Modulation of glucocorticoid receptor-interacting protein 1 (GRIP1) transactivation and co-activation activities through its C-terminal repression and self-association domains

Glucocorticoid receptor-interacting protein 1 (GRIP1), a p160 family nuclear receptor co-activator, possesses at least two autonomous activation domains (AD1 and AD2) in the C-terminal region. AD1 activity appears to be mediated by CBP/p300, whereas AD2 activity is apparently mediated through co-activator-associated arginine methyltransferase 1 (CARM1). The mechanisms responsible for regulating the activities of AD1 and AD2 are not well understood. We provide evidence that the GRIP1 C-terminal region may be involved in regulating its own transactivation and nuclear receptor co-activation activities through primary self-association and a repression domain. We also compared the effects of the GRIP1 C terminus with those of other factors that functionally interact with the GRIP1 C terminus, such as CARM1. Based on our results, we propose a regulatory mechanism involving conformational changes to GRIP1 mediated through its intramolecular and intermolecular interactions, and through modulation of the effects of co-repressors on its repression domains. These are the first results to indicate that the structural components of GRIP1, especially those of the C terminus, might functionally modulate its putative transactivation activities and nuclear receptor co-activator functions.     

H36-alpha 7 is a novel integrin alpha chain that is developmentally regulated during skeletal myogenesis [published erratum appears in J Cell Biol 1992 Jul;118(1):213]

H36 is a 120,000-D membrane glycoprotein that is expressed during the differentiation of skeletal muscle. H36 cDNA clones were isolated from a lambda UniZapXR rat myotube cDNA library and sequenced. The deduced amino acid sequence demonstrates that H36 is a novel integrin alpha chain that shares extensive homology with other alpha integrins that includes: (a) the GFFKR sequence found in all alpha integrins; (b) a single membrane spanning region; (c) conservation of 18 of 22 cysteines; and (d) a protease cleavage site found in the non-I region integrin alpha chains. The cytoplasmic domain of H36 is unique and additional regions of nonhomology further indicate H36 is distinct from all other alpha chains. In keeping with current nomenclature we designate this alpha chain alpha 7. Northern blots demonstrate that expression of H36-alpha 7 mRNA is regulated both early in the development of the myogenic lineage and later, during terminal differentiation. Detection of H36-alpha 7 mRNA coincides with conversion of H36- myogenic precursor cells to H36+ cells. H36-alpha 7 mRNA is present in replicating myoblasts: expression increases upon terminal differentiation and is markedly reduced in developmentally defective myoblasts. In addition, H36-alpha 7 mRNA is not detected in C3H10T1/2 cells. It is in myotubes derived from myoblasts obtained by treatment of 10T1/2 cells with azacytidine or transfection with MRF4. Immunoblots and immunofluorescence demonstrate that the H36-alpha 7 chain is associated with integrin beta 1. Affinity chromatography demonstrates that H36-alpha 7 beta 1 selectively binds to laminin. The expression of H36-alpha 7 on secondary myoblasts during the development of the limb in vivo corresponds with the appearance of laminin in the limb, with the responsiveness of secondary myoblast proliferation to laminin, and with the onset of increased muscle mass, suggesting that H36-alpha 7 modulates this stage in limb development. We conclude that H36-alpha 7 is a novel alpha integrin laminin binding protein whose expression is developmentally regulated during skeletal myogenesis.     

Laminin alpha4 and integrin alpha6 are upregulated in regenerating dy/dy skeletal muscle: comparative expression of laminin and integrin isoforms in muscles regenerating after crush injury

