The basic helix-loop-helix transcription factors myogenin and Id2 mediate specific induction of caveolin-3 gene expression during embryonic development

miRNA mediated downregulation of cyclase-associated protein 1 (CAP1) is required for myoblast fusion

Myoblast fusion is essential for the formation, growth, and regeneration of skeletal muscle, but the molecular mechanisms that govern fusion and myofiber formation remain poorly understood. Past studies have shown an important role of the actin cytoskeleton and actin regulators in myoblast fusion. The Cyclase-Associated Proteins (CAP) 1 and 2 recently emerged as critical regulators of actin treadmilling in higher eukaryotes including mammals. Whilst the role of CAP2 in skeletal muscle development and function is well characterized, involvement of CAP1 in this process remains elusive. Here we report that CAP1, plays a critical role in cytoskeletal remodeling during myoblast fusion and formation of myotubes. Cap1 mRNA and protein are expressed in both murine C2C12 and human LHCN-M2 myoblasts, but their abundance decreases during myogenic differentiation. Perturbing the temporally controlled expression of CAP1 by overexpression or CRISPR-Cas9 mediated knockout impaired actin rearrangement, myoblast alignment, expression of profusion molecules, differentiation into multinucleated myotubes, and myosin heavy chain expression. Endogenous Cap1 expression is post-transcriptionally downregulated during differentiation by canonical myomiRs miR-1, miR-133, and miR-206, which have conserved binding sites at the 3′ UTR of the Cap1 mRNA. Deletion of the endogenous 3′ UTR by CRISPR-Cas9 in C2C12 cells phenocopies overexpression of CAP1 by inhibiting myotube formation. Our findings implicates Cap1 and its myomiR-mediated downregulation in the myoblast fusion process and the generation of skeletal muscle.

Comprehensive analysis of circRNA expression profiles and circRNA-associated competing endogenous RNA networks in the development of mouse thymus

The thymus plays an irreplaceable role as a primary lymphoid organ. However, the complicate processes of its development and involution are incompletely understood. Accumulating evidence indicates that non-coding RNAs play key roles in the regulation of biological development. At present, the studies of the circRNA profiles and of circRNA-associated competing endogenous RNAs (ceRNAs) in the thymus are still scarce. Here, deep-RNA sequencing was used to study the biological mechanisms underlying the development process (from 2-week-old to 6-week-old) and the recession process (from 6-week-old to 3-month-old) of the mouse thymus. It was found that 196 circRNAs, 233 miRNAs and 3807 mRNAs were significantly dysregulated. The circRNA-associated ceRNA networks were constructed in the mouse thymus, which were mainly involved in early embryonic development and the proliferation and division of T cells. Taken together, these results elucidated the regulatory roles of ceRNAs in the development and involution processes of the mouse thymus.

Calcineurin Broadly Regulates the Initiation of Skeletal Muscle-Specific Gene Expression by Binding Target Promoters and Facilitating the Interaction of the SWI/SNF Chromatin Remodeling Enzyme

Calcineurin (Cn) is a calcium-activated serine/threonine protein phosphatase that is broadly implicated in diverse cellular processes, including the regulation of gene expression. During skeletal muscle differentiation, Cn activates the nuclear factor of activated T-cell (NFAT) transcription factor but also promotes differentiation by counteracting the negative influences of protein kinase C beta (PKCβ) via dephosphorylation and activation of Brg1, an enzymatic subunit of the mammalian SWI/SNF ATP-dependent chromatin remodeling enzyme. Here we identified four major temporal patterns of Cn-dependent gene expression in differentiating myoblasts and determined that Cn is broadly required for the activation of the myogenic gene expression program. Mechanistically, Cn promotes gene expression through direct binding to myogenic promoter sequences and facilitating the binding of Brg1, other SWI/SNF subunit proteins, and MyoD, a critical lineage determinant for skeletal muscle differentiation. We conclude that the Cn phosphatase directly impacts the expression of myogenic genes by promoting ATP-dependent chromatin remodeling and formation of transcription-competent promoters.

Inhibitor of DNA binding in heart development and cardiovascular diseases

Id proteins, inhibitors of DNA binding, are transcription regulators containing a highly conserved helix-loop-helix domain. During multiple stages of normal cardiogenesis, Id proteins play major roles in early development and participate in the differentiation and proliferation of cardiac progenitor cells and mature cardiomyocytes. The fact that a depletion of Ids can cause a variety of defects in cardiac structure and conduction function is further evidence of their involvement in heart development. Multiple signalling pathways and growth factors are involved in the regulation of Ids in a cell- and tissue- specific manner to affect heart development. Recent studies have demonstrated that Ids are related to multiple aspects of cardiovascular diseases, including congenital structural, coronary heart disease, and arrhythmia. Although a growing body of research has elucidated the important role of Ids, no comprehensive review has previously compiled these scattered findings. Here, we introduce and summarize the roles of Id proteins in heart development, with the hope that this overview of key findings might shed light on the molecular basis of consequential cardiovascular diseases. Furthermore, we described the future prospective researches needed to enable advancement in the maintainance of the proliferative capacity of cardiomyocytes. Additionally, research focusing on increasing embryonic stem cell culture adaptability will help to improve the future therapeutic application of cardiac regeneration.

