1. Volume 146, Issue 3, Pages 421-434 (August 2011)
Recruitment of Actin Modifiers to TrkA Endosomes Governs Retrograde NGF Signaling and Survival 
Anthony W. Harrington, Coryse St. Hillaire, Larry S. Zweifel, Natalia O. Glebova, Polyxeni Philippidou, Simon Halegoua, David D. Ginty 
Cell 
Volume 146, Issue 3, Pages (August 2011)
DOI: /j.cell
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2. Cell  , DOI: ( /j.cell )
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3. Figure 1
Both NGF and NT3 Activate and Promote Internalization of TrkA, but Only NGF Supports Retrograde TrkA Trafficking and Retrograde Survival
(A) Compartmentalized sympathetic neurons were deprived of NGF for 18 hr, and distal axons were incubated with NGF (100 ng/ml), NT3 (1000 ng/ml), or no neurotrophin. Surviving cells were counted 36–48 hr later.
(B) Mass sympathetic neuron cultures were NGF-deprived overnight and then stimulated with NGF or NT3 for 20 min. Lysates were subject to SDS-PAGE and immunoblotted for the indicated proteins. Upper and lower blots represent different samples.
(C) The Flag-TrkA endosome transport assay was done in compartmentalized sympathetic neurons. The percentage of cell bodies (CBs) containing Flag-TrkA punctae was quantified.
(D) The Flag-TrkA endosome transport assay was performed to assess the extent of internalization of Flag-TrkA and formation of Flag-TrkA endosomes following a 30 min treatment with NGF or NT3 on distal axons of compartmentalized sympathetic neurons. The number of internalized Flag-TrkA endosomes was counted per 10 μm of axon for each condition.
Data in graphs are represented as mean ± standard error of the mean (SEM). Scale bars represent 10 μm. Statistical analysis was done using one-way ANOVA followed by Tukey's post-hoc test. ∗∗p < 0.01, ∗∗∗p < See also Figure S1 and Figure S2.
Cell  , DOI: ( /j.cell )
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4. Figure 2
Actin Depolymerization in Distal Axons Is Essential for Retrograde Transport of TrkA Signaling Endosomes
(A) Protein from purified PC12 TrkA endosomes was analyzed by immunoblot.
(B) Sympathetic neurons were grown in microfluidic chambers and pretreated for 30 min with Latrunculin A (LatA) (10 μM) applied to the distal axon chamber. Retrograde transport of TrkA endosomes to cell bodies was assessed using the Flag-TrkA endosome transport assay. Transported Flag-TrkA endosomes are shown in red, and neurons were counterstained with FITC-phalloidin.
(C) Sympathetic neurons grown in microfluidic chambers were treated with Jasplakinolide (10 μM) for 30 min prior to assessment of Flag-TrkA retrograde transport. Internalized and transported Flag-TrkA endosomes are shown in red; neurons were counterstained with NF200. Retrograde signaling endosome transport to the cell body and internalized TrkA in distal axons is quantified (upper and lower right, respectively).
(D) TrkA endosomes are associated with an F-actin depolymerization activity. Control beads or bead-bound Flag-TrkA endosomes were incubated with 2 μM F-actin. After 15 min, beads were removed, F-actin and G-actin were separated by ultracentrifugation, and protein was analyzed by SDS-PAGE and Coomassie staining.
Scale bars represent 10 μm. Graphical data in (C) are represented as mean ± SEM. Statistical analysis was done using the Student's t test. ∗∗p < See also Figure S3, Figure S4, and Table S1.
Cell  , DOI: ( /j.cell )
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5. Figure 3
The Actin-Severing Protein Cofilin Is Associated with TrkA Signaling Endosomes and Is Required for Their Retrograde Transport
(A) Early endosomes (top panels) were purified from cultured rat sympathetic neurons deprived of NGF for 18 hr then treated for 20 min with NGF (100 ng/ml) or NT3 (1000 ng/ml) or left untreated. Levels of total cofilin, Ser3-phosphorylated cofilin, and Rac1 were assessed by immunoblot. Relative levels of cofilin and inactive cofilin (Ser3 phosphorylated) associated with NGF and NT3 endosomes were quantified. Whole-cell lysates (bottom panels) from sympathetic neurons treated with NGF and NT3 were analyzed for cofilin and phospho-cofilin.
