Association of utrophin and multiple dystrophin short forms with the mammalian M(r) 58,000 dystrophin-associated protein (syntrophin)

Angiomotin isoform 2 promotes binding of PALS1 to KIF13B at primary cilia and regulates ciliary length and signaling

The kinesin-3 motor KIF13B functions in endocytosis, vesicle transport and regulation of ciliary length and signaling. Direct binding of the membrane-associated guanylate kinase (MAGUK) DLG1 to the MAGUK-binding stalk domain of KIF13B relieves motor autoinhibition and promotes microtubule plus-end-directed cargo transport. Here, we characterize angiomotin (AMOT) isoform 2 (p80, referred to as Ap80) as a novel KIF13B interactor that promotes binding of another MAGUK, the polarity protein and Crumbs complex component PALS1, to KIF13B. Live-cell imaging analysis indicated that Ap80 is concentrated at and recruits PALS1 to the base of the primary cilium, but is not a cargo of KIF13B itself. Consistent with a ciliary function for Ap80, its depletion led to elongated primary cilia and reduced agonist-induced ciliary accumulation of SMO, a key component of the Hedgehog signaling pathway, whereas Ap80 overexpression caused ciliary shortening. Our results suggest that Ap80 activates KIF13B cargo binding at the base of the primary cilium to regulate ciliary length, composition and signaling.

Essential role for InSyn1 in dystroglycan complex integrity and cognitive behaviors in mice

Human mutations in the dystroglycan complex (DGC) result in not only muscular dystrophy but also cognitive impairments. However, the molecular architecture critical for the synaptic organization of the DGC in neurons remains elusive. Here, we report Inhibitory Synaptic protein 1 (InSyn1) is a critical component of the DGC whose loss alters the composition of the GABAergic synapses, excitatory/inhibitory balance in vitro and in vivo, and cognitive behavior. Association of InSyn1 with DGC subunits is required for InSyn1 synaptic localization. InSyn1 null neurons also show a significant reduction in DGC and GABA receptor distribution as well as abnormal neuronal network activity. Moreover, InSyn1 null mice exhibit elevated neuronal firing patterns in the hippocampus and deficits in fear conditioning memory. Our results support the dysregulation of the DGC at inhibitory synapses and altered neuronal network activity and specific cognitive tasks via loss of a novel component, InSyn1.

Syntrophins entangled in cytoskeletal meshwork: Helping to hold it all together

Syntrophins are a family of 59 kDa peripheral membrane-associated adapter proteins, containing multiple protein-protein and protein-lipid interaction domains. The syntrophin family consists of five isoforms that exhibit specific tissue distribution, distinct sub-cellular localization and unique expression patterns implying their diverse functional roles. These syntrophin isoforms form multiple functional protein complexes and ensure proper localization of signalling proteins and their binding partners to specific membrane domains and provide appropriate spatiotemporal regulation of signalling pathways. Syntrophins consist of two PH domains, a PDZ domain and a conserved SU domain. The PH1 domain is split by the PDZ domain. The PH2 and the SU domain are involved in the interaction between syntrophin and the dystrophin-glycoprotein complex (DGC). Syntrophins recruit various signalling proteins to DGC and link extracellular matrix to internal signalling apparatus via DGC. The different domains of the syntrophin isoforms are responsible for modulation of cytoskeleton. Syntrophins associate with cytoskeletal proteins and lead to various cellular responses by modulating the cytoskeleton. Syntrophins are involved in many physiological processes which involve cytoskeletal reorganization like insulin secretion, blood pressure regulation, myogenesis, cell migration, formation and retraction of focal adhesions. Syntrophins have been implicated in various pathologies like Alzheimer's disease, muscular dystrophy, cancer. Their role in cytoskeletal organization and modulation makes them perfect candidates for further studies in various cancers and other ailments that involve cytoskeletal modulation. The role of syntrophins in cytoskeletal organization and modulation has not yet been comprehensively reviewed till now. This review focuses on syntrophins and highlights their role in cytoskeletal organization, modulation and dynamics via its involvement in different cell signalling networks.

The root cause of Duchenne muscular dystrophy is the lack of dystrophin in smooth muscle of blood vessels rather than in skeletal muscle per se

Background:The dystrophin protein is part of the dystrophin associated protein complex (DAPC) linking the intracellular actin cytoskeleton to the extracellular matrix. Mutations in the dystrophin gene cause Duchenne and Becker muscular dystrophy (D/BMD). Neuronal nitric oxide synthase associates with dystrophin in the DAPC to generate the vasodilator nitric oxide (NO). Systemic dystrophin deficiency, such as in D/BMD, results in muscle ischemia, injury and fatigue during exercise as dystrophin is lacking, affecting NO production and hence vasodilation. The role of neuregulin 1 (NRG) signaling through the epidermal growth factor family of receptors ERBB2 and ERBB4 in skeletal muscle has been controversial, but it was shown to phosphorylate α-dystrobrevin 1 (α-DB1), a component of the DAPC. The aim of this investigation was to determine whether NRG signaling had a functional role in muscular dystrophy.Methods:Primary myoblasts (muscle cells) were isolated from conditional knock-out mice containing lox P flanked ERBB2 and ERBB4 receptors, immortalized and exposed to Cre recombinase to obtainErbb2/4double knock-out (dKO) myoblasts where NRG signaling would be eliminated. Myotubes, thein vitroequivalent of muscle fibers, formed by fusion of the lox P flankedErbb2/4myoblasts as well as theErbb2/4dKO myoblasts were then used to identify changes in dystrophin expression.Results:Elimination of NRG signaling resulted in the absence of dystrophin demonstrating that it is essential for dystrophin expression. However, unlike the DMD mouse model mdx, with systemic dystrophin deficiency, lack of dystrophin in skeletal muscles ofErbb2/4dKO mice did not result in muscular dystrophy. In these mice, ERBB2/4, and thus dystrophin, is still expressed in the smooth muscle of blood vessels allowing normal blood flow through vasodilation during exercise.Conclusions:Dystrophin deficiency in smooth muscle of blood vessels, rather than in skeletal muscle, is the main cause of disease progression in DMD.

