Modulation of astrocytic metabolic phenotype by proinflammatory cytokines

Astrocytes play an important role in nervous system homeostasis. In particular, they contribute to the regulation of local energy metabolism and to oxidative stress defence. In previous experiments, we showed that long-term treatment with interleukin 1alpha (IL-1alpha) or tumor necrosis factor-alpha (TNFalpha) alone increases glucose utilization in primary culture of mouse astrocytes. In our study, we report that a combination of IL-1beta and TNFalpha exerts a synergistic effect on glucose utilization and markedly modifies the metabolic phenotype of astrocytes. Thus, IL-1beta+TNFalpha treated astrocytes show a marked decrease in glycogen levels, a slight but not significant decrease in lactate release as well as a massive increase in both the pentose phosphate pathway and TCA cycle activities. Glutamate-stimulated glucose utilization and lactate release, a typical feature of astrocyte energy metabolism, are altered after pretreatment with IL-1beta+TNFalpha. As far as mechanisms for oxidative stress defence are concerned, we observed that treatment with IL-1beta+TNFalpha decreases cellular glutathione content and increases glutathione release into the extracellular space while stimulating superoxide anion and nitric oxide production as well as H(2)O(2) release. Interestingly, stimulation of glucose utilization by IL-1beta+TNFalpha is not affected by the antioxidant N-acetyl-L-cysteine, suggesting that cellular stress does not account for this effect. Finally, the effects of cytokines on glucose utilization appear to involve multiple signaling cascades including the phosphoinositide 3-kinase and mitogen-activated protein kinases. Taken together these results establish that a proinflammatory environment such as observed in several neuropathological conditions including Alzheimer's disease, markedly modifies the metabolic phenotype of astrocytes.

Amyloid-beta aggregates cause alterations of astrocytic metabolic phenotype: impact on neuronal viability

Igor Allaman, Thierry Laroche, Hilal Lashuel, Pierre Magistretti, David Viertl

Amyloid-beta (Abeta) peptides play a key role in the pathogenesis of Alzheimer's disease and exert various toxic effects on neurons; however, relatively little is known about their influence on glial cells. Astrocytes play a pivotal role in brain homeostasis, contributing to the regulation of local energy metabolism and oxidative stress defense, two aspects of importance for neuronal viability and function. In the present study, we explored the effects of Abeta peptides on glucose metabolism in cultured astrocytes. Following Abeta(25-35) exposure, we observed an increase in glucose uptake and its various metabolic fates, i.e., glycolysis (coupled to lactate release), tricarboxylic acid cycle, pentose phosphate pathway, and incorporation into glycogen. Abeta increased hydrogen peroxide production as well as glutathione release into the extracellular space without affecting intracellular glutathione content. A causal link between the effects of Abeta on glucose metabolism and its aggregation and internalization into astrocytes through binding to members of the class A scavenger receptor family could be demonstrated. Using astrocyte-neuron cocultures, we observed that the overall modifications of astrocyte metabolism induced by Abeta impair neuronal viability. The effects of the Abeta(25-35) fragment were reproduced by Abeta(1-42) but not by Abeta(1-40). Finally, the phosphoinositide 3-kinase (PI3-kinase) pathway appears to be crucial in these events since both the changes in glucose utilization and the decrease in neuronal viability are prevented by LY294002, a PI3-kinase inhibitor. This set of observations indicates that Abeta aggregation and internalization into astrocytes profoundly alter their metabolic phenotype with deleterious consequences for neuronal viability.

