Utrophin and dystrophin-associated glycoproteins in normal and dystrophin deficient cardiac muscle

Control of nuclear β-dystroglycan content is crucial for the maintenance of nuclear envelope integrity and function

β-Dystroglycan (β-DG) is a plasma membrane protein that has ability to target to the nuclear envelope (NE) to maintain nuclear architecture. Nevertheless, mechanisms controlling β-DG nuclear localization and the physiological consequences of a failure of trafficking are largely unknown. We show that β-DG has a nuclear export pathway in myoblasts that depends on the recognition of a nuclear export signal located in its transmembrane domain, by CRM1. Remarkably, NES mutations forced β-DG nuclear accumulation resulting in mislocalization and decreased levels of emerin and lamin B1 and disruption of various nuclear processes in which emerin (centrosome-nucleus linkage and β-catenin transcriptional activity) and lamin B1 (cell cycle progression and nucleoli structure) are critically involved. In addition to nuclear export, the lifespan of nuclear β-DG is restricted by its nuclear proteasomal degradation. Collectively our data show that control of nuclear β-DG content by the combination of CRM1 nuclear export and nuclear proteasome pathways is physiologically relevant to preserve proper NE structure and activity.

Nuclear import of β-dystroglycan is facilitated by ezrin-mediated cytoskeleton reorganization

The β-dystroglycan (β-DG) protein has the ability to target to multiple sites in eukaryotic cells, being a member of diverse protein assemblies including the transmembranal dystrophin-associated complex, and a nuclear envelope-localised complex that contains emerin and lamins A/C and B1. We noted that the importin α2/β1-recognised nuclear localization signal (NLS) of β-DG is also a binding site for the cytoskeletal-interacting protein ezrin, and set out to determine whether ezrin binding might modulate β-DG nuclear translocation for the first time. Unexpectedly, we found that ezrin enhances rather than inhibits β-DG nuclear translocation in C2C12 myoblasts. Both overexpression of a phosphomimetic activated ezrin variant (Ez-T567D) and activation of endogenous ezrin through stimulation of the Rho pathway resulted in both formation of actin-rich surface protrusions and significantly increased nuclear translocation of β-DG as shown by quantitative microscopy and subcellular fractionation/Western analysis. In contrast, overexpression of a nonphosphorylatable inactive ezrin variant (Ez-T567A) or inhibition of Rho signaling, decreased nuclear translocation of β-DG concomitant with a lack of cell surface protrusions. Further, a role for the actin cytoskeleton in ezrin enhancement of β-DG nuclear translocation was implicated by the observation that an ezrin variant lacking its actin-binding domain failed to enhance nuclear translocation of β-DG, while disruption of the actin cytoskeleton led to a reduction in β-DG nuclear localization. Finally, we show that ezrin-mediated cytoskeletal reorganization enhances nuclear translocation of the cytoplasmic but not the transmembranal fraction of β-DG. This is the first study showing that cytoskeleton reorganization can modulate nuclear translocation of β-DG, with the implication that β-DG can respond to cytoskeleton-driven changes in cell morphology by translocating from the cytoplasm to the nucleus to orchestrate nuclear processes in response to the functional requirements of the cell.

Phosphatase-dead myotubularin ameliorates X-linked centronuclear myopathy phenotypes in mice

Myotubularin MTM1 is a phosphoinositide (PPIn) 3-phosphatase mutated in X-linked centronuclear myopathy (XLCNM; myotubular myopathy). We investigated the involvement of MTM1 enzymatic activity on XLCNM phenotypes. Exogenous expression of human MTM1 in yeast resulted in vacuolar enlargement, as a consequence of its phosphatase activity. Expression of mutants from patients with different clinical progression and determination of PtdIns3P and PtdIns5P cellular levels confirmed the link between vacuolar morphology and MTM1 phosphatase activity, and showed that some disease mutants retain phosphatase activity. Viral gene transfer of phosphatase-dead myotubularin mutants (MTM1(C375S) and MTM1(S376N)) significantly improved most histological signs of XLCNM displayed by a Mtm1-null mouse, at similar levels as wild-type MTM1. Moreover, the MTM1(C375S) mutant improved muscle performance and restored the localization of nuclei, triad alignment, and the desmin intermediate filament network, while it did not normalize PtdIns3P levels, supporting phosphatase-independent roles of MTM1 in maintaining normal muscle performance and organelle positioning in skeletal muscle. Among the different XLCNM signs investigated, we identified only triad shape and fiber size distribution as being partially dependent on MTM1 phosphatase activity. In conclusion, this work uncovers MTM1 roles in the structural organization of muscle fibers that are independent of its enzymatic activity. This underlines that removal of enzymes should be used with care to conclude on the physiological importance of their activity.

