Duality of G protein-coupled mechanisms for beta-adrenergic activation of NKCC activity in skeletal muscle

The Role of Ion-Transporting Proteins on Crosstalk Between the Skeletal Muscle and Central Nervous Systems Elicited by Physical Exercise

A paradigm shift in the understanding of bidirectional interactions between peripheral and central nervous systems is essential for development of rehabilitation and preventive interventions based on physical exercise. Although a causal relationship has not been completely established, modulation of voltage-dependent ion channels (Ca2+, Cl-, K+, Na+, lactate-, H+) in skeletal and neuronal cells provides opportunities to maintain force production during exercise and reduce the risk of disease. However, there are caveats to consider when interpreting the effects of physical exercise on this bidirectional axis, since exercise protocol details (e.g., duration and intensity) have variable effects on this crosstalk. Therefore, an integrative perspective of the skeletal muscle and brain's communication pathway is discussed, and the role of physical exercise on such communication highway is explained in this review.

Gαi2 regulates the adult myogenesis of masticatory muscle satellite cells

Although similar to trunk and limb skeletal muscles, masticatory muscles are believed as unique in both developmental origins and myogenesis. Gαi2 has been demonstrated to promote muscle hypertrophy and muscle satellite cell differentiation in limb muscles. However, the effect of Gαi2 on masticatory muscles is still unexplored. This study aimed to identify the role of Gαi2 in the proliferation and differentiation of masticatory muscle satellite cells, further exploring the metabolic mechanism of masticatory muscles. The proliferation rate, myotube size, fusion index of masticatory muscle satellite cells and Pax7, Myf5, MyoD, Tcf21 and Musculin expressions were significantly decreased by Gαi2 knockdown, while in cells infected with AdV4-Gαi2, the proliferation rate, myotube size, fusion index and Tbx1 expression were significantly increased. Masticatory muscle satellite cells also displayed phenotype transformation as Gαi2 changed. In addition, Gαi2 altered myosin heavy chain (MyHC) isoforms of myotubes with less MyHC-2A expression in siGαi2 group and more MyHC-slow expression in AdV4-Gαi2 group. In conclusion, Gαi2 could positively affect the adult myogenesis of masticatory muscle satellite cells and maintain the superiority of MyHC-slow. Masticatory muscle satellite cells may have their unique Gαi2-regulated myogenic transcriptional networks, although they may share some common characteristics with trunk and limb muscles.

A novel loss-of-function mutation of the voltage-gated potassium channel Kv10.2 involved in epilepsy and autism

Background

Novel developmental mutations associated with disease are a continuous challenge in medicine. Clinical consequences caused by these mutations include neuron and cognitive alterations that can lead to epilepsy or autism spectrum disorders. Often, it is difficult to identify the physiological defects and the appropriate treatments.

Results

We have isolated and cultured primary cells from the skin of a patient with combined epilepsy and autism syndrome. A mutation in the potassium channel protein Kv10.2 was identified. We have characterised the alteration of the mutant channel and found that it causes loss of function (LOF). Primary cells from the skin displayed a very striking growth defect and increased differentiation. In vitro treatment with various carbonic anhydrase inhibitors with various degrees of specificity for potassium channels, (Brinzolamide, Acetazolamide, Retigabine) restored the activation capacity of the mutated channel. Interestingly, the drugs also recovered in vitro the expansion capacity of the mutated skin cells. Furthermore, treatment with Acetazolamide clearly improved the patient regarding epilepsy and cognitive skills. When the treatment was temporarily halted the syndrome worsened again.

Conclusions

By in vitro studying primary cells from the patient and the activation capacity of the mutated protein, we could first, find a readout for the cellular defects and second, test pharmaceutical treatments that proved to be beneficial. The results show the involvement of a novel LOF mutation of a Potassium channel in autism syndrome with epilepsy and the great potential of in vitro cultures of primary cells in personalised medicine of rare diseases.

A century of exercise physiology: key concepts in muscle cell volume regulation

Skeletal muscle cells can both gain and lose volume during periods of exercise and rest. Muscle cells do not behave as perfect osmometers because the cell volume changes are less than predicted from the change in extracellular osmolality. Therefore, there are mechanisms involved in regulating cell volume, and they are different for regulatory volume decreases and regulatory volume increases. Also, after an initial rapid change in cell volume, there is a gradual and partial recovery of cell volume that is effected by ion and water transport mechanisms. The mechanisms have been studied in non-contracting muscle cells, but remain to be fully elucidated in contracting muscle. Changes in muscle cell volume are known to affect the strength of contractile activity as well as anabolic/catabolic signaling, perhaps indicating that cell volume should be a regulated variable in skeletal muscle cells. Muscles contracting at moderate to high intensity gain intracellular volume because of increased intracellular osmolality. Concurrent increases in interstitial (extracellular) muscle volume occur from an increase in osmotically active molecules and increased vascular filtration pressure. At the same time, non-contracting muscles lose cell volume because of increased extracellular (blood) osmolality. This review provides the physiological foundations and highlights key concepts that underpin our current understanding of volume regulatory processes in skeletal muscle, beginning with consideration of osmosis more than 200 years ago and continuing through to the process of regulatory volume decrease and regulatory volume increase.

Beyond Homeostasis: Potassium and Pathogenesis during Bacterial Infections

Potassium is an essential mineral nutrient required by all living cells for normal physiological function. Therefore, maintaining intracellular potassium homeostasis during bacterial infection is a requirement for the survival of both host and pathogen. However, pathogenic bacteria require potassium transport to fulfill nutritional and chemiosmotic requirements, and potassium has been shown to directly modulate virulence gene expression, antimicrobial resistance, and biofilm formation. Host cells also require potassium to maintain fundamental biological processes, such as renal function, muscle contraction, and neuronal transmission; however, potassium flux also contributes to critical immunological and antimicrobial processes, such as cytokine production and inflammasome activation. Here, we review the role and regulation of potassium transport and signaling during infection in both mammalian and bacterial cells and highlight the importance of potassium to the success and survival of each organism.

