Collecting duct cells that lack normal cilia have mislocalized vasopressin-2 receptors

Sex-Dependent Effects of Nephron Ift88 Disruption on BP, Renal Function, and Cystogenesis

BACKGROUND

Primary cilia regulation of renal function and BP in health and disease is incompletely understood. This study investigated the effect of nephron ciliary loss on renal physiology, BP, and ensuing cystogenesis.

METHODS

Mice underwent doxycycline (DOX)-inducible nephron-specific knockout (KO) of the Ift88 gene at 2 months of age using a Cre-LoxP strategy. BP, kidney function, and renal pathology were studied 2 and 9 months after DOX (Ift88 KO) or vehicle (control).

RESULTS

At 2 months post-DOX, male, but not female, Ift88 KO, compared with sex-matched control, mice had reduced BP, enhanced salt-induced natriuresis, increased urinary nitrite and nitrate (NOx) excretion, and increased kidney NOS3 levels, which localized to the outer medulla; the reductions in BP in male mice were prevented by L-NAME. At 9 months post-DOX, male, but not female, Ift88 KO mice had polycystic kidneys, elevated BP, and reduced urinary NOx excretion. No differences were observed in plasma renin concentration, plasma aldosterone, urine vasopressin, or urine PGE₂ between Ift88 KO and control mice at 2 or 9 months post-DOX.

CONCLUSIONS

Nephron cilia disruption in male, but not female, mice (1) reduces BP prior to cyst formation, (2) increases NOx production that may account for the lower BP prior to cyst formation, and (3) induces polycystic kidneys that are associated with hypertension and reduced renal NO production.

Local cyclic adenosine monophosphate signalling cascades-Roles and targets in chronic kidney disease

The molecular mechanisms underlying chronic kidney disease (CKD) are poorly understood and treatment options are limited, a situation underpinning the need for elucidating the causative molecular mechanisms and for identifying innovative treatment options. It is emerging that cyclic 3',5'-adenosine monophosphate (cAMP) signalling occurs in defined cellular compartments within nanometre dimensions in processes whose dysregulation is associated with CKD. cAMP compartmentalization is tightly controlled by a specific set of proteins, including A-kinase anchoring proteins (AKAPs) and phosphodiesterases (PDEs). AKAPs such as AKAP18, AKAP220, AKAP-Lbc and STUB1, and PDE4 coordinate arginine-vasopressin (AVP)-induced water reabsorption by collecting duct principal cells. However, hyperactivation of the AVP system is associated with kidney damage and CKD. Podocyte injury involves aberrant AKAP signalling. cAMP signalling in immune cells can be local and slow the progression of inflammatory processes typical for CKD. A major risk factor of CKD is hypertension. cAMP directs the release of the blood pressure regulator, renin, from juxtaglomerular cells, and plays a role in Na⁺ reabsorption through ENaC, NKCC2 and NCC in the kidney. Mutations in the cAMP hydrolysing PDE3A that cause lowering of cAMP lead to hypertension. Another major risk factor of CKD is diabetes mellitus. AKAP18 and AKAP150 and several PDEs are involved in insulin release. Despite the increasing amount of data, an understanding of functions of compartmentalized cAMP signalling with relevance for CKD is fragmentary. Uncovering functions will improve the understanding of physiological processes and identification of disease-relevant aberrations may guide towards new therapeutic concepts for the treatment of CKD.

Limitations and opportunities in the pharmacotherapy of ciliopathies

Ciliopathies are a family of rather diverse conditions, which have been grouped based on the finding of altered or dysfunctional cilia, potentially motile, small cellular antennae extending from the surface of postmitotic cells. Cilia-related disorders include embryonically arising conditions such as Joubert, Usher or Kartagener syndrome, but also afflictions with a postnatal or even adult onset phenotype, i.e. autosomal dominant polycystic kidney disease. The majority of ciliopathies are syndromic rather than affecting only a single organ due to cilia being found on almost any cell in the human body. Overall ciliopathies are considered rare diseases. Despite that, pharmacological research and the strive to help these patients has led to enormous therapeutic advances in the last decade. In this review we discuss new treatment options for certain ciliopathies, give an outlook on promising future therapeutic strategies, but also highlight the limitations in the development of therapeutic approaches of ciliopathies.

