Role of megalin and cubilin in renal physiology and pathophysiology

Low Density Lipoprotein Receptor-related Protein 2 Expression and Function in Cultured Astrocytes and Microglia

Activation of glial cells, astrocytes and microglia, has been observed in neurodegenerative diseases including Alzheimer's disease (AD). Amyloid β (Aβ), which is aggregated and the aggregation is detected as characteristic pathology in AD brain, is known to be produced by neurons and to activate glial cells. Clearance of Aβ from the brain via active transport system is important to prevent the accumulation and aggregation. Low density lipoprotein receptor-related protein 2 (LRP2/megalin) is an Aβ transporter. However, expression and contribution of LRP2 in astrocytes and microglia remain to be clarified. In the present study, we examined the expression of LRP2 and its roles in cultured astrocytes prepared from rat embryonic brain cortex and mouse microglial cell line BV-2. Both cultured rat astrocytes and BV-2 cells expressed LRP2 mRNA detected by RT-PCR. When lipopolysaccharide (LPS) or all-trans retinoic acid (ATRA) were added to BV-2 cells, LRP2 mRNA expression and uptake of microbeads, Aβ and insulin were increased. On the other hand, LPS decreased LRP2 expression and uptake of Aβ and insulin in cultured astrocytes. Knockdown of LRP2 using siRNA attenuated the LPS- or ATRA-increased uptake of microbeads, Aβ and insulin in BV-2 cells. These results suggest that LRP2 was expressed in both astrocytes and microglia and might be involved in endocytosis activities. Adequate control of LRP2 expression and function in astrocytes and microglia might regulate Aβ and insulin levels in brain and would be a potential target in AD pathology.

β2-microglobulin and colorectal cancer among inpatients: a case-control study

Β2-microglobulin (β2-M) is associated with various malignancies. However, the relationship between β2-M and colorectal cancer (CRC) remains unclear. We explored the association between β2-M and CRC among inpatients who underwent colonoscopy and explored factors that may modify the association. All consecutive inpatients who underwent colonoscopy were enrolled in a tertiary hospital between April 2015 and June 2022. Inpatients with initial CRC or normal colonoscopies were considered eligible as cases or controls, respectively. Baseline characteristics and laboratory indicators of the participants were collected from electronic medical records. Logistic regression analysis, smooth curve fitting, sensitivity analysis, and subgroup analysis were conducted in the present study. After adjusting for baseline clinical characteristics and laboratory parameters, β2-M was positively associated with CRC (odds ratio [OR] 1.32; 95% confidence interval [CI] 1.11-1.58) among inpatients. When the β2-M level was assigned as tertiles, participants in the highest tertile presented with a higher risk of CRC (OR 2.33; 95% CI 1.57-3.48). A positive linear association was observed between β2-M and CRC with smooth curve fitting. In particular, it may be of great importance to monitor β2-M levels for predicting CRC patients.

Renal protein reabsorption impairment related to a myxosporean infection in the grass frog (Rana temporaria L.)

A morphophysiological study of tubular reabsorption and mechanisms of protein endocytosis in the kidney of frogs (Rana temporaria L.) during parasitic infection was carried out. Pseudoplasmodia and spores of myxosporidia, beforehand assigned to the genus Sphaerospora, were detected in Bowman's capsules and in the lumen of individual renal tubules by light and electron microscopy. Remarkable morphological alteration and any signs of pathology in kidney tissue related to this myxosporean infection have not been noted. At the same time, significant changes in protein reabsorption and distribution of molecular markers of endocytosis in the proximal tubule (PT) cells in infected animals were detected by immunofluorescence confocal microscopy. In lysozyme injection experiments, the endocytosed protein and megalin expression in the infected PTs were not revealed. Tubular expression of cubilin and clathrin decreased, but endosomal recycling marker Rab11 increased or remained unchanged. Thus, myxosporean infection resulted in the alterations in lysozyme uptake and expression of the main molecular determinants of endocytosis. The inhibition of receptor-mediated clathrin-dependent protein endocytosis in amphibian kidneys due to myxosporidiosis was shown for the first time. Established impairment of the endocytic process is a clear marker of tubular cell dysfunction that can be used to assess the functioning of amphibian kidneys during adaptation to adverse environmental factors.

