Chronic stress and glucocorticoids: from neuronal plasticity to neurodegeneration

Stress and stress hormones, glucocorticoids (GCs), exert widespread actions in central nervous system, ranging from the regulation of gene transcription, cellular signaling, modulation of synaptic structure, and transmission and glial function to behavior. Their actions are mediated by glucocorticoid and mineralocorticoid receptors which are nuclear receptors/transcription factors. While GCs primarily act to maintain homeostasis by inducing physiological and behavioral adaptation, prolonged exposure to stress and elevated GC levels may result in neuro- and psychopathology. There is now ample evidence for cause-effect relationships between prolonged stress, elevated GC levels, and cognitive and mood disorders while the evidence for a link between chronic stress/GC and neurodegenerative disorders such as Alzheimer's (AD) and Parkinson's (PD) diseases is growing. This brief review considers some of the cellular mechanisms through which stress and GC may contribute to the pathogenesis of AD and PD.The work was supported by Grants “PTDC/SAU-NMC/113934/2009,” funded by FCT, Portuguese Foundation for Science and Technology, and project DoIT, Desenvolvimento e Operacionalização da Investigação de Translação (N° do projeto 13853), funded by Fundo Europeu de Desenvolvimento Regional (FEDER) through the Programa Operacional Fatores de Competitividade (POFC). In addition, this work was also cofinanced by European Union FP7 Project SwitchBox (Nuno Sousa, Osborne F. X. Almeida) and the Portuguese North Regional Operational Program (ON.2 – O Novo Norte) under the National Strategic Reference Framework (QREN), through the European Regional Development Fund (FEDER). Sheela Vyas acknowledges grant support from Foundation de France, Physiopathology of Parkinson, France Parkinson and ANR Grant “ParkStrim” N° 13-BSV1-0013-02. Work in FT research group was supported by Agence Nationale de la Recherche (TIMMS and StressPsyco) and Fondation pour la Recherche Médicale, Grant no. DEQ20140329552

Novel Transgenic Mice for Inducible Gene Overexpression in Pancreatic Cells Define Glucocorticoid Receptor-Mediated Regulations of Beta Cells

Conditional gene deletion in specific cell populations has helped the understanding of pancreas development. Using this approach, we have shown that deleting the glucocorticoid receptor (GR) gene in pancreatic precursor cells leads to a doubled beta-cell mass. Here, we provide genetic tools that permit a temporally and spatially controlled expression of target genes in pancreatic cells using the Tetracycline inducible system. To efficiently target the Tetracycline transactivator (tTA) in specific cell populations, we generated Bacterial Artificial Chromosomes (BAC) transgenic mice expressing the improved Tetracycline transactivator (itTA) either in pancreatic progenitor cells expressing the transcription factor Pdx1 (BAC-Pdx1-itTA), or in beta cells expressing the insulin1 gene (BAC-Ins1-itTA). In the two transgenic models, itTA-mediated activation of reporter genes was efficient and subject to regulation by Doxycycline (Dox). The analysis of a tetracycline-regulated LacZ reporter gene shows that in BAC-Pdx1-itTA mice, itTA is expressed from embryonic (E) day 11.5 in all pancreatic precursor cells. In the adult pancreas, itTA is active in mature beta, delta cells and in few acinar cells. In BAC-Ins1-itTA mice tTA is active from E13.5 and is restricted to beta cells in fetal and adult pancreas. In both lines, tTA activity was suppressed by Dox treatment and re-induced after Dox removal. Using these transgenic lines, we overexpressed the GR in selective pancreatic cell populations and found that overexpression in precursor cells altered adult beta-cell fraction but not glucose tolerance. In contrast, GR overexpression in mature beta cells did not alter beta-cell fraction but impaired glucose tolerance with insufficient insulin secretion. In conclusion, these new itTA mouse models will allow fine-tuning of gene expression to investigate gene function in pancreatic biology and help us understand how glucocorticoid signaling affects on the long-term distinct aspects of beta-cell biology

