An ankyrin-based mechanism for functional organization of dystrophin and dystroglycan

Ankyrin-B is required for the establishment and maintenance of lens cytoarchitecture, mechanics, and clarity

This study illustrates a vital role for ankyrin-B in lens architecture, growth and function through its involvement in membrane protein and spectrin-actin cytoskeletal organization and stability The transparent ocular lens is essential for vision by focusing light onto the retina. Despite recognizing the importance of its unique cellular architecture and mechanical properties, the molecular mechanisms governing these attributes remain elusive. This study aims to elucidate the role of ankyrin-B (AnkB), a membrane scaffolding protein, in lens cytoarchitecture, growth and function using a conditional knockout (cKO) mouse model. AnkB cKO mouse has no defects in lens morphogenesis, but exhibited changes that supported a global role for AnkB in maintenance of lens clarity, size, cytoarchitecture, and stiffness. Notably, absence of AnkB led to nuclear cataract formation, evident from P16. AnkB cKO lens fibers exhibit progressive disruption in membrane organization of the spectrin-actin cytoskeleton, channel proteins, cell-cell adhesion, shape change, loss and degradation of several membrane proteins (e.g., NrCAM. N-cadherin and aquaporin-0) along with a disorganized plasma membrane and impaired ball-and-socket membrane interdigitations. Furthermore, absence of AnkB led to decreased lens stiffness. Collectively, these results illustrate the essential role for AnkB in lens architecture, growth and function through its involvement in membrane protein and cytoskeletal organization.

Physical and functional convergence of the autism risk genes Scn2a and Ank2 in neocortical pyramidal cell dendrites

Dysfunction in sodium channels and their ankyrin scaffolding partners have both been implicated in neurodevelopmental disorders, including autism spectrum disorder (ASD). In particular, the genes SCN2A, which encodes the sodium channel NaV1.2, and ANK2, which encodes ankyrin-B, have strong ASD association. Recent studies indicate that ASD-associated haploinsufficiency in Scn2a impairs dendritic excitability and synaptic function in neocortical pyramidal cells, but how NaV1.2 is anchored within dendritic regions is unknown. Here, we show that ankyrin-B is essential for scaffolding NaV1.2 to the dendritic membrane of mouse neocortical neurons and that haploinsufficiency of Ank2 phenocopies intrinsic dendritic excitability and synaptic deficits observed in Scn2a+/- conditions. These results establish a direct, convergent link between two major ASD risk genes and reinforce an emerging framework suggesting that neocortical pyramidal cell dendritic dysfunction can contribute to neurodevelopmental disorder pathophysiology.

A review on mechanistic insights into structure and function of dystrophin protein in pathophysiology and therapeutic targeting of Duchenne muscular dystrophy

Duchenne Muscular Dystrophy (DMD) is an X-linked recessive genetic disorder characterized by progressive and severe muscle weakening and degeneration. Among the various forms of muscular dystrophy, it stands out as one of the most common and impactful, predominantly affecting boys. The condition arises due to mutations in the dystrophin gene, a key player in maintaining the structure and function of muscle fibers. The manuscript explores the structural features of dystrophin protein and their pivotal roles in DMD. We present an in-depth analysis of promising therapeutic approaches targeting dystrophin and their implications for the therapeutic management of DMD. Several therapies aiming to restore dystrophin protein or address secondary pathology have obtained regulatory approval, and many others are ongoing clinical development. Notably, recent advancements in genetic approaches have demonstrated the potential to restore partially functional dystrophin forms. The review also provides a comprehensive overview of the status of clinical trials for major therapeutic genetic approaches for DMD. In addition, we have summarized the ongoing therapeutic approaches and advanced mechanisms of action for dystrophin restoration and the challenges associated with DMD therapeutics.

Dystrophin- and Utrophin-Based Therapeutic Approaches for Treatment of Duchenne Muscular Dystrophy: A Comparative Review

Duchenne muscular dystrophy is a devastating disease that leads to progressive muscle loss and premature death. While medical management focuses mostly on symptomatic treatment, decades of research have resulted in first therapeutics able to restore the affected reading frame of dystrophin transcripts or induce synthesis of a truncated dystrophin protein from a vector, with other strategies based on gene therapy and cell signaling in preclinical or clinical development. Nevertheless, recent reports show that potentially therapeutic dystrophins can be immunogenic in patients. This raises the question of whether a dystrophin paralog, utrophin, could be a more suitable therapeutic protein. Here, we compare dystrophin and utrophin amino acid sequences and structures, combining published data with our extended in silico analyses. We then discuss these results in the context of therapeutic approaches for Duchenne muscular dystrophy. Specifically, we focus on strategies based on delivery of micro-dystrophin and micro-utrophin genes with recombinant adeno-associated viral vectors, exon skipping of the mutated dystrophin pre-mRNAs, reading through termination codons with small molecules that mask premature stop codons, dystrophin gene repair by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (CRISPR/Cas9)-mediated genetic engineering, and increasing utrophin levels. Our analyses highlight the importance of various dystrophin and utrophin domains in Duchenne muscular dystrophy treatment, providing insights into designing novel therapeutic compounds with improved efficacy and decreased immunoreactivity. While the necessary actin and β-dystroglycan binding sites are present in both proteins, important functional distinctions can be identified in these domains and some other parts of truncated dystrophins might need redesigning due to their potentially immunogenic qualities. Alternatively, therapies based on utrophins might provide a safer and more effective approach.

Spatial positioning of preimplantation mouse embryo cells is regulated by mTORC1 and m7G-cap-dependent translation at the 8- to 16-cell transition

Preimplantation mouse embryo development involves temporal-spatial specification and segregation of three blastocyst cell lineages: trophectoderm, primitive endoderm and epiblast. Spatial separation of the outer-trophectoderm lineage from the two other inner-cell-mass (ICM) lineages starts with the 8- to 16-cell transition and concludes at the 32-cell stages. Accordingly, the ICM is derived from primary and secondary contributed cells; with debated relative EPI versus PrE potencies. We report generation of primary but not secondary ICM populations is highly dependent on temporal activation of mammalian target of Rapamycin (mTOR) during 8-cell stage M-phase entry, mediated via regulation of the 7-methylguanosine-cap (m7G-cap)-binding initiation complex (EIF4F) and linked to translation of mRNAs containing 5' UTR terminal oligopyrimidine (TOP-) sequence motifs, as knockdown of identified TOP-like motif transcripts impairs generation of primary ICM founders. However, mTOR inhibition-induced ICM cell number deficits in early blastocysts can be compensated by the late blastocyst stage, after inhibitor withdrawal; compensation likely initiated at the 32-cell stage when supernumerary outer cells exhibit molecular characteristics of inner cells. These data identify a novel mechanism specifically governing initial spatial segregation of mouse embryo blastomeres, that is distinct from those directing subsequent inner cell formation, contributing to germane segregation of late blastocyst lineages.

Subcellular diversity of Nav1.5 in cardiomyocytes: distinct functions, mechanisms and targets

In cardiomyocytes, the rapid depolarisation of the membrane potential is mediated by the α-subunit of the cardiac voltage-gated Na⁺ channel (Na_V 1.5), encoded by the gene SCN5A. This ion channel allows positively charged Na⁺ ions to enter the cardiomyocyte, resulting in the fast upstroke of the action potential and is therefore crucial for cardiac excitability and electrical propagation. This essential role is underscored by the fact that dysfunctional Na_V 1.5 is associated with high risk for arrhythmias and sudden cardiac death. However, development of therapeutic interventions regulating Na_V 1.5 has been limited due to the complexity of Na_V 1.5 structure and function and its diverse roles within the cardiomyocyte. In particular, research from the last decade has provided us with increased knowledge on the subcellular distribution of Na_V 1.5 as well as the proteins which it interacts with in distinct cardiomyocyte microdomains. We here review these insights, detailing the potential role of Na_V 1.5 within subcellular domains as well as its dysfunction in the setting of arrhythmia disorders. We furthermore provide an overview of current knowledge on the pathways involved in (microdomain-specific) trafficking of Na_V 1.5, and their potential as novel targets. Unravelling the complexity of Na_V 1.5 (dys)function may ultimately facilitate the development of therapeutic strategies aimed at preventing lethal arrhythmias. This is not only of importance for pathophysiological conditions where sodium current is specifically decreased within certain subcellular regions, such as in arrhythmogenic cardiomyopathy and Duchenne muscular dystrophy, but also for other acquired and inherited disorders associated with Na_V 1.5.