Diabetic Proteinuria Revisited: Updated Physiologic Perspectives

Albuminuria, a hallmark of diabetic nephropathy, reflects not only injury and dysfunction of the filtration apparatus, but is also affected by altered glomerular hemodynamics and hyperfiltration, as well as by the inability of renal tubular cells to fully retrieve filtered albumin. Albuminuria further plays a role in the progression of diabetic nephropathy, and the suppression of glomerular albumin leak is a key factor in its prevention. Although microalbuminuria is a classic manifestation of diabetic nephropathy, often progressing to macroalbuminuria or overt proteinuria over time, it does not always precede renal function loss in diabetes. The various components leading to diabetic albuminuria and their associations are herein reviewed, and the physiologic rationale and efficacy of therapeutic interventions that reduce glomerular hyperfiltration and proteinuria are discussed. With these perspectives, we propose that these measures should be initiated early, before microalbuminuria develops, as substantial renal injury may already be present in the absence of proteinuria. We further advocate that the inhibition of the renin–angiotensin axis or of sodium–glucose co-transport likely permits the administration of a normal recommended or even high-protein diet, highly desirable for sarcopenic diabetic patients.

Cubilin-, megalin-, and Dab2-dependent transcription revealed by CRISPR/Cas9 knockout in kidney proximal tubule cells

The multiligand receptors megalin (Lrp2) and cubilin (Cubn) and their endocytic adaptor protein Dab2 (Dab2) play essential roles in maintaining the integrity of the apical endocytic pathway of proximal tubule (PT) cells and have complex and poorly understood roles in the development of chronic kidney disease. Here, we used RNA-sequencing and CRISPR/Cas9 knockout (KO) technology in a well-differentiated cell culture model to identify PT-specific transcriptional changes that are directly consequent to the loss of megalin, cubilin, or Dab2 expression. KO of Lrp2 had the greatest transcriptional effect, and nearly all genes whose expression was affected in Cubn KO and Dab2 KO cells were also changed in Lrp2 KO cells. Pathway analysis and more granular inspection of the altered gene profiles suggested changes in pathways with immunomodulatory functions that might trigger the pathological changes observed in KO mice and patients with Donnai-Barrow syndrome. In addition, differences in transcription patterns between Lrp2 and Dab2 KO cells suggested the possibility that altered spatial signaling by aberrantly localized receptors contributes to transcriptional changes upon the disruption of PT endocytic function. A reduction in transcripts encoding sodium-glucose cotransporter isoform 2 was confirmed in Lrp2 KO mouse kidney lysates by quantitative PCR analysis. Our results highlight the role of megalin as a master regulator and coordinator of ion transport, metabolism, and endocytosis in the PT. Compared with the studies in animal models, this approach provides a means to identify PT-specific transcriptional changes that are directly consequent to the loss of these target genes.NEW & NOTEWORTHY Megalin and cubilin receptors together with their adaptor protein Dab2 represent major components of the endocytic machinery responsible for efficient uptake of filtered proteins by the proximal tubule (PT). Dab2 and megalin expression have been implicated as both positive and negative modulators of kidney disease. We used RNA sequencing to knock out CRISPR/Cas9 cubilin, megalin, and Dab2 in highly differentiated PT cells to identify PT-specific changes that are directly consequent to knockout of each component.

Renal Handling of Albumin-From Early Findings to Current Concepts

Albumin is the main protein of blood plasma, lymph, cerebrospinal and interstitial fluid. The protein participates in a variety of important biological functions, such as maintenance of proper colloidal osmotic pressure, transport of important metabolites and antioxidant action. Synthesis of albumin takes place mainly in the liver, and its catabolism occurs mostly in vascular endothelium of muscle, skin and liver, as well as in the kidney tubular epithelium. Long-lasting investigation in this area has delineated the principal route of its catabolism involving glomerular filtration, tubular endocytic uptake via the multiligand scavenger receptor tandem—megalin and cubilin-amnionless complex, as well as lysosomal degradation to amino acids. However, the research of the last few decades indicates that also additional mechanisms may operate in this process to some extent. Direct uptake of albumin in glomerular podocytes via receptor for crystallizable region of immunoglobulins (neonatal FC receptor) was demonstrated. Additionally, luminal recycling of short peptides into the bloodstream and/or back into tubular lumen or transcytosis of whole molecules was suggested. The article discusses the molecular aspects of these processes and presents the major findings and controversies arising in the light of the research concerning the last decade. Their better characterization is essential for further research into pathophysiology of proteinuric renal failure and development of effective therapeutic strategies.

