Membrane repair of human skeletal muscle cells requires Annexin-A5

Cytotoxic T-Lymphocyte-Associated Protein 4 Fused to a Modified Fragment of IgG1 Reduces Muscle Fiber Damage in a Model of Duchenne Muscular Dystrophy by Attenuating Proinflammatory Gene Expression in Myeloid Lineage Cells

Duchenne muscular dystrophy (DMD) is a lethal, muscle-wasting, genetic disease that is greatly amplified by an immune response to the diseased muscles. The mdx mouse model of DMD was used to test whether the pathology can be reduced by treatment with a cytotoxic T-lymphocyte-associated protein 4 fused to a modified fragment of IgG1 (CTLA4-Ig) fusion protein that blocks costimulatory signals required for activation of T cells. CTLA4-Ig treatment reduced mdx sarcolemma lesions and reduced the numbers of activated T cells, macrophages, and antigen-presenting cells in mdx muscle and reduced macrophage invasion into muscle fibers. In vitro data showed that CTLA4-Ig acts directly on bone marrow cells and macrophages to modify their function and gene expression. CTLA4-Ig treatment of mdx bone marrow cells diminished their mobility and chemotactic response to chemokine ligand-2. Treating mdx macrophages with CTLA4-Ig reduced their cytolysis of muscle cells in vitro. RNA-sequencing analysis of mdx macrophages showed that CTLA4-Ig reduced expression of genes associated with leukocyte chemotaxis, migration, and extravasation; >90% of those affected genes were tumor necrosis factor-α target genes. Comparison of mdx and wild-type macrophages by RNA sequencing showed that 46% of the genes down-regulated by CTLA4-Ig were genes up-regulated in macrophages by the presence of muscular dystrophy in mice. These findings show that CTLA4-Ig is a promising immunotherapeutic for DMD, and many of its beneficial effects may result from direct actions on macrophages that modify their expression of proinflammatory genes.

Human skeletal muscle fiber heterogeneity beyond myosin heavy chains

Skeletal muscle is a heterogenous tissue comprised primarily of myofibers, commonly classified into three fiber types in humans: one "slow" (type 1) and two "fast" (type 2A and type 2X). However, heterogeneity between and within traditional fiber types remains underexplored. We applied transcriptomic and proteomic workflows to 1050 and 1038 single myofibers from human vastus lateralis, respectively. Proteomics was conducted in males, while transcriptomics included ten males and two females. We identify metabolic, ribosomal, and cell junction proteins, in addition to myosin heavy chain isoforms, as sources of multi-dimensional variation between myofibers. Furthermore, whilst slow and fast fiber clusters are identified, our data suggests that type 2X fibers are not phenotypically distinct to other fast fibers. Moreover, myosin heavy chain-based classifications do not adequately describe the phenotype of myofibers in nemaline myopathy. Overall, our data indicates that myofiber heterogeneity is multi-dimensional with sources of variation beyond myosin heavy chain isoforms.

A Novel Assay Reveals the Early Setting-Up of Membrane Repair Machinery in Human Skeletal Muscle Cells

Defect in membrane repair contributes to the development of muscular dystrophies such as limb girdle muscular dystrophy (LGMD) type R2 or R12. Nevertheless, many other muscular dystrophies may also result from a defect in this process. Identifying these pathologies requires the development of specific methods to inflict sarcolemma damage on a large number of cells and rapidly analyze their response. We adapted a protocol hitherto used to study the behavior of cancer cells to mechanical constraint. This method is based on forcing the passage of cells through a thin needle, which induces shear stress. Due to size considerations, this method requires working with mononuclear muscle cells instead of myotubes or muscle fibers. Although functional sarcolemma repair was thought to be restricted to myotubes and muscle fibers, we show here that 24h-differentiated myoblasts express a complete machinery capable of addressing membrane damage. At this stage, muscle cells do not yet form myotubes, revealing that the membrane repair machinery is set up early throughout the differentiation process. When submitted to the shear-stress assay, these cells were observed to repair membrane damage in a Ca2+-dependent manner, as previously reported. We show that this technique is able to identify the absence of membrane resealing in muscle cells from patient suffering from LGMDR2. The proposed technique provides therefore a suitable method for identifying cellular dysregulations in membrane repair of dystrophic human muscle cells.

Calcium influx rapidly establishes distinct spatial recruitments of Annexins to cell wounds

To survive daily damage, the formation of actomyosin ring at the wound edge is required to rapidly close cell wounds. Calcium influx is one of the start signals for these cell wound repair events. Here, we find that the rapid recruitment of all 3 Drosophila calcium-responding and phospholipid-binding Annexin proteins (AnxB9, AnxB10, and AnxB11) to distinct regions around the wound is regulated by the quantity of calcium influx rather than their binding to specific phospholipids. The distinct recruitment patterns of these Annexins regulate the subsequent recruitment of RhoGEF2 and RhoGEF3 through actin stabilization to form a robust actomyosin ring. Surprisingly, while the wound does not close in the absence of calcium influx, we find that reduced calcium influx can still initiate repair processes, albeit leading to severe repair phenotypes. Thus, our results suggest that, in addition to initiating repair events, the quantity of calcium influx is important for precise Annexin spatiotemporal protein recruitment to cell wounds and efficient wound repair.

On the role of dysferlin in striated muscle: membrane repair, t-tubules and Ca2+ handling

Dysferlin is a 237 kDa membrane-associated protein characterised by multiple C2 domains with a diverse role in skeletal and cardiac muscle physiology. Mutations in DYSF are known to cause various types of human muscular dystrophies, known collectively as dysferlinopathies, with some patients developing cardiomyopathy. A myriad of in vitro membrane repair studies suggest that dysferlin plays an integral role in the membrane repair complex in skeletal muscle. In comparison, less is known about dysferlin in the heart, but mounting evidence suggests that dysferlin's role is similar in both muscle types. Recent findings have shown that dysferlin regulates Ca2+ handling in striated muscle via multiple mechanisms and that this becomes more important in conditions of stress. Maintenance of the transverse (t)-tubule network and the tight coordination of excitation-contraction coupling are essential for muscle contractility. Dysferlin regulates the maintenance and repair of t-tubules, and it is suspected that dysferlin regulates t-tubules and sarcolemmal repair through a similar mechanism. This review focuses on the emerging complexity of dysferlin's activity in striated muscle. Such insights will progress our understanding of the proteins and pathways that regulate basic heart and skeletal muscle function and help guide research into striated muscle pathology, especially that which arises due to dysferlin dysfunction.

Annexins-a family of proteins with distinctive tastes for cell signaling and membrane dynamics

Annexins are cytosolic proteins with conserved three-dimensional structures that bind acidic phospholipids in cellular membranes at elevated Ca2+ levels. Through this they act as Ca2+-regulated membrane binding modules that organize membrane lipids, facilitating cellular membrane transport but also displaying extracellular activities. Recent discoveries highlight annexins as sensors and regulators of cellular and organismal stress, controlling inflammatory reactions in mammals, environmental stress in plants, and cellular responses to plasma membrane rupture. Here, we describe the role of annexins as Ca2+-regulated membrane binding modules that sense and respond to cellular stress and share our view on future research directions in the field.

