MT1-MMP down-regulates the glucose 6-phosphate transporter expression in marrow stromal cells: a molecular link between pro-MMP-2 activation, chemotaxis, and cell survival

Bone marrow-derived stromal cells (BMSC) are avidly recruited by experimental vascularizing tumors, which implies that they must respond to tumor-derived growth factor cues. In fact, BMSC chemotaxis and cell survival are regulated, in part, by the membrane type-1 matrix metalloproteinase (MT1-MMP), an MMP also involved in pro-MMP-2 activation and in degradation of the extracellular matrix (ECM). Given that impaired chemotaxis was recently observed in bone marrow cells isolated from a glucose 6-phosphate transporter-deficient (G6PT-/-) mouse model, we sought to investigate the potential MT1-MMP/G6PT signaling axis in BMSC. We show that MT1-MMP-mediated activation of pro-MMP-2 by concanavalin A (ConA) correlated with an increase in the sub-G1 cell cycle phase as well as with cell necrosis, indicative of a decrease in BMSC survival. BMSC isolated from Egr-1-/- mouse or MT1-MMP gene silencing in BMSC with small interfering RNA (siMT1-MMP) antagonized both the ConA-mediated activation of pro-MMP-2 and the induction of cell necrosis. Overexpression of recombinant full-length MT1-MMP triggered necrosis and this was signaled through the cytoplasmic domain of MT1-MMP. ConA inhibited both the gene and protein expression of G6PT, while overexpression of recombinant G6PT inhibited MT1-MMP-mediated pro-MMP-2 activation but could not rescue BMSC from ConA-induced cell necrosis. Cell chemotaxis in response to the tumorigenic growth factor sphingosine 1-phosphate was significantly abrogated in siMT1-MMP BMSC and in chlorogenic acid-treated BMSC. Altogether, we provide evidence for an MT1-MMP/G6PT signaling axis that regulates BMSC survival, ECM degradation, and mobilization. This may lead to optimized clinical applications that use BMSC as a platform for the systemic delivery of therapeutic or anti-cancer recombinant proteins in vivo.     

Microtubule organization requires cell cycle-dependent nucleation at dispersed cytoplasmic sites: polar and perinuclear microtubule organizing centers in the plant pathogen Ustilago maydis

Growth of most eukaryotic cells requires directed transport along microtubules (MTs) that are nucleated at nuclear-associated microtubule organizing centers (MTOCs), such as the centrosome and the fungal spindle pole body (SPB). Herein, we show that the pathogenic fungus Ustilago maydis uses different MT nucleation sites to rearrange MTs during the cell cycle. In vivo observation of green fluorescent protein-MTs and MT plus-ends, tagged by a fluorescent EB1 homologue, provided evidence for antipolar MT orientation and dispersed cytoplasmic MT nucleating centers in unbudded cells. On budding gamma-tubulin containing MTOCs formed at the bud neck, and MTs reorganized with >85% of all minus-ends being focused toward the growth region. Experimentally induced lateral budding resulted in MTs that curved out of the bud, again supporting the notion that polar growth requires polar MT nucleation. Depletion or overexpression of Tub2, the gamma-tubulin from U. maydis, affected MT number in interphase cells. The SPB was inactive in G2 phase but continuously recruited gamma-tubulin until it started to nucleate mitotic MTs. Taken together, our data suggest that MT reorganization in U. maydis depends on cell cycle-specific nucleation at dispersed cytoplasmic sites, at a polar MTOC and the SPB.     

