Erythroid 5-aminolevulinate synthase is required for erythroid differentiation in mouse embryonic stem cells

We have examined the induction of the enzymes of the heme biosynthetic pathway during erythroid differentiation of mouse embryonic stem (ES) cells. Following transfer to appropriate medium all of the pathway enzymes are induced within three days. Unlike differentiating mouse erythroleukemia cells (Lake-Bullock, H. and Dailey, H.A. Mol Cell Biol 13:7122-7132, 1993), all of the enzymes appear to be induced simultaneously and not sequentially in differentiating ES cells. The role of erythroid 5-aminolevulinate synthase (ALAS-2) in this differentiation process was examined by disruption of the ALAS-2 gene. The targeting vector used for disruption replaced all of exons 4 to 6 with a selectable neomycin resistance gene. The resulting genetically modified (ALAS-2 knockout) cells, as well as normal ES cells were used to study induction of heme biosynthesis. Following 10 days of culture in methylcellulose media significant morphological differences between the embryoid bodies (EBs) of the two cell lines were observed. ES cells exhibited morphology of typical EBs with a dark field (blood island) in the center, while ALAS-2 knockout ES cells developed very poorly both in size and shape. At 8 days of differentiation, only 3% of all EBs contained visible erythropoietic cells (i.e., stained positively for hemoglobin) in the ALAS-2 knockout cell line, compared with 50% in ES cells. Most of the genes in the heme synthetic pathway were expressed to a stable level within 3 to 6 days after induction in normal ES cells, while the ALAS-2 knockout cell line failed to significantly increase the level of expression of these genes. Fetal beta-globin mRNA was not detectable in the differentiating ALAS-2 knockout cells, whereas mRNA for this gene was detected in normal ES cells within 3 days of differentiation. These results suggest that ALAS-2 is necessary for ES cell erythroid differentiation and that there is an interrelationship between heme and globin synthesis in differentiating ES cells.