Browsing by Subject "Neurogenesis"
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Item Activation of early neural progenitors is required for traumatic brain injury-induced hippocampal neurogenes(2008-09-19) Yu, Tzong-Shiue; Kernie, Steven G.Traumatic brain injury (TBI) is the most common form of acquired brain injury in both children and adults in the United States. TBI causes neuronal loss and results in a variety of neurological impairments and deficits in hippocampus-dependent functions. However, cognitive recovery commonly occurs though the mechanism is unknown. Exploration of post-natal neurogenesis in the hippocampus raises the possibility that adult-born neurons may contribute to cognitive recovery from TBI. Several studies in animal models that mimic TBI demonstrate there is enhanced generation of adult-born neurons in the dentate gyrus and those adult-born neurons may correlate with cognitive recovery. Due to the limits of current methodology in studying neurogenesis, it remains unclear what relevance injury-induced neurogenesis may have in the recovery process following TBI. In order to explore the relevance of injury-induced neurogenesis, I have characterized a previously generated transgenic mouse line that has rtTA-IRES-eGFP expression under the control of a nestin promoter and also contains a neural progenitor-specific regulatory element. By using this line, I have demonstrated that eGFP-expressing cells represent early neural progenitors in the adult dentate gyrus. Performing unilateral controlled cortical injury (CCI) demonstrates that this injury depletes doublecoritn (Dcx)-expressing late neural progenitors while activating eGFP-expressing early neural progenitors. To address whether the subsequent recovery of Dcx-expressing late progenitors was derived from activation of early neural progenitors, I generated a transgenic line that expresses modified herpes simplex viral thymidine kinase (delta-HSV-TK) under the control of the neural progenitor-specific regulatory element of the nestin gene. This allows for temporally regulated ablation of dividing neural progenitors by exposing the animal to ganciclovir. Using this line, I demonstrate that ablation of dividing GFP-expressing early neural progenitors in neurogenic areas occurs only in the presence of ganciclovir. CCI on these mice, reveals that no newly born Dcx-expressing late neural progenitors are observed seven days after injury when exposed to ganciclovir. However, the repopulation of Dcx-expressing cells is apparent when ganciclovir was removed one day before injury. Four weeks after injury, those newly born Dcx-expressing cells became mature NeuN-expressing neurons. This suggests that injury-induced activation of early neural progenitors is required for the recovery of injured hippocampal neurons.Item EPH-B and Ephrin-B Signaling In Migration and Proliferation of Stem Cells(2011-08-26T17:34:14Z) Catchpole, Timothy; Hinkimeyer, MarkeIn this dissertation, I investigate the role of Eph-ephrin signaling in the Dentate Gyrus (DG). The DG is a distinctive neuronal structure located in the hippocampus and is one of two areas in the mature brain where stem and progenitor cells reside to continuously produce new neurons throughout adulthood. While Eph-ephrin signaling has been linked to other stem cell populations in the adult, involvement in the progenitor population residing in the hippocampus had not been demonstrated. Here, I establish the expression of B subclass Eph receptors in both embryonic and adult progenitors in the hippocampus . Analysis of EphB1 -/-mutant mice shows that this receptor tyrosine kinase is involved in the regulation of proliferation and polarity of progenitor cells in the neurogenic niche of the DG. Ephrin-B3 acts as a ligand to regulate some aspects of EphB1 activity in the DG. I also show that the EphB2 receptor tyrosine kinase is critical for normal formation of a specific region of the DG known as the lateral suprapyramidal blade (LSB) during late embryonic and early postnatal development. Analysis of intracellular truncation and single amino acid point mutations demonstrates that the tyrosine kinase catalytic activity of EphB2 is essential for LSB formation. This activity is consistent with specific expression of EphB2 in the neural progenitor cells that migrate in a medial direction from the dentate notch of the lateral ventricles to populate and form the DG near the midline of the brain. I further show that ephrin-B1 alone acts as the ligand to activate EphB2 forward signaling in these migrating neural progenitors to contribute to the formation of this vitally important structure. Finally I briefly describe the role of EphB2 forward signaling in stem cell populations beyond the hippocampus. This data demonstrates that Eph-ephrin signaling is intimately involved in both the formation of the neurogenic niche and in the regulation of progenitor cells that occupy that niche.Item Neurogenesis and Gliogenesis from Ascl1 (Mash1) Expressing Progenitors in the Central Nervous System(2010-05-14) Kim, Euiseok Joshua; Johnson, JaneFor the functional architecture of the central nervous system, a small population of neural stem cells generates the correct numbers and types of neurons, oligodendrocytes