The effect of aryl hydrocarbon receptor ligands on gentamicin-induced nephrotoxicity in rats

Polycyclic aromatic hydrocarbons (PAHs)/aryl hydrocarbon receptor (AhR) regulate the expression of target genes, including drug transporter genes which harbor xenobiotic response element (XRE) in their promoter regions. Thus, PAHs/AhR could alter the toxicokinetic profile of many nephrotoxic drugs, including aminoglycosides. In the current study, we investigated the expression and localization of AhR and megalin in rat kidney. Furthermore, we investigated whether AhR and its ligands could modulate the expression of megalin and consequently the gentamicin-induced nephrotoxicity (GN) in rats. Both megalin and AhR receptors are expressed in the proximal tubules of the rat kidney. Treatment with AhR agonist benzo(a)pyrene aggravated GN as indicated by a significant increase in serum creatinine, BUN, KIM1, NAGL, CD-86, and urinary albumin/creatinine ratio. On the other hand, treatment with AhR antagonist resveratrol ameliorated GN as manifested by a pronounced decrease in the aforementioned parameters. The effects of AhR ligands on GN were associated with altered expression of megalin receptor.

Lysosome-Rich Enterocytes Mediate Protein Absorption in the Vertebrate Gut

The guts of neonatal mammals and stomachless fish have a limited capacity for luminal protein digestion, which allows oral acquisition of antibodies and antigens. However, how dietary protein is absorbed during critical developmental stages when the gut is still immature is unknown. Here, we show that specialized intestinal cells, which we call lysosome-rich enterocytes (LREs), internalize dietary protein via receptor-mediated and fluid-phase endocytosis for intracellular digestion and trans-cellular transport. In LREs, we identify a conserved endocytic machinery, composed of the scavenger receptor complex Cubilin/Amnionless and Dab2, that is required for protein uptake by LREs and for growth and survival of larval zebrafish. Moreover, impairing LRE function in suckling mice, via conditional deletion of Dab2, leads to stunted growth and severe protein malnutrition reminiscent of kwashiorkor, a devastating human malnutrition syndrome. These findings identify digestive functions and conserved molecular mechanisms in LREs that are crucial for vertebrate growth and survival.

A Review of the Clinical Pharmacokinetics of Polymyxin B

Polymyxin B remains an antibiotic of last resort because of its toxicities. Although newer therapies are becoming available, it is anticipated that resistance to these agents will continue to emerge, and understanding the safest and most efficacious manner to deliver polymyxin B will remain highly important. Recent data have demonstrated that polymyxin B may be less nephrotoxic than colistin. Pharmacokinetically, polymyxin B is primarily eliminated via non-renal pathways, and most do not recommend adjusting the dose for renal impairment. However, some recent studies suggest a weak relationship between polymyxin B clearance and patient creatinine clearance. This review article will describe the clinical pharmacokinetics of polymyxin B and address relevant issues in chemistry and assays available.

Aminoglycoside-Induced Cochleotoxicity: A Review

Aminoglycoside antibiotics are used as prophylaxis, or urgent treatment, for many life-threatening bacterial infections, including tuberculosis, sepsis, respiratory infections in cystic fibrosis, complex urinary tract infections and endocarditis. Although aminoglycosides are clinically-essential antibiotics, the mechanisms underlying their selective toxicity to the kidney and inner ear continue to be unraveled despite more than 70 years of investigation. The following mechanisms each contribute to aminoglycoside-induced toxicity after systemic administration: (1) drug trafficking across endothelial and epithelial barrier layers; (2) sensory cell uptake of these drugs; and (3) disruption of intracellular physiological pathways. Specific factors can increase the risk of drug-induced toxicity, including sustained exposure to higher levels of ambient sound, and selected therapeutic agents such as loop diuretics and glycopeptides. Serious bacterial infections (requiring life-saving aminoglycoside treatment) induce systemic inflammatory responses that also potentiate the degree of ototoxicity and permanent hearing loss. We discuss prospective clinical strategies to protect auditory and vestibular function from aminoglycoside ototoxicity, including reduced cochlear or sensory cell uptake of aminoglycosides, and otoprotection by ameliorating intracellular cytotoxicity.

Design and implementation of a custom next generation sequencing panel for selected vitamin D associated genes

Background

Biologically active vitamin D has an important regulatory role within the genome. It binds the vitamin D receptor (VDR) in order to control the expression of a wide range of genes as well as interacting with the epigenome to modify chromatin and methylation status. Vitamin D deficiency is associated with several human diseases including end-stage renal disease.

