Different Mechanisms Must Be Considered to Explain the Increase in Hippocampal Neural Precursor Cell Proliferation by Physical Activity

Mice Mutated in the First Fibronectin Domain of Adhesion Molecule L1 Show Brain Malformations and Behavioral Abnormalities

The X-chromosome-linked cell adhesion molecule L1 (L1CAM), a glycoprotein mainly expressed by neurons in the central and peripheral nervous systems, has been implicated in many neural processes, including neuronal migration and survival, neuritogenesis, synapse formation, synaptic plasticity and regeneration. L1 consists of extracellular, transmembrane and cytoplasmic domains. Proteolytic cleavage of L1's extracellular and transmembrane domains by different proteases generates several L1 fragments with different functions. We found that myelin basic protein (MBP) cleaves L1's extracellular domain, leading to enhanced neuritogenesis and neuronal survival in vitro. To investigate in vivo the importance of the MBP-generated 70 kDa fragment (L1-70), we generated mice with an arginine to alanine substitution at position 687 (L1/687), thereby disrupting L1's MBP cleavage site and obliterating L1-70. Young adult L1/687 males showed normal anxiety and circadian rhythm activities but enhanced locomotion, while females showed altered social interactions. Older L1/687 males were impaired in motor coordination. Furthermore, L1/687 male and female mice had a larger hippocampus, with more neurons in the dentate gyrus and more proliferating cells in the subgranular layer, while the thickness of the corpus callosum and the size of lateral ventricles were normal. In summary, subtle mutant morphological changes result in subtle behavioral changes.

[Movement as a Neuromodulator: How Physical Activity Influences the Physiology of Adolescent Depression]

Movement as a Neuromodulator: How Physical Activity Influences the Physiology of Adolescent Depression Abstract: In the context of adolescent depression, physical activity is becoming increasingly recognized for its positive effects on neuropathology. Current scientific findings indicate that physical training affects the biological effects of depression during adolescence. Yet the pathophysiology of adolescent depression is not yet fully understood. Besides psychosocial and genetic influences, various neurobiological factors are being discussed. One explanation model describes a dysfunction of the hypothalamus-pituitary-adrenal axis (HPA axis) with a sustained elevation in cortisol concentration. Recent studies highlight neuroimmunological processes and a reduced concentration of growth factors as causative factors. These changes appear to lead to a dysregulation of the excitation and inhibition balance of the cerebral cortex as well as to cerebral morphological alterations. Regular physical training can potentially counteract the dysregulation of the HPA axis and normalize cortisol levels. The release of proinflammatory cytokines is inhibited, and the expression of growth factors involved in adult neurogenesis is stimulated. One should ensure the synergistic interaction of biological and psychosocial factors when designing the exercise schedule (endurance or strength training, group or individual sports, frequency, duration, and intensity). Addressing these open questions is essential when integrating physical activity into the guidelines for treating depressive disorders in children and adolescents.

Memantine increases the dendritic complexity of hippocampal young neurons in the juvenile brain after cranial irradiation

Introduction

Cranial irradiation (IR) negatively regulates hippocampal neurogenesis and causes cognitive dysfunctions in cancer survivors, especially in pediatric patients. IR decreases proliferation of neural stem/progenitor cells (NSPC) and consequently diminishes production of new hippocampal neurons. Memantine, an NMDA receptor antagonist, used clinically to improve cognition in patients suffering from Alzheimer's disease and dementia. In animal models, memantine acts as a potent enhancer of hippocampal neurogenesis. Memantine was recently proposed as an intervention to improve cognitive impairments occurring after radiotherapy and is currently under investigation in a number of clinical trials, including pediatric patients. To date, preclinical studies investigating the mechanisms underpinning how memantine improves cognition after IR remain limited, especially in the young, developing brain. Here, we investigated whether memantine could restore proliferation in the subgranular zone (SGZ) or rescue the reduction in the number of hippocampal young neurons after IR in the juvenile mouse brain.

Methods

Mice were whole-brain irradiated with 6 Gy on postnatal day 20 (P20) and subjected to acute or long-term treatment with memantine. Proliferation in the SGZ and the number of young neurons were further evaluated after the treatment. We also measured the levels of neurotrophins associated with memantine improved neural plasticity, brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF).

Results

We show that acute intraperitoneal treatment with a high, non-clinically used, dose of memantine (50 mg/kg) increased the number of proliferating cells in the intact brain by 72% and prevented 23% of IR-induced decrease in proliferation. Long-term treatment with 10 mg/kg/day of memantine, equivalent to the clinically used dose, did not impact proliferation, neither in the intact brain, nor after IR, but significantly increased the number of young neurons (doublecortin expressing cells) with radial dendrites (29% in sham controls and 156% after IR) and enhanced their dendritic arborization. Finally, we found that long-term treatment with 10 mg/kg/day memantine did not affect the levels of BDNF, but significantly reduced the levels of NGF by 40%.

Conclusion

These data suggest that the enhanced dendritic complexity of the hippocampal young neurons after treatment with memantine may contribute to the observed improved cognition in patients treated with cranial radiotherapy.

