Glucocorticoid-related molecular signaling pathways regulating hippocampal neurogenesis

Early life adversities, psychopathologies and novel pharmacological strategies

Exposure to adversities during early life stages (early life adversities - ELA), ranging from pregnancy to adolescence, represents a major risk factor for the vulnerability to mental disorders. Hence, it is important to understand the molecular and functional underpinning of such relationship, in order to develop strategies aimed at reducing the psychopathologic burden associated with ELA, which may eventually lead to a significant improvement in clinical practice. In this review, we will initially recapitulate clinical and preclinical evidence supporting the link between ELA and psychopathology and we will primarily discuss the main biological mechanisms that have been described as potential mediators of the effects of ELA on the psychopathologic risk, including the role for genetic factors as well as sex differences. The knowledge emerging from these studies may be instrumental for the development of novel therapeutic strategies aimed not only at correcting the deficits that emerge from ELA exposure, but also in preventing the manifestation of a full-blown psychopathologic condition. With this respect, we will specifically focus on adolescence as a key time frame for disease onset as well as for early therapeutic intervention. We believe that incorporating clinical and preclinical research data in the context of early life adversities can be instrumental to elucidate the mechanisms contributing to the risk for psychopathology or that may promote resilience. This will ultimately allow the identification of 'at risk' individuals who may benefit from specific forms of interventions that, by interfering with disease trajectories, could result in more benign clinical outcomes.

Postbiotics as Molecules Targeting Cellular Events of Aging Brain-The Role in Pathogenesis, Prophylaxis and Treatment of Neurodegenerative Diseases

Aging is the most prominent risk factor for neurodegeneration occurrence. The most common neurodegenerative diseases (NDs), Alzheimer's (AD) and Parkinson's (PD) diseases, are characterized by the incidence of proteinopathy, abnormal activation of glial cells, oxidative stress, neuroinflammation, impaired autophagy and cellular senescence excessive for the patient's age. Moreover, mitochondrial disfunction, epigenetic alterations and neurogenesis inhibition, together with increased blood-brain barrier permeability and gut dysbiosis, have been linked to ND pathogenesis. Since NDs still lack curative treatment, recent research has sought therapeutic options in restoring gut microbiota and supplementing probiotic bacteria-derived metabolites with beneficial action to the host-so called postbiotics. The current review focuses on literature explaining cellular mechanisms involved in ND pathogenesis and research addressing the impact that postbiotics as a whole mixture and particular metabolites, such as short-chain fatty acids (SCFAs), lactate, polyamines, polyphenols, tryptophan metabolites, exopolysaccharides and bacterial extracellular vesicles, have on the ageing-associated processes underlying ND occurrence. The review also discusses the issue of implementing postbiotics into ND prophylaxis and therapy, depicting them as compounds addressing senescence-triggered dysfunctions that are worth translating from bench to pharmaceutical market in response to "silver consumers" demands.

Exploring mood disorders and treatment options using human stem cells

Despite their global prevalence, the mechanisms for mood disorders like bipolar disorder and major depressive disorder remain largely misunderstood. Mood stabilizers and antidepressants, although useful and effective for some, do not have a high responsiveness rate across those with these conditions. One reason for low responsiveness to these drugs is patient heterogeneity, meaning there is diversity in patient characteristics relating to genetics, etiology, and environment affecting treatment. In the past two decades, novel induced pluripotent stem cell (iPSC) research and technology have enabled the use of human-derived brain cells as a new model to study human disease that can help account for patient variance. Human iPSC technology is an emerging tool to better understand the molecular mechanisms of these disorders as well as a platform to test novel treatments and existing pharmaceuticals. This literature review describes the use of iPSC technology to model bipolar and major depressive disorder, common medications used to treat these disorders, and novel patient-derived alternative treatment methods for non-responders stemming from past publications, as well as presenting new data derived from these models.

Dissecting the networks underlying diverse brain disorders after prenatal glucocorticoid overexposure

New human life begins in the uterus in a period of both extreme plasticity and sensitivity to environmental disturbances. The fetal stage is also a vital period for central nervous system development, with experiences at this point profoundly and permanently shaping brain structure and function. As such, some brain disorders may originate in utero. Glucocorticoids, a class of essential stress hormones, play indispensable roles in fetal development, but overexposure may have lasting impacts on the brain. In this review, we summarize data from recent clinical and non-clinical studies regarding alterations in fetal brains due to prenatal glucocorticoid overexposure that are associated with nervous system disorders. We discuss relevant changes to brain structure and cellular functions and explore the underlying molecular mechanisms. In addition, we summarize factors that may cause differential outcomes between varying brain regions, and outline clinically feasible intervention strategies that are expected to minimize negative consequences arising from fetal glucocorticoid overexposure. Finally, we highlight the need for experimental evidence aided by new technologies to clearly determine the effects of excessive prenatal glucocorticoid exposure. This review consolidates diverse findings to help researchers better understand the relationship between the prenatal glucocorticoid overexposure and the effects it has on various fetal brain regions, promoting further development of critical intervention strategies.

CGG repeats in the human FMR1 gene regulate mRNA localization and cellular stress in developing neurons

The human genome has many short tandem repeats, yet the normal functions of these repeats are unclear. The 5' untranslated region (UTR) of the fragile X messenger ribonucleoprotein 1 (FMR1) gene contains polymorphic CGG repeats, the length of which has differing effects on FMR1 expression and human health, including the neurodevelopmental disorder fragile X syndrome. We deleted the CGG repeats in the FMR1 gene (0CGG) in human stem cells and examined the effects on differentiated neurons. 0CGG neurons have altered subcellular localization of FMR1 mRNA and protein, and differential expression of cellular stress proteins compared with neurons with normal repeats (31CGG). In addition, 0CGG neurons have altered responses to glucocorticoid receptor (GR) activation, including FMR1 mRNA localization, GR chaperone HSP90α expression, GR localization, and cellular stress protein levels. Therefore, the CGG repeats in the FMR1 gene are important for the homeostatic responses of neurons to stress signals.

Research advances in huntingtin-associated protein 1 and its application prospects in diseases

Huntingtin-associated protein 1 (HAP1) was the first protein discovered to interact with huntingtin. Besides brain, HAP1 is also expressed in the spinal cord, dorsal root ganglion, endocrine, and digestive systems. HAP1 has diverse functions involving in vesicular transport, receptor recycling, gene transcription, and signal transduction. HAP1 is strongly linked to several neurological diseases, including Huntington's disease, Alzheimer's disease, epilepsy, ischemic stroke, and depression. In addition, HAP1 has been proved to participate in cancers and diabetes mellitus. This article provides an overview of HAP1 regarding the tissue distribution, cell localization, functions, and offers fresh perspectives to investigate its role in diseases.

Psilocybin for dementia prevention? The potential role of psilocybin to alter mechanisms associated with major depression and neurodegenerative diseases

Major depression is an established risk factor for subsequent dementia, and depression in late life may also represent a prodromal state of dementia. Considering current challenges in the clinical development of disease modifying therapies for dementia, the focus of research is shifting towards prevention and modification of risk factors to alter the neurodegenerative disease trajectory. Understanding mechanistic commonalities underlying affective symptoms and cognitive decline may reveal biomarkers to aid early identification of those at risk of progressing to dementia during the preclinical phase of disease, thus allowing for timely intervention. Adult hippocampal neurogenesis (AHN) is a phenomenon that describes the birth of new neurons in the dentate gyrus throughout life and it is associated with spatial learning, memory and mood regulation. Microglia are innate immune system macrophages in the central nervous system that carefully regulate AHN via multiple mechanisms. Disruption in AHN is associated with both dementia and major depression and microgliosis is a hallmark of several neurodegenerative diseases. Emerging evidence suggests that psychedelics promote neuroplasticity, including neurogenesis, and may also be immunomodulatory. In this context, psilocybin, a serotonergic agonist with rapid-acting antidepressant properties has the potential to ameliorate intersecting pathophysiological processes relevant for both major depression and neurodegenerative diseases. In this narrative review, we focus on the evidence base for the effects of psilocybin on adult hippocampal neurogenesis and microglial form and function; which may suggest that psilocybin has the potential to modulate multiple mechanisms of action, and may have implications in altering the progression from major depression to dementia in those at risk.

