Tau mediates microtubule bundle architectures mimicking fascicles of microtubules found in the axon initial segment.

Peter J Chung,Stuart C Feinstein,Joanna Deek,Cyrus R Safinya,Herbert P Miller,Myung Chul Choi,Youli Li,Chaeyeon Song,Leslie Wilson

doi:10.1038/ncomms12278

Abstract

Tau, an intrinsically disordered protein confined to neuronal axons, binds to and regulates microtubule dynamics. Although there have been observations of string-like microtubule fascicles in the axon initial segment (AIS) and hexagonal bundles in neurite-like processes in non-neuronal cells overexpressing Tau, cell-free reconstitutions have not replicated either geometry. Here we map out the energy landscape of Tau-mediated, GTP-dependent ‘active' microtubule bundles at 37 °C, as revealed by synchrotron SAXS and TEM. Widely spaced bundles (wall-to-wall distance Dw–w≈25–41 nm) with hexagonal and string-like symmetry are observed, the latter mimicking bundles found in the AIS. A second energy minimum (Dw–w≈16–23 nm) is revealed under osmotic pressure. The wide spacing results from a balance between repulsive forces, due to Tau's projection domain (PD), and a stabilizing sum of transient sub-kBT cationic/anionic charge–charge attractions mediated by weakly penetrating opposing PDs. This landscape would be significantly affected by charge-altering modifications of Tau associated with neurodegeneration.

Highlights

Tau, an intrinsically disordered protein confined to neuronal axons, binds to and regulates microtubule dynamics
Tau consists of an amino-terminal tail (NTT) containing a projection domain (PD) and proline-rich region, followed by the microtubule-binding region (MTBR) and a carboxyl-terminal tail (CTT)
The osmotic pressure data and, the abrupt transition above Pc are consistent with the onset of antiparallel dimerization between fully interpenetrating dipolar Tau PDs on opposing microtubule surfaces (Fig. 4c)

Summary

Introduction

An intrinsically disordered protein confined to neuronal axons, binds to and regulates microtubule dynamics. Before the discovery of Tau, groundbreaking electron microscopy studies of the fine structure of axons in mature rat hippocampal neurons revealed[5,6] that the axon initial segment (AIS) contained widely spaced (Dw–wE25–30 nm) string-like microtubule bundles (‘fascicles of microtubules’, Fig. 1c). Seminal studies[8,9] of non-neuronal cells overexpressing transfected Tau cDNA had concluded that the Tau PD determines inter-microtubule distances in observed widely spaced hexagonally ordered microtubule arrays (Fig. 1d), the studies could not discern whether bundles result from the presence of Tau-mediated attractions or due to a repulsive lattice under confinement. Synchrotron SAXS studies under osmotic pressure allows us to map out the energy landscape of Tau-mediated, GTP-dependent active microtubule bundles at 37 °C. SAXS and TEM reveal widely spaced bundles (energy minimum at microtubule wall-to-wall distance Dw–wE25–41 nm) with hexagonal and string-like symmetry, the latter mimicking bundles found in the AIS. We find that contrary to current dogma, the CTT alone is able to mediate relatively wide spacings in the absence of the NTT

Methods

Results

Conclusion