The cell cycle–apoptosis connection revisited in the adult brain

Adult neurogenesis is studied in vivo using thymidine analogues such as bromodeoxyuridine (BrdU) to label DNA synthesis during the S phase of the cell cycle. However, BrdU may also label DNA synthesis events not directly related to cell proliferation, such as DNA repair and/or abortive reentry into the cell cycle, which can occur as part of an apoptotic process in postmitotic neurons. In this study, we used three well-characterized models of injury-induced neuronal apoptosis and the combined visualization of cell birth (BrdU labeling) and death (Tdt-mediated dUTP-biotin nick end labeling) to investigate the specificity of BrdU incorporation in the adult mouse brain in vivo. We present evidence that BrdU is not significantly incorporated during DNA repair and that labeling is not detected in vulnerable or dying postmitotic neurons, even when a high dose of BrdU is directly infused into the brain. These findings have important implications for a controversy surrounding adult neurogenesis: the connection between cell cycle reactivation and apoptosis of terminally differentiated neurons

Mécanismes de contrôle de la neurogenèse in vivo dans la voie olfactive de mammifère adulte

Mes travaux de thèse ont visé à identifier les messages extracellulaires contrôlant in vivo la neurogenèse qui persiste chez les mammifères adultes dans la voie olfactive. Au niveau de l'épithélium olfactif, j'ai utilisé conjointement le modèle in vivo de la bulbectomie (ablation du bulbe olfactif, déclanchant séquentiellement l'apoptose des neurones sensoriels puis la stimulation mitotique de leurs progéniteurs, une approche biochimique (RT-PCR semi-quantitative et western-blot) et l'étude d'une souche de souris knock-out, pour démontrer que la cytokine (Leukemia Inhibitory Factor) est indispensable à la prolifération post-lésionnelle des progéniteurs neuronaux, sans qu'elle semble intervenir en conditions basales. Par RT-PCR in situ, le LIF est détecté dès 8 heures post-lésion dans les neurones sensoriels qui vont mourir par apoptose. Ainsi, cet exemple montre d'une part que des cellules induites à entrer apoptose peuvent émettre un signal mitogène paracrine pour initier leur propre remplacement et d'autre part que des mécanismes peuvent différer entre une réaction post-lésionnelle et la situation physiologique normale, ce qui présente un grand intérêt pour les recherches sur le cancer et la thérapie cellulaire. Au niveau du bulbe olfactif, j'ai montré en collaboration avec l'Institut de Recherche Pierre Fabre que le blocage des récepteurs alpha-2-noradrénergiques entraîne une augmentation de neurogenèse ainsi qu'une réduction d'apoptose, sans induire de variations dans la zone sous-ventriculaire (ZSV). De façon intéressante, un agoniste des mêmes récepteurs ne provoque aucun effet dans le bulbe olfactif, mais induit une diminution de prolifération et d'apoptose dans la ZSV. Ces résultats indiquent que la neurotransmission noradrénergique contribue au contrôle de la neurogenèse chez l'adulte par la mise en oeuvre de processus cellulaires distincts, et que l'apoptose constitue un des mécanismes de contrôle de la neurogenèse adulte.LYON1-BU.Sciences (692662101) / SudocSudocFranceF

The neuropoietic cytokine family in development, plasticity, disease and injury

Neuropoietic cytokines are well known for their role in the control of neuronal, glial and immune responses to injury or disease. Since this discovery, it has emerged that several of these proteins are also involved in nervous system development, in particular in the regulation of neurogenesis and stem cell fate. Recent data indicate that these proteins have yet more functions, as key modulators of synaptic plasticity and of various behaviours. In addition, neuropoietic cytokines might be a factor in the aetiology of psychiatric disorders

Transient microstructural brain anomalies and epileptiform discharges in mice defective for epilepsy and language-related NMDA receptor subunit gene Grin2a

