Several lung pathologies, notably respiratory distress syndrome (RDS) in the premature infant, are related to fetal lung immaturity. RDS is incontrovertibly linked to a deficiency in pulmonary surfactant, which is a complex mixture of phospholipids and four lung-specific apoproteins that have been designated SP-A, SP-B, SP-C and SP-D. Pulmonary surfactant, which is synthesized and secreted by alveolar type II cells, functions by lowering surface tension at the air-liquid interface, thereby preventing alveolar collapse at low lung volumes. The hydrophobic proteins SP-B and SP-C are importantly involved in the surface activities of surfactant, while SP-A appears to be more involved in regulation of surfactant metabolism in the alveolus. The function of SP-D is not fully understood, although it appears to be involved in non-immune host defense in the lung. Given their importance to normal surfactant function and metabolism, much effort has been expended on studying the regulation of SP-A, SP-B and SP- C. An important recent observation is that the expression of SP-A, SP-B and SP-C begins much earlier in lung development than was previously thought. Epithelial-mesenchymal interactions, which have heretofore been documented to be essential for normal branching morphogenesis in the lung, also appear to be involved in specifying the differentiated distal epithelial cell phenotype in the early lung, as demonstrated by the ability of early fetal lung mesenchyme to reprogram early fetal tracheal epithelium to express the alveolar type II cell phenotype. We have developed several systems that we will use for investigating the basis of epithelial-mesenchymal interactions. In order to ascertain the range of epithelial cell fates in tissue recombinations, we will use markers of distal lung and tracheal epithelial differentiation to characterize the response of fetal lung and fetal tracheal epithelia to the influences of both lung and tracheal mesenchyme. We have demonstrated that the inductive influence of lung mesenchyme on tracheal epithelium is mediated by diffusible molecule(s). We will expand these studies to determine the size of these factors, how quickly they are effective, and whether they must be continuously present to sustain distal lung epithelial development. We have developed a complex culture system in which we have been able to induce expression of SP-C, a specific marker of the distal lung epithelium, in fetal tracheal epithelial cells. We will thoroughly characterize this system. We will then determine the critical components of this medium, test additional growth factors and hormones, and develop a defined extracellular matrix substratum for this induction. The induction of expression of a new cell phenotype in tracheal epithelium by lung mesenchyme is accompanied by the expression of new genes. Using the technique of differential display reverse transcription polymerase chain reaction, we will identify and isolate some these genes. This will provide new insight and reagents for the study of mechanisms of epithelial cell determination and differentiation in the fetal lung. At the completion of this grant we expect to have substantially increased our knowledge of the factors that regulate distal lung epithelial cell differentiation and proliferation in the developing fetus. This knowledge will prove useful in the prevention and treatment of disease resulting from lung immaturity.
