PTH-induced internalization of apical membrane NaPi2a: role of actin and myosin VI

The Intricacies of Renal Phosphate Reabsorption-An Overview

To maintain an optimal body content of phosphorus throughout postnatal life, variable phosphate absorption from food must be finely matched with urinary excretion. This amazing feat is accomplished through synchronised phosphate transport by myriads of ciliated cells lining the renal proximal tubules. These respond in real time to changes in phosphate and composition of the renal filtrate and to hormonal instructions. How they do this has stimulated decades of research. New analytical techniques, coupled with incredible advances in computer technology, have opened new avenues for investigation at a sub-cellular level. There has been a surge of research into different aspects of the process. These have verified long-held beliefs and are also dramatically extending our vision of the intense, integrated, intracellular activity which mediates phosphate absorption. Already, some have indicated new approaches for pharmacological intervention to regulate phosphate in common conditions, including chronic renal failure and osteoporosis, as well as rare inherited biochemical disorders. It is a rapidly evolving field. The aim here is to provide an overview of our current knowledge, to show where it is leading, and where there are uncertainties. Hopefully, this will raise questions and stimulate new ideas for further research.

The Measurement and Interpretation of Fibroblast Growth Factor 23 (FGF23) Concentrations

Two decades after the discovery of the hormone FGF23, we know more about phosphate homeostasis as it turned out that FGF23 is the central hormone that regulates this. Hereditary hypophosphatemic rickets and tumor-induced osteomalacia could by then be explained, by autonomous FGF23 production, and the nephrology field was excited by this new marker as it turned out to be independently associated with mortality in people treated by hemodialysis. This led to the development of several immunoassays to be able to measure FGF23 in blood. In the past years we learned that FGF23 is a rather stable peptide, the precision of the assays is acceptable but assays are not standardized and therefore not comparable. This means that reference values and cutoff values need to be assay specific. For several assays reference values have been established and gender and age did not seem of high importance. The phosphate content of the diet, which can be culturally dependent, however, should be taken into account when interpreting results, but to what extent is not totally clear. Currently, clinical application of the immunoassays is established in the diagnosis of hereditary hypophosphatemic rickets and diagnosis and follow-up of tumor-induced osteomalacia. Definite conclusions on the usefulness of the FGF23 measurement in people with CKD either as a marker for risk prediction or a as target for treatment remains to be determined. The latter applications would require dedicated prospective clinical trials, which may take years, before providing answers. To improve the standardization of the FGF23 assays and to shed light on the biological functions that fragments might have we might aim for an LC-MS/MS-based method to quantify both intact and fragmented FGF23. In this literature review we will summarize the current knowledge on the physiological role of FGF23, its quantification, and the clinical usefulness of its determination.

Targeted Disruption of a Proximal Tubule-Specific TMEM174 Gene in Mice Causes Hyperphosphatemia and Vascular Calcification

BACKGROUND

The proximal tubules play a critical role in phosphate (Pi) homeostasis by reabsorbing Pi via sodium-dependent Pi cotransporters. NPT2A is a major proximal-specific Pi cotransporter, whose expression is regulated by circulating hormones, such as parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23). In this study, we aimed to find a novel regulator in Pi homeostasis.

METHODS

Using RNA-seq and RT-qPCR analysis, we identified proximal tubule cell-enriched genes. We next used RNAi screening of the identified proximal tubular cell-enriched genes to identify a novel proximal tubule-specific gene that contributes to FGF23- and PTH-mediated inhibition of Pi uptake and NPT2 reduction. We created mice lacking this novel regulator of Pi homeostasis to examine whether the novel regulator contributes to Pi homeostasis in vivo.

RESULTS

We identified 54 kidney-enriched genes, 19 of which are expressed in renal primary proximal tubule cells. One of the proximal tubule-specific genes, TMEM174, interacted with NPT2A, and its knockdown blocked the reduction of NPT2A protein by FGF23 and PTH treatments in human and opossum proximal tubule cells. TMEM174 KO mice had significantly increased levels of serum Pi, FGF23, and PTH, resulting in vascular calcification.

CONCLUSIONS

TMEM174 is a novel regulator of Pi homeostasis that interacts with NPT2A.

A phasor-based approach to improve optical sectioning in any confocal microscope with a tunable pinhole

Confocal fluorescence microscopy is a well‐established imaging technique capable of generating thin optical sections of biological specimens. Optical sectioning in confocal microscopy is mainly determined by the size of the pinhole, a small aperture placed in front of a point detector. In principle, imaging with a closed pinhole provides the highest degree of optical sectioning. In practice, the dramatic reduction of signal‐to‐noise ratio (SNR) at smaller pinhole sizes makes challenging the use of pinhole sizes significantly smaller than 1 Airy Unit (AU). Here, we introduce a simple method to “virtually” perform confocal imaging at smaller pinhole sizes without the dramatic reduction of SNR. The method is based on the sequential acquisition of multiple confocal images acquired at different pinhole aperture sizes and image processing based on a phasor analysis. The implementation is conceptually similar to separation of photons by lifetime tuning (SPLIT), a technique that exploits the phasor analysis to achieve super‐resolution, and for this reason we call this method SPLIT‐pinhole (SPLIT‐PIN). We show with simulated data that the SPLIT‐PIN image can provide improved optical sectioning (i.e., virtually smaller pinhole size) but better SNR with respect to an image obtained with closed pinhole. For instance, two images acquired at 2 and 1 AU can be combined to obtain a SPLIT‐PIN image with a virtual pinhole size of 0.2 AU but with better SNR. As an example of application to biological imaging, we show that SPLIT‐PIN improves confocal imaging of the apical membrane in an in vitro model of the intestinal epithelium.

