Genetic control of adult neurogenesis: interplay of differentiation, proliferation and survival modulates new neurons function, and memory circuits

Immunological perturbations, psychiatric disorders and associated therapeutics: a new era for psychiatry?

The three main theories explaining major mental illness, namely mood disorders, psychoses and dementias, have been partially discredited. Alongside this, there are emerging links between perturbations of the immune system and the onset and phenotypic features of these disorders. This article outlines the alternative pathophysiology and suggests potential treatments which could improve disease burden and avoid the need for psychotropic medication, with their associated side effects and relapse following withdrawal.

Discovery of small-molecule activators of nicotinamide phosphoribosyltransferase (NAMPT) and their preclinical neuroprotective activity

The decline of nicotinamide adenine dinucleotide (NAD) occurs in a variety of human pathologies including neurodegeneration. NAD-boosting agents can provide neuroprotective benefits. Here, we report the discovery and development of a class of potent activators (NATs) of nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme in the NAD salvage pathway. We obtained the crystal structure of NAMPT in complex with the NAT, which defined the allosteric action of NAT near the enzyme active site. The optimization of NAT further revealed the critical role of K189 residue in boosting NAMPT activity. NATs effectively increased intracellular levels of NAD and induced subsequent metabolic and transcriptional reprogramming. Importantly, NATs exhibited strong neuroprotective efficacy in a mouse model of chemotherapy-induced peripheral neuropathy (CIPN) without any overt toxicity. These findings demonstrate the potential of NATs in the treatment of neurodegenerative diseases or conditions associated with NAD level decline.

Transcriptome Analysis in a Mouse Model of Premature Aging of Dentate Gyrus: Rescue of Alpha-Synuclein Deficit by Virus-Driven Expression or by Running Restores the Defective Neurogenesis

The dentate gyrus of the hippocampus and the subventricular zone are neurogenic niches where neural stem and progenitor cells replicate throughout life to generate new neurons. The Btg1 gene maintains the stem cells of the neurogenic niches in quiescence. The deletion of Btg1 leads to an early transient increase of stem/progenitor cells division, followed, however, by a decrease during adulthood of their proliferative capability, accompanied by apoptosis. Since a physiological decrease of neurogenesis occurs during aging, the Btg1 knockout mouse may represent a model of neural aging. We have previously observed that the defective neurogenesis of the Btg1 knockout model is rescued by the powerful neurogenic stimulus of physical exercise (running). To identify genes responsible for stem and progenitor cells maintenance, we sought here to find genes underlying this premature neural aging, and whose deregulated expression could be rescued by running. Through RNA sequencing we analyzed the transcriptomic profiles of the dentate gyrus isolated from Btg1 wild-type or Btg1 knockout adult (2-month-old) mice submitted to physical exercise or sedentary. In Btg1 knockout mice, 545 genes were deregulated, relative to wild-type, while 2081 genes were deregulated by running. We identified 42 genes whose expression was not only down-regulated in the dentate gyrus of Btg1 knockout, but was also counter-regulated to control levels by running in Btg1 knockout mice, vs. sedentary. Among these 42 counter-regulated genes, alpha-synuclein (Snca), Fos, Arc and Npas4 showed significantly greater differential regulation. These genes control neural proliferation, apoptosis, plasticity and memory and are involved in aging. In particular, Snca expression decreases during aging. We tested, therefore, whether an Snca-expressing lentivirus, by rescuing the defective Snca levels in the dentate gyrus of Btg1 knockout mice, could also reverse the aging phenotype, in particular the defective neurogenesis. We found that the exogenous expression of Snca reversed the Btg1 knockout-dependent decrease of stem cell proliferation as well as the increase of progenitor cell apoptosis. This indicates that Snca has a functional role in the process of neural aging observed in this model, and also suggests that Snca acts as a positive regulator of stem cell maintenance.

