Mechanisms of iron acquisition from siderophores by microorganisms and plants

Most bacteria, fungi, and some plants respond to Fe stress by the induction of high-affinity Fe transport systems that utilize biosyrthetic chelates called siderophores. To competitively acquire Fe, some microbes have transport systems that enable them to use other siderophore types in addition to their own. Bacteria such as Escherichia coli achieve this ability by using a combination of separate siderophore receptors and transporters, whereas other microbial species, such as Streptomyces pilosus, use a low specificity, high-affinity transport system that recognizes more than one siderophore type. By either strategy, such versatility may provide an advantage under Fe-limiting conditions; allowing use of siderophores produced at another organism's expense, or Fe acquisition from siderophores that could otherwise sequester Fe in an unavailable form.Plants that use microbial siderophores may also be more Fe efficient by virtue of their ability to use a variety of Fe sources under different soil conditions. Results of our research examining Fe transport by oat indicate parity in plant and microbial requirements for Fe and suggest that siderophores produced by root-colonizing microbes may provide Fe to plants that can use the predominant siderophore types. In conjunction with transport mechanisms, ecological and soil chemical factors can influence the efficacy of siderophores and phytosiderophores. A model presented here attempts to incorporate these factors to predict conditions that may govern competition for Fe in the plant rhizosphere. Possibly such competition has been a factor in the evolution of broad transport capabilities for different siderophores by microorganisms and plants.     

Predator-mediated coexistence: an equilibrium interpretation

I have constructed a spatially distributed analytical model of predators, superior prey competitors, and inferior prey competitors, based on the limiting deterministic version of a simulation model by Caswell (1978). Persistence regions for the three populations are mapped in parameter space. Conceptually shrinking the system from infinite size (i.e. infinitely many spatial “cells”) to some finite size introduces demographic stochasticity, increasing the chance of extinction of one or more populations within a given time interval. But some of the finite (stochastic) system's behavior, such as any tendency to damp perturbations, can be related to the behavior of the deterministic system at the same location in parameter space.     

Neuronal expression of myeloperoxidase is increased in Alzheimer's disease

Myeloperoxidase, a heme protein expressed by professional phagocytic cells, generates an array of oxidants which are proposed to contribute to tissue damage during inflammation. We now report that enzymatically active myeloperoxidase and its characteristic amino acid oxidation products are present in human brain. Further, expression of myeloperoxidase is increased in brain tissue showing Alzheimer's neuropathology. Consistent with expression in phagocytic cells, myeloperoxidase immunoreactivity was present in some activated microglia in Alzheimer brains. However, the majority of immunoreactive material in brain localized with amyloid plaques and, surprisingly, neurons including granule and pyramidal neurons of the hippocampus. Confirming neuronal localization of the enzyme, several neuronal cell lines as well as primary neuronal cultures expressed myeloperoxidase protein. Myeloperoxidase mRNA was also detected in neuronal cell lines. These results reveal the unexpected presence of myeloperoxidase in neurons. The increase in neuronal myeloperoxidase expression we observed in Alzheimer disease brains raises the possibility that the enzyme contributes to the oxidative stress implicated in the pathogenesis of the neurodegenerative disorder.     

Relative risk trees for censored survival data.

A method is developed for obtaining tree-structured relative risk estimates for censored survival data. The first step of a full likelihood estimation procedure is used in a recursive partitioning algorithm that adopts most aspects of the widely used Classification and Regression Tree (CART) algorithm of Breiman et al. (1984, Classification and Regression Trees, Belmont, California: Wadsworth). The performance of the technique is investigated through stimulation and compared to the tree-structured survival methods proposed by Davis and Anderson (1989, Statistics in Medicine 8, 947-961) and Therneau, Grambsch, and Fleming (1990, Biometrika 77, 147-160).     