Na+ -K+ -2Cl- Cotransporter (NKCC) Physiological Function in Nonpolarized Cells and Transporting Epithelia

Two genes encode the Na+ -K+ -2Cl- cotransporters, NKCC1 and NKCC2, that mediate the tightly coupled movement of 1Na+ , 1K+ , and 2Cl- across the plasma membrane of cells. Na+ -K+ -2Cl- cotransport is driven by the chemical gradient of the three ionic species across the membrane, two of them maintained by the action of the Na+ /K+ pump. In many cells, NKCC1 accumulates Cl- above its electrochemical potential equilibrium, thereby facilitating Cl- channel-mediated membrane depolarization. In smooth muscle cells, this depolarization facilitates the opening of voltage-sensitive Ca2+ channels, leading to Ca2+ influx, and cell contraction. In immature neurons, the depolarization due to a GABA-mediated Cl- conductance produces an excitatory rather than inhibitory response. In many cell types that have lost water, NKCC is activated to help the cells recover their volume. This is specially the case if the cells have also lost Cl- . In combination with the Na+ /K+ pump, the NKCC's move ions across various specialized epithelia. NKCC1 is involved in Cl- -driven fluid secretion in many exocrine glands, such as sweat, lacrimal, salivary, stomach, pancreas, and intestine. NKCC1 is also involved in K+ -driven fluid secretion in inner ear, and possibly in Na+ -driven fluid secretion in choroid plexus. In the thick ascending limb of Henle, NKCC2 activity in combination with the Na+ /K+ pump participates in reabsorbing 30% of the glomerular-filtered Na+ . Overall, many critical physiological functions are maintained by the activity of the two Na+ -K+ -2Cl- cotransporters. In this overview article, we focus on the functional roles of the cotransporters in nonpolarized cells and in epithelia. © 2018 American Physiological Society. Compr Physiol 8:871-901, 2018.

Role of cyclic AMP sensor Epac1 in masseter muscle hypertrophy and myosin heavy chain transition induced by β2-adrenoceptor stimulation

The predominant isoform of β-adrenoceptor (β-AR) in skeletal muscle is β2-AR and that in the cardiac muscle is β1-AR. We have reported that Epac1 (exchange protein directly activated by cAMP 1), a new protein kinase A-independent cAMP sensor, does not affect cardiac hypertrophy in response to pressure overload or chronic isoproterenol (isoprenaline) infusion. However, the role of Epac1 in skeletal muscle hypertrophy remains poorly understood. We thus examined the effect of disruption of Epac1, the major Epac isoform in skeletal muscle, on masseter muscle hypertrophy induced by chronic β2-AR stimulation with clenbuterol (CB) in Epac1-null mice (Epac1KO). The masseter muscle weight/tibial length ratio was similar in wild-type (WT) and Epac1KO at baseline and was significantly increased in WT after CB infusion, but this increase was suppressed in Epac1KO. CB treatment significantly increased the proportion of myosin heavy chain (MHC) IIb at the expense of that of MHC IId/x in both WT and Epac1KO, indicating that Epac1 did not mediate the CB-induced MHC isoform transition towards the faster isoform. The mechanism of suppression of CB-mediated hypertrophy in Epac1KO is considered to involve decreased activation of Akt signalling. In addition, CB-induced histone deacetylase 4 (HDAC4) phosphorylation on serine 246 mediated by calmodulin kinase II (CaMKII), which plays a role in skeletal muscle hypertrophy, was suppressed in Epac1KO. Our findings suggest that Epac1 plays a role in β2-AR-mediated masseter muscle hypertrophy, probably through activation of both Akt signalling and CaMKII/HDAC4 signalling.

Temporal changes in ERK phosphorylation are harmonious with 4E-BP1, but not p70S6K, during clenbuterol-induced hypertrophy in the rat gastrocnemius

Extracellular signal-regulated kinase (ERK) is required for clenbuterol (CB)-dependent fast-type myofibril enlargement; however, its contribution to translation control is unclear. ERK mediates translational regulation through mammalian target of rapamycin complex 1 (mTORC1) activation and (or) mTORC1-independent pathways. In this study, we aimed to investigate the role of ERK in translational control during CB-induced muscular hypertrophy by measuring time-dependent changes in the phosphorylation statuses of ERK, p70 ribosomal S6 kinase (p70S6K; an indicator of mTORC1 activity), 4E-binding protein 1 (4E-BP1), eukaryotic elongation factor 2 (eEF2), and other related signaling molecules in rat gastrocnemius muscles. Five-day administration of CB induced phenotypes associated with muscular hypertrophy (significant increases in wet weight and isometric ankle flexion torque in the gastrocnemius muscle), but was not accompanied by elevated ERK or p70S6K phosphorylation. One-day administration of CB caused significant increases in the phosphorylation of ERK, p70S6K, and 4E-BP1. In contrast, 3-day administration of CB caused significant increases in the phosphorylation of ERK and 4E-BP1, but not p70S6K. In addition, positive correlations were observed between ERK and 4E-BP1 on days 1 and 3, whereas a correlation between ERK and p70S6K was only observed on day 1. eEF2 phosphorylation was unchanged on both days 1 and 3. These findings suggest that ERK accelerates the initiation of translation, but does not support the involvement of ERK in translational elongation. Furthermore, ERK may play a major role in promoting translational initiation by mediating the phosphorylation of 4E-BP1, and may contribute to the initial activation of mTORC1 during CB administration.

