bluuebunny/crossref_metadata_2025_split huggingface dataset:
The permeability properties of septate junctions in Malpighian tubules of Rhodnius
Authors: Le H. , Skaer B. , Maddrell S. H. P. , Harrison J. B. 
Published: April 2021

ABSTRACT
This paper describes the structural characteristics and permeability properties of the smooth septate junctions between the upper Malpighian tubule cells of a blood-sucking bug, Rhodnius prolixus. The permeability of the paracellular route was tested only for solutes that could be demonstrated not to cross the epithelium via the cellular route. The intercellular clefts were readily permeated by sucrose, inulin and polyethylene glycol (PEG), showing a higher permeability to molecules of smaller radius (PEG versus sucrose). Negatively charged molecules permeated the clefts more readily than positively charged ones. The effects of pH, urea and luminal flow rate on permeability were studied. The results are discussed in relation to the physiological tightness of the Malpighian tubules to certain solutes and to its function as an excretory epithelium.
Strategies and mechanisms for the control of transcriptional initiation of mammalian protein-coding genes
Authors: Melton David W. 
Published: April 2021

ABSTRACT
An ability to exercise precise control over a large number of genes, with widely differing patterns of expression, is a requirement for all organisms and this ability reaches its most sophisticated level in mammalian systems. Although there are numerous examples of control being exerted at every stage along the expression pathway, the key control point in most instances appears to be at the first stage of the process - the transcriptional initiation event itself. The purpose of this article is to provide a brief overview of current ideas on the control of transcriptional initiation in mammalian cells, both in the context of recent developments in mammalian systems and by analogy with more advanced studies in prokaryotes and yeast. (See Ptashne, 1986; Struhl, 1987, for reviews on transcriptional initiation in bacteria and yeast, respectively.)
Structure of the generative cell wall complex after freeze substitution in pollen tubes of Nicotiana and Impatiens
Authors: Cresti M. , Lancelle S. A. , Hepler P. K. 
Published: April 2021

ABSTRACT
The mature generative cell in pollen grains and pollen tubes is surrounded by a wall complex that includes two plasma membranes, one facing the generative cell cytoplasm and one facing the vegetative cell cytoplasm, and usually some intervening wall material. After conventional chemical fixation, the two plasma membranes are very uneven and often appear to be joined, giving the impression that numerous plasmodesmata connect the vegetative and generative cells. These areas alternate with swollen, distorted areas, which give the wall complex the appearance of being composed of a chain of vesicles. Utilizing rapid freeze fixation and freeze substitution, we have re-examined the ultrastructure of the generative cell wall complex from pollen tubes grown in vitro, and the differences are striking. The two plasma membranes are very smooth and closely appressed to a layer of wall material. Occasionally the wall complex contains swollen areas, or varicosities, and these may contain pockets of lightly stained material, but again the surrounding plasma membranes are tightly appressed to these areas. Plasmodesmata are not seen, but this does not eliminate the possibility that they may exist at an earlier stage of development.
Effects of calcium antagonists on cyclic AMP phosphodiesterase induction in Dictyostelium discoideum
Authors: Coukell M. B. , Cameron A. M. 
Published: April 2021

ABSTRACT
Previous studies have suggested that cyclic GMP and/or Ca2+ might function as second messengers in the induction by exogenous cyclic AMP of the cyclic AMP phosphodiesterase (PD) in Dictyostelium discoideum. To assess further the role of Ca2+ in PD induction we examined the effect on this process of a number of putative Ca2+-channel blockers. At relatively low micromolar concentrations, TMB-8, nicardipine, nifedipine, diltiazem and verapamil all altered cell shape and inhibited PD induction in a similar dose-dependent fashion. Concentrations of these drugs that abolished PD induction had no effect on cell viability; however, higher concentrations reduced viability and caused cell lysis. All effects of these compounds on the cells were antagonized at least partially by 5–10mM-Ca2+. Other cations tested were considerably less effective. Like the organic inhibitors, La3+ also altered cell shape, inhibited PD induction and reduced cell viability at elevated concentrations, but its effect on the cells appeared to be more complex. Inhibition of PD induction by the organic antagonists could not be, attributed solely to an impaired uptake of extracellular Ca2+, a reduction of ATP pools in the cells or a direct effect on calmodulin. Concentrations of TMB-8 that inhibited PD induction had little effect on the cyclic GMP response. Therefore, this compound did not inhibit PD induction indirectly by blocking cyclic GMP synthesis. While the sites of action of these drugs could not be determined precisely, the results are consistent with the hypothesis that intracellular Caz+ functions as a regulator of PD induction in this organism.
adaptation to chemotactic cyclic amp signals in Dictyostelium involves the g-protein
Authors: Small Nadine V. , Europe-Finner G. Nicholas , Newell Peter C. 
Published: April 2021