Loss of primary cilia increases polycystin-2 and TRPV4 and the appearance of a nonselective cation channel in the mouse cortical collecting duct

Flow-related bending of cilia results in Ca²⁺ influx through a polycystin-1 (Pkd1) and polycystin-2 (Pkd2) complex, both of which are members of the transient receptor potential (TRP) family (TRPP1 and TRPP2, respectively). Deletion of this complex as well as cilia result in polycystic kidney disease. The Ca²⁺ influx pathway has been previously characterized in immortalized collecting duct cells without cilia and found to be a 23-pS channel that was a multimere of TRPP2 and TRPV4. The purpose of the present study was to determine if this TRPP2 and TRPV4 multimere exists in vivo. Apical channel activity was measured using the patch-clamp technique from isolated split-open cortical collecting ducts from adult conditional knockout mice with (Ift88^flox/flox) or without (Ift88^-/-) cilia. Single tubules were isolated for measurements of mRNA for Pkd1, Pkd2, Trpv4, and epithelial Na⁺ channel subunits. The predominant channel activity from Ift88^flox/flox mice was from epithelial Na⁺ channel [5-pS Na⁺-selective channels with long mean open times (475.7 ± 83.26 ms) and open probability > 0.2]. With the loss of cilia, the predominant conductance was a 23-pS nonselective cation channel (reversal potential near 0) with a short mean open time (72 ± 17 ms), open probability < 0.08, and a characteristic flickery opening. Loss of cilia increased mRNA levels for Pkd2 and Trpv4 from single isolated cortical collecting ducts. In conclusion, 23-pS channels exist in vivo, and activity of this channel is elevated with loss of cilia, consistent with previous finding of an elevated-unregulated Ca²⁺-permeable pathway at the apical membrane of collecting duct cells that lack cilia.

Inhibition of non-receptor tyrosine kinase Src induces phosphoserine 256-independent aquaporin-2 membrane accumulation

KEY POINTS

Aquaporin-2 (AQP2) is crucial for water homeostasis, and vasopressin (VP) induces AQP2 membrane trafficking by increasing intracellular cAMP, activating PKA and causing phosphorylation of AQP2 at serine 256, 264 and 269 residues and dephosphorylation of serine 261 residue on the AQP2 C-terminus. It is thought that serine 256 is the master regulator of AQP2 trafficking, and its phosphorylation has to precede the change of phosphorylation state of other serine residues. We found that Src inhibition causes serine 256-independent AQP2 membrane trafficking and induces phosphorylation of serine 269 independently of serine 256. This targeted phosphorylation of serine 269 is important for Src inhibition-induced AQP2 membrane accumulation; without serine 269, Src inhibition exerts no effect on AQP2 trafficking. This result helps us better understand the independent pathways that can target different AQP2 residues, and design new strategies to induce or sustain AQP2 membrane expression when VP signalling is defective.

ABSTRACT

Aquaporin-2 (AQP2) is essential for water homeostasis. Upon stimulation by vasopressin, AQP2 is phosphorylated at serine 256 (S256), S264 and S269, and dephosphorylated at S261. It is thought that S256 is the master regulator of AQP2 trafficking and membrane accumulation, and that its phosphorylation has to precede phosphorylation of other serine residues. In this study, we found that VP reduces Src kinase phosphorylation: by suppressing Src using the inhibitor dasatinib and siRNA, we could increase AQP2 membrane accumulation in cultured AQP2-expressing cells and in kidney collecting duct principal cells. Src inhibition increased exocytosis and inhibited clathrin-mediated endocytosis of AQP2, but exerted its effect in a cAMP, PKA and S256 phosphorylation (pS256)-independent manner. Despite the lack of S256 phosphorylation, dasatinib increased phosphorylation of S269, even in S256A mutant cells in which S256 phosphorylation cannot occur. To confirm the importance of pS269 in AQP2 re-distribution, we expressed an AQP2 S269A mutant in LLC-PK1 cells, and found that dasatinib no longer induced AQP2 membrane accumulation. In conclusion, Src inhibition causes phosphorylation of S269 independently of pS256, and induces AQP2 membrane accumulation by inhibiting clathrin-mediated endocytosis and increasing exocytosis. We conclude that S269 can be phosphorylated without pS256, and pS269 alone is important for AQP2 apical membrane accumulation under some conditions. These data increase our understanding of the independent pathways that can phosphorylate different residues in the AQP2 C-terminus, and suggest new strategies to target distinct AQP2 serine residues to induce membrane expression of this water channel when VP signalling is defective.

