Detection of mutations in transgenic fish carrying a bacteriophage lambda cII transgene target

Exploration of space to achieve scientific breakthroughs

Living organisms adapt to changing environments using their amazing flexibility to remodel themselves by a process called evolution. Environmental stress causes selective pressure and is associated with genetic and phenotypic shifts for better modifications, maintenance, and functioning of organismal systems. The natural evolution process can be used in complement to rational strain engineering for the development of desired traits or phenotypes as well as for the production of novel biomaterials through the imposition of one or more selective pressures. Space provides a unique environment of stressors (e.g., weightlessness and high radiation) that organisms have never experienced on Earth. Cells in the outer space reorganize and develop or activate a range of molecular responses that lead to changes in cellular properties. Exposure of cells to the outer space will lead to the development of novel variants more efficiently than on Earth. For instance, natural crop varieties can be generated with higher nutrition value, yield, and improved features, such as resistance against high and low temperatures, salt stress, and microbial and pest attacks. The review summarizes the literature on the parameters of outer space that affect the growth and behavior of cells and organisms as well as complex colloidal systems. We illustrate an understanding of gravity-related basic biological mechanisms and enlighten the possibility to explore the outer space environment for application-oriented aspects. This will stimulate biological research in the pursuit of innovative approaches for the future of agriculture and health on Earth.

An Effective Protocol for Proteome Analysis of Medaka (Oryzias latipes) after Acute Exposure to Ionizing Radiation

All terrestrial organisms are subject to evolutionary pressures associated with natural sources of ionizing radiation (IR). The legacy of human-induced IR associated with energy, weapons production, medicine, and research has changed the distribution and magnitude of these evolutionary pressures. To date, no study has systematically examined the effects of environmentally relevant doses of radiation exposure across an organismal proteome. This void in knowledge has been due, in part, to technological deficiencies that have hampered quantifiable environmentally relevant IR doses and sensitive detection of proteomic responses. Here, we describe a protocol that addresses both needs, combining quantifiable IR delivery with a reliable method to yield proteomic comparisons of control and irradiated Medaka fish. Exposures were conducted at the Savannah River Ecology Laboratory (SREL, in Aiken, SC), where fish were subsequently dissected into three tissue sets (carcasses, organs and intestines) and frozen until analysis. Tissue proteins were extracted, resolved by Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis (SDS-PAGE), and each sample lane was divided into ten equal portions. Following in-gel tryptic digestion, peptides released from each gel portion were identified and quantified by Liquid Chromatography-Mass Spectrometry (LC-MS/MS) to obtain the most complete, comparative study to date of proteomic responses to environmentally relevant doses of IR. This method provides a simple approach for use in ongoing epidemiologic studies of chronic exposure to environmentally relevant levels of IR and should also serve well in physiological, developmental, and toxicological studies.

The architecture of intra-organism mutation rate variation in plants

Given the disposability of somatic tissue, selection can favor a higher mutation rate in the early segregating soma than in germline, as seen in some animals. Although in plants intra-organismic mutation rate heterogeneity is poorly resolved, the same selectionist logic can predict a lower rate in shoot than in root and in longer-lived terminal tissues (e.g., leaves) than in ontogenetically similar short-lived ones (e.g., petals), and that mutation rate heterogeneity should be deterministic with no significant differences between biological replicates. To address these expectations, we sequenced 754 genomes from various tissues of eight plant species. Consistent with a selectionist model, the rate of mutation accumulation per unit time in shoot apical meristem is lower than that in root apical tissues in perennials, in which a high proportion of mutations in shoots are themselves transmissible, but not in annuals, in which somatic mutations tend not to be transmissible. Similarly, the number of mutations accumulated in leaves is commonly lower than that within a petal of the same plant, and there is no more heterogeneity in accumulation rates between replicate branches than expected by chance. High mutation accumulation in runners of strawberry is, we argue, the exception that proves the rule, as mutation transmission patterns indicate that runner has a restricted germline. However, we also find that in vitro callus tissue has a higher mutation rate (per unit time) than the wild-grown comparator, suggesting nonadaptive mutational "fragility". As mutational fragility does not obviously explain why the shoot-root difference varies with plant longevity, we conclude that some mutation rate variation between tissues is consistent with selectionist theory but that a mechanistic null of mutational fragility should be considered.

Subchronic perfluorooctanesulfonate (PFOS) exposure induces elevated mutant frequency in an in vivo λ transgenic medaka mutation assay

Perfluorooctanesulfonate (PFOS) has been widely detected in the environment, wildlife and humans, but few studies have ever examined its mutagenic effect in vivo. In the present study, we use a transgenic fish model, the λ transgenic medaka, to evaluate the potential mutagenicity of PFOS in vivo following a subchronic exposure of 30 days. The mutant frequency of cII target gene was 3.46 × 10^-5 in liver tissue from control fish, which increased by 1.4-fold to 4.86 × 10^-5 in fish exposed to 6.7 μg/L PFOS, 1.55-fold to 5.36 × 10^-5 in fish exposed to 27.6 μg/L PFOS, and 2.02-fold to 6.99 × 10^-5 in fish exposed to 87.6 μg/L PFOS. This dose-dependent increase of mutant frequency was also accompanied with mutational spectrum changes associated with PFOS exposure. In particular, PFOS-induced mutation was characterized by +1 frameshift mutations, which increased from 0% in control fish to 13.2% in fish exposed to 27.6 μg/L PFOS and 14.6% in fish exposed to 87.6 μg/L PFOS. Our findings provide the first evidence of PFOS's mutagenicity in an aquatic model system. Given the fact that most conventional mutagenic assays were negative for PFOS, we propose that PFOS-induced mutation in liver tissue of λ transgenic medaka may be mediated through compromised liver function.

Comparative Sigma Factor-mRNA Levels in Mycobacterium marinum under Stress Conditions and during Host Infection

We have used RNASeq and qRT-PCR to study mRNA levels for all σ-factors in different Mycobacterium marinum strains under various growth and stress conditions. We also studied their levels in M. marinum from infected fish and mosquito larvae. The annotated σ-factors were expressed and transcripts varied in relation to growth and stress conditions. Some were highly abundant such as sigA, sigB, sigC, sigD, sigE and sigH while others were not. The σ-factor mRNA profiles were similar after heat stress, during infection of fish and mosquito larvae. The similarity also applies to some of the known heat shock genes such as the α-crystallin gene. Therefore, it seems probable that the physiological state of M. marinum is similar when exposed to these different conditions. Moreover, the mosquito larvae data suggest that this is the state that the fish encounter when infected, at least with respect to σ-factor mRNA levels. Comparative genomic analysis of σ-factor gene localizations in three M. marinum strains and Mycobacterium tuberculosis H37Rv revealed chromosomal rearrangements that changed the localization of especially sigA, sigB, sigD, sigE, sigF and sigJ after the divergence of these two species. This may explain the variation in species-specific expression upon exposure to different growth conditions.

