Cadmium-induced metallothionein expression during embryonic and early larval development of the mollusc Crassostrea virginica.

ABSTRACT

Newly fertilized eggs of the oyster Crassostrea virginica were exposed to 0.2 microM Cd and sampled during the first 24 hr of embryonic and larval development for determination of the temporal patterns of total bioaccumulated and metallothionein (MT)-bound Cd concentrations and the concentrations of MT mRNA. In comparison with controls, exposure to this concentration of Cd resulted in delayed development to the D-stage veliger larval stage. Maternal MT mRNA, which was carried over into the egg, declined immediately after fertilization. In controls, levels of MT mRNA were stable after this initial decline, and little or no detectable Cd was bound to MTs. Accumulation of new MT mRNA was observed in the exposed group after 12 hr of exposure when increases due to induction by Cd had occurred. Total accumulated Cd concentrations, which ranged from 0.04 to 0.1 pmol Cd/microg DNA in unexposed individuals, increased to 13.5 pmol Cd/microg DNA after 24 hr in Cd-exposed individuals. There was an increase in MT-bound Cd in exposed individuals prior to MT induction, which may be explained in part as Cd binding to maternal MT. This was detected as binding to CvNAcMT, the expected N-acetylated form of the oyster MT. Amounts of Cd bound to this form of MT increased, thereafter, and was followed by increases in binding to CvMT, the unexpected, nonacetylated form of the oyster MT, which has been associated with Cd toxicity, possibly in relation to the effects of Cd on normal cotranslational processing.