Changes in cellular composition of kidney collecting duct cells in rats with lithium-induced NDI

Lithium-induced Nephrogenic Diabetes Insipidus with Efficacy of Desmopressin in Combination with Thiazide Diuretics and Non-steroidal Anti-inflammatory Drugs: A Case Report with a Review of the Literature

Nephrogenic diabetes insipidus (NDI) is characterized by excessive urination and an inability to concentrate urine. Lithium is the most common cause of acquired NDI. Treatment typically involves thiazide diuretics and nonsteroidal anti-inflammatory drugs (NSAIDs). However, the efficacy of desmopressin in NDI remains unclear. We herein report a case of lithium-induced NDI in a 71-year-old woman with lithium-induced NDI. Thiazide diuretics and NSAIDs reduced the urine output by approximately 40% compared to pretreatment, while the addition of desmopressin reduced it by approximately 70%. This case suggests that desmopressin can be a viable treatment option for lithium-induced NDI.

Low exposition to lithium prevents nephrogenic diabetes insipidus but not microcystic dilations of the collecting ducts in long-term rat model

Lithium induces nephrogenic diabetes insipidus (NDI) and microcystic chronic kidney disease (CKD). As previous clinical studies suggest that NDI is dose-dependent and CKD is time-dependent, we investigated the effect of low exposition to lithium in a long-term experimental rat model. Rats were fed with a normal diet (control group), with the addition of lithium (Li+ group), or with lithium and amiloride (Li+/Ami group) for 6 months, allowing obtaining low plasma lithium concentrations (0.25 ± 0.06 and 0.43 ± 0.16 mmol/L, respectively). Exposition to low concentrations of plasma lithium levels prevented NDI but not microcystic dilations of kidney tubules, which were identified as collecting ducts (CDs) on immunofluorescent staining. Both hypertrophy, characterized by an increase in the ratio of nuclei per tubular area, and microcystic dilations were observed. The ratio between principal cells and intercalated cells was higher in microcystic than in hypertrophied tubules. There was no correlation between AQP2 messenger RNA levels and cellular remodeling of the CD. Additional amiloride treatment in the Li+/Ami group did not allow consistent morphometric and cellular composition changes compared to the Li+ group. Low exposition to lithium prevented overt NDI but not microcystic dilations of the CD, with differential cellular composition in hypertrophied and microcystic CDs, suggesting different underlying cellular mechanisms.

Kidney collecting duct cell type composition is regulated by Notch signaling via modulation of mTORC1

The plasticity and diversity of cell types with specialized functions likely defines the capacity of multicellular organisms to adapt to physiologic stressors. The kidney collecting ducts contribute to water, electrolyte, and pH homeostasis and are composed of mature intermingled epithelial cell types that are susceptible to transdifferentiate. The conversion of kidney collecting duct principal cells to intercalated cells is actively inhibited by Notch signaling to ensure urine concentrating capability. Here we identify Hes1, a target of Notch signaling, allows for maintenance of functionally distinct epithelial cell types within the same microenvironment by regulating mechanistic target of rapamycin complex 1 (mTORC1) activity. Hes1 directly represses the expression of insulin receptor substrate 1 ( Irs1 ), an upstream component of mTOR pathway and suppresses mTORC1 activity in principal cells. Genetic inactivation of tuberous sclerosis complex 2 ( Tsc2 ) to increase mTORC1 activity in mature principal cells is sufficient to promote acquisition of intercalated cell properties, while inhibition of mTORC1 in adult kidney epithelia suppresses intercalated cell properties. Considering that mTORC1 integrates environmental cues, the linkage of functionally distinct epithelial cell types to mTORC1 activity levels likely allows for cell plasticity to be regulated by physiologic and metabolic signals and the ability to sense/transduce these signals.

Tubuloid differentiation to model the human distal nephron and collecting duct in health and disease

Organoid technology is rapidly gaining ground for studies on organ (patho)physiology. Tubuloids are long-term expanding organoids grown from adult kidney tissue or urine. The progenitor state of expanding tubuloids comes at the expense of differentiation. Here, we differentiate tubuloids to model the distal nephron and collecting ducts, essential functional parts of the kidney. Differentiation suppresses progenitor traits and upregulates genes required for function. A single-cell atlas reveals that differentiation predominantly generates thick ascending limb and principal cells. Differentiated human tubuloids express luminal NKCC2 and ENaC capable of diuretic-inhibitable electrolyte uptake and enable disease modeling as demonstrated by a lithium-induced tubulopathy model. Lithium causes hallmark AQP2 loss, induces proliferation, and upregulates inflammatory mediators, as seen in vivo. Lithium also suppresses electrolyte transport in multiple segments. In conclusion, this tubuloid model enables modeling of the human distal nephron and collecting duct in health and disease and provides opportunities to develop improved therapies.

Absence of E-Cadherin and β-Catenin in the Basal Plasma Membrane of Collecting Duct Cells During NDI Development and Recovery

Lithium (Li) induces severe polyuria and polydipsia in up to 40% of patients undergoing Li treatment. In rats, Li treatment induces a reversible cellular remodeling of the collecting duct (CD), decreasing the fraction of principal-to-intercalated cells. To investigate the potential role of adherens junction proteins, we performed immunohistochemistry on kidney cross-sections from rats treated with Li as well as rats undergoing recovery on a normal diet following 4 weeks of Li-treatment. We performed immunoelectron microscopy on cryosections to determine the ultrastructural localizations. Immunohistochemistry showed that E-cadherin and β-catenin were present in both the lateral and basal plasma membrane domains of CD cells. Immunoelectron microscopy confirmed that β-catenin was localized both to the lateral and the basal plasma membrane. The basal localization of both proteins was absent from a fraction of mainly principal cells after 10 and 15 days of Li-treatment. After 4 weeks of Li-treatment few to no cells were absent of E-cadherin and β-catenin at the basal plasma membrane. After 12 and 19 days of recovery some cells exhibited an absence of basal localization of both proteins. Thus, the observed localizational changes of E-cadherin and β-catenin appear before the cellular remodeling during both development and recovery from Li-NDI.

Tubular Diseases and Stones Seen From Pediatric and Adult Nephrology Perspectives

The tubular system of the kidneys is a complex series of morphologic and functional units orchestrating the content of tubular fluid as it flows along the nephron and collecting ducts. Renal tubules maintain body water, regulate electrolytes and acid-base balance, reabsorb precious organic solutes, and eliminate specific metabolites, toxins, and drugs. In addition, decisive mechanisms to adjust blood pressure are governed by the renal tubules. Genetic as well as acquired disorders of these tubular functions may cause serious diseases that manifest both in childhood and adulthood. This article addresses a selection of tubulopathies and the underlying pathomechanisms, while highlighting the important differences in pediatric and adult nephrology care. These range from rare monogenic conditions such as nephrogenic diabetes insipidus, cystinosis, and Bartter syndrome that present in childhood, to the genetic and acquired tubular pathologies causing hypertension or nephrolithiasis that are more prevalent in adults. Both pediatric and adult nephrologists must be aware of these conditions and the age-dependent manifestations that warrant close interaction between the two subspecialties.

Tamoxifen Affects Aquaporin-3 Expression and Subcellular Localization in Rat and Human Renal Collecting Ducts

Sex hormones play an important role in the regulation of water homeostasis, and we have previously shown that tamoxifen (TAM), a selective estrogen receptor modulator (SERM), affects the regulation of aquaporin (AQP)-2. In this study, we investigated the effect of TAM on the expression and localization of AQP3 in collecting ducts using various animal, tissue, and cell models. The impact of TAM on AQP3 regulation was studied in rats subjected to 7 days of unilateral ureteral obstruction (UUO), with the rats fed a lithium-containing diet to induce nephrogenic diabetes insipidus (NDI), as well as in human precision-cut kidney slices (PCKS). Moreover, intracellular trafficking of AQP3 after TAM treatment was investigated in Madin-Darby Canine Kidney (MDCK) cells stably expressing AQP3. In all models, the expression of AQP3 was evaluated by Western blotting, immunohistochemistry and qPCR. TAM administration attenuated UUO-induced downregulation of AQP3 and affected the localization of AQP3 in both the UUO model and the lithium-induced NDI model. In parallel, TAM also affected the expression profile of other basolateral proteins, including AQP4 and Na/K-ATPase. In addition, TGF-β and TGF-β+TAM treatment affected the localization of AQP3 in stably transfected MDCK cells, and TAM partly attenuated the reduced AQP3 expression in TGF-β exposed human tissue slices. These findings suggest that TAM attenuates the downregulation of AQP3 in a UUO model and a lithium-induced NDI model and affects the intracellular localization in the collecting ducts.

