Zeta potential distribution on calcium oxalate crystal and Tamm-Horsfall protein surface analyzed with Doppler electrophoretic light scattering

4-PBA rescues hyperoxaluria induced nephrolithiasis by modulating urinary glycoproteins: Cross talk between endoplasmic reticulum, calcium homeostasis and mitochondria

AIM

Urinary glycoproteins such as Tamm Horsfall Protein (THP) and Osteopontin (OPN) are well established key regulators of renal stone formation. Additionally, recent revelations have highlighted the influence of Endoplasmic Reticulum (ER) and mitochondria of crucial importance in nephrolithiasis. However, till date conclusive approach highlighting the influence of ER stress on urinary glycoproteins and chaperone in nephrolithiasis remains elusive. Therefore, the present study was focussed on deciphering the possible effect of 4-PBA mitigating ER stress on urinary glycoproteins and calnexin (chaperone) with emphasis on interlinking calcium homeostasis in hyperoxaluric rats.

MATERIAL AND METHODS

Post 9 days of treatment, animals were sacrificed, and renal tissues were investigated for urinary glycoproteins, calnexin, calcium homeostasis, ER environment, redox status, and mitochondrial linkage.

KEY FINDINGS

4-PBA appreciably reversed the altered levels of THP, OPN, and calnexin observed along with curtailing the disrupted calcium homeostasis when assessed for SERCA activity and intra-cellular calcium levels. Additionally, significant improvement in the perturbed ER environment as verified by escalated ER stress markers, disturbed protein folding-aggregation-degradation (congo red assay) pathway, and redox status was found post 4-PBA intervention. Interestingly, linkage of ER stress and mitochondria was established under hyperoxaluric conditions when assessed for protein levels of VDAC1 and GRP75.

SIGNIFICANCE

4-PBA treatment resulted in rectifying the repercussions of ER-mitochondrial caused distress when assessed for protein folding/aggregation/degradation events along with disturbed calcium homeostasis. The present study advocates the necessity to adopt a holistic vision towards hyperoxaluria with emphasis on glycoproteins and ER environment.

Label-Free Protein Detection by Micro-Acoustic Biosensor Coupled with Electrical Field Sorting. Theoretical Study in Urine Models

Diagnostic devices for point-of-care (POC) urine analysis (urinalysis) based on microfluidic technology have been actively developing for several decades as an alternative to laboratory based biochemical assays. Urine proteins (albumin, immunoglobulins, uromodulin, haemoglobin etc.) are important biomarkers of various pathological conditions and should be selectively detected by urinalysis sensors. The challenge is a determination of different oligomeric forms of the same protein, e.g., uromodulin, which have similar bio-chemical affinity but different physical properties. For the selective detection of different types of proteins, we propose to use a shear bulk acoustic resonator sensor with an additional electrode on the upper part of the bioliquid-filled channel for protein electric field manipulation. It causes modulation of the protein concentration over time in the near-surface region of the acoustic sensor, that allows to distinguish proteins based on their differences in diffusion coefficients (or sizes) and zeta-potentials. Moreover, in order to improve the sensitivity to density, we propose to use structured sensor interface. A numerical study of this approach for the detection of proteins was carried out using the example of albumin, immunoglobulin, and oligomeric forms of uromodulin in model urine solutions. In this contribution we prove the proposed concept with numerical studies for the detection of albumin, immunoglobulin, and oligomeric forms of uromodulin in urine models.

