ReviewPhysiological roles of Rho and Rho effectors in mammals
Introduction
Rho was first identified and cloned in 1985 as a Ras homolog (Rho is the abbreviation of Ras homolog) (Madaule and Axel, 1985). Like Ras, Rho is a binary molecular switch GTPase. Rho is activated upon binding GTP and inactivated upon GTP hydrolysis to GDP. There are 7 Rho GTPases in S. cerevisae, 9 Rho GTPases in D. melanogaster, and 23 Rho GTPases in mammals (Bustelo et al., 2007). Of those Rho GTPases, the best characterized is the Rho subfamily, including ubiquitously expressed RhoA and its homologs, RhoB and RhoC. A work by Ridley and Hall in 1992 showed that Rho is important for the formation of actin stress fibers and focal adhesion complexes in fibroblasts (Ridley and Hall, 1992). Utilizing botulinum C3 exoenzyme, a specific inhibitor of Rho, Kishi et al. and Mabuchi et al. elucidated the crucial role of Rho in cytokinesis (Kishi et al., 1993, Mabuchi et al., 1993). Subsequent works have further determined the involvement of Rho in several other fundamental processes of mammalian cells as described below.
Rho is a small protein of 21 kDa without any clear protein domain that could directly interact with the actin cytoskeleton. It was therefore hypothesized in the early 90s that Rho exerts its function through binding to downstream effector molecules. This led to the cloning of multiple Rho effectors (Fig. 1) during the mid 90s, which include Rhotekin (Reid et al., 1996), Rhophilin (Watanabe et al., 1996), PKN (Amano et al., 1996a, Watanabe et al., 1996), Citron (Di Cunto et al., 1998, Madaule et al., 1995), ROCK (Ishizaki et al., 1996, Leung et al., 1995, Matsui et al., 1996) and mDia (Watanabe et al., 1997). Extensive biochemical and cell biological studies were then carried out in cultured cells and clarified roles of these Rho effectors in vitro. However, their physiological roles in vivo have remained mostly unknown until recently. In this article, we briefly describe studies on the function of Rho and its effectors in vitro and then review and discuss recent mouse studies on Rho and its effectors utilizing gene-targeting technology. These works have not only validated the roles of these molecules suggested by in vitro studies but also elucidated their novel functions in the body.
Section snippets
RhoA
It is known that RhoA plays important roles in regulating the formation of actin stress fibers and focal adhesion complexes in fibroblasts (Ridley and Hall, 1992). About the same time, utilizing botulinum C3 exoenzyme that specifically inactivates Rho (Morii et al., 1988), it was also reported that Rho activity is critical for cytokinesis (Kishi et al., 1993, Mabuchi et al., 1993). Studies utilizing C3 exoenzyme, dominant active and dominant negative mutants of RhoA in cultured cells
ROCK
RHO-associated, Coiled-coil Containing Protein Kinase or ROCK (also called Rho-kinase) is one of the most important Rho effectors, known to mediate Rho-dependent actomyosin contractility through the phosphorylation of myosin light chain and myosin phosphatase (Amano et al., 1996b, Kimura et al., 1996, Kureishi et al., 1997). There are two isoforms of ROCK in mammals named ROCK-I and ROCK-II (Nakagawa et al., 1996). Studies of in vitro functions of ROCK are far advanced compared to other Rho
Discussions and perspectives
There are many lessons that we can learn from knockout mice studies of Rho and its effectors. Current availability of different knockout mice and their corresponding phenotypes are summarized in Table 1, Table 2.
Acknowledgements
This work was supported by Grants-in-Aid for Scientific Research from MEXT of the Japanese Government (DT and SN) and grants from Fujiwara Foundation (DT). SW is a recipient of pre- and post-doctoral fellowship from the Japan Society for Promotion of Science. We apologize to the many authors of papers relevant to this article whose work could not be cited due to space limitations.
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Present address: Division of Biological Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan.