Yuko Oyama

Evidence for megalin-mediated proximal tubular uptake of L-FABP, a carrier of potentially nephrotoxic molecules.

Liver-type fatty acid binding protein (L-FABP) binds with high affinity to hydrophobic molecules including free fatty acid, bile acid and bilirubin, which are potentially nephrotoxic, and is involved in their metabolism mainly in hepatocytes. L-FABP is released into the circulation, and patients with liver damage have an elevated plasma L-FABP level. L-FABP is also present in renal tubules; however, the precise localization of L-FABP and its potential role in the renal tubules are not known. In this study, we examined the cellular and subcellular localization of L-FABP in the rat kidney and tried to determine from where the L-FABP in kidney tissues had originated. Immunohistochemical studies of kidney sections localized L-FABP in the lysosomes of proximal tubule cells (PTC). In rats with carbon tetrachloride (CCl4)-induced acute liver injury, we detected high levels of L-FABP in the circulation and in the kidney compared with those in the control rat by immunoblotting, while reverse transcription-polymerase chain reaction showed that the level of L-FABP mRNA expression in the kidney of CCl4-treated rats was low and did not differ from that in the control rat. When 35S-L-FABP was intravenously administered to rats, the kidneys took up 35S-L-FABP more preferentially than the liver and heart, and histoautoradiography of kidney sections revealed that 35S-L-FABP was internalized via the apical domains of PTC. Quartz-crystal microbalance analysis revealed that L-FABP bound to megalin, a multiligand endocytotic receptor on PTC, in a Ca2+-dependent manner. Degradation assays using megalin-expressing rat yolk sac tumor-derived L2 cells demonstrated that megalin mediated the cellular uptake and catabolism of 125I-L-FABP. In conclusion, circulatory L-FABP was found to be filtered by glomeruli and internalized by PTC probably via megalin-mediated endocytosis. These results suggest a novel renal uptake pathway for L-FABP, a carrier of hydrophobic molecules, some of which may exert nephrotoxic effects.

Ultrastructural characteristics of diabetic nephropathy

Diabetic nephropathy is a major cause of chronic renal failure in Japan, and the prevalence rate has markedly increased during the past decade. Diabetic nephropathy shows various specific histological changes not only in glomeruli but also in the interstitial region. Nodular, diffuse, and exudative lesions, so-called diabetic glomerulosclerosis, are well known as glomerular lesions. At first, they were historically evaluated only by light microscopy, and thus which components of the glomeruli were modified was not sufficiently clear. Subsequent electron microscopic studies clarified that the expansion of the mesangial matrix was the true form of nodular and diffuse lesions, and that insudated serum substance was the real appearance of an exudative lesion. Interstitial lesions also exhibit specific features in diabetic nephropathy. In electron microscopic studies, it was proved that the size of mitochondria and thickness of the tubular basement membrane were increased in diabetic nephropathy. In this review, we introduce typical electron microscopic findings in diabetic nephropathy and recent opinions on the progression of diabetic nephropathy.

Bioengineered Implantation of Megalin-Expressing Cells: A Potential Intracorporeal Therapeutic Model for Uremic Toxin Protein Clearance in Renal Failure

Patients who have renal failure and are on dialysis therapy experience serious complications caused by low-molecular-weight uremic toxin proteins normally filtered by glomeruli and metabolized by proximal tubule cells (PTC). Dialysis-related amyloidosis is one such complication induced by systemic deposition of amyloid proteins derived from 12-kD beta(2)-microglobulin (beta(2)-m). Despite the use of high-flux membrane hemodialysis devices and direct absorbent columns, the removal of beta(2)-m is suboptimal, because the effects are transient and insufficient. Megalin is expressed in the apical membranes of PTC and recognized as a multiligand endocytic receptor that binds numerous low-molecular-weight proteins, including beta(2)-m. This study tested the feasibility of an intracorporeal therapeutic model of continuous beta(2)-m removal using megalin-expressing cell implantation. By cell association and degradation assays, rat yolk sac-derived L2 cells were identified to internalize and degrade beta(2)-m via megalin. The cells were effectively implanted within the subcutaneous tissues of nude mice using a type I collagen scaffold and a method inducing local angiogenesis. After nephrectomy and intraperitoneal injection with (125)I-beta(2)-m, it was found that the implanted cells took up the labeled ligand, efficiently removing it from the blood. Bioengineered implantation of megalin-expressing cells may represent a new supportive therapy for dialysis patients to compensate for the loss of renal protein metabolism and remove uremic toxin proteins.

