Mechanism Of Albuminuria Research Articles

Inhibition of MAPK P38 Decreases Hyperglycemia-Induced Nephrin Endocytosis and Protects Against Proteinuria

Diabetic nephropathy is the leading cause of end stage renal disease. In diabetes albuminuria is an early sign of a leaky renal filter and leads to an accelerated loss of renal function. Despite intensive research there is still no specific antiproteinuric therapy available. Thus, deeper understanding of the molecular mechanisms of albuminuria is desirable. MAPK p38 signaling has been shown to be involved in the development of hyperglycemia-induced albuminuria; however, it remains unclear how increased p38 activity leads to loss of albumin in the urine. We recently demonstrated that acute hyperglycemia triggers endocytosis of nephrin, the key molecule of the slit diaphragm, and induces albuminuria. Here, we identify p38 as pivotal regulator of hyperglycemia-induced nephrin endocytosis and proteinuria. Activated p38 phosphorylates the nephrin c-terminus at serine 1146, facilitating the interaction of PKCα with nephrin. PKCα phosphorylates nephrin at threonine residues 1120 and 1125, mediating the binding of β-arrestin2 to nephrin. β-arrestin2 triggers endocytosis of nephrin by coupling it to the endocytotic machinery, leading to a leaky glomerular filter. Pharmacological inhibition of p38 preserves nephrin surface expression and significantly attenuates albuminuria. Our data highlight the pivotal role of p38 in the genesis of diabetic proteinuria and the potential use of p38 inhibitors to slow down progression of kidney disease in patients with proteinuria.

Injury to the endothelial surface layer induces glomerular hyperfiltration rats with early-stage diabetes.

Glomerular endothelial surface layer (ESL) may play a role in the mechanisms of albuminuria in diabetic nephropathy, which lack evidence in vivo. The effects of high glucose on the passage of albumin across the glomerular ESL were analysed in streptozotocin-induced diabetic Sprague-Dawley rats for 4 weeks. Albuminuria and glomerular mesangial matrix were significantly increased in diabetic rats. The passage of albumin across the ESL, as measured by albumin-colloid gold particle density in the glomerular basement membrane (GBM), was increased significantly in diabetic rats. The thickness of the glomerular ESL, examined indirectly by infusing Intralipid into vessels using an electron microscope, was significantly decreased and the GBM exhibited little change in diabetic rats. In summary, the glomerular ESL may play a role in the pathogenesis of albuminuria in rats with early-stage diabetes.

Significance of Urinary Full-Length and Ectodomain Forms of Megalin in Patients With Type 2 Diabetes

OBJECTIVEMegalin, an endocytic receptor in proximal tubule cells, is involved in the mechanisms of albuminuria in diabetic nephropathy (DN). To develop efficient novel biomarkers associated with the pathogenesis of DN, we investigated urinary megalin excretion in type 2 diabetes.RESEARCH DESIGN AND METHODSSandwich enzyme-linked immunosorbent assay systems were established with monoclonal antibodies against the NH2 (amino [A]-megalin assay) and COOH (C-megalin assay) termini of megalin to analyze urinary forms of megalin in 68 patients with type 2 diabetes.RESULTSThe A-megalin assay mainly detected a megalin ectodomain form in the soluble urinary fraction, whereas the C-megalin assay identified a full-length form in both soluble and insoluble fractions. Urinary C-megalin levels were significantly high in patients with normoalbuminuria, were elevated in line with increased albuminuria, and showed a better association with estimated glomerular filtration rate (eGFR) (<60 mL/min/1.73 m2) than did urinary albumin. In contrast, urinary A-megalin levels were increased in patients with normo- and microalbuminuria but not in those with macroalbuminuria. Urinary C-megalin levels were also positively associated with plasma inorganic phosphate and negatively with hemoglobin levels in those showing no features of bleeding and not taking vitamin D analogs, phosphate binders, or erythropoiesis-stimulating agents.CONCLUSIONSUrinary full-length megalin excretion as measured by the C-megalin assay is well associated with reduced eGFR and linked to the severity of DN, phosphate dysregulation, and anemia, whereas urinary excretion of megalin ectodomain as measured by the A-megalin assay may be associated with distinctive mechanisms of earlier DN in type 2 diabetes.

