Magnetic resonance spectroscopy identifies neural progenitor cells in the live human brain

Neurogenesis and pattern separation: time for a divorce

The generation of new neurons in the adult mammalian brain has led to numerous theories as to their functional significance. One of the most widely held views is that adult neurogenesis promotes pattern separation, a process by which overlapping patterns of neural activation are mapped to less overlapping representations. While a large body of evidence supports a role for neurogenesis in high interference memory tasks, it does not support the proposed function of neurogenesis in mediating pattern separation. Instead, the adult-generated neurons seem to generate highly overlapping and yet distinct distributed representations for similar events. One way in which these immature, highly plastic, hyperactive neurons may contribute to novel memory formation while avoiding interference is by virtue of their extremely sparse connectivity with incoming perforant path fibers. Another intriguing proposal, awaiting empirical confirmation, is that the young neurons' recruitment into memory formation is gated by a novelty/mismatch mechanism mediated by CA3 or hilar back-projections. Ongoing research into the intriguing link between neurogenesis, stress-related mood disorders, and age-related neurodegeneration may lead to promising neurogenesis-based treatments for this wide range of clinical disorders. WIREs Cogn Sci 2017, 8:e1427. doi: 10.1002/wcs.1427 For further resources related to this article, please visit the WIREs website.

The dynamics of adult neurogenesis in human hippocampus

The phenomenon of adult neurogenesis is now an accepted occurrence in mammals and also in humans. At least two discrete places house stem cells for generation of neurons in adult brain. These are olfactory system and the hippocampus. In animals, newly generated neurons have been directly or indirectly demonstrated to generate a significant amount of new neurons to have a functional role. However, the data in humans on the extent of this process is still scanty and such as difficult to comprehend its functional role in humans. This paper explores the available data on as extent of adult hippocampal neurogenesis in humans and makes comparison to animal data.

Re-evaluating the link between neuropsychiatric disorders and dysregulated adult neurogenesis

People diagnosed with neuropsychiatric disorders such as depression, anxiety, addiction or schizophrenia often have dysregulated memory, mood, pattern separation and/or reward processing. These symptoms are indicative of a disrupted function of the dentate gyrus (DG) subregion of the brain, and they improve with treatment and remission. The dysfunction of the DG is accompanied by structural maladaptations, including dysregulation of adult-generated neurons. An increasing number of studies using modern inducible approaches to manipulate new neurons show that the behavioral symptoms in animal models of neuropsychiatric disorders can be produced or exacerbated by the inhibition of DG neurogenesis. Thus, here we posit that the connection between neuropsychiatric disorders and dysregulated DG neurogenesis is beyond correlation or epiphenomenon, and that the regulation of adult-generated DG neurogenesis merits continued and focused attention in the ongoing effort to develop novel treatments for neuropsychiatric disorders.

Physical exercise induces hippocampal neurogenesis and prevents cognitive decline

Accumulating evidence from animal and human research indicate that adult hippocampal neurogenesis plays a key role in cognition. Meanwhile, cognitive decline is well known to associate with ageing-related neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). Therefore, prevention of hippocampal neurogenesis reduction should be critical for these diseases. Physical exercise, a potent enhancer of adult hippocampal neurogenesis, has emerged as a potential therapy or an adjunctive therapeutic strategy for cognitive decline. In this review, we discuss the recent findings on hippocampal neurogenesis and the incorporation of new born neurons into the neuronal network in humans and in rodents. By focusing on hippocampal neurogenesis, we illustrate the role and possible mechanisms of physical exercise in cognition preservation.

Adult Stem Cell Therapy for Stroke: Challenges and Progress

Stroke is one of the leading causes of death and physical disability among adults. It has been 15 years since clinical trials of stem cell therapy in patients with stroke have been conducted using adult stem cells like mesenchymal stem cells and bone marrow mononuclear cells. Results of randomized controlled trials showed that adult stem cell therapy was safe but its efficacy was modest, underscoring the need for new stem cell therapy strategies. The primary limitations of current stem cell therapies include (a) the limited source of engraftable stem cells, (b) the presence of optimal time window for stem cell therapies, (c) inherited limitation of stem cells in terms of growth, trophic support, and differentiation potential, and (d) possible transplanted cell-mediated adverse effects, such as tumor formation. Here, we discuss recent advances that overcome these hurdles in adult stem cell therapy for stroke.

Regulation of adult neural progenitor cell functions by purinergic signaling

Extracellular purines are signaling molecules in the neurogenic niches of the brain and spinal cord, where they activate cell surface purinoceptors at embryonic neural stem cells (NSCs) and adult neural progenitor cells (NPCs). Although mRNA and protein are expressed at NSCs/NPCs for almost all subtypes of the nucleotide-sensitive P2X/P2Y, and the nucleoside-sensitive adenosine receptors, only a few of those have acquired functional significance. ATP is sequentially degraded by ecto-nucleotidases to ADP, AMP, and adenosine with agonistic properties for distinct receptor-classes. Nucleotides/nucleosides facilitate or inhibit NSC/NPC proliferation, migration and differentiation. The most ubiquitous effect of all agonists (especially of ATP and ADP) appears to be the facilitation of cell proliferation, usually through P2Y1Rs and sometimes through P2X7Rs. However, usually P2X7R activation causes necrosis/apoptosis of NPCs. Differentiation can be initiated by P2Y2R-activation or P2X7R-blockade. A key element in the transduction mechanism of either receptor is the increase of the intracellular free Ca²⁺ concentration, which may arise due to its release from intracellular storage sites (G protein-coupling; P2Y) or due to its passage through the receptor-channel itself from the extracellular space (ATP-gated ion channel; P2X). Further research is needed to clarify how purinergic signaling controls NSC/NPC fate and how the balance between the quiescent and activated states is established with fine and dynamic regulation. GLIA 2017;65:213-230.

Identification of Gastric Cancer Biomarkers Using 1H Nuclear Magnetic Resonance Spectrometry

Existing gastric cancer diagnosing methods were invasive, hence, a reliable non-invasive gastric cancer diagnosing method is needed. As a starting point, we used 1H NMR for identifying gastric cancer biomarkers using a panel of gastric cancer spheroids and normal gastric spheroids. We were able to identify 8 chemical shift biomarkers for gastric cancer spheroids. Our data suggests that the cancerous and non-cancerous spheroids significantly differ in the lipid composition and energy metabolism. These results encourage the translation of these biomarkers into in-vivo gastric cancer detection methodology using MRI-MS.

