Integration Of Newborn Neurons Research Articles

Chronic chemogenetic activation of hippocampal progenitors enhances adult neurogenesis and modulates anxiety-like behavior and fear extinction learning

Adult hippocampal neurogenesis is a lifelong process that involves the integration of newborn neurons into the hippocampal network, and plays a role in cognitive function and the modulation of mood-related behavior. Here, we sought to address the impact of chemogenetic activation of adult hippocampal progenitors on distinct stages of progenitor development, including quiescent stem cell activation, progenitor turnover, differentiation and morphological maturation. We find that hM3Dq-DREADD-mediated activation of nestin-positive adult hippocampal progenitors recruits quiescent stem cells, enhances progenitor proliferation, increases doublecortin-positive newborn neuron number, accompanied by an acceleration of differentiation and morphological maturation, associated with increased dendritic complexity. Behavioral analysis indicated anxiolytic behavioral responses in transgenic mice subjected to chemogenetic activation of adult hippocampal progenitors at timepoints when newborn neurons are predicted to integrate into the mature hippocampal network. Furthermore, we noted an enhanced fear memory extinction on a contextual fear memory learning task in transgenic mice subjected to chemogenetic activation of adult hippocampal progenitors. Our findings indicate that hM3Dq-DREAD-mediated chemogenetic activation of adult hippocampal progenitors impacts distinct aspects of hippocampal neurogenesis, associated with the regulation of anxiety-like behavior and fear memory extinction.

Differential adhesion during development establishes individual neural stem cell niches and shapes adult behaviour in Drosophila.

Neural stem cells (NSCs) reside in a defined cellular microenvironment, the niche, which supports the generation and integration of newborn neurons. The mechanisms building a sophisticated niche structure around NSCs and their functional relevance for neurogenesis are yet to be understood. In the Drosophila larval brain, the cortex glia (CG) encase individual NSC lineages in membranous chambers, organising the stem cell population and newborn neurons into a stereotypic structure. We first found that CG wrap around lineage-related cells regardless of their identity, showing that lineage information builds CG architecture. We then discovered that a mechanism of temporally controlled differential adhesion using conserved complexes supports the individual encasing of NSC lineages. An intralineage adhesion through homophilic Neuroglian interactions provides strong binding between cells of a same lineage, while a weaker interaction through Neurexin-IV and Wrapper exists between NSC lineages and CG. Loss of Neuroglian results in NSC lineages clumped together and in an altered CG network, while loss of Neurexin-IV/Wrapper generates larger yet defined CG chamber grouping several lineages together. Axonal projections of newborn neurons are also altered in these conditions. Further, we link the loss of these 2 adhesion complexes specifically during development to locomotor hyperactivity in the resulting adults. Altogether, our findings identify a belt of adhesions building a neurogenic niche at the scale of individual stem cell and provide the proof of concept that niche properties during development shape adult behaviour.

Ipsilesional Hippocampal GABA Is Elevated and Correlates With Cognitive Impairment and Maladaptive Neurogenesis After Cortical Stroke in Mice.

Cognitive dysfunction is a frequent stroke sequela, but its pathogenesis and treatment remain unresolved. Involvement of aberrant hippocampal neurogenesis and maladaptive circuitry remodeling has been proposed, but their mechanisms are unknown. Our aim was to evaluate potential underlying molecular/cellular events implicated. Stroke was induced by permanent occlusion of the middle cerebral artery occlusion in 2-month-old C57BL/6 male mice. Hippocampal metabolites/neurotransmitters were analyzed longitudinally by in vivo magnetic resonance spectroscopy. Cognitive function was evaluated with the contextual fear conditioning test. Microglia, astrocytes, neuroblasts, interneurons, γ-aminobutyric acid (GABA), and c-fos were analyzed by immunofluorescence. Approximately 50% of mice exhibited progressive post-middle cerebral artery occlusion cognitive impairment. Notably, immature hippocampal neurons in the impaired group displayed more severe aberrant phenotypes than those from the nonimpaired group. Using magnetic resonance spectroscopy, significant bilateral changes in hippocampal metabolites, such as myo-inositol or N-acetylaspartic acid, were found that correlated, respectively, with numbers of glia and immature neuroblasts in the ischemic group. Importantly, some metabolites were specifically altered in the ipsilateral hippocampus suggesting its involvement in aberrant hippocampal neurogenesis and remodeling processes. Specifically, middle cerebral artery occlusion animals with higher hippocampal GABA levels displayed worse cognitive outcome. Implication of GABA in this setting was supported by the amelioration of ischemia-induced memory deficits and aberrant hippocampal neurogenesis after blocking pharmacologically GABAergic neurotransmission, an intervention which was ineffective when neurogenesis was inhibited. These data suggest that GABA exerts its detrimental effect, at least partly, by affecting morphology and integration of newborn neurons into the hippocampal circuits. Hippocampal GABAergic neurotransmission could be considered a novel diagnostic and therapeutic target for poststroke cognitive impairment.