The Complexities of Organ Crosstalk in Phosphate Homeostasis: Time to Put Phosphate Sensing Back in the Limelight

Phosphate homeostasis is essential for health and is achieved via interaction between the bone, kidney, small intestine, and parathyroid glands and via intricate processes involving phosphate transporters, phosphate sensors, and circulating hormones. Numerous genetic and acquired disorders are associated with disruption in these processes and can lead to significant morbidity and mortality. The role of the kidney in phosphate homeostasis is well known, although it is recognized that the cellular mechanisms in murine models and humans are different. Intestinal phosphate transport also appears to differ in humans and rodents, with recent studies demonstrating a dominant role for the paracellular pathway. The existence of phosphate sensing has been acknowledged for decades; however, the underlying molecular mechanisms are poorly understood. At least three phosphate sensors have emerged. PiT2 and FGFR1c both act as phosphate sensors controlling Fibroblast Growth Factor 23 secretion in bone, whereas the calcium-sensing receptor controls parathyroid hormone secretion in response to extracellular phosphate. All three of the proposed sensors are expressed in the kidney and intestine but their exact function in these organs is unknown. Understanding organ interactions and the mechanisms involved in phosphate sensing requires significant research to develop novel approaches for the treatment of phosphate homeostasis disorders.

The Molecular Basis of Calcium and Phosphorus Inherited Metabolic Disorders

Calcium (Ca) and Phosphorus (P) hold a leading part in many skeletal and extra-skeletal biological processes. Their tight normal range in serum mirrors their critical role in human well-being. The signalling “voyage” starts at Calcium Sensing Receptor (CaSR) localized on the surface of the parathyroid glands, which captures the “oscillations” of extracellular ionized Ca and transfers the signal downstream. Parathyroid hormone (PTH), Vitamin D, Fibroblast Growth Factor (FGF23) and other receptors or ion-transporters, work synergistically and establish a highly regulated signalling circuit between the bone, kidneys, and intestine to ensure the maintenance of Ca and P homeostasis. Any deviation from this well-orchestrated scheme may result in mild or severe pathologies expressed by biochemical and/or clinical features. Inherited disorders of Ca and P metabolism are rare. However, delayed diagnosis or misdiagnosis may cost patient’s quality of life or even life expectancy. Unravelling the thread of the molecular pathways involving Ca and P signaling, we can better understand the link between genetic alterations and biochemical and/or clinical phenotypes and help in diagnosis and early therapeutic intervention.

A guide to plasma membrane solute carrier proteins

This review aims to serve as an introduction to the solute carrier proteins (SLC) superfamily of transporter proteins and their roles in human cells. The SLC superfamily currently includes 458 transport proteins in 65 families that carry a wide variety of substances across cellular membranes. While members of this superfamily are found throughout cellular organelles, this review focuses on transporters expressed at the plasma membrane. At the cell surface, SLC proteins may be viewed as gatekeepers of the cellular milieu, dynamically responding to different metabolic states. With altered metabolism being one of the hallmarks of cancer, we also briefly review the roles that surface SLC proteins play in the development and progression of cancer through their influence on regulating metabolism and environmental conditions.

Conditional Myh9 and Myh10 inactivation in adult mouse renal epithelium results in progressive kidney disease

Actin-associated nonmuscle myosin II (NM2) motor proteins play critical roles in a myriad of cellular functions, including endocytosis and organelle transport pathways. Cell type–specific expression and unique subcellular localization of the NM2 proteins, encoded by the Myh9 and Myh10 genes, in the mouse kidney tubules led us to hypothesize that these proteins have specialized functional roles within the renal epithelium. Inducible conditional knockout (cKO) of Myh9 and Myh10 in the renal tubules of adult mice resulted in progressive kidney disease. Prior to overt renal tubular injury, we observed intracellular accumulation of the glycosylphosphatidylinositol-anchored protein uromodulin (UMOD) and gradual loss of Na⁺ K⁺ 2Cl^– cotransporter from the apical membrane of the thick ascending limb epithelia. The UMOD accumulation coincided with expansion of endoplasmic reticulum (ER) tubules and activation of ER stress and unfolded protein response pathways in Myh9&10-cKO kidneys. We conclude that NM2 proteins are required for localization and transport of UMOD and loss of function results in accumulation of UMOD and ER stress–mediated progressive renal tubulointerstitial disease. These observations establish cell type–specific role(s) for NM2 proteins in regulation of specialized renal epithelial transport pathways and reveal the possibility that human kidney disease associated with MYH9 mutations could be of renal epithelial origin.

Adult mouse renal epithelium specific knockout of Myh9 and Myh10 genes result in kidney disease.

Advances in fluorescence microscopy techniques to study kidney function

Fluorescence microscopy, in particular immunofluorescence microscopy, has been used extensively for the assessment of kidney function and pathology for both research and diagnostic purposes. The development of confocal microscopy in the 1950s enabled imaging of live cells and intravital imaging of the kidney; however, confocal microscopy is limited by its maximal spatial resolution and depth. More recent advances in fluorescence microscopy techniques have enabled increasingly detailed assessment of kidney structure and provided extraordinary insights into kidney function. For example, nanoscale precise imaging by rapid beam oscillation (nSPIRO) is a super-resolution microscopy technique that was originally developed for functional imaging of kidney microvilli and enables detection of dynamic physiological events in the kidney. A variety of techniques such as fluorescence recovery after photobleaching (FRAP), fluorescence correlation spectroscopy (FCS) and Förster resonance energy transfer (FRET) enable assessment of interaction between proteins. The emergence of other super-resolution techniques, including super-resolution stimulated emission depletion (STED), photoactivated localization microscopy (PALM), stochastic optical reconstruction microscopy (STORM) and structured illumination microscopy (SIM), has enabled functional imaging of cellular and subcellular organelles at ≤50 nm resolution. The deep imaging via emission recovery (DIVER) detector allows deep, label-free and high-sensitivity imaging of second harmonics, enabling assessment of processes such as fibrosis, whereas fluorescence lifetime imaging microscopy (FLIM) enables assessment of metabolic processes.

Mechanisms of phosphate transport

Over the past 25 years, successive cloning of SLC34A1, SLC34A2 and SLC34A3, which encode the sodium-dependent inorganic phosphate (P_i) cotransport proteins 2a-2c, has facilitated the identification of molecular mechanisms that underlie the regulation of renal and intestinal P_i transport. P_i and various hormones, including parathyroid hormone and phosphatonins, such as fibroblast growth factor 23, regulate the activity of these P_i transporters through transcriptional, translational and post-translational mechanisms involving interactions with PDZ domain-containing proteins, lipid microdomains and acute trafficking of the transporters via endocytosis and exocytosis. In humans and rodents, mutations in any of the three transporters lead to dysregulation of epithelial P_i transport with effects on serum P_i levels and can cause cardiovascular and musculoskeletal damage, illustrating the importance of these transporters in the maintenance of local and systemic P_i homeostasis. Functional and structural studies have provided insights into the mechanism by which these proteins transport P_i, whereas in vivo and ex vivo cell culture studies have identified several small molecules that can modify their transport function. These small molecules represent potential new drugs to help maintain P_i homeostasis in patients with chronic kidney disease - a condition that is associated with hyperphosphataemia and severe cardiovascular and skeletal consequences.

