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It is believed that mitochondrial dynamics is coordinated with endosomal traffic rates during cytoskeletal remodeling, but the mechanisms involved are largely unknown. The adenovirus early region 4 ORF4 protein (E4orf4) subverts signaling by Src family kinases (SFK) to perturb cellular morphology, membrane traffic, and organellar dynamics and to trigger cell death. Using E4orf4 as a model, we uncovered a functional connection between mitochondria-shaping proteins and the small GTPase Rab11a, a key regulator of polarized transport via recycling endosomes. We found that E4orf4 induced dramatic changes in the morphology of mitochondria along with their mobilization at the vicinity of a polarized actin network typifying E4orf4 action, in a manner controlled by SFK and Rab11a. Mitochondrial remodeling was associated with increased proximity between Rab11a and mitochondrial membranes, changes in fusion-fission dynamics, and mitochondrial relocalization of the fission factor dynamin-related protein 1 (Drp1), which was regulated by the Rab11a effector protein FIP1/RCP. Knockdown of FIP1/RCP or inhibition of Drp1 markedly impaired mitochondrial remodeling and actin assembly, involving Rab11a-mediated mitochondrial dynamics in E4orf4-induced signaling. A similar mobilization of mitochondria near actin-rich structures was mediated by Rab11 and Drp1 in viral Src-transformed cells and contributed to the biogenesis of podosome rosettes. These findings suggest a role for Rab11a in the trafficking of Drp1 to mitochondria upon SFK activation and unravel a novel functional interplay between Rab11a and mitochondria during reshaping of the cell cytoskeleton, which would facilitate mitochondria redistribution near energy-requiring actin-rich structures.
Mitochondrial dynamics, including changes in shape and distribution, have emerged as crucial determinants of cell fate regulation. Processes involving extensive cellular remodeling from mitotic division, cell death, to senescence have been linked to changes in the morphology and position of mitochondria, which appear to cluster at many sites of high ATP requirement in several cell types (1). Cycles of membrane fusion and fission seem to be coordinated with cytoskeleton-based transport of mitochondria and allow the exchange of damaged mitochondrial constituents with those of healthy mitochondria to maintain network function and respond to cellular energy demand (2). Accordingly, mitochondrial dynamics have been found to influence chemotaxis (3), cell cycle progression (4–6), Ca2+ signaling and signaling at the immune synapse (7, 8), generation of reactive oxygen species (9), migration of cancer cells (10), neurotransmission (11), and apoptotic signaling (12), perhaps by promoting spatial compartmentalization and polarization at the single cell level. Indeed, relocalization of mitochondria could foster the asymmetric segregation of signaling complexes and lipid domains by providing the ATP required for the proper functioning of motors, kinases, and GTPases controlling membrane-cytoskeletal dynamics. For instance, synaptic mitochondria appear to feed the myosin ATPase required to mobilize reserve vesicles at Drosophila neuromuscular junctions (13).
Mitochondrial distribution is regulated by their movement along microtubules and is coordinated with shape transitions, allowing division of the mitochondrial network into smaller organelles that can be transported by motor proteins (14, 15). Several large GTPases control mitochondrial shape transitions and form the core morphogenesis machinery (16). Although dynamin-related protein 1 (Drp1) is the main regulator of fission (17), the mitofusins (Mfn1 and Mfn2) and optic atrophy type 1 (OPA1) catalyze fusion of the outer and inner mitochondrial membrane, respectively (18, 19). Activation and trafficking of the cytoplasmic fission factor Drp1 is emerging as a crucial regulatory step. This is modulated by several post-translational modifications that influence Drp1 localization to the mitochondrial outer membrane and its higher order assembly into helical oligomers circumscribing mitochondrial tubules at fission foci (20). Thus mitochondrial shape and networks are a result of precise balancing of fusion and fission events that appear to have a direct impact on key cellular functions. Yet the mechanisms whereby mitochondrial dynamic changes are integrated to a variety of cellular processes are incompletely understood and are likely to involve a repertoire of Drp1-interacting proteins within key inter-organelle signaling pathways.
Although it is generally accepted that reshaping of the cell membranes and cytoskeleton requires a concerted dialog between actin, vesicular transport, and organellar dynamics, little is known about the mechanisms promoting communication between the mitochondria and other organelles. Rather, it is generally assumed that mitochondria are completely disconnected from vesicular transport routes. Still, recent evidence suggests that mitochondria can make close contacts with organelles other than the endoplasmic reticulum, including Golgi membranes and endo-lysosomal vesicles, which are functionally relevant (21, 22). For instance, an early coalescence of organelles to a perinuclear region has been found to promote organelle membrane “mixing” between mitochondria, Golgi, and endosomes upon stimulation of death receptors and could favor the transfer of death-promoting factors between organelles (23, 24). A growing number of classic vesicular components are found at mitochondria, where they could regulate some form of inter-organellar communication and perhaps cargo exchanges between mitochondria and other membrane compartments. In support of this notion, vesicular transport routes from mitochondria to peroxisomes and lysosomes have been reported, one of which involves the retromer complex classically associated with endosome-to-Golgi membrane traffic (25–28). Thus far, evidence suggests that cross-talks between mitochondria and other organelles are regulated by trafficking proteins, and the mechanisms involved have emerged as a fundamental issue.
Infection with human viruses instigates extensive remodeling of the host cells. Many viral proteins have evolved to subvert core elements of the eukaryotic cell machinery that control actin dynamics and membrane trafficking to optimize viral infection (29–31). Studying such interactions of viral factors has provided major advances in our knowledge of the role of the cytoskeleton in key biological function. Human adenovirus type 2 early region 4 ORF4 (hereafter named E4orf4) is a 14-kDa early viral protein that has been shown to manipulate actin dynamics in many cell types and is believed to facilitate the exodus and spreading of newly assembled virions by contributing to the late disruption of infected cells (32, 33). Ectopic expression of E4orf4 outside the viral context induces a nonclassical caspase-independent form of tumor cell-selective death typified by dramatic changes in actin dynamics (34–37). Not only the distinctive changes in actin dynamics typify killing by E4orf4, they also contribute to cell killing per se by engaging the death machinery. Indeed, drugs that inhibit myosin II ATPase or actin polymerization strongly impair E4orf4-induced cell death (38, 39). Early cell polarization is observed concomitantly with assembly of a peculiar juxtanuclear actin-myosin network upon E4orf4 expression. These cytoskeletal changes are associated with a chronic increase in cell tension, cellular blebbing, and nuclear condensation (39, 40). It has been further shown that E4orf4 perturbs Rho GTPase signaling and polarized membrane trafficking via Rab11a-positive recycling endosomes, leading to an impairment of organellar structure and integrity (39, 41). For instance, a chronic increase in the retrograde transport of Rab11a endosomes to the Golgi has been linked to Golgi membrane scattering and caspase-independent cell death in response to E4orf4, but also in response to the general apoptotic trigger staurosporine in tumor cells (33, 41). These findings illustrate the value of using E4orf4 as a probing tool to uncover signaling mechanisms controlling noncanonical death pathways and inter-organellar communication of potential significance for tumor cell survival.
