Glutamine as an energy substrate in cultured neurons during glucose deprivation

Abstract

During glucose deprivation an increase in aspartate formation from glutamine has been observed in different brain preparations, including synaptosomes and cultured astrocytes. To what extent this reaction, which provides a substantial amount of energy, occurs in different types of neurons is unknown. The present study shows that (14)CO(2) formation from [U-(14)C]glutamine in cerebellar granule neurons, a glutamatergic preparation, increased by 60% during glucose deprivation, indicating enhanced aspartate formation or increased complete oxidative degradation of glutamine. In primary cultures of cerebrocortical interneurons, a GABAergic preparation, the rate of (14)CO(2) production from [U-(14) C] glutamine was four times lower and not stimulated by glucose deprivation. During incubation with glutamine (0.8 mM) as the only metabolic substrate, cerebellar granule cells maintained an oxygen consumption rate of 12 nmol/min/mg protein, corresponding to an aspartate formation of 8 nmol/min/mg protein (three oxidations occur between glutamine and aspartate) or to a total oxidative degradation of 3 nmol/min/mg protein. During glucose deprivation, the rate of aspartate formation increased, and during a 20-min incubation in phosphate-buffered saline it amounted to 3.3 nmol/min/mg protein at 0.2 mM glutamine, which might have been more if measured at 0.8 mM glutamine. These values are consistent with the rate of glutamine utilization calculated based on oxygen consumption and leaves open the possibility that some glutamine is completely degraded oxidatively, as has been shown by other authors based on pyruvate recycling and labeling of lactate from aspartate in cerebellar granule neurons.

Estimation of intracellular fluxes in cerebellar neurons after hypoglycemia: Importance of the pyruvate recycling pathway and glutamine oxidation

“…Glutamine consumption rates were also in the range of values reported for cerebellar neurons (Olstad et al,2007b) and increased significantly in cells subjected to hypoglycemia. Similar findings have been reported for primary cultures of cerebellar neurons together with a 60% increase in [ 14 CO 2 ] production measured during 1 hr of glucose deprivation in the presence of 0.2 mM [U‐ 14 C]glutamine (Peng et al,2007). The same study also demonstrated that, during hypoglycemia, glutamine was able to maintain a significant rate of oxygen consumption, confirming that glutamine oxidation is an important energy source for cerebellar neurons.…”

“…The values obtained for glutamine pools are much larger than those reported by Olstad et al (2007a; between 57 and 84 nmol/mg protein in the presence of 0.5 mM glutamine), probably because of the higher concentration of glutamine in the culture medium (2 mM). Glutamate pool sizes were much lower than those observed for glutamine but in the range of values previously reported for incubations in medium without glutamate (Olstad et al,2007b; Peng et al,2007).…”

Multifunctional role of astrocytes as gatekeepers of neuronal energy supply

“…Astrocytes metabolize glutamate to TCA cycle intermediates (Schousboe et al, 1993; Hertz and Zielke, 2004; Hertz et al, 2007), which diminishes the glutamate pool, and may drive astrocytic glucose consumption, ATP production and de novo glutamate synthesis (Hertz, 2011). Neurons also utilize glutamine and/or glutamate as energy substrates during glucose deprivation in vitro (Peng et al, 2007) or ischemia in vivo (Pascual et al, 1998). They similarly use glutamine or glutamate to replenish intermediates of the TCA cycle during metabolism of other substrates in vitro (Shokati et al, 2005).…”

Inhibition of Mitochondrial Pyruvate Transport by Zaprinast Causes Massive Accumulation of Aspartate at the Expense of Glutamate in the Retina

“…During glucose deprivation, neurons turn to glutamine and glutamate for energy (48). Zaprinast does not affect glucose uptake or glycolysis (as assessed by lactate production), but its block of pyruvate transport mimics hypoglycemic conditions because less glucose-derived pyruvate is available to mitochondria.…”