β2-Adrenergic agonists and the treatment of skeletal muscle wasting disorders

β2-Agonists are traditionally used for the treatment of bronchospasm associated with asthma and the treatment of symptomatic patients with COPD. However, β2-agonists are also powerful anabolic agents that trigger skeletal muscle hypertrophy. Investigating the effects of β2-agonists in skeletal muscle over the past 30 years in different animal models has led to the identification of potential therapeutic applications in several muscle wasting disorders, including neuromuscular diseases, cancer cachexia, sepsis or thermal injury. In these conditions, numerous studies indicate that β2-agonists can attenuate and/or reverse the decrease in skeletal muscle mass and associated weakness in animal models of muscle wasting but also in human patients. The purpose of this review is to present the biological and clinical significance of β2-agonists for the treatment of skeletal muscle wasting. After the description of the molecular mechanisms involved in the hypertrophy and anti-atrophy effect of β2-agonists, we will review the anti-atrophy effects of β2-agonist administration in several animal models and human pathologies associated with or leading to skeletal muscle wasting. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

cAMP signaling in skeletal muscle adaptation: hypertrophy, metabolism, and regeneration

Among organ systems, skeletal muscle is perhaps the most structurally specialized. The remarkable subcellular architecture of this tissue allows it to empower movement with instructions from motor neurons. Despite this high degree of specialization, skeletal muscle also has intrinsic signaling mechanisms that allow adaptation to long-term changes in demand and regeneration after acute damage. The second messenger adenosine 3',5'-monophosphate (cAMP) not only elicits acute changes within myofibers during exercise but also contributes to myofiber size and metabolic phenotype in the long term. Strikingly, sustained activation of cAMP signaling leads to pronounced hypertrophic responses in skeletal myofibers through largely elusive molecular mechanisms. These pathways can promote hypertrophy and combat atrophy in animal models of disorders including muscular dystrophy, age-related atrophy, denervation injury, disuse atrophy, cancer cachexia, and sepsis. cAMP also participates in muscle development and regeneration mediated by muscle precursor cells; thus, downstream signaling pathways may potentially be harnessed to promote muscle regeneration in patients with acute damage or muscular dystrophy. In this review, we summarize studies implicating cAMP signaling in skeletal muscle adaptation. We also highlight ligands that induce cAMP signaling and downstream effectors that are promising pharmacological targets.

Contribution of the extracellular cAMP-adenosine pathway to dual coupling of β2-adrenoceptors to Gs and Gi proteins in mouse skeletal muscle

β(2)-Adrenoceptor (β(2)-AR) agonists increase skeletal muscle contractile force via activation of G(s) protein/adenylyl cyclases (AC) and increased generation of cAMP. Herein, we evaluated the possible dual coupling of β(2)-AR to G(s) and G(i) proteins and the influence of the β(2)-AR/G(s)-G(i)/cAMP signaling cascade on skeletal muscle contraction. Assuming that the increment of intracellular cAMP is followed by cAMP efflux and extracellular generation of adenosine, the contribution of the extracellular cAMP-adenosine pathway on the β(2)-AR inotropic response was also addressed. The effects of clenbuterol/fenoterol (β(2)-AR agonists), forskolin (AC activator), cAMP/8-bromo-cAMP, and adenosine were evaluated on isometric contractility of mouse diaphragm muscle induced by supramaximal direct electrical stimulation (0.1 Hz, 2 ms duration). Clenbuterol/fenoterol (10-1000 μM), 1 μM forskolin, and 20 μM rolipram induced transient positive inotropic effects that peaked 30 min after stimulation onset, declining to 10 to 20% of peak levels in 30 min. The late descending phase of the β(2)-AR agonist inotropic effect was mimicked by either cAMP or adenosine and abolished by preincubation of diaphragm with pertussis toxin (PTX) (G(i) signaling inhibitor) or the organic anion transporter inhibitor probenecid, indicating a delayed coupling of β(2)-AR to G(i) protein which depends on cAMP efflux. Remarkably, the PTX-sensitive β(2)-AR inotropic effect was inhibited by the A(1) adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine and ecto-5'-phosphodiesterase inhibitor α,β-methyleneadenosine 5'-diphosphate sodium salt, indicating that β(2)-AR coupling to G(i) is indirect and dependent on A(1) receptor activation. The involvement of the extracellular cAMP-adenosine pathway in β(2)-AR signaling would provide a negative feedback loop that may limit stimulatory G protein-coupled receptor positive inotropism and potential deleterious effects of excessive contractile response.

Expression of protease-activated receptor (PAR)-2, but not other PARs, is regulated by inflammatory cytokines in rat astrocytes

Protease-activated receptors (PARs) are widely expressed in the central nervous system (CNS) and are believed to play an important role in normal brain functioning as well as in development of various inflammatory and neurodegenerative disorders. Pathological conditions cause altered expression of PARs in brain cells and therefore altered responsiveness to PAR activation. The exact mechanisms of regulation of PAR expression are not well studied. Here, we evaluated in rat astrocytes the influence of LPS, pro-inflammatory cytokines TNFα and IL-1β and continuous PAR activation by PAR agonists on the expression levels of PARs. These stimuli are important in inflammatory and neurological disorders, where their levels are increased. We report that LPS as well as cytokines TNFα and IL-1β affected only the PAR-2 level, but their effects were opposite. LPS and TNFα increased the functional expression of PAR-2, whereas IL-1β down-regulated the functional response of PAR-2. Agonists of PAR-1 specifically increased mRNA level of PAR-2, but not protein level. Transcript levels of other PARs were not changed after PAR-1 activation. Stimulation of the cells with PAR-2 or PAR-4 agonists did not alter PAR levels. We found that up-regulation of PAR-2 is dependent on PKC activity, mostly via its Ca²⁺-sensitive isoforms. Two transcription factors, NFκB and AP-1, are involved in up-regulation of PAR-2. These findings provide new information about the regulation of expression of PAR subtypes in brain cells. This is of importance for targeting PARs, especially PAR-2, for the treatment of CNS disorders.