Two major signaling systems engaged during cellular transformation are involved in E4orf4 killing as follows: the Src family kinases (SFK)7 (37, 38) and the protein phosphatase 2A (36, 42). E4orf4 physically interacts with the heterotrimeric protein phosphatase 2A and with SFK via distinct domains, and both interactions are required for optimal induction of cell death in cultured mammalian cells (43). E4orf4 binding to the kinase domain of Src promotes its tyrosine phosphorylation and increases the phosphorylation of a subset of proteins having a common ability to regulate actin dynamics (37, 40, 43, 44). Moreover, cellular transformation by v-Src can sensitize cells to E4orf4-induced killing, suggesting that E4orf4 hijacks key effectors of Src signaling in cellular transformation.8 An elegant study in Drosophila has provided evidence that both the E4orf4-protein phosphatase 2A and the E4orf4-SFK interaction also contribute to the induction of a distinctive caspase-independent mode of cell death upon expression of E4orf4 in whole organism (45). Notwithstanding, it appears that E4orf4 simultaneously inhibits classic apoptotic pathways in whole organism, suggesting that damage to normal cells might be minimized in normal cellular contexts where canonical apoptotic pathways are functional. Thus, the mechanistic underpinnings of E4orf4-induced signaling may be significant for cancer therapy.
In this study, we show that the cytoskeletal rearrangements leading to cell death in response to E4orf4 depend upon changes in mitochondrial shape and distribution. We have examined a functional connection between mitochondrial dynamics and the small GTPase Rab11a, which mediates SFK-dependent polarized membrane trafficking from recycling endosomes in response to E4orf4. We have further explored this connection during cellular transformation by RSV v-Src. We provide evidence for a conserved pathway activated by SFK, which could coordinate changes in endocytic recycling with mitochondrial dynamics to regulate cytoskeletal-membrane dynamics and perhaps to influence tumor cell invasive properties.
FLAG-E4orf4-mCherry was produced by PCR amplification using the primers 5′-CAG CTC GAG GCT AGC GTC TCT AAG GGC GAG GAA-3′ and 5′-CTC GGA TCC GAA TTC TTA TTT GTA CAG TTC ATC-3′ and mCherry-pJI as a template (obtained from DNA2.0, Inc.). The DNA fragment encoding mCherry was subcloned into the XhoI/BamHI sites of the previously described FLAG-E4orf4 construct into pCDNA 3.1 (37). The adenovirus vector AdFLAG-E4orf4-mCherry was generated by Welgen Inc., Worcester, MA, by subcloning cDNA for FLAG-E4orf4-mCherry into the pENTCMV-teto shuttle vector that contains a bacterial tetracycline resistance operon (TetO) sequence close to the CMV promoter. The resulting recombinant adenovirus was amplified in the HEK293V cell line that expresses the tetracycline repressor and inhibits E4orf4 expression to prevent its toxic action and allow efficient replication of the virus (a generous gift from Philip E. Branton, McGill University). The following DNA constructs were described before: FLAG-Ad2E4orf4-mRFP (WT, 6R-A, and 4Y-F) (39); FLAG-Ad2E4orf4 (43); OCT-YFP/OCT-PAGFP, YFP-Drp1, and CFP-Drp1 (K38E) (46); Mfn2(1–703)YFP (47); GFP-Rab11a (WT/S25N) (48); GFP-FIP1/RCP, GFP-FIP5/Rip11, and GFP-RBDFIP (49); GFP-FIP3/GFP-FIP4 (50); and ts72 v-Src (51). The GTP mutant of GFP-Rab11a (Q70L) was generated by PCR-mediated site-directed mutagenesis using GFP-Rab11a as template and the following primers designed to replace a glutamine residue at the position 70 with a leucine: 5′-GGG ACA CAG CAG GGC TAG AGC GAT ATC GAG CT-3′ and 5′-AGC TCG ATA TCG CTC TAG CCC TGC TGT GTC CC-3′; boldface indicates nucleotide substitution. All DNA constructs were verified by DNA sequence analyses.
The following antibodies and drugs were used: anti-β-actin (AC15 or AC-74), anti-FLAG (M2), anti-Myc (9E10), anti-v-Src (Ab1), mouse monoclonal anti-Drp1 (anti-DMN1L clone 3B5), and rabbit polyclonal anti-FIP5/Rip11 were obtained from Sigma; monoclonal anti-calreticulin, monoclonal anti-cytochrome c (7H8.2C12), monoclonal anti-cytochrome c (6H2.B4), monoclonal anti-Fyn (clone 25), monoclonal anti-Drp1 (clone 8/DLP1), monoclonal anti-GM130 (clone 35), and monoclonal anti-paxillin were obtained from BD Biosciences; rabbit polyclonal anti-Cdc42 (P1) and rabbit polyclonal anti-TOM20 (FL-145) were obtained from Santa Cruz Biotechnology; monoclonal anti-GAPDH (clone 6C5) was obtained from Fitzgerald Industries International; monoclonal anti-GFP (3E6) was obtained from Molecular Probes/EMD Millipore Corp.; anti-HA (Ha.11) was obtained from The Jackson Laboratory; monoclonal anti-phosphotyrosines (PY20) was obtained from ICN/MP Biochemicals; rabbit polyclonal anti-Rab11a and monoclonal anti-TfR (clone H68.4) were obtained from Thermo Fisher Scientific; rabbit polyclonal anti-Src(Tyr(P)) (416) was obtained from Cell Signaling Technology. The rabbit polyclonal anti-FIP1/RCP was a kind gift from Rytis Prekeris (University of Colorado, Denver) and was described previously (52). DMSO, oligomycin, Mdivi-1, and gelatin were obtained from Sigma; blebbistatin, SKI-1, and PP2 were obtained from EMD Millipore Corp.; Alexa Fluor® 488 phalloidin, Texas Red phalloidin, Alexa Fluor® 647 phalloidin, Mitotracker Deep Red and gelatin were from pig skin; Oregon Green® 488 was obtained from Molecular Probes/Thermo Fisher Scientific. For experiments on E4orf4-expressing cells, chemical inhibitors SKI-1 (10 μm), PP2 (10 μm), blebbistatin (50 μm), Mdivi-1 (50 μm), oligomycin (5 μm) or appropriate vehicle was added to the culture medium during transfection before the onset of E4orf4 expression or otherwise as indicated in the figure legends.
HeLa (53) and MCF7 (54) cell lines were maintained in α-minimal essential medium and 10% fetal bovine serum (FBS); MDA-MB-231 cells (55) were cultured in Roswell Park Memorial Institute medium (RPMI 1640) supplemented with 10% FBS, and 293T cells (56) were grown in Dulbecco's modified Eagle's medium (DMEM) and 10% FBS. The MDCK tsv-Src clonal cell line Pi34 stably expresses a temperature-sensitive v-Src mutant ts72-v-Src and was described before (57). These cells were cultured at 40.5 °C (restrictive temperature) or 35 °C (permissive temperature) in DMEM supplemented with 10% newborn calf serum. The HeLa-(GFP)Rab11a cell line stably expresses GFP-Rab11a and was obtained by selecting stable transfected cells with G418 (400 μg/ml) for a 3-week period. All cell lines were grown in a humidified atmosphere with 5% CO2. MCF7 and HeLa cells were transfected with Lipofectamine 2000 (Invitrogen), according to the manufacturer's recommendations. 293T cells were seeded on poly-l-lysine (Sigma)-coated dishes for transfection by the calcium-phosphate method in the presence of chloroquine (25 μm; 6 h) and were analyzed 24 h after transfection (37). MDA-MB-231 cells were infected with a control adenovirus AdLacZ or with AdE4orf4-mCherry at a multiplicity of infection of 75 plaque-forming units per cell. For knockdown experiments, HeLa were transfected with 50 nm siRNA duplexes overnight using the calcium-phosphate method, split for subsequent transfection 48 h later, and analyzed 60–72 h later by Western blot or immunofluorescence. The siRNA duplexes were based on human sequences and were purchased from Qiagen (HPP grade siRNA) or from Thermo Fisher Scientific (standard A4 grade). Sequences of the sense strands are as follows: Rab11a 1 (5′-UGUCAGACAGACGCGAAAA-3′), Qiagen (41, 50); Rab11a 3 (5′-GGCAUUGUAGAGAUCUGAATT-3′), Qiagen (Hs_ RAB11A_2); Rab11a 4 (5′-GAGUACAGUGAGAGGUUAAUU-3′), Thermo Fisher Scientific siRab11-11; FIP1/RCP 12 (5′-GGUUAAUGAUUACAAUUAATT-3′), Qiagen (Hs_RAB11FIP1_12); FIP1/RCP 14 (5′-CGCACUCGCUAAUACAGUUTT-3′), Qiagen (Hs_RAB11FIP1_14); FIP5/Rip11 5 (5′-CCAUCCAGUUCACGCGCAATT-3′), Qiagen (Hs_RAB11 FIP5_5); and FIP5/Rip11 6 (5′-GGAACGCGGCGAGAUUGAATT-3′), Qiagen (Hs_RAB11 FIP5_6).
