DNA hypomethylation perturbs the function and survival of CNS neurons in postnatal animals

DNA methyltransferase I (Dnmt1), the maintenance enzyme for DNA cytosine methylation, is expressed at high levels in the CNS during embryogenesis and after birth. Because embryos deficient for Dnmt1 die at gastrulation, the role of Dnmt1 in the development and function of the nervous system could not be studied by using this mutation. We therefore used the cre/loxP system to produce conditional mutants that lack Dnmt1 in neuroblasts of embryonic day 12 embryos or in postmitotic neurons of the postnatal animal. Conditional deletion of the Dnmt1 gene resulted in rapid depletion of Dnmt1 proteins, indicating that the enzyme in postmitotic neurons turns over quickly. Dnmt1 deficiency in postmitotic neurons neither affected levels of global DNA methylation nor influenced cell survival during postnatal life. In contrast, Dnmt1 deficiency in mitotic CNS precursor cells resulted in DNA hypomethylation in daughter cells. Whereas mutant embryos carrying 95% hypomethylated cells in the brain died immediately after birth because of respiratory distress, mosaic animals with 30% hypomethylated CNS cells were viable into adulthood. However, these mutant cells were eliminated quickly from the brain within 3 weeks of postnatal life. Thus, hypomethylated CNS neurons were impaired functionally and were selected against at postnatal stages.

Developmental and spatial expression pattern of syntaxin 13 in the mouse central nervous system

Harald Hirling, Juan-Carlos Sarria

Vesicular transport involves SNARE (soluble- N-ethylmaleimide-sensitive-factor-attachment-protein-receptor) proteins on transport vesicles and on target membranes. Syntaxin 13 is a SNARE enriched in brain, associated with recycling endosomes; its overexpression in PC12 cells promotes neurite outgrowth. This suggests an important role for receptor recycling during neuronal differentiation. Here we describe the spatiotemporal pattern of syntaxin 13 expression during mouse brain development. During early embryogenesis (E12-E15), it was found in the forebrain ventricular zone and in primary motor and sensory neurons in the brainstem, spinal cord and sensory ganglia. In the forebrain at E15, syntaxin 13 was not detected in neuroblasts in the intermediate zone of the embryonic hemispheric wall, while there was labeling in cortical neurons in deeper layers starting at E15-18, and progressively in later-generated neurons up to layer II around P6. Syntaxin 13 reached maximal expression in all brain divisions at about P7, followed by a decrease, with heterogeneous neuron populations displaying various staining intensities in adult brain. While usually restricted to the soma of neurons, we transiently detected syntaxin 13 in dendrites of pyramidal neurons during the first postnatal week. In conclusion, the developmentally regulated syntaxin 13 expression in various neuronal populations is consistent with its involvement in endocytic trafficking and neurite outgrowth.

2002

Feeding active neurons: (re)emergence of a nursing role for astrocytes

Pierre Magistretti, Luc Pellerin

Despite unquestionable evidence that glucose is the major energy substrate for the brain, data collected over several decades with different approaches suggest that lactate may represent a supplementary metabolic substrate for neurons. Starting with the pioneering work of McIlwain in the early 1950s which showed that lactate can sustain the respiratory rate of small brain tissue pieces, this idea receives confirmation with more recent studies using nuclear magnetic resonance spectroscopy undoubtedly demonstrating that lactate is efficiently oxidized by neurons, both in vitro and in vivo. Not only is lactate able to maintain ATP levels and promote neuronal survival but it was also found to support neuronal activity, at least if low levels of glucose are present. Despite the early suggestion for a role of astrocytes in metabolic supply to neurons, it is only recently however that they have been considered as a potential source of lactate for neurons. Moreover, it has been proposed that astrocytes might provide lactate to neurons in response to enhanced synaptic activity by a well-characterized mechanism involving glutamate uptake. The description of specific transporters for lactate on both astrocytes and neurons further suggest that there exist a coordinated mechanism of lactate exchange between the two cell types. Thus it is proposed that astrocytes play a nursing role toward neurons by providing lactate as an additional energy substrate especially during periods of enhanced synaptic activity. The importance of this metabolic cooperation within the central nervous system, although not unique if compared to other organs, still remains to be explored.