Actin microfilaments regulate vacuolar structures and dynamics: dual observation of actin microfilaments and vacuolar membrane in living tobacco BY-2 Cells

Actin microfilaments (MFs) participate in many fundamental processes in plant growth and development. Here, we report the co-localization of the actin MF and vacuolar membrane (VM), as visualized by vital VM staining with FM4-64 in living tobacco BY-2 cells stably expressing green fluorescent protein (GFP)-fimbrin (BY-GF11). The MFs were intensively localized on the VM surface and at the periphery of the cytoplasmic strands rather than at their center. The co-localization of MFs and VMs was confirmed by the observation made using transient expression of red fluorescent protein (RFP)-fimbrin in tobacco BY-2 cells stably expressing GFP-AtVam3p (BY-GV7) and BY-2 cells stably expressing gamma-tonoplast intrinsic protein (gamma-TIP)-GFP fusion protein (BY-GG). Time-lapse imaging revealed dynamic movement of MF structures which was parallel to that of cytoplasmic strands. Disruption of MF structures disorganized cytoplasmic strand structures and produced small spherical vacuoles in the VM-accumulating region. Three-dimensional reconstructions of the vacuolar structures revealed a disconnection of these small spherical vacuoles from the large vacuoles. Real-time observations and quantitative image analyses demonstrated rapid movements of MFs and VMs near the cell cortex, which were inhibited by the general myosin ATPase inhibitor, 2,3-butanedion monoxime (BDM). Moreover, both bistheonellide A (BA) and BDM treatment inhibited the reorganization of the cytoplasmic strands and the migration of daughter cell nuclei at early G1 phase, suggesting a requirement for the acto-myosin system for vacuolar morphogenesis during cell cycle progression. These results suggest that MFs support the vacuolar structures and that the acto-myosin system plays an essential role in vacuolar morphogenesis.     

db—cAMP对转化细胞钙调素基因表达与细胞骨架的影响

We have demonstrated that the distribution of microtubules (MT), microfilaments (MF) and fibronectin (FN) were diminished, while the gene expression of the calmodulin and c-fos enhanced in the transformed C3 H10 T1/2 cells. After treatment with 1 mM db-cAMP for 1 hr. and 2 hrs., there was an early and rapidly reduced in gene expression of calmodulin and c-fos respectively. After db-cAMP treatment for 4-5 days, the number of Capping cells of ConA binding decreased significantly and the cell surface microvilli decreased also. The growth of treated cells was inhibited markedly. By using 4F1 cDNA probe, which is preferentially expressed in G1 phase, we have found that the db-cAMP treated cells were accumulated at G1 phase. Of particular interest is the fact that the distribution of microtubules, microfilaments and fibronectin were recovered after treatment with 1 mM db-cAMP for 6 days. It is suggested that the inhibition of proliferation, alteration of phenotype and recovery of cytoskeleton in transformed cells after treatment with db-cAMP are related to the inhibition of gene expression of calmodulin.     

Calmodulin and wound healing in the coenocytic green alga Ernodesmis verticillata (Kützing) Børgesen: Ultrastructure of the cortical cytoskeleton and immunogold labeling

Ultrastructural changes in the cortical cytoskeleton during wound-induced cytoplasmic contraction were examined in the coenocytic green alga Ernodesmis verticillata. Both calmodulin (CaM) and actin were localized in intact and contracting cells by immunogold labeling. Within 5 min after wounding, compact microfilament (MF) bundles were observed which increase in diameter as cytoplasmic contraction proceeds. Calmodulin labeling is associated with amorphous material studding the MF bundles, whereas actin labeling occurs along the individual MFs. No MF bundles were ever observed during contraction that were not also labeled with anti-CaM antibodies. In cells treated with the CaM antagonist W-7 (N-[6-aminohexyl]-5-chloro-1-naphtha-lenesulfonamide), MF bundles do not form, and the formation of loosely arranged MFs (similar to nascent bundles in untreated cells) is greatly retarded. We propose that CaM binds indirectly to actin by activating an actin-binding regulatory protein which functions in early stages of the transduction sequence leading to functional MF bundles. Additionally, ultrastructural evidence is presented for a plasma-membrane skeleton or undercoating in this alga.Abbreviations CaM calmodulin - MF(s) microfilament(s) - MT(s) microtubule(s) - W-7 N-[6-aminohexyl]-5-chloro-1-naphthalenesulfonamide We are especially grateful to Dr. J. A. West (University of California, Berkeley, USA) for the original algal isolates and to Dr. L. Van Eldik (Vanderbilt University School of Medicine, Vanderbilt, Tenn., USA) for the generous gift of CaM antibodies. Portions of this work were supported by National Science Foundation grant DCB 84-02345 and U. S. Department of Agriculture grant 87-CRCR-1-2545 to J.W.L.     

