Beyond the tubule: pathological variants of LRP2, encoding the megalin receptor, result in glomerular loss and early progressive chronic kidney disease

Pathogenic variants in the LRP2 gene, encoding the multiligand receptor megalin, cause a rare autosomal recessive syndrome: Donnai-Barrow/Facio-Oculo-Acoustico-Renal (DB/FOAR) syndrome. Because of the rarity of the syndrome, the long-term consequences of the tubulopathy on human renal health have been difficult to ascertain, and the human clinical condition has hitherto been characterized as a benign tubular condition with asymptomatic low-molecular-weight proteinuria. We investigated renal function and morphology in a murine model of DB/FOAR syndrome and in patients with DB/FOAR. We analyzed glomerular filtration rate in mice by FITC-inulin clearance and clinically characterized six families, including nine patients with DB/FOAR and nine family members. Urine samples from patients were analyzed by Western blot analysis and biopsy materials were analyzed by histology. In the mouse model, we used histological methods to assess nephrogenesis and postnatal renal structure and contrast-enhanced magnetic resonance imaging to assess glomerular number. In megalin-deficient mice, we found a lower glomerular filtration rate and an increase in the abundance of injury markers, such as kidney injury molecule-1 and N-acetyl-β-d-glucosaminidase. Renal injury was validated in patients, who presented with increased urinary kidney injury molecule-1, classical markers of chronic kidney disease, and glomerular proteinuria early in life. Megalin-deficient mice had normal nephrogenesis, but they had 19% fewer nephrons in early adulthood and an increased fraction of nephrons with disconnected glomerulotubular junction. In conclusion, megalin dysfunction, as present in DB/FOAR syndrome, confers an increased risk of progression into chronic kidney disease.

Three-dimensional Growth of Renal Epithelial Cells inVitro:A Tool in Toxicity Testing

The tissue culture of multicellular spheroids from the renal epithelial cell line LLC-PK1 (proximal tubule) is described. This represents a biological system of intermediate complexity between renal tissue in vivo and simple monolayer cultures. The multicellular structures, which show many similarities to kidney tubules in vivo, including a vectorial water transport, should prove useful for studying the potential nephrotoxicity of drugs and chemicals in vitro.

In addition, the propagation of renal epithelial cells as multicellular spheroids in serum-free culture may provide information on the release of specific biological parameters, which may be suppressed or masked in serum-supplemented media.

Serum uromodulin-a marker of kidney function and renal parenchymal integrity

Background

An ELISA to analyse uromodulin in human serum (sUmod) was developed, validated and tested for clinical applications.

Methods

We assessed sUmod, a very stable antigen, in controls, patients with chronic kidney disease (CKD) stages 1-5, persons with autoimmune kidney diseases and recipients of a renal allograft by ELISA.

Results

Median sUmod in 190 blood donors was 207 ng/mL (women: men, median 230 versus 188 ng/mL, P = 0.006). sUmod levels in 443 children were 193 ng/mL (median). sUmod was correlated with cystatin C (rs = -0.862), creatinine (rs = -0.802), blood urea nitrogen (BUN) (rs = -0.645) and estimated glomerular filtration rate (eGFR)-cystatin C (rs  =  0.862). sUmod was lower in systemic lupus erythematosus-nephritis (median 101 ng/mL), phospholipase-A2 receptor- positive glomerulonephritis (median 83 ng/mL) and anti-glomerular basement membrane positive pulmorenal syndromes (median 37 ng/mL). Declining sUmod concentrations paralleled the loss of kidney function in 165 patients with CKD stages 1-5 with prominent changes in sUmod within the 'creatinine blind range' (71-106 µmol/L). Receiver-operating characteristic analysis between non-CKD and CKD-1 was superior for sUmod (AUC 0.90) compared with eGFR (AUC 0.39), cystatin C (AUC 0.39) and creatinine (AUC 0.27). sUmod rapidly recovered from 0 to 62 ng/mL (median) after renal transplantation in cases with immediate graft function and remained low in delayed graft function (21 ng/mL, median; day 5-9: relative risk 1.5-2.9, odds ratio 1.5-6.4). Immunogold labelling disclosed that Umod is transferred within cytoplasmic vesicles to both the apical and basolateral plasma membrane. Umod revealed a disturbed intracellular location in kidney injury.

Conclusions

We conclude that sUmod is a novel sensitive kidney-specific biomarker linked to the structural integrity of the distal nephron and to renal function.

Enzyme Replacement Therapy During Pregnancy in Fabry Patients : Review of Published Cases of Live Births and a New Case of a Severely Affected Female with Fabry Disease and Pre-eclampsia Complicating Pregnancy

Fabry disease (FD) is an X-linked, lysosomal storage disease. Mutations in the gene coding for alpha-galactosidase A lead to globotriaosylceramide (Gb-3) accumulation in lysosomes and in placenta and umbilical cord. Impact of FD and treatment with enzyme replacement (ERT) on foetal development is undisclosed.A 38-year-old primigravida with FD (G85N) is reported. She has 50% reduced alpha-galactosidase A activity and elevated plasma and urine-Gb-3. She was severely affected with ischaemic stroke at age 23, hypertension, albuminuria and moderately reduced renal function. ERT was initiated at age 23 years in 2001 and continued during spontaneous pregnancy at age 38. In third trimester she developed moderate-to-severe pre-eclampsia, successfully managed by methyldopa. Chorion villus sampling revealed a male foetus without the maternal gene mutation. Planned Caesarean section was performed without complications at gestational age week 38 + 6, delivering a healthy boy. Histopathological placental examination showed no sign of Gb-3 accumulation. Literature survey disclosed a total of 12 cases, 8 were treated with ERT during pregnancy and 5 infants inherited the family mutation. All outcomes were successful. In the six cases with available placental histopathological examination, Gb-3 accumulation was only seen on the foetal side if the foetus had the inherited mutation.In conclusion, the present case, describing the first data from a severely affected FD patient receiving ERT during pregnancy complicated by pre-eclampsia, together with all other published cases, has emphasized that ERT is safe during pregnancy and resulting in successful foetal outcome; despite this, ERT is by the health authorities advised against during pregnancy.

