A caveolin-3 mutant that causes limb girdle muscular dystrophy type 1C disrupts Src localization and activity and induces apoptosis in skeletal myotubes

Molecular cloning, tissue expression and polymorphism analysis of the Caveolin-3 gene in ducks

1. Duck meat is considered a delicacy, but choosing the best meat is problematic. Caveolin-3 (CAV-3) is a muscle-specific protein marker in animals. The goal of the current study was to detect the characteristics of CAV-3 gene in ducks. 2. Full-length CAV-3 was acquired from ducks (Anas platyrhynchos) using reverse transcriptase PCR and rapid amplification of cDNA ends. DNAMAN software was used for homology comparisons. Quantitative reverse transcription-PCR, PCR-single-strand conformation polymorphism, and sequencing were used to determine CAV-3 expression and polymorphism of a single nucleotide, respectively. The study examined four types of ducks, including Jinding, Chaohu, Cherry Valley, and Gaoyou ducks. 3. The study acquired 1066 bp of CAV-3 cDNA sequences, including a 456 bp complete open reading frame encoding 151 amino acids. Both coding sequences (CDSs) and translated amino acids exhibited highest homology with Gallus gallus (CDS homology 91.67%, amino acids 94.04%), followed by mammalian species (CDS homology 79.0%, amino acids 78.0%). Single nucleotide polymorphism analysis revealed five mutations in exons (A489G, G501A, A557G, T563A, and A602G), and a C805T mutation in an intron. Among amplified polymorphic loci detected using primer 2, allele frequency was higher for A (489A501G507A563T602A) than B (489G501G507G563T602C) or C (489G501A507G563A602C). The highest occurred in Cherry Valley ducks (0.7587). Using primer 4, the M allele frequency was higher than that of the N allele. CAV-3 was most highly expressed in the heart, followed by skeletal muscles. Additionally, CAV-3 had higher expression in heart and breast muscle of overfed Muscovy ducks than control ducks, but no difference was seen in thigh muscle. 4. CAV-3 in ducks had the highest homology with Gallus gallus CAV-3, and it could be used as a marker for muscle quality in ducks.

Caveolins and cavins in the trafficking, maturation, and degradation of caveolae: implications for cell physiology

Caveolins (Cavs) are ~20 kDa scaffolding proteins that assemble as oligomeric complexes in lipid raft domains to form caveolae, flask-shaped plasma membrane (PM) invaginations. Caveolae ("little caves") require lipid-lipid, protein-lipid, and protein-protein interactions that can modulate the localization, conformational stability, ligand affinity, effector specificity, and other functions of proteins that are partners of Cavs. Cavs are assembled into small oligomers in the endoplasmic reticulum (ER), transported to the Golgi for assembly with cholesterol and other oligomers, and then exported to the PM as an intact coat complex. At the PM, cavins, ~50 kDa adapter proteins, oligomerize into an outer coat complex that remodels the membrane into caveolae. The structure of caveolae protects their contents (i.e., lipids and proteins) from degradation. Cellular changes, including signal transduction effects, can destabilize caveolae and produce cavin dissociation, restructuring of Cav oligomers, ubiquitination, internalization, and degradation. In this review, we provide a perspective of the life cycle (biogenesis, degradation), composition, and physiologic roles of Cavs and caveolae and identify unanswered questions regarding the roles of Cavs and cavins in caveolae and in regulating cell physiology.¹.

Attenuated rapid onset vasodilation with greater force production in skeletal muscle of caveolin-2-/- mice

Caveolin-2 (Cav2) is a major protein component of caveolae in membranes of vascular smooth muscle and endothelium, yet its absence alters the ultrastructure of skeletal muscle fibers. To gain insight into Cav2 function in skeletal muscle, we tested the hypothesis that genetic deletion of Cav2 would alter microvascular reactivity and depress contractile function of skeletal muscle in vivo. In the left gluteus maximus muscle (GM) of anesthetized Cav2(-/-) and wild-type (WT) male mice (age, 6 mo), microvascular responses to physiological agonists and to GM contractions were studied at 34°C. For feed arteries (FA), first- (1A), second- (2A) and third-order (3A) arterioles, respective mean diameters at rest (45, 35, 25, 12 μm) and during maximal dilation (65, 55, 45, 30 μm) were similar between groups. Cumulative dilations to ACh (10(-9) to 10(-5) M) and constrictions to norepinephrine (10(-9) to 10(-5) M) were also similar between groups, as were steady-state dilations during rhythmic twitch contractions (2 and 4 Hz; 30 s). For single tetanic contractions (100 Hz; 100, 250, and 500 ms), rapid onset vasodilation (ROV) increased with contraction duration throughout networks in GM of both groups but was reduced by nearly half in Cav2(-/-) mice compared with WT mice (P < 0.05). Nevertheless, maximal force during tetanic contraction was ∼40% greater in GM of Cav2(-/-) vs. WT mice (152 ± 14 vs. 110 ± 3 mN per square millimeter, respectively; P < 0.05). Thus, while structural and functional properties of resistance networks are well maintained in the GM of Cav2(-/-) mice, diminished ROV with greater force production reveals novel physiological roles for Cav2 in skeletal muscle.

