Interaction of α1-syntrophin with multiple isoforms of heterotrimeric G protein α subunits

The Laminin-Induced Phosphorylation of PKCδ Regulates AQP4 Distribution and Water Permeability in Rat Astrocytes

In astrocytes, the water-permeable channel aquaporin-4 (AQP4) is concentrated at the endfeet that abut the blood vessels of the brain. The asymmetric distribution of this channel is dependent on the function of dystroglycan (DG), a co-expressed laminin receptor, and its associated protein complex. We have demonstrated that the addition of laminin to astrocytes in culture causes the clustering of AQP4, DG, and lipid rafts. The last, in particular, have been associated with the initiation of cell signaling. As laminin binding to DG in muscle cells can induce the tyrosine phosphorylation of syntrophin and laminin requires tyrosine kinases for acetylcholine receptor clustering in myotubes, we asked if signal transduction might also be involved in AQP4 clustering in astrocytes. We analyzed the timecourse of AQP4, DG, and monosialotetrahexosylganglioside (GM1) clustering in primary cultures of rat astrocytes following the addition of laminin, and determined that the clustering of DG precedes that of AQP4 and GM1. We also showed that laminin induces the formation of phosphotyrosine-rich clusters and that the tyrosine kinase inhibitor, genistein, disrupts the laminin-induced clustering of both β-DG and AQP4. Using the Kinexus antibody microarray chip, we then identified protein-serine kinase C delta (PKCδ) as one of the main proteins exhibiting high levels of tyrosine phosphorylation upon laminin treatment. Selective inhibitors of PKC and siRNA against PKCδ disrupted β-DG and AQP4 clustering, and also caused water transport to increase in astrocytes treated with laminin. Our results demonstrate that the effects of laminin on AQP4 localization and function are relayed, at least in part, through PKC signaling.

Dystrophin-glycoprotein complex regulates muscle nitric oxide production through mechanoregulation of AMPK signaling

Patients deficient in dystrophin, a protein that links the cytoskeleton to the extracellular matrix via the dystrophin-glycoprotein complex (DGC), exhibit muscular dystrophy, cardiomyopathy, and impaired muscle nitric oxide (NO) production. We used live-cell NO imaging and in vitro cyclic stretch of isolated adult mouse cardiomyocytes as a model system to investigate if and how the DGC directly regulates the mechanical activation of muscle NO signaling. Acute activation of NO synthesis by mechanical stretch was impaired in dystrophin-deficient mdx cardiomyocytes, accompanied by loss of stretch-induced neuronal NO synthase (nNOS) S1412 phosphorylation. Intriguingly, stretch induced the acute activation of AMP-activated protein kinase (AMPK) in normal cardiomyocytes but not in mdx cardiomyocytes, and specific inhibition of AMPK was sufficient to attenuate mechanoactivation of NO production. Therefore, we tested whether direct pharmacologic activation of AMPK could bypass defective mechanical signaling to restore nNOS activity in dystrophin-deficient cardiomyocytes. Indeed, activation of AMPK with 5-aminoimidazole-4-carboxamide riboside or salicylate increased nNOS S1412 phosphorylation and was sufficient to enhance NO production in mdx cardiomyocytes. We conclude that the DGC promotes the mechanical activation of cardiac nNOS by acting as a mechanosensor to regulate AMPK activity, and that pharmacologic AMPK activation may be a suitable therapeutic strategy for restoring nNOS activity in dystrophin-deficient hearts and muscle.