Role of megalin, a proximal tubular endocytic receptor, in the pathogenesis of diabetic and metabolic syndrome-related nephropathies: protein metabolic overload hypothesis

SUMMARY:  Megalin is an endocytic receptor on the apical membranes of proximal tubule cells (PTC) in the kidney, and is involved in the reabsorption and metabolism of various proteins that have been filtered by glomeruli. Patients with diabetes, especially type 2 diabetes, or metabolic syndrome are likely to have elevated serum levels of advanced glycation end products, liver‐type fatty acid binding protein, angiotensin II, insulin and leptin, and renal metabolism of these proteins is potentially overloaded. Some of these proteins are themselves nephrotoxic, while others are carriers of nephrotoxic molecules. Megalin is involved in the proximal tubular uptake of these proteins. We hypothesize that megalin‐mediated metabolic overload in PTC leads to compensatory cellular hypertrophy and sustained Na+ reabsorption, causing systemic hypertension and glomerular hyperfiltration via tubuloglomerular feedback, and named this as ‘protein metabolic overload hypothesis’. Impaired metabolism of bioactive proteins such as angiotensin II and insulin in PTC may enhance hypertrophy of PTC and/or Na+ reabsorption. Sleep apnoea syndrome, a frequent complication of diabetes and metabolic syndrome, may cause renal hypoxia and result in relative overload of protein metabolism in the kidneys. The development of strategies to identify patients with diabetes or metabolic syndrome who are at high risk for renal metabolic overload would allow intensive treatment of these patients in an effort to prevent the development of nephropathy. Further studies on the intracellular molecular signalling associated with megalin‐mediated metabolic pathways may lead to the development of novel strategies for the treatment of nephropathies related to diabetes and metabolic syndrome.

Pretreatment plasma intact parathyroid hormone and serum calcium levels, but not serum phosphate levels, predict the response to maxacalcitol therapy in dialysis patients with secondary hyperparathyroidism

BackgroundThe treatment strategy for secondary hyperparathyroidism is generally determined empirically with regards to present parathyroid function and serum calcium (Ca) and inorganic phosphate (Pi) levels. More evidence is needed to avoid the aimless continuation of active vitamin D therapy.MethodsNondiabetic dialysis patients whose plasma intact parathyroid hormone (iPTH) levels were greater than 300 pg/ml were included in the study. Maxacalcitol was intravenously injected three times a week. The treatment was continued for 48 weeks, unless the iPTH level was reduced to less than 300 pg/ml or unfavorable events occurred. The patients whose plasma iPTH levels were below 300 pg/ml within 48 weeks were defined as those who had been successfully treated.ResultsFindings for 146 patients were analyzed, and 96 patients were successfully treated. Serum Pi levels did not significantly increase during the therapy. The pretreatment plasma iPTH levels and serum Ca levels were lower in the patients who were successfully treated with maxacalcitol. A logistic regression study and classifying by stratum analyses revealed that the pretreatment serum Ca levels and plasma iPTH levels were significantly related to the result of maxacalcitol therapy, while the serum Pi levels were not. Analyses using a receiver-operating characteristic curve revealed that the areas under curves obtained for iPTH and Ca were significantly greater than those obtained for Pi (P < 0.0001).ConclusionsSerum Ca levels and parathyroid function were correlated with the results of maxacalcitol therapy. Pretreatment serum Pi levels could not predict the result.

Posterior Reversible Encephalopathy Syndrome in a Patient with Severe Uremia without Hypertension.

A 28-year-old man was admitted to our hospital with nausea, headache and weakness of the left hand. He had severe uremia without hypertension due to recurrent/chronic pyelonephritis. Brain magnetic resonance imaging showed reversible vasogenic edema in the brainstem and bilateral frontal centrum semiovale. All of his neurological symptoms immediately improved after the introduction of hemodialysis. When a patient with uremia presents with neurological symptoms, posterior reversible encephalopathy syndrome should be considered in the differential diagnosis even if high blood pressure is not observed. Brain magnetic resonance imaging may be helpful in such a case, and an appropriate therapy could be subsequently initiated.

