Concentrations of metallothionein-like proteins and heavy metals in the freshwater snail Lymnaea stagnalis exposed to different levels of waterborne cadmium

Sublethal cadmium exposure in the freshwater snail Lymnaea stagnalis meets a deficient, poorly responsive metallothionein system while evoking oxidative and cellular stress

The Great Pond snail Lymnaea stagnalis (Gastropoda, Hygrophila) is a wide-spread freshwater gastropod, being considered as a model organism for research in many fields of biology, including ecotoxicology. The aim of the present study was to explore the Cd sensitivity of L. stagnalis through the measurement of a biomarker battery for oxidative, toxic and cellular stress. The interpretation of biomarker parameters occurred against the background of a truncated metallothionein protein with a limited Cd-binding capacity. Individuals of L. stagnalis were exposed through 14 days to uncontaminated water (controls) or to low (30 μg · L^-1) or high (50 μg · L^-1) Cd concentrations. The digestive gland of control and low-Cd exposed snails was processed for transcriptional analysis of the Metallothionein (MT) gene expression, and for determination of biomarkers for oxidative stress, toxicity and cellular stress. Digestive gland supernatants of high-Cd exposed snails were subjected to chromatography and subsequent analysis by spectrophotometry. It was shown that the MT system of L. stagnalis is functionally deficient, with a poor Cd responsiveness at both, the transcriptional and the protein expression levels. Instead, L. stagnalis appears to rely on alternative detoxification mechanisms such as Cd binding by phytochelatins and metal inactivation by compartmentalization within the lysosomal system. In spite of this, however, traces of Cd apparently leak out of the pre-determined detoxification pathways, leading to adverse effects, which is clearly indicated by biomarkers of oxidative and cellular stress.

Cadmium Pathways in Snails Follow a Complementary Strategy between Metallothionein Detoxification and Auxiliary Inactivation by Phytochelatins

Metal detoxification is crucial for animals to cope with environmental exposure. In snails, a pivotal role in protection against cadmium (Cd) is attributed to metallothioneins (MTs). Some gastropod species express, in a lineage-specific manner, Cd-selective MTs devoted exclusively to the binding and detoxification of this single metal, whereas other species of snails possess non-selective MTs, but still show a high tolerance against Cd. An explanation for this may be that invertebrates and in particular snails may also synthetize phytochelatins (PCs), originally known to be produced by plants, to provide protection against metal or metalloid toxicity. Here we demonstrate that despite the fact that similar mechanisms for Cd inactivation exist in snail species through binding of the metal to MTs, the actual detoxification pathways for this metal may follow different traits in a species-specific manner. In particular, this depends on the detoxification capacity of MTs due to their Cd-selective or non-specific binding features. In the terrestrial slug Arion vulgaris, for example, Cd is solely detoxified by a Cd-selective MT isoform (AvMT1). In contrast, the freshwater snail Biomphalaria glabrata activates an additional pathway for metal inactivation by synthesizing phytochelatins, which compensate for the insufficient capacity of its non-selective MT system to detoxify Cd. We hypothesize that in other snails and invertebrate species, too, an alternative inactivation of the metal by PCs may occur, if their MT system is not Cd-selective enough, or its Cd loading capacity is exhausted.

Cadmium in vivo exposure alters stress response and endocrine-related genes in the freshwater snail Physa acuta. New biomarker genes in a new model organism

The freshwater snail Physa acuta is a sensitive organism to xenobiotics that is appropriate for toxicity testing. Cadmium (Cd) is a heavy metal with known toxic effects on several organisms, which include endocrine disruption and activation of the cellular stress responses. There is scarce genomic information on P. acuta; hence, in this work, we identify several genes related to the hormonal system, the stress response and the detoxification system to evaluate the effects of Cd. The transcriptional activity of the endocrine-related genes oestrogen receptor (ER), oestrogen-related receptor (ERR), and retinoid X receptor (RXR), the heat shock proteins genes hsp70 and hsp90 and a metallothionein (MT) gene was analysed in P. acuta exposed to Cd. In addition, the hsp70 and hsp90 genes were also evaluated after heat shock treatment. Real-time reverse transcriptase-polymerase chain reaction (qRT-PCR) analysis showed that Cd presence induced a significant increase in the mRNA levels of ER, ERR and RXR, suggesting a putative mode of action that could explain the endocrine disruptor activity of this heavy metal at the molecular level on Gastropoda. Moreover, the hsp70 gene was upregulated after 24-h Cd treatment, but the hsp90 gene expression was not affected. In contrast, the hsp70 and hsp90 genes were strongly upregulated during heat shock response. Finally, the MT gene expression showed a non-significant variability after Cd exposure. In conclusion, this study provides, for the first time, information about the effects of Cd on the endocrine system of Gastropoda at the molecular level and offers new putative biomarker genes that could be useful in ecotoxicological studies, risk assessment and bioremediation.