Glucocorticoid effects on the brain: from adaptive developmental plasticity to allostatic overload

Exposure to stress during early life may alter the developmental trajectory of an animal by a mechanism known as adaptive plasticity. For example, to enhance reproductive success in an adverse environment, it is known that animals accelerate their growth during development. However, these short-term fitness benefits are often associated with reduced longevity, a phenomenon known as the growth rate-lifespan trade-off. In humans, early life stress exposure compromises health later in life and increases disease susceptibility. Glucocorticoids (GCs) are major stress hormones implicated in these processes. This Review discusses the evidence for GC-mediated adaptive plasticity in development, leading to allostatic overload in later life. We focus on GC-induced effects on brain structure and function, including neurogenesis; highlight the need for longitudinal studies; and discuss approaches to identify molecular mechanisms mediating GC-induced alteration of the brain developmental trajectory leading to adult dysfunctions. Further understanding of how stress and GC exposure can alter developmental trajectories at the molecular and cellular level is of critical importance to reduce the burden of mental and physical ill health across the life course.

Prednisolone improves hippocampal regeneration after trimethyltin-induced neurodegeneration in association with prevention of T lymphocyte infiltration

The endogenous regenerative capacity of the brain is quite weak; however, a regenerative reaction, the production of new neurons (neurogenesis), has been reported to occur in brain lesions. In addition, leukocytes are well known to infiltrate brain lesions. Therefore, leukocytes would also have a link with regenerative neurogenesis; however, their role has not been fully elucidated. In this study, we investigated leukocyte infiltration and its influence on brain tissue regeneration in a trimethyltin (TMT)-injected mouse model of hippocampal regeneration. Immunohistochemically, CD3-positive T lymphocytes were found in the hippocampal lesion of TMT-injected mice. Prednisolone (PSL) treatment inhibited T lymphocyte infiltration and increased neuronal nuclei (NeuN)-positive mature neurons and doublecortin (DCX)-positive immature neurons in the hippocampus. Investigation of bromodeoxyuridine (BrdU)-labeled newborn cells revealed the percentage of BrdU/NeuN- and BrdU/DCX-positive cells increased by PSL treatment. These results indicate that infiltrated T lymphocytes prevent brain tissue regeneration by inhibiting hippocampal neurogenesis.

Resilience to Early Life Adversity Effects on Stress Reactivity by Postnatal Knockdown of 5-HT 1A Autoreceptors

Early Life Adversity (ELA) predisposes to stress hypersensitivity in adulthood, but neurobiological mechanisms that can protect from long-lasting effects of ELA are poorly understood. Serotonin 1A (5HT 1A ) autoreceptors in the raphé nuclei regulate adult stress vulnerability, but if 5HT 1A could be targeted to prevent ELA effects on susceptibility to future stressors is unknown. Here, we exposed mice with postnatal knockdown of 5HT 1A autoreceptors to the limited bedding and nesting model of ELA from postnatal day (P)3-10. We then tested behavioral, neuroendocrine, neurogenic, and neuroinflammatory responses to an acute swim stress in male and female mice in adolescence (P35) and in adulthood (P56). In ELA-exposed females, adult swim stress exposure increased passive coping and despair-like behavior, corticosterone levels at baseline and after stress, and neuronal activity and corticotropin releasing hormone levels in the paraventricular nucleus of the hypothalamus. ELA also reduced neurogenesis and increased microglia activation in the ventral dentate gyrus (DG) of the hippocampus - an important mediator of individual differences in stress susceptibility. These effects of ELA were specific to females, but not males, and manifested predominantly in adulthood, but not earlier on in adolescence. Postnatal 5HT 1A autoreceptor knockdown prevented ELA effects on stress reactivity and on neurogenesis and neuroinflammation in the DG, indicating that reducing 5HT 1A autoreceptors confers resilience to ELA. Our findings demonstrate that ELA induces long-lasting and sex-specific impairments in stress reactivity and ventral DG function across development, and identify 5HT 1A autoreceptors as potential targets to prevent these persistent effects of ELA.

A Glucocorticoid-Sensitive Hippocampal Gene Network Moderates the Impact of Early-Life Adversity on Mental Health Outcomes

BACKGROUND

Early stress increases the risk for psychiatric disorders. Glucocorticoids are stress mediators that regulate transcriptional activity and morphology in the hippocampus, which is implicated in the pathophysiology of multiple psychiatric conditions. We aimed to establish the relevance of hippocampal glucocorticoid-induced transcriptional activity as a mediator of the effects of early life on later psychopathology in humans.

METHODS

RNA sequencing was performed with anterior and posterior hippocampal dentate gyrus from adult female macaques (n = 12/group) that were chronically treated with betamethasone (glucocorticoid receptor agonist) or vehicle. Coexpression network analysis identified a preserved gene network in the posterior hippocampal dentate gyrus that was strongly associated with glucocorticoid exposure. The single nucleotide polymorphisms in the genes in this network were used to create an expression-based polygenic score in humans.

RESULTS

The expression-based polygenic score significantly moderated the association between early adversity and psychotic disorders in adulthood (UK Biobank, women, n = 44,519) and on child peer relations (ALSPAC [Avon Longitudinal Study of Parents and Children], girls, n = 1666 for 9-year-olds and n = 1594 for 11-year-olds), an endophenotype for later psychosis. Analyses revealed that this network was enriched for glucocorticoid-induced epigenetic remodeling in human hippocampal cells. We also found a significant association between single nucleotide polymorphisms from the expression-based polygenic score and adult brain gray matter density.

CONCLUSIONS

We provide an approach for the use of transcriptomic data from animal models together with human data to study the impact of environmental influences on mental health. The results are consistent with the hypothesis that hippocampal glucocorticoid-related transcriptional activity mediates the effects of early adversity on neural mechanisms implicated in psychiatric disorders.

Hippocampal subfield volumes predict treatment response to oral ketamine in people with suicidality

Ongoing stress results in hippocampal neuro-structural alterations which produce pathological consequences, including depression and suicidality. Ketamine may ameliorate stress related illnesses, including suicidality, via neuroplasticity processes. This novel study sought to determine whether oral ketamine treatment specifically affects hippocampal (whole and subfield) volumes in patients with chronic suicidality and MDD. It was hypothesised that oral ketamine treatment would differentially alter hippocampal volumes in trial participants categorised as ketamine responders, versus those who were non-responders. Twenty-eight participants received 6 single, weekly doses of oral ketamine (0.5-3 mg/kg) and underwent MRI scans at pre-ketamine (week 0), post-ketamine (week 6), and follow up (week 10). Hippocampal subfield volumes were extracted using the longitudinal pipeline in FreeSurfer. Participants were grouped according to ketamine response status and then compared in terms of grey matter volume (GMV) changes, among 10 hippocampal regions, over 6 and 10 weeks. Mixed ANOVAs were used to analyse interactions between time and group. Post treatment analysis revealed a significant main effect of group for three left hippocampal GMVs as well in the left and right whole hippocampus. Ketamine acute responders (Week 6) showed increased GMVs in both left and right whole hippocampus and in three subfields compared to acute non-responders, across all three timepoints, suggesting that pre-treatment increased hippocampal GMVs (particularly left hemisphere) may be predictive biomarkers of acute treatment response. Future studies should further investigate the potential of hippocampal volumes as a biomarker of ketamine treatment response.

Relation of hippocampal volume and SGK1 gene expression to treatment remission in major depression is moderated by childhood maltreatment: A CAN-BIND-1 report

Preclinical research implicates stress-induced upregulation of the enzyme, serum- and glucocorticoid-regulated kinase 1 (SGK1), in reduced hippocampal volume. In the current study, we tested the hypothesis that greater SGK1 mRNA expression in humans would be associated with lower hippocampal volume, but only among those with a history of prolonged stress exposure, operationalized as childhood maltreatment (physical, sexual, and/or emotional abuse). Further, we examined whether baseline levels of SGK1 and hippocampal volume, or changes in these markers over the course of antidepressant treatment, would predict treatment outcomes in adults with major depression [MDD]. We assessed SGK1 mRNA expression from peripheral blood, and left and right hippocampal volume at baseline, as well as change in these markers over the first 8 weeks of a 16-week open-label trial of escitalopram as part of the Canadian Biomarker Integration Network in Depression program (MDD [n = 161] and healthy comparison participants [n = 91]). Childhood maltreatment was assessed via contextual interview with standardized ratings. In the full sample at baseline, greater SGK1 expression was associated with lower hippocampal volume, but only among those with more severe childhood maltreatment. In individuals with MDD, decreases in SGK1 expression predicted lower remission rates at week 16, again only among those with more severe maltreatment. Decreases in hippocampal volume predicted lower week 16 remission for those with low childhood maltreatment. These results suggest that both glucocorticoid-related neurobiological mechanisms of the stress response and history of childhood stress exposure may be critical to understanding differential treatment outcomes in MDD. ClinicalTrials.gov: NCT01655706 Canadian Biomarker Integration Network for Depression Study.