FOS Rescues Neuronal Differentiation of Sox2-Deleted Neural Stem Cells by Genome-Wide Regulation of Common SOX2 and AP1(FOS-JUN) Target Genes

The transcription factor SOX2 is important for brain development and for neural stem cells (NSC) maintenance. Sox2-deleted (Sox2-del) NSC from neonatal mouse brain are lost after few passages in culture. Two highly expressed genes, Fos and Socs3, are strongly downregulated in Sox2-del NSC; we previously showed that Fos or Socs3 overexpression by lentiviral transduction fully rescues NSC's long-term maintenance in culture. Sox2-del NSC are severely defective in neuronal production when induced to differentiate. NSC rescued by Sox2 reintroduction correctly differentiate into neurons. Similarly, Fos transduction rescues normal or even increased numbers of immature neurons expressing beta-tubulinIII, but not more differentiated markers (MAP2). Additionally, many cells with both beta-tubulinIII and GFAP expression appear, indicating that FOS stimulates the initial differentiation of a "mixed" neuronal/glial progenitor. The unexpected rescue by FOS suggested that FOS, a SOX2 transcriptional target, might act on neuronal genes, together with SOX2. CUT&RUN analysis to detect genome-wide binding of SOX2, FOS, and JUN (the AP1 complex) revealed that a high proportion of genes expressed in NSC are bound by both SOX2 and AP1. Downregulated genes in Sox2-del NSC are highly enriched in genes that are also expressed in neurons, and a high proportion of the "neuronal" genes are bound by both SOX2 and AP1.

Gastrodiae rhizoma attenuates brain aging via promoting neuritogenesis and neurodifferentiation

BACKGROUND

Gastrodiae Rhizoma (Tianma), the dried tuber of Gastrodia elata Bl. (Orchidaceae), is listed as a top-grade herbal medicine in Shen-nong Ben-ts'ao Jing and has been used for treating headaches, dizziness, vertigo and convulsion. It has a neuroprotective effect and extends the lifespan in mouse models of Huntington's disease and Niemann-Pick type C disease. However, its effect on senescence remains unknown.

PURPOSE

This study aimed to investigate the anti-aging effects and the underlying mechanism of Gastrodiae Rhizoma.

METHODS

D-galactose (D-gal)- and BeSO₄-induced cellular senescence and senescence-associated β-galactosidase (SA-β-gal) activity were evaluated in SH-SY5Y and PC12 cells. D-gal-induced aging mice were used as an in vivo model. Animal behaviors including nesting and burrowing and Morris water maze were conducted. Neurogenesis in the hippocampus was assessed by immunohistochemistry and confocal microscopy, and the aging-related proteins were assessed by Western blot analysis. The potential neuritogenesis activity of the partially purified fraction of Gastrodiae Rhizoma (TM-2) and its major ingredients were investigated in PC12 cells.

RESULTS

TM-2 could improve D-gal-induced learning and memory impairement by inhibiting oxidative stress, increasing hippocampal neurogenesis and regulating the SH2B1-Akt pathway. Moreover, N⁶-(4-hydroxybenzyl)adenine riboside (T1-11) and parishins A and B, three constituents of TM-2, had anti-aging activity, as did T1-11 and parishin A induced neuritogenesis.

CONCLUSION

Our data suggested that TM-2 slowed down D-gal-induced cellular and mouse brain aging. These results indicate that Gastrodiae Rhizoma has a beneficial effect on senescence. It may be used for neuroprotection and promoting neurogenesis.

Interaction Between Neurogenic Stimuli and the Gene Network Controlling the Activation of Stem Cells of the Adult Neurogenic Niches, in Physiological and Pathological Conditions

