The Mineralocorticoid Agonist Fludrocortisone Promotes Survival and Proliferation of Adult Hippocampal Progenitors

Mineralocorticoid receptor agonist aldosterone rescues hippocampal neural stem cell proliferation defects and improves postoperative cognitive function in aged mice

OBJECTIVES

Hippocampal neurogenesis is closely related to learning and memory, and hippocampal neurogenesis disorders are involved in the development of many neurodegenerative diseases. Mineralocorticoid receptor (MR) plays a vital role in regulating stress response, neuroendocrine and cognitive functions, and is involved in regulating the integrity and stability of neural networks. However, the potential role of MR in the pathogenesis of postoperative cognitive dysfunction (POCD) is unclear. Therefore, this study evaluated the effect and mechanism of MR activation on postoperative hippocampal neurogenesis and cognitive function in aged mice.

METHODS

18-month-old male Kunming mice were randomly divided into Control group (C group), Surgery group (S group), Surgery+ Aldosterone group (S+Aldo group), Surgery + Wortmannin group (S+Wort group), Surgery + Aldosterone + Wortmannin group (S+Aldo+Wort group). Laparotomy was used to establish an animal model of postoperative cognitive dysfunction. After surgery, mice were intraperitoneally injected with aldosterone (100 ug/kg,150 ug/kg,200 ug/kg) and / or wortmannin (1 mg/kg); One day before the sacrifice, mice were injected intraperitoneally with BrdU (100 mg / kg / time, 3 times in total). Mice were subjected to Morris water maze and field tests at 1, 3, 7, and 14 days after surgery. Immunofluorescence was used to detect the number of BrdU +, Nestin +, BrdU/Nestin + positive cells in the hippocampal dentate gyrus of mice at 1, 3, 7 and 14 days after surgery. Western-blot was used to detect PI3K/Akt/GSK-3β signaling pathway related proteins Akt, p-Akt, GSK-3β, P-GSK-3β expression.

RESULTS

Stress impairs the performance of aged mice in water maze and open field tests, reduces the number of BrdU/Nestin+ cells in the hippocampal dentate gyrus, and inhibits the phosphorylation of Akt and GSK-3β proteins in the hippocampus. Aldosterone treatment promotes P-Akt, P-GSK-3β protein expression and hippocampal neural stem cell proliferation, and improves postoperative cognitive dysfunction. However, wortmannin treatment significantly reversed these effects of aldosterone.

CONCLUSIONS

The mineralocorticoid receptor agonist aldosterone promotes the proliferation of hippocampal neural stem cells and improves cognitive dysfunction in aged mice after surgery, and the mechanism may be related to activation of PI3K/Akt/GSK-3β signaling.

Effect of eplerenone on cognitive impairment in spontaneously hypertensive rats

OBJECTIVE

The present study aimed to determine the effect of blocking brain mineralocorticoid receptor on cognitive impairment in spontaneously hypertensive rats and its intracellular changes.

METHODS

12-week-old male spontaneous hypertensive rats (SHR) and Wistar Kyoto (WKY) rats were given eplerenone (EPL, 30 mg/Kg/d or 100 mg/Kg/d) or pure water via oral gavage daily for 8 weeks. Effects of blocking brain mineralocorticoid receptor (MR) on cognitive function were examined through cognitive behavioral experiments. The morphology of hippocampal neurons was observed. Synaptic proteins and autophagy levels were detected by western blot.

RESULTS

The results showed decreases in both short-term working memory and long-term spatial learning and memory ability, hippocampal neuron damage, and reduced expression of synaptic proteins in the SHR-Veh group. Impaired autophagy was found in the SHR-Veh group as evidenced by decreased expression levels of Beclin-1 protein and a defect in P62 degradation. These abnormalities were reversed by eplerenone, either the high dosage or low dosage. Reduced cognitive dysfunction and enhanced autophagy in hippocampal neurons in both SHR-EPL30 group and SHR-EPL100 group were independent of lowering blood pressure.

CONCLUSION

Eplerenone improves cognitive deficits observed in SHRs, and increases autophagy in hippocampal neurons of SHRs, which suggests a new site of MR antagonists in treatment of hypertension-related cognitive impairment.

