Adult neurogenesis is a conserved feature of the central nervous system across the animal kingdom. This process takes place in restricted neurogenic niches of the brain, where active populations of adult stem/progenitor cells are capable of producing newborn neurons. The niche is tightly controlled by intrinsic signals within the microenvironment and from stimuli arising from the external world, which together determine the cellular behaviour of the niche and neuronal output. Currently, our understanding of the biological properties of adult neurogenesis rests mainly on two niches of the vertebrate forebrain. To broaden our view of the diversity of this trait comparative models and new niches must be explored. Here, I have taken advantage of the robust neurogenic capacity of the adult zebrafish brain to examine differences in forebrain and sensory neurogenic niches in regards to cytoarchitectural organization, neurogenic plasticity, and regulation. Five principle findings emerge: (1) up to six morphologically distinct cell types compose forebrain and sensory niches, and are devoid of ependymal cells; (2) heterogeniety in the phenotype of the stem/progenitor cell exists across niches; some having radial glial characteristics; (3) active populations of proliferating stem/progenitor cells reside within primary sensory structures of the adult brain, forming a “sensory neurogenic niche”; different from other models of adult neurogenesis; (4) changes in the social environment induce neurogenic plasticity in sensory niches more readily than integrative niches of the forebrain, and occur independently of cortisol levels; (5) modality-specific stimulation influences stages of adult neurogenesis exclusively in corresponding primary sensory niches as a result of sensory-dependent neurogenic plasticity. Additionally, I have shown that Fibroblast Growth Factor signalling may not be involved in maintaining cell proliferation in sensory niches. These studies showcase the diverse properties of forebrain and sensory neurogenic niches and provide a new perspective concerning the functional role of adult neurogenesis.

Throughout adulthood, the brain continuously generates new neurons in two neurogenic regions: the subgranular zone of the hippocampus and the subventricular zone on the lateral wall of the lateral ventricles. These neurons have been shown to integrate into hippocampal and olfactory bulb circuitry, respectively. Nevertheless, their specific contribution to hippocampal or olfactory function remains unclear. Previous studies have tried to assess adult born neuron contribution to memory function by suppressing neurogenesis and examining the impact on memory acquisition. Although ablation of neurogenesis has been shown to impair performance in hippocampus dependent and olfactory tasks, many studies fail to see an effect. Compensation from residual cells in either system after ablation may underlie these contradictory findings. Thus, a more direct approach to answer this question would be to ablate adult born neurons after their incorporation into the memory trace. To do this, we established a double transgenic strategy to tag and selectively ablate adult born neurons with temporal control. Ablation of a population of predominantly mature, adult generated dentate granule cells did not prevent acquisition of contextual fear conditioning or Morris Water Maze memories. Removal of that same population of cells after training, however, led to memory degradation in three hippocampus dependent tasks. Similarly, post-training ablation of a population of adult generated olfactory interneurons iii impaired performance in an associative odour memory task, whereas pre-training ablation had no impact. Together, these data show that adult generated neurons form a crucial component of both hippocampal and olfactory memory traces.

Although once thought to be incapable of regeneration, the adult mammalian brain generates new neurons in two regions: the SVZ of the lateral ventricle and the DG of the hippocampus. While the cell types involved in adult neurogenesis have been broadly characterized, the transcriptional regulation of this process remains poorly understood. Here, we demonstrate that transcription factor Cux2 is important for normal postnatal hippocampal neurogenesis. Cux2neo/neo mutant mice generated fewer Dcx-positive neuroblasts, Tbr2-positive transit amplifers, and Calretinin-positive immature neurons, without affecting gliogenesis. Moreover, we show that Cux2 is principally expressed in Type1/Type2a cells. Using cultured embryonic NPCs we show that Cux2 mutants generate fewer neurons. Indeed, Cux2 plays a pro-neuronal role in both the postnatal hippocampus and in cultured embryonic NPCs. Cux2 may thus serve as an important regulator of the neuronal fate and may be a novel marker for neuronally committed Type 1/2a NPCs in the postnatal DG.

Adult neurogenesis is a conserved feature of the central nervous system across the animal kingdom. This process takes place in restricted neurogenic niches of the brain, where active populations of adult stem/progenitor cells are capable of producing newborn neurons. The niche is tightly controlled by intrinsic signals within the microenvironment and from stimuli arising from the external world, which together determine the cellular behaviour of the niche and neuronal output. Currently, our understanding of the biological properties of adult neurogenesis rests mainly on two niches of the vertebrate forebrain. To broaden our view of the diversity of this trait comparative models and new niches must be explored. Here, I have taken advantage of the robust neurogenic capacity of the adult zebrafish brain to examine differences in forebrain and sensory neurogenic niches in regards to cytoarchitectural organization, neurogenic plasticity, and regulation. Five principle findings emerge: (1) up to six morphologically distinct cell types compose forebrain and sensory niches, and are devoid of ependymal cells; (2) heterogeniety in the phenotype of the stem/progenitor cell exists across niches; some having radial glial characteristics; (3) active populations of proliferating stem/progenitor cells reside within primary sensory structures of the adult brain, forming a “sensory neurogenic niche”; different from other models of adult neurogenesis; (4) changes in the social environment induce neurogenic plasticity in sensory niches more readily than integrative niches of the forebrain, and occur independently of cortisol levels; (5) modality-specific stimulation influences stages of adult neurogenesis exclusively in corresponding primary sensory niches as a result of sensory-dependent neurogenic plasticity. Additionally, I have shown that Fibroblast Growth Factor signalling may not be involved in maintaining cell proliferation in sensory niches. These studies showcase the diverse properties of forebrain and sensory neurogenic niches and provide a new perspective concerning the functional role of adult neurogenesis.