Wound Repair of the Cell Membrane: Lessons from Dictyostelium Cells

The cell membrane is frequently subjected to damage, either through physical or chemical means. The swift restoration of the cell membrane's integrity is crucial to prevent the leakage of intracellular materials and the uncontrolled influx of extracellular ions. Consequently, wound repair plays a vital role in cell survival, akin to the importance of DNA repair. The mechanisms involved in wound repair encompass a series of events, including ion influx, membrane patch formation, endocytosis, exocytosis, recruitment of the actin cytoskeleton, and the elimination of damaged membrane sections. Despite the absence of a universally accepted general model, diverse molecular models have been proposed for wound repair in different organisms. Traditional wound methods not only damage the cell membrane but also impact intracellular structures, including the underlying cortical actin networks, microtubules, and organelles. In contrast, the more recent improved laserporation selectively targets the cell membrane. Studies on Dictyostelium cells utilizing this method have introduced a novel perspective on the wound repair mechanism. This review commences by detailing methods for inducing wounds and subsequently reviews recent developments in the field.

Towards multi-target glioblastoma therapy: Structural, distribution, and functional insights into protein target candidates

Glioblastoma is the most commonly occurring and most lethal primary brain tumor. Treatment options are limited in number and therapeutic development remains a major challenge. However, substantial progress has been made in better understanding the underlying biology of the disease. A recent proteomic meta-analysis revealed that 270 proteins were commonly dysregulated in glioblastoma, highlighting the complexity of the disease. This motivated us to explore potential protein targets which may be collectively inhibited, based on common upregulation, as part of a multi-target therapeutic strategy. Herein, we identify and characterize structural attributes relevant to the druggability of six protein target candidates. Computational analysis of crystal structures revealed druggable cavities in each of these proteins, and various parameters of these cavities were determined. For proteins with inhibitor-bound structures available, inhibitor compounds were found to overlap with the computationally determined cavities upon structural alignment. We also performed bioinformatic analysis for normal transcriptional expression distribution of these proteins across various brain regions and various tissues, as well as gene ontology curation to gain functional insights, as this information is useful for understanding the potential for off-target adverse effects. Our findings represent initial steps towards the development of multi-target glioblastoma therapy and may aid future work exploring similar therapeutic strategies.

Inhibition of the membrane repair protein annexin-A2 prevents tumor invasion and metastasis

Cancer cells are exposed to major compressive and shearing forces during invasion and metastasis, leading to extensive plasma membrane damage. To survive this mechanical stress, they need to repair membrane injury efficiently. Targeting the membrane repair machinery is thus potentially a new way to prevent invasion and metastasis. We show here that annexin-A2 (ANXA2) is required for membrane repair in invasive breast and pancreatic cancer cells. Mechanistically, we show by fluorescence and electron microscopy that cells fail to reseal shear-stress damaged membrane when ANXA2 is silenced or the protein is inhibited with neutralizing antibody. Silencing of ANXA2 has no effect on proliferation in vitro, and may even accelerate migration in wound healing assays, but reduces tumor cell dissemination in both mice and zebrafish. We expect that inhibiting membrane repair will be particularly effective in aggressive, poor prognosis tumors because they rely on the membrane repair machinery to survive membrane damage during tumor invasion and metastasis. This could be achieved either with anti-ANXA2 antibodies, which have been shown to inhibit metastasis of breast and pancreatic cancer cells, or with small molecule drugs.

Calcium influx rapidly establishes distinct spatial recruitments of Annexins to cell wounds

To survive daily damage, the formation of actomyosin ring at the wound periphery is required to rapidly close cell wounds. Calcium influx is one of the start signals for these cell wound repair events. Here, we find that rapid recruitment of all three Drosophila calcium responding and phospholipid binding Annexin proteins (AnxB9, AnxB10, AnxB11) to distinct regions around the wound are regulated by the quantity of calcium influx rather than their binding to specific phospholipids. The distinct recruitment patterns of these Annexins regulate the subsequent recruitment of RhoGEF2 and RhoGEF3 through actin stabilization to form a robust actomyosin ring. Surprisingly, we find that reduced extracellular calcium and depletion of intracellular calcium affect cell wound repair differently, despite these two conditions exhibiting similar GCaMP signals. Thus, our results suggest that, in addition to initiating repair events, both the quantity and sources of calcium influx are important for precise Annexin spatiotemporal protein recruitment to cell wounds and efficient wound repair.

When Size Really Matters: The Eccentricities of Dystrophin Transcription and the Hazards of Quantifying mRNA from Very Long Genes

At 2.3 megabases in length, the dystrophin gene is enormous: transcription of a single mRNA requires approximately 16 h. Principally expressed in skeletal muscle, the dystrophin protein product protects the muscle sarcolemma against contraction-induced injury, and dystrophin deficiency results in the fatal muscle-wasting disease, Duchenne muscular dystrophy. This gene is thus of key clinical interest, and therapeutic strategies aimed at eliciting dystrophin restoration require quantitative analysis of its expression. Approaches for quantifying dystrophin at the protein level are well-established, however study at the mRNA level warrants closer scrutiny: measured expression values differ in a sequence-dependent fashion, with significant consequences for data interpretation. In this manuscript, we discuss these nuances of expression and present evidence to support a transcriptional model whereby the long transcription time is coupled to a short mature mRNA half-life, with dystrophin transcripts being predominantly nascent as a consequence. We explore the effects of such a model on cellular transcriptional dynamics and then discuss key implications for the study of dystrophin gene expression, focusing on both conventional (qPCR) and next-gen (RNAseq) approaches.

Leveraging Plasma Membrane Repair Therapeutics for Treating Neurodegenerative Diseases

Plasma membrane repair is an essential cellular mechanism that reseals membrane disruptions after a variety of insults, and compromised repair capacity can contribute to the progression of many diseases. Neurodegenerative diseases are marked by membrane damage from many sources, reduced membrane integrity, elevated intracellular calcium concentrations, enhanced reactive oxygen species production, mitochondrial dysfunction, and widespread neuronal death. While the toxic intracellular effects of these changes in cellular physiology have been defined, the specific mechanism of neuronal death in certain neurodegenerative diseases remains unclear. An abundance of recent evidence indicates that neuronal membrane damage and pore formation in the membrane are key contributors to neurodegenerative disease pathogenesis. In this review, we have outlined evidence supporting the hypothesis that membrane damage is a contributor to neurodegenerative diseases and that therapeutically enhancing membrane repair can potentially combat neuronal death.

Repair of traumatic lesions to the plasmalemma of neurons and other cells: Commonalities, conflicts, and controversies

Neuroscientists and Cell Biologists have known for many decades that eukaryotic cells, including neurons, are surrounded by a plasmalemma/axolemma consisting of a phospholipid bilayer that regulates trans-membrane diffusion of ions (including calcium) and other substances. Cells often incur plasmalemmal damage via traumatic injury and various diseases. If the damaged plasmalemma is not rapidly repaired within minutes, activation of apoptotic pathways by calcium influx often results in cell death. We review publications reporting what is less-well known (and not yet covered in neuroscience or cell biology textbooks): that calcium influx at the lesion sites ranging from small nm-sized holes to complete axonal transection activates parallel biochemical pathways that induce vesicles/membrane-bound structures to migrate and interact to restore original barrier properties and eventual reestablishment of the plasmalemma. We assess the reliability of, and problems with, various measures (e.g., membrane voltage, input resistance, current flow, tracer dyes, confocal microscopy, transmission and scanning electron microscopy) used individually and in combination to assess plasmalemmal sealing in various cell types (e.g., invertebrate giant axons, oocytes, hippocampal and other mammalian neurons). We identify controversies such as plug versus patch hypotheses that attempt to account for currently available data on the subcellular mechanisms of plasmalemmal repair/sealing. We describe current research gaps and potential future developments, such as much more extensive correlations of biochemical/biophysical measures with sub-cellular micromorphology. We compare and contrast naturally occurring sealing with recently-discovered artificially-induced plasmalemmal sealing by polyethylene glycol (PEG) that bypasses all natural pathways for membrane repair. We assess other recent developments such as adaptive membrane responses in neighboring cells following injury to an adjacent cell. Finally, we speculate how a better understanding of the mechanisms involved in natural and artificial plasmalemmal sealing is needed to develop better clinical treatments for muscular dystrophies, stroke and other ischemic conditions, and various cancers.

