Department of Molecular Biology, Cell Biology and Biochemistry, Brown University, Providence, RI 02912, USA.
Center for Translational Neuroscience, Carney Institute for Brain Science and Brown Institute for Translational Science, Brown University, Providence, RI 02912, USA.
Hum Mol Genet. 2022 Feb 21;31(4):587-603. doi: 10.1093/hmg/ddab269.
The metabolic needs for postnatal growth of the human nervous system are vast. Recessive loss-of-function mutations in the mitochondrial enzyme glutamate pyruvate transaminase 2 (GPT2) in humans cause postnatal undergrowth of brain, and cognitive and motor disability. We demonstrate that GPT2 governs critical metabolic mechanisms in neurons required for neuronal growth and survival. These metabolic processes include neuronal alanine synthesis and anaplerosis, the replenishment of tricarboxylic acid (TCA) cycle intermediates. We performed metabolomics across postnatal development in Gpt2-null mouse brain to identify the trajectory of dysregulated metabolic pathways: alterations in alanine occur earliest; followed by reduced TCA cycle intermediates and reduced pyruvate; followed by elevations in glycolytic intermediates and amino acids. Neuron-specific deletion of GPT2 in mice is sufficient to cause motor abnormalities and death pre-weaning, a phenotype identical to the germline Gpt2-null mouse. Alanine biosynthesis is profoundly impeded in Gpt2-null neurons. Exogenous alanine is necessary for Gpt2-null neuronal survival in vitro but is not needed for Gpt2-null astrocytes. Dietary alanine supplementation in Gpt2-null mice enhances animal survival and improves the metabolic profile of Gpt2-null brain but does not alone appear to correct motor function. In surviving Gpt2-null animals, we observe smaller upper and lower motor neurons in vivo. We also observe selective death of lower motor neurons in vivo with worsening motor behavior with age. In conclusion, these studies of the pathophysiology of GPT2 Deficiency have identified metabolic mechanisms that are required for neuronal growth and that potentially underlie selective neuronal vulnerabilities in motor neurons.
人类神经系统产后生长的代谢需求巨大。人类中线粒体酶谷氨酸丙酮酸转氨酶 2（GPT2）的隐性失活功能突变导致大脑产后生长不良，并伴有认知和运动障碍。我们证明 GPT2 控制着神经元中关键的代谢机制，这些代谢机制对于神经元的生长和存活是必需的。这些代谢过程包括神经元丙氨酸合成和补料作用，即三羧酸（TCA）循环中间产物的补充。我们在 Gpt2 基因敲除小鼠的出生后发育过程中进行代谢组学研究，以确定失调代谢途径的轨迹：最早发生的是丙氨酸的改变；随后 TCA 循环中间产物和丙酮酸减少；随后糖酵解中间产物和氨基酸升高。在小鼠中特异性敲除神经元 GPT2 足以导致运动异常和断奶前死亡，这一表型与生殖系 Gpt2 基因敲除小鼠完全相同。GPT2 基因敲除神经元中的丙氨酸生物合成受到严重阻碍。在体外，GPT2 基因敲除神经元的生存需要外源性丙氨酸，但 Gpt2 基因敲除星形胶质细胞不需要。GPT2 基因敲除小鼠的饮食中补充丙氨酸可提高动物的存活率，并改善 Gpt2 基因敲除大脑的代谢谱，但单独补充丙氨酸似乎并不能纠正运动功能。在存活的 Gpt2 基因敲除动物中，我们观察到体内的上运动神经元和下运动神经元变小。我们还观察到随着年龄的增长，运动行为恶化，体内下运动神经元选择性死亡。总之，这些对 GPT2 缺乏症的病理生理学的研究确定了神经元生长所必需的代谢机制，这些机制可能是运动神经元中选择性神经元易损性的基础。
