"Avian-type" renal medullary tubule organization causes immaturity of urine-concentrating ability in neonates

Phylogeny and ontogeny of the renin-angiotensin system: Current view and perspectives

The renin-angiotensin system (RAS) evolved early among vertebrates and remains functioning throughout the vertebrate phylogeny and has adapted to various environments. The RAS is crucial for the regulation of blood pressure, fluid-electrolyte balance and tissue homeostasis. The RAS is also expressed during early ontogeny in renal and extra-renal tissues, and exerts unique vascular growth and differentiation functions. In this brief review, we describe advances from molecular-genetic and whole animal approaches and discuss similarities and unique aspects of the RAS in the context of embryonic development and vertebrates' phylogeny.

The role of prostaglandin E2 receptor EP1 in 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced neonatal hydronephrosis in mice

Prostaglandin E₂ (PGE₂) is a critical factor in the pathogenesis of dioxin-induced neonatal hydronephrosis. Since the PGE₂ receptor has four subtypes, EP1 - EP4, this study was aimed to challenge the hypothesis that at least one of the four subtypes is responsible for the pathogenesis of dioxin-induced hydronephrosis. To this end, we used mouse pups, with a C57BL/6 J background, genetically lacking EP1, EP2, or EP3, and wild-type pups in whom EP4 was suppressed by administering ONO-AE3-208 (ONO), an EP4 antagonist, from postnatal day 1 (PND 1) to PND 13. To expose the pups to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) via lactation, the dams were administered TCDD at an oral dose of 20 μg/kg on PND 1. The pups' urine and kidneys were collected on PND 14 for urinalysis and histological examination, respectively. We found that the incidence of hydronephrosis was 80% in the EP1^+/+ group, but was markedly reduced to 28.6% in the EP1^-/- group despite the fact that PGE₂ concentration in the urine was similarly increased in the both groups. In contrast, the incidence of hydronephrosis was 80% and 100% in the EP2^+/+ and EP2^-/-groups, respectively, and 88.9% and 100% in the EP3^+/+ and EP3^-/- groups, respectively. With regard to EP4, the incidence of hydronephrosis in vehicle (saline)-treated groups and ONO-treated was 88.9% and 100%, respectively. Therefore, we concluded that among PGE₂ receptor subtypes, EP1 plays a predominant role in the onset of TCDD-induced neonatal hydronephrosis in mouse pups.

Polyuria-associated hydronephrosis induced by xenobiotic chemical exposure in mice

Hydronephrosis is a commonly found disease state characterized by the dilation of renal calices and pelvis, resulting in the loss of kidney function in the severest cases. A generally accepted etiology of hydronephrosis involves the obstruction of urine flow along the urinary tract. In the recent years, we have developed a mouse model of hydronephrosis induced by lactational exposure to dioxin and demonstrated a lack of anatomical obstruction in this model. We also showed that prostaglandin E2 synthesis system plays a critical role in the onset of hydronephrosis. In the present study, we found that neonatal hydronephrosis was not likely to be associated with functional obstruction (impaired peristalsis) but was found to be associated with polyuria and low urine osmolality with the downregulation of proteins involved in the urine concentrating process. The administration of an antidiuretic hormone analog to the dioxin-exposed pups not only suppressed the increased urine output but also decreased the incidence and severity of hydronephrosis. In contrast to the case in pups, administration of dioxin to adult mice failed to induce polyuria and upregulation of prostaglandin E2 synthesis system, and the adult mice were resistant to develop hydronephrosis. These findings suggest the possibility that polyuria could induce hydronephrosis in the absence of anatomical or functional obstruction of the ureter. It is concluded that the present animal model provides a unique example of polyuria-associated type of hydronephrosis, suggesting a need to redefine this disease state.

Glomerular protein separation as a mechanism for powering renal concentrating processes

Various models have been proposed to explain the urine concentrating mechanism in mammals, however uncertainty remains regarding the origin of the energy required for the production of concentrated urine. We propose a novel mechanism for concentrating urine. We postulate that the energy for the concentrating process is derived from the osmotic potentials generated by the separation of afferent blood into protein-rich efferent blood and protein-deplete filtrate. These two streams run in mutual juxtaposition along the length of the nephron and are thus suitably arranged to provide the osmotic potential to concentrate the urine. The proposed model is able to qualitatively explain the production of various urine concentrations under different clinical conditions. An approach to testing the feasibility of the hypothesis is proposed.

Organ-specific development characterizes circadian clock gene Per2 expression in rats

To explore developmental changes in circadian organization of central and peripheral oscillators, circadian rhythms in clock gene expression were examined in 12 organs in transgenic rats carrying a bioluminescence reporter for Per2. Organ slices were obtained from different developmental stages starting at postnatal day 5 and tissue was cultured for more than 6 days. In addition, four organs were examined from embryonic day 20. Robust circadian rhythms in bioluminescence were detected in all organs examined. The circadian period in vitro was specific to each organ and remained essentially the same during development. The circadian peak phase on the first day of culture was significantly different not only among organs but also in the same organ. Three patterns in circadian phase were detected during development. Thus, during development, circadian phase did not change in the suprachiasmatic nucleus, adrenal gland, and liver, whereas delay shifts were seen in the pineal, lung, heart, kidney, spleen, thymus, and testis. Finally, circadian phase advanced at postnatal day 10–15 and subsequently delayed in skeletal muscle and stomach.Circadian amplitude also showed developmental changes in several organs. These findings indicate that the temporal orders of physiological functions of various organs change during development. Such age-dependent and organ-specific changes in the phase relationship among circadian clocks most likely reflect entrainment to organ-specific time cues at different developmental stages.

