Nonsynaptic GABA signaling in postnatal subventricular zone controls proliferation of GFAP-expressing progenitors

Deficient brain GABA metabolism leads to widespread impairments of astrocyte and oligodendrocyte function

The neurometabolic disorder succinic semialdehyde dehydrogenase (SSADH) deficiency leads to great neurochemical imbalances and severe neurological manifestations. The cause of the disease is loss of function of the enzyme SSADH, leading to impaired metabolism of the principal inhibitory neurotransmitter GABA. Despite the known identity of the enzymatic deficit, the underlying pathology of SSADH deficiency remains unclear. To uncover new mechanisms of the disease, we performed an untargeted integrative analysis of cerebral protein expression, functional metabolism, and lipid composition in a genetic mouse model of SSADH deficiency (ALDH5A1 knockout mice). Our proteomic analysis revealed a clear regional vulnerability, as protein alterations primarily manifested in the hippocampus and cerebral cortex of the ALDH5A1 knockout mice. These regions displayed aberrant expression of proteins linked to amino acid homeostasis, mitochondria, glial function, and myelination. Stable isotope tracing in acutely isolated brain slices demonstrated an overall maintained oxidative metabolism of glucose, but a selective decrease in astrocyte metabolic activity in the cerebral cortex of ALDH5A1 knockout mice. In contrast, an elevated capacity of oxidative glutamine metabolism was observed in the ALDH5A1 knockout brain, which may serve as a neuronal compensation of impaired astrocyte glutamine provision. In addition to reduced expression of critical oligodendrocyte proteins, a severe depletion of myelin-enriched sphingolipids was found in the brains of ALDH5A1 knockout mice, suggesting degeneration of myelin. Altogether, our study highlights that impaired astrocyte and oligodendrocyte function is intimately linked to SSADH deficiency pathology, suggesting that selective targeting of glial cells may hold therapeutic potential in this disease.

The bidirectional role of GABAA and GABAB receptors during the differentiation process of neural precursor cells of the subventricular zone

The intricate process of neuronal differentiation integrates multiple signals to induce transcriptional, morphological, and electrophysiological changes that reshape the properties of neural precursor cells during their maturation and migration process. An increasing number of neurotransmitters and biomolecules have been identified as molecular signals that trigger and guide this process. In this sense, taurine, a sulfur-containing, non-essential amino acid widely expressed in the mammal brain, modulates the neuronal differentiation process. In this study, we describe the effect of taurine acting via the ionotropic GABAA receptor and the metabotropic GABAB receptor on the neuronal differentiation and electrophysiological properties of precursor cells derived from the subventricular zone of the mouse brain. Taurine stimulates the number of neurites and favors the dendritic complexity of the neural precursor cells, accompanied by changes in the somatic input resistance and the strength of inward and outward membranal currents. At the pharmacological level, the blockade of GABAA receptors inhibits these effects, whereas the stimulation of GABAB receptors has no positive effects on the taurine-mediated differentiation process. Strikingly, the blockade of the GABAB receptor with CGP533737 stimulates neurite outgrowth, dendritic complexity, and membranal current kinetics of neural precursor cells. The effects of taurine on the differentiation process involve Ca2+ mobilization and the activation of intracellular signaling cascades since chelation of intracellular calcium with BAPTA-AM, and inhibition of the CaMKII, ERK1/2, and Src kinase inhibits the neurite outgrowth of neural precursor cells of the subventricular zone.

Maternal taurine as a modulator of Cl- homeostasis as well as of glycine/GABAA receptors for neocortical development

During brain and spinal cord development, GABA and glycine, the inhibitory neurotransmitters, cause depolarization instead of hyperpolarization in adults. Since glycine and GABAA receptors (GABAARs) are chloride (Cl-) ion channel receptor, the conversion of GABA/glycine actions during development is influenced by changes in the transmembrane Cl- gradient, which is regulated by Cl- transporters, NKCC1 (absorption) and KCC2 (expulsion). In immature neurons, inhibitory neurotransmitters are released in a non-vesicular/non-synaptic manner, transitioning to vesicular/synaptic release as the neuron matures. In other word, in immature neurons, neurotransmitters generally act tonically. Thus, the glycine/GABA system is a developmentally multimodal system that is required for neurogenesis, differentiation, migration, and synaptogenesis. The endogenous agonists for these receptors are not fully understood, we address taurine. In this review, we will discuss about the properties and function of taurine during development of neocortex. Taurine cannot be synthesized by fetuses or neonates, and is transferred from maternal blood through the placenta or maternal milk ingestion. In developing neocortex, taurine level is higher than GABA level, and taurine tonically activates GABAARs to control radial migration as a stop signal. In the marginal zone (MZ) of the developing neocortex, endogenous taurine modulates the spread of excitatory synaptic transmission, activating glycine receptors (GlyRs) as an endogenous agonist. Thus, taurine affects information processing and crucial developmental processes such as axonal growth, cell migration, and lamination in the developing cerebral cortex. Additionally, we also refer to the possible mechanism of taurine-regulating Cl- homeostasis. External taurine is uptake by taurine transporter (TauT) and regulates NKCC1 and KCC2 mediated by intracellular signaling pathway, with-no-lysine kinase 1 (WNK1) and its subsequent kinases STE20/SPS1-related proline-alanine-rich protein kinase (SPAK) and oxidative stress response kinase-1 (OSR1). Through the regulation of NKCC1 and KCC2, mediated by the WNK-SPAK/OSR1 signaling pathway, taurine plays a role in maintaining Cl- homeostasis during normal brain development.

Cortical regulation of neurogenesis and cell proliferation in the ventral subventricular zone

Neurogenesis and differentiation of neural stem cells (NSCs) are controlled by cell-intrinsic molecular pathways that interact with extrinsic signaling cues. In this study, we identify a circuit that regulates neurogenesis and cell proliferation in the lateral ventricle-subventricular zone (LV-SVZ). Our results demonstrate that direct glutamatergic projections from the anterior cingulate cortex (ACC), as well as inhibitory projections from calretinin+ local interneurons, modulate the activity of cholinergic neurons in the subependymal zone (subep-ChAT+). Furthermore, in vivo optogenetic stimulation and inhibition of the ACC-subep-ChAT+ circuit are sufficient to control neurogenesis in the ventral SVZ. Both subep-ChAT+ and local calretinin+ neurons play critical roles in regulating ventral SVZ neurogenesis and LV-SVZ cell proliferation.

Neuromodulation for Brain Tumors: Myth or Reality? A Narrative Review

In recent years, research on brain cancers has turned towards the study of the interplay between the tumor and its host, the normal brain. Starting from the establishment of a parallelism between neurogenesis and gliomagenesis, the influence of neuronal activity on the development of brain tumors, particularly gliomas, has been partially unveiled. Notably, direct electrochemical synapses between neurons and glioma cells have been identified, paving the way for new approaches for the cure of brain cancers. Since this novel field of study has been defined "cancer neuroscience", anticancer therapeutic approaches exploiting these discoveries can be referred to as "cancer neuromodulation". In the present review, we provide an up-to-date description of the novel findings and of the therapeutic neuromodulation perspectives in cancer neuroscience. We focus both on more traditional oncologic approaches, aimed at modulating the major pathways involved in cancer neuroscience through drugs or genetic engineering techniques, and on electric stimulation proposals; the latter is at the cutting-edge of neuro-oncology.

