Thesis (Ph. D.)--University of Chicago, Dept. of Physics, August 2006. / "This work was published in The European Physical Journal E - Soft Matter in February 2006 (DOI: 10.1140/epje/i2005-10054-2). Includes bibliographical references.

No description available.

A key process enabling the correct functioning of neural circuits involves the formation of multi‐layered membranous myelin sheaths around axons. Myelin sheaths, made by specialised glial cells called oligodendrocytes in the central nervous system (CNS), metabolically support underlying axons and speed up electrical impulse conduction, aiding efficient communication between neurons. As only a subset of axons in the CNS are myelinated, with unique patterns developed therein, it raises the questions: how does an oligodendrocyte choose which axon to myelinate and what regulates the amount of myelin made? The production of myelin sheaths by the oligodendrocyte, is under strong influence from of a range of signals including those mediated by G protein‐coupled receptor (GPR) superfamily members. One GPR, Endothelin receptor B (EDNRB), best known for regulating blood flow, had previously been demonstrated to both positively and negatively influence myelination. I have investigated how EDNRB regulates myelination using an in vitro myelination assay, alongside in vivo analysis in zebrafish and mice. These systems identified a direct signalling role for EDNRB in the promotion of myelin sheath number. Furthermore, profiling the protein signalling cascade downstream of this receptor identified a range of known and novel factors involved in the regulation of myelin sheath number including the MAPK pathway, Src family kinases, ErbB receptors, protein kinase C ε, NMDAR and AMPAR. Functional analyses of a subset of these factors elucidate how EDNRB signalling, potentially connecting signals from a range of cell types, ensures correct adequate myelination in the CNS.

No description available.

Thesis (Ph.D.) - University of Queensland, 2004. / Includes bibliography.

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 114-137).

The nodes of Ranvier are short, periodical interruptions in the myelin sheath of myelinated axons, at which voltage-gated sodium channels are highly concentrated. The correct targeting of sodium channels to the nodes of Ranvier permits rapid propagation of action potentials in myelinated axons. The nodes of Ranvier contain a unique set of ion channels, cell-adhesion molecules, and cytoplasmic adaptor proteins. Neurofascins are cell adhesion molecules of the immunoglobulin superfamily and previous work has shown they are involved in the assembly of the node of Ranvier. The Neurofascin (Nfasc) gene is subject to extensive alternative splicing. RT-PCR studies have suggested that there were several different Neurofascin (Nfasc) transcripts. Thus far, research on the Neurofascins has concentrated on two isoforms, Nfasc186 and Nfasc155, which are expressed in neurons and glia respectively. A third Neurofascin isoform, Nfasc140, lacking the Mucin domain and two of the fibronectin repeats was originally identified in the laboratory of V. Bennett. However, neither the location nor function of this protein was known. By RT-PCR I successfully cloned the Nfasc140 cDNA and determined its domain composition, which was confirmed by a series of Western blots using domain-specific antibodies. The developmental expression of Nfasc140 revealed that it is the predominate isoform of Neurofascin during the embryonic stage. Using cell-type-specific conditional Neurofascin knock-out mice, I have also found that Nfasc140 is a neuronal isoform, like Nfasc186. I have used transgenic mouse lines to characterize the location and function of Nfasc140. Like Nfasc186, Nfasc140 is targeted to the nodes of Ranvier and axonal initial segment. Also Nfasc140 alone can reconstitute the nodal complex in Neurofascin knock-out mice in CNS and PNS in the absence of Nfasc186 and Nfasc155. It can also partially restore the electrophysiological function of PNS nerves. In order to address the role of the paranodes in sodium channel clustering, I generated a new neuronal-Cre-expressing transgenic line which, when bred with floxed Nfasc mice, generated early neuronal Neurofascin knock-out mice. Using those animals I have shown that after the ablation of all neuronal Neurofascins, when only glial Nfasc155 is presented, sodium channels can still target to the nodes of Ranvier in both PNS and CNS. These conditional knock-out mice have a longer life span than pan-Neurofascin knock-out mice. This indicates the importance of paranodal junctions, in addition to nodal neuronal Neurofascins, in clustering sodium channels at the node.

Close interaction between oligodendrocytes and axons is essential to initiate myelination and to form specialised domains along myelinated fibres. These domains are characterised by the assembly of protein complexes at the axon-glia interface and key components of these complexes are the Neurofascins. Neurofascins are transmembrane glycoproteins belonging to the L1 subgroup of the Immunoglobulin (Ig) superfamily of cell adhesion molecules. The Neurofascin (Nfasc) gene is subject to extensive alternative splicing. Two of the best characterised isoforms are Nfasc155 and Nfasc186, which are expressed in glia and neurons respectively. In myelinated fibres, Nfasc186 is the predominant isoform expressed at nodes of Ranvier and axon initial segments (AIS) in both the central and peripheral nervous system (CNS and PNS), whereas Nfasc155 resides on the glial side of the paranodal axoglial junction. The Neurofascin gene has been inactivated by homologous recombination and Neurofascin-null mice die within the first week of postnatal life. The main focus of this work was to investigate the role of the Neurofascins in the developing CNS. Similarly to what has been previously observed in the PNS, this study shows that in myelinated fibres of the spinal cord, nodal and paranodal markers are mislocalised and axoglial junctions do not form in the absence of the Neurofascins. In contrast to the PNS, where ensheathment of axons is unaffected, myelin proteins in the CNS are greatly reduced in the mutant. This appears to be due to the reduced ability of oligodendrocyte myelinating processes to extend along axons. This work also shows that the role of Nfasc186 is to maintain the long term stability of the AIS rather than its assembly. In the PNS, Nfasc186 was found to play an essential role in node assembly. However, PNS and CNS nodes are likely to assemble by different mechanisms. To investigate the relative contribution of the Neurofascin isoforms in CNS node assembly, this work made use of transgenic lines in which either neuronal Nfasc186 or glial Nfasc155 was expressed on a Neurofascin null background. Expression of either isoform was found to independently rescue the nodal complex and a model of how the Neurofascins cooperate in the assembly of the CNS node of Ranvier is proposed.

