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FEBS Letters 585 (2011) 723–729

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Review IQGAP1 in microbial pathogenesis: Targeting the ⇑ Hugh Kim a, Colin D. White b, David B. Sacks c,

a Department of Translational Medicine, Brigham and Women’s Hospital and Harvard Medical School, 1 Blackfan Circle, Boston, MA 02115, USA b Department of Pathology, Beth Israel Deaconess Medical Center and Harvard Medical School, 3 Blackfan Circle, Boston, MA 02115, USA c Department of Laboratory Medicine, National Institutes of Health, 10 Center Drive, Bethesda, MD 20892, USA

article info abstract

Article history: Microbial pathogens cause widespread morbidity and mortality. Central to the pathogens’ virulence Received 28 November 2010 is manipulation of the host cell’s cytoskeleton, which facilitates microbial invasion, multiplication, Revised 25 January 2011 and avoidance of the innate immune response. IQGAP1 is a ubiquitously expressed scaffold Accepted 26 January 2011 that integrates diverse signaling cascades. Research has shown that IQGAP1 binds to and modulates Available online 2 February 2011 the activity of multiple that participate in bacterial invasion. Here, we review data that sup- Edited by Renee Tsolis port a role for IQGAP1 in infectious disease via its ability to regulate the actin cytoskeleton. In addi- tion, we explore other mechanisms by which IQGAP1 may be exploited by microbial pathogens. Published by Elsevier B.V. on behalf of the Federation of European Biochemical Societies. Keywords: EPEC IQGAP1 Microbial pathogenesis Salmonella

1. Introduction a homology domain, a WW domain, four IQ motifs (IQ motifs bind ), and a region with significant sequence Microbial pathogens are a major cause of morbidity and mortal- similarity to the catalytic domain of Ras GTPase-activating proteins ity worldwide. In the United States alone, an estimated 76 million (GAPs) [5]. Each of these domains serves to mediate the interaction foodborne illnesses (caused primarily by Salmonella enterica of IQGAP1 with multiple distinct proteins [6]. By regulating the serovar typhimurium, Campylobacter jejuni, Shigella flexneri, function of its binding partners, IQGAP1 participates in diverse cel- Cryptosporidium parvum and Escherichia coli) occur annually, and lular functions, ranging from small GTPase signaling to control of account for an estimated treatment cost of up to 83 billion US dol- cell proliferation and [6,7]. Of particular relevance in the lars [1]. Despite considerable variation in the manner by which they context of this article is the role of IQGAP1 in the maintenance of produce disease, most microbial pathogens exert and sustain their cytoskeletal architecture. IQGAP1 binds actin directly [8], enhances effects by usurping a relatively limited number of signaling actin polymerization in vitro [9,10], and colocalizes with actin in pathways inside the host cell. In particular, microbes frequently lamellipodia [11]. Moreover, IQGAP1 stimulates actin assembly manipulate the cytoskeleton of the host cell, thereby facilitating by forming complexes with N-WASP (neuronal Wiskott Aldrich their attachment and entry [2–4]. Microbial pathogens also employ Syndrome protein) and Arp2/3 (actin-related protein 2/3) [12]. several survival strategies, allowing them to migrate within the By controlling the activity of the small GTPases Rac1 and Cdc42, host cell and avoid bactericidal defense mechanisms [2–4]. Control IQGAP1 also modulates the cytoskeleton indirectly. (Note that, de- of the host cell’s cytoskeleton is also integral to each of these stages spite its name, IQGAP1 is not a GAP and actually stabilizes Rac1 of infection, therefore proteins that govern cytoskeletal remodeling and Cdc42 in their active forms [11,13].) The role of IQGAP1 in cel- participate in microbial pathogenesis. In this review, we summarize lular signaling and cytoskeletal dynamics has been the focus of recent evidence that strongly supports a role for the several excellent reviews [5–7,14,15]. Here, we focus only on those IQGAP1 in infectious disease. IQGAP1 functions germane to microbial pathogenesis.