Seeds of the annual grasses Schizachyrium spp. as a food resource for tropical granivorous birds

Seed dynamics of the annual tropical grasses Schizachyrium fragile (R. Br.) A. Camus, S. pachyarthron C. Gardner and S. pseudeulalia (Hosok.) S.T. Blake were studied with the aim of documenting fluxes in granivore food resources. In S. fragile, seed production began in the early dry season, and seed output was primarily influenced by seedling survival. Following seed-fall, there were 651 S. fragile seeds/m² (393 kernels/m²⁾ and 1014 S. pachyarthron seeds/m² (593 kernels/m²) across the study area, with a combined kernel biomass of 14.1 × 10³ g/ha. Seed densities remained stable through the dry season, then declined rapidly after wet season rain. Initial wet season rains of up to 25 mm, 40 mm and 50 mm resulted in seed-bank germination of < 5%, 57%, and 93%, respectively. Some seeds were buried by rain and resurfaced or germinated later, but about 30% was lost. Widespread and abrupt depletion of the seed-bank is likely to occur at the start of about 8% of wet seasons on central Cape York Peninsula, leaving little seed, either for subsequent germination or as food for granivores. Burning early in the dry season, when most seeds were still on the plants, reduced seed densities by 85%. The proportion of seeds with sound kernels was reduced in areas burnt by dry season fires, both directly after the fires and, subsequently, as a result of preferential granivore activity. Areas burnt in the dry season were thus depleted of seed earlier than were unburnt areas. Food availability for granivores should therefore be optimized by fire regimes that include a range of burning histories, including fires in both early dry and early wet seasons as well as keeping other areas unburnt.     

Stable carbon and nitrogen isotope enrichment in primate tissues

Isotopic studies of wild primates have used a wide range of tissues to infer diet and model the foraging ecologies of extinct species. The use of mismatched tissues for such comparisons can be problematic because differences in amino acid compositions can lead to small isotopic differences between tissues. Additionally, physiological and dietary differences among primate species could lead to variable offsets between apatite carbonate and collagen. To improve our understanding of the isotopic chemistry of primates, we explored the apparent enrichment (ε*) between bone collagen and muscle, collagen and fur or hair keratin, muscle and keratin, and collagen and bone carbonate across the primate order. We found that the mean ε* values of proteinaceous tissues were small (≤1‰), and uncorrelated with body size or phylogenetic relatedness. Additionally, ε* values did not vary by habitat, sex, age, or manner of death. The mean ε* value between bone carbonate and collagen (5.6 ± 1.2‰) was consistent with values reported for omnivorous mammals consuming monoisotopic diets. These primate-specific apparent enrichment values will be a valuable tool for cross-species comparisons. Additionally, they will facilitate dietary comparisons between living and fossil primates.     

Transforming growth factor-beta signaling helps specify tumor type in DMBA and hormone-induced mammary cancers

To determine the role of transforming growth factor-beta (TGF-beta) signaling in mammary development and tumor formation, we previously generated transgenic mice that expressed a dominant-negative form of the TGF-beta type II receptor (DNIIR) under the control of DNA regulatory elements from the metallothionein promoter (MT-DNIIR-28). In this report, we tested the hypothesis that loss of TGF-beta signaling in the mammary gland alters the development of chemically or hormonally induced tumors in mice. Four groups of mice were used in the study: wild-type and MT-DNIIR-28 mice on zinc with pituitary isograft, and wild-type and MT-DNIIR-28 mice on zinc with pituitary isograft treated with the carcinogen, 7,12-dimethylbenz[a]anthracene (DMBA). Tumor-free survival over time, tumor growth rate, and tumor pathology were measured. Statistically significant differences in tumor free survival over time or tumor growth rate were not detected in wild-type versus transgenic mice treated with DMBA. In contrast, tumor-free survival was significantly altered in transgenic mice that were treated with the pituitary isograft alone with MT-DNIIR mice developing tumors more quickly. Alterations in the types of tumors that formed in wild-type versus MT-DNIIR DMBA-treated mice were detected. In wild-type mice, tumors with squamous differentiation or bicellular adenomyoepitheliomas were most common. Adenomyoepitheliomas were not detected in transgenic mice. Furthermore, there was reduced staining for alpha smooth muscle actin and keratin 14, markers for myoepithelial cells, in the glandular portion of tumors in transgenic mice. The pathology of tumors induced by pituitary isograft alone was also markedly different in wild-type and transgenic mice. All the tumors classified from wild-type mice demonstrated some form of squamous differentiation, whereas squamous differentiation was not detected in the pituitary-induced transgenic tumors. The results suggest that TGF-beta acts as a tumor suppressor for hormone-induced cancers and that TGF-beta has a role in determining tumor pathology by regulating myoepithelial or squamous differentiation, maintenance, or transformation.     

