Characterization of the Protein Components of Matrix Stones Sheds Light on S100-A8 and S100-A9 Relevance in the Inflammatory Pathogenesis of These Rare Renal Calculi

Among the different types of kidney stones, matrix stones are uncommon urinary calculi composed of a soft, pliable, amorphous substance with little crystalline content. To gain insight into the pathogenesis we investigated the protein component by analyzing the proteomic profiles of surgically removed matrix stones.A total of 5 stones were harvested from 4 patients who underwent surgery for medical reasons at 3 clinical centers during a 7-year period. Matrix stone proteome characterization was performed by mass spectrometry based techniques using an integrated top-down/bottom-up proteomic platform.We identified 142 nonredundant proteins and peptides across all samples. Neutrophil defensin 1, and proteins S100-A8 and S100-A9 were the main components of these renal calculi.The abundance of identified inflammatory molecules points to an inflammatory process as the event that initializes soft calculi formation rather than as a consequence of such formation. The post-translational oxidative changes in S100-A8 and A9, and the presence of thymosin β-4, granulins and ubiquitin also suggest the intervention of host defenses through a superimposed, vigorous counter inflammatory process. The post-translational changes seen in the proteins and peptides, and the known self-assembling capability of S100-A8 and S100-A9 probably explain the gelatinous consistency of these stones.