Loss of albumin and megalin binding to renal cubilin in rats results in albuminuria after total body irradiation

Albumin Uptake and Processing by the Proximal Tubule: Physiologic, Pathologic and Therapeutic Implications

For nearly 50 years the proximal tubule (PT) has been known to reabsorb, process, and either catabolize or transcytose albumin from the glomerular filtrate. Innovative techniques and approaches have provided insights into these processes. Several genetic diseases, nonselective PT cell defects, chronic kidney disease (CKD), and acute PT injury lead to significant albuminuria, reaching nephrotic range. Albumin is also known to stimulate PT injury cascades. Thus, the mechanisms of albumin reabsorption, catabolism, and transcytosis are being reexamined with the use of techniques that allow for novel molecular and cellular discoveries. Megalin, a scavenger receptor, cubilin, amnionless, and Dab2 form a nonselective multireceptor complex that mediates albumin binding and uptake and directs proteins for lysosomal degradation after endocytosis. Albumin transcytosis is mediated by a pH-dependent binding affinity to the neonatal Fc receptor (FcRn) in the endosomal compartments. This reclamation pathway rescues albumin from urinary losses and cellular catabolism, extending its serum half-life. Albumin that has been altered by oxidation, glycation, or carbamylation or because of other bound ligands that do not bind to FcRn traffics to the lysosome. This molecular sorting mechanism reclaims physiological albumin and eliminates potentially toxic albumin. The clinical importance of PT albumin metabolism has also increased as albumin is now being used to bind therapeutic agents to extend their half-life and minimize filtration and kidney injury. The purpose of this review is to update and integrate evolving information regarding the reabsorption and processing of albumin by proximal tubule cells including discussion of genetic disorders and therapeutic considerations.

Emerging Technologies to Study the Glomerular Filtration Barrier

Kidney disease is characterized by loss of glomerular function with clinical manifestation of proteinuria. Identifying the cellular and molecular changes that lead to loss of protein in the urine is challenging due to the complexity of the filtration barrier, constituted by podocytes, glomerular endothelial cells, and glomerular basement membrane. In this review, we will discuss how technologies like single cell RNA sequencing and bioinformatics-based spatial transcriptomics, as well as in vitro systems like kidney organoids and the glomerulus-on-a-chip, have contributed to our understanding of glomerular pathophysiology. Knowledge gained from these studies will contribute toward the development of personalized therapeutic approaches for patients affected by proteinuric diseases.

Megalin: A Novel Endocytic Receptor for Prorenin and Renin

Megalin is an endocytic receptor contributing to protein reabsorption. Impaired expression or trafficking of megalin increases urinary renin and allowed the detection of prorenin, which normally is absent in urine. Here, we investigated (pro)renin uptake by megalin, using both conditionally immortalized proximal tubule epithelial cells and Brown Norway Rat yolk sac cells (BN16). To distinguish binding and internalization, cells were incubated with recombinant human (pro)renin at 4°C and 37°C, respectively. (Pro)renin levels were assessed by immunoradiometric assay. At 4°C, BN16 cells bound 3× more prorenin than renin, suggestive for a higher affinity of prorenin. Similarly, at 37°C, prorenin accumulated at 3- to 4-fold higher levels than renin in BN16 cells. Consequently, depletion of medium prorenin (but not renin) content occurred after 24 hours. No such differences were observed in conditionally immortalized proximal tubule epithelial cells, and M6P (mannose-6-phosphate) greatly reduced conditionally immortalized proximal tubule epithelial cells (pro)renin uptake, suggesting that these cells accumulate (pro)renin largely via M6P receptors. M6P did not affect (pro)renin uptake in BN16 cells. Yet, inhibiting megalin expression with siRNA greatly reduced (pro)renin binding and internalization by BN16 cells. Furthermore, treating BN16 cells with albumin, an endogenous ligand of megalin, also decreased binding and internalization of (pro)renin, while deleting the (pro)renin receptor affected the latter only. Exposing prorenin's prosegment with the renin inhibitor aliskiren dramatically increased prorenin binding, while after prosegment cleavage with trypsin prorenin binding was identical to that of renin. In conclusion, megalin might function as an endocytic receptor for (pro)renin and displays a preference for prorenin. Megalin-mediated endocytosis requires the (pro)renin receptor.