In the adult mammalian brain new neurons are continuously generated throughout life in two niches, the dentate gyrus of the hippocampus and the subventricular zone. This process, called adult neurogenesis, starts from stem cells, which are activated and enter the cell cycle. The proliferative capability of stem cells progressively decreases during aging. The population of stem cells is generally quiescent, and it is not clear whether the potential for stem cells to expand is limited, or whether they can expand and then return to quiescence, remaining available for further activation. Certain conditions may deregulate stem cells quiescence and self-renewal. In fact we discuss the possibility of activation of stem cells by neurogenic stimuli as a function of the intensity of the stimulus (i.e., whether this is physiological or pathological), and of the deregulation of the system (i.e., whether the model is aged or carrying genetic mutations in the gene network controlling quiescence). It appears that when the system is aged and/or carrying mutations of quiescence-maintaining genes, preservation of the quiescent state of stem cells is more critical and stem cells can be activated by a neurogenic stimulus which is ineffective in normal conditions. Moreover, when a neurogenic stimulus is in itself a cause of brain damage (e.g., kainic acid treatment) the activation of stem cells occurs bypassing any inhibitory control. Plausibly, with strong neurogenic stimuli, such as kainic acid injected into the dentate gyrus, the self-renewal capacity of stem cells may undergo rapid exhaustion. However, the self-renewal capability of stem cells persists when normal stimuli are elicited in the presence of a mutation of one of the quiescence-maintaining genes, such as p16Ink4a, p21Cip1 or Btg1. In this case, stem cells become promptly activated by a neurogenic stimulus even during aging. This indicates that stem cells retain a high proliferative capability and plasticity, and suggests that stem cells are protected against the response to stimulus and are resilient to exhaustion. It will be interesting to assess at which functional degree of deregulation of the quiescence-maintaining system, stem cells will remain responsive to repeated neurogenic stimuli without undergoing exhaustion of their pool.

Theanine, the Main Amino Acid in Tea, Prevents Stress-Induced Brain Atrophy by Modifying Early Stress Responses

Chronic stress can impair the health of human brains. An important strategy that may prevent the accumulation of stress may be the consumption of functional foods. When senescence-accelerated mice prone 10 (SAMP10), a stress-sensitive strain, were loaded with stress using imposed male mouse territoriality, brain volume decreased. However, in mice that ingested theanine (6 mg/kg), the main amino acid in tea leaves, brain atrophy was suppressed, even under stress. On the other hand, brain atrophy was not clearly observed in a mouse strain that aged normally (Slc:ddY). The expression level of the transcription factor Npas4 (neuronal PAS domain protein 4), which regulates the formation and maintenance of inhibitory synapses in response to excitatory synaptic activity, decreased in the hippocampus and prefrontal cortex of stressed SAMP10 mice, but increased in mice that ingested theanine. Lipocalin 2 (Lcn2), the expression of which increased in response to stress, was significantly high in the hippocampus and prefrontal cortex of stressed SAMP10 mice, but not in mice that ingested theanine. These data suggest that Npas4 and Lcn2 are involved in the brain atrophy and stress vulnerability of SAMP10 mice, which are prevented by the consumption of theanine, causing changes in the expression of these genes.

Endogenous neural precursor cells in health and disease

Neurogenesis persists in the adult brain of mammals in the subventricular zone (SVZ) of the lateral ventricles and in the subgranular zone (SGZ) of the dentate gyrus (DG). The complex interactions between intrinsic and extrinsic signals provided by cells in the niche but also from distant sources regulate the fate of neural stem/progenitor cells (NPCs) in these sites. This fine regulation is perturbed in aging and in pathological conditions leading to a different NPC behavior, tailored to the specific physio-pathological features. Indeed, NPCs exert in physiological and pathological conditions important neurogenic and non-neurogenic regulatory functions and participate in maintaining and protecting brain tissue homeostasis. In this review, we discuss intrinsic and extrinsic signals that regulate NPC activation and NPC functional role in various homeostatic and non-homeostatic conditions.