Structure and function of the thin limbs of the loop of Henle

The thin limbs of the loop of Henle, which comprise the intermediate segment, connect the proximal tubule to the distal tubule and lie entirely within the renal medulla. The descending thin limb consists of at least two or three morphologically and functionally distinct subsegments and participates in transepithelial transport of NaCl, urea, and water. Only one functionally distinct segment is recognized for the ascending thin limb, which carries out transepithelial transport of NaCl and urea in the reabsorptive and/or secretory directions. Membrane transporters involved with passive transcellular Cl, urea, and water fluxes have been characterized for thin limbs; however, these pathways do not account for all transepithelial fluid and solute fluxes that have been measured in vivo. The paracellular pathway has been proposed to play an important role in transepithelial Na and urea fluxes in defined thin-limb subsegments. As the transport pathways become clearer, the overall function of the thin limbs is becoming better understood. Primary and secondary signaling pathways and protein-protein interactions are increasingly recognized as important modulators of thin-limb cell function and cell metabolism. These functions must be investigated under diverse extracellular conditions, particularly for those cells of the deep inner medulla that function in an environment of wide variation in hyperosmolality. Transgenic mouse models of several key water and solute transport proteins have provided significant insights into thin-limb function. An understanding of the overall architecture of the medulla, including juxtapositions of thin limbs with collecting ducts, thick ascending limbs, and vasa recta, is essential for understanding the role of the kidney in maintaining Na and water homeostasis, and for understanding the urine concentrating mechanism.

Aquaporins in avian kidneys: function and perspectives

For terrestrial vertebrates, water economy is a prerequisite for survival, and the kidney is their major osmoregulatory organ. Birds are the only vertebrates other than mammals that can concentrate urine in adaptation to terrestrial environments. Aquaporin (AQP) and glyceroporin (GLP) are phylogenetically old molecules and have been found in plants, microbial organisms, invertebrates, and vertebrates. Currently, 13 AQPs/aquaGLPs and isoforms are known to be present in mammals. AQPs 1, 2, 3, 4, 6, 7, 8, and 11 are expressed in the kidney; of these, AQPs 1, 2, 3, 4, and 7 are shown to be involved in fluid homeostasis. In avian kidneys, AQPs 1, 2, 3, and 4 have been identified and characterized. Also, gene and/or amino acid sequences of AQP5, AQP7, AQP8, AQP9, AQP11, and AQP12 have been reported in birds. AQPs 2 and 3 are expressed along cortical and medullary collecting ducts (CDs) and are responsible, respectively, for the water inflow and outflow of CD epithelial cells. While AQP4 plays an important role in water exit in the CD of mammalian kidneys, it is unlikely to participate in water outflow in avian CDs. This review summarizes current knowledge on structure and function of avian AQPs and compares them to those in mammalian and nonmammalian vertebrates. Also, we aim to provide input into, and perspectives on, the role of renal AQPs in body water homeostasis during ontogenic and phylogenetic advancement.

Comparative physiology and architecture associated with the mammalian urine concentrating mechanism: role of inner medullary water and urea transport pathways in the rodent medulla

Comparative studies of renal structure and function have potential to provide insights into the urine-concentrating mechanism of the mammalian kidney. This review focuses on the tubular transport pathways for water and urea that play key roles in fluid and solute movements between various compartments of the rodent renal inner medulla. Information on aquaporin water channel and urea transporter expression has increased our understanding of functional segmentation of medullary thin limbs of Henle's loops, collecting ducts, and vasa recta. A more complete understanding of membrane transporters and medullary architecture has identified new and potentially significant interactions between these structures and the interstitium. These interactions are now being introduced into our concept of how the inner medullary urine-concentrating mechanism works. A variety of regulatory pathways lead directly or indirectly to variable patterns of fluid and solute movements among the interstitial and tissue compartments. Animals with the ability to produce highly concentrated urine, such as desert species, are considered to exemplify tubular structure and function that optimize urine concentration. These species may provide unique insights into the urine-concentrating process.(1)

Renal progenitors: an evolutionary conserved strategy for kidney regeneration

Following kidney injury, repair can result in functional tissue becoming a patch of cells and disorganized extracellular matrix--a scar--or it can recapitulate the original tissue architecture through the process of regeneration. Regeneration can potentially occur in all animal species and humans. Indeed, the repair of portions of the existing nephron after tubular damage, a response that has been designated classically as cellular regeneration, is conserved in all animal species from the ancestral phases of evolution. By contrast, another type of regenerative response--nephron neogenesis--has been described in lower branches of the animal kingdom, but does not occur in adult mammals. Converging evidence suggests that a renal progenitor system is present in the adult kidney across different stages of evolution, with renal progenitors having been identified as the main drivers of kidney regenerative responses in fish, insects, rodents and humans. In this Review, we describe similarities and differences between the renal progenitor systems through evolution, and propose explanations for how progressive kidney adaptation to environmental changes both required and permitted neonephrogenesis to be given up and for cellular regeneration to be retained as the main regenerative strategy. Understanding the mechanisms that drive renal progenitor growth and differentiation represent the key step for modulating this potential for therapeutic purposes.