Non-canonical roles for caveolin in regulation of membrane repair and mitochondria: implications for stress adaptation with age

Many different theories of ageing have been proposed but none has served the unifying purpose of defining a molecular target that can limit the structural and functional decline associated with age that ultimately leads to the demise of the organism. We propose that the search for a molecule with these unique properties must account for regulation of the signalling efficiency of multiple cellular functions that degrade with age due to a loss of a particular protein. We suggest caveolin as one such molecule that serves as a regulator of key processes in signal transduction. We define a particular distinction between cellular senescence and ageing and propose that caveolin plays a distinct role in each of these processes. Caveolin is traditionally thought of as a membrane-localized protein regulating signal transduction via membrane enrichment of specific signalling molecules. Ultimately we focus on two non-canonical roles for caveolin - membrane repair and regulation of mitochondrial function - which may be novel features of stress adaptation, especially in the setting of ageing. The end result of preserving membrane structure and mitochondrial function is maintenance of homeostatic signalling, preserving barrier function, and regulating energy production for cell survival and resilient ageing.

CAV3 gene sequence variations: National Genome Database and clinics

INTRODUCTION

Caveolinopathies are a group of untreatable, degenerative muscle diseases associated with caveolin 3 (CAV3) gene mutations.

OBJECTIVES

The goal of this study was to characterize the role of the CAV3 gene in patients with limb-girdle muscular dystrophy, hyperCKemia, cardiomyopathies, as well as utilization of the National Genome Database in clinical applications.

MATERIALS AND METHODS

We sequenced the coding region and exon/intron boundaries of CAV3 gene in 81 neuromuscular disorder patients, a sample group from the National Genome Database, consisting of 97 individuals with cardiomyopathies, and also random selection of 100 persons. Immunohistochemical staining of muscle biopsy was performed to verify findings in one case, as the setup for the project was to use less invasive molecular biology methods.

RESULTS

We identified three novel sequence variations (c.183C>G, p.S61R; c.220C>A, p.R74S; c.220C>T, p.R74C) and found evidence that one was associated with hypercreatine kinase-emia. Two previously reported mutations in families with limb-girdle muscular dystrophy were found. No mutations were identified in the cohort of patients with cardiomyopathies.

DISCUSSION

CAV3 gene encodes muscle-specific protein with dominant negative type of missense mutations in it causing various phenotypes. Our study confirmed CAV3 gene involvement in neuromuscular disorders, but found no evidence in the group of patients with cardiomyopathies. Persons included in the National Genome Database could be screened for late onset Mendelian diseases.

MURC/cavin-4 Is Co-Expressed with Caveolin-3 in Rhabdomyosarcoma Tumors and Its Silencing Prevents Myogenic Differentiation in the Human Embryonal RD Cell Line

The purpose of this study was to investigate whether MURC/cavin-4, a plasma membrane and Z-line associated protein exhibiting an overlapping distribution with Caveolin-3 (Cav-3) in heart and muscle tissues, may be expressed and play a role in rhabdomyosarcoma (RMS), an aggressive myogenic tumor affecting childhood. We found MURC/cavin-4 to be expressed, often concurrently with Cav-3, in mouse and human RMS, as demonstrated through in silico analysis of gene datasets and immunohistochemical analysis of tumor samples. In vitro expression studies carried out using human cell lines and primary mouse tumor cultures showed that expression levels of both MURC/cavin-4 and Cav-3, while being low or undetectable during cell proliferation, became robustly increased during myogenic differentiation, as detected via semi-quantitative RT-PCR and immunoblotting analysis. Furthermore, confocal microscopy analysis performed on human RD and RH30 cell lines confirmed that MURC/cavin-4 mostly marks differentiated cell elements, colocalizing at the cell surface with Cav-3 and labeling myosin heavy chain (MHC) expressing cells. Finally, MURC/cavin-4 silencing prevented the differentiation in the RD cell line, leading to morphological cell impairment characterized by depletion of myogenin, Cav-3 and MHC protein levels. Overall, our data suggest that MURC/cavin-4, especially in combination with Cav-3, may play a consistent role in the differentiation process of RMS.

Oxidative stress induces caveolin 1 degradation and impairs caveolae functions in skeletal muscle cells

Increased level of oxidative stress, a major actor of cellular aging, impairs the regenerative capacity of skeletal muscle and leads to the reduction in the number and size of muscle fibers causing sarcopenia. Caveolin 1 is the major component of caveolae, small membrane invaginations involved in signaling and endocytic trafficking. Their role has recently expanded to mechanosensing and to the regulation of oxidative stress-induced pathways. Here, we increased the amount of reactive oxidative species in myoblasts by addition of hydrogen peroxide (H₂O₂) at non-toxic concentrations. The expression level of caveolin 1 was significantly decreased as early as 10 min after 500 μM H₂O₂ treatment. This reduction was not observed in the presence of a proteasome inhibitor, suggesting that caveolin 1 was rapidly degraded by the proteasome. In spite of caveolin 1 decrease, caveolae were still able to assemble at the plasma membrane. Their functions however were significantly perturbed by oxidative stress. Endocytosis of a ceramide analog monitored by flow cytometry was significantly diminished after H₂O₂ treatment, indicating that oxidative stress impaired its selective internalization via caveolae. The contribution of caveolae to the plasma membrane reservoir has been monitored after osmotic cell swelling. H₂O₂ treatment increased membrane fragility revealing that treated cells were more sensitive to an acute mechanical stress. Altogether, our results indicate that H₂O₂ decreased caveolin 1 expression and impaired caveolae functions. These data give new insights on age-related deficiencies in skeletal muscle.