2002

Psychobiological Vulnerability to Stress

Jorge Eduardo Castro Cifuentes, Maria del Carmen Sandi Perez

Several psychobiological factors are associated with vulnerability/resilience to stress-induced depression. Emerging evidence indicates behavioral traits, such as anxiety, can be related to depression-like symptoms after exposure to chronic stress. However, single traits cannot explain the variability observed in individual's vulnerability to stress, highlighting that a combination of behavioral traits might provide a better characterization of the individual's vulnerability to development of depression following prolonged stress. Hippocampal neurogenesis has been highlighted as a cellular mechanism involved in the reduction on hippocampal volume under conditions of stress and depression, suggesting that a lack of new cells and connections may underlie the cognitive and emotional symptoms of mood disorders, and modulating neurogenesis is regarded as a target/mechanism for antidepressants actions. Macrophage migration inhibitory factor (MIF) is a pro-inflammatory cytokine that is expressed and potentially involved in cellular proliferation in the central nervous system. MIF was shown to induce cell cycle progression via activation of the extracellular signal-regulated kinase (ERK) cascade. Stress has been shown to alter the degree of activation of the multifunctional ERK 1/2 -as indicated by its phosphorylation (p) pERK1/2- and pharmacological manipulations of the ERK pathway affected anxiety and coping behavior. Nonetheless, other mediating mechanisms are involved in the course of both depression and antidepressant actions. Even though the amygdala has been related to the etiology and recovery of depression through imaging studies showing correlations between these mood conditions and amygdala's volume and functional activation, the number of studies devoted to this brain area is still scarce. We hypothesize here an essential role of the amygdala on depression and antidepressant actions based on the implication of the basolateral complex (BLA) in emotional arousal and stress-induced modulation of cognitive processes, and on recent data showing activation of corticotropin releasing factor receptors (CRFR1) as a substrate for stress-induced alterations. Interestingly, alterations in the functional connectivity between the amygdala, the hippocampus and frontal areas have been identified in individuals with genetic vulnerability to depression. Looking for such psychobiological interactions is particularly important today because of increasingly incidence of depression and stress related disorders'. Identifying and understanding psychobiological vulnerability to stress is crucial for best oriented and more effective prevention and intervention strategies. Therefore, the aim of this thesis was: i) to identify in rats behavioral traits capable of predicting differences in vulnerability/resilience to the behavioral, endocrine, neurogenic and brain activity effects of stress, ii) to explore the role of the amygdala and personality traits in the antidepressants' effects on depression-like behavior and hippocampal neurogenesis, and iii) to study the role of MIF expression in hippocampal neurogenesis and anxiety- and depression- like behaviors. Our first study indicates that both the direction and intensity of the impact of stress on behavior corresponds to different patterns of neurogenesis impairment and brain activity; moreover, these effects are dependent on the interaction between personality traits and the length of stress exposure. Chronic stress induced progressively depressive-like symptoms associated with an increase in cell proliferation and a decrease of cell survival in the hippocampus and with a decrease in ERK1/2 activity in the prefrontal cortex, the hippocampus and a U-shape effect in the amygdala. Personality-like profiles of high exploration seem to play a resilient role for the development of stress-induced depressive-like symptoms and the combination of high anxiety and low exploration seems to lead to higher vulnerability to develop depression-like behaviors. Whereas sub chronic stress increased cell proliferation in the combined profile of high anxiety and low exploration, profiles of low exploration showed a decrease in cell survival after exposure to chronic stress. Interestingly, ERK1/2 activity was decreased in control animals of the personality profile high anxiety and low exploration and subsequently increased after sub-chronic and chronic stress in the paraventricular nucleus, the amygdala and the hippocampus but not in the orbitofrontal cortex. The second study provides evidence for an important role for the amygdala on fluoxetine-stimulated cell proliferation and survival, as well as on the establishment of a link between cell proliferation and depression-like behavior. We also show an important role for anxiety in mediating the effects of basolateral amygdala and antidepressant activity on cell proliferation and survival. Chronic fluoxetine treatment had a positive effect on hippocampal cell survival only when the BLA was lesioned. Anxiety was related to hippocampal cell survival in opposite ways in sham- and BLA-lesioned animals (i.e., negatively in sham- and positively in BLA-lesioned animals). Both BLA lesions and low anxiety were critical factors to enable a negative relationship between cell proliferation and depression-like behavior. Therefore, our study highlights a role for the amygdala on fluoxetine-stimulated cell survival and on the establishment of a link between cell proliferation and depression-like behavior. It also reveals an important modulatory role for anxiety on cell proliferation involving both BLA-dependent and -independent mechanisms. The third study demonstrates the relevance of MIF expression for adult hippocampal neurogenesis. We identify MIF expression in neurogenic cells (in stem cells, cells undergoing proliferation and in newly proliferated cells undergoing maturation) in the subgranular zone of the rodent dentate gyrus. A causal role for MIF in cell proliferation was demonstrated using genetic and pharmacological approaches. Behaviorally, genetic deletion of MIF resulted in increased anxiety- and depression-like behaviors, as well as of impaired hippocampus-dependent memory. Together, our studies provide evidence supporting a pivotal role for MIF in both basal and antidepressant-stimulated adult hippocampal cell proliferation. Moreover, loss of MIF results in a behavioral phenotype that to a large extent corresponds with alterations predicted to arise from reduced hippocampal neurogenesis. These findings underscore MIF as a potentially relevant molecular target for the development of treatments linked to deficits in neurogenesis, as well as to problems related to anxiety, depression and cognition. Altogether the results presented support the idea of high anxiety trait individuals being a particularly prone phenotype and exposure to stressful life events as a requirement for individuals with this personality trait to develop depression.

EPFL2012

Psychobiological Vulnerability to Stress

Jorge Eduardo Castro Cifuentes, Maria del Carmen Sandi Perez

EPFL2012