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Vascular Biology |
From the Departments of Plasma Proteins (M.G.R., R.B., A.K., K.A.G., E.S., J.A.v.M., J.V.) and Molecular Cell Biology (M.F-B.), Sanquin Research and Landsteiner Laboratory, and the Department of Vascular Medicine (J.A.v.M.), Academic Medical Centre, University of Amsterdam, The Netherlands.
Correspondence to Jan Voorberg, Department of Plasma Proteins, Sanquin Research at CLB and Landsteiner Laboratory, Academic Medical Centre, University of Amsterdam, Plesmanlaan 125, 1066 CX Amsterdam, The Netherlands. E-mail j.voorberg{at}sanquin.nl
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Methods and Results Our results demonstrate that the clustered WPBs are localized at the microtubule organizing center and that cluster formation depends on an intact microtubule network. Disruption of the microtubules by nocodazole completely abolished clustering, whereas treatment with the actin depolymerizing compound cytochalasin B had no effect on WPB clustering. Interfering with the dyneindynactin interaction by overexpression of the p50 dynamitin subunit or the CC1 domain of the p150glued subunit of the dynactin complex completely inhibited perinuclear clustering of WPBs, suggesting that dynein activity mediates this process. Furthermore, we found that inhibition of dephosphorylation resulted in an increase in clustering, whereas inhibition of protein kinase A (PKA) markedly reduced WPB clustering.
Conclusions These results suggest that perinuclear clustering of WPBs involves PKA-dependent regulation of the dyneindynactin complex. Endothelial cell stimulation with epinephrine results in retrograde movement of a subset of WPBs to the microtubule organizing center. This minus-end directed transport requires an intact microtubular network and is mediated by the motor protein dynein. Together, our results suggest that epinephrine-induced clustering of WPBs involves PKA-dependent regulation of the dyneindynactin complex.
Perinuclear clustering is observed for several different organelles and illustrates dynamic regulation of the secretory pathway and organelle distribution. Previously, we observed that a subset of Weibel-Palade bodies (WPBs), endothelial cellspecific storage organelles, undergo centralization when endothelial cells are stimulated with cAMP-raising agonists of von Willebrand factor secretion. In this study, we investigated this phenomenon of WPB clustering in more detail.
Key Words: organelle trafficking dynein Weibel-Palade bodies PKA endothelial cells
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See cover
| Methods |
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| Results |
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To study the localization of WPB clusters in relation to the actin cytoskeleton and microtubules, HUVECs were incubated for 1 hour with SF medium or forskolin/IBMX. After stimulation, the cells were fixed and vWF, F actin, and microtubules were visualized by immunostaining with polyclonal anti-VWF phalloidin, and monoclonal antityrosinated tubulin, and then analyzed by fluorescence confocal microscopy. Unstimulated cells showed WPBs dispersed throughout the cytoplasm codistributing with actin filaments at the cell periphery (Figure 2A, detail). Forskolin stimulation induced WPB clustering at the perinuclear region. These clustered WPBs never associated with the actin cytoskeleton (Figure 2B, detail). In unstimulated cells, WPBs were localized along microtubules (Figure 2C, detail) confirming previous findings.9 On stimulation with forskolin, clustered WPBs appeared to be localized to areas at the perinuclear region where microtubules emerge, corresponding to the MTOC (Figure 2D, right panel arrow). To investigate the role of the microtubules and the actin cytoskeleton in WPB clustering in more detail, we analyzed forskolin-induced WPB clustering in the absence and presence of either the microtubule-disrupting agent nocodazole or the actin-depolarizing compound cytochalasin B. Microtubule disruption by nocodazole completely abolished forskolin-induced WPB clustering, whereas cytochalasin B had no affect on forskolin-induced clustering of WPBs (Figure 2E). These results indicate that cAMP induces microtubule-dependent minus-enddirected transport of WPBs to the MTOC.
