Urinary concentration defects and mechanisms underlying nephronophthisis

NPHP3 splice acceptor site variant is associated with infantile nephronophthisis and asphyxiating thoracic dystrophy; A rare combination

Nephronophthisis (NPHP) is a group of rare inherited ciliopathy disorders characterized by the multicystic dysplastic kidney, oligohydramnios, and tubulointerstitial nephritis that progresses to end-stage renal disease (ESRD). NPHP is a clinically and genetically heterogeneous disorder with extrarenal symptoms including skeletal deformities, nervous system anomalies, and ophthalmologic features. Three clinical subtypes, infantile, juvenile, and adolescent, have been recognized based on age of onset of ESRD. Infantile nephronophthisis with asphyxiating thoracic dystrophy is a very rare association. Here, we investigated a consanguineous family having two neonates with a clinical phenotype of lethal infantile NPHP associated with asphyxiating thoracic dystrophy. Whole exome sequence data analysis identified a splice acceptor site variant (Chr3-132408107-CCT-C; NM_153240.4: c.2694-2_2694-1del) in the NPHP3 gene. The segregation of a variant in the family was confirmed by Sanger sequencing. The lethal phenotype in our case might be due to respiratory insufficiency secondary to a severely restricted thoracic cage. Present work is an exclusive depiction of lethal infantile NPHP phenotype in association with asphyxiating thoracic dystrophy that has not been reported before in families segregating NPHP3 mutations. Moreover, this work expands the phenotypic spectrum of NPHP3 variants. Overall, our findings add to the increasing body of evidence that mutations in ciliary genes/proteins show pleiotropic effects with phenotypic overlap between related disorders and apparently unrelated clinical entities.

Atypical histological abnormalities in an adult patient with nephronophthisis harboring NPHP1 deletion: a case report

BACKGROUND

Nephronophthisis (NPHP) is a chronic tubular interstitial disorder that exhibits an autosomal recessive genetic form and causes progressive renal failure in children. Patients with NPHP rarely show urinary abnormalities, edema, or hypertension. Thus, NPHP is often detected only when renal failure becomes advanced. NPHP can be divided into three types based on the age of end-stage renal failure, i.e., infant type (approximately 5 years old), juvenile type (approximately 13-14 years old), and adolescent type (approximately 19 years old). Here, we report a case of NPHP diagnosed by genetic analysis at 26 years of age with atypical histological abnormalities.

CASE PRESENTATION

A 26-year-old woman showed no growth disorders or urinary abnormalities in annual school physical examinations. However, at a check-up at 26 years old, she exhibited renal dysfunction (eGFR 26 mL/min/1.73 m2). Urine tests indicated low specific gravity of urine, but not proteinuria or microscopic hematuria. Urinary β2-microglobulin was high (805 μg/L), and renal biopsy was performed for definitive diagnosis. Histological findings showed no significant findings in glomeruli. However, moderate fibrosis was observed in the interstitial area, and moderate atrophy was observed in the tubules. There were no significant findings in immunofluorescence analysis, and no electron dense deposits were detected by electron microscopy. Although cyst-like expansion of the tubules was unclear, tubular atrophy was dominantly found in the distal tubule by cytokeratin 7 staining. Genetic analysis of the NPHP1 gene showed complete deletion of this gene, leading to a definitive diagnosis of NPHP.

CONCLUSIONS

NPHP is not merely a pediatric disease and is relatively high incidence in patients with adult onset end-stage of renal disease. In this case, typical histological abnormalities, such as cyst-like expansion of the tubular lesion, were not observed, and diagnosis was achieved by genetic analysis of the NPHP1 gene, which is responsible for the onset of NPHP. In patients with renal failure with tubular interstitial disease dominantly in the distal tubules, it is necessary to discriminate NPHP, even in adult cases.