Megalin dependent urinary cystatin C excretion in ischemic kidney injury in rats

Background

Cystatin C, a marker of kidney injury, is freely filtered in the glomeruli and reabsorbed by the proximal tubules. Megalin and cubilin are endocytic receptors essential for reabsorption of most filtered proteins. This study examines the role of these receptors for the uptake and excretion of cystatin C and explores the effect of renal ischemia/reperfusion injury on renal cystatin C uptake and excretion in a rat model.

Methods

Binding of cystatin C to megalin and cubilin was analyzed by surface plasmon resonance analysis. ELISA and/or immunoblotting and immunohistochemistry were used to study the urinary excretion and tubular uptake of endogenous cystatin C in mice. Furthermore, renal uptake and urinary excretion of cystatin C was investigated in rats exposed to ischemia/reperfusion injury.

Results

A high affinity binding of cystatin C to megalin and cubilin was identified. Megalin deficient mice revealed an increased urinary excretion of cystatin C associated with defective uptake by endocytosis. In rats exposed to ischemia/reperfusion injury urinary cystatin C excretion was increased and associated with a focal decrease in proximal tubule endocytosis with no apparent change in megalin expression.

Conclusions

Megalin is essential for the normal tubular recovery of endogenous cystatin C. The increase in urinary cystatin C excretion after ischemia/reperfusion injury is associated with decreased tubular uptake but not with reduced megalin expression.

Megalin and cubilin in proximal tubule protein reabsorption: from experimental models to human disease

Proximal tubule protein uptake is mediated by 2 receptors, megalin and cubilin. These receptors rescue a variety of filtered ligands, including biomarkers, essential vitamins, and hormones. Receptor gene knockout animal models have identified important functions of the receptors and have established their essential role in modulating urinary protein excretion. Rare genetic syndromes associated with dysfunction of these receptors have been identified and characterized, providing additional information on the importance of these receptors in humans. Using various disease models in combination with receptor gene knockout, the implications of receptor dysfunction in acute and chronic kidney injury have been explored and have pointed to potential new roles of these receptors. Based on data from animal models, this paper will review current knowledge on proximal tubule endocytic receptor function and regulation, and their role in renal development, protein reabsorption, albumin uptake, and normal renal physiology. These findings have implications for the pathophysiology and diagnosis of proteinuric renal diseases. We will examine the limitations of the different models and compare the findings to phenotypic observations in inherited human disorders associated with receptor dysfunction. Furthermore, evidence from receptor knockout mouse models as well as human observations suggesting a role of protein receptors for renal disease will be discussed in light of conditions such as chronic kidney disease, diabetes, and hypertension.

Screening, diagnosis, and management of patients with Fabry disease: conclusions from a "Kidney Disease: Improving Global Outcomes" (KDIGO) Controversies Conference

Patients with Fabry disease (FD) are at a high risk for developing chronic kidney disease and cardiovascular disease. The availability of specific but costly therapy has elevated the profile of this rare condition. This KDIGO conference addressed controversial areas in the diagnosis, screening, and management of FD, and included enzyme replacement therapy and nonspecific standard-of-care therapy for the various manifestations of FD. Despite marked advances in patient care and improved overall outlook, there is a need to better understand the pathogenesis of this glycosphingolipidosis and to determine the appropriate age to initiate therapy in all types of patients. The need to develop more effective specific therapies was also emphasized.

Megalin in acute kidney injury: foe and friend

The kidney proximal tubule is a key target in many forms of acute kidney injury (AKI). The multiligand receptor megalin is responsible for the normal proximal tubule uptake of filtered molecules, including nephrotoxins, cytokines, and markers of AKI. By mediating the uptake of nephrotoxins, megalin plays an essential role in the development of some types of AKI. However, megalin also mediates the tubular uptake of molecules implicated in the protection against AKI, and changes in megalin expression have been demonstrated in AKI in animal models. Thus, modulation of megalin expression in response to AKI may be an important part of the tubule cell adaption to cellular protection and regeneration and should be further investigated as a potential target of intervention. This review explores current evidence linking megalin expression and function to the development, diagnosis, and progression of AKI as well as renal protection against AKI.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

Cubilin is expressed in rat and human glomerular podocytes

BACKGROUND

The bulk of proteins filtered in the glomeruli are reabsorbed in the proximal tubule by endocytosis mediated by two multiligand receptors operating in concert, megalin and cubilin. Podocytes can also internalize protein and megalin; this was initially reported in rat proximal tubular and glomerular epithelial cells and has recently also been demonstrated in human podocytes. Cubilin, crucial for albumin reabsorption in the proximal tubule, has not been identified in glomerular epithelial cells.

