Adverse outcome pathways (AOPs) for radiation-induced reproductive effects in environmental species: state of science and identification of a consensus AOP network

BACKGROUND

Reproductive effects of ionizing radiation in organisms have been observed under laboratory and field conditions. Such assessments often rely on associations between exposure and effects, and thus lacking a detailed mechanistic understanding of causality between effects occurring at different levels of biological organization. The Adverse Outcome Pathway (AOP), a conceptual knowledge framework to capture, organize, evaluate and visualize the scientific knowledge of relevant toxicological effects, has the potential to evaluate the causal relationships between molecular, cellular, individual, and population effects. This paper presents the first development of a set of consensus AOPs for reproductive effects of ionizing radiation in wildlife. This work was performed by a group of experts formed during a workshop organized jointly by the Multidisciplinary European Low Dose Initiative (MELODI) and the European Radioecology Alliance (ALLIANCE) associations to present the AOP approach and tools. The work presents a series of taxon-specific case studies that were used to identify relevant empirical evidence, identify common AOP components and propose a set of consensus AOPs that could be organized into an AOP network with broader taxonomic applicability.

CONCLUSION

Expert consultation led to the identification of key biological events and description of causal linkages between ionizing radiation, reproductive impairment and reduction in population fitness. The study characterized the knowledge domain of taxon-specific AOPs, identified knowledge gaps pertinent to reproductive-relevant AOP development and reflected on how AOPs could assist applications in radiation (radioecological) research, environmental health assessment, and radiological protection. Future advancement and consolidation of the AOPs is planned to include structured weight of evidence considerations, formalized review and critical assessment of the empirical evidence prior to formal submission and review by the OECD sponsored AOP development program.

Unusual evolution of tree frog populations in the Chernobyl exclusion zone

Despite the ubiquity of pollutants in the environment, their long‐term ecological consequences are not always clear and still poorly studied. This is the case concerning the radioactive contamination of the environment following the major nuclear accident at the Chernobyl nuclear power plant. Notwithstanding the implications of evolutionary processes on the population status, few studies concern the evolution of organisms chronically exposed to ionizing radiation in the Chernobyl exclusion zone. Here, we examined genetic markers for 19 populations of Eastern tree frog (Hyla orientalis) sampled in the Chernobyl region about thirty years after the nuclear power plant accident to investigate microevolutionary processes ongoing in local populations. Genetic diversity estimated from nuclear and mitochondrial markers showed an absence of genetic erosion and higher mitochondrial diversity in tree frogs from the Chernobyl exclusion zone compared to other European populations. Moreover, the study of haplotype network permitted us to decipher the presence of an independent recent evolutionary history of Chernobyl exclusion zone's Eastern tree frogs caused by an elevated mutation rate compared to other European populations. By fitting to our data a model of haplotype network evolution, we suspected that Eastern tree frog populations in the Chernobyl exclusion zone have a high mitochondrial mutation rate and small effective population sizes. These data suggest that Eastern tree frog populations might offset the impact of deleterious mutations because of their large clutch size, but also question the long‐term impact of ionizing radiation on the status of other species living in the Chernobyl exclusion zone.

Epidemiological studies of leukaemia in children and young adults around nuclear facilities: a critical review

The existence of an increased risk of childhood leukaemia near nuclear installations is a recurrent issue. A review of the related epidemiological literature is presented here. Results for 198 nuclear sites throughout 10 countries were included in the review. In addition to local studies, 25 multi-site studies have been published for eight countries. A large variability was noticed in the quality of the data as well as in the definition of the study population and in the methods of analysis. Many studies present important limits that make the results difficult to interpret. The review confirms that some clusters of childhood leukaemia cases exist locally. However, results based on multi-site studies around nuclear installations do not indicate an increased risk globally. Many studies were launched to investigate possible origins of the observed clusters around specific sites, but up to now, none of the proposed hypotheses have explained them.

Involvement of phospholipids in resistance and adaptation of Escherichia coli to acid conditions and to long-term survival

In Escherichia coli membranes, three major phospholipids are formed: phosphatidylethanolamine (PE), phosphatidylglycerol (PG) and cardiolipin (CL). We report here the survival of mutants lacking either PE or both PG and CL at an acid pHo and during long-term survival experiments. Stationary phase cultures of E. coli lacking PE are much more sensitive to acid shock (pHo 3) than the wild-type strain. Moreover, in the strain lacking PE, long-term survival in stationary phase is impaired and after 5 days no viable cells are recovered. The survival of an exponential phase culture to acid shock is known to be increased if the culture is exposed to moderately acid conditions (pHo 5) prior to a shift to pHo 3. If either PE or both PG and CL are missing, the exposure to pHo 5 does not increase the survival at pHo 3.