Human ureteric bud organoids recapitulate branching morphogenesis and differentiate into functional collecting duct cell types

Directed differentiation of human pluripotent stem cells (hPSCs) into functional ureteric and collecting duct (CD) epithelia is essential to kidney regenerative medicine. Here we describe highly efficient, serum-free differentiation of hPSCs into ureteric bud (UB) organoids and functional CD cells. The hPSCs are first induced into pronephric progenitor cells at 90% efficiency and then aggregated into spheres with a molecular signature similar to the nephric duct. In a three-dimensional matrix, the spheres form UB organoids that exhibit branching morphogenesis similar to the fetal UB and correct distal tip localization of RET expression. Organoid-derived cells incorporate into the UB tips of the progenitor niche in chimeric fetal kidney explant culture. At later stages, the UB organoids differentiate into CD organoids, which contain >95% CD cell types as estimated by single-cell RNA sequencing. The CD epithelia demonstrate renal electrophysiologic functions, with ENaC-mediated vectorial sodium transport by principal cells and V-type ATPase proton pump activity by FOXI1-induced intercalated cells.

Reduction of lithium induced interstitial fibrosis on co-administration with amiloride

Long-term administration of lithium is associated with chronic interstitial fibrosis that is partially reduced with exposure to amiloride. We examined potential pathways of how amiloride may reduce interstitial fibrosis. Amiloride was administered to a rat model of lithium induced interstitial fibrosis over a long term (6 months), as well as for short terms of 14 and 28 days. Kidney cortical tissue was subjected to RNA sequencing and microRNA expression analysis. Gene expression changes of interest were confirmed using immunohistochemistry on kidney tissue. Pathways identified by RNA sequencing of kidney tissue were related to 'promoting inflammation' for lithium and 'reducing inflammation' for amiloride. Validation of candidate genes found amiloride reduced inflammatory components induced by lithium including NF-κB/p65^Ser536 and activated pAKT^Ser473, and increased p53 mediated regulatory function through increased p21 in damaged tubular epithelial cells. Amiloride also reduced the amount of Notch1 positive PDGFrβ pericytes and infiltrating CD3 cells in the interstitium. Thus, amiloride attenuates a multitude of pro-inflammatory components induced by lithium. This suggests amiloride could be repurposed as a possible anti-inflammatory, anti-fibrotic agent to prevent or reduce the development of chronic interstitial fibrosis.

MRI-based kidney radiomic analysis during chronic lithium treatment

BACKGROUND

Lithium therapy during bipolar disorder is associated with an increased risk of chronic kidney disease (CKD) that is slowly progressive and undetectable at early stages. We aimed at identifying kidney image texture features as possible imaging biomarkers of decreased measured glomerular filtration rate (mGFR) using radiomics of T2-weighted magnetic resonance imaging (MRI).

METHODS

One hundred and eight patients treated with lithium were evaluated including mGFR and kidney MRI, with T2-weighted sequence single-shot fast spin-echo. Computed radiomic analysis was performed after kidney segmentation. Significant features were selected to build a radiomic signature using multivariable Cox analysis to detect an mGFR <60 ml/min/1.73 m². The texture index was validated using a training and a validation cohort.

RESULTS

Texture analysis index was able to detect an mGFR decrease, with an AUC of 0.85 in the training cohort and 0.71 in the validation cohort. Patients with a texture index below the median were older (59 [42-66] vs. 46 [34-54] years, p = .001), with longer treatment duration (10 [3-22] vs. 6 [2-10] years, p = .02) and a lower mGFR (66 [46-84] vs. 83 [71-94] ml/min/1.73m², p < .001). Texture analysis index was independently and negatively associated with age (β = -.004 ± 0.001, p < .001), serum vasopressin (-0.005 ± 0.002, p = .02) and lithium treatment duration (-0.01 ± 0.003, p = .001), with a significant interaction between lithium treatment duration and mGFR (p = .02).

CONCLUSIONS

A renal texture index was developed among patients treated with lithium associated with a decreased mGFR. This index might be relevant in the diagnosis of lithium-induced renal toxicity.

Nephrogenic diabetes insipidus: a comprehensive overview

Nephrogenic diabetes insipidus (NDI) is characterized by the inability to concentrate urine that results in polyuria and polydipsia, despite having normal or elevated plasma concentrations of arginine vasopressin (AVP). In this study, we review the clinical aspects and diagnosis of NDI, the various etiologies, current treatment options and potential future developments. NDI has different clinical manifestations and approaches according to the etiology. Hereditary forms of NDI are mainly caused by mutations in the genes that encode key proteins in the AVP signaling pathway, while acquired causes are normally associated with specific drug exposure, especially lithium, and hydroelectrolytic disorders. Clinical manifestations of the disease vary according to the degree of dehydration and hyperosmolality, being worse when renal water losses cannot be properly compensated by fluid intake. Regarding the diagnosis of NDI, it is important to consider the symptoms of the patient and the diagnostic tests, including the water deprivation test and the baseline plasma copeptin measurement, a stable surrogate biomarker of AVP release. Without proper treatment, patients may developcomplications leading to high morbidity and mortality, such as severe dehydration and hypernatremia. In that sense, the treatment of NDI consists in decreasing the urine output, while allowing appropriate fluid balance, normonatremia, and ensuring an acceptable quality of life. Therefore, therapeutic options include nonpharmacological interventions, including sufficient water intake and a low-sodium diet, and pharmacological treatment. The main medications used for NDI are thiazide diuretics, nonsteroidal anti-inflammatory drugs (NSAIDs), and amiloride, used isolated or in combination.

Sulfatide with ceramide composed of phytosphingosine (t18:0) and 2-hydroxy fatty acids in renal intercalated cells

Diverse molecular species of sulfatide with differences in FA lengths, unsaturation degrees, and hydroxylation statuses are expressed in the kidneys. However, the physiological functions of specific sulfatide species in the kidneys are unclear. Here, we evaluated the distribution of specific sulfatide species in the kidneys and their physiological functions. Electron microscopic analysis of kidneys of Cst-deficient mice lacking sulfatide showed vacuolar accumulation in the cytoplasm of intercalated cells in the collecting duct, whereas the proximal and distal tubules were unchanged. Immunohistochemical analysis revealed that vacuolar H⁺-ATPase-positive vesicles were accumulated in intercalated cells in sulfatide-deficient kidneys. Seventeen sulfatide species were detected in the murine kidney by iMScope MALDI-MS analysis. The distribution of the specific sulfatide species was classified into four patterns. Although most sulfatide species were highly expressed in the outer medullary layer, two unique sulfatide species of m/z 896.6 (predicted ceramide structure: t18:0-C22:0h) and m/z 924.6 (predicted ceramide structure: t18:0-C24:0h) were dispersed along the collecting duct, implying expression in intercalated cells. In addition, the intercalated cell-enriched fraction was purified by fluorescence-activated cell sorting using the anti-vacuolar H⁺-ATPase subunit 6V0A4, which predominantly contained sulfatide species (m/z 896.6 and 924.6). The Degs2 and Fa2h genes, which are responsible for ceramide hydroxylation, were expressed in the purified intercalated cells. These results suggested that sulfatide molecular species with ceramide composed of phytosphingosine (t18:0) and 2-hydroxy FAs, which were characteristically expressed in intercalated cells, were involved in the excretion of NH₃ and protons into the urine.

