Increased caveolin-3 levels in mdx mouse muscles

Why is early-onset atrial fibrillation uncommon in patients with Duchenne muscular dystrophy? Insights from the mdx mouse

AIMS

A reduction in both dystrophin and neuronal nitric oxide synthase (NOS1) secondary to microRNA-31 (miR-31) up-regulation contributes to the atrial electrical remodelling that underpins human and experimental atrial fibrillation (AF). In contrast, patients with Duchenne muscular dystrophy (DMD), who lack dystrophin and NOS1 and, at least in the skeletal muscle, have raised miR-31 expression, do not have increase susceptibility to AF in the absence of left ventricular (LV) dysfunction. Here, we investigated whether dystrophin deficiency is also associated with atrial up-regulation of miR-31, loss of NOS1 protein, and increased AF susceptibility in young mdx mice.

METHODS AND RESULTS

Echocardiography showed normal cardiac structure and function in 12-13 weeks mdx mice, with no indication by assay of hydroxyproline that atrial fibrosis had developed. The absence of dystrophin in mdx mice was accompanied by an overall reduction in syntrophin and a lower NOS1 protein content in the skeletal muscle and in the left atrial and ventricular myocardium, with the latter occurring alongside reduced Nos1 transcript levels (exons 1-2 by quantitative polymerase chain reaction) and an increase in NOS1 polyubiquitination [assessed using tandem polyubiquitination pulldowns; P < 0.05 vs. wild type (WT)]. Neither the up-regulation of miR-31 nor the substantial reduction in NOS activity observed in the skeletal muscle was present in the atrial tissue of mdx mice. At difference with the skeletal muscle, the mdx atrial myocardium showed a reduction in the constitutive NOS inhibitor, caveolin-1, coupled with an increase in NOS3 serine1177 phosphorylation, in the absence of differences in the protein content of other NOS isoforms or in the relative expression NOS1 splice variants. In line with these findings, transoesophageal atrial burst pacing revealed no difference in AF susceptibility between mdx mice and their WT littermates.

CONCLUSION

Dystrophin depletion is not associated with atrial miR-31 up-regulation, reduced NOS activity, or increased AF susceptibility in the mdx mouse. Compared with the skeletal muscle, the milder atrial biochemical phenotype may explain why patients with DMD do not exhibit a higher prevalence of atrial arrhythmias despite a reduction in NOS1 content.

A Role for Caveolin-3 in the Pathogenesis of Muscular Dystrophies

Caveolae are the cholesterol-rich small invaginations of the plasma membrane present in many cell types including adipocytes, endothelial cells, epithelial cells, fibroblasts, smooth muscles, skeletal muscles and cardiac muscles. They serve as specialized platforms for many signaling molecules and regulate important cellular processes like energy metabolism, lipid metabolism, mitochondria homeostasis, and mechano-transduction. Caveolae can be internalized together with associated cargo. The caveolae-dependent endocytic pathway plays a role in the withdrawal of many plasma membrane components that can be sent for degradation or recycled back to the cell surface. Caveolae are formed by oligomerization of caveolin proteins. Caveolin-3 is a muscle-specific isoform, whose malfunction is associated with several diseases including diabetes, cancer, atherosclerosis, and cardiovascular diseases. Mutations in Caveolin-3 are known to cause muscular dystrophies that are collectively called caveolinopathies. Altered expression of Caveolin-3 is also observed in Duchenne’s muscular dystrophy, which is likely a part of the pathological process leading to muscle weakness. This review summarizes the major functions of Caveolin-3 in skeletal muscles and discusses its involvement in the pathology of muscular dystrophies.

Muscle and cardiac therapeutic strategies for Duchenne muscular dystrophy: past, present, and future

BACKGROUND

Duchenne muscular dystrophy (DMD) is a severe X-linked neuromuscular childhood disorder that causes progressive muscle weakness and degeneration and results in functional decline, loss of ambulation and early death of young men due to cardiac or respiratory failure. Although the major cause of the disease has been known for many years-namely mutation in the DMD gene encoding dystrophin, one of the largest human genes-DMD is still incurable, and its treatment is challenging.

METHODS

A comprehensive and systematic review of literature on the gene, cell, and pharmacological experimental therapies aimed at restoring functional dystrophin or to counteract the associated processes contributing to disease progression like inflammation, fibrosis, calcium signaling or angiogenesis was carried out.

RESULTS

Although some therapies lead to satisfying effects in skeletal muscle, they are highly ineffective in the heart; therefore, targeting defective cardiac and respiratory systems is vital in DMD patients. Unfortunately, most of the pharmacological compounds treat only the symptoms of the disease. Some drugs addressing the underlying cause, like eteplirsen, golodirsen, and ataluren, have recently been conditionally approved; however, they can correct only specific mutations in the DMD gene and are therefore suitable for small sub-populations of affected individuals.

CONCLUSION

In this review, we summarize the possible therapeutic options and describe the current status of various, still imperfect, strategies used for attenuating the disease progression.

TRPCs: Influential Mediators in Skeletal Muscle

Ca²⁺ itself or Ca²⁺-dependent signaling pathways play fundamental roles in various cellular processes from cell growth to death. The most representative example can be found in skeletal muscle cells where a well-timed and adequate supply of Ca²⁺ is required for coordinated Ca²⁺-dependent skeletal muscle functions, such as the interactions of contractile proteins during contraction. Intracellular Ca²⁺ movements between the cytosol and sarcoplasmic reticulum (SR) are strictly regulated to maintain the appropriate Ca²⁺ supply in skeletal muscle cells. Added to intracellular Ca²⁺ movements, the contribution of extracellular Ca²⁺ entry to skeletal muscle functions and its significance have been continuously studied since the early 1990s. Here, studies on the roles of channel proteins that mediate extracellular Ca²⁺ entry into skeletal muscle cells using skeletal myoblasts, myotubes, fibers, tissue, or skeletal muscle-originated cell lines are reviewed with special attention to the proposed functions of transient receptor potential canonical proteins (TRPCs) as store-operated Ca²⁺ entry (SOCE) channels under normal conditions and the potential abnormal properties of TRPCs in muscle diseases such as Duchenne muscular dystrophy (DMD).

Sarcolipin overexpression impairs myogenic differentiation in Duchenne muscular dystrophy

Reduction in the expression of sarcolipin (SLN), an inhibitor of sarco(endo)plasmic reticulum (SR) Ca²⁺-ATPase (SERCA), ameliorates severe muscular dystrophy in mice. However, the mechanism by which SLN inhibition improves muscle structure remains unclear. Here, we describe the previously unknown function of SLN in muscle differentiation in Duchenne muscular dystrophy (DMD). Overexpression of SLN in C₂C₁₂ resulted in decreased SERCA pump activity, reduced SR Ca²⁺ load, and increased intracellular Ca²⁺ (Cai2+) concentration. In addition, SLN overexpression resulted in altered expression of myogenic markers and poor myogenic differentiation. In dystrophin-deficient dog myoblasts and myotubes, SLN expression was significantly high and associated with defective Cai2+ cycling. The dystrophic dog myotubes were less branched and associated with decreased autophagy and increased expression of mitochondrial fusion and fission proteins. Reduction in SLN expression restored these changes and enhanced dystrophic dog myoblast fusion during differentiation. In summary, our data suggest that SLN upregulation is an intrinsic secondary change in dystrophin-deficient myoblasts and could account for the Cai2+ mishandling, which subsequently contributes to poor myogenic differentiation. Accordingly, reducing SLN expression can improve the Cai2+ cycling and differentiation of dystrophic myoblasts. These findings provide cellular-level supports for targeting SLN expression as a therapeutic strategy for DMD.

Targeting angiogenesis in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) represents one of the most devastating types of muscular dystrophies which affect boys already at early childhood. Despite the fact that the primary cause of the disease, namely the lack of functional dystrophin is known already for more than 30 years, DMD still remains an incurable disease. Thus, an enormous effort has been made during recent years to reveal novel mechanisms that could provide therapeutic targets for DMD, especially because glucocorticoids treatment acts mostly symptomatic and exerts many side effects, whereas the effectiveness of genetic approaches aiming at the restoration of functional dystrophin is under the constant debate. Taking into account that dystrophin expression is not restricted to muscle cells, but is present also in, e.g., endothelial cells, alterations in angiogenesis process have been proposed to have a significant impact on DMD progression. Indeed, already before the discovery of dystrophin, several abnormalities in blood vessels structure and function have been revealed, suggesting that targeting angiogenesis could be beneficial in DMD. In this review, we will summarize current knowledge about the angiogenesis status both in animal models of DMD as well as in DMD patients, focusing on different organs as well as age- and sex-dependent effects. Moreover, we will critically discuss some approaches such as modulation of vascular endothelial growth factor or nitric oxide related pathways, to enhance angiogenesis and attenuate the dystrophic phenotype. Additionally, we will suggest the potential role of other mediators, such as heme oxygenase-1 or statins in those processes.