and astrocytes in a precisely coordinated manner. Basic helix-loop-helix (bHLH) transcription factors play central roles in determining distinct neural cell fates and thus contribute to mechanisms controlling neural cell type diversity during the embryogenesis. Fundamental to understanding nervous system formation is to uncover links between early cell type specification mechanisms, the developmental dynamics of each lineage, and the contributions of specific molecules to these processes to form the mature nervous system structures. Ascl1 (previously Mash1) is a bHLH transcription factor essential for neuronal differentiation and neural sub-type specification. Ascl1 is present in proliferating progenitor cells but these cells are actively differentiating as evidenced by their rapid migration out of germinal zones. Although it has been studied for its role in several neural lineages, the full complement of lineages arising from Ascl1 progenitor cells and the molecular mechanism of Ascl1’s functions are not completely understood. Using an inducible Cre-flox genetic fate-mapping strategy, Ascl1 lineages were determined in both the embryonic and adult central nervous system. In chapter two, the fate of Ascl1+ progenitor cells throughout the brain was described. Depending on the temporal and spatial context during embryogenesis, Ascl1+ cells contribute to distinct neuronal and glial cells in each major brain division. In chapter three, by labeling Ascl1+ progenitor cells at distinct phases of their development, I delineated the temporal lineage relationship of distinct subtypes of neurons and glia in the developing spinal cord. Two spatially and temporally distinct Ascl1+ progenitor populations contribute differentially to inhibitory dILA and excitatory dILB neurons in the dorsal spinal cord. At later stages of embryogenesis, Ascl1+ progenitors are restricted to glial lineages giving rise to both astrocytes and oligodendrocytes. Analysis of conditional mutants of Ascl1 demonstrated that Ascl1 is required for only one division of each lineage. Loss of Ascl1 results in a reduction of inhibitory dILA neurons and oligodendrocytes, but not excitatory dILB neurons and astrocytes. In chapter four, the physiological functions of Nicastrin in gliogenesis were investigated. Nicastrin is a requisite subunit of the !-secretase complex essential for activating Notch signaling pathway. Conditional mutant of Nicastrin leads to the increased level of oligodendrocytes lineage markers in the neural tube, the opposite phenotype of that for Ascl1. Thus, I propose that Notch signaling in constraining levels of Ascl1 is required in oligodendrogenesis. In chapter five, I revealed that Ascl1 is a common molecular marker of early progenitors of both neurons and oligodendrocytes in the adult brain, and these Ascl1 defined progenitors mature with distinct dynamics in different brain regions. In this thesis, I define Ascl1 as a neural differentiation factor crucial for neural cell type diversification, playing important roles in cell differentiation and subtype specification at several different nodes of cell fate decisions throughout neurodevelopment.Item Regulation and Function of PTF1a in the Developing Nervous System(2012-08-15) Meredith, David Miles; Johnson, Jane E.Basic helix-loop-helix transcription factors serve many roles in development, including regulation of neurogenesis. Many of these factors are activated in naive neural progenitors and function to promote neuronal differentiation and cell-type specification. Ptf1a is a basic helix-loop-helix protein that is required for proper inhibitory neuron formation in several regions of the developing nervous system, including the spinal cord, cerebellum, retina, and hypothalamus. In addition, Ptf1a is essential for proper pancreas formation and exocrine function. In both the nervous system and pancreas, Ptf1a functions as a switch in cell fate determination. In the absence of Ptf1a, inhibitory neurons are lost and those cells instead adopt an excitatory identity. Similarly, endodermal progenitors will assume duodenal characteristics in place of a pancreatic identity when Ptf1a is lost. Like most other tissue-specific basic-helix-loop-helix factors, Ptf1a dimerizes with E-proteins and binds a degenerate hexameric E-box motif (CANNTG). Ptf1a is unique, however, in that it also requires the presence of Rbpj(l) to form an active transcription complex, PTF1. This interaction is central to Ptf1a function, as disruption of Ptf1a?s ability to bind Rbpj in vivo phenocopies the Ptf1a null in the nervous system and pancreas. Similarly, all targets described thus far for Ptf1a require an intact PTF1 binding site, which includes both an E-box and Rbpj binding site. In order to understand how a factor such as Ptf1a is capable of giving rise to such disparate organs, I wanted to place it in context of a larger regulatory network that directs a multipotent progenitor into a mature inhibitory neuron. Thus, I examine two regulatory schemes controlling Ptf1a expression during development in Chapters two and three. I then investigate direct Ptf1a targets in a genome-wide fashion using massively parallel sequencing technology in Chapters four and five. These efforts