The proposed mechanism of action during different pain management techniques on expression of cytolytic molecule perforin in patients after colorectal cancer surgery

The postoperative period is accompanied with neuroendocrine, metabolic and immune alteration which is caused by tissue damage, anesthesia, postoperative pain and psychological stress. Postoperative pain contributes to dysfunction of immune response as a result of interaction between central nervous and immune system. The postoperatively activated hypotalamo-pituitary-adrenocortical axis, sympathic and parasympathic nerve systems are important modulators of immune response. According to bidirectional communication of immune and nervous system, appropriate postoperative pain management could affect immune response in postoperative period. Although the postoperative suppression of immune response has been reported, a very little are known about the influences of different pain management techniques on cytotoxic function of immune cells in patients with colorectal cancer in early postoperative period. Perforin is a cytotoxic molecule expressed by activated lymphocytes which has a crucial role in elimination of tumor cells and virus-infected cells, mostly during the effector's phase of immune response. Immune compromise during the postoperative period could affect the healing processes, incidence of postoperative infections and rate and size of tumor metastases disseminated during operation. The pharmacological management of postoperative pain in patients with malignancies uses very different analgesic techniques whose possible influence on cytotoxic functions of immune cells are still understood poor. For decades the most common way of treating postoperative pain after colorectal cancer surgery was intravenous analgesia with opiods. In the last decade many investigations pointed out that opiods can also contribute to postoperative suppression of immune response. Epidural analgesia is a regional anesthesia technique that acts directly on the origin of pain impulses and pain relief can be achieved with small doses of opiods combined with local anesthetics. Local anesthetics potentate analgesic properties of opiods but per se are also acting as antiinflammatory drugs. Afferent neural blockade by epidural analgesia attenuates neuroendocrine stress response. We propose that epidural analgesia could be more convenient that intravenous analgesia in maintenance of immunological homeostasis that is altered by surgical stress, tumor growth and pain.

Defective coupling of apical PTH receptors to phospholipase C prevents internalization of the Na+-phosphate cotransporter NaPi-IIa in Nherf1-deficient mice

Phosphate reabsorption in the renal proximal tubule occurs mostly via the type IIa Na(+)-phosphate cotransporter (NaP(i)-IIa) in the brush border membrane (BBM). The activity and localization of NaP(i)-IIa are regulated, among other factors, by parathyroid hormone (PTH). NaP(i)-IIa interacts in vitro via its last three COOH-terminal amino acids with the PDZ protein Na(+)/H(+)-exchanger isoform 3 regulatory factor (NHERF)-1 (NHERF1). Renal phosphate reabsorption in Nherf1-deficient mice is altered, and NaP(i)-IIa expression in the BBM is reduced. In addition, it has been proposed that NHERF1 and NHERF2 are important for the coupling of PTH receptors (PTHRs) to phospholipase C (PLC) and the activation of the protein kinase C pathway. We tested the role of NHERF1 in the regulation of NaP(i)-IIa by PTH in Nherf1-deficient mice. Immunohistochemistry and Western blotting demonstrated that stimulation of apical and basolateral receptors with PTH-(1-34) led to internalization of NaP(i)-IIa in wild-type and Nherf1-deficient mice. Stimulation of only apical receptors with PTH-(3-34) failed to induce internalization in Nherf1-deficient mice. Expression and localization of apical PTHRs were similar in wild-type and Nherf1-deficient mice. Activation of the protein kinase C- and A-dependent pathways with 1,2-dioctanoyl-sn-glycerol or 8-bromo-cAMP induced normal internalization of NaP(i)-IIa in wild-type, as well as Nherf1-deficient, mice. Stimulation of PLC activity due to apical PTHRs was impaired in Nherf1-deficient mice. These data suggest that NHERF1 in the proximal tubule is important for PTH-induced internalization of NaP(i)-IIa and, specifically, couples the apical PTHR to PLC.

Proximal tubular epithelial cells are generated by division of differentiated cells in the healthy kidney

We searched for evidence for a contribution of stem cells in growth of the proximal S3 segments of healthy rats. According to the stem cell model, stem cells are undifferentiated and slow cycling; the bulk of cycling cells are transit amplifying, rapidly cycling cells. We show the following. 1) By continuous application of a thymidine analog (ThA) for 7 days, S3 proximal epithelial cells in healthy kidneys display a high-cycling rate. 2) Slow-cycling cells, identified by lack of ThA uptake during 14 days of continuous ThA application up to death and by expression of the cell cycle protein Ki67 at death, have the same degree of differentiation as quiescent cells. 3) To detect rapidly cycling cells, rats were killed at various time points after injection of a ThA. Double immunofluorescence for ThA and a cell cycle marker was performed, with colocalization indicating successive divisions. During one week after division, daughter cells display a very low proliferation rate, indicating the absence of rapidly cycling cells. 4) Labeling with cyclin D1 showed that this low proliferation rate is due to cycle arrest. 5) More than 50% of the S3 cells entered the cell cycle 36 h after a potent proliferative stimulus (lead acetate injection). We conclude that generation of new cells in the proximal tubule relies on division of differentiated, normally slow-cycling cells. These may rapidly enter the cycle under an adequate stimulus.

