Fluorescence resonance energy transfer (FRET)-based subcellular visualization of pathogen-induced host receptor signaling
© Buntru et al; licensee BioMed Central Ltd. 2009
Received: 29 October 2009
Accepted: 25 November 2009
Published: 25 November 2009
Bacteria-triggered signaling events in infected host cells are key elements in shaping the host response to pathogens. Within the eukaryotic cell, signaling complexes are spatially organized. However, the investigation of protein-protein interactions triggered by bacterial infection in the cellular context is technically challenging. Here, we provide a methodological approach to exploit fluorescence resonance energy transfer (FRET) to visualize pathogen-initiated signaling events in human cells.
Live-cell microscopy revealed the transient recruitment of the Src family tyrosine kinase Hck upon bacterial engagement of the receptor carcinoembryonic antigen-related cell adhesion molecule 3 (CEACAM3). In cells expressing a CEACAM3 variant lacking the cytoplasmic domain, the Src homology 2 (SH2) domain of Hck (Hck-SH2) was not recruited, even though bacteria still bound to the receptor. FRET measurements on the basis of whole cell lysates revealed intimate binding between Hck-SH2 (using enhanced yellow fluorescent protein (YPet)-Hck-SH2) and the tyrosine-phosphorylated enhanced cyan fluorescent protein-labeled cytoplasmic domain of wild-type CEACAM3 (CEACAM3 WT-CyPet) and a flow cytometry-based FRET approach verified this association in intact cells. Using confocal microscopy and acceptor photobleaching, FRET between Hck-SH2 and CEACAM3 was localized to the sites of bacteria-host cell contact.
These data demonstrate not only the intimate binding of the SH2 domain of Hck to the tyrosine-phosphorylated cytoplasmic domain of CEACAM3 in intact cells, but furthermore, FRET measurements allow the subcellular localization of this process during bacterial infection. FRET-based assays are valuable tools to resolve bacteria-induced protein-protein interactions in the context of the intact host cell.
Pathogenic bacteria tightly interact with their host, often exploiting adhesin-mediated engagement of eukaryotic surface receptors to trigger intracellular signaling events . As bacteria-induced responses are of critical importance during the initiation and progression of the infection, signaling processes in the host cell are usually studied in molecular detail. Both biochemical and genetic approaches have shed light on protein-protein interactions and signaling connections that occur in infected eukaryotic cells. However, widely used biochemical approaches to investigate protein-protein interactions such as glutathione S-transferase (GST)-pull-down assays or coimmunoprecipitation from cell lysates have two major drawbacks: firstly, it is always possible that the two associated proteins are not directly interacting, but rather are linked by a third protein; secondly, biochemical approaches disrupt the cellular context and therefore lack spatial resolution. Similarly, genetic methods such as yeast two-hybrid screens, although applicable even in a high-throughput format, do not provide any information on where these processes occur under physiological conditions at the subcellular level.
By contrast, the introduction of green fluorescent protein (GFP) from Aequorea victoria, has greatly facilitated the microscopic investigation of proteins in living cells. Though the use of GFP and its spectral variants allows the observation of colocalization of multiple proteins in real time, the resolution of light microscopes (about 250 nm) is too low to prove a direct interaction of two colocalized putative binding partners. Therefore, there is a need for methods that combine the power of biochemical studies to pinpoint molecular interactions with the ability to study the subcellular context as provided by fluorescence microscopy.
Within the last few years, the phenomenon of fluorescence resonance energy transfer (FRET), first described by Förster in 1948, has garnered increasing interest as a method to address protein-protein interactions in the context of the cell [2–4]. During FRET, energy is transferred from a donor fluorophore in its excited state in a non-radiative way by dipole-dipole interactions to an acceptor molecule . The efficiency of fluorescence resonance energy transfer is defined by: E = 1/[1 + (r/R0)6]. Apparently, the efficiency of energy transfer depends on the sixth power of the distance 'r' separating the donor and the acceptor molecule. Therefore, FRET only takes place to a significant extent if molecules are spaced within a few nm (about 1 to 10 nm) . The additional parameter 'R0', called the Förster radius, is defined as the distance where efficiency of energy transfer from donor to acceptor is 50%. R0 is FRET-pair specific and is influenced by the spectral overlap of donor emission and acceptor excitation, the quantum yield of the donor, the absorption coefficient of the acceptor and the relative orientation of donor and acceptor. As a consequence, FRET is only likely to occur if two proteins labeled with a donor and an appropriate acceptor molecule are in direct contact.
