Inhibition of neurogenic precursor proliferation by antisense alpha thyroid hormone receptor oligonucleotides

Thyroid hormone 3,5,3'-triiodo-L-thyronine (T3) is required for normal brain development in vertebrates. T3 acts through two classes of nuclear receptors (TR alpha and TR beta) that have distinct developmental spatial and temporal distributions suggesting different functions during neuronal development. One possibility is that TR alpha, which is expressed early in embryogenesis, is involved in neuroblast proliferation. To test this hypothesis we used the embryonic chick optic lobe, as we found that T3 stimulates [3H]thymidine incorporation in this tissue both in vivo and in vitro during embryonic days 6-9. We applied oligonucleotides (ODNs) against TR alpha and TR beta to primary cultures of chick optic lobes. By employing a cationic lipid vector we could use very low ODN concentrations (< 150 nM). Antisense ODNs against TR alpha significantly inhibited [3H]thymidine incorporation, whereas antisense TR beta had no significant effect. However, both ODNs inhibited expression of TRs, as they blocked transcription from a T3-activated reporter gene. Random ODNs used as controls had no significant effect on [3H]thymidine incorporation or on T3-dependent transcription. These observations suggest that TR alpha is implicated in neuroblast proliferation and add credence to the hypothesis that the multiplicity of nuclear receptors allows for specific actions of T3 during development.