Renal Ciliopathies: Sorting Out Therapeutic Approaches for Nephronophthisis

Nephronophthisis (NPH) is an autosomal recessive ciliopathy and a major cause of end-stage renal disease in children. The main forms, juvenile and adult NPH, are characterized by tubulointerstitial fibrosis whereas the infantile form is more severe and characterized by cysts. NPH is caused by mutations in over 20 different genes, most of which encode components of the primary cilium, an organelle in which important cellular signaling pathways converge. Ciliary signal transduction plays a critical role in kidney development and tissue homeostasis, and disruption of ciliary signaling has been associated with cyst formation, epithelial cell dedifferentiation and kidney function decline. Drugs have been identified that target specific signaling pathways (for example cAMP/PKA, Hedgehog, and mTOR pathways) and rescue NPH phenotypes in in vitro and/or in vivo models. Despite identification of numerous candidate drugs in rodent models, there has been a lack of clinical trials and there is currently no therapy that halts disease progression in NPH patients. This review covers the most important findings of therapeutic approaches in NPH model systems to date, including hypothesis-driven therapies and untargeted drug screens, approached from the pathophysiology of NPH. Importantly, most animal models used in these studies represent the cystic infantile form of NPH, which is less prevalent than the juvenile form. It appears therefore important to develop new models relevant for juvenile/adult NPH. Alternative non-orthologous animal models and developments in patient-based in vitro model systems are discussed, as well as future directions in personalized therapy for NPH.

The ubiquitin ligase NEDD4-2/NEDD4L regulates both sodium homeostasis and fibrotic signaling to prevent end-stage renal disease

Kidney disease progression can be affected by Na⁺ abundance. A key regulator of Na⁺ homeostasis is the ubiquitin ligase NEDD4-2 and its deficiency leads to increased Na⁺ transport activity and salt-sensitive progressive kidney damage. However, the mechanisms responsible for high Na⁺ induced damage remain poorly understood. Here we show that a high Na⁺ diet compromised kidney function in Nedd4-2-deficient mice, indicative of progression toward end-stage renal disease. Injury was characterized by enhanced tubule dilation and extracellular matrix accumulation, together with sustained activation of both Wnt/β-catenin and TGF-β signaling. Nedd4-2 knockout in cortical collecting duct cells also activated these pathways and led to epithelial-mesenchymal transition. Furthermore, low dietary Na⁺ rescued kidney disease in Nedd4-2-deficient mice and silenced Wnt/β-catenin and TGF-β signaling. Our study reveals the important role of NEDD4-2-dependent ubiquitination in Na⁺ homeostasis and protecting against aberrant Wnt/β-catenin/TGF-β signaling in progressive kidney disease.

A case report of NPHP1 deletion in Chinese twins with nephronophthisis

Background

Nephronophthisis (NPHP) is a rare autosomal recessive inherited disorder with high heterogeneity. The majority of NPHP patients progress to end-stage renal disease (ESRD) within the first three decades of life. As an inherited disorder with highly genetic heterogeneity and clinical presentations, NPHP still poses a challenging task for nephrologists without special training to make a well-judged decision on its precise diagnosis, let alone its mechanism and optimal therapy.

Case presentation

A Chinese family with NPHP was recruited in current study. The clinical characteristics (including findings from renal biopsy) of NPHP patients were collected from medical records and the potential responsible genes were explored by the whole exome sequencing (WES). A homozygous deletion of NPHP1 (1–20 exons) was found in both affected patients, which was further confirmed by quantitative PCR.

Conclusions

Homozygous full gene deletion of the NPHP1 gene was identified in a Chinese family with NPHP, which was the molecular pathogenic basis of this disorder. Furthermore, identification of the pathogenic genes for those affected patients can help to have a full knowledge on NPHP’s molecular mechanism and precise treatment.

A bell-shaped pattern of urinary aquaporin-2-bearing extracellular vesicle release in an experimental model of nephronophthisis

