Hippocampal epigenetic modification at the doublecortin gene is involved in the impairment of neurogenesis with aging

Molecular and histological correlates of cognitive decline across age in male C57BL/6J mice

INTRODUCTION

Increasing age is the number one risk factor for developing cognitive decline and neurodegenerative disease. Aged humans and mice exhibit numerous molecular changes that contribute to a decline in cognitive function and increased risk of developing age-associated diseases. Here, we characterize multiple age-associated changes in male C57BL/6J mice to understand the translational utility of mouse aging.

METHODS

Male C57BL/6J mice from various ages between 2 and 24 months of age were used to assess behavioral, as well as, histological and molecular changes across three modalities: neuronal, microgliosis/neuroinflammation, and the neurovascular unit (NVU). Additionally, a cohort of 4- and 22-month-old mice was used to assess blood-brain barrier (BBB) breakdown. Mice in this cohort were treated with a high, acute dose of lipopolysaccharide (LPS, 10 mg/kg) or saline control 6 h prior to sacrifice followed by tail vein injection of 0.4 kDa sodium fluorescein (100 mg/kg) 2 h later.

RESULTS

Aged mice showed a decline in cognitive and motor abilities alongside decreased neurogenesis, proliferation, and synapse density. Further, neuroinflammation and circulating proinflammatory cytokines were increased in aged mice. Additionally, we found changes at the BBB, including increased T cell infiltration in multiple brain regions and an exacerbation in BBB leakiness following chemical insult with age. There were also a number of readouts that were unchanged with age and have limited utility as markers of aging in male C57BL/6J mice.

CONCLUSIONS

Here we propose that these changes may be used as molecular and histological readouts that correspond to aging-related behavioral decline. These comprehensive findings, in the context of the published literature, are an important resource toward deepening our understanding of normal aging and provide an important tool for studying aging in mice.

A Mixture of Ginkgo biloba L. Leaf and Hericium erinaceus (Bull.) Pers. Fruit Extract Attenuates Scopolamine-Induced Memory Impairments in Mice

Alzheimer's disease (AD) is a neurodegenerative disease that is characterized by loss of memory and cognitive impairment via dysfunction of the cholinergic nervous system. In cholinergic dysfunction, it is well known that impaired cAMP response element-binding protein (CREB) and brain-derived neurotrophic factor (BDNF) signaling are major pathological markers and are some of the strategies for the development of AD therapy. Therefore, this study is aimed at evaluating whether a mixture comprising Ginkgo biloba L. leaf (GL) and Hericium erinaceus (Bull.) Pers. (HE) fruit extract (GH mixture) alleviated cognitive impairment induced in a scopolamine-induced model. It was discovered that GH reduced neuronal apoptosis and promoted neuronal survival by activating BDNF signaling in an in vitro assay. In addition, the GH (p.o. 240 mg/kg) oral administration group significantly restored the cognitive deficits of the scopolamine-induced mouse group (i.p. 1.2 mg/kg) in the behavior tests such as Y-maze and novel object recognition task (NORT) tests. This mixture also considerably enhanced cholinergic system function in the mouse brain. Furthermore, GH markedly upregulated the expressed levels of extracellular signal-regulated kinase (ERK), CREB, and BDNF protein levels. These results demonstrated that GH strongly exerted a neuroprotective effect on the scopolamine-induced mouse model, suggesting that an optimized mixture of GL and HE could be used as a good material for developing functional foods to aid in the prevention of neurodegenerative diseases, including AD.

Short-Term Environmental Enrichment is a Stronger Modulator of Brain Glial Cells and Cervical Lymph Node T Cell Subtypes than Exercise or Combined Exercise and Enrichment

Physical exercise (PE) and environmental enrichment (EE) can modulate immunity. However, the differential effects of short-term PE, EE, and PE + EE on neuroimmune mechanisms during normal aging has not been elucidated. Hence, a cohort of 3-, 8-, and 13-month-old immunologically unchallenged C57BL/6 wild-type mice were randomly assigned to either Control, PE, EE, or PE + EE groups and provided with either no treatment, a running wheel, a variety of plastic and wooden objects alone or in combination with a running wheel for seven weeks, respectively. Immunohistochemistry and 8-color flow cytometry were used to determine the numbers of dentate gyrus glial cells, and the proportions of CD4+ and CD8+ T cell numbers and their subsets from cervical lymph nodes, respectively. An increase in the number of IBA1+ microglia in the dentate gyrus at 5 and 10 months was observed after EE, while PE and PE + EE increased it only at 10 months. No change in astroglia number in comparison to controls were observed in any of the treatment groups. Also, all treatments induced significant differences in the proportion of specific T cell subsets, i.e., CD4+ and CD8+ T naïve (TN), central memory (TCM), and effector memory (TEM) cells. Our results suggest that in the short-term, EE is a stronger modulator of microglial and peripheral T cell subset numbers than PE and PE + EE, and the combination of short-term PE and EE has no additive effects.

