Molecular diversity of diencephalic astrocytes reveals adult astrogenesis regulated by Smad4.

Astrocytes regulate brain-wide functions and also show region-specific differences, but little is known about how general and region-specific functions are aligned at the single-cell level. To explore this, we isolated adult mouse diencephalic astrocytes by ACSA-2-mediated magnetic-activated cell sorting (MACS). Single-cell RNA-seq revealed 7 gene expression clusters of astrocytes, with 4 forming a supercluster. Within the supercluster, cells differed by gene expression related to ion homeostasis or metabolism, with the former sharing gene expression with other regions and the latter being restricted to specific regions. All clusters showed expression of proliferation-related genes, and proliferation of diencephalic astrocytes was confirmed by immunostaining. Clonal analysis demonstrated low level of astrogenesis in the adult diencephalon, but not in cerebral cortex grey matter. This led to the identification of Smad4 as a key regulator of diencephalic astrocyte in vivo proliferation and in vitro neurosphere formation. Thus, astrocytes show diverse gene expression states related to distinct functions with some subsets being more widespread while others are more regionally restricted. However, all share low-level proliferation revealing the novel concept of adult astrogenesis in the diencephalon.

Psychological stress increases histone H3 phosphorylation in adult dentate gyrus granule neurons: involvement in a glucocorticoid receptor-dependent behavioural response.

Chromatin remodelling associated with transcriptional activation of silent genes involves phosphorylation at Serine-10 and acetylation at Lysine-14 in the N-terminal tails of the nucleosomal protein histone H3. We have identified neurons predominantly in the dentate gyrus showing a speckled nuclear immunoreactivity pattern for phosphorylated histone H3 [i.e. P(Ser10)-H3] and phospho-acetylated histone H3 [i.e. P(Ser10)-Ac(Lys14)-H3]. Forced swimming increased the number of P(Ser10)-H3-positive [P(Ser10)-H3+] neurons in the rat and mouse dentate gyrus. Exposure of mice to a predator had a similar effect, but exposing rats to ether vapour or a cold environment evoked no change in the number of P(Ser10)-H3+ dentate neurons, indicating that the effect of stress on histone H3 phosphorylation is stressor-specific. The forced swimming-induced increase in dentate P(Ser10)-H3+ neurons peaked at 8-24 h, was restricted to NeuN+ (i.e. mature) neurons, and occurred mainly in the middle and superficial aspects of the granular cell layer. Moreover, this increase showed stimulus strength dependency (i.e. swimming at 19 degrees C produced a larger increase than swimming at 25 degrees C) and could be blocked by the glucocorticoid receptor (GR) antagonists RU 38486 and ORG 34517. Under these experimental conditions, when the forced swimming-induced behavioural immobility response was determined in a re-test 24 h after the initial forced swim test, striking correlations were observed between the phosphorylation of histone H3 in dentate gyrus granule neurons and the acquired immobility response. Our data indicate that stressful events with a strong psychological component such as forced swimming evoke distinct GR-dependent histone modifications in mature dentate gyrus granule neurons that may participate in the behavioural adaptation of the organism to this event.

Effects of long-term voluntary exercise on the mouse hypothalamic-pituitary-adrenocortical axis.

We studied the effects of long-term (i.e. 4 wk) voluntary exercise on the hypothalamic-pituitary-adrenocortical (HPA) axis in male mice. Voluntary exercise was provided by giving mice access to a running wheel, in which they indeed ran for about 4 km/d. Exercising mice showed similar body weights as control animals but presented less abdominal fat, lighter thymuses, and heavier adrenal glands. Exercise resulted in asymmetric structural changes in the adrenal glands. Whereas control mice had larger left than right adrenals, this condition was abolished in exercising animals, mainly because of enlargement of the right adrenal cortex. Tyrosine hydroxylase mRNA expression in the adrenal medullas of exercising mice was increased. In exercising mice, early-morning baseline plasma ACTH levels were decreased, whereas plasma corticosterone levels at the start of the dark phase were twice as high as those in control animals. To forced swimming and restraint stress, exercising mice responded with higher corticosterone levels than those of the control animals but with similar ACTH levels. However, if exposed to a novel environment, then exercising mice presented decreased ACTH responses. Interestingly, exercising mice showed a decreased corticosterone response to novelty only when the novel environment contained a functioning running wheel. Glucocorticoid receptor levels were unchanged, whereas mineralocorticoid receptor levels were decreased, in hippocampus of exercising animals. Corticotropin-releasing factor mRNA levels in the paraventricular nucleus were lower in exercising mice. Thus, voluntary exercise results in complex, adaptive changes at various levels within the HPA axis as well as in sympathoadrenomedullary and limbic/neocortical afferent control mechanisms. These changes seem to underlie the differential responsiveness of the HPA axis to physical vs. emotional challenges.