Regulation of acetylcholinesterase in cultured muscle by chemical agents and electrical stimulation

Termination and beyond: acetylcholinesterase as a modulator of synaptic transmission

Termination of synaptic transmission by neurotransmitter hydrolysis is a substantial characteristic of cholinergic synapses. This unique termination mechanism makes acetylcholinesterase (AChE), the enzyme in charge of executing acetylcholine breakdown, a key component of cholinergic signaling. AChE is now known to exist not as a single entity, but rather as a combinatorial complex of protein products. The diverse AChE molecular forms are generated by a single gene that produces over ten different transcripts by alternative splicing and alternative promoter choices. These transcripts are translated into six different protein subunits. Mature AChE proteins are found as soluble monomers, amphipatic dimers, or tetramers of these subunits and become associated to the cellular membrane by specialized anchoring molecules or members of other heteromeric structural components. A substantial increasing body of research indicates that AChE functions in the central nervous system go far beyond the termination of synaptic transmission. The non-enzymatic neuromodulatory functions of AChE affect neurite outgrowth and synaptogenesis and play a major role in memory formation and stress responses. The structural homology between AChE and cell adhesion proteins, together with the recently discovered protein partners of AChE, predict the future unraveling of the molecular pathways underlying these multileveled functions.

Sarcoplasmic-reticulum biogenesis in contraction-inhibited skeletal-muscle cultures

We have previously shown that inhibition of the spontaneous contractile activity of cultured embryonic-chick skeletal-muscle fibres with tetrodotoxin (TTX) leads to decreased sarcoplasmic-reticulum Ca(2+)-transport rates and steady-state concentrations of the high-energy Ca(2+)-ATPase phosphoenzyme intermediate [Charuk & Holland (1983) Exp. Cell Res. 144, 143-157]. In the present study we used a monoclonal antibody to the Ca(2+)-ATPase to show that there is a decreased amount of enzyme accumulated by contraction-inhibited myotubes. Indirect immunofluorescence microscopy using the monoclonal antibody to the Ca(2+)-ATPase also revealed a disordered subcellular organization of the sarcotubular system in contraction-inhibited myotubes. The biogenesis of sarcoplasmic-reticulum proteins in TTX-paralysed myofibres was studied by labelling cells with [35S]methionine before isolation of the active Ca(2+)-pump membrane fraction. Protein turnover was selectively increased in that fraction from TTX-treated muscle cultures. Electrophoretic analysis and quantitative fluorography confirmed that decreased accumulation of the Ca(2+)-ATPase enzyme in contraction-inhibited myotubes was associated with increased turnover of this protein. The present results demonstrate that biogenesis of the sarcoplasmic-reticulum Ca(2+)-ATPase is regulated by the contractile activity of skeletal-muscle fibres.

Calcium and ionophore A23187 stimulates deposition of extracellular matrix and acetylcholinesterase release in cultured myotubes

Calcium (Ca2+) and calcium-transporting ionophores stimulate protein secretion in many cellular systems. We demonstrate here than increases in intracellular calcium concentration induce a time- and concentration-dependent deposition of extracellular matrix and an increase in acetylcholinesterase secretion. Scanning and transmission electron-microscopy revealed that treatment with the calcium ionophore A23187, or high extracellular Ca2+ levels (5 mM to 15 mM) produce significant deposits of extracellular matrix around the myotubes, as well as a marked increase in the acetylcholinesterase reaction-product. Blocking muscle contraction was not necessary for the induction of AChE secretory activity. Sucrose density-gradients of media conditioned by muscle cells revealed 3 separate acetylcholinesterase molecular forms. However, incubation with A23187 increased only the 4.5 S and the 7.2 S molecular forms, whereas the 12.0 S form showed no significant differences from controls. Polyacrylamide gel electrophoresis, and autoradiography using [3H]diisopropyl fluorophosphate revealed a broad band at 65,000 daltons. This band was broader than for controls when medium was obtained from A23187-treated cells. Our results show that increasing intracellular Ca2+ concentration induces marked deposition of extracellular matrix and increased acetylcholinesterase secretion, with an apparent selectivity for the monomeric and dimeric acetylcholinesterase molecular forms.

Contractile activity is required for the expression of neonatal myosin heavy chain in embryonic chick pectoral muscle cultures

The expression of neonatal myosin heavy chain (MHC) was examined in developing embryonic chicken muscle cultures using a monoclonal antibody (2E9) that has been shown to be specific for that isoform (Bandman, E., 1985, Science (Wash. DC), 227: 780-782). After 1 wk in vitro some myotubes could be stained with the antibody, and the number of cells that reacted with 2E9 increased with time in culture. All myotubes always stained with a second monoclonal antibody that reacted with all MHC isoforms (AG19) or with a third monoclonal antibody that reacted with the embryonic but not the neonatal MHC (EB165). Quantitation by ELISA of an extract from 2-wk cultures demonstrated that the neonatal MHC represented between 10 and 15% of the total myosin. The appearance of the neonatal isoform was inhibited by switching young cultures to medium with a higher [K+] which has been shown to block spontaneous contractions of myotubes in culture. Furthermore, if mature cultures that reacted with the neonatal antibody were placed into high [K+] medium, neonatal MHC disappeared from virtually all myotubes within 3 d. The effect of high [K+] medium was reversible. When cultures maintained in high [K+] medium for 2 wk were placed in standard medium, which permitted the resumption of contractile activity, within 24 h cells began to react with the neonatal specific antibody, and by 72 h many myotubes were strongly positive. Since similar results were also obtained by inhibiting spontaneous contractions with tetrodotoxin, we suggest that the development of contractile activity is not only associated with the maturation of myotubes in culture, but may also be the signal that induces the expression of the neonatal MHC.