Born this way: Hippocampal neurogenesis across the lifespan

The capability of the mammalian brain to generate new neurons through the lifespan has gained much attention for the promise of new therapeutic possibilities especially for the aging brain. One of the brain regions that maintains a neurogenesis-permissive environment is the dentate gyrus of the hippocampus. Here, new neurons are generated from a pool of multipotent neural progenitor cells to become fully functional neurons that are integrated into the brain circuitry. A growing body of evidence points to the fact that neurogenesis in the adult hippocampus is necessary for certain memory processes, and in mood regulation, while alterations in hippocampal neurogenesis have been associated with a myriad of neurological and psychiatric disorders. More recently, evidence has come to light that new neurons may differ in their vulnerability to environmental and disease-related influences depending on the time during the life course at which they are exposed. Thus, it has been the topic of intense research in recent years. In this review, we will discuss the complex process and associated functional relevance of hippocampal neurogenesis during the embryonic/postnatal period and in adulthood. We consider the implications of hippocampal neurogenesis during the developmentally critical periods of adolescence and older age. We will further consider the literature surrounding hippocampal neurogenesis and its functional role during these critical periods with a view to providing insight into the potential of harnessing neurogenesis for health and therapeutic benefit.

Adult Hippocampal Neurogenesis: A Coming-of-Age Story

What has become standard textbook knowledge over the last decade was a hotly debated matter a decade earlier: the proposition that new neurons are generated in the adult mammalian CNS. The early discovery by Altman and colleagues in the 1960s was vulnerable to criticism due to the lack of technical strategies for unequivocal demonstration, quantification, and physiological analysis of newly generated neurons in adult brain tissue. After several technological advancements had been made in the field, we published a paper in 1996 describing the generation of new neurons in the adult rat brain and the decline of hippocampal neurogenesis during aging. The paper coincided with the publication of several other studies that together established neurogenesis as a cellular mechanism in the adult mammalian brain. In this Progressions article, which is by no means a comprehensive review, we recount our personal view of the initial setting that led to our study and we discuss some of its implications and developments that followed. We also address questions that remain regarding the regulation and function of neurogenesis in the adult mammalian brain, in particular the existence of neurogenesis in the adult human brain.

Understanding the Role of Antiviral Cytokines and Chemokines on Neural Stem/Progenitor Cell Activity and Survival

Viral infections of the central nervous system are accompanied by the expression of cytokines and chemokines that can be critical for the control of viral replication in the brain. The outcomes of cytokine/chemokine signaling in neural cells vary widely, with cell-specific effects on cellular activity, proliferation, and survival. Neural stem/progenitor cells (NSPCs) are often altered during viral infections, through direct infection by the virus or by the influence of immune cell activity or cytokine/chemokine signaling. However, it has been challenging to dissect the contribution of the virus and specific inflammatory mediators during an infection. In addition to initiating an antiviral program in infected NSPCs, cytokines/chemokines can induce multiple changes in NSPC behavior that can perturb NSPC numbers, differentiation into other neural cells, and migration to sites of injury, and ultimately brain development and repair. The focus of this review was to dissect the effects of common antiviral cytokines and chemokines on NSPC activity, and to consider the subsequent pathological consequences for the host from changes in NSPC function.

Endothelial Cells Exposed to Fluid Shear Stress Support Diffusion Based Maturation of Adult Neural Progenitor Cells

Introduction

The neural stem cell (NSC) niche is a highly complex cellular and biochemical milieu supporting proliferating NSCs and neural progenitor cells (NPCs) with close apposition to the vasculature, primarily comprised of endothelial cells (ECs). Current in vitro models of the niche incorporate EC-derived factors, but do not reflect the physiologically relevant hemodynamic state of the ECs or the spatial resolution observed between cells within the niche.

Methods

In this work, we developed a novel in vitro model of the niche that (1) incorporates ECs cultured with fluid shear stress and (2) fosters paracrine cytokine gradients between ECs and NSCs in a spatiotemporal configuration mimicking the cytoarchitecture of the subventricular niche. A modified cone and plate viscometer was used to generate a shear stress of 10 dynes cm^-2 for ECs cultured on a membrane, while statically cultured NPCs are 10 or 1000 μm below the ECs.

Results

NPCs cultured within 10 μm of dynamic ECs exhibit increased PSA-NCAM⁺ and OLIG2⁺ cells compared to progenitors in all other culture regimes and the hemodynamic EC phenotype results in distinct progeny phenotypes. This co-culture regime yields greater release of pro-neurogenic factors, suggesting a potential mechanism for the observed progenitor maturation.

Conclusions

Based on these results, models incorporating ECs exposed to shear stress allow for paracrine signaling gradients and regulate NPC lineage progression with appropriate niche spatial resolution occurring at 10 μm. This model could be used to evaluate cellular or pharmacological interactions within the healthy, diseased, or aged brain.

