Rodent Renal Structure Differs among Species

Comparative Analysis of Hypertensive Tubulopathy in Animal Models of Hypertension and Its Relevance to Human Pathology

Assessment of hypertensive tubulopathy for more than fifty animal models of hypertension in experimental pathology employs criteria that do not correspond to lesional descriptors for tubular lesions in clinical pathology. We provide a critical appraisal of experimental hypertension with the same approach used to estimate hypertensive renal tubulopathy in humans. Four models with different pathogenesis of hypertension were analyzed-chronic angiotensin (Ang) II-infused and renin-overexpressing (TTRhRen) mice, spontaneously hypertensive (SHR), and Goldblatt two-kidney one-clip (2K1C) rats. Mouse models, SHR, and the nonclipped kidney in 2K1C rats had no regular signs of hypertensive tubulopathy. Histopathology in animals was mild and limited to variations in the volume density of tubular lumen and epithelium, interstitial space, and interstitial collagen. Affected kidneys in animals demonstrated lesion values that are significantly different compared with healthy controls but correspond to mild damage if compared with hypertensive humans. The most substantial human-like hypertensive tubulopathy was detected in the clipped kidney of 2K1C rats. For the first time, our study demonstrated the regular presence of chronic progressive nephropathy (CPN) in relatively young mice and rats with induced hypertension. Because CPN may confound the assessment of rodent models of hypertension, proliferative markers should be used to verify nonhypertensive tubulopathy.

Modeling normal and nephrotic axial uptake of albumin and other filtered proteins along the proximal tubule

KEY POINTS

We used new and published data to develop a mathematical model that predicts the profile of albumin uptake in the mouse proximal tubule (PT) in normal and nephrotic states, and partially accounts for competitive inhibition of uptake by normally filtered and pathologic ligands. Three pathways, consisting of high-affinity uptake by cubilin receptors, low-affinity uptake by megalin receptors, and fluid phase uptake, contribute to the overall retrieval of filtered proteins. The axial profile and efficiency of protein uptake depend on the initial filtrate composition and the individual protein affinities for megalin and cubilin. Under normal conditions, the majority of albumin is retrieved in S1 but shifts to S2 under nephrotic conditions. Other proteins exhibit different uptake profiles. Our model explains how tubular proteinuria can occur despite a large excess in potential PT uptake capacity.

ABSTRACT

Recent studies indicate that filtered albumin is retrieved in the proximal tubule (PT) via three pathways: receptor-mediated endocytosis via cubilin (high affinity) and megalin (low affinity), and fluid-phase uptake. Expression of megalin is required to maintain all three pathways, making it challenging to determine their respective contributions. Moreover, uptake of filtered molecules varies between the sub-segments (S1, S2, and S3) that make up the PT. Here we used new and published data to develop a mathematical model that predicts the rates of albumin uptake in mouse PT sub-segments in normal and nephrotic states, and partially accounts for competition by β2-microglobulin (β2m) and Immunoglobulin G (IgG). Our simulations indicate that receptor-mediated, rather than fluid-phase uptake, accounts for the vast majority of ligand recovery. Our model predicts that ∼75% of normally filtered albumin is reabsorbed via cubilin; however, megalin-mediated uptake predominates under nephrotic conditions. Our results also suggest that ∼80% of albumin is normally recovered in S1, whereas nephrotic conditions or knockout of cubilin shifts the bulk of albumin uptake to S2. The model predicts β2m and IgG axial recovery profiles qualitatively similar to those of albumin under normal conditions. In contrast with albumin however, the bulk of IgG and β2m uptake still occurs in S1 under nephrotic conditions. Overall, our model provides a kinetic rationale for why tubular proteinuria can occur even though a large excess in potential PT uptake capacity exists, and suggests testable predictions to expand our understanding of the recovery profile of filtered proteins along the PT. Abstract figure legend. Data from mouse models and from cultured proximal tubule (PT) cells were used to create a mathematical model that predicts the uptake profile of albumin and other filtered ligands along the mouse PT in normal and nephrotic states. The distinct contributions of cubilin receptors (magenta), megalin receptors (green), and fluid phase uptake (blue) to total albumin retrieval (black) in S1, S2, and S3 subsegments of the PT are delineated. Under normal conditions, albumin is primarily recovered in the S1 segment by cubilin, whereas the majority is retrieved in S2 under nephrotic conditions. Other proteins exhibit strikingly different uptake profiles. Our model explains how the distribution and capacity of high-affinity and low-affinity uptake pathways enable uptake of albumin over a broad range of filtered concentrations, and how tubular proteinuria can occur despite a large excess in potential PT uptake capacity. Created with BioRender.com. This article is protected by copyright. All rights reserved.

Female cotton rats (Sigmodon hispidus) develop chronic anemia with renal inflammation and cystic changes

The cotton rat (Sigmodon hispidus) is a laboratory rodent that has been used for studies on human infectious diseases. In the present study, we observed that female cotton rats, not the male cotton rats, developed chronic anemia characterized by reduced red blood cell, hemoglobin, and hematocrit levels from 5 to 9 months of age without any changes in the mean corpuscular hemoglobin and volume levels. In peripheral blood, the reticulocyte count did not increase in response to anemia in female cotton rats, and no extramedullary hematopoiesis was observed in the liver or spleen. Further, the serum levels of urea nitrogen and creatinine increased from 5 to 9 months of age in female cotton rats compared to male cotton rats, and these increases became more prominent from 10 months of age onward, indicating chronic kidney disease. Histopathologically, female cotton rats manifested tubulointerstitial lesions characterized by the infiltration of mononuclear cells, including plasma cells and CD3(+) T-cells, as well as the dilation of calbindin-D28k(+) distal tubules from 5 to 9 months of age. The severity of these lesions progressed from 10 months of age onward, and renal fibrotic features and numerous tubular cysts appeared without any obvious glomerular lesions. A significant decrease in the erythropoietin protein levels was observed in the kidney of aged female cotton rats, and significant correlations were detected between anemia and tubulointerstitial damage. These results suggest that aged female cotton rats chronically develop renal anemia, and this rodent may serve as a novel model to elucidate its pathogenesis.

In vitro generation of three-dimensional renal structures

End-stage renal disease is currently being treated effectively by transplantation. However, increasing demand and donor shortage make this treatment challenging. Recent advances in cell-based therapies have provided potential opportunities to alleviate the current challenges of donor shortage. In this study we developed a system to generate renal structures in vitro using primary kidney cells. This system involves the cultivation of expanded primary renal cells in a three-dimensional collagen-based culture system. After one week of growth, individual renal cells began to form renal structures resembling tubules and glomeruli. Histologically, these structures show phenotypic resemblance to native kidney structures. The reconstituted tubules stained positively for Tamm-Horsfall protein, which is expressed in the thick ascending limb of Henle's Loop and distal convoluted tubules. These results show that renal structures can be reconstituted in a three-dimensional culture system, which may eventually be used for renal cell therapy applications.

Species Specific Differences in the Ratio of Short to Long Loop Nephrons in the Kidneys of Laboratory Rodents

The ratio of short to long loop nephrons (SLNs and LLNs, respectively) in laboratory rodents (mice, rats, hamsters, gerbils, and guinea pigs) was investigated using the air cast method. In mice and rats, the percentage of SLNs was significantly higher than that of LLNs, while in hamsters and gerbils, the reverse was true (% of LLNs >% of SLNs). In guinea pigs, no significant difference in the percentages of LLNs and SLNs was noted.