Involvement of Cellular Membranes and their Lipids in Nucleation of Stone Forming Crystals

Abstract

AbstractMore than 80% of human kidney stones consist of calcium oxalate and/or calcium phosphate. Human urine is generally metastable with respect to these salts and their nucleation is heterogeneous. Based on: 1. ultrastructural and immunohistochemical studies of stones in which cellular degradation products and lipids were commonly seen in association with calcific crystals and 2. in vivo studies of nephrolithiasis in rat models where calcium oxalate (CaOx) and calcium phosphate (CaP) crystals almost always formed and seen in association with cell membranes, we proposed that membranes and their lipids are involved in crystallization of these salts. To test our hypothesis we isolated organic matrix of kidney stones, its lipid contents and membrane vesicles from epithelial cells of rat kidney and incubated them in metastable solution of CaOx. Both membrane vesicles and matrix from the stones supported crystallization of CaOx and crystals formed in association with the membranes. Lipids of the stone matrix appeared better nucleators than whole matrix. Urine spends only minutes within the kidneys thus any nucleation which can lead to stone formation has to occur rapidly. In studies described here, we demonstrate that under specific circumstances relevant to conditions in the kidney, membrane vesicle- supported CaOx crystallization can occur within seconds, demonstrating the possibility of such events happening in the kidneys. We also studied CaOx monohydrate (COM) precipitation at Langmuir monolayers of dipalmitoylphosphatidylglecerol (DPPG), dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylserine (DPPS) showed precipitation to be heterogeneous and selective with a majority of crystals orienting with the 101 face of COM facing the monolayer. Our results show that membrane lipids can initiate nucleation of calcium oxalate crystals in solutions similar to those present in the kidneys. In addition these crystals form within the time urine spends inside the renal tubules demonstrating for the first time the likelihood of occurrence of such a phenomenon in the kidneys during stone formation.