Pulmonary hypoplasia in mice lacking tumor necrosis factor-alpha converting enzyme indicates an indispensable role for cell surface protein shedding during embryonic lung branching morphogenesis

Many membrane-bound protein precursors, including cytokines and growth fact ors, are proteolytically shed to yield soluble intercellular regulatory lig ands. The responsible protease, tumor necrosis factor-a converting enzyme ( TACE/ADAM-17), is a transmembrane metalloprotease-disintegrin that cleaves multiple cell surface proteins, although it was initially identified for th e enzymatic release of tumor necrosis factor-alpha (TNF-alpha). Mammalian l ung growth and development are tightly controlled by cytokines and peptide growth factors. However, the biological function of the cell shedding mecha nism during lung organogenesis is not understood. We therefore evaluated th e role of TACE as a "sheddase" during lung morphogenesis by analyzing the d evelopmental phenotypes of lungs in mice with an inactive TACE gene in both in vivo and ex vivo organ explant culture. Neonatal TACE-deficient mice ha d visible respiratory distress and their lungs failed to form normal saccul ar structures. These newborn mutant lungs had fewer peripheral epithelial s acs with deficient septation and thick-walled mesenchyme, resulting in redu ced surface for gas exchange. At the canalicular stage of E16.5, the lungs of TACE mutant mice were impaired in branching morphogenesis, inhibited in epithelial cell proliferation and differentiation, and delayed in vasculoge nesis. Embryonic TACE knockout mouse lungs (E12) branched poorly compared t o wild-type lungs, when placed into serumless organ culture. Gene expressio n of both surfactant protein-C and aquaporin-5 were inhibited in cultured T ACE-mutant embryonic lungs, indicating defects in both branching and periph eral epithelial cytodifferentiation in the absence of TACE protein. Further more, both the hypoplastic phenotype and the delayed cytodifferentiation in TACE-deficient lungs were rescued by exogenous addition of soluble stimula tory factors including either TNF-a or epidermal growth factor in embryonic lung culture. Thus, the impaired lung branching and maturation without TAC E suggest a broad role for TACE in the processing of multiple membrane-anch ored proteins, one or more of which is essential for normal lung morphogene sis. Taken together, our data indicate that the TACE-mediated proteolytic m echanism which enzymatically releases membrane-tethered proteins plays an i ndispensable role in lung morphogenesis, and its inactivation leads to abno rmal lung development. (C) 2001 Academic Press.