Plexin B Regulates Rho through the Guanine Nucleotide Exchange Factors Leukemia-associated Rho GEF (LARG) and PDZ-RhoGEF

Abstract

Plexins represent a novel family of transmembrane receptors that transduce attractive and repulsive signals mediated by the axon-guiding molecules semaphorins. Emerging evidence implicates Rho GTPases in these biological events. However, Plexins lack any known catalytic activity in their conserved cytoplasmic tails, and how they transduce signals from semaphorins to Rho is still unknown. Here we show that Plexin B2 associates directly with two members of a recently identified family of Dbl homology/pleckstrin homology containing guanine nucleotide exchange factors for Rho, PDZ-Rho-GEF, and Leukemia-associated Rho GEF (LARG). This physical interaction is mediated by their PDZ domains and a PDZ-binding motif found only in Plexins of the B family. In addition, we show that ligand-induced dimerization of Plexin B is sufficient to stimulate endogenous RhoA potently and to induce the reorganization of the cytoskeleton. Moreover, overexpression of the PDZ domain of PDZ-RhoGEF but not its regulator of G protein signaling domain prevents cell rounding and neurite retraction of differentiated PC12 cells induced by activation of endogenous Plexin B1 by semaphorin 4D. The association of Plexins with LARG and PDZ-RhoGEF thus provides a direct molecular mechanism by which semaphorins acting on Plexin B can control Rho, thereby regulating the actin-cytoskeleton during axonal guidance and cell migration.The Rho family of small GTP-binding proteins, which includes Rho, Rac, and Cdc42, participate in a number of key cellular events, such as the regulation of cell morphology, cell aggregation, tissue polarity, cytokinesis, cell motility, smooth muscle contraction, and cell growth control (reviewed in Refs. 1 and 2). In vivo, the activity of small GTP-binding proteins of the Rho family is tightly regulated. Guanine nucleotide exchange factors (GEFs) 1 act as activators by promoting the conversion of the inactive GDP-bound to the active GTP-bound species. All known GEFs for GTP-binding proteins have in common a 250-amino acid stretch of significant similarity with Dbl, called Dbl homology (DH) domain, that is critical for their ability to stimulate nucleotide exchange toward GTPases of the Rho family and is often adjacent to a pleckstrin-homology (PH) domain (reviewed in Ref.3). In turn, these GEFs link a variety of cell surface receptors to the activation of Rho proteins, thereby regulating the dynamic remodeling of actin-containing cytostructures and gene expression.Recently, a new family of DH-domain containing RhoGEFs, including PDZ-RhoGEF, leukemia-associated RhoGEF (LARG), and p115RhoGEF, has been identified (4 -6). In addition to their DH/PH domains, these GEFs exhibit a number of structural motifs suggestive of a role in signal transduction. Indeed, these GEFs associate with G␣ 12 and G␣ 13 through a regulators of G protein signaling (RGSs)-like domain (5, 7, 8), thus providing a direct link from these heterotrimeric G protein ␣-subunits and their coupled cell surface receptors to Rho. Of interest, PDZ-RhoGEF and LARG, but n...

Semaphorin signaling in angiogenesis, lymphangiogenesis and cancer

“…Similarly, Sema4D can be released from platelets by the action of metalloprotease ADAM17 (TACE), thereby acting on endothelial cells as well as platelets that also express Sema4D receptors [72]. The pro-angiogenic effect of Sema4D is mediated by the activation of a small GTPase RhoA through Rho-specific GEFs, leukemiaassociated RhoGEF, and PDZ-RhoGEF, which bind to the C-terminal PDZ-binding motif of plexin-B1 [67,[73][74][75][76]. Upon ligation of Sema4D, PDZ-RhoGEF and leukemia-associated RhoGEF are recruited to plexin-B1, thus stimulating RhoA and its downstream effector Rho kinase, and Rho kinase activation promotes angiogenic response through a chain of events involving myosin light chain phosphorylation, stress fiber contaction, nonreceptor tyrosine kinase activation, and activation of the Akt and Erk pathways [77] (Figure 3).…”

Regulation of immune cell responses by semaphorins and their receptors

“…In the nervous system, semaphorin-plexin signaling has been shown to mediate diverse neural functions by regulating GTPase activities and cytoplasmic/receptor-type protein kinases. 11,31,32 These plexin-mediated signals are involved in integrin-mediated attachment, 15,33,34 actomyosin contraction [35][36][37][38] and microtubule destabilization. [39][40][41] In addition, plexins can associate with different coreceptors in distinct tissues to allow semaphorins to exert pleiotropic functions.…”

Control of cell mechanics by RhoA and calcium fluxes during epithelial scattering

“…99 PDZRhoGEF, p190RhoGEF, and leukemia-associated Rho GEF (LARG) are examples of Rho-specific GEFs that could link pyk2 and RhoA. 98,[100][101][102] Calcium may also act on myosin independently of RhoA. Myosin light chain kinase (MLCK) is a Ca 2C / Calmodulin dependent kinase that phosphorylates the regulatory light chains of myosin II isoforms.…”