METHODS

In the present study, we used immunocytochemistry and reverse transcription-polymerase chain reaction on laser-captured glomeruli to demonstrate synthesis and expression of cubilin in rat and human glomeruli. In parallel experiments, the expression of cubilin was studied in cultured podocytes.

RESULTS

This study identifies cubilin in rat and human glomeruli according to a pattern similar to that reported for megalin. Cubilin revealed a surface expression but also intracellular expression in the podocytes.

CONCLUSION

Our findings show that the podocytes display the two endocytic receptors which are responsible for the only documented process for protein reabsorption in proximal tubule cells.

Proneurotrophin-3 may induce Sortilin-dependent death in inner ear neurons

The precursor of the neurotrophin (NT) nerve growth factor (NGF) (proNGF) serves physiological functions distinct from its mature counterpart as it induces neuronal apoptosis through activation of a p75 NT receptor (p75(NTR) ) and Sortilin death-signalling complex. The NTs brain-derived nerve growth factor (BDNF) and NT3 provide essential trophic support to auditory neurons. Injury to the NT-secreting cells in the inner ear is followed by irreversible degeneration of spiral ganglion neurons with consequences such as impaired hearing or deafness. Lack of mature NTs may explain the degeneration of spiral ganglion neurons, but another mechanism is possible as unprocessed proNTs released from the injured cells may contribute to the degeneration by induction of apoptosis. Recent studies demonstrate that proBDNF, like proNGF, is a potent inducer of Sortilin:p75(NTR) -mediated apoptosis. In addition, a coincident upregulation of proBDNF and p75(NTR) has been observed in degenerating spiral ganglion neurons, but the Sortilin expression in the inner ear is unresolved. Here we demonstrate that Sortilin and p75(NTR) are coexpressed in neurons of the neonatal inner ear. Furthermore, we establish that proNT3 exhibits high-affinity binding to Sortilin and has the capacity to enhance cell surface Sortilin:p75(NTR) complex formation as well as to mediate apoptosis in neurons coexpressing p75(NTR) and Sortilin. Based on the examination of wildtype and Sortilin-deficient mouse embryos, Sortilin does not significantly influence the developmental selection of spiral ganglion neurons. However, our results suggest that proNT3 and proBDNF may play important roles in the response to noise-induced injuries or ototoxic damage via the Sortilin:p75(NTR) death-signalling complex.

LRP2 in ependymal cells regulates BMP signaling in the adult neurogenic niche

The microenvironment of growth factors in the subependymal zone (SEZ) of the adult brain provides the instructive milieu for neurogenesis to proceed in this germinal niche. In particular, tight regulation of bone morphogenetic protein (BMP) signaling is essential to balance proliferative and non-proliferative cell fate specification. However, the regulatory pathways that control BMP signaling in the SEZ are still poorly defined. We demonstrate that LRP2, a clearance receptor for BMP4 is specifically expressed in ependymal cells of the lateral ventricles in the adult brain. Intriguingly, expression is restricted to the ependyma that faces the stem cell niche. Expression is not seen in ependyma elsewhere in the lateral ventricles or in the dentate gyrus, the second major neurogenic zone of the adult brain. We further show that lack of LRP2 expression in adult mice results in impaired proliferation of neural precursor cells in the SEZ resulting in decreased numbers of neuroblasts reaching the olfactory bulb. Reduced neurogenesis coincides with increased BMP4 expression and enhanced activation of downstream mediators phospho-SMAD1/5/8 and ID3 in the stem cell niche. Our findings suggest a novel mechanism whereby LRP2-mediated catabolism of BMP4 in the ependyma modulates the microenvironment of the SEZ and enables adult neurogenesis to proceed.

Proteinuria and events beyond the slit

The origin of proteinuria is found in either the glomerular filtration device or the proximal tubular reabsorption machinery. During equilibrium, small amounts of predominantly low molecular weight proteins are filtered and reabsorbed by the receptor complex megalin/cubilin/amnionless. This results in a protein-free filtrate passing further down the tubule. During glomerular damage, the reabsorption machinery in the proximal tubule is challenged due to elevated amounts of proteins passing the glomerular filtration slits. Even though it is considered to be a high-capacity system, several conditions result in proteinuria, thus exposing the cells in the rest of the nephron to a protein-rich environment. The impact on cells in the more distal part of the nephron is uncertain, but studies support an involvement in fibrosis development. Protein accumulation in lysosomes of the proximal tubule, due to increased protein internalization, is thought to mediate inflammation and fibrosis, eventually leading to renal failure. In contrast, low molecular weight proteinuria develops when the endocytic machinery is malfunctioning either by direct or indirect causes such as in Imerslund-Gräsbeck syndrome (IGS) or Dent's disease, respectively. This review discusses the origin of proteinuria and describes the structural fundament for protein reabsorption in the proximal tubule as well as conditions resulting in low molecular weight proteinuria.

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.

