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Tumor necrosis factor-[alpha] converting enzyme (TACE) is a membrane-anchored, Zn-dependent metalloprotease, which belongs to the ADAM (a disintegrin and metalloprotease) family. TACE functions as a membrane sheddase to release the ectodomain portions of many transmembrane proteins, including the precursors of TNF[alpha] , TGF[alpha], several other cytokines, as well as the receptors for TNF[alpha], and neuregulin (ErbB4). Mice with [TACE.sup.[delta]Zn/[delta]Zn] null mutation die at birth with phenotypic changes, including failure of eyelid fusion, hair and skin defects, and abnormalities of lung development. Abnormal fetal heart development was not previously described. Herein, we report that [TACE.sup.[delta]Zn/[delta]Zn] null mutant mice by late gestation exhibit markedly enlarged fetal hearts with increased myocardial trabeculation and reduced cell compaction, mimicking the pathological changes of noncompaction of ventricular myocardium. In addition, larger cardiomyocyte cell size and increased cell proliferation were observed in ventricles of [TACE.sup.[delta]Zn/[delta]Zn] knockout mouse hearts. At the molecular level, reduced expression of epidermal growth factor receptor, attenuated protein cleavage of ErbB4, and changes in MAPK activation were also detected in [TACE.sup.[delta]Zn/[delta]Zn] knockout heart tissues. The data suggest that TACE-mediated cell surface protein ectodomain shedding plays an essential and a novel regulatory role during cardiac development and modeling. Keywords: TACE; Heart development; Cardiomyocyte

TGF-[Beta]1 is a known inhibitor of branching morphogenesis when added exogenously to mouse embryonic lungs in culture. However, the issue of whether endogenous TGF-[Beta] signaling has a function in the process of lung organogenesis is not completely resolved. We utilized immunoperturbation and antisense oligodeoxynucleotide inhibitory strategies to abrogate TGF-[Beta] type II receptor function in embryonic mouse lungs undergoing branching morphogenesis in serumless explant culture. Antisera directed against a TGF-[Beta] type II receptor N-terminal peptide that perturbs TGF-[Beta] ligand-receptor binding increased branching by 70%. Similarly, antisense TGF-[Beta] type II receptor oligodeoxynucleotides (40/[[micro]molar]) resulted in a 58% increase in branching, compared to scrambled and mismatched sequence controls, while TGF-[Beta] type II receptor mRNA and its protein expression levels were suppressed by 95 and 84%, respectively. Addition of exogenous TGF-[Beta]1 did not overcome the stimulatory effects either of TGF-[Beta] type II receptor immunoperturbation or of antisense oligodeoxynucleotide treatment on lung branching morphogenesis. Using in situ hybridization and immunohistochemistry, both TGF-[Beta] type II receptor mRNA and protein were localized to the epithelium lining the developing airways, and to the surrounding mesenchyme, indicating that TGF-[Beta] type II receptor is an important regulator of epithelial-mesenchymal interaction. Exogenous TGF-[Beta]1 decreased cyclin A mRNA levels in control embryonic lung explants, while TGF-[Beta] type II receptor antisense oligodeoxynucleotides prevented the downregulation of cyclin A mRNA expression by exogenous TGF-[Beta]1. In addition, PCNA immunostaining of the primitive bronchial epithelium was increased in the presence of TGF-[Beta] type II receptor antisense oligodeoxynucleotides either alone or together with exogenous TGF-[Beta]1, whereas TGF-[Beta]1 alone decreased PCNA staining. Thus, abrogation of TGF-[Beta] type II receptor expression prevented TGF-[Beta]1-induced epithelial cell [G.sub.1] arrest. These results demonstrate, for the first time, that abrogation of the TGF-[Beta] type II receptor stimulates embryonic lung organogenesis in culture and reverses the negative influence of endogenous TGF-[Beta] signaling upon epithelial cell cycle progression.

Many membrane-bound protein precursors, including cytokines and growth factors, are proteolytically shed to yield soluble intercellular regulatory ligands. The responsible protease, tumor necrosis factor- [Alpha] converting enzyme (TACE/ADAM-17), is a transmembrane metalloprotease-disintegrin that cleaves multiple cell surface proteins, although it was initially identified for the enzymatic release of tumor necrosis factor-[Alpha] (TNF-[Alpha]). Mammalian lung growth and development are tightly controlled by cytokines and peptide growth factors. However, the biological function of the cell shedding mechanism during lung organogenesis is not understood. We therefore evaluated the role of TACE as a 'sheddase' during lung morphogenesis by analyzing the developmental phenotypes of lungs in mice with an inactive TACE gene in both in vivo and ex vivo organ explant culture. Neonatal TACE-deficient mice had visible respiratory distress and their lungs failed to form normal saccular structures. These newborn mutant lungs had fewer peripheral epithelial sacs with deficient septation and thick-walled mesenchyme, resulting in reduced 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 vasculogenesis. Embryonic TACE knockout mouse lungs (E12) branched poorly compared to wild-type lungs, when placed into serumless organ culture. Gene expression of both surfactant protein-C and aquaporin-5 were inhibited in cultured TACE-mutant embryonic lungs, indicating defects in both branching and peripheral epithelial cytodifferentiation in the absence of TACE protein. Furthermore, both the hypoplastic phenotype and the delayed cytodifferentiation in TACE-deficient lungs were rescued by exogenous addition of soluble stimulatory factors including either TNF-[Alpha] or epidermal growth factor in embryonic lung culture. Thus, the impaired lung branching and maturation without TACE suggest a broad role for TACE in the processing of multiple membrane-anchored proteins, one or more of which is essential for normal lung morphogenesis. Taken together, our data indicate that the TACE-mediated proteolytic mechanism which enzymatically releases membrane-tethered proteins plays an indispensable role in lung morphogenesis, and its inactivation leads to abnormal lung development. [C] 2001 Academic Press Key Words: TACE; TNF-[Alpha]; EGF; ectodomain shedding; lung branching morphogenesis; lung explant culture; SP-C; vasculogenesis.