The DBA/2-FG pcy (pcy) mouse is a model of human nephronophthisis, a recessive cystic kidney disease. Renal expression of aquaporin-2 (AQP2), a water channel protein, has been shown to be altered in pcy mice. However, the relationship between the renal expression and its release in urinary extracellular vesicles (uEV-AQP2), which account for most urinary AQP2, in pcy mice has remained largely unknown. In this study, we examined age-related alterations of this relationship in pcy mice. In comparison with control mice, pcy mice after the age of 14 weeks showed defective urinary concentration ability with an increase in urinary volume. Interestingly, the release of uEV-AQP2 increased progressively up to the age of 16 weeks, but at 21 weeks the release did not significantly differ from that in control mice (i.e., a bell-shaped pattern was evident). Similar results were obtained for uEV marker proteins, including tumor susceptibility gene 101 (TSG101) protein and apoptosis-linked gene 2-interacting protein X (Alix). Immunoblot analysis revealed that renal AQP2 expression increased progressively from 11 weeks, and immunohistochemistry showed that this increase was possibly due to an increase in the number of AQP2-positive cells. Analysis of mRNAs for seven types of AQP expressed in the kidney supported this notion. These data suggest that the level of uEV-AQP2 does not simply mirror the renal expression of AQP2 and that the altered release of uEV-AQP2 in pcy mice depends on the numbers of both renal AQP2-positive cells and EVs released into the urine.

Juvenile nephronophthisis and dysthyroidism: a rare association

Nephronophthisis, an autosomal recessive kidney disease, represents the most frequent genetic cause of end-stage kidney disease in the first three decades of life. A 27-year-old male was presented with gait imbalance, sever pruritus since 10 days prior time of admission. In past medical history, he had bilateral cataract, torsional nystagmus, and bilateral optic nerve atrophy since 2 years of age. He was also mentioned history of multinodular goiter with dysfunctional thyroid state since 2 years before admission. At admission bilateral blindness, torsional nystagmus, asymmetric thyromegaly with nodularity was found in physical examination. Laboratory tests showed elevated urea and creatinine (200, 10.7 mg/dl), hypomagnesemia (1.1 mEq/l), decreased thyroid stimulating hormone (<0.004 mIU/l). Ophthalmologist consultation confirmed retinitis pigmentosa. Renal sonography showed small-sized kidneys. Brain magnetic resonance imaging did not reveal molar tooth sign. Genetic testing performed and a large homozygous deletion at the NPHP1 gene locus was found. The patient was diagnosed with juvenile nephronophthisis and consideration of dysthyroidism as extrarenal manifestation of nephronophthisis is suggested in this case. Furthermore, loss of function mutation in SLC41A1 gene that leads to magnesium depletion must be noted in patients with suspected to nephronophthisis.

The Kidney in Bardet-Biedl Syndrome: Possible Pathogenesis of Urine Concentrating Defect

BACKGROUND

The ciliopathies are a growing number of disorders caused by mutations in genes involved in the function of the primary cilium. Bardet-Biedl syndrome (BBS) belongs to this group of disorders. In this setting, kidney dysfunction is highly variable, and urine concentrating defect, a common feature of multiple ciliopathies, has been described as the most frequent defect. Here we review the mechanism of urine concentration and describe the possible mechanism underling this defect in ciliopathies and especially BBS, based on the current body of literature.

SUMMARY

Active Na⁺ absorption along the thick ascending limb of the loop of Henle (TAL) is critical for generating the corticomedullary osmotic gradient, and the countercurrent anatomical arrangement of the 2 branches of the loop of Henle enhances this gradient. The vasa recta, paralleling the loop of Henle, operate into the countercurrent mechanism, minimizing washout of solutes from the interstitium. Final water reabsorption is mediated by the aquaporin 2 (AQP2) water channels along the distal nephron, and it is under hormonal control. Several studies demonstrated that hyposthenuria in BBS patients relies on kidney resistance to desmopressin, suggesting a renal origin. We recently showed that the majority of hyposthenuric BBS patients have also a defect regarding maximal urine dilution. Independent studies showed that BBS10 deficiency caused AQP2 mistrafficking in vitro; accordingly, we demonstrated impaired urinary AQP2 excretion in BBS patients with combined concentrating and diluting defect. Whether receptor signaling pathways or downstream events cause AQP2 deregulation is still unclear. In addition, reduced urinary uromodulin excretion in BBS patients opens the possibility that TAL dysfunction may also play a pathogenic role.

KEY MESSAGE

Impaired water handling in BBS is associated with AQP2 mistrafficking. The potential role of additional factors, such as the dissipation of the medullary osmotic gradient due to TAL dysfunction and/or structural anomalies, remains to be elucidated.

