Notch1 and Notch2 receptor-mediated signaling appear to have important and unique roles in lymphoid lineage commitment. Notch1 is required for T cell development, while Notch2 is essential for marginal zone B cell development. This specificity is not completely explained by differential expression patterns of Notch1 and 2 or Notch ligands, suggesting that there are other genes that contribute to specifying Notch receptor functions. We have previously shown that the MAML family of transcriptional co-activators is essential for Notch-induced transcriptional events, and functions by forming ternary complexes with Notch and the transcription factor CSL in the nucleus. This MAML family currently consists of three members, MAML1-3, all of which can function as co-activators for Notch receptors in vitro . In this study, we investigated the possibility that MAML1 co-activator contributes to determining Notch receptor function by generating mice deficient in the Maml1 gene. Maml1 -deficient mice fail to thrive and die within 10 days of birth. The morphology of marrow, nodes, and spleen was grossly intact. The ability of Maml1-deficient stem cells to generate different T and B lineages of lymphoid cells was determined by transplanting fetal liver cells isolated from E14.5 embryos into lethally irradiated wild-type recipient mice and analyzing donor-derived lymphoid cells 12 weeks post-transplantation. We found that the deletion of Maml1 results in complete loss of marginal zone B cell lineage (MZB, defined by B220+CD21hiCD23lo). Moreover, the number of MZB cells was reduced to about 50% in Maml1 -heterozygous fetal liver chimeras as compared to wild type controls. However, T cell development was largely unaffected, with only a modest but significant increase in the number of γδ T cells (about 2 fold) in both the thymus and the spleen. Therefore, these results suggest the unexpected finding that targeted deletion of Maml1 in hematopoietic cells is similar to the targeted deletion of either Notch2 or the Notch ligand, Delta-like 1 (Dll1) resulting in the loss of marginal zone B cells and minimal effects on T cell development. Moreover, the number of marginal zone B cells is correlated with Maml1 gene dosage, indicating haploinsufficiency. These data suggest that the Notch ligand Dll1 activates Notch2 signaling resulting in a Notch2/MAML1/CSL complex that is essential for marginal zone B cell development. Further studies with respect to relative expression levels of various MAML genes and interactions of MAML co-activators and Notch receptors may shed additional light into understanding how different Notch receptors regulate cell fate decisions in hematopoiesis.

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