Serotonin-induced regulation of the actin network for learning-related synaptic growth requires Cdc42, N-WASP, and PAK in Aplysia sensory neurons.

Application of Clostridium difficile toxin B, an inhibitor of the Rho family of GTPases, at the Aplysia sensory to motor neuron synapse blocks long-term facilitation and the associated growth of new sensory neuron varicosities induced by repeated pulses of serotonin (5-HT). We have isolated cDNAs encoding Aplysia Rho, Rac, and Cdc42 and found that Rho and Rac had no effect but that overexpression in sensory neurons of a dominant-negative mutant of ApCdc42 or the CRIB domains of its downstream effectors PAK and N-WASP selectively reduces the long-term changes in synaptic strength and structure. FRET analysis indicates that 5-HT activates ApCdc42 in a subset of varicosities contacting the postsynaptic motor neuron and that this activation is dependent on the PI3K and PLC signaling pathways. The 5-HT-induced activation of ApCdc42 initiates reorganization of the presynaptic actin network leading to the outgrowth of filopodia, some of which are morphological precursors for the learning-related formation of new sensory neuron varicosities.

Vibrio parahaemolyticus inhibition of Rho family GTPase activation requires a functional chromosome I type III secretion system.

Vibrio parahaemolyticus is a leading cause of seafood-borne gastroenteritis; however, its virulence mechanisms are not well understood. The identification of type III secreted proteins has provided candidate virulence factors whose functions are still being elucidated. Genotypic strain variability contributes a level of complexity to understanding the role of different virulence factors. The ability of V. parahaemolyticus to inhibit Rho family GTPases and cause cytoskeletal disruption was examined with HeLa cells. After HeLa cells were infected, intracellular Rho activation was inhibited in response to external stimuli. In vitro activation of Rho, Rac, and Cdc42 isolated from infected HeLa cell lysates was also inhibited, indicating that the bacteria were specifically targeting GTPase activation. The inhibition of Rho family GTPase activation was retained for clinical and environmental isolates of V. parahaemolyticus and was dependent on a functional chromosome I type III secretion system (CI-T3SS). GTPase inhibition was independent of hemolytic toxin genotype and the chromasome II (CII)-T3SS. Rho inhibition was accompanied by a shift in the total actin pool to its monomeric form. These phenotypes were abrogated in a mutant strain lacking the CI-T3S effector Vp1686, suggesting that the inhibiting actin polymerization may be a downstream effect of Vp1686-dependent GTPase inhibition. Although Vp1686 has been previously characterized as a potential virulence factor in macrophages, our findings reveal an effect on cultured HeLa cells. The ability to inhibit Rho family GTPases independently of the CII-T3SS and the hemolytic toxins may provide insight into the mechanisms of virulence used by strains lacking these virulence factors.

Spatial and temporal analysis of Rac activation during live neutrophil chemotaxis.

The ability of cells to recognize and respond with directed motility to chemoattractant agents is critical to normal physiological function. Neutrophils represent the prototypic chemotactic cell in that they respond to signals initiated through the binding of bacterial peptides and other chemokines to G protein-coupled receptors with speeds of up to 30 microm/min. It has been hypothesized that localized regulation of cytoskeletal dynamics by Rho GTPases is critical to orchestrating cell movement. Using a FRET-based biosensor approach, we investigated the dynamics of Rac GTPase activation during chemotaxis of live primary human neutrophils. Rac has been implicated in establishing and maintaining the leading edge of motile cells, and we show that Rac is dynamically activated at specific locations in the extending leading edge. However, we also demonstrate activated Rac in the retracting tail of motile neutrophils. Rac activation is both stimulus and adhesion dependent. Expression of a dominant-negative Rac mutant confirms that Rac is functionally required both for tail retraction and for formation of the leading edge during chemotaxis. These data establish that Rac GTPase is spatially and temporally regulated to coordinate leading-edge extension and tail retraction during a complex motile response, the chemotaxis of human neutrophils.

Purification and biochemical properties of Rac1, 2, 3 and the splice variant Rac1b.