Role of calcium-sensor proteins in cell membrane repair

Cell membrane repair is a critical process used to maintain cell integrity and survival from potentially lethal chemical, and mechanical membrane injury. Rapid increases in local calcium levels due to a membrane rupture have been widely accepted as a trigger for multiple membrane-resealing models that utilize exocytosis, endocytosis, patching, and shedding mechanisms. Calcium-sensor proteins, such as synaptotagmins (Syt), dysferlin, S100 proteins, and annexins, have all been identified to regulate, or participate in, multiple modes of membrane repair. Dysfunction of membrane repair from inefficiencies or genetic alterations in these proteins contributes to diseases such as muscular dystrophy (MD) and heart disease. The present review covers the role of some of the key calcium-sensor proteins and their involvement in membrane repair.

Hydrodynamic dissection of Stentor coeruleus in a microfluidic cross junction

Stentor coeruleus, a single-cell ciliated protozoan, is a model organism for wound healing and regeneration studies. Despite Stentor's large size (up to 2 mm in extended state), microdissection of Stentor remains challenging. In this work, we describe a hydrodynamic cell splitter, consisting of a microfluidic cross junction, capable of splitting Stentor cells in a non-contact manner at a high throughput of ∼500 cells per minute under continuous operation. Introduction of asymmetry in the flow field at the cross junction leads to asymmetric splitting of the cells to generate cell fragments as small as ∼8.5 times the original cell size. Characterization of cell fragment viability shows reduced 5-day survival as fragment size decreases and as the extent of hydrodynamic stress imposed on the fragments increases. Our results suggest that cell fragment size and composition, as well as mechanical stress, play important roles in the long-term repair of Stentor cells and warrant further investigations. Nevertheless, the hydrodynamic splitter can be useful for studying phenomena immediately after cell splitting, such as the closure of wounds in the plasma membrane which occurs on the order of 100-1000 seconds in Stentor.

Skeletal Muscle Cells Derived from Induced Pluripotent Stem Cells: A Platform for Limb Girdle Muscular Dystrophies

Limb girdle muscular dystrophies (LGMD), caused by mutations in 29 different genes, are the fourth most prevalent group of genetic muscle diseases. Although the link between LGMD and its genetic origins has been determined, LGMD still represent an unmet medical need. Here, we describe a platform for modeling LGMD based on the use of human induced pluripotent stem cells (hiPSC). Thanks to the self-renewing and pluripotency properties of hiPSC, this platform provides a renewable and an alternative source of skeletal muscle cells (skMC) to primary, immortalized, or overexpressing cells. We report that skMC derived from hiPSC express the majority of the genes and proteins that cause LGMD. As a proof of concept, we demonstrate the importance of this cellular model for studying LGMDR9 by evaluating disease-specific phenotypes in skMC derived from hiPSC obtained from four patients.

The C2 domains of dysferlin: roles in membrane localization, Ca2+ signalling and sarcolemmal repair

Dysferlin is an integral membrane protein of the transverse tubules of skeletal muscle that is mutated or absent in limb girdle muscular dystrophy 2B and Miyoshi myopathy. Here we examine the role of dysferlin's seven C2 domains, C2A through C2G, in membrane repair and Ca2+ release, as well as in targeting dysferlin to the transverse tubules of skeletal muscle. We report that deletion of either domain C2A or C2B inhibits membrane repair completely, whereas deletion of C2C, C2D, C2E, C2F or C2G causes partial loss of membrane repair that is exacerbated in the absence of extracellular Ca2+ . Deletion of C2C, C2D, C2E, C2F or C2G also causes significant changes in Ca2+ release, measured as the amplitude of the Ca2+ transient before or after hypo-osmotic shock and the appearance of Ca2+ waves. Most deletants accumulate in endoplasmic reticulum. Only the C2A domain can be deleted without affecting dysferlin trafficking to transverse tubules, but Dysf-ΔC2A fails to support normal Ca2+ signalling after hypo-osmotic shock. Our data suggest that (i) every C2 domain contributes to repair; (ii) all C2 domains except C2B regulate Ca2+ signalling; (iii) transverse tubule localization is insufficient for normal Ca2+ signalling; and (iv) Ca2+ dependence of repair is mediated by C2C through C2G. Thus, dysferlin's C2 domains have distinct functions in Ca2+ signalling and sarcolemmal membrane repair and may play distinct roles in skeletal muscle. KEY POINTS: Dysferlin, a transmembrane protein containing seven C2 domains, C2A through C2G, concentrates in transverse tubules of skeletal muscle, where it stabilizes voltage-induced Ca2+ transients and participates in sarcolemmal membrane repair. Each of dysferlin's C2 domains except C2B regulate Ca2+ signalling. Localization of dysferlin variants to the transverse tubules is not sufficient to support normal Ca2+ signalling or membrane repair. Each of dysferlin's C2 domains contributes to sarcolemmal membrane repair. The Ca2+ dependence of membrane repair is mediated by C2C through C2G. Dysferlin's C2 domains therefore have distinct functions in Ca2+ signalling and sarcolemmal membrane repair.

Sex differences in the involvement of skeletal and cardiac muscles in myopathic Ano5-/- mice

Limb-girdle muscular dystrophy R12 (LGMD-R12) is caused by recessive mutations in the Anoctamin-5 gene (ANO5, TMEM16E). Although ANO5 myopathy is not X-chromosome linked, we performed a meta-analysis of the research literature and found that three-quarters of patients with LGMD-R12 are males. Females are less likely to present with moderate to severe skeletal muscle and/or cardiac pathology. Because these sex differences could be explained in several ways, we compared males and females in a mouse model of LGMD-R12. This model recapitulates the sex differences in human LGMD-R12. Only male Ano5-/- mice had elevated serum creatine kinase after exercise and exhibited defective membrane repair after laser injury. In contrast, by these measures, female Ano5-/- mice were indistinguishable from wild type. Despite these differences, both male and female Ano5-/- mice exhibited exercise intolerance. Although exercise intolerance of male mice can be explained by skeletal muscle dysfunction, echocardiography revealed that Ano5-/- female mice had features of cardiomyopathy that may be responsible for their exercise intolerance. These findings heighten concerns that mutations of ANO5 in humans may be linked to cardiac disease.

The cellular response to plasma membrane disruption for nanomaterial delivery

Delivery of nanomaterials into cells is of interest for fundamental cell biological research as well as for therapeutic and diagnostic purposes. One way of doing so is by physically disrupting the plasma membrane (PM). Several methods that exploit electrical, mechanical or optical cues have been conceived to temporarily disrupt the PM for intracellular delivery, with variable effects on cell viability. However, apart from acute cytotoxicity, subtler effects on cell physiology may occur as well. Their nature and timing vary with the severity of the insult and the efficiency of repair, but some may provoke permanent phenotypic alterations. With the growing palette of nanoscale delivery methods and applications, comes a need for an in-depth understanding of this cellular response. In this review, we summarize current knowledge about the chronology of cellular events that take place upon PM injury inflicted by different delivery methods. We also elaborate on their significance for cell homeostasis and cell fate. Based on the crucial nodes that govern cell fitness and functionality, we give directions for fine-tuning nano-delivery conditions.

