Developmental Biology and Regenerative Medicine Program, Saban Research Institute, Children's Hospital Los Angeles, Keck School of Medicine of University of Southern California, 4650 Sunset Boulevard MS35, Los Angeles, CA 90027, USA.
Dev Biol. 2011 May 15;353(2):242-58. doi: 10.1016/j.ydbio.2011.02.031. Epub 2011 Mar 6.
Six1 is a member of the six-homeodomain family of transcription factors. Six1 is expressed in multiple embryonic cell types and plays important roles in proliferation, differentiation and survival of precursor cells of different organs, yet its function during lung development was hitherto unknown. Herein we show that Six1(-/-) lungs are severely hypoplastic with greatly reduced epithelial branching and increased mesenchymal cellularity. Six1 is expressed at the distal epithelial tips of branching tubules as well as in the surrounding distal mesenchyme. Six1(-/-) lung epithelial cells show increased expression of differentiation markers, but loss of progenitor cell markers. Six1 overexpression in MLE15 lung epithelial cells in vitro inhibited cell differentiation, but increases the expression of progenitor cell markers. In addition, Six1(-/-) embryos and newborn mice exhibit mesenchymal overproliferation, decreased Fgf10 expression and severe defects in the smooth muscle component of the bronchi and major pulmonary vessels. These defects lead to rupture of major vessels in mutant lungs after birth. Treatment of Six1(-/-) epithelial explants in culture with recombinant Fgf10 protein restores epithelial branching. As Shh expression is abnormally increased in Six1(-/-) lungs, we also treated mutant mesenchymal explants with recombinant Shh protein and found that these explants were competent to respond to Shh and continued to grow in culture. Furthermore, inhibition of Shh signaling with cyclopamine stimulated Six1(-/-) lungs to grow and branch in culture. This study provides the first evidence for the requirement of Six1 in coordinating Shh-Fgf10 signaling in embryonic lung to ensure proper levels of proliferation and differentiation along the proximodistal axis of epithelial, mesenchymal and endothelial cells. These findings uncover novel and essential functions for Six1 as a critical coordinator of Shh-Fgf10 signaling during embryonic lung development. We propose that Six1 is hence critical for coordination of proper lung epithelial, mesenchymal and vascular development.
Six1 是转录因子六类同源结构域家族的成员。Six1 在多种胚胎细胞类型中表达，在不同器官前体细胞的增殖、分化和存活中发挥重要作用，但它在肺发育过程中的功能尚不清楚。在此，我们发现 Six1(-/-) 肺严重发育不良，上皮分支减少，间质细胞增多。Six1 在分支小管的远端上皮尖端以及周围的远端间质中表达。Six1(-/-) 肺上皮细胞中分化标志物的表达增加，但祖细胞标志物的表达减少。体外在 MLE15 肺上皮细胞中过表达 Six1 抑制细胞分化，但增加祖细胞标志物的表达。此外，Six1(-/-) 胚胎和新生小鼠表现出间质过度增殖、Fgf10 表达减少以及支气管和平滑肌成分严重缺陷和主要肺血管。这些缺陷导致突变肺中的主要血管在出生后破裂。在培养中用重组 Fgf10 蛋白处理 Six1(-/-) 上皮外植体可恢复上皮分支。由于 Six1(-/-) 肺中的 Shh 表达异常增加，我们还在用重组 Shh 蛋白处理突变的间充质外植体，发现这些外植体能够对 Shh 做出反应，并在培养中继续生长。此外，用环巴胺抑制 Shh 信号可刺激 Six1(-/-) 肺在培养中生长和分支。本研究首次证明了 Six1 在协调胚胎肺中 Shh-Fgf10 信号以确保上皮、间充质和内皮细胞沿近-远轴的适当增殖和分化水平方面的必要性。这些发现揭示了 Six1 在胚胎肺发育过程中作为 Shh-Fgf10 信号的关键协调因子的新的和重要的功能。我们提出，Six1 对于协调肺上皮、间充质和血管的正常发育至关重要。
