alphaENaC-mediated lithium absorption promotes nephrogenic diabetes insipidus

Lithium: a versatile tool for understanding renal physiology

By virtue of its unique interactions with kidney cells, lithium became an important research tool in renal physiology and pathophysiology. Investigators have uncovered the intricate relationships of lithium with the vasopressin and aldosterone systems, and the membrane channels or transporters regulated by them. While doing so, their work has also led to 1) questioning the role of adenylyl cyclase activity and prostaglandins in lithium-induced suppression of aquaporin-2 gene transcription; 2) unraveling the role of purinergic signaling in lithium-induced polyuria; and 3) highlighting the importance of the epithelial sodium channel (ENaC) in lithium-induced nephrogenic diabetes insipidus (NDI). Lithium-induced remodeling of the collecting duct has the potential to shed new light on collecting duct remodeling in disease conditions, such as diabetes insipidus. The finding that lithium inhibits glycogen synthase kinase-3β (GSK3β) has opened an avenue for studies on the role of GSK3β in urinary concentration, and GSK isoforms in renal development. Finally, proteomic and metabolomic profiling of the kidney and urine in rats treated with lithium is providing insights into how the kidney adapts its metabolism in conditions such as acquired NDI and the multifactorial nature of lithium-induced NDI. This review provides state-of-the-art knowledge of lithium as a versatile tool for understanding the molecular physiology of the kidney, and a comprehensive view of how this tool is challenging some of our long-standing concepts in renal physiology, often with paradigm shifts, and presenting paradoxical situations in renal pathophysiology. In addition, this review points to future directions in research where lithium can lead the renal community.

Genetic ablation of aquaporin-2 in the mouse connecting tubules results in defective renal water handling

Body water balance is regulated via the water channel aquaporin-2 (AQP2), which is expressed in the renal connecting tubule (CNT) and collecting duct (CD). The relative roles of AQP2 in the CNT and CD are not fully understood. To study the role of AQP2 in the CNT we generated a mouse model with CNT-specific AQP2 deletion (AQP2-CNT-knockout (KO)). Confocal laser scanning microscopy and immunogold electron microscopy demonstrated an absence of AQP2 in the CNT of AQP2-CNT-KO mice. Twenty-four hour urine output was significantly increased (KO: 3.0 ± 0.3 ml (20 g body weight (BW))(-1); wild-type (WT): 1.9 ± 0.3 ml (20 g BW)(-1)) and urine osmolality decreased (KO: 1179 ± 107 mosmol kg(-1); WT: 1790 ± 146 mosmol kg(-1)) in AQP2-CNT-KO mice compared with controls. After 24 h water restriction, urine osmolality was still significantly lower in AQP2-CNT-KO mice (KO: 2087 ± 169 mosmol kg(-1); WT: 2678 ± 144 mosmol kg(-1)). A significant difference in urine osmolality between groups before desmopressin (dDAVP) (KO: 873 ± 129 mosmol kg(-1); WT: 1387 ± 163 mosmol kg(-1)) was not apparent 2 h after injection, with urine osmolality increased significantly in both groups (KO: 2944 ± 41 mosmol kg(-1); WT: 3133 ± 66 mosmol kg(-1)). Cortical kidney fractions from AQP2-CNT-KO mice had significantly reduced AQP2, with no compensatory changes in sodium potassium chloride cotransporter (NKCC2), AQP3 or AQP4. Lithium chloride treatment increased urine volume and decreased osmolality in both WT and AQP2-CNT-KO mice. After 8 days of treatment, the AQP2-CNT-KO mice still had a significantly higher urine volume and lower urine osmolality, suggesting that the CNT does not play a significant role in the pathology of lithium-induced nephrogenic diabetes insipidus. Our studies indicate that the CNT plays a role in regulating body water balance under basal conditions, but not for maximal concentration of the urine during antidiuresis.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

Tissue injury after lithium treatment in human and rat postnatal kidney involves glycogen synthase kinase-3β-positive epithelium

It was hypothesized that lithium causes accelerated and permanent injury to the postnatally developing kidney through entry into epithelial cells of the distal nephron and inhibition of glycogen synthase kinase-3β (GSK-3β). GSK-3β immunoreactivity was associated with glomeruli, the thick ascending limb of Henle's loop, and collecting ducts in the developing and adult human and rat kidney. In rats, the abundance of inactive, phosphorylated GSK-3β (pGSK-3β) protein decreased during postnatal development. After feeding of dams with litters lithium [50 mmol Li/kg chow, postnatal (P) days 7-28], the offspring showed plasma lithium concentration of 1.0 mmol/l. Kidneys from lithium-treated rat pups exhibited dilated distal nephron segments with microcysts. Stereological analysis showed reduced cortex and outer medullary volumes. Lithium increased pGSK-3β and the proliferation marker proliferating cell nuclear antigen (PCNA) protein abundances in the cortex and medulla. After lithium treatment, pGSK-3β-immunopositive cells exhibited restricted distribution and were associated primarily with subsets of cells in dilated and microcystic segments of cortical collecting ducts. After 6 wk of lithium discontinuation, adult rats exhibited attenuated urine concentration capacity and diminished outer medullary volume. Histological sections of two nephrectomy samples and a biopsy from three long-term lithium-treated patients showed multiple cortical microcysts that originated from normally appearing tubules. Microcysts were lined by a cuboidal PCNA-, GSK-3β-, and pGSK-3β-immunopositive epithelium. The postnatal rat kidney may serve as an experimental model for the study of lithium-induced human kidney injury. The data are compatible with a causal relationship between epithelial entry of lithium into cells of the aldosterone-sensitive distal nephron, inactivation of GSK-3β, proliferation, and microcysts.