Translocator protein in the rise and fall of central nervous system neurons

Translocator protein (TSPO), a 18 kDa protein found in the outer mitochondrial membrane, has historically been associated with the transport of cholesterol in highly steroidogenic tissues though it is found in all cells throughout the mammalian body. TSPO has also been associated with molecular transport, oxidative stress, apoptosis, and energy metabolism. TSPO levels are typically low in the central nervous system (CNS), but a significant upregulation is observed in activated microglia during neuroinflammation. However, there are also a few specific regions that have been reported to have higher TSPO levels than the rest of the brain under normal conditions. These include the dentate gyrus of the hippocampus, the olfactory bulb, the subventricular zone, the choroid plexus, and the cerebellum. These areas are also all associated with adult neurogenesis, yet there is no explanation of TSPO's function in these cells. Current studies have investigated the role of TSPO in microglia during neuron degeneration, but TSPO's role in the rest of the neuron lifecycle remains to be elucidated. This review aims to discuss the known functions of TSPO and its potential role in the lifecycle of neurons within the CNS.

The neuroscience of cancer

The nervous system regulates tissue stem and precursor populations throughout life. Parallel to roles in development, the nervous system is emerging as a critical regulator of cancer, from oncogenesis to malignant growth and metastatic spread. Various preclinical models in a range of malignancies have demonstrated that nervous system activity can control cancer initiation and powerfully influence cancer progression and metastasis. Just as the nervous system can regulate cancer progression, cancer also remodels and hijacks nervous system structure and function. Interactions between the nervous system and cancer occur both in the local tumour microenvironment and systemically. Neurons and glial cells communicate directly with malignant cells in the tumour microenvironment through paracrine factors and, in some cases, through neuron-to-cancer cell synapses. Additionally, indirect interactions occur at a distance through circulating signals and through influences on immune cell trafficking and function. Such cross-talk among the nervous system, immune system and cancer-both systemically and in the local tumour microenvironment-regulates pro-tumour inflammation and anti-cancer immunity. Elucidating the neuroscience of cancer, which calls for interdisciplinary collaboration among the fields of neuroscience, developmental biology, immunology and cancer biology, may advance effective therapies for many of the most difficult to treat malignancies.

Interdependencies of the Neuronal, Immune and Tumor Microenvironment in Gliomas

Gliomas are the most common primary brain malignancy and are universally fatal. Despite significant breakthrough in understanding tumor biology, treatment breakthroughs have been limited. There is a growing appreciation that major limitations on effective treatment are related to the unique and highly complex glioma tumor microenvironment (TME). The TME consists of multiple different cell types, broadly categorized into tumoral, immune and non-tumoral, non-immune cells. Each group provides significant influence on the others, generating a pro-tumor dynamic with significant immunosuppression. In addition, glioma cells are highly heterogenous with various molecular distinctions on the cellular level. These variations, in turn, lead to their own unique influence on the TME. To develop future treatments, an understanding of this complex TME interplay is needed. To this end, we describe the TME in adult gliomas through interactions between its various components and through various glioma molecular phenotypes.

Cortical projection to the subventricular zone and its effect on adult neurogenesis in mice

Various brain regions/nuclei project axons to the subventricular zone (SVZ), a postnatal neurogenic niche. In adults, neurogenesis is controlled by neuronal activity, via neurotransmitters. Glutamate is a major excitatory neurotransmitter, and glutamate receptors are expressed in SVZ cells. Although the cerebral cortex is a major source of glutamate and the medial cortex projects axons to the medial striatum next to the SVZ, it remains unclear whether cortical neurons regulate adult neurogenesis in vivo. First, to analyze axonal projection, plasmid vector expressing DsRed was introduced to the medial cortex by in utero electroporation. At the adult stage, DsRed-labeled axons were detected in the dorsolateral, striatal, and septal areas of the SVZ, and where they were in contact with neuroblasts. Furthermore, maturation of the cortical projection and the SVZ appeared to synchronize during postnatal stages. Next, stab injuries were made in the bilateral medial cortex to interrupt cortical input to the SVZ. At 17 days post-injury, cell proliferation in the SVZ and tangential migration of neuroblasts to the olfactory bulb were not significantly affected. There were clusters of neuroblasts in the striatum close to the SVZ in all experimental groups, but the number and size of neuroblast clusters were significantly larger in the medial cortex-injured group compared with the other experimental groups. These neuroblast clusters had a morphology of tangentially migrating cells to the olfactory bulb. These results suggest that cortical input to the SVZ inhibits the radial migration of neuroblasts to converge with the migration pathway in vivo.

Taurine Promotes Differentiation and Maturation of Neural Stem/Progenitor Cells from the Subventricular Zone via Activation of GABAA Receptors

Neurogenesis, the formation of new neurons in the brain, occurs throughout the lifespan in the subgranular zone of the dentate gyrus and subventricular zone (SVZ) lining the lateral ventricles of the mammal brain. In this process, gamma-aminobutyric acid (GABA) and its ionotropic receptor, the GABA_A receptor (GABA_AR), play a critical role in the proliferation, differentiation, and migration process of neural stem/progenitor cells (NPC). Taurine, a non-essential amino acid widely distributed throughout the central nervous system, increases the proliferation of SVZ progenitor cells by a mechanism that may involve GABA_AR activation. Therefore, we characterized the effects of taurine on the differentiation process of NPC expressing GABA_AR. Preincubation of NPC-SVZ with taurine increased microtubule-stabilizing proteins assessed with the doublecortin assay. Taurine, like GABA, stimulated a neuronal-like morphology of NPC-SVZ and increased the number and length of primary, secondary, and tertiary neurites compared with control NPC of the SVZ. Furthermore, neurite outgrowth was prevented when simultaneously incubating cells with taurine or GABA and the GABA_AR blocker, picrotoxin. Patch-clamp recordings revealed a series of modifications in the NPCs' passive and active electrophysiological properties exposed to taurine, including regenerative spikes with kinetic properties similar to the action potentials of functional neurons.

GABAergic regulation of cell proliferation within the adult mouse spinal cord

Manipulation of neural stem cell proliferation and differentiation in the postnatal CNS is receiving significant attention due to therapeutic potential. In the spinal cord, such manipulations may promote repair in conditions such as multiple sclerosis or spinal cord injury, but may also limit excessive cell proliferation contributing to tumours such as ependymomas. We show that when ambient γ-aminobutyric acid (GABA) is increased in vigabatrin-treated or decreased by GAD67 allele haplodeficiency in glutamic acid decarboxylase67-green fluorescent protein (GAD67-GFP) mice of either sex, the numbers of proliferating cells respectively decreased or increased. Thus, intrinsic spinal cord GABA levels are correlated with the extent of cell proliferation, providing important evidence for manipulating these levels. Diazepam binding inhibitor, an endogenous protein that interacts with GABA receptors and its breakdown product, octadecaneuropeptide, which preferentially activates central benzodiazepine (CBR) sites, were highly expressed in spinal cord, especially in ependymal cells surrounding the central canal. Furthermore, animals with reduced CBR activation via treatment with flumazenil or Ro15-4513, or with a G2F77I mutation in the CBR binding site had greater numbers of Ethynyl-2'-deoxyuridine positive cells compared to control, which maintained their stem cell status since the proportion of newly proliferated cells becoming oligodendrocytes or astrocytes was significantly lower. Altering endogenous GABA levels or modulating GABAergic signalling through specific sites on GABA receptors therefore influences NSC proliferation in the adult spinal cord. These findings provide a basis for further study into how GABAergic signalling could be manipulated to enable spinal cord self-regeneration and recovery or limit pathological proliferative activity.

