Interplay between DISC1 and GABA signaling regulates neurogenesis in mice and risk for schizophrenia

How extrinsic stimuli and intrinsic factors interact to regulate continuous neurogenesis in the postnatal mammalian brain is unknown. Here we show that regulation of dendritic development of newborn neurons by Disrupted-in-Schizophrenia 1 (DISC1) during adult hippocampal neurogenesis requires neurotransmitter GABA-induced, NKCC1-dependent depolarization through a convergence onto the AKT-mTOR pathway. In contrast, DISC1 fails to modulate early-postnatal hippocampal neurogenesis when conversion of GABA-induced depolarization to hyperpolarization is accelerated. Extending the period of GABA-induced depolarization or maternal deprivation stress restores DISC1-dependent dendritic regulation through mTOR pathway during early-postnatal hippocampal neurogenesis. Furthermore, DISC1 and NKCC1 interact epistatically to affect risk for schizophrenia in two independent case control studies. Our study uncovers an interplay between intrinsic DISC1 and extrinsic GABA signaling, two schizophrenia susceptibility pathways, in controlling neurogenesis and suggests critical roles of developmental tempo and experience in manifesting the impact of susceptibility genes on neuronal development and risk for mental disorders.     

Control of early events in olfactory processing by adult neurogenesis

The mature brain needs to have flexible control over behavior in the face of ever-changing needs. It achieves this control through morphological and physiological changes at the level of molecules, spines, dendrites, and axons and through processes of adult neurogenesis, entire cells. The functional maturation of newly generated cells in the adult forebrain involves the expression of neurotransmitter receptors before synaptic activity and excitatory gamma-aminobutyric acid (GABAergic) influences prior to glutamatergic input. The production of new cells for incorporation into neural circuits that are already up and running gives rise to a unique situation that may require epigenetic regulation. However, once mature, new neurons must carve out a niche among more established cells to be useful. How do they survive and what are they used for? Recent studies have revealed that adult neurogenesis alters the olfactory bulb at all levels, from single cells to the network and system levels. It has also been suggested that cell turnover may be particularly beneficial for the processing of new information in dynamic networks. However, elucidating the functional meaning of adult neurogenesis must wait for the development of new paradigms to eliminate the pool of newly generated neurons but sparing the preexisting ones. Nevertheless, there is already considerable correlative evidence to indicate that adult neurogenesis is a plastic mechanism by which the performance of the brain can be optimized in a given environment.     

Adult neurogenesis in the short-lived teleost Nothobranchius furzeri: localization of neurogenic niches, molecular characterization and effects of aging

We studied adult neurogenesis in the short‐lived annual fish Nothobranchius furzeri and quantified the effects of aging on the mitotic activity of the neuronal progenitors and the expression of glial fibrillary acid protein (GFAP) in the radial glia. The distribution of neurogenic niches is substantially similar to that of zebrafish and adult stem cells generate neurons, which persist in the adult brain. As opposed to zebrafish, however, the N. furzeri genome contains a doublecortin (DCX) gene. Doublecortin is transiently expressed by newly generated neurons in the telencephalon and optic tectum (OT). We also analyzed the expression of the microRNA miR‐9 and miR‐124 and found that they have complementary expression domains: miR‐9 is expressed in the neurogenic niches of the telencephalon and the radial glia of the OT, while miR‐124 is expressed in differentiated neurons. The main finding of this paper is the demonstration of an age‐dependent decay in adult neurogenesis. Using unbiased stereological estimates of cell numbers, we detected an almost fivefold decrease in the number of mitotically active cells in the OT between young and old age. This reduced mitotic activity is paralleled by a reduction in DCX labeling. Finally, we detected a dramatic up‐regulation of GFAP in the radial glia of the aged brain. This up‐regulation is not paralleled by a similar up‐regulation of S100B and Musashi‐1, two other markers of the radial glia. In summary, the brain of N. furzeri replicates two typical hallmarks of mammalian aging: gliosis and reduced adult neurogenesis.     