Trafficking of Annexins during Membrane Repair in Human Skeletal Muscle Cells

Defects in membrane repair contribute to the development of muscular dystrophies, such as Miyoshi muscular dystrophy 1, limb girdle muscular dystrophy (LGMD), type R2 or R12. Deciphering membrane repair dysfunctions in the development of muscular dystrophies requires precise and detailed knowledge of the membrane repair machinery in healthy human skeletal muscle cells. Using correlative light and electron microscopy (CLEM), we studied the trafficking of four members of the annexin (ANX) family, in myotubes damaged by laser ablation. Our data support a model in which ANXA4 and ANXA6 are recruited to the disruption site by propagating as a wave-like motion along the sarcolemma. They may act in membrane resealing by proceeding to sarcolemma remodeling. On the other hand, ANXA1 and A2 exhibit a progressive cytoplasmic recruitment, likely by interacting with intracellular vesicles, in order to form the lipid patch required for membrane resealing. Once the sarcolemma has been resealed, ANXA1 is released from the site of the membrane injury and returns to the cytosol, while ANXA2 remains accumulated close to the wounding site on the cytoplasmic side. On the other side of the repaired sarcolemma are ANXA4 and ANXA6 that face the extracellular milieu, where they are concentrated in a dense structure, the cap subdomain. The proposed model provides a basis for the identification of cellular dysregulations in the membrane repair of dystrophic human muscle cells.

Fusion and beyond: Satellite cell contributions to loading-induced skeletal muscle adaptation

Satellite cells support adult skeletal muscle fiber adaptations to loading in numerous ways. The fusion of satellite cells, driven by cell-autonomous and/or extrinsic factors, contributes new myonuclei to muscle fibers, associates with load-induced hypertrophy, and may support focal membrane damage repair and long-term myonuclear transcriptional output. Recent studies have also revealed that satellite cells communicate within their niche to mediate muscle remodeling in response to resistance exercise, regulating the activity of numerous cell types through various mechanisms such as secretory signaling and cell-cell contact. Muscular adaptation to resistance and endurance activity can be initiated and sustained for a period of time in the absence of satellite cells, but satellite cell participation is ultimately required to achieve full adaptive potential, be it growth, function, or proprioceptive coordination. While significant progress has been made in understanding the roles of satellite cells in adult muscle over the last few decades, many conclusions have been extrapolated from regeneration studies. This review highlights our current understanding of satellite cell behavior and contributions to adaptation outside of regeneration in adult muscle, as well as the roles of satellite cells beyond fusion and myonuclear accretion, which are gaining broader recognition.

Plasma membrane disruption (PMD) formation and repair in mechanosensitive tissues

Mammalian cells employ an array of biological mechanisms to detect and respond to mechanical loading in their environment. One such mechanism is the formation of plasma membrane disruptions (PMD), which foster a molecular flux across cell membranes that promotes tissue adaptation. Repair of PMD through an orchestrated activity of molecular machinery is critical for cell survival, and the rate of PMD repair can affect downstream cellular signaling. PMD have been observed to influence the mechanical behavior of skin, alveolar, and gut epithelial cells, aortic endothelial cells, corneal keratocytes and epithelial cells, cardiac and skeletal muscle myocytes, neurons, and most recently, bone cells including osteoblasts, periodontal ligament cells, and osteocytes. PMD are therefore positioned to affect the physiological behavior of a wide range of vertebrate organ systems including skeletal and cardiac muscle, skin, eyes, the gastrointestinal tract, the vasculature, the respiratory system, and the skeleton. The purpose of this review is to describe the processes of PMD formation and repair across these mechanosensitive tissues, with a particular emphasis on comparing and contrasting repair mechanisms and downstream signaling to better understand the role of PMD in skeletal mechanobiology. The implications of PMD-related mechanisms for disease and potential therapeutic applications are also explored.

Annexins and Membrane Repair Dysfunctions in Muscular Dystrophies

Muscular dystrophies constitute a group of genetic disorders that cause weakness and progressive loss of skeletal muscle mass. Among them, Miyoshi muscular dystrophy 1 (MMD1), limb girdle muscular dystrophy type R2 (LGMDR2/2B), and LGMDR12 (2L) are characterized by mutation in gene encoding key membrane-repair protein, which leads to severe dysfunctions in sarcolemma repair. Cell membrane disruption is a physiological event induced by mechanical stress, such as muscle contraction and stretching. Like many eukaryotic cells, muscle fibers possess a protein machinery ensuring fast resealing of damaged plasma membrane. Members of the annexins A (ANXA) family belong to this protein machinery. ANXA are small soluble proteins, twelve in number in humans, which share the property of binding to membranes exposing negatively-charged phospholipids in the presence of calcium (Ca²⁺). Many ANXA have been reported to participate in membrane repair of varied cell types and species, including human skeletal muscle cells in which they may play a collective role in protection and repair of the sarcolemma. Here, we discuss the participation of ANXA in membrane repair of healthy skeletal muscle cells and how dysregulation of ANXA expression may impact the clinical severity of muscular dystrophies.

Muscle Proteomic and Transcriptomic Profiling of Healthy Aging and Metabolic Syndrome in Men

(1) Background: Aging is associated with a progressive decline in muscle mass and function. Aging is also a primary risk factor for metabolic syndrome, which further alters muscle metabolism. However, the molecular mechanisms involved remain to be clarified. Herein we performed omic profiling to decipher in muscle which dominating processes are associated with healthy aging and metabolic syndrome in old men. (2) Methods: This study included 15 healthy young, 15 healthy old, and 9 old men with metabolic syndrome. Old men were selected from a well-characterized cohort, and each vastus lateralis biopsy was used to combine global transcriptomic and proteomic analyses. (3) Results: Over-representation analysis of differentially expressed genes (ORA) and functional class scoring of pathways (FCS) indicated that healthy aging was mainly associated with upregulations of apoptosis and immune function and downregulations of glycolysis and protein catabolism. ORA and FCS indicated that with metabolic syndrome the dominating biological processes were upregulation of proteolysis and downregulation of oxidative phosphorylation. Proteomic profiling matched 586 muscle proteins between individuals. The proteome of healthy aging revealed modifications consistent with a fast-to-slow transition and downregulation of glycolysis. These transitions were reduced with metabolic syndrome, which was more associated with alterations in NADH/NAD+ shuttle and β-oxidation. Proteomic profiling further showed that all old muscles overexpressed protein chaperones to preserve proteostasis and myofiber integrity. There was also evidence of aging-related increases in reactive oxygen species but better detoxifications of cytotoxic aldehydes and membrane protection in healthy than in metabolic syndrome muscles. (4) Conclusions: Most candidate proteins and mRNAs identified herein constitute putative muscle biomarkers of healthy aging and metabolic syndrome in old men.

Plasma membrane integrity: implications for health and disease

Plasma membrane integrity is essential for cellular homeostasis. In vivo, cells experience plasma membrane damage from a multitude of stressors in the extra- and intra-cellular environment. To avoid lethal consequences, cells are equipped with repair pathways to restore membrane integrity. Here, we assess plasma membrane damage and repair from a whole-body perspective. We highlight the role of tissue-specific stressors in health and disease and examine membrane repair pathways across diverse cell types. Furthermore, we outline the impact of genetic and environmental factors on plasma membrane integrity and how these contribute to disease pathogenesis in different tissues.

