Trends in Microbiology
Cortactin: an Achilles' heel of the actin cytoskeleton targeted by pathogens
Introduction
Many microbial pathogens manipulate the actin cytoskeleton within their eukaryotic hosts for their own benefit 1, 2, 3. Actin assembly plays a fundamental role in many vital cellular processes, such as membrane dynamics, migration, adhesion and transport. Thus, by targeting actin, pathogens can radically and rapidly ‘reprogram’ their host target cell to enable processes, such as attachment and entry into cells, movement within and between cells, vacuole formation and remodeling, and avoidance of phagocytosis, to take place. Our current understanding of these mechanisms reveals a striking diversity in the strategies that are used by different pathogens.
Subversion of the host actin cytoskeleton by pathogens is a complex multi-factorial process that involves, for example, the Rho-family GTPases, phosphoinositides and actin-binding proteins. Among the numerous proteins involved in the actin cytoskeletal network, cortactin appears to be one of the favorite targets of microbes. Discovered more than a decade ago and named after its cortical localization and actin-binding properties [4], cortactin plays a crucial role in the regulation of actin dynamics in eukaryotic cells. Studies on the molecular basis of the infections afflicted by pathogens, including enteropathogenic Escherichia coli (EPEC), Shigella, Helicobacter and vaccinia virus, have shown that cortactin plays a crucial role in invasion, actin-based motility, pedestal formation and cell scattering (Figure 1; Table 1). Importantly, this frequently coincides with a change in the cellular localization and/or tyrosine phosphorylation of cortactin. Thus, cortactin is emerging as another important cellular target of microbes, suggesting that it can be regarded as an Achilles' heel of the actin cytoskeleton. Here, we review the fundamental aspects of cortactin function during microbial infection. We begin with a description of the domain structure and cellular functions of cortactin. Then we address, in the light of exciting new findings by Martinez-Quiles et al. [5] and Bougneres et al. [6], the long-standing question of how the activities of cortactin can be regulated by phosphorylation. Finally, we discuss the implications of these findings for microbial pathogenesis by revisiting the various infection models in more detail.
Section snippets
Cortactin domains and functions
The reason for the popularity of cortactin as a target for a growing number of microbial pathogens lies in its central role in cellular signal transduction 7, 8; cortactin has several domains that mediate binding and regulation of other cellular proteins (Figure 2). The N-terminal acidic (NTA) domain binds and contributes to the activation of the Arp2/3 complex 9, 10, a process involved in the dynamic assembly of branched actin networks (Box 1). A region of 6.5 tandem repeats was shown to
Cortactin regulation
It is important to understand how cortactin function is regulated; one part of this regulation involves the subcellular localization. Recruitment of cortactin to sites of cortical actin polymerization has been shown to depend on the small Rho-family GTPase Rac1 and on the serine-threonine kinase Pak1 12, 13. Moreover, cortactin is also recruited by several pathogens (Table 1).
