Tyro3 family-mediated cell entry of Ebola and Marburg viruses

Filoviruses, represented by the genera Ebolavirus and Marburgvirus, cause a lethal hemorrhagic fever in humans and in nonhuman primates. Although filovirus can replicate in various tissues or cell types in these animals, the molecular mechanisms of its broad tropism remain poorly understood. Here we show the involvement of members of the Tyro3 receptor tyrosine kinase family-Axl, Dtk, and Mer-in cell entry of filoviruses. Ectopic expression of these family members in lymphoid cells, which otherwise are highly resistant to filovirus infection, enhanced infection by pseudotype viruses carrying filovirus glycoproteins on their envelopes. This enhancement was reduced by antibodies to Tyro3 family members, Gas6 ligand, or soluble ectodomains of the members. Live Ebola viruses infected both Axl- and Dtk-expressing cells more efficiently than control cells. Antibody to Axl inhibited infection of pseudotype viruses in a number of Axl-positive cell lines. These results implicate each Tyro3 family member as a cell entry factor in filovirus infection.     

Optimal use of leaf nitrogen explains seasonal changes in leaf nitrogen content of an understorey evergreen shrub

Background and Aims

Understorey evergreen species commonly have a higher leaf nitrogen content in winter than in summer. Tested here is a hypothesis that such changes in leaf nitrogen content maximize nitrogen-use efficiency, defined as the daily carbon gain per unit nitrogen, under given temperature and irradiance levels.

Methods

The evergreen shrub Aucuba japonica growing naturally at three sites with different irradiance regimes in Japan was studied. Leaf photosynthetic characteristics, Rubisco and leaf nitrogen with measurements of temperature and irradiance monthly at each site were determined. Daily carbon gain was determined as a function of leaf nitrogen content to calculate the optimal leaf nitrogen content that maximized daily nitrogen-use efficiency.

Key Results

As is known, the optimal leaf nitrogen content increased with increasing irradiance. The optimal leaf nitrogen content also increased with decreasing temperature because the photosynthetic capacity per Rubisco decreased. Across sites and months, the optimal leaf nitrogen content was close to the actual leaf nitrogen content and explained the variation in actual leaf nitrogen by 64 %. Sensitivity analysis showed that the effect of temperature on optimal nitrogen content was similar in magnitude to that of irradiance.

Conclusions

Understorey evergreen species regulate leaf nitrogen content so as to maximize nitrogen-use efficiency in daily carbon gain under changing irradiance and temperature conditions.     

Identification of a novel streptococcal adhesin P (SadP) protein recognizing galactosyl-α1-4-galactose-containing glycoconjugates: convergent evolution of bacterial pathogens to binding of the same host receptor

Bacterial adhesion is often a prerequisite for infection, and host cell surface carbohydrates play a major role as adhesion receptors. Streptococci are a leading cause of infectious diseases. However, only few carbohydrate-specific streptococcal adhesins are known. Streptococcus suis is an important pig pathogen and a zoonotic agent causing meningitis in pigs and humans. In this study, we have identified an adhesin that mediates the binding of S. suis to galactosyl-α1-4-galactose (Galα1-4Gal)-containing host receptors. A functionally unknown S. suis cell wall protein (SSU0253), designated here as SadP (streptococcal adhesin P), was identified using a Galα1-4Gal-containing affinity matrix and LC-ESI mass spectrometry. Although the function of the protein was not previously known, it was recently identified as an immunogenic cell wall protein in a proteomic study. Insertional inactivation of the sadP gene abolished S. suis Galα1-4Gal-dependent binding. The adhesin gene sadP was cloned and expressed in Escherichia coli. Characterization of its binding specificity showed that SadP recognizes Galα1-4Gal-oligosaccharides and binds its natural glycolipid receptor, GbO(3) (CD77). The N terminus of SadP was shown to contain a Galα1-Gal-binding site and not to have apparent sequence similarity to other bacterial adhesins, including the E. coli P fimbrial adhesins, or to E. coli verotoxin or Pseudomonas aeruginosa lectin I also recognizing the same Galα1-4Gal disaccharide. The SadP and E. coli P adhesins represent a unique example of convergent evolution toward binding to the same host receptor structure.     

Plant responses to elevated CO<Subscript>2</Subscript> concentration at different scales: leaf,whole plant,canopy, and population