Microfluidic guillotine reveals multiple timescales and mechanical modes of wound response in Stentor coeruleus

Background

Wound healing is one of the defining features of life and is seen not only in tissues but also within individual cells. Understanding wound response at the single-cell level is critical for determining fundamental cellular functions needed for cell repair and survival. This understanding could also enable the engineering of single-cell wound repair strategies in emerging synthetic cell research. One approach is to examine and adapt self-repair mechanisms from a living system that already demonstrates robust capacity to heal from large wounds. Towards this end, Stentor coeruleus, a single-celled free-living ciliate protozoan, is a unique model because of its robust wound healing capacity. This capacity allows one to perturb the wounding conditions and measure their effect on the repair process without immediately causing cell death, thereby providing a robust platform for probing the self-repair mechanism.

Results

Here we used a microfluidic guillotine and a fluorescence-based assay to probe the timescales of wound repair and of mechanical modes of wound response in Stentor. We found that Stentor requires ~ 100–1000 s to close bisection wounds, depending on the severity of the wound. This corresponds to a healing rate of ~ 8–80 μm²/s, faster than most other single cells reported in the literature. Further, we characterized three distinct mechanical modes of wound response in Stentor: contraction, cytoplasm retrieval, and twisting/pulling. Using chemical perturbations, active cilia were found to be important for only the twisting/pulling mode. Contraction of myonemes, a major contractile fiber in Stentor, was surprisingly not important for the contraction mode and was of low importance for the others.

Conclusions

While events local to the wound site have been the focus of many single-cell wound repair studies, our results suggest that large-scale mechanical behaviors may be of greater importance to single-cell wound repair than previously thought. The work here advances our understanding of the wound response in Stentor and will lay the foundation for further investigations into the underlying components and molecular mechanisms involved.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-00970-0.

Quantitative Proteomics Analysis for the Identification of Differential Protein Expression in Calf Muscles between Young and Old SD Rats Using Mass Spectrometry

Aging is associated with loss of muscle mass and strength that leads to a condition termed sarcopenia. Impaired conditions, morbidity, and malnutrition are the factors of devaluation of muscle fibers in aged animals. Satellite cells play an important role in maintaining muscle homeostasis during tissue regeneration and repair. Proteomic profiling on the skeletal muscle tissues of different age group rats helps to determine the differentially expressed (DE) proteins, which may eventually lead to the development of biomarkers in treating the conditions of sarcopenia. In this study, nanoscale liquid chromatography coupled to tandem mass spectrometry (nano-LC-MS/MS) analysis was implemented in the calf tissues of young and old groups of rats. The mass spectrometry (MS) analysis revealed the presence of 335 differentially expressed proteins between the two different age conditions, among which those based on log-fold change 25 proteins were upregulated and 77 were downregulated. The protein-protein interaction network analysis revealed 18 upregulated proteins with three distinct interconnected networks and 57 downregulated proteins with two networks. Further, gene ontology (GO) enrichment analysis showed the biological process, cellular component, and molecular function of the differential proteins. Pathway enrichment analysis of the DE proteins identified nine significantly enriched pathways with a list of eight significant genes (Cryab, Hspb2, Acat1, Ak1, Adssl1, Anxa5, Gys1, Ogdh, Gc, and Adssl1). Quantification of significant genes by quantitative real-time polymerase chain reaction (qRT-PCR) confirmed the downregulation at the mRNA level. Western blot analysis of their protein expression showed concordant results on two candidate proteins (Ogdh and annexin 5) confirming their differential regulation between the two age groups of rats. Thus, these proteomic approaches on young and aged rats provide insights into the development of protein targets in the treatment of sarcopenia (muscle loss).

Plasma membrane wound repair is characterized by extensive membrane lipid and protein rearrangements in vascular endothelial cells

Vascular endothelial cells are subject to mechanical stress resulting from blood flow and interactions with leukocytes. Stress occurs at the apical, vessel-facing cell surface and leads to membrane ruptures that have to be resealed to ensure cell survival. To mimic this process, we developed a laser ablation protocol selectively inducing wounds in the apical plasma membrane of endothelial cells. We show that Ca²⁺-dependent membrane resealing is initiated following this wounding protocol and that the process is accompanied by substantial membrane lipid dynamics at the wound site. Specifically, phosphatidylinositol (4,5)-bisphosphate, phosphatidylserine and phosphatidic acid rapidly accumulate at membrane wounds forming potential interaction platforms for Ca²⁺/phospholipid binding proteins of the annexin (Anx) family that are also recruited within seconds after wounding. Depletion of one annexin, AnxA2, and its putative binding partner S100A11 interferes with membrane resealing suggesting that Ca²⁺-dependent annexin-phospholipid interactions are required for efficient membrane wound repair in endothelial cells.

ANO5 ensures trafficking of annexins in wounded myofibers

Mutations in ANO5 (TMEM16E) cause limb-girdle muscular dystrophy R12. Defective plasma membrane repair is a likely mechanism. Using myofibers from Ano5 knockout mice, we show that trafficking of several annexin proteins, which together form a cap at the site of injury, is altered upon loss of ANO5. Annexin A2 accumulates at the wound to nearly twice the level observed in WT fibers, while annexin A6 accumulation is substantially inhibited in the absence of ANO5. Appearance of annexins A1 and A5 at the cap is likewise diminished in the Ano5 knockout. These changes are correlated with an alteration in annexin repair cap fine structure and shedding of annexin-positive vesicles. We conclude that loss of annexin coordination during repair is disrupted in Ano5 knockout mice and underlies the defective repair phenotype. Although ANO5 is a phospholipid scramblase, abnormal repair is rescued by overexpression of a scramblase-defective ANO5 mutant, suggesting a novel, scramblase-independent role of ANO5 in repair.

Plasma membrane integrity in health and disease: significance and therapeutic potential

Maintenance of plasma membrane integrity is essential for normal cell viability and function. Thus, robust membrane repair mechanisms have evolved to counteract the eminent threat of a torn plasma membrane. Different repair mechanisms and the bio-physical parameters required for efficient repair are now emerging from different research groups. However, less is known about when these mechanisms come into play. This review focuses on the existence of membrane disruptions and repair mechanisms in both physiological and pathological conditions, and across multiple cell types, albeit to different degrees. Fundamentally, irrespective of the source of membrane disruption, aberrant calcium influx is the common stimulus that activates the membrane repair response. Inadequate repair responses can tip the balance between physiology and pathology, highlighting the significance of plasma membrane integrity. For example, an over-activated repair response can promote cancer invasion, while the inability to efficiently repair membrane can drive neurodegeneration and muscular dystrophies. The interdisciplinary view explored here emphasises the widespread potential of targeting plasma membrane repair mechanisms for therapeutic purposes.

Induction of Ca2+-Dependent Exocytotic Processes by Laser Ablation of Endothelial Cells

Ca²⁺ regulates a variety of cellular processes that are essential to maintain cell integrity and function. Different methods have been used to study these processes by increasing intracellular Ca²⁺ levels. Here, we describe a protocol to initiate Ca²⁺-dependent membrane-related events, using laser ablation by near-infrared irradiation. This creates a rupture in the plasma membrane that allows the extracellular Ca²⁺ to enter the cell and thereby induce a receptor-independent Ca²⁺ increase. We report laser ablation protocols to study two different Ca²⁺-induced processes in human endothelial cells-membrane resealing and exocytosis of secretory granules called Weibel-Palade bodies (WPBs). Thus, laser ablation represents a technique that permits the analysis of different Ca²⁺-regulated processes at high spatiotemporal resolution in a controlled manner.

