A balanced evaluation of the evidence for adult neurogenesis in humans: implication for neuropsychiatric disorders

Adult Hippocampal Neurogenesis in the Human Brain: Updates, Challenges, and Perspectives

The existence of neurogenesis in the adult human hippocampus has been under considerable debate within the past three decades due to the diverging conclusions originating mostly from immunohistochemistry studies. While some of these reports conclude that hippocampal neurogenesis in humans occurs throughout physiologic aging, others indicate that this phenomenon ends by early childhood. More recently, some groups have adopted next-generation sequencing technologies to characterize with more acuity the extent of this phenomenon in humans. Here, we review the current state of research on adult hippocampal neurogenesis in the human brain with an emphasis on the challenges and limitations of using immunohistochemistry and next-generation sequencing technologies for its study.

Significance of the anterior cingulate cortex in neurogenesis plasticity: Connections, functions, and disorders across postnatal and adult stages

The anterior cingulate cortex (ACC) is a complex and continually evolving brain region that remains a primary focus of research due to its multifaceted functions. Various studies and analyses have significantly advanced our understanding of how the ACC participates in a wide spectrum of memory and cognitive processes. However, despite its strong connections to brain areas associated with hippocampal and olfactory neurogenesis, the functions of the ACC in regulating postnatal and adult neurogenesis in these regions are still insufficiently explored. Investigating the intricate involvement of the ACC in neurogenesis could enhance our comprehension of essential aspects of brain plasticity. This involvement stems from its complex circuitry with other relevant brain regions, thereby exerting both direct and indirect impacts on the neurogenesis process. This review sheds light on the promising significance of the ACC in orchestrating postnatal and adult neurogenesis in conditions related to memory, cognitive behavior, and associated disorders.

5-Bromo-2'-deoxyuridine labeling: historical perspectives, factors influencing the detection, toxicity, and its implications in the neurogenesis

The halopyrimidine 5-bromo-2'-deoxyuridine (BrdU) is an exogenous marker of DNA synthesis. Since the introduction of monoclonal antibodies against BrdU, an increasing number of methodologies have been used for the immunodetection of this synthesized bromine-tagged base analogue into replicating DNA. BrdU labeling is widely used for identifying neuron precursors and following their fate during the embryonic, perinatal, and adult neurogenesis in a variety of vertebrate species including birds, reptiles, and mammals. Due to BrdU toxicity, its incorporation into replicating DNA presents adverse consequences on the generation, survival, and settled patterns of cells. This may lead to false results and misinterpretation in the identification of proliferative neuroblasts. In this review, I will indicate the detrimental effects of this nucleoside during the development of the central nervous system, as well as the reliability of BrdU labeling to detect proliferating neuroblasts. Moreover, it will show factors influencing BrdU immunodetection and the contribution of this nucleoside to the study of prenatal, perinatal, and adult neurogenesis. Human adult neurogenesis will also be discussed. It is my hope that this review serves as a reference for those researchers who focused on detecting cells that are in the synthetic phase of the cell cycle.

Neurogenesis in primates versus rodents and the value of non-human primate models

Neurogenesis, the process of generating neurons from neural stem cells, occurs during both embryonic and adult stages, with each stage possessing distinct characteristics. Dysfunction in either stage can disrupt normal neural development, impair cognitive functions, and lead to various neurological disorders. Recent technological advancements in single-cell multiomics and gene-editing have facilitated investigations into primate neurogenesis. Here, we provide a comprehensive overview of neurogenesis across rodents, non-human primates, and humans, covering embryonic development to adulthood and focusing on the conservation and diversity among species. While non-human primates, especially monkeys, serve as valuable models with closer neural resemblance to humans, we highlight the potential impacts and limitations of non-human primate models on both physiological and pathological neurogenesis research.

Adult neurogenesis and "immature" neurons in mammals: an evolutionary trade-off in plasticity?

Neuronal plasticity can vary remarkably in its form and degree across animal species. Adult neurogenesis, namely the capacity to produce new neurons from neural stem cells through adulthood, appears widespread in non-mammalian vertebrates, whereas it is reduced in mammals. A growing body of comparative studies also report variation in the occurrence and activity of neural stem cell niches between mammals, with a general trend of reduction from small-brained to large-brained species. Conversely, recent studies have shown that large-brained mammals host large amounts of neurons expressing typical markers of neurogenesis in the absence of cell division. In layer II of the cerebral cortex, populations of prenatally generated, non-dividing neurons continue to express molecules indicative of immaturity throughout life (cortical immature neurons; cINs). After remaining in a dormant state for a very long time, these cINs retain the potential of differentiating into mature neurons that integrate within the preexisting neural circuits. They are restricted to the paleocortex in small-brained rodents, while extending into the widely expanded neocortex of highly gyrencephalic, large-brained species. The current hypothesis is that these populations of non-newly generated "immature" neurons might represent a reservoir of developmentally plastic cells for mammalian species that are characterized by reduced stem cell-driven adult neurogenesis. This indicates that there may be a trade-off between various forms of plasticity that coexist during brain evolution. This balance may be necessary to maintain a "reservoir of plasticity" in brain regions that have distinct roles in species-specific socioecological adaptations, such as the neocortex and olfactory structures.

Neuropsychiatric events associated with montelukast in patients with asthma: a systematic review

BACKGROUND

The United States Food and Drug Administration issued a black box warning on the mental health adverse effects of montelukast in 2020. Age-related effects on the risk of developing specific neuropsychiatric events in montelukast users remain largely unknown.

OBJECTIVE

To describe the risk of neuropsychiatric events associated with montelukast in adults and children with asthma.