Neuronal differentiation and inhibition of glial differentiation of murine neural stem cells by pHPMA hydrogel for the repair of injured spinal cord

Currently, several therapeutic methods of treating the effects of spinal cord injury (SCI) are being considered. On the one hand, transplantation of stem cells (SCs), in particular, neural stem/progenitor cells (NSPCs), is promising, as these cells have the potential to differentiate into nervous tissue cells, able to enhance endogenous regeneration and prevent the development of inflammatory processes. On the other hand, it is quite promising to replace the damaged nervous tissue with synthetic matrices, in particular hydrogels, which can create artificial conditions for the regenerative growth of injured nerve fibers through the spinal cord injury area, i.e. stimulate and support axonal regeneration and myelination. In this work, we combined both of these novel approaches by populating (injecting or rehydrating) a heteroporous pHPMA hydrogel (NeuroGel) with murine hippocampal NSPCs. Being inside the hydrogel (10 days of cultivation), NSPCs were more differentiated into neurons: 19.48% ± 1.71% (the NSPCs injection into the hydrogel) and 36.49% ± 4.20% (the hydrogel rehydration in the NSPCs suspension); in control cultures, the level of differentiation in neurons was only 2.40% ± 0.31%. Differentiation of NSPCs into glial cells, in particular into oligodendrocyte progenitor cells, was also observed - 8.89% ± 2.15% and 6.21% ± 0.80% for injection and rehydration variants, respectively; in control - 28.75% ± 2.08%. In the control NSPCs culture, there was a small number of astrocytes - 2.11% ± 0.43%. Inside the hydrogel, NSPCs differentiation in astrocytes was not observed. In vitro data showed that the hydrogel promotes the differentiation of NSPCs into neurons, and inhibits the differentiation into glial cells. And in vivo showed post-traumatic recovery of rat spinal cord tissue after injury followed by implantation of the hydrogel+NSPCs complex (approximately 7 months after SCI). The implant area was closely connected with the recipient tissue, and the recipient cells freely grew into the implant itself. Inside the implant, a formed dense neuronal network was visible. In summary, the results are primarily an experimental ground for further studies of implants based on pHPMA hydrogel with populated different origin SCs, and the data also indicate the feasibility and efficiency of using an integrated approach to reduce possible negative side effects and facilitate the rehabilitation process after a SCI.

Neurotrophic effects of intermittent fasting, calorie restriction and exercise: a review and annotated bibliography

In the last decades, important progress has been achieved in the understanding of the neurotrophic effects of intermittent fasting (IF), calorie restriction (CR) and exercise. Improved neuroprotection, synaptic plasticity and adult neurogenesis (NSPAN) are essential examples of these neurotrophic effects. The importance in this respect of the metabolic switch from glucose to ketone bodies as cellular fuel has been highlighted. More recently, calorie restriction mimetics (CRMs; resveratrol and other polyphenols in particular) have been investigated thoroughly in relation to NSPAN. In the narrative review sections of this manuscript, recent findings on these essential functions are synthesized and the most important molecules involved are presented. The most researched signaling pathways (PI3K, Akt, mTOR, AMPK, GSK3β, ULK, MAPK, PGC-1α, NF-κB, sirtuins, Notch, Sonic hedgehog and Wnt) and processes (e.g., anti-inflammation, autophagy, apoptosis) that support or thwart neuroprotection, synaptic plasticity and neurogenesis are then briefly presented. This provides an accessible entry point to the literature. In the annotated bibliography section of this contribution, brief summaries are provided of about 30 literature reviews relating to the neurotrophic effects of interest in relation to IF, CR, CRMs and exercise. Most of the selected reviews address these essential functions from the perspective of healthier aging (sometimes discussing epigenetic factors) and the reduction of the risk for neurodegenerative diseases (Alzheimer's disease, Huntington's disease, Parkinson's disease) and depression or the improvement of cognitive function.

Mechanisms underlying the effect of voluntary running on adult hippocampal neurogenesis

Adult hippocampal neurogenesis is important for preserving learning and memory-related cognitive functions. Physical exercise, especially voluntary running, is one of the strongest stimuli to promote neurogenesis and has beneficial effects on cognitive functions. Voluntary running promotes exit of neural stem cells (NSCs) from the quiescent stage, proliferation of NSCs and progenitors, survival of newborn cells, morphological development of immature neuron, and integration of new neurons into the hippocampal circuitry. However, the detailed mechanisms driving these changes remain unclear. In this review, we will summarize current knowledge with respect to molecular mechanisms underlying voluntary running-induced neurogenesis, highlighting recent genome-wide gene expression analyses. In addition, we will discuss new approaches and future directions for dissecting the complex cellular mechanisms driving change in adult-born new neurons in response to physical exercise.

Computational analysis of crosstalk between transcriptional regulators and RNA-binding proteins suggests mutual regulation of polycomb proteins and SRSF1 influencing adult hippocampal neurogenesis

BACKGROUND

Adult hippocampal neurogenesis (AHN) is a clinically significant neural phenomenon. Understanding its molecular regulation would be important. In this regard, most studies have focused on transcriptional regulators (TRs), epigenetic modifiers, or non-coding RNAs. RNA-binding proteins (RBPs) have emerged as dominant molecular regulators. It would be significant to understand the potential cross-talk between RBPs and TRs, which could influence AHN.

METHODS

The present study employed computational analyses to identify RBPs and TRs regulating AHN, followed by the analysis of their interaction networks and detection of hub proteins. Next, the potential mutual regulation of hub TRs and RBPs was analyzed. Additionally, hippocampal genes differentially expressed upon exercise were analyzed for potential regulation by the identified TRs and RBPs.

RESULTS

105 TRs and 26 RBPs were found to influence AHN, which could also form interactive networks. Polycomb complex proteins were among the TR network hubs, while HNRNP and SRSF family members were among the hub RBPs. Further, the polycomb complex proteins and SRSF1 could have a mutual regulatory relationship, suggesting a cross-talk between epigenetic/transcriptional and post-transcriptional regulatory pathways. A number of exercise-induced hippocampal genes were also found to be potential targets of the identified TRs and RBPs.

CONCLUSION

SRSF1 may influence post-transcriptional stability, localization, and alternative splicing patterns of polycomb complex transcripts, and the polycomb proteins may in turn epigenetically influence the SRSF1. Further experimental validation of these regulatory loops/networks could provide novel insights into the molecular regulation of AHN, and unravel new targets for disease-treatment.

