Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, C104
Vascular signalling in the fetal lung is regulated by the FGF-10/FGFR2b/Spry2 airway branching periodicity clock.
C. L. Scott1, D. Walker1, E. Cwiklinski1, A. R. Tee2, S. C. Land1
1. Centre for Cardiovascular and Lung Biology, Dundee University, Dundee, United Kingdom. 2. Institute for Medical Genetics, University of Cardiff, Cardiff, Wales, United Kingdom.
Development of the respiratory tree requires co-ordinated signalling between airway and vascular tissues but the link between these processes remains unclear. The mammalian target of rapamycin complex-1 (mTORC1) can amplify hypoxia inducible factor-1α (HIF-1α)-mediated vasculogenesis by interacting with an N-terminal mTOR binding (TOS) motif (Land & Tee, 2007). We hypothesised that mTORC1 regulation by the fibroblast growth factor (FGF)-10/FGF-receptor2b/Sprouty2 pathway which regulates airway branching periodicity (Metzger et al., 2008) links vasculogenic HIF-1α activity to airway growth. Bicameral cultures of human bronchial epithelial cells (16HBE14o- or “HBE”) and undifferentiated human embryonic lung mesenchyme cells (HEL12469 or “HELM”) were maintained at fetal or alveolar PO2 to study the induction of vasculogenic signalling by FGF-10 and regulators of mTORC1. Results were compared to the pattern of vasculogenesis in gestation day 12.5 wild-type and TSC1-/- mice and vasculogenic signalling in isolated rat fetal distal lung epithelium (FDLE). mTORC1 activation in HBE at fetal PO2 evoked a 5-fold, rapamycin-sensitive, amplification of HIF-1α above endogenous levels and induced vascular endothelial growth factor (VEGF) release and vascular differentiation (Flk1, sprouting) in HELM co-cultures. This was abolished by null-mutation of the HIF-1α-mTORC1 docking motif. mTORC1 activity in vivo occurred in pseudoglandular epithelial lung buds and hyper-activation in E12.5 murine TSC1-/- fetuses raised VEGF expression and disaggregated Tie2-positive vascular bundles. This intrinsic mTORC1 activity was responsive to airway branching regulators as, in both HBE and FLDE primary cultures cultured at fetal PO2, FGF-10 induced a concentration-dependent activation of rapamycin-sensitive HIF-1α which was accompanied by degradation of sprouty2. This destabilisation of sprouty2 was linked mechanistically to mTORC-1-evoked HIF-1α activity as graded over-expression of sprouty2 abolished the induction of HIF-1α by either FGF-10 or Rheb, the upstream activating G-protein of mTORC1. We postulate a model of co-ordinated airway and vascular branching morphogenesis whereby mTORC1 integrates HIF-1α-driven vasculogenesis with the FGF-10/FGFR2b/Sprouty2 airway branching periodicity regulator.
Where applicable, experiments conform with Society ethical requirements