Regulation of the G protein Galphai2 by growth and development in fetal airway epithelium

Functional expression of GABAB receptors in airway epithelium

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system and exerts its actions via both ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors. The GABA(B) receptor is a dimer composed of R1 and R2 components and classically couples to the heterotrimeric G(i) protein. In addition to their location on neurons, GABA and functional GABA(B) receptors have been detected in peripheral tissue such as airway smooth muscle. We questioned whether airway epithelium expresses receptors that could respond to GABA. We detected the mRNA encoding multiple-splice variants of the GABA(B)R1 and GABA(B)R2 in total RNA isolated from native human and guinea pig airway epithelium and human airway epithelial cell lines (BEAS-2B and H441). Immunoblots identified the GABA(B)R1 and GABA(B)R2 proteins in both guinea pig airway epithelium and BEAS-2B cells. The expression of GABA(B)R1 protein was immunohistochemically localized to basal mucin-secreting and ciliated columnar epithelial cells in guinea pig trachea. Baclofen inhibited adenylyl cyclase activity, induced ERK phosphorylation and cross-regulated phospholipase C, leading to increased inositol phosphates in BEAS-2B cells in a pertussis toxin-sensitive manner, implicating G(i) protein coupling. Thus, these receptors couple to G(i) and cross-regulate the phospholipase C/inositol phosphate pathway. The second messengers of these pathways, cyclic AMP and calcium, play pivotal roles in airway epithelial cell primary functions of mucus clearance. Furthermore, the enzyme that synthesizes GABA, glutamic acid decarboxylase (GAD65/67), was also localized to airway epithelium. GABA may modulate an uncharacterized signaling cascade via GABA(B) receptors coupled to G(i) protein in airway epithelium.

Distribution of ERK1/2 and ERK3 during normal rat fetal lung development

The extracellular regulated kinases-1 and -2 (ERK1/2) are well-characterized mitogen-activated protein kinases (MAPK) that play critical roles in proliferation and differentiation, whereas the function(s) of MAPK ERK3 are currently unknown. To understand better the roles of these kinases in development, the temporal distribution of ERK1, -2, and -3 proteins were investigated in multiple tissues. The ERK3 protein, in contrast to ERK1/2 varied both between and within individual organs over time. To characterize this variability in greater detail, the temporal and spatial distributions of activated ERK1/2 and ERK3 during rat fetal lung development were investigated. The diphosphorylated (activated) forms of ERK1/2 (dp-ERK1/2), ERK3, and its phosphorylated form (P-ERK3) decreased from embryonic day 17 (E17) through E21 while both ERK1 and ERK2 total proteins remained unchanged, indicating that ERK1/2 and ERK3 proteins are expressed independently during fetal lung development. In addition, characterization of the distribution of these proteins by fluorescent immunohistochemistry indicated that phosphorylated ERK1/2 and total ERK1/2 were distributed throughout multiple cell types, with the phosphorylated ERK1/2 colocalizing with prophase mitotic cells. In contrast, ERK3 was restricted to the distal lung epithelium during the pseudoglandular phase (E17) but shifted to the proximal airways, particularly Clara cells during the saccular stage (E21). The P-ERK3 colocalized with the mitotic marker P-histone H3 in fetal lung and in NIH3T3 and HeLa cells, implicating a potential role for P-ERK3 in mitosis. Thus, expression of ERK1/2 and ERK3 and their phosphorylated forms are expressed independently and are temporally and spatially localized during fetal lung morphogenesis. These observations will facilitate detailed functional analysis of these kinases to assess their roles in pulmonary development and diseases.

Retinoic acid decreases fetal lung mesenchymal cell proliferation in vivo and in vitro

Retinoic acid (RA) is an important coordinator of mammalian organogenesis. RA is implicated in critical lung developmental events. Cell proliferation is precisely regulated during development. We investigated the effect of RA on proliferating mesenchymal cells in both whole organ lung cultures and cell cultures. The potential pathways required for the response were studied in cultures of lung mesenchymal cells from embryonic day (e) 12. We observed an RA-dependent reduction in proliferation of mesenchymal cells in both whole organ and in cell culture. In mesenchymal cell cultures, RA decreased proliferation in lung mesenchymal cells by 72%. This was associated with a decrease of erk-1/2 activity by 68%. Mesenchymal cell proliferation is erk-1/2 dependent. Erk-1/2 can be activated by G-protein coupled receptors (GPCR) or tyrosine kinase receptors (RTK). RA treatment altered both the RTK and the GPCR pathways in primary lung mesenchymal cells. The Epidermal Growth Factor (EGF) dependent erk-1/2 activation was increased by 35% whereas the G(i)-protein cascade was inhibited by 44% in cells treated with RA. Our results suggest that RA decreases proliferation of lung mesenchyme via a G(i)-protein and the erk-1/2 signaling cascade.

