Activity-dependent and independent mechanisms of visual map formation – VISIODEV

The strengh of our project is to combine elegant mouse genetic methods, a unique retinal electroporation technique allowing the expression of various molecules of interest, together with cell culture, tracing and histological methods. We will also cover different aspects of map formation, ranging from axon guidance,fasciculation, branch formation and retraction that allow the correct organization of the visual map, reflected mainly in vivo in the topographic and eye-specific organization of retinal axons within their targets.

We have demonstrated that presynaptic release is necessary for the establishment of eye-specific map but not for the retinotopic map. It is the first time that these phenomenon are uncoupled as general perturbation of retinal activity impact both maps. This result will help to determine the relative contribution of the different activity-dependent mechanisms on visual map formation. Furthermore, we showed that 2 guidance molecules, Sema6D and its receptor Plexin-A1 are important for the organization of retinal axons in their target. We now seek to understand how these two molecules act in this phenomenon.

Understanding how connections are established and refined during brain development is crucial to apprehend and circumvent the defects happening in neurodevelopmental diseases. Furthermore, as the correct establishment of visual maps is crucial for proper vision, the results of our research will also provide information on how visual defects occur in critical visual and oculomotor pathologies such as albinism, amblyopia, strabismus and nystagmus and how to promote proper axonal targeting in regenerative strategies after neurodegeneration or nerve injury.

Since 2014, the results obtained were presented at scientific meetings as oral presentation (ISER meeting, Tokyo, Japon) and posters (ISDN, Nice et Society for Neuroscience meeting, Wahsington DC, USA) and are the basis of a manuscript in revision and of another manuscript in preparation.

The establishment of neural connections requires activity-dependent and independent mechanisms. The well-understood organization of the visual system comprises an accessible model for analyzing the contribution of each of these factors in the formation of connectivity. Retinal projections form a visual map in their target organized in a topographic and eye-specific manner. This specific organization is crucial for vision. The goal of this project is to understand how activity and molecules shape the visual map during development of the visual system in the mouse. We have recently identified a novel role for PlexinA1, a semaphorin receptor usually studied in axon guidance, in the formation of the visual map. Our preliminary results show that PlexinA1 is required for the fasciculation and proper targeting of retinal axons. We propose to use various tracing and anatomical techniques to determine the origin of these abnormalities and to identify which ligand of PlexinA1 is responsible for the defects observed in PlexinA1-KO mice. We will then focus on the role of activity on the formation of visual maps. Before eye opening, retinal waves are critical for the proper refinement of visual maps, both topographic and eye-specific, yet whether calcium-dependent synaptic release is required has not been investigated. Here, we propose to use a conditional deletion of RIM proteins to strongly reduce calcium-dependent synaptic release specifically in different RGCs by mouse genetics and in utero retinal electroporation techniques. This will allow us to study the importance of calcium-dependent synaptic release in a general manner but also at the level of axon-axon interaction and to determine if this effect is cell autonomous or not. In addition, we will use the same genetic mouse model to study the consequences of the reduced calcium-dependent synaptic release on gene expression, and hope to identify possible activity regulated candidates involved in map formation. Finally, we will focus on the role of serotonin on visual map formation. Increased levels of serotonin perturb visual map formation and 5HT1B receptor and SERT transporter are expressed transiently in the visual system during the period of map formation. Based on these results, we will first investigate if serotonin has a role on retinal axon growth and guidance using in vitro assays in combination with pharmacological treatment. Furthermore, we will study the role of serotonin on map refinement. Our working hypothesis is that the specific expression of SERT on ipsilateral RGCs will allow the local modulation of serotonin concentration by clearing serotonin from the ipsilateral synapses. As the 5HT1B receptor is expressed in all RGCs, the 5HT1B receptor will be sujected to differential concentrations of serotonin and thus will modulate differently glutamatergic transmission and/or the response to various guidance cues. We aim to perturb in vivo serotonin signaling by using in utero electroporation of SERT or the 5HT1B receptor, to see if the retinogeniculate axon projection is perturbed. This will allow us to determine the precise role of serotonin on retinal axon guidance and map refinement at the target and thus better understand the possible consequences on fetal development of selective serotonin reuptake inhibitors (SSRIs) used to treat depression in pregnant and breast-feeding women. Finally, we will study the interplay between these different mechanisms, looking for example whether serotonin acts through modulation of synaptic release or whether activity can regulate the expression of molecules known to be involved in visual map formation. Understanding the cellular and molecular mechanisms of visual map formation will provide insight into how vision is established. More generally, this project will help decipher how neural connections are formed and refined to better comprehend the mechanisms at hand in neurodevelopmental diseases such as schizophrenia and autism.