Neural precursor cells tune striatal connectivity through the release of IGFBPL1

The adult brain retains over life endogenous neural stem/precursor cells (eNPCs) within the subventricular zone (SVZ). Whether or not these cells exert physiological functions is still unclear. In the present work, we provide evidence that SVZ-eNPCs tune structural, electrophysiological, and behavioural aspects of striatal function via secretion of insulin-like growth factor binding protein-like 1 (IGFBPL1). In mice, selective ablation of SVZ-eNPCs or selective abrogation of IGFBPL1 determined an impairment of striatal medium spiny neuron morphology, a higher failure rate in GABAergic transmission mediated by fast-spiking interneurons, and striatum-related behavioural dysfunctions. We also found IGFBPL1 expression in the human SVZ, foetal and induced-pluripotent stem cell-derived NPCs. Finally, we found a significant correlation between SVZ damage, reduction of striatum volume, and impairment of information processing speed in neurological patients. Our results highlight the physiological role of adult SVZ-eNPCs in supporting cognitive functions by regulating striatal neuronal activity.

Inhibition of GABAA receptors in intestinal stem cells prevents chemoradiotherapy-induced intestinal toxicity

Lethal intestinal tissue toxicity is a common side effect and a dose-limiting factor in chemoradiotherapy. Chemoradiotherapy can trigger DNA damage and induce P53-dependent apoptosis in LGR5+ intestinal stem cells (ISCs). Gamma-aminobutyric acid (GABA) and its A receptors (GABAAR) are present in the gastrointestinal tract. However, the functioning of the GABAergic system in ISCs is poorly defined. We found that GABAAR α1 (GABRA1) levels increased in the murine intestine after chemoradiotherapy. GABRA1 depletion in LGR5+ ISCs protected the intestine from chemoradiotherapy-induced P53-dependent apoptosis and prolonged animal survival. The administration of bicuculline, a GABAAR antagonist, prevented chemoradiotherapy-induced ISC loss and intestinal damage without reducing the chemoradiosensitivity of tumors. Mechanistically, it was associated with the reduction of reactive oxygen species-induced DNA damage via the L-type voltage-dependent Ca2+ channels. Notably, flumazenil, a GABAAR antagonist approved by the U.S. Food and Drug Administration, rescued human colonic organoids from chemoradiotherapy-induced toxicity. Therefore, flumazenil may be a promising drug for reducing the gastrointestinal side effects of chemoradiotherapy.

Loss of functional System x-c uncouples aberrant postnatal neurogenesis from epileptogenesis in the hippocampus of Kcna1-KO mice

Mutations in Kv1.1 (Kcna1) voltage-gated potassium channels in humans and mice generate network hyperexcitability, enhancing aberrant postnatal neurogenesis in the dentate subgranular zone, resulting in epilepsy and hippocampal hypertrophy. While Kcna1 loss stimulates proliferation of progenitor cell subpopulations, the identity of extrinsic molecular triggers linking network hyperexcitability to aberrant postnatal neurogenesis remains incomplete. System x-c (Sxc) is an inducible glutamate/cysteine antiporter that regulates extracellular glutamate. Here, we find that the functional unit of Sxc, xCT (Slc7a11), is upregulated in regions of Kcna1 knockout (KO) hippocampus, suggesting a contribution to both hyperplasia and epilepsy. However, Slc7a11 KO suppressed and rescued hippocampal enlargement without altering seizure severity in Kcna1-Slc7a11-KO mice. Microglial activation, but not astrocytosis, was also reduced. Our study identifies Sxc-mediated glutamate homeostasis as an essential non-synaptic trigger coupling aberrant postnatal neurogenesis and neuroimmune crosstalk, revealing that neurogenesis and epileptogenesis in the dentate gyrus are not mutually contingent events.

Chloride imbalance in Fragile X syndrome

Developmental changes in ionic balance are associated with crucial hallmarks in neural circuit formation, including changes in excitation and inhibition, neurogenesis, and synaptogenesis. Neuronal excitability is largely mediated by ionic concentrations inside and outside of the cell, and chloride (Cl-) ions are highly influential in early neurodevelopmental events. For example, γ-aminobutyric acid (GABA) is the main inhibitory neurotransmitter of the mature central nervous system (CNS). However, during early development GABA can depolarize target neurons, and GABAergic depolarization is implicated in crucial neurodevelopmental processes. This developmental shift of GABAergic neurotransmission from depolarizing to hyperpolarizing output is induced by changes in Cl- gradients, which are generated by the relative expression of Cl- transporters Nkcc1 and Kcc2. Interestingly, the GABA polarity shift is delayed in Fragile X syndrome (FXS) models; FXS is one of the most common heritable neurodevelopmental disorders. The RNA binding protein FMRP, encoded by the gene Fragile X Messenger Ribonucleoprotein-1 (Fmr1) and absent in FXS, appears to regulate chloride transporter expression. This could dramatically influence FXS phenotypes, as the syndrome is hypothesized to be rooted in defects in neural circuit development and imbalanced excitatory/inhibitory (E/I) neurotransmission. In this perspective, we summarize canonical Cl- transporter expression and investigate altered gene and protein expression of Nkcc1 and Kcc2 in FXS models. We then discuss interactions between Cl- transporters and neurotransmission complexes, and how these links could cause imbalances in inhibitory neurotransmission that may alter mature circuits. Finally, we highlight current therapeutic strategies and promising new directions in targeting Cl- transporter expression in FXS patients.

Tuning the neurogenesis channel

Earlier work has implicated the neurotransmitter GABA in controlling forebrain progenitor proliferation. In this issue of Neuron, Everlien et al. (2022) demonstrate that diazepam binding inhibitor acts to keep the neurogenesis-promoting effect of GABA at bay.

Neurotransmitters: Potential Targets in Glioblastoma

Simple Summary

Aiming to discover potential treatments for GBM, this review connects emerging research on the roles of neurotransmitters in the normal neural and the GBM microenvironments and sheds light on the prospects of their application in the neuropharmacology of GBM. Conventional therapy is blamed for its poor effect, especially in inhibiting tumor recurrence and invasion. Facing this dilemma, we focus on neurotransmitters that modulate GBM initiation, progression and invasion, hoping to provide novel therapy targeting GBM. By analyzing research concerning GBM therapy systematically and scientifically, we discover increasing insights into the regulatory effects of neurotransmitters, some of which have already shown great potential in research in vivo or in vitro. After that, we further summarize the potential drugs in correlation with previously published research. In summary, it is worth expecting that targeting neurotransmitters could be a promising novel pharmacological approach for GBM treatment.

Abstract

For decades, glioblastoma multiforme (GBM), a type of the most lethal brain tumor, has remained a formidable challenge in terms of its treatment. Recently, many novel discoveries have underlined the regulatory roles of neurotransmitters in the microenvironment both physiologically and pathologically. By targeting the receptors synaptically or non-synaptically, neurotransmitters activate multiple signaling pathways. Significantly, many ligands acting on neurotransmitter receptors have shown great potential for inhibiting GBM growth and development, requiring further research. Here, we provide an overview of the most novel advances concerning the role of neurotransmitters in the normal neural and the GBM microenvironments, and discuss potential targeted drugs used for GBM treatment.