Involvement of LIN28A in Wnt-dependent regulation of hippocampal neurogenesis in the aging brain.

SummaryHippocampal neurogenesis declines with aging. Wnt ligands and antagonists within the hippocampal neurogenic niche regulate the proliferation of neural progenitor cells and the development of new neurons, and the changes of their levels in the niche mediate aging-associated decline of neurogenesis. We found that RNA-binding protein LIN28A remained existent in neural progenitor cells and granule neurons in the adult hippocampus and that it decreased with aging. Lin28a knockout inhibited the responsiveness of neural progenitor cells to niche Wnt agonists and reduced neurogenesis, thus impairing pattern separation. Overexpression of Lin28a increased the proliferation of neural progenitor cells, promoted the functional integration of newborn neurons, restored neurogenesis in Wnt-deficient dentate gyrus, and rescued the impaired pattern separation in aging mice. Our data suggest that LIN28A regulates adult hippocampal neurogenesis as an intracellular mechanism by responding to niche Wnt signals, and its decrease is involved in aging-associated decline of hippocampal neurogenesis and related cognitive functions.

Astrocytes Mediate Cholinergic Regulation of Adult Hippocampal Neurogenesis and Memory Through M1 Muscarinic Receptor

BackgroundIn the neurogenic niches of the adult hippocampus, new functional neurons are continuously generated throughout life, and generation of these neurons has been implicated in learning and memory. Astrocytes, as components of the neurogenic niches, are critical in the regulation of adult hippocampal neurogenesis (AHN). However, little is known about how astrocytes receive and respond to extrinsic cues to regulate AHN. MethodsBy using a transgenic strategy to conditionally delete astrocytic CRHM1 in mice and AAV (adeno-associated virus)–mediated overexpression of astrocytic CHRM1 specifically in the hippocampal dentate gyrus, we systematically investigated the role of astrocytic CHRM1 in the regulation of AHN and the underlying mechanisms using the combined approaches of immunohistochemistry, retrovirus labeling, electrophysiology, primary astrocyte cultures, immunoblotting, and behavioral assays. ResultsWe report that genetic ablation of CHRM1 in astrocytes led to defects in neural stem cell survival, neuronal differentiation, and maturation and integration of newborn neurons in the dentate gyrus. Astrocytic CHRM1-mediated modulation of AHN was mediated by BDNF (brain-derived neurotrophic factor) signaling. Furthermore, CHRM1 ablation in astrocytes impaired contextual fear memory. These impairments in both AHN and memory were rescued by overexpression of astrocytic CHRM1 in the dentate gyrus. ConclusionsOur findings reveal a critical role for astrocytes in mediating cholinergic regulation of AHN and memory through CHRM1.

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.

Intracellular A53T Mutant α-Synuclein Impairs Adult Hippocampal Newborn Neuron Integration.