Tsc2 mutation induces renal tubular cell nonautonomous disease

TSC renal cystic disease is poorly understood and has no approved treatment. In a new principal cell-targeted murine model of Tsc cystic disease, the renal cystic epithelium is mostly composed of type A intercalated cells with an intact Tsc2 gene confirmed by sequencing, although these cells exhibit a Tsc-mutant disease phenotype. We used a newly derived targeted murine model in lineage tracing and extracellular vesicle (EV) characterization experiments and a cell culture model in EV characterization and cellular induction experiments to understand TSC cystogenesis. Using lineage tracing experiments, we found principal cells undergo clonal expansion but contribute very few cells to the cyst. We determined that cystic kidneys contain more interstitial EVs than noncystic kidneys, excrete fewer EVs in urine, and contain EVs in cyst fluid. Moreover, the loss of Tsc2 gene in EV-producing cells greatly changes the effect of EVs on renal tubular epithelium, such that the epithelium develops increased secretory and proliferative pathway activity. We demonstate that the mTORC1 pathway activity is independent form the EV production, and that the EV effects for a single cell line can vary significantly. TSC cystogenesis involves significant contribution from genetically intact cells conscripted to the mutant phenotype by mutant cell derived EVs.

A new view of macula densa cell protein synthesis

Macula densa (MD) cells, a chief sensory cell type in the nephron, are endowed with unique microanatomic features including a high density of protein synthetic organelles and secretory vesicles in basal cell processes ("maculapodia") that suggest a so far unknown high rate of MD protein synthesis. This study aimed to explore the rate and regulation of MD protein synthesis and their effects on glomerular function using novel transgenic mouse models, newly established fluorescence cell biology techniques, and intravital microscopy. Sox2-tdTomato kidney tissue sections and an O-propargyl puromycin incorporation-based fluorescence imaging assay showed that MD cells have the highest level of protein synthesis within the kidney cortex followed by intercalated cells and podocytes. Genetic gain of function of mammalian target of rapamycin (mTOR) signaling specifically in MD cells (in MD-mTORgof mice) or their physiological activation by low-salt diet resulted in further significant increases in the synthesis of MD proteins. Specifically, these included both classic and recently identified MD-specific proteins such as cyclooxygenase 2, microsomal prostaglandin E2 synthase 1, and pappalysin 2. Intravital imaging of the kidney using multiphoton microscopy showed significant increases in afferent and efferent arteriole and glomerular capillary diameters and blood flow in MD-mTORgof mice coupled with an elevated glomerular filtration rate. The presently identified high rate of MD protein synthesis that is regulated by mTOR signaling is a novel component of the physiological activation and glomerular hemodynamic regulatory functions of MD cells that remains to be fully characterized.NEW & NOTEWORTHY This study discovered the high rate of protein synthesis in macula densa (MD) cells by applying direct imaging techniques with single cell resolution. Physiological activation and mammalian target of rapamycin signaling played important regulatory roles in this process. This new feature is a novel component of the tubuloglomerular cross talk and glomerular hemodynamic regulatory functions of MD cells. Future work is needed to elucidate the nature and (patho)physiological role of the specific proteins synthesized by MD cells.

Updates and Perspectives on Aquaporin-2 and Water Balance Disorders

Ensuring the proper amount of water inside the body is essential for survival. One of the key factors in the maintenance of body water balance is water reabsorption in the collecting ducts of the kidney, a process that is regulated by aquaporin-2 (AQP2). AQP2 is a channel that is exclusively selective for water molecules and impermeable to ions or other small molecules. Impairments of AQP2 result in various water balance disorders, including nephrogenic diabetes insipidus (NDI), which is a disease characterized by a massive loss of water through the kidney and consequent severe dehydration. Dysregulation of AQP2 is also a cause of water retention with hyponatremia in heart failure, hepatic cirrhosis, and syndrome of inappropriate antidiuretic hormone secretion (SIADH). Antidiuretic hormone vasopressin is an upstream regulator of AQP2. Its binding to the vasopressin V2 receptor promotes AQP2 targeting to the apical membrane and thus enables water reabsorption. Tolvaptan, a vasopressin V2 receptor antagonist, is effective and widely used for water retention with hyponatremia. However, there are no studies showing improvement in hard outcomes or long-term prognosis. A possible reason is that vasopressin receptors have many downstream effects other than AQP2 function. It is expected that the development of drugs that directly target AQP2 may result in increased treatment specificity and effectiveness for water balance disorders. This review summarizes recent progress in studies of AQP2 and drug development challenges for water balance disorders.

Atorvastatin does not ameliorate nephrogenic diabetes insipidus induced by lithium or potassium depletion in mice

Acquired forms of nephrogenic diabetes insipidus (NDI) include lithium (Li)-induced and hypokalemia-induced NDI. Both forms are associated with AQP2 downregulation and collecting duct (CD) cellular remodeling. Statins are cholesterol-lowering drugs appearing to increase AQP2 membrane-translocation and improve urine concentration in other NDI models. We have investigated if statins are able to prevent or rescue the Li-induced changes in mice and in a mouse cortical CD cell line (mCCDc1l ). Biotinylation assays showed that acute (1hr) atorvastatin, simvastatin, or fluvastatin increased AQP2 membrane accumulation in mCCDc1l cells showing that the cell line responds to acute statin treatment. To see whether chronic statin treatment abolish the Li effects, mCCDc1l cells were treated with 48 h Li, combined Li/atorvastatin or combined Li/simvastatin. Li reduced AQP2, but combined Li/atorvastatin or Li/simvastatin did not prevent AQP2 downregulation. In mice, chronic (21 days) Li increased urine output and reduced urine osmolality, but combined Li/atorvastatin did not prevent these effects. In inner medulla (IM), Li reduced total AQP2 and increased pS261-AQP2. Combined Li/atorvastatin did not abolish these changes. Atorvastatin did not prevent a Li-induced increase in intercalated cells and proliferation in IM. In mice with already established NDI, atorvastatin had no effect on the Li-induced changes either. Mice subjected to 14 days of potassium-deficient diet developed polyuria and AQP2 downregulation in IM. Co-treatment with atorvastatin did not prevent this. In conclusion, atorvastatin does not appear to be able to prevent or rescue Li-NDI or to prevent hypokalemic-induced NDI.

Renal cystic disease in tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is associated with TSC1 or TSC2 gene mutations resulting in hyperactivation of the mTORC1 pathway. This mTORC1 activation is associated with abnormal tissue development and proliferation such that in the kidney there are both solid tumors and cystic lesions. This review summarizes recent advances in tuberous sclerosis complex nephrology and focuses on the genetics and cell biology of tuberous sclerosis complex renal disease, highlighting a role of extracellular vesicles and the innate immune system in disease pathogenesis.

