Imaging Mass Spectrometry Reveals Acyl-Chain- and Region-Specific Sphingolipid Metabolism in the Kidneys of Sphingomyelin Synthase 2-Deficient Mice

Obesity was reported to cause kidney injury by excessive accumulation of sphingolipids such as sphingomyelin and ceramide. Sphingomyelin synthase 2 (SMS2) is an important enzyme for hepatic sphingolipid homeostasis and its dysfunction is considered to result in fatty liver disease. The expression of SMS2 is also high in the kidneys. However, the contribution of SMS2 on renal sphingolipid metabolism remains unclear. Imaging mass spectrometry is a powerful tool to visualize the distribution and provide quantitative data on lipids in tissue sections. Thus, in this study, we analyzed the effects of SMS2 deficiency on the distribution and concentration of sphingomyelins in the liver and kidneys of mice fed with a normal-diet or a high-fat-diet using imaging mass spectrometry and liquid chromatography/electrospray ionization-tandem mass spectrometry. Our study revealed that high-fat-diet increased C18–C22 sphingomyelins, but decreased C24-sphingomyelins, in the liver and kidneys of wild-type mice. By contrast, SMS2 deficiency decreased C18–C24 sphingomyelins in the liver. Although a similar trend was observed in the whole-kidneys, the effects were minor. Interestingly, imaging mass spectrometry revealed that sphingomyelin localization was specific to each acyl-chain length in the kidneys. Further, SMS2 deficiency mainly decreased C22-sphingomyelin in the renal medulla and C24-sphingomyelins in the renal cortex. Thus, imaging mass spectrometry can provide visual assessment of the contribution of SMS2 on acyl-chain- and region-specific sphingomyelin metabolism in the kidneys.已有研究表明，肥胖可通过鞘脂类（sphingolipids）、鞘磷脂（sphingomyelin）与神经酰胺（ceramide）等物质的过度蓄积引发肾损伤。鞘磷脂合酶2（sphingomyelin synthase 2，SMS2）是维持肝脏鞘脂稳态的关键酶，其功能异常被认为可诱发脂肪肝。该酶在肾脏中也呈现高表达，但目前SMS2对肾脏鞘脂代谢的调控作用仍不明确。成像质谱（imaging mass spectrometry）是一种可直观展示组织切片中脂质分布并获取其定量数据的高效分析工具。因此，本研究采用成像质谱与液相色谱-电喷雾串联质谱（liquid chromatography/electrospray ionization-tandem mass spectrometry）技术，分析了正常饮食与高脂饮食喂养小鼠的肝脏、肾脏中SMS2缺失对鞘磷脂分布与含量的影响。本研究结果显示，高脂饮食可使野生型小鼠肝脏与肾脏中的C18~C22鞘磷脂水平升高，同时降低C24鞘磷脂的含量。与之相反，SMS2缺失会降低小鼠肝脏内C18~C24鞘磷脂的水平。尽管在全肾脏组织中也观察到了类似趋势，但该影响相对微弱。有趣的是，成像质谱分析结果显示，肾脏中不同酰基链长度的鞘磷脂呈现出特异性的定位分布。进一步研究发现，SMS2缺失主要会降低肾髓质中的C22鞘磷脂以及肾皮质中的C24鞘磷脂水平。综上，成像质谱可直观评估SMS2对肾脏中基于酰基链特异性与区域特异性的鞘磷脂代谢的调控作用。