Length control of primary cilia: analysis of monociliate and multiciliate PtK1 cells

Lengths of primary cilia in cultured PtK1 cells, on both a population basis and within individual (multiciliate) cells, have been compared. The latter examines the degree of discrepancy between cilia arising within the same cell and using a common precursor pool, on the hypothesis that a better correlation would be expected between cilia issuing from the same centrosome than between those in the population in general. To obtain accurate et plentiful measurements, a 'flow-fixation' technique was devised, which flattens the long primary cilia of cultured PtK1 cells (a kidney epithelial cell line from Potorous tridactylus, the kangaroo rat), prior to immunostaining with an antibody directed against detyrosinated tubulin (ID5). Comparisons of the flow-fixed measurements with a through-focus procedure for upright cilia in conventionally fixed cultures showed reasonable agreement, but not as closely as with measurements made on the living cells using the edge-on method of Roth et al. (J. Cell Sci. 89 (1988) 457). The incidence of multiciliation of confluent PtK1 cells cultures was approximately 5%, of which the majority were biciliates. Although shaft length in general varied considerably, biciliates and multiciliates showed a greater internal consistency, with discrepancies of < 25% in 70% of the cases. On both accounts, this consistency is far poorer than in, for comparison, Chlamydomonas, where its two flagella were < 5% different in length and within 10% tolerance throughout the whole population. Thus, length of primary cilia in PtK1 cell populations is considerably less stringently controlled than in PtK1 cells bearing 9 + 2 cilia, but those issuing from a single multiciliated cell tend to show better correspondence.

Ultrastructural, tomographic and confocal imaging of the chondrocyte primary cilium in situ

Hyaline cartilage chondrocytes express one primary cilium per cell, but its function remains unknown. We examined the ultrastructure of chick embryo sternal chondrocyte cilia and their interaction with extracellular matrix molecules by transmission electron microscopy (TEM) and, for the first time, double-tilt electron tomography. Ciliary bending was also examined by confocal immunohistochemistry. Tomography and TEM showed the ciliary axoneme to interdigitate amongst collagen fibres and condensed proteoglycans. TEM also revealed the presence of electron-opaque particles in the proximal axoneme which may represent intraciliary-transport (ICT) particles. We observed a wide range of ciliary bending patterns. Some conformed to a heavy elastica model associated with shear stress. Others were acutely deformed, suggesting ciliary deflection by collagen fibres and proteoglycans with which the cilia make contact. We conclude that mechanical forces transmitted through these matrix macromolecules bend the primary cilium, identifying it as a potential mechanosensor involved in skeletal patterning and growth.

Reactivation of cell surface transport in Reticulomyxa

Granuloreticulosean protists transport particles (e.g., bacteria, algae, and sand grains) along the outer surfaces of their pseudopodia. This cell surface transport plays a vital role in feeding, reproduction, shell construction, and locomotion and can be visualized by the movements of extracellularly adherent polystyrene microspheres (i.e., latex beads). Our videomicroscopic analyses of transport associated with the pseudopodia of Reticulomyxa filosa revealed two distinct types of both intracellular and cell surface transport: (1) saltatory, bidirectional transport of individual or clustered organelles and/or surface-attached particles, and (2) continuous, unidirectional bulk or "resolute" motion of aggregated organelles and/or surface-bound particles. Organelles and surface-attached polystyrene microspheres remained firmly attached to the microtubule cytoskeletons of detergent-extracted pseudopodia. Both saltatory and resolute organelle and surface transport reactivated upon the addition of 0.01-1.0 mM ATP. At 1 mM ATP, the velocities of reactivated saltatory transport were indistinguishable from those observed in vivo. The reactivated transport was microtubule-dependent and was not inhibited by incubation with Ca(2+)-gelsolin under conditions that abolish rhodamine-phalloidin detection of actin filaments. These findings provide further support that both intracellular organelle and membrane surface transport are mediated by a common mechanism, and establish Reticulomyxa as a unique model system to further study the mechanochemistry of cell surface transport in vitro.