Sinus node dysfunction: current understanding and future directions

The sinoatrial node (SAN) is the primary pacemaker of the heart. Normal SAN function is crucial in maintaining proper cardiac rhythm and contraction. Sinus node dysfunction (SND) is due to abnormalities within the SAN, which can affect the heartbeat frequency, regularity, and the propagation of electrical pulses through the cardiac conduction system. As a result, SND often increases the risk of cardiac arrhythmias. SND is most commonly seen as a disease of the elderly given the role of degenerative fibrosis as well as other age-dependent changes in its pathogenesis. Despite the prevalence of SND, current treatment is limited to pacemaker implantation, which is associated with substantial medical costs and complications. Emerging evidence has identified various genetic abnormalities that can cause SND, shedding light on the molecular underpinnings of SND. Identification of these molecular mechanisms and pathways implicated in the pathogenesis of SND is hoped to identify novel therapeutic targets for the development of more effective therapies for this disease. In this review article, we examine the anatomy of the SAN and the pathophysiology and epidemiology of SND. We then discuss in detail the most common genetic mutations correlated with SND and provide our perspectives on future research and therapeutic opportunities in this field.

Pleiotropic Ankyrins: Scaffolds for Ion Channels and Transporters

The ankyrin proteins (Ankyrin-R, Ankyrin-B, and Ankyrin-G) are a family of scaffolding, or membrane adaptor proteins necessary for the regulation and targeting of several types of ion channels and membrane transporters throughout the body. These include voltage-gated sodium, potassium, and calcium channels in the nervous system, heart, lungs, and muscle. At these sites, ankyrins recruit ion channels, and other membrane proteins, to specific subcellular domains, which are then stabilized through ankyrin's interaction with the submembranous spectrin-based cytoskeleton. Several recent studies have expanded our understanding of both ankyrin expression and their ion channel binding partners. This review provides an updated overview of ankyrin proteins and their known channel and transporter interactions. We further discuss several potential avenues of future research that would expand our understanding of these important organizational proteins.

The role of the dystrophin glycoprotein complex in muscle cell mechanotransduction

Dystrophin is the central protein of the dystrophin-glycoprotein complex (DGC) in skeletal and heart muscle cells. Dystrophin connects the actin cytoskeleton to the extracellular matrix (ECM). Severing the link between the ECM and the intracellular cytoskeleton has a devastating impact on the homeostasis of skeletal muscle cells, leading to a range of muscular dystrophies. In addition, the loss of a functional DGC leads to progressive dilated cardiomyopathy and premature death. Dystrophin functions as a molecular spring and the DGC plays a critical role in maintaining the integrity of the sarcolemma. Additionally, evidence is accumulating, linking the DGC to mechanosignalling, albeit this role is still less understood. This review article aims at providing an up-to-date perspective on the DGC and its role in mechanotransduction. We first discuss the intricate relationship between muscle cell mechanics and function, before examining the recent research for a role of the dystrophin glycoprotein complex in mechanotransduction and maintaining the biomechanical integrity of muscle cells. Finally, we review the current literature to map out how DGC signalling intersects with mechanical signalling pathways to highlight potential future points of intervention, especially with a focus on cardiomyopathies.

A review of the function of the Dystrophic Glycoprotein Complex (DGC) in mechanosignaling provides an overview of the various components of DGC and potential mechanopathogenic mechanisms, particularly as they relate to muscular dystrophy.

Mechanisms underlying the role of ankyrin-B in cardiac and neurological health and disease

The ANK2 gene encodes for ankyrin-B (ANKB), one of 3 members of the ankyrin family of proteins, whose name is derived from the Greek word for anchor. ANKB was originally identified in the brain (B denotes “brain”) but has become most widely known for its role in cardiomyocytes as a scaffolding protein for ion channels and transporters, as well as an interacting protein for structural and signaling proteins. Certain loss-of-function ANK2 variants are associated with a primarily cardiac-presenting autosomal-dominant condition with incomplete penetrance and variable expressivity characterized by a predisposition to supraventricular and ventricular arrhythmias, arrhythmogenic cardiomyopathy, congenital and adult-onset structural heart disease, and sudden death. Another independent group of ANK2 variants are associated with increased risk for distinct neurological phenotypes, including epilepsy and autism spectrum disorders. The mechanisms underlying ANKB's roles in cells in health and disease are not fully understood; however, several clues from a range of molecular and cell biological studies have emerged. Notably, ANKB exhibits several isoforms that have different cell-type–, tissue–, and developmental stage– expression profiles. Given the conservation within ankyrins across evolution, model organism studies have enabled the discovery of several ankyrin roles that could shed important light on ANKB protein-protein interactions in heart and brain cells related to the regulation of cellular polarity, organization, calcium homeostasis, and glucose and fat metabolism. Along with this accumulation of evidence suggesting a diversity of important ANKB cellular functions, there is an on-going debate on the role of ANKB in disease. We currently have limited understanding of how these cellular functions link to disease risk. To this end, this review will examine evidence for the cellular roles of ANKB and the potential contribution of ANKB functional variants to disease risk and presentation. This contribution will highlight the impact of ANKB dysfunction on cardiac and neuronal cells and the significance of understanding the role of ANKB variants in disease.

Roles and mechanisms of ankyrin-G in neuropsychiatric disorders

Ankyrin proteins act as molecular scaffolds and play an essential role in regulating cellular functions. Recent evidence has implicated the ANK3 gene, encoding ankyrin-G, in bipolar disorder (BD), schizophrenia (SZ), and autism spectrum disorder (ASD). Within neurons, ankyrin-G plays an important role in localizing proteins to the axon initial segment and nodes of Ranvier or to the dendritic shaft and spines. In this review, we describe the expression patterns of ankyrin-G isoforms, which vary according to the stage of brain development, and consider their functional differences. Furthermore, we discuss how posttranslational modifications of ankyrin-G affect its protein expression, interactions, and subcellular localization. Understanding these mechanisms leads us to elucidate potential pathways of pathogenesis in neurodevelopmental and psychiatric disorders, including BD, SZ, and ASD, which are caused by rare pathogenic mutations or changes in the expression levels of ankyrin-G in the brain.

Mutations affecting the production, distribution, or function of the ankyrin-G protein may contribute to a variety of different neuropsychiatric disorders. Ankyrin-G is typically observed at the synapses between neurons, and contributes to intercellular adhesion and signaling along with other important functions. Peter Penzes and colleagues at Northwestern University, Chicago, USA, review the biology of this protein and identify potential mechanisms by which ankyrin-G mutations might impair healthy brain development. Mutations in the gene encoding this protein are strongly linked with bipolar disorder, but have also been tentatively connected to autism spectrum disorders and schizophrenia. The authors highlight physiologically important interactions with a diverse array of other brain proteins, which can in turn be modulated by various chemical modifications to ankyrin-G, and conclude that drugs that influence these modifications could have potential therapeutic value.

