Characterization and localization of a 77 kDa protein related to the dystrophin gene family

The role of the dystrophin glycoprotein complex in muscle cell mechanotransduction

Dystrophin is the central protein of the dystrophin-glycoprotein complex (DGC) in skeletal and heart muscle cells. Dystrophin connects the actin cytoskeleton to the extracellular matrix (ECM). Severing the link between the ECM and the intracellular cytoskeleton has a devastating impact on the homeostasis of skeletal muscle cells, leading to a range of muscular dystrophies. In addition, the loss of a functional DGC leads to progressive dilated cardiomyopathy and premature death. Dystrophin functions as a molecular spring and the DGC plays a critical role in maintaining the integrity of the sarcolemma. Additionally, evidence is accumulating, linking the DGC to mechanosignalling, albeit this role is still less understood. This review article aims at providing an up-to-date perspective on the DGC and its role in mechanotransduction. We first discuss the intricate relationship between muscle cell mechanics and function, before examining the recent research for a role of the dystrophin glycoprotein complex in mechanotransduction and maintaining the biomechanical integrity of muscle cells. Finally, we review the current literature to map out how DGC signalling intersects with mechanical signalling pathways to highlight potential future points of intervention, especially with a focus on cardiomyopathies.

A review of the function of the Dystrophic Glycoprotein Complex (DGC) in mechanosignaling provides an overview of the various components of DGC and potential mechanopathogenic mechanisms, particularly as they relate to muscular dystrophy.

Dystrophin Dp71 and the Neuropathophysiology of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is caused by frameshift mutations in the DMD gene that prevent the body-wide translation of its protein product, dystrophin. Besides a severe muscle phenotype, cognitive impairment and neuropsychiatric symptoms are prevalent. Dystrophin protein 71 (Dp71) is the major DMD gene product expressed in the brain and mutations affecting its expression are associated with the DMD neuropsychiatric syndrome. As with dystrophin in muscle, Dp71 localises to dystrophin-associated protein complexes in the brain. However, unlike in skeletal muscle; in the brain, Dp71 is alternatively spliced to produce many isoforms with differential subcellular localisations and diverse cellular functions. These include neuronal differentiation, adhesion, cell division and excitatory synapse organisation as well as nuclear functions such as nuclear scaffolding and DNA repair. In this review, we first describe brain involvement in DMD and the abnormalities observed in the DMD brain. We then review the gene expression, RNA processing and functions of Dp71. We review genotype-phenotype correlations and discuss emerging cellular/tissue evidence for the involvement of Dp71 in the neuropathophysiology of DMD. The literature suggests changes observed in the DMD brain are neurodevelopmental in origin and that their risk and severity is associated with a cumulative loss of distal DMD gene products such as Dp71. The high risk of neuropsychiatric syndromes in Duchenne patients warrants early intervention to achieve the best possible quality of life. Unravelling the function and pathophysiological significance of dystrophin in the brain has become a high research priority to inform the development of brain-targeting treatments for Duchenne.

Evidence of the involvement of dystrophin Dp71 in corneal angiogenesis

PURPOSE

The aim of this study was to define the role of dystrophin Dp71 in corneal angiogenesis.

METHODS

Inflammation-induced corneal neovascularization experiments were performed in Dp71-null mice and C57BL/6J wild-type mice.

RESULTS

The corneal neovascular area covered by neovascularization was larger in the injured corneas of the Dp71-null mice compared to the corneas of the wild-type mice: 40.72% versus 26.33%, respectively (p<0.005). Moreover, increased angiogenesis was associated with a high expression of vascular endothelial growth factor (VEGF). Similarly, aortic ring assays showed a significant enhancement of the neovascular area.

CONCLUSIONS

These results suggest that dystrophin Dp71 could play an important role as a negative regulator of corneal angiogenesis.

The SLMAP/Striatin complex: An emerging regulator of normal and abnormal cardiac excitation-contraction coupling

The excitation-contraction (E-C) module involves a harmonized correspondence between the sarcolemma and the sarcoplasmic reticulum. This is provided by membrane proteins, which primarily shape the caveolae, the T-tubule/Sarcoplasmic reticulum (TT/SR) junction, and the intercalated discs (ICDs). Distortion of either one of these structures impairs myocardial contraction, and subsequently translates into cardiac failure. Thus, detailed studies on the molecular cues of the E-C module are becoming increasingly necessary to pharmacologically eradicate cardiac failure Herein we reviewed the organization of caveolae, TT/SR junctions, and the ICDs in the heart, with special attention to the Sarcolemma Membrane Associated Protein (SLMAP) and striatin (STRN) in cardiac membranes biology and cardiomyocyte contraction. We emphasized on their in vivo and in vitro signaling in cardiac function/dysfunction. SLMAP is a cardiac membrane protein that plays an important role in E-C coupling and the adrenergic response of the heart. Similarly, STRN is a dynamic protein that is also involved in cardiac E-C coupling and ICD-related cardiomyopathies. Both SLMAP and STRN are linked to cardiac conditions, including heart failure, and their role in cardiomyocyte function was elucidated in our laboratory. They interact together in a protein complex that holds therapeutic potentials for cardiac dysfunction. This review is the first of its kind to conceptualize the role of the SLMAP/STRN complex in cardiac function and failure. It provides in depth information on the signaling of these two proteins and projects their interaction as a novel therapeutic target for cardiac failure.