International audienceObjective: The epilepsy-aphasia spectrum (EAS) is a heterogeneous group of age-dependent childhood disorders characterized by sleep-activated discharges associated with infrequent seizures and language, cognitive, and behavioral deficits. Defects in the GRIN2A gene, encoding a subunit of glutamate-gated N-methyl-D-aspartate (NMDA) receptors, represent the most important cause of EAS identified so far. Neocortical or thalamic lesions were detected in a subset of severe EAS disorders, and more subtle anomalies were reported in patients with so-called "benign" phenotypes. However, whether brain structural alterations exist in the context of GRIN2A defects is unknown. Methods: Magnetic resonance diffusion tensor imaging (MR-DTI) was used to perform longitudinal analysis of the brain at 3 developmental timepoints in living mice genetically knocked out (KO) for Grin2a. In addition, electroencephalogra-phy (EEG) was recorded using multisite extracellular electrodes to characterize the neocortical activity in vivo. Results: Microstructural alterations were detected in the neocortex, the corpus callosum, the hippocampus, and the thalamus of Grin2a KO mice. Most MR-DTI alterations were detected at a specific developmental stage when mice were aged 30 days, but not at earlier (15 days) or later (2 months) ages. EEG analysis detected epileptiform discharges in Grin2a KO mice in the third postnatal week. Significance: Grin2a KO mice replicated several anomalies found in patients with EAS disorders. Transient structural alterations detected by MR-DTI recalled the age-dependent course of EAS disorders, which in humans start during childhood and show variable outcome at the onset of adolescence. Together with the epileptiform discharges detected in young Grin2a KO mice, our data suggested the existence of early anomalies in the maturation of the neocortical and thalamocortical systems. Whereas the possible relationship of those anomalies with sleep warrants further investigations, our data suggest that Grin2a KO mice may serve as an animal model to study the neu-ronal mechanisms of EAS disorders and to design new therapeutic strategies. K E Y W O R D S brain structure, EEG, epilepsy-aphasia, mouse model, MR-DTI Salmi and Bolbos contributed equally to the study

Inactivation of Socs3 in the hypothalamus enhances the hindbrain response to endogenous satiety signals via oxytocin signaling

Leptin is an adipocyte-derived hormone that controls energy balance by acting primarily in the CNS, but its action is lost in common forms of obesity due to central leptin resistance. One potential mechanism for such leptin resistance is an increased hypothalamic expression of Suppressor of cytokine signaling 3 (Socs3), a feedback inhibitor of the Jak-Stat pathway that prevents Stat3 activation. Ample studies have confirmed the important role of Socs3 in leptin resistance and obesity. However, the degree to which Socs3 participates in the regulation of energy homeostasis in nonobese conditions remains largely undetermined. In this study, using adult mice maintained under standard diet, we demonstrate that Socs3 deficiency in the mediobasal hypothalamus (MBH) reduces food intake, protects against body weight gain, and limits adiposity, suggesting that Socs3 is necessary for normal body weight maintenance. Mechanistically, MBH Socs3-deficient mice display increased hindbrain sensitivity to endogenous, meal-related satiety signals, mediated by oxytocin signaling. Thus, oxytocin signaling likely mediates the effect of hypothalamic leptin on satiety circuits of the caudal brainstem. This provides an anatomical substrate for the effect of leptin on meal size, and more generally, a mechanism for how the brain controls short-term food intake as a function of the energetic stores available in the organism to maintain energy homeostasis. Any dysfunction in this pathway could potentially lead to overeating and obesity

Pathogenic MTOR somatic variant causing focal cortical dysplasia drives hyperexcitability via overactivation of neuronal GluN2C N‐methyl‐D‐aspartate receptors

International audienceAbstract Objective Genetic variations in proteins of the mechanistic target of rapamycin (mTOR) pathway cause a spectrum of neurodevelopmental disorders often associated with brain malformations and with intractable epilepsy. The mTORopathies are characterized by hyperactive mTOR pathway and comprise tuberous sclerosis complex (TSC) and focal cortical dysplasia (FCD) type II. How hyperactive mTOR translates into abnormal neuronal activity and hypersynchronous network remains to be better understood. Previously, the role of upregulated GluN2C‐containing glutamate‐gated N‐methyl‐D‐aspartate receptors (NMDARs) has been demonstrated for germline defects in the TSC genes. Here, we questioned whether this mechanism would expand to other mTORopathies in the different context of a somatic genetic variation of the MTOR protein recurrently found in FCD type II. Methods We used a rat model of FCD created by in utero electroporation of neural progenitors of dorsal telencephalon with expression vectors encoding either the wild‐type or the pathogenic MTOR variant (p.S2215F). In this mosaic configuration, patch‐clamp whole‐cell recordings of the electroporated, spiny stellate neurons and extracellular recordings of the electroporated areas were performed in neocortical slices. Selective inhibitors were used to target mTOR activity and GluN2C‐mediated currents. Results Neurons expressing the mutant protein displayed an excessive activation of GluN2C NMDAR‐mediated spontaneous excitatory postsynaptic currents. GluN2C‐dependent increase in spontaneous spiking activity was detected in the area of electroporated neurons in the mutant condition and was restricted to a critical time window between postnatal days P9 and P20. Significance Somatic MTOR pathogenic variant recurrently found in FCD type II resulted in overactivation of GluN2C‐mediated neuronal NMDARs in neocortices of rat pups. The related and time‐restricted local hyperexcitability was sensitive to subunit GluN2C‐specific blockade. Our study suggests that GluN2C‐related pathomechanisms might be shared in common by mTOR‐related brain disorders