Role of SNTA1 in Rac1 activation, modulation of ROS generation, and migratory potential of human breast cancer cells

Background:

Alpha-1-syntrophin (SNTA1) has been implicated in the activation of Rac1. However, the underlying mechanism has not yet been explored. Here, we show that a novel complex, involving SNTA1, P66shc, and Grb2 proteins, is involved in Rac1 activation.

Methods:

Co-immunoprecipitation assays were used to show the complex formation, while siRNAs and shRNAs were used to downregulate expression of these proteins. Various Rac1 activation assays and functional assays, such as migration assays, in vitro wound healing assays, cell proliferation assays, and ROS generation assays, were also performed.

Results:

The results showed a significant increase in activation of Rac1 when SNTA1 and P66shc were overexpressed, whereas depletion of SNTA1 and P66shc expression effectively reduced the levels of active Rac1. The results indicated a significant displacement of Sos1 protein from Grb2 when SNTA1 and P66shc are overexpressed in breast cancer cell lines, resulting in Sos1 predominantly forming a complex with Eps8 and E3b1. In addition, the SNTA1/P66shc-mediated Rac1 activation resulted in an increase in reactive oxygen species (ROS) production and migratory potential in human breast cancer cells.

Conclusion:

Together, our results present a possible mechanism of Rac1 activation involving SNTA1 and emphasise its role in ROS generation, cell migration, and acquisition of malignancy.

The ZZ domain of dystrophin in DMD: making sense of missense mutations

Duchenne muscular dystrophy (DMD) is associated with the loss of dystrophin, which plays an important role in myofiber integrity via interactions with β-dystroglycan and other members of the transmembrane dystrophin-associated protein complex. The ZZ domain, a cysteine-rich zinc-finger domain near the dystrophin C-terminus, is implicated in forming a stable interaction between dystrophin and β-dystroglycan, but the mechanism of pathogenesis of ZZ missense mutations has remained unclear because not all such mutations have been shown to alter β-dystroglycan binding in previous experimental systems. We engineered three ZZ mutations (p.Cys3313Phe, p.Asp3335His, and p.Cys3340Tyr) into a short construct similar to the Dp71 dystrophin isoform for in vitro and in vivo studies and delineated their effect on protein expression, folding properties, and binding partners. Our results demonstrate two distinct pathogenic mechanisms for ZZ missense mutations. The cysteine mutations result in diminished or absent subsarcolemmal expression because of protein instability, likely due to misfolding. In contrast, the aspartic acid mutation disrupts binding with β-dystroglycan despite an almost normal expression at the membrane, confirming a role for the ZZ domain in β-dystroglycan binding but surprisingly demonstrating that such binding is not required for subsarcolemmal localization of dystrophin, even in the absence of actin binding domains.

Syntrophin proteins as Santa Claus: role(s) in cell signal transduction

Syntrophins are a family of cytoplasmic membrane-associated adaptor proteins, characterized by the presence of a unique domain organization comprised of a C-terminal syntrophin unique (SU) domain and an N-terminal pleckstrin homology (PH) domain that is split by insertion of a PDZ domain. Syntrophins have been recognized as an important component of many signaling events, and they seem to function more like the cell's own personal 'Santa Claus' that serves to 'gift' various signaling complexes with precise proteins that they 'wish for', and at the same time care enough for the spatial, temporal control of these signaling events, maintaining overall smooth functioning and general happiness of the cell. Syntrophins not only associate various ion channels and signaling proteins to the dystrophin-associated protein complex (DAPC), via a direct interaction with dystrophin protein but also serve as a link between the extracellular matrix and the intracellular downstream targets and cell cytoskeleton by interacting with F-actin. They play an important role in regulating the postsynaptic signal transduction, sarcolemmal localization of nNOS, EphA4 signaling at the neuromuscular junction, and G-protein mediated signaling. In our previous work, we reported a differential expression pattern of alpha-1-syntrophin (SNTA1) protein in esophageal and breast carcinomas. Implicated in several other pathologies, like cardiac dys-functioning, muscular dystrophies, diabetes, etc., these proteins provide a lot of scope for further studies. The present review focuses on the role of syntrophins in membrane targeting and regulation of cellular proteins, while highlighting their relevance in possible development and/or progression of pathologies including cancer which we have recently demonstrated.

Molecular scaffolds underpinning macroglial polarization: an analysis of retinal Müller cells and brain astrocytes in mouse

Key roles of macroglia are inextricably coupled to specialized membrane domains. The perivascular endfoot membrane has drawn particular attention, as this domain contains a unique complement of aquaporin-4 (AQP4) and other channel proteins that distinguishes it from perisynaptic membranes. Recent studies indicate that the polarization of macroglia is lost in a number of diseases, including temporal lobe epilepsy and Alzheimer's disease. A better understanding is required of the molecular underpinning of astroglial polarization, particularly when it comes to the significance of the dystrophin associated protein complex (DAPC). Here, we employ immunofluorescence and immunogold cytochemistry to analyze the molecular scaffolding in perivascular endfeet in macroglia of retina and three regions of brain (cortex, dentate gyrus, and cerebellum), using AQP4 as a marker. Compared with brain astrocytes, Müller cells (a class of retinal macroglia) exhibit lower densities of the scaffold proteins dystrophin and α-syntrophin (a DAPC protein), but higher levels of AQP4. In agreement, depletion of dystrophin or α-syntrophin--while causing a dramatic loss of AQP4 from endfoot membranes of brain astrocytes--had only modest or insignificant effect, respectively, on the AQP4 pool in endfoot membranes of Müller cells. In addition, while polarization of brain macroglia was less affected by dystrophin depletion than by targeted deletion of α-syntrophin, the reverse was true for retinal macroglia. These data indicate that the molecular scaffolding in perivascular endfeet is more complex than previously assumed and that macroglia are heterogeneous with respect to the mechanisms that dictate their polarization.