For isolation of heavy/light membranes, cells were suspended at 50 × 106 cells/ml in sucrose buffer (20 mm HEPES, pH 7.5, 250 mm sucrose, 10 mm KCl, 1.5 mm MgCl2, 1 mm EDTA, 1 mm EGTA, 15 μg/ml leupeptin, 5 μg/ml aprotinin, 1 μg/ml pepstatin A, 1 mm PMSF, 1 mm Na3VO4), swollen on ice for 1 h, and forced through a 27-gauge needle 50–60 times (39). PNS were pelleted at 700 × g (P1) and further fractionated by serial centrifugation (heavy membranes (P2), 8000 × g; light membranes (P3), 170,000 × g). To obtain a 10–30% linear gradient of Opti-Prep (iodixanol), the homogenized cells were centrifuged at 700 × g, and PNS (1 ml) were mixed with Opti-Prep (1.5 ml of 50% solution) to reach a final concentration of 30% iodixanol. The mixtures were layered under 1.3 ml of 20% iodixanol and 1.2 ml of 10% iodixanol, respectively, as described (58). The gradient was spun at 360,000 × g for 3 h at 4 °C and collected into 20 fractions. Equal volumes of fractions were loaded on SDS-polyacrylamide gels, and Western blots were performed as described previously (37). Protein concentrations were determined with the DC protein assay from Bio-Rad, and densitometric analyses were performed from FluorS MAX MultiImager-captured images using Quantity 1-D software version 4.5.0 (Bio-Rad). For immunoprecipitation analyses, cells from 10-cm plates were transferred to ice, washed with ice-cold PBS, and scraped into 0.5 ml of native immunoprecipitation buffer (0.1 m MES-NaOH, pH 6.5, 1 mm magnesium acetate, 0.5 mm EGTA, 200 μm sodium vanadate, 1% (w/v) digitonin with protease inhibitors), as described (59). Cell lysates were transferred into a 1.5-ml tube on ice, centrifuged at 10,000 × g for 5 min, and transferred to a fresh tube. Cell lysates were incubated for 60-min with 8 μg per 1 ml of lysate of anti-GFP (mouse monoclonal clone 3E6; Molecular Probes/Thermo Fisher Scientific, Ottawa, Ontario, Canada) on ice, followed by addition of 40 μl of Dynabeads® protein G (InVitrogenTM/Thermo Fisher Scientific, Ontario, Canada) per 1 ml of lysate and incubation at 4 °C for an additional 30-min period. Immune complexes were collected using a magnetic stand (EMD Millipore Corp., Billerica, MA), washed three times in lysis buffer, and transferred to a fresh tube. Equal amounts of immune complexes were resolved on SDS-PAGE, transferred onto nitrocellulose, and processed for immunoblotting as described before (43). Please note that two different anti-Drp1 antibodies were used for immune complex analyses as follows: the mouse monoclonal anti-Drp1 from BD Biosciences (clone 8/DLP1), and the mouse monoclonal anti-DNM1L from Sigma (clone 3B5).
DNA was stained with cell-permeable Hoechst prior to cell fixation. Cell fixation was performed in 3.7% formaldehyde in Luftig buffer (0.2 m sucrose, 35 mm PIPES, pH 7.4, 5 mm EGTA, 5 mm MgSO4) for 20 min at 37 °C, and fixation-induced fluorescence was quenched with 50 mm NH4Cl for 15 min at room temperature. Cells were then permeabilized with 0.2% Triton X-100 in PBS for 10 min or with 0.4% saponin in PBS for 30 min. Immunolabeling was performed as described previously (37) using the indicated primary antibodies followed by Alexa Fluor® goat anti-rabbit or goat anti-mouse antibodies and Alexa Fluor® phalloidin for F-actin labeling (Molecular Probes/Thermo Fisher Scientific, Ottawa, Ontario, Canada). The cellular phenotypes were monitored routinely by at least two independent investigators by visual inspection of fixed specimens using a ×60 or ×100x objective lens. Polarized remodeling of the mitochondrial network was estimated based on the relocalization of short, discontinuous mitochondrial units (representing >50% of the mitochondrial units) to a juxtanuclear region. These cells were showing a very distinctive early polarization phenotype, i.e. nuclear translocation to one pole of the cell next to the remodeled mitochondrial network, along with stress fiber formation (typical of E4orf4 expression (39)). Mitochondrial remodeling was estimated in MDCK tsv-Src by scoring the number of cells that exhibited mitochondrial clustering at the vicinity of podosome rosettes detected by F-actin. Typical remodeling of focal adhesions and actin was estimated from fixed specimens seeded on fibronectin-coated dishes, by scoring the number of E4orf4-expressing cells exhibiting loss of perinuclear paxillin vesicular staining along with increased paxillin staining at focal adhesions and/or showing a juxtanuclear actin network and/or blebbing, as described before (39, 40), or by scoring the number of MDCK tsv-Src cells exhibiting loss of stress fibers along with assembly of podosome rosettes. The Volocity software versions 5.0/6.0 (Quorum Technologies) and ImageJ 1.43 (National Institute of Health) were used for processing of entire images before cropping to emphasize the main point of the image; processing was limited to background subtraction and brightness/contrast adjustments, unless otherwise indicated.
The mitochondrial aspect ratio (ARmit) was estimated by a computer-assisted morphometric analysis of confocal images using the “ObjectJ” plugin in ImageJ software (Amsterdam University, The Netherlands). Images were processed with a median filter to remove point noise, and objects were defined manually to determine the length and the width of individual mitochondria. ARmit values (length/width) were calculated for 30–40 individual mitochondria within a cell and averaged to obtain a mean ARmit per cell. The coincidence between fluorescent subcellular structures within a cell was analyzed using confocal image stacks or single plane images that have been selected to include the highest density of Rab11a-labeled structures by two different approaches. Confocal images were cleared of background using a region of interest (ROI) outside cells with the “subtract background from ROI” function and filtered with a median filter to remove point noise. The “colocalization threshold” plugin of ImageJ or Volocity software (Quorum Technologies) was used to estimate single channel-specific threshold-adjusted Mander's coefficients (tM), which may even be used if the intensities in both channels are different from one another (60). As a complementary approach, we used the Volocity 5.0/6.0 (Quorum Technologies) to perform an object-based analysis as described (41, 61). Briefly, fluorescent objects in both channels were found by applying a threshold set at 800 arbitrary unit of intensity or greater, and objects were delineated using a segmentation procedure based on a watershed algorithm. The intersecting area between objects with a size volume ≥3 voxels was determined and was expressed as a ratio in percent over the total area of voxels within individual channels (% overlap). A similar object-based analysis was performed to estimate the number of mitochondrial Drp1 foci/μm2 from a fixed ROI (40 μm2) that had been selected to include the area within a cell showing the highest density of Drp1 foci. Single plane images showing the highest Drp1 staining were chosen for every cell and were cleared of background. Fluorescent objects in both channels (Drp1-YFP and mitochondrial cytochrome c in red) were found by applying a threshold set at 150 a.u. or greater. Drp1 objects with a size of ≥3 pixels intersecting with mitochondrial staining were scored and divided by the area. To measure mitochondrial connectivity, HeLa cells were transfected with siRNAs and then with E4orf4-mRFP or the vector together with OCT-PAGFP, and cells were imaged 24 h after transfection of E4orf4 using an FV1000 confocal laser scanning microscope driven by FluoView software (Olympus). Quantitative assessment of OCT-PAGFP dilution was performed according to Ref. 62. Briefly, a region of perinuclear mitochondria within a fixed ROI was stimulated by irradiation at 405 nm (scanning mode, SIM Tornado; ROI, 45 pixels by 45 pixels; laser, 5%, 12.5 μs/pixel), and then fluorescence emission at 488 nm was measured at 10-min intervals by acquiring confocal z-sections covering the entire cell, and the entire mitochondrial network was finally stimulated by irradiation at 405 nm. Image analyses were performed using ImageJ.
