Our long-term goal has been the elucidation of regulatory mechanisms and pathways associated with macromolecule transport across alveolar epithelium. Preliminary studies on protein transport across primary cultured cell monolayers, which exhibit phenotypic and morphological traits of in vivo alveolar epithelial Type I (ATl) cells, uncovered important information on alveolar epithelial protein transport, in that some serum proteins (e.g., albumin (Alb) and immunoglobulins G (IgG)) are absorbed via saturable processes at rates greater than those predicted by passive diffusional mechanisms, whereas dimeric immunoglobulin A (dIgA) is secreted into alveolar fluid via a polymeric immunoglobulin receptor (pIgR)-mediated process. We also found very recently that a glucocorticoid analog, dexamethasone, decreases net absorption of IgG across alveolar epithelium, implicating possible regulation of alveolar epithelial transport of serum proteins by soluble (e.g., humoral) factors. As a logical extension of these functional transport studies on these three key proteins (i.e., Alb, IgG, dIgA) across alveolar epithelium, regulation of transport mechanisms will be investigated. Our central hypotheses include i) that these three key proteins traverse the alveolar epithelial barrier predominantly utilizing regulable vesicular transport, ii) that endocytosis of Alb and dIgA is predominantly mediated by their cognate receptors clustered in caveolin-coated structures (i.e., caveolae) and clathrincoated pits, respectively, while IgG is internalized by fluid-phase endocytosis (i.e., without binding receptors at the cell surface due to non-optimal fluid pH), and iii) transport of these proteins is regulated differentially in alveolar epithelium. In order to test these hypotheses, we will study the following four Specific Aims: i) differential properties of transalveolar epithelial transport of specific proteins, ii) characterization of binding / internalization of specific proteins, iii) regulation of trafficking of specific proteins across alveolar epithelium, and iv) regulation of protein transport across injured alveolar epithelium. We will utilize primary cultured alveolar epithelial cell monolayers as an in vitro model to provide pertinent mechanistic information on differential regulation of alveolar epithelial transport of these three key proteins. Regulation of alveolar epithelial expression of cognate receptors for these key proteins will be investigated where appropriate. Results obtained from these studies will help elucidate specific mechanisms (and associated pathways) for translocation of proteins across lung alveolar epithelium and roles of the cognate receptors for the key proteins in alveolar epithelial protein transport in health and disease.
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