Unchanged expression of the sodium-dependent phosphate cotransporter NaPi-IIa despite diurnal changes in renal phosphate excretion

Renal phosphate excretion is subjected to circadian rhythmicity. The bulk of filtered inorganic phosphate (P(i)) is reabsorbed by the sodium-dependent phosphate cotransporter NaPi-IIa. The regulation of proximal tubular phosphate reabsorptive capacity is largely attributed to the altered abundance of NaPi-IIa residing in the brush border membrane (BBM) of proximal tubular cells. Therefore, we examined if the diurnal rise in renal phosphate excretion is accompanied by a corresponding change in NaPi-IIa expression. Renal phosphate excretion, creatinine clearance, and serum phosphate were determined at consecutive time points in rats, starting from 8 a.m. until 5 p.m. During this period, renal phosphate excretion (fractional P(i) excretion) increased more than eightfold until 5 p.m. compared to the morning values at 8 a.m. In addition, serum phosphate and creatinine clearance as well as the calculated tubular phosphate threshold increased. Neither immunoblot analysis of BBMs nor immunohistochemical staining for NaPi-IIa yielded evidence for a lower abundance of NaPi-IIa in kidneys collected in the afternoon compared to those in the morning. However, kidneys sampled in the afternoon showed a small decrease (14%) in (32)P uptakes into BBM vesicles (BBMVs). Thus, the diurnal rise in renal phosphate excretion was associated with a mild reduction in the sodium-dependent phosphate transport rate in proximal tubular BBMs. There was no apparent downregulation of NaPi-IIa abundance and only a small reduction in Na(+)-dependent Pi-transport activity. Thus, the diurnal changes in urinary phosphate excretion appear to be mainly related to changes in serum phosphate and tubular threshold but not to NaPi-IIa expression.

The renal Na+/phosphate cotransporter NaPi-IIa is internalized via the receptor-mediated endocytic route in response to parathyroid hormone

The major renal Na(+)/phosphate cotransporter, NaPi-IIa, is regulated by a number of factors including parathyroid hormone (PTH), dopamine, and dietary phosphate intake. PTH induces the acute internalization of NaPi-IIa from the brush border membrane (BBM) and its routing to and subsequent degradation in lysosomes. Previous work indicated that megalin, part of the apical receptor-mediated endocytic apparatus, may play a role in the PTH-induced removal of NaPi-IIa. Here we examined in rats the time-dependent internalization route of NaPi-IIa after acute PTH application using immunohistochemistry and markers of several endocytic compartments. NaPi-IIa removal from the BBM was detectable as early as 5 min after PTH injection. After 10-15 min, NaPi-IIa was localized in subapical compartments positive for clathrin. Shortly thereafter, NaPi-IIa appeared in endosomes stained for EEA1 (early endosomal antigen 1). After 45-60 min, NaPi-IIa was found in late endosomes/lysosomes marked with lgp120. In contrast, no change in the subcellular localization of megalin and the Na(+)/H(+) exchanger NHE3 was detected up to 60 min after PTH injection. To further characterize the internalization route, insulin, as a marker for receptor-mediated endocytosis, and horseradish peroxidase (HRP) and fluorescein isothiocyanate (FITC)-dextran (10 kDa), as markers for fluid-phase mediated endocytosis, were used. NaPi-IIa colocalized with insulin 5-30 min after PTH injection but did not overlap with HRP or FITC-dextran. These results demonstrate a distinct internalization route of NaPi-IIa in response to acute PTH application that may involve the receptor-mediated endocytic pathway including clathrin-coated vesicles and EEA1-positive early endosomes, and routes NaPi-IIa to lysosomes for degradation.

Regulation of sodium-proton exchanger isoform 3 (NHE3) by PKA and exchange protein directly activated by cAMP (EPAC)

The Na(+)/H(+) exchanger 3 (NHE3) is expressed in the brush border membrane (BBM) of proximal tubules (PT). Its activity is down-regulated on increases in intracellular cAMP levels. The aim of this study was to investigate the contribution of the protein kinase A (PKA) and the exchange protein directly activated by cAMP (EPAC) dependent pathways in the regulation of NHE3 by adenosine 3',5'-cyclic monophosphate (cAMP). Opossum kidney cells and murine kidney slices were treated with cAMP analogs, which selectively activate either PKA or EPAC. Activation of either pathway resulted in an inhibition of NHE3 activity. The EPAC-induced effect was independent of PKA as indicated by the lack of activation of the kinase and the insensitivity to the PKA inhibitor H89. Both PKA and EPAC inhibited NHE3 activity without inducing changes in the expression of the transporter in BBM. Activation of PKA, but not of EPAC, led to an increase of NHE3 phosphorylation. In contrast, activation of PKA, but not of EPAC, inhibited renal type IIa Na(+)-coupled inorganic phosphate cotransporter (NaPi-IIa), another Na-dependent transporter expressed in proximal BBM. PKA, but not EPAC, induced the retrieval of NaPi-IIa from BBM. Our results suggest that EPAC activation may represent a previously unrecognized mechanism involved in the cAMP regulation of NHE3, whereas regulation of NaPi-IIa is mediated by PKA but not by EPAC.

