Establishment of an unfed strain of Paramecium bursaria and analysis of associated bacterial communities controlling its proliferation

The ciliate Paramecium bursaria harbors several hundred symbiotic algae in its cell and is widely used as an experimental model for studying symbiosis between eukaryotic cells. Currently, various types of bacteria and eukaryotic microorganisms are used as food for culturing P. bursaria; thus, the cultivation conditions are not uniform among researchers. To unify cultivation conditions, we established cloned, unfed strains that can be cultured using only sterile medium without exogenous food. The proliferation of these unfed strains was suppressed in the presence of antibiotics, suggesting that bacteria are required for the proliferation of the unfed strains. Indeed, several kinds of bacteria, such as Burkholderiales, Rhizobiales, Rhodospirillales, and Sphingomonadales, which are able to fix atmospheric nitrogen and/or degrade chemical pollutants, were detected in the unfed strains. The genetic background of the individually cloned, unfed strains were the same, but the proliferation curves of the individual P. bursaria strains were very diverse. Therefore, we selected multiple actively and poorly proliferating individual strains and compared the bacterial composition among the individual strains using 16S rDNA sequencing. The results showed that the bacterial composition among actively proliferating P. bursaria strains was highly homologous but different to poorly proliferating strains. Using unfed strains, the cultivation conditions applied in different laboratories can be unified, and symbiosis research on P. bursaria will make great progress.

ZIP kinase phosphorylated and activated by Rho kinase/ROCK contributes to cytokinesis in mammalian cultured cells

Cytokinesis of animal cells requires contraction of a contractile ring, composed of actin filaments and myosin II filaments. Phosphorylation of myosin II regulatory light chain (MRLC) promotes contraction of the actomyosin ring by activating myosin II motor activity. Both Rho-associated coiled-coil kinase (Rho kinase/ROCK) and Zipper-interacting protein kinase (ZIP kinase/ZIPK) have been reported to phosphorylate MRLC at the contractile ring. However, it remains unclear whether these kinases function independently of each other. Here, we clarified that ROCK colocalizes and forms a complex with ZIPK at telophase. As ROCK is reported to phosphorylate and activate ZIPK in vitro, we hypothesized that ZIPK phosphorylated by ROCK contributes to control cytokinesis. To address this, we expressed EGFP-ZIPK wild type (WT), a non-phosphorylatable mutant (T265A) or a phosphorylation-mimicking mutant (T265D) in HeLa cells and treated these cells with a ROCK inhibitor. Decrease in phosphorylated MRLC and a delay of furrow ingression by the ROCK inhibitor were rescued by the expression of EGFP-ZIPK-T265D, but not EGFP-ZIPK-WT or -T265A. This suggests that ROCK regulates MRLC phosphorylation followed by furrow ingression, through ZIPK phosphorylation.

Effects of the fungicide ortho-phenylphenol (OPP) on the early development of sea urchin eggs

In this study, we assessed the impact of imidazole fungicide ortho-phenylphenol (OPP) on the early development of a marine invertebrate, the sea urchin, a marine bioindicator. Fungicides are widely used and have been reported to accumulate not only in farm soil but also in freshwater and seawater sediments. Therefore, it is essential to clarify the effects of OPP on marine environments. Toxicity was estimated as the inhibition ratio of the 120 min-embryo and/or the 24 h-embryo development. The addition of OPP to embryos of the two sea urchin species, Scaphechinus mirabilis (S. mirabilis) and Strongylocentrotus nudus (S. nudus), at 0.1 mM or higher, resulted in acute toxicity (cell death). The IC₅₀ value of the 120 min-embryos or the 24 h-embryos for S. mirabilis and S. nudus with OPP was around 0.06 mM, indicating that fertilized eggs and embryos of the sea urchin are more sensitive to OPP than higher vertebrates. In addition, in the presence of OPP (0.005-0.05 mM), the proportion (%) of the gastrula keeping the fertilization membrane increased, suggesting that OPP (0.005-0.05 mM) inhibited the hatching process, possibly by affecting the hatching enzyme activity.

Aurora B but not rho/MLCK signaling is required for localization of diphosphorylated myosin II regulatory light chain to the midzone in cytokinesis

Non-muscle myosin II is stimulated by monophosphorylation of its regulatory light chain (MRLC) at Ser19 (1P-MRLC). MRLC diphosphorylation at Thr18/Ser19 (2P-MRLC) further enhances the ATPase activity of myosin II. Phosphorylated MRLCs localize to the contractile ring and regulate cytokinesis as subunits of activated myosin II. Recently, we reported that 2P-MRLC, but not 1P-MRLC, localizes to the midzone independently of myosin II heavy chain during cytokinesis in cultured mammalian cells. However, the mechanism underlying the distinct localization of 1P- and 2P-MRLC during cytokinesis is unknown. Here, we showed that depletion of the Rho signaling proteins MKLP1, MgcRacGAP, or ECT2 inhibited the localization of 1P-MRLC to the contractile ring but not the localization of 2P-MRLC to the midzone. In contrast, depleting or inhibiting a midzone-localizing kinase, Aurora B, perturbed the localization of 2P-MRLC to the midzone but not the localization of 1P-MRLC to the contractile ring. We did not observe any change in the localization of phosphorylated MRLC in myosin light-chain kinase (MLCK)-inhibited cells. Furrow regression was observed in Aurora B- and 2P-MRLC-inhibited cells but not in 1P-MRLC-perturbed dividing cells. Furthermore, Aurora B bound to 2P-MRLC in vitro and in vivo. These results suggest that Aurora B, but not Rho/MLCK signaling, is essential for the localization of 2P-MRLC to the midzone in dividing HeLa cells.