Characterization of SPACR, a sialoprotein associated with cones and rods present in the interphotoreceptor matrix of the human retina: immunological and lectin binding analysis

Rod and cone photoreceptors project from the outer retinal surface into a carbohydrate-rich interphotoreceptor matrix (IPM). Unique IPM glycoconjugates are distributed around rods and cones. Wheat germ agglutinin (WGA) strongly decorates the rod matrix domains and weakly decorates the cone matrix domains. This study characterizes the major WGA-binding glycoprotein in the human IPM, which we refer to as SPACR (sialoprotein associated with cones and rods). SPACR, which has a molecular weight of 147 kDa, was isolated and purified from the IPM by lectin affinity chromatography. A polyclonal antibody to SPACR was prepared that colocalizes in tissue preparations with WGA-binding domains in the IPM. Sequential digestion of SPACR with N- and O- glycosidases results in a systematic increase in electrophorectic mobility, indicating the presence of both N- and O-linked glycoconjugates. Complete deglycosylation results in a reduction in the relative molecular mass of SPACR by about 30%. Analysis of lectin binding allowed us to identify some of the structural characteristics of SPACR glycoconjugates. Treatment with neuraminidase exposes Galbeta1- 3GalNAc disaccharide as indicated by positive peanut agglutinin (PNA) staining, accompanied by the loss of WGA staining. Maackia amurensis agglutinins (MAA-1 and MAA-2), specific for sialic acid in alpha2-3 linkage to Gal, bind SPACR, while Sambucus nigra agglutinin (SNA), specific for alpha2-6 linked sialic acid, does not, indicating that the dominant glycoconjugate determinant on SPACR is the O-linked carbohydrate, NeuAcalpha2-3Galbeta1-3GalNAc. The abundance of sialic acid in SPACR suggests that this glycoprotein may contribute substantially to the polyanionic nature of the IPM. The carbohydrate chains present on SPACR could also provide sites for extensive crosslinking and participate in the formation of the ordered IPM lattice that surrounds the elongate photoreceptors projecting from the outer retinal surface.      

The mechanism of excitation-contraction coupling in phenylephrine-stimulated human saphenous vein

The human saphenous vein (HSV) is the most widely used graft in coronary artery revascularization procedures and is susceptible to spasm perioperatively. The aim of this study is to elucidate the mechanism(s) of agonist-induced excitation-contraction coupling in this vessel. Isometric contraction experiments were combined with in situ smooth muscle intracellular Ca(2+) concentration ([Ca(2+)](i)) imaging by confocal microscopy of intact undistended HSV segments during activation with phenylephrine (PE; 50 microM). Stimulation with PE produced a sustained contraction. Preincubation with 5 microM nifedipine, a blocker of the L-type voltage-operated Ca(2+) channel, or 50 microM SKF-96365, a blocker of both the voltage- and receptor-operated channels, reduced force generation by 25-30%. Ca(2+) imaging revealed that PE elicited only a transient rise in [Ca(2+)](i), suggesting that Ca(2+) plays only a minor role. However, a requirement for basal Ca(2+) levels was demonstrated when PE contractions could not be maintained in Ca(2+)-free medium. In light of the transient Ca(2+) response, it appears that signals other than Ca(2+) must maintain the tonic contraction elicited by PE, such as those that sensitize the myofilaments to Ca(2+). Application of HA-1077 (a Rho kinase inhibitor) at the peak of the contraction completely abolished the plateau phase of the response, whereas application of genistein (a tyrosine kinase inhibitor) reduced this phase by approximately 50%. The foregoing results suggest that, whereas the transient Ca(2+) signal can contribute to the development of force, maintenance of the plateau phase of the PE contraction in the HSV is the result of myofilament Ca(2+) sensitization by Rho kinase and tyrosine phosphorylation. The elucidation of the mechanisms of excitation-contraction coupling in the HSV may be useful for the development of therapeutic strategies for the alleviation of vein graft spasm.