Methods

This article describes the design and testing of a custom, targeted next generation sequencing (NGS) panel for selected vitamin D associated genes. Sequencing runs were used to determine the effectiveness of the panel for variant calling, to compare efficiency and data across different sequencers, and to perform representative, proof of principle association analyses. These analyses were underpowered for significance testing. Amplicons were designed in two pools (163 and 166 fragments respectively) and used to sequence two cohorts of renal transplant recipients on the Ion Personal Genome Machine (PGM)™ and Ion S5™ XL desktop sequencers.

Results

Coverage was provided for 43.8 kilobases across seven vitamin D associated genes (CYP24A1, CUBN, VDR, GC, NADSYN1, CYP27B1, CYP2R1) as well as 38 prioritised SNPs. Sequencing runs provided sufficient sequencing quality, data output and validated the effective library preparation and panel design.

Conclusions

This novel, custom-designed, validated panel provides a fast, cost effective, and specific approach for the analysis of vitamin D associated genes in a wide range of patient cohorts. This article does not report results from a controlled health-care intervention.

Electronic supplementary material

The online version of this article (doi:10.1186/s13104-017-2664-z) contains supplementary material, which is available to authorized users.

Role of Renal Drug Exposure in Polymyxin B-Induced Nephrotoxicity

Despite dose-limiting nephrotoxic potentials, polymyxin B has reemerged as the last line of therapy against multidrug-resistant Gram-negative bacterial infections. However, the handling of polymyxin B by the kidneys is still not thoroughly understood. The objectives of this study were to evaluate the impact of renal polymyxin B exposure on nephrotoxicity and to explore the role of megalin in renal drug accumulation. Sprague-Dawley rats (225 to 250 g) were divided into three dosing groups, and polymyxin B was administered (5 mg/kg, 10 mg/kg, and 20 mg/kg) subcutaneously once daily. The onset of nephrotoxicity over 7 days and renal drug concentrations 24 h after the first dose were assessed. The effects of sodium maleate (400 mg/kg intraperitoneally) on megalin homeostasis were evaluated by determining the urinary megalin concentration and electron microscopic study of renal tissue. The serum/renal pharmacokinetics of polymyxin B were assessed in megalin-shedding rats. The onset of nephrotoxicity was correlated with the daily dose of polymyxin B. Renal polymyxin B concentrations were found to be 3.6 ± 0.4 μg/g, 9.9 ± 1.5 μg/g, and 21.7 ± 4.8 μg/g in the 5-mg/kg, 10-mg/kg, and 20-mg/kg dosing groups, respectively. In megalin-shedding rats, the serum pharmacokinetics of polymyxin B remained unchanged, but the renal exposure was attenuated by 40% compared to that of control rats. The onset of polymyxin B-induced nephrotoxicity is correlated with the renal drug exposure. In addition, megalin appears to play a pivotal role in the renal accumulation of polymyxin B, which might contribute to nephrotoxicity.

Neutrophil Gelatinase-Associated Lipocalin as an Early Marker of Contrast-Induced Nephropathy After Elective Invasive Cardiac Procedures

BACKGROUND

Contrast-induced nephropathy (CIN) is an acute kidney injury (AKI) defined as serum creatinine (sCr) increase 48 to 72 hours after contrast administration. Because most subjects undergoing invasive cardiac procedures are discharged within 24 hours, sCr is unsuitable for CIN detection.

HYPOTHESIS

In the present study we tested the hypothesis that neutrophil gelatinase-associated lipocalin (NGAL) is superior compared with sCr and other established nephropathy markers in early CIN diagnosis after elective invasive cardiac procedures.

METHODS

Serum creatinine, urine creatinine, serum cystatin C, urine albumin, urine NGAL (uNGAL), and plasma NGAL were measured at 0, 6, 24, and 48 hours after contrast administration in 100 elective invasive cardiac procedures. Estimated glomerular filtration rate and albumin-to-creatinine ratio were calculated. Changes from baseline were considered statistically significant at P < 0.05 and clinically significant when > the biomarker's reference change value. Participants were divided into those with and without clinically significant uNGAL changes (uNGAL positive and negative for AKI, respectively).

