Identification of dystrophin-binding protein(s) in membranes from Torpedo electrocyte and rat muscle

Alpha1-syntrophin-deficient skeletal muscle exhibits hypertrophy and aberrant formation of neuromuscular junctions during regeneration

Alpha1-syntrophin is a member of the family of dystrophin-associated proteins; it has been shown to recruit neuronal nitric oxide synthase and the water channel aquaporin-4 to the sarcolemma by its PSD-95/SAP-90, Discs-large, ZO-1 homologous domain. To examine the role of alpha1-syntrophin in muscle regeneration, we injected cardiotoxin into the tibialis anterior muscles of alpha1-syntrophin-null (alpha1syn-/-) mice. After the treatment, alpha1syn-/- muscles displayed remarkable hypertrophy and extensive fiber splitting compared with wild-type regenerating muscles, although the untreated muscles of the mutant mice showed no gross histological change. In the hypertrophied muscles of the mutant mice, the level of insulin-like growth factor-1 transcripts was highly elevated. Interestingly, in an early stage of the regeneration process, alpha1syn-/- mice showed remarkably deranged neuromuscular junctions (NMJs), accompanied by impaired ability to exercise. The contractile forces were reduced in alpha1syn-/- regenerating muscles. Our results suggest that the lack of alpha1-syntrophin might be responsible in part for the muscle hypertrophy, abnormal synapse formation at NMJs, and reduced force generation during regeneration of dystrophin-deficient muscle, all of which are typically observed in the early stages of Duchenne muscular dystrophy patients.

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

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

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)

The torpedo electrocyte: a model system to study membrane-cytoskeleton interactions at the postsynaptic membrane

Many aspects of the organization of the electromotor synapse of electric fish resemble the nerve-muscle junction. In particular, the postsynaptic membrane in both systems share most of their proteins. As a remarquable source of cholinergic synapses, the Torpedo electrocyte model has served to identify the most important components involved in synaptic transmission such as the nicotinic acetylcholine receptor and the enzyme acetylcholinesterase, as well as proteins associated with the subsynaptic cytoskeleton and the extracellular matrix involved in the assembly of the postsynaptic membrane, namely the 43-kDa protein-rapsyn, the dystrophin/utrophin complex, agrin, and others. This review encompasses some representative experiments that helped to clarify essential aspects of the supramolecular organization and assembly of the postsynaptic apparatus of cholinergic synapses.

alpha1-syntrophin gene disruption results in the absence of neuronal-type nitric-oxide synthase at the sarcolemma but does not induce muscle degeneration

alpha1-Syntrophin is a member of the family of dystrophin-associated proteins and is strongly expressed in the sarcolemma and the neuromuscular junctions. All three syntrophin isoforms have a PDZ domain that appears to participate in protein-protein interactions at the plasma membrane. alpha1-Syntrophin has additionally been shown to associate with neuronal nitric-oxide synthase (nNOS) through PDZ domains in vitro. These observations suggest that alpha1-syntrophin may work as a modular adaptor protein that can link nNOS or other signaling enzyme to the sarcolemmal dystrophin complex. In the sarcolemma, nNOS regulates the homeostasis of reactive free radical species and may contribute to the oxidative damage to muscle protein in muscle disease such as Duchenne muscular dystrophy. In this study, we generated alpha1-syntrophin knock-out mice to clarify the interaction between alpha1-syntrophin and nNOS in the skeletal muscle. We observed that nNOS, normally expressed in the sarcolemma, was largely absent from the sarcolemma, but considerably remained in the cytosol of the knock-out mice. Even though the distribution of nNOS was altered, the knock-out mice displayed no gross histological changes in the skeletal muscle. We also discovered that muscle contractile properties have not been influenced in the knock-out mice.

