Reelin Secreted by GABAergic Neurons Regulates Glutamate Receptor Homeostasis

BACKGROUND: Reelin is a large secreted protein of the extracellular matrix that has been proposed to participate to the etiology of schizophrenia. During development, reelin is crucial for the correct cytoarchitecture of laminated brain structures and is produced by a subset of neurons named Cajal-Retzius. After birth, most of these cells degenerate and reelin expression persists in postnatal and adult brain. The phenotype of neurons that bind secreted reelin and whether the continuous secretion of reelin is required for physiological functions at postnatal stages remain unknown. METHODOLOGY/PRINCIPAL FINDINGS: Combining immunocytochemical and pharmacological approaches, we first report that two distinct patterns of reelin expression are present in cultured hippocampal neurons. We show that in hippocampal cultures, reelin is secreted by GABAergic neurons displaying an intense reelin immunoreactivity (IR). We demonstrate that secreted reelin binds to receptors of the lipoprotein family on neurons with a punctate reelin IR. Secondly, using calcium imaging techniques, we examined the physiological consequences of reelin secretion blockade. Blocking protein secretion rapidly and reversibly changes the subunit composition of N-methyl-D-aspartate glutamate receptors (NMDARs) to a predominance of NR2B-containing NMDARs. Addition of recombinant or endogenously secreted reelin rescues the effects of protein secretion blockade and reverts the fraction of NR2B-containing NMDARs to control levels. Therefore, the continuous secretion of reelin is necessary to control the subunit composition of NMDARs in hippocampal neurons. CONCLUSIONS/SIGNIFICANCE: Our data show that the heterogeneity of reelin immunoreactivity correlates with distinct functional populations: neurons synthesizing and secreting reelin and/or neurons binding reelin. Furthermore, we show that continuous reelin secretion is a strict requirement to maintain the composition of NMDARs. We propose that reelin is a trans-neuronal messenger secreted by GABAergic neurons that regulates NMDARs homeostasis in postnatal hippocampus. Defects in reelin secretion could play a major role in the development of neuropsychiatric disorders, particularly those associated with deregulation of NMDARs such as schizophrenia

Rôle de la reelin au cours de la maturation postnatale des neurones d'hippocampe in vitro

Le reelin est une protéine sécrétée de la matrice extracellulaire qui régule la migration neuronale au cours du développement embryonnaire des structures embryonnaires des structures cérébrales laminées, dont l'hippocampe. Les cellules sécrétrices de reelin dans l'hippocampe meurent à la naissance, malgré cela, elle est toujours exprimée dans le cerveau adulte, où son rôle reste à définir clairement. Le but de ma thèse a été d'étudier le rofil d'expression et la fonction de la reelin au cours de la maturation postnatale des neurones d'hippocampe in vitro. J'ai pu mettre en évidence la coexistence de deux types de cellules exprimant la reelin : des neurones présentant un marquage périnucléaire intense, identifiés comme GABAergiques et sécréteurs de reelin, et des neurones caractérisés paar un marquage léger et ponctiforme. J'ai également démontré que la reelin contrôle par l'intermédiaire de ses récepteurs aux lipoprotéines, la composition en sous-unités des récepteurs NMDA somatiques au cours de la maturation postnatale hippocampique in vitro.Reelin is a secreted extracellular matrix protein that regulates neuronal migration during embryonic devrlopment of cerebral laminated structures. Despite the disappearance of reelin secreting cells in hippocampus soon after birth, this protein is still expressed in adult brain, where its precise role is still unclear. During my Ph.D, I studied the expression profile and the function of reelin during postnatal maturation of hippocampal neurons in vitro. I showed that two types of reelin expressing cells coexist in cultured hippocampal neurons : neurons with an intense perinuclear labelling, identified as GABAergic cells responsible for reelin secretion, and neurons with a light punctate labelling. I also demonstrated that Reelin and its lipoprotein receptors ApoER2 and VLDLR control the subunit composition of somatic NMDA receptors during hippocampal postnatal maturation in vitro.BORDEAUX2-BU Santé (330632101) / SudocSudocFranceF