Rho guanine nucleotide exchange factors: regulators of Rho GTPase activity in development and disease

Abstract

The aberrant activity of Ras homologous (Rho) family small GTPases (20 human members) has been implicated in cancer and other human diseases. However, in contrast to the direct mutational activation of Ras found in cancer and developmental disorders, Rho GTPases are activated most commonly in disease by indirect mechanisms. One prevalent mechanism involves aberrant Rho activation via the deregulated expression and/or activity of Rho family guanine nucleotide exchange factors (RhoGEFs). RhoGEFs promote formation of the active GTP-bound state of Rho GTPases. The largest family of RhoGEFs is comprised of the Dbl family RhoGEFs with 70 human members. The multitude of RhoGEFs that activate a single Rho GTPase reflects the very specific role of each RhoGEF in controlling distinct signaling mechanisms involved in Rho activation. In this review, we summarize the role of Dbl RhoGEFs in development and disease, with a focus on Ect2 (epithelial cell transforming squence 2), Tiam1 (T-cell lymphoma invasion and metastasis 1), Vav and P-Rex1/2 (PtdIns(3,4,5)P3 (phosphatidylinositol (3,4,5)-triphosphate)-dependent Rac exchanger).

Figure 1. Rho GTPase regulation

The human Rho GTPase family is comprised of 20 members. The majority cycle between GDP-bound inactive and GTP-bound active states. Unlike RhoGEFs and RhoGAPs that possess shared domains and sequence identity, allowing precise determination of the number encoded in the human genome, the majority of Rho GTPase effectors lack a common well-defined recognition domain/motif. The information here applies mainly to RhoA, Rac1, Cdc42 and related isoforms. Not all Rho GTPases are regulated by GEFs, GAPs and/or GDIs, not all are posttranslationally modified by a geranylgeranyl isoprenoid lipid, and some do not have known membrane targeting elements.

Figure 2. Key discoveries linking Dbl RhoGEFs to human disease

We highlight representative discoveries in the study of RhoGEFs that link their aberrant function to human disease. Gain-of-function, GOF; loss-of-function, LOF.

Figure 3. The human Dbl RhoGEF family

The 72 DH domains found within the 70 members of human Dbl family RhoGEFs were aligned using ClustalX and a phylogenetic tree was subsequently drawn using the program FigTree (http://tree.bio.ed.ac.uk/software/figtree/). The protein domain architecture for each full-length Dbl protein is shown as annotated by SMART (http://smart.embl.de/) or described in the published literature. Black arrowheads indicate sites of genetic truncation known to activate RhoGEF function, while bracketed arrowheads designate biologically or catalytically active fragments of a GEF. Numbers within braces next to a Dbl protein indicate the reported Rho GTPase specificity of the DH domain. Please refer to Supplementary Table 1 for other names utilized for these RhoGEFs.

Figure 4. Mechanisms of RhoGEF activation

Shown here are the “canonical” isoforms as identified in UniProt (http://www.uniprot.org/). Domain structures were determined, in part, by SMART (http://smart.embl-heidelberg.de/). Additional domains not identified in SMART were added based on previous sequence analyses of Ect2, Tiam1, P-Rex and Vav proteins. The number at the right end of each protein indicates the number of amino acids in the full length protein. Solid triangles indicate the position of N-terminal truncations that result in the formation of constitutively activated and transforming variants of Ect2, Tiam1, and Vav1-3. The arrow indicates the site of initiation of the shorter Tiam2S splice variant. The % values refer to amino acid sequence identity for the full length protein (overall) or the isolated DH domain, when compared to the first isoform shown. Gain-of-function P-Rex2a truncation (Δ) or missense (*) transforming mutations. Abbreviations: Ac, acidic domain; CC, coiled-coil; CH, calponin homology; Clb6 region, homology to yeast Clb6 B-type cyclin; Ex, extra region; NLS, nuclear localization sequence; PIP3, phosphatidylinositol (3,4,5)-triphosphate; XRCC1, homology to human repair protein XRCC1.