Christopher W. Ward

Angiotensin II type 1 receptor blockade attenuates TGF-β–induced failure of muscle regeneration in multiple myopathic states

Skeletal muscle has the ability to achieve rapid repair in response to injury or disease. Many individuals with Marfan syndrome (MFS), caused by a deficiency of extracellular fibrillin-1, exhibit myopathy and often are unable to increase muscle mass despite physical exercise. Evidence suggests that selected manifestations of MFS reflect excessive signaling by transforming growth factor (TGF)-β (refs. 2,3). TGF-β is a known inhibitor of terminal differentiation of cultured myoblasts; however, the functional contribution of TGF-β signaling to disease pathogenesis in various inherited myopathic states in vivo remains unknown. Here we show that increased TGF-β activity leads to failed muscle regeneration in fibrillin-1–deficient mice. Systemic antagonism of TGF-β through administration of TGF-β–neutralizing antibody or the angiotensin II type 1 receptor blocker losartan normalizes muscle architecture, repair and function in vivo. Moreover, we show TGF-β–induced failure of muscle regeneration and a similar therapeutic response in a dystrophin-deficient mouse model of Duchenne muscular dystrophy. NOTE: In the version of this article initially published, the same panels were inadvertently used to show negative pSmad2/3 and periostin staining in muscle of Fbn1C1039G/+ mice treated with TGF-β‐neutralizing antibody in both the steady-state (Fig. 1a, right column, second and third rows, respectively) and muscle-regeneration (Fig. 1b, right column, third and fourth rows, respectively) experiments. In reality, these images only relate to the steady-state experiment (Fig. 1a). The intended images for Figure 1b are provided (red, pSmad2/3 staining; green, periostin staining). As both sets of images show negative staining in neutralizing antibody–treated Fbn1C1039G/+ mice, this does not alter any observations or conclusions discussed in the manuscript. The error has been corrected in the HTML and PDF versions of the article.

X-ROS Signaling: Rapid Mechano-Chemo Transduction in Heart

Reactive oxygen species produced by NADPH oxidase link mechanical stretching to calcium signaling in mammalian myocytes. We report that in heart cells, physiologic stretch rapidly activates reduced-form nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 (NOX2) to produce reactive oxygen species (ROS) in a process dependent on microtubules (X-ROS signaling). ROS production occurs in the sarcolemmal and t-tubule membranes where NOX2 is located and sensitizes nearby ryanodine receptors (RyRs) in the sarcoplasmic reticulum (SR). This triggers a burst of Ca2+ sparks, the elementary Ca2+ release events in heart. Although this stretch-dependent “tuning” of RyRs increases Ca2+ signaling sensitivity in healthy cardiomyocytes, in disease it enables Ca2+ sparks to trigger arrhythmogenic Ca2+ waves. In the mouse model of Duchenne muscular dystrophy, hyperactive X-ROS signaling contributes to cardiomyopathy through aberrant Ca2+ release from the SR. X-ROS signaling thus provides a mechanistic explanation for the mechanotransduction of Ca2+ release in the heart and offers fresh therapeutic possibilities.

A functional insulin‐like growth factor receptor is not necessary for load‐induced skeletal muscle hypertrophy

Increasing the mechanical load on skeletal muscle results in increased expression of insulin‐like growth factor I (IGF‐I), which is thought to be a critical step in the induction of muscle hypertrophy. To determine the role of the IGF‐I receptor in load‐induced skeletal muscle hypertrophy, we utilized a transgenic mouse model (MKR) that expresses a dominant negative IGF‐I receptor specifically in skeletal muscle. Skeletal muscle hypertrophy was induced in the plantaris muscle using the functional overload (FO) model, a model which has previously been shown to induce significant elevations of IGF‐I expression in skeletal muscle. Adult male wild‐type (WT) and MKR mice were subjected to 0, 7 or 35 days of FO. In control or unchallenged animals, the plantaris mass was 11% greater in WT compared to the MKR mice (P < 0.05). After 7 days of FO, plantaris mass increased significantly by 26% and 62% in WT and MKR mice, respectively (P < 0.05). After 35 days of FO, WT and MKR mice demonstrated significant increases of 100% and 122%, respectively, in plantaris mass (P < 0.05). Further, at no time point was the degree of hypertrophy significantly different between the WT and MKR mice. Previous research suggests that IGF‐I induces muscle growth through activation of the Akt–mTOR signalling pathway; therefore, we measured the phosphorylation status of Akt and p70s6k in the WT and MKR mice after 7 days of FO. Significant increases of ∼100% and ∼200% in Akt (Ser‐473) and p70s6k (Thr‐389) phosphorylation were measured in overloaded plantaris from both WT and MKR mice, respectively. Moreover, no differences were detected between the WT and MKR mice. These data suggest that increased mechanical load can induce muscle hypertrophy and activate the Akt and p70s6k independent of a functioning IGF‐I receptor.

Antagonism of CXCR3 Inhibits Lung Metastasis in a Murine Model of Metastatic Breast Cancer

Tumor cells aberrantly express chemokines and/or chemokine receptors, and some may promote tumor growth and metastasis. We examined the expression and function of chemokine receptor CXCR3 in a syngeneic murine model of metastatic breast cancer. By flow cytometry, CXCR3 was detected in all murine mammary tumor cell lines examined. All human breast cancer cell lines examined also expressed CXCR3, as did the immortalized but nontumorigenic MCF-10A cell line. Interaction of CXCR3 ligands, CXCL9, CXCL10, and CXCL11, with CXCR3 on the highly malignant murine mammary tumor cell line 66.1 resulted in intracellular calcium mobilization and chemotaxis in vitro. To test the hypothesis that tumor metastasis is facilitated by CXCR3 expressed by tumor cells, we employed a small molecular weight antagonist of CXCR3, AMG487. 66.1 tumor cells were pretreated with AMG487 prior to i.v. injection into immune-competent female mice. Antagonism of CXCR3 on 66.1 tumor cells inhibited experimental lung metastasis, and this antimetastatic activity was compromised in mice depleted of natural killer cells. Systemic administration of AMG487 also inhibited experimental lung metastasis. In contrast to the antimetastatic effect of AMG487, local growth of 66.1 mammary tumors was not affected by receptor antagonism. These studies indicate that murine mammary tumor cells express CXCR3 which facilitates the development of lung metastases. These studies also indicate for the first time that a small molecular weight antagonist of CXCR3 has the potential to inhibit tumor metastasis.

Axial Stretch of Rat Single Ventricular Cardiomyocytes Causes an Acute and Transient Increase in Ca2+ Spark Rate

We investigate acute effects of axial stretch, applied by carbon fibers (CFs), on diastolic Ca2+ spark rate in rat isolated cardiomyocytes. CFs were attached either to both cell ends (to maximize the stretched region), or to the center and one end of the cell (to compare responses in stretched and nonstretched half-cells). Sarcomere length was increased by 8.01±0.94% in the stretched cell fraction, and time series of XY confocal images were recorded to monitor diastolic Ca2+ spark frequency and dynamics. Whole-cell stretch causes an acute increase of Ca2+ spark rate (to 130.7±6.4%) within 5 seconds, followed by a return to near background levels (to 104.4±5.1%) within 1 minute of sustained distension. Spark rate increased only in the stretched cell region, without significant differences in spark amplitude, time to peak, and decay time constants of sparks in stretched and nonstretched areas. Block of stretch-activated ion channels (2 &mgr;mol/L GsMTx-4), perfusion with Na+/Ca2+-free solution, and block of nitric oxide synthesis (1 mmol/L L-NAME) all had no effect on the stretch-induced acute increase in Ca2+ spark rate. Conversely, interference with cytoskeletal integrity (2 hours of 10 &mgr;mol/L colchicine) abolished the response. Subsequent electron microscopic tomography confirmed the close approximation of microtubules with the T-tubular–sarcoplasmic reticulum complex (to within ≈10-8m). In conclusion, axial stretch of rat cardiomyocytes acutely and transiently increases sarcoplasmic reticulum Ca2+ spark rate via a mechanism that is independent of sarcolemmal stretch-activated ion channels, nitric oxide synthesis, or availability of extracellular calcium but that requires cytoskeletal integrity. The potential of microtubule-mediated modulation of ryanodine receptor function warrants further investigation.

Losartan restores skeletal muscle remodeling and protects against disuse atrophy in sarcopenia.