Aerobic Exercise as a Tool to Improve Hippocampal Plasticity and Function in Humans: Practical Implications for Mental Health Treatment

Aerobic exercise (AE) has been widely praised for its potential benefits to cognition and overall brain and mental health. In particular, AE has a potent impact on promoting the function of the hippocampus and stimulating neuroplasticity. As the evidence-base rapidly builds, and given most of the supporting work can be readily translated from animal models to humans, the potential for AE to be applied as a therapeutic or adjunctive intervention for a range of human conditions appears ever more promising. Notably, many psychiatric and neurological disorders have been associated with hippocampal dysfunction, which may underlie the expression of certain symptoms common to these disorders, including (aspects of) cognitive dysfunction. Augmenting existing treatment approaches using AE based interventions may promote hippocampal function and alleviate cognitive deficits in various psychiatric disorders that currently remain untreated. Incorporating non-pharmacological interventions into clinical treatment may also have a number of other benefits to patient well being, such as limiting the risk of adverse side effects. This review incorporates both animal and human literature to comprehensively detail how AE is associated with cognitive enhancements and stimulates a cascade of neuroplastic mechanisms that support improvements in hippocampal functioning. Using the examples of schizophrenia and major depressive disorder, the utility and implementation of an AE intervention to the clinical domain will be proposed, aimed to reduce cognitive deficits in these, and related disorders.

Targeting cancer stem-like cells in glioblastoma and colorectal cancer through metabolic pathways

Cancer stem-like cells (CSCs) are thought to be the main cause of tumor occurrence, progression and therapeutic resistance. Strong research efforts in the last decade have led to the development of several tailored approaches to target CSCs with some very promising clinical trials underway; however, until now no anti-CSC therapy has been approved for clinical use. Given the recent improvement in our understanding of how onco-proteins can manipulate cellular metabolic networks to promote tumorigenesis, cancer metabolism research may well lead to innovative strategies to identify novel regulators and downstream mediators of CSC maintenance. Interfering with distinct stages of CSC-associated metabolics may elucidate novel, more efficient strategies to target this highly malignant cell population. Here recent discoveries regarding the metabolic properties attributed to CSCs in glioblastoma (GBM) and malignant colorectal cancer (CRC) were summarized. The association between stem cell markers, the response to hypoxia and other environmental stresses including therapeutic insults as well as developmentally conserved signaling pathways with alterations in cellular bioenergetic networks were also discussed. The recent developments in metabolic imaging to identify CSCs were also summarized. This summary should comprehensively update basic and clinical scientists on the metabolic traits of CSCs in GBM and malignant CRC.

Reduced miR-512 and the Elevated Expression of Its Targets cFLIP and MCL1 Localize to Neurons With Hyperphosphorylated Tau Protein in Alzheimer Disease

The cause for the neurofibrillary tangles and plaques in Alzheimer disease likely relates to an abnormal accumulation of their key components, which include β-amyloid and hyperphosphorylated tau protein. We segregated Alzheimer brain sections from people with end-stage disease into those with abundant hyperphosphorylated tau protein and those without and compared each to normal brains for global microRNA patterns. A significant reduced expression of several microRNAs, including miR-512, was evident in the Alzheimer brain sections with abundant hyperphosphorylated tau. Immunohistochemistry documented that 2 known targets of microRNA-512, cFLIP and MCL1, were significantly over expressed and each colocalized to neurons with the abnormal tau protein. Analysis for apoptosis including activated caspase-3, increased caspase-4 and caspase-8, apoptosis initiating factor, APAF-1 activity, and the TUNEL assay was negative in the areas where neurons showed hyperphosphorylated tau. MCM2 expression, a marker of neuroprogenitor cells, was significantly reduced in the Alzheimer sections that contained the hyperphosphorylated tau. These results suggest that a basic defect in Alzheimer disease may be the reduced microRNA-driven increased expression of proteins that may alter the apoptotic/antiapoptotic balance of neurons. This, in turn, could lead to the accumulation of key Alzheimer proteins such as hyperphosphorylated tau that ultimately prevent normal neuronal function and lead to disease symptomatology.

Role of miR-211 in Neuronal Differentiation and Viability: Implications to Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is an age-related irreversible neurodegenerative disorder characterized by extracellular β Amyloid(Aβ) deposition, intracellular neurofibrillary tangles and neuronal loss. The dysfunction of neurogenesis and increased degeneration of neurons contribute to the pathogenesis of AD. We now report that miR-211-5p, a small non-coding RNA, can impair neurite differentiation by directly targeting NUAK1, decrease neuronal viability and accelerate the progression of Aβ-induced pathologies. In this study, we observed that during embryonic development, the expression levels of miR-211-5p were down-regulated in the normal cerebral cortexes of mice. However, in APPswe/PS1ΔE9 double transgenic adult mice, it was up-regulated from 9 months of age compared to that of the age-matched wild type mice. Studies in primary cortical neuron cultures demonstrated that miR-211-5p can inhibit neurite growth and branching via NUAK1 repression and decrease mature neuron viability. The impairments were more obvious under the action of Aβ. Our data showed that miR-211-5p could inhibit cortical neuron differentiation and survival, which may contribute to the synaptic failure, neuronal loss and cognitive dysfunction in AD.

Transgenic mouse models for studying adult neurogenesis

The mammalian hippocampus shows a remarkable capacity for continued neurogenesis throughout life. Newborn neurons, generated by the radial neural stem cells (NSCs), are important for learning and memory as well as mood control. During aging, the number and responses of NSCs to neurogenic stimuli diminish, leading to decreased neurogenesis and age-associated cognitive decline and psychiatric disorders. Thus, adult hippocampal neurogenesis has garnered significant interest because targeting it could be a novel potential therapeutic strategy for these disorders. However, if we are to use neurogenesis to halt or reverse hippocampal-related pathology, we need to understand better the core molecular machinery that governs NSC and their progeny. In this review, we summarize a wide variety of mouse models used in adult neurogenesis field, present their advantages and disadvantages based on specificity and efficiency of labeling of different cell types, and review their contribution to our understanding of the biology and the heterogeneity of different cell types found in adult neurogenic niches.

