Involvement of microtubules and 10-nm filaments in the movement and positioning of nuclei in syncytia.

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Abstract
Previous studies showed that infection of baby hamster kidney (BHK 21-F) cells with the parainfluenza virus SV[simian virus]5 causes extensive cell fusion, that nuclei migrate in the syncytial cytoplasm and align in tightly-packed rows, and that microtubules are involved in nuclear movement and alignment. The role of microtubules, 10-nm filaments and actin-containing microfilaments in this process was investigated by immunofluorescence microscopy using specific antisera, time-lapse cinematography and EM. During cell fusion, microtubules and 10-nm filaments from many cells form large bundles which are localized between rows of nuclei. No organized bundles of actin fibers were detected in these areas, although actin fibers were observed in regions away from the aligned nuclei. Although colchicine disrupts microtubules and inhibits nuclear movement, cytochalasin B (CB; 20-50 .mu.g/ml) does not inhibit cell fusion or nuclear movement. CB alters the shape of the syncytium, resulting in long filamentous processes extending from a central region. When these processes from neighboring cells make contact, fusion occurs and nuclei migrate through the channels which are formed. EM and immunofluorescence microscopy reveal bundles of microtubules and 10-nm filaments in parallel arrays within these processes, but no bundles of microfilaments were detected. The effect of CB on the structural integrity of microfilaments at this high concentration (20 .mu.g/ml) was demonstrated by the disappearance of filaments interacting with heavy meromyosin. Cycloheximide (20 .mu.g/ml) inhibits protein synthesis but does not affect cell fusion, the formation of microtubules and 10-nm filament bundles, or nuclear migration and alignment; continued protein synthesis is not required. The association of microtubules and 10-nm filaments with nuclear migration and alignment suggests that microtubules and 10-nm filaments are 2 components in a system which serves both cytoskeletal and force-generating functions in intracellular movement and position of nuclei.