ABSTRACT
In the course of analyzing the chemical composition of Alzheimer's disease neuritic and vascular amyloid, we have purified stable dimeric and trimeric components of Abeta peptides. These peptides (molecular mass 9.0 and 13.5 kDa) were separated by size exclusion chromatography in the presence of 80% formic acid or 5 guanidine thiocyanate, pH 7.4. The average ratio of monomers, dimers, and trimers was 55:30:15, respectively. Similar structures were produced over time upon incubation of synthetic Abeta-(1-42) at pH 7.4. The stability of these oligomeric forms was also demonstrated by Western blot and mass spectrometry. Atomic force microscopy and electron microscopy rotary shadowing revealed that the monomers polymerized into 8-10-nm filaments, whereas the dimers generated prolate ellipsoids measuring 3-4 nm in diameter. The pathogenic effects of the dimeric Abeta-(1-40/42) were tested in cultures of rat hippocampal neuron glia cells. Only in the presence of microglia did the dimer elicit neuronal killing. It is possible that these potentially pathogenic Abeta-(1-40/42) dimers and trimers from Alzheimer's disease amyloid represent the soluble oligomers of Abeta recently described in Alzheimer's disease brains (Kuo, Y.-M., Emmerling, M. R., Vigo-Pelfrey, C., Kasunic, T. C., Kirkpatrick, J. B., Murdoch, G. H., Ball, M. J., and Roher, A. E. (1996) J. Biol. Chem., 271, 4077-4081).
Subject(s)
Alzheimer Disease/pathology , Amyloid beta-Peptides/toxicity , Peptide Fragments/toxicity , Amyloid beta-Peptides/isolation & purification , Amyloid beta-Peptides/ultrastructure , Animals , Blood Vessels/chemistry , Brain Chemistry , Cells, Cultured , Microscopy, Atomic Force , Neurotoxins/chemistry , Peptide Fragments/isolation & purification , RatsABSTRACT
Carbon isotope ratios of mature leaves from the C(3) angiosperm root hemiparasites Striga hermonthica (Del.) Benth (-26.7 per thousand) and S. asiatica (L.) Kuntze (-25.6 per thousand) were more negative than their C(4) host, sorghum (Sorghum bicolor [L.] Moench cv CSH1), (-13.5 per thousand). However, in young photosynthetically incompetent plants of S. hermonthica this difference was reduced to less than 1 per thousand. Differences between the carbon isotope ratios of two C(3)-C(3) associations, S. gesnerioides (Willd.) Vatke-Vigna unguiculata (L.) Walp. and Oryza sativa L.-Rhamphicarpa fistulosa (Hochst.) Benth differed by less than 1 per thousand. Theoretical carbon isotope ratios for mature leaves of S. hermonthica and S. asiatica, calculated from foliar gas exchange measurements, were -31.8 and -32.0 per thousand, respectively. This difference between the measured and theoretical delta(13)C-values of 5 to 6 per thousand suggests that even in mature, photosynthetically active plants, there is substantial input of carbon from the C(4) host. We estimate this to be approximately 28% of the total carbon in S. hermonthica and 35% in S. asiatica. This level of carbon transfer contributes to the host's growth reductions observed in Striga-infected sorghum.
ABSTRACT
Gas exchange characteristics are reported for both members of the sorghum-Striga host-parasite association. Both Striga hermonthica (Del.) Benth and Striga asiatica (L.) Kuntze had transpiration rates considerably in excess of those of sorghum (Sorghum bicolor (L.) Moench, cv CSH1). Stomatal conductance in both Striga spp. showed little response to periods of darkness and moderate water stress. Low rates of net CO(2) fixation and high rates of dark respiration led to no net daily (24 hours) C gain, and Striga would appear to be reliant on its host for photosynthate. Infection of sorghum plants with either S. hermonthica or S. asiatica reduced host photosynthetic capacity. Infected sorghum plants were also more prone to water stress, but reduced rates of CO(2) fixation could not be accounted for in terms of lower stomatal conductance. Lower stomatal conductances were associated with an increase in water use efficiency (WUE) in uninfected sorghum; however, Striga-infected sorghum plants had lower WUE than those of uninfected plants. We suggest that Striga exerts a specific effect on processes affecting C acquisition in sorghum leaves. The water relations of S. hermonthica and S. asiatica are not characteristic of plants growing in semiarid environments and are more likely to reflect the nature of the parasitic life-style. Despite transfer of water and solutes from host to parasite, the reduction in C fixation observed in infected sorghum plants appears to be the major determinant of growth reductions observed in sorghum supporting Striga.