A novel long-range enhancer regulates postnatal expression of Zeb2: implications for Mowat-Wilson syndrome phenotypes

The zinc-finger, E-box-binding homeobox-2 (Zeb2) gene encodes a SMAD-interacting transcription factor that has diverse roles in development and disease. Mutations at the hZeb2 locus cause Mowat-Wilson syndrome (MWS), a genetic disorder that is associated with mental retardation and other, case- and sex-dependent clinical features. Recent studies have detailed microRNA-mediated control of Zeb2, but little is known about the genomic context of this gene or of enhancer sequences that may direct its diverse functions. Here, we describe a novel transgenic rodent model in which Zeb2 regulatory sequence has been disrupted, resulting in a postnatal developmental phenotype that is autosomal dominant. The phenotype exhibits a genotype-by-sex interaction and manifests primarily as an acute attenuation of postnatal kidney development in males. Other aspects of embryonic and neonatal development, including neuronal, are unaffected. The transgene insertion site is associated with a 12 kb deletion, 1.2 Mb upstream of Zeb2, within a 4.1 Mb gene desert. A conserved sequence, derived from the deleted region, enhanced Zeb2 promoter activity in transcription assays. Tissue and temporal restriction of this enhancer activity may involve postnatal changes in proteins that bind this sequence. A control human/mouse VISTA enhancer (62 kb upstream of Zeb2) also up-regulated the Zeb2 promoter, providing evidence of a string of conserved distal enhancers. The phenotype arising from deletion of one copy of the extreme long-range enhancer indicates a critical role for this enhancer at one developmental stage. Haploinsufficiency of Zeb2 in this developmental context reflects inheritance of MWS and may underlie some sex-dependent, non-neural characteristics of this human inherited disorder.

Ontogeny of the renal urotensin II system in the rat

Urotensin II (UII), a peptide hormone which influences glomerular filtration rate and urine concentration, and its receptor, UT, are expressed in the adult rat kidney. The ability of the kidney to reabsorb sodium and water starts to develop in utero and matures during early postnatal life in the rat, yet little is known about the ontogeny of the renal UII system. This study mapped renal expression of the urotensin system during the fetal and postnatal periods and determined renal activity of UII in the immature rat. Urotensin II peptide and mRNA were present in Sprague-Dawley (SD) rat metanephroi from the earliest stage examined, embyonic day 19 (E19; rat gestation 22 days); levels increased to peak at 4 weeks of age. In contrast, UT protein and mRNA expression declined rapidly between E19 and birth and remained at a similar level postnatally. Infusion of rat UII [6-60 pmol min(-1) (100 g body weight)(-1)] or rat urotensin-related peptide [6 pmol min(-1) (100 g body weight)(-1)] in anaesthetized 4-week-old SD rats had no influence on measured renal parameters; however, infusion of UT antagonist, SB-706375 (0.01 mg kg(-1) min(-1)), provoked a pronounced diuresis [vehicle 23.5 ± 1.9 versus antagonist 75.3 ± 12.5 μl min(-1) (100 g body weight)(-1); P < 0.001] and natriuresis, accompanied by modest increases in effective renal blood flow and glomerular filtration rate [vehicle 0.4 ± 0.1 versus antagonist 1.1 ± 0.2 ml min(-1) (100 g body weight)(-1); P < 0.0001] and a significant increase in fractional sodium excretion. These results indicate that the endogenous rat UII system may influence renal sodium and water excretion before the onset of full urine concentrating capacity in the SD rat.

Urine concentrating mechanism in the inner medulla of the mammalian kidney: role of three-dimensional architecture

The urine concentrating mechanism in the mammalian renal inner medulla (IM) is not understood, although it is generally considered to involve countercurrent flows in tubules and blood vessels. A possible role for the three-dimensional relationships of these tubules and vessels in the concentrating process is suggested by recent reconstructions from serial sections labelled with antibodies to tubular and vascular proteins and mathematical models based on these studies. The reconstructions revealed that the lower 60% of each descending thin limb (DTL) of Henle's loops lacks water channels (aquaporin-1) and osmotic water permeability and ascending thin limbs (ATLs) begin with a prebend segment of constant length. In the outer zone of the IM (i) clusters of coalescing collecting ducts (CDs) form organizing motif for loops of Henle and vasa recta; (ii) DTLs and descending vasa recta (DVR) are arrayed outside CD clusters, whereas ATLs and ascending vasa recta (AVR) are uniformly distributed inside and outside clusters; (iii) within CD clusters, interstitial nodal spaces are formed by a CD on one side, AVR on two sides, and an ATL on the fourth side. These spaces may function as mixing chambers for urea from CDs and NaCl from ATLs. In the inner zone of the IM, cluster organization disappears and half of Henle's loops have broad lateral bends wrapped around terminal CDs. Mathematical models based on these findings and involving solute mixing in the interstitial spaces can produce urine slightly more concentrated than that of a moderately antidiuretic rat but no higher.