Transforming growth factor-β1 and diabetic nephropathy

Transforming growth factor-β1 (TGF-β1) is established to be involved in the pathogenesis of diabetic nephropathy. The diabetic milieu enhances oxidative stress and induces the expression of TGF-β1. TGF-β1 promotes cell hypertrophy and extracellular matrix accumulation in the mesangium, which decreases glomerular filtration rate and leads to chronic renal failure. Recently, TGF-β1 has been demonstrated to regulate urinary albumin excretion by both increasing glomerular permeability and decreasing reabsorption in the proximal tubules. TGF-β1 also increases urinary excretion of water, electrolytes and glucose by suppressing tubular reabsorption in both normal and diabetic conditions. Although TGF-β1 exerts hypertrophic and fibrogenic effects in diabetic nephropathy, whether suppression of the function of TGF-β1 can be an option to prevent or treat the complication is still controversial. This is partly because adrenal production of mineralocorticoids could be augmented by the suppression of TGF-β1. However, differentiating the molecular mechanisms for glomerulosclerosis from those for the suppression of the effects of mineralocorticoids by TGF-β1 may assist in developing novel therapeutic strategies for diabetic nephropathy. In this review, we discuss recent findings on the role of TGF-β1 in diabetic nephropathy.

Low TGFβ1 expression prevents and high expression exacerbates diabetic nephropathy in mice

Nephropathy develops in many but not all patients with long-standing type 1 diabetes. Substantial efforts to identify genotypic differences explaining this differential susceptibility have been made, with limited success. Here, we show that the expression of the transforming growth factor β1 gene (Tgfb1) affects the development of diabetic nephropathy in mice. To do this we genetically varied Tgfb1 expression in five steps, 10%, 60%, 100%, 150%, and 300% of normal, in mice with type 1 diabetes caused by the Akita mutation in the insulin gene (Ins2(Akita)). Although plasma glucose levels were not affected by Tgfb1 genotype, many features of diabetic nephropathy (mesangial expansion, elevated plasma creatinine and urea, decreased creatinine clearance and albuminuria) were progressively ameliorated as Tgfb1 expression decreased and were progressively exacerbated when expression was increased. The diabetic 10% hypomorphs had comparable creatinine clearance and albumin excretion to wild-type mice and no harmful changes in renal morphology. The diabetic 300% hypermorphs had ∼1/3 the creatinine clearance of wild-type mice, >20× their albumin excretion, ∼3× thicker glomerular basement membranes and severe podocyte effacement, matching human diabetic nephropathy. Switching Tgfb1 expression from low to high in the tubules of the hypomorphs increased their albumin excretion more than 10-fold but creatinine clearance remained high. Switching Tgfb1 expression from low to high in the podocytes markedly decreased creatinine clearance, but minimally increased albumin excretion. Decreasing expression of Tgfb1 could be a promising option for preventing loss of renal function in diabetes.

The proximal tubule and albuminuria: really!

Recent data highlight the role of the proximal tubule (PT) in reabsorbing, processing, and transcytosing urinary albumin from the glomerular filtrate. Innovative techniques and approaches have provided exciting insights into these processes, and numerous investigators have shown that selective PT cell defects lead to significant albuminuria, even reaching nephrotic range in animal models. Thus, the mechanisms of albumin reabsorption and transcytosis are undergoing intense study. Working in concert with megalin and cubilin, a nonselective multireceptor complex that predominantly directs proteins for lysosomal degradation, the neonatal Fc receptor (FcRn) located at the brush border of the apical membrane has been implicated as the "receptor" mediating albumin transcytosis. The FcRn pathway facilitates reabsorption and mediates transcytosis by its pH-dependent binding affinity in endosomal compartments. This also allows for selective albumin sorting within the PT cell. This reclamation pathway minimizes urinary losses and catabolism of albumin, thus prolonging its serum half-life. It may also serve as a molecular sorter to preserve and reclaim normal albumin while allowing "altered" albumin to be catabolized via lysosomal pathways. Here, we critically review the data supporting this novel mechanism.

