Evidence from neuronal heterokaryons for a trans-acting factor suppressing Thy-1 expression during neuronal development.

The expression of Thy-1 on developing sensory neurons cultured in vitro is strictly regulated and follows a temporal course identical to that observed in situ. We have investigated the role of trans-acting factors in this innate regulation by constructing heterokaryons between mature Thy-1.1+ expressing neurons and immature Thy-1.2- neurons. It has been observed by immunofluorescence that within 16 hr of fusion, Thy-1.1 expression is suppressed in such heterokaryons. However, this suppression is reversible, and after 3-4 days in vitro, at the time at which Thy-1.2 is normally expression, there is re-expression of the Thy-1.1 molecule producing Thy-1.1+/Thy-1.2+ co-expressing heterokaryons. As nuclear fusion does not occur, it appears that a developmentally regulated diffusible suppressor molecule is responsible for these observations.