Periodic Reporting for period 4 - SynID (Activity-dependent modulation of synapse identity)
Okres sprawozdawczy: 2022-04-01 do 2023-03-31
Cognitive functions and behaviors in mammals depend on the formation of complex and precise neural circuits during development. There could be several hundred or even thousands of types of neurons with very different structural and functional properties in mammals. Synapses, which are the connections between neurons that allow the passage of information, also differ in structure and function. The formation of the complex nervous system in mammals therefore requires the production of a wide variety of different synaptic types. Our goal was to understand the molecular and cellular mechanisms that control the formation of these specific connections. Another goal was to understand how neuronal activity during development affects these mechanisms and thus what role modulation of the environment could play. Our work highlights the fundamental mechanisms regulating brain development, but also sheds light on the etiology of neurodevelopmental diseases such as autism or schizophrenia. Indeed, mutations in genes producing the components of synapses have been found in these diseases, leading to the term "synaptopathies" or synapse diseases.
The originality of our approach is to dissect the brain at the molecular level to determine the constituents of each type of synapses established on a single neuronal type. For this we use sophisticated methods that combine the use of genetically modified mice with biochemistry and molecular biology techniques. Our studies have shown that there are specific constituents controlling the formation of each type of synapses established on a given neuronal target, the Purkinje cell. These results support the existence of a "molecular barcode" defining the identity of each type of synapse in the mammalian brain. Our work has identified a molecular combination, made of secreted proteins, that constitutes this barcode for two types of synapses formed on the Purkinje cell. We have also shown that contrary to previous belief, this code is not already generated when neurons differentiate but is acquired step-by-step during postnatal development. We haveperformed functional studies using neuron-specific Crispr/Cas9 loss of function experiments to show that this code is made of partially non redundant signaling pathways (Paul, Sigoillot et al. https://doi.org/10.1101/2023.01.03.521946(odnośnik otworzy się w nowym oknie)). We have also shown that some neurons depend on a default code while others have to add on, during postnatal development, molecules that define their identity on the target neuron. The next goal was to understand how this barcode is generated during postnatal development. Neuronal activity is known to contribute to the development of neuronal circuits and to control gene expression. We have tested how activity acts in the olivocerebellar system using a pharmacological and a genetic approach. We have first used a pharmacological model of schizophrenia. Our analysis showed that very precise modifications of the circuits are long-lasting following the transient change in neuronal activity, highlighting that there is a period during postnatal development that is very sensitive to changes in neuronal activity ( doi: 10.1073/pnas.2122544119.). We have also used neuron-specific modulation of activity using an intersectional approach between genetically modified mouse lines and viral drivers to show how the activity of inferior olivary neurons during postnatal development is essential for their proper contact on Purkinje cells (Paul, Sigoillot et al. https://doi.org/10.1101/2023.01.03.521946(odnośnik otworzy się w nowym oknie)).
This project performed the first comprehensive characterization of the molecular code controlling the formation of a specific circuit in the mammalian brain using the cerebellar Purkinje cells as a moel. Our data have shown that the acquisition of this code is complex, progressive and depends on the type of synapses. This work provides clues as to the interaction between genes and environment for the formation of a complex neuronal circuit and the disturbances that could occur in synaptopathies such as autism or schizophrenia.