Obscurin regulates ankyrin macromolecular complex formation

Obscurin is a large scaffolding protein in striated muscle that maintains sarcolemmal integrity and aligns the sarcoplasmic reticulum with the underlying contractile machinery. Ankyrins are a family of adaptor proteins with some isoforms that interact with obscurin. Previous studies have examined obscurin interacting with individual ankyrins. In this study, we demonstrate that two different ankyrins interact with obscurin's carboxyl terminus via independent ankyrin-binding domains (ABDs). Using in-vitro binding assays, co-precipitation assays, and FLIM-FRET analysis, we show that obscurin interacts with small ankyrin 1.5 (sAnk1.5) and the muscle-specific ankyrin-G isoform (AnkG107). While there is no direct interaction between sAnk1.5 and AnkG107, obscurin connects the two ankyrins both in vitro and in cells. Moreover, AnkG107 recruits β-spectrin to this macromolecular protein complex and mutating obscurin's ABDs disrupts complex formation. To further characterize AnkG107 interaction with obscurin, we measure obscurin-binding to different AnkG107 isoforms expressed in the heart and find that the first obscurin-binding domain in AnkG107 principally mediates this interaction. We also find that AnkG107 does not bind to filamin-C and displays minimal binding to plectin-1 compared to obscurin. Finally, both sAnk1.5-GFP and AnkG107-CTD-RFP are targeted to the M-lines of ventricular cardiomyocytes and mutating their obscurin-binding domains disrupts the M-line localization of these ankyrin constructs. Altogether, these findings support a model in which obscurin can interact via independent binding domains with two different ankyrin protein complexes to target them to the sarcomeric M-line of ventricular cardiomyocytes.

Circulating small RNA signatures differentiate accurately the subtypes of muscular dystrophies: small-RNA next-generation sequencing analytics and functional insights

Muscular dystrophies are a group of rare and severe inherited disorders mainly affecting the muscle tissue. Duchene Muscular Dystrophy, Myotonic Dystrophy types 1 and 2, Limb Girdle Muscular Dystrophy and Facioscapulohumeral Muscular Dystrophy are some of the members of this family of disorders. In addition to the current diagnostic tools, there is an increasing interest for the development of novel non-invasive biomarkers for the diagnosis and monitoring of these diseases. miRNAs are small RNA molecules characterized by high stability in blood thus making them ideal biomarker candidates for various diseases. In this study, we present the first genome-wide next-generation small RNA sequencing in serum samples of five different types of muscular dystrophy patients and healthy individuals. We identified many small RNAs including miRNAs, lncRNAs, tRNAs, snoRNAs and snRNAs, that differentially discriminate the muscular dystrophy patients from the healthy individuals. Further analysis of the identified miRNAs showed that some miRNAs can distinguish the muscular dystrophy patients from controls and other miRNAs are specific to the type of muscular dystrophy. Bioinformatics analysis of the target genes for the most significant miRNAs and the biological role of these genes revealed different pathways that the dysregulated miRNAs are involved in each type of muscular dystrophy investigated. In conclusion, this study shows unique signatures of small RNAs circulating in five types of muscular dystrophy patients and provides a useful resource for future studies for the development of miRNA biomarkers in muscular dystrophies and for their involvement in the pathogenesis of the disorders.

Mechanisms of microtubule organization in differentiated animal cells

Microtubules are polarized cytoskeletal filaments that serve as tracks for intracellular transport and form a scaffold that positions organelles and other cellular components and modulates cell shape and mechanics. In animal cells, the geometry, density and directionality of microtubule networks are major determinants of cellular architecture, polarity and proliferation. In dividing cells, microtubules form bipolar spindles that pull chromosomes apart, whereas in interphase cells, microtubules are organized in a cell type-specific fashion, which strongly correlates with cell physiology. In motile cells, such as fibroblasts and immune cells, microtubules are organized as radial asters, whereas in immotile epithelial and neuronal cells and in muscles, microtubules form parallel or antiparallel arrays and cortical meshworks. Here, we review recent work addressing how the formation of such microtubule networks is driven by the plethora of microtubule regulatory proteins. These include proteins that nucleate or anchor microtubule ends at different cellular structures and those that sever or move microtubules, as well as regulators of microtubule elongation, stability, bundling or modifications. The emerging picture, although still very incomplete, shows a remarkable diversity of cell-specific mechanisms that employ conserved building blocks to adjust microtubule organization in order to facilitate different cellular functions.

Understanding the molecular basis of cardiomyopathy

Inherited cardiomyopathies are a major cause of mortality and morbidity worldwide and can be caused by mutations in a wide range of proteins located in different cellular compartments. The present review is based on Dr. Ju Chen's 2021 Robert M. Berne Distinguished Lectureship of the American Physiological Society Cardiovascular Section, in which he provided an overview of the current knowledge on the cardiomyopathy-associated proteins that have been studied in his laboratory. The review provides a general summary of the proteins in different compartments of cardiomyocytes associated with cardiomyopathies, with specific focus on the proteins that have been studied in Dr. Chen's laboratory.

Impaired Intracellular Ca2+ Dynamics, M-Band and Sarcomere Fragility in Skeletal Muscles of Obscurin KO Mice

Obscurin is a giant sarcomeric protein expressed in striated muscles known to establish several interactions with other proteins of the sarcomere, but also with proteins of the sarcoplasmic reticulum and costameres. Here, we report experiments aiming to better understand the contribution of obscurin to skeletal muscle fibers, starting with a detailed characterization of the diaphragm muscle function, which we previously reported to be the most affected muscle in obscurin (Obscn) KO mice. Twitch and tetanus tension were not significantly different in the diaphragm of WT and Obscn KO mice, while the time to peak (TTP) and half relaxation time (HRT) were prolonged. Differences in force-frequency and force-velocity relationships and an enhanced fatigability are observed in an Obscn KO diaphragm with respect to WT controls. Voltage clamp experiments show that a sarcoplasmic reticulum's Ca2+ release and SERCA reuptake rates were decreased in muscle fibers from Obscn KO mice, suggesting that an impairment in intracellular Ca2+ dynamics could explain the observed differences in the TTP and HRT in the diaphragm. In partial contrast with previous observations, Obscn KO mice show a normal exercise tolerance, but fiber damage, the altered sarcomere ultrastructure and M-band disarray are still observed after intense exercise.

Molecular and cellular basis of genetically inherited skeletal muscle disorders

Neuromuscular disorders comprise a diverse group of human inborn diseases that arise from defects in the structure and/or function of the muscle tissue - encompassing the muscle cells (myofibres) themselves and their extracellular matrix - or muscle fibre innervation. Since the identification in 1987 of the first genetic lesion associated with a neuromuscular disorder - mutations in dystrophin as an underlying cause of Duchenne muscular dystrophy - the field has made tremendous progress in understanding the genetic basis of these diseases, with pathogenic variants in more than 500 genes now identified as underlying causes of neuromuscular disorders. The subset of neuromuscular disorders that affect skeletal muscle are referred to as myopathies or muscular dystrophies, and are due to variants in genes encoding muscle proteins. Many of these proteins provide structural stability to the myofibres or function in regulating sarcolemmal integrity, whereas others are involved in protein turnover, intracellular trafficking, calcium handling and electrical excitability - processes that ensure myofibre resistance to stress and their primary activity in muscle contraction. In this Review, we discuss how defects in muscle proteins give rise to muscle dysfunction, and ultimately to disease, with a focus on pathologies that are most common, best understood and that provide the most insight into muscle biology.

Ankyrin-dependent Na+ channel clustering prevents neuromuscular synapse fatigue

Skeletal muscle contraction depends on activation of clustered acetylcholine receptors (AchRs) and muscle-specific Na⁺ channels (Nav1.4). Some Nav1.4 channels are highly enriched at the neuromuscular junction (NMJ), and their clustering is thought to be essential for effective muscle excitation. However, this has not been experimentally tested, and how NMJ Na⁺ channels are clustered is unknown. Here, using muscle-specific ankyrinR, ankyrinB, and ankyrinG single, double, and triple-conditional knockout mice, we show that Nav1.4 channels fail to cluster only after deletion of all three ankyrins. Remarkably, ankyrin-deficient muscles have normal NMJ morphology, AchR clustering, sarcolemmal levels of Nav1.4, and muscle force, and they show no indication of degeneration. However, mice lacking clustered NMJ Na⁺ channels have significantly reduced levels of motor activity and their NMJs rapidly fatigue after repeated nerve-dependent stimulation. Thus, the triple redundancy of ankyrins facilitates NMJ Na⁺ channel clustering to prevent neuromuscular synapse fatigue.

