Utrophin Binds Laterally along Actin Filaments and Can Couple Costameric Actin with Sarcolemma When Overexpressed in Dystrophin-deficient Muscle

Abstract

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.

Ets-2 Repressor Factor Silences Extrasynaptic Utrophin by N-Box–mediated Repression in Skeletal Muscle

“…Dystrophin-related protein or utrophin is considered the autosomal homologue of dystrophin, because it shares extensive sequence homology as well as its size and abundance in muscle (Love et al, 1989;Khurana et al, 1990;Tinsley et al, 1992). Utrophin and dystrophin also share functional properties such as the ability to associate with the dystroglycan complex and bind F-actin (Matsumura et al, 1992;Winder and Kendrick, 1995;Rybakova et al, 2002). Direct evidence for the ability of utrophin to functionally substitute comes from experiments demonstrating that utrophin overexpression driven either by transgenic means Rafael et al, 1998;Tinsley et al, 1998;Fisher et al, 2001) or viral vectors (Gilbert et al, 1999;Wakefield et al, 2000;Cerletti et al, 2003) can rescue dystrophin-deficient muscle.…”

Specific Interaction of the Actin-binding Domain of Dystrophin with Intermediate Filaments Containing Keratin 19

“…Dystrophin and its associated proteins are enriched at all these sites (Porter et al, 1992;Williams and Bloch, 1999a;Williams et al, 2000). When it is present, the dystrophin complex helps to link the contractile apparatus to the sarcolemma and through the membrane to the extracellular matrix (Ibraghimov-Beskrovnaya et al, 1992;Ervasti and Campbell, 1993;Rybakova et al, 2000;Rybakova et al, 2002; Michele et al, 2003). When dystrophin is absent, links between the contractile apparatus and the sarcolemma can weaken and eventually break (Porter et al, 1992;Ehmer et al, 1997;Williams and Bloch, 1999b;Rybakova et al, 2000), damaging the sarcolemma (Mokri and Engel, 1975), and, ultimately, killing the myofiber (reviewed in Emery, 1993).…”

Cytoplasmic γ-actin expression in diverse animal models of muscular dystrophy

“…Transgenic expression of the dystrophin homolog utrophin restored the stable association of costameric actin with the sarcolemma [5]. Most recently, we demonstrated that γ cyto -actin protein levels were elevated 10-fold in striated muscle from the dystrophin-deficient mdx mouse [6].…”

“…The actin binding proteins γ-filamin, talin, vinculin, plectin and utrophin are all upregulated and/or redistributed to costameres in dystrophin-deficient muscle [4,5,[32][33][34]. With the exception of utrophin, all of these proteins interact with integrins in vivo and α7β1 integrin is also upregulated in dystrophin-deficient muscle [35,36].…”