Clathrin light chain A drives selective myosin VI recruitment to clathrin-coated pits under membrane tension

Clathrin light chains (CLCa and CLCb) are major constituents of clathrin-coated vesicles. Unique functions for these evolutionary conserved paralogs remain elusive, and their role in clathrin-mediated endocytosis in mammalian cells is debated. Here, we find and structurally characterize a direct and selective interaction between CLCa and the long isoform of the actin motor protein myosin VI, which is expressed exclusively in highly polarized tissues. Using genetically-reconstituted Caco-2 cysts as proxy for polarized epithelia, we provide evidence for coordinated action of myosin VI and CLCa at the apical surface where these proteins are essential for fission of clathrin-coated pits. We further find that myosin VI and Huntingtin-interacting protein 1-related protein (Hip1R) are mutually exclusive interactors with CLCa, and suggest a model for the sequential function of myosin VI and Hip1R in actin-mediated clathrin-coated vesicle budding.

The MYO6 interactome: selective motor-cargo complexes for diverse cellular processes

Myosins of class VI (MYO6) are unique actin-based motor proteins that move cargo towards the minus ends of actin filaments. As the sole myosin with this directionality, it is critically important in a number of biological processes. Indeed, loss or overexpression of MYO6 in humans is linked to a variety of pathologies including deafness, cardiomyopathy, neurodegenerative diseases as well as cancer. This myosin interacts with a wide variety of direct binding partners such as for example the selective autophagy receptors optineurin, TAX1BP1 and NDP52 and also Dab2, GIPC, TOM1 and LMTK2, which mediate distinct functions of different MYO6 isoforms along the endocytic pathway. Functional proteomics has recently been used to identify the wider MYO6 interactome including several large functionally distinct multi-protein complexes, which highlight the importance of this myosin in regulating the actin and septin cytoskeleton. Interestingly, adaptor-binding not only triggers cargo attachment, but also controls the inactive folded conformation and dimerisation of MYO6. Thus, the C-terminal tail domain mediates cargo recognition and binding, but is also crucial for modulating motor activity and regulating cytoskeletal track dynamics.

The potential role of myosin motor proteins in mediating the subcellular distribution of NHE3 in the renal proximal tubule

Isoform 3 of the Na⁺/H⁺ exchanger (NHE3) is responsible for the majority of the reabsorption of NaCl, NaHCO₃, and, consequently, water in the renal proximal tubule. As such, this transporter plays an essential role in acid-base balance and extracellular fluid volume homeostasis and determining systemic arterial blood pressure levels. NHE3 activity is modulated by a number of mechanisms, including the redistribution of the transporter between the body of the microvilli (where NHE3 is active) and the base of the microvilli (where NHE3 is less active). Although the physiological, pathophysiological, and pharmacological importance of the subcellular distribution of NHE3 has been well established, the exact mechanism whereby NHE3 is translocated along microvilli microdomains of the proximal tubule apical membrane is unknown. Nonmuscle myosin IIA and unconventional myosin VI move cargoes in anterograde and retrograde directions, respectively, and are known to redistribute along with NHE3 in the proximal tubule in response to a variety of natriuretic and antinatriuretic stimuli, including stimulation or inhibition of the renin-angiotensin system, high dietary Na⁺ intake, and high blood pressure. Therefore, this review aims to discuss the current evidence that suggests a potential role of myosin IIA and myosin VI in mediating the subcellular distribution of NHE3 along the kidney proximal tubule microvilli and their possible contribution in modifying NHE3-mediated Na⁺ reabsorption under both physiological and pathophysiological conditions.

Visualizing the regulation of SLC34 proteins at the apical membrane

The cloning of the renal NaPi-2a (SLC34A1) and NaPi-2c (SLC34A3) phosphate transporters has made it possible to characterize the molecular and biophysical regulation of renal proximal tubular reabsorption of inorganic phosphate (Pi). Dietary factors, such as Pi and K, and several hormones and phosphatonins, including parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and glucocorticoids, regulate the transporters through various transcriptional, translational, and post-translational mechanisms that involve acute trafficking via endocytosis or exocytosis, interactions with PDZ domain proteins, lipid microdomains, and diffusion and clustering in the apical brush border membrane. The visualization of these trafficking events by means of novel microscopy techniques that includes fluorescence lifetime imaging microscopy (FLIM), Förster resonance energy transfer (FRET), fluctuation correlation spectroscopy (FCS), and modulation tracking (MT), is the primary focus of this review.

Physiological regulation of phosphate by vitamin D, parathyroid hormone (PTH) and phosphate (Pi)

Inorganic phosphate (Pi) is an abundant element in the body and is essential for a wide variety of key biological processes. It plays an essential role in cellular energy metabolism and cell signalling, e.g. adenosine and guanosine triphosphates (ATP, GTP), and in the composition of phospholipid membranes and bone, and is an integral part of DNA and RNA. It is an important buffer in blood and urine and contributes to normal acid-base balance. Given its widespread role in almost every molecular and cellular function, changes in serum Pi levels and balance can have important and untoward effects. Pi homoeostasis is maintained by a counterbalance between dietary Pi absorption by the gut, mobilisation from bone and renal excretion. Approximately 85% of total body Pi is present in bone and only 1% is present as free Pi in extracellular fluids. In humans, extracellular concentrations of inorganic Pi vary between 0.8 and 1.2 mM, and in plasma or serum Pi exists in both its monovalent and divalent forms (H2PO4- and HPO42-). In the intestine, approximately 30% of Pi absorption is vitamin D regulated and dependent. To help maintain Pi balance, reabsorption of filtered Pi along the renal proximal tubule (PT) is via the NaPi-IIa and NaPi-IIc Na+-coupled Pi cotransporters, with a smaller contribution from the PiT-2 transporters. Endocrine factors, including, vitamin D and parathyroid hormone (PTH), as well as newer factors such as fibroblast growth factor (FGF)-23 and its coreceptor α-klotho, are intimately involved in the control of Pi homeostasis. A tight regulation of Pi is critical, since hyperphosphataemia is associated with increased cardiovascular morbidity in chronic kidney disease (CKD) and hypophosphataemia with rickets and growth retardation. This short review considers the control of Pi balance by vitamin D, PTH and Pi itself, with an emphasis on the insights gained from human genetic disorders and genetically modified mouse models.

