Erythroid heterokaryons: a system for investigating the functional role of trans-acting factors in developmental hemoglobin switching.

We have shown that erythroid cells from widely divergent species such as amphibians and mammals can be efficiently fused using either calcium phosphate bridges or polyethylene glycol. Transient heterokaryons of adult mouse erythroid (MEL) cells and Rana catesbeiana (bullfrog) tadpole erythroid cells produce adult Rana alpha globin mRNA and adult Rana hemoglobin (Hb) tetramers. Rana tadpole/adult Xenopus erythroid heterokaryons also exhibit this switch to adult Rana globin gene expression. These results indicate that trans-acting factors--and the globin gene regulatory mechanism of which they are a part--are conserved in vertebrate phylogeny. We also wish to know whether the reciprocal Hb switch occurs in each of these two types of heterokaryons, i.e., whether embryonic or fetal globin genes are reactivated in the adult nucleus. Experiments to answer these questions are in progress and are briefly discussed. The influence of stage of erythroid differentiation of the larval and adult donor cells on the cross-inductions is also being explored. These types of experiments should indicate which cells will be the best sources of stimulatory and inhibitory factors that are globin-gene specific. This system may be useful as an in situ assay for the function of purified trans-factors, which could be encapsulated within RBC ghosts and delivered via cell fusion.