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

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

Fluoxetine or Sox2 reactivate proliferation-defective stem and progenitor cells of the adult and aged dentate gyrus

The dentate gyrus of the hippocampus and the subventricular zone are neurogenic niches where the production of new neurons from glia-like stem cells continues throughout adult life. It is not clear whether the pool of stem cells is fated to be exhausted or is conserved until old age. We observed that the antiproliferative gene Btg1 maintains the quiescence of stem cells, and its ablation causes an increase of stem/progenitor cells proliferation in neonatal mice followed by progressive loss of proliferation during adulthood. Fluoxetine is an antidepressant, which exerts a powerful neurogenic effect on dentate gyrus progenitor cells, but is ineffective on stem cells. Here we show that adult dentate gyrus stem cells in the Btg1 knockout mice, with reduced self-renewal and proliferative capability, can be reactivated by fluoxetine, which increases their number greatly above the level of control or fluoxetine-treated wild-type mice. The increase of mitotic index above wild-type in Btg1 knockout fluoxetine-treated stem cells indicates that fluoxetine forces quiescent stem cells to enter the cycle. Stem cell proliferation undergoes continuous reactivation until fluoxetine is administered. Remarkably, fluoxetine reactivates proliferation-defective stem cells also in aged Btg1 knockout mice (15-month-old), an effect absent in wild-type aged mice. Moreover, overexpression of Sox2 retrovirally transduced in Btg1 knockout dentate gyrus cells significantly increases the number of neuroblasts, indicating that Sox2 is able to promote the self-renewal of proliferation-defective stem cells. Overall, the deletion of an antiproliferative gene, such as Btg1, reveals that dentate gyrus stem cells retain a hidden plasticity for self-renewal also in old age, in agreement with a model of permanent self-renewal.

The Long Run: Neuroprotective Effects of Physical Exercise on Adult Neurogenesis from Youth to Old Age

BACKGROUND

The rapid lengthening of life expectancy has raised the problem of providing social programs to counteract the age-related cognitive decline in a growing number of older people. Physical activity stands among the most promising interventions aimed at brain wellbeing, because of its effective neuroprotective action and low social cost. The purpose of this review is to describe the neuroprotective role exerted by physical activity in different life stages. In particular, we focus on adult neurogenesis, a process which has proved being highly responsive to physical exercise and may represent a major factor of brain health over the lifespan.

METHODS

The most recent literature related to the subject has been reviewed. The text has been divided into three main sections, addressing the effects of physical exercise during childhood/ adolescence, adulthood and aging, respectively. For each one, the most relevant studies, carried out on both human participants and rodent models, have been described.

RESULTS

The data reviewed converge in indicating that physical activity exerts a positive effect on brain functioning throughout the lifespan. However, uncertainty remains about the magnitude of the effect and its biological underpinnings. Cellular and synaptic plasticity provided by adult neurogenesis are highly probable mediators, but the mechanism for their action has yet to be conclusively established.

CONCLUSION

Despite alternative mechanisms of action are currently debated, age-appropriate physical activity programs may constitute a large-scale, relatively inexpensive and powerful approach to dampen the individual and social impact of age-related cognitive decline.

Pattern of Notch2, Numb, and Cas8 Gene Expression in Relevant Structures of the Rat Brain during Formation of Spatial Memory

The expression of Notch2, Numb, and Cas8 genes, whose protein products are involved in regulation of neurogenesis/neuroapoptosis processes, was studied in the relevant cerebral structures of male Wistar rats trained in a spatial habit. The formation of long-term spatial memory was found to be associated with the formation of a specific pattern of transcription activity of the studied genes in different brain structures. The maximum expression of Notch2 gene was found in the hippocampus and cerebellum, the maximum expression of Numb was detected in the prefrontal cortex and cerebellum, and the maximum expression of Cas8 was revealed in the prefrontal cortex of trained animals.

Terminal Differentiation of Adult Hippocampal Progenitor Cells Is a Step Functionally Dissociable from Proliferation and Is Controlled by Tis21, Id3 and NeuroD2