Diazepam binding inhibitor governs neurogenesis of excitatory and inhibitory neurons during embryonic development via GABA signaling

Of the neurotransmitters that influence neurogenesis, gamma-aminobutyric acid (GABA) plays an outstanding role, and GABA receptors support non-synaptic signaling in progenitors and migrating neurons. Here, we report that expression levels of diazepam binding inhibitor (DBI), an endozepine that modulates GABA signaling, regulate embryonic neurogenesis, affecting the long-term outcome regarding the number of neurons in the postnatal mouse brain. We demonstrate that DBI is highly expressed in radial glia and intermediate progenitor cells in the germinal zones of the embryonic mouse brain that give rise to excitatory and inhibitory cells. The mechanism by which DBI controls neurogenesis involves its action as a negative allosteric modulator of GABA-induced currents on progenitor cells that express GABA_A receptors containing γ₂ subunits. DBI's modulatory effect parallels that of GABA_A-receptor-mediating signaling in these cells in the proliferative areas, reflecting the tight control that DBI exerts on embryonic neurogenesis.

Drugs and Endogenous Factors as Protagonists in Neurogenic Stimulation

Neurogenesis is a biological process characterized by new neurons formation from stem cells. For decades, it was believed that neurons only multiplied during development and in the postnatal period but the discovery of neural stem cells (NSCs) in mature brain promoted a revolution in neuroscience field. In mammals, neurogenesis consists of migration, differentiation, maturation, as well as functional integration of newborn cells into the pre-existing neuronal circuit. Actually, NSC density drops significantly after the first stages of development, however in specific places in the brain, called neurogenic niches, some of these cells retain their ability to generate new neurons and glial cells in adulthood. The subgranular (SGZ), and the subventricular zones (SVZ) are examples of regions where the neurogenesis process occurs in the mature brain. There, the potential of NSCs to produce new neurons has been explored by new advanced methodologies and in neuroscience for the treatment of brain damage and/or degeneration. Based on that, this review highlights endogenous factors and drugs capable of stimulating neurogenesis, as well as the perspectives for the use of NSCs for neurological and neurodegenerative diseases.

The neural stem cell secretome across neurodevelopment

Neural stem cell (NSC) based therapies are at the forefront of regenerative medicine strategies to combat illness and injury of the central nervous system (CNS). In addition to their ability to produce new cells, NSCs secrete a variety of products, known collectively as the NSC secretome, that have been shown to ameliorate CNS disease pathology and promote recovery. As pre-clinical and clinical research to harness the NSC secretome for therapeutic purposes advances, a more thorough understanding of the endogenous NSC secretome can provide useful insight into the functional capabilities of NSCs. In this review, we focus on research investigating the autocrine and paracrine functions of the endogenous NSC secretome across life. Throughout development and adulthood, we find evidence that the NSC secretome is a critical component of how endogenous NSCs regulate themselves and their niche. We also find gaps in current literature, most notably in the clinically-relevant domain of endogenous NSC paracrine function in the injured CNS. Future investigations to further define the endogenous NSC secretome and its role in CNS tissue regulation are necessary to bolster our understanding of NSC-niche interactions and to aid in the generation of safe and effective NSC-based therapies.

A New Hypothesis for Alzheimer’s Disease: The Lipid Invasion Model

This paper proposes a new hypothesis for Alzheimer’s disease (AD)—the lipid invasion model. It argues that AD results from external influx of free fatty acids (FFAs) and lipid-rich lipoproteins into the brain, following disruption of the blood-brain barrier (BBB). The lipid invasion model explains how the influx of albumin-bound FFAs via a disrupted BBB induces bioenergetic changes and oxidative stress, stimulates microglia-driven neuroinflammation, and causes anterograde amnesia. It also explains how the influx of external lipoproteins, which are much larger and more lipid-rich, especially more cholesterol-rich, than those normally present in the brain, causes endosomal-lysosomal abnormalities and overproduction of the peptide amyloid-β (Aβ). This leads to the formation of amyloid plaques and neurofibrillary tangles, the most well-known hallmarks of AD. The lipid invasion model argues that a key role of the BBB is protecting the brain from external lipid access. It shows how the BBB can be damaged by excess Aβ, as well as by most other known risk factors for AD, including aging, apolipoprotein E4 (APOE4), and lifestyle factors such as hypertension, smoking, obesity, diabetes, chronic sleep deprivation, stress, and head injury. The lipid invasion model gives a new rationale for what we already know about AD, explaining its many associated risk factors and neuropathologies, including some that are less well-accounted for in other explanations of AD. It offers new insights and suggests new ways to prevent, detect, and treat this destructive disease and potentially other neurodegenerative diseases.

Immunotherapeutic Approaches for Glioblastoma Treatment

Glioblastoma remains a challenging disease to treat, despite well-established standard-of-care treatments, with a median survival consistently of less than 2 years. In this review, we delineate the unique disease-specific challenges for immunotherapies, both brain-related and non-brain-related, which will need to be adequately overcome for the development of effective treatments. We also review current immunotherapy treatments, with a focus on clinical applications, and propose future directions for the field of GBM immunotherapy.

Neural Signaling in Cancer

Nervous system activity regulates development, homeostasis, and plasticity of the brain as well as other organs in the body. These mechanisms are subverted in cancer to propel malignant growth. In turn, cancers modulate neural structure and function to augment growth-promoting neural signaling in the tumor microenvironment. Approaching cancer biology from a neuroscience perspective will elucidate new therapeutic strategies for presently lethal forms of cancer. In this review, we highlight the neural signaling mechanisms recapitulated in primary brain tumors, brain metastases, and solid tumors throughout the body that regulate cancer progression. Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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."

Adult Neurogenesis: A Story Ranging from Controversial New Neurogenic Areas and Human Adult Neurogenesis to Molecular Regulation

The generation of new neurons in the adult brain is a currently accepted phenomenon. Over the past few decades, the subventricular zone and the hippocampal dentate gyrus have been described as the two main neurogenic niches. Neurogenic niches generate new neurons through an asymmetric division process involving several developmental steps. This process occurs throughout life in several species, including humans. These new neurons possess unique properties that contribute to the local circuitry. Despite several efforts, no other neurogenic zones have been observed in many years; the lack of observation is probably due to technical issues. However, in recent years, more brain niches have been described, once again breaking the current paradigms. Currently, a debate in the scientific community about new neurogenic areas of the brain, namely, human adult neurogenesis, is ongoing. Thus, several open questions regarding new neurogenic niches, as well as this phenomenon in adult humans, their functional relevance, and their mechanisms, remain to be answered. In this review, we discuss the literature and provide a compressive overview of the known neurogenic zones, traditional zones, and newly described zones. Additionally, we will review the regulatory roles of some molecular mechanisms, such as miRNAs, neurotrophic factors, and neurotrophins. We also join the debate on human adult neurogenesis, and we will identify similarities and differences in the literature and summarize the knowledge regarding these interesting topics.

GABAA receptors: structure, function, pharmacology, and related disorders

Background

γ-Aminobutyric acid sub-type A receptors (GABA_ARs) are the most prominent inhibitory neurotransmitter receptors in the CNS. They are a family of ligand-gated ion channel with significant physiological and therapeutic implications.

