Gliosis-dependent expression of complement factor H truncated variants attenuates retinal neurodegeneration following ischemic injury

Inherited, age-related, and acute retinal diseases are often exacerbated by an aberrant or excessive activity of the complement system. Consequently, cells not directly affected by an acute event or genetic variants may degenerate, resulting in enhanced visual impairment. The therapeutic potential of supplementation of complement factor H (FH), a key regulator of the complement cascade, is therefore particularly promising in the context of retinal diseases caused by complement activation. In this study, we engineered adeno-associated viruses (AAVs) containing sequences of two truncated human FH variants. The expression of these variants was regulated by the glial fibrillary acidic protein (GFAP) promoter, which is selectively active in gliotic Müller cells. Both FH variants consisted of FH domains 19-20, which were connected to domains 1-4 and 1-7, respectively, by a polyglycine linker. These AAVs were intravitreally injected following ischemic injury of C57BL/6J mouse retinas. We observed transgene expression in gliotic Müller cells and to some extent in astrocytes. The expression correlated directly with damage severity. Interventions resulted in decreased complement activation, accelerated normalization of microglia activity and morphological improvements. Reduced levels of C3 transcripts and C3d protein in conjunction with higher transcript levels of inhibitory regulators like Cfi and Cfh, hinted at attenuated complement activity. This study demonstrates the great potential of complement regulatory gene addition therapy. With further in vivo testing it could be applied to treat a wide range of retinal diseases where no causative therapies are available.

As in Real Estate, Location Matters: Cellular Expression of Complement Varies Between Macular and Peripheral Regions of the Retina and Supporting Tissues

The cellular events that dictate the initiation of the complement pathway in ocular degeneration, such as age-related macular degeneration (AMD), is poorly understood. Using gene expression analysis (single cell and bulk), mass spectrometry, and immunohistochemistry, we dissected the role of multiple retinal and choroidal cell types in determining the complement homeostasis. Our scRNA-seq data show that the cellular response to early AMD is more robust in the choroid, particularly in fibroblasts, pericytes and endothelial cells. In late AMD, complement changes were more prominent in the retina especially with the expression of the classical pathway initiators. Notably, we found a spatial preference for these differences. Overall, this study provides insights into the heterogeneity of cellular responses for complement expression and the cooperation of neighboring cells to complete the pathway in healthy and AMD eyes. Further, our findings provide new cellular targets for therapies directed at complement.

PDGF Receptor Alpha Signaling Is Key for Müller Cell Homeostasis Functions

Müller cells, the major retinal macroglia, are key to maintaining vascular integrity as well as retinal fluid and ion homeostasis. Although platelet derived growth factor (PDGF) receptor expression in Müller glia has been reported earlier, their actual role for Müller cell function and intimate interaction with cells of the retinal neurovascular unit remains unclear. To close this gap of knowledge, Müller cell-specific PDGF receptor alpha (PDGFRα) knockout (KO) mice were generated, characterized, and subjected to a model of choroidal neovascularization (CNV). PDGFRα-deficient Müller cells could not counterbalance hypoosmotic stress as efficiently as their wildtype counterparts. In wildtypes, the PDGFRα ligand PDGF-BB prevented Müller cell swelling induced by the administration of barium ions. This effect could be blocked by the PDGFR family inhibitor AC710. PDGF-BB could not restore the capability of an efficient volume regulation in PDGFRα KO Müller cells. Additionally, PDGFRα KO mice displayed reduced rod and cone-driven light responses. Altogether, these findings suggest that Müller glial PDGFRα is central for retinal functions under physiological conditions. In contrast, Müller cell-specific PDGFRα KO resulted in less vascular leakage and smaller lesion areas in the CNV model. Of note, the effect size was comparable to pharmacological blockade of PDGF signaling alone or in combination with anti-vascular endothelial growth factor (VEGF) therapy—a treatment regimen currently being tested in clinical trials. These data imply that targeting PDGF to treat retinal neovascular diseases may have short-term beneficial effects, but may elicit unwarranted side effects given the putative negative effects on Müller cell homeostatic functions potentially interfering with a long-term positive outcome.

Cell-Type-Specific Complement Expression in the Healthy and Diseased Retina

Complement dysregulation is a feature of many retinal diseases, yet mechanistic understanding at the cellular level is limited. Given this knowledge gap about which retinal cells express complement, we performed single-cell RNA sequencing on ~92,000 mouse retinal cells and validated our results in five major purified retinal cell types. We found evidence for a distributed cell-type-specific complement expression across 11 cell types. Notably, Müller cells are the major contributor of complement activators c1s, c3, c4, and cfb. Retinal pigment epithelium (RPE) mainly expresses cfh and the terminal complement components, whereas cfi and cfp transcripts are most abundant in neurons. Aging enhances c1s, cfb, cfp, and cfi expression, while cfh expression decreases. Transient retinal ischemia increases complement expression in microglia, Müller cells, and RPE. In summary, we report a unique complement expression signature for murine retinal cell types suggesting a well-orchestrated regulation of local complement expression in the retinal microenvironment.

Overshooting complement activity contributes to retinal degeneration. Pauly et al. demonstrate a distinct complement expression profile of retinal cell types that changes with aging and during retinal degeneration. This prompts the intriguing concept of a local retinal complement activation possibly independent of the systemic components typically produced by the liver.

Regulation of phosphate transport in proximal tubules

Homeostasis of inorganic phosphate (P(i)) is primarily an affair of the kidneys. Reabsorption of the bulk of filtered P(i) occurs along the renal proximal tubule and is initiated by apically localized Na(+)-dependent P(i) cotransporters. Tubular P(i) reabsorption and therefore renal excretion of P(i) is controlled by a number of hormones, including phosphatonins, and metabolic factors. In most cases, regulation of P(i) reabsorption is achieved by changing the apical abundance of Na(+)/Pi cotransporters. The regulatory mechanisms involve various signaling pathways and a number of proteins that interact with Na(+)/P(i) cotransporters.