Losartan improves muscle remodeling and protects against immobilization atrophy by mediating pathways critical for muscle homeostasis. Losartan Comes of Age The Little Old Lady from Pasadena describes a diminutive woman of advanced years who aggressively drives her Dodge around a southern California city. In popular culture, people link long life spans with being “little”; yet, shortened stature is only one physical change associated with aging. Another, less jocular, transformation is loss of muscle mass and strength—called sarcopenia—which can cause disability and predicts impending death in older adults. Burks et al. now identify losartan, an angiotensin II receptor antagonist commonly used to treat high blood pressure, as a new drug candidate for treating sarcopenia. Although the causes of sarcopenia are poorly understood, transforming growth factor–β (TGF-β) may contribute to faulty repair in aged muscle. Burks et al. used losartan to antagonize TGF-β signaling in an aged mouse model of sarcopenia. Losartan treatment improved muscle remodeling after injury and protected sarcopenic muscle from further loss of muscle mass caused by immobilization; these effects were mediated via two signaling circuits critical for skeletal muscle homeostasis: the TGF-β and insulin-like growth factor 1 (IGF-1)/Akt/mammalian target of rapamycin (mTOR) pathways. These observations suggest that treatment with losartan, a Food and Drug Administration (FDA)–approved drug, may benefit sarcopenia patients and allow little old ladies everywhere to continue their street racing for many years to come. Go granny go. Sarcopenia, a critical loss of muscle mass and function because of the physiological process of aging, contributes to disability and mortality in older adults. It increases the incidence of pathologic fractures, causing prolonged periods of hospitalization and rehabilitation. The molecular mechanisms underlying sarcopenia are poorly understood, but recent evidence suggests that increased transforming growth factor–β (TGF-β) signaling contributes to impaired satellite cell function and muscle repair in aged skeletal muscle. We therefore evaluated whether antagonism of TGF-β signaling via losartan, an angiotensin II receptor antagonist commonly used to treat high blood pressure, had a beneficial impact on the muscle remodeling process of sarcopenic mice. We demonstrated that mice treated with losartan developed significantly less fibrosis and exhibited improved in vivo muscle function after cardiotoxin-induced injury. We found that losartan not only blunted the canonical TGF-β signaling cascade but also modulated the noncanonical TGF-β mitogen-activated protein kinase pathway. We next assessed whether losartan was able to combat disuse atrophy in aged mice that were subjected to hindlimb immobilization. We showed that immobilized mice treated with losartan were protected against loss of muscle mass. Unexpectedly, this protective mechanism was not mediated by TGF-β signaling but was due to an increased activation of the insulin-like growth factor 1 (IGF-1)/Akt/mammalian target of rapamycin (mTOR) pathway. Thus, blockade of the AT1 (angiotensin II type I) receptor improved muscle remodeling and protected against disuse atrophy by differentially regulating the TGF-β and IGF-1/Akt/mTOR signaling cascades, two pathways critical for skeletal muscle homeostasis. Thus, losartan, a Food and Drug Administration–approved drug, may prove to have clinical benefits to combat injury-related muscle remodeling and provide protection against disuse atrophy in humans with sarcopenia.

Microtubules underlie dysfunction in duchenne muscular dystrophy.

Decreasing microtubule density in a mouse model of muscular dystrophy reduces contraction-induced muscle injury. Microtubules and Muscle Dysfunction Duchenne muscular dystrophy (DMD) is an inherited, progressive, and eventually fatal degenerative muscle disorder that is caused by the absence of the microtubule-associated protein dystrophin. Increased Ca2+ influx and enhanced production of reactive oxygen species (ROS) are detected in DMD muscle, although it is not known how these cellular events are connected to the microtubule network and the pathology of DMD. Khairallah et al. used mice that are a model for DMD (mdx mice). They found that a modest stretch of adult mdx muscle, but not of wild-type or young mdx muscle, stimulated the production of ROS by NADPH oxidase 2 (NOX2) through the dense microtubule network as the mechanotransduction element. This pathway, known as X-ROS, triggered Ca2+ influx through stretch-activated channels. Ca2+ influx, X-ROS production, and contraction-induced muscle injury were decreased in adult mdx muscle by treatments that reduced the density of the microtubule network or that inhibited NOX2. X-ROS production was increased in young mdx muscle by treatments that increased microtubule density. Transcriptome analysis revealed increased expression of X-ROS–related genes in human DMD skeletal muscle, suggesting that drugs that reduce microtubule density or block the activity of NOX2 could slow the progression of DMD. Duchenne muscular dystrophy (DMD) is a fatal X-linked degenerative muscle disease caused by the absence of the microtubule-associated protein dystrophin, which results in a disorganized and denser microtubule cytoskeleton. In addition, mechanotransduction-dependent activation of calcium (Ca2+) and reactive oxygen species (ROS) signaling underpins muscle degeneration in DMD. We show that in muscle from adult mdx mice, a model of DMD, a brief physiologic stretch elicited microtubule-dependent activation of NADPH (reduced-form nicotinamide adenine dinucleotide phosphate) oxidase–dependent production of ROS, termed X-ROS. Further, X-ROS amplified Ca2+ influx through stretch-activated channels in mdx muscle. Consistent with the importance of the microtubules to the dysfunction in mdx muscle, muscle cells with dense microtubule structure, such as those from adult mdx mice or from young wild-type mice treated with Taxol, showed increased X-ROS production and Ca2+ influx, whereas cells with a less dense microtubule network, such as young mdx or adult mdx muscle treated with colchicine or nocodazole, showed little ROS production or Ca2+ influx. In vivo treatments that disrupted the microtubule network or inhibited NADPH oxidase 2 reduced contraction-induced injury in adult mdx mice. Furthermore, transcriptome analysis identified increased expression of X-ROS–related genes in human DMD skeletal muscle. Together, these data show that microtubules are the proximate element responsible for the dysfunction in Ca2+ and ROS signaling in DMD and could be effective therapeutic targets for intervention.