Lithium carbonate: Updated reproductive and developmental toxicity assessment using scientific literature and guideline compliant studies

The current publication describes most recent so far unpublished (key) guideline and GLP compliant reproductive and developmental toxicity studies of lithium carbonate in rats, including their interpretation and conclusions in terms of human hazard assessment when compared to existing literature. Particular attention was paid to the target organs and dose response of lithium ion related effects to differentiate between a primary (pharmacokinetic/pharmacodynamic) action and secondary effects as a result of systemic and target organ toxicity. In the key two-generation reproduction toxicity (OECD TG 416) study in rats, doses of 5, 15 and 45 mg/kg bw/d (0.95, 2.9 and 8.6 mg Li⁺/kg bw/d) were given by oral gavage, resulting in clear NOAELs of 15 mg/kg bw/d (2.9 mg Li⁺/kg bw/d) for systemic parental toxicity and 45 mg/kg bw/d (8.6 mg Li⁺/kg bw/d) for reproductive toxicity and fetal toxicity. Target organ changes were consistently observed in liver (cytoplasmic rarefaction) and kidney (dilated tubuli). In the key developmental toxicity (OECD TG 414) study in rats, doses given by oral gavage were 10, 30 and 90 mg/kg bw/d (1.9, 5.7 and 17.1 mg Li⁺/kg bw/d) was investigated resulting in NO(A)ELs of 30 mg/kg bw/d (5.7 mg Li⁺/kg bw/d) (maternal toxicity) and 90 mg/kg bw/d (17 mg Li⁺/kg bw/d) (fetal toxicity and teratogenicity). The highest dose of 90 mg/kg bw/day resulted in clear signs of toxicity and peak plasma concentrations at the toxic range (>1.0 mEq lithium/L). Toxic effects of lithium carbonate were not seen in the reproductive and developmental organs. No adverse effects on sperm (total motility, progressive motility and morphology of testicular and cauda epididymal sperm) were observed in the two-generation rat reproduction toxicity study. There was also no impact on fertility indices or on litter sizes in this study, nor were there any fetal effects in the two-generation reproduction toxicity and developmental toxicity study at doses causing already systemic toxicity in the dams. Secondary effects such as decreased weight (gain) and food consumption were reported in the developmental toxicity study. The absence of any reproductive/developmental findings at dose levels causing clear systemic toxicity in the test animals in these key mammalian studies, does not suggest an immediate concern for possible human reproductive or developmental toxicity effects from exposure to lithium during drug use.

Genetic background determines renal response to chronic lithium treatment in female mice

Background Chronic lithium treatment for bipolar disease causes mainly side effects in the kidney. A subset of lithium users develops nephrogenic diabetes insipidus (NDI), a urinary concentrating disorder, and chronic kidney disease (CKD). Age, lithium dose and duration of treatment are important risk factors, while genetic background might also play an important role. Methods In order to investigate the role of genetics, female mice of 29 different inbred strains were treated for one year with control or lithium chow and urine, blood and kidneys were analysed. Results Chronic lithium treatment increased urine production and/or reduced urine osmolality in 21 strains. Renal histology showed that lithium increased interstitial fibrosis and/or tubular atrophy in eight strains, while in none of the strains glomerular injury was induced. Interestingly, lithium did not elevate urinary albumin-creatinine ratio (ACR) in any strain, while eight strains even demonstrated a lowered ACR. The protective effect on ACR coincided with a similar decrease in urinary IgG levels, a marker of glomerular function, while the adverse effect of lithium on interstitial fibrosis/tubular atrophy coincided with a severe increase in urinary β2-microglobulin (B2M) levels, an indicator of proximal tubule damage. Conclusion Genetic background plays an important role in the development of lithium-induced NDI and chronic renal pathology in female mice. The strong correlation of renal pathology with urinary B2M levels indicates B2M as a promising biomarker for chronic renal damage induced by lithium.

Targeting the Trafficking of Kidney Water Channels for Therapeutic Benefit

The ability to regulate water movement is vital for the survival of cells and organisms. In addition to passively crossing lipid bilayers by diffusion, water transport is also driven across cell membranes by osmotic gradients through aquaporin water channels. There are 13 aquaporins in human tissues, and of these, aquaporin-2 (AQP2) is the most highly regulated water channel in the kidney: The expression and trafficking of AQP2 respond to body volume status and plasma osmolality via the antidiuretic hormone, vasopressin (VP). Dysfunctional VP signaling in renal epithelial cells contributes to disorders of water balance, and research initially focused on regulating the major cAMP/PKA pathway to normalize urine concentrating ability. With the discovery of novel and more complex signaling networks that regulate AQP2 trafficking, promising therapeutic targets have since been identified. Several strategies based on data from preclinical studies may ultimately translate to the care of patients with defective water homeostasis.

Identification of Acer2 as a First Susceptibility Gene for Lithium-Induced Nephrogenic Diabetes Insipidus in Mice

BACKGROUND

Lithium, mainstay treatment for bipolar disorder, causes nephrogenic diabetes insipidus and hypercalcemia in about 20% and 10% of patients, respectively, and may lead to acidosis. These adverse effects develop in only a subset of patients treated with lithium, suggesting genetic factors play a role.

METHODS

To identify susceptibility genes for lithium-induced adverse effects, we performed a genome-wide association study in mice, which develop such effects faster than humans. On day 8 and 10 after assigning female mice from 29 different inbred strains to normal chow or lithium diet (40 mmol/kg), we housed the animals for 48 hours in metabolic cages for urine collection. We also collected blood samples.

RESULTS

In 17 strains, lithium treatment significantly elevated urine production, whereas the other 12 strains were not affected. Increased urine production strongly correlated with lower urine osmolality and elevated water intake. Lithium caused acidosis only in one mouse strain, whereas hypercalcemia was found in four strains. Lithium effects on blood pH or ionized calcium did not correlate with effects on urine production. Using genome-wide association analyses, we identified eight gene-containing loci, including a locus containing Acer2, which encodes a ceramidase and is specifically expressed in the collecting duct. Knockout of Acer2 led to increased susceptibility for lithium-induced diabetes insipidus development.

CONCLUSIONS

We demonstrate that genome-wide association studies in mice can be used successfully to identify susceptibility genes for development of lithium-induced adverse effects. We identified Acer2 as a first susceptibility gene for lithium-induced diabetes insipidus in mice.

Extracellular Nucleotides and P2 Receptors in Renal Function

The understanding of the nucleotide/P2 receptor system in the regulation of renal hemodynamics and transport function has grown exponentially over the last 20 yr. This review attempts to integrate the available data while also identifying areas of missing information. First, the determinants of nucleotide concentrations in the interstitial and tubular fluids of the kidney are described, including mechanisms of cellular release of nucleotides and their extracellular breakdown. Then the renal cell membrane expression of P2X and P2Y receptors is discussed in the context of their effects on renal vascular and tubular functions. Attention is paid to effects on the cortical vasculature and intraglomerular structures, autoregulation of renal blood flow, tubuloglomerular feedback, and the control of medullary blood flow. The role of the nucleotide/P2 receptor system in the autocrine/paracrine regulation of sodium and fluid transport in the tubular and collecting duct system is outlined together with its role in integrative sodium and fluid homeostasis and blood pressure control. The final section summarizes the rapidly growing evidence indicating a prominent role of the extracellular nucleotide/P2 receptor system in the pathophysiology of the kidney and aims to identify potential therapeutic opportunities, including hypertension, lithium-induced nephropathy, polycystic kidney disease, and kidney inflammation. We are only beginning to unravel the distinct physiological and pathophysiological influences of the extracellular nucleotide/P2 receptor system and the associated therapeutic perspectives.

Diabetes insipidus

Diabetes insipidus (DI) is a disorder characterized by excretion of large amounts of hypotonic urine. Central DI results from a deficiency of the hormone arginine vasopressin (AVP) in the pituitary gland or the hypothalamus, whereas nephrogenic DI results from resistance to AVP in the kidneys. Central and nephrogenic DI are usually acquired, but genetic causes must be evaluated, especially if symptoms occur in early childhood. Central or nephrogenic DI must be differentiated from primary polydipsia, which involves excessive intake of large amounts of water despite normal AVP secretion and action. Primary polydipsia is most common in psychiatric patients and health enthusiasts but the polydipsia in a small subgroup of patients seems to be due to an abnormally low thirst threshold, a condition termed dipsogenic DI. Distinguishing between the different types of DI can be challenging and is done either by a water deprivation test or by hypertonic saline stimulation together with copeptin (or AVP) measurement. Furthermore, a detailed medical history, physical examination and imaging studies are needed to ensure an accurate DI diagnosis. Treatment of DI or primary polydipsia depends on the underlying aetiology and differs in central DI, nephrogenic DI and primary polydipsia.

