Label-Free Non-Linear Optics for the Study of Tubulin-Dependent Defects in Central Myelin

The satisfactory visualization of cytoskeletal components in the brain is challenging. The ubiquitous distribution of the networks of microtubules, microfilaments, and intermediate filaments in all the neural tissues, together with the variability in the outcomes of fluorescent protein fusion strategies and the limited applicability to dynamic studies of antibodies and drugs as chromophore vehicles, make classical optical approaches not as effective as for other proteins. When tubulin needs to be studied, the label-free generation of second harmonics is a very suitable option due to the non-centrosymmetric organization of the molecule. This technique, when conjugated to microscopy, can qualitatively describe the volumetric distribution of parallel bundles of microtubules in biological samples, with the additional advantage of working with fresh tissues that are unfixed and unpermeabilized. This work describes how to image tubulin with a commercial second harmonic generation microscopy setup to highlight microtubules in the tubulin-enriched structures of the oligodendrocytes, as in hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC) tubulinopathy, a recently described myelin disorder.