Effects of Environmental Enrichment on Doublecortin and BDNF Expression along the Dorso-Ventral Axis of the Dentate Gyrus

Adult hippocampal neurogenesis (AHN) in the dentate gyrus is known to respond to environmental enrichment, chronic stress, and many other factors. The function of AHN may vary across the septo-temporal axis of the hippocampus, as different subdivisions are responsible for different functions. The dorsal pole regulates cognitive-related behaviors, while the ventral pole mediates mood-related responses through the hypothalamic-pituitary-adrenal (HPA) axis. In this study, we investigate different methods of quantifying the effect of environmental enrichment on AHN in the dorsal and ventral parts of the dentate gyrus (dDG and vDG). To this purpose, 11-week-old female CD-1 mice were assigned for 8 days to one of two conditions: the Environmental Enrichment (E) group received (i) running wheels, (ii) larger cages, (iii) plastic tunnels, and (iv) bedding with male urine, while the Control (C) group received standard housing. Dorsal CA (Cornu Ammonis) and DG regions were larger in the E than the C animals. Distance run linearly predicted the volume of the dorsal hippocampus, as well as of the intermediate and ventral CA regions. In the dDG, the amount of Doublecortin (DCX) immunoreactivity was significantly higher in E than in C mice. Surprisingly, this pattern was the opposite in the vDG (C > E). Real-time PCR measurement of Dcx mRNA and DCX protein analysis using ELISA showed the same pattern. Brain Derived Neurotrophic Factor (BDNF) immunoreactivity and mRNA displayed no difference between E and C, suggesting that upregulation of DCX was not caused by changes in BDNF levels. BDNF levels were higher in vDG than in dDG, as measured by both methods. Bdnf expression in vDG correlated positively with the distance run by individual E mice. The similarity in the patterns of immunoreactivity, mRNA and protein for differential DCX expression and for BDNF distribution suggests that the latter two methods might be effective tools for more rapid quantification of AHN.

Factors Released from Endothelial Cells Exposed to Flow Impact Adhesion, Proliferation, and Fate Choice in the Adult Neural Stem Cell Lineage

The microvasculature within the neural stem cell (NSC) niche promotes self-renewal and regulates lineage progression. Previous work identified endothelial-produced soluble factors as key regulators of neural progenitor cell (NPC) fate and proliferation; however, endothelial cells (ECs) are sensitive to local hemodynamics, and the effect of this key physiological process has not been defined. In this study, we evaluated adult mouse NPC response to soluble factors isolated from static or dynamic (flow) EC cultures. Endothelial factors generated under dynamic conditions significantly increased neuronal differentiation, while those released under static conditions stimulated oligodendrocyte differentiation. Flow increases EC release of neurogenic factors and of heparin sulfate glycosaminoglycans that increase their bioactivity, likely underlying the enhanced neuronal differentiation. Additionally, endothelial factors, especially from static conditions, promoted adherent growth. Together, our data suggest that blood flow may impact proliferation, adhesion, and the neuron-glial fate choice of adult NPCs, with implications for diseases and aging that reduce flow.

Neuroprotection in Schizophrenia and Its Therapeutic Implications

Schizophrenia is a chronic and debilitating mental disorder. The persisting negative and cognitive symptoms that are unresponsive to pharmacotherapy reveal the impairment of neuroprotective aspects of schizophrenia. In this review, of the several neuroprotective factors, we mainly focused on neuroinflammation, neurogenesis, and oxidative stress. We conducted a narrative and selective review. Neuroinflammation is mainly mediated by pro-inflammatory cytokines and microglia. Unlike peripheral inflammatory responses, neuroinflammation has a role in various neuronal activities such as neurotransmission neurogenesis. The cross-talk between neuroinflammation and neurogenesis usually has beneficial effects in the CNS under physiological conditions. However, uncontrolled and chronic neuroinflammation exert detrimental effects such as neuronal loss, inhibited neurogenesis, and excessive oxidative stress. Neurogenesis is also a major component of neuroprotection. Adult neurogenesis mainly occurs in the hippocampal region, which has an important role in memory formation and processing. Impaired neurogenesis and an ineffective response to antipsychotics may be thought to indicate a deteriorating course of schizophrenia. Oxidative stress and excessive dopaminergic neurotransmission may create a vicious cycle and consequently disturb NMDA receptor-mediated glutamatergic neurotransmission. Based on the current evidences, several neuroprotective therapeutic approaches have been reported to be efficacious for improving psychopathology, but further longitudinal and large-sample based studies are needed.

The effects of aging in the hippocampus and cognitive decline

Aging is a natural process that is associated with cognitive decline as well as functional and social impairments. One structure of particular interest when considering aging and cognitive decline is the hippocampus, a brain region known to play an important role in learning and memory consolidation as well as in affective behaviours and mood regulation, and where both functional and structural plasticity (e.g., neurogenesis) occur well into adulthood. Neurobiological alterations seen in the aging hippocampus including increased oxidative stress and neuroinflammation, altered intracellular signalling and gene expression, as well as reduced neurogenesis and synaptic plasticity, are thought to be associated with age-related cognitive decline. Non-invasive strategies such as caloric restriction, physical exercise, and environmental enrichment have been shown to counteract many of the age-induced alterations in hippocampal signalling, structure, and function. Thus, such approaches may have therapeutic value in counteracting the deleterious effects of aging and protecting the brain against age-associated neurodegenerative processes.