Novel in situ normal streaming potential device for characterizing electrostatic properties of confluent cells

The characteristics of transport across confluent cell monolayers may often be attributed to its electrostatic properties. While tangential streaming potential is often used to quantify these electrostatic properties, this method is not effective for transport normal to the apical cell surface where the charge properties along the basolateral sides may be important (i.e., confluent cells with leaky tight junctions). In addition, even when cells have a uniform charge distribution, the shear stress generated by the conventional tangential flow device may dislodge cells from their confluent state. Here we introduce a novel streaming potential measurement device to characterize the normal electrostatic properties of confluent cells. The streaming potential device encompasses a 24 mm cell-seeded Transwell(®) with two AgCl electrodes on either side of the cell-seeded Transwell. Phosphate buffered saline is pressurized transversal to the Transwell and the resultant pressure gradient induces a potential difference. Confluent monolayers of HEK and EA926 cells are used as examples. The corresponding zeta potential of the cell-membrane configuration is calculated using the Helmholtz-Smoluchowski equation and the zeta potential of the confluent cell layer is deconvolved from the overall measurements. For these test models, the zeta potential is consistent with that determined using a commercial dispersed-cell device. This novel streaming potential device provides a simple, easy, and cost-effective methodology to determine the normal zeta potential of confluent cells cultured on Transwell systems while keeping the cells intact. Furthermore, its versatility allows periodic measurements of properties of the same cell culture during transient studies.

Calcium oxalate monohydrate aggregation induced by aggregation of desialylated Tamm-Horsfall protein

Tamm-Horsfall protein (THP) is thought to protect against calcium oxalate monohydrate (COM) stone formation by inhibiting COM aggregation. Several studies reported that stone formers produce THP with reduced levels of glycosylation, particularly sialic acid levels, which leads to reduced negative charge. In this study, normal THP was treated with neuraminidase to remove sialic acid residues, confirmed by an isoelectric point shift to higher pH. COM aggregation assays revealed that desialylated THP (ds-THP) promoted COM aggregation, while normal THP inhibited aggregation. The appearance of protein aggregates in solutions at ds-THP concentrations ≥1 μg/mL in 150 mM NaCl correlated with COM aggregation promotion, implying that ds-THP aggregation induced COM aggregation. The aggregation-promoting effect of the ds-THP was independent of pH above its isoelectric point, but was substantially reduced at low ionic strength, where protein aggregation was much reduced. COM aggregation promotion was maximized at a ds-THP to COM mass ratio of ~0.025, which can be explained by a model wherein partial COM surface coverage by ds-THP aggregates promotes crystal aggregation by bridging opposing COM surfaces, whereas higher surface coverage leads to repulsion between adsorbed ds-THP aggregates. Thus, desialylation of THP apparently abrogates a normal defensive action of THP by inducing protein aggregation, and subsequently COM aggregation, a condition that favors kidney stone formation.

Stabilization of calcium oxalate suspension by urinary macromolecules, probably an efficient protection from stone formation

INTRODUCTION

Crystal aggregation (AGN) destabilizes crystal suspensions and during crystalluria probably favors crystal apposition to kidney calcifications and preexisting stones. We analyzed inhibition of AGN and stabilization of calcium oxalate suspensions by urinary macromolecules (UM), urine and solutions with urinary citrate concentration.

MATERIALS AND METHODS

Solutions of UM (UMS) were obtained by a hemofiltration procedure from urine of 6 healthy men. Calcium oxalate suspensions were prepared in all solutions and urine by adjusting Ca2+ to 1.5 mM and by an oxalate titration to 1.0 mM. Crystallization was monitored measuring optical density (OD). Stability of suspensions (SS) was defined as the time without sedimentation and zeta potential (ZP) of crystals was measured. AGN was visualized by scanning electron microscopy and quantified by maximal OD.

RESULTS

UMS inhibited AGN and increased ZP and SS. Most inhibitory activity of urine could be attributed to UM. 3.3-fold dilution of UM reduced SS only by 30%.

CONCLUSIONS

During crystalluria, UM of healthy men are supposed to protect from stone formation by inhibiting AGN and stabilizing crystal suspensions. As a probably important aspect, this protection was found to be limited in time and may favorably be influenced by an increase of diuresis.

Comparison of urinary proteins in calcium stone formers and healthy individuals: a case-control study

OBJECTIVE

This study aimed at comparing the urinary protein levels in calcium stone formers with those of healthy individuals.