Global Exploration of RNA-Binding Proteins in Exercise-Induced Adult Hippocampal Neurogenesis: A Transcriptome Meta-analysis and Computational Study

Voluntary physical exercise is a robust enhancer of adult hippocampal neurogenesis (AHN). A complete understanding of the molecular regulation of AHN is important in order to exploit the benefits of the process toward therapeutic approaches. Several factors such as epigenetic modifiers, non-coding RNAs, and transcription factors have been reported to regulate AHN. However, there is a limited understanding of the impact of RNA-binding proteins (RBPs) on exercise-mediated AHN, in spite of their well-documented significance in embryonic neurogenesis. The present study is the first global analysis to catalog the potential RBPs influencing exercise-mediated AHN. Here, a transcriptome meta-analysis was conducted to study exercise-mediated gene expression modulation in hippocampi of adult mice. Next, potential RBPs influencing transcriptome-wide expression changes via untranslated regions (UTRs) were identified. Among other RBPs, MATR3, Musashi, TIA1, and FXR2 (known critical modulators of neurogenesis) were found to potentially regulate gene expression patterns. Subsequently, binding sites of known neurogenesis-regulating RBPs were identified in the UTRs of AHN-associated genes modulated by exercise. Finally, a number of RBPs including RBFOX1, RBFOX3, and QKI (known regulators of neurogenesis) were found to be highly expressed in mouse hippocampal formation and also potentially interact with other RBPs, suggesting their combinatorial functioning in exercise-induced AHN. Thus, the present meta-analysis-based computational study identified several RBPs potentially important in exercise-induced AHN, which could form a foundation for further experiments to unravel RBP-mediated regulation of AHN.

Could a Different View of Quiescence Help Us Understand How Neurogenesis Is Regulated?

The majority of adult neural stem cells (aNSCs) are in a distinct metabolic state of reversible cell cycle exit also known as quiescence. The rate of aNSC activation determines the number of new neurons generated and directly influences the long-term maintenance of neurogenesis. Despite its relevance, it is still unclear how aNSC quiescence is regulated. Many factors contribute to this, like aNSC heterogeneity, the lack of reliable quiescence markers, the complexity of the neurogenic niches or the intricacy of the transcriptional and post-transcriptional mechanisms involved. In this perspective article I discuss possible solutions to these problems. But, first and foremost, I believe we require a model that goes beyond a simple transition toward activation. Instead, we must acknowledge the full complexity of aNSC states, which include not only activation but also differentiation and survival as behavioural outcomes. I propose a model where aNSCs dynamically transition through a cloud of highly interlinked cellular states driven by intrinsic and extrinsic cues. I also show how a new perspective enables us to integrate current results into a coherent framework leading to the formulation of new testable hypothesis. This model, like all others, is still far from perfect and will be reshaped by future findings. I believe that having a more complete view of aNSC transitions and embracing their complexity will bring us closer to understand how aNSC activity and neurogenesis are controlled throughout life.

Selenium mediates exercise-induced adult neurogenesis and reverses learning deficits induced by hippocampal injury and aging

Although the neurogenesis-enhancing effects of exercise have been extensively studied, the molecular mechanisms underlying this response remain unclear. Here, we propose that this is mediated by the exercise-induced systemic release of the antioxidant selenium transport protein, selenoprotein P (SEPP1). Using knockout mouse models, we confirmed that SEPP1 and its receptor low-density lipoprotein receptor-related protein 8 (LRP8) are required for the exercise-induced increase in adult hippocampal neurogenesis. In vivo selenium infusion increased hippocampal neural precursor cell (NPC) proliferation and adult neurogenesis. Mimicking the effect of exercise through dietary selenium supplementation restored neurogenesis and reversed the cognitive decline associated with aging and hippocampal injury, suggesting potential therapeutic relevance. These results provide a molecular mechanism linking exercise-induced changes in the systemic environment to the activation of quiescent hippocampal NPCs and their subsequent recruitment into the neurogenic trajectory.

ROS Dynamics Delineate Functional States of Hippocampal Neural Stem Cells and Link to Their Activity-Dependent Exit from Quiescence

Cellular redox states regulate the balance between stem cell maintenance and activation. Increased levels of intracellular reactive oxygen species (ROS) are linked to proliferation and lineage specification. In contrast to this general principle, we here show that in the hippocampus of adult mice, quiescent neural precursor cells (NPCs) maintain the highest ROS levels (hiROS). Classifying NPCs on the basis of cellular ROS content identified distinct functional states. Shifts in ROS content primed cells for a subsequent state transition, with lower ROS content marking proliferative activity and differentiation. Physical activity, a physiological activator of adult hippocampal neurogenesis, recruited hiROS NPCs into proliferation via a transient Nox2-dependent ROS surge. In the absence of Nox2, baseline neurogenesis was unaffected, but the activity-induced increase in proliferation disappeared. These results provide a metabolic classification of NPC functional states and describe a mechanism linking the modulation of cellular ROS by behavioral cues to the activation of adult NPCs.