NF-κB and GATA-Binding Factor 6 Repress Transcription of Caveolins in Bladder Smooth Muscle Hypertrophy

Caveolins (CAVs) are structural proteins of caveolae that function as signaling platforms to regulate smooth muscle contraction. Loss of CAV protein expression is associated with impaired contraction in obstruction-induced bladder smooth muscle (BSM) hypertrophy. In this study, microarray analysis of bladder RNA revealed down-regulation of CAV1, CAV2, and CAV3 gene transcription in BSM from models of obstructive bladder disease in mice and humans. We identified and characterized regulatory regions responsible for CAV1, CAV2, and CAV3 gene expression in mice with obstruction-induced BSM hypertrophy, and in men with benign prostatic hyperplasia. DNA affinity chromatography and chromatin immunoprecipitation assays revealed a greater increase in binding of GATA-binding factor 6 (GATA-6) and NF-κB to their cognate binding motifs on CAV1, CAV2, and CAV3 promoters in obstructed BSM relative to that observed in control BSM. Knockout of NF-κB subunits, shRNA-mediated knockdown of GATA-6, or pharmacologic inhibition of GATA-6 and NF-κB in BSM increased CAV1, CAV2, and CAV3 transcription and promoter activity. Conversely, overexpression of GATA-6 decreased CAV2 and CAV3 transcription and promoter activity. Collectively, these data provide new insight into the mechanisms by which CAV gene expression is repressed in hypertrophied BSM in obstructive bladder disease.

Protein 4.1R Influences Myogenin Protein Stability and Skeletal Muscle Differentiation

Protein 4.1R (4.1R) isoforms are expressed in both cardiac and skeletal muscle. 4.1R is a component of the contractile apparatus. It is also associated with dystrophin at the sarcolemma in skeletal myofibers. However, the expression and function of 4.1R during myogenesis have not been characterized. We now report that 4.1R expression increases during C2C12 myoblast differentiation into myotubes. Depletion of 4.1R impairs skeletal muscle differentiation and is accompanied by a decrease in the levels of myosin heavy and light chains and caveolin-3. Furthermore, the expression of myogenin at the protein, but not mRNA, level is drastically decreased in 4.1R knockdown myocytes. Similar results were obtained using MyoD-induced differentiation of 4.1R^-/- mouse embryonic fibroblast cells. von Hippel-Lindau (VHL) protein is known to destabilize myogenin via the ubiquitin-proteasome pathway. We show that 4.1R associates with VHL and, when overexpressed, reverses myogenin ubiquitination and stability. This suggests that 4.1R may influence myogenesis by preventing VHL-mediated myogenin degradation. Together, our results define a novel biological function for 4.1R in muscle differentiation and provide a molecular mechanism by which 4.1R promotes myogenic differentiation.

From embryonic development to human diseases: The functional role of caveolae/caveolin

Caveolae, an almost ubiquitous, structural component of the plasma membrane, play a critical role in many functions essential for proper cell function, including membrane trafficking, signal transduction, extracellular matrix remodeling, and tissue regeneration. Three main types of caveolin proteins have been identified from caveolae since the discovery of caveolin-1 in the early 1990s. All three (Cav-1, Cav-2, and Cav-3) play crucial roles in mammalian physiology, and can effect pathogenesis in a wide range of human diseases. While many biological activities of caveolins have been uncovered since its discovery, their role and regulation in embryonic develop remain largely poorly understood, although there is increasing evidence that caveolins may be linked to lung and brain birth defects. Further investigations are clearly needed to decipher how caveolae/caveolins mediate cellular functions and activities of normal embryogenesis and how their perturbations contribute to developmental disorders.

Caveolin-3 Promotes a Vascular Smooth Muscle Contractile Phenotype

Epidemiological studies have demonstrated the importance of cardiovascular diseases in Western countries. Among the cell types associated with a dysfunctional vasculature, smooth muscle (SM) cells are believed to play an essential role in the development of these illnesses. Vascular SM cells are key regulators of the vascular tone and also have an important function in the development of atherosclerosis and restenosis. While in the normal vasculature, contractile SM cells are predominant, in atherosclerotic vascular lesions, synthetic cells migrate toward the neointima, proliferate, and synthetize extracellular matrix proteins. In the present study, we have examined the role of caveolin-3 in the regulation of SM cell phenotype. Caveolin-3 is expressed in vivo in normal arterial SM cells, but its expression appears to be lost in cultured SM cells. Our data show that caveolin-3 expression in the A7r5 SM cell line is associated with increased expression of contractility markers such as SM α-actin, SM myosin heavy chain but decreased expression of the synthetic phenotype markers such as p-Elk and Klf4. Moreover, we also show that caveolin-3 expression can reduce proliferation upon treatment with LDL or PDGF. Finally, we show that caveolin-3-expressing SM cells are less sensitive to apoptosis than control cells upon treatment with oxidized LDL. Taken together, our data suggest that caveolin-3 can regulate the phenotypic switch between contractile and synthetic SM cells. A better understanding of the factors regulating caveolin-3 expression and function in this cell type will permit the development of a better comprehension of the factors regulating SM function in atherosclerosis and restenosis.

Myogenin is a positive regulator of MEGF10 expression in skeletal muscle

MEGF10 is known to function as a myogenic regulator of satellite cells in skeletal muscle. Mutations in MEGF10 gene cause a congenital myopathy called early onset myopathy, areflexia, respiratory distress and dysphagia (EMARDD). Despite its biological importance in muscle physiology, transcriptional regulation of the MEGF10 gene is unknown. Here, we characterized the 5' flanking region of the human MEGF10 gene and showed that the role of myogenic basic helix-loop-helix factor (bHLH) myogenin in MEGF10 transcription in muscle cells. Myogenin was found to share a similar expression pattern with MEGF10 during muscle regeneration and to increase the promoter activity of the MEGF10 gene in C2C12 cells. Overexpression of myogenin led to upregulation of MEGF10 mRNA in C2C12 cells. Site-directed mutagenesis assays revealed that the conserved E-box element at the region -114/-108 serves as a myogenin-binding motif. Promoter enzyme immunoassays and chromatin immunoprecipitation analysis showed direct interaction between myogenin and the myogenin-binding motif in the MEGF10 promoter. Taken together, these results indicate that myogenin is a positive regulator in transcriptional regulation of MEGF10 in skeletal muscle.