PATIENTS AND METHODS

From January 2002 until June 2004, 100 calcium stone formers (mean age 38.6 +/- 10.3 years), who had at least two episodes of calcium stone formation, were compared with 100 healthy individuals (mean age 33.8 +/- 9.7 years). Their 24-hour urinary protein levels, using SDS-PAGE, were measured.

RESULTS

The mean 24-hour urinary Tamm-Horsfall protein (THP) levels were 3.3 +/- 0.8 mg in the case group and 4.6 +/- 1.9 mg in the controls, and the difference was not statistically significant (p = 0.53). However, the THP levels in individuals with and without bacteriuria were significantly different (15.8 +/- 3.3 mg vs. 2.6 +/- 1.0 mg, p = 0.0001). The mean 24-hour urinary albumin concentrations were 163.31 +/- 15.1 mg in the case group and 74.26 +/- 4.6 mg in the controls. The mean 24-hour urinary transferrin levels were 8.09 +/- 2.7 mg in the case group and 0.40 +/- 0.3 mg in the controls. The differences were statistically significant for both albumin and transferrin (p < 0.0001 and p = 0.0063, respectively). There were no significant differences in any other mean urinary protein concentrations between cases and controls.

CONCLUSIONS

The THP level in the urine of stone formers is not quantitatively different from that of healthy individuals, but it increases in association with bacteriuria. Albumin and transferrin may play a presumptive role in stone formation.

N-Glycans carried by Tamm-Horsfall glycoprotein have a crucial role in the defense against urinary tract diseases

Tamm-Horsfall glycoprotein (THGP), produced exclusively by renal cells from the thick ascending limb of Henle's loop, is attached by a glycosyl-phosphatidylinositol (GPI)-anchor to the luminal face of the cells. Urinary excretion of THGP (50-100 mg/day) occurs upon proteolytic cleavage of the large ectodomain of the GPI-anchored form. N-Glycans, consisting of a large repertoire of sialylated polyantennary chains and high-mannose structures, account for approximately 30% of the weight of human urinary THGP. We describe: (i) the involvement of urinary THGP high-mannose glycans in defense against infections of the urinary tract, caused by type-1 fimbriated Escherichia coli, which recognize high-mannose structures, (ii) the role of GalNAcbeta1-4(NeuAcalpha2-3)Galbeta1-4GlcNAcbeta1-3Gal (Sd(a) determinant) carried by human THGP in protecting the distal nephron from colonization of type-S fimbriated E. coli which recognise NeuAcalpha2-3Gal, (iii) the inhibitory effect of sialylated THGP on crystal aggregation of calcium oxalate and calcium phosphate, thus preventing nephrolithiasis. Finally, we outline the importance of N-glycans in promoting the polymerization of THGP, a process resulting in the formation of homopolymers with an M(r) of several million in urine. Since THGP defense against diseases of the urinary tract mainly consists in binding damaging agents, its ability to behave as a multivalent ligand significantly enhances this protective role.

Brewster angle microscopy of calcium oxalate monohydrate precipitation at phospholipid monolayer phase boundaries

The precipitation of calcium oxalate monohydrate (COM) at phospholipid monolayers confined to the air/water interface is observed in situ with the aid of Brewster angle microscopy. COM crystals appear as bright objects that are easily identified and quantified to assess the effects of different conditions on crystallization. Crystal precipitation was monitored at monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in liquid condensed (LC) and liquid expanded (LE) phases. Within the LC phase, higher pressures reduce the incidence of crystallization at the interface, implying that within this phase precipitation is enhanced by higher compressibility or fluidity of the monolayer. Precipitation at biphasic LC/LE and LE/gas (G) monolayers was also studied. COM appears preferentially at phase boundaries of the DPPC LC/LE and LE/G monolayers. However, when an LC/LE phase boundary is created by two different phospholipids that are phase segregated, such as DPPC and 1,2-dimyristoyl-sn-glycero-3-phosphocholine, crystal formation occurs away from the interface within the DPPC LC phase. It is suggested that COM growth at phase boundaries is preferred only when there is molecular exchange between the phases.