Intestinal phosphate absorption in a model of chronic renal failure

Hyperphosphatemia is an important consequence of chronic renal failure (CRF). Lowering of the plasma phosphate concentration is believed to be critical in the management of patients with CRF, especially those on dialysis. Reports of the effect of CRF on the intestinal handling of phosphate in vitro have been conflicting; but what happens in vivo has not been studied. What effect a reduction in the dietary phosphate intake has on intestinal phosphate absorption in CRF in vivo is unclear. In this study, we have used the in situ intestine loop technique to determine intestinal phosphate absorption in the 5/6-nephrectomy rat model of CRF under conditions of normal and restricted dietary phosphate intake. In this model of renal disease, we found that there is no significant change in the phosphate absorption in either the duodenum or jejunum regardless of the dietary phosphate intake. There was also no change in the expression of the messenger RNA of the major intestinal phosphate carrier the sodium-dependent-IIb transporter. Furthermore, we found no change in the intestinal villus length or in the location of phosphate uptake along the villus. Our results indicate that in CRF, unlike the kidney, there is no reduction in phosphate transport across the small intestine. This makes intestinal phosphate absorption a potential target in the prevention and treatment of hyperphosphatemia.

Ifosfamide metabolites CAA, 4-OH-Ifo and Ifo-mustard reduce apical phosphate transport by changing NaPi-IIa in OK cells

Renal Fanconi syndrome occurs in about 1-5% of all children treated with Ifosfamide (Ifo) and impairment of renal phosphate reabsorption in about 20-30% of them. Pathophysiological mechanisms of Ifo-induced nephropathy are ill defined. The aim has been to investigate whether Ifo metabolites affect the type IIa sodium-dependent phosphate transporter (NaPi-IIa) in viable opossum kidney cells. Ifo did not influence viability of cells or NaPi-IIa-mediated transport up to 1 mM/24 h. Incubation of confluent cells with chloroacetaldehyde (CAA) and 4-hydroperoxyIfosfamide (4-OH-Ifo) led to cell death by necrosis in a concentration-dependent manner. At low concentrations (50-100 microM/24 h), cell viability was normal but apical phosphate transport, NaPi-IIa protein, and -mRNA expression were significantly reduced. Coincubation with sodium-2-mercaptoethanesulfonate (MESNA) prevented the inhibitory action of CAA but not of 4-OH-Ifo; DiMESNA had no effect. Incubation with Ifosfamide-mustard (Ifo-mustard) did alter cell viability at concentrations above 500 microM/24 h. At lower concentrations (50-100 microM/24 h), it led to significant reduction in phosphate transport, NaPi-IIa protein, and mRNA expression. MESNA did not block these effects. The effect of Ifo-mustard was due to internalization of NaPi-IIa. Cyclophosphamide-mustard (CyP-mustard) did not have any influence on cell survival up to 1000 microM, but the inhibitory effect on phosphate transport and on NaPi-IIa protein was the same as found after Ifo-mustard. In conclusion, CAA, 4-OH-Ifo, and Ifo- and CyP-mustard are able to inhibit sodium-dependent phosphate cotransport in viable opossum kidney cells. The Ifo-mustard effect took place via internalization and reduction of de novo synthesis of NaPi-IIa. Therefore, it is possible that Ifo-mustard plays an important role in pathogenesis of Ifo-induced nephropathy.

Proximal tubular handling of phosphate: A molecular perspective

Members of the SLC34 gene family of solute carriers encode for three Na+-dependent phosphate (P i) cotransporter proteins, two of which (NaPi-IIa/SLC34A1 and NaPi-IIc/SLC34A3) control renal reabsorption of P i in the proximal tubule of mammals, whereas NaPi-IIb/SCLC34A2 mediates P i transport in organs other than the kidney. The P i transport mechanism has been extensively studied in heterologous expression systems and structure-function studies have begun to reveal the intricacies of the transport cycle at the molecular level using techniques such as cysteine scanning mutagenesis, and voltage clamp fluorometry. Moreover, sequence differences between the three types of cotransporters have been exploited to obtain information about the molecular determinants of hormonal sensitivity and electrogenicity. Renal handling of P i is regulated by hormonal and non-hormonal factors. Changes in urinary excretion of P i are almost invariably mirrored by changes in the apical expression of NaPi-IIa and NaPi-IIc in proximal tubules. Therefore, understanding the mechanisms that control the apical expression of NaPi-IIa and NaPi-IIc as well as their functional properties is critical to understanding how an organism achieves P i homeostasis.

Functionally important residues in the predicted 3(rd) transmembrane domain of the type IIa sodium-phosphate co-transporter (NaPi-IIa)

The type IIa Na(+)/P(i), cotransporter (NaPi-IIa) mediates electrogenic transport of three Na(+) and one divalent P(i) ion (and one net positive charge) across the cell membrane. Sequence comparison of electrogenic NaPi-IIa and IIb isoforms with the electroneutral NaPi-IIc isoform pointed to the third transmembrane domain (TMD-3) as a possibly significant determinant of substrate binding. To elucidate the role of TMD-3 in the topology and mechanism underlying NaPi-IIa function we subjected it to cysteine scanning mutagenesis. The constructs were expressed in Xenopus oocytes and P(i) transport kinetics were assayed by electrophysiology and radiotracer uptake. Cys substitution resulted in only marginally altered kinetics of P(i) transport in those mutants providing sufficient current for analysis. Only one site, at the extracellular end of TMD-3, appeared to be accessible to methanethiosulfonate reagents. However, additional mutations carried out at D224 (replaced by E, G or N) and N227 (replaced by D or Q) resulted in markedly altered voltage and substrate dependencies of the P(i)-dependent currents. Replacing Asp-224 (highly conserved in electrogenic a and b isoforms) with Gly (the residue found in the electroneutral c isoform) resulted in a mutant that mediated electroneutral Na(+)-dependent P(i) transport. Since electrogenic NaPi-II transports 3 Na(+)/transport cycle, whereas electroneutral NaPi-IIc only transports 2, we speculate that this loss of electrogenicity might result from the loss of one of the three Na(+) binding sites in NaPi-IIa.