Confocal microscopy of live and fixed cells was performed with an Olympus FV1000 Confocal (100× oil 1.4 NA) driven by FluoView software (Olympus) or with a PerkinElmer Life Sciences Ultraview Spinning Disc Confocal (100× oil 1.4 NA, 60× oil 1.4 NA, or 40× oil 1.3NA with 1.5× optivar), equipped with an EMCCD cooled charge-coupled camera at −50 °C (Hamamatsu Photonics K.K.) and driven by Volocity software version 6.01. Both systems were equipped with a humidified 5% CO2/thermoregulated chamber. For quantitative cellular imaging, acquisitions were taken on separate channels using the same parameters (gain and laser power) optimized to keep fluorescent signals in the dynamic range. For live cell imaging, HeLa(GFP)-Rab11a cells were seeded on fibronectin-coated glass dishes (MatTek Corp.), transfected with E4orf4-mRFP or the vector, and incubated with 25 nm MitoTracker Deep Red for 20 min at 37 °C before imaging using spinning disc confocal microscopy and 60× oil 1.4 NA objective. Confocal z-sections (5–8 z-steps of 0.5 μm) were acquired on separate channels (green, Rab11a-GFP; far red, MitoTracker) at 7.5-s intervals for a 3-min period. The numbers of Rab11a-mitochondrial interactions at hot spots were estimated by visual inspection of three-dimensional time sequences of deconvolved image stacks using Volocity 5.0 software by scoring contacts between Rab11a-GFP-positive structures and mitochondria only if they met the following two criteria: first, if they occurred more than once at the same site (region) on a defined mitochondrial tubule over the 3-min period; and second, if they were associated in time and space with remodeling of the mitochondrial tubule (i.e. stretching/constriction or contact/merging with another mitochondrial unit). Mitochondrial motility was estimated from three-dimensional time sequences of Mito-Tracker-labeled mitochondria using a method adapted from Ref. 63. Briefly, two single plane confocal images acquired 7.5-s apart were overlaid; the yellow pixels were subtracted, and the number of green and red pixels were scored to estimate the number of pixels changing over time (disappearing or appearing into the focal plane). The mean mitochondrial movement per cell was estimated from eight time points. One-way analysis of variance tests were used with p values of <0.05 considered significant (*, p < 0.05; **, p < 0.01; ***, p < 0.001). Prism 5.0 software (GraphPad software) was used to compare mean values of individual experiments, whereas SAS/STAT 9.1 software (SAS Institute) was used to compare all single cell measurements. Two-way analysis of variance tests were performed using Prism 6.0 to compare measures from single cells within distinct groups and independent experiments, with p values of <0.05 considered significant.
While studying the cytoskeletal transitions that promote cell killing in response to E4orf4, we were intrigued by the recurrence of dramatic changes in mitochondrial network organization that followed recruitment of Rab11a-positive recycling endosomes to the juxtanuclear region. To characterize these mitochondrial changes, HeLa cells were cotransfected with E4orf4-mRFP and OCT-YFP, a marker of the mitochondrial matrix, or were processed for staining of Tom20, a marker of the outer mitochondrial membrane. Mitochondria from cells transfected with the vector only were interconnected and distributed homogeneously, showing a tubular and elongated shape (Fig. 1, A, Z view, and C, Ctl). In contrast, E4orf4-expressing cells showed a very distinctive polar redistribution of mitochondria next to the nucleus that was typically relocated to one side of the cell before the onset of nuclear condensation (polarization at the single cell level). In these cells, mitochondria adopted more heterogeneous punctate shapes and accumulated at the vicinity of the juxtanuclear actin ring (Fig. 1A, E4orf4-mRFP, Z view), where Rab11a-positive recycling endosomes were also recruited (Fig. 1B, red arrow). Such a phenotype was observed in ~35% of E4orf4-expressing cells early after transfection (Fig. 2B, polarized remodeling, E4orf4 WT) and was associated with mitochondrial fragmentation (Fig. 1C, insets 2.5x). This was reflected by a significant decrease of the aspect ratio of individual mitochondria in E4orf4-expressing cells (ARmit = length/width, ~2.0–2.5-fold; Fig. 1D) and by a marked loss of the population of long mitochondria, as seen by electron microscopy.9 However, in contrast to classic apoptotic changes and in agreement with our previous work, we found that mitochondrial fragmentation was not accompanied by outer membrane permeabilization and cytochrome c release and could not be prevented by overexpression of Bcl-2 in E4orf4-expressing cells (data not shown) (38). Rather, these cells exhibited a progressive and late loss of mitochondrial transmembrane potential that predominated within the pool of smaller mitochondria being recruited at the juxtanuclear region (data not shown). This was consistent with previous work showing that the caspase-independent death activity of E4orf4 is associated with a late disruption of mitochondrial bioenergetics (34, 41).
Similar changes in mitochondrial shape and position were observed in MCF7 and 293T cells expressing E4orf4 (data not shown), two additional cell lines for which a key role for SFK in E4orf4-mediated cytoskeletal transitions has been well established (37, 39, 41). To explore the dependence of mitochondrial changes on the SFK-dependent activity of E4orf4, we first used E4orf4 mutant proteins that are unable to subvert SFK signaling (E4orf4(4Y-F) and E4orf4(6R-A)) (38, 43, 44). Alternatively, cells expressing wild type E4orf4 were incubated in the presence of SFK inhibitors (SKI-1, PP2). As shown in Fig. 2, A–C, E4orf4 mutants were severely defective in induction of mitochondrial remodeling, just as wild type E4orf4 in the presence of SFK inhibitors, indicating that changes in organellar dynamics were linked to SFK-dependent signaling. We next explored the role of Rab11a that is required downstream of SFK to mobilize the traffic of recycling endosomes, remodel actin, and trigger cell death in response to E4orf4 (39, 41). To that end, we used siRNA sequences achieving >75% depletion of Rab11a in HeLa cells (Fig. 2E). Remarkably, depletion of Rab11a reduced the ability of E4orf4 to translocate and reorganize the mitochondrial network by ~55–60%, mimicking the block induced by SFK inhibitors (Fig. 2, A and B, right graph). Under these conditions, E4orf4-induced mitochondrial fragmentation was completely inhibited (Fig. 2C, ARmit, red bars) (41). In marked contrast, control siRNA had no effect on E4orf4-induced changes in mitochondrial shape and distribution (Fig. 2, B, right graph, and C). The mitochondrial phenotype was inhibited by three distinct siRNA sequences targeting Rab11a, although it was rescued by cotransfecting an siRNA-resistant Rab11a-GFP, ruling out off-target effects (Fig. 2, A, right panels, B, right graph, and E, Rab11a-GFP). Further quantitative assessment of the mitochondrial fission-fusion balance using mitochondrial photoactivatable GFP (OCT-PAGFP) (62) revealed a marked decrease in the dilution of OCT-PAGFP fluorescence within the remodeled network that reflected a loss of mitochondrial connectivity; such decrease was alleviated by transfection of Rab11a-specific siRNA (Fig. 2D). Together, the data strongly suggested that E4orf4 could alter the mitochondrial fission-fusion balance by exploiting some novel functions of Rab11a activated downstream of SFK.