Cubilin and megalin co-localize in the neonatal inner ear

Cubilin and megalin are multifunctional endocytotic receptors expressed in many absorptive epithelia. The receptors have separate functions but may act in concert in several tissues including the small intestine, the visceral yolk sac and the renal proximal tubule to perform significant physiological functions essential to homeostasis in the extracellular fluid compartments. Because of the importance of fluid homeostasis in the inner ear, we investigated the expression of cubilin and megalin in this structure. Furthermore, we examined the binding properties of 6 different aminoglycosides (AGs) to cubilin and compared the results to data obtained for megalin, which is a well-known receptor for AGs and other polybasic substances, including several ototoxic drugs. In the cochlea, immunohistochemical labelling for cubilin showed expression corresponding to the apical surface of the strial marginal cells, to epithelial cells at the spiral prominence and to epithelial cells of Reissner's membrane facing the cochlear duct. In the vestibular apparatus, positive labelling was found in dark cells of the utricle and those flanking the crista ampullaris of the semicircular canals. Exactly the same tissue distribution was found for megalin. These findings support the prevailing view that cubilin and megalin constitute a dual-receptor complex facilitating the function of each other. The physiological role of this receptor complex in the inner ear remains unknown, although several established ligands for both cubilin and megalin are present in the inner ear fluids. Receptor-ligand interaction analysis showed that all 6 AGs bind to both receptors and with approximately the same affinity. The results demonstrate a novel role for cubilin as a drug receptor for AGs and possibly other ototoxic substances.

The signal peptide anchors apolipoprotein M in plasma lipoproteins and prevents rapid clearance of apolipoprotein M from plasma

Lipoproteins consist of lipids solubilized by apolipoproteins. The lipid-binding structural motifs of apolipoproteins include amphipathic alpha-helixes and beta-sheets. Plasma apolipoprotein (apo) M lacks an external amphipathic motif but, nevertheless, is exclusively associated with lipoproteins (mainly high density lipoprotein). Uniquely, however, apoM is secreted to plasma without cleavage of its hydrophobic NH(2)-terminal signal peptide. To test whether the signal peptide serves as a lipoprotein anchor for apoM in plasma, we generated mice expressing a mutated apoM(Q22A) cDNA in the liver (apoM(Q22A)-Tg mice (transgenic mice)) and compared them with mice expressing wild-type human apoM (apoM-Tg mice). The substitution of the amino acid glutamine 22 with alanine in apoM(Q22A) results in secretion of human apoM without a signal peptide. The human apoM mRNA level in liver and the amount of human apoM protein secretion from hepatocytes were similar in apoM-Tg and apoM(Q22A)-Tg mice. Nevertheless, human apoM was not detectable in plasma of apoM(Q22A)-Tg mice, whereas it was easily measured in the apoM-Tg mice. To examine the plasma metabolism, recombinant apoM lacking the signal peptide was produced in Escherichia coli and injected into wild-type mice. The apoM without signal peptide did not associate with lipoproteins and was rapidly cleared in the kidney. Accordingly, ligation of the kidney arteries in apoM(Q22A)-Tg mice resulted in rapid accumulation of human apoM in plasma. The data suggest that hydrophobic signal peptide sequences, if preserved upon secretion, can anchor plasma proteins in lipoproteins. In the case of apoM, this mechanism prevents rapid loss by filtration in the kidney.

A pivotal role of the human kidney in catabolism of HDL protein components apolipoprotein A-I and A-IV but not of A-II

Renal handling of major HDL components was studied by analyzing urine from patients with Fanconi syndrome, a rare renal proximal tubular reabsorption failure, including dysfunction of the kidney HDL receptor, cubilin. A high urinary excretion of apolipoprotein A-I and A-IV corresponding to a major part of the metabolism of these proteins was measured. In contrast, no urinary excretion of apolipoprotein A-II which is more hydrophobic and tighter bound to HDL was found. Control urines displayed absence of the three apolipoproteins. Urinary excretion of phospholipids, triglycerides, cholesterol and cholesterol esters in patients was as low as in controls. In conclusion, these data indicate that the human kidney is a major site for filtered nascent apolipoprotein A-I and A-IV but not for HDL particles.

Controversies in nephrology: renal albumin handling, facts, and artifacts!

In this article, we discuss and contradict a recent publication by Russo et al., which suggests that the filtration of large amounts of albumin followed by transtubular transport of intact albumin is a physiological phenomenon.

Immunocytochemistry of renal membrane proteins on epoxy sections

Immunocytochemistry performed on paraffin or cryosections is often hampered by poor morphology. Epoxy sections, in contrast, generally retain well-preserved tissue architecture. Immunocytochemistry, however, on epoxy-embedded sections is difficult due in part to the plastic itself and to the fixation conditions. Here, we present a technique for visualization of membrane proteins by immunocytochemistry on epoxy sections of kidneys fixed with glutaraldehyde without or with osmium post-fixation. Semithin sections were obtained from Epon 812-embedded mouse and rat kidney blocks. Before immunoperoxidase or immunofluorescence labeling, the sections were etched with the epoxy solvent, methanolic potassium hydroxide, followed by antigen retrieval using microwave heating. The sections were then treated with the primary antibody followed by secondary antibodies as usual. The distribution and expression patterns of a variety of membrane proteins, such as aquaporin (AQP)-1, AQP-2, and megalin, were identical to those observed by traditional immunocytochemical procedures on paraffin or cryosections. The advantages of our novel method include not only enhanced morphological quality but also the feasibility for investigators to visualize antigens of interest using archival specimens in Epon blocks.