Dp71 gene disruption alters the composition of the dystrophin-associated protein complex and neuronal nitric oxide synthase expression in the hypothalamic supraoptic and paraventricular nuclei

DP71 is the major cerebral dystrophin isoform and exerts its multiple functions via the dystrophin-associated protein complex (DAPC), also comprised of β-dystroglycan (β-DG) and α1-syntrophin (α1-Syn). Since DP71 disruption leads to impairment in the central control of the osmoregulatory axis, we investigated: 1) the DAPC composition in the hypothalamic supraoptic nucleus (SON) and paraventricular nucleus (PVN) of Dp71-null mice; and 2) the expression and activity of neuronal nitric oxide synthase (nNOS), because it is a potential partner of the DAPC and a functional index of osmoregulatory axis activity. In wild-type mice, dystrophins and their autosomal homologs the utrophins, β-DG, and α1-Syn were localized in astrocyte end feet. In Dp71-null mice, the levels of β-DG and α1-Syn were lower and utrophin expression did not change. The location of the DAPC in astrocytic end feet suggests that it could be involved in hypothalamic osmosensitivity, which adapts the osmotic response. The altered composition of the DAPC in Dp71-null mice could thus explain why these mice manifest an hypo-osmolar status. In the SON and PVN neurons of Dp71-null mice, nNOS expression and activity were increased. Although we previously established that DP140 is expressed de novo in these neurons, the DAPC remained incomplete due to the low levels of β-DG and α1-Syn produced in these cells. Our data reveal the importance of DP71 for the constitution of a functional DAPC in the hypothalamus. Such DAPC disorganization may lead to modification of the microenvironment of the SON and PVN neurons and thus may result in a perturbed osmoregulation.

Neuregulin/ErbB regulate neuromuscular junction development by phosphorylation of α-dystrobrevin

Neuregulin/ErbB signaling maintains high efficacy of synaptic transmission by stabilizing the postsynaptic apparatus via phosphorylation of α-dystrobrevin1.

Neuregulin (NRG)/ErbB signaling is involved in numerous developmental processes in the nervous system, including synapse formation and function in the central nervous system. Although intensively investigated, its role at the neuromuscular synapse has remained elusive. Here, we demonstrate that loss of neuromuscular NRG/ErbB signaling destabilized anchoring of acetylcholine receptors (AChRs) in the postsynaptic muscle membrane and that this effect was caused by dephosphorylation of α-dystrobrevin1, a component of the postsynaptic scaffold. Specifically, in mice in which NRG signaling to muscle was genetically or pharmacologically abolished, postsynaptic AChRs moved rapidly from the synaptic to the perisynaptic membrane, and the subsynaptic scaffold that anchors the AChRs was impaired. These defects combined compromised synaptic transmission. We further show that blockade of NRG/ErbB signaling abolished tyrosine phosphorylation of α-dystrobrevin1, which reduced the stability of receptors in agrin-induced AChR clusters in cultured myotubes. Our data indicate that NRG/ErbB signaling maintains high efficacy of synaptic transmission by stabilizing the postsynaptic apparatus via phosphorylation of α-dystrobrevin1.

Deletion of aquaporin-4 changes the perivascular glial protein scaffold without disrupting the brain endothelial barrier

Expression of the water channel aquaporin-4 (AQP4) at the blood-brain interface is dependent upon the dystrophin associated protein complex. Here we investigated whether deletion of the Aqp4 gene affects the molecular composition of this protein scaffold and the integrity of the blood-brain barrier. High-resolution immunogold cytochemistry revealed that perivascular expression of α-syntrophin was reduced by 60% in Aqp4(-/-) mice. Additionally, perivascular AQP4 expression was reduced by 88% in α-syn(-/-) mice, in accordance with earlier reports. Immunofluorescence showed that Aqp4 deletion also caused a modest reduction in perivascular dystrophin, whereas β-dystroglycan labeling was unaltered. Perivascular microglia were devoid of AQP4 immunoreactivity. Deletion of Aqp4 did not alter the ultrastructure of capillary endothelial cells, the expression of tight junction proteins (claudin-5, occludin, and zonula occludens 1), or the vascular permeability to horseradish peroxidase and Evans blue albumin dye. We conclude that Aqp4 deletion reduces the expression of perivascular glial scaffolding proteins without affecting the endothelial barrier. Our data also indicate that AQP4 and α-syntrophin are mutually dependent upon each other for proper perivascular expression.

Dystrophin-associated protein scaffolding in brain requires alpha-dystrobrevin

Dystrophin and the alpha-dystrobrevins bind directly to the adapter protein syntrophin to form membrane-associated scaffolds. At the blood-brain barrier, alpha-syntrophin colocalizes with dystrophin and the alpha-dystrobrevins in perivascular glial endfeet and is required for localization of the water channel aquaporin-4. Earlier we have shown that localization of the scaffolding proteins gamma2-syntrophin, alpha-dystrobrevin-2, and dystrophin to glial endfeet is also dependent on the presence of alpha-syntrophin. In this study, we show that the expression levels of alpha-syntrophin, gamma2-syntrophin, and dystrophin at the blood-brain barrier are reduced in alpha-dystrobrevin-null mice. This is the first demonstration in which assembly of an astroglial protein scaffold containing syntrophin and dystrophin in perivascular astrocytes is dependent on the presence of alpha-dystrobrevin.