Main body

GABA_ARs are heteropentamers formed from a selection of 19 subunits: six α (alpha1-6), three β (beta1-3), three γ (gamma1-3), three ρ (rho1-3), and one each of the δ (delta), ε (epsilon), π (pi), and θ (theta) which result in the production of a considerable number of receptor isoforms. Each isoform exhibits distinct pharmacological and physiological properties. However, the majority of GABA_ARs are composed of two α subunits, two β subunits, and one γ subunit arranged as γ2β2α1β2α1 counterclockwise around the center. The mature receptor has a central chloride ion channel gated by GABA neurotransmitter and modulated by a variety of different drugs. Changes in GABA synthesis or release may have a significant effect on normal brain function. Furthermore, The molecular interactions and pharmacological effects caused by drugs are extremely complex. This is due to the structural heterogeneity of the receptors, and the existence of multiple allosteric binding sites as well as a wide range of ligands that can bind to them. Notably, dysfunction of the GABAergic system contributes to the development of several diseases. Therefore, understanding the relationship between GABA_A receptor deficits and CNS disorders thus has a significant impact on the discovery of disease pathogenesis and drug development.

Conclusion

To date, few reviews have discussed GABA_A receptors in detail. Accordingly, this review aims to summarize the current understanding of the structural, physiological, and pharmacological properties of GABA_ARs, as well as shedding light on the most common associated disorders.

In vivo live imaging of postnatal neural stem cells

Neural stem cells (NSCs) are maintained in specific regions of the postnatal brain and contribute to its structural and functional plasticity. However, the long-term renewal potential of NSCs and their mode of division remain elusive. The use of advanced in vivo live imaging approaches may expand our knowledge of NSC physiology and provide new information for cell replacement therapies. In this Review, we discuss the in vivo imaging methods used to study NSC dynamics and recent live-imaging results with respect to specific intracellular pathways that allow NSCs to integrate and decode different micro-environmental signals. Lastly, we discuss future directions that may provide answers to unresolved questions regarding NSC physiology.

Locomotion dependent neuron-glia interactions control neurogenesis and regeneration in the adult zebrafish spinal cord

Physical exercise stimulates adult neurogenesis, yet the underlying mechanisms remain poorly understood. A fundamental component of the innate neuroregenerative capacity of zebrafish is the proliferative and neurogenic ability of the neural stem/progenitor cells. Here, we show that in the intact spinal cord, this plasticity response can be activated by physical exercise by demonstrating that the cholinergic neurotransmission from spinal locomotor neurons activates spinal neural stem/progenitor cells, leading to neurogenesis in the adult zebrafish. We also show that GABA acts in a non-synaptic fashion to maintain neural stem/progenitor cell quiescence in the spinal cord and that training-induced activation of neurogenesis requires a reduction of GABA_A receptors. Furthermore, both pharmacological stimulation of cholinergic receptors, as well as interference with GABAergic signaling, promote functional recovery after spinal cord injury. Our findings provide a model for locomotor networks’ activity-dependent neurogenesis during homeostasis and regeneration in the adult zebrafish spinal cord.

The mechanisms stimulating adult neurogenesis are unclear. Here, the authors show the contribution of cholinergic and GABAergic signalling within the locomotor network to spinal cord neurogenesis during homeostasis and regeneration, showing neurogenesis depends on circuit activity in the adult zebrafish.

The Structural E/I Balance Constrains the Early Development of Cortical Network Activity

Neocortical networks have a characteristic constant ratio in the number of glutamatergic projection neurons (PN) and GABAergic interneurons (IN), and deviations in this ratio are often associated with developmental neuropathologies. Cultured networks with defined cellular content allowed us to ask if initial PN/IN ratios change the developmental population dynamics, and how different ratios impact the physiological excitatory/inhibitory (E/I) balance and the network activity development. During the first week in vitro, the IN content modulated PN numbers, increasing their proliferation in networks with higher IN proportions. The proportion of INs in each network set remained similar to the initial plating ratio during the 4 weeks cultivation period. Results from additional networks generated with more diverse cellular composition, including early-born GABA neurons, suggest that a GABA-dependent mechanism may decrease the survival of additional INs. A large variation of the PN/IN ratio did not change the balance between isolated spontaneous glutamatergic and GABAergic postsynaptic currents charge transfer (E/I balance) measured in PNs or INs. In contrast, the E/I balance of multisynaptic bursts reflected differences in IN content. Additionally, the spontaneous activity recorded by calcium imaging showed that higher IN ratios were associated with increased frequency of network bursts combined with a decrease of participating neurons per event. In the 4th week in vitro, bursting activity was stereotypically synchronized in networks with very few INs but was more desynchronized in networks with higher IN proportions. These results suggest that the E/I balance of isolated postsynaptic currents in single cells may be regulated independently of PN/IN proportions, but the network bursts E/I balance and the maturation of spontaneous network activity critically depends upon the structural PN/IN ratio.

Neuro-Signals from Gut Microbiota: Perspectives for Brain Glioma

Glioblastoma (GBM) is the most aggressive form of glioma tumor in adult brain. Among the numerous factors responsible for GBM cell proliferation and invasion, neurotransmitters such as dopamine, serotonin and glutamate can play key roles. Studies performed in mice housed in germ-free (GF) conditions demonstrated the relevance of the gut-brain axis in a number of physiological and pathological conditions. The gut-brain communication is made possible by vagal/nervous and blood/lymphatic routes and pave the way for reciprocal modulation of functions. The gut microbiota produces and consumes a wide range of molecules, including neurotransmitters (dopamine, norepinephrine, serotonin, gamma-aminobutyric acid [GABA], and glutamate) that reach their cellular targets through the bloodstream. Growing evidence in animals suggests that modulation of these neurotransmitters by the microbiota impacts host neurophysiology and behavior, and affects neural cell progenitors and glial cells, along with having effects on tumor cell growth. In this review we propose a new perspective connecting neurotransmitter modulation by gut microbiota to glioma progression.

Single-Cell Profiling and SCOPE-Seq Reveal Lineage Dynamics of Adult Ventricular-Subventricular Zone Neurogenesis and NOTUM as a Key Regulator

In the adult ventricular-subventricular zone (V-SVZ), neural stem cells (NSCs) generate new olfactory bulb (OB) neurons and glia throughout life. To map adult neuronal lineage progression, we profiled >56,000 V-SVZ and OB cells by single-cell RNA sequencing (scRNA-seq). Our analyses reveal the molecular diversity of OB neurons, including fate-mapped neurons, lineage progression dynamics, and an NSC intermediate enriched for Notum, which encodes a secreted WNT antagonist. SCOPE-seq technology, which links live-cell imaging with scRNA-seq, uncovers cell-size transitions during NSC differentiation and preferential NOTUM binding to proliferating neuronal precursors. Consistently, application of NOTUM protein in slice cultures and pharmacological inhibition of NOTUM in slice cultures and in vivo demonstrated that NOTUM negatively regulates V-SVZ proliferation. Timely, context-dependent neurogenesis demands adaptive signaling among neighboring progenitors. Our findings highlight a critical regulatory state during NSC activation marked by NOTUM, which attenuates WNT-stimulated proliferation in NSC progeny.

