Final Report Summary - SYNTWOGLIOTS (In the brain, at the level of a single synapse an individual astrocyte releases several gliotransmitters)
Brain processing is based on transmission of information from one neuron to another at the level of the synapse. For a long time, it has been considered that the synapse was made of only two neuronal components: a presynaptic terminal releasing neurotransmitters and a postsynaptic element receiving and integrating information. Whereas this bipartite organization is the basis of the synaptic communication, a third element, the astrocytic process, localized in close apposition to the two neuronal elements, has to be considered. Indeed during synaptic transmission, astrocyte, a type of glial cells, detects neurotransmitters (chemical signals) released at the synapse and in turn regulates its efficacy of transmission by releasing active substances, named gliotransmitters (Figure1).
Whereas astrocytes regulate synaptic transmission in several structures of the brain, the hippocampus, a structure important for memory, has proven to be an exquisite model. In this structure, it is clear that astrocytes regulate excitatory glutamatergic transmission during a wide array of synaptic activity. To do that, these glial cells release two main gliotransmitters that are purines (Pascual et al., 2005; Serrano et al., 2006; Panatier et al. 2011) and D-serine (Henneberger et al., 2010; Mothet et al., 2000; Yang et al., 2003). In juvenile animals, astrocytes release purines to increase the efficacy of transmission at individual glutamatergic synapses (Panatier et al., 2011). In addition, it has been shown in adult that these glial cells release D-serine, to control the activity of glutamatergic NMDA receptors, receptors playing a key role in memory (Henneberger et al., 2010; Yang et al., 2003).
A possibility could be that distinct populations of astrocytes release distinct gliotransmitters. However each astrocyte occupies an exclusive territory called domain that encloses around 100 000 synapses (Bushong et al., 2002). This makes it very unlikely that a given gliotransmitter is associated with one astrocytic domain. However, because all studies focused on one gliotransmitter, it is not known whether an individual astrocyte can release several gliotransmitters at the same synapse.
The ability of an individual astrocyte to release several gliotransmitters would allow it to modulate adequately transfer of information that occurs in a wide diversity of synaptic conditions. Hence, we hypothesized that an individual astrocyte releases several gliotransmitters at the level of individual glutamatergic synapses.
To validate this project a prerequisite was to show whether a functional synapse requires the presence of a third element, the astrocytic process. Until four years ago, the functional study of individual bipartite and tripartite synapses was not possible because of the lack of spatial resolution of traditional light microscopic approaches in living brain slices. Nevertheless, when I came back in France, in the Neurocentre Magendie (Bordeaux), I had the unique opportunity to combine stimulated emission depletion (STED) microscopy, developed in Pr. Valentin Nägerl’s laboratory (Interdisciplinary Institute for Neuroscience, Bordeaux, France) with tools I developed during my PhD and post-doctoral training. Using STED microscopy we have shown that:
- 1) It is possible to image at the nanoscale the morphological interaction of astrocytic processes with the two neuronal elements of the synapse in living brain slices using STED microscopy (Panatier et al., 2014).
- 2) The majority of synapses are in contact with an astrocytic process. Importantly, synapses without an astrocytic process in close apposition were immature-like synapses.
- 3) The interaction of the astrocytic process with the two neuronal elements of synapses is dynamic and can be modified, in function of the efficacy of transmission.
Then, once tripartite, the main question emerges: does a single astrocyte release several gliotransmitters at the single synapse level? Our data show that:
- 4) As in juvenile, astrocytes in hippocampal adult rats (9 to 12 weeks) up-regulate basal synaptic transmission through the release of purines. This part was in fact mandatory as Henneberger and collaborators (Henneberger et al., 2010) have shown that astrocytes release D-serine at the level of an individual astrocyte in adults.
- 5) We have uncovered the astrocytic receptor implicated in the release of D-serine.
Purines and D-serine are released in the same structure, by the same cells. Thus we are in a good track to further show that both gliotransmitters are released at the level of a single synapse as our data show that 1) astrocytes are well positioned at the synapse; 2) purines and D-serine are released in the same area during basal synaptic transmission in adult hippocampus and can be studied using exactly the same technique at the same time.
This project will have strong consequences on our current vision of the tripartite synapse and thus on the processing of information in the brain. With a short-term perspective this work will have consequences in fundamental research but also perhaps with a long-term perspective in clinical research.
