The endocytosis receptor megalin: From bench to bedside

Protein handling in kidney tubules

The kidney proximal tubule reabsorbs and degrades filtered plasma proteins to reclaim valuable nutrients and maintain body homeostasis. Defects in this process result in proteinuria, one of the most frequently used biomarkers of kidney disease. Filtered proteins enter proximal tubules via receptor-mediated endocytosis and are processed within a highly developed apical endo-lysosomal system (ELS). Proteinuria is a strong risk factor for chronic kidney disease progression and genetic disorders of the ELS cause hereditary kidney diseases, so deepening understanding of how the proximal tubule handles proteins is crucial for translational nephrology. Moreover, the ELS is both an entry point for nephrotoxins that induce tubular damage and a target for novel therapies to prevent it. Cutting-edge research techniques, such as functional intravital imaging and computational modelling, are shedding light on spatial and integrative aspects of renal tubular protein processing in vivo, how these are altered under pathological conditions and the consequences for other tubular functions. These insights have potentially important implications for understanding the origins of systemic complications arising in proteinuric states, and might lead to the development of new ways of monitoring and treating kidney diseases.

Protamine protects against vancomycin-induced kidney injury

Vancomycin causes kidney injury by accumulating in the proximal tubule, likely mediated by megalin uptake. Protamine is a putative megalin inhibitor that shares binding sites with heparin and is approved for the treatment of heparin overdose. We employed a well-characterized Sprague-Dawley rat model to assess kidney injury and function in animals that received vancomycin, protamine alone, or vancomycin plus protamine over 5 days. Urinary KIM-1 was used as the primary measure for kidney injury, while plasma iohexol clearance was calculated to assess kidney function. Animals had samples drawn pre-treatment in order to serve as their own controls. Additionally, since protamine is not a known nephrotoxin, the protamine group also served as a control. Cellular inhibition studies were performed to assess the ability of protamine to inhibit organic anion transporter (OAT1 and OAT3) and organic cation transporter-2 (OCT2). Rats that received vancomycin alone had significantly increased urinary KIM-1 on day 2 (24.9 ng/24 h, 95% CI 1.87-48.0) compared to the protamine alone group. By day 4, animals that received protamine with their vancomycin had KIM-1 amounts that were elevated compared to protamine alone as a base comparison (KIM-1 29.0 ng/24 h, 95% CI 5.0-53.0). No statistically observed differences were identified for iohexol clearance changes between drug groups or when comparing clearance change from baseline (P > 0.05). No substantial inhibition of OAT1, OAT3, or OCT2 was observed with protamine. IC50 values for protamine were 0.1 mM for OAT1 and OAT3 and 0.043 mM for OCT2. Protamine, when added to vancomycin therapy, delays vancomycin-induced kidney injury as defined by urinary KIM-1 in the rat model by 1-3 days. Protamine putatively acts through the blockade of megalin and does not appear to have significant inhibition on OAT1, OAT3, or OCT2. Since protamine is an approved FDA medication, it has clinical potential as a therapeutic to reduce vancomycin-related kidney injury; however, greater utility may be found by pursuing compounds with fewer adverse event liabilities.

Megalin: A Sidekick or Nemesis of the Kidney?

Megalin is an endocytic receptor in the proximal tubules that reabsorbs filtered proteins in the kidneys. Recycling of megalin after endocytosis and its expression on the apical plasma membrane of the proximal tubule are critical for its function. The expression of megalin in the kidney undergoes dynamic changes under physiologic and pathophysiologic conditions. Receptors and various effector signaling components regulate megalin expression and, potentially, function. Genetic manipulation and rare mutations in megalin suggest that a lack of or deficiency in megalin expression/function promotes tubular proteinuria and albuminuria. However, the role of megalin in kidney diseases associated with obesity, diabetes, hypertension, and nephrotoxicity remains unclear. To address these questions, animal and human studies have indicated megalin as a protective, injurious, and potentially urinary marker of nephropathy. This article reviews the literature on the regulation of megalin expression and the role of megalin in the pathophysiology of the kidney under experimental and clinical conditions. Moreover, this review articulates the need for studies that can clarify whether megalin can serve as a therapeutic target, in one way or the other, to treat kidney disease.