Throughout adulthood, the brain continuously generates new neurons in two neurogenic regions: the subgranular zone of the hippocampus and the subventricular zone on the lateral wall of the lateral ventricles. These neurons have been shown to integrate into hippocampal and olfactory bulb circuitry, respectively. Nevertheless, their specific contribution to hippocampal or olfactory function remains unclear. Previous studies have tried to assess adult born neuron contribution to memory function by suppressing neurogenesis and examining the impact on memory acquisition. Although ablation of neurogenesis has been shown to impair performance in hippocampus dependent and olfactory tasks, many studies fail to see an effect. Compensation from residual cells in either system after ablation may underlie these contradictory findings. Thus, a more direct approach to answer this question would be to ablate adult born neurons after their incorporation into the memory trace. To do this, we established a double transgenic strategy to tag and selectively ablate adult born neurons with temporal control. Ablation of a population of predominantly mature, adult generated dentate granule cells did not prevent acquisition of contextual fear conditioning or Morris Water Maze memories. Removal of that same population of cells after training, however, led to memory degradation in three hippocampus dependent tasks. Similarly, post-training ablation of a population of adult generated olfactory interneurons iii impaired performance in an associative odour memory task, whereas pre-training ablation had no impact. Together, these data show that adult generated neurons form a crucial component of both hippocampal and olfactory memory traces.

Adult neurogenesis is a process that produces neurons in the adult brain and garners potential for the development of novel therapeutic interventions to combat neurodegenerative and other brain related diseases. With the hope of increasing neurogenesis, active investigations are defining the cellular and molecular mechanisms that regulate adult neural precursor cell (NPC) survival, and thus maintain neurogenesis. Recently, autophagy, an intracellular recycling pathway, has been implicated in regulating adult NPCs in embryonic knockout mice models. Whether autophagy has a similar effect within the adult and how autophagy regulates development of adult NPC remains unknown. Here, we investigate the role of Beclin1, a gene responsible for autophagy induction, in adult hippocampal NPC function in mice. Retroviral-mediated removal of Beclin1 from proliferating adult NPCs in vivo led to a reduction in the survival of adult-born neurons. In addition, Beclin1 was removed specifically from nestin-expressing adult neural stem- and progenitor-cells through the development of a Beclin1 nestin-inducible knockout mouse. Beclin1 nKO mice had a reduction in NPC proliferation and development, and overall fewer adult-generated neurons. Together, these findings reveal Beclin1 is required for adult hippocampal neurogenesis through regulating the proliferation and survival of the NPCs, in the absence of changing NPC fate.

Stroke is the leading cause of long-term neurological disability worldwide, signifying the need for viable therapeutic options. Pre-clinical and post-mortem stroke studies have demonstrated that stroke increases the number of newborn progenitor cells (PCs) in the adult brain that can migrate to the site of injury. While there is a positive correlation between increasing neurogenesis and improvements in stroke recovery, methods used to increase PCs and neurogenesis also alter many other forms of plasticity, making it difficult to determine the function of PCs per se. To investigate whether specifically enhancing PC survival is sufficient to improve recovery, the iBax transgenic mouse model was used to remove the pro-apoptotic gene Bax inducibly from nestin-expressing PCs either before or after focal strokes induced by photothrombosis. Increasing PC survival before or after stroke in the iBax mice increased the number of PCs in the peri-infarct region. Interestingly, the majority of the cells that migrated to the peri-infarct region expressed the glial fibrillary acidic protein (GFAP) which is found in astrocytes when Bax was removed prior to stroke, yet when Bax was removed after stroke the majority of the cells expressed doublecortin (DCX) which is expressed in neuroblasts. Irrespective of this significant increase in the different populations of surviving PCs following stroke, there was no change in long-term behavioural deficits on the adhesive removal, horizontal ladder, and cylinder tasks up to 90 days post stroke. Additionally, enhancing PC survival before or after stroke resulted in a significant increase in adult-generated neurons within the dentate gyrus, which was associated with a modest change in spatial learning on the Barnes maze. Together, these experiments suggest strategies that enhance the survival of the PCs by preventing cell death will, by themselves, be insufficient to promote sensorimotor recovery following stroke.