PERM1 interacts with the MICOS-MIB complex to connect the mitochondria and sarcolemma via ankyrin B

Skeletal muscle subsarcolemmal mitochondria (SSM) and intermyofibrillar mitochondria subpopulations have distinct metabolic activity and sensitivity, though the mechanisms that localize SSM to peripheral areas of muscle fibers are poorly understood. A protein interaction study and complexome profiling identifies PERM1 interacts with the MICOS-MIB complex. Ablation of Perm1 in mice reduces muscle force, decreases mitochondrial membrane potential and complex I activity, and reduces the numbers of SSM in skeletal muscle. We demonstrate PERM1 interacts with the intracellular adaptor protein ankyrin B (ANKB) that connects the cytoskeleton to the plasma membrane. Moreover, we identify a C-terminal transmembrane helix that anchors PERM1 into the outer mitochondrial membrane. We conclude PERM1 functions in the MICOS-MIB complex and acts as an adapter to connect the mitochondria with the sarcolemma via ANKB.

Duchenne muscular dystrophy

Duchenne muscular dystrophy is a severe, progressive, muscle-wasting disease that leads to difficulties with movement and, eventually, to the need for assisted ventilation and premature death. The disease is caused by mutations in DMD (encoding dystrophin) that abolish the production of dystrophin in muscle. Muscles without dystrophin are more sensitive to damage, resulting in progressive loss of muscle tissue and function, in addition to cardiomyopathy. Recent studies have greatly deepened our understanding of the primary and secondary pathogenetic mechanisms. Guidelines for the multidisciplinary care for Duchenne muscular dystrophy that address obtaining a genetic diagnosis and managing the various aspects of the disease have been established. In addition, a number of therapies that aim to restore the missing dystrophin protein or address secondary pathology have received regulatory approval and many others are in clinical development.

Cargo hold and delivery: Ankyrins, spectrins, and their functional patterning of neurons

The highly polarized, typically very long, and nonmitotic nature of neurons present them with unique challenges in the maintenance of their homeostasis. This architectural complexity serves a rich and tightly controlled set of functions that enables their fast communication with neighboring cells and endows them with exquisite plasticity. The submembrane neuronal cytoskeleton occupies a pivotal position in orchestrating the structural patterning that determines local and long-range subcellular specialization, membrane dynamics, and a wide range of signaling events. At its center is the partnership between ankyrins and spectrins, which self-assemble with both remarkable long-range regularity and micro- and nanoscale specificity to precisely position and stabilize cell adhesion molecules, membrane transporters, ion channels, and other cytoskeletal proteins. To accomplish these generally conserved, but often functionally divergent and spatially diverse, roles these partners use a combinatorial program of a couple of dozens interacting family members, whose code is not fully unraveled. In a departure from their scaffolding roles, ankyrins and spectrins also enable the delivery of material to the plasma membrane by facilitating intracellular transport. Thus, it is unsurprising that deficits in ankyrins and spectrins underlie several neurodevelopmental, neurodegenerative, and psychiatric disorders. Here, I summarize key aspects of the biology of spectrins and ankyrins in the mammalian neuron and provide a snapshot of the latest advances in decoding their roles in the nervous system.

Cardiac regeneration and remodelling of the cardiomyocyte cytoarchitecture

Adult mammals are unable to regenerate their hearts after cardiac injury, largely due to the incapacity of cardiomyocytes (CMs) to undergo cell division. However, mammalian embryonic and fetal CMs, similar to CMs from fish and amphibians during their entire life, exhibit robust replicative activity, which stops abruptly after birth and never significantly resumes. Converging evidence indicates that formation of the highly ordered and stable cytoarchitecture of mammalian mature CMs is coupled with loss of their proliferative potential. Here, we review the available information on the role of the cardiac cytoskeleton and sarcomere in the regulation of CM proliferation. The actin cytoskeleton, the intercalated disc, the microtubular network and the dystrophin-glycoprotein complex each sense mechanical cues from the surrounding environment. Furthermore, they participate in the regulation of CM proliferation by impinging on the yes-associated protein/transcriptional co-activator with PDZ-binding motif, β-catenin and myocardin-related transcription factor transcriptional co-activators. Mastering the molecular mechanisms regulating CM proliferation would permit the development of innovative strategies to stimulate cardiac regeneration in adult individuals, a hitherto unachieved yet fundamental therapeutic goal.

Ankyrins in human health and disease - an update of recent experimental findings

Ankyrins are adaptor molecules that in eukaryotic cells form complexes with ion channel proteins, cell adhesion and signalling molecules and components of the cytoskeleton. They play a pivotal role as scaffolding proteins, in the structural anchoring to the muscle membrane, in muscle development, neurogenesis and synapse formation. Dysfunction of ankyrins is implicated in numerous diseases such as hereditary spherocytosis, neurodegeneration of Purkinje cells, cardiac arrhythmia, Brugada syndrome, bipolar disorders and schizophrenia, congenital myopathies and congenital heart disease as well as cancers. Detecting either down- or over-expression of ankyrins and ergo their use as biomarkers can provide a new paradigm in the diagnosis of these diseases. This paper provides an outline of knowledge about the structure of ankyrins, and by making use of recent experimental research studies critically discusses their role in several health disorders. Moreover, therapeutic options utilizing engineered ankyrins, designed ankyrin repeat proteins (DARPins), are discussed.

Skeletal muscle: A review of molecular structure and function, in health and disease

Decades of research in skeletal muscle physiology have provided multiscale insights into the structural and functional complexity of this important anatomical tissue, designed to accomplish the task of generating contraction, force and movement. Skeletal muscle can be viewed as a biomechanical device with various interacting components including the autonomic nerves for impulse transmission, vasculature for efficient oxygenation, and embedded regulatory and metabolic machinery for maintaining cellular homeostasis. The "omics" revolution has propelled a new era in muscle research, allowing us to discern minute details of molecular cross-talk required for effective coordination between the myriad interacting components for efficient muscle function. The objective of this review is to provide a systems-level, comprehensive mapping the molecular mechanisms underlying skeletal muscle structure and function, in health and disease. We begin this review with a focus on molecular mechanisms underlying muscle tissue development (myogenesis), with an emphasis on satellite cells and muscle regeneration. We next review the molecular structure and mechanisms underlying the many structural components of the muscle: neuromuscular junction, sarcomere, cytoskeleton, extracellular matrix, and vasculature surrounding muscle. We highlight aberrant molecular mechanisms and their possible clinical or pathophysiological relevance. We particularly emphasize the impact of environmental stressors (inflammation and oxidative stress) in contributing to muscle pathophysiology including atrophy, hypertrophy, and fibrosis. This article is categorized under: Physiology > Mammalian Physiology in Health and Disease Developmental Biology > Developmental Processes in Health and Disease Models of Systems Properties and Processes > Cellular Models.

Misplaced Golgi Elements Produce Randomly Oriented Microtubules and Aberrant Cortical Arrays of Microtubules in Dystrophic Skeletal Muscle Fibers

Differentiated mammalian cells and tissues, such as skeletal muscle fibers, acquire an organization of Golgi complex and microtubules profoundly different from that in proliferating cells and still poorly understood. In adult rodent skeletal muscle, the multinucleated muscle fibers have hundreds of Golgi elements (GE), small stacks of cisternae that serve as microtubule-organizing centers. We are interested in the role of the GE in organizing a peculiar grid of microtubules located in the fiber cortex, against the sarcolemma. Modifications of this grid in the mdx mouse model of Duchenne muscular dystrophy have led to identifying dystrophin, the protein missing in both human disease and mouse model, as a microtubule guide. Compared to wild-type (WT), mdx microtubules are disordered and more dense and they have been linked to the dystrophic pathology. GE themselves are disordered in mdx. Here, to identify the causes of GE and microtubule alterations in the mdx muscle, we follow GFP-tagged microtubule markers in live mdx fibers and investigate the recovery of GE and microtubules after treatment with nocodazole. We find that mdx microtubules grow 10% faster but in 30% shorter bouts and that they begin to form a tangled network, rather than an orthogonal grid, right after nucleation from GE. Strikingly, a large fraction of microtubules in mdx muscle fibers seem to dissociate from GE after nucleation. Moreover, we report that mdx GE are mispositioned and increased in number and size. These results were replicated in WT fibers overexpressing the beta-tubulin tubb6, which is elevated in Duchenne muscular dystrophy, in mdx and in regenerating muscle. Finally, we examine the association of GE with ER exit sites and ER-to-Golgi intermediate compartment, which starts during muscle differentiation, and find it persisting in mdx and tubb6 overexpressing fibers. We conclude that GE are full, small, Golgi complexes anchored, and positioned through ER Exit Sites. We propose a model in which GE mispositioning, together with the absence of microtubule guidance due to the lack of dystrophin, determines the differences in GE and microtubule organization between WT and mdx muscle fibers.