(B) Colocalization of internalized Flag-TrkA endosomes (red) and cofilin (green) was assessed by immunocytochemistry of distal axons of sympathetic neurons that were untreated or treated with either NGF or NT3.
(C) Lentivirus-infected sympathetic neurons were grown in microfluidic chambers and the retrograde transport of Flag-TrkA endosomes was assessed. Neurons were counterstained with FITC-phalloidin.
(D) The effect of cofilin knockdown on NGF-dependent retrograde survival was assessed for sympathetic neurons grown in microfluidic chambers. Neurons were allowed to project axons into distal chambers and then were infected with either a control lentivirus or a virus expressing an shRNA for cofilin. Survival was monitored 36 hr to 48 hr later under conditions in which NGF was exposed to both cell body and distal axon compartments, or NGF was exposed exclusively to the distal axon compartment.
(E) Inhibition of actin polymerization by LatA rescues the TrkA transport defect in cofilin knockdown neurons. Neurons were treated as in (D) with the addition of a 30 min pretreatment of vehicle or LatA in distal axons prior to performing the Flag-TrkA retrograde transport assay.
Scale bars represent 10 μm. All data in graphs are represented as mean ± SEM. Statistical analysis was done using Student's t test with the exception of that in (D), which was done using one-way ANOVA followed by Tukey's post-hoc test. ∗p < 0.05, ∗∗p < 0.01, ∗∗∗p <
Cell  , DOI: ( /j.cell )
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6. Figure 4
Rac1 Is Activated by NGF but Not NT3 and Is Associated with NGF/TrkA-Containing Endosomes
(A) Dissociated sympathetic neurons were stimulated with NGF (100 ng/ml) or NT3 (1000 ng/ml) for 20 min. Cell lysates were subjected to GST-PAK pull-down assay as a measure of Rac1 activity. Densitometry was performed and signal intensities were normalized against total Rac1. Statistical analysis was done using Student's t test. ∗∗p < 0.01.
(B) Compartmentalized sympathetic neurons were subjected to the Flag-TrkA endosome assay and distal axons were incubated with vehicle, NGF, or NT3 for 3 hr and then processed with the GST-PAK immunoassay to simultaneously visualize Flag-TrkA (red) and Rac1-GTP (green). Confocal images of distal axons (DA), middle axons (MA), and cell bodies (CB) are shown.
Scale bars represent 10 μm. Quantification of total and colocalized signal is shown below representative pictures. Data in graphs in panels (A) and (B) are represented as mean ± SEM. See also Figure S5 and Figure S6.
Cell  , DOI: ( /j.cell )
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7. Figure 5
Rac1 Is Necessary and Sufficient for Retrograde TrkA Trafficking and Retrograde Survival
(A) Compartmentalized sympathetic neurons were infected with control or RacN17 virus and subjected to the Flag-TrkA transport assay in the presence or absence of NGF for 3 hr. Confocal images of cell bodies are shown.
(B) Percentage of cell bodies containing Flag-TrkA punctae following distal axon incubation with vehicle or NGF.
(C) Sympathetic neurons were grown as above and infected with a control or RacN17 virus. Neuronal survival was measured.
(D) Compartmentalized sympathetic neurons were infected with a control virus or a virus encoding a constitutively active form of Rac1 (RacV12). Neurons were subjected to the Flag-TrkA transport assay and stimulated on distal axons for 3 hr with vehicle, NGF, or NT3. Confocal images of cell bodies are shown.
(E) Percentage of cell bodies containing Flag-TrkA punctae following distal axon stimulation with vehicle, NGF, or NT3.
(F) Compartmentalized neurons were infected with control or RacV12 virus and were treated as indicated, and neuronal survival was measured.
Scale bars represent 10 μm. All graphical data are presented as mean ± SEM. ∗∗∗p < , ∗∗p < 0.01, ∗p < Statistical analysis was done using one-way ANOVA followed by Tukey's post-hoc test.
