Acetylcholinesterase activity in developing skeletal muscle cells

Ultrastructure of sites of cholinesterase activity on amphibian embryonic muscle cells cultured without nerve

Muscle cells, derived from 1-day-old embryos of Xenopus laevis, were cultured in the absence of nerve and then examined histochemically for cholinesterase activity. Virtually all of the cells exhibited one or more discrete patches of cholinesterase activity on their surface. The patches were located not only on the lower cell surface which was apposed to the floor of the culture dish but also on the lateral and upper surfaces which were not apposed to any solid substrate. At many of the cholinesterase patches the cell surface was invaginated and the invaginations also became filled with the histochemical reaction product. Surface sites of cholinesterase activity were often associated with a thickened plasma membrane and overlying basal lamina-like material. Regions of the cell surface without cholinesterase were typically smooth and ultrastructurally unspecialized. The cholinesterase patches and their associated ultrastructural features developed in the absence of electrical and contractile activity. The similarities between the muscle specializations at cholinesterase sites on cultured, noninnervated, Xenopus muscle cells and those at the neuromuscular and myotendinous junctions of normal skeletal muscle are discussed.

The regulation of acetylcholinesterase by cis-elements within intron I in cultured contracting myotubes

The onset of spontaneous contraction in rat primary muscle cultures coincides with an increase in acetylcholinesterase (AChE) activity. In order to establish whether contractile activity modulates the rate of AChE transcript synthesis, and what elements of the gene are determinant, we examined the promoter and intron I in contracting muscle cultures. Ache genomic fragments attached to a luciferase reporter were transfected into muscle cultures that were either electrically stimulated or paralyzed with tetrodotoxin to enhance or inhibit contractions, respectively. Cultures transfected with intron I-containing constructs showed a 2-fold increase in luciferase activity following electrical stimulation, compared to tetrodotoxin treatment, suggesting that this region contains elements responding to contractile activity. Deleting a 780 bp distal region within intron I, containing an N-box element at +890 bp, or introducing a 2-bp mutation within its core sequence, eliminated the contraction-induced response. In contrast, mutating an N-box element at +822 bp had no effect on the response. Furthermore, co-transfecting a dominant negative GA-binding protein (GABP), a transcription factor known to selectively bind N-box elements, reduced the stimulation-mediated increase. Our results suggest that the N-box within intron I at +890 bp is a regulatory element important in the transcriptional response of Ache to contractile activity in muscle.

Regulation of NCAM by growth factors in serum-free myotube cultures

Regulation of the neural cell adhesion molecule (NCAM) was examined in primary cultures of chick skeletal muscle grown in serum-free defined medium. Relative levels of NCAM (per microgram protein) increased 20-30% in myotubes grown on Matrigel, a reconstituted basement membrane preparation, compared to those grown on collagen; total NCAM levels on Matrigel were increased 40-55% due to the additional increase in total protein. A dose dependent increase in relative NCAM levels in myotubes grown on Matrigel in defined medium was observed with the addition of adsorbed horse serum, while relative NCAM levels in myotubes grown on collagen were unaffected by altering the serum concentration. Thus, extracellular matrix molecules and soluble factors exert trophic effects on myotube NCAM expression. Similar developmental changes in the expression of the different molecular size forms of NCAM occurred in myotubes grown on collagen and Matrigel: levels of 150K and 135K Mr forms decreased during development, while 125K remained prominent in older myotubes. Relative NCAM levels were specifically enhanced 11-26% by several factors: nerve growth factor, thyroxine, insulin-like growth factor II, dibutyryl cyclic AMP, veratridine (a sodium ion channel agonist), and nisoldipine (a calcium ion channel agonist). Total protein and overall myotube development in serum-free cultures were enhanced by fetuin, insulin-like growth factor II, acidic fibroblast growth factor, calcitonin gene-related peptide, dibutyryl cyclic AMP, and veratridine. Thus, changes in extracellular matrix, intracellular calcium, and sodium ions, as well as extracellular trophic factors, such as nerve growth factor, thyroxine, and insulin-like growth factor II, may regulate muscle NCAM expression during embryonic development.