Cardiac T-Tubule Microanatomy and Function

Unique to striated muscle cells, transverse tubules (t-tubules) are membrane organelles that consist of sarcolemma penetrating into the myocyte interior, forming a highly branched and interconnected network. Mature t-tubule networks are found in mammalian ventricular cardiomyocytes, with the transverse components of t-tubules occurring near sarcomeric z-discs. Cardiac t-tubules contain membrane microdomains enriched with ion channels and signaling molecules. The microdomains serve as key signaling hubs in regulation of cardiomyocyte function. Dyad microdomains formed at the junctional contact between t-tubule membrane and neighboring sarcoplasmic reticulum are critical in calcium signaling and excitation-contraction coupling necessary for beat-to-beat heart contraction. In this review, we provide an overview of the current knowledge in gross morphology and structure, membrane and protein composition, and function of the cardiac t-tubule network. We also review in detail current knowledge on the formation of functional membrane subdomains within t-tubules, with a particular focus on the cardiac dyad microdomain. Lastly, we discuss the dynamic nature of t-tubules including membrane turnover, trafficking of transmembrane proteins, and the life cycles of membrane subdomains such as the cardiac BIN1-microdomain, as well as t-tubule remodeling and alteration in diseased hearts. Understanding cardiac t-tubule biology in normal and failing hearts is providing novel diagnostic and therapeutic opportunities to better treat patients with failing hearts.

Altered astrocyte morphology and vascular development in dystrophin-Dp71-null mice

Understanding retinal vascular development is crucial because many retinal vascular diseases such as diabetic retinopathy (in adults) or retinopathy of prematurity (in children) are among the leading causes of blindness. Given the localization of the protein Dp71 around the retinal vessels in adult mice and its role in maintaining retinal homeostasis, the aim of this study was to determine if Dp71 was involved in astrocyte and vascular development regulation. An experimental study in mouse retinas was conducted. Using a dual immunolabeling with antibodies to Dp71 and anti-GFAP for astrocytes on retinal sections and isolated astrocytes, it was found that Dp71 was expressed in wild-type (WT) mouse astrocytes from early developmental stages to adult stage. In Dp71-null mice, a reduction in GFAP-immunopositive astrocytes was observed as early as postnatal day 6 (P6) compared with WT mice. Using real-time PCR, it was showed that Dp71 mRNA was stable between P1 and P6, in parallel with post-natal vascular development. Regarding morphology in Dp71-null and WT mice, a significant decrease in overall astrocyte process number in Dp71-null retinas at P6 to adult age was found. Using fluorescence-conjugated isolectin Griffonia simplicifolia on whole mount retinas, subsequent delay of developing vascular network at the same age in Dp71-null mice was found. An evidence that the Dystrophin Dp71, a membrane-associated cytoskeletal protein and one of the smaller Duchenne muscular dystrophy gene products, regulates astrocyte morphology and density and is associated with subsequent normal blood vessel development was provided.

Novel Nuclear Protein Complexes of Dystrophin 71 Isoforms in Rat Cultured Hippocampal GABAergic and Glutamatergic Neurons

The precise functional role of the dystrophin 71 in neurons is still elusive. Previously, we reported that dystrophin 71d and dystrophin 71f are present in nuclei from cultured neurons. In the present work, we performed a detailed analysis of the intranuclear distribution of dystrophin 71 isoforms (Dp71d and Dp71f), during the temporal course of 7-day postnatal rats hippocampal neurons culture for 1h, 2, 4, 10, 15 and 21 days in vitro (DIV). By immunofluorescence assays, we detected the highest level of nuclear expression of both dystrophin Dp71 isoforms at 10 DIV, during the temporal course of primary culture. Dp71d and Dp71f were detected mainly in bipolar GABAergic (≥60%) and multipolar Glutamatergic (≤40%) neurons, respectively. We also characterized the existence of two nuclear dystrophin-associated protein complexes (DAPC): dystrophin 71d or dystrophin 71f bound to β-dystroglycan, α1-, β-, α2-dystrobrevins, α-syntrophin, and syntrophin-associated protein nNOS (Dp71d-DAPC or Dp71f-DAPC, respectively), in the hippocampal neurons. Furthermore, both complexes were localized in interchromatin granule cluster structures (nuclear speckles) of neuronal nucleoskeleton preparations. The present study evinces that each Dp71's complexes differ slightly in dystrobrevins composition. The results demonstrated that Dp71d-DAPC was mainly localized in bipolar GABAergic and Dp71f-DAPC in multipolar Glutamatergic hippocampal neurons. Taken together, our results show that dystrophin 71d, dystrophin 71f and DAP integrate protein complexes, and both complexes were associated to nuclear speckles structures.

Induction of dystrophin Dp71 expression during neuronal differentiation: opposite roles of Sp1 and AP2alpha in Dp71 promoter activity

In this study, we delineated the molecular mechanisms that modulate Dp71 expression during neuronal differentiation, using the N1E-115 cell line. We demonstrated that Dp71 expression is up-regulated in response to cAMP-mediated neuronal differentiation of these cells, and that this induction is controlled at promoter level. Functional deletion analysis of the Dp71 promoter revealed that a 5'-flanking 159-bp DNA fragment that contains Sp1 and AP2 binding sites is necessary and sufficient for basal expression of this TATA-less promoter, as well as for its induction during neuronal differentiation. Electrophoretic mobility shift and chromatin immunoprecipitation assays revealed that Sp1 and AP2alpha bind to their respective DNA elements within the Dp71 basal promoter. Overall, mutagenesis assays on the Sp1 and AP2 binding sites, over-expression of Sp1 and AP2alpha, as well as knock-down experiments on Sp1 and AP2alpha gene expression established that Dp71 basal expression is controlled by the combined action of Sp1 and AP2alpha, which act as activator and repressor, respectively. Furthermore, we demonstrated that induction of Dp71 expression in differentiated cells is the result of the maintenance of positive regulation exerted by Sp1, as well as of the loss of AP2alpha binding, which ultimately releases the promoter from repression.

Role of mental retardation-associated dystrophin-gene product Dp71 in excitatory synapse organization, synaptic plasticity and behavioral functions

Background

Duchenne muscular dystrophy (DMD) is caused by deficient expression of the cytoskeletal protein, dystrophin. One third of DMD patients also have mental retardation (MR), likely due to mutations preventing expression of dystrophin and other brain products of the DMD gene expressed from distinct internal promoters. Loss of Dp71, the major DMD-gene product in brain, is thought to contribute to the severity of MR; however, the specific function of Dp71 is poorly understood.