Dystrophin-glycoprotein complex regulates muscle nitric oxide production through mechanoregulation of AMPK signaling

Patients deficient in dystrophin, a protein that links the cytoskeleton to the extracellular matrix via the dystrophin-glycoprotein complex (DGC), exhibit muscular dystrophy, cardiomyopathy, and impaired muscle nitric oxide (NO) production. We used live-cell NO imaging and in vitro cyclic stretch of isolated adult mouse cardiomyocytes as a model system to investigate if and how the DGC directly regulates the mechanical activation of muscle NO signaling. Acute activation of NO synthesis by mechanical stretch was impaired in dystrophin-deficient mdx cardiomyocytes, accompanied by loss of stretch-induced neuronal NO synthase (nNOS) S1412 phosphorylation. Intriguingly, stretch induced the acute activation of AMP-activated protein kinase (AMPK) in normal cardiomyocytes but not in mdx cardiomyocytes, and specific inhibition of AMPK was sufficient to attenuate mechanoactivation of NO production. Therefore, we tested whether direct pharmacologic activation of AMPK could bypass defective mechanical signaling to restore nNOS activity in dystrophin-deficient cardiomyocytes. Indeed, activation of AMPK with 5-aminoimidazole-4-carboxamide riboside or salicylate increased nNOS S1412 phosphorylation and was sufficient to enhance NO production in mdx cardiomyocytes. We conclude that the DGC promotes the mechanical activation of cardiac nNOS by acting as a mechanosensor to regulate AMPK activity, and that pharmacologic AMPK activation may be a suitable therapeutic strategy for restoring nNOS activity in dystrophin-deficient hearts and muscle.

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.

Alpha-dystrobrevin-1 recruits alpha-catulin to the alpha1D-adrenergic receptor/dystrophin-associated protein complex signalosome

α(1D)-Adrenergic receptors (ARs) are key regulators of cardiovascular system function that increase blood pressure and promote vascular remodeling. Unfortunately, little information exists about the signaling pathways used by this important G protein-coupled receptor (GPCR). We recently discovered that α(1D)-ARs form a "signalosome" with multiple members of the dystrophin-associated protein complex (DAPC) to become functionally expressed at the plasma membrane and bind ligands. However, the molecular mechanism by which the DAPC imparts functionality to the α(1D)-AR signalosome remains a mystery. To test the hypothesis that previously unidentified molecules are recruited to the α(1D)-AR signalosome, we performed an extensive proteomic analysis on each member of the DAPC. Bioinformatic analysis of our proteomic data sets detected a common interacting protein of relatively unknown function, α-catulin. Coimmunoprecipitation and blot overlay assays indicate that α-catulin is directly recruited to the α(1D)-AR signalosome by the C-terminal domain of α-dystrobrevin-1 and not the closely related splice variant α-dystrobrevin-2. Proteomic and biochemical analysis revealed that α-catulin supersensitizes α(1D)-AR functional responses by recruiting effector molecules to the signalosome. Taken together, our study implicates α-catulin as a unique regulator of GPCR signaling and represents a unique expansion of the intricate and continually evolving array of GPCR signaling networks.

The neurobiology of the dystrophin-associated glycoprotein complex

While the function of dystrophin in muscle disease has been thoroughly investigated, dystrophin and associated proteins also have important roles in the central nervous system. Many patients with Duchenne and Becker muscular dystrophies (D/BMD) have cognitive impairment, learning disability, and an increased incidence of some neuropsychiatric disorders. Accordingly, dystrophin and members of the dystrophin-associated glycoprotein complex (DGC) are found in the brain where they participate in macromolecular assemblies that anchor receptors to specialized sites within the membrane. In neurons, dystrophin and the DGC participate in the postsynaptic clustering and stabilization of some inhibitory GABAergic synapses. During development, alpha-dystroglycan functions as an extracellular matrix receptor controlling, amongst other things, neuronal migration in the developing cortex and cerebellum. Several types of congenital muscular dystrophy caused by impaired alpha-dystroglycan glycosylation cause neuronal migration abnormalities and mental retardation. In glial cells, the DGC is involved in the organization of protein complexes that target water-channels to the plasma membrane. Finally, mutations in the gene encoding epsilon-sarcoglycan cause the neurogenic movement disorder, myoclonus-dystonia syndrome implicating epsilon-sarcoglycan in dopaminergic neurotransmission. In this review we describe the recent progress in defining the role of the DGC and associated proteins in the brain.