Membrane-bound acetylcholinesterase: an early differentiation marker for skeletal myoblasts

Cell-bound acetylcholinesterase (AChE) was found to be an early differentiation marker on embryonic chick skeletal myoblasts in mixed primary cell cultures. AChE biosynthesis was detected and characterized by (a) a sensitive microtiter assay, (b) use of selective inhibitors, and (c) with mono- and polyclonal antibodies. Both secreted and cell-bound AChE appeared on the first day in culture, at a time when no muscle cell fusion was observed. Characterization of this enzyme revealed that true AChE was bound and secreted by myoblasts. BW284c51, which permeates cell membranes poorly, inhibited all the cell-associated AChE activity on myoblasts, suggesting that the activity measured was on the outer cell surface. On the other hand, fibroblasts appeared to have no or very little bound enzyme and the low level of secreted enzyme activity had the characteristics of pseudo-, or butyrylcholinesterase. Polyclonal anti-Torpedo californica electroplax AChE antibody and several monoclonal antibodies were found to bind specifically to chick myoblasts. Since the cells had not been made permeable before antibody binding, a membrane-bound form of the enzyme was most likely being detected. The cell-bound true AChE was present in identifiable quantities from the first day of culture. Membrane-bound AChE can thus serve as an early differentiation marker for embryonic chick myoblasts in mixed primary cultures.

A monoclonal antibody specific for avian early myogenic cells and differentiated muscle

A monoclonal antibody raised in mouse in response to homogenates of Remark ganglia and dorsal mesentery of chicken embryos was found to exhibit a unique reactivity towards myogenic cells, heart, striated muscles, and smooth muscles in chicken and quail. Indirect immunofluorescence assays were performed at different stages of chicken and quail embryonic development and, after hatching, on tissue sections and cultured cells. They revealed that the cytoplasmic marker recognized by 13F4 is expressed in early embryonic heart, in somitic myotome (from stage 14 onward), in the skeletal muscles in limbs and trunk, in all muscles in the head and the branchial arches, in the smooth muscles of the digestive tract and blood vessels. In myofibrils of striated muscles, the antigen is localized in the Z lines. The antigenicity of the molecule recognized by 13F4 is not associated with a glycolipid or a glycoprotein. It is of peptidic nature and its molecular weight is 54 kDa. We stress the value of this cell-type-specific marker in studies on ontogenesis and differentiation of all muscular structures, namely, of myocardium and striated muscles, which express 13F4 antigenicity from an early developmental stage.

Ontogenesis of autonomic receptors and AChE activity in the rat vas deferens

The study was undertaken to obtain some information about the development of the autonomic receptors and the AChE activity in the rat vas deferens. The results suggest that the adrenoceptors were fully developed at birth. The M1-ACh receptors were developed before the M2-ACh receptors. The AChE activity developed before the ACh muscarinic receptors of the rat vas deferens.

Oligosaccharyltransferase activity is markedly increased during differentiation of a nonfusing myoblast cell line

We have studied several aspects of glycoprotein synthesis in myoblast differentiation by using a nonfusing myoblast cell line, BC3H1. Previous studies showed that transfer of proliferating undifferentiated BC3H1 cells to mitogen-depleted medium results in the cells' withdrawal from the cell cycle and induction of a variety of muscle-specific gene products [E. N. Olson, L. Glaser, J. P. Merlie, R. Sebane, and J. Lindstrom (1983) J. Biol. Chem. 258, 13946-13953]. Because cell surface glycoproteins have been implicated in myoblast differentiation, in the present study we measured the amount of oligosaccharyltransferase in microsomes isolated from BC3H1 cells at various stages of differentiation. By using an acceptor peptide containing the sequence-Asn-Leu-Thr-, enzyme activity was measured by formation of [3H]glycopeptide. In addition, active enzyme protein was measured with a 125I-labeled photoreactive derivative of the acceptor tripeptide. Both of these independent assay methods revealed a marked increase in oligosaccharyltransferase when differentiation was induced by serum depletion. Moreover, mitogenic stimulation of differentiated cells resulted in a return of oligosaccharyltransferase to near basal levels. This reversible increase in this key enzyme in protein glycosylation occurred despite the fact that both total protein and glycoprotein synthesis were depressed during differentiation. These data indicate that during myogenesis the level of oligosaccharyltransferase is regulated in parallel with a number of muscle-specific gene products. These results are discussed in the context of regulation of the pathway of glycoprotein synthesis.