Dystrophin and utrophin isoforms are expressed in glia, but not neurons, of the avian parasympathetic ciliary ganglion

Muscular dystrophy patients often show cognitive impairment, in addition to muscle degeneration caused by dystrophin gene defects. The cognitive impairments lead to speculation that the dystrophin protein family may play a key role at neuronal synapses. Dystrophin regulates the stability of selected GABA(A) receptor subtypes and alpha3-containing nicotinic acetylcholine receptors (nAChRs) at a subset of central GABAergic and peripheral sympathetic nicotinic neuron synapses. Similarly, utrophin, the autosomal homologue of dystrophin, is not required for clustering but indirectly stabilizes muscle-type nAChRs at the neuromuscular junction. We examined dystrophin and utrophin expression and localization in the avian parasympathetic ciliary ganglion (CG) to determine whether these proteins play a general role at neuronal nicotinic synapses. We have determined that full-length utrophin and dystrophin and the short dystrophin isoform Dp116 are the major isoforms expressed in the CG based on immunoblotting and immunolabeling. Unexpectedly, the cytoskeletal proteins were not detected at nicotinic synapses or in CG neurons. They are expressed in myelinating and non-myelinating Schwann cells. Further, utrophin expression developmentally precedes that of dystrophin. The proteins show partially overlapping distributions, but also differential accumulation along the surface membrane of Schwann cells adjacent to neuronal somata versus axonal processes. Our findings are consistent with reports that dystrophin protein family members function in the maintenance of cell-cell interactions and myelination by anchoring the Schwann cell surface membrane to the basal lamina. In contrast, our results differ from those in skeletal muscle and a subset of sympathetic neurons where utrophin and dystrophin localize at nicotinic synapses.

Characterization of a novel Dp71 dystrophin-associated protein complex (DAPC) present in the nucleus of HeLa cells: members of the nuclear DAPC associate with the nuclear matrix

Dystrophin is an essential component in the assembly and maintenance of the dystrophin-associated protein complex (DAPC), which includes members of the dystroglycan, syntrophin, sarcoglycan and dystrobrevin protein families. Distinctive complexes have been described in the cell membrane of different tissues and cultured cells. In this work, we report the identification and characterization of a novel DAPC present in the nuclei of HeLa cells, which contains dystrophin Dp71 as a key component. Using confocal microscopy and cell fractionation analyses, we found the presence of Dp71, beta-sarcoglycan, beta-dystroglycan, alpha- and beta-syntrophin, alpha1- and beta-dystrobrevin and nNOS in the nuclei of HeLa cells. Furthermore, we demonstrated by co-immunoprecipitation experiments that most of these proteins form a complex in the nuclear compartment. Next, we analyze the possible association of the nuclear DAPC with the nuclear matrix. We found the presence of Dp71, beta-dystroglycan, nNOS, beta-sarcoglycan, alpha/beta syntrophin, alpha1-dystrobrevin and beta-dystrobrevin in the nuclear matrix protein fractions and in situ nuclear matrix preparations from HeLa cells. Moreover, we found that Dp71, beta-dystroglycan and beta-dystrobrevin co-immunoprecipitated with the nuclear matrix proteins lamin B1 and actin. The association of members of the nuclear DAPC with the nuclear matrix indicates that they may work as scaffolding proteins involved in nuclear architecture.