Sub-lethal cadmium exposure increases phytochelatin concentrations in the aquatic snail Lymnaea stagnalis

Phytochelatins are metal-binding metabolites found in almost all plant species and some animal groups, including nematodes and annelids, where they can play an important role in detoxifying metals such as cadmium. Species from several other taxa contain a phytochelatin synthase (PCS) gene orthologue, including molluscs, indicating they may have the potential to synthesize phytochelatins. However, the presence of a gene alone does not demonstrate that it plays a functional role in metal detoxification. In the present study, we show that the aquatic snail Lymnaea stagnalis produced both penta- and heptapeptide phytochelatins (i.e. phytochelatin-2 and phytochelatin-3), and their levels increased in response to sub-lethal levels of cadmium.

Dietary Uptake of Cu Sorbed to Hydrous Iron Oxide is Linked to Cellular Toxicity and Feeding Inhibition in a Benthic Grazer

Whereas feeding inhibition caused by exposure to contaminants has been extensively documented, the underlying mechanism(s) are less well understood. For this study, the behavior of several key feeding processes, including ingestion rate and assimilation efficiency, that affect the dietary uptake of Cu were evaluated in the benthic grazer Lymnaea stagnalis following 4-5 h exposures to Cu adsorbed to synthetic hydrous ferric oxide (Cu-HFO). The particles were mixed with a cultured alga to create algal mats with Cu exposures spanning nearly 3 orders of magnitude at variable or constant Fe concentrations, thereby allowing first order and interactive effects of Cu and Fe to be evaluated. Results showed that Cu influx rates and ingestion rates decreased as Cu exposures of the algal mat mixture exceeded 10(4) nmol/g. Ingestion rate appeared to exert primary control on the Cu influx rate. Lysosomal destabilization rates increased directly with Cu influx rates. At the highest Cu exposure where the incidence of lysosomal membrane damage was greatest (51%), the ingestion rate was suppressed 80%. The findings suggested that feeding inhibition was a stress response emanating from excessive uptake of dietary Cu and cellular toxicity.

Mechanisms of waterborne Cu toxicity to the pond snail Lymnaea stagnalis: physiology and Cu bioavailability

We examined the mechanisms of toxicity of waterborne Cu to the freshwater pond snail Lymnaea stagnalis. The snail is one of the most sensitive species to acute Cu exposure (96 h LC(50), LC(20): 24.9, 18.0 μgl(-1)); they are not protected by the water quality criteria of the US EPA. Tissue Na and Ca were also reduced by Cu in the acute exposure. In contrast, during 28 d chronic exposures to Cu in the presence of food, which resulted in higher DOC concentrations, there was no significant mortality but an inhibition of growth, which may reflect a re-allocation of resources to detoxification. Cu detoxification was evidenced in chronic exposure by increases in metallothionein-like protein concentrations and Cu binding to metal-rich granules, decreases in thiobarbituric acid-reactive substances, and changes in the subcellular distribution in the soft tissues. Our results demonstrated that apart from external Cu bioavailability, compartmentalization of metals within the cells can alter toxicity of Cu to the snails.

Review on methods for determination of metallothioneins in aquatic organisms

One aspect of environmental degradation in coastal areas is pollution from toxic metals, which are persistent and are bioaccumulated by marine organisms, with serious public health implications. A conventional monitoring system of environmental metal pollution includes measuring the level of selected metals in the whole organism or in respective organs. However, measuring only the metal content in particular organs does not give information about its effect at the subcellular level. Therefore, the evaluation of biochemical biomarker metallothionein may be useful in assessing metal exposure and the prediction of potential detrimental effects induced by metal contamination. There are some methods for the determination of metallothioneins including spectrophotometric method, electrochemical methods, chromatography, saturation-based methods, immunological methods, electrophoresis, and RT-PCR. In this paper, different methods are discussed briefly and the comparison between them will be presented.