Markers of Accelerated Skeletal Muscle Regenerative Response in Murphy Roths Large Mice: Characteristics of Muscle Progenitor Cells and Circulating Factors

The "super-healing" Murphy Roths Large (MRL/MpJ) mouse possesses a superior regenerative capacity for repair of many tissues, which makes it an excellent animal model for studying molecular and cellular mechanisms during tissue regeneration. As the role of muscle progenitor cells (MPCs) in muscle-healing capacity of MRL/MpJ mice has not been previously studied, we investigated the muscle regenerative capacity of MRL/MpJ mice following muscle injury, and the results were compared to results from C57BL/6J (B6) age-matched control mice. Our results show that muscle healing upon cardiotoxin injury was accelerated in MRL/MpJ mice and characterized by reduced necrotic muscle area, reduced macrophage infiltration, and more regenerated myofibers (embryonic myosin heavy chain+/centronucleated fibers) at 3, 5, and 12 days postinjury, when compared to B6 age-matched control mice. These observations were associated with enhanced function of MPCs, including improved cell proliferation, differentiation, and resistance to stress, as well as increased muscle regenerative potential when compared to B6 MPCs. Mass spectrometry of serum proteins revealed higher levels of circulating antioxidants in MRL/MpJ mice when compared to B6 mice. Indeed, we found relatively higher gene expression of superoxide dismutase 1 (Sod1) and catalase (Cat) in MRL/MpJ MPCs. Depletion of Sod1 or Cat by small interfering RNA impaired myogenic potential of MRL/MpJ MPCs, indicating a role for these antioxidants in muscle repair. Taken together, these findings provide evidence that improved function of MPCs and higher levels of circulating antioxidants play important roles in accelerating muscle-healing capacity of MRL/MpJ mice. Stem Cells 2019;37:357-367.

Therapeutic strategies to address neuronal nitric oxide synthase deficiency and the loss of nitric oxide bioavailability in Duchenne Muscular Dystrophy

Duchenne Muscular Dystrophy is a rare and fatal neuromuscular disease in which the absence of dystrophin from the muscle membrane induces a secondary loss of neuronal nitric oxide synthase and the muscles capacity for endogenous nitric oxide synthesis. Since nitric oxide is a potent regulator of skeletal muscle metabolism, mass, function and regeneration, the loss of nitric oxide bioavailability is likely a key contributor to the chronic pathological wasting evident in Duchenne Muscular Dystrophy. As such, various therapeutic interventions to re-establish either the neuronal nitric oxide synthase protein deficit or the consequential loss of nitric oxide synthesis and bioavailability have been investigated in both animal models of Duchenne Muscular Dystrophy and in human clinical trials. Notably, the efficacy of these interventions are varied and not always translatable from animal model to human patients, highlighting a complex interplay of factors which determine the downstream modulatory effects of nitric oxide. We review these studies herein.

Attempting to Compensate for Reduced Neuronal Nitric Oxide Synthase Protein with Nitrate Supplementation Cannot Overcome Metabolic Dysfunction but Rather Has Detrimental Effects in Dystrophin-Deficient mdx Muscle

Duchenne muscular dystrophy arises from the loss of dystrophin and is characterized by calcium dysregulation, muscular atrophy, and metabolic dysfunction. The secondary reduction of neuronal nitric oxide synthase (nNOS) from the sarcolemma reduces NO production and bioavailability. As NO modulates glucose uptake, metabolism, and mitochondrial bioenergetics, we investigated whether an 8-week nitrate supplementation regimen could overcome metabolic dysfunction in the mdx mouse. Dystrophin-positive control (C57BL/10) and dystrophin-deficient mdx mice were supplemented with sodium nitrate (85 mg/l) in drinking water. Following the supplementation period, extensor digitorum longus and soleus were excised and radioactive glucose uptake was measured at rest (basal) and during contraction. Gastrocnemius was excised and mitochondrial respiration was measured using the Oroboros Oxygraph. Tibialis anterior was analyzed immunohistochemically for the presence of dystrophin, nNOS, nitrotyrosine, IgG and CD45+ cells, and histologically to assess areas of damage and regeneration. Glucose uptake in the basal and contracting states was normal in unsupplemented mdx muscles but was reduced following nitrate supplementation in mdx muscles only. The mitochondrial utilization of substrates was also impaired in mdx gastrocnemius during phosphorylating and maximal uncoupled respiration, and nitrate could not improve respiration in mdx muscle. Although nitrate supplementation reduced mitochondrial hydrogen peroxide emission, it induced mitochondrial uncoupling in red gastrocnemius, increased muscle fiber peroxynitrite (nitrotyrosine), and promoted skeletal muscle damage. Our novel data suggest that despite lower nNOS protein expression and likely lower NO production in mdx muscle, enhancing NO production with nitrate supplementation in these mice has detrimental effects on skeletal muscle. This may have important relevance for those with DMD.

Absence of Dystrophin Disrupts Skeletal Muscle Signaling: Roles of Ca2+, Reactive Oxygen Species, and Nitric Oxide in the Development of Muscular Dystrophy

Dystrophin is a long rod-shaped protein that connects the subsarcolemmal cytoskeleton to a complex of proteins in the surface membrane (dystrophin protein complex, DPC), with further connections via laminin to other extracellular matrix proteins. Initially considered a structural complex that protected the sarcolemma from mechanical damage, the DPC is now known to serve as a scaffold for numerous signaling proteins. Absence or reduced expression of dystrophin or many of the DPC components cause the muscular dystrophies, a group of inherited diseases in which repeated bouts of muscle damage lead to atrophy and fibrosis, and eventually muscle degeneration. The normal function of dystrophin is poorly defined. In its absence a complex series of changes occur with multiple muscle proteins showing reduced or increased expression or being modified in various ways. In this review, we will consider the various proteins whose expression and function is changed in muscular dystrophies, focusing on Ca(2+)-permeable channels, nitric oxide synthase, NADPH oxidase, and caveolins. Excessive Ca(2+) entry, increased membrane permeability, disordered caveolar function, and increased levels of reactive oxygen species are early changes in the disease, and the hypotheses for these phenomena will be critically considered. The aim of the review is to define the early damage pathways in muscular dystrophy which might be appropriate targets for therapy designed to minimize the muscle degeneration and slow the progression of the disease.

Lipogenesis mitigates dysregulated sarcoplasmic reticulum calcium uptake in muscular dystrophy

Muscular dystrophy is accompanied by a reduction in activity of sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) that contributes to abnormal Ca(2+) homeostasis in sarco/endoplasmic reticulum (SR/ER). Recent findings suggest that skeletal muscle fatty acid synthase (FAS) modulates SERCA activity and muscle function via its effects on SR membrane phospholipids. In this study, we examined muscle's lipid metabolism in mdx mice, a mouse model for Duchenne muscular dystrophy (DMD). De novo lipogenesis was ~50% reduced in mdx muscles compared to wildtype (WT) muscles. Gene expressions of lipogenic and other ER lipid-modifying enzymes were found to be differentially expressed between wildtype (WT) and mdx muscles. A comprehensive examination of muscles' SR phospholipidome revealed elevated phosphatidylcholine (PC) and PC/phosphatidylethanolamine (PE) ratio in mdx compared to WT mice. Studies in primary myocytes suggested that defects in key lipogenic enzymes including FAS, stearoyl-CoA desaturase-1 (SCD1), and Lipin1 are likely contributing to reduced SERCA activity in mdx mice. Triple transgenic expression of FAS, SCD1, and Lipin1 (3TG) in mdx myocytes partly rescued SERCA activity, which coincided with an increase in SR PE that normalized PC/PE ratio. These findings implicate a defect in lipogenesis to be a contributing factor for SERCA dysfunction in muscular dystrophy. Restoration of muscle's lipogenic pathway appears to mitigate SERCA function through its effects on SR membrane composition.

Revisiting the dystrophin-ATP connection: How half a century of research still implicates mitochondrial dysfunction in Duchenne Muscular Dystrophy aetiology

Duchenne Muscular Dystrophy (DMD) is a fatal neuromuscular disease that is characterised by dystrophin-deficiency and chronic Ca(2+)-induced skeletal muscle wasting, which currently has no cure. DMD was once considered predominantly as a metabolic disease due to the myriad of metabolic insufficiencies evident in the musculature, however this aspect of the disease has been extensively ignored since the discovery of dystrophin. The collective historical and contemporary literature documenting these metabolic nuances has culminated in a series of studies that importantly demonstrate that metabolic dysfunction exists independent of dystrophin expression and a mild disease phenotype can be expressed even in the complete absence of dystrophin expression. Targeting and supporting metabolic pathways with anaplerotic and other energy-enhancing supplements has also shown therapeutic value. We explore the hypothesis that DMD is characterised by a systemic mitochondrial impairment that is central to disease aetiology rather than a secondary pathophysiological consequence of dystrophin-deficiency.