The G Protein 3 Subunit C825T Polymorphism Is Associated With Microalbuminuria in Hypertensive Women and Cardiovascular Disease in Hypertensive Men

The aim of the study was to examine the association between the G protein β3 subunit C825T polymorphism, associated with cardiovascular risk factors like hypertension (HT) and obesity and microalbuminuria (MA), reflecting the endothelial dysfunction in the atherosclerotic process. Second, we wanted to examine the association between the polymorphism and cardiovascular disease (CVD). In the large population-based, cross-sectional Nord-Trøndelag Health Study, HUNT 1995-1997, a total of 5,755 treated hypertensive individuals attended the MA substudy. A randomly selected sample of 1,000 of these was genotyped. A total of 402 men and 540 women were included in the final analyses. MA was measured as albumin/creatinine ratio (ACR) in three urine samples. Logistic regression was used to calculate odds ratios (ORs) for the association between MA or CVD and the genotype. The study demonstrated a positive association between the TT genotype and MA in women (OR 3.2, 95% confidence interval (CI): 1.1-8.7, P = 0.03), but not in men. The association became stronger with increasing number of positive urine samples and with increasing cutoff value in women with TT genotype compared to the CC genotype. Additionally, there was a positive association between TT genotype and CVD in men and postmenopausal women without hormone therapy. Increased MA in TT homozygous women might be explained by other mechanisms of albuminuria or inflammation than the atherosclerotic process. We postulate that the association between CVD and the genotype are mediated through mechanisms other than the classical risk factors and endothelial dysfunction, reflected by MA, which have to be further investigated.

Mechanisms of microalbuminuria

There is currently a major debate on the mechanisms of albuminuria. 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. In conclusion, 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.

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

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

Diabetes and chronic kidney disease: lessons from the Pima Indians

Although diabetic nephropathy is a very rare cause of kidney failure during childhood, the underlying events leading to progressive kidney injury begin during childhood in many patients with type 1 diabetes mellitus (T1DM) and in increasing numbers of children with type 2 diabetes mellitus (T2DM). The Pima Indians of Arizona represent an exceptionally thoroughly studied population suffering from very high rates of T2DM and diabetic nephropathy (T2DN). This population well illustrates the often inexorable progression from glomerular hyperfiltration to microalbuminuria to overt proteinuria and loss of glomerular filtration rate (GFR), paralleled by the accumulation of mesangial matrix and basement membrane, glomerular hypertrophy, loss of podocytes and eventual glomerular sclerosis and interstitial fibrosis. Structural changes quantitatively account for the loss of GFR in T2DN. The mechanism of albuminuria (and its relationship to GFR loss) is much less clear. There is strong functional and structural evidence for defects in glomerular size-selectivity (shunts) due to podocyte pathology, but only beginning at relatively high levels of proteinuria (albumin/creatinine ratios > 3000 mg/g). Podocyte loss accompanies, and may underlie, the loss of glomeruli to sclerosis. At this point, most evidence in humans suggests detachment of intact podocytes from the glomerular basement membrane, rather than apoptosis, as the predominant mechanism of podocyte loss.

Disease-dependent mechanisms of albuminuria

The mechanism of albuminuria is perhaps one of the most complex yet important questions in renal physiology today. Recent studies have directly demonstrated that the normal glomerulus filters substantial amounts of albumin and that charge selectivity plays little or no role in preventing this process. This filtered albumin is then processed by proximal tubular cells by two distinct pathways; dysfunction in either one of these pathways gives rise to discrete forms of albuminuria. Most of the filtered albumin is returned to the peritubular blood supply by a retrieval pathway. 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 degradation 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 and hypertensive environments. Dysfunction in this degradation pathway leads to albuminuria below the nephrotic range. These new insights into albumin filtration and processing argue for a reassessment of the role of podocytes and the slit diaphragm as major direct determinants governing albuminuria, provide information on how glomerular morphology and "tubular" albuminuria may be interrelated, and offer a new rationale for drug development.

Controversies in nephrology: Response to ‘renal albumin handling, facts, and artifacts’

For 40 years indirect measurements of the glomerular sieving coefficient of albumin yielded very low values. The first direct measurement by 2-photon microscopy by Russo et al (Kidney Int (2007) 71, 504-513) gives values 50-times higher. This demonstrated that relatively large quantities of albumin are normally filtered based on size selectivity alone. Most of this albumin is retrieved and returned to the blood supply. These new discoveries represent a paradigm shift in our understanding of albumin processing by the kidney. They also serve to explain several anomalous aspects of previous studies on glomerular filtration and mechanism of albuminuria and support the fact that glomerular charge selectivity is not a major factor controlling glomerular permselectivity.

Mechanisms of albuminuria in the chronic nitric oxide inhibition model.

Chronic nitric oxide (NO) inhibition causes hypertension and renal injury. Concomitant salt overload promotes massive albuminuria. We investigated the mechanisms whereby these treatments impair glomerular permselectivity. Adult male Munich-Wistar rats received either a standard-salt (SS; 0.5% Na) or high-salt (HS; 3.1% Na) diet and either no treatment or the NO inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME). At 30 days, albuminuria was moderate, the density of fixed anionic sites at the glomerular basement membrane (GBM), estimated by cationic ferritin binding, declined by approximately 35%, and the fractional clearance of 70-kDa neutral dextran (phi) rose moderately in rats receiving L-NAME and SS. Rats given L-NAME and HS exhibited massive albuminuria, whereas phi was nearly tripled. Depletion of GBM anionic sites was also seen in these rats. The GBM was thickened in both L-NAME-treated groups. These abnormalities were largely reversed after cessation of treatments. These results indicate that chronic L-NAME treatment promotes reversible albuminuria by impairing both glomerular size and charge selectivity. These effects likely reflect functional rather than structural disruption of the glomerular wall.