Myelin is essential for neuronal survival and maintenance of normal functions of the nervous system. Demyelinating disorders are debilitating and are often associated with failure of resident oligodendrocyte precursor cells (OPCs) to differentiate into mature, myelinating oligodendrocytes. Derivation of OPCs, from a safe source that evades ethical issues offers a solution to remyelination therapy. We therefore hypothesized that bone marrow stromal cells (BMSCs) harbour neural progenitor cells at a pre-commitment stage and that in vitro conditions can be exploited to direct differentiation of these cells along the oligodendroglial lineage. For the current study, adult rat BMSCs used were >90% immunopositive for CD90, CD73, STRO-1 (stromal cell markers), 10% for nestin (neural progenitor marker) but negligible for CD45 (haematopoietic cell marker) as measured by flow cytometry. Transfer of the culture from a highly adhesive substratum to a moderately adhesive substratum resulted in increase in proportion of p75-positive cells but CD49b-positive cells remained at 97% and Sox 10-positive cells remained negligible. Transfer of the culture to a non-adherent substratum fostered the generation of neurospheres comprising cells that were positive for the neural stem/progenitor cell (NP) marker, nestin, and for the neural crest markers CD49b, p75 and Sox10. Prior to this stage, the BMSCs were not yet committed to the neural lineage even though transient upregulation of occasional marker may suggest a bias towards the neural crest cell lineage. The BM-NPs were then maintained in adherent culture supplemented with beta-Heregulin (β-Her), basic fibroblast growth factor (bFGF) and platelet-derived growth factor-AA (PDGF-AA) to direct differentiation along the oligodendroglial lineage. Within two weeks of glial induction, cells expressing the OPC markers - NG2, Olig2, PDGFRa and Sox10, were detectable and these could be expanded in culture for up to 3 months with no observable decline in marker expression. These BM-OPCs matured into myelinating oligodendrocytes after 2 weeks in co-culture with either dorsal root ganglion neurons or cortical neurons. In vivo myelination by BM-OPCs was demonstrated by exploitation of the non-myelinated axons of retinal ganglion cells of adult rats. By 8 weeks post-injection of BM-OPCs into the retina, myelin basic protein-positive processes were also observable along the retinal axons. The results not only suppport our hypothesis, but also provide pointers to the adult bone marrow as a safe and accessible source for the derivation of OPCs towards transplantation therapy in acute demyelinating disorders. / published_or_final_version / Biochemistry / Doctoral / Doctor of Philosophy

In the peripheral nervous system (PNS), Schwann cells ensheathe and myelinate axons to promote saltatory conduction of nerve impulses. Close interactions between Schwann cells and axons, and Schwann cells and the basal lamina are essential for the regulation of Schwann cell development and function. Myelinating Schwann cells are highly polarized radially and longitudinally for specifying distinct domains in the axon, which is required for fast action potential propagation. In addition, the Schwann cell cytoplasm is organized into discrete compartments, called Cajal bands, which contain different dystrophin-glycoprotein complexes that are believed to segregate the Schwann cell plasma membrane into appositions between the outer surface of the myelin sheath and the cytoplasmic face of the Schwann cell plasma membrane. Periaxin is expressed in myelinating Schwann cells, and homodimerizes at its PDZ domain to form a transmembrane complex with dystrophin-related protein 2 (DRP2) and dystroglycan. This PDG complex is concentrated at the appositions, and is essential for myelin sheath maintenance and stability in the mature PNS. In mice lacking Periaxin, an intact myelin sheath is formed but subsequently becomes unstable. Periaxin-null Schwann cells are also shorter, which has been proposed to result in a reduction in nerve conduction velocity. This thesis is a study of how Periaxin PDZ domain dimerization contributes to the regulation of PDG complex stability, apposition maintenance, Schwann cell internodal distance and myelin stability. I have studied the function of Periaxin by generating a conditional mutant mouse that lacks the PDZ domain, which is predicted to abrogate dimerization. In these mutants, DRP2 is severely depleted and appositions containing DRP2 fail to form. Mutant Schwann cells also have disrupted Cajal bands and shorter internodal lengths. In the mature peripheral nerves, mutant mice display a peripheral neuropathy characterized by hypermyelination with focally folded myelin. Nerve conduction velocity, motor coordination and sensory function were also studied in these mutant mice. Taken together, these data suggest that dimerization of the Periaxin PDZ domain is required for the stabilization of the PDG and appositions, and regulation of Schwann cell elongation and myelin maintenance. By analyzing a tamoxifen-inducible conditional mouse lacking Periaxin’s PDZ domain in mature myelinating Schwann cells, this work also shows that Periaxin dimerization is essential for maintaining Schwann cell compartmentalization and myelin stability in adult nerves. Finally, studies of single amino acid mutations of the Periaxin PDZ reveal that subtle changes in the structure of the PDZ domain can abrogate dimerization,and a possible mechanism for PDZ-PDZ homodimerization of Periaxin is proposed.