An approach to identify SNPs in the gene encoding acetyl-CoA acetyltransferase-2 (ACAT-2) and their proposed role in metabolic processes in pig

The novel liver protein acetyl-CoA acetyltransferase-2 (ACAT2) is involved in the beta-oxidation and lipid metabolism. Its comprehensive relative expression, in silico non-synonymous single nucleotide polymorphism (nsSNP) analysis, as well as its annotation in terms of metabolic process with another protein from the same family, namely, acetyl-CoA acyltransferase-2 (ACAA2) was performed in Sus scrofa. This investigation was conducted to understand the most important nsSNPs of ACAT2 in terms of their effects on metabolic activities and protein conformation. The two most deleterious mutations at residues 122 (I to V) and 281 (R to H) were found in ACAT2. Validation of expression of genes in the laboratory also supported the idea of differential expression of ACAT2 and ACAA2 conceived through the in silico analysis. Analysis of the relative expression of ACAT2 and ACAA2 in the liver tissue of Jeju native pig showed that the former expressed significantly higher (P<0.05). Overall, the computational prediction supported by wet laboratory analysis suggests that ACAT2 might contribute more to metabolic processes than ACAA2 in swine. Further associations of SNPs in ACAT2 with production traits might guide efforts to improve growth performance in Jeju native pigs.

Identification and functional analysis of a new putative caveolin-3 variant found in a patient with sudden unexplained death

Background

Sudden cardiac death (SCD) is the clinical outcome of a lethal arrhythmia that can develop on the background of unrecognized channelopathies or cardiomyopathies. Several susceptibility genes have been identified for the congenital forms of these cardiac diseases, including caveolin-3 (Cav-3) gene. In the heart Cav-3 is the main component of caveolae, plasma membrane domains that regulate multiple cellular processes highly relevant for cardiac excitability, such as trafficking, calcium homeostasis, signal transduction and cellular response to injury. Here we characterized a new putative Cav-3 variant, Cav-3 V82I, found in a patient with SCD.

Results

In heterologous systems Cav-3 V82I was expressed at significantly higher level than Cav-3 WT and accumulated within the cells. Cells expressing Cav-3 V82I exhibited a decreased activation of extracellular-signal-regulated kinases (ERKs) and were more vulnerable to sub-lethal osmotic stress.

Conclusion

Considering that abnormal loss of myocytes can play a mechanistic role in lethal cardiac diseases, we suggest that the detrimental effect of Cav-3 V82I variant on cell viability may participate in determining the susceptibility to cardiac death.

Palmitate diet-induced loss of cardiac caveolin-3: a novel mechanism for lipid-induced contractile dysfunction

Obesity is associated with an increased risk of cardiomyopathy, and mechanisms linking the underlying risk and dietary factors are not well understood. We tested the hypothesis that dietary intake of saturated fat increases the levels of sphingolipids, namely ceramide and sphingomyelin in cardiac cell membranes that disrupt caveolae, specialized membrane micro-domains and important for cellular signaling. C57BL/6 mice were fed two high-fat diets: palmitate diet (21% total fat, 47% is palmitate), and MCT diet (21% medium-chain triglycerides, no palmitate). We established that high-palmitate feeding for 12 weeks leads to 40% and 50% increases in ceramide and sphingomyelin, respectively, in cellular membranes. Concomitant with sphingolipid accumulation, we observed a 40% reduction in systolic contractile performance. To explore the relationship of increased sphingolipids with caveolins, we analyzed caveolin protein levels and intracellular localization in isolated cardiomyocytes. In normal cardiomyocytes, caveolin-1 and caveolin-3 co-localize at the plasma membrane and the T-tubule system. However, mice maintained on palmitate lost 80% of caveolin-3, mainly from the T-tubule system. Mice maintained on MCT diet had a 90% reduction in caveolin-1. These data show that caveolin isoforms are sensitive to the lipid environment. These data are further supported by similar findings in human cardiac tissue samples from non-obese, obese, non-obese cardiomyopathic, and obese cardiomyopathic patients. To further elucidate the contractile dysfunction associated with the loss of caveolin-3, we determined the localization of the ryanodine receptor and found lower expression and loss of the striated appearance of this protein. We suggest that palmitate-induced loss of caveolin-3 results in cardiac contractile dysfunction via a defect in calcium-induced calcium release.

Caveolae, caveolin, and cavins: potential targets for the treatment of cardiac disease

Caveolae are omega-shaped membrane invaginations present in essentially all cell types of the cardiovascular system, including endothelial cells, smooth muscle cells, macrophages, cardiac myocytes, and fibroblasts. Numerous functions have been ascribed to this omega-shaped structure. Caveolae are enriched with different signaling molecules and ion channel regulatory proteins and function both in protein trafficking and signal transduction in these cell types. Caveolins are the structural proteins that are necessary for the formation of caveola membrane domains. Mechanistically, caveolins interact with a variety of downstream signaling molecules, as, for example, Src-family tyrosine kinase, p42/44 mitogen-activated protein (MAP) kinase, and endothelial nitric oxide synthase (eNOS) and hold the signal transducers in the inactive condition until activated with proper stimulus. Caveolae are gradually acquiring increasing attention as cellular organelles contributing to the pathogenesis of several structural and functional processes including cardiac hypertrophy, atherosclerosis, and heart failure. At present, very little is known about the role of caveolae in cardiac function and dysfunction, although recent studies with caveolin knock-out mouse have shown that caveolae and caveolins play a pivotal role in various human pathobiological conditions. This review will discuss the possible role and mechanism of action of caveolae and caveolins in different cardiac diseases.