Muscle specific kinase protects dystrophic mdx mouse muscles from eccentric contraction-induced loss of force-producing capacity

KEY POINTS

Adeno-associated viral vector was used to elevate the expression of muscle specific kinase (MuSK) and rapsyn (a cytoplasmic MuSK effector protein) in the tibialis anterior muscle of wild-type and dystrophic (mdx) mice. In mdx mice, enhanced expression of either MuSK or rapsyn ameliorated the acute loss of muscle force associated with strain injury. Increases in sarcolemmal immunolabelling for utrophin and β-dystroglycan suggest a mechanism for the protective effect of MuSK in mdx muscles. MuSK also caused subtle changes to the structure and function of the neuromuscular junction, suggesting novel roles for MuSK in muscle physiology and pathophysiology.

ABSTRACT

Muscle specific kinase (MuSK) has a well-defined role in stabilizing the developing mammalian neuromuscular junction, but MuSK might also be protective in some neuromuscular diseases. In the dystrophin-deficient mdx mouse model of Duchenne muscular dystrophy, limb muscles are especially fragile. We injected the tibialis anterior muscle of 8-week-old mdx and wild-type (C57BL10) mice with adeno-associated viral vectors encoding either MuSK or rapsyn (a cytoplasmic MuSK effector protein) fused to green fluorescent protein (MuSK-GFP and rapsyn-GFP, respectively). Contralateral muscles injected with empty vector served as controls. One month later mice were anaesthetized with isoflurane and isometric force-producing capacity was recorded from the distal tendon. MuSK-GFP caused an unexpected decay in nerve-evoked tetanic force, both in wild-type and mdx muscles, without affecting contraction elicited by direct electrical stimulation of the muscle. Muscle fragility was probed by challenging muscles with a strain injury protocol consisting of a series of four strain-producing eccentric contractions in vivo. When applied to muscles of mdx mice, eccentric contraction produced an acute 27% reduction in directly evoked muscle force output, affirming the susceptibility of mdx muscles to strain injury. mdx muscles overexpressing MuSK-GFP or rapsyn-GFP exhibited significantly milder force deficits after the eccentric contraction challenge (15% and 14%, respectively). The protective effect of MuSK-GFP in muscles of mdx mice was associated with increased immunolabelling for utrophin and β-dystroglycan in the sarcolemma. Elevating the expression of MuSK or rapsyn revealed several distinct synaptic and extrasynaptic effects, suggesting novel roles for MuSK signalling in muscle physiology and pathophysiology.

Cardiac microtubules in health and heart disease

Cardiomyocytes are large (∼40,000 µm3), rod-shaped muscle cells that provide the working force behind each heartbeat. These highly structured cells are packed with dense cytoskeletal networks that can be divided into two groups—the contractile (i.e. sarcomeric) cytoskeleton that consists of filamentous actin-myosin arrays organized into myofibrils, and the non-sarcomeric cytoskeleton, which is composed of β- and γ-actin, microtubules, and intermediate filaments. Together, microtubules and intermediate filaments form a cross-linked scaffold, and these networks are responsible for the delivery of intracellular cargo, the transmission of mechanical signals, the shaping of membrane systems, and the organization of myofibrils and organelles. Microtubules are extensively altered as part of both adaptive and pathological cardiac remodeling, which has diverse ramifications for the structure and function of the cardiomyocyte. In heart failure, the proliferation and post-translational modification of the microtubule network is linked to a number of maladaptive processes, including the mechanical impediment of cardiomyocyte contraction and relaxation. This raises the possibility that reversing microtubule alterations could improve cardiac performance, yet therapeutic efforts will strongly benefit from a deeper understanding of basic microtubule biology in the heart. The aim of this review is to summarize the known physiological roles of the cardiomyocyte microtubule network, the consequences of its pathological remodeling, and to highlight the open and intriguing questions regarding cardiac microtubules.

Impact statement

Advancements in cell biological and biophysical approaches and super-resolution imaging have greatly broadened our view of tubulin biology over the last decade. In the heart, microtubules and microtubule-based transport help to organize and maintain key structures within the cardiomyocyte, including the sarcomere, intercalated disc, protein clearance machinery and transverse-tubule and sarcoplasmic reticulum membranes. It has become increasingly clear that post translational regulation of microtubules is a key determinant of their sub-cellular functionality. Alterations in microtubule network density, stability, and post-translational modifications are hallmarks of pathological cardiac remodeling, and modified microtubules can directly impede cardiomyocyte contractile function in various forms of heart disease. This review summarizes the functional roles and multi-leveled regulation of the cardiac microtubule cytoskeleton and highlights how refined experimental techniques are shedding mechanistic clarity on the regionally specified roles of microtubules in cardiac physiology and pathophysiology.

βII-spectrin promotes mouse brain connectivity through stabilizing axonal plasma membranes and enabling axonal organelle transport

βII-spectrin is the generally expressed member of the β-spectrin family of elongated polypeptides that form micrometer-scale networks associated with plasma membranes. We addressed in vivo functions of βII-spectrin in neurons by knockout of βII-spectrin in mouse neural progenitors. βII-spectrin deficiency caused severe defects in long-range axonal connectivity and axonal degeneration. βII-spectrin-null neurons exhibited reduced axon growth, loss of actin-spectrin-based periodic membrane skeleton, and impaired bidirectional axonal transport of synaptic cargo. We found that βII-spectrin associates with KIF3A, KIF5B, KIF1A, and dynactin, implicating spectrin in the coupling of motors and synaptic cargo. βII-spectrin required phosphoinositide lipid binding to promote axonal transport and restore axon growth. Knockout of ankyrin-B (AnkB), a βII-spectrin partner, primarily impaired retrograde organelle transport, while double knockout of βII-spectrin and AnkB nearly eliminated transport. Thus, βII-spectrin promotes both axon growth and axon stability through establishing the actin-spectrin-based membrane-associated periodic skeleton as well as enabling axonal transport of synaptic cargo.

ANK2 autism mutation targeting giant ankyrin-B promotes axon branching and ectopic connectivity

Giant ankyrin-B (ankB) is a neurospecific alternatively spliced variant of ANK2, a high-confidence autism spectrum disorder (ASD) gene. We report that a mouse model for human ASD mutation of giant ankB exhibits increased axonal branching in cultured neurons with ectopic CNS axon connectivity, as well as with a transient increase in excitatory synapses during postnatal development. We elucidate a mechanism normally limiting axon branching, whereby giant ankB localizes to periodic axonal plasma membrane domains through L1 cell-adhesion molecule protein, where it couples microtubules to the plasma membrane and prevents microtubule entry into nascent axon branches. Giant ankB mutation or deficiency results in a dominantly inherited impairment in selected communicative and social behaviors combined with superior executive function. Thus, gain of axon branching due to giant ankB-deficiency/mutation is a candidate cellular mechanism to explain aberrant structural connectivity and penetrant behavioral consequences in mice as well as humans bearing ASD-related ANK2 mutations.