METHODS

A systematic search of all studies investigating neuropsychiatric events in montelukast users was performed in PubMed, the Cochrane Library and Embase from inception to 7 September 2022. Animal studies and conference abstracts were excluded.

RESULTS

59 studies (21 pharmacovigilance studies, four reviews from 172 randomised controlled trials, 20 observational studies, 10 case reports and four case series) evaluating neuropsychiatric events in patients with asthma on montelukast were reviewed. No significant association was shown between montelukast and suicide-related events in six of the observational studies. No association was found for depression as defined by the International Classification of Diseases 10th revision codes in three observational studies and a review of randomised clinical trials. However, findings from four studies using antidepressant prescriptions as the outcome identified significant associations. Consistent with nine pharmacovigilance studies, two large-scale observational studies revealed possible associations of montelukast with anxiety and sleeping disorders in adult patients with asthma, respectively. However, the results were not replicated in two observational studies on children.

CONCLUSION

Montelukast is not associated with suicide- and depression-related events in asthma patients. Older adults may be particularly susceptible to anxiety and sleeping disorders.

Reorganization in the macaque interoceptive-allostatic network following anterior cingulate cortex damage

Accumulating evidence indicates that the adult brain is capable of significant structural change following damage-a capacity once thought to be largely limited to developing brains. To date, most existing research on adult plasticity has focused on how exteroceptive sensorimotor networks compensate for damage to preserve function. Interoceptive networks-those that represent and process sensory information about the body's internal state-are now recognized to be critical for a wide range of physiological and psychological functions from basic energy regulation to maintaining a sense of self, but the extent to which these networks remain plastic in adulthood has not been established. In this report, we used detailed histological analyses to pinpoint precise changes to gray matter volume in the interoceptive-allostatic network in adult rhesus monkeys (Macaca mulatta) who received neurotoxic lesions of the anterior cingulate cortex (ACC) and neurologically intact control monkeys. Relative to controls, monkeys with ACC lesions had significant and selective unilateral expansion of the ventral anterior insula and significant relative bilateral expansion of the lateral nucleus of the amygdala. This work demonstrates the capacity for neuroplasticity in the interoceptive-allostatic network which, given that changes included expansion rather than atrophy, is likely to represent an adaptive response following damage.

How Widespread Are the “Young” Neurons of the Mammalian Brain?

After the discovery of adult neurogenesis (stem cell-driven production of new neuronal elements), it is conceivable to find young, undifferentiated neurons mixed with mature neurons in the neural networks of the adult mammalian brain. This “canonical” neurogenesis is restricted to small stem cell niches persisting from embryonic germinal layers, yet, the genesis of new neurons has also been reported in various parenchymal brain regions. Whichever the process involved, several populations of “young” neurons can be found at different locations of the brain. Across the years, further complexity emerged: (i) molecules of immaturity can also be expressed by non-dividing cells born during embryogenesis, then maintaining immature features later on; (ii) remarkable interspecies differences exist concerning the types, location, amount of undifferentiated neurons; (iii) re-expression of immaturity can occur in aging (dematuration). These twists are introducing a somewhat different definition of neurogenesis than normally assumed, in which our knowledge of the “young” neurons is less sharp. In this emerging complexity, there is a need for complete mapping of the different “types” of young neurons, considering their role in postnatal development, plasticity, functioning, and interspecies differences. Several important aspects are at stake: the possible role(s) that the young neurons may play in maintaining brain efficiency and in prevention/repair of neurological disorders; nonetheless, the correct translation of results obtained from laboratory rodents. Hence, the open question is: how many types of undifferentiated neurons do exist in the brain, and how widespread are they?

Effect of Escitalopram on the Number of DCX-Positive Cells and NMUR2 Receptor Expression in the Rat Hippocampus under the Condition of NPSR Receptor Blockade

BACKGROUND

Neuropeptide S (NPS) is a multifunctional regulatory factor that exhibits a potent anxiolytic activity in animal models. However, there are no reports dealing with the potential molecular interactions between the activity of selective serotonin reuptake inhibitors (SSRIs) and NPS signaling, especially in the context of adult neurogenesis and the expression of noncanonical stress-related neuropeptides such as neuromedin U (NMU). The present work therefore focused on immunoexpression of neuromedin U receptor 2 (NMUR2) and doublecortin (DCX) in the rat hippocampus after acute treatment with escitalopram and in combination with selective neuropeptide S receptor (NPSR) blockade.

METHODS

Studies were carried out on adult, male Sprague-Dawley rats that were divided into five groups: animals injected with saline (control) and experimental individuals treated with escitalopram (at single dose 10 mg/kg daily), escitalopram + SHA-68, a selective NPSR antagonist (at single dose 40 mg/kg), SHA-68 alone, and corresponding vehicle control. All animals were sacrificed under halothane anaesthesia. The whole hippocampi were quickly excised, fixed, and finally sliced for general qualitative immunohistochemical assessment of the NPSR and NMUR2 expression. The number of immature neurons was enumerated using immunofluorescent detection of doublecortin (DCX) expression within the subgranular zone (SGZ).

RESULTS

Acute escitalopram administration affects the number of DCX and NMUR2-expressing cells in the adult rat hippocampus. A decreased number of DCX-expressing neuroblasts after treatment with escitalopram was augmented by SHA-68 coadministration.

CONCLUSIONS

Early pharmacological effects of escitalopram may be at least partly connected with local NPSR-related alterations of neuroblast maturation in the rat hippocampus. Escitalopram may affect neuropeptide and DCX-expression starting even from the first dose. Adult neurogenesis may be regulated via paracrine neuropeptide S and NMU-related signaling.