Luminal kidney and intestine SLC6 amino acid transporters of B0AT-cluster and their tissue distribution in Mus musculus

The B degrees transport system mediates the Na(+)-driven uptake of a broad range of neutral amino acids into epithelial cells of small intestine and kidney proximal tubule. A corresponding transporter was identified in 2004 (A. Broer, K. Klingel, S. Kowalczuk, J. E. Rasko, J. Cavanaugh, and S. Broer. J Biol Chem 279: 24467-24476, 2004) within the SLC6 family and named B degrees AT1 (SLC6A19). A phylogenetically related transporter known as XT3 in human (SLC6A20) and XT3s1 in mouse was shown to function as an imino acid transporter, to localize also to kidney and small intestine and renamed SIT1 or Imino(B). Besides these two transporters with known functions, there are two other gene products belonging to the same phylogenetic B degrees AT-cluster, XT2 (SLC6A18) and rodent XT3 that are still "orphans." Quantitative real-time RT-PCR showed that the mRNAs of the four B degrees AT-cluster members are abundant in kidney, whereas only those of B degrees AT1 and XT3s1/SIT1 are elevated in small intestine. In brain, the XT3s1/SIT1 mRNA is more abundant than the other B degrees AT-cluster mRNAs. We show here by immunofluorescence that all four mouse B degrees AT-cluster transporters localize, with differential axial gradients, to the brush-border membrane of proximal kidney tubule and, with the possible exception of XT3, also of intestine. Deglycosylation and Western blotting of brush-border proteins demonstrated the glycosylation and confirmed the luminal localization of B degrees AT1, XT2, and XT3. In summary, this study shows the luminal brush-border localization of the Na(+)-dependent amino and imino acid transporters B degrees AT1 and XT3s1/SIT1 in kidney and intestine. It also shows that the structurally highly similar orphan transporters XT2 and XT3 have the same luminal but a slightly differing axial localization along the kidney proximal tubule.

Secreted frizzled-related protein-4 reduces sodium-phosphate co-transporter abundance and activity in proximal tubule cells

The phosphatonin, secreted frizzled-related protein-4 (sFRP-4), induces phosphaturia and inhibits 25-hydroxyvitamin D 1alpha-hydroxylase activity normally induced in response to hypophosphatemia. To determine the mechanism by which sFRP-4 alters renal phosphate (P(i)) transport, we examined the effect of sFRP-4 on renal brush border membrane (BBMV) Na(+)-dependent P(i) uptake, and the abundance and localization of the major Na(+)-P(i)-IIa co-transporter in proximal tubules and opossum kidney (OK) cells. Infusion of sFRP-4 increased renal fractional excretion of P(i) and decreased renal beta-catenin concentrations. The increase in renal P(i) excretion with sFRP-4 infusion was associated with a 21.9 +/- 3.4% decrease in BBMV Na(+)-dependent P(i) uptake (P < 0.001) compared with a 39.5 +/- 2.1% inhibition of Na(+)-dependent P(i) transport in renal BBMV induced by PTH (P < 0.001). sFRP-4 infusion was associated with a 30.7 +/- 4.8% decrease in Na(+)-P(i)-IIa co-transporter protein abundance (P < 0.01) assessed by immunoblotting methods compared to a 45.4 +/- 8.8% decrease induced by PTH (P < 0.001). In OK cells, sFRP-4 reduced surface expression of a heterologous Na(+)-P(i)-IIa co-transporter. We conclude that sFRP-4 increases renal P(i) excretion by reducing Na(+)-P(i)-IIa transporter abundance in the brush border of the proximal tubule through enhanced internalization of the protein.

Parathyroid hormone treatment induces dissociation of type IIa Na+-P(i) cotransporter-Na+/H+ exchanger regulatory factor-1 complexes

The type IIa Na+-P(i) cotransporter (NaP(i)-IIa) and the Na+/H+ exchanger regulatory factor-1 (NHERF1) colocalize in the apical membrane of proximal tubular cells. Both proteins interact in vitro. Herein the interaction between NaP(i)-IIa and NHERF1 is further documented on the basis of coimmunoprecipitation and co-pull-down assays. NaP(i)-IIa is endocytosed and degraded in lysosomes upon parathyroid hormone (PTH) treatment. To investigate the effect of PTH on the NaP(i)-IIa-NHERF1 association, we first compared the localization of both proteins after PTH treatment. In mouse proximal tubules and OK cells, NaP(i)-IIa was removed from the apical membrane after hormonal treatment; however, NHERF1 remained at the membrane. Moreover, PTH treatment led to degradation of NaP(i)-IIa without changes in the amount of NHERF1. The effect of PTH on the NaP(i)-IIa-NHERF1 interaction was further studied using coimmunoprecipitation. PTH treatment reduced the amount of NaP(i)-IIa coimmunoprecipitated with NHERF antibodies. PTH-induced internalization of NaP(i)-IIa requires PKA and PKC; therefore, we next analyzed whether PTH induces changes in the phosphorylation state of either partner. NHERF1 was constitutively phosphorylated. Moreover, in mouse kidney slices, PTH induced an increase in NHERF1 phosphorylation; independent activation of PKA or PKC also resulted in increased phosphorylation of NHERF1 in kidney slices. However, NaP(i)-IIa was not phosphorylated either basally or after exposure to PTH. Our study supports an interaction between NHERF1 and NaP(i)-IIa on the basis of their brush-border membrane colocalization and in vitro coimmunoprecipitation/co-pull-down assays. Furthermore, PTH weakens this interaction as evidenced by different in situ and in vivo behavior. The PTH effect takes place in the presence of increased phosphorylation of NHERF1.

