Showing papers in "Molecular Biology of the Cell in 2003"
TL;DR: The results suggest that the regulation of autophagy is organ dependent and the role of Aut7/Apg8 is not restricted to the starvation response, and this transgenic mouse model is a useful tool to study mammalian autophagic regulation.
Abstract: Macroautophagy mediates the bulk degradation of cytoplasmic components. It accounts for the degradation of most long-lived proteins: cytoplasmic constituents, including organelles, are sequestered into autophagosomes, which subsequently fuse with lysosomes, where degradation occurs. Although the possible involvement of autophagy in homeostasis, development, cell death, and pathogenesis has been repeatedly pointed out, systematic in vivo analysis has not been performed in mammals, mainly because of a limitation of monitoring methods. To understand where and when autophagy occurs in vivo, we have generated transgenic mice systemically expressing GFP fused to LC3, which is a mammalian homologue of yeast Atg8 (Aut7/Apg8) and serves as a marker protein for autophagosomes. Fluorescence microscopic analyses revealed that autophagy is differently induced by nutrient starvation in most tissues. In some tissues, autophagy even occurs actively without starvation treatments. Our results suggest that the regulation of autophagy is organ dependent and the role of autophagy is not restricted to the starvation response. This transgenic mouse model is a useful tool to study mammalian autophagy.
2,238 citations
TL;DR: Transcriptional responses of the fission yeast Schizosaccharomyces pombe to various environmental stresses were explored and promoter motifs associated with some of the groups of coregulated genes were identified.
Abstract: We explored transcriptional responses of the fission yeast Schizosaccharomyces pombe to various environmental stresses. DNA microarrays were used to characterize changes in expression profiles of all known and predicted genes in response to five stress conditions: oxidative stress caused by hydrogen peroxide, heavy metal stress caused by cadmium, heat shock caused by temperature increase to 39°C, osmotic stress caused by sorbitol, and DNA damage caused by the alkylating agent methylmethane sulfonate. We define a core environmental stress response (CESR) common to all, or most, stresses. There was a substantial overlap between CESR genes of fission yeast and the genes of budding yeast that are stereotypically regulated during stress. CESR genes were controlled primarily by the stress-activated mitogen-activated protein kinase Sty1p and the transcription factor Atf1p. S. pombe also activated gene expression programs more specialized for a given stress or a subset of stresses. In general, these “stress-specific” responses were less dependent on the Sty1p mitogen-activated protein kinase pathway and may involve specific regulatory factors. Promoter motifs associated with some of the groups of coregulated genes were identified. We compare and contrast global regulation of stress genes in fission and budding yeasts and discuss evolutionary implications.
776 citations
TL;DR: It is demonstrated that HSF-1 acts in multiple tissues to regulate longevity, and could represent an important molecular strategy to couple the regulation of longevity with an ancient genetic switch that governs the ability of cells to sense and respond to stress.
Abstract: The correlation between longevity and stress resistance observed in long-lived mutant animals suggests that the ability to sense and respond to environmental challenges could be important for the regulation of life span. We therefore examined the role of heat shock factor (HSF-1), a master transcriptional regulator of stress-inducible gene expression and protein folding homeostasis, in the regulation of longevity. Down-regulation of hsf-1 by RNA interference suppressed longevity of mutants in an insulin-like signaling (ILS) pathway that functions in the nervous system of Caenorhabditis elegans to influence aging. hsf-1 was also required for temperature-induced dauer larvae formation in an ILS mutant. Using tissue-specific expression of wild-type or dominant negative HSF-1, we demonstrated that HSF-1 acts in multiple tissues to regulate longevity. Down-regulation of individual molecular chaperones, transcriptional targets of HSF-1, also decreased longevity of long-lived mutant but not wild-type animals. However, suppression by individual chaperones was to a lesser extent, suggesting an important role for networks of chaperones. The interaction of ILS with HSF-1 could represent an important molecular strategy to couple the regulation of longevity with an ancient genetic switch that governs the ability of cells to sense and respond to stress.
710 citations
TL;DR: It is found that overexpression of a catalytically inactive, dominant-negative mutant of Aurora B impaired the localization of the entire Aurora B/INCENP/survivin complex to centromeres and the central spindle and severely disturbed mitotic progression.
Abstract: The function of the Aurora B kinase at centromeres and the central spindle is crucial for chromosome segregation and cytokinesis, respectively. Herein, we have investigated the regulation of human Aurora B by its complex partners inner centromere protein (INCENP) and survivin. We found that overexpression of a catalytically inactive, dominant-negative mutant of Aurora B impaired the localization of the entire Aurora B/INCENP/survivin complex to centromeres and the central spindle and severely disturbed mitotic progression. Similar results were also observed after depletion, by RNA interference, of either Aurora B, INCENP, or survivin. These data suggest that Aurora B kinase activity and the formation of the Aurora B/INCENP/survivin complex both contribute to its proper localization. Using recombinant proteins, we found that Aurora B kinase activity was stimulated by INCENP and that the C-terminal region of INCENP was sufficient for activation. Under identical assay conditions, survivin did not detectably influence kinase activity. Human INCENP was a substrate of Aurora B and mass spectrometry identified three consecutive residues (threonine 893, serine 894, and serine 895) containing at least two phosphorylation sites. A nonphosphorylatable mutant (TSS893-895AAA) was a poor activator of Aurora B, demonstrating that INCENP phosphorylation is important for kinase activation.