To exploit this methodology in the study of pathogen-induced host cell signaling and to provide a general framework on how to approach FRET analysis in the context of receptor-initiated signaling cascades, we have used the example of carcinoembryonic antigen-related cell adhesion molecule (CEACAM)-mediated contact with the Gram-negative pathogen Neisseria gonorrhoeae. Over the last few years, our group and others have demonstrated that CEACAM3, a granulocyte-expressed member of this receptor family, functions as an opsonin-independent phagocytic receptor [7, 8]. CEACAM3 recognizes colony opacity associated (Opa) proteins of N. gonorrhoeae (Ngo) as well as additional outer membrane adhesins of other Gram-negative bacteria and, upon binding of bacteria, initiates an intracellular signaling cascade .
Efficient uptake of CEACAM3-bound bacteria depends on an immunoreceptor tyrosine-based activation motif (ITAM)-like sequence in the cytoplasmic part of the receptor, which is phosphorylated within minutes of receptor engagement . Biochemical analyses have demonstrated that Src homology 2 (SH2) domains of several signaling molecules, including the protein tyrosine kinases (PTKs) c-Src and Hck, are able to bind to the tyrosine-phosphorylated cytoplasmic domain of CEACAM3 . As well as the SH2 domain of Src PTKs, the SH2 domains of phosphatidylinositol-3 kinase, phospholipase Cγ, and Syk have also been found to colocalize with the receptor upon bacterial binding [11, 12]. However, it is unclear if this colocalization is indeed due to direct interaction of the respective signaling molecule with the phosphorylated receptor, or if there are additional molecules involved.
Results and Discussion
Bacterial engagement of CEACAM3 is accompanied by Hck-SH2 recruitment
Receptor engagement by bacterial pathogens is known to trigger intracellular signaling cascades in the infected eukaryotic cell. Our group and others have shown previously that Src kinases are critical for CEACAM3-mediated uptake of N. gonorrhoeae [10, 14, 15]. Biochemically, GST-pull-down assays have demonstrated the ability of the Src kinase Hck to interact with the phosphorylated cytoplasmic domain (CT) of CEACAM3. To demonstrate the recruitment of Hck to CEACAM3 upon bacterial binding and to investigate the kinetics of this process, 293T cells were cotransfected with the cDNA of Hck-SH2-far-red fluorescent protein (mKate) together with CEACAM3 WT-GFP or CEACAM3 ΔCT-GFP, respectively (Figure 1a). At 2 days later, the cells were infected with OpaCEA-expressing N. gonorrhoeae and imaged once per min for 2 h using confocal microscopy (see Additional file 1).
Representative images of live cells during the infection process are shown in Figure 1b. Whereas Hck-SH2 strongly colocalizes with CEACAM3 WT at sites of bacterial contact, the unligated receptor does not recruit the kinase SH2 domain. In cells expressing CEACAM3 ΔCT, a mutant form of the receptor that lacks the complete cytoplasmic domain and that is not phosphorylated upon bacterial infection, Hck-SH2 is distributed evenly in the cytoplasm, even though the bacteria bind to the extracellular domain (Figure 1b and Additional file 2). Clearly, Hck-SH2 recruitment to the bacteria-bound CEACAM3 WT is transient (see Additional file 1). Within 5 to 10 min, the SH2 domain disappears from cell-associated bacteria suggesting that the CEACAM3-initiated signaling complex is changing its composition during bacterial internalization. It is also interesting to note that cells expressing CEACAM3 WT seem to polarize with regard to bacterial uptake: Hck-SH2 is recruited to one side of the cell, where efficient receptor clustering and internalization takes place. If this represents direct binding of the Hck-SH2 domain to the phosphorylated cytoplasmic domain of CEACAM3 or if the recruitment of Hck-SH2 is due to some other phosphoprotein, which is found in the vicinity of the receptor, remains unresolved. However, the results clearly demonstrate that the Hck SH2 domain is effectively recruited to CEACAM3-enriched parts of the cell membrane upon receptor engagement by bacteria.