Chronic dietary exposure of zebrafish to PAH mixtures results in carcinogenic but not genotoxic effects

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous contaminants that can be present at high levels as mixtures in polluted aquatic environments. Many PAHs are potent mutagens and several are well-known carcinogens. Despite numerous studies on individual compounds, little is known about the toxicity of PAHs mixtures that are encountered in environmental situations. In the present work, zebrafish were continuously fed from 5 days post-fertilisation to 14 months post-fertilisation (mpf) with a diet spiked with fractions of either pyrolytic (PY), petrogenic light oil (LO), or petrogenic heavy oil (HO) origin at three concentrations. A decrease in survival was identified after 3 mpf in fish fed with the highest concentration of HO or LO, but not for PY. All PAH fractions caused preneoplastic and neoplastic disorders in long-term-exposed animals. Target tissues were almost exclusively of epithelial origin, with the bile duct epithelium being the most susceptible to chronic exposure to all PAH fractions, and with germ cells being the second most responsive cells. Significantly higher incidences of neoplasms were observed with increasing PAH concentration and exposure duration. The most severe carcinogenic effects were induced by dietary exposure to HO compared to exposure to LO or PY (45, 30 and 7 %, respectively, after 9 to 10 months of exposure to an intermediate concentration of PAHs). In contrast, earliest carcinogenic effects were detected as soon as 3 mpf after exposure to LO, including the lowest concentration, or to PY. PAH bioactivation and genotoxicity in blood was assessed by ethoxyresorufin-O-deethylase activity quantification and comet and micronuclei assays, respectively, but none of these were positive. Chronic dietary exposure of zebrafish to PAH mixtures results in carcinogenotoxic events that impair survival and physiology of exposed fish.

Quantifying in vivo somatic mutations using transgenic mouse model systems

This chapter describes the use of the bacteriophage cII positive selection somatic mutational assay with the Muta™Mouse transgenic model system. The assay is similar to others involving a transgenic target, including the cII and lacI assays in the Big Blue(®) Mouse, lacZ in the MutaMouse, and the gpt delta assay. Briefly, high-molecular-weight DNA is purified from the tissue of interest and used as substrate during in vitro packaging reactions, where the λ transgenes are excised from the genome and assembled into viable phage. Phage containing the mutational targets is then adsorbed into an appropriate bacterial host, and mutations sustained in vivo are detected and quantified by either standard recombinant screening or selection assays. Mutant frequencies are reported as the ratio of mutant phage to total phage units analyzed. The λ-based transgenic mouse assays are used to study and characterize in vivo mutagenesis as well as for mutagenicity assessment of chemicals and other agents. These models permit the enumeration of mutations sustained in virtually any tissue of the mouse and are both sensitive and robust. Application of the assays is simple, not requiring resources beyond those commonly found in most academic laboratories.

Neoplasia and neoplasm-associated lesions in laboratory colonies of zebrafish emphasizing key influences of diet and aquaculture system design

During the past decade, the zebrafish has emerged as a leading model for mechanistic cancer research because of its sophisticated genetic and genomic resources, its tractability for tissue targeting of transgene expression, its efficiency for forward genetic approaches to cancer model development, and its cost effectiveness for enhancer and suppressor screens once a cancer model is established. However, in contrast with other laboratory animal species widely used as cancer models, much basic cancer biology information is lacking in zebrafish. As yet, data are not published regarding dietary influences on neoplasm incidences in zebrafish. Little information is available regarding spontaneous tumor incidences or histologic types in wild-type lines of zebrafish. So far, a comprehensive database documenting the full spectrum of neoplasia in various organ systems and tissues is not available for zebrafish as it is for other intensely studied laboratory animal species. This article confirms that, as in other species, diet and husbandry can profoundly influence tumor incidences and histologic spectra in zebrafish. We show that in many laboratory colonies wild-type lines of zebrafish exhibit elevated neoplasm incidences and neoplasm-associated lesions such as heptocyte megalocytosis. We present experimental evidence showing that certain diet and water management regimens can result in high incidences of neoplasia and neoplasm-associated lesions. We document the wide array of benign and malignant neoplasms affecting nearly every organ, tissue, and cell type in zebrafish, in some cases as a spontaneous aging change, and in other cases due to carcinogen treatment or genetic manipulation.

The challenges of implementing pathogen control strategies for fishes used in biomedical research

Over the past several decades, a number of fish species, including the zebrafish, medaka, and platyfish/swordtail, have become important models for human health and disease. Despite the increasing prevalence of these and other fish species in research, methods for health maintenance and the management of diseases in laboratory populations of these animals are underdeveloped. There is a growing realization that this trend must change, especially as the use of these species expands beyond developmental biology and more towards experimental applications where the presence of underlying disease may affect the physiology animals used in experiments and potentially compromise research results. Therefore, there is a critical need to develop, improve, and implement strategies for managing health and disease in aquatic research facilities. The purpose of this review is to report the proceedings of a workshop entitled "Animal Health and Disease Management in Research Animals" that was recently held at the 5th Aquatic Animal Models for Human Disease in September 2010 at Corvallis, Oregon to discuss the challenges involved with moving the field forward on this front.

Expression of common fluorescent reporters may modulate virulence for Mycobacterium marinum: dramatic attenuation results from Gfp over-expression

Mycobacterium marinum is an established surrogate pathogen for Mycobacterium tuberculosis because of its strong conservation of thousands of orthologous genes, lower risk to researchers and similar pathology in fish. This pathogen causes TB-like chronic disease in a wide variety of fish species. As in human TB, the microbe grows within the host macrophages, can mount life-long chronic infections and produces granulomatous lesions in target organs. One of the fish species known to manifest chronic "fish TB" is the small laboratory fish, Japanese ricefish (medaka; Oryzias latipes). Our laboratory is currently characterizing the disease progression in medaka using fluorescent reporter systems that are introduced into engineered strains of M. marinum. While conducting these studies we observed differences in growth, plasmid stability, and virulence depending on which fluorescent reporter construct was present. Here, we describe large negative effects on virulence and organ colonization that occurred with a commonly used plasmid pG13, that expresses green fluorescent protein (Gfp). The studies presented here, indicate that Gfp over-expression was the basis for the reduced virulence in this reporter construct. We also show that these negative effects could be reversed by significantly reducing Gfp expression levels or by using low-expression constructs of Rfp.

Neoplasia and neoplasm-associated lesions in laboratory colonies of zebrafish emphasizing key influences of diet and aquaculture system design

During the past decade, the zebrafish has emerged as a leading model for mechanistic cancer research because of its sophisticated genetic and genomic resources, its tractability for tissue targeting of transgene expression, its efficiency for forward genetic approaches to cancer model development, and its cost effectiveness for enhancer and suppressor screens once a cancer model is established. However, in contrast with other laboratory animal species widely used as cancer models, much basic cancer biology information is lacking in zebrafish. As yet, data are not published regarding dietary influences on neoplasm incidences in zebrafish. Little information is available regarding spontaneous tumor incidences or histologic types in wild-type lines of zebrafish. So far, a comprehensive database documenting the full spectrum of neoplasia in various organ systems and tissues is not available for zebrafish as it is for other intensely studied laboratory animal species. This article confirms that, as in other species, diet and husbandry can profoundly influence tumor incidences and histologic spectra in zebrafish. We show that in many laboratory colonies wild-type lines of zebrafish exhibit elevated neoplasm incidences and neoplasm-associated lesions such as heptocyte megalocytosis. We present experimental evidence showing that certain diet and water management regimens can result in high incidences of neoplasia and neoplasm-associated lesions. We document the wide array of benign and malignant neoplasms affecting nearly every organ, tissue, and cell type in zebrafish, in some cases as a spontaneous aging change, and in other cases due to carcinogen treatment or genetic manipulation.