Pathological pattern of Mdx mice diaphragm correlates with gradual expression of the short utrophin isoform Up71

Utrophin gene is transcribed in a large mRNA of 13 kb that codes for a protein of 395 kDa. It shows amino acid identity with dystrophin of up to 73% and is widely expressed in muscle and non-muscle tissues. Up71 is a short utrophin product of the utrophin gene with the same cysteine-rich and C-terminal domains as full-length utrophin (Up395). Using RT-PCR, Western blots analysis, we demonstrated that Up71 is overexpressed in the mdx diaphragm, the most pathological muscle in dystrophin-deficient mdx mice, compared to wild-type C57BL/10 or other mdx skeletal muscles. Subsequently, we demonstrated that this isoform displayed an increased expression level up to 12 months, whereas full-length utrophin (Up395) decreased. In addition, beta-dystroglycan, the transmembrane glycoprotein that anchors the cytoplasmic C-terminal domain of utrophin, showed similar increase expression in mdx diaphragm, as opposed to other components of the dystrophin-associated protein complex (DAPC) such as alpha-dystrobrevin1 and alpha-sarcoglycan. We demonstrated that Up71 and beta-dystroglycan were progressively accumulated along the extrasynaptic region of regenerating clusters in mdx diaphragm. Our data provide novel functional insights into the pathological role of the Up71 isoform in dystrophinopathies.

A novel isoform of delta-sarcoglycan is localized at the sarcoplasmic reticulum of mouse skeletal muscle

The sarcoglycan-sarcospan complex (alpha-, beta-, gamma-, delta-, epsilon-, and zeta-SG-SSPN), a component of the dystrophin-associated glycoprotein complex (DAGC), is located at the sarcolemma of muscle fibers where it contributes to maintain cell integrity during contraction-relaxation cycles; gamma- and delta-SG are also expressed in the sarcoplasmic reticulum (SR). In this study, we report the identification of a novel isoform of murine delta-SG produced by alternative splicing that we named delta-SG3. This isoform is present at transcript level in several tissues, with its highest expression in skeletal and cardiac muscle. The delta-SG3 protein lacks the last 122 amino acids at the C-terminal, which are replaced by 10 new amino acids (EGFLNMQLAG). Interestingly, double immunofluorescence analysis for delta-SG3 and the dihydropyridine receptor (DHPR) shows a close localization of these two proteins. We propose the subcellular distribution of this novel delta-SG3 isoform at the SR and its involvement in intracellular calcium concentration regulation.

The sarcoglycan-sarcospan complex localization in mouse retina is independent from dystrophins

The sarcoglycan-sarcospan (SG-SSPN) complex is part of the dystrophin-glycoprotein complex that has been extensively characterized in muscle. To establish the framework for functional studies of sarcoglycans in retina here, we quantified sarcoglycans mRNA levels with real-time reverse transcriptase-polymerase chain reaction (RT-PCR) and performed immunohistochemistry to determine their cellular and subcellular distribution. We showed that the beta-, delta-, gamma-, epsilon-sarcoglycans and sarcospan are expressed in mouse retina. They are localized predominantly in the outer and the inner limiting membranes, probably in the Müller cells and also in the ganglion cells axons where the expression of dystrophins have never been reported. We also investigated the status of the sarcoglycans in the retina of mdx(3cv) mutant mice for all Duchene Muscular Dystrophy (DMD) gene products. The absence of dystrophin did not produce any change in the sarcoglycan-sarcospan components expression and distribution.

Utrophin deficiency worsens cardiac contractile dysfunction present in dystrophin-deficient mdx mice

