Role of Prox1 in the Transforming Ascending Thin Limb of Henle's Loop during Mouse Kidney Development

The homeobox transcription factor Prox1 is critical to the development of many embryonic organs and tissues, although current understanding of its expression in the developing renal medulla is limited. We examined the functional role of Prox1 during mouse kidney development with particular emphasis on the developing loop of Henle. Our data show that Prox1 is expressed in the transdifferentiating region from the NKCC2-positive thick ascending limb, into the CLC-K1-positive ascending thin limb of Henle’s loop beginning at embryonic day 18. From 1 to 14 days of age, Prox1-positive cells gradually disappeared from the papillary tip, and remained in the initial part of inner medulla after 21 days. In this transforming area, no Prox1 was observed in cells undergoing apoptosis but was expressed strongly in the remaining cells, which differentiated into ascending thin limb epithelial cells. In vitro and in vivo approaches showed that Prox1 expression increases where the osmolality is near optimal range, but decreases at below- or above-optimal ranges. Renal hypoosmolality induced by furosemide (NKCC2 inhibitor) inhibited Prox1 expression and delayed maturation of the ascending limb of Henle’s loop. Together, these studies suggest that Prox1 appears to be a critical stage specific regulator of specifying ascending thin limb cell fate and that its expression is regulated by osmolality.

同源框转录因子Prox1（homeobox transcription factor Prox1）对多种胚胎器官及组织的发育至关重要，但目前学界对其在发育中肾髓质内的表达认知仍较为有限。本研究围绕小鼠肾脏发育过程中Prox1的功能角色展开探究，重点聚焦于发育中的亨利袢（loop of Henle）。实验数据显示，自胚胎第18天（embryonic day 18）起，Prox1在由NKCC2阳性髓袢升支粗段（NKCC2-positive thick ascending limb）向CLC-K1阳性亨利袢升支细段（CLC-K1-positive ascending thin limb of Henle’s loop）转分化的区域中表达。在出生后1至14天期间，Prox1阳性细胞逐渐从肾乳头顶端消失；至出生21天后，仅在内髓质的初始区域仍可检测到该阳性细胞。在该转分化区域内，发生凋亡（apoptosis）的细胞不表达Prox1，而剩余最终分化为升支细段上皮细胞的细胞则呈Prox1强表达。体内外实验均证实，当渗透压（osmolality）处于最优范围附近时，Prox1的表达会上调；而当渗透压低于或高于最优区间时，Prox1的表达则会下调。由呋塞米（furosemide，NKCC2抑制剂）诱导的肾内低渗透压，可抑制Prox1的表达并延缓亨利袢升支的成熟进程。综上，本研究提示Prox1或是调控亨利袢升支细段细胞命运（cell fate）的关键阶段特异性调控因子，且其表达受渗透压的调控。