Adult neurogenesis in the central olfactory pathway of dendrobranchiate and caridean shrimps: New insights into the evolution of the deutocerebral proliferative system in reptant decapods

Persistent neurogenesis in the central olfactory pathway characterizes many reptant decapods such as lobsters, crayfish and crabs. In these animals, the deutocerebral proliferative system generates new neurons which integrate into the neuronal network of the first order processing neuropil of the olfactory system, the deutocerebral chemosensory lobes (also called olfactory lobes). However, differences concerning the phenotype and the mechanisms that drive adult neurogenesis were reported in crayfish versus spiny lobsters. While numerous studies have focussed on these mechanisms and regulation of adult neurogenesis, investigations about the phylogenetic distribution are missing. To contribute an evolutionary perspective on adult neurogenesis in decapods, we investigated two representatives of basally diverging lineages, the dendrobranchiate Penaeus vannamei and the caridean Crangon crangon using the thymidine analogue Bromodeoxyuridine (BrdU) as marker for the S phase of cycling cells. Compared to reptant decapods, our results suggest a simpler mechanism of neurogenesis in the adult brain of dendrobranchiate and caridean shrimps. Observed differences in the rate of proliferation and spatial dimensions are suggested to correlate with the complexity of the olfactory system. We assume that a more complex and mitotically more active proliferative system in reptant decapods evolved with the emergence of another processing neuropil, the accessory lobes. © 2018 Wiley Periodicals, Inc. Develop Neurobiol, 2018     

Understanding the regulation and function of adult neurogenesis: contribution from an insect model, the house cricket

Since the discovery of adult neurogenesis, a major issue is the role of newborn neurons and the function-dependent regulation of adult neurogenesis. We decided to use an animal model with a relatively simple brain to address these questions. In the adult cricket brain as in mammals, new neurons are produced throughout life. This neurogenesis occurs in the main integrative centers of the insect brain, the mushroom bodies (MBs), where the neuroblasts responsible for their formation persist after the imaginal molt. The rate of production of new neurons is controlled not only by internal cues such as morphogenetic hormones but also by external environmental cues. Adult crickets reared in an enriched sensory environment experienced an increase in neuroblast proliferation as compared with crickets reared in an impoverished environment. In addition, unilateral sensory deprivation led to reduced neurogenesis in the MB ipsilateral to the lesion. In search of a functional role for the new cells, we specifically ablated MB neuroblasts in young adults using brain-focused gamma ray irradiation. We developed a learning paradigm adapted to the cricket, which we call the "escape paradigm." Using this operant associative learning test, we showed that crickets lacking neurogenesis exhibited delayed learning and reduced memory retention of the task when olfactory cues were used. Our results suggest that environmental cues are able to influence adult neurogenesis and that, in turn, newly generated neurons participate in olfactory integration, optimizing learning abilities of the animal, and thus its adaptation to its environment. Nevertheless, odor learning in adult insects cannot always be attributed to newly born neurons because neurogenesis is completed earlier in development in many insect species. In addition, many of the irradiated crickets performed significantly better than chance on the operant learning task.     

Neural stem cells as therapeutic agents for age-related brain repair

Neurogenesis occurs in two germinal centres of the adult brain and persists with increasing age, although at a reduced level. This observation, that the mature brain can support neurogenesis, has given rise to the hope that neural stem cells could be used to repair the brain by repopulating regions suffering from neuronal loss as a result of injury or disease. The aging brain is vulnerable to mild cognitive impairment, increasing incidence of stroke, and a variety of neurodegenerative diseases. However, most studies to date have focused on the young adult brain, and relatively little information is available about the regulation of neurogenesis in the aged brain or the potential of using neural stem cells to repair the aged brain. This review summarizes the current state of knowledge on neurogenesis in the young adult brain and discusses the information available on age-related changes in neurogenesis. Possible therapeutic strategies using neural stem cells for repair of the aging brain are considered.     

Mechanisms of action of antidepressants: from neurotransmitter systems to signaling pathways

Antidepressants are commonly used in the treatment of anxiety and depression, medical conditions that affect approximately 17-20% of the population. The clinical effects of antidepressants take several weeks to manifest, suggesting that these drugs induce adaptive changes in brain structures affected by anxiety and depression. In order to develop shorter-acting and more effective drugs for the treatment of anxiety and depression, it is important to understand how antidepressants bring about their beneficial effects. Recent reports suggest that antidepressants can induce neurogenesis in the adult brain, although the mechanisms involved are not clearly understood. In this review, we describe the different neurotransmitter systems that are affected by anxiety and depression and how they are modulated by antidepressant treatment with a focus on signaling molecules and pathways that are activated during neurotransmitter receptor induced neurogenesis.     