Brain Mineralocorticoid receptor in health and disease: from molecular signaling to cognitive and emotional function

Brain mineralocorticoid receptors (MR) mediate effects of glucocorticoid hormones in stress adaptation, as well as the effects of aldosterone itself in relation to salt homeostasis. Brain stem MRs respond to aldosterone, whereas forebrain MRs mediate rapid and delayed glucocorticoid effects in conjunction with the glucocorticoid receptor (GR). MR‐mediated effects depend on age, gender, genetic variations, and environmental influences. Disturbed MR activity through chronic stress, certain (endocrine) diseases or during glucocorticoid therapy can cause deleterious effects on affective state, cognitive and behavioural function in susceptible individuals. Considering the important role MR plays in cognition and emotional function in health and disease, MR modulation by pharmacological intervention could relieve stress‐ and endocrine‐related symptoms. Here, we discuss recent pharmacological interventions in the clinic and genetic developments in the molecular underpinnings of MR signalling. Further understanding of MR‐dependent pathways may help to improve psychiatric symptoms in a diversity of settings.

Sulfoquinovosyl oleoyl palmitoyl glycerol (SQDG) and hexane extract of Sargassum plagyophylum prevent depression induced by dexamethasone or stress in mice

Introduction: M Glucocorticoids and stress are a leading cause of depression by dysregulation of hypothalamic hypophyseal adrenal axis. Sargassum plagyophylum hexane extract (HxE) has proven antidepressant-like effects in mice. We aimed at evaluating HxE and sulfoquinovosyl oleoyl palmitoyl glycerol (SQDG) isolated compound antidepressant effects following dexamethasone (Dex) or water avoidance stress (WAS) induced depression in mice. Methods: The HxE was prepared and fractionated by different chromatography methods to isolate active compounds. Depression was induced in male mice by Dex single dose or by four days of WAS. After the locomotor test, depression was assessed by measuring the immobility time during the forced swimming test (FST) and sucrose preference test. Results: 6-Deoxy-6-sulpho-α-D-glucopyranosyl-1,2-O-diacyl-glycerol was isolated and elucidated from the seaweed. The manipulations did not cause important changes in animals’ locomotor activity. During FST, immobility time increased dramatically by Dex (193 ± 13 s vs control 109 ± 7 s) or WAS (189 ± 13 s vs sham 86 ± 14 s), that indicated depression. HxE 40 mg/kg reduced the immobility time when it was administered with Dex (110 ± 28 s, P < 0.01 vs Dex alone) or together with WAS (86 ± 11 s, P < 0.001 vs WAS). SQDG 40 mg/kg reduced the immobility time when co-administered with Dex (22 ± 9 s, P < 0.001 vs Dex alone) and when it was administered along with WAS (68 ± 16 s, P < 0.001 vs WAS). The results of the sucrose preference test were in line with FST results as sucrose preference below 65% was considered for anhedonia. Conclusion: SQDG and probably the steroid content in S. plagyophylum HxE prevented depression in mice; thus, they should be considered for further clinical evaluations.

Glucocorticoid-mediated mechanisms of hippocampal damage: Contribution of subgranular neurogenesis

A comprehensive overview of the interplay between glucocorticoids (GCs) and adult hippocampal neurogenesis (AHN) is presented, particularly, in the context of a diseased brain. The effectors of GCs in the dentate gyrus neurogenic niche of the hippocampal are reviewed, and the consequences of the GC signaling on the generation and integration of new neurons are discussed. Recent findings demonstrating how GC signaling mediates impairments of the AHN in various brain pathologies are overviewed. GC-mediated effects on the generation and integration of adult-born neurons in the hippocampal dentate gyrus depend on the nature, severity, and duration of the acting stress factor. GCs realize their effects on the AHN primarily via specific glucocorticoid and mineralocorticoid receptors. Disruption of the reciprocal regulation between the hypothalamic-pituitary-adrenal (HPA) axis and the generation of the adult-born granular neurons is currently considered to be a key mechanism implicating the AHN into the pathogenesis of numerous brain diseases, including those without a direct hippocampal damage. These alterations vary from reduced proliferation of stem and progenitor cells to increased cell death and abnormalities in morphology, connectivity, and localization of young neurons. Although the involvement of the mutual regulation between the HPA axis and the AHN in the pathogenesis of cognitive deficits and mood impairments is evident, several unresolved critical issues are stated. Understanding the details of GC-mediated mechanisms involved in the alterations in AHN could enable the identification of molecular targets for ameliorating pathology-induced imbalance in the HPA axis/AHN mutual regulation to conquer cognitive and psychiatric disturbances.