The β2-adrenoceptor agonist formoterol improves structural and functional regenerative capacity of skeletal muscles from aged rat at the early stages of postinjury

Skeletal muscles from old rats fail to completely regenerate following injury. This study investigated whether pharmacological stimulation of β2-adrenoceptors in aged muscles following injury could improve their regenerative capacity, focusing on myofiber size recovery. Young and aged rats were treated with a subcutaneous injection of β2-adrenergic agonist formoterol (2 μg/kg/d) up to 10 and 21 days after soleus muscle injury. Formoterol-treated muscles from old rats evaluated at 10 and 21 days postinjury showed reduced inflammation and connective tissue but a similar number of regenerating myofibers of greater caliber when compared with their injured controls. Formoterol minimized the decrease in tetanic force and increased protein synthesis and mammalian target of rapamycin phosphorylation in old muscles at 10 days postinjury. Our results suggest that formoterol improves structural and functional regenerative capacity of regenerating skeletal muscles from aged rats by increasing protein synthesis via mammalian target of rapamycin activation. Furthermore, formoterol may have therapeutic benefits in recovery following muscle damage in senescent individuals.

Muscle plasticity and β₂-adrenergic receptors: adaptive responses of β₂-adrenergic receptor expression to muscle hypertrophy and atrophy

We discuss the functional roles of β₂-adrenergic receptors in skeletal muscle hypertrophy and atrophy as well as the adaptive responses of β₂-adrenergic receptor expression to anabolic and catabolic conditions. β₂-Adrenergic receptor stimulation using anabolic drugs increases muscle mass by promoting muscle protein synthesis and/or attenuating protein degradation. These effects are prevented by the downregulation of the receptor. Endurance training improves oxidative performance partly by increasing β₂-adrenergic receptor density in exercise-recruited slow-twitch muscles. However, excessive stimulation of β₂-adrenergic receptors negates their beneficial effects. Although the preventive effects of β₂-adrenergic receptor stimulation on atrophy induced by muscle disuse and catabolic hormones or drugs are observed, these catabolic conditions decrease β₂-adrenergic receptor expression in slow-twitch muscles. These findings present evidence against the use of β₂-adrenergic agonists in therapy for muscle wasting and weakness. Thus, β₂-adrenergic receptors in the skeletal muscles play an important physiological role in the regulation of protein and energy balance.

Signaling pathways of isoproterenol-induced ERK1/2 phosphorylation in primary cultures of astrocytes are concentration-dependent

Stimulation of β-adrenoceptors activates the canonical adenylate cyclase pathway (via G(s) protein) but can also evoke phosphorylation of extracellular-regulated kinases 1 and 2 (ERK(1/2) ) via G(s)/G(i) switching or β-arrestin-mediated recruitment of Src. In primary cultures of mouse astrocytes, activation of the former of these pathways required micromolar concentrations of the β(1)/β(2) -adrenergic agonist isoproterenol, that acted on β(1)-adrenoceptors, whereas the latter was activated already by nanomolar concentrations, acting on β(2) receptors. Protein kinase A activity was required for G(s)/G(i) switching, which was followed by Ca(2+) release from intracellular stores and G(iα)- and metalloproteinase-dependent transactivation of the epidermal growth factor receptor (EGFR; at its Y1173 phophorylation site), via its receptor-tyrosine kinase, β-arrestin 1/2 recruitment, and MAPK/ERK kinase-dependent ERK(1/2) phosphorylation. ERK(1/2) phosphorylation by Src activation depended on β-arrestin 2, but not β-arrestin 1, was accompanied by Src/EGFR co-precipitation and phosphorylation of the EGFR at the Src-phosphorylated Y845 site and the Y1045 autophosphorylation site; it was independent of transactivation but dependent on MAPK/ERK kinase activity, suggesting EGFR phosphorylation independently of the receptor-tyrosine kinase or activation of Ras or Raf directly from Src. Most astrocytic consequences of activating either pathway (or both) are unknown, but morphological differentiation and increase in glial fibrillary acidic protein in response to dibutyryl cAMP-mediated increase in cAMP depend on G(s)/G(i) switching and transactivation.

Potassium-transporting proteins in skeletal muscle: cellular location and fibre-type differences

Abstract Potassium (K(+)) displacement in skeletal muscle may be an important factor in the development of muscle fatigue during intense exercise. It has been shown in vitro that an increase in the extracellular K(+) concentration ([K(+)](e)) to values higher than approx. 10 mm significantly reduce force development in unfatigued skeletal muscle. Several in vivo studies have shown that [K(+)](e) increases progressively with increasing work intensity, reaching values higher than 10 mm. This increase in [K(+)](e) is expected to be even higher in the transverse (T)-tubules than the concentration reached in the interstitium. Besides the voltage-sensitive K(+) (K(v)) channels that generate the action potential (AP) it is suggested that the big-conductance Ca(2+)-dependent K(+) (K(Ca)1.1) channel contributes significantly to the K(+) release into the T-tubules. Also the ATP-dependent K(+) (K(ATP)) channel participates, but is suggested primarily to participate in K(+) release to the interstitium. Because there is restricted diffusion of K(+) to the interstitium, K(+) released to the T-tubules during AP propagation will be removed primarily by reuptake mediated by transport proteins located in the T-tubule membrane. The most important protein that mediates K(+) reuptake in the T-tubules is the Na(+),K(+)-ATPase alpha(2) dimers, but a significant contribution of the strong inward rectifier K(+) (Kir2.1) channel is also suggested. The Na(+), K(+), 2Cl(-) 1 (NKCC1) cotransporter also participates in K(+) reuptake but probably mainly from the interstitium. The relative content of the different K(+)-transporting proteins differs in oxidative and glycolytic muscles, and might explain the different [K(+)](e) tolerance observed.