ABSTRACT
Amoebae of Dictyostelium discoideum show adaptation towards a chemotactic cyclic AMP signal. Within a few seconds of receipt of the signal they are inhibited for a period of 1–2 min from further chemotactic responses to subsequent cyclic AMP signals of similar or smaller magnitude. The site of this adaptation mechanism in the chemotactic transduction pathway was investigated by addition of components of the transduction chain (GTP analogues, myo-inositol-1,4,5-trisphosphate (InsP3) and Ca2+) to permeabilized cells followed by determination of the amount of cyclic GMP formed as a measure of the chemotactic response. This approach was made possible by finding that permeabilization of amoebae with saponin did not uncouple the cell surface cyclic AMP receptors from stimulation of cyclic GMP formation.
It was found that InsP3 and Ca2+ were ‘downstream’ from the adaptation mechanism: they could trigger a cyclic GMP response in cyclic AMP-adapted amoebae but could not themselves induce adaptation. In contrast, GTPγS was unable to trigger a cyclic GMP response in cyclic AMP-adapted cells, although it could trigger multiple cyclic GMP responses in non-adapted cells. We deduce that the site of adaptation to cyclic AMP stimulation is at the G-protein involved in this signalling pathway. Moreover, as GTPγS was found to be unable to induce adaptation, we conclude that the mechanism of adaptation involves an action of the cyclic AMP receptor on the G-protein that is distinct from its commonly reported action of stimulating G-protein binding of GTP.
Effect of calcium on oxytocin-induced contraction of mammary gland myoepithelium as visualized by nbd-phallicidin
Authors: Moore David M. , Vogl A. Wayne , Baimbridge Kenneth , Emerman Joanne T. 
Published: April 2021

ABSTRACT
The effect of calcium on oxytocin-induced contraction of myoepithelial cells was visualized with NBD-phallacidin, a fluorescent stain for filamentous actin. In the absence of oxytocin, the cells appeared relaxed; long, branching processes radiated from the cell bodies. In the presence of 50nM-oxytocin, myoepithelial cells contracted into smaller spoke-shaped bodies in which the arms were shorter and thicker. Electron microscopy confirmed the morphological differences between oxytocin-treated and untreated myoepithelium. To determine a role for extracellular calcium, tissue was incubated in EGTA, then exposed to oxytocin, with or without added calcium. Contraction occurred in the presence of oxytocin plus additional calcium but not in the absence of calcium. When the tissue was incubated with the calmodulin antagonist trifluoperazine (TFP) in calcium-containing medium, oxytocin did not induce myoepithelial cell contraction. These data support previous results obtained with a myosin light-chain phosphorylation assay implicating calcium and calmodulin in oxytocin-induced contraction. Furthermore, NBD-phallicidin visualization of myoepithelial cells demonstrates that the effect of calcium on contraction is physiologically significant.
Cell spreading on serum is not identical to spreading on fibronectin
Authors: Mcinnes Colin , Knox Peter , Winterbourne David J. 
Published: April 2021

ABSTRACT
Adhesion and spreading of cell lines on dishes coated with serum-derived proteins were studied after removal of cell-surface proteoglycans. A mixture of glycosaminoglycans lyases from heparin-induced Flavobacterium heparinum removed 80% of the [35S] sulphate-labelled glycosaminoglycans from the surface of attached cells within 30 min, but this had little effect on cell morphology. The rate of cell attachment to dishes coated with serum was unaffected by prior treatment of cells with this mixture of glycosaminogly can lyases. While a heparan sulphate lyase preparation abolished cell spreading in response to fibronectin there was no effect of the enzyme on the spreading mediated by vitronectin. These results suggest that, although heparan sulphate is required for spreading on purified fibronectin, the spreading stimulated by serum under routine culture conditions requires neither cellular heparan sulphate nor serum-derived fibronectin.
Functional monopolar spindles caused by mutation in MGR, a cell division gene of Drosophila melanogaster
Authors: González Cayetano , Casal José , Ripoll Pedro 
Published: April 2021

