The YAP Transcriptional Co-Activator Is Not Required for Mouse Hematopoiesis, at Steady State or After 5FU Treatment.

Abstract 1592

The Yes-associated protein (YAP) is a transcriptional co-activator that interacts with many transcription factors, including RUNX2, p73, ERBB4, PEBP2α, p53BP2, SMAD7 and the TEAD/TEF proteins. Recently, it has been established that YAP is the major downstream molecule of the evolutionarily conserved Hippo signaling pathway. First elucidated in Drosophila, the Hippo signaling pathway controls organ size by regulating apoptosis and proliferation. Beginning a kinase cascade, the MST1/MST2 kinases (fly ortholog: Hippo) cooperate with the WW domain-containing SAV1 (fly ortholog: Salvador) to phosphorylate and activate the LATS1/LATS2 kinases (fly ortholog: Warts). In turn, activated LATS1/LATS2 phosphorylate and inactivate YAP (fly ortholog: Yorkie). Inactivated YAP binds to 14-3-3 and is kept in the cytosol. In contrast, unphosphorylated YAP binds to TEAD family transcription factors (fly ortholog: Scalloped) and the complex transactivates genes including cell cycle regulators (e.g. cyclin) and cell death inhibitors (e.g. IAPs, inhibitor of apoptosis). Therefore, inactivation of MST or LATS kinase, or overexpression of YAP results in organ/tissue overgrowth characterized by excessive cell proliferation and diminished apoptosis in both fly and mammals. In two transgenic mouse models, overexpression of YAP1 in liver results in reversible increase in liver size due to increased proliferation and decreased cell death of hepatocytes.

Several pieces of evidence suggest that YAP regulates stem cell self-renewal and differentiation. First, transcriptional profiling of mouse stem cells (hematopoietic, neuronal and embryonic) identified both YAP1 and TEAD2 as 2 of the only 14 transcription factors commonly expressed in all 3 types of stem cell. Second, in many adult human tissues (e.g. intestine, lung, pancreas), YAP1 is preferentially expressed in stem-progenitor cell compartments. Third, in mouse intestine and chick neural tubes, overexpression of YAP results in expansion of intestinal or neural stem-progenitor cells, as well as inhibition of progenitor cell differentiation in vivo. Finally, it has been shown that YAP expression decreases during mouse ES cell differentiation. Ectopic expression of YAP can maintain ES cell pluripotency and prevent differentiation both in vitro and in vivo. Given that YAP has a proven role in regulating stem-progenitor cells in multiple tissue and multiple organisms, we set out to investigate whether YAP also regulate hematopoetic stem-progenitors in mouse and human.

In conditional YAP knockout mice with a specific deletion of YAP expression in the hematopoetic system, we found no abnormalities at steady state in hemtaopoietic lineages, as assessed by CBC (complete blood counts) or immunophenotypic analysis. In addition, the YAP-null hematopietic stem-progenitors (HSPCs) had no change in vitro hematopoietic colony-forming cells. Furthermore, 5-fluorouracil treatment did not reveal a significant difference in blood cell numbers or types, between wild type and YAP-null mice. Although these results in mice suggest that YAP appears not to be required in hematopoiesis, overexpression of YAP, which promotes stem-progenitor cell proliferation in other tissues, may still provide an excellent opportunity to drive HSPC expansion.