Perez-Catalan, Nelson A.2020-08-172020-08-172020-06https://hdl.handle.net/1794/25559Submitted to the Undergraduate Library Research Award scholarship competition: 2020. 94 pages.Neurons are electrically excitable cells that transmit information throughout the nervous system with high speed and accuracy. This is largely facilitated by their specialized morphology, with dendrites receiving diverse information, to axons propagating the message to specific neighboring neuronal partners. During development, neuronal networks undergo rapid changes, ranging from short-term changes on the order of milliseconds, to long-term modifications in neural architecture that could last as long as the lifetime of the organism. This ‘plasticity’ ensures that neuronal networks, or circuits, undergo constant checks during development, while also facilitating a degree of adaptability that acts as the basis for learning and memory. The mechanisms that the nervous system employs to establish the correct connections and regulate plasticity remain a poorly understood topic in neuroscience. Research in both mammals and invertebrates, including Drosophila, have defined that glial cells are capable of instructing neurons to find partners to form synapses, a specialized chemical junction between two neurons where electrical signals propagate. More specifically, studies in astrocytes, the most abundant glial cell subtype in the central nervous system, have demonstrated that while neurogenesis precedes astrogenesis in the cortex, neuronal synapses only begin to form after astrocytes have been generated (explored in Chapter 1). Astrocyte development is crucial for circuit formation in the nervous system, and their dysfunction can lead to neurodevelopmental, neurodegenerative, neuroimmune, and neoplastic diseases, such as ALS and Alzheimer’s. This thesis explores a subset of the mechanisms employed by the nervous system to regulate circuit plasticity and circuit establishment during development, with a specific focus on astroglia. iv My first goal was to characterize plasticity within a model neural circuit during development. In the second chapter of this thesis, I use the highly specialized genetic toolkit available for Drosophila to characterize the structural dynamics of motor neuron dendrites during development in vivo by utilizing fluorescence microscopy. By manipulating neuronal activity in my model motor neurons, I show that the presence of stable microtubule populations within dendrites is directly correlated with structurally stable arbors. Furthermore, overexpression of the cell adhesion molecule Neurexin in motor neurons led to the increased stability of microtubule populations within dendritic arbors. Finally, I demonstrate that astrocytes are required to restrict motor dendrite plasticity to newly hatched larva. Interestingly, astrocytes robustly express Neuroligins, which are binding partners for Neurexin, suggesting that astrocyte-secreted proteins are capable of directly regulating neuronal morphology and plasticity. Previous studies in vitro have shown that in addition to regulating circuit plasticity, astrocyte-derived secreted and cell surface molecules (CSMs) can modify synaptogenesis during circuit development. In a separate line of questioning, I explore the role of astrocyte-secreted and cell surface proteins in the formation of excitatory cholinergic synapses in vivo (described in Chapter 3). Specifically, I took part in a reverse genetic screen to knock down astrocyte-derived proteins using commercially available RNAi lines. Concurrently, we labeled both neuronal membranes and their presynaptic sites (Brp+) using Synaptic Tagging with Recombination (STaR) to assess non-cell autonomous changes in synapse number. We performed two parallel screens, the first labeled individual dorsal bipolar dendritic (Dbd) sensory neurons. The second targeted neurons that generate synapses localized in the mushroom body, a memory and v learning center in the Drosophila brain. Excitingly, the major astrocyte-secreted molecules that induce synapse formation (e.g. TGF-β) or inhibit synapse development (e.g. SPARC) in vertebrates are conserved in fly, and we identified fourteen novel genes (of 245 tested) required in astrocytes for synaptogenesis. In sum, this work further characterizes dendritic dynamics during a critical period in Drosophila development. My data shows that altered neuronal activity in aCC/RP2 motor neurons within a critical period of motor circuit plasticity causes significant dendritic remodeling within minutes, and that astrocytes are required for proper critical period closure. Further, I demonstrate that the ablation of astrocytes postcritical period induces abnormal period of heightened plasticity. Finally, this work provides direct evidence of the key regulatory function of astrocytes in synaptogenesis, and their role in regulating global synapse formation in the central nervous system.enCreative Commons BY-NC-ND 4.0-USThesis / Dissertation