Changes in cell surface antigens during in vitro lizard myogenesis

Indirect immunofluorescence has been used to examine surface antigens of lizard myogenic cells during in vitro differentiation. At least two developmental stage-specific surface alterations have been identified. One of these is a compositional change and involves the appearance of a cell-surface antigen(s) as the cells differentiate. This antigen(s) (Ag1422) is muscle specific and is characteristic of some rounded-up G0 myosin-positive myocytes, all stretched-back, G0 myosin-positive myocytes, and all identifiable myotubes. The antigen is not found on proliferating myoblasts, extended G1 (myosin-negative) cell-cycle-competent myoblasts or newly differentiated rounded-up, G0 myosin-positive myocytes. Pretreatment of cells with neuraminidase, trypsin, or proteinase K indicates the antigen is not present in "masked" form on normally nonreactive cells. Proteinase K is effective in the removal or destruction of the antigen, indicating it is at least partially protein in nature. The antigen is expressed in a similar developmental stage-specific fashion on early-passage myogenic cells taken from both adult lizard tail regenerates and embryonic muscle. The antibodies identifying Ag1422 can be removed by adsorption with homogenates of mature skeletal muscle. Therefore, Ag1422 is not an artifact due to in vitro conditions or the expression of a transformation antigen unique to the continuous culture line. The second alteration is an apparent restriction in the mobility of surface components (antigens and lectin receptors). Upon treatment with multivalent ligands, undifferentiated myosin-negative myoblasts exhibit rapid patching and capping of cell surface components while well-differentiated myocytes and myotubes do not. This mobility restriction is evident after the appearance of Ag1422. Treatment with cytochalasin B (15 micrograms/ml) and/or colchicine (100 microM) does not alter the restricted mobility of surface components seen on differentiated cells. Therefore, neither microfilaments nor microtubules seem to be involved in the mobility restriction. These observations are discussed in relation to current views of myogenesis.

Exercise effects on recovery of muscle acetylcholinesterase from reduced neuromuscular activity

In order to investigate the effects of reduced and subsequently increased neuromuscular activity on muscle acetylcholinesterase (AChE), rats had one hindlimb immobilized with plaster casts for 4 weeks and were killed either at the end of immobilization (group I), after 4 weeks of resumed normal activity following cast removal (group R), or after 4 weeks of resumed activity supplemented with a daily treadmill-walking task (group E). Immobilization resulted in a decrease in adductor longus muscle weight to 66.4% of control; total muscle end-plate-associated AChE was decreased to 51.4%. Total muscle ACh hydrolysis was not significantly affected. Mild daily exercise during recovery increased total muscle end-plate AChE to control levels after 4 weeks, while in group R the corresponding level was significantly lower (84.4%). Decreased neuromuscular activity has different effects on end-plate AChE and non-end-plate AChE. Mild endurance-type overload during recovery from immobilization can accelerate recovery of end-plate AChE activity to normal.

Neurotransmitter metabolizing enzymes and plasma butyrylcholinesterase in mice exposed to trichloroethylene

Acetylcholinesterase, glutamine synthetase, acid phosphatase and glutamate dehydrogenase activity in brain and cholinesterase activity in blood were investigated in mice exposed to 170 p.p.m. trichloroethylene (TCE) during 30 days. The neuronal enzymes remained unaffected which suggests that no general damage occurred to either the glia or the nerve cell populations. In accordance with this no effect was seen on acid phosphatase. In contrast, plasma butyrylcholinesterase increased twofold in exposed male mice while it was unaffected in females. Liver weight in males and females increased with a factor of 1.5 and 1.9, respectively.