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.

Monitoring of deiodinase deficiency based on transcriptomic responses in SH-SY5Y cells

Iodothyronine deiodinase types I, II, and III (D1, D2, and D3, respectively), which constitute a family of selenoenzymes, activate and inactivate thyroid hormones through the removal of specific iodine moieties from thyroxine and its derivatives. These enzymes are important in the biological effects mediated by thyroid hormones. The expression of activating and inactivating deiodinases plays a critical role in a number of cell systems, including the neuronal system, during development as well as in adult vertebrates. To investigate deiodinase-disrupting chemicals based on transcriptomic responses, we examined differences in gene expression profiles between T3-treated and deiodinase-knockdown SH-SY5Y cells using microarray analysis and quantitative real-time RT-PCR. A total of 1,558 genes, consisting of 755 upregulated and 803 downregulated genes, were differentially expressed between the T3-treated and deiodinase-knockdown cells. The expression levels of 10 of these genes (ID2, ID3, CCL2, TBX3, TGOLN2, C1orf71, ZNF676, GULP1, KLF9, and ITGB5) were altered by deiodinase-disrupting chemicals (2,3,7,8-tetrachlorodibenzo-p-dioxin, polychlorinated biphenyls, propylthiouracil, iodoacetic acid, methylmercury, β-estradiol, methimazole, 3-methylcholanthrene, aminotriazole, amiodarone, cadmium chloride, dimethoate, fenvalerate, octylmethoxycinnamate, iopanoic acid, methoxychlor, and 4-methylbenzylidene-camphor). These genes are potential biomarkers for detecting deiodinase deficiency and predicting their effects on thyroid hormone production.

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.

Inhibitor of DNA binding 2 promotes sensory axonal growth after SCI

This study investigated whether neuronal inhibitor of DNA binding 2 (Id2), a regulator of basic helix-loop-helix (bHLH) transcription factors, can activate the intrinsic neuritogenetic mode of dorsal root ganglion (DRG) neurons in adult mice following spinal cord injury (SCI). First, the Id2 developmental expression profile of DRG neurons, along with the correlated activity of Cdh1-anaphase promoting complex (Cdh1-APC), was characterized. Next, a D-box mutant Id2 (Id2DBM) adenoviral vector, resistant to Cdh1-APC degradation, was developed to enhance neuronal Id2 expression. After the vector was introduced into DRG neurons, the effect of Id2 on neurite outgrowth of cultured DRG neurons and sensory axonal regeneration following spinal cord dorsal hemisection was evaluated. The expression of Id2 in DRG neurons was high in the embryonic stage, downregulated after birth, and significantly reduced in the adult. Expression of Cdh1-APC was opposite to Id2, which may be responsible for Id2 degradation during DRG maturation. Overexpression of Id2DBM in DRG neurons enhanced neuritogenesis on both permissive and inhibitory substrates. Following spinal cord dorsal hemisection, overexpression of Id2DBM reduced axon dieback and increased the number and length of regenerative fibers into the lesion gap. Reprogramming the intrinsic growth status of quiescent adult DRG neurons by enhancing Id2 expression results in active neuritogenesis following SCI. Id2 may be a novel target for enhancing sensory axonal regeneration following injuries to the adult spinal cord.

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.

Left ventricular dysfunction in murine models of heart failure and in failing human heart is associated with a selective decrease in the expression of caveolin-3

BACKGROUND

Caveolins are scaffolding proteins that are integral components of caveolae, flask-shaped invaginations in the membranes of all mammalian cells. Caveolin-1 and -2 are expressed ubiquitously, whereas caveolin-3 is found only in muscle. The role of caveolin-3 in heart muscle disease is controversial.

METHODS AND RESULTS

The present study was undertaken to assess the effects of left ventricular dysfunction on the expression of caveolin proteins using 2 well characterized models of murine heart failure and failing human heart. Transgenic mice with constitutive overexpression of A(1)-adenosine receptor (A(1)-TG) demonstrated cardiac dilatation and decreased left ventricular function at 10 weeks of age. This was accompanied by a marked decrease in caveolin-3 mRNA and protein levels compared with non-TG control mice. The change in caveolin-3 expression was selective, because levels of caveolin-1 and -2 did not change. Confocal imaging of myocytes isolated from A(1)-TG mice demonstrated a loss of the plate-like appearance of T tubules. Caveolin-3 levels were also reduced in hearts from mice overexpressing tumor necrosis factor α. There was a direct relationship between caveolin-3 expression and fractional shortening in all mice that were studied (r = 0.65; P < .001). Although we could not demonstrate a significant decrease in caveolin-3 levels in failing human heart, we did find a direct correlation (r = 0.7; P < .05) between levels of caveolin-3 protein and Ca(2+)-adenosine triphosphatase, a marker of the heart failure phenotype.

CONCLUSIONS

These results suggest a relationship between left ventricular dysfunction and caveolin-3 levels and suggest that caveolin-3 may provide a novel target for heart failure therapy.