A mathematical model of the urine concentrating mechanism in the rat renal medulla. II. Functional implications of three-dimensional architecture

In a companion study [Layton AT. A mathematical model of the urine concentrating mechanism in the rat renal medulla. I. Formulation and base-case results. Am J Physiol Renal Physiol. (First published November 10, 2010). 10.1152/ajprenal.00203.2010] a region-based mathematical model was formulated for the urine concentrating mechanism in the renal medulla of the rat kidney. In the present study, we investigated model sensitivity to some of the fundamental structural assumptions. An unexpected finding is that the concentrating capability of this region-based model falls short of the capability of models that have radially homogeneous interstitial fluid at each level of only the inner medulla (IM) or of both the outer medulla and IM, but are otherwise analogous to the region-based model. Nonetheless, model results reveal the functional significance of several aspects of tubular segmentation and heterogeneity: 1) the exclusion of ascending thin limbs that reach into the deep IM from the collecting duct clusters in the upper IM promotes urea cycling within the IM; 2) the high urea permeability of the lower IM thin limb segments allows their tubular fluid urea content to equilibrate with the surrounding interstitium; 3) the aquaporin-1-null terminal descending limb segments prevent water entry and maintain the transepithelial NaCl concentration gradient; 4) a higher thick ascending limb Na(+) active transport rate in the inner stripe augments concentrating capability without a corresponding increase in energy expenditure for transport; 5) active Na(+) reabsorption from the collecting duct elevates its tubular fluid urea concentration. Model calculations predict that these aspects of tubular segmentation and heterogeneity promote effective urine concentrating functions.

A mathematical model of the urine concentrating mechanism in the rat renal medulla. I. Formulation and base-case results

A new, region-based mathematical model of the urine concentrating mechanism of the rat renal medulla was used to investigate the significance of transport and structural properties revealed in anatomic studies. The model simulates preferential interactions among tubules and vessels by representing concentric regions that are centered on a vascular bundle in the outer medulla (OM) and on a collecting duct cluster in the inner medulla (IM). Particularly noteworthy features of this model include highly urea-permeable and water-impermeable segments of the long descending limbs and highly urea-permeable ascending thin limbs. Indeed, this is the first detailed mathematical model of the rat urine concentrating mechanism that represents high long-loop urea permeabilities and that produces a substantial axial osmolality gradient in the IM. That axial osmolality gradient is attributable to the increasing urea concentration gradient. The model equations, which are based on conservation of solutes and water and on standard expressions for transmural transport, were solved to steady state. Model simulations predict that the interstitial NaCl and urea concentrations in adjoining regions differ substantially in the OM but not in the IM. In the OM, active NaCl transport from thick ascending limbs, at rates inferred from the physiological literature, resulted in a concentrating effect such that the intratubular fluid osmolality of the collecting duct increases ~2.5 times along the OM. As a result of the separation of urea from NaCl and the subsequent mixing of that urea and NaCl in the interstitium and vasculature of the IM, collecting duct fluid osmolality further increases by a factor of ~1.55 along the IM.

Maximum urine concentrating capability in a mathematical model of the inner medulla of the rat kidney

In a mathematical model of the urine concentrating mechanism of the inner medulla of the rat kidney, a nonlinear optimization technique was used to estimate parameter sets that maximize the urine-to-plasma osmolality ratio (U/P) while maintaining the urine flow rate within a plausible physiologic range. The model, which used a central core formulation, represented loops of Henle turning at all levels of the inner medulla and a composite collecting duct (CD). The parameters varied were: water flow and urea concentration in tubular fluid entering the descending thin limbs and the composite CD at the outer-inner medullary boundary; scaling factors for the number of loops of Henle and CDs as a function of medullary depth; location and increase rate of the urea permeability profile along the CD; and a scaling factor for the maximum rate of NaCl transport from the CD. The optimization algorithm sought to maximize a quantity E that equaled U/P minus a penalty function for insufficient urine flow. Maxima of E were sought by changing parameter values in the direction in parameter space in which E increased. The algorithm attained a maximum E that increased urine osmolality and inner medullary concentrating capability by 37.5% and 80.2%, respectively, above base-case values; the corresponding urine flow rate and the concentrations of NaCl and urea were all within or near reported experimental ranges. Our results predict that urine osmolality is particularly sensitive to three parameters: the urea concentration in tubular fluid entering the CD at the outer-inner medullary boundary, the location and increase rate of the urea permeability profile along the CD, and the rate of decrease of the CD population (and thus of CD surface area) along the cortico-medullary axis.

Severe toxicity and cyclooxygenase (COX)-2 mRNA increase by lithium in the neonatal mouse kidney

Functions of the kidney of mammals are immature during the neonatal period, and the neonatal kidney could be susceptible to chemicals, including drugs and environmental toxicants. Among these chemicals, cyclooxygenase (COX)-inducing chemicals should be given attentions as the potential kidney toxicants during the period, and we hypothesized that lithium chloride (LiCl) has such toxicity. Neonatal mice of C57BL/J strain were intraperitoneally injected with LiCl (2 mmol/kg body weight) daily until 21 days of age, and examined on 7 days and 21 days of age. Neonatal treatment of LiCl caused a significant increase in COX-2 mRNA and a decrease in mRNAs of aquaporins on day 7 of age. Osmolarity of urine from LiCl-treated neonates was significantly lower than that of control neonate. Most of the LiCl-treated neonates died during the second week of age. Histological examination revealed renal cysts on day 7 and hydronephrosis on day 21. in the surviving neonates. The present results showed that the kidney of mouse neonates is vulnerable to lithium, and suggested the possibility that COX-2 upregulation is responsible for the severe renal toxicity including hydronephrosis.