Diphosphorylated but not monophosphorylated myosin II regulatory light chain localizes to the midzone without its heavy chain during cytokinesis

Myosin II is activated by the monophosphorylation of its regulatory light chain (MRLC) at Ser19 (1P-MRLC). Its ATPase activity is further enhanced by MRLC diphosphorylation at Thr18/Ser19 (2P-MRLC). As these phosphorylated MRLCs are colocalized with their heavy chains at the contractile ring in dividing cells, we believe that the phosphorylated MRLC acts as a subunit of the activated myosin II during cytokinesis. However, the distinct role(s) of 1P- and 2P-MRLC during cytokinesis has not been elucidated. In this study, a monoclonal antibody (4F12) specific for 2P-MRLC was raised and used to examine the roles of 2P-MRLC in cultured mammalian cells. Our confocal microscopic observations using 4F12 revealed that 2P-MRLC localized to the contractile ring, and, unexpectedly, to the midzone also. Interestingly, 2P-MRLC did not colocalize with 1P-MRLC, myosin II heavy chain, and F-actin at the midzone. These results suggest that 2P-MRLC has a role different from that of 1P-MRLC at the midzone, and is not a subunit of myosin II.

Proline-rich domain in dynamin-2 has a low microtubule-binding activity: how is this activity controlled during mitosis in HeLa cells?

The large GTPase dynamin is strongly accumulated in the constricted area including midzonal microtubules of dividing cells. The proline-rich domain (PRD) of dynamin has been considered as a microtubule-binding domain. However, it remains unclear how PRD controls dynamin-microtubule interaction in mitotic cells. Here, we found that the microtubule-binding activity of PRD is low in dynamin-2. One of the mitosis-specific kinase activities to PRD in HeLa cells was identified as cyclin B-Cdc2 kinase. The kinase phosphorylated PRD at Ser(764) and/or Thr(766) and reduced the microtubule-binding activity of PRD. These results suggest that phosphorylation of PRD by cyclin B-Cdc2 kinase plays an important role to control dynamin-2-microtubule interaction in mitotic HeLa cells.

Life cycle analysis of unicellular algae

Unicellular green alga is a very convenient object for flow cytometric characterization. Flow cytometry has been proposed as a quick and reliable tool for studying life cycle and growth of unicellular algae. Cell size of vegetating algae can be monitored in association with their DNA and endogenous chlorophyll content. Cells of interest (e.g., group of cells of a certain stage of the life cycle) in an asynchronously proliferating cell population can be sorted out for further microscopical or molecular biology studies. This methodological approach can be helpful for researchers who are interested in algal proliferation.

Direct evidence for roles of phosphorylated regulatory light chain of myosin II in furrow ingression during cytokinesis in HeLa cells

Phosphorylation of myosin II is thought to play an important role in cytokinesis. Although it is well known that phosphorylated regulatory light chain of myosin II (P-MRLC) localizes along the contractile ring, it is not clear how P-MRLC controls myosin II and F-actin in furrow ingression during cytokinesis. To elucidate roles of P-MRLC in furrow ingression, HeLa cells transfected with EGFP-tagged wild-type or each MRLC mutant were observed using a live-imaging microscope. Time-lapse observation revealed that a delay of furrow ingression was observed in the nonphosphorylatable form of MRLC (AA-MRLC)-expressing cell but not in the wild-type or phospho-mimic MRLC-expressing cell. Among each form of MRLC-expressing cell, the total amount of P-MRLC including phospho-mimic MRLCs was smallest in the cell expressing AA-MRLC. However, the amount of F-actin and myosin II at the contractile ring in the AA-MRLC-expressing cell was the same as that in the normal cell. Interestingly, delay of furrow ingression by a Rho-kinase inhibitor, Y27632, was rescued by phospho-mimic MRLCs. These results suggest that the P-MRLC is essential for the progress of furrow ingression but not the retainment of F-actin and myosin II in the contractile ring of dividing HeLa cells.

Characterization of the interface between normal and transformed epithelial cells

In most cancers, transformation begins in a single cell in an epithelial cell sheet. However, it is not known what happens at the interface between non-transformed (normal) and transformed cells once the initial transformation has occurred. Using Madin-Darby canine kidney (MDCK) epithelial cells that express constitutively active, oncogenic Ras (Ras(V12)) in a tetracycline-inducible system, we investigated the cellular processes arising at the interface between normal and transformed cells. We show that two independent phenomena occur in a non-cell-autonomous manner: when surrounded by normal cells, Ras(V12) cells are either apically extruded from the monolayer, or form dynamic basal protrusions and invade the basal matrix. Neither apical extrusion nor basal protrusion formation is observed when Ras(V12) cells are surrounded by other Ras(V12) cells. We show that Cdc42 and ROCK (also known as Rho kinase) have vital roles in these processes. We also demonstrate that E-cadherin knockdown in normal cells surrounding Ras(V12) cells reduces the frequency of apical extrusion, while promoting basal protrusion formation and invasion. These results indicate that Ras(V12)-transformed cells are able to recognize differences between normal and transformed cells, and consequently leave epithelial sheets either apically or basally, in a cell-context-dependent manner.