Dendritic dysfunction is an early event in α-synuclein (α-syn) mediated neurodegeneration. Altered postsynaptic potential and loss of dendritic spines have been observed in different in vitro and in vivo models of synucleinopathies. The integration of newborn neurons into the hippocampus offers the possibility to study dendrite and spine formation in an adult environment. Specifically, survival of hippocampal adult newborn neurons is regulated by synaptic input and was reduced in a mouse model transgenic for human A53T mutant α-syn. We thus hypothesized that dendritic integration of newborn neurons is impaired in the adult hippocampus of A53T mice. We analyzed dendritic morphology of adult hippocampal neurons 1 month after retroviral labeling. Dendrite length was unchanged in the dentate gyrus of A53T transgenic mice. However, spine density and mushroom spine density of newborn neurons were severely decreased. In this mouse model, transgenic α-syn was expressed both within newborn neurons and within their environment. To specifically determine the cell autonomous effects, we analyzed cell-intrinsic overexpression of A53T α-syn using a retrovirus. Since A53T α-syn overexpressing newborn neurons exhibited decreased spine density 1 month after labeling, we conclude that cell-intrinsic A53T α-syn impairs postsynaptic integration of adult hippocampal newborn neurons. Our findings further support the role of postsynaptic degeneration as an early feature in synucleinopathies and provide a model system to study underlying mechanisms.

Molecular and Cellular Mechanisms Regulating Quiescence and Division of Hippocampal Stem Cells

Abstract—Neurogenesis in the dentate gyrus of the hippocampus occurs throughout the life of mammals, including humans. A common view is that integration of newborn neurons provides a high level of plasticity in the hippocampal neuronal network. This is important for cognitive brain function, memory formation, and emotions. Neurogenesis is supported by the pool of stem cells located in the subgranular zone of the dentate gyrus. These cells spend most of their life in a state of mitotic quiescence. During aging, the reservoir of stem cells gradually decreases due to division and irreversible differentiation into astrocytes. The kinetics of spending the pool of stem cells is determined by the balance between quiescence and division. The question on the molecular mechanisms regulating this balance is one of the key issues in the biology of brain stem cells. This is important not only in order to understand the fundamental principles of regulation of neurogenesis, but also in developing new strategies for using the internal reservoir of stem cells to ameliorate hippocampal plasticity deficit in various brain pathologies and aging. To date a lot of new and interesting data have been accumulated on multiple mechanisms involved in the regulation of stem cell quiescence and division. In this review, we overview current knowledge on the signal molecules and cellular receptors participating in the maintenance of the quiescent state of stem cells, the mechanisms of the shift from the quiescent state to proliferation at the level of gene transcription, and the roles of components of the stem cell niche, such as neuronal connections, macro- and microglia, and the extracellular matrix, in the regulation of the balance between quiescence and division of stem cells and in the maintenance and exhaustion of their pool.

Hippocampal neural stem cells and microglia response to experimental inflammatory bowel disease (IBD).

Inflammatory bowel disease (IBD), including Crohn's disease (CD) and ulcerative colitis (UC), is a disease associated with dysbiosis, resulting in compromised intestinal epithelial barrier and chronic mucosal inflammation. Patients with IBD present with increased incidence of psychiatric disorders and cognitive impairment. Hippocampus is a brain region where adult neurogenesis occurs with functional implications in mood control and cognition. Using a well-established model of experimental colitis based on the administration of dextran sodium sulfate (DSS) in the drinking water, we sought to characterize the short and long-term effects of colitis on neurogenesis and glia responses in the hippocampus. We show that acute DSS colitis enhanced neurogenesis but with deficits in cell cycle kinetics of proliferating progenitors in the hippocampus. Chronic DSS colitis was characterized by normal levels of neurogenesis but with deficits in the migration and integration of newborn neurons in the functional circuitry of the DG. Notably, we found that acute DSS colitis-induced enhanced infiltration of the hippocampus with macrophages and inflammatory myeloid cells from the periphery, along with elevated frequencies of inflammatory M1-like microglia and increased release of pro-inflammatory cytokines. In contrast, increased percentages of tissue-repairing M2-like microglia, along with elevated levels of the anti-inflammatory cytokine, IL-10 were observed in the hippocampus during chronic DSS colitis. These findings uncover key effects of acute and chronic experimental colitis on adult hippocampal neurogenesis and innate immune cell responses, highlighting the potential mechanisms underlying cognitive and mood dysfunction in patients with IBD.

Neurodegeneration and Neuro-Regeneration-Alzheimer's Disease and Stem Cell Therapy.