The Santa Barbara Basin is a symbiosis oasis

It is generally agreed that the origin and initial diversification of Eucarya occurred in the late Archaean or Proterozoic Eons when atmospheric oxygen levels were low and the risk of DNA damage due to ultraviolet radiation was high. Because deep water provides refuge against ultraviolet radiation and early eukaryotes may have been aerotolerant anaerobes, deep-water dysoxic environments are likely settings for primeval eukaryotic diversification. Fossil evidence shows that deep-sea microbial mats, possibly of sulphur bacteria similar to Beggiatoa, existed during that time. Here we report on the eukaryotic community of a modern analogue, the Santa Barbara Basin (California, USA). The Beggiatoa mats of these severely dysoxic and sulphidic sediments support a surprisingly abundant protistan and metazoan meiofaunal community, most members of which harbour prokaryotic symbionts. Many of these taxa are new to science, and both microaerophilic and anaerobic taxa appear to be represented. Compared with nearby aerated sites, the Santa Barbara Basin is a 'symbiosis oasis' offering a new source of organisms for testing symbiosis hypotheses of eukaryogenesis.

Molecular evidence that Reticulomyxa filosa is a freshwater naked foraminifer

Reticulomyxa filosa is a freshwater protist possessing fine granular, branching and anastomosing pseudopodia and therefore traditionally placed in the class Granuloreticulosea, order Athalamida, as a sister group to the order Foraminiferida. Recent studies have revealed remarkable similarities in pseudopodial motility and ultrastructure between R. filosa and foraminifera (e.g. Allogromia laticollaris), prompting us to conduct a molecular phylogenetic analysis of these seemingly disparate organisms. We sequenced the complete small-subunit of the ribosomal DNA of the cultured strain of R. filosa and compared it to the corresponding sequences of other protists including 12 species of foraminifera. We also sequenced and analyzed the actin coding genes from R. filosa and two species of foraminifera, Allogromia sp. and Ammonia sp. The analysis of both data sets clearly shows that R. filosa branches within the clade of foraminifera, suggesting that R. filosa is in fact a freshwater naked foraminiferan.

Regulation of arachidonic acid release and prostaglandin E2 production in thymic epithelial cells by ATPgammaS and transforming growth factor-alpha

The arachidonic acid metabolites produced by thymic epithelial cells play a pivotal role in thymocyte development. We have discovered that ATP and TGF-alpha regulate the arachidonic acid metabolism in TEA3A1 rat thymic epithelial cells by activating phospholipase A2 enzymatic activity. Our present study demonstrates that ATP and its nonhydrolyzable analog ATPgammaS stimulate both prostaglandin E2 production and Ca2+ influx in TEA3A1 cells. The stimulation of prostaglandin E2 production and Ca2+ influx by ATP is inhibited by pertussis toxin treatment, indicating that ATP mediates its effect by binding to a G-protein-coupled purinergic receptor. Treatment of cells with ATPgammaS and transforming growth factor-alpha results in a synergistic activation of phospholipase A2 and stimulation of prostaglandin E2 production. Results from experiments using an inhibitor of receptor-mediated Ca2+ influx indicate that the synergistic stimulation of prostaglandin E2 production by ATPgammaS and transforming growth factor-alpha requires ATPgammaS-mediated Ca2+ influx. The inhibitor of tyrosine kinase genistein also blocked both ATPgammaS- and ATPgammaS plus transforming growth factor-alpha-mediated stimulation of prostaglandin E2 production, indicating that the activation of phospholipase A2 may involve a protein tyrosine phosphorylation step.

Analysis and modeling of the primary cilium bending response to fluid shear

Since a nonmotile, primary (9 + 0) cilium projects from most mammalian kidney epithelial cells into the tubule lumen, where it is exposed to fluid motion, the present study examined primary cilium response to fluid shear stress. The reversible, large-angle bending of the primary cilium upon exposure to fluid shear forces (10(-11)-10(-10) N.m2 = 10(-8)-10(-7) dyn/cm) was characterized in vitro using videomicroscopic side views of PtK1 cells, and the cilium was then mathematically modeled as a cantilevered beam. The flexural rigidity of the primary cilium was calculated to be 3.1 +/- 0.8 x 10(-23) N.m2 with a corrected quadruple integration approach and 1.4-1.6 x 10(-23) N.m2 with the "heavy elastica" theory. Comparison of theoretical profiles to the experimental bending responses of cilia established the validity of the "heavy elastica" model; this model, in turn, was used to predict primary cilium bending behavior under representative conditions in the rat nephron. The results of the study are consistent with the hypothesis that primary cilia serve a mechanosensory function in kidney epithelial cells.