Endocytosis mediated by megalin and cubilin is involved in enamel development

BACKGROUND

Endocytosis of enamel matrix proteins (EMPs) by ameloblasts is a key process in the mineralization of enamel during the maturation stage of amelogenesis. However, the relevant receptor mediating endocytosis of EMPs is still unclear. The aim of this study was to explore potential endocytic receptors involved in this process.

RESULTS

Two endocytic receptors, megalin, and cubilin, were found to be distributed in ameloblasts of mouse incisors and molars during the secretory and maturation stages. Megalin was located at the distal end of ameloblasts during the maturation stage when proteolysis and recycling were the most active. Megalin and cubilin were also expressed in an ameloblast-lineage cell (ALC) line. The immunoelectron microscopy results showed that megalin was positively labeled on the vesicle structures of ALC, where endocytosis happened. Immunofluorescence showed that megalin and cubilin were colocalized with amelogenin, and the absorption of amelogenin was significantly reduced when megalin and cubilin were inhibited by their inhibitor, receptor-associated protein (RAP). Knockdown of megalin and cubilin with siRNA also reduced the ability of ALC to absorb amelogenin.

CONCLUSIONS

The results of this study suggest that megalin and cubilin are involved in the absorption process of ameloblasts during amelogenesis.

Lipophorin receptor knockdown reduces hatchability of kissing bug Rhodnius prolixus eggs

Lipophorin is the primary lipoprotein present in the hemolymph of insects, responsible for the lipids' transport between organs. It interacts with specific sites on cell membranes in an essential process for transferring lipids. The lipophorin receptor is the protein responsible for the interaction between lipophorin and cell membranes. In the kissing bug Rhodnius prolixus, much information on the interaction of lipophorin with organs is available. However, molecular data on the lipophorin receptor and its functions is still needed. Here, we explored lipophorin receptor gene expression and functions using a functional genomics approach. The R. prolixus genome encodes seven genes from the low-density lipoprotein receptor family, including a single ortholog of the lipophorin receptor. All organs analyzed (anterior and posterior midguts, fat body, ovaries, and flight muscle) expressed this gene. In the fat body, blood-feeding strongly reduced lipophorin receptor gene expression. Lipophorin receptor knockdown by RNA interference delayed egg laying and reduced the triacylglycerol in laid eggs without altering lipid stores in the fat body or lipid levels in the hemolymph. In the ovaries, lipophorin receptor knockdown reduces the expression of acetyl-CoA carboxylase and a fatty acid synthase while altered the gene expression profile in the fat body, causing an increase in the expression of carnitine palmitoyltransferase 1 and a reduction in Brummer lipase and vitellogenin 2. RNA interference treatment reduced the hatching of the eggs, causing the collapse and darkening of the laid eggs, in addition to the hatching of deformed first-stage nymphs. Furthermore, the structure of the chorion showed distortions in patterns and cracks and reduced hydrocarbon levels. These results show that the lipophorin receptor alone is not essential for lipid physiology in R. prolixus. However, this protein plays a fundamental role in the viability of eggs and, consequently, in insect reproduction.

Development of sandwich enzyme-linked immunosorbent assays quantifying mouse urinary megalin, a novel proximal tubular biomarker

