Understanding the links between neuronal plasticity which underlies memory and energy metabolism is a major goal of brain studies. The brain is a main energy consumer and the central regulator of energy homeostasis, and it prioritizes its own supply over peripheral organs. However we have shown in Drosophila that the brain is also able to regulate its own activity under energy shortage to favor survival. I will describe how our integrated strategy to study at the molecular, circuit and behavioral levels how the brain energy status is transmitted to the drosophila olfactory memory center, and how this information is used to regulate long-term memory formation. Finally, I will present recent data linking sexual activity with memory capacity.

Body and organ size are intrinsic properties of living organisms and are intimately linked to the developmental program to produce fit individuals with proper proportions. The regulation of organ size integrates both systemic and organ-specific processes and deregulation of these processes leads to severe medical conditions including cancer. We study these regulations in the context of Drosophila development, where the merge between genetic and physiological approaches allows deciphering the principles of organ growth with a high level of precision.

Synaptic organizers regulate formation, elimination and maintenance of synaptic connections throughout life. Although excitatory and inhibitory synaptic imbalance underlies certain neuropsychiatric and neurological disorders, no tools are currently available to directly modulate the balance. Recently, a new class of synaptic organizers, termed extracellular scaffolding proteins (ESPs), are reported to acutely and potently modulate specific synapses by directly binding to certain pre- and postsynaptic membrane proteins. Here, to expand the repertoire of ESPs with a variety of pre- and postsynaptic specificities, we developed a new synthetic ESP, Cbln1–neuronal pentraxin 1 (NP1) chimera (CPTX), by exploiting the structure of NP1 and Cbln1. Unlike original Cbln1, CPTX induced excitatory synapses by recruiting AMPA glutamate receptors in vitro. Furthermore, CPTX restored excitatory synapses and synaptic plasticity in vivo, as well as spatial and contextual memories in Alzheimer’s disease model mice. In this talk, I would like to discuss a possible toolkit of ESPs with a variety of pre- and postsynaptic specificities to modify neuronal circuits.

The nervous system has an extraordinary capacity to repair damage by regenerating axons and synaptic connections. I will discuss the molecular and cellular basis of axon regeneration in C. elegans, and describe how regenerated synapses and circuits differ from those generated during development. Finally, I will present new data on a novel ncRNA pathway that controls regenerations.

The serotonin 5-HT3 receptor is a pentameric ligand-gated ion channel. It belongs to a large family of receptors that transduce signals across the plasma membrane​: upon binding of neurotransmitter molecules to extracellular sites, the receptors undergo complex conformational transitions, which result in transient opening of a pore permeable ​to ions. 5-HT3 receptors are therapeutic targets for emesis and nausea, irritable bowel syndrome and depression. I will present structures of the 5-HT3 receptor obtained by crystallography or cryo-electron microscopy. The structures, obtained in complex with inhibitors or agonists, represent snapshots in different states: inhibited, pre-active, active. Together with molecular dynamics simulations and functional recordings, they reveal the molecular mechanism of the fast neurotransmission mediated by 5HT3 receptors.

One of the main goals of developmental biology is to understand how the different cell types that constitute a multicellular organism are specified during its development. Luisa Cochella is a young PI (awarded for both ERC starting Grant and EMBO Young investigator program) who is currently exploring the different mechanisms of gene expression regulation that control this process. Her lab investigates how the transcriptional history of a cell influences its fate as well as how post- transcriptional mechanisms contribute to the diversification of the genetic programs that control cell fate. Luisa is more specifically interested in both neuronal and muscle differentiation.

Prenatal inflammation and dysfunction of microglia, the brain resident macrophages, have both been associated with the etiology of several neuropsychiatric disorders, including schizophrenia and autism spectrum disorders. Consistently, microglia were shown to regulate neurogenesis, synaptic remodeling and maturation at postnatal stages. However, microglia invade the brain during mid-embryogenesis and could thus exert earlier prenatal and perinatal roles during normal and pathological brain wiring. Here we show that embryonic microglia, which display a transient uneven distribution, regulate the wiring of forebrain circuits. By taking advantage of multiple mouse models, including cell-depletion approaches, we found that perturbing microglia activity affects the development of neocortical inhibitory interneurons, which constitute main actors in neuropsychiatric diseases. In particular, absence, prenatal inflammation or functional perturbation of microglia affects the timely positioning of specific subsets of interneurons as well as their subsequent functional integration in the neocortex. We furthermore found that responses of microglia to environmental signals, including the ones from the microbiome, are sexually dimorphic in males and females. This remarkable finding has major implications for our comprehension of sexual biases in the occurrence of microglia-related diseases, such as the prevalence in males of neurodevelopmental disorders. Our work reveals key roles for immune cells during the normal assembly of cortical circuits and provides novel insights onto how microglia dysfunction or immune risks lead to pathological brain wiring.