Intestinal and renal adaptation to a low-Pi diet of type II NaPi cotransporters in vitamin D receptor- and 1αOHase-deficient mice

Intake of a low-phosphate diet stimulates transepithelial transport of Pi in small intestine as well as in renal proximal tubules. In both organs, this is paralleled by a change in the abundance of the apically localized NaPi cotransporters NaPi type IIa (NaPi-IIa) and NaPi type IIb (NaPi-IIb), respectively. Low-Pi diet, via stimulation of the activity of the renal 25-hydroxyvitamin-D3-1α-hydroxylase (1αOHase), leads to an increase in the level of 1,25-dihydroxy-vitamin D3 [1,25(OH)2D]. Regulation of the intestinal absorption of Pi and the abundance of NaPi-IIb by 1,25(OH)2D has been supposed to involve the vitamin D receptor (VDR). In this study, we investigated the adaptation to a low-Pi diet of NaPi-IIb in small intestine as well as NaPi-IIa in kidneys of either VDR- or 1αOHase-deficient mice. In both mouse models, upregulation by a low-Pi diet of the NaPi cotransporters NaPi-IIa and NaPi-IIb was normal, i.e., similar to that observed in the wild types. Also, in small intestines of VDR- and 1αOHase-deficient mice, the same changes in NaPi-IIb mRNA found in wild-type mice were observed. On the basis of the results, we conclude that the regulation of NaPi cotransport in small intestine (via NaPi-IIb) and kidney (via NaPi-IIa) by low dietary intake of Pi cannot be explained by the 1,25(OH)2D-VDR axis.

Activation of dopamine D1-like receptors induces acute internalization of the renal Na+/phosphate cotransporter NaPi-IIa in mouse kidney and OK cells

The Na(+)/phosphate cotransporter NaPi-IIa (SLC34A1) is the major transporter mediating the reabsorption of P(i) in the proximal tubule. Expression and activity of NaPi-IIa is regulated by several factors, including parathyroid hormone, dopamine, metabolic acidosis, and dietary P(i) intake. Dopamine induces natriuresis and phosphaturia in vivo, and its actions on several Na(+)-transporting systems such as NHE3 and Na(+)-K(+)-ATPase have been investigated in detail. Using freshly isolated mouse kidney slices, perfused proximal tubules, and cultured renal epithelial cells, we examined the acute effects of dopamine on NaPi-IIa expression and localization. Incubation of isolated kidney slices with the selective D(1)-like receptor agonists fenoldopam (10 microM) and SKF-38393 (10 microM) for 1 h induced NaPi-IIa internalization and reduced expression of NaPi-IIa in the brush border membrane (BBM). The D(2)-like selective agonist quinpirole (1 microM) had no effect. The D(1) and D(2) agonists did not affect the renal Na(+)/sulfate cotransporter NaSi in the BBM of the proximal tubule. Studies with isolated perfused proximal tubules demonstrated that activation of luminal, but not basolateral, D(1)-like receptors caused NaPi-IIa internalization. In kidney slices, inhibition of PKC (1 microM chelerythrine) or ERK1/2 (20 microM PD-098089) pathways did not prevent the fenoldopam-induced internalization. Inhibition with the PKA blocker H-89 (10 microM) abolished the effect of fenoldopam. Immunoblot demonstrated a reduction of NaPi-IIa protein in BBMs from kidney slices treated with fenoldopam. Incubation of opossum kidney cells transfected with NaPi-IIa-green fluorescent protein chimera shifted fluorescence from the apical membrane to an intracellular pool. In summary, dopamine induces internalization of NaPi-IIa by activation of luminal D(1)-like receptors, an effect that is mediated by PKA.

Potential role of ABCA7 in cellular lipid efflux to apoA-I

ABCA7 is homologous to ABCA1 and has recently been shown in cell culture to bind apolipoprotein A-I (apoA-I) and to promote the efflux of phospholipids. However, it is not known if ABCA7 promotes lipid efflux in vivo. When expressed in HEK293 cells, both human and mouse ABCA7 promoted phospholipid efflux to apoA-I but no detectable cholesterol efflux. However, genetic knockdown of ABCA7 in mouse peritoneal macrophages did not affect phospholipid or cholesterol efflux to apoA-I. Moreover, in ABCA1-knockout macrophages, there was no detectable apoA-I-stimulated phospholipid efflux, inconsistent with a residual role of ABCA7. In contrast to plasma membrane localization of ABCA7 in transfected embryonic kidney cells, immunofluorescence microscopy of endogenous ABCA7 in macrophages showed a predominantly intracellular localization of the protein. Strikingly, immunofluorescence studies of adult mouse kidney revealed an apical brush border membrane localization of ABCA7 in the proximal tubule, suggesting that ABCA7 may come in contact with apoA-I in the glomerular filtrate. Although ABCA7 does not contribute to apolipoprotein-mediated lipid efflux in resting macrophages, its cell surface location in the kidney suggests that it could serve such a role in tissue microenvironments.