2010

Lentiviral Transgenesis as a Tool to Study the Role of Uncoupling Protein Isoforms in the Nervous System

Hélène Danielle Perreten Lambert

A tight coupling exists between synaptic activity and glucose utilization by astrocytes. Metabolic cooperation between neurons and astrocytes mediates this coupling. During synaptic activation, glutamate that is released in the synaptic cleft as a neurotransmitter by neurons is rapidly cleared by an active uptake into astrocytes. One glutamate is co-transported with three Na+. Intracellular astrocytic Na+ homeostasis is re-established by the Na+/K+-ATPase which requires ATP provided mainly by glycolysis. The resulting lactate is released in extracellular space and contributes to the energetic balance of activated neurons. This coupling between astrocytes and neurons is known as the astrocyte-neuron lactate shuttle hypothesis (ANLSH). The role of mitochondria in this coupling remains to be determined and more specifically the role of uncoupling proteins (UCP) which have been recently identified in neural tissues. UCPs belong to a family of mitochondrial carriers which are present in the inner mitochondrial membrane. There are five isoforms named UCP1 to UCP5. The well-known characterized is UCP1, also named thermogenin, which is present in the brown adipose tissue (BAT) of the small rodents and is responsible of non-shivering thermogenesis. It dissipates the proton gradient across the inner mitochondrial membrane, thereby producing heat instead of ATP. UCP2 is ubiquitous and is implicated in protection against excessive reactive oxygen (ROS) production. The expression of UCP3 is restricted to skeletal muscle where it is linked to fatty acid metabolism. The role of the brain isoforms UCP4 and UCP5 is still poorly understood. Initially, we set out to evaluate their function in energy metabolism of astrocytes and neurons. We used the lentiviral strategy to overexpress or silence the UCPs isoforms in primary cultures of astrocytes and neurons. We found that only UCP4 and UCP5 had an uncoupling activity in brain cells. Indeed, we observed a reduced mitochondrial potential and a reduced ATP/ADP ratio in astrocytes as well as in neurons overexpressing UCP4 or UCP5. UCP4 reduced the oxidative pathway of astrocytes without modifying the basal glucose metabolism and without having a lethal effect on the cell. Oxygen consumption rate (OCR) of brain cells ovexpressing UCPs was reduced. Moreover, UCP4 increased lactate release by astrocytes, suggesting an implication in the astrocyte-neuron coupling. We also brought evidence that both UCP4 and UCP5 partially protect brain cells from oxidative damage. All these results were confirmed by experiments on UCPs silencing. In a second step, we used a co-culture model to study the impact of astrocytic uncoupling protein on neuronal survival and we demonstrated that the presence of uncoupling protein in astrocytes prevents neuronal death after glutamate excitotoxicity. Taken together, all these results support an important role of uncoupling proteins in the regulation of astrocytic energy production, thus promoting local export of lactate which can be used by neurons.

EPFL2011

Differential effects of pro- and anti-inflammatory cytokines alone or in combinations on the metabolic profile of astrocytes

Igor Allaman, Pierre Magistretti

We have previously reported that the pro-inflammatory cytokines tumor necrosis factor-α (TNFα) and interleukin-1β (IL-1β) induce profound modifications of the metabolic profile of astrocytes. The present study was undertaken to further characterize the effects of cytokines in astrocytes and to determine whether similar effects could also be observed in neurons. To do so, selected pro-inflammatory (IL-6 and interferon-γ, in addition to the above-mentioned TNFα and IL-1β) and anti-inflammatory cytokines (IL-4, IL-10, transforming growth factor-β1 and interferon-β) were applied to primary neuronal and astrocytic cultures, and key metabolic parameters were assessed. As a general pattern, we observed that pro-inflammatory cytokines increased glucose utilization in astrocytes while the anti-inflammatory cytokines IL-4 and IL-10 decreased astrocytic glucose utilization. In contrast, no significant change could be observed in neurons. When pairs of pro-inflammatory cytokines were co-applied in astrocytes, several additive or synergistic modifications could be observed. In contrast, IL-10 partially attenuated the effects of pro-inflammatory cytokines. Finally, the modifications of the astrocytic metabolism induced by TNFα + IL-1β and interferon-γ modulated neuronal susceptibility to an excitotoxic insult in neuron-astrocyte co-cultures. Together, these results suggest that pro- and anti-inflammatory cytokines differentially affect the metabolic profile of astrocytes, and that these changes have functional consequences for surrounding neurons.

Wiley-Blackwell2011

Lentiviral Transgenesis as a Tool to Study the Role of Uncoupling Protein Isoforms in the Nervous System

Hélène Danielle Perreten Lambert

EPFL2011