New function of the proline rich domain in dynamin-2 to negatively regulate its interaction with microtubules in mammalian cells

Microtubule reorganization is necessary for many cellular functions such as cell migration, cell polarity and cell division. Dynamin was originally identified as a microtubule-binding protein. Previous limited digestion experiment revealed that C-terminal 100-amino acids proline rich domain (PRD) of dynamin is responsible for microtubule binding in vitro. However, as obvious localization of dynamin along microtubules is only observed at the spindle midzone during mitosis but not in interphase cells, it remains unclear how dynamin interacts with microtubules in vivo. Here, we report that GFP-dynamin-2-(1-786), a truncated mutant lacking a C-terminal portion of the PRD, localized along microtubules in interphase HeLa cells. GFP-dynamin-2-wild type (WT) and GFP-dynamin-2-(1-745), a construct that was further truncated to remove the entire PRD, localized in discrete punctate structures but not along microtubules. These data suggest that the N-terminal (residues 746-786) but not the entire PRD is necessary for the interaction of dynamin-2 with microtubules in the cell and that the C-terminus of PRD (787-870) negatively regulate this interaction. Microtubules in cells expressing GFP-dynamin-2-(1-786) were stabilized against exposure to cold. These results provide a first evidence for a regulated interaction of dynamin-2 with microtubules in cultured mammalian cells.

mDia2 induces the actin scaffold for the contractile ring and stabilizes its position during cytokinesis in NIH 3T3 cells

mDia proteins are mammalian homologues of Drosophila diaphanous and belong to the formin family proteins that catalyze actin nucleation and polymerization. Although formin family proteins of nonmammalian species such as Drosophila diaphanous are essential in cytokinesis, whether and how mDia proteins function in cytokinesis remain unknown. Here we depleted each of the three mDia isoforms in NIH 3T3 cells by RNA interference and examined this issue. Depletion of mDia2 selectively increased the number of binucleate cells, which was corrected by coexpression of RNAi-resistant full-length mDia2. mDia2 accumulates in the cleavage furrow during anaphase to telophase, and concentrates in the midbody at the end of cytokinesis. Depletion of mDia2 induced contraction at aberrant sites of dividing cells, where contractile ring components such as RhoA, myosin, anillin, and phosphorylated ERM accumulated. Treatment with blebbistatin suppressed abnormal contraction, corrected localization of the above components, and revealed that the amount of F-actin at the equatorial region during anaphase/telophase was significantly decreased with mDia2 RNAi. These results demonstrate that mDia2 is essential in mammalian cell cytokinesis and that mDia2-induced F-actin forms a scaffold for the contractile ring and maintains its position in the middle of a dividing cell.

Arrest of cytoplasmic streaming induces algal proliferation in green paramecia

A green ciliate Paramecium bursaria, bearing several hundreds of endosymbiotic algae, demonstrates rotational microtubule-based cytoplasmic streaming, in which cytoplasmic granules and endosymbiotic algae flow in a constant direction. However, its physiological significance is still unknown. We investigated physiological roles of cytoplasmic streaming in P. bursaria through host cell cycle using video-microscopy. Here, we found that cytoplasmic streaming was arrested in dividing green paramecia and the endosymbiotic algae proliferated only during the arrest of cytoplasmic streaming. Interestingly, arrest of cytoplasmic streaming with pressure or a microtubule drug also induced proliferation of endosymbiotic algae independently of host cell cycle. Thus, cytoplasmic streaming may control the algal proliferation in P. bursaria. Furthermore, confocal microscopic observation revealed that a division septum was formed in the constricted area of a dividing paramecium, producing arrest of cytoplasmic streaming. This is a first report to suggest that cytoplasmic streaming controls proliferation of eukaryotic cells.

Aurora B kinase activity is required to prevent polar cortical ingression during cytokinesis

Cytokinesis requires proper regulation of microtubule dynamics. It has been suggested that dynamic astral microtubules prevent cortical ingression. However, it remains unknown how astral microtubules maintain their dynamic state. Here we show that aurora B kinase, a component of the chromosome passenger complex, is required to sustain the dynamic state of astral microtubules during cytokinesis. Treatment of HeLa cells with Hesperadin, an inhibitor of aurora B kinase, caused abnormal cortical protrusion, leading to cortical ingression in the protruding region and cytokinesis failure. Actin filaments, myosin II, and RhoA failed to localize at the equator but instead distributed along the lateral and/or polar cortex in cells treated with Hesperadin. Time-lapse analyses of microtubule dynamics showed that, in cells treated with Hesperadin, abnormally bundled astral microtubules targeted the protruding region. Mitotic kinesin-like protein 1 (MKLP1), a component of the spindle midzone required for bundling of microtubules, was not detected along bundled astral microtubules in cells treated with Hesperadin, suggesting that factors other than MKLP1 may be involved in this process. Our results suggest that aurora B kinase activity is required for proper regulation of microtubule dynamics to ensure that cytokinesis occurs precisely at the cell equator.

ROCK1 phosphorylates and activates zipper-interacting protein kinase

Zipper-interacting protein kinase (ZIPK) regulates Ca(2+)-independent phosphorylation of both smooth muscle (to regulate contraction) and non-muscle myosin (to regulate non-apoptotic cell death) through either phosphorylation and inhibition of myosin phosphatase, the myosin phosphatase inhibitor CPI17, or direct phosphorylation of myosin light chain. ZIPK is regulated by multisite phosphorylation. Phosphorylation at least three sites Thr-180, Thr-225, and Thr-265 has been shown to be essential for full activity, whereas phosphorylation at Thr-299 regulates its intracellular localization. Herein we utilized an unbiased proteomics screen of smooth muscle extracts with synthetic peptides derived from the sequence of the regulatory phosphorylation sites of the enzyme to identify the protein kinases that might regulate ZIPK activity in vivo. Discrete kinase activities toward Thr-265 and Thr-299 were defined and identified by mass spectrometry as Rho kinase 1 (ROCK1). In vitro, ROCK1 showed a high degree of substrate specificity toward native ZIPK, both stoichiometrically phosphorylating the enzyme at Thr-265 and Thr-299 as well as bringing about activation. In HeLa cells, coexpression of ZIPK with ROCK1 altered the ROCK-induced phenotype of focused stress fiber pattern to a Rho-like phenotype of parallel stress fiber pattern. This effect was also dependent upon phosphorylation at Thr-265. Our findings provide a new regulatory pathway in smooth muscle and non-muscle cells whereby ROCK1 phosphorylates and regulates ZIP kinase.

