Constitutively active Notch4 promotes early human hematopoietic progenitor cell maintenance while inhibiting differentiation and causes lymphoid abnormalities in vivo.

Notch transmembrane receptors are known to play a critical role in cell-fate decisions, with Notch1 shown to enhance self-renewal of hematopoietic stem cells and cause T-cell leukemia. Four Notch receptors exist, and the extent of redundancy and overlap in their function is unknown. Notch4 is structurally distinct from Notch1 through Notch3 and has not been extensively studied in hematopoiesis. By polymerase chain reaction (PCR) we find Notch4 transcript expression in human marrow cells and in both CD34(+) and CD34(-) populations. When constitutively active Notch1 or Notch4 was overexpressed in normal human marrow or cord cells, we found reduced colony-forming and short-term proliferative ability while the primitive progenitor content of myeloid long-term cultures was significantly increased. Notch4-intracellular domain (Notch4-IC)-transduced cord cells transplanted into beta(2)-microglobulin(-/-) nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice resulted in significantly higher levels of engraftment of both green fluorescent protein-positive (GFP(+)) and GFP(-) populations as compared with controls. GFP(+) cells in bone marrow and spleen of animals that had received transplants gave rise to an immature CD4(+)CD8(+) T-cell population, whereas B-cell development was blocked. These results indicate that activation of Notch4 results in enhanced stem cell activity, reduced differentiation, and altered lymphoid development, suggesting it may influence both stem cells and the fate of the common lymphoid progenitor.