Methodology/Principal Findings

Complementary approaches were used to explore the role of Dp71 in neuronal function and identify mechanisms by which Dp71 loss may impair neuronal and cognitive functions. Besides the normal expression of Dp71 in a subpopulation of astrocytes, we found that a pool of Dp71 colocalizes with synaptic proteins in cultured neurons and is expressed in synaptic subcellular fractions in adult brains. We report that Dp71-associated protein complexes interact with specialized modular scaffolds of proteins that cluster glutamate receptors and organize signaling in postsynaptic densities. We then undertook the first functional examination of the brain and cognitive alterations in the Dp71-null mice. We found that these mice display abnormal synapse organization and maturation in vitro, altered synapse density in the adult brain, enhanced glutamatergic transmission and reduced synaptic plasticity in CA1 hippocampus. Dp71-null mice show selective behavioral disturbances characterized by reduced exploratory and novelty-seeking behavior, mild retention deficits in inhibitory avoidance, and impairments in spatial learning and memory.

Conclusions/Significance

Results suggest that Dp71 expression in neurons play a regulatory role in glutamatergic synapse organization and function, which provides a new mechanism by which inactivation of Dp71 in association with that of other DMD-gene products may lead to increased severity of MR.

Dp71, utrophin and beta-dystroglycan expression and distribution in PC12/L6 cell cocultures

Function of dystrophin Dp71 isoforms is unknown but seems related to neurite outgrowth and synapse formation. To evaluate Dp71 role in myoneural synapses, we established a coculture model using PC12 cells and L6 myotubes and analyzed expression and localization of Dp71 and related proteins, utrophin and beta-dystroglycan, in PC12 cells. Confocal microscopy showed Dp71d isoform in PC12 nuclei, golgi-complex-like and endoplasmic reticulum-like structures, whereas Dp71ab concentrates at neurite tips and cytoplasm, colocalizing with beta-dystroglycan, utrophin, synaptophysin and acetylcholine receptors. Evidences suggest that Dp71ab isoform, unlike Dp71d, may take part in neurite-related processes. This is the first work on Dp and members of Dp-associated protein complex roles in a cell-line based coculturing system, which may be useful in determining Dp71 isoforms associations.

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.

Dystrophin Dp71f associates with the beta1-integrin adhesion complex to modulate PC12 cell adhesion

Dystrophin Dp71 is the main product of the Duchenne muscular dystrophy gene in the brain; however, its function is unknown. To study the role of Dp71 in neuronal cells, we previously generated by antisense treatment PC12 neuronal cell clones with decreased Dp71 expression (antisense-Dp71 cells). PC12 cells express two different splicing isoforms of Dp71, a cytoplasmic variant called Dp71f and a nuclear isoform called Dp71d. We previously reported that antisense-Dp71 cells display deficient adhesion to substrate and reduced immunostaining of beta1-integrin in the cell area contacting the substrate. In this study, we isolated additional antisense-Dp71 clones to analyze in detail the potential involvement of Dp71f isoform with the beta1-integrin adhesion system of PC12 cells. Immunofluorescence analyses as well as immunoprecipitation assays demonstrated that the PC12 cell beta1-integrin adhesion complex is composed of beta1-integrin, talin, paxillin, alpha-actinin, FAK and actin. In addition, our results showed that Dp71f associates with most of the beta1-integrin complex components (beta1-integrin, FAK, alpha-actinin, talin and actin). In the antisense-Dp71 cells, the deficiency of Dp71 provokes a significant reduction of the beta1-integrin adhesion complex and, consequently, the deficient adhesion of these cells to laminin. In vitro binding experiments confirmed the interaction of Dp71f with FAK and beta1-integrin. Our data indicate that Dp71f is a structural component of the beta1-integrin adhesion complex of PC12 cells that modulates PC12 cell adhesion by conferring proper complex assembly and/or maintenance.

Phosphorylation of dystrophin Dp71d by Ca2+/calmodulin-dependent protein kinase II modulates the Dp71d nuclear localization in PC12 cells

We have shown that the splicing isoform of Dp71 (Dp71d) localizes to the nucleus of PC12 cells, an established cell line derived from a rat pheochromocytoma; however, the mechanisms governing its nuclear localization are unknown. As protein phosphorylation modulates the nuclear import of proteins, and as Dp71d presents several potential sites for phosphorylation, we analyzed whether Dp71d is phosphorylated in PC12 cells and the role of phosphorylation on its nuclear localization. We demonstrated that Dp71d is phosphorylated under basal conditions at serine and threonine residues by endogenous protein kinases. Dp71d phosphorylation was activated by 2-O-tetradecanoyl phorbol 13-acetate (TPA), but this effect was blocked by EGTA. Supporting the role of intracellular calcium on Dp71d phosphorylation, we observed that the stimulation of calcium influx by cell depolarization increased Dp71d phosphorylation, and that the calcium-calmodulin inhibitor N-(6-aminohexyl)-1-naphthalenesulfonamide (W-7) blocked such induction. The blocking action of bisindolylmaleimide I (Bis I), a specific inhibitor for Ca2+/diacylglicerol-dependent protein kinase (PKC), on Dp71d phosphorylation suggested the participation of PKC in this event. In addition, transfection experiments with Ca2+/calmodulin-dependent protein kinase II (CaMKII) expression vectors as well as the use of KN-62, a CaMKII-specific inhibitor, demonstrated that CaMKII is also involved in Dp71d phosphorylation. Stimulation of Dp71d phosphorylation by cell depolarization and/or the overexpression of CaMKII favored the Dp71d nuclear accumulation. Overall, our results indicate that CAMKII-mediated Dp71d phosphorylation modulates its nuclear localization.

