Periodic Reporting for period 4 - EnergyMemo (Dynamic Interplay between Energy and Memory)
Reporting period: 2022-04-01 to 2023-03-31
The EnergyMemo project addressed the coupling between memory formation and brain energy metabolism, in the brain of the fruitfly Drosophila melanogaster. The metabolic syndrome, an emerging pathology linked to abnormal energy management by the body, is associated with an increased risk of diabetes, cardio vascular disease, but its impact on brain function remained under-investigated. Given the fast spreading of this syndrome, and given that the link between neurodegenerative diseases and defects in brain metabolism also appears increasingly clear, it is crucial to delineate the mechanisms by which the allocation and regulation of neuronal energy fluxes shape memorization abilites.
Using Drosophila for our studies gives access to a plethora of versatile genetically expressed tools to manipulate energy metabolism pathways in the course of memory formation, but also to monitor the use of energy by neurons and their neighbouring glial cells through fluorescence imaging experiments. In addition, previous work for our lab and others in the field provided a precise description of the neural networks involved in the encoding of memories over different time scales in the fly’s brain.
With this in hand, we developed research thematics at the interface of learning and memory and energy metabolism fields, which gives our team an original positioning within the international research landscape. Our results highlight that timely regulations of energy supply to neurons is key to proper encoding of long-term memory, and to identify the network and molecular mechanisms that underlie those regulations, with a particular emphasis on neuron-glia interaction.
Using Drosophila for our studies gives access to a plethora of versatile genetically expressed tools to manipulate energy metabolism pathways in the course of memory formation, but also to monitor the use of energy by neurons and their neighbouring glial cells through fluorescence imaging experiments. In addition, previous work for our lab and others in the field provided a precise description of the neural networks involved in the encoding of memories over different time scales in the fly’s brain.
With this in hand, we developed research thematics at the interface of learning and memory and energy metabolism fields, which gives our team an original positioning within the international research landscape. Our results highlight that timely regulations of energy supply to neurons is key to proper encoding of long-term memory, and to identify the network and molecular mechanisms that underlie those regulations, with a particular emphasis on neuron-glia interaction.
The starting point of our project was the discovery that dopamine input on the mushroom body, the brain area that integrates olfactory signals with salient stimuli of positive or negative valence, triggers an increase in pyruvate consumption by mitochondria in these neurons, which is necessary and sufficient to initiate the formation of long-term memory. Since then, an important part of our work consisted in identifying how this dopamine input circuit was controlled. First, we have identified a new pair of neurons that uses the serotonin neuromodulator to activate the energy-modulating dopamine input. We additionally showed that the phosphodiesterase-encoding gene dunce must is inhibited in these serotonergic neurons to allow for long-term memory encoding, thus establishing this gene as a so-called ‘memory checkpoint’ gene (Scheunemann et al., Neuron 2018). Strikingly, we further found that this neuron is subjected to a post-mating regulation that enables dunce inhibition, which explained why virgin females could not properly form long-term memory (Scheunemann et al., Science Advances 2019). Second, we also identified another GABA-ergic feedback circuit that allows the proper shaping over time of the activity of the energy-regulating dopamine neurons (Pavlowsky et al., Current Biology, 2018).
In parallel, we initiated a systematic study of neuron-glia interaction during long-term memory formation. We showed that a particular type of glial cells, that enwraps the neuronal cell bodies, is activated during memory formation, and we delineated the full molecular mechanism of the neuron-to-glia and glia-to-neuron metabolic dialogue (de Tredern et al., Cell Reports 2021).
Developing a pioneer expertise in the fluorescent imaging of energy metabolism fostered collaboration with the group of Irene Miguel-Aliaga, on a project focused on inter-organ communication in Drosophila. Using fluorescent sensors that we provided, this group showed that carbohydrate metabolism in the intestine is sexually dimorphic and that gut-derived citrate promotes food intake and sperm production (Hudry et al., Cell 2019).
Finally, we have discovered a new source of energy required for memory formation under starvation (Silva et al, Nature Metabolism 2022). Thus, we have shown that glial cells (non neuronal cells of the brain), use fat to produce small energetic molecules called ketone bodies, which are transferred to neurons and sustain their activity. Before our work, it was thought that ketone bodies used by the brain in mammals originated only from the liver.
In parallel, we initiated a systematic study of neuron-glia interaction during long-term memory formation. We showed that a particular type of glial cells, that enwraps the neuronal cell bodies, is activated during memory formation, and we delineated the full molecular mechanism of the neuron-to-glia and glia-to-neuron metabolic dialogue (de Tredern et al., Cell Reports 2021).
Developing a pioneer expertise in the fluorescent imaging of energy metabolism fostered collaboration with the group of Irene Miguel-Aliaga, on a project focused on inter-organ communication in Drosophila. Using fluorescent sensors that we provided, this group showed that carbohydrate metabolism in the intestine is sexually dimorphic and that gut-derived citrate promotes food intake and sperm production (Hudry et al., Cell 2019).
Finally, we have discovered a new source of energy required for memory formation under starvation (Silva et al, Nature Metabolism 2022). Thus, we have shown that glial cells (non neuronal cells of the brain), use fat to produce small energetic molecules called ketone bodies, which are transferred to neurons and sustain their activity. Before our work, it was thought that ketone bodies used by the brain in mammals originated only from the liver.
The most cutting-edge aspect of our project is the implementation in Drosophila of genetically-encoded fluorescent biosensors for the in vivo imaging of brain energy metabolism. Many sensors already existed before the start of our project, but their usage had been mostly limited to ex-vivo or cell culture applications. Studies employing these sensors truly in vivo have remained scarce. We started by developing a protocol to measure brain mitochondrial metabolism in vivo using a pyruvate sensor. Then, we devised another protocol to measure cellular glucose consumption in neurons and glia. This approach was key to the success of our project and to decipher the metabolic regulations underlying memory. We pursued by developing additional approaches to monitor other parameters of brain metabolism, such as ATP, lactate or H2O2. Thanks to the EnergyMemo project, our team has progressively emerged as a key player in the study of brain energy metabolism in vivo, in particular in connection with memory formation.
In the first half of the EnergyMemo project our studies have been mostly focused on long-term memory so far, which is the most stable memory phase described in Drosophila, as originally planned in the proposal. In the second half of the project we further deciphered the dynamic interplay between energy metabolism and memory as we studied the metabolic regulations that engage mushroom body neurons into the formation of other, less stable, memory phases. We also investigated the state-dependent alteration of memory ability under the angle of the modifications of neuron-glia interaction. Our EnergyMemo project provides a comprehensive picture of metabolic plasticity occurring in the olfactory memory center, the mushroom body, from learning to long-term memory formation.
In the first half of the EnergyMemo project our studies have been mostly focused on long-term memory so far, which is the most stable memory phase described in Drosophila, as originally planned in the proposal. In the second half of the project we further deciphered the dynamic interplay between energy metabolism and memory as we studied the metabolic regulations that engage mushroom body neurons into the formation of other, less stable, memory phases. We also investigated the state-dependent alteration of memory ability under the angle of the modifications of neuron-glia interaction. Our EnergyMemo project provides a comprehensive picture of metabolic plasticity occurring in the olfactory memory center, the mushroom body, from learning to long-term memory formation.