A new spiked sediment assay using embryos of the Japanese medaka specifically designed for a reliable toxicity assessment of hydrophobic chemicals

Despite their low water solubility, hydrophobic pollutants are widespread in the aquatic environment and could represent a threat for living organisms. EU regulations on chemicals require accurate and reliable data on chemical toxicity. Current normalised fish toxicity assays, in particular those advocated by OECD guidelines, do not allow reliable toxicity assessment of hydrophobic compounds due to their low water solubility. In order to accurately evaluate the toxicity of this kind of compounds, a new spiked sediment assay using embryos of the Japanese medaka was developed. It consists of directly exposing fertilised eggs, during their entire embryonic development, onto the reference sediment spiked with the test compound. A large set of lethal or sublethal effects in embryos and newly hatched larvae, including non-invasive endpoints is analysed in order to maximise the sensitivity of the test. The approach was validated using four model pollutants with different modes of action: DMBA, PCB126, PCB153 and 4-nonylphenol (NP). All compounds, except PCB153, induced a dose-dependent increase in toxic effects. In fact, lethal effects only occurred at the highest tested concentration. In contrast, sub-lethal effects including skeletal deformations, cardiac activity modulation, body length reduction and hatching delay were observed at low to moderate concentrations of DMBA and PCB126. NP induced subtle effects in embryos, altering cardiac activity and hatching success but only at high concentrations. Although a few more improvements would make it a fully standardised assay, this spiked sediment assay using medaka embryos proves to be sensitive enough to measure hydrophobic chemical toxicity using an environmentally realistic mode of exposure.

Tolerance of spermatogonia to oxidative stress is due to high levels of Zn and Cu/Zn superoxide dismutase

Background

Spermatogonia are highly tolerant to reactive oxygen species (ROS) attack while advanced-stage germ cells such as spermatozoa are much more susceptible, but the precise reason for this variation in ROS tolerance remains unknown.

Methodology/Principal Findings

Using the Japanese eel testicular culture system that enables a complete spermatogenesis in vitro, we report that advanced-stage germ cells undergo intense apoptosis and exhibit strong signal for 8-hydroxy-2′-deoxyguanosine, an oxidative DNA damage marker, upon exposure to hypoxanthine-generated ROS while spermatogonia remain unaltered. Activity assay of antioxidant enzyme, superoxide dismutase (SOD) and Western blot analysis using an anti-Copper/Zinc (Cu/Zn) SOD antibody showed a high SOD activity and Cu/Zn SOD protein concentration during early spermatogenesis. Immunohistochemistry showed a strong expression for Cu/Zn SOD in spermatogonia but weak expression in advanced-stage germ cells. Zn deficiency reduced activity of the recombinant eel Cu/Zn SOD protein. Cu/Zn SOD siRNA decreased Cu/Zn SOD expression in spermatogonia and led to increased oxidative damage.

Conclusions/Significance

These data indicate that the presence of high levels of Cu/Zn SOD and Zn render spermatogonia resistant to ROS, and consequently protected from oxidative stress. These findings provide the biochemical basis for the high tolerance of spermatogonia to oxidative stress.

Evolution of the mutation rate

Understanding the mechanisms of evolution requires information on the rate of appearance of new mutations and their effects at the molecular and phenotypic levels. Although procuring such data has been technically challenging, high-throughput genome sequencing is rapidly expanding knowledge in this area. With information on spontaneous mutations now available in a variety of organisms, general patterns have emerged for the scaling of mutation rate with genome size and for the likely mechanisms that drive this pattern. Support is presented for the hypothesis that natural selection pushes mutation rates down to a lower limit set by the power of random genetic drift rather than by intrinsic physiological limitations, and that this has resulted in reduced levels of replication, transcription, and translation fidelity in eukaryotes relative to prokaryotes.

Isolated spermatozoa as indicators of mutations transmitted to progeny

Spermatozoa comprise a large and homogeneous population of cells that may serve as an alternative to resource-intensive assays of transmissible mutations based on progeny. To evaluate mutagenic responses in spermatozoa derived from germ cells exposed to a mutagen at different stages of spermatogenesis, we compared cII mutant frequencies (MFs) in spermatozoa collected from male lambda transgenic medaka exposed to ethylnitrosourea (ENU) as either post-meiotic or pre-meiotic germ cells. cII MFs in spermatozoa exposed to ENU as spermatogonial stem cells were induced significantly, 9-fold, compared to controls, whereas, cII MFs in spermatozoa exposed as spermatozoa/late spermatids were not elevated. To directly compare responses in spermatozoa with those in progeny, we analyzed cII MFs directly in spermatozoa and in the offspring produced from identical sperm samples of ENU-exposed males. cII MFs in isolated spermatozoa exposed to ENU as post-meiotic germ cells were not significantly elevated, whereas 11-30% of the progeny derived from the identically exposed germ cells exhibited significantly elevated cII MFs, approximately 2-fold to >130-fold, compared to controls. The contradictory responses between spermatozoa and progeny analyses can be attributed to induced pre-mutational lesions that remain intact in spermatozoa but were not detected as mutations. Progeny analyses, by contrast, revealed mutant individuals with elevated cII mutant frequencies because persistent DNA damage in the spermatozoa was fixed as mutations in cells of the early stage embryo. Spermatozoa exposed to a mutagen as spermatogonial stem cells can provide an efficient means to detect the portion of transmissible mutations that were fixed as mutations in spermatozoa. The caveat is that direct analyses of mutations in spermatozoa excludes the contribution of mutations that arise from post-fertilization processes in cells of early stage embryos, and therefore may underestimate the actual frequency of mutant offspring.