Targeting of acetylcholine receptor and 43 kDa rapsyn to the postsynaptic membrane in Torpedo marmorata electrocyte

In this study we have investigated the intracellular routing of two major components of the postsynaptic membrane in Torpedo electrocytes, the nicotinic acetylcholine receptor and the extrinsic 43 kDa protein rapsyn, and of a protein from the non-innervated membrane, the Na+,K+ ATPase. We isolated subpopulations of post-Golgi vesicles (PGVs) enriched either in AChR or in Na+,K+ ATPase. Rapsyn was associated to AChR-containing PGVs suggesting that both AChR and rapsyn are targeted to intracellular organelles in the secretory pathway before delivery to the postsynaptic membrane. In vitro assays further show that rapsyn-containing PVGs do bind more efficiently to microtubules compared to Na+,K+ ATPase-enriched PVGs. These data provide evidence in favor of the contribution of the secretory pathway to the delivery of synaptic components.

Evidence for in situ and in vitro association between beta-dystroglycan and the subsynaptic 43K rapsyn protein. Consequence for acetylcholine receptor clustering at the synapse

The accumulation of dystrophin and associated proteins at the postsynaptic membrane of the neuromuscular junction and their co-distribution with nicotinic acetylcholine receptor (AChR) clusters in vitro suggested a role for the dystrophin complex in synaptogenesis. Co-transfection experiments in which alpha- and beta-dystroglycan form a complex with AChR and rapsyn, a peripheral protein required for AChR clustering (Apel, D. A., Roberds, S. L., Campbell, K. P., and Merlie, J. P. (1995) Neuron 15, 115-126), suggested that rapsyn functions as a link between AChR and the dystrophin complex. We have investigated the interaction between rapsyn and beta-dystroglycan in Torpedo AChR-rich membranes using in situ and in vitro approaches. Cross-linking experiments were carried out to study the topography of postsynaptic membrane polypeptides. A cross-linked product of 90 kDa was labeled by antibodies to rapsyn and beta-dystroglycan; this demonstrates that these polypeptides are in close proximity to one another. Affinity chromatography experiments and ligand blot assays using rapsyn solubilized from Torpedo AChR-rich membranes and constructs containing beta-dystroglycan C-terminal fragments show that a rapsyn-binding site is present in the juxtamembranous region of the cytoplasmic tail of beta-dystroglycan. These data point out that rapsyn and dystroglycan interact in the postsynaptic membrane and thus reinforce the notion that dystroglycan could be involved in synaptogenesis.

Polarized sorting of nicotinic acetylcholine receptors to the postsynaptic membrane in Torpedo electrocyte

Several regulatory mechanisms contribute to the accumulation and maintenance of high concentrations of acetylcholine receptors (AChR) at the postsynaptic membrane of the neuromuscular junction, including compartmentalized gene transcription, targeting, clustering and anchoring to the cytoskeleton. The targeting of the AChR to the postsynaptic membrane is likely to involve a polarized sorting in the exocytic pathway. In this work, we used the electrocyte of Torpedo marmorata electric organ to study the intracellular trafficking of neosynthesized AChR and its delivery to the postsynaptic membrane. Gradient centrifugation and immunoisolation techniques have led to the isolation of two populations of post-Golgi transport vesicles (PGVs) enriched in proteins of either the innervated (AChR) or non-innervated (Na,K-ATPase) membrane domains of the cell. Immunolabelling of these vesicles at the EM level disclosed that very few PGVs contained both proteins. In AChR-enriched vesicles, high sialylation of AchR molecules, an expected post-translational modification of proteins exiting the trans-Golgi network, and the presence of a marker of the exocytic pathway (Rab6p), indicate that these vesicles are carriers engaged in the Golgi-to-plasma membrane transport. These data suggest that AChR and Na,K-ATPase are sorted intracellularly most likely within the trans-Golgi network. Furthermore, EM analysis and immunogold-labelling experiments provided in situ evidence that the AChR-containing PGVs are conveyed to the postsynaptic membrane, possibly by a microtubule-dependent transport mechanism. Our data therefore provide the first evidence that the targeting of receptors for neurotransmitters to synaptic sites could be contributed by intracellular sorting and polarized delivery in the exocytic pathway.

