Browsing by Subject "Neuron"
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Item Actin Isoforms in neuronal structure and function(2011-07) Cheever, Thomas R.The actin cytoskeleton plays critical roles in nearly every aspect of neuronal development and function. During these processes, the localized polymerization of actin is one mechanism employed to carryout crucial tasks for normal neuronal function. While the activity of actin binding proteins is generally thought to be the primary mediator of spatially restricted actin polymerization, another prominent mechanism involves the local translation of β-actin, one of two actin isoforms expressed in neurons. The localized translation of β-actin has been shown previously to be essential for growth cone guidance in cultured neurons. Additionally, defects in the localization of β-actin have been implicated in the motor neuron disease Spinal Muscular Atrophy (SMA). However, no study to date has directly examined the role of β-actin in a mammalian in vivo system. Although the functions of β-actin were thought to be critical for all neurons, the work described in this thesis indicates that specific functions of β-actin are surprisingly confined to select populations in the central nervous system (CNS). β-actin is not required for motor axon regeneration or motor neuron function, but is required for the proper structure of the hippocampus, cerebellum, and corpus callosum, as well as hippocampal-associated behaviors. Thus, the work described here provides the first direct demonstration of specific roles for β-actin in vivo and presents a model to translate provocative findings in cell culture to the mammalian CNS.Item Investigating neurogenesis and cell type specification in the mammalian thalamus.(2012-06) Bluske, Krista K.The thalamus mediates a variety of important brain functions that are critical for behavior and survival. A key feature that enables the thalamus to perform such diverse functions is its parcellation into anatomically and functionally distinct groups of neurons called nuclei. The purpose of this project was to identify the origin of neuronal diversity within the thalamus by investigating the process of neurogenesis. During neurogenesis, proliferating progenitor cells begin to divide asymmetrically to generate neurons. The central hypothesis of the research presented herein is that thalamic organization requires the appropriate number and types of neurons to be generated and that these critical processes are regulated during neurogenesis. This work has characterized the different types of progenitor cells present during thalamic neurogenesis. We confirmed the existence of a special population of thalamic progenitor cells, intermediate (or basal) progenitor cells, and identified transcription factors that regulate their formation and/or maintenance. We also addressed the origin of distinct subtypes of neurons. The spatial organization of thalamic progenitor cells into two distinct progenitor domains during neurogenesis is thought to drive the formation of different subtypes of thalamic neurons. Signaling molecules have been proposed to induce the formation of distinct progenitor domains in numerous brain areas, including the thalamus. We provided a detailed characterization of components of the Wnt/β-catenin-mediated transcriptional pathway during thalamic neurogenesis. Based on the pattern of signaling activity, we hypothesized that Wnt/β-catenin-mediated transcription has a function in forming the two progenitor domains during thalamic neurogenesis. Using conditional genetic manipulations of β-catenin, we found that β-catenin-mediated transcription is required for the specification of thalamic progenitor domains. Furthermore, we found that the Wnt/β-catenin signaling pathway functions in parallel with the sonic hedgehog (Shh) signaling pathway, which had been previously shown to specify thalamic progenitor identity in an opposing manner, by independently regulating transcriptional networks in thalamic progenitor cells. Collectively, the process of neurogenesis involves the generation of the correct number of neurons by regulating asymmetric progenitor divisions and generation of appropriate neuronal subtypes through the functions of signaling pathways and transcriptional networks. These mechanisms provide a broad map for the generation and positioning of appropriate types of neurons in the correct locations within the thalamus.Item On the formation and functions of the neurons in the spinal cord that project axons to the thalamus, in rodent and primate.