Analysis of binding interactions of pepsin inhibitor-3 to mammalian and malarial aspartic proteases

The nematode Ascaris suum primarily infects pigs, but also causes disease in humans. As part of its survival mechanism in the intestinal tract of the host, the worm produces a number of protease inhibitors, including pepsin inhibitor-3 (PI3), a 17 kDa protein. Recombinant PI3 expressed in E. coli has previously been shown to be a competitive inhibitor of a subgroup of aspartic proteinases: pepsin, gastricsin and cathepsin E. The previously determined crystal structure of the complex of PI3 with porcine pepsin (p. pepsin) showed that there are two regions of contact between PI3 and the enzyme. The first three N-terminal residues (QFL) bind into the prime side of the active site cleft and a polyproline helix (139-143) in the C-terminal domain of PI3 packs against residues 289-295 that form a loop in p. pepsin. Mutational analysis of both inhibitor regions was conducted to assess their contributions to the binding affinity for p. pepsin, human pepsin (h. pepsin) and several malarial aspartic proteases, the plasmepsins. Overall, the polyproline mutations have a limited influence on the Ki values for all the enzymes tested, with the values for p. pepsin remaining in the low-nanomolar range. The largest effect was seen with a Q1L mutant, with a 200-fold decrease in Ki for plasmepsin 2 from Plasmodium falciparum (PfPM2). Thermodynamic measurements of the binding of PI3 to p. pepsin and PfPM2 showed that inhibition of the enzymes is an entropy-driven reaction. Further analysis of the Q1L mutant showed that the increase in binding affinity to PfPM2 was due to improvements in both entropy and enthalpy.     

Use of polymyxin B, levallorphan, and tetracaine to isolate novel envelope mutants of Escherichia coli.

Mutants of Escherichia coli were isolated by their resistance to the bacteriocidal effects of the membrane-active drugs polymyxin B, levallorphan, and tetracaine. The mutants were examined for additional changes in cellular physiology evoked by the lesions; many polymyxin-resistant strains had a concomitant increased sensitivity to anionic detergents, and several strains of each type had concomitant alterations in generation time and morphology. Mutants of each class (polymyxin resistant, tetracaine resistant, and levallorphan resistant) were transduced into recipient strains. The levallorphan resistance site (lev) was located at approximately 9 min on the E. coli chromosome. Polymyxin (pmx) and tetracaine (tec) resistance loci were also transduced. The lev and tec strains had a slight prolongation of generation time, in contrast with their isogenic wild-type strains. The tec transductant produced long filaments in the absence of tetracaine and had an altered colonial morphology, it reverted at high frequency, with the morphological abnormalities reverting along with the tetracaine resistance. The pmx transductant had an increased sensitivity to levallorphan and to anionic detergents. In contrast, both lev and tec mutants were more resistant to acriflavine than was the wild type or the pmx transductant. The pmx, lev, and tec loci differed in sensitivity to mitomycin C; the lev strain was more resistant, the tec strain was more sensitive, and the pmx strain was much more sensitive than the wild type. There was no difference in sensitivity to several other dyes and detergents, colicins, or T bacteriophage between the transductant and isogenic wild-type strains. Thus, lev, tec, and pmx loci confer more subtle alterations in the permeability barrier than do lipopolysaccharide-deficient mutants previously studied.     

Lipid and lipopolysaccharide composition of Escherichia coli surface-altered mutants selected for resistance to levallorphan, tetracaine, and polymyxin.

Certain mutants of Escherichia coli with an altered permeability barrier have an essentially normal lipopolysaccharide, fatty acid, and phospholipid content, with a slight increase in the membrane protein:lipid ratio. The phospholipid metabolism of the lev and tec strains shows an abnormal response to growth in the selective agents levallorphan and tetracaine, respectively.     