Metformin pretreatment rescues olfactory memory associated with subependymal zone neurogenesis in a juvenile model of cranial irradiation

Cranial irradiation (IR) is an effective adjuvant therapy in the treatment of childhood brain tumors but results in long-lasting cognitive deficits associated with impaired neurogenesis, as evidenced in rodent models. Metformin has been shown to expand the endogenous neural stem cell (NSC) pool and promote neurogenesis under physiological conditions and in response to neonatal brain injury, suggesting a potential role in neurorepair. Here, we assess whether metformin pretreatment, a clinically feasible treatment for children receiving cranial IR, promotes neurorepair in a mouse cranial IR model. Using immunofluorescence and the in vitro neurosphere assay, we show that NSCs are depleted by cranial IR but spontaneously recover, although deficits to proliferative neuroblasts persist. Metformin pretreatment enhances the recovery of neurogenesis, attenuates the microglial response, and promotes recovery of long-term olfactory memory. These findings indicate that metformin is a promising candidate for further preclinical and clinical investigations of neurorepair in childhood brain injuries.

Therapeutic Potential of GABAergic Signaling in Myelin Plasticity and Repair

Oligodendrocytes (OLs) produce myelin to insulate axons. This accelerates action potential propagation, allowing nerve impulse information to synchronize within complex neuronal ensembles and promoting brain connectivity. Brain plasticity includes myelination, a process that starts early after birth and continues throughout life. Myelin repair, followed by injury or disease, requires new OLs differentiated from a population derived from oligodendrocyte precursor cells (OPCs) that continue to proliferate, migrate and differentiate to preserve and remodel myelin in the adult central nervous system. OPCs represent the largest proliferative neural cell population outside the adult neurogenic niches in the brain. OPCs receive synaptic inputs from glutamatergic and GABAergic neurons throughout neurodevelopment, a unique feature among glial cells. Neuron-glia communication through GABA signaling in OPCs has been shown to play a role in myelin plasticity and repair. In this review we will focus on the molecular and functional properties of GABA_A receptors (GABA_ARs) expressed by OPCs and their potential role in remyelination.

Adult Neurogenesis in Epileptogenesis: An Update for Preclinical Finding and Potential Clinical Translation

Epileptogenesis refers to the process in which a normal brain becomes epileptic, and is characterized by hypersynchronous spontaneous recurrent seizures involving a complex epileptogenic network. Current available pharmacological treatment of epilepsy is generally symptomatic in controlling seizures but is not disease-modifying in epileptogenesis. Cumulative evidence suggests that adult neurogenesis, specifically in the subgranular zone of the hippocampal dentate gyrus, is crucial in epileptogenesis. In this review, we describe the pathological changes that occur in adult neurogenesis in the epileptic brain and how adult neurogenesis is involved in epileptogenesis through different interventions. This is followed by a discussion of some of the molecular signaling pathways involved in regulating adult neurogenesis, which could be potential druggable targets for epileptogenesis. Finally, we provide perspectives on some possible research directions for future studies.

Critical aspects of neurodevelopment

Organisms have the unique ability to adapt to their environment by making use of external inputs. In the process, the brain is shaped by experiences that go hand-in-hand with optimisation of neural circuits. As such, there exists a time window for the development of different brain regions, each unique for a particular sensory modality, wherein the propensity of forming strong, irreversible connections are high, referred to as a critical period of development. Over the years, this domain of neurodevelopmental research has garnered considerable attention from many scientists, primarily because of the intensive activity-dependent nature of development. This review discusses the cellular, molecular, and neurophysiological bases of critical periods of different sensory modalities, and the disorders associated in cases the regulators of development are dysfunctional. Eventually, the neurobiological bases of the behavioural abnormalities related to developmental pathologies are discussed. A more in-depth insight into the development of the brain during the critical period of plasticity will eventually aid in developing potential therapeutics for several neurodevelopmental disorders that are categorised under critical period disorders.

Tonic GABAergic inhibition, via GABAA receptors containing αβƐ subunits, regulates excitability of ventral tegmental area dopamine neurons

The activity of midbrain dopamine neurons is strongly regulated by fast synaptic inhibitory γ‐Aminobutyric acid (GABA)ergic inputs. There is growing evidence in other brain regions that low concentrations of ambient GABA can persistently activate certain subtypes of GABA_A receptor to generate a tonic current. However, evidence for a tonic GABAergic current in midbrain dopamine neurons is limited. To address this, we conducted whole‐cell recordings from ventral tegmental area (VTA) dopamine neurons in brain slices from mice. We found that application of GABA_A receptor antagonists decreased the holding current, indicating the presence of a tonic GABAergic input. Global increases in GABA release, induced by either a nitric oxide donor or inhibition of GABA uptake, further increased this tonic current. Importantly, prolonged inhibition of the firing activity of local GABAergic neurons abolished the tonic current. A combination of pharmacology and immunohistochemistry experiments suggested that, unlike common examples of tonic inhibition, this current may be mediated by a relatively unusual combination of α4βƐ subunits. Lastly, we found that the tonic current reduced excitability in dopamine neurons suggesting a subtractive effect on firing activity.

The postnatal GABA shift: A developmental perspective

GABA is the major inhibitory neurotransmitter that counterbalances excitation in the mature brain. The inhibitory action of GABA relies on the inflow of chloride ions (Cl^-), which hyperpolarizes the neuron. In early development, GABA signaling induces outward Cl^- currents and is depolarizing. The postnatal shift from depolarizing to hyperpolarizing GABA is a pivotal event in brain development and its timing affects brain function throughout life. Altered timing of the postnatal GABA shift is associated with several neurodevelopmental disorders. Here, we argue that the postnatal shift from depolarizing to hyperpolarizing GABA represents the final shift in a sequence of GABA shifts, regulating proliferation, migration, differentiation, and finally plasticity of developing neurons. Each developmental GABA shift ensures that the instructive role of GABA matches the circumstances of the developing network. Sensory input may be a crucial factor in determining proper timing of the postnatal GABA shift. A developmental perspective is necessary to interpret the full consequences of a mismatch between connectivity, activity and GABA signaling during brain development.

The Strange Case of Jekyll and Hyde: Parallels Between Neural Stem Cells and Glioblastoma-Initiating Cells

During embryonic development, radial glial precursor cells give rise to neural lineages, and a small proportion persist in the adult mammalian brain to contribute to long-term neuroplasticity. Neural stem cells (NSCs) reside in two neurogenic niches of the adult brain, the hippocampus and the subventricular zone (SVZ). NSCs in the SVZ are endowed with the defining stem cell properties of self-renewal and multipotent differentiation, which are maintained by intrinsic cellular programs, and extrinsic cellular and niche-specific interactions. In glioblastoma, the most aggressive primary malignant brain cancer, a subpopulation of cells termed glioblastoma stem cells (GSCs) exhibit similar stem-like properties. While there is an extensive overlap between NSCs and GSCs in function, distinct genetic profiles, transcriptional programs, and external environmental cues influence their divergent behavior. This review highlights the similarities and differences between GSCs and SVZ NSCs in terms of their gene expression, regulatory molecular pathways, niche organization, metabolic programs, and current therapies designed to exploit these differences.