Characterization of embryonic cholinesterase in chick limb bud by colorimetry and disk electrophoresis

In the chondrogenic blastema of the chick limb bud an histochemical cholinesterase activity related to aggregation of the chondroblasts was described by Drews and Drews (1972). This cholinesterase activity was termed "embryonic cholinesterase" (Drews 1975). In the present study embryonic cholinesterase from the limb bud has been characterized by colorimetry and disk electrophoresis and compared to cholinesterase from the myotomes of the same embryos. Embryonic cholinesterase comprises both acetylcholinesterase and butyrylcholinesterase activity. The banding patterns of embryonic cholinesterase from limb bud and of cholinesterase from myotomes are identical. These findings support the hypothesis that the cholinergic system is involved in the regulation of embryonic development.

Manipulation of myogenesis in vitro: reversible inhibition by DMSO

A system has been developed for the detailed analysis of the transition from proliferative myoblast to differentiated muscle cell. Dimethylsulfoxide (DMSO) prevents the terminal differentiation of L8 myoblasts in vitro, and its effect is reversible. DMSO (2%) inhibits the fusion of myoblasts to form multinucleate myotubes, the normal increases in activity of creatine phosphokinase (CPK) and acetylcholinesterase, and the synthesis of alpha-actin and acetylcholine receptor protein. Upon removal of DMSO from the medium, a lag precedes the onset of differentiation. The potential to inhibit muscle differentiation reversibly is not specific to DMSO, but is shared by a number of compounds, including dimethylformamide, hexamethylbisacetamide and butyric acid, all potent inducers of gene expression in Friend erythroleukemia cells. L8 cells routinely cease DNA synthesis and initiate fusion and muscle protein synthesis once they are confluent. In the presence of DMSO, however, nearly all cells continue DNA synthesis, even several days after reaching confluence. Protein synthetic patterns of DMSO-inhibited cells are almost indistinguishable from those of untreated myoblasts and distinct from differentiated myotubes. It appears that cells exposed to DMSO are locked indefinitely in a proliferative myoblast stage of development and are unable to enter the Go phase of the cell cycle necessary for initiation of differentiation. DMSO coordinately inhibits all the differentiative parameters measured. In contrast, cytochalasin B uncouples normally linked differentiative events so that fusion is inhibited while muscle-specific protein synthesis proceeds. DMSO has similar effects on both cytochalasin B-treated and fusing control cultures, suggesting that its primary effect is exerted not at the level of fusion but earlier in the differentiative time-table. Once fusion and the synthesis of muscle-specific proteins are well under way, the addition of DMSO is ineffective and differentiation continues in its presence. The potential to manipulate muscle gene expression in vitro makes this system particularly useful for the detailed analysis of the processes involved in the transition to the differentiated state and for determining the linkage of developmental events.

Acetylcholinesterase differentiation during myogenesis in early chick embryonic cells caused by an inducer RNA

In the stage 4 chick blastoderm, an area located 0.6 mm posterior to Hensen's node, the post-nodal piece (PNP), consists of an undifferentiated population of cells, since the explants when cultivated in vitro in a variety of media do not develop into any histologically identifiable structures. However, addition of a specific low molecular weight RNA isolated from the 16-day-old chick embryonic heart promotes the appearance of a distinct mode of morphological and biochemical changes that is similar to that of embryonic cardiogenic process. The RNA-induced changes in the PNP also include a marked increase in acetylcholinesterase activity. The increase in enzymatic activity can be measured biochemically, as well as visualized histochemically.

Neurotrophic protein regulates muscle acetylcholinesterase in culture

Skeletal muscles lose acetylcholinesterase in culture as a result of denervation. A protein fraction isolated from peripheral nerves maintained the level of acetylcholinesterase in cultures of aneural embryonic muscle or denervated adult chicken muscle. These results indicate that trophic regulation of muscle acetylcholinesterase might be mediated by a protein produced by nerves.