Many Genes-One Disease? Genetics of Nephronophthisis (NPHP) and NPHP-Associated Disorders

Nephronophthisis (NPHP) is a renal ciliopathy and an autosomal recessive cause of cystic kidney disease, renal fibrosis, and end-stage renal failure, affecting children and young adults. Molecular genetic studies have identified more than 20 genes underlying this disorder, whose protein products are all related to cilia, centrosome, or mitotic spindle function. In around 15% of cases, there are additional features of a ciliopathy syndrome, including retinal defects, liver fibrosis, skeletal abnormalities, and brain developmental disorders. Alongside, gene identification has arisen molecular mechanistic insights into the disease pathogenesis. The genetic causes of NPHP are discussed in terms of how they help us to define treatable disease pathways including the cyclic adenosine monophosphate pathway, the mTOR pathway, Hedgehog signaling pathways, and DNA damage response pathways. While the underlying pathology of the many types of NPHP remains similar, the defined disease mechanisms are diverse, and a personalized medicine approach for therapy in NPHP patients is likely to be required.

Renal phenotype in Bardet-Biedl syndrome: a combined defect of urinary concentration and dilution is associated with defective urinary AQP2 and UMOD excretion

The renal phenotype in Bardet-Biedl syndrome (BBS) is highly variable. The present study describes renal findings in 41 BBS patients and analyzes the pathogenesis of hyposthenuria, the most common renal dysfunction. Five of 41 patients (12%) showed an estimated glomerular filtration rate < 60 ml·min-1·1.73 m-2 Urine protein and urine albumin-to-creatinine ratio were over 200 and 30 mg/g in 9/24 and 7/23 patients, respectively. Four of 41 patients showed no renal anomalies on ultrasound. Twenty of 34 patients had hyposthenuria in the absence of renal insufficiency. In all 8 of the hyposthenuric patients studied, dDAVP failed to elevate urine osmolality (Uosm), suggesting a nephrogenic origin. Interestingly, water loading (WL) did not result in a significant reduction of Uosm, indicating combined concentrating and diluting defects. dDAVP infusion induced a significant increase of plasma Factor VIII and von Willebrand Factor levels, supporting normal function of the type 2 vasopressin receptor at least in endothelial cells. While urinary aquaporin 2 (u-AQP2) abundance was not different between patients and controls at baseline, the dDAVP-induced increased u-AQP2 and the WL-induced reduction of u-AQP2 were blunted in patients with a combined concentrating and diluting defect, suggesting a potential role of AQP2 in the defective regulation of water absorption. Urine Uromodulin excretion was reduced in all hyposthenuric patients, suggesting a thick ascending limb defect. Interestingly, renal Na, Cl, Ca, but not K handling was impaired after acute WL but not at basal. In summary, BBS patients show combined urinary concentration and dilution defects; a thick ascending limb and collecting duct tubulopathy may underlie impaired water handling.

Identification of an NPHP1 deletion causing adult form of nephronophthisis

AIMS

Nephronophthisis (NPHP) is an autosomal recessive cystic disease of the kidney with main characteristic features of polyuria/polydipsia, mild or absent proteinuria, interstitial fibrosis, and tubular cysts. NPHP is responsible for 5-10 % of inheritable end-stage renal disease (ESRD) cases. We investigated the clinical features and genetic cause of NPHP in a Persian family with three siblings affected by tubulointerstitial nephropathy reaching ESRD in adulthood.

METHODS

Uromodulin (UMOD), known to be involved in adult medullary cystic kidney disease, and nephronophthisis 1 (NPHP1) were investigated in the genomic DNA of the probands using DNA sequencing, multiplex ligation-dependent probe amplification (MLPA) analysis and molecular karyotyping.

RESULTS

No mutation was detected in UMOD. Copy number variation analysis of the NPHP1 gene using the commercially available MLPA kit identified a recurrent large homozygous deletion encompassing all NPHP1 exons. The parents were heterozygous for this deletion. Whole genome array-CGH analysis confirmed a homozygous deletion on chromosome 2q13, NPHP1 site, and revealed that the size of the copy number loss was approximately 102 Kbp.

