Mammalian Rho GTPases: new insights into their functions from in vivo studies

There are 20 members of the Rho GTPase family in mammals; all eukaryotes have several Rho GTPases.Rho GTPases regulate cytoskeletal dynamics, cell polarity, cell migration, cell-cycle progression, vesicle trafficking and cytokinesis.Most studies into the function of Rho GTPases in mammalian systems have used cultured cells that express constitutively active and/or dominant-negative mutants.Knockout mice for five Rho GTPases — RhoB, RhoC, RAC2, RAC3 and RhoH — are viable and fertile, allowing studies of their functions in vivo and in cells purified from tissues. So far, these studies have mostly shown that each of these GTPases has a similar role in vivo to that predicted from in vitro studies.Knockout mice for Rac1 and Cdc42 die early in embryogenesis, and hence conditional knockout alleles have been generated to study their function.The effects of knocking out Rac1 or Cdc42 have been investigated in multiple tissues and cell types in mice, from haematopoietic cells to neurons, glial cells, and epithelial cells; these studies have revealed novel functions for RAC1 and CDC42 that had not been predicted from in vitro analysis of cell lines.In some cases, knockout of Rac1 or Cdc42 gives a different phenotype to expression of dominant negative RAC1 or dominant-negative CDC42; for example, dominant-negative CDC42 inhibits filopodium extension, but CDC42-null fibroblastoid cells can still make filopodia. However, CDC42-null neurons have a reduced number of filopodia. These results suggest that CDC42 contribution to filopodium extension is cell-type-specific.