Regulation of myosin II dynamics by phosphorylation and dephosphorylation of its light chain in epithelial cells

Nonmuscle myosin II, an actin-based motor protein, plays an essential role in actin cytoskeleton organization and cellular motility. Although phosphorylation of its regulatory light chain (MRLC) is known to be involved in myosin II filament assembly and motor activity in vitro, it remains unclear exactly how MRLC phosphorylation regulates myosin II dynamics in vivo. We established clones of Madin Darby canine kidney II epithelial cells expressing MRLC-enhanced green fluorescent protein or its mutants. Time-lapse imaging revealed that both phosphorylation and dephosphorylation are required for proper dynamics of myosin II. Inhibitors affecting myosin phosphorylation and MRLC mutants indicated that monophosphorylation of MRLC is required and sufficient for maintenance of stress fibers. Diphosphorylated MRLC stabilized myosin II filaments and was distributed locally in regions of stress fibers where contraction occurs, suggesting that diphosphorylation is involved in the spatial regulation of myosin II assembly and contraction. We further found that myosin phosphatase or Zipper-interacting protein kinase localizes to stress fibers depending on the activity of myosin II ATPase.

Traction forces of fibroblasts are regulated by the Rho-dependent kinase but not by the myosin light chain kinase

Adhesive cells show complex mechanical interactions with the substrate, however the exact mechanism of such interactions, termed traction forces, is still unclear. To address this question we have measured traction forces of fibroblasts treated with agents that affect the myosin II-dependent contractile mechanism. Using the potent myosin II inhibitor blebbistatin, we demonstrate that traction forces are strongly dependent on a functional myosin II heavy chain. Since myosin II is regulated by both the myosin light chain kinase (MLCK) and, directly or indirectly, the Rho-associated kinase (ROCK), we examined the effects of inhibitors against these kinases. Interestingly, inhibition of the myosin light chain kinase had no detectable effect, while inhibition of the Rho-dependent kinase caused strong inhibition of traction forces. Our results indicate that ROCK and MLCK play non-redundant roles in regulating myosin II functions, and that a subset of myosin II, regulated by the Rho small GTPase, may be responsible for the regulation of traction forces in migrating fibroblasts.

Myosin II activity is not essential for recruitment of myosin II to the furrow in dividing HeLa cells

To elucidate whether phosphorylation of myosin II regulatory light chain (MRLC) is essential for myosin II recruitment to the furrow during cytokinesis, HeLa cells transfected with three types of GFP-tagged recombinant MRLCs, wild-type MRLC, non-phosphorylated form of MRLC, and phosphorylated form of MRLC, were examined. Living cell-imaging showed that both phosphorylated and non-phosphorylated form of MRLCs were recruited to the equator at the same time after anaphase onset, suggesting that phosphorylation of MRLC is not responsible for recruitment of myosin II to the equator. Moreover, the treatment with an inhibitor of myosin II activity, blebbistatin, induced no effect on recruitment of those three recombinant MRLCs. During cytokinesis, phosphorylated but not non-phosphorylated form of MRLC was retained in the equator. These results suggest that phosphorylation of MRLC is essential for retainment of myosin II in the furrow but not for initial recruitment of myosin II to the furrow in dividing HeLa cells.

Adenovirus E4orf4 hijacks rho GTPase-dependent actin dynamics to kill cells: a role for endosome-associated actin assembly

The adenovirus early region 4 ORF4 protein (E4orf4) triggers a novel death program that bypasses classical apoptotic pathways in human cancer cells. Deregulation of the cell cytoskeleton is a hallmark of E4orf4 killing that relies on Src family kinases and E4orf4 phosphorylation. However, the cytoskeletal targets of E4orf4 and their role in the death process are unknown. Here, we show that E4orf4 translocates to cytoplasmic sites and triggers the assembly of a peculiar juxtanuclear actin-myosin network that drives polarized blebbing and nuclear shrinkage. We found that E4orf4 activates the myosin II motor and triggers de novo actin polymerization in the perinuclear region, promoting endosomes recruitment to the sites of actin assembly. E4orf4-induced actin dynamics requires interaction with Src family kinases and involves a spatial regulation of the Rho GTPases pathways Cdc42/N-Wasp, RhoA/Rho kinase, and Rac1, which make distinct contributions. Remarkably, activation of the Rho GTPases is required for induction of apoptotic-like cell death. Furthermore, inhibition of actin dynamics per se dramatically impairs E4orf4 killing. This work provides strong support for a causal role for endosome-associated actin dynamics in E4orf4 killing and in the regulation of cancer cell fate.

Actomyosin tension is required for correct recruitment of adherens junction components and zonula occludens formation

The adherens junction (AJ) densely associated with actin filaments is a major cell-cell adhesion structure. To understand the importance of actin filament association in AJ formation, we first analyzed punctate AJs in NRK fibroblasts where one actin cable binds to one AJ structure unit. The accumulation of AJ components such as the cadherin/catenin complex and vinculin, as well as the formation of AJ-associated actin cables depended on Rho activity. Inhibitors for the Rho target, ROCK, which regulates myosin II activity, and for myosin II ATPase prevented the accumulation of AJ components, indicating that myosin II activity is more directly involved than Rho activity. Depletion of myosin II by RNAi showed similar results. The inhibition of myosin II activity in polarized epithelial MTD-1A cells affected the accumulation of vinculin to circumferential AJ (zonula adherens). Furthermore, correct zonula occludens (tight junction) formation along the apicobasal axis that requires cadherin activity was also impaired. Although MDCK cells which are often used as typical epithelial cells do not have a typical zonula adherens, punctate AJs formed dependently on myosin II activity by inducing wound closure in a MDCK cell sheet. These findings suggest that tension generated by actomyosin is essential for correct AJ assembly.