Age-associated Cognitive Decline: Insights into Molecular Switches and Recovery Avenues

Age-associated cognitive decline is an inevitable phenomenon that predisposes individuals for neurological and psychiatric disorders eventually affecting the quality of life. Scientists have endeavored to identify the key molecular switches that drive cognitive decline with advancing age. These newly identified molecules are then targeted as recovery of cognitive aging and related disorders. Cognitive decline during aging is multi-factorial and amongst several factors influencing this trajectory, gene expression changes are pivotal. Identifying these genes would elucidate the neurobiological underpinnings as well as offer clues that make certain individuals resilient to withstand the inevitable age-related deteriorations. Our laboratory has focused on this aspect and investigated a wide spectrum of genes involved in crucial brain functions that attribute to senescence induced cognitive deficits. We have recently identified master switches in the epigenome regulating gene expression alteration during brain aging. Interestingly, these factors when manipulated by chemical or genetic strategies successfully reverse the age-related cognitive impairments. In the present article, we review findings from our laboratory and others combined with supporting literary evidences on molecular switches of brain aging and their potential as recovery targets.

Aging alters histone H3 lysine 4 methylation in mouse germinal vesicle stage oocytes

Decreasing oocyte competence with maternal aging is a major factor in mammalian infertility. One of the factors contributing to this infertility is changes to chromatin modifications, such as histone acetylation in old MII stage oocytes. Recent studies indicate that changes in histone acetylation at MII arise at the germinal vesicle (GV) stage. We hypothesised that histone methylation could also change in old GV oocytes. To test this hypothesis, we examined mono-, di- and trimethylation of histone H3 lysine 4 (H3K4 me1, me2 and me3, respectively) in young and older oocytes from 6-8- and 42-44-week-old mice, respectively. We found that H3K4 me2 and me3 decreased in older compared with young GV oocytes (100% vs. 81% and 100% vs. 87%, respectively; P<0.05). H3K4 me2 later increased in older MII oocytes (21% vs. 56%; P<0.05). We also examined the expression of genes encoding the H3K4 demethylases lysine (K)-specific demethylase 1A (Kdm1a) and retinol binding protein 2 (Rbp2). Expression of Kdm1a increased at both the mRNA and protein levels in older GV oocytes, but decreased in older MII oocytes (P<0.05), and was negatively correlated with H3K4 me2 levels. Conversely, expression of Rbp2 mRNA and protein decreased in older GV oocytes (P<0.05), and this was not correlated with H3K4 me3 levels. Finally, we showed that inhibition of Kdm1a of older oocytes at the GV stage restored levels of H3K4 me2 at the MII stage to those seen in 'young' oocytes (41% vs. 38%; P>0.05). These results suggest that changes in expression of H3K4 me2 and Kdm1a in older GV oocytes may represent a molecular mechanism underlying human infertility caused by aging.

TAM receptor deficiency affects adult hippocampal neurogenesis

The Tyro3, Axl and Mertk (TAM) subfamily of receptor protein tyrosine kinases functions in cell growth, differentiation, survival, and most recently found, in the regulation of immune responses and phagocytosis. All three receptors and their ligands, Gas6 (growth arrest-specific gene 6) and protein S, are expressed in the central nervous system (CNS). TAM receptors play pivotal roles in adult hippocampal neurogenesis. Loss of these receptors causes a comprised neurogenesis in the dentate gyrus of adult hippocampus. TAM receptors have a negative regulatory effect on microglia and peripheral antigen-presenting cells, and play a critical role in preventing overproduction of pro-inflammatory cytokines detrimental to the proliferation, differentiation, and survival of adult neuronal stem cells (NSCs). Besides, these receptors also play an intrinsic trophic function in supporting NSC survival, proliferation, and differentiation into immature neurons. All these events collectively ensure a sustained neurogenesis in adult hippocampus.

Neural stem cells in Parkinson's disease: a role for neurogenesis defects in onset and progression

Parkinson's disease (PD) is the second most common neurodegenerative disorder, leading to a variety of motor and non-motor symptoms. Interestingly, non-motor symptoms often appear a decade or more before the first signs of motor symptoms. Some of these non-motor symptoms are remarkably similar to those observed in cases of impaired neurogenesis and several PD-related genes have been shown to play a role in embryonic or adult neurogenesis. Indeed, animal models deficient in Nurr1, Pitx3, SNCA and PINK1 display deregulated embryonic neurogenesis and LRRK2 and VPS35 have been implicated in neuronal development-related processes such as Wnt/β-catenin signaling and neurite outgrowth. Moreover, adult neurogenesis is affected in both PD patients and PD animal models and is regulated by dopamine and dopaminergic (DA) receptors, by chronic neuroinflammation, such as that observed in PD, and by differential expression of wild-type or mutant forms of PD-related genes. Indeed, an increasing number of in vivo studies demonstrate a role for SNCA and LRRK2 in adult neurogenesis and in the generation and maintenance of DA neurons. Finally, the roles of PD-related genes, SNCA, LRRK2, VPS35, Parkin, PINK1 and DJ-1 have been studied in NSCs, progenitor cells and induced pluripotent stem cells, demonstrating a role for some of these genes in stem/progenitor cell proliferation and maintenance. Together, these studies strongly suggest a link between deregulated neurogenesis and the onset and progression of PD and present strong evidence that, in addition to a neurodegenerative disorder, PD can also be regarded as a developmental disorder.

