Disease-dependent mechanisms of albuminuria

Modeling transport in the kidney: investigating function and dysfunction

Mathematical models of water and solute transport in the kidney have significantly expanded our understanding of renal function in both health and disease. This review describes recent theoretical developments and emphasizes the relevance of model findings to major unresolved questions and controversies. These include the fundamental processes by which urine is concentrated in the inner medulla, the ultrastructural basis of proteinuria, irregular flow oscillation patterns in spontaneously hypertensive rats, and the mechanisms underlying the hypotensive effects of thiazides. Macroscopic models of water, NaCl, and urea transport in populations of nephrons have served to test, confirm, or refute a number of hypotheses related to the urine concentrating mechanism. Other macroscopic models focus on the mechanisms, role, and irregularities of renal hemodynamic control and on the regulation of renal oxygenation. At the mesoscale, models of glomerular filtration have yielded significant insight into the ultrastructural basis underlying a number of disorders. At the cellular scale, models of epithelial solute transport and pericyte Ca2+ signaling are being used to elucidate transport pathways and the effects of hormones and drugs. Areas where further theoretical progress is conditional on experimental advances are also identified.

The impact of stopping inhibitors of the renin-angiotensin system in patients with advanced chronic kidney disease

BACKGROUND

Inhibition of the renin-angiotensin-aldosterone system (RAAS) has shown to slow chronic kidney disease (CKD) progression. This is most notable at the earlier stages of diabetic and proteinuric nephropathies. Objective. Here, we observed the impact of discontinuation of angiotensin converting enzyme inhibitors (ACEi)/angiotensin receptors blockers (ARB) in patients with advanced kidney disease.

METHODS

52 patients (21 females and 31 males) with advanced CKD (stages 4 and 5), who attended our low clearance clinic (LCC) in preparation for renal replacement therapy (RRT). Mean age was 73.3 ± 1.8 years with an estimated glomerular filtration rate (eGFR) of 16.38 ± 1 ml/min/1.73 m(2). Baseline urine protein:creatinine ratio (PCR) was 77 ± 20 mg/mmol. 46% suffered from diabetes mellitus. Patients were followed for at least 12 months before and after ACEi/ARB were stopped.

RESULTS

12 months after discontinuation of ACEi/ARB eGFR increased significantly to 26.6 ± 2.2 ml/min/ 1.73 m(2) (p = 0.0001). 61.5% of patients had more than a 25% increase in eGFR, whilst 36.5% had an increase exceeding 50%. There was a significant decline in the eGFR slope -0.39 ± 0.07 in the 12 months preceding discontinuation. The negative slope was reversed +0.48 ± 0.1 (p = 0.0001). Mean arterial blood pressure (MAP) increased from 90 ± 1.8 mmHg to 94 ± 1.3 mmHg (p = 0.02), however ≥50% of patients remained within target. Overall proteinuria was not affected (PCR before = 77 ± 20 and after = 121.6 ± 33.6 mg/mmol).

CONCLUSION

Discontinuation of ACEi/ARB has undoubtedly delayed the onset of RRT in the majority of those studied. This observation may justify a rethink of our approach to the inhibition of the RAAS in patients with advanced CKD who are nearing the start of RRT.

Measuring morbidity associated with urinary schistosomiasis: assessing levels of excreted urine albumin and urinary tract pathologies

Background

Urinary schistosomiasis is responsible for a variety of debilitating conditions; foremost perhaps are urinary tract pathologies (UTPs). Although portable ultrasonography can be used to detect UTPs visually, there is still a need for rapid morbidity assessment (henceforth referred to as RaMA) tools that can be deployed in the field during implementation, monitoring and evaluation of control programmes. We therefore aimed to determine associations between excreted urine-albumin, as measured using a HemoCue photometer, and UTPs, as detected by ultrasonography, in children and adults from an urinary schistosomiasis endemic area in Zanzibar.

Methodology/Principal Findings

In a survey of 140 school-children of both sexes (aged 9 to 15 yr) and 47 adult males (≥16 yr) on the island of Unguja, the prevalence of egg-patent urinary schistosomiasis was 36.4% (CI₉₅ 28.5–45.0%) and 46.8% (CI₉₅ 32.1–61.9%) (P = 0.14), and that of UTPs was 39.4% (CI₉₅ 31.0–48.3%) and 64.4% (CI₉₅ 48.8–78.1%) (P = 0.006), respectively. In school-children, raised urine-albumin concentrations (>40 mg/L) were associated, albeit non-significantly, with prevalence of infection (OR = 3.1, P = 0.070), but more specifically and significantly with the prevalence of micro-haematuria (OR = 76.7, P<0.0001). In adults, elevated urine-albumin excretion was associated with UTPs, particularly lesions of the bladder wall (OR = 8.4, P = 0.013). Albuminuria showed promising diagnostic performance, especially in school-aged children with sensitivity of 63.3% and specificity of 83.1% at detecting lower UTPs, i.e. bladder-wall lesions (ultrasonography as ‘gold standard’).