Experimental Arthritis Inhibits Adult Hippocampal Neurogenesis in Mice

Background: Adult-born neurons of the hippocampal dentate gyrus play a role in specific forms of learning, and disturbed neurogenesis seems to contribute to the development of neuropsychiatric disorders, such as major depression. Neuroinflammation inhibits adult neurogenesis, but the effect of peripheral inflammation on this form of neuroplasticity is ambiguous. Objective: Our aim was to investigate the influence of acute and chronic experimental arthritis on adult hippocampal neurogenesis and to elucidate putative regulatory mechanisms. Methods: Arthritis was triggered by subcutaneous injection of complete Freund’s adjuvant (CFA) into the hind paws of adult male mice. The animals were killed either seven days (acute inflammation) or 21 days (chronic inflammation) after the CFA injection. Behavioral tests were used to demonstrate arthritis-related hypersensitivity to painful stimuli. We used in vivo bioluminescence imaging to verify local inflammation. The systemic inflammatory response was assessed by complete blood cell counts and by measurement of the cytokine/chemokine concentrations of TNF-α, IL-1α, IL-4, IL-6, IL-10, KC and MIP-2 in the inflamed hind limbs, peripheral blood and hippocampus to characterize the inflammatory responses in the periphery and in the brain. In the hippocampal dentate gyrus, the total number of newborn neurons was determined with quantitative immunohistochemistry visualizing BrdU- and doublecortin-positive cells. Microglial activation in the dentate gyrus was determined by quantifying the density of Iba1- and CD68-positive cells. Results: Both acute and chronic arthritis resulted in paw edema, mechanical and thermal hyperalgesia. We found phagocytic infiltration and increased levels of TNF-α, IL-4, IL-6, KC and MIP-2 in the inflamed hind paws. Circulating neutrophil granulocytes and IL-6 levels increased in the blood solely during the acute phase. In the dentate gyrus, chronic arthritis reduced the number of doublecortin-positive cells, and we found increased density of CD68-positive macrophages/microglia in both the acute and chronic phases. Cytokine levels, however, were not altered in the hippocampus. Conclusions: Our data suggest that acute peripheral inflammation initiates a cascade of molecular and cellular changes that eventually leads to reduced adult hippocampal neurogenesis, which was detectable only in the chronic inflammatory phase.

Adult Neural Stem Cell Regulation by Small Non-coding RNAs: Physiological Significance and Pathological Implications

The adult neurogenic niches are complex multicellular systems, receiving regulatory input from a multitude of intracellular, juxtacrine, and paracrine signals and biological pathways. Within the niches, adult neural stem cells (aNSCs) generate astrocytic and neuronal progeny, with the latter predominating in physiological conditions. The new neurons generated from this neurogenic process are functionally linked to memory, cognition, and mood regulation, while much less is known about the functional contribution of aNSC-derived newborn astrocytes and adult-born oligodendrocytes. Accumulating evidence suggests that the deregulation of aNSCs and their progeny can impact, or can be impacted by, aging and several brain pathologies, including neurodevelopmental and mood disorders, neurodegenerative diseases, and also by insults, such as epileptic seizures, stroke, or traumatic brain injury. Hence, understanding the regulatory underpinnings of aNSC activation, differentiation, and fate commitment could help identify novel therapeutic avenues for a series of pathological conditions. Over the last two decades, small non-coding RNAs (sncRNAs) have emerged as key regulators of NSC fate determination in the adult neurogenic niches. In this review, we synthesize prior knowledge on how sncRNAs, such as microRNAs (miRNAs) and piwi-interacting RNAs (piRNAs), may impact NSC fate determination in the adult brain and we critically assess the functional significance of these events. We discuss the concepts that emerge from these examples and how they could be used to provide a framework for considering aNSC (de)regulation in the pathogenesis and treatment of neurological diseases.

Effect of soluble amyloid precursor protein-alpha on adult hippocampal neurogenesis in a mouse model of Alzheimer's disease

Soluble amyloid precursor protein-alpha (sAPPα) is a regulator of neuronal and memory mechanisms, while also having neurogenic and neuroprotective effects in the brain. As adult hippocampal neurogenesis is impaired in Alzheimer’s disease, we tested the hypothesis that sAPPα delivery would rescue adult hippocampal neurogenesis in an APP/PS1 mouse model of Alzheimer’s disease. An adeno-associated virus-9 (AAV9) encoding murine sAPPα was injected into the hippocampus of 8-month-old wild-type and APP/PS1 mice, and later two different thymidine analogues (XdU) were systemically injected to label adult-born cells at different time points after viral transduction. The proliferation of adult-born cells, cell survival after eight weeks, and cell differentiation into either neurons or astrocytes was studied. Proliferation was impaired in APP/PS1 mice but was restored to wild-type levels by viral expression of sAPPα. In contrast, sAPPα overexpression failed to rescue the survival of XdU⁺-labelled cells that was impaired in APP/PS1 mice, although it did cause a significant increase in the area density of astrocytes in the granule cell layer across both genotypes. Finally, viral expression of sAPPα reduced amyloid-beta plaque load in APP/PS1 mice in the dentate gyrus and somatosensory cortex. These data add further evidence that increased levels of sAPPα could be therapeutic for the cognitive decline in AD, in part through restoration of the proliferation of neural progenitor cells in adults.

