Previous studies indicate involvement of the multifunctional Ca2+/calmodulin-dependent protein kinase II (CaMKII) in vascular smooth muscle (VSM) cell migration. In the present study, molecular loss-of-function studies were used specifically to assess the role of the predominant CaMKIIδ2 isoform on VSM cell migration using a scratch wound healing assay. Targeted CaMKIIδ2 knockdown using siRNA or inhibition of activity by overexpressing a kinase-negative mutant resulted in attenuation of VSM cell migration. Temporal and spatial assessments of kinase autophosphorylation indicated rapid and transient activation in response to wounding, in addition to a sustained activation in the leading edge of migrating and spreading cells. Furthermore, siRNA-mediated suppression of CaMKIIδ2 resulted in the inhibition of wound-induced Rac activation and Golgi reorganization, and disruption of leading edge morphology, indicating an important function for CaMKIIδ2 in regulating VSM cell polarization. Numerous previous reports link activation of CaMKII to ERK1/2 signaling in VSM. Wound-induced ERK1/2 activation was also found to be dependent on CaMKII; however, ERK activity did not account for effects of CaMKII in regulating Golgi polarization, indicating alternative mechanisms by which CaMKII affects the complex events involved in cell migration. Wounding a VSM cell monolayer results in CaMKIIδ2 activation, which positively regulates VSM cell polarization and downstream signaling, including Rac and ERK1/2 activation, leading to cell migration.
- Ca2+/calmodulin-dependent protein kinase II
- calcium signaling
vascular smooth muscle cells (VSM) found within the medial wall of the vasculature are quiescent and express a differentiated phenotype. Differentiated VSM cells function to maintain vascular tone, which is regulated primarily by increases in free intracellular Ca2+ and/or signaling pathways affecting the balance of myosin light chain kinase and myosin phosphatase activities (14, 37). In response to injury or disease, VSM cells may become proliferative and migrate across the internal elastic lamina, resulting in neointima formation (46). Although this phenotypic transition correlates with changes in expression of the ion channels and mechanisms regulating Ca2+ signals (4, 52), several studies have demonstrated dependence of VSM cell proliferation and migration on Ca2+-dependent regulatory pathways (17, 22, 31, 44). In contrast to Ca2+-dependent regulation of differentiated VSM function, our understanding of the cellular mechanisms and intracellular targets of Ca2+ signals in the regulation of VSM cell proliferation and migration is still unclear.
Calcium/calmodulin-dependent protein kinase II (CaMKII) is a ubiquitous multifunctional serine/threonine protein kinase, with complex structural and autoregulatory properties (23). CaMKII has been implicated in the regulation of VSM cell migration (2, 38, 40). However, existing studies are not entirely consistent, with divergent results apparently dependent on the specific pharmacological or molecular approaches used to manipulate CaMKII activity. For example, attenuation of CaMKII activity with the pharmacological inhibitors KN-93 and KN-62 has been reported to block VSM cell migration in a Transwell assay (38), and VSM cells stably overexpressing constitutively active CaMKII α-subunits demonstrated enhanced migration (2). On the other hand, transiently overexpressed constitutively active CaMKIIδ2, the predominant endogenous CaMKII isoform in cultured VSM cells, was found to inhibit cell migration, and, conversely, overexpression of kinase-negative CaMKIIδ2 enhanced VSM cell migration (40). Additional approaches, such as loss-of-function small interfering RNA (siRNA) silencing, are needed to resolve the functional importance of endogenous CaMKII isoforms in regulating VSM cell migration.
Cell migration on a surface is a multifaceted process with an initial step of cell polarization and extension of a leading edge toward the direction of migration. New focal adhesions are formed and mature to stabilize the cell, followed by retraction of the cell body, resulting in net translocation (41). Interpretation of existing studies of CaMKII involvement in VSM migration is further complicated by the complexity of the modified Boyden chamber or Transwell assays commonly used to model the process. Concerted regulation of cell attachment, spreading, matrix degradation, and invasion through a pore involves processes in addition to those already-complicated events involved in moving across a surface. It is noteworthy in this regard that simple adherence and spreading of cultured VSM cells on extracellular matrices such as fibronectin or collagen also result in the activation of CaMKII and subsequent downstream signaling involving ERK1/2 (29). The migration process itself might be more directly assayed using a scratch wound healing model, which also provides the opportunity for visualizing protein dynamics using fluorescence microscopy approaches.