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.

Src family kinases mediate epidermal growth factor receptor signaling from lipid rafts in breast cancer cells

Activation of the epidermal growth factor receptor (EGFR) regulates cellular proliferation, survival, and migration of breast cancer cells. In particular, EGFR recruits signaling proteins to the cell membrane leading to their phosphorylation and activation. However, EGFR also localizes to other cellular structures, including endosomes, mitochondrion, and nuclei. Recently, we demonstrated that lipid raft localization of EGFR in triple-negative breast cancer cell lines promotes EGFR protein-dependent, EGFR kinase-independent activation of Akt. Here, we further define the mechanism by which lipid rafts regulate EGFR signaling to Akt. Specifically, we show that the non-receptor tyrosine kinase c-Src co-localizes and co-associates with EGFR and lipid rafts. Breast cancer cells resistant to treatment with EGFR inhibitors, were also resistant to treatment with Src family kinase (SFK) inhibitors; however, the combination of EGFR and SFK inhibitors synergistically decreases cell viability. We found that this decrease in cell viability observed with EGFR and SFK inhibitor co-treatment correlates with loss of Akt phosphorylation. In addition, we found that in breast cancer cell lines with EGFR and c-Src co-localized to lipid rafts, phospho-inositide 3 kinase (PI3K) was also associated with lipid rafts. Together, the data herein suggest that lipid rafts provide a platform for the interaction of EGFR, c-Src, and PI3K, leading to activation of cellular survival signaling in breast cancer cells.

Lipid raft in cardiac health and disease

Lipid rafts are sphingolipid and cholesterol rich micro-domains of the plasma membrane that coordinate and regulate varieties of signaling processes. Lipid rafts are also present in cardiac myocytes and are enriched in signaling molecules and ion channel regulatory proteins. Lipid rafts are receiving increasing attention as cellular organelles contributing to the pathogenesis of several structural and functional processes including cardiac hypertrophy and heart failure. At present, very little is known about the role of lipid rafts in cardiac function and dysfunction. This review will discuss the possible role of lipid rafts in cardiac health and disease.

Caveolin-3 undergoes SUMOylation by the SUMO E3 ligase PIASy: sumoylation affects G-protein-coupled receptor desensitization

Caveolin (Cav) proteins in the plasma membrane have numerous binding partners, but the determinants of these interactions are poorly understood. We show here that Cav-3 has a small ubiquitin-like modifier (SUMO) consensus motif (ΨKX(D/E, where Ψ is a hydrophobic residue)) near the scaffolding domain and that Cav-3 is SUMOylated in a manner that is enhanced by the SUMO E3 ligase PIASy (protein inhibitor of activated STAT-y). Site-directed mutagenesis revealed that the consensus site lysine is the preferred SUMOylation site but that mutation of all lysines is required to abolish SUMOylation. Co-expression of a SUMOylation-deficient mutant of Cav-3 with β-adrenergic receptors (βARs) alters the expression level of β(2)ARs but not β(1)ARs following agonist stimulation, thus implicating Cav-3 SUMOylation in the mechanisms for β(2)AR but not β(1)AR desensitization. Expression of endothelial nitric-oxide synthase (NOS3) was not altered by the SUMOylation-deficient mutant. Thus, SUMOylation is a covalent modification of caveolins that influence the regulation of certain signaling partners.

Caveolin-3 is a direct molecular partner of the Cav1.1 subunit of the skeletal muscle L-type calcium channel

Caveolin-3 is the striated muscle specific isoform of the scaffolding protein family of caveolins and has been shown to interact with a variety of proteins, including ion channels. Mutations in the human CAV3 gene have been associated with several muscle disorders called caveolinopathies and among these, the P104L mutation (Cav-3(P104L)) leads to limb girdle muscular dystrophy of type 1C characterized by the loss of sarcolemmal caveolin. There is still no clear-cut explanation as to specifically how caveolin-3 mutations lead to skeletal muscle wasting. Previous results argued in favor of a role for caveolin-3 in dihydropyridine receptor (DHPR) functional regulation and/or T-tubular membrane localization. It appeared worth closely examining such a functional link and investigating if it could result from the direct physical interaction of the two proteins. Transient expression of Cav-3(P104L) or caveolin-3 specific siRNAs in C2C12 myotubes both led to a significant decrease of the L-type Ca(2+) channel maximal conductance. Immunolabeling analysis of adult skeletal muscle fibers revealed the colocalization of a pool of caveolin-3 with the DHPR within the T-tubular membrane. Caveolin-3 was also shown to be present in DHPR-containing triadic membrane preparations from which both proteins co-immunoprecipitated. Using GST-fusion proteins, the I-II loop of Ca(v)1.1 was identified as the domain interacting with caveolin-3, with an apparent affinity of 60nM. The present study thus revealed a direct molecular interaction between caveolin-3 and the DHPR which is likely to underlie their functional link and whose loss might therefore be involved in pathophysiological mechanisms associated to muscle caveolinopathies.

