RHOD

RhoD is a Rho-family small GTPase that couples actin cytoskeleton remodeling to early endosome dynamics, cell migration, cytokinesis, and axon guidance. In its GTP-bound state, RhoD recruits the formin splice variant hDia2C to early endosomes, aligning them along actin filaments and reducing their motility through activation of c-Src kinase (PMID:12577064), and independently engages the nucleation-promoting factor WHAMM to drive Arp2/3-dependent actin dynamics and FILIP1 to link to filamin A, thereby coordinating migration and adhesion (PMID:23087206). RhoD antagonizes RhoA-mediated stress fiber formation and focal adhesion assembly, suppressing cell migration and cytokinesis (PMID:10229194), and competitively binds the cytoplasmic domains of Plexin-A1 and Plexin-B1 to block Rnd1-induced plexin activation and Semaphorin 3A-mediated axon repulsion (PMID:11784792, PMID:17916560).

1. 1996 High

   The discovery that activated RhoD reorganizes the actin cytoskeleton and governs early endosome motility and distribution established it as a Rho-family GTPase linking membrane traffic to the cytoskeleton.

   Evidence Expression of constitutively active RhoD in cultured cells with live imaging and subcellular fractionation

   PMID:8945468

   Open questions at the time

   • Effector proteins mediating the endosome–cytoskeleton coupling were unknown
   • Whether the endosome phenotype required Rho effector signaling or direct membrane binding was untested
2. 1999 High

   Demonstrating that GTP-bound RhoD disassembles stress fibers and focal adhesions, reduces migration, and blocks cytokinesis revealed a functional antagonism with RhoA and a requirement for GTPase cycling.

   Evidence Microinjection of active/dominant-negative RhoD in fibroblasts; phagokinetic track migration assay; Xenopus embryo cytokinesis assay

   PMID:10229194

   Open questions at the time

   • The downstream effectors mediating stress-fiber disassembly were unidentified
   • Mechanism of cytokinesis inhibition was not resolved
3. 2002 High

   Identification of competitive binding of RhoD and Rnd1 to Plexin-A1, with RhoD blocking Rnd1-driven plexin activation, extended RhoD function to axon guidance and showed it acts as a negative regulator of semaphorin signaling.

   Evidence Co-immunoprecipitation and pulldown of GTPases with plexin cytoplasmic domain; axon collapse and turning assays in sympathetic neurons

   PMID:11784792

   Open questions at the time

   • Structural basis of competitive plexin binding was undefined
   • In vivo relevance of RhoD in axon guidance was not tested
4. 2002 Medium

   Showing that RhoD switches PAR-1/thrombin signaling from a RhoA/ROCK to a Gαq/PLC/CaM-MLCK pathway to promote invasion clarified that RhoD–RhoA antagonism extends to G-protein-coupled receptor signaling.

   Evidence Pharmacological inhibitors (C3 exoenzyme, Y-27632), dominant-negative RhoA, activated RhoD transfection, Boyden chamber invasion assay

   PMID:11919159

   Open questions at the time

   • Single-lab study without independent replication
   • Direct binding between RhoD and GPCR-associated signaling components was not shown
5. 2003 High

   Discovery that RhoD recruits the formin hDia2C to early endosomes and activates c-Src kinase defined a RhoD→hDia2C→c-Src signaling axis that mechanistically explains how RhoD reduces endosome motility and aligns endosomes on actin filaments.

   Evidence Yeast two-hybrid, co-immunoprecipitation, live-cell endosome tracking, c-Src kinase activity assay

   PMID:12577064

   Open questions at the time

   • Whether hDia2C actin-nucleating activity per se is required or whether c-Src is the sole downstream mediator was untested
   • Physiological cargo affected by slowed endosome motility was not identified
6. 2007 High

   Atomic-resolution structural analysis revealed that RhoD binds plexin-B1 through a non-canonical interface on the ubiquitin-like fold domain, and that GTPase binding destabilizes the plexin effector-domain homodimer, providing a structural mechanism for RhoD-mediated plexin regulation.

   Evidence Solution NMR and 2.0 Å X-ray crystallography of plexin-B1 cytoplasmic domain; in vitro GTPase-binding assays

   PMID:17916560

   Open questions at the time

   • Whether RhoD and Rnd1 compete for the same binding surface on plexin-B1 in cells was not directly shown
   • Functional consequences of dimer destabilization on downstream R-Ras GAP activity were not tested
7. 2012 High

   Identification of WHAMM and FILIP1 as RhoD effectors connected RhoD to Arp2/3-dependent actin nucleation and filamin A scaffolding, revealing dual parallel mechanisms by which RhoD controls migration and adhesion.

   Evidence Reciprocal co-immunoprecipitation, siRNA knockdown of RhoD and WHAMM, cell migration and attachment assays

   PMID:23087206

   Open questions at the time

   • Whether WHAMM and hDia2C operate on distinct or overlapping actin pools downstream of RhoD is unknown
   • In vivo physiological relevance of the FILIP1–filamin A axis downstream of RhoD has not been tested

• A complete in vivo model integrating RhoD's endosomal, migratory, and axon-guidance functions is lacking; knockout or conditional deletion phenotypes in mammals have not been reported, and the GEFs and GAPs that regulate RhoD cycling remain poorly defined.
• No mammalian knockout phenotype reported
• GEFs and GAPs specific to RhoD are largely unidentified
• Relationship between endosome and migration phenotypes in vivo is unresolved