Nitric oxide synthase deficiency and the pathophysiology of muscular dystrophy

The secondary loss of neuronal nitric oxide synthase (nNOS) that occurs in dystrophic muscle is the basis of numerous, complex and interacting features of the dystrophic pathology that affect not only muscle itself, but also influence the interaction of muscle with other tissues. Many mechanisms through which nNOS deficiency contributes to misregulation of muscle development, blood flow, fatigue, inflammation and fibrosis in dystrophic muscle have been identified, suggesting that normalization in NO production could greatly attenuate diverse aspects of the pathology of muscular dystrophy through multiple regulatory pathways. However, the relative importance of the loss of nNOS from the sarcolemma versus the importance of loss of total nNOS from dystrophic muscle remains unknown. Although most current evidence indicates that nNOS localization at the sarcolemma is not required to achieve NO-mediated reductions of pathology in muscular dystrophy, the question remains open concerning whether membrane localization would provide a more efficient rescue from features of the dystrophic phenotype.

Caveolae regulation of mechanosensitive channel function in myotubes

Mutations that lead to muscular dystrophy often create deficiencies in cytoskeletal support of the muscle sarcolemma causing hyperactive mechanosensitive cation channel (MSC) activity and elevated intracellular Ca(2+). Caveolae are cholesterol-rich microdomains that form mechanically deformable invaginations of the sarcolemma. Mutations to caveolin-3, the main scaffolding protein of caveolae in muscle, cause Limbe-Girdle muscular dystrophy. Using genetic and acute chemical perturbations of developing myotubes we investigated whether caveolae are functionally linked to MSCs. MSC sensitivity was assayed using suction application to patches and probe-induced indentation during whole-cell recordings. Membrane mechanical stress in patches was monitored using patch capacitance/impedance. Cholesterol depletion disrupted caveolae and caused a large increase in MSC current. It also decreased the membrane mechanical relaxation time, likely reflecting cytoskeleton dissociation from the bilayer. Reduction of Cav3 expression with miRNA also increased MSC current and decreased patch relaxation time. In contrast Cav3 overexpression produced a small decrease in MSC currents. To acutely and specifically inhibit Cav3 interactions, we made a chimeric peptide containing the antennapedia membrane translocation domain and the Cav3 scaffolding domain (A-CSD3). A-CSD3 action was time dependent initially producing a mild Ca(2+) leak and increased MSC current, while longer exposures decreased MSC currents coinciding with increased patch stiffening. Images of GFP labeled Cav3 in patches showed that Cav3 doesn't enter the pipette, showing patch composition differed from the cell surface. However, disruption via cholesterol depletion caused Cav3 to become uniformly distributed over the sarcolemma and Cav3 appearance in the patch dome. The whole-cell indentation currents elicited under the different caveolae modifying conditions mirror the patch response supporting the role of caveolae in MSC function. These studies show that normal expression levels of Cav3 are mechanoprotective to the sarcolemma through multiple mechanisms, and Cav3 upregulation observed in some dystrophies may compensate for other mechanical deficiencies.

Caveolins in rhabdomyosarcoma

Caveolins are scaffolding proteins that play a pivotal role in numerous processes, including caveolae biogenesis, vesicular transport, cholesterol homeostasis and regulation of signal transduction. There are three different isoforms (Cav-1, -2 and -3) that form homo- and hetero-aggregates at the plasma membrane and modulate the activity of a number of intracellular binding proteins. Cav-1 and Cav-3, in particular, are respectively expressed in the reserve elements (e.g. satellite cells) and in mature myofibres of skeletal muscle and their expression interplay characterizes the switch from muscle precursors to differentiated elements. Recent findings have shown that caveolins are also expressed in rhabdomyosarcoma, a group of heterogeneous childhood soft-tissue sarcomas in which the cancer cells seem to derive from progenitors that resemble myogenic cells. In this review, we will focus on the role of caveolins in rhabdomyosarcomas and on their potential use as markers of the degree of differentiation in these paediatric tumours. Given that the function of Cav-1 as tumour conditional gene in cancer has been well-established, we will also discuss the relationship between Cav-1 and the progression of rhabdomyosarcoma.

Ex vivo stretch reveals altered mechanical properties of isolated dystrophin-deficient hearts

Duchenne muscular dystrophy (DMD) is a progressive and fatal disease of muscle wasting caused by loss of the cytoskeletal protein dystrophin. In the heart, DMD results in progressive cardiomyopathy and dilation of the left ventricle through mechanisms that are not fully understood. Previous reports have shown that loss of dystrophin causes sarcolemmal instability and reduced mechanical compliance of isolated cardiac myocytes. To expand upon these findings, here we have subjected the left ventricles of dystrophin-deficient mdx hearts to mechanical stretch. Unexpectedly, isolated mdx hearts showed increased left ventricular (LV) compliance compared to controls during stretch as LV volume was increased above normal end diastolic volume. During LV chamber distention, sarcomere lengths increased similarly in mdx and WT hearts despite greater excursions in volume of mdx hearts. This suggests that the mechanical properties of the intact heart cannot be modeled as a simple extrapolation of findings in single cardiac myocytes. To explain these findings, a model is proposed in which disruption of the dystrophin-glycoprotein complex perturbs cell-extracellular matrix contacts and promotes the apparent slippage of myocytes past each other during LV distension. In comparison, similar increases in LV compliance were obtained in isolated hearts from β-sarcoglycan-null and laminin-α₂ mutant mice, but not in dysferlin-null mice, suggesting that increased whole-organ compliance in mdx mice is a specific effect of disrupted cell-extracellular matrix contacts and not a general consequence of cardiomyopathy via membrane defect processes. Collectively, these findings suggest a novel and cell-death independent mechanism for the progressive pathological LV dilation that occurs in DMD.

Delineating the role of alterations in lipid metabolism to the pathogenesis of inherited skeletal and cardiac muscle disorders: Thematic Review Series: Genetics of Human Lipid Diseases

As the specific composition of lipids is essential for the maintenance of membrane integrity, enzyme function, ion channels, and membrane receptors, an alteration in lipid composition or metabolism may be one of the crucial changes occurring during skeletal and cardiac myopathies. Although the inheritance (autosomal dominant, autosomal recessive, and X-linked traits) and underlying/defining mutations causing these myopathies are known, the contribution of lipid homeostasis in the progression of these diseases needs to be established. The purpose of this review is to present the current knowledge relating to lipid changes in inherited skeletal muscle disorders, such as Duchenne/Becker muscular dystrophy, myotonic muscular dystrophy, limb-girdle myopathic dystrophies, desminopathies, rostrocaudal muscular dystrophy, and Dunnigan-type familial lipodystrophy. The lipid modifications in familial hypertrophic and dilated cardiomyopathies, as well as Barth syndrome and several other cardiac disorders associated with abnormal lipid storage, are discussed. Information on lipid alterations occurring in these myopathies will aid in the design of improved methods of screening and therapy in children and young adults with or without a family history of genetic diseases.

Caveolae respond to cell stretch and contribute to stretch-induced signaling

Caveolae are invaginations of the plasma membrane that are formed by caveolins. Caveolar membranes are also enriched in cholesterol, glycosphingolipids and signaling enzymes such as Src kinase. Here we investigate the effect of cell stretch upon caveolar dynamics and signaling. Transfection of C2 myoblasts with caveolin-3-YFP led to the formation of caveolae-like membrane pits 50-100 nm in diameter. Glycosphingolipids became immobilized and tightly packed together within caveolin-rich regions of the plasma membrane. Fluorescence resonance energy transfer (FRET) was used to assess the degree of glycosphingolipid packing. Myoblasts were subjected to a brief (1 minute) stretch on an elastic substratum. Stretch caused a reduction in glycosphingolipid FRET, consistent with a reversible unfolding of caveolar pits in response to membrane tension. Cells expressing caveolin-3-YFP also displayed an enhanced stretch-induced activation of Src kinase, as assessed by immunofluorescence. Repeated stretches resulted in the trafficking and remodeling of caveolin-3-rich membrane domains and accelerated turnover of membrane glycosphingolipids. The stretch-induced unfolding of caveolae, activation of Src and redistribution of caveolin and glycosphingolipids might reflect mechanisms of the cellular adaptation to mechanical stresses.

