Astrocytic release of ATP through type 2 inositol 1,4,5‐trisphosphate receptor calcium signaling and social dominance behavior in mice

Brain mitochondrial function is critical for numerous neuronal processes. We recently identified a link between brain energy and social dominance, where higher levels of mitochondrial function resulted in increased social competitive ability. The underlying mechanism of this link, however, remains unclear. Here, we investigated the contribution of astrocytic release of adenosine triphosphate (ATP) through the type 2 inositol 1,4,5‐triphosphate receptor to social dominance behavior. Mice lacking the type 2 inositol 1,4,5‐triphosphate receptor were characterized for their social dominance behavior, as well as their performance on a nonsocial task, the Morris Water Maze. In parallel, we also examined mitochondrial function in the medial prefrontal cortex, nucleus accumbens, and hippocampus to investigate how deficiencies in astrocytic ATP could modulate overall mitochondrial function. While knockout mice showed similar competitive ability compared with their wild‐type littermates, dominant knockout mice exhibited a significant delay in exerting their dominance during the initial encounter. Otherwise, there were no differences in anxiety and exploratory traits, spatial learning and memory, or brain mitochondrial function in either light or dark circadian phases. Our findings point to a marginal role of astrocytic ATP through IP3R2 in social competition, suggesting that, under basal conditions, the neuronal compartment is predominant for social dominance exertion.

Medium chain triglyceride diet reduces anxiety-like behaviors and enhances social competitiveness in rats

Carlos Canto Alvarez, Fiona Hollis, Maria del Carmen Sandi Perez

Medium-chain triglycerides (MCT) are emerging as unique dietary supplements that are potentially relevant for the amelioration of brain dysfunctions. MCT are converted into ketones and free medium chain fatty acids that, in the brain, are highly effective energy sources to mitochondria and potentially less harmful than glucose metabolism to neurons. Given the recently established link between mitochondrial dysfunction and high anxiety and depression, we performed this study to investigate the effectiveness of an MCT-enriched diet to ameliorate anxiety- and depression-related behaviors in rats. Male rats were distributed into groups, according to their anxiety-like behaviors in the elevated plus maze. Each group was given either MCT-supplemented diet or an isocaloric control diet for fifteen days. Starting from the eighth day of diet, rats were exposed to different behavioral tests. MCT-fed rats exhibited reduced anxiety-like behaviors and enhanced social competitiveness, while their coping responses in the forced swim test were not affected by the treatment. When evaluated at the end of the two-week MCT diet, mitochondrial respiration was reduced in the medial prefrontal cortex (mPFC) while unchanged in the nucleus accumbens. In the mPFC, enzymes related to glycolysis and oxidative phosphorylation were also decreased by MCT diet, while proteins controlling glucose and glutamate transport were increased. Altogether, our findings strongly suggest the effectiveness of MCT diet to exert anxiolytic effects. In the brain, our results point to the mPFC as a brain region in which MCT supplementation improves transport and control of energy substrates.

2018

The Impact of Sleep Fragmentation on Sleep Homeostasis, Brain and Peripheral Energy Metabolism and Spatial Learning

