Myosin I is required for hypha formation in Candida albicans

The pathogenic yeast Candida albicans can undergo a dramatic change in morphology from round yeast cells to long filamentous cells called hyphae. We have cloned the CaMYO5 gene encoding the only myosin I in C. albicans. A strain with a deletion of both copies of CaMYO5 is viable but cannot form hyphae under all hypha-inducing conditions tested. This mutant exhibits a higher frequency of random budding and a depolarized distribution of cortical actin patches relative to the wild-type strain. We found that polar budding, polarized localization of cortical actin patches, and hypha formation are dependent on a specific phosphorylation site on myosin I, called the “TEDS-rule” site. Mutation of this serine 366 to alanine gives rise to the null mutant phenotype, while a S366D mutation, the product of which mimics a phosphorylated serine, allows hypha formation. However, the S366D mutation still causes a depolarized distribution of cortical actin patches in budding cells, similar to that in the null mutant. The localization of CaMyo5-GFP together with cortical actin patches at the bud and hyphal tips is also dependent on serine 366. Intriguingly, the cortical actin patches in the majority of the hyphae of the mutant expressing Camyo5^S366D were depolarized, suggesting that although their distribution is dependent on myosin I localization, polarized cortical actin patches may not be required for hypha formation.     

Relationship of actin organization to growth in the two forms of the dimorphic yeastCandida tropicalis

Summary Candida tropicalis is a dimorphic yeast capable of growing both as a budding yeast and as filamentous hyphae depending upon the source of the carbon used in the culture medium. The organization of F-actin during growth of the yeast form (Y-form) and the hyphal form (H-form) was visualized by rhodamine-conjugated phalloidin by using a conventional fluorescence microscope as well as a laser scanning confocal fluorescence microscope. In single cells without a bud or non-growing hyphae, actin dots were evenly distributed throughout the cytoplasm. Before the growth of the bud or hypha, the actin dots were concentrated at one site. During bud growth, actin dots were located solely in the bud. They filled the small bud and then filled the apical two-thirds of the cytoplasm of the middlesized bud. During growth of the large bud, actin dots which had filled the apical half of the cytoplasm gradually moved to the tip of the bud. In the formation of the septum, actin dots were arranged in two lines at the conjunction of the bud and the mother cell. During hyphal growth, the majority of actin dots were concentrated at the hyphal apex. A line of clustered spots or a band of actin was observed only at the site where the formation of a new septum was imminent. This spatial and temporal organization of actin in both categories of cells was demonstrated to be closely related to the growth and local deposition of new cell wall material by monitoring the mode of growth with Calcofluor staining. Treatment of both forms of cells with cytochalasin A (CA) confirmed the close relationship between actin and new cell wall deposition. CA treatment revealed lightly stained unlocalized actin which was associated with abnormal cell wall deposition as well as changes in morphology. These results suggest that actin is required for proper growth and proper deposition of cell wall material and also for maintaining the morphology of both forms of cells.Abbrevations FM fluorescence microscopy - EM electron microscopy - rh rhodamine - CA cytochalasin A - CD cytochalasin D - PBS phosphate-buffered saline - DMSO dimethylsulfoxide - GA glutaraldehyde     

Relationship of actin and tubulin distribution to bud growth in wild-type and morphogenetic-mutant Saccharomyces cerevisiae

The distribution of actin in wild-type cells and in morphogenetic mutants of the budding yeast Saccharomyces cerevisiae was explored by staining cells with fluorochrome-labeled phallotoxins after fixing and permeabilizing the cells by several methods. The actin appeared to be localized in a set of cortical spots or patches, as well as in a network of cytoplasmic fibers. Bundles of filaments that may possibly correspond to the fibers visualized by fluorescence were observed with the electron microscope. The putative actin spots were concentrated in small and medium-sized buds and at what were apparently the sites of incipient bud formation on unbudded cells, whereas the putative actin fibers were generally oriented along the long axes of the mother-bud pairs. In several morphogenetic mutants that form multiple, abnormally elongated buds, the actin patches were conspicuously clustered at the tips of most buds, and actin fibers were clearly oriented along the long axes of the buds. There was a strong correlation between the occurrence of active growth at particular bud tips and clustering of actin spots at those same tips. Near the end of the cell cycle in wild- type cells, actin appeared to concentrate (as a cluster of spots or a band) in the neck region connecting the mother cell to its bud. Observations made using indirect immunofluorescence with a monoclonal anti-yeast-tubulin antibody on the morphogenetic mutant cdc4 (which forms multiple, abnormally elongated buds while the nuclear cycle is arrested) revealed the surprising occurrence of multiple bundles of cytoplasmic microtubules emanating from the one duplicated spindle-pole body per cell. It seems that most or all of the buds contain one or more of these bundles of microtubules, which often can be seen to extend to the very tips of the buds. These observations are consistent with the hypotheses that actin, tubulin, or both may be involved in the polarization of growth and localization of cell-wall deposition that occurs during the yeast cell cycle.     