Single-molecule labeling for studying trafficking of renal transporters

The ability to detect and track single molecules presents the advantage of visualizing the complex behavior of transmembrane proteins with a time and space resolution that would otherwise be lost with traditional labeling and biochemical techniques. Development of new imaging probes has provided a robust method to study their trafficking and surface dynamics. This mini-review focuses on the current technology available for single-molecule labeling of transmembrane proteins, their advantages, and limitations. We also discuss the application of these techniques to the study of renal transporter trafficking in light of recent research.

Nonmuscle myosin 2 proteins encoded by Myh9, Myh10, and Myh14 are uniquely distributed in the tubular segments of murine kidney

The diverse epithelial cell types of the kidneys are segregated into nephron segments and the collecting ducts in order to endow each tubular segment with unique functions. The rich diversity of the epithelial cell types is highlighted by the unique membrane channels and receptors expressed within each nephron segment. Our previous work identified a critical role for Myh9 and Myh10 in mammalian endocytosis. Here, we examined the expression patterns of Nonmuscle myosin 2 (NM2) heavy chains encoded by Myh9, Myh10, and Myh14 in mouse kidneys as these genes may confer unique nephron segment‐specific membrane transport properties. Interestingly, we found that each segment of the renal tubules predominately expressed only two of the three NM2 isoforms, with isoform‐specific subcellular localization, and different levels of expression within a nephron segment. Additionally, we identify Myh14 to be restricted to the intercalated cells and Myh10 to be restricted to the principal cells within the collecting ducts and connecting segments. We speculate that the distinct expression pattern of the NM2 proteins likely reflects the diversity of the intracellular trafficking machinery present within the different renal tubular epithelial segments.

Myosin-Driven Intracellular Transport

The delivery of intracellular material within cells is crucial for maintaining normal function. Myosins transport a wide variety of cargo, ranging from vesicles to ribonuclear protein particles (RNPs), in plants, fungi, and metazoa. The properties of a given myosin transporter are adapted to move on different actin filament tracks, either on the disordered actin networks at the cell cortex or along highly organized actin bundles to distribute their cargo in a localized manner or move it across long distances in the cell. Transport is controlled by selective recruitment of the myosin to its cargo that also plays a role in activation of the motor.

Sexual Dimorphic Pattern of Renal Transporters and Electrolyte Homeostasis

Compared with males, females have lower BP before age 60, blunted hypertensive response to angiotensin II, and a leftward shift in pressure natriuresis. This study tested the concept that this female advantage associates with a distinct sexual dimorphic pattern of transporters along the nephron. We applied quantitative immunoblotting to generate profiles of transporters, channels, claudins, and selected regulators in both sexes and assessed the physiologic consequences of the differences. In rats, females excreted a saline load more rapidly than males did. Compared with the proximal tubule of males, the proximal tubule of females had greater phosphorylation of Na+/H+ exchanger isoform 3 (NHE3), distribution of NHE3 at the base of the microvilli, and less abundant expression of Na+/Pi cotransporter 2, claudin-2, and aquaporin 1. These changes associated with less bicarbonate reabsorption and higher lithium clearance in females. The distal nephrons of females had a higher abundance of total and phosphorylated Na+/Cl- cotransporter (NCC), claudin-7, and cleaved forms of epithelial Na+ channel (ENaC) α and γ subunits, which associated with a lower baseline plasma K+ concentration. A K+-rich meal increased the urinary K+ concentration and decreased the level of renal phosphorylated NCC in females. Notably, we observed similar abundance profiles in female versus male C57BL/6 mice. These results define sexual dimorphic phenotypes along the nephron and suggest that lower proximal reabsorption in female rats expedites excretion of a saline load and enhances NCC and ENaC abundance and activation, which may facilitate K+ secretion and set plasma K+ at a lower level.

ISN Forefronts Symposium 2015: Maintaining Balance Under Pressure-Hypertension and the Proximal Tubule

Renal control of effective circulating volume (ECV) is key for circulatory performance. When renal sodium excretion is inadequate, blood pressure rises and serves as a homeostatic signal to drive natriuresis to re-establish ECV. Recognizing that hypertension involves both renal and vascular dysfunction, this report concerns proximal tubule sodium hydrogen exchanger 3 (NHE3) regulation during acute and chronic hypertension. NHE3 is distributed in tall microvilli (MV) in the proximal tubule, where it reabsorbs a significant fraction of the filtered sodium. NHE3 redistributes, in the plane of the MV membrane, between the MV body, where NHE3 is active, and the MV base, where NHE3 is less active. A high-salt diet and acute hypertension both retract NHE3 to the base and reduce proximal tubule sodium reabsorption independent of a change in abundance. The renin angiotensin system provokes NHE3 redistribution independent of blood pressure: The angiotensin-converting enzyme (ACE) inhibitor captopril redistributes NHE3 to the base and subsequent angiotensin II (AngII) infusion returns NHE3 to the body of the MV and restores reabsorption. Chronic AngII infusion presents simultaneous AngII stimulation and hypertension; that is, NHE3 remains in the body of the MV, due to the high local AngII level and inflammation, and exhibits a compensatory decrease in abundance driven by the hypertension. Genetically modified mice with blunted hypertensive responses to chronic AngII infusion (due to lack of the proximal tubule AngII receptors interleukin-17A or interferon-γ expression) exhibit reduced local AngII accumulation and inflammation and larger decreases in NHE3 abundance, which improves the pressure natriuresis response and reduces the need for elevated blood pressure to facilitate circulating volume balance.

