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PubMedDate not supplied

Retinal light adaptation--evidence for a feedback mechanism.

Light adaptation is the adjustment of retinal response properties to variations in ambient illumination. It enables the encoding of visual information over a millionfold intensity range, from moonlight to broad daylight, despite the relatively small dynamic range of response of visual neurones. We have studied the effects of light adaptation on the dynamics and sensitivity of visual responses of neurones in the turtle retina, by measuring the responses of horizontal cells in the retina to light which was modulated with a sinusoidal time course around various mean levels. As a quantitative measure of the transduction from light to neural signals, we calculated the gain of response at each frequency. Gain is defined as the amplitude of the modulated response component divided by the amplitude of light modulation. We report here that the gain (mV photon-1) at low temporal frequencies decreased as the mean light level increased. Over a 2 log-unit range of mean light levels, low-frequency gain was inversely proportional to the mean light level, as in Weber's law. However, at high temporal frequencies, the gain was almost independent of mean light level. Our results are reminiscent of Kelly's results on human temporal-frequency sensitivity in various states of light adaptation. We found that a family of horizontal-cell temporal frequency responses, measured at various mean light levels, could be accounted for by a negative feedback model in which the feedback strength is proportional to mean light level.

Adaptation, Physiological↗
PubMed1999

Characterisation of a novel gene family of putative cyclic nucleotide- and calmodulin-regulated ion channels in Arabidopsis thaliana.

In plants, cyclic GMP is involved in signal transduction in response to light and gibberellic acid. For cyclic AMP, a potential role during the plant cell cycle was recently reported. However, cellular targets for cyclic nucleotides in plants are largely unknown. Here we report on the identification and characterisation of a new gene family in Arabidopsis, which share features with cyclic nucleotide-gated channels from animals and inward-rectifying K+ channels from plants. The identified gene family comprises six members (Arabidopsis thaliana cyclic nucleotide-gated channels, AtCNGC1-6) with significant homology among the deduced proteins. Hydrophobicity analysis predicted six membrane-spanning domains flanked by hydrophilic amino and carboxy termini. A putative cyclic nucleotide binding domain (CNBD) which contains several residues that are invariant in other CNBDs was located in the carboxy terminus. This domain overlaps with a predicted calmodulin (CaM) binding site, suggesting interaction between cyclic nucleotide and CaM regulation. We demonstrated interaction of the carboxy termini of AtCNGC1 and AtCNGC2 with CaM in yeast, indicating that the CaM binding sites are functional. Furthermore, it was shown that both AtCNGC1 and AtCNGC2 can partly complement the K(+)-uptake-deficient yeast mutant CY162. Therefore, we propose that the identified genes constitute a family of plant cyclic nucleotide- and CaM-regulated ion channels.

Amino Acid Sequence↗
PubMed1999

Rapid blue light regulation of a Trichoderma harzianum photolyase gene.

Photolyases and blue light receptors belong to a superfamily of flavoproteins that make use of blue and UVA light either to catalyze DNA repair or to control development. We have isolated a DNA photolyase gene (phr1) from Trichoderma harzianum, a common soil fungus that is of interest as a biocontrol agent against soil-borne plant pathogens and as a model for the study of light-dependent development. The sequence of phr1 is similar to other Class I Type I eukaryotic photolyase genes. Low fluences of blue light rapidly induced phr1 expression both in vegetative mycelia, which lack photoprotective pigments, and, to a greater extent, in conidiophores. Thus, visible light induces the development of pigmented, resistant spores as well as the expression of phr1, perhaps announcing in this way the imminent exposure to the more damaging short wavelengths of sunlight. Light induction of phr1 in non-sporulating mutants shows that a complete sporulation pathway is not required for photoregulation. The light requirements for photoinduction of phr1 were not altered in dimY photoperception mutants. This suggests that photoinduction of sporulation and of photolyase expression is distinct in their photoreceptor system or in the transduction of the blue light signal.