The loss of dystrophin in patients with Duchenne muscular dystrophy (DMD) causes devastating skeletal muscle degeneration and cardiomyopathy. Dystrophin-deficient (mdx) mice have a much milder phenotype, whereas double knockout (DKO) mice lacking both dystrophin and its homolog, utrophin, exhibit the clinical signs observed in DMD patients. We have previously shown that DKO and mdx mice have similar severities of histological features of cardiomyopathy, but no contractile functional measurements of DKO heart have ever been carried out. To investigate whether DKO mice display cardiac dysfunction at the tissue level, contractile response of the myocardium was tested in small, unbranched, ultrathin, right ventricular muscles. Under near physiological conditions, peak isometric active developed tension (F(dev), in mN/mm2) at a stimulation frequency of 4 Hz was depressed in DKO mice (15.3 +/- 3.7, n = 8) compared with mdx mice (24.2 +/- 5.4, n = 7), which in turn were depressed compared with wild-type (WT) control mice (33.2 +/- 4.5, n = 7). This reduced Fdev was also observed at frequencies within the murine physiological range; at 12 Hz, Fdev was (in mN/mm2) 11.4 +/- 1.8 in DKO, 14.5 +/- 4.2 in mdx, and 28.8 +/- 5.4 in WT mice. The depression of Fdev was observed over the entire frequency range of 4-14 Hz and was significant between DKO versus mdx mice, as well as between DKO or mdx mice versus WT mice. Under beta-adrenergic stimulation (1 micromol/l isoproterenol), Fdev in DKO preparations was only (in mN/mm2) 14.7 +/- 5.1 compared with 30.9 +/- 8.9 in mdx and 41.0 +/- 4.9 in WT mice. These data show that cardiac contractile dysfunction of mdx mice is generally worsened in mice also lacking utrophin.

alpha7B integrin changes in mdx mouse muscles after L-arginine administration

Muscle fibers attach to laminin in the basal lamina using two mechanisms, i.e., dystrophin with its associated proteins and alpha7beta1 integrin. In humans, gene-mutation defects in one member of these complexes result in muscular dystrophies. This study revealed changes after L-arginine treatment of utrophin-associated proteins and the alpha7B integrin subunit in mdx mouse, a dystrophin-deficient animal model. In the two studied muscles (cardiac muscle and diaphragm), the alpha7B integrin subunit was increased in 5-week-old treated mice. Interestingly, the diaphragm histopathological appearance was significantly improved by L-arginine administration. These results highlight a possible way to compensate for dystrophin deficiency via alpha7beta1 integrin.

The heart in human dystrophinopathies

Dystrophinopathies are due to mutations in the dystrophin gene on chromosome Xp21.1 and comprise the allelic entities Duchenne muscular dystrophy (DMD), Becker muscular dystrophy (BMD) and X-linked dilative cardiomyopathy (XLDCM). In all three entities, the heart is affected to various degrees, depending on the stage of the disease and the type of the mutation (cardiac involvement, CI). The pathoanatomic evidence of CI in dystrophinopathies is the replacement of myocardium by connective tissue or fat. In DMD/BMD, the left ventricular posterobasal and lateral walls are most extensively affected, sparing the right ventricle and the atrium. Degree and dynamics of CI vary among the three entities. In DMD/BMD, CI usually remains subclinical in the early stages of the disease. Typical initial manifestations of CI in DMD/BMD are sinus tachycardia, tall R1 in V1, prominent Q in I, aVL, V6 or in II, III, and aVF, increased QT dispersion and possibly autonomic dysfunction. Initially, echocardiography is normal or shows regional wall motion abnormalities in areas of fibrosis. With spreading of fibrosis, left ventricular dysfunction and ventricular arrhythmias additionally occur. In the final stages of the disease, systolic function may lead to heart failure and sudden death. Subclinical or clinical CI is present in about 90% of the DMD/BMD patients but is the cause of death in only 20% of the DMD and 50% of the BMD patients. XLDCM is a rapidly progressive, almost exclusively myocardial disorder, starting in teenage males as heart failure due to dilative cardiomyopathy (CMP), leading to death from intractable heart failure within 1-2 years after diagnosis. Therapy of arrhythmias and CMP in all three disorders follows the established cardiological recommendations. Due to its protective effect, ACE inhibitors are recommended already at the early stages of the disease. Beta-blockers may be an additional option if indicated.