Conclusion/Significance

This study indicates that albuminuria assays could be used as a RaMA tool for monitoring UTP prevalence during control programmes, as well as a tool for selecting those with more chronic bladder-wall lesions without resorting to ultrasonography.

Urinary schistosomiasis is a debilitating disease caused by a parasitic worm that dwells in the blood vessels, particularly those surrounding the human bladder wall. Although not directly associated with high patient mortality, this disease is linked to both short-term morbidity, e.g. visible blood in urine (acute), as well as long-term sequelae, e.g. urinary tract pathologies (chronic). Numerous control programmes based upon chemotherapy have been implemented in sub-Saharan Africa in an attempt to reduce the burden of disease inflicted, particularly in children. Although there are rapid tests to assess the prevalence of acute manifestations of disease (i.e. blood in urine), namely urine-reagent strips, monitoring of chronic manifestations (i.e. urinary tract pathologies) is still rather laborious, time-consuming and requires specialised equipment, e.g. portable ultrasonography, as well as highly trained staff. This study has attempted to evaluate associations between albuminuria (albumin in urine, a new application for the HemoCue photometer) and urinary tract pathologies, and consequently assess this new biochemical marker as a potential rapid proxy of chronic disease sequelae typical in children in areas where urinary schistosomiasis is of public health importance.

Mammalian target of rapamycin inhibition prevents glomerular hypertrophy in a model of renal mass reduction

BACKGROUND

Sirolimus (SRL) is a potent and specific immunosuppressive drug used in organ transplantation, as basic therapy or in combination with calcineurin inhibitors. Although SRL is a nonnephrotoxic drug, many reports have related its use with the development of proteinuria, especially after conversion. Therefore, the aim of this study was to elucidate the interrelation between early and late SRL administration on the development of glomerular hypertrophy and proteinuria in a model of renal mass reduction (RMR).

METHODS

Rats underwent 2/3 cryoablation of the left kidney and subsequent right nephrectomy (n=42) or sham operations (n=29). Two weeks before (early study) or 12 weeks after (late study) surgery, SRL or vehicle was administered three times weekly. Creatinine clearance and proteinuria were determined throughout the study, and a complete histologic analysis was performed at the end of the study.

RESULTS

Treatment with SRL had no effect on creatinine clearance, independently of the administration time. Four weeks after RMR, a significant increase in proteinuria was observed. Proteinuria was stabilized after early and late SRL administration, whereas vehicle-treated animals showed a further increase in proteinuria. Glomerular hypertrophy was strongly associated with proteinuria, and early SRL introduction prevented glomerular enlargement. The histologic analysis showed less structural damage in the two groups of animals treated with SRL than in the control group.

CONCLUSION

Although early SRL introduction blocked glomerular hypertrophy, SRL treatment revealed the potential to halt progression of proteinuria and histologic damage at any time of administration in a model of RMR.

The glomerular filter: an imperfect barrier is required for perfect renal function

PURPOSE OF REVIEW

There is currently a major debate on the mechanisms of albuminuria, and this review appraises recent studies in this area.

RECENT FINDINGS

The traditional view of albuminuria is that it is the result of damage to an essentially impermeable glomerular barrier. However, over the years, critical evidence for this traditional model has been shown to be flawed. An alternative explanation has evolved in which the glomerular filter governs albumin permeability by size selectivity alone. This means that the filter offers a significant barrier to albumin, but it is imperfect - the barrier leaks albumin. The virtue of this leakage is that it endows the filter an in-built anticlogging mechanism. The filtered albumin, if not rescued, would be excreted at nephrotic levels in the urine. There is evidence that proximal tubular cells participate in retrieving most of this filtered albumin to return it back to the blood supply intact. A small amount of the filtered albumin is not retrieved but directed toward lysosomal degradation, and the peptide products are exocytosed into the tubular lumen and excreted.

SUMMARY

In acquired and chemically induced kidney disease, albuminuria is the result of dysfunction in proximal tubular cell processing of albumin rather than alterations in glomerular permeability.

Microalbuminuria in type 1 diabetes is associated with enhanced excretion of the endocytic multiligand receptors megalin and cubilin

OBJECTIVE

Proteinuria is the hallmark of diabetic nephropathy; yet, glomerular histology does not fully explain mechanisms contributing to proteinuria. Our objective was to identify proteins in the urine of individuals with type 1 diabetes and microalbuminuria that might implicate a mechanistic pathway operative in proteinuria.

RESEARCH DESIGN AND METHODS

Using a GeLC/MS platform proteomics approach, we compared the urine proteome from 12 healthy nondiabetic individuals, 12 subjects with type 1 diabetes yet normal urinary albumin excretion rates, and 12 subjects with type 1 diabetes and microalbuminuria (type 1 diabetes + microalbuminuria).