Hck-SH2 binds to phosphorylated cytoplasmic domain of CEACAM3
The method of measuring FRET between two potential interaction partners in cell lysates is less labor intensive than other biochemical approaches such as GST-pull-down assays or coimmunoprecipitations. Nevertheless, the sensitized emission generated by FRET in these samples cannot be observed directly. This is mostly due to the fact that the signal in the FRET channel is contaminated by spectral bleed through of donor emission and direct excitation of the acceptor at the excitation wavelength of the donor. Therefore, the signal has to be adjusted with specific correction factors derived from equivalent lysates containing either the donor or the acceptor only . The sensitized emission calculated by linear unmixing of the signal in the FRET channel is further normalized to acceptor intensity to obtain FRET efficiency that can be compared between different samples .
To further assure that the calculated FRET efficiency between CEACAM3 WT-CyPet and YPet-Hck SH2 is due to an SH2 domain-mediated molecular interaction, we introduced an additional internal control. In this regard, we took advantage of a recombinant glutathione S-transferase fusion protein of the c-Src-SH2 (GST-Src SH2) that has shown strong binding to phosphorylated CEACAM3 WT in in vitro GST-pull-down assays . We reasoned that an excess of non-fluorescent GST-Src SH2 added to the lysates should act as a specific competitive inhibitor displacing YPet-Hck-SH2 from the phosphorylated CEACAM3 WT. In line with this assumption, FRET between CEACAM3 WT-CyPet and YPet-Hck SH2 was almost completely abolished upon addition of the c-Src SH2 domain (Figure 2c). As a further control, similar amounts of GST or GST-Src SH2 were added to lysates of cells expressing CEACAM3 WT-CyPet, YPet-Hck SH2 and v-Src. Only in the case of GST-Src SH2 was a dramatic decrease of FRET efficiency observed, whereas GST alone had no effect on FRET (Figure 2d). These results demonstrate that FRET observed in whole cell lysates is due to a specific interaction between the YPet-tagged SH2 domain of Hck and the tyrosine-phosphorylated cytoplasmic domain of CEACAM3 WT-CyPet. Accordingly, we confirmed a tight binding between the SH2 domain of the Src family protein tyrosine kinase (PTK) Hck and CEACAM3.
FRET between CEACAM3 and Hck-SH2 occurs in intact cells
FRET acceptor bleaching measurements reveal direct association between Hck-SH2 and CEACAM3 at sites of bacterial contact
FRET measurements based on sensitized emission have been recently used to detect phosphatidylinositol-3' kinase activation in response to bacterial infection . The approach was based on the simultaneous recruitment of differentially labeled pleckstrin homology (PH) domains to the cell membrane upon generation of 3'-phosphoinositides. However, it was not designed to reveal protein-protein interactions. In our case, we employed a different methodology to microscopically detect FRET by acceptor photobleaching. In contrast to FRET determination based on sensitized emission, which requires extensive controls to exclude artifacts arising from variable concentrations and stoichiometry of acceptor or donor fluorophores in transiently transfected cells, acceptor bleaching displays FRET in a straightforward way. This is due to the fact that acceptor bleaching causes a positive signal due to an increase of donor fluorescence intensity following photochemical destruction of the acceptor. One reported caveat of acceptor photobleaching is the photoconversion of different YFP variants into a CFP-like species . However, the photoconverted YFP with CFP-like properties appears to get excited primarily at 405 nm and to a lesser extent at 458 nm as used in the current investigation. In fact, we did not observe an increase of CyPet fluorescence in bleached regions without CEACAM3-bound bacteria (Figure 4b). Furthermore, we bleached cells expressing only the YPet acceptor construct and did not observe an increase of the signal in the CyPet channel upon excitation at 458 nm (data not shown). One clear advantage of acceptor bleaching is that it generates additional internal controls: first, donor intensity in regions with unbleached acceptor fluorophore should be unaffected; second, in bleached areas, where no protein-protein interactions take place, no alteration of donor fluorescence should be observed. Indeed, upon comparison of prebleaching and postbleaching pictures of the same region, an increase of donor