526 citations
TL;DR: A mechanism for the patterning of podosomes in osteoclasts is provided and a turnover of actin inside the podosome is revealed, showing that an additional step of differentiation, requiring microtubule integrity, stabilizes the podsomes at the periphery of the cell in large circular patterns.
Abstract: Podosomes, small actin-based adhesion structures, differ from focal adhesions in two aspects: their core structure and their ability to organize into large patterns in osteoclasts. To address the mechanisms underlying these features, we imaged live preosteoclasts expressing green fluorescent protein-actin during their differentiation. We observe that podosomes always form inside or close to podosome groups, which are surrounded by an actin cloud. Fluorescence recovery after photobleaching shows that actin turns over in individual podosomes in contrast to cortactin, suggesting a continuous actin polymerization in the podosome core. The observation of podosome assemblies during osteoclast differentiation reveals that they evolve from simple clusters into rings that expand by the continuous formation of new podosomes at their outer ridge and inhibition of podosome formation inside the rings. This self-organization of podosomes into dynamic rings is the mechanism that drives podosomes at the periphery of the cell in large circular patterns. We also show that an additional step of differentiation, requiring microtubule integrity, stabilizes the podosome circles at the cell periphery to form the characteristic podosome belt pattern of mature osteoclasts. These results therefore provide a mechanism for the patterning of podosomes in osteoclasts and reveal a turnover of actin inside the podosome.
443 citations
TL;DR: It is reported that NV junctions in yeast promote piecemeal microautophagy of the nucleus (PMN), a novel mode of selective microautophile that targets nonessential components of the yeast nucleus for degradation and recycling in the vacuole.
Abstract: Nucleus-vacuole (NV) junctions in Saccharomyces cerevisiae are formed through specific interactions between Vac8p on the vacuole membrane and Nvj1p in the nuclear envelope. Herein, we report that N...
424 citations
TL;DR: It is concluded that GSNO-attenuated prolyl hydroxylase activity accounts for HIF-1alpha accumulation under conditions of NO formation during normoxia and that PHD activity is subject to regulation by NO.
Abstract: Hypoxia inducible factor-1 (HIF-1) is the master regulator of metabolic adaptation to hypoxia. It is appreciated that HIF-1α accumulation is achieved under normoxic conditions by e.g., nitric oxide. We determined molecular mechanisms of HIF-1α accumulation under the impact of S-nitrosoglutathione (GSNO). In human embryonic kidney cells GSNO provoked nuclear accumulation of HIF-1α. This appeared unrelated to gene transcription and protein translation, thus pointing to inhibition of HIF-1α degradation. Indeed, GSNO as well as the hypoxia mimic CoCl2 decreased ubiquitination of HIF-1α and GSNO-induced HIF-1α failed to coimmunoprecipitate with pVHL (von Hippel Lindau protein). Considering that HIF-1α-pVHL interactions require prolyl hydroxylation of HIF-1α, we went on to demonstrate inhibition of HIF-1α prolyl hydroxylases (PHDs) by GSNO. In vitro HIF-1α-pVHL interactions revealed that GSNO dose-dependently inhibits PHD activity but not the interaction of a synthetic peptide resembling the hydroxylated oxygen-dependent degradation domain of HIF-1α with pVHL. We conclude that GSNO-attenuated prolyl hydroxylase activity accounts for HIF-1α accumulation under conditions of NO formation during normoxia and that PHD activity is subject to regulation by NO.
405 citations
TL;DR: Results indicate that endocytosis of junctional proteins is a clathrin-mediated process leading into a unique storage compartment and may mediate the disruption of intercellular contacts during normal tissue remodeling and in pathology.
Abstract: The adherens junction (AJ) and tight junction (TJ) are key regulators of epithelial polarity and barrier function. Loss of epithelial phenotype is accompanied by endocytosis of AJs and TJs via unknown mechanisms. Using a model of calcium depletion, we defined the pathway of internalization of AJ and TJ proteins (E-cadherin, p120 and β-catenins, occludin, JAM-1, claudins 1 and 4, and ZO-1) in T84 epithelial cells. Proteinase protection assay and immunocytochemistry revealed orchestrated internalization of AJs and TJs into a subapical cytoplasmic compartment. Disruption of caveolae/lipid rafts did not prevent endocytosis, nor did caveolin-1 colocalize with internalized junctional proteins. Furthermore, AJ and TJ proteins did not colocalize with the macropinocytosis marker dextran. Inhibitors of clathrin-mediated endocytosis blocked internalization of AJs and TJs, and junctional proteins colocalized with clathrin and α-adaptin. AJ and TJ proteins were observed to enter early endosomes followed by movement to organelles that stained with syntaxin-4 but not with markers of late and recycling endosomes, lysosomes, or Golgi. These results indicate that endocytosis of junctional proteins is a clathrin-mediated process leading into a unique storage compartment. Such mechanisms may mediate the disruption of intercellular contacts during normal tissue remodeling and in pathology.
