Developmental reorganization of the skeletal framework and its surface lamina in fusing muscle cells

Triton X-100 extraction of P815 tumor cells: evidence for a plasma membrane skeleton structure

It has been shown that a Triton X-100-insoluble protein matrix can be isolated from the plasma membranes of P815 tumor cells and murine lymphoid cells (Mescher, M. F., M. J. L. Jose and S. P. Balk, 1981, Nature (Lond.), 289:139-144). The properties of the matrix suggested that this set of proteins might form a membrane skeletal structure, stable in the absence of the lipid bilayer. Since purification of plasma membrane results in yields of only 20 to 40%, it was not clear whether the matrix was associated with the entire plasma membrane. To determine if a detergent-insoluble structure was present over the entire cell periphery and stable in the absence of the membrane bilayer or cytoskeletal components, we have examined extraction of whole cells with Triton X-100. Using the same conditions as those used for isolation of the matrix from membranes, we found that extraction of intact cells resulted in structures consisting of a continuous layer of protein at the periphery, a largely empty cytoplasmic space, and a nuclear remnant. Little or no lipid bilayer structure was evident in association with the peripheral layer, and no filamentous cytoskeletal structures could be seen in the cytoplasmic space by thin-section electron microscopy. Analysis of these Triton shells showed them to retain approximately 15% of the total cell protein, most of which was accounted for by low molecular weight nuclear proteins. 5'-Nucleotidase, a cell surface enzyme that remains associated with the plasma membrane matrix, was quantitatively recovered with the shells. Included among the polypeptides present in the shells was a set with mobilities identical to those of the set that makes up the plasma membrane matrix. The polypeptide composition of the shells further confirmed that cytoskeletal proteins were present to a very low extent, if at all, after the extraction. The results demonstrate that a detergent-insoluble protein matrix associated with the periphery of these cells forms a continuous, intact macrostructure whose stability is independent of the membrane bilayer or filamentous cytoskeletal elements, and thus has the properties of a membrane skeletal structure. Although not yet directly demonstrated, the results also strongly suggest that this peripheral layer is composed of the previously described set of plasma membrane matrix proteins. This article discusses possible roles for this proposed membrane skeletal structure in stabilizing the membrane bilayer and affecting the dynamics of other membrane proteins.

Epithelial structure revealed by chemical dissection and unembedded electron microscopy

Cytoskeletal structures obtained after extraction of Madin-Darby canine kidney epithelial cell monolayers with Triton X-100 were examined in transmission electron micrographs of cell whole mounts and unembedded thick sections. The cytoskeleton, an ordered structure consisting of a peripheral plasma lamina, a complex network of filaments, and chromatin-containing nuclei, was revealed after extraction of intact cells with a nearly physiological buffer containing Triton X-100. The cytoskeleton was further fractionated by extraction with (NH4)2SO4, which left a structure enriched in intermediate filaments and desmosomes around the nuclei. A further digestion with nuclease and elution with (NH4)2SO4 removed the chromatin. The stable structure that remained after this procedure retained much of the epithelial morphology and contained essentially all of the cytokeratin filaments and desmosomes and the chromatin-depleted nuclear matrices. This structural network may serve as a scaffold for epithelial organization. The cytoskeleton and the underlying nuclear matrix intermediate filament scaffold, when examined in both conventional embedded thin sections and in unembedded whole mounts and thick sections, showed the retention of many of the detailed morphological aspects of the intact cells, which suggests a structural continuum linking the nuclear matrix, the intermediate filament network, and the intercellular desmosomal junctions. Most importantly, the protein composition of each of the four fractions obtained by this sequential procedure was essentially unique. Thus, the proteins constituting the soluble fraction, the cytoskeleton, the chromatin fraction, and the underlying nuclear matrix-intermediate filament scaffold are biochemically distinct.

Ultrastructural study of developing rabbit diaphragm

Diaphragms obtained from forty rabbit fetuses during the last third of gestation and from five rabbits one week old were studied ultrastructurally. Four gestational ages were chosen for the investigation. The diaphragm at 20 days gestation revealed early myogenesis which was characterised by many myogenic cells in the process of mitosis and by fusion of myoblasts to form myotubes. A large number of lipid droplets and a moderate quantity of glycogen were observed in the myotubes. These myotubes contained a few myofibrils peripherally. At 22 days of gestation, the myotubes exhibited a relatively large number of myofibrils and large amounts of glycogen. They also showed sarcolemmal modifications which appeared to be developing motor end plates. Mitosis of the myogenic cells persisted even after fusion. At 25 days of gestation, connective tissue sheaths appeared around the muscle fibres as well as further differentiation of the muscle fibres. At 30 days of gestation, the muscle fibres exhibited nearly complete differentiation, with the formation of myotendinous junctions. At one week post partum, the diaphragm showed full development of its muscle fibres. These morphological observations suggest a very rapid functional differentiation of the rabbit diaphragm during the last third of gestation.