The discovery of a novel R-phycoerythrin from an antarctic red alga

A novel biliprotein, named R-phycoerythrin IV, has been discovered. It absorbs blue light better than any other known red algal biliprotein. The protein was found in Phyllophora antarctica, a benthic macroalga, which grows beneath the coastal waters of McMurdo Sound, Antarctica. Fluorescence emission and fluorescence excitation polarization spectroscopy demonstrated that R-phycoerythrin IV behaved as a typical R-phycoerythrin in the functioning of energy migration and has an emission maximum at 577 nm. The circular dichroism (CD) spectrum of the chromophores was compared with visible absorption spectrum, and both were deconvoluted. This process showed the energy states of various individual chromophores. The molecular weight of the protein suggested a alpha6beta6gamma polypeptide structure, and far UV CD studies revealed polypeptides with highly alpha-helical secondary structures. Dynamic light scattering indicated that the protein had a 5.54 nm radius, and its shape was nonspherical. R-phycoerythrin was also purified from a second benthic Antarctic red alga, Iridaea cordata. Its spectroscopic properties were similar to those of some R-phycoerythrins from nonpolar regions. The unique spectroscopic properties of R-phycocerythrin IV may help enable the alga to occupy its niche deeper in the water column than the red alga that has the typical R-phycoerythrin.

Morphological and proliferative responses of endothelial cells to hydrostatic pressure: role of fibroblast growth factor

Subconfluent bovine pulmonary artery endothelial cells on rigid substrates were exposed to 1.5-15 cm H2O sustained hydrostatic pressure for up to 7 days and exhibited elongation, cytoskeletal rearrangement, increased cell proliferation, and bilayering. The role of basic fibroblast growth factor (bFGF) in the mechanism(s) of these endothelial cell responses to sustained hydrostatic pressure was investigated. Evidence that bFGF was released from endothelial cells exposed to sustained hydrostatic pressure or compression was provided by the following experimental results: 1) Cells exposed to control (3 mm H2O) pressure displayed intense nuclear and cytoplasmic bFGF staining by immunocytochemical techniques; this staining was absent in cells exposed to 10 cm H2O for 7 days. 2) Conditioned medium from endothelial cells exposed to 10 cm H2O for 7 days contained a transferable, growth-promoting activity exhibiting heparin-Sepharose affinity, lability to both heat and freeze/thawing, and neutralization by anti-bovine bFGF. 3) Suramin (0.1 mM), a growth-factor receptor inhibitor, abrogated the proliferative and morphological responses of endothelial cells exposed to sustained hydrostatic pressure. Endothelial cells exposed to elevated hydrostatic pressure demonstrated no detectable decrement in cell viability as assessed by Trypan blue exclusion. The results of the present study indicate that hydrostatic pressure or compression can induce bFGF release from endothelial cells independent of cell injury or death; bFGF is subsequently responsible for the morphological, proliferative, and bilayering responses of endothelial cells to hydrostatic pressure.

Estrogen-stimulation of postconfluent cell accumulation and foci formation of human MCF-7 breast cancer cells

Foci, nodules of cellular overgrowth, that appear after confluence are an in vitro characteristic of malignant transformation. A well-studied in vitro model of estrogen-dependent tumors is the MCF-7 cell line, derived from a pleural metastasis of a human breast adenocarcinoma. We report that cultivation of MCF-7 cells, using routine methods, results in extensive estrogen-stimulated postconfluent cell accumulation characterized by discrete three-dimensional arrays. Side view Nomarski optical sections revealed these to be principally multicellular foci with occasional domes and pseudoacinar vacuoles. This effect on MCF-7 cell growth occurs in media containing fetal bovine serum but not with calf serum or charcoal-dextran-treated fetal bovine serum unless supplemented with estrogens. Foci formation starts 5-6 days after confluence, and the number of foci generated is a function of the concentration of added estrogens. Foci formation is suppressed by the antiestrogens Tamoxifen and LY 156758. Addition of progesterone, testosterone, or dexamethasone had little or no effect, while various estrogens (ethinyl estradiol, diethylstilbestrol, and moxestrol) induced foci development. Clones derived from single cells of the initial MCF-7 population revealed a wide variance in estrogen-induced foci formation, demonstrating heterogeneity of this tumor cell line. The postconfluent cell growth of the estrogen receptor-deficient cell line, MDA-MB-231, contrasted with MCF-7 by developing an extensive multilayer morphology devoid of discrete structures. The tumorigenic potential of the MCF-7 cells used in our experiments was confirmed by their estrogen-dependent growth in immunosuppressed male BDF1 mice. These data suggest an estrogen receptor-based mechanism for the development of multicellular foci during postconfluent growth of MCF-7 cells. After confluence, foci, in contrast to the quiescent surrounding monolayer, retain proliferating cells. Focus formation, therefore, reflects the heterogeneous responsiveness of these cells to estrogens and should provide a model permitting in vitro comparisons between the progenitor cells of multicellular foci and the monolayer population.

