Identification and characterisation of transcript and protein of a new short N-terminal utrophin isoform

The crystal structures of dystrophin and utrophin spectrin repeats: implications for domain boundaries

Dystrophin and utrophin link the F-actin cytoskeleton to the cell membrane via an associated glycoprotein complex. This functionality results from their domain organization having an N-terminal actin-binding domain followed by multiple spectrin-repeat domains and then C-terminal protein-binding motifs. Therapeutic strategies to replace defective dystrophin with utrophin in patients with Duchenne muscular dystrophy require full-characterization of both these proteins to assess their degree of structural and functional equivalence. Here the high resolution structures of the first spectrin repeats (N-terminal repeat 1) from both dystrophin and utrophin have been determined by x-ray crystallography. The repeat structures both display a three-helix bundle fold very similar to one another and to homologous domains from spectrin, α-actinin and plectin. The utrophin and dystrophin repeat structures reveal the relationship between the structural domain and the canonical spectrin repeat domain sequence motif, showing the compact structural domain of spectrin repeat one to be extended at the C-terminus relative to its previously defined sequence repeat. These structures explain previous in vitro biochemical studies in which extending dystrophin spectrin repeat domain length leads to increased protein stability. Furthermore we show that the first dystrophin and utrophin spectrin repeats have no affinity for F-actin in the absence of other domains.

Tissue expression and actin binding of a novel N-terminal utrophin isoform

Utrophin and dystrophin present two large proteins that link the intracellular actin cytoskeleton to the extracellular matrix via the C-terminal-associated protein complex. Here we describe a novel short N-terminal isoform of utrophin and its protein product in various rat tissues (N-utro, 62 kDa, amino acids 1–539, comprising the actin-binding domain plus the first two spectrin repeats). Using different N-terminal recombinant utrophin fragments, we show that actin binding exhibits pronounced negative cooperativity (affinity constants K₁ = ∼5 × 10⁶ and K₂ = ∼1 × 10⁵M⁻¹) and is Ca²⁺-insensitive. Expression of the different fragments in COS7 cells and in myotubes indicates that the actin-binding domain alone binds exlusively to actin filaments. The recombinant N-utro analogue binds in vitro to actin and in the cells associates to the membranes. The results indicate that N-utro may be responsible for the anchoring of the cortical actin cytoskeleton to the membranes in muscle and other tissues.

Dystrophins, utrophins, and associated scaffolding complexes: role in mammalian brain and implications for therapeutic strategies

Two decades of molecular, cellular, and functional studies considerably increased our understanding of dystrophins function and unveiled the complex etiology of the cognitive deficits in Duchenne muscular dystrophy (DMD), which involves altered expression of several dystrophin-gene products in brain. Dystrophins are normally part of critical cytoskeleton-associated membrane-bound molecular scaffolds involved in the clustering of receptors, ion channels, and signaling proteins that contribute to synapse physiology and blood-brain barrier function. The utrophin gene also drives brain expression of several paralogs proteins, which cellular expression and biological roles remain to be elucidated. Here we review the structural and functional properties of dystrophins and utrophins in brain, the consequences of dystrophins loss-of-function as revealed by numerous studies in mouse models of DMD, and we discuss future challenges and putative therapeutic strategies that may compensate for the cognitive impairment in DMD based on experimental manipulation of dystrophins and/or utrophins brain expression.

Utrophin is a calpain substrate in muscle cells

Calpains are Ca2+ -dependent cytosolic cysteine proteases that participate in the pathology of Duchenne muscular dystrophy (DMD). Utrophin is a functional homolog of dystrophin that partially compensates for dystrophin deficiency in myofibers of mdx mice. In this study, we investigated the susceptibility of utrophin to cleavage by calpain in vitro and in muscle cells. We found that utrophin is a direct in vitro substrate of purified calpain I and II. Cleavage of utrophin by calpain I or II generates specific degradation products that are also found in cultured control and DMD myotubes under conditions with elevated intracellular Ca2+ levels. In addition, we showed that activation of cellular calpains by Ca2+ ionophore treatment reduces utrophin protein levels in muscle cells and that calpain inhibition prevents this Ca2+ -induced reduction in utrophin levels. These observations suggest that, beside its known effect on general muscle protein degradation, calpain contributes to DMD pathology by specifically degrading the compensatory protein utrophin.

Dystroglycan receptor is involved in integrin activation in intestinal epithelia

The dystroglycans (alpha-DG and beta-DG), which play important roles in the formation of basement membranes, have been well studied in skeletal muscle and nerve, but their expression and localization in intestinal epithelial cells has not been previously investigated. Here, we demonstrated that the DG complex, composed of alpha-DG, beta-DG, and utrophin, is specifically expressed in the basolateral membrane of the Caco-2-BBE monolayer. The DG complex coprecipitated with beta(1)-integrin, suggesting a possible interaction among these proteins. In addition, we observed that activation of DG receptors by laminin-1 enhanced the interaction between beta(1)-integrin and laminin-1, whereas activation of DG receptors by laminin-2 reduced the interaction between beta(1)-integrin and laminin-2. Finally, we demonstrated that the intracellular COOH-terminal tail of beta-DG and its binding to the DG binding domain of utrophin are crucial for the interactions between laminin-1/-2 and beta(1)-integrin. Collectively, these novel results indicate that dystroglycans play important roles in the regulation of interactions between intestinal epithelial cells and the extracellular matrix.

