Date

11-14-2011

UMMS Affiliation

Graduate School of Biomedical Sciences, Program in Neuroscience

Document Type

Dissertation, Doctoral

Subjects

Dissertations, UMMS; Neuroglia; Neurons; Drosophila Proteins; Axons

Disciplines

Life Sciences | Medicine and Health Sciences | Neuroscience and Neurobiology

Abstract

The nervous system is made up of two major cell types, neurons and glia. The major distinguishing feature between neuronal cells and glial cells is that neurons are capable of transmitting action potentials while glial cells are electrically incompetent. For over a century glial cells were neglected and it was thought they existed merely to provide trophic and structural support to neurons. However, in the past few decades it has become increasingly clear that glial cell functions underlie almost all aspects of nervous system development, maintenance, and health. During development, glia act as permissive substrates for axons, provide guidance cues, regulate axon bundling, facilitate synapse formation, refine synaptic connections, and promote neuronal survival. In the mature nervous system glial cells regulate adult neurogenesis through phagocytosis, act as the primary immune cell, and contribute to complex processes such as learning and memory. In recent years, glial cells have also become a primary focus in the study of neurodegenerative diseases. Mounting evidence shows that glial cells exert both beneficial as well as detrimental effects in the pathology of several nervous system disorders, and modulation of glial activity is emerging as a viable therapeutic strategy for many diseases. Although glial cells are critical to the proper development and functioning of the nervous system, there is still relatively little known about the molecular mechanisms used by glial cells, how they exert their effects on neurons, and how glia and neurons communicate.

Despite the relative simplicity and small size of the Drosophila nervous system, glial cell organization and function in flies shows a remarkable complexity similar to vertebrate glial cells. In this study I use Drosophila as a model organism to study cellular and molecular mechanisms of glial clearance of axonal debris after acute axotomy. In chapter two of this thesis, I characterize three distinct subtypes of glial cells in the adult brain; cell body glia which ensheath neuronal cell bodies in the cortex region of the brain, astrocyte like glial cells which bear striking morphological similarity to mammalian astrocytes and share common molecular components, and ensheathing glial cells which I show act as the primary phagocytic cell type in the neuropil region of the brain. In addition, I identify dCed-6, the ortholog of mammalian GULP, as a necessary component of the glial phagocytic machinery.

In chapter three of this thesis, I perform a candidate based, in vivo, RNAi screen to identify novel genes involved in the glial engulfment of degenerating axon material. The Gal4/UAS system was used to drive UAS-RNAi for approximately 300 candidate genes with the glial specific repo-Gal4 driver. Two assays were used as a readout in this screen, clearance of axon material five days after injury, and Draper upregulation one day after maxillary palp or antennal injury. Overall, I identified 20 genes which, when knocked down specifically in glial cells, result in axon clearance defects after injury. Finally, in chapter four I identify Stat92E as a novel glial gene required for glial phagocytic function. I show that Stat92E regulates both basal and injury induced Draper expression. Injury-induced Draper expression is transcriptionally regulated through a Stat92E dependent non-canonical signaling mechanism whereby signaling through the Draper receptor activates Stat92E which in turn transcriptionally activates draper through a binding site located in the first intron of Draper. Draper represents only the second receptor known to positively regulate Stat92E transcriptional activity under normal physiological conditions.