CYTOCHALASIN B INFLUENCES DICTYOSOMAL VESICLE PRODUCTION AND MORPHOGENESIS IN THE DESMID EUASTRUM¹

Abstract

Cytochalasin B (CB) applied to young developing cells of the desmid Euastrum oblongum Ralfs ex Ralfs, at concentrations that do not entirely inhibit cytoplasmic streaming, retarded cell growth and caused malformations of cell shape. While the basic symmetry of the cell was maintained, only the first indentations were formed and the cell body appeared to be swollen. Electron microscopic investigations revealed that vesicle production at the dictyosomes was disturbed by cytochalasin. In contrast to untreated control cells, where vesicles with electron‐dense contents (“dark vesicles”) were formed during primary wall formation, vesicles pinched off by the dictyosomes during CB treatment exhibited an “empty” appearance. These vesicles, which correspond to the “dark vesicles” in size, were accumulated around the dictyosomes without being transported to the plasma membrane and were frequently connected to the trans‐cisternae of the Golgi bodies. We speculate that CB may influence the transfer of products from the endoplasmic reticulum (ER) to the dictyosomes via transition vesicles, which results in a disturbed vesicle production at the Golgi bodies. CB also causes a shift in ER and dictyosome distribution. Moreover, a cortical actin system appears to be involved in the cell shaping of Euastrum. The arrangement of microtubules around the nucleus is not affected by the drug.

Microinjection of profilins from different sources into the green algaMicrasterias causes transient inhibition of cell growth

“…The fact that Micrasterias cells can recover from profilin injections within 2 h is possibly due to the continuous synthesis of G-actin which fills up the pool and allows new F-actin to be polymerized and then continue its normal function. Microinjection of profilin or gelsolin does not cause any significant malformation of cell shape in Micrasterias whereas cytochalasins markedly alter morphogenesis (Noguchi and Ueda 1981, Lehtonen 1983, Url et al 1993). This contrasts with the findings of Ft~chtbauer et al (1983) who described similar effects of cytochalasins and microinjected F-actin-capping proteins in other organisms.…”

“…a Normal accumulation of dictyosomes in the central cytoplasm, b Cross-sectioned microtubules (arrows) close to the nuclear membrane. Bars: 0.5 gm like in cytochalasin-treated cells (e.g., Url et al 1993). Changes in the distribution of the cortical actin filament network in Micrasterias after profilin injection, could be visualized by double injection of fluorescently labelled phalloidin and profilin and would yield important information, but this has not yet been performed successfully without extremely harming the cells.…”

“…In Micrasterias denticulata cessation of growth at an expanding lobe causes the next indentation to develop at the tip of a former growth maximum. Various factors controlling this process have been elucidated (for a summary, see Meindl 1993). For example, it has been demonstrated that multipolar tip growth in Micrasterias is not controlled by extended gradients of the cytosolic free Ca2+-concentration (Holzinger et al 1995) although inward ionic currents and the accumulation of membrane-associated calcium have been observed in the growing areas of the cell (Meindl 1982, Troxell andScheffey 1991).…”

Changes of the actin filament system in the green algaMicrasterias denticulata induced by different cytoskeleton inhibitors

“…The breakdown of the actin cytoskeleton correlates with the severe morphological and ultrastructural changes occurring as a consequence of both drugs in desmids including growth retardation or inhibition, aberrations of the cell shape, dislocation of organelles, abnormal accumulation of vesicles, and aberrant growth of the septum wall (Tippit and Pickett-Heaps 1974, Noguchi and Ueda 1981, Url et al 1993, H6ftberger and Ltitz.-Meindl 1999. The actin clusters and fragments of actin filaments are no longer capable of guiding secretory 'vesicles to their destinations at the plasma membrane properly.…”

Profilin is localized in the nucleus-associated microtubule and actin system and is evenly distributed in the cytoplasm of the green algaMicrasterias denticulata

“…The prominent role of the actin cytoskeleton for the realization of the complicated cell pattern in desmids has been well established by studies with inhibitor drugs (Tippit and Picket-Heaps 1974, Noguchi and Ueda 1981, Lehtonen 1983, Meindl 1993, Url et al 1993, Holzinger and Meindl 1997, H6ftberger and L~tz-Meindl 1999. F-actin is distributed in living M. denticulata cells as a dense cortical network covering the inner surface of the growing and nongrowing semicell and in a system associated with the migrating nucleus (Meindl et al 1994).…”