Adult hippocampal neurogenesis: pharmacological mechanisms of antidepressant active ingredients in traditional Chinese medicine

Depression is characterized by prominent indicators and manifestations, such as anhedonia, which refers to the inability to experience pleasure, and persistent feelings of hopelessness. In clinical practice, the primary treatment approach involves the utilization of selective serotonin reuptake inhibitors (SSRIs) and related pharmacological interventions. Nevertheless, it is crucial to recognize that these agents are associated with significant adverse effects. Traditional Chinese medicine (TCM) adopts a multifaceted approach, targeting diverse components, multiple targets, and various channels of action. TCM has potential antidepressant effects. Anomalies in adult hippocampal neurogenesis (AHN) constitute a pivotal factor in the pathology of depression, with the regulation of AHN emerging as a potential key measure to intervene in the pathogenesis and progression of this condition. This comprehensive review presented an overview of the pharmacological mechanisms underlying the antidepressant effects of active ingredients found in TCM. Through examination of recent studies, we explored how these ingredients modulated AHN. Furthermore, we critically assessed the current limitations of research in this domain and proposed novel strategies for preclinical investigation and clinical applications in the treatment of depression in future.

Acute and long-term effects of adolescence stress exposure on rodent adult hippocampal neurogenesis, cognition, and behaviour

Adolescence represents a critical period for brain and behavioural health and characterised by the onset of mood, psychotic and anxiety disorders. In rodents, neurogenesis is very active during adolescence, when is particularly vulnerable to stress. Whether stress-related neurogenesis changes influence adolescence onset of psychiatric symptoms remains largely unknown. A systematic review was conducted on studies investigating changes in hippocampal neurogenesis and neuroplasticity, hippocampal-dependent cognitive functions, and behaviour, occurring after adolescence stress exposure in mice both acutely (at post-natal days 21-65) and in adulthood. A total of 37 studies were identified in the literature. Seven studies showed reduced hippocampal cell proliferation, and out of those two reported increased depressive-like behaviours, in adolescent rodents exposed to stress. Three studies reported a reduction in the number of new-born neurons, which however were not associated with changes in cognition or behaviour. Sixteen studies showed acutely reduced hippocampal neuroplasticity, including pre- and post-synaptic plasticity markers, dendritic spine length and density, and long-term potentiation after stress exposure. Cognitive impairments and depressive-like behaviours were reported by 11 of the 16 studies. Among studies who looked at adolescence stress exposure effects into adulthood, seven showed that the negative effects of stress observed during adolescence on either cell proliferation or hippocampal neuroplasticity, cognitive deficits and depressive-like behaviour, had variable impact in adulthood. Treating adolescent mice with antidepressants, glutamate receptor inhibitors, glucocorticoid antagonists, or healthy diet enriched in omega-3 fatty acids and vitamin A, prevented or reversed those detrimental changes. Future research should investigate the translational value of these preclinical findings. Developing novel tools for measuring hippocampal neurogenesis in live humans, would allow assessing neurogenic changes following stress exposure, investigating relationships with psychiatric symptom onset, and identifying effects of therapeutic interventions.

Suppressive effects of bilobalide on depression-like behaviors induced by chronic unpredictable mild stress in mice

Background: Depression is a psychiatric disorder with depressed mood and even suicide attempts as the main clinical symptoms, and its pathogenesis has not yet been fully elucidated. Brain derived neurotrophic factor (BDNF) plays an important role in the pathogenesis of depression. Purpose: The main aim of the present study was to evaluate the effectiveness and reveal the potential mechanisms of bilobalide (BB) intervention in alleviating depression-like behaviors by using chronic unpredictable mild stress (CUMS) mice via mediating the BDNF pathway. Methods: Behavioral assessments were carried out by using the sucrose preference test (SPT), tail suspension test (TST), and forced swimming test (FST). CUMS mice were randomly divided into 5 groups: CUMS + solvent, CUMS + BB low, CUMS + BB medium, CUMS + BB high and CUMS + fluoxetine. Total serum levels of tumor necrosis factor (TNF-α) and interleukin-6 (IL-6) were measured by ELISA. Expression of TNF-α, IL-6, AKT, GSK3β, β-catenin, Trk-B and BDNF in the mouse hippocampus was assessed by western blotting. Results: BB treatment reduced the levels of pro-inflammatory cytokines (IL-6 and TNF-α) and increased the protein expression of BDNF in the hippocampus region of the CUMS mice. Moreover, BB treatment enhanced the AKT/GSK3β/β-catenin signaling pathway which is downstream of the BDNF receptor Trk-B in the hippocampus of these mice. Conclusions: Overall, the experimental results indicated that BB reverses CUMS-induced depression-like behavior. BB exerts antidepressant-like effects by inhibiting neuroinflammation and enhancing the function of neurotrophic factors.

Neural Progenitor Cells and the Hypothalamus

Neural progenitor cells (NPCs) are multipotent neural stem cells (NSCs) capable of self-renewing and differentiating into neurons, astrocytes and oligodendrocytes. In the postnatal/adult brain, NPCs are primarily located in the subventricular zone (SVZ) of the lateral ventricles (LVs) and subgranular zone (SGZ) of the hippocampal dentate gyrus (DG). There is evidence that NPCs are also present in the postnatal/adult hypothalamus, a highly conserved brain region involved in the regulation of core homeostatic processes, such as feeding, metabolism, reproduction, neuroendocrine integration and autonomic output. In the rodent postnatal/adult hypothalamus, NPCs mainly comprise different subtypes of tanycytes lining the wall of the 3^rd ventricle. In the postnatal/adult human hypothalamus, the neurogenic niche is constituted by tanycytes at the floor of the 3^rd ventricle, ependymal cells and ribbon cells (showing a gap-and-ribbon organization similar to that in the SVZ), as well as suprachiasmatic cells. We speculate that in the postnatal/adult human hypothalamus, neurogenesis occurs in a highly complex, exquisitely sophisticated neurogenic niche consisting of at least four subniches; this structure has a key role in the regulation of extrahypothalamic neurogenesis, and hypothalamic and extrahypothalamic neural circuits, partly through the release of neurotransmitters, neuropeptides, extracellular vesicles (EVs) and non-coding RNAs (ncRNAs).

The role of Smo-Shh/Gli signaling activation in the prevention of neurological and ageing disorders

Sonic hedgehog (Shh) signaling is an essential central nervous system (CNS) pathway involved during embryonic development and later life stages. Further, it regulates cell division, cellular differentiation, and neuronal integrity. During CNS development, Smo-Shh signaling is significant in the proliferation of neuronal cells such as oligodendrocytes and glial cells. The initiation of the downstream signalling cascade through the 7-transmembrane protein Smoothened (Smo) promotes neuroprotection and restoration during neurological disorders. The dysregulation of Smo-Shh is linked to the proteolytic cleavage of GLI (glioma-associated homolog) into GLI3 (repressor), which suppresses target gene expression, leading to the disruption of cell growth processes. Smo-Shh aberrant signalling is responsible for several neurological complications contributing to physiological alterations like increased oxidative stress, neuronal excitotoxicity, neuroinflammation, and apoptosis. Moreover, activating Shh receptors in the brain promotes axonal elongation and increases neurotransmitters released from presynaptic terminals, thereby exerting neurogenesis, anti-oxidation, anti-inflammatory, and autophagy responses. Smo-Shh activators have been shown in preclinical and clinical studies to help prevent various neurodegenerative and neuropsychiatric disorders. Redox signalling has been found to play a critical role in regulating the activity of the Smo-Shh pathway and influencing downstream signalling events. In the current study ROS, a signalling molecule, was also essential in modulating the SMO-SHH gli signaling pathway in neurodegeneration. As a result of this investigation, dysregulation of the pathway contributes to the pathogenesis of various neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD).Thus, Smo-Shh signalling activators could be a potential therapeutic intervention to treat neurocomplications of brain disorders.