Expression and regulation of the renal Na/phosphate cotransporter NaPi-IIa in a mouse model deficient for the PDZ protein PDZK1

Inorganic phosphate (P(i)) is reabsorbed in the renal proximal tubule mainly via the type-IIa sodium-phosphate cotransporter (NaPi-IIa). This protein is regulated tightly by different factors, among them dietary P(i) intake and parathyroid hormone (PTH). A number of PDZ-domain-containing proteins have been shown to interact with NaPi-IIa in vitro, such as Na(+)/H(+) exchanger-3 regulatory factor-1 (NHERF1) and PDZK1. PDZK1 is highly abundant in kidney and co-localizes with NaPi-IIa in the brush border membrane of proximal tubules. Recently, a knock-out mouse model for PDZK1 (Pdzk1(-/-)) has been generated, allowing the role of PDZK1 in the expression and regulation of the NaPi-IIa cotransporter to be examined in in vivo and in ex vivo preparations. The localization of NaPi-IIa and other proteins interacting with PDZK1 in vitro [Na(+)/H(+) exchanger (NHE3), chloride-formate exchanger (CFEX)/putative anion transporter-1 (PAT1), NHERF1] was not altered in Pdzk1(-/-) mice. The abundance of NaPi-IIa adapted to acute and chronic changes in dietary P(i) intake, but steady-state levels of NaPi-IIa were reduced in Pdzk1(-/-) under a P(i) rich diet. This was paralleled by a higher urinary fractional P(i) excretion. The abundance of the anion exchanger CFEX/PAT1 (SLC26A6) was also reduced. In contrast, NHERF1 abundance increased in the brush border membrane of Pdzk1(-/-) mice fed a high-P(i) diet. Acute regulation of NaPi-IIa by PTH in vivo and by PTH and activators of protein kinases A, C and G (PKA, PKC and PKG) in vitro (kidney slice preparation) was not altered in Pdzk1(-/-) mice. In conclusion, loss of PDZK1 did not result in major changes in proximal tubule function or NaPi-IIa regulation. However, under a P(i)-rich diet, loss of PDZK1 reduced NaPi-IIa abundance indicating that PDZK1 may play a role in the trafficking or stability of NaPi-IIa under these conditions.

Kidney-specific inactivation of the megalin gene impairs trafficking of renal inorganic sodium phosphate cotransporter (NaPi-IIa)

Renal reabsorption of inorganic phosphate is mediated by the type IIa sodium phosphate cotransporter (NaPi-IIa) of the proximal tubule. Changes in renal phosphate handling are mainly attributable to altered NaPi-IIa brush border membrane (BBM) expression. Parathyroid hormone (PTH) induces inactivation of NaPi-IIa by endocytic membrane retrieval and degradation. The key elements triggering this process are not clear to date. Megalin serves as a receptor for the endocytosis of multiple ligands and is coexpressed with NaPi-IIa in the proximal tubule. Investigated was the role of megalin in the regulation of NaPi-IIa in steady state and during inactivation. Kidneys and tubular BBM fractions from mice with a renal-specific megalin gene defect and from controls were analyzed by light and electron microscopic histochemical techniques and Western blot test. Steady-state levels of NaPi-IIa in BBM were significantly enhanced, mRNA levels preserved, and phosphaturia reduced in the absence of megalin. Fluid-phase endocytosis was prevented and the apical endocytic apparatus markedly reduced. Systemic administration of PTH resulted in a defective retrieval and impaired degradation of NaPi-IIa. In vitro, the application of various stimuli of the PTH-induced signaling cascade had no effect either. Adequate steady-state expression of NaPi-IIa and the capacity of the proximal tubule cell to react on PTH-driven inactivation of NaPi-IIa by endocytosis and intracellular translocation require the presence of megalin.

Novel aspects in regulated expression of the renal type IIa Na/Pi-cotransporter

Proximal tubular phosphate (P(i)) reabsorption is a key element in overall phosphate homeostasis; physiologic/pathophysiologic alterations are related to the control of brush border membrane expression (regulated endocytosis) of the type IIa sodium (Na)/phosphate(P(i))-cotransporter (NaPi-IIa). The carboxy terminus of NaPi-IIa contains sequences important for its apical delivery/expression; the last three amino acids are involved in PSD95/DglA/ZO-1 (PDZ) interactions involving NaPi-IIa, Na/H exchanger-regulatory factor 1 (NHERF1/2), and PDZK1/2 (apical scaffold). Regulated endocytosis of NaPi-IIa [e.g., parathyroid hormone (PTH)-induced] is reduced in megalin-deficient mice; internalization occurs via clathrin-coated structures, early endosomes, and finally leads to lysosomal degradation. NaPi-IIa contains, in the third intracellular loop, a sequence motif required for internalization. Different hormonal [e.g., PTH, atrial natriuretic peptide (ANP), also nitric oxide (NO)] and nonhormonal factors activate a variety of intracellular signaling cascades [protein kinase A (PK-A), protein kinase C (PK-C), protein kinase G (PK-G), extracellular receptor kinase (ERK)-1/2] leading (by unknown mechanisms) to NaPi-IIa internalization. Different phosphatonins [e.g., fibroblast growth factor (FGF)-23, frizzled related protein (FRP)-4, matrix extracellularphosphoglycoprotein (MEPE)], associated with different pathophysiologic states of renal P(i)-handling, seem also to control apical expression of NaPi-IIa. Internalization of NaPi-IIa first requires its removal from the apical scaffold. This scaffold can also be considered as a regulatory scaffold containing also protein kinase A (PK-A)-anchoring proteins (AKAPs, ezrin) and the apical PTH receptor. The role of the different components of the regulatory scaffold in regulated endocytosis of NaPi-IIa is at present unknown.