Microdensitometer-computer correlation analysis of two distinct, spatially segregated classes of microtubule bridges in Allogromia pseudopodia

Previous video-light microscopic studies have shown that the microtubule bundles in the pseudopodia of foraminiferan protists display several types of movements in vivo, including active bending, zipping/splaying, and axial translocations. To gain insight into the types and arrangement of microtubule-associated proteins (e.g., mechanoenzymes, crosslinkers) in such a highly dynamic system, we employed microdensitometric-computer correlation methods to analyze, quantitatively, intermicrotubule bridges in thin-section electron micrographs of Allogromia laticollaris and Allogromia sp. (strain NF). Two distinct bridges occupying mutually exclusive zones between adjacent microtubules were identified. Type I bridges displayed a single axial repeat (34 nm for A. laticollaris and 28 nm for Allogromia sp.) and Type II bridges showed a typical 12-dimer helical superlattice pattern. In A. laticollaris, the two types of bridges were morphologically distinct: Type I bridges were aligned perpendicular to the microtubule wall and were 23-nm wide with an electron-lucent core; Type II bridges were irregular filaments projecting from the microtubules at various angles. When compared with the known distribution of microtubule-associated proteins in other systems, our findings indicate that, in vivo, Allogromia pseudopodial microtubules are decorated with MAP2-like bridges interrupted by discrete clusters of a dynein-like component.

Kinetochores capture astral microtubules during chromosome attachment to the mitotic spindle: direct visualization in live newt lung cells

When viewed by light microscopy the mitotic spindle in newt pneumocytes assembles in an optically clear area of cytoplasm, virtually devoid of mitochondria and other organelles, which can be much larger than the forming spindle. This unique optical property has allowed us to examine the behavior of individual microtubules, at the periphery of asters in highly flattened living prometaphase cells, by video-enhanced differential interference-contrast light microscopy and digital image processing. As in interphase newt pneumocytes (Cassimeris, L., N. K. Pryer, and E. D. Salmon. 1988. J. Cell Biol. 107:2223-2231), centrosomal (i.e., astral) microtubules in prometaphase cells appear to exhibit dynamic instability, elongating at a mean rate of 14.3 +/- 5.1 microns/min (N = 19) and shortening at approximately 16 microns/min. Under favorable conditions the initial interaction between a kinetochore and the forming spindle can be directly observed. During this process the unattached chromosome is repeatedly probed by microtubules projecting from one of the polar regions. When one of these microtubules contacts the primary constriction the chromosome rapidly undergoes poleward translocation. Our observations on living mitotic cells directly demonstrate, for the first time, that chromosome attachment results from an interaction between astral microtubules and the kinetochore.

Diffuse kinetochores and holokinetic anaphase chromatin movement during mitosis in the hemipteran Agallia constricta (leafhopper) cell line AC-20

Mitosis in the hemipteran Agallia constricta (leafhopper) cell line AC-20 was examined by light microscopy of living and fixed cells. During early prometaphase the numerous small (0.30-3.0-microns) chromosomes appear as discrete units that lack a primary constriction. However, by late prometaphase the chromosomes are tightly packed at the spindle equator and are no longer clearly resolvable as individuals. When viewed from the side the metaphase chromatin appears as a 2-3-microns wide band that spans the width of the spindle; when viewed from the pole it appears as a fenestrated disk. The metaphase chromatin splits at anaphase into two sister chromatin plates, each of which exhibits holokinetic poleward movement, i.e., all parts of each plate move as a single unit with the same velocity. In many early-to-mild anaphase cells the separating sister plates are connected by chromatin-containing bridges that break as anaphase progresses. Ultrastructural analyses of serial thick and thin sections from cells fixed by conventional, OsO4/KFeCN, or high pressure rapid freezing methods, reveal that by metaphase all of the chromosomes are interconnected to form a large, irregularly shaped fenestrated disk of chromatin. Similar analyses reveal that adjacent chromatids remain interconnected throughout anaphase. Each disk of metaphase and anaphase chromatin contains numerous kinetochores recessed within its pole-facing surface. Kinetochores consist of a fine, faintly staining fibrillar material arranged along the chromatin surface as thin (0.1-0.3 micron dia.) rods varying considerably (0.15-2.3 microns) in length. From these observations we conclude that the polycentric metaphase chromatin of A. constricta, and its holokinetic behavior during anaphase, arises from the aggregation or cohesion of smaller prometaphase chromosomes, each of which contains a single, diffuse kinetochore.