Non-neural agrin codistributes with acetylcholine receptors during early differentiation of Torpedo electrocytes

Agrin, an extracellular matrix protein synthesized by nerves and muscles is known to promote the clustering of acetylcholine receptors and other synaptic proteins in cultured myotubes. This observation suggests that agrin may provide at least part of the signal for synaptic specialization in vivo. The extracellular matrix components agrin, laminin and merosin bind to alpha-dystroglycan, a heavily glycosylated peripheral protein part of the dystrophin-glycoprotein complex, previously characterized in the sarcolemma of skeletal and cardiac muscles and at the neuromuscular junction. In order to understand further the function of agrin and alpha DG in the genesis of the acetylcholine receptor-rich membrane domain, the settling of components of the dystrophin-glycoprotein complex and agrin was followed by immunofluorescence localization in developing Torpedo marmorata electrocytes. In 40–45 mm Torpedo embryos, a stage of development at which the electrocytes exhibit a definite structural polarity, dystrophin, alpha/beta-dystroglycan and agrin accumulated concomitantly with acetylcholine receptors at the ventral pole of the cells. Among these components, agrin appeared as the most intensely concentrated and sharply localized. The scarcity of the nerve-electrocyte synaptic contacts at this stage of development, monitored by antibodies against synaptic vesicles, further indicates that before innervation, the machinery for acetylcholine receptor clustering is provided by electrocyte-derived agrin rather than by neural agrin. These observations suggest a two-step process of acetylcholine receptor clustering involving: (i) an instructive role of electrocyte-derived agrin in the formation of a dystrophin-based membrane scaffold upon which acetylcholine receptor molecules would accumulate according to a diffusion trap model; and (ii) a maturation and/or stabilization step controlled by neural agrin. In the light of these data, the existence of more than one agrin receptor is postulated to account for the action of agrin variants at different stages of the differentiation of the postsynaptic membrane in Torpedo electrocytes.

The three human syntrophin genes are expressed in diverse tissues, have distinct chromosomal locations, and each bind to dystrophin and its relatives

The syntrophins are a biochemically heterogeneous group of 58-kDa intracellular membrane-associated dystrophin-binding proteins. We have cloned and characterized human acidic (alpha 1-) syntrophin and a second isoform of human basic (beta 2-) syntrophin. Comparison of the deduced amino acid structure of the three human isoforms of syntrophin (together with the previously reported human beta 1-syntrophin) demonstrates their overall similarity. The deduced amino acid sequences of human alpha 1- and beta 2-syntrophin are nearly identical to their homologues in mouse, suggesting a strong functional conservation among the individual isoforms, Much like beta 1-syntrophin, human beta 2-syntrophin has multiple transcript classes and is expressed widely, although in a distinct pattern of relative abundance. In contrast, human alpha 1-syntrophin is most abundant in heart and skeletal muscle, and less so in other tissues. Somatic cell hybrids and fluorescent in situ hybridization were both used to determine their chromosomal locations: beta 2-syntrophin to chromosome 16q22-23 and alpha 1-syntrophin to chromosome 20q11.2. Finally, we used in vitro translated proteins in an immunoprecipitation assay to show that, like beta 1-syntrophin, both beta 2- and alpha 1-syntrophin interact with peptides encoding the syntrophin-binding region of dystrophin, utrophin/dystrophin related protein, and the Torpedo 87K protein.

Direct binding of Torpedo syntrophin to dystrophin and the 87 kDa dystrophin homologue

Syntrophin, a 58-kDa membrane-associated protein, is one component of a protein complex associated with dystrophin and other members of the dystrophin family, including the 87-kDa homologue (87K protein). To characterize interactions between synthrophin and 87K protein, we used an in vitro overlay binding assay. We demonstrate that purified Torpedo syntrophin binds directly to dystrophin and 87K. By expressing overlapping regions of the 87K protein as bacterial fusion proteins for binding targets, we show that a 52-amino acid region of 87K (residues 375-426) is sufficient for binding syntrophin.

