World Congress of Malacology Antwerp ... - Unitas Malacologica

Gastropod species as model organisms in ecotoxicology ?
Introducing the molecular view
Dallinger, Reinhard
Institute of Zoology, University of Innsbruck, Austria,
Email: reinhard.dallinger@uibk.ac.at
During their evolutionary history, gastropods have been successful in colonizing different
environments with some of the most adverse habitats on the earth. One of the preconditions for this
adaptational capacity of gastropods has been their huge physiological plasticity, along with their high
ability to cope with stressful conditions of different origin. In particular, this remarkable potential for
adaptation is based, among other features, on an energy metabolism with a variety of anaerobic
pathways. This enables some gastropod species to survive in habitats with limited energy resources,
and to outlast periods of adverse environmental conditions by reducing their activity and energy
demand to a minimum. In addition, a range of stress compensation and detoxification mechanisms
ensures the organism’s survival even under the influence of stressful physical and chemical
environmental factors, including adverse human impact. All these remarkable features make
gastropods potential candidates for fundamental and applied research in ecotoxicology. In the present
talk, I would like to substantiate this hypothesis by focusing on the exceptional ability of some
pulmonate species to cope with metal stress, and perhaps with more than this.
Some terrestrial pulmonates (e.g. Helix pomatia, Cornu aspersum, Arianta arbustorum) possess
mechanisms of metal detoxification, which are much more specific compared to those found in most
other animal species. The primary reason for this is the fact that these pulmonates rely on metal
binding Metallothioneins (MT) consisting of metal- and function-specific isoforms. In the Roman
snail (Helix pomatia), for example, one MT isoform is devoted to Cu homeostasis, while a second
isoform deals with detoxification of Cd. The adaptation of the two peptides to their metal-related
tasks represents an unique example of structural and functional diversification of different isoforms
within a protein family. Because of its metal-specific features, the Cd-MT isoform of Helix pomatia
and some other related pulmonate snails has successfully been used as a biomarker of exposure in
metal-contaminated terrestrial environments.
Another unique feature of the MT system of terrestrial pulmonates is the complexity of their MT
genes. The Cd-MT gene of Helix pomatia, for example, is by far the largest and most complex MT
gene known throughout all organismic kingdoms. Its nucleotide structure suggests that at the
molecular level, adaptation of terrestrial pulmonates to metal stress is achieved by a broad potential
of response patterns to a diversity of extrinsic and intrinsic stress stimuli, rather than strict metal
specificity. This is confirmed by the fact that the Cd-MT isoform gene of Helix pomatia contains,
apart from Metal-Responsive Elements (MREs) and nuclear activators and silencers, a manifold array
of binding sites for various stress-related transcription factors. These include, among others,
transcription factors involved in heat shock response, xenobiotic metabolism, and immune defense.
The multiplicity and diversity of these transcription factor binding sites probably allows, apart from
metal responsiveness, fine-tuning of transcriptional activation of the Cd-MT gene in dependence of
many other evnironmental stressors. In fact, the induction rate of the Cd-MT gene of Helix pomatia
due to metal exposure is much higher at the transcriptional level than at the protein level, a feature
which has also been observed for other stress-related proteins. The high complexity of the Cd-MT
gene of Helix pomatia and its sensitivity to transcriptional induction and regulation suggests that it
could be used as a complex biomarker for stress response, which includes, apart from metals, many
other environmental stressors. We are currently testing this hypothesis by transfection of reporter
gene constructs from Helix pomatia into embryonic cells of the freshwater pulmonate Biomphalaria
glabrata.
45