Toward a molecular equivalent dose: use of the medaka model in comparative risk assessment

Recent changes in the risk assessment landscape underscore the need to be able to compare the results of toxicity and dose-response testing between a growing list of animal models and, quite possibly, an array of in vitro screening assays. How do we compare test results for a given compound between vastly different species? For example, what dose level in the ambient water of a small fish model would be equivalent to 10 ppm of a given compound in the rat's drinking water? Where do we begin? To initially address these questions, and in order to compare dose-response tests in a standard rodent model with a fish model, we used the concept of molecular dose. Assays that quantify types of DNA damage that are directly relevant to carcinogenesis integrate the factors such as chemical exposure, uptake, distribution, metabolism, etc. that tend to vary so widely between different phyletic levels. We performed parallel exposures in F344 rats and Japanese medaka (Oryzias latipes) to the alkylating hepatocarcinogen, dimethylnitrosamine (DMN). In both models, we measured the DNA adducts 8-hydroxyguanine, N(7)-methylguanine and O(6)-methylguanine in the liver; mutation frequency using lambda cII transgenic medaka and lambda cII transgenic (Big Blue(R)) rats; and early morphological changes in the livers of both models using histopathology and immunohistochemistry. Pulse dose levels in fish were 0, 10, 25, 50, or 100 ppm DMN in the ambient water for 14 days. Since rats are reported to be especially sensitive to DMN, they received 0, 0.1, 1, 5, 10, or 25 ppm DMN in the drinking water for the same time period. While liver DNA adduct concentrations were similar in magnitude, mutant frequencies in the DMN-exposed medaka were up to 20 times higher than in the Big Blue rats. Future work with other compounds will generate a more complete picture of comparative dose response between different phyletic levels and will help guide risk assessors using "alternative" models.

Chronic Mycobacterium marinum infection acts as a tumor promoter in Japanese Medaka (Oryzias latipes)

An accumulating body of research indicates there is an increased cancer risk associated with chronic infections. The genus Mycobacterium contains a number of species, including M. tuberculosis, which mount chronic infections and have been implicated in higher cancer risk. Several non-tuberculosis mycobacterial species, including M. marinum, are known to cause chronic infections in fish and like human tuberculosis, often go undetected. The elevated carcinogenic potential for fish colonies infected with Mycobacterium spp. could have far reaching implications because fish models are widely used to study human diseases. Japanese medaka (Oryzias latipes) is an established laboratory fish model for toxicology, mutagenesis, and carcinogenesis; and produces a chronic tuberculosis-like disease when infected by M. marinum. We examined the role that chronic mycobacterial infections play in cancer risk for medaka. Experimental M. marinum infections of medaka alone did not increase the mutational loads or proliferative lesion incidence in all tissues examined. However, we showed that chronic M. marinum infections increased hepatocellular proliferative lesions in fish also exposed to low doses of the mutagen benzo[a]pyrene. These results indicate that chronic mycobacterial infections of medaka are acting as tumor promoters and thereby suggest increased human risks for cancer promotion in human populations burdened with chronic tuberculosis infections.

Effects of post-Hurricane Katrina New Orleans (LA, USA) sediments on early development of the Japanese medaka (Oryzias latipes)

When Hurricane Katrina struck the U.S. Gulf Coast, levees surrounding New Orleans, Louisiana, USA, were breached, leading to widespread flooding of the city and potential contamination from industrial spills, residential sources, and redistribution of pre-existing pollutants. We chemically characterized sediment samples from five New Orleans locations and used early development and mutagenesis in Japanese medaka (Oryzias latipes) as metrics of the toxic effects of these sediments. Sediment samples were analyzed for organohalogen pesticides, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and metals. One month after Hurricane Katrina, four of the five sites had unsafe concentrations of arsenic and one or more pesticides, pesticide metabolites, or polycyclic aromatic hydrocarbons. Medaka embryonic mortality and time to hatching both increased during exposure to aqueous extracts of sediments, with the greatest toxicity observed for the most heavily contaminated sediment. Exposure to sediment extracts did not, however, result in significantly elevated rates of mutagenesis. When the most contaminated site was resampled 4.5 months later, the sediment had lower contaminant concentrations and fewer deleterious effects on medaka development. Using the medaka bioassay, therefore, we demonstrate toxic effects of post-Hurricane Katrina sediments immediately following the storm, with some amelioration over time of contaminant concentrations and their negative biological effects.

Transgenic lambda medaka as a new model for germ cell mutagenesis

To address the need for improved approaches to study mutations transmitted to progeny from mutagen-exposed parents, we evaluated lambda transgenic medaka, a small fish that carries the cII mutation target gene, as a new model for germ cell mutagenesis. Mutations in the cII gene in progeny derived from ethyl-nitrosourea (ENU)-exposed males were readily detected. Frequencies of mutant offspring, proportions of mosaic or whole body mutant offspring, and mutational spectra differed according to germ cell stage exposed to ENU. Postmeiotic germ cells (spermatozoa/late spermatids) generated a higher frequency of mutant offspring (11%) compared to premeiotic germ cells (3.5%). Individuals with cII mutant frequencies (MF) elevated more than threefold above the spontaneous MF (3 x 10(-5)) in the range of 10(-4) to 10(-3) were mosaic mutant offspring, whereas those with MFs approaching 1 x 10(-2) were whole body mutant offspring. Mosaic mutant offspring comprised the majority of mutant offspring derived from postmeiotic germ cells, and unexpectedly, from spermatogonial stem cells. Mutational spectra comprised of two different mutations, but at identical sites were unusual and characteristic of delayed mutations, in which fixation of a second mutation was delayed following fertilization. Delayed mutations and prevalence of mosaic mutant offspring add to growing evidence that implicates germ cells in mediating processes postfertilization that contribute to genomic instability in progeny. This model provides an efficient and sensitive approach to assess germ cell mutations, expands opportunities to increase understanding of fundamental mechanisms of mutagenesis, and provides a means for improved assessment of potential genetic health risks.

Fundulus as the premier teleost model in environmental biology: opportunities for new insights using genomics

A strong foundation of basic and applied research documents that the estuarine fish Fundulus heteroclitus and related species are unique laboratory and field models for understanding how individuals and populations interact with their environment. In this paper we summarize an extensive body of work examining the adaptive responses of Fundulus species to environmental conditions, and describe how this research has contributed importantly to our understanding of physiology, gene regulation, toxicology, and ecological and evolutionary genetics of teleosts and other vertebrates. These explorations have reached a critical juncture at which advancement is hindered by the lack of genomic resources for these species. We suggest that a more complete genomics toolbox for F. heteroclitus and related species will permit researchers to exploit the power of this model organism to rapidly advance our understanding of fundamental biological and pathological mechanisms among vertebrates, as well as ecological strategies and evolutionary processes common to all living organisms.

Characterization of toxic effects of sediment-associated organic pollutants using the lambda transgenic medaka

A novel sediment-contact assay using embryos of the transgenic medaka was developed to fully characterize the toxic effects induced by exposure to a mixture of organic pollutants in sediments. Embryos of the lambda transgenic medaka were exposed for 10 days to a clean reference sediment spiked with either the solvent alone, benzo[a]pyrene (B[a]P), or three concentrations (0.3x, 1x, and 2x) of an organic extract (OE) of sediments from the Seine estuary. The 1 x OE-spiked sediment contained concentrations of polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls similar to those in field-collected sediment. Exposure to this sediment, but not to the B[a]P-spiked sediment, significantly increased embryo-larval mortality and prevalence of spinal deformities. Mutant frequency at the cII mutation target gene in the liver of 10-week-old medaka was significantly increased following exposure to either B[a]P or the three doses of OE. The predominant OE-induced liver mutations were G:C to T:A transversions, consistent with PAHs being the major contributors to the mutation induction. Liver and gonadal tumors were observed in 35-week-old medaka exposed to either B[a]P (1/25) or to the 1 x OE (1/24). The benefits of medaka as a fish model for toxicological assessment and the benefits of the cII mutation assay for mutation detection combine to provide comprehensive assessment of a wide range of genotoxic and nongenotoxic effects of aquatic pollutants.