Understanding epigenetic changes in aging stem cells--a computational model approach

During aging, a decline in stem cell function is observed in many tissues. This decline is accompanied by complex changes of the chromatin structure among them changes in histone modifications and DNA methylation which both affect transcription of a tissue-specific subset of genes. A mechanistic understanding of these age-associated processes, their interrelations and environmental dependence is currently lacking. Here, we discuss related questions on the molecular, cellular, and population level. We combine an individual cell-based model of stem cell populations with a model of epigenetic regulation of transcription. The novel model enables to simulate age-related changes of trimethylation of lysine 4 at histone H3 and of DNA methylation. These changes entail expression changes of genes that induce age-related phenotypes (ARPs) of cells. We compare age-related changes of regulatory states in quiescent stem cells occupying a niche with those observed in proliferating cells. Moreover, we analyze the impact of the activity of the involved epigenetic modifiers on these changes. We find that epigenetic aging strongly affects stem cell heterogeneity and that homing at stem cell niches retards epigenetic aging. Our model provides a mechanistic explanation how increased stem cell proliferation can lead to progeroid phenotypes. Adapting our model to properties observed for aged hematopoietic stem cell (HSC) clones, we predict that the hematopoietic ARP activates young HSCs and thereby retards aging of the entire HSC population. In addition, our model suggests that the experimentally observed high interindividual variance in HSC numbers originates in a variance of histone methyltransferase activity.

Jmjd3 is essential for the epigenetic modulation of microglia phenotypes in the immune pathogenesis of Parkinson's disease

Classical activation (M1 phenotype) and alternative activation (M2 phenotype) are the two polars of microglial activation states that can produce either detrimental or beneficial effects in the central nervous system (CNS). Harnessing the beneficial properties of microglia cells by modulating their polarization states provides great potential for the treatment of Parkinson's disease (PD). However, the epigenetic mechanism that regulates microglia polarization remains elusive. Here, we reported that histone H3K27me3 demethylase Jumonji domain containing 3 (Jmjd3) was essential for M2 microglia polarization. Suppression of Jmjd3 in N9 microglia inhibited M2 polarization and simultaneously exaggerated M1 microglial inflammatory responses, which led to extensive neuron death in vitro. We also observed that the suppression of Jmjd3 in the substantia nigra (SN) in vivo dramatically caused microglial overactivation and exacerbated dopamine (DA) neuron death in 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP)-intoxicated mouse model of PD. Moreover, we showed that the Jmjd3 level was lower in the midbrain of aged mice, which was accompanied by an elevated level of H3K27me3 and an increased ratio of M1 to M2 markers, suggesting that aging is an important factor in switching the microglia phenotypes. Overall, our studies indicate that Jmjd3 is able to enhance the polarization of M2 microglia by modifying histone H3K27me3, and therefore it has a pivotal role in the switch of microglia phenotypes that may contribute to the immune pathogenesis of PD.

Select 3',5'-cyclic nucleotide phosphodiesterases exhibit altered expression in the aged rodent brain

3',5'-cyclic nucleotide phosphodiesterases (PDEs) are the only known enzymes to compartmentalize cAMP and cGMP, yet little is known about how PDEs are dynamically regulated across the lifespan. We mapped mRNA expression of all 21 PDE isoforms in the adult rat and mouse central nervous system (CNS) using quantitative polymerase chain reaction (qPCR) and in situ hybridization to assess conservation across species. We also compared PDE mRNA and protein in the brains of old (26 months) versus young (5 months) Sprague-Dawley rats, with select experiments replicated in old (9 months) versus young (2 months) BALB/cJ mice. We show that each PDE isoform exhibits a unique expression pattern across the brain that is highly conserved between rats, mice, and humans. PDE1B, PDE1C, PDE2A, PDE4A, PDE4D, PDE5A, PDE7A, PDE8A, PDE8B, PDE10A, and PDE11A showed an age-related increase or decrease in mRNA expression in at least 1 of the 4 brain regions examined (hippocampus, cortex, striatum, and cerebellum). In contrast, mRNA expression of PDE1A, PDE3A, PDE3B, PDE4B, PDE7A, PDE7B, and PDE9A did not change with age. Age-related increases in PDE11A4, PDE8A3, PDE8A4/5, and PDE1C1 protein expression were confirmed in hippocampus of old versus young rodents, as were age-related increases in PDE8A3 protein expression in the striatum. Age-related changes in PDE expression appear to have functional consequences as, relative to young rats, the hippocampi of old rats demonstrated strikingly decreased phosphorylation of GluR1, CaMKIIα, and CaMKIIβ, decreased expression of the transmembrane AMPA regulatory proteins γ2 (a.k.a. stargazin) and γ8, and increased trimethylation of H3K27. Interestingly, expression of PDE11A4, PDE8A4/5, PDE8A3, and PDE1C1 correlate with these functional endpoints in young but not old rats, suggesting that aging is not only associated with a change in PDE expression but also a change in PDE compartmentalization.