376 citations
TL;DR: It appears that Dnf1p, DnF2p and Drs2p each help regulate the transbilayer lipid arrangement in the plasma membrane, and that this regulation is critical for budding endocytic vesicles.
Abstract: Plasma membranes in eukaryotic cells display asymmetric lipid distributions with aminophospholipids concentrated in the inner and sphingolipids in the outer leaflet This asymmetry is maintained by
360 citations
TL;DR: In vivo time-lapse imaging of nucleoids show they are dynamic structures able to divide and redistribute in the mitochondrial network and suggest that nucleoids are the mitochondrial units of inheritance.
Abstract: The organization of multiple mitochondrial DNA (mtDNA) molecules in discrete protein-DNA complexes called nucleoids is well studied in Saccharomyces cerevisiae. Similar structures have recently been observed in human cells by the colocalization of a Twinkle-GFP fusion protein with mtDNA. However, nucleoids in mammalian cells are poorly characterized and are often thought of as relatively simple structures, despite the yeast paradigm. In this article we have used immunocytochemistry and biochemical isolation procedures to characterize the composition of human mitochondrial nucleoids. The results show that both the mitochondrial transcription factor TFAM and mitochondrial single-stranded DNA-binding protein colocalize with Twinkle in intramitochondrial foci defined as nucleoids by the specific incorporation of bromodeoxyuridine. Furthermore, mtDNA polymerase POLG and various other as yet unidentified proteins copurify with mtDNA nucleoids using a biochemical isolation procedure, as does TFAM. The results demonstrated that mtDNA in mammalian cells is organized in discrete protein-rich structures within the mitochondrial network. In vivo time-lapse imaging of nucleoids show they are dynamic structures able to divide and redistribute in the mitochondrial network and suggest that nucleoids are the mitochondrial units of inheritance. Nucleoids did not colocalize with dynamin-related protein 1, Drp1, a protein of the mitochondrial fission machinery.
357 citations
TL;DR: A novel role for the stroma is established in influencing EMT in colon carcinoma, and a selective advantage is identified to the stromal presence of infiltrating leukocytes in regulating malignant tumor progression.
Abstract: An epithelial-mesenchymal transition (EMT) characterizes the progression of many carcinomas and it is linked to the acquisition of an invasive phenotype. Given that the tumor microenvironment is an active participant in tumor progression, an important issue is whether a reactive stroma can modulate this process. Using a novel EMT model of colon carcinoma spheroids, we demonstrate that their transforming-growth factor-beta1 (TGF-beta)-induced EMT is accelerated dramatically by the presence of activated macrophages, and we identify tumor necrosis factor-alpha (TNF-alpha) as the critical factor produced by macrophages that accelerates the EMT. A synergy of TNF-alpha and TGF-beta signaling promotes a rapid morphological conversion of the highly organized colonic epithelium to dispersed cells with a mesenchymal phenotype, and this process is dependent on enhanced p38 MAPK activity. Moreover, exposure to TNF-alpha stimulates a rapid burst of ERK activation that results in the autocrine production of this cytokine by the tumor cells themselves. These results establish a novel role for the stroma in influencing EMT in colon carcinoma, and they identify a selective advantage to the stromal presence of infiltrating leukocytes in regulating malignant tumor progression.
TL;DR: Using a BMP-responsive promoter-reporter construct in mammalian cells, it is found that Smurf1 cooperated with I-Smad in inhibiting BMP signaling and that the inhibitory activity of Smirf1 was not necessarily correlated with its ability to bind to Smad1/5 directly.
Abstract: Smad ubiquitin regulatory factor (Smurf) 1 binds to receptor-regulated Smads for bone morphogenetic proteins (BMPs) Smad1/5 and promotes their degradation. In addition, Smurf1 associates with transforming growth factor-beta type I receptor through the inhibitory Smad (I-Smad) Smad7 and induces their degradation. Herein, we examined whether Smurf1 negatively regulates BMP signaling together with the I-Smads Smad6/7. Smurf1 and Smad6 cooperatively induced secondary axes in Xenopus embryos. Using a BMP-responsive promoter-reporter construct in mammalian cells, we found that Smurf1 cooperated with I-Smad in inhibiting BMP signaling and that the inhibitory activity of Smurf1 was not necessarily correlated with its ability to bind to Smad1/5 directly. Smurf1 bound to BMP type I receptors via I-Smads and induced ubiquitination and degradation of these receptors. Moreover, Smurf1 associated with Smad1/5 indirectly through I-Smads and induced their ubiquitination and degradation. Smurf1 thus controls BMP signaling with and without I-Smads through multiple mechanisms.