2. IQGAP1: a key modulator of cytoskeletal function 3. IQGAP1 and microbial pathogenesis

IQGAP1 is a ubiquitously expressed 189-kDa scaffold protein Early evidence to implicate the involvement of IQGAP1 in that contains several protein-interacting domains. These include microbial pathogenesis was derived by profiling. Microarray analysis revealed that <3.5% of 3500 genes in a human monocyte ⇑ Corresponding author. Fax: +1 301 402 1885. cell line, U937, had altered expression following infection with E-mail address: [email protected] (D.B. Sacks). Mycobacteria. One of the identified was IQGAP1, which was

0014-5793/$36.00 Published by Elsevier B.V. on behalf of the Federation of European Biochemical Societies. doi:10.1016/j.febslet.2011.01.041 724 H. Kim et al. / FEBS Letters 585 (2011) 723–729 downregulated 5.6-fold [16]. Proteomic analysis later showed re- bacterial toxins directly into host cells [23]. Among the injected duced IQGAP1 expression in murine splenic tissue after infection effectors are SopE and SopE2, which act as guanine nucleotide ex- by Yersinia pestis [17], suggesting that IQGAP1 may be a target change factors (GEFs). In their catalytically inactive forms, Rac1 for pathogen-induced changes in the host cell. Consistent with this and Cdc42 are bound to guanosine diphosphate (GDP). GEFs cata- postulate, IQGAP1 is known to interact with numerous proteins lyze the substitution of GDP by guanosine-50-triphosphate (GTP), that functionally link pathogenic microbes to host cell invasion resulting in Rac1 and Cdc42 activation [24]. Once activated, Rac1 (Table 1). For example, IQGAP1 binding to Dia1, a Diaphanous- and Cdc42 activate N-WASP and the Arp2/3 complex, thereby pro- related formin that assembles actin filaments, is required for phag- moting actin polymerization and actin filament elongation at the S. ocytic cup formation [18], an essential step in microbial invasion typhimurium-host cell interface [25]. These molecular events result into host cells [19]. IQGAP1 also binds directly to selected bacterial in the formation of membrane ruffles that facilitate S. typhimurium proteins with defined roles in pathogen invasion, including the internalization. E. coli-derived Tir and Ibe, and the S. typhimurium-derived SseI Recent published data indicate S. typhimurium modulates [20–22] (Table 1). The functional consequences of these interac- IQGAP1 to gain entry into host cells [26]. IQGAP1 is recruited to tions are discussed in more detail in the following paragraphs. sites of S. typhimurium attachment to HeLa cells, and siRNA- mediated knockdown of IQGAP1 reduces ruffle formation and de- 4. IQGAP1 is a target for S. typhimurium pathogenesis creases S. typhimurium infection by 33%. The magnitude of this ef- fect may be limited by residual IQGAP1 in the siRNA-treated cells, 4.1. Regulation of IQGAP1 for S. typhimurium invasion since S. typhimurium entry into IQGAP1-null mouse embryonic fibroblasts (MEFs) is reduced by 65% compared to control MEFs As is characteristic of many cell-invasive pathogens, S. typhimu- [26]. These data suggest that IQGAP1 is usurped by S. typhimurium rium employs an elaborate molecular apparatus, called a type III to enter host cells. The molecular mechanisms underlying these secretion system (T3SS), to facilitate its infection by injecting observations have begun to be characterized. Overexpression of

Table 1 Host- and pathogen-derived IQGAP1 binding partners relevant to microbial pathogenesis.

