Epithelium Lining Rat Renal Papilla: Nomenclature and Association with Chronic Progressive Nephropathy (CPN)

Reduced urine volume and changed renal sphingolipid metabolism in P2ry14-deficient mice

The UDP-glucose receptor P2RY14, a rhodopsin-like G protein-coupled receptor (GPCR), was previously described as receptor expressed in A-intercalated cells of the mouse kidney. Additionally, we found P2RY14 is abundantly expressed in mouse renal collecting duct principal cells of the papilla and epithelial cells lining the renal papilla. To better understand its physiological function in kidney, we took advantage of a P2ry14 reporter and gene-deficient (KO) mouse strain. Morphometric studies showed that the receptor function contributes to kidney morphology. KO mice had a broader cortex relative to the total kidney area than wild-type (WT) mice. In contrast, the area of the outer stripe of the outer medulla was larger in WT compared to KO mice. Transcriptome comparison of the papilla region of WT and KO mice revealed differences in the gene expression of extracellular matrix proteins (e.g., decorin, fibulin-1, fibulin-7) and proteins involved in sphingolipid metabolism (e.g., small subunit b of the serine palmitoyltransferase) and other related GPCRs (e.g., GPR171). Using mass spectrometry, changes in the sphingolipid composition (e.g., chain length) were detected in the renal papilla of KO mice. At the functional level, we found that KO mice had a reduced urine volume but an unchanged glomerular filtration rate under normal chow and salt diets. Our study revealed P2ry14 as a functionally important GPCR in collecting duct principal cells and cells lining the renal papilla and the possible involvement of P2ry14 in nephroprotection by regulation of decorin.

Methyl-tert-butyl ether (MTBE): integration of rat and mouse carcinogenicity data with mode of action and human and rodent bioassay dosimetry and toxicokinetics indicates MTBE is not a plausible human carcinogen

Methyl-tert-butyl ether (MTBE) is a fuel oxygenate used in non-United States geographies. Multiple health reviews conclude that MTBE is not a human-relevant carcinogen, and this review provides updated mode of action (MOA), exposure, dosimetry and risk perspectives supporting those conclusions. MTBE is non-genotoxic and has large margins of exposure between blood concentrations at the overall rat 400 ppm inhalation NOAEL and blood concentrations in typical workplace or general population exposures. Non-cancer and threshold cancer hazard quotients range from a high of 0.046 for fuel-pump gasoline station attendants and are 100-1,000-fold lower for general population exposures. Cancer risks conservatively assuming genotoxicity for these same scenarios are all less than 1 × 10^-6. The onset of MTBE nonlinear toxicokinetics (TK) in rats at inhalation exposures less than 3,000 ppm, a dose that is also not practically achievable in fuel-use scenarios, indicates that high-dose specific male rat kidney and testes (3,000 and 8,000 ppm) and female mouse liver tumors (8000 ppm) are not quantitatively relevant to humans. Mode of action analyses also indicate MTBE male rat kidney tumors, and lesser so female mouse liver tumors, are not qualitatively relevant to humans. Thus, an integrated analysis of the toxicology, exposure/dosimetry, TK, and MOA data indicates that MTBE presents minimal human cancer and non-cancer risks.

Confounders for kidney carcinogenesis in rodent cancer bioassays

In the long-term safety testing of chemicals for carcinogenicity the toxicologist needs to be aware of a number of scenarios where renal tubule tumors, or their precursors, arise that are not due to a carcinogenic action of the test article. Situations producing false positive results in the kidney include exacerbation of chronic progressive nephropathy (CPN) in rats, confusion of atypical tubule hyperplasia (the obligate precursor of renal tubule tumor) with foci of benign CPN-related renal tubule cell proliferation, inclusion of spontaneous tumor entities, such as the amphophilic-vacuolar tumor, in the test article tumor count, the possibility of a link between spontaneous forms of tubule dilatation and renal tubule tumor formation in mice, and the supposed predictivity of chemically-induced karyomegaly for renal carcinogenicity in both rats and mice. Examples of these misleading situations are described and discussed.