Megalin, a type I transmembrane protein, serves as a multi-ligand endocytic receptor in the apical membrane of proximal tubules. Its ectodomain and full-length forms are excreted into human urine, with the former being more abundant. We previously developed two types of sandwich enzyme-linked immunosorbent assays (ELISAs) utilizing monoclonal antibodies that target the amino-terminal ligand-binding domain-I and the carboxyl-terminal cytoplasmic region of human megalin, respectively. The former, termed "A-megalin" ELISA, primarily identifies ectodomains of megalin, whereas the latter, "C-megalin" ELISA, specifically recognizes full-length megalin originating from urinary extracellular vesicles. This study developed novel sandwich ELISAs to assess mouse urinary A-megalin and C-megalin, thereby facilitating studies involving these biomarkers in mouse disease models. Immunoblotting and immunohistochemistry of monoclonal antibodies against human megalin were performed to assess their compatibility with mouse megalin in novel sandwich ELISAs, which were constructed and validated using human assay protocols. Immunoblot analysis of megalin in urinary extracellular vesicles and supernatant was performed to investigate the ratio of ectodomain to full-length forms in mouse urine. Stable measurements having a precision and accuracy within 15 % were achieved in the measurement of quality control samples. A-megalin and C-megalin were detectable in the urine of C57BL/6 mice, whereas most urine samples from kidney-specific conditional megalin-knockout mice were below detection limits. Ectodomain forms of megalin were at least approximately 70 times more abundant than the full-length form, even in mouse urine. In conclusion, we successfully developed sandwich ELISAs for assessing mouse urinary A-megalin and C-megalin to evaluate primarily ectodomain and full-length forms of megalin, respectively.

Combined nephrotoxicity of Polymyxins and Vancomycin: a study on adverse event reporting for monotherapy versus combinations using the FDA adverse event reporting system (FAERS)

BACKGROUND

Multidrug-resistant (MDR) infections pose a global public health crisis with significant mortality and economic burdens. Combination of polymyxins and vancomycin has shown effectiveness against MDR infections. However, their combined nephrotoxicity complicates clinical use. Given these concerns, we conducted a pharmacovigilance analysis using the FDA Adverse Event Reporting System (FAERS) to assess the nephrotoxicity of combinations of polymyxins and vancomycin compared to monotherapy.

RESEARCH DESIGN AND METHODS

In this retrospective study, data from FAERS reports (2012 Q4 to 2023 Q2) were deduplicated and analyzed for adverse events (AEs) related to vancomycin, polymyxin B, and colistin. Disproportionality analyses were performed to evaluate the association between drugs and nephrotoxicity.

RESULTS

A total of 9,796,784 adverse event reports, including 73,009 reports associated with nephrotoxicity, were included. All three drugs showed significant associations with nephrotoxicity. In combination therapy, polymyxin B-vancomycin exhibited a stronger association with nephrotoxicity compared to monotherapy, whereas colistin-vancomycin demonstrated a lower association with nephrotoxicity than colistin monotherapy.

CONCLUSIONS

This study found that combining vancomycin with colistin alleviated colistin-induced nephrotoxicity, while combining vancomycin with polymyxin B worsened polymyxin B-induced nephrotoxicity.

Podocyte-specific KLF6 primes proximal tubule CaMK1D signaling to attenuate diabetic kidney disease

Diabetic kidney disease (DKD) is the main cause of chronic kidney disease worldwide. While injury to the podocytes, visceral epithelial cells that comprise the glomerular filtration barrier, drives albuminuria, proximal tubule (PT) dysfunction is the critical mediator of DKD progression. Here, we report that the podocyte-specific induction of human KLF6, a zinc-finger binding transcription factor, attenuates podocyte loss, PT dysfunction, and eventual interstitial fibrosis in a male murine model of DKD. Utilizing combination of snRNA-seq, snATAC-seq, and tandem mass spectrometry, we demonstrate that podocyte-specific KLF6 triggers the release of secretory ApoJ to activate calcium/calmodulin dependent protein kinase 1D (CaMK1D) signaling in neighboring PT cells. CaMK1D is enriched in the first segment of the PT, proximal to the podocytes, and is critical to attenuating mitochondrial fission and restoring mitochondrial function under diabetic conditions. Targeting podocyte-PT signaling by enhancing ApoJ-CaMK1D might be a key therapeutic strategy in attenuating the progression of DKD.