Dystrophin splice variants are distinctly localized in the hippocampus

It has previously been demonstrated that Dp71, the most abundant dystrophin protein in the brain, is mainly localized in the postsynaptic densities. Here we show the localization of Dp71f, one of the splice variants of this protein, within the CA3 region of the hippocampus. Immunopositivity occurs in the postsynaptic density of small asymmetrical axospinous and axodendritic synapses, while it is absent in the postsynaptic densities of the axospinous synapses of the large mossy fiber terminals. Dp71f immunoreactivity was found to be attached to the membranes of the mossy fibers in the stratum lucidum of the CA3 area. In a certain population of thin myelinated axons the protein seems to be present within the axon proper. These data support the notion of a physiological role of Dp71f distinct from other dystrophin isoforms present in the central nervous system.

Glutamate regulates dystrophin-71 levels in glia cells

Glial glutamate receptors are likely to be involved in neuronal differentiation, migration, and plasticity. Dystrophin, the protein defective in Duchenne muscular dystrophy (DMD) is widely expressed in the Central Nervous System. Activation of internal promoters of the DMD gene leads to the production of several proteins, the Dystrophin-71 (Dp-71) being the most abundant in the encephalon. This protein is known to stabilize neurotransmitter receptors in clusters and its absence has been correlated with cognitive deficits in a mouse model. Using cultured chick Bergmann glia cells and mouse cerebellar fusiform astrocytes, we demonstrate here that glutamate receptor activation results in a time and dose dependent decrease of Dp-71 levels. This effect is mediated through alphaamino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. The present results suggest an involvement of Dp-71 in glutamate receptor signaling and possibly clustering and further support the notion of an active role of glia in the physiology of glutamatergic transmission.

Dystrophin Dp71 in PC12 cell adhesion

Previously, we reported that PC12 cells with decreased Dp71 expression (antisense-Dp71 cells) display deficient nerve-growth-factor-induced neurite outgrowth. In this study, we show that disturbed neurite outgrowth of antisense-Dp71 cells is accompanied by decreased adhesion activity on laminin, collagen and fibronectin. In wild-type cells, the immunostaining of Dp71 and beta1-integrin overlaps in the basal area contacting the substrate, but staining of both proteins decrease in the antisense-Dp71 cells. Morphology of antisense-Dp71 cells at the electron microscopic level is characterized by the lack of filopodia, cellular projections involved in adhesion. Our findings suggest that Dp71 is required for the efficient PC12 cell attachment to beta1-integrin-dependent substrata and that decreased adhesion activity of the antisense-Dp71 cells could determine their deficiency to extend neurites.

Dystroglycan is involved in laminin-1-stimulated motility of Müller glial cells: combined velocity and directionality analysis

We investigate the role of dystroglycan, a major laminin-1 receptor and central member of the dystrophin-glycoprotein complex, in the laminin-1 induced motility of cultured Muller glial cells. Binding of laminin-1 to dystroglycan was prevented by IIH6, a function-blocking monoclonal antibody against alpha-dystroglycan. As an alternative means of inhibition, we used heparin to mask the dystroglycan binding site of the laminin-1, known to overlap with heparin binding sites. Cell motility was characterized in a two-dimensional motility assay based on computer-controlled videomicroscopy and statistical analysis of cellular trajectories. We obtained data on both the cell velocity and the diffusion index, a measure of direction-changing frequency. Both means of inhibition of dystroglycan function led to a significant decrease in the ability of laminin-1 to stimulate cell migration. At the same time, dystroglycan function does not appear to be involved in laminin-1-dependent increase in process dynamism and direction-changing activity.

Formation of multiple complexes between beta-dystroglycan and dystrophin family products

Beta-dystroglycan is expressed in a wide variety of tissues and has generally been reported with an Mr of 43 kDa, sometimes accompanied with a 31 kDa protein assumed to be a truncated product. This molecule was recently identified as the anomalous beta-dystroglycan expressed in various carcinoma cell lines. We produced and characterized a G5 polyclonal antibody specific to beta-dystroglycan that is directed against the C-terminal portion of the molecule. We provide evidence that beta-dystroglycan may vary in size and properties by studying different Xenopus tissues. Besides normal beta-dystroglycan with an Mr of 43 kDa in smooth and cardiac muscle and sciatic nerve extracts, we found it in skeletal muscle and brain proteins with an Mr of 38 and 65 kDa, respectively. Glycosylation properties and proteolytic susceptibilities of these different beta-dystroglycans are analysed and compared in this work. Crosslinking experiments with various beta-dystroglycan preparations obtained from skeletal and cardiac muscles and brain gave rise to specific new covalent products with Mr of 125 kDa (doublet band), or 120 and 130 kDa, or 140 and 240 kDa, respectively. We provide evidence, using various similar beta-dystroglycan preparations, that the immunoprecipitation procedure with G5 specific polyclonal antibody allows consistent pelleting of various dystrophin-family isoforms. Skeletal muscles from Xenopus reveals the presence of two distinct beta-dystroglycan complexes, one with dystrophin and another one which involves alpha-dystrobrevin. Cardiac muscle and brain from Xenopus are shown to contain three beta-dystroglycan complexes related to various dystrophin-family isoforms. Dystrophin or alpha-dystrobrevin or Dp71 were found in cardiac muscle and dystrophin or Dp180 or Up71 in brain. This variability in the relationship between beta-dystroglycan and dystrophin-family isoforms suggests that each protein--currently known as dystrophin associated protein--could not be present in each of these complexes.