Tamm-Horsfall glycoprotein: biology and clinical relevance

Tamm-Horsfall glycoprotein (THP) is the most abundant urinary protein in mammals. Urinary excretion occurs by proteolytic cleavage of the large ectodomain of the glycosyl phosphatidylinositol-anchored counterpart exposed at the luminal cell surface of the thick ascending limb of Henle's loop. We describe the physical-chemical structure of human THP and its biosynthesis and interaction with other proteins and leukocytes. The clinical relevance of THP reported here includes: (1) involvement in the pathogenesis of cast nephropathy, urolithiasis, and tubulointerstitial nephritis; (2) abnormalities in urinary excretion in renal diseases; and (3) the recent finding that familial juvenile hyperuricemic nephropathy and autosomal dominant medullary cystic kidney disease 2 arise from mutations of the THP gene. We critically examine the literature on the physiological role and mechanism(s) that promote urinary excretion of THP. Some lines of research deal with the in vitro immunoregulatory activity of THP, termed uromodulin when isolated from urine of pregnant women. However, an immunoregulatory function in vivo has not yet been established. In the most recent literature, there is renewed interest in the capacity of urinary THP to compete efficiently with urothelial cell receptors, such as uroplakins, in adhering to type 1 fimbriated Escherichia coli. This property supports the notion that abundant THP excretion in urine is promoted in the host by selective pressure to obtain an efficient defense against urinary tract infections caused by uropathogenic bacteria.

Effect of experimental hyperoxaluria on renal calcium oxalate monohydrate binding proteins in the rat

OBJECTIVE

To determine the functional role of calcium oxalate binding proteins in the nucleation, aggregation and retention of calcium oxalate crystals under physiological and hyperoxaluric conditions. Materials and methods Hyperoxaluria was induced in rats using 0.75% of ethylene glycol in drinking water. Calcium oxalate binding proteins were isolated and fractionated by cellulose column chromatography. Three major protein peak fractions were obtained (73 kDa in Tris-HCl buffer, 20 kDa in 0.05 mol/L NaCl buffer and 23 kDa in 0.3 mol/L buffer). Oxalate binding and the inhibition of crystal nucleation and aggregation by these fractions were determined.

RESULTS

The adsorption of calcium oxalate monohydrate (COM) was ubiquitous in rat tissues and subcellular organelles, but the percentage adsorption varied; maximum absorption occurred in kidneys and pancreas, with microsomes showing maximal adsorption in the kidney. Hyperoxaluric rat tissues showed a greater percentage adsorption. Microsomes were enriched with the 20 kDa protein, while nuclei contained the 23 kDa protein in higher concentrations. COM-binding proteins derived from hyperoxaluric rat kidney had a greater content of 74 kDa and 23 kDa proteins with increased oxalate-binding activities. In the crystal-growth studies, the 74 kDa protein was a promoter, while the other protein fractions inhibited crystallization. In hyperoxaluria, the crystal-growth promoting activity of the 74 kDa protein was further increased, while the inhibition by the 20 and 23 kDa proteins was decreased. The 74 kDa protein derived from control rats formed single COM crystals in a crystal growth system, while the hyperoxaluric rat fraction induced the aggregation of COM crystals.

CONCLUSION

COM-binding proteins (the 74 and 23 kDa fractions) were expressed more in hyperoxaluric rats. In hyperoxaluria the 74 kDa protein tended to promote crystal nucleation and aggregation, and the 20 and 23 kDa proteins were less inhibitory, which increases the risk of stone formation.