Downregulation of protease-activated receptor-1 in human lung fibroblasts is specifically mediated by the prostaglandin E receptor EP2 through cAMP elevation and protein kinase A

Many cellular functions of lung fibroblasts are controlled by protease-activated receptors (PARs). In fibrotic diseases, PAR-1 plays a major role in controlling fibroproliferative and inflammatory responses. Therefore, in these diseases, regulation of PAR-1 expression plays an important role. Using the selective prostaglandin EP2 receptor agonist butaprost and cAMP-elevating agents, we show here that prostaglandin (PG)E(2), via the prostanoid receptor EP2 and subsequent cAMP elevation, downregulates mRNA and protein levels of PAR-1 in human lung fibroblasts. Under these conditions, the functional response of PAR-1 in fibroblasts is reduced. These effects are specific for PGE(2). Activation of other receptors coupled to cAMP elevation, such as beta-adrenergic and adenosine receptors, does not reproduce the effects of PGE(2). PGE(2)-mediated downregulation of PAR-1 depends mainly on protein kinase A activity, but does not depend on another cAMP effector, the exchange protein activated by cAMP. PGE(2)-induced reduction of PAR-1 level is not due to a decrease of PAR-1 mRNA stability, but rather to transcriptional regulation. The present results provide further insights into the therapeutic potential of PGE(2) to specifically control fibroblast function in fibrotic diseases.

Role of beta-adrenoceptor signaling in skeletal muscle: implications for muscle wasting and disease

The importance of beta-adrenergic signaling in the heart has been well documented, but it is only more recently that we have begun to understand the importance of this signaling pathway in skeletal muscle. There is considerable evidence regarding the stimulation of the beta-adrenergic system with beta-adrenoceptor agonists (beta-agonists). Although traditionally used for treating bronchospasm, it became apparent that some beta-agonists could increase skeletal muscle mass and decrease body fat. These so-called "repartitioning effects" proved desirable for the livestock industry trying to improve feed efficiency and meat quality. Studying beta-agonist effects on skeletal muscle has identified potential therapeutic applications for muscle wasting conditions such as sarcopenia, cancer cachexia, denervation, and neuromuscular diseases, aiming to attenuate (or potentially reverse) the muscle wasting and associated muscle weakness, and to enhance muscle growth and repair after injury. Some undesirable cardiovascular side effects of beta-agonists have so far limited their therapeutic potential. This review describes the physiological significance of beta-adrenergic signaling in skeletal muscle and examines the effects of beta-agonists on skeletal muscle structure and function. In addition, we examine the proposed beneficial effects of beta-agonist administration on skeletal muscle along with some of the less desirable cardiovascular effects. Understanding beta-adrenergic signaling in skeletal muscle is important for identifying new therapeutic targets and identifying novel approaches to attenuate the muscle wasting concomitant with many diseases.

CC family chemokines directly regulate myoblast responses to skeletal muscle injury

Chemokines have been implicated in the promotion of leucocyte trafficking to diseased muscle. The purpose of this study was to determine whether a subset of inflammatory chemokines are able to directly drive myoblast proliferation, an essential early component of muscle regeneration, in a manner which is entirely independent of leucocytes. Cultured myoblasts (C2C12) were exposed to monocyte chemoattractant protein-1 (MCP-1; CCL2), macrophage inflammatory protein-1alpha (MIP-1alpha; CCL3) or MIP-1beta (CCL4). All chemokines induced phosphorylation of extracellular signal-regulated kinase (ERK)1/2 mitogen-activated protein kinase (MAPK) and greatly increased myoblast proliferative responses. Chemokine-induced myoblast proliferation was abolished by pertussis toxin and the MEK1/2 inhibitor U0126, implicating both Galphai-coupled receptors and ERK1/2-dependent signalling. Myoblasts expressed receptors for all of the chemokines tested, and mitogenic responses were specifically inhibited by antibodies directed against CC family chemokine receptors 2 and 5 (CCR2 and CCR5). Within an in vitro myogenic wound healing assay devoid of leucocytes, all chemokines significantly accelerated the time course of myoblast wound closure after mechanical injury. Injections of MCP-1 into cardiotoxin-injured skeletal muscles in vivo also suppressed expression of the differentiation marker myogenin, consistent with a mitogenic effect. Taken together, our results indicate that CC chemokines have potent and direct effects on myoblast behaviour, thus indicating a novel role in muscle repair beyond leucocyte chemoattraction. Therefore, interventions aimed at modulating the balance between myoblast and leucocyte effects of CC chemokines in injured muscle could represent a novel strategy for the treatment of destructive muscle pathologies.

Attenuation of age-related muscle wasting and weakness in rats after formoterol treatment: therapeutic implications for sarcopenia

We investigated the potential of the beta(2)-adrenoceptor agonist formoterol to increase mass and force-producing capacity of extensor digitorum longus (EDL) and soleus muscles from young, adult, and old rats. In addition, we examined the result of formoterol withdrawal. Young (3 month), adult (16 month), and old (27 month) F344 rats were treated with either formoterol (25 microg/kg/day, i.p.) or saline vehicle for 4 weeks. Another group of rats (for each age) was similarly treated with formoterol, followed by a withdrawal period of 4 weeks. Formoterol treatment increased EDL muscle mass and the force-producing capacity of both EDL and soleus muscles, without a concomitant increase in heart mass in adult and old rats. The hypertrophy and increased force-producing capacity of EDL muscles persisted 4 weeks after withdrawal of treatment. The findings have major implications for potential clinical trials utilizing beta(2)-agonists for sarcopenia.

