Dexamethasone inhibits the maturation of newly formed neurons and glia supplemented with polyunsaturated fatty acids.

Stress bears a negative impact on adult neurogenesis. High levels of corticoids have been shown to inhibit neural stem cell proliferation, and are considered responsible for the loss of neural precursors. Their effects on the differentiation of the glial and neuronal lineages have been less studied. We examined the effect of dexamethasone (Dex), a synthetic glucocorticoid, on the differentiation of rat neural stem cells in vitro. Dex had no effect on the differentiation of cells cultured under standard conditions. Since we previously determined that NSC, when cultured under classical conditions, were deprived of polyunsaturated fatty acids (PUFA), and displayed phospholipid compositions very different from the in vivo figures [1], we examined the effect of Dex under PUFA supplementation. Dex impaired neuron and oligodendrocyte maturation in PUFA-supplemented cells, demonstrated by the reduction of neurite lengths and oligodendrocyte sizes. This effect was mediated by the glucocorticoid receptor (GR), since it was eliminated by mifepristone, a GR antagonist, and could be relayed by a reduction of ERK phosphorylation. We determined that GR was associated with PPAR ß and α under basal conditions, and that this association was disrupted when PUFA were added in combination with Dex. We assumed that this effect on the receptor status enabled the effect of Dex on PUFA supplemented cells, since we determined that the binding to the glucocorticoid response element was higher in cells incubated with PUFA and Dex. In conclusion, corticoids can impair NSC differentiation, and consequently impact the entire process of neurogenesis.

The role of PPARß/δ in the regulation of glutamatergic signaling in the hamster suprachiasmatic nucleus.

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily and function as transcription factors that regulate gene expression in numerous biological processes. Although the PPARß/δ subtype is highly expressed in the brain, its physiological roles in neuronal function remain to be elucidated. In this study, we examined the presence of PPARß/δ in the master circadian clock of the Syrian hamster and investigated its putative functional role in this structure. In mammals, the central circadian clock, located in the suprachiasmatic nucleus (SCN), is entrained by the light-dark (LD) cycle via photic6 signals conveyed by a direct pathway whose terminals release glutamate. Using immunocytochemical and qRT-PCR analysis, we demonstrated that the rhythmic expression of PPAR ß/δ within the SCN of hamsters raised under an LD cycle was detectable only at the transcriptional level when the hamsters were maintained under constant darkness (DD). The increase in the number of immunoreactive PPARß/δ cells observed under DD after light stimulation during the early subjective night (CT14), but not during the subjective day (CT06), demonstrated that the expression of PPARß/δ can be up-regulated according to the photosensitive phase of the circadian clock. All of the PPARß/δ-positive cells in the SCN also expressed the glutamate receptor NMDAR1. Moreover, we demonstrated that at the photosensitive point (CT14), the administration of L-16504, a specific agonist of PPARß/δ, amplified the phase delay of the locomotor response induced by a light pulse. Taken together, these data suggest that PPARß/δ activation modulates glutamate release that mediates entrainment of the circadian clock by light.

Rat neural stem cell proliferation and differentiation are durably altered by the in utero polyunsaturated fatty acid supply.

We isolated neural stem cells/neural progenitors (NSC) from 1-day-old rat pups born to mothers fed diets that were deficient or supplemented with n-3 polyunsaturated fatty acids (PUFAs) and compared their proliferation and differentiation in vitro. The cells isolated from the n-3PUFA-deficient pups consistently proliferated more slowly than cells that were isolated from n-3PUFA-supplemented pups, despite the fact that both were cultured under the same conditions. The differences in the proliferation rates were evaluated up until 40 days of culture and were highly significant. When the cells were allowed to differentiate, the deficient cells exhibited a higher degree of neuronal maturation in response to the addition of PUFAs in the medium, as demonstrated by an increase in neurite length, whereas the neurons derived from the supplemented pups showed no change. This result was consistent, regardless of the age of the culture. The properties of the NSC were durably modified throughout the length of the culture, although the membrane phospholipid compositions were similar. We examined the differential expression of selected mRNAs and micro RNAs. We found significant differences in the gene expression of proliferating and differentiating cells, and a group of genes involved in neurogenesis was specifically modified by n-3 PUFA treatment. We conclude that n-3 PUFA levels in the maternal diet can induce persistent modifications of the proliferation and differentiation of NSCs and of their transcriptome. Therefore, the n-3 supply received in utero may condition on a long-term basis cell renewal in the brain.

Astrocytes in culture require docosahexaenoic acid to restore the n-3/n-6 polyunsaturated fatty acid balance in their membrane phospholipids.

Docosahexaenoic acid (DHA), the main n-3 polyunsaturated fatty acid (PUFA) in membranes, is particularly abundant in brain cells. Decreased cerebral concentrations of DHA, resulting from dietary n-3 deficiency, are associated with impaired cognitive function. Because the cellular causes of this impairment are still unknown, we need in vitro models that mimic the variations in n-3/n-6 PUFA seen in vivo. We have compared the PUFA profiles of hamster astrocytes cultured in medium supplemented with long-chain PUFA [DHA and/or arachidonic acid (AA)] with those of brain tissue from hamsters fed an n-6/n-3 PUFA-balanced diet or one lacking n-3 PUFA. Astrocytes were obtained from the brain cortex of newborn hamsters and cultured in minimum essential medium + 5% fetal calf serum (FCS) supplemented with DHA and/or AA for 10 days. The astrocytes cultured in medium + FCS had low n-3 PUFA contents, comparable to those of brain tissue from hamsters fed an n-3-deficient diet. We have shown that astrocytes grown in medium supplemented with DHA and/or AA, plus alpha-tocopherol to prevent lipid peroxidation, incorporated large amounts of these long-chain PUFA, so that the n-6/n-3 PUFA compositions of the phosphatidylethanolamine and phosphatidylcholine, the two main classes of membrane phospholipids, were greatly altered. Astrocytes cultured in medium plus DHA had a more physiological n-3 status, grew better, and retained their astrocyte phenotype. Thus astrocytes in culture are likely to be physiologically relevant only when provided with adequate DHA. This reliable method of altering membrane phospholipid composition promises to be useful for studying the influence of n-6/n-3 imbalance on astrocyte function.