Low noise electron microscopy by merging multiple images digitized from conventional films with reference to the mouse kidney

In a conventional transmission electron microscope system, the resolution is regarded as an absolute limitation, that is, 0.2 nm in theory and 0.6 nm in sections of biological materials. However, in an oversampled system, this limitation can be broken. In the present study, 60-nm-thick Epon sections from a mouse kidney were used. From these sections tight junctions located in the distal tubule were selected as test objects. Sets of up to 15 electron microscope images of the same target were recorded on negatives at x10,000, x13,000, and x63,000, respectively. The recorded films were digitized using a light microscope equipped with a digital camera. In each set the images were expanded, aligned, and merged into a more highly resolved output image. Each output image revealed details in the tight junction, which were not visible at the original magnifications. Two different sizes of colloidal gold particles (10 nm and 1 nm) conjugated with an immunoglobin G (IgG) served as references. With this improvement of resolution, it becomes possible to inspect some barely visible biologic (virus) particles and structures, such as glycogen and free ribosomes in their native environment.

Characterization of proteinuria and tubular protein uptake in a new model of oral L-lysine administration in rats

Intravenous infusion of basic amino acids is used experimentally and pharmacologically to prevent renal proximal tubular uptake of filtered proteins. Intravenously injected L-lysine is rapidly cleared from plasma and the effect on tubular protein reabsorption is transient. To obtain a more sustained effect, we developed a model of oral L-lysine administration and characterized this model by analyzing urinary protein excretion and proximal tubule uptake of filtered proteins. Rats placed in metabolic cages were treated with 20 mmol/kg/6 h of L-lysine, glycine, or water. Urines were analyzed for proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting, and radioimmunoassay. Proximal tubule uptake of proteins and expression of apical membrane receptors were investigated by immunocytochemistry. In vitro uptake and receptor expression were studied using a yolk sac cell line. L-lysine administration produced increased urinary excretion of a large number of proteins while the effect on tubular accumulation of selected proteins was variable. L-lysine treatment induced changes in the localization of two receptors responsible for tubular endocytosis of filtered proteins. In conclusion, oral L-lysine treatment induced proteinuria, in particular albuminuria, as efficiently as previous reports on intravenous infusion. The effect on tubular protein accumulation was variable suggesting differential effects on tubular reabsorption and degradation of filtered proteins. Changes in tubular protein handling were accompanied by changes in the localization of the endocytic receptors, megalin, and cubilin. In vitro experiments supported the in vivo observations. The findings suggest that L-lysine may affect receptor trafficking in addition to possible effects on the direct binding of ligands to the receptors.

Renal albumin absorption in physiology and pathology

Albumin is the most abundant plasmaprotein serving multiple functions as a carrier of metabolites, hormones, vitamins, and drugs, as an acid/base buffer, as antioxidant and by supporting the oncotic pressure and volume of the blood. The presence of albumin in urine is considered to be the result of the balance between glomerular filtration and tubular reabsorption. Albuminuria has been accepted as an independent risk factor and a marker for renal as well as cardiovascular disease, and during the past decade, evidence has suggested that albumin itself may cause progression of renal disease. Thus, the reduction of proteinuria and, in particular, albuminuria has become a target in itself to prevent deterioration of renal function. Studies have shown albumin and its ligands to induce expression of inflammatory and fibrogenic mediators, and it has been hypothesized that increased filtration of albumin causes excessive tubular reabsorption, resulting in inflammation and fibrosis, resulting in the loss of renal function. In addition, it is known that tubular dysfunction in itself may cause albuminuria owing to decreased reabsorption of filtered albumin, and, recently, it has been suggested that significant amounts of albumin fragments are excreted in the urine as a result of tubular degradation. Thus, although both tubular and glomerular dysfunction influences renal handling of albumin, it appears that tubular reabsorption plays a central role in mediating the effects of albumin on renal function. The present paper will review the mechanisms for tubular albumin uptake and the possible implications for the development of renal disease.

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.

Mutually dependent localization of megalin and Dab2 in the renal proximal tubule

Disabled-2 (Dab2) is a cytoplasmic adaptor protein that binds to the cytoplasmic tail of the multiligand endocytic receptor megalin, abundantly expressed in renal proximal tubules. Deletion of Dab2 induces a urinary increase in specific plasma proteins such as vitamin D binding protein and retinol binding protein (Morris SM, Tallquist MD, Rock CO, and Cooper JA. EMBO J 21: 1555-1564, 2002). However, the subcellular localization of Dab2 in the renal proximal tubule and its function have not been fully elucidated yet. Here, we report the characterization of Dab2 in the renal proximal tubule. Immunohistocytochemistry revealed colocalization with megalin in coated pits and vesicles but not in dense apical tubules and the brush border. Kidney-specific megalin knockout almost abolished Dab2 staining, indicating that Dab2 subcellular localization requires megalin in the proximal tubule. Reciprocally, knockout of Dab2 led to a redistribution of megalin from endosomes to microvilli. In addition, there was an overall decrease in levels of megalin protein observed by immunoblotting but no decrease in clathrin or alpha-adaptin protein levels or in megalin mRNA. In rat yolk sac epithelial BN16 cells, Dab2 was present apically and colocalized with megalin. Introduction of anti-Dab2 antibody into BN16 cells decreased the internalization of 125I-labeled receptor-associated protein, substantiating the role of Dab2 in megalin-mediated endocytosis. The present study shows that Dab2 is localized in the apical endocytic apparatus of the renal proximal tubule and that this localization requires megalin. Furthermore, the study suggests that the urinary loss of megalin ligands observed in Dab2 knockout mice is caused by suboptimal trafficking of megalin, leading to decreased megalin levels.