Proposed function(s) of interaction with IQGAP1 Relevance of binding partner to microbial pathogenesis Reference(s) Host-derived IQGAP1 binding partner Actin Cross-links actin filaments Intricately involved in many aspects of bacterial invasion [8,26,74] Arf6 Links Arf6 to Rac1 activation and Controls bacterial invasion by modulating host cell actin [75] remodeling Arp2/3 Links growth factor signaling to actin assembly Promotes host ruffling; essential for [12] S. typhimurium, S. flexneri and Rickettsia conorii invasion c-Src c-Src catalyzes tyrosine phosphorylation of IQGAP1 and Promotes bacterial invasion [76] bridges IQGAP1 to VEGFR2 Calmodulin Regulates IQGAP1 function EPEC-induced actin pedestals are formed [20,41] via a Ca2+/calmodulin-dependent pathway CaMKII Unknown; may be involved in the regulation of cell Phosphorylates vimentin at Ser82, thereby positively regulating [77] adhesion EPEC invasion CD44 Links hyaluronan to actin cytoskeleton Recruited to the bacterial attachment site during EPEC infection [78] Cortactin Necessary for hyperoxia-induced tyrosine phosphorylation Tyrosine phosphorylation of cortactin is required for invasion of [79] of Src, and cortactin and ROS generation biliary epithelium by C. parvum ERK1/2 MAPK Scaffold Stimulated by S. typhimurium infection; involved in C. jejuni [53,71] internalization Exo70 Necessary for correct localization of the exocyst; regulates Recruitment of the exocyst to localized areas of the host cell [80] protein synthesis, exocytosis and secretion membrane promotes S. typhimurium invasion MEK1/2 MAPK Scaffold MAPK signaling targeted by cell-invasive bacteria; MEK activation [51,54,55] required for efficient invasion of L. monocytogenes, S. typhimurium, Chlamydophila pneumonia and Pseudomonas aeruginosa N-WASP Activates N-WASP and stimulates actin assembly; Promotes host cell actin polymerization; essential for EPEC and [12,81] necessary for N-WASP localization at lamellipodia S. flexneri invasion

PtdIns(4,5)P2 Maintains integrity of PIP2-containing lipid rafts PIP2 is metabolized at sites of S. typhimurium invasion [49] PLD2 Necessary for hyperoxia-induced tyrosine phosphorylation PLD activity correlates with Acinetobacter baumannii pathogenesis [79] of Src, and cortactin and ROS generation Rac1/Cdc42 Inhibits intrinsic GTPase activity, thereby stabilizing active Involved in host cell membrane ruffling; essential [74,82,83] form for S. typhimurium invasion ShcA Unknown; may be important in cytoskeletal Affects S. typhimurium adherence to host cell; involved [84] reorganization in response to activation of growth factor in C. pneumoniae invasion receptors WAVE2 Involved in lamellipodia formation in response to HGF Required for invasion of L. monocytogenes; regulates invasion [85] of S. typhimurium Pathogen-derived IQGAP1 binding partnera Gag (M-MuLV) Necessary for virus trafficking and replication Promotes viral replication [48] Ibe (EPEC) Appears to be necessary for Ibe function during infection Promotes EPEC invasion [21] SseI (S. typhimurium) Necessary for SseI inhibition of cell migration in primary Disables host macrophage function [22] macrophages and dendritic cells Tir (EPEC) Regulates actin pedestal formation by EPEC Promotes EPEC invasion [20]

Abbreviations: Arf6, ADP-ribosylation factor 6; CaMKII, Ca2+/calmodulin-dependent protein kinase II; EPEC, enteropathogenic E. coli; ERK, extracellular-regulated kinase; HGF, hepatocyte growth factor; Ibe, IQGAP1-binding effector protein; M-MuLV, moloney murine leukemia virus; MAPK, mitogen-activated protein kinase; MEK, MAPK kinase;

PtdIns(4,5)P2, phosphatidylinositol 4,5-bisphosphate; PLD2, Phospholipase D2; ROS, reactive oxygen species; Tir, translocated intimin receptor; VEGFR2, vascular endothelial growth factor receptor-2; WAVE, WASP family Verprolin-homologous protein. a The pathogen from which each protein is derived is indicated in parentheses. H. Kim et al. / FEBS Letters 585 (2011) 723–729 725