Vasopressin antagonists in polycystic kidney disease

Increased cell proliferation and fluid secretion, probably driven by alterations in intracellular calcium homeostasis and cyclic adenosine 3,5-phosphate, play an important role in the development and progression of polycystic kidney disease. Hormone receptors that affect cyclic adenosine monophosphate and are preferentially expressed in affected tissues are logical treatment targets. There is a sound rationale for considering the arginine vasopressin V2 receptor as a target. The arginine vasopressin V2 receptor antagonists OPC-31260 and tolvaptan inhibit the development of polycystic kidney disease in cpk mice and in three animal orthologs to human autosomal recessive polycystic kidney disease (PCK rat), autosomal dominant polycystic kidney disease (Pkd2/WS25 mice), and nephronophthisis (pcy mouse). PCK rats that are homozygous for an arginine vasopressin mutation and lack circulating vasopressin are markedly protected. Administration of V2 receptor agonist 1-deamino-8-D-arginine vasopressin to these animals completely recovers the cystic phenotype. Administration of 1-deamino-8-D-arginine vasopressin to PCK rats with normal arginine vasopressin aggravates the disease. Suppression of arginine vasopressin release by high water intake is protective. V2 receptor antagonists may have additional beneficial effects on hypertension and chronic kidney disease progression. A number of clinical studies in polycystic kidney disease have been performed or are currently active. The results of phase 2 and phase 2-3 clinical trials suggest that tolvaptan is safe and well tolerated in autosomal dominant polycystic kidney disease. A phase 3, placebo-controlled, double-blind study in 18- to 50-yr-old patients with autosomal dominant polycystic kidney disease and preserved renal function but relatively rapid progression, as indicated by a total kidney volume >750 ml, has been initiated and will determine whether tolvaptan is effective in slowing down the progression of this disease.

Role of three-dimensional architecture in the urine concentrating mechanism of the rat renal inner medulla

Recent studies of three-dimensional architecture of rat renal inner medulla (IM) and expression of membrane proteins associated with fluid and solute transport in nephrons and vasculature have revealed structural and transport properties that likely impact the IM urine concentrating mechanism. These studies have shown that 1) IM descending thin limbs (DTLs) have at least two or three functionally distinct subsegments; 2) most ascending thin limbs (ATLs) and about half the ascending vasa recta (AVR) are arranged among clusters of collecting ducts (CDs), which form the organizing motif through the first 3-3.5 mm of the IM, whereas other ATLs and AVR, along with aquaporin-1-positive DTLs and urea transporter B-positive descending vasa recta (DVR), are external to the CD clusters; 3) ATLs, AVR, CDs, and interstitial cells delimit interstitial microdomains within the CD clusters; and 4) many of the longest loops of Henle form bends that include subsegments that run transversely along CDs that lie in the terminal 500 microm of the papilla tip. Based on a more comprehensive understanding of three-dimensional IM architecture, we distinguish two distinct countercurrent systems in the first 3-3.5 mm of the IM (an intra-CD cluster system and an inter-CD cluster system) and a third countercurrent system in the final 1.5-2 mm. Spatial arrangements of loop of Henle subsegments and multiple countercurrent systems throughout four distinct axial IM zones, as well as our initial mathematical model, are consistent with a solute-separation, solute-mixing mechanism for concentrating urine in the IM.

Urine concentration and avian aquaporin water channels

Although birds and mammals have evolved from primitive tetrapods and advanced divergently, both can conserve water by producing hyperosmotic urine. Unique aspects in the avian system include the presence of loopless and looped nephrons, lack of the thin ascending limb of Henle's loop, a corticomedullary osmotic gradient primarily consisting of NaCl without contribution of urea, and significant postrenal modification of final urine. The countercurrent multiplier mechanism operates between the descending and ascending limbs of Henle via recycling of a single solute (NaCl) with no water accompaniment, forming an osmotic gradient along the medullary cone. Bird kidneys and developing rat kidneys share morphological and functional characteristics. Avian kidneys express aquaporin (AQP) 1, 2, and 4 homologues that share considerable homology with mammalian counterparts, but their distribution and function may not be the same. AQP2 expression in Japanese quail (q) evolves in the collecting duct of early metanephric kidneys and continues to increase in intensity and distribution during nephrogenesis and maturation. qAQP2 mRNA and protein are increased by arginine vasotocin (avian ADH), but vasotocin-induced enhancement of cAMP production and water permeability are less marked than in mammalian kidneys. Nephrogenesis is delayed by insufficient nutrition in avian embryos and newborns and results in fewer nephrons and an impaired water balance in adults. Diabetes insipidus quail with homozygous autosomal recessive mutation and an unaffected vasotocin system have low AQP2 expression, underdeveloped medullary cones. Comparative studies will provide important insight into integrative, cellular, and molecular mechanisms of epithelial water transport and its control by humoral, neural, and hemodynamic mechanisms.

