Fetal Lung Development and Injury
Lung development is a finely tuned arrangement of biological events that involves the regulation of many signaling pathways that interact to regulate gene expression. Disturbance in these events can interfere with lung morphogenesis and result in incorrect differentiation of pulmonary cells, leading to lung diseases. A well-defined sequence of histologically characterized events during lung development involves mesenchymal-epithelial and epithelial-mesenchymal communication to regulate the timely progression of structural and functional lung development. Antenatal and postnatal events such as chorioamnionitis, hypoxia, hyperoxia, and mechanical ventilation, nutritional deficits, prolonged exposure to glucocorticoids, and pro-inflammatory mediators each disrupt the developmentally regulated orchestration of growth factor signaling in lung development resulting in bronchopulmonary dysplasia (BPD) in preterm born infants (Laughon et al, 2009 ).
Figure 1. Preterm infants must overcome many challenges including problems related to lung development.
NRG/ ErbB4 Regulate Fetal Lung Development
Our group was the first to describe that NRG1β, a growth factor, endogenously released from the mature fetal lung fibroblast, stimulates the onset of fetal surfactant synthesis (Dammann et al, 2003 ). NRG1β is getting increasing attention for its potential neuroprotective role in the fetal-neonatal brain (Dammann et al, 2008 ). Less is known about its role in fetal neonatal lung protection. NRG1β signals its effect through ErbB receptors, ErbB3, and ErbB4. There are four ErbB receptors, ErbB1 (or EGF receptor), ErbB2, ErbB3, and ErbB4, each of which forms homodimers or heterodimers with each other upon ligand binding. The signaling diversity of this group of receptors is due to the dimerization and the subsequent transactivation of the dimerization partner, leading to the activation of different intracellular signaling pathways and resulting in one of a variety of different cellular effects, including differentiation, proliferation, apoptosis, and cell migration ― depending upon the cell type, age, and surrounding circumstances. In the fetal lung, ErbB4 is the most prominent dimer partner (Liu et al, 2007 ), prominently expressed in epithelia of developing columnar and saccular units in late gestation, and critically important for fetal surfactant synthesis (Zscheppang et al, 2007 ). Finally, ErbB4 deletion leads to a hyper-reactive airway system, structural alveolar simplification, and signs of chronic inflammation in the adult lung (Purevdorj et al, 2008 ) and delay in the timely progression of fetal structural (Figure1) and functional lung development (Liu et al, 2010 ). ErbB4 is the member of the ErbB receptor family, which is mostly involved in maturational processes, e.g. in the neurosystem. ErbB4 is a unique member in his family, being the only member with transcription factor like properties. The role of ErbB receptors in transmembrane signaling is well established. Recently, ligand-dependent translocation of ErbB receptors to the nucleus has been reported and linked to differentiation in the brain and the breast.
Figure 2. Histological pictures of E17 control HER4heart+/- (a, c) and HER4heart-/- (b, d) lungs. Representative light microscopic images from cryosections stained with hematoxilin and eosin. HER4heart-/- (b) have more transitory ducts and saccules compared to HER4heart+/- control lungs. A higher resolution shows more epithelial cells surrounding the air spaces in control lungs (c). HER4heart-/- (d) lungs have less sacculi and show accumulation of tissue containing mesenchyme cells (arrow). Morphometric evaluations revealed an increased volume density of the septal tissue and a decreased volume density of the airspace in HER4heart-/- lungs.
Fetal Type II Cell Behavior
The maturation of the fetal type II cells plays a critical role in lung development. ErbB4 is prominently expressed in the fetal type II cell and downregulation of ErbB4 downregulates fetal surfactant production and changes the cellular localization of its main dimerization partner ErbB1 (Figure 2). Re-expression of full-length ErbB4 in primary fetal type II epithelial cells, isolated from ErbB4 deleted transgenic mice, led to an increased Sftpb mRNA expression. This stimulatory effect required nuclear translocation of ErbB4 and transactivation of Stat5a leading to direct binding to and activation of the Sftpb promoter. Studies are ongoing to further delineate the exact interactions with transcription factors involved in type II cell development.
Figure 3. Effects of ErbB4 down regulation on cellular ErbB4 and ErbB1 receptor localization. Confocal microscopy was used to study the co-localization (orange color) of ErbB4 (green dye) and ErbB1 (red dye) in primary d19 fetal rat type II cells. The top panels show localization signals of ErbB4 and ErbB1 in type II cells treated with scrambled siRNA alone (left) , or also stimulated with NRG- (middle), or FCM (right). The bottom panels show localization signals of both receptors in type II cells treated with ErbB4 siRNA cocktail alone (left), or also stimulated with NRG- (middle), or FCM (right).
Epithelial to mesenchymal transition (EMT) is strongly associated with fibrotic processes, giving an explanation for the origin of some fibroblasts during fibrotic diseases, such as chronic obstructive pulmonary disease (COPD) and bronchopulmonary dysplasia (BPD). EMT describes the loss of epithelial properties and the gain of a mesenchymal phenotype. Transforming growth factor-1β (TGF-β1) and epidermal growth factor (EGF) are known to induce EMT. Fetal tissue is capable to repair without fibrosis or scar formation. We have recently shown that TGF-β1 and EGF did not induce EMT in fetal type II cells, as it does in the adult type II. Further studies are ongoing to enhance our knowledge why fetal type II cells that differ from adult cells in their age-related phenotypical and functionally unique properties. Further knowledge will permit the development of novel in utero therapeutic approaches to enhance lung maturation and prevent injury by decreasing the incidence of surfactant deficiency, but also preventing the development of BPD.