We reasoned that the dependence of mitochondrial remodeling on Rab11a could reveal a novel interaction between the trafficking factor and mitochondria. To determine whether this was so, we first examined the cellular distribution of YFP-Rab11a or GFP-Rab11a relative to mitochondria probed by immunofluorescence with anti-Tom20 or by cotransfection of OCT-Cherry in cells transfected with E4orf4-mRFP, or the vector only, by quantitative analyses of the degree of colocalization using two different methods (60, 61). Although there was little overlap between markers in control HeLa cells (<10%), E4orf4 markedly promoted the codistribution of punctated Rab11a structures with mitochondrial markers, which increased ~2.6–3.1-fold relative to control cells (Fig. 3, A and B). Notably, only wild type Rab11a, and not GDP or GTP mutants of Rab11a (S25N or Q70L), was relocalized at or close to mitochondrial membranes in response to E4orf4 (Fig. 3C). This was observed in another set of experiments in which different forms of GFP-Rab11a were expressed alone, or together with E4orf4, for a short period of time to avoid toxicity induced as a result of deregulated Rab11a-dependent trafficking. This suggested that mitochondrial relocalization of Rab11a and changes in mitochondrial shape require cycling of Rab11a between its GTP-bound and GDP-bound states, just as Rab11a mediated endosomal recycling through various pathways (64–66). Besides, overexpression of GDP or GTP mutants of Rab11a interfered with E4orf4-induced polarization of recycling endosomes and mitochondrial shape changes (data not shown). To further analyze the impact of E4orf4 on the distribution of endogenous Rab11a and mitochondria, post-nuclear supernatants from 293T cells transfected with E4orf4 were fractionated using a self-generated Opti-Prep gradient. As shown previously, E4orf4 promoted the translocation of Rab11a, along with transferrin receptor-positive endosomes, to fractions of intermediate density relative to control cells (41). This phenomenon likely reflected the enhanced retrograde transport of Rab11a endosomes to the Golgi in E4orf4-expressing cells. We observed that mitochondrial markers (Tom20 and cytochrome c), but not Fyn, also shifted to these intermediate density fractions upon E4orf4 transfection and showed greater codistribution with Rab11a (Fig. 3D, fractions circumscribed by red dashed lines). This was consistent with increased proximity between Rab11a and mitochondrial membranes and fragmentation of the organelles as seen in our single cell analyses of E4orf4-expressing cells.
Inspection of three-dimensional reconstructions of image stacks revealed that Rab11a was frequently localized at sites of mitochondrial constriction (Fig. 3E, insets, arrowheads) or between two mitochondria in an end-to-end configuration (Fig. 3E, insets, arrows), suggesting that Rab11a could be involved in mitochondrial fission/fusion dynamics. To follow the behavior of Rab11a relative to mitochondrial dynamics, we performed four-dimensional microscopic analyses in HeLa-GFP-Rab11a cells transfected with E4orf4-mRFP after labeling mitochondria with MitoTracker, by using spinning disk confocal microscopy. The precise outcome of perinuclear GFP-Rab11a dynamics on mitochondrial morphology could not be determined, given the coclustering of markers and the dynamicity of mitochondria. Nonetheless, we observed what appeared as recurring contacts between more peripheral mitochondrial tubules and GFP-Rab11a at defined sites, which were spatially and temporally associated with remodeling of the mitochondrial tubules at, or proximal to, the interaction sites (Fig. 3F). Remarkably, such transient and recurrent “contacts” between Rab11a and mitochondria occurred at what appeared as preferential sites (hot spots) on mitochondrial tubules (Fig. 3F, arrow in frames 0:52, 1:45, 2:00). The behavior of some Rab11a-positive structures was reminiscent of tubular endosomes encircling the mitochondrion at sites of remodeling (supplemental Movie). Besides, increased proximity events between Rab11a and mitochondrial membranes were followed shortly by mitochondrial constriction/fission (Fig. 3F, frames 0.52–1.07; frames 2:00–2:07) or contact between the two fragments of the original mitochondrion restoring the initial shape (Fig. 3F, frames 1:37–2:00). Although similar Rab11a dynamics were observed in control HeLa-GFP-Rab11a cells transfected with the vector only, they were stimulated ~2.0-fold in E4orf4-expressing cells (Fig. 3G), consistent with a >2-fold increase in mitochondria-Rab11a colocalization as measured in fixed cells. Thus, we concluded that E4orf4 promoted the trafficking of Rab11a or Rab11a-positive endosomes to the mitochondria leading to changes in mitochondrial dynamics.
Mitochondrial movement is thought to be highly coordinated with changes in their shape (fission/fusion dynamics) for producing more “movable” mitochondria (14). Indeed, we noticed that small mitochondrial fragments often budded from the remodeled network and showed long distance movement (Fig. 4A, arrowheads in frames 0:30–1:30). To further evaluate mitochondrial motility in cells undergoing organelle remodeling, time sequences of MitoTracker-labeled mitochondria were analyzed using a method adapted from Ref. 63. Two images obtained 60 s apart from each other were colored green and red, respectively, and were subsequently overlaid to visualize mitochondria that maintained their position (yellow pixels) and those that moved (green and red pixels) (Fig. 4B). Image subtraction allowed for visualization of the sites of movement (t = 60–t = 0; Fig. 4, bottom panels). In control cells, green and red pixels were mostly side-by-side and reflected lateral wiggling movements of the organelles (Fig. 4B, insets 1a and 1b). Longitudinal movements of the organelles and single green and red pixels were more abundant in E4orf4-expressing cells and likely manifested directional movement into or out of the focal plane (Fig. 4B, insets 2 and 3). Assessment of mitochondrial movement over a 1-min time period by subtraction of sequential images (7.5-s interval) revealed that the overall motility of the mitochondrial network was increased ~1.5-fold in E4orf4-expressing cells (% mitochondrial movement = motile pixels/total pixels; Fig. 4C). Together, the results suggested a functional connection between the mitochondrial localization of Rab11a and changes in fission-fusion dynamics that would promote mitochondrial redistribution.
The above-described mitochondrial dynamics associated with Rab11a were reminiscent of transient fusions, which have been characterized as the mitochondrial version of the “kiss and run” phenomenon (67). These events appear to promote mitochondrial motility and rely on both fission (Drp1) and fusion (Mnfs, OPA1) factors that can interact at the fusion/fission sites to regulate the pairing of fusion/fission events (67–69). Actually, we found that transfection of either a dominant-negative Drp1 construct lacking GTPase activity (CFPDrp1K38E) (70, 71) or of a mutant Mfn2(1–703)YFP lacking the C-terminal coiled-coil domain essential for mitochondrial fusion (47, 72) markedly inhibited the distinctive rearrangement of mitochondria in E4orf4-expressing cells along with early nuclear polarization (Fig. 5A, ~53% to ~58% inhibition, respectively). In agreement with a role for mitochondrial division, cotransfection of dominant-negative Drp1 inhibited mitochondrial fragmentation and restored the aspect ratio of individual mitochondria in E4orf4-expressing cells to near control values (Fig. 5A, ARmit). The Drp1 inhibitor Mdivi-1 also reduced the number of E4orf4-expressing cells displaying polarized mitochondrial remodeling (Fig. 5A, ~62% inhibition) and inhibited the E4orf4-mediated increase in mitochondrial motility (Fig. 4C), further supporting a strong dependence on mitochondrial division (73). Thus, the results were consistent with a role for mitochondria-shaping proteins.