The role of actin, fimbrin and endocytosis in growth of hyphae in Aspergillus nidulans

Filamentous fungi are ideal systems to study the process of polarized growth, as their life cycle is dominated by hyphal growth exclusively at the cell apex. The actin cytoskeleton plays an important role in this growth. Until now, there have been no tools to visualize actin or the actin-binding protein fimbrin in live cells of a filamentous fungus. We investigated the roles of actin (ActA) and fimbrin (FimA) in hyphal growth in Aspergillus nidulans . We examined the localization of ActA::GFP and FimA::GFP in live cells, and each displayed a similar localization pattern. In actively growing hyphae, cortical ActA::GFP and FimA::GFP patches were highly mobile throughout the hypha and were concentrated near hyphal apices. A patch-depleted zone occupied the apical 0.5 μm of growing hypha. Both FimA::GFP and Act::GFP also localize transiently to septa. Movement and later localization of both was compromised after cytochalasin treatment. Disruption of fimA resulted in delayed polarity establishment during conidium germination, abnormal hyphal growth and endocytosis defects in apolar cells. Endocytosis was severely impaired in apolar fimA disruption cells. Our data support a novel apical recycling model which indicates a critical role for actin patch-mediated endocytosis to maintain polarized growth at the apex.     

Growth and the inducibility of mycelium formation in Candida albicans: a single-cell analysis using a perfusion chamber

In Candida albicans, cells actively growing in the budding form cannot be immediately induced to form a mycelium until they enter stationary phase. However, if exponential phase cells are starved for a minimum of 10 to 20 min, they are inducible. Using a video-monitored perfusion chamber, we found that starved cells were able to form mycelia regardless of their position in the budding cycle. When starved exponential cells were released into fresh nutrient medium at high temperature and pH, conditions conducive to mycelium formation, unbudded cells evaginated after an average lag period of 75 min and then grew exclusively in the mycelial form. Depending upon the volume, or maturity, of the bud, budded cells entered two different avenues of outgrowth leading to mycelium formation. If the daughter bud was small, growth resumed by apical elongation of the bud, leading to a 'shmoo' shape which tapered into an apical mycelium. If the daughter bud was large, the cell underwent a sequence of evaginations: first, the mother cell evaginated after an average period of 75 min; then the daughter bud evaginated 40 min later. Both evaginations then grew in the mycelial form. In this latter sequence, the evagination on the mother cell was positioned non-randomly, occurring in the majority of cells adjacent to the bud. All buds undergoing evagination contained a nucleus, but roughly 20% of buds undergoing apical elongation did not.     

Structural rearrangements of tubulin and actin during the cell cycle of the yeast Saccharomyces

The distribution of actin and tubulin during the cell cycle of the budding yeast Saccharomyces was mapped by immunofluorescence using fixed cells from which the walls had been removed by digestion. The intranuclear mitotic spindle was shown clearly by staining with a monoclonal antitubulin; the presence of extensive bundles of cytoplasmic microtubules is reported. In cells containing short spindles still entirely within the mother cells, one of the bundles of cytoplasmic microtubules nearly always extended to (or into) the bud. Two independent reagents (anti-yeast actin and fluorescent phalloidin) revealed an unusual distribution of actin: it was present as a set of cortical dots or patches and also as distinct fibers that were presumably bundles of actin filaments. Double labeling showed that at no stage in the cell cycle do the distributions of actin and tubulin coincide for any significant length, and, in particular, that the mitotic spindle did not stain detectably for actin. However, both microtubule and actin staining patterns change in a characteristic way during the cell cycle. In particular, the actin dots clustered in rings about the bases of very small buds and at the sites on unbudded cells at which bud emergence was apparently imminent. Later in the budding cycle, the actin dots were present largely in the buds and, in many strains, primarily at the tips of these buds. At about the time of cytokinesis the actin dots clustered in the neck region between the separating cells. These aspects of actin distribution suggest that it may have a role in the localized deposition of new cell wall material.     