TRIM72, a novel negative feedback regulator of myogenesis, is transcriptionally activated by the synergism of MyoD (or myogenin) and MEF2

TRIM72 is known to be involved in the negative feedback regulation of myogenesis by targeting insulin receptor substrate-1. Here, we found that TRIM72 was more highly expressed in oxidative muscle with the higher activity of MEF2, compared to glycolytic muscle. Indeed, TRIM72 promoter contained an evolutionarily conserved MEF2 site juxtaposed to E-box. TRIM72 promoter activity was decreased by the site-directed mutagenesis of either E-boxes or a MEF2 site and synergistically enhanced by MyoD (or myogenin) and MEF2, which were associated with proximal E-box, and MEF2 site of the TRIM72 promoter, respectively. Taken together all these data, we concluded that the synergism of MyoD (or myogenin) and MEF2 is necessary for TRIM72 expression during C2C12 differentiation.

Prolyl hydroxylase EGLN3 regulates skeletal myoblast differentiation through an NF-kappaB-dependent pathway

The egg-laying abnormal-9 (EGLN) prolyl hydroxylases have been shown to regulate the stability and thereby the activity of the alpha subunits of hypoxia-inducible factor (HIF) through its ability to catalyze their hydroxylation. We have previously shown that EGLN3 promotes differentiation of C2C12 skeletal myoblasts. However, the mechanism underlying this effect remains to be fully elucidated. Here, we report that exposure of C2C12 cells to dimethyl oxalylglycine (DMOG), desferrioxamine, and hypoxia, all inhibitors of prolyl hydroxylase activity, led to repression of C2C12 myogenic differentiation. Inactivation of HIF by expression of a HIF dominant-negative mutant or deletion of HIF-1alpha by RNA interference did not affect the inhibitory effect of DMOG, suggesting that the effect of DMOG is HIF-independent. Pharmacologic inactivation of EGLN3 hydroxylase resulted in activation of the canonical NF-kappaB pathway. The inhibitory effect of DMOG on myogenic differentiation was markedly impaired in C2C12 cells expressing a dominant-negative mutant of IkappaBalpha. Exogenous expression of wild-type EGLN3, but not its catalytically inactive mutant, significantly inhibited NF-kappaB activation induced by overexpressed TRAF2 or IkappaB kinase 2. In contrast, deletion of EGLN3 by small interfering RNAs led to activation of NF-kappaB. These data suggest that EGLN3 is a negative regulator of NF-kappaB, and its prolyl hydroxylase activity is required for this effect. Furthermore, wild-type EGLN3, but not its catalytically inactive mutant, potentiated myogenic differentiation. This study demonstrates a novel role for EGLN3 in the regulation of NF-kappaB and suggests that it is involved in mediating myogenic differentiation, which is HIF-independent.

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.

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.

Nitric Oxide Down-regulates Caveolin-3 Levels through the Interaction with Myogenin, Its Transcription Factor

Certain patients suffering from chronic diseases such as AIDS or cancer experience a constant cellular secretion of tumor necrosis factor alpha and other pro-inflammatory cytokines that results in a continuous release of nitric oxide (*NO) to the bloodstream. One immediate consequence of the deleterious action of *NO is weight loss and the progressive destruction of muscular mass in a process known as cachexia. We have previously reported that caveolin-3, a specific marker of muscle cells, becomes down-regulated by the action of *NO on muscular myotubes. We describe herein that the changes observed in caveolin-3 levels are due to the alteration of the DNA binding activity of the muscular transcription factor myogenin. In the presence of *NO, the binding of transcription factors from cell nuclear extracts of muscular tissues to the E boxes present in the caveolin-3 promoter become substantially reduced. When we purified recombinant myogenin and treated it with *NO donors, we could detect its S-nitrosylation by three independent methods, suggesting that very likely one of the cysteine residues of the molecule is being modified. Given the role of myogenin as a regulatory protein that determines the level of multiple muscle genes expressed during late myogenesis, our results might represent a novel mode of regulation of muscle development under conditions of nitric oxide-mediated toxicity.

Enhancer of polycomb1, a novel homeodomain only protein-binding partner, induces skeletal muscle differentiation

Homeodomain only protein, Hop, is an unusual small protein that modulates target gene transcription without direct binding to DNA. Here we show that Hop interacts with Enhancer of Polycomb1 (Epc1), a homolog of a Drosophila polycomb group gene product that regulates transcription, to induce the skeletal muscle differentiation. Yeast two-hybrid assay with the human adult heart cDNA library revealed that Hop can associate with Epc1. The amino-terminal domain of Epc1 as well as full Epc1 physically interacted with Hop in mammalian cells and in yeast. Epc1 is highly expressed in the embryonic heart and adult skeletal muscles. Serum deprivation induced differentiation of H9c2, a myoblast cell line, into skeletal myocytes, and Epc1 was up-regulated. Differentiation of H9c2 was induced by Epc1 overexpression, although it was severely impaired in Epc1-knockdown cells. Co-transfection of Hop potentiated Epc1-induced transactivation of myogenin and myotube formation. Hop knock-out mice elicited a decrease in myosin heavy chain and myogenin expressions in skeletal muscle and showed delay in hamstring muscle healing after injury. Differentiation was impaired in skeletal myoblasts from Hop knock-out mice. These results suggest that Epc1 plays a role in the initiation of skeletal muscle differentiation, and its interaction with Hop is required for the full activity.