Sythesis of tropomyosin in cultures of differentiating muscle cells

The accumulation of tropomyosin in cultures of differentiating muscle cells was quantitatively measured. Tropomyosin was isolated from cultured cells during and after myoblast fusion; both alpha- and beta-subunits were present in myotube cultures. During fusion small amounts of tropomyosin were detectable, but, as fusion approached a maximum, tropomyosin accumulation began to increase. The increased synthesis of tropomyosin after the initiation of muscle cell fusion is consistent with the increased synthesis of other proteins characteristic of muscle, including myosin.

Enzymatic activities of muscle fibres differentiated, in vitro, from pectoralis major (white) and adductor magnus (red) muscles of chick embryos

Specific activities of NADP isocitrate dehydrogenase and acetylcholinesterase were significantly higher in muscle fibres differentiated, in vitro, from myoblasts of adductor magnus (red) than pectoralis major (white) muscles 10-day-old chick embryos. This is evidence, as far as enzyme activities are concerned, that myoblasts from different types of skeletal muscles are able to give, in tissue culture, muscle fibres of different properties, even in the absence of nerve supply.

A new myasthenic syndrome with end-plate acetylcholinesterase deficiency, small nerve terminals, and reduced acetylcholine release

A new myasthenic syndrome is described in a patient whose symptoms began soon after birth and included generalized weakness increased by exertion, easy fatigability, hyporeflexia, and refractoriness to anticholinesterase drugs. Electromyography showed a decremental response at all frequencies of stimulation and a repetitive response to single nerve stimulation. Miniature end-plate potentials (mepps) were of normal amplitude but of decreased frequency. The mepp duration and half-decay time were prolonged, and prostigmine was without any addtitional effect. The quantum content of the end-plate potential was decreased due to a reduced store of quanta immediately available for release, but the probability of release was normal. Quantitative electron microscopy demonstrated a 3-fold to 4-fold decrease of nerve terminal size and reduced postsynaptic membrane density. The postsynaptic folds showed focal degeneration, and many were distended by labyrinthine membranous networks that communicated with the synaptic space. Degenerating nuclei were found in the junctional sarcoplasm. The ultrastructural localization of the acetylcholine receptor protein was normal. Acetylcholinesterase (AChE) was absent from the motor end-plates by histochemical and electron cytochemical criteria. Biochemical studies indicated total absence of the end-plate-specific 16 S species of AChE and marked decrease in total muscle AChE. A congenital defect in the molecular assembly of AChE or in its attachment to the postsynaptic membrane might represent the basic abnormality and condition the morphological and physiological alterations.

Alteration in cell surface LETS protein during myogenesis

Cell surface alterations during myogenesis have been investigated in Yaffe's myogenic cell line L8, using indirect immunofluorescence with an antibody against the large external transformation-sensitive (LETS) protein. The immunofluorescent technique reveals a susbstantial alteration in the distribution of this surface antigen. With the prefused myoblasts, LETS protein is dispersed all over the cell surface; following myoblast fusion, this pattern is markedly changed. All of the fibril-like surface LETS protein disappears, and in some myotubes, discrete clusters of LETS protein become conspicuous. By use of radioimmunological assay, the total LETS protein is quantitatively reduced upon myoblast fusion.

Acetylcholinesterase isozymes in developing mouse tissues

Several isozymes of acetylcholinesterase are separated by 10% acrylamide gel electrophoresis of mouse blood, brain, heart, muscle and tongue tissues. Two isozymes migrating near the origin are described which show changes in relative activity during development. The faster of the two bands is proportionately higher in concentration in embryonic tissues and is highly specific for the acetylthiocholine iodide substrate. This isozyme corresponds to the erythrocyte membrane AChE in electrophortic mobility and substrate specificity. The slower of the two bands is predominant in adult tissues and exhibits considerable cross reaction with the butyrylthiocholine iodide substrate. During embryonic and postnatal developmental stages there is a gradual shift from the faster migrating isozyme toward a predominance of the slower migrating isozyme.