Dystrophin Dp71 is required for neurite outgrowth in PC12 cells

To determine the role of Dp71 in neuronal cells, we generated PC12 cell lines in which Dp71 protein levels were controlled by stable transfection with either antisense or sense constructs. Cells expressing the antisense Dp71 RNA (antisense-Dp71 cells) contained reduced amounts of the two endogenous Dp71 isoforms. Antisense-Dp71 cells exhibited a marked suppression of neurite outgrowth upon the induction with NGF or dibutyryl cyclic AMP. Early responses to NGF-induced neuronal differentiation, such as the cessation of cell division and the activation of ERK1/2 proteins, were normal in the antisense-Dp71 cells. On contrary, the induction of MAP2, a late differentiation marker, was disturbed in these cells. Additionally, the deficiency of Dp71 correlated with an altered expression of the dystrophin-associated protein complex (DAPC) members alpha and beta dystrobrevins. Our results indicate that normal expression of Dp71 is essential for neurite outgrowth in PC12 cells and constitute the first direct evidence implicating Dp71 in a neuronal function.

Subcellular localization of components of the dystrophin glycoprotein complex in cultured retinal muller glial cells

The dystrophin glycoprotein complex (DGC) is a membrane-associated protein complex binding extracellular matrix (ECM) molecules, such as laminin and forming a bridge towards the cytoskeleton. The molecular composition of the DGC is cell type dependent and it is involved in cell adhesion and motility. Here we present immunocytochemical localization of beta-dystroglycan, the central member of the DGC, utrophin and Dp71f, the spliced 71 kDa dystrophin protein product of the DMD gene, in cultured retinal Muller glial cells. It is shown that beta-dystroglycan and utrophin are colocalized in clusters in all parts of Muller cells including the lamellipodium and leading edge of migrating cells. As a contrast, Dp71f labels are distinct from beta-dystroglycan and confined to the perinuclear cytoplasm of Muller cells indicating that Dp71f is not a member of the DGC in cultured Muller cells.

Synaptic vesicle morphology and recycling are altered in myenteric neurons of mice lacking dystrophin (mdx mice)

Several dystrophin isoforms are known. The full-length isoform is present in striated and smooth muscles and neurons and its lack causes Duchenne Muscular Dystrophy, a progressive myopathy accompanied by mild cognitive deficits and gastrointestinal dismotility. An ultrastructural study was undertaken in the colon of mice lacking full-length dystrophin and maintaining shorter isoforms (mdx mice) to ascertain whether myenteric neurons have an altered morphology. Results showed a significant increase in the size of synaptic vesicle and in the number of recycling vesicles. An enlargement of endoplasmic reticulum cisternae in a subpopulation of neurons was also seen. Immunohistochemistry confirmed that the shorter isoforms were expressed in mdx mice myenteric neurons. These findings indicate the presence of a neuropathy at the myenteric plexus which might justify the defective neuronal control of gastrointestinal motility reported for these animals and which might be correlated with full-length dystrophin loss, since the shorter isoforms are present.

Differential expression and subcellular distribution of dystrophin Dp71 isoforms during differentiation process

Dp71 is the major product of the Duchenne muscular dystrophy gene in the brain. In order to study the function of Dp71 in the nervous system we examined the expression of Dp71 isoforms in PC12 rat pheochromocytoma cell line, a well-established system to study neuronal differentiation. We show by reverse transcriptase-polymerase chain reaction and Western blot assays that PC12 cells express two Dp71 isoforms. One isoform lacks exon 71 and the other isoform lacks exons 71 and 78 (Dp71d and Dp71f isoforms respectively). Nerve growth factor-induced neuronal differentiation of PC12 cells results in differential regulation of the expression and subcellular localization of Dp71 isoforms: a) the amount of Dp71f protein increases nine-fold in total extracts while Dp71d increases up to seven-fold in nuclear extracts; b) Dp71f relocates from the cytoplasm to neuritic processes, being prominent at varicosities and the growth cone; c) Dp71d relocates almost entirely to the nucleus and is detected to a lower extent in the cytoplasm and neuritic processes. Dp71f co-localizes with beta-dystroglycan and synaptophysin while Dp71d co-localizes with beta-dystroglycan in the nucleus. Dp71d accumulates at cell-cell contacts where Dp71f is absent. These results suggest that Dp71d and Dp71f associate with different subcellular complexes and therefore may have distinct functions in PC12 cells.

Dystrophin isoform Dp7l is present in lamellipodia and focal complexes in human astrocytoma cells U-373 MG

Dp71 is the most abundant product of the dmd gene in the brain. There are at least 2 isoforms derived from alternative splicing of exon 78 (Dp71d, which contains exon 78 and Dp71f, the spliced isoform) but the precise localization and function of each isoform is still unknown. In the present study, we demonstrate by RT-PCR that the Dp71f isoform is present in an astrocytoma cell line U-373 MG, and its subcellular localization was determined in the cytoplasm, particularly in perinuclear areas, with lower amounts towards the periphery but increasing in the leader borders of lamellipodia and focal complexes. Double labeling indirect immunofluorescence showed that Dp71f colocalized with actin-like beta-dystroglycan and beta-1 integrin. We also demonstrated by triple labeling that Dp71f was colocalized with actin and two members of integrin complexes, alpha-actinin and vinculin, in focal complexes. Ventral plasma membranes were enriched and in those containing focal complex proteins, we found colocalization of Dp71f, actin and vinculin. It is concluded that U-373 MG cells express Dp71f as part of lamellipodia and focal complex proteins, and possibly connected via distroglycan complexes to integrin complexes.

Comparative distribution of short dystrophin superfamily products in various guinea pig spermatozoa domains

In this study, the presence and cellular distribution of dystrophin family products (i.e. Dp71d, Dp71f-like protein and dystrobrevin) was examined by indirect immunofluorescence and Western blotting in guinea pig spermatozoa. Two dystrophin-associated proteins, beta-dystroglycan and alpha-syntrophin, and nNOS a protein frequently associated with alpha-syntrophin, were determined. In spermatozoa lacking plasma membrane and acrosome, Dp71f-like protein was found in the postacrosomal perinuclear theca and also in the middle piece of the flagellum. In the flagellum, Dp71f-like protein is localized together with alpha-syntrophin and nNOS. Dp71d was present in the plasma membrane of the middle piece with beta-dystroglycan, alpha-syntrophin and nNOS. Dp71d was also present in plasma membrane of the post acrosomal region, but only with nNOS. Finally, dystrobrevin was located all along skeletal flagellum structures and in the subacrosomal hemisphere of the perinuclear theca. This distinct and complementary distribution in various domains of spermatozoa may reveal a specific function for each short dystrophin family product, in the stabilization of the domains where they are located.