Collectively, discrepancies using various pharmacological and molecular approaches and the complexity of potential underlying mechanisms have thwarted a definitive conclusion as to the role of CaMKII in VSM cell migration. In this study, we used loss-of-function molecular approaches and a wound healing assay to evaluate the activation and function of the endogenous CaMKIIδ2 isoforms in VSM cell migration. CaMKII was found to be acutely activated at the wound edge and to contribute net positively to migration and wound closure. Moreover, we have for the first time demonstrated that CaMKII is activated in the leading edge of migrating VSM cells and promotes cellular polarization by regulating Golgi reorientation and leading edge dynamics, the latter inferred by changes in leading edge morphology. Both wound-induced ERK1/2 and Rac activation were found to be dependent on CaMKII activation, suggesting potential mechanisms by which CaMKII could exert effects on VSM cell polarization and migration.
MATERIALS AND METHODS
VSM cells were enzymatically dispersed from thoracic aortas of 200- to 300-g male Sprague-Dawley rats as previously described (15, 47). Cells were cultured in DMEM/F-12 medium supplemented with 10% fetal bovine serum (FBS) at 37°C and 5% CO2. Confluent cultures from passages 3–9 were used for each experiment to minimize phenotypic variations. Protocols involving use of experimental animals for these procedures were reviewed and approved by the Albany Medical Center Institutional Animal Care and Use Committee.
Antibodies and materials.
Creation and specificity of the anti-peptide polyclonal antibody against the δ2-specific isoform of CaMKII and phosphorylated CaMKII (Thr287) were described previously (15, 51). Other antibodies used include anti-Rac (Upstate), anti-GM130 (BD Transduction), β-actin (Sigma), phospho-p44/42 MAP kinase (Cell Signaling), ERK2 (BD Biosciences), and FITC-conjugated phalloidin (Sigma). All cell culture media and supplies were from Fisher Scientific (Pittsburgh, PA) unless otherwise specified. Platelet-derived growth factor and ERK inhibitor U0126 were from Calbiochem (La Jolla, CA).
Ambion Silencer Predesigned siRNA (ID no. 1998162, standard purity) and Dharmacon ON-TARGETplus SMARTpool (catalog no. L-099520-00-0010; Thermo Fisher Scientific, Lafayette, CO) siRNA specifically targeting rat CaMKIIδ protein were electroporated in VSM cells. Controls included Silencer Negative Control No. 1 siRNA (Ambion) and ON-TARGETplus siCONTROL Nontargeting siRNA No. 1. Rac siRNA was Dharmacon ON-TARGETplus SMARTpool (catalog no. L-080171-00-0010). Electroporation was performed as follows: subconfluent VSM cells were removed from the culture dish by addition of trypsin and pelleted. The pellet was resuspended in siPORT siRNA Electroporation Buffer (Ambion) at a concentration of 2 × 105 cells/cuvette with 1.5 μg of the respective siRNA. Cells were electroporated with one 0.15-ms pulse of 300 V (Gene Pulser II, Bio-Rad). After an additional incubation for 10 min at 37°C, cells were plated on the appropriate culture dishes. Knockdown was confirmed by Western blot after 48 h of exposure.
Adenoviral short hairpin RNA.
We have developed successful CaMKIIδ2 knockdown in our cultured rat aortic smooth muscle cells using a GFP-tagged adenoviral vector to deliver short hairpin RNA (shRNA) constructs (20, 29). Target sequences for suppressing rat and mouse CaMKIIδ2 were selected using the siRNA Target Finder and Design Tool (Ambion) online program. To confirm specificity, the potential siRNAs were subjected to basic local alignment search tool (BLAST) searches against expressed sequence tag libraries. The sequence targeting the translated region of the CaMKIIδ2 isoform is 5′-ATAAACCAATCCACACTAT-3′, nt 1543-1562. Control virus was constructed to target a region of the firefly luciferase with 5′-CGUACGCGGAAUACUUCGATT-3′ as previously described (11). The target sequences were subcloned into the AdTrackHP vector (generous gift from Dr. J. L. Zhang), and positive colonies were purified (Qiagen) and electroporated into electrocompetent AdEasy cells (Stratagene) as previously described (19).