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A ROS gradient delineates cell types in the course of adult hippocampal neurogenesis

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Quiescent hippocampal stem cells have unusually high intracellular ROS

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Physical activity recruits quiescent stem cells in a ROS-dependent manner

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NOX2 dependency distinguishes this recruitment from baseline proliferation

Neural Stem Cells and Cannabinoids in the Spotlight as Potential Therapy for Epilepsy

Epilepsy is one of the most common brain diseases worldwide, having a huge burden in society. The main hallmark of epilepsy is the occurrence of spontaneous recurrent seizures, having a tremendous impact on the lives of the patients and of their relatives. Currently, the therapeutic strategies are mostly based on the use of antiepileptic drugs, and because several types of epilepsies are of unknown origin, a high percentage of patients are resistant to the available pharmacotherapy, continuing to experience seizures overtime. Therefore, the search for new drugs and therapeutic targets is highly important. One key aspect to be targeted is the aberrant adult hippocampal neurogenesis (AHN) derived from Neural Stem Cells (NSCs). Indeed, targeting seizure-induced AHN may reduce recurrent seizures and shed some light on the mechanisms of disease. The endocannabinoid system is a known modulator of AHN, and due to the known endogenous antiepileptic properties, it is an interesting candidate for the generation of new antiepileptic drugs. However, further studies and clinical trials are required to investigate the putative mechanisms by which cannabinoids can be used to treat epilepsy. In this manuscript, we will review how cannabinoid-induced modulation of NSCs may promote neural plasticity and whether these drugs can be used as putative antiepileptic treatment.

Quantitative characterization of proliferative cells subpopulations in the hilus of the hippocampus of adult Wistar rats: an integrative study

The hilus plays an important role modulating the excitability of the hippocampal dentate gyrus (DG). It also harbors proliferative cells whose proliferation rate is modified during pathological events. However, the characterization of these cells, in terms of cellular identity, lineage, and fate, as well as the morphology and proportion of each cell subpopulation has been poorly studied. Therefore, a deeper investigation of hilar proliferative cells might expand the knowledge not only in the physiology, but in the pathophysiological processes related to the hippocampus too. The aim of this work was to perform an integrative study characterizing the identity of proliferative cells populations harbored in the hilus, along with morphology and proportion. In addition, this study provides comparative evidence of the subgranular zone (SGZ) of the DG. Quantified cells included proliferative, neural precursor, Type 1, oligodendrocyte progenitor (OPCs), neural progenitor (NPCs), and proliferative mature astrocytes in the hilus and SGZ of Wistar adult rats. Our results showed that 84% of the hilar proliferative cells correspond to neural precursor cells, OPCs and NPCs being the most abundant at 54 and 45%, respectively, unlike the SGZ, where OPCs represent only 11%. Proliferative mature astrocytes and Type 1-like cells were rarely observed in the hilus. Together, our results lay the basis for future studies focused on the lineage and fate of hilar proliferative cells and suggest that the hilus could be relevant to the formation of new cells that modulate multiple physiological processes governed by the hippocampus.

Environmental enrichment ameliorates perinatal brain injury and promotes functional white matter recovery

Hypoxic damage to the developing brain due to preterm birth causes many anatomical changes, including damage to the periventricular white matter. This results in the loss of glial cells, significant disruptions in myelination, and thereby cognitive and behavioral disabilities seen throughout life. Encouragingly, these neurological morbidities can be improved by environmental factors; however, the underlying cellular mechanisms remain unknown. We found that early and continuous environmental enrichment selectively enhances endogenous repair of the developing white matter by promoting oligodendroglial maturation, myelination, and functional recovery after perinatal brain injury. These effects require increased exposure to socialization, physical activity, and cognitive enhancement of surroundings-a complete enriched environment. Using RNA-sequencing, we identified oligodendroglial-specific responses to hypoxic brain injury, and uncovered molecular mechanisms involved in enrichment-induced recovery. Together, these results indicate that myelin plasticity induced by modulation of the neonatal environment can be targeted as a therapeutic strategy for preterm birth.

Impacts of exercise interventions on different diseases and organ functions in mice

Background

In recent years, much evidence has emerged to indicate that exercise can benefit people when performed properly. This review summarizes the exercise interventions used in studies involving mice as they are related to special diseases or physiological status. To further understand the effects of exercise interventions in treating or preventing diseases, it is important to establish a template for exercise interventions that can be used in future exercise-related studies.

Methods

PubMed was used as the data resource for articles. To identify studies related to the effectiveness of exercise interventions for treating various diseases and organ functions in mice, we used the following search language: (exercise [Title] OR training [Title] OR physical activity [Title]) AND (mice [title/abstract] OR mouse [title/abstract] OR mus [title/abstract]). To limit the range of search results, we included 2 filters: one that limited publication dates to "in 10 years" and one that sorted the results as "best match". Then we grouped the commonly used exercise methods according to their similarities and differences. We then evaluated the effectiveness of the exercise interventions for their impact on diseases and organ functions in 8 different systems.

Results

A total of 331 articles were included in the analysis procedure. The articles were then segmented into 8 systems for which the exercise interventions were used in targeting and treating disorders: motor system (60 studies), metabolic system (45 studies), cardio-cerebral vascular system (58 studies), nervous system (74 studies), immune system (32 studies), respiratory system (7 studies), digestive system (1 study), and the system related to the development of cancer (54 studies). The methods of exercise interventions mainly involved the use of treadmills, voluntary wheel-running, forced wheel-running, swimming, and resistance training. It was found that regardless of the specific exercise method used, most of them demonstrated positive effects on various systemic diseases and organ functions. Most diseases were remitted with exercise regardless of the exercise method used, although some diseases showed the best remission effects when a specific method was used.

Conclusion

Our review strongly suggests that exercise intervention is a cornerstone in disease prevention and treatment in mice. Because exercise interventions in humans typically focus on chronic diseases, national fitness, and body weight loss, and typically have low intervention compliance rates, it is important to use mice models to investigate the molecular mechanisms underlying the health benefits from exercise interventions in humans.

