Rapid Turnover of the Cardiac L-Type CaV1.2 Channel by Endocytic Recycling Regulates Its Cell Surface Availability

Figures

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  Figure 1. CaV1.2 Distributes along Microtubules Extending from the Perinuclear Region to the Cell Periphery Independent of an Intact Actin Cytoskeleton (A) Schema of the intracellular anterograde and retrograde trafficking of the calcium channel. The channel is packed into vesicles that walks along tubulin and actin filaments to be delivered to the plasma membrane from where it can be removed by endocytosis during retrograde traffic. (B) Laser scanning confocal images of representative HL-1 cells fluorescently stained for CaV1.2, tubulin and actin, using antibody coupled to ATTO 488, antibody coupled to Cy3 and phalloidin coupled to Alexa Fluor 647, respectively. Scale bar: 25 μm. (C) Laser scanning confocal images of HL-1 cells immunostained for CaV1.2 and tubulin in the presence of 10 μM of the actin filament disrupter cytochalasin D and box plot summarizing the colocalization analysis between CaV1.2 and tubulin according to Manders overlap coefficient in untreated and cytochalasin D treated HL-1 cells. The right panel shows a control cell exposed to the same cytochalasin D treatment and stained for actin using phalloidin-647. Scale bar: 25 μm. Numbers below each box correspond to the…

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  Figure 2. Long-term Stable Expression of L-type Currents in HL-1 Cells Is Independent of Microtubule-Based Transport (A) Laser scanning confocal images of control HL-1 cells immunostained for tubulin and CaV1.2 (top panels) or cells following incubation with 33 μM of nocodazole for 2 hr (middle panels) or 18–22 hr (bottom panels). Scale bar: 25 μm. (B) Representative L-type current traces from cells under the three conditions shown in A elicited by voltage steps from a holding potential of −40 mV to +60 mV in 10 mV increments (protocol pulse indicated above). (C) Average current density to voltage (I-V) curves (upper panel) and fraction of activated channels versus voltage plot (activation curves, lower panel) for L-type currents (IL-type) from different cells as shown in B (n = 5 for control cells and n = 6 and 5, for cells exposed to nocodazole for 2 hr and >18 hr, respectively). The continuous line represents the fit to a Boltzmann function for the control dataset.

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  Figure 3. Internalization of CaV1.2 Channels in HL-1 Cells Occurs within a Few Minutes (A) Experimental design to follow the time course of CaV1.2 internalization. HL-1 cells expressing extracellularly HA-epitope tagged CaV1.2 are incubated with fluorescently labeled anti-HA-antibody at room temperature, washed, and immediately transferred to the cell microscope stage top incubator set at 37°C for live cell imaging using the spinning disk confocal microscopy. (B) Representative time-lapse image sequence of the course of internalization of fluorescently labeled CaV1.2 in HL-1 cells. The time after completion of the labeling reaction at which each frame was acquired is indicated. The corresponding fluorescence intensity plots along the dotted line drawn by eye across the middle of the cell are shown in the lower panels. The white arrowheads in the image shown at t = 0 indicate HA-labeled channels accumulated at the cell center and at the plasma membrane and correspond to the black arrowheads in the fluorescence intensity profile shown below. Over time the fluorescence intensity in the plasma membrane decreases, whereas the fluorescence intensity in the inner part of the cell increases. Scale…

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  Figure 4. Endogenous CaV1.2 Colocalizes with Clathrin-Endocytic Vesicles and Rab11a-Recycling Endosomes but Not with Lysosomes (A) Illustration showing a general endocytic pathway with the endosomal markers used in this study. HL-1 cells were immunostained for CaV1.2, clathrin, Rab7 (late endosomes, LE), and Rab11a (recycling endosomes and endocytic recycling compartment, RE/ERC). Lysosomes were labeled using a cDNA encoding for LAMP1 fused to monomeric red fluorescent protein (mRFP). PM, plasma membrane. (B) Laser scanning confocal images of HL-1 cells fluorescently labeled for endogenous CaV1.2 (green) and the indicated endocytic pathway marker (magenta). The corresponding merge images with an enlarged view are shown at the right. Overlap pixels appear in white. Scale bar: 10 μm. (C) Box plot summarizing the colocalization analysis between CaV1.2 and the indicated markers according to Manders' overlap coefficient in HL-1 cells. Numbers above each box represent the number of cells analyzed (n size). Each box represents the interquartile range (25th and 75th quartile) with the median indicated by a line. Whiskers above and below indicate the 95% confidence interval. CaV1.2 colocalizes with…

