Decreased urinary peptide excretion in patients with renal disease

Spatiotemporal organisation of protein processing in the kidney

The kidney regulates plasma protein levels by eliminating them from the circulation. Proteins filtered by glomeruli are endocytosed and degraded in the proximal tubule and defects in this process result in tubular proteinuria, an important clinical biomarker. However, the spatiotemporal organization of renal protein metabolism in vivo was previously unclear. Here, using functional probes and intravital microscopy, we track the fate of filtered proteins in real time in living mice, and map specialized processing to tubular structures with singular value decomposition analysis and three-dimensional electron microscopy. We reveal that degradation of proteins requires sequential, coordinated activity of distinct tubular sub-segments, each adapted to specific tasks. Moreover, we leverage this approach to pinpoint the nature of endo-lysosomal disorders in disease models, and show that compensatory uptake in later regions of the proximal tubule limits urinary protein loss. This means that measurement of proteinuria likely underestimates severity of endocytotic defects in patients.

Polesel et al. visualize plasma protein filtration, uptake and metabolism in the kidneys of living mice in real-time. They reveal coordinated activity of different specialized tubular segments, with major compensatory adaptations occurring in disease states.

Exposure sources, amounts and time course of gluten ingestion and excretion in patients with coeliac disease on a gluten-free diet

BACKGROUND

A major deficit in understanding and improving treatment in coeliac disease (CD) is the lack of empiric data on real world gluten exposure.

AIMS

To estimate gluten exposure on a gluten-free diet (GFD) using immunoassays for gluten immunogenic peptides (GIP) and to examine relationships among GIP detection, symptoms and suspected gluten exposures METHODS: Adults with biopsy-confirmed CD on a GFD for 24 months were recruited from a population-based inception cohort. Participants kept a diary and collected urine samples for 10 days and stools on days 4-10. 'Doggie bags' containing ¼ portions of foods consumed were saved during the first 7 days. Gluten in food, stool and urine was quantified using A1/G12 ELISA.

RESULTS

Eighteen participants with CD (12 female; age 21-70 years) and three participants on a gluten-containing diet enrolled and completed the study. Twelve out of 18 CD participants had a median 2.1 mg gluten per exposure (range 0.2 to >80 mg). Most exposures were asymptomatic and unsuspected. There was high intra-individual variability in the interval between gluten ingestion and excretion. Participants were generally unable to identify the food.

CONCLUSIONS

Gluten exposure on a GFD is common, intermittent, and usually silent. Excretion kinetics are highly variable among individuals. The amount of gluten varied widely, but was typically in the milligram range, which was 10-100 times less than consumed by those on an unrestricted diet. These findings suggest that a strict GFD is difficult to attain, and specific exposures are difficult to detect due to variable time course of excretion.

Peptiduria: a potential early predictor of diabetic kidney disease

BACKGROUND

To protect the kidney effectively with medication in type 2 diabetics, it is crucial to identify such at-risk patients early for treatment. We investigated whether peptiduria precedes proteinuria (the earliest urinary marker in our model), and thereby serve as an early predictor of diabetic nephropathy.

METHODS

A longitudinal study was performed in a rat model of diabetic nephropathy. Peptides, defined as degradation products of proteins of < 13 kD size, were quantified by a previously validated method using a combination of Lowry and Biorad protein assays. Peptides in urine were also confirmed by chromatographically separating low molecular weight fractions from urine and quantifying albumin fragments in these fractions by enzyme immunoassay. Also, the mechanism of peptiduria was addressed by measuring acid phosphatase, a marker of lysosomal activity, in urine and on kidney sections (histochemically).

RESULTS

In rats with diabetic nephropathy, proteinuria occurred after 12 weeks of diabetes, while peptiduria occurred as early as 2 weeks after diabetes. Peptiduria was confirmed by showing that the chromatographically separated low molecular weight fractions of urine containing albumin fragments is in proportion to the level of peptiduria. The time course of peptiduria paralleled the increase in urinary acid phosphatase suggesting that the mechanism of early peptiduria could be due to upregulation of lysosomal enzyme activity in the tubules.

CONCLUSIONS

Our results showing that peptiduria precedes proteinuria in diabetic nephropathy provide a compelling rationale to perform a prospective human clinical trial to investigate whether peptiduria can serve as an early predictor of diabetic nephropathy.