Rac proteins (Rac1, 1b, 2, 3) belong to the GTP-binding proteins (or GTPases) of the Ras superfamily and thus act as molecular switches cycling between an active GTP-bound and an inactive GDP-bound form through nucleotide exchange and hydrolysis. Like most other GTPases, these proteins adopt different conformations depending on the bound nucleotide, the main differences lying in the conformation of two short and flexible loop structures designated as the switch I and switch II region. The three distinct mammalian Rac isoforms, Rac1, 2 and 3, share a very high sequence identity (up to 90%), with Rac1b being an alternative splice variant of Rac1 with a 19 amino acid insertion in vicinity to the switch II region. We have demonstrated that Rac1 and Rac3 are very closely related with respect to their biochemical properties, such as effector interaction, nucleotide binding, and hydrolysis. In contrast, Rac2 displays a slower nucleotide association and is more efficiently activated by the Rac-GEF Tiam1. Modeling and normal mode analysis corroborate the hypothesis that the altered molecular dynamics of Rac2, in particular at the switch I region, may be responsible for different biochemical properties. On the other hand, our structural and biochemical analysis of Rac1b has shown that, compared with Rac1, Rac1b has an accelerated GEF-independent GDP/GTP-exchange and an impaired GTP-hydrolysis, accounting for a self-activating GTPase. This chapter discusses the use of fluorescence spectroscopic methods, allowing real-time monitoring of the interaction of nucleotides, regulators, and effectors with the Rac proteins at submicromolar concentrations and quantification of the kinetic and equilibrium constants.

In vitro guanine nucleotide exchange activity of DHR-2/DOCKER/CZH2 domains.

Rho family GTPases regulate a large variety of biological processes, including the reorganization of the actin cytoskeleton. Like other members of the Ras superfamily of small GTP-binding proteins, Rho GTPases cycle between a GDP-bound (inactive) and a GTP-bound (active) state, and, when active, the GTPases relay extracellular signals to a large number of downstream effectors. Guanine nucleotide exchange factors (GEFs) promote the exchange of GDP for GTP on Rho GTPases, thereby activating them. Most Rho-GEFs mediate their effects through their signature domain known as the Dbl Homology-Pleckstrin Homology (DH-PH) module. Recently, we and others identified a family of evolutionarily conserved, DOCK180-related proteins that also display GEF activity toward Rho GTPases. The DOCK180-family of proteins lacks the canonical DH-PH module. Instead, they rely on a novel domain, termed DHR-2, DOCKER, or CZH2, to exchange GDP for GTP on Rho targets. In this chapter, the experimental approach that we used to uncover the exchange activity of the DHR-2 domain of DOCK180-related proteins will be described.

Biochemical characterization of the Cool (Cloned-out-of-Library)/Pix (Pak-interactive exchange factor) proteins.

The Cool (Cloned out of Library)/Pix (Pak interactive exchange factor) proteins have been implicated in a diversity of biological activities, ranging from pathways initiated by growth factors and chemoattractants to X-linked mental retardation. Initially discovered through yeast two-hybrid and biochemical analyses as binding partners for the Cdc42/Rac-target/effector, Pak (p21 activated kinase), the sequences for the Cool/Pix proteins revealed a DH (Dbl homology) domain. Because the DH domain is the limit functional unit for stimulating guanine nucleotide exchange on Rho family GTP-binding proteins, it was assumed that the Cool/Pix proteins would act as guanine nucleotide exchange factors (GEFs) for the Rho proteins. Of the three known isoforms, (p50Cool-1, p85Cool-1/beta-Pix, and 90Cool-2/alpha-Pix), only Cool-2/alpha-Pix has exhibited significant GEF activity. A number of experimental techniques have been used to characterize Cool-2, and in vitro analysis has revealed that its GEF activity is under tight control through intramolecular interactions involving several binding partners. Here we describe the biochemical methods used to study the Cool/Pix proteins and, in particular, the regulation of the GEF activity of Cool-2/alpha-Pix.

Assay and properties of the GIT1/p95-APP1 ARFGAP.

GIT1/p95-APP1 is an adaptor protein with an aminoterminal ARFGAP domain involved in the regulation of ARF6 function. GIT1/p95-APP1 forms stable complexes with a number of proteins including downstream effectors and exchanging factors for members of the Rho family of small GTPases. This protein can also interact with other adaptor proteins implicated in the regulation of cell adhesion and synapse formation. The stability of the endogenous and reconstituted complexes after cell lysis allows the biochemical identification and characterization of the GIT1 complexes that can be isolated from different cell types. This article presents methods for the identification of the endogenous and reconstituted GIT1 complexes that can be utilized for the biochemical and functional characterization of the complexes from different tissue and cell types.

Purification, crystallization and preliminary X-ray crystallographic analysis of a rice Rac/Rop GTPase, OsRac1.

