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theses

Axo-glial interactions are critical for myelination and the domain organization of myelinated fibers. Cell adhesion molecules belonging to the Nectin like family, and in particular Necl-1 (axonal) and its heterophilic binding partner Necl-4 (Schwann cell), mediate these interactions along the internode. Using targeted shRNA-mediated knockdown, we show that the removal of axonal Necl-1 promotes Schwann cell myelination in the in vitro DRG neuron/Schwann cell myelinating system. Conversely, over-expressing Necl-1 on the surface of DRG neuron axons results in an almost complete inability by Schwann cells to form myelin segments. Axons of superior cervical ganglion (SCG) neurons, which do not normally support the formation of myelin segments by Schwann cells, express higher levels of Necl-1 compared to DRG neurons. Knocking down Necl-1 in SCG neurons promotes myelination. Finally, the extracellular domain of Necl-1 interferes in a dose-dependent manner with the activation of ErbB3 and of the pro-myelinating PI3K/Akt pathway, but does not interfere with the activation of the Mek/Erk1/2 pathway. While not in direct contradiction, these in vitro results shed lights on the apparent lack of phenotype that was reported from in vivo studies of Necl-1-/- mice. Our results suggest that Necl-1 may act as a negative regulator of PNS myelination, potentially through the selective regulation of the signaling cascades activated in Schwann cells by axonal contact, and in particular by type III Nrg-1. Further analyses of peripheral nerves in the Necl-1-/- mice will be needed to determine the exact role of axonal Necl-1 in PNS myelination. This opposite effect that Necl-1 in regulating myelination has led me to investigate the function of it’s another binding partner Necl-2. I have shown that in vitro Necl-2 expressed by Schwann cells is inhibitory to myelination. Strikingly, upon axonal contact, Necl-2 perturbs PDK1 and Akt activation (pro-myelinating signal) in direct correlation with the observed myelination results, suggesting the regulatory role of Necl-2 in axo-glial interaction. I have also observed that Necl-2 regulates actin cytoskeleton rearrangement in Schwann cells that is potentially due to regulating Rac1 activity. These findings provide another mechanism of internodal cell adhesion molecule in fine-tuning PNS myelination, both at signaling and cytoskeleton level.

ETD_8461

Ph.D.

Includes bibliographical references

by Ming-Shuo Chen

doi:10.7282/T3WD43NK

ETD doctoral

The author owns the copyright to this work.