Defective membrane repair machinery impairs survival of invasive cancer cells

Cancer cells are able to reach distant tissues by migration and invasion processes. Enhanced ability to cope with physical stresses leading to cell membrane damages may offer to cancer cells high survival rate during metastasis. Consequently, down-regulation of the membrane repair machinery may lead to metastasis inhibition. We show that migration of MDA-MB-231 cells on collagen I fibrils induces disruptions of plasma membrane and pullout of membrane fragments in the wake of cells. These cells are able to reseal membrane damages thanks to annexins (Anx) that are highly expressed in invasive cancer cells. In vitro membrane repair assays reveal that MDA-MB-231 cells respond heterogeneously to membrane injury and some of them possess a very efficient repair machinery. Finally, we show that silencing of AnxA5 and AnxA6 leads to the death of migrating MDA-MB-231 cells due to major defect of the membrane repair machinery. Disturbance of the membrane repair process may therefore provide a new avenue for inhibiting cancer metastasis.

An interaction of heart disease-associated proteins POPDC1/2 with XIRP1 in transverse tubules and intercalated discs

Background

Popeye domain-containing proteins 1 and 2 (POPDC1 and POPDC2) are transmembrane proteins involved in cyclic AMP-mediated signalling processes and are required for normal cardiac pacemaking and conduction. In order to identify novel protein interaction partners, POPDC1 and 2 proteins were attached to beads and compared by proteomic analysis with control beads in the pull-down of proteins from cultured human skeletal myotubes.

Results

There were highly-significant interactions of both POPDC1 and POPDC2 with XIRP1 (Xin actin binding repeat-containing protein 1), actin and, to a lesser degree, annexin A5. In adult human skeletal muscle, both XIRP1 and POPDC1/2 were present at the sarcolemma and in T-tubules. The interaction of POPDC1 with XIRP1 was confirmed in adult rat heart extracts. Using new monoclonal antibodies specific for POPDC1 and POPDC2, both proteins, together with XIRP1, were found mainly at intercalated discs but also at T-tubules in adult rat and human heart.

Conclusions

Mutations in human POPDC1, POPDC2 and in human XIRP1, all cause pathological cardiac arrhythmias, suggesting a possible role for POPDC1/2 and XIRP1 interaction in normal cardiac conduction.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12860-020-00329-3.

Annexin-A6 in Membrane Repair of Human Skeletal Muscle Cell: A Role in the Cap Subdomain

Defects in membrane repair contribute to the development of some muscular dystrophies, highlighting the importance to decipher the membrane repair mechanisms in human skeletal muscle. In murine myofibers, the formation of a cap subdomain composed notably by annexins (Anx) is critical for membrane repair. We applied membrane damage by laser ablation to human skeletal muscle cells and assessed the behavior of annexin-A6 (AnxA6) tagged with GFP by correlative light and electron microscopy (CLEM). We show that AnxA6 was recruited to the site of membrane injury within a few seconds after membrane injury. In addition, we show that the deficiency in AnxA6 compromises human sarcolemma repair, demonstrating the crucial role played by AnxA6 in this process. An AnxA6-containing cap-subdomain was formed in damaged human myotubes in about one minute. Through transmission electron microscopy (TEM), we observed that extension of the sarcolemma occurred during membrane resealing, which participated in forming a dense lipid structure in order to plug the hole. By properties of membrane folding and curvature, AnxA6 helped in the formation of this tight structure. The compaction of intracellular membranes-which are used for membrane resealing and engulfed in extensions of the sarcolemma-may also facilitate elimination of the excess of lipid and protein material once cell membrane has been repaired. These data reinforce the role played by AnxA6 and the cap subdomain in membrane repair of skeletal muscle cells.

News about non-secretory exocytosis: mechanisms, properties, and functions

The fusion by exocytosis of many vesicles to the plasma membrane induces the discharge to the extracellular space of their abundant luminal cargoes. Other exocytic vesicles, however, do not contain cargoes, and thus, their fusion is not followed by secretion. Therefore, two distinct processes of exocytosis exist, one secretory and the other non-secretory. The present review deals with the knowledge of non-secretory exocytosis developed during recent years. Among such developments are the dual generation of the exocytic vesicles, initially released either from the trans-Golgi network or by endocytosis; their traffic with activation of receptors, channels, pumps, and transporters; the identification of their tethering and soluble N-ethylmaleimide-sensitive factor attachment protein receptor complexes that govern membrane fusions; the growth of axons and the membrane repair. Examples of potential relevance of these processes for pathology and medicine are also reported. The developments presented here offer interesting chances for future progress in the field.

Comparative label-free mass spectrometric analysis of temporal changes in the skeletal muscle proteome after impact trauma in rats

Proteomics offers the opportunity to identify and quantify many proteins and to explore how they correlate and interact with each other in biological networks. This study aimed to characterize changes in the muscle proteome during the destruction, repair, and early-remodeling phases after impact trauma in male Wistar rats. Muscle tissue was collected from uninjured control rats and rats that were euthanized between 6 h and 14 days after impact injury. Muscle tissue was analyzed using unbiased, data-independent acquisition LC-MS/MS. We identified 770 reviewed proteins in the muscle tissue, 296 of which were differentially abundant between the control and injury groups (P ≤ 0.05). Around 50 proteins showed large differences (≥10-fold) or a distinct pattern of abundance after injury. These included proteins that have not been identified previously in injured muscle, such as ferritin light chain 1, fibrinogen γ-chain, fibrinogen β-chain, osteolectin, murinoglobulin-1, T-kininogen 2, calcium-regulated heat-stable protein 1, macrophage-capping protein, retinoid-inducible serine carboxypeptidase, ADP-ribosylation factor 4, Thy-1 membrane glycoprotein, and ADP-ribosylation factor-like protein 1. Some proteins increased between 6 h and 14 days, whereas other proteins increased in a more delayed pattern at 7 days after injury. Bioinformatic analysis revealed that various biological processes, including regulation of blood coagulation, fibrinolysis, regulation of wound healing, tissue regeneration, acute inflammatory response, and negative regulation of the immune effector process, were enriched in injured muscle tissue. This study advances the understanding of early muscle healing after muscle injury and lays a foundation for future mechanistic studies on interventions to treat muscle injury.

Reduced Annexin A3 in schizophrenia

The cellular and molecular mechanisms underlying onset and development of schizophrenia have not yet been completely elucidated, but the association of disturbed neuroplasticity and inflammation has gained particular relevance recently. These mechanisms are linked to annexins functions. ANXA3, particularly, is associated to inflammation and membrane metabolism cascades. The aim was to determine the ANXA3 levels in first-onset drug-naïve psychotic patients. We investigated by western blot the protein expression of annexin A3 in platelets of first-onset, drug-naïve psychotic patients (diagnoses according to DSM-IV: 28 schizophrenia, 27 bipolar disorder) as compared to 30 age- and gender-matched healthy controls. Annexin A3 level was lower in schizophrenia patients as compared to healthy controls (p < 0.001) and to bipolar patients (p < 0.001). Twenty out of 28 schizophrenic patients had undetectable annexin A3 levels, as compared to none from the bipolar and none from the control subjects. ANXA3 was reduced in drug-naïve patients with schizophrenia. ANXA3 affects neuroplasticity, inflammation and apoptosis, as well as it modulates membrane phospholipid metabolism. All these processes have been discussed in regard to the biology of schizophrenia. In face of these data, we feel that further studies with larger samples are warranted to investigate the possible role of reduced ANXA3 as a possible risk marker for schizophrenia.