Functional substitution by TAT-utrophin in dystrophin-deficient mice

BACKGROUND

The loss of dystrophin compromises muscle cell membrane stability and causes Duchenne muscular dystrophy and/or various forms of cardiomyopathy. Increased expression of the dystrophin homolog utrophin by gene delivery or pharmacologic up-regulation has been demonstrated to restore membrane integrity and improve the phenotype in the dystrophin-deficient mdx mouse. However, the lack of a viable therapy in humans predicates the need to explore alternative methods to combat dystrophin deficiency. We investigated whether systemic administration of recombinant full-length utrophin (Utr) or DeltaR4-21 "micro" utrophin (muUtr) protein modified with the cell-penetrating TAT protein transduction domain could attenuate the phenotype of mdx mice.

METHODS AND FINDINGS

Recombinant TAT-Utr and TAT-muUtr proteins were expressed using the baculovirus system and purified using FLAG-affinity chromatography. Age-matched mdx mice received six twice-weekly intraperitoneal injections of either recombinant protein or PBS. Three days after the final injection, mice were analyzed for several phenotypic parameters of dystrophin deficiency. Injected TAT-muUtr transduced all tissues examined, integrated with members of the dystrophin complex, reduced serum levels of creatine kinase (11,290+/-920 U versus 5,950+/-1,120 U; PBS versus TAT), the prevalence of muscle degeneration/regeneration (54%+/-5% versus 37%+/-4% of centrally nucleated fibers; PBS versus TAT), the susceptibility to eccentric contraction-induced force drop (72%+/-5% versus 40%+/-8% drop; PBS versus TAT), and increased specific force production (9.7+/-1.1 N/cm(2) versus 12.8+/-0.9 N/cm(2); PBS versus TAT).

CONCLUSIONS

These results are, to our knowledge, the first to establish the efficacy and feasibility of TAT-utrophin-based constructs as a novel direct protein-replacement therapy for the treatment of skeletal and cardiac muscle diseases caused by loss of dystrophin.

Dp71f modulates GSK3-beta recruitment to the beta1-integrin adhesion complex

Previously, it was shown that Dp71f binds to the beta1-integrin adhesion complex to modulate PC12 cell adhesion. The absence of Dp71f led to a failure in the beta1-integrin adhesion complex formation. One of the structural proteins which links the beta1-integrin cytoplasmic domain to the actin cytoskeleton is ILK. GSK3-beta is an ILK substrate and the carboxi-terminal region of dystrophin 427 is a substrate for hierarchical phosphorylation by GSK3-beta. Dp71f contains the carboxi-terminal domain present in dystrophin 427. By using co-immunoprecipitation assays, in the present work it is demonstrated that in the neuronal PC12 cell line an interaction between Dp71f and GSK3-beta occurs. This interaction was corroborated by in vitro pulldown assays. We show that GSK3-beta is recruited to the beta1-integrin complex and that a reduced expression of Dp71f induces a reduced GSK3-beta recruitment to the beta1-integrin complex. In addition, the present work establishes that adhesion of PC12 cells to laminin does not influence the phosphorylation status of Dp71f.

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.

The ABCA1 cholesterol transporter associates with one of two distinct dystrophin-based scaffolds in Schwann cells

Cytoskeletal scaffolding complexes help organize specialized membrane domains with unique functions on the surface of cells. In this study, we define the scaffolding potential of the Schwann cell dystrophin glycoprotein complex (DGC) by establishing the presence of four syntrophin isoforms, (alpha1, beta1, beta2, and gamma2), and one dystrobrevin isoform, (alpha-dystrobrevin-1), in the abaxonal membrane. Furthermore, we demonstrate the existence of two separate DGCs in Schwann cells that divide the abaxonal membrane into spatially distinct domains, the DRP2/periaxin rich plaques and the Cajal bands that contain Dp116, utrophin, alpha-dystrobrevin-1 and four syntrophin isoforms. Finally, we show that the two different DGCs can scaffold unique accessory molecules in distinct areas of the Schwann cell membrane. Specifically, the cholesterol transporter ABCA1, associates with the Dp116/syntrophin complex in Cajal bands and is excluded from the DRP2/periaxin rich plaques.

Differential targeting of nNOS and AQP4 to dystrophin-deficient sarcolemma by membrane-directed α-dystrobrevin

α-Dystrobrevin associates with and is a homologue of dystrophin, the protein linked to Duchenne and Becker muscular dystrophies. We used a transgenic approach to restore α-dystrobrevin to the sarcolemma in mice that lack dystrophin (mdx mice) to study two interrelated functions: (1) the ability of α-dystrobrevin to rescue components of the dystrophin complex in the absence of dystrophin and (2) the ability of sarcolemmal α-dystrobrevin to ameliorate the dystrophic phenotype. We generated transgenic mice expressing α-dystrobrevin-2a linked to a palmitoylation signal sequence and bred them onto the α-dystrobrevin-null and mdx backgrounds. Expression of palmitoylated α-dystrobrevin prevented the muscular dystrophy observed in the α-dystrobrevin-null mice, demonstrating that the altered form of α-dystrobrevin was functional. On the mdx background, the palmitoylated form of α-dystrobrevin was expressed on the sarcolemma but did not significantly ameliorate the muscular dystrophy phenotype. Palmitoylated dystrobrevin restored α-syntrophin and aquaporin-4 (AQP4) to the mdx sarcolemma but was unable to recruit β-dystroglycan or the sarcoglycans. Despite restoration of sarcolemmal α-syntrophin, neuronal nitric oxide synthase (nNOS) was not localized to the sarcolemma, suggesting that nNOS requires both dystrophin and α-syntrophin for correct localization. Thus, although nNOS and AQP4 both require interaction with the PDZ domain of α-syntrophin for sarcolemmal association, their localization is regulated differentially.