Epithelial cytoskeletal framework and nuclear matrix-intermediate filament scaffold: three-dimensional organization and protein composition

Madin-Darby canine kidney (MDCK) cells grow as differentiated, epithelial colonies that display tissue-like organization. We examined the structural elements underlying the colony morphology in situ using three consecutive extractions that produce well-defined fractions for both microscopy and biochemical analysis. First, soluble proteins and phospholipid were removed with Triton X-100 in a physiological buffer. The resulting skeletal framework retained nuclei, dense cytoplasmic filament networks, intercellular junctional complexes, and apical microvillar structures. Scanning electron microscopy showed that the apical cell morphology is largely unaltered by detergent extraction. Residual desmosomes, as can be seen in thin sections, were also well-preserved. The skeletal framework was visualized in three dimensions as an unembedded whole mount that revealed the filament networks that were masked in Epon-embedded thin sections of the same preparation. The topography of cytoskeletal filaments was relatively constant throughout the epithelial sheet, particularly across intercellular borders. This ordering of epithelial skeletal filaments across contiguous cell boundaries was in sharp contrast to the more independent organization of networks in autonomous cells such as fibroblasts. Further extraction removed the proteins of the salt-labile cytoskeleton and the chromatin as separate fractions, and left the nuclear matrix-intermediate filament (NM-IF) scaffold. The NM-IF contained only 5% of total cellular protein, but whole mount transmission electron microscopy and immunofluorescence showed that this scaffold was organized as in the intact epithelium. Immunoblots demonstrate that vimentin, cytokeratins, desmosomal proteins, and a 52,000-mol-wt nuclear matrix protein were found almost exclusively in the NM-IF scaffold. Vimentin was largely perinuclear while the cytokeratins were localized at the cell borders. The 52,000-mol-wt nuclear matrix protein was confined to the chromatin-depleted matrix and the desmosomal proteins were observed in punctate polygonal arrays at intercellular junctions. The filaments of the NM-IF were seen to be interconnected, via the desmosomes, over the entire epithelial colony. The differentiated epithelial morphology was reflected in both the cytoskeletal framework and the NM-IF scaffold.

Filament-directed intercellular contacts during differentiation of cultured chick myoblasts

Detergent-extracted, critical point dried chicken myoblasts at early stages of development in tissue culture were observed by electron microscopy. It was found that the organization of filaments within these cells changes significantly during development. A particular specialization of the cellular filament framework is the formation of microprocesses; long extensions of the cellular filament system. These microprocesses appear to be involved in cell-to-cell contact. The filaments of these processes appear to integrate with the filament system of a contacted cell, and possibly transmit tension from one cell to another. The role of these structures in effecting muscle differentiation by forming cytoplasmic connections and the implications for muscle gene expression are discussed.

Acetylcholinesterase of mammalian neuromuscular junctions: presence of tailed asymmetric acetylcholinesterase in synaptic basal lamina and sarcolemma

A sarcolemma-rich fraction can be isolated after subcellular fractionation of mouse intercostal muscles by sedimentation on a discontinuous sucrose gradient. The quantitative recovery of the acetylcholine receptor in this fraction is about 50%, which indicates the presence of a high proportion of postsynaptic membranes. Acetylcholinesterase (AcChoEase; EC 3.1.1.7) is found mainly in three different layers: the top layer, which contains soluble AcChoEase, the intermediate layer (fraction A), and the last, AcChoR-rich, layer (fraction C). The relative proportions of the molecular forms of AcChoEase are different in the three layers. The "16S" AcChoEase is in a higher proportion in both types of membrane fractions (A and C) compared to soluble AcChoEase. Both total AcChoEase and 16S AcChoEase are enriched in the A and C fractions. In the C fraction, the sequential use of homogenizations in the presence of detergent and high ionic strength allows the "solubilization" of two distinct AcChoEase pools. One is detergent-soluble and mainly composed of slow-sedimenting forms; the other one is detergent-insoluble, high-ionic strength-soluble, and composed mainly of collagen-like, tailed, asymmetric (16S) AcChoEase. Thus, most of the asymmetric AcChoEase is specifically localized in the synaptic extracellular matrix of the mammalian muscle fiber. However, in the A fraction, most of the 16S AcChoEase found is solubilized by detergent alone, suggesting an association with microsomal membranes. It may mean that at least some of the basal lamina-embedded 16S AcChoEase is preassembled intracellularly in the sarcoplasmic reticulum.

