Stand-scale gap-phase dynamics is generally viewed as the main driver of development in mesic deciduous forests of the temperate biome. Soil charcoal of temperate forests in eastern North America are unnoticed in most surveys, thus explaining why fire is undervalued as a driver of forest succession. The extent to which gap-phase, fire, or other processes are responsible for the regeneration and maintenance of mesic deciduous forests is unknown because paleoecological evidence is lacking. We tested the fire-driven succession hypothesis on the development of this major forest type. Based on charcoal 14C dates of two sites, 44 and 55 fires occurred since early Holocene, with a mean interval of 170 to 215 years. The vegetation of both sites followed comparable post-glacial trajectories consisting of three distinct periods. Conifers dominated the two first periods during 5200–6000 years and were replaced by hardwoods–conifers over the last 3500 years. The first period was represented by boreal conifers, whereas the second period, dominated by white pine (Pinus strobus) forests, persisted during 3000–4300 years. The third period marked the development of hardwood (sugar maple, Acer saccharum) forests. Fires occurred continuously on the sites since early Holocene likely under dry conditions during the conifer periods and cooler and moister conditions during the hardwood–conifer period. Recurrent fires appear with climate as key drivers of the long-term dynamics of several temperate forests in eastern North America. Similar studies on other temperate forests should be pursued to test the hypothesis of climate–fire interactions influencing tree composition change.
Impaired sensory, motor and central processing systems combining with biomechanical changes are risk of fall factors in the elderly population. The aim of this study was to assess the auto-adaptation and the regulation of the dynamic control of equilibrium in age-related adaptive strategies, by using a seated position on a seesaw. 15 young adults and 12 healthy middle-aged adults were asked to actively maintain a sitting posture as stable as possible during 12.8 s, on a 1-degree of freedom seesaw (auto-stabilization paradigm), with and without vision. The seesaw was placed in order to allow roll or pitch oscillations. We determine length and surfaces CoP shifts, mean positions and variability, a Postural Performance Index (PI) and a Strategy Organization Ratio (SOR). Our results shows that adopted strategies are plane-dependant during auto-stabilization (parallel and perpendicular axes control is impacted) and age-dependant. PIx during roll seated auto-stabilization tasks appears as the most relevant parameter of aged-related instability. The visual effect, during pitch auto-stabilization, characterizes the postural sensory-motor human behavior. The quantitative and qualitative postural assessment, thanks to seated auto-stabilization task, need to be promoted for long-term health care and probably for the rehabilitation of various disorders. 相似文献
HeLa cell line stably transfected with the tat gene from human immunodeficiency virus type 1 has a decreased antioxidant potential. In this work, we used this model to investigate the effect of a high glucose level (20 mM) on the glucose induced cytotoxicity and on the antioxidant system. In comparison to cell culture under control medium, HeLa-wild cell cultured under 20 mM glucose did not exhibit necrosis or apoptosis, contrary to HeLa-tat cell presenting a significant increase in necrotic or apoptotic state. Moreover after 48 h culture under high glucose level the HeLa-tat proliferation rate was not higher than the one of HeLa-wild cells. In HeLa-wild cell high glucose level resulted in an induction of glutathione reductase activity in opposition to HeLa-tat cells where no change was observed. High glucose level resulted in 20% increase in GSSG/GSH ratio in HeLa-wild cells and 38% increase in HeLa-tat cells. Moreover, high glucose level resulted in a dramatic cytosolic thiol decrease and an important lipid peroxidation in HeLa-tat cells. No significant change of these two parameters was observed in HeLa-wild cells. In both cell lines, high glucose resulted in an increase of total SOD activity, as a consequence of the increase in Cu,Zn-SOD activity. High glucose did not result in an increase of Mn-SOD activity in both cell lines. As a consequence of tat tranfection Mn-SOD activity was 50% lower in HeLa-tat cells in comparison to HeLa-wild cells. This work emphasizes the importance of the antioxidant system in the glucose induced cytotoxicity. 相似文献
The DNA‐binding protein TRF2 is essential for telomere protection and chromosome stability in mammals. We show here that TRF2 expression is activated by the Wnt/β‐catenin signalling pathway in human cancer and normal cells as well as in mouse intestinal tissues. Furthermore, β‐catenin binds to TRF2 gene regulatory regions that are functional in a luciferase transactivating assay. Reduced β‐catenin expression in cancer cells triggers a marked increase in telomere dysfunction, which can be reversed by TRF2 overexpression. We conclude that the Wnt/β‐catenin signalling pathway maintains a level of TRF2 critical for telomere protection. This is expected to have an important role during development, adult stem cell function and oncogenesis. 相似文献
Among the identified risk factors of age-related macular degeneration, sunlight is known to induce cumulative damage to the retina. A photosensitive derivative of the visual pigment, N-retinylidene-N-retinylethanolamine (A2E), may be involved in this phototoxicity. The high energy visible light between 380 nm and 500 nm (blue light) is incriminated. Our aim was to define the most toxic wavelengths in the blue-green range on an in vitro model of the disease. Primary cultures of porcine retinal pigment epithelium cells were incubated for 6 hours with different A2E concentrations and exposed for 18 hours to 10 nm illumination bands centered from 380 to 520 nm in 10 nm increments. Light irradiances were normalized with respect to the natural sunlight reaching the retina. Six hours after light exposure, cell viability, necrosis and apoptosis were assessed using the Apotox-Glo Triplex™ assay. Retinal pigment epithelium cells incubated with A2E displayed fluorescent bodies within the cytoplasm. Their absorption and emission spectra were similar to those of A2E. Exposure to 10 nm illumination bands induced a loss in cell viability with a dose dependence upon A2E concentrations. Irrespective of A2E concentration, the loss of cell viability was maximal for wavelengths from 415 to 455 nm. Cell viability decrease was correlated to an increase in cell apoptosis indicated by caspase-3/7 activities in the same spectral range. No light-elicited necrosis was measured as compared to control cells maintained in darkness. Our results defined the precise spectrum of light retinal toxicity in physiological irradiance conditions on an in vitro model of age-related macular degeneration. Surprisingly, a narrow bandwidth in blue light generated the greatest phototoxic risk to retinal pigment epithelium cells. This phototoxic spectrum may be advantageously valued in designing selective photoprotection ophthalmic filters, without disrupting essential visual and non-visual functions of the eye. 相似文献
Bonobos (Pan paniscus) inhabit regions south of the Congo River including all areas between its southerly tributaries. To investigate the genetic diversity and evolutionary relationship among bonobo populations, we sequenced mitochondrial DNA from 376 fecal samples collected in seven study populations located within the eastern and western limits of the species’ range. In 136 effective samples from different individuals (range: 7–37 per population), we distinguished 54 haplotypes in six clades (A1, A2, B1, B2, C, D), which included a newly identified clade (D). MtDNA haplotypes were regionally clustered; 83 percent of haplotypes were locality-specific. The distribution of haplotypes across populations and the genetic diversity within populations thus showed highly geographical patterns. Using population distance measures, seven populations were categorized in three clusters: the east, central, and west cohorts. Although further elucidation of historical changes in the geological setting is required, the geographical patterns of genetic diversity seem to be shaped by paleoenvironmental changes during the Pleistocene. The present day riverine barriers appeared to have a weak effect on gene flow among populations, except for the Lomami River, which separates the TL2 population from the others. The central cohort preserves a high genetic diversity, and two unique clades of haplotypes were found in the Wamba/Iyondji populations in the central cohort and in the TL2 population in the eastern cohort respectively. This knowledge may contribute to the planning of bonobo conservation. 相似文献