Biology of the striated muscle dystrophin-glycoprotein complex

Since its first description in 1990, the dystrophin-glycoprotein complex has emerged as a critical nexus for human muscular dystrophies arising from defects in a variety of distinct genes. Studies in mammals widely support a primary role for the dystrophin-glycoprotein complex in mechanical stabilization of the plasma membrane in striated muscle and provide hints for secondary functions in organizing molecules involved in cellular signaling. Studies in model organisms confirm the importance of the dystrophin-glycoprotein complex for muscle cell viability and have provided new leads toward a full understanding of its secondary roles in muscle biology.

Dp71ab/DAPs complex composition changes during the differentiation process in PC12 cells

PC12 cells express different Dp71 isoforms originated from alternative splicing; one of them, Dp71ab lacks exons 71 and 78. To gain insight into the function of Dp71 isoforms we identified dystrophin associated proteins (DAPs) that associate in vivo with Dp71ab during nerve growth factor (NGF) induced differentiation of PC12 cells. DAPs expression was analyzed by RT-PCR, Western blot and indirect immunofluorescence, showing the presence of each mRNA and protein corresponding to alpha-, beta-, gamma-, delta-, and epsilon-sarcoglycans as well as zeta-sarcoglycan mRNA. Western blot analysis also revealed the expression of beta-dystroglycan, alpha1-syntrophin, alpha1-, and beta-dystrobrevins. We have established that Dp71ab forms a complex with beta-dystroglycan, alpha1-syntrophin, beta-dystrobrevin, and alpha-, beta- and gamma-sarcoglycans in undifferentiated PC12 cells. In differentiated PC12 cells, the complex composition changes since Dp71ab associates only with beta-dystroglycan, alpha1-syntrophin, beta-dystrobrevin, and delta-sarcoglycan. Interestingly, neuronal nitric oxide synthase associates with the Dp71ab/DAPs complex during NGF treatment, raising the possibility that Dp71ab may be involved in signal transduction events during neuronal differentiation.

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.

gamma-Syntrophin scaffolding is spatially and functionally distinct from that of the alpha/beta syntrophins

The syntrophins are a family of scaffolding proteins with multiple protein interaction domains that link signaling proteins to dystrophin family members. Each of the three most characterized syntrophins (alpha, beta1, beta2) contains a PDZ domain that binds a unique set of signaling proteins including kinases, ion and water channels, and neuronal nitric oxide synthase (nNOS). The PDZ domains of the gamma-syntrophins do not bind nNOS. In vitro pull-down assays show that the gamma-syntrophins can bind dystrophin but have unique preferences for the syntrophin binding sites of dystrophin family members. Despite their ability to bind dystrophin in vitro, neither gamma-syntrophin isoform co-localizes with dystrophin in skeletal muscle. Furthermore, gamma-syntrophins do not co-purify with dystrophin isolated from mouse tissue. These data suggest that the interaction of gamma-syntrophin with dystrophin is transient and potentially subject to regulatory mechanisms. gamma1-Syntrophin is highly expressed in brain and is specifically localized in hippocampal pyramidal neurons, Purkinje neurons in cerebellum, and cortical neurons. gamma2-Syntrophin is expressed in many tissues including skeletal muscle where it is found only in the subsynaptic space beneath the neuromuscular junction. In both neurons and muscle, gamma-syntrophin isoforms localize to the endoplasmic reticulum where they may form a scaffold for signaling and trafficking.

Assembly of a perivascular astrocyte protein scaffold at the mammalian blood–brain barrier is dependent on α-syntrophin

alpha-Syntrophin, a member of the dystrophin-associated protein complex, is required for proper localization of the water channel aquaporin-4 at the blood-brain barrier. Mice lacking alpha-syntrophin have reduced levels of aquaporin-4 in perivascular astroglial endfeet. Consequently, they exhibit reduced edema and infarct volume in brain trauma models and reduced K+ clearance from the neuropil, leading to increased seizure susceptibility. We have used the alpha-syntrophin null mice to investigate whether alpha-syntrophin is required for proper localization of other components of the dystrophin complex at the blood-brain barrier. We find that alpha-syntrophin is required for the full recruitment of gamma2-syntrophin and alpha-dystrobrevin-2 to glial endfeet in adult cerebellum. In contrast, the localization of beta1- and beta2-syntrophin and alpha-dystrobrevin-1 at the blood-brain barrier is not dependent on the presence of alpha-syntrophin. The localization patterns of alpha-dystrobrevin-1 and -2 in wild type cerebellum are strikingly different; while alpha-dystrobrevin-1 is present in glial endfeet throughout the cerebellum, alpha-dystrobrevin-2 is restricted to glial endfeet in the granular layer alone. Finally, we show that the enrichment of dystrophin in glial endfeet depends on the presence of alpha-syntrophin. This finding is the first demonstration that dystrophin localization is dependent on syntrophin. Since the localization of gamma2-syntrophin, alpha-dystrobrevin-2, and dystrophin is contingent on alpha-syntrophin, we conclude that alpha-syntrophin is a central organizer of the astrocyte dystrophin complex, an important molecular scaffold for localization of aquaporin-4 at the blood-brain barrier.

Cognitive impairment in neuromuscular disorders

Several studies have suggested the presence of central nervous system involvement manifesting as cognitive impairment in diseases traditionally confined to the peripheral nervous system. The aim of this review is to highlight the character of clinical, genetic, neurofunctional, cognitive, and psychiatric deficits in neuromuscular disorders. A high correlation between cognitive features and cerebral protein expression or function is evident in Duchenne muscular dystrophy, myotonic dystrophy (Steinert disease), and mitochondrial encephalomyopathies; direct correlation between tissue-specific protein expression and cognitive deficits is still elusive in certain neuromuscular disorders presenting with or without a cerebral abnormality, such as congenital muscular dystrophies, congenital myopathies, amyotrophic lateral sclerosis, adult polyglucosan body disease, and limb-girdle muscular dystrophies. No clear cognitive deficits have been found in spinal muscular atrophy and facioscapulohumeral dystrophy.