Inadequate Expression and Activation of Mineralocorticoid Receptor Aggravates Spatial Memory Impairment after Traumatic Brain Injury

The administration of glucocorticoids (GCs) for the treatment of traumatic brain injury (TBI) is controversial. Both protective and deleterious effects of GCs on the brain have been reported in previous studies, while the mechanisms are unclear. Most experimental studies have reported glucocorticoid receptor (GR)-mediated deleterious effects after TBI. Sufficient mineralocorticoid receptor (MR) activation was reported to be indispensable for normal function and survival of hippocampal neurons, but changes in MR expression and activation and the roles of MRs in the survival of neurons after TBI remain unclear. We hypothesized that inadequate MR expression and activation caused by TBI aggravates posttraumatic hippocampal apoptosis but that restoration by restoring MRs promotes the survival of neurons. Using a rat controlled cortical impact model, we examined plasma corticosterone, MR expression and activation, neuronal apoptosis in the hippocampus, and spatial memory on day 3 after injury with and without fludrocortisone (1 mg/kg) treatment. Plasma corticosterone levels were significantly reduced after TBI. In addition, both MR expression and activation were inhibited. Fludrocortisone treatment significantly increased both the expression and activation of MRs, reduced the number of apoptotic neurons and cell loss in the ipsilateral hippocampus, and subsequently improved spatial memory. Its protective effects were counteracted by the MR antagonist spironolactone. The results suggest that adequate expression and activation of MRs is crucial for the survival of neurons after TBI and that fludrocortisone protects hippocampal neurons via promoting MR expression and activation.

Adult neurogenesis in the mammalian dentate gyrus

Earlier observations in neuroscience suggested that no new neurons form in the mature central nervous system. Evidence now indicates that new neurons do form in the adult mammalian brain. Two regions of the mature mammalian brain generate new neurons: (a) the border of the lateral ventricles of the brain (subventricular zone) and (b) the subgranular zone (SGZ) of the dentate gyrus of the hippocampus. This review focuses only on new neuron formation in the dentate gyrus of the hippocampus. During normal prenatal and early postnatal development, neural stem cells (NSCs) give rise to differentiated neurons. NSCs persist in the dentate gyrus SGZ, undergoing cell division, with some daughter cells differentiating into functional neurons that participate in learning and memory and general cognition through integration into pre-existing neural networks. Axons, which emanate from neurons in the entorhinal cortex, synapse with dendrites of the granule cells (small neurons) of the dentate gyrus. Axons from granule cells synapse with pyramidal cells in the hippocampal CA3 region, which send axons to synapse with CA1 hippocampal pyramidal cells that send their axons out of the hippocampus proper. Adult neurogenesis includes proliferation, differentiation, migration, the death of some newly formed cells and final integration of surviving cells into neural networks. We summarise these processes in adult mammalian hippocampal neurogenesis and discuss the roles of major signalling molecules that influence neurogenesis, including neurotransmitters and some hormones. The recent controversy raised concerning whether or not adult neurogenesis occurs in humans also is discussed.

Corticosterone Replacement Alleviates Hippocampal Neuronal Apoptosis and Spatial Memory Impairment Induced by Dexamethasone via Promoting Brain Corticosteroid Receptor Rebalance after Traumatic Brain Injury

The balance of mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) is indispensable for maintaining the normal function and structure of the hippocampus. However, changes in GR/MR and their effect on the survival of hippocampal neurons after traumatic brain injury (TBI) are still unclear. Previous studies have indicated that high-dose glucocorticoids (GC) aggravate hippocampal neuronal damage after TBI. We hypothesize that the imbalance of GR/MR expression and activation caused by injury and irrational use of dexamethasone (DEX) aggravates post-traumatic hippocampal apoptosis and spatial memory dysfunction, but that restoration by refilling MR and inhibiting GR promotes the survival of neurons. Using rat controlled cortical impact model, we examined the plasma corticosterone (CORT), corticosteroid receptor expression, apoptosis, and cell loss in the hippocampus, and, accordingly, the spatial memory after TBI and GC treatment within 7 days. Plasma CORT, MR, and GR expression level were significantly reduced at 2 days after TBI. Accordingly, the number of apoptotic cells also peaked at 2 days. Compared with the TBI control group, DEX treatment (5 mg/kg) significantly reduced plasma CORT, upregulated GR expression, and increased the number of apoptotic cells and cell loss, whereas CORT replacement (0.3 mg/kg) upregulated MR expression, inhibited apoptosis, and improved spatial memory. The deleterious and protective effects of DEX and CORT were counteracted by spironolactone and mifepristone respectively. The results suggest that inhibition of GR by RU486 or the refilling of MR by CORT protects hippocampal neurons and alleviates spatial memory impairment via promoting GR/MR rebalancing after TBI.