Cathepsin B localizes to plasma membrane caveolae of differentiating myoblasts and is secreted in an active form at physiological pH

Our in vitro studies support a functional link between the induction of cathepsin B gene expression and the catabolic restructuring associated with myotube formation during myogenesis in vivo. We have tested two predictions that are basic to this hypothesis: (1) that active cathepsin B is localized to plasma membrane caveolae of fusing myoblasts; and (2) that active cathepsin B is secreted from fusing myoblasts at physiological pH. During differentiation, L6 rat myoblasts demonstrated a fusion-related increase in activity associated with the 25/26-kDa, fully processed, active form of cathepsin B. Immunocytochemical studies demonstrated a redistribution of lysosomal cathepsin B protein toward the membrane of fusing myoblasts, and a colocalization of cathepsin B with caveolin-3, the muscle-specific structural protein of membrane caveolae. Sucrose density fractionation and Western blot analysis demonstrated that an active form of cathepsin B localizes to caveolar fractions along with caveolin-3, annexin-VII, beta-dystroglycan and dystrophin. Finally, 'real-time' activity assays and Western blot analysis demonstrated that active cathepsin B is secreted from fusing myoblasts at physiological pH. Collectively, these studies support an association of active cathepsin B with plasma membrane caveolae and the secretion of active cathepsin B from differentiating myoblasts during myoblast fusion.

Molecular characterization and expression analysis of the porcine caveolin-3 gene

Caveolin-3 is the muscle-specific form of the caveolin protein family and plays an important role in modulating both the morphological appearance and function of caveolae. In this study, we cloned and characterized caveolin-3 from porcine muscle. The promoter of porcine caveolin-3 contained three consensus E box elements and one RORalpha2 monomeric binding motif. The deduced amino acid sequence of porcine caveolin-3 contains a WW domain. This gene was mapped to SSC13 q23-q24 by the SCHP and the IMpRH panel. RT-PCR analyses showed that caveolin-3 was expressed specifically in skeletal muscle and heart. And we provide the first evidence that caveolin-3 has a certain regulated expression pattern during the prenatal period of the porcine skeletal muscle development. This result suggests that the caveolin-3 gene might be a candidate gene of meat production trait and provides some information for establishing of an animal model using pig to study human caveolinopathies.

Absence of dysferlin alters myogenin expression and delays human muscle differentiation "in vitro"

Mutations in dysferlin cause a type of muscular dystrophy known as dysferlinopathy. Dysferlin may be involved in muscle repair and differentiation. We compared normal human skeletal muscle cultures expressing dysferlin with muscle cultures from dysferlinopathy patients. We quantified the fusion index of myoblasts as a measure of muscle development and conducted optic and electronic microscopy, immunofluorescence, Western blot, flow cytometry, and real-time PCR at different developmental stages. Short interference RNA was used to corroborate the results obtained in dysferlin-deficient cultures. A luciferase reporter assay was performed to study myogenin activity in dysferlin-deficient cultures. Myoblasts fusion was consistently delayed as compared with controls whereas the proliferation rate did not change. Electron microscopy showed that control cultured cells at 10 days were fusiform, whereas dysferlin-deficient cells were star-shaped and large. After 15 days the normal multinucleated appearance and structured myofibrils were not present in dysferlin-deficient cells. Strikingly, myogenin was not detected in myotubes from dysferlin-deficient cultures using Western blot, and mRNA analysis showed low levels (p < 0.05) compared with controls. Flow cytometry and immunofluorescence also showed reduced levels of myogenin in dysferlin-deficient cultures. When the dysferlin gene was knocked down ( approximately 80%), myogenin mRNA leveled down to approximately 70%. MyoD and desmin mRNA levels in controls and dysferlin-deficient cultures were similar. The reporter luciferase assay demonstrated a low myogenin activity in dysferlin-deficient cultures. These results point to a functional link between dysferlin and myogenin, and both proteins may share a new signaling pathway involved in differentiation of skeletal muscle in vitro.

The amphoterin (HMGB1)/receptor for advanced glycation end products (RAGE) pair modulates myoblast proliferation, apoptosis, adhesiveness, migration, and invasiveness. Functional inactivation of RAGE in L6 myoblasts results in tumor formation in vivo

We reported that RAGE (receptor for advanced glycation end products), a multiligand receptor of the immunoglobulin superfamily expressed in myoblasts, when activated by its ligand amphoterin (HMGB1), stimulates rat L6 myoblast differentiation via a Cdc42-Rac-MKK6-p38 mitogen-activated protein kinase pathway, and that RAGE expression in skeletal muscle tissue is developmentally regulated. We show here that inhibition of RAGE function via overexpression of a signaling deficient RAGE mutant (RAGE delta cyto) results in increased myoblast proliferation, migration, and invasiveness, and decreased apoptosis and adhesiveness, whereas myoblasts overexpressing RAGE behave the opposite, compared with mock-transfected myoblasts. These effects are accompanied by a decreased induction of the proliferation inhibitor, p21(Waf1), and increased induction of cyclin D1 and extent of Rb, ERK1/2, and JNK phosphorylation in L6/RAGE delta cyto myoblasts, the opposite occurring in L6/RAGE myoblasts. Neutralization of culture medium amphoterin negates effects of RAGE activation, suggesting that amphoterin is the RAGE ligand involved in RAGE-dependent effects in myoblasts. Finally, mice injected with L6/RAGE delta cyto myoblasts develop tumors as opposed to mice injected with L6/RAGE or L6/mock myoblasts that do not. Thus, the amphoterin/RAGE pair stimulates myoblast differentiation by the combined effect of stimulation of differentiation and inhibition of proliferation, and deregulation of RAGE expression in myoblasts might contribute to their neoplastic transformation.

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.

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.

