Intracellular traffic of the MHC class I-like IgG Fc receptor, FcRn, expressed in epithelial MDCK cells.

Transfer of passive immunity from mother to the fetus or newborn involves the transport of IgG across several epithelia. Depending on the species, IgG is transported prenatally across the placenta and yolk sac or is absorbed from colostrum and milk by the small intestine of the suckling newborn. In both cases apical to basolateral transepithelial transport of IgG is thought to be mediated by FcRn, an IgG Fc receptor with homology to MHC class I antigens. We have now expressed the human FcRn in polarized MDCK cells and analyzed the intracellular routing of the receptor. FcRn showed a predominant intracellular localization at steady state. Newly synthesized FcRn was delivered in a non-vectorial fashion to both the apical and basolateral surfaces of MDCK cell monolayers. Following internalization from the apical or basolateral domain, the receptor transcytosed to the opposite surface. These findings provide direct evidence for the transepithelial transport function of FcRn and indicate that the receptor undergoes multiple rounds of transcytosis.

Nonvectorial surface transport, endocytosis via a Di-leucine-based motif, and bidirectional transcytosis of chimera encoding the cytosolic tail of rat FcRn expressed in Madin-Darby canine kidney cells.

Transfer of passive immunity from the mother to the fetus or newborn involves the transport of IgG across several epithelia. Depending on the species, IgG is transported prenatally across the placenta and yolk sac or is absorbed from colostrum and milk by the small intestine of the suckling newborn. In both cases apical to basolateral transepithelial transport of IgG is thought to be mediated by FcRn, an IgG Fc receptor with homology to major histocompatibility class I antigens. Here, we analyzed the intracellular routing of chimera encoding the rat FcRn tail fused to the ecto- and transmembrane domain of the macrophage FcgammaRIIb. Newly synthesized chimera were delivered in a nonvectorial manner to the apical and basolateral cell surface, from where the chimera were able to internalize and transcytose. Apical to basolateral and basolateral to apical transcytosis were differently regulated. This intracellular routing of the chimera is similar to that of the native FcRn, indicating that the cytosolic tail of the receptor is necessary and sufficient to endow an unrelated FcR with the intracellular transport behavior of FcRn. Furthermore, the di-leucine motif in the cytosolic domain of FcRn was required for rapid and efficient endocytosis but not for basolateral sorting of the chimera.

Inhibition of influenza-induced membrane fusion by lysophosphatidylcholine.

Lysolipids have been reported to inhibit various membrane fusion events, and it was suggested that inhibition was due to their "inverted cone" shape, which hinders the formation of intermediate lipid structures required for fusion (Chernomordik, L. V., Vogel, S. S., Sokoloff, A., Onaran, H. O., Leikina, E. A., and Zimmerberg, J. (1993) FEBS Lett. 318, 71-76). Here, the effect of lysophosphatidylcholine (LPC) on fusion mediated by the hemagglutinin (HA) of influenza virus was investigated. Virus-liposome fusion was inhibited by LPC if the lysolipid was added to the membranes from an aqueous stock solution but not if LPC was symmetrically distributed over both leaflets of the liposomal bilayer. These findings would be consistent with an effect of LPC on lipid intermediate formation, but inhibition increased with increasing acyl chain length and thus a less pronounced inverted cone shape of the lysolipids suggesting that the mechanism of inhibition might be different. At low pH, due to the exposure of the fusion peptide of HA, followed by its insertion into the liposomal membrane, virus acquires the ability to bind to zwitterionic liposomes lacking receptors for HA. This type of binding was inhibited by LPC. Moreover, leakage of calcein from receptor-containing liposomes, induced by purified HA at low pH, was inhibited by LPC. Therefore, the inhibition of influenza-induced fusion by LPC was caused by the binding of LPC to fusion peptides, thereby preventing their interaction with the target membrane rather than an effect on intermediate lipid structures.