ABSTRACT
Mutation in the gene merry-go-round (mgr) of Drosophila causes a variety of phenotypic traits in somatic and germinal tissues, such as poly-ploid cells, metaphasic arrest, postmeiotic cysts with 16 nuclei, and spermatids with four times the normal chromosome content. The most characteristic phenotype is the appearance of mitotic and meiotic figures where all chromosomes are arranged in a circle. Treatment with anti-mitotic drugs and the phenotype of double mutants mgr asp (asp being a mutation altering the spindle) show that these circular figures need a functional spindle for their formation. These abnormal figures are caused by monopolar spindles similar to those observed after different treatments in several organisms. All mutant traits indicate that mgr performs a function necessary for the correct behaviour of centrosomes, thus opening this or-ganelle to genetic analysis.
Structure of myofibrils at extra-junctional membrane attachment sites in cultured cardiac muscle cells
Authors: Atherton Blair T. , Behnke Monica M. 
Published: April 2021

ABSTRACT
The composition and organization of myofibrils at extra-junctional membrane attachment sites in cultured neonatal rat cardiac muscle cells were analysed by immunofluorescence and electron microscopy. When myofibril terminals attached to the cell membrane via focal contacts at regions of the sarcolemma that lacked intercalated discs, they appeared to be non-striated and resembled thick actin cables. Although the non-striated terminals contained actin, myosin and alpha-actinin, the proteins were not organized into recognizable sarcomeres at the light microscopic level. Analysis of the structure of the terminals in the electron microscope confirmed that the usual sarcomeric organization and attachments to the sarcolemma were markedly modified. The non-striated myofibril terminals differed in structure from both stress fibres in non-muscle cells and stress fibre-like structures present in embryonic heart cells in culture. Non-striated myofibril terminals attached to the cell membrane by lateral contact with extra-junctional electron-dense membrane plaques rather than by insertion by their ends into the fascia adherens. It is proposed that the structure and composition of membrane-attachment points for myofibrils may have an influence on the structure, organization or stability of contractile elements in cardiac muscle.
DNA replication and repair of Tilapia cells II. Effects of temperature on DNA replication and ultraviolet repair in Tilapia ovary cells
Authors: Chen J. Don , Yew F. H. 
Published: April 2021

ABSTRACT
TO-2 is a fish cell line derived from the Tilapia ovary. It grows over a wide range of temperature (15–34°C). While most fish cells lack DNA excision repair and are hypersensitive to ultraviolet light (u.v.), Tilapia cells are more u.v.-resistant than mammalian cells. In this paper we report the effects of temperature on DNA replication and u.v. repair in TO-2 cells.
When the cells were moved from 31 °C to the sublethal high temperature of 37°C, the rate of DNA synthesis first decreased to 60%, then speedy recovery soon set in, and after 8h at 37°C the rate of DNA synthesis overshot the 31 °C control level by 180%. When moved to low temperature (18°C) Tilapia cells also showed an initial suppression of DNA synthesis before settling at 30% of the control level. u.v. reduced but could not block DNA synthesis completely. The inhibition was overcome in 3 h at 37, 31 and 25°C, but not at 18°C. Initiation of nascent DNA synthesis was blocked at 4 Jm−2 in TO-2 cells compared with ≤ljm−2 in mammalian cells. After 9Jm−2 u.v. irradiation, low molecular weight DNA replication intermediates started to accumulate, and they could be chased into high molecular weight DNA with little delay.
TO-2 cells showed low levels of u.v.-induced excision repair; but this was prominent compared with other fish cells. The u.v.-induced incision rate has been measured at various temperatures, and the activation energy of incision estimated to be 13 kcal mol−1 (1 cal ≈ 4·I84 J).
Response of aggregating chick corneal cells to modifiers of n-linked oligosaccharides, endoglycosidase h and deoxymannojirimycin
Authors: Overton Jane 
Published: April 2021

