Objective To assess the effectiveness of L-cystine dimethyl ester (CDME), an inhibitor of cystine crystal growth, for the treatment of cystine urolithiasis in a Slc3a1 knockout mouse model of cystinuria. Methods CDME (200 μg per mouse) or water was delivered by gavage daily for four weeks. Higher doses by gavage or in the water supply were administered to assess organ toxicity. Urinary amino acids and cystine stones were analyzed to assess drug efficacy using several analytical methods. Results Treatment with CDME led to a significant decrease in stone size compared with the water group (p = 0.0002), but the number of stones was greater (p = 0.005). The change in stone size distribution between the two groups was evident by micro computed tomography. Overall, cystine excretion in urine was the same between the two groups (p = 0.23), indicating that CDME did not interfere with cystine metabolism. SEM analysis of cystine stones from the CDME group demonstrated a change in crystal habit, with numerous small crystals. L-cysteine methyl ester was detected by UPLC-MS in stones from the CDME group only, indicating that a CDME metabolite was incorporated into the crystal structure. No pathological changes were observed at the doses tested. Conclusions These data demonstrate that CDME promotes formation of small stones but does not prevent stone formation, consistent with the hypothesis that CDME inhibits cystine crystal growth. Combined with the lack of observed adverse effects, our findings support the use of CDME as a viable treatment for cystine urolithiasis.

Oxidative stress has been implicated in the pathogenesis of methylmercury (MeHg) neurotoxicity. Studies of mature neurons suggest that the mitochondrion may be a major source of MeHg-induced reactive oxygen species and a critical mediator of MeHg-induced neuronal death, likely by activation of apoptotic pathways. It is unclear, however, whether the mitochondria of developing and mature neurons are equally susceptible to MeHg. Murine embryonal carcinoma (EC) cells, which differentiate into neurons following exposure to retinoic acid, were used to compare the differentiation-dependent effects of MeHg on ROS production and mitochondrial depolarization. EC cells and their neuronal derivatives were pre-incubated with the ROS indicator 2’,7’-dichlorofluoroscin diacetate or tetramethylrhodamine methyl ester, an indicator of mitochondrial membrane potential, with or without cyclosporin A (CsA), an inhibitor of mitochondrial permeability transition pore opening, and examined by laser scanning confocal microscopy in the presence of 1.5 μM MeHg. To examine consequences of mitochondrial perturbation, immunohistochemical localization of cytochrome c (cyt c) was determined after incubation of cells in MeHg for 4 hours. MeHg treatment induced earlier and significantly higher levels of ROS production and more extensive mitochondrial depolarization in neurons than in undifferentiated EC cells. CsA completely inhibited mitochondrial depolarization by MeHg in EC cells but only delayed this response in the neurons. In contrast, CsA significantly inhibited MeHg-induced neuronal ROS production. Cyt c release was also more extensive in neurons, with less protection afforded by CsA. These data indicate that neuronal differentiation state influences mitochondrial transition pore dynamics and MeHg-stimulated production of ROS.

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