Ontogeny of adult neural stem cells in the mammalian brain

Neural stem cells (NSCs) persist into adulthood in the subgranular zone (SGZ) of the dentate gyrus in the hippocampus and in the ventricular-subventricular zone (V-SVZ) of the lateral ventricles, where they generate new neurons and glia cells that contribute to neural plasticity. A better understanding of the developmental process that enables NSCs to persist beyond development will provide insight into factors that determine the size and properties of the adult NSC pool and thus the capacity for life-long neurogenesis in the adult mammalian brain. We review current knowledge regarding the developmental origins of adult NSCs and the developmental process by which embryonic NSCs transition into their adult form. We also discuss potential mechanisms that might regulate proper establishment of the adult NSC pool, and propose future directions of research that will be key to unraveling how NSCs transform to establish the adult NSC pool in the mammalian brain.

Neural stem cell pools in the vertebrate adult brain: Homeostasis from cell-autonomous decisions or community rules?

Adult stem cell populations must coordinate their own maintenance with the generation of differentiated cell types to sustain organ physiology, in a spatially controlled manner and over long periods. Quantitative analyses of clonal dynamics have revealed that, in epithelia, homeostasis is achieved at the population rather than at the single stem cell level, suggesting that feedback mechanisms coordinate stem cell maintenance and progeny generation. In the central nervous system, however, little is known of the possible community processes underlying neural stem cell maintenance. Recent work, in part based on intravital imaging made possible in the adult zebrafish, conclusively highlights that homeostasis in neural stem cell pools may rely on population asymmetry and long-term spatiotemporal coordination of neural stem cell states and fates. These results suggest that neural stem cell assemblies in the vertebrate brain behave as self-organized systems, such that the stem cells themselves generate their own intrinsic niche.

Calcium Channels in Adult Brain Neural Stem Cells and in Glioblastoma Stem Cells

The brain of adult mammals, including humans, contains neural stem cells (NSCs) located within specific niches of which the ventricular-subventricular zone (V-SVZ) is the largest one. Under physiological conditions, NSCs proliferate, self-renew and produce new neurons and glial cells. Several recent studies established that oncogenic mutations in adult NSCs of the V-SVZ are responsible for the emergence of malignant primary brain tumors called glioblastoma. These aggressive tumors contain a small subpopulation of cells, the glioblastoma stem cells (GSCs), that are endowed with proliferative and self-renewal abilities like NSCs from which they may arise. GSCs are thus considered as the cells that initiate and sustain tumor growth and, because of their resistance to current treatments, provoke tumor relapse. A growing body of studies supports that Ca²⁺ signaling controls a variety of processes in NSCs and GSCs. Ca²⁺ is a ubiquitous second messenger whose fluctuations of its intracellular concentrations are handled by channels, pumps, exchangers, and Ca²⁺ binding proteins. The concerted action of the Ca²⁺ toolkit components encodes specific Ca²⁺ signals with defined spatio-temporal characteristics that determine the cellular responses. In this review, after a general overview of the adult brain NSCs and GSCs, we focus on the multiple roles of the Ca²⁺ toolkit in NSCs and discuss how GSCs hijack these mechanisms to promote tumor growth. Extensive knowledge of the role of the Ca²⁺ toolkit in the management of essential functions in healthy and pathological stem cells of the adult brain should help to identify promising targets for clinical applications.

Environmental regulation of the chloride transporter KCC2: switching inflammation off to switch the GABA on?

Chloride homeostasis, the main determinant factor for the dynamic tuning of GABAergic inhibition during development, has emerged as a key element altered in a wide variety of brain disorders. Accordingly, developmental disorders such as schizophrenia, Autism Spectrum Disorder, Down syndrome, epilepsy, and tuberous sclerosis complex (TSC) have been associated with alterations in the expression of genes codifying for either of the two cotransporters involved in the excitatory-to-inhibitory GABA switch, KCC2 and NKCC1. These alterations can result from environmental insults, including prenatal stress and maternal separation which share, as common molecular denominator, the elevation of pro-inflammatory cytokines. In this review we report and systemize recent research articles indicating that different perinatal environmental perturbations affect the expression of chloride transporters, delaying the developmental switch of GABA signaling, and that inflammatory cytokines, in particular interleukin 1β, may represent a key causal factor for this phenomenon. Based on literature data, we provide therefore a unifying conceptual framework, linking environmental hits with the excitatory-to-inhibitory GABA switch in the context of brain developmental disorders.

Woo E, Patten A, Yau SY, Gil-Mohapel J. Beyond the Hippocampus and the SVZ: Adult Neurogenesis Throughout the Brain

Convincing evidence has repeatedly shown that new neurons are produced in the mammalian brain into adulthood. Adult neurogenesis has been best described in the hippocampus and the subventricular zone (SVZ), in which a series of distinct stages of neuronal development has been well characterized. However, more recently, new neurons have also been found in other brain regions of the adult mammalian brain, including the hypothalamus, striatum, substantia nigra, cortex, and amygdala. While some studies have suggested that these new neurons originate from endogenous stem cell pools located within these brain regions, others have shown the migration of neurons from the SVZ to these regions. Notably, it has been shown that the generation of new neurons in these brain regions is impacted by neurologic processes such as stroke/ischemia and neurodegenerative disorders. Furthermore, numerous factors such as neurotrophic support, pharmacologic interventions, environmental exposures, and stem cell therapy can modulate this endogenous process. While the presence and significance of adult neurogenesis in the human brain (and particularly outside of the classical neurogenic regions) is still an area of debate, this intrinsic neurogenic potential and its possible regulation through therapeutic measures present an exciting alternative for the treatment of several neurologic conditions. This review summarizes evidence in support of the classic and novel neurogenic zones present within the mammalian brain and discusses the functional significance of these new neurons as well as the factors that regulate their production. Finally, it also discusses the potential clinical applications of promoting neurogenesis outside of the classical neurogenic niches, particularly in the hypothalamus, cortex, striatum, substantia nigra, and amygdala.

Lysosomes and signaling pathways for maintenance of quiescence in adult neural stem cells

Quiescence is a cellular strategy for maintaining somatic stem cells in a specific niche in a low metabolic state without senescence for a long period of time. During development, neural stem cells (NSCs) actively proliferate and self-renew, and their progeny differentiate into both neurons and glial cells to form mature brain tissues. On the other hand, most NSCs in the adult brain are quiescent and arrested in G0/G1 phase of the cell cycle. Quiescence is essential in order to avoid the precocious exhaustion of NSCs, ensuring a sustainable source of available stem cells in the brain throughout the lifespan. After receiving activation signals, quiescent NSCs reenter the cell cycle and generate new neurons. This switching between quiescence and proliferation is tightly regulated by diverse signaling pathways. Recent studies suggest significant involvement of cellular proteostasis (homeostasis of the proteome) in the quiescent state of NSCs. Proteostasis is the result of integrated regulation of protein synthesis, folding, and degradation. In this review, we discuss regulation of quiescence by multiple signaling pathways, especially bone morphogenetic protein and Notch signaling, and focus on the functional involvement of the lysosome, an organelle governing cellular degradation, in quiescence of adult NSCs.

Neural Stem Cell Niche and Adult Neurogenesis

Adult neurogenesis is a process that generates new and functional neurons from neural stem cells (NSCs) in a specialized neurogenic niche throughout life. Misregulated neurogenesis is detrimental to normal brain functions. To ensure proper neurogenesis, the niche cells must respond to extrinsic cues while fulfilling the intrinsic requirements of the neurogenic program and adapting their roles accordingly to influence NSC behavior. Understanding how the neurogenic niche executes its functions may guide strategies to maintain its integrative process and provide a permissive milieu for neurogenesis. In this review, we summarize the recent discoveries of interactive regulation of NSCs and neurogenesis by neurogenic niche and its implications in functional integrity of adult brain and neurological disorders.