Identification of spectrin-like repeats required for high affinity utrophin-actin interaction

Most studies aimed at characterizing the utrophinactin interaction have focused on the amino-terminal tandem calponin homology domain. However, we recently reported evidence suggesting that spectrin-like repeats of utrophin also participate in binding to actin. Here we expressed several recombinant fragments encoding the utrophin amino-terminal domain alone or in combination with various numbers of spectrin-like repeats. We further quantitatively characterized the actin binding properties of each recombinant utrophin fragment using a high-speed sedimentation assay. To evaluate the capacity of each protein to stabilize actin filaments, we compared the effect of utrophin recombinant fragments and full-length utrophin on 6-propionyl-2-(N,N-dimethylamino)naphthalene actin depolymerization. Our results suggest that, whereas the amino-terminal domain is essential for primary interaction between utrophin and actin, spectrin-like repeats have additive effects on the affinity and stoichiometry of binding. Our data indicate that the amino-terminal domain and first 10 consecutive spectrin-like repeats recapitulate the actin binding activity of full-length utrophin more faithfully than the amino-terminal domain alone. These findings support the model for lateral association of utrophin along the actin filament and provide the molecular basis for designing the most effective utrophin "mini-genes" for treatment of dystrophinopathies.

Molecular heterogeneity of the dystrophin-associated protein complex in the mouse kidney nephron: differential alterations in the absence of utrophin and dystrophin

The dystrophin-associated protein complex (DPC) consisting of syntrophin, dystrobrevin, and dystroglycan isoforms is associated either with dystrophin or its homolog utrophin. It is present not only in muscle cells, but also in numerous tissues, including kidney, liver, and brain. Using high-resolution immunofluorescence imaging and Western blotting, we have investigated the effects of utrophin and dystrophin gene deletion on the formation and membrane anchoring of the DPC in kidney epithelial cells, which co-express utrophin and low levels of the C-terminal dystrophin isoform Dp71. We show that multiple, molecularly distinct DPCs co-exist in the nephron; these DPCs have a segment-specific distribution and are only partially associated with utrophin in the basal membrane of tubular epithelial cells. In utrophin-deficient mice, a selective reduction of beta2-syntrophin has been observed in medullary tubular segments, whereas alpha1-syntrophin and beta1-syntrophin are retained, concomintant with an upregulation of beta-dystroglycan, beta-dystrobrevin, and Dp71. These findings suggest that beta2-syntrophin is dependent on utrophin for association with the DPC, and that loss of utrophin is partially compensated by Dp71, allowing the preservation of the DPC in kidney epithelial cells. This hypothesis is confirmed by the almost complete loss of all DPC proteins examined in mice lacking full-length utrophin and all C-terminal dystrophin isoforms (utrophin(0/0)/mdx(3Cv)). The DPC thus critically depends on these proteins for assembly and/or membrane localization in kidney epithelial cells.

An atomic model for actin binding by the CH domains and spectrin-repeat modules of utrophin and dystrophin

Utrophin and dystrophin link cytoskeletal F-actin filaments to the plasmalemma. Genetic strategies to replace defective dystrophin with utrophin in individuals with muscular dystrophy requires full characterization of these proteins. Both contain homologous N-terminal actin-binding motifs composed of a pair of calponin-homology (CH) domains (CH1 and CH2) that are connected by spectrin-repeat modules to C-terminal membrane-binding sequences. Here, electron microscopy and 3D reconstruction of F-actin decorated with utrophin and dystrophin actin-binding constructs were performed using Utr261 (utrophin's CH domain pair), Utr416 (utrophin's CH domains and first spectrin-repeat) and Dys246 (dystrophin's CH domain pair). The lozenge-like utrophin CH domain densities localized to the upper surface of actin subdomain 1 and extended azimuthally over subdomain 2 toward subdomains 3 and 4. The cylinder-shaped spectrin-repeat was located at the end of the CH domain pair and was aligned longitudinally along the cleft between inner and outer actin domains, where tropomyosin is present when on thin filaments. The connection between the spectrin-repeat module and the CH domains defined the orientation of CH1 and CH2 on actin. Resolution of utrophin's CH domains and spectrin-repeats permitted docking of crystal structures into respective EM densities, leading to an atomic model where both CH and spectrin-domains bind actin. The CH domain-actin interaction for dystrophin was found to be more complex than for utrophin. Binding assays showed that Utr261 and Utr416 interacted with F-actin as monomers, whereas Dys246 appeared to associate as a dimer, consistent with a bilobed Dys246 structure observed on F-actin in electron microscope reconstructions. One of the lobes was similar in shape, position and orientation to the monomeric CH domains of Utr261, while the other lobe apparently represented a second set of CH domains in the dimeric Dys246. The extensive contact made by dystrophin on actin may be used in vivo to help muscles dissipate mechanical stress from the contractile apparatus to the extracellular matrix.

