Fetal oxygen tension promotes tenascin‐C–dependent lung branching morphogenesis

Tenascin‐C (TN‐C) is a mesenchyme‐derived extracellular matrix (ECM) glycoprotein required for fetal lung branching morphogenesis. Given that the low oxygen (O 2 ) environment of the fetus is also essential for normal lung branching morphogenesis, we determined whether fetal O 2 tension supports this process by promoting TN‐C expression. Initial studies showed that 15‐day fetal rat lung explants cultured for 2 days at 3% O 2 not only branched well, but they also expressed higher levels of TN‐C when compared to lungs maintained at 21% O 2 , which branched poorly. Antisense oligonucleotide studies demonstrated that TN‐C produced in response to 3% O 2 was essential for lung branching morphogenesis. As well, exogenous TN‐C protein was shown to promote branching of lung epithelial rudiments cultured at 21% O 2 . Because ECM‐degrading proteinases are capable of catabolizing TN‐C protein, we reasoned that 3% O 2 might promote TN‐C deposition by limiting the activity of these enzymes within the fetal lung. Consistent with this idea, gelatin zymography showed that the activity of a 72‐kDa gelatinase, identified as matrix metalloproteinase‐2 (MMP‐2), was lower at 3% O 2 vs. 21% O 2 . Furthermore, pharmacologic inhibition of MMP‐2 activity in fetal lung explants cultured at 21% O 2 resulted in increased TN‐C deposition within the mesenchyme, as well as enhanced branching morphogenesis. Collectively, these studies indicate that fetal O 2 tension promotes TN‐C–dependent lung epithelial branching morphogenesis by limiting the proteolytic turnover of this ECM component within the adjacent mesenchyme. Developmental Dynamics 234:1–10, 2005. © 2005 Wiley‐Liss, Inc.