RESULTS

Thirty-three individuals were uNGAL positive for AKI. Serum cystatin C changes were statistically and clinically nonsignificant in both groups. Serum creatinine and plasma NGAL were statistically but not clinically elevated 48 hours postcatheterization in the AKI group. Except for contrast volume (higher in AKI group), groups were comparable at baseline (P not significant) regarding cardiovascular risk factors, coronary heart disease, coronary interventions performed, and renal biomarkers. Baseline uNGAL was significantly correlated to estimated glomerular filtration rate and albumin-to-creatinine ratio.

CONCLUSIONS

Urine NGAL is potentially superior compared with conventional nephropathy markers in early CIN diagnosis after elective invasive cardiac procedures. Definition of clinically significant uNGAL changes with reference change value is probably a valuable supplement to statistically defined significant variations.

Urinalysis in children and adolescents

BACKGROUND

Urinalysis is the most commonly performed biochemical test in infancy and early childhood. The urine sample should be correctly obtained, age-specific aspects should be considered, and age-dependent reference values should be used.

METHOD

This review is based on a selective literature search in electronic databases, textbooks, and guidelines from Germany and abroad on the acquisition of urine samples and the performance of urinalysis in infancy and early childhood.

RESULTS

The timing and mode of acquisition of the urine sample affect the assessment of hematuria, proteinuria, leukocyturia, nitrituria, and the uropathogenic bacterial colony count in the urine culture. Dipstick tests can be used for targeted screening for these features. The test results should be interpreted together with the findings of urine microscopy, the medical history, and the physical examination. Proteinuria should be quantified and differentiated; both of these things can be done either from collected urine or (especially in infants and young children) from a spontaneously voided urine sample, by determination of the protein/creatinine quotient. Orthostatic proteinuria in an adolescent requires no further evaluation or treatment. Hematuria should be characterized as either glomerular or non-glomerular erythrocyturia. Asymptomatic, isolated microhematuria in childhood is not uncommon and often transient; in the absence of a family history, it usually does not require an extensive work-up. Proteinuria combined with hematuria should arouse the suspicion of glomerulonephritis.

CONCLUSION

Urinalysis in infancy and early childhood is a simple and informative diagnostic test as long as the urine sample has been obtained properly and the results are interpreted appropriately for this age group.

Proteinuria Increases Plasma Phosphate by Altering Its Tubular Handling

Proteinuria and hyperphosphatemia are cardiovascular risk factors independent of GFR. We hypothesized that proteinuria induces relative phosphate retention via increased proximal tubule phosphate reabsorption. To test the clinical relevance of this hypothesis, we studied phosphate handling in nephrotic children and patients with CKD. Plasma fibroblast growth factor 23 (FGF-23) concentration, plasma phosphate concentration, and tubular reabsorption of phosphate increased during the proteinuric phase compared with the remission phase in nephrotic children. Cross-sectional analysis of a cohort of 1738 patients with CKD showed that albuminuria≥300 mg/24 hours is predictive of higher phosphate levels, independent of GFR and other confounding factors. Albuminuric patients also displayed higher plasma FGF-23 and parathyroid hormone levels. To understand the molecular mechanisms underlying these observations, we induced glomerular proteinuria in two animal models. Rats with puromycin-aminonucleoside-induced nephrotic proteinuria displayed higher renal protein expression of the sodium-phosphate co-transporter NaPi-IIa, lower renal Klotho protein expression, and decreased phosphorylation of FGF receptor substrate 2α, a major FGF-23 receptor substrate. These findings were confirmed in transgenic mice that develop nephrotic-range proteinuria resulting from podocyte depletion. In vitro, albumin did not directly alter phosphate uptake in cultured proximal tubule OK cells. In conclusion, we show that proteinuria increases plasma phosphate concentration independent of GFR. This effect relies on increased proximal tubule NaPi-IIa expression secondary to decreased FGF-23 biologic activity. Proteinuria induces elevation of both plasma phosphate and FGF-23 concentrations, potentially contributing to cardiovascular disease.

