Tripodi Farida, Maffioli Elisa, Sacchi Silvia, Rabattoni Valentina, Motta Zoraide, Bearzi Claudia, Tedeschi Gabriella, Pollegioni Loredano, Coccetti Paola
Department of Biotechnology and Biosciences, University of Milano-Bicocca, Milan, Italy.
Department of Veterinary Medicine and Animal Science (DIVAS), University of Milan, Lodi, Italy.
Front Cell Neurosci. 2025 Jul 21;19:1616911. doi: 10.3389/fncel.2025.1616911. eCollection 2025.
Astrocytes are the major source of L-serine (L-Ser) in the brain: the glycolytic intermediate D-3-phosphoglycerate is converted into L-Ser through the phosphorylated pathway (PP) made up of three enzymes, phosphoglycerate dehydrogenase (PHGDH), phosphoserine aminotransferase (PSAT) and phosphoserine phosphatase (PSP), recently proposed to generate a metabolic assembly named serinosome. In the central nervous system, L-Ser is used for a number of functions, including the synthesis of glycine (Gly) and D-serine (D-Ser), the two key NMDAR co-agonists.
Here, we used iPSC-derived human astrocytes as a cellular model to evaluate the impact on cell metabolism of the overexpression of each of the three enzymes of the PP as GFP-tagged proteins.
The subcellular cytosolic localization of PP enzymes remains unchanged compared to endogenous proteins, while the complex formation is increased in all cases. Notably, among the factors involved, the overexpression of PHGDH appears to play a pivotal role in promoting the serinosome assembly and/or stabilization, highlighting the critical importance of this multi-domain protein. Particularly, the overexpression of each enzyme of the PP alters the cellular metabolism in a specific way. The L-Ser and Gly levels increase more in PHGDH overexpressing cells, in agreement with the known kinetics of the PP. A consistent increase in the TCA cycle, as well as in mitochondrial activities, serine-glycine-one carbon pathway, asparagine, arginine, purine and pyrimidines metabolism is also observed.
Peculiar alterations are observed when each enzyme of the PP is overexpressed, strongly supporting the use of human iPSC-derived astrocytes overexpressing the PP pathway enzymes as a valuable cellular model for understanding how Ser glial metabolism occurs in a non-tumor system under both physiological and pathological conditions.
星形胶质细胞是大脑中L-丝氨酸（L-Ser）的主要来源：糖酵解中间产物D-3-磷酸甘油酸通过由磷酸甘油酸脱氢酶（PHGDH）、磷酸丝氨酸氨基转移酶（PSAT）和磷酸丝氨酸磷酸酶（PSP）三种酶组成的磷酸化途径（PP）转化为L-Ser，最近有人提出该途径会生成一种名为丝氨酸体的代谢组件。在中枢神经系统中，L-Ser具有多种功能，包括合成甘氨酸（Gly）和D-丝氨酸（D-Ser）这两种关键的NMDAR共激动剂。
在此，我们使用诱导多能干细胞衍生的人星形胶质细胞作为细胞模型，以评估作为绿色荧光蛋白标记蛋白的PP三种酶中每种酶的过表达对细胞代谢的影响。
与内源性蛋白质相比，PP酶的亚细胞胞质定位保持不变，而在所有情况下复合物形成均增加。值得注意的是，在所涉及的因素中，PHGDH的过表达似乎在促进丝氨酸体组装和/或稳定方面起关键作用，突出了这种多结构域蛋白的至关重要性。特别是，PP的每种酶的过表达都以特定方式改变细胞代谢。与PP的已知动力学一致，在过表达PHGDH的细胞中L-Ser和Gly水平升高更多。还观察到三羧酸循环以及线粒体活性、丝氨酸-甘氨酸-一碳途径、天冬酰胺、精氨酸、嘌呤和嘧啶代谢持续增加。
当PP的每种酶过表达时会观察到特殊的变化，有力地支持了将过表达PP途径酶的人诱导多能干细胞衍生的星形胶质细胞用作有价值的细胞模型，以了解在生理和病理条件下非肿瘤系统中Ser胶质细胞代谢是如何发生的。