Aquaporin-2 water channel in developing quail kidney: possible role in programming adult fluid homeostasis

Avian kidneys have loopless and looped nephrons; a countercurrent multiplier mechanism operates in the latter by NaCl recycling. We identified an aquaporin-2 (AQP2) homolog in apical/subapical regions of cortical and medullary collecting duct (CD) cells in kidneys of Japanese quail (q), Coturnix japonica. We investigated whether undernutrition during the embryonic/maturation period retards kidney and AQP2 development in quail and programs impaired volume regulation in adults. Protocols included 1) time course and 2) effects of 5-10% egg white withdrawal (EwW) or 48-h post-hatch food deprivation (FD) on nephron growth and qAQP2 mRNA expression, and 3) effects of EwW and FD on qAQP2 mRNA responses to 72-h water deprivation in adults. In metanephric kidneys, qAQP2 mRNA is expressed in medullary CDs at embryonic day 10; distribution and intensity increase during maturation. The number and size of glomeruli continue to increase after birth, whereas nephrogenic zones decrease. In EwW embryos, qAQP2 mRNA expression is initially delayed, then restored; birth weight and hatching rate are lower than in controls. Adults from EwW embryos and FD chicks have fewer (P < 0.01) glomeruli. Water deprivation reduces body weight more in EwW birds than in controls. The results suggest that qAQP2 evolved in metanephric kidneys and that undernutrition may retard nephrogenesis, leading to impaired adult water homeostasis.

Gestational length affects a change in the transepithelial voltage and the rNKCC2 expression pattern in the ascending thin limb of Henle's loop

To examine whether the functional and morphologic conversion of the neonatal ascending thin limb (ATL) of Henle's loop is related to gestational length, we evaluated the transepithelial voltages (Vts) of ATLs in perinatal mouse, hamster, rabbit, and rat kidneys. In isolated microperfused tubule preparations, Vts of neonatal ATLs were 23.8 +/- 1.4 in mouse, 25.7 +/- 2.2 in hamster, and 18.2 +/- 1.6 mV in rabbit. The influence of gestational length on the Vts and rat Na-K-Cl cotransporter (rNKCC2) expression pattern was also examined in perinatal rats subjected to a prolonged gestation due to either a daily s.c. injection of 5 mg progesterone or ligation of the extremities of the uterine horn. Vts of d 3 neonates were 2.9 +/- 1.0 (p < 0.0001 versus d 0); Vts of d 23 fetuses subjected to ligation were 4.9 +/- 0.8 (p < 0.005 versus d 0); and Vts of d 23 fetuses given progesterone were 3.4 +/- 1.7 mV (p < 0.001 versus d 0). rNKCC2 expression tended to disappear in the renal papillae of d 23 fetuses. Our data demonstrate that the perinatal conversion of the ATL is a phenomenon commonly observed among rodents; furthermore, it is dependent on the gestational length, but unrelated to the birth process.

CLC chloride channels and transporters: a biophysical and physiological perspective

Chloride-transporting proteins play fundamental roles in many tissues in the plasma membrane as well as in intracellular membranes. They have received increasing attention in the last years because crucial, and often unexpected and novel, physiological functions have been disclosed with gene-targeting approaches, X-ray crystallography, and biophysical analysis. CLC proteins form a gene family that comprises nine members in mammals, at least four of which are involved in human genetic diseases. The X-ray structure of the bacterial CLC homolog, ClC-ec1, revealed a complex fold and confirmed the anticipated homodimeric double-barreled architecture of CLC-proteins with two separate Cl-ion transport pathways, one in each subunit. Four of the mammalian CLC proteins, ClC-1, ClC-2, ClC-Ka, and ClC-Kb, are chloride ion channels that fulfill their functional roles-stabilization of the membrane potential, transepithelial salt transport, and ion homeostasisin the plasma membrane. The other five CLC proteins are predominantly expressed in intracellular organelles like endosomes and lysosomes, where they are probably important for a proper luminal acidification, in concert with the V-type H+-ATPase. Surprisingly, ClC-4, ClC-5, and probably also ClC-3, are not Cl- ion channels but exhibit significant Cl-/H+ antiporter activity, as does the bacterial homolog ClC-ec1 and the plant homolog AtCLCa. The physiological significance of the Cl-/H+ antiport activity remains to be established.

Phylogenetic, ontogenetic, and pathological aspects of the urine-concentrating mechanism

The urine-concentrating mechanism is one of the most fundamental functions of avian and mammalian kidneys. This particular function of the kidneys developed as a system to accumulate NaCl in birds and as a system to accumulate NaCl and urea in mammals. Based on phylogenetic evidence, the mammalian urine-concentrating mechanism may have evolved as a modification of the renal medulla's NaCl accumulating system that is observed in birds. This qualitative conversion of the urine-concentrating mechanism in the mammalian inner medulla of the kidneys may occur during the neonatal period. Human kidneys have several suboptimal features caused by the neonatal conversion of the urine-concentrating mechanism. The urine-concentrating mechanism is composed of various functional molecules, including water channels, solute transporters, and vasopressin receptors. Abnormalities in water channels aquaporin (AQP)1 and AQP2, as well as in the vasopressin receptor V2R, are known to cause nephrogenic diabetes insipidus. An analysis of the pathological mechanism involved in nephrogenic diabetes insipidus suggests that molecular chaperones may improve the intracellular trafficking of AQP2 and V2R, and, in the near future, such chaperones may become a new clinical tool for treating nephrogenic diabetes insipidus.