Negative regulation of TLX by IL-1β correlates with an inhibition of adult hippocampal neural precursor cell proliferation

Adult hippocampal neurogenesis is modulated by a number of intrinsic and extrinsic factors including local signalling molecules, exercise, aging and inflammation. Inflammation is also a major contributor to several hippocampal-associated disorders. Interleukin-1beta (IL-1β) is the most predominant pro-inflammatory cytokine in the brain, and an increase in its concentration is known to decrease the proliferation of both embryonic and adult hippocampal neural precursor cells (NPCs). Recent research has focused on the role of nuclear receptors as intrinsic regulators of neurogenesis, and it is now established that the orphan nuclear receptor TLX is crucial in maintaining the NPC pool in neurogenic brain regions. To better understand the involvement of TLX in IL-1β-mediated effects on hippocampal NPC proliferation, we examined hippocampal NPC proliferation and TLX expression in response to IL-1β treatment in an adult rat hippocampal neurosphere culture system. We demonstrate that IL-1β reduced the proliferation of hippocampal NPCs and TLX expression in a dose and time-dependent manner and that co-treatment with IL-1β receptor antagonist or IL-1 receptor siRNA prevented these effects. We also report a dose-dependent effect of IL-1β on the composition of cell phenotypes in the culture and on expression of TLX in these cells. This study thus provides evidence of an involvement of TLX in IL-1β-induced changes in adult hippocampal neurogenesis, and offers mechanistic insight into disorders in which neuroinflammation and alterations in neurogenesis are characteristic features.

Ginsenoside Rd maintains adult neural stem cell proliferation during lead-impaired neurogenesis

Lead exposure attracts a great deal of public attention due to its harmful effects on human health. Even low-level lead (Pb) exposure reduces the capacity for neurogenesis. It is well known that microglia-mediated neurotoxicity can impair neurogenesis. Despite this, few in vivo studies have been conducted to understand the relationship between acute Pb exposure and microglial activation. We investigated whether the acute Pb exposure altered the expression of a marker of activated microglial cells (Iba-1), and markers of neurogenesis (BrdU and doublecortin) in aging rats. As compared to controls, Pb exposure significantly enhanced the expression of Iba-1 immunoreactivity; increased the expression levels of IL-1β, IL-6, and TNF-α and decreased the numbers of BrdU(+) and doublecortin(+) cells. Our prior work demonstrated that ginsenoside Rd (Rd), one of the major active ingredients in Panax ginseng, was neuroprotective in a variety of paradigms involving anti-inflammatory mechanisms. Thus, we further examined whether Rd could attenuate Pb-induced phenotypes. Compared with the Pb exposure group, Rd pretreatment indeed attenuated the effects of Pb exposure. These results suggest that Rd may be neuroprotective in old rats following acute Pb exposure, which involves limitation of microglial activation and maintenance of NSC proliferation.

Estriol preserves synaptic transmission in the hippocampus during autoimmune demyelinating disease

Cognitive deficits occur in over half of multiple sclerosis patients, with hippocampal-dependent learning and memory commonly impaired. Data from in vivo MRI and post-mortem studies in MS indicate that the hippocampus is targeted. However, the relationship between structural pathology and dysfunction of the hippocampus in MS remains unclear. Hippocampal neuropathology also occurs in experimental autoimmune encephalomyelitis (EAE), the most commonly used animal model of MS. Although estrogen treatment of EAE has been shown to be anti-inflammatory and neuroprotective in the spinal cord, it is unknown if estrogen treatment may prevent hippocampal pathology and dysfunction. In the current study we examined excitatory synaptic transmission during EAE and focused on pathological changes in synaptic protein complexes known to orchestrate functional synaptic transmission in the hippocampus. We then determined if estriol, a candidate hormone treatment, was capable of preventing functional changes in synaptic transmission and corresponding hippocampal synaptic pathology. Electrophysiological studies revealed altered excitatory synaptic transmission and paired-pulse facilitation (PPF) during EAE. Neuropathological experiments demonstrated that there were decreased levels of pre- and post-synaptic proteins in the hippocampus, diffuse loss of myelin staining and atrophy of the pyramidal layers of hippocampal cornu ammonis 1 (CA1). Estriol treatment prevented decreases in excitatory synaptic transmission and lessened the effect of EAE on PPF. In addition, estriol treatment prevented several neuropathological alterations that occurred in the hippocampus during EAE. Cross-modality correlations revealed that deficits in excitatory synaptic transmission were significantly correlated with reductions in trans-synaptic protein binding partners known to modulate excitatory synaptic transmission. To our knowledge, this is the first report describing a functional correlate to hippocampal neuropathology in any MS model. Furthermore, a treatment was identified that prevented both deficits in synaptic function and hippocampal neuropathology.