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Disruption of the DyneinDynactin Complex Abolishes Perinuclear Clustering of WPBs
The increase in minus-enddirected transport on epinephrine and forskolin stimulation suggests a role for the minus-enddirected motor protein dynein in this process. Because dynein is associated with vesicles via the anchoring complex dynactin, we hypothesized that disruption of this dynactin complex would result in the inhibition of minus-enddirected transport of WPBs, thereby reducing forskolin-induced cluster formation at the MTOC. To test this hypothesis, we overexpressed GFP-tagged dynamitin, the 50-kDa subunit of the dynactin complex (GFP-p50), which is known to lead to dissociation of the p150glued subunit from the rest of the complex resulting in inhibition of dynein-dependent vesicle transport.24,25 GFP-expressing cells, which were taken as controls, showed perinuclear clustering on forskolin stimulation similar to untransfected cells (Figure 3A through 3D and 3I). In contrast, in GFP-p50expressing cells, the clustering of WPBs was completely abolished (Figure 3E through 3I). In cells that do not express GFP-p50, WPB clustering was still observed on forskolin stimulation (Figure 3G and 3H, arrows). Furthermore, the residual number of WPBs in GFP-p50expressing cells stimulated with forskolin was reduced compared with unstimulated GFP-p50expressing cells, suggesting that exocytosis of WPBs in response to forskolin is unaffected by expression of GFP-p50. Moreover, no difference in the residual number of WPBs on forskolin stimulation was observed in GFP-p50expressing cells compared with GFP-expressing cells suggesting that exocytosis of WPBs is not increased when clustering is blocked (data not shown). To exclude the possibility that the inhibitory effect of p50 is attributable to interference with pathways other than the dyneindynactin pathway,26 we expressed RFP-CC1, a fusion protein of DsRed1 and the CC1 part of the dynein-interactive domain of p150glued, in HUVECs. Expression of this construct competitively inhibits dyneindynactin binding and, thus, prevents dynein-mediated transport.27 Consistent with the GFP-p50 data, RFP-CC1 markedly inhibited WPB clustering in response to forskolin stimulation (Figure 3J). In contrast, overexpression of GFP-KLC2-TPR, which is based on the tetra-trico peptide repeats (TPR) cargobinding domain of kinesin light chain 2 and was shown previously to block conventional kinesin-dependent movement,28 does not impair forskolin-induced WPB clustering (Figure 3K). These results demonstrate that disruption of the dyneindynactin complex inhibits WPB cluster formation at the MTOC, indicating that the clustering of WPBs is mediated by the dyneindynactin complex.
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WPB Clustering Is Controlled by Protein Phosphorylation Events
Phosphorylation events have been implicated in the regulation of vesicular, bidirectional transport. To determine whether phosphorylation events play a role in the minus-enddirected movement of WPBs, we investigated whether okadaic acid, a broad-spectrum inhibitor of protein phosphatases, affected perinuclear clustering of WPBs. HUVECs were preincubated with okadaic acid for 1 hour and then incubated in SF medium with or without forskolin. The inhibition of dephosphorylation by okadaic acid resulted in WPB clustering in nonstimulated HUVECs. Moreover, the clustering of WPBs induced by forskolin was additionally enhanced by the presence of okadaic acid (Figure II, available online at http://atvb.ahajournals.org). The observation that inhibition of dephosphorylation results in a clustering of WPBs suggests that in resting HUVECs, proteins involved in clustering are continuously phosphorylated and dephosphorylated with the majority of target proteins in the dephosphorylated state. We hypothesize that inhibition of dephosphorylation and stimulation with cAMP-raising agonists shifts this equilibrium toward phosphorylation and promotes the clustering of WPBs. The most common effector of cAMP is PKA, which is known to play a role in vesicle transport. A number of observations support a role for PKA in the regulation of motor proteins in melanophores, the pigment cells of fish and amphibians.1 To test whether the observed clustering of WPBs in human endothelial cells is PKA dependent, HUVECs were incubated for 1 hour with SF medium or forskolin/IBMX in the absence or presence of increasing concentrations of the PKA inhibitor H89 or the cAMP-analogue Rp-8CPT-cAMPS. The presence of H89 inhibited perinuclear WPB clustering in a concentration-dependent manner (Figure 4A), and also Rp-8CPT-cAMPS markedly reduced forskolin-induced WPB clustering (Figure 4B). In addition, we used a genetic approach to establish the involvement of PKA in WPB clustering. Overexpression of pNP210 encoding a recombinant PKA inhibitor prevented forskolin-induced WPB clustering, whereas an inactive variant encoded by plasmid pNP211 did not affect WPB clustering (Figure 4B). In combination with our data on phosphatase inhibition by okadaic acid, these results suggest that PKA-dependent phosphorylation of currently unknown target proteins induces the formation of WPB clusters.
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| Acknowledgments |
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Received May 23, 2005; accepted October 5, 2005.
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