Enhanced excitation-coupled calcium entry in myotubes expressing malignant hyperthermia mutation R163C is attenuated by dantrolene.

Dantrolene is the drug of choice for the treatment of malignant hyperthermia (MH) and is also useful for treatment of spasticity or muscle spasms associated with several clinical conditions. The current study examines the mechanisms of dantrolenes action on skeletal muscle and shows that one of dantrolenes mechanisms of action is to block excitation-coupled calcium entry (ECCE) in both adult mouse flexor digitorum brevis fibers and primary myotubes. A second important new finding is that myotubes isolated from mice heterozygous and homozygous for the ryanodine receptor type 1 R163C MH susceptibility mutation show significantly enhanced ECCE rates that could be restored to those measured in wild-type cells after exposure to clinical concentrations of dantrolene. We propose that this gain of ECCE function is an important etiological component of MH susceptibility and possibly contributes to the fulminant MH episode. The inhibitory potency of dantrolene on ECCE found in wild-type and MH-susceptible muscle is consistent with the drugs clinical potency for reversing the MH syndrome and is incomplete as predicted by its efficacy as a muscle relaxant.

In Vivo Restoration of Physiological Levels of Truncated TrkB.T1 Receptor Rescues Neuronal Cell Death in a Trisomic Mouse Model

Imbalances in neurotrophins or their high-affinity Trk receptors have long been reported in neurodegenerative diseases. However, a molecular link between these gene products and neuronal cell death has not been established. In the trisomy 16 (Ts16) mouse there is increased apoptosis in the cortex, and hippocampal neurons undergo accelerated cell death that cannot be rescued by administration of brain-derived neurotrophic factor (BDNF). Ts16 neurons have normal levels of the TrkB tyrosine kinase receptor but an upregulation of the TrkB.T1 truncated receptor isoform. Here we show that restoration of the physiological level of the TrkB.T1 receptor by gene targeting rescues Ts16 cortical cell and hippocampal neuronal death. Moreover, it corrects resting Ca2+ levels and restores BDNF-induced intracellular signaling mediated by full-length TrkB in Ts16 hippocampal neurons. These data provide a direct link between neuronal cell death and abnormalities in Trk neurotrophin receptor levels.

Malformed mdx myofibers have normal cytoskeletal architecture yet altered EC coupling and stress-induced Ca2+ signaling

Skeletal muscle function is dependent on its highly regular structure. In studies of dystrophic (dy/dy) mice, the proportion of malformed myofibers decreases after prolonged whole muscle stimulation, suggesting that the malformed myofibers are more prone to injury. The aim of this study was to assess morphology and to measure excitation-contraction (EC) coupling (Ca(2+) transients) and susceptibility to osmotic stress (Ca(2+) sparks) of enzymatically isolated muscle fibers of the extensor digitorum longus (EDL) and flexor digitorum brevis (FDB) muscles from young (2-3 mo) and old (8-9 mo) mdx and age-matched control mice (C57BL10). In young mdx EDL, 6% of the myofibers had visible malformations (i.e., interfiber splitting, branched ends, midfiber appendages). In contrast, 65% of myofibers in old mdx EDL contained visible malformations. In the mdx FDB, malformation occurred in only 5% of young myofibers and 11% of old myofibers. Age-matched control mice did not display the altered morphology of mdx muscles. The membrane-associated and cytoplasmic cytoskeletal structures appeared normal in the malformed mdx myofibers. In mdx FDBs with significantly branched ends, an assessment of global, electrically evoked Ca(2+) signals (indo-1PE-AM) revealed an EC coupling deficit in myofibers with significant branching. Interestingly, peak amplitude of electrically evoked Ca(2+) release in the branch of the bifurcated mdx myofiber was significantly decreased compared with the trunk of the same myofiber. No alteration in the basal myoplasmic Ca(2+) concentration (i.e., indo ratio) was seen in malformed vs. normal mdx myofibers. Finally, osmotic stress induced the occurrence of Ca(2+) sparks to a greater extent in the malformed portions of myofibers, which is consistent with deficits in EC coupling control. In summary, our data show that aging mdx myofibers develop morphological malformations. These malformations are not associated with gross disruptions in cytoskeletal or t-tubule structure; however, alterations in myofiber Ca(2+) signaling are evident.