Another aspect of regulation is directly associated with phosphorylation induced by growth factor receptor stimuli (
Actin pedestal formation
EPEC and enterohaemorrhagic E. coli (EHEC) establish intimate adhesion to host cells via specialized actin-rich protrusions called pedestals [19]. This is accomplished by delivering a bacterial effector protein, Tir, into the host cells using a type III secretion system (T3SS; Box 2). Tir of EPEC becomes phosphorylated on tyrosine residues by redundant tyrosine kinases of the Src and Abl families 20, 21 and acts as a receptor for the adhesin intimin, hence its name (for translocated intimin
Global actin rearrangements
The gastric pathogen H. pylori uses a type IV secretion system (T4SS; Box 2) to inject a protein known as CagA into the cytoplasm of gastric epithelial cells. Some molecular data have recently emerged that describe how CagA is able to induce a global rearrangement of the host-cell actin cytoskeleton that is characterized by cellular elongation and cell scattering [29]. First, translocated CagA is phosphorylated by Src tyrosine kinases 30, 31. Secondly, CagA interacts with several cellular
Intracellular and intercellular motility
A group of unrelated pathogens has evolved similar strategies to exploit the actin cytoskeleton to enable movement intracellularly and intercellularly within the cytoplasm of infected cells [40]. Localized actin polymerization at one pole of the pathogen generates a branched actin bundle (‘comet tail’), which pushes the organism forward. The Listeria monocytogenes protein ActA activates the Arp2/3 complex directly [41]. Shigella flexneri stimulates actin polymerization by the surface protein
Host cell invasion
Several pathogens can enter non-phagocytic cells to establish an intracellular niche [3]. Typically, they trigger a signalling cascade, which leads to local actin assembly, phagocytic cup closure and ultimately bacterial internalization. For example, S. flexneri uses a T3SS that injects Ipa and Ipg effector proteins to direct cytoskeletal reorganization 52, 53. The first link between cortactin and host cell invasion resulted from pioneering work on Shigella [54]. Cortactin was found to be
Concluding remarks
Although many of the molecular mechanisms behind cortactin function remain to be fully understood, recent findings have important implications for its role during infection with a variety of bacterial, viral and protozoan pathogens. The observation that serine and tyrosine phosphorylation modulates cortactin activity in vitro is fascinating [5]. It is not yet clear, however, if this also holds true within the complex environment of a living cell. Probably additional factors, such as Crk [6],
Acknowledgements
We are grateful to Jürgen Wehland, Klemens Rottner (both from GBF Braunschweig, Germany) and Terry Kwok (OvG University Magdeburg) for critical comments on the manuscript and Christof Hauck (University Wuerzburg) for providing unpublished data on Staphylococcus aureus. The work of Steffen Backert is supported by the Priority Program SPP1150 of the Deutsche Forschungsgemeinschaft (Ba1671/3–1) and NBL3-Magdeburger Forschungsverbund (PFG4).
Glossary
- ADF:
- actin depolymerizing factor.
- Arp2/3:
- actin-related proteins 2 and 3 of the so-called Arp2/3 complex (see Box 1 in the main text). Box 1
- CagA:
- translocated virulence factor of Helicobacter pylori encoded by the cytotoxin-associated gene A.
- Crk:
- SH2/SH3 domain-containing adapter protein.
- Erk1–2:
- extracellular signal-regulated kinases 1 and 2.
- EspFU:
- Escherichia coli secreted protein FU.
- Grb2:
- growth factor receptor-bound protein 2, another SH2/SH3 domain-containing adapter protein.
- Ipa:
- invasion plasmid
References (70)
Cortactin promotes and stabilizes Arp2/3-induced actin filament network formation
Curr. Biol.
(2001)Cdc42/Rac1-dependent activation of the p21-activated kinase (Pak) regulates human platelet lamellipodia spreading: implication of the cortical-actin binding protein cortactin
Blood
(2002)The role of tyrosine phosphorylation of cortactin in the locomotion of endothelial cells
J. Biol. Chem.
(1998)- et al.
Tails of two Tirs: actin pedestal formation by enteropathogenic E. coli and enterohemorrhagic E. coli O157:H7
Curr. Opin. Microbiol.
(2003) EspFU is a translocated EHEC effector that interacts with Tir and N-WASP and promotes Nck-independent actin assembly
Dev. Cell.
(2004)Src is the kinase of the Helicobacter pylori CagA protein in vitro and in vivo
J. Biol. Chem.
(2002)Attenuation of Helicobacter pylori CagA x SHP-2 signaling by interaction between CagA and C-terminal Src kinase
J. Biol. Chem.
(2003)Grb2 is a key mediator of Helicobacter pylori CagA protein activities
Mol. Cell.
(2002)- et al.
Surfing pathogens and the lessons learned for actin polymerization
Trends Cell Biol.
(2001) Grb2 and Nck act cooperatively to promote actin-based motility of vaccinia virus
Curr. Biol.
(2002)
Sequential interaction of actin-related proteins 2 and 3 (Arp2/3) complex with neural Wiscott-Aldrich syndrome protein (N-WASP) and cortactin during branched actin filament network formation
J. Biol. Chem.
Cryptosporidium parvum invasion of biliary epithelia requires host cell tyrosine phosphorylation of cortactin via c-Src
Gastroenterology
Integration of signals to the Arp2/3 complex
Curr. Opin. Cell Biol.
