INHIBITION OF NEUROGENIC PRECURSOR PROLIFERATION BY ANTISENSE ALPHA-THYROID HORMONE-RECEPTOR OLIGONUCLEOTIDES

Thyroid hormone 3,5,3'-triiodo-L-thyronine (T-3) is required for norma l brain development in vertebrates. T-3 acts through two classes of nu clear receptors (TR alpha and TR beta) that have distinct developmenta l spatial and temporal distributions suggesting different functions du ring neuronal development. One possibility is that TR alpha, which is expressed early in embryogenesis, is involved in neuroblast proliferat ion. To test this hypothesis we used the embryonic chick optic lobe, a s we found that T-3 stimulates [H-3]thymidine incorporation in this ti ssue both in vivo and in vitro during embryonic days 6-9. We applied o ligonucleotides (ODNs) against TR alpha and TR beta to primary culture s 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 [H-3]thymidine incorporation, whereas an tisense TR beta had no significant effect, However, both ODNs inhibite d expression of TRs, as they blocked transcription from a T-3-activate d reporter gene. Random ODNs used as controls had no significant effec t on [H-3]thymidine incorporation or on T-3-dependent transcription. T hese observations suggest that TR alpha is implicated in neuroblast pr oliferation and add credence to the hypothesis that the multiplicity o f nuclear receptors allows for specific actions of T-3 during developm ent.