Flexible-substratum technique for viewing cells from the side: some in vivo properties of primary (9+0) cilia in cultured kidney epithelia

Cells cultured on thin plastic (e.g. Formvar, Teflon, polycarbonate) membranes can be clearly imaged from the side in vivo by video microscopy. We have used this flexible-substratum technique to examine the behaviour and properties of primary cilia in confluent cultures of the kidney epithelial cell lines PtK1, PtK2, LLC-PK1, MDCK and BSC-40. In these cells primary cilia appear as rigid rods, up to 55 micron long, which project at various angles from the dorsal cell surface. The length distribution of primary cilia in confluent cultures is a distinct characteristic of each established kidney cell line examined, with LLC-PK1 exhibiting three distinct length populations. Primary cilia of kidney cell lines bend passively in response to flow but do not display propagated bending or vortical motions. Up to 26% of the cilia in the cell types examined possess one or more conspicuous swellings along the ciliary shaft. Treatment with 0.05% trypsin, which is sufficient to cause cell rounding, does not induce the resorption or shedding of the cilium. These direct observations demonstrate that kidney epithelial-cell primary cilia are non-motile and longer than previously thought, and suggest that their length represents a phenotypic marker for each cell line.

Microtubules associate with actin-containing filaments at discrete sites along the ventral surface of Allogromia reticulopods

We have investigated the distribution of actin and microtubules in pseudopodial networks (reticulopods) of the protozoan Allogromia sp., strain NF, in order to help elucidate the respective roles these components play in network organization and motility. Double-label fluorescence studies with tubulin antibodies and tetramethyl-rhodamine (TMR)-phalloidin reveal that microtubules and filamentous actin co-localize in regions where trunk pseudopods contact the substratum and splay to form the pseudopodial network; distal to these regions the network contains numerous microtubules but little or no F-actin. Similar results were obtained using various commercial actin antibodies. Correlative anti-actin immunofluorescence and high-voltage electron microscopy of serial 0.25 micron sections reveal that actin is contained within discrete electron-opaque, fan-shaped structures distributed along the cytoplasmic aspect of the ventral reticulopodial membrane. Electron microscopy of serial 100 nm sections from conventionally fixed specimens confirms that these actin-rich plaques are composed of a felt of roughly parallel, 5 nm diameter filaments. A subset of parallel and often bundled microtubules is enmeshed within, or contacts the periphery of, these filament plaques. Upon leaving a plaque, bundled microtubules frequently splay into smaller bundles. These observations are consistent with the hypothesis that interactions between microtubules and actin-containing microfilaments, particularly at substratum adhesion points, are involved in various aspects of reticulopodial motility, particularly network morphogenesis and cell body locomotion.

Optical approaches to the study of foraminiferan motility

Microtubules are the major cytoskeletal component of foraminiferan reticulopodia. Video-enhanced differential interference contrast light microscopy has demonstrated that the microtubules serve as the intracellular tracks along which rapid bidirectional organelle transport and cell surface motility occurs. Microtubules appear to move, both axially and laterally within the pseudopodial cytoplasm, and these microtubule translocations appear to drive the various reticulopodial movements. F-actin is localized to discrete filament plaques form at sites of pseudopod-substrate adhesion. Correlative immunofluorescence and electron microscopy reveals a structural interaction between microtubules and the actin-containing filament plaques. Our recent data on reticulopodial motility are discussed in an historical context, and a model for foram motility, based on motile microtubules, is presented.

Primary cilia cycle in PtK1 cells: effects of colcemid and taxol on cilia formation and resorption

The effects of colcemid (0.16-1.0 microM) and taxol (10 microM) on the primary cilia cycle in PtK1 cells were studied by antitubulin immunofluorescence microscopy and by high-voltage electron microscopy of serial 0.25-micron sections. Although these drugs induce a fully characteristic rearrangement (taxol) or disassembly (colcemid) of cytoplasmic microtubules, neither affects the structure of primary cilia formed prior to the treatment or the resorption of primary cilia during the initial stages of mitosis. Cells arrested in mitosis by taxol or colcemid remain in mitosis for 5-7 h at 37 degrees C and then form 4N "micronucleated" restitution nuclei. Formation of primary cilia in these micronucleated cells is blocked by colcemid in a concentration-dependent fashion: normal cilia with expanded (ie, bulbed) distal ends form at the lower (0.16-0.25 microM) concentrations, while both cilia formation and centriole replication are inhibited at the higher (greater than or equal to 1.0 microM) concentrations. However, even in the presence of 1.0 microM colcemid, existing centrioles acquire the appendages characteristically associated with ciliating centrioles and attach to the dorsal cell surface. Continuous treatment with colcemid thus produces a population of cells enriched for the early stages of primary cilia formation. Micronucleated cells formed from a continuous taxol treatment contain two normal centriole pairs, and one or both parenting centrioles possess a primary cilium. Taxol, which has been reported to stabilize microtubules in vitro, does not inhibit the cell-cycle-dependent assembly and disassembly of axonemal microtubules in vivo.

