Search

Membrane type 1-matrix metalloprotease (MT1-MMP or MMP-14) is a major activator of pro-MMP-2 and is essential for skeletal development. We show here that it is required for branching morphogenesis of the submandibular gland but not the lung. Instead, in the lung, it is essential for postnatal development of alveolar septae. Lung development in Mmp14-/- mice is arrested at the prealveolar stage with compensatory hyperinflation of immature saccules. Mmp2-/- mice lacked comparable detects in the lung and submandibular gland, suggesting that MT1-MMP acts via mechanisms independent of pro-MMP-2 activation. Since the developmental defects in the lung are first manifest around the time of initial vascularization (E16.5), we investigated the behavior of pulmonary endothelial cells from Mmp14+/+ and Mmp14-/- mice. Endothelial cells from lungs of 1-week-old Mmp14-/- mice show reduced migration and formation of three-dimensional structures on Matrigel. Since pulmonary septal development requires capillary growth, the underlying mechanism of pulmonary hypoplasia in Mmp14-/- mice may be defective angiogenesis, supporting a model in which angiogenesis is a critical rate-limiting step for acquisition of pulmonary parenchymal mass. Keywords: MT1-MMP; Branching morphogenesis; Alveolization; Lung; Submandibular gland; Angiogenesis; MMP; TIMP; Septation; Pulmonary

Light microscopic study of the ability of pulmonary mesenchyme to stimulate cytodifferentiation and morphogenesis with variations in fetal trachea by transplanting isolated rat fetal pulmonary mesenchyme to a segment of tracheal epithelium reveals that the constitutive cells of the induced tracheal epithelium are morphologically similar to control lung explant cells. Reverse transcription polymerase chain reaction analysis of the induced tracheal epithelium indicates that type II cell differentiation markers, mRNAs for the surfactant proteins SP-A, -B, and -C, are present, indicating the initiation of type II cell differentiation by the transplanted pulmonary epithelium.

Epithelial-mesenchymal interactions are important for lung morphogenesis and cytodifferentiation. Molecules such as the FGFs, BMP4, and SHH have been shown to mediate signaling that is important to these processes. We have employed a culture system to study the effects of these molecules on lung epithelial differentiation without the influence of mesenchyme. Tracheal epithelium, which in the rat can be reprogrammed to express a distal lung epithelial phenotype, was used to test the ability of these molecules to promote growth and specify distal lung differentiation. Different FGFs exhibited distinct abilities to induce epithelial growth and expression of the distal lung epithelial marker, surfactant protein C (SP-C), while all FGFs appeared to be able to induce BMP4 expression. Expression of SHH was not upregulated under conditions known to induce SP-C expression. FGF1 induced the strongest expression of SP-C, as measured by whole-mount in situ hybridization and quantitative real-time PCR (SmartCycler, Cepheid). Addition of BMP4 with FGF1 resulted in inhibition of both growth and expression of SP-C; inhibition of BMP4 signaling was able to reverse these effects. Tracheal epithelium cultured under conditions containing FGF10 failed to express SP-C and showed little growth. These data suggest that BMP4 and SHH may not be involved in specification of distal lung epithelial differentiation. (Supported by a Franklin D. Roosevelt Fellowship from the March of Dimes and a Pulmonary and Cardiovascular Development Training Grant (T32 HL07752) from NHLBI.)