Elevated CO₂ enhances photosynthesis and growth of plants, but the enhancement is strongly influenced by the availability of nitrogen. In this article, we summarise our studies on plant responses to elevated CO₂. The photosynthetic capacity of leaves depends not only on leaf nitrogen content but also on nitrogen partitioning within a leaf. In Polygonum cuspidatum, nitrogen partitioning among the photosynthetic components was not influenced by elevated CO₂ but changed between seasons. Since the alteration in nitrogen partitioning resulted in different CO₂-dependence of photosynthetic rates, enhancement of photosynthesis by elevated CO₂ was greater in autumn than in summer. Leaf mass per unit area (LMA) increases in plants grown at elevated CO₂. This increase was considered to have resulted from the accumulation of carbohydrates not used for plant growth. With a sensitive analysis of a growth model, however, we suggested that the increase in LMA is advantageous for growth at elevated CO₂ by compensating for the reduction in leaf nitrogen concentration per unit mass. Enhancement of reproductive yield by elevated CO₂ is often smaller than that expected from vegetative growth. In Xanthium canadense, elevated CO₂ did not increase seed production, though the vegetative growth increased by 53%. As nitrogen concentration of seeds remained constant at different CO₂ levels, we suggest that the availability of nitrogen limited seed production at elevated CO₂ levels. We found that leaf area development of plant canopy was strongly constrained by the availability of nitrogen rather than by CO₂. In a rice field cultivated at free-air CO₂ enrichment, the leaf area index (LAI) increased with an increase in nitrogen availability but did not change with CO₂ elevation. We determined optimal LAI to maximise canopy photosynthesis and demonstrated that enhancement of canopy photosynthesis by elevated CO₂ was larger at high than at low nitrogen availability. We also studied competitive asymmetry among individuals in an even-aged, monospecific stand at elevated CO₂. Light acquisition (acquired light per unit aboveground mass) and utilisation (photosynthesis per unit acquired light) were calculated for each individual in the stand. Elevated CO₂ enhanced photosynthesis and growth of tall dominants, which reduced the light availability for shorter subordinates and consequently increased size inequality in the stand.     

Dependence of functional traits related to growth rates and their CO<Subscript>2</Subscript> response on multiple habitat climate factors across <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> populations

The values of many plant traits are often different even within a species as a result of local adaptation. Here, we studied how multiple climate variables influence trait values in Arabidopsis thaliana grown under common conditions. We examined 9 climate variables and 29 traits related to vegetative growth rate in 44 global A. thaliana accessions grown at ambient or elevated CO₂ concentration ([CO₂]) and applied a multiple regression analysis. We found that genetic variations in the traits related to growth rates were associated with various climate variables. At ambient [CO₂], plant size was positively correlated with precipitation in the original habitat. This may be a result of larger biomass investment in roots at the initial stage in plants adapting to a lower precipitation. Stomatal conductance and photosynthetic nitrogen use efficiency were negatively correlated with vapor pressure deficit, probably as a result of the trade-off between photosynthetic water- and nitrogen-use efficiency. These results suggest that precipitation and air humidity influence belowground and aboveground traits, respectively. Elevated [CO₂] altered climate dependences in some of the studied traits. The CO₂ response of relative growth rate was negatively correlated with altitude, indicating that plants inhabiting a higher altitude have less plasticity to changing [CO₂]. These results are useful not only for understanding evolutionary process but also to predict the plant species that are favored under future global change.     

Small-scale distributional dynamics of the yellow water-lily and its herbivore Galerucella nymphaeae (Coleoptera: Chrysomelidae)

Summary I followed the within-generation distribution of a chewing specialist herbivore, the water-lily beetle, on individually marked leaves of the yellow water-lily. Yellow water-lilies produced new leaves steadily throughout the growth season. Average leaf longevity was 3–4 weeks, much shorter than the developmental time of the beetle. The average egg-load of leaves was about 120. Leaf longevity was independent of egg density, but migration rate of the first instar larvae was density-dependent. Beetles occurred in every leaf, but consumed only a fraction (17%) of the available leaf area. However, this caused the leaf to lose its floating ability, so even this low rate of consumption made the leaf unavailable to herbivores. The herbivore population had to redistribute itself throughout the summer, escaping from the drowning leaves to fresh ones. No beetle could survive from egg to adult on a single leaf. The small-scale redistribution of the herbivore strongly affected the damage experienced by the host plant. In general, ability to redistribute depends on the dispersal ability of the herbivore, and thus migration ability may strongly affect the plant-herbivore interaction.     

The soluble serum protein Gas6 bridges virion envelope phosphatidylserine to the TAM receptor tyrosine kinase Axl to mediate viral entry

Virus entry into cells is typically initiated by binding of virally encoded envelope proteins to specific cell surface receptors. Studying infectivity of lentivirus pseudotypes lacking envelope binding, we still observed high infectivity for some cell types. On further investigation, we discovered that this infectivity is conferred by the soluble bovine protein S in fetal calf serum, or Gas6, its human homolog. Gas6 enhances native infectivity of pseudotypes of multiple viral envelope proteins. Gas6 mediates binding of the virus to target cells by bridging virion envelope phosphatidylserine to Axl, a TAM receptor tyrosine kinase on target cells. Phagocytic clearance of apoptotic cells is known to involve bridging by Gas6. Replication of vaccinia virus, which was previously reported to use apoptotic mimicry to enter cells, is also enhanced by Gas6. These results reveal an alternative molecular mechanism of viral entry that can broaden host range and enhance infectivity of enveloped viruses.