SMITH, T.M.*; MANZON, R.G.; Univ. of Regina, Saskatchewan, Canada; Univ. of Regina, Saskatchewan, Canada: Cloning and developmental expression of a putative sea lamprey (Petromyzon marinus) transthyretin cDNA.

Unlike all other metamorphosing vertebrates, the onset of sea lamprey (Petromyzon marinus) metamorphosis coincides with a rapid drop in total serum thyroid hormone (TH) levels, TH levels remain at their lowest throughout metamorphosis, and metamorphosis can be induced with goitrogens and inhibited with exogenous TH treatments. Serum thyroid hormone binding proteins (THBP), such as transthyretin (TTR), are essential for the transport and distribution of TH to target tissues. To date, studies have not examined THBP or free serum TH concentrations in sea lampreys. Using semi-degenerate RT-PCR and RACE, we have isolated a 486 bp cDNA encoding a putative TTR from the livers of larval sea lamprey. Our cDNA encodes for the N-terminal 134 amino acids and this protein sequence is 65-70 % identical to the TTRs of other vertebrates. A single 1500 bp transcript was detected by Northern blotting of larval liver RNA; low levels of this transcript were also found in the larval nephric fold and intestine. Temporal expression analyses indicate that liver TTR mRNA levels are lowest in larvae and stage 1 metamorphosing animals, begin to increase rapidly by stage 2 of metamorphosis, peak at stage 5 of metamorphosis then slowly decrease to low levels in adults. These data indicate that TTR transcript levels are increasing when total TH levels are low and suggest that a decrease in TTR-bound TH could not account for an increase in free TH levels, thus, it is unlikely that free TH levels are elevated during lamprey metamorphosis when total levels are at nadir. Collectively, our data further support the notion that reduced TH levels are required for the onset and progression of lamprey metamorphosis. Supported by NSERC Grant # 261948 to RGM.