Hyphal Elongation Is Regulated Independently of Cell Cycle in Candida albicans

The mechanism for apical growth during hyphal morphogenesis in Candida albicans is unknown. Studies from Saccharomyces cerevisiae indicate that cell morphogenesis may involve cell cycle regulation by cyclin-dependent kinase. To examine whether this is the mechanism for hyphal morphogenesis, the temporal appearance of different spindle pole body and spindle structures, the cell cycle-regulated rearrangements of the actin cytoskeleton, and the phosphorylation state of the conserved Tyr19 of Cdc28 during the cell cycle were compared and found to be similar between yeast and serum-induced hyphal apical cells. These data suggest that hyphal elongation is not mediated by altering cell cycle progression or through phosphorylation of Tyr19 of Cdc28. We have also shown that germ tubes can evaginate before spindle pole body duplication, chitin ring formation, and DNA replication. Similarly, tip-associated actin polarization in each hypha occurs before the events of the G(1)/S transition and persists throughout the cell cycle, whereas cell cycle-regulated actin assemblies come and go. We have also shown that cells in phases other than G(1) can be induced to form hyphae. Hyphae induced from G(1) cells have no constrictions, and the first chitin ring is positioned in the germ tube at various distances from the base. Hyphae induced from budded cells have a constriction and a chitin ring at the bud neck, beyond which the hyphae continue to elongate with no further constrictions. Our data suggest that hyphal elongation and cell cycle morphogenesis programs are uncoupled, and each contributes to different aspects of cell morphogenesis.     

A Highlights from MBoC Selection: Dynamics of Multiple Nuclei in Ashbya gossypii Hyphae Depend on the Control of Cytoplasmic Microtubules Length by Bik1, Kip2, Kip3, and Not on a Capture/Shrinkage Mechanism

Ashbya gossypii has a budding yeast-like genome but grows exclusively as multinucleated hyphae. In contrast to budding yeast where positioning of nuclei at the bud neck is a major function of cytoplasmic microtubules (cMTs), A. gossypii nuclei are constantly in motion and positioning is not an issue. To investigate the role of cMTs in nuclear oscillation and bypassing, we constructed mutants potentially affecting cMT lengths. Hyphae lacking the plus (+)end marker Bik1 or the kinesin Kip2 cannot polymerize long cMTs and lose wild-type nuclear movements. Interestingly, hyphae lacking the kinesin Kip3 display longer cMTs concomitant with increased nuclear oscillation and bypassing. Polymerization and depolymerization rates of cMTs are 3 times higher in A. gossypii than in budding yeast and cMT catastrophes are rare. Growing cMTs slide along the hyphal cortex and exert pulling forces on nuclei. Surprisingly, a capture/shrinkage mechanism seems to be absent in A. gossypii. cMTs reaching a hyphal tip do not shrink, and cMT +ends accumulate in hyphal tips. Thus, differences in cMT dynamics and length control between budding yeast and A. gossypii are key elements in the adaptation of the cMT cytoskeleton to much longer cells and much higher degrees of nuclear mobilities.     

CaSPA2 is important for polarity establishment and maintenance in Candida albicans

Saccharomyces cerevisiae Spa2p is a component of polarisome that controls cell polarity. Here, we have characterized the role of its homologue, CaSpa2p, in the polarized growth in Candida albicans. During yeast growth, GFP-tagged CaSpa2p localized to distinct growth sites in a cell cycle-dependent manner, while during hyphal growth it persistently localized to hyphal tips throughout the cell cycle. Persistent tip localization of the protein was also observed in Catup1Delta and Canrg1Delta, mutants constitutive for filamentous growth. Caspa2Delta exhibited defects in polarity establishment and maintenance, such as random budding and failure to confine growth to a small surface area leading to round cells with wide, elongated bud necks and markedly thicker hyphae. It was also defective in nuclear positioning, presumably a result of defective interactions between cytoplasmic microtubules with certain polarity determinants. The highly conserved SHD-I and SHD-V domains were found to be important and responsible for different aspects of CaSpa2p function. Caspa2Delta exhibited no virulence in the mouse systemic candidiasis model. Because of the existence of distinct growth forms and the easy control of the switch between them in vitro, C. albicans may serve as a useful model in cell polarity research.     