Cell proliferation and differentiation are interdependent processes. Here, we have asked to what extent the two processes of neural progenitor cell amplification and differentiation are functionally separated. Thus, we analyzed whether it is possible to rescue a defect of terminal differentiation in progenitor cells of the dentate gyrus, where new neurons are generated throughout life, by inducing their proliferation and/or their differentiation with different stimuli appropriately timed. As a model we used the Tis21 knockout mouse, whose dentate gyrus neurons, as demonstrated by us and others, have an intrinsic defect of terminal differentiation. We first tested the effect of two proliferative as well as differentiative neurogenic stimuli, one pharmacological (fluoxetine), the other cognitive (the Morris water maze (MWM) training). Both effectively enhanced the number of new dentate gyrus neurons produced, and fluoxetine also reduced the S-phase length of Tis21 knockout dentate gyrus progenitor cells and increased the rate of differentiation of control cells, but neither factor enhanced the defective rate of differentiation. In contrast, the defect of terminal differentiation was fully rescued by in vivo infection of proliferating dentate gyrus progenitor cells with retroviruses either silencing Id3, an inhibitor of neural differentiation, or expressing NeuroD2, a proneural gene expressed in terminally differentiated dentate gyrus neurons. This is the first demonstration that NeuroD2 or the silencing of Id3 can activate the differentiation of dentate gyrus neurons, complementing a defect of differentiation. It also highlights how the rate of differentiation of dentate gyrus neurons is regulated genetically at several levels and that a neurogenic stimulus for amplification of neural stem/progenitor cells may not be sufficient in itself to modify this rate.

Neural Stem Cell Plasticity: Advantages in Therapy for the Injured Central Nervous System

The physiological and pathological properties of the neural germinal stem cell niche have been well-studied in the past 30 years, mainly in animals and within given limits in humans, and knowledge is available for the cyto-architectonic structure, the cellular components, the timing of development and the energetic maintenance of the niche, as well as for the therapeutic potential and the cross talk between neural and immune cells. In recent years we have gained detailed understanding of the potentiality of neural stem cells (NSCs), although we are only beginning to understand their molecular, metabolic, and epigenetic profile in physiopathology and, further, more can be invested to measure quantitatively the activity of those cells, to model in vitro their therapeutic responses or to predict interactions in silico. Information in this direction has been put forward for other organs but is still limited in the complex and very less accessible context of the brain. A comprehensive understanding of the behavior of endogenous NSCs will help to tune or model them toward a desired response in order to treat complex neurodegenerative diseases. NSCs have the ability to modulate multiple cellular functions and exploiting their plasticity might make them into potent and versatile cellular drugs.

HDAC1, HDAC4, and HDAC9 Bind to PC3/Tis21/Btg2 and Are Required for Its Inhibition of Cell Cycle Progression and Cyclin D1 Expression

PC3/Tis21 is a transcriptional cofactor that inhibits proliferation in several cell types, including neural progenitors. Here, we report that PC3/Tis21 associates with HDAC1, HDAC4, and HDAC9 in vivo, in fibroblast cells. Furthermore, when HDAC1, HDAC4, or HDAC9 are silenced in fibroblasts or in a line of cerebellar progenitor cells, the ability of PC3/Tis21 to inhibit proliferation is significantly reduced. Overexpression of HDAC1, HDAC4, or HDAC9 in fibroblasts and in cerebellar precursor cells synergizes with PC3/Tis21 in inhibiting the expression of cyclin D1, a cyclin selectively inhibited by PC3/Tis21. Conversely, the depletion of HDAC1 or HDAC4 (but not HDAC9) in fibroblasts and in cerebellar precursor cells significantly impairs the ability of PC3/Tis21 to inhibit cyclin D1 expression. An analysis of HDAC4 deletion mutants shows that both the amino-terminal moiety and the catalytic domain of HDAC4 associate to PC3/Tis21, but neither alone is sufficient to potentiate the inhibition of cyclin D1 by PC3/Tis21. As a whole, our findings indicate that PC3/Tis21 inhibits cell proliferation in a way dependent on the presence of HDACs, in fibroblasts as well as in neural cells. Considering that several reports have demonstrated that HDACs can act as transcriptional corepressors on the cyclin D1 promoter, our data suggest that the association of PC3/Tis21 to HDACs is functional to recruit them to target genes, such as cyclin D1, for repression of their expression. J. Cell. Physiol. 232: 1696-1707, 2017. © 2016 Wiley Periodicals, Inc.