Radial glial cell‐specific ablation in the adult Zebrafish brain

The zebrafish brain can continue to produce new neurons in widespread neurogenic brain regions throughout life. In contrast, neurogenesis in the adult mammalian brain is restricted to the subventricular zone (SVZ) and dentate gyrus (DG). In neurogenic regions in the adult brain, radial glial cells (RGCs) are considered to function as neural stem cells (NSCs). We generated a Tg(gfap:Gal4FF) transgenic zebrafish line, which enabled us to express specific genes in RGCs. To study the function of RGCs in neurogenesis in the adult zebrafish brain, we also generated a Tg(gfap: Gal4FF; UAS:nfsB‐mcherry) transgenic zebrafish line, which allowed us to induce cell death exclusively within RGCs upon addition of metronidazole (Mtz) to the media. RGCs expressing nitroreductase were specifically ablated by the Mtz treatment, decreasing the number of proliferative RGCs. Using the Tg(gfap:Gal4FF; UAS:nfsB‐mcherry) transgenic zebrafish line, we found that RGCs were specifically ablated in the adult zebrafish telencephalon. The Tg(gfap:Gal4FF) line could be useful to study the function of RGCs. genesis 53:431–439, 2015. © 2015 Wiley Periodicals, Inc.     

Adult Neurogenesis: Basic Concepts of Signaling

Findings over the past decades demonstrating persistent neurogenesis in the adult brain havechallenged the view of a fixed circuitry in normally functioning brain and raised hopes for self-renewalfollowing brain injury. In addition to providing insights for repair, studying adult neurogenesis mayimprove our understanding of embryonic development assuming that fundamental mechanisms aresimilar. It is argued here, using examples of cell:cell communication, that parallels can be drawnbetween adult and embryonic neurogenesis. Paradoxically, cell:cell communication in neurogenicregions resembles that in a mature neuroglial network. This suggests that differences in the integrativeproperties of cells and the extracellular matrix molecules may constitute a neurogenic environment or“niche”. While reasons for persistent adult neurogenesis in humans remains obscure, recent findingsregarding the environmental and activity-driven control of neurogenesis reinforce the original conceptof a role for neurogenesis in motor memory formation and refinement of information processing.     

Targeted Deletion of the ERK5 MAP Kinase Impairs Neuronal Differentiation,Migration, and Survival during Adult Neurogenesis in the Olfactory Bulb

Recent studies have led to the exciting idea that adult-born neurons in the olfactory bulb (OB) may be critical for complex forms of olfactory behavior in mice. However, signaling mechanisms regulating adult OB neurogenesis are not well defined. We recently reported that extracellular signal-regulated kinase (ERK) 5, a MAP kinase, is specifically expressed in neurogenic regions within the adult brain. This pattern of expression suggests a role for ERK5 in the regulation of adult OB neurogenesis. Indeed, we previously reported that conditional deletion of erk5 in adult neurogenic regions impairs several forms of olfactory behavior in mice. Thus, it is important to understand how ERK5 regulates adult neurogenesis in the OB. Here we present evidence that shRNA suppression of ERK5 in adult neural stem/progenitor cells isolated from the subventricular zone (SVZ) reduces neurogenesis in culture. By contrast, ectopic activation of endogenous ERK5 signaling via expression of constitutive active MEK5, an upstream activating kinase for ERK5, stimulates neurogenesis. Furthermore, inducible and conditional deletion of erk5 specifically in the neurogenic regions of the adult mouse brain interferes with cell cycle exit of neuroblasts, impairs chain migration along the rostral migratory stream and radial migration into the OB. It also inhibits neuronal differentiation and survival. These data suggest that ERK5 regulates multiple aspects of adult OB neurogenesis and provide new insights concerning signaling mechanisms governing adult neurogenesis in the SVZ-OB axis.     

Hormones,neuroblasts and the adult insect

Insect neurogenesis has been subjected to extensive study and as a result is regarded as being well understood. It is, therefore, all the more surprising when a fundamentally novel aspects of the process is uncovered. Until recently it was thought that the production of central neurons ceased before the emergence of the adult. Recently, however, Cayre et al. have shown that neurogenesis also occurs in the adult brain. Their studies also show that the rate at which adult neuroblasts divide is controlled by hormones, suggesting that hormones may play a more important role in regulating neurogenesis than previously suspected.     