Indices of dentate gyrus neurogenesis are unaffected immediately after or following withdrawal from morphine self-administration compared to saline self-administering control male rats

Opiates - including morphine - are powerful analgesics with high abuse potential. In rodents, chronic opiate exposure or self-administration negatively impacts hippocampal-dependent function, an effect perhaps due in part to the well-documented opiate-induced inhibition of dentate gyrus (DG) precursor proliferation and neurogenesis. Recently, however, intravenous (i.v.) morphine self-administration (MSA) was reported to enhance the survival of new rat DG neurons. To reconcile these disparate results, we used rat i.v. MSA to assess 1) whether a slightly-higher dose MSA paradigm also increases new DG neuron survival; 2) how MSA influences cells in different stages of DG neurogenesis, particularly maturation and survival; and 3) if MSA-induced changes in DG neurogenesis persist through a period of abstinence. To label basal levels of proliferation, rats received the S-phase marker bromodeoxyuridine (BrdU, i.p.) 24 -h prior to 21 days (D) of i.v. MSA or saline self-administration (SSA). Either immediately after SA (0-D) or after 4 weeks in the home cage (28-D withdrawal), stereology was used to quantify DG proliferating precursors (or cells in cell cycle; Ki67+ cells), neuroblast/immature neurons (DCX+ cells), and surviving DG granule cells (BrdU+ cells). Analysis revealed the number of DG cells immunopositive for these neurogenesis-relevant markers was similar between MSA and SSA rats at the 0-D or 28-D timepoints. These negative data highlight the impact experimental parameters, timepoint selection, and quantification approach have on neurogenesis results, and are discussed in the context of the large literature showing the negative impact of opiates on DG neurogenesis.

Newborn Neurons in the Aged Hippocampus-Scarce and Slow but Highly Plastic

In this issue of Cell Reports, Trinchero et al. (2017) demonstrate that newborn neurons in the aged hippocampus are delayed in development but are highly susceptible to stimuli improving neuronal activity. This plasticity is mediated cell-intrinsically by neurotrophin signaling.

MiR-135a-5p Is Critical for Exercise-Induced Adult Neurogenesis

Physical exercise stimulates adult hippocampal neurogenesis and is considered a relevant strategy for preventing age-related cognitive decline in humans. The underlying mechanisms remains controversial. Here, we show that exercise increases proliferation of neural precursor cells (NPCs) of the mouse dentate gyrus (DG) via downregulation of microRNA 135a-5p (miR-135a). MiR-135a inhibition stimulates NPC proliferation leading to increased neurogenesis, but not astrogliogenesis, in DG of resting mice, and intriguingly it re-activates NPC proliferation in aged mice. We identify 17 proteins (11 putative targets) modulated by miR-135 in NPCs. Of note, inositol 1,4,5-trisphosphate (IP3) receptor 1 and inositol polyphosphate-4-phosphatase type I are among the modulated proteins, suggesting that IP3 signaling may act downstream miR-135. miR-135 is the first noncoding RNA essential modulator of the brain's response to physical exercise. Prospectively, the miR-135-IP3 axis might represent a novel target of therapeutic intervention to prevent pathological brain aging.

Exercise-Induced Activated Platelets Increase Adult Hippocampal Precursor Proliferation and Promote Neuronal Differentiation

Physical activity is a strong positive physiological modulator of adult neurogenesis in the hippocampal dentate gyrus. Although the underlying regulatory mechanisms are still unknown, systemic processes must be involved. Here we show that platelets are activated after acute periods of running, and that activated platelets promote neurogenesis, an effect that is likely mediated by platelet factor 4. Ex vivo, the beneficial effects of activated platelets and platelet factor 4 on neural precursor cells were dentate gyrus specific and not observed in the subventricular zone. Moreover, the depletion of circulating platelets in mice abolished the running-induced increase in precursor cell proliferation in the dentate gyrus following exercise. These findings demonstrate that platelets and their released factors can modulate adult neural precursor cells under physiological conditions and provide an intriguing link between running-induced platelet activation and the modulation of neurogenesis after exercise.

Depression and adult neurogenesis: Positive effects of the antidepressant fluoxetine and of physical exercise

Of wide interest for health is the relation existing between depression, a very common psychological illness, accompanied by anxiety and reduced ability to concentrate, and adult neurogenesis. We will focus on two neurogenic stimuli, fluoxetine and physical exercise, both endowed with the ability to activate adult neurogenesis in the dentate gyrus of the hippocampus, known to be required for learning and memory, and both able to counteract depression. Fluoxetine belongs to the class of selective serotonin reuptake inhibitor (SSRI) antidepressants, which represent the most used pharmacological therapy; physical exercise has also been shown to effectively counteract depression symptoms in rodents as well as in humans. While there is evidence that the antidepressant effect of fluoxetine requires its pro-neurogenic action, exerted by promoting proliferation, differentiation and survival of progenitor cells of the hippocampus, on the other hand fluoxetine exerts also neurogenesis-independent antidepressant effects by influencing the plasticity of the new neurons generated. Similarly, the antidepressant action of running also correlates with an increase of hippocampal neurogenesis and plasticity, although the gene pathways involved are only partially coincident with those of fluoxetine, such as those involved in serotonin metabolism and synapse formation. We further discuss how extra-neurogenic actions are also suggested by the fact that, unlike running, fluoxetine is unable to stimulate neurogenesis during aging, but still displays antidepressant effects. Moreover, in specific conditions, fluoxetine or running activate not only progenitor but also stem cells, which normally are not stimulated; this fact reveals how stem cells have a long-term, hidden ability to self-renew and, more generally, that neurogenesis is subject to complex controls that may play a role in depression, such as the type of neurogenic stimulus or the state of the local niche. Finally, we discuss how fluoxetine or running are effective in counteracting depression originated from stress or neurodegenerative diseases.