Expression of visinin-like protein-3 in mouse kidney

In renal proximal brush borders the Na/Pi cotransporter NaPi-IIa is part of a heteromultimeric complex including the PDZ proteins PDZK1 and NHERF1, which interact with the C terminus of NaPi-IIa. In this study, a yeast two-hybrid screen against the N terminus of the Na/Pi cotransporter NaPi-IIa was performed. Thereby we identified visinin-like protein-3 (VILIP-3), a member of neuronal calcium sensors. In this study, expression and protein localization of VILIP-3 in the mouse kidney was performed by immunofluorescence and RT-PCR using laser-assisted microdissected nephron segments. VILIP-3 was found to be abundant in distal and collecting ducts where it partly colocalized with calbindin D28K. In addition VILIP-3 was observed in the brush borders of proximal tubular S1 and S3 segments of both superficial and deep nephrons.

PDZK1: II. an anchoring site for the PKA-binding protein D-AKAP2 in renal proximal tubular cells

BACKGROUND

PDZK1, a multiple PDZ protein, was recently found to interact with the type IIa Na/Pi cotransporter (NaPi-IIa) in renal proximal tubular cells. In a preceding study, yeast two-hybrid screens using single PDZ domains of PDZK1 as baits were performed. Among the identified proteins, a C-terminal fragment of the dual-specific A-kinase anchoring protein 2 (D-AKAP2) was obtained by screening PDZ domain 4.

METHODS

After its molecular cloning by means of RACE, the renal expression of D-AKAP2 was analyzed by real-time polymerase chain reaction (PCR) and immunohistochemistry. Protein interactions were characterized by overlays, pull-downs, and immunoprecipitations from transfected opossum kidney (OK) cells.

RESULTS

Based on 5'-RACE and PDZK1 overlays of mouse kidney cortex separated by two-dimensional electrophoresis, it was suggested that the renal isoform of D-AKAP2 in mouse comprises 372 amino acids and exists as a protein of >40 kD. Immunohistochemistry and real-time PCR localized D-AKAP2 only to the subapical pole of proximal tubular cells in mouse kidney. In pull-down experiments, D-AKAP2 tightly bound PDZK1 as well as N+/H+ exchanger regulator factor (NHERF-1), but the latter with an apparent fourfold lower affinity. Similarly, His-tagged D-AKAP2 specifically retained PDZK1 from mouse kidney cortex homogenate. In addition, myc-tagged D-AKAP2 and HA-tagged PDZK1 co-immunoprecipitated from transfected OK cells.

CONCLUSION

We conclude that D-AKAP2 anchors protein kinase A (PKA) to PDZK1 and to a lesser extent to NHERF-1. Since PDZK1 and NHERF-1 both sequester NaPi-IIa to the apical membrane, D-AKAP2 may play an important role in the parathyroid hormone (PTH)-mediated regulation of NaPi-IIa by compartmentalization of PKA.

Segment-specific expression of sodium-phosphate cotransporters NaPi-IIa and -IIc and interacting proteins in mouse renal proximal tubules

Sodium-dependent phosphate cotransport in renal proximal tubules (PTs) is heterogeneous with respect to proximal tubular segmentation (S1 vs. S3) and nephron generation (superficial vs. juxtamedullary). In the present study, S1 and S3 segments of superficial and juxtamedullary nephrons were laser-microdissected and mRNA and protein expression of the Na/Pi-cotransporters NaPi-IIa and NaPi-IIc and the PDZ proteins NHERF-1 and PDZK1 determined. Expression of NaPi-IIa mRNA decreased axially in juxtamedullary nephrons. There was no effect of dietary Pi content on NaPi-lla mRNA expression in any proximal tubular segment. The abundance of the NaPi-IIa cotransporter in the brush-border membrane showed inter- and intranephron heterogeneity and increased in response to a low-Pi diet (5 days), suggesting that up-regulation of NaPi-lla occurs via post-transcriptional mechanisms. In contrast, NaPi-IIc mRNA and protein was up-regulated by the low-Pi diet in all nephron generations analysed. NHERF-1 and PDZK1, at both mRNA and protein levels, were distributed evenly along the PTs and did not change after a low-Pi diet.

Dopamine acutely decreases apical membrane Na/H exchanger NHE3 protein in mouse renal proximal tubule

BACKGROUND

Dopamine is a principal natriuretic hormone in mammalian Na+ homeostasis. Dopamine acutely alters glomerular filtration rate (GFR) and decreases Na+ absorption in both the proximal and distal nephron. Proximal tubule natriuresis is effected through inhibition of the apical membrane Na/H exchanger NHE3.

METHODS

We examined whether dopamine directly and acutely decreases apical membrane NHE3 protein using renal tissue in two in vitro systems: renal cortical slices and in vitro perfused single tubules. After incubation with dopamine, NHE3 activity was measured by 22Na flux and NHE3 antigen was measured by immunoblot in apical membrane and total cellular membranes.

RESULTS

Direct application of dopamine to either cortical slices or microperfused tubules acutely decreases NHE3 activity and antigen at the apical membrane of the proximal tubule. No change in total cellular NHE3 was detected.

CONCLUSION

One mechanism by which dopamine causes natriuresis is via direct and acute reduction of NHE3 protein at the apical membrane via changes in NHE3 protein trafficking.