The mitochondrial recruitment of Drp1 is a key event in the regulation of mitochondrial dynamics (17, 67, 74). Cytoplasmic dynein regulates the correct targeting of Drp1 to the outer mitochondrial membrane, and it has been proposed that Drp1 itself, or Drp1-decorated membrane vesicles, are cargos for dynein (14). Given the spatiotemporal dynamics of Rab11a at or close to mitochondria and the strong dependence of mitochondrial changes on Drp1, we postulated that Rab11a could modulate the mitochondrial recruitment of Drp1 (directly or via regulation of vesicle trafficking). Because of the asynchronous onset of E4orf4 expression, we first performed single cell analyses to visualize the localization of YFP-Drp1 cotransfected with E4orf4-mRFP so as to focus our attention on the early steps of the remodeling process. We observed that E4orf4 induced an early decrease in diffuse, cytoplasmic YFP-Drp1 along with an increase in membrane-associated punctate Drp1 foci (Fig. 5, B, siCtl + E4orf4, and C, no siRNA) that were enriched at sites of mitochondrial constriction/fission (Fig. 5D, arrows). A similar impact on Drp1 localization was visualized at the level of the endogenous protein (data not shown). Importantly, although control siRNA did not perturb the mitochondrial recruitment of Drp1, which was increased ~1.5-fold in the early stages of E4orf4 expression, Rab11a-specific siRNAs ablated the E4orf4-dependent increase in mitochondrial Drp1 foci (Fig. 5C, E4orf4 + siRab11a).
Rab11 family-interacting proteins (Rab11-FIP1s, hereafter named FIPs) are effectors of Rab11 GTPases, which regulate their different functions in endosomal recycling by assembling mutually exclusive targeting complexes on recycling endosomes (75, 76). We next sought to explore whether the Rab11a-dependent mitochondrial recruitment of Drp1 could involve specific FIPs. To first analyze the impact of E4orf4 on Rab11/FIPs-targeting complexes, native immunoprecipitations of FIPs were performed in HeLa cells coexpressing various FIPs fused to GFP and E4orf4-mRFP using a MES-digitonin lysis buffer, as described (59, 77, 78). As expected from a protein that mobilizes Rab11a-regulated trafficking, E4orf4 was detected in immune complexes of several FIPs, in particular with FIP1C/RCP, FIP4, and FIP5/RIP11 (Fig. 6A). Single cell analyses further revealed a marked recruitment of FIP1C/RCP(GFP)- and FIP5/RIP11(GFP)-labeled vesicles to the juxtanuclear region, where Rab11a and typical recycling cargos (TfR) accumulate upon E4orf4 expression (Fig. 6C, arrows) (41). This suggested that E4orf4 could perturb the polarized trafficking of Rab11a vesicles by diverting FIPs, in particular FIP1C/RCP and FIP5/RIP11 (33, 41).
To next determine whether Rab11a-mediated recruitment of Drp1 could involve a specific FIP, we assessed the presence of Drp1 in FIP immune complexes. Anti-Drp1 antibodies detected bands of ~100 and ~160 kDa only within FIP1C/RCP(GFP)-immune complexes, and furthermore, these bands were more abundant in E4orf4-containing complexes (Fig. 6, A and B, red asterisk), suggesting that a specific interaction between FIP1C-RCP and Drp1 was enhanced by E4orf4. Because monomeric Drp1 essentially migrated as a doublet of ~80 kDa (Fig. 6, A and B, TL), the Drp1 species associated with FIP1C/RCP might represent modified forms (SUMO1- or ubiquitin-modified forms) and/or dimers of Drp1 incompletely disassembled by SDS-PAGE, respectively. All forms were observed previously along with Drp1 mitochondrial targeting and organelle fission (46, 79–81). A complex picture is emerging regarding the multiple covalent modifications of Drp1 controlling its localization, protein interactions, higher order assembly, and GTPase activity, which are likely to influence one another dynamically (20). Notwithstanding the uncertainty regarding specific modification(s), the identity of the ~100-kDa band in FIP1C/RCP-immune complexes was validated as Drp1 using two distinct anti-Drp1 antibodies (compare Fig. 6, A and B, asterisks), and similar results were obtained using 293T cells.10 As expected, Rab11a was also detected in FIP1C/RCP-immune complexes but not in GFP-immune complexes. We further found that a fraction of FIP1C/RCP(GFP) was localized to mitochondria or in vesicles tightly associated with mitochondria, the proportion of which was significantly increased in E4orf4-expressing cells (~1.5–1.8-fold; Fig. 6, D and E). In these cells, FIP1C/RCP(GFP)-punctae could be observed at discrete sites along mitochondrial tubules (Fig. 6F). Together, the data were consistent with a potential role for FIP1C/RCP in E4orf4-mediated and Rab11a-dependent recruitment of Drp1 to the mitochondria.
To directly address the functional relevance of FIP1C/RCP and FIP5/RIP11, we used four different siRNA sequences that depleted FIP1/RCP or FIP5/RIP11 by ~60–75 or ~85–90%, respectively, in HeLa cells (Fig. 7A). Depletion of FIP1/RCP or FIP5/RIP11 inhibited the number of cells exhibiting polarized recruitment of Rab11a-positive endosomes in response to E4orf4 by ~48–60% (Fig. 7B). However, only FIP1/RCP knockdown could prevent the mitochondrial targeting of Drp1 (Fig. 7C, siFIP1/RCP) that remained unaffected by transfection with FIP5/RIP11-specific siRNAs (Fig. 7C, siFIP5/RIP11). Likewise, FIP1/RCP siRNAs, but not FIP5/RIP11 siRNAs, reduced mitochondrial remodeling by ~37–57% in response to E4orf4 (Fig. 7D), consistent with a specific interaction between FIP1C and Drp1. Under such conditions, Golgi membrane scattering, which is a result of a chronic increase in the transport of Rab11a endosomes to the Golgi (41), was also inhibited by ~60% (data not shown). This was in agreement with a recently described role for FIP1C/RCP in retrograde transport via the early/recycling endosomes-to-trans-Golgi network pathway (82). In marked contrast, silencing either FIP1/RCP or FIP5/RIP11 similarly impaired actin assembly in E4orf4-expressing cells (Fig. 7E, arrowhead). We concluded that although FIP1/RCP could regulate organelle dynamics, in part by controlling Drp1 trafficking, both FIP1/RCP and FIP5/RIP11 contributed to the cytoskeletal changes induced by E4orf4.
We next attempted to determine the functional relevance of mitochondrial dynamic changes to the cytoskeletal transitions driving E4orf4-induced cell death, including focal adhesions remodeling, assembly of the polarized actin network, and ultimately, cellular blebbing (39–41). Remarkably, interfering with mitochondrial fission/fusion dynamics by transfection of mutant constructs of mitochondria-shaping proteins or by treating cells with the Drp1 inhibitor Mdivi-1 severely impaired paxillin recruitment to the atypically enlarged focal adhesions (Fig. 8A, arrowhead, ~53–67% inhibition), just as it inhibited assembly of the juxtanuclear actin ring in E4orf4-expressing cells (Fig. 8B, arrowhead, ~38–56% inhibition). The Drp1 inhibitor Mdivi-1 also impaired Rab11a recruitment on large tubulovesicular structures (data not shown), suggesting that polarized endosomal transport via Rab11a endosomes might be regulated by Drp1-mediated mitochondrial dynamics. Intriguingly, all treatments that impaired mitochondrial changes in response to E4orf4 also interfered at a very early step in the process of cell polarization; this was reflected by a loss of nuclear translocation to one side of the cell as typically observed in HeLa cells (Figs. 5A and and8,8, B and C). Furthermore, the dramatic cellular blebbing phenotype that characterizes E4orf4 tumoricidal action in breast cancer cells (MDA-MB-231) and depends upon a chronic increase in myosin II ATPase (37, 39) was reverted by a short treatment with Mdivi-1 (Fig. 8C, ~75% inhibition), just as it was impaired by the mitochondrial ATP synthase inhibitor oligomycin (Fig. 8C, oligomycin, ~71% inhibition). The results were consistent with a distinctive role for mitochondria in early E4orf4-induced signaling and reinforced long standing evidence that E4orf4-induced cell death does not rely on a classic mitochondrial pathway involving an early loss of mitochondrial functions (38). We concluded that under these experimental conditions, the cell demolition program induced by E4orf4 depended upon mitochondrial dynamics and bioenergetic metabolism.