intensity can only be observed where bacteria contact the host cell (Figure 4b). These results demonstrate that early host signaling in this situation is confined to the sites of tight pathogen binding to the receptor CEACAM3. It can be envisioned that such an approach would be also ideal to visualize and localize signaling events in response to other pathogenic bacteria. In particular receptor-dependent protein-protein interactions in the infected host cell that are induced upon binding of pathogens to host integrins [20–22], cadherins , tetraspanins , or proteoglycans  could yield valuable insight into the spatial organization of host responses. Furthermore, it would be highly desirable to study the subcellular distribution and place of action of bacterially secreted effector molecules [26, 27]. This would require tagging of the secreted bacterial protein (for example, by tetracysteine motifs that could be selectively labeled by biarsenical dyes ) and would allow the validation of biochemically determined protein-protein interactions between bacterial effector molecules and host factors in the context of the intact infected cell.
As is often the case in signaling pathways, the recruitment of Hck-SH2 to CEACAM3 in living cells was transient (Additional file 1). Therefore, it would be of interest to analyze the kinetics of the Hck-SH2-CEACAM3 association by FRET in live infected cells. However, acceptor photobleaching destroys the acceptor fluorophore, preventing the continuous analysis of protein-protein interactions. Therefore, additional methodological approaches such as fluorescence lifetime imaging microscopy (FLIM) would be required. Measuring FRET according to the lifetime of the donor is independent of fluorophore concentrations and is non-destructive. However, the hardware requirements restrict the use of FLIM to dedicated microscope facilities that might be cautious of introducing live bacterial pathogens. This might limit the widespread use of FLIM in the realm of infection biology and makes the described approach of acceptor bleaching a valuable and feasible method to validate and subcellularly localize pathogen triggered signaling events in infected host cells.
Elucidation of protein-protein interactions is of intense interest to understand signaling pathways in cells. Numerous biochemical and genetic approaches such as GST-pull-down assays, coimmunoprecipitation, yeast two-hybrid screens and protein microarrays are well established techniques for analyzing protein-protein interactions. However, utilization of FRET techniques not only allows the determination of intimate binding of two proteins, but also enables the verification of protein-protein interactions in the physiological context of the cell. Measuring FRET by acceptor bleaching allowed us to localize the biochemically established interaction between phosphorylated CEACAM3 and the Src PTK Hck to the sites of pathogen binding in infected host cells. Our results demonstrate that receptor-initiated signaling events are not only transient, as observed by the recruitment of the Hck SH2 domain in living cells, but the pathogen-induced protein-protein interaction is also spatially confined within the infected cell.
Therefore, this study refines our understanding of bacteria-induced signaling in eukaryotic cells and provides a rational framework to harness the potential of FRET in other close encounters between specialized microbes or their translocated effectors and the host organism.
The coding sequences of CyPet and YPet were kindly provided by Patrick Daugherty (University of California, Santa Barbara, CA, USA). Upon polymerase chain reaction (PCR) amplification of CyPet (primers 5'-ACTACCGGTCGTGGTGAGCAAGGGAGAG-3' and 5'-ACTGCGGCCGCTTATTTGTACAGTTCGTCC-3'), the coding sequence was inserted via AgeI/NotI (restriction sites in primers are in bold) into pLPS-3' enhanced green fluorescent protein (EGFP) expression vector (Clontech, Mountain View, CA), thereby replacing the EGFP coding sequence and yielding pLPS-3'CyPet. YPet cDNA was PCR amplified (primers 5'-ACTACCGGTACCATGGTGAGCAAAG-3' and 5'-ATCCTCGAGACTTATAGAGCTCGTTCATGC-3') and inserted in pEGFP-C1 loxP  via AgeI/XhoI replacing EGFP and yielding pYPet loxP. Similarly, the cDNA of mKate (kindly provided by Dmitriy Chudakov, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Moscow, Russia) was amplified with PCR primers 5'-ATCACCGGTACCATGAGCGAGCTGATCAAG-3' and 5'-ACTCTCGAGTCTTGTGCCCCAGTTTGC-3' and inserted to obtain pmKate loxP.
