NH4+ triggers the release of astrocytic lactate via mitochondrial pyruvate shunting

Abstract

Neural activity is accompanied by a transient mismatch between local glucose and oxygen metabolism, a phenomenon of physiological and pathophysiological importance termed aerobic glycolysis. Previous studies have proposed glutamate and K + as the neuronal signals that trigger aerobic glycolysis in astrocytes. Here we used a panel of genetically encoded FRET sensors in vitro and in vivo to investigate the participation of NH + 4 , a by-product of catabolism that is also released by active neurons. Astrocytes in mixed cortical cultures responded to physiological levels of NH + 4 with an acute rise in cytosolic lactate followed by lactate release into the extracellular space, as detected by a lactate-sniffer. An acute increase in astrocytic lactate was also observed in acute hippocampal slices exposed to NH + 4 and in the somatosensory cortex of anesthetized mice in response to i.v. NH + 4 . Unexpectedly, NH + 4 had no effect on astrocytic glucose consumption. Parallel measurements showed simultaneous cytosolic pyruvate accumulation and NADH depletion, suggesting the involvement of mitochondria. An inhibitor-stop technique confirmed a strong inhibition of mitochondrial pyruvate uptake that can be explained by mitochondrial matrix acidification. These results show that physiological NH + 4 diverts the flux of pyruvate from mitochondria to lactate production and release. Considering that NH + 4 is produced stoichiometrically with glutamate during excitatory neurotransmission, we propose that NH + 4 behaves as an intercellular signal and that pyruvate shunting contributes to aerobic lactate production by astrocytes.B rain tissue is almost exclusively energized by the oxidation of glucose. However, during neuronal activation, there is a larger increase in local glucose consumption relative to oxygen consumption (1). As this mismatch occurs in the presence of normal or augmented oxygen levels, it has been termed aerobic glycolysis, paralleling the signal detected by functional magnetic resonance imaging (2). Aerobic glycolysis and its associated lactate surge are causally linked to diverse functions of the brain in health and disease (3-10). Two signals are known to trigger aerobic glycolysis in brain tissue: glutamate and K + , which are released by active neurons and stimulate glycolysis in astrocytes (11,12).Neurons produce as much NH + 4 as they produce glutamate, both molecules being stoichiometrically linked in the glutamateglutamine cycle (13). Brain tissue NH + 4 increases within seconds of neural activation (14-16) and is quickly released to the interstitium (17, 18) to be captured by astrocytes through K + channels and transporters (19). It is well established that chronic exposure to pathological levels of NH + 4 such as those observed during liver failure has a major impact on brain metabolism, but it is not known whether this molecule may affect energy metabolism at physiological levels, particularly within the time scale of synaptic transmission. A previous study showed a reversible rise in brain tissue la...

Ammonia Mediates Cortical Hemichannel Dysfunction in Rodent Models of Chronic Liver Disease

“…There is recent evidence that in astrocytes, ammonia may acutely divert the flux of pyruvate to lactate production, contributing to the net aerobic lactate production. (7) However, the effects of chronic ammonia exposure on astrocytes are unknown. We investigated the role of ammonia by treating BDL rats with OP, a drug known to lower systemic and brain ammonia.…”

“…(6) facilitated in the presence of ammonia. (7) This appears to be due to acidification of the mitochondrial matrix resulting in a direct inhibition of mitochondrial pyruvate uptake. Increased brain lactate levels have also been reported in hyperammonemic conditions such as in acute liver failure (ALF), which is thought to be due to inhibition of the tricarboxylic acid cycle enzyme aketoglutarate dehydrogenase, suggesting a reduction in oxidative metabolism.…”

Nanomolar nitric oxide concentrations quickly and reversibly modulate astrocytic energy metabolism

“…Moreover, perisynaptic astrocytic processes possess very few mitochondria compared with larger, more distant, astrocytic processes (43). These considerations speak against inhibition of astrocytic metabolism by neuronal NO, but neurons may still modulate the metabolism of astrocytic mitochondria via other signals such as glutamate and NH 4 ϩ (18,44,45). In contrast, there are multiple lines of evidence pointing to endothelial NO.…”

“…Acting in synergy with NH 4 ϩ , which also inhibits mitochondrial pyruvate consumption (18), endothelial NO is expected to contribute to the standing astrocytic lactate reservoir (17,57,58) that is acutely released to the interstitium "on demand" via a K ϩ -activated lactate channel (17). Lactate is a metabolic fuel and also an intercellular signal that modulates multiple functions in brain tissue, from vascular tone to memory processing (59 -61), and therefore the effect of NO on astrocytic mitochondria may be understood as a modulator of these functions.…”

“…A distinctive advantage of these tools over standard isotopebased methodologies is that they can detect functional changes before they are obscured by desensitization or compensatory adaptations. Using these tools we were able to show that K ϩ and NH 4 ϩ , which are released by active neurons, trigger acute changes in astrocytic glucose consumption and lactate production (15)(16)(17)(18). The aim of this study was to investigate whether NO is capable of modulating energy metabolism in astrocytes with the speed, reversibility, and potency expected for a physiological signal.…”