Ya-Jing Liu

Cultured rat cortical astrocytes synthesize melatonin: absence of a diurnal rhythm.

Abstract:  Melatonin not only plays a major role in the regulation of circadian rhythms, but is also involved in antioxidative defense and immunomodulation. Circulating melatonin levels are derived primarily from the pineal gland while other sources of melatonin have also been reported. Here, we show for the first time that astrocytes from the rat cortex and glioma C6 cell line synthesize melatonin in vitro. In addition, we show the presence of serotonin, the precursor of melatonin and the two key enzymes in the pathway of melatonin synthesis, i.e. N‐acetyltransferase and hydroxyndole‐O‐methyltransferase in the cultured rat cortical astrocytes. Release of melatonin into the culture medium showed no diurnal changes. These point to astrocytes as a local source of melatonin in the rat brain. Its exact physiological function remains a topic for future studies.

Increased hippocampal tau phosphorylation and axonal mitochondrial transport in a mouse model of chronic stress

Corticotropin-releasing hormone (CRH) is considered the driving force of the hypothalamo-pituitary-adrenal (HPA) axis and plays an important role in mood regulation. The HPA axis is reported to be closely related to acute stress-induced tau phosphorylation in the rodent hippocampus. However, the relationship between the hyperactive HPA axis and tau phosphorylation in the hippocampus and hence the functional implications for chronic stress are not fully understood. In this study, we aimed to examine tau phosphorylation and the effect on axonal transport of mitochondria in the hippocampus of a chronic stress model. A mouse model was created by neonatal isolation before weaning, followed by chronic mild stress by social isolation after weaning. Behavioural tests showed that the model had a typical depression/anxiety-like behaviour accompanied by increased plasma corticosterone level and hypothalamic CRH mRNA expression. Phosphorylated tau increased significantly, accompanied by increased synaptosomal mitochondrial levels in hippocampus of the chronic stress model. CRH receptor 1 antagonist (CP154,526) treatment, not glucocorticoid receptor antagonist (RU486) treatment, decreased tau phosphorylation and synaptosomal mitochondrial levels in the hippocampus of the mouse model. Consistent with an in-vivo model, when hyperphosphorylated tau was inhibited by lithium in cultured primary hippocampal neurons, mitochondrial transport monitored by live imaging was also decreased. We show here for the first time that phosphorylated tau in the hippocampus of a chronic stress model, accompanied by increased mitochondrial transport, was mediated by CRH receptor 1, not by glucocorticoid receptors, which suggests that centrally derived CRH may be involved in the process of mitochondrial axon transport and hence play an important role in hippocampus of a chronic stress model.

Apolipoprotein E influences melatonin biosynthesis by regulating NAT and MAOA expression in C6 cells

Abstract: Previous studies have demonstrated that apolipoprotein E (ApoE) genotype and melatonin are closely associated with Alzheimer’s disease (AD). However, the relationship between ApoE genotype and melatonin remains unclear. Recently, we reported that cultured rat cortical astrocytes and glioma C6 cells synthesize melatonin. In the current study, we investigated the effect of ApoE genotype on melatonin biosynthesis. C6 cells with stable expression of ApoE isoforms (ApoE 2, 3 and 4) were established. A higher level of melatonin was demonstrated in cultured ApoE4‐C6 cells than that in ApoE3‐C6 cells. In addition, we found that N‐acetyltransferase (NAT) protein level was up‐regulated in ApoE4‐C6 cells compared with ApoE3‐C6 cells. Further study suggested that mRNA expression of monoamine oxidase A (MAOA) and monoamine oxidase B (MAOB) decreased in ApoE4‐C6 cells. In conclusion, the increased melatonin level in ApoE4‐C6 cells results from up‐regulation of NAT expression, a key enzyme for melatonin synthesis, and down‐regulation of MAOA and MAOB expression, the metabolic enzyme for its precursor serotonin.

Microglial activation mediates host neuronal survival induced by neural stem cells.

The rational of neural stem cells (NSCs) in the therapy of neurological disease is either to replace dead neurons or to improve host neuronal survival, the latter of which has got less attention and the underlying mechanism is as yet little known. Using a transwell co‐culture system, we reported that, in organotypic brain slice cultures, NSCs significantly improved host neuronal viability. Interestingly, this beneficial effect of NSCs was abrogated by a microglial inhibitor minocycline, while it was mimicked by a microglial agonist, Toll‐like receptor 9 (TLR9) ligand CpG‐ODN, which supports the pro‐vital mediation by microglia on this NSCs‐improved neuronal survival. Moreover, we showed that NSCs significantly induced host microglial movement and higher expression of a microglial marker IBA‐1, the latter of which was positively correlated with TLR9 or extracellular‐regulated protein kinases 1/2 (ERK1/2) activation. Real‐time PCR revealed that NSCs inhibited the expression of pro‐inflammatory molecules, but significantly increased the expression of molecules associated with a neuroprotective phenotype such as CX3CR1, triggering receptor expressed on myeloid cells‐2 (TREM2) and insulin growth factor 1 (IGF‐1). Similarly, in the microglia cells, NSCs induced the same microglial response as that in the slices. Further treatment with TLR9 ligand CpG‐ODN, TLR9 inhibitor chloroquine (CQ) or ERK1/2 inhibitor U0126 demonstrated that TLR9‐ERK1/2 pathway was involved in the NSCs‐induced microglial activation. Collectively, this study indicated that NSCs improve host neuronal survival by switching microglia from a detrimental to a neuroprotective phenotype in adult mouse brain, and the microglial TLR9‐ERK1/2 pathway seems to participate in this NSCs‐mediated rescue action.

The influence of chronic stress on anxiety-like behavior and cognitive function in different human GFAP-ApoE transgenic adult male mice

Abstract The apolipoprotein E (ApoE) ɛ4 allele (ApoE4) is an important genetic risk factor for the pathogenesis of Alzheimer’s disease (AD). In addition to genetic factors, environmental factors such as stress may play a critical role in AD pathogenesis. This study was designed to investigate the anxiety-like behavioral and cognitive changes in different human glial fibrillary acidic protein (GFAP)-ApoE transgenic adult male mice under chronic stress conditions. On the open field test, anxiety-like behavior was increased in the non-stressed GFAP-ApoE4 transgenic mice relative to the corresponding GFAP-ApoE3 (ApoE ɛ3 allele) mice. Anxiety-like behavior was increased in the stressed GFAP-ApoE3 mice relative to non-stressed GFAP-ApoE3 mice, but was unexpectedly decreased in the stressed GFAP-ApoE4 mice relative to non-stressed GFAP-ApoE4 mice. On the novel object recognition task, both GFAP-ApoE4 and GFAP-ApoE3 mice exhibited long-term non-spatial memory impairment after chronic stress. Interestingly, short-term non-spatial memory impairment (based on the novel object recognition task) was observed only in the stressed GFAP-ApoE4 male mice relative to non-stressed GFAP-ApoE4 transgenic mice. In addition, short-term spatial memory impairment was observed in the stressed GFAP-ApoE3 transgenic male mice relative to non-stressed GFAP-ApoE3 transgenic male mice; however, short-term spatial memory performance of GFAP-ApoE4 transgenic male mice was not reduced compared to non-stressed control mice based on the Y-maze task. In conclusion, our findings suggested that chronic stress affects anxiety-like behavior and spatial and non-spatial memory in GFAP-ApoE transgenic mice in an ApoE isoform-dependent manner.