Association of ghrelin receptor gene polymorphism with bulimia nervosa in a Japanese population

Eating disorders (EDs) have a highly heterogeneous etiology and multiple genetic factors might contribute to their pathogenesis. Ghrelin, a novel growth hormone-releasing peptide, enhances appetite and increases food intake, and human ghrelin plasma levels are inversely correlated with body mass index. In the present study, we examined the 171T/C polymorphism of the ghrelin receptor (growth hormone secretagogue receptor, GHSR) gene in patients diagnosed with EDs, because the subjects having ghrelin gene polymorphism (Leu72Met) was not detected in a Japanese population, previously. In addition, beta3 adrenergic receptor gene polymorphism (Try64Arg) and cholecystokinin (CCK)-A receptor (R) gene polymorphism (-81A/G, -128G/T), which are both associated with obesity, were investigated. The subjects consisted of 228 Japanese patients with EDs [96 anorexia nervosa (AN), 116 bulimia nervosa (BN) and 16 not otherwise specified (NOS)]. The age- and gender-matched control group consisted of 284 unrelated Japanese subjects. The frequency of the CC type of the GHSR gene was significantly higher in BN subjects than in control subjects (chi(2) = 4.47, p = 0.035, odds ratio = 2.05, Bonferroni correction: p = 0.070), while the frequency in AN subjects was not different from that in controls. The distribution of neither beta3 adrenergic receptor gene nor CCK-AR polymorphism differed between EDs and control subjects. Therefore, the CC type of GHSR gene polymorphism (171T/C) is a risk factor for BN, but not for AN.

Dissecting the role of Rho-mediated signaling in contractile ring formation

In anaphase, microtubules provide a specification signal for positioning of the contractile ring. However, the nature of the signal remains unknown. The small GTPase Rho is a potent regulator of cytokinesis, but the involvement of Rho in contractile ring formation is disputed. Here, we show that Rho serves as a microtubule-dependent signal that specifies the position of the contractile ring. We found that Rho translocates to the equatorial region before furrow ingression. The Rho-specific inhibitor C3 exoenzyme and small interfering RNA to the Rho GDP/GTP exchange factor ECT2 prevent this translocation and disrupt contractile ring formation, indicating that active Rho is required for contractile ring formation. ECT2 forms a complex with the GTPase-activating protein MgcRacGAP and the kinesinlike protein MKLP1 at the central spindle, and the localization of ECT2 at the central spindle depends on MgcRacGAP and MKLP1. In addition, we show that the bundled microtubules direct Rho-mediated signaling molecules to the furrowing site and regulate furrow formation. Our study provides strong evidence for the requirement of Rho-mediated signaling in contractile ring formation.

Green paramecia as an evolutionary winner of oxidative symbiosis: a hypothesis and supportive data

A single cell of the green paramecia (Paramecium bursaria) harbors several hundreds of endo-symbiotic Chlorella-like algae in its cytoplasm. Removal of algae from the host organism and re-association of ex-symbiotic host paramecia with ex-symbiotic algae can be experimentally demonstrated in the laboratory. However, the mechanism precisely governing the alga-protozoan association is not fully understood, and the origin of symbiosis in the evolutionary view has not been given. Here, we propose the possible biochemical models (models 1 and 2) explaining the co-evolution between Paramecium species and algal symbionts by pointing out that algal photosynthesis in the host paramecia plays a dual role providing the energy source and the risk of oxidative damage to the host. Model 1 lays stress on the correlation between the (re)greening ability of the paramecia and the tolerance to oxidative stress whereas model 2 emphasizes the cause of evolutionary selection leading to the emergence of Paramecium species tolerant against reactive oxygen species.

Myosin II regulatory light chain is required for trafficking of bile salt export protein to the apical membrane in Madin-Darby canine kidney cells

BSEP, MDR1, and MDR2 ATP binding cassette transporters are targeted to the apical (canalicular) membrane of hepatocytes, where they mediate ATP-dependent secretion of bile acids, drugs, and phospholipids, respectively. Sorting to the apical membrane is essential for transporter function; however, little is known regarding cellular proteins that bind ATP binding cassette proteins and regulate their trafficking. A yeast two-hybrid screen of a rat liver cDNA library identified the myosin II regulatory light chain, MLC2, as a binding partner for BSEP, MDR1, and MDR2. The interactions were confirmed by glutathione S-transferase pulldown and co-immunoprecipitation assays. BSEP and MLC2 were overrepresented in a rat liver subcellular fraction enriched in canalicular membrane vesicles, and MLC2 colocalized with BSEP in the apical domain of hepatocytes and polarized WifB, HepG2, and Madin-Darby canine kidney cells. Expression of a dominant negative, non-phosphorylatable MLC2 mutant reduced steady state BSEP levels in the apical domain of polarized Madin-Darby canine kidney cells. Pulse-chase studies revealed that Blebbistatin, a specific myosin II inhibitor, severely impaired delivery of newly synthesized BSEP to the apical surface. These findings indicate that myosin II is required for BSEP trafficking to the apical membrane.

Is myosin light-chain phosphorylation a regulatory signal for the osmotic activation of the Na+-K+-2Cl- cotransporter?