Structure of the split PH domain and distinct lipid-binding properties of the PH-PDZ supramodule of alpha-syntrophin

Pleckstrin homology (PH) domains play diverse roles in cytoskeletal dynamics and signal transduction. Split PH domains represent a unique subclass of PH domains that have been implicated in interactions with complementary partial PH domains 'hidden' in many proteins. Whether partial PH domains exist as independent structural units alone and whether two halves of a split PH domain can fold together to form an intact PH domain are not known. Here, we solved the structure of the PH(N)-PDZ-PH(C) tandem of alpha-syntrophin. The split PH domain of alpha-syntrophin adopts a canonical PH domain fold. The isolated partial PH domains of alpha-syntrophin, although completely unfolded, remain soluble in solution. Mixing of the two isolated domains induces de novo folding and yields a stable PH domain. Our results demonstrate that two complementary partial PH domains are capable of binding to each other to form an intact PH domain. We further showed that the PH(N)-PDZ-PH(C) tandem forms a functionally distinct supramodule, in which the split PH domain and the PDZ domain function synergistically in binding to inositol phospholipids.

{alpha}-Syntrophin regulates ARMS localization at the neuromuscular junction and enhances EphA4 signaling in an ARMS-dependent manner

EphA4 signaling has recently been implicated in the regulation of synapse formation and plasticity. In this study, we show that ankyrin repeat-rich membrane spanning (ARMS; also known as a kinase D–interacting substrate of 220 kD), a substrate for ephrin and neurotrophin receptors, was expressed in developing muscle and was concentrated at the neuromuscular junction (NMJ). Using yeast two-hybrid screening, we identified a PDZ (PSD-95, Dlg, ZO-1) domain protein, α-syntrophin, as an ARMS-interacting protein in muscle. Overexpression of α-syntrophin induced ARMS clustering in a PDZ domain–dependent manner. Coexpression of ARMS enhanced EphA4 signaling, which was further augmented by the presence of α-syntrophin. Moreover, the ephrin-A1–induced tyrosine phosphorylation of EphA4 was reduced in C2C12 myotubes after the blockade of ARMS and α-syntrophin expression by RNA interference. Finally, α-syntrophin–null mice exhibited a disrupted localization of ARMS and EphA4 at the NMJ and a reduced expression of ARMS in muscle. Altogether, our findings suggest that ARMS may play an important role in regulating postsynaptic signal transduction through the syntrophin-mediated localization of receptor tyrosine kinases such as EphA4.

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.

Laminin-alpha1 globular domains 3 and 4 induce heterotrimeric G protein binding to alpha-syntrophin's PDZ domain and alter intracellular Ca2+ in muscle

Alpha-syntrophin is a component of the dystrophin glycoprotein complex (DGC). It is firmly attached to the dystrophin cytoskeleton via a unique COOH-terminal domain and is associated indirectly with alpha-dystroglycan, which binds to extracellular matrix laminin. Syntrophin contains two pleckstrin homology (PH) domains and one PDZ domain. Because PH domains of other proteins are known to bind the betagamma-subunits of the heterotrimeric G proteins, whether this is also a property of syntrophin was investigated. Isolated syntrophin from rabbit skeletal muscle binds bovine brain Gbetagamma-subunits in gel blot overlay experiments. Laminin-1-Sepharose or specific antibodies against syntrophin, alpha- and beta-dystroglycan, or dystrophin precipitate a complex with Gbetagamma from crude skeletal muscle microsomes. Bacterially expressed syntrophin fusion proteins and truncation mutants allowed mapping of Gbetagamma binding to syntrophin's PDZ domain; this is a novel function for PDZ domains. When laminin-1 is bound, maximal binding of Gsalpha and Gbetagamma occurs and active Gsalpha, measured as GTP-gamma35S bound, decreases. Because intracellular Ca2+ is elevated in Duchenne muscular dystrophy and Gsalpha is known to activate the dihydropyridine receptor Ca2+ channel, whether laminin also altered intracellular Ca2+ was investigated. Laminin-1 decreases active (GTP-gammaS-bound) Gsalpha, and the Ca2+ channel is inhibited by laminin-1. The laminin alpha1-chain globular domains 4 and 5 region, the region bound by DGC alpha-dystroglycan, is sufficient to cause an effect, and an antibody that specifically blocks laminin binding to alpha-dystroglycan inhibits Gbeta binding by syntrophin in C2C12 myotubes. These observations suggest that DGC is a matrix laminin, G protein-coupled receptor.

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

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

The potassium channel Kir4.1 associates with the dystrophin-glycoprotein complex via alpha-syntrophin in glia

One of the major physiological roles of potassium channels in glial cells is to promote "potassium spatial buffering" in the central nervous system, a process necessary to maintain an optimal potassium concentration in the extracellular environment. This process requires the precise distribution of potassium channels accumulated at high density in discrete subdomains of glial cell membranes. To obtain a better understanding of how glial cells selectively target potassium channels to discrete membrane subdomains, we addressed the question of whether the glial inwardly rectifying potassium channel Kir4.1 associates with the dystrophin-glycoprotein complex (DGC). Immunoprecipitation experiments revealed that Kir4.1 is associated with the DGC in mouse brain and cultured cortical astrocytes. In vitro immunoprecipitation and pull-down assays demonstrated that Kir4.1 can bind directly to alpha-syntrophin, requiring the presence of the last three amino acids of the channel (SNV), a consensus PDZ domain-binding motif. Furthermore, Kir4.1 failed to associate with the DGC in brains from alpha-syntrophin knockout mice. These results suggest that Kir4.1 is localized in glial cells by its association with the DGC through a PDZ domain-mediated interaction with alpha-syntrophin and suggest an important role for the DGC in central nervous system physiology.

