Manfred Schliwa

Microsurgical removal of centrosomes blocks cell reproduction and centriole generation in BSC-1 cells

We have removed the centrosome from cultured BSC-1 cells by microsurgery, leaving enough cytoplasm with the nucleated cell fragment (karyoplast) to ensure survival and growth. In each experiment, we followed the fate of the karyoplast as well as the anucleate cell fragment (cytoplast) containing the original pair of centrioles. Experimental karyoplasts reestablish a juxtanuclear microtubule-organizing center, an astral array of microtubules, and a compact Golgi apparatus. They enter and presumably complete S phase, and they grow beyond the size of an average BSC-1 cell. However, they do not regenerate centrioles in time periods equivalent to more than 10 cell cycles and do not undergo cell division. Control-operated cells with centrosomes left in the karyoplast progress through the cell cycle, duplicate the centrosome, and form clonal cell colonies. We conclude that the removal of centrioles uncouples cell growth from cell reproduction and impedes centriole biogenesis and centrosome duplication.

Mechanisms of Intracellular Organelle Transport

All eukaryotic cells exhibit some form of intracellular motility. The spectrum of motile activities observed inside living cells is extraordinarily broad, ranging from processes barely detectable even in time-lapse recordings, to the breathtaking bulk transport of cytoplasm best observed by slow-motion analysis. On the basis of their phenomenology, motile activities of cytoplasmic constituents may be subdivided into three broad categories: 1. Bulk movement of cytoplasm in association with, or as a consequence of, cell deformation. Good examples include the extension and retraction of cell processes (pseudopodia) in amoeboid cells, an activity accompanied by flow of cytoplasm into or out of these cell extensions. Cellular inclusions are carried along in the cytoplasm in a seemingly passive manner. Organelle translocation ceases as the protrusive or retractive activity of the cell comes to a standstill. 2. Uniform, continuous transport of organelles and cytoplasm along more or less defined pathways in the absence of cell deformation. Prime examples include the rotational or vectorial “streaming” of endoplasm in many plant cells, and shuttle streaming in slime molds. 3. Discontinuous, erratic, “saltatory” movements of particles and organelles. This form of transport is observed in a wide variety of eukaryotic cells and includes such diverse phenomena as organelle movements in protists and fast transport of materials in neurons.

Melanophore death and disappearance produces color metamorphosis in the polychromatic Midas cichlid (Cichlasoma citrinellum)

SummaryWe describe the histological basis of color metamorphosis in the polychromatic Midas cichlid, Cichlasoma citrinellum. Eight percent of the individuals in a natural population transform from gray with black markings to orange, simultaneously losing their ability to adjust coloration in response to background and social context. This trait is inherited. Light- and electron microscopy revealed that this transformation is a two-step process. First, the melanophores die, then macrophage-like cells remove the debris. As a result of this initial process, the underlying xanthophores become visible, producing the orange coloration. A similar process may occur in individuals that further transform to white, or go directly from gray to white.

Reticulomyxa: a new model system of intracellular transport.

SUMMARY Reticulomyxa is a large multinucleated freshwater protozoan that provides a new model system in which to study intracellular transport and cytoskeletal dynamics. Within the cell body and reticulopodial network, rapid, visually striking saltatory organelle motility as well as bulk cytoplasmic streaming can be readily observed. In addition, the cytoskeletal elements within these strands undergo dynamic splaying and fusing rearrangements, which can be visualized by video-enhanced light microscopy. A reactivatable lysed cell model has been developed that appears to preserve, and therefore permits examination of, these three forms of motility in a more controlled environment. Individual organelle movements are microtubule-based and have similarities to, but also differences from, the recently described kinesin-based transport. This lysed cell model can be further manipulated to provide native, ordered, completely exposed networks of either microtubules or microfilaments, or a combination of both, and thus may serve as a versatile motility assay system in which to examine the movement of exogenously added isolated organelles or latex beads.

Relationship between the organization of actin bundles and vinculin plaques

SummaryThe temporal pattern of the formation and dissolution of vinculin patches during experimental manipulation of the state of actin within the cell was studied. Cytochalasin D-induced retraction and disappearance of stress fibers is followed, with a brief delay, by the dissolution of vinculin-containing patches and the coordinated redistribution of both actin and vinculin into newly formed amorphous aggregates or foci. Recovery from cytochalasin treatment begins with a transformation of these foci into doughnut-shaped assemblies in which actin and vinculin are precisely co-localized. The emergence and growth of filament bundles is paralleled by the appearance of faint vinculin patches that gradually increase in size in parallel with the stress fibers. If stress fibers are stabilized by microinjected rhodamine-phalloidin against stimuli that normally induce a coordinated redistribution of actin and vinculin, also the vinculin patches persist. These observations indicate that treatments influencing the state of actin in the cell have corresponding effects on the stability of vinculin patches and suggest a strong interdependency of actin and vinculin organization.