Tissue distribution and metabolism of benzo[a]pyrene in embryonic and larval medaka (Oryzias latipes)

The need to understand chemical uptake, distribution, and metabolism in embryonic and larval fish derives from the fact that these early life stages often exhibit greater sensitivity to xenobiotic compounds than do adult animals. In this study, a 6-h acute waterborne exposure immediately after fertilization was used to quickly load the egg with benzo[a]pyrene (BaP). This exposure was used to mimic the initial egg concentration of a persistent bioaccumulative toxicant that could result from maternal transfer. We used multiphoton laser scanning microscopy (MPLSM) in combination with conventional analytical chemistry methods to characterize the tissue distribution of BaP and its principal metabolites in medaka embryos and post-hatch larvae. Embryonic metabolism of BaP was evident by MPLSM prior to liver formation or heart development. A major product of this metabolism was identified by liquid chromatography/mass spectrometry as BaP-3-glucuronide. MPLSM showed that metabolites were sequestered within the yolk, biliary system, and gastrointestinal tract. When the gastrointestinal tract became patent a few days after hatch, the metabolites were rapidly eliminated. These findings indicate that some of the earliest embryonic tissues are metabolically competent and that redistribution of BaP and its metabolic products occurs throughout development. Rapid metabolism of BaP substantially reduces the body burden of parent chemical in the developing embryo, potentially reducing toxicity. It remains unclear whether metabolism of BaP in medaka embryos leads to the formation of DNA adducts associated with genotoxic effects or yields metabolites that later lead to other toxicity in juveniles or adults.

Mycobacterium marinum produces long-term chronic infections in medaka: a new animal model for studying human tuberculosis

Human infection by Mycobacterium tuberculosis is endemic, with approximately 2 billion infected and is the most common cause of adult death due to an infectious agent. Because of the slow growth rate of M. tuberculosis and risk to researchers, other species of Mycobacterium have been employed as alternative model systems to study human tuberculosis (TB). Mycobacterium marinum may be a good surrogate pathogen, conferring TB-like chronic infections in some fish. Medaka (Oryzias latipes) has been established for over five decades as a laboratory fish model for toxicology, genotoxicity, teratogenesis, carcinogenesis, classical genetics and embryology. We are investigating if medaka might also serve as a host for M. marinum in order to model human TB. We show that both acute and chronic infections are inducible in a dose dependent manner. Colonization of target organs and systemic granuloma formation has been demonstrated through the use of histology. M. marinum expressing green fluorescent protein (Gfp) was used to monitor bacterial colonization of these organs in fresh tissues as well as in intact animals. Moreover, we have employed the See-Through fish line, a variety of medaka devoid of major pigments, to monitor real-time disease progression, in living animals. We have also compared the susceptibility of another prominent fish model, zebrafish (Danio rerio), to our medaka-M. marinum model. We determined the course of infections in zebrafish is significantly more severe than in medaka. Together, these results indicate that the medaka-M. marinum model provides unique advantages for studying chronic mycobacteriosis.

Uptake, metabolism, mutant frequencies and mutational spectra in lambda transgenic medaka embryos exposed to benzo[alpha]pyrene dosed sediments

The goal of this study was to provide data supporting the use of lambda transgenic medaka (Oryzias latipes) embryos to evaluate mutagens in sediments. Embryos incubated directly on sediments dosed with the reference mutagen, benzo[alpha]pyrene (BaP), were examined for BaP uptake and metabolism. Mutant frequency and mutational spectrum were assessed in the cII transgene recovered from adult medaka livers exposed as embryos. Embryos rapidly accumulated 14C-BaP and metabolized BaP to polar metabolites, indicating sediment-sorbed BaP is available for bioaccumulation and medaka embryos are capable of bioactivating this mutagen. Exposure of embryos to BaP dosed sediments significantly induced cII transgene mutant frequencies with mutations predominantly being in G:C base pairs, consistent with known mechanisms of BaP mutagenesis in transgenic mice and fish.

Sub-chronic exposure to 1,1-dichloropropene induces frameshift mutations in lambda transgenic medaka

1,1-Dichloropropene (1,1-DCP) is a contaminant present in both ground and surface waters used as sources for drinking water. Structural similarity to several compounds with known mutagenicity and carcinogenicity, and recent demonstration of mutagenicity in vitro, suggest this compound may be similarly mutagenic in vivo. A transgenic fish model, the lamda transgenic medaka, was used to evaluate the potential mutagenicity of this contaminant in vivo following sub-chronic exposure for 6 weeks. Mutant frequencies of the cII target gene (MF) increased six-fold in the livers of fish exposed to the lowest 1,1-DCP exposure concentration (0.44 mg/L, MF = 18.4 x 10(-5), and increased with each treatment, culminating in a 32-fold induction in fish from the highest 1,1-DCP treatment (16.60 mg/L, MF = 96.3 x 10(-5). Mutations recovered from treated fish showed a distinctive mutational spectrum comprised predominantly of +1 frameshift mutations, induced 166-fold above that of untreated animals. The majority of frameshifts were +1 insertions at thiamine and adenine. These results represent the first evidence of mutagenicity of 1,1-DCP in vivo, and of the highly characteristic spectrum of induced mutations dominated by +1 frameshift mutations. Based upon results from previous in vitro studies, the similar role of glutathione S-transferase (GSTT1-1) in the activation of 1,1-DCP to a mutagen in vivo is also suggested. This study further illustrates the utility of the lamda transgenic medaka as a model for identifying and characterizing potential genetic health risks associated with chemical exposures in the environment.

Antimutagenicity of green tea polyphenols in the liver of transgenic medaka

We examined the ability of a mixture of the predominant green tea polyphenolic compounds (GTP) to reduce benzo[a]pyrene (B[a]P)-induced mutations in the cII gene of the lambda transgenic medaka. Fish were treated with 50 ppb B[a]P for 24 hr, followed by exposure to 2 ppm or 10 ppm GTP for 28 days. cII mutations in livers of fish exposed to B[a]P were increased significantly, 2.6-fold above controls. In contrast, the addition of GTP significantly reduced the frequency of cII mutants by 84%, comparable to that of controls. The frequencies of mutations at G:C basepairs, mutations that are highly characteristic of B[a]P exposure, were elevated significantly in treated fish. By comparison, B[a]P-exposed fish also treated with GTP showed reductions in these mutations, demonstrating a protective effect of GTP against B[a]P-induced mutagenesis. The antioxidant mechanism of GTP possibly played an important role in the reduction of B[a]P mutagenicity. These results corroborate findings from rodent models, showing that the protective effects of green tea extend to different species, and suggesting that similar mechanisms of B[a]P mutagenesis and GTP antimutagenesis are shared among the models. These studies illustrate the utility of lambda transgenic medaka for in vivo mutation analyses and suggest that this fish may be a valuable model in chemoprevention studies.