Rac and Rho activity assays.
Rac and Rho activity was determined from whole cell lysates and isolated leading edge of migrating VSM cells using a commercially available enzyme-linked colorimetric assay (ELISA)-based Rac or Rho activity assay (G-LISA; Cytoskeleton, Denver, CO). Protein was isolated using lysis buffer provided, snap-frozen, and processed according to the G-LISA protocol. The lysates were incubated in microwells to which the p21-activated kinase or Rhotekin binding domain peptide was bound to capture active Rac or Rho, respectively. Active protein was detected using indirect immunodetection followed by a colorimetric reaction measured by absorbance at 490 nm.
Leading edge isolation.
The leading edge and whole cell lysate was isolated using a modified Boyden chamber assay as previously described (6). Control cells or cells deficient in CaMKIIδ2 because of RNA interference were plated onto a modified Boyden chamber (3-μM pores, 6-well format; BD BioCoat) coated with 10 μg/ml fibronectin (Sigma) at a density of 1.5 × 106 cells/well and allowed to adhere for 2 h at 37°C in an incubation chamber with 5% CO2 in media containing 0.4% FBS. Platelet-derived growth factor (PDGF)-BB (10 ng/ml) was then added into the lower chamber to stimulate migration for 120 min. The tops of the membranes are swabbed to remove the cell body. The remaining cell was lysed and processed for immunoblotting or relative Rac or Rho activity.
Cells plated on collagen-coated glass coverslips or cell culture dishes were wounded using a 10-μl pipette tip, washed, and placed back into a 37°C incubator in the presence of 10% FBS for the indicated time. Cells were fixed using 4% paraformaldehyde for 20 min and permeabilized with 0.2% Triton X-100 in PBS for 5 min. Nonspecific binding was blocked with 5% fish gelatin in PBS plus 0.1% Triton X-100 followed by a 1-h incubation at room temperature with the described anti-CaMKII, anti-P-CaMKIIThr287 or anti-GM130 antibodies diluted to 1:100–1:250 in blocking buffer. This was followed by washes and a 1-h incubation at room temperature with the appropriate fluorochrome-conjugated secondary antibodies. F-actin was labeled using rhodamine- or FITC-conjugated phalloidin diluted 1:250. Coverslips were mounted onto slides using Vectashield with 4′,6-diamidino-2-phenylindole (DAPI; Vector Laboratories, Burlingame, CA). Cells were imaged on a Zeiss LSM 510 META confocal microscope (Carl Zeiss, Thornwood, NY) or a Leica DM IRB (Leica Microsystems, Bannockburn, IL).
Cell lysates and immunoblotting.
Cells were maintained at 37°C in 5% CO2 during the pretreatment. Reactions were stopped by removal of HBSS and transfer of the dishes to ice, and cells were lysed (0.5 ml/60-mm dish or 1 ml/100-mm dish) in a modified RIPA buffer composed of 10 mM Tris (pH 7.4), 100 mM NaCl, 1 mM EDTA, 1 mM EGTA, 1 mM NaF, 20 mM Na4P2O7, 2 mM Na3VO4, 1% Triton X-100, 0.1% SDS, 0.5% deoxycholate, 10% glycerol, 1 mM DTT, 0.1 mM phenylmethylsulfonyl fluoride (PMSF), and 0.2 U/ml aprotinin. The lysates were collected into 1.5-ml tubes and cleared by centrifugation at 14,000 rpm at 4°C for 10 min.
The multiwound assay was performed by dragging a multitoothed comb (0.5 μM per tooth, 0.5 μM apart) through a VSM monolayer, placing the dish on a 37°C warming plate for the indicated time. Cells were lysed using appropriate buffer for the specific subsequent assay.
Lysates were resolved on an SDS-PAGE gel and transferred to nitrocellulose. The membranes were blocked in Tris-buffered saline containing 0.2% Tween 20 (TBST) and 5% nonfat dry milk. After blocking, the membranes were incubated in primary antibody for 1 h at 22°C, washed three times with TBST, and incubated with horseradish peroxidase-conjugated secondary antibody (Amersham) for 1 h at 22°C followed by washing (3 times) with TBST. Membranes were developed using chemiluminescence substrate (Amersham); signal intensity was measured with a Fuji LAS4000 Imaging Station, and band intensity was compared using Multi Gauge V3.1. All blots shown are representative of at least three experiments.