Amino Acid Sequence↗
PubMed1988

Cross-linking of IgE-receptor complexes by rigid bivalent antigens greater than 200 A in length triggers cellular degranulation.

We have examined the effect of cross-linking IgE-receptor complexes with variable receptor-receptor distances on the transmembrane signaling that leads to degranulation of rat basophilic leukemia cells. Linear polymers of the biotin-binding protein avidin were generated with bis biotin-1,12-diamidododecane, and a dinitrophenyl-biotin conjugate was bound at each end of the polymers to form a series of rigid bivalent haptens of well-defined length. The polymers were fractionated by size with nondenaturing PAGE, electro-eluted, and tested for their ability to stimulate degranulation of rat basophilic leukemia cells sensitized with anti-DNP IgE. We found that hexamers of avidin (of length greater than or equal to 240 A) were as effective in triggering degranulation as dimers (of length approximately 80 A), while the monomeric avidin antigen (of length approximately 40 A) elicited a poorer degranulation response from the cells. The mechanism by which aggregation of cell surface receptors can initiate signal transduction is discussed in light of these results.

Animals↗
PubMed2002

Phytochrome in cotyledons regulates the expression of genes in the hypocotyl through auxin-dependent and -independent pathways.

To elucidate the mechanism of plant responses to shading, we identified three promoter/enhancer trap lines (M812, J53, J59) that exhibited reporter expression in the hypocotyl in response to the end-of-day far-red light treatment. Interestingly, we found auxin-responsive genes in the vicinities of the reporter insertion sites in M812 and J53. We examined the effects of auxin on the reporter expression in these lines together with a previously identified N35 line. The results indicated that the reporter expression was induced by exogenous auxin in N35 and J53. Furthermore, an auxin transport inhibitor inhibited the responses of these lines to the end-of-day far-red light treatment, suggesting the involvement of auxin in the responses of plants to shading. By contrast, neither auxin nor the transport inhibitor affected the response in M812 and J59. Interestingly, J59 responded to ABA. Hence, ABA might be involved in the response as well. Analysis of the photoreceptive sites for the responses revealed the cotyledons, not the hypocotyl, are the major photoreceptive sites both in the auxin-responsive and ABA-responsive lines. Hence, some signals appeared to be transmitted from the cotyledons to the hypocotyl.

Abscisic Acid↗
PubMed2002

Supplementary ultraviolet-B radiation induces a rapid reversal of the diadinoxanthin cycle in the strong light-exposed diatom Phaeodactylum tricornutum.

A treatment of the diatom Phaeodactylum tricornutum with high light (HL) in the visible range led to the conversion of diadinoxanthin (Dd) to diatoxanthin (Dt). In a following treatment with HL plus supplementary ultraviolet (UV)-B, the Dt was rapidly epoxidized to Dd. Photosynthesis of the cells was inhibited under HL + UV-B. This is accounted for by direct damage by UV-B and damage because of the UV-B-induced reversal of the Dd cycle and the associated loss of photoprotection. The reversal of the Dd cycle by UV-B was faster in the presence of dithiothreitol, an inhibitor of the Dd de-epoxidase. Our results imply that the reversal of the Dd cycle by HL + UV-B was caused by an increase in the rate of gross Dt epoxidation, whereas the de-epoxidation of Dd was unaffected by UV-B. This is further supported by our finding that the in vitro de-epoxidation activity and the affinity toward the cosubstrate ascorbic acid of the Dd de-epoxidase were both unaffected by UV-B pretreatment of intact cells. We provide evidence that Dt epoxidation is normally down-regulated by a high pH gradient under HL. It is proposed that supplementary UV-B affected the pH gradient across the thylakoid membrane, which disrupted the down-regulation of Dt epoxidation and led to the observed increase in the rate of Dt epoxidation.

Adaptation, Physiological↗
PubMed1992

Role of sucrose-phosphate synthase in sucrose metabolism in leaves.

