Thyroid Hormone Receptor Repression Linked to Type I Pneumocyte Associated Respiratory Distress Syndrome

Although the lung is a defining feature of air-breathing animals, the pathway controlling the formation of the type I pneumocyte, the cell that mediates gas exchange, is poorly understood. In contrast, glucocorticoids and their cognate receptor (GR) have long been known to promote type II pneumocyte maturation; prenatal administration of glucocorticoids is commonly used to attenuate the severity of infant respiratory distress syndrome (RDS). Here we show that knock-in mutations of the nuclear corepressor SMRT in C57Bl6 mice (SMRT(mRID)) produces a novel respiratory distress syndrome due to prematurity of the type I pneumocyte. Though unresponsive to glucocorticoids, treatment with anti-thyroid hormone drugs (propylthiouracil or methimazole) completely rescues the SMRT-induced RDS, suggesting an unrecognized and essential role for the thyroid hormone receptor (TR) in lung development. We show that TR and SMRT control type I pneumocyte differentiation through Klf2, which in turn appears to directly activate the type I pneumocyte gene program. Conversely, mice without lung Klf2 lack mature type I pneumocytes and die shortly after birth, closely recapitulating the SMRT(mRID) phenotype. These results identify a second nuclear receptor, the TR, in type I pneumocyte differentiation and suggest a new type of therapeutic option in the treatment of glucocorticoid non-responsive RDS.