Dexras1 is a homeostatic regulator of exercise-dependent proliferation and cell survival in the hippocampal neurogenic niche

Adult hippocampal neurogenesis is highly responsive to exercise, which promotes the proliferation of neural progenitor cells and the integration of newborn granule neurons in the dentate gyrus. Here we show that genetic ablation of the small GTPase, Dexras1, suppresses exercise-induced proliferation of neural progenitors, alters survival of mitotic and post-mitotic cells in a stage-specific manner, and increases the number of mature newborn granule neurons. Dexras1 is required for exercise-triggered recruitment of quiescent neural progenitors into the cell cycle. Pharmacological inhibition of NMDA receptors enhances SGZ cell proliferation in wild-type but not dexras1-deficient mice, suggesting that NMDA receptor-mediated signaling is dependent on Dexras1. At the molecular level, the absence of Dexras1 abolishes exercise-dependent activation of ERK/MAPK and CREB, and inhibits the upregulation of NMDA receptor subunit NR2A, bdnf, trkB and vegf-a expression in the dentate gyrus. Our study reveals Dexras1 as an important stage-specific regulator of exercise-induced neurogenesis in the adult hippocampus by enhancing pro-mitogenic signaling to neural progenitor cells and modulating cell survival.

Movement maintains forebrain neurogenesis via peripheral neural feedback in larval zebrafish

The postembryonic brain exhibits experience-dependent development, in which sensory experience guides normal brain growth. This neuroplasticity is thought to occur primarily through structural and functional changes in pre-existing neurons. Whether neurogenesis also mediates the effects of experience on brain growth is unclear. Here, we characterized the importance of motor experience on postembryonic neurogenesis in larval zebrafish. We found that movement maintains an expanded pool of forebrain neural precursors by promoting progenitor self-renewal over the production of neurons. Physical cues associated with swimming (bodily movement) increase neurogenesis and these cues appear to be conveyed by dorsal root ganglia (DRG) in the zebrafish body: DRG-deficient larvae exhibit attenuated neurogenic responses to movement and targeted photoactivation of DRG in immobilized larvae expands the pallial pool of proliferative cells. Our results demonstrate the importance of movement in neurogenic brain growth and reveal a fundamental sensorimotor association that may couple early motor and brain development.

Sensory experiences early in life help the brain to grow and develop. For example, raising animals in complete darkness stops the visual areas of their brain from forming properly. Previous studies have shown that sensory input helps to strengthen the connections between already existing brain cells, but it is unclear if it affects the actual creation of new brain cells.

Conditions that reduce the mobility of young children, such as muscular disease, are often accompanied by learning difficulties. This suggests that physical movement may be important for healthy brain development. Scientists have previously found a link between exercise and an increased production of new brain cells in adults. However, such a link has not been established earlier in life, when the brain is developing the most.

To address this, Hall and Tropepe studied how movement affects the brain development in zebrafish larvae, at an age when many of their brain cells are created. Restraining the larvae decreased their physical movement, while making them swim against a current increased their movement. Hall et al. looked at how this affected the larvae’s number of so called progenitor cells – the cells from which brain cells are created. When the larvae moved less, the number of progenitor cells decreased. But when they moved more frequently, the amount of progenitor cells increased.

The experiments also showed that some sensory cells, which detect sensations associated with movement of the body during swimming, are linked to brain cell production. Blocking the development of these sensory cells prevented the rise in progenitor cells seen with increased movement in the larvae. However, activating these sensory cells in immobilised larvae increased the number of progenitor cells, even though the larvae could not move. These findings suggest that measures to increase physical movement in young children could be used to help the brain develop normally.

Identifying molecular mediators of environmentally enhanced neurogenesis

Adult hippocampal neurogenesis occurs throughout life and supports healthy brain functions. The production of new neurons decreases with age, and deficiencies in adult neurogenesis are associated with neurodevelopmental and degenerative disease. The rate of neurogenesis is dynamically sensitive to an individual's environmental conditions and experiences, and certain stimuli are known robustly to enhance neurogenesis in rodent models, including voluntary exercise, enriched environment, and electroconvulsive shock. In these models, information about an organism's environment and physiological state are relayed to neurogenic cell types within the hippocampus through a series of tissue and cellular interfaces, ultimately eliciting a neurogenic response from neural stem cells and newborn neurons. Therefore, an understanding of the way that novel genes and proteins act in specific cell types within this circuit-level context is of scientific and therapeutic value. Several well-studied neurotrophic factors have been implicated in environmentally enhanced neurogenesis. This review highlights recently discovered, novel molecular mediators of neurogenesis in response to environmental cues and summarizes the contribution of advanced, large-scale gene expression and function assessment technology to past, present, and future efforts aimed at elucidating cell-type-specific molecular mediators of environmentally enhanced neurogenesis.

Effects of Strain and Species on the Septo-Temporal Distribution of Adult Neurogenesis in Rodents

The functional septo-temporal (dorso-ventral) differentiation of the hippocampus is accompanied by gradients of adult hippocampal neurogenesis (AHN) in laboratory rodents. An extensive septal AHN in laboratory mice suggests an emphasis on a relation of AHN to tasks that also depend on the septal hippocampus. Domestication experiments indicate that AHN dynamics along the longitudinal axis are subject to selective pressure, questioning if the septal emphasis of AHN in laboratory mice is a rule applying to rodents in general. In this study, we used C57BL/6 and DBA2/Crl mice, wild-derived F1 house mice and wild-captured wood mice and bank voles to look for evidence of strain and species specific septo-temporal differences in AHN. We confirmed the septal > temporal gradient in C57BL/6 mice, but in the wild species, AHN was low septally and high temporally. Emphasis on the temporal hippocampus was particularly strong for doublecortin positive (DCX+) young neurons and more pronounced in bank voles than in wood mice. The temporal shift was stronger in female wood mice than in males, while we did not see sex differences in bank voles. AHN was overall low in DBA and F1 house mice, but they exhibited the same inversed gradient as wood mice and bank voles. DCX+ young neurons were usually confined to the subgranular zone and deep granule cell layer. This pattern was seen in all animals in the septal and intermediate dentate gyrus. In bank voles and wood mice however, the majority of temporal DCX+ cells were radially dispersed throughout the granule cell layer. Some but not all of the septo-temporal differences were accompanied by changes in the DCX+/Ki67+ cell ratios, suggesting that new neuron numbers can be regulated by both proliferation or the time course of maturation and survival of young neurons. Some of the septo-temporal differences we observe have also been found in laboratory rodents after the experimental manipulation of the molecular mechanisms that control AHN. Adaptations of AHN under natural conditions may operate on these or similar mechanisms, adjusting neurogenesis to the requirements of hippocampal function.