An optimization algorithm for a distributed-loop model of an avian urine concentrating mechanism

To better understand how the avian kidney's morphological and transepithelial transport properties affect the urine concentrating mechanism (UCM), an inverse problem was solved for a mathematical model of the quail UCM. In this model, a continuous, monotonically decreasing population distribution of tubes, as a function of medullary length, was used to represent the loops of Henle, which reach to varying levels along the avian medullary cones. A measure of concentrating mechanism efficiency - the ratio of the free-water absorption rate (FWA) to the total NaCl active transport rate (TAT) - was optimized by varying a set of parameters within bounds suggested by physiological experiments. Those parameters include transepithelial transport properties of renal tubules, length of the prebend enlargement of the descending limb (DL), DL and collecting duct (CD) inflows, plasma Na(+) concentration, length of the cortical thick ascending limbs, central core solute diffusivity, and population distribution of loops of Henle and of CDs along the medullary cone. By selecting parameter values that increase urine flow rate (while maintaining a sufficiently high urine-to-plasma osmolality ratio (U/P)) and that reduce TAT, the optimization algorithm identified a set of parameter values that increased efficiency by approximately 60% above base-case efficiency. Thus, higher efficiency can be achieved by increasing urine flow rather than increasing U/P. The algorithm also identified a set of parameters that reduced efficiency by approximately 70% via the production of a urine having near-plasma osmolality at near-base-case TAT. In separate studies, maximum efficiency was evaluated as selected parameters were varied over large ranges. Shorter cones were found to be more efficient than longer ones, and an optimal loop of Henle distribution was found that is consistent with experimental findings.

Two fluid compartments in the renal inner medulla: a view through the keyhole of the concentrating process

Approximately four decades ago, the countercurrent theory became influential in studies on the concentrating process in the mammalian kidney. The theory successfully represented the concentrating process in the outer medulla, but the problem of the concentrating mechanism in the inner medulla, as defined by Homer Smith has remained essentially intractable. In a recent comprehensive review by Knepper and coworkers of various theories and models, attention was refocused on the possible role of hyaluronate (HA) in the inner medullary concentrating process. The authors proposed a hypothesis that HA can convert hydrostatic pressure to concentrating work.Here, we briefly survey the earlier ideas on the role imputed to HA and present a new hypothesis which is different from that of Knepper and coworkers. We estimate that the hydrostatic pressures available in the inner medulla can account only for a very small fraction of the concentrating work. We hypothesize that the role of HA is tied up with extravasated plasma albumin and suggest that owing to the property of HA solutions to exclude other macromolecules, extravasated plasma albumin and HA constitute two fluid compartments in the interstitium in the inner medulla. In this proposed two-compartment model, the Gibbs-Donnan distribution influences the movement of ions and water between the HA and the extravasated albumin compartment. To relate the hypothetical role of HA to the concentrating process, we briefly describe new results obtained by other investigators on the accumulation of urea in the inner medulla. This subject has been critically reviewed recently by Yang & Bankir.Many processes have been identified as contributing to the concentrating process in the mammalian inner medulla. We speculate that among these many processes, the primary responsibility for the final concentration of the excreted urine may be portioned out differently in different mammalian species.

Postnatal expression of KLF12 in the inner medullary collecting ducts of kidney and its trans-activation of UT-A1 urea transporter promoter

Maturation of the inner medulla of the kidney occurs after birth and is vital for mammals to acquire maximal urinary concentrating ability. During this process, expression of several kidney transporters and channels involved in urine concentrating mechanisms is known to be regulated. We previously isolated KLF15 as a transcription factor that regulates the expression of the ClC-K1 chloride channel. We have now found that another KLF transcription factor, KLF12, is expressed in the kidney from around 15 days after birth. To gain insight into its involvement in the maturation process of the inner medulla, we first determined the expression site of KLF12 within the kidney by in situ hybridization. By comparing the AQP2 immunolocalization in sequential sections, KLF12 was found to be expressed in the collecting ducts. Because expression of the urea transporter UT-A1 and amiloride-sensitive epithelial sodium channels ENaC is known to be tightly regulated in the collecting ducts after birth, we tested whether KLF12 has a regulatory role in the promoter activities of these genes. KLF12 is able to increase UT-A1 but not ENaC promoter activity through the binding to CACCC motif. These results suggest that KLF12 is involved in the maturation processes of collecting ducts after birth, and that UT-A1 is a target gene of KLF12.

Mechanoregulation of intracellular Ca2+ concentration is attenuated in collecting duct of monocilium-impaired orpk mice

Autosomal recessive polycystic kidney disease (ARPKD) is characterized by the progressive dilatation of collecting ducts, the nephron segments responsible for the final renal regulation of sodium, potassium, acid-base, and water balance. Murine models of ARPKD possess mutations in genes encoding cilia-associated proteins, including Tg737 in orpk mice. New findings implicate defects in structure/function of primary cilia as central to the development of polycystic kidney disease. Our group (Liu W, Xu S, Woda C, Kim P, Weinbaum S, and Satlin LM, Am J Physiol Renal Physiol 285: F998-F1012, 2003) recently reported that increases in luminal flow rate in rabbit collecting ducts increase intracellular Ca(2+) concentration ([Ca(2+)](i)) in cells therein. We thus hypothesized that fluid shear acting on the apical membrane or hydrodynamic bending moments acting on the cilium increase renal epithelial [Ca(2+)](i). To further explore this, we tested whether flow-induced [Ca(2+)](i) transients in collecting ducts from mutant orpk mice, which possess structurally abnormal cilia, differ from those in controls. Isolated segments from 1- and 2-wk-old mice were microperfused in vitro and loaded with fura 2; [Ca(2+)](i) was measured by digital ratio fluorometry before and after the rate of luminal flow was increased. All collecting ducts responded to an increase in flow with an increase in [Ca(2+)](i), a response that appeared to be dependent on luminal Ca(2+) entry. However, the magnitude of the increase in [Ca(2+)](i) in 2- but not 1-wk-old mutant orpk animals was blunted. We speculate that this defect in mechano-induced Ca(2+) signaling in orpk mice leads to aberrant structure and function of the collecting duct in ARPKD.

