TTF-1 Phosphorylation Is Required for Peripheral Lung Morphogenesis, Perinatal Survival, and Tissue-specific Gene Expression*

Thyroid transcription factor-1 (TTF-1) is a 43-kDa, phosphorylated member of the Nkx2 family of homeodomain-containing proteins expressed selectively in lung, thyroid, and the central nervous system. To assess the role of TTF-1 and its phosphorylation during lung morphogenesis, mice bearing a mutant allele, in which seven serine phosphorylation sites were mutated, Titf1PM/PM, were generated by homologous recombination. Although heterozygous Titf1PM/+ mice were unaffected, homozygous Titf1PM/PM mice died immediately following birth. In contrast to Titf1 null mutant mice, which lack peripheral lung tissues, bronchiolar and peripheral acinar components of the lung were present in the Titf1PM/PM mice. Although lobulation and early branching morphogenesis were maintained in the mutant mice, abnormalities in acinar tubules and pulmonary hypoplasia indicated defects in lung morphogenesis later in development. Although TTF-1PM protein was readily detected within the nuclei of pulmonary epithelial cells at sites and abundance consistent with that of endogenous TTF-1, expression of a number of known TTF-1 target genes, including surfactant proteins and secretoglobulin 1A, was variably decreased in the mutant mice. Vascular endothelial growth factor mRNA was decreased in association with decreased formation of peripheral pulmonary blood vessels. Genes mediating surfactant homeostasis, vasculogenesis, host defense, fluid homeostasis, and inflammation were highly represented among those regulated by TTF-1. Thus, in contrast to the null Titf1 mutation, the Titf1PM/PM mutant substantially restored lung morphogenesis. Direct and indirect transcriptional targets of TTF-1 were identified that are likely to play important roles in lung formation and function.