RNA-Seq and protein mass spectrometry in microdissected kidney tubules reveal signaling processes initiating lithium-induced nephrogenic diabetes insipidus

Lithium salts, used for treating bipolar disorder, frequently induce nephrogenic diabetes insipidus (NDI) thereby limiting therapeutic success. NDI is associated with loss of expression of the gene coding for the molecular water channel, aquaporin-2, in the renal collecting duct (CD). Here, we use systems biology methods in a well-established rat model of lithium-induced NDI to identify signaling pathways activated at the onset of polyuria. Using single-tubule RNA-Seq, full transcriptomes were determined in microdissected cortical collecting ducts (CCDs) of rats after 72 hours without or with initiation of lithium chloride administration. Transcriptome-wide changes in mRNA abundances were mapped to gene sets associated with curated canonical signaling pathways, showing evidence for activation of NF-κB signaling with induction of genes coding for multiple chemokines and most components of the Major Histocompatibility Complex Class I antigen-presenting complex. Administration of anti-inflammatory doses of dexamethasone to lithium chloride-treated rats countered the loss of aquaporin-2. RNA-Seq also confirmed prior evidence of a shift from quiescence into the cell cycle with arrest. Time course studies demonstrated an early (12 hour) increase in multiple immediate early response genes including several transcription factors. Protein mass spectrometry in microdissected CCDs provided corroborative evidence and identified decreased abundance of several anti-oxidant proteins. Thus, in the context of prior observations, our study can be best explained by a model in which lithium increases ERK activation leading to induction of NF-κB signaling and an inflammatory-like response that represses Aqp2 transcription.

Salt-sensitive transcriptome of isolated kidney distal tubule cells

In the distal kidney tubule, the steroid hormone aldosterone regulates sodium reabsorption via the epithelial sodium channel (ENaC). Most studies seeking to identify ENaC-regulating aldosterone-induced proteins have used transcriptional profiling of cultured cells. To identify salt-sensitive transcripts in an in vivo model, we used low-NaCl or high-NaCl diet to stimulate or suppress endogenous aldosterone, in combination with magnetic- and fluorescence-activated cell sorting to isolate distal tubule cells from mouse kidney for transcriptional profiling. Of the differentially expressed transcripts, 162 were more abundant in distal tubule cells isolated from mice fed low-NaCl diet, and 161 were more abundant in distal tubule cells isolated from mice fed high-NaCl diet. Enrichment analysis of Gene Ontology biological process terms identified multiple statistically overrepresented pathways among the differentially expressed transcripts that were more abundant in distal tubule cells isolated from mice fed low-NaCl diet, including ion transmembrane transport, regulation of growth, and negative regulation of apoptosis. Analysis of Gene Ontology molecular function terms identified differentially expressed transcription factors, transmembrane transporters, kinases, and G protein-coupled receptors. Finally, comparison with a recently published study of gene expression changes in distal tubule cells in response to administration of aldosterone identified 18 differentially expressed genes in common between the two experiments. When expression of these genes was measured in cortical collecting ducts microdissected from mice fed low-NaCl or high-NaCl diet, eight were differentially expressed. These genes are likely to be regulated directly by aldosterone and may provide insight into aldosterone signaling to ENaC in the distal tubule.

[Chronic metabolic and renal disorders related to lithium salts treatment]

Lithium salts are the main treatment of bipolar disorder, which is characterized by potentially life-threatening maniac and/or depressive episodes. They have proven efficient in the prevention and treatment of acute episodes as well as in the prevention of suicidal risk. However, this efficacy is counterbalanced by a narrow therapeutic range that can lead to potentially harmful overdose, and by adverse long-term events. Nevertheless, they remain first-line treatment, notwithstanding therapeutic alternatives. In this review, we will describe toxic effects of long-term treatment at therapeutic levels of lithium salts. Regarding renal effects, early-impaired urine concentrating ability might lead to polyuria and polydipsia, and even to hypernatremia if free access to water is compromised. Long-term lithium treatment might also lead to chronic kidney disease, characterized by tubulo-interstitial multicystic nephropathy. End-stage renal disease requiring renal replacement therapy is a rare complication. Major extra-renal toxic effects are hypercalcemia and hypothyroidism. Treatment cessation due to these adverse events should be a multidisciplinary and case-by-case decision based on the benefit/risk ratio. Since these toxic effects are mild and display slow progression, treatment cessation is uncommon. However, regular medical and biological check-up is needed in order to prevent these disorders, and patients might be referred to nephrologists and/or endocrinologists once the disorders are established.

Nephrogenic Diabetes Insipidus

Nephrogenic diabetes insipidus (NDI) results from the inability of the late distal tubules and collecting ducts to respond to vasopressin. The lack of ability to concentrate urine results in polyuria and polydipsia. Primary and acquired forms of NDI exist in children. Congenital NDI is a result of mutation in AVPR2 or AQP2 genes. Secondary NDI is associated with electrolyte abnormalities, obstructive uropathy, or certain medications. Management of NDI can be difficult with only symptomatic treatment available, using low-solute diet, diuretics, and prostaglandin inhibitors.

Genetic deletion of ADP-activated P2Y12 receptor ameliorates lithium-induced nephrogenic diabetes insipidus in mice

AIM

Therapeutic use of lithium in bipolar disorder is limited by the development of nephrogenic diabetes insipidus (NDI). We reported that pharmacological blockade of P2Y₁₂ receptor (R) with clopidogrel or prasugrel significantly ameliorated lithium-induced NDI in rodents. Using mice genetically lacking P2Y₁₂ -R we evaluated whether the observed amelioration is mediated through P2Y₁₂ -R METHODS: P2ry12^-/- mouse line (C57/BL6) was rederived from cryopreserved embryos of the knockout (KO) mice generated by Deltagen Inc. Syngeneic wild type (WT) mice obtained by heterozygous crossing were inbred. Groups of adult WT and KO mice were fed lithium-added (40 mmol LiCl/kg food) or regular diet, and euthanized after 2 or 4 weeks. Twenty-four hour urine samples and terminal blood and kidney samples were analyzed.

RESULTS

At both time points, lithium-induced polyuria and decrease in aquaporin-2 (AQP2) protein abundance in the kidney medulla were less marked in KO vs WT mice. Immunofluorescence microscopy revealed that lithium-induced alterations in the cellular disposition of AQP2 protein in the medullary collecting ducts of WT mice were blunted in KO mice. Serum lithium, sodium and osmolality were similar in both genotypes after lithium treatment. After 2 weeks, lithium induced marked increases in urinary excretion of Na, K, and arginine vasopressin in WT mice but not in KO mice.

CONCLUSION

Taken together, our data show that similar to pharmacological blockade, deletion of P2Y₁₂ -R significantly ameliorates lithium-induced NDI, without reducing serum lithium levels. Hence, targeting P2Y₁₂ -R with currently available drugs in the market offers a novel and safer method for treating NDI.