The cDNA encoding the SH2 domain of human Hck was transferred by Cre-mediated recombination from pDNR-dual into pYPet loxP as described previously . GST and the GST fusion protein of the SH2 domain of human c-Src, the v-Src and the CEACAM3-GFP expression constructs, as well as the hemagglutinin (HA)-tagged CEACAM3 variants in pBluescript were described previously [8, 10, 29]. CEACAM3 wild type (CEACAM3 WT) and CEACAM3 lacking the cytoplasmic domain (CEACAM3 ΔCT) were amplified with primers 5'-GAAGTTATCAGTCGATACCATGGGGCCCCCCTCAGCC-3' and 5'-ATGGTCTAGAAAGCTTGCAGCGTAATCTGGAACGTCATATGG-3' from the respective cDNA in pBluescript and subcloned in pDNR-Dual using the InFusion kit (Clontech). The cDNAs were subsequently transferred to pLPS-3'CyPet by Cre-mediated recombination to yield CEACAM3 WT-HA-CyPet or CEACAM3 ΔCT-HA-CyPet, respectively .
Cell culture and transfection
The human embryonic kidney cell line 293T (293T cells) was grown in Dulbecco modified Eagle medium (DMEM)/10% calf serum (CS) at 37°C, 5% CO2. Cells were subcultured every 2 to 3 days. Transfection with expression vectors for CEACAM3, SH2 domains, v-Src or the empty control vector (pCDNA) was accomplished by standard calcium phosphate coprecipitation using a total amount of 6 μg plasmid/10 cm culture dish as previously described . Cells were used 2 days after transfection. Expression was verified by western blotting as described previously.
The murine fibroblast cell line NIH 3T3 was grown in DMEM/10% fetal calf serum (FCS) supplemented with non-essential amino acids and sodium pyruvate on gelatine-coated culture dishes at 37°C, 5% CO2. Cells were subcultured every 2 to 3 days. NIH 3T3 cells were transfected with expression vectors using Metafectene Pro (Cambio, Cambridge, UK) according to the manufacturer's instructions.
OpaCEA-expressing (Opa52), non-piliated N. gonorrhoeae MS11-B2.1 (strain N309) was obtained from T. F. Meyer (MPI Infektionsbiologie, Berlin, Germany). Bacteria were grown at 37°C, 5% CO2 on GC-Agar (Gibco BRL, Paisley, UK) supplemented with vitamins and appropriate antibiotics. For labeling, bacteria (2 × 108/ml) were washed with sterile phosphate-buffered saline (PBS) and suspended in AlexaFluor647-NHS (Invitrogen, Karlsruhe, Germany) in PBS. Suspensions were incubated at 37°C for 30 min in the dark under constant shaking. Prior to use, bacteria were washed three times with PBS.
FRET measurements in whole cell lysates
Transfected cells were washed (160 mM 3-(N-morpholino)propanesulfonic acid (MOPS), 1 mM ethyleneglycol tetra-acetic acid (EGTA), pH 7,4) and lysed for 10 min using the same buffer supplemented with 1% Triton X-100, 1 mM NaVO3 and complete protease inhibitors. A total of 100 μl of each lysate were transferred to a 96-well plate and the fluorescence in the following channels was recorded using a Varioskan Flash (Thermo Scientific, Waltham, MA): donor channel (excitation (Ex)/emission (Em): 435 nm/477 nm), acceptor channel (Ex/Em: 500 nm/530 nm), FRET channel (Ex/Em: 435 nm/530 nm). Raw data were processed by subtracting the background fluorescence signals obtained from lysates of untransfected cells. Signal in the FRET channel (DA) was corrected for spectral bleedthrough of the donor (α) and cross-excitation of the acceptor (β) with samples expressing donor or acceptor construct only. Afterwards sensitized emission was normalized to acceptor signal. In brief, FRET efficiency was calculated as follows: EAapp = (DA-α·DD-β·AA)/AA where DD = signal donor channel and AA = signal acceptor channel.