Trifluoperazine, a calmodulin antagonist, inhibits muscle cell fusion

We investigated the effect of trifluoperazine (TFP), a calmodulin antagonist, on the fusion of chick skeletal myoblasts in culture. TFP was found to inhibit myoblast fusion. This effect occurs at concentrations that have been reported to inhibit Ca2+-calmodulin in vitro, and is reversed upon removal of TFP. In addition, other calmodulin antagonists, including chlorpromazine, N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide (W7), and N-(6-aminohexyl)-1-naphthalene-sulfonamide (W5), inhibit fusion at doses that correspond closely to the antagonistic effects of these drugs on calmodulin. The expression of surface acetylcholine receptor, a characteristic aspect of muscle differentiation, is not impaired in TFP-arrested myoblasts. Myoblasts inhibited from fusion by 10 microM TFP display impaired alignment. In the presence of the Ca2+ ionophore A23187, the fusion block by 10 microM TFP is partially reversed and myoblast alignment is restored. The presence and distribution of calmodulin in both prefusional myoblasts and fused muscle cells was established by immunofluorescence. We observed an apparent redistribution of calmodulin staining that is temporally correlated with the onset of myoblast fusion. Our findings suggest a possible role for calmodulin in the regulation of myoblast fusion.

Inhibition of the formation of myotubes in vitro by inhibitors of transglutaminase

The effects of competitive inhibitors of transglutaminase on the formation of myotubes by the fusion of myoblasts in vitro has been investigated. Myotube formation was inhibited when myoblasts from 11-day-old chick embryos were cultured in vitro in the presence of 10 mM histamine or 0.2 mM dansyl cadaverine. The inhibitions observed were reversed when the treated cells were subsequently cultured in normal medium. Glycine methyl ester also inhibited myotube formation but sarcosine methyl ester, which is not a competitive inhibitor of transglutaminase, had little if any inhibitory action. The formation of myotubes was not inhibited by cultivation in normal medium adjusted to pH 8.0-8.1, indicating that the observed effects of histamine and of dansyl cadaverine were not mediated by a lysosomotropic effect. Inhibition of myotube formation in the presence of histamine was accompanied by the production of abnormal multinucleated cells, indicating that myoblast fusion occurred in the treated cultures but that the fused cells failed to elongate into normal myotubes. Transglutaminase activity has been found in cell-free lysates of embryonic chick myoblasts and it is concluded that a transglutaminase enzyme, activated by an increase in the concentration of intracellular Ca2+, plays an important role in stabilising the cytoskeletal network of developing myotubes.

Myofibrillogenesis in vitro as seen with the scanning electron microscope

The present study describes an experimental approach whereby myofibrillogenesis and the three-dimensional arrangement of myofibrils present within cultured skeletal muscle cells can be examined using the scanning electron microscope. This procedure uses cells that have been cultured on gold-coated coverslips, and treated with Triton X-100 to extract the cell membrane and the soluble cytoplasm. Subsequent electroconductive staining by treatment with thiocarbohydrazide and osmium allows the myofibrils to be visualized. The images of myofibrils in various states of development observed by this method generally accords to those previously reported by transmission electron microscopy. Cell elongation and adhesion to the substrate causes mechanical stress from different directions which meet at branchings of the cultured myotubes. Many myofibrils are observed to run in the direction of the inferred stress lines.

Mitotic architecture of the cell: the filament networks of the nucleus and cytoplasm

The skeletal framework of cells at the various stages of mitosis are prepared by extraction with nonionic detergent and examined by stereoscopic whole mount electron microscopy. The insoluble filament network remaining after the detergent-extraction and the depolymerization of microtubules is shown. The nonchromatin filament network of the nucleus, or nuclear matrix, becomes visible as the chromatin condenses at prophase. Filaments are associated with the chromosomes throughout mitosis. Parts of the chromosomes are associated with or are near the nuclear lamina at early stages. The nuclear lamina disappears at metaphase while chromosomes remain associated with filaments now continuous with the cytoplasmic network. Microtubules appear to be unnecessary for maintaining the chromosome position in these preparations since comparison of cells with and without microtubules shows no gross change in chromosome arrangement. The cellular filament network at metaphase and anaphase appears continuous from the plasma lamina to the chromosomes. The filament networks visualized here may be responsible for the prometaphase chromosome movement and participate in the formation of the midbody.

