Copper-induced calcium release from ER involves the activation of ryanodine-sensitive and IP3-sensitive channels in Ulva compressa

The marine alga Ulva compressa (Chlorophyta) showed a triphasic release of intracellular calcium with maximal levels at 2, 3 and 12 h and a biphasic accumulation of intracellular hydrogen peroxide with peaks at 3 and 12 h when cultivated with copper excess. Intracellular hydrogen peroxide originated exclusively in organelles. In this work, we analyzed the intracellular origin of calcium release and the type of calcium channels activated in response to copper excess. U. compressa was treated with thapsigargin, an inhibitor of endoplasmic reticulum (ER) calcium ATPase, ryanodine, an inhibitor of ryanodine-sensitive channels and xestospongin C, an inhibitor of inositol 1, 4, 5-triphosphate (IP₃)-sensitive channels. Thapsigargin induced the depletion of calcium stored in ER at 75 min and completely inhibited calcium release at 2, 3 and 12 h of copper exposure indicating that calcium release originated in ER. In addition, ryanodine and xestospogin C inhibited calcium release at 2 and 3 h of copper exposure whereas the peak at 12 h was only inhibited by ryanodine. Thus, copper induced the activation of ryanodine-sensitive and IP₃-sensitive calcium channels in ER of U. compressa.Key words: calcium release, endoplasmic reticulum, calcium channels, marine alga, Ulva compressaPlants showed common responses to biotic and abiotic stresses, mainly the accumulation of reactive oxygen species (ROS), in particular hydrogen peroxide, and the release of intracellular calcium.^1,2 Regarding abiotic stress, it has been shown that ozone triggers a NADPH oxidase-dependent biphasic oxidative burst in Arabidopsis thaliana that activates antioxidant and defense enzymes.^3,4 In addition, cadmium induced a NADPH oxidase-dependent monophasic accumulation of extracellular hydrogen peroxide in tobacco cells.⁵ On the other hand, ozone as well as absicic acid treatment, dessication, cold, heat, salinity, UV light and anoxia induce intracellular calcium release and the activation of antioxidant enzymes.^6–8 Regarding abiotic stress in algae, copper induced a monophasic increase of intracellular hydrogen peroxide at 2 h of copper exposure in the brown seaweeds Lessonia nigrecsens and Scytosiphon lomentaria.⁹ On the other hand, strontium induced calcium release in the green microalga Eremosphaera viridis as did osmotic stress in the zygote of the brown macroalga Fucus serratus.^10,11 U. compressa is a cosmopolitan marine macroalga (Chlorophyta) growing in copper-impacted coastal areas in northern Chile.¹² U. compressa cultivated in seawater with copper excess (10 µM) showed co-occuring increases of intracellular calcium and hydrogen peroxide.¹³ Copper induced a triphasic release of calcium with maximal levels at 2, 3 and 12 h and a biphasic production of hydrogen peroxide with peaks at 3 and 12 h. Interestingly, the production of hydrogen peroxide occurred exclusively in organelles, i.e., mitochondria and chloroplasts. In addition, calcium and hydrogen peroxide act as signals in the differential activation of antioxidant and defense enzymes.¹³ In this work, we analyzed the intracellular origin of copper-induced calcium release and the type of calcium channels activated in response to copper excess in U. compressa.     

Habitat and climate heterogeneity maintain beta-diversity of birds among landscapes within ecoregions

Aim  Habitat and climate heterogeneity may affect patterns of species diversity from the relatively local scale of communities to the broad biogeographical scale of continents. However, the effects of heterogeneity on species diversity have not been studied as widely at intermediate scales although differences among landscapes in local climate and habitat should maintain beta-diversity.
Location  Bailey ecoregions in the USA.
Methods  Using a geographically extensive dataset on bird distribution and abundance in 35 ecoregions, we tested for the effects of habitat and climate heterogeneity on beta-diversity at two discrete spatial scales: among sample points within landscapes, and among landscapes within ecoregions.
Results  Landscape-level beta-diversity typically accounted for 50% or more of gamma-diversity and was significantly and positively related to habitat heterogeneity (elevational range within an ecoregion) and climate heterogeneity (variation in potential evapotranspiration). Contrary to predictions, point-level beta-diversity was negatively related to habitat and climate heterogeneity, perhaps because heterogeneity constrains alpha-diversity at the landscape level. The geographical spatial separation of landscapes within an ecoregion did not significantly affect beta-diversity at either scale.
Main conclusions  Our results suggest that habitat selection and adaptation to local climate may be the primary processes structuring bird diversity among landscapes within ecoregions, and that dispersal limitation has a lesser role in influencing beta-diversity among landscapes.     

Partitioning species diversity across landscapes and regions: a hierarchical analysis of alpha, beta, and gamma diversity

Species diversity may be additively partitioned within and among samples (alpha and beta diversity) from hierarchically scaled studies to assess the proportion of the total diversity (gamma) found in different habitats, landscapes, or regions. We developed a statistical approach for testing null hypotheses that observed partitions of species richness or diversity indices differed from those expected by chance, and we illustrate these tests using data from a hierarchical study of forest-canopy beetles. Two null hypotheses were implemented using individual- and sample-based randomization tests to generate null distributions for alpha and beta components of diversity at multiple sampling scales. The two tests differed in their null distributions and power to detect statistically significant diversity components. Individual-based randomization was more powerful at all hierarchical levels and was sensitive to departures between observed and null partitions due to intraspecific aggregation of individuals. Sample-based randomization had less power but still may be useful for determining whether different habitats show a higher degree of differentiation in species diversity compared with random samples from the landscape. Null hypothesis tests provide a basis for inferences on partitions of species richness or diversity indices at multiple sampling levels, thereby increasing our understanding of how alpha and beta diversity change across spatial scales.     