Over-expression of BCL2 rescues muscle weakness in a mouse model of oculopharyngeal muscular dystrophy

Oculopharyngeal muscular dystrophy (OPMD) is a late-onset muscular dystrophy caused by a polyalanine expansion mutation in the coding region of the poly-(A) binding protein nuclear 1 (PABPN1) gene. In unaffected individuals, (GCG)(6) encodes the first 6 alanines in a homopolymeric stretch of 10 alanines. In most patients, this (GCG)(6) repeat is expanded to (GCG)(8-13), leading to a stretch of 12-17 alanines in mutant PABPN1, which is thought to confer a toxic gain of function. Thus, OPMD has been modelled by expressing mutant PABPN1 transgenes in the presence of endogenous copies of the gene in cells and mice. In these models, increased apoptosis is seen, but it is unclear whether this process mediates OPMD. The role of apoptosis in the pathogenesis of different muscular dystrophies is unclear. Blocking apoptosis ameliorates muscle disease in some mouse models of muscular dystrophy such as laminin α-2-deficient mice, but not in others such as dystrophin-deficient (mdx) mice. Here we demonstrate that apoptosis is not only involved in the pathology of OPMD but also is a major contributor to the muscle weakness and dysfunction in this disease. Genetically blocking apoptosis by over-expressing BCL2 ameliorates muscle weakness in our mouse model of OPMD (A17 mice). The effect of BCL2 co-expression on muscle weakness is transient, since muscle weakness is apparent in mice expressing both A17 and BCL2 transgenes at late time points. Thus, while apoptosis is a major pathway that causes muscle weakness in OPMD, other cell death pathways may also contribute to the disease when apoptosis is inhibited.

Caveolinopathies: translational implications of caveolin-3 in skeletal and cardiac muscle disorders

Caveolae are specialized lipid rafts localized on the cytoplasmic surface of the sarcolemmal membrane. Caveolae contribute to the maintenance of plasma membrane integrity, constitute specific macromolecular complexes that provide highly localized regulation of ion channels, and regulate vesicular trafficking and signal transduction. In skeletal muscle, the main structural assembly of caveolae is mediated by caveolin-3. Another family of adapter proteins, the cavins, is involved in the regulation of caveolae function and in the trafficking of caveolin-derived structures. Caveolin-3 defects lead to four distinct skeletal muscle disease phenotypes: limb-girdle muscular dystrophy, rippling muscle disease, distal myopathy, and hyperCKemia. Many patients show an overlap of these symptoms, and the same mutation can be linked to different clinical phenotypes. An ever-growing interest is also focused on the association between caveolin-3 mutations and heart disorders. Indeed, caveolin-3 mutants have been described in a patient with hypertrophic cardiomyopathy and two patients with dilated cardiomyopathy, and mutations in the caveolin-3 gene (CAV3) have been identified in patients affected by congenital long QT syndrome. Although caveolin-3 deficiency represents the primary event, multiple secondary molecular mechanisms lead to muscle tissue damage. Among these, sarcolemmal membrane alterations, disorganization of skeletal muscle T-tubule network, and disruption of distinct cell signaling pathways have been determined.

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.

Membrane repair defects in muscular dystrophy are linked to altered interaction between MG53, caveolin-3, and dysferlin

Defective membrane repair can contribute to the progression of muscular dystrophy. Although mutations in caveolin-3 (Cav3) and dysferlin are linked to muscular dystrophy in human patients, the molecular mechanism underlying the functional interplay between Cav3 and dysferlin in membrane repair of muscle physiology and disease has not been fully resolved. We recently discovered that mitsugumin 53 (MG53), a muscle-specific TRIM (Tri-partite motif) family protein (TRIM72), contributes to intracellular vesicle trafficking and is an essential component of the membrane repair machinery in striated muscle. Here we show that MG53 interacts with dysferlin and Cav3 to regulate membrane repair in skeletal muscle. MG53 mediates active trafficking of intracellular vesicles to the sarcolemma and is required for movement of dysferlin to sites of cell injury during repair patch formation. Mutations in Cav3 (P104L, R26Q) that cause retention of Cav3 in Golgi apparatus result in aberrant localization of MG53 and dysferlin in a dominant-negative fashion, leading to defective membrane repair. Our data reveal that a molecular complex formed by MG53, dysferlin, and Cav3 is essential for repair of muscle membrane damage and also provide a therapeutic target for treatment of muscular and cardiovascular diseases that are linked to compromised membrane repair.

Stem cell antigen-1 localizes to lipid microdomains and associates with insulin degrading enzyme in skeletal myoblasts

Stem cell antigen-1 (Sca-1, Ly6A/E) is a glycosylphosphotidylinositol-anchored protein that identifies many tissue progenitor cells. We originally identified Sca-1 as a marker of myogenic precursor cells and subsequently demonstrated that Sca-1 regulates proliferation of activated myoblasts, suggesting an important role for Sca-1 in skeletal muscle homeostasis. Beyond its functional role in regulating proliferation, however, little is known about the mechanism(s) that drive Sca-1-mediated events. We now report that lipid microdomain organization is essential for normal myogenic differentiation, and that Sca-1 constitutively localizes to these domains during myoblast proliferation and differentiation. We also demonstrate that Sca-1 associates with insulin degrading enzyme (IDE), a catalytic protein responsible for the cleavage of mitogenic peptides, in differentiating myoblasts. We show that chemical inhibition of IDE as well as RNAi knockdown of IDE mRNA recapitulates the phenotype of Sca-1 interference, that is, sustained myoblast proliferation and delayed myogenic differentiation. These findings identify the first signaling protein that physically and functionally associates with Sca-1 in myogenic precursor cells, and suggest a potential pathway for Sca-1-mediated signaling. Future efforts to manipulate this pathway may lead to new strategies for augmenting the myogenic proliferative response, and ultimately muscle repair.