Cardiomyopathic features associated with muscular dystrophy are independent of dystrophin absence in cardiovasculature

The loss of dystrophin results in skeletal muscle degeneration and cardiomyopathy in patients with Duchenne muscular dystrophy. In skeletal muscle, dystrophin strengthens the myofiber membrane by linking the submembranous cytoskeleton and extracellular matrix through its direct interaction with the dystroglycan/sarcoglycan complex. In limb-girdle muscular dystrophy, the loss of the sarcoglycans in cardiovasculature leads to cardiomyopathy. It is unknown whether the absence of dystrophin in cardiomyocytes or cardiovasculature leads to the cardiomyopathy associated with primary dystrophin deficiency. We show here that the cardiomyopathic features of the utrophin/dystrophin-deficient mouse can be prevented by the presence of dystrophin in cardiomyocytes but not in cardiovasculature. Furthermore, restoration of the dystroglycans and sarcoglycans to the cardiomyocyte membrane is not sufficient to prevent cardiomyopathy. These data provide the first evidence that dystrophin plays a mechanical role in cardiomyocytes similar to its role in skeletal muscle. These results indicate that treatment of cardiomyocytes but not cardiovasculature is essential in dystrophinopathies.

Variations in dystrophin complex in red and white caudal muscles from Torpedo marmorata

We present an up-to-date study on the nature, at the protein level, of various members of the dystrophin complex at the muscle cell membrane by comparing red and white caudal muscles from Torpedo marmorata. Our investigations involved immunodetection approaches and Western blotting analysis. We determined the presence or absence of different molecules belonging to the dystrophin family complex by analyzing their localization and molecular weight. Specific antibodies directed against dystrophin, i.e., DRP2 alpha-dystrobrevin, beta-dystroglycan, alpha-syntrophin, alpha-, beta-, gamma-, and delta-sarcoglycan, and sarcospan, were used. The immunofluorescence study (confocal microscopy) showed differences in positive immunoreactions at the sarcolemmal membrane in these slow-type and fast-type skeletal muscle fibers. Protein extracts from T. marmorata red and white muscles were analyzed by Western blotting and confirmed the presence of dystrophin and associated proteins at the expected molecular weights. Differences were confirmed by comparative immunoprecipitation analysis of enriched membrane preparations with anti-beta-dystroglycan polyclonal antibody. These experiments revealed clear complex or non-complex formation between members of the dystrophin system, depending on the muscle type analyzed. Differences in the potential function of these various dystrophin complexes in fast or slow muscle fibers are discussed in relation to previous data obtained in corresponding mammalian tissues. (J Histochem Cytochem 49:857-865, 2001)

Absence of utrophin in intercalated discs of human cardiac muscle

Utrophin is the autosomal homologue of dystrophin. In normal skeletal muscle it is localised only to neuromuscular and myotendinous junctions, nerves and vascular tissue. In Xp21 muscular dystrophies, utrophin is also detected on the sarcolemma of skeletal and cardiac muscle, while dystrophin is absent or reduced. In normal cardiac muscle, some reports have demonstrated utrophin at intercalated discs and T-tubules. We have re-examined the distribution of utrophin in normal human cardiac muscle using a panel of eight monoclonal antibodies against different epitopes in N- and C-terminal domains. In contrast to previous studies, utrophin was not detected at the intercalated discs or T-tubules, although labelling of blood vessels was strong. We conclude that the primary location of utrophin in normal heart is in the vascular system. In addition, our results show that the utrophin on cardiac blood vessels is full length, similar to that of skeletal muscle blood vessels.

Dystrophin-associated proteins in obliquely striated muscle of the leech Pontobdella muricata (Annelida, Hirudinea)

The distribution of dystrophin-associated proteins (beta-dystroglycan, alpha-, beta-, gamma- and delta-sarcoglycan, alpha-syntrophin and sarcospan) were studied in obliquely striated muscle of the leech Pontobdella muricata. Western blot analysis and immunohistochemical electron microscopy, using various polyclonal antibodies, were employed. Western blot analysis of all of these antibodies showed a single band, with approximately the same molecular weights as similar proteins detected in vertebrate muscles. The immunoelectron microscopy study confirmed specific immunogold labelling in the membrane of muscle cells. Since all dystrophin complex components have similar molecular weights and the same localisation in leech as in vertebrate skeletal muscle, we assume that these proteins have similar properties in leech and vertebrate muscle. The presence of these molecules in annelid muscles, together with a short version of dystrophin (previously described as IDLp-140) is of particular interest since phylogenetic and functional studies on this material could help to shed new light on the role and function of this complex in the muscle membrane.