Myosin light-chain (MLC) kinase (MLCK)-dependent increase in MLC phosphorylation has been proposed to be a key mediator of the hyperosmotic activation of the Na+-K+-2Cl- cotransporter (NKCC). To address this hypothesis and to assess whether MLC phosphorylation plays a signaling or permissive role in NKCC regulation, we used pharmacological and genetic means to manipulate MLCK, MLC phosphorylation, or myosin ATPase activity and followed the impact of these alterations on the hypertonic stimulation of NKCC in porcine kidney tubular LLC-PK1 epithelial cells. We found that the MLCK inhibitor ML-7 suppressed NKCC activity independently of MLC phosphorylation. Notably, ML-7 reduced both basal and hypertonically stimulated NKCC activity without influencing MLC phosphorylation under these conditions, and it inhibited NKCC activation by Cl- depletion, a treatment that did not increase MLC phosphorylation. Furthermore, prevention of the osmotically induced increase in MLC phosphorylation by viral induction of cells with a nonphosphorylatable, dominant negative MLC mutant (AA-MLC) did not affect the hypertonic activation of NKCC. Conversely, a constitutively active MLC mutant (DD-MLC) that mimics the diphosphorylated form neither stimulated isotonic nor potentiated hypertonic NKCC activity. Furthermore, a depolarization-induced increase in endogenous MLC phosphorylation failed to activate NKCC. However, complete abolition of basal MLC phosphorylation by K252a or the inhibition of myosin ATPase by blebbistatin significantly reduced the osmotic stimulation of NKCC without suppressing its basal or Cl- depletion-triggered activity. These results indicate that an increase in MLC phosphorylation is neither a sufficient nor a necessary signal to stimulate NKCC in tubular cells. However, basal myosin activity plays a permissive role in the optimal osmotic responsiveness of NKCC.

A new approach for the assessment of acrylamide toxicity using a green paramecium

Exposure to acrylamide induces neurotoxic effects in humans. In addition, it induces genotoxic, reproductive and carcinogenic effects in laboratory animals. However, no convenient bioassay system for assessing acrylamide toxicity to animal and plant cells has been proposed to date. The present study aims to evaluate acrylamide toxicity to a green paramecium, Paramecium bursaria, bearing many endosymbiotic algae, because some chemicals are highly toxic to paramecia or microalgae, and some protozoa are already used for evaluation of environmental contaminations. Results showed that high acrylamide concentrations (> or = 1500 mg/l) have a lethal effect on P. bursaria. Although low acrylamide concentrations (< or = 150 mg/l) induced less change on the paramecium growth, the number of endosymbiotically growing algal cells drastically decreased. The acrylamide concentration required to induce a 50% decrease in the cell number (IC(50)) was determined to be 7.8 mg/l for endosymbiotic algae, indicating that the algal sensitivity to acrylamide was 7 and 15 times higher than that of Syrian hamster embryo (SHE) cells and the host cells, respectively. Here, we propose the use of P. bursaria being a convenient and sensitive bioindicator as a new approach for the assessment of acrylamide toxicity.

Protective role of HSP27 against UVC-induced cell death in human cells

It is an intriguing problem whether heat shock proteins (HSPs) play a protective role in UVC-induced cell death in human cells, and the problem has not been solved. To search for the HSPs involved in UVC resistance, gene expression profiles using cDNA array were compared between UVC-sensitive human RSa cells and their UVC-resistant variant AP(r)-1 cells. The expression levels of heat shock protein 27 (HSP27) were lower in RSa cells than in AP(r)-1 cells. RSa cells transfected with sense HSP27 cDNA showed slightly lower sensitivity to UVC-induced cell death than the control cells transfected with a vector alone and much lower sensitivity than RSa cells transfected with the antisense HSP27 cDNA. Furthermore, the removal capacities of the two major types of UVC-damaged DNA (thymine dimers and (6-4)photoproducts) in the cells with the up-regulation of HSP27 were moderately elevated compared with those in the control cells, while those in the cells with down-regulation were remarkably suppressed. These results suggest that HSP27 is involved in the UVC-resistance of human cells, at least those tested, possibly via functioning in nucleotide excision repair.

Malformation of immature starfish oocytes by theonellapeptolide Ie, a Tridecapeptide lactone from a marine sponge Petrosia species, through disturbance of cortical F-actin distribution

Theonellapeptolide Ie (Tp), an oligopeptide lactone isolated from a marine sponge, Petrosia sp., was shown to induce an unprecedented morphological change in the immature oocytes of the starfish Asterina pectinifera. The cortical F-actin was disturbed and assembled to form dots and rings, as evidenced by staining with rhodamine-conjugated phalloidin. The oocyte eventually became malformed. When Tp was added to an immature oocyte which had been pretreated with cytochalasin B or D, inhibitors of actin polymerization, no malformation was observed. When Tp was added to an oocyte which had been induced to mature by 1-methyladenine (1-MeAde), a maturation-inducing substance in starfishes, no morphological changes were observed in the maturing oocytes which reached the first meiotic prometaphase 40 min after the start of 1-MeAde treatment. This is the first description of a chemical that induces aberrant redistribution of F-actin-based cytoskeleton in an animal oocyte which is arrested at the first meiotic prophase.

Flow cytometric studies of the host-regulated cell cycle in algae symbiotic with green paramecium

Paramecium bursaria (green paramecium) possesses endosymbiotically growing chlorella-like green algae. An aposymbiotic cell line of P. bursaria (MBw-1) was prepared from the green MB-1 strain with the herbicide paraquat. The SA-2 clone of symbiotic algae was employed to reinfect MBw-1 cells and thus a regreened cell line (MBr-1) was obtained. The regreened paramecia were used to study the impact of the host's growth status on the life cycle of the symbiotic algae. Firstly, the relationship between the timing of algal propagation and the host cell division was investigated by counting the algal cells in single host cells during and after the host cell division and also in the stationary phase. Secondly, the changes in the endogenous chlorophyll level, DNA content, and cell size in the symbiotic algae were monitored by flow cytometry and fluorescence microscopy. The number of algae was shown to be doubled prior to or during the host cell division and the algal population in the two daughter cells is maintained at constant level until the host cell cycle reenters the cytokinesis, suggesting that algal propagation and cell cycle are dependent on the host's cell cycle. During the host's stationary growth, unicellular algal vegetatives with low chlorophyll content were dominant. In contrast, complexes of algal cells called sporangia (containing 1-4 autospores) were present in the logarithmically growing hosts, indicating that algal cell division leading to the formation of sporangia with multiple autospores is active in the dividing paramecia.