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.

Identification of Dp71 isoforms in the platelet membrane cytoskeleton. Potential role in thrombin-mediated platelet adhesion

Utrophin is a component of the platelet membrane cytoskeleton and participates in cytoskeletal reorganization (Earnest, J. P., Santos, G. F., Zuerbig, S., and Fox, J. E. B. (1995) J. Biol. Chem. 270, 27259-27265). Although platelets do not contain dystrophin, the identification of smaller C-terminal isoforms of dystrophin, including Dp71, which are expressed in a wide range of nonmuscle tissues and cell lines, has not been investigated. In this report, we have identified Dp71 protein variants of 55-60 kDa (designated Dp71Delta(110)) in the membrane cytoskeleton of human platelets. Both Dp71Delta(110) and utrophin sediment from lysed platelets along with the high speed detergent-insoluble pellet, which contains components of the membrane cytoskeleton. Like the membrane cytoskeletal proteins vinculin and spectrin, Dp71Delta(110) and utrophin redistributed from the high speed detergent-insoluble pellet to the integrin-rich low speed pellet of thrombin-stimulated platelets. Immunoelectron microscopy provided further evidence that Dp71Delta(110) was localized to the submembranous cytoskeleton. In addition to Dp71Delta(110), platelets contained several components of the dystrophin-associated protein complex, including beta-dystroglycan and syntrophin. To better understand the potential function of Dp71Delta(110), collagen adhesion assays were performed on platelets isolated from wild-type or Dp71-deficient (mdx(3cv)) mice. Adhesion to collagen in response to thrombin was significantly decreased in platelets isolated from mdx(3cv) mice, compared with wild-type platelets. Collectively, our results provide evidence that Dp71Delta(110) is a component of the platelet membrane cytoskeleton, is involved in cytoskeletal reorganization and/or signaling, and plays a role in thrombin-mediated platelet adhesion.

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.

Dystrophin-glycoprotein complex and Ras and Rho GTPase signaling are altered in muscle atrophy

The dystrophin-glycoprotein complex (DGC) is a sarcolemmal complex whose defects cause muscular dystrophies. The normal function of this complex is not clear. We have proposed that this is a signal transduction complex, signaling normal interactions with matrix laminin, and that the response is normal growth and homeostasis. If so, the complex and its signaling should be altered in other physiological states such as atrophy. The amount of some of the DGC proteins, including dystrophin, beta-dystroglycan, and alpha-sarcoglycan, is reduced significantly in rat skeletal muscle atrophy induced by tenotomy. Furthermore, H-Ras, RhoA, and Cdc42 decrease in expression levels and activities in muscle atrophy. When the small GTPases were assayed after laminin or beta-dystroglycan depletion, H-Ras, Rac1, and Cdc42 activities were reduced, suggesting a physical linkage between the DGC and the GTPases. Dominant-negative Cdc42, introduced with a retroviral vector, resulted in fibers that appeared atrophic. These data support a putative role for the DGC in transduction of mechanical signals in muscle.

Utrophin binds laterally along actin filaments and can couple costameric actin with sarcolemma when overexpressed in dystrophin-deficient muscle

Dystrophin is widely thought to mechanically link the cortical cytoskeleton with the muscle sarcolemma. Although the dystrophin homolog utrophin can functionally compensate for dystrophin in mice, recent studies question whether utrophin can bind laterally along actin filaments and anchor filaments to the sarcolemma. Herein, we have expressed full-length recombinant utrophin and show that the purified protein is fully soluble with a native molecular weight and molecular dimensions indicative of monomers. We demonstrate that like dystrophin, utrophin can form an extensive lateral association with actin filaments and protect actin filaments from depolymerization in vitro. However, utrophin binds laterally along actin filaments through contribution of acidic spectrin-like repeats rather than the cluster of basic repeats used by dystrophin. We also show that the defective linkage between costameric actin filaments and the sarcolemma in dystrophin-deficient mdx muscle is rescued by overexpression of utrophin. Our results demonstrate that utrophin and dystrophin are functionally interchangeable actin binding proteins, but that the molecular epitopes important for filament binding differ between the two proteins. More generally, our results raise the possibility that spectrin-like repeats may enable some members of the plakin family of cytolinkers to laterally bind and stabilize actin filaments.

Function and genetics of dystrophin and dystrophin-related proteins in muscle

The X-linked muscle-wasting disease Duchenne muscular dystrophy is caused by mutations in the gene encoding dystrophin. There is currently no effective treatment for the disease; however, the complex molecular pathology of this disorder is now being unravelled. Dystrophin is located at the muscle sarcolemma in a membrane-spanning protein complex that connects the cytoskeleton to the basal lamina. Mutations in many components of the dystrophin protein complex cause other forms of autosomally inherited muscular dystrophy, indicating the importance of this complex in normal muscle function. Although the precise function of dystrophin is unknown, the lack of protein causes membrane destabilization and the activation of multiple pathophysiological processes, many of which converge on alterations in intracellular calcium handling. Dystrophin is also the prototype of a family of dystrophin-related proteins, many of which are found in muscle. This family includes utrophin and alpha-dystrobrevin, which are involved in the maintenance of the neuromuscular junction architecture and in muscle homeostasis. New insights into the pathophysiology of dystrophic muscle, the identification of compensating proteins, and the discovery of new binding partners are paving the way for novel therapeutic strategies to treat this fatal muscle disease. This review discusses the role of the dystrophin complex and protein family in muscle and describes the physiological processes that are affected in Duchenne muscular dystrophy.