Neural Stem Cells in Adult Mammals are not Astrocytes

At the turn of the 21st century studies of the cells that resided in the adult mammalian subventricular zone (SVZ) characterized the neural stem cells (NSCs) as a subtype of astrocyte. Over the ensuing years, numerous studies have further characterized the properties of these NSCs and compared them to parenchymal astrocytes. Here we have evaluated the evidence collected to date to establish whether classifying the NSCs as astrocytes is appropriate and useful. We also performed a meta-analysis with 4 previously published datasets that used cell sorting and unbiased single-cell RNAseq to highlight the distinct gene expression profiles of adult murine NSCs and niche astrocytes. On the basis of our understanding of the properties and functions of astrocytes versus the properties and functions of NSCs, and from our comparative transcriptomic analyses we conclude that classifying the adult mammalian NSC as an astrocyte is potentially misleading. From our vantage point, it is more appropriate to refer to the cells in the adult mammalian SVZ that retain the capacity to produce new neurons and macroglia as NSCs without attaching the term "astrocyte-like."

A Baldwin interpretation of adult hippocampal neurogenesis: from functional relevance to physiopathology

Hippocampal adult neurogenesis has been associated to many cognitive, emotional, and behavioral functions and dysfunctions, and its status as a selected effect or an "appendix of the brain" has been debated. In this review, we propose to understand hippocampal neurogenesis as the process underlying the "Baldwin effect", a particular situation in evolution where fitness does not rely on the natural selection of genetic traits, but on "ontogenetic adaptation" to a changing environment. This supports the view that a strong distinction between developmental and adult hippocampal neurogenesis is made. We propose that their functions are the constitution and the lifelong adaptation, respectively, of a basic repertoire of cognitive and emotional behaviors. This lifelong adaptation occurs through new forms of binding, i.e., association or dissociation of more basic elements. This distinction further suggests that a difference is made between developmental vulnerability (or resilience), stemming from dysfunctional (or highly functional) developmental hippocampal neurogenesis, and adult vulnerability (or resilience), stemming from dysfunctional (or highly functional) adult hippocampal neurogenesis. According to this hypothesis, developmental and adult vulnerability are distinct risk factors for various mental disorders in adults. This framework suggests new avenues for research on hippocampal neurogenesis and its implication in mental disorders.

An assessment of the existence of adult neurogenesis in humans and value of its rodent models for neuropsychiatric diseases

In sub-mammalian vertebrates like fishes, amphibians, and reptiles, new neurons are produced during the entire lifespan. This capacity diminishes considerably in birds and even more in mammals where it persists only in the olfactory system and hippocampal dentate gyrus. Adult neurogenesis declines even more drastically in nonhuman primates and recent evidence shows that this is basically extinct in humans. Why should such seemingly useful capacity diminish during primate evolution? It has been proposed that this occurs because of the need to retain acquired complex knowledge in stable populations of neurons and their synaptic connections during many decades of human life. In this review, we will assess critically the claim of significant adult neurogenesis in humans and show how current evidence strongly indicates that humans lack this trait. In addition, we will discuss the allegation of many rodent studies that adult neurogenesis is involved in psychiatric diseases and that it is a potential mechanism for human neuron replacement and regeneration. We argue that these reports, which usually neglect significant structural and functional species-specific differences, mislead the general population into believing that there might be a cure for a variety of neuropsychiatric diseases as well as stroke and brain trauma by genesis of new neurons and their incorporation into existing synaptic circuitry.

The PSA-NCAM-Positive "Immature" Neurons: An Old Discovery Providing New Vistas on Brain Structural Plasticity

Studies on brain plasticity have undertaken different roads, tackling a wide range of biological processes: from small synaptic changes affecting the contacts among neurons at the very tip of their processes, to birth, differentiation, and integration of new neurons (adult neurogenesis). Stem cell-driven adult neurogenesis is an exception in the substantially static mammalian brain, yet, it has dominated the research in neurodevelopmental biology during the last thirty years. Studies of comparative neuroplasticity have revealed that neurogenic processes are reduced in large-brained mammals, including humans. On the other hand, large-brained mammals, with respect to rodents, host large populations of special "immature" neurons that are generated prenatally but express immature markers in adulthood. The history of these "immature" neurons started from studies on adhesion molecules carried out at the beginning of the nineties. The identity of these neurons as "stand by" cells "frozen" in a state of immaturity remained un-detected for long time, because of their ill-defined features and because clouded by research ef-forts focused on adult neurogenesis. In this review article, the history of these cells will be reconstructed, and a series of nuances and confounding factors that have hindered the distinction between newly generated and "immature" neurons will be addressed.

Brain Plasticity in Humans and Model Systems: Advances, Challenges, and Future Directions

Plasticity, and in particular, neurogenesis, is a promising target to treat and prevent a wide variety of diseases (e.g., epilepsy, stroke, dementia). There are different types of plasticity, which vary with age, brain region, and species. These observations stress the importance of defining plasticity along temporal and spatial dimensions. We review recent studies focused on brain plasticity across the lifespan and in different species. One main theme to emerge from this work is that plasticity declines with age but that we have yet to map these different forms of plasticity across species. As part of this effort, we discuss our recent progress aimed to identify corresponding ages across species, and how this information can be used to map temporal variation in plasticity from model systems to humans.

Characterization of substantia nigra neurogenesis in homeostasis and dopaminergic degeneration: beneficial effects of the microneurotrophin BNN-20

Background

Loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) underlines much of the pathology of Parkinson’s disease (PD), but the existence of an endogenous neurogenic system that could be targeted as a therapeutic strategy has been controversial. BNN-20 is a synthetic, BDNF-mimicking, microneurotrophin that we previously showed to exhibit a pleiotropic neuroprotective effect on the dopaminergic neurons of the SNpc in the “weaver” mouse model of PD. Here, we assessed its potential effects on neurogenesis.