Congenital Unilateral Renal Aplasia in a Cynomolgus Monkey (Macaca fascicularis) With Investigation Into Potential Pathogenesis

We describe and characterize unilateral renal aplasia in a cynomolgus monkey (Macaca fascicularis) from a chronic toxicology study adding to the limited histopathology reports of congenital renal anomalies in macaques. In the current case, the affected kidney was macroscopically small and characterized microscopically by a thin cortex with an underdeveloped medulla and an absent papilla. The remnant medulla lacked a corticomedullary junction and contained only a few irregular collecting duct-like structures. The cortex had extensive interstitial mature collagen deposition with fibromuscular collar formation around Bowman's capsules. Due to parenchymal collapse, mature glomeruli were condensed together with occasional atrophic and sclerotic glomeruli. The majority of the cortical tubules were poorly differentiated with only small islands of fully developed cortical tubules present. Histochemical and immunohistochemical stains were utilized to demonstrate key diagnostic features of this congenital defect, to assist with differentiating it from renal dysplasia, and to provide potential mechanistic pathways. Immunostaining (S100, paired box gene 2 [PAX2], aquaporins) of the medulla was compatible with incomplete maturation associated with aplasia, while the immunostaining profile for the cortex (vimentin, calbindin, PAX2-positive cortical tubules, and smooth muscle actin-positive fibromuscular collars) was most compatible with dedifferentiation secondary to degenerative changes.

Identification and classification of epithelial cells in nephron segments by actin cytoskeleton patterns

The basic functional unit in a kidney is the nephron, which is a long and morphologically segmented tubule. The nephron begins with a cluster of capillaries called glomerulus through which the blood is filtered into the Bowman's space. The filtrate flows through the nephron segments. During this flow, electrolytes and solutes are reabsorbed by channels and transport systems into the capillaries wrapped around the nephron. Many questions related to renal function focus on identifying the sites of expression of these systems. In this study, we mapped whole kidney sections by confocal microscopic imaging of fluorescent phalloidin, which binds to actin filaments. In tile scans (composed of hundreds of images) of these sections, the cortex and the medullary regions (outer and inner stripes of the outer medulla, and inner medulla) could be easily identified by their cytoskeletal patterns. At a higher resolution, we identified distinct features of the actin cytoskeleton in the apical, basal, and lateral borders of the cells. These features could be used to identify segments of a nephron (the proximal tubule, thin and thick segments of Henle's loop, and distal tubule), the collecting duct system, the papillary ducts in the papilla, and the urothelium that covers the pelvis. To verify our findings, we used additional markers, including aquaporin isoforms, cytokeratin 8‐18, and WGA lectin. This study highlights the power of high‐resolution confocal microscopy for identifying specific cell types using the simple probe of F‐actin‐binding phalloidin.

Intrarenal Renin–Angiotensin System Involvement in the Pathogenesis of Chronic Progressive Nephropathy—Bridging the Informational Gap Between Disciplines

Chronic progressive nephropathy (CPN) is the most commonly encountered spontaneous background finding in laboratory rodents. Various theories on its pathogenesis have been proposed, but there is a paucity of data regarding specific mechanisms or physiologic pathways involved in early CPN development. The current CPN mechanism of action for tumorigenesis is largely based on its associated increase in tubular cell proliferation without regard to preceding subcellular degenerative changes. Combing through the published literature from multiple biology disciplines provided insight into the preceding cellular events. Mechanistic pathways involved in the progressive age-related decline in rodent kidney function and several key inflexion points have been identified. These critical pathway factors were then connected using data from renal models from multiple rodent strains, other species, and mechanistic work in humans to form a cohesive picture of pathways and protein interactions. Abundant data linked similar renal pathologies to local events involving hypoxia (hypoxia-inducible factor 1α), altered intrarenal renin–angiotensin system (RAS), oxidative stress (nitric oxide), and pro-inflammatory pathways (transforming growth factor β), with positive feedback loops and downstream effectors amplifying the injury and promoting scarring. Intrarenal RAS alterations seem to be central to all these events and may be critical to CPN development and progression.

