Download or read book Cofilactin Bundling in Neurodevelopment and Degeneration written by Ryan Hylton and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Neurons are the primary cellular unit of the nervous system, and their structure and function are the fundamental basis for all human movement, sensation, and thought. Neurons communicate through cell-to-cell contacts called synapses, whose location is determined during neuronal development. Here, neurites are guided toward their eventual synaptic partners through the action of a subcellular compartment at their distal tip called the growth cone. Chemical and mechanical cues in the environment attract or repel growth cones, resulting in turning and advance towards their final destination. Later in the life cycle, and especially during certain disease processes, neurons and their synapses degenerate. In both neuronal development and degeneration, the motility and morphology of neurons is determined in part by the activity of filamentous actin (F-actin). This is especially true in the growth cone, where its most distal "peripheral domain" is comprised of actin-rich spikes called filopodia, which act as the growth cone's antennae, and lamellipodia where F-actin polymerization pushes the growth cone forward. F-actin dynamics are regulated by a series of actin binding proteins that alter its polymerization and depolymerization rates, as well as the ultrastructure of F-actin networks. The focus of this dissertation is the actin binding protein cofilin, whose chief function is the severing of actin filaments. It does so by binding to "old", ADP-bound F-actin in a cooperative manner, where it induces a hyper-twisted conformation of the filament, forming a structure called cofilactin. Because of the difference in helical pitch between actin and cofilactin, longitudinal inter-subunit interactions between actin monomers are disrupted at the boundary between the two, resulting in filament severing at this location. This altered twist also confers increased bending and torsional flexibility to the filament. Among other roles, cofilin activity is crucial to normal growth cone motility. Here, it is traditionally described with regards to its F-actin severing activity in the lamellipodia for the sake of actin turnover. Its role in filopodial dynamics is not as well understood. In the chapters below, however, we reveal a novel structural role for cofilin in growth cone filopodia. Using light and electron microscopy, we show that cofilactin clusters into bundles at the proximal base of growth cone filopodia, whereas their distal tips are comprised of normal F-actin crosslinked by the actin binding protein fascin. Immunofluorescence and tomographic analysis suggest that cofilin sterically excludes fascin from binding to actin in the filopodial base. Live-cell imaging of neurons demonstrates that lateral movements of filopodia hinge upon a transition region between these two domains where cofilactin and fascin-linked F-actin coexist. Based on examination of transition region tomograms, we hypothesize that there is a reduction of filament-to-filament crosslinking in this area, which gives it a high degree of flexibility. Conversely, the actin-rich tip is crosslinked by fascin and cofilactin in the base is connected through a currently unknown mechanism. Interestingly, cofilactin also forms bundles in a neurodegenerative context. In particular, multiple forms of cellular stress induce the formation of cofilin-actin rods: rod-shaped inclusions of cofilin and actin that have been shown to disrupt neuritic transport as well as synapse structure and function. These rods have been implicated in multiple forms of neurodegeneration and were previously found in the post-mortem brains of Alzheimer's disease patients. We produced these rods in cultured neurites and performed a structural investigation which revealed noteworthy similarities between them and cofilactin bundles from growth cone filopodia. Many structural and functional assays are still needed to fully understand the interconnected relationship between these two bundles of cofilactin. There are many open questions regarding the initiation, filament crosslinking, and dissolution of each bundle type. However, any similarities or differences between the two would help explain how cofilactin can bundle during neuronal development, and how this action can turn pathological during cofilin-actin rod formation. Future studies will also guide attempts at producing therapies aimed at eliminating cofilin-actin rods, which may be critical to alleviating multiple forms of neurodegeneration.