The L1 cell adhesion molecule is induced in renal cancer cells and correlates with metastasis in clear cell carcinomas

PURPOSE

The L1 cell adhesion molecule is overexpressed in many human carcinomas. The objectives of the study were to provide a comprehensive description of L1 distribution in human kidney and to establish the prognostic relevance of L1 expression in renal cell carcinomas (RCC).

EXPERIMENTAL DESIGN

Using two antibodies to the extracellular part and the cytoplasmic domain, respectively, we first compared L1 expression in normal kidney and renal tumors of diverse histopathologic origin, then we studied L1 expression together with tumor stage, grade, molecular prognostic biomarkers, and metastatic behavior.

RESULTS

In normal kidney, L1 immunoreactive with both antibodies was expressed in all epithelial cells originating from the ureteric bud except for intercalated cells. In renal tumors, L1 was mainly detected in those originating from cells that do not express L1 in the normal kidney [i.e., 33 of 72 clear cell RCC (ccRCC) and 25 of 88 papillary RCC (papRCC)]. Both in ccRCC and papRCC, L1 reacted only with the antibody to the extracellular domain, suggesting that the protein was truncated. In these carcinomas, L1 expression was strongly correlated with Ki-67 proliferation index (ccRCC, P = 0.0059; papRCC, P = 0.0039), but only in ccRCC, the presence of L1 was associated with the risk of metastasis (P = 0.0121). This risk was higher if cyclin D1 was concurrently absent in tumor cells (P < 0.0001). The L1(+)/cyclin D1(-) profile was an independent prognostic factor of metastasis occurrence in multivariate analysis (P = 0.0023).

CONCLUSION

We have found a combination of markers that can serve to identify a subgroup of high-risk patients with ccRCC that may require more aggressive therapies.

The L1 Cell Adhesion Molecule Is Induced in Renal Cancer Cells and Correlates with Metastasis in Clear Cell Carcinomas

Purpose: The L1 cell adhesion molecule is overexpressed in many human carcinomas. The objectives of the study were to provide a comprehensive description of L1 distribution in human kidney and to establish the prognostic relevance of L1 expression in renal cell carcinomas (RCC).

Experimental Design: Using two antibodies to the extracellular part and the cytoplasmic domain, respectively, we first compared L1 expression in normal kidney and renal tumors of diverse histopathologic origin, then we studied L1 expression together with tumor stage, grade, molecular prognostic biomarkers, and metastatic behavior.

Results: In normal kidney, L1 immunoreactive with both antibodies was expressed in all epithelial cells originating from the ureteric bud except for intercalated cells. In renal tumors, L1 was mainly detected in those originating from cells that do not express L1 in the normal kidney [i.e., 33 of 72 clear cell RCC (ccRCC) and 25 of 88 papillary RCC (papRCC)]. Both in ccRCC and papRCC, L1 reacted only with the antibody to the extracellular domain, suggesting that the protein was truncated. In these carcinomas, L1 expression was strongly correlated with Ki-67 proliferation index (ccRCC, P = 0.0059; papRCC, P = 0.0039), but only in ccRCC, the presence of L1 was associated with the risk of metastasis (P = 0.0121). This risk was higher if cyclin D1 was concurrently absent in tumor cells (P &lt; 0.0001). The L1+/cyclin D1− profile was an independent prognostic factor of metastasis occurrence in multivariate analysis (P = 0.0023).

Conclusion: We have found a combination of markers that can serve to identify a subgroup of high-risk patients with ccRCC that may require more aggressive therapies.

Caveolae, caveolin and cav-p60 in smooth muscle and renin-producing cells in the rat kidney

AIMS

In vascular smooth muscle cells caveolae are important for signalling mechanisms regulating vascular contraction. In smooth muscle layer of the renal afferent arteriole juxtaglomerular cells (JG cells) are non-contractile renin producing cells that have the capacity to change their phenotype into smooth muscle cells and back again by metaplastic transformation. Signalling mechanisms in JG cells are not fully understood and we therefore investigated if caveolae were present, and thereby could be involved as integrators of cellular signalling in both of these phenotypes of smooth muscle cells.

METHODS

Using electron microscopy we compared the number of caveolae in JG cells and smooth muscle cells in the afferent arteriole of the rat kidney. The expression of caveolin and cav-p60 was examined using a combination of immunogold electron microscopy and immunofluorescence microscopy.

RESULTS

We found that JG cells have sixfold less caveolae per cell surface sectional length than smooth muscle cells. The expression of cavolin-1 and cav-p60 correlated with the number of caveolae. An examination of the general distribution of caveolae, cav-p60 and caveolins in the rat kidney showed that cav-p60, like caveolin-1, is a specific maker of caveolae.

CONCLUSION

The number of caveolae in JG cells is very low, and this makes it unlikely that caveolae are of major importance for the renin secretion specific for JG cells.

Diversity in rat tissue accumulation of vitamin B12 supports a distinct role for the kidney in vitamin B12 homeostasis

BACKGROUND

Vitamin B(12) in plasma is complexed to the carrier proteins transcobalamin (TC) and haptocorrin. The TC-B(12) complex is filtered in the glomeruli and reabsorbed in the renal tubules by receptor-mediated endocytosis, providing a route for a significant renal accumulation of vitamin B(12). The present study investigates the role of the rodent kidney in B(12) homeostasis by examining the distribution of vitamin B(12) in rats during vitamin B(12) depletion or B(12) load, and compares kidney accumulation with the vitamin distribution in other tissues including brain, liver, testes, intestine, spleen and plasma.