Citrate determines calcium oxalate crystallization kinetics and crystal morphology-studies in the presence of Tamm-Horsfall protein of a healthy subject and a severely recurrent calcium stone former

BACKGROUND

The aim of this study was to measure the effects of normal (nTHP) and abnormal stone former Tamm-Horsfall protein (SF-THP) on calcium oxalate (CaOx) nucleation and aggregation as well as on crystal morphology, in presence or absence of citrate.

METHODS

Nucleation and aggregation of CaOx crystals from a supersaturated, stirred solution (200 mM NaCl, 10 mM Na-acetate, pH 5.70, 5 mM Ca and 0.5 mM Ox) were studied by spectrophotometric time-course measurements of OD at 620 nm (OD(620)). Measured parameters were induction time t(I) (time to induce formation of detectable particles), S(N), (slope of increase of OD(620), mainly due to crystal nucleation), and S(A), (slope of decrease of OD(620) after equilibrium has been reached, due to crystal aggregation). Effects of citrate, nTHP and SF-THP on these parameters were measured, and scanning electron microscopy (SEM) was performed.

RESULTS

At 1.5, 2.5 and 3.5 mM, citrate increased t(I) and inhibited crystal nucleation (by 78-87%) as well as aggregation (by 63-70%), and smaller CaOx crystals (length/width ratio 1.7+/-0.1) than under standard conditions (length/width 3.9+/-0.5) were visible (P<0.001). Normal THP at 30 and 40 mg/l inhibited crystal nucleation and, more strongly, aggregation (inhibition 76-81%). SEM revealed a decrease in length/width ratio to 2.6+/-0.4 (P=0.051 vs standard conditions) and less aggregation than without nTHP. At all concentrations tested, SF-THP reduced t(I) (P=0.0001 vs standard conditions) and promoted aggregation (inhibition -48 to -33%); crystals were elongated with a length/width ratio of 4.3+/-0.6 (P<0. 05 vs nTHP). When simultaneously present with nTHP, citrate enhanced the inhibitory effects of nTHP, producing the smallest (length/width 1.5+/-0.1) and least aggregated crystals. Finally, 3.5 mM citrate turned promotory SF-THP into a crystallization inhibitor with abundant small and clustered, but not aggregated crystals.

CONCLUSION

Citrate appears to be the main determinant of CaOx crystallization rates and crystal morphology in the presence of nTHP as well as SF-THP. Its effects appear to predominate over those of THP, since even promotory SF-THP is turned into a crystallization inhibitor in the presence of citrate. This re-emphasizes at a morphological level what has been concluded from functional as well from clinical studies, namely that citrate is needed in urine at equimolar concentrations to calcium in order to prevent the formation of large crystal aggregates in presence of abnormal THP.

Uric acid-binding proteins in calcium oxalate stone formers and their effect on calcium oxalate crystallization

OBJECTIVES

To study the effect of urinary uric acid-binding proteins of controls and stone formers on calcium oxalate crystal nucleation and aggregation.

MATERIALS AND METHODS

Urine samples were collected over 24 h from 20 stone formers and from 20 age-matched normal controls. Uric acid crystallization was induced by adding equal volumes of 2.5 mmol/L uric acid. The bound proteins were separated on a cellulose column, and by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The effect of the separated fractions on calcium oxalate crystal nucleation and aggregation was assessed.

RESULTS

The protein bound to unit mass of uric acid crystals was higher in hyperoxaluric urine than in control urine. On cellulose-column separation, the uric acid-crystal binding proteins produced three major protein peaks, i.e. fraction I (buffer), fraction II (0.05 mol/L sodium chloride in Tris-HCl buffer) and fraction III (0.3 mol/L sodium chloride in buffer), with a minor peak obtained on elution with increasing concentrations of sodium chloride in Tris-HCl buffer (pH 7.0). Fraction I derived from either stone formers or controls promoted calcium oxalate crystallization. Fraction II from the control samples was a strong inhibitor, whereas hyperoxaluric fraction II was less inhibitory.

