Department of Neurobiology; Freeman Lab
First Thesis Advisor
Astrocytes, Drosophila, Neurogenesis, Neuroglia, Neurons, Synapses
Dissertations, UMMS; Astrocytes; Drosophila; Neurogenesis; Neuroglia; Neurons; Synapses
The nervous system is composed of neurons and glia. Glial cells have been neglected and thought to have only a supportive role in the nervous system, even though ~60% of the mammalian brain is composed of glia. Yet, in recent years, it has been shown that glial cells have several important functions during the development, maintenance and function of the nervous system. Glial cells regulate both pre and post mitotic neuronal survival during normal development and maintenance of the nervous system as well as after injury, are necessary for axon guidance, proper axon fasciculation, and myelination during development, promote synapse formation, regulate ion balance in the extracellular space, are required for normal synaptic function, and have immune functions in the brain. Although glia have crucial roles in nervous system development and function, there are still much unknown about the underlying molecular mechanisms in glial development, function and glial-neuronal communication.
Drosophila offers great opportunity to study glial biology, with its simple yet sophisticated and stereotypic nervous system. Glial cells in flies show great complexity similar to the mammalian nervous system, and many cellular and molecular functions are conserved between flies and mammals. In this study, I use Drosophila as a model organism to study the function of one subtype of glia: astrocytes. The role of astrocytes in synapse formation, function and maintenance has been a focus of study. However, their role in engulfment and clearance of neuronal debris during development remains unexplored.
I generated a driver line that enables the study of astrocytes in Drosophila.In chapter two of this thesis, I characterize astrocytes during metamorphosis, when extensive neuronal remodeling takes place. I found that astrocytes turn into phagocytes in a cell-autonomous, steroid-dependent manner, by upregulating the phagocytic receptor Draper and forming acidic phagolysosomal structures. I show that astrocytes clear neuronal debris during nervous system remodeling and that this is a novel function for astrocytes during the development of nervous system. I analyzed two different neuronal populations: MB γ neurons that prune their neurites and vCrz+ neurons that undergo apoptosis. I discovered that MB γ axons are engulfed by astrocytes using the Draper and Crk/Mbc/dCed-12 pathways in a partially redundant way. Interestingly, Draper is required for clearance of vCrz+ cell bodies, while Crk/Mbc/dCed-12, but not Draper, are required for clearance of vCrz+ neurites. Surprisingly, I also found that loss of Draper delayed vCrz+ neurite degeneration, suggesting that glia facilitate neurite destruction through engulfment signaling.
Taken together, my work identifies a novel function for astrocytes in the clearance of synaptic and neuronal debris during developmental remodeling of the nervous system. Additionally, I show that Crk/Mbc/dCed-12 act as a new glial signaling pathway required for pruning, and surprisingly, that glia use different engulfment pathways to clear neuronal debris generated by cell death versus local pruning.
Tasdemir-Yilmaz, OE. Role of Astrocytes in Sculpting Neuronal Circuits in the Drosophila CNS: A Dissertation. (2014). University of Massachusetts Medical School. GSBS Dissertations and Theses. Paper 729. DOI: 10.13028/M23P4G. http://escholarship.umassmed.edu/gsbs_diss/729
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