Renal Papillary Rarefaction: An Artifact Mimicking Papillary Necrosis

In histopathology, the presence of a tissue change that does not represent the tissue's normal appearance can often lead to an incorrect diagnosis and interpretation. These changes are collectively known as "artifacts" resulting from postmortem autolysis, improper fixation, problems with tissue handling or slide preparation procedures. Most tissue artifacts are obvious, yet some artifacts may be subtle, occur in relatively well-fixed tissue, and demand careful observation to avoid confusion with real biological lesions. The kidney often contains artifacts that may be observed throughout all regions of the renal parenchyma. Cortical tubule artifacts present the greatest challenge when discerning an artifact versus an induced lesion following exposure to a xenobiotic. However, confounding artifacts observed at the tip of the renal papilla may also be problematic for the pathologist. An uncommon artifact involving tinctorial alteration and rarefaction affecting the papillary tip of the rat kidney is described here and differentiated from treatment induced lesions of renal papillary necrosis.

Histopathology re-examination of the NTP toxicity/carcinogenicity studies of tert-butyl alcohol to identify renal tumor and toxicity modes of action

Tert-butyl alcohol (TBA) targets the rat kidney following repeated exposures, including renal tubule tumors. The mode of action (MOA) of these tumors, concluded by a pathology working group, involves both alpha2u-globulin nephropathy (α2u-gN) and exacerbated chronic progressive nephropathy (CPN), but has been disputed and an undefined MOA proposed. This study further reviews the histology slides of male and female rat kidneys from the NTP drinking water 13-week toxicity and 2-year carcinogenicity studies, including the 15-month interim sacrifice group. The papillary epithelial lining alteration formerly referred to as "transitional cell hyperplasia" develops as part of advanced CPN and does not represent a separate toxicity. No changes were observed in the kidney pelvis urothelium. The only alterations in subchronic male rats involved α2u-gN and CPN, without test article-related alterations in females. Focused examination of areas of parenchyma unaffected by CPN in TBA-treated male and female rats of the chronic studies revealed no renal tubule abnormalities other than from the effects of α2u-gN and CPN. Unrelated to toxicity were spontaneous amphophilic or vacuolar tubule proliferative lesions. All observed TBA-associated non-neoplastic and neoplastic histopathological changes in the kidney can be explained by α2u-gN or enhanced CPN, neither of which are relevant to humans.

Brief Synopsis: Review of Renal Tubule and Interstitial Anatomy and Physiology and Renal INHAND, SEND, and DIKI Nomenclature

This article provides a synopsis of the first two presentations from the second scientific session of the 37th Annual Symposium of the Society of Toxicologic Pathology in Indianapolis, Indiana, on June 18, 2018; the session focused on acute kidney injury. The first presentation, given by Dr. Kevin McDorman, focused on "Fundamentals of Renal Tubule and Interstitial Anatomy and Physiology." Several common background findings from toxicity studies were additionally discussed. Lastly, factors that impact the relevance and usefulness of historical control data, such as quality and consistency of histopathology, were discussed. The second presentation, given by Dr. Torrie Crabbs, provided a review of International Harmonization of Nomenclature and Diagnostic Criteria (INHAND), Standard for Exchange of Nonclinical Data (SEND), and drug-induced kidney injury (DIKI) nomenclature. INHAND is a global collaborative project that provides internationally accepted standardized nomenclature and diagnostic criteria for proliferative and nonproliferative changes in laboratory animals in toxicity and carcinogenicity studies. SEND is currently a required standard for data submission to the Food and Drug Administration (FDA). Since the FDA has indicated its preference for INHAND nomenclature, SEND will predominately use INHAND terminology; thus, familiarity with INHAND terminology is critical for toxicologic pathologists. The diagnostic features of three common DIKI findings, in addition to several complicated INHAND terminologies, were reviewed.