Flow cytometric FRET measurements
Transfected 293T cells were trypsin treated, suspensions were washed with ice-cold PBS and the cells resuspended in fluorescence-activated cell sorting (FACS) buffer (PBS, 1% FCS, 0.05% NaN3). Samples were kept on ice in the dark until measured. The cell population was gated by forward and sideward scatter, and 104 cells were analyzed using a LSR II flow cytometer (Becton Dickinson, Heidelberg, Germany). CyPet was excited at 405 nm and emission detected at 450/50 nm. YPet was excited at 488 nm and emission detected at 525/50 nm. Cells transfected with the empty vector (pCDNA) were used for background correction. Cells expressing donor or acceptor construct only were used to compensate the signal in the FRET channel (excitation: 405 nm, emission: 525/50 nm) for spectral bleedthrough and cross-excitation. Cotransfected cells were identified on the basis of CyPet and YPet fluorescence and the fluorescence intensity of double-positive cells was determined in the FRET channel.
For colocalization experiments, transfected 293T cells were seeded in 3.5 cm culture dishes with a coverslip bottom (MatTek, Ashland, MA, USA) at 1.5 × 105 cells/dish 1 day before infection. The culture dishes had been coated with a combination of human fibronectin (4 μg/ml) and poly-L-Lysine (10 μg/ml) in PBS at 37°C for 2 h. Cells were infected with AlexaFluor647-labeled N. gonorrhoeae and the infection process was monitored for 2 h with a TCS SP5 confocal laser scanning microscope (Leica, Wetzlar, Germany) using a 63 ×, 1.4 NA PLAPO oil immersion objective lens. Fluorescence signals of labeled specimens were serially recorded with appropriate excitation wavelengths and emission bands for EGFP, mKate and AlexaFluor647, respectively, to avoid bleedthrough. Images were processed with ImageJ (NIH, Bethesda, MD, USA).
For FRET acceptor bleaching studies, transfected NIH 3T3 cells were seeded on coated glass coverslips at 3 × 104 cells/well in 24-well plates 1 day before infection. Cells were infected with AlexaFluor647-labeled N. gonorrhoeae for 30 min and fixed with 4% paraformaldehyde in PBS. Following three washes, samples were embedded in mounting medium (Dako, Glostrup, Denmark). Acceptor bleaching was accomplished with a TCS SP5 using the implemented FRET acceptor bleaching wizard. Prebleach and postbleach images were serially recorded with excitation of CyPet at 458 nm and YPet at 514 nm with an argon laser and appropriate emission bands. Low laser intensities were used to avoid acquisition bleaching. The acceptor was bleached with high intensity at the 514 nm line. Cells expressing donor construct only were used to exclude donor bleaching under these conditions. Images were processed with ImageJ. To calculate FRET efficiency, donor prebleach (Dpre) and postbleach (Dpost) images were smoothed by median filtering. Next, images were background subtracted and thresholded on fluorescence intensity. FRET efficiency (E) was calculated on a pixel-by-pixel basis as E = 1-(Dpre/Dpost). Donor prebleach and postbleach images as well as FRET image are presented in pseudocolor for better visualization.
We thank Dave Daugherty (University of California, Santa Barbara, CA, USA) for CyPet/YPet cDNAs, Dmitriy Chudakov (Shemyakin and Ovchinnikov Institute of Bioorganic Chemistry, Moscow, Russia) for mKate cDNA, Andreas Zumbusch and Elisa May (Universität Konstanz, Germany) for support with microscopy and image analysis, and S Feindler-Boeckh and R Hohenberger-Bregger for expert technical assistance. AB acknowledges support from the Landesgraduiertenförderung and Konstanz Research School, Chemical Biology. This study was supported by funds from the Landesstiftung BW (P-LS-Prot/66) to CRH.
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