Direct Nanoscale Imaging of Calcium Oxalate Crystallization on Brushite Reveals the Mechanisms Underlying Stone Formation

A mixture of calcium oxalate and calcium phosphate is a source of chronic human disease, forming kidney stones. However, the mechanisms of pathological biomineralization and its modulation by natural inhibitors such as osteopontin (OPN) proteins are poorly defined at the nanoscale. Here, the in vitro formation of calcium oxalate monohydrate (COM) concretions having brushite nidi is observed using in situ atomic force microscopy (AFM) in a simulated acidic urinary milieu. We quantify the dissolution kinetics of the [101]Cc, [1̅00]Cc, and [101̅]Cc steps on the brushite (CaHPO4·2H2O, DCPD) (010) surfaces by oxalate, two urinary constituents. In support of clinical observations, we further demonstrate the inhibitory effect of phosphorylated OPN peptides on the step retreat rates through step-specific interactions, this in turn regulating the kinetics of COM nucleation and aggregation at the expense of brushite crystals by means of the interfacial mineral replacement reactions. The definition of respective roles for DCPD and OPN peptides thereby offers general insights concerning the control of kidney stone formation and the mechanisms through which aberrant crystallization kinetics is inhibited.