METHODS

Fifteen rats were fed on a diet containing different concentrations of B(12) supplemented with s.c. injections of B(12). Twenty four hours prior to sacrifice, all animals were injected with [(57)Co]B(12). The vitamin contents of kidneys, liver, spleen, brain, testis, intestine, skeletal muscle, serum and urine were analysed. Both total tissue vitamin B(12) accumulation and [(57)Co]B(12) were determined to compare steady-state B(12) and the distribution of an acutely injected dose. In the kidney, free and protein-bound B(12) was determined by gel filtration.

RESULTS

The rat kidneys accumulated more B(12) during normal and loaded conditions than any other tissue. A 110-fold increase in vitamin content was observed from the deficient to the loaded conditions in the kidney compared with a 3.5-fold increase in the liver. In contrast to all other organs, significantly smaller amounts of acutely injected B(12) accumulated in the kidneys in the vitamin-deprived state compared with both the normal and the vitamin-loaded condition.

CONCLUSIONS

The present study suggests a significant role for the rodent kidney in vitamin B(12) metabolism. We propose a model for rat tissue uptake consistent with the presence of two different TC-B(12) receptors and renal uptake following filtration of TC-B(12) in the glomeruli. The presented model allows for the reduced renal uptake and accumulation in vitamin-deprived conditions, thus reserving the vitamin for other tissues, including nerve tissue and bone marrow, which are more sensitive to vitamin B(12) deficiency.

Pathophysiology of protein and vitamin handling in the proximal tubule

The two membrane receptors megalin and cubilin are highly expressed in the endocytic pathway of the renal proximal tubule. Numerous ligands have been identified, and these receptors appear to be the main players responsible for the tubular clearance of proteins filtered in the glomeruli. Cubilin does not have an endocytosis signalling sequence and, since it binds to megalin, it appears that megalin in addition to internalizing its own ligands is also responsible for internalization of cubilin and its ligands. The importance of the receptors is underscored by the proteinuria observed in megalin-deficient mice and in dogs lacking functional cubilin.

The tandem endocytic receptors megalin and cubilin are important proteins in renal pathology

The molecular mechanisms controlling proximal tubule reabsorption of proteins have been much elucidated in recent years. Megalin and cubilin constitute two important endocytic receptor proteins involved in this process. Although structurally very different the two receptor proteins interact to mediate the reabsorption of a large number of filtered proteins, including carrier proteins important for transport and cellular uptake of several vitamins, lipids and other nutrients. Dysfunction of either protein results in tubular proteinuria and is associated with specific changes in vitamin metabolism due to the defective proximal tubular reabsorption of carrier proteins. Additional focus on the two receptors is attracted by the possible pathogenic role of excessive tubular protein uptake during conditions of increased filtration of proteins, and by recent findings implicating members of the low density lipoprotein-receptor family, which includes megalin, in the transduction of signals by association with cytoplasmic proteins.

Megalin and cubilin: multifunctional endocytic receptors

The ability to take up substances from the surrounding environment not only provides cells with vital nutrients, but also enables the selective transport of substances from one compartment to another. Megalin and cubilin are two structurally different endocytic receptors that interact to serve such functions. Evidence has accumulated in recent years to indicate that these receptors have important functions in both normal physiology and pathology.

Megalin deficiency offers protection from renal aminoglycoside accumulation

Aminoglycosides are antibiotics commonly used to treat life-threatening Gram-negative bacterial infections. However, their use is hampered by their severe nephrotoxicity due to accumulation in renal proximal tubules. Several pathways have been implicated in the renal uptake of aminoglycosides including megalin, an endocytic receptor in proximal tubular cells. Here, we have used mouse models with genetic or functional megalin deficiency to explore the contribution of megalin and other pathways to renal aminoglycoside uptake in vivo. We demonstrate that the uptake of aminoglycosides into the kidney directly correlates with renal megalin activity and is completely eliminated in mice lacking the receptor. Thus, our studies provide unequivocal evidence that megalin is the only major pathway responsible for renal aminoglycoside accumulation and that the receptor represents a unique drug target to prevent aminoglycoside-induced nephrotoxicity in patients.

Cubilin dysfunction causes abnormal metabolism of the steroid hormone 25(OH) vitamin D(3)

Steroid hormones are central regulators of a variety of biological processes. According to the free hormone hypothesis, steroids enter target cells by passive diffusion. However, recently we demonstrated that 25(OH) vitamin D(3) complexed to its plasma carrier, the vitamin D-binding protein, enters renal proximal tubules by receptor-mediated endocytosis. Knockout mice lacking the endocytic receptor megalin lose 25(OH) vitamin D(3) in the urine and develop bone disease. Here, we report that cubilin, a membrane-associated protein colocalizing with megalin, facilitates the endocytic process by sequestering steroid-carrier complexes on the cellular surface before megalin-mediated internalization of the cubilin-bound ligand. Dogs with an inherited disorder affecting cubilin biosynthesis exhibit abnormal vitamin D metabolism. Similarly, human patients with mutations causing cubilin dysfunction exhibit urinary excretion of 25(OH) vitamin D(3). This observation identifies spontaneous mutations in an endocytic receptor pathway affecting cellular uptake and metabolism of a steroid hormone.

