Action of phospholipase C at the neuromuscular junction in rodent skeletal muscles

Gene Expression Profile at the Motor Endplate of the Neuromuscular Junction of Fast-Twitch Muscle

The neuromuscular junction (NMJ) is a prototypic chemical synapse between the spinal motor neuron and the motor endplate. Gene expression profiles of the motor endplate are not fully elucidated. Collagen Q (ColQ) is a collagenic tail subunit of asymmetric forms of acetylcholinesterase and is driven by two distinct promoters. pColQ1 is active throughout the slow-twitch muscle, whereas pColQ1a is active at the motor endplate of fast-twitch muscle. We made a transgenic mouse line that expresses nuclear localization signal (NLS)-attached Cre recombinase under the control of pColQ1a (pColQ1a-Cre mouse). RiboTag mouse expresses an HA-tagged ribosomal subunit, RPL22, in cells expressing Cre recombinase. We generated pColQ1a-Cre:RiboTag mouse, and confirmed that HA-tagged RPL22 was enriched at the NMJ of tibialis anterior (TA) muscle. Next, we confirmed that Chrne and Musk that are specifically expressed at the NMJ were indeed enriched in HA-immunoprecipitated (IP) RNA, whereas Sox10 and S100b, markers for Schwann cells, and Icam1, a marker for vascular endothelial cells, and Pax3, a marker for muscle satellite cells, were scarcely detected. Gene set enrichment analysis (GSEA) of RNA-seq data showed that “phosphatidylinositol signaling system” and “extracellular matrix receptor interaction” were enriched at the motor endplate. Subsequent analysis revealed that genes encoding diacylglycerol kinases, phosphatidylinositol kinases, phospholipases, integrins, and laminins were enriched at the motor endplate. We first characterized the gene expression profile under translation at the motor endplate of TA muscle using the RiboTag technique. We expect that our gene expression profiling will help elucidate molecular mechanisms of the development, maintenance, and pathology of the NMJ.

Electric organ polyamines and their effects on the acetylcholine receptor

1. The electric organ of Torpedo nobiliana contained putrescine (PUT), spermidine (SPD), spermine (SPM), and cadaverine (CAD). Traces of acetylated SPD and SPM were occasionaly seen. 2. Upon fractionation of the tissue by differential centrifugation, the polyamines (PA) were found predominantly in the soluble fraction. The postsynaptic membrane fraction, containing a high concentration of acetylcholine receptor (AChR), was proportionally enriched in SPM. The molar ratio of SPM to AChR was approximately two in these membranes. 3. The effect of exogeneous PA on AChR function was studied by two methods: carbamoylcholine (CCh)-dependent 86Rb+ influx into receptor-rich membrane vesicles and [alpha-125I]bungarotoxin (Bgt) binding to the AChR. 4. SPM inhibited both ion influx and the rate of Bgt binding at concentrations above 1 mM, and therefore it appears to act as a competitive antagonist of the AChR. 5. At submicromolar concentrations, and only after preincubation with the receptor-rich membrane, SPM and PUT increased the ion influx by about 20% over control values. 6. Preincubation with 100 nM SPM did not affect the equilibrium binding of iodinated toxin or the rate of toxin binding, and therefore SPM was not uncovering new receptors. 7. By measuring the initial rate of toxin binding after different periods of preincubation with 1 microM CCh, the rate of the slow phase of receptor desensitization was determined. This rate was not changed by 100 nM SPM. 8. Although these results suggest that at low concentrations SPM is a positive modulator of the AChR, the precise mechanism of action is not determined yet.

Positive modulators of muscle acetylcholine receptor

A solution of succinic anhydride (SA) in buffer N-tris(hydroxymethyl)methyl-2-aminoethane sulfonic acid (TES), the SATES solution, potentiates the effect of carbonyl containing agonists on frog muscle (del Castillo et al., Br. J. Pharmac. 84: 275-288, 1985). Here we report that the main compound in the SATES solution is a monosuccinyl ester of TES (MST). This compound is not an agonist of the acetylcholine (ACh) receptor nor is it an inhibitor of ACh esterase, yet MST potentiates the ACh-induced tension and depolarization in frog muscle to a new maximum. It increases the amplitude of miniature endplate potentials but has no effect on the time course of miniature endplate currents. The acetylated analogue of MST (mono-acetyl-TES: MAT) had similar but more pronounced effects on frog muscle. Neither MST nor MAT affect [3H]Ach binding to receptor-rich membranes from the electric organ of Torpedo californica, or the affinity state transition induced by agonists. Radiolabeled MST does not bind to these membranes and MAT does not alter agonist-induced ion flux. Therefore, these compounds seem to act as positive modulators of muscle ACh receptor but are inactive in Torpedo vesicles.

Effect of soluble factors from nerve and muscle on alpha-bungarotoxin binding sites in isolated sarcolemmal membranes of the rat

1. Sarcolemmal membranes were isolated from normal and from denervated muscles and the specific binding of [125I]-alpha-bungarotoxin to the membranes was determined. 2. Cytosol prepared from either slow (soleus) or fast (extensor digitorum longus) muscle increased the toxin binding. Similar effects were seen with cytosol from muscles which had been denervated 5 to 7 days previously and with cytosol from sciatic nerve. A purified preparation of a bacterial phospholipase C also produced this effect. 3. Bathing medium in which normal or denervated muscles had been incubated, increased the toxin binding to normal membranes, but the normal bathing medium decreased the binding to membranes prepared from denervated muscles. 4. The possibility that pre-existing latent acetylcholine receptors can be activated by intracellular factors released from muscles is discussed.