From Ligands to Behavioral Outcomes: Understanding the Role of Mineralocorticoid Receptors in Brain Function

Stress is a normal response to situational pressures or demands. Exposure to stress activates the hypothalamic-pituitary-adrenal (HPA) axis and leads to the release of corticosteroids, which act in the brain via two distinct receptors: mineralocorticoid receptors (MR) and glucocorticoid receptors (GR). Persistent HPA axis overactivation or dysregulation can disrupt an individual's homeostasis, thereby contributing to an increased risk for mental illness. On the other hand, successful coping with stressful events involves adaptive and cognitive processes in the brain that render individuals more resilient to similar stressors in the future. Here we review the role of the MR in these processes, starting with an overview of the physiological structure, ligand binding, and expression of MR, and further summarizing its role in the brain, its relevance to psychiatric disorders, and related rodent studies. Given the central role of MR in cognitive and emotional functioning, and its importance as a target for promoting resilience, future research should investigate how MR modulation can be used to alleviate disturbances in emotion and behavior, as well as cognitive impairment, in patients with stress-related psychiatric disorders.

In vitro modeling of the neurobiological effects of glucocorticoids: A review

Hypothalamic-pituitary adrenal (HPA)axis dysregulation has long been implicated in stress-related disorders such as major depression and post-traumatic stress disorder. Glucocorticoids (GCs) are released from the adrenal glands as a result of HPA-axis activation. The release of GCs is implicated with several neurobiological changes that are associated with negative consequences of chronic stress and the onset and course of psychiatric disorders. Investigating the underlying neurobiological effects of GCs may help to better understand the pathophysiology of stress-related psychiatric disorders. GCs impact a plethora of neuronal processes at the genetic, epigenetic, cellular, and molecular levels. Given the scarcity and difficulty in accessing human brain samples, 2D and 3D in vitro neuronal cultures are becoming increasingly useful in studying GC effects. In this review, we provide an overview of in vitro studies investigating the effects of GCs on key neuronal processes such as proliferation and survival of progenitor cells, neurogenesis, synaptic plasticity, neuronal activity, inflammation, genetic vulnerability, and epigenetic alterations. Finally, we discuss the challenges in the field and offer suggestions for improving the use of in vitro models to investigate GC effects.

Social Enhancement of Adult Neurogenesis in Zebrafish is Not Regulated by Cortisol

In Mammals adult neurogenesis is influenced by environmental conditions, and the glucocorticoid hormones (GC) play a major role in this regulation. In contrast in fish, the study of the effects of cortisol on the regulation of environmental driven adult neurogenesis has produced conflicting results. While in some species elevated cortisol levels impair cell proliferation, in others, it promotes cell proliferation and differentiation. This lack of consistency may be explained by methodological differences across studies, namely in the stimuli and/or cortisol treatments used. Here, we tested the effects of the social environment on adult neurogenesis, considering a positive and a negative social context, and different durations of cortisol exposure. We hypothesise that there is an interaction between the valence of the social environment and cortisol, such that elevated acute cortisol experienced during social interactions only have a detrimental effect on neurogenesis in negative social contexts. Therefore, fish were exposed to a positive (conspecific shoal) or negative (predator) social experience, and the interaction between the valence of the social context and cortisol exposure (acute and chronic) was tested. Our results indicate that adult neurogenesis is modulated by the social environment, with the number of newly generated cells being dependent on the valence of the social information (positive > negative). These effects were independent of cortisol, either for acute or chronic exposure, highlighting the social environment as a key factor in the modulation of cell proliferation in the adult zebrafish brain, and rejecting a role for cortisol in this modulation.

Prenatal arsenic exposure induces immunometabolic alteration and renal injury in rats

Arsenic (As) exposure is progressively associated with chronic kidney disease (CKD), a leading public health concern present worldwide. The adverse effect of As exposure on the kidneys of people living in As endemic areas have not been extensively studied. Furthermore, the impact of only prenatal exposure to As on the progression of CKD also has not been fully characterized. In the present study, we examined the effect of prenatal exposure to low doses of As 0.04 and 0.4 mg/kg body weight (0.04 and 0.4 ppm, respectively) on the progression of CKD in male offspring using a Wistar rat model. Interestingly, only prenatal As exposure was sufficient to elevate the expression of profibrotic (TGF-β1) and proinflammatory (IL-1α, MIP-2α, RANTES, and TNF-α) cytokines at 2-day, 12- and 38-week time points in the exposed progeny. Further, alteration in adipogenic factors (ghrelin, leptin, and glucagon) was also observed in 12- and 38-week old male offspring prenatally exposed to As. An altered level of these factors coincides with impaired glucose metabolism and homeostasis accompanied by progressive kidney damage. We observed a significant increase in the deposition of extracellular matrix components and glomerular and tubular damage in the kidneys of 38-week-old male offspring prenatally exposed to As. Furthermore, the overexpression of TGF-β1 in kidneys corresponds with hypermethylation of the TGF-β1 gene-body, indicating a possible involvement of prenatal As exposure-driven epigenetic modulations of TGF-β1 expression. Our study provides evidence that prenatal As exposure to males can adversely affect the immunometabolism of offspring which can promote kidney damage later in life.

Role of stress-related glucocorticoid changes in astrocyte-oligodendrocyte interactions that regulate myelin production and maintenance

Repeated activation of stress responses and elevated corticosteroids result in alterations of neuronal physiology and metabolism, and lead to disturbances of normal connectivity between neurons in various brain regions. In addition, stress responses are also associated with anomalies in the function of glial cells, particularly astrocytes and oligodendrocytes, which in turn may further contribute to the mechanisms of neuronal dysfunction. The actions of corticosteroids on astrocytes are very likely mediated by the presence of intracellular and cell membrane-bound CORT receptors. Although apparently less abundant than in astrocytes, activation of CORT receptors in oligodendrocytes also leads to structural changes that are reflected in myelin maintenance and plasticity. The close interactions between astrocytes and oligodendrocytes through extracellular matrix molecules, soluble factors and astrocyte-oligodendrocyte gap junctions very likely mediate part of the disturbances in myelin structure, leading to plastic myelin adaptations or pathological myelin disruptions that may significantly influence brain connectivity. Likewise, the intimate association of the tips of some astrocytes processes with a majority of nodes of Ranvier in the white matter suggest that stress and overexposure to corticosteroids may lead to remodeling of node of Ranvier and their specific extracellular milieu.

Inhibition of FKBP51 induces stress resilience and alters hippocampal neurogenesis

Stress-related psychiatric disorders such as depression are among the leading causes of morbidity and mortality. Considering that many individuals fail to respond to currently available antidepressant drugs, there is a need for antidepressants with novel mechanisms. Polymorphisms in the gene encoding FK506-binding protein 51 (FKBP51), a co-chaperone of the glucocorticoid receptor, have been linked to susceptibility to stress-related psychiatric disorders. Whether this protein can be targeted for their treatment remains largely unexplored. The aim of this work was to investigate whether inhibition of FKBP51 with SAFit2, a novel selective inhibitor, promotes hippocampal neuron outgrowth and neurogenesis in vitro and stress resilience in vivo in a mouse model of chronic psychosocial stress. Primary hippocampal neuronal cultures or hippocampal neural progenitor cells (NPCs) were treated with SAFit2 and neuronal differentiation and cell proliferation were analyzed. Male C57BL/6 mice were administered SAFit2 while concurrently undergoing a chronic stress paradigm comprising of intermittent social defeat and overcrowding, and anxiety and depressive -related behaviors were evaluated. SAFit2 increased neurite outgrowth and number of branch points to a greater extent than brain derived neurotrophic factor (BDNF) in primary hippocampal neuronal cultures. SAFit2 increased hippocampal NPC neurogenesis and increased neurite complexity and length of these differentiated neurons. In vivo, chronic SAFit2 administration prevented stress-induced social avoidance, decreased anxiety in the novelty-induced hypophagia test, and prevented stress-induced anxiety in the open field but did not alter adult hippocampal neurogenesis in stressed animals. These data warrant further exploration of inhibition of FKBP51 as a strategy to treat stress-related disorders.

