Patterns of metallothionein expression in oyster embryos

Metallothioneins (MT) play important roles in amelioration of metal toxicity and also in Zn and Cu regulation during normal growth and development. Metal-rich proteins of oyster embryos (Crassostrea virginica) that eluted in the same position as mammalian, crab, and adult oyster MTs after size-exclusion HPLC were further purified by anion exchange HPLC. The patterns of MT expression under conditions that did not perturb embryogenesis (control; 20 μg/liter Cd; 10 μg/liter Cu; 10 μg/liter Cd and Cu simultaneously), and during exposure to a concentration of Cu (20 μg/liter) that interfered with calcification were determined. In control embryos, four constitutive MTs (Zn or ZnCu isoforms) were observed. During exposure to Cd, two additional isoforms were produced: one major Cd-binding MT and a second minor component. During Cu exposures, excess Cu was associated with a peak that eluted at the same position as the second Cd peak, and probably represents a MT isoform that is inducible by Cu and Cd (CuCdMT). Simultaneous exposure to Cd and Cu resulted in inhibition of expression of the first CdMT, and increased binding of Cd to Cu MTs. Under these non-perturbed conditions, constitutive MTs did not change. However, constitutive MTs of embryos exposed to 20 μg/liter Cu were significantly reduced. These results indicate that bivalve embryos may represent a valuable experimental system for elucidating the functional significance of MT isoforms, discovering roles of MTs in distinct physiological processes, studying regulation of MT expression, and distinguishing patterns of MT expression that signal toxicity.