The role of volume-sensitive ion transport systems in regulation of epithelial transport

This review focuses on using the knowledge on volume-sensitive transport systems in Ehrlich ascites tumour cells and NIH-3T3 cells to elucidate osmotic regulation of salt transport in epithelia. Using the intestine of the European eel (Anguilla anguilla) (an absorptive epithelium of the type described in the renal cortex thick ascending limb (cTAL)) we have focused on the role of swelling-activated K+- and anion-conductive pathways in response to hypotonicity, and on the role of the apical (luminal) Na+-K+-2Cl- cotransporter (NKCC2) in the response to hypertonicity. The shrinkage-induced activation of NKCC2 involves an interaction between the cytoskeleton and protein phosphorylation events via PKC and myosin light chain kinase (MLCK). Killifish (Fundulus heteroclitus) opercular epithelium is a Cl(-)-secreting epithelium of the type described in exocrine glands, having a CFTR channel on the apical side and the Na+/K+ ATPase, NKCC1 and a K+ channel on the basolateral side. Osmotic control of Cl- secretion across the operculum epithelium includes: (i) hyperosmotic shrinkage activation of NKCC1 via PKC, MLCK, p38, OSR1 and SPAK; (ii) deactivation of NKCC by hypotonic cell swelling and a protein phosphatase, and (iii) a protein tyrosine kinase acting on the focal adhesion kinase (FAK) to set levels of NKCC activity.

Therapeutic approaches for muscle wasting disorders

Muscle wasting and weakness are common in many disease states and conditions including aging, cancer cachexia, sepsis, denervation, disuse, inactivity, burns, HIV-acquired immunodeficiency syndrome (AIDS), chronic kidney or heart failure, unloading/microgravity, and muscular dystrophies. Although the maintenance of muscle mass is generally regarded as a simple balance between protein synthesis and protein degradation, these mechanisms are not strictly independent, but in fact they are coordinated by a number of different and sometimes complementary signaling pathways. Clearer details are now emerging about these different molecular pathways and the extent to which these pathways contribute to the etiology of various muscle wasting disorders. Therapeutic strategies for attenuating muscle wasting and improving muscle function vary in efficacy. Exercise and nutritional interventions have merit for slowing the rate of muscle atrophy in some muscle wasting conditions, but in most cases they cannot halt or reverse the wasting process. Hormonal and/or other drug strategies that can target key steps in the molecular pathways that regulate protein synthesis and protein degradation are needed. This review describes the signaling pathways that maintain muscle mass and provides an overview of some of the major conditions where muscle wasting and weakness are indicated. The review provides details on some therapeutic strategies that could potentially attenuate muscle atrophy, promote muscle growth, and ultimately improve muscle function. The emphasis is on therapies that can increase muscle mass and improve functional outcomes that will ultimately lead to improvement in the quality of life for affected patients.

Histamine receptors in mammalian retinas

Mammalian retinas are innervated by histaminergic axons that originate from perikarya in the posterior hypothalamus. To identify the targets of these retinopetal axons, we localized histamine receptors (HR) in monkey and rat retinas by light and electron microscopy. In monkeys, puncta containing HR3 were found at the tips of ON-bipolar cell dendrites in cone pedicles and rod spherules, closer to the photoreceptors than the other neurotransmitter receptors. This is the first ultrastructural localization of any histamine receptor and the first direct evidence that HR3 is present on postsynaptic membranes in the central nervous system. In rat retinas, most HR1 were localized to dopaminergic amacrine cells. The differences in histamine receptor localization may reflect the differences in the activity patterns of the two species.

SPAK and OSR1, key kinases involved in the regulation of chloride transport

Reversible phosphorylation by protein kinases is probably one of the most important examples of post-translational modification of ion transport proteins. Ste20-related proline alanine-rich kinase (SPAK) and oxidative stress response kinase (OSR1) are two serine/threonine kinases belonging to the germinal centre-like kinase subfamily VI. Genetic analysis suggests that OSR1 evolved first, with SPAK arising following a gene duplication in vertebrate evolution. SPAK and OSR1 are two recently discovered kinases which have been linked to several key cellular processes, including cell differentiation, cell transformation and proliferation, cytoskeleton rearrangement, and most recently, regulation of ion transporters. Na-K-2Cl cotransporter activity is regulated by phosphorylation. Pharmacological evidence has identified several kinases and phosphatases which alter cotransporter function, however, no direct linkage between these enzymes and the cotransporter has been demonstrated. This article will review some of the physical and physiological properties of SPAK and OSR1, and present new evidence of a direct interaction between the Na-K-Cl cotransporter and the stress kinases.

Peptide and non-peptide G-protein coupled receptors (GPCRs) in skeletal muscle

G-protein coupled receptors (GPCRs) represent a large class of cell surface receptors that mediate a multitude of functions. Over the years, a number of GPCRs and ancillary proteins have been shown to be expressed in skeletal muscle. Unlike the case with other muscle tissues like cardiac and vascular smooth muscle cells, there has been little attempt at systematically analyzing GPCRs in skeletal muscle. Here we have compiled all the GPCRs that are expressed in skeletal muscle. In addition, we review the known function of these receptors in both skeletal muscle tissue and in cultured skeletal muscle cells.

Activation of BK channels in GH3 cells by a c-PLA2-dependent G-protein signaling pathway