Exercise protects against methamphetamine-induced aberrant neurogenesis

While no effective therapy is available for the treatment of methamphetamine (METH)-induced neurotoxicity, aerobic exercise is being proposed to improve depressive symptoms and substance abuse outcomes. The present study focuses on the effect of exercise on METH-induced aberrant neurogenesis in the hippocampal dentate gyrus in the context of the blood-brain barrier (BBB) pathology. Mice were administered with METH or saline by i.p. injections for 5 days with an escalating dose regimen. One set of mice was sacrificed 24 h post last injection of METH, and the remaining animals were either subjected to voluntary wheel running (exercised mice) or remained in sedentary housing (sedentary mice). METH administration decreased expression of tight junction (TJ) proteins and increased BBB permeability in the hippocampus. These changes were preserved post METH administration in sedentary mice and were associated with the development of significant aberrations of neural differentiation. Exercise protected against these effects by enhancing the protein expression of TJ proteins, stabilizing the BBB integrity, and enhancing the neural differentiation. In addition, exercise protected against METH-induced systemic increase in inflammatory cytokine levels. These results suggest that exercise can attenuate METH-induced neurotoxicity by protecting against the BBB disruption and related microenvironmental changes in the hippocampus.

Jujube-containing herbal decoctions induce neuronal differentiation and the expression of anti-oxidant enzymes in cultured PC12 cells

ETHNOPHARMACOLOGICAL RELEVANCE

The fruit of Ziziphus jujuba (Mill.), known as Jujuba Fructus (JF) or jujube, is a well-known Traditional Chinese Medicine (TCM) for blood nourishment and sedation effect. Apart from prescribing as single herb alone, JF is very often being included in multi-herbal decoctions to prolong, enhance and harmonize pharmaceutical effects of decoctions while at the same time reducing toxicity. Here, we aimed to compare the protective and differentiating activities of three chemically standardized jujube-containing decoctions, including Guizhi Tang (GZT), Neibu Dangguijianzhong Tang (NDT) and ZaoTang (ZOT) in cultured PC12 cells.

MATERIALS AND METHODS

The protein expressions of neurofilaments, including NF68, NF160 and NF200, under the herbal treatment were revealed by western blot. The determination of neurite outgrowth in cultured PC12 cells upon the treatment of herbal extracts was performed by light microscope equipped with a phase-contrast condenser and SPOT imaging software. The protective effect against tBHP-induced cytotoxicity under the herbal treatment was measured by MTT assay. A luciferase reporter construct carrying four repeats of anti-oxidant response element (ARE) and a downstream luciferase reporter gene luc2P was transfected into PC12 cells to study the transcriptional activation of ARE. The mRNA expression of antioxidant enzymes under the herbal treatment was analyzed by quantitative real-time PCR.

RESULTS

These jujube-containing decoctions processed similar neuro-protective and brain beneficial properties. The herbal treatment induced the protein expression of neurofilaments. Neurite outgrowth was observed under the herbal treatment. In parallel, the pre-treatment of herbal extracts protected PC 12 cells against oxidative stress-induced apoptosis in a dose-dependent manner. Moreover, the herbal treatments triggered the mRNA expressions of relevant anti-oxidation genes, i.e. glutamate-cysteine ligase catalytic subunit (GCLC), glutamate-cysteine ligase modulatory subunit (GCLM), glutathione S-transferase (GST) and NAD(P)H quinone oxidoreductase (NQO1) via the activation of anti-oxidant response element (ARE).

CONCLUSION

The results therefore demonstrated neuro-protective and differentiating properties of the three closely related decoctions, and which subsequently illustrated the enhancement function of jujube within a multi-herbal decoction.