Self-Assembling P38 Peptide Inhibitor Nanoparticles Ameliorate the Transition from Acute to Chronic Kidney Disease by Suppressing Ferroptosis

Accumulating evidence highlights p38 as a crucial factor highly activated during the process of acute kidney injury (AKI), but the application of p38 inhibitor in AKI is quite limited due to the low efficiency and poor kidney-targeting ability. Herein, a novel self-assembling peptide nanoparticle with specific p38-inhibiting activity is constructed, which linked mitogen-activated protein kinase kinase 3b (MKK3b), the functional domain of p38, with the cell-penetrating TAT sequence, ultimately self-assembling into TAT-MKK3b nanoparticles (TMNPs) through tyrosinase oxidation. Subsequent in vitro and in vivo studies demonstrated that TMNPs preferably accumulated in the renal tubular epithelial cells (RTECs) through forming protein coronas by binding to albumin, and strongly improved the reduced renal function of ischemia-reperfusion injury (IRI)-induced AKI and its transition to chronic kidney disease (CKD). Mechanically, TMNPs inhibited ferroptosis via its solute carrier family 7 member 11 (SLC7A11)/glutathione peroxidase 4 (GPX4) axis-inducing capacity and synergistic potent antioxidant property in AKI. The findings indicated that the multifunctional TMNPs exhibited renal targeting, ROS-scavenging, and ferroptosis-mitigating capabilities, which may serve as a promising therapeutic agent for the treatment of AKI and its progression to CKD.

Immunoexpression Patterns of Megalin, Cubilin, Caveolin-1, Gipc1 and Dab2IP in the Embryonic and Postnatal Development of the Kidneys in Yotari (Dab1-/-) Mice

Our study examines the immunoexpression patterns of Megalin, Cubilin, Caveolin-1, Gipc1 and Dab2IP in the embryonic development (E) and postnatal (P) mouse kidney, with a focus on differentiating patterns between wild-type (wt) and yotari, Dab1-/- (yot) mice. Immunofluorescence revealed raised immunoexpression of receptors Megalin and Cubilin at the ampulla/collecting ducts and convoluted tubules across all developmental stages, with the most prominent immunoexpression observed in the convoluted tubules and the parietal epithelium of the Bowman's capsule. Quantitative analysis showed a higher percentage of Megalin and Cubilin in wt compared to yot mice at E13.5. Co-expression of Megalin and Cubilin was observed at the apical membrane of convoluted tubules and the parietal layer of the Bowman's capsule. The staining intensity of Megalin varied across developmental stages, with the strongest reactivity observed at the ampulla and collecting ducts at embryonic day (E) 13.5 in wt mice. In contrast, Caveolin-1 exhibited high immunoexpression in the metanephric mesenchyme, blood vessels, and the border area between the metanephric mesenchyme and renal vesicle, with a decrease in immunoexpression as development progressed. Gipc1 showed diffuse cytoplasmic staining in metanephric mesenchyme, convoluted tubules and collecting ducts, with significant differences in immunoexpression between wild-type and yot mice at both investigated embryonic time points. Dab2IP immunofluorescent staining was most prominent in renal vesicle/glomeruli and ampulla/collecting ducts at E13.5, with mild staining intensity observed in the distal convoluted tubules postnatally. Our findings elucidate distinct immunoexpression of patterns and potential parts of these proteins in the development and function of the kidney, highlighting the importance of further investigation into their regulatory mechanisms.

The multifaceted links between hearing loss and chronic kidney disease

Hearing loss affects nearly 1.6 billion people and is the third-leading cause of disability worldwide. Chronic kidney disease (CKD) is also a common condition that is associated with adverse clinical outcomes and high health-care costs. From a developmental perspective, the structures responsible for hearing have a common morphogenetic origin with the kidney, and genetic abnormalities that cause familial forms of hearing loss can also lead to kidney disease. On a cellular level, normal kidney and cochlea function both depend on cilial activities at the apical surface, and kidney tubular cells and sensory epithelial cells of the inner ear use similar transport mechanisms to modify luminal fluid. The two organs also share the same collagen IV basement membrane network. Thus, strong developmental and physiological links exist between hearing and kidney function. These theoretical considerations are supported by epidemiological data demonstrating that CKD is associated with a graded and independent excess risk of sensorineural hearing loss. In addition to developmental and physiological links between kidney and cochlear function, hearing loss in patients with CKD may be driven by specific medications or treatments, including haemodialysis. The associations between these two common conditions are not commonly appreciated, yet have important implications for research and clinical practice.