Alzheimer's Disease and Hippocampal Adult Neurogenesis; Exploring Shared Mechanisms

New neurons incorporate into the granular cell layer of the dentate gyrus throughout life. Neurogenesis is modulated by behavior and plays a major role in hippocampal plasticity. Along with older mature neurons, new neurons structure the dentate gyrus, and determine its function. Recent data suggest that the level of hippocampal neurogenesis is substantial in the human brain, suggesting that neurogenesis may have important implications for human cognition. In support of that, impaired neurogenesis compromises hippocampal function and plays a role in cognitive deficits in Alzheimer's disease mouse models. We review current work suggesting that neuronal differentiation is defective in Alzheimer's disease, leading to dysfunction of the dentate gyrus. Additionally, alterations in critical signals regulating neurogenesis, such as presenilin-1, Notch 1, soluble amyloid precursor protein, CREB, and β-catenin underlie dysfunctional neurogenesis in Alzheimer's disease. Lastly, we discuss the detectability of neurogenesis in the live mouse and human brain, as well as the therapeutic implications of enhancing neurogenesis for the treatment of cognitive deficits and Alzheimer's disease.

Cell proliferation is reduced in the hippocampus in schizophrenia

OBJECTIVE

The molecular and cellular basis of structural and functional abnormalities of the hippocampus found in schizophrenia is currently unclear. Postnatal neurogenesis contributes to hippocampal function in animal models and is correlated with hippocampal volume in primates. Reduced hippocampal cell proliferation has been previously reported in schizophrenia, which may contribute to hippocampal dysfunction.

METHOD

We measured the cell proliferation marker, Ki67, in post-mortem hippocampal tissue from patients with schizophrenia (n = 10) and matched controls (n = 16). Ki67-labelled cells were counted within the dentate gyrus and hilus on sections taken from the anterior hippocampus.

RESULTS

We replicated the finding of a significant reduction in Ki67+ cells/mm² in schizophrenia cases compared to controls (t24 = 2.1, p = 0.023). In our relatively small sample, we did not find a relationship between Ki67+ cells and age overall, or between Ki67 + cells and duration of illness or antipsychotic treatment in people with schizophrenia.

CONCLUSION

Our results confirm that reduced hippocampal cell proliferation may be present in schizophrenia. Restoring hippocampal neurogenesis may be a potential therapeutic target for the treatment of hippocampal dysfunction in schizophrenia.

Virtual Environmental Enrichment through Video Games Improves Hippocampal-Associated Memory

The positive effects of environmental enrichment and their neural bases have been studied extensively in the rodent (van Praag et al., 2000). For example, simply modifying an animal's living environment to promote sensory stimulation can lead to (but is not limited to) enhancements in hippocampal cognition and neuroplasticity and can alleviate hippocampal cognitive deficits associated with neurodegenerative diseases and aging. We are interested in whether these manipulations that successfully enhance cognition (or mitigate cognitive decline) have similar influences on humans. Although there are many "enriching" aspects to daily life, we are constantly adapting to new experiences and situations within our own environment on a daily basis. Here, we hypothesize that the exploration of the vast and visually stimulating virtual environments within video games is a human correlate of environmental enrichment. We show that video gamers who specifically favor complex 3D video games performed better on a demanding recognition memory task that assesses participants' ability to discriminate highly similar lure items from repeated items. In addition, after 2 weeks of training on the 3D video game Super Mario 3D World, naive video gamers showed improved mnemonic discrimination ability and improvements on a virtual water maze task. Two control conditions (passive and training in a 2D game, Angry Birds), showed no such improvements. Furthermore, individual performance in both hippocampal-associated behaviors correlated with performance in Super Mario but not Angry Birds, suggesting that how individuals explored the virtual environment may influence hippocampal behavior.

SIGNIFICANCE STATEMENT

The hippocampus has long been associated with episodic memory and is commonly thought to rely on neuroplasticity to adapt to the ever-changing environment. In animals, it is well understood that exposing animals to a more stimulating environment, known as environmental enrichment, can stimulate neuroplasticity and improve hippocampal function and performance on hippocampally mediated memory tasks. Here, we suggest that the exploration of vast and visually stimulating environments within modern-day video games can act as a human correlate of environmental enrichment. Training naive video gamers in a rich 3D, but not 2D, video game, resulted in a significant improvement in hippocampus-associated cognition using several behavioral measures. Our results suggest that modern day video games may provide meaningful stimulation to the human hippocampus.

Depression as a risk factor for Alzheimer's disease: Genes, steroids, cytokines and neurogenesis - What do we need to know?

Depression (MDD) is prodromal to, and a component of, Alzheimer's disease (AD): it may also be a trigger for incipient AD. MDD is not a unitary disorder, so there may be particular subtypes of early life MDD that pose independent high risks for later AD, though the identification of these subtypes is problematical. There may either be a common pathological event underlying both MDD and AD, or MDD may sensitize the brain to a second event ('hit') that precipitates AD. MDD may also accelerate brain ageing, including altered DNA methylation, increased cortisol but decreasing DHEA and thus the risk for AD. So far, genes predicting AD (e.g. APOEε4) are not risk factors for MDD, and those implicated in MDD (e.g. SLC6A4) are not risks for AD, so a common genetic predisposition looks unlikely. There is as yet no strong indication that an epigenetic event occurs during some forms of MDD that predisposes to later AD, though the evidence is limited. Glucocorticoids (GCs) are disturbed in some cases of MDD and in AD. GCs have marked degenerative actions on the hippocampus, a site of early β-amyloid deposition, and rare genetic variants of GC-regulating enzymes (e.g. 11β-HSD) predispose to AD. GCs also inhibit hippocampal neurogenesis and plasticity, and thus episodic memory, a core symptom of AD. Disordered GCs in MDD may inhibit neurogenesis, but the contribution of diminished neurogenesis to the onset or progression of AD is still debated. GCs and cytokines also reduce BDNF, implicated in both MDD and AD and hippocampal neurogenesis, reinforcing the notion that those cases of MDD with disordered GCs may be a risk for AD. Cytokines, including IL1β, IL6 and TNFα, are increased in the blood in some cases of MDD. They also reduce hippocampal neurogenesis, and increased cytokines are a known risk for later AD. Inflammatory changes occur in both MDD and AD (e.g. raised CRP, TNFα). Both cytokines and GCs can have pro-inflammatory actions in the brain. Inflammation (e.g. microglial activation) may be a common link, but this has not been systematically investigated. We lack substantial, rigorous and comprehensive follow-up studies to better identify possible subtypes of MDD that may represent a major predictor for later AD. This would enable specific interventions during critical episodes of these subtypes of MDD that should reduce this substantial risk.

