Lymphangiogenesis in the developing lung promoted by VEGF-A.

Understanding the basic processes of late-stage pulmonary vascular development is essential as this period corresponds to the stage when preterm infants have increased chance of survival. During this period, refinement of the gas exchange unit leads to close apposition of the capillary vasculature and airway epithelium through thinning of the mesenchyme, formation of alveolar septae and functional adaptation of endothelial cells into vessels including pulmonary lymphatics. The pulmonary lymphatic network promotes efficient gas exchange through maintaining interstitial fluid balance. Through conditional transgene regulation, we found that a modest, pathologically relevant increase in vascular endothelial growth factor A (VEGF-A) in distal lung during only the perinatal period adversely affected final refinement of the gas exchange unit. VEGF-A induction disrupted the established vascular network, increased endothelial cell number, altered endothelial ultrastructure and reduced mesenchymal thinning. In addition, VEGF-A induction caused a 3-fold increase in small vessels identified as lymphatics in distal lung. mRNA levels of lymphangiogenic factors VEGF-D/-C were unchanged, while levels of the cognate receptor VEGFR-3 increased. The responses to VEGF-A induction in the perinatal period differ from those during early lung development when endothelial migration, but not proliferation altered initial vascular patterning (Akeson, A.L., Greenberg, J.M., Cameron, J.E., Thompson, F.Y., Brooks, S.K., Wiginton, D., Whitsett, J.A., 2003. Temporal and spatial regulation of VEGF-A controls vascular patterning in the embryonic lung. Dev. Biol. 264, 443-455). The late-stage response resembles that of adult lung to VEGF-inducing stimuli including injury and disease. These data suggest that VEGF-A influences the balance between development of blood and lymphatic vasculature during lung organogenesis.

Mesenchymal expression of vascular endothelial growth factors D and A defines vascular patterning in developing lung.

The lung has specific vascular patterning requirements for effective gas exchange at birth, including alignment of airways and blood vessels and lymphatic vessels. Vascular endothelial growth factors (VEGF) are potent effectors of vascular development. We examined the temporal and spatial expression of VEGF-D and specific VEGF-A isoforms at each stage of lung development. VEGF-D, expressed only by cadherin-11-positive cells of the mesenchyme, is first detected at embryonic day (E) 13.5, a period of active vasculogenesis. VEGFR-3, its cognate receptor, is detected earlier on days E11.5 to E14.5, in both blood vessels and lymphatic vessels and later, on day E17.5, in only lymphatic vessels. VEGF-A is expressed in the mesenchyme throughout lung development and also by the epithelium midway through organogenesis. Before E14, the predominant forms of VEGF-A are the soluble isoforms, VEGF-A120 and 164. Not until E14.5 do epithelial cells at the tips of expanding airways express VEGF-A, including VEGF-A188, an isoform with high affinity for extracellular matrix. Our results demonstrate unique temporal and spatial expression of VEGF-D and specific VEGF-A isoforms during lung development and suggest these related factors have distinct functions in vascular and lymphatic patterning of the lung.