Acrylamide sensitization of the heat response of the cytoskeleton and cytotoxicity in attaching and well-spread synchronous Chinese hamster ovary cells

The vimentin intermediate filament (VIMF) network is more sensitive to heat-induced disruption than either the microtubule (MT) or microfilament (MF) cytoskeletal (CSK) arrays in G1 Chinese hamster ovary (CHO) cells (Coss and Wachsberger: Radiation Research, 1987). We therefore investigated the effect of the VIMF disruptive agent, acrylamide (Eckert: European Journal of Cell Biology 37:169-174, 1985), on the heat response of synchronous CHO cells. Cells, either in the process of spreading (G1 or S phase) or in the well-spread state (S phase), were exposed to a nontoxic concentration of 5 mM acrylamide, heated, and processed for immunofluorescence microscopy 30 min or 20 hr following the heat shock. Recovery from CSK disruption was related to cell survival. CHO cells, either in the process of spreading or in the well-spread state, were sensitized to heat-induced CSK disruption and cytotoxicity by acrylamide. Recovery from CSK disruption correlated with surviving fractions of cells treated in the G1 phase but not with surviving fractions of cells treated in the S phase and was independent of the degree of cell spreading. This correlation suggests that damage to CSK structures may contribute to the death of cells treated in G1 but not necessarily to the death of cells treated in S phase. The degree of acrylamide sensitization of heat-induced CSK disruption was greater for cells exposed to acrylamide prior to spreading than for well-spread cells. Furthermore, normal spreading of cells was prevented when they were plated into medium containing acrylamide, suggesting that acrylamide interferes with the initial stages of attachment and spreading of these cells. These observations are interpreted in relation to the possible role that VIMFs, together with cortical MFs, may play in mediating cell surface focal contacts in the initial stages of cell attachment and spreading.     

db-cAMP对转化细胞钙调素基因表达与细胞骨架的影响

转化的C_3H_(10)T_(1/2)细胞表现增殖速度加快、表面微绒毛增加,细胞变圆,叠层生长,ConA受体呈帽状分布,微管、微丝、纤粘蛋白分布明显减少。与增殖有关的癌基因c-fos表达增强,同时发现与细胞增殖、转化和细胞骨架调节有关的钙调素(CaM)基因表达加强。用1mmo/Ldb-cAMP处理转化细胞,观察到CaM基因和原癌基因c-fos的表达分别在处理后1小时和2小时急剧下降。处理后4—5天,转化细胞表型趋正常化,大部分细胞恢复单层生长。细胞表面微绒毛和泡状物减少,ConA受体帽状分布消失,恢复分散分布在细胞膜上的特点。细胞生长明显被抑制,用优先在G_1期表达的4F_1 cDNA为探针进行分子杂交,证实了经db-cAMP处理后的细胞被阻抑在G_1期。经db-cAMP处理6天的转化细胞中微管、微丝、纤粘蛋白基本恢复正常分布。实验表明CaM的表达增强与转化细胞表型变化和细胞骨架组装减弱密切相关,db-cAMP作用后CaM表达下降是抑制转化细胞增殖并使细胞表型和细胞骨架分布趋于正常的关键事件之一。     

Isolation and partial characterization of human cell mutants differing in sensitivity to killing and mutation by methylnitrosourea and N-methyl-N'-nitro-N-nitrosoguanidine

