**Author details**

to control bone homeostasis by negatively regulating both osteoclast and osteoblast differen‐ tiation [265]. Other studies showed that downregulation of miR‐204, miR‐205, miR‐133a, or miR‐30b/c in VSMCs occurs prior to calcification and upregulates Runx2 expression [266, 267]. Micro‐RNA‐125b, which targets Ets1 and osterix, was found downregulated 21 days after exposing VSMCs to osteogenic medium [268]. Another set of miRs, miR‐135a(*n*), miR‐762, miR‐714, miR‐712(*n*), that target the calcium flux proteins NCX1, PMCA1, and NCKX4, have also been implicated in VSMC calcification [266]. It is still not clear, however, whether these miRs are really important in VSMC differentiation to an osteoblast‐like phenotype, or whether this process is associated with changes in the expression of a panel of miRs targeting several

30 Updates and Advances in Nephrolithiasis - Pathophysiology, Genetics, and Treatment Modalities

It is worth considering the possibility of ectopic renal calcification being an osteogenic‐like process. Evidence to support the notion that resident renal cells could be prompted to trans‐ differentiate, or differentiate along an osteogenic lineage, comes from the following observa‐ tions. Madin‐Darby canine kidney (MDCK) cells grown in monolayers directly on a plastic dish, or a dish coated with collagen gel, developed small blisters/domes/nodules after 21 days that became more prominent after 30 days [269, 270]. Microscopic examination showed that the nodules were CaP crystals. MDCK cells grown in agar produced spherical colonies in which layers of epithelial cells, with their apical surface on the outside, enclosed CaP crystal

Kumar et al. [71] found that rat inner medullary collecting duct cells grown in a calcifying medium formed calcifying nodules that were positive for typical bone proteins. Miyazawa et al. [273, 274] reported finding that CaOx crystals upregulated vimentin (VIM) in normal rat kidney proximal cells and that other genes, such as OPN, fibronectin (FN), cathepsins B and L, and mitogen‐activated protein kinase, related to the pathogenesis of stone formation. Using MDCK cells grown for 28 days in the presence of 10 mM β‐glycerophosphate, Azari et al. identified a mineralization process with an increased ALP activity and the presence of small aggregates of hydroxyapatite crystals within membrane‐bounded vesicles [275]. Other related osteogenic genes (RUNX1 and 2, osterix, BMP2 and 7, bone morphogenetic protein receptor 2, collagen, OCN, osteonectin (ON), OPN, MGP, OPG, cadherins, FN, and VIM) were found upregulated in the kidney of hyperoxaluric rats [276, 277]. Khan et al., again, showed a pronounced expression of MGP, together with that of collagen, OPN and FN, in renal medul‐ lary peritubular vessels of hyperoxaluric rats [111], confirming that the tubular epithelial cells of hyperoxaluric kidneys acquired a number of osteoblastic features, and suggesting a dedif‐

Mezzabotta et al. were the first to provide evidence of human renal cells transdifferentiating into an osteogenic‐like phenotype, producing CaP deposits [64]. They found spontaneous instances of calcification phenomena in primary papillary renal cells derived from a patient with medullary sponge kidney (MSK) and medullary nephrocalcinosis, who carried a muta‐ tion in the GDNF gene. To investigate whether this spontaneous mineralization was merely a

proteins important for calcification.

**7. Evidence of cell‐driven renal calcification**

deposits on the basal side of the epithelium [90, 93, 270–272].

ferentiation of epithelial cells to the osteogenic phenotype [278].

Giovanna Priante, Monica Ceol, Liliana Terrin, Lisa Gianesello, Federica Quaggio, Dorella Del Prete and Franca Anglani\*

\*Address all correspondence to: franca.anglani@unipd.it

Laboratory of Histomorphology and Molecular Biology of the Kidney, Nephrology Unit, Department of Medicine DIMED, University of Padua, Padua, Italy
