Control by calcium of the contractility of Labyrinthula slimeways and of the translocation of Labyrinthula cells

Bothrosome Formation in Schizochytrium aggregatum (Labyrinthulomycetes, Stramenopiles) during Zoospore Settlement

Labyrinthulomycetes are characterized by the presence of ectoplasmic nets originating from an organelle known as the bothrosome, whose evolutionary origin is unclear. To address this issue, we investigated the developmental process from a zoospore to a vegetative cell in Schizochytrium aggregatum. After disappearance of the flagellum during zoospore settlement, the bothrosome emerged at the anterior-ventral pole of the cells. A new Golgi body also appeared at this stage, and the bothrosome was positioned close to both the new and the old Golgi bodies. This observation suggested that the Golgi body is related to the formation of the bothrosome. Actin appeared as a spot in the same location as the newly appeared bothrosome, as determined by immunofluorescence labeling. An immunoelectron microscopic analysis revealed that actin was present in the ectoplasmic nets and in the cytoplasm around the bothrosome, indicating that the electron-dense materials of the bothrosome are not the polar center of F-actin. This suggests that actin filaments pull the endoplasmic reticulum to the bothrosome and induce the membrane to become evaginated within ectoplasmic nets.

Labyrinthula terrestris: a new pathogen of cool-season turfgrasses

ABSTRACT A new disease of turfgrass known as rapid blight is caused by Labyrinthula terrestris, a newly described member of the Labyrinthulids. This unusual group of microorganisms previously has been found in marine systems, and L. terrestris is the only member of the genus Labyrinthula known to be a pathogen of terrestrial plants. Members of this genus are defined by the formation of ectoplasmic networks in which the unicellular somatic cells move or 'glide'. Infections of cool-season turfgrasses occur most commonly when they are irrigated with suboptimal irrigation water with elevated salinity. Disease has been observed in 11 states in the United States and in the United Kingdom. A phylogenetic study indicates that isolates from turfgrass in the United States fall firmly within a clade containing other Labyrinthula spp. and that they came from a common lineage. Its rapid emergence as a turfgrass pathogen may coincide with increased use of relatively high-salinity water or reclaimed water for irrigation. L. terrestris is a potential problem in susceptible turfgrass varieties wherever soil salinity is allowed to accumulate as a result of poor soil structure or suboptimal quality irrigation water is used for irrigation.

Cellular and Molecular Mechanics of Gliding Locomotion in Eukaryotes

Gliding is a form of substrate-dependent cell locomotion exploited by a variety of disparate cell types. Cells may glide at rates well in excess of 1 microm/sec and do so without the gross distortion of cellular form typical of amoeboid crawling. In the absence of a discrete locomotory organelle, gliding depends upon an assemblage of molecules that links cytoplasmic motor proteins to the cell membrane and thence to the appropriate substrate. Gliding has been most thoroughly studied in the apicomplexan parasites, including Plasmodium and Toxoplasma, which employ a unique assortment of proteins dubbed the glideosome, at the heart of which is a class XIV myosin motor. Actin and myosin also drive the gliding locomotion of raphid diatoms (Bacillariophyceae) as well as the intriguing form of gliding displayed by the spindle-shaped cells of the primitive colonial protist Labyrinthula. Chlamydomonas and other flagellated protists are also able to abandon their more familiar swimming locomotion for gliding, during which time they recruit a motility apparatus independent of that driving flagellar beating.

Actin-Based Motility in the Net Slime MouldLabyrinthula: Evidence for the Role of Myosin in Gliding Movement

In contrast to crawling movement (e.g. in amoebae and tissue cells) the other major class of substratum-associated motility in eukaryotes, gliding, has received relatively little attention. The net slime mold Labyrinthula provides a useful laboratory model for studying this process since it exhibits a particular kind of gliding in its plasmodial stage. Here nucleated spindle cells glide along self-established cytoplasmic trackways in a predominantly unidirectional manner, at 1-2 microm/s. These trackways, upon which gliding is dependent, are held by filopodial tethers some distance off the well-developed reticulopodial mesh anchoring the plasmodium onto the substratum. Reflection interference microscopy resolves this matrix in live plasmodia. The axially disposed cytoskeletal elements of the trackways are revealed by rhodamine-labelled phalloidin to be rich in F-actin. A weft of peripheral, rapidly extending filopodia (50 microm/min) typifies the expanding regions of the plasmodium. Here spindle cells are recruited before emigrating into newly differentiated trackways. Immunoblotting whole plasmodia or a sucrose-soluble cytoplasmic extract reveals a single actin-positive band of Mr 48 kDa. Polyclonal antibodies to two distinct myosin peptide sequences identify a single myosin HC (Mr 96 kDa) in immunoblots. Gliding was reversibly blocked by 10 mM 2,3-butanedione-2-monoxime, a myosin ATPase inhibitor, but it was insensitive to the actin-binding drugs cytochalasin D and phalloidin. We suggest that the force (>50 pN) for gliding motility results from interaction of myosin molecules, associated with the spindle cells, with trackway F-actin via the bothrosomes.

Roles of actin filaments and three second-messenger systems in short-term regulation of chick dorsal root ganglion neurite outgrowth

In a previous study (J. Cell Biol. 109: 1229-1243, 1989), we reported that conditions which increased growth cone calcium levels and induced neurite retraction in cultured chick DRG neurons also resulted in an apparent loss of actin filaments in the growth cone periphery. We further showed that the actin-stabilizing drug phalloidin could block or reverse calcium-ionophore-induced neurite retraction, indicating that the behavioral changes were mediated, at least in part, by changes in actin filament stability. In this study, we have further characterized the calcium sensitivity of growth cone behavior to identify which features of calcium-induced behavioral effects can be attributed to effects on actin filaments alone, and to assess whether two other second-messenger systems, cAMP and protein kinase C, might influence neurite outgrowth by altering calcium levels or actin stability. The results indicated that growth cone behavior was highly sensitive to small changes in calcium concentrations. Neurite outgrowth was only observed in calcium-permeabilized cells when extracellular calcium concentrations were between 200 and 300 nM, and changes as small as 50 nM commonly produced detectable changes in behavior. Furthermore, low doses of cytochalasins mimicked all of the grossly observable features of growth cone responses to elevation of intracellular calcium, including the apparent preferential destruction of lamellipodial actin filaments and sparing of filopodial actin, suggesting that the behavioral effects of calcium elevation could be explained by loss of actin filaments alone. The effects of cAMP elevation and protein kinase C activation on growth cone behavior, ultrastructure, and fura2-AM-measured calcium levels indicated that the effects of cAMP manipulations could be partially explained by a cAMP-induced lowering of growth cone calcium levels and concomitant increased stabilization of actin filaments, but protein kinase C appeared to act through an independent mechanism.