AT1 receptors in the collecting duct directly modulate the concentration of urine

Mice lacking AT(1) angiotensin receptors have an impaired capacity to concentrate the urine, but the underlying mechanism is unknown. To determine whether direct actions of AT(1) receptors in epithelial cells of the collecting duct regulate water reabsorption, we used Cre-Loxp technology to specifically eliminate AT(1A) receptors from the collecting duct in mice (CD-KOs). Although levels of AT(1A) receptor mRNA in the inner medulla of CD-KO mice were significantly reduced, their kidneys appeared structurally normal. Under basal conditions, plasma and urine osmolalities and urine volumes were similar between CD-KO mice and controls. The increase in urine osmolality in response to water deprivation or vasopressin administration, however, was consistently attenuated in CD-KO mice. Similarly, levels of aquaporin-2 protein in inner and outer medulla after water deprivation were significantly lower in CD-KO mice compared with controls, despite its normal localization to the apical membrane. In summary, these results demonstrate that AT(1A) receptors in epithelial cells of the collecting duct directly modulate aquaporin-2 levels and contribute to the concentration of urine.

Genetic deletion of the P2Y2 receptor offers significant resistance to development of lithium-induced polyuria accompanied by alterations in PGE2 signaling

Lithium (Li)-induced polyuria is due to resistance of the medullary collecting duct (mCD) to the action of arginine vasopressin (AVP), apparently mediated by increased production of PGE(2). We previously reported that the P2Y(2) receptor (P2Y(2)-R) antagonizes the action of AVP on the mCD and may play a role in Li-induced polyuria by enhancing the production of PGE(2) in mCD. Hence, we hypothesized that genetic deletion of P2Y(2)-R should ameliorate Li-induced polyuria. Wild-type (WT) or P2Y(2)-R knockout (KO) mice were fed normal or Li-added diets for 14 days and euthanized. Li-induced polyuria, and decreases in urine osmolality and AQP2 protein abundance in the renal medulla, were significantly less compared with WT mice despite the lack of differences in Li intake or terminal serum or inner medullary tissue Li levels. Li-induced increased urinary excretion of PGE(2) was not affected in KO mice. However, prostanoid EP(3) receptor (EP3-R) protein abundance in the renal medulla of KO mice was markedly lower vs. WT mice, irrespective of the dietary regimen. The protein abundances of other EP-Rs were not altered across the groups irrespective of the dietary regimen. Ex vivo stimulation of mCD with PGE(2) generated significantly more cAMP in Li-fed KO mice (130%) vs. Li-fed WT mice (100%). Taken together, these data suggest 1) genetic deletion of P2Y(2)-R offers significant resistance to the development of Li-induced polyuria; and 2) this resistance is apparently due to altered PGE(2) signaling mediated by a marked decrease in EP3-R protein abundance in the medulla, thus attenuating the EP3-mediated decrease in cAMP levels in mCD.

Lithium reduces aquaporin-2 transcription independent of prostaglandins

Vasopressin (AVP)-stimulated translocation and transcription of aquaporin-2 (AQP2) water channels in renal principal cells is essential for urine concentration. Twenty percent of patients treated with lithium develop nephrogenic diabetes insipidus (NDI), a disorder in which the kidney is unable to concentrate urine. In vivo and in mouse collecting duct (mpkCCD) cells, lithium treatment coincides with decreased AQP2 abundance and inactivation of glycogen synthase kinase (Gsk) 3β. This is paralleled in vivo by an increased renal cyclooxygenase 2 (COX-2) expression and urinary prostaglandin PGE(2) excretion. PGE(2) reduces AVP-stimulated water reabsorption, but its precise role in lithium-induced downregulation of AQP2 is unclear. Using mpkCCD cells, we here investigated whether prostaglandins contribute to lithium-induced downregulation of AQP2. In these cells, lithium application reduced AQP2 abundance, which coincided with Gsk3β inactivation and increased COX-2 expression. Inhibition of COX by indomethacin, leading to reduced PGE(2) and PGF(2α) levels, or dexamethasone-induced downregulation of COX-2 both increased AQP2 abundance, while PGE(2) addition reduced AQP2 abundance. However, lithium did not change the prostaglandin levels, and indomethacin and dexamethasone did not prevent lithium-induced AQP2 downregulation. Further analysis revealed that lithium decreased AQP2 protein abundance, mRNA levels and transcription, while PGE(2) reduced AQP2 abundance by increasing its lysosomal degradation, but not by reducing AQP2 gene transcription. In conclusion, our data reveal that in mpkCCD cells, prostaglandins decrease AQP2 protein stability by increasing its lysosomal degradation, indicating that in vivo paracrine-produced prostaglandins might have a role in lithium-induced NDI via this mechanism. However, lithium affects also AQP2 gene transcription, which is prostaglandin independent.