a b s t r a c tInhaled aerosol dose models play critical roles in medicine, the regulation of air pollutants and basic research. The models fall into several categories: traditional, computational fluid dynamical (CFD), physiologically based pharmacokinetic (PBPK), empirical, semi-empirical, and "reference". Each type of model has its strengths and weaknesses, so multiple models are commonly used for practical applications. Aerosol dose models combine information on aerosol behavior and the anatomy and physiology of exposed human and laboratory animal subjects. Similar models are used for in-vitro studies. Several notable advances have been made in aerosol dose modeling in the past 80 years. The pioneers include Walter Findeisen, who in 1935 published the first traditional model and established the structure of modern models. His model combined aerosol behavior with simplified respiratory tract structures. Ewald Weibel established morphometric techniques for the lung in 1963 that are still used to develop data for modeling today. Advances in scanning techniques have similarly contributed to the knowledge of respiratory tract structure and its use in aerosol dose modeling. Several scientists and research groups have developed and advanced traditional, CFD, and PBPK models. Current issues under study include understanding individual and species differences; examining localized particle deposition; modeling non-ideal aerosols and nanoparticle behavior; linking the regions of the respiratory tract airways from nasal-oral to alveolar; and developing sophisticated supporting software. Although a complete history of inhaled aerosol dose modeling is far too extensive to cover here, selected highlights are described in this paper.

The immutans (im) mutant of Arabidopsis shows a variegated phenotype comprising albino and green somatic sectors. We have cloned the IM gene by transposon tagging and show that even stable null alleles give rise to a variegated phenotype. The gene product has amino acid similarity to the mitochondrial alternative oxidase. We show that the IM protein is synthesized as a precursor polypeptide that is imported into chloroplasts and inserted into the thylakoid membrane. The albino sectors of im plants contain reduced levels of carotenoids and increased levels of the caro-tenoid precursor phytoene. The data presented here are consistent with a role for the IM protein as a cofactor for caro-tenoid desaturation. The suggested terminal oxidase function of IM appears to be essential to prevent photooxidative damage during early steps of chloroplast formation. We propose a model in which IM function is linked to phytoene de-saturation and, possibly, to the respiratory activity of the chloroplast.

The immutans ( im ) mutant of Arabidopsis shows a variegated phenotype comprising albino and green somatic sectors. We have cloned the IM gene by transposon tagging and show that even stable null alleles give rise to a variegated phenotype. The gene product has amino acid similarity to the mitochondrial alternative oxidase. We show that the IM protein is synthesized as a precursor polypeptide that is imported into chloroplasts and inserted into the thylakoid membrane. The albino sectors of im plants contain reduced levels of carotenoids and increased levels of the carotenoid precursor phytoene. The data presented here are consistent with a role for the IM protein as a cofactor for carotenoid desaturation. The suggested terminal oxidase function of IM appears to be essential to prevent photooxidative damage during early steps of chloroplast formation. We propose a model in which IM function is linked to phytoene desaturation and, possibly, to the respiratory activity of the chloroplast. INTRODUCTIONIn plant cells, plastid differentiation is intimately linked to organogenesis and is affected by both developmental regulatory mechanisms and environmental conditions. The complex mechanisms involved in the phototransformation of the plastids of dark-grown seedlings (etioplasts) to photosynthetically active plastids (chloroplasts) have been described extensively (reviewed in Mullet, 1988;Taylor, 1989;Chory and Susek, 1994). Alternatively, developing grass tissues provide a unique system in which a gradient of developmental stages of cells and plastids (proplastids to chloroplasts) is present (Mullet, 1988;Bilang and Bogorad, 1996; Inada et al., 1996).Mutants visibly impaired in chloroplast differentiation can be selected by their pale green, yellow, or albino color. The chlorophyll-deficient mutant olive of Antirrhinum has this phenotype (Hudson et al., 1993). Not all of these mutants are altered directly in pigment synthesis. Putative functions can be proposed for mutated genes based on the similarity of their coding sequences to known proteins. The Arabidopsis mutant albino3 (Sundberg et al., 1997) is impaired in chloroplast membrane biogenesis, as suggested by amino acid sequence similarity between ALBINO3 and the yeast mitochondrial OXA1 protein involved in mitochondrial biogenesis. The Arabidopsis cla1 mutation (Mandel et al., 1996) identifies an enzyme also present in cyanobacteria. Mutations also have been described that affect the proplastid-tochloroplast transition. The dag mutant from Antirrhinum (Chatterjee et al., 1996) and dcl1 from tomato (Keddie et al., 1996) do not contain chloroplasts but rather plastids resembling nondifferentiated proplastids. These genes together with the PALE CRESS gene from Arabidopsis (Reiter et al., 1994) affect both chloroplast development and leaf architecture.One class of mutations gives rise to variegated plants that have a mutant phenotype in some sectors and a wild-type phenotype in others. The iojap (Han et al., 1992) mutant of maize or the chloroplast mutator in ma...