Glucocorticoid Negative Feedback in Regulation of the Hypothalamic-Pituitary-Adrenal Axis in Rhesus Monkeys With Various Types of Adaptive Behavior: Individual and Age-Related Differences

The study of the mechanisms underlying the increased vulnerability of the individual to stressful environmental factors in different age periods is of great relevance for prevention and effective treatment of stress-dependent diseases that are widespread in the population of aging individuals. The purpose of our study was to investigate the individual and age-related features of the glucocorticoid negative feedback in regulation of the hypothalamic-pituitary-adrenal (HPA) axis, the key adaptive neuroendocrine system, in experiments with physically healthy young and old female rhesus monkeys with administration of mineracorticoid receptor (fludrocortisone) and glucocorticoid receptor (dexamethasone) agonists. We studied the monkeys with increased trait anxiety and depression-like behavior (DAB) characterized, as previously was shown, by the increased vulnerability to acute stress and the animals with normal standard behavior (SB) as the control. The pronounced individual differences in the reaction of HPA axis to fludrocortisone and dexamethasone in young animals were found. Young animals with DAB showed a lower sensitivity of HPA axis to the inhibitory effect of both fludrocortisone and dexamethasone compared with young animals with SB. At the same time, there were no significant intergroup differences in the concentration of ACTH and cortisol in response to placebo injection, i.e., in basal conditions. The old individuals with DAB demonstrated the essential relative resistance of HPA axis to fludrocortisone test and higher basal plasma levels of cortisol and ACTH in the evening (the period of HPA axis low circadian activity) compared to old SB animals. In the same time, the intergroup differences in the response of HPA axis to dexamethasone administration were leveled due to age-related increase in sensitivity of HPA axis to dexamethasone in animals with DAB. These data testify the pronounced intergroup and age differences in the feedback regulation of HPA axis, presumably resulting from unequal individual, and age-related changes in the activity of mineralcorticoid and glucocorticoid receptors in the brain structures supporting the functions of HPA axis. The maximum age disorders in functioning of the negative feedback mechanism in the regulation of HPA axis are characteristic of animals with DAB, which, apparently, underlie the increased vulnerability of these animals to stress exposure.

Steroid and Xenobiotic Receptor Signalling in Apoptosis and Autophagy of the Nervous System

Apoptosis and autophagy are involved in neural development and in the response of the nervous system to a variety of insults. Apoptosis is responsible for cell elimination, whereas autophagy can eliminate the cells or keep them alive, even in conditions lacking trophic factors. Therefore, both processes may function synergistically or antagonistically. Steroid and xenobiotic receptors are regulators of apoptosis and autophagy; however, their actions in various pathologies are complex. In general, the estrogen (ER), progesterone (PR), and mineralocorticoid (MR) receptors mediate anti-apoptotic signalling, whereas the androgen (AR) and glucocorticoid (GR) receptors participate in pro-apoptotic pathways. ER-mediated neuroprotection is attributed to estrogen and selective ER modulators in apoptosis- and autophagy-related neurodegenerative diseases, such as Alzheimer’s and Parkinson’s diseases, stroke, multiple sclerosis, and retinopathies. PR activation appeared particularly effective in treating traumatic brain and spinal cord injuries and ischemic stroke. Except for in the retina, activated GR is engaged in neuronal cell death, whereas MR signalling appeared to be associated with neuroprotection. In addition to steroid receptors, the aryl hydrocarbon receptor (AHR) mediates the induction and propagation of apoptosis, whereas the peroxisome proliferator-activated receptors (PPARs) inhibit this programmed cell death. Most of the retinoid X receptor-related xenobiotic receptors stimulate apoptotic processes that accompany neural pathologies. Among the possible therapeutic strategies based on targeting apoptosis via steroid and xenobiotic receptors, the most promising are the selective modulators of the ER, AR, AHR, PPARγ agonists, flavonoids, and miRNAs. The prospective therapies to overcome neuronal cell death by targeting autophagy via steroid and xenobiotic receptors are much less recognized.