Podocyte-specific soluble epoxide hydrolase deficiency in mice attenuates acute kidney injury

Podocytes play an important role in maintaining glomerular function, and podocyte injury is a significant component in the pathogenesis of proteinuria. Soluble epoxide hydrolase (sEH) is a cytosolic enzyme whose genetic deficiency and pharmacological inhibition have beneficial effects on renal function, but its role in podocytes remains unexplored. The objective of this study was to investigate the contribution of sEH in podocytes to lipopolysaccharide (LPS)-induced kidney injury. We report increased sEH transcript and protein expression in murine podocytes upon LPS challenge. To determine the function of sEH in podocytes in vivo we generated podocyte-specific sEH-deficient (pod-sEHKO) mice. Following LPS challenge, podocyte sEH-deficient mice exhibited lower kidney injury, proteinuria, and blood urea nitrogen concentrations than controls suggestive of preserved renal function. Also, renal mRNA and serum concentrations of inflammatory cytokines IL-6, IL-1β, and TNFα were significantly lower in LPS-treated pod-sEHKO than control mice. Moreover, podocyte sEH deficiency was associated with decreased LPS-induced NF-κB and MAPK activation and attenuated endoplasmic reticulum stress. Furthermore, the protective effects of podocyte sEH deficiency in vivo were recapitulated in E11 murine podocytes treated with a selective sEH pharmacological inhibitor. Altogether, these findings identify sEH in podocytes as a contributor to signaling events in acute renal injury and suggest that sEH inhibition may be of therapeutic value in proteinuria.

ENZYMES

Soluble epoxide hydrolase: EC 3.3.2.10.

Determination of urinary peptides in patients with proteinuria

Although considered useful in the diagnosis and prognosis of renal diseases, proteinuria can only be detected after significant renal paranchymal changes. There is considerable interest in the estimation of urinary peptides as an early marker of renal disease. In the current study, we have estimated urinary peptides in patients with different grades of proteinuria. Twenty-four hour urine samples were collected from 138 subjects and classified into three groups based on the urine protein excreted: group I (normoproteinuria, 0–150 mg/day, n = 37), group II (microproteinuria, 150–300 mg/day, n = 31), and group III (macroproteinuria, > 300 mg/day, n = 70). Urine proteins were determined using Bradford's method and urinary peptide levels were determined by subtracting Bradford's value from the Lowry value of the same sample. There was a significant decrease in the levels of urinary peptides in group III compared to group I (P < 0.01), however, there was no difference in peptides between groups I and II. The percentage of urinary peptides was decreased in both groups II and III compared to group I (P < 0.01), and there was a significant difference in % urinary peptide content in group II compared to group III (P < 0.01). On correlation, % urinary peptides correlated negatively with urinary proteins/g creatinine (r = - 0.782, P < 0.01) and positively with urinary peptides/g creatinine (r = 0.238, P < 0.01). Our data suggest that there is a marked decrease in urinary peptide levels with an increase in proteinuria. This may suggest impaired tubular protein reabsorption and degradation capacity of renal tubules.

Proteinuria in diabetic kidney disease: a mechanistic viewpoint

Proteinuria is the hallmark of diabetic kidney disease (DKD) and is an independent risk factor for both renal disease progression, and cardiovascular disease. Although the characteristic pathological changes in DKD include thickening of the glomerular basement membrane and mesangial expansion, these changes per se do not readily explain how patients develop proteinuria. Recent advances in podocyte and glomerular endothelial cell biology have shifted our focus to also include these cells of the glomerular filtration barrier in the development of proteinuria in DKD. This review describes the pathophysiological mechanisms at a cellular level which explain why patients with DKD develop proteinuria.

Clinical Utility of Cardiac Troponin I in the Diagnosis of Acute Coronary Syndrome in Patients With Renal Failure

