Year: 2020
Type: dissertation
Abstract: Neurons are electrically excitable cells that transmit information throughout the \nnervous system with high speed and accuracy. This is largely facilitated by their \nspecialized morphology, with dendrites receiving diverse information, to axons \npropagating the message to specific neighboring neuronal partners. During \ndevelopment, neuronal networks undergo rapid changes, ranging from short-term \nchanges on the order of milliseconds, to long-term modifications in neural architecture \nthat could last as long as the lifetime of the organism. This ‘plasticity’ ensures that \nneuronal networks, or circuits, undergo constant checks during development, while also \nfacilitating a degree of adaptability that acts as the basis for learning and memory. The \nmechanisms that the nervous system employs to establish the correct connections and \nregulate plasticity remain a poorly understood topic in neuroscience. Research in both \nmammals and invertebrates, including Drosophila, have defined that glial cells are \ncapable of instructing neurons to find partners to form synapses, a specialized chemical \njunction between two neurons where electrical signals propagate. More specifically, \nstudies in astrocytes, the most abundant glial cell subtype in the central nervous system, \nhave demonstrated that while neurogenesis precedes astrogenesis in the cortex, neuronal \nsynapses only begin to form after astrocytes have been generated (explored in Chapter \n1). Astrocyte development is crucial for circuit formation in the nervous system, and \ntheir dysfunction can lead to neurodevelopmental, neurodegenerative, neuroimmune, \nand neoplastic diseases, such as ALS and Alzheimer’s. This thesis explores a subset of \nthe mechanisms employed by the nervous system to regulate circuit plasticity and \ncircuit establishment during development, with a specific focus on astroglia. \niv \nMy first goal was to characterize plasticity within a model neural circuit during \ndevelopment. In the second chapter of this thesis, I use the highly specialized genetic \ntoolkit available for Drosophila to characterize the structural dynamics of motor neuron \ndendrites during development in vivo by utilizing fluorescence microscopy. By \nmanipulating neuronal activity in my model motor neurons, I show that the presence of \nstable microtubule populations within dendrites is directly correlated with structurally \nstable arbors. Furthermore, overexpression of the cell adhesion molecule Neurexin in \nmotor neurons led to the increased stability of microtubule populations within dendritic \narbors. Finally, I demonstrate that astrocytes are required to restrict motor dendrite \nplasticity to newly hatched larva. Interestingly, astrocytes robustly express Neuroligins, \nwhich are binding partners for Neurexin, suggesting that astrocyte-secreted proteins are \ncapable of directly regulating neuronal morphology and plasticity. \nPrevious studies in vitro have shown that in addition to regulating circuit \nplasticity, astrocyte-derived secreted and cell surface molecules (CSMs) can modify \nsynaptogenesis during circuit development. In a separate line of questioning, I explore \nthe role of astrocyte-secreted and cell surface proteins in the formation of excitatory \ncholinergic synapses in vivo (described in Chapter 3). Specifically, I took part in a \nreverse genetic screen to knock down astrocyte-derived proteins using commercially \navailable RNAi lines. Concurrently, we labeled both neuronal membranes and their presynaptic \nsites (Brp+) using Synaptic Tagging with Recombination (STaR) to assess \nnon-cell autonomous changes in synapse number. We performed two parallel screens, \nthe first labeled individual dorsal bipolar dendritic (Dbd) sensory neurons. The second \ntargeted neurons that generate synapses localized in the mushroom body, a memory and \nv \nlearning center in the Drosophila brain. Excitingly, the major astrocyte-secreted \nmolecules that induce synapse formation (e.g. TGF-β) or inhibit synapse development \n(e.g. SPARC) in vertebrates are conserved in fly, and we identified fourteen novel genes \n(of 245 tested) required in astrocytes for synaptogenesis. \nIn sum, this work further characterizes dendritic dynamics during a critical \nperiod in Drosophila development. My data shows that altered neuronal activity in \naCC/RP2 motor neurons within a critical period of motor circuit plasticity causes \nsignificant dendritic remodeling within minutes, and that astrocytes are required for \nproper critical period closure. Further, I demonstrate that the ablation of astrocytes postcritical \nperiod induces abnormal period of heightened plasticity. Finally, this work \nprovides direct evidence of the key regulatory function of astrocytes in synaptogenesis, \nand their role in regulating global synapse formation in the central nervous system.
Author: Nelson A. Perez-Catalan
Language: English
Subfield: Cellular and Molecular Neuroscience
Field: Neuroscience
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W3094695138