The dystrophin-glycoprotein complex, cellular signaling, and the regulation of cell survival in the muscular dystrophies

Mutations of different components of the dystrophin-glycoprotein complex (DGC) cause muscular dystrophies that vary in terms of severity, age of onset, and selective involvement of muscle groups. Although the primary pathogenetic processes in the muscular dystrophies have clearly been identified as apoptotic and necrotic muscle cell death, the pathogenetic mechanisms that lead to cell death remain to be determined. Studies of components of the DGC in muscle and in nonmuscle tissues have revealed that the DGC is undoubtedly a multifunctional complex and a highly dynamic structure, in contrast to the unidimensional concept of the DGC as a mechanical component in the cell. Analysis of the DGC reveals compelling analogies to two other membrane-associated protein complexes, namely integrins and caveolins. Each of these complexes mediates signal transduction cascades in the cell, and disruption of each complex causes muscular dystrophies. The signal transduction cascades associated with the DGC, like those associated with integrins and caveolins, play important roles in cell survival signaling, cellular defense mechanisms, and regulation of the balance between cell survival and cell death. This review focuses on the functional components of the DGC, highlighting the evidence of their participation in cellular signaling processes important for cell survival. Elucidating the link between these functional components and the pathogenetic processes leading to cell death is the foremost challenge to understanding the mechanisms of disease expression in the muscular dystrophies due to defects in the DGC.

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.

In vivo requirement of the alpha-syntrophin PDZ domain for the sarcolemmal localization of nNOS and aquaporin-4

alpha-Syntrophin is a scaffolding adapter protein expressed primarily on the sarcolemma of skeletal muscle. The COOH-terminal half of alpha-syntrophin binds to dystrophin and related proteins, leaving the PSD-95, discs-large, ZO-1 (PDZ) domain free to recruit other proteins to the dystrophin complex. We investigated the function of the PDZ domain of alpha-syntrophin in vivo by generating transgenic mouse lines expressing full-length alpha-syntrophin or a mutated alpha-syntrophin lacking the PDZ domain (Delta PDZ). The Delta PDZ alpha-syntrophin displaced endogenous alpha- and beta 1-syntrophin from the sarcolemma and resulted in sarcolemma containing little or no syntrophin PDZ domain. As a consequence, neuronal nitric oxide synthase (nNOS) and aquaporin-4 were absent from the sarcolemma. However, the sarcolemmal expression and distribution of muscle sodium channels, which bind the alpha-syntrophin PDZ domain in vitro, were not altered. Both transgenic mouse lines were bred with an alpha-syntrophin-null mouse which lacks sarcolemmal nNOS and aquaporin-4. The full-length alpha-syntrophin, not the Delta PDZ form, reestablished nNOS and aquaporin-4 at the sarcolemma of these mice. Genetic crosses with the mdx mouse showed that neither transgenic syntrophin could associate with the sarcolemma in the absence of dystrophin. Together, these data show that the sarcolemmal localization of nNOS and aquaporin-4 in vivo depends on the presence of a dystrophin-bound alpha-syntrophin PDZ domain.

Mouse alpha1-syntrophin binding to Grb2: further evidence of a role for syntrophin in cell signaling

Syntrophins have been proposed to serve as adapter proteins. Syntrophins are found in the dystrophin glycoprotein complex (DGC); defects in the constituents of this complex are linked to various muscular dystrophies. Blot overlay experiments demonstrate that alpha-dystroglycan, beta-dystroglycan, and syntrophins all bind Grb2, the growth factor receptor bound adapter protein. Mouse alpha1-syntrophin sequences were produced as chimeric fusion proteins in bacteria and found to also bind Grb2 in a Ca2+-independent manner. This binding was localized to the proline rich sequences adjacent to and overlapping with the N-terminal pleckstrin homology domain (PH1). Grb2 bound syntrophin with an apparent KD of 563 +/- 15 nM. Grb2-C-SH3 domain bound syntrophin with slightly higher affinity than Grb2-N-SH3 domain. Crk-L, an SH2/SH3 protein of similar domain structure but different specificity, does not bind these syntrophin sequences.

Interaction of γ1-Syntrophin with Diacylglycerol Kinase-ζ

Syntrophins are modular adapter proteins that link ion channels and signaling proteins to dystrophin and its homologues. A yeast two-hybrid screen of a human brain cDNA library using the PDZ domain of gamma 1- syntrophin, a recently identified brain-specific isoform, yielded overlapping clones encoding the C terminus of diacylglycerol kinase-zeta (DGK-zeta), an enzyme that converts diacylglycerol into phosphatidic acid. In biochemical assays, the C terminus of DGK-zeta, which contains a consensus PDZ-binding motif, was found to be necessary and sufficient for association with gamma 1-syntrophin. When coexpressed in HeLa cells, DGK-zeta and gamma 1-syntrophin formed a stable complex that partitioned between the cytoplasm and nucleus. DGK-zeta translocates from the cytosol to the nucleus, a process negatively regulated by protein kinase C phosphorylation. We found that DGK-zeta recruits gamma 1-syntrophin into the nucleus and that the PDZ-binding motif is required. Disrupting the interaction altered the intracellular localization of both proteins; DGK-zeta accumulated in the nucleus, whereas gamma 1-syntrophin remained in the cytoplasm. The level of endogenous syntrophins in the nucleus of HeLa cells also reflected the amount of nuclear DGK-zeta. In the brain, DGK-zeta and gamma 1-syntrophin were colocalized in cell bodies and dendrites of cerebellar Purkinjie neurons and other neuronal cell types, suggesting that their interaction is physiologically relevant. Moreover, coimmunoprecipitation and pull-down experiments from brain extracts and cells suggest that DGK-zeta, gamma 1-syntrophin, and dystrophin form a ternary complex. Collectively, our results suggest that gamma 1-syntrophin participates in regulating the subcellular localization of DGK-zeta to ensure correct termination of diacylglycerol signaling.

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.