Persistent upregulation of the β-tubulin tubb6, linked to muscle regeneration, is a source of microtubule disorganization in dystrophic muscle

In healthy adult skeletal muscle fibers microtubules form a three-dimensional grid-like network. In the mdx mouse, a model of Duchenne muscular dystrophy (DMD), microtubules are mostly disordered, without periodicity. These microtubule defects have been linked to the mdx mouse pathology. We now report that increased expression of the beta 6 class V β-tubulin (tubb6) contributes to the microtubule changes of mdx muscles. Wild-type muscle fibers overexpressing green fluorescent protein (GFP)-tubb6 (but not GFP-tubb5) have disorganized microtubules whereas mdx muscle fibers depleted of tubb6 (but not of tubb5) normalize their microtubules, suggesting that increasing tubb6 is toxic. However, tubb6 increases spontaneously during differentiation of mouse and human muscle cultures. Furthermore, endogenous tubb6 is not uniformly expressed in mdx muscles but is selectively increased in fiber clusters, which we identify as regenerating. Similarly, mdx-based rescued transgenic mice that retain a higher than expected tubb6 level show focal expression of tubb6 in subsets of fibers. Tubb6 is also upregulated in cardiotoxin-induced mouse muscle regeneration, in human myositis and DMD biopsies, and the tubb6 level correlates with that of embryonic myosin heavy chain, a regeneration marker. In conclusion, modulation of a β-tubulin isotype plays a role in muscle differentiation and regeneration. Increased tubb6 expression and microtubule reorganization are not pathological per se but reflect a return to an earlier developmental stage. However, chronic elevation of tubb6, as occurs in the mdx mouse, may contribute to the repeated cycles of regeneration and to the pathology of the disease.

Variable rescue of microtubule and physiological phenotypes in mdx muscle expressing different miniaturized dystrophins

Delivery of miniaturized dystrophin genes via adeno-associated viral vectors is one leading approach in development to treat Duchenne muscular dystrophy. Here we directly compared the functionality of five mini- and micro-dystrophins via skeletal muscle-specific transgenic expression in dystrophin-deficient mdx mice. We evaluated their ability to rescue defects in the microtubule network, passive stiffness and contractility of skeletal muscle. Transgenic mdx mice expressing the short dystrophin isoform Dp116 served as a negative control. All mini- and micro-dystrophins restored elevated detyrosinated α-tubulin and microtubule density of mdx muscle to values not different from C57BL/10, however, only mini-dystrophins restored the transverse component of the microtubule lattice back to C57BL/10. Passive stiffness values in mdx muscles expressing mini- or micro-dystrophins were not different from C57BL/10. While all mini- and micro-dystrophins conferred significant protection from eccentric contraction-induced force loss in vivo and ex vivo compared to mdx, removal of repeats two and three resulted in less protection from force drop caused by eccentric contraction ex vivo. Our data reveal subtle yet significant differences in the relative functionalities for different therapeutic constructs of miniaturized dystrophin in terms of protection from ex vivo eccentric contraction-induced force loss and restoration of an organized microtubule lattice.

Syntrophins entangled in cytoskeletal meshwork: Helping to hold it all together

Syntrophins are a family of 59 kDa peripheral membrane-associated adapter proteins, containing multiple protein-protein and protein-lipid interaction domains. The syntrophin family consists of five isoforms that exhibit specific tissue distribution, distinct sub-cellular localization and unique expression patterns implying their diverse functional roles. These syntrophin isoforms form multiple functional protein complexes and ensure proper localization of signalling proteins and their binding partners to specific membrane domains and provide appropriate spatiotemporal regulation of signalling pathways. Syntrophins consist of two PH domains, a PDZ domain and a conserved SU domain. The PH1 domain is split by the PDZ domain. The PH2 and the SU domain are involved in the interaction between syntrophin and the dystrophin-glycoprotein complex (DGC). Syntrophins recruit various signalling proteins to DGC and link extracellular matrix to internal signalling apparatus via DGC. The different domains of the syntrophin isoforms are responsible for modulation of cytoskeleton. Syntrophins associate with cytoskeletal proteins and lead to various cellular responses by modulating the cytoskeleton. Syntrophins are involved in many physiological processes which involve cytoskeletal reorganization like insulin secretion, blood pressure regulation, myogenesis, cell migration, formation and retraction of focal adhesions. Syntrophins have been implicated in various pathologies like Alzheimer's disease, muscular dystrophy, cancer. Their role in cytoskeletal organization and modulation makes them perfect candidates for further studies in various cancers and other ailments that involve cytoskeletal modulation. The role of syntrophins in cytoskeletal organization and modulation has not yet been comprehensively reviewed till now. This review focuses on syntrophins and highlights their role in cytoskeletal organization, modulation and dynamics via its involvement in different cell signalling networks.

Independent variability of microtubule perturbations associated with dystrophinopathy

Absence of the protein dystrophin causes Duchenne muscular dystrophy. Dystrophin directly binds to microtubules in vitro, and its absence in vivo correlates with disorganization of the subsarcolemmal microtubule lattice, increased detyrosination of α-tubulin, and altered redox signaling. We previously demonstrated that the dystrophin homologue utrophin neither binds microtubules in vitro nor rescues microtubule lattice organization when overexpressed in muscles of dystrophin-deficient mdx mice. Here, we fine-mapped the dystrophin domain necessary for microtubule binding to spectrin-like repeats 20–22. We show that transgenic mdx mice expressing a full-length dystrophin/utrophin chimera completely lacking microtubule binding activity are surprisingly rescued for all measured dystrophic phenotypes, including full restoration of microtubule lattice organization. Conversely, despite the presence of dystrophin at the sarcolemma, β-sarcoglycan-deficient skeletal muscle presents with a disorganized and densified microtubule lattice. Finally, we show that the levels of α-tubulin detyrosination remain significantly elevated to that of mdx levels in transgenic mdx mice expressing nearly full-length dystrophin. Our results demonstrate that the microtubule-associated perturbations of mdx muscle are distinct, separable, and can vary independently from other parameters previously ascribed to dystrophin deficiency.

Impaired muscle relaxation and mitochondrial fission associated with genetic ablation of cytoplasmic actin isoforms

While α-actin isoforms predominate in adult striated muscle, skeletal muscle-specific knockouts (KOs) of nonmuscle cytoplasmic β_cyto - or γ_cyto -actin each cause a mild, but progressive myopathy effected by an unknown mechanism. Using transmission electron microscopy, we identified morphological abnormalities in both the mitochondria and the sarcoplasmic reticulum (SR) in aged muscle-specific β_cyto - and γ_cyto -actin KO mice. We found β_cyto - and γ_cyto -actin proteins to be enriched in isolated mitochondrial-associated membrane preparations, which represent the interface between mitochondria and sarco-endoplasmic reticulum important in signaling and mitochondrial dynamics. We also measured significantly elongated and interconnected mitochondrial morphologies associated with a significant decrease in mitochondrial fission events in primary mouse embryonic fibroblasts lacking β_cyto - and/or γ_cyto -actin. Interestingly, mitochondrial respiration in muscle was not measurably affected as oxygen consumption was similar in skeletal muscle fibers from 12 month-old muscle-specific β_cyto - and γ_cyto -actin KO mice. Instead, we found that the maximal rate of relaxation after isometric contraction was significantly slowed in muscles of 12-month-old β_cyto - and γ_cyto -actin muscle-specific KO mice. Our data suggest that impaired Ca²⁺ re-uptake may presage development of the observed SR morphological changes in aged mice while providing a potential pathological mechanism for the observed myopathy.