Oral biomarkers in exercise-induced neuroplasticity in Parkinson's disease

In this article, we review candidate biomarkers for Parkinson's disease (PD) in oral cavity, potential of oral biomarkers as markers of neuroplasticity, and literature on the effects of exercise on oral cavity biomarkers in PD. We first describe how pathophysiological pathways of PD may be transduced from brain stem and ganglia to oral cavity through the autonomic nervous system or transduced by a reverse path. Next we describe the effects of exercise in PD and potential impact on oral cavity. We propose that biomarkers in oral cavity may be useful targets for describing exercise-induced brain neuroplasticity in PD. Nevertheless, much research remains to be carried out before applying these biomarkers for the determination of disease state and therapeutic response to develop strategies to mitigate motor or non-motor symptoms in PD.

Linking adult hippocampal neurogenesis with human physiology and disease

We here review the existing evidence linking adult hippocampal neurogenesis and human brain function in physiology and disease. Furthermore, we aim to point out where evidence is missing, highlight current promising avenues of investigation, and suggest future tools and approaches to foster the link between life-long neurogenesis and human brain function. Developmental Dynamics 245:702-709, 2016. © 2016 Wiley Periodicals, Inc.

Post-transcriptional regulation of gene expression in neural stem cells

Expression of each gene can be controlled at several steps during the flow of genetic information from DNA to protein. Tight regulation of gene expression is especially important for stem cells because of their greater ripple effects, compared with terminally differentiated cells. Dysregulation of gene expression arising in stem cells can be perpetuated within the stem cell pool via self-renewal throughout life. In addition, transcript profiles within stem cells can determine the selective advantage or disadvantage of each cell, leading to changes in cell fate, such as a tendency for proliferation, death, and differentiation. The identification of neural stem/progenitor cells (NSPCs) and greater understanding of their cellular physiology have raised the possibility of using NSPCs to replace damaged or injured neurons. However, an accurate grasp of gene expression control must take precedence in order to use NSPCs in therapies for neurological diseases. Recently, accumulating evidence has demonstrated the importance of post-transcriptional regulation in NSPC fate decisions. In this review, we will summarize and discuss the recent findings on key mRNA modulators and their vital roles in NSPC homeostasis. Copyright © 2016 John Wiley & Sons, Ltd.

Live Imaging of Adult Neural Stem Cells in Rodents

The generation of cells of the neural lineage within the brain is not restricted to early development. New neurons, oligodendrocytes, and astrocytes are produced in the adult brain throughout the entire murine life. However, despite the extensive research performed in the field of adult neurogenesis during the past years, fundamental questions regarding the cell biology of adult neural stem cells (aNSCs) remain to be uncovered. For instance, it is crucial to elucidate whether a single aNSC is capable of differentiating into all three different macroglial cell types in vivo or these distinct progenies constitute entirely separate lineages. Similarly, the cell cycle length, the time and mode of division (symmetric vs. asymmetric) that these cells undergo within their lineage progression are interesting questions under current investigation. In this sense, live imaging constitutes a valuable ally in the search of reliable answers to the previous questions. In spite of the current limitations of technology new approaches are being developed and outstanding amount of knowledge is being piled up providing interesting insights in the behavior of aNSCs. Here, we will review the state of the art of live imaging as well as the alternative models that currently offer new answers to critical questions.

Regional brain volumes, diffusivity, and metabolite changes after electroconvulsive therapy for severe depression

OBJECTIVE

To investigate the role of hippocampal plasticity in the antidepressant effect of electroconvulsive therapy (ECT).

METHOD

We used magnetic resonance (MR) imaging including diffusion tensor imaging (DTI) and proton MR spectroscopy (¹ H-MRS) to investigate hippocampal volume, diffusivity, and metabolite changes in 19 patients receiving ECT for severe depression. Other regions of interest included the amygdala, dorsolateral prefrontal cortex (DLPFC), orbitofrontal cortex, and hypothalamus. Patients received a 3T MR scan before ECT (TP1), 1 week (TP2), and 4 weeks (TP3) after ECT.

RESULTS

Hippocampal and amygdala volume increased significantly at TP2 and continued to be increased at TP3. DLPFC exhibited a transient volume reduction at TP2. DTI revealed a reduced anisotropy and diffusivity of the hippocampus at TP2. We found no significant post-ECT changes in brain metabolite concentrations, and we were unable to identify a spectral signature at ≈1.30 ppm previously suggested to reflect neurogenesis induced by ECT. None of the brain imaging measures correlated to the clinical response.

CONCLUSION

Our findings show that ECT causes a remodeling of brain structures involved in affective regulation, but due to their lack of correlation with the antidepressant effect, this remodeling does not appear to be directly underlying the antidepressant action of ECT.

Adult Hippocampal Neurogenesis, Fear Generalization, and Stress

The generalization of fear is an adaptive, behavioral, and physiological response to the likelihood of threat in the environment. In contrast, the overgeneralization of fear, a cardinal feature of posttraumatic stress disorder (PTSD), manifests as inappropriate, uncontrollable expression of fear in neutral and safe environments. Overgeneralization of fear stems from impaired discrimination of safe from aversive environments or discernment of unlikely threats from those that are highly probable. In addition, the time-dependent erosion of episodic details of traumatic memories might contribute to their generalization. Understanding the neural mechanisms underlying the overgeneralization of fear will guide development of novel therapeutic strategies to combat PTSD. Here, we conceptualize generalization of fear in terms of resolution of interference between similar memories. We propose a role for a fundamental encoding mechanism, pattern separation, in the dentate gyrus (DG)-CA3 circuit in resolving interference between ambiguous or uncertain threats and in preserving episodic content of remote aversive memories in hippocampal-cortical networks. We invoke cellular-, circuit-, and systems-based mechanisms by which adult-born dentate granule cells (DGCs) modulate pattern separation to influence resolution of interference and maintain precision of remote aversive memories. We discuss evidence for how these mechanisms are affected by stress, a risk factor for PTSD, to increase memory interference and decrease precision. Using this scaffold we ideate strategies to curb overgeneralization of fear in PTSD.