PTH and Vitamin D

PTH and Vitamin D are two major regulators of mineral metabolism. They play critical roles in the maintenance of calcium and phosphate homeostasis as well as the development and maintenance of bone health. PTH and Vitamin D form a tightly controlled feedback cycle, PTH being a major stimulator of vitamin D synthesis in the kidney while vitamin D exerts negative feedback on PTH secretion. The major function of PTH and major physiologic regulator is circulating ionized calcium. The effects of PTH on gut, kidney, and bone serve to maintain serum calcium within a tight range. PTH has a reciprocal effect on phosphate metabolism. In contrast, vitamin D has a stimulatory effect on both calcium and phosphate homeostasis, playing a key role in providing adequate mineral for normal bone formation. Both hormones act in concert with the more recently discovered FGF23 and klotho, hormones involved predominantly in phosphate metabolism, which also participate in this closely knit feedback circuit. Of great interest are recent studies demonstrating effects of both PTH and vitamin D on the cardiovascular system. Hyperparathyroidism and vitamin D deficiency have been implicated in a variety of cardiovascular disorders including hypertension, atherosclerosis, vascular calcification, and kidney failure. Both hormones have direct effects on the endothelium, heart, and other vascular structures. How these effects of PTH and vitamin D interface with the regulation of bone formation are the subject of intense investigation.

Characterization of FGF23-Dependent Egr-1 Cistrome in the Mouse Renal Proximal Tubule

Fibroblast growth factor 23 (FGF23) is a potent regulator of phosphate (Pi) and vitamin D homeostasis. The transcription factor, early growth response 1 (egr-1), is a biomarker for FGF23-induced activation of the ERK1/2 signaling pathway. We have shown that ERK1/2 signaling blockade suppresses renal egr-1 gene expression and prevents FGF23-induced hypophosphatemia and 1,25-dihydroxyvitamin D (1,25(OH)2D) suppression in mice. To test whether egr-1 itself mediates these renal actions of FGF23, we administered FGF23 to egr-1-/- and wild-type (WT) mice. In WT mice, FGF23 induced hypophosphatemia and suppressed expression of the renal Na/Pi cotransporters, Npt2a and Npt2c. In FGF23-treated egr-1-/- mice, hypophosphatemic response was greatly blunted and Na/Pi cotransporter expression was not suppressed. In contrast, FGF23 induced equivalent suppression of serum 1,25(OH)2D concentrations by suppressing renal cyp27b1 and stimulating cyp24a1 mRNA expression in both groups of mice. Thus, downstream of receptor binding and ERK1/2 signaling, we can distinguish the effector pathway that mediates FGF23-dependent inhibition of Pi transport from the pathway that mediates inhibition of 1,25(OH)2D synthesis in the kidney. Furthermore, we demonstrate that the hypophosphatemic effect of FGF23 is significantly blunted in Hyp/egr-1-/- mice; specifically, serum Pi concentrations and renal Npt2a and Npt2c mRNA expression are significantly higher in Hyp/egr-1-/- mice than in Hyp mice. We then characterized the egr-1 cistrome in the kidney using ChIP-sequencing and demonstrate recruitment of egr-1 to regulatory DNA elements in proximity to several genes involved in Pi transport. Thus, our data demonstrate that the effect of FGF23 on Pi homeostasis is mediated, at least in part, by activation of egr-1.

Real-time monitoring of NKCC2 endocytosis by total internal reflection fluorescence (TIRF) microscopy

The apical Na-K-2Cl cotransporter (NKCC2) mediates NaCl reabsorption by the thick ascending limb (TAL). The amount of NKCC2 at the apical membrane of TAL cells is determined by exocytic delivery, recycling, and endocytosis. Surface biotinylation allows measurement of NKCC2 endocytosis, but it has low time resolution and does not allow imaging of the dynamic process of endocytosis. We hypothesized that total internal reflection fluorescence (TIRF) microscopy imaging of labeled NKCC2 would allow monitoring of NKCC2 endocytosis in polarized Madin-Darby canine kidney (MDCK) and TAL cells. Thus we generated a NKCC2 construct containing a biotin acceptor domain (BAD) sequence between the transmembrane domains 5 and 6. Once expressed in polarized MDCK or TAL cells, surface NKCC2 was specifically biotinylated by exogenous biotin ligase (BirA). We also demonstrate that expression of a secretory form of BirA in TAL cells induces metabolic biotinylation of NKCC2. Labeling biotinylated surface NKCC2 with fluorescent streptavidin showed that most apical NKCC2 was located within small discrete domains or clusters referred to as "puncta" on the TIRF field. NKCC2 puncta were observed to disappear from the TIRF field, indicating an endocytic event which led to a decrease in the number of surface puncta at a rate of 1.18 ± 0.16%/min in MDCK cells, and a rate 1.09 ± 0.08%/min in TAL cells (n = 5). Treating cells with a cholesterol-chelating agent (methyl-β-cyclodextrin) completely blocked NKCC2 endocytosis. We conclude that TIRF microscopy of labeled NKCC2 allows the dynamic imaging of individual endocytic events at the apical membrane of TAL cells.

Myosin 1b Regulates Amino Acid Transport by Associating Transporters with the Apical Plasma Membrane of Kidney Cells

Amino acid transporters (AATers) in the brush border of the apical plasma membrane (APM) of renal proximal tubule (PT) cells mediate amino acid transport (AAT). We found that the membrane-associated class I myosin myosin 1b (Myo1b) localized at the apical brush border membrane of PTs. In opossum kidney (OK) 3B/2 epithelial cells, which are derived from PTs, expressed rat Myo1b-GFP colocalized in patched microvilli with expressed mouse V5-tagged SIT1 (SIT1-V5), which mediates neutral amino acid transport in OK cells. Lentivirus-mediated delivery of opossum Myo1b-specific shRNA resulted in knockdown (kd) of Myo1b expression, less SIT1-V5 at the APM as determined by localization studies, and a decrease in neutral AAT as determined by radioactive uptake assays. Myo1b kd had no effect on Pi transport or noticeable change in microvilli structure as determined by rhodamine phalloidin staining. The studies are the first to define a physiological role for Myo1b, that of regulating renal AAT by modulating the association of AATers with the APM.

