the mTOR route, although there is almost no information on its activity in aged tissues. Aging also induces Ca²⁺ signal alterations, but information regarding the mechanisms for these changes is almost inexistent. We investigated the possible involvement of the mTOR pathway in the age-dependent changes on Ca²⁺ stores mobilization in colonic smooth muscle cells of young (4 month old) and aged (24 month old) guinea pigs. mTORC1 activity was enhanced in aged smooth muscle, as revealed by phosphorylation of mTOR and its direct substrates S6K1 and 4E-BP1. Mobilization of intracellular Ca²⁺ stores through IP₃R or RyR channels was impaired in aged cells, and it was facilitated by mTOR and by FKBP12, as indicated by the inhibitory effects of KU0063794 (a direct mTOR inhibitor), rapamycin (a FKBP12-mediated mTOR inhibitor) and FK506 (an FKBP12 binding immunosuppressant). Aging suppressed the facilitation of the Ca²⁺ mobilization by FKBP12 but not by mTOR, without changing the total expression of FKBP12 protein. In conclusion, or study shows that in smooth muscle aging enhances the constitutive activity of mTORC1 pathway and impairs Ca²⁺ stores mobilization by suppression of the FKBP12-induced facilitation of Ca²⁺ release.

Beclin 1 interactome controls the crosstalk between apoptosis, autophagy and inflammasome activation: impact on the aging process

Autophagy and apoptosis are crucial cellular housekeeping and tissue survival mechanisms. There is emerging evidence of important crosstalk between apoptosis and autophagy which can be linked to inflammasome activation. Beclin 1 is a platform protein which assembles an interactome consisting of diverse proteins which control the initiation of autophagocytosis and distinct phases in endocytosis. Recent studies have demonstrated that the anti-apoptotic Bcl-2 family members can interact with Beclin 1 and inhibit autophagy. Consequently, impaired autophagy can trigger inflammasome activation. Interestingly, the hallmarks of the ageing process include a decline in autophagy, increased resistance to apoptosis and a low-grade inflammatory phenotype. Age-related stresses, e.g. genotoxic, metabolic and environmental insults, enhance the expression of NF-κB-driven anti-apoptotic Bcl-2 proteins which repress the Beclin 1-dependent autophagy. Suppression of autophagocytosis provokes inflammation including NF-κB activation which further potentiates anti-apoptotic defence. In a context-dependent manner, this feedback defence mechanism can enhance the aging process or provoke tumorigenesis or cellular senescence. We will review the role of Beclin 1 interactome in the crosstalk between apoptosis, autophagy and inflammasomes emphasizing that disturbances in Beclin 1-dependent autophagy can have a crucial impact on the aging process.

Mild stress of caffeine increased mtDNA content in skeletal muscle cells: the interplay between Ca2+ transients and nitric oxide

Caffeine increases mitochondrial biogenesis in myotubes by evoking Ca(2+) transients. Nitric oxide (NO) also induces mitochondrial biogenesis in skeletal muscle cells via upregulation of AMP-activated protein kinase (AMPK) activity and PGC-1α. However, the interplay and timing sequence between Ca(2+) transients and NO releases remain unclear. Herein, we tested the hypothesis that caffeine-evoked Ca(2+) transients triggered NO production to increase mtDNA in skeletal muscle cells. Ca(2+) transients were recorded with Fura-2 AM and confocal microscopy; mtDNA staining, mitochondrial membrane potential and NO level were determined using fluorescent probes PicoGreen, tetramethylrhodamine methyl ester (TMRM) and DAF-FM, respectively. In primary cultured myotubes, a subtle and moderate stress of caffeine increased mtDNA exclusively. Mitochondrial membrane potential and mtDNA were increased by 1 mM as well as 5 mM caffeine, whereas 10 mM caffeine did not change the fluorescence intensity of PicoGreen and TMRM. NO level in myocytes increased gradually following the first jump of Ca(2+) transients evoked by caffeine (5 mM) till the end of recording, when Fura-2 indicated that Ca(2+) transients recovered partly and even disappeared. Importantly, nitric oxide synthase (NOS) inhibitor (L-NAME) suppressed caffeine-induced mtDNA biogenesis, whereas NO donor (DETA-NO) increased mtDNA content. These data strongly suggest that caffeine-induced mtDNA biogenesis is dose-sensitive and dependent on a certain level of stress. Further, an increasing level of NO following Ca(2+) transients is required for caffeine-induced mtDNA biogenesis. Additionally, Ca(2+) transients, a usual and first response to caffeine, was either suppressed or attenuated by L-NAME, DETA-NO, AICAR and U0126, suggesting an inability to control [Ca(2+)](i) in these treated cells. There may be an important interplay between NO and Ca(2+) transients in intracellular signaling system involving NOS, AMPK and MEK.