Understanding treatment-resistant depression using "omics" techniques: A systematic review

BACKGROUND

Treatment-resistant depression (TRD) results in huge healthcare costs and poor patient clinical outcomes. Most studies have adopted a "candidate mechanism" approach to investigate TRD pathogenesis, however this is made more challenging due to the complex and heterogeneous nature of this condition. High-throughput "omics" technologies can provide a more holistic view and further insight into the underlying mechanisms involved in TRD development, expanding knowledge beyond already-identified mechanisms. This systematic review assessed the information from studies that examined TRD using hypothesis-free omics techniques.

METHODS

PubMed, MEDLINE, Embase, APA PsycInfo, Scopus and Web of Science databases were searched on July 2022. 37 human studies met the eligibility criteria, totalling 17,518 TRD patients, 571,402 healthy controls and 62,279 non-TRD depressed patients (including antidepressant responders and untreated MDD patients).

RESULTS

Significant findings were reported that implicate the role in TRD of various molecules, including polymorphisms, genes, mRNAs and microRNAs. The pathways most commonly reported by the identified studies were involved in immune system and inflammation, neuroplasticity, calcium signalling and neurotransmitters.

LIMITATIONS

Small sample sizes, variability in defining TRD, and heterogeneity in study design and methodology.

CONCLUSIONS

These findings provide insight into TRD pathophysiology, proposing future research directions for novel drug targets and potential biomarkers for clinical staging and response to antidepressants (citalopram/escitalopram in particular) and electroconvulsive therapy (ECT). Further validation is warranted in large prospective studies using standardised TRD criteria. A multi-omics and systems biology strategy with a collaborative effort will likely deliver robust findings for translation into the clinic.

In vitro modeling of glucocorticoid mechanisms in stress-related mental disorders: Current challenges and future perspectives

In the last decade, in vitro models has been attracting a great deal of attention for the investigation of a number of mechanisms underlying neurological and mental disorders, including stress-related disorders, for which human brain material has rarely been available. Neuronal cultures have been extensively used to investigate the neurobiological effects of stress hormones, in particular glucocorticoids. Despite great advancements in this area, several challenges and limitations of studies attempting to model and investigate stress-related mechanisms in vitro exist. Such experiments often come along with non-standardized definitions stress paradigms in vitro, variations in cell models and cell types investigated, protocols with differing glucocorticoid concentrations and exposure times, and variability in the assessment of glucocorticoid-induced phenotypes, among others. Hence, drawing consensus conclusions from in-vitro stress studies is challenging. Addressing these limitations and aligning methodological aspects will be the first step towards an improved and standardized way of conducting in vitro studies into stress-related disorders, and is indispensable to reach the full potential of in vitro neuronal models. Here, we consider the most important challenges that need to be overcome and provide initial guidelines to achieve improved use of in vitro neuronal models for investigating mechanisms underlying the development of stress-related mental disorders.

Sex differences in the vulnerability of the hippocampus to prenatal stress

Distressing events during pregnancy that engage activity of the body's endocrine stress response have been linked with later life cognitive deficits in offspring and associated with developmental changes in cognitive-controlling neural regions. Interestingly, prenatal stress (PS)-induced alterations have shown some sex specificity. Here, we review the literature of animal studies examining sex-specific effect of physical PS on the function and structure of the hippocampus as hippocampal impairments likely underlie PS-associated deficits in learning and memory. Furthermore, the connectivity between the hypothalamic-pituitary-adrenal (HPA) axis and the hippocampus as well as the heavy presence of glucocorticoid receptors (GRs) in the hippocampus suggests this structure plays an important role in modulation of activity within stress circuitry in a sex-specific pattern. We hope that better understanding of sex-specific, PS-related hippocampal impairment will assist in uncovering the molecular mechanisms behind sex-based risk factors in PS populations across development, and perhaps contribute to greater precision in management of cognitive disturbances in this vulnerable population.

Imbalance in the ratio between mineralocorticoid and glucocorticoid receptors and neurodegeneration in the dentate gyrus of aged dogs

Background and Aim

Cortisol binds to mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) found in the hippocampus. The balanced expression of these receptors is essential to neuronal survival as MR and GR activations have antiapoptotic and proapoptotic effects, respectively. Given the aging changes in dogs' dentate gyrus (DG) and the possible involvement of cortisol receptors in this process, this study aimed to evaluate the expression of MR and GR and neuronal degeneration in this hippocampal region of aged dogs.

Materials and Methods

This study included cadaveric histologic hippocampus sections from six dogs aged 10 years and older (AG group) and 12 young/adult dogs aged up to 8 years (YAd group). Nissl staining and immunohistochemistry were performed to identify cells and investigate MR and GR expression, respectively. Furthermore, fluorescent labeling (fluoro-Jade B) was used to detect degenerating neurons.

Results

The AG group's polymorphic layer of the DG had a lower cell count (16%) and more degenerating neurons than the YAd group. In addition to these cellular changes, the AG group had lower MR immunoreactivity and MR-to-GR ratio. Furthermore, the lowest MR expression was associated with neuronal degeneration in the polymorphic layer of the DG of dogs.

Conclusion

An imbalance in the MR-to-GR ratio was observed in the polymorphic layer of the DG of aged dogs, along with lower MR expression and a greater number of degenerating neurons. These findings have clinical implications for understanding the decline in hippocampal memory formation associated with cognitive changes in aged dogs.

Hippocampal BDNF mediated anxiety-like behaviours induced by obesogenic diet withdrawal

Obesogenic diets (ODs) consumption is associated with anxiety-like behaviour and negative changes in hippocampal BDNF. At the same time, interrupting OD intake, OD withdrawal (WTD), can bring health benefits, but previous studies reported the development of anxiety-like behaviours. The present work aimed to assess the relationship between anxiety-like behaviour with hippocampal BDNF in a WTD rodent model. Male Wistar rats (60d old) were fed a high-sugar/high-fat (HSHF) diet for 30d (n=32), and half of them were transitioned to a control diet for 48h (n=16) afterwards. The control group (n=16) was fed a control diet across the whole experiment. Besides increasing anxiety-like behaviours and lowering sociability, the WTD led to an increase in BDNF in the dentate gyrus and the CA1 of the hippocampus. It also decreased locomotor activity in both OF and EPM, however, they did not significantly interfere with the other behavioural parameters analysed. Western blotting analysis revealed that the increase in BDNF likely occurred in the mature forms (14kD monomer and 28kD dimer). The mediation models analyses suggested that the effect of WTD on anxiety-like behaviour was driven by hippocampal BDNF, this mediation of effect was region-dependent. Our results also suggested that mature BDNF forms (14kD and 28kD) were responsible. The present work brought light to a possible new role for mature BDNF, although it is generally associated with beneficial features, it can also be part of the genesis of anxiety-like behaviours and sociability aspects on WTD models.

Early life adversity shapes neural circuit function during sensitive postnatal developmental periods

Early life adversity (ELA) is a major risk factor for mental illness, but the neurobiological mechanisms by which ELA increases the risk for future psychopathology are still poorly understood. Brain development is particularly malleable during prenatal and early postnatal life, when complex neural circuits are being formed and refined through an interplay of excitatory and inhibitory neural input, synaptogenesis, synaptic pruning, myelination, and neurogenesis. Adversity that influences these processes during sensitive periods of development can thus have long-lasting and pervasive effects on neural circuit maturation. In this review, we will discuss clinical and preclinical evidence for the impact of ELA on neural circuit formation with a focus on the early postnatal period, and how long-lasting impairments in these circuits can affect future behavior. We provide converging evidence from human and animal studies on how ELA alters the functional development of brain regions, neural circuits, and neurotransmitter systems that are crucial for cognition and affective behavior, including the hippocampus, the hypothalamus-pituitary-adrenal (HPA) axis, neural networks of fear responses and cognition, and the serotonin (5-HT) system. We also discuss how gene-by-environment (GxE) interactions can determine individual differences in susceptibility and resilience to ELA, as well as molecular pathways by which ELA regulates neural circuit development, for which we emphasize epigenetic mechanisms. Understanding the molecular and neurobiological mechanisms underlying ELA effects on brain function and psychopathology during early postnatal sensitive periods may have great potential to advance strategies to better treat or prevent psychiatric disorders that have their origin early in life.