External-pH-dependent expression of the maltose regulon and ompF gene in Escherichia coli is affected by the level of glycerol kinase, encoded by glpK

The expression of the maltose system in Escherichia coli is regulated at both transcriptional and translational levels by the pH of the growth medium (pHo). With glycerol as the carbon source, transcription of malT, encoding the transcriptional activator of the maltose regulon, is weaker in acidic medium than in alkaline medium. malT transcription became high, regardless of the pHo, when glycerol-3-phosphate or succinate was used as the carbon source. Conversely, malT expression was low, regardless of the pHo, when maltose was used as the carbon source. The increase in malT transcription, associated with the pHo, requires the presence of glycerol in the growth medium and the expression of the glycerol kinase (GlpK). Changes in the level of glpK transcription had a great effect on malT transcription. Indeed, a glpFKX promoter-down mutation has been isolated, and in the presence of this mutation, malT expression was increased. When glpK was expressed from a high-copy-number plasmid, the glpK-dependent reduced expression of the maltose system became effective regardless of the pHo. Analysis of this repression showed that a malTp1 malTp10 promoter, which is independent of the cyclic AMP (cAMP)-cAMP receptor protein (CRP) complex, was no longer repressed by glpFKX amplification. Thus, GlpK-dependent repression of the maltose system requires the cAMP-CRP complex. We propose that the pHo may affect a complex interplay between GlpK, the phosphotransferase-mediated uptake of glucose, and the adenylate cyclase.

Involvement of carbon source and acetyl phosphate in the external-pH-dependent expression of porin genes in Escherichia coli

The porin composition of the Escherichia coli cell envelope was analyzed during growth at different external pHs (pHo) as a function of the acetyl phosphate (AcP) level (DeltaackA pta or ackA mutant, pyruvate or glucose as the carbon source) in the presence or absence of EnvZ. Our results indicate that the AcP level is influenced by the pHo, leading to modulation of the amount of OmpR-P and subsequent pHo-dependent expression of ompF and ompC. We also propose the existence of a specific signal, independent of EnvZ and AcP, leading to OmpR phosphorylation in response to pyruvate.

Analysis of the effect exerted by extracellular pH on the maltose regulon in Escherichia coli K-12

The Escherichia coli maltose regulon consists of five operons under the control of the MalT transcriptional activator. lac operon fusions were constructed in vitro with the MalT-dependent promoter and with the malT promoter itself. beta-Galactosidase activity displayed by these fusions during growth at different external pH (pHo) revealed that growth at a pHo higher than 6 stimulates the transcription of malT- and MalT-controlled genes in the absence or presence of maltose. Using a malTp1 malTp10 promoter that is cAMP-CRP (cAMP receptor protein)-independent, it was demonstrated that CRP is essential for malT pHo regulation and that the pHo-dependent activity of malKp is a direct consequence of malT regulation. The pHo regulation displayed by a deleted but still functional malT promoter fused to lacZ demonstrates that this minimal promoter contains all the regulatory regions for establishing pHo regulation. In the absence of MIc, a repressor of malT expression, the pHo regulation of malT was still effective. It is proposed that binding of cAMP-CRP at malTp may be affected by malTp topology induced by pHo or that a pHo-dependent effector may act in concert with the cAMP-CRP complex.

Characterization of IS199 from Streptococcus mutans V403

The complete nucleotide sequence of the insertion sequence IS199 has been determined. This novel element was found in Streptococcus mutans V403 where DNA hybridization studies suggested that it was present in multiple copies on the genome. IS199 is 1220 bp and appears unique to the mutans streptococci. A limited epidemiological survey revealed this element to occur infrequently in this group (4/50 strains examined). IS199 belongs to the IS3 family of procaryotic insertion sequence elements. It contained tandem open reading frames (ORFs) reminiscent of the structure of the IS3 class. Additionally, the larger of the two ORF products (ORFB) exhibited amino acid sequence similarities to comparable proteins from IS3 and other elements of this family. Transposition of IS199 gave rise to a 3-bp target site duplication. We were able to insert an antibiotic resistance gene into IS199 enabling us to follow the transposition of this element in S. mutans V403.

Immunologic characteristics of a Streptococcus mutans glucosyltransferase B sucrose-binding site peptide-cholera toxin B-subunit chimeric protein

Glucosyltranferases (Gtfs) produced by the mutans streptococci are recognized as virulence factors in dental caries, and the inhibition of Gtfs by secretory immunoglobulin A is predicted to provide protection against this disease. The basis of such mucosal immunity is linked to the ability to reliably stimulate production of secretory immunoglobulin A against Gtfs. In this regard, we are exploring the immunogenicities of various Gtf peptides genetically fused to the B subunit of cholera toxin (CTB), a known mucosal adjuvant. In this work, we have created a gene fusion linking the GtfB active-site (AS) peptide DANFDSIRVDAVDNVDADLLQIA to the amino terminus of CTB. This sequence, deduced from the nucleotide sequence of gtfB from Streptococcus mutans GS5, has been found to be strongly conserved in Gtfs from several mutans streptococci. We have purified this recombinant protein (AS:CTB) from Escherichia coli carrying the fusion gene under the control of the lactose operon promoter. This protein was immunogenic in rabbits and produced specific serum antibodies against both the Gtf peptide and the CTB moiety. The antiserum was tested for its ability to inhibit GtfB activity obtained from a mutant of S. mutans able to make only this enzyme and none of the other usual Gtfs or fructosyltransferase. Approximately 50% of the GtfB activity was inhibited in such assays. These results suggest that the AS of this enzyme is accessible to antibody binding and that this region of the protein may be considered a vulnerable target for vaccine design and development. The AS:CTB was able to bind GM1, ganglioside in enzyme-linked immunosorbent assays, indicating that the recombinant protein retained this property, which is though to be critical to the mucosal immunoadjuvant properties of CTB. Thus, this protein may be promising as a candidate anticaries vaccinogen alone or in combination with other Gtf peptides or conjugates.