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.

NaPi-IIa and interacting partners

Regulation of renal proximal tubular reabsorption of phosphate (Pi) is one of the critical steps in Pi homeostasis. Experimental evidence suggests that this regulation is achieved mainly by controlling the apical expression of the Na+-dependent Pi cotransporter type IIa (NaPi-IIa) in proximal tubules. Only recently have we started to obtain information regarding the molecular mechanisms that control the apical expression of NaPi-IIa. The first critical observation was the finding that truncation of only its last three amino acid residues has a strong effect on apical expression. A second major finding was the observation that the last intracellular loop of NaPi-IIa contains sequence information that confers parathyroid hormone (PTH) sensitivity. The use of the above domains of the cotransporter in yeast two-hybrid (Y2H) screening allowed the identification of proteins interacting with NaPi-IIa. Biochemical and morphological, as well as functional, analyses have allowed us to obtain insights into the physiological roles of such interactions, although our present knowledge is still far from complete.

Regulation of intestinal phosphate cotransporter NaPi IIb by ubiquitin ligase Nedd4-2 and by serum- and glucocorticoid-dependent kinase 1

Serum and glucocorticoid-inducible kinase 1 (SGK1) is highly expressed in enterocytes. The significance of the kinase in regulation of intestinal function has, however, remained elusive. In Xenopus laevis oocytes, SGK1 stimulates the epithelial Na(+) channel by phosphorylating the ubiquitin ligase Nedd4-2, which regulates channels by ubiquitination leading to subsequent degradation of the channel protein. Thus the present study has been performed to explore whether SGK1 regulates transport systems expressed in intestinal epithelial cells, specifically type IIb sodium-phosphate (Na(+)-P(i)) cotransporter (NaPi IIb). Immunohistochemistry in human small intestine revealed SGK1 colocalization with Nedd4-2 in villus enterocytes. For functional analysis cRNA encoding NaPi IIb, the SGK isoforms and/or the Nedd4-2 were injected into X. laevis oocytes, and transport activity was quantified as the substrate-induced current (I(P)). Exposure to 3 mM phosphate induces an I(P) in NaPi IIb-expressing oocytes. Coinjection of Nedd4-2, but not the catalytically inactive mutant (C938S)Nedd4-2, significantly downregulates I(P), whereas the coinjection of (S422D)SGK1 markedly stimulates I(P) and even fully reverses the effect of Nedd4-2 on I(P). The effect of (S422D)SGK1 on NaPi IIb is mimicked by wild-type SGK3 but not by wild-type SGK2, constitutively active (T308D,S473D)PKB, or inactive (K127N)SGK1. Moreover, (S422D)SGK1 and SGK3 phosphorylate Nedd4-2. In conclusion, SGK1 stimulates the NaPi IIb, at least in part, by phosphorylating and thereby inhibiting Nedd4-2 binding to its target. Thus the present study reveals a novel signaling pathway in the regulation of intestinal phosphate transport, which may be important for regulation of phosphate balance.

Expression of the Na/P(i)-cotransporter type IIb in Sf9 cells: functional characterization and purification

In mammals the type IIb Na/P(i)-cotransporter is expressed in various tissues such as intestine, brain, lung and testis. The type IIb cotransporter shows 51% homology with the renal type IIa Na/P(i)-cotransporter, for which a detailed model of the secondary structure has emerged based on recent structure/function studies. To make the type IIb Na/P(i)-cotransporter available for future structural studies, we have expressed this cotransporter in Sf9 cells. Sf9 cells were infected with recombinant baculovirus containing 6His NaPi-IIb. Infected cells expressed a polypeptide of approximately 90 kDa, corresponding to a partially glycosylated form of the type IIb cotransporter. Transport studies demonstrated that the type IIb protein expressed in Sf9 cells mediates transport of phosphate in a Na-dependent manner with similar kinetic characteristics (apparent K(m)s for sodium and phosphate and pH dependence) as previously described. Solubilization experiments demonstrated that, in contrast to the type IIa cotransporter, the type IIb can be solubilized by nonionic detergents and that solubilized type IIb Na/P(i)-cotransporter can be purified by Ni-NTA chromatography.

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.

Functional studies on a split type II Na/P(i)-cotransporter

Analysis of rat and mouse proximal tubular brush-border membrane expression of the type IIa Na/P(i)-cotransporter provides evidence for its cleavage in the large extracellular loop (ECL-2). To study functional properties and membrane distribution of this split NaP(i)-IIa transporter we followed two strategies. In one strategy we expressed the transporter as two complementary parts (p40 and p45) in Xenopus laevis oocytes and as another strategy we cleaved the WT protein with trypsin. Both strategies resulted in a split NaP(i)-IIa protein located in the plasma membrane. The two domains were tied together by a disulfide bridge, most likely involving the cysteines 306 and 334. Surface expression of the NaP(i)-IIa fragments was dependent on the presence of both domains. If both domains were coexpressed, the transporter was functional and transport characteristics were identical to those of the WT-NaP(i)-IIa protein. Corresponding to this, the transporter cleaved by trypsin also retains its transport capacity. These data indicate that cleavage of the type IIa Na/P(i)-cotransporter at ECL-2 is compatible with its cotransport function.

