Fig. 8

Loss of GPT2 leads to decreased pre-synaptic glutamate and synaptic transmission which is ameliorated by alpha-ketoglutarate supplementation. Based on the data, we propose the following model: (1) GPT2 is the main alanine aminotransferase and the main mediator of glutamine to alanine and glutamate to alanine interconversions (Fig. 1). (2) Loss of GPT2 leads to decreased TCA cycle intermediates in synaptosomes (Fig. 7). (3) Glutamate dehydrogenase (GDH) activity is increased (Fig. 6), likely to compensate for the deficit in the TCA cycle and to increase the flow of glutamate back into the TCA cycle. Double heavy nitrogen labeled glutamine pool is enriched (Fig. 6), likely to clear extra ammonia resulting from increased GDH activity, and concomitantly total glutamine levels are increased in Gpt2-null synaptosomes (Fig. 6). (4) We observe decreases in total glutamate in Gpt2-null synaptosomes as well as decreased glutamate levels released upon stimulation (Figs. 3 and 5). (5) We see increases in VGLUT1 protein levels in synaptosomes (Fig. S1) and the average size of synaptic vesicles in asymmetric glutamatergic spines in Gpt2-null CA1 stratum radiatum was also increased (Fig. 4), possibly to allow more glutamate entry into the synaptic vesicles. (6) Alpha-ketoglutarate supplementation in Gpt2-null synaptosomes increases released glutamate upon stimulation back to wild-type levels (Fig. 5)