Two cell-wall-associated aminopeptidases from Lactobacillus helveticus and the purification and characterization of APII from strain ITGL1

Lactobacillus helveticus ITGL1 is able to hydrolyse many amino-acyl and dipeptidyl-p-nitroanilides. Analysis of heat inactivation kinetics, metal ion and protease inhibitor effects, and the subcellular location of aminopeptidase activities in both the parental strain and mutants deficient in lysyl-p-nitroanilide hydrolysis, led to the characterization of two cell-wall-associated aminopeptidases, APII and APIV. APII, which catalysed L-lysine p-nitroanilide hydrolysis, was purified about 28-fold to homogeneity from cell-wall extracts of L. helveticus ITGL1 and characterized. The purified enzyme appeared to be monomeric, with a molecular mass of 97 kDa. Aminopeptidase activity was greatest at pH 6.5 and 50 degrees C. APII was completely inhibited by bestatin, chelating agents such as EDTA or 1,10-phenanthroline and the divalent cations Zn2+ and Cu2+. The activity of the EDTA-treated enzyme was restored by Co2+, Ca2+ or Mn2+. Although APII was able to degrade several dipeptides and tripeptides with hydrophobic N-terminal amino acid (Leu, Ala), it was inactive on peptides containing Pro or Gly, and may thus contribute to the development of cheese flavour by processing bitter peptides.

Cell-wall-associated proteinase of Lactobacillus delbrueckii subsp. bulgaricus CNRZ 397: differential extraction, purification and properties of the enzyme

Whole cells of Lactobacillus delbrueckii subsp. bulgaricus CNRZ 397 were able to hydrolyse alpha- and beta-caseins. Irrespective of the growth medium used, milk or De Man-Rogosa-Sharpe (MRS) broth, identical patterns of alpha- and beta-casein hydrolytic products, respectively, were visualized by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. A soluble proteinase present in cell-wall extracts was active on caseins and displayed the same hydrolytic patterns as whole cells. It was purified from cell-wall extract to homogeneity by ultrafiltration and ion exchange chromatography. The enzyme is a monomer with a molecular mass of 170 kDa, an optimum temperature of 42 degrees C and an optimum pH of 5.5. It was strongly activated by dithiothreitol and partially inhibited by E-64. These properties indicate that cysteine residues play an important role in the enzyme mechanism. The purified proteinase was not able to hydrolyse di- or tripeptides.

X-Prolyl-Dipeptidyl Aminopeptidase of Lactobacillus delbrueckii subsp. bulgaricus : Characterization of the Enzyme and Isolation of Deficient Mutants

Lactobacillus delbrueckii subsp. bulgaricus CNRZ 397 is able to hydrolyze X-proline- para -nitroanilides and X-proline-β-naphthylamides (X for alanyl- or glycyl-). A single metal-independent cytoplasmic enzyme with a molecular weight estimated to be 82,000 is responsible for these activities and was named X-prolyl-dipeptidyl aminopeptidase (X-Pro-DPAP). Isolation and analysis of mutants totally deficient for X-Pro-DPAP activity showed that a total lack of this enzyme induces (i) a decrease in the growth rate; (ii) an increase in cell wall proteinase activity; (iii) the loss of three cell wall proteins with respective molecular masses of 16, 40, and 52 kilodaltons; and (iv) enhancement of a cell wall protein with a molecular mass of 150 kilodaltons. The involvement of X-Pro-DPAP in casein catabolism is discussed.

Isolation and Characterization of Aminopeptidase-Deficient Lactobacillus bulgaricus Mutants

Lactobacillus bulgaricus CNRZ 397 is able to hydrolyze many amino-acyl- and dipeptidyl-β-naphthylamides. Analysis of heat inactivation kinetics, protease inhibitor effects, and the subcellular location of aminopeptidase (AP) activities from the parental strain and mutant derivatives dificient in alanyl- or leucyl-β-naphthylamide hydrolysis pointed out the existence of four APs. All mutants isolated were totally deficient in AP II, a cell wall metallo-enzyme with a broad substrate specificity but that is specifically responsible for lysyl-AP activity and is characterized by a molecular mass of 95,000 daltons. AP I and AP III are cytoplasmic enzymes that exhibit arginyl-AP activity; both enzymes are inducible during growth in rich peptide MRS medium (Difco Laboratories, Detroit, Mich.). The existence of a fourth AP (AP IV) that is involved in leucyl-AP activity was suggested. Moreover, we showed that X-prolyl-dipeptidyl-AP activity, which was not catalyzed by an AP, involved an enzyme(s) that is controlled by a regulatory mechanism that is common to that of AP II.