Radiolabeled 15-mer peptide internalization is mediated by megalin (LRP2 receptor) in a CRISPR/Cas9-based LRP2 knockout human kidney cell model

BACKGROUND

Megalin (LRP2 receptor) mediates the endocytosis of radiolabeled peptides into proximal tubular kidney cells, which may cause nephrotoxicity due to the accumulation of a radioactive tracer. The study aimed to develop a cellular model of human kidney HK2 cells with LRP2 knockout (KO) using CRISPR/Cas9 technique. This model was employed for the determination of the megalin-mediated accumulation of 68Ga- and 99mTc-labeled 15-mer peptide developed to target the vascular endothelial growth factor (VEGF) receptor in oncology radiodiagnostics.

RESULTS

The gene editing in the LRP2 KO model was verified by testing two well-known megalin ligands when higher viability of KO cells was observed after gentamicin treatment at cytotoxic concentrations and lower FITC-albumin internalization by the KO cells was detected in accumulation studies. Fluorescent-activated cell sorting was used to separate genetically modified LRP2 KO cell subpopulations. Moreover, flow cytometry with a specific antibody against megalin confirmed LRP2 knockout. The verified KO model identified both 68Ga- and 99mTc-radiolabeled 15-mer peptides as megalin ligands in accumulation studies. We found that both radiolabeled 15-mers enter LRP2 KO HK2 cells to a lesser extent compared to parent cells. Differences in megalin-mediated cellular uptake depending on the radiolabeling were not observed. Using biomolecular docking, the interaction site of the 15-mer with megalin was also described.

CONCLUSION

The CRISPR/Cas9 knockout of LRP2 in human kidney HK2 cells is an effective approach for the determination of radiopeptide internalization mediated by megalin. This in vitro method provided direct molecular evidence for the cellular uptake of radiolabeled anti-VEGFR 15-mer peptides via megalin.

Polybrominated diphenyl ethers in dust, hair and urine: Exposure, excretion

Polybrominated diphenyl ethers (PBDEs) have been detected in various environmental media and human tissues. PBDEs concentrations in dust from college buildings and homes and in paired hair and urine samples from students were determined. This is of great significance to explore the accumulation and excretion patterns of PBDEs in the human body. The median PBDEs concentrations in the dust (College: 84.59 ng/g; Home: 170.32 ng/g) and hair (undergraduate: 6.16 ng/g; Home: 3.25 ng/g) samples were generally lower than were found in the majority of previous studies. The PBDEs concentrations in the hair and urine samples were subjected to principal component analysis, and the results combined with the PBDEs detection rates confirmed that hair is a useful non-invasive sampling medium for assessing PBDEs exposure and the risks posed. Body mass indices (BMIs) were used to divide students who had not been exposed to large amounts of PBDEs into groups. Body fat percentage is an important factor affecting the accumulation of PBDE in the human body. Environmental factors were found to affect the PBDEs concentrations in the hair and urine samples less for normal-weight students (BMI≤24) than overweight students (BMI>24). Short-term environmental changes to more readily affect the PBDEs concentrations in the tissues of the normal-weight than overweight students. PBDEs with seven or more bromine substituents were found not to be readily excreted in urine. Performing molecular docking simulations of the binding of isomers BDE-99 and BDE-100 to megalin. The binding energy was higher for BDE-100 and megalin than for BDE-99 and megalin, meaning BDE-99 would be more readily excreted than BDE-100.