Guest Editors: Jean Claude Hervé and Nicolas Bourmeyster
Factors influencing the activities of the Rho family of GTPases
Major effector proteins for Rho GTPases
Cellular signaling networks and Rho GTPases: Major crossroads
Tissue signaling networks involving Rho GTPases
Rho GTPase signaling in the development and progression of disease
Rho GTPase as pathogen targets; therapeutic perspectives
Multiple cytoskeletal pathways and PI3K signaling mediate CDC-42-induced neuronal protrusion in C. elegans
Jamie K Alan, Eric C Struckhoff and Erik A Lundquist
Rho GTPases are key regulators of cellular protrusion and are involved in many developmental events including axon guidance during nervous system development. Rho GTPase pathways display functional redundancy in developmental events, including axon guidance. Therefore, their roles can often be masked when using simple loss-of-function genetic approaches. As a complement to loss-of-function genetics, we constructed a constitutively activated CDC-42(G12V) expressed in C. elegans neurons. CDC-42(G12V) drove the formation of ectopic lamellipodial and filopodial protrusions in the PDE neurons, which resembled protrusions normally found on migrating growth cones of axons. We then used a candidate gene approach to identify molecules that mediate CDC-42(G12V)-induced ectopic protrusions by determining if loss of function of the genes could suppress CDC-42(G12V). Using this approach, we identified 3 cytoskeletal pathways previously implicated in axon guidance, the Arp2/3 complex, UNC-115/abLIM and UNC-43/Ena. We also identified the Nck-interacting kinase MIG-15/NIK and p21-activated kinases (PAKs), also implicated in axon guidance. Finally, PI3K signaling was required, specifically the Rictor/mTORC2 branch but not the mTORC1 branch that has been implicated in other aspects of PI3K signaling including stress and aging. Our results indicate that multiple pathways can mediate CDC-42-induced neuronal protrusions that might be relevant to growth cone protrusions during axon pathfinding. Each of these pathways involves Rac GTPases, which might serve to integrate the pathways and coordinate the multiple CDC-42 pathways. These pathways might be relevant to developmental events such as axon pathfinding as well as disease states such as metastatic melanoma.
IQGAP1 is a key node within the small GTPase network
Guillaume Jacquemet and Martin J Humphries
Coordination of the activity of multiple small GTPases is required for the regulation of many physiological processes, including cell migration. There are now several examples of functional interplay between small GTPase pairs, but the mechanisms that control GTPase activity in time and space are only partially understood. Here, we build on the hypothesis that small GTPases are part of a large, integrated network and propose that key proteins within this network integrate multiple signaling events and coordinate multiple small GTPase activities. Specifically, we identify the scaffolding protein IQGAP1 as a master regulator of multiple small GTPases, including Cdc42, Rac1, Rap1 and RhoA. In addition, we demonstrate that IQGAP1 promotes Arf6 activation downstream of β1 integrin engagement. Furthermore, following literature-curated searches and recent mass spectrometric analysis of IQGAP1-binding partners, we report that IQGAP1 recruits other small GTPases, including RhoC, Rac2, M-Ras, RhoQ, Rab10 and Rab5, small GTPase regulators, including Tiam1, RacGAP1, srGAP2 and HERC1, and small GTPase effectors, including PAK6, N-WASP, several sub-units of the Arp2/3 complex and the formin mDia1. Therefore, we propose that IQGAP1 acts as a small GTPase scaffolding platform within the small GTPase network, and recruits and/or regulates small GTPases, small GTPase regulators and effectors to orchestrate cell behavior. Finally, to identify other putative key regulators of small GTPase cross-talk, we have assembled a small GTPase network using protein-protein interaction databases.
A novel anti-microbial function for a familiar Rab GTPase
Stefania Spanò and Jorge E Galán
Salmonella enterica is a bacterial pathogen that closely interacts with its host and replicates intracellularly. It has evolved the ability to create an intracellular membrane vacuole where it can survive and replicate. The nature of the Salmonella vacuole is still poorly understood, and although it has some features in common with lysosomes, it serves as a suitable niche for its survival. In contrast to broad-host Salmonella enterica serovars, Salmonella enterica serovar Typhi (S. Typhi) is a host-adapted pathogen that does not have the ability to replicate in any species other than humans. Such host adaptation is manifested at the single cell level since this pathogen is unable to survive in non-human macrophages. We recently reported that a pathway dependent on the Rab GTPase Rab32 and its guanine-nucleotide exchange factor BLOC-3 restricts the growth and survival of S. Typhi in non-permissive macrophages. We also found that broad host Salmonellae, such as S. Typhimurium, are able to antagonize this pathway by delivering a bacterial effector protein that specifically cleaves Rab32 resulting in its degradation.
The Rac-specific exchange factors Dock1 and Dock5 are dispensable for the establishment of the glomerular filtration barrier in vivo
Mélanie Laurin, Annie Dumouchel, Yoshinori Fukui and Jean-François Côté
Podocytes are specialized kidney cells that form the kidney filtration barrier through the connection of their foot processes. Nephrin and Neph family transmembrane molecules at the surface of podocytes interconnect to form a unique type of cell-cell junction, the slit diaphragm, which acts as a molecular sieve. The cytoplasmic tails of Nephrin and Neph mediate cytoskeletal rearrangement that contributes to the maintenance of the filtration barrier. Nephrin and Neph1 orthologs are essential to regulate cell-cell adhesion and Rac-dependent actin rearrangement during Drosophila myoblast fusion. We hypothesized here that molecules regulating myoblast fusion in Drosophila could contribute to signaling downstream of Nephrin and Neph1 in podocytes. We found that Nephrin engagement promoted recruitment of the Rac exchange factor Dock1 to the membrane. Furthermore, Nephrin overexpression led to lamellipodia formation that could be blocked by inhibiting Rac1 activity. We generated in vivo mouse models to investigate whether Dock1 and Dock5 contribute to the formation and maintenance of the kidney filtration barrier. Our results indicate that while Dock1 and Dock5 are expressed in podocytes, their functions are not essential for the development of the glomerular filtration barrier. Furthermore, mice lacking Dock1 were not protected from LPS-induced podocyte effacement. Our data suggest that Dock1 and Dock5 are not the important exchange factors regulating Rac activity during the establishment and maintenance of the glomerular barrier.