Anxiety and hippocampal neuronal activity: Relationship and potential mechanisms

The hippocampus has been implicated in modulating anxiety. It interacts with a variety of brain regions, both cortical and subcortical areas regulating emotion and stress responses, including prefrontal cortex, amygdala, hypothalamus, and the nucleus accumbens, to adjust anxiety levels in response to a variety of stressful conditions. Growing evidence indicates that anxiety is associated with increased neuronal excitability in the hippocampus, and alterations in local regulation of hippocampal excitability have been suggested to underlie behavioral disruptions characteristic of certain anxiety disorders. Furthermore, studies have shown that some anxiolytics can treat anxiety by altering the excitability and plasticity of hippocampal neurons. Hence, identifying cellular and molecular mechanisms and neural circuits that regulate hippocampal excitability in anxiety may be beneficial for developing targeted interventions for treatment of anxiety disorders particularly for the treatment-resistant cases. We first briefly review a role of the hippocampus in fear. We then review the evidence indicating a relationship between the hippocampal activity and fear/anxiety and discuss some possible mechanisms underlying stress-induced hippocampal excitability and anxiety-related behavior.

Natural Products for the Treatment of Post-stroke Depression

Post-stroke depression (PSD) is the most frequent and important neuropsychiatric consequence of stroke. It is strongly associated with exacerbated deterioration of functional recovery, physical and cognitive recoveries, and quality of life. However, its mechanism is remarkably complicated, including the neurotransmitters hypothesis (which consists of a monoaminergic hypothesis and glutamate-mediated excitotoxicity hypothesis), inflammation hypothesis, dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis, and neurotrophic hypothesis and neuroplasticity. So far, the underlying pathogenesis of PSD has not been clearly defined yet. At present, selective serotonin reuptake inhibitors (SSRIs) have been used as the first-line drugs to treat patients with PSD. Additionally, more than SSRIs, a majority of the current antidepressants complied with multiple side effects, which limits their clinical application. Currently, a wide variety of studies revealed the therapeutic potential of natural products in the management of several diseases, especially PSD, with minor side effects. Accordingly, in our present review, we aim to summarize the therapeutic targets of these compounds and their potential role in-clinic therapy for patients with PSD.

Nitric Oxide Synthase inhibition counteracts the stress-induced DNA methyltransferase 3b expression in the hippocampus of rats

It has been postulated that the activation of NMDA receptors (NMDAr) and nitric oxide (NO) production in the hippocampus is involved in the behavioral consequences of stress. Stress triggers NMDAr-induced calcium influx in limbic areas, such as the hippocampus, which in turn activates neuronal NO synthase (nNOS). Inhibition of nNOS or NMDAr activity can prevent stress-induced effects in animal models, but the molecular mechanisms behind this effect are still unclear. In this study, cultured hippocampal neurons treated with NMDA or dexamethasone showed an increased of DNA methyltransferase 3b (DNMT3b) mRNA expression, which was blocked by pre-treatment with nNOS inhibitor nω -propyl-l-arginine (NPA). In rats submitted to the Learned Helplessness paradigm (LH), we observed that inescapable stress increased DNMT3b mRNA expression at 1h and 24h in the hippocampus. The NOS inhibitors 7-NI and aminoguanidine (AMG) decreased the number of escape failures in LH and counteracted the changes in hippocampal DNMT3b mRNA induced in this behavioral paradigm. Altogether, our data suggest that NO produced in response to NMDAr activation following stress upregulates DNMT3b in the hippocampus.

Spironolactone affects cardiovascular and craniofacial development in zebrafish embryos (Danio rerio)

Spironolactone, a potassium-sparing diuretic and aldosterone antagonist, is a mineralocorticoid hormone commonly prescribed to patients suffering from heart failure, hirsutism, dermatological afflictions, and hypertension. Interestingly, relatively little work has been done on the development of vertebrate embryos after exposure to this compound. Here, we treat zebrafish embryos with spironolactone at 10^-6 M, 10^-7 M, or 10^-8 M, and observe them after three to seven days of exposure. While no effect was observed in mortality, we did detect differences in cardiovascular development at 3 dpf and craniofacial development at 5 dpf. At 10^-6 M, smaller atria, ventricles, and blood vessels were observed. The highest concentrations also caused a longer ceratohyal/Meckel's distance, longer palatoquadrate, and smaller angles between the palatoquadrate and both the ceratohyal and Meckel's. Further research of spironolactone's effects on embryonic development could lead to a better understanding of the compound resulting in improved public and environmental health.

The therapeutic potential of traditional Chinese medicine in depression: Targeting adult hippocampal neurogenesis

BACKGROUND

Depression is a common mental disorder characterized by persistent sadness and lack of interest or pleasure in previously rewarding or enjoyable activities. Understandably, the causes of depression are complex. Nevertheless, the understanding of depression pathophysiology has progressed considerably and numerous studies indicate that hippocampal neurogenesis plays a pivotal role. However, no drugs specifically targeting hippocampal neurogenesis yet exist. Meanwhile, the effects of traditional Chinese medicine (TCM) on hippocampal neurogenesis have received increasing attention in the field of antidepressant treatment because of its multi-ingredient, multi-target, and holistic view. However, the effects and mechanisms of TCM on hippocampal neurogenesis in clinical trials and pharmaceutical studies remain to be comprehensively delineated.

PURPOSE

To summarize the importance of hippocampal neurogenesis in depression and illustrate the targets and mechanisms of hippocampal neurogenesis regulation that underlie the antidepressant effects of TCM.

METHOD

A systematic review of clinical trials and studies ending by January 2022 was performed across eight electronic databases (Web of Science, PubMed, SciFinder, Research Gate, ScienceDirect, Google Scholar, Scopus and China Knowledge Infrastructure) according to the PRISMA criteria, using the search terms 'traditional Chinese medicine' "AND" 'depression' "OR" 'hippocampal neurogenesis' "OR" 'multi-ingredient' "OR" 'multi-target'.

RESULTS

Numerous studies show that hippocampal neurogenesis is attenuated in depression, and that antidepressants act by enhancing hippocampal neurogenesis. Moreover, compound Chinese medicine (CCM), Chinese meteria medica (CMM), and major bioactive components (MBCs) can promote hippocampal neurogenesis exerting antidepressant effects through modulation of neurotransmitters and receptors, neurotrophins, the hypothalamic-pituitary-adrenal axis, inflammatory factors, autophagy, and gut microbiota.

CONCLUSION

We have comprehensively summarized the effect and mechanism of TCM on hippocampal neurogenesis in depression providing a unique perspective on the use of TCM in the antidepressant field. TCM has the characteristics and advantages of multiple targets and high efficacy, showing great potential in the field of depression treatment.

Brain pathology and cognitive scores prior to onset of late-life depression

OBJECTIVES

Understanding the biological changes that occur prior to onset of late-life depression (LLD) is key to its prevention. To investigate potential predictors of LLD, we assessed cognitive scores and neurodegenerative and vascular biomarkers in healthy older adults who later developed depression.

METHODS

Longitudinal data from the Alzheimer's Disease Neuroimaging Initiative of 241 cognitively unimpaired and non-depressed older adults aged 56-90 at baseline with at least 4 years of follow-up were included. Participants were classified based on whether they developed an incident depression (n = 96) or not (n = 145). Cognitive measures of memory, executive functioning, and language, and biomarkers proposed to be related to LLD: hippocampal volume, white matter hyperintensity volume (WMH), and cortical and cerebrospinal fluid (CSF) amyloid beta levels, were compared between the incident depression and the never-depressed groups at four time points: at baseline, the visit prior to onset, at onset, and after the onset of depression.

RESULTS

In the incident depression group, there was a mild decline in cognitive scores from baseline to the visit before depression onset compared with the never-depressed group. The cognitive differences between the groups became more marked after depression onset. Baseline cortical amyloid burden, CSF amyloid beta levels, and WMH were significant predictors of incident depression. Compared to the non-depressed group, hippocampal volume was not reduced before onset, but was reduced following depression.

CONCLUSIONS

Amyloid pathology and WMH can predict future development of LLD in cognitively unimpaired individuals and may be involved in precipitating vulnerability for depression in older adults.