The spatial distribution of termites in shortgrass steppe: a geostatistical approach

The broad-scale distribution of subterranean termites (Reticulitermestibialis) was studied in a shortgrass-steppe ecosystem in northern Colorado, United States. Termite occurrence and abundance was measured over 4 months at 10-m intervals along a 900-m transect that spanned a topographic gradient. Geostatistics were used to model the probability of termite occurrence along the transect, and to identify the distributional extent and potential roles of termites in shortgrass steppe. Semivariance was calculated between sample pairs of differing distances and kriging was used to interpolate the probability of termite occurrence along the transect. The semivariogram showed spatial dependence in termite distribution between samples 10–330 m apart and converged on the population variance at distances >330 m, which suggested that spatial dependence explained much of the broad-scale variation in termite distribution. A relatively large nugget variance, however, indicated there was spatial dependence below the 10-m sampling resolution. Termites were most frequently found on a south-facing slope and in a lowland swale. Four-wing saltbush (Atriplexcanescens) was also common in these areas and is important in the production of woody litter. The distribution of termites was significantly associated with proximity to saltbush, which showed a strong spatial dependence at scales <500 m. Kriged probabilities of occurrence and cross-correlation between termites and shrubs showed that peak termite occurrence was shifted upslope 100 m from areas of closest shrub proximity. Other factors, such as soil temperature, texture, or organic matter, are therefore likely to influence termite distributions in shortgrass steppe. The geostatistical approach used here provides a basis for further study on termites in shortgrass steppe, where their roles in decomposition and nutrient cycling are unknown. Geostatistics could also be used to describe distribution patterns on other soil arthropods sampled from traps or soil cores along transects that span topographic or land-use changes. Received: 4 April 1997 / Accepted: 4 November 1997     

B lymphocyte precursors in human bone marrow: an analysis of normal individuals and patients with antibody-deficiency states.

Lymphoid cells containing cytoplasmic IgM but lacking stable surface IgM are believed to be the direct precursors of B lymphocytes. We have characterized these pre-B cells in the bone marrow of normal individuals and patients with a variety of immunoglobulin deficiencies or hematologic disorders by using immunofluorescence and autoradiography. Pre-B cells comprised 5.8 +/- 5.7% of lymphoid cells in normal bone marrow. Eleven patients with infantile X-linked agammaglobulinemia (X-LA) lacked B lymphocytes but had a normal frequency (3.8 +/- 3.6%) of bone marrow pre-B cells. A smaller proportion of marrow pre-B cells from patients with X-LA were engaged in spontaneous DNA synthesis than was found for normal controls. In individuals other than the group with X-LA, the number of circulating B cells was positively correlated with the frequency of marrow pre-B cells. These results indicate that patients with X-LA have a defect in maturation of pre-B cells, and suggest that some patients with acquired B lymphocyte deficiency may have lost the capacity to generate pre-B cells from stem cells.     

Mitochondrial growth and division during the cell cycle in HeLa cells

The growth and division of mitochondria during the cell cycle was investigated by a morphometric analysis of electron micrographs of synchronized HeLa cells. The ratio of total outer membrane contour length to cytoplasmic area did not vary significantly during the cell cycle, implying a continuous growth of the mitochondrial outer membrane. The mean fraction of cytoplasmic area occupied by mitochondrial profiles was likewise found to remain constant, indicating that the increase in total mitochondrial volume per cell occurs continuously during interphase, in such a way that the mitochondrial complement occupies a constant fraction( approximately 10-11(percent)) of the volume of the cytoplasm. The mean area, outer membrane contour length, and axis ratio of the mitochondrial profiles also did not vary appreciably during the cell cycle; furthermore, the close similarity of the frequency distributions of these parameters for the six experimental time-points suggested a stable mitochondrial shape distribution. The constancy of both the mean mitochondrial profile area and the number of mitochondrial profiles per unit of cytoplasmic area was interpreted to indicate the continuous division of mitochondria at the level of the cell population. Furthermore, no evidence was found for the occurrence of synchronous mitochondrial growth and division within individual cells. Thus, it appears that, in HeLa cells, there is no fixed temporal relationship between the growth and division of mitochondria and the events of the cell cycle. A number of statistical methods were developed for the purpose of making numerical estimates of certain three-dimensional cellular and mitochondrial parameters. Mean cellular and cytoplasmic volumes were calculated for the six time-points; both exhibited a nonlinear, approx. twofold increase. A comparison of the axis ratio distributions of the mitochondrial profiles with theoretical distributions expected from random sectioning of bodies of various three-dimensional shapes allowed the derivation of an "average" mitochondrial shape. This, in turn, permitted calculations to be made which expressed the two-dimensional results in three-dimensional terms. Thus, the estimated values for the number of mitochondria per unit of cytoplasmic volume and for the mean mitochondrial volume were found to remain constant during the cell cycle, while the estimated number of mitochondria per cell increase approx. twofold in an essentially continuous manner.