Gamma-tubulin in basal land plants: characterization, localization, and implication in the evolution of acentriolar microtubule organizing centers

Although seed plants have gamma-tubulin, a ubiquitous component of centrosomes associated with microtubule nucleation in algal and animal cells, they do not have discrete microtubule organizing centers (MTOCs) comparable to animal centrosomes, and the organization of microtubule arrays in plants has remained enigmatic. Spindle development in basal land plants has revealed a surprising variety of MTOCs that may represent milestones in the evolution of the typical diffuse acentrosomal plant spindle. We have isolated and characterized the gamma-tubulin gene from a liverwort, one of the extant basal land plants. Sequence similarity to the gamma-tubulin gene of higher plants suggests that the gamma-tubulin gene is highly conserved in land plants. The G9 antibody to fission yeast gamma-tubulin recognized a single band of 55 kD in immunoblots from bryophytes. Immunohistochemistry with the G9 antibody clearly documented the association of gamma-tubulin with various MTOC sites in basal land plants (e.g., discrete centrosomes with and without centrioles and the plastid surface in monoplastidic meiosis of bryophytes). Changes in the distribution of gamma-tubulin occur in a cell cycle-specific manner during monoplastidic meiosis in the liverwort Dumortiera hirsuta. gamma-Tubulin changes its localization from the plastid surface in prophase I to the spindle, from the spindle to phragmoplasts and the nuclear envelope in telophase I, and back to the plastid surfaces in prophase II. In vitro experiments show that gamma-tubulin is detectable on the surface of isolated plastids and nuclei of D. hirsuta, and microtubules can be repolymerized from the isolated plastids. gamma-Tubulin localization patterns on plastid and nuclear surfaces are not affected by the destruction of microtubules by oryzalin. We conclude that gamma-tubulin is a highly conserved protein associated with microtubule nucleation in basal land plants and that it has a cell cycle-dependent distribution essential for the orderly succession of microtubule arrays.

Uncoordinated regulation of stress fibers and focal adhesions by DAP kinase

Death-associated protein kinase (DAP kinase) is a proapoptotic, calcium/calmodulin-dependent serine/threonine kinase. Here, we report that DAP kinase phosphorylates the regulatory light chain of myosin II (MLC) both in vitro and in vivo, and that this phosphorylation occurs preferentially at residue Ser19. In quiescent fibroblasts, DAP kinase stabilizes stress fibers through phosphorylation of MLC, but it is dispensable for the formation of peripheral microfilament bundles. This cytoskeletal effect of DAP kinase occurs before the onset of apoptosis and does not require an intact death domain. In addition, DAP kinase is required for serum-induced stress-fiber formation, which is associated with the upregulation of its catalytic activity. Despite being both sufficient and necessary for the assembly or maintenance of stress fibers, DAP kinase is incapable of stimulating the formation of focal adhesions in quiescent cells. Moreover, it promotes the disassembly of focal adhesions but not stress fibers in cells receiving serum factors. Together, our results identify a novel and unique function of DAP kinase in the uncoupling of stress fibers and focal adhesions. Such uncoupling would lead to a perturbation of the balance between contractile and adhesion forces and subsequent cell detachment, which might contribute to its pro-apoptotic activity.

Identification and characterization of novel components of a Ca2+/calmodulin-dependent protein kinase cascade in HeLa cells

In this report, we cloned a novel calmodulin-kinase (CaM-KIdelta) from HeLa cells and characterized its activation mechanism. CaM-KIdelta exhibits Ca(2+)/CaM-dependent activity that is enhanced (approximately 30-fold) in vitro by phosphorylation of its Thr180 by CaM-K kinase (CaM-KK)alpha, consistent with detection of CaM-KIdelta-activating activity in HeLa cells. We also identified a novel CaM-KKbeta isoform (CaM-KKbeta-3) in HeLa cells whose activity was highly Ca(2+)/CaM-independent. Transiently expressed CaM-KIdelta exhibited enhanced protein kinase activity in HeLa cells without ionomycin stimulation. This sustained activation of CaM-KIdelta was completely abolished by Thr180Ala mutation and inhibited by CaM-KK inhibitor, STO-609, indicating a functional CaM-KK/CaM-KIdelta cascade in HeLa cells.

Determination of human myosin III as a motor protein having a protein kinase activity

The class III myosin is the most divergent member of the myosin superfamily, having a domain with homology to a protein kinase. However, the function of class III myosin at a molecular level is not known at all, and it has been questioned whether it is actually an actin-based motor molecule. Here, we showed that human myosin III has an ATPase activity that is significantly activated by actin (20-fold) with Kactin of 112 microm and Vmax of 0.34 s-1, indicating the mechanoenzymatic activity of myosin III. Furthermore, we found that human myosin III has actin translocating activity (0.11 +/- 0.05 microm/s) using an in vitro actin gliding assay, and it moves toward the plus end of actin filaments. Myosin III containing calmodulin as the light chain subunit showed a protein kinase activity and underwent autophosphorylation. The autophosphorylation was the intramolecular process, and the sites were at the C-terminal end of the motor domain. Autophosphorylation significantly activated the kinase activity, although it did not affect the ATPase activity. The present study is the first report that clearly demonstrates that the class III myosin is an actin-based motor protein having a protein kinase activity.

Glycosylation of the alpha and beta tubulin by sialyloligosaccharides

To examine whether alpha and beta tubulin are glycoproteins, we used a pyridylamino labeling method and a monoclonal antibody, SG3-1, raised against NeuAcalpha2-3Gal structure. Alpha and beta tubulin from both pig brain and HeLa cells were positive for the SG3-1 antibody by immunoblot assay. Sialidase treatment reduced the reactivity of the SG3-1 antibody to alpha and beta tubulin molecules. N-linked oligosaccharide analysis also showed that alpha and beta tubulin are glycosylated. Moreover, immunofluorescence analysis showed that the filamentous structure recognized by the SG3-1 antibody was overlapped with microtubules, especially in the vicinity of the nucleus. These results indicate that alpha and beta tubulin are glycosylated with sialyloligosaccharides.