Isogenic variants resistant to alkylating agents have been isolated from the human lymphoblast cell line TK6. The cell lines may be divided into four classes on the basis of resistance to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The sensitive TK6 parental line shows a 37% survival after 45-min exposure to 0.04 microM MNNG; the three classes of more resistant mutants show 37% survival after 45-min exposure to 2 microM (MF lines), 6 microM (MT lines), and greater than or equal to 10 microM (MX line) MNNG. A representative MF line, MF1, is resistant to both killing and mutation by MNNG or N-methyl-N-nitrosourea. An MT clone, MT1, is highly resistant to killing but hypermutable by MNNG. The MT1 line, like the parental TK6, does not remove O6-methylguanine adducts from the DNA. Our data are consistent with the hypothesis that the MT1 line possesses a nonexcision pathway of defense against killing by alkylating agents. Rather than preventing alkylation of DNA or removing alkylated adducts, the MT1 cells appear to be tolerant of the adducts that are not removed from the DNA.     

Activation of progelatinase A (MMP-2) by neutrophil elastase, cathepsin G, and proteinase-3: a role for inflammatory cells in tumor invasion and angiogenesis

Gelatinase A (MMP-2), a matrix metalloproteinase (MMP) involved in tumor invasion and angiogenesis, is secreted as an inactive zymogen (proMMP-2) and activated by proteolytic cleavage. Here we report that polymorphonuclear neutrophil (PMN)-derived elastase, cathepsin G, and proteinase-3 activate proMMP-2 through a mechanism that requires membrane-type 1 matrix metalloproteinase (MT1-MMP) expression. Immunoprecipitation of human PMN-conditioned medium with a mixture of antibodies to elastase, cathepsin G, and proteinase-3 abolished proMMP-2 activation, whereas individual antibodies were ineffective. Incubation of HT1080 cells with either purified PMN elastase or cathepsin G or proteinase-3 resulted in dose-and time-dependent proMMP-2 activation. Addition of PMN-conditioned medium to MT1-MMP expressing cells resulted in increased proMMP-2 activation and in vitro invasion of extracellular matrix (ECM), but had no effect with cells that express no MT1-MMP. MMP-2 activation by PMN-conditioned medium or purified elastase was blocked by the elastase inhibitor alpha(1)-antitrypsin but not by Batimastat, an MMP inhibitor, showing that elastase activation of MMP-2 is not mediated by MMP activities. The PMN-conditioned medium-induced increase in cell invasion was blocked by Batimastat as well as by alpha(1)-antitrypsin, showing that PMN serine proteinases trigger a proteinase cascade that entails proMMP-2 activation: this gelatinase is the downstream effector of the proinvasive activity of PMN proteinases. These findings indicate a novel role for PMN-mediated inflammation in a variety of tissue remodeling processes including tumor invasion and angiogenesis.     

Ca2+ signaling in mouse mesenteric small arteries: myogenic tone and adrenergic vasoconstriction

Arteries that have developed myogenic tone (MT) are in a markedly different physiological state compared with those that have not, with higher cytosolic [Ca(2+)] and altered activity of several signal transduction pathways. In this study, we sought to determine whether alpha(1)-adrenoceptor-induced Ca(2+) signaling is different in pressurized arteries that have spontaneously developed MT (the presumptive physiological state) compared with those that have not (a common experimental state). At 32 degrees C and intraluminal pressure of 70 mmHg, cytoplasmic [Ca(2+)] was steady in most smooth muscle cells (SMCs). In a minority of cells (34%), however, at least one propagating Ca(2+) wave occurred. alpha(1)-Adrenoceptor activation (phenylephrine, PE; 0.1-10.0 microM) caused strong vasoconstriction and markedly increased the frequency of Ca(2+) waves (in virtually all cells). However, when cytosolic [Ca(2+)] was elevated experimentally in these arteries ([K(+)] 20 mM), PE failed to elicit Ca(2+) waves, although it did elevate [Ca(2+)] (F/F(0)) further and caused further vasoconstriction. During development of MT, the cytosolic [Ca(2+)] (F/F(0)) in individual SMCs increased, Ca(2+) waves disappeared (from SMCs that had them), and small Ca(2+) ripples (frequency approximately 0.05 Hz) appeared in approximately 13% of cells. PE elicited only spatially uniform increases in [Ca(2+)] and a smaller change in diameter (than in the absence of MT). Nevertheless, when cytosolic [Ca(2+)] and MT were decreased by nifedipine (1 microM), PE did elicit Ca(2+) waves. Thus alpha(1)-adrenoceptor-mediated Ca(2+) signaling is markedly different in arteries with and without MT, perhaps due to the elevated [Ca(2+)], and may have a different molecular basis. alpha(1)-Adrenoceptor-induced vasoconstriction may be supported either by Ca(2+) waves or by steady elevation of cytoplasmic [Ca(2+)], depending on the amount of MT.     