Conserved and Divergent Features of Adult Neurogenesis in Zebrafish

Adult neurogenesis, i.e., the generation of neurons from neural stem cells (NSCs) in the adult brain, contributes to brain plasticity in all vertebrates. It varies, however, greatly in extent, location and physiological characteristics between species. During the last decade, the teleost zebrafish (D. rerio) was increasingly used to study the molecular and cellular properties of adult NSCs, in particular as a prominent NSC population was discovered at the ventricular surface of the dorsal telencephalon (pallium), in territories homologous to the adult neurogenic niches of rodents. This model, for its specific features (large NSC population, amenability to intravital imaging, high regenerative capacity) allowed rapid progress in the characterization of basic adult NSC features. We review here these findings, with specific comparisons with the situation in rodents. We specifically discuss the cellular nature of NSCs (astroglial or neuroepithelial cells), their heterogeneities and their neurogenic lineages, and the mechanisms controlling NSC quiescence and fate choices, which all impact the neurogenic output. We further discuss the regulation of NSC activity in response to physiological triggers and non-physiological conditions such as regenerative contexts.

GABAergic Input From the Basal Forebrain Promotes the Survival of Adult-Born Neurons in the Mouse Olfactory Bulb

A unique feature of the olfactory system is the continuous generation and integration of new neurons throughout adulthood. Adult-born neuron survival and integration is dependent on activity and sensory experience, which is largely mediated by early synaptic inputs that adult-born neurons receive upon entering the olfactory bulb (OB). As in early postnatal development, the first synaptic inputs onto adult-born neurons are GABAergic. However, the specific sources of early synaptic GABA and the influence of specific inputs on adult-born neuron development are poorly understood. Here, we use retrograde and anterograde viral tracing to reveal robust GABAergic projections from the basal forebrain horizontal limb of the diagonal band of Broca (HDB) to the granule cell layer (GCL) and glomerular layer (GL) of the mouse OB. Whole-cell electrophysiological recordings indicate that these projections target interneurons in the GCL and GL, including adult-born granule cells (abGCs). Recordings from birth-dated abGCs reveal a developmental time course in which HDB GABAergic input onto abGCs emerges as the neurons first enter the OB, and strengthens throughout the critical period of abGC development. Finally, we show that removing GABAergic signaling from HDB neurons results in decreased abGC survival. Together these data show that GABAergic projections from the HDB synapse onto immature abGCs in the OB to promote their survival through the critical period, thus representing a source of long-range input modulating plasticity in the adult OB.

Epigenetic Modifications of GABAergic Interneurons Contribute to Deficits in Adult Hippocampus Neurogenesis and Depression-Like Behavior in Prenatally Stressed Mice

BACKGROUND

Prenatal stress (PRS) is considered a risk factor for depressive disorder. Adult hippocampal neurogenesis is believed to play a role in the regulation of affective behaviors. GABAergic interneuron is a key modulator in adult hippocampal neurogenesis. Growing evidence indicates that PRS has adverse effects on adult hippocampal neurogenesis and DNA epigenetic modifications of the GABAergic system. The aim of this study was to investigate whether epigenetic GABAergic dysfunction participates in the negative impact of PRS on adult hippocampal neurogenesis and related emotional behaviors.

METHODS

Behavioral tests were used to explore PRS-induced depression-like behaviors of adult female mice. Immunohistochemistry staining, real-time reverse transcription-polymerase chain reaction, western blot, and chromatin immunoprecipitation were employed to detect adult neurogenesis and epigenetic changes of the GABAergic system in the hippocampus of PRS mice.

RESULTS

PRS mice developed a depression phenotype accompanied by the inhibited maturation of hippocampal newborn neurons. Compared with control mice, PRS mice showed decreased expression of glutamic acid decarboxylase 67 at the mRNA and protein levels. GABAA receptor agonist phenobarbital could rectify the decrease of 5-bromo-2-deoxyuridine/neuronal nuclei double-positive (BrdU+/NeuN+) cells in PRS mice. PRS mice also showed increased expression of DNA methyltransferase 1 and increased binding of DNA methyltransferase 1 to glutamic acid decarboxylase 67 promoter region. The treatment with DNA methyltransferase 1 inhibitor 5-aza-deoxycytidine restored the decrease of BrdU+/NeuN+ cells and depression-like behaviors in PRS mice via improving GABAergic system.

CONCLUSIONS

The present results indicate that epigenetic changes of the GABAergic system are responsible for adult hippocampus neurogenesis and depression-like behaviors in PRS mice.

Therapeutic Plasticity of Neural Stem Cells

Neural stem cells (NSCs) have garnered significant scientific and commercial interest in the last 15 years. Given their plasticity, defined as the ability to develop into different phenotypes inside and outside of the nervous system, with a capacity of almost unlimited self-renewal, of releasing trophic and immunomodulatory factors, and of exploiting temporal and spatial dynamics, NSCs have been proposed for (i) neurotoxicity testing; (ii) cellular therapies to treat CNS diseases; (iii) neural tissue engineering and repair; (iv) drug target validation and testing; (v) personalized medicine. Moreover, given the growing interest in developing cell-based therapies to target neurodegenerative diseases, recent progress in developing NSCs from human-induced pluripotent stem cells has produced an analog of endogenous NSCs. Herein, we will review the current understanding on emerging conceptual and technological topics in the neural stem cell field, such as deep characterization of the human compartment, single-cell spatial-temporal dynamics, reprogramming from somatic cells, and NSC manipulation and monitoring. Together, these aspects contribute to further disentangling NSC plasticity to better exploit the potential of those cells, which, in the future, might offer new strategies for brain therapies.

GABAA Receptors are Selectively Expressed in NG2 Glia of the Cerebellar White Matter

The cerebellum is involved in the coordination of movement. Its cellular composition is dominated by GABAergic neuronal types, and glial cells are known to express functional receptors. GABAergic signaling regulates cell proliferation, differentiation, and migration during neurodevelopment. However, little is known about the functional expression of GABA receptors in the cerebellar white matter (WM). Thus, the aim of this study was to test whether glial cells express functional GABA receptors during postnatal development (P7-P9) of cerebellar WM. Immunofluorescence showed that half of the astrocytes express GAD67, suggesting that glial cells synthesize GABA. Calcium imaging in cerebellar slices revealed that GABA and the GABA_A agonist muscimol evoked calcium transients in sulforhodamine B negative cells, whereas the GABA_B agonist baclofen failed to evoke responses in cerebellar WM. Whole-cell patch-clamp recordings of GFAP⁺ cells showed dye coupling and a passive current-voltage relation typical of astrocytes. Surprisingly, these cells did not respond to muscimol. Two additional populations were identified as GFAP^- cells. The first population showed dye coupling, slow decaying inward and outward currents with no voltage dependence, and did not respond to GABA_A agonists. The second population showed an outward-rectifying current-voltage relationship and responded to muscimol, but dye coupling was absent. These cells received synaptic input and were NG2⁺, but evoked calcium waves failed to modulate the frequency of spontaneous postsynaptic currents (sPSCs) or signaling into NG2 glia. We conclude that GABA_A receptor-mediated signaling is selective for NG2 glia in the WM of the cerebellum.