Actions of Brain-Derived Neurotrophic Factor and Glucocorticoid Stress in Neurogenesis

Altered neurogenesis is suggested to be involved in the onset of brain diseases, including mental disorders and neurodegenerative diseases. Neurotrophic factors are well known for their positive effects on the proliferation/differentiation of both embryonic and adult neural stem/progenitor cells (NSCs/NPCs). Especially, brain-derived neurotrophic factor (BDNF) has been extensively investigated because of its roles in the differentiation/maturation of NSCs/NPCs. On the other hand, recent evidence indicates a negative impact of the stress hormone glucocorticoids (GCs) on the cell fate of NSCs/NPCs, which is also related to the pathophysiology of brain diseases, such as depression and autism spectrum disorder. Furthermore, studies including ours have demonstrated functional interactions between neurotrophic factors and GCs in neural events, including neurogenesis. In this review, we show and discuss relationships among the behaviors of NSCs/NPCs, BDNF, and GCs.

A Refill for the Brain Mineralocorticoid Receptor: The Benefit of Cortisol Add-On to Dexamethasone Therapy

Some serious medical conditions require life-saving treatment with high doses of synthetic glucocorticoids such as dexamethasone. A substantial number of patients subjected to this treatment develops psychosis, mood disturbances, or sleep problems. A recent clinical trial demonstrated that dexamethasone therapy for young patients with acute lymphoblastic leukemia caused severe adverse psychological effects and sleep disturbances in about 30% of these patients. These side effects were ameliorated by coadministration of a low dose of the naturally occurring glucocorticoid hormone cortisol. This paradoxical finding was predicted by the idea that the synthetic glucocorticoid targets the glucocorticoid receptor, causing suppression of cortisol secretion and, thus, depletion of the brain mineralocorticoid receptor (MR) of its endogenous ligand. The refill of the unoccupied brain MR with physiological amounts of cortisol ameliorates the dexamethasone-induced psychological side effects. In the present report, we discuss the mechanistic underpinning of the MR refill concept in glucocorticoid therapy.

Glucocorticoid-Induced Leucine Zipper in Central Nervous System Health and Disease

The central nervous system (CNS) is a large network of intercommunicating cells that function to maintain tissue health and homeostasis. Considerable evidence suggests that glucocorticoids exert both neuroprotective and neurodegenerative effects on the CNS. Glucocorticoids act by binding two related receptors in the cytoplasm, the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR). The glucocorticoid receptor complex mediates cellular responses by transactivating target genes and by protein: protein interactions. The paradoxical effects of glucocorticoids on neuronal survival and death have been attributed to the concentration and the ratio of mineralocorticoid to glucocorticoid receptor activation. Glucocorticoid-induced leucine zipper (GILZ) is a recently identified protein transcriptionally upregulated by glucocorticoids. Constitutively, expressed in many tissues including brain, GILZ mediates many of the actions of glucocorticoids. It mimics the anti-inflammatory and anti-proliferative effects of glucocorticoids but exerts differential effects on stem cell differentiation and lineage development. Recent experimental data on the effects of GILZ following induced stress or trauma suggest potential roles in CNS diseases. Here, we provide a short overview of the role of GILZ in CNS health and discuss three potential rationales for the role of GILZ in Alzheimer's disease pathogenesis.

Effective PI3K modulators for improved therapy against malignant tumors and for neuroprotection of brain damage after tumor therapy (Review)

Due to the key role in various cellular processes including cell proliferation and cell survival on many cell types, dysregulation of the PI3K/AKT pathway represents a crucial step of the pathogenesis in many diseases. Furthermore, the tumor suppressor PTEN negatively regulates the PI3K/AKT pathway through its lipid phosphatase activity, which is recognized as one of the most frequently deleted and/or mutated genes in human cancer. Given the pervasive involvement of this pathway, the development of the molecules that modulate this PI3K/AKT signaling has been initiated in studies which focus on the extensive effective drug discovery. Consequently, the PI3K/AKT pathway appears to be an attractive pharmacological target both for cancer therapy and for neurological protection necessary after the therapy. A better understanding of the molecular relations could reveal new targets for treatment development. We review recent studies on the features of PI3K/AKT and PTEN, and their pleiotropic functions relevant to the signaling pathways involved in cancer progress and in neuronal damage by the therapy.