Srinivas S, Wu Z, Chen C M, D'Agati V, Costantini F
Department of Genetics and Development and Department of Pathology, College of Physicians and Surgeons, Columbia University, New York, NY 10032, USA.
Development. 1999 Apr;126(7):1375-86. doi: 10.1242/dev.126.7.1375.
During kidney development, factors from the metanephric mesenchyme induce the growth and repeated branching of the ureteric bud, which gives rise to the collecting duct system and also induces nephrogenesis. One signaling pathway known to be required for this process includes the receptor tyrosine kinase RET and co-receptor GFR(&agr;)-1, which are expressed in the ureteric bud, and the secreted ligand GDNF produced in the mesenchyme. To examine the role of RET signaling in ureteric bud morphogenesis, we produced transgenic mice in which the pattern of RET expression was altered, or in which a ligand-independent form of RET kinase was expressed. The Hoxb7 promoter was used to express RET throughout the ureteric bud branches, in contrast to its normal expression only at the bud tips. This caused a variable inhibition of ureteric bud growth and branching reminiscent of, but less severe than, the RET knockout phenotype. Manipulation of the level of GDNF, in vitro or in vivo, suggested that this defect was due to insufficient rather than excessive RET signaling. We propose that RET receptors expressed ectopically on ureteric bud trunk cells sequester GDNF, reducing its availability to the normal target cells at the bud tips. When crossed to RET knockout mice, the Hoxb7/RET transgene, which encoded the RET9 isoform, supported normal kidney development in some RET-/- animals, indicating that the other major isoform, RET51, is not required in this organ. Expression of a Hoxb7/RET-PTC2 transgene, encoding a ligand-independent form of RET kinase, caused the development of abnormal nodules, outside the kidney or at its periphery, containing branched epithelial tubules apparently formed by deregulated growth of the ureteric bud. This suggests that RET signaling is not only necessary but is sufficient to induce ureteric bud growth, and that the orderly, centripetal growth of the bud tips is controlled by the spatially and temporally regulated expression of GDNF and RET.
在肾脏发育过程中，后肾间充质中的因子诱导输尿管芽的生长和反复分支，输尿管芽形成集合管系统并诱导肾发生。已知该过程所需的一条信号通路包括在输尿管芽中表达的受体酪氨酸激酶RET和共受体GFR（α）-1，以及间充质中产生的分泌配体GDNF。为了研究RET信号在输尿管芽形态发生中的作用，我们制备了转基因小鼠，其中RET表达模式发生改变，或者表达一种不依赖配体的RET激酶形式。与正常仅在芽尖表达不同，Hoxb7启动子用于在整个输尿管芽分支中表达RET。这导致输尿管芽生长和分支受到可变抑制，类似于但比RET基因敲除表型轻。体内外对GDNF水平的操纵表明，这种缺陷是由于RET信号不足而非过度所致。我们提出，在输尿管芽主干细胞上异位表达的RET受体会隔离GDNF，降低其对芽尖正常靶细胞的可用性。当与RET基因敲除小鼠杂交时，编码RET9异构体的Hoxb7/RET转基因在一些RET-/-动物中支持正常的肾脏发育，表明另一种主要异构体RET51在该器官中不是必需的。编码不依赖配体的RET激酶形式的Hoxb7/RET-PTC2转基因的表达导致在肾脏外或其周边出现异常结节，其中含有明显由输尿管芽失控生长形成的分支上皮小管。这表明RET信号不仅是诱导输尿管芽生长所必需的，而且是充分的，并且芽尖有序的向心生长受GDNF和RET在空间和时间上的调控表达控制。