T-tubule biogenesis and triad formation in skeletal muscle and implication in human diseases

In skeletal muscle, the excitation-contraction (EC) coupling machinery mediates the translation of the action potential transmitted by the nerve into intracellular calcium release and muscle contraction. EC coupling requires a highly specialized membranous structure, the triad, composed of a central T-tubule surrounded by two terminal cisternae from the sarcoplasmic reticulum. While several proteins located on these structures have been identified, mechanisms governing T-tubule biogenesis and triad formation remain largely unknown. Here, we provide a description of triad structure and plasticity and review the role of proteins that have been linked to T-tubule biogenesis and triad formation and/or maintenance specifically in skeletal muscle: caveolin 3, amphiphysin 2, dysferlin, mitsugumins, junctophilins, myotubularin, ryanodine receptor, and dihydhropyridine Receptor. The importance of these proteins in triad biogenesis and subsequently in muscle contraction is sustained by studies on animal models and by the direct implication of most of these proteins in human myopathies.

Skeletal muscle NADPH oxidase is increased and triggers stretch-induced damage in the mdx mouse

Recent studies have shown that oxidative stress contributes to the pathogenesis of muscle damage in dystrophic (mdx) mice. In this study we have investigated the role of NADPH oxidase as a source of the oxidative stress in these mice. The NADPH oxidase subunits gp91^phox, p67^phox and rac 1 were increased 2–3 fold in tibilais anterior muscles from mdx mice compared to wild type. Importantly, this increase occurred in 19 day old mice, before the onset of muscle necrosis and inflammation, suggesting that NADPH oxidase is an important source of oxidative stress in mdx muscle. In muscles from 9 week old mdx mice, gp91^phox and p67^phox were increased 3–4 fold and NADPH oxidase superoxide production was 2 times greater than wild type. In single fibers from mdx muscle NADPH oxidase subunits were all located on or near the sarcolemma, except for p67^phox,which was expressed in the cytosol. Pharmacological inhibition of NADPH oxidase significantly reduced the intracellular Ca²⁺ rise following stretched contractions in mdx single fibers, and also attenuated the loss of muscle force. These results suggest that NADPH oxidase is a major source of reactive oxygen species in dystrophic muscle and its enhanced activity has a stimulatory effect on stretch-induced Ca²⁺ entry, a key mechanism for muscle damage and functional impairment.

Accumulation of caveolin-3 protein is limited in damaged muscle in chicken muscular dystrophy

Members of the caveolin family are the main component of caveolae, and caveolin-3 is a muscle-specific protein. Caveolin-3 deficiency induces a muscular dystrophic phenotype, while its overexpression is also harmful to muscle cells. Increased caveolae were observed in chicken muscular dystrophy; however, the underlying mechanism causing the onset remains unclear. Therefore, the current study analyzes the expression of caveolin-3 and other caveola-related proteins in dystrophic chickens. Western blotting and semi-quantitative RT-PCR analysis revealed that (1) caveolin-3 is highly expressed in the damaged muscle of dystrophic chickens (7.12-fold); (2) the amount of caveolin-3 protein is regulated in posttranslational modification, since no significant increase is observed at the mRNA level (1.09-fold); and (3) the expression pattern of other caveola-related proteins is similar to that of caveolin-3. These results suggest that the accumulation of caveolin-3 protein may be associated with the causative process of chicken muscular dystrophy.

Therapeutic potential of proteasome inhibition in Duchenne and Becker muscular dystrophies

Duchenne muscular dystrophy (DMD) and its milder allelic variant, Becker muscular dystrophy (BMD), result from mutations of the dystrophin gene and lead to progressive muscle deterioration. Enhanced activation of proteasomal degradation underlies critical steps in the pathogenesis of the DMD/BMD dystrophic process. Previously, we demonstrated that treatment with the proteasome inhibitor MG-132 rescues the cell membrane localization of dystrophin and the dystrophin glycoprotein complex in mdx mice, a natural genetic mouse model of DMD. The current work aims to thoroughly define the therapeutic potential in dystrophinopathies of Velcade, a drug that selectively blocks the ubiquitin-proteasome pathway. Velcade is particularly intriguing since it has been approved for the treatment of multiple myeloma. Therefore, its side effects in humans have been explored. Velcade effects were analyzed through two independent methodological approaches. First, we administered the drug systemically in mdx mice over a 2-week period. In this system, Velcade restores the membrane expression of dystrophin and dystrophin glycoprotein complex members and improves the dystrophic phenotype. In a second approach, we treated with the compound explants from muscle biopsies of DMD or BMD patients. We show that the inhibition of the proteasome pathway up-regulates dystrophin, alpha-sarcoglycan, and beta-dystroglycan protein levels in explants from BMD patients, whereas it increases the proteins of the dystrophin glycoprotein complex in DMD cases.

Calcium and the damage pathways in muscular dystrophyThis article is one of a selection of papers published in this special issue on Calcium Signaling

Duchenne muscular dystrophy (DMD) is a severe muscle-wasting disease caused by the absence of the cytoskeletal protein dystrophin. Experiments on the mdx mouse, a model of DMD, have shown that mdx muscles are particularly susceptible to stretch-induced damage. In this review, we discuss evidence showing that a series of stretched contractions of mdx muscle fibres causes a prolonged increase in resting intracellular calcium concentration ([Ca2+]i). The rise in [Ca2+]i is caused by Ca2+ entry through a class of stretch-activated channels (SACNSC) for which one candidate gene is TRPC1. We review the evidence for activation of SACNSC in muscle by reactive oxygen species (ROS) and suggest that stretch-induced ROS production is part of the pathway that triggers increased channel activity. When the TRPC1 gene was transfected into C2 myoblasts, expression occurred throughout the cell. Only when the TRPC1 gene was coexpressed with caveolin-3 did the TRPC1 protein express in the membrane. When TRPC1 was expressed in the membrane, it could be activated by ROS to produce Ca2+ entry and this entry was inhibited by PP2, an inhibitor of src kinase. These results suggest that stretched contractions activate ROS production, which activates src kinase. Activity of this kinase causes opening of SACNSC and allows Ca2+ entry. This pathway appears to be a significant cause of muscle damage in DMD.

Muscular dystrophy begins early in embryonic development deriving from stem cell loss and disrupted skeletal muscle formation

Examination of embryonic myogenesis of two distinct, but functionally related, skeletal muscle dystrophy mutants (mdx and cav-3(-/-)) establishes for the first time that key elements of the pathology of Duchenne muscular dystrophy (DMD) and limb-girdle muscular dystrophy type 1C (LGMD-1c) originate in the disruption of the embryonic cardiac and skeletal muscle patterning processes. Disruption of myogenesis occurs earlier in mdx mutants, which lack a functional form of dystrophin, than in cav-3(-/-) mutants, which lack the Cav3 gene that encodes the protein caveolin-3; this finding is consistent with the milder phenotype of LGMD-1c, a condition caused by mutations in Cav3, and the earlier [embryonic day (E)9.5] expression of dystrophin. Myogenesis is severely disrupted in mdx embryos, which display developmental delays; myotube morphology and displacement defects; and aberrant stem cell behaviour. In addition, the caveolin-3 protein is elevated in mdx embryos. Both cav-3(-/-) and mdx mutants (from E15.5 and E11.5, respectively) exhibit hyperproliferation and apoptosis of Myf5-positive embryonic myoblasts; attrition of Pax7-positive myoblasts in situ; and depletion of total Pax7 protein in late gestation. Furthermore, both cav-3(-/-) and mdx mutants have cardiac defects. In cav-3(-/-) mutants, there is a more restricted phenotype comprising hypaxial muscle defects, an excess of malformed hypertrophic myotubes, a twofold increase in myonuclei, and reduced fast myosin heavy chain (FMyHC) content. Several mdx mutant embryo pathologies, including myotube hypotrophy, reduced myotube numbers and increased FMyHC, have reciprocity with cav-3(-/-) mutants. In double mutant (mdxcav-3(+/-)) embryos that are deficient in dystrophin (mdx) and heterozygous for caveolin-3 (cav-3(+/-)), whereby caveolin-3 is reduced to 50% of wild-type (WT) levels, these phenotypes are severely exacerbated: intercostal muscle fibre density is reduced by 71%, and Pax7-positive cells are depleted entirely from the lower limbs and severely attenuated elsewhere; these data suggest a compensatory rather than a contributory role for the elevated caveolin-3 levels that are found in mdx embryos. These data establish a key role for dystrophin in early muscle formation and demonstrate that caveolin-3 and dystrophin are essential for correct fibre-type specification and emergent stem cell function. These data plug a significant gap in the natural history of muscular dystrophy and will be invaluable in establishing an earlier diagnosis for DMD/LGMD and in designing earlier treatment protocols, leading to better clinical outcome for these patients.