Supramolecular aggregation of aquaporin-4 is different in muscle and brain: correlation with tissue susceptibility in neuromyelitis optica

Neuromyelitis optica (NMO) is an autoimmune demyelinating disease of the central nervous system (CNS) caused by autoantibodies (NMO‐IgG) against the water channel aquaporin‐4 (AQP4). Though AQP4 is also expressed outside the CNS, for example in skeletal muscle, patients with NMO generally do not show clinical/diagnostic evidence of skeletal muscle damage. Here, we have evaluated whether AQP4 supramolecular organization is at the basis of the different tissue susceptibility. Using immunofluorescence we found that while the sera of our cohort of patients with NMO gave typical perivascular staining in the CNS, they were largely negative in the skeletal muscle. This conclusion was obtained using human, rat and mouse skeletal muscle including the AQP4‐KO mouse. A biochemical analysis using a new size exclusion chromatography approach for AQP4 suprastructure fractionation revealed substantial differences in supramolecular AQP4 assemblies and isoform abundance between brain and skeletal muscle matching a lower binding affinity of NMO‐IgG to muscle compared to the brain. Super‐resolution microscopy analysis with g‐STED revealed different AQP4 organization in native tissues, while in the brain perivascular astrocyte endfoot membrane AQP4 was mainly organized in large interconnected and raft‐like clusters, in the sarcolemma of fast‐twitch fibres AQP4 aggregates often appeared as small, relatively isolated linear entities. In conclusion, our results provide evidence that AQP4 supramolecular structure is different in brain and skeletal muscle, which is likely to result in different tissues susceptibility to the NMO disease.

Dual AAV Gene Therapy for Duchenne Muscular Dystrophy with a 7-kb Mini-Dystrophin Gene in the Canine Model

Dual adeno-associated virus (AAV) technology was developed in 2000 to double the packaging capacity of the AAV vector. The proof of principle has been demonstrated in various mouse models. Yet, pivotal evidence is lacking in large animal models of human diseases. Here we report expression of a 7-kb canine ΔH2-R15 mini-dystrophin gene using a pair of dual AAV vectors in the canine model of Duchenne muscular dystrophy (DMD). The ΔH2-R15 minigene is by far the most potent synthetic dystrophin gene engineered for DMD gene therapy. We packaged minigene dual vectors in Y731F tyrosine-modified AAV-9 and delivered to the extensor carpi ulnaris muscle of a 12-month-old affected dog at the dose of 2 × 10¹³ viral genome particles/vector/muscle. Widespread mini-dystrophin expression was observed 2 months after gene transfer. The missing dystrophin-associated glycoprotein complex was restored. Treatment also reduced muscle degeneration and fibrosis and improved myofiber size distribution. Importantly, dual AAV therapy greatly protected the muscle from eccentric contraction-induced force loss. Our data provide the first clear evidence that dual AAV therapy can be translated to a diseased large mammal. Further development of dual AAV technology may lead to effective therapies for DMD and many other diseases in human patients.

CaMKII inhibition mitigates ischemia/reperfusion-elicited calpain activation and the damage to membrane skeleton proteins in isolated rat hearts

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) has been implicated in myocardial ischemia/reperfusion (IR) injury. The aim of this study was to determine the effect of CaMKII on the damage to membrane skeleton proteins, which is an important cause of IR injury. Isolated rat hearts were subjected to 45-min global ischemia/2-h reperfusion. Both KN-62 and KN-93 were used to inhibit CaMKII. Compared with controls, the hearts in the IR group exhibited remarkable myocardial injury area, LDH release, cell apoptosis and contractile dysfunction, along with an increase in the phosphorylation of CaMKII and its substrate phospholamban. Treatment with either KN-62 or KN-93 mitigated both the heart injury and the phosphorylation of CaMKII and phospholamban. The analysis of cell skeleton proteins revealed that IR injury resulted in an increase in the 150-kDa fragments resulting from the degradation of α-fodrin and dystrophin translocating from the sarcolemmal membrane to the cytosol and a decrease in the 220-kDa isoform of ankyrin-B. As expected, Evans blue dye staining showed an increase in membrane permeability or membrane rupture in the IR group. All of these alterations were alleviated by treatment with either KN-62 or KN-93. In addition, both KN-62 and KN-93 blocked the activity and membrane recruitment of calpain, a key protease responsible for destroying cell skeleton proteins during IR injury. In conclusion, our data provide evidence that damage to membrane skeleton proteins via calpain is a destructive downstream event of CaMKII activation in the setting of myocardial IR injury.

Obscure functions: the location-function relationship of obscurins

The obscurin family of polypeptides is essential for normal striated muscle function and contributes to the pathogenesis of fatal diseases, including cardiomyopathies and cancers. The single mammalian obscurin gene, OBSCN, gives rise to giant (∼800 kDa) and smaller (∼40-500 kDa) proteins that are composed of tandem adhesion and signaling motifs. Mammalian obscurin proteins are expressed in a variety of cell types, including striated muscles, and localize to distinct subcellular compartments where they contribute to diverse cellular processes. Obscurin homologs in Caenorhabditis elegans and Drosophila possess a similar domain architecture and are also expressed in striated muscles. The long sought after question, "what does obscurin do?" is complex and cannot be addressed without taking into consideration the subcellular distribution of these proteins and local isoform concentration. Herein, we present an overview of the functions of obscurins and begin to define the intricate relationship between their subcellular distributions and functions in striated muscles.

Exercise-induced alterations and loss of sarcomeric M-line organization in the diaphragm muscle of obscurin knockout mice

We recently reported that skeletal muscle fibers of obscurin knockout (KO) mice present altered distribution of ankyrin B (ankB), disorganization of the subsarcolemmal microtubules, and reduced localization of dystrophin at costameres. In addition, these mice have impaired running endurance and increased exercise-induced sarcolemmal damage compared with wild-type animals. Here, we report results from a combined approach of physiological, morphological, and structural studies in which we further characterize the skeletal muscles of obscurin KO mice. A detailed examination of exercise performance, using different running protocols, revealed that the reduced endurance of obscurin KO animals on the treadmill depends on exercise intensity and age. Indeed, a mild running protocol did not evidence significant differences between control and obscurin KO mice, whereas comparison of running abilities of 2-, 6-, and 11-mo-old mice exercised at exhaustion revealed a progressive age-dependent reduction of the exercise tolerance in KO mice. Histological analysis indicated that heavy exercise induced leukocyte infiltration, fibrotic connective tissue deposition, and hypercontractures in the diaphragm of KO mice. On the same line, electron microscopy revealed that, in the diaphragm of exercised obscurin KO mice, but not in the hindlimb muscles, both M-line and H-zone of sarcomeres appeared wavy and less defined. Altogether, these results suggest that obscurin is required for the maintenance of morphological and ultrastructural integrity of skeletal muscle fibers against damage induced by intense mechanical stress and point to the diaphragm as the skeletal muscle most severely affected in obscurin-deficient mice.

Ankyrin-B is a PI3P effector that promotes polarized α5β1-integrin recycling via recruiting RabGAP1L to early endosomes

Endosomal membrane trafficking requires coordination between phosphoinositide lipids, Rab GTPases, and microtubule-based motors to dynamically determine endosome identity and promote long-range organelle transport. Here we report that ankyrin-B (AnkB), through integrating all three systems, functions as a critical node in the protein circuitry underlying polarized recycling of α5β1-integrin in mouse embryonic fibroblasts, which enables persistent fibroblast migration along fibronectin gradients. AnkB associates with phosphatidylinositol 3-phosphate (PI3P)-positive organelles in fibroblasts and binds dynactin to promote their long-range motility. We demonstrate that AnkB binds to Rab GTPase Activating Protein 1-Like (RabGAP1L) and recruits it to PI3P-positive organelles, where RabGAP1L inactivates Rab22A, and promotes polarized trafficking to the leading edge of migrating fibroblasts. We further determine that α5β1-integrin depends on an AnkB/RabGAP1L complex for polarized recycling. Our results reveal AnkB as an unexpected key element in coordinating polarized transport of α5β1-integrin and likely of other specialized endocytic cargos.

Triadin and CLIMP-63 form a link between triads and microtubules in muscle cells

In skeletal muscle, the triad is a structure comprising a transverse (T)-tubule and sarcoplasmic reticulum (SR) cisternae. Triads constitute the basis of excitation-contraction coupling as the cradle of the Ca²⁺ release complex. We have shown previously that triadin, a member of this complex, has shaping properties on reticulum membrane and is indirectly involved in a link between triads and microtubules. We have identified here that CLIMP-63 (also known as CKAP4), as the partner of triadin, is responsible for this association of triads and microtubules. Triadin and CLIMP-63 interact through their respective luminal domains and the shaping properties of triadin depend on the capacity of CLIMP-63 to bind microtubules with its cytosolic portion. In skeletal muscle, CLIMP-63 is localized in the SR, including triads, and is associated with the Ca²⁺ release complex through its interaction with triadin. Knockout of triadin in muscles results in the delocalization of CLIMP-63 from triads, its dissociation from the Ca²⁺ release complex and a disorganization of the microtubule network. Our results suggest that the association of triadin and CLIMP-63 could be involved in the shaping of SR terminal cisternae and in the guidance of microtubules close to the triads.

