Determining K+ channel activation curves from K+ channel currents

Potassium ion channels are generally believed to have current-voltage (IV) relations which are linearly related to driving force ( V - E(K)), where V is membrane potential and E(K) is the potassium ion equilibrium potential. Consequently, activation curves for K+ channels have often been measured by normalizing voltage-clamp families of macroscopic K+ currents with (V - E(K)), where V is the potential of each successive step in the voltage clamp sequence. However, the IV relation for many types of K+ channels actually has a non-linear dependence upon driving force which is well described by the Goldman-Hodgkin-Katz relation. When the GHK dependence on (V - E(K)) is used in the normalization procedure, a very different voltage dependence of the activation curve is obtained which may more accurately reflect this feature of channel gating. Novel insights into the voltage dependence of the rapidly inactivating I(A) channels Kv1.4 and Kv4.2 have been obtained when this procedure was applied to recently published results.     

Buried polar interactions and conformational stability in the simian immunodeficiency virus (SIV) gp41 core

For human (HIV) and simian (SIV) immunodeficiency viruses, the gp41 envelope protein undergoes a receptor-activated conformational change from a labile native structure to an energetically more stable fusogenic conformation, which then mediates viral-cell membrane fusion. The core structure of fusion-active gp41 is a six-helix bundle in which three antiparallel carboxyl-terminal helices are packed against an amino-terminal trimeric coiled coil. Here we show that a recombinant model of the SIV gp41 core, designated N36(L6)C34, forms an alpha-helical trimer that exhibits a cooperative two-state folding-unfolding transition. We investigate the importance of buried polar interactions in determining the overall fold of the gp41 core. We have replaced each of four polar amino acids at the heptad a and d positions of the coiled coil in N36(L6)C34 with a representative hydrophobic amino acid, isoleucine. The Q565I, T582I, and T586I variants form six-helix bundle structures that are significantly more stable than that of the wild-type peptide, whereas the Q575I variant misfolds into an insoluble aggregate under physiological conditions. Thus, the buried polar residues within the amino-terminal heptad repeat are important determinants of the structural specificity and stability of the gp41 core. We suggest that these conserved buried polar interactions play a role in governing the conformational state of the gp41 molecule.     

EXPERIMENTAL EVIDENCE FOR HOST RACES IN MISTLETOE (PHORADENDRON TOMENTOSUM)

In order to test the hypothesis that mistletoes (Phoradendron tomentosum) are differentially adapted for the host species that they occur on, mistletoe seeds from the three most common hosts in central Texas (hackberry, Celtis occidentalis, elm, Ulmus crassifolia, and mesquite, Prosopsis glandulosa) were planted onto different individuals of each of the three hosts. Germination of seeds and formation of haustorial disks by seedlings were followed in the subsequent 17 months. Germination of seeds was very high for all nine combinations of maternal (source) and seed (experimental) hosts (range 82%–98%). There were no significant differences in seed germination between the two groups when source and experimental hosts were the same species and when they were different species. In contrast, development of haustorial disks when source and experimental hosts were the same species was significantly greater than when experimental host and source host species were different. The data suggest that populations of mistletoes are genetically differentiated such that early seedling development is greatest when there is correspondence between maternal and seedling host species.     

Distribution, abundance, and conservation of Vinaceous Amazons (Amazona vinacea) in Argentina and Paraguay

ABSTRACT.   Vinaceous Amazons ( Amazona vinacea ) are endemic to the Atlantic forest of southeastern Brazil, eastern Paraguay, and the province of Misiones in Argentina. We searched for Vinaceous Amazons throughout the western part of its range in Argentina and Paraguay during 1639 days of fieldwork from 1997 to 2006. These parrots have disappeared from most areas where they were historically recorded in these countries, and are now limited to a few sites in northeastern Paraguay and central Misiones (Argentina). We estimate the minimum remaining populations at 220 individuals in Paraguay and 203 individuals in Argentina. Important sites for the species are (1) the farming area from San Pedro to Tobuna (Misiones, Argentina) and (2) the Itaipú reserves complex and Reserva Natural Privada Itabó (Paraguay). In our surveys, Vinaceous Amazons were absent from the largest tracts of forest in Misiones, and were most often observed feeding, roosting, and nesting in small forest remnants and in agricultural areas that included forest fragments and isolated trees. Threats to amazons in these areas include nest poaching, forest clearing, and being shot as a crop pest. We confirmed 40 Vinaceous Amazons kept as pets in 35 homes between San Pedro and Tobuna. Environmental education and law enforcement are urgently needed to reduce threats in populated areas, and subsistence farmers need technical and logistical support to slow or stop the conversion of forest into cropland. Finally, additional study is needed to determine this amazon's habitat preferences, nest site requirements, and demography in different habitats.     