Modulation of renal type IIa Na+/Pi cotransporter kinetics by the arginine modifier phenylglyoxal

The effects of the arginine-modifying reagent phenylglyoxal on the kinetics of the type IIa Na + /Pi cotransporter expressed in Xenopus, oocytes were studied by means of 32Pi uptake and electrophysiology. Phenylglyoxal incubation induced up to 60% loss of cotransport function but only marginally altered the Na+-leak. Substrate activation and pH dependency remained essentially unaltered, whereas the voltage dependency of Pi-induced change in electrogenic response was significantly reduced. Presteady-state charge movements were suppressed and the equilibrium charge distribution was shifted slightly towards hyperpolarizing potentials. Charge movements in the absence of external Na+ were also suppressed, which indicated that the empty-carrier kinetics were modified. These effects were incorporated into an ordered alternating access model for NaPi-IIa, whereby the arginine modification by phenylglyoxal was modeled as altered apparent electrical distances moved by mobile charges, together with a slower rate of translocation of the electroneutral, fully loaded carrier.

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.

Molecular aspects in the regulation of renal inorganic phosphate reabsorption: the type IIa sodium/inorganic phosphate co-transporter as the key player

The type IIa sodium/inorganic phosphate co-transporter is the rate-limiting inorganic phosphate transport pathway in renal brush-border membranes, and is thus a key player in overall inorganic phosphate homeostasis. Its regulation is mostly associated with membrane retrieval/reinsertion (traffic) of the transport protein. This membrane traffic is controlled by specific 'motifs' at the level of the transporter protein and probably involves interacting proteins (e.g. for scaffolding, regulation or sorting). The intracellular signaling mechanisms (e.g. the involvement of kinases) and the involvement of the cytoskeleton are not yet understood. Hereditary alterations in renal inorganic phosphate handling can be associated with factors controlling the expression of the brush-border type IIa sodium/inorganic phosphate co-transporter.

Molecular determinants for apical expression and regulatory membrane retrieval of the type IIa Na/Pi cotransporter

Renal inorganic phosphate (Pi) reabsorption is a key process in Pi homeostasis. Type IIa Na/Pi cotransporters, located at the apical membrane of renal proximal tubular cells, guarantee the vectorial transport of Pi. Renal Pi reabsorption can be modulated by controlling the number of cotransporters expressed at the apical membrane. Indeed, factors that increase Pi reabsorption induce the expression of type IIa cotransporters at the apical membrane, whereas factors that decrease Pi reabsorption lead to their retrieval. Therefore, proper sorting of this type of cotransporters is an essential step in Pi homeostasis. The relevance of polarization has been highlighted by the finding that improper sorting of transporters can cause disease. Here we describe the identification of signals involved in apical expression of newly synthesized type IIa cotransporters and in their hormonal-induced endocytosis.

Molecular determinants for apical expression of the renal type IIa Na+/Pi-cotransporter

Type IIa and IIb Na+/Pi-cotransporters have different patterns of expression in vivo: IIa is expressed in apical membranes of renal proximal tubules, and IIb in intestinal and lung epithelia. They are found in different subcellular locations when transfected in epithelial cells: IIa is apically expressed in renal proximal cells (OK), but mostly intracellularly in intestinal cells (CaCo2); IIb is apical in both cell types. To identify the domains responsible for the different expression of both cotransporters (in CaCo2), as well as those responsible for the apical expression of IIa (in OK), mutated cotransporters were fused to the Enhanced Green Fluorescent Protein (EGFP), and their expression analyzed by confocal microscopy. We conclude that the apical expression information for CaCo2 is contained within the C-terminal tail of IIb, but is not contained within IIa. From analysis of mutated IIa cotransporters we identified residues, within the C-terminal tail, involved in the apical expression of these cotransporters in OK cells: internal PR-residues and terminal TRL-residues. These signals are functional in OK but not in CaCo2-cells, supporting the concept that polarized targeting can be protein and cell specific.

The opossum kidney cell type IIa Na/P(i) cotransporter is a phosphoprotein

BACKGROUND/AIM

Parathyroid hormone (PTH)-dependent inhibition of proximal tubular P(i) reabsorption is mediated by protein kinase A and/or C and is associated with reduced border membrane expression of the type IIa Na/P(i) cotransporter. The aim of this study was to analyze phosphorylation of the type IIa cotransporter protein.

METHODS

Opossum kidney cells were used as a 'proximal tubular' cell model. Protein phosphorylation was determined by immunoprecipitation of the type IIa Na/P(i) cotransporter, followed by autoradiography. The transporter protein content was evaluated by Western blotting and transport activity by tracer P(i) uptake.

RESULTS

Under control conditions (no PTH) the transporter was phosphorylated; upon treatment with PTH, a decrease in phosphorylation was observed. A protein phosphatase inhibitor (okadaic acid) was unable to prevent PTH-induced Na/P(i) cotransporter inhibition but reduced transporter degradation.

CONCLUSION

The type IIa Na/P(i) cotransporter is a phosphoprotein, but alterations in its phosphorylation seem not to be involved in P(i) transport inhibition.

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.