Glucocorticoid-mediated regulation of utrophin levels in human muscle fibers

Previous studies on transgenic mice indicate that upregulation of utrophin protein may offer a potential treatment strategy for Duchenne muscular dystrophy. We have analyzed the effect of the glucocorticoid 6alpha-methylprednisolone-21 sodium succinate on utrophin protein levels, using a cell-based assay with differentiated human myotubes, derived from biopsies of healthy individuals or Duchenne muscular dystrophy patients. We found that within 5-7 days 6alpha-methylprednisolone-21 sodium succinate increases utrophin protein up to approximately 40% in both normal and dystrophin-deficient myotubes compared to untreated control cultures. When analyzed in promoter-reporter assays 6alpha-methylprednisolone-21 sodium succinate activated a utrophin promoter A-fragment but did not activate a utrophin promoter B-fragment. Surprisingly, endogenous levels of utrophin mRNA in 6alpha-methylprednisolone-21 sodium succinate-treated muscle cells were unaltered indicating that the utrophin-inducing effect of glucocorticoids may be a result of post-transcriptional mechanisms. We have also analyzed 66 glucocorticoids for their effect on utrophin protein levels and found that glucocorticoids in general are able to induce utrophin protein in human myotubes.

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.

The utrophin actin-binding domain binds F-actin in two different modes: implications for the spectrin superfamily of proteins

Utrophin, like its homologue dystrophin, forms a link between the actin cytoskeleton and the extracellular matrix. We have used a new method of image analysis to reconstruct actin filaments decorated with the actin-binding domain of utrophin, which contains two calponin homology domains. We find two different modes of binding, with either one or two calponin-homology (CH) domains bound per actin subunit, and these modes are also distinguishable by their very different effects on F-actin rigidity. Both modes involve an extended conformation of the CH domains, as predicted by a previous crystal structure. The separation of these two modes has been largely dependent upon the use of our new approach to reconstruction of helical filaments. When existing information about tropomyosin, myosin, actin-depolymerizing factor, and nebulin is considered, these results suggest that many actin-binding proteins may have multiple binding sites on F-actin. The cell may use the modular CH domains found in the spectrin superfamily of actin-binding proteins to bind actin in manifold ways, allowing for complexity to arise from the interactions of a relatively few simple modules with actin.

Different splice variants of filamin-B affect myogenesis, subcellular distribution, and determine binding to integrin [beta] subunits

Integrins connect the extracellular matrix with the cell interior, and transduce signals through interactions of their cytoplasmic tails with cytoskeletal and signaling proteins. Using the yeast two-hybrid system, we isolated a novel splice variant (filamin-Bvar-1) of the filamentous actin cross-linking protein, filamin-B, that interacts with the cytoplasmic domain of the integrin beta1A and beta1D subunits. RT-PCR analysis showed weak, but wide, expression of filamin-Bvar-1 and a similar splice variant of filamin-A (filamin-Avar-1) in human tissues. Furthermore, alternative splice variants of filamin-B and filamin-C, from which the flexible hinge-1 region is deleted (DeltaH1), were induced during in vitro differentiation of C2C12 mouse myoblasts. We show that both filamin-Avar-1 and filamin-Bvar-1 bind more strongly than their wild-type isoforms to different integrin beta subunits. The mere presence of the high-affinity binding site for beta1A is not sufficient for targeting the filamin-Bvar-1 construct to focal contacts. Interestingly, the simultaneous deletion of the H1 region is required for the localization of filamin-B at the tips of actin stress fibers. When expressed in C2C12 cells, filamin-Bvar-1(DeltaH1) accelerates their differentiation into myotubes. Furthermore, filamin-B variants lacking the H1 region induce the formation of thinner myotubes than those in cells containing variants with this region. These findings suggest that specific combinations of filamin mRNA splicing events modulate the organization of the actin cytoskeleton and the binding affinity for integrins.

Structural and functional aspects of filamins

Filamins are a family of high molecular mass cytoskeletal proteins that organize filamentous actin in networks and stress fibers. Over the past few years it has become clear that filamins anchor various transmembrane proteins to the actin cytoskeleton and provide a scaffold for a wide range of cytoplasmic signaling proteins. The recent cloning of three human filamins and studies on filamin orthologues from chicken and Drosophila revealed unexpected complexity of the filamin family, the biological implications of which have just started to be addressed. Expression of dysfunctional filamin-A leads to the genetic disorder of ventricular heterotopia and gives reason to expect that abnormalities in the other isogenes may also be connected with human disease. In this review aspects of filamin structure, its splice variants, binding partners and biological function will be discussed.