Ca2+-Calmodulin Dependent Wound Repair in Dictyostelium Cell Membrane

Wound repair of cell membrane is a vital physiological phenomenon. We examined wound repair in Dictyostelium cells by using a laserporation, which we recently invented. We examined the influx of fluorescent dyes from the external medium and monitored the cytosolic Ca²⁺ after wounding. The influx of Ca²⁺ through the wound pore was essential for wound repair. Annexin and ESCRT components accumulated at the wound site upon wounding as previously described in animal cells, but these were not essential for wound repair in Dictyostelium cells. We discovered that calmodulin accumulated at the wound site upon wounding, which was essential for wound repair. The membrane accumulated at the wound site to plug the wound pore by two-steps, depending on Ca²⁺ influx and calmodulin. From several lines of evidence, the membrane plug was derived from de novo generated vesicles at the wound site. Actin filaments also accumulated at the wound site, depending on Ca²⁺ influx and calmodulin. Actin accumulation was essential for wound repair, but microtubules were not essential. A molecular mechanism of wound repair will be discussed.

Multiple poloxamers increase plasma membrane repair capacity in muscle and nonmuscle cells

Various previous studies established that the amphiphilic tri-block copolymer known as poloxamer 188 (P188) or Pluronic-F68 can stabilize the plasma membrane following a variety of injuries to multiple mammalian cell types. This characteristic led to proposals for the use of P188 as a therapeutic treatment for various disease states, including muscular dystrophy. Previous studies suggest that P188 increases plasma membrane integrity by resealing plasma membrane disruptions through its affinity for the hydrophobic lipid chains on the lipid bilayer. P188 is one of a large family of copolymers that share the same basic tri-block structure consisting of a middle hydrophobic propylene oxide segment flanked by two hydrophilic ethylene oxide moieties [poly(ethylene oxide)80-poly(propylene oxide)27-poly(ethylene oxide)80]. Despite the similarities of P188 to the other poloxamers in this chemical family, there has been little investigation into the membrane-resealing properties of these other poloxamers. In this study we assessed the resealing properties of poloxamers P181, P124, P182, P234, P108, P407, and P338 on human embryonic kidney 293 (HEK293) cells and isolated muscle from the mdx mouse model of Duchenne muscular dystrophy. Cell membrane injuries from glass bead wounding and multiphoton laser injury show that the majority of poloxamers in our panel improved the plasma membrane resealing of both HEK293 cells and dystrophic muscle fibers. These findings indicate that many tri-block copolymers share characteristics that can increase plasma membrane resealing and that identification of these shared characteristics could help guide design of future therapeutic approaches.

Annexin-V stabilizes membrane defects by inducing lipid phase transition

Annexins are abundant cytoplasmic proteins, which bind to membranes that expose negatively charged phospholipids in a Ca²⁺-dependent manner. During cell injuries, the entry of extracellular Ca²⁺ activates the annexin membrane-binding ability, subsequently initiating membrane repair processes. However, the mechanistic action of annexins in membrane repair remains largely unknown. Here, we use high-speed atomic force microscopy (HS-AFM), fluorescence recovery after photobleaching (FRAP), confocal laser scanning microscopy (CLSM) and molecular dynamics simulations (MDSs) to analyze how annexin-V (A5) binds to phosphatidylserine (PS)-rich membranes leading to high Ca²⁺-concentrations at membrane, and then to changes in the dynamics and organization of lipids, eventually to a membrane phase transition. A5 self-assembly into lattices further stabilizes and likely structures the membrane into a gel phase. Our findings are compatible with the patch resealing through vesicle fusion mechanism in membrane repair and indicate that A5 retains negatively charged lipids in the inner leaflet in an injured cell.

Annexins are cytoplasmic proteins, which bind to membranes exposing negatively charged phospholipids in a Ca²⁺-dependent manner. Here the authors use high-speed atomic force microscopy and other techniques to show that annexin-V self-assembles into highly structured lattices that lead to a membrane phase transition on PS-rich membranes.

Cardiomyocyte damage control in heart failure and the role of the sarcolemma

The cardiomyocyte plasma membrane, termed the sarcolemma, is fundamental for regulating a myriad of cellular processes. For example, the structural integrity of the cardiomyocyte sarcolemma is essential for mediating cardiac contraction by forming microdomains such as the t-tubular network, caveolae and the intercalated disc. Significantly, remodelling of these sarcolemma microdomains is a key feature in the development and progression of heart failure (HF). However, despite extensive characterisation of the associated molecular and ultrastructural events there is a lack of clarity surrounding the mechanisms driving adverse morphological rearrangements. The sarcolemma also provides protection, and is the cell's first line of defence, against external stresses such as oxygen and nutrient deprivation, inflammation and oxidative stress with a loss of sarcolemma viability shown to be a key step in cell death via necrosis. Significantly, cumulative cell death is also a feature of HF, and is linked to disease progression and loss of cardiac function. Herein, we will review the link between structural and molecular remodelling of the sarcolemma associated with the progression of HF, specifically considering the evidence for: (i) Whether intrinsic, evolutionary conserved, plasma membrane injury-repair mechanisms are in operation in the heart, and (ii) if deficits in key 'wound-healing' proteins (annexins, dysferlin, EHD2 and MG53) may play a yet to be fully appreciated role in triggering sarcolemma microdomain remodelling and/or necrosis. Cardiomyocytes are terminally differentiated with very limited regenerative capability and therefore preserving cell viability and cardiac function is crucially important. This review presents a novel perspective on sarcolemma remodelling by considering whether targeting proteins that regulate sarcolemma injury-repair may hold promise for developing new strategies to attenuate HF progression.

Actin dynamics and myosin contractility during plasma membrane repair and restoration: Does one ring really heal them all?

In order to survive daily insults, cells have evolved various mechanisms that detect, stabilize and repair damages done to their plasma membrane and cytoskeletal structures. Damage to the PM endangers wounded cells by exposing them to uncontrolled exchanges with the extracellular milieu. The processes and molecular machinery enabling PM repair are therefore at the center of the bulk of the investigations into single-cell repair program. Wounds are repaired by dynamically remodeling the composition and shape of the injured area through exocytosis-mediated release of intracellular membrane components to the wounded area, endocytosis-mediated removal of the injured area, or the shedding of the injury. The wound healing program of Xenopus oocytes and early Drosophila embryos is by contrast, mostly characterized by the rapid formation of a large membrane patch over the wound that eventually fuse with the plasma membrane which restores plasma membrane continuity and lead to the shedding of patch material into the extracellular space. Formation and contraction of actomyosin ring restores normal plasma membrane composition and organizes cytoskeletal repairs. The extend of the contributions of the cytoskeleton to the wound healing program of somatic cells have comparatively received little attention. This review offers a survey of the current knowledge on how actin dynamics, myosin-based contraction and other cytoskeletal structures affects PM and cortical cytoskeleton repair of somatic cells.

