Browsing by Subject "Dentate Gyrus"
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Item EPH-B and Ephrin-B Signaling in Migration and Proliferation of Stem Cells(2011-08-10) Catchpole, Timothy; Henkemeyer, MarkIn 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 Exploring the Role and Sensitivity of the Hippocampal Dentate Gyrus: From Addiction-Relevant Memories to the Influence of Space Radiation on Hippocampal Neurogenesis(2015-04-09) Rivera, Phillip Daniel; German, Dwight; Eisch, Amelia J.; Chen, Benjamin P.; Powell, Craig M.; Zhang, Chun-LiThe hippocampus and its subregion the dentate gyrus (DG) are involved in learning and memory. Adult hippocampal neurogenesis, which takes place in the DG, is also thought to contribute to learning and memory. Understanding the neural basis of learning and memory could help in a wide range of situations, from helping addicts break the cycle of substance abuse to ensuring appropriate astronaut action during spaceflight missions. This doctoral dissertation spans this wide range by using animal models relevant to addiction and spaceflight to improve understanding of the DG and adult neurogenesis, and obliquely, of learning and memory. After an introductory chapter, I show morphine-context reward memories are established via drug/context associations (D/CA, Chapter 2), and require adult neurogenesis for extinction of young reward memories (Chapter 3). Using conditioned place preference, a behavioral test classically used to assess drug strength, and the immediate early gene cFos as an indirect marker of neuronal activity, I found that morphine-paired mice sequestered to a morphine-paired context had more DG cFos+ cells than those sequestered to a saline-paired context or other controls. Thus, the retrieval of D/CA memory is accompanied by activation of hippocampal DG neurons. Surprisingly, image-guided cranial irradiation (IG-IR) prevented extinction of young, but not old, morphine D/CA memories without affecting retrieval. These data suggest that deficits in adult neurogenesis may contribute to stronger D/CA reward memory. The second section of my dissertation (Chapter 4) examines the influence of space radiation on adult neurogenesis. I find acute and fractionated space radiation similarly diminish adult neurogenesis, but neither decrease neural stem cell number, the putative source of new neurons. Thus, while spaceflight mission success may be hampered by space radiation due to diminished neurogenesis, my data raise the possibility that neurogenesis may recover overtime. Taken together, my data show an impaired DG (and perhaps neurogenesis) diminishes extinction of morphine-context reward memories, and that adult neurogenesis is decreased (perhaps reversibly) by space radiation. In my final chapter (Chapter 5), I discuss implications of these data for the fields of learning/memory and neuroscience in general, and suggest future directions that may help addicts recover and allow astronauts to perform optimally during spaceflight missions.Item The Impact of Chronic Morphine on Adult Hippocampal Progenitor Cells and the Neurogenic Niche(2009-06-15) Arguello, Amy; Eisch, Amelia J.The birth of new neurons persists in the adult hippocampal subgranular zone (SGZ). Adult neurogenesis is dynamically regulated and thought to be important for certain types of spatial learning and memory. SGZ proliferation and neurogenesis are decreased by chronic morphine, yet how this alteration occurs is unknown. It is unclear if morphine causes alterations in cell cycle progression, progenitor cell maturation, or indirectly inhibits progenitor cells by altering the hippocampal neurogenic niche. I first examined a time course of morphine's effect on the progenitor cell cycle, cell death and immature SGZ neurons. I found that S phase cycling cells were vulnerable to morphine at early time points with a concurrent increase in cell death. I found that although the total population of SGZ immature neurons remained unchanged, the proportion of progenitor cells that progressed to a more mature stage decreased. I next asked whether decreased levels of proliferation resulted from shortened S phase length. Using a modified double injection paradigm of halogenated thymidine analogs, I found that chronic morphine did not alter the length of S phase of progenitor cells. Next, I asked if chronic morphine could have an indirect inhibitory effect on progenitor cells by altering growth factors and neurovasculature within the hippocampal neurogenic niche. I found that protein levels of factors within the niche were maintained or upregulated (e.g. vascular endothelial growth factor) to compensate for the morphine-induced decrease in proliferation. Lastly, I asked whether chronic morphine would decrease proliferation in an inducible nestin-CreERT2/R26R-yellow fluorescent protein transgenic mouse. I found that proliferation in this transgenic mouse was not altered after a particular paradigm of morphine exposure. Together these findings suggest that morphine alters adult hippocampal proliferation through multiple effects: both on the progenitor cells themselves (cell cycle, maturation) and indirectly by alteration of the neurogenic niche. Additional work is needed to understand the mechanism of the morphine-induced changes in progenitor cell cycle and the neurogenic niche. The present findings will benefit both the addiction field by offering new avenues for treatment and neural stem cell biology by demonstrating stages of neurogenesis that are more vulnerable to exogenous stimuli.Item The Role of Glutamate Transporters in Early Postnatal Hippocampal Neurogenesis(2011-02-01) Gilley, Jennifer A.; Kernie, Steven G.Adult 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 Simulated Space Radiation: Effects on Murine Behavior and Neurogenesis(2018-04-17) Whoolery, Cody William; German, Dwight; Stowe, Ann; Eisch, Amelia J.; Shay, Jerry W.; Burma, SandeepAn unavoidable consequence of deep space exploration is exposure to high-atomic number, high-energy (HZE) particles, such as 28Si or 56Fe, that comprise galactic cosmic radiation (GCR). It is widely believed that GCR is damaging to the brain, as HZE particle exposure decreases rodent hippocampal dentate gyrus neurogenesis as well as function (e.g. learning and memory). While this raises concern that GCR will compromise astronaut health and mission success, most data have been collected using a specific set of parameters: 2 month old mice or rats (age-equivalent to a teenage human) irradiated with 56Fe. For this dissertation, I have filled three major knowledge gaps with regards to space radiation, brain, and behavior, which are described in the five chapters of my dissertation. In my introductory chapter, Chapter 1, I provide the essential background needed to understand the research chapters, including an overview of the hippocampus, the process of neurogenesis and how radiation affects it, and in-depth look at the published literature on how space radiation influences the central nervous system (CNS). In Chapter 2, I present my published work (Whoolery et al. 2017) on how dentate gyrus neurogenesis is changed at two timepoints post-28Si irradiation (24 h and 3 months), and explain how 28Si-induced changes in dentate gyrus neurogenesis compares to the effects seen after exposure to other previously-studied ions. In Chapter 3, I present my submitted work on how dentate gyrus-dependent behaviors are changed after 6 month old mice are exposed to mission-relevant doses of 56Fe. I test mice on many behavioral paradigms, but the most striking results come from two pattern separation tasks: the aversive Contextual Discrimination Fear Conditioning (CDFC) task and the appetitive Location Discrimination (LD) task which is performed on the Lafayette Bussey touchscreen platform. As I show in Chapter 3, mice exposed to 56Fe radiation display surprisingly improved performance on both CDFC and LD, findings which are discussed in relation to mission-critical behaviors and prior results on space radiation-induced changes in behavior. In Chapter 4, I expand on the operant touchscreen data provided. Specifically, I investigate whether the 56Fe-induced improvement in pattern separation is transient and if it is sex-specific. In my conclusion chapter, Chapter 5, I summarize the main conclusions of Chapters 2-4, provided future directions for each project, possible mechanisms that may underlie this improvement in pattern separation, and end with my thoughts on remaining challenges in the field and obstacles that need to be overcome.