Megalin-dependent cubilin-mediated endocytosis is a major pathway for the apical uptake of transferrin in polarized epithelia

Cubilin is a 460-kDa protein functioning as an endocytic receptor for intrinsic factor vitamin B(12) complex in the intestine and as a receptor for apolipoprotein A1 and albumin reabsorption in the kidney proximal tubules and the yolk sac. In the present study, we report the identification of cubilin as a novel transferrin (Tf) receptor involved in catabolism of Tf. Consistent with a cubilin-mediated endocytosis of Tf in the kidney, lysosomes of human, dog, and mouse renal proximal tubules strongly accumulate Tf, whereas no Tf is detectable in the endocytic apparatus of the renal tubule epithelium of dogs with deficient surface expression of cubilin. As a consequence, these dogs excrete increased amounts of Tf in the urine. Mice with deficient synthesis of megalin, the putative coreceptor colocalizing with cubilin, also excrete high amounts of Tf and fail to internalize Tf in their proximal tubules. However, in contrast to the dogs with the defective cubilin expression, the megalin-deficient mice accumulate Tf on the luminal cubilin-expressing surface of the proximal tubule epithelium. This observation indicates that megalin deficiency causes failure in internalization of the cubilin-ligand complex. The megalin-dependent, cubilin-mediated endocytosis of Tf and the potential of the receptors thereby to facilitate iron uptake were further confirmed by analyzing the uptake of (125)I- and (59)Fe-labeled Tf in cultured yolk sac cells.

Transcellular Transport of Vitamin B12in LLC-PK1 Renal Proximal Tubule Cells

Abstract. The transcobalamin-vitamin B12complex is responsible for the transport of B12from plasma and into the tissues. The complex is filtered in the renal glomeruli and is a high-affinity ligand for the endocytic receptor megalin expressed in the proximal tubule. This study shows by the use of the proximal tubule LLC-PK1 cell line that transcobalamin-B12is internalized by megalin-mediated endocytosis. After endocytosis and accumulation in endosomes, transcobalamin is degraded and the B12molecule is released from the cells in complex with newly synthesized proteins. The release is polarized in such a way that vitamin in the apical medium is bound to proteins with the size of haptocorrin, whereas the B12released at the basolateral side is complexed to two different proteins with the sizes of transcobalamin and haptocorrin. Furthermore, transcobalamin mRNA was identified by reverse transcription-PCR in LLC-PK1 cells and human and pig kidney, whereas haptocorrin mRNA was identified only in LLC-PK1 cells. The results strongly suggest that megalin located in the proximal tubule cells is important for receptor-mediated tubular reabsorption followed by transcellular transport and release of vitamin B12complexed to newly synthesized carrier proteins. This mechanism is likely to play a significant role in the maintenance of B12homeostasis by returning filtered B12to the pool of circulating vitamin.

A two-receptor pathway for catabolism of Clara cell secretory protein in the kidney

Clara cell secretory protein (CCSP) is a transport protein for lipophilic substances in bronchio-alveolar fluid, plasma, and uterine secretion. It acts as a carrier for steroid hormones and polychlorinated biphenyl metabolites. Previously, the existence of receptors for uptake of CCSP.ligand complexes into the renal proximal tubules had been suggested. Using surface plasmon resonance analysis, we demonstrate that CCSP binds to cubilin, a peripheral membrane protein on the surface of proximal tubular cells. Binding to cubilin results in uptake and lysosomal degradation of CCSP in cultured cells. Surprisingly, internalization of CCSP is blocked not only by cubilin antagonists but also by antibodies directed against megalin, an endocytic receptor that does not bind CCSP but associates with cubilin. Consistent with a role of both receptors in renal uptake of CCSP in vivo, patients deficient for cubilin or mice lacking megalin exhibit a defect in tubular uptake of the protein and excrete CCSP into the urine. These findings identify a cellular pathway consisting of a CCSP-binding protein (cubilin) and an endocytic coreceptor (megalin) responsible for tissue-specific uptake of CCSP and associated ligands.

Megalin and cubilin: synergistic endocytic receptors in renal proximal tubule

The multiligand, endocytic receptors megalin and cubilin are colocalized in the renal proximal tubule. They are heavily expressed in the apical endocytic apparatus. Megalin is a 600-kDa transmembrane protein belonging to the low-density lipoprotein-receptor family. The cytoplasmic tail contains three NPXY motifs that mediate the clustering in coated pits and are possibly involved in signaling functions. Cubilin, also known as the intestinal intrinsic factor-cobalamin receptor, is a 460-kDa receptor with no transmembrane domain and no known signal for endocytosis. Because the two receptors bind each other with high affinity and colocalize in several tissues, it is highly conceivable that megalin mediates internalization of cubilin and its ligands. Both receptors are important for normal tubular reabsorption of proteins, including albumin. Among the proteins normally filtered in the glomeruli, cubilin has been shown to bind albumin, immunoglobulin light chains, and apolipoprotein A-I. The variety of filtered ligands identified for megalin include vitamin-binding proteins, hormones, enzymes, apolipoprotein H, albumin, and beta(2)- and alpha(1)-microglobulin. Loss of these proteins and vitamins in the urine of megalin-deficient mice illustrates the physiological importance of this receptor.