Plasma Membrane Integrity During Cell-Cell Fusion and in Response to Pore-Forming Drugs Is Promoted by the Penta-EF-Hand Protein PEF1 in Neurospora crassa

Plasma membrane damage commonly occurs during cellular growth and development. To counteract these potentially lethal injuries, membrane repair mechanisms have evolved, which promote the integrity of the lipid bilayer. Although the membrane of fungi is the target of important clinical drugs and agricultural fungicides, the molecular mechanisms which mediate membrane repair in these organisms remain elusive. Here we identify the penta-EF-hand protein PEF1 of the genetic model fungus Neurospora crassa as part of a cellular response mechanism against different types of membrane injury. Deletion of the pef1 gene in the wild type and different lysis-prone gene knockout mutants revealed a function of the protein in maintaining cell integrity during cell-cell fusion and in the presence of pore-forming drugs, such as the plant defense compound tomatine. By fluorescence and live-cell imaging we show that green fluorescent protein (GFP)-tagged PEF1 accumulates at the sites of membrane injury in a Ca2+-dependent manner. Site-directed mutagenesis identified Ca2+-binding domains essential for the spatial dynamics and function of the protein. In addition, the subcellular localization of PEF1 revealed that the syncytial fungal colony undergoes compartmentation in response to antifungal treatment. We propose that plasma membrane repair in fungi constitutes an additional line of defense against membrane-disturbing drugs, thereby expanding the current model of fungal drug resistance mechanisms.

Annexins and plasma membrane repair

Plasma membrane wound repair is a cell-autonomous process that is triggered by Ca²⁺ entering through the site of injury and involves membrane resealing, i.e., re-establishment of a continuous plasma membrane, as well as remodeling of the cortical actin cytoskeleton. Among other things, the injury-induced Ca²⁺ elevation initiates the wound site recruitment of Ca²⁺-regulated proteins that function in the course of repair. Annexins are a class of such Ca²⁺-regulated proteins. They associate with acidic phospholipids of cellular membranes in their Ca²⁺ bound conformation with Ca²⁺ sensitivities ranging from the low to high micromolar range depending on the respective annexin protein. Annexins accumulate at sites of plasma membrane injury in a temporally controlled manner and are thought to function by controlling membrane rearrangements at the wound site, most likely in conjunction with other repair proteins such as dysferlin. Their role in membrane repair, which has been evidenced in several model systems, will be discussed in this chapter.

Mechanisms protecting host cells against bacterial pore-forming toxins

Pore-forming toxins (PFTs) are key virulence determinants produced and secreted by a variety of human bacterial pathogens. They disrupt the plasma membrane (PM) by generating stable protein pores, which allow uncontrolled exchanges between the extracellular and intracellular milieus, dramatically disturbing cellular homeostasis. In recent years, many advances were made regarding the characterization of conserved repair mechanisms that allow eukaryotic cells to recover from mechanical disruption of the PM membrane. However, the specificities of the cell recovery pathways that protect host cells against PFT-induced damage remain remarkably elusive. During bacterial infections, the coordinated action of such cell recovery processes defines the outcome of infected cells and is, thus, critical for our understanding of bacterial pathogenesis. Here, we review the cellular pathways reported to be involved in the response to bacterial PFTs and discuss their impact in single-cell recovery and infection.

Into the breach: how cells cope with wounds

Repair of wounds to individual cells is crucial for organisms to survive daily physiological or environmental stresses, as well as pathogen assaults, which disrupt the plasma membrane. Sensing wounds, resealing membranes, closing wounds and remodelling plasma membrane/cortical cytoskeleton are four major steps that are essential to return cells to their pre-wounded states. This process relies on dynamic changes of the membrane/cytoskeleton that are indispensable for carrying out the repairs within tens of minutes. Studies from different cell wound repair models over the last two decades have revealed that the molecular mechanisms of single cell wound repair are very diverse and dependent on wound type, size, and/or species. Interestingly, different repair models have been shown to use similar proteins to achieve the same end result, albeit sometimes by distinctive mechanisms. Recent studies using cutting edge microscopy and molecular techniques are shedding new light on the molecular mechanisms during cellular wound repair. Here, we describe what is currently known about the mechanisms underlying this repair process. In addition, we discuss how the study of cellular wound repair—a powerful and inducible model—can contribute to our understanding of other fundamental biological processes such as cytokinesis, cell migration, cancer metastasis and human diseases.

Biochemical and pathological changes result from mutated Caveolin-3 in muscle

Background

Caveolin-3 (CAV3) is a muscle-specific protein localized to the sarcolemma. It was suggested that CAV3 is involved in the connection between the extracellular matrix (ECM) and the cytoskeleton. Caveolinopathies often go along with increased CK levels indicative of sarcolemmal damage. So far, more than 40 dominant pathogenic mutations have been described leading to several phenotypes many of which are associated with a mis-localization of the mutant protein to the Golgi. Golgi retention and endoplasmic reticulum (ER) stress has been demonstrated for the CAV3 p.P104L mutation, but further downstream pathophysiological consequences remained elusive so far.

Methods

We utilized a transgenic (p.P104L mutant) mouse model and performed proteomic profiling along with immunoprecipitation, immunofluorescence and immunoblot examinations (including examination of α-dystroglycan glycosylation), and morphological studies (electron and coherent anti-Stokes Raman scattering (CARS) microscopy) in a systematic investigation of molecular and subcellular events in p.P104L caveolinopathy.

Results

Our electron and CARS microscopic as well as immunological studies revealed Golgi and ER proliferations along with a build-up of protein aggregates further characterized by immunoprecipitation and subsequent mass spectrometry. Molecular characterization these aggregates showed affection of mitochondrial and cytoskeletal proteins which accords with our ultra-structural findings. Additional global proteomic profiling revealed vulnerability of 120 proteins in diseased quadriceps muscle supporting our previous findings and providing more general insights into the underlying pathophysiology. Moreover, our data suggested that further DGC components are altered by the perturbed protein processing machinery but are not prone to form aggregates whereas other sarcolemmal proteins are ubiquitinated or bind to p62. Although the architecture of the ER and Golgi as organelles of protein glycosylation are altered, the glycosylation of α-dystroglycan presented unchanged.

Conclusions

Our combined data classify the p.P104 caveolinopathy as an ER-Golgi disorder impairing proper protein processing and leading to aggregate formation pertaining proteins important for mitochondrial function, cytoskeleton, ECM remodeling and sarcolemmal integrity. Glycosylation of sarcolemmal proteins seems to be normal. The new pathophysiological insights might be of relevance for the development of therapeutic strategies for caveolinopathy patients targeting improved protein folding capacity.

Electronic supplementary material

The online version of this article (10.1186/s13395-018-0173-y) contains supplementary material, which is available to authorized users.

When a transmembrane channel isn't, or how biophysics and biochemistry (mis)communicate

Annexins are a family of soluble proteins that bind to acidic phospholipids such as phosphatidylserine in a calcium-dependent manner. The archetypical member of the annexin family is annexin A5. For many years, its function remained unknown despite the availability of a high-resolution structure. This, combined with the observations of specific ion conductance in annexin-bound membranes, fueled speculations about the possible membrane-spanning forms of annexins that functioned as ion channels. The channel hypothesis remained controversial and did not gather sufficient evidence to become accepted. Yet, it continues to draw attention as a framework for interpreting indirect (e.g., biochemical) data. The goal of the mini-review is to examine the data on annexin-lipid interactions from the last ~30 years from the point of view of the controversy between the two lines of inquiry: the well-characterized peripheral assembly of the annexins at membranes vs. their putative transmembrane insertion. In particular, the potential role of lipid rearrangements induced by annexin binding is highlighted.