Probing in-vitro dynamics of microtubules in living neurons with second harmonic imaging

Microtubules are the most important cytoskeleton elements in neurons providing the cells with shape sustaining structural integrity and a means of intracellular cargo transport. Microtubules are composed of non-centrosymmetric crystalline lattices and as such posses an axial polarity. This axial polarity originates from the organization of α- and β- tubulin subunits along a preferred direction1. In neurons axons and dendrites are the two most studied structures that are composed of microtubules. Axons and dendrites differ in their complement of cytoplasmic organelles and in their organization of microtubule arrays. Axons present a uniform polarity, meaning that all the microtubules are aligned with their positive ends distal to the soma, and dendrites have a mixed polarity2. Thanks to their noncentosymmetric structures, microtubules can be detected by means of second harmonic generation (SHG), a label-free optical process that is forbidden in centrosymmetric media. Thus, the polarity of microtubule assembly within axons and dendrites has a direct impact on the strength of the emitted SHG signal. It has been reported in static SHG imaging that dendrites do not exhibit SHG signal, since their microtubule assembly is not uniformly organized and that the source of SHG signal in axons comes from the uniform polarity of axonal microtubules 3.