Phosphorylation of myosin II regulatory light chain is necessary for migration of HeLa cells but not for localization of myosin II at the leading edge

To investigate the role of phosphorylated myosin II regulatory light chain (MRLC) in living cell migration, these mutant MRLCs were engineered and introduced into HeLa cells. The mutant MRLCs include an unphosphorylatable form, in which both Thr-18 and Ser-19 were substituted with Ala (AA-MRLC), and pseudophosphorylated forms, in which Thr-18 and Ser-19 were replaced with Ala and Asp, respectively (AD-MRLC), and both Thr-18 and Ser-19 were replaced with Asp (DD-MRLC). Mutant MRLC-expressing cell monolayers were mechanically stimulated by scratching, and the cells were forced to migrate in a given direction. In this wound-healing assay, the AA-MRLC-expressing cells migrated much more slowly than the wild-type MRLC-expressing cells. In the case of DD-MRLC- and AD-MRLC-expressing cells, no significant differences compared with wild-type MRLC-expressing cells were observed in their migration speed. Indirect immunofluorescence staining showed that the accumulation of endogenous diphosphorylated MRLC at the leading edge was not observed in AA-MRLC-expressing cells, although AA-MRLC was incorporated into myosin heavy chain and localized at the leading edge. In conclusion, we propose that the phosphorylation of MRLC is required to generate the driving force in the migration of the cells but not necessary for localization of myosin II at the leading edge.

The carboxy-terminal pleckstrin homology domain of ROCK interacts with filamin-A

The small GTPase Rho and its effector ROCK/Rho-kinase regulate actin cytoskeletal reorganization through phosphorylation of the regulatory light chain of myosin II. We previously reported that ROCK co-purified with the actin-binding protein filamin-A from HeLa cells. Here, we show that the pleckstrin homology (PH) domain of ROCK, but not the kinase or coiled-coil domain, interacts with filamin-A. We also determined that the PH domain of ROCK binds to the carboxy-terminal region of filamin-A containing the last 24th repeat. ROCK co-localized with filamin-A at the protrusive cell membranes of HeLa cells.

Myosin motors and not actin comets are mediators of the actin-based Golgi-to-endoplasmic reticulum protein transport

We have previously reported that actin filaments are involved in protein transport from the Golgi complex to the endoplasmic reticulum. Herein, we examined whether myosin motors or actin comets mediate this transport. To address this issue we have used, on one hand, a combination of specific inhibitors such as 2,3-butanedione monoxime (BDM) and 1-[5-isoquinoline sulfonyl]-2-methyl piperazine (ML7), which inhibit myosin and the phosphorylation of myosin II by the myosin light chain kinase, respectively; and a mutant of the nonmuscle myosin II regulatory light chain, which cannot be phosphorylated (MRLC2(AA)). On the other hand, actin comet tails were induced by the overexpression of phosphatidylinositol phosphate 5-kinase. Cells treated with BDM/ML7 or those that express the MRLC2(AA) mutant revealed a significant reduction in the brefeldin A (BFA)-induced fusion of Golgi enzymes with the endoplasmic reticulum (ER). This delay was not caused by an alteration in the formation of the BFA-induced tubules from the Golgi complex. In addition, the Shiga toxin fragment B transport from the Golgi complex to the ER was also altered. This impairment in the retrograde protein transport was not due to depletion of intracellular calcium stores or to the activation of Rho kinase. Neither the reassembly of the Golgi complex after BFA removal nor VSV-G transport from ER to the Golgi was altered in cells treated with BDM/ML7 or expressing MRLC2(AA). Finally, transport carriers containing Shiga toxin did not move into the cytosol at the tips of comet tails of polymerizing actin. Collectively, the results indicate that 1) myosin motors move to transport carriers from the Golgi complex to the ER along actin filaments; 2) nonmuscle myosin II mediates in this process; and 3) actin comets are not involved in retrograde transport.

Spatial localization of mono-and diphosphorylated myosin II regulatory light chain at the leading edge of motile HeLa cells

Nonmuscle myosin II activity is regulated by phosphorylation of the myosin II regulatory light chain (MRLC) at Ser19 or at both Thr18 and Ser19, and the phosphorylation of MRLC promotes the contractility and stability of actomyosin. To analyze the states of MRLC phosphorylation at the leading edge in the motile HeLa cells, we have examined the subcellular distribution of monophosphorylated or diphosphorylated form of MRLC using a confocal microscope. The cross-sectional imaging revealed that monophosphorylated MRLC distributed throughout the cortical region and the leading edge, but its fluorescent signal was much stronger at the leading edge. This distribution pattern of monophosphorylated MRLC was almost identical to those of myosin II and F-actin. On the other hand, diphosphorylated MRLC is localized at the base of leading edge, spatially very close to the substrate, and colocalized with F-actin in part at the base of filopodia. Diphosphorylated MRLC was hardly detectable at the tip of filopodia and the cell cortical region, where monophosphorylated MRLC was clearly detected. These localization patterns suggest that myosin II is activated at the leading edge, especially at the base but not the tip of filopodia in motile cells. Next, we analyzed the cells expressing GFP-tagged recombinant MRLCs. Expression of GFP-tagged diphosphorylatable and monophosphorylatable MRLCs led to a significant increase in the filopodial number, compared with the cells expressing nonphosphorylatable MRLC. This result indicated that expression of phosphorylatable MRLC enhances the formation of filopodia at the wound edge.