Aging causes many changes in the human body, and is a high risk for various diseases. Dementia, a common age-related disease, is a clinical disorder triggered by neurodegeneration. Brain damage caused by neuronal death leads to cognitive decline, memory loss, learning inabilities and mood changes. Numerous disease conditions may cause dementia; however, the most common one is Alzheimer’s disease (AD), a futile and yet untreatable illness. Adult neurogenesis carries the potential of brain self-repair by an endogenous formation of newly-born neurons in the adult brain; however it also declines with age. Strategies to improve the symptoms of aging and age-related diseases have included different means to stimulate neurogenesis, both pharmacologically and naturally. Finally, the regulatory mechanisms of stem cells neurogenesis or a functional integration of newborn neurons have been explored to provide the basis for grafted stem cell therapy. This review aims to provide an overview of AD pathology of different neural and glial cell types and summarizes current strategies of experimental stem cell treatments and their putative future use in clinical settings.

Neurogenesis and antidepressant action.

A theoretical framework is proposed to gain insight into the pathogenesis of major depressive disorder (MDD). Despite being a relatively weak argument, the neurogenesis theory is suggested to compensate for the limitations of the monoamine theory. In the adult hippocampus, neurogenesis is functionally related to regulation of the hypothalamic-pituitary-adrenal (HPA) axis, inflammatory processes, cognitive functions and other aspects that contribute to etiological factors that lead to MDD and promote recovery from MDD. Despite a lack of investigation into neurogenesis and antidepressant action, it is proposed that chronic administration of antidepressant(s) can induce the recruitment and integration of newborn neurons into the dentate gyrus and, ultimately, lead to the remission of MDD. The extant body of literature indicates that the suppression of neurogenesis per se may be associated with an impaired response to antidepressant treatment rather than with the induction of depressive-like behaviors. Moreover, recent studies have shown that increasing the survival rate and incorporation of new neurons can alleviate depressive-like behaviors and promote stress resilience. According to the neurogenic reserve hypothesis, hippocampal neurogenesis supports specific cortical functions, including executive functions, pattern separation and contextual information processing, control over the HPA axis and behavioral coping mechanisms in response to stressful situations. Therefore, hippocampal neurogenesis may be a promising biological indicator of stress resilience and antidepressant response in patients with MDD.

Functional Integration of Newborn Neurons in the Zebrafish Optic Tectum.

Neurogenesis persists during adulthood in restricted parts of the vertebrate brain. In the optic tectum (OT) of the zebrafish larva, newborn neurons are continuously added and contribute to visual information processing. Recent studies have started to describe the functional development and fate of newborn neurons in the OT. Like the mammalian brain, newborn neurons in the OT require sensory inputs for their integration into local networks and survival. Recent findings suggest that the functional development of newborn neurons requires both activity-dependent and hard-wired mechanisms for proper circuit integration. Here, we review these findings and argue that the study of neurogenesis in non-mammalian species will help elucidate the general mechanisms of circuit assembly following neurogenesis.

A role for the orphan nuclear receptor TLX in the interaction between neural precursor cells and microglia.

Microglia are an essential component of the neurogenic niche in the adult hippocampus and are involved in the control of neural precursor cell (NPC) proliferation, differentiation and the survival and integration of newborn neurons in hippocampal circuitry. Microglial and neuronal cross-talk is mediated in part by the chemokine fractalkine/chemokine (C-X3-C motif) ligand 1 (CX3CL1) released from neurons, and its receptor CX3C chemokine receptor 1 (CX3CR1) which is expressed on microglia. A disruption in this pathway has been associated with impaired neurogenesis yet the specific molecular mechanisms by which this interaction occurs remain unclear. The orphan nuclear receptor TLX (Nr2e1; homologue of the Drosophila tailless gene) is a key regulator of hippocampal neurogenesis, and we have shown that in its absence microglia exhibit a pro-inflammatory activation phenotype. However, it is unclear whether a disturbance in CX3CL1/CX3CR1 communication mediates an impairment in TLX-related pathways which may have subsequent effects on neurogenesis. To this end, we assessed miRNA expression of up- and down-stream signalling molecules of TLX in the hippocampus of mice lacking CX3CR1. Our results demonstrate that a lack of CX3CR1 is associated with altered expression of TLX and its downstream targets in the hippocampus without significantly affecting upstream regulators of TLX. Thus, TLX may be a potential participant in neural stem cell (NSC)–microglial cross-talk and may be an important target in understanding inflammatory-associated impairments in neurogenesis.