CONCLUSION

This is the first report of determination of an NPHP1 deletion size using routine diagnostic methods. The results of this study expand the knowledge about the genotype-phenotype correlations in NPHP1, and have implications for genetic counseling and family planning advice for other affected families. This is the first molecular analysis of NPHP1 in an Iranian kindred.

Vasopressin-2 receptor signaling and autosomal dominant polycystic kidney disease: from bench to bedside and back again

Blockade of the vasopressin-2 receptor (V2R) in the kidney has recently emerged as a promising therapeutic strategy in autosomal dominant polycystic kidney disease. The pathophysiologic basis of V2R-dependent cyst proliferation and disease progression, however, is not fully understood. Recent evidence suggests that polycystic kidney disease is characterized by defects in urinary concentrating mechanisms and subsequent deregulation of vasopressin excretion by the neurohypophysis. On the cellular level, several recent studies revealed unexpected crosstalk of signaling pathways downstream of V2R activation in the kidney epithelium. This review summarizes some of the unexpected roles of V2R signaling and suggests that vasopressin signaling itself may contribute crucially to loss of polarity and enhanced proliferation in cystic kidney epithelium.

Functional modelling of a novel mutation in BBS5

Background

Bardet-Biedl syndrome (BBS) is an autosomal recessive ciliopathy disorder with 18 known causative genes (BBS1-18). The primary clinical features are renal abnormalities, rod-cone dystrophy, post-axial polydactyly, learning difficulties, obesity and male hypogonadism.

Results

We describe the clinical phenotype in three Saudi siblings in whom we have identified a novel mutation in exon 12 of BBS5 (c.966dupT; p.Ala323CysfsX57). This single nucleotide duplication creates a frame shift results in a predicted elongated peptide. Translation blocking Morpholino oligonucleotides were used to create zebrafish bbs5 morphants. Morphants displayed retinal layering defects, abnormal cardiac looping and dilated, cystic pronephric ducts with reduced cilia expression. Morphants also displayed significantly reduced dextran clearance via the pronephros compared to wildtype embryos, suggesting reduced renal function in morphants. The eye, kidney and heart defects reported in morphant zebrafish resemble the human phenotype of BBS5 mutations. The pathogenicity of the novel BBS5 mutation was determined. Mutant mRNA was unable to rescue pleiotropic phenotypes of bbs5 morphant zebrafish and in cell culture we demonstrate a mislocalisation of mutant BBS5 protein which fails to localise discretely with the basal body.

Conclusions

We conclude that this novel BBS5 mutation has a deleterious function that accounts for the multisystem ciliopathy phenotype seen in affected human patients.

The ciliary protein nephrocystin-4 translocates the canonical Wnt regulator Jade-1 to the nucleus to negatively regulate β-catenin signaling

Nephronophthisis (NPH) is an autosomal-recessive cystic kidney disease and represents the most common genetic cause for end-stage renal disease in children and adolescents. It can be caused by the mutation of genes encoding for the nephrocystin proteins (NPHPs). All NPHPs localize to primary cilia, classifying this disease as a "ciliopathy." The primary cilium is a critical regulator of several cell signaling pathways. Cystogenesis in the kidney is thought to involve overactivation of canonical Wnt signaling, which is negatively regulated by the primary cilium and several NPH proteins, although the mechanism remains unclear. Jade-1 has recently been identified as a novel ubiquitin ligase targeting the canonical Wnt downstream effector β-catenin for proteasomal degradation. Here, we identify Jade-1 as a novel component of the NPHP protein complex. Jade-1 colocalizes with NPHP1 at the transition zone of primary cilia and interacts with NPHP4. Furthermore, NPHP4 stabilizes protein levels of Jade-1 and promotes the translocation of Jade-1 to the nucleus. Finally, NPHP4 and Jade-1 additively inhibit canonical Wnt signaling, and this genetic interaction is conserved in zebrafish. The stabilization and nuclear translocation of Jade-1 by NPHP4 enhances the ability of Jade-1 to negatively regulate canonical Wnt signaling. Loss of this repressor function in nephronophthisis might be an important factor promoting Wnt activation and contributing to cyst formation.