Methods

We quantified total numbers of dopaminergic neurons in the SNpc of wild-type and “weaver” mice, with or without administration of BNN-20, and we employed BrdU labelling and intracerebroventricular injections of DiI to evaluate the existence of dopaminergic neurogenesis in the SNpc and to assess the origin of newborn dopaminergic neurons. The in vivo experiments were complemented by in vitro proliferation/differentiation assays of adult neural stem cells (NSCs) isolated from the substantia nigra and the subependymal zone (SEZ) stem cell niche to further characterize the effects of BNN-20.

Results

Our analysis revealed the existence of a low-rate turnover of dopaminergic neurons in the normal SNpc and showed, using three independent lines of experiments (stereologic cell counts, BrdU and DiI tracing), that the administration of BNN-20 leads to increased neurogenesis in the SNpc and to partial reversal of dopaminergic cell loss. The newly born dopaminergic neurons, that are partially originated from the SEZ, follow the typical nigral maturation pathway, expressing the transcription factor FoxA2. Importantly, the pro-cytogenic effects of BNN-20 were very strong in the SNpc, but were absent in other brain areas such as the cortex or the stem cell niche of the hippocampus. Moreover, although the in vitro assays showed that BNN-20 enhances the differentiation of NSCs towards glia and neurons, its in vivo administration stimulated only neurogenesis.

Conclusions

Our results demonstrate the existence of a neurogenic system in the SNpc that can be manipulated in order to regenerate the depleted dopaminergic cell population in the “weaver” PD mouse model. Microneurotrophin BNN-20 emerges as an excellent candidate for future PD cell replacement therapies, due to its area-specific, pro-neurogenic effects.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02398-3.

Incorporation of 5-Bromo-2'-deoxyuridine into DNA and Proliferative Behavior of Cerebellar Neuroblasts: All That Glitters Is Not Gold

The synthetic halogenated pyrimidine analog, 5-bromo-2'-deoxyuridine (BrdU), is a marker of DNA synthesis. This exogenous nucleoside has generated important insights into the cellular mechanisms of the central nervous system development in a variety of animals including insects, birds, and mammals. Despite this, the detrimental effects of the incorporation of BrdU into DNA on proliferation and viability of different types of cells has been frequently neglected. This review will summarize and present the effects of a pulse of BrdU, at doses ranging from 25 to 300 µg/g, or repeated injections. The latter, following the method of the progressively delayed labeling comprehensive procedure. The prenatal and perinatal development of the cerebellum are studied. These current data have implications for the interpretation of the results obtained by this marker as an index of the generation, migration, and settled pattern of neurons in the developing central nervous system. Caution should be exercised when interpreting the results obtained using BrdU. This is particularly important when high or repeated doses of this agent are injected. I hope that this review sheds light on the effects of this toxic maker. It may be used as a reference for toxicologists and neurobiologists given the broad use of 5-bromo-2'-deoxyuridine to label dividing cells.

Formation and integration of new neurons in the adult hippocampus

Neural stem cells (NSCs) generate new neurons throughout life in the mammalian brain. Adult-born neurons shape brain function, and endogenous NSCs could potentially be harnessed for brain repair. In this Review, focused on hippocampal neurogenesis in rodents, we highlight recent advances in the field based on novel technologies (including single-cell RNA sequencing, intravital imaging and functional observation of newborn cells in behaving mice) and characterize the distinct developmental steps from stem cell activation to the integration of newborn neurons into pre-existing circuits. Further, we review current knowledge of how levels of neurogenesis are regulated, discuss findings regarding survival and maturation of adult-born cells and describe how newborn neurons affect brain function. The evidence arguing for (and against) lifelong neurogenesis in the human hippocampus is briefly summarized. Finally, we provide an outlook of what is needed to improve our understanding of the mechanisms and functional consequences of adult neurogenesis and how the field may move towards more translational relevance in the context of acute and chronic neural injury and stem cell-based brain repair.

The "missing heritability"-Problem in psychiatry: Is the interaction of genetics, epigenetics and transposable elements a potential solution?

Psychiatric disorders exhibit an enormous burden on the health care systems worldwide accounting for around one-third of years lost due to disability among adults. Their etiology is largely unknown and diagnostic classification is based on symptomatology and course of illness and not on objective biomarkers. Most psychiatric disorders are moderately to highly heritable. However, it is still unknown what mechanisms may explain the discrepancy between heritability estimates and the present data from genetic analysis. In addition to genetic differences also epigenetic modifications are considered as potentially relevant in the transfer of susceptibility to psychiatric diseases. Though, whether or not epigenetic alterations can be inherited for many generations is highly controversial. In the present article, we will critically summarize both the genetic findings and the results from epigenetic analyses, including also those of noncoding RNAs. We will argue that one possible solution to the "missing heritability" problem in psychiatry is a potential role of retrotransposons, the exploration of which is presently only in its beginnings.

Benefits of animal models to understand the pathophysiology of depressive disorders

Major depressive disorder (MDD) is a potentially life-threatening mental disorder imposing severe social and economic burden worldwide. Despite the existence of effective antidepressant treatment strategies the exact pathophysiology of the disease is still unknown. Large number of animal models of MDD have been developed over the years, but all of them suffer from significant shortcomings. Despite their limitations these models have been extensively used in academic research and drug development. The aim of this review is to highlight the benefits of animal models of MDD. We focus here on recent experimental data where animal models were used to examine current theories of this complex disease. We argue, that despite their evident imperfections, these models provide invaluable help to understand cellular and molecular mechanisms contributing to the development of MDD. Furthermore, animal models are utilized in research to find clinically useful biomarkers. We discuss recent neuroimaging and microRNA studies since these investigations yielded promising candidates for biomarkers. Finally, we briefly summarize recent progresses in drug development, i.e. the FDA approval of two novel antidepressant drugs: S-ketamine and brexanolone (allopregnanolone). Deeper understanding of the exact molecular and cellular mechanisms of action responsible for the antidepressant efficacy of these rapid acting drugs could aid us to design further compounds with similar effectiveness, but less side effects. Animal studies are likely to provide valuable help in this endeavor.