Detection of calcium sparks in intact and permeabilized skeletal muscle fibers

Ca(2+) sparks are the elementary units of Ca(2+) signaling in striated muscle fibers that appear as highly localized Ca(2+) release events through ryanodine receptor (RyR) Ca(2+) release channels in the sarcoplasmic reticulum (SR). While these events are commonly observed in resting cardiac myocytes, they are rarely seen in resting skeletal muscle fibers. Since Ca(2+) spark analysis can provide extensive data on the Ca(2+) handling characteritsics of normal and diseased striated muscle, there has been interest in developing methods for observing Ca(2+) sparks in skeletal muscle. Previously, we discovered that stress generated by osmotic pressure changes induces a robust Ca(2+) spark response confined in close spatial proximity to the sarcolemmal membrane in wild-type intact mammalian muscles. Our studies showed these peripheral Ca(2+) sparks (PCS) were altered in dystrophic or aged skeletal muscles. Other methods to induce Ca(2+) sparks include permeabilization of the sarcolemmal membrane with detergents, such as saponin. In this chapter, we will discuss the methods for isolation of muscle fibers, the techniques for inducing Ca(2+) sparks in these isolated fibers, and provide guidance on the analysis of data from these experiments.

Vitamin B3, the nicotinamide adenine dinucleotides and aging

Organism aging is a process of time and maturation culminating in senescence and death. The molecular details that define and determine aging have been intensely investigated. It has become appreciated that the process is partly an accumulation of random yet inevitable changes, but it can be strongly affected by genes that alter lifespan. In this review, we consider how NAD(+) metabolism plays important roles in the random patterns of aging, and also in the more programmatic aspects. The derivatives of NAD(+), such as reduced and oxidized forms of NAD(P)(+), play important roles in maintaining and regulating cellular redox state, Ca(2+) stores, DNA damage and repair, stress responses, cell cycle timing and lipid and energy metabolism. NAD(+) is also a substrate for signaling enzymes like the sirtuins and poly-ADP-ribosylpolymerases, members of a broad family of protein deacetylases and ADP-ribosyltransferases that regulate fundamental cellular processes such as transcription, recombination, cell division, proliferation, genome maintenance, apoptosis, stress resistance and senescence. NAD(+)-dependent enzymes are increasingly appreciated to regulate the timing of changes that lead to aging phenotypes. We consider how metabolism, specifically connected with Vitamin B3 and the nicotinamide adenine dinucleotides and their derivatives, occupies a central place in the aging processes of mammals.

Calcium signaling via two-pore channels: local or global, that is the question

Recently, we identified, for the first time, two-pore channels (TPCs, TPCN for gene name) as a novel family of nicotinic acid adenine dinucleotide phosphate (NAADP)-gated, endolysosome-targeted calcium release channels. Significantly, three subtypes of TPCs have been characterized, TPC1-3, with each being targeted to discrete acidic calcium stores, namely lysosomes (TPC2) and endosomes (TPC1 and TPC3). That TPCs act as NAADP-gated calcium release channels is clear, given that NAADP binds to high- and low-affinity sites associated with TPC2 and thereby induces calcium release and homologous desensitization, as observed in the case of endogenous NAADP receptors. Moreover, NAADP-evoked calcium signals via TPC2 are ablated by short hairpin RNA knockdown of TPC2 and by depletion of acidic calcium stores with bafilomycin. Importantly, however, NAADP-evoked calcium signals were biphasic in nature, with an initial phase of calcium release from lysosomes via TPC2, being subsequently amplified by calcium-induced calcium release (CICR) from the endoplasmic reticulum (ER). In marked contrast, calcium release via endosome-targeted TPC1 induced only spatially restricted calcium signals that were not amplified by CICR from the ER. These findings provide new insights into the mechanisms that cells may utilize to "filter" calcium signals via junctional complexes to determine whether a given signal remains local or is converted into a propagating global signal. Essentially, endosomes and lysosomes represent vesicular calcium stores, quite unlike the ER network, and TPCs do not themselves support CICR or, therefore, propagating regenerative calcium waves. Thus "quantal" vesicular calcium release via TPCs must subsequently recruit inositol 1,4,5-trisphoshpate receptors and/or ryanodine receptors on the ER by CICR to evoke a propagating calcium wave. This may call for a revision of current views on the mechanisms of intracellular calcium signaling. The purpose of this review is, therefore, to provide an appropriate framework for future studies in this area.