The expression of laminin isoforms and laminin-binding integrin receptors known to occur in muscle was investigated during myogenic regeneration after crush injury. Comparisons were made between dystrophic 129ReJ dy/dy mice, which have reduced laminin alpha2 expression, and their normal littermates. The overall histological pattern of regeneration after crush injury was similar in dy/dy and control muscle, but proceeded faster in dy/dy mice. In vitro studies revealed a greater yield of mononuclear cells extracted from dy/dy muscle and a reduced proportion of desmin-positive cells upon in vitro cultivation, reflecting the presence of inflammatory cells and "preactivated" myoblasts due to ongoing regenerative processes within the endogenous dystrophic lesions. Laminin alpha1 was not detectable in skeletal muscle. Laminin alpha2 was present in basement membranes of mature myofibers and newly formed myotubes in control and dy/dy muscles, albeit weaker in dy/dy. Laminin alpha2-negative myogenic cells were detected in dy/dy and control muscle, suggesting the involvement of other laminin alpha chains in early myogenic differentiation, such as laminin alpha4 and alpha5 which were both transiently expressed in basement membranes of newly formed myotubes of dy/dy and control mice. Integrin beta1 was expressed on endothelial cells, muscle fibers, and peripheral nerves in uninjured muscle and broadened after crush injury to the interstitium where it occurred on myogenic and nonmyogenic cells. Integrin alpha3 was not expressed in uninjured or regenerating muscle, while integrin alpha6 was expressed mainly on endothelial cells and peripheral nerves in uninjured muscle. Upon crush injury integrin alpha6 increased in the interstitium mainly on nonmyogenic cells, including infiltrating leukocytes, endothelial cells, and fibroblasts. In dy/dy muscle, integrin alpha6 occurred on some newly formed myotubes. Integrin alpha7 was expressed on muscle fibers at the myotendinous junction and showed weak and irregular expression on muscle fibers. After crush injury, integrin alpha7 expression extended to the newly formed myotubes and some myoblasts. However, many myoblasts and newly formed myotubes were integrin alpha7 negative. No marked difference was observed in integrin alpha7 expression between dy/dy and control muscle, either uninjured or after crush injury. Only laminin alpha4 and integrin alpha6 expression patterns were notably different between dy/dy and control muscle. Expression of both molecules was more extensive in dy/dy muscle, especially in the interstitium of regenerating areas and on newly formed myotubes. In view of the faster myogenic regeneration observed in dy/dy mice, the data suggest that laminin alpha4 and integrin alpha6 support myogenic regeneration. However, whether these accelerated myogenic effects are a direct consequence of the reduced laminin alpha2 expression in dy/dy mice, or an accentuation of the ongoing regenerative events in focal lesions in the muscle, requires further investigation.     

Expression of the murine alpha B-crystallin gene in lens and skeletal muscle: identification of a muscle-preferred enhancer.

The alpha B-crystallin gene is expressed at high levels in lens and at lower levels in some other tissues, notably skeletal and cardiac muscle, kidney, lung, and brain. A promoter fragment of the murine alpha B-crystallin gene extending from positions -661 to +44 and linked to the bacterial chloramphenicol acetyltransferase (CAT) gene showed preferential expression in lens and skeletal muscle in transgenic mice. Transfection experiments revealed that a region between positions -426 and -257 is absolutely required for expression in C2C12 and G8 myotubes, while sequences downstream from position -115 appear to be determinants for lens expression. In association with a heterologous promoter, a -427 to -259 fragment functions as a strong enhancer in C2C12 myotubes and less efficiently in myoblasts and lens. Gel shift and methylation interference studies demonstrated that nuclear proteins from C2C12 myoblasts and myotubes specifically bind to the enhancer.     

p130Cas-dependent actin remodelling regulates myogenic differentiation

Actin dynamics are implicated in various cellular processes, not only through the regulation of cytoskeletal organization, but also via the control of gene expression. In the present study we show that the Src family kinase substrate p130Cas (Cas is Crk-associated substrate) influences actin remodelling and concomitant muscle-specific gene expression, thereby regulating myogenic differentiation. In C2C12 myoblasts, silencing of p130Cas expression by RNA interference impaired F-actin (filamentous actin) formation and nuclear localization of the SRF (serum-response factor) co-activator MAL (megakaryocytic acute leukaemia) following the induction of myogenic differentiation. Consequently, formation of multinucleated myotubes was abolished. Re-introduction of wild-type p130Cas, but not its phosphorylation-defective mutant, into p130Cas-knockdown myoblasts restored F-actin assembly, MAL nuclear localization and myotube formation. Depletion of the adhesion molecule integrin β3, a key regulator of myogenic differentiation as well as actin cytoskeletal organization, attenuated p130Cas phosphorylation and MAL nuclear localization during C2C12 differentiation. Moreover, knockdown of p130Cas led to the activation of the F-actin-severing protein cofilin. The introduction of a dominant-negative mutant of cofilin into p130Cas-knockdown myoblasts restored muscle-specific gene expression and myotube formation. The results of the present study suggest that p130Cas phosphorylation, mediated by integrin β3, facilitates cofilin inactivation and promotes myogenic differentiation through modulating actin cytoskeleton remodelling.