Department of Anatomy and Neuroscience, University of Melbourne, Melbourne, Victoria, Australia; Murdoch Children's Research Institute, Parkville, Victoria, Australia.
Murdoch Children's Research Institute, Parkville, Victoria, Australia.
Kidney Int. 2018 Mar;93(3):589-598. doi: 10.1016/j.kint.2017.09.015. Epub 2017 Dec 6.
The regulation of final nephron number in the kidney is poorly understood. Cessation of nephron formation occurs when the self-renewing nephron progenitor population commits to differentiation. Transcription factors within this progenitor population, such as SIX2, are assumed to control expression of genes promoting self-renewal such that homozygous Six2 deletion results in premature commitment and an early halt to kidney development. In contrast, Six2 heterozygotes were assumed to be unaffected. Using quantitative morphometry, we found a paradoxical 18% increase in ureteric branching and final nephron number in Six2 heterozygotes, despite evidence for reduced levels of SIX2 protein and transcript. This was accompanied by a clear shift in nephron progenitor identity with a distinct subset of downregulated progenitor genes such as Cited1 and Meox1 while other genes were unaffected. The net result was an increase in nephron progenitor proliferation, as assessed by elevated EdU (5-ethynyl-2'-deoxyuridine) labeling, an increase in MYC protein, and transcriptional upregulation of MYC target genes. Heterozygosity for Six2 on an Fgf20-/- background resulted in premature differentiation of the progenitor population, confirming that progenitor regulation is compromised in Six2 heterozygotes. Overall, our studies reveal a unique dose response of nephron progenitors to the level of SIX2 protein in which the role of SIX2 in progenitor proliferation versus self-renewal is separable.
肾脏中终末肾单位数量的调节机制尚未完全阐明。当自我更新的肾单位祖细胞群体开始分化时，肾单位的形成就会停止。这个祖细胞群体中的转录因子，如 SIX2，被认为可以控制促进自我更新的基因的表达，因此 SIX2 纯合缺失会导致过早的分化，并导致肾脏发育早期停止。相比之下，SIX2 杂合子被认为不受影响。通过定量形态计量学方法，我们发现 SIX2 杂合子的输尿管分支和终末肾单位数量增加了 18%，尽管 SIX2 蛋白和转录本水平降低。这伴随着肾单位祖细胞特征的明显转变，下调的祖细胞基因（如 Cited1 和 Meox1）明显减少，而其他基因不受影响。其净结果是肾单位祖细胞增殖增加，这可以通过 EdU（5-ethynyl-2'-deoxyuridine）标记的升高、MYC 蛋白的增加以及 MYC 靶基因的转录上调来评估。在 Fgf20-/-背景下，Six2 的杂合性导致祖细胞群体的过早分化，这证实了 Six2 杂合子中祖细胞的调节受到了损害。总的来说，我们的研究揭示了肾单位祖细胞对 SIX2 蛋白水平的独特剂量反应，其中 SIX2 在祖细胞增殖与自我更新中的作用是可分离的。