Genetic Deletion of P2Y2 Receptor Offers Long-Term (5 Months) Protection Against Lithium-Induced Polyuria, Natriuresis, Kaliuresis, and Collecting Duct Remodeling and Cell Proliferation

Chronic lithium administration for the treatment of bipolar disorder leads to nephrogenic diabetes insipidus (NDI), characterized by polyuria, natriuresis, kaliuresis, and collecting duct remodeling and cell proliferation among other features. Previously, using a 2-week lithium-induced NDI model, we reported that P2Y2 receptor (R) knockout mice are significantly resistant to polyuria, natriuresis, kaliuresis, and decrease in AQP2 protein abundance in the kidney relative to wild type mice. Here we show this protection is long-lasting, and is also associated with significant amelioration of lithium-induced collecting duct remodeling and cell proliferation. Age-matched wild type and knockout mice were fed regular (n = 5/genotype) or lithium-added (40 mmol/kg chow; n = 10/genotype) diet for 5 months and euthanized. Water intake, urine output and osmolality were monitored once in every month. Salt blocks were provided to mice on lithium-diet to prevent sodium loss. At the end of 5 months mice were euthanized and serum and kidney samples were analyzed. There was a steady increase in lithium-induced polyuria, natriuresis and kaliuresis in wild type mice over the 5-month period. Increases in these urinary parameters were very low in lithium-fed knockout mice, resulting in significantly widening differences between the wild type and knockout mice. Terminal AQP2 and NKCC2 protein abundances in the kidney were significantly higher in lithium-fed knockout vs. wild type mice. There were no significant differences in terminal serum lithium or sodium levels between the wild type and knockout mice. Confocal immunofluorescence microscopy revealed that lithium-induced marked remodeling of collecting duct with significantly increased proportion of [H+]-ATPase-positive intercalated cells and decreased proportion of AQP2-positive principal cells in the wild type, but not in knockout mice. Lithium-induced collecting duct cell proliferation (indicated by Ki67 labeling), was significantly lower in knockout vs. wild type mice. This is the first piece of evidence that purinergic signaling is potentially involved in lithium-induced collecting duct remodeling and cell proliferation. Our results demonstrate that genetic deletion of P2Y2-R protects against the key structural and functional alterations in Li-induced NDI, and underscore the potential utility of targeting this receptor for the treatment of NDI in bipolar patients on chronic lithium therapy.

Endogenous Notch Signaling in Adult Kidneys Maintains Segment-Specific Epithelial Cell Types of the Distal Tubules and Collecting Ducts to Ensure Water Homeostasis

BACKGROUND

Notch signaling is required during kidney development for nephron formation and principal cell fate selection within the collecting ducts. Whether Notch signaling is required in the adult kidney to maintain epithelial diversity, or whether its loss can trigger principal cell transdifferentiation (which could explain acquired diabetes insipidus in patients receiving lithium) is unclear.

METHODS

To investigate whether loss of Notch signaling can trigger principal cells to lose their identity, we genetically inactivated Notch1 and Notch2, inactivated the Notch signaling target Hes1, or induced expression of a Notch signaling inhibitor in all of the nephron segments and collecting ducts in mice after kidney development. We examined renal function and cell type composition of control littermates and mice with conditional Notch signaling inactivation in adult renal epithelia. In addition, we traced the fate of genetically labeled adult kidney collecting duct principal cells after Hes1 inactivation or lithium treatment.

RESULTS

Notch signaling was required for maintenance of Aqp2-expressing cells in distal nephron and collecting duct segments in adult kidneys. Fate tracing revealed mature principal cells in the inner stripe of the outer medulla converted to intercalated cells after genetic inactivation of Hes1 and, to a lesser extent, lithium treatment. Hes1 ensured repression of Foxi1 to prevent the intercalated cell program from turning on in mature Aqp2⁺ cell types.

CONCLUSIONS

Notch signaling via Hes1 regulates maintenance of mature renal epithelial cell states. Loss of Notch signaling or use of lithium can trigger transdifferentiation of mature principal cells to intercalated cells in adult kidneys.

Chronic lithium treatment induces novel patterns of pendrin localization and expression

Prolonged lithium treatment is associated with various renal side effects and is known to induce inner medullary collecting duct (IMCD) remodeling. In animals treated with lithium, the fraction of intercalated cells (ICs), which are responsible for acid-base homeostasis, increases compared with renal principal cells (PCs). To investigate the intricacies of lithium-induced IMCD remodeling, male Sprague-Dawley rats were fed a lithium-enriched diet for 0,1, 2, 3, 6, 9, or 12 wk. Urine osmolality was decreased at 1 wk, and from 2 to 12 wk, animals were severely polyuric. After 6 wk of lithium treatment, approximately one-quarter of the cells in the initial IMCD expressed vacuolar H+-ATPase, an IC marker. These cells were localized in portions of the inner medulla, where ICs are not normally found. Pendrin, a Cl-/[Formula: see text] exchanger, is normally expressed only in two IC subtypes found in the convoluted tubule, the cortical collecting duct, and the connecting tubule. At 6 wk of lithium treatment, we observed various patterns of pendrin localization and expression in the rat IMCD, including a novel phenotype wherein pendrin was coexpressed with aquaporin-4. These observations collectively suggest that renal IMCD cell plasticity may play an important role in lithium-induced IMCD remodeling.

A differential expression of uncoupling protein-2 associates with renal damage in stroke-resistant spontaneously hypertensive rat/stroke-prone spontaneously hypertensive rat-derived stroke congenic lines

OBJECTIVES

Uncoupling protein-2 (UCP2), a mitochondrial anion transporter involved in mitochondrial uncoupling, limiting reactive oxygen species formation, is significantly downregulated in kidneys of high-salt-fed stroke-prone spontaneously hypertensive rat (SHRSP), where it associates with increased renal damage occurrence.

METHODS

We aimed at establishing whether UCP2 differential expression associates with renal damage in two stroke-resistant spontaneously hypertensive rat (SHRSR)/SHRSP-derived stroke congenic lines. For this purpose, SHRSR, SHRSP, and two reciprocal stroke congenic lines carrying the (D1Rat134-Mt1pa) segment of chromosome 1 were fed with Japanese style diet for 8 weeks. At 4, 6, and 8 weeks of Japanese diet, kidneys were removed and analyzed for UCP2 gene and protein expression [UCP2 maps within (D1Rat134-Mt1pa)]; nuclear factor kappa-light-chain-enhancer of activated B cells protein expression; oxidized total protein levels; mitochondrial function; gene expression of cubulin, megalin, and nephrin. At 6 and 8 weeks of Japanese diet, histological damage and percentage of high molecular weight urinary proteins excretion were assessed.

RESULTS

Introgression of UCP2 in the SHRSP configuration within the SHRSR genome led to UCP2 downregulation upon Japanese diet, as compared with the SHRSR, with significantly reduced ATP levels, increased rate of inflammation, oxidative stress, renal damage, and excretion of high molecular weight proteins. The opposite phenomena were observed in the reciprocal congenic line, compared with the SHRSP. In vitro, high-NaCl medium led to UCP2 downregulation, increased apoptosis/necrosis, and reduced viability in primary renal proximal tubular epithelial cells isolated from SHRSP. Exposure of the SHRSP/proximal tubular epithelial cells to recombinant UCP2 rescued the high-salt-dependent deleterious effects.

CONCLUSION

A differential UCP2 expression associates with different degree of renal damage upon Japanese diet in two SHRSR/SHRSP-derived stroke congenic lines through modulation of mitochondrial function, inflammation, and oxidative stress.

Tamoxifen attenuates development of lithium-induced nephrogenic diabetes insipidus in rats

Lithium is widely used in treatment of bipolar affective disorders but often causes nephrogenic diabetes insipidus (NDI), a disorder characterized by severe urinary-concentrating defects. Lithium-induced NDI is caused by lithium uptake by collecting duct principal cells and altered expression of aquaporin-2 (AQP2), which are essential for water reabsorption of tubular fluid in the collecting duct. Sex hormones have previously been shown to affect the regulation of AQP2, so we tested whether tamoxifen (TAM), a selective estrogen receptor modulator, would attenuate lithium-induced alterations on renal water homeostasis. Rats were treated for 14 days with lithium, and TAM treatment was initiated 1 wk after onset of lithium administration. Lithium treatment resulted in severe polyuria and reduced AQP2 expression, which were ameliorated by TAM. Consistent with this, TAM attenuated downregulation of AQP2 and increased phosphorylation of the cAMP-responsive element-binding protein, which induced AQP2 expression in freshly isolated inner-medullary collecting duct suspension prepared from lithium-treated rats. In conclusion, TAM attenuated polyuria dose dependently and impaired urine concentration and downregulation of AQP2 protein expression in rats with lithium-induced NDI. These findings suggest that TAM is likely to be a novel therapeutic option for lithium-induced NDI.