Nucleotide excision repair activity varies among murine spermatogenic cell types

Germ cells perform a unique and critical biological function: they propagate the DNA that will be used to direct development of the next generation. Genetic integrity of germ cell DNA is essential for producing healthy and reproductively fit offspring, and yet germ cell DNA is damaged by endogenous and exogenous agents. Nucleotide excision repair (NER) is an important mechanism for coping with a variety of DNA lesions. Little is known about NER activity in spermatogenic cells. We expected that germ cells would be more efficient at DNA repair than somatic cells, and that this efficiency may be reduced with age when the prevalence of spontaneous mutations increases. In the present study, NER was measured in defined spermatogenic cell types, including premeiotic cells (A and B type spermatogonia), meiotic cells (pachytene spermatocytes), and postmeiotic haploid cells (round spermatids) and compared with NER in keratinocytes. Global genome repair and transcription-coupled repair subpathways of NER were examined. All spermatogenic cell types from young mice displayed good repair of (6-4) pyrimidone photoproducts, although the repair rate was slower than in primary keratinocytes. In aged mice, repair of 6-4 pyrimidone photoproducts was depressed in postmeiotic cells. While repair of cyclobutane pyrimidine dimers was not detected in spermatogenic cells or in keratinocytes, the transcribed strands of active genes were repaired with greater efficiency than nontranscribed strands or inactive genes in keratinocytes and in meiotic and postmeiotic cells; spermatogonia displayed low to moderate ability to repair cyclobutane pyrimidine dimers on both DNA strands regardless of transcriptional status. Overall, the data suggest cell type-specific NER activity during murine spermatogenesis, and our results have possible implications for germ cell aging.

MNNG-induced mutations in the adult gill and hepatopancreas and in embryos of rpsL transgenic zebrafish

To evaluate the feasibility of a mutagenicity assay using adult rpsL transgenic zebrafish, 4- to 8-month-old females were exposed to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) (0, 15 or 30 mg/L in a water bath for 2 h). At 2 weeks after exposure, MNNG showed a concentration-dependent significant increase in mutant frequency (MF) of 8 x 10(-5), 18 x 10(-5), and 51 x 10(-5), respectively, in the gill. DNA sequencing revealed that 60-74% of the induced mutations were G:C to A:T transitions, consistent with the known mutagenic effects of MNNG. A marginal but significant increase in MF was observed in the hepatopancreas only in the group exposed to 30 mg/L, with the induction of some G:C to A:T transitions. A time-course of the appearance of mutations was determined in fish treated with 15 mg/L MNNG. In both, the gill and hepatopancreas, a higher MF was observed at 3 weeks than at 2 weeks, suggesting that an expression time of at least 3 weeks is preferable for the assay. When embryos (29 h post-fertilization) were exposed to MNNG (0, 50, and 150 mg/L) for 1 h, MFs increased significantly with an increase in the concentration of MNNG (5 x 10(-5), 40 x 10(-5), and 144 x 10(-5), respectively) at 3 days after exposure. G:C to A:T transitions were the predominant mutations, and these occurred at the same sites in the rpsL gene as in adult tissues. Thus, MNNG induces typical mutations in the gill and hepatopancreas of adult fish, and in embryos, suggesting that the rpsL zebrafish is a useful tool for monitoring genotoxicity caused by water-borne mutagens.

Achieving immortality in the C. elegans germline

Germline immortality is a topic that has intrigued theoretical biologists interested in aging for over a century. The germ cell lineage can be passed from one generation to the next, indefinitely. In contrast, somatic cells are typically only needed for a single generation and are then discarded. Germ cells may, therefore, harbor rejuvenation mechanisms that enable them to proliferate for eons. Such processes are thought to be either absent from or down-regulated in somatic cells, although cell non-autonomous forms of rejuvenation are formally possible. A thorough description of mechanisms that foster eternal youth in germ cells is lacking. The mysteries of germline immortality are being addressed in the nematode Caenorhabditis elegans by studying mutants that reproduce normally for several generations but eventually become sterile. The mortal germline mutants probably become sterile as a consequence of accumulating various forms of heritable cellular damage. Such mutants are abundant, indicating that several different biochemical pathways are required to rejuvenate the germline. Thus, forward genetics should help to define mechanisms that enable the germline to achieve immortality.

Resolving mechanisms of toxicity while pursuing ecotoxicological relevance?

In this age of modern biology, aquatic toxicological research has pursued mechanisms of action of toxicants. This has provided potential tools for ecotoxicologic investigations. However, problems of biocomplexity and issues at higher levels of biological organization remain a challenge. In the 1980s and 1990s and continuing to a lesser extent today, organisms residing in highly contaminated field sites or exposed in the laboratory to calibrated concentrations of individual compounds were carefully analyzed for their responses to priority pollutants. Correlation of biochemical and structural analyses in cultured cells and tissues, as well as the in vivo exposures led to the production and application of biomarkers of exposure and effect and to our awareness of genotoxicity and its chronic manifestations, such as neoplasms, in wild fishes. To gain acceptance of these findings in the greater environmental toxicology community, "validation of the model" versus other, better-established often rodent models, was necessary and became a major focus. Resultant biomarkers were applied to heavily contaminated and reference field sites as part of effects assessment and with investigations following large-scale disasters such as oil spills or industrial accidents. Over the past 15 years, in the laboratory, small aquarium fish models such as medaka (Oryzias latipes), zebrafish (Danio rerio), platyfish (Xiphophorus species), fathead minnow (Pimephales promelas), and sheepshead minnow (Cyprinodon variegatus) were increasingly used establishing mechanisms of toxicants. Today, the same organisms provide reliable information at higher levels of biological organization relevant to ecotoxicology. We review studies resolving mechanisms of toxicity and discuss ways to address biocomplexity, mixtures of contaminants, and the need to relate individual level responses to populations and communities.

A second life in science—working after the age of 65

I was born in January, 1921 and was fortunate in working for a research organization that had no fixed retirement age. I was permitted to continue Science as long as there were some resources to support research that had some relevance to the organization's goals. A number of projects on which I worked were continuations of ones begun before the age of 65 (1986) and several new ones were based on both previous interests and ideas and some on new ideas. A number of the ideas arose from participation on Committees of the US National Research Council. I was able to extend my earlier interests in DNA repair to include experiments on the variations in DNA repair among apparently normal humans. In collaborations with other researchers we showed that the repair abilities following exposures to chemicals or to ionizing or ultraviolet (UV) radiation did not follow Poisson distributions. I participated in experiments, using a fish model to estimate the wavelength ranges in sunlight responsible for inducing melanoma and another fish model to estimate the germ cell mutations that might arise from exposures to the heavily ionizing particles in cosmic rays beyond low Earth orbit. A transgenic fish model was used to investigate the possibilities of using the fish to assay for mutagens in sediments in Long Island Sound. These Reflections summarize the atmosphere necessary for a second life and the scientific results of this life.

