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The role of DiGeorge Critical Region 2 in cortical circuit formation

Defense Thèse de doctorat : Univ. Genève et Lausanne, 2016 - Neur. 184 - 2016/10/07
Abstract The generation, migration and integration of different subtypes of neurons in cortical circuits has emerged as a key process in the pathophysiology of a variety of developmental disorders including autism spectrum disorders (ASD) and schizophrenia (SZ). Historical candidate-gene studies, as well as recent genome-wide association studies identified numerous genetic loci associated with schizophrenia. Many of these risk genes regulate embryonic cellular events involved in the formation of neural circuits. However, the functional role that schizophrenia-risk genes plays in the development of the disease is still largely unknown. During my PhD, I investigated the impact of a novel risk gene for schizophrenia, Di-George Critical Region 2 (Dgcr2) on cortical circuit formation. Dgcr2 is expressed throughout brain development and encodes for an activity-dependent adhesion protein. This gene is located in the 22q11.2 locus, in the minimal critical region that produces - when heterozygously deleted in humans - the 22q11 micro-deletion syndrome, also known as Di-George syndrome. This genetic micro-deletion is one of the highest known risk factors for schizophrenia, and DGCR2 has been highlighted as an important susceptibility gene from this locus, conferring increased risk for schizophrenia to 22q11 micro-deletion carriers. Additionally, exome sequencing of idiopathic schizophrenia family trios identified a rare de novo mutation in DGCR2 strongly associated with the disease, supporting the idea that DGCR2 is an important schizophrenia risk gene per se. However, evidence is still lacking about the function of DGCR2 in the pathophysiology of the disease. Here I aimed to describe the molecular mechanisms by which Dgcr2 impacts the construction of cortical microcircuits. In order to generate a developmental mouse model for schizophrenia, I performed in utero electroporation targeted to upper-layer pyramidal neurons (PN) and assessed the consequences of the shRNA-mediated Knock-Down (KD) of Dgcr2 on different steps of cortical circuit formation such as the proliferation of neuronal progenitors, neuronal migration and dendritogenesis. KD of the expression of mouse (m)Dgcr2 affected the laminar positioning of PN in a persistent manner in the somatosensory cortex. PN mispositioning due to Dgcr2 KD could be fully rescued by overexpressing the shRNA-resistant human (h)DGCR2 but not the (h)DGCR2 containing the SZ-risk mutation P429R, indicating a deleterious impact of this SZ-risk mutation on the migratory function of DGCR2. Surprisingly, I discovered that the P429R mutation is associated to a drastic reduction in the protein concentration of DGCR2, thus leading to a state of Dgcr2 haplo-insufficiency in a similar fashion as in the 22q11.2 microdeletion syndrome. Live-imaging techniques revealed that Dgcr2 KD significantly affected the dynamic locomotion of PN, indicating that Dgcr2 is likely to regulate key molecular pathways involved in the migration of PN. Using co-immunoprecipitation and proximity ligation assays, I discovered that DGCR2 is part of the REELIN complex, a key pathway regulating cortical neuron migration.(17) Moreover, Dgcr2 KD affected the REELIN-dependent phosphorylation of Akt and the expression of REELIN-dependent genes. This results allowed me to uncover the molecular and cellular mechanisms through which the SZ-risk gene Dgcr2 acts on early steps of cortical circuit formation. As a next step and in a collaborative manner, I aimed to assess the impact of Dgcr2 KD on the later functional maturation of upper-layer PN in the medial prefrontal cortex (mPFC), a key brain region involved in schizophrenia. At more mature time point (postnatal day 30), we found that Dgcr2 KD alters the morphology of the apical dendritic arbors of PN. Overall, this study revealed novel molecular mechanisms involved in schizophrenia-related developmental alterations, and opens the way to study in more details the functional impact of such early cortical circuits impairments on schizophrenia-related phenotypes.
Keywords NeuroscienceDevelopmentCellular migrationNeuronal migrationSchizophreniaGenetics22q11DGCR2REELINDigeorgeCorticogenesis
URN: urn:nbn:ch:unige-917304
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Research group Groupe Dayer Alexandre (Formation du circuit cortical) (875)
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MOLINARD-CHENU, Aude. The role of DiGeorge Critical Region 2 in cortical circuit formation. Université de Genève. Thèse, 2016. https://archive-ouverte.unige.ch/unige:91730

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Deposited on : 2017-02-06

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