PDZK1: I. A major scaffolder in brush borders of proximal tubular cells11See Editorial by Moe, p. 1916

BACKGROUND

In proximal tubular cells, PDZK1 (NaPi-Cap1) has been implicated in apical expression of the Na+-dependent phosphate cotransporter (NaPi-IIa) via interaction with its C-terminus. PDZK1 represents a multidomain protein consisting of four PDZ domains and thus is believed to have a broader specificity besides NaPi-IIa.

METHODS

We subjected single PDZ domains derived from PDZK1 either to yeast two-hybrid screens or yeast trap assays. Different pull-down assays and blot overlays were applied to corroborate the PDZK1-mediated interactions in vitro. Co-localization of interacting proteins with PDZK1 in proximal tubular cells was assessed by immunohistochemistry.

RESULTS

In the yeast screens, the most abundant candidate protein to interact with PDZK1 was the membrane-associated protein of 17 kD (MAP17). Besides MAP17, C-terminal parts of following transporters were also identified: NaPi-IIa, solute carrier SLC17A1 (NaPi-I), Na+/H+ exchanger (NHE-3), organic cation transporter (OCTN1), chloride-formate exchanger (CFEX), and urate-anion exchanger (URAT1). In addition, other regulatory factors were found among the clones, such as a protein kinase A (PKA)-anchoring protein (D-AKAP2) and N+/H+ exchanger regulator factor (NHERF-1). All interactions of itemized proteins with PDZK1 were affirmed by in vitro techniques. Apart from PDZK1, strong in vitro interactions of NHERF-1 were also observed with the solute transporters (excluding MAP17) and D-AKAP2. All identified proteins were immunolocalized in proximal tubular cells, wherein all membrane proteins co-localized with PDZK1 in brush borders.

CONCLUSION

We hypothesize that PDZK1 and NHERF-1 establish an extended network beneath the apical membrane to which membrane proteins and regulatory components are anchored.

Interactions of MAP17 with the NaPi-IIa/PDZK1 protein complex in renal proximal tubular cells

An essential role in phosphate homeostasis is played by Na/Pi cotransporter IIa that is localized in the brush borders of renal proximal tubular cells. Recent studies identified several PDZ proteins interacting with the COOH-terminal tail of NaPi-IIa, such as PDZK1 and NHERF-1. Here, by using yeast two-hybrid screen of mouse kidney cDNA library, we attempted to find proteins interacting with the NH2-terminal part of NaPi-IIa. We identified MAP17, a 17-kDa membrane protein that has been described to be associated with various human carcinomas, but it is also expressed in normal kidneys. Results obtained by various in vitro analyses suggested that MAP17 interacts with the fourth domain of PDZK1 but not with other PDZ proteins localized in proximal tubular brush borders. As revealed by immunofluorescence, MAP17 was abundant in S1 but almost absent in S3 segments. No alterations of the apical abundance of MAP17 were observed after maneuvers undertaken to change the content of NaPi-IIa (parathyroid hormone treatment, different phosphate diets). In agreement, no change in the amount of MAP17 mRNA was observed. Results obtained from transfection studies using opossum kidney cells indicated that the apical localization of MAP17 is independent of PDZK1 but that MAP17 is required for apical localization of PDZK1. In summary, we conclude that MAP17 1) interacts with PDZK1 only, 2) associates with the NH2 terminus of NaPi-IIa within the PDZK1/NaPi-IIa/MAP17 complex, and 3) acts as an apical anchoring site for PDZK1.

Impaired PTH-induced endocytotic down-regulation of the renal type IIa Na+/Pi-cotransporter in RAP-deficient mice with reduced megalin expression

Inorganic phosphate (P(i)) reabsorption in the renal proximal tubule occurs mostly via the Na(+)/P(i) cotransporter type IIa (NaP(i)-IIa) located in the brush-border membrane (BBM) and is regulated, among other factors, by dietary P(i) intake and parathyroid hormone (PTH). The PTH-induced inhibition of P(i) reabsorption is mediated by endocytosis of Na/P(i)-IIa from the BBM and subsequent lysosomal degradation. Megalin is involved in receptor-mediated endocytosis of proteins from the urine in the renal proximal tubule. The recently identified receptor-associated protein (RAP) is a novel type of chaperone responsible for the intracellular transport of endocytotic receptors such as megalin. Gene disruption of RAP leads to a decrease of megalin in the BBM and to a disturbed proximal tubular endocytotic machinery. Here we investigated whether the distribution of NaP(i)-IIa and/or its regulation by dietary P(i) intake and PTH is affected in the proximal tubules of RAP-deficient mice as a model for megalin loss. In RAP-deficient mice megalin expression was strongly reduced and restricted to a subapical localization. NaP(i)-IIa protein distribution and abundance in the kidney was not altered. The localization and abundance of the NaP(i)-IIa interacting proteins MAP17, PDZK-1, D-AKAP2, and NHE-RF1 were also normal. Other transport proteins expressed in the BBM such as the Na(+)/H(+) exchanger NHE-3 and the Na(+)/sulphate cotransporter NaSi were normally expressed. In whole animals and in isolated fresh kidney slices the PTH-induced internalization of NaP(i)-IIa was strongly delayed in RAP-deficient mice. PTH receptor expression in the proximal tubule was not affected by the RAP knock-out. cAMP, cGMP or PKC activators induced internalization which was delayed in RAP-deficient mice. In contrast, both wildtype and RAP-deficient mice were able to adapt to high-, normal, and low-P(i) diets appropriately as indicated by urinary P(i) excretion and NaP(i)-IIa protein abundance.