MX [3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone], a drinking-water carcinogen, does not induce mutations in the liver of cII transgenic medaka (Oryzias latipes)

Mutagenicity assays with Salmonella have shown that 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX), a drinking-water disinfection by-product, is a potent mutagen, accounting for about one-third of the mutagenic potency/potential of chlorinated drinking water. The ability of MX to induce mutations was investigated in the liver of medaka (Oryzias latipes), a small fish model, utilizing the cII transgenic medaka strain that allows detection of in vivo mutations. Methylazoxymethanol acetate (MAMAc), a carcinogen in medaka, served as a positive control. Fish were exposed to MX at 0, 1, 10, or 30 mg/L for 96 h, whereas the MAMAc exposures were for 2 h at 0, 0.1, 1, or 10 mg/L. Both exposures were conducted under static water conditions and with fasted medaka. Following exposure, fish were returned to regular culture conditions to allow mutation expression for 15 or 40 d for MX or for 15 or 32 d for MAMAc. Mutations were not induced in medaka exposed to MX for 96 h. However, a concentration- and time-dependent increase in mutations was observed from the livers of fish exposed to 1 and 10 mg/L MAMAc. In conclusion, mutation induction was not observed in the livers of cII medaka exposed to MX for 96 h; however, studies are planned to examine mutation induction in the gills and skin to explore the possibility that MX-induced DNA damage occurs primarily in the tissues of initial contact.

Low Dose Rate Irradiation Facility: initial study on chronic exposures to medaka

Uncertainties associated with the effects from chronic low-level exposures to radiation prompted us to construct a Low Dose Rate Irradiation Facility (LoDIF). The facility was designed specifically to test the appropriateness of the 10 mGy d(-1) guideline often espoused as acceptable for protection of aquatic biota from ionizing radiation. Scientists at the 0.4 ha facility use 40 outdoor mesocosms and 137Cs irradiators of three different source strengths to research the effects of chronic low-level irradiation at different levels of biological organization. A description of the facility is included along with results from a pilot study in which Japanese medaka (a small fish native to Asia) were chronically irradiated at the highest dose rate possible within the facility (350+/-150 mGy d(-1)). Irradiated fish produced fewer eggs per day (p=0.03); had a lower percentage of viable eggs (p=0.04), and produced a lower percentage of hatchlings (p=0.05). Although these data are not surprising based on the relatively high dose rates, they are important to future work at the LoDIF because they confirm the utility of our chosen model organism for detecting population-level responses, and they illustrate the statistical power achieved from using replicated mesocosms, in that statistical significance was achieved with few replicates per treatment. Future directions for the LoDIF are presented, as well as an invitation for interested researchers to participate in our studies.

Molecular markers of endocrine disruption in aquatic organisms

A wide range of organic contaminant compounds prevalent in the aquatic environment has been shown to exhibit hormone-disrupting activity. The actual potency of such compounds are low compared with endogenous hormones, such as 17beta-estradiol, but may still produce detrimental biological effects. Induced hormone levels are routinely measured using commercial testing kits, though these fail to relate to actual effects. Field and laboratory studies on the biological effects of environmental estrogens have, in the past, largely relied on assays of vitellogenin (vtg) induction in male fish, reduced growth in testes formation, and intersex incidence. Here, we critically review the current and potential application of molecular techniques in assessing the adverse biological reproductive effects of endocrine-disrupting chemicals in aquatic organisms. The role of fish (estrogen, androgen, and progestogen) hormone receptors and invertebrate (ecdysone) hormone receptor, egg production (vtg and chorion) proteins, steroid biosynthesis enzymes (aromatase, sulfotransferase, and hydroxysteroid dehydrogenase), DNA damage, apoptosis, and their potential development as biomarkers are discussed in turn. In each case, the sequences characterized are presented and homologies across species are highlighted. Molecular methods of gauging vtg and zona radiata (ZR) expression and protein concentrations have included immunoassay and reverse transcription polymerase chain reaction (RT-PCR). Suggestions for the isolation for key gene expression products (aromatase, ZR, and vtg, for instance), from a wider range of fish species using degenerate primers, are given. Endocrine disruption in invertebrates has received less attention compared with fish, partly because the knowledge regarding invertebrate endocrinology is limited. Here we review and suggest alternate isolation strategies for key players in the imposex induction process: vitellin (Vn), aromatase, and Ala-Pro-Gly-Trp (APGW) amide neurohormone. Current molecular-level techniques rely on ligand-binding assays, enzyme-linked immunosorbent assay (ELISA), and, more recently, gene expression. In the future, more reliance will be placed on the development of gene expression assays using reporter systems combined with cross-species PCR-based or polyclonal antibody-based assays. We discuss the use of recombinant receptors as a means of primary screening of environmental samples for estrogenicity and antiestrogenicity, which avoids species and seasonal variation in receptor response to ligand binding, a recognized problem of earlier bioassays. Most exciting is the potential that microarray and proteomics approaches have to offer. Such techniques are now used routinely in medical research to identify specific genes and proteins affected by treatment with endocrine disruptors, including estradiol. The technique has yet to be used to screen aquatic organisms, but it has the potential to implicate previously unsuspected estradiol-sensitive genes that may later become molecular markers of endocrine disruption.

Fish can be first--advances in fish transgenesis for commercial applications

Over the past 15 years researchers have generated stable lines of several species of transgenic fish important for aquaculture. 'All-fish' growth hormone (GH) gene constructs and antifreeze protein (AFP) genes have been successfully introduced into the fish genome resulting in a significant acceleration of growth rate and an increase in cold and freeze tolerance. However, neither gene modification is completely understood; there are still questions to be resolved. Expression rates are still low, producing variable growth enhancement rates and less than desired levels of freeze resistance. Transgene strategies are also being developed to provide improved pathogen resistance and modified metabolism for better utilization of the diet. Additional challenges are to tailor the genetically modified fish strains to prevent release of the modified genes into the environment.

Acridine mutagenesis of zebrafish (Danio rerio)

Mutagenesis screening, in which heritable traits are isolated following damage to the genome, is a powerful approach for investigating gene function. Among vertebrate model organisms, the zebrafish (Danio rerio) is ideally suited to mutagenesis screens. The success of large-scale screens is dependent on the way in which changes are identified. The type of damage induced is also pivotal. Single base coding region deletions and insertions are suited to abolition of gene function whilst inducing a small physical alteration to the genome. Such mutations are not commonly found following mutagenesis schemes reported to date. Here, we show that an acridine mutagen, ICR191, which in other model organisms frequently induces single base deletions and insertions, is mutagenic in zebrafish. ICR191 induces hallmark phenotypes associated with genetic damage in treated embryos. Alterations are heritable. Offspring of mutagenised fish had mutations in a marker gene and were found to produce offspring with abnormal development. Using an adaptation of a molecular mutation detection method, fluorescent arbitrary primed PCR, we identified an induced alteration directly. The estimated frequency of induced mutations was sufficiently high to make it feasible to employ this approach for mutagenesis screening.