The ER and ageing II: calcium homeostasis

Increase in intracellular Ca(2+) concentration occurs by Ca(2+) influx through the plasma membrane and by Ca(2+) release from intracellular stores. The ER is the most important Ca(2+) store. Its stress, characterized by the impairment of Ca(2+) homeostasis and by the accumulation of misfolded proteins, can be induced by different factors. In turn, it induces defense mechanisms such as unfolded protein response, and when it is severe and prolonged, activation of the apoptotic pathway. Damage to the ER, impairment of its function, and a decreased level of its Ca(2+)-handling proteins might all play a role in physiological ageing by handicapping the ER stress response. Thus, healthy ageing is accompanied by subtle alterations of Ca(2+) homeostasis and signaling, including alterations in the ER Ca(2+) load and release. The expression and/or function of ryanodine receptors, IP3 receptors, and SERCA Ca(2+) pumps located in the ER membrane, and Ca(2+)-binding proteins within ER lumen all seem to be affected in aged cells. Data are presented on age-dependent, tissue-specific changes in ER-related Ca(2+) homeostasis in skeletal, cardiac and smooth muscles, as well as in the nervous and immune systems. Disturbances of Ca(2+) homeostasis and of signaling are potential targets for intervention in aged humans.

DHPR activation underlies SR Ca2+ release induced by osmotic stress in isolated rat skeletal muscle fibers

Changes in skeletal muscle volume induce localized sarcoplasmic reticulum (SR) Ca(2+) release (LCR) events, which are sustained for many minutes, suggesting a possible signaling role in plasticity or pathology. However, the mechanism by which cell volume influences SR Ca(2+) release is uncertain. In the present study, rat flexor digitorum brevis fibers were superfused with isoosmotic Tyrode's solution before exposure to either hyperosmotic (404 mOsm) or hypoosmotic (254 mOsm) solutions, and the effects on cell volume, membrane potential (E(m)), and intracellular Ca(2+) ([Ca(2+)](i)) were determined. To allow comparison with previous studies, solutions were made hyperosmotic by the addition of sugars or divalent cations, or they were made hypoosmotic by reducing [NaCl](o). All hyperosmotic solutions induced a sustained decrease in cell volume, which was accompanied by membrane depolarization (by 14-18 mV; n = 40) and SR Ca(2+) release. However, sugar solutions caused a global increase in [Ca(2+)](i), whereas solutions made hyperosmotic by the addition of divalent cations only induced LCR. Decreasing osmolarity induced an increase in cell volume and a negative shift in E(m) (by 15.04 +/- 1.85 mV; n = 8), whereas [Ca(2+)](i) was unaffected. However, on return to the isoosmotic solution, restoration of cell volume and E(m) was associated with LCR. Both global and localized SR Ca(2+) release were abolished by the dihydropyridine receptor inhibitor nifedipine by sustained depolarization of the sarcolemmal or by the addition of the ryanodine receptor 1 inhibitor tetracaine. Inhibitors of the Na-K-2Cl (NKCC) cotransporter markedly inhibited the depolarization associated with hyperosmotic shrinkage and the associated SR Ca(2+) release. These findings suggest (1) that the depolarization that accompanies a decrease in cell volume is the primary event leading to SR Ca(2+) release, and (2) that volume-dependent regulation of the NKCC cotransporter contributes to the observed changes in E(m). The differing effects of the osmotic agents can be explained by the screening of fixed charges by divalent ions.