Apical sorting of influenza hemagglutinin by transcytosis in retinal pigment epithelium.

The retinal pigment epithelium is endowed with a unique distribution of certain plasma membrane proteins. Na+,K+-ATPase, for instance, is polarized to the apical surface of RPE, rather than to the basolateral surface as in most other epithelia. To study the sorting pathways of RPE cells, we used temperature sensitive mutants of influenza and vesicular stomatitis virus (VSV) to synchronize the transport of hemagglutinin (HA) and VSV G protein (VSV G) along the biosynthetic pathway of the RPE cell line RPE-J. After HA and VSV G accumulated in the trans-Golgi network of RPE-J cells kept at 20 degrees C, transfer to the permissive temperature (32 degrees C) resulted in the transport of both HA and VSV G to the basolateral plasma membrane. Later, while VSV G remained basolateral, HA progressively reversed its polarity, eventually becoming apical. Further analysis demonstrated that the reversal of HA polarity was due to transcytosis of HA from the basolateral to the apical surface of RPE-J cells. To determine whether HA followed a transcytotic route in RPE in vivo, influenza and VSV were injected into the subretinal space of rat eyes. Again, both HA and VSV G were initially observed at the basolateral surface of RPE cells. However, whereas VSV G remained there, HA progressively redistributed to the apical surface. These findings demonstrated that RPE cells use a transcytotic pathway for the targeting of at least some apical proteins to their destination.