Amino acids involved in sodium interaction of murine type II Na(+)-P(i) cotransporters expressed in Xenopus oocytes

Type IIa and IIb Na+-Pi cotransporters are highly conserved proteins expressed in brush border membranes of proximal tubules and small intestine, respectively. The kinetics of IIa and IIb differ significantly: type IIb is saturated at lower concentrations of Na+ and Pi. To define the domain responsible for the difference in Na+ affinity we constructed several mouse IIa-IIb chimeras as well as site-directed mutagenized cotransporters. Pi uptake activity was determined after injection of cRNAs into Xenopus laevis oocytes. From the chimera experiments we concluded that the domain containing part of the second intracellular loop, the fifth transmembrane domain (TD) and part of the third extracellular loop determines the specific Na+ activation properties for both types of cotransporter. Within this domain only a few residues located in the fifth TD are not conserved between type IIa and IIb. Site-directed mutagenesis on non-conserved residues was performed. Substitution of F402 of IIa by the corresponding L418 from IIb yielded a cotransporter that behaved like the IIb. On the other hand, substitution of the specific L418 of IIb by the corresponding F402 of IIa produced a cotransporter with a Na+ activation similar to IIa. (Single letter amino acid nomenclature is used throughout the paper.) These data suggest that the specific Na+ activation properties exhibited by type IIa and type IIb Na+-Pi cotransporters are at least in part due to the presence of a specific amino acid (F402 in IIa, and L418 in IIb) within the fifth TD of the protein.

Investigating the surface expression of the renal type IIa Na+/Pi-cotransporter in Xenopus laevis oocytes

We have combined a functional assay, surface labeling and immunocytochemical methods to compare total and surface-exposed renal type IIa Na+/Pi cotransporter protein. The wild-type type cotransporter (NaPi-IIa) and its functionally comparable cysteine mutant S460C were expressed in Xenopus oocytes. S460C contains a novel cysteine residue that, when modified by preincubation with methanethiosulfonate reagents, leads to complete suppression of cotransport function. This allowed surface labeling of the S460C using MTSEA-Biotin and confirmation by electrophysiology on the same cell. Protein was analyzed by Western blotting before and after streptavidin precipitation and by immunocytochemistry and immunogold electronmicroscopy. MTSEA-Biotin treatment resulted in a complete inhibition of S460C-mediated Na+/Pi-cotransport activity, which indicated that all transporters at the surface were biotinylated. After biotinylation, only a small fraction of total S460C protein was precipitated by streptavidin compared with the total amount of S460C protein detected in the lysate. Light- and electron-microscopy analysis of oocytes showed a large amount of WT and S460C transporter protein beneath the oocyte membrane. These data indicate that the apparent weak labeling efficiencies of surface-biotinylation-based assays of membrane proteins heterologously expressed in oocytes can be related to diminished incorporation of the protein in the oolemma.

Molecular mechanisms in proximal tubular and small intestinal phosphate reabsorption (plenary lecture)

Renal and small intestinal (re-)absorption contribute to overall phosphate(Pi)-homeostasis. In both epithelia, apical sodium (Na+)/Pi-cotransport across the luminal (brush border) membrane is rate limiting and the target for physiological/pathophysiological alterations. Three different Na/Pi-cotransporters have been identified: (i) type I cotransporter(s)--present in the proximal tubule--also show anion channel function and may play a role in secretion of organic anions; in the brain, it may serve vesicular glutamate uptake functions; (ii) type II cotransporter(s) seem to serve rather specific epithelial functions; in the renal proximal tubule (type Ila) and in the small intestine (type IIb), isoform determines Na+-dependent transcellular Pi-movements; (iii) type III cotransporters are expressed in many different cells/tissues where they could serve housekeeping functions. In the small intestine, alterations in Pi-absorption and, thus, apical expression of IIb protein are mostly in response to longer term (days) situations (altered Pi-intake, levels of 1.25 (OH2) vitamin D3, growth, etc), whereas in renal proximal tubule, in addition, hormonal effects (e.g. Parathyroid Hormone, PTH) acutely control (minutes/hours) the expression of the IIa cotransporter. The type II Na/Pi-cotransporters operate (as functional monomers) in a 3 Na+:1 Pi stoichiometry, including transfer of negatively charged (-1) empty carriers and electroneutral transfers of partially loaded carriers (1 Na+, slippage) and of the fully loaded carriers (3 Na+, 1 Pi). By a chimera (IIa/IIb) approach, and by site-directed mutagenesis (including cysteine-scanning), specific sequences have been identified contributing to either apical expression, PTH-induced membrane retrieval, Na+-interaction or specific pH-dependence of the IIa and IIIb cotransporters. For the COOH-terminal tail of the IIa Na/Pi-cotransporter, several interacting PDZ-domain proteins have been identified which may contribute to either its apical expression (NaPi-Cap1) or to its subapical/lysosomal traffic (NaPi-Cap2).

Dietary phosphate and parathyroid hormone alter the expression of the calcium-sensing receptor (CaR) and the Na+-dependent Pi transporter (NaPi-2) in the rat proximal tubule

Dietary phosphate (Pi) intake and parathyroid hormone (PTH) are essential regulators of proximal tubular (PT) Pi reabsorption; both factors are associated with adaptive changes in PT apical brush border membrane (BBM) Na/Pi-cotransport activity and specific transporter protein (NaPi-2) content. Urinary Pi excretion is also inversely correlated with luminal Ca2+ concentration ([Ca2+]) both in a PTH-dependent and -independent fashion. A cell-surface, Ca2+(/polyvalent cation)-sensing receptor (CaR) has been localized to the PT BBM with unknown function. To investigate whether PTH and/or dietary Pi intake could affect the distribution or the expression of the CaR, we evaluated their effects on rat kidney CaR and the NaPi-2 expression by Western blot analysis and immunofluorescence microscopy. A chronic high-Pi (1.2%) versus low-Pi (0.1%) diet and acute PTH (1-34) infusion significantly reduced the PT BBM expression of both NaPi-2 and CaR proteins. CaR-specific immunoreactivity in nephron segments other than the PT was not affected by PTH or Pi intake. These results suggest that reduced renal PT CaR expression by a high-Pi diet and by increased circulating PTH levels could contribute to the local control of PT handling of Ca2+ and Pi.

