Hippocampal CysLT1R overexpression or activation accelerates memory deficits, synaptic dysfunction, and amyloidogenesis in young APP/PS1 transgenic mice

Background

Our previous studies demonstrated that cysteinyl leukotrienes receptor 1 (CysLT₁R) knockout, pharmacological blockade, or hippocampus knockdown produced beneficial effects against Alzheimer’s disease (AD); however, whether CysLT₁R upregulation has deleterious effects on AD remains elusive.

Methods

In this study, we investigated the changes in behaviors, hippocampal amyloidogenesis, and synapse plasticity after CysLT₁R overexpression by microinfusion of the lentiviral vector, containing its coding sequence of mouse (LV-CysLT₁R), into the bilateral dentate gyri (DG) of the hippocampus or CysLT₁R activation by repeated systemic administration of its agonist YM-17690 (0.1 mg/kg, once a day, i.p., for 28 d).

Results

The behavior data showed that overexpression of CysLT₁R in hippocampal DG or administration of YM-17690 deteriorated behavioral performance in Morris water maze (MWM), Y-maze tests, and novel object recognition (NOR) in young APP/PS1 mice. The further studies showed that these treatments significantly destroyed synaptic function, as evidenced by impaired hippocampal long-term potentiation (LTP), decreased spine density, low number of synapses, and decreased postsynaptic protein (PSD95), and promoted the generation of amyloid β (Aβ) through increased expression of BACE1 and PS1 in the hippocampus of young APP/PS1 mice.

Conclusions

Together, our results indicate that CysLT₁R upregulation accelerates memory impairment in young APP/PS1 mice, which is associated with promoting synaptic dysfunction and amyloidogenesis in the hippocampus.

Takeda G Protein-Coupled Receptor 5 Modulates Depression-like Behaviors via Hippocampal CA3 Pyramidal Neurons Afferent to Dorsolateral Septum

BACKGROUND

Takeda G protein-coupled receptor 5 (TGR5) is recognized as a promising target for type 2 diabetes and metabolic syndrome; its expression has been demonstrated in the brain and is thought to be neuroprotective. Here, we hypothesize that dysfunction of central TGR5 may contribute to the pathogenesis of depression.

METHODS

In well-established chronic social defeat stress (CSDS) and chronic restraint stress (CRS) models of depression, we investigated the functional roles of TGR5 in CA3 pyramidal neurons (PyNs) and underlying mechanisms of the neuronal circuit in depression (for in vivo studies, n = 10; for in vitro studies, n = 5-10) using fiber photometry; optogenetic, chemogenetic, pharmacological, and molecular profiling techniques; and behavioral tests.

RESULTS

Both CSDS and CRS most significantly reduced TGR5 expression of hippocampal CA3 PyNs. Genetic overexpression of TGR5 in CA3 PyNs or intra-CA3 infusion of INT-777, a specific agonist, protected against CSDS and CRS, exerting significant antidepressant-like effects that were mediated via CA3 PyN activation. Conversely, genetic knockout or TGR5 knockdown in CA3 facilitated stress-induced depression-like behaviors. Re-expression of TGR5 in CA3 PyNs rather than infusion of INT-777 significantly improved depression-like behaviors in Tgr5 knockout mice exposed to CSDS or CRS. Silencing and stimulation of CA3 PyNs→somatostatin-GABAergic (gamma-aminobutyric acidergic) neurons of the dorsolateral septum circuit bidirectionally regulated depression-like behaviors, and blockade of this circuit abrogated the antidepressant-like effects from TGR5 activation of CA3 PyNs.

CONCLUSIONS

These findings indicate that TGR5 can regulate depression via CA3 PyNs→somatostatin-GABAergic neurons of dorsolateral septum transmission, suggesting that TGR5 could be a novel target for developing antidepressants.

