Cellular and secreted forms of acetylcholinesterase in mouse muscle cultures

In Vitro Innervation as an Experimental Model to Study the Expression and Functions of Acetylcholinesterase and Agrin in Human Skeletal Muscle

Acetylcholinesterase (AChE) and agrin, a heparan-sulfate proteoglycan, reside in the basal lamina of the neuromuscular junction (NMJ) and play key roles in cholinergic transmission and synaptogenesis. Unlike most NMJ components, AChE and agrin are expressed in skeletal muscle and α-motor neurons. AChE and agrin are also expressed in various other types of cells, where they have important alternative functions that are not related to their classical roles in NMJ. In this review, we first focus on co-cultures of embryonic rat spinal cord explants with human skeletal muscle cells as an experimental model to study functional innervation in vitro. We describe how this heterologous rat-human model, which enables experimentation on highly developed contracting human myotubes, offers unique opportunities for AChE and agrin research. We then highlight innovative approaches that were used to address salient questions regarding expression and alternative functions of AChE and agrin in developing human skeletal muscle. Results obtained in co-cultures are compared with those obtained in other models in the context of general advances in the field of AChE and agrin neurobiology.

Local control of acetylcholinesterase gene expression in multinucleated skeletal muscle fibers: individual nuclei respond to signals from the overlying plasma membrane

Nuclei in multinucleated skeletal muscle fibers are capable of expressing different sets of muscle-specific genes depending on their locations within the fiber. Here we test the hypothesis that each nucleus can behave autonomously and responds to signals generated locally on the plasma membrane. We used acetylcholinesterase (AChE) as a marker because its transcripts and protein are concentrated at the neuromuscular and myotendenous junctions. First, we show that tetrodotoxin (TTX) reversibly suppresses accumulation of cell surface AChE clusters, whereas veratridine or scorpion venom (ScVn) increase them. AChE mRNA levels are also regulated by membrane depolarization. We then designed chambered cultures that allow application of sodium channel agonists or antagonists to restricted regions of the myotube surface. When a segment of myotube is exposed to TTX, AChE cluster formation is suppressed only on that region. Conversely, ScVn increases AChE cluster formation only where in contact with the muscle surface. Likewise, both the synthesis and secretion of AChE are shown to be locally regulated. Moreover, using in situ hybridization, we show that the perinuclear accumulation of AChE transcripts also depends on signals that each nucleus receives locally. Thus AChE can be up- and downregulated in adjacent regions of the same myotubes. These results indicate that individual nuclei are responding to locally generated signals for cues regulating gene expression.

Calcitonin gene-related peptide decreases expression of acetylcholinesterase in mammalian myotubes

Nerve-derived trophic factors are known to modulate expression of acetylcholinesterase (AChE) in skeletal muscle fibers, yet the precise identity of these factors remains elusive. In the present study, we treated mouse C2 myotubes with calcitonin gene-related peptide (CGRP). Compared to non-treated myotubes, cell-associated AChE activity levels were decreased by approximately 60% after 48 h of treatment. A parallel reduction in AChE total protein levels was also observed as determined by Western blot analysis. The reduction in AChE activity was due to a decrease in the levels of the G1 molecular form and to an elimination of G1. By contrast, levels of secreted AChE remained unchanged following CGRP treatment. Finally, the overall decrease in AChE activity was accompanied by a reduction in AChE transcripts which could not be attributed to changes in the transcriptional rate of the ACHE gene.

Acetylcholinesterase mRNA level and synaptic activity in rat muscles depend on nerve-induced pattern of muscle activation

Acetylcholinesterase (AChE) mRNA levels are severalfold higher in fast rat muscles compared with slow. We hypothesized that AChE mRNA levels and AChE activity in the neuromuscular junction depend on a specific nerve-induced pattern of motor unit activation. Chronic low-frequency stimulation, mimicking the activation pattern in slow muscles, was applied to fast muscles in rats. Molecular forms of AChE were analyzed by velocity sedimentation, and AChE mRNA levels were analyzed by Northern blots. AChE mRNA levels in stimulated fast muscles dropped to 10-20% of control after 1 week and became comparable to those in slow soleus muscles. The activity of the junctional A12 AChE form in 35 d stimulated fast muscles decreased to 56% of control value, reaching that in the soleus muscle. Therefore, synaptic AChE itself depends on the muscle activation pattern. Complete inactivity after denervation also decreased the AChE mRNA level in fast muscles to <10% in 48 hr. In contrast, profuse fibrillations observed in noninnervated immature regenerating muscles maintain AChE mRNA levels at 80% of that in the innervated fast muscles. If protein synthesis was inhibited by cycloheximide, AChE mRNA levels in 3-d-old regenerating muscle, still containing myoblasts, increased approximately twofold. No significant increase after cycloheximide application was observed either in denervated mature fast muscles or in normal slow muscles. Low AChE mRNA levels observed in those muscles are probably not caused by decreased stability of AChE mRNA as demonstrated in myoblasts.