Juvenile nephropathy in a Boxer dog resembling the human nephronophthisis-medullary cystic kidney disease complex

A juvenile nephropathy in a 4-year-old male Boxer dog, closely resembling the Nephronophthisis (NPHP)-Medullary Cystic Kidney Disease Complex (MCKD) in humans is described. Gross examination of the kidneys revealed several multiple cysts at the corticomedullary junction and in the medulla. Histological examination was characterized by a widespread tubular atrophy and dilatation, with a marked thickening of the tubular basement membrane, interstitial lymphocytic infiltration and fibrosis. Ultrastructural studies revealed dilated tubules with irregular basement membrane thickening and splitting. Lectin histochemistry investigation revealed that the cysts originated in the distal convoluted tubule and collecting duct. Having excluded all other known cystic diseases of the kidney, and based on the lectin histochemistry results, the macroscopic and histological findings of our case are highly compatible with a diagnosis of the NPHP-MCKD complex. To our knowledge, this is the first report describing this particular lesion.

Nephronophthisis: a genetically diverse ciliopathy

Nephronophthisis (NPHP) is an autosomal recessive cystic kidney disease and a leading genetic cause of established renal failure (ERF) in children and young adults. Early presenting symptoms in children with NPHP include polyuria, nocturia, or secondary enuresis, pointing to a urinary concentrating defect. Renal ultrasound typically shows normal kidney size with increased echogenicity and corticomedullary cysts. Importantly, NPHP is associated with extra renal manifestations in 10-15% of patients. The most frequent extrarenal association is retinal degeneration, leading to blindness. Increasingly, molecular genetic testing is being utilised to diagnose NPHP and avoid the need for a renal biopsy. In this paper, we discuss the latest understanding in the molecular and cellular pathogenesis of NPHP. We suggest an appropriate clinical management plan and screening programme for individuals with NPHP and their families.

Missing-in-metastasis MIM/MTSS1 promotes actin assembly at intercellular junctions and is required for integrity of kidney epithelia

MIM/MTSS1 is a tissue-specific regulator of plasma membrane dynamics, whose altered expression levels have been linked to cancer metastasis. MIM deforms phosphoinositide-rich membranes through its I-BAR domain and interacts with actin monomers through its WH2 domain. Recent work proposed that MIM also potentiates Sonic hedgehog (Shh)-induced gene expression. Here, we generated MIM mutant mice and found that full-length MIM protein is dispensable for embryonic development. However, MIM-deficient mice displayed a severe urinary concentration defect caused by compromised integrity of kidney epithelia intercellular junctions, which led to bone abnormalities and end-stage renal failure. In cultured kidney epithelial (MDCK) cells, MIM displayed dynamic localization to adherens junctions, where it promoted Arp2/3-mediated actin filament assembly. This activity was dependent on the ability of MIM to interact with both membranes and actin monomers. Furthermore, results from the mouse model and cell culture experiments suggest that full-length MIM is not crucial for Shh signaling, at least during embryogenesis. Collectively, these data demonstrate that MIM modulates interplay between the actin cytoskeleton and plasma membrane to promote the maintenance of intercellular contacts in kidney epithelia.

NPHP4 is necessary for normal photoreceptor ribbon synapse maintenance and outer segment formation, and for sperm development

Nephronophthisis (NPHP) is an autosomal recessive kidney disease that is often associated with vision and/or brain defects. To date, 11 genes are known to cause NPHP. The gene products, while structurally unrelated, all localize to cilia or centrosomes. Although mouse models of NPHP are available for 9 of the 11 genes, none has been described for nephronophthisis 4 (Nphp4). Here we report a novel, chemically induced mutant, nmf192, that bears a nonsense mutation in exon 4 of Nphp4. Homozygous mutant Nphp4(nmf192/nmf192) mice do not exhibit renal defects, phenotypes observed in human patients bearing mutations in NPHP4, but they do develop severe photoreceptor degeneration and extinguished rod and cone ERG responses by 9 weeks of age. Photoreceptor outer segments (OS) fail to develop properly, and some OS markers mislocalize to the inner segments and outer nuclear layer in the Nphp4(nmf192/nmf192) mutant retina. Despite NPHP4 localization to the transition zone in the connecting cilia (CC), the CC appear to be normal in structure and ciliary transport function is partially retained. Likewise, synaptic ribbons develop normally but then rapidly degenerate by P14. Finally, Nphp4(nmf192/nmf192) male mutants are sterile and show reduced sperm motility and epididymal sperm counts. Although Nphp4(nmf192/nmf192) mice fail to recapitulate the kidney phenotype of NPHP, they will provide a valuable tool to further elucidate how NPHP4 functions in the retina and male reproductive organs.