uncovered that Ptf1a employs several mechanisms to achieve proper cell-type specification, including initiation of transcription factor cascades, direct activation of inhibitory neuron machinery, and direct suppression of the excitatory neuron program. Furthermore, I identify novel binding modes and potential co-regulatory factors that could impart tissue-specific function. [Keywords: Ptf1a; neural tube; pancreas; Rbpj; bHLH; ChlP-Seq; RNA-Seq; transcription; development; GABAergic]Item Regulation of Adult Hippocampal Neurogenesis: Insights from Mouse Models of Dementia and Depression(2009-05-13) Donovan, Michael Harry; Eisch, Amelia J.While neurogenesis is largely complete by birth, the subgranular zone (SGZ) in the adult hippocampus continues to produce functional young neurons. The last decade has produced a multitude of research demonstrating that the process of SGZ neurogenesis is dynamically regulated. Stimuli that negatively impact SGZ neurogenesis include stress, depression models, aging and models of neurodegenerative disease. Positive regulators of SGZ neurogenesis include antidepressants and hippocampal-dependent learning. These results have sparked tremendous speculation, both scientific and popular, that adult hippocampal neurogenesis might be critical for mood regulation and/or memory, and might be a promising target for the treatment of depression and dementia. However, we still know little about underlying mechanisms of how increases and decreases in SGZ neurogenesis occur. Here, I examine several manipulations of adult hippocampal neurogenesis, focusing on potential neuromechanisms underlying alterations in SGZ neurogenesis. First, in a mouse model of dementia, I find that in addition to agedependant decline in SGZ proliferation, these mice have retarded migration and maturation of new SGZ neurons and ectopic proliferation in a normally non-neurogenic region. Second, I explore how the antidepressant fluoxetine increases SGZ neurogenesis. I show that the increase occurs only after chronic administration and is not preceded by changes in cell death, cell-cycle or proliferating cell lineage. I next address the capacity of proliferating SGZ cells to respond to brain-derived neurotrophic factor (BDNF), a neurochemical implicated in antidepressant action and neurogenesis regulation. I find that most proliferating cells do not contain the necessary TrkB receptors in vivo, and thus BDNF action is likely indirect or through type-1 stem cells, which contain TrkB. Finally, I look at changes in neurogenesis in a social-defeat depression model. I find that, like other models of repeated stress, social-defeat stress appears to produce a stress-induced decrease in S-phase cells. However, closer analysis reveals that this decrease does not indicate decreased proliferation, and mice that are behaviorally sensitive to the stress actually show an increase in neurogenesis overall. Taken together, these results emphasize the complexity of the processes that comprise adult hippocampal neurogenesis, highlighting the importance of further investigation into the neuromechanisms of changes in neurogenesis.Item The Role of Adult Neurogenesis in Cocaine Addiction(2009-01-14) Noonan, Michele Ann; Eisch, AmeliaNew neurons are born in the adult hippocampus in a region known as the subgranular zone (SGZ). This process is dynamically regulated and new neurons are thought to be important for certain types of spatial learning and memory. Proliferation of SGZ neural progenitors is decreased by drugs of abuse, yet it is not clear how the type and amount of drug as well as the pattern of administration changes long-term effects on neurogenesis. In addition, it is unclear what role if any SGZ neurogenesis plays in initiating drug-taking or relapse behaviors, or whether changes in neurogenesis are merely side effects of drug-taking. I first examined effects of chronic cocaine self-administration and withdrawal on the different stages of neurogenesis. I found an early deficit in proliferation of neural progenitors, as well as a 4 week delayed increase in doublecortin-positive (DCX+) immature neurons which were common to both rats in withdrawal or those continuing to self-administer cocaine. I next asked the question of the functional consequence of changes in adult hippocampal neurogenesis to the acquisition and maintenance of drug-taking, as well as relapse to drug-taking. I found that reduced adult neurogenesis via cranial irradiation prior to cocaine-taking was associated with increased acquisition of drug-taking and increased motivation for cocaine, but not sucrose, while reduced adult neurogenesis after rats have acquired cocaine self-administration was associated with increased resistance to extinction of drug-seeking behavior. Finally, I asked if formation of drug-context associations would be altered in rodents with reduced neurogenesis in a passive drug exposure paradigm. I found that a transgenic mouse with reduced adult neurogenesis has impaired long-term drug-context memory in the cocaine conditioned place preference paradigm (CPP). Together these findings suggest that reduced adult hippocampal neurogenesis is a risk factor for drug addiction, that decreased proliferation after chronic drug intake