LIPID TRAFFIC IN THE LUNG

Surfactant constituents in the lung are synthesized by alveolar epithelial Type II (AEC II) cell and secreted into the aveolar lumen. Components of the surfactant are subsequently internalized and selectively transported to the lamellar bodies (LB) of AEC II cells in a process termed "recycling." To learn more about this complex process, we are studying the internalization and sorting of fluorescent analogs of sphingomyelin (SM) and lactosylceramide (LacCer) in AEC II cells in vitro, and in the intact lung. Portions of this work are being carried out in collaboration with Dr. Hubmayr's group in the Thoracic Diseases Research Unit.

• Studies with AEC II cells in vitro. When cells are pulse labeled with the fluorescent sphingolipid analogs, the SM analog is targeted to the lamellar bodies (LB) while the LacCer analog is targeted to endosomes and lysosomes. LB targeting by SM has been confirmed by co-localization studies using a monoclonal antibody to a LB membrane protein. No degradation of the fluorescent SM occurs under these incubation conditions. Internalization and targeting of the fluorescent SM occurs under these incubation conditions. Internalization and targeting of SM is termperature-dependent with SM reaching the LBs within 10 minutes at 37°C; however, LB targetingis also observed after prolonged incubation at 4°C. We are currently trying to elucidate the mechanism responsible for targeting of SM to the LB. Our results to date suggest that the internalization and targeting of SM analog
• to the LB is a selective, protein-mediated, and non-vesicular process involving transbilayer movement of SM at the plasma membrane.
  
  
• Studies with intact lung. In parallel studies, we are studying the distribution of fluorescent sphingolipid (SL) analogs in isolated rat lung by laser confocal fluorescence microscopy, following vascular perfusion or intratracheal administration of the lipid analogs to anaesthetized rats. In these studies, the SM analog also accumulates in the LB of the AEC II cells located at the corners of the alveoli. Studies are underway to examine the effects of lung injury and certain disease states on the traficking of these SL analogs in vivo.