Fear and anxiety-related disorders such as post-traumatic stress disorder (PTSD) are among the most frequent psychiatric conditions and represent a major public health concern. Such disorders are commonly viewed as impairment of fear memory, its consolidation in particular. Pavlovian fear conditioning in mice models one key aspect of panic attacks and PSTD: the spontaneous recall of fear memory. To date, the circuits and mechanisms responsible for fear memory consolidation remain unknown. From our preliminary data and a large body of literature, the present project proposes to study the ventral hippocampus - prefrontal cortex network as the core circuit for fear memory consolidation. To this end, we will combine state of the art behaviour, electrophysiology and optogenetic techniques to thoroughly dissect and manipulate this circuit and its mechanisms. Beyond physiology, interfering with fear memory circuit mechanisms would be a novel and promising therapeutic strategy.

Current views on systems memory consolidation posit that long-term memories are reliant on the neocortex. However, the circuit-level and cellular mechanisms allowing to allocate these memories to specific cortical neural assemblies, the so-called engram cells, are still elusive. Recent data indicate that anterior thalamic nuclei (ATN) functionally connect the hippocampal formation with the retrosplenial cortex (RSC), which is thought to act as a permanent repository for remote memories. In ThalaGram, we propose that a dynamic interplay between the ATN and the RSC is required for the formation of cortical engram cells. To test this functional hypothesis, we have built a 3-partner consortium within Bordeaux Neurocampus with complementary expertise aimed at addressing the functional dynamics of thalamocortical interactions during the course of memory formation. We will first clarify the functional architecture of the ATN-RSC circuit by revealing the cellular identity of cortical cells contacted by thalamic inputs and their synaptic and cellular properties, as well as the returning connections from cortical layer 5/6 neurons to the ATN. We will pay special attention to presynaptic plasticity at the output ATN-RSC synapses and take advantage of a mouse model to conditionally suppress synaptic facilitation, therefore targeting selectively information transfer at cortico-thalamic connections. We will next systematically assess the effects of specific acute/opto- or chronic/chemogenetic inhibition of projection-defined ATN and RSC neurons at each phase (encoding, consolidation, retrieval) of a contextual fear paradigm. We will then track the formation of cortical engram cells and will address how they connect with the ATN by using genetically encoded activity reporters. Finally, we will question how new contextual memories can be assimilated into existing knowledge by relying on an innovative behavioral paradigm enabling the animal to form pre-existing cortical mental schemas. We will address the contribution of ATN-RSC interactions in this process and explore whether thalamic inputs influence the content (gist-like or richly detailed) of the cortical memory being retrieved. Altogether, we hope to unravel a novel role for the ATN-RSC circuit in the dynamics of memory engram formation and organization.

Functional sensory maps in the cerebral cortex reorganize in response to peripheral injury, with active modalities gaining cortical space at the expense of less active regions. Map expansion drives homeostatic and activity-dependent strengthening and weakening of pre-existing synapses to promote the recovery of altered sensorimotor skills. While most studies have classically focused on the growth and stabilization of synapses, whether spine elimination also serves critical function during cortical remapping remains unknown. The project syTune grounds on solid preliminary results and combine state-of-the-art imaging, electrophysiological and genetic tools in vivo to address for the first time to what extent the activity-dependent removal of spines in the barrel cortex causes cortical map expansion and the recovery of altered sensorimotor skills upon sensory deafferentation. A better understanding of synapse elimination should pave the way for new studies related to brain diseases.

