Rac-king up new roles in haematopoiesis

Rac1 and Rac2 guanosine triphosphatases (GTPases) are crucial signalling regulators in eukaryotic cells, acting downstream of various cellular receptors. Two studies published in Science have used conditional gene targeting to clarify the roles of Rac1 and Rac2 in particular aspects of haematopoietic-cell differentiation.

Gu et al. looked at the differential activity of Rac1 and Rac2 in haematopoietic stem/progenitor cells (HSC/Ps). As Rac1 deficiency is embryonic lethal, they generated conditional Rac1-knockout mice. Deficiency of Rac1 inhibited the ability of HSC/Ps to reconstitute haematopoiesis in an engraftment model (which requires adhesion and proliferation in the bone marrow), and the absence of both Rac1 and Rac2 led to the movement of progenitor cells out of the bone marrow and into the peripheral circulation, as a result of decreased adhesion to fibronectin. Rac1 and Rac2 were also shown to have roles in regulating the cytoskeletal changes that are required for engraftment and mobilization.

Rac1^−/−, and to a greater extent Rac1^−/−Rac2^−/−, HSC/Ps had reduced proliferation in response to growth factors in vitro, correlating with decreased levels of cyclin D1 and phosphorylation of extracellular signal-regulated kinase (ERK), and increased levels of the cyclin-dependent kinase inhibitor KIP1. By contrast, Rac2^−/− cells were more susceptible to apoptosis after growth-factor stimulation, associated with reduced Akt activation. Therefore, it seems that Rac1 regulates entry into the cell cycle, whereas Rac2 mainly regulates cell survival.

Walmsley et al. looked further downstream at B-cell development by generating, for the first time, mice with a conditional deletion of Rac1 in the B-cell lineage (Rac1^B mice). Rac1^BRac2^−/− mice had normal numbers of pro-B, pre-B and immature B cells in the bone marrow, but reduced numbers of B cells at later developmental stages in the spleen and lymph nodes. In the absence of both Rac1 and Rac2, B-cell development seems to be blocked at the transitional type 1 B-cell stage in the spleen.

Recent studies have shown activation of Rac1 after B-cell receptor (BCR) stimulation, and signals from the BCR are known to be crucial for B-cell development, so are BCR signalling defects responsible for the observed phenotype of Rac1^BRac2^−/− mice? Rac2^−/− and Rac1^−/+Rac2^−/− mature B cells stimulated with an IgM-specific antibody had decreased survival compared with wild-type B cells, which correlated with decreased induction of the anti-apoptotic protein Bcl-X_L. These B cells also proliferated less than wild-type cells. This was shown to be due to decreased induction of the cell-cycle regulator cyclin D2 after stimulation. So, Rac1 and Rac2 transduce BCR signals for B-cell survival and entry into the cell cycle. Finally, in contrast to wild-type cells, BAFF could not increase the survival of Rac1^BRac2^−/− immature B cells, and this was probably the result of decreased expression of BAFF receptor (BAFFR). Rac1 and Rac2 were shown to transduce BCR signals leading to the upregulation of expression of BAFFR messenger RNA.

These two studies show that in both B cells and HSC/Ps, Rac1 and Rac2 have essential roles in controlling proliferation and survival. Furthermore, despite their sequence similarity, the two GTPases seem to have partially non-redundant physiological functions. Gu et al. confirmed this functional distinction in neutrophils, in which Rac2 seems to be the main GTPase regulating directed migration and superoxide generation, whereas Rac1 regulates neutrophil shape.