Grape seed proanthocyanidins ameliorates cadmium-induced renal injury and oxidative stress in experimental rats through the up-regulation of nuclear related factor 2 and antioxidant responsive elements

Cadmium (Cd) preferentially accumulates in the kidney, the major target for Cd-related toxicity. Cd-induced reactive oxygen species (ROS) have been considered crucial mediators for renal injury. The biologically significant ionic form of cadmium (Cd+) binds to many bio-molecules, and these interactions underlie the toxicity mechanisms of Cd. The present study was hypothesized to explore the protective effect of grape seed proanthocyanidins (GSP) on Cd-induced renal toxicity and to elucidate the potential mechanism. Male Wistar rats were treated with Cd as cadmium chloride (CdCl2, 5 mg·kg−1 bw, orally) and orally pre-administered with GSP (100 mg·kg−1 bw) 90 min before Cd intoxication for 4 weeks to evaluate renal damage of Cd and antioxidant potential of GSP. Serum renal function parameters (blood urea nitrogen and creatinine) levels in serum and urine, renal oxidative stress (lipid peroxidation, protein carbonylation, enzymatic, and non-enzymatic antioxidants), inflammatory (NF-κB p65, NO, TNF-α, IL-6), apoptotic (caspase-3, caspase-9, Bax, Bcl-2), membrane bound ATPases, and Nrf2 (HO-1, keap1, γ-GCS, and μ-GST) markers were evaluated in Cd-treated rats. Pretreatment with GSP revealed a significant improvement in renal oxidative stress markers in kidneys of Cd-treated rats. In addition, GSP treatment decreases the amount of iNOS, NF-κB, TNF-α, caspase-3, and Bax and increases the levels Bcl-2 protein expression. Similarly, mRNA and protein analyses substantiated that GSP treatment notably normalizes the renal expression of Nrf2/Keap1 and its downstream regulatory proteins in the Cd-treated rats. Histopathological and ultra-structural observations also demonstrated that GSP effectively protects the kidney from Cd-induced oxidative damage. These findings suggest that GSP ameliorates renal dysfunction and oxidative stress through the activation of Nrf2 pathway in Cd-intoxicated rats.

β2-Microglobulin-mediated signaling as a target for cancer therapy

β2-microglobulin (β2-m) has become the focus of intense scrutiny since the discovery of its undesirable roles promoting osteomimicry and cancer progression. β2-m is a well-known housekeeping protein that forms complexes with the heavy chain of major histocompatibility complex class I molecules, which are heterodimeric cell surface proteins that present antigenic peptides to cytotoxic T cells. On recognition of foreign peptide antigens on cell surfaces, T cells actively bind and lyse antigen-presenting cancer cells. In addition to its roles in tumor immunity, β2-m has two different functions in cancer cells, either tumor promoting or tumor suppressing, in cancer cell context-dependent manner. Our studies have demonstrated that β2-m is involved extensively in the functional regulation of growth, survival, apoptosis, and even metastasis of cancer cells. We found that β2-m is a soluble growth factor and a pleiotropic signaling molecule which interacts with its receptor, hemochromatosis protein, to modulate epithelial-to-mesenchymal transition (EMT) through iron-responsive pathways. Specific antibodies against β2-m have remarkable tumoricidal activity in cancer, through β2-m action on iron flux, alterations of intracellular reactive oxygen species, DNA damage and repair enzyme activities, β-catenin activation and cadherin switching, and tumor responsiveness to hypoxia. These novel functions of β2-m and β2-m signaling may be common to several solid tumors including human lung, breast, renal, and prostate cancers. Our experimental results could lead to the development of a novel class of antibody-based pharmaceutical agents for cancer growth control. In this review, we briefly summarize the recent data regarding β2-m as a promising new cancer therapeutic target and discuss antagonizing this therapeutic target with antibody therapy for the treatment of localized and disseminated cancers.

LRP-1 and LRP-2 receptors function in the membrane neuron. Trafficking mechanisms and proteolytic processing in Alzheimer's disease

Low density lipoprotein receptor-related protein (LRP) belongs to the low-density lipoprotein receptor family, generally recognized as cell surface endocytic receptors, which bind and internalize extracellular ligands for degradation in lysosomes. Neurons require cholesterol to function and keep the membrane rafts stable. Cholesterol uptake into the neuron is carried out by ApoE via LRPs receptors on the cell surface. In neurons the most important are LRP-1 and LRP-2, even it is thought that a causal factor in Alzheimer's disease (AD) is the malfunction of this process which cause impairment intracellular signaling as well as storage and/or release of nutrients and toxic compounds. Both receptors are multifunctional cell surface receptors that are widely expressed in several tissues including neurons and astrocytes. LRPs are constituted by an intracellular (ICD) and extracellular domain (ECD). Through its ECD, LRPs bind at least 40 different ligands ranging from lipoprotein and protease inhibitor complex to growth factors and extracellular matrix proteins. These receptors has also been shown to interact with scaffolding and signaling proteins via its ICD in a phosphorylation-dependent manner and to function as a co-receptor partnering with other cell surface or integral membrane proteins. Thus, LRPs are implicated in two major physiological processes: endocytosis and regulation of signaling pathways, which are both involved in diverse biological roles including lipid metabolism, cell growth processes, degradation of proteases, and tissue invasion. Interestingly, LRPs were also localized in neurons in different stages, suggesting that both receptors could be implicated in signal transduction during embryonic development, neuronal outgrowth or in the pathogenesis of AD.