Mouse alpha 1- and beta 2-syntrophin gene structure, chromosome localization, and homology with a discs large domain

The syntrophin family of dystrophin-associated proteins consists of three isoforms, alpha 1, beta 1, and beta 2, each encoded by a distinct gene. We have cloned and characterized the mouse alpha 1- and beta 2-syntrophin genes. The mouse alpha 1-syntrophin gene ( > 24 kilobases) is comprised of eight exons. The mouse beta 2-syntrophin gene ( > 33 kilobases) contains seven exons, all of which have homologues at the corresponding position in the alpha 1-syntrophin gene. Primer extension analysis reveals two transcription initiation sites in the alpha 1-syntrophin gene and a single site in the beta 2-syntrophin gene. The sequence immediately 5' of the transcription start sites of both genes lacks a TATA box but is GC-rich and has multiple putative SP1 binding sites. The alpha 1-syntrophin gene is located on human chromosome 20 and mouse chromosome 2, while the beta 2-syntrophin gene is on human chromosome 16 and mouse chromosome 8. Analysis of the amino acid sequence of the syntrophins reveals the presence of four conserved domains. The carboxyl-terminal 56 amino acids are highly conserved and constitute a syntrophin unique domain. Two pleckstrin homology domains are located at the amino-terminal end of the protein. The first pleckstrin homology domain is interrupted by a domain homologous to repeated sequences originally found in the Drosophila discs-large protein.

Actin interaction with purified dystrophin from electric organ of Torpedo marmorata: possible resemblance with filamin-actin interface

We have purified dystrophin from Torpedo marmorata electric tissue by means of alkaline extraction in conjunction with an affinity chromatography column using anti-peptide antibodies. Using solution (cosedimentation) and solid phase experiments (sedimentation with Sepharose filamentous actin and ELISA), we have demonstrated that purified dystrophin is able to bind filamentous and monomeric actin. Using ELISA coupled with biotin labelled peptides and taking advantage of strong affinity binding of streptavidin-biotin complex, we have identified two exposed sequences of the actin molecule implicated in dystrophin binding: fragment 40-113, further restricted to peptide 75-106 and peptide 360-372. In a previous study, we have shown that fragment 40-113 displays binding site(s) for filamin but probably not for alpha-actinin. Moreover, we have recently reported that alpha-actinin and filamin display divergent behaviours towards conformational changes of actin. In this study, we have demonstrated that, similarly to filamin, dystrophin binding is insensitive to the locking of actin in a monomeric conformation. Taken together, these results lead us to favour the idea that dystrophin could share properties in common with filamin in its binding of actin.

Association of aciculin with dystrophin and utrophin

Aciculin is a recently identified 60-kDa cytoskeletal protein, highly homologous to the glycolytic enzyme phosphoglucomutase type 1, (Belkin, A. M., Klimanskaya, I. V., Lukashev, M. E., Lilley, K., Critchley, D., and Koteliansky, V. E. (1994) J. Cell Sci. 107, 159-173). Aciculin expression in skeletal muscle is developmentally regulated, and this protein is particularly enriched at cell-matrix adherens junctions of muscle cells (Belkin, A. M., and Burridge, K. (1994) J. Cell Sci. 107, 1993-2003). The purpose of our study was to identify cytoskeletal protein(s) interacting with aciculin in various cell types. Using immunoprecipitation from cell lysates of metabolically labeled differentiating C2C12 muscle cells with anti-aciculin-specific antibodies, we detected a high molecular weight band (M(r) approximately 400,000), consistently coprecipitating with aciculin. We showed that this 400 kDa band comigrated with dystrophin and immunoblotted with anti-dystrophin antibodies. The association between aciculin and dystrophin in C2C12 cells was shown to resist Triton X-100 extraction and the majority of the complex could be extracted only in the presence of ionic detergents. In the reverse immunoprecipitation experiments, aciculin was detected in the precipitates with different anti-dystrophin antibodies. Immunodepletion experiments with lysates of metabolically labeled C2C12 myotubes showed that aciculin is a major dystrophin-associated protein in cultured skeletal muscle cells. Double immunostaining of differentiating and mature C2C12 myotubes with antibodies against aciculin and dystrophin revealed precise colocalization of these two cytoskeletal proteins throughout the process of myodifferentiation in culture. In skeletal muscle tissue, both proteins are concentrated at the sarcolemma and at myotendinous junctions. In contrast, utrophin, an autosomal homologue of dystrophin, was not codistributed with aciculin in muscle cell cultures and in skeletal muscle tissues. Analytical gel filtration experiments with purified aciculin and dystrophin showed interaction of these proteins in vitro, indicating that their association in skeletal muscle is due to direct binding. Whereas dystrophin was shown to be a major aciculin-associated protein in skeletal muscle, immunoblotting of anti-aciculin immunoprecipitates with antibodies against utrophin showed that aciculin is associated with utrophin in cultured A7r5 smooth muscle cells and REF52 fibroblasts. Immunodepletion experiments performed with lysates of metabolically labeled A7r5 cells demonstrated that aciculin is a major utrophin-binding protein in this cell type. Taken together, our data show that aciculin is a novel dystrophin- and utrophin-binding protein. Association of aciculin with dystrophin (utrophin) in various cell types might provide an additional cytoskeletal-matrix transmembrane link at sites where actin filaments terminate at the plasma membrane.