Retinoid-mediated regulation of mood: possible cellular mechanisms

Vitamin A and its derivatives, the retinoids, have long been studied for their ability to alter central nervous system (CNS) development. Increasingly, it is recognized that sufficient levels of retinoids may also be required for adult CNS function. However, excess dietary vitamin A, due to the consumption of supplements or foods rich in vitamin A, has been reported to induce psychosis. In addition, 13-cis-retinoic acid (13-cis-RA, isotretinoin), the active ingredient in the acne treatment Accutane, has been reported to cause adverse psychiatric events, including depression and suicidal ideation. Nevertheless, epidemiological studies have reported no consistent link between Accutane use and clinical depression in humans. Using an animal model, we have recently shown that 13-cis-RA induces an increase in depression-related behavior. Impairments in spatial learning and memory have also been demonstrated following 13-cis-RA treatment in mice. This review focuses on the behavioral and possible cellular effects of retinoid deficiency or excess in the adult brain in relation to altered mood. Specifically, we discuss the effect of retinoids on depression-related behaviors and whether norepinephrinergic, dopaminergic, or serotonergic neurotransmitter systems may be impaired. In addition, we consider the evidence that adult neurogenesis, a process implicated in the pathophysiology of depression, is reduced by retinoid signaling. We suggest that 13-cis-RA treatment may induce depression-related behaviors by decreasing adult neurogenesis and/or altering the expression of components of serotonergic neurotransmitter system, thereby leading to impaired serotonin signaling.     

Sensory inputs stimulate progenitor cell proliferation in an adult insect brain

Although most brain neurons are produced during embryonic and early postnatal development, recent studies clearly demonstrated in a wide range of species from invertebrates to humans that new neurons are added to specific brain structures throughout adult life. Hormones, neurotransmitters, and growth factors as well as environmental conditions modulate this neurogenesis. In this study, we address the role of sensory inputs in the regulation of adult neural progenitor cell proliferation in an insect model. In some insect species, adult neurogenesis occurs in the mushroom bodies, the main sensory integrative centers of the brain, receiving multimodal information and often considered as the analog of the vertebrate hippocampus. We recently showed that rearing adult crickets in enriched sensory and social conditions enhanced neuroblast proliferation in the mushroom bodies. Here, by manipulating hormonal levels and affecting olfactory and/or visual inputs, we show that environmental regulation of neurogenesis is in direct response to olfactory and visual stimuli rather than being mediated via hormonal control. Experiments of unilateral sensory deprivation reveal that neuroblast proliferation can be inhibited in one brain hemisphere only. These results, obtained in a relatively simple brain, emphasize the role of sensory inputs on stem cell division.     

An identified serotonergic neuron regulates adult neurogenesis in the crustacean brain

New neurons are born and integrated into functional circuits in the brains of many adult organisms. In virtually all of these systems, serotonin is a potent regulator of neuronal proliferation. Specific neural pathways underlying these serotonergic influences have not, however, been identified and manipulated. The goal of this study was to test whether adult neurogenesis in the crustacean brain is influenced by electrical activity in the serotonergic dorsal giant neurons (DGNs) innervating the primary olfactory processing areas, the olfactory lobes, and higher order centers, the accessory lobes. Adult‐born neurons occur in two interneuronal cell clusters that are part of the olfactory pathway. This study demonstrates that neurogenesis also continues in these areas in a dissected, perfused brain preparation, although the rate of neuronal production is lower than in brains from intact same‐sized animals. Inclusion of 10^?9 M serotonin in the perfusate delivered to the dissected brain preparation restores the rate of neurogenesis to in vivo levels. Although subthreshold stimulation of the DGN does not significantly alter the rate of neurogenesis, electrical activation of a single DGN results in significant increases in neurogenesis in Cluster 10 on the same side of the brain, when compared with levels on the contralateral, unstimulated side. Measurements of serotonin levels in the perfusate using high‐performance liquid chromatography established that serotonin levels are elevated about 10‐fold during DGN stimulation, confirming that serotonin is released during DGN activity. This is the first identified neural pathway through which adult neurogenesis has been directly manipulated. © 2009 Wiley Periodicals, Inc. Develop Neurobiol 2009     