Novel extracellular matrix and microtubule cables associated with pseudopodia of Astrammina rara, a carnivorous Antarctic foraminifer

Astrammina rara is a benthic foraminiferan protozoan which uses an extensive network of fine, branching, and anastomosing pseudopodia to capture and digest metazoans up to 12 mm long. During such predation the pseudopodia appear remarkably elastic and tensile. Electron microscopy has revealed a novel extracellular matrix of thin branching fibers associated with A. rara's pseudopodia, which are otherwise typical in appearance. These fibers are structurally associated with the pseudopodial glycocalyx. Cytoplasmic microtubules are often found in close juxtaposition to the plasma membrane and overlying extracellular fibers. In the main pseudopodial trunks bundles of 30-300 microtubules are surrounded by a lightly staining matrix and linked by occasional bridges. The microtubules follow straight trajectories in distal filopodia, but in pseudopodial trunks they appear to coil around one another to form cables. These microtubular cables and extracellular fibers are novel features of A. rara's pseudopodia and may provide the structural basis for their tensile strength.

A new model of reticulopodial motility and shape: evidence for a microtubule-based motor and an actin skeleton

Cytoskeletal inhibitors were used as probes to test the involvement of microtubules and actin microfilaments in the development, motility, and shape maintenance of the pseudopodial networks (i e, reticulopodia) of the foraminifers Allogromia sp strain NF and Allogromia laticollaris. Agents that disassemble cytoplasmic microtubules (cold, colchicine, and nocodazole) arrest all movement but have variable effects on reticulopodial shape. Electron microscopy reveals a granulofibrillar matrix but few, if any, microtubules in these motility-arrested reticulopods. Allogromiids treated with cytochalasin B or D lose substrate adhesion and undergo dramatic changes in shape and motile behavior, highlighted by the coalescence of reticulopodial cytoplasm into irregularly shaped bodies with chaotic motility. Serial semithick sections of such preparations, viewed by high-voltage electron microscopy, document a striking rearrangement of microtubules within these cytochalasin-induced bodies. All aspects of cytochalasin-altered motility are completely inhibited by colchicine. Actin is present in reticulopodia, as determined by staining with rhodamine-phalloidin; this staining is not observed in cytochalasin-treated organisms. These data provide compelling evidence that microtubules are required for reticulopodial motility. An actin-based cytoskeleton is thought to play a role in maintaining shape, mediating pseudopod/substrate adhesion, and coordinating the various microtubule-dependent processes.

Naturally occurring tubulin-containing paracrystals in Allogromia: immunocytochemical identification and functional significance

Bundles of microtubules (MTs) are readily visualized in vivo by videomicroscopy in highly flattened reticulopodia of the foraminiferan protozoan Allogromia sp. strain NF. In this report we use videomicroscopy, immunocytochemistry, and high-voltage electron microscopy to characterize the dynamic changes that occur in this extensive MT cytoskeleton, and in the associated cytoplasmic transport, during induced withdrawal and subsequent reextension of reticulopodia. Within seconds after application of the withdrawal stimulus (seawater substitute made hypertonic with MgCl2) intracellular bidirectional transport along linear MT-containing fibrils ceases and is replaced by an inward, constant-velocity flow of cytoplasm along the fibrils. As withdrawal continues, most fibrils become wavy and coalesce to form phase-dense pools. These wavy fibrils and phase-dense pools contain a paracrystalline material and few if any MTs. Same-section correlative immunofluorescence and high-voltage electron microscopy reveal that the paracrystalline material contains tubulin. During recovery linear fibrils (MTs) rapidly extend from the phase-dense pools (paracrystals), which concurrently shrink in size, thus reestablishing normal network morphology and motility. We conclude that the MT cytoskeleton in Allogromia reticulopodia is transformed during withdrawal into a tubulin-containing paracrystal, which serves as a temporary reservoir of MT protein and an initiation site for MT regrowth.

Microtubule-dependent reticulopodial motility: is there a role for actin?