Identification of alpha-syntrophin binding to syntrophin triplet, dystrophin, and utrophin

Syntrophin represents three cytoplasmic components of the dystrophin-glycoprotein complex that links the cytoskeleton to the extracellular matrix in skeletal muscle. alpha-Syntrophin has now been translated in vitro and shown to associate directly with all three components of the syntrophin triplet and with dystrophin. The in vitro translated 71-kDa non-muscle dystrophin isoform, containing the cystein-rich/C-terminal domain, can also interact with the syntrophin triplet. The syntrophin binding motif in dystrophin was localized to exons 73 and 74 including amino acids 3447-3481 by comparing the interactions of alpha-syntrophin and seven overlapping human dystrophin fusion proteins. More than one syntrophin interaction site in this binding motif was suggested. alpha-Syntrophin also interacts directly with a C-terminal utrophin fusion protein. alpha-Syntrophin is localized to the muscle sarcolemma as well as to the neuromuscular junction in control mouse muscle. However, similar to utrophin, alpha-syntrophin is only present at the neuromuscular junction in mdx mouse muscle in which dystrophin is absent. Our data suggest that alpha-syntrophin binds all syntrophin isoforms, and syntrophin directly interacts with dystrophin through more than one binding site in dystrophin exons 73 and 74 including amino acids 3447-3481.

Direct involvement of a lamin-B-related (54 kDa) protein in the association of intermediate filaments with the postsynaptic membrane of the Torpedo marmorata electrocyte

Mechanisms by which motor innervation induces postsynaptic membrane differentiation and functional compartmentalization of the subneural sarcoplasm in skeletal muscle fibres are still poorly understood. However, transmembrane control of cytoskeletal activities by the nerve terminal may be considered. Here, we examine several properties of a 54 kDa protein, previously identified in the postsynaptic membrane of the Torpedo marmorata electrocyte with anti-lamin B antibodies, in order to study its role in the assembly of the subneural intermediate filament meshwork. Using a ligand blot assay, we show that this protein binds desmin, a type III intermediate filaments protein, at micromolar concentrations. Moreover, purified acetylcholine receptor-rich membrane fragments are able to generate arrays of desmin filaments in vitro. Immunofluorescence experiments indicate that the 54 kDa protein becomes associated with the acetylcholine receptor-rich membrane at an early stage of development of the electrocyte, and that a polarized desmin network develops concomitantly from the postsynaptic membrane. Taken together, these data show that, like karyoskeletal lamin B, the 54 kDa protein is involved in the organization of the subneural intermediate filament meshwork. Control of the assembly of the subneural cytoskeleton by components of the postsynaptic membrane may thus be a prerequisite for the functional compartmentalization of the muscle fibre triggered by motor innervation.

Membrane interactions with the actin cytoskeleton

Recent advances have been made in our understanding of the direct binding of actin to integral membrane proteins. New information has been obtained about indirect actin-membrane associations through spectrin superfamily members and through proteins at the cytoplasmic surfaces of focal contacts and adherens junctions.