Figure 2.

The polarization of TfR to the dendritic plasma membrane parallels the exclusion of TfR-containing carrier vesicles from the axon. TfR-GFP was expressed by using a defective herpesvirus. Cell surface TfR was assessed by staining living cells with an anti-TfR antibody, whereas GFP fluorescence served as a measure of all expressed TfR, including that associated with intracellular vesicles. (a and b) On day 2, the polarization of TfR varied somewhat from cell to cell. In some cells (a), surface staining for TfR (Center) was absent from the axon (arrows), which was paralleled by the absence of axonal TfR-GFP fluorescence associated with intracellular vesicles (Right). Staining in dendritic processes (arrowheads) was readily observed with both labels. In other cells (b), surface staining and TfR-GFP fluorescence were present in the distal axon (arrows) at a level comparable to that in the dendrites (arrowheads). The GFP fluorescence illustrates all TfR present in cells, including carrier vesicles in dendritic and axonal processes. (Bar, 20 μm.) (c) On the basis of a cell-by-cell comparison, there was a close correlation between the degree of polarization of cell surface TfR and TfR-GFP fluorescence. The total fluorescence in all dendritic processes was expressed as a percentage of the total fluorescence in all neurites including the axon. Cells in culture for 1 day were infected with replication-defective herpesvirus encoding TfR-GFP. After 18 h, living cells were stained with antibody to detect protein expression on the cell surface.