Are Anxiety Disorders Associated with Accelerated Aging? A Focus on Neuroprogression

Anxiety disorders (AnxDs) are highly prevalent throughout the lifespan, with detrimental effects on daily-life functioning, somatic health, and quality of life. An emerging perspective suggested that AnxDs may be associated with accelerated aging. In this paper, we explored the association between AnxDs and hallmarks of accelerated aging, with a specific focus on neuroprogression. We reviewed animal and human findings that suggest an overlap between processes of impaired neurogenesis, neurodegeneration, structural, functional, molecular, and cellular modifications in AnxDs, and aging. Although this research is at an early stage, our review suggests a link between anxiety and accelerated aging across multiple processes involved in neuroprogression. Brain structural and functional changes that accompany normal aging were more pronounced in subjects with AnxDs than in coevals without AnxDs, including reduced grey matter density, white matter alterations, impaired functional connectivity of large-scale brain networks, and poorer cognitive performance. Similarly, molecular correlates of brain aging, including telomere shortening, Aβ accumulation, and immune-inflammatory and oxidative/nitrosative stress, were overrepresented in anxious subjects. No conclusions about causality or directionality between anxiety and accelerated aging can be drawn. Potential mechanisms of this association, limitations of the current research, and implications for treatments and future studies are discussed.

Neurocognitive Aging and the Hippocampus across Species

There is extensive evidence that aging is associated with impairments in episodic memory. Many of these changes have been ascribed to neurobiological alterations to the hippocampal network and its input pathways. A cross-species consensus is beginning to emerge suggesting that subtle synaptic and functional changes within this network may underlie the majority of age-related memory impairments. In this review we survey convergent data from animal and human studies that have contributed significantly to our understanding of the brain-behavior relationships in this network, particularly in the aging brain. We utilize a cognitive as well as a neurobiological perspective and synthesize data across approaches and species to reach a more detailed understanding of age-related alterations in hippocampal memory function.

Brain maturation in neonatal rodents is impeded by sevoflurane anesthesia

BACKGROUND

A wealth of data shows neuronal demise after general anesthesia in the very young rodent brain. Herein, the authors apply proton magnetic resonance spectroscopy (1HMRS), testing the hypothesis that neurotoxic exposure during peak synaptogenesis can be tracked via changes in neuronal metabolites.

METHODS

1HMRS spectra were acquired in the brain (thalamus) of neonatal rat pups 24 and 48 h after sevoflurane exposure on postnatal day (PND) 7 and 15 and in unexposed, sham controls. A repeated measure ANOVA was performed to examine whether changes in metabolites were different between exposed and unexposed groups. Sevoflurane-induced neurotoxicity on PND7 was confirmed by immunohistochemistry.

RESULTS

In unexposed PND7 pups (N = 21), concentration of N-acetylaspartate (NAA; [NAA]) increased by 16% from PND8 to PND9, whereas in exposed PND7 pups (N = 19), [NAA] did not change and concentration of glycerophosphorylcholine and phosphorylcholine ([GPC + PCh]) decreased by 25%. In PND15 rats, [NAA] increased from PND16 to PND17 for both the exposed (N = 14) and the unexposed (N = 16) groups. Two-way ANOVA for PND7 pups demonstrated that changes over time observed in [NAA] (P = 0.031) and [GPC + PCh] (P = 0.024) were different between those two groups.

CONCLUSIONS

The authors demonstrated that normal [NAA] increase from PND8 to PND9 was impeded in sevoflurane-exposed rats when exposed at PND7; however, not impeded when exposed on PND15. Furthermore, the authors showed that noninvasive 1HMRS is sufficiently sensitive to detect subtle differences in developmental time trajectory of [NAA]. This is potentially clinically relevant because 1HMRS can be applied across species and may be useful in providing evidence of neurotoxicity in the human neonatal brain.

Neurocognitive Effects Following Cranial Irradiation for Brain Metastases

About 90% of patients with brain metastases have impaired neurocognitive function at diagnosis and up to two-thirds will show further declines within 2-6 months of whole brain radiotherapy. Distinguishing treatment effects from progressive disease can be challenging because the prognosis remains poor in many patients. Omitting whole brain radiotherapy after local therapy in good prognosis patients improves verbal memory at 4 months, but the effect of higher intracranial recurrence and salvage therapy rates on neurocognitive function beyond this time point is unknown. Hippocampal-sparing whole brain radiotherapy and postoperative stereotactic radiosurgery are investigational techniques intended to reduce toxicity. Here we describe the changes that can occur and review technological, pharmacological and practical approaches used to mitigate their effect in clinical practice.

In Vivo Targeted Magnetic Resonance Imaging of Endogenous Neural Stem Cells in the Adult Rodent Brain

Neural stem cells in the adult mammalian brain have a significant level of neurogenesis plasticity. In vivo monitoring of adult endogenous NSCs would be of great benefit to the understanding of the neurogenesis plasticity under normal and pathological conditions. Here we show the feasibility of in vivo targeted MR imaging of endogenous NSCs in adult mouse brain by intraventricular delivery of monoclonal anti-CD15 antibody conjugated superparamagnetic iron oxide nanoparticles. After intraventricular administration of these nanoparticles, the subpopulation of NSCs in the anterior subventricular zone and the beginning of the rostral migratory stream could be in situ labeled and were in vivo visualized with 7.0-T MR imaging during a period from 1 day to 7 days after the injection. Histology confirmed that the injected targeted nanoparticles were specifically bound to CD15 positive cells and their surrounding extracellular matrix. Our results suggest that in vivo targeted MR imaging of endogenous neural stem cells in adult rodent brain could be achieved by using anti-CD15-SPIONs as the molecular probe; and this targeting imaging strategy has the advantage of a rapid in vivo monitoring of the subpopulation of endogenous NSCs in adult brains.