Spousal caregiving, widowhood, and cognition: A systematic review and a biopsychosocial framework for understanding the relationship between interpersonal losses and dementia risk in older adulthood

Accumulating research suggests that stressful life events, especially those that threaten close intimate bonds, are associated with an increased risk of dementia. Grieving the loss of a spouse, whether in the form of caregiving or after the death, ranks among 'life's most significant stressors', evoking intense psychological and physiological distress. Despite numerous studies reporting elevated dementia risk or poorer cognition among spousal caregivers and widow(er)s compared to controls, no review has summarized findings across cognitive outcomes (i.e., dementia incidence, cognitive impairment rates, cognitive performance) or proposed a theoretical model for understanding the links between partner loss and abnormal cognitive decline. The current systematic review summarizes findings across 64 empirical studies. Overall, both cross-sectional and longitudinal studies revealed an adverse association between partner loss and cognitive outcomes. In turn, we propose a biopsychosocial model of cognitive decline that explains how caregiving and bereavement may position some to develop cognitive impairment or Alzheimer's disease and related dementias. More longitudinal studies that focus on the biopsychosocial context of caregivers and widow(er)s are needed.

Targeting the Erk1/2 and autophagy signaling easily improved the neurobalst differentiation and cognitive function after young transient forebrain ischemia compared to old gerbils

The hippocampal neurogenesis occurs constitutively throughout adulthood in mammalian species, but declines with age. In this study, we overtly found that the neuroblast proliferation and differentiation in the subgranular zone and the maturation into fully functional and integrated neurons in the granule-cell layer in young gerbils following cerebral ischemia/reperfusion was much more than those in old gerbils. The neurological function and cognitive and memory-function rehabilitation in the young gerbils improved faster than those in the old one. These results demonstrated that, during long term after cerebral ischemia/reperfusion, the ability of neurogenesis and recovery of nerve function in young animals were significantly higher than that in the old animals. We found that, after 14- and 28-day cerebral ischemia/reperfusion, the phosphorylation of MEK1/2, ERK1/2, p90RSK, and MSK1/2 protein levels in the hippocampus of young gerbils was significantly much higher than that of old gerbils. The levels of autophagy-related proteins, including Beclin-1, Atg3, Atg5, and LC3 in the hippocampus were effectively maintained and elevated at 28 days after cerebral ischemia/reperfusion in the young gerbils compared with those in the old gerbils. These results indicated that an increase or maintenance of the phosphorylation of ERK1/2 signal pathway and autophagy-related proteins was closely associated with the neuroblast proliferation and differentiation and the process of maturation into neurons. Further, we proved that neuroblast proliferation and differentiation in the dentate gyrus and cognitive function were significantly reversed in young cerebral ischemic gerbils by administering the ERK inhibitor (U0126) and autophagy inhibitor (3MA). In brief, following experimental young ischemic stroke, the long-term promotion of the neurogenesis in the young gerbil’s hippocampal dentate gyrus by upregulating the phosphorylation of ERK signaling pathway and maintaining autophagy-related protein levels, it overtly improved the neurological function and cognitive and memory function.

The Influence of Gut Microbiota on Neurogenesis: Evidence and Hopes

Adult neurogenesis (i.e., the life-long generation of new neurons from undifferentiated neuronal precursors in the adult brain) may contribute to brain repair after damage, and participates in plasticity-related processes including memory, cognition, mood and sensory functions. Among the many intrinsic (oxidative stress, inflammation, and ageing), and extrinsic (environmental pollution, lifestyle, and diet) factors deemed to impact neurogenesis, significant attention has been recently attracted by the myriad of saprophytic microorganismal communities inhabiting the intestinal ecosystem and collectively referred to as the gut microbiota. A growing body of evidence, mainly from animal studies, reveal the influence of microbiota and its disease-associated imbalances on neural stem cell proliferative and differentiative activities in brain neurogenic niches. On the other hand, the long-claimed pro-neurogenic activity of natural dietary compounds endowed with antioxidants and anti-inflammatory properties (such as polyphenols, polyunsaturated fatty acids, or pro/prebiotics) may be mediated, at least in part, by their action on the intestinal microflora. The purpose of this review is to summarise the available information regarding the influence of the gut microbiota on neurogenesis, analyse the possible underlying mechanisms, and discuss the potential implications of this emerging knowledge for the fight against neurodegeneration and brain ageing.

Gut feelings: the microbiota-gut-brain axis on steroids

The intricate connection between central and enteric nervous systems is well established with emerging evidence linking gut microbiota function as a significant new contributor to gut-brain axis signaling. Several microbial signals contribute to altered gut-brain communications, with steroids representing an important biological class that impacts central and enteric nervous system function. Neuroactive steroids contribute pathologically to neurological disorders, including dementia and depression, by modulating the activity of neuroreceptors. However, limited information is available on the influence of neuroactive steroids on the enteric nervous system and gastrointestinal function. In this review, we outline how steroids can modulate enteric nervous system function by focusing on their influence on different receptors that are present in the intestine in health and disease. We also highlight the potential role of the gut microbiota in modulating neuroactive steroid signaling along the gut-brain axis.

Steroid hormones and hippocampal neurogenesis in the adult mammalian brain

Hippocampal neurogenesis persists across the lifespan in many species, including rodents and humans, and is associated with cognitive performance and the pathogenesis of neurodegenerative disease and psychiatric disorders. Neurogenesis is modulated by steroid hormones that change across development and differ between the sexes in rodents and humans. Here, we discuss the effects of stress and glucocorticoid exposure from gestation to adulthood as well as the effects of androgens and estrogens in adulthood on neurogenesis in the hippocampus. Throughout the review we highlight sex differences in the effects of steroid hormones on neurogenesis and how they may relate to hippocampal function and disease. These data highlight the importance of examining age and sex when evaluating the effects of steroid hormones on hippocampal neurogenesis.

Reviewing the Role of the Endocannabinoid System in the Pathophysiology of Depression

Major depressive disorder is a high-impact, debilitating disease and it is currently considered the most prevalent mental illness. It is associated with disability, as well as increased morbidity and mortality. Despite its significant repercussions in our society, its exact pathophysiology remains unclear and therefore, available antidepressant treatment options are limited and, in some cases, ineffective. In the past years, research has focused on the development of a multifactorial theory of depression. Simultaneously, evidence supporting the role of the endocannabinoid system in the neurobiology of neuropsychiatric diseases has emerged. Studies have shown that the endocannabinoid system strongly impacts neurotransmission, and the neuroendocrine and neuroimmune systems, which are known to be dysfunctional in depressive patients. Accordingly, common antidepressants were shown to have a direct impact on the expression of cannabinoid receptors throughout the brain. Therefore, the relationship between the endocannabinoid system and major depressive disorder is worth consideration. Nevertheless, most studies focus on smaller pieces of what is undoubtedly a larger mosaic of interdependent processes. Therefore, the present review summarizes the existing literature regarding the role of the endocannabinoid system in depression aiming to integrate this information into a holistic picture for a better understanding of the relationship between the two.

Glucocorticoid production in the thymus and brain: Immunosteroids and neurosteroids

Glucocorticoids (GCs) regulate a myriad of physiological systems, such as the immune and nervous systems. Systemic GC levels in blood are often measured as an indicator of local GC levels in target organs. However, several extra-adrenal organs can produce and metabolize GCs locally. More sensitive and specific methods for GC analysis (i.e., mass spectrometry) allow measurement of local GC levels in small tissue samples with low GC concentrations. Consequently, is it now apparent that systemic GC levels often do not reflect local GC levels. Here, we review the use of systemic GC measurements in clinical and research settings, discuss instances where systemic GC levels do not reflect local GC levels, and present evidence that local GC levels provide useful insights, with a focus on local GC production in the thymus (immunosteroids) and brain (neurosteroids). Lastly, we suggest key areas for further research, such as the roles of immunosteroids and neurosteroids in neonatal programming and the potential clinical relevance of local GC modulators.