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)

Biochemical and histochemical analysis of 71 kDa dystrophin isoform (Dp71f) in rat brain

Dp71 is a member of the dystrophin family and the most abundant dmd gene product in the brain. In the present study, we focused on a short dystrophin transcript named Dp71f, which is alternatively spliced when exon 78 is absent The topographic localization of this protein in the encephalon has not been properly described yet, nor its cellular or subcellular localization, and even less its functions. Dp71f was found to be a cytoplasmic 70 kDa protein and localized in all encephalon regions studied. Double labeling using specific markers for various cell types confirmed Dp71f distribution in the cytoplasm of all cell types studied. Labeling was more conspicuous near the nucleus and diminished towards the periphery of cells. In some cases, we observed cells that were positive for actin and Dp71f in regions corresponding to lamellipodia-like structures. Dp71f and Dp71d isoforms were differently distributed. Our study is the first specific and unambiguous description of the topography and cellular localization patterns of Dp71f in brain, suggesting that Dp71f is a ubiquitous protein.

Expression of dystrophin in the mouse myenteric neurones

Dystrophin, a membrane-associated protein, plays relevant roles in cell functions. Its lack or trunkated expression results in Duchenne muscular dystrophy (DMD), a pathology associated with alterations in gastrointestinal motility considered to be neural in origin. No data are available on the presence of dystrophin in myenteric neurones. We labelled mouse myenteric neurones with DYS1-, DYS2-, DYS3-antibodies; staining was located on the perikarya and processes, with no differences in distribution or intensity among the antibodies; the western immunoblot analysis indicated that myenteric neurones express several dystrophin isoforms; anti-dystrophins/anti-neuronal specific enolase double-labeling confirmed that all neurones express dystrophin. Dystrophin in myenteric neurones might play a role in cytoskeletal organization, axonal transport and signal pathways; its lack might cause the intestinal motor abnormalities reported in DMD patients.

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.

Dystrophin and utrophin complexed with different associated proteins in cardiac Purkinje fibres

Abnormal dystrophin expression is directly responsible for Duchenne and Becker muscular dystrophies. In skeletal muscle, dystrophin provides a link between the actin network and the extracellular matrix via the dystrophin-associated protein complex. In mature skeletal muscle, utrophin is a dystrophin-related protein localized mainly at the neuromuscular junction, with the same properties as dystrophin in terms of linking the protein complex. Utrophin could potentially overcome the absence of dystrophin in dystrophic skeletal muscles. In cardiac muscle, dystrophin and utrophin were both found to be present with a distinct subcellular distribution in Purkinje fibres, i.e. utrophin was limited to the cytoplasm, while dystrophin was located in the cytoplasmic membrane. In this study, we used this particular characteristic of cardiac Purkinje fibres and demonstrated that associated proteins of dystrophin and utrophin are different in this structure. We conclude, contrary to skeletal muscle, dystrophin-associated proteins do not form a complex in Purkinje fibres. In addition, we have indirect evidence of the presence of two different 400 kDa dystrophins in Purkinje fibres.

The demonstration of immunoreactive dystrophin and its developmental expression in perivascular astrocytes

Immunoreactivity of dystrophin family proteins was observed in the astrocytes of the adult and immature rat hippocampus and cerebral cortex by using Dys2, a monoclonal antibody recognizing both 427 kDa and short dystrophin isoforms. As revealed by light and electron microscopy, immunostaining of the ribosomal apparatus and of pericapillary endfeet was particularly pronounced in the adult. In the pericapillary astrocyte processes immunostaining appeared between postnatal days 10 and 20, and reached the intensity seen in the adult by postnatal day 30. In the pericapillary astrocyte process, the membrane facing the endothelial basal lamina was the earliest structure to show the immunoreaction. At later stages, the pericapillary astrocyte process was gradually filled up with immunoprecipitate. Findings suggest that dystrophins are expressed coinciding with the development of the blood-brain barrier, and it is assumed that they contribute to the formation of this system.

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

In this study, various members of the dystrophin family (dystrophin, the short dystrophin product Dp 71, utrophin and DRP2), and different members of the dystrophin-associated glycoprotein (DAG) complex (beta-dystroglycan, alpha-, beta-, gamma- and delta-sarcoglycans) were localized in bovine cardiac muscle using a battery of specific antibodies. We have established that dystrophin is exclusively associated with beta-dystroglycan and both alpha- and delta-sarcoglycans in cardiac muscle cell membranes. In contrast, utrophin is a specific component of intercalated disks together with beta- and gamma-sarcoglycans, while beta-dystroglycan, alpha- and delta-sarcoglycans are not present. Dp 71 is mainly localized at the T tubule transverse area. In dystrophin deficient cardiac muscle, utrophin and beta-sarcoglycan were observed in intercalated disks and at the sarcolemma of each cardiocyte. Our results revealed that complexes of associated glycoproteins differ in cardiac muscle when associated with dystrophin or utrophin. Despite the described sequence homologies between dystrophin and utrophin, the present results indicate that these proteins have different roles in some specific cardiac cell areas.