Seed-induced Aβ deposition is modulated by microglia under environmental enrichment in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) is characterized by severe neuronal loss as well as the accumulation of amyloid‐β (Aβ), which ultimately leads to plaque formation. Although there is now a general agreement that the aggregation of Aβ can be initiated by prion‐like seeding, the impact and functional consequences of induced Aβ deposits (Aβ seeding) on neurons still remain open questions. Here, we find that Aβ seeding, representing early stages of plaque formation, leads to a dramatic decrease in proliferation and neurogenesis in two APP transgenic mouse models. We further demonstrate that neuronal cell death occurs primarily in the vicinity of induced Aβ deposits culminating in electrophysiological abnormalities. Notably, environmental enrichment and voluntary exercise not only revives adult neurogenesis and reverses memory deficits but, most importantly, prevents Aβ seeding by activated, phagocytic microglia cells. Our work expands the current knowledge regarding Aβ seeding and the consequences thereof and attributes microglia an important role in diminishing Aβ seeding by environmental enrichment.

Impact of diet-derived signaling molecules on human cognition: exploring the food-brain axis

The processes that define mammalian physiology evolved millions of years ago in response to ancient signaling molecules, most of which were acquired by ingestion and digestion. In this way, evolution inextricably linked diet to all major physiological systems including the nervous system. The importance of diet in neurological development is well documented, although the mechanisms by which diet-derived signaling molecules (DSMs) affect cognition are poorly understood. Studies on the positive impact of nutritive and non-nutritive bioactive molecules on brain function are encouraging but lack the statistical power needed to demonstrate strong positive associations. Establishing associations between DSMs and cognitive functions like mood, memory and learning are made even more difficult by the lack of robust phenotypic markers that can be used to accurately and reproducibly measure the effects of DSMs. Lastly, it is now apparent that processes like neurogenesis and neuroplasticity are embedded within layers of interlocked signaling pathways and gene regulatory networks. Within these interdependent pathways and networks, the various transducers of DSMs are used combinatorially to produce those emergent adaptive gene expression responses needed for stimulus-induced neurogenesis and neuroplasticity. Taken together, it appears that cognition is encoded genomically and modified by epigenetics and epitranscriptomics to produce complex transcriptional programs that are exquisitely sensitive to signaling molecules from the environment. Models for how DSMs mediate the interplay between the environment and various neuronal processes are discussed in the context of the food–brain axis.

Early Postnatal but Not Late Adult Neurogenesis Is Impaired in the Pitx3-Mutant Animal Model of Parkinson's Disease

The generation of new neurons in the adult dentate gyrus has functional implications for hippocampal formation. Reduced hippocampal neurogenesis has been described in various animal models of hippocampal dysfunction such as dementia and depression, which are both common non-motor-symptoms of Parkinson's disease (PD). As dopamine plays an important role in regulating precursor cell proliferation, the loss of dopaminergic neurons in the substantia nigra (SN) in PD may be related to the reduced neurogenesis observed in the neurogenic regions of the adult brain: subventricular zone (SVZ) and dentate gyrus (DG). Here we examined adult hippocampal neurogenesis in the Pitx3-mutant mouse model of PD (aphakia mice), which phenotypically shows a selective embryonic degeneration of dopamine neurons within the SN and to a smaller extent in the ventral tegmental area (VTA). Proliferating cells were labeled with BrdU in aphakia mice and healthy controls from 3 to 42 weeks of age. Three weeks old mutant mice showed an 18% reduction of proliferating cells in the DG and of 26% in the SVZ. Not only proliferation but also the number of new neurons was impaired in young aphakia mice resulting in 33% less newborn cells 4 weeks after BrdU-labeling. Remarkably, however, the decline in the number of proliferating cells in the neurogenic regions vanished in older animals (8–42 weeks) indicating that aging masks the effect of dopamine depletion on adult neurogenesis. Region specific reduction in precursor cells proliferation correlated with the extent of dopaminergic degeneration in mesencephalic subregions (VTA and SN), which supports the theory of age- and region-dependent regulatory effects of dopaminergic projections. Physiological stimulation of adult neurogenesis by physical activity (wheel running) almost doubled the number of proliferating cells in the dentate gyrus of 8 weeks old aphakia mice to a number comparable to that of wild-type mice, abolishing the slight reduction of baseline neurogenesis at this age. The described age-dependent susceptibility of adult neurogenesis to PD-like dopaminergic degeneration and its responsiveness to physical activity might have implications for the understanding of the pathophysiology and treatment of non-motor symptoms in PD.