Sucrose is formed in the cytoplasm of leaf cells from triose phosphates exported from the chloroplast. Flux control is shared among key enzymes of the pathway, one of which is sucrose-phosphate synthase (SPS). Regulation of SPS by protein phosphorylation is important in vivo and may explain diurnal changes in SPS activity and carbon partitioning. The signal transduction pathway mediating the light activation of SPS in vivo appears to involve metabolites and novel "coarse" control of the protein phosphatase that dephosphorylates and activates SPS. Regulation of the phosphorylation of SPS may provide a general mechanism whereby sucrose formation is coordinated with the rate of photosynthesis and the rate of nitrate assimilation. There are apparent differences among species in the properties of SPS that may reflect different strategies for the control of carbon partitioning. The SPS gene has recently been cloned from maize; results of preliminary studies with transgenic tomato plants expressing high levels of maize SPS support the postulate that SPS activity can influence the partitioning of carbon between starch and sucrose.

Journal Article↗
PubMed1991

Rotation and switching of the flagellar motor assembly in Halobacterium halobium.

Halobacterium halobium swims with a polarly inserted motor-driven flagellar bundle. The swimming direction of the cell can be reserved by switching the rotational sense of the bundle. The switch is under the control of photoreceptor and chemoreceptor proteins that act through a branched signal chain. The swimming behavior of the cells and the switching process of the flagellar bundle were investigated with a computer-assisted motion analysis system. The cells were shown to swim faster by clockwise than by counterclockwise rotation of the flagellar bundle. From the small magnitude of speed fluctuations, it is concluded that the majority, if not all, of the individual flagellar motors of a cell rotate in the same direction at any given time. After stimulation with light (blue light pulse or orange light step-down), the cells continued swimming with almost constant speed but then slowed before they reversed direction. The cells passed through a pausing state during the change of the rotational sense of the flagellar bundle and then exhibited a transient acceleration. Both the average length of the pausing period and the transient acceleration were independent of the stimulus size and thus represent intrinsic properties of the flagellar motor assembly. The average length of the pausing period of individual cells, however, was not constant. The time course of the probability for spontaneous motor switching was calculated from frequency distribution and shown to be independent of the rotational sense. The time course further characterizes spontaneous switching as a stochastic rather than an oscillator-triggered event.

Flagella↗
PubMed1992

Regulation of gene expression by vitamin A: the role of nuclear retinoic acid receptors.

Over the past five years, a wealth of information has accumulated concerning the molecular mechanisms mediating RA effects on gene expression. The molecular cloning of the 3 retinoic acid receptors (RARs), of their 16 or so different isoforms, and of the 3 retinoid X receptors (RXRs) as well as the identification of at least 2 different active isomers of RA (all trans- and 9-cis-RA) and of several different CRABPs and CRBPs now provide the essential tools to explain the pleiotropy that has become associated with RA effects. In the years to come, a concentrated effort to delineate the complex interactions between the various components of the retinoid signal transduction system should shed light on the mechanisms underlying pattern formation during vertebrate development and point to new ways in which retinoids can be exploited therapeutically.

Amino Acid Sequence↗
PubMedDate not supplied

Bright light during nighttime: effects on the circadian regulation of alertness and performance.

The present studies evaluated to what extent duration (all-night or 4-hour exposures) and timing of nocturnal bright light (BL) (beginning or end of the night) modulate effects on vigilance. The results showed that all-night BL exposure is able to alleviate the nocturnal decrements in alertness and performance. However, under certain circumstances, this continuous BL exposure may induce adverse effects on mood and finally reveal to be counterproductive. Shorter BL exposure (4 h) during nighttime helps improve mood and performance, although the effects of short BL pulses were less efficacious than all-night BL exposure. The latter part of the night appears the best time for using the alerting effect of BL. The immediate alerting effect of BL seems to be mediated by a global activation of the central nervous system.

Adult↗
PubMed2001

[ICAM-1 expression of rat brain microvascular endothelial cells caused by fluid shear force].

