Dynamics of Dystrophin's Actin-Binding Domain

Structural Dynamics of Protein Interactions Using Site-Directed Spin Labeling of Cysteines to Measure Distances and Rotational Dynamics with EPR Spectroscopy

Here we review applications of site-directed spin labeling (SDSL) with engineered cysteines in proteins, to study the structural dynamics of muscle and non-muscle proteins, using and developing the electron paramagnetic resonance (EPR) spectroscopic techniques of dipolar EPR, double electron electron resonance (DEER), saturation transfer EPR (STEPR), and orientation measured by EPR. The SDSL technology pioneered by Wayne Hubbell and collaborators has greatly expanded the use of EPR, including the measurement of distances between spin labels covalently attached to proteins and peptides. The Thomas lab and collaborators have applied these techniques to elucidate dynamic interactions in the myosin-actin complex, myosin-binding protein C, calmodulin, ryanodine receptor, phospholamban, utrophin, dystrophin, β-III-spectrin, and Aurora kinase. The ability to design and engineer cysteines in proteins for site-directed covalent labeling has enabled the use of these powerful EPR techniques to measure distances, while showing that they are complementary with optical spectroscopy measurements.

Therapeutic Strategies for Dystrophin Replacement in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked hereditary disease characterized by progressive muscle wasting due to modifications in the DMD gene (exon deletions, nonsense mutations, intra-exonic insertions or deletions, exon duplications, splice site defects, and deep intronic mutations) that result in a lack of functional dystrophin expression. Many therapeutic approaches have so far been attempted to induce dystrophin expression and improve the patient phenotype. In this manuscript, we describe the relevant updates for some therapeutic strategies for DMD aiming to restore dystrophin expression. We also present and analyze in vitro and in vivo ongoing experimental approaches to treat the disease.

CRISPR-Cas9 Gene Therapy for Duchenne Muscular Dystrophy

Discovery of the CRISPR-Cas (clustered regularly interspaced short palindromic repeat, CRISPR-associated) system a decade ago has opened new possibilities in the field of precision medicine. CRISPR-Cas was initially identified in bacteria and archaea to play a protective role against foreign genetic elements during viral infections. The application of this technique for the correction of different mutations found in the Duchenne muscular dystrophy (DMD) gene led to the development of several potential therapeutic approaches for DMD patients. The mutations responsible for Duchenne muscular dystrophy mainly include exon deletions (70% of patients) and point mutations (about 30% of patients). The CRISPR-Cas 9 technology is becoming increasingly precise and is acquiring diverse functions through novel innovations such as base editing and prime editing. However, questions remain about its translation to the clinic. Current research addressing off-target editing, efficient muscle-specific delivery, immune response to nucleases, and vector challenges may eventually lead to the clinical use of the CRISPR-Cas9 technology. In this review, we present recent CRISPR-Cas9 strategies to restore dystrophin expression in vitro and in animal models of DMD.

The Dystrophin Node as Integrator of Cytoskeletal Organization, Lateral Force Transmission, Fiber Stability and Cellular Signaling in Skeletal Muscle

The systematic bioanalytical characterization of the protein product of the DMD gene, which is defective in the pediatric disorder Duchenne muscular dystrophy, led to the discovery of the membrane cytoskeletal protein dystrophin. Its full-length muscle isoform Dp427-M is tightly linked to a sarcolemma-associated complex consisting of dystroglycans, sarcoglyans, sarcospan, dystrobrevins and syntrophins. Besides these core members of the dystrophin-glycoprotein complex, the wider dystrophin-associated network includes key proteins belonging to the intracellular cytoskeleton and microtubular assembly, the basal lamina and extracellular matrix, various plasma membrane proteins and cytosolic components. Here, we review the central role of the dystrophin complex as a master node in muscle fibers that integrates cytoskeletal organization and cellular signaling at the muscle periphery, as well as providing sarcolemmal stabilization and contractile force transmission to the extracellular region. The combination of optimized tissue extraction, subcellular fractionation, advanced protein co-purification strategies, immunoprecipitation, liquid chromatography and two-dimensional gel electrophoresis with modern mass spectrometry-based proteomics has confirmed the composition of the core dystrophin complex at the sarcolemma membrane. Importantly, these biochemical and mass spectrometric surveys have identified additional members of the wider dystrophin network including biglycan, cavin, synemin, desmoglein, tubulin, plakoglobin, cytokeratin and a variety of signaling proteins and ion channels.

Structural Characteristics, Binding Partners and Related Diseases of the Calponin Homology (CH) Domain

The calponin homology (CH) domain is one of the most common modules in various actin-binding proteins and is characterized by an α-helical fold. The CH domain plays important regulatory roles in both cytoskeletal dynamics and signaling. The CH domain is required for stability and organization of the actin cytoskeleton, calcium mobilization and activation of downstream pathways. The CH domain has recently garnered increased attention due to its importance in the onset of different diseases, such as cancers and asthma. However, many roles of the CH domain in various protein functions and corresponding diseases are still unclear. Here, we review current knowledge about the structural features, interactome and related diseases of the CH domain.

Steric regulation of tandem calponin homology domain actin-binding affinity

Tandem calponin homology (CH1-CH2) domains are common actin-binding domains in proteins that interact with and organize the actin cytoskeleton. Despite regions of high sequence similarity, CH1-CH2 domains can have remarkably different actin-binding properties, with disease-associated point mutants known to increase as well as decrease affinity for F-actin. To investigate features that affect CH1-CH2 affinity for F-actin in cells and in vitro, we perturbed the utrophin actin-binding domain by making point mutations at the CH1-CH2 interface, replacing the linker domain, and adding a polyethylene glycol (PEG) polymer to CH2. Consistent with a previous model describing CH2 as a steric negative regulator of actin binding, we find that utrophin CH1-CH2 affinity is both increased and decreased by modifications that change the effective "openness" of CH1 and CH2 in solution. We also identified interface mutations that caused a large increase in affinity without changing solution "openness," suggesting additional influences on affinity. Interestingly, we also observe nonuniform subcellular localization of utrophin CH1-CH2 that depends on the N-terminal flanking region but not on bulk affinity. These observations provide new insights into how small sequence changes, such as those found in diseases, can affect CH1-CH2 binding properties.