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  Figure 5. Targeting of Endogenous CaV1.2 to Rab11a-recycling Compartment in HL-1 Cells Depends on an Intact Actin Cytoskeleton but Not on Microtubules (A) Laser scanning confocal images of HL-1 expressing Rab11a-mRFP (magenta) and immunostained for CaV1.2 (green) after 2 hr exposure to the indicated pharmacological agents: dynasore to inhibit clathrin-mediated endocytosis, nocodazole and cytochalasin D to disrupt the microtubule- and actin-based cytoskeleton, respectively. The overlapping pixels appear in white in the merged images shown at the right. Inset shows an enlarged view of the corresponding merged image. Scale bars: 10 μm. (B) Box plot of the normalized fluorescence intensity at the plasma membrane from cells treated as described in panel (A). For better visualization of the cell membrane, the fluorescence and transmission images of representative cells are shown below. Scale bars: 10 μm. untr, Untreated cells; Dyn, dynasore; Noc, nocodazole; CytD, cytochalasin D. Numbers above each box represent the n size. Each box represents the interquartile range (IQR) (25th and 75th quartile) with the median indicated by a line. Whiskers above and below indicate the 95% confidence interval.…

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  Figure 6. Disruption of the Actin Cytoskeleton Re-routes CaV1.2 to Lysosomes in HL-1 Cells (A) Time-lapse sequences of the sorting of internalized CaV1.2 in HL-1 cells expressing either fluorescently labeled Rab11a or LAMP1 for staining RE/ERC and lysosomes, respectively. CaV1.2-HA channels at the cell surface were labeled as in Figure 3. The HA-mediated fluorescence signal of the cell surface CaV1.2 channels is shown in green, and the signal for Rab11a-mRFP and LAMP1-mRFP, in magenta. Only the merged images are shown with the overlapping pixels in white. Scale bar: 10 μm. (B) Time-lapse sequences of the sorting of internalized CaV1.2 in HL-1 cells as in A, but after pharmacological disruption of the actin cytoskeleton using cytochalasin D. Post-endocytic accumulation of CaV1.2 in the RE/ERC is severely impaired in the absence of an intact actin cytoskeleton. Scale bar: 10 μm. (C) Box plot of the colocalization analysis according to Manders' overlap coefficient between CaV1.2 with either Rab11a or LAMP1 in HL-1 cells in the presence (dark gray boxes) and absence (light gray boxes) of cytochalasin D. MOC was calculated from the images acquired at t = 0. Numbers above each box represent the…

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  Figure 7. Model for Trafficking Pathway and Regulation of Cell Surface Expression of CaV1.2 in HL-1 Cells (A) CaV1.2 in HL-1 cells is internalized via clathrin-dynamin-dependent endocytosis and mainly recycled back to the plasma membrane via a Rab11a-mediated pathway. The delivery of the channel from sorting endosomes to the endocytic recycling compartment (ERC) and back to the plasma membrane is regulated by actin filaments (blue arrows). Traffic of defective channels from sorting endosomes to lysosomes, as well as of de novo channels from the biosynthetic secretory pathway, is mediated by microtubules (green arrows). (B) The secretory and endosomal transport of CaV1.2 is depicted as interwoven trafficking pathways. The secretory traffic aids the refilling of the ERC pool upon removal of the channel from the recycling circuit by degradation, via a yet-to-be discovered crosstalk (black arrow).