Slit and robo: expression patterns in lung development

First described as an axonal guidance cue through its repulsive effect on neurons expressing its receptor Roundabout (Robo), the Slit ligand has effects on cell migration, axon branching and elongation. Indirect evidence implicates Slit and Robo in lung development. We now demonstrate that Slit-2 and Slit-3 are developmentally regulated in embryonic murine lung. Immunohistochemistry demonstrates Slit-2 and Slit-3 expression by the pulmonary mesenchyme and airway epithelium. Robo-1 and Robo-2 are also expressed by the developing mesenchyme and airway epithelium. As lung development progresses, Robo-1 and Robo-2 expression localizes to only the airway epithelium. We conclude Slit/Robo are expressed in temporo-spatially adjacent domains suggesting interactive roles in pulmonary bronchiolar development.

MEK-1/2 inhibition reduces branching morphogenesis and causes mesenchymal cell apoptosis in fetal rat lungs

The roles of the mitogen-activated protein (MAP) kinases extracellular signal-regulated kinases-1 and -2 (ERK-1/2) in fetal lung development have not been extensively characterized. To determine if ERK-1/2 signaling plays a role in fetal lung branching morphogenesis, U-0126, an inhibitor of the upstream kinase MAP ERK kinase (MEK), was added to fetal lung explants in vitro. Morphometry as measured by branching, area, perimeter, and complexity were significantly reduced in U-0126-treated lungs. At the same time, U-0126 treatment reduced ERK-1/2, slightly increased p38 kinase, but did not change c-Jun NH(2)-terminal kinase activities, indicating that U-0126 specifically inhibited the ERK-1/2 enzymes. These changes were associated with increased apoptosis as measured by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling and immunofluorescent labeling of anti-active caspase-3 in the mesenchyme of explants after U-0126 treatment compared with the control. Mitosis characterized by immunolocalization of proliferating cell nuclear antigen was found predominantly in the epithelium and was reduced in U-0126-treated explants. Thus U-0126 causes specific inhibition of ERK-1/2 signaling, diminished branching morphogenesis, characterized by increased mesenchymal apoptosis, and decreased epithelial proliferation in fetal lung explants.

Decreased mitogen activated protein kinase activities in congenital diaphragmatic hernia-associated pulmonary hypoplasia

BACKGROUND/PURPOSE

The mechanisms that cause pulmonary hypoplasia associated with congenital diaphragmatic hernia (CDH) currently are unknown. The authors proposed that the reduced size and immaturity of these lungs may be associated with differences in the levels of mitogen activated protein (MAP) kinase phosphorylation (extracellular signal regulated protein kinases, ERK-1 and -2).

METHODS

ERK-1 activities were measured using immune-complex kinase assays on fetal whole-lung lysates obtained from both nitrofen and olive oil-treated (control) pregnant rats. In addition, ERK-1 and ERK-2 functional activities were estimated by semiquantitative Western blot analysis, using an antibody specific for the diphosphorylated (dp-ERK, activated) forms of the enzymes.

RESULTS

ERK-1 activities, measured using immune-complex kinase assays, were reduced in CDH lungs compared with olive oil-treated controls (P <.02). In addition, dp-ERK-1 and dp-ERK-2 levels were found to be reduced in CDH lungs compared with controls (dp-ERK-1, P =.003; dp-ERK-2, P =.04), whereas ERK-1 and ERK-2 protein levels were unchanged.

CONCLUSIONS

The lower values of ERK-1 activity and reduced amounts of dp-ERK-1 and dp-ERK-2 in lung tissue from CDH animals, suggests that ERK-1 and ERK-2 activities are reduced in pulmonary hypoplasia associated with CDH. The observed reduction in ERK-1 and ERK-2 activities implicates attenuated cell signaling upstream of the ERK-1 and -2 enzymes.