Genetic dissection of peripheral glia and glial sheath development

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Overview: The long-term goal of my research program is to study the protein interactions that underlies the development of the glial sheath that ensheaths and insulates the peripheral nerve. The peripheral nerve responds to environment cues and controls muscle contraction and are surrounded by layers of glial cells, the major insulator of the nervous system. The formation of a glial sheath around axons is an essential component of the development and function of the nervous system in all animals. Little is known about the mechanisms that trigger glial cells to migrate along and then wrap their associated axons. Both glial migration and the formation of the glial wrap around axons involves protein adhesion and communication between cells leading to extensive rearrangements of the cytoskeleton. Glial cells must differentiate from elongated migrating cells into cells with elaborate extensions that encompass the axons they wrap. In Drosophila three distinct glial layers contribute to the mature peripheral nerve. In later stages the glial sheath must be maintained and expand to match animal growth to ensure complete insulation of the underlying axons. Loss of the glial sheath leads to a disruption of neuronal function and lethality. Our previous work funded by NSERC has shown that a class of adhesion proteins called the integrins and their binding partners within the cell play a critical role in establishing and maintaining the glial sheath. We found that integrin communication between the distinct glia layers in the peripheral nervous system (PNS) is critical to maintain glia function and insulating glia wrap. But how the integrin complex functions to control glia sheath formation is not known. We have evidence within the core of the peripheral nerve, integrin mediated adhesion is not through extracellular matrix components but rather through membrane bound ligands. Our investigation of potential integrin ligands identified Basigin, a transmembrane Ig domain protein, as critical for glia-glial adhesion to maintain the glial wrap and the glial layers. Our NSERC funded research has also uncovered a critical role of the gap junction protein, Innexin 2, in the development or maintenance of the glial sheath and these data also point to an essential role for glia-glia communication. But how Innexin 2 contributes to sheath formation either as a gap junction or an adhesion protein is not known. Therefore our research program is consolidating on the central theme that the development and maintenance of the glia sheath around axons is a function of not just of glia to axon communication but also of glia to glia communication. Our short terms goals will address the mechanisms that underlie this communication. Approach: My laboratory has extensive experience analyzing the molecules and developmental dynamics of the glia of the peripheral nervous system using a combination of molecular and cell biology paired with high resolution imaging. We have generated a collection of cellular and genetic markers that allow us to manipulate the genes expressed in the peripheral glia and then assess the results during the development. Therefore we can use the Drosophila model system to isolate and characterize the global fundamental mechanisms that underlie glial cell development and sheath formation in all animals. Objectives: We will investigate the molecular mechanisms that underlie integrin mediated glial sheath formation and maintenance, and the role of the Ig domain proteins including Basigin in this process. We will use a loss of function and rescue approach to determine how innexin 2 functions in the glial sheath, as a gap junction and through what signaling mechanism.