Long-term maintenance of adult neural stem cells (NSCs) is an intricate process of activation, expansion, and differentiation while preserving the stem cell pool. Several regulatory mechanisms underlie the delicate balance in the choice between quiescence versus activation for lifelong NSC maintenance and continuous neurogenesis. Perturbations in this dynamic process result in disease manifestation. The quiescence/activation of NSC was shown to be regulated by the Rb/E2F axis through a molecular program mediated by REST (RE1 Silencing Transcription Factor). Loss of Rb family increased NSC activation at the expense of quiescence through activator E2F transcription factors. The activation and neurogenesis of NSCs were impaired by the loss of effector E2Fs, as well as loss of Opa1, the latter indicating that mitochondrial dynamics is important to maintain stem cell state. Single-cell transcriptome analysis from NSC lineages isolated from adult mouse hippocampus revealed that stem cell progenies are uniquely affected in Opa1-KO leading to impairments in NSC activation and differentiation. Unbiased transcriptional profiling suggested a mitochondrial dysfunction in Opa1-KO that results in activation of classic cellular stress response pathway genes (Atf4, Slc7a11 and Chac1). Thus, the regulatory gene network comprising quiescence (Rb) and activation (E2Fs) programs, and mitochondrial metabolism (Opa1) and their interplay ensures the maintenance of the molecular program of NSC, particularly revealing how it enables stem cells survive stress.

In the mature dentate gyrus (DG) new neurons are continuously produced in a process known as adult neurogenesis. These neurons are thought to contribute to learning and memory processes, but their precise function is still not fully understood. The rate of neuronal production and the development of young neurons are affected by stimuli such as physical exercise and learning events. The neurotransmitter γ-aminobutyric acid (GABA) has recently emerged as a possible key mediator of this activity-dependent regulation. My study examined the role of a subtype of GABAA receptors that contains the δ subunit (δGABA(A)R) in the regulation of adult hippocampal neurogenesis. Mice lacking the δ subunit (Gabrd−/− or δ-null mice) displayed decreased maturation, migration and dendritic complexity of adult-born neurons. Conversely, following treatment with a selective δGABA(A)R agonist, neuronal maturation was promoted in wild-type, but not δ-null mice. These results indicate a key role for δGABA(A)R in activity-dependent regulation of adult neurogenesis. When administered to rats, δGABA(A)R agonists promoted neuronal survival as well as maturation. The effect on maturation was blocked by the N-Methyl-D-aspartate (NMDA) blocker AP-5, suggesting that some aspects of δGABA(A)R-mediated regulation require the activation of NMDA receptors. To further understand the contribution of adult born neurons to memory function, neurogenesis in rats was alternatively suppressed using ionizing radiation, or enhanced by allowing the rats to engage in running. The rats were then trained and tested in a behavioral paradigm designed to assess their susceptibility to memory interference, a phenomenon which occurs when similar memories are not sufficiently distinguished from one another. Irradiated rats exhibited increased susceptibility to memory interference, indicating an important role for adult-born neurons in the encoding and/or retrieval of distinct memories. Remarkably, irradiated rats that engaged in running exhibited increased neuronal growth and a complete reversal of the memory impairment,emphasizing the beneficial effects of physical exercise on cognitive functions. Taken together, the findings reported in this dissertation offer novel information about the process of adult neurogenesis and its physiological significance.

In the mature dentate gyrus (DG) new neurons are continuously produced in a process known as adult neurogenesis. These neurons are thought to contribute to learning and memory processes, but their precise function is still not fully understood. The rate of neuronal production and the development of young neurons are affected by stimuli such as physical exercise and learning events. The neurotransmitter γ-aminobutyric acid (GABA) has recently emerged as a possible key mediator of this activity-dependent regulation. My study examined the role of a subtype of GABAA receptors that contains the δ subunit (δGABA(A)R) in the regulation of adult hippocampal neurogenesis. Mice lacking the δ subunit (Gabrd−/− or δ-null mice) displayed decreased maturation, migration and dendritic complexity of adult-born neurons. Conversely, following treatment with a selective δGABA(A)R agonist, neuronal maturation was promoted in wild-type, but not δ-null mice. These results indicate a key role for δGABA(A)R in activity-dependent regulation of adult neurogenesis. When administered to rats, δGABA(A)R agonists promoted neuronal survival as well as maturation. The effect on maturation was blocked by the N-Methyl-D-aspartate (NMDA) blocker AP-5, suggesting that some aspects of δGABA(A)R-mediated regulation require the activation of NMDA receptors. To further understand the contribution of adult born neurons to memory function, neurogenesis in rats was alternatively suppressed using ionizing radiation, or enhanced by allowing the rats to engage in running. The rats were then trained and tested in a behavioral paradigm designed to assess their susceptibility to memory interference, a phenomenon which occurs when similar memories are not sufficiently distinguished from one another. Irradiated rats exhibited increased susceptibility to memory interference, indicating an important role for adult-born neurons in the encoding and/or retrieval of distinct memories. Remarkably, irradiated rats that engaged in running exhibited increased neuronal growth and a complete reversal of the memory impairment,emphasizing the beneficial effects of physical exercise on cognitive functions. Taken together, the findings reported in this dissertation offer novel information about the process of adult neurogenesis and its physiological significance.