A dibasic motif involved in parathyroid hormone-induced down-regulation of the type IIa NaPi cotransporter

Type II NaPi cotransporters are expressed in the apical membrane of P(i)-(re)absorbing epithelia: the type IIa in renal proximal tubule and the type IIb in small intestine. Parathyroid hormone (PTH) leads to a retrieval from the apical membrane of the type IIa NaPi cotransporter. The type IIa cotransporter is also expressed in opossum kidney (OK) cells, and its expression is under the control of PTH. In the present study, we identified the molecular "domains" involved in the PTH-induced retrieval of the type IIa NaPi cotransporter. Wild-type mouse type IIa (mIIa) and type IIb (mIIb) as well as several mIIa-mIIb chimeras and site-directed mutants were fused to the enhanced green fluorescent protein and transfected into OK cells. We found that mIIa but not mIIb was internalized and degraded after incubation with 1-34 (or 3-34) PTH. Using chimeras, we found that the N and C termini were not required in this effect, whereas a "domain" located between residues 216 and 658 seemed to be necessary. This region contains two putative intracellular loops with highly conserved sequences between mIIa and mIIb; in the last intracellular loop, two charged amino acids of type IIa (K(503)R(504)) are replaced by uncharged residues in type IIb (N(520)I(521)). We generated two mutants in which these residues were interchanged: mIIaNI and mIIbKR. Similarly to mIIa, the mIIbKR mutant was endocytosed in response to 1-34 PTH; in contrast, mIIaNI behaved as mIIb and was not internalized. In conclusion, a dibasic amino acid motif (K(503)R(504)) located in the last intracellular loop of the type IIa NaPi cotransporter is essential for its PTH-induced retrieval.

Molecular characteristics of phosphate transporters and their regulation

A key process in overall P(i)-homeostasis is renal proximal tubular reabsorption of inorganic phosphate (P(i)), which involves secondary active sodium/phosphate (Na(+)/P(i)) cotransport reabsorption at the brush border membrane. Among the two different molecularly identified Na(+)/P(i) cotransporters, the type-IIa Na(+)/P(i) cotransporter (NaPi-IIa) accounts for up to 70% of brush border membrane transport. Regulation of renal P(i) reabsorption centers around brush border membrane insertion and retrieval of transporter protein under the influence of hormonal and nonhormonal factors. Immunohistochemical and fluorescence techniques have provided new insights into the tissue distribution and the regulation processes. The intrinsic electrogenicity of NaPi-IIa, has allowed detailed studies of the transport kinetics of NaPi-IIa and, combined with mutagenesis methods, structure-function information at the protein level is emerging.

Proximal tubular phosphate reabsorption: molecular mechanisms

Renal proximal tubular reabsorption of P(i) is a key element in overall P(i) homeostasis, and it involves a secondary active P(i) transport mechanism. Among the molecularly identified sodium-phosphate (Na/P(i)) cotransport systems a brush-border membrane type IIa Na-P(i) cotransporter is the key player in proximal tubular P(i) reabsorption. Physiological and pathophysiological alterations in renal P(i) reabsorption are related to altered brush-border membrane expression/content of the type IIa Na-P(i) cotransporter. Complex membrane retrieval/insertion mechanisms are involved in modulating transporter content in the brush-border membrane. In a tissue culture model (OK cells) expressing intrinsically the type IIa Na-P(i) cotransporter, the cellular cascades involved in "physiological/pathophysiological" control of P(i) reabsorption have been explored. As this cell model offers a "proximal tubular" environment, it is useful for characterization (in heterologous expression studies) of the cellular/molecular requirements for transport regulation. Finally, the oocyte expression system has permitted a thorough characterization of the transport characteristics and of structure/function relationships. Thus the cloning of the type IIa Na-P(i )cotransporter (in 1993) provided the tools to study renal brush-border membrane Na-P(i) cotransport function/regulation at the cellular/molecular level as well as at the organ level and led to an understanding of cellular mechanisms involved in control of proximal tubular P(i) handling and, thus, of overall P(i) homeostasis.

The functional unit of the renal type IIa Na+/Pi cotransporter is a monomer

The composition of the functional unit of the rat renal type IIa Na(+)/P(i) cotransporter (NaPi-IIa) was investigated by using two approaches based on the differential sensitivities of the wild type (WT) and mutant S460C proteins to 2-aminoethylmethanethiosulfonate hydrobromide (MTSEA), a charged cysteine modifier. Transport activity of S460C is completely blocked after incubation in MTSEA, whereas that of the WT remains unaffected. First, Xenopus laevis oocytes were coinjected with cRNAs coding for the WT and S460C in different proportions, and the transport inhibition after MTSEA incubation was assayed by electrophysiology. The relationship between MTSEA inhibition and proportion of cRNA was consistent with that for a functional monomer. Second, concatameric proteins were constructed that either comprised two WT proteins (WT-WT), two S460C mutants (S460C-S460C), or one of each (WT-S460C). Western blots of oocytes injected with fusion protein cRNA showed bands at approximately 200 kDa, whereas a main band at approximately 90 kDa was obtained for the WT cRNA alone. The kinetic properties of concatamers were the same as for the single proteins. Transport activity of the WT-WT concatamer was unaffected by MTSEA incubation, fully inhibited for S460C-S460C, but 50% inhibited for WT-S460C. This behavior was also consistent with NaPi-IIa being a functional monomer.