We summarize our recent immunocytochemical characterization of the reticulopodial cytoskeleton of two allogromiid foraminifers and our pharmacologic dissection of its motility. The reticulopodial microtubule cytoskeleton stained with an antiserum to brain microtubule-associated protein 2. Polymeric actin was localized in the reticulopodia by rhodamine-phalloidin staining. Microtubule inhibitors reversibly inhibited all aspects of motility; cytochalasins induced altered morphology and disorganization of motility but did not inhibit pseudopodial movements or intracellular transport. Simultaneous application of KCN and salicylhydroxamic acid (an alternative oxidase inhibitor) rapidly blocked all movement, indicating that motility is dependent on metabolic energy and that an alternative oxidative pathway functions in allogromiids. Micromanipulation and laser microsurgical experiments revealed tension throughout the reticulopodium. Our results suggest that microtubules are active components of the reticulopodial motile machinery. Actin may mediate substrate adhesion, whole-cell locomotion, pseudopodial tension, and coordination of the microtubule-based motility.

Rapid intracellular motility and dynamic membrane events in an Antarctic foraminifer

Some properties of cytoplasmic transport in a cold-adapted (Antarctic) organism are reported for the first time. Phase-contrast light microscopy of Astrammina rara, an arenaceous foraminiferan protozoan, reveals that the saltatory transport of cytoplasmic granules in reticulopods occurs bidirectionally and at rates up to 7.5-micron/s. Extracellularly attached latex microspheres are rapidly translocated on the reticulopodial surface, thus demonstrating membrane fluidity at low (-1.8 degrees C) ambient temperatures. Rapid extension/withdrawal and branching/fusing of pseudopodia further illustrate dynamic plasma membrane activity at subzero temperatures. Immunofluorescence microscopy with an antibody monospecific for tubulin shows that these pseudopods contain microtubules. The motility of this cold-adapted foraminifer therefore appears fully comparable to the motility of allogromiid foraminifers from temperate waters.

Evidence that cell surface motility in Allogromia is mediated by cytoplasmic microtubules

We have previously shown that reticulopods of Allogromia sp. (strain NF) and Allogromia laticollaris display rapid, bidirectional saltatory transport of plasma membrane surface markers (i.e., polystyrene microspheres). Correlative video microscopic and electron microscopic methods were used to determine whether cytoskeletal components are involved in this surface transport. Such transport was observed only where the plasma membrane overlay cytoplasmic fibrils, which have been shown to be involved in organelle transport. Ultrastructural analysis indicated that these fibrils contain microtubules and an associated flocculent fibrillar material. In studies with nonionic detergents the surface marker particles remained bound to these microtubule-containing fibrils, even after the plasma membrane had been removed. Surface transport was inhibited when reticulopods were treated with agents that induce microtubule disassembly. Together these observations provide strong evidence that surface motility in Allogromia is mediated by labile cytoplasmic microtubules.

Correlative immunofluorescence and electron microscopy on the same section of epon-embedded material

Semithick (0.25-0.50 micron) sections, cut from cells stained with fluorescein isothiocyanate (FITC)-conjugated antibodies prior to embedding in Epon, show high resolution patterns of immunofluorescence against a background void of autofluorescence. These same sections can then be viewed, after uranyl and lead staining, in the electron microscope. We clearly establish the specificity of this same-section correlative immunofluorescence-electron microscopy approach by showing that the immunofluorescent patterns observed in such sections of cells, stained prior to embedding for the indirect immunofluorescent localization of tubulin, follows the distribution of microtubules within the same sections as determined by electron microscopy. We then use this method to demonstrate for the first time that the 57 kD core protein of wound tumor virus is associated, at the ultrastructural level, with two distinct cellular inclusions in virally infected AC-20 cells. In some instances the fidelity in the correlation between the distribution of immunofluorescently labeled antigens and the ultrastructure in the same section eliminates the need to employ more complex procedures for labeling antigens for ultrastructural detection. This technique, therefore, provides a rapid and simple first approach to many problems that require the ultrastructural localization of specific antigens.