ABSTRACT
Chick corneal epithelium takes on its mature conformation between 11 and 16 days of incubation. Earlier work has shown that desmosome frequency increases during this period, reaching its highest rate at 154 days. In the present report aggregation rates of cells from embryos of 11 days and those of 154 days are compared. Younger cells, which form fewer desmosomes, aggregate at a more moderate rate than older cells. In addition, younger cells bind less concanavalin A (ConA) than older cells. To determine if increase in ConA binding could be related to these cellular responses, aggregating cells were exposed to endoglycosidase H (EndoH) and to deoxymannojirimycin. This treatment should permit comparison of the response of cells that have a normal complement of N-linked oligosaccharides with those that have reduced high-mannose or complex type sugars. The effectiveness of EndoH under the conditions used was confirmed by failure of treated glycoprotein after separation by SDS-PAGE and electroblotting to bind ConA. Aggregation rates of both older and younger cells were unaffected, as measured by disappearance of single cells, though older cells formed somewhat smaller aggregates at the highest dosage used. Desmosome formation was markedly reduced in the presence of the enzyme, even in the absence of other changes in the fine structure. At the highest dose of the enzyme the fine structure of older but not younger cells showed indications of blockage of transport. Deoxymannojirimycin appears to cause a build-up of high-mannose groups, since treated cells showed increased incorporation of [3H]mannose. In addition, SDS-PAGE of an NP40-extracted cell fraction showed a similar distribution of proteins, but incorporation of [3H] mannose differed between inhibitor-treated cells and controls. There was no effect on cell fine structure and junctional frequency was normal. Thus N-linked sugars appear to be important in this system to permit re-formation of normal cell contacts, but there is no evidence that processing is required.
Fibronectin observed in the nuclear matrix of hela tumour cells
Authors: Zerlauth Gerold , Wesierska-Gadek Józefa , Sauermann Georg 
Published: April 2021

ABSTRACT
We have investigated the intracellular distribution of insoluble fibronectin in HeLa tumour cells. By indirect immunofluorescence microscopy fibronectin was detected in the nuclear region, but not in the region of the cell surface. Isolated nuclei and isolated nuclear matrices also stained for fibronectin. By quantitative enzyme-linked immunosorbent assay (ELISA), fibronectin was found almost exclusively in the subcellular fraction of isolated nuclear matrices. Using immunoblotting techniques fibronectin was detected in nuclear matrices isolated from cells grown in standard and in fibronectin-depleted medium. The data demonstrate that fibronectin in HeLa tumour cells is preferentially associated with the nuclear matrix.
Intracellular diaphragmed fenestrae in cultured capillary endothelial cells
Authors: Montesano Roberto , Orci Lelio 
Published: April 2021

ABSTRACT
The endothelium of visceral capillaries is characterized by the occurrence of numerous fenestrae, which are usually bridged by a thin, singlelayered diaphragm. Both in vivo and in vitro, diaphragmed fenestrae perforate the endothelial cell cytoplasm in the most attenuated regions of the cell. We report here that in capillary endothelial cells grown under experimental conditions promoting the development of intracellular lumina (for example, suspension within a threedimensional collagen matrix), diaphragmed fenestrae can form in a unique, previously undescribed intracellular location - that is, within thin cytoplasmic septa separating contiguous luminal compartments.
Flexible-substratum technique for viewing cells from the side: Some in vivo properties of primary (9+0) cilia in cultured kidney epithelia
Authors: Roth Karen E. , Rieder Conly L. , Bowser Samuel S. 
Published: April 2021