Expression of neurogenic markers in Alzheimer's disease: a systematic review and metatranscriptional analysis

Alzheimer's disease (AD) is the most common form of dementia characterized by substantial neuronal loss and progressive brain atrophy. Animal studies have suggested that the process of adult neurogenesis might be altered at the earliest phases of disease onset. The relationship between AD progression and adult neurogenesis in the human brain is, however, not well understood. Here, we present a systematic review of the postmortem studies that investigated changes in human adult neurogenesis in the AD brain. We present findings from 11 postmortem studies that were identified by a systematic search within the literature, focusing on what markers of neurogenesis were used, which stages of AD were investigated, and whether the studies had any confounding information that could potentially hinder clear interpretation of the presented data. In addition, we also review studies that examined transcriptomic changes in human AD postmortem brains and reveal upregulated expression of neural progenitor and proliferation markers and downregulated expression of later neurogenic markers in AD. Taken together, the existing literature seems to suggest that the overall level of human adult neurogenesis is reduced during the later stages of AD, potentially due to failed maturation and integration of new-born neurons. Further investigations using complementary methods such as in vitro disease modeling will be helpful to understand the exact molecular mechanisms underlying such pattern of change and to determine whether neurogenesis can be an effective therapeutic target for early intervention.

Functional Interactions between Newborn and Mature Neurons Leading to Integration into Established Neuronal Circuits

SummaryFrom development up to adulthood, the vertebrate brain is continuously supplied with newborn neurons that integrate into established mature circuits. However, how this process is coordinated during development remains unclear. Using two-photon imaging, GCaMP5 transgenic zebrafish larvae, and sparse electroporation in the larva’s optic tectum, we monitored spontaneous and induced activity of large neuronal populations containing newborn and functionally mature neurons. We observed that the maturation of newborn neurons is a 4-day process. Initially, newborn neurons showed undeveloped dendritic arbors, no neurotransmitter identity, and were unresponsive to visual stimulation, although they displayed spontaneous calcium transients. Later on, newborn-labeled neurons began to respond to visual stimuli but in a very variable manner. At the end of the maturation period, newborn-labeled neurons exhibited visual tuning curves (spatial receptive fields and direction selectivity) and spontaneous correlated activity with neighboring functionally mature neurons. At this developmental stage, newborn-labeled neurons presented complex dendritic arbors and neurotransmitter identity (excitatory or inhibitory). Removal of retinal inputs significantly perturbed the integration of newborn neurons into the functionally mature tectal network. Our results provide a comprehensive description of the maturation of newborn neurons during development and shed light on potential mechanisms underlying their integration into a functionally mature neuronal circuit.

Adult neural stem cell dysfunction in the subventricular zone of the lateral ventricle leads to diabetic olfactory defects.

Sensitive smell discrimination is based on structural plasticity of the olfactory bulb, which depends on migration and integration of newborn neurons from the subventricular zone. In this study, we examined the relationship between neural stem cell status in the subventricular zone and olfactory function in rats with diabetes mellitus. Streptozotocin was injected through the femoral vein to induce type 1 diabetes mellitus in Sprague-Dawley rats. Two months after injection, olfactory sensitivity was decreased in diabetic rats. Meanwhile, the number of BrdU-positive and BrdU+/DCX+ double-labeled cells was lower in the subventricular zone of diabetic rats compared with age-matched normal rats. Western blot results revealed downregulated expression of insulin receptor β, phosphorylated glycogen synthase kinase 3β, and β-catenin in the subventricular zone of diabetic rats. Altogether, these results indicate that diabetes mellitus causes insulin deficiency, which negatively regulates glycogen synthase kinase 3β and enhances β-catenin degradation, with these changes inhibiting neural stem cell proliferation. Further, these signaling pathways affect proliferation and differentiation of neural stem cells in the subventricular zone. Dysfunction of subventricular zone neural stem cells causes a decline in olfactory bulb structural plasticity and impairs olfactory sensitivity in diabetic rats.