It has been shown that E4orf4 action largely relies on its ability to subvert SFK signaling in the context of transformed cells (37, 43, 44). The above-described findings suggest that E4orf4 might exploit a pathway rewired during cellular transformation for supporting cellular energy and metabolic needs, presumably to stimulate high cytoskeletal dynamics. To investigate further the relationship between oncogenic SFK signaling, Rab11, and mitochondrial dynamics, we exploited MDCK cells expressing a thermosensitive mutant of viral Src (MDCK-tsv-Src) that are rapidly and reversibly transformed at the permissive temperature of 35 °C and form actin-rich structures called podosomes and rosettes. These organelles are matrix-degrading adhesion structures related to the invadopodia found in some cancer cells. Such adhesive structures, collectively called “invadosomes,” are believed to regulate protease-driven invasive cell migration and represent a powerful model to study the convergence of cytoskeletal and membrane trafficking pathways (83–85).
On switching MDCK-tsv-Src cells to the permissive temperature, cells accumulated high levels of tyrosine-phosphorylated proteins and exhibited morphological changes typical of an epithelial-mesenchymal transition along with assembly of actin-rich, dot-like podosome structures on a fibronectin matrix (Fig. 9A, white arrowheads; data not shown). These organelles were organized into polarized circular clusters called rosettes (Fig. 9A, black arrowheads), which required sustained v-Src signaling at 35 °C (Fig. 9A, 35 °C O/N + Rev) and exhibited high matrix degrading activities (data not shown). Remarkably, on v-Src activation, mitochondria exhibited a more punctate morphology and were redistributed near rosettes that formed at the vicinity of the perinuclear GFP-Rab11a-containing endosomal compartment (Fig. 9, B, 35 °C 2h, and C, black arrowheads). Changes in mitochondrial shape and distribution were observed in ~70% of v-Src-transformed cells (Fig. 9D, 35 °C O/N) and could be partially reverted by switching the cells back to the restrictive temperature of 40.5 °C (Fig. 9, B, 35 °C O/N + Rev, and D, reverted), suggesting that they were related to oncogenic Src signaling.
To assess functional relationships between Rab11a, mitochondrial dynamics, and rosette biogenesis, we overexpressed dominant-negative constructs that block recycling via Rab11a-positive endosomes (Rab11aS25N-GFP) or impair mitochondrial dynamics (Drp1K38E, Mnf2(1–703)). Remarkably, Rab11aS25N-GFP could impair both the early changes in mitochondrial shape and rosette formation upon v-Src activation (Fig. 9, D and E, ~40–64%, respectively), involving Rab11a in the biogenesis of rosettes. Importantly, mutants of mitochondria-shaping proteins drastically impaired rosette formation on v-Src activation, just like the Drp1 inhibitor Mdivi-1 (Fig. 9, E and F, ~96–90% inhibition). The mitochondrial ATPase inhibitor oligomycin had a partial inhibitory effect on rosette formation (~46% inhibition, Fig. 9E), suggesting that v-Src-transformed cells require mitochondrial ATP production at the vicinity of rosettes to support the high dynamicity of these organelles. Finally, a 4-h treatment with Mdivi-1 at 35 °C could revert v-Src-induced rosette formation and morphological transformation to the same extent as switching the cells back to the restrictive temperature for 4 h (Fig. 9E, 35 °C O/N + Mdivi-1, O/N + reverted), indicating a strong dependence of cell transformation on mitochondrial division. We concluded that E4orf4 exploits an SFK- and Rab11a-regulated pathway that serves to coordinate mitochondrial functions with membrane trafficking during cellular transformation.
It is believed that E4orf4 has evolved the ability to hijack SFK signaling for promoting a concerted dialog between actin, vesicular transport, and organellar dynamics, for reasons that are not entirely clear (33). Yet it provides a unique probing tool for studying the mechanisms involved, which appear to control a noncanonical, tumor cell-selective death pathway. The results presented in this study convey two original and interrelated messages as follows: first, that a signaling pathway regulated by Rab11a and FIP1C/RCP makes connections with mitochondria to modulate the mitochondrial recruitment of the fission factor Drp1; and second, that the Rab11a-regulated pathway is modulated by SFK and mediates some of the signaling to mitochondria required for mobilizing these organelles near energy-requiring actin-rich structures. These findings were substantiated by taking advantage of E4orf4 as a prototype model, and of v-Src, to confirm the relevance of our findings in the context of Src-transformed cells. They have important implications in inter-organelle signaling and oncogene-induced cytoskeletal transformations that are discussed below.
The small GTPase Rab11a mainly localizes to the tubulovesicular recycling compartment and functions in the slow retrieval of internalized membranes and signaling molecules to the plasma membrane or to the Golgi via retrograde membrane transport (86). Recycling endosomes also appear to provide an intracellular reservoir of membranes and signaling molecules regulating actin dynamics, which would be mobilized during dynamic rearrangements of the cell (87–89). For instance, polarized membrane trafficking of Rab11 vesicles derived from recycling endosomes to the intercellular bridge plays a crucial role in the completion of cytokinesis (90, 91). Likewise, polarized membrane trafficking of Rab11a vesicles is required for E4orf4-induced actin assembly and cell death (41). Although the trafficking of Rab11 vesicles has been associated with SFK signaling (33, 92), this is the first report, to our knowledge, of a role for Rab11a in mitochondrial dynamics that would be regulated by SFK signaling. Thus, Rab11a may function as a converging regulator of polarized membrane trafficking and mitochondrial dynamics during rearrangements of the cell, to help coordinate organellar functions with cytoskeletal dynamics. The detailed molecular mechanism whereby Rab11a regulates mitochondrial dynamics remains to be clarified, but evidence was obtained involving FIP1C/RCP, an effector of Rab11a in endosome-to-Golgi transport, in the mitochondrial recruitment of Drp1. Although we observed a specific FIP1C-Drp1 association, it is complicated to discriminate whether such interaction is taking place before or following their mitochondrial recruitment at fission sites. In theory, there are several possibilities for functional interactions between Rab11a-FIP1C and Drp1, because Drp1 can also be found on membrane vesicles and on the Golgi in some but not all cell lines, where it has been proposed to regulate the apical sorting of proteins (93–97). The question of whether Drp1 is cotransported together with Rab11a-FIP1C/RCP to the mitochondria could not be resolved with the resolution achieved in our live-cell imaging analyses. Other than modulating the trafficking of Drp1, Rab11a-FIP1C could promote the stable association of Drp1 with mitochondrial membranes by contributing to the necessary membrane remodeling underlying the stable assembly of Drp1 oligomers at fission foci. It is intriguing that FIP1C was reported to bind another member of the dynamin family of proteins (82), suggesting that it may function to scaffold oligomeric complexes controlling membrane scission in various organelles (endosomes, Golgi, and mitochondria). Although future work and emerging super-resolution imaging techniques will be needed to elucidate the exact function of Rab11a-FIP1C at the mitochondria (see below), this study provides a novel functional link between mitochondrial dynamics and trafficking factors controlling vesicle traffic at the endosome-Golgi interface.