Cysteine residues and the structure of the rat renal proximal tubular type II sodium phosphate cotransporter (rat NaPi IIa)

The rat renal Na/P(i) cotransporter type IIa (rat NaP(i) IIa) is a 637 amino acid protein containing 12 cysteine residues. We examined the effect of different cysteine modifying methanethiosulfonate (MTS)-reagents and the disulfide bond reducing agent tris(2-carboxyethyl)phosphine (TCEP) on the transport activity of wild-type and 12 single cysteine substitution mutants of rat NaPi IIa expressed in Xenopus laevis oocytes. The transport activity of the wild-type protein was resistant to three membrane impermeant MTS-reagents (MTSEA, MTSET and MTSES). In contrast, membrane permeant methyl methanethiosulfonate (MMTS) and TCEP inhibited the transport activity of both the wild-type, as well as all the single mutant proteins. This indicated the existence of more than one functionally important cysteine residue, not accessible extracellularly, and at least 2 disulfide bridges. To identify the disulfide bridges, three double mutants lacking 2 of the 3 cysteine residues predicted to be extracellular in different combinations were examined. This led to the identification of one disulfide bridge between C306 and C334; reconsideration of the topological model predictions suggested a second disulfide bridge between C225 and C520. Evaluation of a fourth double mutant indicated that at least one of two disulfide bridges (C306 and C334; C225 and C520) has to be formed to allow the surface expression of a functional cotransporter. A revised secondary structure is proposed which includes two partially repeated motifs that are connected by disulfide bridges formed between cysteine pairs C306-C334 and C225-C520.

Cleavage of disulfide bonds leads to inactivation and degradation of the type IIa, but not type IIb sodium phosphate cotransporter expressed in Xenopus laevis oocytes

Tris(2-carboxyethyl)phosphine (TCEP) reduces (cleaves) disulfide bonds of the renal proximal tubule type IIa Na/Pi- cotransporter (rat NaPi IIa) and thereby inhibits its function. We tested the effect of TCEP on the murine type IIa Na/P(i)-cotransporter and the corresponding IIb intestinal isoform both expressed in Xenopus laevis oocytes. After incubation with TCEP the function of NaPi IIa was inhibited and protein amount was decreased. Injection of the lysosomal inhibitor leupeptin prevented degradation of the protein. Exposure of oocytes to TCEP at 0 degrees C led to a reduction in transport function without concomitant loss in Na/Pi IIa protein. In contrast to NaPi type IIa, the type IIb isoform was neither inhibited, nor degraded after incubation with TCEP. These results suggest that cleavage of disulfide bonds led to changes within the confirmation of the type IIa transporter that result in (i) inhibition of the transport activity and (ii) internalization and subsequent lysosomal degradation of transporter protein. Sequence comparisons suggest the involvement/presence of different disulfide bonds in type IIa and type IIb Na/P(i)-cotransporters.

Proton-sensitive transitions of renal type II Na(+)-coupled phosphate cotransporter kinetics

In the kidney proximal tubule, acidification of the glomerular filtrate leads to an inhibition of inorganic phosphate (P(i)) reabsorption by type II Na(+)-coupled cotransporters (NaPi-II). As external pH also alters the divalent/monovalent P(i) ratio, it has been difficult to separate putative proton interactions with the cotransporter from direct titration of divalent P(i), the preferred species transported. To distinguish between these possibilities and identify pH-sensitive transitions in the cotransport cycle, the pH-dependent kinetics of two NaPi-II isoforms, expressed in Xenopus laevis oocytes, were investigated electrophysiologically. At -50 mV, both isoforms showed >70% suppression of electrogenic response for an external pH change from 8.0 to 6.2, not attributable to titration of divalent P(i). This was accompanied by a progressive removal of steady-state voltage dependence. The NaPi-II-related uncoupled slippage current was unaffected by a pH change from 7.4 to 6.2, with no shift in the reversal potential, which suggested that protons do not function as substrate. The voltage-dependence of pre-steady-state relaxations was shifted to depolarizing potentials in 100 mM and 0 mM Na(ext)(+) and two kinetic components were resolved, the slower of which was pH-dependent. The changes in kinetics are predicted by a model in which protons interact with the empty carrier and final Na(+) binding step.

Luminal and contraluminal action of 1-34 and 3-34 PTH peptides on renal type IIa Na-P(i) cotransporter

Parathyroid hormone (PTH) inhibits proximal tubular reabsorption of P(i) by retrieval of type IIa Na-P(i) cotransporters (NaPi-IIa) from the brush-border membrane (BBM). We analyzed by immunohistochemistry whether PTH analogs, signaling through either protein kinase A (PKA) and C (PKC; 1-34 PTH) or only PKC (3-34 PTH), elicit in rat kidney in vivo or in the perfused murine proximal tubule in vitro a retrieval of NaPi-IIa and whether pharmacological agonists or inhibitors of these kinases are able to either mimic or interfere with these PTH effects. Treatment with either 1-34 or 3-34 PTH downregulated NaPi-IIa in rat kidney. In isolated murine proximal tubules 1-34 PTH was effective when added to either the apical or basolateral perfusate, whereas 3-34 PTH acted only via the luminal perfusate. These effects were mimicked by an activation of PKA with 8-bromoadenosine 3',5'-cyclic monophosphate or PKC with 1, 2-dioctanoylglycerol. The luminal action of both PTH peptides was blocked by inhibition of the PKC pathway (calphostin C), whereas the basolateral effect of 1-34 PTH was completely abolished by inhibiting both pathways (H-89 and calphostin C). These results suggest that 1) NaPi-IIa can be internalized by cAMP-dependent and -independent signaling mechanisms; 2) functional PTH receptors are located in both membrane domains; and 3) apical PTH receptors may preferentially initiate the effect through a PKC-dependent mechanism.