Mutation analysis of membrane type-1 matrix metalloproteinase (MT1-MMP). The role of the cytoplasmic tail Cys(574), the active site Glu(240), and furin cleavage motifs in oligomerization, processing, and self-proteolysis of MT1-MMP expressed in breast carcinoma cells

Membrane type-1 matrix metalloproteinase (MT1-MMP) is a key enzyme in the activation pathway of matrix prometalloproteinase-2 (pro-MMP-2). Both activation and autocatalytic maturation of pro-MMP-2 in trans suggest that MT1-MMP should exist as oligomers on the cell surface. To better understand the functions of MT1-MMP, we designed mutants with substitutions in the active site (E240A), the cytoplasmic tail (C574A), and the RRXR furin cleavage motifs (R89A, ARAA, and R89A/ARAA) of the enzyme. The mutants were expressed in MCF7 breast carcinoma cells that are deficient in both MMP-2 and MT1-MMP. Our results supported the existence of MT1-MMP oligomers and demonstrated that a disulfide bridge involving the Cys(574) of the enzyme's cytoplasmic tail covalently links MT1-MMP monomers on the MCF7 cell surface. The presence of MT1-MMP oligomers also was shown for the enzyme naturally expressed in HT1080 fibrosarcoma cells. The single (R89A and ARAA) and double (R89A/ARAA) furin cleavage site mutants of MT1-MMP were processed in MCF7 cells into the mature proteinase capable of activating pro-MMP-2 and stimulating cell locomotion. This suggested that furin cleavage is not a prerequisite for the conversion of pro-MT1-MMP into the functionally active enzyme. A hydroxamate class inhibitor (GM6001, or Ilomastat) blocked activation of MT1-MMP in MCF7 cells but not in HT1080 cells. This implied that a matrixin-like proteinase sensitive to hydroxamates could be involved in a furin-independent, alternative pathway of MT1-MMP activation in breast carcinoma cells. The expression of the wild type MT1-MMP enhanced cell invasion and migration, indicating a direct involvement of this enzyme in cell locomotion. In contrast, both the C574A and E240A mutations render MT1-MMP inefficient in stimulating cell migration and invasion. In addition, the C574A mutation negatively affected cell adhesion, thereby indicating critical interactions involving the cytosolic part of MT1-MMP and the intracellular milieu.     

Transport of newly synthesized ribosome precedes initiation of S phase in root meristem cells in Helianthus annuus L

According to earlier results, cold treatment blocks most of cells in G1 phase; after 3 h of postincubation at 20 degrees C these cells initiate S phase. Simultaneous cytophotometric (DNA stained with Methyl Green) and autoradiographic (3H-uridine, 3H-arginine) analyses of cold pretreated cells have shown that transport of 3H-RNA into cytoplasm is faster in G2 than in G1 cells: radioactivity of cytoplasm is faster in G2 cells becomes higher than that of nucleoli as early as between 2 h and 2.5 h postincubation, while in G1 cells--not until between 2.5. and 3.0 h. Simultaneous cytophotometric measurements of DNA (Feulgen) and protein (Naphthol Yellow S) contents demonstrate the considerable increase in cytoplasmic protein contents during the 2nd h of postincubation at 20 degrees C; therefore it precedes the export of more than 50% of 3H-RNA synthesized after cold treatment. The results of these experiments indicate two separate events in G1 cells: increase in cytoplasmic protein amounts (up to 2nd h of postincubation) and enhanced transport of newly-synthesized ribosome (2.5 h of postincubation), the later event immediately precede the entry of cells into S phase.     