Effect of tetrodotoxin relaxation of cultured skeletal muscle on the sarcoplasmic reticulum Ca2+-transport ATPase

The spontaneous contractions of cultured chick skeletal muscle fibers were abolished by growth of cultures in the presence of tetrodotoxin (TTX). Inhibition of the contractile activity of cultured myofibers was associated with a marked reduction in the rate of azide-insensitive, ATP-dependent Ca2+ uptake by the total particulate fraction of cell homogenates and by purified sarcoplasmic reticulum. Myosin heavy chain (MHC) accumulation and azide-insensitive, ATP-dependent Ca2+ uptake into a total cell membrane fraction were measured simultaneously in the same culture dish. A decrease in the activity of the ATP-dependent Ca2+ uptake system preceded a significant reduction in MHC content of contraction-inhibited cultures. The reduced rate of Ca2+ uptake observed in the sarcoplasmic reticulum from TTX-treated cultures paralleled a decrease in the amount of enzymatically active Ca2+-transport ATPase. The cellular concentration of the ATPase was estimated from a measurement of the concentration of the Ca2+-dependent, hydroxylamine-sensitive, steady state level of phosphorylated intermediate formed in culture microsomes. In contrast to the changes observed in activity of the sarcoplasmic reticulum ATPase and MHC content of TTX-treated cultures, neither the specific activity of creatine kinase nor the accumulation of the MM isoenzyme were affected. It is therefore concluded that the contractile activity of muscle has a selective effect on the maintenance of the adult skeletal muscle phenotype.

Structure of developing frog neuromuscular junctions

Developing neuromuscular junctions in the cutaneous pectoris muscle from tadpoles and and postmetamorphic frogs were studied in the light microscope. Presynaptic nerve terminals and postsynaptic acetylcholinesterase (AChE) activity simultaneously demonstrated at these developing junctions using the NBT-AChE method. The earliest nerve contacts studied were small enlargements at the ends of unmyelinated axons. As development progressed, single nerve contacts, often with growth cones, grew in length, generally parallel to the long axis of the myotube. Further endplate maturation was characterized by individual terminal processes developing secondary and tertiary branches, ultimately leading to highly complex terminal arborizations. The initial synaptic contact at developing neuromuscular junctions was made by a single axon, but with further development these same synaptic sites became multiply innervated. The occurrence of multiple innervation was a transient phenomena; the multiple synaptic inputs were eliminated during further maturation. The time course of synapse elimination was protracted, with some multiple innervation even persisting in relatively large adult frogs. The first nerve contacts were generally devoid of observable endplate AChE activity. Early appearance of AChE activity was sometimes graded and in some cases portions of the nerve terminal processes were associated with AChE while other regions of the same terminal arborization were not.

Diazepam inhibits myoblast fusion and expression of muscle specific protein synthesis

The presence of diazepam in culutres of chicken embryo myoblasts arrests normal muscle cell differentiation. High concentrations of the drug reversibly prevent myoblasts from fusing to form multinucleated myotubes. Lower concentrations of diazepam allow cell fusion to occur, but inhibit the synthesis and accumulation of myosin heavy chain, implying that cell fusion does not obligate myoblasts to synthesize and accumulate large quantities of muscle specific protein. The effect of diazepam on muscle cells in culture is direct and specific.

Regulation of acetylcholinesterase in chick muscle cultures after treatment with diisopropylphosphorofluoridate: Ribonucleic acid and protein synthesis

Previous studies have established that short treatments with diisoprophylphosphorofluoridate of chick embryo muscle cultures irreversibly inhibit the enzyme acetylcholinesterase and that the enzyme's levels in the cells rapidly recover due to synthesis of new protein. In addition, it has been shown that acetylcholine, acetyl-beta-methylcholine and choline increase the acetylcholinesterase content of the cultures. In the experiments presented here, actinomycin D and cycloheximide were used to study the relationships between the synthesis of ribonucleic acid and of protein, the recovery of acetylcholinesterase levels after diisopropylphosphorofluoridate treatment and the increase of acetylcholinesterase levels evoked by choline and its esters. Both recovery of acetylcholinesterase and the increase in its activity with acetyl-beta-methylcholine occurred in the absence of ribonucleic acid synthesis but required protein synthesis. The results suggest that transcription of desoxyribonucleic acid and a change in the degradation rate of acetylcholinesterase are not involved. An additional finding was that the level of newly synthesized acetylcholinesterase activity continued to increase for a short period of time after synthesis of protein had ceased, as if some previously synthesized protein was being transformed into active enzyme.