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8. Figure 6
Rac1 Activation Is Required for Localization of Cofilin to Signaling Endosomes
Sympathetic neurons were grown in microfluidic chambers and infected with a RacN17 virus or a control virus. Two days later, neurons were subjected to the Flag-TrkA labeling assay in the presence of NGF (100 ng/ml). The effect of RacN17 on cofilin localization was examined by immunocytochemistry; internalized Flag-TrkA endosomes are labeled red and cofilin is labeled green. The percentage of cofilin-positive Flag-TrkA endosomes was quantified. Shown are means ± SEM (n = 5). Scale bars represent 10 μm. Statistical analysis was done using Student's t test. ∗∗p < 0.01.
Cell  , DOI: ( /j.cell )
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9. Figure 7
Blocking Endosome Acidification Enables NT3-Mediated Retrograde Transport
(A) PC12 cells were stimulated with NGF or NT3 for 10 min in DMEM buffered at pH 7.0, pH 6.0, or pH 5.5. Cells were subsequently lysed and subjected to immunoblotting for TrkA phosphorylated on tyrosine 490 (P-TrkA). Densitometric analysis was performed and P-TrkA levels were normalized against the amount of actin.
(B) Radioligand binding assays were performed in which COS7 cells in 24-well plates, either untransfected or transfected with full-length TrkA, were incubated with 1 nM 125I-NGF or 125I-NT3 in media buffered to the pH values indicated. Data are represented as mean ± SEM, (n = 3). Similar results were obtained in experiments using 0.2 nM 125I-NGF and 125I-NT3 (not shown).
(C) Compartmentalized sympathetic neurons were NGF-deprived overnight, and the Flag-TrkA transport assay was performed. Distal axons were stimulated with NGF or NT3 alone or with NGF or NT3 in the presence of NH4Cl or bafilomycin for 3 hr. Similar experiments were done using neurons that were infected with either a control or RacN17 virus.
(D) Quantification of (C).
(E) Colocalization of internalized Flag-TrkA endosomes (red) and cofilin (green) was assessed by immunocytochemistry of distal axons of sympathetic neurons treated for 20 min with NT3 in the presence of either DMSO (vehicle control) or bafilomycin. The number of cofilin-positive Flag-TrkA endosomes in distal axons was quantified.
(F) Distal axons of compartmentalized sympathetic neurons were stimulated with NGF or NT3 in the presence or absence of bafilomycin for 3 hr. Neurons were subsequently fixed and subjected to the GST-PAK immunoassay to visualize Rac1-GTP punctae. Quantification represents number of Rac1-GTP punctae per cell body or per 30 μm of axon.
Scale bars represent 10 μm. All graphical data shown are presented as means ± SEM. Statistical analysis was done using Student's t test (E) or one-way ANOVA followed Tukey's post-hoc test (A). ∗∗p < 0.01, ∗p < See also Figure S7.
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10. Figure S1
NGF and NT3 Similarly Activate TrkA in PC12 Cells, Related to Figure 1
(A) PC12 cells were stimulated for 20 min with either NGF or NT3. Lysates were subjected to immunoblotting with the indicated antibodies. Densitometric analysis indicates that NGF and NT3 similarly promote phosphorylation and activation of TrkA. Data are represented as mean ± SEM, (n = 3), ∗p < 0.05.
(B) Mass cultures of sympathetic neurons were NGF-deprived, subjected to the Flag-TrkA transport assay, and incubated for 20 min with either NGF or NT3 to assess internalization of Flag-TrkA endosomes. Quantification of Flag-TrkA punctae reveals that both NGF and NT3 can promote TrkA internalization to a similar extent. Scale bars represent 10 μm.
(C) PC12 cells were stimulated with NGF or NT3 and subjected to cell fractionation followed by immunoblotting. NGF and NT3 can promote internalization of TrkA to a similar extent.
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11. Figure S2
NGF- and NT3-Formed TrkA Endosomes Colocalize with Rab5, Related to Figure 1
Sympathetic neurons were deprived of NGF for 18 hr and subjected to the Flag-TrkA transport assay following exposure of distal axons to either NGF or NT3. Flag-TrkA and Rab5 immunocytochemistry was done to visualize the extent of colocalization of the early endosome marker Rab5 and Flag-TrkA endosomes.