Dystrophin isoforms DP71 and DP427 have distinct roles in myogenic cells

Duchenne muscular dystrophy is caused by mutations in the dystrophin gene, a complex gene that generates a family of distinct isoforms. In immature muscle cells, two dystrophin isoforms are expressed, Dp427 and Dp71. To characterize the function of Dp71 in myogenesis, we have examined the expression of Dp71 in myogenic cells. The localization of Dp71 in these cells is distinct from the localization of Dp427. Whereas Dp427 localizes to focal adhesions and surface membrane during myogenesis, Dp71 localizes to stress fiberlike structures in myogenic cells. Biochemical fractionation of myogenic cells demonstrates that Dp71 cosediments with the actin bundles thus confirming this interaction. Furthermore, transfection of C2C12 myoblasts with constructs encoding Dp71 fused to green fluorescent protein targeted the protein to the actin microfilament bundles. These results demonstrate involvement of Dp71 with the actin cytoskeleton during myogenesis and suggest a role for Dp71 that is distinct from Dp427.

Different utrophin and dystrophin properties related to their vascular smooth muscle distributions

Monoclonal antibodies used to distinguish between dystrophin and utrophin were systematically applied to skeletal muscles containing arteries and veins. Small arteries were found to contain long forms of both utrophin and dystrophin, while small veins contained only long forms of utrophin. In addition, all sizes of vascular smooth muscles were demonstrated to contain another related Mr 80 kDa protein (possibly a short utrophin transcript). Regardless of their tissue distributions, we assumed that each of these molecules had distinct properties, i.e. dystrophin with a mechanical function and utrophin with an architectural function. This difference in the roles of dystrophin and utrophin could reduce the efficiency of protection against muscle membrane degeneration when utrophin overexpression is programmed.

Expression of a new M(r) 70-kDa dystrophin-related protein in the axon of peripheral nerves from Torpedo marmorata

By comparison with localizations of dystrophin family products in rabbit peripheral nerves, we investigated the potential existence and distribution of similar products in peripheral nerves from Torpedo marmorata. In immunofluorescence studies, a specific set of monoclonal antibodies directed against dystrophin family proteins clearly stained a thin rim surrounding each Schwann cell-axon unit both in T. marmorata and rabbit peripheral nerves. In contrast when using the dystrophin/utrophin monoclonal H'3E7 antibody, we found a clear difference between rabbit and T. marmorata peripheral nerves according to fluorescent labeling detected within Torpedo nerve axons. Further differences were noted following western blot analyses of T. marmorata peripheral nerve extracts, highlighting the presence of a new and specific M(r) 70-kDa protein band belonging to the dystrophin family, which is localized within axons in addition to: (1) an M(r)400-kDa protein band detected with dystrophin/utrophin antibodies; and (2) an M(r) 116-kDa doublet protein band corresponding to Dp116 and Up116 isoforms. All of these products, detected according to the specificities of the monoclonal antibodies used, are discussed in terms of their potential identities as short and long dystrophin or utrophin mammalian products.

An improved permeabilization protocol for the introduction of peptides into cardiac myocytes. Application to protein kinase C research

We have developed an improved, less disruptive procedure for the transient permeabilization of neonatal cardiac myocytes using saponin. The method allows delivery of peptides to a high percentage of cells in culture without effects on long-term cell viability. Permeation was confirmed microscopically by cellular uptake of a fluorescently labeled peptide and biochemically by uptake of 125I-labeled calmodulin and a 20-kD protein kinase C epsilon fragment into the cells. The intracellular molar concentration of the introduced peptide was approximately 10% of that applied outside. We found no significant effects of permeabilization on spontaneous, phorbol ester-modulated, or norepinephrine-modulated contraction rates. Similarly, the expression of c-fos mRNA (measured 30 minutes after permeabilization) and the incorporation of [-14C]phenylalanine following agonist stimulation (measured 3 days after permeabilization) were not altered by saponin permeabilization. Finally, permeabilization of cells in the presence of a protein kinase C pseudosubstrate peptide, but not a control peptide, inhibited phorbol ester-induced [14C]phenylalanine incorporation into proteins by 80%. Our results demonstrate a methodology for the introduction of peptides into neonatal cardiac myocytes that allows study of their actions without substantial compromises in cell integrity.

Expression of dystrophin Dp71 during PC12 cell differentiation

The expression of dystrophin-protein 71 (Dp71) was investigated during nerve growth factor (NGF) induced differentiation of PC12 cells. A semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) assay was designed to measure Dp71 mRNA, whereas the Dp71 protein amount was evaluated by immunoblot analysis using an anti-dystrophin monoclonal antibody. Comparison with control cultures showed that Dp71 mRNA and protein levels increased in parallel with NGF treatment peaking with increments of 60% and 1.4 times, respectively. The upregulation of Dp71 expression during PC12 cells differentiation point at PC12 cells as a suitable model for studying the function of Dp71 in neuronal cells.

Phosphorylation of the carboxyl-terminal region of dystrophin

Dystrophin is a protein product of the gene responsible for Duchenne and Becker muscular dystrophy. The protein is localized to the inner surface of sarcolemma and is associated with a group of membrane (glyco)proteins. Dystrophin links cytoskeletal actins via the dystrophin-associated protein complex to extracellular matrix protein, laminin. This structural organization implicates the role of dystrophin in stabilizing the sarcolemma of muscle fibers. Precisely how dystrophin functions is far from clear. The presence of an array of isoforms of the C-terminal region of dystrophin suggests that dystrophin may have functions other than structural. In agreement, many potential phosphorylation sites are found in the C-terminal region of dystrophin, and the C-terminal region of dystrophin is phosphorylated both in vitro and in vivo by many protein kinases, including MAP kinase, p34cdc2 kinase, CaM kinase, and casein kinase, and is dephosphorylated by calcineurin. The C-terminal domain of dystrophin is also a substrate for hierarchical phosphorylation by casein kinase-2 and GSK-3. These observations, in accordance with the finding that the cysteine-rich region binds to Ca2+, Zn2+, and calmodulin, suggest an active involvement of dystrophin in transducing signals across muscle sarcolemma. Phosphorylation-dephosphorylation of the C-terminal region of dystrophin may play a role in regulating dystrophin-protein interactions and (or) transducing signal from the extracellular matrix via the dystrophin molecule to the cytoskeleton.