As the society is aging, the prevalence of neurodegenerative disorders is becoming more and more important. Since most of these pathologies are associated with impairment of synaptic functions, it is of uttermost importance to understand how synapses work under physiological conditions to develop appropriate therapeutically strategies.
I believe that understanding information processing in the nervous system will only be possible when the function of the tripartite synapse will be fully unraveled.
Whereas astrocytes regulate synaptic transmission in several structures of the brain, the hippocampus, a structure important for memory, has proven to be an exquisite model. In this structure, it is clear that astrocytes regulate excitatory glutamatergic transmission during a wide array of synaptic activity. To do that, these glial cells release two main gliotransmitters that are purines (Pascual et al., 2005; Serrano et al., 2006; Panatier et al. 2011) and D-serine (Henneberger et al., 2010; Mothet et al., 2000; Yang et al., 2003). In juvenile animals, astrocytes release purines to increase the efficacy of transmission at individual glutamatergic synapses (Panatier et al., 2011). In addition, it has been shown in adult that these glial cells release D-serine, to control the activity of glutamatergic NMDA receptors, receptors playing a key role in memory (Henneberger et al., 2010; Yang et al., 2003).
A possibility could be that distinct populations of astrocytes release distinct gliotransmitters. However each astrocyte occupies an exclusive territory called domain that encloses around 100 000 synapses (Bushong et al., 2002). This makes it very unlikely that a given gliotransmitter is associated with one astrocytic domain. However, because all studies focused on one gliotransmitter, it is not known whether an individual astrocyte can release several gliotransmitters at the same synapse.
The ability of an individual astrocyte to release several gliotransmitters would allow it to modulate adequately transfer of information that occurs in a wide diversity of synaptic conditions. Hence, we hypothesized that an individual astrocyte releases several gliotransmitters at the level of individual glutamatergic synapses.
To validate this project a prerequisite was to show whether a functional synapse requires the presence of a third element, the astrocytic process. Until four years ago, the functional study of individual bipartite and tripartite synapses was not possible because of the lack of spatial resolution of traditional light microscopic approaches in living brain slices. Nevertheless, when I came back in France, in the Neurocentre Magendie (Bordeaux), I had the unique opportunity to combine stimulated emission depletion (STED) microscopy, developed in Pr. Valentin Nägerl’s laboratory (Interdisciplinary Institute for Neuroscience, Bordeaux, France) with tools I developed during my PhD and post-doctoral training. Using STED microscopy we have shown that:
- 1) It is possible to image at the nanoscale the morphological interaction of astrocytic processes with the two neuronal elements of the synapse in living brain slices using STED microscopy (Panatier et al., 2014).
- 2) The majority of synapses are in contact with an astrocytic process. Importantly, synapses without an astrocytic process in close apposition were immature-like synapses.
- 3) The interaction of the astrocytic process with the two neuronal elements of synapses is dynamic and can be modified, in function of the efficacy of transmission.
Then, once tripartite, the main question emerges: does a single astrocyte release several gliotransmitters at the single synapse level? Our data show that:
- 4) As in juvenile, astrocytes in hippocampal adult rats (9 to 12 weeks) up-regulate basal synaptic transmission through the release of purines. This part was in fact mandatory as Henneberger and collaborators (Henneberger et al., 2010) have shown that astrocytes release D-serine at the level of an individual astrocyte in adults.
- 5) We have uncovered the astrocytic receptor implicated in the release of D-serine.
Purines and D-serine are released in the same structure, by the same cells. Thus we are in a good track to further show that both gliotransmitters are released at the level of a single synapse as our data show that 1) astrocytes are well positioned at the synapse; 2) purines and D-serine are released in the same area during basal synaptic transmission in adult hippocampus and can be studied using exactly the same technique at the same time.
This project will have strong consequences on our current vision of the tripartite synapse and thus on the processing of information in the brain. With a short-term perspective this work will have consequences in fundamental research but also perhaps with a long-term perspective in clinical research.
As the society is aging, the prevalence of neurodegenerative disorders is becoming more and more important. Since most of these pathologies are associated with impairment of synaptic functions, it is of uttermost importance to understand how synapses work under physiological conditions to develop appropriate therapeutically strategies.
I believe that understanding information processing in the nervous system will only be possible when the function of the tripartite synapse will be fully unraveled.