BK-channels in GH3 cells are activated by arachidonic acid produced by c-PLA2. beta-adrenergic agonists also activate BK channels and were presumed to do so via production of cAMP. We, however, show for the first time in GH3 cells that a beta-adrenergic agonist activates a pertussis-toxin-sensitive G protein that activates c-PLA2. The arachidonic acid produced by c-PLA2 then activates BK channels. We examined BK channels in cell-attached patches and in excised patches from untreated GH3 cells and from GH3 cells exposed to c-PLA2 antisense oligonucleotides. For the cell-attached patch experiments, physiologic pipette and bath solutions were used. For the excised patches, 150 mM KCl was used in both the pipette and bath solutions, and the cytosolic surface contained 1 microM free Ca2+ (buffered with 5 mM K2EGTA). Treatment of GH3 cells with the G protein activator, fluoroaluminate, (AlF4-) produced an increase in the Po of BK channels of 177 +/- 41% (mean +/- SD) in cell-attached patches. Because G proteins are membrane associated, we also added an activator of G proteins, 100 microM GTP-gamma-S, to the cytosolic surface of excised patches. This treatment leads to an increase in Po of 50 +/- 9%. Similar treatment of excised patches with GDP-beta-S had no effect on Po. Isoproterenol (1 microM), an activator of beta-adrenergic receptors and, consequently, some G proteins, increased BK channel activity 229 +/- 37% in cell-attached patches from cultured GH3 cells. Western blot analysis showed that GH3 cells have beta-adrenergic receptor protein and that isoproterenol acts through these receptors because the beta-adrenergic receptor antagonist, propanolol, blocks the action of isoproterenol. To test whether G protein activation of BK channels involves c-PLA2, we studied the effects of GTP-gamma-S on excised patches and isoproterenol on cell attached patches from GH3 cells previously treated with c-PLA2 antisense oligonucleotides or pharmacological inhibitors of c-PLA2. Neither isoproterenol nor GTP-gamma-S had any effect on Po in these patches. Similarly, neither isoproterenol nor GTP-gamma-S had any effect on Po in cultured GH3 cells pretreated with pertussis toxin. Isoproterenol also significantly increased the rate of arachidonic production in GH3 cells. These results show that some receptor-linked, pertussis-toxin-sensitive G protein in GH3 cells can activate c-PLA2 to increase the amount of arachidonic acid present and ultimately increase BK-channel activity.

Laminin-alpha1 globular domains 3 and 4 induce heterotrimeric G protein binding to alpha-syntrophin's PDZ domain and alter intracellular Ca2+ in muscle

Alpha-syntrophin is a component of the dystrophin glycoprotein complex (DGC). It is firmly attached to the dystrophin cytoskeleton via a unique COOH-terminal domain and is associated indirectly with alpha-dystroglycan, which binds to extracellular matrix laminin. Syntrophin contains two pleckstrin homology (PH) domains and one PDZ domain. Because PH domains of other proteins are known to bind the betagamma-subunits of the heterotrimeric G proteins, whether this is also a property of syntrophin was investigated. Isolated syntrophin from rabbit skeletal muscle binds bovine brain Gbetagamma-subunits in gel blot overlay experiments. Laminin-1-Sepharose or specific antibodies against syntrophin, alpha- and beta-dystroglycan, or dystrophin precipitate a complex with Gbetagamma from crude skeletal muscle microsomes. Bacterially expressed syntrophin fusion proteins and truncation mutants allowed mapping of Gbetagamma binding to syntrophin's PDZ domain; this is a novel function for PDZ domains. When laminin-1 is bound, maximal binding of Gsalpha and Gbetagamma occurs and active Gsalpha, measured as GTP-gamma35S bound, decreases. Because intracellular Ca2+ is elevated in Duchenne muscular dystrophy and Gsalpha is known to activate the dihydropyridine receptor Ca2+ channel, whether laminin also altered intracellular Ca2+ was investigated. Laminin-1 decreases active (GTP-gammaS-bound) Gsalpha, and the Ca2+ channel is inhibited by laminin-1. The laminin alpha1-chain globular domains 4 and 5 region, the region bound by DGC alpha-dystroglycan, is sufficient to cause an effect, and an antibody that specifically blocks laminin binding to alpha-dystroglycan inhibits Gbeta binding by syntrophin in C2C12 myotubes. These observations suggest that DGC is a matrix laminin, G protein-coupled receptor.

Levodopa with carbidopa diminishes glycogen concentration, glycogen synthase activity, and insulin-stimulated glucose transport in rat skeletal muscle

We hypothesized that levodopa with carbidopa, a common therapy for patients with Parkinson's disease, might contribute to the high prevalence of insulin resistance reported in patients with Parkinson's disease. We examined the effects of levodopa-carbidopa on glycogen concentration, glycogen synthase activity, and insulin-stimulated glucose transport in skeletal muscle, the predominant insulin-responsive tissue. In isolated muscle, levodopa-carbidopa completely prevented insulin-stimulated glycogen accumulation and glucose transport. The levodopa-carbidopa effects were blocked by propranolol, a beta-adrenergic antagonist. Levodopa-carbidopa also inhibited the insulin-stimulated increase in glycogen synthase activity, whereas propranolol attenuated this effect. Insulin-stimulated tyrosine phosphorylation of insulin receptor substrate (IRS)-1 was reduced by levodopa-carbidopa, although Akt phosphorylation was unaffected by levodopa-carbidopa. A single in vivo dose of levodopa-carbidopa increased skeletal muscle cAMP concentrations, diminished glycogen synthase activity, and reduced tyrosine phosphorylation of IRS-1. A separate set of rats was treated intragastrically twice daily for 4 wk with levodopa-carbidopa. After 4 wk of treatment, oral glucose tolerance was reduced in rats treated with drugs compared with control animals. Muscles from drug-treated rats contained at least 15% less glycogen and approximately 50% lower glycogen synthase activity compared with muscles from control rats. The data demonstrate beta-adrenergic-dependent inhibition of insulin action by levodopa-carbidopa and suggest that unrecognized insulin resistance may exist in chronically treated patients with Parkinson's disease.