Down-regulation of mitochondrial mRNAs in the mdx mouse model for Duchenne muscular dystrophy

In our search for genes up- or down-regulated genes in the mdx mouse model for Duchenne muscular dystrophy, we isolated a down-regulated mitochondrial DNA clone. In addition to this clone, all protein-coding mitochondrial genes tested had tissue-specific and age independent down-regulated expression. This implied mechanisms at the RNA level since no change in the mitochondrial DNA contents were detected. Cytochrome c oxidase activity showed the same range of down-regulated expression. These data provide a molecular basis for energetic metabolism modifications in mdx mice.

Re-evaluation of the distributions of dystrophin and utrophin in sciatic nerve

Differential expression of proteins belonging to the dystrophin family was analysed in peripheral nerves. In agreement with previous reports, no full-size dystrophin was detectable, only Dp116, one of the short dystrophin products of the Duchenne muscular dystrophy (DMD) gene. We used specific monoclonal antibodies to fully investigate the presence of utrophin, a dystrophin homologue encoded by a gene located on chromosome 6q24. Evidence is presented here of the presence of two potential isoforms of full-length utrophin in different nerve structures, which may differ by alternative splicing of the 3'-terminal part of the utrophin gene according to the specificities of the monoclonal antiobodies used. One full-length utrophin was co-localized with Dp116 in the sheath around each separate Schwann cell-axon unit, but the other utrophin isoform was found to be perineurium-specific. We also highlighted a potential 80 kDa utrophin-related protein. The utrophin distribution in peripheral nerves was re-evaluated and utrophin isoforms were detected at the protein level. This preliminary indication will require more concrete molecular evidence to confirm the presence of these two utrophin isoforms as well as the potential 80 kDa utrophin isoform, but the results strongly suggest that each isoform must have a specialized role and function within each specific nervous structure.

Identification and characterization of the dystrophin anchoring site on beta-dystroglycan

Dystrophin, the product of the Duchenne muscular dystrophy gene, is tightly associated with the sarcolemmal membrane to a large glycoprotein complex. One function of the dystrophin-glycoprotein complex is to link the cytoskeleton to the extracellular matrix in skeletal muscle. However, the molecular interactions of dystrophin with the membrane components of the dystrophin-glycoprotein complex are still elusive. Here, we demonstrate and characterize a specific interaction between beta-dystroglycan and dystrophin. We show that skeletal muscle and brain dystrophin as well as brain dystrophin isoforms specifically bind to beta-dystroglycan. To localize and characterize the dystrophin and beta-dystroglycan interaction domains, we reconstituted the interaction in vitro using dystrophin fusion proteins and in vitro translated beta-dystroglycan. We demonstrated that the 15 C-terminal amino acids of beta-dystroglycan constituted a unique binding site for the second half of the hinge 4 and the cysteine-rich domain of dystrophin (amino acids 3054-3271). This dystrophin binding site is located in a proline-rich environment of beta-dystroglycan within amino acids 880-895. The identification of the interaction sites in dystrophin and beta-dystroglycan provides further insight into the structure and the molecular organization of the dystrophin-glycoprotein complex at the sarcolemma membrane and will be helpful for studying the pathogenesis of Duchenne muscular dystrophy.

Dystroglycan expression in the wild type and mdx mouse neural retina: synaptic colocalization with dystrophin, dystrophin-related protein but not laminin

Alpha- and beta-dystroglycan (alpha- and beta-DG) are members of a dystrophin-associated glycoprotein complex (DGC) in skeletal muscle which binds to agrin and laminin, and has been postulated to be involved in myoneural snyapse formation. The absence of functional dystrophin in Duchenne muscular dystrophy (DMD) and in one of its animal models, the mdx mouse, leads to a reduction of alpha- and beta-DG in muscle, and is often associated with mental retardation and abnormal retinal synaptic transmission in DMD. Using immunohistochemistry, we find that alpha- and beta-DG are expressed in the outer plexiform layer of both wild type and mdx retina, where both dystrophin and dystrophin-related protein (DRP), but not laminin are present. In situ hybridization identifies two neuronal populations, photoreceptors and retinal ganglion cells, that express DG mRNA. Alpha- and beta-DG are also expressed in the inner limiting membrane and around blood vessels where they colocalize with laminin and DRP. Western blot analysis revealed the expression of several dystrophin isoforms in wild type and mdx retina, possibly explaining the unaltered expression of alpha- and beta-dystroglycan in the mdx central nervous system (CNS). Our results support the hypothesis that alpha- and beta-DG can interact with dystrophin and DRP in the CNS and perform functions analogous to those of the DGC in muscle.

A mutation in the dystrophin gene selectively affecting dystrophin expression in the heart

We have previously shown in a large X-linked pedigree that a deletion removing the dystrophin muscle promoter, the first muscle exon and part of intron 1 caused a severe dilated cardiomyopathy with no associated muscle weakness. Dystrophin expression was present in the muscle of affected males and transcription studies indicated that this dystrophin originated from the brain and Purkinje cell isoforms, upregulated in this skeletal muscle. We have now studied dystrophin transcription and expression in the heart of one member of this family. In contrast to the skeletal muscle, dystrophin transcription and expression were absent in the heart, with the exception of the distal Dp71 dystrophin isoform, normally present in the heart. The 43- and 50-kD dystrophin-associated proteins were severely reduced in the heart, despite the presence of Dp71, but not in skeletal muscle. The absence of dystrophin and the down-regulation of the dystrophin-associated proteins in the heart accounted for the severe cardiomyopathy in this family. The mutation present in these males selectively affects dystrophin expression in the heart; this could be secondary to the removal of cardiac-specific regulatory sequences. This family may represent the first example of a mutation specifically affecting the cardiac expression of a gene, present physiologically in both the skeletal and cardiac muscles.