Biochemical and molecular characterization of acetylcholinesterase from the hagfish Myxine glutinosa

To obtain information about the evolution of the cholinesterases, we investigated the cholinesterase activity of an agnathan vertebrate, the hagfish Myxine glutinosa. On the basis of evidence from enzymology, pharmacology, and molecular biology, we conclude that the cholinesterase activity is due to acetylcholinesterase (AChE). The enzyme hydrolyzes acetylthiocholine preferentially and exhibits substrate inhibition. The hydrolysis of both acetylthiocholine and butyrylthiocholine are inhibited in parallel by cholinesterase inhibitors, with the AChE-specific drug BW284c51 being the most potent; however, this drug and propidium, a peripheral anionic site ligand, are much weaker inhibitors of the hagfish enzyme than of Torpedo AChE. We used sequential extraction, collagenase digestion, and velocity sedimentation on sucrose gradients to determine that the AChE from the skeletal muscle of the hagfish is present in both globular and asymmetric forms. We also used the polymerase chain reaction with degenerate oligonucleotide probes and genomic DNA to obtain a 1 kb gene fragment for hagfish AChE. The enzyme has an acyl binding site typical of other vertebrate AChE, but lacks two aromatic residues implicated in the function of the peripheral anionic subsite. We discuss the relevance of our findings to the evolution of the cholinesterases in the vertebrates.

Regulation of the expression of acetylcholinesterase by muscular activity in avian primary cultures

Primary cultures of avian muscle cells express both globular and asymmetric molecular forms of acetylcholinesterase (AChE) when grown in a simple defined culture medium. Under these conditions, we analyzed the role of various agents interfering with muscular activity: tetrodotoxin (TTX) and veratridine, as well as a depolarizing concentration of KCl. These treatments caused the complete cessation of contractions in mature myotubes. We observed no influence on cellular AChE activity. The paralyzing treatments induced different effects on AChE secretion: TTX increased the secretion by approximately 25%, whereas KCl and veratridine reduced it by approximately 30%. The proportions of secreted molecular forms (mostly hydrophilic G4 and G2) were not modified significantly. TTX did not affect the pattern of molecular forms of cellular AChE (in particular, the proportion of A forms was not changed). Depolarization by veratridine or KCl induced an increase in the proportion of A forms in mature myotubes by a factor of 2-3. Similar results were obtained with quail myotubes cultured under the same conditions. This study shows that, in avian muscle cultures, the ionic balance across myotube membranes, rather than muscular activity per se, can regulate the level of A forms and the rate of AChE secretion. These results do not exclude the possible involvement of other factors, such as Ca2+ and/or peptidic factors. In addition, taking together our results and data from the literature. we conclude that the expression of AChE molecular forms depends both on the species and on the culture conditions used.

Generation of subunit-specific antibody probes for Torpedo acetylcholinesterase: cross-species reactivity and use in cell-free translations

The assembly of the collagen tailed A12 form of acetylcholinesterase (AChE) is regulated by muscle contraction. To begin to study this regulation, we derived antibody probes for the three subunits (100 kd, catalytic, and collagen tail) of AChE purified from Torpedo californica electric tissue. These included a polyclonal antiserum that recognizes all 3 subunits and 19 monoclonal antibodies; 16 of the monoclonals recognized the catalytic subunit, 2 recognized the tail subunit, and 1 recognized the 100 kd subunit on Western blots. We used immunohistochemical procedures to show that several of the anticatalytic and one of the antitail monoclonals cross-reacted with frog muscle AChE and Western blotting to show that several of the anticatalytic monoclonals cross-react with rat brain AChE. These antibodies were then used to immunoprecipitate AChE precursors from a cell-free translation system. There were generally three primary translation products, corresponding to the three enzyme subunits. Therefore, each subunit is probably derived from a separate mRNA. Occasionally there were two translation products corresponding to the catalytic subunit alone. The catalytic subunit was glycosylated following addition of canine microsomal membranes to the translation mix. The mRNA coding for this subunit appeared to be present in the poly(A)- RNA pool.