Dissecting the role of glucocorticoids on pancreas development

To determine whether glucocorticoids are involved in pancreas development, glucocorticoid treatment of rat pancreatic buds in vitro was combined with the analysis of transgenic mice lacking the glucocorticoid receptor (GR) in specific pancreatic cells. In vitro treatment of embryonic pancreata with dexamethasone, a glucocorticoid agonist, induced a decrease of insulin-expressing cell numbers and a doubling of acinar cell area, indicating that glucocorticoids favored acinar differentiation; in line with this, expression of Pdx-1, Pax-6, and Nkx6.1 was downregulated, whereas the mRNA levels of Ptf1-p48 and Hes-1 were increased. The selective inactivation of the GR gene in insulin-expressing beta-cells in mice (using a RIP-Cre transgene) had no measurable consequences on beta- or alpha-cell mass, whereas the absence of GR in the expression domain of Pdx-1 (Pdx-Cre transgene) led to a twofold increased beta-cell mass, with increased islet numbers and size but normal alpha-cell mass in adults. These results demonstrate that glucocorticoids play an important role in pancreatic beta-cell lineage, acting before hormone gene expression onset and possibly also modulating the balance between endocrine and exocrine cell differentiation

Characterization of the Spinal Nucleus of the Bulbocavernosus Neuromuscular System in Male Mice Lacking Androgen Receptor in the Nervous System

International audienceMotoneurons in the spinal nucleus of the bulbocavernosus (SNB) and their target bulbocavernosus (BC) and levator ani (LA) muscles play a role in male copulation and fertility. Testosterone (T) induces sexual differentiation of this SNB neuromuscular system during development and maintains its activation in adulthood. In the rat, T-induced effects mostly involve the androgen receptor (AR). However, the role of central AR in T-induced effects remains to be studied with pertinent genetic models. We addressed this question by using specific motoneuron immunolabeling and retrograde tracing in mice selectively disrupted for AR in the nervous system. This work reveals that nervous system AR is not required either for T-induced development of BC-LA muscles and perinatal sparing of SNB motoneurons from atrophy or for adult sensitivity of BC-LA muscles to T. By contrast, loss of AR expression in the nervous system resulted in SNB motoneurons having smaller somata and shorter dendrites than controls. We studied the effects of adult castration and T supplementation on SNB cell morphology in control and mutant males; these experiments showed that central AR is involved in the developmental regulation of soma size and dendritic length and in the adult maintenance of soma size of SNB motoneurons. T seemed to act indirectly through BC-LA muscles to maintain dendritic length in adulthood. Our results also suggest that central AR functions may contribute to normal activity of SNB motoneurons and perineal muscles because mutant mice displayed diminished copulatory behavior and fertility

The MAPK pathway and Egr-1 mediate stress-related behavioral effects of glucocorticoids

International audienceMany of the behavioral consequences of stress are mediated by the activation of the glucocorticoid receptor by stress-induced high levels of glucocorticoid hormones. To explore the molecular mechanisms of these effects, we combined in vivo and in vitro approaches. We analyzed mice carrying a brain-specific mutation (GR(NesCre)) in the glucocorticoid receptor gene (GR, also called Nr3c1) and cell lines that either express endogenous glucocorticoid receptor or carry a constitutively active form of the receptor (DeltaGR) that can be transiently induced. In the hippocampus of the wild-type [corrected] mice after stress, as well as in the cell lines, activation of glucocorticoid receptors greatly increased the expression and enzymatic activity of proteins in the MAPK signaling pathway and led to an increase in the levels of both Egr-1 mRNA and protein. In parallel, inhibition of the MAPK pathway within the hippocampus abolished the increase in contextual fear conditioning induced by glucocorticoids. The present results provide a molecular mechanism for the stress-related effects of glucocorticoids on fear memories

A Subpopulation of Serotonergic Neurons That Do Not Express the 5-HT1A Autoreceptor

International audience5-HT neurons are topographically organized in the hindbrain, and have been implicated in the etiology and treatment of psychiatric diseases such as depression and anxiety. Early studies suggested that the raphe 5-HT neurons were a homogeneous population showing similar electrical properties, and feedback inhibition mediated by 5-HT1A autoreceptors. We utilized histochemistry techniques in ePet1-eGFP and 5-HT1A-iCre/R26R mice to show that a subpopulation of 5-HT neurons do not express the somatodendritic 5-HT1A autoreceptor mRNA. In addition, we performed patch-clamp recordings followed by single-cell PCR in ePet1-eGFP mice. From 134 recorded 5-HT neurons located in the dorsal, lateral, and median raphe, we found lack of 5-HT1A mRNA expression in 22 cells, evenly distributed across raphe subfields. We compared the cellular characteristics of these neuronal types and found no difference in passive membrane properties and general excitability. However, when injected with large depolarizing current, 5-HT1A-negative neurons fired more action potentials, suggesting a lack of autoinhibitory action of local 5-HT release. Our results support the hypothesis that the 5-HT system is composed of subpopulations of serotonergic neurons with different capacity for adaptation