Maxime Olivier Baud

The quality of sleep has recently come to the forefront of public health concerns in industrialized nations. Indeed, voluntary sleep curtailment is widespread, sleep disorders are increasingly recognized and both correlate with the current epidemiology of diabetes and obesity (Van Cauter and Knutson, 2008). Sleep fragmentation (SF) is a periodic disruption found in highly prevalent sleep-related breathing and movement disorders, where the sleeper might be aroused several hundred times per night. By roughly summing the prevalence of obstructive sleep apnea (OSA) syndromes and periodic limb movement disorders, two of the most common sleep disorders, we predict a total prevalence for SF of nearly 15% in the European active population. Overlooked cases and insufficient concern for the problematic bear a major socioeconomic burden because of the broad impact of excessive daytime sleepiness on productivity (Hossain and Shapiro, 2002), motor vehicle accidents (Aldrich, 1989) and medical complications (Spiegel et al., 2009). Recurrent interference with the natural architecture of sleep leads to sleepiness, difficulty in concentration and cognitive impairments such as decreased reaction times and poor working memory equivalent to a full night of sleep deprivation in severe cases (Bonnet, 1985, 1989; Stepanski, 2002). In addition, recent experimental evidence in humans has unraveled the unprecedented link between sleep of poor quality and dramatic endocrine imbalances such as deregulation of appetite controlling hormones, glucocorticoids and increase in sympathetic tone, ultimately leading to the development of insulin resistance (Spiegel et al., 2009; Stamatakis and Punjabi, 2010). Current animal models for the study of sleep pathology mostly involve short intervention often biased by stress induction and usually involving a complete period of sleep deprivation. These models do not tightly reflect the most common clinical pattern characterized by sleep perturbation over long periods of time. Therefore, paying attention to maximally avoid methodological stress, we designed a new device aiming at performing instrumental SF for fourteen days in mice. Our model mimics SF observed in sleep disorders since it has no major impact on circadian timing and total amount of sleep but clearly shortens individual sleep episodes. With this method, we approached three problematics that were insufficiently addressed in the literature. In our first study, we addressed the question of whether SF could trigger electrophysiological and molecular sleep homeostasis mechanisms in the short-term and whether these effects could be maintained in the long-term. We showed that SF leads to an anticipated homeostatic regulation of Slow Wave Activity (SWA), the sleep EEG hallmark of sleep homeostasis. Furthermore, EEG spectral analysis revealed an unexpected power increase in 4-40 Hz frequencies during sleep with a clear increase in the amplitude of slow waves and spindles. We concluded that sustained sleep pressure in SF leads to a global increase in cortical synchronization. We next explored whether these electrophysiological changes could impact on the expression of transcripts known to be regulated by sleep homeostasis (immediate early genes, clock-genes and heat shock proteins). Only heat shock proteins showed a consistent induction after one day but normalized after fourteen days of SF. These results point to the fact that electrophysiological and molecular aspects of sleep homeostasis are dissociable and that long-term sleep pressure triggers allostatic cellular processes of adaptation. In our second study, we explored the effects of long-term SF on general metabolism. We showed that SF led to a general imbalance in energy intake and expenditure, possibly through a slight elevation of brain temperature at rest. We also observed that glucocorticoids were slightly elevated at given circadian points and were accompanied by glucose intolerance. Altogether, this study brings evidence for the participation of SF in addition to intermittent hypoxia in the development of a metabolic syndrome in OSA patients. In the third study, we combined our chronic model with a spatial learning task spanning over several days (Morris Water Maze) in order to study the impact of SF on memory and metabolic plasticity. We showed that SF impaired proper memory consolidation supporting the idea that continuous sleep is required for specific mechanisms of memory trace post-processing. We also provide preliminary evidence that brain energy metabolism is not optimally responsive to neural activation after SF and that this effect might underlie the observed cognitive impairment. In conclusion, we showed that our animal SF model is suitable for studying the specific mechanisms underlying cognitive impairments and metabolic imbalance observed in sleep medicine. Our results underscore the existence of a global allostatic load in a wide range of systems during SF. Sensitization of the population to the prevalence of SF and its stealthy mode of presentation might improve its detection. Increased recognition of the problematic would be especially beneficial to cognitive development in school children with sleep apnea and might reduce accidents related to sleepiness. More specifically, we propose that screening sleep quality and effective correction of SF in patients with endocrine disorders has the potential to improve glucose metabolism and energy balance.

EPFL2011

Deciphering the astrocyte-neuron metabolic cooperation in vivo

Manuel Raphael Zenger

Brain energy consumption is high and a very tightly regulated energy metabolism ensures correct performance. Neurons, the cells responsible for transmitting information via synapses, interact very closely with star shaped glial cells called astrocytes. Astrocytes play a pivotal role in the regulation of brain energy metabolism and for neurotransmission. Especially, in the case of glutamate, the main excitatory neurotransmitter in the brain, astrocytes are contributing to the clearing of otherwise potentially toxic levels of glutamate in the synaptic cleft. In addition, astrocytes are capable of providing neurons with lactate, which can be used as an additional source of energy in periods of increased demand, such as during neurotransmission. Lactate released by astrocytes is also a signal for neuronal plasticity. In this work, we addressed different aspects of the astrocyte-neuron metabolic coupling. Uncoupling proteins (UCPs) are proteins of the inner mitochondrial membrane that are capable of dissipating a proton gradient (uncoupling ATP production from respiration). Five different UCP isoforms have been described. UCP1 is essentially found in brown adipose tissue and is implicated in non-shivering thermogenesis, UCP2 is widely expressed and protecting against reactive oxygen species and UCP3 is mainly expressed in skeletal muscle tissue where a possible role in fatty acid metabolism has been described. UCP4 and 5 are essentially expressed in the brain, but their precise function still needs to be determined. In the present study, we showed that in neurons, UCP4 and UCP5 are predominantly expressed, as compared to the other UCPs. In astrocytes, UCP5 is the most abundant isoform, but also UCP2 and 4 are significantly expressed indicating an important role for these three UCPs in astrocytes. The role of UCP4 was further elucidated. For this purpose, a lentiviral overexpressing UCP4 construct was produced and characterized. This construct overexpressing UCP4 at the mitochondria allowed to demonstrate a neuro-protective role of UCP4 expression in astrocytes. In a complementary approach to study neuron-astrocyte metabolic coupling, the role of different astrocytic and neuronal genes in memory and learning was explored in a spatial learning paradigm. Mice were trained 1 to 9 days in a radial maze. The gene analysis showed that genes involved in lactate production and utilization such as lactate dehydrogenases were highly increased. A gene involved in the shuttling of lactate from astrocytes to neurons, such as monocarboxylate transporter 1 was also upregulated. An unexpected decrease in glucose transporter 1 and decrease in hexokinase 2 expression were observed. Finally, genes involved in glycogen metabolism were upregulated. Na+/K+-ATPase alpha2 isoform (ATP1A2), which plays a key role in re-establishing Na+ homeostasis disrupted by the co-transport of Na+ along with glutamate during glutamate uptake, was upregulated following spatial learning. Using a lentiviral transgenesis method, an attempt was made to downregulate the expression of the ATP1A2 in vivo. However, this technique did not allow us to produce sufficient knock down to study the role of ATP1A2 in vivo. Preliminary data for an alternative method are shown. Overall, the set of results presented here provides additional support for the existence of a dynamically regulated neuron-astrocyte metabolic coupling.