Requirement of a leucine residue for (apical) membrane expression of type IIb NaPi cotransporters

Type II NaPi cotransporters mediate epithelial phosphate (P(i)) reabsorption. In mammals the type IIb protein is expressed in the small intestinal apical membrane and other epithelia; it is not expressed in the renal proximal tubule where we find the type IIa isoform. To look for molecular determinant(s) involved in apical expression of type IIb cotransporters, we have made deletion mutations within the C-terminal tails of mouse IIb (mIIb) and human IIb (hIIb) transporter proteins. The constructs were fused to the enhanced green fluorescent protein and transiently transfected into intestinal CaCo2-cells. Both mIIb and hIIb were located exclusively in the apical membrane of the cells. For mIIb, the removal of a cysteine cluster or the last three amino acids (TVF) had no effect on the location of the protein. However, truncation at the level of the conserved L691/689 prevented the apical membrane expression of both mIIb and hIIb, respectively, and the mutated proteins were located in endosomal and lysosomal structures. A similar expression pattern of the mIIb and hIIb constructs was found in renal proximal tubular opossum kidney cells. Our data suggest that L691/689 is involved in mechanisms leading to an apical expression of type IIb NaPi cotransporters.

[Kidney function from the molecular viewpoint]

Over the last 10-15 years the use of cellular and molecular biology permitted the analysis of kidney function at a cellular/molecular level. These advances allow a more detailed understanding of physiological/pathophysiological mechanisms. By taking examples from our studies on the renal handling of inorganic phosphate this gain in knowledge' is illustrated.

Molecular determinants of pH sensitivity of the type IIa Na/P(i) cotransporter

Type II Na/P(i) cotransporters play key roles in epithelial P(i) transport and thereby contribute to overall P(i) homeostasis. Renal proximal tubular brush border membrane expresses the IIa isoform, whereas the IIb isoform is preferentially expressed in small intestinal brush border membrane of mammals. IIa and IIb proteins are predicted to contain eight transmembrane domains with the N- and C-terminal tails facing the cytoplasm. They differ in their pH dependences: the activity of IIa increases at higher pH, whereas the IIb shows no or a slightly opposite pH dependence. To determine the structural domains responsible for the difference in pH sensitivity, mouse IIa and IIb chimeras were constructed, and their pH dependence was characterized. A region between the fourth and fifth transmembrane domains was required for conferring pH sensitivity to the IIa-mediated Na/P(i) cotransport. Sequence comparison (IIa versus IIb) of the third extracellular loops revealed a stretch of three charged amino acids in IIa (REK) replaced by uncharged residues in IIb (GNT). Introduction of the uncharged GNT sequence (by REK) in IIa abolished its pH dependence, whereas introduction of the charged REK stretch in IIb (by GNT) led to a pH dependence similar to IIa. These findings suggest that charged residues within the third extracellular loop are involved in the pH sensitivity of IIa Na/P(i) cotransporter.

Asymmetrical targeting of type II Na-P(i) cotransporters in renal and intestinal epithelial cell lines

Targeting of newly synthesized transporters to either the apical or basolateral domains of polarized cells is crucial for the function of epithelia, such as in the renal proximal tubule or in the small intestine. Recently, different sodium-phosphate cotransporters have been identified. Type II cotransporters can be subdivided into two groups: type IIa and type IIb. Type IIa is predominantly expressed in renal proximal tubules, whereas type IIb is located on the intestinal and lung epithelia. To gain some insights into the polarized targeting of the type II cotransporters, we have transiently expressed type IIa and type IIb cotransporters in several epithelial cell lines: two lines derived from renal proximal cells (opossum kidney and LLC-PK(1)), one from renal distal cells (Madin-Darby canine kidney), and one from colonic epithelium (CaCo-2). We studied the expression of the transporters fused to the enhanced green fluorescent protein. Our data indicate that the polarized targeting is dependent on molecular determinants most probably located at the COOH terminus of the cotransporters as well as on the cellular context.

Internalization of proximal tubular type II Na-P(i) cotransporter by PTH: immunogold electron microscopy

Physiological/pathophysiological alterations in proximal tubular P(i) reabsorption are associated with an altered brush-border membrane (BBM) expression of type II Na-P(i) cotransporter molecules. Reduction is achieved by an internalization and lysosomal degradation and an increase in P(i) reabsorption by new synthesis and BBM insertion of type II Na-P(i) cotransporters. In the present study, we investigated by immunohistochemistry and immunogold electron microscopy the routing of internalized rat type II Na-P(i) cotransporters (NaPi-2). In kidney of rats on a chronic low-P(i) diet, NaPi-2 is mainly localized in the BBM, in cisterns of the Golgi apparatus and sparsely also in large endocytotic vacuoles and lysosomes. Fifteen minutes after the injection of the 1-34 analog of parathyroid hormone (PTH), the amount of NaPi-2 was decreased in the BBM and increased in endocytotic vesicles. NaPi-2 molecules colocalized with horseradish peroxidase injected prior to the injection of PTH. Vesicles labeled for NaPi-2 were occasionally also labeled for clathrin or the adaptor protein AP2. We conclude that NaPi-2 molecules enter the subapical compartment from where NaPi-2-containing vesicles are segregated off and directed to the lysosomes. A clathrin-mediated pathway may contribute to the PTH-induced internalization of NaPi-2.