Fetal PGC-1 alpha Overexpression Programs Adult Pancreatic beta-Cell Dysfunction

International audienceAdult beta-cell dysfunction, a hallmark of type 2 diabetes, can be programmed by adverse fetal environment. We have shown that fetal glucocorticoids (GCs) participate in this programming through inhibition of beta-cell development. Here we have investigated the molecular mechanisms underlying this regulation. We showed that GCs stimulate the expression of peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1 alpha), a coregulator of the GCs receptor (GR), and that the overexpression of PGC-1 alpha represses genes important for beta-cell development and function. More precisely, PGC-1 alpha inhibited the expression of the key beta-cell transcription factor pancreatic duodenal homeobox 1 (Pdx1). This repression required the GR and was mediated through binding of a GR/PGC-1 alpha complex to the Pdx1 promoter. To explore PGC-1 alpha function, we generated mice with inducible beta-cell PGC-1 alpha overexpression. Mice overexpressing PGC-1 alpha exhibited at adult age impaired glucose tolerance associated with reduced insulin secretion, decreased beta-cell mass, and beta-cell hypotrophy. Interestingly, PGC-1 alpha expression in fetal life only was sufficient to impair adult beta-cell function whereas beta-cell PGC-1 alpha overexpression from adult age had no consequence on beta-cell function. Altogether, our results demonstrate that the GR and PGC-1 alpha participate in the fetal programming of adult beta-cell function through inhibition of Pdx1 expression. Diabetes 62:1206-1216, 201

Chronic Stress Triggers Social Aversion via Glucocorticoid Receptor in Dopaminoceptive Neurons

International audienceRepeated traumatic events induce long-lasting behavioral changes that are key to organism adaptation and that affect cognitive, emotional, and social behaviors. Rodents subjected to repeated instances of aggression develop enduring social aversion and increased anxiety. Such repeated aggressions trigger a stress response, resulting in glucocorticoid release and activation of the ascending dopamine (DA) system. We bred mice with selective inactivation of the gene encoding the glucocorticoid receptor (GR) along the DA pathway, and exposed them to repeated aggressions. GR in dopaminoceptive but not DA-releasing neurons specifically promoted social aversion as well as dopaminergic neurochemical and electrophysiological neuroadaptations. Anxiety and fear memories remained unaffected. Acute inhibition of the activity of DA-releasing neurons fully restored social interaction in socially defeated wild-type mice. Our data suggest a GR-dependent neuronal dichotomy for the regulation of emotional and social behaviors, and clearly implicate GR as a link between stress resiliency and dopaminergic tone

Hepatocyte proliferation during liver regeneration is impaired in mice with liver-specific IGF-1R knockout

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Genetic disruption of mineralocorticoid receptor leads to impaired neurogenesis and granule cell degeneration in the hippocampus of adult mice

To dissect the effects of corticosteroids mediated by the mineralocorticoid (MR) and the glucocorticoid receptor (GR) in the central nervous system, we compared MR(–/–) mice, whose salt loss syndrome was corrected by exogenous NaCl administration, with GR(–/–) mice having a brain-specific disruption of the GR gene generated by the Cre/loxP-recombination system. Neuropathological analyses revealed a decreased density of granule cells in the hippocampus of adult MR(–/–) mice but not in mice with disruption of GR. Furthermore, adult MR(–/–) mice exhibited a significant reduction of granule cell neurogenesis to 65% of control levels, possibly mediated by GR due to elevated corticosterone plasma levels. Neurogenesis was unaltered in adult mice with disruption of GR. Thus, we could attribute long-term trophic effects of adrenal steroids on dentate granule cells to MR. These MR-related alterations may participate in the pathogenesis of hippocampal changes observed in ageing, chronic stress and affective disorders