Regulation of hormone-sensitive renal phosphate transport

Phosphate is essential for growth and maintenance of the skeleton and for generating high-energy phosphate compounds. Evolutionary adaptation to high dietary phosphorous in humans and other terrestrial vertebrates involves regulated mechanisms assuring the efficient renal elimination of excess phosphate. These mechanisms prominently include PTH, FGF23, and Vitamin D, which directly and indirectly regulate phosphate transport. Disordered phosphate homeostasis is associated with pathologies ranging from kidney stones to kidney failure. Chronic kidney disease results in hyperphosphatemia, an elevated calcium×phosphate product with considerable morbidity and mortality, mostly associated with adverse cardiovascular events. This chapter highlights recent findings and insights regarding the hormonal regulation of renal phosphate transport along with imbalances of phosphate balance due to acquired or inherited diseases states.

Renal transporter activation during angiotensin-II hypertension is blunted in interferon-γ-/- and interleukin-17A-/- mice

Ample genetic and physiological evidence establishes that renal salt handling is a critical regulator of blood pressure. Studies also establish a role for the immune system, T-cell infiltration, and immune cytokines in hypertension. This study aimed to connect immune cytokines, specifically interferon-γ (IFN-γ) and interleukin-17A (IL-17A), to sodium transporter regulation in the kidney during angiotensin-II (Ang-II) hypertension. C57BL/6J (wild-type) mice responded to Ang-II infusion (490 ng/kg per minute, 2 weeks) with a rise in blood pressure (170 mm Hg) and a significant decrease in the rate of excretion of a saline challenge. In comparison, mice that lacked the ability to produce either IFN-γ (IFN-γ(-/-)) or IL-17A (IL-17A(-/-)) exhibited a blunted rise in blood pressure (<150 mm Hg), and both the genotypes maintained baseline diuretic and natriuretic responses to a saline challenge. Along the distal nephron, Ang-II infusion increased abundance of the phosphorylated forms of the Na-K-2Cl cotransporter, Na-Cl cotransporter, and Ste20/SPS-1-related proline-alanine-rich kinase, in both the wild-type and the IL-17A(-/-) but not in IFN-γ(-/-) mice; epithelial Na channel abundance increased similarly in all the 3 genotypes. In the proximal nephron, Ang-II infusion significantly decreased abundance of Na/H-exchanger isoform 3 and the motor myosin VI in IL-17A(-/-) and IFN-γ(-/-), but not in wild-type; the Na-phosphate cotransporter decreased in all the 3 genotypes. Our results suggest that during Ang-II hypertension both IFN-γ and IL-17A production interfere with the pressure natriuretic decrease in proximal tubule sodium transport and that IFN-γ production is necessary to activate distal sodium reabsorption.

Renal control of calcium, phosphate, and magnesium homeostasis

Calcium, phosphate, and magnesium are multivalent cations that are important for many biologic and cellular functions. The kidneys play a central role in the homeostasis of these ions. Gastrointestinal absorption is balanced by renal excretion. When body stores of these ions decline significantly, gastrointestinal absorption, bone resorption, and renal tubular reabsorption increase to normalize their levels. Renal regulation of these ions occurs through glomerular filtration and tubular reabsorption and/or secretion and is therefore an important determinant of plasma ion concentration. Under physiologic conditions, the whole body balance of calcium, phosphate, and magnesium is maintained by fine adjustments of urinary excretion to equal the net intake. This review discusses how calcium, phosphate, and magnesium are handled by the kidneys.

Sphingolipids affect fibrinogen-induced caveolar transcytosis and cerebrovascular permeability

Inflammation-induced vascular endothelial dysfunction can allow plasma proteins to cross the vascular wall, causing edema. Proteins may traverse the vascular wall through two main pathways, the paracellular and transcellular transport pathways. Paracellular transport involves changes in endothelial cell junction proteins, while transcellular transport involves caveolar transcytosis. Since both processes are associated with filamentous actin formation, the two pathways are interconnected. Therefore, it is difficult to differentiate the prevailing role of one or the other pathway during various pathologies causing an increase in vascular permeability. Using a newly developed dual-tracer probing method, we differentiated transcellular from paracellular transport during hyperfibrinogenemia (HFg), an increase in fibrinogen (Fg) content. Roles of cholesterol and sphingolipids in formation of functional caveolae were assessed using a cholesterol chelator, methyl-β-cyclodextrin, and the de novo sphingolipid synthesis inhibitor myriocin. Fg-induced formation of functional caveolae was defined by association and colocalization of Na+-K+-ATPase and plasmalemmal vesicle-associated protein-1 with use of Förster resonance energy transfer and total internal reflection fluorescence microscopy, respectively. HFg increased permeability of the endothelial cell layer mainly through the transcellular pathway. While MβCD blocked Fg-increased transcellular and paracellular transport, myriocin affected only transcellular transport. Less pial venular leakage of albumin was observed in myriocin-treated HFg mice. HFg induced greater formation of functional caveolae, as indicated by colocalization of Na+-K+-ATPase with plasmalemmal vesicle-associated protein-1 by Förster resonance energy transfer and total internal reflection fluorescence microscopy. Our results suggest that elevated blood levels of Fg alter cerebrovascular permeability mainly by affecting caveolae-mediated transcytosis through modulation of de novo sphingolipid synthesis.

Myosin VI and its cargo adaptors - linking endocytosis and autophagy

The coordinated trafficking and tethering of membrane cargo within cells relies on the function of distinct cytoskeletal motors that are targeted to specific subcellular compartments through interactions with protein adaptors and phospholipids. The unique actin motor myosin VI functions at distinct steps during clathrin-mediated endocytosis and the early endocytic pathway - both of which are involved in cargo trafficking and sorting - through interactions with Dab2, GIPC, Tom1 and LMTK2. This multifunctional ability of myosin VI can be attributed to its cargo-binding tail region that contains two protein-protein interaction interfaces, a ubiquitin-binding motif and a phospholipid binding domain. In addition, myosin VI has been shown to be a regulator of the autophagy pathway, because of its ability to link the endocytic and autophagic pathways through interactions with the ESCRT-0 protein Tom1 and the autophagy adaptor proteins T6BP, NDP52 and optineurin. This function has been attributed to facilitating autophagosome maturation and subsequent fusion with the lysosome. Therefore, in this Commentary, we discuss the relationship between myosin VI and the different myosin VI adaptor proteins, particularly with regards to the spatial and temporal regulation that is required for the sorting of cargo at the early endosome, and their impact on autophagy.