(2009-07) Davidson, SteveMammals and other advanced vertebrates possess a population of neurons located in the spinal cord that put forth axons to the thalamus. These cells are responsive to somatosensory stimuli and in humans are required for the normal perception of mechanical, thermal, and chemical stimuli. The studies contained in this thesis examine both the development of this pathway, called the spinothalamic tract (STT), and its physiological responses to stimuli that evoke somatosensory experiences. Experiments in adult mice show that the whole STT is made up of about 7000 cells and these are located in a pattern homologous to the STT in rat, cat and monkey. Experiments in neonatal and embryonic mice show that the axons of the STT reach the thalamus before birth. A study of the physiological characteristics of STT neurons located in the marginal zone of the spinal cord dorsal horn in the adult rat suggests that STT axons are topographically organized within the ventrobasal complex of the thalamus according to their responses to thermal stimuli. Studies in primates show that axons from the STT that project to the nuclei of the posterior thalamus are responsive to multiple modalities of somatosensory stimuli but differ from neurons projecting to VPL in some functional properties. A special focus of this thesis explores the poorly understood sensation of itch. Evidence is provided for at least two pathways for itch; one that is activated by cutaneous administered histamine and another that is activated by the protease contained within the spicules of the tropical legume cowhage. Despite this specificity for types of itch, each of these pathways is also responsive to noxious and/or innocuous mechanical, thermal and/or chemical stimuli. The population of cells that is responsive to histamine is transiently inhibited by scratching the skin during the histamine response, suggesting a mechanism for the well known relief from itch that is produced by scratching. The main conclusions from the studies in this thesis are that the STT is extant before the time of birth; that, in adults, it is a complex pathway that can contribute to the encoding of specific somatosensory sensations from cells that are responsive to multimodal stimuli.Item Select antiretrovirals from multiple classes induce excitatory synapse loss in primary rodent hippocampal cultures(2024-02) McMullan, HannahThe introduction of antiretroviral drugs (ARVs) has radically improved the prognoses for people living with HIV (PLWH) and is estimated to have averted almost 21 million deaths. Despite the widespread use of highly suppressive ARV treatments, approximately half of PLWH still suffer from HIV-associated neurocognitive disorders (HAND), an overall rate virtually unchanged from before their introduction. Additionally, ARVs have been identified in causing their own neuropsychiatric adverse effects, raising the question of whether they may be contributing to the persistence of HAND in virologically suppressed PLWH. As synapse loss is correlated with cognitive decline in HAND, it seems possible that ARVs themselves induce synaptic deficits that contribute both to their own neuropsychiatric effects and to the persistence of HAND. This theory is particularly bolstered by work showing that efavirenz, an ARV well known for its adverse neuropsychiatric effects, induces dendritic spine loss.I have used an automated, high-content imaging assay and cultured rat hippocampal neurons expressing PSD95-eGFP to label excitatory synapses and mCherry to fill neuronal structures to assess synaptic changes induced by a 24 hour exposure to 10 µM concentrations of 25 clinically used ARVs and one neurotoxic ARV metabolite across 6 mechanistic classes. Five ARVs elicited significant synapse loss over 24 hours: the integrase strand transfer inhibitor bictegravir, the non-nucleoside reverse transcriptase inhibitors etravirine and the 8-OH metabolite of efavirenz, the capsid inhibitor lenacapavir, and the protease inhibitors nelfinavir and saquinavir. Concentration-response assays indicated that, of the compounds assessed, only lenacapavir induced synapse loss at putative drug concentrations free in the plasma, although all four “hit” compounds evoked synapse loss at maximal total plasma concentrations, suggesting that drug interactions could raise free plasma concentrations to toxic levels. Time course studies indicated that synapse loss induced by these drugs is fully reached by 24 hours and can persist for at least three days. Therapeutically relevant antiretroviral combinations did not induce synergistic synapse loss in my study. Investigations into the mechanisms underlying synapse loss revealed that bictegravir-induced synapse loss is partially through activation of L-type voltage gated calcium channels and that bictegravir, etravirine, and lenacapavir produced synapse loss through ionotropic glutamate receptors. These results suggest that some ARVs are synaptotoxic at micromolar concentrations, and that lenacapacvir is synaptotoxic at nanomolar concentrations, indicating that if given in combination with drugs that also bind serum proteins or in disease states in which excitatory synaptic function is already potentiated, select ARVs may contribute to neuropsychiatric effects and neurocognitive deficits. The high content imaging assay used in this study provides an efficient means to evaluate new drugs and drug combinations for potential neuro- and synaptotoxicity as part of preclinical drug development.