Rac GTPases play multiple roles in erythropoiesis

The four members of the Rac family of GTPases –Rac1, Rac2, Rac3 and RhoG – are members of the Rho superfamily that regulates the organization, dynamics, and function of the actin cytoskeleton. Rac GTPases play significant roles in many cellular processes including migration, cytokinesis, lamellipodia formation, and cell polarity.1 Genetically modified mice deficient in each of the Racs are available;2–6 deficiency of Rac1 causes intrauterine death, whereas mice defective in Rac2, Rac3 or RhoG develop fairly normally. Rac proteins may have redundant functions in certain types of cells and unique functions in others.

As shown by single- and double knock-outs of Rac genes, the Rac GTPases play important roles in many hematopoietic cells.7 Rac2 is specifically expressed in hematopoietic cells, and is directly involved in chemotaxis and superoxide production in neutrophils and macrophages.3,8–11 In addition, Rac2, together with Rac1, mediates B-cell receptor signaling pathways.12 T-cell activation is also affected in Rac2-deficient mice13 and hematopoietic stem cells from Rac2 mice show defective long-term engraftment.14

In contrast, Rac1 is ubiquitously expressed and plays essential roles in several organ systems, including hematopoietic cells. Using a hematopoietic cell-specific knockout of Rac1, Gu et al. demonstrated that depletion of Rac1 blocks the ability of hematopoietic stem cells to engraft irradiated recipient mice.15 When crossed with Rac2 mice, a hematopoietic-specific deletion of Rac1 results in a massive egress of hematopoietic stem cells into the blood from the bone marrow.15 Rac regulates signaling pathways downstream of integrins and c-kit in mast cells and hematopoietic stem cells15–17 and presumably these defects cause loss of adhesion of the hematopoietic stem cells to the bone marrow stroma.

Rac GTPases play essential roles in erythropoiesis

There is growing evidence that Rac GTPases are essential for erythropoiesis. In this issue of the Journal, Kalfa et al. demonstrate that Rac1 and Rac2 are required for early stages of erythropoiesis in the bone marrow but – surprisingly – not in the spleen.18 Using hematopoietic tissue-specific Rac1 knock-out mice in a total Rac2 background, they showed that deficiency of both Rac1 and Rac2 blocks early stages of erythropoiesis in the bone marrow without affecting cell survival or proliferation. Abnormalities in the morphology of erythroid burst-forming units resembled alterations seen previously in Rac1;Rac2 myeloid colonies, and suggested an impairment in cell migration and/or proliferation. These results indicated that Rac1 and Rac2 have redundant but essential roles in the early erythroid progenitor stages of erythropoiesis in the bone marrow.

The same group previously reported that Rac1;Rac2erythrocytes have an unstable cytoskeleton; deficiency of Rac1 and Rac2 alters actin assembly and decreases erythrocyte deformability, and generates a hemolytic anemia.19 Thus, it was not surprising that in these Rac1 Rac2 mice there was compensatory erythropoiesis in the spleen. What was surprising was that splenic erythroid progenitors somehow circumvented the deficiency of the Rac1 and Rac2 GTPases.

While Kalfa’s study focused on early stages of erythropoiesis, we showed that Rac1 and Rac2 are required for enucleation of late stage erythroblasts.20 Deregulation of Rac GTPases during the late stages of erythropoiesis completely blocked enucleation of cultured mouse fetal erythroblasts without affecting their normal proliferation and differentiation. The contractile actin ring that forms on the plasma membrane of late-stage erythroblasts at the boundary between the cytoplasm and nucleus of enucleating cells was disrupted when Rac GTPases were inhibited in late stages of erythropoiesis. This effect of Rac GTPases was mediated by their downstream target mDia2, a formin protein required for nucleation of unbranched actin filaments. This function of Rac GTPases in enucleation is specific to Rac1 and Rac2 since RhoA and Cdc42 are not involved in this process.

The current work by Kalfa et al.18 is important because it reveals that Rac1 and Rac2 are required during early stages of erythropoiesis as well as for enucleation and the integrity of the red cell cytoskeleton. Like all good papers it raises more interesting questions than it answers. For instance, we do not know whether Rac3 or RhoG play any role in red cell formation or function. Since there are no significant erythropoietic phenotypes in Rac3 and RhoG knock-out mice,4–6 it is likely that other Rac GTPases family proteins compensate for any possible negative effects of deletion of these proteins. In this aspect, in vitro studies of cultured mouse erythroblasts could elucidate whether Rac3 and RhoG have functions in erythropoiesis.

Figure 1.Rac GTPases play essential roles in erythrocytes.

Kalfa et al. show that the major disorder of Rac1;Rac2 hematopoietic cells is the defective proliferation of myelo-erythroid progenitor cells in the bone marrow, but there is a normal or possibly increased survival and/or proliferation of these progenitors in the spleen. Presumably, as noted by the authors, this derives from differences in the bone marrow and splenic microenvironment, but we do not know which cytokines or other factors produced by stromal cells may be responsible for this crucial difference. Bone marrow transplantation studies of Rac1;Rac2 hematopoietic progenitors into normal recipients, and vice versa, could begin to separate the hematopoietic cell autonomous functions of Rac1 and Rac2 from those of the stromal populations.

We would also like to know how Rac1 and Rac2 regulate differentiation of erythroid progenitors. As discussed by the authors, cytokine-mediated signaling pathways may be involved. In other cell types Rac GTPases regulate gene expression and cell transformation in multiple pathways, such as in the Jun N-terminal kinase21,22 and Ras signaling pathways,23 and these or other pathways could mediate the effects of Rac GTPases in early stages of erythropoiesis.

Footnotes

• Dr. Lodish is a Professor of Biology at the Massachusetts Institute of Technology and a member of the Whitehead Institute for Biomedical Research.