Salt-deficient diet exacerbates cystogenesis in ARPKD via epithelial sodium channel (ENaC)

Background

Autosomal Recessive Polycystic Kidney Disease (ARPKD) is marked by cyst formation in the renal tubules, primarily in the collecting duct (CD) system, ultimately leading to end-stage renal disease. Patients with PKD are generally advised to restrict their dietary sodium intake. This study was aimed at testing the outcomes of dietary salt manipulation in ARPKD.

Methods

PCK/CrljCrlPkhd1pck/CRL (PCK) rats, a model of ARPKD, were fed a normal (0.4% NaCl; NS), high salt (4% NaCl; HS), and sodium-deficient (0.01% NaCl; SD) diets for 8 weeks. Immunohistochemistry, GFR measurements, balance studies, and molecular biology approaches were applied to evaluate the outcomes of the protocol. Renin-angiotensin-aldosterone system (RAAS) levels were assessed using LC-MS/MS, and renal miRNA profiles were studied.

Findings

Both HS and SD diets resulted in an increase in cystogenesis. However, SD diet caused extensive growth of cysts in the renal cortical area, and hypertrophy of the tissue; RAAS components were enhanced in the SD group. We observed a reduction in epithelial Na⁺ channel (ENaC) expression in the SD group, accompanied with mRNA level increase. miRNA assay revealed that renal miR-9a-5p level was augmented in the SD group; we showed that this miRNA decreases ENaC channel number in CD cells.

Interpretation

Our data demonstrate a mechanism of ARPKD progression during salt restriction that involves activity of ENaC. We further show that miR-9a-5p potentially implicated in this mechanism and that miR-9a-5p downregulates ENaC in cultured CD cells. Our findings open new therapeutic possibilities and highlight the importance of understanding salt reabsorption in ARPKD.

Ciliogenesis is reciprocally regulated by PPARA and NR1H4/FXR through controlling autophagy in vitro and in vivo

The primary cilia are evolutionarily conserved microtubule-based cellular organelles that perceive metabolic status and thus link the sensory system to cellular signaling pathways. Therefore, ciliogenesis is thought to be tightly linked to autophagy, which is also regulated by nutrient-sensing transcription factors, such as PPARA (peroxisome proliferator activated receptor alpha) and NR1H4/FXR (nuclear receptor subfamily 1, group H, member 4). However, the relationship between these factors and ciliogenesis has not been clearly demonstrated. Here, we present direct evidence for the involvement of macroautophagic/autophagic regulators in controlling ciliogenesis. We showed that activation of PPARA facilitated ciliogenesis independently of cellular nutritional states. Importantly, PPARA-induced ciliogenesis was mediated by controlling autophagy, since either pharmacological or genetic inactivation of autophagy significantly repressed ciliogenesis. Moreover, we showed that pharmacological activator of autophagy, rapamycin, recovered repressed ciliogenesis in ppara^-/- cells. Conversely, activation of NR1H4 repressed cilia formation, while knockdown of NR1H4 enhanced ciliogenesis by inducing autophagy. The reciprocal activities of PPARA and NR1H4 in regulating ciliogenesis were highlighted in a condition where de-repressed ciliogenesis by NR1H4 knockdown was further enhanced by PPARA activation. The in vivo roles of PPARA and NR1H4 in regulating ciliogenesis were examined in greater detail in ppara^-/- mice. In response to starvation, ciliogenesis was facilitated in wild-type mice via enhanced autophagy in kidney, while ppara^-/- mice displayed impaired autophagy and kidney damage resembling ciliopathy. Furthermore, an NR1H4 agonist exacerbated kidney damage associated with starvation in ppara^-/- mice. These findings indicate a previously unknown role for PPARA and NR1H4 in regulating the autophagy-ciliogenesis axis in vivo.