Matrix metalloproteinase-9 inhibition ameliorates pathogenesis and improves skeletal muscle regeneration in muscular dystrophy

Duchenne muscular dystrophy (DMD) is a fatal X-linked genetic disorder of skeletal muscle caused by mutation in dystrophin gene. Although the degradation of skeletal muscle extracellular matrix, inflammation and fibrosis are the common pathological features in DMD, the underlying mechanisms remain poorly understood. In this study, we have investigated the role and the mechanisms by which increased levels of matrix metalloproteinase-9 (MMP-9) protein causes myopathy in dystrophin-deficient mdx mice. The levels of MMP-9 but not tissue inhibitor of MMPs were drastically increased in skeletal muscle of mdx mice. Besides skeletal muscle, infiltrating macrophages were found to contribute significantly to the elevated levels of MMP-9 in dystrophic muscle. In vivo administration of a nuclear factor-kappa B inhibitory peptide, NBD, blocked the expression of MMP-9 in dystrophic muscle of mdx mice. Deletion of Mmp9 gene in mdx mice improved skeletal muscle structure and functions and reduced muscle injury, inflammation and fiber necrosis. Inhibition of MMP-9 increased the levels of cytoskeletal protein beta-dystroglycan and neural nitric oxide synthase and reduced the amounts of caveolin-3 and transforming growth factor-beta in myofibers of mdx mice. Genetic ablation of MMP-9 significantly augmented the skeletal muscle regeneration in mdx mice. Finally, pharmacological inhibition of MMP-9 activity also ameliorated skeletal muscle pathogenesis and enhanced myofiber regeneration in mdx mice. Collectively, our study suggests that the increased production of MMP-9 exacerbates dystrophinopathy and MMP-9 represents as one of the most promising therapeutic targets for the prevention of disease progression in DMD.

Amelioration of muscular dystrophy by transgenic expression of Niemann-Pick C1

Duchenne muscular dystrophy (DMD) and other types of muscular dystrophies are caused by the loss or alteration of different members of the dystrophin protein complex. Understanding the molecular mechanisms by which dystrophin-associated protein abnormalities contribute to the onset of muscular dystrophy may identify new therapeutic approaches to these human disorders. By examining gene expression alterations in mouse skeletal muscle lacking alpha-dystrobrevin (Dtna(-/-)), we identified a highly significant reduction of the cholesterol trafficking protein, Niemann-Pick C1 (NPC1). Mutations in NPC1 cause a progressive neurodegenerative, lysosomal storage disorder. Transgenic expression of NPC1 in skeletal muscle ameliorates muscular dystrophy in the Dtna(-/-) mouse (which has a relatively mild dystrophic phenotype) and in the mdx mouse, a model for DMD. These results identify a new compensatory gene for muscular dystrophy and reveal a potential new therapeutic target for DMD.

TRPC1 binds to caveolin-3 and is regulated by Src kinase - role in Duchenne muscular dystrophy

Transient receptor potential canonical 1 (TRPC1), a widely expressed calcium (Ca(2+))-permeable channel, is potentially involved in the pathogenesis of Duchenne muscular dystrophy (DMD). Ca(2+) influx through stretch-activated channels, possibly formed by TRPC1, induces muscle-cell damage in the mdx mouse, an animal model of DMD. In this study, we showed that TRPC1, caveolin-3 and Src-kinase protein levels are increased in mdx muscle compared with wild type. TRPC1 and caveolin-3 colocalised and co-immunoprecipitated. Direct binding of TRPC1-CFP to caveolin-3-YFP was confirmed in C2 myoblasts by fluorescence energy resonance transfer (FRET). Caveolin-3-YFP targeted TRPC1-CFP to the plasma membrane. Hydrogen peroxide, a reactive oxygen species (ROS), increased Src activity and enhanced Ca(2+) influx, but only in C2 myoblasts co-expressing TRPC1 and caveolin-3. In mdx muscle, Tiron, a ROS scavenger, and PP2, a Src inhibitor, reduced stretch-induced Ca(2+) entry and increased force recovery. Because ROS production is increased in mdx/DMD, these results suggest that a ROS-Src-TRPC1/caveolin-3 pathway contributes to the pathogenesis of mdx/DMD.

Targeting of acetylcholinesterase to lipid rafts of muscle

Despite the great progress made in setting the basis for the molecular diversity of acetylcholinesterase (AChE), an explanation for the existence of two types of amphiphilic subunits, with and without glicosylphosphatidylinositol (GPI) (Types I and II), has not been provided yet. In searching whether, as for the deficiency of dystrophin, that of merosin (laminin-alpha2 chain) alters the number of caveolae in muscle, a high increase in caveolin-3 (Cav3) was observed in the Triton X-100-resistant membranes (TRM) isolated from muscle of merosin-deficient dystrophic mice (Lama2dy). The rise in Cav3 was accompanied by that of non-caveolar lipid rafts, as showed by the greater ecto-5'-nucleotidase (eNT) activity, a marker of non-caveolar rafts, in TRM of dystrophic muscle. The observation of AChE activity in TRM, the increased levels of rafts and raft-bound AChE activity in merosin-deficient muscle and the presence of phospholipase C-sensitive AChE dimers in TRM supported targeting of glypiated AChE to rafts. This issue and the involvement of TRM in conveying nicotinic receptors to the neuromuscular junction and particular muscarinic receptors to cardiac sarcolemma strongly support a role for lipid rafts in targeting ACh receptors and glypiated AChE. Their nearby location in the surface membrane may provide cells with a fine tuning for regulating cholinergic responses.

N-Acetylcysteine ameliorates skeletal muscle pathophysiology in mdx mice

Duchenne muscular dystrophy (DMD) is a severe degenerative muscle disease caused by a mutation in the gene encoding dystrophin, a protein linking the cytoskeleton to the extracellular matrix. In this study we investigated whether the antioxidant N-acetylcysteine (NAC) provided protection against dystrophic muscle damage in the mdx mouse, an animal model of DMD. In isolated mdx muscles, NAC prevented the increased membrane permeability and reduced the force deficit associated with stretch-induced muscle damage. Three-week-old mdx mice were treated with NAC in the drinking water for 6 weeks. Dihydroethidium staining showed that NAC treatment reduced the concentration of reactive oxygen species (ROS) in mdx muscles. This was accompanied by a significant decrease in centrally nucleated fibres in muscles from NAC-treated mdx mice. Immunoblotting showed that NAC treatment decreased the nuclear protein expression of NF-kappaB, a transcription factor involved in pro-inflammatory cytokine expression. Finally, we show that NAC treatment reduced caveolin-3 protein levels and increased the sarcolemmal expression of beta-dystroglycan and the dystrophin homologue, utrophin. Taken together, our findings suggest that ROS play an important role in the dystrophic pathogenesis, both in terms of activating damage pathways and in regulating the expression of some dystrophin-associated membrane proteins. These results offer the prospect that antioxidants such as NAC could have therapeutic potential for DMD patients.

Mechanosensitive channel properties and membrane mechanics in mouse dystrophic myotubes

Muscular dystrophy is associated with increased activity of mechanosensitive channels (MSCs) and increased cell calcium levels. MSCs in patches from mdx mouse myotubes have higher levels of resting activity, compared to patches from wild-type mice, and a pronounced latency of activation and deactivation. Measurements of patch capacitance and geometry reveal that the differences are linked to cortical membrane mechanics rather than to differences in channel gating. We found unexpectedly that patches from mdx mice are strongly curved towards the pipette tip by actin pulling normal to the membrane. This force produces a substantial tension (approximately 5 mN m(-1)) that can activate MSCs in the absence of overt stimulation. The inward curvature of patches from mdx mice is eliminated by actin inhibitors. Applying moderate suction to the pipette flattens the membrane, reducing tension, and making the response appear to be stretch inactivated. The pronounced latency to activation in patches from mdx mice is caused by the mechanical relaxation time required to reorganize the cortex from inward to outward curvature. The increased latency is equivalent to a three-fold increase in cortical viscosity. Disruption of the cytoskeleton by chemical or mechanical means eliminates the differences in kinetics and curvature between patches from wild-type and mdx mice. The stretch-induced increase in specific capacitance of the patch, approximately 80 fF microm(-2), far exceeds the specific capacitance of bilayers, suggesting the presence of stress-sensitive access to large pools of membrane, possibly caveoli, T-tubules or portions of the gigaseal. In mdx mouse cells the intrinsic gating property of fast voltage-sensitive inactivation is lost. It is robust in wild-type mouse cells (observed in 50% of outside-out patches), but never observed in mdx cells. This link between dystrophin and inactivation may lead to increased background cation currents and Ca2+ influx. Spontaneous Ca2+ transients in mdx mouse cells are sensitive to depolarization and are inhibited by the specific MSC inhibitor GsMTx4, in both the D and L forms.