RESULTS

The abundance of megalin and cubilin, two multiligand receptors expressed in kidney proximal tubule cells and involved with the reuptake of filtered albumin and megalin/cubilin ligands, was significantly increased in type 1 diabetes + microalbuminuria urine, compared with both nonalbuminuric groups.

CONCLUSIONS

Aberrant shedding of megalin and cubilin could contribute to albuminuria in diabetes and to deficiency states of important vitamins and hormones.

Receptor-mediated endocytosis in renal proximal tubule

Proteins filtered in renal glomeruli are removed from the tubular fluid by endocytosis in the proximal tubule mediated by the two receptors megalin and cubilin. After endocytic uptake, the proteins are transferred to lysosomes for degradation, while the receptors are returned to the apical cell membrane by receptor recycling in dense apical tubules. In the renal proximal tubule, there is no significant transcellular transport of protein. The reabsorptive process is extremely efficient as evidenced by the virtual protein free urine in humans. The two receptors bind a variety of ligands. The process serves not only to remove the proteins from the ultrafiltrate but also to conserve a variety of essential substances such as vitamins and trace elements carried by plasma proteins. The endocytic apparatus is highly developed in the proximal tubule demonstrating the high capacity of the cells; however, under certain circumstances like diseases affecting the glomeruli, the system is overloaded resulting in proteinuria.

Glomerular permeability to macromolecules in the Necturus kidney

Many aspects of the glomerular filtration of macromolecules remain controversial, including the location of the major filtration barrier, the effects of electrical charge, and the reason the filtration barrier does not clog. We examined these issues in anesthetized Necturus maculosus, using fluorescently labeled probes and a two-photon microscope. With the high resolution of this system and the extraordinary width ( approximately 3.5 mum) of the glomerular basement membrane (GBM) in this salamander, we were able to visualize fluorescent molecules in the GBM in vivo. GBM/plasma concentration ratios for myoglobin, ovalbumin, and serum albumin did not differ from that of inulin, indicating that the GBM does not discriminate among these molecules. The GBM/plasma concentration ratios for fluoresceinated dextran 500 and 2,000 kDa were significantly below that of inulin. Glomerular sieving coefficients (GSCs) for various macromolecules decreased as molecular mass increased, and the GSCs for bovine or human serum albumin were extremely low. The effect of electrical charge on filterability of a macromolecule was also examined. The GSCs for native (anionic) and neutral human serum albumin were not significantly different, nor did GSCs for anionic and neutral dextran 40 kDa differ, indicating that charge has no detectable effect on filterability of these macromolecules. These studies indicate that the main filtration barrier to albumin is the podocyte slit diaphragm. Electron microscopic studies revealed many cell processes within the GBM. Macromolecules that penetrated the GBM were taken up by mesangial cells and endothelial cells, suggesting that these cells help to prevent clogging of the filter.

Adapting mass spectrometry-based platforms for clinical proteomics applications: The capillary electrophoresis coupled mass spectrometry paradigm

Single biomarker detection is common in clinical laboratories due to the currently available method spectrum. For various diseases, however, no specific single biomarker could be identified. A strategy to overcome this diagnostic void is to shift from single analyte detection to multiplexed biomarker profiling. Mass spectrometric methods were employed for biomarker discovery in body fluids. The enormous complexity of biofluidic proteome compartments implies upstream fractionation. For this reason, mass spectrometry (MS) was coupled to two-dimensional gel electrophoresis, liquid chromatography, surface-enhanced laser desorption/ionization, or capillary electrophoresis (CE). Differences in performance and operating characteristics make them differentially suited for routine laboratory applications. Progress in the field of clinical proteomics relies not only on the use of an adequate technological platform, but also on a fast and efficient proteomic workflow including standardized sample preparation, proteomic data processing, statistical validation of biomarker selection, and sample classification. Based on CE-MS analysis, we describe how proteomic technology can be implemented in a clinical laboratory environment. In the last part of this review, we give an overview of CE-MS-based clinical studies and present information on identity and biological significance of the identified peptide biomarkers providing evidence of disease-induced changes in proteolytic processing and posttranslational modification.

Where does albuminuria come from in diabetic kidney disease?

The classic mechanism to explain albumin excretion in diabetes has been permeability defects in the glomerular filter. However, a new concept has emerged that albuminuria can be explained by the two major pathways the proximal tubular cell uses to process filtered albumin. Specifically, albumin permeability through the glomerular filter is only governed by size selectivity. Most of the filtered albumin is retrieved by the proximal tubular cell and returned to the peritubular blood supply. Albuminuria in the nephrotic range would arise from retrieval pathway dysfunction. The small quantities of filtered albumin that are not retrieved undergo obligatory lysosomal degradation before urinary excretion as small peptide fragments. This pathway is sensitive to metabolic factors responsible for hypertrophy and fibrosis, particularly molecules such as angiotensin II and transforming growth factor-beta1, whose production is stimulated by hyperglycemic environments. Dysfunction in this degradation pathway may lead to albuminuria below the nephrotic range.