Oxidative stress results when the balance between the production of reactive oxygen species (ROS) overrides the antioxidant capability of the target cell; oxidative damage from the interaction of reactive oxygen with critical cellular macromolecules may occur. ROS may interact with and modify cellular protein, lipid, and DNA, which results in altered target cell function. The accumulation of oxidative damage has been implicated in both acute and chronic cell injury including possible participation in the formation of cancer. Acute oxidative injury may produce selective cell death and a compensatory increase in cell proliferation. This stimulus may result in the formation of newly initiated preneoplastic cells and/or enhance the selective clonal expansion of latent initiated preneoplastic cells. Similarly, sublethal acute oxidative injury may produce unrepaired DNA damage and result in the formation of new mutations and, potentially, new initiated cells. In contrast, sustained chronic oxidative injury may lead to a nonlethal modification of normal cellular growth control mechanisms. Cellular oxidative stress can modify intercellular communication, protein kinase activity, membrane structure and function, and gene expression, and result in modulation of cell growth. We examined the role of oxidative stress as a possible mechanism by which nongenotoxic carcinogens may function. In studies with the selective mouse liver carcinogen dieldrin, a species-specific and dose-dependent decrease in liver antioxidant concentrations with a concomitant increase in ROS formation and oxidative damage was seen. This increase in oxidative stress correlated with an increase in hepatocyte DNA synthesis. Antioxidant supplementation prevented the dieldrin-induced cellular changes. Our findings suggest that the effect of nongenotoxic carcinogens (if they function through oxidative mechanisms) may be amplified in rodents but not in primates because of rodents' greater sensitivity to ROS. These results and findings reported by others support a potential role for oxidative-induced injury in the cancer process specifically during the promotion stage.ImagesFigure 5Figure 6Figure 9

Transportation stress increases serum concentrations of oxidative stress biomarkers that are related to episodes of BRD and mortality in calves.

Widespread antibiotic resistance is a major public health concern, and plants represent an emerging antibiotic exposure route. Recent studies indicate that crop plants fertilized with antibiotic-laden animal manure accumulate antibiotics; however, the molecular mechanisms of antibiotic entry and subcellular partitioning within plant cells remain unknown. Here, we report that mutations in the Arabidopsis (Arabidopsis thaliana) locus Multiple Antibiotic Resistance1 (MAR1) confer resistance, while MAR1 overexpression causes hypersensitivity to multiple aminoglycoside antibiotics. Additionally, yeast expressing MAR1 are hypersensitive to the aminoglycoside G418. MAR1 encodes a protein with 11 putative transmembrane domains with low similarity to ferroportin1 from Danio rerio. A MAR1:yellow fluorescent protein fusion localizes to the chloroplast, and chloroplasts from plants overexpressing MAR1 accumulate more of the aminoglycoside gentamicin, while mar1-1 mutant chloroplasts accumulate less than the wild type. MAR1 overexpression lines are slightly chlorotic, and chlorosis is rescued by exogenous iron. MAR1 expression is also down-regulated by low iron. These data suggest that MAR1 is a plastid transporter that is likely to be involved in cellular iron homeostasis and allows opportunistic entry of multiple antibiotics into the chloroplast.

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SummaryWe have used a universal adaptor ampli®cation procedure to isolate random Mutator-tagged fragments from Mutator-active maize plants. Direct sequence characterization of 761 Mutator-tagged fragments indicated that a signi®cant number were homologous to sequences within the public databases. The ability of Mutator-tagged fragments to detect homology was not related to the length of the sequence within the range 100±400 bp. However, fragments above this size did show an increased chance of detecting homology to either expressed sequence tags or genes. Characterization of the insertion sites of the Mutator elements suggested that while it does target transcribed regions, Mutator does not appear to have any site preference within the transcription unit. Hybridization of previously unidenti®ed Mutator-tagged fragments to arrayed cDNA libraries con®rmed that many of these also showed homology to transcribed regions of the genome. Examination of back-crossed progeny con®rmed that all the insertions examined were germinal; however, in all but one case, sel®ng ®ve individual Mutatortagged lines failed to reveal an obvious phenotype. This study suggests that the random sequencing of Mutator-tagged fragments is capable of producing both a signi®cant number of interesting transposon tagged genes and mutant plant lines, all of which could be extremely valuable in future gene discovery and functional genomics programmes.