Cadmium Detoxification in Earthworms: From Genes to Cells

Cadmium pollution has clear ecotoxicological consequences as it is readily bio-available and has a reported tendency to bio-accumulate in soil biota. Understanding the mechanisms of cadmium trafficking pathways within sentinel terrestrial invertebrates, such as the earthworm, is therefore considered to be of importance. Using X-ray microanalysis, quantitative polymerase chain reaction, and immunohistochemical techniques, we were able to demonstrate that the earthworm has the intrinsic capacity to efficiently sequester and compartmentalize cadmium via a metallothionein-mediated trafficking pathway. There is evidence that wMT-2, rather than wMT-1, is the major isoform implicated in the detoxification of cadmium and the identification of three independent wMT-2 loci (totalling over 25 kb of genomic sequence) has revealed a complex genomic organization. Complementary in silico analysis of over 6500 expressed sequence tags has identified a third metallothionein isoform, wMT-3, found to be highly enriched in embryonic tissue. In summary, this paper provides a detailed dissection of the genetic, molecular, and cellular basis of a sophisticated pathway that facilitates the uptake, accumulation, transport, and excretion of cadmium.