Rho signaling research: history, current status and future directions

Abstract

One of the main research areas in biology from the mid-1980s through the 1990s was the elucidation of signaling pathways governing cell responses. These studies brought, among other molecules, the small GTPase Rho to the epicenter. Rho signaling research has since expanded to all areas of biology and medicine. Here, we describe how Rho emerged as a key molecule governing cell morphogenesis and movement, how it was linked to actin reorganization, and how the study of Rho signaling has expanded from cultured cells to whole biological systems. We then give an overview of the current research status of Rho signaling in development, brain, cardiovascular system, immunity and cancer, and discuss the future directions of Rho signaling research, with emphasis on one Rho effector, ROCK*. *The Rho GTPase family. Rho family GTPases have now expanded to contain 20 members. Amino acid sequences of 20 Rho GTPases found in human were aligned and the phylogenetic tree was generated by ClustalW2 software (EMBL-EBI) based on NJ algorithm. The subfamilies of the Rho GTPases are highlighted by the circle and labeled on the right side. Rho cited in this review refers to the original members of Rho subfamily, RhoA, RhoB and RhoC, that are C3 substrates, and, unless specified, not to other members of the Rho subfamily such as Rac, Cdc42, and Rnd.

Keywords: C3 exoenzyme; ROCK; Rho; SRF; Y-27632; actin; actomyosin; myosin.

Figure 1

(A) Simplified scheme depicting the actions of mDia and ROCK in Rho‐mediated actin remodeling. Upon the activation by Rho, mDia promotes actin nucleation and polymerization to form actin filaments and ROCK activates myosin to bundles actin filaments. The upper right box shows the FH1FH2 of mDia1‐catalyzed actin polymerization in vitro. Red arrowheads indicate the mDia1‐free barbed end of F‐actin growing at slow rate, and blue arrowheads indicate the barbed end undergoing mDia1 (FH1FH2)‐dependent fast growth. Times are indicated in seconds. Scale bar, 5 μm. Modified from Yamashiro S, et al. MBoC 25, 1010–1024 (2014). The lower right box shows F‐actin staining of HeLa cells overexpressing vector control or active ROCK‐I. Note that F‐actin bundles are extensively induced in active ROCK‐I overexpressed cells. Modified from Ishizaki T, et al. FEBS Lett. 404, 118–124 (1997). (B) Crystal structures of Y‐27632‐bound (left) and fasudil‐bound (right) kinase domain of ROCK‐I. Modified from Jacobs M, et al. JBC 281, 260–268 (2006).

Figure 2

Examples of the ROCK actions in development. The left box shows the impaired eyelid closure phenotype of ROCK‐I^−/− embryos. Scanning electron micrographs of the eyes of the WT and ROCK‐I^−/− embryos are shown on the top and whole‐mount F‐actin staining of the eyelids of WT and ROCK‐I^−/− embryos are shown on the bottom. Note that F‐actin bundles encircling the eye (arrowheads) are impaired in ROCK‐I^−/− embryos. Modified from Shimizu Y, et al. JCB 168, 941–953 (2005). The right box shows the role of ROCK in neural tube closure. H&E staining of mouse embryo neural tube is shown on the top left (H. Kamijo, T. Ishizaki, D. Thumkeo, S. Narumiya, et al., unpublished results). The upper three panels show immunofluorescence micrographs of chick embryo neural tube during neural tube closure. Modified from Nishimura T, et al. Development 141, 1987–1998 (2008) and Nishimura T, et al. Cell 149, 1084–1097 (2012). Note concentration of ROCK‐I and pMLC on the apical surface as marked by ZO‐1 staining. The lower panels show stereomicroscope micrographs of 9.5 dpc mouse embryo neural tube (H. Kamijo, T. Ishizaki, D. Thumkeo, S. Narumiya, et al., unpublished results). Note impaired neural tube closure of ROCK‐I^+/−; ROCK‐II^−/− mouse embryo (white arrow). A model proposes the role of ROCK‐mediated actomyosin on the apical surface of neuroepithelium during neural tube closure is shown on the bottom right.

Figure 3

Milestone discoveries in Rho signaling research.