Heterotrimeric G protein signaling in polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a signalopathy of renal tubular epithelial cells caused by naturally occurring mutations in two distinct genes, polycystic kidney disease 1 (PKD1) and 2 (PKD2). Genetic variants in PKD1, which encodes the polycystin-1 (PC-1) protein, remain the predominant factor associated with the pathogenesis of nearly two-thirds of all patients diagnosed with PKD. Although the relationship between defective PC-1 with renal cystic disease initiation and progression remains to be fully elucidated, there are numerous clinical studies that have focused upon the control of effector systems involving heterotrimeric G protein regulation. A major regulator in the activation state of heterotrimeric G proteins are G protein-coupled receptors (GPCRs), which are defined by their seven transmembrane-spanning regions. PC-1 has been considered to function as an unconventional GPCR, but the mechanisms by which PC-1 controls signal processing, magnitude, or trafficking through heterotrimeric G proteins remains to be fully known. The diversity of heterotrimeric G protein signaling in PKD is further complicated by the presence of non-GPCR proteins in the membrane or cytoplasm that also modulate the functional state of heterotrimeric G proteins within the cell. Moreover, PC-1 abnormalities promote changes in hormonal systems that ultimately interact with distinct GPCRs in the kidney to potentially amplify or antagonize signaling output from PC-1. This review will focus upon the canonical and noncanonical signaling pathways that have been described in PKD with specific emphasis on which heterotrimeric G proteins are involved in the pathological reorganization of the tubular epithelial cell architecture to exacerbate renal cystogenic pathways.

Molecular pathways and therapies in autosomal-dominant polycystic kidney disease

Autosomal-dominant polycystic kidney disease (ADPKD) is the most prevalent inherited renal disease, characterized by multiple cysts that can eventually lead to kidney failure. Studies investigating the role of primary cilia and polycystins have significantly advanced our understanding of the pathogenesis of PKD. This review will present clinical and basic aspects of ADPKD, review current concepts of PKD pathogenesis, evaluate potential therapeutic targets, and highlight challenges for future clinical studies.

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.

Activation of the intrarenal renin-angiotensin-system in murine polycystic kidney disease

The mechanism for early hypertension in polycystic kidney disease (PKD) has not been elucidated. One potential pathway that may contribute to the elevation in blood pressure in PKD is the activation of the intrarenal renin-angiotensin-system (RAS). For example, it has been shown that kidney cyst and cystic fluid contains renin, angiotensin II (AngII), and angiotensinogen (Agt). Numerous studies suggest that ciliary dysfunction plays an important role in PKD pathogenesis. However, it is unknown whether the primary cilium affects the intrarenal RAS in PKD. The purpose of this study was to determine whether loss of cilia or polycystin 1 (PC1) increases intrarenal RAS in mouse model of PKD. Adult Ift88 and Pkd1 conditional floxed allele mice with or without cre were administered tamoxifen to induce global knockout of the gene. Three months after tamoxifen injection, kidney tissues were examined by histology, immunofluorescence, western blot, and mRNA to assess intrarenal RAS components. SV40 immortalized collecting duct cell lines from hypomorphic Ift88 mouse were used to assess intrarenal RAS components in collecting duct cells. Mice without cilia and PC1 demonstrated increased kidney cyst formation, systolic blood pressure, prorenin, and kidney and urinary angiotensinogen levels. Interestingly immunofluorescence study of the kidney revealed that the prorenin receptor was localized to the basolateral membrane of principal cells in cilia (−) but not in cilia (+) kidneys. Collecting duct cAMP responses to AngII administration was greater in cilia (−) vs. cilia (+) cells indicating enhanced intrarenal RAS activity in the absence of cilia. These data suggest that in the absence of cilia or PC1, there is an upregulation of intrarenal RAS components and activity, which may contribute to elevated blood pressure in PKD.

A defect in vasopressin secretion in autosomal dominant polycystic kidney disease

A nephrogenic defect in urine concentration is well established in patients with polycystic kidney disease, but Ho et al. report a defect in the increase of plasma vasopressin in response to dehydration. On a cellular level, transient receptor potential channels responsible for osmoperception could interact with TRPPs encoded by the polycystic genes PKD1 and PKD2.

Nedd9 restrains renal cystogenesis in Pkd1-/- mice

Mutations inactivating the cilia-localized Pkd1 protein result in autosomal dominant polycystic kidney disease (ADPKD), a serious inherited syndrome affecting ∼ 1 in 500 people, in which accumulation of renal cysts eventually destroys kidney function. Severity of ADPKD varies throughout the population, for reasons thought to involve differences both in intragenic Pkd1 mutations and in modifier alleles. The scaffolding protein NEDD9, commonly dysregulated during cancer progression, interacts with Aurora-A (AURKA) kinase to control ciliary resorption, and with Src and other partners to influence proliferative signaling pathways often activated in ADPKD. We here demonstrate Nedd9 expression is deregulated in human ADPKD and a mouse ADPKD model. Although genetic ablation of Nedd9 does not independently influence cystogenesis, constitutive absence of Nedd9 strongly promotes cyst formation in the tamoxifen-inducible Pkd1fl/fl;Cre/Esr1(+) mouse model of ADPKD. This cystogenic effect is associated with striking morphological defects in the cilia of Pkd1(-/-);Nedd9(-/-) mice, associated with specific loss of ciliary localization of adenylase cyclase III in the doubly mutant genotype. Ciliary phenotypes imply a failure of Aurora-A activation: Compatible with this idea, Pkd1(-/-);Nedd9(-/-) mice had ciliary resorption defects, and treatment of Pkd1(-/-) mice with a clinical Aurora-A kinase inhibitor exacerbated cystogenesis. In addition, activation of the ADPKD-associated signaling effectors Src, Erk, and the mTOR effector S6 was enhanced, and Ca(2+) response to external stimuli was reduced, in Pkd1(-/-);Nedd9(-/-) versus Pkd1(-/-) mice. Together, these results indicated an important modifier action of Nedd9 on ADPKD pathogenesis involving failure to activate Aurora-A.

Epidermal growth factor-induced proliferation of collecting duct cells from Oak Ridge polycystic kidney mice involves activation of Na+/H+ exchanger

Epidermal growth factor (EGF) is linked to the pathogenesis of polycystic kidney disease (PKD). We explored signaling pathways activated by EGF in orpk cilia (-) collecting duct cell line derived from a mouse model of PKD (hypomorph of the Tg737/Ift88 gene) with severely stunted cilia, and in a control orpk cilia (+) cell line with normal cilia. RT-PCR demonstrated mRNAs for EGF receptor subunits ErbB1, ErbB2, ErbB3, ErbB4, and mRNAs for Na(+)/H(+) exchangers (NHE), NHE-1, NHE-2, NHE-3, NHE-4, and NHE-5 in both cell lines. EGF stimulated proton efflux in both cell lines. This effect was significantly attenuated by MIA, 5-(n-methyl-N-isobutyl) amiloride, a selective inhibitor of NHE-1 and NHE-2, and orpk cilia (-) cells were more sensitive to MIA than control cells (P < 0.01). EGF significantly induced extracellular signal-regulated kinase (ERK) phosphorylation in both cilia (+) and cilia (-) cells (63.3 and 123.6%, respectively), but the effect was more pronounced in orpk cilia (-) cells (P < 0.01). MIA significantly attenuated EGF-induced ERK phosphorylation only in orpk cilia (-) cells (P < 0.01). EGF increased proliferation of orpk cilia (+) cells and orpk cilia (-) cells, respectively, and MIA at 1-5 μM attenuated EGF-induced proliferation in orpk cilia (-) cells without affecting proliferation of orpk cilia (+) cells. EGF-induced proliferation of both cell lines was significantly decreased by the EGFR tyrosine kinase inhibitor AG1478 and MEK inhibitor PD98059. These results suggest that EGF exerts mitogenic effects in the orpk cilia (-) cells via activation of growth-associated amiloride-sensitive NHEs and ERK.

Peroxiredoxin VI oxidation in cerebrospinal fluid correlates with traumatic brain injury outcome