α1-AR-mediated activation of NKCC in rat cardiomyocytes involves ERK-dependent phosphorylation of the cotransporter

We studied molecular and functional characteristics as well as hormonal regulation of the Na-K-2Cl cotransporter (NKCC) in the isolated rat heart and cardiomyocytes. NKCC activity was measured as bumetanide-sensitive86Rb+influx in isolated perfused rat hearts and isolated cardiomyocytes. Stimulation of α1-adrenoceptors (AR) by phenylephrine (30 μM) increased86Rb+influx. The NKCC inhibitor bumetanide (50 μM) reduced the response to phenylephrine by 45 ± 13% ( n = 12, P < 0.01). PD-98059 (10 μM), an inhibitor of the activation of the mitogen-activated protein kinases extracellular signal-regulated protein kinase 1 and 2 (ERK1/2), reduced the total response to phenylephrine by 51 ± 13% ( n = 10, P < 0.01) and eliminated the bumetanide-sensitive component, indicating that α1-AR mediated stimulation of NKCC is dependent on activation of ERK1/2. Inhibitors of protein kinase C or phosphatidylinositol 3-kinase had no effect. The presence of NKCC mRNA and protein was demonstrated in isolated rat cardiomyocytes. Phosphorylation of NKCC after α1-AR stimulation was shown by immunoprecipitation of the phosphoprotein from32Piprelabeled cardiomyocytes. Increased phosphorylation of the NKCC protein was also abolished by PD-98059. We conclude that the NKCC is present in rat cardiomyocytes and that ion transport by the cotransporter is regulated by α1-AR stimulation through phosphorylation of this protein involving the ERK pathway.

Riding the tides: K+ concentration and volume regulation by muscle Na+-K+-2Cl- cotransport activity

Until recently, the existence of a Na+-K+-2Cl− cotransporter (NKCC) in skeletal muscle was unclear. Recent evidence shows that the NKCC is strongly expressed and provides both K+ and water transport functions in resting and contracting skeletal muscle. The contribution of NKCC activity to K+ and volume regulation in skeletal muscle has potential consequences for muscle contractility and metabolism.

ATP-sensitive potassium channels mediate hyperosmotic stimulation of NKCC in slow-twitch muscle

In mildly hyperosmotic medium, activation of the Na+ -K+ -2Cl- cotransporter (NKCC) counteracts skeletal muscle cell water loss, and compounds that stimulate protein kinase A (PKA) activity inhibit the activation of the NKCC. The aim of this study was to determine the mechanism for PKA inhibition of NKCC activity in resting skeletal muscle. Incubation of rat slow-twitch soleus and fast-twitch plantaris muscles in isosmotic medium with the PKA inhibitors H-89 and KT-5720 caused activation of the NKCC only in the soleus muscle. NKCC activation caused by PKA inhibition was insensitive to MEK MAPK inhibitors and to insulin but was abolished by the PKA stimulators isoproterenol and forskolin. Furthermore, pinacidil [an ATP-sensitive potassium (KATP) channel opener] or inhibition of glycolysis increased NKCC activity in the soleus muscle but not in the plantaris muscle. Preincubation of the soleus muscle with glibenclamide (a KATP channel inhibitor) prevented the NKCC activation by hyperosmolarity, PKA inhibition, pinacidil, and glycolysis inhibitors. In contrast, glibenclamide stimulated NKCC activity in the plantaris muscle. In cells stably transfected with the Kir6.2 subunit of the of KATP channel, inhibition of glycolysis activated potassium current and NKCC activity. We conclude that activation of KATP channels in slow-twitch muscle is necessary for activation of the NKCC and cell volume restoration in hyperosmotic conditions.

The GTP-binding protein RhoA mediates Na,K-ATPase exocytosis in alveolar epithelial cells

The purpose of this study was to define the role of the Rho family of small GTPases in the beta-adrenergic regulation of the Na,K-ATPase in alveolar epithelial cells (AEC). The beta-adrenergic receptor agonist isoproterenol (ISO) increased the Na,K-ATPase protein abundance at the plasma membrane and activated RhoA in a time-dependent manner. AEC pretreated with mevastatin, a specific inhibitor of prenylation, or transfected with the dominant negative RhoAN19, prevented ISO-mediated Na,K-ATPase exocytosis to the plasma membrane. The ISO-mediated activation of RhoA in AEC occurred via beta2-adrenergic receptors and involved Gs-PKA as demonstrated by incubation with the protein kinase A (PKA)-specific inhibitors H89 and PKI (peptide specific inhibitor), and Gi, as incubation with pertussis toxin or cells transfected with a minigene vector for Gi inhibited the ISO-mediated RhoA activation. However, cells transfected with minigene vectors for G12 and G13 did not prevent RhoA activation by ISO. Finally, the ISO-mediated Na,K-ATPase exocytosis was regulated by the Rho-associated kinase (ROCK), as preincubation with the specific inhibitor Y-27632 or transfection with dominant negative ROCK, prevented the increase in Na,K-ATPase at the plasma membrane. Accordingly, ISO regulates Na,K-ATPase exocytosis in AEC via the activation of beta2-adrenergic receptor, Gs, PKA, Gi, RhoA, and ROCK.

NKCC activity restores muscle water during hyperosmotic challenge independent of insulin, ERK, and p38 MAPK

In isosmotic conditions, insulin stimulation of PI 3-K/Akt and p38 MAPK pathways in skeletal muscle inhibits Na(+)-K(+)-2Cl(-) cotransporter (NKCC) activity induced by the ERK1,2 MAPK pathway. Whether these signaling cascades contribute to NKCC regulation during osmotic challenge is unknown. Increasing osmolarity by 20 mosM with either glucose or mannitol induced NKCC-mediated (86)Rb uptake and water transport into rat soleus and plantaris skeletal muscle in vitro. This NKCC activity restored intracellular water. In contrast to mannitol, hyperosmolar glucose increased ERK1,2 and p38 MAPK phosphorylation. Glucose, but not mannitol, impaired insulin-stimulated phosphorylation of Akt and p38 MAPK in the plantaris and soleus muscles, respectively. Hyperosmolarity-induced NKCC activation was insensitive to insulin action and pharmacological inhibition of ERK1,2 and p38 MAPK pathways. Paradoxically, cAMP-producing agents, which stimulate NKCC activity in isosmotic conditions, suppressed hyperosmolar glucose- and mannitol-induced NKCC activity and prevented restoration of muscle cell volume in hyperosmotic media. These results indicate that NKCC activity helps restore muscle cell volume during hyperglycemia. Moreover, hyperosmolarity activates NKCC regulatory pathways that are insensitive to insulin inhibition.