Background and objectives: Calcium oxalate kidney stones affect almost 10% of the population nowadays. We described a dramatic increase in the proportion of calcium oxalate stones nucleated on Randall’s plaque over the past decades. Randall’s plaques are calcium phosphate (apatite) deposits which may form at the tip of renal papillae and promote nucleation of calcium oxalate crystals at the surface of the papilla. Incipient plaques appear in the interstitial tissue. To date, there is no murine model of Randall’s plaque. Abcc6 deficient mice develop soft tissues calcifications with age, especially in the kidney and recent studies suggest that Abcc6 may promote pyrophosphate production and thereby protect against tissue calcifications. We hypothesized that Abcc6-/- mice would develop nephrocalcinosis or Randall’s plaques.
Methods: We analyzed the topography and the composition of kidney calcifications in Abcc6-/-, Abcc6+/-, and Abcc6+/+ control mice aged from 11 to 131 weeks (generated by Gorgels TG and Bergen AA). The topography of the microcalcifications has been determined by Von Kossa staining and scanning electron microscopy and the crystalline phases present in the kidney have been characterized by µ-Fourier transform infrared imaging (µ-FTIR). The precise topography of incipient plaques has been assessed by immunostaining of the various segments of the nephron coupled to Von Kossa. In addition, pyrophosphate level has been assessed in both urine and blood of Abcc6-/- and Abcc6+/+ mice.
Results and Discussion: Only Abcc6-/- mice develop renal calcifications with age, especially after 6 months. Almost no calcifications were observed around vessels, in the cortex or in the medulla, with the exception of the tip of the papilla. Calcifications start at the most distal part of the papilla and remain close to the deepest part of nephrons. Two types of calcifications were observed: tubular plugs and interstitial round shaped structures surrounding tubular structures made of apatite and amorphous carbonated calcium phosphate. This phenotype mimics human Randall’s plaque and differs from nephrocalcinosis. Abcc6-/- mice have low levels of pyrophosphate in blood and urine, suggesting that pyrophosphate deficiency may be the promoter of plaque formation.
Conclusion: Abcc6-/- mouse is a murine model of Randall’s plaque, highlighting the potential role of pyrophosphate deficiency in the events preceding the onset of kidney stone disease.