Traumatic brain injury (TBI) patients would benefit from the identification of reliable biomarkers to predict outcomes and treatment strategies. In our study, cerebrospinal fluid (CSF) from patients with severe TBI was evaluated for oxidant stress-mediated damage progression after hospital admission and subsequent ventriculostomy placement. Interestingly, substantial levels of peroxiredoxin VI (Prdx6), a major antioxidant enzyme normally found in astrocytes, were detected in CSF from control and TBI patients and were not associated with blood contamination. Functionally, Prdx6 and its associated binding partner glutathione S-transferase Pi (GSTP1-1, also detected in CSF) act in tandem to detoxify lipid peroxidation damage to membranes. We found Prdx6 was fully active in CSF of control patients but becomes significantly inactivated (oxidized) in TBI. Furthermore, significant and progressive oxidation of "buried" protein thiols in CSF of TBI patients (compared to those of nontrauma controls) was detected over a 24-h period after hospital admission, with increased oxidation correlating with severity of trauma. Conversely, recovery of Prdx6 activity after 24h indicated more favorable patient outcome. Not only is this the first report of an extracellular form of Prdx6 but also the first report of its detection at a substantial level in CSF. Taken together, our data suggest a meaningful correlation between TBI-initiated oxidation of Prdx6, its specific phospholipid hydroperoxide peroxidase activity, and severity of trauma outcome. Consequently, we propose that Prdx6 redox status detection has the potential to be a biomarker for TBI outcome and a future indicator of therapeutic efficacy.

ADAM17 promotes proliferation of collecting duct kidney epithelial cells through ERK activation and increased glycolysis in polycystic kidney disease

Polycystic kidney disease (PKD) is a common genetic disorder leading to cyst formation in the kidneys and other organs that ultimately results in kidney failure and death. Currently, there is no therapy for slowing down or stopping the progression of PKD. In this study, we identified the disintegrin metalloenzyme 17 (ADAM17) as a key regulator of cell proliferation in kidney tissues of conditional knockout Ift88(-/-) mice and collecting duct epithelial cells from Ift88°(rpk) mice, animal models of autosomal recessive polycystic kidney disease (ARPKD). Using Western blotting, an enzyme activity assay, and a growth factor-shedding assay in the presence or absence of the specific ADAM17 inhibitor TMI-005, we show that increased expression and activation of ADAM17 in the cystic kidney and in collecting duct epithelial cells originating from the Ift88°(rpk) mice (designated as PKD cells) lead to constitutive shedding of several growth factors, including heparin-binding EGF-like growth factor (HB-EGF), amphiregulin, and transforming growth factor-α (TGF-α). Increased growth factor shedding induces activation of the EGFR/MAPK/ERK pathway and maintains higher cell proliferation rate in PKD cells compared with control cells. PKD cells also displayed increased lactate formation and extracellular acidification indicative of aerobic glycolysis (Warburg effect), which was blocked by ADAM17 inhibition. We propose that ADAM17 is a key promoter of cellular proliferation in PKD cells by activating the EGFR/ERK axis and a proproliferative glycolytic phenotype.

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.

Ductuli efferentes of the male Golden Syrian hamster reproductive tract

Efferent ductules are responsible for the transportation of spermatozoa from the testis to the epididymis and their epithelium is responsible for the reabsorption of over 90% of the luminal fluid. The purpose of this research was to characterize the gross morphology and histology of efferent ductules in the male Golden Syrian hamster. The efferent ductules emerge from rete testis with a unique polarity at the apex or cephalic pole of the testis. The number of efferent ductules varied from 3 to 10 with an average of 6.0 and blind ending ducts were observed in approximately 56% of the males. The ductules merged into a single common duct prior to entering the caput epididymidis. The proximal efferent ductule lumen was wider than the distal (conus and common ducts), consistent with reabsorption of most of the luminal fluid, as was morphology of the ductal epithelium. Non-ciliated cells in the proximal region had prominent endocytic apparatuses, showing both coated pits and apical tubules in the apical cytoplasm. Large basolateral, intercellular spaces were also present in the epithelium of the proximal region. Distal non-ciliated cells had an abundance of large endosomes and lysosomal granules. Localisation of sodium/hydrogen exchanger-3 (NHE3; SLC9A3) and aquaporins 1 and 9 (AQP1, AQP9) along the microvillus border was also consistent with ion transport and fluid reabsorption by this epithelium. In comparison, the caput epididymidis epithelium expressed only AQP9 immunostaining. Another unusual feature of the hamster efferent ductules was the presence of glycogen aggregates in the basal cytoplasm of small groups of epithelial cells, but only in the proximal ducts near the rete testis. Androgen (AR), estrogen (ESR1 and ESR2) and vitamin D receptors (VDR) were also abundant in epithelial nuclei of proximal and distal efferent ductules. In comparison, caput epididymidis showed very little immunostaining for ESR1.