Involvement of doublecortin-expressing cells in the arcuate nucleus in body weight regulation

Hypothalamic functions, including feeding behavior, show a high degree of plasticity throughout life. Doublecortin (DCX) is a marker of plasticity and neuronal migration expressed in the hypothalamus. Therefore, we wanted to map the fate of DCX(+) cells in the arcuate nucleus (ARC) of the hypothalamus. For this purpose, we generated a BAC transgenic mouse line that expresses the inducible recombinase CreER(T2) under control of the DCX locus. Crossing this line with the Rosa26 or Ai14 reporter mouse lines, we found reporter(+) cells in the ARC upon tamoxifen treatment. They were born prenatally and expressed both DCX and the plasticity marker TUC-4. Immediately after labeling, reporter(+) cells had an enlarged soma that normalized over time, suggesting morphological remodeling. Reporter(+) cells expressed β-endorphin and BSX, neuronal markers of the feeding circuit. Furthermore, leptin treatment led to phosphorylation of STAT3 in reporter(+) cells in accordance with the concept that they are part of the feeding circuits. Indeed, we found a negative correlation between the number of reporter(+) cells and body weight and epididymal fat pads. Our data suggest that DCX(+) cells in the ARC represent a cellular correlate of plasticity that is involved in controlling energy balance in adult mice.

Interleukin-1β: a new regulator of the kynurenine pathway affecting human hippocampal neurogenesis

Increased inflammation and reduced neurogenesis have been associated with the pathophysiology of major depression. Here, we show for the first time how IL-1β, a pro-inflammatory cytokine shown to be increased in depressed patients, decreases neurogenesis in human hippocampal progenitor cells. IL-1β was detrimental to neurogenesis, as shown by a decrease in the number of doublecortin-positive neuroblasts (-28%), and mature, microtubule-associated protein-2-positive neurons (-36%). Analysis of the enzymes that regulate the kynurenine pathway showed that IL-1β induced an upregulation of transcripts for indolamine-2,3-dioxygenase (IDO), kynurenine 3-monooxygenase (KMO), and kynureninase (42-, 12- and 30-fold increase, respectively, under differentiating conditions), the enzymes involved in the neurotoxic arm of the kynurenine pathway. Moreover, treatment with IL-1β resulted in an increase in kynurenine, the catabolic product of IDO-induced tryptophan metabolism. Interestingly, co-treatment with the KMO inhibitor Ro 61-8048 reversed the detrimental effects of IL-1β on neurogenesis. These observations indicate that IL-1β has a critical role in regulating neurogenesis whereas affecting the availability of tryptophan and the production of enzymes conducive to toxic metabolites. Our results suggest that inhibition of the kynurenine pathway may provide a new therapy to revert inflammatory-induced reduction in neurogenesis.

Inflammation and depression

Major depressive disorder (MDD) is a complex illness and it is likely that alterations in several interacting systems underlie its pathogenesis. Numerous hypotheses have been proposed to elucidate its origins. The inflammatory hypothesis emphasises the role of psycho-neuroimmunological dysfunctions. This is based on several observations: subsets of MDD patients have an altered peripheral immune system, with impaired cellular immunity and increased levels of proinflammatory cytokines; cytokines can influence neurotransmitter metabolism, neuroendocrine function and regional brain activity, all of which are relevant to depression; acute administration of cytokines causes sickness behaviour which shares features with depression, and patients undergoing cytokine treatment develop depressive symptoms. In this chapter, we discuss the evidence linking inflammation and MDD, looking at data from clinical and animal studies, the role of stress, possible mechanisms and the involvement of genetic polymorphisms. Further understanding of pathways involved is still needed. This will be vital for the identification of new drug targets and preventative strategies.

Aging microglia: relevance to cognition and neural plasticity

Over the years it has become evident that the immune system can affect the function of the central nervous system (CNS), including altering cognitive processes. The impact of immune activation on the CNS is particularly important for aged individuals, as the brain's resident immune cells, microglia, acquire a pro-inflammatory profile. The low-grade chronic neuroinflammation that develops with normal aging likely contributes to the susceptibility to cognitive deficits and a host of age-related pathologies. Understanding why microglia show increased inflammatory activity (i.e., neuroinflammation) and identifying effective treatments to reduce microglia activation is expected to have beneficial effects on cognitive performance and measures of neural plasticity. However, microglia also promote regeneration after injury. Therefore, effective treatments must dampen inflammatory activity while preserving microglia's neuroprotective function. Discovering factors that induce neuroinflammation and investigating potential preventative therapies is expected to uncover the ways of maintaining normal microglia activity in the aged brain.