Cell surface changes accompanying myoblast differentiation

Myoblasts are mononucleated cells and associated with differentiation undergo cell fusion and become multinucleated. The current studies have examined cell surface dynamic changes of Concanavalin A lectin receptor mobility and the role of hormones in modulating myoblast differentiation. A uniform distribution of Con-A receptors is observed in undifferentiated cells when reacted with Con-A at 37 degrees C. Cells from differentiating cultures or fully differentiated myotubes reacted similarly at 37 degrees C show a significant redistribution of Con-A into patches, "caps," and endocytic vesicles containing Con-A. If undifferentiated and differentiated cells are first prefixed with glutaraldehyde then reacted with Con-A continuous distribution of Con-A is seen across the cell surface. This suggests redistribution of Con-A and its receptors occurs in differentiated cells reacted with lectin at 37 degrees C. It is further shown that insulin (10 microgram/ml) significantly enhances myoblast differentiation but that this occurs after an apparent stimulation of proliferation. In contrast to insulin, dexamethasone (10 micron and 100 micron) profoundly inhibits myoblast differentiation while having different effects on proliferation; 10 micron dex stimulates cell growth while 100 micron dex suppresses cell proliferation. Lastly, an extracellular filamentous matrix which binds Con-A is observed at the ultrastructural level in high density cultures. No significant redistribution of Con-A is observed on this matrix in distinction to the redistribution observed on the cell membrane in differentiated cells.

Appearance and disappearance of acetycholine receptor during differentiation of chick skeletal muscle in vitro

During differentiation of embryonic chick skeletal muscle in culture, elaboration of acetylcholine receptor (AChR) and acetylcholinesterase occurs shortly after myoblast fusion. During further development, AChR was found to decrease markedly on the myotube surface, while acetylcholinesterase continued to increase. Surface distribution of AChR, as followed by autoradiography using 125I-alpha-bungarotoxin, was homogeneous in newly fused myotubes. With further differentiation, clusters of AChR appeared on the surface of the myotubes, and their subsequent disappearance paralleled a decrease in overall AChR levels. Quantitative autoradiography showed a reduction of over 75% in the density of AChR on the surface of well differentiated, cross-striated myotubes. Thus the appearance of AChR on the cell surface, its condensation into clusters, and finally its depletion seem to be sequential events in the differentiation of skeletal muscle in culture in the absence of direct neuronal influence.

Creatine kinase, myokinase, and acetylcholinesterase activities in muscle-forming primary cultures of mouse teratocarcinoma cells

Multipotential mouse teratocarcinoma cells in embryoid bodies were explanted on plastic or collagen substrates. Various modes of cell determination, including myogenesis, occurred. The predominant avenue of differentiation soon became myogenesis: many multinucleated myotubes formed and yielded an extensive network of skeletal muscle fibers. The process does not proceed to normal completion, as the fibers have a paucity of striations and are not contractile. Activities of several enzymes ordinarily associated with muscle differentiation were examined. Acetylcholinesterase activity increases, especially during myotube formation, as in normal myogenesis. However, creatine kinase activity rises during myotube formation and then drops abnormally, and myokinase activity fails to increase appreciably. The fetal isozymic form of creatine kinase is expressed in the cultures, although well differentiated solid tumors taken from mice show attainment of the adult muscle isozyme type if skeletal muscle is demonstrably present. The results are consistent with the interpretation that coordinately regulated changes in gene expressions controlling these functions may be required for later stages of myogenesis.

