The Rho Family GTPase Cdc42 Regulates Multiple Hematopoietic Stem/Progenitor Cell Functions and Erythropoiesis.

The Rho family GTPase Cdc42 has emerged as a key signal transducer in cell regulation. To investigate its physiologic function in hematopoiesis, we have generated mice carrying a gene targeted null allele of cdc42gap, a major negative regulatory gene of Cdc42 and mice with conditional targeted cdc42 allele (cdc42^flox/flox). Deletion of the respective gene products in mice was confirmed by PCR genotyping and Western blotting. Low-density fetal liver or bone marrow cells from Cdc42GAP^−/− mice displayed ~3 fold elevated Cdc42 activity and normal RhoA, Rac1 or Rac2 activity, indicating that cdc42gap deletion has a specific effect on Cdc42 activity. The Cdc42GAP-deficient hematopoietic stem/progenitor cells (HSC/Ps, Lin⁻c-Kit⁺) generated from Cdc42GAP^−/− E14.5 fetal liver and the Cdc42^−/− HSC/Ps derived by in vitro expression of Cre via a retrovirus vector from Cdc42^flox/flox low density bone marrow showed a growth defect in liquid culture that was associated with increased apoptosis but normal cell cycle progression. Cdc42GAP-deficient HSC/Ps displayed impaired cortical F-actin assembly with extended actin protrusions upon exposure to SDF–1 in vitro and a punctuated actin structure after SCF stimulation while Cdc42^−/− but not wild type HSC/Ps responded to SDF-1 in inducing membrane protrusions. Both Cdc42^−/− and Cdc42GAP^−/− HSC/Ps were markedly decreased in adhesion to fibronectin. Moreover, both Cdc42^−/− and Cdc42GAP^−/− HSC/Ps showed impaired migration in response to SDF-1. These results demonstrate that Cdc42 regulation is essential for multiple HSC/P functions. To understand the in vivo hematopoietic function of Cdc42, we have characterized the Cdc42GAP^−/− mice further. The embryos and newborns of homozygous showed a ~30% reduction in hematopoietic organ (i.e. liver, bone marrow, thymus and spleen) cellularity, consistent with the reduced sizes of the animals. This was attributed to the increased spontaneous apoptosis associated with elevated Cdc42/JNK/Bid activities but not to a proliferative defect as revealed by in vivo TUNEL and BrdU incorporation assays. ~80% of Cdc42GAP^−/− mice died one week after birth, and the surviving pups attained adulthood but were anemic. Whereas Cdc42GAP^−/− mice contained small reduction in the frequency of HSC markers and normal CFU-G, CFU-M, and CFU-GM activities, the frequency of BFU-E and CFU-E were significantly reduced. These results suggest an important role of Cdc42 in erythropoiesis in vivo. Taken together, we propose that Cdc42 is essential for multiple HSC/P functions including survival, actin cytoskeleton regulation, adhesion and migration, and that deregulation of its activity can have a significant impact on erythropoiesis.

Cdc42 regulates HSC/P functions and erythropoiesis