A central challenge in developmental neurobiology is to understand how the myriad types of neurons in the human nervous system are generated. Stochastic gene expression mechanisms are crucial to differentiate neuronal subtypes and expand function. During stochastic fate specification, individual neurons randomly choose between different fates, resulting in unique patterns but consistent proportions of cell types among genetically identical organisms. My lab studies the stochastic mechanisms that specify the color-detecting photoreceptors in the fly and human retina. Fruit flies have a well-characterized retina and an abundance of genetic tools that enable molecular analyses of gene regulatory mechanisms. To overcome the challenges associated with human studies, we have developed a human retinal organoid system that recapitulates retinal development and photoreceptor specification. With these systems, we are interrogating how DNA elements, trans factors, and chromatin architecture control random on/off gene expression. Our molecular approaches are complemented by quantitative genetics to determine how natural variation in the genome impacts gene expression and photoreceptor specification. Finally, we conduct behavioral and functional assays to measure differences in color perception when photoreceptor fates are altered. By studying highly divergent organisms from multiple angles, we aim to define the unifying principles underlying stochastic fate specification during nervous system development.

Neuroblastoma is an embryonal neoplasm arising from the peripheral nervous system that accounts for 15% of cancer deaths in childhood. It is an enigmatic tumor presenting with a great genetic and clinical heterogeneity, both in terms of presentation and outcome. The characterization of the genetic alterations observed in neuroblastoma led to the identification of major players of neuroblastoma oncogenesis that has considerably improved our understanding of the biology of this pediatric cancer. More recently, the analysis of the super-enhancer landscape allowed to decipher the core regulatory circuitries controlling the gene expression program of neuroblastoma. Distinct transcription factor networks predicate different tumor identities, corresponding to sympathetic noradrenergic or mesenchymal/neural-crest cell like identities. Cells of mesenchymal identity are more resistant to chemotherapeutic agents. Moreover, some neuroblastoma cells exhibit plasticity and are able to shift between the NCC-like and noradrenergic identities. The understanding of cell identity, heterogeneity and plasticity in neuroblastoma has strong implications with respect to the development of new therapeutic strategies to eradicate tumor cells in neuroblastoma patients.

The formation, targeting, and maintenance of axon and dendrites are critical for proper brain development and synaptic function. Deficits in synapse establishment and maturation can lead to neurodevelopmental, neurodegenerative, and psychiatric disorders. The neuronal cytoskeleton regulates the architecture and dynamics of synaptic processes by providing structural support and the tracks for motor protein-based synaptic transport. The latter is particularly important for the establishment of long axonal projections, which requires coordinated long-range organelle transport. The membrane associated adaptor ankyrin-B (AnkB) promotes fast axonal transport and elongation by coupling dynactin to multiple organelles through binding to phosphatidylinositol 3-phosphate lipids in these cargos. Additionally, AnkB directly binds βII-spectrin, which, in turn, controls the formation of a ring-shaped membrane periodic skeleton (MPS) in axons and mature dendrites. Interestingly, βII-spectrin also associates with molecular motors. I will show that AnkB and βII-spectrin are key elements in independent and overlapping pathways responsible for the transport of synaptic cargo and other organelles, and are essential for establishing proper brain structural and functional connectivity.

Behaviour is a striking phenotype and often one of the first things we notice about an animal. Broadly speaking, we are interested in understanding how genes affect behaviour, but despite rapid advances in technology for sequencing and engineering genomes, it is still a challenge to associate particular genes with heritable behavioural differences because behaviour is time consuming to measure and difficult to quantify. We are using automated imaging to record the behaviour of freely moving nematode worms and developing new analysis methods to extract relevant features. I will discuss unsupervised methods to quantify behavioural repertoires, and how making connections to language processing and data compression can give insight into the structure of behaviour. Finally I will show how these new representations can advance the study of behavioural genetics and phenotypic drug screening.

Although a variety of primary sensory neurons are implicated in the detection and transmission of different sensory modalities, how they arise during development remains poorly understood. The process of neuronal specification is the acquisition of definitive phenotypic characteristics for a given subclass of neurons during embryonic development. This acquisition can be divided into several interdependent and sequential phases, from the time point when progenitor cells exit the cell cycle toward the newly formed and perfectly differentiated neuron. Using mouse genetics, Alexandre demonstrated that transcriptions factors of Maf and Zeb families control the specification and differentiation of specific sensory neuron sub-types. His work contributed to uncover the complex developmental sequence ensuring the formation of the peripheral sensory system and to highlight the progenitor diversity that underlies the developmental plasticity of sensory neuron generation.