Depletion of natural killer cells does not result in neurologic disease due to Sarcocystis neurona in mice with severe combined immunodeficiency

Sarcocystis neurona is an apicomplexan parasite that is the primary etiologic agent of equine protozoal myeloencephalitis in horses. Protective immune responses in horses have not been determined, but interferon-gamma (IFN-gamma) is considered critical for protection from neurologic disease in mice. The role of adaptive and innate immune responses in control of parasites was explored by infecting BALB/c, IFN-gamma knockout (GKO), and severe combined immune deficient (SCID) mice with S. neurona (10(4) sporocysts/mouse). Immune competent BALB/c mice eliminated parasites within 30 days, with no sign of neurologic disease, whereas GKO mice developed fulminant neurologic disease. In contrast, SCID mice remained healthy throughout the experimental period despite the persistence of parasite at low levels in some mice. Treatment with anti-IFN-gamma antibody resulted in neurologic disease in infected SCID mice. Although SCID mice lack adaptive immune responses, they have natural killer (NK) cells capable of producing significant quantities of IFN-gamma. Therefore, SCID mice were infected with sporocysts of S. neurona and treated with anti-asialo GM1. Depletion of NK cells, confirmed by flow cytometry, did not result in neurologic disease in SCID mice. These results indicate that IFN-gamma mediates protection from neurologic disease in SCID mice. Protective levels of IFN-gamma may originate from a low number of nondepleted NK cells or from a non-T cell, non-NK cell population.     

Bivalve carrying capacity in coastal ecosystems

The carrying capacity of suspension feeding bivalvesin 11 coastal and estuarine ecosystems is examined. Bivalve carrying capacity is defined in terms of watermass residence time, primary production time andbivalve clearance time. Turnover times for the 11ecosystems are compared both two and threedimensionally. Fast systems, e.g., Sylt and NorthInlet, have turnover times of days or less, while,slow systems, e.g., Delaware Bay, have turnover timesof months and years. Some systems,Marennes-Oléron, South San Francisco Bay and NorthInlet, require a net influx of phytoplankton in orderto support their bivalve populations. Three systems,Carlingford Lough, Chesapeake Bay and Delaware Bay,have very long bivalve clearance times due to small orreduced bivalve filter feeder populations. Carlingford Lough stands out because it is a naturallyplanktonic system now being converted to bivalveculture with an adherently stronger benthic-pelagiccoupling. Existing models of bivalve carrying capacity arereviewed. The Herman model is utilized as anappropriate ecosystem level model to examine carryingcapacity because it includes the three major turnovertime elements of water mass residence time, primaryproduction time and bivalve filter feeder clearancetime. The graphical analysis suggests that massive andsuccessful bivalve filter feeder populations are foundin systems with relatively short residence times(<40 days) and short primary production times (<4days) in order to sustain a high bivalve biomass withits associated rapid clearance times. Outliersystems are constrained by long water mass residencetimes, extended primary production times, and longclearance times. This revised version was published online in August 2006 with corrections to the Cover Date.     

A review of the feedbacks between bivalve grazing and ecosystem processes

This paper gives an overview of interactions betweenbivalve grazing and ecosystem processes, that mayaffect the carrying capacity of ecosystems for bivalvesuspension feeders. These interactions consist of anumber of positive and negative feedbacks.Bivalve grazing can result in local food depletion,which may negatively influence bivalve growth. On alarger scale, it may induce a top-down control ofphytoplankton biomasss, and structural shifts inphytoplankton composition. In the case of harmfulalgal blooms, phytoplankton may negatively affectbivalve grazing rates.The processing of large amounts of particulate mattermay change nutrient cycling on the scale of estuaries,and can result in changes in the inorganic nutrientpool available for phytoplankton, through regenerationand reduced storage of nutrients in algal biomass.This can reduce nutrient limitation of thephytoplankton and stimulate algal growth rates.Observations from mesocosm studies suggest that apositive feedback from bivalve grazing onphytoplankton growth may also change the physiologicalstate of the algae and improve food quality. This revised version was published online in August 2006 with corrections to the Cover Date.     

UNUSUAL POPULATION GENETICS OF A PARASITIC NEMATODE: mtDNA VARIATION WITHIN AND AMONG POPULATIONS

Very little is known about the distribution of genetic variance within and among populations of parasitic helminths. In this study we used mitochondrial DNA (mtDNA) restriction fragment analysis to describe the population genetic structure of Ostertagia ostertagi, a nematode parasite of cattle, in the United States. Estimates of within-population mtDNA diversity are 5 to 10 times greater than typical estimates reported for species in other taxa. Although populations are genetically differentiated for a key life–history trait, greater than 98% of the total genetic diversity is partitioned within populations, and the geographic distribution of individual mtDNA haplotypes suggests high gene flow among populations.