Lithium increases ammonium excretion leading to altered urinary acid-base buffer composition

Previous reports identify a voltage dependent distal renal tubular acidosis (dRTA) secondary to lithium (Li⁺) salt administration. This was based on the inability of Li⁺-treated patients to increase the urine-blood (U-B) pCO₂ when challenged with NaHCO₃ and, the ability of sodium neutral phosphate or Na₂SO₄ administration to restore U-B pCO₂ in experimental animal models. The underlying mechanisms for the Li⁺-induced dRTA are still unknown. To address this point, a 7 days time course of the urinary acid-base parameters was investigated in rats challenged with LiCl, LiCitrate, NaCl, or NaCitrate. LiCl induced the largest polyuria and a mild metabolic acidosis. Li⁺-treatment induced a biphasic response. In the first 2 days, proper urine volume and acidification occurred, while from the 3rd day of treatment, polyuria developed progressively. In this latter phase, the LiCl-treated group progressively excreted more NH₄⁺ and less pCO₂, suggesting that NH₃/NH₄⁺ became the main urinary buffer. This physiological parameter was corroborated by the upregulation of NBCn1 (a marker of increased ammonium recycling) in the inner stripe of outer medulla of LiCl treated rats. Finally, by investigating NH₄⁺ excretion in ENaC-cKO mice, a model resistant to Li⁺-induced polyuria, a primary role of the CD was confirmed. By definition, dRTA is characterized by deficient urinary ammonium excretion. Our data question the presence of a voltage-dependent Li⁺-induced dRTA in rats treated with LiCl for 7 days and the data suggest that the alkaline urine pH induced by NH₃/NH₄⁺ as the main buffer has lead to the interpretation dRTA in previous studies.

The soluble (Pro) renin receptor does not influence lithium-induced diabetes insipidus but does provoke beiging of white adipose tissue in mice

Earlier we reported that the recombinant soluble (pro) renin receptor sPRR-His upregulates renal aquoporin-2 (AQP2) expression, and attenuates polyuria associated with nephrogenic diabetes insipidus (NDI) induced by vasopressin type 2 receptor (V2R) antagonism. Patients that receive lithium therapy develop polyuria associated NDI that might be secondary to downregulation of renal AQP2. We hypothesized that sPRR-His attenuates indices of NDI associated with lithium treatment. Eight-week-old male C57/BL6 mice consumed chow supplemented with LiCl (40 mmol/kg diets) for 14 days. For the last 7 days mice received either sPRR-His [30 μg/(kg day), i.v.; sPRR] or vehicle (Veh) via minipump. Control (Con) mice consumed standard chow for 14 days. Compared to Con mice, 14-d LiCl treatment elevated water intake and urine volume, and decreased urine osmolality, regardless of sPRR-His or Veh administration. These data indicate that sPRR-His treatment does not attenuate indices of NDI evoked by lithium. Unexpectedly, epididymal fat mass was lower, adipocyte UCP1 mRNA and protein expression were higher, and multilocular lipid morphology was enhanced, in LiCl-fed mice treated with sPRR-His versus vehicle. The beiging of white adipose tissue is a novel metabolic benefit of manipulating the sPRR in the context of lithium-induced NDI.

Lithium induces aerobic glycolysis and glutaminolysis in collecting duct principal cells

Lithium, given to bipolar disorder patients, causes nephrogenic diabetes insipidus (Li-NDI), a urinary-concentrating defect. Li-NDI occurs due to downregulation of principal cell AQP2 expression, which coincides with principal cell proliferation. The metabolic effect of lithium on principal cells, however, is unknown and investigated here. In earlier studies, we showed that the carbonic anhydrase (CA) inhibitor acetazolamide attenuated Li-induced downregulation in mouse-collecting duct (mpkCCD) cells. Of the eight CAs present in mpkCCD cells, siRNA and drug treatments showed that downregulation of CA9 and to some extent CA12 attenuated Li-induced AQP2 downregulation. Moreover, lithium induced cell proliferation and increased the secretion of lactate. Lithium also increased urinary lactate levels in wild-type mice that developed Li-NDI but not in lithium-treated mice lacking ENaC, the principal cell entry site for lithium. Inhibition of aerobic glycolysis with 2-deoxyglucose (2DG) attenuated lithium-induced AQP2 downregulation in mpkCCD cells but did not attenuate Li-NDI in mice. Interestingly, NMR analysis demonstrated that lithium also increased the urinary succinate, fumarate, citrate, and NH₄⁺ levels, which were, in contrast to lactate, not decreased by 2DG. Together, our data reveal that lithium induces aerobic glycolysis and glutaminolysis in principal cells and that inhibition of aerobic glycolysis, but not the glutaminolysis, does not attenuate Li-NDI.

Long-term aldosterone administration increases renal Na+-Cl− cotransporter abundance in late distal convoluted tubule

Renal Na+-Cl− cotransporter (NCC) is expressed in early distal convoluted tubule (DCT) 1 and late DCT (DCT2). NCC activity can be stimulated by aldosterone administration, and the mechanism is assumed to depend on the enzyme 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2), which inactivates glucocorticoids that would otherwise occupy aldosterone receptors. Because 11β-HSD2 in rat may only be abundantly expressed in DCT2 cells and not in DCT1 cells, it has been speculated that aldosterone specifically stimulates NCC activity in DCT2 cells. In mice, however, it is debated if 11β-HSD2 is expressed in DCT2 cells. The present study examined whether aldosterone administration in mice stimulates NCC abundance and phosphorylation in DCT2 cells but not in DCT1 cells. B6/C57 male mice were administered 100 µg aldosterone·kg body weight−1·24 h−1 for 6 days and euthanized during isoflurane inhalation. Western blotting of whole kidney homogenate showed that aldosterone administration stimulated NCC and pT58-NCC abundances ( P < 0.001). In DCT1 cells, confocal microscopy detected no effect of the aldosterone administration on NCC and pT58-NCC abundances. By contrast, NCC and pT58-NCC abundances were stimulated by aldosterone administration in the middle of DCT2 ( P < 0.001 and <0.01, respectively) and at the junction between DCT2 and CNT ( P < 0.001 and <0.05, respectively). In contrast to rat, immunohistochemistry in mouse showed no/very weak 11β-HSD2 expression in DCT2 cells. Collectively, long-term aldosterone administration stimulates mouse NCC and pT58-NCC abundances in DCT2 cells and presumably not in DCT1 cells.

Transcription factor TFCP2L1 patterns cells in the mouse kidney collecting ducts

Although most nephron segments contain one type of epithelial cell, the collecting ducts consists of at least two: intercalated (IC) and principal (PC) cells, which regulate acid-base and salt-water homeostasis, respectively. In adult kidneys, these cells are organized in rosettes suggesting functional interactions. Genetic studies in mouse revealed that transcription factor Tfcp2l1 coordinates IC and PC development. Tfcp2l1 induces the expression of IC specific genes, including specific H⁺-ATPase subunits and Jag1. Jag1 in turn, initiates Notch signaling in PCs but inhibits Notch signaling in ICs. Tfcp2l1 inactivation deletes ICs, whereas Jag1 inactivation results in the forfeiture of discrete IC and PC identities. Thus, Tfcp2l1 is a critical regulator of IC-PC patterning, acting cell-autonomously in ICs, and non-cell-autonomously in PCs. As a result, Tfcp2l1 regulates the diversification of cell types which is the central characteristic of 'salt and pepper' epithelia and distinguishes the collecting duct from all other nephron segments.