Migration assay and quantification.
Two days post-siRNA treatment, an artificial wound was made in the monolayer by scraping a 10-μl pipette tip across the bottom of the dish. The wound was extensively washed, medium containing 10% FBS was replaced, and cells were allowed to migrate for the appropriate time in a 37°C incubation chamber with 5% CO2. Cells were subsequently fixed with 4% paraformaldehyde and stained with Coomassie blue. Images were taken with a Leica DM IRB at ×10 using brightfield microscopy. The remaining open area of the wound was measured using Image J as previously described, with some modifications (9). Images were cropped to specified size, and, with the “area calculator” in Image J (National Institutes of Health), the total area absent of Coomassie-stained cells could be measured by setting a threshold using Image J's threshold tool for selection of this open area only. From this setting, the open area for an image can be calculated in arbitrary units.
Golgi reorientation analysis.
Analysis of Golgi reorientation was carried out, with some modifications, as described previously (36). Cells were fixed at 0, 2, and 4 h following wounding of the confluent monolayer with a pipette tip, before VSM cell migration, which initiates ∼5–6 h postwounding. The Golgi was stained with an antibody directed against the Golgi-specific membrane protein GM130, actin stained with rhodamine-conjugated phalloidin, and the nucleus stained with DAPI. To measure Golgi orientation, 120° angles were drawn (Image J) from the center of the nucleus on cells that lined the edge of the wound, creating three sectors. The angles were drawn such that one of the 120° sectors faced the edge of the wound (sector A). If the Golgi encompassed a sector away from the wound edge or spanned any two sectors, it was categorized as polarized in a different direction away from the wound edge. Golgi found within all three sectors were classified as nonpolarized. All of the transduced cells at the wound edge in three fields were categorized (120–150 total cells) per siRNA treatment and per time point. Data are expressed as means ± SE. Mean values of groups were analyzed using GraphPad PRISM version 4.00 and compared by ANOVA with post hoc comparisons using Bonferroni's multiple comparison test. For all comparisons, P < 0.05 was considered statistically significant.
CaMKII silencing inhibits VSM cell migration.
Molecular loss-of-function approaches were used to suppress endogenous CaMKIIδ2 expression (siRNA) or activity (kinase-negative CaMKIIδ2 mutant), and the effects on VSM cell migration were assessed using a scratch wound assay (Fig. 1). CaMKIIδ2 expression was consistently suppressed by at least 80% 48 h postelectroporation using two separate commercial pools of siRNA duplexes (Fig. 1C). Confluent monolayers of either control siRNA-treated or CaMKIIδ2 siRNA-treated cells were scratch “wounded” with a 10-μl pipette tip and placed in serum-containing media (10% FBS) for 12 h. Cell proliferation over this period is expected to be minimal; therefore, wound closure under these conditions primarily reflects VSM cell spreading and migration. CaMKIIδ2 suppression using siRNA inhibited wound closure (Fig. 1A), quantified by threshold analysis of the open wound area (Fig. 1B), as previously described (9) (see materials and methods). Adenoviral transduction of a kinase-negative CaMKIIδ2 has been shown by our laboratory to inhibit CaMKIIδ2 catalytic activity, both in vitro and in intact cells (40). Similar to siRNA-mediated knockdown of CaMKII, overexpression of the kinase-negative CaMKIIδ2 mutant also suppressed VSM cell migration in the scratch wound (Fig. 1, A and B). Together, these data indicate a positive role for CaMKIIδ2 in regulating cell migration.