We studied the effect of shear stress (SS) on intercellular adhesion molecule-1 (ICAM-1) expression of rat brain microvascular endothelial cells (RBMECs) using the parallel plate flow chamber method. It was demonstrated that RBMECs showed a time-dependent, but not a force-dependent, upregulation in ICAM-1 expression. Endothelial cell surface expression of ICAM-1 in the supernatants of RBMECs exposed to SS was not changed, thus excluding the possibility that the upregulated expression is due to the factors synthesized by these cells. These data throw light on understanding the signal transduction pathway inside the endothelial cells under the effect of SS.

Animals↗
PubMed1999

Epstein-Barr Virus Encoded Latent Membrane Protein 1 Increases Expression of Immunoglobulin kappa Light Chain through NFkappaB in a Nasopharyngeal Carcinoma Cell Line.

NFkappaB activity analysis was performed with pNFkappaB-luc reporter plasmid. Expression of NFkappaB in nuclear and expression of Igkappa in intact cells were analyzed by Western Blot to identify whether Epstein-Barr virus encoded latent membrane protein 1 (LMP1) increases the expression of immunoglobulin kappa light chain through NFkappaB signal transduction pathway in nasopharyngeal carcinoma. Results showed that LMP1 could increase NFkappaB activity and promote the accumulation of NFkappaB p65 in nucleus, and LMP1 also increased Igkappaexpression. In addition, phosphorothioate oligonucleotides to antisense NFkappaB p65 and p50 inhibited the Igkappaexpression. These imply that LMP1 can regulate Igkappa expression via NFkappaB signal transduction pathway in nasopharyngeal carcinoma.

Journal Article↗
PubMed1993

Developmentally regulated changes in 1,2-diacylglycerol in Dictyostelium. Regulation by light and G proteins.

We have measured 1,2-diacylglycerol (DG) mass during Dictyostelium development. DG levels are initially high in vegetative cells, decrease upon starvation, increase during aggregation, and rise dramatically during culmination, concomitant with SpiA (a spore cell-specific gene) expression. These results are consistent with DG being involved in culmination-stage morphological changes and cell-type differentiation. Mutant analysis shows that the rise in DG during aggregation requires cAMP signaling pathways but is not directly regulated through these processes but via developmental programs induced through cAMP. DG accumulation during aggregation is approximately 8-fold higher than would be expected from inositol lipid hydrolysis (1), suggesting that DG is produced from sources in addition to phosphatidylinositol 4,5-bisphosphate. Our data suggest that during aggregation, although some DG is formed through phospholipase D activity, other pathways (e.g. de novo synthesis) may be more important regulators of DG accumulation. During culmination, DG accumulation correlated with the formation of phosphatidic acid and phosphatidylethanol suggesting the activation of phospholipase D. During this time, the [3H]palmitate labeling of a number of phospholipids decreased rapidly, suggesting a rapid metabolism of phospholipids at this time. Exposure of slugs developed in the dark to light, which initiates culmination, causes rapid DG accumulation, suggesting the activation of phospholipid hydrolysis. The temporal pattern and level of DG accumulation is altered in G alpha 1 null and overexpressing strains, suggesting that G alpha 1 is upstream from DG formation during culmination. These results demonstrate that specific pathways of DG formation are under developmental control and suggest a possible link between light, the activation of DG production, and induction of culmination.

Animals↗
PubMed1996

From seed germination to flowering, light controls plant development via the pigment phytochrome.

Plant growth and development are regulated by interactions between the environment and endogenous developmental programs. Of the various environmental factors controlling plant development, light plays an especially important role, in photosynthesis, in seasonal and diurnal time sensing, and as a cue for altering developmental pattern. Recently, several laboratories have devised a variety of genetic screens using Arabidopsis thaliana to dissect the signal transduction pathways of the various photoreceptor systems. Genetic analysis demonstrates that light responses are not simply endpoints of linear signal transduction pathways but are the result of the integration of information from a variety of photoreceptors through a complex network of interacting signaling components. These signaling components include the red/far-red light receptors, phytochromes, at least one blue light receptor, and negative regulatory genes (DET, COP, and FUS) that act downstream from the photoreceptors in the nucleus. In addition, a steroid hormone, brassinolide, also plays a role in light-regulated development and gene expression in Arabidopsis. These molecular and genetic data are allowing us to construct models of the mechanisms by which light controls development and gene expression in Arabidopsis. In the future, this knowledge can be used as a framework for understanding how all land plants respond to changes in their environment.