Small GTPases regulate a large variety of key cellular processes. Plant small Rac/Rop GTPases have recently received broad attention as it is becoming clear that these enzymes regulate various plant cellular processes. OsRac1, a rice Rac/Rop protein, is a key regulator of reactive oxygen species (ROS) production and induces immune responses. Although four structures of plant small GTPases have been reported, all of these were of the inactive form. Here, OsRac1 was purified and co-crystallized with the GTP analogue 5'-guanylyl imidodiphosphate (GMPPNP). The crystal belonged to space group P2(1)2(1)2(1) and a complete data set was collected to 1.9 Šresolution.

Rho GTPase activation assays.

The Rho GTPase family of signaling proteins controls a wide range of highly dynamic cellular processes. Activation of Rho GTPases can be investigated and quantified in cell extracts using so-called pull-down assays. Proteins that bind specifically to the activated form of the Rho GTPase are used to capture it onto a bead support. Western blotting of the captured samples with specific antibodies then allows for quantification of the level of Rho GTPase activation in the sample. This unit describes the techniques for preparing the reagents required for assays of RhoA, Rac, and Cdc42 and gives practical tips for the successful application of the assay in a range of situations.

Characterization of IQGAP1-containing complexes in NK-like cells: evidence for Rac 2 and RACK1 association during homotypic adhesion.

IQGAP1 is a scaffolding protein that binds to a diverse array of signaling and structural molecules that are often associated with cell polarization and adhesion. Through interaction with its target proteins, IQGAP1 participates in multiple cellular functions, including Ca2+-calmodulin signaling, definition of cytoskeletal architecture, regulation of Cdc42 and Rac1 dependent cytoskeletal changes, and control of E-cadherin mediated intercellular adhesion. These analysis have been largely restricted to cells of epithelial and fibroblast origin. The present studies were initiated to examine the role of IQGAP1 in cellular interactions involving the lymphoid cells. A mass spectrometric based analysis of IQGAP1 containing complexes isolated from the human NK-like cell line, YTS, identified several known and new potential IQGAP1 interaction partners including receptor of activated C kinase 1 (RACK1) and the small GTPase, Rac2. Immunofluorescence analysis of YTS cells indicated that a minor component of IQGAP1 was localized at the cell membrane with the remainder diffusely distributed through out the cytoplasm. However, at sites of cellular contact, there was a marked accumulation of IQGAP1. Staining for RACK1 and Rac2 revealed that both of these proteins accumulated these contact sites. Antibody-based studies suggested that a subset of RACK1 was associated in an IQGAP1-containing complex, which prevented recognition of RACK1 by monoclonal antibody. These results suggest that RACK1, Rac2, and IQGAP1 are components of complexes involved in NK cell homotypic adhesion.

Sensory neurons of the Atonal lineage pioneer the formation of glomeruli within the adult Drosophila olfactory lobe.

The first centers for processing of odor information by animals lie in the olfactory lobe. Sensory neurons from the periphery synapse with interneurons in anatomically recognizable units, termed glomeruli, seen in both insects and vertebrates. The mechanisms that underlie the formation of functional maps of the odor-world in the glomeruli within the olfactory lobe remains unclear. We address the basis of sensory targeting in the fruitfly Drosophila and show that one class of sensory neurons, those of the Atonal lineage, plays a crucial role in glomerular patterning. Atonal-dependent neurons pioneer the segregation of other classes of sensory neurons into distinct glomeruli. Furthermore, correct sensory innervation is necessary for the arborization of projection neurons into glomeruli and for the elaboration of processes of central glial cells into the lobe.

Control of morphogenesis and actin localization by the Penicillium marneffei RAC homolog.

Rac proteins control polarized growth in many organisms but the specific function of these proteins remains undefined. In this study, we describe the cloning and functional characterization of a RAC homolog, cflB, from the dimorphic fungus Penicillium marneffei. P. marneffei produces asexual spores on complex structures (conidiophores) and switches between hyphal and yeast growth. CflB colocalizes with actin at the tips of vegetative hyphal cells and at sites of cell division. Deletion of cflB results in cell division (septation) and growth defects in both vegetative hyphal and conidiophore cell types such that cells become depolarized, exhibit inappropriate septation and the actin cytoskeleton is severely disrupted. This data suggests that Rac proteins play a crucial role in actin dependent polarized growth and division. The CDC42 ortholog in P. marneffei, cflA, controls vegetative hyphal and yeast growth polarization but does not affect asexual development. By contrast, CflB affects cellular polarization during asexual development and hyphal growth but not during yeast growth. This shows that these two GTPases have both overlapping and distinct roles during growth and development. RAC orthologs are not found in less morphologically complex eukaryotes such as Saccharomyces cerevisiae, suggesting that RAC genes might have evolved with increasing cellular complexity.