Ankyrin-B in lens architecture and biomechanics: Just not tethering but more

The ankyrins are a family of well-characterized metazoan adaptor proteins that play a key role in linking various membrane-spanning proteins to the underlying spectrin-actin cytoskeleton; a mechanistic understanding of their role in tissue architecture and mechanics, however, remains elusive. Here we comment on a recent study demonstrating a key role for ankyrin-B in maintaining the hexagonal shape and radial alignment of ocular lens fiber cells by regulating the membrane organization of periaxin, dystrophins/dystroglycan, NrCAM and spectrin-actin network of proteins, and revealing that ankyrin-B deficiency impairs fiber cell shape and mechanical properties of the ocular lens. These observations indicate that ankyrin-B plays an important role in maintaining tissue cytoarchitecture, cell shape and biomechanical properties via engaging in key protein: protein interactions required for membrane anchoring and organization of the spectrin-actin skeleton, scaffolding proteins and cell adhesive proteins.

Dysfunction of the β2-spectrin-based pathway in human heart failure

β2-Spectrin is critical for integrating membrane and cytoskeletal domains in excitable and nonexcitable cells. The role of β2-spectrin for vertebrate function is illustrated by dysfunction of β2-spectrin-based pathways in disease. Recently, defects in β2-spectrin association with protein partner ankyrin-B were identified in congenital forms of human arrhythmia. However, the role of β2-spectrin in common forms of acquired heart failure and arrhythmia is unknown. We report that β2-spectrin protein levels are significantly altered in human cardiovascular disease as well as in large and small animal cardiovascular disease models. Specifically, β2-spectrin levels were decreased in atrial samples of patients with atrial fibrillation compared with tissue from patients in sinus rhythm. Furthermore, compared with left ventricular samples from nonfailing hearts, β2-spectrin levels were significantly decreased in left ventricle of ischemic- and nonischemic heart failure patients. Left ventricle samples of canine and murine heart failure models confirm reduced β2-spectrin protein levels. Mechanistically, we identify that β2-spectrin levels are tightly regulated by posttranslational mechanisms, namely Ca(2+)- and calpain-dependent proteases. Furthermore, consistent with this data, we observed Ca(2+)- and calpain-dependent loss of β2-spectrin downstream effector proteins, including ankyrin-B in heart. In summary, our findings illustrate that β2-spectrin and downstream molecules are regulated in multiple forms of cardiovascular disease via Ca(2+)- and calpain-dependent proteolysis.

The Dystrophin Complex: Structure, Function, and Implications for Therapy

The dystrophin complex stabilizes the plasma membrane of striated muscle cells. Loss of function mutations in the genes encoding dystrophin, or the associated proteins, trigger instability of the plasma membrane, and myofiber loss. Mutations in dystrophin have been extensively cataloged, providing remarkable structure-function correlation between predicted protein structure and clinical outcomes. These data have highlighted dystrophin regions necessary for in vivo function and fueled the design of viral vectors and now, exon skipping approaches for use in dystrophin restoration therapies. However, dystrophin restoration is likely more complex, owing to the role of the dystrophin complex as a broad cytoskeletal integrator. This review will focus on dystrophin restoration, with emphasis on the regions of dystrophin essential for interacting with its associated proteins and discuss the structural implications of these approaches.

Structural basis for the membrane association of ankyrinG via palmitoylation

By clustering various ion channels and transporters, ankyrin-G (AnkG) configures the membrane-excitation platforms in neurons and cardiomyocytes. AnkG itself localizes to specific areas on the plasma membrane via s-palmitoylation of Cys. However, the structural mechanism by which AnkG anchors to the membrane is not understood. In this study, we solved the crystal structures of the reduced and oxidized forms of the AnkG s-palmitoylation domain and used multiple long-term coarse-grained molecular dynamics simulations to analyze their membrane association. Here we report that the membrane anchoring of AnkG was facilitated by s-palmitoylation, defining a stable binding interface on the lipid membrane, and that AnkG without s-palmitoylation also preferred to stay near the membrane but did not have a unique binding interface. This suggests that AnkG in the juxtamembrane region is primed to accept lipid modification at Cys, and once that happens AnkG constitutes a rigid structural base upon which a membrane-excitation platform can be assembled.

An Adaptable Spectrin/Ankyrin-Based Mechanism for Long-Range Organization of Plasma Membranes in Vertebrate Tissues

Ankyrins are membrane-associated proteins that together with their spectrin partners are responsible for micron-scale organization of vertebrate plasma membranes, including those of erythrocytes, excitable membranes of neurons and heart, lateral membrane domains of columnar epithelial cells, and striated muscle. Ankyrins coordinate functionally related membrane transporters and cell adhesion proteins (15 protein families identified so far) within plasma membrane compartments through independently evolved interactions of intrinsically disordered sequences with a highly conserved peptide-binding groove formed by the ANK repeat solenoid. Ankyrins are coupled to spectrins, which are elongated organelle-sized proteins that form mechanically resilient arrays through cross-linking by specialized actin filaments. In addition to protein interactions, cellular targeting and assembly of spectrin/ankyrin domains also critically depend on palmitoylation of ankyrin-G by aspartate-histidine-histidine-cysteine 5/8 palmitoyltransferases, as well as interaction of beta-2 spectrin with phosphoinositide lipids. These lipid-dependent spectrin/ankyrin domains are not static but are locally dynamic and determine membrane identity through opposing endocytosis of bulk lipids as well as specific proteins. A partnership between spectrin, ankyrin, and cell adhesion molecules first emerged in bilaterians over 500 million years ago. Ankyrin and spectrin may have been recruited to plasma membranes from more ancient roles in organelle transport. The basic bilaterian spectrin-ankyrin toolkit markedly expanded in vertebrates through gene duplications combined with variation in unstructured intramolecular regulatory sequences as well as independent evolution of ankyrin-binding activity by ion transporters involved in action potentials and calcium homeostasis. In addition, giant vertebrate ankyrins with specialized roles in axons acquired new coding sequences by exon shuffling. We speculate that early axon initial segments and epithelial lateral membranes initially were based on spectrin-ankyrin-cell adhesion molecule assemblies and subsequently served as "incubators," where ion transporters independently acquired ankyrin-binding activity through positive selection.

Ankyrin-B directs membrane tethering of periaxin and is required for maintenance of lens fiber cell hexagonal shape and mechanics

Periaxin (Prx), a PDZ domain protein expressed preferentially in myelinating Schwann cells and lens fibers, plays a key role in membrane scaffolding and cytoarchitecture. Little is known, however, about how Prx is anchored to the plasma membrane. Here we report that ankyrin-B (AnkB), a well-characterized adaptor protein involved in linking the spectrin-actin cytoskeleton to integral membrane proteins, is required for membrane association of Prx in lens fibers and colocalizes with Prx in hexagonal fiber cells. Under AnkB haploinsufficiency, Prx accumulates in the soluble fraction with a concomitant loss from the membrane-enriched fraction of mouse lenses. Moreover, AnkB haploinsufficiency induced age-dependent disruptions in fiber cell hexagonal geometry and radial alignment and decreased compressive stiffness in mouse lenses parallel to the changes observed in Prx null mouse lens. Both AnkB- and Prx-deficient mice exhibit disruptions in membrane organization of the spectrin-actin network and the dystrophin-glycoprotein complex in lens fiber cells. Taken together, these observations reveal that AnkB is required for Prx membrane anchoring and for maintenance of lens fiber cell hexagonal geometry, membrane skeleton organization, and biomechanics.