Alterations in renal degradation of albumin in early experimental diabetes in the rat: a new factor in the mechanism of albuminuria

1. Albumin is normally excreted as a mixture of intact protein and fragments that are produced during renal passage. The purpose of this study was to investigate the ratio of intact versus degraded forms of excreted albumin to ascertain whether changes in this ratio could account for the apparent increase in albumin excretion seen in diabetes, as measured by standard radioimmunoassay techniques. 2. Four-week male Sprague-Dawley rats with streptozotocin-induced diabetes and age-matched control rats were intravenously injected with [3H]albumin. Urine collected over 2 h was analysed by size exclusion chromatography and radioimmunoassay. A standard radioimmunoassay found a 7-fold increase in albumin excretion rate in diabetic rats, whereas there was only a 2-fold increase in albumin excretion (intact plus fragments). Urine analysed by size exclusion chromatography showed severe degradation for control rats (% monomer=4+/-2%); in diabetic rats there was a significant amount of monomer albumin excreted, along with moderately degraded and heavily degraded albumin (% monomer=17+/-5%). 3. This study has shown that the radioimmunoassay, which specifically detects intact albumin, considerably underestimates the amount of total urinary albumin which consists of intact and degraded material. The increase in albumin excretion rate observed in diabetes as measured by radioimmunoassay is mainly due to a change in the amount of intact albumin excreted and this is specifically due to the inhibition of albumin degradation at a post-glomerular site and not due to the onset of any type of glomerular 'shunt' pathway.

Alterations in renal degradation of albumin in early experimental diabetes in the rat: a new factor in the mechanism of albuminuria

1.Albumin is normally excreted as a mixture of intact protein and fragments that are produced during renal passage. The purpose of this study was to investigate the ratio of intact versus degraded forms of excreted albumin to ascertain whether changes in this ratio could account for the apparent increase in albumin excretion seen in diabetes, as measured by standard radioimmunoassay techniques. 2.Four-week male Sprague–Dawley rats with streptozotocin-induced diabetes and age-matched control rats were intravenously injected with [3H]albumin. Urine collected over 2 h was analysed by size exclusion chromatography and radioimmunoassay. A standard radioimmunoassay found a 7-fold increase in albumin excretion rate in diabetic rats, whereas there was only a 2-fold increase in albumin excretion (intact plus fragments). Urine analysed by size exclusion chromatography showed severe degradation for control rats (% monomer = 4±2%); in diabetic rats there was a significant amount of monomer albumin excreted, along with moderately degraded and heavily degraded albumin (% monomer = 17±5%). 3.This study has shown that the radioimmunoassay, which specifically detects intact albumin, considerably underestimates the amount of total urinary albumin which consists of intact and degraded material. The increase in albumin excretion rate observed in diabetes as measured by radioimmunoassay is mainly due to a change in the amount of intact albumin excreted and this is specifically due to the inhibition of albumin degradation at a post-glomerular site and not due to the onset of any type of glomerular ‘shunt' pathway.

Glomerular structural changes in type 1 (insulin-dependent) diabetes mellitus: causes, consequences, and prevention.

Diabetic nephropathy is caused primarily by advanced glomerulopathy, the renal expression of diabetic microangiopathy. With stereological methods a quantitative description of the structural changes is achieved. The glomerulopathy is characterized by an increase in basement membrane material: thickening of the capillary wall and an increase in mesangial volume relative to glomerular volume, comprising increase in matrix. Among groups of patients conformity between renal function stage and structure exists. The parameters measuring glomerulopathy are normal at the onset of diabetes; patients with normoalbuminuria may show slight basement membrane thickening, or normal parameters; the microalbuminuric group shows a measurable, but moderate glomerulopathy; patients with overt nephropathy have advanced lesions; at this stage heterogeneity among glomeruli makes the estimates weaker. Recent data indicate that the changes in peripheral basement membrane and in mesangial matrix develop in concert and both contribute to the early stage of glomerulopathy in patients with microalbuminuria. As to the consequences of the structural changes the mechanism of albuminuria is not clear. It is suggested that the early glomerulopathy entails other structural modifications, including formation of new vessels which may be the site of leakage. The marked deviations in glomerular filtration rate correspond well with estimates of filtration surface area: in the early hyperfunction state it is increased; in advanced nephropathy it is decreased, due to advanced glomerulopathy in conjunction with glomerular occlusion. The diabetic state is the necessary condition for the glomerulopathy. In relating structural changes to presumed contributing causes no supporting evidence of a relationship with glomerular hyperfunction or hypertrophy was observed. The structural parameters may be useful tools in clinical trials aiming at arresting the development of glomerulopathy, and thereby providing a prevention of diabetic nephropathy.