ABSTRACT
Cells cultured on thin plastic (e.g. Formvar, Teflon, polycarbonate) membranes can be clearly imaged from the side in vivo by video microscopy. We have used this flexible-substratum technique to examine the behaviour and properties of primary cilia in confluent cultures of the kidney epithelial cell lines PtK1 PtK2, LLC-PK1, MDCK and BSC-40. In these cells primary cilia appear as rigid rods, up to 55 um long, which project at various angles from the dorsal cell surface. The length distribution of primary cilia in confluent cultures is a distinct characteristic of each established kidney cell line examined, with LLC-PKi exhibiting three distinct length populations. Primary cilia of kidney cell lines bend passively in response to flow but do not display propagated bending or vortical motions. Up to 26% of the cilia in the cell types examined possess one or more conspicuous swellings along the ciliary shaft. Treatment with 0·05% trypsin, which is sufficient to cause cell rounding, does not induce the resorption or shedding of the cilium. These direct observations demonstrate that kidney epithelial-cell primary cilia are non-motile and longer than previously thought, and suggest that their length represents a phenotypic marker for each cell line.
Tight-junctional strands first appear in regions where three cells meet in differentiating olfactory epithelium: A freeze-fracture study
Authors: Menco Bert Ph. M. 
Published: April 2021

ABSTRACT
Tight junctions of the olfactory epithelium of rat embryos were studied at the 14th day of gestation and during their subsequent development. Two different epithelial morphologies could be distinguished at the 14th gestational day. In one group of embryos the epithelial surface appeared undifferentiated, with tight-junctional strands found exclusively in regions where three cells met. The main orientation of these strands is in a direction parallel to the longitudinal orientation of the epithelial cells. These junctions resemble tight junctions that interconnect three cells, i.e. tricellular tight junctions, in that respect. However, unlike these the junctions mainly have single strands of particles, whereas tricellular junctions usually consist of paired strands of particles. Tight-junctional strands were completely absent in areas where two cells met. These areas, i.e. those of incipient bicellular tight junctions, had gap-junction-like aggregates of intra-membranous particles. Another group of 14-day-old embryos displayed a differentiating olfactory epithelial surface with bicellular as well as tricellular tight-junctional strands. The latter ones were paired. Here too the tight-junctional belts displayed some gap-junction-like aggregates of particles, but there were considerably fewer of these than earlier. As one or the other tight-junctional appearance was always seen in a single freeze-fracture replica, it is reasonable to assume that the two tight-junctional appearances reflect a sequential pattern of differentiation peculiar to the whole surface of the olfactory epithelium, i.e. to surfaces of receptor cells as well as to surfaces of supporting cells. It would appear that, at the onset of olfactory epithelial differentiation, tight junctions first interconnect cells in regions where three cells meet and that tricellular strand formation precedes the formation of bicellular strands. When strands were present at the 14th day of embryonic development, their numbers were lower than those found later. However, strand packing, expressed as the density per micrometre of strands parallel to the epithelial surface, increased beginning at the 16th day of embryonic development.
Induction of fibroblast spreading by Mn2+: A possible role for unusual binding sites for divalent cations in receptors for proteins containing arg-gly-asp
Authors: Edwards J. G. , Hameed H. , Campbell G. 
Published: April 2021

ABSTRACT
Mn2+ at low (μM) concentrations fulfils the divalent cation requirement for spreading of BHK21 cells on fibronectin. At much higher concentrations, Mn2+ (and to a small extent also Mg2+) induces spreading on haemoglobin, not normally an adhesive protein. Since high Mn2+ also induces spreading of BHK variants unresponsive to exogenous fibronectin, it is unlikely to be acting as a cofactor for secreted cellular fibronectin or by stimulating its secretion. High Ca2+, but not Mg2+, inhibits the induction of spreading by Mn2+ on haemoglobin.
Pre-treatment of cells with high concentrations of trypsin decreases the rate of spreading induced by Mg2+ on fibronectin, and by Mn2+ on haemoglobin, to similar extents.
High and low Mn2+ could induce spreading, either by different mechanisms or through a common pathway. In the second case, at both concentrations, Mn2+ could act by binding to Ca2+/Mg2+ sites in one or more receptors for adhesion proteins. This would require binding of Mn2+ or Mg2+ to these sites to activate the receptors in the absence of adhesion proteins, and the effect of such proteins to be to increase the affinity of the sites for metal ions.
The sites in question may be formed by sequences homologous to those found in the extracellular domains of the vitronectin receptor and platelet membrane glycoprotein lib. Although very similar to the Ca2+-binding loop in the EF hand of calmodulin, these sequences more closely resemble bacterial galactose-binding protein in lacking one of the conserved co-ordinating sidechains.