Altered caveolin-3 expression disrupts PI(3) kinase signaling leading to death of cultured muscle cells

Caveolae and their coat proteins, caveolins, co-ordinate multiple signaling pathways. Caveolin-3 is a muscle-specific caveolin isoform that is deficient in limb girdle muscular dystrophy type 1 C (LGMD1C). Paradoxically, overexpression of this protein also causes muscle degeneration in vivo. We hypothesize that altered membrane expression of caveolin-3 in muscle cells causes a degenerative phenotype by disrupting the co-ordination of signaling pathways that are critical to the maintenance of cell survival. Here, we show for the first time that, in normal muscle cells subjected to oxidative stress, the phosphatidylinositol (3) kinase (PI(3) kinase)-associated proteins PDK1 and Akt associate with caveolae where they bind to caveolin-3, and that normal activation of this pathway promotes cell survival. Either increased or decreased expression of caveolin-3 at the membrane caused an increased susceptibility to oxidative stress, and myotube survival was markedly improved by PI(3) kinase inhibition. This occurred concomitantly with altered phosphorylation of the pro-apoptotic proteins GSK3beta and Bad, despite normal levels of Akt activation. Taken together, our results demonstrate that altered caveolin-3 expression can change the outcome of PI(3) kinase activation from cell survival to cell death. These findings indicate that normal expression and localization of caveolin-3 are required to appropriately co-ordinate PI(3) kinase/Akt-mediated cell survival signaling, and suggest that this pathway may be an effective therapeutic target for the treatment of muscular dystrophies associated with caveolin-3 mutations.

Caveolin-3 Is a Sensitive and Specific Marker for Rhabdomyosarcoma

Caveolin-3 (Cav-3) is a principal structural protein of caveolae membrane domains. Animal studies have revealed that Cav-3 is expressed in skeletal and cardiac myocytes but absent in other types of cells. Recent studies have shown that abnormalities in the Cav-3 gene are associated with some forms of muscular dystrophy, while skeletal muscle abnormalities have been observed in Cav-3 transgenic and knockout mice. In this study the authors evaluated the distribution of Cav-3 in normal human tissues and compared the expression of Cav-3 with that of myogenin and myoD1 in rhabdomyosarcoma (RMS), malignant mixed mullerian tumor (MMMT), and an array of neoplasms that mimic RMS to assess the utility of Cav-3 as a diagnostic marker for tumors with skeletal muscle differentiation. In nonneoplastic human tissues, crisp membrane staining for Cav-3 was present in cardiac and skeletal myocytes and occasionally in arterial smooth muscle cells and prostatic stromal cells, while other cell types were negative for Cav-3. Eighty-eight percent (21/24) of RMS studied were positive for Cav-3. Positive staining was generally observed in the more maturely differentiated tumor cells but not the primitive tumor cells. Eight of nine cases of MMMT stained strongly with Cav-3 in their rhabdomyosarcomatous component but not in other components. Fifty-four other neoplasms (13 leiomyosarcomas, 8 neuroblastomas, 5 lymphomas, 6 Wilms tumors without skeletal muscle differentiation, 5 Ewing sarcomas, 4 malignant fibrous histiocytomas, 4 angiosarcomas, 6 malignant melanomas, and 3 synovial sarcomas) were negative for Cav-3 expression. Nearly all (96% [23/24]) cases of RMS were positive for myogenin, while 88% (21/24) were positive for myoD1. Primitive tumor cells showed significantly increased expression of myoD1 and myogenin; conversely, more differentiated tumor cells were negative or weakly stained. The rhabdomyosarcomatous component of MMMT stained focally with myogenin and myoD1, in contrast to the strong Cav-3 labeling in these cells. These results demonstrate that Cav-3 is specifically expressed in human cardiac and skeletal myocytes. Furthermore, its high specificity and relatively high sensitivity (88%) for tumors with skeletal muscle differentiation suggest that Cav-3 is a valuable marker for these tumors and may be used to assess the degree of differentiation of RMS and to identify residual tumor cells in post-chemotherapy specimens.

Getting rid of caveolins: phenotypes of caveolin-deficient animals

The elucidation of the role of caveolae has been the topic of many investigations which were greatly enhanced after the discovery of caveolin, the protein marker of these flask-shaped plasma membrane invaginations. The generation of mice deficient in the various caveolin genes (cav-1, cav-2 and cav-3) has provided physiological models to unravel the role of caveolins or caveolae at the whole organism level. Remarkably, despite the essential role of caveolins in caveolae biogenesis, all knockout mice are viable and fertile. However, lack of caveolae or caveolins leads to a wide range of phenotypes including muscle, pulmonary or lipid disorders, suggesting their implication in many cellular processes. The aim of this review is to give a broad overview of the phenotypes described for the caveolin-deficient mice and to link them to the numerous functions so far assigned to caveolins/caveolae.

Zebrafish as a model for caveolin-associated muscle disease; caveolin-3 is required for myofibril organization and muscle cell patterning

Caveolae are an abundant feature of many animal cells. However, the exact function of caveolae remains unclear. We have used the zebrafish, Danio rerio, as a system to understand caveolae function focusing on the muscle-specific caveolar protein, caveolin-3 (Cav3). We have identified caveolin-1 (alpha and beta), caveolin-2 and Cav3 in the zebrafish. Zebrafish Cav3 has 72% identity to human CAV3, and the amino acids altered in human muscle diseases are conserved in the zebrafish protein. During embryonic development, cav3 expression is apparent by early segmentation stages in the first differentiating muscle precursors, the adaxial cells and slightly later in the notochord. cav3 expression appears in the somites during mid-segmentation stages and then later in the pectoral fins and facial muscles. Cav3 and caveolae are located along the entire sarcolemma of late stage embryonic muscle fibers, whereas beta-dystroglycan is restricted to the muscle fiber ends. Down-regulation of Cav3 expression causes gross muscle abnormalities and uncoordinated movement. Ultrastructural analysis of isolated muscle fibers reveals defects in myoblast fusion and disorganized myofibril and membrane systems. Expression of the zebrafish equivalent to a human muscular dystrophy mutant, CAV3P104L, causes severe disruption of muscle differentiation. In addition, knockdown of Cav3 resulted in a dramatic up-regulation of eng1a expression resulting in an increase in the number of muscle pioneer-like cells adjacent to the notochord. These studies provide new insights into the role of Cav3 in muscle development and demonstrate its requirement for correct intracellular organization and myoblast fusion.

Reduction of caveolin-3 expression does not inhibit stretch-induced phosphorylation of ERK2 in skeletal muscle myotubes

Mechanotransduction is critical to the maintenance and growth of skeletal muscle, but the mechanism by which cellular deformations are converted to biochemical signals remains unclear. Among the earliest and most ubiquitous responses to mechanical stimulation is the phosphorylation and activation of mitogen-activated protein kinases, in particular ERK2. Caveolin-3 (CAV-3) binds ERK2 and its upstream activators in inactive states on the caveolae of resting muscle. Caveolae are deformed by stretch, and it was hypothesized that this deformation might disrupt the CAV-3-dependent inhibition of ERK2 to affect stretch-induced activation. Stretch-induced phosphorylation of ERK2 in myotubes was both amplitude and velocity dependent, consistent with a viscoelastic mechanism, such as deformation of caveolae. Chemical disruption of caveolae by cholesterol depletion increased ERK2 activation by up to 176%. Small interfering RNA oligomers were then used to knock down expression of CAV-3 in cultured myotubes before mechanical stimulation, with the expectation that reducing CAV-3 expression would eliminate the stretch-induced activation of ERK2. Knockdown reduced CAV-3 protein content by 55% but did not significantly alter the stretch-induced increase in ERK2 phosphorylation, suggesting that CAV-3 is not an essential element of the mechanotransduction pathway, although the limited extent of knockdown limits the strength of this conclusion.

Role of Caveolae and Caveolins in Health and Disease

Although they were discovered more than 50 years ago, caveolae have remained enigmatic plasmalemmal organelles. With their characteristic “flasklike” shape and virtually ubiquitous tissue distribution, these interesting structures have been implicated in a wide range of cellular functions. Similar to clathrin-coated pits, caveolae function as macromolecular vesicular transporters, while their unique lipid composition classifies them as plasma membrane lipid rafts, structures enriched in a variety of signaling molecules. The caveolin proteins (caveolin-1, -2, and -3) serve as the structural components of caveolae, while also functioning as scaffolding proteins, capable of recruiting numerous signaling molecules to caveolae, as well as regulating their activity. That so many signaling molecules and signaling cascades are regulated by an interaction with the caveolins provides a paradigm by which numerous disease processes may be affected by ablation or mutation of these proteins. Indeed, studies in caveolin-deficient mice have implicated these structures in a host of human diseases, including diabetes, cancer, cardiovascular disease, atherosclerosis, pulmonary fibrosis, and a variety of degenerative muscular dystrophies. In this review, we provide an in depth summary regarding the mechanisms by which caveolae and caveolins participate in human disease processes.