Metallothionein in the freshwater gastropod Melanopsis dufouri chronically exposed to cadmium: a methodological approach

Previous studies have demonstrated that the use of differential pulse polarography (DPP) for metallothionein (MT) determination in marine gastropod tissues, particularly the digestive gland, requires taking into account the presence of heat-stable high molecular weight compounds that exhibit polarographic signal. In the present paper, similar compounds were identified in tissues from the freshwater snail Melanopsis dufouri which also interfere with MT determination by DPP and, due to their silver binding capacity, also interfere in the silver assay for MT quantification. Ultrafiltration seems to be effective in removing these high molecular weight compounds from heat-denatured homogenate supernatant allowing direct MT quantification by DPP. A fully validated procedure for metallothionein determination in M. dufouri is described. In spite of a considerable accumulation of cadmium in the visceral complex of M. dufouri following exposure to 100 microg CdL(-1) for 8 weeks (up to 37 microgg(-1)) only a small increase in MT concentration was found.

Exposure routes of copper: short term effects on survival, weight, and uptake in Florida apple snails (Pomacea paludosa)

The uptake and effects (survival, weight) of copper (Cu) on Florida apple snails (Pomacea paludosa) via exposures to copper-enriched agricultural soil-water and water-only treatments were investigated. Soils were collected from citrus sites in south Florida and flooded with laboratory freshwater for 14d. Neonate apple snails (96-h-old) were then exposed to either Cu from a soil-overlying water (i.e., flooded agricultural soils) treatment or overlying water-only (i.e., equilibrated overlying water produced from 14d flooding of agriculture soils) treatment for 14d under standard laboratory conditions. Survival, weight (dry, wet), and whole body Cu uptake were measured. Copper exposure via soil-water exposures resulted in higher mortality and whole body Cu uptake than water-only exposures, indicating Cu uptake from soils. However, snail wet and dry weights were higher in soil-water treatments than in water-only treatments. Micronutrients from soils may be consumed by snails increasing weights. Survival, apple snail dry weight, and whole body Cu concentrations were significantly correlated with soil and water Cu concentrations in soil-water treatments. Survival was significantly correlated with the concentration of Cu(CO3)2(2-) in water-only treatments. This suggests that Cu(CO3)2(2-) is toxic to apple snails. Whole body Cu concentrations were higher in surviving snails than dead snails, suggesting that apple snails have the ability to detoxify accumulated Cu (e.g., through metallothionein induction, granules).

Effects of cadmium exposure on embryogenesis of Stagnicola elodes (Mollusca, Gastropoda): potential consequences for parasite transmission

Experiments on the toxicity of cadmium (Cd(2+)) to the embryonic development of Stagnicola elodes (Mollusca, Gastropoda), obligatory first intermediate host of numerous trematodes of pathogenic importance, were carried out as part of a study on the effects of metal pollution on host-parasite relationships. Freshly laid snail eggs were exposed to Cd concentrations of 0, 0.02, 0.2, and 2.0 mg Cd(2+)/L, and survival and embryogenesis were examined for 30 days. Mean survival time (+/- SD) of the control group was 23.1 (+/- 5.3) days compared with 10.1 (+/- 3.2) at 0.02 mg Cd(2+)/L, 3.9 (+/- 0.7) at 0.2 mg Cd(2+)/L, and 1.1 (+/- 0.08) at 2.0 mg Cd(2+)/L. Mortality patterns of all test groups differed significantly from each other, demonstrating that the percentage of surviving individuals at any given time was inversely related to Cd concentration. Concentration-dependent effects of Cd exposure on snail embryogenesis were noted. While embryos of the control group developed properly and started hatching on day 16, eggs exposed to 0.02 mg Cd(2+)/L exhibited a prolonged gastrula period and failed to hatch. Eggs in the 0.2 mg Cd(2+)/L group were blocked in the gastrula stage on day 5, whereas individuals exposed to 2.0 mg Cd(2+)/L died in the morula stage on the second day. Data showed that Cd severely affects S. elodes embryogenesis. By implication, Cd contamination at concentrations >or=0.02 mg Cd(2+)/L will thus decrease transmission success of various trematodes by decreasing intermediate host snail abundance.

