Mechanisms of stone crystal attachment to inner medullary collecting duct cells

The formation of a clinically significant kidney stone requires a number of predisposing events including: urine supersaturation, microcrystal formation, further crystal growth, and maturation of the kidney stone. Although crystalluria is associated with stone formation, not all patients with crystalluria form stones. Thus, the retention of microcrystals by the urothelium is thought to be a necessary and critical event in the growth of renal calculi. Observations in both rat models of stone disease and in humans suggest that crystal retention involves the urothelial cell membrane. This attachment process appears to be mediated by specific interactions between molecular structures on the surface of stone crystals and molecular arrays on the surfaces of cell membranes, yet the exact nature of this crystal-membrane interaction is unknown.; The involvement of cell membrane phospholipids in stone crystal attachment is suggested by their presence in the matrix of kidney stones. This hypothesis was first investigated using the red blood cell as a model membrane. These studies quantitated crystal-membrane interactions following selective changes in the RBC membrane phospholipid composition utilizing a calcium oxalate monohydrate (COM) crystal-induced membranolytic assay. Membrane enrichment with anionic phospholipids was found to greatly increase crystal-membrane interactions. Crystal-membrane interaction was associated with an increase in the negative charge on the RBC membrane surface. This relationship was further investigated by examining COM crystal attachment to inner medullary collecting duct (IMCD) cells following selective changes in cell membrane phospholipid composition. Alone among the major membrane phospholipids examined, enrichment with phosphatidylserine (PS) was found to greatly increase the attachment of crystals to the cells and this increased attachment correlated with the exposure of PS on the exofacial leaflet of the cell membrane.; We also hypothesized that for effective stone crystal attachment to the epithelium there must be cell membrane rearrangement that would allow for long-range bonding between the stone crystal and the cell membrane. This rearrangement may be influenced by the physical state of the membrane since increases in membrane fluidity are known to facilitate molecular movement within the membrane. This question was investigated by quantitating COM crystal attachment to IMCD cells following changes in cell membrane fluidity. Crystal attachment to IMCD cells was directly correlated to changes in membrane fluidity and this correlation was consistently observed regardless of the method used to alter the cell membrane. The results are consistent with the theory that the ability to form a crystal attachment region on the cell surface may be related to the ease of rearrangement of membrane components on the cell surface. The effect of membrane organization was further investigated by measuring stone crystal attachment to six different cultured kidney epithelial cell lines. Loss of cell membrane organization seems to increase COM attachment. This increase in COM attachment to continuously cultured kidney epithelial cells compared to primary cell culture might be a result of increased cell proliferation in the continuous cell cultures. Stone crystal retention in the kidney tubule might be connected to cell/tissue injury and the subsequent regeneration with its high rate of cell division and consequent loss of rigorous membrane asymmetry. Variations in the urothelial cell environment during certain pathological conditions in the kidney could induce these perturbations in membrane organization and prime kidney epithelial cells at or near the papillary tip to bind COM crystals, possibly leading to the initiating event in urolithiasis.