title: Transcytosis of the polymeric immunoglobulin receptor in cultured hippocampal neurons.
abstract: BACKGROUND
  A wide variety of proteins are transported across epithelial cells by vesicular carriers. This process, transcytosis, is used to generate cell surface polarity and to transport macromolecules between the luminal and serosal sides of the epithelial layer. The polymeric immunoglobulin receptor is a well-characterized transcytotic molecule in epithelia. It binds to its ligand, polymeric immunoglobulin, at the basolateral surface, and the receptor-ligand complex is transcytosed to the apical surface, where the ligand is released. Our previous studies have shown that hippocampal neurons may employ mechanisms similar to those of epithelial cells to sort proteins to two plasma membrane domains. The machinery used for axonal delivery recognizes proteins that are targeted apically in epithelia, whereas basolaterally destined proteins are delivered to the dendrites. It has not been clear, however, whether transcytosis occurs in neurons.
  
  
  RESULTS
  We report expression of the polymeric immunoglobulin receptor in cultured hippocampal neurons, using a Semliki Forest Virus expression system, and show by immunofluorescence microscopy that the newly synthesized receptor is targeted from the Golgi complex predominantly to the dendrites - only about 20% of the infected neurons display axonal immunofluorescence. Addition of ligand leads to significant redistribution of the receptor to the axons, shown by an approximately three-fold increase in axonal immunoreactivity with the anti-receptor antibodies.
  
  
  CONCLUSIONS
  Our results suggest that a transcytotic route, analogous to that in epithelia, exists in neurons, where it transports proteins from the somatodendritic to the axonal domain. Cultured neurons expressing the polymeric immunoglobulin receptor offer an experimental system that should be useful for further characterization of this novel neuronal pathway at the molecular and functional level.
authors.affiliation: Cell Biology Program, European Molecular Biology Laboratory, Meyerhofstrasse 1, Postfach 10.2209, D-69012 Heidelberg, Germany.
authors.forename: E
authors.initials: E
authors.lastname: Ikonen
authors.forename: R G
authors.initials: RG
authors.lastname: Parton
authors.forename: W
authors.initials: W
authors.lastname: Hunziker
authors.forename: K
authors.initials: K
authors.lastname: Simons
authors.forename: C G
authors.initials: CG
authors.lastname: Dotti