Regulation of signaling at regions of cell-cell contact by endoplasmic reticulum-bound protein-tyrosine phosphatase 1B

Abstract

Protein-tyrosine phosphatase 1B (PTP1B) is a ubiquitously expressed PTP that is anchored to the endoplasmic reticulum (ER). PTP1B dephosphorylates activated receptor tyrosine kinases after endocytosis, as they transit past the ER. However, PTP1B also can access some plasma membrane (PM)-bound substrates at points of cell-cell contact. To explore how PTP1B interacts with such substrates, we utilized quantitative cellular imaging approaches and mathematical modeling of protein mobility. We find that the ER network comes in close proximity to the PM at apparently specialized regions of cell-cell contact, enabling PTP1B to engage substrate(s) at these sites. Studies using PTP1B mutants show that the ER anchor plays an important role in restricting its interactions with PM substrates mainly to regions of cell-cell contact. In addition, treatment with PTP1B inhibitor leads to increased tyrosine phosphorylation of EphA2, a PTP1B substrate, specifically at regions of cell-cell contact. Collectively, our results identify PM-proximal sub-regions of the ER as important sites of cellular signaling regulation by PTP1B.

Figure 1. PTP1B localizes to regions of cell-cell contact.

Randomly growing PTP1B-null fibroblasts transiently expressing PTP1B WT-GFP or PTP1B D/A-GFP were fixed and co-stained with β-catenin antibodies and DAPI (top and middle panels). The bottom panel shows fibroblasts co-expressing PTP1B D/A-GFP and the plasma membrane marker Grp43-mCherry in live cells. Regions of cell-cell contact are indicated with arrows. Scale bars correspond to 5 µm.

Figure 2. The endoplasmic reticulum lies in close proximity to the plasma membrane at regions of cell-cell contact.

(a) MDCK cells co-expressing PTP1B D/A-mCherry and a general marker for the endoplasmic reticulum, stress-related ER protein (SREP-YFP); or (b) PTP1B D/A-mCherry Sec61-YFP, a marker for the rough ER. Arrows indicate region of cell-cell contact. Scale bars correspond to 10 µm. (c) Immunogold electron microscopy shows PTP1B D/A localization in the ER (arrows). No significant labeling was detected in PTP1B-null cells (left bottom image). Significant immunogold labeling was also detected at regions of cell-cell contact (right image). The boxed area, which is magnified below, shows the region of cell-cell contact that reveals labeling at the ER (arrows) proximal to the PM (arrowheads). Also see Fig. S2. Scale bars correspond to 0.2 µm.

Figure 3. Diffusive motion of photoactivated PTP1B on the ER.

(a) PTP1B-null fibroblasts co-expressing PTP1B WT-PhAc (upper panel) and PTP1B WT-RFP (lower panel) were photoactivated in the ER (region indicated with a circle), and the photoactivated pool was followed for the indicated times. (b) Cells were photoactivated at a region of interest (marked “1”), and PTP1B fluorescence intensity at this and distal regions (marked “2–4”) was quantified. The lower panel represents the change in fluorescence ratio (WT-PhAc/WT-RFP) at the four highlighted regions over time. (c) Cells co-expressing PTP1B D/A-PhAc and PTP1B D/A-RFP were photoactivated in the ER (circle). For each set of figures, the region in the boxed area of the top panel is shown magnified in the lower panel, with an arrow indicating the PM-proximal ER at regions of cell-cell contact (note that the adjacent cell is not transfected). (d) Cells co-expressing PTP1B D/A-PhAc and PTP1B D/A-RFP were photoactivated at regions of cell-cell contact (rectangle), revealing slower mobility compared to PTP1B D/A in the ER. Scale bars correspond to 5 µm.

Figure 4. Assessment of PTP1B mobility using Fluorescence Recovery after Photobleaching.

(a) PTP1B-null fibroblasts expressing PTP1B WT-GFP (upper panel) were photobleached in the indicated regions at the ER (circle), and fluorescence recovery was monitored for up to a minute (images are shown for the first 20 sec). Cells expressing PTP1B D/A-GFP (middle and lower panels) were photobleached either at the ER (circle) or at regions of cell-cell contact (rectangle). Scale bars correspond to 5 µm. (b) Quantification of intracellular fluorescence recovery in multiple cells expressing PTP1B WT, PTP1B D/A, TC PTP or SHP2 (see Fig. S2 a–d). Also shown is the fluorescence recovery of PTP1B D/A at regions of cell-cell contact (see Fig. S3e). For each PTP, actual data points are presented in color, whereas solid black lines represent fits to our mathematical model. See main text and Supplementary Materials for a detailed discussion of modeling and fitting.