TL;DR: Quantitative analysis of colocalization of EGF-rhodamine conjugate and coated pits labeled with yellow-fluorescent-protein-tagged beta2 subunit of clathrin adaptor complex AP-2 revealed that EGFR mutants lacking Grb2 binding sites do not efficiently enter coated pits.
Abstract: The molecular mechanisms of clathrin-dependent internalization of epidermal growth factor receptor (EGFR) are not well understood and, in particular, the sequence motifs that mediate EGFR interactions with coated pits have not been mapped. We generated a panel of EGFR mutants and stably expressed these mutants in porcine aortic endothelial (PAE) cells. Interestingly, mutations of tyrosine phosphorylation sites 1068 and 1086 that interact with growth-factor-receptor-binding protein Grb2 completely abolished receptor internalization in PAE cells. Quantitative analysis of colocalization of EGF-rhodamine conjugate and coated pits labeled with yellow-fluorescent-protein–tagged β2 subunit of clathrin adaptor complex AP-2 revealed that EGFR mutants lacking Grb2 binding sites do not efficiently enter coated pits. The depletion of Grb2 from PAE as well as HeLa cells expressing endogenous EGFRs by RNA interference substantially reduced the rate of EGFR internalization through clathrin-dependent pathway, thus providing the direct evidence for the important role of Grb2 in this process. Overexpression of Grb2 mutants, in which the SH3 domains were either deleted or inactivated by point mutations, significantly inhibited EGFR internalization in both PAE and HeLa cells. These findings indicate that Grb2, in addition to its key function in signaling through Ras, has a major regulatory role at the initial steps of EGFR internalization through clathrin-coated pits. Furthermore, the EGFR mutant lacking Grb2 binding sites did not efficiently recruit c-Cbl and was not polyubiquitinated. The data are consistent with the model whereby Grb2 participates in EGFR internalization through the recruitment of Cbl to the receptor, thus allowing proper ubiquitylation of EGFR and/or associated proteins at the plasma membrane.
TL;DR: Although estrogen receptor was expressed in both the ovarian and breast cancers, genes that are coregulated with the estrogen receptor in breast cancers did not show a similar pattern of coexpression in the ovarian cancers.
Abstract: We used DNA microarrays to characterize the global gene expression patterns in surface epithelial cancers of the ovary. We identified groups of genes that distinguished the clear cell subtype from other ovarian carcinomas, grade I and II from grade III serous papillary carcinomas, and ovarian from breast carcinomas. Six clear cell carcinomas were distinguished from 36 other ovarian carcinomas (predominantly serous papillary) based on their gene expression patterns. The differences may yield insights into the worse prognosis and therapeutic resistance associated with clear cell carcinomas. A comparison of the gene expression patterns in the ovarian cancers to published data of gene expression in breast cancers revealed a large number of differentially expressed genes. We identified a group of 62 genes that correctly classified all 125 breast and ovarian cancer specimens. Among the best discriminators more highly expressed in the ovarian carcinomas were PAX8 (paired box gene 8), mesothelin, and ephrin-B1 (EFNB1). Although estrogen receptor was expressed in both the ovarian and breast cancers, genes that are coregulated with the estrogen receptor in breast cancers, including GATA-3, LIV-1, and X-box binding protein 1, did not show a similar pattern of coexpression in the ovarian cancers.
TL;DR: Evidence that HSF1 does not regulate the induction of every transcript that accumulates after heat shock is provided, and the results suggest that an independent posttranscriptional mechanism regulates the accumulation of a significant number of transcripts.
Abstract: Previous work has implicated heat shock transcription factor 1 (HSF1) as the primary transcription factor responsible for the transcriptional response to heat stress in mammalian cells. We characterized the heat shock response of mammalian cells by measuring changes in transcript levels and assaying binding of HSF1 to promoter regions for candidate heat shock genes chosen by a combination of genome-wide computational and experimental methods. We found that many heat-inducible genes have HSF1 binding sites (heat shock elements, HSEs) in their promoters that are bound by HSF1. Surprisingly, for 24 heat-inducible genes, we detected no HSEs and no HSF1 binding. Furthermore, of 182 promoters with likely HSE sequences, we detected HSF1 binding at only 94 of these promoters. Also unexpectedly, we found 48 genes with HSEs in their promoters that are bound by HSF1 but that nevertheless did not show induction after heat shock in the cell types we examined. We also studied the transcriptional response to heat shock in fibroblasts from mice lacking the HSF1 gene. We found 36 genes in these cells that are induced by heat as well as they are in wild-type cells. These results provide evidence that HSF1 does not regulate the induction of every transcript that accumulates after heat shock, and our results suggest that an independent posttranscriptional mechanism regulates the accumulation of a significant number of transcripts.
