Dystroglycan is an important cell adhesion receptor linking the actin cytoskeleton, via utrophin and dystrophin, to laminin in the extracellular matrix. To identify adhesion‐related signalling molecules associated with dystroglycan, we conducted a yeast two‐hybrid screen and identified mitogen‐activated protein (MAP) kinase kinase 2 (MEK2) as a β‐dystroglycan interactor. Pull‐down experiments and localization studies substantiated a physiological link between β‐dystroglycan and MEK and localized MEK with dystroglycan in membrane ruffles. Moreover, we also identified active extracellular signal‐regulated kinase (ERK), the downstream kinase from MEK, as another interacting partner for β‐dystroglycan and localized both active ERK and dystroglycan to focal adhesions in fibroblast cells. These studies suggest a role for dystroglycan as a multifunctional adaptor or scaffold capable of interacting with components of the ERK–MAP kinase cascade including MEK and ERK. These findings have important implications for our understanding of the role of dystroglycan in normal cellular processes and in disease states such as muscular dystrophy.

Dystroglycan is a ubiquitously expressed heterodimeric adhesion receptor. The extracellular α-subunit makes connections with a number of laminin G domain ligands including laminins, agrin and perlecan in the extracellular matrix and the transmembrane β-subunit makes connections to the actin filament network via cytoskeletal linkers including dystrophin, utrophin, ezrin and plectin, depending on context. Originally discovered as part of the dystrophin glycoprotein complex of skeletal muscle, dystroglycan is an important adhesion molecule and signalling scaffold in a multitude of cell types and tissues and is involved in several diseases. Dystroglycan has emerged as a multifunctional adhesion platform with many interacting partners associating with its short unstructured cytoplasmic domain. Two particular hotspots are the cytoplasmic juxtamembrane region and at the very carboxy terminus of dystroglycan. Regions which between them have several overlapping functions: in the juxtamembrane region; a nuclear localisation signal, ezrin/radixin/moesin protein, rapsyn and ERK MAP Kinase binding function, and at the C terminus a regulatory tyrosine governing WW, SH2 and SH3 domain interactions. We will discuss the binding partners for these motifs and how their interactions and regulation can modulate the involvement of dystroglycan in a range of different adhesion structures and functions depending on context. Thus dystroglycan presents as a multifunctional scaffold involved in adhesion and adhesion-mediated signalling with its functions under exquisite spatio-temporal regulation.

Dystroglycans are essential transmembrane adhesion receptors for laminin. α-Dystroglycan is a highly glycosylated extracellular protein that interacts with laminin in the extracellular matrix and the transmembrane region of β-dystroglycan. β-Dystroglycan, via its cytoplasmic tail, interacts with dystrophin and utrophin and also with the actin cytoskeleton. As a part of the dystrophin–glycoprotein complex of muscles, dystroglycan is also important in maintaining sarcolemmal integrity. Mutations in dystrophin that lead to Duchenne muscular dystrophy also lead to a loss of dystroglycan from the sarcolemma, and chimaeric mice lacking muscle dystroglycan exhibit a severe muscular dystrophy phenotype. Using yeast two-hybrid analysis and biochemical and cell biological studies, we show, in the present study, that the cytoplasmic tail of β-dystroglycan interacts directly with F-actin and, furthermore, that it bundles actin filaments and induces an aberrant actin phenotype when overexpressed in cells.

Background: SM22 has long been studied as an actin-associated protein. Interestingly, levels of SM22 are often reduced in tumour cell lines, while they are increased during senescence possibly indicating a role for SM22 in cell fate decisions via its interaction with actin. In this study we aimed to determine whether reducing levels of SM22 could actively contribute to a tumourigenic phenotype.Results: We demonstrate that in REF52 fibroblasts, decreased levels of SM22 disrupt normal actin organization leading to changes in the motile behaviour of cells. Interestingly, SM22 depletion also led to an increase in the capacity of cells to spontaneously form podosomes with a concomitant increase in the ability to invade Matrigel. In PC3 prostate epithelial cancer cells by contrast, where SM22 is undetectable, re-expression of SM22 reduced the ability to invade Matrigel. Furthermore SM22 depleted cells also had reduced levels of reactive oxygen species when under serum starvation stress.Conclusions: These findings suggest that depletion of SM22 could contribute to tumourigenic properties of cells. Reduction in SM22 levels would tend to promote cell survival when cells are under stress, such as in a hypoxic tumour environment, and may also contribute to increases in actin dynamics that favour metastatic potential.

Dystroglycan is a ubiquitously expressed cell adhesion protein. Its principal role has been determined as a component of the dystrophin-glycoprotein complex of muscle, where it constitutes a key component of the costameric cell adhesion system. To investigate more fundamental aspects of dystroglycan function in cell adhesion, we examined the role of dystroglycan in the dynamics and assembly of cellular adhesions in myoblasts. We show that β-dystroglycan is recruited to adhesion structures and, based on staining for vinculin, that overexpression or depletion of dystroglycan affects both size and number of fibrillar adhesions. Knockdown of dystroglycan increases the size and number of adhesions, whereas overexpression decreases the number of adhesions. Dystroglycan knockdown or overexpression affects the ability of cells to adhere to different substrates, and has effects on cell migration that are consistent with effects on the formation of fibrillar adhesions. Using an SH3 domain proteomic screen, we identified vinexin as a binding partner for dystroglycan. Furthermore, we show that dystroglycan can interact indirectly with vinculin by binding to the vinculin-binding protein vinexin, and that this interaction has a role in dystroglycan-mediated cell adhesion and spreading. For the first time, we also demonstrate unequivocally that β-dystroglycan is a resident of focal adhesions.

Dystroglycan is a ubiquitously expressed cell adhesion receptor best understood in its role as part of the dystrophin glycoprotein complex of mature skeletal muscle. Less is known of the role of dystroglycan in more fundamental aspects of cell adhesion in other cell types, nor of its role in myoblast cell adhesion. We have examined the role of dystroglycan in the early stages of myoblast adhesion and spreading and found that dystroglycan initially associates with other adhesion proteins in large puncta morphologically similar to podosomes. Using a human SH3 domain phage display library we identified Tks5, a key regulator of podosomes, as interacting with β-dystroglycan. We verified the interaction by immunoprecipitation, GST-pulldown and immunfluorescence localisation. Both proteins localise to puncta during early phases of spreading, but importantly following stimulation with phorbol ester, also localise to structures indistinguishable from podosomes. Dystroglycan overexpression inhibited podosome formation by sequestering Tks5 and Src. Mutation of dystroglycan tyrosine 890, previously identified as a Src substrate, restored podosome formation. We propose therefore, that Src-dependent phosphorylation of β-dystroglycan results in the formation of a Src/dystroglycan complex that drives the SH3-mediated association between dystroglycan and Tks5 which together regulate podosome formation in myoblasts.