To analyze sensitivity and specificity of cardiac troponin I (cTnI) in detecting obstructive coronary artery disease in African American population with renal insufficiency presenting with acute coronary syndrome. Retrospective analysis of 108 patients who underwent coronary angiography over a 3-year period in a single institution. A troponin I level of 0.1 ng/mL or higher was considered abnormal troponin I. Renal insufficiency was defined as creatinine of 1.2 mg/dL or higher. Obstructive coronary artery disease (CAD) was defined as luminal diameter reduction of 70% or more (or total occlusion) in at least 1 coronary artery. Patients were divided into group 1 (renal insufficiency without need for hemodialysis, n = 76, mean age = 65) and group 2 (patients requiring hemodialysis, n = 32, mean age = 60). Access Accu TnI method was used to quantitate cTnI where murine monoclonal antibodies specifically bind to the C-terminal end of cTnI. In group 1, 41 (54%) patients had abnormal troponin of whom 37 (90%) had CAD and 4 (10%) had normal angiogram; 35 (46%) patients had normal troponin, of whom 25 (71%) had CAD and 10 (29%) had normal angiogram yielding a sensitivity of 60% and specificity of 71% (P = 0.003; 95% confidence interval). In group 2, 20 (63%) had abnormal troponin of whom 19 (95%) had CAD and 1(5%) had normal angiogram; 12 (38%) had normal troponin of whom 7 (59%) had CAD and 5 (41%) had normal angiogram yielding a sensitivity of 73% and specificity of 83% (P = 0.06; 95% confidence interval). cTnI has a sensitivity of 60% and specificity of 71% in acute coronary syndrome patients with renal insufficiency. In patients on hemodialysis, troponin I has a sensitivity of 73% and specificity of 83% for detection of obstructive CAD.

Characterization of the urinary albumin degradation pathway in the isolated perfused rat kidney

This study examines the existence of the urinary albumin degradation pathway and the proposed role of receptor-mediated endocytosis in this process using the isolated perfused rat kidney (IPK) model. Albumin-derived peptides in IPK urine are analyzed in terms of their relative size distribution using radioactivity and absorbance at 214 nm, and their susceptibility to trypsin digestion. The effects of perfusing kidneys with concanamycin A and myristoyl trimethyl ammonium bromide (MTMAB), inhibitors of the receptor-mediated endocytosis regulators vacuolar-type H(+) ATPase (v-ATPase) and dynamin GTPase, respectively, are examined. Normal IPK urine contains mildly degraded (defined as approximately 10-40 kDa; 43.0 +/- 8.3%) and heavily degraded (defined as <10 kDa; 22.6 +/- 7.7%) albumin peptides as well as intact albumin (34.5 +/- 4.1%). The relative size distribution of the peptides is similar by radioactivity and absorbance at 214 nm, and both profiles are reduced to very small peptides following trypsin digestion. Administration of concanamycin A or MTMAB causes a significant increase in the proportion of intact albumin (concanamycin A: 55.8 +/- 11.6%; MTMAB: 50.0 +/- 11.9%) excreted compared with normal IPK urine. This coincides with a reduction in the proportion of mildly (concanamycin A: 27.6 +/- 9.8%; MTMAB: 39.9 +/- 11.5%) and heavily degraded (concanamycin A: 16.6 +/- 7.4%; MTMAB: 10.0 +/- 2.5%) albumin present and is not associated with changes in glomerular permeability to albumin because no significant change is observed in the fractional clearance of Ficoll (radius range 20-60 A) in the presence of concanamycin A. This study demonstrates the existence of albumin peptides in IPK urine and suggests that receptor-mediated endocytosis plays a role in urinary albumin degradation.

Cross-Reactivity of Amino Acids and Other Compounds in the Biuret Reaction: Interference with Urinary Peptide Measurements

Background: Biuret assays for total protein measurement are considered to react with all peptides longer than 2 residues. Some studies using biuret assays of urine suggest that small peptides generally are more abundant than proteins in urine, but it is not clear whether this is a problem of assay specificity.

Methods: We analyzed the specificity and kinetics of a biuret reaction for solutions of amino acids, organic compounds, peptides, proteins, and ultrafiltered urine specimens and compared the results with standard clinical assays for protein measurement.

Results: The biuret assay cross-reacted with several amino acids, dipeptides, and other organic compounds able to form 5- or 6-member ring chelation complexes with copper. Reactions with amino acids and dipeptides had higher absorbance maxima (blue color) than with larger peptides and proteins (purple). Compounds forming potential 4-, 7-, 8-, or 9-member ring complexes with copper had low reactivity. Amino acid amides, dipeptides, and longer peptides had substantial reactivity, except those containing proline. Proteins and polypeptides had similar biuret reactivities per peptide bond, but reaction kinetics were slower for proteins than peptides. Urine specimens ultrafiltered through 3-kDa–cutoff membranes had substantial biuret reactivity, but absorbance maxima were consistent with cross-reactive amino acids rather than peptides.

Conclusions: Many compounds, including amino acids, amino acid derivatives, and dipeptides, cross-react in biuret assays. Our studies improve understanding of the specificity of endpoint and kinetic biuret assays widely used in clinical laboratories. Amino acids, urea, and creatinine contribute to overestimation of urinary peptide content by biuret assays.