IQGAP1 increases the amount of active Rac1 and Cdc42 in cells, maintaining them in their GTP-bound form. IQGAP1 binding to while reducing the amount of endogenous IQGAP1 markedly de- Rac1-GTP and Cdc42-GTP enhances actin polymerization, and re- creases the activity of both GTPases [13,26]. During S. typhimurium sults in the recruitment of additional IQGAP1 (which is also bound infection of HeLa cells, the levels of active Rac1 and Cdc42 increase to active Rac1 and Cdc42) to the site of S. typhimurium attachment. >2-fold [26]. However, in IQGAP1-null MEFs, Rac1 and Cdc42 acti- The presence of increased Rac1-GTP and Cdc42-GTP bound to vation is abrogated and S. typhimurium invasion is decreased [26]. IQGAP1 further augments actin polymerization, inducing the These findings imply that regulation of Rac1 and Cdc42 by IQGAP1 formation of membrane ruffles and facilitating S. typhimurium is important for S. typhimurium entry. Consistent with this hypoth- internalization. IQGAP1 therefore functionally links Rac1 and esis, S. typhimurium infection is increased in cells transfected with Cdc42 to actin, augments actin polymerization, and promotes wild-type IQGAP1, but not in cells transfected with an IQGAP1 bacterial invasion. mutant that lacks Rac1 and Cdc42 binding [26]. Interestingly, an IQGAP1 mutant that does not bind actin (termed IQGAP1G75Q 4.2. S. typhimurium targets IQGAP1 to establish chronic infection [27]) also fails to promote S. typhimurium entry [26]. Moreover, in contrast to wild-type IQGAP1, IQGAP1G75Q does not translo- An important aspect of S. typhimurium pathogenicity is the abil- cate to sites of S. typhimurium infection. Based on the data de- ity of certain species to establish chronic, long-term infection in scribed above, S. typhimurium invasion into host cells appears the host by evading the host’s immune response [28,29]. The contingent on IQGAP1 binding to both Rac1/Cdc42 and actin. capacity of S. typhimurium to survive and replicate in macrophages Based on research from our laboratory, we propose a model that is therefore of major importance to the ability of the pathogen to integrates the observations described above (Fig. 1). As previously cause disease. Macrophages are key participants in cell-mediated highlighted, S. typhimurium attachment to the host cell results in immunity as they destroy foreign pathogens and function as anti- the injection of effectors via the T3SS. Following their injection, gen-presenting cells [30]. The S. typhimurium pathogenicity island SopE and SopE2 catalyze the exchange of GDP on Rac1 and 2 (SPI2)-coded T3SS injects proteins that enable S. typhimurium to Cdc42 for GTP. This facilitates actin polymerization. IQGAP1 bound create a S. typhimurium-containing vacuole (SCV) in which it can to actin translocates to the site of S. typhimurium attachment on replicate [31]. Moreover, SPI2-secreted effectors allow intracellular the host cell. Here, IQGAP1 binds to active Rac1 and Cdc42, bacteria to avoid the bactericidal properties of macrophages and