Potential marker enzymes and metal-metal interactions in Helisoma duryi and Lymnaea natalensis exposed to cadmium

In this study, we investigate the effects of exposure to cadmium and copper on Lymnaea natalensis and Helisoma duryi. The snails were dosed with Cd2+ or Cu2+ for a period of 96h. Snails dosed with Cd accumulated the metal significantly (P<0.05) in tissues but not in shells. Mortality was observed at approximately 1mg Cd/l of culture water. In tissues and shells of snails dosed with Cd or Cu, synergistic and antagonistic metal-metal interactions involving Cd, Cu, Zn, and Pb were observed and these may affect metal toxicity. Glutamate dehydrogenase, aspartate aminotransferase, and alanine aminotransferase were assayed in whole snail tissue sub-cellular fractions of Cd-dosed snails. Generally, enzyme activity significantly increased at lower concentrations of Cd but decreased at high concentrations of the metal. However, mitochondrial alanine aminotransferase activity progressively declined with increasing Cd concentration. The changes in some of the enzymes' activities suggest biomarker potential.

Chronic toxicity of tributyltin to development and reproduction of the European freshwater snail Lymnaea stagnalis (L.)

Chronic toxicity, growth and reproduction were measured in the freshwater gastropod Lymnaea stagnalis exposed to waterborne bis(tri-n-butyltin) oxide (TBTO) over a range of four nominal concentrations (0-10microg TBTl(-1)). Egg development was completely inhibited at 10microg TBTl(-1), whilst abnormal embryonic development was observed at 1microg TBTl(-1). For the solvent control and the 0.01microg TBTl(-1)treatment group, normal development of L. stagnalis was observed. Survivorship of hatchlings was significantly reduced by TBT at 1microgl(-1) while inhibition of shell growth of L. stagnalis was also observed at this concentration. The data were used to determine intrinsic growth rates (r) using two theoretical approaches (the Euler-Lotka equation and a Leslie Matrix). Both approaches showed that survival, fecundity and population growth rate were reduced at 1microg TBTl(-1). Interestingly, at 0.01microg TBTl(-1) snails showed a higher fecundity and growth rate than in the solvent control. The TBT concentration at which the r would equal zero (ECr(0)) and the population NOEC (No Observed Effect Concentration) were estimated. The population NOEC was defined as either the lower 95% confidence or lower 95% pointwise percentile limit of the ECr(0). Values obtained using the two different approaches were similar and thus a geometric mean was calculated to obtain a final representative population NOEC value for L. stagnalis of 2745ng TBTl(-1). The present data together with chronic toxicity TBT data for freshwater organisms, obtained from peer-reviewed literature, were used to construct a species sensitivity distribution (SSD). A predicted no effect concentration was then derived from the SSD (hazardous concentration at 5%, i.e., HC5 or 95% protection level). This SSD was compared with the SSD derived from saltwater species datasets. The HC5 value for saltwater species (3.55ng TBTl(-1); lower confidence limit: 1.93ng TBTl(-1)) was significantly lower than that for freshwater species (30.13ng TBTl(-1); lower confidence limit: 9.23ng TBTl(-1)), indicating that saltwater species are probably more susceptible to TBT than their freshwater counterparts.

Metallothioneins and trace metals in the dogwhelk Nucella lapillus (L.) collected from Icelandic coasts

Different sizes of the dogwhelk Nucella lapillus were collected from eight locations along the southwest and north coasts of Iceland. Concentrations of total metallothioneins (MTs) and heavy metals (Cd, Cu, Cr, Mn, Ni, and Zn) were analysed using the silver saturation method and inductively coupled plasma-atomic emission spectrometry. The level of tributyltin (TBT) contamination was also assessed using imposex indices, the vas deferens stage index (VDSI) and relative penis size index (RPSI). Gufunes N. lapillus presented the highest values of VDSI (4.0) and RPSI (11.1), followed by Grenivík individuals (VDSI = 3.0; RPSI = 0.9), while the Strandakirkja population showed the lowest VDSI (0.3) and zero RPSI. At a standardised size (0.25 g dry soft-body weight), Grenivík N. lapillus exhibited significantly higher concentrations of all metals whereas overall metal concentrations were significantly lower in individuals from Strandakirkja and Garethskagi compared to other study sites. Partial correlation analyses with size correction indicated that MT concentrations were better correlated with Cd and Cu concentrations than with other metals. At the standard size, the pattern of MT concentrations in N. lapillus from different sites was, however, very different from those of metal profiles. Such discrepancies between the patterns of MT and metals in N. lapillus might be explained by the fact that MT induction could be influenced by various factors such as temperature, dietary metal intake, growth rate and co-existence of other MT-inducing chemicals.