Evaluating Exercise as a Therapeutic Intervention for Methamphetamine Addiction-Like Behavior

The need for effective treatments for addiction and dependence to the illicit stimulant methamphetamine in primary care settings is increasing, yet no effective medications have been FDA approved to reduce dependence [1]. This is partially attributed to the complex and dynamic neurobiology underlying the various stages of addiction [2]. Therapeutic strategies to treat methamphetamine addiction, particularly the relapse stage of addiction, could revolutionize methamphetamine addiction treatment. In this context, preclinical studies demonstrate that voluntary exercise (sustained physical activity) could be used as an intervention to reduce methamphetamine addiction. Therefore, it appears that methamphetamine disrupts normal functioning in the brain and this disruption is prevented or reduced by engaging in exercise. This review discusses animal models of methamphetamine addiction and sustained physical activity and the interactions between exercise and methamphetamine behaviors. The review highlights how methamphetamine and exercise affect neuronal plasticity and neurotoxicity in the adult mammalian striatum, hippocampus, and prefrontal cortex, and presents the emerging mechanisms of exercise in attenuating intake and in preventing relapse to methamphetamine seeking in preclinical models of methamphetamine addiction.

Regulation of Adult Neurogenesis and Plasticity by (Early) Stress, Glucocorticoids, and Inflammation

Exposure to stress is one of the best-known negative regulators of adult neurogenesis (AN). We discuss changes in neurogenesis in relation to exposure to stress, glucocorticoid hormones, and inflammation, with a particular focus on early development and on lasting effects of stress. Although the effects of acute and mild stress on AN are generally brief and can be quickly overcome, chronic exposure or more severe forms of stress can induce longer lasting reductions in neurogenesis that can, however, in part, be overcome by subsequent exposure to exercise, drugs targeting the stress system, and some antidepressants. Exposure to stress, particularly during the sensitive period of early life, may (re)program brain plasticity, in particular, in the hippocampus. This may increase the risk to develop cognitive or anxiety symptoms, common to brain diseases like dementia and depression in which plasticity changes occur, and a normalization of neurogenesis may be required for a successful treatment response and recovery.

Preventive brain radio-chemotherapy alters plasticity associated metabolite profile in the hippocampus but seems to not affect spatial memory in young leukemia patients

BACKGROUND

Neuronal plasticity leading to evolving reorganization of the neuronal network during entire lifespan plays an important role for brain function especially memory performance. Adult neurogenesis occurring in the dentate gyrus of the hippocampus represents the maximal way of network reorganization. Brain radio-chemotherapy strongly inhibits adult hippocampal neurogenesis in mice leading to impaired spatial memory.

METHODS

To elucidate the effects of CNS radio-chemotherapy on hippocampal plasticity and function in humans, we performed a longitudinal pilot study using 3T proton magnetic resonance spectroscopy ((1)H-MRS) and virtual water-maze-tests in 10 de-novo patients with acute lymphoblastic leukemia undergoing preventive whole brain radio-chemotherapy. Patients were examined before, during and after treatment.

RESULTS

CNS radio-chemotherapy did neither affect recall performance in probe trails nor flexible (reversal) relearning of a new target position over a time frame of 10 weeks measured by longitudinal virtual water-maze-testing, but provoked hippocampus-specific decrease in choline as a metabolite associated with cellular plasticity in (1)H-MRS.

CONCLUSION

Albeit this pilot study needs to be followed up to definitely resolve the question about the functional role of adult human neurogenesis, the presented data suggest that (1)H-MRS allows the detection of neurogenesis-associated plasticity in the human brain.

Adult Neurogenesis in Humans

Adult neurogenesis appears very well conserved among mammals. It was, however, not until recently that quantitative data on the extent of this process became available in humans, largely because of methodological challenges to study this process in man. There is substantial hippocampal neurogenesis in adult humans, but humans appear unique among mammals in that there is no detectable olfactory bulb neurogenesis but continuous addition of new neurons in the striatum.

A quantitative analysis of (1)H-MR spectroscopy at 3.0 T of three brain regions from childhood to middle age

OBJECTIVE

To study age-related metabolic changes in different brain regions.

METHODS

Point-resolved spectroscopy (repetition time/echo time = 2000 ms/30 ms) was performed in the left and right hippocampus, the left thalamus and the left centrum semiovale of 80 healthy subjects (37 females and 43 males aged 7-64 years). Analysis of covariance and linear regression were used for statistical analysis. Both metabolite concentration ratios with respect to total creatine (tCr) and absolute metabolite concentrations were included for analysis.

RESULTS

Ins (myo-inositol)/tCr (p < 0.001) and absolute Ins concentration (p = 0.031) were significantly increased with age after adolescence. NAA (N-acetylaspartic acid)/tCr (p < 0.001) and absolute NAA concentration (p = 0.010) significantly declined with age after adolescence.

CONCLUSION

Age-related increase of Ins and decline of NAA are found in all three regions, especially at the hippocampus, indicating possible gliosis in the ageing brain.

ADVANCES IN KNOWLEDGE

We could use NAA/tCr and Ins/tCr as an indicator to estimate the neurons-to-glial cells ratio at the thalamus. This may be an index to distinguish normal tissues from gliosis.

Dysplasticity, metaplasticity, and schizophrenia: Implications for risk, illness, and novel interventions

In this paper, we review the history of the concept of neuroplasticity as it relates to the understanding of neuropsychiatric disorders, using schizophrenia as a case in point. We briefly review the myriad meanings of the term neuroplasticity, and its neuroscientific basis. We then review the evidence for aberrant neuroplasticity and metaplasticity associated with schizophrenia as well as the risk for developing this illness, and discuss the implications of such understanding for prevention and therapeutic interventions. We argue that the failure and/or altered timing of plasticity of critical brain circuits might underlie cognitive and deficit symptoms, and may also lead to aberrant plastic reorganization in other circuits, leading to affective dysregulation and eventually psychosis. This "dysplastic" model of schizophrenia can suggest testable etiology and treatment-relevant questions for the future.

In vivo and ex vivo magnetic resonance spectroscopy of the infarct and the subventricular zone in experimental stroke

Ex vivo high-resolution magic-angle spinning (HRMAS) provides metabolic information with higher sensitivity and spectral resolution than in vivo magnetic resonance spectroscopy (MRS). Therefore, we used both techniques to better characterize the metabolic pattern of the infarct and the neural progenitor cells (NPCs) in the ipsilateral subventricular zone (SVZi). Ischemic stroke rats were divided into three groups: G0 (non-stroke controls, n = 6), G1 (day 1 after stroke, n = 6), and G7 (days 6 to 8 after stroke, n = 12). All the rats underwent MRS. Three rats per group were analyzed by HRMAS. The remaining rats were used for immunohistochemical studies. In the infarct, both techniques detected significant metabolic changes. The most relevant change was in mobile lipids (2.80 ppm) in the G7 group (a 5.53- and a 3.95-fold increase by MRS and HRMAS, respectively). In the SVZi, MRS did not detect any significant metabolic change. However, HRMAS detected a 2.70-fold increase in lactate and a 0.68-fold decrease in N-acetylaspartate in the G1 group. None of the metabolites correlated with the 1.37-fold increase in NPCs detected by immunohistochemistry in the G7 group. In conclusion, HRMAS improves the metabolic characterization of the brain in experimental ischemic stroke. However, none of the metabolites qualifies as a surrogate biomarker of NPCs.

Analytical strategies for studying stem cell metabolism

Owing to their capacity for self-renewal and pluripotency, stem cells possess untold potential for revolutionizing the field of regenerative medicine through the development of novel therapeutic strategies for treating cancer, diabetes, cardiovascular and neurodegenerative diseases. Central to developing these strategies is improving our understanding of biological mechanisms responsible for governing stem cell fate and self-renewal. Increasing attention is being given to the significance of metabolism, through the production of energy and generation of small molecules, as a critical regulator of stem cell functioning. Rapid advances in the field of metabolomics now allow for in-depth profiling of stem cells both in vitro and in vivo, providing a systems perspective on key metabolic and molecular pathways which influence stem cell biology. Understanding the analytical platforms and techniques that are currently used to study stem cell metabolomics, as well as how new insights can be derived from this knowledge, will accelerate new research in the field and improve future efforts to expand our understanding of the interplay between metabolism and stem cell biology.

Detrimental role of prolonged sleep deprivation on adult neurogenesis

Adult mammalian brains continuously generate new neurons, a phenomenon called adult neurogenesis. Both environmental stimuli and endogenous factors are important regulators of adult neurogenesis. Sleep has an important role in normal brain physiology and its disturbance causes very stressful conditions, which disrupt normal brain physiology. Recently, an influence of sleep in adult neurogenesis has been established, mainly based on sleep deprivation studies. This review provides an overview on how rhythms and sleep cycles regulate hippocampal and subventricular zone neurogenesis, discussing some potential underlying mechanisms. In addition, our review highlights some interacting points between sleep and adult neurogenesis in brain function, such as learning, memory, and mood states, and provides some insights on the effects of antidepressants and hypnotic drugs on adult neurogenesis.

The current state of the neurogenic theory of depression and anxiety

Newborn neurons are continuously added to the adult hippocampus. Early studies found that adult neurogenesis is impaired in models of depression and anxiety and accelerated by antidepressant treatment. This led to the theory that depression results from impaired adult neurogenesis and restoration of adult neurogenesis leads to recovery. Follow up studies yielded a complex body of often inconsistent results, and the veracity of this theory is uncertain. We propose five criteria for acceptance of this theory, we review the recent evidence for each criterion, and we draw the following conclusions: Diverse animal models of depression and anxiety have impaired neurogenesis. Neurogenesis is consistently boosted by antidepressants in animal models only when animals are stressed. Ablation of neurogenesis in animal models impairs cognitive functions relevant to depression, but only a minority of studies find that ablation causes depression or anxiety. Recent human neuroimaging and postmortem studies are consistent with the neurogenic theory, but they are indirect. Finally, a novel drug developed based on the neurogenic theory is promising in animal models.

In vivo imaging of endogenous neural stem cells in the adult brain

The discovery of endogenous neural stem cells (eNSCs) in the adult mammalian brain with their ability to self-renew and differentiate into functional neurons, astrocytes and oligodendrocytes has raised the hope for novel therapies of neurological diseases. Experimentally, those eNSCs can be mobilized in vivo, enhancing regeneration and accelerating functional recovery after, e.g., focal cerebral ischemia, thus constituting a most promising approach in stem cell research. In order to translate those current experimental approaches into a clinical setting in the future, non-invasive imaging methods are required to monitor eNSC activation in a longitudinal and intra-individual manner. As yet, imaging protocols to assess eNSC mobilization non-invasively in the live brain remain scarce, but considerable progress has been made in this field in recent years. This review summarizes and discusses the current imaging modalities suitable to monitor eNSCs in individual experimental animals over time, including optical imaging, magnetic resonance tomography and-spectroscopy, as well as positron emission tomography (PET). Special emphasis is put on the potential of each imaging method for a possible clinical translation, and on the specificity of the signal obtained. PET-imaging with the radiotracer 3’-deoxy-3’-[¹⁸F]fluoro-L-thymidine in particular constitutes a modality with excellent potential for clinical translation but low specificity; however, concomitant imaging of neuroinflammation is feasible and increases its specificity. The non-invasive imaging strategies presented here allow for the exploitation of novel treatment strategies based upon the regenerative potential of eNSCs, and will help to facilitate a translation into the clinical setting.

Persistent modification of forebrain networks and metabolism in rats following adolescent exposure to a 5-HT7 receptor agonist

RATIONALE

The serotonin 7 receptor (5-HT7-R) is part of a neuro-transmission system with a proposed role in neural plasticity and in mood, cognitive or sleep regulation.

OBJECTIVES

We investigated long-term consequences of sub-chronic treatment, during adolescence (43-45 to 47-49 days old) in rats, with a novel 5-HT7-R agonist (LP-211, 0 or 0.250 mg/kg/day).

METHODS

We evaluated behavioural changes as well as forebrain structural/functional modifications by in vivo magnetic resonance (MR) in a 4.7 T system, followed by ex vivo histology.

RESULTS

Adult rats pre-treated during adolescence showed reduced anxiety-related behaviour, in terms of reduced avoidance in the light/dark test and a less fragmented pattern of exploration in the novel object recognition test. Diffusion tensor imaging (DTI) revealed decreased mean diffusivity (MD) in the amygdala, increased fractional anisotropy (FA) in the hippocampus (Hip) and reduced axial (D||) together with increased radial (D⊥) diffusivity in the nucleus accumbens (NAcc). An increased neural dendritic arborization was confirmed in the NAcc by ex vivo histology. Seed-based functional MR imaging (fMRI) identified increased strength of connectivity within and between "limbic" and "cortical" loops, with affected cross-correlations between amygdala, NAcc and Hip. The latter displayed enhanced connections through the dorsal striatum (dStr) to dorso-lateral prefrontal cortex (dl-PFC) and cerebellum. Functional connection also increased between amygdala and limbic elements such as NAcc, orbito-frontal cortex (OFC) and hypothalamus. MR spectroscopy (1H-MRS) indicated that adolescent LP-211 exposure increased glutamate and total creatine in the adult Hip.