Neurogenic Processes Are Induced by Very Short Periods of Voluntary Wheel-Running in Male Mice

Even in the adult mammalian brain progenitor cells proliferate and give rise to young neurons which integrate into the neuronal network. The dentate gyrus possesses such a neurogenic niche reactive to external stimuli like physical activity. In most studies mice or rats have been exposed to wheel running for periods of several weeks to activate neurogenesis while early neurogenic processes induced by very short running periods are less well understood. To address this issue, we allowed male C57Bl/6 mice free access to a running wheel for 2 or 7 days. We injected bromodeoxyuridine (BrdU) before the last running night, respectively, and quantified cell proliferation with immunocytochemistry for BrdU and Ki-67. Furthermore, we performed immunocytochemistry for doublecortin (DCX) and real-time RT-qPCR for NeuroD1 to characterize and quantify changes in neurogenesis on the protein and mRNA level. Real-time RT-qPCR for neurogenic niche factors (BDNF, FGF-2, BMP4, Noggin) was used to detect changes in the molecular composition of the neurogenic niche. Interestingly, we observed that cell proliferation was already affected after 2 days of running showing a transient decrease, which was followed by a rebound with increased proliferation after 7 days. Neurogenesis was stimulated after 2 days of running, reflected by elevated NeuroD1 mRNA levels, and it was significantly increased after 7 days as indicated by DCX immunostaining. On the level of niche factors we observed changes in expression in favor of neuronal differentiation (increased BDNF mRNA expression) and proliferation (decreased BMP4 mRNA expression) already after 2 days, although increased proliferation is reflected on the cellular level only later. In summary, our data show that 2 days of running are sufficient to activate neurogenic processes and we hypothesize that a strong pressure toward differentiation privileges neurogenesis while proliferation lags behind.

Skeletal myofiber vascular endothelial growth factor is required for the exercise training-induced increase in dentate gyrus neuronal precursor cells

KEY POINTS

Peripheral vascular endothelial growth factor (VEGF) is necessary for exercise to stimulate hippocampal neurogenesis. Here we report that skeletal myofiber VEGF directly or indirectly regulates exercise-signalled proliferation of neuronal precursor cells. Our results found skeletal myofiber VEGF to be necessary for maintaining blood flow through hippocampal regions independent of exercise training state. This study demonstrates that skeletal myofiber VEGF is required for the hippocampal VEGF response to acute exercise. These results help to establish the mechanisms by which exercise, through skeletal myofiber VEGF, affects the hippocampus.

ABSTRACT

Exercise signals neurogenesis in the dentate gyrus of the hippocampus. This phenomenon requires vascular endothelial growth factor (VEGF) originating from outside the blood-brain barrier, but no cellular source has been identified. Thus, we hypothesized that VEGF produced by skeletal myofibers plays a role in regulating hippocampal neuronal precursor cell proliferation following exercise training. This was tested in adult conditional skeletal myofiber-specific VEGF gene-ablated mice (VEGF^HSA-/- ) by providing VEGF^HSA-/- and non-ablated (VEGF^f/f ) littermates with running wheels for 14 days. Following this training period, hippocampal cerebral blood flow (CBF) was measured by functional magnetic resonance imaging (fMRI), and neuronal precursor cells (BrdU+/Nestin+) were detected by immunofluorescence. The VEGF^f/f trained group showed improvements in both speed and endurance capacity in acute treadmill running tests (P < 0.05). The VEGF^HSA-/- group did not. The number of proliferating neuronal precursor cells was increased with training in VEGF^f/f (P < 0.05) but not in VEGF^HSA-/- mice. Endothelial cell (CD31+) number did not change in this region with exercise training or skeletal myofiber VEGF gene deletion. However, resting blood flow through the hippocampal region was lower in VEGF^HSA-/- mice, both untrained and trained, than untrained VEGF^f/f mice (P < 0.05). An acute hypoxic challenge decreased CBF (P < 0.05) in untrained VEGF^f/f , untrained VEGF^HSA-/- and trained VEGF^HSA-/- mice, but not trained VEGF^f/f mice. VEGF^f/f , but not VEGF^HSA-/- , mice were able to acutely run on a treadmill at an intensity sufficient to increase hippocampus VEGF levels. These data suggest that VEGF expressed by skeletal myofibers may directly or indirectly regulate both hippocampal blood flow and neurogenesis.

Physical exercise rescues defective neural stem cells and neurogenesis in the adult subventricular zone of Btg1 knockout mice

Adult neurogenesis occurs throughout life in the dentate gyrus (DG) and the subventricular zone (SVZ), where glia-like stem cells generate new neurons. Voluntary running is a powerful neurogenic stimulus triggering the proliferation of progenitor cells in the DG but, apparently, not in the SVZ. The antiproliferative gene Btg1 maintains the quiescence of DG and SVZ stem cells. Its ablation causes intense proliferation of DG and SVZ stem/progenitor cells in young mice, followed, during adulthood, by progressive decrease of the proliferative capacity. We have previously observed that running can rescue the deficit of DG Btg1-null neurogenesis. Here, we show that in adult Btg1-null SVZ stem and neuroblast cells, the reduction of proliferation is associated with a longer cell cycle and a more frequent entry into quiescence. Notably, running increases proliferation in Btg1-null SVZ stem cells highly above the levels of sedentary wild-type mice and restores normal values of cell cycle length and quiescence in stem and neuroblast cells, without affecting wild-type cells. Btg1-null SVZ neuroblasts show also increased migration throughout the rostral migratory stream and a deficiency of differentiated neurons in the olfactory bulb, possibly a consequence of premature exit from the cycle; running, however, normalizes migration and differentiation, increasing newborn neurons recruited to the olfactory circuitry. Furthermore, running increases the self-renewal of Btg1-null SVZ-derived neurospheres and, remarkably, in aged Btg1-null mice almost doubles the proliferating SVZ stem cells. Altogether, this reveals that SVZ stem cells are endowed with a hidden supply of self-renewal capacity, coupled to cell cycle acceleration and emerging after ablation of the quiescence-maintaining Btg1 gene and following exercise.