The spectrum of congenital myasthenic syndromes

The past decade saw remarkable advances in defining the molecular and genetic basis of the congenital myasthenic syndromes. These advances would not have been possible without antecedent clinical observations, electrophysiologic analysis, and careful morphologic studies that pointed to candidate genes or proteins. For example, a kinetic abnormality of the acetylcholine receptor (AChR) detected at the single channel level pointed to a kinetic mutation in an AChR subunit; endplate AChR deficiency suggested mutations residing in an AChR subunit or in rapsyn; absence of acetylcholinesterase (AChE) from the endplate predicted mutations in the catalytic or collagen-tailed subunit of this enzyme; and a history of abrupt episodes of apnea associated with a stimulation dependent decrease of endplate potentials and currents implicated proteins concerned with ACh resynthesis or vesicular filling. Discovery of mutations in endplate-specific proteins also prompted expression studies that afforded proof of pathogenicity, provided clues for rational therapy, lead to precise structure function correlations, and highlighted functionally significant residues or molecular domains that previous systematic mutagenesis studies had failed to detect. An overview of the spectrum of the congenital myasthenic syndromes suggests that most are caused by mutations in AChR subunits, and particularly in the ɛ subunit. Future studies will likely uncover new types of CMS that reside in molecules governing quantal release, organization of the synaptic basal lamina, and expression and aggregation of AChR on the postsynaptic junctional folds.     

Mesdc2 plays a key role in cell-surface expression of Lrp4 and postsynaptic specialization in myotubes

Low-density lipoprotein receptor-related protein 4 (Lrp4) is essential for pre- and post-synaptic specialization at the neuromuscular junction (NMJ), an indispensable synapse between a motor nerve and skeletal muscle. Muscle-specific receptor tyrosine kinase MuSK must form a complex with Lrp4 to organize postsynaptic specialization at NMJs. Here, we show that the chaperon Mesdc2 binds to the intracellular form of Lrp4 and promotes its glycosylation and cell-surface expression. Furthermore, knockdown of Mesdc2 suppresses cell-surface expression of Lrp4, activation of MuSK, and postsynaptic specialization in muscle cells. These results suggest that Mesdc2 plays an essential role in NMJ formation by promoting Lrp4 maturation.     

Phospholipase A inhibition of acetylcholine receptor function in Torpedo californica membrane vesicles

A protein isolated from $\text{Naja naja siamensis}$ venom on the basis of its phospholipase A activity inhibits acetylcholine receptor function in post-synaptic membrane vesicles from $\text{Torpedo californica}$. Specifically, the phospholipase A prevents the large increase in sodium efflux that can normally be induced by carbamylcholine, a receptor agonist. The phospholipase A inhibition shows the following properties: 1) it occurs at concentrations 50 times lower than the concentrations required for inhibition by α-neurotoxins; 2) the phospholipase A has no effect on the binding properties of the receptor; 3) the inhibition is abolished by removal of calcium ions; and 4) some phospholipid hydrolysis accompanies inhibition. It is suggested that the phospholipase A acts enzymatically to uncouple ligand binding from ion permeability in the receptor containing membrane vesicles.     

Inhibition of the mitochondrial unfolded protein response by acetylcholine alleviated hypoxia/reoxygenation-induced apoptosis of endothelial cells

The mitochondrial unfolded protein response (UPR^mt) is involved in numerous diseases that have the common feature of mitochondrial dysfunction. However, its pathophysiological relevance in the context of hypoxia/reoxygenation (H/R) in endothelial cells remains elusive. Previous studies have demonstrated that acetylcholine (ACh) protects against cardiomyocyte injury by suppressing generation of mitochondrial reactive oxygen species (mtROS). This study aimed to explore the role of UPR^mt in endothelial cells during H/R and to clarify the beneficial effects of ACh. Our results demonstrated that H/R triggered UPR^mt in endothelial cells, as evidenced by the elevation of heat shock protein 60 and LON protease 1 protein levels, and resulted in release of mitochondrial pro-apoptotic proteins, including cytochrome C, Omi/high temperature requirement protein A 2 and second mitochondrial activator of caspases/direct inhibitor of apoptosis-binding protein with low PI, from the mitochondria to cytosol. ACh administration markedly decreased UPR^mt by inhibiting mtROS and alleviating the mitonuclear protein imbalance. Consequently, ACh alleviated the release of pro-apoptotic proteins and restored mitochondrial ultrastructure and function, thereby reducing the number of terminal deoxynucleotidyl transferase mediated dUTP-biotin nick end labeling (TUNEL)-positive cells. Intriguingly, 4-diphenylacetoxy-N-methylpiperidine methiodide, a type-3 muscarinic ACh receptor (M3AChR) inhibitor, abolished the ACh-elicited attenuation of UPR^mt and TUNEL positive cells, indicating that the salutary effects of ACh were likely mediated by M3AChR in endothelial cells. In conclusion, our studies demonstrated that UPR^mt might be essential for triggering the mitochondrion-associated apoptotic pathway during H/R. ACh markedly suppressed UPR^mt by inhibiting mtROS and alleviating the mitonuclear protein imbalance, presumably through M3AChR.     

Electric field regulated signaling pathways

Physiological electric field (EF) is a potent guidance cue for many physiological development and pathological conditions. The EF induced cellular responses such as migration and proliferation, are considered to be regulated by multiple signaling pathways in a coordinated way. Unlike the signaling transduction regulating the cellular responses toward chemical gradients, the signaling network involved in electric stimulation shows a unique manner, combining the regulation of ion channels, membrane receptors and associated intracellular signaling pathways. This review shall discuss the cellular responses in EF, and summarize the primary signaling network activated during the EF-induced cellular response.