Brain assembly is hypothesized to begin when pioneer axons extend over non-neuronal cells, forming tracts guiding follower axons. Yet, the identities of the pioneer-neurons and of their guidance-substrates and their interactions, are not well understood. Here, using time-lapse embryonic imaging, genetics, protein-interaction, and functional studies, we uncover the early events of C. elegans brain assembly. We demonstrate that C. elegans possesses radial-glia-like cells key for assembly initiation. Glia guide pioneer and follower axons using distinct signals. Pioneer neurons we identify, with unique growth properties, anatomy, and innervation, cooperate with glia to guide follower axons. We identified a CHIN-1/Chimaerin- KPC-1/Furin double mutant that severely disrupts assembly, unlike previously known mutants. CHIN-1/Chimaerin and KPC-1/Furin cooperate non-canonically in glia and pioneer neurons for guidance-cue trafficking. We exploit this genetic bottleneck to define a guidance-gene network governing assembly, with specific glia and pioneer-neuron contributions. Our studies reveal previously-unknown roles for glia in pioneer-axon guidance, and suggest conserved principles of brain formation.

In my group, we study the neural crest, a unique cell population that emerges from the primitive neural field and which has a multi-systemic and structural contribution to vertebrate development. Over the last decade, I have been dedicating myself to the cellular and molecular background of the observation I made in 2004, that the cephalic neural crest (CNC), exerts an autonomous and prominent control on forebrain development. This notion has broken the traditional view of how the brain develops. By using exquisite grafting experiments in combination with focal spatially and temporally controlled transgenesis, we have discovered the unexpected and potent “paracrine role that the CNC exerts on forebrain growth and patterning early in development and documented this mechanism at the level of cell interaction, signalling and gene expression. We are now following this exiting line of research, which revisits fundamental concepts in Neurosciences. This notion provides also a conceptual renewal, which is biomedically relevant. The mechanisms identified so far in our model are conserved across tetrapodes, but some social behavioural features are specific to amniotes. Our ongoing project and future directions are to explore the aetiology of neural disorders and behavioural impairments in Humans and in the light of CNC dysfunctions.

Pentameric channel-receptors, including nicotinic acetylcholine, glycine and GABAA receptors, play a key role in fast excitatory and inhibitory transmission in the nervous system and are the target of numerous therapeutic and addictive drugs. They carry several neurotransmitter binding sites which govern the opening of a transmembrane ion channel. Extensively expressed in animals, they were found in several bacteria, especially the homolog from the cyanobacteria Gloeobacter violaceus (GLIC) which functions as a proton-gated ion channel. The simplified architecture of this archaic homologue, as well as its prokaryotic origin, allowed solving its X-ray structure in several conformations. Those static structures suggest that channel opening occurs through symmetrical quaternary twist and “blooming” motions, together with tertiary deformation. We further engineered multiple fluorescent reporters on the structure, allowing investigating the dynamics of the allosteric reorganizations and showing that activation involves a key pre-active conformation. Finally, the GLIC system was exploited to solve the structure of human receptors through the generation of functional chimeras. Overall, our work gives insights into the mechanism of gating and pharmacological regulation of this important family of neurotransmitter receptors.

Synapses are organized subcellular structures that transmit information within the nervous system and to other parts of our body. Our studies use C. elegans have uncovered multiple pathways controlling synapse formation, maintenance and function. Using a genetic model mimicking the physiological state of seizures, our recent work have identified novel regulatory themes affecting presynaptic release machinery. We also discovered that a novel immunoglobulin superfamily (IgSF) transmembrane protein mediates synapse and non-neuronal tissue interaction in synapse maintenance. These findings have implications to our understanding of circuit malfunction under disease conditions.

Any type of neurons can be easily identified based on its electrophysiological activity, such as its pattern of spontaneous activity, the shape of its action potential, its dendritic integration, etc. How is stability of such electrical phenotype achieved, what are its molecular principles, and what is the degree of robustness of electrical phenotype in the face of different perturbations are questions only very partially answered. We studied these questions on dopaminergic neurons of the substantia nigra pars compacta. Our work involved characterizing the electrical phenotype of these neurons and measuring its post-natal development and its stability at mature stages. We also characterized the specific relationships of electrophysiological parameters underlying the electrical phenotype. In order to determine how complex electrical phenotype is achieved, we then investigated the networks of co-regulation of ion channels at the genetic and at the protein levels. Our results suggest that ion channel gene expression and protein interactions display a modular structure that may be involved in stabilizing phenotype. We also show that electrical phenotype also presents such a modular structure. Our ultimate goal is to provide a systems-level approach to robustness of electrical phenotype.

AnIn the brain distinct population of inhibitory GABAergic interneurons innervate principal glutamatergic neurons to regulate various aspects of brain function. At the postsynaptic compartment, specific GABAAR subunits are segregated to different neuronal compartments to recieve specific inputs from different interneurons. The correct interpretation of the incoming signal requires functional coupling between the presynaptic neurotransmitter GABA, postsynaptic GABAARs, and downstream signaling by postsynaptic density proteins. The main postsynaptic density protein at inhibitory synapse is gephyrin. In the past decade we have identified diverse signaling cascades that converge on gephyrin scaffold to regulate its scaffolding property, and in turn GABAergic neurotransmission. These studies have shed light into mechanisms that underlie dynamic changes in inhibitory neurotransmission, and how excitation shapes inhibition.