Behavior of Caveolae and Caveolin-3 During the Development of Myocyte Hypertrophy

Recent studies have indicated that caveolae are enriched in a variety of signaling molecules, some of which are associated with cardiomyocyte hypertrophy. Caveolin-3, a major constituent of cardiac caveolae, has been suggested to interact with several signaling molecules. We investigated the morphologic changes of caveolae and caveolin-3 expression in hypertrophied cardiomyocytes induced by an alpha1-adrenergic agonist. Cultured rat neonatal cardiomyocytes were used for the experiments. Phenylephrine induced cellular hypertrophy associated with an increase of the number of caveolae and an up-regulation of caveolin-3. Although PMA increased the number of caveolae and the caveolin-3 expression, the extent of these up-regulations was less than that by phenylephrine. Moreover, ionomycin increased the number of caveolae and up-regulated caveolin-3 as much as phenylephrine. Phenylephrine-induced up-regulations of caveolae and caveolin-3 expression were inhibited by BAPTA, suggesting that the intracellular Ca2+ is involved in those regulations. Inhibitors of calcineurin and Ca2+calmodulin-dependent kinase II attenuated the phenylephrine-induced up-regulation of caveolin-3. In pressure-overloaded rat hearts, caveolin-3 protein levels were increased compared with sham-operated rats. In conclusion, the number of caveolae and the expression of caveolin-3 were up-regulated in rat hypertrophied cardiomyocytes, possibly via the alterations of intracellular Ca2+ and protein kinase C.

Expression and localization of caveolins during postnatal development in rat heart: implication of thyroid hormone

Caveolins modulate signaling pathways involved in cardiac development. Caveolin-1 exists in two isoforms: the beta-isoform derivates from an alternative translational start site that creates a protein truncated by 31 amino acids, mainly expressed in endothelial cells, whereas caveolin-3 is present in muscle cells. Our aim was to define caveolin distribution and expression during cardiac postnatal development using immunofluorescence and Western blotting. Caveolin-3 sarcolemmal labeling appeared as dotted lines from days 1 to 5 and as continuous lines after 14 days of age. Caveolin-3 expression, low at birth, increased (4-fold) to reach a maximum (P < 0.05) by day 5 and then decreased to stabilize in adults. Total caveolin-1 and its alpha-isoform were codistributed at birth in endothelial and smooth muscle cells; afterward, only the caveolin-1alpha labeling became limited to endothelium. Quantitative analysis indicated a similar temporal pattern of both total caveolin-1 and caveolin-1alpha expression, suggesting that caveolin-1alpha and -1beta are coregulated; the caveolin-1alpha levels increased fourfold by day 5 to reach a maximum by day 14 (P < 0.05). Tyrosine-14-caveolin-1 phosphorylation, low at birth, increased suddenly around day 14 (8-fold vs. day 1) and returning afterward to basal level. Because the T3/T4 level is maximal by day 14, caveolin-1 expression/phosphorylation profiles were analyzed in hypothyroid heart. The levels of caveolin-1alpha and consequently tyrosine-14-caveolin-1 phosphorylation, but not that of caveolin-3, decreased (50%) in hypothyroid 14-day-old rats. Our data demonstrate that, during postnatal cardiac growth, 1) caveolins are distinctly regulated, and 2) thyroid hormones are involved in caveolin-1alpha expression.

Induction of nitric oxide synthase-2 proceeds with the concomitant downregulation of the endogenous caveolin levels

Several cell types express inducible nitric oxide synthase (NOS2) in response to exogenous insults such as bacterial lipopolysaccharide (LPS) or proinflammatory cytokines. For instance, muscular cells treated with LPS and interferon gamma (IFN-gamma) respond by increasing the mRNA and protein levels of NOS2, and synthesize large amounts of nitric oxide. We show here that transcriptional induction of NOS2 in muscular cells proceeds with a concomitant decrease in the levels of caveolin-1, -2 and -3. Addition of *NO-releasing compounds to C2C12 muscle cells reveals that this downregulation of the caveolin (cav) levels is due to the presence of *NO itself in the case of caveolin-3 and to the action of the LPS/IFN-gamma in the case of cav-1 and cav-2. Likewise, muscle cells obtained from NOS2(-/-) knockout mice challenged with LPS/IFN-gamma could downregulate their levels of cav-1 but not of cav-3, unlike wild-type animals, in which both cav-1 and cav-3 levels diminished in the presence of the proinflammatory insult. Laser confocal immunofluorescence analysis proves that *NO exerts autocrine and paracrine actions, hence diminishing the cav-3 levels. When the induced NOS2 was purified using an affinity resin or immunoprecipitated from muscular tissues, it appears strongly bound not only to calmodulin but also to cav-1, and marginally to cav-2 and cav-3. When the cav levels where reduced using antisense oligonucleotides, an increase in the NOS2-derived.NO levels could be measured, demonstrating the inhibitory role of the three cav isoforms. Our results show that cells expressing NOS2 diminish their cav levels when the synthesis of *NO is required.