Proteomic profiling of the effect of metabolic acidosis on the apical membrane of the proximal convoluted tubule

The physiological response to the onset of metabolic acidosis requires pronounced changes in renal gene expression. Adaptations within the proximal convoluted tubule support the increased extraction of plasma glutamine and the increased synthesis and transport of glucose and of NH(4)(+) and HCO(3)(-) ions. Many of these adaptations involve proteins associated with the apical membrane. To quantify the temporal changes in these proteins, proteomic profiling was performed using brush-border membrane vesicles isolated from proximal convoluted tubules (BBMV(PCT)) that were purified from normal and acidotic rats. This preparation is essentially free of contaminating apical membranes from other renal cortical cells. The analysis identified 298 proteins, 26% of which contained one or more transmembrane domains. Spectral counts were used to assess changes in protein abundance. The onset of acidosis produced a twofold, but transient, increase in the Na(+)-dependent glucose transporter and a more gradual, but sustained, increase (3-fold) in the Na(+)-dependent lactate transporter. These changes were associated with the loss of glycolytic and gluconeogenic enzymes that are contained in the BBMV(PCT) isolated from normal rats. In addition, the levels of γ-glutamyltranspeptidase increased twofold, while transporters that participate in the uptake of neutral amino acids, including glutamine, were decreased. These changes could facilitate the deamidation of glutamine within the tubular lumen. Finally, pronounced increases were also observed in the levels of DAB2 (3-fold) and myosin 9 (7-fold), proteins that may participate in endocytosis of apical membrane proteins. Western blot analysis and accurate mass and time analyses were used to validate the spectral counting.

A PDMS-based biochip with integrated sub-micrometre position control for TIRF microscopy of the apical cell membrane

A poly(dimethylsiloxane) (PDMS)-based biochip with an integrated pressure controlled positioning system with sub-micrometre precision was realized. The biochip was easy and cheap to manufacture and enabled positioning in a wet environment. It allowed the application of total internal reflection fluorescence (TIRF) microscopy at the dorsal cell membrane, which is not adhering to a support. Specifically, the chip enabled TIRF microscopy at the apical membrane of polarized epithelial cells. Thereby, the device allowed us for the first time to monitor individual fusion events of GPI-GFP bearing vesicles at the apical membrane in live Madin-Darby canine kidney II (MDCK II) cells. Moreover, a mapping of fusion sites became feasible and revealed that the whole apical membrane is fusion competent. In total, the biochip offers an all-in-one solution for apical TIRF microscopy and contributes a novel tool to study trafficking processes close to the apical plasma membrane in polarized epithelial cells.

Off the beaten renin-angiotensin-aldosterone system pathway: new perspectives on antiproteinuric therapy

CKD is a major public health problem in the developed and the developing world. The degree of proteinuria associated with renal failure is a generally well accepted marker of disease severity. Agents with direct antiproteinuric effects are highly desirable therapeutic strategies for slowing, or even halting, progressive loss of kidney function. We review progress on therapies acting further downstream of the renin-angiotensin-aldosterone system pathway (e.g., transforming growth factor-beta antagonism, endothelin antagonism) and on those acting independent of the renin-angiotensin-aldosterone system pathway. In all, we discuss 26 therapeutic targets or compounds and 2 lifestyle changes (dietary modification and weight loss) that have been used clinically for diabetic or nondiabetic kidney disease. These therapies include endogenous molecules (estrogens, isotretinoin), biologic antagonists (monoclonal antibodies, soluble receptors), and small molecules. Where mechanistic data are available, these therapies have been shown to exert favorable effects on glomerular cell phenotype. In some cases, recent work has indicated surprising new molecular pathways for some therapies, such as direct effects on the podocyte by glucocorticoids, rituximab, and erythropoietin. It is hoped that recent advances in the basic science of kidney injury will prompt development of more effective pharmaceutical and biologic therapies for proteinuria.

Nanometer-scale imaging by the modulation tracking method

We developed an optical imaging method based on a feedback principle in which the specific scan pattern is adapted according to the shape of the sample. The feedback approach produces nanometer-resolved 3D images of very small and moving features in live cells in seconds. We show images of microvilli in live cultured opossum kidney cells expressing NaPi co-transporter proteins with different GFP constructs and images of cell protrusions in a collagen matrix with a resolution of about 20 nm. We found that in the microvilli the NaPi proteins can be found clustered. Along cell protrusions in 3D we identified cellular adhesions to the extracellular matrix. Our approach to super-resolution and to 3D nanoimaging is different than other proposed methods that break the diffraction limit using non-linear effects or are based on single molecule localization.

Differential modulation of the molecular dynamics of the type IIa and IIc sodium phosphate cotransporters by parathyroid hormone

The kidney is a key regulator of phosphate homeostasis. There are two predominant renal sodium phosphate cotransporters, NaPi2a and NaPi2c. Both are regulated by parathyroid hormone (PTH), which decreases the abundance of the NaPi cotransporters in the apical membrane of renal proximal tubule cells. The time course of PTH-induced removal of the two cotransporters from the apical membrane, however, is markedly different for NaPi2a compared with NaPi2c. In animals and in cell culture, PTH treatment results in almost complete removal of NaPi2a from the brush border (BB) within 1 h whereas for NaPi2c this process in not complete until 4 to 8 h after PTH treatment. The reason for this is poorly understood. We have previously shown that the unconventional myosin motor myosin VI is required for PTH-induced removal of NaPi2a from the proximal tubule BB. Here we demonstrate that myosin VI is also necessary for PTH-induced removal of NaPi2c from the apical membrane. In addition, we show that, while at baseline the two cotransporters have similar diffusion coefficients within the membrane, after PTH addition the diffusion coefficient for NaPi2a initially exceeds that for NaPi2c. Thus NaPi2c appears to remain "tethered" in the apical membrane for longer periods of time after PTH treatment, accounting, at least in part, for the difference in response times to PTH of NaPi2a versus NaPi2c.