Digestion of prey in foraminifera is not anomalous: a correlation of light microscopic, cytochemical, and HVEM technics to study phagotrophy in two allogromiids

Correlative light, high-voltage electron and conventional electron microscopic methods were used to investigate digestion in two allogromiid foraminiferans, Allogromia sp., strain NF, and A. laticollaris Arnold. Microscopic observations showed that bacterial prey are phagocytosed by reticulopodia and are transported to the allogromiid cell body within blister-like phagosomes. Larger prey (algae, diatoms) are transported along the reticulopodial surface and are either stored extrathalamously or phagocytosed at the oral opening (peduncle). Studies of allogromiids optimally fixed and labeled with an extracellular-space label (colloidal thorium) showed that phagocytosed prey are completely enclosed by a plasma membrane envelope; this finding was corroborated by a serial-section three-dimensional reconstruction of the oral zone of one allogromiid. Cytochemical staining for acid phosphatase showed that lysosomes are absent from reticulopods but abundant in the cell body, particularly in the oral zone cytoplasm. We conclude that digestion in allogromiid foraminiferans is accomplished by a vacuole-based digestive apparatus and not by extracellular digestion within a lacunary system, as has been suggested in earlier studies.

Surface transport properties of reticulopodia: do intracellular and extracellular motility share a common mechanism?

The reticulopodial networks of the foraminiferan protozoans Allogromia sp., strain NF, and A. laticollaris display rapid (up to 11 microns/second) and bidirectional saltatory transport of membrane surface markers (polystyrene microspheres). Electron microscopy shows that microspheres adhere directly to the reticulopodial surface glycocalyx. A videomicroscopic analysis of this phenomenon reveals that microsphere movement is typically independent of pseudopod extension/withdrawal and that particles of different sizes and surface properties display similar motile characteristics. The motile properties of surface-associated microspheres appear identical to those of saltating intracellular organelles. Indeed, in some instances the surface-attached microspheres appear transiently linked in motion to these underlying organelles. Our observations suggest that, in reticulopodia, surface transport of microspheres and intracellular transport of organelles are driven by a common mechanism.

Multiple fission in Allogromia sp., strain NF (Foraminiferida): release, dispersal, and ultrastructure of offspring

The release, dispersal, and ultrastructure of juveniles arising through multiple fission in the benthic foraminiferan Allogromia sp., strain NF (Lee & Pierce, 1963) has been examined by light and electron microscopy. An extensive reticulopodial network participates in the dispersal of fully differentiated young as they emerge from the fragmented parental test. During the earliest stages of release, offspring are of two classes--aroused and unaroused. Unaroused juveniles, which have not extended pseudopods, attach externally to the network and are transported bidirectionally along its surface. Aroused juveniles, which have extended pseudopods and are in protoplasmic continuity with the network, move quickly to the periphery of the network. Within 24 h, juveniles establish a communal "feeding reticulum" in which dispersed individuals are in protoplasmic continuity with neighbors via a common reticulopodial network. At the ultrastructural level, the cell body cytoplasm of unaroused juveniles contains numerous patches of a paracrystalline material, which disappears as their pseudopodia are extended to join the communal feeding reticulum. This paracrystalline material therefore appears to be a temporary reservoir of precursors required for pseudopod construction.

Evidence against surf-riding as a general mechanism for surface motility

The mechanism responsible for the energy-dependent movement of membrane components (ie, surface motility) is unknown. Recently a potentially unifying model, termed "surf-riding" [Hewitt, 1979] or "surf-boarding" [Berlin and Oliver, 1982], has been proposed to explain surface motility. Using phase-contrast light microscopy and membrane surface markers (polystyrene microspheres), we have tested the surf-riding/surf-boarding hypothesis on two protozoan systems: the axopodia of the heliozoan Echinosphaerium nucleofilum and the reticulopodial networks of the allogromiid foraminiferans Allogromia laticollaris and Allogromia sp, strain NF. Our evidence indicates that surface motility, as displayed by these organisms, does not occur by a surf-riding/surf-boarding mechanism. Previous observations on surface motility associated with the Chlamydomonas flagellum indicate that this system is also incompatible with the surf-boarding/surf-riding hypothesis.

Factors which influence light microscopic visualization of biological material in sections prepared for electron microscopy

Epoxy-embedded biological material, sectioned for conventional or high-voltage electron microscopy, can be visualized within the section with good contrast and detail by phase-contrast or dark-field light microscopy. The (phase) contrast of such material is not substantially influenced by the type of embedding resin or section support substrate. It is, however, influenced by the type of fixation, by heavy metal (uranyl and lead) staining and by the section thickness. After screening ultrathin and semithin sections for content with the light microscope, one need stain and examine only those grids containing sections of interest. This approach eliminates the need to screen sections with the electron microscope and, in some cases, the need to stain non-useful sections. This time-saving procedure is particularly useful for studies requiring ultrastructural examination of a selected area or structure which is large enough to be visualized with the light microscope but which comprises only a small volume of the embedded material.