蜜环菌侵染天麻皮层过程中酸性磷酸酶的细胞化学研究

被蜜环菌(Armillaria mellea)侵染的天麻(Gastrodia elata B1.)皮层中,由外至内形成三种类型的染菌细胞:菌丝结细胞、空腔细胞和消化细胞。外部两类细胞中的酸性磷酸酶定位显示,一些位于空腔细胞或衰老的菌丝结细胞中的菌丝内部逐渐产生大量酸性磷酸酶,随后菌丝发生自溶。这两类细胞中未发现明显的释放水解酶消化菌丝的现象。当菌丝进入消化细胞以后,情况与此不同,大量包含酸性磷酸酶的微小颗粒出现在菌丝周围,随后这些酶颗粒相互融合,形成包围菌丝的消化泡,菌丝被溶酶体水解酶所消化。最后消化泡变为包含代谢废物的残体。     

A comparison of volume growth during bud and mycelium formation in Candida albicans: a single cell analysis

When stationary phase cells of the dimorphic yeast Candida albicans are diluted into fresh medium at pH 4.5 (low pH), they synchronously form ellipsoidal buds, but when diluted into the same medium at pH 6.7 (high pH), they synchronously form elongate mycelia. Using a perfusion chamber to monitor single cells, we have compared the rates of volume growth between budding and mycelium-forming cells. Results are presented which demonstrate that: (1) after release from stationary phase into medium of low or high pH, each original sphere grows in volume to the time of initial evagination, but does not grow subsequently; (2) successive budding on the original mother cell occurs without interruption resulting in continuous volume growth; however, an interruption in volume growth of the initial bud (B1) occurs before it in turn evaginates; and (3) the rate of volume growth of the first bud at low pH is identical to the rate of volume growth of the mycelium at high pH even though the surface to volume ratios are quite different. The last result is unexpected and is therefore considered in relation to cell wall deposition.     

Bud8p and Bud9p, proteins that may mark the sites for bipolar budding in yeast

The bipolar budding pattern of a/alpha Saccharomyces cerevisiae cells appears to depend on persistent spatial markers in the cell cortex at the two poles of the cell. Previous analysis of mutants with specific defects in bipolar budding identified BUD8 and BUD9 as potentially encoding components of the markers at the poles distal and proximal to the birth scar, respectively. Further genetic analysis reported here supports this hypothesis. Mutants deleted for BUD8 or BUD9 grow normally but bud exclusively from the proximal and distal poles, respectively, and the double-mutant phenotype suggests that the bipolar budding pathway has been totally disabled. Moreover, overexpression of these genes can cause either an increased bias for budding at the distal (BUD8) or proximal (BUD9) pole or a randomization of bud position, depending on the level of expression. The structures and localizations of Bud8p and Bud9p are also consistent with their postulated roles as cortical markers. Both proteins appear to be integral membrane proteins of the plasma membrane, and they have very similar overall structures, with long N-terminal domains that are both N- and O-glycosylated followed by a pair of putative transmembrane domains surrounding a short hydrophilic domain that is presumably cytoplasmic. The putative transmembrane and cytoplasmic domains of the two proteins are very similar in sequence. When Bud8p and Bud9p were localized by immunofluorescence and tagging with GFP, each protein was found predominantly in the expected location, with Bud8p at presumptive bud sites, bud tips, and the distal poles of daughter cells and Bud9p at the necks of large-budded cells and the proximal poles of daughter cells. Bud8p localized approximately normally in several mutants in which daughter cells are competent to form their first buds at the distal pole, but it was not detected in a bni1 mutant, in which such distal-pole budding is lost. Surprisingly, Bud8p localization to the presumptive bud site and bud tip also depends on actin but is independent of the septins.     

Septin function in Candida albicans morphogenesis

The septin proteins function in the formation of septa, mating projections, and spores in Saccharomyces cerevisiae, as well as in cell division and other processes in animal cells. Candida albicans septins were examined in this study for their roles in morphogenesis of this multimorphic, opportunistically pathogenic fungus, which can range from round budding yeast to elongated hyphae. C. albicans green fluorescent protein labeled septin proteins localized to a tight ring at the bud and pseudohyphae necks and as a more diffuse array in emerging germ tubes of hyphae. Deletion analysis demonstrated that the C. albicans homologs of the S. cerevisiae CDC3 and CDC12 septins are essential for viability. In contrast, the C. albicans cdc10Delta and cdc11Delta mutants were viable but displayed conditional defects in cytokinesis, localization of cell wall chitin, and bud morphology. The mutant phenotypes were not identical, however, indicating that these septins carry out distinct functions. The viable septin mutants could be stimulated to undergo hyphal morphogenesis but formed hyphae with abnormal curvature, and they differed from wild type in the selection of sites for subsequent rounds of hyphal formation. The cdc11Delta mutants were also defective for invasive growth when embedded in agar. These results further extend the known roles of the septins by demonstrating that they are essential for the proper morphogenesis of C. albicans during both budding and filamentous growth.     

Roles of the CDC24 gene product in cellular morphogenesis during the Saccharomyces cerevisiae cell cycle

Temperature-sensitive yeast mutants defective in gene CDC24 continued to grow (i.e., increase in cell mass and cell volume) at restrictive temperature (36 degrees C) but were unable to form buds. Staining with the fluorescent dye Calcofluor showed that the mutants were also unable to form normal bud scars (the discrete chitin rings formed in the cell wall at budding sites) at 36 degrees C; instead, large amounts of chitin were deposited randomly over the surfaces of the growing unbudded cells. Labeling of cell-wall mannan with fluorescein isothiocyanate-conjugated concanavalin A suggested that mannan incorporation was also delocalized in mutant cells grown at 36 degrees C. Although the mutants have well-defined execution points just before bud emergence, inactivation of the CDC24 gene product in budded cells led both to selective growth of mother cells rather than of buds and to delocalized chitin deposition, indicating that the CDC24 gene product functions in the normal localization of growth in budded as well as in unbudded cells. Growth of the mutant strains at temperatures less than 36 degrees C revealed allele-specific differences in behavior. Two strains produced buds of abnormal shape during growth at 33 degrees C. Moreover, these same strains displayed abnormal localization of budding sites when growth at 24 degrees C (the normal permissive temperature for the mutants); in each case, the abnormal pattern of budding sites segregated with the temperature sensitivity in crosses. Thus, the CDC24 gene product seems to be involved in selection of the budding site, formation of the chitin ring at that site, the subsequent localization of new cell wall growth to the budding site and the growing bud, and the balance between tip growth and uniform growth of the bud that leads to the normal cell shape.     

Polarized hyphal growth in Candida albicans requires the Wiskott-Aldrich Syndrome protein homolog Wal1p

The yeast-to-hypha transition is a key feature in the cell biology of the dimorphic human fungal pathogen Candida albicans. Reorganization of the actin cytoskeleton is required for this dimorphic switch in Candida. We show that C. albicans WAL1 mutants with both copies of the Wiskott-Aldrich syndrome protein (WASP) homolog deleted do not form hyphae under all inducing conditions tested. Growth of the wild-type and wal1 mutant strains was monitored by in vivo time-lapse microscopy both during yeast-like growth and under hypha-inducing conditions. Isotropic bud growth produced round wal1 cells and unusual mother cell growth. Defects in the organization of the actin cytoskeleton resulted in the random localization of actin patches. Furthermore, wal1 cells exhibited defects in the endocytosis of the lipophilic dye FM4-64, contained increased numbers of vacuoles compared to the wild type, and showed defects in bud site selection. Under hypha-inducing conditions wal1 cells were able to initiate polarized morphogenesis, which, however, resulted in the formation of pseudohyphal cells. Green fluorescent protein (GFP)-tagged Wal1p showed patch-like localization in emerging daughter cells during the yeast growth phase and at the hyphal tips under hypha-inducing conditions. Wal1p-GFP localization largely overlapped with that of actin. Our results demonstrate that Wal1p is required for the organization of the actin cytoskeleton and hyphal morphogenesis in C. albicans as well as for endocytosis and vacuole morphology.     

The involvement of cell wall expansion in the two modes of mycelium formation of Candida albicans

When budding cells of Candida albicans are starved for 20 min and then diluted into fresh nutrient medium at 37 degrees C, pH 6.7, they form mycelia by two alternative modes. For cells with small buds, the bud expands apically, resulting in a transiently tapered daughter cell. With continued growth, the daughter cell tapers into an elongated mycelium. For cells with large buds, the bud completes expansion in the budding form, the mother cell and then the daughter bud evaginate, and the evaginations grow as mycelia. The present study investigates whether the temporal and spatial changes in the zones of wall expansion during bud growth are involved in the two modes of mycelium formation. Data are presented which demonstrate that the transition circumference which determines the two modes of mycelium formation and the transition circumference at which the active apical expansion zone shuts down are both 7 micron. This exact correlation suggests that starved cells with buds with a circumference of less than 7 micron form mycelia in the tapering mode due to the reactivation of the still present apical expansion zone, and that starved cells with buds with a circumference greater than 7 micron complete bud growth by general expansion due to the absence of the apical expansion zone at the time of starvation.     

Transmission of the eect of an antifungal agent within a single hypha

A micro-compartment culture method was devised in which a single hypha of Rhizopus stolonifer growing on an agar section traversed an antifungal non-diffusible barrier to another agar section; thus the local environment of the distal or proximal part of the hypha could be controlled independently. The responses in terms of hyphal extension of the test fungus to local application of amphotericin B in this culture system were estimated by using an automatic analysing system. After hyphae had traversed the barrier, distal application of amphotericin B caused no appreciable effect on the proximal hyphae. In contrast, proximal application of amphotericin B caused inhibition of the extension of distal hyphae. The reversal of polarized cytoplasmic streaming also occured during the inhibition of distal hyphal extension. The extents of inhibition of the distal hyphal extension and the cytoplasmic streaming were dependent upon the hyphal distance between the amphotericin B application site and the hyphal tip. These results show that the effect of an antifungal agent on a hypha depends on the region of the hypha exposed. Cytoplasmic streaming may play key role in the transmission of antifungal effects within a single hypha.     

Studies concerning the temporal and genetic control of cell polarity in Saccharomyces cerevisiae

The establishment of cell polarity was examined in the budding yeast, S. cerevisiae. The distribution of a polarized protein, the SPA2 protein, was followed throughout the yeast cell cycle using synchronized cells and cdc mutants. The SPA2 protein localizes to a patch at the presumptive bud site of G1 cells. Later it concentrates at the bud tip in budded cells. At cytokinesis, the SPA2 protein is at the neck between the mother and daughter cells. Analysis of unbudded haploid cells has suggested a series of events that occurs during G1. The SPA2 patch is established very early in G1, while the spindle pole body residues on the distal side of the nucleus. Later, microtubules emanating from the spindle pole body intersect the SPA2 crescent, and the nucleus probably rotates towards the SPA2 patch. By middle G1, most cells contain the SPB on the side of the nucleus proximal to the SPA2 patch, and a long extranuclear microtubule bundle intersects this patch. We suggest that a microtubule capture site exists in the SPA2 staining region that stabilizes the long microtubule bundle; this capture site may be responsible for rotation of the nucleus. Cells containing a polarized distribution of the SPA2 protein also possess a polarized distribution of actin spots in the same region, although the actin staining is much more diffuse. Moreover, cdc4 mutants, which form multiple buds at the restrictive temperature, exhibit simultaneous staining of the SPA2 protein and actin spots in a subset of the bud tips. spa2 mutants contain a polarized distribution of actin spots, and act1-1 and act1-2 mutants often contain a polarized distribution of the SPA2 protein suggesting that the SPA2 protein is not required for localization of the actin spots and the actin spots are not required for localization of the SPA2 protein. cdc24 mutants, which fail to form buds at the restrictive temperature, fail to exhibit polarized localization of the SPA2 protein and actin spots, indicating that the CDC24 protein is directly or indirectly responsible for controlling the polarity of these proteins. Based on the cell cycle distribution of the SPA2 protein, a "cytokinesis tag" model is proposed to explain the mechanism of the non-random positioning of bud sites in haploid yeast cells.