Microaerophilic conditions permit to mimic in vitro events occurring during in vivo Helicobacter pylori infection and to identify Rho/Ras-associated proteins in cellular signaling
J. Biol. Chem.
Type III secretion machines: bacterial devices for protein delivery into host cells
Science
Microbial pathogenesis and cytoskeletal function
Nature
Bacterial invasion: the paradigms of enteroinvasive pathogens
Science
Cortactin, an 80/85-kilodalton pp60src substrate, is a filamentous actin-binding protein enriched in the cell cortex
J. Cell Biol.
Erk/Src phosphorylation of cortactin acts as a switch on-switch off mechanism that controls its ability to activate N-WASP
Mol. Cell. Biol.
Cortactin and Crk cooperate to trigger actin polymerization during Shigella invasion of epithelial cells
J. Cell Biol.
Cortactin: coupling membrane dynamics to cortical actin assembly
Oncogene
Cortactin signaling and dynamic actin networks
Biochem. J.
Activation of Arp2/3 complex-mediated actin polymerization by cortactin
Nat. Cell Biol.
Cortactin localization to sites of actin assembly in lamellipodia requires interactions with F-actin and the Arp2/3 complex
J. Cell Biol.
Translocation of cortactin to the cell periphery is mediated by the small GTPase Rac1
J. Cell Sci.
Hepatocyte Growth Factor/scatter factor-induces phosphorylation of cortactin in A431 cells in a Src kinase-independent manner
Oncogene
Actin depolymerization-induced tyrosine phosphorylation of cortactin: the role of Fer kinase
Biochem. J.
Signaling pathways and structural domains required for phosphorylation of EMS1/cortactin
Cancer Res.
Essential role of neural Wiskott-Aldrich syndrome protein in podosome formation and degradation of extracellular matrix in src-transformed fibroblasts
Cancer Res.
Phosphorylation of the enteropathogenic E. coli receptor by the Src-family kinase c-Fyn triggers actin pedestal formation
Nat. Cell Biol.
Enteropathogenic Escherichia coli use redundant tyrosine kinases to form actin pedestals
Mol. Biol. Cell.
Interaction of enteropathogenic or enterohemorrhagic Escherichia coli with HeLa cells results in translocation of cortactin to the bacterial adherence site
Infect. Immun.
Enteropathogenic Escherichia coli (EPEC) attachment to epithelial cells: exploiting the host cell cytoskeleton from the outside
Cell. Microbiol.
Enterohaemorrhagic and enteropathogenic Escherichia coli use different mechanisms for actin pedestal formation that converge on N-WASP
Cell. Microbiol.
Enteropathogenic E. coli Tir binds Nck to initiate actin pedestal formation in host cells
Nat. Cell Biol.
Cited by (91)
Characterization of the Atl-mediated staphylococcal internalization mechanism
2020, International Journal of Medical MicrobiologyCortactin: Cell Functions of A Multifaceted Actin-Binding Protein
2018, Trends in Cell BiologyScreening host proteins required for bacterial adherence after H9N2 virus infection
2018, Veterinary MicrobiologyCitation Excerpt :Cortactin is currently identified as an integrator of signaling in actin cytoskeleton. Studies proved that cortactin is involved in the reorganization of cortical actin cytoskeleton which is necessary for bacterial adhesion (Selbach and Backert, 2005). Additionally, cortactin is implicated in the formation of actin tails, which are used by pathogens for intracellular and intercellular movement.
CagA-mediated pathogenesis of Helicobacter pylori
2016, Microbial PathogenesisCitation Excerpt :CagAP also dephosphorylates some of Src kinase substrates such as cortactin, vinculin and ezrin, deactivation of which causes the elongation of infected host cells. [97] ; [63,98,95]. The phosphorylation-independent activities of CagA mainly disrupt cellular mitogenic signals, cell-to-cell junctions and trigger the inflammatory responses.