Osteogenic changes in kidneys of hyperoxaluric rats

Abstract

Many calcium oxalate (CaOx) kidney stones develop attached to renal papillary subepithelial deposits of calcium phosphate (CaP), called Randall’s plaque (RP). Pathogenesis of the plaques is not fully understood. We hypothesize that abnormal urinary environment in stone forming kidneys leads to epithelial cells losing their identity and becoming osteogenic. To test our hypothesis male rats were made hyperoxaluric by administration of hydroxy-l-proline (HLP). After 28 days, rat kidneys were extracted. We performed genome wide analyses of differentially expressed genes and determined changes consistent with dedifferentiation of epithelial cells into osteogenic phenotype. Selected molecules were further analyzed using quantitative-PCR and immunohistochemistry. Genes for runt related transcription factors (RUNX1 and 2), zinc finger protein Osterix, bone morphogenetic proteins (BMP2 and 7), bone morphogenetic protein receptor(BMPR2), collagen, osteocalcin, osteonectin, osteopontin (OPN), matrix-gla-protein (MGP), osteoprotegrin (OPG), cadherins, fibronectin (FN) and vimentin (VIM) were up regulated while those for alkaline phosphatase (ALP) and cytokeratins 10 and 18 were down regulated. In conclusion, epithelial cells of hyperoxaluric kidneys acquire a number of osteoblastic features but without CaP deposition, perhaps a result of down regulation of ALP and up regulation of OPN and MGP. Plaque formation may additionally require localized increases in calcium and phosphate and decrease in mineralization inhibitory potential.

Kidney stones

“…Indeed, Madin–Darby canine kidney cells produce CaP microliths on the basal side when grown in monolayers 161,162 , and exposure of these cells to high oxalate levels, as well as CaOx and CaP crystals, leads to the activation of NADPH oxidase and the production of reactive oxygen species 60,163–167 , resulting in an osteogenic phenotype. Evidence has shown that genes considered to be involved in epithelial transformation and bone morphogenesis — including those that encode RUNX2, osterix, BMP2, BMP7, BMP receptor type 2, collagen, osteocalcin, osteonectin, osteopontin, matrix-gla-protein, osteoprotegrin, cadherin, fibronectin and vimentin — are upregulated in hyperoxaluric rats 168 , all of which are markers of the osteogenic phenotype. Overall, some evidence supports that the abnormal urinary conditions of hyperoxaluria, hypercalciuria, hypocitraturia and renal oxidative stress trigger the transformation of renal epithelial cells into osteoblastic phenotypes.…”

“…Another possibility is that renal epithelial cells, when exposed to CaP and/or CaOx, produce reactive oxygen species and, probably, a range of factors associated with osteogenesis, such as Runt-related transcription factor 2 (RUNX2), osterix (also known as Sp7), bone morphogenetic protein 2 (BMP2), BMP7, BMP receptor type 2 (BMPR2), collagen and osteopontin. Epithelial cells produce matrix vesicles on the basal side (step 5) followed by their calcification (step 6; REFS 88,168). b | Once CaP crystals have been deposited in the basement membrane of the loop of Henle and/or collecting ducts, mineralization continues.…”

Role of Osteogenesis in the Formation of Randall's Plaques

“…Selected molecules were further analyzed using quantitative-PCR and immunohistochemistry. Genes for runt related transcription factors (RUNX1 and 2), zinc finger protein Osterix, bone morphogenetic proteins (BMP2 and 7), bone morphogenetic protein receptor (BMPR2), collagen, osteocalcin, osteonectin, OPN, matrix-gla-protein (MGP), osteoprotegrin (OPG), cadherins, fibronectin (FN), and VIM were up regulated while those for ALP and cytokeratins 10 and 18 were down regulated in kidneys of hyperoxaluric rats (Khan et al, 2014b;Joshi et al, 2015). All elements of the kidney including renal interstitium and most renal tubules showed vimentin staining, with the strongest expressions being in the renal tubules that contained calcium oxalate crystals.…”

Histological aspects of the “fixed-particle” model of stone formation: animal studies

“…Alternatively, renal tubular epithelial cells or vascular endothelial cells of vasa recta may undergo transformation into osteogenic cells and produce CaP deposits on their basal side [97]. Transformation of vascular smooth muscle cells into osteoblast/chondrocyte phenotype appears to be the first step in vascular calcification in the kidneys [98,99].…”

“…The co-existence of idiopathic stone formation and various cardiovascular diseases such as hypertension, myocardial infarction, carotid artery atherosclerosis, coronary heart disease,[9,100-102] leads us to suggest that vascular calcification and RP formation may involve similar molecules and pathways [101,103]. Interestingly, animal models of both CaOx [97] and CaP [104] nephrolithiasis have shown transformation of renal epithelial cells into osteoblast like cells with increased expression of bone morphogenetic protein, runt-related transcription factor and Osterix.…”