TL;DR: CRB3, another Crumbs homologue that is preferentially expressed in epithelial tissues and skeletal muscles in human, plays a role in apical membrane morphogenesis and tight junction regulation.
Abstract: Crumbs is an apical transmembrane protein crucial for epithelial morphogenesis in Drosophila melanogaster embryos. A protein with all the characteristics for a Crumbs homologue has been identified from patients suffering from retinitis pigmentosa group 12, but this protein (CRB1) is only expressed in retina and some parts of the brain, both in human and mouse. Here, we describe CRB3, another Crumbs homologue that is preferentially expressed in epithelial tissues and skeletal muscles in human. CRB3 shares the conserved cytoplasmic domain with other Crumbs but exhibits a very short extracellular domain without the EGF- and laminin A-like G repeats present in the other Crumbs. CRB3 is localized to the apical and subapical area of epithelial cells from the mouse and human intestine, suggesting that it could play a role in epithelial morphogenesis. Indeed, expression of CRB3 or of a chimera containing the extracellular domain of the neurotrophin receptor p75NTR and the transmembrane and cytoplasmic domains of CRB3 led to a slower development of functional tight junctions in Madin-Darby canine kidney cells. This phenotype relied on the presence of CRB3 four last amino acids (ERLI) that are involved in a direct interaction with Par6, a regulator of epithelial polarity and tight junction formation. Thus, CRB3, through its cytoplasmic domain and its interactors, plays a role in apical membrane morphogenesis and tight junction regulation.
TL;DR: A diversity of gene expression patterns in gastric cancer is found, reflecting variation in intrinsic properties of tumor and normal cells and variation in the cellular composition of these complex tissues.
Abstract: Gastric cancer is the world's second most common cause of cancer death. We analyzed gene expression patterns in 90 primary gastric cancers, 14 metastatic gastric cancers, and 22 nonneoplastic gastric tissues, using cDNA microarrays representing ∼30,300 genes. Gastric cancers were distinguished from nonneoplastic gastric tissues by characteristic differences in their gene expression patterns. We found a diversity of gene expression patterns in gastric cancer, reflecting variation in intrinsic properties of tumor and normal cells and variation in the cellular composition of these complex tissues. We identified several genes whose expression levels were significantly correlated with patient survival. The variations in gene expression patterns among cancers in different patients suggest differences in pathogenetic pathways and potential therapeutic strategies.
TL;DR: It is concluded that the formation and stability of supermature FAs depends on a high alpha-SMA-mediated contractile activity of myofibroblast stress fibers and that fibroblast adhesion is further increased correlating with a "supermaturation" of FAs.
Abstract: Cultured myofibroblasts are characterized by stress fibers, containing alpha-smooth muscle actin (alpha-SMA) and by supermature focal adhesions (FAs), which are larger than FAs of alpha-SMA-negative fibroblasts. We have investigated the role of alpha-SMA for myofibroblast adhesion and FA maturation. Inverted centrifugation reveals two phases of initial myofibroblast attachment: during the first 2 h of plating microfilament bundles contain essentially cytoplasmic actin and myofibroblast adhesion is similar to that of alpha-SMA-negative fibroblasts. Then, myofibroblasts incorporate alpha-SMA in stress fibers, develop mature FAs and their adhesion capacity is significantly increased. When alpha-SMA expression is induced in 5 d culture by TGFbeta or low serum levels, fibroblast adhesion is further increased correlating with a "supermaturation" of FAs. Treatment of myofibroblasts with alpha-SMA fusion peptide (SMA-FP), which inhibits alpha-SMA-mediated contractile activity, reduces their adhesion to the level of alpha-SMA negative fibroblasts. With the use of flexible micropatterned substrates and EGFP-constructs we show that SMA-FP application leads to a decrease of myofibroblast contraction, shortly followed by disassembly of paxillin- and beta3 integrin-containing FAs; alpha5 integrin distribution is not affected. FRAP of beta3 integrin-EGFP demonstrates an increase of FA protein turnover following SMA-FP treatment. We conclude that the formation and stability of supermature FAs depends on a high alpha-SMA-mediated contractile activity of myofibroblast stress fibers.
TL;DR: Examples of 3-D tomographic reconstructions of whole yeast are shown and the power of this technology to obtain quantifiable information from whole, hydrated cells is demonstrated.
Abstract: We examined the yeast, Saccharomyces cerevisiae, using X-ray tomography and demonstrate unique views of the internal structural organization of these cells at 60-nm resolution. Cryo X-ray tomography is a new imaging technique that generates three-dimensional (3-D) information of whole cells. In the energy range of X-rays used to examine cells, organic material absorbs approximately an order of magnitude more strongly than water. This produces a quantifiable natural contrast in fully hydrated cells and eliminates the need for chemical fixatives or contrast enhancement reagents to visualize cellular structures. Because proteins can be localized in the X-ray microscope using immunogold labeling protocols (Meyer-Ilse et al., 2001. J. Microsc. 201, 395-403), tomography enables 3-D molecular localization. The time required to collect the data for each cell shown here was <15 min and has recently been reduced to 3 min, making it possible to examine numerous yeast and to collect statistically significant high-resolution data. In this video essay, we show examples of 3-D tomographic reconstructions of whole yeast and demonstrate the power of this technology to obtain quantifiable information from whole, hydrated cells.
