Expression of osteogenic proteins in kidneys of cats with nephrocalcinosis

Background: Nephrocalcinosis is a common pathological finding in cats with chronic kidney disease and nephrolithiasis. Understanding its pathogenesis may identify future therapeutic targets.

Hypothesis: Nephrocalcinosis is associated with expression of an osteogenic phenotype.

Animals: Kidneys with medullary mineralization were obtained from 18 cats (10 with and 8 without nephroliths) undergoing necropsy.

Methods: Cross-sectional study. Microradiography and histopathology (modified von Kossa stain) were used to confirm parenchymal mineralization. Immunohistochemistry for 5 osteogenic markers was performed to determine their co-localization with nephrocalcinosis. The proportion of kidneys with stronger immunointensity in mineralized versus non-mineralized regions was analyzed using 1-tailed sign tests. The proportion of kidneys with co-localization of nephrocalcinosis and each marker was compared between kidneys with and without nephroliths using Fisher's exact tests.

Results: Nephrocalcinosis co-localized with osteopontin immunoreactivity in all 18 cats (100%) and with osteocalcin in 12 cats (67%). Both osteogenic markers had stronger immunointensity in mineralized regions compared with non-mineralized regions. Limited co-localization was observed with other markers: bone morphogenic protein-2 in 2 kidneys (both with nephroliths) and tissue non-specific alkaline phosphatase in 1 kidney (without nephroliths); runt-related transcription factor-2 was undetected. No statistically significant differences were found in the co-localization of nephrocalcinosis with osteogenic proteins between kidneys with and without nephroliths.

Conclusions and clinical importance: Expression of osteogenic proteins in areas of nephrocalcinosis indicates that nephrocalcinosis is associated with the development of an osteogenic phenotype. Targeting these processes could offer a novel approach to prevent nephrolithiasis at its origin.