likely contributes to drug-taking and drug-seeking behaviors, and that the delayed increase in immature neurons after drug-taking is likely protective against relapse. In sum, increases in adult hippocampal neurogenesis are beneficial both to the naïve and addicted brain, and therapeutics specifically increasing adult neurogenesis could aid in preventing initial addiction as well preventing future relapse.Item The Role of Glutamate Transporters in Early Postnatal Hippocampal Neurogenesis(2011-02-01T19:33:49Z) Gilley, Jennifer A.; Kernie, StevenAdult neurogenesis has been well-characterized in the subgranular zone (SGZ) of the hippocampal dentate gyrus however, early postnatal development of the dentate gyrus and changes in the neurogenic niche during this time have not been well-studied. Using a well-characterized transgenic mouse that labels early neural progenitor cells with green fluorescent protein (GFP), we created a developmental profile of the dentate gyrus from postnatal day seven (P7) to six months of age. In addition, we determined that early progenitor populations within the developing dentate gyrus exhibit age-dependent changes in proliferation and differentiation which are controlled by cell-autonomous cues. To identify potential regulators of these phenotypes, we performed microarrays and identified several differentially expressed genes within the progenitor pools of different aged mice. GltI, a glutamate transporter, was identified as a candidate which was upregulated 10 fold in progenitors from older animals. In astrocytes, GltI and Glast are required to maintain low levels of glutamate to prevent overstimulation of glutamate receptors. In neural precursors it has been suggested that glutamate stimulates proliferation by activating metabotropic glutamate receptors (mGluRs) which leads to increased intracellular calcium, however the function of glutamate transporters on these progenitors has not been identified. To elucidate the functional role of GltI and Glast, we performed in vitro experiments in glutamate-free media. By misexpressing GltI and Glast, we show that glutamate transporters negatively regulate calcium-dependent proliferation by controlling glutamate availability to mGluRs. To address the in vivo function of glutamate transporters in injury-induced neurogenesis, we characterized their expression after hypoxic-ischemic (HI) injury and noticed prolonged upregulation of both transporters on type I cells suggesting they may be involved in hypoxic preconditioning. To address this we induced expression of glutamate transporters with HI before exposing mice to traumatic brain injury (TBI). Compared to animals only injured with TBI, mice with both injuries displayed decreased progenitor proliferation suggesting an impaired capacity for repair. We have therefore identified a novel and clinically relevant role for GltI and Glast in progenitor proliferation during development and after injury.Item Role of Mash1-E Protein Heterodimers in Mash1 Function in the Developing Neural Tube(2003-05-01) Collisson, Tandi Louise; Lu, Qing RichardNeural-specific Class II bHLH transcription factors heterodimerize with ubiquitous Class I bHLH E proteins to form complexes required for neural differentiation. There are four known E proteins, HEB, E12, E47 and E2.2, in the mammalian nervous system, which potentially form heterodimers with Mash1 in the neural tube. To test the relevance of particular Mash1-E protein heterodimer combinations in vivo, I constructed tethered Mash1-E protein heterodimers for over-expression in the chick neural tube. By comparing overexpression of Mash1 with over-expression of these Mash1-E protein heterodimers, their abilities to effect neural differentiation and cell-type specification were analyzed. Mash1-E protein heterodimers are interchangeable in the function of driving neurogenesis in the chick neural tube. The effects of Mash1-E protein heterodimers on cell-type specificity were different, suggesting non-redundant functions in effecting dorsal interneuron populations. Furthermore, additional Mash1 heterodimer partners may be required for the cell-type specification function of Mash1.Item Transcriptional Regulation of Adult Neurogenesis by NRSF/REST and NeuroD1(2011-08-26T17:35:21Z) Ure, Kerstin Maria; Hsieh, JennyNeurogenesis in the adult brain is a complex and lifelong process that is regulated by multiple pathways and is sensitive to many external stimuli. Two critical regulatory factors in this process are NRSF/REST and NeuroD1. NRSF/REST, a transcriptional repressor that binds a specific NRSE site and recruits corepressors and chromatin remodeling machinery to repress its target genes, is critical for maintenance of the neural stem cell pool and for proper pacing of neuronal differentiation. NeuroD1, a bHLH transcription factor, is necessary for the terminal differentiation, maturation, and survival of newborn neurons. In addition, both factors are necessary for the neurogenic response to both physiological and pathological stimuli, which may induce neurogenesis through different pathways. Thus, NRSF/REST and NeuroD1 are necessary for neurogenesis to occur correctly, to persist throughout the organism’s lifespan, and to respond to external stimuli.