Diabetic albuminuria is due to a small fraction of nephrons distinguished by albumin-stained tubules and glomerular adhesions

Abstract

OVE26 diabetic mice develop severe albuminuria. Immunohistochemical analysis revealed a pattern of intense albumin staining in a small subset of OVE26 tubules. Immunostaining was strikingly heterogeneous; some tubules stained intensely for albumin, but most tubules had weak or no staining. Serial sectioning showed that staining patterns were distinctive for each nephron. Electron microscopy revealed that albumin accumulated in villi and at the base of the brush border. Tubule cell injury, as shown by loss of villi, tubule dilation, and cellular protrusions into the tubule lumen, was unambiguously associated with albumin staining. Examination of albumin staining of proteinuric human kidneys also showed a heterogeneous pattern of staining. Analysis of OVE26 serial sections indicated that all glomeruli connected to albumin-positive tubules were identified by albumin-stained lesions in the tuft that adhered to Bowman's capsule, implicating this as a critical feature of heavy albumin leakage. These results indicate that albumin accumulation provides a marker of damaged nephrons, and confirm that albumin leakage produces significant tubular damage. This study shows that that formation of sclerotic glomerular adhesions is a critical step leading to severe albuminuria.

Figure 1

Albumin accumulates in tubule clusters of severely albuminuric OVE26 mice. Albumin immunohistochemistry was performed on kidney sections from OVE26 mice with albuminuria over 10 mg/day (Hi OVE), less than 4 mg/day (Low OVE), or non-diabetic FVB mice (FVB). A–C: Typical low power (original magnification, ×4X) images from Hi OVE, Low OVE, and FVB mice. Examples of some tubule clusters have been circled in (A) and all tubule clusters are circled in (B). D: Average daily urine albumin for the mice used in this analysis. E and F: Semiquantitative comparisons for the volume of albumin staining (area multiplied by intensity of staining) and the number of stained clusters normalized to renal cortical area. Asterisks in D–F indicate that all groups are significantly different from one another (P < 0.05) by one-way analysis of variance on ranks (n = 4 mice per group). Vertical bars are the SE.

Figure 2

Consistency of albumin staining in serial sections. A–D: Albumin staining in a series of four adjacent 3-μm sections. The same tubules stain in each section and they display similar morphology and the same pattern of staining in each section. The red arrows point to the same dilated tubule with mostly brush border staining. The black arrows point to the same dilated tubule with brush border and cytoplasmic staining. The white arrow points to the same non-dilated tubule with only cytoplasmic staining.

Figure 3

Diabetic models with low levels of albuminuria have weak tubular albumin staining. A: Typical staining from an OVE26Nmt kidney at 5 months of age with albuminuria of 0.3 mg per day and blood glucose over 600 mg/dl. B: Typical staining in a db/db diabetic mouse on the C57BLKS background. Despite blood glucose values over 550 mg/dl tubule staining was weak. C: Strong albumin staining of one OVE26Nmt mouse at 7 months of age with high albuminuria (11 mg per day). Observations in (A) and (B) were from three OVE26Nmt mice and three db/db mice between 4 and 5 months of age.

Figure 4

Albumin staining is in a subset of proximal tubules of OVE26 mice. A–C: Serial or near serial sections stained for albumin (A), the proximal tubule marker LTA (B), and the distal tubule marker peanut agglutinin (C). All albumin staining appears to be in LTA-stained tubules, but not all LTA-stained tubules stain for albumin.

Figure 5

Trichrome and albumin staining of tubules in OVE26 serial sections. A and B: ×200 images of albumin stained tubules that are dilated, thin-walled, and contain casts. C and D: ×1000 images of the tubules indicated by arrows in (A) and (B). E and F: Two albumin-stained tubules (marked by arrows) that appear frayed or mottled, as compared with surrounding tubules. In trichrome staining these tubules are brighter red than surrounding tubules. G and H: ×1000 images of the lower right tubule.

Figure 6

Albumin-stained OVE26 tubules also stain for IgG and C3. OVE26 (A, C, E, G) and FVB (B, D, F, H) stained tubules were double stained for albumin and mouse IgG (A–F) or for albumin and mouse complement component C3 (G–H) as described in Materials and Methods. Images are typical of examples from at least three mice of each type. The antibody and the type of mouse are indicated on each panel. Side by side panels show double staining of the section. Original magnification, ×400.

Figure 7

Electron microscopy reveals albumin-associated tubule damage. A: FVB tubules did not have positive albumin staining and they had a dense brush border surrounding the lumen (original magnification, ×3800, scale bar = 2 μm). B: In most OVE26 tubules there was no positive albumin staining and the brush border, and lumen appear normal (original magnification, ×1900, scale bar = 5 μm). C: In many OVE26 tubules the brush border stained strongly for albumin (original magnification, ×3800, scale bar = 2 μm). In these positively stained tubules the brush border density is reduced or absent. D: In many positively stained tubules, the cytoplasm protrudes into the tubule lumen (original magnification, ×2750, scale bar = 2 μm). E: Tubular protrusions are released into the tubular lumen (original magnification, ×2750, scale bar = 2 μm). F: Thin-walled ragged tubules sit adjacent to positively stained proximal tubules (original magnification, ×1475, scale bar = 5 μm).

Figure 8

Tubules of human proteinuric patients stain with human albumin antibody. Sections are from a chronic diabetic nephropathy patient (A), two nephrotic patients (B and C), and a non-proteinuric patient (D). Original magnification, ×200.

Figure 9

Features of glomeruli leading to albumin-stained tubules. A: Example of the method used to find glomeruli that lead to positively stained tubules. The asterisk in each panel marks the mapped tubule and its originating glomerulus (original magnification, ×400). B: Typical examples of glomeruli that have been connected to albumin-stained tubules by serial sectioning. An albumin-stained adhesion is evident in each glomerulus. The left most panel also shows a typical OVE26 control glomerulus that does not lead to a stained tubule nor contains an adhesion (original magnification, ×100). C: Quantitation of fully sectioned glomeruli that contain albumin-stained adhesions from 23 glomeruli leading to albumin-stained tubules (designated as Impaired OVE26) and 23 glomeruli that do not lead to stained tubules (designated as Control OVE26); *P < 0.01 by χ² test. D–F: Glomerular area, glomerular tuft area, and the area of Bowman’s space for the center section of each of the Impaired or Control OVE26 glomeruli (area is in μm², *P < 0.01 by student’s t-test). Results are from three severely albuminuric OVE26 mice.

Figure 10

Characterization of OVE26 glomeruli by trichrome staining. In a typical comparison (A and B), it is apparent that OVE26 glomeruli had more fibrous material and were much larger than FVB glomeruli (original magnification, ×400). C and D: Consecutive sections of an impaired OVE26 glomerulus stained for albumin and trichrome, respectively (original magnification, ×100). Albumin adhesions often co-localized with fibrous areas (indicated by arrows). In OVE26 sections the frequency of fibrotic glomeruli (E) and the frequency of glomeruli with nodules (F) were higher than in FVB glomeruli (*P < 0.03 by Mann-Whitney Rank Sum Test) and much more frequent than albumin adhesions.