Neurons have a very complex and dynamic morphology, which is believed to hold the key to understanding the ability of the mammalian brain to process and store huge amounts of information. While the branching patterns of dendrites are largely stable over the lifetime of an animal, neuronal synapses are highly plastic and constantly subject to activity-dependent remodelling, which may be an important substrate of learning and memory. However, regular 2-photon microscopy does not have enough spatial resolution to properly visualize functionally crucial details of spine and dendrite anatomy; in many instances it even fails to distinguish individual spines, leading to serious errors in reporting their density and dynamics. Hence, our insights into the complexity and plasticity of these nanoscale structures, let alone their impact on synaptic signalling, circuit function and ultimately animal behaviour, remain fragmentary and circumstantial. While the concept of dendritic spines as distinct anatomical and biochemical compartments is firmly established, there is little consensus on whether and how spines influence electrical signaling. Evidence indicates that spine size correlates with quantal synaptic currents, but very little is known about the amplitude and time course of the voltage deflection in the spine during a synaptic event and how this voltage might be boosted by the activation of ion channels as it spreads into the dendrite and towards the soma. However, such knowledge is absolutely essential for understanding how synaptic activity from across many synapses is integrated by the dendritic tree and converted into postsynaptic spiking. Even less is known about how spine and dendrite morphology, in particular the spine neck, exerts electrotonic amplification and filtering of synaptic voltages, which has been a long-standing question in the plasticity field, because structural changes may represent a powerful mechanism for tuning synaptic potentials, encoding synaptic memory and influencing higher integrative functions of dendrites and circuits. Our objective is to close the knowledge gap between nanoscale neuronal morphology and postsynaptic integration of synaptic potentials within a relevant behavioral context using an array of cutting-edge experimental techniques and mathematical modeling. Our approach will overcome the technical bottlenecks that impeded the measurement of rapid voltage signals inside small compartments and visualization of neuronal morphology with sufficiently high spatial resolution in live tissue. The consortium brings together research teams with strong and complementary expertise in STED microscopy and synaptic plasticity (Partner 1: Nägerl), dendritic integration and voltage-sensitive dye (VSD) imaging (Partner 2: DiGregorio), in vivo electrophysiology of hippocampal neurons (Partner 2: Schmidt-Hieber), and biophysical modeling and numerical simulations (Partner 3: Cattaert), using acute slices and intact brains in vivo as experimental preparations. Our overarching hypothesis is that nanoscale features of neuronal morphology (like spine necks and dendritic constrictions) and their spatial distribution in the dendritic tree exert a powerful influence on postsynaptic electrical signalling, shaping postsynaptic potentials in the spine head and their dendritic integration, which ultimately influence the neuronal representations of space. Specifically, through this proposal (NanoDend) we will 1) identify the nano-anatomical dendritic motifs that might underlie non-linear dendritic integration, 2) investigate the influence of spine morphology on spine and dendritic potentials and plasticity, and 3) establish the anatomical principles that influence nonlinear dendritic integration and place field formation.

Focal cortical Dysplasia (FCD) is one of the major causes of focal onset seizures refractory to antiepileptic drugs (AEDs). Only one-third of these patients are eligible for epilepsy surgery (i.e. the removal of the epileptogenic area), which offers the chance of seizure remission for 30%-40% of patients. There is a clear need to develop new and more efficient therapy. Gene therapy constitutes a promising alternative to conventional therapies as it can be applied focally using appropriate viral vectors and is less invasive than surgical resection, likely reducing undesirable changes in brain function. Various ongoing preclinical projects currently investigate the benefits of therapies aimed at increasing the expression of inhibitory peptides or ion channels or using chemogenetic compounds for the treatment of epilepsy. In this project we propose a different strategy, aimed at reducing the expression of kainate receptors (GluK2-containing KARs), a subclass of glutamate receptors involved in epileptogenesis, through local viral transduction of selective miRNAs. One of the main advantages of miRNAs is that it locally inhibits specific neuronal proteins without altering the genetic code. In a preclinical project, we recently proved that this strategy was efficient in reducing epileptiform events in temporal lobe epilepsy without affecting normal synaptic transmission (Boileau et al, under review). The aim of our project, supported by preliminary data, is to evaluate if the GluK2-containing KAR miRNA strategy can be proposed for treating focal cortical epilepsies and, more specifically, type II FCD.