CONCLUSIONS

Persistent MR-detectable modifications indicate a rearrangement within forebrain networks, accounting for long-lasting behavioural changes as a function of developmental 5-HT7-R stimulation.

Role of adult hippocampal neurogenesis in cognition in physiology and disease: pharmacological targets and biomarkers

Adult hippocampal neurogenesis is a remarkable form of brain structural plasticity by which new functional neurons are generated from adult neural stem cells/precursors. Although the precise role of this process remains elusive, adult hippocampal neurogenesis is important for learning and memory and it is affected in disease conditions associated with cognitive impairment, depression, and anxiety. Immature neurons in the adult brain exhibit an enhanced structural and synaptic plasticity during their maturation representing a unique population of neurons to mediate specific hippocampal function. Compelling preclinical evidence suggests that hippocampal neurogenesis is modulated by a broad range of physiological stimuli which are relevant in cognitive and emotional states. Moreover, multiple pharmacological interventions targeting cognition modulate adult hippocampal neurogenesis. In addition, recent genetic approaches have shown that promoting neurogenesis can positively modulate cognition associated with both physiology and disease. Thus the discovery of signaling pathways that enhance adult neurogenesis may lead to therapeutic strategies for improving memory loss due to aging or disease. This chapter endeavors to review the literature in the field, with particular focus on (1) the role of hippocampal neurogenesis in cognition in physiology and disease; (2) extrinsic and intrinsic signals that modulate hippocampal neurogenesis with a focus on pharmacological targets; and (3) efforts toward novel strategies pharmacologically targeting neurogenesis and identification of biomarkers of human neurogenesis.

The hippocampus and exercise: histological correlates of MR-detected volume changes

Growing evidence indicates that physical exercise increases hippocampal volume. This has consistently been shown in mice and men using magnetic resonance imaging. On the other hand, histological studies have reported profound alterations on a cellular level including increased adult hippocampal neurogenesis after exercise. A combined investigation of both phenomena has not been documented so far although a causal role of adult neurogenesis for increased hippocampal volume has been suggested before. We investigated 20 voluntary wheel running and 20 sedentary mice after a period of 2 month voluntary wheel running. Half of each group received focalized hippocampal irradiation to inhibit neurogenesis prior to wheel running. Structural MRI and histological investigations concerning newborn neurons (DCX), glial cells (GFAP), microglia, proliferating and pyknotic cells, neuronal activation, as well as blood vessel density and arborisation were performed. In a regression model, neurogenesis was the marker best explaining hippocampal gray matter volume. Individual analyses showed a positive correlation of gray matter volume with DCX-positive newborn neurons in the subgroups, too. GFAP-positive cells significantly interacted with gray matter volume with a positive correlation in sham-irradiated mice and no correlation in irradiated mice. Although neurogenesis appears to be an important marker of higher hippocampal gray matter volume, a monocausal relationship was not indicated, requesting further investigations.

Mental and Physical (MAP) Training: a neurogenesis-inspired intervention that enhances health in humans

New neurons are generated in the hippocampus each day and their survival is greatly enhanced through effortful learning (Shors, 2014). The numbers of cells produced can be increased by physical exercise (van Praag, Kempermann, & Gage, 1999). These findings inspired us to develop a clinical intervention for humans known as Mental and Physical Training, or MAP Training. Each session consists of 30min of mental training with focused attention meditation (20min sitting and 10min walking). Meditation is an effortful training practice that involves learning about the transient nature of thoughts and thought patterns, and acquiring skills to recognize them without necessarily attaching meaning and/or emotions to them. The mental training component is followed by physical training with 30min of aerobic exercise performed at moderate intensity. During this component, participants learn choreographed dance routines while engaging in aerobic exercise. In a pilot "proof-of-concept" study, we provided supervised MAP Training (2 sessions per week for 8weeks) to a group of young mothers in the local community who were recently homeless, most of them having previously suffered from physical and sexual abuse, addiction, and depression. Preliminary data suggest that MAP Training improves dependent measures of aerobic fitness (as assessed by maximal rate of oxygen consumed) while decreasing symptoms of depression and anxiety. Similar changes were not observed in a group of recently homeless women who did not participate in MAP Training. It is not currently possible to determine whether new neurons in the human brain increase in number as a result of MAP Training. Rather these preliminary results of MAP Training illustrate how neuroscientific research can be translated into novel clinical interventions that benefit human health and wellness.

A ventral view on antidepressant action: roles for adult hippocampal neurogenesis along the dorsoventral axis

Adult hippocampal neurogenesis is implicated in antidepressant action, stress responses, and cognitive functioning. The hippocampus is functionally segregated along its longitudinal axis into dorsal (dHi) and ventral (vHi) regions in rodents, and analogous posterior and anterior regions in primates, whereby the vHi preferentially regulates stress and anxiety, while the dHi preferentially regulates spatial learning and memory. Given the role of neurogenesis in functions preferentially regulated by the dHi or vHi, it is plausible that neurogenesis is preferentially regulated in either the dHi or vHi depending upon the stimulus. We appraise here the literature on the effects of stress and antidepressants on neurogenesis along the hippocampal longitudinal axis and explore whether preferential regulation of neurogenesis in the vHi/anterior hippocampus contributes to stress resilience and antidepressant action.

The Adult Brain Makes New Neurons, and Effortful Learning Keeps Them Alive

The brain continues to produce new neurons throughout life. For instance, the hippocampus (a brain region necessary for select learning processes) produces thousands of new neurons each day. However, a significant number of them die and do so within just a few weeks of their birth. Laboratory animals that are trained to learn a new skill between one and two weeks after the new cells are generated retain most cells that would have otherwise died. The types of skills that keep new cells alive are not limited to those that depend on the hippocampus but rather include those that are effortful to learn, requiring more training trials or time spent training. Importantly, training alone is not sufficient to increase cell survival; animals that are trained but do not learn do not retain more cells than animals that are not trained. Therefore, learning increases the survival of newly generated cells in the hippocampus as long as the learning experience is new, effortful, and successful. Once rescued, the vast majority of these cells differentiate into neurons, thereby forming synapses and generating action potentials as they become incorporated into the existing architecture and functional circuitry of the adult brain.