Caveolae as organizers of pharmacologically relevant signal transduction molecules

Caveolae, a subset of membrane (lipid) rafts, are flask-like invaginations of the plasma membrane that contain caveolin proteins, which serve as organizing centers for cellular signal transduction. Caveolins (-1, -2, and -3) have cytoplasmic N and C termini, palmitolylation sites, and a scaffolding domain that facilitates interaction and organization of signaling molecules so as to help provide coordinated and efficient signal transduction. Such signaling components include upstream entities (e.g., G protein-coupled receptors (GPCRs), receptor tyrosine kinases, and steroid hormone receptors) and downstream components (e.g., heterotrimeric and low-molecular-weight G proteins, effector enzymes, and ion channels). Diseases associated with aberrant signaling may result in altered localization or expression of signaling proteins in caveolae. Caveolin-knockout mice have numerous abnormalities, some of which may reflect the impact of total body knockout throughout the life span. This review provides a general overview of caveolins and caveolae, signaling molecules that localize to caveolae, the role of caveolae/caveolin in cardiac and pulmonary pathophysiology, pharmacologic implications of caveolar localization of signaling molecules, and the possibility that caveolae might serve as a therapeutic target.

Caveolin-3 T78M and T78K missense mutations lead to different phenotypes in vivo and in vitro

Caveolins are the principal protein components of caveolae, invaginations of the plasma membrane involved in cell signaling and trafficking. Caveolin-3 (Cav-3) is the muscle-specific isoform of the caveolin family and mutations in the CAV3 gene lead to a large group of neuromuscular disorders. In unrelated patients, we identified two distinct CAV3 mutations involving the same codon 78. Patient 1, affected by dilated cardiomyopathy and limb girdle muscular dystrophy (LGMD)-1C, shows an autosomal recessive mutation converting threonine to methionine (T78M). Patient 2, affected by isolated familiar hyperCKemia, shows an autosomal dominant mutation converting threonine to lysine (T78K). Cav-3 wild type (WT) and Cav-3 mutations were transiently transfected into Cos-7 cells. Cav-3 WT and Cav-3 T78M mutant localized at the plasma membrane, whereas Cav-3 T78K was retained in a perinuclear compartment. Cav-3 T78K expression was decreased by 87% when compared with Cav-3 WT, whereas Cav-3 T78M protein levels were unchanged. To evaluate whether Cav-3 T78K and Cav-3 T78M mutants behaved with a dominant negative pattern, Cos-7 cells were cotransfected with green fluorescent protein (GFP)-Cav-3 WT in combination with either mutant or WT Cav-3. When cotransfected with Cav-3 WT or Cav-3 T78M, GFP-Cav-3 WT was localized at the plasma membrane, as expected. However, when cotransfected with Cav-3 T78K, GFP-Cav-3 WT was retained in a perinuclear compartment, and its protein levels were reduced by 60%, suggesting a dominant negative action. Accordingly, Cav-3 protein levels in muscles from a biopsy of patient 2 (T78K mutation) were reduced by 80%. In conclusion, CAV3 T78M and T78K mutations lead to distinct disorders showing different clinical features and inheritance, and displaying distinct phenotypes in vitro.

G-protein-coupled receptor-signaling components in membrane raft and caveolae microdomains

The efficiency of signal transduction in cells derives in part from subcellular, in particular plasma membrane, microdomains that organize signaling molecules and signaling complexes. Two related plasma membrane domains that compartmentalize G-protein coupled receptor (GPCR) signaling complexes are lipid (membrane) rafts, domains that are enriched in certain lipids, including cholesterol and sphingolipids, and caveolae, a subset of lipid rafts that are enriched in the protein caveolin. This review focuses on the properties of lipid rafts and caveolae, the mechanisms by which they localize signaling molecules and the identity of GPCR signaling components that are organized in these domains.

Increasing alpha 7 beta 1-integrin promotes muscle cell proliferation, adhesion, and resistance to apoptosis without changing gene expression

The dystrophin-glycoprotein complex maintains the integrity of skeletal muscle by associating laminin in the extracellular matrix with the actin cytoskeleton. Several human muscular dystrophies arise from defects in the components of this complex. The alpha(7)beta(1)-integrin also binds laminin and links the extracellular matrix with the cytoskeleton. Enhancement of alpha(7)-integrin levels alleviates pathology in mdx/utrn(-/-) mice, a model of Duchenne muscular dystrophy, and thus the integrin may functionally compensate for the absence of dystrophin. To test whether increasing alpha(7)-integrin levels affects transcription and cellular functions, we generated alpha(7)-integrin-inducible C2C12 cells and transgenic mice that overexpress the integrin in skeletal muscle. C2C12 myoblasts with elevated levels of integrin exhibited increased adhesion to laminin, faster proliferation when serum was limited, resistance to staurosporine-induced apoptosis, and normal differentiation. Transgenic expression of eightfold more integrin in skeletal muscle did not result in notable toxic effects in vivo. Moreover, high levels of alpha(7)-integrin in both myoblasts and in skeletal muscle did not disrupt global gene expression profiles. Thus increasing integrin levels can compensate for defects in the extracellular matrix and cytoskeleton linkage caused by compromises in the dystrophin-glycoprotein complex without triggering apparent overt negative side effects. These results support the use of integrin enhancement as a therapy for muscular dystrophy.