Genetic manipulation of adult-born hippocampal neurons rescues memory in a mouse model of Alzheimer's disease

In adult mammals, neural progenitors located in the dentate gyrus retain their ability to generate neurons and glia throughout lifetime. In rodents, increased production of new granule neurons is associated with improved memory capacities, while decreased hippocampal neurogenesis results in impaired memory performance in several memory tasks. In mouse models of Alzheimer's disease, neurogenesis is impaired and the granule neurons that are generated fail to integrate existing networks. Thus, enhancing neurogenesis should improve functional plasticity in the hippocampus and restore cognitive deficits in these mice. Here, we performed a screen of transcription factors that could potentially enhance adult hippocampal neurogenesis. We identified Neurod1 as a robust neuronal determinant with the capability to direct hippocampal progenitors towards an exclusive granule neuron fate. Importantly, Neurod1 also accelerated neuronal maturation and functional integration of new neurons during the period of their maturation when they contribute to memory processes. When tested in an APPxPS1 mouse model of Alzheimer's disease, directed expression of Neurod1 in cycling hippocampal progenitors conspicuously reduced dendritic spine density deficits on new hippocampal neurons, to the same level as that observed in healthy age-matched control animals. Remarkably, this population of highly connected new neurons was sufficient to restore spatial memory in these diseased mice. Collectively our findings demonstrate that endogenous neural stem cells of the diseased brain can be manipulated to become new neurons that could allow cognitive improvement.

Neurogenesis of the spiral ganglion cells in the cochlea requires the transcriptional cofactor TIS21

The molecular mechanisms controlling the proliferation and differentiation of spiral ganglion cells (SGCs) in the inner ear are still largely unknown. TIS21 is a transcriptional cofactor that shows antiproliferative, antiapoptotic, and prodifferentiative effects on neural progenitor cells. To investigate the function of TIS21 during SGC development, we analyzed SGC neurogenesis from embryonic day 13.5 (E13.5) to postnatal day 4 (P4) in Tis21-GFP knock-in mice, in which the protein-encoding exon of the Tis21 gene was replaced by EGFP. Through E13.5 to P4, we found fewer SGCs in homozygous Tis21-GFP knock-in mice than in wild-type mice. Our results suggest that TIS21 is required for development of SGCs. Deleting Tis21 may affect progenitor cells or neuroblasts at the beginning of cochlear-vestibular ganglion formation and would consequently lead to a decrease in the number of SGCs.

Tis21 is required for adult neurogenesis in the subventricular zone and for olfactory behavior regulating cyclins, BMP4, Hes1/5 and Ids

Bone morphogenic proteins (BMPs) and the Notch pathway regulate quiescence and self-renewal of stem cells of the subventricular zone (SVZ), an adult neurogenic niche. Here we analyze the role at the intersection of these pathways of Tis21 (Btg2/PC3), a gene regulating proliferation and differentiation of adult SVZ stem and progenitor cells. In Tis21-null SVZ and cultured neurospheres, we observed a strong decrease in the expression of BMP4 and its effectors Smad1/8, while the Notch anti-neural mediators Hes1/5 and the basic helix-loop-helix (bHLH) inhibitors Id1-3 increased. Consistently, expression of the proneural bHLH gene NeuroD1 decreased. Moreover, cyclins D1/2, A2, and E were strongly up-regulated. Thus, in the SVZ Tis21 activates the BMP pathway and inhibits the Notch pathway and the cell cycle. Correspondingly, the Tis21-null SVZ stem cells greatly increased; nonetheless, the proliferating neuroblasts diminished, whereas the post-mitotic neuroblasts paradoxically accumulated in SVZ, failing to migrate along the rostral migratory stream to the olfactory bulb. The ability, however, of neuroblasts to migrate from SVZ explants was not affected, suggesting that Tis21-null neuroblasts do not migrate to the olfactory bulb because of a defect in terminal differentiation. Notably, BMP4 addition or Id3 silencing rescued the defective differentiation observed in Tis21-null neurospheres, indicating that they mediate the Tis21 pro-differentiative action. The reduced number of granule neurons in the Tis21-null olfactory bulb led to a defect in olfactory detection threshold, without effect on olfactory memory, also suggesting that within olfactory circuits new granule neurons play a primary role in odor sensitivity rather than in memory.