Cyclooxygenase-1 is involved in the inhibition of hippocampal neurogenesis after lipopolysaccharide-induced neuroinflammation

Growing evidence indicates that neuroinflammation can alter adult neurogenesis by mechanisms as yet unclear. We have previously demonstrated that the neuroinflammatory response and neuronal damage after lipopolysaccharide (LPS) injection is reduced in cyclooxygenase-1 deficient (COX-1(-/-)) mice. In this study, we investigated the role of COX-1 on hippocampal neurogenesis during LPS-induced neuroinflammation, using COX-1(-/-) and wild type (WT) mice. We found that LPS-induced neuroinflammation resulted in the decrease of proliferation, survival and differentiation of hippocampal progenitor cells in WT but not in COX-1(-/-) mice. Thus, we demonstrate for the first time that COX-1 is involved in the inhibition of BrdU progenitor cells in proliferation and hippocampal neurogenesis after LPS. These results suggest that COX-1 may represent a viable therapeutic target to reduce neuroinflammation and promote neurogenesis in neurodegenerative diseases with a strong inflammatory component.

Physical exercise alleviates debilities of normal aging and Alzheimer's disease

Both healthy aging and the pathologic incidence of disorders associated with aging involve an array of debilities. Physical exercise harnesses implicit and inherent biologic characteristics amenable to the putative interventional influences under clinical, institutional or laboratory conditions. The neurodegenerative and pathophysiologic progressions that constitute Alzheimer's disease (AD), amnestic mild cognitive impairment (aMCI), normal aging, and different animal models of AD have shown the existence of several putative mechanisms. A large variety of moderating factors have demonstrated that the ever-proliferating plethora of neurotrophic factors, neurogenesis as observed through generality of expression and neuronal arborization. The insistent efficacy of brain vascular angiogenesis may delay also the comorbid incidence of depressive disorders with dementia pathology. The pathogenesis of aging may be contained by selective treatments: these diverse conditions, linked to the basis of the aging concept, have been shown, to greater or lesser extents, to respond to a variety of scheduled applications of physical exercise. The range of reports that provide accounts of the mechanisms mediating the positive progressive response to exercise intervention is far-ranging; these studies indicate that subtle changes at molecular, neuronal, vascular and epigenetic levels may exert notable consequence at functional expression and, perhaps most essentially, offer convincing expectancy of significant benefits.

Enhanced IL-1beta production in response to the activation of hippocampal glial cells impairs neurogenesis in aged mice

A variety of mechanisms that contribute to the accumulation of age-related damage and the resulting brain dysfunction have been identified. Recently, decreased neurogenesis in the hippocampus has been recognized as one of the mechanisms of age-related brain dysfunction. However, the molecular mechanism of decreased neurogenesis with aging is still unclear. In the present study, we investigated whether aging decreases neurogenesis accompanied by the activation of microglia and astrocytes, which increases the expression of IL-1beta in the hippocampus, and whether in vitro treatment with IL-1beta in neural stem cells directly impairs neurogenesis. Ionized calcium-binding adaptor molecule 1 (Iba1)-positive microglia and glial fibrillary acidic protein (GFAP)-positive astrocytes were increased in the dentate gyrus of the hippocampus of 28-month-old mice. Furthermore, the mRNA level of IL-1beta was significantly increased without related histone modifications. Moreover, a significant increase in lysine 9 on histone H3 (H3K9) trimethylation at the promoter of NeuroD (a neural progenitor cell marker) was observed in the hippocampus of aged mice. In vitro treatment with IL-1beta in neural stem cells prepared from whole brain of E14.5 mice significantly increased H3K9 trimethylation at the NeuroD promoter. These findings suggest that aging may decrease hippocampal neurogenesis via epigenetic modifications accompanied by the activation of microglia and astrocytes with the increased expression of IL-1beta in the hippocampus.

Modulators of cytoskeletal reorganization in CA1 hippocampal neurons show increased expression in patients at mid-stage Alzheimer's disease

During the progression of Alzheimer's disease (AD), hippocampal neurons undergo cytoskeletal reorganization, resulting in degenerative as well as regenerative changes. As neurofibrillary tangles form and dystrophic neurites appear, sprouting neuronal processes with growth cones emerge. Actin and tubulin are indispensable for normal neurite development and regenerative responses to injury and neurodegenerative stimuli. We have previously shown that actin capping protein beta2 subunit, Capzb2, binds tubulin and, in the presence of tau, affects microtubule polymerization necessary for neurite outgrowth and normal growth cone morphology. Accordingly, Capzb2 silencing in hippocampal neurons resulted in short, dystrophic neurites, seen in neurodegenerative diseases including AD. Here we demonstrate the statistically significant increase in the Capzb2 expression in the postmortem hippocampi in persons at mid-stage, Braak and Braak stage (BB) III-IV, non-familial AD in comparison to controls. The dynamics of Capzb2 expression in progressive AD stages cannot be attributed to reactive astrocytosis. Moreover, the increased expression of Capzb2 mRNA in CA1 pyramidal neurons in AD BB III-IV is accompanied by an increased mRNA expression of brain derived neurotrophic factor (BDNF) receptor tyrosine kinase B (TrkB), mediator of synaptic plasticity in hippocampal neurons. Thus, the up-regulation of Capzb2 and TrkB may reflect cytoskeletal reorganization and/or regenerative response occurring in hippocampal CA1 neurons at a specific stage of AD progression.