Serum from Older Adults Increases Apoptosis and Molecular Aging Markers in Human Hippocampal Progenitor Cells

Age-related alteration in neural stem cell function is linked to neurodegenerative conditions and cognitive decline. In rodents, this can be reversed by exposure to a young systemic milieu and conversely, the old milieu can inhibit stem cell function in young rodents. In this study, we investigated the in vitro effect of the human systemic milieu on human hippocampal progenitor cells (HPCs) using human serum from early adulthood, mid-life and older age. We showed that neuroblast number following serum treatment is predictive of larger dentate gyrus, CA3, CA4 and whole hippocampus volumes and that allogeneic human serum from asymptomatic older individuals induced a two-fold increase in apoptotic cell death of HPCs compared with serum from young adults. General linear models revealed that variability in markers of proliferation and differentiation was partly attributable to use of antihypertensive medication and very mild cognitive decline among older subjects. Finally, using an endophenotype approach and whole-genome expression arrays, we showed upregulation of established and novel ageing molecular hallmarks in response to old serum. Serum from older subjects induced a wide range of cellular and molecular phenotypes, likely reflecting a lifetime of environmental exposures. Our findings support a role for the systemic enviroment in neural stem cell maintenance and are in line with others highlighting a distinction between neurobiological and chronological ageing. Finally, the herein described serum assay can be used by future studies to further analyse the effect of environmental exposures as well as to determine the role of the systemic environment in health and disease.

Mouse Neural Stem Cell Differentiation and Human Adipose Mesenchymal Stem Cell Transdifferentiation Into Neuron- and Oligodendrocyte-like Cells With Myelination Potential

Stem cell therapy is an interesting approach for neural repair, once it can improve and increase processes, like angiogenesis, neurogenesis, and synaptic plasticity. In this regard, adult neural stem cells (NSC) are studied for their mechanisms of proliferation, differentiation and functionality in neural repair. Here, we describe novel neural differentiation methods. NSC from adult mouse brains and human adipose-derived stem cells (hADSC) were isolated and characterized regarding their neural differentiation potential based on neural marker expression profiles. For both cell types, their capabilities of differentiating into neuron-, astrocyte- and oligodendrocytes-like cells (NLC, ALC and OLC, respectively) were analyzed. Our methodologies were capable of producing NLC, ALC and OLC from adult murine and human transdifferentiated NSC. NSC showed augmented gene expression of NES, TUJ1, GFAP and PDGFRA/Cnp. Following differentiation induction into NLC, OLC or ALC, specific neural phenotypes were obtained expressing MAP2, GalC/O4 or GFAP with compatible morphologies, respectively. Accordingly, immunostaining for nestin⁺ in NSC, GFAP⁺ in astrocytes and GalC/O4⁺ in oligodendrocytes was detected. Co-cultured NLC and OLC showed excitability in 81.3% of cells and 23.5% of neuron/oligodendrocyte marker expression overlap indicating occurrence of in vitro myelination. We show here that hADSC can be transdifferentiated into NSC and distinct neural phenotypes with the occurrence of neuron myelination in vitro, providing novel strategies for CNS regeneration therapy. Superior Part: Schematic organization of obtaining and generating hNSC from hADSC and differentiation processes and phenotypic expression of neuron, astrocyte and oligodendrocyte markers (MAP2, GFAP and O4, respectively) and stem cell marker (NES) of differentiating hNSC 14 days after induction. The nuclear staining in blue corresponds to DAPI. bar = 100 μm. Inferior part: Neural phenotype fates in diverse differentiation media. NES: nestin; GFAP: Glial fibrillary acidic protein. MAP2: Microtubule-associated protein 2. TUJ1: β-III tubulin. PDGFRA: PDGF receptor alpha. Two-way ANOVA with Bonferroni post-test with n = 3. * p < 0.05 and ** p < 0.01: (NSCiM1 NSC induction medium 1) vs differentiation media.

Prenatal stress leads to deficits in brain development, mood related behaviors and gut microbiota in offspring

Early exposure to stressful and adverse life events at fetal and neonatal stages is one of crucial risk factors for mood disorders such as anxiety and depressive disorder in adulthood. Intergenerational effects of prenatal stress on offspring are still not fully understood. We here uncover a significant negative impact of prenatal stress on brain development in embryos and newborns, and on mood-related behaviors and gut microbiota in adult offspring. Prenatal stress leads to reduced numbers in neural progenitors and newborn neurons, and altered gene expression profiles in the mouse embryonic cerebral cortex. Adult mouse offspring exposed to prenatal stress displays altered gene expression in the cortex and elevated responses in anxiety- and depression-like behaviors. Interestingly, prenatal stress has an enduring effect on gut microbiota, as specific microbial community structure is altered in adult F1 offspring treated with prenatal stress, compared to that of the control. Our results highlight the essential impact of prenatal stress on cortical neurogenesis, gene expression patterns, mood-related behaviors, and even gut microbiota in the next generation.

The role of AQP4 in the pathogenesis of depression, and possible related mechanisms

Modulation of the aquaporin 4 (AQP4) water-regulatory channel or production of autoantibodies against this protein have been implicated in a variety of neuropsychiatric conditions, and possible mechanisms have been proposed. However, the nature of the interaction between AQP4 expression and its implications in depression remain elusive. To our knowledge, this is the first review summarising data for the involvement of AQP4 in the context of depression and related mechanisms across a wide range of experimental studies: pre-clinical (KO and wild-type), post-mortem, ex vivo, and clinical studies in depression. Overall, preclinical AQP4 wild-type studies showed that exposure to stress or inflammation, used as models of depression, decreased AQP4 protein and gene expression in various brain regions, including prefrontal cortex (PFC), choroid plexus and, especially, hippocampus. In preclinical AQP4 KO studies, AQP4 expression is necessary to prevent the effect of stress and inflammation on reduced neurogenesis and gliogenesis, and increased apoptosis and depressive-like behaviours. While in post-mortem and ex vivo studies of depression AQP4 expression was usually decreased in the hippocampus, prefrontal cortex and locus coeruleus, in clinical studies, where mRNA AQP4 expression or serum AQP4 autoantibodies were measured, there were no differences in depressed patients when compared with controls. In the future, studies should further investigate the mechanisms underlying the action of AQP4, and continue exploring if AQP4 autoantibodies are either contributing or underlying mechanisms of depression, or whether they are simply a mechanism underlying other autoimmune conditions where depression is present.

Omega-3 polyunsaturated fatty acids protect against inflammation through production of LOX and CYP450 lipid mediators: relevance for major depression and for human hippocampal neurogenesis

Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) can exert antidepressant, anti-inflammatory and neuroprotective properties, but the exact molecular mechanism underlying their effects is still not fully understood. We conducted both in vitro and clinical investigations to test which EPA or DHA metabolites are involved in these anti-inflammatory, neuroprotective and antidepressant effects. In vitro, we used the human hippocampal progenitor cell line HPC0A07/03C, and pre-treated cells with either EPA or DHA, followed by interleukin 1beta (IL1β), IL6 and interferon-alpha (IFN-α). Both EPA and DHA prevented the reduction in neurogenesis and the increase in apoptosis induced by these cytokines; moreover, these effects were mediated by the lipoxygenase (LOX) and cytochrome P450 (CYP450) EPA/DHA metabolites, 5-hydroxyeicosapentaenoic acid (HEPE), 4-hydroxydocosahexaenoic acid (HDHA), 18-HEPE, 20-HDHA, 17(18)-epoxyeicosatetraenoic acid (EpETE) and 19(20)-epoxydocosapentaenoic acid (EpDPA), detected here for the first time in human hippocampal neurones using mass spectrometry lipidomics of the supernatant. In fact, like EPA/DHA, co-treatment with these metabolites prevented cytokines-induced reduction in neurogenesis and apoptosis. Moreover, co-treatment with 17(18)-EpETE and 19(20)-EpDPA and the soluble epoxide hydroxylase (sEH) inhibitor, TPPU (which prevents their conversion into dihydroxyeicosatetraenoic acid (DiHETE)/ dihydroxydocosapentaenoic acid (DiHDPA) metabolites) further enhanced their neurogenic and anti-apoptotic effects. Interestingly, these findings were replicated in a sample of n = 22 patients with a DSM-IV Major Depressive Disorder, randomly assigned to treatment with either EPA (3.0 g/day) or DHA (1.4 g/day) for 12 weeks, with exactly the same LOX and CYP450 lipid metabolites increased in the plasma of these patients following treatment with their precursor, EPA or DHA, and some evidence that higher levels of these metabolites were correlated with less severe depressive symptoms. Overall, our study provides the first evidence for the relevance of LOX- and CYP450-derived EPA/DHA bioactive lipid metabolites as neuroprotective molecular targets for human hippocampal neurogenesis and depression, and highlights the importance of sEH inhibitors as potential therapeutic strategy for patients suffering from depressive symptoms.