Aquaporins in skeletal muscle: reassessment of the functional role of aquaporin‐4

Aquaporin-4 (AQP4) is the major water channel of the neuromuscular system, but its physiological function in both perivascular astrocytes and skeletal muscle sarcolemma is unclear. The purpose of this study was to assess the following in skeletal muscle: a) the expression of all cloned water cannels; b) the functional role of AQP4 using sarcolemma vesicles purified by means of several fractionation methods, and c) the functional effect of AQP4 reduction in mdx mice, the animal model of Duchenne muscular dystrophy (DMD). Immunofluorescence and immunoblot experiments performed with affinity purified antibodies revealed that only AQP1 and AQP4 are expressed in mouse skeletal muscle: AQP1 in endothelial cells of continuous capillaries and AQP4 on the plasma membrane of muscle fiber. Plasma membrane vesicle purification was performed with a procedure extensively used to purify and characterize dystrophin-associated proteins (DAPs) from rabbit skeletal muscle. Western blot analysis showed strong co-enrichment of the analyzed DAPs and AQP4, indicating that the membrane vesicle preparation was highly enriched in sarcolemma. Stopped-flow light-scattering measurements showed high osmotic water permeability of sarcolemma vesicles (approximately 150 microm/s) compatible with the AQP-mediated pathway for water movement. Sarcolemma vesicles prepared from mdx mice revealed, in parallel with AQP4 disappearance from the plasma membrane, a strong reduction in water permeability compared with wild-type mice. Altogether, these results demonstrate high AQP4-mediated water permeability of the skeletal muscle sarcolemma. Expression of sarcolemmal AQP4 together with that of vascular AQP1 may be responsible for the fast water transfer from the blood into the muscle during intense activity. These data imply an important role for aquaporins in skeletal muscle physiology as well as an involvement of AQP4 in the molecular alterations that occur in the muscle of DMD patients.

The Caveolin genes: from cell biology to medicine

Caveolae are vesicular organelles (50-100-nm in diameter) that are particularly abundant in cells of the cardiovascular system, including endothelial cells, smooth muscle cells, macrophages, cardiac myocytes and fibroblasts. In these cell types, caveolae function both in protein trafficking and signal transduction, as well as in cholesterol homeostasis. Caveolins are the structural proteins that are both necessary and sufficient for the formation of caveolae membrane domains. Caveolins 1 and 2 are co-expressed in most cell types, while the expression of caveolin-3 is muscle-specific. Thus, endothelial cells and fibroblasts are rich in caveolins 1 and 2, while cardiac myocytes and skeletal muscle fibers express caveolin-3. In contrast, smooth muscle cells express all three caveolins (Cav-1, -2, and -3). Mechanistically, caveolins interact with a variety of downstream signaling molecules, including Src-family tyrosine kinases, p42/44 mitogen activated protein (MAP) kinase, and endothelial nitric oxide synthase (eNOS), and hold these signal transducers in the inactive conformation until activation by an appropriate stimulus. In many ways, caveolins serve both to compartmentalize and regulate signaling. Recent studies using caveolin-deficient mouse models dramatically show that caveolae and caveolins play a prominent role in various human patho-biological conditions, especially those related to the cardiovascular system. These disease phenotypes include: atherosclerosis, cardiac hypertrophy, cardiomyopathy, pulmonary hypertension, and neointimal hyperplasia (smooth muscle cell proliferation). In addition, caveolins play a significant role in other disease phenotypes, such as cancer, diabetes, bladder dysfunction, and muscular dystrophy, as we discuss in this review. Thus, caveolin-deficient mice will serve as important new animal models to dissect the intricate role of caveolae and caveolins in the pathogenesis of human diseases.

Proteasome inhibitor (MG-132) treatment of mdx mice rescues the expression and membrane localization of dystrophin and dystrophin-associated proteins

Dystrophin, the protein product of the Duchenne muscular dystrophy (DMD) gene, is absent in the skeletal muscle of DMD patients and mdx mice. At the plasma membrane of skeletal muscle fibers, dystrophin associates with a multimeric protein complex, termed the dystrophin-glycoprotein complex (DGC). Protein members of this complex are normally absent or greatly reduced in dystrophin-deficient skeletal muscle fibers, and are thought to undergo degradation through an unknown pathway. As such, we reasoned that inhibition of the proteasomal degradation pathway might rescue the expression and subcellular localization of dystrophin-associated proteins. To test this hypothesis, we treated mdx mice with the well-characterized proteasomal inhibitor MG-132. First, we locally injected MG-132 into the gastrocnemius muscle, and observed the outcome after 24 hours. Next, we performed systemic treatment using an osmotic pump that allowed us to deliver different concentrations of the proteasomal inhibitor, over an 8-day period. By immunofluorescence and Western blot analysis, we show that administration of the proteasomal inhibitor MG-132 effectively rescues the expression levels and plasma membrane localization of dystrophin, beta-dystroglycan, alpha-dystroglycan, and alpha-sarcoglycan in skeletal muscle fibers from mdx mice. Furthermore, we show that systemic treatment with the proteasomal inhibitor 1) reduces muscle membrane damage, as revealed by vital staining (with Evans blue dye) of the diaphragm and gastrocnemius muscle isolated from treated mdx mice, and 2) ameliorates the histopathological signs of muscular dystrophy, as judged by hematoxylin and eosin staining of muscle biopsies taken from treated mdx mice. Thus, the current study opens new and important avenues in our understanding of the pathogenesis of DMD. Most importantly, these new findings may have clinical implications for the pharmacological treatment of patients with DMD.

Modulation of myoblast fusion by caveolin-3 in dystrophic skeletal muscle cells: implications for Duchenne muscular dystrophy and limb-girdle muscular dystrophy-1C

Caveolae are vesicular invaginations of the plasma membrane. Caveolin-3 is the principal structural component of caveolae in skeletal muscle cells in vivo. We have recently generated caveolin-3 transgenic mice and demonstrated that overexpression of wild-type caveolin-3 in skeletal muscle fibers is sufficient to induce a Duchenne-like muscular dystrophy phenotype. In addition, we have shown that caveolin-3 null mice display mild muscle fiber degeneration and T-tubule system abnormalities. These data are consistent with the mild phenotype observed in Limb-girdle muscular dystrophy-1C (LGMD-1C) in humans, characterized by a approximately 95% reduction of caveolin-3 expression. Thus, caveolin-3 transgenic and null mice represent valid mouse models to study Duchenne muscular dystrophy (DMD) and LGMD-1C, respectively, in humans. Here, we derived conditionally immortalized precursor skeletal muscle cells from caveolin-3 transgenic and null mice. We show that overexpression of caveolin-3 inhibits myoblast fusion to multinucleated myotubes and lack of caveolin-3 enhances the fusion process. M-cadherin and microtubules have been proposed to mediate the fusion of myoblasts to myotubes. Interestingly, we show that M-cadherin is downregulated in caveolin-3 transgenic cells and upregulated in caveolin-3 null cells. For the first time, variations of M-cadherin expression have been linked to a muscular dystrophy phenotype. In addition, we demonstrate that microtubules are disorganized in caveolin-3 null myotubes, indicating the importance of the cytoskeleton network in mediating the phenotype observed in these cells. Taken together, these results propose caveolin-3 as a key player in myoblast fusion and suggest that defects of the fusion process may represent additional molecular mechanisms underlying the pathogenesis of DMD and LGMD-1C in humans.

Caveolin: a key target for modulating nitric oxide availability in health and disease

The endothelial layer is a key component of the cardiovascular system. Recent evidence indicates that strategies aimed at preserving the endothelium may have important implications in the battle against cardiovascular disease. Nitric oxide remains the critical factor determinant of endothelial function. Understanding the regulatory components involved in nitric oxide production may elucidate novel targets for improving compromised vascular function. The caveolae/caveolin system has recently become of interest due to its ability to regulate endothelial nitric oxide synthase activity. The caveolae/caveolin system is a multifaceted structure in the plasma membrane, which plays an integral role in cellular signaling. Recognizing the potential of this specialized domain may provide the fundamental knowledge to target the endothelium in disease.