Saikosaponin-d impedes hippocampal neurogenesis and causes cognitive deficits by inhibiting the survival of neural stem/progenitor cells via neurotrophin receptor signaling in mice

Neural stem/progenitor cells (NPCs) are multipotent stem cells in the central nervous system. Damage to NPCs has been demonstrated to cause adverse effects on neurogenesis and to contribute to neurological diseases. Our previous research suggested that saikosaponin-d (SSd), a cytostatic drug belonging to the bioactive triterpenoid saponins, exhibited neurotoxicity by inhibiting hippocampal neurogenesis, but the underlying mechanism remained elusive. This study was performed to clarify the role of SSd in cognitive function and the mechanism by which SSd induced damage to hippocampal neurogenesis and NPCs. Our results indicated that SSd caused hippocampus-dependent cognitive deficits and inhibited hippocampal neurogenesis by reducing the numbers of newborn neurons in mice. RNA sequencing analysis revealed that SSd-induced neurotoxicity in the hippocampus involved neurotrophin receptor-interacting MAGE (NRAGE)/neurotrophin receptor interacting factor (NRIF)/p75NTR -associated cell death executor (NADE) cell signaling activated by the p75 neurotrophin receptor (p75NTR ). Mechanistic studies showed that a short hairpin RNA targeting p75NTR intracellular domain reversed SSd-increased NRAGE/NRIF/NADE signaling and the c-Jun N-terminal kinase/caspase apoptotic pathway, subsequently contributing to the survival of NPCs, as well as cell proliferation and differentiation. The addition of recombinant brain-derived neurotrophic factor (BDNF) ameliorated the SSd-induced inhibition of BDNF/Tyrosine kinase receptor B (TrkB) neurotrophic signaling, but did not affect SSd-activated pro-BDNF/p75NTR signaling. Moreover, the SSd-induced elevation of cytosolic Ca2+ concentration was responsible for damage to NPCs. The extracellular Ca2+ chelator ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA), rather than the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) (BAPTA/AM), attenuated SSd-induced cytosolic Ca2+ dysfunction and SSd-disordered TrkB/p75NTR signaling. Overall, this study demonstrated a new mechanism for the neurotoxic effect of SSd, which has emerging implications for pharmacological research of SSd and provides a better understanding of neurotoxicity induced by cytostatic drugs.

N,N-dimethyltryptamine compound found in the hallucinogenic tea ayahuasca, regulates adult neurogenesis in vitro and in vivo

N,N-dimethyltryptamine (DMT) is a component of the ayahuasca brew traditionally used for ritual and therapeutic purposes across several South American countries. Here, we have examined, in vitro and vivo, the potential neurogenic effect of DMT. Our results demonstrate that DMT administration activates the main adult neurogenic niche, the subgranular zone of the dentate gyrus of the hippocampus, promoting newly generated neurons in the granular zone. Moreover, these mice performed better, compared to control non-treated animals, in memory tests, which suggest a functional relevance for the DMT-induced new production of neurons in the hippocampus. Interestingly, the neurogenic effect of DMT appears to involve signaling via sigma-1 receptor (S1R) activation since S1R antagonist blocked the neurogenic effect. Taken together, our results demonstrate that DMT treatment activates the subgranular neurogenic niche regulating the proliferation of neural stem cells, the migration of neuroblasts, and promoting the generation of new neurons in the hippocampus, therefore enhancing adult neurogenesis and improving spatial learning and memory tasks.

Inducible and Conditional Stimulation of Adult Hippocampal Neurogenesis Rescues Cadmium-Induced Impairments of Adult Hippocampal Neurogenesis and Hippocampus-Dependent Memory in Mice

Cadmium (Cd) is a heavy metal and an environmental pollutant. However, the full spectrum of its neurotoxicity and the underlying mechanisms are not completely understood. Our previous studies demonstrated that Cd exposure impairs adult hippocampal neurogenesis and hippocampus-dependent memory in mice. This study aims to determine if these adverse effects of Cd exposure can be mitigated by genetically and conditionally enhancing adult neurogenesis. To address this issue, we utilized the transgenic constitutive active MEK5 (caMEK5) mouse strain we previously developed and characterized. This mouse strain enables us to genetically and conditionally activate adult neurogenesis by administering tamoxifen to induce expression of a caMEK5 in adult neural stem/progenitor cells, which stimulates adult neurogenesis through activation of the endogenous extracellular signal-regulated kinase 5 mitogen-activated protein kinase pathway. The caMEK5 mice were exposed to 0.6 mg/l Cd through drinking water for 38 weeks. Once impairment of memory was confirmed, tamoxifen was administered to induce caMEK5 expression and to activate adult neurogenesis. Behavior tests were conducted at various time points to monitor hippocampus-dependent memory. Upon completion of the behavior tests, brain tissues were collected for cellular studies of adult hippocampal neurogenesis. We report here that Cd impaired hippocampus-dependent spatial memory and contextual fear memory in mice. These deficits were rescued by the tamoxifen induction of caMEK5 expression. Furthermore, Cd inhibition of adult hippocampal neurogenesis was also reversed. This rescue experiment provides strong evidence for a direct link between Cd-induced impairments of adult hippocampal neurogenesis and hippocampus-dependent memory.

