This item is under embargo and not available online per the author's request. For access information, please visit http://libanswers.wustl.edu/faq/5640.
Date of Award
Doctor of Philosophy (PhD)
Myelin, the lipid-rich sheath that insulates axons to facilitate rapid conduction of action potentials, is generated by specialized glial cells oligodendrocytes (OLs) in the central nervous system (CNS) and Schwann cells (SCs) in the peripheral nervous system (PNS). The importance of myelin is underscored by the devastating symptoms experienced by patients who suffer from demyelinating diseases of the PNS (e.g., Charcot-Marie-Tooth disease) and CNS (e.g., multiple sclerosis, MS). The molecular mechanisms governing myelination are not fully understood, yet several studies implicate extracellular matrix (ECM) proteins and their receptors as important regulators of these processes (reviewed in Chapter 1 of this thesis). In my thesis work, I used both forward and reverse genetics to define two new regulators of myelination with links to the ECM: GPR56 and ADAMTS9.
GPR56 belongs to a specialized class of G protein-coupled receptors (GPCRs) called the adhesion GPCRs (aGCPRs). aGPCRs mediate both cell-cell and cell-matrix interactions, and thus could play a role in myelinating glial cells. I used zebrafish and mouse models to define the function of GPR56 in both OL and SC development and myelination. First, I explore the function of Gpr56 in the CNS in zebrafish and show that gpr56 is robustly expressed in early stages of OL development. In addition, I demonstrate that impaired Gpr56 function results in a significant reduction of OL number and of myelination due to decreased proliferation of OL precursor cells, and that these functions of Gpr56 are mediated via interactions with G12/13 proteins and RhoA activation (Ackerman et al. 2015, Nat Commun; Chapter 2 of this thesis). In contrast, diminished GPR56 function in the PNS of zebrafish and mouse mutants does not result in hypomyelination. Instead, loss of Gpr56 causes defects in axon-SC interactions during development, myelin structural abnormalities, and impaired myelin maintenance. Importantly, these developmental defects result in axon degeneration and progressive neuropathy in adult Gpr56 mutants (Ackerman et al., in revision; Chapter 3 of this thesis). As mutations in GPR56 in humans cause neurological disorders that can present with PNS pathology, my work defines Gpr56 as a novel, clinically relevant regulator of nervous system health.
In a separate yet complimentary line of work, I conducted a large scale forward genetic screen in zebrafish in collaboration with members of the Monk and Solnica-Krezel laboratories and identified the secreted matrix metalloprotease ADAMTS9 as a critical new regulator of OL myelination. ADAMTS9 normally functions to cleave chondroitin sulfate proteoglycans (CSPGs) in the ECM, and accumulation of CSPGs is a major cause of failed axon regrowth and remyelination after CNS injury and in demyelinating plaques of MS brains. Excitingly, I find that myelin is lost in adamts9 zebrafish mutants concurrently with engulfment of OL lineage cells (including mature, myelinating OLs) by phagocytic immune cells, and that these phenotypes can be ameliorated upon removal of CSPGs from the ECM (Ackerman et al., in preparation; Chapter 4 of this thesis). MS is a complex, autoimmune disease in which the immune system attacks the OL myelin sheath, and the cause of this attack is unknown. Thus, the identification of ADAMTS9 as a critical regulator of OL-immune cell interactions represents a major advancement in our understanding of CNS demyelination, and may present a new therapeutic target for MS.
In sum, my thesis work defined the roles of two previously unappreciated regulators of myelination, GPR56 and ADAMTS9, and further emphasized the importance of the extracellular matrix in modulating the development of myelinating glial cells.
Chair and Committee
Kelly R. Monk
James B. Skeath, Liliana Solnica-Krezel, Joseph D. Dougherty, Valeria Cavalli,
Ackerman, Sarah Degenova, "Mechanisms of Myelination, It's All About the Matrix" (2016). Arts & Sciences Electronic Theses and Dissertations. 745.
Available for download on Friday, May 15, 2116