To gain insight into potential mechanisms by which CaMKII exerts its effect on the complex process of cell migration, temporal and spatial activation of CaMKIIδ2 was first assessed using an antibody that specifically recognizes the Thr287 autophosphorylated form of the kinase (Fig. 2). Previous studies have validated the utility of this antibody using Western blotting approaches (16, 29, 40), and control experiments using Ca2+ depletion protocols and siRNA knockdown of CaMKII were used to validate usefulness of the antibody using immunofluorescence approaches (Supplemental Fig. S1; supplemental data are available at the online version of this article). Scratch wounding produced a rapid increase in CaMKII Thr287 autophosphorylation, reflective of CaMKII activation, apparent in cells near the wound edge (Fig. 2A). Staining in cells away from the wound edge serve as an internal control and reflect patterns in unwounded monolayers. As an alternative approach, Western blot analysis of lysates from VSM monolayers with multiple parallel scratch wounds also indicated transient activation of CaMKIIδ2 within 30 s postwounding (Fig. 2B). Ionomycin, a calcium ionophore, was used as a positive control to induce maximal CaMKII activation. Quantification of the phospho-Thr287 signal normalized to total CaMKII expression indicated a peak 2.5-fold increase in active CaMKII under these conditions, returning to levels not different from control by 45 min (Fig. 2C). These data indicate acute activation of CaMKIIδ2 in response to wounding, consistent with a regulation of early events involved in the initiation of VSM cell migration.
CaMKII activation in the leading edge.
Effects of a multifunctional protein kinase like CaMKII may in part be dependent on localized activation (23). Knowledge of the spatial distribution of active CaMKII could provide insight into potential cellular processes and proteins targeted by the kinase. To investigate the localization of active CaMKII in spreading VSM cells, indirect immunofluorescence of phospho-Thr287 or total CaMKIIδ2 antibody distribution was visualized using confocal microscopy, and distribution of FITC-conjugated phalloidin was used to localize filamentous actin. The results indicated a distinct distribution of activated CaMKII in lamellipodia with a pattern that was nonoverlapping with either cortical or stress fiber filamentous actin (Fig. 3A). This pattern contrasts with the distribution of total CaMKIIδ2, which localizes throughout the cell with a perinuclear concentration (Ref. 25 and Fig. 3B). Biochemical analysis of a leading edge fraction isolated using a 3-μm-pore Transwell apparatus confirmed activated CaMKII in this cellular fraction, judged by comparing the Western blot signals of phospho-Thr287 CaMKII vs. total CaMKII (Fig. 3C). Quantitation of immunoblots from three separate experiments indicates a phospho-CaMKII:total CaMKIIδ2 signal ratio of 1.22 ± 0.26 in the leading edge vs. 0.64 ± .06 for the cell lysate, reflecting an approximate twofold increase in CaMKII activation in the leading edge.
VSM cell polarization, with formation of a leading edge, is apparent within 4–6 h following scratch wounding. Figure 4A demonstrates localization of activated CaMKII in the leading edge of VSM cells at the wound edge 6 h after wounding. Total CaMKII distribution in the migrating cells was concentrated in the perinuclear area, similar to that observed in spreading (Fig. 3B) or stationary cells (25). These results indicate selective activation of a pool of CaMKIIδ2 in the leading edge of spreading or migrating VSM and are consistent with previous biochemical results demonstrating that adhesion and spreading of VSM cells on multiple extracellular matrix-coated substrates are a stimulus for CaMKII activation and downstream signaling (29). Repeated cytosolic calcium transients have been demonstrated to occur during migration of VSM cells (45) and may account for the increase in autophosphorylated CaMKII at this 6-h time point.
CaMKIIδ2 regulates VSM polarization.
Cells undergoing directional migration maintain a polar morphology with a leading lamella, driven by actin polymerization and focal adhesion dynamics, and a retracting trailing edge regulated by cytoskeletal contraction and dissolution of focal adhesions. In many cells, polarization of intracellular membrane traffic accompanies and promotes cellular polarity (27, 50). Polarized activation of CaMKII in the leading edge raised the possibility that it could act to regulate VSM cell polarity, accounting for positive function in regulating VSM migration. Figure 5 demonstrates the effect of CaMKII suppression using siRNA on VSM cell polarity in cells at the wound edge. Control cells typically exhibit a defined leading edge oriented toward the open area of the wound and polarization of the Golgi apparatus, typically to a position between the nucleus and leading lamellipodia (Fig. 5, A and B). In CaMKIIδ2-deficient cells, the leading edge was characteristically irregular, with multiple protrusions, and the Golgi apparatus appeared poorly organized and/or localized in directions away from the wound edge (Fig. 5, C and D). These single-section confocal images suggest less total Golgi staining with CaMKII suppression; however, Golgi content was not quantified in these experiments. Figure 5, insets, illustrates nonwounded VSM cells with the same staining regimen. This experiment suggests a role for CaMKIIδ2 in regulating cellular polarization as measured by both Golgi reorientation and leading edge formation.
