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Rac GTPases differentially integrate signals regulating hematopoietic stem cell localization

Nature Medicine volume 11, pages 886–891 (2005)Cite this article

Abstract

The molecular events that regulate engraftment and mobilization of hematopoietic stem cells and progenitors (HSC/Ps) are still incompletely defined1,2. We have examined the role of the Rho GTPases Rac1 and Rac2 in HSC engraftment and mobilization. Rac1, but not the hematopoietic-specific Rac2, is required for the engraftment phase of hematopoietic reconstitution, because Rac1−/− HSCs did not rescue in vivo hematopoiesis after transplantation, but deletion of Rac1 after engraftment did not impair steady-state hematopoiesis. Rac1−/− HSC/Ps showed impaired spatial localization to the endosteum but near-normal homing to the medullary cavity in vivo. Interaction with the bone marrow microenvironment in vitro was markedly altered. Whereas post-engraftment deletion of Rac1 alone did not impair hematopoiesis, deficiency of both Rac1 and Rac2 led to massive mobilization of HSCs from the marrow associated with ineffective hematopoiesis and intense selection for Rac-expressing HSCs. This mobilization was reversible by re-expression of Rac1. In addition, a rationally designed, reversible small-molecule inhibitor of Rac activation led to transient mobilization of engraftable HSC/Ps. Rac proteins thus differentially regulate engraftment and mobilization phenotypes, suggesting that these biological processes and steady-state hematopoiesis are biochemically separable and that Rac proteins may be important molecular targets for stem cell modification.

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Figure 1: Effect of Rac deficiency on steady-state hematopoiesis, homing and transendothelial migration of HSC/Ps.
Figure 2: CAFC frequency and spatial distribution of HSC/Ps of wild-type and Rac-deficient bone marrow cells.
Figure 3: Mobilization of HSC/Ps in Rac-deficient mice.
Figure 4: Effect of NSC23766 on mobilization of HSC/Ps.

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Acknowledgements

This work was supported by US National Institutes of Health grant numbers R01 DK62752 (to D.A.W.) and R01 GM60523 (to Y.Z.). J.A.C. is a recipient of an award from the National Blood Foundation (United States). The authors want to thank M.-D. Filippi, H. Geiger and J. Mulloy for critical review of the manuscript; D. Witte for assistance in histologic analysis of organs; N. Krishnan, Y. Yamada, J. Bailey, V. Summey-Harner, C. Harris, F. Sadique, S. Homan and L. McMillan for technical assistance, and M. O'Leary, J. Hayden and E. Meunier for providing assistance in manuscript editing. We thank the Experimental Hematology Division Mouse and Flow Cores for services and Amgen, Inc. and Takara Bio for reagents.

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Authors and Affiliations

  1. Hoxworth Blood Center, University of Cincinnati Medical Center, 3130 Highland Avenue, Cincinnati, 45267, Ohio, USA

    Jose A Cancelas

  2. Division of Experimental Hematology, Cincinnati Children's Research Foundation, 3333 Burnet Avenue, Cincinnati, 45229, Ohio, USA

    Jose A Cancelas, Andrew W Lee, Rethinasamy Prabhakar, Yi Zheng & David A Williams

  3. Division of Pathology, Cincinnati Children's Research Foundation, 3333 Burnet Avenue, Cincinnati, 45229, Ohio, USA

    Keith F Stringer

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  1. Jose A Cancelas
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Correspondence to David A Williams.

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The authors have a pending patent on Rac inhibitors.

Supplementary information

Supplementary Fig. 1

Lineage reconstitution in peripheral blood of polyI:C treated mice (Rac-deficient) cells. (PDF 173 kb)

Supplementary Fig. 2

Content of HSC/P in BM and spleen of Rac-deficient mice. (PDF 267 kb)

Supplementary Fig. 3

Mobilization of HSC/P in Rac-deficient mice. (PDF 325 kb)

Supplementary Methods (PDF 43 kb)

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Cancelas, J., Lee, A., Prabhakar, R. et al. Rac GTPases differentially integrate signals regulating hematopoietic stem cell localization. Nat Med 11, 886–891 (2005). https://doi.org/10.1038/nm1274

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