A Case of Transforming Growth Factor-β-Induced Gene-Related Oculorenal Syndrome: Granular Corneal Dystrophy Type II with a Unique Nephropathy

Many types of inherited renal diseases have ocular features that occasionally support a diagnosis. The following study describes an unusual example of a 40-year-old woman with granular corneal dystrophy type II complicated by renal involvement. These two conditions may coincidentally coexist; however, there are some reports that demonstrate an association between renal involvement and granular corneal dystrophy type II. Granular corneal dystrophy type II is caused by a mutation in the transforming growth factor-β-induced (TGFBI) gene. The patient was referred to us because of the presence of mild proteinuria without hematuria that was subsequently suggested to be granular corneal dystrophy type II. A kidney biopsy revealed various glomerular and tubular basement membrane changes and widening of the subendothelial space of the glomerular basement membrane by electron microscopy. However, next-generation sequencing revealed that she had no mutation in a gene that is known to be associated with monogenic kidney diseases. Conversely, real-time polymerase chain reaction, using a simple buccal swab, revealed TGFBI heteromutation (R124H). The TGFBI protein plays an important role in cell-collagen signaling interactions, including extracellular matrix proteins which compose the renal basement membrane. This mutation can present not only as corneal dystrophy but also as renal disease. TGFBI-related oculorenal syndrome may have been unrecognized. It is difficult to diagnose this condition without renal electron microscopic studies. To the best of our knowledge, this is the first detailed report of nephropathy associated with a TGFBI mutation.

Diverse Renal Phenotypes Observed in a Single Family with a Genetic Mutation in Paired Box Protein 2.

A common renal phenotype of paired box protein 2 (PAX2) mutations is renal coloboma syndrome. We report a single family with diverse renal phenotypes associated with PAX2 mutation. The proband presented steroid-resistant focal segmental glomerulosclerosis with optic coloboma, whereas his two sons showed severe renal hypoplasia with end-stage renal disease, with or without optic coloboma. In all three cases, a heterozygous PAX2 genetic mutation was identified (exon 2; NM_003987.3:c.76dupG, p.Val26Glyfs*28). Based on histopathological findings of the proband, we hypothesized that autophagic dysfunction was associated with the pathophysiology of the focal segmental glomerulosclerosis with PAX2 mutation. Detailed funduscopic examination - including the optic disc - might be useful for the diagnosis of renal anomalies associated with PAX2 mutation.

Ectopic ossification in the cranial dura mater in dialysis patients with secondary hyperparathyroidism

We report the case of a 69-year-old woman with secondary hyperparathyroidism who underwent maintenance haemodialysis therapy for 17 years and who presented with severe dural calcification and right subdural haematoma. Her dura mater displayed a rock barnacle-like appearance, and cerebral superficial arteries adhered to the sclerotic lesions. On the microscopic observation, calcified tissue with a clear lamellar structure and osteopontin immunoreactivity was observed. Tartrate-resistant acid phosphatase immunoreactive polynucleated giant cells infiltrated around the tissue. Such morphological properties are specific to the calcified tissue formed through a bone formation-like mechanism that is often observed in arterial media in patients with chronic kidney disease.

Emphysematous cystitis complicated by unilateral hydronephrosis

A 66-year-old woman, whose hemoglobin A1c level was 7.3%, with CKD stageG5/A3 due to diabetic nephropathy was admitted with chill and lower abdominal pain. Computed tomography (CT) imaging demonstrated a right hydronephrosis without ureteral stenosis (Fig. 1a, b) and distension of the urinary bladder with diffuse intramural gas (Fig. 1c, d). A Foley catheter was placed and intravenous antibiotics initiated. Subsequently, urine cultures and blood cultures were positive for Escherichia coli. A diagnosis of emphysematous cystitis (EC) with right hydronephrosis and sepsis was made, and she recovered well and was discharged 18 days after admission. EC is a rare gas-forming infection of the bladder wall, and potentially fatal [1]. EC may subsequently develop into pyelonephritis, including emphysematous pyelonephritis [1]; however, it is unusual to complicate by unilateral hydronephrosis [1, 2]. Even with unilateral hydronephrosis, EC should be considered for differential diagnosis, and it requires prompt treatment.