Membrane cholesterol modulates dihydropyridine receptor function in mice fetal skeletal muscle cells

Caveolae and transverse (T-) tubules are membrane structures enriched in cholesterol and glycosphingolipids. They play an important role in receptor signalling and myogenesis. The T-system is also highly enriched in dihydropyridine receptors (DHPRs), which control excitation-contraction (E-C) coupling. Recent results have shown that a depletion of membrane cholesterol alters caveolae and T-tubules, yet detailed functional studies of DHPR expression are lacking. Here we studied electrophysiological and morphological effects of methyl-beta-cyclodextrin (MbetaCD), a cholesterol-sequestering drug, on freshly isolated fetal skeletal muscle cells. Exposure of fetal myofibres to 1-3 mM MbetaCD for 1 h at 37 degrees C led to a significant reduction in caveolae and T-tubule areas and to a decrease in cell membrane electrical capacitance. In whole-cell voltage-clamp experiments, the L-type Ca(2+) current amplitude was significantly reduced, and its voltage dependence was shifted approximately 15 mV towards more positive potentials. Activation and inactivation kinetics were slower in treated cells than in control cells and stimulation by a saturating concentration of Bay K 8644 was enhanced. In addition, intramembrane charge movement and Ca(2+) transients evoked by a depolarization were reduced without a shift of the midpoint, indicating a weakening of E-C coupling. In contrast, T-type Ca(2+) current was not affected by MbetaCD treatment. Most of the L-type Ca(2+) conductance reduction and E-C coupling weakening could be explained by a decrease of the number of DHPRs due to the disruption of caveolae and T-tubules. However, the effects on L-type channel gating kinetics suggest that membrane cholesterol content modulates DHPR function. Moreover, the significant shift of the voltage dependence of L-type current without any change in the voltage dependence of charge movement and Ca(2+) transients suggests that cholesterol differentially regulates the two functions of the DHPR.

Peroxisome proliferator-activated receptor-gamma upregulates caveolin-1 and caveolin-2 expression in human carcinoma cells

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a nuclear receptor for eicosanoids that promotes differentiation of human epithelial and mesenchymal cells in vitro and in vivo. PPARgamma was proposed as a target for drug-induced differentiation therapy of cancer. Caveolin-1 is a constituent of plasma membrane caveolae in epithelial cells that is often downregulated upon oncogenic transformation. Caveolin-1 has growth-inhibitory activities and its disruption is sufficient to induce transformation in fibroblasts. Herein we have tested the hypothesis that caveolins are transcriptional target genes for PPARgamma. In human HT-29 colon carcinoma cells, thiazolidinedione PPARgamma ligands increased the levels of caveolin-1 and caveolin-2 proteins two to fivefold in a concentration-dependent manner within 24 h. In human MCF-7 breast adenocarcinoma cells, nonthiazolidinedione PPARgamma ligands elevated caveolin-2 protein three to fourfold, while the thiazoli-dinediones were less effective. Caveolin-1 mRNA levels were found to be upregulated by PPARgamma ligands already after 3 h in both the cell lines. Ectopic expression of a dominant-negative PPARgamma construct attenuated ligand-induced upregulation of caveolins in both HT-29 and HEK-293T cells, indicating that ligand action is mediated by PPARgamma. Ligand-treated MCF-7 cells exhibited a differentiated phenotype, as evinced by analysis of cell-specific differentiation markers: protein levels of maspin were elevated and perinuclear lipid droplets accumulated. In contrast, in HT-29 cells, caveolin expression was not correlated with differentiation. Interestingly, PPARgamma partially cofractionated in lipid rafts and could be coimmunoprecipitated from cell lysates with caveolin-1, indicating that PPARgamma and caveolin-1 may coexist in a complex. Our data indicate that PPARgamma participates in the regulation of caveolin gene expression in human carcinoma cells and suggest that caveolin-1 may mediate some of the phenotypic changes induced by this nuclear receptor in cancer cells. These findings may have potentially important functional implications in the context of cancer differentiation therapy and multidrug resistance.

Id2 expression during apoptosis and satellite cell activation in unloaded and loaded quail skeletal muscles

Inhibitor of differentiation-2 (Id2) is a basic helix-loop-helix protein that acts as a negative regulator of the myogenic regulatory transcription factor family, but Id2 has also been implicated in apoptosis in several cell lines. In this study, we tested the hypothesis that Id2 has a role in both apoptosis-associated muscle atrophy and muscle hypertrophy. A weight corresponding to 12% of the body weight was attached to one wing of Japanese quail to induce hypertrophy in the patagialis (PAT) muscle. Birds in group 1 were killed after 5 (n = 8), 7 (n = 10), or 14 days (n = 10) of loading. The left wing was loaded for 14 days in group 2 birds, and then the weight was removed and the PAT was examined after 7 (n = 10), 14 (n = 10), or 21 (n = 5) days of unloading. A time-released bromodeoxyuridine (BrdU) pellet was implanted subcutaneously with wing weighting to identify activated satellite cells during loading. The left wing was loaded for 14 days, unloaded for 14 days, and then the weight was reattached for a subsequent 7 (n = 10) or 14 days (n = 10) in group 3 birds. BrdU was implanted on the second loading phase in this group. Id2 mRNA as measured by kinetic PCR increased by 3.9-, 2.7-, and 1.6-fold, relative to control levels after 7, 14, and 21 days of unloading (group 2). Id2 protein as estimated by Western blots increased by 1.5-, 1.4-, and 0.75-fold after 7, 14, and 21 days of unloading (group 2). Muscle unloading induced apoptosis, because poly(ADP-ribose) polymerase-(PARP)-positive nuclei increased and caspase 8 levels increased by 2.6- and 1.7-fold after 7 or 14 days of unloading, respectively (group 2). Although BrdU-positive nuclei increased during loading (groups 1 and 3), 50% failed to survive during unloading (group 2). Id2 mRNA increased by 2.2- and 1.8-fold after 5 and 7 days of loading, respectively, but decreased to control levels by 14 days of loading in group 1. Id2 protein levels increased 2.1-fold after 5 days of loading (group 1). In contrast, Id2 did not increase in reloaded muscles of group 3 birds. These data suggest that Id2 may have a role in apoptosis-associated atrophy of skeletal muscles, but its role in muscle hypertrophy is less clear.