Albuminuria increases cystatin C excretion: implications for urinary biomarkers

BACKGROUND

Low-molecular weight (LMW) proteins, including albumin and novel urinary biomarkers of acute kidney injury (AKI) such as cystatin C and neutrophil gelatinase-associated lipocalin (NGAL), are normally absorbed from the glomerular filtrate by receptor-mediated transport. We evaluated the effect of albuminuria on urinary excretion of novel biomarkers.

METHODS

Sprague-Dawley rats given four injections over 2 days of 5 mg/g body wt/day bovine serum albumin (BSA) in saline were compared with controls given saline alone. Urinary cystatin C, albumin and protein excretion rates were compared prior to treatment (Day -1), after treatment (Day 2) and 4 days later (Day 6). A preliminary assessment of the clinical effect of proteinuria on the filtered urinary biomarkers cystatin C and NGAL was made by comparison with the effect on urinary interleukin-18 (IL-18) that is not absorbed from the glomerular filtrate, in a cohort of intensive care unit patients.

RESULTS

BSA induced transient increases in albuminuria, proteinuria and cystatinuria (P < 0.01, P < 0.001 and P < 0.001, respectively). Beyond a threshold 6-fold increase in albuminuria, cystatin C absorption was reduced by competitive inhibition. The excretion rates of all analytes returned to preinjection levels by Day 6. Clinical proteinuria was associated with increasing cystatin C and NGAL concentrations (n = 90, P < 0.0001) but not IL-18 (P = 0.12).

CONCLUSIONS

Proteinuria may increase the threshold for detection of AKI by increasing the excretion of LMW protein biomarkers.

The role of cubilin gene polymorphisms and their influence on 25(OH)D3 and 1,25(OH)2D3 plasma levels in type 1 diabetes patients

BACKGROUND

Megalin and cubilin bind and internalize the 25(OH)D3-DBP complex in the kidney. Once the complex is internalized, 25(OH)D3 is released and activated to 1,25(OH)2D3 the ligand for the vitamin D receptor (VDR). Supporting the important role of cubilin in this process recent findings showed that cubilin deficiency results in decrease of 25(OH)D3 and 1,25(OH)2D3 plasma levels.

METHODS

Two hundred T1D patients and healthy controls (n=200) were genotyped for five polymorphisms (rs3740168, rs3740165, rs1801233, rs1801229 and rs2796835) within the cubilin gene. The polymorphisms were analyzed by RFLP or real time PCR. Statistic analyses were performed by using allele-wise and genotype-wise chi2 testing by using BiAS software. A p-value<0.05 was considered as significant.

RESULTS

We found that the genotype "AA" of the rs3740165 was more frequent in T1D patients compared to healthy controls (26.7% vs. 5.1%, p=4x10(-7)). Nevertheless no association between the rs3740165 polymorphism and the 25(OH)D3 or 1,25(OH)2D3 plasma levels was found. No association with the other studied polymorphisms was observed.

CONCLUSION

Thus our findings reveal a novel association of the cubilin rs3740165 polymorphism with type 1 diabetes. Nevertheless how exactly this polymorphism could increase the risk to develop type 1 diabetes is subject for further investigations.

The effect of progressive glomerular disease on megalin-mediated endocytosis in the kidney

BACKGROUND

A well-characterized dog model of the X-linked collagen disease Alport syndrome (XLAS) was used to study the effect of progressive glomerular disease on megalin-mediated endocytosis. In XLAS, altered structure and function of the glomerular basement membrane induces a progressive proteinuric nephropathy.

METHODS

The investigation was performed in male XLAS dogs and age-matched normal male littermates. The urine profile and megalin-mediated endocytosis in the proximal tubule of six healthy and six XLAS dogs were examined at 2, 4, 6, 8 and 10 months of age using SDS-PAGE, immunoblotting and immunohistochemistry.