Function of chloride channels in the kidney

Numerous Cl- channels have been identified in the kidney using physiological approaches and thus are thought to be involved in a range of physiological processes, including vectorial transepithelial Cl- transport, cell volume regulation, and vesicular acidification. In addition, expression of genes from several Cl- channel gene families has also been observed. However, the molecular characteristics of a number of Cl- channels within the kidney are still unknown, and the physiological roles of Cl- channels identified by molecular means remain to be determined. A gene knockout approach using mice might shed further light on the characteristics of these various Cl- channels. In addition, study of diseases involving Cl- channels (channelopathies) might clarify the physiological role of specific Cl- channels. To date, more is known about CLC Cl- channels than any other Cl- channels within the kidney. This review focuses on the physiological roles of CLC Cl- channels within the kidney, particularly kidney-specific ClC-K Cl- channels, as well as the recently identified maxi anion channel in macula densa, which is involved in tubulo-glomerular feedback.

Development of water transport in the collecting duct

The ability of the immature kidney to concentrate urine is lower than in adults. This can lead to severe water and electrolyte disorders, especially in premature babies. Resistance to AVP and lower tonicity of the medullary interstitium seem to be the major factors limiting urine concentration in newborns. AVP-stimulated cAMP generation is impaired. This is the result of inhibition of the production by PGE(2) acting through EP3 receptors and increased degradation by phosphodiesterase IV. The expression of aquaporin-2 (AQP2) in the immature kidney is low; however, under conditions of water deprivation and after stimulation with DDAVP, it rises to adult levels. The expression of AQP3 and AQP4 is intact at birth and does not seem to contribute to the hyporesponsiveness to AVP. Low sodium transport by thick ascending loops of Henle, immaturity of the medullary architecture, and adaptations in the transport of urea contribute to the lower tonicity of the medullary interstitium. This paper reviews the alterations in the AVP signal transduction pathway in the immature kidney.

Regulation of water movement across vertebrate renal tubules

Kidneys play an essential role in fluid-ion balance, but the mechanisms of renal handling of water vary depending on structural organization of kidneys and the environment. Fishes and amphibians in a hypoosmotic environment excrete excess water by forming dilute urine, whereas terrestrial tetrapods require water conservation by the kidney for survival. Diluting segments operated by a luminal Na(+)-K(+)-2Cl(-) cotransporter coupled with a basolateral Na(+)-K(+) pump are essential in forming dilute urine. In birds and mammals, the diluting segment that has the same transport characteristics now serves, with the development of additional architectural organization, for countercurrent urine concentration and water conservation. Recently, a number of aquaporin (AQP) water channels have been identified in various transporting epithelia. AQPs conserve the NPA (asparagine-proline-alanine) motif, forming pores selective to water. Although all vertebrate kidneys presumably possess AQP water channels, AQP homologues have been cloned only from amphibian, avian and mammalian renal systems. Studies on expression sites, function and regulation of AQPs will provide important insight into cellular and molecular mechanisms of epithelial water transport and its control by humoral, neural and hemodynamic mechanisms.

Expression of urea transporters in the developing rat kidney

Urea transport in the kidney is mediated by a family of transporter proteins that includes renal urea transporters (UT-A) and erythrocyte urea transporters (UT-B). Because newborn rats are not capable of producing concentrated urine, we examined the time of expression and the distribution of UT-A and UT-B in the developing rat kidney by light and electron microscopic immunocytochemistry. Kidneys from 16-, 18-, and 20-day-old fetuses, 1-, 4-, 7-, 14-, and 21-day-old pups, and adult animals were studied. In the adult kidney, UT-A was expressed intensely in the inner medullary collecting duct (IMCD) and terminal portion of the short-loop descending thin limb (DTL) and weakly in long-loop DTL in the outer part of the inner medulla. UT-A immunoreactivity was not present in the fetal kidney but was observed in the IMCD and DTL in 1-day-old pups. The intensity of UT-A immunostaining in the IMCD gradually increased during postnatal development. In 4- and 7-day-old pups, UT-A immunoreactivity was present in the DTL at the border between the outer and inner medulla. In 14- and 21-day-old pups, strong UT-A immunostaining was observed in the terminal part of short-loop DTL in the outer medulla, and weak labeling remained in long-loop DTL descending into the outer part of the inner medulla. In the adult kidney, there was intense staining for UT-B in descending vasa recta (DVR) and weak labeling of glomeruli. In the developing kidney, UT-B was first observed in the DVR of a 20-day-old fetus. After birth there was a striking increase in the number of UT-B-positive DVR, in association with the formation of vascular bundles. The intensity of immunostaining remained strong in the outer medulla but gradually decreased in the inner medulla. We conclude that the expression of urea transporters in short-loop DTL and DVR coincides with the development of the ability to produce a concentrated urine.