CONCLUSION

Uric acid-binding proteins isolated either from the urine of stone formers or controls modulated calcium oxalate crystal growth. Proteins isolated from stone formers were less inhibitory of crystal nucleation and aggregation. These proteins may act as a bridge, leading to the epitaxial deposition of calcium oxalate over a urate core.

The inhibition of calcium oxalate monohydrate crystal growth by maleic acid copolymers

The crystallization of calcium oxalate monohydrate (COM) was investigated at conditions of constant supersaturation both in the absence and in the presence of synthetic maleic acid copolymers at 37C, 0.15 M NaCl. The dependence of the rates of COM crystallization in the absence of inhibitors was found to be second order at low and first order at higher supersaturations suggesting a surface diffusion controlled mechanism. The presence of all copolymers tested at concentration levels up to 5 ppm retarded the rates of COM crystal growth up to 90%. The decrease of the COM crystal growth rates by the polymers depended on the nature of the comonomer polymerized with maleic acid and the order of inhibition was found to be vinyl acetate > N-vinylpyrrolidone > styrene. Taking into consideration kinetics data published in the literature concerning the inhibition of COM crystal growth, it is suggested that molecular weight also plays a role, with more inhibition at higher molecular weights. The morphology of the COM crystals grown was unaffected yet the crystals growing at lower rates in the presence of the copolymers were larger and their size more uniform. It is concluded that maleic acid copolymers are strong inhibitors of the crystallization of COM, the inhibitory activity being more pronounced in the case of the linear copolymers.

Intratubular crystallization events

Can urolithiasis start as an intratubular event? Under severe hyperoxaluric conditions in animal models at least crystal formation can. Recently models have been presented that assess the chances of crystal formation under more normal conditions. These models describe changes in fluid composition as this passes through the nephron, these conditions being simulated in in vitro experiments. It appears that under naturally occurring intratubular conditions calcium-salt crystallization takes place within the time tubular fluid normally spends in the nephron. Precipitation starts with a calcium-phosphate phase under conditions found in the thin lambs. This crystalline phase then (partly) dissolves when collecting duct conditions are used, thereby inducing formation of calcium oxalates. Under these conditions the latter increase in size by way of crystal growth and agglomeration. Large particle formation and cell adhesion can eventually result in particle retention and subsequent stone formation. Viewing urolithiasis as originally an intratubular event has consequences for in vitro experiments and treatments, which are discussed in this paper.

Isoelectric focusing of Tamm-Horsfall glycoproteins: a simple tool for recognizing recurrent calcium oxalate renal stone formers

Tamm-Horsfall glycoproteins (THPs) from healthy probands and a majority of recurrent calcium oxalate renal stone formers reveal different physicochemical properties when analyzed using isoelectric focusing (IEF). The pI values of THPs from healthy probands are approximately 3.5 while THPs from recurrent renal stone formers have pI values of between 4.5 and 6. The two groups of THPs exhibit completely different protein patterns. The differences in IEF analysis allow differentiation between THPs from healthy probands and recurrent calcium oxalate stone formers and may possibly be used as a simple diagnostic method for the recognition of recurrent calcium oxalate renal stone formers.

A simple diagnostic method for the differentiation of Tamm-Horsfall glycoproteins from healthy probands and those from recurrent calcium oxalate renal stone formers

Tamm-Horsfall glycoproteins (THPs)* from healthy probands, and those from a majority of recurrent calcium oxalate renal stone formers, reveal different properties when analyzed using isoelectric focusing. The pl-values of THPs from healthy probands are approximately 3.5 while THPs from recurrent renal stone formers have pl-values between 4.5 and 6. The two groups of THPs exhibit completely different protein patterns in IEF. This proves the structural difference of these THPs. The differences in IEF analysis allow the differentiation between THPs from healthy probands and those from recurrent calcium oxalate stone formers. These differences could possibly be used as a simple diagnostic method for the recognition of recurrent calcium oxalate renal stone formers.