The physiological and pathophysiological functions of renal and extrarenal vasopressin V2 receptors

The arginine vasopressin (AVP) type 2 receptor (V2R) is unique among AVP receptor subtypes in signaling through cAMP. Its key function is in the kidneys, facilitating the urine concentrating mechanism through the AVP/V2 type receptor/aquaporin 2 system in the medullary and cortical collecting ducts. Recent clinical and research observations strongly support the existence of an extrarenal V2R. The clinical importance of the extrarenal V2R spans widely from stimulation of coagulation factor in the endothelium to as yet untested potential therapeutic targets. These include V2R-regulated membranous fluid turnover in the inner ear, V2R-regulated mitogensis and apoptosis in certain tumor tissues, and numerous other cell types where the physiological role of V2Rs still requires further research. Here, we review current evidence on the physiological and pathophysiological functions of renal and extrarenal V2Rs. These functions of V2R are important, not only in rare diseases with loss or gain of function of V2R but also in relation to the recent use of nonpeptide V2R antagonists to treat hyponatremia and possibly retard the growth of cysts and development of renal failure in autosomal dominant polycystic kidney disease. The main functions of V2R in principal cells of the collecting duct are water, salt, and urea transport by modifying the trafficking of aquaporin 2, epithelial Na(+) channels, and urea transporters and vasodilation and stimulation of coagulation factor properties, mainly seen with pharmacological doses of 1-desamino-8-D-AVP. The AVPR2 gene is located on the X chromosome, in a region with high probability of escape from inactivation; this may lead to phenotypic sex differences, with females expressing higher levels of transcript than males.

New insights into regulated aquaporin-2 function

PURPOSE OF REVIEW

Aquaporin-2 (AQP2) water channels in principal cells of the kidney collecting duct are essential for urine concentration. Due to application of modern technologies, progress in our understanding of AQP2 has accelerated in recent years. In this article, we highlight some of the new insights into AQP2 function that have developed recently, with particular focus on the cell biological aspects of AQP2 regulation.

RECENT FINDINGS

AQP2 is subjected to a number of regulated modifications, including phosphorylation and ubiquitination, which are important for AQP2 function, cellular localization and degradation. AQP2 is likely internalized via clathrin and non-clathrin-mediated endocytosis. Regulation of AQP2 endocytosis, in addition to exocytosis, is a vital mechanism in determining overall AQP2 membrane abundance. AQP2 is associated with regulated membrane microdomains. Studies using membrane cholesterol depleting reagents, for example statins, have supported the role of membrane rafts in regulation of AQP2 trafficking. Noncanonical roles for AQP2, for example in epithelial cell migration, are emerging.

SUMMARY

AQP2 function and thus urine concentration is dependent on a variety of cell signalling mechanisms, posttranslational modification and interplay between AQP2 and its lipid environment. This complexity of regulation allows fine-tuning of AQP2 function and thus body water homeostasis.

Primary cilia and kidney injury: current research status and future perspectives

Cilia, membrane-enclosed organelles protruding from the apical side of cells, can be divided into two classes: motile and primary cilia. During the past decades, motile cilia have been intensively studied. However, it was not until the 1990s that people began to realize the importance of primary cilia as cellular-specific sensors, particularly in kidney tubular epithelial cells. Furthermore, accumulating evidence indicates that primary cilia may be involved in the regulation of cell proliferation, differentiation, apoptosis, and planar cell polarity. Many signaling pathways, such as Wnt, Notch, Hedgehog, and mammalian target of rapamycin, have been located to the primary cilia. Thus primary cilia have been regarded as a hub that integrates signals from the extracellular environment. More importantly, dysfunction of this organelle may contribute to the pathogenesis of a large spectrum of human genetic diseases, named ciliopathies. The significance of primary cilia in acquired human diseases such as hypertension and diabetes has gradually drawn attention. Interestingly, recent reports disclosed that cilia length varies during kidney injury, and shortening of cilia enhances the sensitivity of epithelial cells to injury cues. This review briefly summarizes the current status of cilia research and explores the potential mechanisms of cilia-length changes during kidney injury as well as provides some thoughts to allure more insightful ideas and promotes the further study of primary cilia in the context of kidney injury.