Caveolin-3 knock-out mice develop a progressive cardiomyopathy and show hyperactivation of the p42/44 MAPK cascade

A growing body of evidence suggests that muscle cell caveolae may function as specialized membrane micro-domains in which the dystrophin-glycoprotein complex and cellular signaling molecules reside. Caveolin-3 (Cav-3) is the only caveolin family member expressed in striated muscle cell types (cardiac and skeletal). Interestingly, skeletal muscle fibers from Cav-3 (-/-) knock-out mice show a number of myopathic changes, consistent with a mild-to-moderate muscular dystrophy phenotype. However, it remains unknown whether a loss of Cav-3 affects the phenotypic behavior cardiac myocytes in vivo. Here, we present a detailed characterization of the hearts of Cav-3 knock-out mice. We show that these mice develop a progressive cardiomyopathic phenotype. At four months of age, Cav-3 knock-out hearts display significant hypertrophy, dilation, and reduced fractional shortening, as revealed by gated cardiac MRI and transthoracic echocardiography. Histological analysis reveals marked cardiac myocyte hypertrophy, with accompanying cellular infiltrates and progressive interstitial/peri-vascular fibrosis. Interestingly, loss of Cav-3 expression in the heart does not change the expression or the membrane association of the dystrophin-glycoprotein (DG) complex. However, a marker of the DG complex, alpha-sarcoglycan, was specifically excluded from lipid raft domains in the absence of Cav-3. Because activation of the Ras-p42/44 MAPK pathway in cardiac myocytes can drive cardiac hypertrophy, we next assessed the activation state of this pathway using a phospho-specific antibody probe. We show that p42/44 MAPK (ERK1/2) is hyperactivated in hearts derived from Cav-3 knock-out mice. These results are consistent with previous in vitro data demonstrating that caveolins may function as negative regulators of the p42/44 MAPK cascade. Taken together, our data argue that loss of Cav-3 expression is sufficient to induce a molecular program leading to cardiac myocyte hypertrophy and cardiomyopathy.

Altered aquaporin-4 expression in human muscular dystrophies: a common feature?

Duchenne Muscular Dystrophy (DMD) is a progressive lethal muscle disease that affects young boys. Dystrophin, absent in DMD and reduced in the milder form Becker Muscular Dystrophy (BMD), binds to several membrane-associated proteins known as dystrophin-associated proteins (DAPs). Once this critical structural link is disrupted, muscle fibers become more vulnerable to mechanical and osmotic stress. Recently, we have reported that the expression of aquaporin-4 (AQP4), a water-selective channel expressed in the sarcolemma of fast-twitch fibers and astrocyte end-feet, is drastically reduced in the muscle and brain of the mdx mouse, the animal model of DMD. In the present study, we analyzed the expression of AQP4 in several DMD/BMD patients of different ages with different mutations in the dystrophin gene. Immunofluorescence results indicate that, compared with healthy control children, AQP4 is reduced severely in all the DMD muscular biopsies analyzed and in 50% of the analyzed BMD. Western blot analysis revealed that the deficiency in sarcolemma AQP4 staining is due to a reduction in total AQP4 muscle protein content rather than to changes in immunoreactivity. Double-immunostaining experiments indicate that AQP4 reduction is independent of changes in the fiber myosin heavy chain composition. AQP4 and a-syntrophin analysis of BMD muscular biopsies revealed that the expression and stability of AQP4 in the sarcolemma does not always decrease when a-syntrophin is strongly reduced. Finally, limb-girdle muscular dystrophy biopsies and facioscapulohumeral muscular dystrophy revealed that AQP4 expression was not altered in these forms of muscular dystrophy. These experiments provide the first evidence of AQP4 reduction in a human pathology and show that this deficiency is an important feature of DMD/BMD.

Changes in the distribution and density of caveolin 3 molecules at the plasma membrane of mdx mouse skeletal muscles: a fracture-label electron microscopic study

To analyze the molecular mechanism of the increased caveolin 3 activities in dystrophin-deficient muscles, we investigated three-dimensionally the changes in caveolin 3 molecular distribution and density at the sarcolemma of mdx mice by the fracture-label electron microscopic technique. At the sarcolemma of skeletal muscles from mdx mice, the densities of gold particles associated with caveolae, non-associated with caveolae and arranged circularly without caveolae were higher than those in control mice (P<0.01, P<0.01 and P<0.05 by two-tailed t-test), although in mdx mice, the overall arrangement of gold particles appeared to be irregular. These findings may reflect the active process of caveolar formation and the results of the disrupted protein-protein interaction in dystrophin-deficient muscle plasma membrane.

Dystroglycan is not required for localization of dystrophin, syntrophin, and neuronal nitric-oxide synthase at the sarcolemma but regulates integrin alpha 7B expression and caveolin-3 distribution

Dystroglycan is part of the dystrophin-associated protein complex, which joins laminin in the extracellular matrix to dystrophin within the subsarcolemmal cytoskeleton. We have investigated how mutations in the components of the laminin-dystroglycan-dystrophin axis affect the organization and expression of dystrophin-associated proteins by comparing mice mutant for merosin (alpha(2)-laminin, dy), dystrophin (mdx), and dystroglycan (Dag1) using immunohistochemistry and immunoblots. We report that syntrophin and neuronal nitric-oxide synthase are depleted in muscle fibers lacking both dystrophin and dystroglycan. Some fibers deficient in dystroglycan, however, localize dystrophin at the cell surface at levels similar to that in wild-type muscle. Nevertheless, these fibers have signs of degeneration/regeneration including increased cell surface permeability and central nuclei. In these fibers, syntrophin and nitric-oxide synthase are also localized to the plasma membrane, whereas the sarcoglycan complex is disrupted. These results suggest a mechanism of membrane attachment for dystrophin independent of dystroglycan and that the interaction of sarcoglycans with dystrophin requires dystroglycan. The distribution of caveolin-3, a muscle-specific component of caveolae recently found to bind dystroglycan, was affected in dystroglycan- and dystrophin-deficient mice. We also examined alternative mechanisms of cell-extracellular matrix attachment to elucidate how the muscle basement membrane may subsist in the absence of dystroglycan, and we found the alpha(7B) splice variant of the alpha(7) integrin receptor subunit to be up-regulated. These results support the possibility that alpha(7B) integrin compensates in mediating cell-extracellular matrix attachment but cannot rescue the dystrophic phenotype.

Muscle plasma membrane changes in dystrophin gene exon 52 knockout mouse

Changes in muscle plasma membranes in mice lacking exon 52 of the dystrophin gene (mdx52 mouse) were studied using the freeze-fracture technique. The extensor digitorum longus (EDL) muscle plasma membrane of the mdx52 mouse at 8 weeks of age showed significantly increased caveola density (p < 0.05 by two-tailed t-test) and significantly decreased densities of intramembranous particles (IMPs), orthogonal arrays (OAs) and orthogonal array subunit particles (OASPs) (p < 0.05 by two-tailed t-test, p < 0.01 by Wilcoxon rank-sum test, p < 0.05 by two-tailed t-test, respectively) on the protoplasmic face when compared with those of control EDL muscles. These changes are more similar to those seen in DMD than those in the mdx mouse at the same age as reported previously. Thus, the gene abnormality in the different exon of the mouse dystrophin gene seems to induce somewhat different changes in the muscle plasma membrane.

The dystrophin-glycoprotein complex, cellular signaling, and the regulation of cell survival in the muscular dystrophies

Mutations of different components of the dystrophin-glycoprotein complex (DGC) cause muscular dystrophies that vary in terms of severity, age of onset, and selective involvement of muscle groups. Although the primary pathogenetic processes in the muscular dystrophies have clearly been identified as apoptotic and necrotic muscle cell death, the pathogenetic mechanisms that lead to cell death remain to be determined. Studies of components of the DGC in muscle and in nonmuscle tissues have revealed that the DGC is undoubtedly a multifunctional complex and a highly dynamic structure, in contrast to the unidimensional concept of the DGC as a mechanical component in the cell. Analysis of the DGC reveals compelling analogies to two other membrane-associated protein complexes, namely integrins and caveolins. Each of these complexes mediates signal transduction cascades in the cell, and disruption of each complex causes muscular dystrophies. The signal transduction cascades associated with the DGC, like those associated with integrins and caveolins, play important roles in cell survival signaling, cellular defense mechanisms, and regulation of the balance between cell survival and cell death. This review focuses on the functional components of the DGC, highlighting the evidence of their participation in cellular signaling processes important for cell survival. Elucidating the link between these functional components and the pathogenetic processes leading to cell death is the foremost challenge to understanding the mechanisms of disease expression in the muscular dystrophies due to defects in the DGC.

Association of neuronal nitric oxide synthase (nNOS) with alpha1-syntrophin at the sarcolemma

alpha1-syntrophin is a PDZ-containing dystrophin-associated protein, expressed predominantly in striated muscle and brain. alpha1-syntrophin null mice generated by gene targeting technique showed no overt muscular dystrophic phenotype. Though other dystrophin-associated proteins were localized at the sarcolemma, neuronal nitric oxide synthase (nNOS) was selectively lost from the membrane fraction but remained in the cytoplasm. Thus, the alpha1-syntrophin null mice are useful in the elucidation of the functional importance of nNOS targeting at the sarcolemma. In addition, the mice would facilitate identification of other signaling molecules, which are targeted to dystrophin complex via interaction with alpha1-syntrophin.