Hemoglobin switching across vertebrate classes: exchange of developmental signals by cell fusion

Our aim is to obtain evidence for trans-acting factors that regulate developmental hemoglobin (Hb) switching. Our approach is to fuse erythroid cells that have different developmental programs, allowing the trans-acting factors from the adult cell to have access to the nucleus of the fetal or embryonic cell and vice-versa. After cell fusion, the heteropolykaryons are cultured for six hours, and globin gene expression is assayed at two levels: (1) at the level of mRNAs on dot blots hybridized with globin-specific cDNA probes, and (2) at the level of fully-formed Hb tetramers separated by native polyacrylamide gel electrophoresis (PAGE). Since the donor erythroid cells are from different species, it is easy to determine which globin gene products are from which nucleus. And since there is no nuclear fusion for at least twelve hours, the Hb switching that occurs is due to regulation in trans. Our results show that developmental Hb switching occurs in mouse-frog erythroid cell polykaryons. When DMSO-induced murine erythroleukemia (MEL) cells (which express only adult mouse Hbs) are fused with Rana tadpole RBCs (which express only embryonic and fetal-like Hbs), the resultant heteropolykaryons express adult frog globin mRNA and adult frog Hbs. We conclude that there are developmental stage-specific trans-acting factors for Hb switching, that trans-acting factors from adult mouse erythroid cells can induce expression of adult frog globin genes in a tadpole RBC nucleus, and that Hb switching mechanisms are conserved across vertebrate classes.