Stress,hippocampal neurogenesis and cognition: functional correlations

The brain of many species including humans, harbors stem cells that continue to generate new neurons up into adulthood. This form of structural plasticity occurs in a limited number of brain regions, i.e. the subventricular zone and the hippocampal dentate gyrus and is regulated by environmental and hormonal factors. In this minireview, we provide an overview of the effects of stress and glucocorticoid hormones on adult hippocampal neurogenesis and discuss how these effects may be relevant for cognitive function and possibly, brain disease. While its exact functional role remains elusive, adult neurogenesis has been implicated in learning and memory, fear and mood regulation and recently, adult-born neurons were found to be involved in specific cognitive functions such as pattern separation (i.e. the ability to form unique memory representations) and cognitive flexibility. The process of adult neurogenesis is influenced by several factors; whereas e.g. exercise stimulates, exposure to stress and stress hormones generally inhibit neurogenesis. Effects of acute, mild stress are generally short-lasting and recover quickly, but chronic or severe forms of stress can induce lasting reductions in adult neurogenesis. Some of the inhibitory effects of stress can be rescued by exercise, by allowing a period of recovery from stress, by drugs that target the stress system, or by some, but not all, antidepressants. Stress may, partly through its effects on adult neurogenesis, alter structure and plasticity of the hippocampal circuit. This can lead to subsequent changes in stress responsivity and aspects of memory processing, which may be particularly relevant for stress related psychopathology or brain diseases that involve perturbed memory processing.     

Vascular endothelial growth factor-B (VEGFB) stimulates neurogenesis: evidence from knockout mice and growth factor administration

Vascular endothelial growth factor-B (VEGFB) is an angiogenic and neuroprotective protein that reduces hypoxic and ischemic neuronal injury. To determine if VEGFB also regulates neurogenesis in the adult brain, we studied the effects of VEGFB administration in vitro and in vivo, as well as the effect of VEGFB gene knockout (KO) in mice, on bromodeoxyuridine (BrdU) incorporation and expression of immature neuronal markers in the subgranular zone (SGZ) of the hippocampal dentate gyrus and the forebrain subventricular zone (SVZ). Intracerebroventricular VEGFB administration increased BrdU incorporation into cells of neuronal lineage both in vitro and in vivo, and VEGFB-KO mice showed impaired neurogenesis, consistent with a neurogenesis-promoting effect of VEGFB. In addition, intraventricular administration of VEGFB restored neurogenesis to wild-type levels in VEGFB-KO mice. These results suggest a role for VEGFB in the regulation of adult neurogenesis, which could have therapeutic implications for diseases associated with central neuronal loss.     

Proliferation, neurogenesis and regeneration in the non-mammalian vertebrate brain

Post-embryonic neurogenesis is a fundamental feature of the vertebrate brain. However, the level of adult neurogenesis decreases significantly with phylogeny. In the first part of this review, a comparative analysis of adult neurogenesis and its putative roles in vertebrates are discussed. Adult neurogenesis in mammals is restricted to two telencephalic constitutively active zones. On the contrary, non-mammalian vertebrates display a considerable amount of adult neurogenesis in many brain regions. The phylogenetic differences in adult neurogenesis are poorly understood. However, a common feature of vertebrates (fish, amphibians and reptiles) that display a widespread adult neurogenesis is the substantial post-embryonic brain growth in contrast to birds and mammals. It is probable that the adult neurogenesis in fish, frogs and reptiles is related to the coordinated growth of sensory systems and corresponding sensory brain regions. Likewise, neurons are substantially added to the olfactory bulb in smell-oriented mammals in contrast to more visually oriented primates and songbirds, where much fewer neurons are added to the olfactory bulb. The second part of this review focuses on the differences in brain plasticity and regeneration in vertebrates. Interestingly, several recent studies show that neurogenesis is suppressed in the adult mammalian brain. In mammals, neurogenesis can be induced in the constitutively neurogenic brain regions as well as ectopically in response to injury, disease or experimental manipulations. Furthermore, multipotent progenitor cells can be isolated and differentiated in vitro from several otherwise silent regions of the mammalian brain. This indicates that the potential to recruit or generate neurons in non-neurogenic brain areas is not completely lost in mammals. The level of adult neurogenesis in vertebrates correlates with the capacity to regenerate injury, for example fish and amphibians exhibit the most widespread adult neurogenesis and also the greatest capacity to regenerate central nervous system injuries. Studying these phenomena in non-mammalian vertebrates may greatly increase our understanding of the mechanisms underlying regeneration and adult neurogenesis. Understanding mechanisms that regulate endogenous proliferation and neurogenic permissiveness in the adult brain is of great significance in therapeutical approaches for brain injury and disease.