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Fig. 1. S. typhimurium targets IQGAP1 to invade host cells. (A) S. typhimurium binds to the host cell membrane and injects effector proteins (small circles) that catalyze the activation of Rac1 and Cdc42. (B) Active (GTP-bound) Rac1 and Cdc42 promote localized actin polymerization at the S. typhimurium-host cell interface. IQGAP1 bound to actin translocates to the site of S. typhimurium attachment. (C) IQGAP1 binds Rac1-GTP and Cdc42-GTP, stabilizing the GTPases in their active GTP-bound forms. The active GTPases promote further local actin polymerization. (D) Ongoing actin assembly induces the accumulation of additional IQGAP1, with attached active Rac1 and Cdc42, at the attachment site. This process promotes further actin polymerization, augmenting formation of membrane ruffles that engulf the S. typhimurium. S. typhimurium is internalized within a S. typhimurium-containing vacuole (SCV). 726 H. Kim et al. / FEBS Letters 585 (2011) 723–729 dendritic cells, and to interfere with antigen presentation to T-cells For example, IQGAP1 binds Tir in vitro, and confocal microscopy [32]. One such Salmonella effector, SseI (also known as SrfH), im- reveals that IQGAP1 colocalizes with Tir at EPEC-induced actin pedes the normal migration of macrophages and dendritic cells, pedestals [20]. Subsequently, another bacterial protein, termed thus severely impairing their bactericidal abilities [22]. Conse- IQGAP1-binding effector protein (Ibe), was shown to regulate Tir quently, mice infected with wild-type S. typhimurium continue to phosphorylation and actin pedestal formation [21]. Like Tir, exhibit increased systemic levels of bacteria up to 45 days post- IQGAP1 binds Ibe in vitro, and colocalizes with Ibe at actin pedes- infection, while mice infected with SseI-deficient S. typhimurium tals. Although the mechanism underlying the contribution of Ibe to clear the pathogen more rapidly [22]. Importantly, IQGAP1 may EPEC pathogenesis remains to be defined, these data provide fur- contribute to the ability of S. typhimurium to circumvent the host’s ther evidence that IQGAP1 is a critical component of E. coli immune response and establish chronic infection. IQGAP1 binds infection. SseI directly in vitro, and colocalizes with SseI in S. typhimurium- Research carried out in our laboratory integrates the findings infected macrophages [22]. Moreover, the inhibitory effect of SseI discussed above (Fig. 2). In this model, EPEC binds to the surface on macrophage migration is contingent on IQGAP1 expression; of cells and injects effectors via its T3SS. One of the effectors in- SseI does not impair migration of macrophages lacking IQGAP1 jected, Tir, associates with intimin and is retained at the site of [22]. The S. typhimurium effector SseI therefore exploits IQGAP1 infection. IQGAP1, presumably via its interaction with Tir, also to reduce macrophage motility, thereby suppressing host immu- accumulates at the EPEC injection site. Simultaneously, EPEC in- 2+ nity and promoting a chronic infective state. duces an increase in [Ca ]i, thereby enhancing the association of IQGAP1 with calmodulin, which reduces IQGAP1 binding to Rac1 5. IQGAP1 is a target for E. coli pathogenesis and Cdc42 [41]. Because it is bound to IQGAP1, calmodulin accu- mulates at the site of bacterial adhesion. Recruitment of Nck in- Like S. typhimurium, enteropathogenic E. coli (EPEC) utilizes a duces clustering of N-WASP and Arp2/3 at the cell-microbe T3SS to establish infection in the host [23]. Evidence suggests that interface, thereby promoting polymerization of actin. The presence this is a multi-step process [33]. First, EPEC injects translocated of IQGAP1 and calmodulin, and the interaction of IQGAP1 with Tir, intimin receptor (Tir) into the host cell. Tir is inserted into the host augments this localized actin polymerization, contributing to ped- cell plasma membrane, where it binds intimin, which is embedded estal formation. Additionally, IQGAP1 bundles actin to ensure that in the bacterial outer membrane [34]. Tir-intimin adhesion leads to an ordered structure of parallel filaments is formed. Tir clustering, which recruits the host protein Nck. Binding to inti- min also catalyzes phosphorylation of Tir by the host cell non- 6. Other bacteria regulate IQGAP1 receptor tyrosine kinase c-Fyn, resulting in N-WASP and Arp2/3 recruitment to the cell-microbe interface [35,36]. N-WASP medi- In addition to being manipulated by S. typhimurium and EPEC, ates actin polymerization, thereby creating an ‘‘actin pedestal’’. IQGAP1 is exploited by other pathogens as part of their infective This structure, which is an essential hallmark of E. coli pathogene- mechanisms. For example, a fundamental aspect of Shigella patho- sis [33], elevates and supports the EPEC above the surface of the genesis is the intercellular spread of bacteria within epithelial tis- epithelial cell. sues [42]. Initial data indicate that intercellular spread of S. flexneri EPEC manipulates IQGAP1 to mediate infection. Specifically, is significantly enhanced in IQGAP1-null MEFs, suggesting that EPEC induces the translocation of IQGAP1 to actin pedestals where IQGAP1 may be targeted during S. flexneri infection (R. Lu, D.B. IQGAP1 is necessary for pedestal formation [20]. Moreover, actin Sacks, M.B. Goldberg, unpublished observations). Other findings polymerization induced by EPEC in IQGAP1-null MEFs is signifi- concern the regulation of IQGAP1 during infection of host cells cantly less than that in control cells, and reconstitution of IQGAP1 by Pseudomonas aeruginosa and Helicobacter pylori. In HL60 leuke- into IQGAP1-null MEFs rescues EPEC infection. Interestingly, the mia cells, IQGAP1 is recruited to sites of P. aeruginosa attachment molecular mechanisms underlying these observations are different [43]. Moreover, infection of human antral epithelial cells with H. to those of S. typhimurium. In contrast to S. typhimurium, which pylori increases IQGAP1 mRNA and induces translocation of promotes the binding of IQGAP1 to Rac1 and Cdc42 [26], EPEC IQGAP1 protein from the to intracellular tubulovesicular inhibits the interaction of IQGAP1 with the Rho GTPases [20]. An- structures [44]. Collectively, these findings implicate IQGAP1 as a other important difference between S. typhimurium and EPEC is host cell target for infection by several bacteria. Further work is that the latter modulates association of IQGAP1 with calmodulin likely to identify additional bacteria that manipulate IQGAP1 to [20]. Calmodulin, a 16.7-kDa Ca2+-binding protein, is an important facilitate host cell infection. transducer of Ca2+ signals [37].Ca2+ signaling is necessary for actin remodeling near the cell surface [38], and increases in intracellular 7. IQGAP1 as a target for viral pathogenesis 2+ 2+ free Ca concentrations ([Ca ]i) are required for EPEC-induced ac- tin pedestal formation [39,40]. Importantly, treatment with a cell- Viruses are strictly dependent on host cells for the transcription permeable calmodulin antagonist (CGS9343B) or a cell-permeable of their genomes [45]. Invasion of host cells is therefore an important 2+ Ca chelator (1,2-bis(2-aminophenoxy)ethane-N,N,N0,N0-tetraace- determinant of viral pathogenesis. Gag proteins are major virulence tic acid tetrakis(acetoxymethyl ester) (BAPTA/AM)) abrogates actin factors produced by retroviruses since they promote viral binding to pedestal formation in wild-type but not IQGAP1-null MEFs [20]. the host cell membrane [46]. The matrix protein domains of Gag are 2+ These data indicate that Ca /calmodulin signaling by EPEC to particularly important for the intracellular trafficking of viral pro- induce actin pedestal formation is mediated entirely through teins [47]. Evidence suggests that IQGAP1 may also be a target of vir- IQGAP1. Thus, although both S. typhimurium and E. coli usurp al pathogenesis. The Gag matrix protein of the Moloney murine IQGAP1 function, the bacteria employ different molecular mecha- leukemia virus (M-MuLV) binds IQGAP1, and this binding is essen- nisms and exploit distinct IQGAP1 binding partners to infect host tial for viral replication [48]. Moreover, viruses encoding mutant, cells. non-IQGAP1-binding matrix proteins are replication deficient, and Additional insight into how EPEC manipulates IQGAP1 is de- M-MuLV replication is reduced when IQGAP1 is knocked down in rived from research from both our laboratory and other investiga- host cells with siRNA. Additional studies are needed to determine tors. IQGAP1 interacts directly with EPEC-derived effector proteins whether other viruses also require IQGAP1 for replication, and/or that are injected into the host cell and facilitate infection [20,21]. whether IQGAP1 participates in viral infection in other ways. H. Kim et al. / FEBS Letters 585 (2011) 723–729 727