TIMP-2 Interaction with MT1-MMP Activates the AKT Pathway and Protects Tumor Cells from Apoptosis

Membrane-type 1 matrix metalloproteinase (MT1-MMP), a transmembrane proteinase with an extracellular catalytic domain and a short cytoplasmic tail, degrades a variety of extracellular matrix (ECM) components. In addition, MT1-MMP activates intracellular signaling through proteolysis-dependent and independent mechanisms. We have previously shown that binding of tissue inhibitor of metalloproteinases-2 (TIMP-2) to MT1-MMP controls cell proliferation and migration, as well as tumor growth in vivo by activating the Ras—extracellular signal regulated kinase-1 and -2 (ERK1/2) pathway through a mechanism that requires the cytoplasmic but not the proteolytic domain of MT1-MMP. Here we show that in MT1-MMP expressing cells TIMP-2 also induces rapid and sustained activation of AKT in a dose- and time-dependent manner and by a mechanism independent of the proteolytic activity of MT1-MMP. Fibroblast growth factor receptor-1 mediates TIMP-2 induction of ERK1/2 but not of AKT activation; however, Ras activation is necessary to transduce the TIMP-2-activated signal to both the ERK1/2 and AKT pathways. ERK1/2 and AKT activation by TIMP-2 binding to MT1-MMP protects tumor cells from apoptosis induced by serum starvation. Conversely, TIMP-2 upregulates apoptosis induced by three-dimensional type I collagen in epithelial cancer cells. Thus, TIMP-2 interaction with MT1-MMP provides tumor cells with either pro- or anti-apoptotic signaling depending on the extracellular environment and apoptotic stimulus.     

Prognostic significance of nuclear and cytoplasmic expression of metallothioneins as related to proliferative activity in squamous cell carcinomas of oral cavity

Metallothioneins (MT) are low molecular weight proteins with high metal and cystein contents. This study was designed to test the hypothesis that cytoplasmic and nuclear MT expression are of prognostic importance in patients with squamous cell carcinomas of the oral cavity, treated by surgery with subsequent radiotherapy. The second aim of the study was to test the potential correlation between the nuclear and cytoplasmic MT expressions as compared to expression of proliferation markers and other clinicopathological variables. Material and Methods: The studies were performed on tumor samples from 50 patients with diagnosis of squamous cell carcinoma of the oral cavity floor or of oral part of the tongue. All the patients were subjected to radical surgery, accompanied by removal of lymph nodes and post-operative radiotherapy. Results: No significant correlation could be detected between percentage and intensity of MT expression on one hand and proportions of cells with Mcm-2 (minichromosome maintenance protein 2), Ki-67 expressions, nor the grade of malignancy (G) on the other. A significantly shorter survival was detected among patients with tumors of MT expression rated 9 or 12 according to the Remmele scale and among patients with a high percentage (> 50%) of nuclear MT staining. In mulivariate analyses, only OTT (Overall Treatment Time), lymph node involvement and high expression of Mcm-2 were found to be independent risk factors for decreased patient's survival. Conclusion: This is relevant evidence that MT overexpression could be related to worse prognosis in patients with oral cancer. We have found no relationship between MT expression and proliferative activity.     