The actions of some anticholinesterase drugs on skeletal muscle in culture

The actions of the anticholinesterase drugs, physostigmine, neostigmine and diisopropylfluorophosphate (DFP) on chick embryonic skeletal muscle in culture were studied. None of the anticholinesterases potentiated depolarization responses to acetylcholine. In high concentrations neostigmine and physostigmine produced depolarization. The neostigmine-induced, but not the physostigmine-induced, depolarization was antagonized by tubocurarine. DFP caused an increase in the rate of repolarization during the presence of a cholinomimetic. It is concluded that the cholinesterase present in cultured muscle fibres does not have a physiological role in hydrolysing acetylcholine and that physostigmine and DFP have an action at the ionic channels that are linked to the cholinoreceptor.

Differentiation of cell membranes in cultures of embryonic chick breast muscle

Three components of differentiated muscle membrane, the acetylcholine receptor, acetylcholinesterase (EC 3.1.1.7; acetylcholine hydrolase), and adenylate cyclase [EC 4.6.1.1; ATP pyrophosphate-lyase (cyclizing)], appear simultaneously during myogenesis in cultures of embryonic chick muscle, after the main period of rapid cell fusion. However, unlike the cytoplasmic proteins of differentiated muscle, the elaboration of these membrane components is unaltered when fusion is blocked by lowering the calcium concentration in the medium. These results suggest that membrane differentiation and cytoplasmic differentiation are regulated independently during muscle development.

Regulation of newly synthesized acetylcholinesterase in muscle cultures treated with diisopropylfluorophosphate

Brief treatment with 0.1 mM diisopropylfluorophosphate inhibited an average of 89% of the acetylcholinesterase (EC 3.1.1.7; acetylcholine hydrolase) activity of cultures of chick embryo muscle. As long as protein synthesis occurred, an average of 78% of the activity returned within 4 hr. Newly synthesized acetylcholinesterase did not stain cytochemically, was rapidly and extensively degraded or released in the presence of 10 muM cycloheximide, and consisted mainly of low-molecular-weight forms. Acetylcholinesterase activity first appeared around the nucleus, about 4 hr after treatment with diisopropylfluorophosphate, and then spread to the rest of the cell about the time release of acetylcholinesterase was detected in the medium. With time, more and more of the enzyme was retained in the cells after treatment with cycloheximide, and the proportions of low-molecular-weight forms decreased and high-molecular-weight forms increased. The results suggest that newly synthesized acetylcholinesterase undergoes an orderly process of binding, movement, and assembly in diisopropylfluorophosphate treated, and probably also in untreated, embryo muscle fibers.

Contact-mediated myogenesis and increased acetylcholinesterase activity in primary cultures of mouse teratocarcinoma cells

Transplantable mouse teratomas are known to contain multipotential teratocarcinoma stem cells analogous to early embryo cells and capable of giving rise to a wide variety of specialized cell types in vivo when they attach to a substratum; if they are grown instead in suspension in the body cavity, they form multicellular embryoid bodies which proliferate with little or no cell specialization. Thus, changes initiated at the cell surface may play some role in promoting early cell differentiation. In order to establish an in vitro system for experimental investigation of this hypothesis, embryoid body cells were explanted under conditions of cell attachment versus suspension and maintained in primary culture. Because cell differentiation in previous reports was relatively limited in vitro, the two cellular populations were first compared for genesis of a quantifiable macromolecular phenotype, acetylcholinesterase (AChE) activity, which characterizes several of the cell types most commonly formed in the attached tumors in vivo. The attached cells produced markedly increased levels of AChE activity within a few weeks, while cells in suspension retained basal levels. AChE was histochemically visualized and was found to occur chiefly in cells undergoing myogenesis, especially during myotube formation. Aberrant muscle fibers formed and became predominant in the cultures. When embryoid bodies were first fractionated by increasing size, which reflects their progressive differentiation, the smallest ones, with relatively more multipotential cells and no apparent muscle cells, also showed AChE increase in attached cultures. The results are consistent with the view that attachment of the cell surface to a substratum may play a critical role in initiating some cellular developmental commitments, as well as in sustaining differentiation of cells whose specialization has already been determined. Further experimental modifications of this primary culture system of teratocarcinoma cells should be useful in analyzing cell-substratum relations and cell-surface changes in early mammalian development.