Oleanolic acid decreases SGK1 in the hippocampus in corticosterone-induced mice

Our previous study has demonstrated that oleanolic acid produced an antidepressant-like effect in mice exposed to chronic stress. Considering that serine/threonine-protein kinase 1 (SGK1) is involved in stress response, the present study aimed to evaluate the involvement of SGK1 in the antidepressant-like effects of oleanolic acid in depression-like mice induced by long term corticosterone (CORT) injection. Behaviors, SGK1, brain-derived neurotrophic factor (BDNF) and its downstream targets were assessed after administration with oleanolic for three weeks. The results indicated that oleanolic acid increased the sucrose preference and decreased the immobility time. In addition, oleanolic acid decreased SGK1 and activated BDNF-AKT/mTOR signaling in the hippocampus of CORT-induced animals. However, we found that GSK650394, an inhibitor of SGK1 did not exert any effects on the behaviors, GR levels and BDNF signaling. The number of spines in hippocampal neurons was not changed by GSK650394 as well. Taken together, this study demonstrated that oleanolic acid produced the antidepressant-like effects, which might be related to the down-regulation of SGK1. However, inhibition of SGK1 directly lacks the effects in the treatment of depression.

miR-124 antagonizes the antidepressant-like effects of standardized gypenosides in mice

Our previous study demonstrated that gypenosides produced antidepressant-like effects in mice exposed to chronic mild stress in a brain-derived neurotrophic factor-dependent manner. However, whether other mechanisms are involved in the antidepressant-like effects of gypenosides is not clear. miR-124 is one of the most abundant microRNAs in the hippocampus, and its dysregulation is related to the pathophysiology of depression. The glucocorticoid receptor is dysfunctional in depression, and it is a direct target of miR-124. Therefore, the present study used corticosterone-induced mice as a model to evaluate the role of miR-124 on the antidepressant-like effects of gypenosides. miR-124 agomir was intracerebrally injected prior to administration of gypenosides and corticosterone injection. Sucrose preference and forced swimming tests were performed 21 days later. Proteins related to glucocorticoid receptors and brain-derived neurotrophic factor-tyrosine receptor kinase B signaling in the hippocampus were evaluated. Our results demonstrated that gypenosides reversed the chronic corticosterone injection-induced decreased sucrose preference and increased immobility time. In contrast, this effect was antagonized by miR-124 injection. In addition, gypenosides increased glucocorticoid receptor and tyrosine receptor kinase B expression in the hippocampus, which activated brain-derived neurotrophic factor signaling. miR-124 also blocked these effects. In conclusion, this study demonstrated that a reduction in miR-124 was required for the antidepressant-like effects of gypenosides induced by chronic corticosterone injection in mice.

microRNA-124 targets glucocorticoid receptor and is involved in depression-like behaviors

Dysregulation of microRNA (miRNA) has been shown to be involved in early observations of depression. MicroRNA-124-3p (miR-124) is the most abundant microRNA in the brain. Previous studies have shown that miR-124 plays a major role in depression. Here we showed that miR-124 directly targeted glucocorticoid receptor (GR) in HEK 293 cells. In addition, inhibition of miR-124 by its antagomir (2nmol/every two days) could reverse the decrease of sucrose preference and the increase of immobility time in mice exposed to chronic corticosterone (CORT, 40mg/kg) injection. Moreover, these effects on behavioral improvement were coupled to the activation of brain-derived neurotrophic factor (BDNF), TrkB, ERK, and CREB, as well as the induction of synaptogenesis and neuronal proliferation. Altogether, our study suggests that miR-124 can be served as a biomarker for depression and a novel target for drug development, and demonstrates that inhibition of miR-124 may be a strategy for treating depression by activating BDNF-TrkB signaling pathway in the hippocampus.

Fluoxetine regulates mTOR signalling in a region-dependent manner in depression-like mice

Previous studies have demonstrated that the mammalian target of rapamycin (mTOR) signaling pathway has an important role in ketamine-induced, rapid antidepressant effects despite the acute administration of fluoxetine not affecting mTOR phosphorylation in the brain. However, the effects of long-term fluoxetine treatment on mTOR modulation have not been assessed to date. In the present study, we examined whether fluoxetine, a type of commonly used antidepressant agent, alters mTOR signaling following chronic administration in different brain regions, including the frontal cortex, hippocampus, amygdala and hypothalamus. We also investigated whether fluoxetine enhanced synaptic protein levels in these regions via the activation of the mTOR signaling pathway and its downstream regulators, p70S6K and 4E-BP-1. The results indicated that chronic fluoxetine treatment attenuated the chronic, unpredictable, mild stress (CUMS)-induced mTOR phosphorylation reduction in the hippocampus and amygdala of mice but not in the frontal cortex or the hypothalamus. Moreover, the CUMS-decreased PSD-95 and synapsin I levels were reversed by fluoxetine, and these effects were blocked by rapamycin only in the hippocampus. In conclusion, our findings suggest that chronic treatment with fluoxetine can induce synaptic protein expression by activating the mTOR signaling pathway in a region-dependent manner and mainly in the hippocampus.