Phenotypic behavior of C2C12 myoblasts upon expression of the dystrophy-related caveolin-3 P104L and TFT mutants

Caveolin-3 (Cav-3) is the main scaffolding protein present in myofiber caveolae. We transfected C2C12 myoblasts with dominant negative forms of Cav-3, P104L or DeltaTFT, respectively, which cause the limb-girdle muscular dystrophy 1-C. Both these forms triggered Cav-3 loss during C2C12 cell differentiation. The P104L mutation reduced myofiber formation by impaired AKT signalling, accompanied by dramatic expression of the E3 ubiquitin ligase Atrogin. On the other hand, the DeltaTFT mutation triggered hypertrophic myotubes sustained by prolonged AKT activation, but independent of increased levels of follistatin and interleukin 4 expression. These data suggest that separated mutations within the same dystrophy-related gene may cause muscle degeneration through different mechanisms.

Type-1 cannabinoid receptors colocalize with caveolin-1 in neuronal cells

Type-1 (CB1) and type-2 (CB2) cannabinoid receptors belong to the rhodopsin family of G protein-coupled receptors, and are activated by endogenous lipids termed “endocannabinoids”. Recent reports have demonstrated that CB1R, unlike CB2R and other receptors and metabolic enzymes of endocannabinoids, functions in the context of lipid rafts, i.e. plasma membrane microdomains which may be important in modulating signal transduction. Here, we present novel data based on cell subfractionation, immunoprecipitation and confocal microscopy studies, that show that in C6 cells CB1R co-localizes almost entirely with caveolin-1. We also show that trafficking of CB1R in response to the raft disruptor methyl-β-cyclodextrin (MCD) is superimposable on that of caveolin-1, and that MCD treatment increases the accessibility of CB1R to its specific antibodies. These findings may be relevant for the manifold CB1R-dependent activities of endocannabinoids, like the regulation of apoptosis and of neurodegenerative diseases.

Regulation of insulin response in skeletal muscle cell by caveolin status

Recent studies on the role of caveolin-1 in adipocytes showed that caveolin has emerged as an important regulatory element in insulin signaling but little is known on its role in skeletal muscle cells. In this study, we demonstrate for the first time that caveolin-1 plays a crucial role in insulin dependent glucose uptake in skeletal muscle cells. Differentiation of L6 skeletal muscle cells induce the expression of caveolin-1 and caveolin-3 with partial colocalization. However in contrast to adipocytes, phosphorylation of insulin receptor beta (IRbeta) and Akt/Erk was not affected by the respective downregulation of caveolin-1 or caveolin-3 in the muscle cells. Moreover, the phosphorylation of IRbeta was detected not only in the caveolae but also in the non-caveolae fractions of the muscle cells despite the interaction of IRbeta with caveolin-1 and caveolin-3. These data implicate the lack of relationship between caveolins and IRbeta pathway in the muscle cells, different from the adipocytes. However, glucose uptake was reduced specifically by downregulation of caveolin-1, but not that of caveolin-3. Taken together, these observations suggest that caveolin-1 plays a crucial role in glucose uptake in differentiated muscle cells and that the regulation of caveolin-1 expression may be an important mechanism for insulin sensitivity, implying the role of muscle cells for type 2 diabetes.

Hypertrophy and atrophy inversely regulate Caveolin-3 expression in myoblasts

Caveolin-3 (Cav-3) is a muscle-specific membrane protein crucial for myoblast differentiation, as loss of the protein due to mutations within the gene causes an autosomal dominant form of limb girdle muscular dystrophy 1-c. Here we show that along with p38 activity the PI3-kinase/AKT/mTOR pathway is required for proper Cav-3 up-regulation during muscle differentiation and hypertrophy, as confirmed by the marked increase of Cav-3 expression in hypertrophied C2C12 cells transfected with an activated form of AKT. Accordingly, Cav-3 expression was further increased during hypertrophy of L6C5 myoblasts treated with Arg(8)-vasopressin and in hypertrophic muscles of MLC/mIGF-1 transgenic mice. In contrast, Cav-3 expression was down-regulated in C2C12 myotubes exposed to atrophic stimuli such as starvation or treatment with dexamethasone. This study clearly suggests that Cav-3 expression is causally linked to the maturation of muscle phenotype and it is tightly regulated by hypertrophic and atrophic stimuli.

Localization of Kv1.5 channels in rat and canine myocyte sarcolemma

Voltage-gated potassium (Kv) channel subtypes localize to the plasma membrane of a number of cell types, and the sarcolemma in myocytes. Because many signaling molecules concentrate in subdomains of the plasma membrane, the localization of Kv channels to these sites may have important implications for channel function and regulation. In this study, the association of the voltage-gated potassium channel Kv1.5 with a specific subtype of lipid rafts, caveolae, in rat and canine cardiac myocytes has been investigated. Interactions between caveolin-3 and beta-dystroglycan or eNOS, as well as between Kv1.5 and alpha-actinin were readily detected in co-immunoprecipitation experiments, whereas no association between Kv1.5 and caveolin-3 was evident. Wide-field microscopy and deconvolution techniques revealed that the percent co-localization of Kv1.5 with caveolin-3 was extremely low in atrial myocytes from rat and canine hearts (8+/-1% and 12.2+/-2%, respectively), and limited in ventricular myocytes (11+/-4% and 20+/-3% in rat and canine, respectively). Immunoelectron microscopic imaging of rat atrial and ventricular tissues showed that Kv1.5 and caveolin-3 labeling generally did not overlap. In HEK293 cells stably expressing the channel, Kv1.5 did not target to the low buoyant density raft fraction along with flotillin but instead fractionated along with the non-raft associated transferrin receptor. Taken together, these results suggest that Kv1.5 is not present in caveolae of rat and canine heart.