Enhanced plasticity of mature granule cells reduces survival of newborn neurons in the adult mouse hippocampus.

Dentate granule cells are born throughout life in the mammalian hippocampus. The integration of newborn neurons into the dentate circuit is activity-dependent, and structural data characterizing synapse formation suggested that the survival of adult-born granule cells is regulated by competition for synaptic partners. Here we tested this hypothesis by using a mouse model with genetically enhanced plasticity of mature granule cells through temporally controlled expression of a nuclear inhibitor of protein phosphatase 1 (NIPP1*). Using thymidine analogues and retrovirus-mediated cell labeling, we show that synaptic integration and subsequent survival of newborn neurons is decreased in NIPP1*-expressing mice, suggesting that newborn neurons compete with preexisting granule cells for stable integration. The data presented here provides experimental evidence for a long-standing hypothesis and suggest cellular competition as a key mechanism regulating the integration and survival of newborn granule cells in the adult mammalian hippocampus.

Neuronal Circuitry Mechanisms Regulating Adult Mammalian Neurogenesis.

The adult mammalian brain is a dynamic structure, capable of remodeling in response to various physiological and pathological stimuli. One dramatic example of brain plasticity is the birth and subsequent integration of newborn neurons into the existing circuitry. This process, termed adult neurogenesis, recapitulates neural developmental events in two specialized adult brain regions: the lateral ventricles of the forebrain. Recent studies have begun to delineate how the existing neuronal circuits influence the dynamic process of adult neurogenesis, from activation of quiescent neural stem cells (NSCs) to the integration and survival of newborn neurons. Here, we review recent progress toward understanding the circuit-based regulation of adult neurogenesis in the hippocampus and olfactory bulb.

Age-related effects of X-ray irradiation on mouse hippocampus.

Therapeutic irradiation of pediatric and adult patients can profoundly affect adult neurogenesis, and cognitive impairment manifests as a deficit in hippocampal-dependent functions. Age plays a major role in susceptibility to radiation, and younger children are at higher risk of cognitive decay when compared to adults. Cranial irradiation affects hippocampal neurogenesis by induction of DNA damage in neural progenitors, through the disruption of the neurogenic microenvironment, and defective integration of newborn neurons into the neuronal network. Our goal here was to assess cellular and molecular alterations induced by cranial X-ray exposure to low/moderate doses (0.1 and 2 Gy) in the hippocampus of mice irradiated at the postnatal ages of day 10 or week 10, as well as the dependency of these phenomena on age at irradiation. To this aim, changes in the cellular composition of the dentate gyrus, mitochondrial functionality, proteomic profile in the hippocampus, as well as cognitive performance were evaluated by a multidisciplinary approach. Our results suggest the induction of specific alterations in hippocampal neurogenesis, microvascular density and mitochondrial functions, depending on age at irradiation. A better understanding of how irradiation impairs hippocampal neurogenesis at low and moderate doses is crucial to minimize adverse effects of therapeutic irradiation, contributing also to radiation safety regulations.

Bidirectional Signaling of Neuregulin-2 Mediates Formation of GABAergic Synapses and Maturation of Glutamatergic Synapses in Newborn Granule Cells of Postnatal Hippocampus.

The hippocampal dentate gyrus is one of special brain regions where neurogenesis persists throughout adulthood. Synaptogenesis is a critical step for newborn neurons to be integrated into preexisting neural network. Because neuregulin-2 (NRG2), a growth factor, is intensely expressed in these regions, we investigated whether it plays a role in synaptogenesis and dendritic growth. We found that NRG2 has dual roles in the development of newborn neurons. For GABAergic synaptogenesis, the extracellular domain of NRG2 acts as a ligand for a receptor on GABAergic neurons. In contrast, its intracellular domain was essential for dendritic outgrowth and glutamatergic synapse maturation. These results imply that NRG2 may play a critical role in network integration of newborn neurons.