Actin turnover regulates mechanical properties of oligodendrocytes and myelin formation

The myelin sheath is a specialized membranous structure that facilitates rapid signal conduction along axonal segments. During central nervous system development, it is formed by oligodendrocytes that extend motile and exploratory processes. Upon axonal contact, these processes transform into flat sheets that spread and wrap around the axons to generate a multilayered stack of membranes. In order to drive the leading edge of the forming sheath in between the growing myelin layers and the interface with the axon, mechanical forces are necessary, however, the underlying mechanisms are not known. Thus, to study how the process of myelin formation occurs, we used an interdisciplinary approach that combines morphological and genetic analysis with nano-mechanical experiments. In order to perform mechanical measurements on flat oligodendrocytes with minimum damage, we developed a vertical optical trap and compared its performance with atomic force microscopy. We found that indentation experiments carried out with both instruments yield consistent results for the cell elasticity. I used both complementary techniques to measure the response of fibroblasts over a large range of forces and deformations modes. Experiments on oligodendrocytes pointed to a key role for the actin filament (F-actin) network dynamics in myelin growth. At the onset of myelin biogenesis, F-actin is located primarily at the non-adhesive leading edge, which is propelled around the axon driven by forces generated by F-actin polymerization. Behind the leading edge, F-actin disassembly reduces surface tension which allows membrane spreading and transforming the processes into large flat sheets that adhere to the substrate. Furthermore, we have identified the actin depolymerizing factor ADF/Cofilin1, as necessary regulator of myelin formation. By controlling the local actin dynamics with repetitive cycles of assembly and disassembly, oligodendrocytes can drive the protrusions forwards and eventually spread