Another unresolved question is whether Rab11a and FIP1C are directly recruited at mitochondrial fission foci or if they may be transported on membrane vesicles to the mitochondria. Although speculative, we feel that there is sufficient evidence to support the latter possibility. A precedent exists for the transport of recycling endosomes to mitochondria, which is thought to contribute to mitochondrial iron trafficking. In this paradigm, direct transfer of iron from transferrin-containing recycling endosomes to the mitochondrion is believed to occur via a kiss and run process that would entail molecular motors and docking complexes (98, 99). The interactions between Rab11a-GFP structures bearing resemblance to endocytic tubules and mitochondrial membranes seen here by three-dimensional live-cell microscopy, was reminiscent of a kiss and run phenomenon that could modulate the transfer of factors influencing fission-fusion events at the mitochondrion. This could be part of a mechanism coupling shape transitions of mitochondria to their cytoskeleton-based transport, as suggested previously (67). Such a process would be consistent with transient residency of Rab11a at mitochondrial membranes, at least in the early stages of the remodeling process, which might explain the low level of colocalization measured with standard confocal imaging methods in fixed cells. Notwithstanding, we could repeatedly measure a significant increase in Rab11a localization at or proximal to mitochondrial membranes in response to E4orf4, which was observed only with wild type Rab11a but not with GDP or GTP mutants when expressed together with E4orf4 in similar conditions. This further supports a specific function for cycling of Rab11a at mitochondria, in the transport or recruitment of components modulating fission-fusion dynamics. We believe that super-resolution imaging techniques such as STED or STORM will be required in the future to provide a definitive answer to this issue. Although electron microscopy analyses are in progress to attempt revealing associations between Rab11a endosomes and mitochondria, they may not be suitable to detect “transient” interactions like those suggested by our three-dimensional live cell microscopy analyses. Actually, increased proximity events between Rab11a-GFP and mitochondria of short duration were frequently observed at “hot spots” on a mitochondrial tubule and could reflect the involvement of tethering and docking complexes at the interface of membranes. It was shown that mitochondrial iron trafficking involves Sec15l1, a component of the mammalian exocyst complex that interacts with Rab11a (100, 101). This vesicle-tethering complex is proposed to mediate the initial recognition between secretory vesicles and the target membrane and could in principle contribute to the tethering of Rab11a endosomes to mitochondrial membranes (102). Of much interest here, the small GTPase RalA (RAS-like protein A), another regulator of polarized membrane trafficking that interacts with the exocyst, has been reported to translocate to mitochondria or to vesicles tightly associated with mitochondria at mitosis, where it modulates the recruitment of Drp1 and proper mitochondrial division (58, 103). Although much remains to be clarified regarding the molecular details, including a potential role for RalA, it is intriguing that the Rab11a-mediated mitochondrial dynamics described here were found to rely on a Rab11a-effector protein in endosomal recycling. Whether the observed phenomenon could reflect a transport route connecting some signaling endosomes, maybe en route to the Golgi complex, and the mitochondria remains to be established in future studies.
Finally, taking advantage of both Ad2 E4orf4 (human adenovirus type 2 early region 4 open reading frame 4 protein) and RSV v-Src, we obtained evidence for a role for Rab11a- and Drp1-mediated mitochondrial dynamics in the assembly of polarized actin-rich structures, including podosome rosettes in v-Src-transformed cells, which represent a powerful paradigm to study the convergence of signaling, adhesive, cytoskeletal, and polarized membrane trafficking pathways (83–85). This provides further strength to a model whereby E4orf4 would subvert oncogenic signaling by SFK, leading to an imbalance in Src morphogenic and survival functions in tumor cells (104). We propose that Ad2 E4orf4 and RSV v-Src, and possibly cancer cells, might exploit an SFK-controlled pathway, which regulates a functional interplay between Rab11a-FIP1C and mitochondria-shaping proteins. Such a pathway would contribute to remodel host cells during viral infection and could modulate the invasive properties of tumor cells. Structural adaptations of mitochondria could be coordinated with metabolic reprogramming in malignant cells (the Warburg effect), a process for which SFK are thought to play a role by targeting several glycolytic enzymes (105, 106). Besides, Src can be observed in mitochondria of several cell types, including cancer cells, where it could modulate mitochondrial dynamics and bioenergetics (107, 108). Contrary to early beliefs, mitochondria are functional in cancer cells, and proliferating cells still need mitochondrial functions; actually, cancer cells appear to derive a significant fraction of their ATP through oxidative phosphorylation (105). Although the mechanism whereby SFK are targeted to the mitochondria is not completely understood, it has been linked to the traffic of receptor tyrosine kinase in some systems (109). Likewise, the activation of various SFK members and their effect on actin polymerization are related to their traffic via Rab11 endosomes, which also modulates the trafficking of receptor tyrosine kinases and signaling dynamics (92). Together, evidence suggests that SFK regulate multifunctional signaling platforms at the recycling endosome involving Rab11a, which could promote inter-organellar communication during morphogenic events (33). Based on the results here, we further speculate that mitochondria are critical for fueling polarized transport processes in the context of cancer cells and that dysregulated mitochondrial dynamics could play a pivotal role in tumor invasion downstream of SFK signaling. In support of this notion, a role for Drp1-mediated mitochondrial fission in lamellipodia formation and breast cancer cell migration has been recently reported (10). In fact, dysregulated mitochondrial dynamics might be a general feature of transformed cells, as several cancer cells lines have been shown to exhibit abnormal mitochondrial morphologies (110).
In conclusion, we propose that the ability of E4orf4 to exploit SFK signaling in key pathways controlling vesicle trafficking, cell polarity, and organellar dynamics makes E4orf4 an appealing system to target crucial elements of signaling in cancer metastasis.
OCT-YFP, OCT-PAGFP, YFP-Drp1, and CFP-Drp1(K38E) were gratefully obtained from H. M. McBride (Montreal Neurological Institute, Montreal, Canada); GFP-Rab11a (WT and S25N) was obtained from T. Balla (National Institutes of Health, Bethesda); Mfn2(1–703)YFP was obtained from R. J. Youle (National Institutes of Health, Bethesda); GFP-FIP1, GFP-FIP5, and GFP-RBDFIP were obtained from M. W. McCaffrey (University College Cork, Ireland); GFP-FIP3, GFP-FIP4, and rabbit polyclonal anti-FIP1/RCP were kindly provided by R. Prekeris (University of Massachusetts Medical School, Worcester, MA); and ts72 v-Src was obtained from I. H. Gelman (Roswell Park Cancer Institute, Buffalo, NY). We thank L. Pellegrini (Centre de Recherche de l'Institut Universitaire en Santé Mentale de Québec, Université Laval) for helpful discussion; H. Lambert for dedicated work and support (Centre de Recherche sur le Cancer de l'Université Laval, Lavoie and Landry laboratories), and C. St-Pierre and A. Loranger (Centre de Recherche sur le Cancer de l'Université Laval) for their assistance in microscopic analyses. We are also grateful to Ronald Hancock (Université Laval) for editing this manuscript.
*This work was supported in part by Canadian Institutes of Health Research Operating Grant MOP-49450 (to J. N. L.) and by the Natural Sciences and Engineering Research Council Grant RF:RGPIN371663 (to J. N. L.).
This article contains supplemental Movie.
8M. C. Boulanger and J. N. Lavoie, unpublished data.
9M. C. Landry and J. N. Lavoie, unpublished data.
10M. C. Boulanger and J. N. Lavoie, unpublished data.
7The abbreviations used are: