Secreted acid sphingomyelinase as a potential gene therapy for limb girdle muscular dystrophy 2B

25-Hydroxycholesterol modulates synaptic vesicle endocytosis at the mouse neuromuscular junction

Many synaptic vesicles undergo exocytosis in motor nerve terminals during neuromuscular communication. Endocytosis then recovers the synaptic vesicle pool and presynaptic membrane area. The kinetics of endocytosis may shape neuromuscular transmission, determining its long-term reliability. Here, using fluorescent dyes, the time course of endocytosis induced by intense activity of the phrenic nerve was studied at the mouse diaphragm neuromuscular junction. It was found that a significant portion of endocytic events occurs after the end of tetanic stimulation. Pitstop 2, clathrin inhibitor, and more profoundly dynole 34-2, dynamin antagonist, suppressed endocytic FM1-43 dye uptake both during and after tetanus. Furthermore, synaptic vesicles formed in the presence of the endocytic blockers released FM-dye during subsequent evoked exocytosis at a lower rate. 25-Hydroxycholesterol (25HC) is an oxysterol, ubiquitously synthetized from excessive cholesterol. In addition, its production greatly increases by activated macrophages. 25HC accelerated FM-dye endocytosis and its sequential evoked exocytosis, and dynole (but not pitstop) prevented 25HC-mediated enhancement of endocytic FM-dye uptake. The positive effects of 25HC were interfered with chelation of cytosolic Ca2+ with a slow Ca2+ buffer EGTA-AM, Ca2+ antagonist TMB8, and sphingomyelin-hydrolyzing enzyme. In contrast to amphiphilic FM1-43 dye capture, 25HC reduced uptake of hydrophilic high molecular weight markers (labeled dextrans and toxin), which utilize bulk endocytosis to enter into nerve terminals. Thus, synaptic vesicle endocytosis had a relatively slow kinetics following the tetanic activity and can be accelerated by 25HC. The positive effect of 25HC on endocytosis engages a dynamin-dependent pathway, interconnected with cytoplasmic Ca2+ and sphingomyelin integrity.

Voluntary wheel running improves molecular and functional deficits in a murine model of facioscapulohumeral muscular dystrophy

Endurance exercise training is beneficial for skeletal muscle health, but it is unclear if this type of exercise can target or correct the molecular mechanisms of facioscapulohumeral muscular dystrophy (FSHD). Using the FLExDUX4 murine model of FSHD characterized by chronic, low levels of pathological double homeobox protein 4 (DUX4) gene expression, we show that 6 weeks of voluntary, free wheel running improves running performance, strength, mitochondrial function, and sarcolemmal repair capacity, while slowing/reversing skeletal muscle fibrosis. These improvements are associated with restored transcriptional activity of gene networks/pathways regulating actin cytoskeletal signaling, vascular remodeling, inflammation, fibrosis, and muscle mass toward wild-type (WT) levels. However, FLExDUX4 mice exhibit blunted increases in mitochondrial content with training and persistent transcriptional overactivation of hypoxia, inflammatory, angiogenic, and cytoskeletal pathways. These results identify exercise-responsive and non-responsive molecular pathways in FSHD, while providing support for the use of endurance-type exercise as a non-invasive treatment option.

Early Endosomes Undergo Calcium-Triggered Exocytosis and Enable Repair of Diffuse and Focal Plasma Membrane Injury

Cells are routinely exposed to agents that cause plasma membrane (PM) injury. While pore-forming toxins (PFTs), and chemicals cause nanoscale holes dispersed throughout the PM, mechanical trauma causes focal lesions in the PM. To examine if these distinct injuries share common repair mechanism, membrane trafficking is monitored as the PM repairs from such injuries. During the course of repair, dispersed PM injury by the PFT Streptolysin O activates endocytosis, while focal mechanical injury to the PM inhibits endocytosis. Consequently, acute block of endocytosis prevents repair of diffuse, but not of focal injury. In contrast, a chronic block in endocytosis depletes cells of early endosomes and inhibits repair of focal injury. This study finds that both focal and diffuse PM injury activate Ca2+ -triggered exocytosis of early endosomes. The use of markers including endocytosed cargo, Rab5, Rab11, and VAMP3, all reveal injury-triggered exocytosis of early endosomes. Inhibiting Rab5 prevents injury-triggered early endosome exocytosis and phenocopies the failed PM repair of cells chronically depleted of early endosomes. These results identify early endosomes as a Ca2+ -regulated exocytic compartment, and uncover the requirement of their dual functions - endocytosis and regulated exocytosis, to differentially support PM repair based on the nature of the injury.

Pharmacotherapeutic Approaches to Treatment of Muscular Dystrophies

Muscular dystrophies are a heterogeneous group of genetic muscle-wasting disorders that are subdivided based on the region of the body impacted by muscle weakness as well as the functional activity of the underlying genetic mutations. A common feature of the pathophysiology of muscular dystrophies is chronic inflammation associated with the replacement of muscle mass with fibrotic scarring. With the progression of these disorders, many patients suffer cardiomyopathies with fibrosis of the cardiac tissue. Anti-inflammatory glucocorticoids represent the standard of care for Duchenne muscular dystrophy, the most common muscular dystrophy worldwide; however, long-term exposure to glucocorticoids results in highly adverse side effects, limiting their use. Thus, it is important to develop new pharmacotherapeutic approaches to limit inflammation and fibrosis to reduce muscle damage and promote repair. Here, we examine the pathophysiology, genetic background, and emerging therapeutic strategies for muscular dystrophies.