The structural organization of the cytoskeleton of the mammary secretory cells

A study was made of the arrangement and specialties of distribution in the mammary secretory cells of albino mice of the following cytoskeleton ultrastructures: microtubules (MT), organization centres (MTOC), microfilaments (MF) and intermediate filaments (IF). During the last period of pregnancy and at different stages of lactation, for the alveolar epithelium the presence of a single material centrioles (CN) was shown in the region of the apical surface near dense intercellular contact. During pregnancy and especially at the beginning of lactation (1-2 days) the relatively large density of the MT was observed in both the cytoplasm of the secretory cells and the region near the CN. On the most hard days of lactation (10 day) the lowering of the number of all the observed structures was held in the majority of the cells. The MT was not observed near the CN. It appeared that the elimination of the MTOC activity during the most hard time of lactation is related to the decrease in the numbers of MT, MF and IF. The pattern of cytoskeleton reorganization, associated with the development of the functional activity of the observed cells, enables us to suggest a cooperative contribution of its base components in the formation of the secretory surface and in the carrying out of exocytosis.     

Ultrastructural Observations on the Distribution of the Intra-and Intercellular Cytoskeleton of the Endosperm Cells in Triticum aestivum

By means of Triton X-l00 extraction and DGD (diethylene glycol distearate) embedment-free section method the distribution pattern and characteristics of intra- and intercellular cytoskeleton of endosperm cells of Triticum aestivum L. were studied with electron microscopy. Threedimensional architecture of the cytoskeleton could be recognized as a meshwork mainly composed of microtubules (MT) and microfilaments (MF). Attention was stressed on the interface of the adjoining cytoskeletal frameworks where an attractive phenomenon observed was that the MF extruding from the surface of the cytoskeleton often traversed the whole wall boundary and connected the neighbouring frameworks into an entity. In the endosperm tissue two types of transcellular MF distribution could be distinguished, the MF in bundles traversing the enlarged intercellular channels and the MF individually penetrating the wall boundary; that seemed to coordinate with the co-presence of normal and modified plasmodesmata in the same wall. The above observations demonstrated the intercellular cytoskeletal continuity within the symplast and confirmed that the MF was the main constituent of the traversing cytoplasmic strands, the possibility of MF being organized as a structural element of the normal plasmodesmata was also discussed.     

小麦胚乳细胞内,细胞间胞质骨架分布格局的超微结构观察

以小麦（ＴｒｉｔｉｃｕｍａｅｓｔｉｖｕｍＬ．）幼嫩胚乳为材料,经ＴｒｉｔｏｎＸ１００抽提、ＤＧＤ（ｄｉｅｔｈｙｌｅｎｅｇｌｙｃｏｌｄｉｓｔｅａｒａｔｅ）渗透、包埋,制备去包埋剂超薄切片,对细胞内、细胞间胞质骨架的分布格局与特征进行了电镜观察。由所获图像可见,胞质骨架呈主要由微管、微丝组成的三维网络结构;特别值得注意的是,有不少５～７ｎｍ的微丝在多处从网络表层向胞壁界面方向突出,并时而可见其横贯分界壁连接相邻骨架网络而将相邻细胞骨架联成一体。胚乳组织中微丝的跨胞分布以两种形式存在,直径达１００～２００ｎｍ微丝束的跨越和单个微丝的分散贯穿,看来这与该组织中开放态胞间通道与正常胞间连丝同时并存相吻合。初步讨论了微丝参与正常胞间连丝结构的可能性。     

The difference in murine CDC2 kinase activity between cytoplasmic and nuclear fractions during the cell cycle

The mouse analog of yeast CDC2+ kinase was detected in the cytoplasmic and nuclear fractions of cultured mouse FM3A cells. Its activity in the nuclear fraction increased in the G2/M phase became seven times higher than that in the G1/S phase, while the activity in the cytoplasmic fraction remained was almost constant from the G1/S to G1 phases. The activity in the cytoplasmic fraction was similar to that in the nuclear fraction in the G2/M phase. The amount of the enzyme remained almost constant during the cell cycle in both the nuclear and cytoplasmic fractions. These findings suggest that the cytoplasmic enzyme might play an independent role in the cell cycle.