Muscular atrophy of caveolin-3-deficient mice is rescued by myostatin inhibition

Caveolin-3, the muscle-specific isoform of caveolins, plays important roles in signal transduction. Dominant-negative mutations of the caveolin-3 gene cause autosomal dominant limb-girdle muscular dystrophy 1C (LGMD1C) with loss of caveolin-3. However, identification of the precise molecular mechanism leading to muscular atrophy in caveolin-3-deficient muscle has remained elusive. Myostatin, a member of the muscle-specific TGF-beta superfamily, negatively regulates skeletal muscle volume. Here we report that caveolin-3 inhibited myostatin signaling by suppressing activation of its type I receptor; this was followed by hypophosphorylation of an intracellular effector, Mad homolog 2 (Smad2), and decreased downstream transcriptional activity. Loss of caveolin-3 in P104L mutant caveolin-3 transgenic mice caused muscular atrophy with increase in phosphorylated Smad2 (p-Smad2) as well as p21 (also known as Cdkn1a), a myostatin target gene. Introduction of the myostatin prodomain, an inhibitor of myostatin, by genetic crossing or intraperitoneal administration of the soluble type II myostatin receptor, another inhibitor, ameliorated muscular atrophy of the mutant caveolin-3 transgenic mice with suppression of p-Smad2 and p21 levels. These findings suggest that caveolin-3 normally suppresses the myostatin-mediated signal, thereby preventing muscular atrophy, and that hyperactivation of myostatin signaling participates in the pathogenesis of muscular atrophy in a mouse model of LGMD1C. Myostatin inhibition may be a promising therapy for LGMD1C patients.

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.

Interaction of synthetic peptides corresponding to the scaffolding domain of Caveolin-3 with model membranes

Caveolin-1 and -3 are among the few proteins in which the functional domains are contiguous and modular. The interaction of synthetic peptides spanning the scaffolding domain of caveolin-3 with model membranes has been investigated. The peptides include the scaffolding domain, the aromatic and positively charged residues at the C-terminal end of this domain as well as deletion of three amino acids TFT, observed in certain patients with limb girdle muscular dystrophy. All of the peptides appear to be peripherally bound to the bilayer surface. However, no preferential binding to sphingomyelin and cholesterol-containing lipid vesicles was observed. Deletion of TFT appears to affect the association with lipid vesicles compared with the native sequence. Association with lipids decreases considerably when TFT as well as the aromatic-rich segment YWFYR, which occurs at the extreme C-terminus of the scaffolding domain, are deleted.

Membrane rafts as potential sites of nongenomic hormonal signaling in prostate cancer

Recent evidence indicates that nuclear receptors for steroid hormones can signal by nongenomic mechanisms that operate independently of their transcription function. These signal-transduction processes occur within seconds to minutes after initiation with agonist and involve interactions between nuclear receptors and other signaling proteins in the cytoplasm and at membrane surfaces. This review provides an overview of published information on possible nongenomic activities of the androgen receptor (AR) and other nuclear receptors, focusing on the potential involvement of these processes in prostate cancer. We discuss the hypothesis that the cholesterol-rich lipid-raft compartment(s) of cancer-cell membranes might provide privileged sites for nongenomic signals mediated by the AR.

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.

Muscle-fiber apoptosis in neuromuscular diseases

Muscle-fiber loss is a characteristic of many progressive neuromuscular disorders. Over the past decade, identification of a growing number of apoptosis-associated factors and events in pathological skeletal muscle provided increasing evidence that apoptotic cell-death mechanisms account significantly for muscle-fiber atrophy and loss in a wide spectrum of neuromuscular disorders. It became obvious that there is not one specific pathway for muscle fibers to undergo apoptotic degradation. In contrast, certain neuromuscular diseases seem to involve characteristic expression patterns of apoptosis-related factors and pathways. Furthermore, there are some characteristics of muscle-fiber apoptosis that rely on the muscle fiber itself as an extremely specified cell type. Multinucleated muscle fibers with successive muscle-fiber segments controlled by individual nuclei display some specifics different from apoptosis of mononucleated cells. This review focuses on the expression patterns of apoptosis-associated factors in different primary and secondary neuromuscular disorders and gives a synopsis of current knowledge.

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.

The dystrophin glycoprotein complex: signaling strength and integrity for the sarcolemma

The dystrophin glycoprotein complex (DGC) is a specialization of cardiac and skeletal muscle membrane. This large multicomponent complex has both mechanical stabilizing and signaling roles in mediating interactions between the cytoskeleton, membrane, and extracellular matrix. Dystrophin, the protein product of the Duchenne and X-linked dilated cardiomyopathy locus, links cytoskeletal and membrane elements. Mutations in additional DGC genes, the sarcoglycans, also lead to cardiomyopathy and muscular dystrophy. Animal models of DGC mutants have shown that destabilization of the DGC leads to membrane fragility and loss of membrane integrity, resulting in degeneration of skeletal muscle and cardiomyocytes. Vascular reactivity is altered in response to primary degeneration in striated myocytes and arises from a vascular smooth muscle cell-extrinsic mechanism.

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.