Dietary resistant starch prevents urinary excretion of 25-hydroxycholecalciferol and vitamin D-binding protein in type 1 diabetic rats

Diabetes is a rapidly growing epidemic affecting millions of Americans and has been implicated in a number of devastating secondary complications. We previously demonstrated that type 2 diabetic rats exhibit vitamin D deficiency due to aberrant megalin-mediated endocytosis and excessive urinary excretion of 25-hydroxycholecalciferol (25D3) and vitamin D-binding protein (DBP). Here, we examined whether a model of type 1 diabetes [T1D; streptozotocin (STZ)-treated Sprague-Dawley rats] would similarly excrete abnormally high concentrations of 25D3 and DBP due to renal damage and compromised expression of megalin and its endocytic partner, disabled-2 (Dab2). Moreover, we tested whether feeding diabetic rats starch that is resistant to digestion could alleviate these abnormalities. Control (n = 12) rats were fed a standard, semipurified diet (AIN-93G) containing 55% total dietary starch and STZ-treated rats were fed the AIN-93G diet (n = 12) or a diet containing 55% high-amylose maize that is partially resistant to digestion [20% total dietary resistant starch (RS); n = 12] for 2 and 5 wk. The RS diet attenuated weight loss and polyuria in STZ-treated rats. Histology and immunohistochemistry revealed that dietary RS also attenuated the loss of Dab2 expression in renal proximal tubules. Moreover, urinary concentrations of both 25D3 and DBP were elevated ∼10-fold in STZ-treated rats (5 wk post STZ injection), which was virtually prevented by the RS. We also observed a ∼1.5-fold increase in megalin mRNA expression in STZ-treated rats, which was attenuated by feeding rats the RS diet for 2 wk. Taken together, these studies indicate that consumption of low-glycemic carbohydrates can attenuate disruption of vitamin D homeostasis in T1D through the rescue of megalin-mediated endocytosis in the kidney.

The urine proteome for radiation biodosimetry: effect of total body vs. local kidney irradiation

Victims of nuclear accidents or radiological terrorism are likely to receive varying doses of ionizing radiation inhomogeneously distributed over the body. Early biomarkers may be useful in determining organ-specific doses due to total body irradiation (TBI) or partial body irradiation. The authors used liquid chromatography and mass spectrometry to compare the effect of TBI and local kidney irradiation (LKI) on the rat urine proteome using a single 10-Gy dose of x-rays. Both TBI and LKI altered the urinary protein profile within 24 h with noticeable differences in gene ontology categories. Some proteins, including fetuin-B, tissue kallikrein, beta-glucuronidase, vitamin D-dependent calcium binding protein and chondroitin sulfate proteoglycan NG2, were detected only in the TBI group. Some other proteins, including major urinary protein-1, RNA binding protein 19, neuron navigator, Dapper homolog 3, WD repeat and FYVE domain containing protein 3, sorting nexin-8, ankycorbin and aquaporin were detected only in the LKI group. Protease inhibitors and kidney proteins were more abundant (fraction of total scans) in the LKI group. Urine protein (Up) and creatinine (Uc) (Up/Uc) ratios and urinary albumin abundance decreased in both TBI and LKI groups. Several markers of acute kidney injury were not detectable in either irradiated group. Present data indicate that abundance and number of proteins may follow opposite trends. These novel findings demonstrate intriguing differences between TBI and LKI, and suggest that urine proteome may be useful in determining organ-specific changes caused by partial body irradiation.