Figure 5. Quantification of PTP1B recruitment to regions of cell-cell contact.

(a) Cos-7 cells co-expressing PTP1B D/A-dHcRed and EGFR-GFP were stimulated with EGF (100 ng/ml), and monitored by confocal time lapse microscopy. Ratio images (right panels) were generated by dividing PTP1B D/A by EGFR fluorescence following image processing as described in Materials and Methods. Scale bars correspond to 10 µm. (b) Distribution of PTP1B/EGFR intensity ratios in regions of cell-cell contact versus no contact (n = 21 cells). The difference between the two data sets is highly significant (p<0.001, Kolmogorov-Smirnov test). (c) Cells expressing mCitrine-tagged PTP1B D/A CD, shown at 5 min after EGF stimulation. Note that this mutant form of PTP1B can access substrates along the entire PM (arrows). (d) Cells co-expressing mCitrine-tagged PTP1B DA CD and mCherry-tagged PTP1B-D/A following EGF stimulation (5 min). Peripheral regions (regions without cell-cell contact) that show accumulation of mCitrine-tagged PTP1B D/A CD are indicated by arrows, whereas regions where ER-anchored PTP1B excluded mCitrine-tagged PTP1B D/A CD are indicated by arrowheads. Scale bars in (c) and (d) correspond to 20 µm.

Figure 6. The ER is specifically polarized towards regions of cell-cell contact.

(a) Cos-7 cells co-expressing PTP1B WT-mCitrine and RFP-TK were treated with nocodazole (33 µM) and imaged for 40 min using TIRF microscopy. Two regions of interest “1” and “2” are highlighted in the “Merge” images and magnified in the far right panels. “1” shows a peripheral region of the cell with no cell-cell contact. The PM outline is traced in white, the ER outline is traced in yellow, and ER retraction relative to the PM can be seen as an increase in distance between the two traced outlines after 40 min of nocodazole treatment. “2” shows a region of cell-cell contact (indicated by arrows). Nocodazole treatment leads to ER retraction from peripheral regions but not from areas of cell-cell contact. (b) MDCK cells expressing calreticulin-TFP, PTP1B D/A CD-mCitrine, and RFP-TK. On the right hand side, a magnified region of interest (rectangle) shows peripheral regions of the ER. Lower left panel shows calculated concentration of PTP1B D/A CD in a cell-cell contact region of interest based on fluorescence calibrated imaging . Lower central panel: correlation of the average image intensity measured in the cytoplasm of cells expressing mCitrine-PTP1B D/A CD to the absolute concentration, as determined from FCS measurements. A linear fit (red line) yielded the calibration function used to calculate absolute concentrations from image intensities. Lower right panel: Example of an auto-correlation G(τ) curve of PTP1B D/A CD-mCitrine (black line) fit by a model accounting for anomalous diffusion and a triplet term (red line). (c) MDCK cells co-expressing calreticulin-TFP and PTP1B D/A-RFP-TK were treated with nocodazole and imaged by confocal microscopy for 40 min. “1” shows regions of cell-cell contact; note how the ER does not retract from these points after nocodazole treatment. “2” shows a peripheral region of the PM; note the distance increase between the ER and PM after nocodazole treatment. All scale bars correspond to 10 µm.

Figure 7. PTP1B accesses specific substrates at cell-cell contacts.

(a, b) Cos-7 cells co-expressing EphA2-mCherry (panels ii, v) and dSH2-YFP (panels iii, vi) were incubated for 1 hour at 37°C with PTP1B allosteric inhibitor (539741 PTP1B inhibitor, 250 µM), and monitored by confocal time lapse microscopy. Transmission images (panel i) show the cell confluency and help to define regions of cell-cell contact. Intensity merge images (panel iv, vii) showing the spatial localization of EphA2 (red) an dSH2 (green) before (iv) and after (vii) PTP1B inhibitor treatment. Note the markedly increased co-localization (orange) at regions of cell-cell contact after inhibitor treatment, indicating increased EphA2 phosphorylation specifically in this region. Scale bars correspond to 20 µm.