Role of PDZK1 protein in apical membrane expression of renal sodium-coupled phosphate transporters

The sodium-dependent phosphate (Na/P(i)) transporters NaPi-2a and NaPi-2c play a major role in the renal reabsorption of P(i). The functional need for several transporters accomplishing the same role is still not clear. However, the fact that these transporters show differential regulation under dietary and hormonal stimuli suggests different roles in P(i) reabsorption. The pathways controlling this differential regulation are still unknown, but one of the candidates involved is the NHERF family of scaffolding PDZ proteins. We propose that differences in the molecular interaction with PDZ proteins are related with the differential adaptation of Na/P(i) transporters. Pdzk1(-/-) mice adapted to chronic low P(i) diets showed an increased expression of NaPi-2a protein in the apical membrane of proximal tubules but impaired up-regulation of NaPi-2c. These results suggest an important role for PDZK1 in the stabilization of NaPi-2c in the apical membrane. We studied the specific protein-protein interactions of Na/P(i) transporters with NHERF-1 and PDZK1 by FRET. FRET measurements showed a much stronger interaction of NHERF-1 with NaPi-2a than with NaPi-2c. However, both Na/P(i) transporters showed similar FRET efficiencies with PDZK1. Interestingly, in cells adapted to low P(i) concentrations, there were increases in NaPi-2c/PDZK1 and NaPi-2a/NHERF-1 interactions. The differential affinity of the Na/P(i) transporters for NHERF-1 and PDZK1 proteins could partially explain their differential regulation and/or stability in the apical membrane. In this regard, direct interaction between NaPi-2c and PDZK1 seems to play an important role in the physiological regulation of NaPi-2c.

Dynamic imaging of the sodium phosphate cotransporters

Although in vivo and cell culture studies have provided useful information about the regulation of the sodium phosphate (NaPi) cotransporters, such studies are unable to provide information at the molecular level about interactions between proteins. The NaPi proteins are found within both intestinal and renal brush border microvilli, and previous work has shown that these microvilli contain scaffolding proteins (PDZ proteins) and myosin motors. The recent development of several advanced imaging techniques has allowed detailed analysis of how NaPi proteins interact with scaffolding proteins and myosin motors. Using techniques such as apical total internal reflection fluorescence microscopy, fluorescence correlation spectroscopy, raster image correlation spectroscopy, and fluorescence lifetime imaging-Förster resonance energy transfer, we have found that a myosin motor is involved in trafficking of the NaPi cotransporters and also that Npt2a and Npt2c seem to have different affinities for the PDZ protein Na+/H+ exchanger regulatory factor 1. Further application of these techniques will provide additional insights into NaPi trafficking and regulation.

The regulation of renal phosphate transport

Renal phosphate transport is mediated by the abundance and activity of the sodium-dependent phosphate transporters, Npt2a, Npt2c, and PiT-2, present within the apical brush border membrane of the proximal tubule. Recent studies have demonstrated differential expression and activity of these sodium-dependent phosphate transporters within the proximal tubule. In general, phosphate transport is regulated by a variety of physiological stimuli, including parathyroid hormone, glucocorticoids, vitamin D3, estrogen, and thyroid hormone. Phosphatonins are now recognized as major regulators of phosphate transport activity. Other factors that affect phosphate transport include dopamine, dietary phosphate, acid-base status, lipid composition, potassium deficiency, circadian rhythm, and hypertension. Studies have shown that the PDZ-containing sodium/hydrogen exchanger regulatory factor (NHERF) proteins, specifically NHERF-1 and NHERF-3, play a critical role in the physiological regulation of phosphate transport, particularly in response to dietary phosphate. In addition, recent studies have found that NHERF-1 is also important in both the parathyroid hormone- and dopamine-mediated inhibition of phosphate transport. This review will detail the various hormones and agents involved in the regulation of phosphate transport as well as provide a brief summary of the signaling pathways and cytoskeletal proteins active in the transport of phosphate in the renal proximal tubule.

Sorting it out in endosomes: an emerging concept in renal epithelial cell transport regulation

Ion and water transport by the kidney is continually adjusted in response to physiological cues. Selective endocytosis and endosomal trafficking of ion transporters are increasingly appreciated as mechanisms to acutely modulate renal function. Here, we discuss emerging paradigms in this new area of investigation.

Myosin motor function: the ins and outs of actin-based membrane protrusions

Cells build plasma membrane protrusions supported by parallel bundles of F-actin to enable a wide variety of biological functions, ranging from motility to host defense. Filopodia, microvilli and stereocilia are three such protrusions that have been the focus of intense biological and biophysical investigation in recent years. While it is evident that actin dynamics play a significant role in the formation of these organelles, members of the myosin superfamily have also been implicated as key players in the maintenance of protrusion architecture and function. Based on a simple analysis of the physical forces that control protrusion formation and morphology, as well as our review of available data, we propose that myosins play two general roles within these structures: (1) as cargo transporters to move critical regulatory components toward distal tips and (2) as mediators of membrane-cytoskeleton adhesion.

Mechanisms of proximal tubule sodium transport regulation that link extracellular fluid volume and blood pressure

One-hundred years ago, Starling articulated the interdependence of renal control of circulating blood volume and effective cardiac performance. During the past 25 years, the molecular mechanisms responsible for the interdependence of blood pressure (BP), extracellular fluid volume (ECFV), the renin-angiotensin system (RAS), and sympathetic nervous system (SNS) have begun to be revealed. These variables all converge on regulation of renal proximal tubule (PT) sodium transport. The PT reabsorbs two-thirds of the filtered Na(+) and volume at baseline. This fraction is decreased when BP or perfusion pressure is increased, during a high-salt diet (elevated ECFV), and during inhibition of the production of ANG II; conversely, this fraction is increased by ANG II, SNS activation, and a low-salt diet. These variables all regulate the distribution of the Na(+)/H(+) exchanger isoform 3 (NHE3) and the Na(+)-phosphate cotransporter (NaPi2), along the apical microvilli of the PT. Natriuretic stimuli provoke the dynamic redistribution of these transporters along with associated regulators, molecular motors, and cytoskeleton-associated proteins to the base of the microvilli. The lipid raft-associated NHE3 remains at the base, and the nonraft-associated NaPi2 is endocytosed, culminating in decreased Na(+) transport and increased PT flow rate. Antinatriuretic stimuli return the same transporters and regulators to the body of the microvilli associated with an increase in transport activity and decrease in PT flow rate. In summary, ECFV and BP homeostasis are, at least in part, maintained by continuous and acute redistribution of transporter complexes up and down the PT microvilli, which affect regulation of PT sodium reabsorption in response to fluctuations in ECFV, BP, SNS, and RAS.