Involvement of the MAPK-kinase pathway in the PTH-mediated regulation of the proximal tubule type IIa Na+/Pi cotransporter in mouse kidney

Reabsorption of phosphate in the proximal tubule is mainly mediated by the type IIa Na(+)/P(i) cotransporter (NaPi-IIa) and tightly regulated by a variety of factors including dietary phosphate intake and parathyroid hormone (PTH). PTH signals through both apical and basolateral PTH receptors and induces the rapid internalization and subsequent degradation of NaPi-IIa. At least two signalling cascades can be activated by PTH: the PLC/PKC and the cAMP/PKA pathways. Recent evidence from OK cell culture suggested the involvement of MAPK kinases in the PTH action. Here we used freshly isolated coronal mouse kidney slices and incubated them in a physiological buffer in the absence and presence of PTH with inhibitors and activators of the various signalling cascades to further study the events leading to internalization of NaPi-IIa. No alterations in the pattern of immunostaining for alpha-tubulin, actin and several brush border membrane proteins demonstrated intactness of the slices over the experimental period. Application of PTH (100 nM) induced a strong decrease of NaPi-IIa brush border staining and internalization after 45 min of incubation. The localization of the Na(+)/sulphate cotransporter (NaSi), however, was not affected. The internalization of NaPi-IIa could be completely prevented by the PKC inhibitor chelerythrine (1 micro M) or the MAPK-kinase (ERK1/2) inhibitor PD098059 (20 micro M). Without PTH both inhibitors alone had no effect. PTH induced phosphorylation of the ERK1/2 MAPK-kinases which was prevented by PD 098059. Separate activation of the cAMP/PKA pathway by 8-Br-cAMP was completely prevented by PD098059 whereas activation of the PLC/PKC pathway by the PKC activator 1,2-dioctanoyl-sn-glycerol (DOG) and the PKG pathway by 8-Br-cGMP induced internalization of NaPi-IIa which could be only partly blocked by PD 098059. Inhibition by SB203580 or activation by anisomycin of the p38 kinase pathway had no influence on NaPi-IIa localization under control conditions or after PTH stimulation. Furthermore, the PTH-induced decrease in NaPi-IIa protein could be reduced by PD 098059. These results suggest that the ERK1/2 MAPK kinase pathway plays a central role in the signalling of PTH leading to specific internalization and subsequent degradation of the type II NaPi-IIa cotransporter in the proximal tubule.

Regulation of the renal type IIa Na/Pi cotransporter by cGMP

Inhibition of proximal tubular phosphate (Pi) reabsorption involves, as far as we know, brush border membrane retrieval of the type IIa Na/Pi-cotransporter. The aim of the present study was to analyze whether intracellular cGMP-mediated regulation of Pi reabsorption also involves retrieval of the type IIa Na/Pi-cotransporter, as previously shown for cAMP. Atrial natriuretic peptide (ANP) and nitric oxide (NO) were used to stimulate guanylate cyclase. In vivo perfusion of mice kidneys with either ANP or NO donors resulted in a downregulation of type IIa Na/Pi-cotransporters on the brush border membranes of proximal tubules. These effects were mimicked by activation of protein kinase G with 8Br-cGMP. In in-vitro-perfused mice proximal tubules, ANP was effective when added either to the apical or basolateral perfusate, suggesting the presence of receptors on both membrane sites. The effects of ANP and NO were blocked by the protein kinase G inhibitor LY 83553. Parallel experiments in OK cells, a renal proximal tubule model, provided similar information. Our findings document that cGMP-mediated regulation (ANP and NO) of type IIa Na/Pi-cotransporters also takes place via internalization of the transporter protein.

Interaction of the type IIa Na/Pi cotransporter with PDZ proteins

The type IIa Na(+)-dependent inorganic phosphate (Na/P(i)) cotransporter is localized in the apical membrane of proximal tubular cells and is regulated by an endocytotic pathway. Because molecular processes such as apical sorting, internalization, or subsequent degradation might be assisted by associated proteins, a yeast two-hybrid screen against the C-terminal, cytosolic tail of type IIa cotransporter was designed. Most of the potential proteins found belonged to proteins with multiple PDZ modules and were either identical/related to PDZK1 or identical to NHERF-1. Yeast trap truncation assays confined the peptide-protein association to the C-terminal amino acid residues TRL of type IIa cotransporter and to single PDZ domains of each identified protein, respectively. The specificity of these interactions were confirmed in yeast by testing other apical localized transmembraneous proteins. Moreover, the type IIa protein was recovered in vitro by glutathione S-transferase-fused PDZ proteins from isolated renal brush border membranes or from type IIa-expressing oocytes. Further, these PDZ proteins are immunohistochemically detected either in the microvilli or in the subapical compartment of proximal tubular cells. Our results suggest that the type IIa Na/P(i) cotransporter interacts with various PDZ proteins that might be responsible for the apical sorting, parathyroid hormone controlled endocytosis or the lysosomal sorting of internalized type IIa cotransporter.