The Urine Proteome as a Biomarker of Radiation Injury: Submitted to Proteomics- Clinical Applications Special Issue: "Renal and Urinary Proteomics (Thongboonkerd)"

Terrorist attacks or nuclear accidents could expose large numbers of people to ionizing radiation, and early biomarkers of radiation injury would be critical for triage, treatment and follow-up of such individuals. However, no such biomarkers have yet been proven to exist. We tested the potential of high throughput proteomics to identify protein biomarkers of radiation injury after total body X-ray irradiation in a rat model. Subtle functional changes in the kidney are suggested by an increased glomerular permeability for macromolecules measured within 24 hours after TBI. Ultrastructural changes in glomerular podocytes include partial loss of the interdigitating organization of foot processes. Analysis of urine by LC-MS/MS and 2D-GE showed significant changes in the urine proteome within 24 hours after TBI. Tissue kallikrein 1-related peptidase, cysteine proteinase inhibitor cystatin C and oxidized histidine were found to be increased while a number of proteinase inhibitors including kallikrein-binding protein and albumin were found to be decreased post-irradiation. Thus, TBI causes immediately detectable changes in renal structure and function and in the urinary protein profile. This suggests that both systemic and renal changes are induced by radiation and it may be possible to identify a set of biomarkers unique to radiation injury.

Targeted disruption of cubilin reveals essential developmental roles in the structure and function of endoderm and in somite formation

Background

Cubilin is a peripheral membrane protein that interacts with the integral membrane proteins megalin and amnionless to mediate ligand endocytosis by absorptive epithelia such as the extraembryonic visceral endoderm (VE).

Results

Here we report the effects of the genetic deletion of cubilin on mouse embryonic development. Cubilin gene deletion is homozygous embryonic lethal with death occurring between 7.5–13.5 days post coitum (dpc). Cubilin-deficient embryos display developmental retardation and do not advance morphologically beyond the gross appearance of wild-type 8–8.5 dpc embryos. While mesodermal structures such as the allantois and the heart are formed in cubilin mutants, other mesoderm-derived tissues are anomalous or absent. Yolk sac blood islands are formed in cubilin mutants but are unusually large, and the yolk sac blood vessels fail to undergo remodeling. Furthermore, somite formation does not occur in cubilin mutants. Morphological abnormalities of endoderm occur in cubilin mutants and include a stratified epithelium in place of the normally simple columnar VE epithelium and a stratified cuboidal epithelium in place of the normally simple squamous epithelium of the definitive endoderm. Cubilin-deficient VE is also functionally defective, unable to mediate uptake of maternally derived high-density lipoprotein (HDL).

Conclusion

In summary, cubilin is required for embryonic development and is essential for the formation of somites, definitive endoderm and VE and for the absorptive function of VE including the process of maternal-embryo transport of HDL.

Role of megalin and cubilin in renal physiology and pathophysiology

Megalin and cubilin are endocytic receptors highly expressed in the endocytic apparatus of the renal proximal tubule. These receptors appear to be responsible for the tubular clearance of most proteins filtered in the glomeruli. Cubilin is a peripheral membrane protein, and therefore it does not have an endocytosis signaling sequence. It appears that megalin is responsible for internalization of cubilin and its ligands in addition to internalizing its own ligands. The proteinuria observed in megalin-deficient mice, in dogs lacking functional cubilin, and in patients with distinct mutations of the cubilin gene illustrates the importance of the receptors.