CaMKIIδ2 promotes Golgi polarization.
Given the importance of Golgi polarization as an early step in acquiring and promoting cellular polarity necessary for directional migration (28), additional experiments were performed to quantify these effects and confirm the morphological observations. In these experiments, an adenoviral construct was used to transduce a short hairpin siRNA targeting CaMKIIδ gene products. GFP was also encoded by the construct under a separate promoter, allowing for identification of transduced cells. This construct has been used in previous studies and was demonstrated to efficiently suppress CaMKIIδ2 protein expression 48–96 h following infection (20, 29). After identification of transduced cells at the wound edge, Golgi polarization in those cells was characterized as shown in Fig. 6A and as described in detail in materials and methods. On the basis of an increase in the fraction of polarized cells relative to the wound edge and a decrease in nonpolarized cells, a significant percent of control cells reoriented their Golgi apparatus within 4 h after wounding and before the initiation of migration (Fig. 6B). In contrast, in CaMKIIδ2-deficient cells, there were fewer cells with a polarized morphology and more nonpolarized cells at all time points, and Golgi reorientation toward the wound edge after 4 h was not statistically different from that at 0 h (Fig. 6B). The rather strict criterion for polarization toward the wound edge may have resulted in dropout of cells with leading edges oriented obliquely to the wound edge from the analysis. However, the reciprocal changes in nonpolarized cells vs. cells polarized toward the wound edge suggest that this was not a major factor in minimizing or maximizing effects. There were no statistically significant changes in the percentage of cells that were polarized in directions away from the wound edge between control and knockdown cells (data not shown). This analysis indicates that CaMKIIδ2 promotes Golgi polarization in VSM cells, a process known to be important for the establishment and reinforcement of leading edge dynamics and establishment of cellular polarity necessary for directed migration.
CaMKIIδ2 promotes Rac activation.
The Rho GTPase Rac is active at the leading edge of polarized cells and has been found to be a key factor in regulating diverse aspects of cell polarity and migration (41). To determine whether the effects of CaMKII on regulation of VSM polarity and migration were proximal or distal to Rac activation, we measured scratch wound-induced GTP-bound Rac in VSM cell lysates from control or CaMKIIδ-suppressed cells (Fig. 7A). Wounding resulted in a twofold increase in active Rac, a response that was inhibited ∼70% in CaMKII-depleted cells. As a control, stimulation of VSM cells with PDGF transiently induced a 10-fold increase in active Rac (Fig. 7B), consistent with the literature (5). Suppression of CaMKIIδ2 protein significantly decreased active Rac in response to PDGF, demonstrating a partial dependence on CaMKIIδ2 activity. The lower level of Rac activation in response to wounding compared with PDGF stimulation is most likely explainable by a localized response in those cells at the wound edge.
The Rho family GTPases are implicated in regulation of cytoskeletal dynamics leading to cell polarity (3, 12, 18). We therefore tested the hypothesis that Rac is necessary for Golgi polarization in cultured VSM cells. Cells electroporated with Rac-specific siRNA lost 68 ± 3.3% of their Rac protein within 48 h after introduction (Fig. 7C, inset). Following scratch wounding, control cells increased their proportion of polarized Golgi with increasing time postwounding, whereas Rac-suppressed cells failed to reorient their Golgi (Fig. 7C). These data directly implicate Rac in VSM cell Golgi polarization.
Reciprocal regulation of Rac and Rho activity in the leading edge has been reported to be an important mechanism in maintaining cell polarity during migration (41). These GTPases act in concert to correctly reorganize the cytoskeleton in response to migratory clues. Similar to the analysis of whole cell lysates, Rac activation was significantly suppressed in leading edge fractions from CaMKIIδ2-depleted cells (Fig. 7D). This decrease in active Rac protein was not due to a decrease in total Rac protein levels in the fraction (data not shown). A concomitant 50% increase in Rho activity was observed in leading edge fractions from CaMKIIδ-depleted cells, resulting in a threefold increase in the Rho/ Rac activity ratio. Combined, these experiments indicate that CaMKIIδ2 promotes Rac activation in VSM cells and may be a particularly important factor in regulating wound-induced Rac activation in leading edges, affecting leading edge dynamics.