Impaired water reabsorption in mice deficient in the type VI adenylyl cyclase (AC6)

Adenylyl cyclase (AC) type VI (AC6) is a calcium-inhibitable enzyme which produces cAMP upon stimulation. Herein, we characterized the specific role of AC6 in the kidneys using two AC6-knockout mouse lines. Immunohistochemical staining revealed that AC6 exists in the tubular parts of the nephron and collecting duct. Activities of AC evoked by forskolin or a selective agonist of the V2 vasopressin receptor were lower in the kidneys of AC6-null mice compared to those of wildtype mice. Results of a metabolic cage assay and dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) showed for the first time that AC6 plays a critical role in regulating water homeostasis.

Impaired Wnt-beta-catenin signaling disrupts adult renal homeostasis and leads to cystic kidney ciliopathy

Cystic kidney disease represents a major cause of end-stage renal disease, yet the molecular mechanisms of pathogenesis remain largely unclear. Recent emphasis has been placed on a potential role for canonical Wnt signaling, but investigation of this pathway in adult renal homeostasis is lacking. Here we provide evidence of a previously unidentified canonical Wnt activity in adult mammalian kidney homeostasis, the loss of which leads to cystic kidney disease. Loss of the Jouberin (Jbn) protein in mouse leads to the cystic kidney disease nephronophthisis, owing to an unexpected decrease in endogenous Wnt activity. Jbn interacts with and facilitates beta-catenin nuclear accumulation, resulting in positive modulation of downstream transcription. Finally, we show that Jbn is required in vivo for a Wnt response to injury and renal tubule repair, the absence of which triggers cystogenesis.

Nephronophthisis

Nephronophthisis (NPHP) is an autosomal recessive kidney disorder characterized by chronic tubulointerstitial nephritis and leading to end-stage renal failure. NPHP as a renal entity is often part of a multisystem disorder and has been associated with many syndromes including Joubert syndrome (and related disorders) and Senior-Loken syndrome. Recent molecular genetic advances have allowed identification of several genes underlying NPHP. Most of these genes express their protein products, named nephrocystins, in primary cilial/basal body structures. Some nephrocystins are part of adherens junction and focal adhesion kinase protein complexes. This shared localization suggests that common pathogenic mechanisms within the kidney underlie this disease. Functional studies implicate nephrocystins in planar cell polarity pathways, which may be crucial for renal development and maintenance of tubular architecture.

Nephrocystin-1 interacts directly with Ack1 and is expressed in human collecting duct

Nephronophthisis is characterised by renal fibrosis, tubular basement membrane disruption and corticomedullary cyst formation leading to end stage renal failure. Mutations in NPHP1 account for the underlying genetic defect in 25% of patients with nephronophthisis. Loss of urine concentration ability may be an early feature of nephronophthisis. Using yeast-2-library screening with the SH3 domain of nephrocystin-1 as bait, we identify Ack1 as a novel interaction partner. This interaction is confirmed using exogenous over-expression followed by co-immunoprecipitation. Ack1 is an activated Cdc42-associated kinase, and like nephrocystin-1, is a known interactor of p130Cas. Nephrocystin-1 partially colocalises with Ack1 at cell-cell contacts in IMCD3 cells. In human kidney, nephrocystin-1 expression is limited to cell-cell junctions in renal collecting duct cells. These data define Ack1 as a novel interaction partner of nephrocystin-1 and implicate cell-cell junctions and the renal collecting duct in the pathology of nephronophthisis.