Missense Mutation of POU Domain Class 3 Transcription Factor 3 in Pou3f3L423P Mice Causes Reduced Nephron Number and Impaired Development of the Thick Ascending Limb of the Loop of Henle

During nephrogenesis, POU domain class 3 transcription factor 3 (POU3F3 aka BRN1) is critically involved in development of distinct nephron segments, including the thick ascending limb of the loop of Henle (TAL). Deficiency of POU3F3 in knock-out mice leads to underdevelopment of the TAL, lack of differentiation of TAL cells, and perinatal death due to renal failure. Pou3f3L423P mutant mice, which were established in the Munich ENU Mouse Mutagenesis Project, carry a recessive point mutation in the homeobox domain of POU3F3. Homozygous Pou3f3L423P mutants are viable and fertile. The present study used functional, as well as qualitative and quantitative morphological analyses to characterize the renal phenotype of juvenile (12 days) and aged (60 weeks) homo- and heterozygous Pou3f3L423P mutant mice and age-matched wild-type controls. In both age groups, homozygous mutants vs. control mice displayed significantly smaller kidney volumes, decreased nephron numbers and mean glomerular volumes, smaller TAL volumes, as well as lower volume densities of the TAL in the kidney. No histological or ultrastructural lesions of TAL cells or glomerular cells were observed in homozygous mutant mice. Aged homozygous mutants displayed increased serum urea concentrations and reduced specific urine gravity, but no evidence of glomerular dysfunction. These results confirm the role of POU3F3 in development and function of the TAL and provide new evidence for its involvement in regulation of the nephron number in the kidney. Therefore, Pou3f3L423P mutant mice represent a valuable research model for further analyses of POU3F3 functions, or for nephrological studies examining the role of congenital low nephron numbers.在肾发生（nephrogenesis）过程中，POU结构域3类转录因子3（POU3F3，又名BRN1）在包括亨利袢粗升支（TAL）在内的特定肾单位节段的发育中发挥关键调控作用。基因敲除小鼠体内POU3F3的缺失会导致TAL发育不全、TAL细胞分化缺失，并因肾衰竭引发围产期死亡。由慕尼黑ENU小鼠诱变项目构建的Pou3f3L423P突变小鼠，在POU3F3的同源框结构域携带隐性点突变。纯合子Pou3f3L423P突变小鼠可正常存活且具有生育能力。本研究通过功能学分析，以及定性与定量形态学分析，对幼年（12日龄）与老年（60周龄）的纯合、杂合Pou3f3L423P突变小鼠，以及同年龄野生型对照小鼠的肾脏表型进行了系统表征。在两个年龄组中，与对照小鼠相比，纯合突变小鼠的肾脏体积显著更小，肾单位数量、平均肾小球体积均降低，TAL体积及肾脏内TAL的体积密度也显著下降。纯合突变小鼠未观察到TAL细胞或肾小球细胞的组织学及超微结构损伤。老年纯合突变小鼠的血清尿素浓度升高、尿比重降低，但未出现肾小球功能障碍的相关证据。上述结果证实了POU3F3在TAL发育与功能维持中的核心作用，并为其参与调控肾脏肾单位数量提供了新的实验证据。因此，Pou3f3L423P突变小鼠可作为开展POU3F3功能相关研究，或是探究先天性低肾单位数量在肾脏病学中作用的优质研究模型。