CaMKIIδ2 promotes wound-induced ERK1/2 activation.
A positive role for CaMKII in ERK1/2 activation is well established in VSM cells (16, 29, 35), and ERK activation presents a potential mechanism by which CaMKII may exert functional effects on VSM polarity and migration. Inhibition of ERK activation with the MEK inhibitor U0126 resulted in significant inhibition of VSM cell migration (Fig. 8, A and B), supporting previous work that implicates ERK as a positive modulator of migration (26). Moreover, ERK1/2 activation was increased in response to monolayer wounding maximally within 5 min (Fig. 8C), consistent with previous results in VSM cells (34). CaMKIIδ2 suppression significantly inhibited ERK activation in response to monolayer wounding (Fig. 8D), implicating a role for CaMKIIδ2 as a proximal regulator of ERK activation.
ERK1/2 does not mediate CaMKII-dependent regulation of VSM cell polarization.
To test the function of ERK as a mediator of CaMKII effects on VSM polarization, leading edge morphology and Golgi polarizations were assessed in U0126-treated cells (Fig. 9). Inhibition of ERK activation with U0126 resulted in changes in leading edge morphology (Fig. 9B); however, the phenotype characterized by increased membrane ruffling and filopodia formation was different from that observed in cells depleted of CaMKIIδ2 (Fig. 5, C and D). Additionally, pretreatment of VSM with U0126 had no significant effects on Golgi organization or polarization toward the wound edge (Fig. 9C), in contrast to results observed in CaMKIIδ-depleted cells (Fig. 6B). These data implicate CaMKIIδ2 as a proximal mediator of wound-induced ERK1/2 activation, but this pathway fails to account for the effects of CaMKIIδ2 in promoting VSM polarization.
The overall objective of this study was to understand how multifunctional CaMKII enables Ca2+-dependent control of VSM cell migration. In this study, we used molecular loss-of-function approaches to assess regulation of VSM migration by endogenous CaMKII using a scratch wound healing assay. Suppression of CaMKIIδ2 protein expression in cultured VSM cells using siRNA technology, or inhibiting activity by overexpressing a kinase-negative mutant, inhibited cell migration in this model system. The wound healing assay also provided the opportunity for assessing CaMKII activation dynamics, and it was determined that the kinase is activated both acutely on monolayer wounding and in a sustained manner localized in the leading edge of migrating cells. The results advance understanding of mechanisms underlying CaMKII regulation by demonstrating for the first time a positive function for the kinase in regulating VSM cell polarization assessed by Golgi reorganization, leading edge morphology, and Rac/Rho activity gradients. Although the specific CaMKII protein targets remain to be determined, our results indicate that downstream signaling to ERK1/2 does not account for the observed effects on cell polarization. Overall, these data indicate an important role for CaMKIIδ2 in directing VSM cell migration by regulating leading edge dynamics and cellular polarity.
Rapid activation of CaMKIIδ2 in response to VSM monolayer wounding confirms one previous report using this approach (53) and is also consistent with previously reported wound-induced stimulation of Ca2+ transients and ERK1/2 activation in VSM cells (34). Active ERK1/2 phosphorylates regulatory proteins in various cellular compartments, including the nucleus, cytosol, membrane, and cytoskeleton (48), and has been reported to regulate aspects of cell migration in a number of systems, including VSM (30). This was confirmed in the present study by demonstrating that pharmacological inhibition of ERK1/2 activation results in suppression of VSM migration and disruption of leading edge morphology. In addition, the present results provide new evidence for a mechanistic link between CaMKII activation and ERK1/2 activation in this setting and are consistent with a number of previous reports linking activation of CaMKII to ERK1/2 signaling in both cultured and intact VSM (1, 2, 16). However, ERK activation did not account for effects of CaMKII in regulating Golgi polarization, indicating alternative mechanisms by which CaMKII affects the complex events involved in cell migration.