Links Between the Neurobiology of Oxytocin and Human Musicality

The human species possesses two complementary, yet distinct, universal communication systems—language and music. Functional imaging studies have revealed that some core elements of these two systems are processed in closely related brain regions, but there are also clear differences in brain circuitry that likely underlie differences in functionality. Music affects many aspects of human behavior, especially in encouraging prosocial interactions and promoting trust and cooperation within groups of culturally compatible but not necessarily genetically related individuals. Music, presumably via its impact on the limbic system, is also rewarding and motivating, and music can facilitate aspects of learning and memory. In this review these special characteristics of music are considered in light of recent research on the neuroscience of the peptide oxytocin, a hormone that has both peripheral and central actions, that plays a role in many complex human behaviors, and whose expression has recently been reported to be affected by music-related activities. I will first briefly discuss what is currently known about the peptide’s physiological actions on neurons and its interactions with other neuromodulator systems, then summarize recent advances in our knowledge of the distribution of oxytocin and its receptor (OXTR) in the human brain. Next, the complex links between oxytocin and various social behaviors in humans are considered. First, how endogenous oxytocin levels relate to individual personality traits, and then how exogenous, intranasal application of oxytocin affects behaviors such as trust, empathy, reciprocity, group conformity, anxiety, and overall social decision making under different environmental conditions. It is argued that many of these characteristics of oxytocin biology closely mirror the diverse effects that music has on human cognition and emotion, providing a link to the important role music has played throughout human evolutionary history and helping to explain why music remains a special prosocial human asset. Finally, it is suggested that there is a potential synergy in combining oxytocin- and music-based strategies to improve general health and aid in the treatment of various neurological dysfunctions.

Understanding the Real State of Human Adult Hippocampal Neurogenesis From Studies of Rodents and Non-human Primates

The concept of adult hippocampal neurogenesis (AHN) has been widely accepted, and a large number of studies have been performed in rodents using modern experimental techniques, which have clarified the nature and developmental processes of adult neural stem/progenitor cells, the functions of AHN, such as memory and learning, and its association with neural diseases. However, a fundamental problem is that it remains unclear as to what extent AHN actually occurs in humans. The answer to this is indispensable when physiological and pathological functions of human AHN are deduced from studies of rodent AHN, but there are controversial data on the extent of human AHN. In this review, studies on AHN performed in rodents and humans will be briefly reviewed, followed by a discussion of the studies in non-human primates. Then, how data of rodent and non-human primate AHN should be applied for understanding human AHN will be discussed.

Closing the gap from transcription to the structural connectome enhances the study of connections in the human brain

The brain is composed of a complex web of networks but we have yet to map the structural connections of the human brain in detail. Diffusion MR imaging is a high-throughput method that relies on the principle of diffusion to reconstruct tracts (ie, pathways) across the brain. Although diffusion MR tractography is an exciting method to explore the structural connectivity of the brain in development and across species, the tractography has at times led to questionable interpretations. There are at present few if any alternative methods to trace structural pathways in the human brain. Given these limitations and the potential of diffusion MR imaging to map the human connectome, it is imperative that we develop new approaches to validate neuroimaging techniques. I discuss our recent studies integrating neuroimaging with transcriptional and anatomical variation across humans and other species over the course of development and in adulthood. Developing a novel framework to harness the potential of diffusion MR tractography provides new and exciting opportunities to study the evolution of developmental mechanisms generating variation in connections and bridge the gap between model systems to humans.

Cellular Response of Ventricular-Subventricular Neural Progenitor/Stem Cells to Neonatal Hypoxic-Ischemic Brain Injury and Their Enhanced Neurogenesis

PURPOSE

To elucidate the brain's intrinsic response to injury, we tracked the response of neural stem/progenitor cells (NSPCs) located in ventricular-subventricular zone (V-SVZ) to hypoxic-ischemic brain injury (HI). We also evaluated whether transduction of V-SVZ NSPCs with neurogenic factor NeuroD1 could enhance their neurogenesis in HI.

MATERIALS AND METHODS

Unilateral HI was induced in ICR neonatal mice. To label proliferative V-SVZ NSPCs in response to HI, bromodeoxyuridine (BrdU) and retroviral particles encoding LacZ or NeuroD1/GFP were injected. The cellular responses of NSPCs were analyzed by immunohistochemistry.

RESULTS

Unilateral HI increased the number of BrdU+ newly-born cells in the V-SVZ ipsilateral to the lesion while injury reduced the number of newly-born cells reaching the ipsilateral olfactory bulb, which is the programmed destination of migratory V-SVZ NSPCs in the intact brain. These newly-born cells were directed from this pathway towards the lesions. HI significantly increased the number of newly-born cells in the cortex and striatum by the altered migration of V-SVZ cells. Many of these newly-born cells differentiated into active neurons and glia. LacZ-expressing V-SVZ NSPCs also showed extensive migration towards the non-neurogenic regions ipsilateral to the lesion, and expressed the neuronal marker NeuN. NeuroD1+/GFP+ V-SVZ NSPCs almost differentiated into neurons in the peri-infarct regions.

CONCLUSION

HI promotes the establishment of a substantial number of new neurons in non-neurogenic regions, suggesting intrinsic repair mechanisms of the brain, by controlling the behavior of endogenous NSPCs. The activation of NeuroD1 expression may improve the therapeutic potential of endogenous NSPCs by increasing their neuronal differentiation in HI.