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.

Potential role for Id myogenic repressors in apoptosis and attenuation of hypertrophy in muscles of aged rats

Aging attenuates the overload-induced increase in myogenic regulatory transcription factor (MRF) expression and the extent of muscle enlargement. To identify whether mRNA levels of repressors of the MRFs are greater in overloaded muscles from aged animals, overload was achieved in plantaris muscle of aged (33 mo; n = 14) and adult (9 mo; n = 17) rats. After 14 days, plantaris muscles in the overloaded limb were ~25% and 6% larger in adult and aged rats, respectively, compared with the contralateral limb. Hypertrophied muscles of adult rats had significantly greater levels of mRNA and protein levels for myogenin and MyoD compared with control muscles, but neither MRF increased with overload in muscles of aged rats. Muscles of aged rats had greater Id mRNA (150-700%) and protein repressor (200-6,000%) levels compared with adult rats. BAX and caspase 9 protein levels were 9,500% and 300% greater, respectively, in both control and hypertrophied muscles of aged rats compared with young adult rats. These data are consistent with the hypothesis that aging increases Id transcripts that activate apoptotic pathways involving BAX. This may contribute to sarcopenia by attenuating MRF protein levels in muscles of old animals.

Microarray analysis of gene expression during early adipocyte differentiation

The molecular mechanisms that regulate cellular differentiation during development and throughout life are complex. It is now recognized that precise patterns of differentially expressed genes ultimately direct a particular cell toward a given lineage and many of these are regulated during the earliest stages of differentiation. Using a microarray-based expression analysis, we have examined gene expression profiles during the first 24 h of 3T3-L1 adipocyte differentiation. RNA was isolated at times 0, 2, 8, 16, and 24 h following stimulation of differentiation and hybridized in duplicate to high density Affymetrix microarray gene chips containing a series of 13,179 cDNA/expressed sequence tag (EST) probe sets. Two hundred and eighty-five cDNA/ESTs were shown to have at least a fivefold change in expression levels during this time course and both hierarchical and self-organizing map clustering analysis was performed to categorize them by expression profiles. Several genes known to be regulated during this time period were confirmed and Western blot analysis of the proteins encoded by some of the identified genes revealed expression profiles similar to their mRNA counterparts. As expected, many of the genes identified have not been examined in such a critical time period during adipogenesis and may well represent novel adipogenic mediators.

Upregulation of collagens detected by gene array in a model of flow-induced pulmonary vascular remodeling

We recently reported localized increased pulmonary arterial resistance, neointimal lesions, and medial thickening induced by aortopulmonary anastomosis in young pigs. This model was used to investigate changes in expression of genes potentially involved in pulmonary vascular remodeling employing a high throughput Atlas Human Cardiovascular Array carrying approximately 600 cardiovascular-related cDNA sequences. Data were confirmed by Northern analysis, Western blots, and histological examination. With the use of lower stringency conditions for hybridization, 56% of the 588 human genes on the array showed visible signal after autoradiography. Approximately 10% of the genes with visible hybridization were altered by shunt-induced high flow. Extracellular matrix and cell adhesion molecules were the most highly represented group of upregulated genes. To our knowledge, our data are the first to demonstrate flow-induced changes in gene expression using a combination of cross species cDNA arrays, homologous hybridization, immunospecific protein, and histology. Our observations expand the list of genes as putative candidates in pulmonary vascular remodeling and support the utility of cross-species microarray analysis in such applications.

Role of intronic E- and N-box motifs in the transcriptional induction of the acetylcholinesterase gene during myogenic differentiation

In this study, we examined whether an intronic N-box motif is involved in the expression of acetylcholinesterase (AChE) during myogenesis. We determined that AChE transcripts are barely detectable in cultured myoblasts and that their levels increase dramatically in myotubes. Nuclear run-on assays revealed that this increase was accompanied by a parallel induction in the transcriptional activity of the AChE gene. These changes in transcription were also observed in transfection experiments using AChE promoter-reporter gene constructs. Mutation of the intronic N-box at position +755 base pairs (bp) reduced by more than 70% expression of the reporter gene in myotubes. Disruption of an adjacent E-box, at position +767 bp, also reduced expression of the reporter gene following myogenic differentiation. Co-transfection experiments using AChE promoter-reporter gene constructs and a myogenin expression vector showed that expression of this regulatory factor increased expression of the reporter gene in myotubes. Although the AChE promoter contains multiple E-boxes, mutation of this intronic one was sufficient to prevent the myogenin-induced increase in reporter gene expression. Together, these results indicate that changes in AChE gene transcription occur during myogenesis and highlight the contribution of the intronic N- and E-box motifs in the developmental regulation of the AChE gene in skeletal muscle.

Opposing effects of Ras on p53: transcriptional activation of mdm2 and induction of p19ARF

Mdm2 acts as a major regulator of the tumor suppressor p53 by targeting its destruction. Here, we show that the mdm2 gene is also regulated by the Ras-driven Raf/MEK/MAP kinase pathway, in a p53-independent manner. Mdm2 induced by activated Raf degrades p53 in the absence of the Mdm2 inhibitor p19ARF. This regulatory pathway accounts for the observation that cells transformed by oncogenic Ras are more resistant to p53-dependent apoptosis following exposure to DNA damage. Activation of the Ras-induced Raf/MEK/MAP kinase may therefore play a key role in suppressing p53 during tumor development and treatment. In primary cells, Raf also activates the Mdm2 inhibitor p19ARF. Levels of p53 are therefore determined by opposing effects of Raf-induced p19ARF and Mdm2.