RESULTS

Gradually increasing urinary excretion of proteins over time and a reduced content of the same proteins in proximal tubule cells were found. Besides the glomerular component of the proteinuria, a significant tubular component was seen, which is due to a progressive change in the uptake of low-molecular-weight (LMW) ligands by megalin. Furthermore, the protein overload present in the lumen of the proximal tubule exceeds the reabsorption capacity of megalin and the co-receptor cubilin and results in a combined low- and high-molecular-weight (HMW) proteinuria. Also, a shift in the distribution of lysosomes was seen in the XLAS dogs suggesting changes in the lysosomal degradation pattern in response to the altered endocytosis.

CONCLUSIONS

The present study shows that the increased glomerular permeability and the subsequently altered megalin-mediated and megalin-dependent cubilin-mediated endocytosis lead to a partial LMW proteinuria and partial HMW proteinuria.

Nebivolol attenuates maladaptive proximal tubule remodeling in transgenic rats

BACKGROUND/AIMS

The impact of nebivolol therapy on the renal proximal tubular cell (PTC) structure and function was investigated in a transgenic (TG) rodent model of hypertension and the cardiometabolic syndrome. The TG Ren2 rat develops nephropathy with proteinuria, increased renal angiotensin II levels and oxidative stress, and PTC remodeling. Nebivolol, a beta(1)-antagonist, has recently been shown to reduce albuminuria, in part, through reductions in renal oxidative stress. Accordingly, we hypothesized that nebivolol therapy would attenuate PTC damage and tubulointerstitial fibrosis.

METHODS

Young Ren2 (R2-N) and SD (SD-N) rats were treated with nebivolol (10 mg/kg/day) or vehicle (R2-C; SD-C) for 3 weeks. PTC structure and function were tested using transmission electron microscopy and functional measurements.

RESULTS

Nebivolol treatment decreased urinary N-acetyl-beta-D-glucosaminidase, tubulointerstitial ultrastructural remodeling and fibrosis, NADPH oxidase activity, 3-nitrotyrosine levels, and increased megalin and lysosomal-associated membrane protein-2 immunostaining in PTCs. Ultrastructural abnormalities that were improved with therapy included altered canalicular structure, reduced endosomes/lysosomes and PTC vacuoles, basement membrane thickening, and mitochondrial remodeling/fragmentation.

CONCLUSION

These observations support the notion that nebivolol may improve PTC reabsorption of albumin and other glomerular filtered small molecular weight proteins in association with the attenuation of oxidative stress, tubulointerstitial injury and fibrosis in this rat model of metabolic kidney disease.

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.

Regulation of phosphate transport in proximal tubules

Homeostasis of inorganic phosphate (P(i)) is primarily an affair of the kidneys. Reabsorption of the bulk of filtered P(i) occurs along the renal proximal tubule and is initiated by apically localized Na(+)-dependent P(i) cotransporters. Tubular P(i) reabsorption and therefore renal excretion of P(i) is controlled by a number of hormones, including phosphatonins, and metabolic factors. In most cases, regulation of P(i) reabsorption is achieved by changing the apical abundance of Na(+)/Pi cotransporters. The regulatory mechanisms involve various signaling pathways and a number of proteins that interact with Na(+)/P(i) cotransporters.

Silencing megalin and cubilin genes inhibits myeloma light chain endocytosis and ameliorates toxicity in human renal proximal tubule epithelial cells

Using target-specific short interfering (si) RNAs, we silenced the tandem endocytic receptors megalin and cubilin genes in cultured human renal proximal tubule epithelial cells. Transfection by siRNA resulted in up to 90% suppression of both megalin and cubilin protein and mRNA expression. In HK-2 cells exposed to kappa-light chain for up to 24 h, light chain endocytosis was reduced in either megalin- or cubilin-silenced cells markedly but incompletely. Simultaneous silencing of both the cubilin and megalin genes, however, resulted in near-complete inhibition of light chain endocytosis, as determined by measuring kappa-light chain protein concentration in cell cytoplasm and by flow cytometry using FITC-labeled kappa-light chain. In these cells, light chain-induced cytokine responses (interleukin-6 and monocyte chemoattractant protein-1) and epithelial-to-mesenchymal transition as well as the associated cellular and morphological alterations were also markedly suppressed. The results demonstrate that light chain endocytosis is predominantly mediated by the megalin-cubilin tandem endocytic receptor and identify endocytosis as a key step in light chain cytotoxicity. Blocking light chain endocytosis prevents its nephrotoxic effects on human kidney proximal tubule cells.