Mechanisms of Axonal Sorting of Tau and Influence of the Axon Initial Segment on Tau Cell Polarity.

Tau is a microtubule-associated protein (MAP) that is mainly sorted into the axons in physiological conditions, but missorted in Alzheimer Disease and related tauopathies. The mechanism(s) of axonal targeting of Tau protein are still a matter of debate. Several possibilities for the axonal localization of Tau protein have been proposed: (1) Targeting of Tau mRNA into axons which is then translated locally. (2) Preferred axonal translation of Tau mRNA. (3) Specific dendritic degradation of Tau protein. (4) Active axonal sorting of somatically translated Tau protein. (5) Axonal retention of Tau protein by specific association of Tau protein with axonal structures, namely particularly modified microtubules. (6) Restriction of Tau diffusion by a selective filter function of the Axon Initial Segment (AIS). In our research we focused on the Tau Diffusion Barrier (TDB), located within the AIS, which controls anterograde and retrograde propagation of Tau. It shows both sensitivity to size of the Tau protein isoforms, and to disruption of the molecular structure of the AIS. Here, we review proposed mechanisms of axonal targeting of Tau and potential influences of the TDB/AIS on the subcellular distribution of Tau.