P75-NTR is sorted apically in I-.PE cells independent of the maturation of the apical surface

Purpose. To understand how RPE cells sort proteins to their apical surface. Methods. Adenovirus mediated gene t -ansfer was used to express p75-NTR, an apical marker in other epithelia, in c eveloping RPE cells, and in the RPE cell line RPE-J. The steady state polariiy of p75-NTR was compared to that of RET-PE2/CE9, a protein which switches polarity from basolateral to apical during the postnatal development of rat RPE, and is found on the basolateral surface of other mature epi helia. Changes in the ratio of apical and basolateral membrane areas durinj; postnatal development were measured to determine how the elongation of RPE microvilli might contribute to changes in polarity. Results. p75-NTR expressed in either RPE-J cells or adult RPE cells in vivo was localized on the apical surface of Ihe cells. However RET-PE2/CE9 was basolateral in RPE-J and apical in the adult RPE. During postnatal development p75-NTR was always sorted with an apparently high fidelity to the apical surface of RPE cells as early as postnatal day 2, a time when RETPE2/CE9 has yet to acquire apical polarity. No relationship was found between the polarity of p75-NTR and he ratio of apical to basolateral membrane. Conclusions. RPE cells are capable of a:tively sorting proteins to the apical plasma membrane early in postnatal development regardless of the ratio of apical to basolateral plasma membram. Sorting of P75-NTR is independent of cellular morphology, extracellular contacts, or eye development, whereas, the sorting of RET-PE2/CE9 is dependent upon all of these factors.