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12. Figure S3
Electron Micrographs of Purified Endosomes, Related to Figure 2
Dynabeads obtained following the final step of the TrkA endosome purification procedure were fixed and processed for electron microscopy. High- and low-magnification images are shown for control beads, which lack bound IgG to capture TrkA-positive endosomes, and IgG beads, which show bound TrkA endosomes. Scale bars represent 100 nm.
Cell  , DOI: ( /j.cell )
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13. Figure S4
Morphological Growth Cone Effects following Acute Application of NGF and NT3 and the Effect of Cofilin Knockdown on Rab5/TrkA Colocalization, Related to Figure 2
(A) Cultured rat sympathetic neurons were NGF-deprived and were either treated with NGF (50 ng/ml) or NT3 (1000 ng/ml) or left untreated. Growth cone morphology was observed by rhodamine-phalloidin (red) and tubulin (green) staining. Note that whereas NGF stimulates the formation of both lamellipodia and filopodia, NT3 lacks the ability to promote robust filopodial extensions.
(B) Distal axons of sympathetic neurons infected with either control or shCofilin knockdown viruses were immunostained for newly internalized Flag-TrkA endosomes and Rab5 following surface stripping to remove any non-internalized Flag-TrkA signal. Note that Rab5 is colocalized with Flag-TrkA endosomes under both control and cofilin knockdown conditions. Scale bar represents 10 μm.
(C) Cultured mouse sympathetic neurons were infected with a virus expressing an shRNA to cofilin or a control virus. Neurons were harvested 24 or 36 hr post-infection and levels of cofilin were assessed by western blot. After 36 hr of infection with the shRNA cofilin virus, cofilin levels were reduced by approximately 80%.
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14. Figure S5
Rac1 Is Differentially Activated by NGF and NT3 in PC12 Cells and Sympathetic Neurons, Related to Figure 4
(A) To assess the specificity of GST-PAK-binding sites, the GST-PAK-binding assay was performed in mass cultures of sympathetic neurons that were previously infected with control or Rac1 shRNA virus.
(B) Sympathetic neurons in mass culture were NGF-deprived overnight, stimulated for 20 min with NGF or NT3, and subjected to the GST-PAK immunoassay. Note that NGF, but not NT3, promotes activation of Rac1. Scale bar represents 10 μm.
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15. Figure S6
Rac1 Activity Is Not Required for TrkA Internalization, Related to Figure 4
(A) Mass cultures of SCG neurons were infected with RacN17 or control virus for 2 days and NGF-deprived overnight. Cells were subjected to the Flag-TrkA transport assay and stimulated with vehicle or NGF (100 ng/ml) for 20 min. The number of internalized TrkA endosomes was quantified. Data are represented as mean ± SEM. Scale bars represent 10 μm.
(B) PC12 cells were preincubated with dynasore (80 μM) for 15 min and then treated with either NGF or vehicle for 20 min. Cells were lysed, subjected to the GST-PAK immunoassay, and processed for Rac1 immunoblotting.
(C) Mass cultures of sympathetic neurons were incubated with control virus or K44A dynamin virus for 24 hr and then NGF-deprived overnight. Cells were subsequently stimulated with NGF and subjected to the GST-PAK immunoassay.
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16. Figure S7
Model for Differential NT3 and NGF Signaling during Sympathetic Neuron Development, Related to Figure 7
The diagram illustrates the differential roles of intermediate-target-derived NT3 and final-target-derived NGF during two stages of sympathetic neuron development. The inability of NT3 to bind TrkA under conditions of low endosomal pH leads to the inactivation of pathways required for actin disassembly and endosome maturation, resulting in locally restricted signaling that supports axonal extension but not retrograde survival. In contrast, upon internalization, target-derived NGF remains bound to TrkA keeping the receptor in an active, signaling state even under the acidic conditions of the early endosome. This enables the formation of transport-competent NGF/TrkA signaling endosomes, retrograde transport of these endosomes to the soma, and neuronal survival. This model explains how both NGF and NT3 can support axonal extension, but NGF supports retrograde survival whereas NT3 cannot.
Cell  , DOI: ( /j.cell )
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