Nitrogen fertilizer and landscape position impacts on CO2 and CH4 fluxes from a landscape seeded to switchgrass

This study was conducted to evaluate the impacts of N fertilizer and landscape position on carbon dioxide (CO₂) and methane (CH₄) fluxes from a US Northern Great Plains landscape seeded to switchgrass (Panicum virgatum L.). The experimental design included three N levels (low, 0 kg N ha⁻¹; medium, 56 kg N ha⁻¹; and high, 112 kg N ha⁻¹) replicated four times. The experiment was repeated at shoulder and footslope positions. Soil CO₂ and CH₄ fluxes were monitored once every 2 weeks from May 2010 to October 2012. The CO₂ fluxes were 40% higher at the footslope than the shoulder landscape position, and CH₄ fluxes were similar in both landscape positions. Soil CO₂ and CH₄ fluxes averaged over the sampling dates were not impacted by N rates. Seasonal variations showed highest CO₂ release and CH₄ uptake in summer and fall, likely due to warmer and moist soil conditions. Higher CH₄ release was observed in winter possibly due to increased anaerobic conditions. However, year to year (2010–2012) variations in soil CO₂ and CH₄ fluxes were more pronounced than the variations due to the impact of landscape positions and N rates. Drought conditions reported in 2012, with higher annual temperature and lower soil moisture than long-term average, resulted in higher summer and fall CO₂ fluxes (between 1.3 and 3 times) than in 2011 and 2010. These conditions also promoted a net CH₄ uptake in 2012 in comparison to 2010 when there was net CH₄ release. Results from this study conclude that landscape positions, air temperature, and soil moisture content strongly influenced soil CO₂ fluxes, whereas soil moisture impacted the direction of CH₄ fluxes (uptake or release). However, a comprehensive life cycle analysis would be appropriate to evaluate environmental impacts associated with switchgrass production under local environmental conditions.     

Biochemical,biophysical, and genetic changes of porcine trophoblast‐derived stem‐like cells during differentiation as evaluated using Raman microspectroscopy,Atomic force microscopy,and quantitative polymerase chain reaction

Porcine trophoblast‐derived stem‐like cells grown into serum medium start to differentiate and become senescent within 30 days. However, trophoblast‐derived cells, cultured in vitro in a defined and non‐serum medium, have the regenerative properties, such as indefinite passage and foreign DNA receptivity, similar to stem cells. To evaluate the biochemical, biophysical, and genetic changes of the terminal differentiation of trophoblast derived cells, Raman microspectroscopy, atomic force microscopy, and qPCR were applied. It was found that Raman spectral intensities of characteristic peaks, cell morphology, and Young's modulus can be used to distinguish differentiated and undifferentiated trophoblast cells. In addition, 17 cytoskeleton and extracellular matrix‐related genes were significantly impacted by medium type (non‐serum versus serum). Our findings suggest that Raman microspectroscopy and atomic force microscopy—both considered as label‐free, non‐invasive techniques—can be applied to distinguish differentiated trophoblast cells, and cellular biochemical information and biophysical properties can be indicative of cellular differences during cell differentiation. In addition, most of cytoskeleton‐related genes exhibit similar pattern to that of Young's modulus during trophoblast cell differentiation, indicating the potential connection between cytoskeleton‐related genes and cellular stiffness. genesis 53:749–761, 2015. © 2015 Wiley Periodicals, Inc.