Correlation of cytoplasmic microtubules and 10-nm filaments with the movement of pigment granules in cutaneous melanocytes of Rana pipiens

Relationship between intermediate filaments and microfilaments in cultured fibroblasts: evidence for common foci during cell spreading

Spreading and fully spread chick embryo fibroblasts (CEF) were examined by double-label fluorescence microscopy using the actin-specific probe rhodamine-phalloidin and an antibody directed against CEF intermediate filaments (IF). During midspreading, a striking relationship became discernible: statistical analysis showed that approximately half of the cell population exhibited one or more phase-dense, phalloidin-binding nodules that appeared to act as foci from which IF diverged. Coincidence between actin-containing structures and IF was not limited to these centers; IF could also frequently be seen running in close parallel arrays with stress fibers. Ultrastructural analysis confirmed the presence of non-membrane-bound out-pocketings along the length of stress fibers from which 10-nm IF diverged. These structures varied in size and shape, and displayed a dense, fine fibrillar appearance. IF and microfilaments (MF) were distinguished by size and by decoration of MF with myosin subfragment-1. Other IF-MF interactions were seen in cells of all stages: IF were observed to loop through stress fibers, most frequently at the cell margins. In colchicine-treated cells, IF became redistributed into cables that often ran parallel and appeared to merge with stress fibers. Cytochalasin D-treated CEF exhibited loose aggregates of actin-containing material that appeared to be associated with IF. These results suggest the possibility of an interaction between actin-containing structures and IF, particularly during cell spreading in cultured fibroblasts.

The effects of taxol on cytoskeletal components in cultured fibroblasts and epithelial cells

Taxol promotes microtubule (MT) assembly in vitro and induces the reorganization of the cytoskeleton into unusual MT arrays in cultured cells. The possibility that taxol also has an indirect effect on intermediate filaments (IF) was investigated. In baby hamster kidney (BHK-21) and human skin (ENSON) fibroblasts treated with 1-10 microM taxol for 1-24 h, the drug induces changes which are similar to those produced by colchicine. These include a loss of major cellular extensions, a redistribution of organelles to a perinuclear location, and an inhibition of locomotion. Saltatory particle movements are not inhibited, however. Ruffling and filopod formation continue, indicating that cells are viable up to 24 h. Polarized light microscopy of living fibroblasts treated with taxol reveals the presence of perinuclear birefringent material which has been examined by immunofluorescence. In control cells, IF and MT radiate from a juxtanuclear region and extend to the cell periphery. In taxol-treated cells, MT and IF are excluded from cell margins, forming large central bundles. In the epithelial cell lines PtK2 and PAM, the keratin system of IF does not become redistributed; in PtK2, however, a second fibroblastlike system of IF does become redistributed to a perinuclear position during taxol treatment. Ultrastructural analyses show that taxol-treated fibroblasts contain parallel arrays of cross-bridged MT-IF as well as bundles of MT exclusive of IF. Epithelial cells contain a predominance of IF-free MT bundles which are organized into hexagonally packed arrays. In these bundles MT frequently exhibit hooks or other incomplete MT profiles and are linked by filamentous material.

Differential effect of cytochalasin B on the aggregation of melanosomes in cultured mouse melanoma cells

Mouse B16 melanoma cells in cultures were treated with 2 microgram/ml cytochalasin B. Melanosomes, localized in dendrites as well as those in the peripheral cell body, formed aggregates which then showed centripetal migration. After 24 hr of treatment, melanosomes were found as large aggregates near the nucleus. However, cytochalasim B showed no effect on the distribution of mitochondria and endoplasmic reticulum. When cells were released from the 24 hr-treatment with cytochalasin B, each melanosome began centrifugal migration. In this process, melanosomes were situated in the periphery, and were in close association with microtubules. Side arm-like structures were observed between microtubules and melanosomes, or mitochondria. These results seem to indicate the presence of mechanism(s) specific to melanosome migration, and possible participation of microtubules in this melanosome migration.

Photomechanical migrations of pigment granules along the retinula cells of the crayfish

The light-dependent migrations of proximal pigment granules along the photoreceptors of the crayfish compound-eye were studied in isolated retinas and eyestalks. The extent and kinetics of movement in each direction were found quantitatively equivalent to those observed in the organ in situ. These and other features make these cells to appear as intrinsically independent pigmentary effectors, directly responsive to light. During dark adaptation (DA) the pigment migrates away from the cell nucleus and accumulates along the axon in two distinct steps. Each step constitutes half of the total distance of about 180 microns and proceeds at 0.30 micron/sec. Only prolonged metabolic impairment inhibited the first phase, while the second was blocked by hypoxia, cyanide, colchicine, and D2O. The maintenance of a full DA position was also shown to be highly dependent upon metabolism. Light incidence on DA eyes is followed by an apparently monophasic expansion of the pigment from the axon towards the perikaryl region at 0.38 micron/sec. This movement was not affected by any of the foregoing agents and seems to be a passive relaxation process. Cytochalasin B had no effect on either motion. The migration in either direction has an exponential time course and is temperature dependent. Electron microscopy revealed two separate patterns of cytoplasmic organization corresponding to the cell areas where the two phases of DA occur. In the region close to the nucleus the pigment appears irregularly scattered, whereas in the axon the granules are situated arond a thick longitudinal bundle of microtubules. These results suggest the existence of two different mechanisms of pigment granule translocation operating in two separate regions of the retinula cell.

Neurofibrillary pathology: current status and research perspectives

The neurofibrils seen in the light microscope are shown, by electron microscopy, to be heterogeneous structures, formed of neurotubules and neurofilaments. A variety of pathological conditions (especially, presenile and senile dementia (are characterized by the presence of neurofibrillary tangles, formed either of paired helical filaments or of single filaments. The morphology, distribution and biochemistry of these various fibrillary structures is reviewed. Particular attention is devoted to the assembly of neurotubules, to the mechanism of action of drugs which prevent assembly, and to possible implications for the experimental induction of neurofibrillary pathology. Of central importance in the arguments is the emphasis on the normal neurofibrillary structures being single forms of pleomorphic proteins. The healthy neuron assembles these proteins into the required form as is necessary for the needs of that neuron. Interference with this process in neuronal cell biology may lead to the deposition of neurofibrillary tangles. On the basis of the morphological and biochemical evidence, several approaches to the experimental study of neurofibrillary pathology are proposed.

Ionophore A23187 and dibutyryl cyclic AMP effects on cell shape and morphology of B-16 melanoma

Melanoma cells treated with dibutyryl cyclic AMP (db cAMP) for 24 h resulted in dendritic cells possessing parallel assembled microtubules. A23187 treatments resulted in a biphasic response: Long term effects of the ionophore were characterized by small epitheloid cells while the immediate response produced elongated cells with parallel arranged 10 nm microgilaments, characteristic of dispersive melanocytes.