Electroendocytosis: exposure of cells to pulsed low electric fields enhances adsorption and uptake of macromolecules

This study demonstrates alteration of cell surface, leading to enhanced adsorption of macromolecules (bovine serum albumin (BSA), dextran, and DNA), after the exposure of cells to unipolar pulsed low electric fields (LEF). Modification of the adsorptive properties of the cell membrane also stems from the observation of LEF-induced cell-cell aggregation. Analysis of the adsorption isotherms of BSA-fluorescein isothiocyanate (FITC) to the surface of COS 5-7 cells reveals that the stimulated adsorption can be attributed to LEF-induced increase in the capacity of both specific and nonspecific binding. The enhanced adsorption was consequently followed by increased uptake. At 20 V/cm the maximal binding and subsequent uptake of BSA-FITC attached to specific sites are 6.5- and 7.4-fold higher than in controls, respectively. The nonspecific LEF-induced binding and uptake of BSA are 34- and 5.2-fold higher than in controls. LEF-enhanced adsorption is a temperature-independent process, whereas LEF-induced uptake is a temperature-dependent one that is abolished at 4 degrees C. The stimulation of adsorption and uptake is reversible, revealing similar decay kinetics at room temperature. It is suggested that electrophoretic segregation of charged components in the outer leaflet of the cell membrane is responsible for both enhanced adsorption and stimulated uptake via changes of the membrane elastic properties that enhance budding and fission processes.

Differential distribution of cubilin and megalin expression in the mouse embryo

Cubilin and megalin are cell surface proteins that work cooperatively in many absorptive epithelia to mediate endocytosis of lipoproteins, vitamin carriers, and other proteins. Here we have investigated the coordinate expression of these receptors during mouse development. Our findings indicate that while there are sites where the receptors are co-expressed, there are other tissues where expression is not overlapping. Apical cubilin expression is pronounced in the extraembryonic visceral endoderm (VE) of 6-9.5 days postcoitum (dpc) embryos. By contrast, little megalin expression is evident in the VE at 6 dpc. However, megalin expression in the VE increases as development progresses (7.5-9.5 dpc), although it is not as uniformly distributed as cubilin. Punctate expression of megalin is also apparent in the region of the ectoplacental cone associated with decidual cells, whereas cubilin expression is not seen in association with the ectoplacenta. Strong expression of megalin is observed in the neural ectoderm, neural plate and neural tube (6-8.5 dpc), but cubilin expression is not apparent in any of these tissues. At 8.5 dpc, megalin is expressed in the developing endothelial cells of blood islands, whereas cubilin is absent from these cells. Finally, cubilin, but not megalin, is expressed by a subpopulation of cells dispersed within the 7.5 dpc embryonic endoderm and having a migratory morphology. In summary, the co-expression of cubilin and megalin in the VE is consistent with the two proteins functioning jointly in this tissue. However, the differential distribution pattern indicates that the proteins also function independent of one another. Furthermore, the finding of megalin expression in blood island endothelial cells and cubilin expression in embryonic endoderm highlight potential new developmental roles for these proteins.

Radiation nephropathy

The pronounced radiosensitivity of renal tissue limits the total radiotherapeutic dose that can be applied safely to treatment volumes that include the kidneys. The incidence of clinical radiation nephropathy has increased with the use of total-body irradiation (TBI) in preparation for bone marrow transplantation and as a consequence of radionuclide therapies. The clinical presentation is azotemia, hypertension, and, disproportionately, severe anemia seen several months to years after irradiation that, if untreated, leads to renal failure. Structural features include mesangiolysis, sclerosis, tubular atrophy, and tubulointerstitial scarring. Similar changes are seen in a variety of experimental animal models. The classic view of radiation nephropathy being inevitable, progressive, and untreatable because of DNA damage-mediated cell loss at division has been replaced by a new paradigm in which radiation-induced injury involves not only direct cell kill but also involves complex and dynamic interactions between glomerular, tubular, and interstitial cells. These serve both as autocrine and as paracrine, if not endocrine, targets of biologic mediators that mediate nephron injury and repair. The renin angiotensin system (RAS) clearly is involved; multiple experimental studies have shown that antagonism of the RAS is beneficial, even when not initiated until weeks after irradiation. Recent findings suggest a similar benefit in clinical radiation nephropathy.