Rapid CaMKIIδ2 activation in response to wounding is consistent with a function in initiating early signaling pathways and events important in cell migration. To our knowledge, this study for the first time demonstrates involvement of the kinase in regulation of cell polarity as indicated by a dependence of wound-induced Rac activation, Golgi reorganization, and leading edge morphology on CaMKIIδ2 expression. Mechanistic relationships between these indexes of cell polarization were not determined in the present study, and, based on extensive literature using other cell systems, it is expected that they will be complex. For example, Golgi polarization is tightly linked to lamellipodia formation and reinforcement by providing asymmetric delivery of membrane and membrane proteins to the dynamic leading edge (28).
Rho family GTPases are also known to be intimately involved in regulating multiple aspects of cell migration (18). Rac activity has been shown to be functionally important in growth factor-stimulated VSM cell migration (10), and a number of studies have demonstrated localized Rac activation at the leading edge of migrating cells where it regulates cytoskeleton and focal adhesion dynamics (33). Rac has also been found to positively regulate Golgi polarization in migrating keratinocytes (7, 39), results now confirmed here in VSM. Thus CaMKII-dependent activation of Rac in both isolated leading edge of migrating cells and in whole cell lysates could account for the observed effects of CaMKIIδ suppression on Golgi polarization and leading edge morphology. We also observed an increase in Rho activity concomitant with the CaMKIIδ2-dependent decrease in Rac activity in the leading edge. This is consistent with the literature in that Rac and Rho have an antagonistic relationship and must be tightly regulated to ensure proper signaling for directed cell migration (41). For example, in NIH-3T3 fibroblasts, Rac signaling was shown to antagonize Rho activity directly at the GTPase level, and the balance between Rac and Rho activities determined cellular morphology and migration (43).
In considering potential mechanisms by which CaMKII might activate Rac, Tiam1 (a Rac guanine nucleotide exchange factor) has been shown to be phosphorylated on threonine residues on treatment of fibroblasts with lysophosphatidic acid and PDGF (13). In vitro studies indicated that phosphorylation of Tiam1 by CaMKII, but not PKC, enhanced Rac nucleotide exchange rates, which was abrogated by pretreatment with a protein phosphatase (13). Importantly, threonine phosphorylation of Tiam1 and associated membrane translocation was prevented by pharmacological inhibitors of CaMKII (5). Future experiments using molecular approaches to manipulate CaMKII expression of activity, such as those applied in the present study, could be used to test this specific mechanism in migrating VSM cells.
A key finding in the present study was asymmetric activation of CaMKII in the leading edge region, in a cellular domain that was nonoverlapping with F-actin. The pattern of localization alone suggests a potential role for CaMKIIδ2 in regulating leading edge dynamics, perhaps by regulating actin filament polymerization, contractile events, or focal adhesion stability and turnover. In this regard, CaMKII and/or ERK1/2 has been implicated in regulating cytoskeletal interactions in differentiated VSM (32, 42), and ERK1/2 has been implicated in regulating both actin-myosin interactions (26) and focal adhesion dynamics (8, 24, 49) in several cultured cell systems.
Conflicting data in the literature indicate either net positive (2, 38) or negative (40) functions for CaMKII in regulating VSM cell migration using a modified Boyden chamber assay. Some of the conflicting data may be explained by nonspecific effects of pharmacological approaches or even molecular approaches used to overexpress a kinase that is multifunctional and may exhibit noncatalytic scaffolding activity (23). Regardless, the loss-of-function approaches used here have the advantage of targeting endogenous CaMKIIδ2, which has been found to be specifically upregulated in synthetic phenotype and cultured VSM (20, 21, 47) and to contribute to VSM migration in vivo in response to vascular injury (21). On the basis of the data reported here, we propose a model whereby monolayer wounding results in CaMKIIδ2 activation, which positively regulates VSM cell polarization and downstream signaling events that promote cell migration, including Rac and ERK1/2 activation. Once cell polarity and a leading edge are established, localized activation of CaMKII may further promote leading edge dynamics facilitating VSM cell migration. The present studies provide rationale and groundwork for more detailed studies to evaluate mechanisms of leading edge CaMKII activation and regulation of leading edge dynamics.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
- Copyright © 2008 the American Physiological Society