TL;DR: To the authors' knowledge, this is the first observation of a cell polarization-independent lateral compartmentation in the plasma membrane of a living cell.
Abstract: Different distribution patterns of the arginine/H+ symporter Can1p, the H+ plasma membrane ATPase Pma1p, and the hexose transport facilitator Hxt1p within the plasma membrane of living Saccharomyces cerevisiae cells were visualized using fluorescence protein tagging of these proteins. Although Hxt1p-GFP was evenly distributed through the whole cell surface, Can1p-GFP and Pma1p-GFP were confined to characteristic subregions in the plasma membrane. Pma1p is a well-documented raft protein. Evidence is presented that Can1p, but not Hxt1p, is exclusively associated with lipid rafts, too. Double labeling experiments with Can1p-GFP– and Pma1p-RFP–containing cells demonstrate that these proteins occupy two different nonoverlapping membrane microdomains. The size of Can1p-rich (Pma1p-poor) areas was estimated to 300 nm. These domains were shown to be stable in growing cells for >30 min. To our knowledge, this is the first observation of a cell polarization-independent lateral compartmentation in the plasma membrane of a living cell.
TL;DR: An essential role for Kin Is is revealed in prevention and/or correction of improper kinetochore-microtubule attachments and disruption of MCAK leads to multiple kinetolic, syntelic, and combined merotelic-syntelic attachments.
Abstract: The complex behavior of chromosomes during mitosis is accomplished by precise binding and highly regulated polymerization dynamics of kinetochore microtubules. Previous studies have implicated Kin Is, unique kinesins that depolymerize microtubules, in regulating chromosome positioning. We have characterized the immunofluorescence localization of centromere-bound MCAK and found that MCAK localized to inner kinetochores during prophase but was predominantly centromeric by metaphase. Interestingly, MCAK accumulated at leading kinetochores during congression but not during segregation. We tested the consequences of MCAK disruption by injecting a centromere dominant-negative protein into prophase cells. Depletion of centromeric MCAK led to reduced centromere stretch, delayed chromosome congression, alignment defects, and severe missegregation of chromosomes. Rates of chromosome movement were unchanged, suggesting that the primary role of MCAK is not to move chromosomes. Furthermore, we found that disruption of MCAK leads to multiple kinetochore–microtubule attachment defects, including merotelic, syntelic, and combined merotelic-syntelic attachments. These findings reveal an essential role for Kin Is in prevention and/or correction of improper kinetochore–microtubule attachments.
TL;DR: It is suggested that cellularization can be divided into two distinct processes: furrow ingression, driven by microtubule mediated vesicle delivery, and basal closure, which is mediated by actin/myosin based constriction.
Abstract: The early Drosophila embryo undergoes two distinct membrane invagination events believed to be mechanistically related to cytokinesis: metaphase furrow formation and cellularization. Both involve actin cytoskeleton rearrangements, and both have myosin II at or near the forming furrow. Actin and myosin are thought to provide the force driving membrane invagination; however, membrane addition is also important. We have examined the role of myosin during these events in living embryos, with a fully functional myosin regulatory light-chain-GFP chimera. We find that furrow invagination during metaphase and cellularization occurs even when myosin activity has been experimentally perturbed. In contrast, the basal closure of the cellularization furrows and the first cytokinesis after cellularization are highly dependent on myosin. Strikingly, when ingression of the cellularization furrow is experimentally inhibited by colchicine treatment, basal closure still occurs at the appropriate time, suggesting that it is regulated independently of earlier cellularization events. We have also identified a previously unrecognized reservoir of particulate myosin that is recruited basally into the invaginating furrow in a microfilament-independent and microtubule-dependent manner. We suggest that cellularization can be divided into two distinct processes: furrow ingression, driven by microtubule mediated vesicle delivery, and basal closure, which is mediated by actin/myosin based constriction.
TL;DR: This work reports that VEGFR-2 is localized in endothelial caveolae, associated with caveolin-1, and that this complex is rapidly dissociated upon stimulation with VEGF, and observes that in an overexpression system in which VEG FR-1 is constitutively active, caveolin -1 overeexpression inhibits VEGfr-2 activity but allows VEGf-dependent activation, suggesting that caveolin can confer ligand dependency to a receptor system.
Abstract: The stimulation of vascular endothelial growth factor receptor-2 (VEGFR-2) by tumor-derived VEGF represents a key event in the initiation of angiogenesis In this work, we report that VEGFR-2 is localized in endothelial caveolae, associated with caveolin-1, and that this complex is rapidly dissociated upon stimulation with VEGF The kinetics of caveolin-1 dissociation correlated with those of VEGF-dependent VEGFR-2 tyrosine phosphorylation, suggesting that caveolin-1 acts as a negative regulator of VEGF R-2 activity Interestingly, we observed that in an overexpression system in which VEGFR-2 is constitutively active, caveolin-1 overexpression inhibits VEGFR-2 activity but allows VEGFR-2 to undergo VEGF-dependent activation, suggesting that caveolin-1 can confer ligand dependency to a receptor system Removal of caveolin and VEGFR-2 from caveolae by cholesterol depletion resulted in an increase in both basal and VEGF-induced phosphorylation of VEGFR-2, but led to the inhibition of VEGF-induced ERK activation and endothelial cell migration, suggesting that localization of VEGFR-2 to these domains is crucial for VEGF-mediated signaling Dissociation of the VEGFR-2/caveolin-1 complex by VEGF or cyclodextrin led to a PP2-sensitive phosphorylation of caveolin-1 on tyrosine 14, suggesting the participation of Src family kinases in this process Overall, these results suggest that caveolin-1 plays multiple roles in the VEGF-induced signaling cascade
TL;DR: It is found that different cellular compartments in mammals maintain a system for repair of oxidized methionine residues and that this function is tuned in enzyme- and stereo-specific manner.
Abstract: Methionine residues in proteins are susceptible to oxidation by reactive oxygen species, but can be repaired via reduction of the resulting methionine sulfoxides by methionine-S-sulfoxide reductase (MsrA) and methionine-R-sulfoxide reductase (MsrB). However, the identity of all methionine sulfoxide reductases involved, their cellular locations and relative contributions to the overall pathway are poorly understood. Here, we describe a methionine-R-sulfoxide reduction system in mammals, in which two MsrB homologues were previously described. We found that human and mouse genomes possess three MsrB genes and characterized their protein products, designated MsrB1, MsrB2, and MsrB3. MsrB1 (Selenoprotein R) was present in the cytosol and nucleus and exhibited the highest methionine-R-sulfoxide reductase activity because of the presence of selenocysteine (Sec) in its active site. Other mammalian MsrBs contained cysteine in place of Sec and were less catalytically efficient. MsrB2 (CBS-1) resided in mitochondria. It had high affinity for methionine-R-sulfoxide, but was inhibited by higher concentrations of the substrate. The human MsrB3 gene gave rise to two protein forms, MsrB3A and MsrB3B. These were generated by alternative splicing that introduced contrasting N-terminal and C-terminal signals, such that MsrB3A was targeted to the endoplasmic reticulum and MsrB3B to mitochondria. We found that only mitochondrial forms of mammalian MsrBs (MsrB2 and MsrB3B) could compensate for MsrA and MsrB deficiency in yeast. All mammalian MsrBs belonged to a group of zinc-containing proteins. The multiplicity of MsrBs contrasted with the presence of a single mammalian MsrA gene as well as with the occurrence of single MsrA and MsrB genes in yeast, fruit flies, and nematodes. The data suggested that different cellular compartments in mammals maintain a system for repair of oxidized methionine residues and that this function is tuned in enzyme- and stereo-specific manner.
TL;DR: A similar block in transport of MHCI and Tac was reversibly induced by a PI3-kinase inhibitor, implying that inactivation of Arf6 and acquisition of PI3P are required for convergence of endosomes arising from these two pathways.
Abstract: The trafficking of two plasma membrane (PM) proteins that lack clathrin internalization sequences, major histocompatibility complex class I (MHCI), and interleukin 2 receptor alpha subunit (Tac) was compared with that of PM proteins internalized via clathrin. MHCI and Tac were internalized into endosomes that were distinct from those containing clathrin cargo. At later times, a fraction of these internalized membranes were observed in Arf6-associated, tubular recycling endosomes whereas another fraction acquired early endosomal autoantigen 1 (EEA1) before fusion with the "classical" early endosomes containing the clathrin-dependent cargo, LDL. After convergence, cargo molecules from both pathways eventually arrived, in a Rab7-dependent manner, at late endosomes and were degraded. Expression of a constitutively active mutant of Arf6, Q67L, caused MHCI and Tac to accumulate in enlarged PIP(2)-enriched vacuoles, devoid of EEA1 and inhibited their fusion with clathrin cargo-containing endosomes and hence blocked degradation. By contrast, trafficking and degradation of clathrin-cargo was not affected. A similar block in transport of MHCI and Tac was reversibly induced by a PI3-kinase inhibitor, implying that inactivation of Arf6 and acquisition of PI3P are required for convergence of endosomes arising from these two pathways.
TL;DR: Investigation of the response of the fungal pathogen Candida albicans to temperature and osmotic and oxidative stresses under conditions that permitted >60% survival of the challenged cells noted that the classical hallmarks of the general stress response observed in Saccharomyces cerevisiae are absent from C.Albicans.
Abstract: We used transcriptional profiling to investigate the response of the fungal pathogen Candida albicans to temperature and osmotic and oxidative stresses under conditions that permitted >60% survival of the challenged cells. Each stress generated the transient induction of a specific set of genes including classic markers observed in the stress responses of other organisms. We noted that the classical hallmarks of the general stress response observed in Saccharomyces cerevisiae are absent from C. albicans; no C. albicans genes were significantly induced in a common response to the three stresses. This observation is supported by our inability to detect stress cross-protection in C. albicans. Similarly, in C. albicans there is essentially no induction of carbohydrate reserves like glycogen and trehalose in response to a mild stress, unlike the situation in S. cerevisiae. Thus C. albicans lacks the strong general stress response exhibited by S. cerevisiae.
TL;DR: A model in which Bni1 FH2 dimers nucleate and processively cap the elongating barbed end of the actin filament, and Bud6 and profilin generate a local flux of ATP-actin monomers to promote actin assembly is proposed.
Abstract: Formins have conserved roles in cell polarity and cytokinesis and directly nucleate actin filament assembly through their FH2 domain. Here, we define the active region of the yeast formin Bni1 FH2 ...
TL;DR: Sod5p was found to be necessary for the virulence of C. albicans in a mouse model of infection, and the sod5 mutant strain showed the same resistance to macrophage attack as its parental strain, suggesting that the loss of virulence in not due to an increased sensitivity to macophage attack.
Abstract: Superoxide dismutases (SOD) convert superoxide radicals into less damaging hydrogen peroxide. The opportunistic human pathogen Candida albicans is known to express CuZnSOD (SOD1) and MnSOD (SOD3) i...
TL;DR: The results raise the possibility that Mgm1p regulates fusion of the mitochondrial outer membrane through its interactions with Fzo1p and Ugo1p.
Abstract: In Saccharomyces cerevisiae, mitochondrial fusion requires at least two outer membrane proteins, Fzo1p and Ugo1p. We provide direct evidence that the dynamin-related Mgm1 protein is also required for mitochondrial fusion. Like fzo1 and ugo1 mutants, cells disrupted for the MGM1 gene contain numerous mitochondrial fragments instead of the few long, tubular organelles seen in wild-type cells. Fragmentation of mitochondria in mgm1 mutants is rescued by disrupting DNM1, a gene required for mitochondrial division. In zygotes formed by mating mgm1 mutants, mitochondria do not fuse and mix their contents. Introducing mutations in the GTPase domain of Mgm1p completely block mitochondrial fusion. Furthermore, we show that mgm1 mutants fail to fuse both their mitochondrial outer and inner membranes. Electron microscopy demonstrates that although mgm1 mutants display aberrant mitochondrial inner membrane cristae, mgm1 dnm1 double mutants restore normal inner membrane structures. However, mgm1 dnm1 mutants remain defective in mitochondrial fusion, indicating that mitochondrial fusion requires Mgm1p regardless of the morphology of mitochondria. Finally, we find that Mgm1p, Fzo1p, and Ugo1p physically interact in the mitochondrial outer membrane. Our results raise the possibility that Mgm1p regulates fusion of the mitochondrial outer membrane through its interactions with Fzo1p and Ugo1p.
TL;DR: Results suggest that GSL analogs are selectively internalized via a caveolar-related mechanism in most cell types, whereas CtxB may undergo "pathway switching" when cav-1 levels are low.
Abstract: We studied the endocytosis of fluorescent glycosphingolipid (GSL) analogs in various cell types using pathway-specific inhibitors and colocalization studies with endocytic markers and DsRed caveolin-1 (cav-1). Based on inhibitor studies, all GSLs tested were internalized predominantly (>80%) by a clathrin-independent, caveolar-related mechanism, regardless of cell type. In addition, fluorescent lactosylceramide (LacCer) colocalized with DsRed-cav-1 in vesicular structures upon endocytosis in rat fibroblasts. The internalization mechanism for GSLs was unaffected by varying the carbohydrate headgroup or sphingosine backbone chain length; however, a fluorescent phosphatidylcholine analog was not internalized via caveolae, suggesting that the GSL ceramide core may be important for caveolar uptake. Internalization of fluorescent LacCer was reduced 80-90% in cell types with low cav-1, but was dramatically stimulated by cav-1 overexpression. However, even in cells with low levels of cav-1, residual LacCer internalization was clathrin independent. In contrast, cholera toxin B subunit (CtxB), which binds endogenous GM1, was internalized via clathrin-independent endocytosis in cells with high cav-1 expression, whereas significant clathrin-dependent uptake occurred in cells with low cav-1. Fluorescent GM1, normally internalized by clathrin-independent endocytosis in HeLa cells with low cav-1, was induced to partially internalize via the clathrin pathway in the presence of CtxB. These results suggest that GSL analogs are selectively internalized via a caveolar-related mechanism in most cell types, whereas CtxB may undergo "pathway switching" when cav-1 levels are low.