Hypertonicity and TonEBP promote development of the renal concentrating system.

DESPITE SEVERE HYPEROSMOTIC stress imposed by NaCl and urea on cells of the renal medulla, the elevated and highly variable osmolality in this part of the kidney is necessary for proper function of the urinary concentrating mechanism. This mechanism develops only after birth, and newborn rats are incapable of producing concentrated urine (13). Depending on hydration state, renal medullary osmolality of adult rats ranges from 500 to 2,500 mosmol/kgH2O as a result of elevated salt and urea concentrations. Although increases in salt and urea both lead to hyperosmolality, only increased salt concentration causes hypertonicity (leading to cell shrinkage). The reason for this difference between salt and urea is the high membrane permeability of urea, which is mostly comparable to that of water. Consequently, urea and salt have different effects on kidney cells. In this issue of AJP-Renal Physiology, Han and coworkers (6) show that hypertonicity in the renal medulla represents an important signal for medullary differentiation and development of the urinary concentrating mechanism. Over the years, remarkable progress has been made in our understanding of how kidney cells adapt to the stress of hypertonicity. Cellular accumulation of organic osmolytes such as inositol, betaine, and sorbitol represents a central feature of adaptation to hypertonic stress. Organic osmolytes are compatible with cell function and protect renal cells from deleterious effects of hypertonicity by lowering intracellular ionic strength (2). The transcription factor tonicity-responsive enhancer binding protein (TonEBP) plays a key role in the accumulation of organic osmolytes by stimulating gene expression of membrane transporters (sodium-inositol cotransporter and sodium-chloride-betaine cotransporter) and a biosynthetic enzyme (aldose reductase) that catalyzes production of sorbitol from glucose (14). Interestingly, TonEBP also stimulates transcription of genes encoding vasopressin-activated urea transporters (UT-A1 and UT-A3) in inner medullary collecting duct cells, suggesting that TonEBP contributes to urea accumulation in renal medullary interstitium (12). In addition, TonEBP stimulates expression of the heat shock protein HSP70, which in turn protects proteins from the denaturing effects of urea (15). These wide-ranging transcriptional targets delineate TonEBP as a critical element of osmosensory signal transduction in cells of the renal medulla. The article by Han et al. (6) provides insight into how TonEBP contributes to development of urinary concentrating ability by analysis of expression of TonEBP and its target genes in developing rat kidney. TonEBP expression is first detected in the renal medulla at the fetal age of 16 days. It increases slowly through birth until postnatal day 21, when the renal medulla is fully developed and maximal urinary concentrating ability is achieved. Han et al. observe that, in general, expression of TonEBP in the kidney precedes that of its target genes, such as the sodium-inositol cotransporter, aldose reductase, and the vasopressin-regulated urea transporter. These findings are in agreement with earlier reports on developmental regulation of aldose reductase, the sodium-inositol cotransporter, and the vasopressin-regulated urea transporter. Aldose reductase mRNA is present at very low levels at the time of birth but increases greatly within the first 2–3 wk after birth, consistent with its dependence on TonEBP (1, 7). As a consequence of activation of aldose reductase and organic osmolyte transporters, the levels of organic osmolytes increase greatly 3 wk after birth in rat renal medulla (11). By this time, salt and urea levels have also increased substantially in the renal medulla and maximal urinary concentrating ability is achieved (11). The timing of TonEBP expression also supports a role of this transcription factor for inducing UT-A expression during mouse development. UT-A immunoreactivity is absent from fetal rat kidneys but detectable in the inner medulla of 1-dayold pups. Two to three weeks after birth, UT-A immunostaining is most intense in descending thin limbs of loops of Henle at the border between the inner and outer medulla (9). This observation is of particular importance because it suggests that hypertonicity and TonEBP might be prerequisites for accumulation of urea in renal medullary interstitium and, therefore, be instrumental for establishment of urinary concentrating ability in the developing kidney. Han et al. (6) speculate that TonEBP is stimulated by local hypertonicity in the fetal kidney resulting from Na-K-2Cl cotransporter (NKCC2) activity, which is already detectable before birth at the fetal age of 15 days. This study presents the first experimental evidence, albeit indirect, that hypertonicity (hyperosmotic salt) drives the accumulation of high concentrations of urea in the kidney inner medulla as a result of a sequential chain of events including activation of NKCC2 cotransporters and the renal countercurrent mechanism for salt deposition in the medulla, generation of hypertonicity, activation of TonEBP, and activation of UT-A, resulting in urea deposition in the medulla. Functional knockout experiments demonstrate that UT-A1 and UT-A3 are responsible for a major portion of urine osmolality under antidiuretic conditions (5). Interestingly, a study by Kobayashi et al. (10) found that NKCC2 expression is very strong throughout the inner medulla in neonatal rat kidneys but entirely undetectable in adult rats, suggesting that this cotransporter plays a prominent role in the establishment of countercurrent multiplication, interstitial hypertonicity, and urinary concentrating ability during kidney development. In addition to its putative role as a signal for establishing high concentrations of urea in the medullary interstitium and high intracellular concentrations of organic osmolytes, hypertonicity may also be an important signal for morphological differentiation of the renal inner medulla and for generating Address for reprint requests and other correspondence: D. Kültz, Physiological Genomics Grp., Dept. of Animal Science, Meyer Hall, Univ. of California, 1 Shields Ave., Davis, CA 95616 (E-mail: [email protected]). Am J Physiol Renal Physiol 287: F876–F877, 2004; doi:10.1152/ajprenal.00272.2004.