Sphingomyelinase modulates synaptic vesicle mobilization at the mice neuromuscular junctions

AIMS

Sphingomyelin is an abundant component of the presynaptic membrane and an organizer of lipid rafts. In several pathological conditions, sphingomyelin is hydrolyzed due to an upregulation and release of secretory sphingomyelinases (SMases). Herein, the effects of SMase on exocytotic neurotransmitter release were studied in the diaphragm neuromuscular junctions of mice.

MAIN METHODS

Microelectrode recordings of postsynaptic potentials and styryl (FM) dyes were used to estimate neuromuscular transmission. Membrane properties were assessed with fluorescent techniques.

KEY FINDINGS

Application of SMase at a low concentration (0.01 U ml^-1) led to a disruption of lipid-packing in the synaptic membranes. Neither spontaneous exocytosis nor evoked neurotransmitter release (in response to single stimuli) were affected by SMase treatment. However, SMase significantly increased neurotransmitter release and the rate of fluorescent FM-dye loss from the synaptic vesicles at 10, 20 and 70 Hz stimulation of the motor nerve. In addition, SMase treatment prevented a shift of the exocytotic mode from "full-collapse" fusion to "kiss-and-run" during high-frequency (70 Hz) activity. The potentiating effects of SMase on neurotransmitter release and FM-dye unloading were suppressed when synaptic vesicle membranes were also exposed to this enzyme (i.e., stimulation occurred during SMase treatment).

SIGNIFICANCE

Thus, hydrolysis of the plasma membrane sphingomyelin can enhance mobilization of synaptic vesicles and facilitate full fusion mode of exocytosis, but SMase acting on vesicular membrane had a depressant effect on the neurotransmission. Partially, the effects of SMase can be related with the changes in synaptic membrane properties and intracellular signaling.

Minimal expression of dysferlin prevents development of dysferlinopathy in dysferlin exon 40a knockout mice

Dysferlin is a Ca²⁺-activated lipid binding protein implicated in muscle membrane repair. Recessive variants in DYSF result in dysferlinopathy, a progressive muscular dystrophy. We showed previously that calpain cleavage within a motif encoded by alternatively spliced exon 40a releases a 72 kDa C-terminal minidysferlin recruited to injured sarcolemma. Herein we use CRISPR/Cas9 gene editing to knock out murine Dysf exon 40a, to specifically assess its role in membrane repair and development of dysferlinopathy. We created three Dysf exon 40a knockout (40aKO) mouse lines that each express different levels of dysferlin protein ranging from ~ 90%, ~ 50% and ~ 10-20% levels of wild-type. Histopathological analysis of skeletal muscles from all 12-month-old 40aKO lines showed virtual absence of dystrophic features and normal membrane repair capacity for all three 40aKO lines, as compared with dysferlin-null BLAJ mice. Further, lipidomic and proteomic analyses on 18wk old quadriceps show all three 40aKO lines are spared the profound lipidomic/proteomic imbalance that characterises dysferlin-deficient BLAJ muscles. Collective results indicate that membrane repair does not depend upon calpain cleavage within exon 40a and that ~ 10-20% of WT dysferlin protein expression is sufficient to maintain the muscle lipidome, proteome and membrane repair capacity to crucially prevent development of dysferlinopathy.

Molekulare Therapien erblicher Myopathien im Erwachsenenalter – eine kursive Rundschau

Unterschiedliche Formen der molekularen Therapie sind zu einer neuen Möglichkeit in der Präzisionsbehandlung erblicher neuromuskulärer Erkrankungen geworden. Dieser kursive Überblick über die molekularen Therapien bei hereditären Myopathien wird sich auf ausgewählte aktuelle Phase 1 bis 3 Studien zu häufigen hereditären Myopathien im Erwachsenenalter wie die Dystrophinopathie Becker-Kiener, die Fazioskapulohumerale Muskeldystrophie, Calpainopathie, und die Dysferlinopathie fokussieren. Die Therapieoptionen zum Morbus Pompe dienen als Beispiel für die hereditären metabolischen Myopathien.

Cellular and molecular mechanisms underlying plasma membrane functionality and integrity

The plasma membrane not only protects the cell from the extracellular environment, acting as a selective barrier, but also regulates cellular events that originate at the cell surface, playing a key role in various biological processes that are essential for the preservation of cell homeostasis. Therefore, elucidation of the mechanisms involved in the maintenance of plasma membrane integrity and functionality is of utmost importance. Cells have developed mechanisms to ensure the quality of proteins that inhabit the cell surface, as well as strategies to cope with injuries inflicted to the plasma membrane. Defects in these mechanisms can lead to the development or onset of several diseases. Despite the importance of these processes, a comprehensive and holistic perspective of plasma membrane quality control is still lacking. To tackle this gap, in this Review, we provide a thorough overview of the mechanisms underlying the identification and targeting of membrane proteins that are to be removed from the cell surface, as well as the membrane repair mechanisms triggered in both physiological and pathological conditions. A better understanding of the mechanisms underlying protein quality control at the plasma membrane can reveal promising and unanticipated targets for the development of innovative therapeutic approaches.

Resisting attack by repairing the damage

Removing membrane pores may help cancer cells survive T cell assault.