DOI:http://dx.doi.org/10.7554/eLife.24265.001

Role of adenylyl cyclase 6 in the development of lithium-induced nephrogenic diabetes insipidus

Psychiatric patients treated with lithium (Li⁺) may develop nephrogenic diabetes insipidus (NDI). Although the etiology of Li⁺-induced NDI (Li-NDI) is poorly understood, it occurs partially due to reduced aquaporin-2 (AQP2) expression in the kidney collecting ducts. A mechanism postulated for this is that Li⁺ inhibits adenylyl cyclase (AC) activity, leading to decreased cAMP, reduced AQP2 abundance, and less membrane targeting. We hypothesized that Li-NDI would not develop in mice lacking AC6. Whole-body AC6 knockout (AC6^-/-) mice and potentially novel connecting tubule/principal cell-specific AC6 knockout (AC6^loxloxCre) mice had approximately 50% lower urine osmolality and doubled water intake under baseline conditions compared with controls. Dietary Li⁺ administration increased water intake and reduced urine osmolality in control, AC6^-/-, and AC6^loxloxCre mice. Consistent with AC6^-/- mice, medullary AQP2 and pS256-AQP2 abundances were lower in AC6^loxloxCre mice compared with controls under standard conditions, and levels were further reduced after Li⁺ administration. AC6^loxloxCre and control mice had a similar increase in the numbers of proliferating cell nuclear antigen-positive cells in response to Li⁺. However, AC6^loxloxCre mice had a higher number of H⁺-ATPase B1 subunit-positive cells under standard conditions and after Li⁺ administration. Collectively, AC6 has a minor role in Li-NDI development but may be important for determining the intercalated cell-to-principal cell ratio.

Elf5 is a principal cell lineage specific transcription factor in the kidney that contributes to Aqp2 and Avpr2 gene expression

The mammalian kidney collecting ducts are critical for water, electrolyte and acid-base homeostasis and develop as a branched network of tubular structures composed of principal cells intermingled with intercalated cells. The intermingled nature of the different collecting duct cell types has made it challenging to identify unique and critical factors that mark and/or regulate the development of the different collecting duct cell lineages. Here we report that the canonical Notch signaling pathway components, RBPJ and Presinilin1 and 2, are involved in patterning the mouse collecting duct cell fates by maintaining a balance between principal cell and intercalated cell fates. The relatively reduced number of principal cells in Notch-signaling-deficient kidneys offered a unique genetic leverage to identify critical principal cell-enriched factors by transcriptional profiling. Elf5, which codes for an ETS transcription factor, is one such gene that is down-regulated in kidneys with Notch-signaling-deficient collecting ducts. Additionally, Elf5 is among the earliest genes up regulated by ectopic expression of activated Notch1 in the developing collecting ducts. In the kidney, Elf5 is first expressed early within developing collecting ducts and remains on in mature principal cells. Lineage tracing of Elf5-expressing cells revealed that they are committed to the principal cell lineage by as early as E16.5. Over-expression of ETS Class IIa transcription factors, including Elf5, Elf3 and Ehf, increase the transcriptional activity of the proximal promoters of Aqp2 and Avpr2 in cultured ureteric duct cell lines. Conditional inactivation of Elf5 in the developing collecting ducts results in a small but significant reduction in the expression levels of Aqp2 and Avpr2 genes. We have identified Elf5 as an early maker of the principal cell lineage that contributes to the expression of principal cell specific genes.

EGF Receptor Inhibition by Erlotinib Increases Aquaporin 2-Mediated Renal Water Reabsorption

Nephrogenic diabetes insipidus (NDI) is caused by impairment of vasopressin (VP) receptor type 2 signaling. Because potential therapies for NDI that target the canonical VP/cAMP/protein kinase A pathway have so far proven ineffective, alternative strategies for modulating aquaporin 2 (AQP2) trafficking have been sought. Successful identification of compounds by our high-throughput chemical screening assay prompted us to determine whether EGF receptor (EGFR) inhibitors stimulate AQP2 trafficking and reduce urine output. Erlotinib, a selective EGFR inhibitor, enhanced AQP2 apical membrane expression in collecting duct principal cells and reduced urine volume by 45% after 5 days of treatment in mice with lithium-induced NDI. Similar to VP, erlotinib increased exocytosis and decreased endocytosis in LLC-PK1 cells, resulting in a significant increase in AQP2 membrane accumulation. Erlotinib increased phosphorylation of AQP2 at Ser-256 and Ser-269 and decreased phosphorylation at Ser-261 in a dose-dependent manner. However, unlike VP, the effect of erlotinib was independent of cAMP, cGMP, and protein kinase A. Conversely, EGF reduced VP-induced AQP2 Ser-256 phosphorylation, suggesting crosstalk between VP and EGF in AQP2 trafficking and a role of EGF in water homeostasis. These results reveal a novel pathway that contributes to the regulation of AQP2-mediated water reabsorption and suggest new potential therapeutic strategies for NDI treatment.

Recent advances in understanding renal ammonia metabolism and transport

PURPOSE OF REVIEW

The purpose of this review is to provide a succinct description of the recent findings that advance our understanding of the fundamental renal process of ammonia metabolism and transport in conditions relevant to the clinician.

RECENT FINDINGS

Recent studies advance our understanding of renal ammonia metabolism. Mechanisms through which chronic kidney disease and altered dietary protein intake alter ammonia excretion have been identified. Lithium, although it can acutely cause distal renal tubular acidosis, was shown with long-term use to increase urinary ammonia excretion, and this appeared to be mediated, at least in part, by increased Rhcg expression. Gene deletion studies showed that the ammonia recycling enzyme, glutamine synthetase, has a critical role in normal-stimulated and acidosis-stimulated ammonia metabolism and that the proximal tubule basolateral bicarbonate transporter, NBCe1, is necessary for normal ammonia metabolism. Finally, our understanding of the molecular ammonia species, NH3 versus NH4, transported by Rh glycoproteins continues to be advanced.

SUMMARY

Fundamental studies have been recently published that advance our understanding of the regulation of ammonia metabolism in clinically important circumstances, and our understanding of the mechanisms and regulation of proximal tubule ammonia generation, and the mechanisms through which Rh glycoproteins contribute to ammonia secretion.

4-PBA improves lithium-induced nephrogenic diabetes insipidus by attenuating ER stress

Endoplasmic reticulum (ER) stress has been implicated in some types of glomerular and tubular disorders. The objectives of this study were to elucidate the role of ER stress in lithium-induced nephrogenic diabetes insipidus (NDI) and to investigate whether attenuation of ER stress by 4-phenylbutyric acid (4-PBA) improves urinary concentrating defect in lithium-treated rats. Wistar rats received lithium (40 mmol/kg food), 4-PBA (320 mg/kg body wt by gavage every day), or no treatment (control) for 2 wk, and they were dehydrated for 24 h before euthanasia. Lithium treatment resulted in increased urine output and decreased urinary osmolality, which was significantly improved by 4-PBA. 4-PBA also prevented reduced protein expression of aquaporin-2 (AQP2), pS256-AQP2, and pS261-AQP2 in the inner medulla of kidneys from lithium-treated rats after 24-h dehydration. Lithium treatment resulted in increased expression of ER stress markers in the inner medulla, which was associated with dilated cisternae and expansion of ER in the inner medullary collecting duct (IMCD) principal cells. Confocal immunofluorescence studies showed colocalization of a molecular chaperone, binding IgG protein (BiP), with AQP2 in principal cells. Immunohistochemistry demonstrated increased intracellular expression of BiP and decreased AQP2 expression in IMCD principal cells of kidneys from lithium-treated rats. 4-PBA attenuated expression of ER stress markers and recovered ER morphology. In IMCD suspensions isolated from lithium-treated rats, 4-PBA incubation was also associated with increased AQP2 expression and ameliorated ER stress. In conclusion, in experimental lithium-induced NDI, 4-PBA improved the urinary concentrating defect and increased AQP2 expression, likely via attenuating ER stress in IMCD principal cells.

Renal aquaporins and water balance disorders

Aquaporins (AQPs) are a 13 member family (AQP0-12) of proteins that act as channels, through which water and, for some family members, glycerol, urea and other small solutes can be transported. Aquaporins are highly abundant in kidney epithelial cells where they play a critical role with respect to water balance. In this review we summarize the current knowledge with respect to the localization and function of AQPs within the kidney tubule, and their role in mammalian water homeostasis and the water balance disorders. Overviews of practical aspects with regard to differential diagnosis for some of these disorders, alongside treatment strategies are also discussed.