Characterization of a detergent-resistant surface lamina in cultured human fibroblasts

Treatment of cultured human fibroblasts with 0.5% Triton X-100 produces substratum-anchored cytoskeletal preparations consisting of cytoplasmic filaments, nucleus and a plasma membrane-derived surface lamina. The lamina was visualized in fluorescence microscopy with fluorochrome-coupled wheat germ agglutinin (WGA) as a lace-like structure, extending throughout the cell domain. It displayed a different organization at the ventral and dorsal surfaces of the cell, partially coaligning with bundles of actin and myosin filaments at the dorsal cell surface. At the ventral surface vinculin patches appeared to be included in the surface lamina. Polyacrylamide gel electrophoresis, combined with lectin reactivity studies and lectin affinity chromatography, revealed a 140 kD sialoglycoprotein as the major glycoprotein component of the surface lamina.

140 000 Dalton surface glycoprotein. A plasma membrane component of the detergent-resistant cytoskeletal preparations of cultured human fibroblasts

A 140 000 D glycoprotein (140 kD gp), labelled radioactively with surface-specific techniques, remained as the major cell surface glycoprotein in the detergent-resistant cytoskeletal preparations of cultured human fibroblasts. The 140 kD gp was present also in trypsinized cells and was not affected by treatment of the cells either with collagenase, chymotrypsin or thrombin. In density gradient fractionation of whole cells the 140 kD gp was recovered in the plasma membrane fraction together with small amounts of cytoskeletal components. In fractionation of cytoskeletal preparations, on the other hand, the 140 kD gp could not be dissociated from cytoskeletal proteins and together with vimentin it formed the major component of the oligomeric polypeptide complex generated by treating the surface-labelled cytoskeletal preparations with bifunctional cross-linking reagent, dithiobis succinimidyl propionate (DTPS). Moreover, the 140 kD gp seemed to copurify with vimentin upon reconstitution of intermediate filaments from urea-solubilized cytoskeletal preparations. On the other hand, low ionic-induced degradation of vimentin led to a decrease in the amount of the detergent-resistant 140 kD gp on the cell surface. In electron microscopy, a close apposition between bilayer-like plasma membrane remnants of the adherent cytoskeletons and cytoskeletal elements could be seen. The results indicate that the 140 kD gp is a plasma membrane glycoprotein which closely interacts with the detergent-resistant cytoskeleton of cultured human fibroblast. Possible mechanisms of the association are discussed.

Interaction of the cytoskeletal framework with acetylcholine receptor on th surface of embryonic muscle cells in culture

To monitor the interaction of cell surface acetylcholine (AcCho) receptors with the cytoskeleton, cultured muscle cells were labeled with radioactive or fluorescent alpha-bungarotoxin and extracted with Triton X-100, using conditions that preserve internal structure. A significant population of the AcCho receptors is retained on the skeletal framework remaining after detergent extraction. The skeleton organization responsible for restricting AcCho receptors to a patched region may also result in their retention after detergent extraction.

Effect of phalloidin on liver actin distribution, content, and turnover

Phalloidin increases F-actin microfilament content and actin-directed immunofluorescence in hepatocytes in vivo and also increases actin polymerization and the stability of F-actin in vitro. We studied the sensitivity of immunofluorescent staining of actin to an actin depolymerizing factor (ADF) as well as actin content, degree of polymerization, and turnover in livers of in vivo phalloidin-treated rats. Pretreatment with ADF abolished anti-actin antibody (AAA) staining of normal liver but did not modify staining of livers from phalloidin-treated animals. Planimetric analyses of SDS-polyacrylamide gels showed the percent actin of total protein was increased by approximately 40% and the absolute amount of actin by approximately 43%, ten days after daily phalloidin treatment (50 micrograms/100 gm body weight). Similar but smaller changes could be seen after one day of treatment. Ultracentrifugational analyses of liver extracts indicated no change in the amount or proportion of G-actin but a 194% increase in the proportion of F-actin in ten-day treated animals, changes also apparent in one day animals. Neither the relative fractional rate of actin synthesis nor its synthesis as a percent of total protein synthesis was altered either at one-day or ten-day post-phalloidin treatment. Dualisotope experiments indicated that the rate of actin degradation was decreased selectively in the one- to three-day period following drug treatment. Thus, phalloidin appears to stabilize actin against the depolymerizing actions of ADF, increases the proportion of F-actin without altering the size of the G-actin pool, and causes accumulation of actin by decreasing its relative rate of degradation.