Germination↗
PubMed1999

Signal transduction networks and the biology of plant cells.

The development of plant transformation in the mid-1980s and of many new tools for cell biology, molecular genetics, and biochemistry has resulted in enormous progress in plant biology in the past decade. With the completion of the genome sequence of Arabidopsis thaliana just around the corner, we can expect even faster progress in the next decade. The interface between cell biology and signal transduction is emerging as a new and important field of research. In the past we thought of cell biology strictly in terms of organelles and their biogenesis and function, and researchers focused on questions such as, how do proteins enter chloroplasts? or, what is the structure of the macromolecules of the cell wall and how are these molecules secreted? Signal transduction dealt primarily with the perception of light (photomorphogenesis) or hormones and with the effect such signals have on enhancing the activity of specific genes. Now we see that the fields of cell biology and signal transduction are merging because signals pass between organelles and a single signal transduction pathway usually involves multiple organelles or cellular structures. Here are some examples to illustrate this new paradigm. How does abscisic acid (ABA) regulate stomatal closure? This pathway involves not only ABA receptors whose location is not yet known, but cation and anion channels in the plasma membrane, changes in the cytoskeleton, movement of water through water channels in the tonoplast and the plasma membrane, proteins with a farnesyl tail that can be located either in the cytosol or attached to a membrane, and probably unidentified ion channels in the tonoplast. In addition there are highly localized calcium oscillations in the cytoplasm resulting from the release of calcium stored in various compartments. The activities of all these cellular structures need to be coordinated during ABA-induced stomatal closure. For another example of the interplay between the proteins of signal transduction pathways and cytoplasmic structures, consider how plants mount defense responses against pathogens. Elicitors produced by pathogens bind to receptors on the plant plasma membrane or in the cytosol and eventually activate a large number of genes. This results in the coordination of activities at the plasma membrane (production of reactive oxygen species), in the cytoskeleton, localized calcium oscillations, and the modulation of protein kinases and protein phosphatases whose locations remain to be determined. The movement of transcription factors into the nucleus to activate the defense genes requires their release from cytosolic anchors and passage through the nuclear pore complexes of the nuclear envelope. This review does not cover all the recent progress in plant signal transduction and cell biology; it is confined to the topics that were discussed at a recent (November 1998) workshop held in Santiago at which lecturers from Chile, the USA and the UK presented recent results from their laboratories.

Plant Cells↗
PubMed2002

EDS5, an essential component of salicylic acid-dependent signaling for disease resistance in Arabidopsis, is a member of the MATE transporter family.

The eds5 mutant of Arabidopsis (earlier named sid1) was shown previously to accumulate very little salicylic acid and PR-1 transcript after pathogen inoculation and to be hypersusceptible to pathogens. We have isolated EDS5 by positional cloning and show that it encodes a protein with a predicted series of nine to 11 membrane-spanning domains and a coil domain at the N terminus. EDS5 is homologous with members of the MATE (multidrug and toxin extrusion) transporter family. EDS5 expression is very low in unstressed plants and strongly induced by pathogens and UV-C light. The transcript starts to accumulate 2 hr after inoculation of Arabidopsis with an avirulent strain of Pseudomonas syringae or UV-C light exposure, and it stays induced for approximately 2 days. EDS5 also is expressed after treatments with salicylic acid, indicating a possible positive feedback regulation. EDS5 expression after infection by certain pathogens as well as after UV-C light exposure depends on the pathogen response proteins EDS1, PAD4, and NDR1, indicating that the signal transduction pathways after UV-C light exposure and pathogen inoculation share common elements.

Amino Acid Sequence↗