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theses

Myelination in Peripheral Nervous System (PNS) is established through Schwann cell (SC) migration, proliferation and radial sorting. After a phase of axonal association and SC radial sorting, SCs start to wrap around an axon at regular intervals before myelin compaction. A translational and transcriptional program elicits morphogenetic changes to SCs, which drives cytoskeleton rearrangement. Rho GTPases are the main effectors of dynamic cytoskeletal modification. Several studies implicate Rho GTPases, such as Rac1, Cdc42, and RhoA (and its downstream effector, ROCK) in SC proliferation, migration, radial sorting and compaction of myelin sheaths. Cdc42 is known to regulate SC proliferation, whereas Rac1 and RhoA show critical roles in myelination process such as radial sorting and formation of paranode compartment. Of many highly conserved signaling pathways by which morphogenetic changes can be established in SCs, the Wnt/Planar Cell Polarity (PCP) signaling pathway provides a versatile platform for understanding morphogenetic changes in SCs. Wnt/PCP pathway is known to promote cytoskeletal modifications through the activation of Rho GTPases, especially RhoA and Rac1, as effector modules to regulate actin polymerization. However, there have been no studies to date investigating Wnt/PCP pathway in relation to PNS myelination. In this study, I have characterized the presence of components for Wnt/PCP in SCs and their regulation during PNS myelination. Furthermore, I show that Wnt5a, a known ligand of the Wnt/PCP pathway, can modulate myelination in a culture model of myelination via a β-catenin-independent manner. In the context of purified SC culture, different concentrations of Wnt5a can elicit different responses of activation of Rho GTPases, especially RhoA and Rac1. Notably, 2.5 ng/ml of exogenous Wnt5a treatment showed hypermyelination compared to control, whereas > 200 ng/ml of Wnt5a had an inhibitory effect on myelination. Further investigations using two inhibitory constructs for Daam1, a key upstream regulator for small GTPases in Wnt/PCP pathway, revealed that Daam1/RhoA axis is necessary for the hypermyelination effect of Wnt5a (2.5 ng/ml). This Wnt5a concentration for hypermyelination does not affect the differentiation of Schwann cells into myelinating stage. Additionally, deletion of endogenous Wnt5a from SC resulted in less migration with higher expression of Prx, a non-migrating SC marker. Given the following two things; 1) existence of Wnt5a and machinery for Wnt/PCP signaling during myelination, and 2) cytoskeletal modulating function for Wnt5a without proliferative effect, my data suggest that Wnt/PCP signaling via Wnt5a/Daam1/RhoA axis may be involved in migration-association transition and/or spiral wrapping phase of SC lineage in the PNS.

ETD_8519

Ph.D.

Includes bibliographical references

by Hyosung Kim

doi:10.7282/T3SQ93GD

ETD doctoral

The author owns the copyright to this work.