Germline genomes--a biological fountain of youth?

The fusion of male- and female-derived gametes initiates the phenomenal process of producing a highly complex mammalian organism. Successful reproduction is so important that mammals invoke a battery of protective mechanisms for the germ cell lineages that function to maximize genetic integrity while still allowing genetic diversity and adaptation. Protective mechanisms likely include, but are not limited to, robust DNA repair to safeguard genetic integrity and apoptosis to remove cells with intolerable levels of DNA damage. Analyses of spontaneous mutant frequencies are generally consistent with germline DNA being stringently maintained relative to somatic tissues. Despite the rigorous protection afforded germ cells, genetic integrity is observed to decline with increased maternal and paternal age. It is not yet clear whether cells in the germ line truly age or whether other processes decline or become dysfunctional with age. For example, in a younger animal, the differentiation and/or utilization of germ cells with lower genetic integrity might be disallowed, whereas in an older animal, such cells might slip past these quality-control mechanisms.

Base excision repair is limited by different proteins in male germ cell nuclear extracts prepared from young and old mice

The combined observations of elevated DNA repair gene expression, high uracil-DNA glycosylase-initiated base excision repair, and a low spontaneous mutant frequency for a lacI transgene in spermatogenic cells from young mice suggest that base excision repair activity is high in spermatogenic cell types. Notably, the spontaneous mutant frequency of the lacI transgene is greater in spermatogenic cells obtained from old mice, suggesting that germ line DNA repair activity may decline with age. A paternal age effect in spermatogenic cells is recognized for the human population as well. To determine if male germ cell base excision repair activity changes with age, uracil-DNA glycosylase-initiated base excision repair activity was measured in mixed germ cell (i.e., all spermatogenic cell types in adult testis) nuclear extracts prepared from young, middle-aged, and old mice. Base excision repair activity was also assessed in nuclear extracts from premeiotic, meiotic, and postmeiotic spermatogenic cell types obtained from young mice. Mixed germ cell nuclear extracts exhibited an age-related decrease in base excision repair activity that was restored by addition of apurinic/apyrimidinic (AP) endonuclease. Uracil-DNA glycosylase and DNA ligase were determined to be limiting in mixed germ cell nuclear extracts prepared from young animals. Base excision repair activity was only modestly elevated in pachytene spermatocytes and round spermatids relative to other spermatogenic cells. Thus, germ line short-patch base excision repair activity appears to be relatively constant throughout spermatogenesis in young animals, limited by uracil-DNA glycosylase and DNA ligase in young animals, and limited by AP endonuclease in old animals.

Mutational spectra of benzo[a]pyrene and MeIQx in rpsL transgenic zebrafish embryos

To evaluate the rpsL transgenic zebrafish (Brachydanio rerio) mutation assay, we treated the embryos with benzo[a]pyrene (B[a]P) (10 microg/ml) or 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) (300 microg/ml) for 16h and determined the mutation spectra. These treatments were previously reported to induce mutant frequencies that were 4.3 and 2.4 times the control value, respectively. In the B[a]P-treated group, half of the mutations were single base substitutions, 74% of which occurred at G:C base pairs. Among G:C base pair substitutions, G:C to T:A and G: C to C:G transversions were predominant, suggesting that B[a]P induced mutations in zebrafish embryos by mechanisms previously described in mammalian tissues. In the MeIQx-treated group, about 60% of the mutations were deletions. Some specific mutations were found, but the compound primarily amplified the background mutation level; improvement in the conditions of treatment may be required for elucidating MeIQx-mutagenesis in this system. This study showed that transgenic zebrafish may be a useful tool for detecting mutagens in aquatic environments and for elucidating mutagenic mechanisms.

Trangenic fish as models in environmental toxicology

Historically, fish have played significant roles in assessing potential risks associated with exposure to chemical contamination in aquatic environments. Considering the contributions of transgenic rodent models to biomedicine, it is reasoned that the development of transgenic fish could enhance the role of fish in environmental toxicology. Application of transgenic fish in environmental studies remains at an early stage, but recent introduction of new models and methods demonstrates progress. Rapid advances are most evident in the area of in vivo mutagenesis using fish carrying transgenes that serve as recoverable mutational targets. These models highlight many advantages afforded by fish as models and illustrate important issues that apply broadly to transgenic fish in environmental toxicology. Development of fish models carrying identical transgenes to those found in rodents is beneficial and has revealed that numerous aspects of in vivo mutagenesis are similar between the two classes of vertebrates. Researchers have revealed that fish exhibit frequencies of spontaneous mutations similar to rodents and respond to mutagen exposure consistent with known mutagenic mechanisms. Results have demonstrated the feasibility of in vivo mutation analyses using transgenic fish and have illustrated their potential value as a comparative animal model. Challenges to development and application of transgenic fish relate to the needs for improved efficiencies in transgenic technology and in aspects of fish husbandry and use. By taking advantage of the valuable and unique attributes of fish as test organisms, it is anticipated that transgenic fish will make significant contributions to studies of environmentally induced diseases.

Biomonitoring the genotoxicity of environmental factors with transgenic plants

All organisms must react to constantly changing surroundings. Environmental factors are thus powerful forces continuously shaping the genomes of all species. Induced genetic changes can be followed using a biomonitor - a living organism that reacts to a given compound in the environment. A vital but challenging task is identifying organisms with which to study the influence of changing environmental conditions. Plants are especially valuable biomonitors. Here, we describe the use of transgenic plant systems to evaluate the genotoxicity of chemical and radiological compounds. We evaluate the potential of further transgene-based systems for studying somatic and germ-line mutations.

Just how does the cII selection system work in Muta Mouse?

The lambda CII protein is an essential component in the lytic vs. lysogeny decision a bacteriophage makes upon infection of a host at low temperatures. The protein interacts with numerous phage promoters modulating the expression of the CI repressor, thus providing the mechanism for lysogenization soon after infection. The Big Blue and Muta Mouse are two widely used in vivo mutational model systems. The assays rely on retrievable lambda-based transgenes housing mutational targets (lacI or lacZ, respectively). The transgenes provide an elegant vehicle for the quantification of mutations sustained in virtually any tissue of the rodent. The use of the bacteriophage cII locus as an alternative, or additional mutational target for use with the Big Blue rodent system was first reported by Jakubczak et al. ([1996]: Proc Natl Acad Sci USA 93:9073-9078). More recently, this selection assay has been applied successfully to the Muta Mouse (Swiger et al. [1999]: Environ Mol Mutagen 33:201-207). The use of an Hfl bacterial strain and low temperature allows the determination of mutations sustained at the cII locus in either system, with high fidelity. The cII selection assay in the Big Blue relies on the presence of the lambda repressor protein CI. In contrast, the recombinant construct used to make the Muta Mouse transgene lacks functional CI protein. Nevertheless, we report an excellent system for quantifying mutations at the cII locus in Muta Mouse. Just how does cII selection work in the Muta Mouse? Written in the context of lambda recombinant genetics, this paper explores the question further.