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Fig. 2. IQGAP1 and Ca2+/calmodulin are required for actin pedestal formation by EPEC. (A) EPEC binds to the surface of the host cell and injects effector proteins, including Tir, 2+ 2+ thereby promoting a transient increase in [Ca ]i. (B) IQGAP1 binds Tir directly and is recruited to the site of EPEC adhesion. The increase in [Ca ]i promotes the interaction of IQGAP1 with calmodulin (CaM), with a concomitant disruption of its association with Rac1 and Cdc42. (C) Following its binding to intimin, Tir is clustered and tyrosine- phosphorylated, inducing the recruitment of Nck to the site of EPEC attachment. Nck directs N-WASP and the Arp2/3 complex to clustered Tir, where together they form an IQGAP1-containing complex that promotes actin polymerization. (D) IQGAP1 contributes to ongoing actin polymerization, resulting in the formation of a pedestal structure that supports EPEC. Additionally, IQGAP1 bundles actin to ensure that an ordered structure of parallel filaments is formed.

8. Pathogenic microbes may target other IQGAP1-associated [60] and by S. typhimurium during entry into fibroblasts [55]. Like pathways IQGAP1, the phosphoinositide PI(4,5)P2 (PtdIns(4,5)P2) promotes actin assembly by activating N-WASP and the Arp2/3 complex

We have reviewed evidence which reveals that IQGAP1 inter- [61], and PtdIns(4,5)P2 is enriched in areas undergoing active actin acts with numerous proteins known to be directly involved in host polymerization (such as the S. typhimurium-host cell interface) cell infection and/or microbial survival. In addition, other IQGAP1- [62]. Importantly, a functional interaction between IQGAP1 and interacting partners, such as phosphoinositides [49], microtubules PtdIns(4,5)P2 has been established. IQGAP1 colocalizes with [50] and mitogen-activated protein kinases (MAPKs) [51–53], are PtdIns(4,5)P2 at the leading edge of growth factor-stimulated cells, targeted by microbial pathogens [54–57] (Table 1). Conceivably, and knockdown of IQGAP1 results in fragmentation of these IQGAP1 binding molecules may also be modulated during PtdIns(4,5)P2-enriched lipid rafts [49]. Based on these data, it is microbial pathogenesis in an IQGAP1-regulated manner. Although tempting to speculate that PtdIns(4,5)P2 activity at the cell mem- no experimental evidence has been published to date, we suggest brane may be regulated by IQGAP1, and that this interaction may possible contributions of these IQGAP1 binding partners to infec- play a role in microbial invasion of the host cell. tious disease. 8.2. IQGAP1 and microtubules in bacterial invasion 8.1. IQGAP1, phosphoinositides and microbial pathogenesis Microtubules are elements of the cytoskeleton and are essential Phosphoinositides are cell membrane-based signaling mole- for cell division, cell migration, vesicle transport and cell polarity cules that are critical for cytoskeletal remodeling and intracellular [63]. Some pathogenic bacteria, including S. typhimurium, E. coli, trafficking processes [58]. The seven known phosphoinositides are Shigella, L. monocytogenes and C. jejuni, exploit host microtubule derived from phosphorylation of the precursor molecule, phospha- networks [55,64–67]. For example, pretreatment of fibroblasts tidylinositol (PI). PI is phosphorylated on the inositol ring by PI with microtubule-depolymerizing agents such as colchicine or kinases to produce several distinct phosphoinositides. These sig- nocodazole impairs S. typhimurium invasion, suggesting that intact naling molecules have multiple functions, including regulation of microtubule networks are required for entry of the bacteria [55].In cytoskeletal rearrangement, which is required for invasion by mi- this context, IQGAP1 regulates microtubule function through its crobes. For example, E. coli invades human brain endothelial cells interaction with CLIP-170 and mDia1 [50,68]. Moreover, siRNA- in a PI3-kinase-dependent manner [59]. Similarly, PI3-kinase is mediated knockdown of IQGAP1 results in decreased stability targeted by Listeria monocytogenes during invasion of Vero cells and increased dynamics of microtubules [68]. Additional work is 728 H. 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