Dynamic Changes in the Neurogenic Potential in the Ventricular-Subventricular Zone of Common Marmoset during Postnatal Brain Development

Even after birth, neuronal production continues in the ventricular-subventricular zone (V-SVZ) and hippocampus in many mammals. The immature new neurons ("neuroblasts") migrate and then mature at their final destination. In humans, neuroblast production and migration toward the neocortex and the olfactory bulb (OB) occur actively only for a few months after birth and then sharply decline with age. However, the precise spatiotemporal profiles and fates of postnatally born neurons remain unclear due to methodological limitations. We previously found that common marmosets, small nonhuman primates, share many features of V-SVZ organization with humans. Here, using marmosets injected with thymidine analogue(s) during various postnatal periods, we demonstrated spatiotemporal changes in neurogenesis during development. V-SVZ progenitor proliferation and neuroblast migration toward the OB and neocortex sharply decreased by 4 months, most strikingly in a V-SVZ subregion from which neuroblasts migrated toward the neocortex. Postnatally born neurons matured within a few months in the OB and hippocampus but remained immature until 6 months in the neocortex. While neurogenic activity was sustained for a month after birth, the distribution and/or differentiation diversity was more restricted in 1-month-born cells than in the neonatal-born population. These findings shed light on distinctive features of postnatal neurogenesis in primates.

Brain Structural Plasticity: From Adult Neurogenesis to Immature Neurons

Brain structural plasticity is an extraordinary tool that allows the mature brain to adapt to environmental changes, to learn, to repair itself after lesions or disease, and to slow aging. A long history of neuroscience research led to fascinating discoveries of different types of plasticity, involving changes in the genetically determined structure of nervous tissue, up to the ultimate dream of neuronal replacement: a stem cell-driven “adult neurogenesis” (AN). Yet, this road does not seem a straight one, since mutable dogmas, conflicting results and conflicting interpretations continue to warm the field. As a result, after more than 10,000 papers published on AN, we still do not know its time course, rate or features with respect to other kinds of structural plasticity in our brain. The solution does not appear to be behind the next curve, as differences among mammals reveal a very complex landscape that cannot be easily understood from rodents models alone. By considering evolutionary aspects, some pitfalls in the interpretation of cell markers, and a novel population of undifferentiated cells that are not newly generated [immature neurons (INs)], we address some conflicting results and controversies in order to find the right road forward. We suggest that considering plasticity in a comparative framework might help assemble the evolutionary, anatomical and functional pieces of a very complex biological process with extraordinary translational potential.

Neuron-glia interaction through Serotonin-BDNF-NGFR axis enables regenerative neurogenesis in Alzheimer's model of adult zebrafish brain

It was recently suggested that supplying the brain with new neurons could counteract Alzheimer’s disease (AD). This provocative idea requires further testing in experimental models in which the molecular basis of disease-induced neuronal regeneration could be investigated. We previously found that zebrafish stimulates neural stem cell (NSC) plasticity and neurogenesis in AD and could help to understand the mechanisms to be harnessed for developing new neurons in diseased mammalian brains. Here, by performing single-cell transcriptomics, we found that amyloid toxicity-induced interleukin-4 (IL4) promotes NSC proliferation and neurogenesis by suppressing the tryptophan metabolism and reducing the production of serotonin. NSC proliferation was suppressed by serotonin via down-regulation of brain-derived neurotrophic factor (BDNF)-expression in serotonin-responsive periventricular neurons. BDNF enhances NSC plasticity and neurogenesis via nerve growth factor receptor A (NGFRA)/ nuclear factor 'kappa-light-chain-enhancer' of activated B-cells (NFkB) signaling in zebrafish but not in rodents. Collectively, our results suggest a complex neuron-glia interaction that regulates regenerative neurogenesis after AD conditions in zebrafish.

Can regeneration of lost neurons counteract neurodegenerative disease? This study shows that serotonergic neurons alter neural stem cell proliferation and neurogenesis via a complex neuron-glia interaction involving interleukin-4, BDNF and NGF receptor in a zebrafish model of Alzheimer's disease.

Neuroimmune and epigenetic mechanisms underlying persistent loss of hippocampal neurogenesis following adolescent intermittent ethanol exposure

Alcohol abuse and binge drinking are common during adolescence - a maturational period characterized by heightened hippocampal neuroplasticity and neurogenesis. Preclinical rodent models of adolescent binge drinking (i.e., adolescent intermittent ethanol [AIE]) find unique vulnerability of adolescent hippocampal neurogenesis with reductions persisting into adulthood after ethanol cessation. Recent discoveries implicate increased neuroimmune signaling and decreased neurotrophic support through epigenetic mechanisms in the persistent AIE-induced loss of neurogenesis. Importantly, interventions aimed at rectifying the increased neuroimmune signaling and neurotrophic-epigenetic modifications through physical activity, anti-inflammatory drugs, and histone deacetylase inhibitors protect and recover the loss of neurogenesis and cognitive deficits. The mechanisms underlying the persistent AIE-induced loss of adult hippocampal neurogenesis could contribute to broader neurodegeneration, loss of hippocampal neuroplasticity, and cognitive dysfunction.

Perspective: Of Mice and Men - How Widespread Is Adult Neurogenesis?

These are exciting times for research on adult hippocampal neurogenesis (AHN). Debate and controversy regarding the existence of generation of new neurons in the adult, and even diseased human brain flourishes as articles against and in favor accumulate. Adult neurogenesis in the human brain is a phenomenon that does not share the qualities of quantum mechanics. The scientific community should agree that human AHN exists or does not, but not both at the same time. In this commentary, we discuss the latest research articles about hAHN and what their findings imply for the neurogenesis field.