Reabsorption of horseradish peroxidase by proximal tubules in rats with Heymann nephritis

Rituximab for Idiopathic Membranous Nephropathy: Who Can Benefit?

Rituximab effectively reduces proteinuria in patients with idiopathic membranous nephropathy (IMN), but response to treatment may vary from patient to patient. The association between baseline clinical, laboratory, and histology covariates and proteinuria reduction was evaluated retrospectively by multiple linear regression analysis at 3 mo after rituximab therapy in 14 patients with IMN with proteinuria > 3.5 g/24 h while on angiotensin-converting enzyme inhibition for at least 6 mo and no previous remissions. The association strength was expressed by standardized beta coefficients (SbetaC). Glomerular (SbetaC = 0.48, P = 0.049) and tubulointerstitial (TI) scores (SbetaC = 0.61, P = 0.003) predicted the outcome. Among glomerular and TI score components, tubular atrophy (SbetaC = 0.59, P = 0.003) and interstitial fibrosis (SbetaC = 0.60, P = 0.001) were significantly associated with 3-mo proteinuria. Urinary protein excretion decreased from 9.1 +/- 4.0 to 4.6 +/- 3.5 g/24 h (P < 0.001) in eight patients with TI score 1.7 but did not change in six with a score > or = 1.7. Nine additional patients with IMN then were allocated prospectively to rituximab treatment on the basis of a TI score < 1.7. Three-month proteinuria decreased in all patients from 8.9 +/- 5.3 to 4.9 +/- 3.9 g/24 h (P < 0.001) and serum albumin increased from 2.2 +/- 0.6 to 2.8 +/- 0.5 mg/dl (P < 0.01). Changes in serum albumin and cholesterol were inversely correlated (P < 0.02, r = -0.44). Rituximab achieved CD20 and CD19 depletion in all patients. In patients with IMN and nephrotic proteinuria despite angiotensin-converting enzyme inhibition therapy, renal biopsy findings may help in predicting response to rituximab and defining selection criteria for randomized trials that aim to assess the risk/benefit profile of B cell target therapy as compared with aspecific immunosuppressants and/or conservative therapy alone.

Membrane receptors for endocytosis in the renal proximal tubule

The renal proximal tubule exhibits a very extensive apical endocytic apparatus consisting of an elaborate network of coated pits and small coated and noncoated endosomes. In addition, the cells contain a large number of late endosomes/prelysosomes, lysosomes, and so-called dense apical tubules involved in receptor recycling from the endosomes to the apical plasma membrane. This endocytic apparatus is involved in the reabsorption of molecules filtered in the glomeruli. The process is very effective as demonstrated by the fact that although several grams of protein are filtered daily in the human glomeruli, human urine is virtually devoid of proteins under physiological conditions. Several key receptors appear to be involved in this function, which serves not only to conserve protein as such for the organism but also to reabsorb vital substances such as different vitamins in complex with their binding proteins. Recent research has established megalin, a 600-kDa protein belonging to the LDL receptor family, as probably the most important receptor in this process in the proximal tubule mediating endocytosis of a large variety of ligands and therefore classifying it as a scavenger receptor. More specific receptors like the folate receptor, IGF-II/Man-6-P receptor, and gp280/IFR, identical to the intrinsic factor receptor, are also functioning in the apical endocytic pathway of renal proximal tubules. A better understanding of these receptors will give us new insight into these very important processes for the organism.

Persistent proximal tubule dysfunction late in Heymann nephritis

To determine whether proximal tubule function returned to normal after cessation of active immunological injury in Heymann nephritis, we compared kidney function in an acute stage of the disease, when antibodies were being deposited on the brush border, to a later, chronic stage. Renal blood flow measurements via a flow probe, along with clearance and micropuncture techniques, were used to measure renal plasma flow, glomerular filtration rate, protein and albumin excretion, organic ion (PAH and TEA) extraction and tubule fluid inulin concentration. Proximal tubule fluid reabsorption, which was depressed in the acute stage of injury, returned to normal in chronic Heymann nephritis, but both PAH and TEA extraction continued to be depressed. PAH extraction was also decreased in isolated perfused kidneys from rats with Heymann nephritis. A three fold increase in PAH content of these perfused kidneys indicated that there was a defect in luminal PAH transport. Reconstitution of the proximal tubule brush border in chronic Heymann nephritis was not accompanied by functional recovery of secretory processes.

Studies on cell proliferation and tracer localization in the kidneys of guinea pigs with experimental autoimmune anti-tubular basement membrane nephritis

The mitotic activity in kidneys of guinea pigs with experimental autoimmune anti-tubular basement membrane (TBM) nephritis was investigated using autoradiographic techniques to determine the uptake of [3H]thymidine by actively dividing cells. It was observed in these animals that cells of proximal tubules, distal tubules, cortical and medullary interstitium, medullary collecting ducts, and loops of Henle took up significantly greater amounts of [3H]thymidine when compared with normal animals. In addition, the behaviour of horseradish peroxidase (HRP) and goat anti-HRP IgG in extraglomerular sites in the kidneys of these animals was studied. Contrary to what was expected, these tracers appeared to be less concentrated in the tubules and interstitium of animals with anti-TBM disease, with tracer movement restricted in areas of disrupted TBM. The significance of these observations is discussed.